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GENERAL  ZOOLOGY 


VILLEE  -  WALKER  —  SMIT 


The  authors  call  your  attention  to  these  special  features 

of  GENERAL  ZOOLOGY 

1.  As  an  introduction  to  the  discussion  of  both  invertebrate  and 
vertebrate  types  there  is  a  chapter  on  the  principles  of  compara- 
tive physiology  (pp.  78-112).  This  points  up  the  similarities  in 
the  problems  w'hich  both  types  of  animals  have  had  to  solve  to 
survive.  Although  they  differ  widely  in  structure  they  have  many 
functions  in  common. 

2.  The  evolutionary  origins  of  the  lower  invertebrates  and  their 
relationships  to  higher  animals  are  discussed  in  an  excellent 
chapter  (pp.  236-243) .  This  includes  a  discussion  of  spiral  cleav- 
age and  certain  larval  types  of  evolutionary  interest. 

3.  There  is  a  unique  chapter  on  the  physiology  and  beha\ior  of 
arthropods    (pp.  326-350). 

4.  The  discussion  of  vertebrates  opens  with  a  presentation  of  the 
anatomy  and  physiology  of  the  frog  as  a  representative  vertebrate 
(pp.  393-423) . 

5.  There  are  three  chapters  on  the  evolution  of  the  se\'eral  classes 
of  vertebrates.  Following  these  chapters  the  structure,  function 
and  development  of  each  organ  system  of  the  vertebrates  are  con- 
sidered in  detail. 

6.  The  chapters  on  genetics  explain  not  only  the  simple  aspects  of 
heredity,  but  also  such  interesting  topics  as  population  genetics 
(p.  681)    and  biochemical  genetics    (p.  683) . 

7.  There  is  a  critical  discussion  of  the  theories  of  the  origin  of  life 
(pp.  710-713). 

8.  A  discussion  of  ecological  principles  and  their  practical  implica- 
tions, such  as  conservation,  makes  up  the  final  section,  four  chap- 
ters, of  the  book. 

9.  The  line  drawings  were  especially  prepared  for  this  book.  The 
illustrations  of  animals  and  their  parts  are  realistic  and  generally 
include  the  outline  of  whole  organisms  for  orientation.  There  has 
been  a  conscious  attempt  to  provide  uncluttered  pictures  that  are 
clear  without  distorting  the  material  described. 

A  laboratory  manual  well  suited  in  organization  and 
content  for  use  with  this  book  is 

LABORATORY  EXERCISES  IN  GENERAL  ZOOLOGY  by 
FISHER  AND  KITZMILLER  (W.  B.  Saunders  Co.,  1958) 


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K 


CLAUDE  A.  VILLEE 

Harvard  University 

WARREN  F.  WALKER,  Jr. 

Oberlin  College 

FREDERICK   E.  SMITH 

University  of  Michigan 


GENERAL  ZOOLOGY 


W.   B.   SAUNDERS   COMPANY 

Philadelphia  and  London 

1958 


©  1958,  by  W.  B.  Saunders  Company 

Copyright  under  the  Interriational  Copyright  Union 
All  Rights  Reserved.     This  book  is  protected  by  copy- 
right. Xo  part  of  it  may   be  duplicated  or  reproduced 
in    any    manner   without    written    permission    from    the 
publisher. 

Made  in  the  United  States  of  America 
Press  of  IT'.  B.  Saunders  Company 
Library  of  Congress  Catalog  Card  Nwnher:  5S-6400 


PREFACE 


The  field  of  Zoology,  along  with  all  of  the  biological  sciences,  has 
grown  enormously  in  the  last  few  decades.  To  deal  with  this  vast  array  of 
knowledge  some  courses  are  based  upon  a  thorough  examination  of 
certain  representative  animals.  Other  courses  are  centered  around  dis- 
cussions of  broad  biological  principles.  Each  of  these  has  obvious  merits 
and  we  have  tried  in  writing  this  text  to  blend  the  two.  Neither  method 
can  be  carried  to  extreme,  for  one  cannot  hope  to  teach  principles  without 
concrete  examples,  nor  can  one  teach  animal  types  without  the  synthesis 
provided  by  an  understanding  of  principles. 

The  special  task  of  anyone  writing  a  textbook  is  to  select  with  care 
the  topics  to  be  discussed  so  as  to  present  a  clear  picture  of  the  subject 
without  giving  an  overwhelming  mass  of  detail.  This  text  probably  in- 
cludes some  material  that  the  instructor  will  have  neither  the  time  nor 
the  inclination  to  consider  in  his  course.  Each  instructor,  of  course,  em- 
phasizes those  topics  he  considers  most  important;  the  text  provides  the 
interested  student  with  an  opportunity  to  read  about  subjects  which  may 
be  omitted  or  considered  only  briefly  in  the  lectures  and  laboratory 
exercises.  In  discussing  the  many  subjects  which  comprise  modern  zoology 
we  have  tried  to  distinguish  between  fact  and  theory  and  to  cite  some  of 
the  problems  that  remain  for  future  zoologists  to  solve.  The  conclusions 
presented  and  the  inferences  drawn  represent,  to  the  best  of  our  knowl- 
edge and  ability,  the  current  interpretation  of  the  relevant  observations 
and  experiments. 

The  introductory  chapter  describes  Zoology  and  its  sub-sciences, 
scientific  method,  and  the  sources  of  scientific  knowledge.  The  general 
concepts  basic  to  a  study  of  the  form  and  function  of  both  vertebrate  and 
invertebrate  animals  are  presented  in  Part  One.  Chapters  2  and  3  provide 
some  of  the  chemical  and  physical  background  for  an  understanding  of 
protoplasm,  cells  and  tissues.  The  chemistry  and  physics  are  not  discussed 
separately  but  are  introduced  as  needed  to  understand  the  biological 
material  being  presented.  The  nature  of  enzymes  and  their  role  in 
cellular  physiology  is  discussed  in  Chapter  4.  Vertebrate  and  invertebrate 
animals  have  had  to  solve  the  same  major  problems  in  order  to  survive, 
and  an  examination  of  their  physiological  mechanisms  shows  that  they 
have  much  in  common.  The  principles  of  nutrition,  digestion,  circulation, 
respiration,  excretion,  protection,  sensation,  locomotion,  irritability,  and 
integration  are  discussed  in  Chapter  5  to  provide  a  general  background 
for  the  discussions  of  the  animal  types  which  follow.  The  principles  of 

iii 


yi  PREFACE 

\quarium,  The  Smithsonian  Institution,  ^Vard's  Natural  Science  Estab- 
lishment, Williams  and  AVilkins  Company,  and  the  United  States  Armv. 
Our  special  thanks  are  due  to  members  of  the  staff  of  the  W.  B. 
Saunders  Company  Avho  gaye  us  assistance  and  encouragement  durmg  the 
long  months  of  writing.  Finally  we  want  to  express  our  thanks  to  Miss 
Ann  deNisco,  Janet  Loring  and   Mrs.   Barbara  Waller  who  helped   m 

reading  proof  and  preparing  the  index. 

Claude  A.  Villee 

Warren  F.  Walker,  Jr. 

Frederick  E.  Smith 

March,  1958 


CONTENTS 


Chapter  1 

INTRODUCTION    1 

1.  Zoology  and  Its  Subsciences 1 

2.  The  Scientific  Method 3 

3.  History  of  Zoology 7 

4.  Applications  of  Zoology   12 


Part  I.        General   Concepts 

Chapter  2 

PROTOPLASM     13 

5.  Characteristics  of  Living  Things 14 

6.  Protoplasm 16 

7.  Chemical  Composition  of  Protoplasm   19 

8.  Organic  Compounds  of  Biological  Importance  .  24 

9.  Physical  Characteristics  of  Protoplasm 30 

Chapter  3 

CELLS  AND  TISSUES  33 

10.  The  Cell  and  Its  Contents 33 

11.  Mitosis 39 

12.  The  Study  of  Cellular  Activities 44 

1 3.  Energy 47 

14.  Molecular  Motion 48 

15.  Diffusion     49 

16.  Exchanges  of  Material  between  Cell  and 

Environment  50 

17.  Tissues    ^^ 

18.  Body  Plan  and  Symmetry 62 

vii 


73802 


Viii  CONTENTS 

Chapter  4 

CELL  METABOLISM    64 

19.  Chemical  Reactions 64 

20.  Enzymes    66 

21.  Factors  Affecting  Enzyme  Activity   69 

22.  Respiration  and  Cellular  Energy   71 

23.  The  Dynamic  State  of  Protoplasm 74 

24.  Special  Types  of  Metabolism 75 

Chapter  5 

PRINCIPLES  OF  PHYSIOLOGY 78 

25.  Types  of  Nutrition   78 

26.  Ingestion,  Digestion  and  Absorption 79 

27.  Circulation     84 

28.  Respiration    87 

29.  The  Elimination  of  \Vastes  Other  than 

Carbon  Dioxide   92 

30.  Protection    95 

31.  Motion    98 

32.  Irritability  and  Response 103 

Chapter  6 

REPRODUCTION 114 

33.  Asexual  Reproduction 115 

34.  Sexual  Reproduction 116 

35.  Reproductive  Systems    122 

36.  Fertilization 123 

37.  Embryonic  Development 126 

38.  Protection  of  the  Embryo 131 

39.  The  Control  of  Development 133 


Part  II.        The  Animal   Kingdom 

Chapter  7 

THE  PRINCIPLES  OF  TAXONOMY 139 

40.  The  Science  of  Taxonomy  139 

41.  The  Binomial  System   140 

42.  Higher  Categories    141 

43.  Uses  of  Taxonomy 141 


CONTENTS  ix 

44.  Definitions   142 

45.  The  History  of  Taxonomy 143 

Chapter  8 

THE  PHYLUM  PROTOZOA 148 

46.  Introduction    148 

47.  Organelles    149 

48.  Class  Flagellata 152 

49.  Class  Sarcodina 157 

50.  Class  Ciliata 160 

51.  Class  Suctoria   165 

52.  Class  Sporozoa 165 

53.  Reproduction  in  the  Protozoa 166 

54.  Relationships  among  the  Protozoa 169 

Chapter  9 

THE  PHYLUM  PORIFER.\ 172 

55.  Introduction    172 

56.  General  Characteristics 172 

57.  The  Classes  of  Sponges 175 

58.  Reproduction    178 

Chapter  10 

THE  PHYL.\  COELEXl  ERATA  AND  CTEXOPHORA   181 

59.  Introduction    181 

60.  Gonionemus:  General  Behavior 181 

61.  Gonionemus:  Feeding  and  Digestion 184 

62.  Gonionemus:  Diffusion 186 

63.  Gonionemus:  Nervous  System 187 

64.  Gonionemus:  Reproduction     188 

65.  Classes  of  the  Phylum  Coelenterata 189 

66.  Class  Hydrozoa  190 

67.  Class  Scyphozoa    192 

68.  Class  Anthozoa   195 

69.  FreshAVater  Coelenterates:  Hydra   198 

70.  The  Phylum  Ctenophora   199 

71.  The  Regulation  of  Form 201 

Chapter  11 

THE  PHYLUM  PLATYHELMINTHES  204 

72.  Dugesia:  Habitat  and  Appearance 204 

73.  Dugesia:  Feeding  and  Digestion 204 


X  CONTENTS 

74.  Dugesia:  Sensation  and  Movement   207 

75.  Dugesia:  Water  Balance  and  Excretion 208 

76.  Dugesia:  Reproduction 209 

77.  Dugesia:  Regeneration  and  Polarity 211 

78.  Class  Turbellaria   212 

79.  Class  Trematoda    213 

80.  Class  Cestoda 216 

Chapter  12 

THE   PHYLA  ASCHELAHNTHES  AND   NEMERTEA    220 

81.  Classification  of  the  Aschelminthes 220 

82.  Class  Rotifera   222 

83.  Philodina   222 

84.  Reproduction  in  Rotifers 225 

85.  Cell  Constancy   225 

86.  Senescence    226 

87.  Resistance  to  Desiccation   226 

88.  Class  Nematoda    227 

89.  The  \'inegar  Eel.  Turbatrix  aceti 228 

90.  The  Pig  Roundworm,  Ascaris  lumhricoides  ....  229 

91.  Molting    231 

92.  Parasitism     231 

93.  Class  Gastrotricha    231 

94.  Class  Kinorhyncha  231 

95.  Class  Gordiacea    232 

96.  Class  Acanthocephala    232 

97.  Phylum  Nemertea 232 

Chapter  13 

INTRODUCTION  TO  THE  HIGHER  IN\'ERTEBRATES   ....  236 

98.  Evolutionary  Relationships  of  the  Sponges   ....  236 

99.  Evolutionary  Relationships  of  the  Coelenterates  .  237 

100.  The  Evolution  of  Three  Germ  Layers 237 

101.  The  Evolution  of  the  Coelom 238 

102.  Spiral  Cleavage  and  Its  Evolutionary  Importance  239 

103.  The  Schizocoelomata  and  Enterocoelomata   ....  241 

Chapter  14 

THE  PHYLUM  MOLLUSCA    244 

104.  General  Features  of  the  Molluscs 244 

105.  Class  Amphineura    246 

106.  Class  Gastropoda:  General  Features 247 


CONTENTS  xi 

107.  Busycon    248 

108.  Other  Gastropods 251 

109.  Class  Pelecypoda:  General  Features 252 

1 10.  Venus  mercenaria 253 

111.  Other  Pelecypoda 256 

1 12.  Class  Scaphopoda  258 

113.  Class  Cephalopoda:  General  Features 259 

1 14.  Loliga   260 

1 15.  Other  Cephalopods   265 

Chapter  15 
PHYLUM  ANNELIDA 267 

116.  General  Features  of  the  Annelid  Worms 267 

117.  Classification  of  the  Phylum   268 

118.  Nereis  3.n(\  Lumbricus:  Habitat  and  Habit    ....  270 

119.  Xereis  2Lnd  Litmbricus:  External  Morphology   ..  270 

120.  Xereis  and  Lumbricus:  Body  Wall 273 

121.  Nereis  2ind  Lumbricus:  Nervous  System    274 

122.  Nereis  2ind  Lumbricus:  Digestive   System    275 

123.  Nereis  SiTid  Lumbricus:  Circulatory  System 276 

124.  Nereis  a.nd  Lumbricus:  Excretory  System   277 

125.  Nereis  and  Lumbricus:  Reproduction    277 

126.  Reproductive  Periodicity  and  Palolo  Worms  .  .  .  279 

127.  Earthworms  and  the  Soil   280 

128.  Other  .\nnelid  W^orms 281 

129.  Class  Hirudinea    281 

130.  The  Relationships  of  Annelids,  Molluscs  and 

Arthropods 283 

131.  The  Trochophore  Larva 286 

Chapter  16 
PHYLUM  ARTHROPODA    289 

132.  Classification  of  the  Phylum   289 

133.  Class  Crustacea   292 

134.  Astacus,  a  Crayfish 293 

135.  External  Morphology  of  the  Crayfish 293 

136.  Internal  Anatomy  of  the  Crayfish 297 

137.  Daphnia,  the  Water-Flea   300 

138.  Other  Crustaceans   303 

139.  The  Subphylum  Labiata 305 

140.  Periplaneta  americana,  a  Cockroach  .  .  .  : 307 

141.  External  Morphology  of  the  Cockroach 307 

142.  Internal  Anatomy  of  the  Cockroach 309 


Xii  CONTENTS 

143.  Classification  of  the  Insecta  313 

144.  Metamorphosis    316 

145.  Apis  mellifera,  the  Honeybee 317 

146.  The  Subphylum  Arachnomorpha 320 

147.  Argiope,  an  Orb  Spider 322 

148.  The  Phylum  Onycophora 323 

Chapter  17 

PHYSIOLOGY  AND  BEHAVIOR  OF  THE  ARTHROPODA   ...  326 

149.  Molting 326 

150.  Arthropod  Hormones    328 

151.  Patterns  of  Muscular  Innervation 332 

152.  The  Flight  Mechanism  in  Insects 334 

153.  Vision   ' 336 

154.  Behavior 340 

155.  Social  Mechanisms  in  Insects 344 

156.  Bee  Language   347 

Chapter  18 

MINOR   PHYLA    352 

157.  Mesozoa    352 

158.  Entoprocta 353 

159.  Sipunculoids  and  Echiuroids   353 

160.  The  Priapuloids  354 

161.  The  Phoronids  and  Brachiopods 355 

162.  The  Bryozoa   356 

163.  The  Chaetognatha 357 

Chapter  19 

THE  PHYLA  HEMICHORDATA  AND  ECHINODERM.\TA  ...  360 

164.  The  Phylum  Hemichordata   360 

165.  Classification  of  the  Phylum  Echinodermata   .  .  .  364 

166.  Asterias  forhesi,  a  Typical  Five-rayed  Starfish  .  .  364 

167.  Class  Asteroidea,   the  Starfish   370 

168.  Class  Crinoidea,  the  Sea  Lilies 370 

169.  Class  Holothuroidea,  the  Sea  Cucumbers    372 

170.  Class  Echinoidea,  the  Sea  Urchins,  Heart 

Urchins  and  Sand  Dollars   373 

171.  Class  Ophiuroidea,  the  Brittle  Stars   375 

172.  Relationships  among  Echinoderm  Classes 375 

173.  Relationships  among  the  Hemichordata, 

Echinodermata,  and  Other  Phyla 377 


CONTENTS  xiii 

Chapter  20 

THE  CHORDATES   383 

174.  Chordate  Characteristics 383 

175.  Subphylum  Urochordata 384 

176.  Subphylum   Cephalochordata    387 

177.  Subphylum  Vertebrata    389 

178.  The  Origin  of  Chordates 391 


Part  III.        The  Vertebrate  Life  and   Organization 

Chapter  21 
THE  FROG-A  REPRESENTATIVE  VERTEBRATE 393 

179.  Frogs  and  Other  .Amphibians 393 

180.  External  Features 394 

181.  Skin  and  Coloration 395 

182.  Skeleton    397 

183.  Muscular  System 401 

184.  Body  Cavity  and  Mesenteries 402 

185.  Digestive  System  404 

186.  Respiratory  System 406 

187.  Circulatory  System   408 

188.  Excretory  System    411 

189.  Reproductive  System   412 

190.  Sense  Organs 414 

191.  Nervous  System 415 

192.  Endocrine  Glands    419 

193.  Life  Cycle 420 

Chapter  22 
A  HISTORY  OF  VERTEBRATES:  FISHES  424 

194.  Methods  of  Determining  the  History  of  Animals.  424 

195.  Vertebrate  Beginnings 427 

196.  Living  Jawless  Vertebrates   429 

197.  Jaws  and  Paired  Appendages   431 

198.  Characteristics  of  Cartilaginous  Fishes   433 

199.  Evolution  of  Cartilaginous  Fishes 436 

200.  Lungs  and  Swim  Bladders   437 

201.  Evolution  of  Bony  Fishes 201 


Xi\'  CONTENTS 

Chapter  25 

A  HISTORY  OF  VERTEBRATES:  AMPHIBIANS  AND 

REPTILES  446 

202.  The  Transition  from  Water  to  Land 446 

203.  Evolution  and  Characteristics  of  Amphibians   .  .  447 

204.  Amphibian  Adaptations   448 

205.  C^haracteristics  of  Reptiles 453 

206.  Evokition  and  Adaptations  of  Reptiles 456 

Chapter  24 

A  HISTORY  OF  \  ERTEBRATES:  BIRDS  AND  MAMMALS  ...  468 

207.  Principles  of  Flight   468 

208.  Structure  of  Birds   471 

209.  The  Origin  and  Evolution  of  Birds 478 

210.  The  Bird  Way  of  Life  481 

211.  Characteristics  of  Mammals 486 

212.  Primitive  Mammals 491 

213.  Adapti\e  Radiation  of  Eutherians 492 

Chapter  25 

PROTECTION,  SUPPORT  AND  MOVEMENT    502 

214.  The  Integument 502 

215.  The  Skeleton 506 

216.  Muscles 512 

Chapter  26 

DIGESTION   AND   RESPIRATION    515 

217.  The  xMouth  515 

218.  The  Pharynx  and  Esophagus 518 

219.  The  Stomach 519 

220.  The  Liver  and  Pancreas 520 

221.  The   Intestine   521 

222.  The  Control  of  Digestive  Secretions   524 

223.  Use  of  Absorbed  Materials 525 

224.  Respiratory  Membranes   528 

225.  The  Respiratory  System  of  Fishes 529 

226.  The  Respiratory  System  of  Terrestrial 

Vertebrates    531 

227.  The  Mechanics  and  Control  of  Breathing 533 


CONTENTS  XV 

Chapter  27 
BLOOD  AND  CIRCULATION    537 

228.  Blood  Plasma    538 

229.  Red  Blood  Cells 539 

230.  Platelets  and  Blood  Clotting 541 

231.  White  Blood  Cells 542 

232.  Immunity 542 

233.  Blood  Groups  544 

234.  The  Rh  Factor 545 

235.  Patterns  of  Circulation   545 

236.  The  Fetal  Circulation 549 

237.  Flow  of  Blood  and  Lymph 551 

Chapter  28 

THE  UROGENITAL  SYSTEM-EXCRETION  AND 

REPRODUCTION    559 

238.  Evolution  of  the  Kidneys  and  Their  Ducts 559 

239.  The  Nephron  and  Its  Function 562 

240.  The  Gonads 566 

241.  Reproductive   Passages    568 

242.  Mammalian  Reproduction   571 

Chapter  29 
SENSE  ORGANS  AND  NERVOUS  COORDINATION   574 

243.  The  Eye  576 

244.  The  Lateral  Line  and  Ear 581 

245.  Organization  of  the  Nervous  System 585 

246.  Peripheral  Nervous  System 591 

247.  Central  Nervous  System  596 

Chapter  30 
THE  ENDOCRINE  SYSTEM 605 

248.  Methods  of  Investigating  Endocrines 606 

249.  The  Thyroid 608 

250.  The  Parathyroid  Glands 614 

251.  The  Islet  Cells  of  the  Pancreas 615 

252.  The  Adrenal  Glands 616 

253.  The  Pituitary  Gland 620 

254.  The  Testis  627 

255.  The  Ovaries    628 

256.  Estrous  and  Menstrual  Cycles 631 

257.  The  Hormones  of  Pregnancy 633 


XVI 


CONTENTS 


258.  Other  Endocrine  Glands 634 

259.  Endocrine  Interrelationships  635 

Chapter  31 
THE  DE\'ELOPMEXT  OF  MAMMALS 637 

260.  Early  Stages  of  Mammalian  Development   637 

261.  Formation  of  the  Xotochord  and  Neural  Tube  .  641 

262.  The  Digestive  Tract  and  Its  Derivatives 642 

263.  Differentiation  of  the  Mesoderm   643 

264.  Growth  of  the  Embryo 646 

265.  Twinning 646 


Part   IV.        Genetics  and   Evolution 

Chapter  32 

PRINCIPLES  OF  HEREDITY    649 

266.  History  of  Genetics   649 

267.  Mendel's  Discoveries   650 

268.  Chromosomal  Basis  of  the  Laws  of  Heredity  .  .  .  652 

269.  Allelomorphs 652 

270.  A  Monohybrid  Cross   653 

271.  Laws  of  Probability 655 

272.  Test  Crosses 655 

273.  Incomplete  Dominance 656 

274.  A  Dihybrid  Cross  656 

275.  Problem  Solving 658 

276.  The  Genetic  Determination  of  Sex 660 

277.  Sex-Linked  Characteristics   662 

278.  Linkage  and  Crossing  Over  663 

279.  Chromosome  Maps 666 

Chapter  33 

GENETICS    669 

280.  The  Interactions  of  Genes 669 

281.  Multiple  Factors 674 

282.  Multiple  Alleles   677 

283.  Lethal  Genes 679 

284.  Penetrance  and  Expressivity  of  Genes 680 

285.  Inbreeding  and  Outbreeding 680 

286.  Population  Genetics    681 

287.  Biochemical  Genetics    683 


CONTENTS  xVli 

288.  Changes  in  Genes:  Mutations   685 

289.  Gene  Action    685 

290.  Cytoplasmic  Inheritance 689 

291.  Inheritance  of  Acquired  Characters 690 

292.  Human  Inheritance  691 

293.  Heredity  and  Environment   692 

294.  Medical  Genetics    693 

Chapter  34 
THE  CONCEPT  OF  EVOLUTION   695 

295.  The  Principle  of  Organic  Evolution 695 

296.  Development  of  Ideas  about  Evolution   696 

297.  Background  for  The  Origin  of  Species 698 

298.  The  Theory  of  Natural  Selection 699 

299.  Modern  Changes  in  the  Theory  of  Natural 

Selection 700 

300.  Genetic  Drift 703 

301.  Preadaptation    703 

302.  Mutations,  the  Raw  Material  of  Evolution   ....  704 

303.  Straight-Line  Evolution    707 

304.  The  Origin  of  Species  by  Hybridization 709 

305.  The  Origin  of  Life 710 

306.  Principles  of  Evolution 713 

Chapter  35 
THE  EVIDENCE  FOR  EVOLUTION    716 

307.  The  Fossil  Evidence 716 

308.  The  Geologic  Time  Table 717 

309.  1  he  Geologic  Eras 720 

310.  The  Evidence  from  Taxonomy 726 

311.  The  Evidence  from  Anatomy   727 

312.  Evidence  from  Comparative  Physiology  and 

Biochemistry    728 

3 1 3.  Evidence  from  Embryology 729 

314.  Evidence  from  Genetics  and  Cytology 732 

315.  Evidence   from   the  Geographic   Distribution   of 

Organisms    733 

316.  The  Biogeographical  Realms 735 

Chapter  36 
THE  EVOLUTION  OF  MAN  738 

317.  Primate  Evolution   738 

318.  The  Lemurs    738 


XViii  CONTENTS 


319.  The  Tarsioids 739 

320.  The  Anthropoids   740 

321.  The  Modern  Great  Apes 741 

322.  The  Man  Apes 743 

323.  Fossil  Ape  Men 744 

324.  Modern  Man  (Homo  sapiens)   749 

325.  Cultural  Evolution 751 


Part  V.        Animals  and  Their  Environment 

Chapter  37 

ECOLOGY  753 

326.  Ecosystems    753 

327.  Habitat  and  Ecologic  Niche 755 

328.  The  Cyclic  Use  of  Matter 755 

329.  The  Carbon  Cycle  756 

330.  The  Nitrogen  Cycle 757 

331.  The  Water  Cycle  758 

332.  Mineral  Cycles   758 

333.  The  Energy  Cycle 759 

334.  Physical  Factors  in  the  Environment 759 

335.  Types  of  Interactions  between  Species 763 

336.  Competition    763 

337.  Commensalism    764 

338.  Protocooperation    764 

339.  Mutualism   765 

340.  Amensalism    765 

341.  Parasitism  and  Predation   765 

342.  Intraspecific  Relations 766 

343.  Food  Chains    767 

344.  Communities  and  Populations    768 

345.  Populations  and  Their  Characteristics   769 

346.  Population  Cycles 773 

347.  Population  Dispersal  775 

348.  Biotic  Communities   775 

349.  Community  Succession    777 

350.  The  Dynamic  Balance  of  Nature 779 

Chapter  38 

THE  ADAPTATION  OF  ANIMALS  TO  THE  ENVIRONMENT  781 

351.  Adaptive  Radiation   782 

352.  Convergent  Evolution   783 


CONTENTS  xix 

353.  Structural  Adaptations   784 

354.  Physiologic  and  Chemical  Adaptations 784 

355.  Color  Adaptations   785 

356.  Adaptations  of  Species  to  Species 787 

357.  The  Distribution  of  Animals 787 

358.  Terrestrial  Life  Zones 789 

359.  Marine  Life  Zones  794 

360.  Fresh-Water  Life  Zones 797 

Chapter  39 
PARASITISM    799 

361.  Origin  of  Parasitism 799 

362.  Ectoparasites    802 

363.  Parasites  of  the  Digestive  Tract   806 

364.  Parasites  in  Body  Tissues   808 

365.  Intracellular  Parasites 813 

366.  Adaptations  to  Parasitism 816 

367.  Host  Specificity 819 

368.  Social  Parasites  820 

Chapter  40 
CONSERVATION   822 

369.  .\griculture    822 

370.  Forestry    824 

371.  Wildlife    824 

372.  Marine   Fisheries    826 

373.  Public  Health  828 

374.  Human  Ecology   829 


Appendix 
A  SYNOPSIS  OF  THE  ANIMAL  KINGDOM   831 

BIBLIOGRAPHY 843 


INDEX 


849 


CHAPTER   1 


Introduction 


1.        Zoology  and  Its  Subsciences 

Zoology  is  one  ot  the  biological  sciences,  the  one  dealing  with  the 
many  different  aspects  of  animal  life.  Since  a  "zoo"  is  a  collection  of  ani- 
mals, one  could  easily  guess  that  "zoology"  dealt  with  animals.  A  visit  to 
a  zoo,  interesting  though  it  is,  can  barely  begin  to  suggest  the  enormous 
variety  of  animals  that  are  living  today  (there  are  about  one  million  dif- 
ferent kinds  of  animals!).  In  addition  to  these  there  are  a  host  of  other 
kinds  of  animals  that  have  lived  in  past  ages  but  are  now  extinct. 

Modern  zoology  concerns  itself  with  much  more  than  the  simple 
recognition  and  classification  of  the  many  kinds  of  animals.  It  includes 
the  study  of  the  structure,  function  and  embryonic  development  of 
each  part  of  an  animal's  body;  of  the  nutrition,  health  and  behavior  of 
animals;  of  their  heredity  and  evolution;  and  of  their  relations  to  the 
physical  environment  and  to  the  plants  and  other  animals  of  that  region. 

At  the  present  time  enough  facts  about  animals  and  their  ways  are 
known  to  fill  a  whole  library  of  books,  and  more  information  appears 
every  year  from  the  intensive  researches  of  zoologists  in  the  field  and  in 
the  laboratory.  No  zoologist  today  can  know  more  than  a  small  fraction 
of  this  enormous  body  of  knowledge.  Zoology  is  now  much  too  broad 
a  subject  to  be  treated  thoroughly  in  a  single  textbook  or  to  be  encom- 
passed by  a  single  scientist.  Most  zoologists  are  specialists  in  some  limited 
phase  of  the  subject— in  one  of  the  subdivisions  of  zoology.  The  sciences 
of  anatomy,  physiology  and  embryology  deal  with  the  structure,  func- 
tion and  development,  respectively,  of  an  animal.  Each  of  these  may  be 
further  subdivided  according  to  the  kind  of  animal  investigated,  e.g., 
invertebrate  physiology,  arthropod  physiology,  insect  physiology  or  com- 
parative physiology.  Parasitology  deals  with  those  forms  of  life  that  live 
in  or  on  and  at  the  expense  of  other  organisms.  Cytology  is  concerned 
with  the  structure,  composition  and  function  of  cells  and  their  parts, 
and  histology  is  the  science  of  the  structure,  function  and  composition 
of  tissues.  The  science  of  genetics  investigates  the  mode  of  transmission 
of  characteristics  from  one  generation  to  the  next  and  is  closely  related 
to  the  science  of  evolution,  which  studies  the  way  in  which  new  species 
of  animals  arise  and  how  the  present  kinds  of  animals  are  related  by 
descent  to  previous  animals.  The  study  of  the  classification  of  organisms, 
both  animals  and  plants,  is  called  taxonomy.  One  of  the  newest  biologi- 

1 


2  INTRODUCTION 

cal  sciences  is  ecology,  the  study  of  the  relations  of  a  group  of  organisms 
to  its  environment,  including  both  the  physical  factors  and  the  other 
forms  of  life  which  provide  food  or  shelter  for  it,  compete  with  it  in 
some  way,  or  prey  upon  it. 

Some  zoologists  specialize  in  the  study  of  one  group  of  animals. 
There  are  mammalogists,  ornithologists,  herpetologists  and  ichthyolo- 
gists who  study  mammals,  birds,  reptiles  and  amphibians,  and  fishes, 
respectively;  entomologists,  who  investigate  insects;  protozoologists,  who 
study  the  single-celled  animals,  and  so  on. 

The  science  of  zoology  thus  includes  both  a  tremendous  body  of 
facts  and  theories  about  animals  and  the  means  for  learning  more.  The 
ultimate  source  of  each  fact  is  in  some  carefully  controlled  observation 
or  experiment  made  by  a  zoologist.  In  earlier  times,  some  scientists  kept 
their  discoveries  to  themselves,  but  there  is  now  a  strong  tradition  that 
scientific  discoveries  are  public  property  and  should  be  freely  published. 
In  a  scientific  publication  a  man  must  do  more  than  simply  say  that 
he  has  made  some  particular  discovery;  he  must  give  all  of  the  relevant 
details  of  the  means  by  which  the  discovery  was  made  so  that  others  can 
repeat  the  observation.  It  is  this  criterion  of  repeatability  that  makes 
us  accept  a  certain  observation  or  experiment  as  representing  a  true 
fact;  observations  that  cannot  be  repeated  by  competent  investigators 
are  discarded. 

When  a  scientist  has  made  some  new  observation,  or  carried  out  a 
series  of  experiments  that  add  to  our  knowledge  in  a  field,  he  writes  a 
report,  called  a  "paper,"  in  which  he  describes  his  methods  in  sufficient 
detail  so  that  another  worker  can  repeat  them,  gives  the  results  of  his 
observations,  discusses  the  conclusions  to  be  drawn  from  them,  perhaps 
formulates  a  theory  to  explain  them  or  discusses  how  they  are  explained 
by  a  previous  theory,  and  finally  indicates  the  place  of  these  new  facts 
in  their  particular  field  of  science.  The  knowledge  that  his  discovery  will 
be  subjected  to  the  keen  scrutiny  of  his  colleagues  is  a  strong  stimulus 
for  repeating  the  observations  or  experiments  carefully  before  publish- 
ing them.  He  then  submits  his  paper  for  publication  in  one  of  the 
professional  journals  in  the  particular  field  of  his  discovery.  There  are 
several  thousand  zoological  journals  published  all  over  the  world.  Some 
of  the  more  important  American  ones  are  the  Journal  of  Experimental 
Zoology,  Journal  of  Cellular  and  Comparative  Physiology,  Biological 
Bulletiyi,  Physiological  Zoology,  American  Journal  of  Physiology,  Ana- 
tomical Record,  Ecology  and  the  journals  devoted  to  research  on  a 
particular  group  of  animals,  such  as  the  Journal  of  Mammalogy.  The 
paper  is  read  by  one  or  more  of  the  board  of  editors  of  the  journal,  all 
of  whom  are  experts  in  the  field.  If  it  is  approved,  it  is  published  and 
becomes  part  of  "the  literature"  of  the  subject. 

At  one  time,  when  there  were  fewer  journals,  it  might  have  been 
possible  for  one  man  to  read  them  each  month  as  they  appeared,  but 
this  is  obviously  impossible  now.  Journals  such  as  Biological  Abstracts 
assist  the  hard-pressed  zoologist  by  publishing,  classified  by  fields,  very 
short  summaries  or  abstracts  of  each  paper  published,  giving  the  facts 
found,  the  conclusion  reached,  and  an  exact  reference  to  the  journal  in 


INTRODUCTION  3 

which  the  full  report  appears.  A  considerable  number  of  journals 
devoted  solely  to  reviewing  the  newer  developments  in  particular  fields 
of  science  have  sprung  up  in  the  past  twenty-five  years;  some  of  these 
are  Physiological  Reviews,  Quarterly  Review  of  Biology,  Nutrition  Re- 
vieivs,  Annual  Review  of  Biochemistiy  and  Recent  Progress  in  Vitamins 
and  Hormones.  The  new  fact  or  theory  thus  becomes  widely  known 
through  publication  in  the  appropriate  professional  journal  and  by 
reference  in  abstract  and  review  journals  and  eventually  may  become  a 
sentence  or  two  in  a  textbook. 

The  professional  societies  of  zoologists  and  the  various  special 
branches  of  zoology  have  annual  meetings  at  which  new  discoveries  may 
be  reported.  Two  of  the  largest  annual  meetings  are  those  of  the  Ameri- 
can Institute  of  Biological  Sciences  and  the  Federation  of  American 
Societies  for  Experimental  Biology.  There  are,  in  addition,  national  and 
international  gatherings,  called  symposia,  of  specialists  in  a  given  field 
to  discuss  the  newer  findings  and  the  present  status  of  the  knowledge  in 
that  field.  For  example,  the  discussions  of  the  Cold  Spring  Harbor 
Symposia  in  Quantitative  Biology,  held  each  June  at  the  Long  Island 
Biological  Laboratory  in  Cold  Spring  Harbor,  are  published  and  provide 
an  excellent  review  of  some  particular  field.  A  different  subject  is  dis- 
ctxssed  each  year. 

2.       The  Scientific  Method 

The  ultimate  aim  of  each  science  is  to  reduce  the  apparent  complex- 
ity of  natural  phenomena  to  simple,  fimdamental  ideas  and  relations,  to 
discover  all  of  the  facts,  and  the  relationships  among  them.  The 
Danish  physicist  Niels  Bohr  puts  it  this  way,  "the  task  of  science  is  both 
to  extend  the  range  of  our  experience  and  to  reduce  it  to  order."  There 
is,  however,  no  single  "scientific  method,"  no  regular,  infallible  sequence 
of  events  which  will  reveal  scientific  truths.  Different  scientists  go  about 
their  work  in  different  ways.  George  Sarton,  in  the  Study  of  the  History 
of  Science,  points  out  that  "Even  as  all  kinds  of  men  are  needed  to  build 
up  a  community,  even  so  we  need  all  kinds  of  scientists  to  develop 
science  in  e\'ery  possible  direction.  Some  are  very  sharp  and  narrow- 
minded,  others  broad-minded  and  superficial.  Many  scientists,  like 
Hannibal,  know  how  to  conquer,  but  not  ho^\•  to  use  their  victories. 
Others  are  colonizers  rather  than  explorers.  Others  are  pedagogues. 
Others  want  to  measure  everything  more  accurately  than  it  was  measured 
before.  This  may  lead  them  to  the  making  of  fundamental  discoveries, 
or  they  may  fail,  and  be  looked  upon  as  insufferable  pedants." 

The  ultimate  source  of  all  the  facts  of  science  is  careful,  close 
observation  and  experiment,  free  of  bias  and  done  as  quantitatively  as 
possible.  The  observations  or  experiments  may  then  be  analyzed,  or 
simplified  into  their  constituent  parts,  so  that  some  sort  of  order  can  be 
brought  into  the  observed  phenomena.  Then  the  parts  can  be  reassem- 
bled and  their  interactions  made  clear.  On  the  basis  of  these  observa- 
tions, the  scientist  constructs  a  hypothesis,  a  trial  idea  about  the  nature 
of  the  observation,  or  about  the  connections  between  a  chain  of  events. 


4  INTRODUCTION 

or  even  about  cause  and  effect  relationships  between  different  events. 
It  is  in  this  abihty  to  see  through  a  mass  oi  data  and  construct  a  reason- 
able hypothesis  to  explain  their  relationships  that  scientists  differ  most. 

The  role  of  a  hypothesis  is  to  penetrate  beyond  the  immediate  data 
and  place  it  into  a  ne^v,  larger  context,  so  that  we  can  interpret  the 
unknown  in  terms  of  the  known.  There  is  no  sharp  distinction  between 
the  usage  of  the  words  "hypothesis"  and  "theory,"  but  the  latter  has, 
in  general,  the  connotation  of  greater  certainty  than  a  hypothesis.  A 
theory  is  a  conceptual  scheme  which  tries  to  explain  the  observed 
phenomena  and  the  relationships  between  them,  so  as  to  bring  into 
one  structme  the  observations  and  hypotheses  of  several  different  fields. 
The  theory  of  evolution,  for  example,  provides  a  conceptual  scheme  into 
which  fit  a  host  of  observations  and  hypotheses  from  paleontology, 
anatomy,  physiology,  biochemistry  and  other  sciences. 

A  good  theory  correlates  many  previously  separate  facts  into  a  logi- 
cal, easily  understood  framework.  The  theory,  by  arranging  the  facts 
properly,  suggests  new  relationships  between  the  individual  facts,  and 
suggests  further  experiments  or  observations  which  might  be  made  to 
test  these  relationships.  It  may  predict  new  phenomena  that  ^vill  be 
observed  under  certain  circumstances  and  finally  may  provide  the  solu- 
tion for  practical  problems.  A  good  theory  should  be  simple,  and  should 
not  require  a  separate  proviso  to  explain  each  fact;  it  should  be  flexible, 
able  to  grow  and  undergo  modifications  in  the  light  of  new  data.  A 
theory  is  not  discarded  because  of  the  existence  of  some  isolated  fact 
which  contradicts  it,  but  only  because  some  other  theory  is  better  able 
to  explain  all  of  the  known  data. 

Once  a  hypothesis  has  been  established,  the  rules  of  formal  logic 
can  be  applied  to  deduce  certain  consequences.  In  physics,  and  to  a 
lesser  extent  in  the  biological  sciences,  the  hypotheses  and  deductions  can 
be  stated  in  mathematical  terms,  and  far-reaching  conclusions  may  be 
deduced.  From  these  inferences,  one  can  predict  the  results  of  other 
observations  and  experiments.  Each  hypothesis  is  ultimately  kept, 
amended  or  discarded  on  the  basis  of  its  ability  to  make  valid  predic- 
tions. A  hypothesis  must  be  subject  to  some  sort  of  experimental  test— 
i.e.,  it  must  make  a  prediction  that  can  be  verified  in  some  way— or  it  is 
mere  speculation.  Conversely,  unless  a  prediction  follows  as  the  logical 
outgrowth  of  some  theory  it  is  no  more  than  a  guess. 

The  finding  of  results  contrary  to  those  predicted  by  the  hypothesis 
causes  the  investigator,  after  he  has  assured  himself  of  the  validity  of 
his  observation,  either  to  discard  the  hypothesis  or  to  change  it  to 
account  for  both  the  original  data  and  the  new  data.  Hypotheses  are 
constantly  being  refined  and  elaborated.  There  are  lew  scientists  who 
\\'ould  regard  any  hypothesis,  no  matter  ho^v  many  times  it  may  have 
been  tested,  as  a  statement  of  absolute  and  universal  truth.  It  is  rather 
regarded  as  the  best  available  approximation  to  the  truth  for  some 
finite  range  of  circumstances.  For  example,  the  Law  of  the  Conservation 
of  Matter  was  widelv  adhered  to  until  the  work  of  Einstein  showed 
that  it  had  to  be  modified  to  allow  for  the  possible  interconversion  of 
matter  and  energ)'. 


INTRODUCTION 


Ideally,  the  scientific  method  consists  of  making  careful  observations 
and  arranging  these  observations  so  as  to  bring  order  into  the  phe- 
nomena. Then  one  postulates  a  hypothesis  or  conceptual  scheme  which 
will  explain  the  facts  at  hand  and  make  predictions  about  the  results  of 
further  experiments  or  observations.  Sciences  differ  widely  in  the  extent 
to  which  prediction  is  possible,  and  the  biological  sciences  have  been 
held  bv  some  to  be  not  truly  "scientific,"  for  they  are  not  completely 
predictable.  However,  even  physics,  which  is  generally  regarded  as  the 
most  scientific  of  the  sciences,  is  far  from  completely  predictable. 

The  history  of  science  shows  that  although  many  scientists  have 
made  their  discoveries  by  folloAving  the  precepts  of  the  ideal  scientific 
method,  there  have  been  occasions  on  which  important  and  tar-reaching 
theories  have  resulted  from  making  incorrect  conclusions  from  erroneous 
postulates,  or  from  the  misinterpretation  of  an  improperly  controlled 
experiment!  There  are  instances  in  which,  in  retrospect,  it  seems  clear 
that  all  the  evidence  for  the  formulation  of  the  correct  theory  was  kno^vn, 
yet  no  scientist  put  the  proper  two  and  two  together.  And  there  are 
other  instances  in  which  scientists  have  been  able  to  establish  the  correct 
theory  despite  an  abundance  of  seemingly  contradictory  evidence. 

In  most  scientific  studies  one  of  the  ultimate  goals  is  to  explain  the 
cause  of  some  phenomenon,  but  the  hard-and-fast  proof  that  a  cause 
and  effect  relationship  exists  between  two  events  is  really  very  difficult 
to  obtain.  If  the  circumstances  leading  to  a  certain  event  always  have  a 
certain  factor  in  common  in  a  variety  of  cases,  that  factor  may  be  the 
cause  of  the  event.  The  difficulty,  of  course,  lies  in  making  sure  that  the 
factor  under  consideration  is  the  only  one  common  to  all  the  cases.  It 
would  be  wrong,  for  example,  to  conclude  from  the  observation  that 
drinking  Scotch  and  soda,  bourbon  and  soda,  and  rye  and  soda  all 
produce  intoxication,  that  soda  is  the  only  factor  in  common  and  there- 
fore is  the  cause  of  the  intoxication.  This  method  of  discovering  the 
common  factor  in  a  series  of  cases  that  may  be  the  cause  of  the  event 
(known  as  the  method  of  agreement)  can  seldom  be  used  as  a  valid 
proof  because  of  this  difficulty  of  being  sure  that  it  is  indeed  the  only 
common  factor.  The  snnple  observation  that  all  people  suffering  from 
beriberi  have  diets  ^\hich  are  low  in  thiamine  is  not  proof  that  a  defi- 
ciency of  this  vitamin  causes  the  disease,  for  there  may  be  many  other 
factors  in  common. 

Experiments  based  on  the  method  of  difference  provide  another  way 
of  elucidating  cause  and  effect  relations.  If  two  sets  of  circumstances 
differ  in  only  one  factor,  and  the  one  containing  the  factor  leads  to  an 
event  and  the  other  does  not,  the  factor  may  be  considered  the  cause 
of  the  event.  For  example,  if  t^vo  groups  of  rats  are  fed  diets  which  are 
identical  except  that  one  contains  all  the  vitamins  and  the  second  con- 
tains all  but  thiamine,  and  if  the  first  group  grows  normally  but  the 
second  fails  to  grow  and  ultimately  develops  polyneuritis,  this  would  be 
a  strong  suggestion  (but  would  not  be  acceptable  as  absolute  proof)  that 
polyneuritis,  or  beriberi  in  rats,  is  caused  by  a  deficiency  of  thiamine. 
By  using  an  inbred  strain  of  rats  that  are  as  alike  as  possible  in  inherited 
traits,   and  by  using  litter  mates    (brothers  and  sisters)    of  this  strain, 


6  INTRODUCTION 

one  could  make  certain  that  there  were  no  hereditary  differences  between 
the  controls  (the  ones  getting  the  complete  diet)  and  the  experimentals 
(the  ones  getting  the  thiamme-deficient  diet).  One  might  postulate  that 
the  thiamine-hee  diet  does  not  have  as  attracive  a  taste  as  the  one  with 
thiamine,  and  the  experimental  animals  simply  eat  less  food,  fail  to 
gro^v.  and  develop  the  deficiency  symptoms  because  they  are  partially 
starved.  This  source  of  error  can  be  avoided  by  "pair-feeding,"  by  pairing 
in  some  arbitrary  way  each  control  and  experimental  animal,  then 
weighing  the  food  eaten  each  day  by  each  experimental  animal  and 
giving  only  that  much  food  to  the  corresponding  control  member  of  the 
pair. 

One  of  the  more  useful  methods  of  detecting  cause  and  effect  rela- 
tionships is  the  method  of  concomitant  variation.  If  a  variation  in  the 
amount  of  one  gnen  factor  produces  a  parallel  variation  in  the  effect, 
the  factor  may  be  the  cause.  Thus,  if  several  gioups  of  rats  were  given 
diets  with  varying  amounts  of  thiamine,  and  if  the  amount  of  protection 
against  beriberi  varied  directly  with  the  amount  of  thiamine  in  the  diet, 
one  could  be  reasonably  sure  that  thiamine  deficiency  is  the  cause  of 
beriberi. 

It  must  be  emphasized  that  it  is  seldom  that  we  can  be  more  than 
"reasonably  sure"  that  X  is  the  cause  of  Y.  As  more  experiments  and 
observations  lead  to  the  same  result,  the  probability  increases  that  X  is 
the  cause  of  Y.  When  experiments  or  observations  can  be  made  quanti- 
tative, when  their  results  can  be  counted  or  measured  in  some  way,  the 
methods  of  statistical  analysis  provide  a  means  for  calculating  the  proba- 
bility that  Y  follows  X  simply  as  a  matter  of  chance.  Scientists  are  usu- 
ally satisfied  that  there  is  some  sort  of  cause  and  effect  relationship 
between  X  and  Y  if  they  can  sho^v  that  there  is  less  than  one  chance  in 
a  huntired  that  the  observed  X-Y  relationship  could  be  due  to  chance 
alone.  A  statistical  analysis  of  a  set  of  data  can  never  give  a  flat  yes  or 
no  to  a  question;  it  can  state  only  that  something  is  \ery  probable  or 
very  improbable.  It  can  also  tell  an  investigator  approximately  how 
many  more  times  he  must  repeat  the  experiment  to  shoA\  ^vith  a  given 
probability  that  Y  is  caused  by  X. 

The  proper  design  of  experiments  is  a  science  in  itself,  and  one  for 
which  only  general  rules  can  be  made.  In  all  experiments,  the  scientist 
must  ever  be  on  his  guard  against  bias  in  himself,  bias  m  the  suoject, 
bias  in  his  instrument  and  bias  in  the  wav  the  experiment  is  designed. 

Each  experiment  must  include  the  proper  control  group  (indeed 
some  experiments  require  several  kinds  of  control  groups)  .  The  control 
group  is  one  treated  exactly  like  the  experimental  group  in  all  respects 
but  one,  the  factor  whose  effect  is  Ijeins:  tested.  The  use  of  controls  in 
medical  experiments  raises  the  difficult  question  of  the  moral  justifica- 
tion of  \\ithholding  treatment  from  a  patient  \\"ho  might  be  benefited 
by  it.  If  there  is  sufficient  evidence  that  one  treatment  is  indeed  better 
than  another,  a  physician  -would  hardly  be  justified  in  further  experi- 
mentation. However,  the  medical  literature  is  full  of  treatments  no^v 
known  to  be  useless  or  even  detrimental,  which  were  used  for  man) 
years,  only  to  be  abandoned  finally  as  experience  showed  that  they  were 


INTRODUCTION  J 

ineffective  and  that  the  evidence  which  had  originally  suggested  their 
use  was  improperly  controlled.  There  is  a  time  in  the  development  of 
any  new  treatment  when  the  medical  profession  is  not  only  morally 
justified,  but  really  morally  required,  to  do  carefully  controlled  tests  on 
human  beings  to  be  sure  that  the  new  treatment  is  better  than  the  former 
one. 

In  medical  testing  it  is  not  sufficient  simply  to  give  a  treatment  to 
one  group  of  patients  and  not  to  give  it  to  another,  for  it  is  widely  known 
that  there  is  a  strong  psychologic  effect  in  simply  giving  a  treatment  of 
any  sort.  For  example,  a  group  of  students  in  a  large  western  university 
served  as  subjects  for  a  test  of  the  hypothesis  that  daily  doses  of  extra 
amounts  of  vitamin  C  might  help  prevent  colds.  This  giew-  out  of  the 
observation  that  people  who  drink  lots  of  fruit  juices  seem  to  have  fewer 
colds.  The  group  receiving  the  vitamin  C  showed  a  65  per  cent  reduc- 
tion in  the  number  of  colds  contracted  during  the  winter  in  which  they 
received  treatment  as  compared  to  the  previous  winter  when  they  had 
no  treatment.  There  were  enough  students  in  the  group  (208)  to  make 
this  result  statistically  significant.  In  the  absence  of  controls,  one  would 
have  been  led  to  the  conclusion  that  vitamin  C  does  help  prevent  colds. 
A  second  group  of  students  were  given  "placebos,"  pills  identical  in  size, 
shape,  color  and  taste  to  the  vitamin  C  pills  but  without  any  vitamin  C. 
The  students  were  not  told  who  was  getting  vitamin  C  and  who  was 
not;  they  only  knew  they  were  getting  pills  that  might  help  prevent  colds. 
The  group  getting  the  placebos  reported  that  they  had  a  63  per  cent 
reduction  in  the  number  of  colds!  This  controlled  experiment  thus  shows 
that  vitamin  C  had  nothing  to  do  with  the  decrease  in  the  number  of 
colds  and  that  the  reductions  reported  in  both  groups  were  either  psy- 
chologic effects  or  simply  the  result  of  a  lesser  amount  of  cold  virus  on 
the  campus  that  year.  There  have  been  reports  that  other  substances, 
called  flavonoids,  present  in  fruit  juices  may  have  some  effect  in  pro- 
tecting against  the  common  cold.  Comparable  carefully  controlled  ex- 
periments are  needed  to  substantiate  this  report. 

3.        History  of  Zoology  * 

Man's  interest  in  animals  is  probably  somewhat  older  than  the  hu- 
man race,  for  the  ape-men  and  men-apes  that  preceded  him  in  evolution 
undoubtedly  learned  at  an  early  time  which  animals  were  dangerous, 
which  could  be  hunted  for  food,  clothing  or  shelter,  where  these  were 
to  be  found,  and  so  on.  Some  of  prehistoric  man's  impressions  of  the 
contemporary  animals  have  survived  in  the  cave  paintings  of  France 
and  Spain  (Fig.  1.1).  Some  animals  were  regarded  as  good  or  evil  spirits. 
Later  man  decorated  pottery,  tools,  cloth  and  other  objects  with  animal 
figures. 

The  early  Egyptians  had  a  wealth  of  knowledge  about  animals  and 
had  domesticated  cattle,  sheep,  pigs,  cats,  geese  and  ducks.  The  Greek 
philosophers  of  the  fifth  and  sixth  centuries  b.c,  Anaximander,  Xenoph- 
anes,  Empedocles  and  others,  speculated  on  the  origin  of  the  animals 
of  the  earth.  One  of  the  earliest  classifications  of  animals  is  found  in  a 


8 


INTRODUCTION 


Figure   1.1.     Paintings  by   Upper  Paleolithic  man  from   the  wall  of  the  cavern  at 
Lascaux,  Dordogne,  France.  (Photo  by  Windels  Montignac.)  (Villee:  Biology.) 


Greek  medical  book  of  this  time  which  classifies  animals  primarily  as 
to  whether  or  not  they  are  edible.  Aristotle  (384-322  b.c.)  was  one  of 
the  greatest  Greek  philosophers  and  wrote  on  many  topics.  His  Historia 
animalium  contains  a  lot  of  information  about  the  animals  of  Greece 
and  the  nearby  regions  of  Asia  Minor.  The  descriptions  that  Aristotle 
made  himself  are  quite  good  and  are  recognizable  as  those  of  particular 
animals  living  today.  The  breadth  and  depth  of  his  zoological  interests 
are  impressive— he  made  a  careful  study  of  the  development  of  the  chick 
and  of  the  breeding  of  sharks  and  bees,  and  he  had  notions  about  the 
functions  of  the  human  organs,  some  of  which,  not  too  surprisingly,  were 
quite  wrong.  He  presented  an  elaborate  theory  that  animals  have  gradu- 
ally evolved,  based  on  a  metaphysical  belief  that  nature  strives  to  change 
from  the  simple  and  imperfect  to  the  more  complex  and  perfect.  His 
contributions  to  logic,  such  as  the  development  of  the  system  of  inductive 
reasoning  from  specific  observations  to  a  generalization  which  explains 
them  all,  have  been  of  inestimable  value  to  all  branches  of  science. 

The  Greek  physician,  Galen  (131-200  a.d.),  was  one  of  the  first  to 
do  experiments  and  dissections  of  animals  to  determine  structure  and 
functions.  He  was  the  first  experimental  physiologist  and  made  some 
notable  discoveries  on  the  functions  of  the  brain  and  nerves  and  demon- 
strated that  arteries  carry  blood  and  not  air.  His  descriptions  of  the 
human  body  were  the  unquestioned  authority  for  some  1300  years,  even 
though  they  contained  some  remarkable  errors,  being  based  on  dissec- 
tions of  pigs  and  monkeys  rather  than  of  human  bodies.  Pliny  (23-79 
A.D.)  and  others  in  succeeding  centuries  compiled  encyclopedias  (Pliny's 
Natural  History  was  a  37  volume  work)  regarding  the  kinds  of  animals 


INTRODUCTION  9 

and  where  they  lived,  which  are  remarkable  mixtures  of  fact  and  fiction. 
Some  of  the  ones  written  in  the  Middle  Ages  were  called  "bestiaries." 
The  zoological  books  written  in  the  Middle  Ages  are,  almost  without 
exception,  copied  from  Aristotle,  Galen  and  Pliny;  no  original  observa- 
tions were  made  to  corroborate  or  refute  the  accuracy  of  these 
authorities. 

The  Renaissance  in  science  began  slowly  with  scholars  such  as  Roger 
Bacon  (1214-1294)  and  Albertus  Magnus  (1206-1280)  who  were  inter- 
ested in  all  branches  of  natural  science  and  philosophy.  The  genius 
Leonardo  da  Vinci  (1452-1519)  was  an  anatomist  and  physiologist  as 
well  as  a  painter,  engineer  and  inventor.  He  made  many  original 
observations  in  zoology,  some  of  which  came  to  light  only  much  later, 
when  his  notebooks  were  deciphered. 

One  of  the  first  to  question  the  authority  of  Galen's  descriptions 
of  human  anatomy  was  the  Belgian,  Andreas  Vesalius  (1514-1564),  who 
was  professor  at  the  University  of  Padua  in  Italy.  By  actual  dissections 
and  by  making  detailed,  clear  drawings  of  what  he  saw,  Vesalius  re- 
vealed many  of  the  inaccuracies  in  Galen's  descriptions  of  the  human 
body.  He  published  his  observations  and  illustrations  in  De  Humani 
corporis  fabrica  (On  the  Structure  of  the  Human  Body)  in  1543.  Since 
Vesalius  dared  to  reject  the  authority  of  Galen,  he  was  the  object  of 
much  adverse  criticism  and  was  finally  forced  to  leave  his  professorial 
post. 

Just  as  Vesalius  had  emphasized  the  importance  of  relying  on 
original  observation  rather  than  on  authority  in  anatomy,  so  did  Wil- 
liam Harvey  (1578-1657)  in  physiology.  Harvey  was  an  English  physician 
who  received  his  medical  training  at  the  University  of  Padua,  where 
Vesalius  had  taught.  He  returned  to  England  and  investigated  the 
circulation  of  the  blood.  In  1628  he  published  Exercitatio  anatomica  de 
motu  cordis  et  sangiii)iis  in  anuiialibus  (Anatomical  studies  on  the  mo- 
tion of  the  heart  and  blood  in  animals).  At  that  time  blood  was  believed 
to  be  generated  in  the  liver  from  food  and  to  pass  just  once  to  the 
organs  of  the  body  where  it  was  used  up.  The  heart  was  believed  to  be 
nonmuscular  and  to  be  expanded  passively  by  the  inflowing  blood. 
Harvey  described,  from  direct  observations  on  animals,  how  first  the 
atria  (auricles)  and  then  the  ventricles  fill  and  empty  by  muscular  con- 
traction. He  showed  by  experiment  that  when  an  artery  is  cut  blood 
spurts  from  it  in  rhythm  with  the  beating  of  the  heart,  and  that  when 
a  vein  is  clamped  it  becomes  full  of  blood  on  the  side  away  from  the 
heart  and  empty  on  the  side  toward  the  heart.  He  demonstrated  that 
the  valves  in  the  veins  permit  blood  to  flow  toward  the  heart  but  not  in 
the  reverse  direction.  From  these  experiments  he  concluded  that  blood 
is  carried  away  from  the  heart  in  arteries  and  back  to  the  heart  in  veins. 
Furthermore,  by  measuring  how  much  blood  is  delivered  by  each  beat 
of  the  heart,  and  by  measuring  the  number  of  heartbeats  per  minute,  he 
could  calculate  the  total  flow  of  blood  through  the  heart  per  minute  or 
hour.  This  he  found  to  be  so  great  that  it  could  not  be  generated  anew 
in  the  liver  but  must  be  recirculated,  used  over  and  over  again.  This 


10  INTRODUCTION 

was  the  first  quantitative  physiologic  argument.  He  inferred  that  there 
must  be  small  vessels  connecting  arteries  and  veins  to  complete  the  cir- 
cular path  of  the  blood  but,  lacking  a  microscope,  he  was  unable  to  see 
them.  In  later  years  he  made  a  careful  study  of  the  development  of  the 
chick,  published  in  1G51  as  Exercitationes  de  generatione  animaliuni.  In 
this  he  postulated  that  mammals,  like  the  chick,  develop  from  an  egg. 

The  develoj)ment  of  the  compound  microscope  by  the  Janssens  in 
1590  and  by  Galileo  in  1(310  provided  the  means  lor  attacking  many 
problems  in  zoology  and  botany.  Robert  Hooke  (1G35-1703),  Marcello 
Malpighi  (1628-1691),  Antony  van  Leeuwenhoek  (1632-1723),  and  Jan 
Swammerdam  (1637-1680)  were  some  of  the  first  microscopists.  They 
studied  the  fine  structure  of  plant  and  animal  tissues.  Hooke  was  the 
first  to  describe  the  presence  of  "cells"  in  jjlant  tissue,  Leeuwenhoek  was 
the  first  to  describe  bacteria,  protozoa  and  sperm,  and  Malpighi  was  the 
first  to  describe  the  capillaries  connecting  arteries  with  veins.  The  light 
microscope  has  been  modified  and  improved  greatly  in  the  past  century, 
and  man's  ability  to  see  the  fine  structure  of  cells  has  been  greatly 
extended  by  the  invention  of  the  phase  microscope  and  of  the  electron 
microscope.  The  latter,  with  good  resolution  at  magnifications  as  great 
as  80,000  to  100,000  diameters,  has  revealed  a  whole  new  level  of  com- 
plexity in  the  structure  of  all  kinds  of  cells. 

John  Ray  (1627-1705)  and  Linnaeus  (Karl  von  Linne)  (1707-1778) 
brought  order  into  the  classification  of  animals  and  plants  and  devised 
the  binomial  system  (two  names,  genus  and  species)  for  the  scientific 
naming  of  the  kinds  of  animals  and  plants.  Linnaeus  first  used  this 
binomial  system  consistently  in  the  tenth  edition  of  his  Systema  naturae 
(1758). 

Contributions  to  our  understanding  of  the  embryonic  development 
of  animals  were  made  by  Fabricius,  the  professor  of  Anatomy  at  Padua 
who  taught  William  Harvey,  and  by  Harvey,  Malpighi,  and  Kaspar 
Wolff  (1759).  Wolff  proposed  the  theory  of  epigenesis,  an  external  force 
that  regulated  differentiation  and  development.  Karl  Ernst  von  Baer 
(1792-1876)  established  the  theory  of  germ  layers  and  emphasized  the 
need  for  comparative  studies  of  development  in  different  animals. 

Following  William  Harvey,  physiology  was  advanced  by  Rene 
Descartes  (1596-1650),  who  was  a  philosopher  rather  than  an  experi- 
menter. He  believed  that  "animal  spirits"  are  generated  in  the  heart, 
stored  in  the  brain,  and  pass  through  the  nerves  to  the  muscles,  causing 
contraction  or  relaxation,  according  to  their  quantity.  Charles  Bell 
(1774-1842)  and  Francois  Magendie  (1783-1855)  made  notable  contribu- 
tions to  our  understanding  of  the  function  of  the  brain  and  spinal 
nerves.  Johannes  MuUer  (1801-1858)  studied  the  properties  of  nerves 
and  capillaries;  his  textbook  of  physiology  stimulated  a  great  deal  of 
interest  and  research  in  the  field.  Claude  Bernard  (1813-1878)  was  one 
of  the  great  advocates  of  experimental  physiology,  and  contributed  sig- 
nificantly to  our  understanding  of  the  role  of  the  liver,  heart,  brain  and 
placenta.  Henry  Bowditch  (1840-1911)  discovered  the  "all-or-none" 
principle  of  the  contraction  of  heart  muscle   and  established   the  first 


INTRODUCTION  \  \ 

laboratory  for  teaching  physiology  in  the  United  States.  Ernest  Starling 
(1866-1927)  made  many  contributions  to  the  physiology  of  circulation 
and  the  nature  of  lymph  and  with  William  Bayliss  (1866-1924)  eluci- 
dated the  hormonal  control  of  the  function  of  the  pancreas. 

The  Scottish  anatomist  John  Hunter  (1728-1793)  and  the  French 
anatomist  Georges  Cuvier  (1769-1832)  were  pioneers  in  the  field  of  com- 
parative anatoiny,  studying  the  same  structure  in  different  animals. 
Richard  Owen  (1804-1892)  developed  the  concepts  of  homology  and 
analogy.  Cuvier  was  one  of  the  first  to  study  the  structure  of  fossils  as 
well  as  of  living  animals  and  is  credited  with  founding  the  science  of 
paleontology.  Cuvier  believed  strongly  in  the  unchanging  nature  of 
species  and  carried  on  bitter  debates  with  Lamarck,  who  in  1809  pro- 
posed a  theory  of  evolution  based  on  the  idea  of  the  inheritance  of 
acquired  characters. 

One  of  the  most  important  and  fruitful  concepts  in  biology  is  the 
cell  theory,  which  has  gradually  grown  since  Robert  Hooke  first  saw, 
with  the  newly  invented  microscope,  the  dead  cell  walls  in  a  piece  of 
cork.  The  French  biologist  Rene  Dutrochet  clearly  stated  in  1824  that 
"all  organic  tissues  are  actually  globular  cells  of  exceeding  smallness, 
which  appear  to  be  united  only  by  simple  adhesive  forces;  thus  all 
tissues,  all  animal  organs  are  actually  only  a  cellular  tissue  variously 
modified."  Dutrochet  recognized  that  growth  is  the  result  of  the  increase 
in  the  volume  of  individual  cells  and  of  the  addition  of  new  cells.  The 
German  botanist  M.  J.  Schleiden  and  zoologist  Theodor  Schwann 
studied  many  different  plant  and  animal  tissues  and  are  generally 
credited  with  formulating  the  cell  theory,  for  they  showed  that  cells  are 
the  units  of  structure  in  plants  and  animals,  and  that  organisms  are 
aggregates  of  cells  arranged  according  to  definite  laws.  The  presence  of 
a  nucleus  within  the  cell,  now  recognized  as  an  almost  universal  feature 
of  cells,  was  first  described  by  Robert  Brown  in  1831. 

Zoology,  along  with  the  other  biological  sciences,  has  expanded  at 
a  tremendous  rate  in  the  past  century,  with  the  establishment  of  the 
subsciences  of  cytology,  embryology,  genetics,  evolution,  biochemistry, 
biophysics,  endocrinology  and  ecology.  The  discoveries  and  new  tech- 
niques of  chemistry  and  physics  have  made  possible  new  approaches  to 
the  biological  sciences  that  have  attracted  the  attention  of  many  biolo- 
gists. So  many  men  have  contributed  to  the  growth  of  zoology  in  this  past 
century  that  only  a  few  in  each  field  can  be  mentioned:  Mendel,  deVries, 
Morgan  and  Bridges  in  genetics,  Darwin,  Dobzhansky,  W^right  and  Gold- 
schmidt  in  evolution,  and  Harrison  and  Spemann  in  embryology.  Many 
others  will  be  mentioned  as  these  subjects  are  discussed  in  detail  in  the 
text. 

The  establishment  and  giowth  of  the  marine  biological  laboratories 
such  as  the  ones  at  Naples,  \Voods  Hole  (Mass.),  Pacific  Grove  (Calif.), 
Friday  Harbor  (Wash.),  and  elsewhere  have  played  an  important  role  in 
fostering  research  in  zoological  sciences.  There  are  comparable  stations 
for  the  study  of  fresh-water  biology,  such  as  the  one  at  Douglas  Lake, 
Michigan. 


12  INTRODUCTION 

4.        Applications  of  Zoology 

Some  of  the  practical  uses  of  a  knowledge  of  zoology  will  become  ap- 
parent as  the  student  proceeds  through  this  text.  Zoology  is  basic  in  many 
ways  to  the  fields  of  medicine  and  public  health,  agriculture,  conserva- 
tion and  to  certain  of  the  social  sciences.  There  are  esthetic  values  in 
the  study  of  zoology,  for  a  knowledge  of  the  structure  and  functions  of 
the  major  types  of  animals  will  greatly  increase  the  pleasure  of  a  stroll 
in  the  woods  or  an  excursion  along  the  seashore.  Trips  to  zoos, 
aquariums  and  museums  are  also  rewarding  in  the  glimpses  they  give 
of  the  host  of  different  kinds  of  animals.  Many  of  these  are  beautifully 
colored  and  shaped,  graceful  or  amusing  to  watch,  but  all  will  mean 
more  to  a  person  equipped  with  the  basic  knowledge  of  zoology  which 
enables  him  to  recognize  them  and  understand  the  ways  in  which  they 
are  adapted  to  survive  in  their  native  habitat. 

Questions 

1.  How  would  you  define  "science"  and  "zoology"?  Is  zoology  a  science? 

2.  Contrast  a  hypothesis  and  a  law. 

3.  What  is  the  role  of  theories  in  science? 

4.  How  would  you  catalogue  the  subsciences  of  zoology? 

5.  Describe  in  your  own  words  the  mode  of  operation  of  the  scientific  method. 

6.  Discuss  the  tests  that  would  be  necessary  to  prove  that  event  A  is  the  cause  of  event  B. 

7.  How  may  the  method  of  concomitant  variation  be  used  to  show  cause-and-eftect  re- 
lationships? 

8.  What  is  a  "placebo"?  How  are  they  used  in  medical  experiments? 

9.  How  would  you  go  about  proving  that  "aminodichloro  sneezic  acid"  is  a  cure  for 
hay  fever? 

10.  What  contributions  to  zoology  were  made  by  (a)  Aristotle,  (b)  Galen,  (c)  Vesalius, 
(d)  William  Harvey,  (e)  Leeuwenhoek,  (f)  von  Baer,  (g)  Claude  Bernard,  (h)  Georges 
Cuvier  and  (i)  Richard  Owen? 

Supplementary  Reading 

The  scientific  method  and  its  application  to  research  problems  are  discussed  in 
Conant's  Science  and  Common  Sense  and  Cohen's  Science,  Seiuant  of  Man.  E.  Bright  Wil- 
son's An  Introduction  to  Scientific  Research  gives  an  excellent,  nontechnical  discussion 
of  the  methods  of  science  and  some  of  the  problems  involved  in  conducting  scientific  in- 
vestigations. W.  B.  Cannon's  The  Way  of  an  Investigator  gives  some  interesting  examples 
of  the  scientific  method  in  medical  research. 

The  Scientific  American  has  well  written  and  illustrated  articles  on  many  phases 
of  zoology.  Some  of  the  outstanding  articles  have  been  collected  and  published  in  book 
form  as  The  Physics  and  Chemistry  of  Life. 

There  are  a  number  of  fine  books  on  the  history  of  science.  The  development  of  the 
sciences  in  general  is  described  in  Sedgwick,  Tyler  and  Bigelow's  A  Short  History  of 
Science.  The  early  development  of  zoology  is  interestingly  told  in  Nordenskiold's  and 
Singer's  histories  of  biology.  The  History  of  Medicine  written  by  Douglas  Guthrie  de- 
scribes the  beginnings  of  anatomy,  physiology  and  bacteriology.  Some  of  the  important 
ideas  in  zoology,  presented  by  extensive  quotations  from  the  original  papers,  are  found 
in  Gabriel  and  Vogel's  Great  Experiments  in  Biology  and  in  T.  S.  Hall's  A  Source  Book  of 
Animal  Biology. 


Part  I 


GENERAL   CONCEPTS 


CHAPTER  2 


Protoplasm 


To  DEFINE  the  field  of  zoology,  or  animal  biology,  it  might  seem  a  simple 
task  first  to  differentiate  the  living  from  the  nonliving  and  then  to  sep- 
arate the  living  into  plants  and  animals.  Yet  each  of  these  is  quite 
difficult  to  do  sharply  and  clearly.  Organisms  such  as  cats,  clams  and 
cicadas  are  clearly  recognizable  as  animals,  but  sponges,  for  example, 
were  considered  to  be  plants  until  well  into  the  nineteenth  century,  and 
there  are  single  celled  organisms  which,  even  today,  are  called  animals  by 
zoologists  and  plants  by  botanists.  Even  the  line  between  living  and  non- 
living is  indistinct,  for  the  viruses,  too  small  to  be  seen  with  an  ordinary 
light  microscope,  can  be  considered  either  the  simplest  living  things  or 
very  complex,  but  nonliving,  organic  chemicals. 

Most  biologists  are  agreed  that  all  the  varied  phenomena  of  life 
are  ultimately  explainable  in  terms  of  the  same  physical  and  chemical 
principles  which  define  nonliving  systems.  The  idea  that  there  are  no 
fundamental  differences  between  living  and  nonliving  things  is  some- 
times called  the  mechanistic  theory  of  life.  An  opposite  view,  widely 
held  by  biologists  until  the  present  century,  stated  that  some  unique 
force,  not  explainable  in  terms  of  physics  and  chemistry,  is  associated 
with  and  controls  life.  The  view  that  living  and  nonliving  systems  are 
basically  different  and  obey  different  laws  is  called  vitalism.  Many  of 
the  phenomena  that  appeared  to  be  so  mysterious  when  first  discovered 
have  subsequently  proved  to  be  understandable  without  invoking  a 
unique  life  force,  and  the  vitalistic  theory  of  life  has  lost  supporters. 

13 


14  GENERA  I   CONCEPTS 


Cytoplasm 

Pia.sma  membrane 

Golgi  bodies 

Centriole 

Vactxole 

Mitochondria 

Nucleus 

Cliromatin 

Nucleolus 

Nuclear 

membrane 

Cylopiasm 


Figure  2.1.     Schematic  drawing  of  a  generalized  animal  cell. 


5.        Characteristics  of  Living  Things 

Organization.  Each  kind  of  living  organism  is  recognized  by  its 
characteristic  form  and  appearance;  the  adult  organism  usually  has  a 
characteristic  size.  Nonliving  things  generally  have  much  more  variable 
shapes  and  sizes.  The  fundamental  structural  and  functional  unit  of 
living  things,  both  animals  and  plants,  is  the  cell.  It  is  the  simplest  bit 
of  living  matter  that  can  exist  independently  and  exhibit  all  the  char- 
acteristics of  life.  A  typical  cell,  such  as  a  liver  cell  (Fig.  2.1),  is  polygonal 
in  shape,  with  a  plasma  membrane  separating  the  living  substance,  or 
protoplasm,  from  the  surroundings.  Almost  without  exception,  cells 
have  a  nucleus,  a  specialized  part  of  the  protoplasm  typically  spherical 
or  ovoid  in  shape  and  separated  from  the  rest  of  the  protoplasm  by  a 
nuclear  membrane.  The  protoplasm  that  makes  up  the  nucleus  is  known 
as  nucleoplasm,  that  outside  the  nucleus  as  cytoplasm.  The  nucleus,  as 
we  shall  see  later,  has  a  major  role  in  controlling  and  regulating  the 
cell's  activities.  It  contains  the  hereditary  units  or  genes.  A  cell  experi- 
mentally deprived  of  its  nucleus  usually  dies  in  a  short  time;  even  if 
it  survives  for  several  days  it  is  unable  to  reproduce. 

Irritability.  Living  things  are  irritable;  they  respond  to  stimuli,  to 
physical  or  chemical  changes  in  their  immediate  surroundings.  Stimuli 
which  are  effective  in  evoking  a  response  in  most  animals  and  plants  are 
changes  in  light  (either  in  its  color,  intensity  or  direction),  temperature, 
pressure,  sound,  and  in  the  chemical  composition  of  the  earth,  water, 
or  air  surrounding  the  animal.  In  man  and  other  complex  animals, 
certain  cells  of  the  body  are  highly  specialized  to  respond  to  certain 
types  of  stimuli:  the  lods  and  cones  in  the  retina  of  the  eye  respond  to 
light,  certain  cells  in  the  nose  and  in  the  taste  buds  of  the  tongue  respond 


PROTOPLASM  \  5 

to  chemical  stimuli,  and  special  groups  of  cells  in  the  skin  respond  to 
changes  in  temperature  or  pressure.  In  lower  animals  such  specialized 
cells  may  be  absent,  but  the  whole  organism  responds  to  any  one  of  a 
variety  of  stimuli.  Single-celled  animals  such  as  the  ameba  will  respond 
by  moving  toward  or  away  from  heat  or  cold,  certain  chemical  substances, 
or  the  touch  of  a  microneedle.  Indeed,  many  of  the  cells  of  higher 
animals  have  a  similar  generalized  sensitivity. 

Movement,  A  third  characteristic  of  living  things  is  their  ability 
to  move.  The  movement  of  most  animals  is  quite  obvious— they  wiggle, 
swim,  run  or  fly.  The  movement  of  plants  is  much  slower  and  less  obvi- 
ous, but  is  present  nonetheless.  A  few  animals— sponges,  corals,  hydroids, 
oysters,  certain  parasites— do  not  move  from  place  to  place,  but  most  of 
these  have  microscopic,  hairlike,  cytoplasmic  projections  from  the  cells, 
called  cilia  or  flagella,  to  move  their  surroundings  past  their  bodies 
and  thus  bring  food  and  other  necessities  of  life  to  themselves.  The 
movement  of  an  animal  body  may  be  the  result  of  muscular  contraction, 
of  the  beating  of  cilia  or  flagella,  or  of  the  slow  oozing  of  a  mass  of 
protoplasm  (known  as  ameboid  motion). 

Metabolism.  AH  living  things  carry  on  a  wide  variety  of  chemical 
reactions,  the  sum  of  which  we  call  metabolism.  There  is  no  way  of 
observing  the  occurrence  of  most  of  these  chemical  reactions  without 
the  aid  of  special  apparatus  such  as  respirometers  to  measure  oxygen 
utilization  and  carbon  dioxide  production  and  thermometers  to  measure 
heat  production.  Elaborate  physical  and  chemical  equipment  and  sub- 
stances labeled  with  radioactive  or  stable  isotopes  are  used  to  trace  in 
detail  the  paths  of  metabolism  and  their  respective  quantitative  im- 
portance to  the  animal  or  plant  under  investigation.  Such  studies  have 
shown  that  the  protoplasm  of  all  cells  is  constantly  taking  in  new  sub- 
stances, altering  them  chemically  in  a  multitude  of  ways,  building  new 
protoplasm,  and  transforming  the  potential  energy  of  some  of  the  mole- 
cules taken  in  into  kinetic  energy  and  heat.  The  large  molecules  taken 
in— proteins,  fats,  carbohydrates  and  others— are  broken  down  stepwise 
to  yield  energy  and  simpler  substances.  This  constant  release  and  utiliza- 
tion of  energy  is  one  of  the  unique  and  characteristic  attributes  of  living 
things.  The  rate  of  metabolism  is  affected  by  temperature,  age,  sex, 
general  health  and  nutrition,  by  hormones,  and  by  many  other  factors. 

Those  metabolic  processes  in  which  simpler  substances  are  combined 
to  form  more  complex  substances  and  which  result  in  the  storage  of 
energy  and  the  production  of  new  protoplasm  are  termed  anabolic.  The 
opposite  processes,  in  which  complex  substances  are  broken  down  to 
release  energy  and  which  result  in  the  wearing  out  of  protoplasm,  are 
called  catabolic.  Both  types  of  metabolism  occur  continuously  and  are 
intricately  interdependent  so  that  they  become,  in  practice,  difficult  to 
distinguish.  Complex  compounds  of  one  sort  may  be  broken  down  and 
their  parts  recombined  in  new  ways  to  yield  new  compounds.  Further- 
more, the  synthesis  of  most  molecules  requires  energy,  so  that  some 
catabolic  processes  must  occur  to  supply  the  energy  to  drive  the  anabolic 
reactions  of  these  syntheses. 

Growth.     Both  plants  and  animals  grow;  nonliving  things  do  not. 


16  GENERAL   CONCEPTS 

The  increase  in  mass  may  be  brought  about  by  an  increase  in  the  size 
of  the  individual  cells,  or  by  an  increase  in  the  number  ot  cells.  An 
increase  in  cell  size  may  occur  by  the  simple  uptake  oi  water,  but  this  is 
not  generally  considered  to  be  growth.  The  term,  growth,  is  restricted  to 
those  processes  which  increase  the  amount  of  living  substance  of  the 
body.  This  is  commonly  measured  by  the  amount  of  nitrogen,  of  protein 
or  of  nucleic  acid  (see  p.  29)  present,  but  objections  may  be  raised  to 
the  use  of  any  single  one  of  these  parameters.  Growth  may  be  uniform 
in  the  several  parts  of  an  organism,  or,  perhaps  more  commonly,  growth 
is  differential,  greater  in  some  parts  than  in  others,  so  that  the  body 
proportions  change  as  growth  occurs. 

Growth  may  occur  throughout  the  life  span  of  an  organism  or  may 
be  restricted  to  a  part  of  it.  One  of  the  truly  remarkable  aspects  of  the 
process  is  that  each  organ  continues  to  function  while  undergoing 
growth. 

Reproduction.  Yet  another  characteristic  of  living  things  is  their 
ability  to  reproduce  their  kind.  Since  individual  animals  grow  old  and 
die,  the  survival  of  the  species  depends  upon  the  replacmg  of  these  indi- 
viduals by  new  ones.  Although  at  one  time  worms  were  believed  to  arise 
from  horse  hairs  in  a  trough  of  water,  maggots  from  decaying  meat  and 
frogs  from  the  mud  of  the  Nile,  we  now  know  that  each  can  come  only 
from  previously  existing  ones.  One  of  tlie  fundamental  tenets  of  biology 
is  that  "all  life  comes  only  from  living  things."  The  process  of  repro- 
duction may  be  as  simple  as  the  splitting  ot  one  individual  into  two. 
In  most  animals,  however,  it  involves  the  production  of  specialized  eggs 
and  sperm  which  unite  to  form  the  zygote  or  fertilized  egg,  from  which 
the  new  organism  develops.  In  some  animals,  the  liver  flukes  for  ex- 
ample, reproduction  involves  several  quite  different  forms,  each  of  which 
gives  rise  to  the  next  in  succession  until  the  cycle  is  completed  and  the 
adult  reappears. 

Adaptation.  To  survive,  an  animal  or  plant  must  be  adapted  to 
its  surroundings.  Each  particular  species  can  achieve  adaptation  either 
by  seeking  out  a  suitable  environment  or  by  undergoing  modifications 
to  make  it  more  fitted  to  its  present  surroundings.  This  ability  to  adapt 
is  a  further  characteristic  of  all  living  things.  Adaptation  may  involve 
immediate  changes  which  depend  upon  the  irritability  of  protoplasm, 
or  it  may  be  the  result  of  a  long-term  process  of  mutation  and  selection 
(p.  704).  It  is  obvious  that  no  single  kind  of  organism  can  adapt  to  all 
the  conceivable  kinds  of  environment,  hence  there  will  be  certain  areas 
where  it  cannot  survive.  The  list  of  factors  which  may  limit  the  distribu- 
tion of  a  species  is  almost  endless:  water,  light,  temperature,  food,  preda- 
tors, other  organisms,  and  so  on. 

6.        Protoplasm 

The  living  substance  that  makes  up  each  cell  is  known  as  proto- 
plasm. We  cannot  see  directly  the  protoplasm  of  most  animals,  for  it  is 
hidden  by  a  protective  covering  of  skin,  hair  or  shell.  In  an  animal  such 
as  the  ameba,  however,  we  can  observe  naked  protoplasm  and  find  that 


PROTOPLASM  1 7 

it  is  a  viscid,  jellylike  substance,  slimy  to  the  touch,  which  is  colorless  or 
faintly  yellow  or  pink. 

When  seen  under  the  light  microscope,  protoplasm  may  appear  to 
have  granules  or  fibrils  of  denser  material,  droplets  of  fatty  substances 
or  fluid-filled  vacuoles,  all  suspended  in  the  clear,  continuous,  semifluid 
"ground  substance."  Protoplasm  is  a  complex  colloidal  system  (see  p. 
30),  whose  consistency  varies  from  liquid  (sol)  to  a  firm  jelly  (gel).  The 
change  from  sol  to  gel  is  reversible  and  the  consistency  may  vary  from 
moment  to  moment  and  from  one  part  of  the  cell  to  another.  Some  of 
the  formed  bodies  within  the  protoplasm— mitochondria,  microsomes 
and  Golgi  apparatus— are  specialized  parts  of  the  living  substance;  others 
are  nonliving  accumulations  of  fat,  protein,  carbohydrate  or  pigments. 

Mitochondria.  \V4ien  animal  cells  are  viewed  through  the  electron 
microscope  (Fig.  2.2),  the  mitochondria  are  seen  to  be  large,  round,  oval 
or  sausage-shaped  structures  with  a  double  membrane  separating  the 
mitochondrial  substance  from  the  surrounding  ground  substance.  The 
inner  membrane  is  thrown  into  folds  which  extend  deep  into  the  center 
of  the  mitochondrion.  These  membranes  are  about  50  Angstrom  units 
(A)  thick,  just  about  the  thickness  of  a  single  layer  of  protein  or  of  a 
double  layer  of  lipid.  Mitochondria  from  all  animals  from  protozoa 
to  man  have  the  same  basic  structure.  As  we  shall  see  in  Chapter  4,  there 
is  experimental  evidence  that  the  mitochondria  are  complicated  enzyme 
machines;  it  is  probable  that  these  folds  within  the  mitochondria  are  the 
sites  of  many  of  the  enzymes  which  catalyze  reactions  by  which  the  cell 
obtains  energy  from  foodstuff  molecules. 

Microsomes.  In  addition  to  mitochondria,  cells  contain  smaller 
particles,  not  visible  with  the  light  microscope,  known  as  microsomes. 
The  electron  microscope  reveals  these  to  be  thin  membranes  to  which 
are  attached  spherical  particles  (Fig.  2.2).  There  are  many  such  particle- 
covered  membranes  in  each  cell.  When  cells  are  cut  in  thin  sections  and 
viewed  in  the  electron  microscope,  these  membranes,  called  endoplasmic 
reticulum,  appear  as  long  thin  strands,  like  strands  of  spaghetti.  The 
microsomes  are,  like  mitochondria,  organized  masses  of  enzymes.  The 
enzymes  of  the  microsomes  are  concerned  with  the  synthesis  of  proteins 
and  of  certain  other  complex  molecules  in  the  cell. 

Golgi  Apparatus.  The  cytoplasm  of  most  cells  (mature  sperm  and 
red  blood  cells  are  notable  exceptions)  contains  another  type  of  inclusion 
known  as  the  Golgi  apparatus.  These  are  visible  in  the  light  microscope 
when  the  tissue  section  has  been  properly  stained.  They  may  appear  as 
granules,  threads,  rods  or  canals.  Golgi  bodies  are  stained  by  the  dye 
neutral  red;  mitochondria  take  up  the  dye  Janus  green.  The  Golgi 
bodies  appear  to  play  a  role  in  the  production  of  cellular  secretions. 

Much  has  been  learned  in  recent  years  of  the  role  each  of  these 
particles  plays  in  the  economy  of  the  cell.  Cells  are  homogenized  in  spe- 
cial glass  grinding  tubes  to  break  the  cell  membrane  and  release  the 
intracellular  structures.  Then,  by  subjecting  the  homogenate  to  increas- 
ing amounts  of  centrifugal  force  in  an  ultracentrifuge,  first  the  nuclei, 
then  the  mitochondria,  and  finally  the  microsomes  can  be  sedimented 
separately.  W'hen  these  sedimented  particles  are  examined  in  the  electron 


18  GENERAL  CONCEPTS 


•rTgr  IT    . 


*       '"^P 


^^^..^•S^ 


7  #■;  V' 


^#'^#.jr\*v  jf  f>^-.'^'i.  ^«<<.    -.v^^w.  ---Xr*?-   ■'*^1 

/  y  •  c     J^,    1^  ,-1..     ~  ' -^  ,.*-•■•''■«•  ^7'^^;      ;,  -^ 


Figure  2.2.  An  electron  micrograph  of  a  section  of  a  cell  from  the  pancreas  of  a 
guinea  pig.  A  segment  of  the  nucleus  (n)  surrounded  by  its  nuclear  membrane,  some 
mitochondria  (m),  which  are  sausage-shaped  structures  with  double-layered  transverse 
partitions,  and  the  paired,  spaghetti-like  strands  of  the  endoplasmic  reticulum  or  mi- 
crosomes are  evident.  (Courtesy  of  G.  Palade.)  (Maximow  and  Bloom:  Textbook  of 
Histology.) 


microscope,  they  are  found  to  have  the  same  structure  exhibited  by 
comparable  structures  in  the  intact  cell.  The  separated  particles  can 
then  be  suspended  in  suitable  incubation  media  and  their  metabolism 
can  be  studied.  Such  separated  mitochondria  and  microsomes  will  carry 
out  many  biochemical  reactions,  and  much  is  now  known  about  the 
functions  of  each  of  these  particles.  The  liquid  left  after  the  homogenate 
has  been  subjected  to  high  centrifugal  force  to  sediment  the  microsomes 
contains  many  other  enzymes  which  apparently  exist  in  the  cell  more  or 
less  free  in  the  ground  substance  of  the  protoplasm. 


PROTOPLASM  19 

Where,  you  may  ask,  is  life  localized-in  the  mitochondria?  in  the 
microsomes?  or  in  the  ground  substance?  The  answer,  of  course,  is  that 
life  is  not  a  function  of  any  single  one  of  these  parts  of  protoplasm,  but 
of  the  whole  integrated  system  of  many  component  parts,  organized  in 
the  proper  spatial  relationship  and  interdependent  on  one  another  in 
a  great  variety  of  ways. 

7.        Chemical  Composition  of  Protoplasm 

Chemical  analysis  of  protoplasm  from  any  animal  from  ameba  to 
man  reveals  a  fundamental  similarity  in  composition.  The  four  chemical 
elements,  carbon,  oxygen,  hydrogen  and  nitrogen,  make  up  90  per  cent 
or  more  of  the  substance  of  protoplasm  from  any  animal  or  plant  cell. 
Potassium,  sulfur,  calcium  and  phosphorus  are  four  other  elements 
usually  present  in  protoplasm  to  the  extent  of  one  per  cent  or  more 
each.  Since  bone  is  largely  composed  of  calcium  and  phosphorus,  the 
amount  of  these  elements  is  much  greater  in  a  bony  animal  than  in  a 
completely  soft-bodied  one.  Smaller  amounts  of  sodium,  chlorine,  iron, 
iodine,  magnesium,  copper,  managanese,  cobalt,  zinc  and  a  few  others 
complete  the  list.  The  unique  aliveness  of  protoplasm  does  not  depend 
on  the  presence  of  some  rare  or  unique  element,  for  these  same  elements 
are  abundant  in  the  atmosphere,  in  the  sea  and  in  the  earth's  crust.  The 
phenomenon  of  life  depends,  instead,  upon  the  complexity  of  the  inter- 
relationships of  these  common,  abundant  elements. 

For  convenience  in  writing  chemical  formulas  and  reactions,  chem- 
ists have  assigned  to  each  of  the  elements  a  symbol,  usually  the  first 
letter  of  the  name  of  the  element:  O,  oxygen;  H,  hydrogen;  C,  carbon; 
N,  nitrogen.  A  second  letter  is  added  to  the  symbol  of  those  elements 
with  the  same  initial  letter:  Ca,  calcium;  Na,  sodium  (Latin,  Natrium); 
Co,  cobalt;  CI,  chlorine;  Cu,  copper. 

Atoms  and  Ions.  The  chemical  properties  of  an  element  are  de- 
termined primarily  by  the  number  and  arrangement  of  electrons  (nega- 
tively charged  particles  of  extremely  small  mass)  revolving  in  the  outer- 
most orbit  around  the  atomic  nucleus  and  to  a  lesser  extent  by  the 
number  of  electrons  in  the  inner  orbits.  These  in  turn  depend  upon  the 
number  and  kind  of  particles  in  the  nucleus.  The  number  of  electrons 
in  the  outermost  orbit  varies  from  zero  to  eight  in  different  kinds  of 
atoms  (Fig.  2.3).  An  element  whose  atoms  have  eight  electrons  in  the 
outermost  orbit  is  chemically  inert  and  will  not  combine  with  other 
elements.  W'hen  there  are  fewer  than  eight  electrons,  the  atom  tends  to 
lose  or  gain  electrons  in  an  attempt  to  achieve  an  outer  orbit  of  eight 
electrons.  Since  the  number  of  positively  charged  particles,  protons,  in 
the  nucleus  is  not  changed,  this  loss  or  gain  of  electrons  produces  an 
atom  with  a  net  positive  or  negative  charge.  Such  electrically  charged 
atoms  are  known  as  Ions.  Atoms  with  one,  two  or  three  electrons  in  the 
outer  orbit  tend  to  lose  them  to  other  atoms  and  become  positively 
charged  ions  (e.g.,  Na  +  ,  sodium  ion;  Ca++,  calcium  ion).  These  are 
called  cations  because  they  migrate  to  the  cathode  of  an  electrolytic  cell. 
Atoms  with  five,  six  or  seven  electrons  in  the  outer  orbit  tend  to  gain 


20 


GENERAL   CONCEPTS 


Electron  in  Hs 
orbit 


Electron  in 
outer  orbit  - 
in  inner  orbit   /' 

r 
I 


Proton  In 
the  nucleus 


Hydrogen  Atom 


0" 


^^Q 


Ceu-bon  Atom 


;o 


4 


.^ 


^v 


sT 


Nitrogen  Atom 


Oxygen  Atom. 


Figure  2.3.  Diagrams  of  the  structure  of  the  atoms  of  the  four  chief  elements  of 
protoplasm:  hydrogen,  carbon,  nitrogen  and  oxygen.  The  symbols  used  are  O,  neutron; 
+,  proton;  ©,  electron. 

electrons  from  other  atoms  and  become  negatively  charged  ions  or 
anions  (e.g.,  Cl~,  chloride  ion).  Anions  migrate  to  the  anode  or  posi- 
tively charged  electrode  of  an  electrolytic  cell.  Because  they  bear  opposite 
electric  charges,  anions  and  cations  are  attracted  to  each  other.  Atoms 
such  as  carbon,  which  have  lour  electrons  in  the  outer  orbit,  neither  lose 
nor  gain  electrons,  but  share  them  with  adjacent  atoms. 

Physical  research  has  shown  that  most  of  these  elements  are  com- 
posed of  two  or  more  kinds  of  atoms,  which  differ  in  the  number  of 
neutrons  in  the  atomic  nucleus.  The  different  kinds  of  atoms  of  an 
element  are  called  isotopes  (iso  =i  equal,  tope  =  place),  because  they 
occupy  the  same  place  in  the  periodic  table  of  the  elements.  All  the 
isotopes  of  a  given  element  have  the  same  number  of  electrons  circling 
the  atomic  nucleus.  The  development  of  the  cyclotron  and  nuclear 
reactor  made  possible  the  artificial  production  of  a  host  of  new  isotopes. 
The  availability  of  these  new  isotopes,  in  turn,  made  possible  a  new 
type  of  biologic  research,  that  of  tracing  particular  elements  and  com- 
pounds through  their  many  devious  metabolic  pathways,  and  of  measur- 
ing the  time  required  for  any  given  substance  in  the  body  to  be  replaced 


PROTOPLASM  2 1 

by  new  molecules  of  that  substance.  This  tracing  is  possible  because, 
although  the  several  isotopes  of  an  element  have  the  same  chemical 
properties,  they  have  different  physical  properties.  Some  are  radioactive, 
that  is,  they  emit  rays  or  particles  of  some  sort  which  can  be  detected 
by  an  instrument  such  as  the  Geiger  counter.  Others  are  differentiated 
in  a  mass  spectrometer  by  the  slight  difference  in  the  mass  of  the  atomic 
nucleus  which  residts  from  the  presence  there  of  an  extra  neutron.  Thus, 
with  radioactive  calcium  one  can  study  the  rate  of  formation  of  bone 
(and  the  effects  of  a  host  of  variables  such  as  vitamin  D  intake  or  rate 
of  parathyroid  activity  on  this  process),  or  the  rate  of  secretion  of  shell 
by  a  clam  or  oyster.  Or,  one  can  prepare  sugar  labeled  with  radioactive 
carbon  (C"  or  C'^)  or  heavy  carbon  (C^-^),  inject  it  into  an  experimental 
animal,  and  determine  the  metabolic  paths  of  glucose— its  conversion  to 
glycogen,  fat  and  protein— and  their  respective  amounts.  Many  problems 
in  zoology  and  the  other  biological  sciences  which  could  be  attacked  in 
no  other  way  have  been  solved  by  this  method. 

The  analysis  of  the  human  body  reveals  that  it  contains  about  50 
per  cent  carbon,  20  per  cent  oxygen,  10  per  cent  hydrogen,  9  per  cent 
nitrogen,  4  per  cent  calcium,  2.5  per  cent  phosphorus  (P),  1  per  cent 
potassium  (K),  0.8  per  cent  sulfur  (S),  0.4  per  cent  sodium  (Na),  and 
0.4  per  cent  chlorine  (CI).  Analyses  of  other  animals  would  yield  com- 
parable results.  Such  analyses  are  not  very  informative  unless  the  animal 
has  some  unusual  element.  Tunicates,  for  example,  are  unusual  in  con- 
taining a  large  amount  of  the  element  vanadium  (V). 

Chemical  Compounds.  Most  elements  are  present  in  protoplasm  as 
chemical  compounds,  substances  composed  of  two  or  more  different 
kinds  of  atoms.  The  smallest  particle  of  a  substance  having  the  compo- 
sition and  properties  of  a  larger  part  of  the  substance  is  called  a 
molecule.  The  molecules  of  a  pure  compound  are  always  composed  of 
two  or  more  elements  combined  in  a  fixed  ratio.  Water  molecules,  for 
example,  always  contain  two  atoms  of  hydrogen  and  one  of  oxygen. 
Chemists  state  this  fact  by  writing  the  formula  of  water  as  HoO.  A  chemi- 
cal formula  represents  both  the  kinds  and  the  relative  proportions  of 
the  atoms  present  in  a  molecule. 

A  large  part  of  any  kind  of  protoplasm  is  simply  water.  In  an  ani- 
mal such  as  man,  the  water  content  of  protoplasm  varies  from  about  20 
per  cent  in  bone  to  85  per  cent  in  brain  cells.  The  water  content  is 
greater  in  embryonic  and  young  cells  and  decreases  as  aging  occurs. 
About  70  per  cent  of  our  total  body  weight  is  water;  as  much  as  95  per 
cent  of  jellyfish  protoplasm  is  water.  Water  has  a  number  of  important 
functions  in  protoplasm.  Most  of  the  other  chemicals  present  are  dis- 
solved in  it;  they  must  be  dissolved  in  water  in  order  to  react.  Water 
aids  in  the  removal  of  the  waste  products  of  metabolism  by  dissolving 
them  so  they  can  be  excreted.  Water  has  a  great  capacity  for  absorbing 
heat  with  a  minimal  change  in  its  own  temperature;  thus  it  protects 
protoplasm  against  sudden  thermal  changes.  Since  water  absorbs  a  large 
amount  of  heat  as  it  changes  from  a  liquid  to  a  gas,  the  mammalian 
body  can  dissipate  excess  heat  by  the  evaporation  of  sweat.  Water's 
high   heat   conductivity   makes   possible   the   even   distribution   of   heat 


22  GENERA  t   CONCEPTS 

throughout  a  hirge  mass  of  protoplasm.  Finally,  water  has  an  important 
function  as  a  lubricant.  It  is  present  in  body  fluids  wherever  one  organ 
rubs  against  another  and  in  joints  where  one  bone  moves  on  another. 

A  mixture  is  made  of  two  or  more  kinds  of  atoms  or  molecules  which 
may  be  present  in  varying  proportions.  Air  is  a  mixture  of  oxygen,  nitro- 
gen, carbon  dioxide  and  water  vapor,  plus  certain  rare  gases  such  as 
argon.  The  proportions  of  these  constituents  may  vary  widely.  Thus,  in 
contrast  to  a  pure  compound,  which  has  a  fixed  ratio  of  its  constituents 
and  definite  chemical  and  physical  properties,  a  mixture  has  properties 
which  vary  with  the  relative  abundance  of  its  constituents. 

Molecules  may  be  composed  of  one,  two,  or  many  kinds  of  atoms. 
Those  of  gaseous  oxygen  or  nitrogen  are  made  of  two  of  the  same  kind 
of  atom— O2  and  No.  The  molecules  of  table  salt,  sodium  chloride,  are 
composed  of  one  atom  of  sodium  and  one  of  chlorine  (NaCl).  A  common 
sugar,  of  great  physiologic  importance,  is  glucose,  whose  molecules  con- 
tain six  carbon,  twelve  hydrogen  and  six  oxygen  atoms;  its  formula  is 
written  CgHioOg. 

To  learn  more  about  the  constituents  of  protoplasm,  biochemists 
have  used  very  sensitive  analytical  techniques  and  have  taken  great  pains 
to  preserve  the  extremely  labile  substances  present  in  this  enormously 
complicated  system.  To  prevent  the  disappearance  of  certain  substances 
it  is  necessary  to  quick-freeze  a  bit  of  excised  tissue,  or  even  a  whole  small 
animal,  by  dropping  it  directly  into  liquid  air.  Biochemical  research  has 
made  it  abundantly  clear  that  the  composition  of  the  protoplasm  of  any 
cell  is  constantly  changing,  that  the  cell  constituents  are  in  a  "dynamic 
state."  There  is  a  continuous  synthesis  of  large,  energy-rich  molecules  and 
continual  decomposition  of  these  into  smaller,  energy-poor  ones.  Some 
of  the  most  important  compounds  of  protoplasm  are  present  only  in  ex- 
tremely minute  amounts  at  any  given  time,  although  the  total  amount 
formed  and  used  in  a  24  hour  period  may  be  quite  large.  An  apprecia- 
tion of  this  may  be  gained  from  the  following  consideration:  when  sub- 
stances undergo  chemical  reactions  in  sequence  (and  almost  all  of  the 
reactions  of  importance  biologically  are  sequences  or  "cycles")  such  as 
A  ^.  B  ^.  C  ^^  D,  the  rate  of  the  whole  process  is  controlled  by  the  rate 
of  the  slowest  reaction  in  the  chain.  For  example,  if  reaction  A  ->  B  is 
10  times  as  fast  as  B  -^  C,  and  if  C  ->  D  is  100  times  as  fast  as  B  ->  C, 
then  the  least  reactive  substance,  B,  will  tend  to  accumulate  and  the  most 
reactive  one,  C,  will  be  present  in  the  smallest  amount.  For  this  reason 
many  of  the  most  active  and  important  substances  of  protoplasm  are 
present  in  extremely  minute  amounts.  This,  coupled  with  their  chemical 
instability,  has  made  their  detection  and  isolation  difficult.  There  are 
probably  many  such  intermediate  compounds  that  remain  to  be  dis- 
covered. 

The  compounds  found  in  protoplasm  are  of  two  main  types:  inor- 
ganic and  organic.  The  latter  include  all  the  compounds  (other  than 
carbonates)  that  contain  the  element  carbon.  The  element  carbon  is 
able  to  form  a  much  wider  variety  of  compounds  than  any  other  element 
because  the  outer  orbit  of  the  carbon  atom  contains  four  electrons, 
which  can  be  shared  in  a  number  of  different  ways  with  adjacent  atoms. 


PROTOPLASM  23 

At  one  time  it  was  believed  that  organic  compounds  were  uniquely 
different  from  other  chemical  substances  and  that  they  could  be  pro- 
duced only  by  living  matter.  This  hypothesis  was  disproved  when  the 
German  chemist  W^ohler  succeeded  in  1828  in  synthesizing  urea  (one  of 
the  waste  products  found  in  human  urine)  from  the  inorganic  com- 
pounds ammonium  sulfate  and  potassium  cyanate.  Since  that  time 
thousands  of  organic  compounds  have  been  synthesized,  some  of  which 
are  quite  complex  molecules  of  great  biological  importance  such  as 
vitamins,  hormones,  antibiotics  and  drugs. 

Inorganic  Compounds.  The  inorganic  compounds  important  in 
living  systems  are  acids,  bases  and  salts.  An  acid  is  a  compound  which 
releases  hydrogen  ions  (H  +  )  when  dissolved  in  water.  Acids  turn  blue 
litmus  paper  to  red  and  have  a  sour  taste.  Hydrochloric  (HCl)  and  sul- 
furic (H2SO4)  are  examples  of  inorganic  acids;  lactic  (from  sour  milk) 
and  acetic  (from  vinegar)  are  two  common  organic  acids.  A  base  is  a 
compound  which  releases  hydroxyl  ions  (OH-)  when  dissolved  in  water. 
Bases  turn  red  litmus  paper  blue.  Sodium  hydroxide  (NaOH)  and  am- 
monium hydroxide  (NH4OH)  are  common  inorganic  bases.  For  con- 
venience in  stating  the  degree  of  acidity  or  alkahnity  of  a  fluid,  the 
hydrogen  ion  concentration  may  be  expressed  in  terms  of  pH,  the 
negative  logarithm  of  the  hydrogen  ion  concentration.  On  this  scale,  a 
neutral  solution  has  a  pH  of  7  (its  hydrogen  ion  concentration  is 
0.000,000,1  or  10-7  molar),  alkaline  solutions  have  pH's  ranging  from 
7  to  14  (the  pH  of  1  M  NaOH),  and  acids  have  pH's  from  7  to  0  (the 
pH  of  1  M  HCl).  The  protoplasm  of  most  animal  cells  is  neither 
strongly  acid  nor  alkaline  but  contains  a  mixture  of  acidic  and  basic 
substances;  its  pH  is  about  7.0.  Any  considerable  change  in  the  pH  of 
protoplasm  is  inconsistent  with  life.  Since  the  scale  is  a  logarithmic  one, 
a  solution  with  a  pH  of  6  has  a  hydrogen  ion  concentration  10  times  as 
great  as  that  of  one  with  a  pH  of  7. 

^Vhen  an  acid  and  a  base  are  mixed,  the  hydrogen  ion  of  the  acid 
unites  with  the  hydroxyl  ion  of  the  base  to  form  a  molecule  of  water 
(H2O).  The  remainder  of  the  acid  (anion)  combines  with  the  rest  of 
the  base  (cation)  to  form  a  salt.  For  example,  hydrochloric  acid  (HCl) 
reacts  with  sodium  hydroxide  (NaOH)  to  form  water  and  sodium 
chloride  (XaCl)  or  common  table  salt: 

H  +  Cl-   +   Na+OH-  >  H2O  +  Na+Cl- 

A  salt  may  be  defined  as  a  compound  in  which  the  hydrogen  atom  of  an 
acid  is  replaced  by  some  metal. 

When  a  salt,  an  acid  or  a  base  is  dissolved  in  water  it  separates  into 
its  constituent  ions.  These  charged  particles  can  conduct  an  electric 
current,  hence  these  substances  are  known  as  electrolytes.  Sugars,  alco- 
hols, and  the  many  other  substances  which  do  not  separate  into  charged 
particles  when  dissolved,  and  therefore  do  not  conduct  an  electric  cur- 
rent, are  called  nonelectrolytes. 

In  protoplasm  from  any  sort  of  animal  one  finds  a  variety  of 
mineral  salts,  of  which  sodium,  potassium,  calcium  and  magnesium  are 
the   chief  cations   (positively   charged   ions)   and   chloride,    bicarbonate. 


24  GENERAL   CONCEPTS 

phosphate  and  suliate  aie  the  important  anions  (negatively  charged 
ions).  1  he  body  fluids  ot  land  vertebrates  resemble  sea  water  in  the 
kinds  ol  salts  piesent  and  in  their  relative  proportions,  but  the  total 
concentration  ol  salts  is  only  about  one-filth  as  great  as  in  sea  water. 
Most  biologists  now  believe  that  life  originated  in  the  sea.  The  early 
protoplasm  became  adapted  to  function  optimally  in  the  presence  of 
this  pattern  of  salts.  As  larger  animals  evolved  and  developed  body 
fluids,  this  pattern  of  salts  was  maintained,  even  as  some  of  the  de- 
scendants migrated  into  fresh  water  or  onto  the  land. 

Although  the  concentration  of  salts  in  cells  and  in  the  body  fluids 
is  small,  this  amount  is  of  great  importance  for  normal  cell  functioning. 
The  concentrations  ot  the  respective  cations  and  anions  are  kept  re- 
markably constant  under  normal  conditions;  any  marked  change  results 
in  impaired  function  and  finally  in  death.  A  great  many  of  the  enzymes 
w'hich  mediate  the  chemical  reactions  occurring  in  the  body  require  one 
or  another  of  these  ions— for  example,  magnesium,  manganese,  cobalt, 
potassium— as  cofactors.  These  enzymes  are  completely  unable  to  func- 
tion in  the  absence  of  the  ion.  Normal  nerve  function  requires  a  certain 
concentration  ot  calciiun  in  the  body  fluids;  a  decrease  in  this  results 
in  convulsions  and  death.  Normal  muscle  contraction  requires  certain 
amounts  ot  calcium,  potassium  and  sodium.  If  a  frog  heart,  for  example, 
is  removed  from  the  body  and  placed  in  a  solution  of  sodium  chloride, 
it  soon  stops  beating  and  remains  in  the  relaxed  state.  If  placed  in  a 
solution  of  potassium  chloride,  or  in  a  mixture  of  sodium  and  calcium 
chloride,  it  ceases  beating  in  the  contracted  condition.  But  it  it  is  placed 
in  a  solution  of  the  three  salts  in  proper  proportion  it  will  continue  to 
beat  for  hours.  Under  the  proper  conditions,  the  strength  of  the  heart 
beat  is  proportional  to  the  concentration  ot  calcium  ions  in  the  fluid 
bathing  the  heart;  this  method  is  sensitive  enough  to  be  used  to  measure 
the  concentration  ot  calcium  ions.  In  addition  to  these  several  specific 
effects  ot  particular  cations,  mineral  salts  serve  an  important  function 
in  maintaining  the  osmotic  relationships  between  protoplasm  and  its 
environment. 

8.        Organic  Compounds  of  Biological  Importance 

The  major  types  of  organic  substances  found  in  protoplasm  are 
the  carbohydrates,  proteins,  tats,  nucleic  acids  and  steroids.  Some  of 
these  are  required  tor  the  structural  integrity  of  the  cell,  others  to  supply 
energy  tor  its  functioning,  and  still  others  are  of  prime  importance  in 
regulating  metabolism  within  the  cell.  The  basic  pattern  of  the  types  of 
substances,  and  even  their  relative  proportions,  is  remarkably  similar  for 
cells  from  the  various  parts  of  the  body  and  for  cells  from  different 
animals.  A  bit  of  human  liver  and  the  protoplasm  of  an  ameba  each 
contain  about  80  per  cent  water,  12  per  cent  protein,  2  per  cent  nucleic 
acid,  5  per  cent  fat,  1  per  cent  carbohydrate  and  a  fraction  of  1  per  cent 
of  steroids  and  other  substances.  Certain  specialized  cells,  of  course,  have 
unique  patterns  ot  chemical  constituents;  the  brain,  for  example,  is  rich 
in  certain  kinds  of  fats. 


PROTOPLASM  25 

Carbohydrates.  The  simplest  of  the  organic  substances  are  the  car- 
bohydrates—the sugars,  starches  and  celluloses— which  contain  carbon, 
hydrogen  and  oxygen  in  a  ratio  ot  1  C  :  2  H  :  1  O.  Carbohydrates  are 
found  in  all  living  cells,  usually  in  relatively  small  amounts,  and  are 
important  as  readily  available  sources  of  energy.  Both  glucose  (also 
known  as  dextrose)  and  fructose  (also  called  levulose)  are  simple  sugars 
with  the  formula  CoHjoOg.  However,  the  arrangement  of  the  atoms 
within  the  two  molecules  is  different  and  the  two  sugars  have  somewhat 
different  chemical  properties  and  quite  different  physiologic  roles.  Such 
differences  in  the  molecular  configurations  of  substances  with  the  same 
chemical  formula  are  frequently  found  in  organic  chemistry.  Chemists 
indicate  the  molecular  configuration  of  a  substance  by  a  structural  for- 
mula in  which  the  atoms  are  represented  by  their  symbols— C,  H,  O,  N, 
etc.— and  the  chemical  bonds  or  forces  which  hold  the  atoms  together  are 
indicated  by  lines.  Hydrogen  has  one  such  bond;  oxygen,  two;  nitrogen, 
three;  and  carbon,  four.  The  structural  formulas  of  glucose  and  fructose 
are  compared  in  Figure  2.4.  Note  that  the  lower  four  carbon  atoms  have 
identical  groups  in  the  two  sugars;  only  the  upper  two  show  differences. 

Glucose  is  the  only  simple  sugar  which  occurs  in  any  quantity  in  the 
cells  and  body  fluids  of  both  vertebrates  and  invertebrates.  The  other 
carbohydrates  eaten  by  vertebrates  are  converted  to  glucose  in  the  liver. 
Glucose  is  an  indispensable  component  of  mammalian  blood,  and  is 
normally  present  in  a  concentration  of  about  0.1  per  cent.  No  par- 
ticular harm  results  from  a  simple  increase  in  the  concentration  of 
glucose  in  the  body  fluids,  but  when  the  concentration  is  reduced  to 
0.04  per  cent  or  less,  the  brain  cells  become  hyperirritable.  They  dis- 
charge nerve  impulses  which  result  in  muscular  twitches,  convulsions, 
and  finally  unconsciousness  and  death.  Brain  cells  use  glucose  as  their 
prime  metabolic  fuel,  and  a  certain  minimum  concentration  of  glucose 
in  the  blood  is  required  to  supply  this.  A  complex  physiologic  control 
mechanism,  which  ojjerates  like  the  "feed-back"  controls  of  electronic 
devices,  and  which  involves  the  liver,  pancreas,  pituitary  and  adrenal 
glands,  maintains  the  proper  concentration  of  glucose  in  the  blood. 

The  double  sugars,  with  the  formula  C12H22O11,  consist  of  two 
molecules  of  simple  sugar  joined  by  the  removal  of  a  molecule  of  water. 

H 

I 
H— C— O— H 

c=o 

I 
HO— C— H 

I 
H— C— O— H 

I 
H— C— O— H 

H— C— O— H 

1 

H 
Fructose 

Figure  2.4.     Structural  formulas  of  two  simple  sugars. 


i    0 

H— C— O- 

-H 

O— C— H 

H— C— O- 

-H 

H— C— O- 

-H 

H— C— O- 

1 

-H 

1 
H 

Glucose 

26  GENERAL   CONCEPTS 

Sucrose,  or  table  sugar,  is  a  combination  of  glucose  and  fructose.  Other 
common  double  sugars  are  maltose,  composed  of  two  molecules  of  glu- 
cose, and  lactose,  composed  of  glucose  and  galactose.  Lactose,  found  in 
the  milk  of  all  mammals,  is  an  miportant  item  in  the  diet  of  the  young 
of  these  forms.  Fructose,  the  sweetest  of  the  common  sugars,  is  more  than 
ten  times  sweeter  than  lactose;  sucrose  is  intermediate. 

Most  animal  cells  contain  some  glycogen  or  animal  starch,  the 
molecules  of  which  are  made  of  a  very  large  number— thousands— of 
of  molecides  of  glucose  joined  together  by  the  removal  of  an  H  from 
one  and  an  OH  from  the  next.  Glycogen  is  the  form  in  which  animal 
cells  store  carbohydrate  for  use  as  an  energy  source  in  cell  metabolism. 
The  glycogen  molecules  within  a  living  cell  are  constantly  being  built 
up  and  broken  down.  Glucose  and  other  simple  sugars  are  not  a  suit- 
able storage  form  of  carbohydrate  for,  being  soluble,  they  readily  pass 
out  of  the  cells.  The  molecules  of  glycogen,  which  are  much  larger  and 
less  soluble,  cannot  pass  through  the  plasma  membrane.  Glycogen  is 
typically  stored  within  cytoplasm  as  microscopic  granules,  which  can  be 
made  visible  by  special  stains.  Glycogen  is  readily  converted  into  small 
molecides  such  as  glucose-phosphate  (p.  72)  to  be  metabolized  within  the 
cell. 

Cellulose,  also  composed  of  hundreds  of  molecules  of  glucose,  is  an 
insoluble  carbohydrate  which  is  a  major  constituent  of  the  tough  outer 
wall  of  plant  cells. 

Glucosamine  and  galactosamine  are  nitrogen-containing  derivatives 
of  the  sugars  glucose  and  galactose  and  are  important  constituents  of 
supporting  substances  such  as  connective  tissue  fibers,  cartilage  and 
chitin,  a  constituent  of  the  hard  outer  shell  of  insects,  spiders  and  crabs. 

Fats.  The  term  fat,  or  lipid,  refers  to  a  heterogeneous  group  of 
compounds  which  share  the  property  of  being  soluble  in  chloroform, 
ether  or  benzene,  but  are  only  very  sparingly  soluble  in  water.  True  fats 
are  composed  of  carbon,  hydrogen  and  oxygen,  but  have  much  less 
oxygen  than  carbon.  Each  molecule  of  a  true  fat  contains  one  molecule 
of  glycerol  (C3H5(OH)3)  and  three  molecules  of  some  fatty  acid,  joined 
together  by  the  removal  of  three  molecules  of  water.  The  fats  differ  in 
the  kinds  of  fatty  acids  present.  Oleic  acid,  C17H33COOH,  is  a  common 
fatty  acid,  and  triolein,  the  fat  containing  three  molecules  of  oleic  acid, 
has  the  formula  Cr,7Hi04O6.  Fats  have  a  greasy  or  oily  consistency;  some, 
such  as  beef  tallow  or  bacon  fat,  are  solid  at  ordinary  temperatures, 
others  such  as  whale  oil  or  cod  liver  oil  are  liquid. 

Fats  are  important  in  protoplasm  both  as  fuels  and  as  structural 
constituents.  They  yield  more  than  twice  as  much  energy  per  gram  than 
do  carbohydrates  and  thus  are  a  more  economical  form  for  the  storage 
of  food  reserves.  Carbohydrates  can  be  metabolized  to  release  energy  very 
quickly  and  thus  serve  as  short-term  storage  forms.  Fats  provide  for  a 
longer-term  storage  of  food  reserves.  Carbohydrates  are  readily  con- 
verted by  cells  into  fats  and  may  be  stored  in  this  form.  This,  of  course, 
is  the  explanation  for  the  observation  that  sugars  and  starches  are  "fat- 
tening." The  reverse  process  may  also  occur,  but  to  a  lesser  extent. 
Experiments    with    fats    labeled    with    radioactive    carbon    atoms    have 


PROTOPLASM  27 

shown  that  these  may  be  converted  in  the  animal  body  to  carbohydrates 
such  as  glucose. 

The  nuclear  membrane,  the  plasma  membrane  around  the  cell,  and 
the  membrane  around  the  mitochondria  all  contain  fatty  substances.  The 
myelin  sheath  which  surrounds  nerve  fibers  (p.  61)  is  exceptionally  rich 
in  lipids.  In  some  animals,  such  as  mammals,  there  are  large  deposits 
of  fat  just  under  the  skin  which  serve  as  fuel  reserves  and  as  insulators 
to  decrease  the  loss  of  heat  from  the  body.  The  lipid  stores  of  animals 
such  as  sharks  and  starfish  are  in  the  form  of  oils  found  in  the  liver. 

Related  to  the  true  fats  are  the  phospholipids,  waxes  and  cerebro- 
sides,  all  of  which  contain  fatty  acids.  The  phospholipids,  which  contain 
phosphorus  and  nitrogen  in  addition  to  glycerol  and  fatty  acids,  are 
important  structural  and  functional  components  of  protoplasm  and  are 
especially  found  in  mitochondria  and  microsomes.  Waxes,  such  as  bees- 
wax and  lanolin,  contain  a  fatty  acid  plus  an  alcohol  other  than  gly- 
cerol. Cerebrosides,  as  their  name  indicates,  are  fatty  substances  found 
especially  in  nerve  tissue.  They  contain  galactose,  long  chain  fatty  acids, 
and  a  long  chain  amino  alcohol,  sphingosine.  The  metabolic  roles  of 
these  special  fats  is  not  clear  at  present. 

Sferoids.  Steroids  are  complex  molecules  containing  carbon  atoms 
arranged  in  four  interlocking  rings,  three  of  which  contain  six  carbon 
atoms  each  and  the  fourth  of  which  contains  five.  Vitamin  D,  male  and 
female  sex  hormones,  the  adrenal  cortical  hormones,  bile  salts  and 
cholesterol  are  examples  of  steroids.  Cholesterol  (Fig.  2.5)  is  an  important 
structural  component  of  nervous  tissue  and  other  tissues,  and  the  steroid 
hormones  are  of  great  importance  in  regulating  certain  aspects  of  me- 
tabolism. 

Proteins.  Proteins  differ  from  carbohydrates  and  true  fats  in  that 
they  contain  nitrogen  in  addition  to  carbon,  hydrogen  and  oxygen.  Pro- 
teins typically  contain  sulfur  and  phosphorus  in  addition.  Proteins  are 
always  present  in  protoplasm  and  are  of  prime  importance  as  the  basic 
building  materials  of  living  matter.  Protein  molecules  are  among  the 
largest  found  in  protoplasm  and  they  share  with  nucleic  acids  the  dis- 
tinction of  great  complexity  and  variety.  Hemoglobin,  the  red  pigment 
found  in  the  blood  of  all  vertebrates  and  many  invertebrates,  has  the 
formula  C,o32H48i60872N78oS8Fe4  (Fe  is  the  symbol  for  iron).  Although 
the  hemoglobin  molecule  is  enormous  compared  to  a  glucose  or  triolein 
molecule,  it  is  only  a  small-to-medium-sized  protein.  Many,  indeed  most, 
of  the  proteins  in  protoplasm  are  enzymes,  biological  catalysts  which 
control  the  rates  of  the  many  chemical  processes  of  the  cell. 

CH3   CH2 

CH        CHo  CH3 


CH3  I  I  / 

X\\/\         CH2— CH 

CH,  I  ]  \ 


HO 


CH 


Figure  2.5.     Structural  formula  of  a  steroid,  cholesterol. 


28  GENERAL   CONCEPTS 


H             H            O  H             H            O 

\          1         ^  \          I         ^ 

N— C— C  b.             a.            N— C— C                b. 

/          I         \  /          1         \ 

H             H            OH  H              1             OH 

CH3 

glycine  alanine 


I 


+  H2O 

CH3       OH 

linkage 
glycylalanine 

Figure  2.6.  Structural  formulas  of  the  amino  acids  glycine  and  alanine,  showing,  a, 
the  amino  group  and,  b,  the  acid  (carboxyl)  group.  These  are  joined  in  a  peptide  linkage 
to  form  glycylalanine  by  the  removal  of  water. 

Protein  molecules  are  made  of  simpler  components,  the  amino  acids, 
some  thirty  or  more  of  which  are  known.  Since  each  type  of  protein  con- 
tains hundreds  of  amino  acids,  present  in  a  certain  proportion  and  in  a 
particular  order,  an  almost  infinite  variety  of  different  proteins  is 
possible.  In  recent  years,  powerful  analytical  methods  have  been  de- 
veloped which  permit  one  to  determine  the  arrangement  of  the  amino 
acids  in  a  given  protein  molecule.  This  is  an  extremely  difficult  and 
tedious  task.  Insulin,  the  hormone  secreted  by  the  pancreas  and  used  in 
the  treatment  of  diabetes,  was  the  first  protein  whose  structure  was 
elucidated.  Work  culminating  in  1957  revealed  the  structure  of  the 
enzyme  ribonuclease. 

Each  cell  contains  hundreds  of  different  proteins  and  each  kind  of 
cell  contains  some  proteins  which  are  unique  to  it.  There  is  evidence 
that  each  species  of  animal  and  plant  has  certam  proteins  which  are 
different  from  those  of  all  other  species.  The  degree  of  similarity  of  the 
proteins  of  two  species  is  a  measure  of  their  evolutionary  relationship. 
The  theory  of  species  specificity  states  that  the  protoplasm  of  each 
species  has  a  characteristic  pattern  of  its  constituent  proteins  and  that 
this  pattern  differs  at  least  slightly  from  that  of  related  species  and  more 
markedly  from  those  of  more  distantly  related  species.  Because  of  the 
interactions  of  unlike  proteins,  grafts  of  tissue  removed  from  one  animal 
will  usually  not  grow  when  implanted  on  a  host  of  a  different  species. 

Amino  acids,  the  unit  building  blocks  of  proteins,  differ  in  the 
number  and  arrangement  of  their  constituent  atoms,  but  all  contain  an 
amino  group  (NHo)  and  an  acid  group  (COOH),  whence  their  name. 
The  amino  group  enables  the  amino  acid  to  act  as  a  base  and  combine 
with  acids;  the  acid  group  enables  it  to  combine  with  bases.  For  this 
reason,  amino  acids  and  proteins  are  important  biological  "buffers"  and 
resist  changes  in  acidity  or  alkalinity,  thus  protecting  protoplasm.  Pro- 
tein molecules  are  built  up  by  linkages  (called  peptide  bonds)  between 
the  amino  group  of  one  amino  acid  and  the  acid  group  of  the  adjacent 
one  (Fig.  2.6).  Pure  amino  acids  have  a  rather  sweet  taste.  The  proteins 
eaten  by  an  animal  are  not  incorporated  directly  into  the  protoplasm 


PROTOPLASM  29 

but  are  first  digested  to  the  constituent  amino  acids  to  enter  the  cell. 
Subsequently  each  cell  combines  the  amino  acids  into  the  proteins  which 
are  characteristic  of  that  cell.  Thus,  a  man  eats  beef  proteins  in  a  steak, 
but  breaks  them  down  to  amino  acids  in  the  process  of  digestion,  then 
rebuilds  them  as  human  proteins. 

Proteins  and  amino  acids  may  serve  as  energy  sources  in  addition  to 
their  structural  and  enzymatic  roles.  Most  animals  eat  more  proteins  than 
are  needed  for  the  maintenance  of  protoplasm.  The  extra  amino  acids 
undergo  the  process  of  deamination  in  which  the  amino  group  is  re- 
moved, then  the  remaining  carbon  skeleton  enters  the  same  metabolic 
paths  as  glucose  and  fatty  acids  and  eventually  is  converted  to  carbon 
dioxide  and  water  by  the  Krebs  tricarboxylic  acid  cycle  (p.  72)  and 
associated  paths.  The  amino  group  is  excreted  as  ammonia,  urea,  uric 
acid  or  some  other  nitrogenous  compound,  depending  on  the  kind  of 
animal.  In  prolonged  fasting,  after  the  supply  of  carbohydrates  and  fats 
has  been  exhausted,  the  proteins  of  protoplasm  itself  are  used  as  a  source 
of  energy.  Animal  cells  can  synthesize  some,  but  not  all,  of  the  different 
kinds  of  amino  acids;  different  species  differ  in  their  synthetic  abilities. 
Man,  for  example,  is  apparently  unable  to  synthesize  eight  of  these;  they 
must  either  be  supplied  in  the  food  eaten  or  perhaps  synthesized  by  the 
bacteria  present  in  the  intestine.  Plant  cells  apparently  can  synthesize 
all  of  the  amino  acids.  The  ones  which  an  animal  cannot  synthesize,  but 
must  obtain  in  its  diet,  are  called  essential  amino  acids.  It  must  be  kept 
in  mind  that  these  are  no  more  essential  tor  protein  synthesis  than  any 
other  amino  acid,  but  are  simply  essential  constituents  of  the  diet,  with- 
out which  the  animal  fails  to  grow  and  eventually  dies. 

Nucleic  Acids.  The  biological  importance  of  the  fifth  major 
group  of  organic  compounds  found  in  protoplasm,  the  nucleic  acids,  has 
been  fully  appreciated  only  in  recent  years.  These  complex  molecules, 
as  large  as  or  larger  than  most  proteins,  were  first  discovered  in  1870, 
when  Miescher  isolated  them  fiom  the  nuclei  of  pus  cells.  Nucleic  acid 
molecules  contain  carbon,  hydrogen,  oxygen,  nitrogen  and  phosphorus; 
they  gained  their  name  from  the  fact  that  they  are  acidic  and  were  first 
identified  in  nuclei.  They  contain  nitrogenous  organic  bases  (purines 
and  pyrimidines),  five-carbon  sugars  (ribose  or  desoxyribose)  and  phos- 
phoric acid.  For  a  long  time  it  was  thought  that  there  were  but  two  kinds 
of  nucleic  acid-one  containing  the  sugar  ribose  and  called  ribose  nu- 
cleic acid  or  RXA  and  found  in  cytoplasm,  and  one  containing  de- 
soxyribose and  called  desoxyribonucleic  acid  or  DNA  and  located  in  the 
cell  nucleus.  Since  1948  experiments  have  made  it  clear  that  there  are 
many  different  kinds  of  RNA  and  of  DNA.  RNA  and  DNA  are  now 
used  as  generic  terms  for  a  large  class  of  substances  which  differ  in  their 
details  of  structure  and  specificity. 

It  is  now  clear  that  DNA  is  responsible  for  a  large  part,  perhaps  all, 
of  the  specificity  and  chemical  properties  of  the  genes,  the  units  of 
heredity  located  in  the  nucleus.  Ribonucleic  acid  plays  an  important 
role  in  the  svnthesis  of  protein  and  perhaps  of  other  large  molecules  as 
well.  The  building  blocks  of  nucleic  acids  are  nucleotides,  just  as  amino 
acids   are   the    units   of  protein    molecules.   A   nucleotide   contains   one 


30  GENERA/.   CONCEPTS 

purine  or  pyrimidine  molecule,  one  ribose  or  desoxyribose  molecule  and 
one  phosphoric  acid  molecule.  1  he  nucleotides  differ  in  the  particular 
knid  ot  purine  or  pyrmiichne  present  and  the  nucleic  acids  differ  in  the 
proportions  and  sequences  ot  their  constituent  nucleotides.  Ribonucleic 
acids  are  iound,  linked  to  proteins,  in  all  parts  oi  protoplasm— nucleus, 
mitochondria,  microsomes  and  in  the  liquid  ground  substance. 

9.        Physical  Characteristics  of  Protoplasm 

The  properties  of  protoplasm  depend  not  only  on  the  kinds  and 
quantities  of  substances  present,  but  on  their  physical  state  as  well.  A 
mixture  of  a  substance  with  water,  or  other  liquid,  may  result  in  a  true 
solution,  a  suspension  or  a  colloidal  solution,  differentiated  by  the  size 
of  the  dispersed  particles.  In  a  true  solution,  the  ions  or  molecules  of 
the  dissolved  substance  (called  the  solute)  are  of  extremely  small  size, 
less  than  0.0001  micron  in  diameter.  The  solute  particles  are  either  ions 
or  small  molecules  dispersed  among  the  molecules  of  the  dissolving 
liquid  (called  the  solvent).  A  true  solution  is  transparent  and  has  a 
higher  boiling  point  and  a  lower  freezing  point  than  pure  water.  Most 
acids,  bases,  salts  and  some  nonelectrolytes,  such  as  sugars  and  amino 
acids,  form  true  solutions  in  water. 

The  dispersed  particles  in  a  suspension,  in  contrast,  are  much 
larger  (greater  than  0.1  micron)  and  are  composed  of  aggregations  of 
many  molecules.  They  tend  to  settle  out  if  the  suspension  is  allowed  to 
stand.  Muddy  water,  for  example,  contains  particles  of  clay  in  suspension. 
Suspensions  are  opaque  rather  than  transparent,  and  have  the  same 
boiling  and  freezing  points  as  pure  water. 

A  colloidal  solution  contains  particles  intermediate  in  size  between 
those  of  a  true  solution  and  a  suspension,  particles  from  0.0001  to  0.1 
micron  in  diameter.  A  colloidal  solution,  or  colloid,  is  transparent  or 
translucent,  has  about  the  same  boiling  and  freezing  points  as  pure 
water,  and  is  stable;  it  does  not  tend  to  separate  into  its  constituent  parts 
on  standing.  The  particles  of  a  colloidal  solution  may  have  a  positive 
or  a  negative  charge,  but  usually  they  all  have  the  same  charge  and 
tend  to  repel  each  other.  The  presence  of  the  charge  is  a  factor  which 
tends  to  keep  the  particles  dispersed.  A  colloid  solution  has  the  unique 
property  of  changing  from  a  liquid  state,  or  sol,  to  a  solid  or  semisolid 
state  or  gel  (Fig.  2.7).  A  familiar  example  of  the  change  from  sol  to  gel 
occurs  when  a  package  of  gelatin  is  dissolved  in  hot  water.  The  particles 
of  gelatin  (a  protein)  are  dispersed  through  the  water  and  a  liquid 
colloidal  solution,  a  sol,  results.  As  the  gelatin  cools,  the  gelatin  par- 
ticles aggregate  and  become  the  continuous  phase,  the  water  particles 
become  dispersed  as  small  droplets  in  the  gelatin  and  a  semisolid  gel 
results.  The  gel  can  be  converted  back  to  a  sol  by  reheating.  The 
gelatin-water  mixture  is  a  liquid  sol  when  it  consists  of  particles  of 
gelatin  dispersed  in  water  and  a  solid  gel  when  the  droplets  of  water 
are  dispersed  in  gelatin.  The  sol-gel  change  may  be  effected  by  changing 
the  temperature,  the  pH  or  the  salt  concentration  or  by  mechanical 
agitation  (whipping  cream,  for  example).  The  change  is  reversible,  but 


PROrOPtASAI 


31 


N 


r 


^ 


r 


w  ^ 


.1 


J 


A  B 

Figure  2.7.  Diagram  of  a  colloidal  solution  as  a  sol  (.4)  and  a  gel  (B).  The  sol  con- 
tains water  as  the  continuous  phase  in  which  the  colloidal  particles  (dark  rods)  are  dis- 
persed. In  the  gel  the  colloidal  particles  have  coalesced  to  form  a  continuous  lacy  network 
in  which  the  water  droplets  (light  circles)  are  dispersed. 

if  the  system  is  subjected  to  large  changes  of  temperature,  acidity,  alka- 
linity or  salt  concentration,  the  colloidal  solution  is  destroyed;  the  par- 
ticles coagulate  and  settle  out. 

Many  of  the  properties  of  colloids  are  a  result  of  the  enormous 
amount  of  surface  area  between  the  dissolved  particles  and  the  dissolv- 
ing medium.  For  example,  a  cube  1  cm.  on  each  edge  has  a  total  surface 
area  of  6  cm.,  but  an  equal  volume  of  material  divided  into  particles 
0.01  micron  on  an  edge  has  a  total  surface  area  of  6,000,000  square  cm. 
Many  chemical  reactions  occur  only  at  a  surface,  and  for  this  reason  a 
colloidal  system  is  a  much  better  medium  for  chemical  reactions  than  is 
any  other  type  of  mixture. 

Many  of  the  unique  properties  of  protoplasm  follow  from  the  fact 
that  it  is  a  colloidal  system  composed  of  protein  molecules  in  water.  The 
protein  molecules  are  too  large  to  form  a  true  solution  in  water  and  too 
small  to  settle  out.  Protoplasm  is  constantly  and  rapidly  changing  from 
sol  to  gel  and  back;  one  portion  of  the  protoplasm  of  a  cell  may  be  a 
sol  while  others  are  gels.  The  constant,  rapid  change  from  sol  to  gel  is 
one  expression  of  the  "aliveness"  of  protoplasm.  Any  extreme  of  tem- 
perature, acidity,  alkalinity,  or  the  presence  of  certain  chemicals  will 
cause  an  irreversible  change  to  the  gel  or  sol  state  and  the  protoplasm 
is  no  longer  alive.  Protoplasin  contains  a  large  amount  of  water-80  per 
cent  of  muscle  is  water,  for  example-yet,  because  the  water  is  part  of  a 
colloidal  system,  bound  to  the  proteins  present,  muscle  itself  can  be- 
come quite  solid  during  contraction.  Muscle  contraction,  like  many 
other  biological  phenomena,  involves  a  change  from  the  sol  state  to  the 
gel.  Shortly  after  death  muscle  undergoes  rigor  mortis,  an  irreversible 
change  to  the  gel  state. 

Questions 

1.  Discuss  the  characteristics  of  living  things.  Are  any  of  these  found  in  nonliving 
systems?  Can  you  think  of  any  which  should  be  added  to  the  list?  Any  which  do  not 
seem  essential? 


32  GENERAL   CONCEPTS 

2.  Describe  an  experiment  to  test  the  theory  that  worms  develop  from  horsehairs  in  a 
%vater  trough.  What  observations  do  you  suppose  led  to  this  hypothesis?  Can  you 
supply  an  alternate  hypothesis  that  explains  the  observation  without  invoking  spon- 
taneous generation? 

3.  Discuss  the  ways  in  which  the  following  animals  are  adapted  to  their  mode  of  life: 
honey  bee,  salmon,  frog,  field  mouse. 

4.  What  are  the  distinguishing  characteristics  of  mitochondria,  microsomes  and  Golgi 
bodies?  What  are  the  functions  of  each? 

5.  What  is  the  exact  meaning  of  each  of  the  following  terms:  atom,  isotope,  ion?  Could 
a  single  particle  of  matter  be  all  three  simultaneously? 

6.  In  what  ways  are  isotopes  used  in  zoological  research? 

7.  What  is  the  most  abundant  compound  in  protoplasm?  What  are  its  functions? 

8.  Discuss  what  is  meant  by  the  "dynamic  state"  of  protoplasm. 

9.  What  distinguishes  organic  and  inorganic  compounds? 

10.  What  is  meant  by  the  symbol  pH? 

11.  What  are  the  functions  in  protoplasm  of  each  of  the  following:  salts,  fats,  proteins, 
nucleic  acids,  steroids? 

12.  What  are  the  chief  properties  of  colloidal  solutions?  Describe  three  examples  of 
colloidal  solutions  other  than  the  ones  discussed  in  the  text. 

Supplementary  Reading 

Some  of  the  chemical  aspects  of  protoplasm  are  discussed  in  R.  W.  Gerard's  Unresting 
Cells.  The  Cell  and  Protoplasm,  edited  by  F.  R.  Moulton,  contains  a  series  of  short  papers, 
each  by  an  authority  on  the  subject,  on  a  variety  of  topics  related  to  protoplasm.  The 
subject  of  atoms,  neutrons  and  isotopes  is  discussed  in  A.  K.  Solomon's  Why  Smash  Atoms? 
and  in  H.  B.  Lemon's  From  Galileo  to  the  Nuclear  Age.  Further  discussions  of  acids,  bases, 
salts  and  the  chemical  compounds  found  in  protoplasm  can  be  found  in  any  introductory 
chemistry  text. 


CHAPTER  3 


Cells  and  Tissues 


10.        The  Cell  and  Its  Contents 

The  living  substance  of  all  animals  is  organized  into  units  called 
cells.  A  cell  is  a  mass  ot  protoplasm  containing  a  nucleus  and  surrounded 
by  a  plasma  membrane.  Mammalian  red  blood  cells  lose  their  nucleus  in 
the  process  of  maturation,  and  a  few  types  of  cells  such  as  those  of 
skeletal  muscles  have  several  nuclei  per  cell,  but  these  are  rare  excep- 
tions to  the  general  rule  of  one  nucleus  per  cell.  In  the  simplest  animals, 
the  protozoa,  all  of  the  protoplasm  is  found  within  a  single  plasma 
membrane.  These  animals  may  be  considered  to  be  unicellular,  i.e., 
single-celled,  or  acellular,  with  bodies  not  divided  into  cells.  Many 
protozoa  have  a  high  degree  of  specialization  of  form  and  function 
within  this  single  cell  (Fig.  3.1),  and  the  single  cell  may  be  quite  large, 
larger  tlian  certain  multicellular,  more  complex  organisms.  Thus,  it 
would  be  wrong  to  infer  that  a  single-celled  animal  is  necessarily  smaller 
or  less  complex  than  a  many-celled  animal. 

The  term  "cell"  was  applied  by  Robert  Hooke,  some  300  years  ago, 
to  the  small,  box-like  cavities  he  saw  when  he  examined  cork  and  other 
plant  material  under  the  newly-invented  compound  microscope.  The 
important  part  of  the  cell,  we  now  realize,  is  not  the  cellulose  wall  seen 
by  Hooke,  but  the  cell  contents.  In  1839  the  Bohemian  physiologist, 
Purkinje,  introduced  the  term  "protoplasm"  for  the  living  material  of 
the  cell.  At  this  time  a  German  botanist,  Schleiden,  and  Schwann,  his 


Food. 

vacuolen 


Contractile 
va.cuolen 


Oral  disc 


Retractile 


Mou.th.->^ 


r  1  aminae       Rectum 


•-Plasmasol]  ^^^^ 


Motor 
mass 


Contractile 
vaCLtole 


Endoplasm. 
MicronuclcLLS 

Macronucleus     Ectoplasm 


Region  of 
gelat-ion 

"JEctoplasTTL 

Figure  3.1.     Diagrams  of  an  anieba  (left)  and  Epidinium  (right)  to  illustrate  the 
range  in  complexity  of  the  single-celled  animals. 

33 


34  GENERAL   CONCEPTS 

fellow  countryman  and  a  zoologist,  formulated  the  generalization  which 
has  since  developed  into  the  cell  theory:  The  bodies  of  all  plants  and 
animals  are  composed  of  cells,  the  fundamental  units  of  life.  The  cell 
IS  both  the  structural  and  functional  unit  in  all  organisms,  the  funda- 
mental unit  possessmg  all  the  characteristics  of  living  things.  A  further 
generalization,  first  clearly  stated  by  Virchow  in  1855,  is  tnat  new  cells 
can  come  into  existence  only  by  the  division  of  previously  existing  cells. 
The  corollary  of  this,  that  all  cells  living  today  can  trace  their  ancestry 
back  to  the  earliest  living  things,  was  stated  by  August  VVeismann  about 

1880. 

The  bodies  of  higher  animals  are  made  of  many  cells,  which  are  not 
all  alike,  but  differ  in  size,  shape  and  functions.  A  group  of  cells  which 
are  similar  in  form,  and  specialized  to  perform  one  or  more  particular 
functions,  is  called  a  tissue.  A  tissue  may  contain  nonliving  cell  products 
in  addition  to  the  cells  themselves.  A  group  of  tissues  may  be  associated 
into  an  organ,  and  organs  into  organ  systems.  For  example,  in  a  verte- 
brate, the  digestive  system  is  composed  of  a  number  of  organs:  esophagus, 
stomach,  intestine,  liver,  pancreas,  and  so  on.  Each  organ,  such  as  the 
stomach,  contains  several  kinds  of  tissue-epithelium,  muscle,  connective 
tissue,  nerves-and  each  tissue  is  made  of  many,  perhaps  millions,  of  cells. 
If  a  single  cell  is  placed  in  the  proper  environment  it  will  survive, 
grow,  and  eventually  divide.  For  most  single-celled  animals,  a  drop  of 
sea  water  or  pond  water  will  provide  the  environment  required.  It  is 
more  difficult  to  culture  cells  removed  from  a  multicellular  animal— a 
man,  chick  or  frog.  This  was  first  accomplished  in  1907  by  Ross  Harrison 
of  Yale,  who  was  able  to  grow  cells  from  a  salamander  in  a  drop  of 
nutrient  medium  containing  blood  plasma.  Since  then,  many  different 
kinds  of  cells  from  animals  and  plants  have  been  cultured  in  vitro,*  and 
many  important  facts  about  cell  physiology  have  been  revealed  in  this 
way. 

The  cells  of  diflferent  organs  and  different  animals  present  a  be- 
wildering variety  of  sizes,  shapes,  colors  and  internal  structures,  but  all 
have  certain  features  in  common.  Each  cell  is  surrounded  by  a  plasma 
membrane,  contains  a  nucleus,  and  has  in  its  cytoplasm  mitochondria, 
microsomes,  Golgi  bodies  and  a  centriole. 

Each  cell  is  completely  enclosed  by  a  thin  sheet  of  protoplasm,  the 
plasma  membrane.  This  is  a  living,  functional  part  of  the  cell,  which 
controls  the  entrance  and  exit  of  nutrients,  secretions  and  waste  prod- 
ucts. The  plasma  membrane  is  permeable  to  certain  substances  and  not 
to  others;  in  addition  it  is  capable  of  doing  work  to  "pump"  substances 
into  and  out  of  the  cell.  Very  few  substances  are  found  at  the  same 
concentration  within  the  cell  and  in  the  surrounding  fluid;  some  con- 
centrations are  much  higher,  others  are  lower,  than  in  the  environment. 
The  activities  of  the  plasma  membrane  are  responsible  for  maintaining 
these  difterences.  When  it  fails  to  do  this,  the  cell  dies.  Nearly  all  plant 
cells  have,  in  addition  to  the  plasma  membrane,  a  thick  cell  wall  made 
of  cellulose.  This  nonliving  wall,  lying  outside  the  plasma  membrane,  is 

*  In  vitro,  Latin,  "in  glass."  The  cells  are  removed  from  the  animal  body  and  incu- 
bated in  glass  vessels. 


CELLS   AND   TISSUES 


35 


secreted  by  the  protoplasm.  It  is  pierced  by  fine  holes,  through  which 
substances  may  pass  and  the  cytoplasm  of  one  cell  may  connect  with  that 
of  adjacent  cells.  These  tough,  firm  cell  walls  provide  support  to  the 
plant  body. 

The  nucleus  of  the  cell  is  usually  spherical  or  ovoid.  It  may  have  a 
fixed  position  in  the  center  of  the  cell  or  at  one  side,  or  it  may  be  moved 
around  as  the  cell  moves  and  changes  shape.  The  nucleus  is  separated 
from  the  cytoplasm  by  a  nuclear  membrane  which  controls  the  move- 
ment of  materials  into  and  out  of  the  nucleus  (Fig.  3.2).  Recent  studies 
with  the  electron  microscope  have  shown  that  there  are  extremely  fine 


Figure  3.2.  -i,  Electron  micrograph  of  the 
nucleus  and  surrounding  cytoplasm  of  a  frog 
liver  cell.  The  spaghetti-like  strands  of  the 
microsomes  are  visible  in  the  lower  right  cor- 
ner. Magnified  16,500  X.  B,  High  power  elec- 
tron micrograph  of  mitochondria  and  micro- 
somes within  a  rat  liver  cell.  Granules  of 
ribonucleoprotein  are  seen  on  the  strands  of 
microsomes,  and  structures  with  double  mem- 
branes are  evident  within  the  mitochondria 
in  the  upper  left  corner  and  on  the  right. 
Magnified  65,000  X.  (Electron  micrographs 
courtesy  Dr.  Don  Fawcett.)  (Villee:   Biology.) 


36 


GENERAL   CONCEPTS 


channels  through  the  nuclear  membrane  through  which  the  nucleoplasm 
and  cytoplasm  "are  continuous.  The  nucleus  is  required  for  growth  and 
for  cell  division,  but  some  cells,  the  ameba,  for  example,  can  survive 
for  many  days  after  the  nucleus  has  been  removed  by  a  microsurgical 
operation,  'l^o  demonstrate  that  it  is  the  absence  of  the  nucleus,  not  the 
operation  itself,  that  causes  the  ensuing  death,  one  can  perform  a  sham 
operation.  A  microneedle  is  inserted  into  an  ameba  and  moved  around 
inside  the  cell  to  simulate  the  operation  of  removing  the  nucleus,  but 
the  needle  is  withdrawn  without  actually  removing  the  nucleus.  An 
ameba  subjected  to  this  sham  operation  will  recover,  grow  and  divide. 
A  controlled  experiment  such  as  this,  in  which  two  amebas  are  subjected 
to  the  same  operative  trauma  and  the  one  with  the  nucleus  lives  whereas 
the  one  without  the  nucleus  dies,  provides  strong  evidence  of  the  vital 
role  of  the  nucleus  in  the  metabolic  processes  that  underlie  growth  and 
cell  division. 

A  classic  demonstration  of  the  role  of  the  nucleus  in  the  control  of 


Figure  3.3.     Hammerling's  demonstration  of  the  production  of  an  umbrella-regener- 
ating substance  by  the  nucleus  of  Acctabularia.  See  text  for  discussion.  (Villee:  Biology.) 


CELLS   AND    TISSUES  37 

cell  growth  is  provided  by  the  experiments  of  Hammerling  with  the  sin- 
gle-celled plant  Acetabularia.  This  marine  alga,  which  is  4  to  5  cm.  long, 
is  mushroom-shaped,  with  "roots"  and  a  stalk  surmounted  by  a  flattened, 
disc-shaped  umbrella.  The  single  nucleus  is  located  near  the  base  of  the 
stalk.  Hammerling  cut  across  the  stalk  (Fig.  3.3)  and  found  that  although 
the  lower  part,  containing  the  nucleus,  could  live  and  regenerate  an 
umbrella,  the  upper  part  would  eventually  die  without  regenerating  a 
stalk  and  roots.  In  further  experiments,  Hammerling  first  severed  the 
stalk  just  above  the  nucleus  (cut  1,  Fig.  3.3),  then  made  a  second  cut  just 
below  the  umbrella  (cut  2).  The  section  of  stalk  thus  isolated,  when 
replaced  in  sea  water,  was  able  to  grow  a  partial  or  complete  umbrella. 
This  might  seem  to  show  that  a  nucleus  is  not  necessary  for  regenera- 
tion; however,  when  Hammerling  cut  oft  this  second  uniDrella  the  stalk 
was  unable  to  form  a  new  one.  From  experiments  such  as  these,  Ham- 
merling concluded  that  the  nucleus  supplies  some  substance  necessary 
for  umbrella  formation.  This  substance  passes  up  the  stalk  and  instigates 
umbrella  growth.  In  the  experiments  described  here,  some  of  this  sub- 
stance remained  in  the  stalk  after  cuts  1  and  2,  enough  to  produce  one 
new  umbrella.  After  that  amount  of  "umbrella  substance"  was  exhausted 
by  the  regeneration  of  an  umbrella,  no  second  regeneration  was  possible 
in  the  absence  of  a  nucleus. 

Dr.  Jean  Brachet,  of  the  University  of  Brussels,  found  that  both 
nucleatecl  and  non-nucleated  fragments  of  Acetabularia  kept  in  radio- 
active carbon  dioxide  in  the  ligiu  would  incorporate  the  radioactive 
carbon  into  proteins  at  rates  which  were  identical  lor  the  first  ten  days. 
Even  thirty  days  after  the  removal  of  the  nucleus,  non-nucleated  frag- 
ments synthesized  protein,  as  measured  by  the  incorporation  of  radio- 
active carbon,  at  a  rate  which  was  70  per  cent  as  great  as  that  of  the 
nucleated  fragments.  Dr.  Brachet  concluded  that  the  nuclear  control  of 
protein  synthesis  is  not  an  immediate  one  but  an  indirect  one.  He  be- 
lieves that  protein  synthesis  is  a  function  of  the  microsomes  and  the 
multiplication  of  the  microsomes  is  under  the  control  of  the  nucleus. 

^Vhen  a  cell  has  been  killed  by  fixation  with  the  proper  chemicals, 
and  then  stained  with  the  appropriate  dyes,  several  structures— strands 
of  chromatin  and  one  or  more  nucleoli— are  visible  within  the  nucleus 
(Fig.  3.4).  These  are  difficult  to  see  in  a  living  cell  with  an  ordinary  light 
microscope  but  are  evident  by  phase  microscopy.  Strands  of  chromatin, 
composed  of  nucleoproteins  with  a  strong  affinity  for  basic  dyes,  run 
irregularly  through  the  nucleus  and  exhibit  a  netlike  or  gianular  appear- 
ance. W^hen  the  cell  divides,  the  chromatin  threads  condense  and  form 
the  dark-staining,  rod-shaped  chromosomes  which  contain  the  hereditary 
units  called  genes.  A  nucleolus  is  a  small,  spherical  body  found  within 
the  nucleus.  There  may  be  more  than  one  nucleolus  per  nucleus,  but  the 
cells  of  any  particular  animal  have  the  same  number  of  nucleoli.  The 
nucleolus  disappears  when  a  cell  is  about  to  divide  and  reappears  after 
division  is  complete.  It  has  been  postulated  that  the  nucleolus  plays  some 
role  in  the  synthesis  of  proteins  and  ribonucleic  acids,  but  its  function 
is  not  known. 


38  GENERAL   CONCEPTS 


Figure  3.4.     Tissue  sections  of  human  adrenal  gland  stained  to  show  cellular  details; 
left,  magnified  600X;  right,  magnified  1500X  (courtesy  of  Dr.  Kurt  Benirschke). 


One  or  two  small,  dark-staining  spherical  bodies,  called  centrioles, 
are  found  in  the  cytoplasm  near  the  nucleus  of  animal  cells.  The  cen- 
triole  plays  a  role  in  cell  division  in  determining  the  location  of  the 
spindle  fibers  on  which  the  chromosomes  move  (p.  42).  It  would  appear, 
however,  that  centrioles  are  not  essential  for  cell  division,  for  plant  cells 
are  able  to  divide  without  them. 

The  cytoplasm  may  contain  droplets  of  fat,  and  crystals  or  granules 
of  protein  or  glycogen  which  are  simply  stored  for  future  use.  In  addi- 
tion, it  contains  the  metabolically  active  cell  organelles,  mitochondria, 
microsomes  and  Golgi  bodies.  Microsomes  are  too  small  to  be  seen  with 
an  ordinary  microscope  and  are  invisible  whether  or  not  the  cell  has  been 
stained.  By  centrifuging  cells  at  high  speed  it  can  be  shown  that  mito- 
chondria are  heavier,  and  the  Golgi  bodies  are  lighter,  than  the  ground 
substance  of  protoplasm.  The  Golgi  bodies  are  usually  concentrated  in 
the  part  of  the  cytoplasm  near  the  centrioles  and  appear  to  have  a  role 
in  the  production  of  secretions.  They  may  have  the  appearance  of 
granules,  rods,  threads  or  canals.  The  mitochondria  are  organized  groups 
of  enzymes  by  means  of  which  carbohydrates,  fatty  acids  and  amino  acids 
are  metabolized  to  carbon  dioxide  and  water  with  the  release  of  most 
of  the  energy  required  by  the  cell  for  survival. 

The  cytoplasm  of  certain  cells,  chiefly  those  of  lower  animals,  con- 
tains vacuoles,  cavities  filled  with  fluid  and  separated  from  the  rest  of 
the  cytoplasm  by  a  vacuolar  membrane.  Most  protozoa,  and  the  endo- 


CELLS   AND   TISSUES  39 

derm  cells  of  coelenterates  and  flatworms,  have  food  vacuoles  in  which 
food  is  digested.  Digestive  enzymes  are  secreted  from  the  cytoplasm  into 
the  cavity  of  the  vacuole,  the  food  particles  are  digested  and  the  products 
of  digestion  are  absorbed  through  the  vacuolar  membrane  into  the  cyto- 
plasm. The  protozoa  living  in  fresh  water  have  the  problem  of  eliminat- 
ing the  water  which  enters  the  cell  constantly  by  osmosis  (p.  51).  These 
forms  have  evolved  contractile  vacuoles,  which  alternately  fill  with  water 
from  the  adjacent  cytoplasm  and  then  eject  the  water  to  the  surrounding 
environment. 

Most  animal  cells  are  quite  small,  too  small  to  be  seen  with  the 
naked  eye.  The  diameter  of  the  human  red  blood  cell  is  about  7.5 
microns  (a  micron  is  0.001  millimeter),  but  most  animal  cells  have 
diameters  ranging  from  10  to  50  microns.  There  are  a  few  species  of 
giant  amebas  with  cells  about  1  mm.  in  diameter.  The  largest  cells  are 
the  yolk-filled  eggs  of  birds  and  sharks.  The  egg  cell  of  a  large  bird  such 
as  a  turkey  or  goose  may  be  several  centimeters  across.  Only  the  yolk 
of  a  bird's  egg  is  the  true  egg  cell;  the  egg  white  and  shell  are  noncellu- 
lar  material  secreted  by  the  bird's  oviduct  as  the  egg  passes  through  it. 

The  limit  of  the  size  of  a  cell  is  set  by  the  physical  fact  that  as  a 
sphere  gets  larger,  its  surface  increases  as  the  square  of  the  radius  but  its 
volume  increases  as  the  cube  of  the  radius.  The  metabolic  activities  of 
the  cell  are  roughly  proportional  to  cell  volume.  These  activities  require 
nutrients  and  oxygen,  and  release  carbon  dioxide  and  other  wastes  which 
must  enter  and  leave  the  cell  through  its  surface.  The  upper  limit  of 
cell  size  is  reached  when  the  surface  area  can  no  longer  provide  for  the 
entrance  of  enough  raw  materials  and  the  exit  of  enough  waste  products 
for  cell  metabolism  to  proceed  normally.  When  this  limit  is  reached  the 
cell  must  either  stop  growing  or  divide. 

11.        Mitosis 

Because  of  the  limitation  on  the  size  of  individual  cells,  growth- 
the  increase  in  protoplasmic  mass-is  accomplished  largely  by  an  increase 
in  the  number  of  cells.  When  a  single-celled  protozoan  divides,  the 
resulting  two  cells  are  separate  individuals,  members  of  a  new  generation. 
In  multicellular  animals,  cell  division  results  in  an  increase  in  the  num- 
ber of  cells  per  individual,  but  the  process  of  cell  division  is  funda- 
mentally the  same  in  both.  This  process  of  cell  division,  called  mitosis, 
is  extremely  regular  and  ensures  the  qualitatively  and  quantitatively 
equal  distribution  of  the  hereditary  factors  between  the  two  resulting 
daughter  cells.  Mitotic  divisions  occur  during  embryonic  development 
and  growth,  in  the  replacement  of  cells  that  w^ear  out,  such  as  blood  cells, 
skin,  the  intestinal  lining,  and  so  on,  and  in  the  repair  of  injuries. 

When  a  dividing  cell  is  stained  and  examined  under  the  microscope, 
dark-staining  bodies,  called  chromosomes,  are  visible  within  the  nucleus. 
Each  consists  of  a  central  thread,  the  chromonema,  along  which  lie  the 
chromomeres-small,  beadlike,  dark-staining  swellings.  In  a  cell  which 
is  not  dividing,  chromosomes  are  usually  not  visible  as  separate  entities; 
instead   the   nucleus   contains   an   irregular   network  of   fine   chromatm 


40  GENERAL   CONCEPTS 

threads.  Genetic  and  cytologic  evidence  indicates  that  the  chromosomes 
remain  distinct  physiologic  and  structural  entities  between  successive 
cell  divisions  even  though  they  are  not  evident  by  the  usual  staining 
procedures. 

It  has  been  suggested  that  tlie  chromomeres  are,  or  contain,  the 
genes,  for  breeding  experiments  have  shown  clearly  that  these  hereditary 
units  lie  within  the  chromosome  in  a  linear  order.  However,  the  correla- 
tion between  chromomeres  and  genes  is  not  regular;  some  chromomeres 
contain  several  genes  and  some  genes  have  been  located  between  chrom- 
omeres. Several  theories  have  been  formulated  to  account  for  these 
swellings  of  the  chromosomes,  but  at  present  their  true  significance  is 
not  clear. 

One  of  the  very  regidar  characteristics  of  any  kind  of  animal  or 
plant  is  the  number  of  chromosomes  in  each  nucleus.  Every  cell  in  the 
body  of  every  human  being,  for  example,  has  forty-six  chromosomes. 
There  are  many  other  kinds  of  animals  and  plants  which  happen  to  have 
46  chromosomes  per  cell  as  well;  so  the  factor  of  chief  importance  in 
differentiating  different  kinds  of  animals  is  not  simply  the  number 
of  chromosomes  per  cell  but  the  kind  of  genes  in  the  chromosomes.  The 
chromosome  number  for  most  kinds  of  animals  lies  between  ten  and 
fifty.  One  kind  of  roundworm  has  only  two  chromosomes  per  cell,  certain 
crabs  have  200  and  one  kind  of  radiolarian,  a  marine  protozoan,  has 
1600  or  so  chromosomes  in  its  nucleus. 

Chromosomes  occur  in  pairs;  the  forty-six  chromosomes  of  each 
human  cell  consist  of  two  of  each  of  twenty-three  different  kinds.  The 
chromosomes  differ  in  length,  shape,  and  in  the  presence  of  identifying 
knobs  or  constrictions  along  their  length.  In  most  animals,  the  morpho- 
logic features  of  the  chromosomes  are  distinct  enough  so  that  one  can 
identify  the  individual  pairs. 

Cell  division  must  be  an  extremely  exact  process  to  ensure  that  each 
daughter  cell  receives  exactly  the  right  number  and  kind  of  chromo- 
somes. If  we  tamper  experimentally  with  the  mechanism  of  cell  division, 
and  the  resulting  cells  receive  more  or  less  than  the  proper  number  of 
chromosomes,  marked  abnormalities  of  growth,  and  perhaps  the  death 
of  these  cells,  will  follow.  Mitosis  may  be  defined  as  the  regular  process 
of  cell  division  by  which  each  of  the  two  daughter  cells  receives  exactly 
the  same  number  and  the  same  kind  of  chromosomes  that  the  parent  cell 
contained.  This  process  involves  what  appears  to  be  a  longitudinal  split- 
ting of  each  chromosome  into  two  halves.  There  is  now  abundant 
evidence  that  no  such  splitting  can  indeed  occur;  instead,  each  original 
chromosome  brings  about  the  synthesis  of  an  exact  replica  of  itself 
immediately  beside  itself.  The  new  chromosome  is  made,  some  time 
before  the  visible  mitotic  process  begins,  from  raw  materials  present  in 
the  nucleus.  When  the  process  is  complete,  the  original  and  the  new 
chromosomes  separate  and  become  incorporated  into  different  daughter 
cells.  The  role  of  the  complicated  mitotic  machinery  is  to  separate  the 
"original"  and  "replica"  chromosomes  and  deliver  them  to  opposite  ends 
of  the  dividing  cell  so  they  will  become  incorporated  into  different 
daughter  cells. 


CELLS   AND   TISSUES 


41 


The  mitotic  process  is  a  continuous  one,  but  for  descriptive  pur- 
poses biologists  have  divided  it  into  four  stages:  prophase,  metaphase, 
anaphase  and  telophase  (Fig.  3.5).  Between  mitoses  a  cell  is  said  to  be 
in  the  resting  stage.  It  is  difficult  to  visualize  from  a  description  or 
diagram  of  mitosis,  or  from  examining  a  fixed  and  stained  slide  of  cells, 
just  how  active  a  process  cell  division  is.  Motion  pictures  made  by  phase 
microscopy  reveal  that  a  cell  undergoing  division  bulges  and  changes 
shape  like  a  gunny  sack  filled  with  a  dozen  unfriendly  cats. 

Prophase.  The  chromatin  threads  condense  and  form  visible 
chromosomes,  which  appear  as  a  tangled  mass  of  coiled  threads  within 
the  nucleus.  Early  in  prophase  the  threads  are  stretched  maximally  so 
that  the  individual  chromomeres  are  visible.  Later  in  prophase  the 
chromosomes   shorten   and   thicken    and    the   chromomeres   lie  so   close 


Figure  3.5.  Mitosis  in  a  cell  of  a  hypothetical  animal  with  a  diploid  number  of  six 
(haploid  number  =  3);  one  pair  of  chromosomes  is  short,  one  pair  is  long  and  hooked, 
and  one  pair  is  long  and  knobbed.  A,  Resting  stage.  B,  Early  prophase,  centriole  divided 
and  chromosomes  appearing.  C,  Later  prophase,  centrioles  at  poles,  chromosomes  short- 
ened and  visibly  double.  D,  Later  prophase,  nuclear  membrane  dissolved,  spindle  pres- 
ent. E,  Metaphase,  chromosomes  arranged  on  the  equator  of  the  spindle.  F,  Anaphase, 
chromosomes  migrating  toward  the  poles.  G,  Telophase,  nuclear  membranes  formed; 
chromosomes  elongating;  cytoplasmic  division  beginning.  H,  Daughter  cells,  resting 
phase. 


42 


GENERAL   CONCEPTS 


Figure  3.6.  Photomicrograph  of  the  mitotic  apparatus  isolated  from  dividing  cells  of 
a  sea  urchin  embryo.  Each  mitotic  apparatus  includes  spindle  fibers,  asters  and  chromo- 
somes. A  metaphase  figure  appears  in  the  upper  right  and  two  anaphase  figures  below. 
(Courtesy  of  Daniel  Mazia.)  (Villee:  Biology.) 


together  that  individual  ones  cannot  be  distinguished.  The  reduplication 
of  the  chromosomes  has  occurred  previously  and  in  many  species  ot 
animals  the  double  nature  of  each  chromosome  is  apparent. 

Early  in  prophase  the  centriole,  a  small  granular  structure  in  the 
cytoplasm,  divides  and  the  daughter  centrioles  migrate  to  opposite  sides 
of  the  cell.  Between  the  separating  centrioles  a  spindle  forms.  The 
spindle  is  composed  of  spindle  fibers,  protoplasmic  threads  arranged  like 
two  cones  base  to  base  (Fig.  3.6).  The  spindle  is  broad  at  the  center  or 
equator  of  the  cell  and  narrows  to  a  point  at  either  end  or  pole.  The 
spindle  is  not  some  optical  artifact  but  a  definite  structure  composed  of 
protoplasm  that  is  denser  than  the  surrounding  protoplasm.  With  a 
microneedle  attached  to  a  micromanipulator  the  spindle  can  be  moved 
as  a  unit  from  one  part  of  the  cell  to  another.  At  the  end  of  prophase, 
the  centrioles  have  divided  and  gone  to  the  opposite  poles  of  the  cell,  the 
spindle  has  formed  between  them  and  the  chromosomes  have  become 
short  and  thick. 

Metaphase.  When  the  chromosomes  are  fully  contracted  and  ap- 
pear as  short,  dark-staining  rods,  the  nuclear  membrane  disappears  and 
the  chromosomes  line  up  in  the  equatorial  plane  of  the  spindle.  The 
short  period  during  which  the  chromosomes  are  in  this  equatorial  plane 
is  known  as  the  metaphase.  This  is  much  shorter  than  the  prophase; 
although  times  for  different  cells  vary  considerably,  the  prophase  lasts 


CELLS   AND   TISSUES  43 

from  thirty  to  sixty  minutes  or  more  and  the  metaphase  lasts  only  two 
to  six  minutes. 

Anaphase.  The  chromosomes  immediately  separate  (Fig.  3.5)  and 
one  ot  the  separating  daughter  chromosomes  goes  to  each  pole.  The 
period  during  which  the  separating  chromosomes  move  from  the  equa- 
torial plate  to  the  poles  is  known  as  the  anaphase  and  lasts  some  three 
to  fifteen  minutes.  The  spindle  fibers  apparently  act  as  guide  rails  along 
which  the  chromosomes  move  toward  the  poles.  Without  such  guide  rails 
the  chromosomes  would  merely  be  pushed  randomly  apart  and  many 
would  fail  to  be  incorporated  into  the  proper  daughter  nucleus.  The 
mechanism  by  which  the  chromosomes  are  moved  apart  is  not  clear. 
Experiments  suggest  that  the  protoplasm  between  the  chromosomes  takes 
up  water,  swells,  and  pushes  the  chromosomes  apart.  Other  experiments 
indicate  that  some  of  the  spindle  fibers  are  contractile  and  can  pull  the 
chromosomes  toward  the  poles. 

Telophase.  When  the  chromosomes  have  reached  the  poles  of  the 
cell,  the  last  phase  of  mitosis,  the  telophase,  begins.  Several  processes 
occur  simultaneously  in  this  period:  a  nuclear  membrane  forms  around 
the  group  of  chromosomes  at  each  pole,  the  chromosomes  elongate,  stain 
less  darkly,  and  return  to  the  resting  condition  in  which  only  irregular 
chromatin  threads  are  visible,  and  the  cytoplasm  of  the  cell  begins  to 
divide.  Division  of  the  cytoplasm  is  accomplished  in  animal  cells  by  the 
formation  of  a  furrow  which  circles  the  cell  at  the  equatorial  plate  and 
gradually  deepens  until  the  two  halves  of  the  cell  are  separated  as  inde- 
pendent daughter  cells.  The  events  of  telophase  require  some  thirty  to 
sixty  minutes  for  their  completion. 

The  mitotic  process  results  in  the  formation  of  two  daughter  cells 
from  a  single  parent  cell  with  each  daughter  cell  having  exactly  the  same 
number  and  kind  of  chromosomes,  and  of  the  units  of  heredity  (genes) 
contained  in  these  chromosomes,  as  the  parent  cell.  Since  all  the  cells 
of  the  body  are  formed  by  mitosis  from  a  single  fertilized  egg,  each  cell 
has  the  same  number  and  kind  of  chromosomes,  and  the  same  number 
and  kind  of  genes,  as  every  other  cell. 

The  speed  and  frequency  of  cell  division  vary  greatly  from  tissue  to 
tissue  and  from  one  animal  to  another.  In  the  early  stages  of  embryonic 
development,  there  may  be  only  thirty  minutes  or  so  between  successive 
cell  divisions.  In  certain  advdt  tissues,  notably  the  nervous  system,  mitoses 
are  extremely  rare.  In  other  adult  tissues,  such  as  the  red  bone  marrow, 
where  red  blood  cells  are  produced,  mitotic  divisions  must  occur  fre- 
quently to  supply  the  10,000,000  red  blood  cells  each  human  being 
produces  every  second  of  the  day  and  night. 

Regulation  of  Mitosis.  The  factors  which  initiate  and  control  cell 
division  are  not  certain.  Tlie  possible  role  of  the  ratio  of  cell  surface  to 
cell  volume  was  discussed  previously  (p.  39).  The  ratio  of  nuclear  sur- 
face to  nuclear  volume  may  also  be  important.  Since  normal  cell  function 
requires  the  transport  of  substances  back  and  forth  through  the  nuclear 
membrane,  growth  will  eventually  result  in  a  state  in  which  the  area  of 
the  nuclear  membrane  is  insufficient  to  meet  the  demands  of  the  volume 
of  cytoplasm.  Cell  division,  by  splitting  the  volume  of  cytoplasm  into 


44  GENERAL   CONCEPrS 

two  parts  and  increasing  the  area  of  nuclear  membrane,  will  restore 
optimal  conditions.  There  is  some  evidence  to  suggest  that  the  chromo- 
somes may  release  a  substance  or  substances  which  initiates  first  the 
nuclear  events  of  prophase  and  metaphase,  and  secondly  the  reactions 
in  the  cytoplasm  which  form  a  cleavage  furrow  and  bring  about  the 
division  of  the  cytoplasm. 

Another  theory  postulates  the  initiation  of  mitosis  by  a  "cell  division 
hormone."  The  mitoses  of  the  cells  of  an  egg  undergoing  cleavage  occur 
simultaneously,  which  suggests  that  a  periodically  released  hormone  may 
control  these  divisions.  The  experiments  of  Haberlandt  indicate  that 
dying  cells  release  a  substance  which  stimulates  cell  division.  He  cut  a 
potato  in  half  and  examined  the  cut  edge  for  mitoses.  He  found  that 
if  he  cleaned  the  cut  edge  to  remove  all  cell  debris  few  mitoses  occurred. 
If  he  did  not  clean  the  cut  edge,  cell  divisions  were  more  frequent, 
and  if  he  put  some  mashed  cells  on  the  cut  edge  an  even  greater  number 
of  cell  divisions  resulted.  He  concluded  that  cut  potato  cells  release  a 
"wound  hormone"  which  stimulates  cell  divisions  in  adjacent  cells. 
Marshak  and  Walker  were  able  to  prepare  an  extract  of  the  nuclei  of 
rat  liver  cells  and  then  to  separate  this  into  two  fractions.  One  fraction, 
when  injected  into  other  rats,  increased,  and  the  other  decreased,  the 
rate  of  cell  divisions  in  liver  cells. 

12.        The  Study  of  Cellular  Activities 

Despite  great  differences  in  size,  shape  and  location  in  the  body,  all 
cells  have  many  metabolic  activities  in  common.  Each  cell  has  a  host  of 
enzymes  which  enable  it  to  release  energy  by  converting  sugars,  fats  and 
proteins  to  carbon  dioxide  and  water.  Each  cell  synthesizes  the  structural 
proteins  and  enzymes  of  its  own  protoplasm.  Superimposed  on  this  basic 
pattern  of  metabolism  common  to  all  cells  may  be  other  activities 
peculiar  to  each  type  of  cell.  For  example,  muscle  cells  have  special 
proteins,  myosin  and  actin,  which  are  contractile;  particular  digestive 
enzymes  are  produced  by  the  cells  lining  the  stomach  and  intestine;  and 
the  cells  of  the  pituitary,  adrenal  and  thyroid  glands  manufacture  char- 
acteristic hormones. 

There  are  many  ways  of  studying  cellular  activity  and  each  of  these 
provides  useful  information  about  cell  morphology  and  physiology. 
Living  cells  suspended  in  a  drop  of  fluid  can  be  examined  under  an 
ordinary  microscope  or  with  one  equipped  with  phase  contrast  lenses 
(Fig.  3.7).  In  this  way  one  can  study  the  movement  of  an  ameba  or  a 
white  blood  cell,  or  the  beating  of  the  cilia  on  a  paramecium.  Cells  from 
a  many-celled  animal— a  frog,  chick  or  man— can  be  grown  by  "tissue 
culture"  for  observation  over  a  long  period  of  time.  A  complex  nutritive 
medium,  made  of  blood  plasma,  an  extract  of  embryonic  tissues  and  a 
mixture  of  vitamins,  is  prepared  and  sterilized.  A  drop  of  this  is  placed 
in  a  cavity  on  a  special  micro  slide,  the  cells  to  be  cultured  are  added 
aseptically,  and  the  cavity  is  sealed  with  a  glass  cover  slip.  After  a  few 
days  the  cells  have  exhausted  one  or  more  of  the  nutritive  materials  and 
must  be  transferred  again  to  a  fresh  drop  of  medium.  Cells  transferred 


CEttS   AND    TISSUES  45 

regularly  in  this  fashion  will  grow  indefinitely— tissue  from  a  chick  heart 
was  grown  for  over  twenty  years  at  the  Rockefeller  Institute  in  New 
York.  Such  experiments  revealed  that  cells  in  tissue  culture  do  not  grow 
old,  for  at  the  end  of  the  twenty-year  period  the  cells  were  as  vigorous 
and  grew  as  fast  as  the  original  cells.  Cells  isolated  from  a  sarcoma  (a 
type  of  cancer)  grow  with  unusual  vigor  in  tissue  culture  and  grow  more 
rapidly  in  plasma  from  a  healthy  person  than  in  plasma  from  a  person 
with  a  sarcoma.  This  observation  suggests  that  the  presence  of  sarcoma 
cells  in  the  body  stimulates  certain  healthy  cells  elsewhere  to  produce 
some  substance  which  inhibits  to  some  extent  the  malignant  growth. 

Cell  morphology  may  be  studied  by  using  a  bit  of  tissue  that  has 
been  killed  quickly  with  a  special  "fixative,"  then  sliced  with  a  machine 
called  a  microtome,  and  stained  with  special  dyes.  The  stained  slices, 
mounted  on  a  glass  slide  and  covered  with  a  glass  cover  slip,  are  then 
ready  for  examination  under  the  microscope.  Since  the  nucleus,  mito- 
chondria and  other  specialized  parts  of  the  cell  are  chemically  different, 
they  will  combine  with  different  dyes  and  be  stained  characteristic 
colors  (Fig.  3.4).  For  observation  in  the  electron  microscope  a  bit  of 
tissue  is  fixed  with  osmic  acid,  mounted  in  acrylic  plastic  for  cutting 
in  extremely  thin  sections,  and  then  placed  on  a  fine  grid  to  be  inserted 
into  the  path  of  the  electron  beam.  Both  light  microscopy  and  electron 
microscopy  have  revealed  many  details  about  cell  structure. 

Some  clue  as  to  the  location  and  functioning  of  enzymes  within  cells 
can  be  obtained  by  histochemical  studies,  in  which  a  cell  is  fixed  by 
methods  which  do  not  destroy  en/yme  activity.  Then  the  proper  chemi- 
cal substrate  for  the  enzyme  is  provided  and,  after  a  specified  period  of 
incubation,  some  substance  is  added  which  will  form  a  colored  com- 
pound with  one  of  the  products  of  the  reaction  mediated  by  the  enzyme. 
The  regions  of  the  cell  which  have  the  greatest  enzyme  activity  will 
have  the  largest  amount  of  the  colored  substance  (Fig.  3.8).  Methods  have 
been  worked  out  which  permit  the  demonstration  and  localization  of  a 
wide  variety  of  enzymes.  Such  studies  have  given  an  interesting  insight 
into  the  details  of  cell  function. 

Another  method  of  investigating  cell  function  is  to  measure,  by 
special  microchemical  analyses,  the  amounts  of  chemical  used  up  or 
produced  as  a  bit  of  tissue  is  incubated  in  a  special  enclosed  glass  vessel. 
In  such  experiments  much  has  been  learned  of  the  roles  in  cell  metab- 
olism of  vitamins,  hormones  and  other  chemicals  by  adding  these  sub- 
stances one  by  one  and  observing  the  resulting  effects. 

Every  living  cell,  whether  it  is  an  individual  unicellular  animal,  or 
a  single  component  of  a  multicellular  one,  must  be  supplied  constantly 
with  nutrients  and  oxygen.  These  materials  are  constantly  being  metab- 
olized-used  up— as  the  cell  goes  about  its  business  of  releasing  energy 
from  the  nutrients  to  provide  for  its  myriad  activities.  Some  of  the  sub- 
stances required  by  the  cell  are  brought  to  it  and  taken  in  by  complex 
active  processes  which  require  the  expenditure  of  energy  by  the  cell,  and 
about  which  little  is  known.  Other  substances  are  brought  to  the  cell  by 
the  simpler,  more  easily  understood  physical  process  of  diffusion.  To 
understand  this  process,  so  important  in  many  biologic  phenomena,  we 


46  GENERAL  CONCEPTS 

3.09 


Figure  3.7.     Legend  on  opposite  page. 


CELLS   AND   TISSUES 


47 


Fiyure  3.7.  Stages  in  iiiU()m,>  ul  a  cell  li'oin  a  salaiiiaiRlci  hcait  grown  in  tissue  cul- 
ture aiul  photographed  by  phase  microscopy.  The  numbers  are  clock  readings.  (Courtesy 
of  L.  Wang.)  (Maxiraow  and  Bloom:  Textbook  of  Histolog) .) 

must  first  consider  some  of  the  basic  physical  concepts  of  energy  and 
molecular  motion. 


13.       Energy 

Energy  may  be  defined  as  the  ability  to  do  work,  to  produce  a 
change  in  matter.  It  may  take  the  form  of  heat,  light,  electricity  or  mo- 
tion. Physicists  recognize  two  kinds  of  energy:  potential  energy,  the 
ability  to  do  work  owing  to  the  position  or  state  of  a  body,  and  kinetic 
energy,  the  capacity  to  do  work  possessed  by  a  body  because  of  its 
motion.  A  rock  at  the  top  of  a  hill  has  potential  energy;  as  it  rolls  down- 
hill the  potential  energy  is  converted  to  kinetic  energy. 

Energy  derived  ultimately  from  solar  energy  is  stored  in  the  mole- 


48 


GENERAL   CONCEPTS 


'^i'^'^S'-^ 


Figure  3.8.  Histochemical  demonstration  of  the  location  of  the  enzyme  alkaline 
phosphatase  within  the  cells  of  the  rat's  kidney.  The  tissue  is  carefully  fixed  and  sec- 
tioned by  methods  which  do  not  destroy  the  enzyme's  activity.  The  tissue  section  is  incu- 
bated at  the  proper  pH  with  a  naphthyl  phosphate.  Some  hydrolysis  of  the  naphthyl 
phosphate  occurs  wherever  the  phospliatase  enzyme  is  located.  The  naphthol  released 
by  the  action  of  the  enzyme  couples  with  a  diazonium  salt  to  form  an  intensely  blue, 
insoluble  azo  dye  which  remains  at  the  site  of  the  enzymatic  activity.  The  photomicro- 
graph thus  reveals  the  sites  of  phosphatase  activity,  i.e.,  the  sites  at  which  the  azo  dye 
is  deposited.  The  cells  of  the  proximal  convoluted  tubules  (left)  have  a  lot  of  enzyme, 
those  of  the  loop  of  Henle  (right)  have  little  or  no  activity.  (Courtesy  of  R.  J.  Barrnett.) 
(Villee:  Biology.) 


cules  of  foodstuffs  as  the  chemical  energy  of  the  bonds  connecting  the 
atoms  in  the  food  molecules.  This  chemical  energy  is  a  kind  of  potential 
energy.  When  these  food  molecules  are  taken  within  a  cell,  chemical 
reactions  occur  which  change  this  potential  energy  into  heat,  motion, 
or  some  other  kind  of  kinetic  energy.  All  forms  of  energy  are  at  least 
partially  interconvertible,  and  living  cells  constantly  transform  potential 
energy  into  kinetic  energy  or  the  reverse.  If  the  conditions  are  suitably 
controlled,  the  amount  of  energy  entering  and  leaving  any  given  system 
can  be  measured  and  compared.  Such  experiments  have  shown  that 
energy  is  neither  created  nor  destroyed,  but  simply  transformed  from 
one  form  to  another.  This  is  an  expression  of  one  of  the  fundamental 
laws  of  physics,  the  Law  of  the  Conservation  of  Energy.  Living  things 
as  well  as  nonliving  systeins  obey  this  law. 


14.        Molecular  Motion 

The  constituent  molecules  of  all  substances  are  constantly  in  mo- 
tion. Despite  the  fact  that  wood,  stone  and  steel  seem  very  solid,  their 
component  molecules  vibrate  continuously  within  a  very  restricted  space. 
The  prime  difference  between  solids,  liquids  and  gases  is  the  freedoin 
of  movement  of  the  molecules  present.  The  molecules  of  a  solid  are  very 
closely  packed  and  the  forces  of  attraction  between  the  molecules  permit 
them  to  vibrate  but  not  to  move  around.  In  the  liquid  state  the  molecules 
are  somewhat  farther  apart  and  the  intermolecular  forces  are  weaker,  so 
that   the  molecules   can   move  about  with   considerable   freedom.    The 


CELLS    AND    TISSUES  49 

molecules  in  the  gaseous  state  are  so  far  apart  that  the  intermolecular 
forces  are  negligible  and  molecular  movement  is  restricted  only  by 
external  barriers.  Molecular  movement  in  all  three  states  of  matter  is 
the  result  of  the  inherent  heat  energy  of  the  molecules.  By  increasing 
this  molecular  heat  energy,  one  can  change  matter  from  one  state  to 
another.  W^hen  ice  is  heated  it  becomes  water,  and  when  water  is  heated 
it  is  converted  to  water  vapor. 

If  a  drop  of  water  is  examined  under  the  microscope,  the  motion  of 
its  molecules  is  not  evident.  If  a  drop  of  India  ink  (which  contains  fine 
carbon  particles)  is  added,  the  carbon  particles  move  continvially  in 
aimless  zig-zag  paths,  for  they  are  constantly  being  bumped  by  water 
molecules  and  the  recoil  from  this  bump  imparts  the  motion  to  the 
carbon  particle.  The  motion  of  such  small  particles  is  called  Brownian 
movement,  after  Robert  Brown,  an  English  botanist,  who  first  observed 
the  motion  of  pollen  grains  in  a  drop  of  water. 

15.        DifFusion 

Molecules  in  a  liquid  or  gaseous  state  will  diffuse,  that  is,  move  in 
all  directions  until  they  are  spread  evenly  throughout  the  space  avail- 
able. Diffusion  may  be  defined  as  the  movement  of  moleciUes  from  a 
region  of  high  concentration  to  one  of  lower  concentration  brought 
about  by  their  inherent  heat  energy.  The  rate  of  diffusion  is  a  function 
of  the  size  of  the  molecule  and  the  temperature.  If  a  bit  of  sugar  is 
placed  in  a  beaker  of  water,  the  sugar  will  dissolve  and  the  individual 
sugar  molecules  will  diffuse  and  come  to  be  distributed  evenly  through- 
out the  liquid  (Fig.  3.9).  Each  molecule  tends  to  move  in  a  straight  hue 
until  it  collides  with  another  molecule  or  the  side  of  the  container;  then 
it  rebounds  and  moves  in  another  direction.  By  this  random  movement 
of  molecules,  the  sugar  eventually  becomes  evenly  distributed  through- 
out the  water  in  the  beaker.  This  could  be  demonstrated  by  tasting 
drops  of  liquid  taken  from  different  parts  of  the  beaker.  If  a  colored 
dye  is  used  in  place  of  sugar,  the  process  of  diffusion  can  be  observed 
directly.  The  molecules  of  sugar  or  dye  continue  to  move  after  they  have 
become  evenly  distributed  throughout  the  liquid  in  the  container;  how- 
ever, as  fast  as  some  molecules  move  from  left  to  right,  others  move  from 
right  to  left,  so  that  an  equilibrium  is  maintained. 

Any  number  of  substances  will  diffuse  independently  of  each  other. 
If  a  lump  of  salt  is  placed  in  one  part  of  a  beaker  of  water  and  a  lump  of 
sugar  in  another,  the  molecules  of  each  will  diffuse  independently  of  the 
other  and  each  drop  of  water  in  the  beaker  will  eventually  have  some 
salt  and  some  sugar  molecules. 

The  rate  of  movement  of  a  single  molecule  is  several  hundred 
meters  per  second,  but  each  molecule  can  go  only  a  fraction  of  a  milli- 
micron before  it  bumps  into  another  molecule  and  rebounds.  Thus  the 
progress  of  a  molecule  in  a  straight  line  is  quite  slow.  Diffusion  is  quite 
rapid  over  short  distances  but  it  takes  a  long  time— days  and  even  weeks 
—for  a  substance  to  diffuse  a  distance  measured  in  inches.  This  fact  has 
important  biologic  implications,  for  it  places  a  sharp  limit  on  the  num- 


50 


GENERAL   CONCEPTS 


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o  0  o  o  0^0^. 

f'  ^■-  II  ■    II         ■ -         -  ■■    ■  *^ 

Su^ar#       Salt  O 

Figure  3.9.  Diffusion.  When  a  cube  of  sugar  is  placed  in  water  (A)  it  dissolves  and 
its  molecules  become  uniformly  distributed  throughout  the  water  as  a  result  of  the 
molecular  motion  of  both  sugar  and  water  molecules  (B).  When  lumps  of  sugar  and  salt 
are  placed  in  water  (C),  each  type  of  molecule  diffuses  independently  of  the  other  and 
both  salt  and  sugar  become  uniformly  distribiUed  in  the  water  (D). 

ber  of  molecules  ot  oxygen  and  nutrients  that  can  reach  an  organism 
by  diffusion  alone.  Only  a  very  small  organism  that  requires  relatively 
few  molecules  per  second  can  survive  it  it  remains  in  one  place  and 
allows  molecules  to  come  to  it  by  diffusion.  A  larger  organism  must 
have  some  means  of  moving  to  a  new  region  or  some  means  of  stirring 
its  environment  to  bring  molecules  to  it,  or  it  may  live  in  some  spot 
where  the  environment  is  constantly  moving  past  it— in  a  river,  for  ex- 
ample, or  in  the  intertidal  region  at  the  seashore.  The  larger  land  plants 
have  solved  this  problem  by  developing  an  extensively  branched  system 
of  roots  which  can  tap  a  large  area  of  the  surrounding  environment  for 
the  needed  raw  materials. 


16.        Exchanges  of  Material  between  Cell  and  Environment 

All  nutrients  and  waste  products  must  pass  through  the  plasma 
membrane  to  enter  or  leave  the  cell.  Cells  are  almost  invariably  sur- 
rounded by  a  watery  medium— the  fresh  or  salt  water  in  which  an  organ- 
ism lives,  the  tissue  sap  of  a  higher  plant,  or  the  plasma  or  extracellular 


CELLS    AND   TISSUES  ^\ 

fluid  of  a  higher  animal.  In  general,  only  dissolved  substances  can  pass 
through  the  plasma  membrane,  but  not  all  dissolved  substances  pene- 
trate the  plasma  membrane  with  equal  facility.  The  membrane  behaves 
as  though  it  had  ultramicroscopic  pores  through  which  substances  pass, 
and  these  pores,  like  the  holes  in  a  sieve,  determine  the  maximum  size 
of  molecule  that  can  pass.  Factors  other  than  simple  molecular  size,  such 
as  the  electric  charge,  if  any,  of  the  diffusing  particle,  the  number  of 
water  molecules  bound  to  the  diffusing  particles  and  its  solubility  in 
fatty  substances,  may  also  be  important  in  determining  whether  or  not 
the  substance  can  pass  through  the  plasma  membrane. 

A  membrane  is  said  to  be  permeable  if  it  will  permit  any  substance 
to  pass  through,  impermeable  if  it  will  allow  no  substance  to  pass,  and 
semipermeable,  or  differentially  permeable,  if  it  will  allow  some  but 
not  all  substances  to  diffuse  through.  The  plasma  membranes  of  all 
cells  and  the  membranes  surrounding  food  and  contractile  vacuoles  are 
semipermeable  membranes.  Permeabdity  is  a  property  of  the  membrane, 
not  of  the  diffusing  substance. 

The  diffusion  of  a  dissolved  substance  through  a  semipermeable 
membrane  is  known  as  dialysis.  If  a  pouch  made  of  collodion,  cello- 
phane or  parchment  is  filled  with  a  sugar  solution  and  placed  in  a  beaker 
of  water,  the  sugar  molecules  will  dialyze  through  the  membrane  (if  the 
pores  are  large  enough)  and  eventually  the  concentration  of  sugar  mole- 
cules in  the  water  outside  the  pouch  will  equal  that  within  the  pouch. 
The  molectdes  then  continue  to  diffuse  but  there  is  no  net  change  in 
concentration  for  the  rates  in  the  two  directions  are  equal. 

A  different  type  of  diffusion  is  observed  if  a  membrane  is  prepared 
with  smaller  pores,  so  that  it  is  permeable  to  the  small  water  molecules 
but  not  to  the  larger  sugar  molecules.  A  pouch  may  be  prepared  of  a 
membrane  with  these  properties  and  filled  with  a  sugar  solution,  then 
the  pouch  is  fitted  with  a  cork  and  glass  tube  and  placed  in  a  beaker  of 
water  so  that  the  levels  of  fluid  inside  and  outside  of  the  pouch  are  the 
same.  The  sugar  molectdes  cannot  pass  through  the  membrane  and  so 
must  remain  inside  the  pouch.  The  water  molectdes  diffuse  through  the 
membrane  and  mix  with  the  sugar  solution,  so  that  the  level  of  ffuid 
within  the  pouch  rises.  The  liquid  within  the  pouch  is  5  per  cent  sugar, 
and  therefore  only  95  per  cent  water;  the  liquid  outside  the  membrane 
is  100  per  cent  water.  The  water  molecules  are  moving  in  both  directions 
through  the  membrane  but  there  is  a  greater  movement  from  the  region 
of  higher  concentration  (100  per  cent,  outside  the  potich)  to  the  region  of 
lower  concentration  (95  per  cent,  within  the  pouch).  This  diffusion  of 
water  or  solvent  molecules  through  a  membrane  is  called  osmosis,  and 
is  illustrated  diagrammatically  in   Figure  3.10. 

If  an  amount  of  water  equal  to  that  originally  present  in  the  pouch 
enters,  the  solution  in  the  pouch  will  be  diluted  to  2.5  per  cent  sugar 
and  97.5  per  cent  water,  but  the  concentration  of  water  outside  the 
pouch  will  still  exceed  that  inside  and  osmosis  will  continue.  An 
equilibrium  is  reached  when  the  water  in  the  glass  tube  rises  to  a 
height  such  that  the  weight  of  the  water  in  the  tube  exerts  a  pressure 
just  equal  to  the  tendency  of  the  water  to  enter  the  pouch.  Osmosis  then 


62 


GENERAL   CONCEPTS 


^ 


Membrane 

^ 

O  O  0  o 

o«o.#* 

•  o#o 

O   n  O 

0  ^'  O  ^''— o  "  o  o 

^  0  ^#4^  o  ^  o 

•     o  ^ 

O   _  u   . 

0  -r  0  ^  ^'    o 

Z' 


Figure  3.10.  Diagram  illustrating  osmosis.  When  a  solution  of  sugar  in  water  is 
separated  from  pure  water  by  a  semipermeable  membrane  which  allows  water  but  not 
the  larger  sugar  molecules  to  pass  tlirough,  there  is  a  net  movement  of  water  molecules 
through  the  membrane  to  the  sugar  solution.  The  water  molecules  are  diffusing  from 
a  region  of  higher  concentration  (pure  water)  to  a  region  of  lower  concentration  (the 
sugar  solution). 


occurs  with  equal  speed  in  both  directions  through  the  semipermeable 
membrane  and  there  will  be  no  net  change  in  the  amount  of  water  in 
the  pouch.  The  pressure  of  the  column  of  water  is  called  the  osmotic 
pressure  of  the  sugar  solution.  The  osmotic  pressure  results  from  the 
tendency  of  the  water  molecules  to  pass  through  the  semipermeable 
membrane  and  equalize  the  concentration  of  water  molecules  on  its 
two  sides.  A  more  concentrated  sugar  solution  would  have  a  greater 
osmotic  pressure  and  would  "draw"  water  to  a  higher  level  in  the  tube. 
A  10  per  cent  sugar  solution  would  cause  water  to  rise  approximately 
twice  as  high  in  the  tube  as  a  5  per  cent  solution. 

It  is  evident  from  this  discussion  that  dialysis  and  osmosis  are 
simply  two  special  forms  of  diffusion.  Diffusion  is  the  general  term  for 
the  movement  of  molecules  from  a  region  of  high  concentration  to  a 
region  of  lower  concentration,  brought  about  by  the  inherent  heat 
energy  of  the  molecides.  Dialysis  is  the  diffusion  of  dissolved  molecules 
through  a  semipermeable  membrane  and  osmosis  is  the  diffusion  of 
solvent  molecules  through  a  semipermeable  membrane.  In  biologic  sys- 
tems the  solvent  molecules  are  almost  universally  water. 

The  salts,  sugars  and  other  substances  dissolved  in  the  fluid  within 
each  cell  give  the  intracellular  fluid  a  certain  osmotic  pressure.  When  the 
cell  is  placed  in  a  fluid  with  the  same  osmotic  pressure  as  that  of  its 
intracellular  fluid,  there  is  no  net  entrance  or  exit  of  water,  and  the  cell 
neither  swells  nor  shrinks.  Such  a  fluid  is  said  to  be  isotonic  or  isosmotic 
with  the  intracellular  fluid  of  the  cell.  Normally,  the  blood  plasma  and 
body  fluids  are  isosmotic  with  the  intracellular  fluids  of  the  body  cells. 
If  the  environmental  fluid  contains  more  dissolved  substances  than  the 
fluid  within  the  cell,  water  Avill  tend  to  pass  out  of  the  cell  and  the  cell 
shrinks.  Such  a  fluid  is  said  to  be  hypertonic  to  the  cell.  If  the  environ- 
mental fluid  has  a  lower  concentration  of  dissolved  substances  than  the 
fluid  in  the  cell,  water  tends  to  pass  into  the  cell  and  the  cell  swells.  This 


CELLS   AND   TISSUES  53 

fluid  is  said  to  be  hypotonic  to  the  cell.  A  solution  of  0.9  per  cent 
sodium  chloride,  0.9  gm.  per  100  ml.  of  water,  soinetimes  loosely  called 
"physiological  saline,"  is  isotonic  to  human  cells. 

A  cell  placed  in  a  solution  that  is  not  isotonic  with  it  may  adjust 
to  the  changed  environment  by  undergoing  a  change  in  its  water  con- 
tent, so  that  it  eventually  achieves  the  same  concentration  of  solutes  as 
in  the  environment.  Many  cells  have  the  ability  to  pump  water  or 
certain  solute  molecules  into  or  out  of  the  cell  and  in  this  way  can 
maintain  an  osmotic  pressure  that  differs  from  that  of  the  surrounding 
medium.  Amebae,  paramecia  and  other  protozoa  that  live  in  pond  water, 
which  is  very  hypotonic  to  their  intracellular  fluid,  have  evolved  con- 
tractile vacuoles  (Fig.  3.1)  which  collect  water  from  the  protoplasm  and 
pump  it  to  the  outside.  Without  such  a  mechanism  the  cells  would 
quickly  burst  from  the  water  entering  the  cell. 

The  power  of  certain  cells  to  accumulate  selectively  certain  kinds  of 
molecules  from  the  environmental  fluid  is  truly  phenomenal.  Human 
cells  (and  those  of  vertebrates  in  general)  can  accumulate  amino  acids 
so  that  the  concentration  within  the  cell  is  2  to  50  times  that  in  the 
extracellular  fluid.  Cells  also  have  a  much  higher  concentration  of  po- 
tassium and  magnesium,  and  a  lower  concentration  of  sodium,  than  the 
environmental  fluids.  Certain  primitive  chordates,  the  tunicates  (p.  384), 
can  accumulate  vanadium  so  that  the  concentration  inside  the  cell  is 
some  2,000,000  times  that  in  the  surrounding  sea  water,  and  sea  weeds 
have  a  comparable  ability  to  accumulate  iodine.  The  transfer  of  water 
or  of  solutes  in  or  out  of  the  cell  against  a  concentration  gradient  is 
physical  work  and  requires  the  expenditure  of  energy.  Some  active 
physiologic  process  is  required  to  perform  these  transfers,  hence  a  cell 
can  move  molecules  against  a  giadient  only  as  long  as  it  is  alive.  If  a 
cell  is  treated  with  some  metabolic  poison,  such  as  cyanide,  it  quickly 
loses  its  ability  to  maintain  concentration  differences  on  the  two  sides 
of  its  plasma  membrane. 

17.        Tissues 

In  the  evolution  of  both  plants  and  animals  one  of  the  major  trends 
has  been  toward  the  structural  and  functional  specialization  of  cells. 
The  cells  which  comprise  the  body  of  one  of  the  higher  animals  are  not 
all  alike,  but  are  differentiated  and  specialized  to  perform  certain  func- 
tions more  efficiently  than  an  unspecialized  animal  body  could.  This 
specialization  has  also  had  the  effect  of  making  the  several  parts  of  the 
body  interdependent,  so  that  an  injury  to,  or  the  destruction  of,  cells  in 
one  part  of  the  body  may  result  in  the  death  of  the  whole  organism. 
The  advantages  of  specialization  are  so  great  that  they  more  than  out- 
weigh the  disadvantages.  The  cells  of  the  body  which  are  similarly  spe- 
cialized are  known  as  a  tissue.  A  tissue  may  be  defined  as  a  group  or 
layer  of  similarly  specialized  cells  which  together  perform  certain  special 
functions.  The  study  of  the  structure  and  arrangement  of  tissues  is 
known  as  histology.  Each  tissue  is  composed  of  cells  which  have  a 
characteristic  shape,  size  and  arrangement;  the  different  types  of  tissue 


54  GENERAL   CONCEPTS 

of  the  vertebrate  body  are  readily  recognized  when  examined  micro- 
scopically. Certain  tissues  are  composed  of  nonliving  cell  products  in 
addition  to  the  cells;  connective  tissue  contains  many  fibers  in  addition 
to  the  fibroblasts  or  connective  tissue  cells,  and  bone  and  cartilage  are 
made  largely  of  proteins  and  salts  secreted  by  the  bone  or  cartilage  cells. 

The  cells  of  a  multicellular  animal  such  as  man  may  be  classified 
in  six  major  groups,  each  of  which  has  several  subgroups.  These  are 
epithelial,  connective,  muscular,  blood,  nervous  and  reproductive  tissues. 

Epithelial  Tissues.  Epithelial  tissues  are  composed  of  cells  which 
form  a  compact,  continuous  layer  or  sheet  covering  the  surface  of  the 
body  or  lining  cavities  within  the  body.  There  is  usually  a  noncellular 
basement  membrane  underlying  the  sheet  of  epithelial  cells.  The 
epithelial  cells  in  the  skin  of  vertebrates  are  usually  connected  by  small 
protoplasmic  processes  or  bridges.  The  epithelia  of  the  body  protect 
the  underlying  cells  from  mechanical  injury,  from  harmful  chemicals 
and  bacteria,  and  from  desiccation.  The  epithelial  lining  of  the  diges- 
tive tract  absorbs  water  and  nutrients  for  use  in  the  body.  The  lining 
of  the  digestive  tract  and  a  variety  of  other  epithelia  produce  and  give 
off  a  wide  spectrum  of  substances,  some  of  which  are  ui>ed  elsewhere  in 
the  body,  and  some  of  which  are  waste  products  which  must  be  elim- 
inated. Since  the  entire  body  is  covered  by  an  epithelium,  all  of  the 
sensory  stimuli  must  pass  through  some  epithelium  to  reach  the  specific 
receptors  for  those  stimuli.  The  functions  of  epithelia  are  thus  protec- 
tion, absorption,  secretion  and  sensation.  The  lining  of  the  digestive 
tract,  windpipe,  lungs,  kidney  tubules  and  urinary  bladder,  and  the 
outer  layer  of  the  skin  are  some  familiar  examples  of  epithelial  tissues. 

The  cells  in  epithelial  tissues  may  be  flat,  cuboidal  or  columnar  in 
shape,  they  may  be  arranged  in  a  single  layer  or  in  many  layers,  and 
they  may  have  fine  protoplasmic  hairs  or  cilia  on  the  free  surface.  On 
the  basis  of  these  structural  characteristics  epithelia  are  subdivided 
into  the  following  groups. 

Squamous  epithelium  is  made  of  thin  flattened  cells  the  shape  of 
flagstones  or  tiles  (Fig.  3.11).  It  is  found  on  the  surface  of  the  skin  and 
the  lining  of  the  mouth,  esophagus  and  vagina.  The  endothelium  lining 
the  cavity  of  blood  vessels  and  the  mesothelium  lining  the  coelom  are 
squamous  epithelia.  In  the  lower  animals  the  skin  is  usually  covered 
with  a  single  layer  of  squamous  epithelium,  but  in  man  and  the  higher 
animals  the  outer  layer  of  the  skin  consists  of  stratified  squamous  epi- 
thelium, made  of  several  layers  of  these  flat  cells. 

The  kidney  tubules  are  lined  with  cuboidal  epithelium,  made  of 
cells  that  are  cube-shaped  and  look  like  dice  (Fig.  3.11).  Many  other 
parts  of  the  body,  such  as  the  stomach  and  intestines,  are  lined  by  cells 
that  are  taller  than  they  are  wide.  An  epithelium  composed  of  such 
elongated,  pillarlike  cells  is  known  as  columnar  epithelium  (Fig.  3.11). 
Columnar  epithelium  may  be  simple,  consisting  of  a  single  layer  of 
cells,  or  stratified,  composed  of  several  layers  of  cells. 

Either  cuboidal  or  columnar  epithelial  cells  may  have  cilia  on  their 
free  surface.  Ciliated  cuboidal  epithelium  is  found  in  the  sperm  ducts 
of  earthworms  and  other  animals  and  ciliated  columnar  epithelium  lines 


CELLS  AND  TISSUES 


55 


Columnar 


Glandular 


Figure  3.1 1 .     Diagram  o£  the  types  of  epithelial  tissue  and  their  location  in  the  body. 


the  ducts  of  the  respiratory  system  of  man  and  other  air-breathing 
vertebrates.  The  rhytlimic,  concerted  beating  of  the  cilia  moves  solici 
particles  in  one  direction  through  the  ducts.  Epithelial  cells,  usually 
columnar  ones,  may  be  specialized  to  receive  stimuli.  The  groups  of 
cells  in  the  taste  buds  of  the  tongue  or  the  olfactory  epithelium  in  the 
nose  are  examples  of  sensory  epithelium.  Columnar  or  cuboidal  epithelia 
may  also  be  specialized  for  secreting  certain  products  such  as  milk,  wax, 
saliva,  perspiration  or  mucus.  The  outer  epithelium  of  most  worms 
secretes  a  thin,  continuous,  noncellular  protective  layer,  called  the 
cuticle,  which  covers  the  entire  body.  Insects,  spiders,  crabs  and  other 
arthropods  secrete  a  cuticle  Avhich  may  be  quite  thick  and  strengthened 
with  deposits  of  chitin  and  salts.  The  hard  protective  shell  of  oysters 
and  snails,  composed  of  calcium  carbonate,  is  secreted  by  epithelial  cells 
in  the  mantle  of  these  animals. 

Connective  Tissues.  The  connective  tissues— bone,  cartilage,  ten- 
dons, ligaments,  fibrous  connective  tissue  and  adipose  tissue— support 
and  bind  together  the  other  tissues  and  organs.  Connective  tissue  cells 
characteristically  secrete  a  nonliving  material  called  the  matrix,  and 
the  nature  and  function  of  each  connective  tissue  is  determined  pri- 
marily by  the  nature  of  this  intercellular  matrix.  The  actual  connective 
tissue  cells  may  form  only  a  small  and  inconspicuous  part  of  the  tissue. 
It  is  the  matrix,  rather  than  the  connective  tissue  cells  themselves,  which 
does  the  actual  connecting  and  supporting. 

Fibrous  connective  tissue  consists  of  a  thick,  interlacing,  matted  net- 
work of  fibers  in  which  are  distributed  the  cells  that  secreted  the  fibers 
(Fig.  3.12).  There  are  three  types  of  fibrous  connective  tissue,  widely 
distributed  throughout  the  body,  which  bind  skin  to  muscle,  muscle  to 
bone,   and   so   on.    These   include   very   delicate   reticular  fibers,   thick, 


56 


GENERAL   CONCEPTS 


Adipose,  tissue 


Figure  3.12. 


Fibrous  cartila.^<z. 
(ajrticular) 
Diagram  of  the  types  of  connective  tissue  anci  their  location  in  the  knee 


joint. 


tough,  vinbranched,  flexible,  but  relatively  inelastic  collagen  fibers,  and 
long,  branched,  elastic  fibers.  Adipose  tissue  is  rich  in  tat  cells,  spe- 
cialized connective  tissue  cells  which  store  large  quantities  of  fat  in  a 
single  drop  in  the  cytoplasm.  Ligaments  and  tendons  are  specialized 
kinds  of  fibrous  connective  tissue.  Tendons  are  composed  of  thick,  closely 
packed  bundles  of  collagen  fibers,  which  form  flexible  cables  that  con- 
nect a  muscle  to  a  bone  or  to  another  muscle.  A  ligament  is  funda- 
mentally similar  in  constitution  to  a  tendon  and  connects  one  bone  to 
another.  An  especially  thick  mat  of  fibrous  connective  tissue  is  located 
in  the  lower  layer  of  the  skin  of  most  vertebrates;  when  this  is  chemically 
treated— "tanned"— it  becomes  leather. 

The  supporting  skeleton  of  vertebrates  is  composed  of  cartilage  or 
bone.  In  some,  for  example  the  sharks,  the  skeleton  is  made  entirely  of 
cartilage.  Cartilage  appears  as  the  supporting  skeleton  in  the  embryonic 
stages  of  all  vertebrates,  but  is  largely  replaced  in  the  adult  by  bone. 
Cartilage  can  be  felt  in  man  as  the  supporting  framework  of  the  pinna 
of  the  ear  (the  external  ear  flap)  or  the  tip  of  the  nose.  It  is  made  of  a 


CELLS  AND   TISSUES  57 

firm  but  elastic  matrix  secreted  by  cartilage  cells  which  become  em- 
bedded in  the  matrix  (Fig.  3.12).  These  cartilage  cells  are  alive;  they 
may  secrete  collagenous  fibers  or  elastic  fibers  to  strengthen  the  cartilage. 

Bone  consists  of  a  dense  matrix  composed  of  proteins  and  calcium 
salts  identical  with  the  mineral  aragonite,  Ca3(P04)2'CaC03.  About  65 
per  cent  of  the  bone  is  made  of  this  mineral.  The  bone  cells  (osteoblasts) 
secrete  both  the  protein  and  the  calcium  salts.  The  osteoblasts  become 
surrounded  and  trapped  by  their  own  secretion  and  remain  in  micro- 
scopic cavities  (lacunae)  in  the  bone  as  living  osteocytes  (Fig.  3.12).  The 
protein  is  laid  down  as  minute  fibers  which  contribute  strength  and 
resiliency  and  the  mineral  salts  contribute  hardness  to  bone.  As  one 
grows  older  the  proportion  of  organic  material  in  the  bone  gradually 
decreases;  hence  the  bones  of  elderly  people  are  more  brittle  than  those 
of  youth. 

At  the  surface  of  each  bone  is  a  thin  fibrous  layer  called  the  peri- 
osteum (peri,  around;  osteum,  bone)  to  which  the  muscles  are  attached 
by  tendons.  The  periosteum  contains  cells,  some  of  which  differentiate 
into  osteoblasts  and  secrete  protein  and  salts  to  bring  about  growth 
and  repair.  Most  bones  are  not  solid,  but  have  a  marrow  cavity  in  the 
center.  The  apparently  solid  matrix  of  the  bone  is  pierced  by  many 
microscopic  channels  (Haversian  canals)  in  which  lie  blood  vessels  and 
nerves  to  supply  the  bone  cells.  The  bony  matrix  is  deposited,  usually 
in  concentric  rings  or  lamellae,  around  these  Haversian  canals.  Each 
bone  cell  is  connected  to  the  adjacent  bone  cells  and  to  the  Haversian 
canals  by  protoplasmic  processes  of  the  bone  cells  which  lie  in  minute 
canals  (canaliculi)  in  the  matrix.  The  bone  cells  obtain  oxygen  and 
raw  materials  and  eliminate  wastes  by  way  of  these  canaliculi.  The  de- 
tails of  the  architecture  of  a  bone  can  be  observed  by  grinding  a  slice 
of  bone  extremely  thin  and  mounting  it  on  a  slide  for  inspection  under 
a  microscope.  Bone  contains  not  only  bone-secreting  cells,  but  also  bone- 
destroying  cells.  By  the  action  of  these  two  types  of  cells,  the  shape  of  a 
bone  may  be  altered  to  resist  changing  stresses  and  strains.  Bone  forma- 
tion and  destruction  is  regulated  by  the  availability  of  calcium  and 
phosphate,  by  the  presence  of  vitamin  D  and  by  the  hormone  secreted 
by  the  parathyroid  glands.  The  marrow  cavity  of  the  bone  may  contain 
yellow  marrow  (largely  a  fat  depot)  or  red  marrow,  the  tissue  in  which 
red  and  certain  white  blood  cells  are  formed. 

Muscular  Tissues.  The  movements  of  most  animals  result  from  the 
contraction  of  elongated,  cylindrical  or  spindle-shaped  cells,  each  of 
which  contains  many  tiny,  longitudinal,  parallel,  contractile  fibers  called 
myofibrils.  Muscle  cells  perform  mechanical  work  by  contracting— by 
getting  shorter  and  thicker;  they  are  unable  to  do  work  by  pushing. 
Three  types  of  muscle  tissue  are  found  in  vertebrates:  skeletal,  cardiac 
and  smooth  (Fig.  3.13).  Cardiac  muscle  is  found  only  in  the  walls  of 
the  heart;  smooth  muscle  in  the  walls  of  the  digestive  tract,  the  urinary 
and  genital  tracts,  and  the  walls  of  arteries  and  veins;  and  skeletal  mus- 
cle makes  up  the  muscle  masses  which  are  attached  to  and  move  the 
bones  of  the  body.  Cardiac  and  skeletal  muscle  cells  are  among  the 
exceptions  to   the  rule  that  cells  have  but  one  nucleus;  each  of  these 


58  GENERA t  CONCEPTS 


Nuclei 


Cross  Sfriotions 


A,  SKELETAL    MUSCLE    FIBERS 


Nuclei 


B,   SMOOTH     MUSCLE     FIBERS 


Nuclei 


C,  CARDIAC     MUSCLE     FIBERS 
Figure  3.13.     Types  of  muscle  tissue.  (Villee:  Biology.) 

cells  has  many  nuclei.  The  nuclei  of  skeletal  muscle  cells  have  an  un- 
usual position,  at  the  periphery  of  the  cell,  just  below  the  plasma  mem- 
brane. Skeletal  muscle  cells  are  extremely  long,  an  inch  or  more  in 
length;  indeed,  some  investigators  believe  that  the  muscle  cells  extend 
from  one  end  of  the  muscle  to  the  other,  so  that  their  length  is  equal 
to  that  of  the  muscle.  Muscle  fibers  range  in  thickness  from  10  to  100 
microns;  continued,  strenuous  muscle  activity  increases  the  thickness  of 
the  fiber.  The  myofibrils  of  skeletal  and  cardiac  muscle  have  alternate 
dark  and  light  cross  bands  or  striations.  These  appear  to  have  some 
fundamental  role  in  contraction,  during  which  the  dark  stripes  decrease 
in  width  and  the  light  stripes  increase  in  width.  The  contraction  of 
skeletal  muscles  is  generally  voluntary,  under  the  control  of  the  will, 
that  of  cardiac  and  smooth  muscles  is  involuntary.  Cardiac  muscle  cells 
are  striated  but  have  centrally  located  nuclei.  Smooth  muscle  cells  are 
not  striated,  have  pointed  ends,  and  have  centrally  located  nuclei. 
Smooth  muscle  contracts  slowly  but  can  remain  contracted  for  long 
periods  of  time.  In  some  of  the  invertebrates  the  voluntary  muscles  of 
the  body,  such  as  the  ones  which  close  the  shell  of  an  oyster,  are  smooth 
muscles.  Striated  muscles  can  contract  very  rapidly  but  cannot  remain 
contracted;  a  striated  muscle  fiber  must  relax  and  rest  before  it  is  able  to 
contract  again.  The  muscles  of  insects,  spiders,  crabs  and  other  arthro- 
pods have  cross  striations  and  contract  very  rapidly.  The  distinguishing 
features  of  the  three  types  of  muscle  are  summarized  in  Table  1. 

Vascular  Tissues.  The  blood,  composed  of  a  liquid  part— plasma 
—and  of  several  types  of  formed  elements— red  cells,  white  cells  and 
platelets— may  be  classified  as  a  separate  type  of  tissue  or  as  one  kind  of 


CELLS  AND  T/SSUES  59 

Table  1.     COMPARISON  OF  VERTEBRATE  MUSCLE  TISSUES 


SKELETAL 

SMOOTH 

CARDIAC 

Location 

Attached  to 
ton 

skele- 

Walls  of  viscera: 
stomach,       intes- 
tines, etc. 

Wall  of  heart 

Shape  of  fiber 

Elongate,  cylindri- 
cal, blunt  ends 

Elongate,      spindle- 
shaped,     pointed 
ends 

Elongate,  cylindri- 
cal, fibers  branch 
and  fuse 

Number  of  nuclei 

Many 

One 

Many 

per  cell 

Position  of  nuclei 

Cross  striations 

Peripheral 
Present 

Central 
Absent 

Central 
Present 

Speed  of  contraction 
Ability  to  remain 

Most  rapid 
Least 

Slowest 
Greatest 

Intermediate 
Intermediate 

contracted 

Type  of  control 

Voluntary 

Involuntary 

Involuntary 

connective  tissue.  The  latter  classification  is  based  on  the  fact  that  blood 
cells  and  connective  tissue  cells  originate  from  similar  cells;  however, 
the  adult  cells  are  quite  different  in  structure  and  function.  The  red 
cells  of  vertebrates  contain  the  red  pigment  hemoglobin,  which  has  the 
property  of  combining  easily  and  reversibly  with  oxygen.  Oxygen,  com- 
bined as  oxyhemoglobin,  is  transported  to  the  cells  of  the  body  in  the 
red  cells.  Mammalian  red  cells  are  flattened,  biconcave  discs  without  a 
nucleus;  those  of  other  vertebrates  are  more  typical  cells  with  an  oval 
shape  and  a  nucleus. 

There  are  five  different  kinds  of  white  blood  cells-lymphocytes, 
monocytes,  neutrophils,  eosinophils  and  basophils  (Fig.  3.14).  These 
have  no  hemoglobin  but  move  around  and  engulf  bacteria.  They  can 
slip  through  the  walls  of  blood  vessels  and  enter  the  tissues  of  the  body 
to  engulf  bacteria  there.  The  fluid  plasma  transports  a  great  variety  of 
substances  from  one  part  of  the  body  to  another.  Some  of  the  substances 
transported  are  in  solution,  others  are  bound  to  one  or  another  of  the 
plasma  proteins.  The  plasma  of  vertebrates  is  a  light  yellow  color;  in 


.sSi?^.^, 


^ 


H 


Figure  3.14.  Types  of  white  blood  cells.  A,  basophil;  B,  eosinophil;  C,  neutrophil; 
E-H,  a  variety  of  lymphocytes;  I  and  /,  monocytes;  D,  a  red  blood  cell  drawn  to  the 
same  scale.  (Villee:  Biology.) 


60 


GENERA/.   CONCEPTS 


Mviscle 


Figure  3.15.  Diagrams  of  an  efferent  neuron  (A)  and  an  afferent  neuron  (B).  The 
arrows  indicate  the  direction  of  the  normal  nerve  impulse.  (Millard,  King  and  Showers: 
Human  Anatomy  and  Physiology.) 

certain  invertebrates  the  oxygen-carrying  pigment  is  not  localized  in 
cells,  but  is  dissolved  in  the  plasma  and  colors  it  red  or  blue.  Platelets 
are  small  fragments  broken  off  from  cells  in  the  bone  marrow;  they  play 
a  role  in  the  clotting  of  blood   (p.  541). 

Nervous  Tissues.  Cells  specialized  for  the  reception  of  stimuli  and 
the  transmission  of  impulses  are  called  neurons.  A  neuron  typically  has 
an  enlarged  cell  body,  containing  the  nucleus,  and  two  or  more  cyto- 
plasmic processes,  the  nerve  fibers,  along  which  the  nerve  impulse  travels 
to  the  next  neuron  (Fig.  3.15).  Nerve  fibers  vary  in  width  from  a  few 
microns  to  30  or  40  microns  and  in  length  from  a  millimeter  or  two  to 
several  feet.  The  neurons  are  connected  end  to  end  so  that  impulses 
may  be  transmitted  all  through  the  body.  Two  types  of  nerve  fibers  are 
distinguished:  axons,  which  transmit  impulses  away  from  the  cell  body, 
and  dendrites,  which  transmit  them  to  the  cell  body.  The  junction  be- 
tween the  axon  of  one  neuron  and  the  dendrite  of  the  next  neuron  in 
the  chain  is  called  a  synapse.  At  the  synapse  the  axon  and  dendrite  do 


CEILS    AND    TISSUES  Ql 

not  actually  touch;  there  is  a  small  gap  between  the  two.  Transmission 
of  an  impulse  across  the  synapse  is  by  a  different  mechanism  from  that 
which  passes  an  impulse  along  the  nerve  fiber.  An  impulse  can  travel 
across  the  synapse  only  from  an  axon  to  a  dendrite;  thus  the  synapse 
serves  as  a  valve  to  prevent  the  backflow  of  impulses.  Neurons  show 
widely  diverse  patterns  of  shape  of  the  cell  body,  and  number  and 
length  of  dendrites  and  axons. 

The  cell  bodies  of  neurons  commonly  occur  in  groups;  there  are 
columns  of  cell  bodies  in  the  spinal  cord,  sheets  of  cell  bodies  over  the 
surface  of  parts  of  the  brain,  nodules  of  cell  bodies  ("nuclei")  within 
the  brain,  and  the  ganglia  of  the  cranial  and  spinal  nerves.  A  ganglion 
is  a  group  of  nerve  cell  bodies  located  outside  the  central  nervous  sys- 
tem. A  nerve  consists  of  a  group  of  axons  and  dendrites  bound  together 
by  connective  tissue.  Each  nerve  fiber— axon  or  dendrite— is  surrounded 
by  one  or  two  sheaths,  a  neurilemma  and/or  a  myelin  sheath.  The 
neurilemma  is  a  delicate,  transparent,  tubelike  membrane  made  of  cells 
which  envelop  the  fiber.  The  myelin  sheath  is  made  of  noncellular,  fatty 
material  which  forms  a  glistening  white  coat  between  the  fiber  and 
neurilemma,  llie  myelin  sheath  is  interrupted  at  fairly  regular  intervals 
along  the  nerve  by  constrictions  called  the  nodes  of  Ranvier.  Nerve  fibers 
are  either  "meduUated"  and  have  a  thick  myelin  sheath,  or  "nonmedul- 
lated"  and  have  an  extremely  thin  myelin  sheath.  Nerve  fibers  in  the 
brain  and  spinal  cord  have  a  myelin  sheath  but  no  neurilemma;  those 
in  the  autonomic  nerves  to  the  viscera,  and  the  nerves  of  many  inverte- 
brates, are  nonmyelinated  and  have  a  very  thin  or  no  myelin  sheath  but 
a  neurilemma.  The  nerves  to  the  skin  and  skeletal  muscles  of  vertebrates 
have  both  a  myelin  sheath  and  a  neurilemma  surrounding  them. 

Nervous  tissue  contains,  in  addition  to  neurons,  several  different 
kinds  of  supporting  cells  called  neuroglia.  These  have  many  cytoplasmic 
processes,  and  the  cells  and  their  processes  form  an  extremely  dense 
supporting  framework  in  which  the  neurons  are  suspended.  The  neu- 
roglia are  believed  to  separate  and  insulate  adjacent  neurons,  so  that 
nerve  impulses  can  pass  from  one  neuron  to  the  next  only  over  the 
synapse,  where  the  neuroglial  barrier  is  incomplete. 

Reproducf/ve  Tissues.  The  egg  cells  (ova)  formed  in  the  ovary  of 
the  female  and  the  sperm  cells  produced  by  the  testes  of  the  male  con- 
stitute the  reproductive  tissues— cells  specially  modified  for  the  pro- 
duction of  offspring  (Fig.  3.16).  Egg  cells  are  generally  spherical  or  oval 
and  are  nonmotile.  A  typical  egg  has  a  large  nucleus,  called  the  germinal 
vesicle,  and  a  variable  amount  of  yolk  in  the  cytoplasm.  Shark  and  bird 
eggs  have  enormous  amounts  of  yolk  which  provides  nourishment  for 
the  development  of  the  embryo  until  it  hatches  from  the  shell.  Sperm 
cells  are  small  and  modified  for  motility.  A  typical  sperm  has  a  long 
tail,  the  beating  of  which  propels  the  sperm  to  its  meeting  and  union 
with  the  egg.  The  head  of  the  sperm  contains  the  nucleus  surrounded 
by  a  thin  film  of  cytoplasm.  The  tail  is  connected  to  the  head  by  a 
short  middle  piece.  An  axial  filament,  formed  by  the  centriole  in  the 
middle  piece,  extends  to  the  tip  of  the  tail.  Most  of  the  cytoplasm  is 
sloughed  off  as  the  sperm  matures;  this  presumably  decreases  the  weight 
of  the  sperm  and  renders  it  more  motile. 


62 


GENERAL  CONCEPTS 


Corona.  Rad.ia.ta. 


Nucleolus 
Nucleus 


Zona  Pellucida. 


Middle /^He^d 
piece-  \  I 


Figure  3.16.  Human  egg  and  sperm  magnified  400  X.  Inset,  side  and  top  views  of 
a  sperm,  magnified  about  200  X.  The  egg  is  surrounded  by  other  cells  which  form  the 
corona  radiata. 


1 8.        Body  Plan  and  Symmetry 

To  refer  to  the  regions  of  an  animal  body,  zoologists  use  the  term 
anterior  for  the  head  end  and  posterior  for  the  tail  end;  the  back  side 
is  the  dorsal  side  and  the  belly  side  is  the  ventral  side.  The  midline  of 
the  body  is  medial  and  the  sides  are  lateral.  The  part  of  a  structure 
nearer  the  point  of  reference  is  proximal,  the  part  farther  away  is  distal. 

A  body  is  symmetrical  if  it  can  be  cut  into  two  equivalent  halves. 
A  few  kinds  of  protozoa  can  be  cut  into  two  equal  halves  by  any 
plane  through  the  center;  they  are  said  to  be  spherically  symmetrical. 
Coelenterates  and  echinoderms  are  radially  symmetrical;  they  can  be 
cut  into  two  equal  halves  by  any  plane  which  includes  the  axis  run- 
ning from  top  to  bottom  through  the  center.  In  such  animals  a  top 
and  bottom  side  can  be  distinguished.  Most  other  animals  are  bilaterally 
symmetrical,  and  can  be  cut  into  two  equivalent  halves  only  by  a  plane 
passing  from  anterior  to  posterior  and  from  the  dorsal  to  ventral  sides 
in  the  midline.  In  such  a  bilaterally  symmetrical  animal,  three  types  of 
planes  or  cuts  can  be  made  to  get  different  views:  sagittal,  frontal  and 
transverse  (Fig.  3.17).  A  sagittal  section  is  one  made  by  cutting  in  the 
median  vertical  plane;  thus  it  includes  the  anterior-posterior  axis  and 
the  dorso-ventral  axis  but  is  at  right  angles  to  the  right-left  axis.  A 
frontal  section  is  at  right  angles  to  a  sagittal  section  and  includes  the 
anterior-posterior  axis  and  the  right-left  axis,  but  is  perpendicvdar  to 
the  dorso-ventral  axis.  Transverse  sections  are  cut  at  right  angles  to  the 
anterior-posterior  axis  and  include  a  dorso-ventral  and  a  right-left  axis. 


DORSAL 


CELLS   AND   TISSUES  53 

.Transverse  plane 


ANTERIOR   "^^ 


POSTERIOR 


VENTRAL 

Figure  3.17.     Diagram  to  illustrate  transverse,  sagittal  and  frontal  planes  in  a  bi- 
laterally symmetrical  animal. 


Questions 

1.  How  would  you  define  a  cell?  What  is  meant  by  the  cell  theory? 

2.  Contrast  the  meaning  of  the  term  cell  in  the  time  of  Robert  Hooke,  in  the  time  of 
Schieidcn  and  Schwann,  and  at  present. 

3.  How  would  you  define  a  tissue?  List  and  give  the  distinguishing  characteristics  of  the 
several  types  of  animal  tissues. 

4.  Describe  the  parts  of  a  typical  animal  cell  and  give  the  functions  of  each. 

5.  Describe  the  methods  that  may  be  used  to  investigate  the  functioning  of  an  ameba. 
Of  a  mammalian  liver  cell. 

6.  In  a   human  cell   undergoing  mitosis,  how  many  chromosomes  are  present  in  the 
metaphase?  In  the  anaphase?  In  the  resting  daughter  cell? 

7.  Outline  briefly  the  events  which  occur  in  each  stage  of  mitosis.  Illustrate  your  discus- 
sion with  diagrams  of  mitosis  in  the  cell  of  an  animal  with  four  pairs  of  chromosomes. 

8.  What  factors  may  regulate  cell  division? 

9.  ^Vhat  is  tlie  principle  underlying  histochemical  studies  of  cell  function? 

10.  Differentiate  clearly  between  diffusion,  dialysis  and  osmosis.  Of  what  biological  im- 
portance is  the  process  of  diffusion? 

11.  In  what  ways  do  gases,  liquids  and  solids  differ? 

12.  Define  the  term  energy.  Differentiate  potential  and  kinetic  energy. 

13.  What  is  a  semipermeable  membrane?  Give  some  examples  of  semipermeable  mem- 
branes in  the  hiunan  body. 

14.  How  would  you  measure  tiie  osmotic  pressure  of  the  contents  of  a  red  blood  cell? 

15.  What  kinds  of  tissue  make  up  the  human  tongue,  stomach,  Uver,  heart,  eye? 

16.  Compare  the  matrix  present  in  bone,  cartilage  and  fibrous  connective  tissue. 

17.  How  would  you  describe  the  position  of  a  rhinoceros'  tusks?  Of  a  camel's  hump?  Of  a 
cobra's  hood? 


Supplementary  Reading 

The  development  of  the  cell  theory  is  interestingly  presented  in  Hall's  A  Source  Book 
in  Afiimal  Biology  by  means  of  long  quotations  from  some  of  the  original  scientific  papers. 
Further  discussion  of  the  properties  of  cells  and  protoplasm  will  be  found  in  General 
Cytology  by  De  Robertis,  Nowinski  and  Saez.  Maximow  and  Bloom's  Textbook  of  Histol- 
ogy is  a  detailed,  technical  discussion  of  the  tissues  of  the  human  body.  It  contains  many 
fine  illustrations,  both  at  the  light  microscope  and  electron  microscope  level,  of  each 
type  of  tissue.  Our  knowledge  of  cell  structure  obtained  by  electron  microscope  and  x-ray 
diffraction  studies  is  summarized  in  The  Fine  Structure  of  Cells,  the  proceeduigs  of  a 
Symposium  held  at  Leiden,  Holland,  in  1954. 


CHAPTER  4 


Cell  Metabolism 


An  examination  of  the  properties  of  living  things  reveals  that  chemical 
reactions  are  basic  to  all  of  them.  These  chemical  activities  of  proto- 
plasm, called  metabolism,  provide  for  the  irritability  and  movement  of 
protoplasm  and  for  its  growth,  maintenance,  repair  and  reproduction. 
Modern  biochemical  research  has  shown  that  the  metabolic  activities 
of  animal,  plant  and  bacterial  cells  are  remarkably  similar,  despite  the 
apparent  differences  of  the  organisms  themselves.  In  all  cells,  sugars  and 
related  substances  are  continually  being  metabolized,  via  a  large  num- 
ber of  intermediate  compounds,  to  water  and  carbon  dioxide  with  the 
release  of  energy  which  is  made  available  to  the  cell  for  further  use. 

Green  plants  differ  from  animals  in  their  ability  to  photosynthesize, 
that  is,  to  capture  the  energy  of  sunlight  and  to  use  it  to  synthesize 
complex,  energy-rich  substances  from  simple  raw  materials— water,  car- 
bon dioxide,  nitrates  and  phosphates.  Animal  and  bacterial  cells  have 
the  ability  to  "fix"  carbon  dioxide,  to  incorporate  it  into  any  one  of  a 
number  of  organic  compounds  and  thus  build  a  new  compound  with 
one  more  carbon  atom  in  the  chain.  Only  green  plants  and  a  few  bac- 
teria, however,  can  utilize  radiant  energy  to  fix  carbon  dioxide;  animals 
and  the  rest  of  the  bacteria  must  get  energy  for  the  reaction  from  some 
energy-releasing  process  such  as  the  metabolism  of  glucose. 

19.        Chemical  Reactions 

A  chemical  reaction  is  a  change  involving  the  molecular  structure 
of  one  or  more  substances;  matter  is  changed  from  one  substance,  with 
its  characteristic  properties,  to  another,  with  new  properties,  and  energy 
is  released  or  absorbed.  Hydrochloric  acid,  HCl,  for  example,  reacts 
with  the  base,  sodium  hydroxide,  NaOH,  to  yield  water,  H2O,  and  the 
salt,  sodium  chloride,  NaCl;  in  the  process  energy  is  released  as  heat. 
The  chemical  properties  of  HCl  and  NaOH  are  very  different  from  those 
of  NaCl  and  HoO.  In  chemical  shorthand  a  plus  sign  connects  the  sym- 
bols of  the  reacting  substances,  HCl  and  NaOH,  and  the  products,  NaCl 
and  H2O.  An  arrow  indicates  the  direction  of  the  reaction: 

HCl  +  NaOH^NaCl  +  HoO 

Most  chemical  reactions  are  reversible  and  this  reversibility  is  indicated 
by  a  double  arrow:  :;=i. 

64 


Cell  metabolism        55 

Atoms  are  neither  destroyed  nor  created  in  the  course  oi  a  chemical 
reaction;  thus  the  sum  ot  each  kind  ot  atom  on  one  side  ot  the  arrow 
must  equal  the  sum  ot  that  kind  of  atom  on  the  other  side.  This  is  an 
expression  ol  one  of  the  basic  laws  of  physics,  the  Law  of  the  Conserva- 
tion of  Matter.  The  direction  of  a  reversible  reaction  is  determined  by 
the  energy  relations  of  the  several  chemicals  involved,  their  relative 
concentrations,  and  their  solubility. 

One  of  the  factors  determining  the  rate  of  a  chemical  reaction  is  the 
temperature;  the  reaction  rate  approximately  doubles  with  each  increase 
of  10°  C.  This  is  true  of  the  chemical  reactions  occurring  in  living  cells 
as  well  as  those  in  a  test  tube,  and  is  another  bit  of  evidence  that  the 
chemical  reactions  of  living  things  are  fundamentally  similar  to  those  of 
nonliving  ones. 

The  over-all  formula  for  the  metabolism  of  glucose  in  the  presence 
of  oxygen  is: 

CuHi.Ou  +  6  O.  ^  6  H.O  +  6  CO.  +  energy 

A  census  of  the  carbon,  hydrogen  and  oxygen  atoms  will  reveal  that 
there  are  equal  numbers  of  each  kind  on  the  two  sides  of  the  arrow. 
Energy  is  released  as  the  glucose  molecule  is  broken  down.  To  reverse 
the  reaction,  and  thus  synthesize  glucose,  an  equivalent  amount  of 
energy  must  be  supplied.  In  photosynthesis  the  radiant  energy  of  sun- 
light is  absorbed  by  the  green  pigment  chlorophyll  and  used  to  split 
water  to  yield  oxygen  and  an  unstable  hydrogen  compound,  which  in 
turn  reacts  with  carbon  dioxide  to  begin  the  synthesis  of  carbohydrates. 

There  are  a  number  of  units  of  energy,  including  the  erg,  the  joule 
and  the  foot-pound,  but  the  one  most  widely  used  in  the  biological 
sciences  is  the  Calorie.  The  kilocalorie,  or  Calorie  written  with  a  capital 
C,  is  the  amount  of  heat  required  to  raise  one  kilogram  of  water  one 
degree  Centigrade  (strictly,  from  14.5°  C.  to  15.5°  C).  Other  forms  of 
energy,  such  as  light,  electricity  or  the  energy  of  motion  or  position,  can 
be  converted  to  heat  and  measured  by  the  resulting  increase  in  tempera- 
ture of  a  known  amount  of  water.  Each  gram  of  glucose,  when  metab- 
olized to  carbon  dioxide,  yields  3.74  Calories.  An  easy  figure  to  remember 
is  that  a  gram  of  carbohydrate  yields  about  4  Calor.es. 

Catalysis.  Many  of  the  substances  that  are  rapidly  metabolized  by 
living  cells  are  remarkably  inert  outside  the  body.  A  glucose  solution, 
for  example,  will  keep  indefinitely  in  a  bottle  if  it  is  kept  free  of  bacteria 
and  molds.  It  must  be  subjected  to  high  temperature  or  to  the  action  of 
strong  acids  or  bases  before  it  will  decompose.  Protoplasm  cannot  fur- 
nish conditions  as  extreme  as  these,  for  the  protoplasm  itself  would  be 
destroyed  long  before  the  glucose,  yet  glucose  is  rapidly  decomposed 
within  cytoplasm  at  ordinary  temperatures  and  pressures  and  in  a  solu- 
tion which  is  neither  acidic  nor  basic.  The  reactions  within  the  cell  are 
brought  about  by  special  agents  known  as  enzymes,  which  belong  to  the 
class  of  substances  known  as  catalysts.  A  catalyst  is  an  agent  which  affects 
the  velocity  of  a  chemical  reaction  without  altering  its  end  point  and 
without  being  used  up  in  the  course  of  the  reaction.  The  list  of  sub- 
stances which  may  serve  as  a  catalyst  in  one  or  more  reactions  is  long 


g6  GENERAL  CONCEPTS 

indeed.  Water  is  an  excellent  catalyst  for  many  reactions.  Pure,  dry 
hydrogen  gas  and  dry  chlorine  gas  do  not  react  when  mixed,  but  if  a 
slight  trace  of  water  is  present  they  react  with  explosive  violence  to  form 
hydrogen  chloride.  Metals  such  as  iron,  nickel,  platinum  and  palladium, 
when  ground  into  a  fine  powder,  are  widely  used  as  catalysts  in  indus- 
trial processes  such  as  the  hydrogenation  of  cottonseed  and  other  vege- 
table oils  to  make  margarine  or  the  cracking  of  petroleum  to  make 
gasoline.  A  minute  amount  of  catalyst  will  speed  up  the  reaction  of  vast 
quantities  of  reactants,  for  the  molecules  of  catalyst  are  not  exhausted 
in  the  reaction  but  are  used  again  and  again. 

20.        Enzymes 

The  speed  and  specificity  of  the  myriad  chemical  reactions  that 
occur  in  protoplasm  are  regulated  by  the  catalysts  called  enzymes,  pro- 
duced by  the  cell.  Man  has  used  the  fermenting  of  grape  juice  and  the 
souring  of  milk,  which  are  enzymatic  processes,  for  thousands  of  years. 
Pasteur  showed  about  100  years  ago  that  these  processes  occur  only  when 
specific  microorganisms  are  present  and  inferred  that  the  enzymes  (he 
called  them  "ferments")  were  active  catalysts  only  when  they  were  a  part 
of  the  living  cell.  In  his  experiments  he  was  unable  to  separate  the  active 
catalysts  from  the  living  cell  and  concluded  that  enzymes  were  living 
things  which  lost  activity  when  separated  from  -the  cell.  Liebig,  in  con- 
trast, believed  that  enzymes  were  simply  complex  organic  compounds 
that  did  not  require  a  living  cell  in  order  to  function,  but  he,  too,  was 
unable  to  remove  an  enzyme  from  a  cell  and  have  it  retain  its  activity. 
Pasteur  and  Liebig  had  a  classic,  long-lasting  argument  over  their  diver- 
gent views.  The  question  was  finally  settled,  after  both  Liebig  and 
Pasteur  had  died,  when  Eduard  Buchner  in  1897  extracted  an  enzyme 
preparation  from  yeast  which,  though  completely  devoid  of  cells,  was 
able  to  decompose  glucose.  In  the  succeeding  years,  hundreds  of  other 
enzymes  have  been  extracted  and  shown  to  have  their  activity  unim- 
paired; some  have  been  purified  and  prepared  as  pure  crystalline  sub- 
stances. We  can  now  define  enzymes  as  organic  catalysts  which  are 
produced  by  living  cells  but  which  are  active  independently  of  the  cell. 
Enzyme-controlled  reactions  are  basic  to  all  the  phenomena  of  life: 
respiration,  digestion,  excretion,  growth,  muscle  contraction,  nerve  con- 
duction, and  so  on.  There  is  no  need  to  postulate  some  mysterious  vital 
force,  as  Pasteur  did,  to  account  for  these  phenomena. 

Properties  of  Enzyrries.  AH  of  the  enzymes  that  have  been  isolated 
and  crystallized  to  date  have  proved  to  be  proteins.  They  are  usually 
colorless,  but  may  be  yellow,  green,  blue,  brown  or  red.  Most  enzymes 
are  soluble  in  water  or  dilute  salt  solution,  but  some,  for  example  the 
enzymes  located  in  the  mitochondria,  are  bound  together  by  lipoproteins 
and  are  insoluble  in  water.  Enzymes  are  usually  named  by  adding  the 
suffix  "-ase"  to  the  name  of  the  substance  acted  upon,  called  the  sub- 
strate. Thus,  sucrose  is  split  by  the  enzyme  sucrase  and  urease  is  the 
enzyme  which  attacks  urea. 

The  catalytic  ability  of  enzymes  is  truly  phenomenal;  without  them 


CEll  METABOUSfA  67 

chemical  reactions  would  occur  much  too  slowly  to  permit  life  to  con- 
tinue. Each  molecule  of  the  enzyme  catalase,  extracted  from  beef  liver, 
will  decompose  5,0U(),{)00  molecules  of  hydrogen  peroxide  (HoO^)  per 
minute  at  0°  C.  Hydrogen  peroxide  is  a  poisonous  substance  produced 
as  a  by-product  in  a  number  of  enzyme  reactions.  Catalase  protects  the 
cell  by  decomposing  the  peroxide.  The  number  of  molecules  of  substrate 
acted  upon  per  minute  by  a  molecule  of  enzyme  is  called  the  turnover 
number  of  the  enzyme.  The  turnover  number  of  catalase,  at  0°  C,  is 
5,000,000.  Most  enzymes  have  high  turnover  numbers,  which  explains 
why  they  can  be  so  remarkably  effective  even  though  present  in  proto- 
plasm only  in  minute  amounts. 

Although  enzymes  in  general  catalyze  specific  reactions,  they  do 
differ  in  the  number  of  kinds  of  substrates  they  will  attack.  Urease  is  an 
example  of  an  ezyme  which  is  absolutely  specific.  Urease  decomposes 
urea  to  ammonia  and  carbon  dioxide  and  will  attack  no  substance  other 
than  urea.  Most  enzymes  are  not  quite  so  specific,  and  will  attack  several 
closely  related  substances.  Peroxidase,  for  example,  will  decompose 
several  different  peroxides  in  addition  to  hydrogen  peroxide.  A  few 
enzymes  are  specific  only  in  requiring  that  the  substrate  have  a  certain 
kind  of  chemical  bond.  The  lipase  secreted  by  the  pancreas  will  split 
the  ester  bonds  connecting  the  glycerol  and  fatty  acids  of  a  wide  variety 

of  fats. 

In  theory,  enzyme-controlled  reactions  are  reversible;  the  enzyme 
does  not  determine  the  direction  of  the  reaction  but  simply  accelerates 
the  rate  at  which  the  reaction  reaches  equilibrium.  The  classic  example 
of  this  is  the  action  of  the  enzyme  lipase  on  the  splitting  of  fat,  or  union 
of  glycerol  and  fatty  acids.  If  one  begins  with  a  fat,  the  enzyme  catalyzes 
the  splitting  of  this  to  give  soine  glycerol  and  fatty  acids.  If  one  begins 
with  a  mixture  of  fatty  acids  and  glycerol,  the  enzyme  catalyzes  the 
synthesis  of  some  fat.  When  either  system  has  operated  long  enough, 
the  same  equilibrium  mixture  of  fat,  glycerol  and  fatty  acid  is  reached: 

Fat  ^  glycerol  -f  3  fatty  acids 

The  equilibrium  point  is  determined  by  complex  thermodynamic 
principles,  which  will  not  be  discussed.  Since  reactions  give  off  energy 
when  going  in  one  direction,  it  is  obvious  that  an  equivalent  amount  of 
energy  in  the  proper  form  must  be  supplied  to  make  the  reaction  go  in 
the  opposite  direction. 

To  drive  an  energy-requiring  reaction,  some  energy-yielding  reac- 
tion must  occur  at  about  the  same  time.  In  most  biologic  systems,  energy- 
yielding  reactions  result  in  the  synthesis  of  "energy-rich"  phosphate 
esters,  such  as  the  terminal  bonds  of  adenosine  triphosphate  (abbreviated 
as  ATP).  The  energy  of  these  energy-rich  bonds  is  then  available  for  the 
conduction  of  an  impulse,  the  contraction  of  a  muscle,  the  synthesis  of 
complex  molecules,  and  so  on,  much  as  the  energy  of  a  storage  battery 
made  by  a  generator  is  available  for  light,  heat  or  running  a  motor. 
Biochemists  use  the  term  "coupled  reactions"  for  two  reactions  which 
must  occur  together  so  that  one  can  furnish  the  energy,  or  one  of  the 
reactants,  needed  by  the  other. 


68 


GENERAL   CONCEPTS 


Enzyine—  substrate  complzx. 


This  -molecule  is 
not  a.  Substra.tc. 
It  does  not  fit  on 
tHc  enzyme- 
Surface. 


A  »    B 

Split  products 


Figure  4.1.     Diagram  illustrating  the  concept  of  a  specific  enzyme  surface  which  per- 
mits the  formation  of  a  specific  enzyme-substrate  complex. 

Enzymes  generally  work  in  teams  in  the  cell,  with  the  product  of 
one  enzyme-controlled  reaction  serving  as  the  substrate  tor  the  next.  We 
can  picture  the  inside  of  a  cell  as  a  factory  with  many  different  assembly 
lines  (and  disassembly  lines)  operating  simultaneously.  Each  of  these 
assembly  lines  is  composed  of  a  number  of  enzymes,  each  of  which 
catalyzes  the  reaction  by  which  one  substance  is  converted  into  a  second. 
This  second  substance  is  passed  along  to  the  next  enzyme,  which  con- 
verts it  into  a  third,  and  so  on  along  the  line.  From  germinating  barley 
seeds  one  can  extract  two  enzymes  that  convert  starch  to  glucose.  The 
first,  amylase,  splits  starch  to  maltose  and  the  second,  maltase,  splits  the 
double  sugar  maltose  to  two  molecules  of  the  single  sugar  glucose.  Eleven 
different  enzymes,  working  in  a  series  or  "cycle,"  are  required  to 
convert  glucose  to  lactic  acid.  The  same  series  of  eleven  enzyines  is  found 
in  human  cells,  in  green  leaves  and  in  bacteria. 

Some  enzymes,  such  as  pepsin  and  urease,  have  been  found  to  consist 
solely  of  protein.  Many  others,  however,  consist  of  two  parts,  one  of 
which  is  protein  (called  the  apoenzyme)  and  the  other  (called  a  co- 
enzyme) is  some  smaller  organic  molecule.  Coenzymes  can  usually  be 
separated  from  their  enzymes  and,  when  analyzed,  have  proved  to  con- 
tain some  vitamin— thiamine,  niacin,  riboflavin,  etc.— as  part  of  the  mole- 
cule. This  finding  has  led  to  the  generalization  that  all  vitamins  function 
as  parts  of  coenzymes  in  the  cell.  Neither  the  apoenzyme  nor  the  coen- 
zyme alone  has  catalytic  properties;  only  when  the  two  are  combined  is 
activity  evident.  Certain  enzymes  require  for  activity,  in  addition  to  a 
coenzyme,  the  presence  of  one  or  more  ions.  Magnesium  (Mg++)  is 
required  for  the  activity  of  several  of  the  enzymes  in  the  chain  which 
converts  glucose  to  lactic  acid.  Ptyalin,  the  starch-splitting  enzyme  of 
saliva,  requires  chloride  ion  as  an  activator.  Most,  if  not  all,  of  the  ele- 
ments required  by  plants   and   animals  in  very  small   amounts— the  so- 


CELL   METABOLISM  59 

called    "trace    elements,"    manganese,    copper,    cobalt,    zinc,    iron,    and 
others— serve  as  enzyme  activators. 

Enzymes  may  be  present  in  the  cell  either  dissolved  in  the  liquid 
part  of  protoplasm  or  bound  to,  and  presumably  an  integral  part  of, 
one  of  the  cell  particles.  A  water  extract  of  ground  liver  contains  all 
of  the  eleven  kinds  of  enzymes  necessary  to  convert  glucose  to  lactic  acid. 
The  respiratory  enzymes,  which  catalyze  the  metabolism  of  lactic  acid 
and  the  carbon  chains  of  fatty  acids  and  amino  acids  to  carbon  diox- 
ide and  water,  are  integral  parts  of  the  mitochondria.  The  microsomes 
have  been  shown  to  contain  a  number  of  enzymes  involved  in  the  syn- 
thesis of  proteins,  cholesterol  and  other  complex  molecules. 

The  Mechanism  of  Enzyme  Catalysis.  Many  years  ago  Emil  Fischer, 
the  German  organic  chemist,  suggested  that  the  specificity  of  the  rela- 
tionship of  an  enzyme  to  its  substrate  indicated  that  the  two  must  fit 
together  like  a  lock  and  key  (Fig.  4.1).  The  idea  that  an  enzyme  combines 
with  its  substrate  to  form  a  reactive  intermediate  enzyme-substrate  com- 
plex, which  subsequently  decomposes  to  release  the  free  enzyme  and  the 
reaction  products,  was  formulated  mathematically  by  Leonor  Michaelis 
more  than  forty  years  ago.  By  brilliant  inductive  reasoning,  he  assumed 
that  such  a  complex  does  form,  and  then  calculated  what  relationships 
shoidd  hold  between  enzyme  concentration,  substrate  concentration,  and 
the  velocity  of  the  reaction.  Exactly  these  relationships  are  observed 
experimentally,  which  is  strong  evidence  that  Michaelis'  assumption, 
that  an  enzyme-substrate  complex  forms  as  an  intermediate,  is  correct. 
Direct  evidence  of  the  existence  of  an  enzyme-substrate  complex  was 
obtained  by  David  Keilin  of  Cambridge  University  and  Britton  Chance 
of  the  University  of  Pennsylvania.  Chance  isolated  a  brown-colored 
peroxidase  from  horseradish  and  found  that  when  this  was  mixed  with 
the  substrate,  hydrogen  peroxide,  a  green-colored  enzyme-substrate  com- 
plex formed.  This  in  turn  changed  to  a  second,  pale  red  complex  which 
finally  split  to  give  the  original  brown  enzyme  and  the  products  of  the 
reaction.  By  observing  the  changes  of  color.  Dr.  Chance  was  able  to 
calculate  the  rates  of  formation  and  of  breakdown  of  this  complex. 

It  is  clear  that  when  it  is  part  of  an  enzyme-substrate  complex,  the 
substrate  is  much  more  reactive  than  it  is  when  free.  It  is  not  clear, 
however,  ivhy  this  should  be  true.  One  explanation  postulates  that  the 
enzyme  unites  with  the  substrate  at  two  or  more  places,  and  the  substrate 
molecule  is  held  in  a  position  which  strains  the  bonds  and  renders  them 
more  likely  to  break. 

21.        Factors  Affecting  Enzyme  Activity 

Temperature.  The  velocity  of  most  chemical  reactions  is  approxi- 
mately doubled  by  each  ten  degree  increase  in  temperature,  and,  over 
a  moderate  range  of  temperature,  this  is  true  of  enzyme-catalyzed  reac- 
tions as  well.  Enzymes,  and  proteins  in  general,  are  inactivated  by  high 
temperatures;  the  higher  the  temperature,  the  more  rapidly  the  enzyme 
activity  is  lost.  Native  protein  molecules  are  believed  to  exist  as  spiral 
coils,  or  helices,  and  the  denaturation  process  is  believed  to  involve  the 


70  GENERAL   CONCEPTS 

unwinding  of  this  helix.  Enzyme  inactivation  is  a  reversible  process  if 
the  temperature  is  not  too  high  and  has  not  been  applied  more  than  a 
short  time.  Most  organisms  are  killed  by  exposure  to  heat  because  their 
cellular  enzymes  are  inactivated.  The  processes  of  protein  denaturation 
and  enzyme  inactivation  show  a  striking  parallelism  and  this  is  one  bit 
of  substantiating  evidence  that  enzymes  are  proteins.  The  enzymes  of 
man  and  other  warm-blooded  animals  operate  most  efficiently  at  a  tem- 
perature of  about  37°  C— body  temperature— whereas  those  of  plants 
and  cold-blooded  animals  work  optimally  at  about  25°  C.  Enzymes  are 
generally  not  inactivated  by  freezmg;  their  reactions  continue  slowly,  or 
perhaps  cease  altogether  at  low  temperatures,  but  their  catalytic  activity 
reappears  when  the  temperature  is  again  raised  to  normal. 

Acidity.  All  enzymes  are  sensitive  to  changes  in  the  acidity  and 
alkalinity— the  pH— of  their  environment,  and  will  be  inactivated  if  sub- 
jected to  strong  acids  or  bases.  Most  enzymes  exert  their  greatest  catalytic 
effect  only  when  the  pH  of  their  environment  is  within  a  certain  rather 
narrow  range.  On  either  side  of  this  optimum  pH,  as  the  pH  is  raised 
or  lowered,  enzyme  activity  rapidly  decreases.  The  protein-digesting 
enzyme  of  the  stomach,  pepsin,  is  remarkable  in  that  it  has  a  pH  op- 
timum of  2.0;  it  will  work  only  in  an  extremely  acid  medium.  The 
protein-digesting  enzyme  secreted  by  the  pancreas,  trypsin,  in  contrast, 
has  a  pH  optimum  of  8.5,  well  on  the  alkaline  side  of  neutrality.  Most 
intracellular  enzymes  have  pH  optima  near  neutrality,  pH  7.0.  This 
marked  influence  of  pH  on  the  activity  of  an  enzyme  is  what  would  be 
predicted  from  the  fact  that  enzymes  are  proteins.  The  topic  is  too 
complex  to  be  discussed  in  detail,  but  the  number  of  positive  and  nega- 
tive charges  associated  with  a  protein  molecule,  and  perhaps  the  shape 
of  the  molecular  surface,  are  determined  by  the  pH.  Probably  only  one 
particular  state  of  the  enzyme  molecule,  with  a  particular  number  of 
negative  and  positive  charges,  is  active  as  a  catalyst.  From  these  consid- 
erations it  is  clear  that  the  catalytic  ability  of  a  protein  molecule  would 
be  expected  to  be  strongly  influenced  by  the  pH  of  the  environment. 

Concentration  of  Enzyme,  Substrate  and  Cofactors.  If  the  pH  and 
temperature  of  an  enzyme  system  are  kept  constant,  and  if  an  excess  of 
substrate  is  present,  the  rate  of  the  reaction  is  directly  proportional  to 
the  amount  of  enzyme  present.  This  method  is  used,  indeed,  to  meas- 
ure the  amount  of  some  particular  enzyme  present  in  a  tissue  extract.  If 
the  pH,  temperature  and  enzyme  concentration  of  a  reaction  system  are 
held  constant,  the  initial  reaction  rate  is  proportional  to  the  amount  of 
substrate  present,  up  to  a  limiting  value.  If  the  enzyme  system  requires 
a  coenzyme  or  specific  activator  ion,  the  concentration  of  this  substance 
may,  under  certain  circumstances,  determine  the  over-all  rate  of  the 
enzyme  system. 

Enzyme  Inhibitors.  Enzymes  can  be  inhibited  by  a  variety  of  chemi- 
cals, some  of  which  inhibit  reversibly,  others  irreversibly.  Cytochrome 
oxidase,  one  of  the  "respiratory  enzymes,"  is  inhibited  by  cyanide,  which 
forms  a  complex  with  the  atom  of  iron  present  in  the  enzyme  molecule 
and  prevents  it  from  participating  in  the  catalytic  process.  Cyanide  is 
poisonous  to  man  and  other  animals  because  of  its  action  on  the  cyto- 


CELL   METABOLISM  71 

chrome  enzymes.  One  of  the  enzymatic  steps  in  the  conversion  of  glucose 
to  lactic  acid  is  inhibited  by  fluoride  ion  and  another  by  iodoacetate. 
These  substances,  and  others,  have  been  used  as  tools  by  biochemists  to 
investigate  the  properties  and  sequences  of  enzyme  systems. 

Enzymes  themselves  may  act  as  poisons  if  they  get  into  the  wrong 
place.  As  little  as  1  milligram  of  crystalline  trypsin  injected  intra- 
venously will  kill  a  rat.  Certain  snake,  bee  and  scorpion  venoms  contain 
enzymes  that  destroy  blood  cells  or  other  body  tissues  when  injected  into 
the  body  of  the  prey. 

22.        Respiration  and  Cellular  Energy 

The  term  "respiration"  originally  meant  simply  inhaling  and  ex- 
haling. It  was  thus  a  synonym  of  breathing  and  the  term  "artificial 
respiration"  reflects  this  usage.  Later,  respiration  came  to  mean  the 
exchange  of  gases  between  the  cell  and  its  environment,  the  intake  of 
oxygen  and  the  release  of  carbon  dioxide.  Most  recently,  as  more  of  the 
details  of  cellular  metabolism  have  become  known,  it  has  come  to  mean 
those  enzymatic  reactions  in  which  oxygen  is  utilized  by  the  cell,  the 
reactions  by  wliich  substrates  are  oxidized  and  most  of  the  energy  is 
made  available  to  the  cell.  The  term  "fermentation"  was  originally 
defined  by  Pasteur  as  "life  without  air"  and  is  now  used  to  refer  to  the 
chemical  reactions  of  substrate  molecules  which  occur  in  the  absence 
of  oxygen. 

The  energy  required  by  each  cell  in  an  animal  or  plant  body  must 
be  obtained  by  releasing  the  potential  energy  of  a  foodstuff  molecule  and 
converting  the  energy  into  a  form  that  is  usable  by  the  cell  for  its  various 
physiologic  functions— contraction,  conduction,  secretion,  or  whatever. 
The  energy  is  released  and  converted  into  "energy-rich"  phosphate  com- 
pounds, of  which  adenosine  triphosphate,  ATP,  is  of  prime  importance. 
These  energy-rich  phosphate  compounds  do  not,  in  general,  pass  from 
one  cell  to  another,  but  are  formed  and  used  within  the  same  cell.  Thus, 
the  energy  for  muscle  contraction  is  not  released  from  food  molecules 
in  the  stomach  or  liver  and  carried  as  "energy"  to  the  muscle.  Instead, 
food  molecules,  such  as  glucose,  are  carried  by  the  blood  to  all  the  cells 
of  the  body.  Then,  within  each  cell,  the  glucose  is  metabolized,  first  to 
pyruvic  acid  and  then,  if  there  is  a  supply  of  oxygen,  to  carbon  dioxide 
and  water.  If  there  is  little  or  no  oxygen,  the  pyruvic  acid  is  converted 
to  lactic  acid,  alcohol,  or  some  other  substance. 

As  the  cellular  metabolism  of  such  diverse  things  as  green  plants, 
rats,  yeast,  bacteria  and  sea  urchins  has  been  investigated,  it  has  become 
clear  that  the  fundamental  enzyme  reactions  in  all  cells  are  remarkably 
similar.  The  steps  by  which  glucose  is  converted  to  pyruvic  acid,  called 
the  glycolytic  cycle,  are  the  same  not  only  in  man  and  mouse,  but  in 
moss  and  mold  as  well.  This  similarity  of  enzyme  systems  may  simply 
reflect  the  fact  that  all  living  things  are  related  by  evolutionary  descent; 
the  system  of  glycolytic  enzymes  became  established  in  the  early  forms 
of  life  and  has  been  transmitted  to  all  the  forms  subsequently  derived 
from  these.  Or,  it  may  be  that  the  types  of  chemical  reactions  that  will 


72  GENERAL   CONCEPTS 

support  lile  are  limited  in  number  and  in  the  course  ot  evolution  other 
methods  have  been  tried  but  have  not  been  able  to  persist. 

Glucose,  to  be  metabolized  within  a  cell,  must  first  be  converted  by 
the  enzyme  glucokinase  to  glucose  phosphate.  Other  sugars,  Iructose,  for 
example,  are  also  converted  to  their  respective  phosphates  belore  any 
further  metabolism  can  occur.  The  glucose  phosphate  is  converted  by 
one  enzyme  to  fructose  phosphate,  and  by  a  second  enzyme  to  fructose 
diphosphate  (a  fructose  molecule  with  two  molecules  of  phosphate  at- 
tached). The  fructose  diphosphate  is  then  cleaved  in  the  middle  of  the 
molecide  to  yield  two  molecules,  each  containing  three  carbons  and  one 
phosphate  group  (Fig.  4.2).  Just  as  a  sugar  with  six  carbons  is  known 
as  a  hexose,  one  with  three  carbons  is  called  a  triose,  and  these  substances 
are  known  as  triose  phosphates.  A  series  of  enzyme  reactions  converts 
the  triose  phosphate  into  pyruvic  acid.  In  the  course  of  these  reactions 
two  energy-rich  phosphate  compounds  are  produced  as  each  molecule  of 
triose  phosphate  is  converted  to  pyruvic  acid.  The  phosphate  group  and 
its  associated  energy  is  transferred  to  adenosine  diphosphate  to  convert 
it  to  adenosine  triphosphate.  The  latter  compoinid  is  the  major  currency 
of  biologically  available  energy,  and  is  available  for  any  of  the  many 
energy-requiring  reactions  of  the  cell.  The  energy  derived  in  this  con- 
version of  triose  phosphate  to  pyruvic  acid  represents  only  about  5  per 
cent  of  the  energy  that  is  ultimately  obtainable  when  the  triose  phos- 
phate is  metabolized  to  carbon  dioxide  and  water. 

The  other  95  per  cent  is  obtained  in  the  oxidation  of  pyruvic  acid, 
which  is  mediated  by  a  series  of  enzymes  many  of  which  are  located  in 
the  mitochondria.  The  series  of  reactions  was  postulated  by  the  English 
biochemist,  H.  A.  Krebs,  and  is  known  as  the  Krebs  citric  acid  cycle,  for 
citric  acid  (which  accumulates  in  the  tissues  of  citrus  fruits)  is  the  first 
substance  in  the  series.  To  enter  the  citric  acid  cycle,  pyruvic  acid  must 
first  be  converted  to  an  acetic  acid-coenzyme  A  compoimd.  The  acetyl 
coenzyme  A  (which  contains  two  carbons)  unites  with  oxaloacetic  acid 
(four  carbons)  to  form  citric  acid  (six  carbons).  The  successive  enzymes 
of  the  citric  acid  cycle  then  break  citric  acid  down  stepwise  through 
eight  different  intermediate  compounds  to  oxaloacetic  acid,  which  is 
then  ready  to  combine  with  another  molecule  of  acetyl  coenzyme  A  and 
continue  the  cycle.  In  this  cycle  (Fig.  4.2)  carbon  dioxide  is  given  off  by 
decarboxylases,  hydrogen  atoms  are  removed  by  dehydrogenases,  and 
the  electrons  of  the  hydrogen  atoms  are  transferred  by  the  electron- 
transmitting  enzymes,  the  cytochromes,  to  oxygen,  which  then  unites 
with  the  hydrogen  ions  to  form  water.  As  the  two  molecules  of  pyruvic 
acid  derived  from  each  molecule  of  glucose  are  metabolized  in  the  Krebs 
cycle  and  cytochromes,  about  36  additional  energy-rich  phosphate  com- 
pounds are  formed.  In  this  way  much  more  of  the  energy  originally  in 
the  glucose  molecule  is  made  available,  as  adenosine  triphosphate,  to 
run  the  many  energy-requiring  processes  of  metabolism.  The  Krebs  cycle 
has  been  called  the  "intracellular  energy  wheel";  it  takes  in  molecules 
of  acetyl  coenzyme  A,  spews  forth  carbon  dioxide  and  hydrogen,  and 
traps,  in  the  form  of  ATP,  the  energy  released. 

The  idea  that  we  breathe  in  oxygen   and  breathe  out  carbon  di- 


CELL  METABOLISM 


73 


oxide  is  so  familiar  that  it  is  perhaps  only  natural  to  infer  that  the 
oxygen  atoms  in  the  carbon  dioxide  (CO2)  are  the  same  atoms  that  en- 
tered the  body  as  gaseous  oxygen.  This  is  not  true,  however,  as  an  ex- 
amination of  Figure  4.2  will  make  clear.  The  oxygen  atoms  that  enter 
the  body  as  oxygen  unite  with  hydrogen  to  form  molecules  of  water, 
and  leave  the  body  as  water.  The  oxygen  atoms  that  leave  the  body  in 
carbon  dioxide  entered  the  body,  by  and  large,  in  some  substrate  mole- 

Glycogen  or  Starch 


Glucose 


Amino   acids 


Lactic  acid± 


V" 


2O 

energy    1 


■H, 


.H 


Electron 

transmitting 

enzymes 

(cytochromes) 


-►  Glucose  phosphate 
Fructose  phosphate 
Fructose  diphosphate 
2  Triose  phosphates 


L 


►  energy  ^'^P  —: 

— -  ^m\\^^ 

Phosphoglycenc  ac.d         ^^^^„,,,^^ 

energy  %^P  # 
"Pyruvic  acid 

I 


)gl: 

1 

jvii 

1 


■^   CO2 


Acetyl  coenzyme  A- 

(2carbons)V 


4 


Oxalacetic  acid"^ 
(4  carbons) 


u 


Fatty 
acids 


KREBS 

CITRIC  ACID 

CYCLE 


Citric  acid 
(6  carbons) 


t^t 


CO: 


-T" 


-^ 


NET:    CgHiaOe  +6  Og— ►  6  COg  +  6  HgO  +  energy 


Figure  4.2.  A  diagram  of  some  of  the  steps  in  the  glycolytic  cycle  (glucose  to  pyruvic 
acid),  the  citric  acid  cycle  and  the  cytochrome  system.  The  symbol  ~  P  refers  to  energy- 
rich  phosphate  bonds  such  as  those  in  adenosine  triphosphate  (ATP)  which  can  yield 
their  energy  to  drive  cellular  mechanisms.  From  this  some  appreciation  can  be  gained 
of  the  tremendous  o\ersimplification  involved  in  writing  the  over-all  formula  for  the 
oxidation  of  glucose  given  below. 


74  GENERAL  CONCEPTS 

cule  such  as  glucose.  The  carbon  and  oxygen  atoms  are  removed  from  a 
substrate  molecule  together,  by  a  process  known  as  decarboxylation. 
There  is  one  such  decarboxylation  process  as  pyruvic  acid  (three  car- 
bons) is  converted  to  acetyl  coenzyme  A  (two  carbons)  and  two  more  in 
the  Krebs  cycle  as  citric  acid  (six  carbons)  is  converted  to  oxaloacetic 
acid  (lour  carbons). 

The  conversion  of  glucose  to  pyruvic  acid  in  the  absence  of  air, 
sometimes  referred  to  as  fermentation,  extracts  only  a  small  portion  of 
the  energy  of  the  glucose  molecule.  When  yeast  cells  ferment  glucose, 
they  convert  the  pyruvic  acid  formed  to  alcohol  and  carbon  dioxide. 
The  souring  of  milk  by  bacteria  involves  the  conversion  of  milk  sugar 
(lactose)  through  the  glycolytic  cycle  to  pyruvic  acid,  and  finally  the 
conversion  of  the  pyruvic  acid  to  lactic  acid. 

Further  examination  of  Figure  4.2  will  show  that  the  Krebs  cycle  is 
the  final  common  pathway  for  the  oxidation  of  fatty  acids  and  amino 
acids  as  well  as  for  carbohydrates.  It  is  the  chief  source  of  chemical 
energy  in  the  cell.  The  fatty  acids  most  commonly  found  in  tissues  are 
ones  containing  16  and  18  carbons  in  a  long  chain.  These  long  chains 
are  chopped  into  two  carbon  pieces,  as  acetyl  coenzyme  A,  and  these 
pieces  enter  the  Krebs  cycle  by  uniting  with  oxaloacetic  acid.  Certain 
amino  acids  can  be  transformed  enzymatically  into  pyruvic  acid  and 
others  are  converted  to  other  members  of  the  Krebs  cycle.  By  a  variety 
of  different  pathways,  the  amino  groups  are  removed  and  the  carbon 
chains  of  the  amino  acids  finally  enter  the  Krebs  cycle  and  are  oxidized 
to  yield  carbon  dioxide,  water  and  energy. 

Some  interesting  calculations  of  the  over-all  energy  changes  in- 
volved in  metabolism  in  the  human  body  have  been  made  by  E.  G.  Ball 
of  Harvard  University.  Since  the  conversion  of  oxygen  to  water  in- 
volves the  participation  of  hydrogen  atoms  and  electrons,  the  total  flow 
of  electrons  in  the  human  body  can  be  calculated  and  expressed  in 
amperes.  From  the  oxygen  consumption  of  an  average  70  kg.  man  at 
rest— 264  ml.  per  minute— and  the  fact  that  each  oxygen  atom  requires 
two  hydrogen  atoms  and  two  electrons  to  form  a  molecule  of  water. 
Dr.  Ball  calculated  that  2.86  X  10""  electrons  flow  from  foodstuff,  via 
dehydrogenases  and  the  cytochromes,  to  oxygen  each  minute.  Since  an 
ampere  equals  3.76  X  10""  electrons  per  minute,  this  flow  of  electrons 
amounts  to  76  amperes.  This  is  quite  a  bit  of  current,  for  an  ordinary 
100  watt  light  bulb  uses  just  a  little  less  than  1  ampere.  Then,  from  the 
number  of  calories  used  by  this  70  kg.  man  at  rest— 1.27  calories  per 
minute— Dr.  Ball  calculated  that  88.7  watts  were  being  used.  Since,  in 
electrical  units,  watts  divided  by  amperes  equals  volts,  88.7  divided  by 
76  equals  1.17  volts.  The  body,  then,  uses  energy  at  about  the  same 
rate  as  a  100  watt  light  bulb,  but  differs  from  it  in  having  a  much 
larger  flow  of  electrons  passing  through  a  much  smaller  voltage  change. 

23.       The  Dynamic  State  of  Protoplasm 

The  body  of  an  animal  or  man  appears  to  be  unchanging  as  days  and 
weeks  go  by  and  it  would  seem  reasonable  to  infer  that  the  component 
cells  of  the  body,  and  even  the  component  molecules  of  the  cells,  are 


CELL   METABOLISM  75 

equally  unchanging.  In  the  absence  of  any  evidence  to  the  contrary,  it 
was  generally  held,  until  about  twenty  years  ago,  that  the  constituent 
molecules  of  animal  and  plant  cells  were  relatively  static  and  that,  once 
formed,  they  remained  intact  for  a  long  period  of  time.  A  corollary  of 
this  concept  is  that  the  molecules  of  food  which  are  not  used  to  in- 
crease the  mass  of  protoplasm  are  rapidly  metabolized  to  provide  a 
source  of  energy.  It  followed  from  this  that  one  could  distinguish  two 
kinds  of  molecules:  relatively  static  ones  that  made  up  the  cellular 
"machinery,"  and  ones  that  were  rapidly  metabolized  and  thus  cor- 
respond to  cellular  "fuel." 

However,  in  1938  Rudolf  Schoenheimer  and  his  colleagues  at 
Columbia  University  began  a  series  of  experiments  in  which  amino 
acids,  fats,  carbohydrates  and  water,  each  suitably  labeled  with  some 
"heavy"  or  radioactive  isotope,  were  fed  to  rats.  Schoenheimer's  experi- 
ments, which  have  been  confirmed  many  times  since,  showed  that  the 
labeled  amino  acids  fed  to  the  rats  were  rapidly  incorporated  into  body 
proteins.  Similarly,  labeled  fatty  acids  were  rapidly  incorporated  into 
the  fat  deposits  of  the  body,  even  though  in  each  case  there  was  no  in- 
crease in  the  total  amount  of  protein  or  fat.  Such  experiments  have 
demonstrated  that  the  fats  and  proteins  of  the  body  cells— and  even  the 
substance  of  the  bones— are  constantly  and  rapidly  being  synthesized 
and  broken  down.  In  the  adult  the  rates  of  synthesis  and  degradation 
are  essentially  equal  so  that  there  is  little  or  no  change  in  the  total  mass 
of  the  animal  body.  The  distinction  between  "machinery"  molecules 
and  "fuel"  molecules  becomes  much  less  sharp,  for  some  of  the  ma- 
chinery molecides  are  constantly  being  broken  down  and  used  as  fuel. 
From  the  rate  at  which  the  labeled  atoms  are  incorporated  it  has  been 
calculated  that  one  half  of  all  the  tissue  proteins  of  the  human  body 
are  broken  down  and  rebuilt  every  eighty  days.  The  proteins  of  the  liver 
and  blood  serum  are  replaced  very  rapidly,  one  half  of  them  being 
synthesized  every  ten  days.  The  muscle  proteins,  in  contrast,  are  re- 
placed much  more  slowly,  one  half  of  the  total  number  of  molecules 
being  replaced  every  180  days.  The  celebrated  aphorism  of  Sir  Fred- 
erick Gowland  Hopkins,  the  late  English  biochemist,  sums  up  this  con- 
cept very  succinctly:  "Life  is  a  dynamic  equilibrium  in  a  polyphasic 
system." 

24.        Special  Types  of  Metabolism 

The  metabolic  paths  just  described,  by  which  carbohydrates,  fats  and 
proteins  are  metabolized  to  carbon  dioxide  and  water,  with  the  con- 
comitant release  of  biologically  available  energy,  are  common  to  almost 
all  cells.  Certain  cells  have  in  addition  one  or  more  unique  metabolic 
abilities  such  as  the  enzymatic  shortening  of  certain  kinds  of  protein 
molecules  (i.e.,  muscle  contraction),  the  enzymatic  synthesis  of  sub- 
stances with  specific  biologic  activities  such  as  hormones,  the  produc- 
tion of  electricity  by  specialized  organs  such  as  that  of  the  electric  eel, 
or  the  enzymatic  production  of  light  by  a  variety  of  fish,  insects,  molds 
and  bacteria. 

B/o/um/nescence.     A  number  of  animals,  and  some  molds  and  bac- 


76  GENERAL   CONCEPTS 


Figure  4.3.  Anomalops  katoptron,  a  luminescent  fish  from  the  waters  of  the  Malay 
Archipelago.  The  crescent-shaped  luminescent  organs  below  the  eyes  are  equipped  with 
reflectors.  (After  Steche.)  (Villee;  Biology.) 

teria  as  well,  have  an  enzymatic  mechanism  for  the  production  of  light. 
Luminescent  animals  are  found  among  the  protozoa,  sponges,  coelen- 
terates,  ctenophores,  nemerteans,  annelids,  crustaceans,  centipedes,  milli- 
pedes, beetles,  flies,  echinoderms,  molluscs,  hemichordates,  tunicates  and 
fishes.  From  this  wide  and  irregular  distribution  of  the  light-emitting 
ability,  it  is  clear  that  the  enzymes  for  luminescence  have  appeared  in- 
dependently in  a  number  of  different  evolutionary  lines.  It  is  some- 
times difficult  to  establish  that  a  given  organism  is  itself  luminescent; 
in  a  number  of  instances  animals  once  believed  to  be  luminescent  have 
been  shown  instead  to  contain  luminescent  bacteria.  When  the  bacteria 
are  removed  the  animal  is  no  longer  able  to  emit  light.  Several  different 
exotic  East  Indian  fish  have  light  organs  under  their  eyes  in  which  live 
luminous  bacteria  (Fig.  4.3).  The  light  organs  contain  long,  cylindrical 
cells  which  are  well  provided  with  blood  vessels  to  supply  an  adequate 
amount  of  oxygen  to  the  bacteria.  The  bacteria  emit  light  continuously 
and  the  fish  have  a  black  membrane,  somewhat  similar  to  an  eyelid,  that 
can  be  drawn  up  over  the  light  organ  to  turn  off  the  light.  How  the 
bacteria  come  to  collect  in  the  fish's  light  organ,  as  they  must  in  each 
newly  hatched  fish,  is  a  complete  mystery. 

Some  animals  have  accessory  lenses,  reflectors  and  color  filters  with 
the  light-producing  organ  and  the  whole  complex  assembly  is  like  a 
lantern.  Certain  shrimp  have  such  complicated  light-emitting  organs. 

The  production  of  light  is  an  enzyme-controlled  reaction,  the  de- 
tails of  which  differ  in  different  organisms.  Bacteria  and  fungi  produce 
light  continuously  il  oxygen  is  available.  Most  luminescent  animals,  in 
contrast,  give  out  flashes  of  light  only  when  their  luminescent  organs 
are  stimulated.  The  name  luciferin  has  been  given  to  the  material  which 
is  oxidized  to  produce  light  and  luciferase  to  the  enzyme  which  catalyzes 
the  reaction.  The  luciferin  and  luciferase  from  one  species  may  be  quite 
different  chemically  from  those  in  another.  The  oxidation  of  luciferin 
by  luciferase  can  occur  only  in  the  presence  of  oxygen.  It  is  possible  to 
extract  luciferin  and  luciferase  from  a  firefly,  mix  the  two  in  a  test  tube 
with  added  magnesium  and  adenosine  triphosphate,  and  demonstrate 
the  emission  of  light  in  the  test  tube.  The  energy  for  the  reaction  is 
supplied  by  the  ATP. 


CELL   METABOLISM  77 

The  amount  of  light  produced  by  certain  himinescent  animals  is 
amazing.  Many  fireflies  produce  as  much  light,  in  terms  of  lumens  per 
square  centimeter,  as  do  modern  fluorescent  lamps.  Different  kinds  of 
animals  may  emit  lights  of  different  colors,  red,  green,  yellow  or  blue. 
One  of  the  more  spectacular  luminescent  beasts  is  the  "railroad  worm" 
of  Uruguay,  the  larva  of  a  beetle,  which  has  a  row  of  green  lights  along 
each  side  of  its  body  and  a  pair  of  red  lights  on  its  head.  The  light  pro- 
duced by  luminescent  organisms  is  entirely  in  the  visible  part  of  the 
spectrum;  no  ultraviolet  or  infrared  light  is  produced.  Since  very  little 
heat  is  given  off  in  the  process,  bioluminescence  has  been  called  "cold 
light." 

What  advantage  an  animal  derives  from  the  emission  of  light  can 
only  be  guessed  at.  For  deep  sea  animals,  which  live  in  perpetual  dark- 
ness, light  organs  might  be  useful  to  enable  members  of  a  species  to 
recognize  one  another,  to  serve  as  a  lure  for  prey  or  as  a  warning  for 
woidd-be  predators.  Experiments  have  shown  that  the  light  emitted  by 
fireflies  serves  as  a  signal  to  bring  the  two  sexes  together  for  mating.  The 
light  emitted  by  bacteria  and  fungi  probably  serves  no  useful  purpose 
to  the  organisms,  but  is  simply  a  by-product  of  oxidative  metabolism, 
just  as  heat  is  a  by-product  of  metabolism  in  other  plants  and  animals. 

Questions 

1.  How  would  you  define  the  term  "metabolism"? 

2.  What  factors  affect  the  rate  of  a  chemical  reaction  in  the  test  tube?  In  a  living  cell? 

3.  Define  the  following  terms:  enzyme,  coenzyme,  apoenzyme,  substrate,  turnover  num- 
ber, energy-rich  phosphate,  coupled  reactions. 

4.  What  might  be  the  advantage  to  a  cell  of  having  all  the  enzymes  that  act  in  sequence 
on  a  given  substance  localized  in  a  particular  intracellular  organelle  such  as  a  mito- 
chondrion or  microsome? 

5.  Discuss  the  several  meanings  of  the  term  "respiration." 

6.  Indicate  brieflv  how  the  carbon  chain  of  an  amino  acid  might  become  part  of  (a)  a 
glycogen  molecule  and  (b)  a  fatty  acid  molecule  in  an  animal  cell. 

7.  What  factors  do  you  suppose  have  led  to  the  evolution  of  luminescent  organs  in  ani- 
mals? 

8.  Suppose  you  discovered  a  new  species  of  bioluminescent  worm.  How  could  you  prove 
that  it  was  the  worm  itself  and  not  some  contaminating  bacterium  that  was  producing 
the  light? 

Supplementary  Reading 

A  series  of  articles  on  the  many  different  fields  of  biology  in  which  enzymes  play  a 
role  is  found  in  Enzymes:  Units  of  Biological  Structure  and  Function,  edited  by  O.  A. 
Gaebler.  Baldwin's  Dynamic  Aspects  of  Biochemistry  gives  a  technical  but  extremely 
interesting  account  of  the  details  of  cellular  metabolism.  Rudolf  Schoenheimer  presents  a 
summary  of  his  classic  experiments  demonstrating  the  rapid  renewal  of  the  chemical 
constituents  of  the  body  in  The  Dynamic  State  of  the  Body  Constituents.  The  phenome- 
non of  bioluminescence  is  described  by  E.  \.  Harvey  in  Living  Light  and,  in  a  more 
detailed  fashion,  in  Bioluminescence.  L.  J.  Henderson,  in  his  classic,  The  Fitness  of  the 
Environment,  advanced  the  thesis  that  the  environment  had  to  have  certain  chemical 
and  physical  characteristics  for  life  to  develop.  A  number  of  eminent  biochemists  and 
physiologists  present  their  current  theories  and  findings  in  Currents  in  Biochemical 
Research,  edited  by  D.  E.  Green. 


CHAPTER  5 


Principles  of  Physiology 


From  the  discussion  of  cell  metabolism  in  the  preceding  chapter,  it 
should  be  evident  that  all  animal  cells  are  laced  with  certain  common 
problems.  To  have  survived,  each  animal— vertebrate  or  invertebrate, 
multicellular  or  unicellular— must  have  solved,  in  one  way  or  another, 
the  problems  of  getting  foodstuffs  and  oxygen,  of  eliminating  carbon 
dioxide  and  wastes,  of  responding  suitably  to  stimuli  from  the  environ- 
ment, of  moving  to  new  areas,  and  of  reproducing  its  kind.  A  survey  of 
the  animal  kingdom  will  reveal  that  in  the  course  of  evolution  an  almost 
bewildering  variety  of  solutions  to  these  problems  has  arisen.  At  this 
point  in  our  discussion,  however,  we  want  to  emphasize  what  is  common 
to  the  physiology  and  morphology  of  animals  rather  than  what  differ- 
ences exist.  The  details  of  the  variety  of  animal  forms  will  be  presented 
in  Chapters  8  to  31. 

25.        Types  of  Nutrition 

Organisms  that  can  synthesize  their  own  foodstuffs  are  said  to  be 
autotrophic  (self-nourishing).  An  autotroph  needs  only  water,  carbon 
dioxide,  inorganic  salts  and  a  source  of  energy  to  survive.  Green  plants 
are  autotrophs  which  obtain  energy  from  sunlight  for  the  synthesis  of 
organic  molecules,  a  process  known  as  photosynthesis.  Certain  bacteria 
are  also  autotrophic,  obtaining  the  energy  for  the  synthesis  of  foods 
either  from  sunlight  (the  so-called  purple  bacteria  are  photosynthetic) 
or  from  the  oxidation  of  certain  inorganic  substances— ammonia,  nitrites 
or  hydrogen  sulfide.  No  animal  is  autotrophic;  animals  obtain  their 
foodstuffs  by  eating  autotrophs,  or  by  eating  other  animals  which  ate 
autotrophs.  Ultimately  the  foodstuff  molecules  of  all  animals  are  syn- 
thesized by  energy  obtained  by  these  autotrophic  organisms  either  from 
sunlight  or  from  the  oxidation  of  inorganic  compounds. 

The  organisms  which  cannot  synthesize  their  own  food  from  inor- 
ganic substances,  and  hence  miist  live  either  by  eating  autotrophs  or 
upon  decaying  matter,  are  called  heterotrophs.  All  animals  and  fungi 
(molds),  as  well  as  most  bacteria,  are  heterotrophs.  Three  types  of  hetero- 
trophic nutrition  are  found  in  the  animal  kingdom;  holozoic,  saprozoic 
and  parasitic. 

Holozoic  nutrition  is  the  type  generally  found  in  animals:  food  is 
obtained  as  particles  of  some  size   which  must  be  eaten  and   digested 

78 


PRINCIPLES   OF   PHYSIOLOGY  79 

before  it  can  be  absorbed  into  the  cell.  Holozoic  organisms  must  find 
and  catch  other  organisms;  this  has  required  the  evolution  of  a  variety 
of  sensory,  nervous  and  muscular  structures  to  find  and  catch  food,  and 
some  sort  of  digestive  system  to  convert  the  food  into  molecules  small 
enough  to  be  absorbed.  Animals  that  feed  chiefly  upon  plants  are  termed 
herbivores,  those  that  eat  other  animals  are  called  carnivores  and  those 
that  eat  both  plants  and  animals  are  known  as  omnivores.  The  morphol- 
ogy and  mode  of  functioning  of  the  digestive  system  in  different  kinds 
of  animals  are  correlated  with  the  nature  of  food  eaten,  peculiarities  of 
the  manner  of  life,  and  so  on.  Carnivores,  for  example,  characteristically 
have  strong  proteolytic  (protein-digesting)  enzymes;  whereas  herbivores 
have  weak  proteolytic,  but  strong  carbohydrate-splitting  action. 

Although  such  familiar  protozoa  as  amebas  and  paramecia  do  ingest 
food  particles,  many  protozoa,  as  well  as  yeasts,  molds  and  most  bacteria, 
cannot  ingest  solid  food.  Instead,  the  required  organic  nutrients  are 
absorbed  through  the  cell  membrane  as  dissolved  molecules.  Plants  and 
animals  with  this  type  of  heterotrophic  nutrition  are  known  as  sapro- 
phytic and  saprozoic,  respectively.  Saprophytes  can  grow  only  in  an 
environment  which  contains  decomposing  animal  or  plant  bodies,  or 
plant  or  animal  by-products  which  will  supply  the  necessary  dissolved 
organic  substances. 

A  third  type  of  heterotrophic  nutrition,  parasitism,  occurs  when  one 
organism  (the  parasite)  lives  on  or  within  the  body  of  another  living 
organism  (the  host)  and  obtains  its  food  from  it.  Almost  every  animal 
is  the  host  for  one  or  more  parasites;  these  obtain  their  nutrients  either 
by  ingesting  and  digesting  solid  particles  from  the  host,  or  by  absorbing 
organic  molecules  through  their  cell  walls  from  the  surrounding  body 
fiuids  or  tissues  of  the  host.  Some  parasites  cause  little  or  no  harm  to 
the  host.  Others  harm  the  host  by  destroying  cells,  by  robbing  it  of 
nutrients  or  by  producing  toxic  waste  products,  and  produce  definite 
symptoms  of  disease.  Some  parasites  have  lost  all  traces  of  a  digestive 
system  and  get  nutrients  only  by  absorbing  organic  substances  through 
their  body  wall.  Any  given  parasite  is  usually  restricted  to  one  or  a  tew 
species  of  hosts;  thus,  most  of  the  parasites  that  infect  man  will  not  infect 
other  animals.  In  the  course  of  evolution,  the  parasite  becomes  adapted 
to  the  specific  conditions  of  temperature,  pH,  and  the  concentration  of 
salts,  vitamins  and  other  nutrients  found  in  one  particular  host,  and 
cannot  survive  elsewhere. 

26.        Ingestion,  Digestion  and  Absorption 

The  protozoa  have  no  digestive  system  and  most  protozoans  have  no 
specialized  structure  for  taking  in  food.  Amebas  capture  food  by  extrud- 
ing two  lobes  of  protoplasm,  called  pseudopods,  which  surround  the 
prey  (Fig.  5.1).  The  pseudopods  meet  around  the  prey  and  form  a  food 
vacuole  containing  the  particle  to  be  eaten.  Digestive  enzymes  are  se- 
creted by  the  protoplasm  into  this  food  vacuole,  the  food  particle  is 
digested,  and  the  molecules  of  digested  food  are  absorbed  through  the 
wall  of  the  vacuole  into  the  cytoplasm,  where  they  are  metabolized  to 


Fooa 


Digested 


Food    :•  '^'-l 


:J  Vacuole     )S'iS}'''') 

,  /     T  .-i-y  Enzymes     V--  J"  ^i) 


FORMATION  OF  A  FOOD  VACUOLE  IN  AN  AMEBA 


Food    •^.M-Jir\^az\e 
M. Mouth 


Entoderm 

Enzymes  Secreted 
Food  Absorbed 


Food 
Wostes 


Ectoderm 
Mesoderm 
Entoderm 

Enzymei 
Stomocti 

Absorbed  Food 
Food  Particles 


HYDRA 


FLATWORM 


Food      Ptiorynx 


Crop 


Intestine 


Absorbed  Food 


Moutti  Esoptiogus    Gizzard  Enzymes 

EARTHWORM 


Anus 


Esophagus  Sfomoch  Inhestine  YC\o<ica 


SALAMANDER 

Figure  5.1.     The  digestive  systems  of  anieba,  hydra,  flatworm,  earthworm  and  a  ver- 
tebrate (salamander).  (Partly  from  Villee:  Biology.) 


release  energy  or  to  provide  for  the  maintenance  and  growth  of  the 
animal.  The  paramecium  and  other  ciliates  have  a  permanent  oral 
groove  which  is  lined  by  cilia.  The  beating  of  the  cilia  passes  food 
particles  to  a  cell  mouth  where  they  are  collected  into  food  vacuoles. 
The  canals  of  sponges  are  lined  by  collar  cells,  which  capture  and  ingest 
microscopic  food  particles  in  food  vacuoles.  In  sponges  and  protozoa, 
digestion  is  intracellidar,  occurring  in  food  vacuoles  within  the  cyto- 
plasm of  the  cell. 


PRINCIPLES   Of   PHYSIOLOGY  Q\ 

The  body  of  the  coelenterate  consists  of  two  layers  of  cells;  the 
inner  one  is  specialized  for  digestion  and  absorption.  Food— small  ani- 
mals and  plants  caught  by  the  tentacles— passes  through  the  mouth  and 
enters  the  central  gastrovascular  cavity.  The  endoderm  cells  secrete 
digestive  enzymes  into  this  cavity  and  some  digestion  occurs.  This  is 
extracellular  digestion,  occurring  in  a  special  digestive  cavity,  and  is 
found  in  most  animals.  Some  partly  digested  food  particles  are  taken  up 
by  the  endoderm  cells  in  food  vacuoles  in  which  intracellular  digestion 
occurs.  There  is  no  separate  anal  aperture;  undigested  wastes  leave  the 
gastrovascular  cavity  by  the  mouth.  Digestion  in  the  fiatworms,  such  as 
planaria,  is  similar  to  that  in  the  coelenterates:  food  enters  and  wastes 
leave  the  branched  digestive  tract  via  the  same  opening  and  digestion 
is  partly  extracellular  and  partly  intracellular.  The  gastrovascular  cavity 
of  the  flatworm  is  greatly  branched  and  the  branches  extend  throughout 
most  of  the  body,  thus  facilitating  the  distribution  of  digested  food. 

In  most  of  the  rest  of  the  invertebrates,  and  in  all  the  vertebrates, 
the  digestive  tract  is  a  tube  with  two  apertures;  food  enters  by  the  mouth 
and  any  undigested  residue  leaves  by  the  anus.  The  digestive  tract  may 
be  short  or  long,  straight  or  coiled,  and  subdivided  into  specialized 
organs.  These  organs,  even  though  they  may  have  similar  names  in 
different  kinds  of  animals,  may  be  quite  different,  and  may  even  have 
different  functions.  1  he  digestive  system  of  the  earthworm,  for  example, 
includes  a  mouth,  a  muscular  pharynx  which  secretes  a  mucous  material 
to  lubricate  the  food  particles,  an  esophagus,  a  soft-walled  crop  where 
food  is  stored,  a  thick  muscular  gizzard  where  food  is  ground  against 
small  stones,  and  a  long  straight  intestine  in  which  extracellular  diges- 
tion occurs  and  through  the  wall  of  which  the  food  is  absorbed.  Many 
invertebrates— worms,  squid,  crustacea,  sea  urchins— have  hard,  toothed 
mouthparts  for  tearing  off  and  chewing  bits  of  food. 

The  details  of  the  vertebrate  digestive  system  will  be  given  in  Chap- 
ter 26.  It  is  similar  in  basic  plan  to  that  of  the  earthworm,  but  has 
undergone  further  evolution  and  specialization.  There  is  a  separate 
small  intestine,  Avhere  most  digestion  and  absorption  occurs,  and  a  fol- 
lowing large  intestine  in  which  digestion  and  absorption,  especially  the 
absorption  of  water,  are  completed.  The  vertebrate  digestive  system  also 
includes  the  liver  and  pancreas,  connected  to  the  small  intestine  by 
ducts.  These  large  digestive  glands  produce,  among  other  things,  certain 
enzymes  and  other  substances  required  for  digestion. 

The  Digestive  Process.  Digestion,  whether  in  ameba  or  man,  in- 
volves the  splitting  of  complex  molecules  into  simpler  ones  by  the  addi- 
tion of  water,  a  process  called  hydrolysis.  There  are  specific  hydrolases 
for  the  enzymatic  splitting  of  proteins,  fats  and  carbohydrates.  The 
digestive  enzymes  of  vertebrates  include  the  protein  hydrolases  pepsin, 
secreted  by  the  stomach,  trypsin  and  chymotrypsin,  secreted  by  the  pan- 
creas, and  several  peptidases  secreted  by  the  pancreas  and  intestinal 
mucosa.  Lipases,  which  split  fats,  are  secreted  by  the  pancreas.  The 
carbohydrate-splitting  enzymes  include  ptyalin,  secreted  by  the  salivary 
glands,  amylase,  secreted  by  the  pancreas,  and  maltase,  sucrose  and 
lactase  secreted  by  the  intestinal  mucosa.  Each  enzyme  has  a  specific  pH 


82  GENERAL   CONCEPTS 

optimum,  ranging  from  a  very  acid  one  for  pepsin  to  an  alkaline  one 
for  trypsin.  The  molecules  of  protein,  fat  and  carbohydrate  originally 
present  in  the  food  are  too  large  to  pass  through  the  wall  of  the  digestive 
tract;  the  digestive  process  converts  these  to  amino  acids,  tatty  acids, 
glycerol  and  single  sugars,  which  are  able  to  be  absorbed  through  the 
wall  of  the  digestive  tract  into  the  body. 

Herbivorous  animals  typically  have  a  pouch  in  which  the  cellulose- 
rich  food  is  subjected  to  bacterial  digestion,  for  the  animal  itself  has  no 
enzyme  to  digest  the  cellulose  walls  of  the  plant  cells.  In  the  rabbit  and 
horse  this  pouch  is  the  caecum,  located  at  the  junction  of  the  small  and 
large  intestine.  The  products  of  bacterial  digestion  are  absorbed  into 
the  blood  stream.  The  cow  and  other  ruminants  have  a  large,  complex 
rumen  between  the  esophagus  and  stomach  in  which  the  plants  are 
digested  by  bacteria  and  protozoa  which  were  eaten  along  with  the 
plants.  The  bacteria  convert  cellulose  to  acetic  acid,  and  a  large  part  of 
the  cow's  calories  are  absorbed  as  acetic  acid  directly  from  the  rumen. 
The  bacteria  further  contribute  to  the  cow's  economy  by  synthesizing 
vitamins  and  amino  acids  from  the  material  ingested. 

The  products  of  the  digestive  process  are  taken  up  into  the  proto- 
plasm of  the  body.  In  those  animals  with  intracellular  digestion  occvn- 
ring  within  food  vacuoles,  the  products  of  digestion  are  simply  trans- 
ported across  the  membrane  of  the  food  vacuole  and  are  then  available 
for  the  many  possible  paths  in  cell  metabolism.  In  animals  with  ex- 
tracellular digestion,  the  products  are  generally  taken  through  the  cells 
lining  the  digestive  tract  and  on  into  the  circulatory  system  for  distribu- 
tion to  the  cells  of  the  body.  In  mammals,  the  amino  acids  and  simple 
sugars  are  absorbed  in  part  by  energy-requiring  processes  and  in  part  by 
simple  diffusion.  The  cells  lining  the  intestine  comprise  a  semiperme- 
able membrane  which  permits  the  passage  of  amino  acids  and  simple 
sugars  but  prevents  the  passage  of  intact  proteins  and  complex  sugars. 
In  many  animals,  the  lining  of  the  intestine  is  thrown  into  folds,  which 
increase  the  area  available  for  absorption.  Amino  acids  and  sugars  are 
taken  up  by  the  blood  stream  for  transport;  in  contrast,  the  products  of 
fat  digestion  in  mammals  cross  the  intestinal  mucosa,  are  reformed  into 
fats  and  enter  the  lymph  vessels  (p.  556)  to  be  carried  to  other  parts  of 
the  body. 

A  discussion  of  the  eventual  fate  of  the  absorbed  food  would  involve 
all  the  reactions  of  cell  metabolism,  some  of  which  were  discussed  in 
Chapter  4.  The  amino  acids  serve  as  raw  materials  for  the  synthesis  of 
cell  proteins.  Amino  acids  may  undergo  deamination  (removal  of  the 
amino  group)  and  their  carbon  chains  are  then  used  to  synthesize  gly- 
cogen and  other  carbohydrates,  to  synthesize  fatty  acids,  or  they  are 
metabolized  in  the  Krebs  citric  acid  cycle  to  yield  energy.  The  amino 
group  is  combined  with  carbon  dioxide  by  yet  another  complex  series 
of  enzymatic  reactions  to  form  urea.  This  waste  product  is  synthesized 
largely  in  the  liver,  carried  in  the  blood  to  the  kidneys,  and  excreted 
in  the  urine. 

The  sugars  absorbed  are  converted  into  glycogen   for  storage  pri- 


PRINCIPLES   Of   PHYSIOLOGY  g^ 

marily  in  liver  and  muscle.  Glycogen  synthesis  occurs  to  a  lesser  extent 
in  other  tissues.  Between  meals  the  stored  glycogen  is  broken  down  for 
use.  Liver  glycogen  can  be  converted  enzymatically  into  glucose  and 
secreted  into  the  blood  stream.  One  of  the  prime  functions  of  the 
vertebrate  liver  is  the  maintenance  of  a  constant  level  of  glucose  in 
the  blood.  It  does  this  by  absorbing  glucose  from  the  blood  coming 
from  the  intestine  just  after  a  meal,  when  the  blood  has  a  high  concen- 
tration of  glucose,  and  by  secreting  glucose  into  the  blood  stream  be- 
tween meals.  The  glycogen  in  muscle  and  other  tissues  cannot  be  con- 
verted to  glucose  (one  of  the  enzymes  required  is  absent)  and  hence  must 
be  utilized  locally.  Carbohydrates  are  rapidly  converted  to  fats  if  more 
are  taken  in  than  can  be  used  directly.  These,  plus  the  fats  taken  in  as 
food,  are  stored  for  use  between  meals. 

Nutritive  Requirements.  In  addition  to  proteins,  fats  and  carbohy- 
drates, animals  require  water,  minerals  and  vitamins  to  maintain  health 
and  to  grow.  Minerals  are  constantly  lost  from  the  body  in  urine,  feces 
and  sweat,  and  an  equivalent  amount  must  be  taken  in  with  the  food. 
Most  foods  contain  adequate  supplies  of  minerals,  and  mineral  de- 
ficiencies are  comparatively  rare.  Certain  htmian  deficiency  diseases  may 
be  traced  to  a  lack  of  iron,  copper,  iodine,  calcium  or  phosphorus.  A 
disease  which  was  resulting  in  the  death  of  whole  herds  of  sheep  in 
Australia  was  finally  shown  to  be  due  to  a  deficiency  of  cobalt.  The  soil 
in  that  region,  and  hence  the  grass  eaten  by  the  sheep,  was  very  poor  in 
this  metal  which  is  required  as  a  trace  element  for  normal  metabolism. 

Water  is  required  by  every  animal.  Aquatic  animals  have  no  prob- 
lem about  obtaining  water;  indeed,  their  problem  is  to  prevent  the 
osmotic  inflow  of  water  and  the  consequent  bursting  of  their  cells.  Many 
land  animals  drink  water,  but  others,  certain  desert  animals  for  example, 
obtain  all  they  require  from  the  food  eaten,  and  from  the  water  formed 
when  the  food  molecides  are  metabolized. 

Vitamins  are  organic  substances  required  in  small  amounts  in  the 
diet.  They  differ  widely  in  their  chemical  structure  but  are  similar  in 
that  they  cannot  be  synthesized  by  the  animal  and  hence  must  be  present 
in  the  diet.  What  is  a  vitamin  for  one  animal  is  not  necessarily  one  for 
anot/ier  anitnal.  That  is,  some  species  can  synthesize  certain  of  these 
required  substances  and  hence  do  not  need  them  in  their  food.  It  is 
probable  that  all  plants  and  animals  require  these  vitamin  molecules  for 
similar  metabolic  functions;  organisms  differ,  however,  in  their  ability 
to  synthesize  them.  Only  man,  monkeys  and  guinea  pigs,  for  example, 
require  vitamin  C  in  the  diet;  other  animals  can  make  it  from  some 
other  substance.  The  vitamins  whose  role  in  metabolism  is  known— 
niacin,  thiamine,  riboflavin,  pyridoxine,  pantothenic  acid,  biotin,  folic 
acid  and  cobalamin  (vitamin  Bio)— have  proved  to  be  constituent  parts 
of  one  or  more  coenzyme  molecules.  Vitamin  A  is  a  part  of  the  light- 
sensitive  pigment  of  the  retina  of  the  eye  (p.  580).  A  lack  of  any  one  of 
these  vitamins  produces  a  particular  deficiency  disease  with  characteristic 
symptoms,  e.g.,  scurvy  (lack  of  vitamin  C),  beriberi  (lack  of  thiamine), 
rickets  (lack  of  vitamin  D)  and  pellagra  (lack  of  niacin). 


^4  GENERAL   CONCEPTS 

27.        Circulation 

The  metabolic  processes  of  all  cells  require  a  constant  supply  of 
food  and  oxygen  and  constant  removal  of  wastes.  In  protozoans  the 
transport  of  substances  is  effected  by  the  diffusion  of  the  molecules,  aided 
generally  by  streaming  movements  of  the  cytoplasm  itself.  The  flowing 
of  the  cytoplasm  from  rear  to  front  as  an  ameba  moves,  and  the  circular 
movement  of  the  cytoplasm  in  protozoa  with  a  fixed  shape  (such  as 
paramecia)  are  examples  of  these  (Fig.  5.2).  Transport  from  cell  to  cell 
in  simple  multicellular  animals  such  as  sponges,  coelenterates  and  flat- 
worms  occurs  by  diffusion.  This  is  aided  in  some  animals  by  the  stirring 
of  the  body  fluids  brought  about  by  the  contraction  of  the  muscles  of 
the  body  wall.  Diffusion,  you  will  recall,  is  the  movement  of  molecules 
from  a  region  of  high  concentration  to  a  region  of  lower  concentration. 
The  rate  of  diffusion  is  directly  proportional  to  the  difference  in  con- 
centration in  the  two  regions  and  inversely  proportional  to  the  distance 
separating  them.  From  this  we  can  see  that  an  adequate  supply  of  food 
and  oxygen  can  be  maintained  by  diffusion  alone  only  in  a  small  animal; 
in  a  larger  animal  the  slower  diffusion  rate  over  the  greater  distance 
would  not  suffice.  Such  animals  must  develop  some  system  of  internal 
transport— some  type  of  circulatory  system.  Not  only  absolute  size,  but 
also  the  shape  and  the  activity  of  an  animal  determine  the  need  for  a 
circulatory  system. 

The  proboscis  worms  or  Nemertea  are  the  simplest  living  animals 
to  have  a  distinct  circulatory  system;  it  consists  of  a  dorsal  and  two 
lateral  blood  vessels  which  extend  the  whole  length  of  the  body  and 
are  connected  by  transverse  vessels.  The  earthworm  has  a  more  com- 
plicated circulatory  system:  a  dorsal  vessel,  in  which  blood  flows 
anteriorly,  a  ventral  vessel  and  a  subneural  vessel  in  which  blood  flows 
posteriorly,  and  five  jDairs  of  pulsating  tubes  ("hearts")  at  the  anterior 
end  which  drive  blood  from  the  dorsal  to  the  ventral  vessel  (Fig.  5.2). 
In  other  segments  of  the  body  a  network  of  vessels  connecting  dorsal  and 
ventral  vessels  ramifies  through  the  body  wall  and  the  wall  of  the 
intestine.  The  blood  in  these  vessels  does  not  flow  regularly  in  one  direc- 
tion, but  ebbs  and  flows  as  the  vessels  constrict  and  dilate. 

A  typical  circulatory  system  includes  blood  vessels  and  heart  and 
the  fluid  within  them— the  blood— which  in  turn  is  composed  of  a  fluid— 
plasma— and  blood  cells  or  corpuscles.  Oxygen  is  carried  in  most  cir- 
culatory systems  not  simply  dissolved  in  the  plasma  but  in  combination 
with  a  heme  protein  pigment.  The  one  found  in  the  earthworm  and 
man  is  hemoglobin,  a  red,  iron-containing  pigment.  The  hemoglobin  of 
vertebrate  blood  is  located  in  cells,  the  red  blood  cells.  In  many  inverte- 
brates, the  hemoglobin  or  other  pigment  is  dissolved  in  the  plasma,  and 
whatever  cells  are  present  are  colorless.  The  respiratory  pigment  of  crab 
blood  is  a  different  heme  protein,  blue-green  hemocyanin,  which  con- 
tains copper  in  place  of  iron. 

The  circulatory  system  of  the  annelid  worms  and  the  vertebrates  is 
said  to  be  "closed,"  i.e.,  the  blood  in  the  course  of  circulation  remains 
within  blood  vessels.   In  contrast,   the  circulatory  system  of  arthropods 


PRINCIPLES   OF  PHYSIOLOGY  85 


PARAMECIUM  HYDRA 

Dorsal  vessel-]     Ho^scrhs 


EARTHWORM 


Blooci 
vessds 


NEMERTEA 


num 


rHearb 


Ventral         Subnearal 
•-vessel       '-vessel 

Ostium^     I    rPericardial  sinuS 


■<«>  -^  ~^  ^ 


^    '^^CRAYFlteH'^^ 


Heart 


p^"" 


CAT 


Figure  5.2.     The  circulatory  systems  of  paramecium,  hydra,  nemertea,  earthworm, 
crayfish  and  cat. 


85  GENERAL   CONCEPTS 

and  molluscs  is  "open";  the  blood  vessels  open  to  the  body  cavity,  called 
a  hemocoel,  and  blood  circulates  partly  within  blood  vessels  and  partly 
through  the  cavity  of  the  hemocoel  in  making  a  complete  circuit.  In 
the  typical  arthropod,  the  heart  and  other  organs  lie  free  in  the  hemocoel 
and  are  bathed  in  blood.  In  the  annelid  worm  and  vertebrates,  the 
organs  lie  in  the  coelomic  cavity  and  are  supplied  by  blood  which 
reaches  them  in  closed  vessels.  The  arthropod  heart  is  generally  a  single, 
elongate,  muscular  tube  lying  in  the  dorsal  midline.  In  each  segment  of 
the  body  there  is  a  pair  of  openings,  supplied  with  valves  to  prevent 
backflow.  Blood  enters  the  heart  from  the  pericardial  sinus,  which  is 
part  of  the  hemocoel,  through  these  openings  (ostia)  and  is  moved  for- 
ward by  peristaltic  waves,  waves  of  contraction  preceded  by  waves  of 
relaxation  along  the  tube.  Blood  is  carried  in  vessels  to  the  head  and  to 
other  parts  of  the  body,  whence  it  returns  to  the  heart  through  the 
hemocoel. 

The  hearts  of  most  invertebrates  are  single  muscular  tubes  which 
develop  only  very  low  pressures— a  few  millimeters  of  mercury— as  they 
pump  blood.  In  the  vertebrates,  with  closed  circulatory  systems,  a  higher 
pressure,  as  high  as  100  to  200  mm.  Hg,  is  required  to  drive  the  blood 
through  the  tremendous  number  of  narrow  capillaries.  This  has  led  to 
the  evolution  of  powerful,  thick-walled  hearts.  The  chamber  of  the  verte- 
brate heart  called  the  ventricle  has  quite  thick  walls.  However,  the  mus- 
cular ventricle  requires  a  certain  amount  of  pressure  to  distend  it  and 
cause  the  blood  to  flow  in  and  fill  it  during  the  relaxation  phase  (dias- 
tole). The  low  pressure  in  vertebrate  veins  is  not  sufficient  to  do  this. 
The  vertebrate  heart  has  a  second  chamber,  the  atrium,  with  walls  thin 
enough  to  be  filled  by  the  low  venous  pressure  yet  strong  enough  to 
pump  blood  into  the  ventricle  and  distend  it.  The  octopus,  whose  heart 
is  similarly  arranged  with  two  different  chambers,  has  the  highest  blood 
pressure,  35  to  45  mm.  Hg,  of  any  of  the  invertebrates. 

The  vertebrate  heart  is  enclosed  in  a  special  cavity,  the  pericardial 
cavity,  separated  from  the  rest  of  the  body  by  a  thin,  strong  sheet  of 
connective  tissue,  the  pericardium.  This  cavity  provides  space  for  the 
heart  to  change  in  volume  as  it  beats. 

The  circulatory  systems  of  all  vertebrates  are  essentially  similar:  a 
closed  system  composed  of  heart,  aorta,  arteries,  capillaries  and  veins 
arranged  in  a  basically  similar  plan.  Arteries  carry  blood  away  from  the 
heart  to  the  tissues,  veins  carry  blood  back  to  the  heart  from  the  tissues, 
and  capillaries  are  minute,  thin-walled  vessels  connecting  the  arteries 
to  the  veins  and  completing  the  circuit  from  heart  to  heart.  The  prin- 
cipal changes  in  the  vertebrate  circulatory  system  have  been  associated 
with  the  change  from  gills  to  lungs  as  respiratory  organs.  The  changes 
in  the  pattern  of  circulation  permit  the  delivery  of  oxygen-rich  blood  to 
the  brain  and  muscles.  The  pattern  of  circulation  in  many  lower  verte- 
brates is  such  that  some  mixing  of  oxygen-rich  and  oxygen-poor  blood 
occurs.  Mammals  and  birds  can  be  warm  blooded  because  their  cir- 
culatory systems  supply  enough  oxygen  to  the  tissues  to  support  a 
metabolic  rate  high  enough  to  maintain  a  high  body  temperature  in  cold 
surroundings. 


PRINCIPLES   OF   PHYSIOLOGY  87 

28.        Respiration 

The  energy  requirements  of  cells  are  met  by  the  release  of  energy, 
generally  by  oxidative  processes,  from  foodstuff  molecules.  These  cellu- 
lar oxidative  processes,  which  include  the  removal  of  hydrogen  and 
carbon  dioxide  from  certain  molecules  and  the  combination  of  the 
hydrogen  with  oxygen  to  form  water,  are  the  fundamental  reactions  of 
respiration  at  the  cellular  level.  We  may  define  cellular  respiration  as 
the  sum  of  the  processes  in  which  oxygen  is  utilized  and  carbon  dioxide 
is  produced.  For  these  processes  to  continue,  the  supply  of  oxygen  must 
be  renewed  constantly  and  the  carbon  dioxide  produced  must  be  re- 
moved. 

Animals  differ  tremendously  in  their  general  levels  of  activity  and 
hence  in  their  requirements  for  energy  and  for  oxygen.  As  a  corollary 
of  this,  animals  differ  in  their  susceptibility  to  oxygen  deprivation.  A 
mouse,  which  uses  2,500  cu.  mm.  of  oxygen  per  gram  per  hour  when 
resting,  and  as  much  as  20,000  cu.  mm.  per  gram  per  hour  when  active, 
rapidly  dies  of  suffocation  when  deprived  of  oxygen  or  when  poisoned 
with  carbon  monoxide.  But  an  earthworm,  which  uses  60  cu.  mm.,  or  a 
sea  anemone,  which  uses  only  13  cu.  mm.  of  oxygen  per  gram  per  hour, 
has  a  much  lower  rate  of  metabolism  and  does  not  readily  suffocate. 
"Life"  goes  on  in  these  lower  animals  at  a  much  lower  rate,  in  general, 
than  it  does  in  birds  and  mammals.  There  are  exceptions  to  this  generali- 
zation, and  some  animals  with  low  rates  of  oxygen  consumption  are 
very  sensitive  to  oxygen  deprivation. 

The  transfer  of  gases  across  the  cell  membrane  to  the  surrounding 
body  fluid— or  pond  or  sea  water— is  also  part  of  the  respiratory  process. 
In  the  larger  and  more  complex  animals,  further  exchange  of  gases  must 
occur  between  the  body  fluids— blood  and  interstitial  fluid— and  the  out- 
side environment,  an  exchange  which  usually  involves  some  specialized 
respiratory  surface,  such  as  lungs  or  gills.  The  molecules  of  oxygen  or 
carbon  dioxide,  whether  in  man  or  anieba,  move  simply  by  diffusion, 
from  a  region  of  high  concentration  to  a  region  of  lower  concentration. 
The  diffusion  gradients  are  maintained,  for  oxygen  is  constantly  utilized 
and  carbon  dioxide  is  produced  within  the  cell.  Physiologists  use  the 
terms  partial  pressure  and  tension  of  a  gas  to  describe  these  diffusion 
gradients  quantitatively. 

The  partial  pressure  of  a  gas  is  simply  the  pressure  due  to  that  one 
gas  in  a  mixture  of  gases.  It  is  calculated  by  multiplying  the  total  pres- 
sure of  the  mixture  of  gases  by  the  percentage  of  that  gas  in  the  mixture. 
Air,  for  example,  normally  has  a  pressure  of  about  760  mm.  Hg  and  is 
one-fifth  oxygen.  The  partial  pressure  of  oxygen  in  air  is  760  X  0.20 
or  152  mm.  Gas  molecules  dissolved  in  a  liquid  have  a  certain  tendency 
to  escape,  to  leave  the  liquid  and  enter  the  gaseous  phase.  This  escaping 
tendency  can  be  measured  by  the  pressure  of  that  gas  in  the  gaseous 
phase  in  contact  with  the  liquid  which  is  required  to  prevent  any  net 
loss  of  the  gas,  i.e.,  to  maintain  equilibrium.  When  a  liquid  and  gas 
are  in  contact,  an  equilibrium  is  reached  when  the  rate  at  which  mole- 
cules pass  from  the  liquid  to  the  gas  equals  the  rate  at  which  they  pass 


88 


GENERAL   CONCEPTS 


from  the  gas  to  the  liquid.  This  escaping  tendency,  known  as  the  tension 
ot  the  gas,  is  expressed  numerically  in  terms  oi  the  partial  pressure  of 
the  gas  with  which  it  would  be  in  equilibrium.  Notice  that  the  gas 
tension  is  a  measure  of  the  tendency  of  the  dissolved  gas  to  diffuse  out 
from  the  solution,  and  is  not  a  measure  of  the  qiumtity  of  gas  present. 
The  actual  quantity  of  gas  in  solution  is  a  property  of  both  the  gas 
and  the  liquid,  and  may  vary  considerably  from  one  liquid  to  another. 
Water  and  blood  in  equilibrium  with  air  would  each  have  an  oxygen 
tension  of  152  mm.  Hg,  but  the  water  would  contain  only  0.2  ml.  of 
oxygen  per  100  ml.  and  blood  (because  of  the  presence  of  hemoglobin) 
would  contain  20  ml.  of  oxygen  per  100  ml.  A  solution  of  pure  hemo- 
globin containing  the  same  amount  of  hemoglobin  as  blood  (15  gm.  per 
100  ml.)  would  also  contain  20  ml.  of  oxygen  per  100  ml.  and  have  an 
oxygen  tension  of  152  mm.  Hg. 

The  Respiratory  Surfaces.  The  protozoa  and  the  simpler  inverte- 
brates—sponges, coelenterates  and  flatworms— obtain  oxygen  from  and 
give  off  carbon  dioxide  to  the  surrounding  water.  This  process  is  termed 
direct  respiration,  since  the  body  cells  exchange  oxygen  and  carbon 
dioxide  directly  with  the  surrounding  environment.  The  cells  of  the 
larger,  more  complex  animals  cannot  exchange  gases  directly  with  the 
environment  and  some  form  of  indirect  respiration  occurs:  the  cells 
exchange  gases  with  the  body  fluids  (internal  respiration)  and  the  body 
fluids  exchange  gases  with  the  external  environment  via  a  specialized 


'f  Aine.ba- 


Insect.-Body  wall  Tissue  Cells 

'Spiracle 


sm 


1  Waiter 


Epitheliuin. 

0,  (pa 

CO,    ^^°- 


Trachea 
"L-uTLg  of  vertebrate 


Tracheolc 


%,.v«™.      ™s..^\v^5^ 


EndollieliuTTi     -^^^ 
Fish:' Gill  f  ila-ment 


Figure  5.3.     Respiration  in  an  ameba,  in  the  tracheal  system  of  an  insect,  in  the  gill 
of  a  fish,  and  in  the  lung  of  a  higher  vertebrate. 


PRINCIPLES   OF   PHYSIOLOGY  89 

respiratory  surface  (external  respiration)  (Fig.  5.3).  The  respiratory  sur- 
face for  many  animals  is  simply  the  skin,  or  perhaps  the  lining  of  the 
mouth.  Fishes,  many  amphibia,  molluscs,  crustaceans  and  some  worms 
have  developed  gills— fine  filaments  of  tissues  containing  blood  channels, 
and  covered  with  an  epithelium.  Gases  diffuse  from  the  surrounding 
water  through  the  thin,  moist  membrane  to  the  blood  vessels.  The 
amount  of  dissolved  oxygen  in  sea  water  is  relatively  constant,  but  the 
amount  in  fresh  water  ponds  may  fluctuate  widely. 

Insects  and  certain  other  arthropods  have  openings,  called  spiracles, 
in  each  segment  of  the  body  through  which  air  passes,  via  a  system  of 
branched  air  ducts,  called  tracheae,  to  all  of  the  internal  organs.  The 
ducts  end  in  microscopic,  fluid-filled  tracheoles;  oxygen  and  carbon 
dioxide  pass  by  diffusion  through  the  walls  of  the  tracheoles  to  the 
adjacent  tissue  cells.  The  larger  insects  can  pump  air  through  the  tra- 
cheae by  contraction  of  muscles  in  the  abdominal  walls.  This  is  an  efficient 
system  for  gas  exchange  in  animals  of  the  size  of  insects,  for  the  oxygen 
reaches  the  tissue  cells  and  carbon  dioxide  is  removed  by  diffusion  alone; 
no  energy  need  be  expended,  as  in  the  vertebrates,  in  maintaining  a 
rapid  flow  of  blood  to  keep  the  body  cells  supplied  with  oxygen. 

The  higher  vertebrates  have  developed  lungs  for  external  respira- 
tion. These  are  hollow  spaces,  usually  greatly  subdivided  into  thousands 
of  small  hollow  pockets  (alveoli),  kejn  moist  with  w^ater,  and  richly 
supplied  with  blood  vessels.  The  walls  of  the  alveoli  are  very  thin  and 
supplied  with  a  rich  bed  of  capillaries.  The  network  of  elastic  fibers 
between  the  alveoli  supports  them  and  makes  the  lung  very  pliable.  The 
arrangement  of  the  lung  alveoli,  as  pockets,  tends  to  minimize  the  loss 
of  water  and  thus  keeps  the  alveolar  surface  moist. 

However  different  respiratory  surfaces  may  appear  morphologically, 
they  are  essentially  similar  in  consisting  of  a  thin,  moist  membrane 
richly  supplied  with  blood  vessels  separating  body  fluid  and  external 
environment.  There  is  no  evidence  for  the  hypothesis  that  the  cells  of 
the  lung  do  work  and  actively  secrete  oxygen  into  the  blood  stream.  It 
can  be  calculated  that  diffusion  is  rapid  enough  to  supply  the  oxygen 
required.  Oxygen  molecules  move  from  the  air  to  the  cells  within  the 
body  along  a  steep  diffusion  gradient,  from  a  region  of  high  concentra- 
tion to  a  region  of  lower  concentration.  The  partial  pressure  of  oxygen 
in  air  is  about  150  mm.  Hg,  and  that  of  the  air  in  the  lungs  is  about  105 
mm.  Hg.  The  oxygen  tension  of  blood  going  to  the  tissues  is  about 
100  mm.  Hg  and  that  of  blood  returning  from  tissues  to  lungs  is  about 
40  mm.  Hg.  The  oxygen  tension  in  tissues  may  vary  from  0  to  40 
mm.  Hg. 

Aleans  of  Obtaining  Oxygen.  Air  contains  about  210  ml.  of  oxygen 
per  liter.  Fresh  pond  water  has  dissolved  in  it  about  7  ml.,  and  sea  water 
about  5  ml.,  of  oxygen  per  liter.  An  air-breathing  animal  has  an  obvious 
advantage  over  a  water-breathing  one  with  respect  to  oxygen  supply, 
for  the  solubility  of  oxygen  in  water  is  low  and  its  rate  of  diffusion  is 
much  less  in  water  than  in  air.  To  overcome  this  handicap,  animals 
breathing  water  usually  have  some  mechanism  to  pass  a  fresh  supply  of 


90  GENERAL   CONCEPTS 

water  constantly  over  the  respiratory  surface.  Air-breathing  animals  may 
obtain  sufficient  oxygen  by  diffusion  alone.  The  earthworm,  for  example, 
obtains  enough  oxygen  by  diffusion  from  the  air  in  its  burrow  and  need 
not  stir  up  that  air.  The  marine  worms  which  live  in  burrows  or  tubes, 
in  contrast,  undulate  their  bodies  to  provide  a  current  of  water  through 
the  burrow.  An  even  more  dramatic  example  of  this  is  provided  by  the 
shore  crab,  which  can  live  in  air  or  water.  This  animal  has  a  set  of  gills 
located  in  a  gill  chamber  between  the  upper  shell  and  the  attachment 
of  the  legs.  A  paddle-shaped  part  (the  scaphognathite)  of  one  leg  moves 
back  and  forth  in  the  gill  chamber  to  keep  a  current  of  water  flowing 
over  the  gills.  If  the  scaphognathites  are  paralyzed,  the  crab  will  soon 
die  if  placed  in  sea  water,  but  will  live  indefinitely  in  air,  for  the  rate  of 
diffusion  from  air  is  rapid  enough  to  supply  all  the  oxygen  the  animal 
needs. 

The  ability  of  blood  to  carry  oxygen  and  carbon  dioxide  depends 
to  a  large  extent  on  the  presence  of  a  heme-protein  pigment,  such  as 
hemoglobin.  If  blood  were  water,  it  could  carry  only  about  0.2  ml.  of 
oxygen  and  0.3  ml.  of  carbon  dioxide  in  each  100  ml.  Whole  blood, 
because  of  the  properties  of  hemoglobin,  can  carry  some  20  ml.  of  oxygen 
and  30  to  60  ml.  of  carbon  dioxide  per  100  ml.  Hemoglobin  is  found 
in  all  of  the  major  groups  of  animals  above  the  flatworms;  certain  groups 
—molluscs  and  Crustacea,  for  example— have  other  heme  pigments  such 
as  hemocyanin.  In  the  respiratory  organ,  the  lung  or  gill,  the  heme 
pigment  unites  with  oxygen.  For  example,  hemoglobin  unites  with 
oxygen  to  form  oxyhemoglobin: 

Hb  +  Oo  ^  HbOa 

The  reaction  is  reversible  and  hemoglobin  releases  the  oxygen  when 
it  reaches  a  region  where  the  oxygen  tension  is  low.  The  combination  of 
oxygen  with  hemoglobin  and  the  release  of  oxygen  from  oxyhemoglobin 
are  controlled  by  the  amount  of  oxygen  present  and  by  the  amount  of 
carbon  dioxide  present.  Carbon  dioxide  reacts  with  water  to  form  car- 
bonic acid,  H0CO3,  hence  an  increase  in  the  concentration  of  carbon 
dioxide  results  in  an  increased  acidity  of  the  blood.  The  oxygen-carrying 
capacity  of  hemoglobin  decreases  as  blood  becomes  more  acid;  thus  the 
combination  of  hemoglobin  with  oxygen  is  controlled  indirectly  by 
the  amount  of  carbon  dioxide  present.  This  results  in  an  extremely 
efficient  transport  system:  In  the  capillaries  of  the  tissues,  carbon  dioxide 
concentration  is  high  and  a  large  amount  of  oxygen  is  released  from 
hemoglobin  by  the  combined  action  of  low  oxygen  tension  and  high 
carbon  dioxide  tension.  In  the  capillaries  of  the  lung  or  gill,  carbon 
dioxide  tension  is  lower  and  a  large  amount  of  oxygen  is  taken  up  by 
hemoglobin  by  the  combined  action  of  high  oxygen  tension  and  low 
carbon  dioxide  tension. 

Hemoglobin  plays  an  important  role  in  the  transport  of  carbon 
dioxide  and  in  the  maintenance  of  a  constant  blood  pH;  its  functions 
in  these  and  in  the  transport  of  oxygen  are  intimately  interrelated.  Some 
carbon  dioxide  is  carried  in  a  loose  chemical  union  with  hemoglobin, 


PRINCIPLES   OF   PHYSIOLOGY 


91 


as  carbamino  Hb,  and  a  small  amount  is  present  as  carbonic  acid,  but 
most  of  it  is  transported  as  bicarbonate  ion,  HCOg".  The  COo  produced 
by  cells  dissolves  in  the  tissue  fluid  to  form  HoCOy,  but  the  carbonic  acid 
is  neutralized  to  bicarbonate  by  the  sodium  and  potassium  ions  released 
when  oxyhemoglobin  is  converted  to  hemoglobin.  The  chemical  details 
of  these  processes  are  very  complex.  Oxyhemoglobin  is  a  stronger  acid 
than  reduced  hemoglobin,  hence  some  cations  are  released  when  HbOo 
is  converted  to  Hb.  In  the  process  of  evolution  this  one  molecide  has 
become  endowed  with  all  the  properties  needed  for  the  transport  of 
large  amounts  of  oxygen  and  carbon  dioxide  with  a  change  of  only  a  few 
hundredths  of  a  pH  unit  in  the  blood. 

The  properties  of  the  heme  pigments  are  such  that  the  amount  of 
oxygen  taken  up  by  the  pigment  is  not  directly  proportional  to  the 
oxygen  tension;  a  graph  of  the  relationship  gives  an  S-shaped  curve  (Fig. 
5.4).  The  blood  is  a  more  effective  transporter  of  oxygen  than  it  would 
be  if  the  oxygen  content  were  a  simple  linear  function  of  oxygen  tension. 
The  effect  of  carbon  dioxide  (really  the  change  in  pH  brought  about  by 
changes  in  carbon  dioxide  content)  on  the  combination  of  oxygen  with 
the  pigment  is  shown  in  Figure  5.5.  The  oxygen  dissociation  curves  for 
arterial  blood,  with  low  carbon  dioxide  tension,  and  for  venous  blood, 
with  high  carbon  dioxide  tension,  illustrate  how  much  more  oxygen  is 
delivered  to  the  tissue  by  a  given  amount  of  blood  as  carbon  dioxide 
is  taken  up  in  the  tissue  capillaries.  The  properties  of  the  heme  proteins 
of  different  species  are  quite  different,  and  in  general  are  adapted  to 
the  amount  of  carbon  dioxide  present.  This  is  low  in  water-breathing 
animals  and  high  in  air-breathing  animals.  This  emphasizes  the  point 
that   the  evolution  of  air-breathing  animals  from  water-breathing  ones 


100 -- 

Per  Cent 

°f  75  4- 

Squld 

Hemocydnin 
Oxygenated      ^^  _  _ 


25-- 


50  100 

Oxygen  tension.  CpO^)  in  mm.Hg 

Figure  5.4.  The  amount  of  oxygen  combined  with  hemocyanin  is  related  to  the 
oxygen  tension  (pOo)  by  an  S-shaped  curve  (solid  line).  Because  of  this,  a  greater  amount 
of  oxygen  (A)  is  defivered  to  the  tissue  by  a  given  decrease  in  pOg  than  there  would  be 
(B),  if  the  properties  of  hemocyanin  were  such  that  there  was  a  linear  relationship  be- 
tween the  percentage  of  hemocyanin  oxygenated  and  the  oxygen  tension  (dotted  line). 


92 


GENERAL   CONCEPTS 


100 -- 


75 


Per  cent 
of 

Ham.ocy.anin    „ 
Oxygenated 


25-- 


50  100 

Oxygen  tension  (pOJ  in  mm.  Hg 

Figure  5.5.  The  effect  of  carbon  dioxide  tension  (pCOa)  on  the  delivery  of  oxygen 
to  tissues.  The  dotted  line  A  indicates  the  amount  of  oxygen  delivered  as  the  pOo  falls 
from  that  of  arterial  blood  to  that  of  venous  blood.  The  dotted  line  B  indicates  the  extra 
amount  of  oxygen  delivered  because  the  pC02  increases  at  the  same  time. 

involved  marked  changes  not  only  in  the  morphology  of  the  respiratory 
organs,  but  also  in  the  chemical  properties  of  the  heme  proteins  serving 
as  blood  pigment. 

29.        The  Elimination  of  Wastes  Other  than  Carbon  Dioxide 

In  the  course  of  the  metabolic  processes  by  which  cells  utilize  sub- 
stances for  energy  and  for  growth  and  maintenance  of  the  protoplasm, 
wastes  are  produced  which  must  be  removed.  The  most  important  are 
the  nitrogenous  wastes  which  result  primarily  from  the  deamination  of 
amino  acids.  These  are  of  no  further  use  to  the  animal  and,  being  toxic, 
would  seriously  interfere  with  metabolism  if  they  accumulated.  They 
are  removed  from  the  blood  and  other  body  fluids  of  vertebrates  by  the 
kidneys.  The  role  of  the  vertebrate  kidney,  and  of  the  excretory  organs 
of  most  other  animals,  is  not  limited  to  the  elimination  of  nitrogenous 
wastes,  but  includes  the  regulation  of  the  volume  of  body  fluids— i.e., 
the  water  content  of  the  body-and  the  regulation  of  the  concentration 
of  salts,  acids,  bases  and  organic  substances  in  the  body  fluids.  The 
cells  of  the  body  require  a  constant  environment  for  their  continued 
normal  functioning.  The  kidneys,  by  excreting  certain  substances  and 
conserving  others,  maintain  the  required  constancy  of  the  blood  and 
body  fluids.  The  substances  to  be  excreted  are  in  solution,  generally, 
in  the  intracellular  fluid,  and  the  excretory  process  may  involve  simple 
diffusion  or  active  processes  in  which  energy  is  expended. 

In  most  protozoa,  the  removal  of  wastes  is  accomplished  by  diffusion 
through  the  cell  membrane  into  the  surrounding  water  where  the  con- 
centration is  lower.  Protozoa  living  in  fresh  water  have  the  additional 
problem  of  ridding  the  body  of  the  water  which  constantly  enters  the 


PRINCIPLES   OF   PHYSIOLOGY  93 

cell  by  osmosis  because  the  concentration  of  salts  is  greater  in  the  cell 
than  in  the  surrounding  environment.  These  forms  have  evolved  a 
contractile  vacuole,  which  fills  with  fluid  from  the  surrounding  proto- 
plasm and  then  empties  to  the  exterior.  Sponges  and  coelenterates  have 
no  specialized  excretory  organs  and  their  wastes  simply  diffuse  from  the 
intracellular  fluid  to  the  external  environment. 

The  simplest  animals  with  specialized  excretory  organs  are  the  flat- 
worms  and  nemerteans,  which  have  flame  cells  (Fig.  5.6)  equipped  with 
flagella,  and  a  branching  system  of  excretory  ducts  from  the  flame  cells 
to  the  outside.  The  flame  cells  lie  in  the  fluid  which  bathes  the  cells  of 
the  body,  and  wastes  diffuse  into  the  flame  cells  and  thence  into  the 
excretory  ducts.  The  beating  of  the  flagella  (which  suggests  a  flickering 
flame  when  seen  under  the  microscope)  presumably  moves  fluid  in  the 
ducts  out  through  the  excretory  pores  and  thus  aids  diffusion.  As  in 
the  contractile  vacuoles  of  the  protozoa,  the  chief  role  of  the  flame  cells 
is  probably  the  regulation  of  the  w'ater  content  of  the  animals.  Some  of 
the  metabolic  wastes  are  removed  by  diffusion  through  the  lining  of  the 
gastrovascular  cavity. 

Each  segment  of  the  body  of  an  earthworm  contains  a  pair  of 
specialized  excretory  organs  known  as  nephrldia.  A  nephridium  is  a 
long,  coiled  tubule,  opening  at  one  end  to  the  body  cavity  in  a  funnel- 
shaped  structure  lined  with  cilia,  and  at  the  other  end  to  the  outside  of 
the  body  via  an  excretory  pore.  Fluid  is  moved  through  the  nephridium 
in  part  by  the  beating  of  its  cilia  and  in  part  by  the  contraction  of 
muscles  in  its  wall.  The  earthworm  excretes  a  very  dilute,  copious  urine, 
at  a  rate  of  about  60  per  cent  of  its  total  body  weight  each  day. 

The  crustacean  excretory  organs  are  the  green  glands,  a  pair  of 
large  structures  located  at  the  base  of  the  antennae  and  supplied  with 
blood  vessels.  Each  gland  consists  of  three  parts:  a  coelomic  sac,  a  green- 
ish, glandular  chamber  with  folded  walls,  and  a  canal  which  leads  to  a 
muscular  bladder.  \Vastes  pass  from  the  blood  to  the  coelomic  sac  and 
glandular  chamber;  the  fluid  in  them  is  isotonic  with  the  blood.  Urine 
collects  in  the  bladder  and  then  is  voided  to  the  outside  through  a  pore 
at  the  base  of  the  antenna. 

The  excretory  organs  of  insects,  the  malpighian  tubules,  are  quite 
different  from  those  of  the  crustaceans.  They  lie  within  the  body  cavity 
(hemocoel)  and  empty  into  the  digestive  tract.  \Vastes  diffuse  into  these 
tubules  and  are  excreted  into  the  cavity  of  the  digestive  tract. 

The  kidneys,  the  vertebrate  excretory  organs,  remove  wastes  from 
the  blood  and  regulate  its  content  of  water,  salts  and  organic  substances. 
The  structural  and  functional  unit  of  the  kidney  is  the  kidney  tubule 
(Fig.  5.6).  This  is  in  close  contact  with  the  blood  stream,  for  a  tuft  of 
capillaries  projects  into  the  funnel-shaped  Bowman's  capsule  at  the  end 
of  the  tubule.  The  tubule  may  be  quite  long  and  looped,  and  in  contact 
with  additional  capillaries  along  its  length.  It  eventually  opens  to  the 
outside  of  the  body  via  collecting  ducts  and  other  intermediate  tubes. 
Substances  are  filtered  into  the  kidney  tubules  from  the  blood  capillaries 
in  the  Bowman's  capsules.  Then,  some  substances  are  reabsorbed  into 
the  blood  stream  and  others  are  secreted  from  the  blood  into  the  urnie 


94 


GENERAL   CONCEPTS 


Biadd 


cr 


A  B 

Contra-ctile.  vacaole       Flajne  ceil 


(Ameba) 


(Flat  worm) 


Ne.phrid.IuTn. 
(Earthv/orm) 


DowmdTi'5 
Glomarulu.s 


oximal  convoluted 
tubule 

Secondary  capil- 
la^ry  nctv/orlc 


-Distal  convo- 
lu-tedL  tubuLa. 


Interlobu 
levT  artery, 


Arcuate,  art 
and  vein.  \ 


BrancK.  of  ' 

ren.al 


^-'■'y^/'"  Collecting 
duct        > 


BnancK  of 
enal  vein. 


Kidriey  tubule  (Ve-i^te-brate) 
Figure  5.6.     The  excretory  systems  of  {A)  ameba,  {B)  flatworm,  (C)  earthwonn  and 
(D)  vertebrate. 


PRINCIPLES  OF  PHYSIOLOGY  95 

as  the  liquid  flows  through  the  tubules  and  past  the  additional  capil- 
laries there.  The  kidney  must  expend  energy  to  move  certain  of  the 
substances  excreted  or  reabsorbed  against  a  diffusion  gradient.  The  ex- 
cretory process  would  be  very  wasteful  and  inefficient  if  the  urine  leaving 
the  body  had  the  same  composition  as  the  fluid  in  the  Bowman's  capsule. 
However,  as  the  urine  passes  down  the  kidney  tubule,  water,  sugar,  salts 
and  many  other  substances  are  reabsorbed,  whereas  the  waste  products 
such  as  ammonia  and  urea  are  not.  It  is  by  this  selective  reabsorption 
of  certain  substances,  and  by  the  addition  of  others  to  the  urine,  that 
the  kidney  tubules  regulate  the  composition  of  the  blood  and  body 
fluids.  In  the  higher  animals  such  as  man,  the  lungs,  skin  and  digestive 
tract  also  remove  certain  wastes  from  the  body. 

The  most  important  waste  products  excreted  by  animals  are  the 
nitrogenous  ones  which  result  from  the  deamination  of  amino  acids  and 
from  the  breakdown  of  nucleic  acids.  The  ammonia  formed  by  de- 
amination is  toxic,  but  quite  soluble  and  readily  diffusible.  If  plenty 
of  water  is  available,  as  it  is  for  fresh-water  animals,  the  ammonia 
diffuses  out  as  such,  either  directly  through  the  body  surface,  or  through 
gills  and  excretory  organs  if  these  are  present.  Animals  living  on  land 
cannot  afford  to  excrete  the  amount  of  water  which  would  be  required 
to  eliminate  ammonia.  In  land  animals,  ammonia  is  converted  meta- 
bolically  to  some  other  substance  to  be  excreted.  In  mammals,  the 
nitrogenous  wastes  from  amino  acid  metabolism  are  excreted  largely  as 
urea,  which  is  a  soluble,  small  molecule  that  diffuses  readily  and  is  less 
toxic  than  ammonia.  Urea  requires  a  moderate  amount  of  water  for  its 
excretion.  Reptiles  and  birds  convert  their  nitrogenous  wastes  largely 
to  uric  acid  for  excretion.  This  substance  is  only  slightly  soluble  so  that 
once  it  has  been  formed  and  excreted  into  the  kidney  tubules,  water 
may  be  reabsorbed  and  the  uric  acid  is  excreted  as  a  paste  or  dry 
powder.  Insects,  which  are  largely  terrestrial  animals,  also  excrete  uric 
acid.  The  uric  acid  is  excreted  into  the  malpighian  tubules  whence  it 
leaves  the  body  via  the  digestive  tract  as  a  dry  paste.  Some  animals 
simply  accumulate  precipitated  uric  acid  in  some  organ  of  the  body— 
the  "fat  body"  of  the  insect  is  an  example.  Nitrogenous  wastes  in  this 
form  are  removed  from  the  body  fluids  as  effectively  as  those  actually 
excreted  from  the  body  in  urine. 

30.        Protection 

The  complex  physicochemical  system  we  know  as  protoplasm  requires 
protection  against  the  many  adverse  effects  of  the  surrounding  environ- 
ment. The  ameba  is  an  exception  to  the  general  rule  that  animals  have 
some  protective  device  to  cover  the  protoplasm.  The  ameba's  proto- 
plasm is  separated  from  the  surrounding  environment  only  by  the 
plasma  membrane.  Many  other  protozoa  have  a  tough,  flexible,  non- 
cellular  pellicle  surrounding  the  cell  and  some  secrete  hard,  durable, 
calcareous  or  siliceous  shells.  All  the  multicellular  animals  have  some 
protective  covering  or  skin  over  the  body.  The  skin  may  consist  of  one 
or  of  many  layers  of  cells,  and  may  be  reinforced  by  scales,  hair,  feathers, 


96  GENERAL   CONCEPTS 

shells,  or  secretions  of  mucus  or  cutin.  Hair,  feathers  and  certain  scales, 
such  as  those  of  reptiles,  are  composed  of  very  insoluble  proteins  called 
keratins  derived  from  dead  cells  in  the  skin.  The  skin  has  a  number  of 
functions:  it  protects  the  underlying  protoplasm  against  mechanical 
and  chemical  injuries;  it  prevents  the  entrance  of  disease  organisms;  it 
prevents  excessive  loss  of  water  from  land  animals  and  excessive  uptake 
of  water  by  fresh-water  animals;  and  it  protects  underlying  cells  against 
the  harmful  effects  of  the  ultraviolet  rays  in  sunlight. 

The  skin  is  an  effective  radiator  by  which  the  body  can  eliminate 
the  heat  which  is  constantly  produced  in  cellular  metabolism.  One  of 
the  factors  controlling  the  rate  of  heat  loss  in  higher  vertebrates  is  the 
size  of  the  blood  vessels  in  the  skin.  To  conserve  heat  in  a  cold  en- 
vironment, the  blood  vessels  are  constricted  to  decrease  the  rate  of 
blood  flow.  The  reverse  occurs  in  a  warm  environment  and  the  rate  of 
heat  loss  can  be  increased  by  the  evaporation  of  water,  i.e.,  sweat,  from 
the  surface  of  the  skin. 

A  great  many  animals  have  a  firm  framework  or  skeleton  which 
protects  and  supports  the  body  and  provides  for  the  attachment  of 
muscles.  Some  animals  manage  to  survive  without  a  skeleton  but  these 
are  mostly  aquatic  forms.  The  slug  and  earthworm  are  among  the  few 
exceptions  to  the  rule  that  terrestrial  animals  require  a  skeleton.  To 
raise  part  of  the  body  off  the  ground,  some  stiff,  hard  framework  is  re- 
quired to  support  the  soft  tissues  against  the  pull  of  gravity.  The  ap- 
pendages of  arthropods  and  vertebrates  have  a  hard,  but  jointed  and 
bendable,  skeletal  framework  which  serves  as  levers  for  locomotion.  The 
skeleton  also  covers  and  protects  such  delicate  organs  as  the  brain, 
spinal  cord  and  lungs.  The  marrow  cavities  of  vertebrate  bones  con- 
tain tissues  which  produce  red  blood  cells  and  certain  of  the  white  blood 
cells. 

An  animal's  skeleton  may  be  an  exoskeleton,  located  on  the  outside 
of  the  body,  or  an  endoskeleton,  located  within  the  body.  The  hard 
shells  of  lobsters,  crabs  and  insects,  and  the  calcareous  shells  of  oysters 
and  clams,  are  examples  of  exoskeletons.  An  exoskeleton  provides  ex- 
cellent protection  for  the  body,  and  muscles  can  be  attached  to  its  inner 
surface  so  as  to  move  one  part  with  respect  to  another.  However,  the 
presence  of  an  exoskeleton  usually  interferes  with  growth.  The  arthro- 
pods have  solved  this  problem  by  periodically  shedding  the  shell.  To 
do  this  the  shell  is  first  softened,  that  is,  some  of  the  calcium  salts 
deposited  in  it  are  dissolved,  the  shell  is  split  and  the  animal  crawls  out 
of  the  old  shell.  It  then  undergoes  a  period  of  rapid  giowth  before  the 
new  shell,  which  formed  under  the  old  one,  becomes  hard  by  the  depo- 
sition of  calcium  salts.  During  this  molting  process  the  arthropod  lacks 
protection  and  is  weak  and  barely  able  to  move.  Hard,  calcareous  exo- 
skeletons are  present  in  most  molluscs  and  arthropods,  and  in  corals, 
bryozoa  and  a  variety  of  lesser  invertebrates.  A  clam  or  oyster  secretes 
additional  shell  at  the  margin  as  it  grows;  the  shell  gets  both  larger 
and  thicker  as  the  animal  grows.  Many  marine  worms  secrete  calcareous 
tubes  in  which  they  live.  Though  these  shells  are  not  directly  a  part 


PRINCIPLES   OF   PHYSIOLOGY 


97 


of  the  animal  body,  they  are,  in  certain  respects,  the  functional  equiv- 
alent of  an  exoskeleton. 

The  vertebrate  skeleton,  lying  within  the  soft  tissues  of  the  body, 
provides  an  excellent  framework  for  their  support  and  does  not  inter- 
fere with  their  growth.  The  arrangement  of  the  parts  of  the  skeleton 
is  essentially  the  same  in  all  the  vertebrates.  The  details  of  this  will  be 
discussed  in  Chapter  25. 

The  skeleton  of  vertebrates  is  composed  of  many  individual  bones 
or  cartilages.  The  region  where  two  hard  parts  meet  and  move  one  on 
the  other  is  known  as  a  joint.  The  fundamental  differences  in  the  me- 
chanics of  the  vertebrate  and  arthropod  joints  are  illustrated  in  Figure 
5.7.  The  muscles  of  the  vertebrate  surround  the  bones;  each  is  attached 
by  one  end  to  one  bone  and  by  its  other  end  to  another  bone.  Its  con- 
traction thus  moves  one  bone  with  respect  to  the  other.  The  muscles  of 
the  artliropod  lie  luitliin  the  skeleton  and  are  attached  to  its  inner 
surface.  The  arthropod  exoskeleton  has  certain  regions— joints— in  which 
the  exoskeleton  is  thin  and  ilexible  so  that  movements  may  occur.  The 
muscle  may  stretch  across  the  joint,  so  that  its  contraction  will  move 
one  part  on  the  next.  Or,  the  muscle  may  be  located  entirely  within 
one  section  of  the  body  or  appendage  and  be  attached  at  one  end  to  a 
tough  apodeme,  a  long,  thin,  firm  part  of  the  exoskeleton  extending 
into  that  section  from  the  adjoining  one. 

The  movement  of  the  wings  of  insects  is  achieved  in  a  curious  way: 
the  flight  muscles  are  located  within  the  body  and  are  attached  to  the 
body  wall.  The  wings  are  attached  to  the  body  wall  over  a  fulcrum, 
the  wing  process  (Fig.  5.8).  The  contraction  of  muscles  arranged  dorso- 
ventraliy  pulls  down  the  tergum,  a  plate  on  the  upper  surface  of  the 


Endo  skeleton 


Apod 


Exoskeleton' 


B 


Figure  5.7.     A  comparison  of  the  vertebrate  endoskeleton  (A)  with  the  arthropod 
exoskeleton  (B),  showing  the  arrangement  of  the  muscles  and  skeleton  at  a  joint. 


98 


GENERAL   CONCEPTS 


Longitudinal  muscle.s 

Win^s  dowrn-  /Slj||jl\    Win^S  up- 
Tarrium  /  \  Terbam 


lin^TerOo-    ^''^, 


Sternum. 


Lon_gitudinal  Tardura 

muscle 


Ant. 


sternal      * 
muscles 


TcrOo- sternal  muscle. 


Post. 


Ant 


Post. 


U) 


^^1  Londitudinal  sections 

Figure  5.8.     Diagram  of  the  arrangement  of  the  wing  muscles  of  an  insect. 

body,  but  raises  the  wings,  which  are  on  the  opposite  side  of  the  ful- 
crum. Then  the  contraction  of  muscles  arranged  longitudinally,  like  the 
string  of  a  bow,  causes  the  tergum  to  bulge  upward  and  the  wing  is 
pulled  do^vn  for  the  power  stroke.  The  mo\'ements  of  the  body  w^all  are 
barely  perceptible,  but,  because  the  length  of  the  lever  on  the  two  sides 
of  the  fulcrum  is  so  different,  the  distance  moved  by  the  tips  of  the  wings 
is  several  hundred  times  as  great. 

31.        Motion 

One  of  the  fundamental  properties  of  all  kinds  of  protoplasm  is  the 
ability  to  contract,  a  process  which  involves  the  transformation  of 
chemical  energy  into  mechanical  energy.  The  chemical  energy  of  the 
energy-rich  phosphate  bonds  synthesized  in  glycolysis  and  in  biologic 
oxidation  (p.  73)  is  converted  into  the  mechanical  energy  of  con- 
tractile protein  molecules  such  as  actomyosin.  There  is  reason  to  be- 
lieve that  the  basic  process  for  the  conversion  of  chemical  to  mechan- 
ical energy  is  fundamentally  similar  in  all  protoplasm,  though  the 
nature  of  the  contracting  protein  molecule  may  differ  somewhat.  The 
mechanical  behavior  of  a  contracting  muscle  and  the  energy  used  can 
be  measured  and  compared  with  the  chemical,  electrical  and  thermal 
changes  coincident  with  contraction  to  try  to  understand  the  nature  of 
the  contractile  mechanism. 

Ameboid  motion  is  the  irregular  flowing  of  protoplasm  seen  in 
amebas,  in  the  amebocytes  of  sponges,  in  the  white  blood  cells  of  verte- 
brates and  in  the  general  process  of  protoplasmic  motion  which  occurs 
during  cell  division.  Careful  microscopic  study  of  a  moving  ameba  re- 
veals that  not  all  of  the  protoplasm  streams  simultaneously.  There  is  a 
solid,  nonmoving  layer  at  the  surface  of  the  cell  which  surrounds  a  core 
of  liquid,  flowing  protoplasm.  At  the  rear  of  the  moving  ameba,  pro- 


PRINCIPLES   OF  PHYSIOLOGY  99 

toplasmic  gel  is  converted  to  sol  to  flow  forward.  At  the  front  end,  the 
streaming  protoplasm  bulges  out  in  a  projection  known  as  a  pseudopod 
(false  foot)  and  changes  from  a  sol  to  a  gel.  The  push  for  the  move- 
ment of  the  protoplasmic  sol  is  believed  to  come  from  the  contraction 
of  the  gel  protoplasm  comprising  the  layer  near  the  surface  of  the  cell. 
The  tip  of  the  pseudopod  is  covered  with  a  thinner  gel  layer  than  that 
elsewhere  in  the  cell  and  hence  is  the  part  to  bulge  when  contraction 
occurs.  By  regulating  the  thickness  of  the  local  areas  in  the  cell  wall, 
the   ameba   can   determine   where   a    pseudopod   will    form    and   hence 
which  direction  he  will  move.  The  animal  has  no  permanent  front  and 
rear  ends.  Ameboid  motion  is  a  crawling  motion,  not  a  swimming  one; 
the  cell  must  be  attached  to  some  physical  substrate  in  order  to  move. 
Another  type  of  motion  is  seen  in  the  movable,  slender,  protoplasmic 
processes  which   project  from  certain  cells.  These  projecting  hairs   are 
termed  flagella   if  each  cell  has  one  or  a  few  long,  whip-like  processes 
and  cilia  if  each  cell  has  many  short  processes.  Flagella  are  found  on 
certain  protozoa  (the  flagellates),  on  the  collar  cells  of  sponges  and  on 
certain    cells    lining    the    gastrovascular    cavity    of    coelenterates.    Cells 
equipped  with  cilia  occur  very  widely:  in  certain  protozoa  (called  cili- 
ates),  on  the  body  surfaces  of  ctenophores,  flatworms   and  rotifers,  on 
the  tentacles  of  bryozoa,  certain  Avorms  and  coelenterates,  on  the  gills 
of  clams  and  oysters,   and   lining  certain  ducts   in  the  vertebrate  body 
such  as  the  bronchi  and  oviducts.  The  paramecium  is  an  example  of  a 
ciliate,   with   some  2500  short  cilia   covering  each  single-celled   animal. 
The  protoplasmic  extensions  beat  in  a  coordinated  rhythm,  not  simul- 
taneously but  one  after  another,  so  that  waves  of  movement  pass  along 
the  body  surface.  The  eftect  of  the  combined  effort  of  the  cilia  beating 
backward   is    to   move   the   animal    forward.   The   cilia   beat   somewhat 
obliquely  so  that  the  animal  revolves  on  its  long  axis  and  moves  in  a 
spiral  path.  The  beating  of  the  cilia  is  under  the  control  of  the  animal, 
and  by  reversing  the  ciliary  beat  it  can  back  up  and  turn  around.  The 
beating  of  cilia   and  flagella  is  believed  to  result  from  the  contraction 
of  the  iMoto}ilasm  in  these  projections  but  the  details  of  the  process  are 
quite  unknown.  Cilia  beat  quite  rapidly,  up  to  40  beats  per  second.  In 
the  electron  microscope  a  system  of  fibers  is  visible  extending  down  the 
long  axis  of  the  flagellum  or  cilium  and  this  undoubtedly  plays  some 
role  in  its  beating. 

Muscles.  Motion  in  most  animals  is  a  function  of  the  contraction 
of  specialized  cells,  the  muscle  cells.  The  contractile  mateiial,  actomyo- 
sin,  is  fundamentally  similar  in  smooth,  striated  and  cardiac  muscles  of 
vertebrates  and  in  the  muscles  of  invertebrates  as  well.  Muscles  that 
contract  rapidly  and  briefly,  such  as  the  skeletal  muscles  of  mammals, 
are  striated,  whereas  those  that  contract  slowly  and  remain  contracted 
for  a  long  time,  such  as  those  in  the  walls  of  the  digestive  tract  or  urinary 
bladder,  are  unstriated.  This  basic  physiologic  and  histologic  correla- 
tion is  evident  in  the  contractile  cells  of  coelenterates,  for  those  of  jelly- 
fish, which  contract  in  twitches,  have  microscopic  cross  striations  and 
those  of  sea  anemones,  which  contract  very  slowly,  are  unstriated. 

The  coelenterate  contractile  cells  in  the  ectoderm  are  arranged  at 


100 


GENERAL   CONCEPTS 


Figure  5.9.     The  muscles  and  bones  of  the  forearm,  showing  the  antagonistic  arrange- 
ment of  the  biceps  and  triceps  muscles. 

right  angles  to  those  in  the  endoderm;  contraction  of  one  or  the  other 
decreases  either  the  length  or  the  diameter  of  the  body.  Flatworms 
typically  have  muscle  fibers  oriented  in  three  different  planes,  but  round- 
worms have  only  longitudinal  fibers  in  the  body  wall.  A  roundworm 
can  bend  or  straighten  its  body  but  cannot  twist  or  extend  its  length. 
Segmented  marine  and  earthworms  have  an  outer  layer  of  circular 
fibers  and  an  inner  layer  of  longitudinal  fibers  in  the  body  wall.  Since 
the  body  cavity  is  filled  with  fluid  which  is  incompressible,  the  con- 
traction of  the  circular  muscles  stretches  the  longitudinal  muscles  and 
extends  the  body,  making  it  longer  and  thinner.  The  contraction  of  the 
longitudinal  muscles  makes  it  shorter  and  thicker. 

Molluscs  generally  have  slow,  nonstriated  muscles,  but  the  scallop, 
which  can  swim  actively  by  clapping  its  two  shells  together,  has  two 
muscles  connecting  the  shells.  One  of  these  is  nonstriated  and  contracts 
slowly,  serving  to  keep  the  shells  closed  at  rest,  and  the  other  is  striated 
and  twitches  rapidly  to  power  the  swimming  movements. 

The  arthropods  have  complex  patterns  of  separate  muscles  rather 
than  simple  layers  of  muscles  as  in  the  worms.  These  muscles  vary  in 
size  and  attachment,  and  provide  for  the  movement  of  the  segments  of 
the  body  and  their  many-jointed  appendages.  The  arthropod  muscles 
are  located  within  the  exoskeleton  and  attach  to  its  inner  surface.  A 
lobster  or  grasshopper  has  hundreds  of  separate  muscles. 

The  muscles  of  vertebrates  are  generally  attached  to  bones  or  car- 
tilages as  pairs  which  tend  to  pull  in  opposite  directions  (Fig.  5.9).  Since 
muscles  can  pull  but  cannot  push,  this  antagonistic  arrangement  allows 
for  movement  in  both  directions.  The  end  of  the  muscle  which  remains 


PRINCIPLES   OF   PHYSIOLOGY  \Q\ 

relatively  fixed  when  a  muscle  contracts  is  known  as  its  origin;  the  end 
which  moves  is  called  the  insertion;  and  the  thick  part  between  the  two 
is  called  the  belly  of  the  muscle.  Thus,  the  biceps,  which  bends  or  flexes 
the  forearm,  has  its  origin  on  the  scapula  and  on  the  upper  end  of  the 
humerus,  and  its  insertion  on  the  radius  in  the  forearm,  fts  antagonist, 
the  triceps,  which  straightens  or  extends  the  forearm,  has  its  origin  on  the 
scapula  and  upper  part  of  the  humerus  and  its  insertion  on  the  ulna. 
The  contraction  of  a  muscle  is  stimulated  by  a  nerve  impulse  reaching 
it  via  a  motor  nerve  fiber  from  the  central  nervous  system.  The  drug 
curare,  the  chief  ingredient  of  the  arrow  poison  used  by  the  South  Ameri- 
can Indians,  blocks  the  junction  between  nerve  and  muscle  so  that 
impulses  cannot  pass  and  the  muscle  is  paralyzed.  A  curare-paralyzed 
muscle  can  still  be  caused  to  contract  by  direct  electric  stimulation,  a 
demonstration  that  muscle  is  independently  irritable. 

The  Mechanism  of  Muscular  Contraction.  The  functional  unit  of 
vertebrate  muscles  is  called  the  motor  unit.  This  consists  of  a  single 
motor  neuron  and  the  group  of  muscle  cells  innervated  by  its  axon,  all 
of  which  will  contract  when  an  impulse  travels  down  the  motor  neuron. 
In  man,  it  is  esthnated  that  there  are  some  250,000,000  muscle  cells  but 
only  some  420,000  motor  neurons  in  spinal  nerves.  Obviously,  some 
motor  neurons  must  innervate  more  than  one  muscle  fiber.  The  degree 
of  fine  control  of  a  muscle,  its  delicacy  of  action,  is  inversely  proportional 
to  the  number  of  muscle  fibers  in  the  motor  unit.  The  muscles  of  the 
eyeball,  for  example,  have  as  few  as  three  to  six  fibers  per  motor  unit, 
whereas  the  leg  muscles  have  perhaps  650  fibers  per  unit. 

If  a  single  motor  unit  is  isolated  and  stimulated  with  brief  electric 
shocks  of  increasing  intensity  beginning  with  shocks  too  weak  to  cause 
contraction,  there  will  be  no  response  until  a  certain  intensity  is  reached, 
then  the  response  is  maximal.  This  phenomenon  is  known  as  the  "all 
or  none  effect."  In  contrast,  a  whole  muscle,  made  of  many  individual 
motor  units,  can  respond  in  a  graded  fashion  depending  upon  the  num- 
ber of  motor  units  which  are  contracting  at  any  given  time. 

A  muscle  given  a  single  stimulus,  a  single  electric  shock,  responds 
with  a  single  quick  twitch.  The  changes  which  accompany  a  single  twitch 
are  shown  in  Figure  5.10.  A  twitch  consists  of  (1)  a  very  short  latent 
period,  the  interval  between  the  application  of  the  stimulus  and  the 
beginning  of  the  contraction,  (2)  a  contraction  period,  during  which 
the  muscle  shortens  and  does  work,  and  (3)  a  relaxation  period,  longest 
of  the  three,  during  which  the  muscle  returns  to  its  original  length.  The 
latent  period  represents  the  interval  between  the  conduction  of  the  action 
current  and  the  completion  of  the  changes  in  the  structure  of  the 
actomyosin  which  enable  it  to  contract.  The  first  event  after  the  stimu- 
lation of  a  muscle  is  the  initiation  and  propagation  of  an  electrical 
response,  the  muscle  action  potential,  followed  by  the  changes  in  the 
structure  of  actomyosin  observed  as  a  change  in  the  total  birefringence  of 
the  muscle,  by  its  shortening,  and  by  the  production  of  heat.  Following 
a  twitch  there  is  a  recovery  period  during  which  the  muscle  is  restored 
to  its  original  condition.  If  a  muscle  is  stimulated  repeatedly  at  intervals 
short  enough  so  that  succeeding  contractions  occur  before  the  muscle 


102 


GENERAL   CONCEPTS 


La-bcnt 


traction       Relajx:ation 


jcriod  -V 


Lcnoth  of  muscle 
Action  potential 


BirefrinOence. 


t 


Heat  producecl 
J I 

Time-  (0.01  se-c.  intervals) 

Stimulits 

Figure  5.10.     Diagram  of  the  changes  that  occur  in  a  muscle  during  a  single  muscle 
twitch.  See  text  for  discussion. 

has  fully  recovered  Irom  the  previous  one,  the  muscle  becomes  fatigued 
and  the  twitches  become  feebler  and  finally  cease.  The  fatigued  muscle 
will  regain  its  ability  to  contract  if  allow^ed  to  rest. 

Muscles  do  not  usually  contract  in  individual  twitches  but  in  more 
sustained  contractions  evoked  by  a  volley  of  nerve  impulses  reaching 
them  in  rapid  succession.  This  state  of  sustained  contraction  is  known  as 
tetanus;  the  individual  motor  units  are  stimulated  in  rotation.  Thus 
individual  muscle  fibers  contract  and  relax,  but  do  this  in  rotation  so 
that  the  muscle  as  a  whole  remains  partly  contracted.  The  strength  of 
the  contraction  depends  on  the  fraction  of  the  muscle  fibers  which  con- 
tract at  any  given  moment. 

All  normal  skeletal  muscles  are  in  a  state  of  sustained  partial  con- 
traction, called  tonus,  as  long  as  the  nerves  to  the  muscle  are  intact. 
Tonus,  then,  is  a  state  of  mild  tetanus,  maintained  by  a  constant  flow 
of  nerve  impulses  to  the  muscle. 

The  problem  of  how  protoplasm  can  exert  a  pull  is  far  from  settled, 
but  it  is  now  believed  that  the  molecules  of  actomyosin  shorten  by  fold- 
ing and  thus  produce  the  tension  of  muscle  contraction.  The  energy  for 
the  contraction  is  derived  from  the  energy-rich  phosphate  bonds  of 
adenosine  triphosphate  and  phosphocreatine  and  these  are  renewed  by 
the  energy  derived  from  the  glycolysis  of  glycogen  to  lactic  acid.  Tins 


PRINCIPLES    OF   PHYSIOLOGY  JQS 

latter  process,  which  can  occur  without  utihzing  oxygen,  provides  energy 
for  the  resynthesis  of  adenosine  triphosphosphate  and  phosphocreatine. 

Capturing  food  or  evading  enemies  may  call  for  prolonged  bursts  of 
muscular  activity.  Although  both  the  rate  of  breathing  and  the  rate  of 
the  heart  beat  may  increase  markedly  during  prolonged  exertion,  these 
changes  could  not  supply  the  muscles  with  enough  oxygen  to  enable 
them  to  contract  repeatedly  if  the  contraction  process  itself  required 
oxygen.  That  muscle  contraction,  and  part  of  the  recovery  process,  occur 
without  the  utilization  of  oxygen  is  clearly  important  for  survival.  Dur- 
ing violent  exercise  glycogen  is  converted  to  lactic  acid  faster  than  the 
lactic  acid  can  be  oxidized.  Lactic  acid  accumulates  and  the  muscle  is  said 
to  have  incurred  an  "oxygen  debt,"  which  is  repaid  after  the  period  of 
exertion  by  continued  rapid  breathing.  This  supplies  enough  extra 
oxygen  to  oxidize  part  of  the  accumulated  lactic  acid.  Some  of  the  energy 
released  by  the  oxidation  of  the  lactic  acid  in  the  Krebs  cycle  and  the 
electron  transmitter  system  (Fig.  4.2)  is  used  to  resynthesize  glycogen 
from  the  remainder  of  the  lactic  acid  and  to  restore  the  energy-rich  com- 
pounds, ATP  and  phosphocreatine,  to  their  normal  condition.  A  muscle 
that  has  contracted  many  times,  has  depleted  its  stores  of  energy-rich 
phosphates  and  glycogen  and  has  accumulated  a  lot  of  lactic  acid,  is 
unable  to  contract  again  and  is  said  to  be  fatigued. 

One  theory  of  muscle  contraction  states  that  the  energy  for  contrac- 
tion is  transferred  from  the  ATP  to  the  actomyosin  at  the  moment  of 
contraction,  and  that  after  this  energy  has  been  used  in  the  physical  short- 
ening of  the  muscle  fiber,  the  muscle  fiber  simply  relaxes  passively.  The 
second  view,  which  is  more  widely  held  at  present,  states  that  contraction 
is  analogous  to  the  releasing  of  a  stretched  spring  and  that  energy  must 
be  put  into  the  system  to  bring  about  the  relaxation— the  stretching— 
of  the  muscle  fiber.  The  stimulation  of  the  muscle  by  a  nerve  impulse,  in 
this  theory,  is  like  the  releasing  of  a  trigger  which  has  been  holding  the 
stretched  spring. 

It  was  noted  in  Figure  5.10  that  an  action  potential  was  associated 
with  muscle  contraction.  Muscles  in  general  are  arranged  with  their 
fibers  in  parallel,  so  that  the  voltage  difference  in  a  large  muscle  is  no 
greater  than  that  of  a  single  fiber.  In  the  electric  organ  of  the  electric 
eel,  however,  the  electric  plates  are  modified  muscle  cells  (motor  end 
plates)  arranged  in  series.  Although  each  plate  has  a  potential  difference 
of  about  0.1  volt,  the  discharge  of  the  entire  organ,  made  of  several 
thousand  plates,  amounts  to  several  hundred  volts. 

32.        Irritability  and  Response 

The  muscles  just  described,  together  with  cilia,  glands,  nematocysts 
and  so  on,  are  eflFectors-they  do  things.  To  ensure  that  these  effectors 
do  the  right  things  at  the  right  time,  animals  are  equipped  with  re- 
ceptors—a variety  of  sense  organs-and  with  nervous  and  endocrine  sys- 
tems to  coordinate  the  activity  of  the  effectors. 

Irritability  or  excitability  is  a  fundamental  property  of  all  proto- 
plasm. AVaves  of  excitation  are  conducted,  although  very  slowly,  by  the 


104  GENERAL   CONCEPTS 

protoplasm  of  eggs  and  plant  cells.  Many  of  the  ciliates  have  a  network 
of  neurofibrils  which  connect  the  bases  of  the  cilia,  together  with  special 
fibrils  to  the  gullet  and  other  special  structures  of  the  body.  It  would 
appear  that  this  net  conducts  impulses  which  coordinate  the  beating  of 
the  cilia  and  the  functioning  of  the  special  organelles,  for  coordination 
is  lost  when  the  net  is  cut  by  a  microneedle.  There  are  no  nerve  cells  in 
sponges,  but  waves  of  excitation  can  be  conducted  from  cell  to  cell,  at 
about  1  cm.  per  minute.  There  are  spindle-shaped  contractile  cells 
around  the  openings  of  the  pores.  These  have  been  termed  "independent 
effectors"  because  they  respond  to  touch  by  contracting  and  thus  com- 
bine sensory  and  motor  functions. 

The  simplest  special  coordinating  system  is  the  nerve  net  found  in 
coelenterates.  The  coelenterate  nerve  fibers  are  found  all  over  the  body 
in  a  diffuse  network;  a  few  sea  anemones  and  medusae  have  rudimentary 
nerve  trunks  composed  of  aggregations  of  nerve  fibers.  Conduction  in 
the  nerve  net  progresses  in  all  directions;  the  fibers  are  not  actually  fused 
together,  but  impulses  pass  from  one  fiber  to  an  adjacent  one  in  either 
direction. 

The  Nerve  Impulse.  Galvani,  in  the  eighteenth  century,  first 
showed  that  a  muscle  contracts  when  an  electric  shock  is  applied  to  the 
nerve  leading  to  it.  DuBois-Reymond  in  the  nineteenth  century  showed 
that  when  a  stimulus  is  applied  to  a  sense  organ  electrical  disturbances 
in  the  efferent  nerves  can  be  detected.  With  the  development  of  im- 
proved instruments  for  detecting  these  weak  currents,  the  electrical 
disturbances  in  nerve  fibers  were  shown  to  have  a  potential  of  about  0.05 
volt,  to  last  for  a  very  short  time,  about  0.0005  second,  and  to  travel 
along  the  nerve  at  speeds  as  great  as  100  yards  per  second. 

The  transmission  of  a  nerve  impulse  is  not  simply  an  electrical 
phenomenon,  like  the  passage  of  a  current  in  a  wire.  It  is  a  physico- 
chemical  process,  which  uses  oxygen  and  produces  carbon  dioxide  and 
heat.  The  transmission  of  a  nerve  impulse  obeys  the  "all-or-none  law"; 
The  conduction  of  the  impulse  is  independent  of  the  nature  or  strength 
of  the  stimulus  starting  it,  provided  that  the  stimulus  is  strong  enough 
to  start  any  impulse.  The  energy  for  the  conduction  of  the  impulse 
comes  from  the  nerve,  not  from  the  stimulus,  so  that,  although  the  speed 
of  the  conducted  impulse  is  independent  of  the  strength  of  the  stimulus, 
it  is  affected  by  the  state  of  the  nerve  fiber.  Drugs  or  low  temperature 
can  retard  or  prevent  the  transmission  of  an  impulse.  The  impulses 
transmitted  by  all  types  of  neurons  are  believed  to  be  essentially  alike. 
That  one  impulse  results  in  a  sensation  of  light,  another  in  a  sensation  of 
pain,  and  a  third  in  the  contraction  of  a  muscle  is  a  function  of  the  way 
the  nerve  fibers  are  connected,  and  not  of  any  special  property  of  the 
impulses. 

According  to  the  generally  accepted  theory  of  the  nature  of  the 
nerve  impulse,  the  semipermeable  membrane  surrounding  each  nerve 
fiber  allows  certain  ions  but  not  others  to  penetrate  it.  The  metabolic 
activities  of  the  nerve  cell  keep  the  membrane  polarized,  with  an  excess 
of  cations  on  the  outside  and  an  excess  of  anions  on  the  inside  (Fig.  5.11). 
The  potential  across  the  membrane  due  to  the  excess  of  positive  ions  out- 


PRINCIPLES   OF  PHYSIOLOGY  105 

side  and  negative  ions  inside  is  from  0.03  to  0.06  volt.  When  the  nerve 
is  stimulated,  its  permeability  is  increased,  the  ions  move  through  the 
membrane,  and  the  membrane  is  depolarized.  Ions  from  the  adjacent, 
not-yet-activated  region  pass  through  the  depolarized  region  and  neu- 
tralize each  other.  This  depolarizes  the  adjacent  region  and  makes  it 
permeable  to  the  migration  of  ions  from  the  next  region,  and  so  on.  The 
nerve  impulse  moves  along  the  surface  of  the  nerve  fiber  as  a  wave  of 
depolarization.  It  seems  probable  that  certain  chemical  reactions  must 
occur  in  the  depolarized  membrane  to  make  it  permeable  and  other 
reactions  must  occur  during  the  refractory  and  recovery  periods  to  re- 
charge, repolarize  the  membrane  and  enable  it  to  be  depolarized  by  the 
next  impulse.  This  theory  provides  an  explanation  for  the  all-or-none 
phenomenon  of  nerve  transmission,  for  no  matter  what  the  strength  of 
the  stimulus,  the  depolarization  can  go  only  to  zero. 

+    +    +    +    -f-i--(-    +    -t--»--f-h    +   +    + 

A  /  RESTING    NERVE    FIBER 

+       ++-t--h+     +      +      +     +      -t--t--h-|--t--<- 

^+     +     +     +     +     +     4-H-f     +     +4-l-      + 
B  /  ACTIVE    NERVE  / 

f       +-!-      +      -(■      +     +       +4-      +      +-(-+      +     + 
Depolarized    region 


^+     +-t-     +     -t--(--(-+     +     -t-+     +     + 


C 


■M-.^.-l./tt'V 


+         + 


_^4      +     +     +      +      ++44 


4    4      4      +     +     + -^4     +     +      +      + 


IMi 


.  -■  V  .-'i<:»>Ww<*^r;.l!:Lij-^«;ft>ii 


+     4     +     +      +     +      + 


K^f£aaittV-jr;i^iiAft'»:t/;i>ii:-^/i*/>v;»*:«X 


Figure  5.11.  Diagram  illustrating  the  membrane  theory  of  nerve  transmission.  A, 
Resting  nerve,  showing  the  polarization  of  the  membrane  with  positive  charges  on  the 
outside  and  negati\e  ones  inside.  B,  Nerve  conducting  an  impulse,  showing,  from  left  to 
right,  the  depolarized  region  where  the  impulse  is,  and  the  polarized  region  ahead  of  the 
impulse.  C,  Stages  in  the  passage  of  the  impulse  along  the  nerve.  (Villee:  Biology.) 


106  GENERAL   CONCEPTS 

Experiments  by  the  English  physiologist  Adrian,  published  in  1926, 
provided  the  explanation  as  to  how  the  nervous  system  transmits  differ- 
ences in  intensity.  By  applying  graded  stimuli  to  an  isolated  sense  organ 
and  amplifying  and  measuring  the  impulses  in  its  nerve,  Adrian  showed 
that  variations  in  the  intensity  of  a  stimulus  lead  to  variations  in  the 
frequeticy  with  which  impulses  are  transmitted:  the  stronger  the  stim- 
ulus, the  more  impulses  per  second.  This  principle  is  true  in  both 
vertebrate  and  invertebrate  sensory  nerves.  In  contrast,  a  single  impulse 
in  a  vertebrate  motor  nerve  elicits  a  single  twitch  of  all  the  muscle  fibers 
in  the  motor  unit.  The  differences  in  the  strength  of  contraction  of  the 
muscle  as  a  whole  are  due  to  variations  in  the  total  number  of  motor 
units  actively  contracting  at  any  given  moment.  In  the  motor  nerves  of 
invertebrates,  however,  the  frequency  of  the  impulses  does  affect  the 
strength  of  contraction  of  the  muscle  innervated.  A  single  nerve  impulse 
will,  in  general,  not  stimulate  the  muscle  to  contract.  At  least  two  suc- 
cessive impulses  are  required,  and  the  strength  of  contraction  is  inversely 
proportional  to  the  interval  between  the  two.  In  many  arthropods  all 
the  muscle  fibers  in  a  given  muscle  are  innervated  by  branches  of  a 
single  nerve  fiber  (axon).  A  single  impulse  in  the  axon  will  not  produce 
contraction  but  repeated  impulses  will;  the  tension  in  the  muscle  in- 
creases with  the  frequency  of  the  stimulation.  It  would  appear  that, 
although  the  arrival  of  a  single  impulse  at  the  nerve-muscle  junction 
is  unable  to  bring  about  muscle  contraction,  it  does  affect  the  junction 
in  such  a  way  as  to  make  it  possible  for  a  second  impulse  to  do  this  if  it 
arrives  soon  enough  after  the  first.  This  phenomenon  is  known  as 
facilitation. 

The  speed  of  propagation  of  the  nerve  impulse  varies  considerably 
from  one  nerve  to  another,  and  even  more  from  one  animal  to  another. 
Conduction  is,  in  general,  more  rapid  in  those  neurons  with  greater 
diameters.  A  number  of  animals— squid,  lobsters  and  earthworms— have 
special  giant  axons  which  conduct  impulses  many  times  faster  than  the 
adjacent  small  fibers.  Conduction  is  more  rapid  in  those  nerves  sur- 
rounded by  a  thick  myelin  sheath.  The  speed  of  conduction  is  greater 
in  those  nerves  in  which  the  myelin  sheath  is  interrupted  periodically 
by  nodes  of  Ranvier. 

Transmission  at  the  Synapse.  Where  the  tip  of  the  axon  of  one 
nerve  comes  close  to  the  tip  of  the  dendrite  of  the  adjoining  nerve  is  a 
region,  called  the  synapse,  across  which  impulses  travel  from  one  nerve 
to  the  other.  Transmission  across  the  synapse  is  slower  than  transmission 
along  a  nerve  fiber.  The  mechanism  by  which  an  impulse  arriving  at 
the  tip  of  one  axon  stimulates  an  impulse  in  the  adjacent  dendrite  is 
not  clear.  There  are  three  hypotheses  to  explain  synaptic  transmission: 
by  the  secretion  of  a  neurohumor,  acetylcholine  or  sympathin,  by  changes 
in  the  concentration  of  cations  in  the  synaptic  region,  or  by  the  trans- 
mission of  an  electric  current.  When  a  nerve  impulse  reaches  the  tip  of 
certain  vertebrate  nerves  it  stimulates  the  secretion  of  acetylcholine.  This 
diffuses  across  the  synaptic  junction  and  stimulates  a  nerve  impulse  in 
the  second  neuron.  Tissues  contain  a  powerful  cholinesterase,  an  en- 
zyme which  specifically  splits  acetylcholine  to  its  constituents,  which  are 


PRINCIPLES   OF   PHYSIOLOGY  \QJ 

inactive,  and  thus  the  continued  stimulation  of  the  adjacent  neuron  is 
prevented. 

The  mechanism  of  synaptic  transmission  in  other  types  of  nerves  is 
the  subject  of  controversy.  There  is  evidence  tliat  acetylchohne  plays 
some  role,  perhaps  the  major  one,  in  synaptic  conduction  in  the  central 
nervous  system  of  vertebrates  and  certain  invertebrates.  Synaptic  trans- 
mission is  greatly  affected  by  the  concentration  of  cations  such  as  potas- 
sium and  calcium,  and  these  ions  may  play  some  direct  role  in 
transmission.  There  is  evidence  from  certain  types  of  nerves  that  the 
electrical  disturbance  which  accompanies  the  nerve  impulse  in  one 
neuron  may  be  sufficient  in  itself  to  elicit  a  nerve  impulse  in  the  next 
neuron.  Each  of  these  agents  may,  under  certain  conditions,  be  shown 
to  stimulate  a  nerve  cell;  which  one  or  ones  actually  function  in  the 
intact  animal  is  not  yet  clear.  One  currently  popular  theory  states  that 
transmission  along  the  axon  and  across  the  synapse  are  fundamentally 
the  same  sort  of  electrical  phenomenon  and  that  liberation  of  acetyl- 
choline is  an  essential  part  of  the  transmission  mechanism  of  each. 
Synapses  are  important  functionally  because  they  are  points  at  which 
the  flow  of  impulses  through  the  nervous  system  is  regulated.  Not  every 
impulse  reaching  a  synapse  is  transmitted  to  the  next  neuron.  The 
synapses,  by  regulating  the  route  of  nerve  impulses  through  the  nervous 
system,  determine  the  response  of  the  organism  to  a  specific  stimulus. 

The  important  details  of  the  arrangement  of  the  neurons  to  form 
the  central  nervous  systems  of  the  higher  invertebrates  and  of  the  verte- 
brates will  be  discussed  in  later  chapters.  The  invertebrate  nervous 
system  consists  of  one  or  more  pairs  of  ganglia— collections  of  nerve  cell 
bodies— at  the  anterior  end  of  the  body  and  one  or  more  nerve  cords 
extending  posteriorly.  The  invertebrate  nerve  cord  is  solid  and  is 
typically  located  on  the  ventral  side  of  the  body;  the  vertebrate  nerve 
cord  is  single,  hollow,  and  located  on  the  dorsal  side  of  the  body. 

Sense  Organs.  Physiologic  experiments  show  that  nerve  fibers  can 
be  stimulated  directly  by  a  variety  of  treatments,  by  electric  shocks,  by 
the  application  of  chemicals,  or  by  mechanical  cutting  or  crushing. 
In  the  intact  organism,  of  course,  sensory  nerve  fibers  are  activated  by 
the  sense  organs  to  which  they  are  connected.  Sense  organs,  like  nerve 
fibers,  respond  to  a  variety  of  treatments,  but  each  is  specialized  so  that 
it  is  extremely  sensitive  to  one  particular  kind  of  stimulus.  The  negli- 
gible amount  of  vinegar  which  can  be  tasted,  or  the  least  amount  of 
vanillin  which  can  be  smelled,  has  no  effect  when  applied  directly  to 
a  nerve. 

Sense  organs  may  be  classified  according  to  the  type  of  stimulus  to 
which  they  are  sensitive.  We  can  distinguish  (1)  chemoreceptors— smell 
and  taste;  (2)  mechanoreceptors-touch,  pressure,  hearing  and  balance; 
(3)  photoreceptors-sight;  (1)  thermoreceptors-hot  and  cold;  and  (5) 
undifferentiated  nerve  endings  which  serve  the  pain  sense.  Sense  organs 
may  also  be  classified  by  the  location  of  the  stimulus:  thus  extero- 
ceptors  supply  information  about  the  surface  of  the  body  (touch,  pres- 
sure, taste,  heat,  cold);  proprioceptors  supply  information  about  the 
position  of  the  body  (stretch  receptors  in  muscles  and  joints,  equilibrium 


108  GENERAL   CONCEPTS 

organs  which  sense  orientation  in  the  field  of  gravity);  distance  receptors 
report  on  objects  away  from  the  body  (sight,  smell  and  hearing),  and 
interoceptors  provide  sensations  of  pain,  fullness,  and  so  on  from 
internal  organs. 

When  a  sense  organ  is  stimulated  continuously  it  may  either  give 
off  a  continuous  stream  of  nerve  impulses  or  it  may  quickly  cease  re- 
sponding to  tlie  stimulus.  The  proprioceptors  of  the  body  are  generally 
of  the  first  type,  nonadaptive,  whereas  the  exteroceptors  are  generally 
adaptive,  and  soon  become  nonresponsive  to  a  continuing  stimulus. 
The  advantage  of  sense  organ  adaptation  is  clear:  it  prevents  a  continual 
train  of  nerve  impulses  nnpinging  on  the  brain  from  all  the  body's 
sense  organs,  yet  does  not  interfere  with  the  body's  responding  to  changes 
in  the  pattern  of  stimuli  which  are  likely  to  be  important  for  survival. 

The  actual  excitation  of  the  sensitive  cells  of  the  sense  organ  is 
either  via  mechanical  stress,  via  chemical  stimulation  by  contact  of  the 
molecules  of  some  substance  from  the  environment,  or  via  some  chemical 
process  induced  in  the  sense  cell  by  the  stimulus.  An  example  of  the 
latter  is  the  chemical  reaction  induced  by  light  falling  on  the  sensitive 
cells  of  the  retina  of  the  eye. 

The  functioning  of  a  sense  organ  in  animals  other  than  man  can  be 
deduced  from  its  morphology  and  nerve  connections.  It  can  be  investi- 
gated by  connecting  the  efferent  nerve  to  an  amplifier  and  oscilloscope, 
applying  stimuli  to  the  sense  organ,  and  measurnig  the  resulting  nerve 
impulses.  It  can  also  be  investigated  at  the  behavioral  level,  by  training 
the  animal  to  associate  one  situation  with  a  given  stimulus  and  a  second 
situation  with  a  different  stimulus,  and  then  observing  its  ability  to 
distinguish  between  the  first  and  second  stimuli  as  they  are  gradually 
changed  to  resemble  each  other. 

Chemoreceptors.  Our  own  senses  of  taste  and  smell  can  be  dis- 
tinguished, for  the  taste  buds  are  organs  in  the  lining  of  the  mouth 
which  respond  to  substances  in  watery  solution,  whereas  the  olfactory 
epithelium  is  in  the  lining  of  the  nose  and  responds  to  substances  which 
enter  as  gases.  In  most  lower  animals,  the  distinction  between  taste  and 
smell  is  blurred,  for  chemoreceptors  are  found  over  much  of  the  surface 
of  the  head  and  part  of  the  body  in  fish,  and  insects  have  chemoreceptors 
in  their  feet.  Chemoreceptors  are  sensitive  to  remarkably  small  amounts 
of  certain  chemicals.  Most  people  can  detect  ionone,  synthetic  violet 
odor,  at  a  concentration  of  one  part  in  30  billion  parts  of  air.  Certain 
male  insects  can  detect  the  odor  given  off  by  the  female  of  the  species 
over  a  distance  of  two  miles.  Several  thousand  different  odors  can  be 
recognized  by  man,  but  there  is  no  clear  correlation  between  the  chemi- 
cal composition  of  a  substance  and  its  smell. 

Chemoreceptors  are  probably  the  most  primitive  of  the  distance 
receptors,  and  many  kinds  of  animals  depend  solely  upon  them  for 
finding  food,  avoiding  predators  and  meeting  mates. 

Mechanoreceptors.  The  skin  of  man  and  other  mammals  contains 
several  kinds  of  sense  organs.  By  making  a  survey  of  a  small  area  of  skin, 
point  by  point,  and  testing  for  regions  sensitive  to  touch,  pressure,  tem- 
perature and  pain,  it  has  been  found  that  receptors  for  each  of  these 


PRINCIPLES   OF   PHYSIOLOGY  109 

sensations  are  located  in  different  spots.  Then,  by  comparing  the  dis- 
tribution of  the  types  of  sense  organs  and  the  types  of  sensations,  it  has 
been  possible  to  identify  the  sense  organ  for  each  stimulus.  In  lower 
animals  the  sensory  organs  are  less  differentiated  and  the  identification 
of  a  particular  nerve  ending  with  a  given  sensitivity  is  usually  mipossible. 

The  sense  cells  at  the  base  of  the  bristles  of  insects  are  clearly 
mechanoreceptors,  and  indeed  it  has  been  possible  to  record  impulses  in 
the  efferent  nerves  when  the  bristle  is  moved. 

The  mammalian  ear  is  a  remarkably  complex  organ  which  contains 
the  senses  of  hearing  and  equilibrium.  It  can  detect  the  direction  of  the 
force  of  gravity  or  of  linear  acceleration,  because  it  contains  otoliths, 
masses  of  calcium  carbonate,  attached  to  slender  processes  of  cells  in 
such  a  way  that  the  weight  of  the  otolith  will  pull  or  push  on  these 
processes.  Motion  of  the  head  about  any  of  its  axes  is  detected  by  the 
motion  of  the  fluid  in  the  semicircular  canals,  which  moves  clumps  of 
hair-like  processes  attached  to  sense  cells  in  the  walls  of  the  canals.  The 
detection  and  analysis  of  sound  waves  involves  the  conversion  of  the 
sound  waves  to  mechanical  vibrations  of  the  ear  drum  and  middle  ear 
bones,  and  then  to  waves  of  motion  in  the  liquid  filling  the  cochlea  of 
the  inner  ear.  The  cochlea  contains  many  sense  cells  with  fibers  of  differ- 
ing lengths  which  respond  to  sounds  of  different  frequencies.  The  ear 
is  basically  a  mechanoreceptor  responding  to  the  mechanical  displace- 
ment of  sense  cells,  or  their  fibers  or  hairs,  produced  by  sound  waves  or 
by  changes  in  position. 

Organs  of  balance,  called  otocysts  or  statocysts,  are  found  in  most 
phyla  of  animals,  even  in  coelenterates.  These  are  usually  hollow  spheres 
of  sense  cells,  in  the  middle  of  which  is  a  statolith,  a  particle  of  sand  or 
calcium  carbonate,  pressed  by  gravity  against  certain  sense  cells.  As  the 
animal's  body  changes  position,  the  statolith  is  pressed  against  different 
sense  cells  and  the  animal  is  then  stimulated  to  regain  its  orientation 
with  respect  to  gravity. 

Many  arthropods,  especially  insects,  have  sense  organs  which  respond 
to  sound  waves;  these  organs  consist  of  a  fine  membrane  stretched  in 
such  a  way  that  it  is  free  to  respond  to  the  vibrations  of  sound  waves. 
The  nerve  from  the  sound-sensitive  organ  of  the  locust  has  been  tapped 
and  recordings  of  the  nerve  impulses  from  it  show  that  it  can  respond 
to  sound  waves  of  between  500  and  10,000  cycles  per  second.  The  human 
ear  responds  to  frequencies  between  20  and  20,000  c.p.s.,  dogs  are  sensi- 
tive to  sounds  as  high  as  40,000  c.p.s.,  and  the  sensitivity  of  the  bat  ear 
extends  to  high-pitched  80,000  c.p.s.  noises. 

Certain  insects  have  balance  organs  which  have  evolved  from  the 
second  pair  of  wings.  These  club-shaped  structures,  called  halteres,  beat 
up  and  down  as  the  wings  do,  and  serve  as  "gyroscopes."  When  the 
direction  of  the  beat  is  changed,  sense  organs  in  the  base  of  the  haltere 
are  stimulated  and  give  off  nerve  impulses.  This  has  been  shown  by 
recording    the    nerve    impulses   passing    through    the    nerves    from    the 

halteres. 

Photoreceptors.  Almost  all  animals  are  sensitive  to  light  and 
respond  to  variations  in  light  intensity.  Even  protozoa  which  have  no 


1  10  GENERAL  CONCEPTS 

special  light-sensitive  organ  show  a  generalized  ability  to  respond  to 
light.  Many  higher  animals— usually  the  burrowing  ones— have  no  recog- 
nizable  "eyes"  but  have  a  general  sensitivity  to  light  over  all  or  a  large 
part  of  the  body.  Clams,  tor  example,  respond  to  sudden  changes  in 
light  intensity  by  drawing  in  their  siphon,  and  earthworms  withdraw 
into  their  burrows  when  the  light  intensity  is  increased. 

Most  animals,  even  coelenterates,  have  some  sort  of  specialized 
structure  for  the  perception  of  light.  A  simple  invertebrate  eye  usually 
consists  of  a  cup-shaped  layer  of  pigment  cells  which  screen  the  light- 
sensitive  cells  from  light  coming  from  all  directions  but  one.  Light-sensi- 
tive cells  are  embedded  between  these  pigment  cells. 

The  cephalopods— the  octopus,  squid,  and  relatives— alone  among 
the  invertebrates  have  well  developed  camera  eyes  which  are  super- 
ficially similar  to  vertebrate  eyes,  with  retina,  lens,  iris,  cornea,  and  a 
mechanism  for  focusing  for  near  and  far  vision.  Although  it  is  difficult 
to  determine  how  well  an  octopus  can  see,  we  can  infer  from  the  struc- 
ture of  the  eye  that  it  should  be  the  functional  equivalent  of  the  verte- 
brate eye. 

The  eyes  of  arthropods— insects  and  crabs— are  mosaic  eyes,  com- 
posed of  many,  perhaps  thousands,  of  visual  units  called  ommatidia. 
Each  ommatidium  has  a  clear  outer  cornea,  under  which  is  a  lens  which 
focuses  the  light  on  the  end  of  the  light-sensitive  element  made  of  eight 
or  so  retinal  cells.  These  are  believed  to  respond  as  a  unit.  Each  om- 
matidium is  separated  from  the  adjacent  ones  by  rings  of  pigment  cells, 
so  that  it  is  a  tube  with  light-sensitive  elements  at  the  base  which  can 
be  reached  only  by  light  parallel  to  the  axis  of  the  tube.  A  mosaic  eye 
presumably  forms  a  very  poor  image  composed  of  a  series  of  rather  large 
dots  like  a  poor  newspaper  photograph.  But  a  mosaic  eye  is  particularly 
sensitive  to  the  motion  of  objects  in  its  surroundings,  for  any  movement 
would  change  the  amount  of  light  falling  on  one  or  more  of  the  om- 
matidia. 

Thermoreceptors.  Temperature-sensitive  cells  are  found  in  a  wide 
variety  of  animals,  from  the  lowest  to  the  highest  levels  of  evolution. 
Ciliates  such  as  paramecia  will  avoid  warm  or  cold  water  and  will  collect 
in  a  region  where  the  temperature  is  intermediate.  Some  insects  have 
thermoreceptors,  either  in  the  antennae  or  all  over  the  body.  Insects 
that  suck  blood  from  warm-blooded  animals  are  attracted  to  their  prey 
by  the  temperature  gradients  nearby.  This  has  been  shown  experi- 
mentally, for  blood-sucking  bugs  are  much  less  able  to  find  their  prey 
after  their  antennae  have  been  removed.  Fish  apparently  have  fairly 
sensitive  thermoreceptors,  for  a  change  of  only  0.5°  C.  will  change  the 
behavior  of  sharks  and  bony  fish. 

As  far  as  we  know,  all  nerve  impulses  are  qualitatively  similar.  The 
impulse  set  up  by  the  ringing  of  a  bell  is  exactly  like  the  impulse 
initiated  by  the  pressure  of  a  pin  against  the  skin,  or  the  impulse  in  the 
optic  nerve  which  results  from  light  falling  on  the  retina.  The  qualita- 
tive differentiation  of  stimuli  must  depend  upon  the  pattern  of  connec- 
tions between  sense  organ  and  brain.  The  ability  to  distinguish  red  from 
green,  hot  from  cold,  or  red  from  cold  is  due  to  the  fact  that  particular 


PRINCIPLES   Of   PHYSIOLOGY  \\l 

sense  organs  and  their  individual  sensitive  cells  are  connected  to  particu- 
lar parts  of  the  brain. 

Coordination  and  Integration.  The  activities  of  the  several  parts 
of  a  many-celled  organism  must  be  coordinated  if  that  organism  is  to 
survive,  and  the  greater  the  degree  of  complexity,  the  greater  the 
specialization  of  the  parts,  the  greater  is  the  need  for  precise  integration 
of  their  separate  functions.  Coordination  of  activity  is  achieved  by  two 
major  systems,  nervous  and  endocrine.  The  nerves  and  sense  organs 
provide  for  rapid  and  precise  adaptation  to  environmental  factors.  The 
endocrine  system,  the  glands  of  internal  secretion  which  secrete  sub- 
stances into  the  blood  stream  (or  its  equivalent  in  lower  animals),  pro- 
vides for  less  rapid,  but  longer  lasting  adaptations  such  as  general  body 
growth,  differentiation,  development  of  sex  organs  and  mating  behavior, 
responses  to  stress,  control  of  tissue  metabolism  and  regulation  of  pig- 
mentation. The  nervous  mechanisms  such  as  reflexes  by  which  coordina- 
tion and  integration  are  achieved  will  be  discussed  in  Chapter  29. 

The  substances  secreted  by  endocrine  glands,  called  hormones,  can- 
not be  defined  as  belonging  to  any  particular  class  of  chemicals;  some 
are  proteins,  some  are  amino  acids  and  some  are  steroids.  They  are 
distinguished  as  a  group  as  being  substances  secreted  by  cells  in  one  part 
of  the  body  which  are  carried  by  the  blood  stream  to  some  other  part 
where  they  affect  cell  activities  in  a  definite  and  characteristic  fashion. 
Acetylcholine  and  sympathin  fit  this  definition  of  a  hormone  and  are 
sometimes  referred  to  as  neurohormones  to  emphasize  this.  AV^hether  a 
hormone  will  affect  a  specific  tissue,  and  the  nature  of  the  effect  pro- 
duced, is  a  function  of  the  tissue;  each  tissue  will  respond  only  to  certain 
hormones.  In  general,  hormones  produced  in  one  animal  will  affect  the 
cells  of  other  animals  in  related  species,  orders  and  even,  in  some  cases, 
classes.  The  endocrine  glands  of  the  vertebrates  will  be  discussed  in 
Chapter  30. 

The  processes  under  endocrine  control  in  invertebrates  include 
molting,  pujKition  and  metamorphosis  in  arthropods,  pigmentation  in 
molluscs  antl  arthropods,  and  growth  and  differentiation  of  secondary 
sex  characteristics  in  annelids  and  arthropods.  The  development  of 
insects,  by  a  series  of  molts  and  metamorphoses,  is  controlled  by  two 
hormones,  the  "growth  and  differentiation"  hormone  (GDH)  and  "ju- 
venile hormone"  (JH).  GDH  is  secreted  by  certain  cells  in  the  dorsal 
mid-region  of  the  insect  brain  and  induces  molting  accompanied  by 
metamorphosis;  juvenile  hormone  is  secreted  by  the  corpus  allatum,  a 
single  median  gland  in  the  posterior  head  region,  and  inhibits  meta- 
morphosis. Transplantation  of  corpora  allata  into  developing  insects 
prevents  metamorphosis  for  several  successive  molts,  so  that  giant  adults 
eventually  result.  In  moths  such  as  the  silkworm,  Platysamia  cecropia, 
the  situation  is  even  more  complicated:  a  hormone  secreted  by  the  brain 
stimulates  the  prothoracic  glands  to  secrete  a  second  hormone  which 
ends  the  pupal  period  and  brings  about  metamorphosis  by  stimulating 
the  cytochrome  system  of  enzymes. 

The  molting  of  crabs  and  other  crustaceans  is  a  complex  process 
involving    many    biochemical    processes    which    must    occur    in    proper 


\  12  GENERAL   CONCEPTS 

sequence.  1  he  removal  ot  the  eyestalk  resuhs  in  premature  moUing  and 
in  more  frequent  successive  molts.  11  sinus  glands  Irom  other  crabs  are 
transplanted  to  crabs  without  eyestalks,  molting  is  delayed.  Thus,  the 
sinus  gland  in  the  eyestalk  produces  a  hormone  which  inhibits  and 
delays  molting. 

There  is  evidence  for  the  hormonal  control  of  the  development  of 
secondary  sex  characters  in  members  of  many  different  invertebrate 
phyla.  When  the  gonads  are  removed  surgically,  or  destroyed  by  para- 
sites, the  sex  characters  either  fail  to  form  or  regress  if  present  initially. 
There  is  some  evidence  that  the  sinus  gland  of  crustaceans  and  the 
corpora  allata  glands  of  insects  secrete  hormones  which  regulate  the 
activity  of  the  ovaries  and  thus  are  analogous  to  the  gonadotropic  hor- 
mones secreted  by  the  vertebrate  pituitary  gland  (p.  626). 

Certain  aspects  of  tissue  metabolism  in  some  invertebrates  appear 
to  be  regulated  by  hormones,  but  there  is  no  clear  evidence  as  yet  of 
any  effect  of  a  vertebrate  hormone  on  invertebrate  tissue  metabolism. 
The  sinus  gland  of  crabs  secretes  a  hormone  which  decreases  basal  meta- 
bolic rate,  for  there  is  an  increase  in  oxygen  consumption  following 
removal  of  the  sinus  gland  and  a  return  to  the  normal  rate  following 
injection  of  extracts  of  the  glands.  The  sinus  gland  hormone  produces 
an  increase  in  blood  sugar  concentration  when  injected  into  crabs,  pro- 
viding another  interesting  parallel  between  the  sinus  gland  secretions 
and  those  of  the  vertebrate  pituitary. 

Hormones  play  a  role  in  determining  pigmentation  in  the  octopus, 
squid,  crabs,  insects,  fish,  amphibia  and  reptiles.  In  most  animals,  color 
changes  are  produced  by  streaming  movements  of  the  pigment-laden 
cytoplasm  of  the  color  cells  (chromatophores).  The  chromatophore  cell 
of  the  cephalopod  has  smooth  muscle  fibers  attached  in  such  a  way  that 
their  contraction  spreads  out  the  pigment-containing  cytoplasm.  Crus- 
taceans can  be  separated  into  two  major  groups,  those  that  darken  and 
those  that  lighten  when  the  eyestalk  is  removed.  Injection  of  eyestalk 
extracts  has  diametrically  opposite  effects  in  the  two  types,  because  of 
basic  differences  in  the  responses  of  the  chromatophore  cells.  More 
recent  experiments  have  shown  that  there  are  at  least  three  different 
chromatophore-regulating  hormones  in  crustaceans. 

A  number  of  endocrine  organs  are  very  closely  associated  with  the 
nervous  system  and  undoubtedly  evolved  from  such  tissue;  others  evolved 
independently  of  the  nervous  system.  It  would  seem  useless  to  try  to 
argue  which  is  the  more  "primitive"  coordinating  system— nervous  or 
endocrine.  Both  had  their  earliest  traces  in  very  primitive,  single-celled 
animals  and  each  type  evolved  independently  of  the  other  to  their 
present  state. 

Questions 

1.  Distinguish  the  types  of  animal  nutrition.  Give  an  example  of  each. 

2.  Discuss  the  similarities  and  differences  of  the  process  of  digestion  in  ameba,  planaria, 
earthworm  and  man. 

3.  What  is  the  function  of:  the  rumen,  the  gizzard,  the  pancreas,  the  atrium  and  the 
hemocoel? 


PRINCIPLES   OF   PHYSIOLOGY  \\^ 

i.  How  would  you  define  a  vitamin?  What  difficulty  is  involved  in  formulating  this 
definition? 

5.  Compare  the  circulatory  systems  of  a  proboscis  worm,  an  earthworm  and  a  caterpillar. 

6.  Define  "partial  pressure"  and  "tension"  of  a  gas. 

7.  Contrast  direct  and  indirect  respiration.  What  are  the  characteristics  of  an  effective 
respiratory  surface? 

8.  Discuss  briefly  the  role  of  hemoglobin  in  the  transport  of  oxygen  and  carbon  dioxide. 

9.  Compare  the  excretion  of  nitrogenous  wastes  in  ameba,  earthworm,  insect  and  man. 

10.  Discuss  the  advantages  and  disadvantages  of  exoskeletons  and  endoskeletons. 

11.  What  functions  may  be  served  by  the  skin  of  an  animal? 

12.  Compare  the  processes  of  ameboid,  ciliary  and  muscular  motion. 

13.  What  is  the  explanation  of  the  "all-or-none  '  response  of  a  motor  unit  to  stimulation? 

14.  Describe  the  sequence  of  events  in  a  single  muscle  twitch. 

15.  What  is  meant  by  tetanus,  tonus  and  oxygen  debt? 

16.  Compare  the  transmission  of  an  impulse  along  a  nerve  fiber  and  across  a  synapse. 

17.  Compare  the  physiologic  properties  of  the  two  major  coordinating  systems  of  verte- 
brates. To  what  extent  are  these  present  in  invertebrates? 

Supplementary  Reading 

The  subjects  and  concepts  discussed  in  Chapters  4  and  5  are  covered  in  much  greater 
detail  and  at  a  more  technical  level  in  L.  V.  Heilbrunn's  An  Outline  of  General  Physiol- 
ogy and  P.  H.  Mitchell's  General  Physiology.  A  wealth  of  information  about  the  physio- 
logic adaptations  of  both  vertebrate  and  invertebrate  animals  is  to  be  found  in  C.  L. 
Prosser's  Comparative  Animal  Physiology,  in  B.  T.  Scheer's  Comparative  Physiology  and 
in  E.  B.  Baldwin's  An  Introduction  to  Comparative  Biochemistry.  The  lectures  given  in 
the  Phvsiology  course  at  the  Marine  Biological  Laboratory  have  been  collected  as  Modern 
Trends  in  Physiology  and  Biochemistry,  edited  by  E.  S.  G.  Barron.  The  papers  given  in 
a  symposium  on  certain  aspects  of  comparative  neurophysiology  have  been  published  as 
Physiological  Triggers,  edited  by  T.  H.  Bullock. 


CHAPTER  6 


Reproduction 


The  processes  needed  for  the  day-to-day  survival  of  the  organism— nu- 
trition, respiration,  excretion,  coordination,  and  the  rest— were  discussed 
in  the  preceding  chapter.  The  survival  of  the  species  as  a  whole  requires 
that  its  individual  members  multiply,  that  they  produce  new  individuals 
to  replace  the  ones  killed  by  predators,  parasites  or  old  age.  One  of  the 
fundamental  tenets  of  biology,  "omne  vivum  ex  vivo"  (all  life  comes  only 
from  living  things),  is  an  expression  of  this  basic  characteristic  of  all 
living  things,  their  ability  to  reproduce  their  kind. 

For  centuries  it  was  believed  that  many  animals  could  arise  from 
nonliving  material  by  "spontaneous  generation."  For  example,  maggots 
and  flies  were  thought  to  originate  from  dead  animals,  and  frogs  and 
rats  to  come  from  river  mud.  The  classic  experiments  which  disproved 
the  theory  of  spontaneous  generation  were  performed  by  Francesco 
Redi  about  1670.  By  the  simple  expedient  of  placing  a  piece  of  meat 
in  each  of  three  jars,  leaving  one  uncovered,  covering  the  second  with 
fine  gauze  and  the  third  with  parchment,  he  demonstrated  that  although 
all  three  pieces  of  meat  decayed,  maggots  appeared  only  on  the  un- 
covered meat.  Maggots  do  not  come  from  decaying  meat,  but  hatch 
from  eggs  laid  on  the  meat  by  blowflies.  With  the  development  of 
lenses  and  microscopes,  and  the  subsequent  increase  in  knowledge  of 
eggs  and  larval  forms,  we  now  know  that  no  animal  arises  by  spontane- 
ous generation. 

The  process  of  reproduction  varies  tremendously  from  one  kind  of 
animal  to  another,  but  we  can  distinguish  two  basic  types:  asexual  and 
sexual.  In  asexual  reproduction  a  single  parent  splits,  buds  or  fragments 
to  give  rise  to  two  or  more  offspring  which  have  hereditary  traits  iden- 
tical with  those  of  the  parent.  Sexual  reproduction  involves  two  indi- 
viduals; each  supplies  a  specialized  reproductive  cell,  a  gamete.  The 
male  gamete,  the  sperm,  subsequently  fuses  with  the  female  gamete,  the 
egg,  to  form  the  zygote  or  fertilized  egg.  The  egg  is  typically  large, 
nonmotile,  and  contains  yolk  which  supplies  nutrients  for  the  embryo 
which  results  if  the  egg  is  fertilized.  The  sperm  is  typically  much 
smaller  and  motile,  adapted  to  swim  actively  to  the  egg  by  the  lashing 
movements  of  its  long,  filamentous  tail.  Sexual  reproduction  is  advan- 
tageous biologically  for  it  makes  possible  the  recombination  of  the  best 
inherited  characteristics  of  the  two  parents  and  provides  for  the  possi- 

114 


REPRODUCTION 


115 


bility  that  some  of  the  offspring  may  be  better  adapted  to  survive  than 
either  parent  was. 

33.        Asexual  Reproduction 

Asexual  reproduction  occurs  commonly  in  plants,  protozoa,  coelen- 
terates,  bryozoa  and  tunicates,  but  may  occur  even  in  the  highest  animals. 
The  production  of  identical  twins  by  the  splitting  of  a  single  fertilized 
egg  is  a  kind  of  asexual  reproduction.  The  splitting  of  the  body  of  the 
parent  into  two  more  or  less  equal  daughter  parts,  which  become  new 
whole  organisms,  is  called  fission.  Fission  occurs  chiefly  among  single- 


Zygote 


Sexrual  ^eneraJbion— \ 


Sporula-tion. 


Saiivajy  6la-nd. 
Sporozoite 


a-metocytc 


Sporula.tion 


Figure  6.1.  A  diagram  of  the  life  cycle  of  the  malaria  parasite,  Plasmodium.  An  in- 
fected mosquito  bites  a  man  and  injects  some  Plasmodium  sporozoites  into  his  blood 
stream.  These  reproduce  asexually  by  sporulation  within  the  red  blood  cells  of  the  host. 
The  infected  red  cells  rupture  and  the  new  crop  of  merozoites  released  then  infects  other 
red  cells.  The  bursting  of  the  red  cells  releases  toxic  substances  which  cause  the  periodic 
fever  and  chill.  In  time  some  merozoites  become  gametocytes  which  can  infect  a  mosquito 
if  one  bites  the  man.  The  gametocytes  develop  into  eggs  and  sperm  and  undergo  sexual 
reproduction  in  the  mosquito,  and  the  zygote,  by  sporulation,  produces  sporozoites  which 
migrate  to  the  salivary  glands. 


115  GENERAL   CONCEPTS 

celled  animals  and  plants;  the  cell  division  involved  is  mitotic.  Coe- 
lenterates  typically  reproduce  by  budding;  a  small  part  of  the  parent's 
body  becomes  differentiated  and  separate  from  the  rest.  It  develops 
into  a  complete  new  individual  and  may  take  up  independent  existence, 
or  the  buds  from  a  single  parent  may  remain  attached  as  a  colony  of 
many  individuals. 

Salamanders,  lizards,  starfish  and  crabs  can  grow  a  new  tail,  leg  or 
other  organ  if  the  original  one  is  lost.  When  this  ability  to  regenerate 
the  whole  from  a  part  is  extremely  marked  it  becomes  a  method  of  re- 
production. The  body  of  the  parent  may  break  into  several  pieces  and 
each  piece  then  develops  into  a  whole  animal  by  regenerating  the  miss- 
ing parts.  A  whole  starfish  can  be  regenerated  from  a  single  arm. 

One  class  of  protozoa,  the  Sporozoa,  characteristically  reproduce 
asexually  by  means  of  spores,  special  cells  with  resistant  coverings  which 
withstand  unfavorable  environmental  conditions.  An  interesting  ex- 
ample of  reproduction  by  spore  formation  is  the  parasitic  protozoan, 
Plasmodium,  which  causes  malaria.  The  organism  has  a  complex  life 
cycle  involving  man  and  the  Anopheles  mosquito  (Fig.  6.1).  The  malaria 
organism  enters  the  human  blood  stream  when  the  mosquito  bites  the 
man,  and  attacks  and  enters  the  red  blood  cells.  Within  the  red  cell 
each  Plasmodium  divides  into  12  to  24  spores,  each  of  which  is  released 
when  the  red  cell  bursts  later  on.  The  released  spores  infect  new  red 
cells  and  the  process  is  repeated.  The  simultaneous  bursting  of  billions 
of  red  cells  causes  the  malarial  chill,  followed  by  fever  as  the  toxic 
substances  released  penetrate  to  other  organs  of  the  body.  If  a  second, 
uninfected  mosquito  bites  the  man,  it  will  suck  up  some  Plasmodiwn 
spores  along  with  its  drink  of  blood.  A  complicated  process  of  sexual 
reproduction  ensues  within  the  mosquito's  stomach  and  new  spores  are 
formed,  some  of  which  migrate  into  the  mosquito's  salivary  glands  and 
are  ready  to  infect  the  next  man  bitten. 

34.        Sexual  Reproduction 

Sexual  reproduction  is  characterized  by  the  development  of  a  new  in- 
dividual from  a  zygote,  or  fertilized  egg,  produced  in  turn  by  the  fusion 
of  two  sex  cells,  an  egg  and  a  sperm.  Certain  protozoa  have  a  compli- 
cated process  of  sexual  reproduction  in  which  two  individuals  come  to- 
gether and  fuse  temporarily  along  their  oral  surfaces.  The  nucleus  of 
each  one  divides  several  times  before  one  of  the  resulting  daughter 
nuclei  migrates  across  to  the  other  animal  and  fuses  with  one  of  its 
nuclei.  Following  this  the  two  animals  separate  and  each  reproduces 
asexually  by  fission.  Paramecia  are  not  differentiated  morphologically 
into  sexes,  but  T.  M.  Sonneborn  has  shown  that  there  are  distinct, 
genetically  determined  mating  types.  A  member  of  one  mating  group 
will  mate  only  with  some  member  of  another  group. 

Meiosis.  The  mitotic  process  of  cell  division  is  remarkably  con- 
stant and  ensures  that  the  number  of  chromosomes  per  cell  will  remain 
unchanged  through  successive  cell  generations.  The  fusion  of  an  egg  and 
a   sperm   to  form  a   fertilized  egg  would  result  in   a  doubling  of  the 


REPRODUCTION  WJ 

chromosome  number  in  each  successive  generation  if  all  cell  divisions 
occurred  by  mitosis.  However,  at  some  point  in  the  succession  of  cell 
divisions  which  constitute  the  life  cycle  of  an  individual,  from  the  origi- 
nal fertilized  egg  through  development,  growth  and  maturation  to  the 
production  of  the  fertilized  egg  in  the  next  generation,  there  occurs  a 
different  type  of  cell  division,  called  meiosis.  In  the  higher  animals,  and 
in  most  of  the  lower  ones,  meiotic  divisions  occur  during  the  formation 
of  gametes.  Meiosis  is  essentially  a  pair  of  cell  divisions  during  which 
the  chromosome  number  is  reduced  to  half  (Fig.  6.2).  Thus  the  gametes 
contain  only  half  as  many  chromosomes  as  the  somatic  cells,  and  when 
two  gametes  unite  at  fertilization,  the  normal  chromosome  number  is 
reconstituted. 

The  reduction  in  chromosome  number  occurs  in  a  very  regular 
way.  Chromosomes  occur  in  pairs  of  similar  chromosomes  in  somatic 
cells.  As  a  result  of  meiosis,  each  gamete  contains  one  and  only  one  of 
each  kind  of  chromosome,  i.e.,  one  complete  set  of  chromosomes.  This 
is  accomplished  by  the  synapsis,  or  longitudinal  pairing,  of  like  chromo- 
somes and  the  subsequent  separation  of  the  members  of  the  pair,  one 
going  to  each  pole.  The  like  chromosomes  which  undergo  synapsis  dur- 
ing meiosis  are  called  homologous  chromosomes.  They  are  identical  in 
size  and  shape,  have  identical  chromomeres  along  their  length  and 
contain  similar  hereditary  factors.  .\  set  of  one  of  each  kind  of  chromo- 
some is  called  the  haploid  number  (n);  a  set  of  two  of  each  kind  is 
called  the  diploid  number  (2n).  Gametes  have  the  haploid  number  (e.g., 
23  in  man)  and  fertilized  eggs  and  all  the  cells  of  the  body  have  the 
diploid  number  (46  for  man).  A  fertilized  egg  gets  exactly  half  of  its 
chromosomes  (and  half  of  its  genes)  from  its  mother,  and  half  from  its 
father.  Only  the  last  two  cell  divisions  which  result  in  mature,  func- 
tional eggs  or  sperm  are  meiotic;  all  other  ones  are  mitotic. 

Each  of  the  meiotic  divisions  has  the  same  four  stages,  prophase, 
metaphase,  anaphase  and  telophase,  found  in  mitosis.  The  chief  dif- 
ferences between  mitotic  and  meiotic  divisions  are  seen  in  the  prophase 
of  the  first  meiotic  division.  Chromosomes  appear  as  long  thin  threads 
which  begin  to  contract  and  get  thicker.  The  homologous  chromosomes 
undergo  synapsis,  they  pair  longitudinally  and  come  to  lie  side  by  side 
along  their  entire  length,  twisting  around  each  other.  Each  then  be- 
comes visibly  double,  as  in  mitosis,  so  that  it  consists  of  two  threads.  By 
synapsis  and  doubling,  a  bundle  of  four  homologous  chromosomes,  called 
a  tetrad,  is  formed. 

The  tetrads  then  line  up  on  the  equatorial  plate;  this  constitutes  the 
metaphase  of  the  first  meiotic  division.  The  homologous  chromosomes 
now  separate  from  one  another  and  move  to  the  poles.  The  chromosomes 
moving  to  the  poles  during  anaphase  of  the  first  meiotic  division  are 
double,  and  at  telophase  each  pole  has  received  the  haploid  number 
of  double  chromosomes.  Typically,  there  is  no  interphase  between  first 
and  second  meiotic  divisions,  but  new  spindles  form  (at  right  angles  to 
the  axis  of  the  original  spindle)  and  the  haploid  number  of  double 
chromosomes  lines  up  on  the  equator  of  this  spindle.  Thus,  the  telo- 
phase of  the  first  meiotic  division  and  the  prophase  of  the  second  are 


118 


GENERAL  CONCEPTS 


A 


3 


D 


Figure  6.2.  Meiosis  in  a  hypothetical  animal  with  a  diploid  chromosome  number  of 
six.  It  has  three  pairs  of  chromosomes,  of  which  one  is  short,  one  is  long  with  a  hook  at 
the  end,  and  one  is  long  and  knobbed.  A,  Early  prophase  of  the  first  meiotic  division: 
chromosomes  begin  to  appear.  B,  Synapsis:  the  pairing  of  the  homologous  chromosomes. 
C,  Apparent  doubling  of  the  synapsed  chromosomes  to  form  groups  of  four  identical 
chromosomes,  tetrads.  D,  Metaphase  of  the  first  meiotic  division,  with  the  tetrads  lined 
up  at  the  equator  of  the  spindle.  E,  Anaphase  of  the  first  meiotic  division:  the  chromo- 
somes migrating  toward  the  poles.  F,  Telophase  of  the  first  meiotic  division.  G,  Prophase 
of  the  second  meiotic  division.  H,  Metaphase  of  the  second  meiotic  division.  /,  Anaphase 
of  the  second  meiotic  division.  /,  Mature  gametes,  each  of  which  contains  only  one  of 
each  kind  of  chromosome.  (Villee:  Biology.) 


REPRODUCTION  119 

short  and  blurred  together.  The  lining  up  of  the  chromosomes  on  the 
spindle  constitutes  the  metaphase  of  the  second  division.  There  is  no 
further  doubUng  of  the  chromosomes;  they  simply  separate  and  pass  to 
the  poles  so  that  in  the  anaphase  of  the  second  meiotic  division  a  hap- 
loid  set  of  single  chromosomes  passes  to  each  pole.  In  the  telophase, 
the  cytoplasm  divides,  the  chromosomes  become  longer,  thinner  and  less 
easily  seen,  and  a  nuclear  membrane  forms  around  them.  The  net  re- 
sult of  the  two  meiotic  divisions  is  a  group  of  four  cells,  each  of  which 
contains  the  haploid  number  of  chromosomes,  that  is,  one  and  only  one 
of  each  kind  of  chromosome.  These  cells  are  mature  gametes  and  do 
not  undergo  any  further  mitotic  or  meiotic  divisions. 

The  term  gonad  refers  to  the  glands  which  produce  gametes,  the 
testis  of  the  male  and  the  ovary  of  the  female.  The  meiotic  process  is 
fundamentally  the  same  in  ovary  and  testis  but  there  are  a  few  differ- 
ences in  detail. 

Spermatogenesis.  A  typical  testis  consists  of  thousands  of  cylindri- 
cal sperm  tubules,  in  each  of  which  develop  billions  of  sperm.  The  walls 
of  the  sperm  tubules  are  lined  with  unspecialized  germ  cells  called 
spermatogonia.  Throughout  development,  the  spermatogonia  divide  by 
mitosis  and  give  rise  to  additional  spermatogonia  to  provide  for  the 
growth  of  the  testis.  After  sexual  maturity,  some  spermatogonia  begin 
to  undergo  spermatogenesis,  which  includes  the  two  meiotic  divisions 
followed  by  the  cellular  changes  which  result  in  mature  sperm.  Other 
spermatogonia  continue  to  divide  mitotically  and  produce  additional 
spermatogonia  for  spermatogenesis  at  a  later  time.  In  most  wild  animals, 
there  is  a  breeding  season,  either  in  spring  or  fall,  during  which  the 
testis  increases  in  size  and  spermatogenesis  occurs.  Between  breeding 
seasons  the  testis  is  usually  smaller  and  contains  only  spermatogonia. 
In  other  animals,  including  man  and  most  domestic  animals,  spermato- 
genesis continues  throughout  the  year  once  sexual  maturity  has  been 
attained. 

The  first  step  in  spermatogenesis  is  the  growth  of  the  spermato- 
gonia into  larger  cells,  the  primary  spermatocytes  (Fig.  6.3).  Each 
primary  spermatocyte  divides,  by  the  first  meiotic  division,  into  two  cells 
of  equal  size,  the  secondary  spermatocytes.  These  in  turn  divide  by 
the  second  meiotic  division  to  yield  four  spermatids.  The  spermatid 
is  a  spherical  cell  with  quite  a  bit  of  cytoplasm.  Although  it  is  a  mature 
gamete  (it  has  the  haploid  number  of  chromosomes),  further  changes 
(but  no  cell  division)  are  required  to  convert  it  into  a  functional 
spermatozoan.  The  nucleus  shrinks  in  size,  becomes  more  dense,  and 
forms  the  head  of  the  sperm  (Fig.  6.4).  Most  of  the  cytoplasm  is  shed, 
but  some  of  the  Golgi  bodies  aggregate  at  the  anterior  end  of  the 
sperm  and  form  a  point  which  may  be  of  some  value  in  puncturing  the 
cell  membrane  of  the  egg.  A  bit  of  the  cytoplasm  is  converted  into  a 
long  flexible  tail,  the  beating  of  which  drives  the  sperm  forward.  The 
mitochondria  aggregate  at  the  junction  of  the  head  and  tail  to  form 
the  middle  piece  which  is  believed  to  supply  the  energy  for  the  beating 
of  the  tail. 

The  mature  spermatozoa  of  different  species  exhibit  a  wide  range 


120 


GENERAL   CONCEPTS 


SPERMATOGENESIS 

A 


OOGENESIS 


Spermatogonia  in 
V  testis  and  oogonia 
''^  in  ova.ry  (divide         * 

meuiy  times  by 

mitosis. 


J 


Primat-y  — — 
Spermato  cy  t  s. 


A  sperm  alo^onium 
Orows  into  a 


1iir 


odary - 


beconc 
Sperina.toc_yt£ 


FIRST 

MEIOTIC 

DIVISION 


Spsrmalids 


I 

4 


SECOND 

MEIOTIC 

DIVISION 


/    \ 


r\. 


V 


An  oogonium, 
brows  into  a. 

i 


m  m 


Primary 
oocyte 


First 
polocyte 

Se-conda-ry 
oocyte 


iO 


\  i  \ 


Second 
polocyte 


Zygote 
Figure  6.3.     Comparison  of  the  formation  of  sperm  and  eggs. 


of  sizes  and  shapes  (Fig.  6.5).  The  sperm  of  a  few  animals,  such  as  the 
parasitic  roundworm  Ascaris,  lack  tails  and  crawl  along  by  ameboid 
motion.  Crabs  and  lobsters  have  curious  tailless  sperm  with  three  pointed 
projections  on  the  head.  These  hold  the  sperm  in  position  on  the  sur- 
face of  the  egg  while  the  middle  piece  uncoils  like  a  spring  and  pushes 
the  sperm  nucleus  into  the  egg  cytoplasm,  thereby  accomplishing  fer- 
tilization. 

Oogenesis.  The  immature  sex  cells  in  the  ovary  are  known  as 
oogonia.  These  undergo  successive  mitotic  divisions  to  form  additional 
oogonia  during  development.  When  the  individual  reaches  sexual  ma- 
turity, oogonia  develop  into  large  primary  oocytes.  These  are  typically 
much  larger  than  the  corresponding  primary  spermatocytes  and  con- 
tain yolk,  which  will  serve  as  food  in  the  event  the  egg  is  fertilized. 
Some  of  the  "morphogenetic  substances"  which  subsequently  regulate 
the  development  of  the  fertilized  egg  are  formed  at  this  time.  When  it 
has  completed  its  growth  phase  the  primary  oocyte  divides  by  the  first 
meiotic  division  (Fig.  6.3).  The  two  daughter  ceils,  however,  are  not 
of  equal  size.  One,  the  secondary  oocyte,  receives  essentially  all  of  the 


REPRODUCTION 


121 


Primary 

sptrmotocyle  r         /. 

dividing  /       .   ' 


Spermotogonia 


Primory    tpirmotocylt 


% 


Figure  6.4.  Diagram  of  part  of  a  section  of  a  human  seminiferous  tubule  to  show 
the  stages  in  spermatogenesis  and  in  the  transformation  of  a  spermatid  into  a  mature 
sperm.  (Villee:  Biology.) 


ASCARIS 


Figure  6.5.     Eggs  and  sperm  from  a  variety  of  animals,  illustrating  the  differences  in 
size  and  shape.  (Partly  after  Retzius,  from  Hunter  and  Hunter:  College  Zoology.) 


122  GENERAL   CONCEPTS 

cytoplasm  and  yolk  while  the  other,  the  first  polocyte  or  polar  body,  is 
essentially  a  bare  nucleus. 

The  secondary  oocyte  divides  by  the  second  meiotic  division,  again 
with  an  unequal  division  of  cytoplasm,  to  yield  a  large  ootid,  with 
essentially  all  of  the  yolk  and  cytoplasm,  and  a  small  second  polocyte. 
(The  first  polocyte  may  divide  at  about  the  same  time  into  two  addi- 
tional polocytes.)  The  ootid  undergoes  further  changes  (but  no  cell 
division)  and  becomes  a  mature  ovum  (egg).  The  polocytes  disintegrate 
and  disappear,  so  that  each  primary  oocyte  forms  a  single  ovum,  in  con- 
trast to  the  four  sperm  derived  from  each  primary  spermatocyte.  The 
formation  of  the  polocytes  is  a  device  to  enable  the  maturing  egg  to  get 
rid  of  its  excess  chromosomes,  and  the  unequal  division  of  the  cytoplasm 
insures  the  mature  egg  enough  cytoplasm  and  yolk  to  survive  and  de- 
velop if  it  is  fertilized. 

The  union  of  a  haploid  set  of  chromosomes  from  the  sperm  with 
another  haploid  set  from  the  egg  during  fertilization  reestablishes  the 
diploid  chromosome  number.  The  fertilized  egg,  and  all  the  body  cells 
which  develop  from  it,  have  the  diploid  number,  two  of  each  kind. 
Each  individual  gets  half  of  his  chromosomes  (and  half  of  his  genes) 
from  his  father  and  half  from  his  mother.  Because  of  the  nature  of  gene 
interaction,  the  offspring  may  resemble  one  parent  much  more  than 
the  other,  but  the  two  parents  make  equal  contributions  to  its  inheri- 
tance. 


35.        Reproductive  Systems 

In  some  of  the  simpler  invertebrates,  such  as  the  coelenterates,  the 
testes  and  ovaries  are  the  only  sex  structures  present,  and  eggs  and  sperm 
are  released  directly  from  the  gonads  into  the  surrounding  water.  Most 
animals,  however,  have  a  system  of  ducts  and  glands  which  serve  to 
carry  gametes  from  the  gonad  to  the  exterior  of  the  body  and  to  pro- 
tect and  nourish  them  during  the  process. 

Many  of  the  lower  animals  are  hermaphroditic;  both  ovaries  and 
testes  are  present  in  the  same  individual  and  it  produces  both  eggs  and 
sperm.  Some  hermaphroditic  animals,  the  parasitic  tapeworms,  for  ex- 
ample, are  capable  of  self-fertilization.  Since  a  particular  host  animal 
may  be  infected  with  but  one  parasite,  hermaphroditism  is  an  im- 
portant adaptation  for  the  survival  of  the  parasitic  species.  Most  her- 
maphrodites, however,  do  not  reproduce  by  self-fertilization;  in  the 
earthworm,  for  example,  two  animals  copulate  and  each  inseminates 
the  other.  In  certain  other  species,  e.g.,  the  oyster,  self-fertilization  is 
impossible  because  the  testes  and  ovaries  produce  gametes  at  different 
times. 

The  reproductive  systems  of  different  species  have  a  fundamentally 
similar  plan,  but  many  variations  on  the  theme  are  evident.  The  gonads 
and  their  ducts  may  be  single,  paired  or  multiple,  perhaps  present  in 
several  segments  of  the  body. 

The  male  reproductive  system  typically  comprises  the  testes,  vasa 
efferentia  and  vas  deferens.  Sperm  are  produced  in  the  coiled  seminifer- 


REPRODUCTION  \2S 

ous  tubules  of  the  testis.  Nurse  cells  are  present  in  the  walls  of  the 
tubules  to  nourish  the  sperm  as  they  develop  from  round  spermatids 
into  mature,  tailed  spermatozoa.  Each  tubule  is  connected  by  a  fine 
tube,  the  vas  eflFerens,  with  the  complexly  coiled  epididymis,  where 
sperm  are  stored.  From  each  epididymis  a  vas  deferens  passes  to  the 
exterior  either  directly  or  through  a  copulatory  organ  or  penis.  The 
seminal  fluid,  in  which  the  sperm  are  suspended,  protects,  nourishes  and 
activates  the  sperm.  It  is  secreted  by  glands  associated  with  the  repro- 
ductive tract;  in  mammals  these  are  the  seminal  vesicles,  a  pair  of 
glands  whose  ducts  empty  into  the  vas  deferens,  the  prostate  glands, 
at  the  junction  of  the  vas  deferens  and  the  urethra,  and  Cowper's  glands, 
which  empty  into  the  urethra  at  the  base  of  the  penis.  Seminal  fluid 
contains  glucose  and  fructose  which  the  sperm  metabolize,  inorganic 
salts  which  act  as  buffers  to  protect  the  sperm  from  the  acids  normally 
present  in  the  urethra  and  female  tract,  and  mucous  materials  which 
lubricate  the  passages  through  which  the  sperm  travel. 

In  many  vertebrates  the  urinary  and  genital  systems  have  one  or 
more  structures  in  common  and  the  two  are  sometimes  considered  to- 
gether as  the  urogenital  system.  In  the  male  mammal,  for  example,  the 
vasa  deferentia  empty  into  the  urethra,  which  also  carries  urine  from 
the  bladder  to  the  outside.  The  urethra  of  mammals  is  surrounded  by 
the  external  reproductive  organ,  the  penis.  This  consists  of  three  col- 
umns of  erectile  tissue— spongy  venous  spaces  which  become  filled  with 
blood  during  sexual  excitement  to  produce  an  erection  of  the  penis. 

Eggs  are  produced  in  the  ovaries  of  the  female  and  are  typically 
surrounded  and  nourished  by  nurse  cells  during  their  development.  At 
the  time  of  ovulation,  the  eggs  are  released  from  the  ovary  into  the 
abdominal  cavity,  whence  they  pass  into  the  funnel-shaped  end  of  the 
oviduct.  Eggs  are  moved  along  the  oviduct  by  the  peristaltic  contractions 
of  its  muscular  wall  or  by  the  beating  of  cilia  lining  the  lumen  of  the 
duct.  The  yolk  of  the  egg  is  formed  while  the  egg  is  still  within  the 
ovary,  but  the  egg  white  and  shell  are  added  by  glands  in  the  wall  of 
the  oviduct.  The  oviducts  may  open  directly  to  the  exterior  or  they  may 
expand  into  a  terminal  duct,  the  uterus,  which  is  a  thick-walled  mus- 
cular pouch  in  which  the  young  develop.  In  mammals  the  uterus  is 
connected  with  the  exterior  by  the  vagina,  which  is  adapted  to  receive 
the  penis  of  the  male  during  copulation.  Female  mammals  have  a 
clitoris,  the  homologue  of  the  male  penis,  just  anterior  to  the  opening 
of  the  vagina;  it  contains  sense  organs  and  erectile  tissue  which  becomes 
engorged  with  blood  during  sexual  excitement. 

36.        Fertilization 

The  union  of  an  egg  and  sperm  is  called  fertilization.  Most  aquatic 
animals  deliver  their  eggs  and  sperm  directly  into  the  surrounding 
water  and  the  union  of  egg  and  sperm  occurs  there  by  chance  meeting. 
This  primitive  and  rather  uncertain  method  of  uniting  the  gametes  is 
called  external  fertilization.  Such  animals  usually  have  no  accessory  sex 
structures. 


124 


GENERAL   CONCEPTS 


■TalL 


-Entra-ncc  Cone 

Hca.d  ajid middle 
.piece  of  sperm. 


-Entra-ncc  pa-tK 


Copula-tion  patK 


t  and  2n(i  pola.i' 
bodies 


^^l^ 


division- 
Figure  6.6.     Diagram  of  the  stages  in  the  process  of  fertiUzation,  the  union  of  the 
egg  and  sperm. 

In  Other  animals,  fertilization  occurs  within  the  body  of  the  female, 
usually  in  the  oviduct,  after  the  sperm  have  been  transferred  from  the 
male  to  the  female  by  copulation  or  by  some  other  means.  This  method 
of  internal  fertilization  requires  some  cooperation  between  the  two 
sexes,  and  many  species  have  evolved  elaborate  patterns  of  mating  be- 
havior to  insure  its  occurrence.  The  male  salamander,  for  example, 
mounts  and  clasps  the  female,  stroking  her  nose  with  his  chin.  He  then 
dismounts  in  front  of  her  and  deposits  a  spermatophore,  a  packet  of 
sperm.  She  picks  up  the  spermatophore  and  stuffs  it  into  her  cloaca, 
where  the  packet  breaks,  the  sperm  are  released,  and  fertilization  fol- 
lows. 

Fertilization  involves  not  only  the  penetration  of  the  egg  by  the 
sperm,  but  the  union  of  the  egg  and  sperm  nuclei  and  the  activation  of 
the  egg  to  undergo  cleavage  and  development  (Fig.  6.6).  The  egg  may 
be  in  any  stage  from  primary  oocyte  to  mature  ovum  at  the  time  of 
sperm  penetration,  but  the  fusion  of  the  sperm  and  egg  nuclei  occurs 
only  after  the  egg  has  matured.  There  is  experimental  evidence  that 
the  eggs  of  some  species  secrete  a  substance,  fertilizin,  which  is  an  im- 
portant constituent  of  the  jelly  coat  surrounding  the  egg.  Fertilizin 
causes  the  sperm  to  clump  together  and  stick  to  the  surface  of  the  egg. 
Other  extracts  of  the  egg  jelly,  which  may  be  identical  with  fertilizin, 
stimulate  sperm  motility  and  respiration  and  prolong  sperm  viability. 

After  the  entrance  of  one  sperm,  a  fertilization  membrane  forms 
around  the  eggs  of  some  species  which  prevents  the  entrance  of  other 
sperm.  This  prevents  polyspermy  and  the  possibility  of  the  fusion  of 
more  than  one  sperm  nucleus  with  the  egg  nucleus.  It  can  be  shown 
experimentally  that  such  fusion  of  two  or  more  sperm  nuclei  with  one 
egg  nucleus  leads  to  abnormal  development. 

Eggs  can  be  stimulated  to  cleave  and  develop  without  fertilization. 
The  development  of  an  unfertilized  egg  into  an  adult  is  known  as 
parthenogenesis  (virgin  birth).  Some  species  of  arthropods  have  been 
found  which  apparently  consist  solely  of  females  which  reproduce 
parthenogenetically.  In  other  species,  parthenogenesis  occurs  for  several 
generations,  then  some  males  are  produced  which  develop  and  mate 
with  the  females.  The  queen  honeybee  is  fertilized  by  a  male  just  once 


REPRODUCTION  125 

during  her  lifetime,  in  her  "nuptial  flight."  The  sperm  are  stored  in  a 
pouch  connected  with  the  genital  tract  and  closed  by  a  muscular  valve. 
If  sperm  are  released  from  the  pouch  as  she  lays  eggs,  fertilization  occurs 
and  the  eggs  develop  into  females— queens  and  workers.  If  the  eggs  are 
not  fertihzed  they  develop  into  males— drones. 

Changes  in  temperature,  in  pH  or  in  the  salt  content  of  the  sur- 
rounding water,  or  chemical  or  mechanical  stimulation  of  the  egg  itself 
will  stimulate  many  eggs  to  parthenogenetic  development.  A  variety 
of  marine  invertebrates,  frogs,  salamanders,  and  even  rabbits  have  been 
produced  parthenogenetically.  The  resulting  adult  animals  are  gen- 
erally weaker  and  smaller  than  normal,  and  are  infertile. 

The  females  of  all  birds,  most  insects,  and  many  aquatic  inverte- 
brates lay  eggs  from  which  the  young  eventually  hatch;  such  animals  are 
said  to  be  oviparous  (egg-bearing).  In  contrast,  mammals  produce  small 
eggs  which  are  kept  in  the  uterus  and  provided  with  nutrients  from  the 
mother's  blood  until  development  has  pioceeded  to  the  stage  where  they 
can  exist  independently,  to  some  extent  at  least.  Such  animals  are  said 
to  be  viviparous  (live-bearing).  In  certain  other  forms— some  insects, 
sharks,  lizards  and  certain  snakes— the  female  is  ovoviviparous,  she  pro- 
duces large,  yolk-filled  eggs  which  are  retained  within  the  female 
reproductive  tract  for  a  considerable  period  of  development.  The  de- 
veloping embryo  forms  no  close  connection  with  the  wall  of  the  oviduct 
or  uterus  and  receives  no  nourishment  from  the  mother. 

The  number  of  eggs  produced  by  each  female  of  a  given  species 
and  the  chance  that  any  particular  egg  will  survive  to  maturity  are  in- 
versely related.  In  the  evolution  of  the  vertebrates  from  fish  to  mam- 
mals, the  trend  has  been  towards  the  production  of  fewer  eggs,  and  the 
development  of  instincts  for  better  parental  care  of  the  young.  Fish 
such  as  the  cod  or  salmon  produce  millions  of  eggs  each  year,  but  only 
a  small  number  of  these  ever  become  adult  fish;  in  contrast,  mammals 
have  few  offspring  but  take  good  care  of  them  so  that  the  majority 
attain  maturity.  Fish  and  amphibia  generally  take  no  care  of  develop- 
ing eggs,  which  are  simply  deposited  in  water  and  left  to  complete  de- 
velopment unaided.  The  eggs  of  reptiles  are  usually  laid  in  earth  or 
sand  and  develop  there  without  parental  care,  warmed  by  the  sun. 
Birds,  in  contrast,  have  a  complex  behavior  pattern  for  nest-building, 
incubating  the  eggs  by  sitting  on  them  and  caring  for  the  newly 
hatched  youngsters.  The  mammalian  egg  develops  within  the  mother's 
uterus  where  it  is  safe  from  predatois  and  from  harmful  factors  in  the 
environment.  Most  mammals  have  a  strong  "maternal  instinct"  to  take 
care  of  the  newborn  until  they  can  shift  for  themselves. 

Many  animals  have  other  special  types  of  instinctive  behavior,  or 
"breeding  habits"  to  insure  successful  reproduction.  A  number  of 
vertebrate  and  invertebrate  species  have  characteristic  courting  and 
mating  behavior  patterns  which  may  be  dangerous  or  even  fatal  to  the 
individual,  yet  insure  the  continuation  of  the  species.  Salmon  swim 
hundreds  of  miles  upstream  to  spawn  and  die,  male  spiders  are  fre- 
quently eaten  by   the  females  after  fertilizing  them,  and  so  on. 


126  GENERAL   CONCEPTS 

37.        Embryonic  Development 

The  division,  growth  and  differentiation  of  a  fertihzed  egg  into  the 
remarkably  complex  and  interdependent  system  of  organs  which  is  the 
adult  animal  is  certainly  one  of  the  most  fascinating  of  all  biologic 
phenomena.  Not  only  are  the  organs  complicated,  and  reproduced  in 
each  new  individual  with  extreme  fidelity  of  pattern,  but  many  of  these 
organs  begin  to  function  while  they  are  still  developing.  The  human 
heart  begins  to  beat,  for  example,  during  the  fourth  week  of  gestation, 
long  before  its  development  is  completed. 

The  early  stages  of  development  of  practically  all  multicellular 
animals  are  fundamentally  similar;  differences  in  development  become 
evident  somewhat  later. 

When  fertilization  has  been  accomplished,  the  zygote  divides  re- 
peatedly by  mitosis,  forming  a  ball  of  smaller  cells  known  as  a  blastula. 
These  early  cell  divisions  by  which  a  many-celled  embryo  is  formed  are 
called  cleavage.  The  pattern  of  cell  division  is  determined  largely  by 
the  amount  of  yolk  present  in  the  egg.  An  isolecithal  egg  has  a  relatively 
small  amount  of  yolk  distributed  more  or  less  evenly  throughout  the 
cytoplasm.  Telolecithal  eggs  have  a  large  amount  of  yolk  which  is  more 
concentrated  at  the  lower  or  vegetal  pole  of  the  egg;  the  active  cyto- 
plasm is  concentrated  at  the  upper  or  animal  pole.  The  frog  egg  is 
about  half  yolk  and  a  bird  egg  is  more  than  95  per  cent  yolk;  the 
cytoplasm  of  the  latter  is  restricted  to  a  small  disc  at  the  animal  pole. 
The  insect  egg  is  an  example  of  a  centroleclthal  one;  the  yolk  accumu- 
lates in  the  center  of  the  egg  and  is  surrounded  by  a  thin  layer  of  cyto- 
plasm. 

The  line  of  the  first  division  in  the  cleavage  of  an  isolecithal  egg 
passes  through  the  animal  and  vegetal  poles  of  the  egg  and  forms  two 
equal  cells,  called  blastomeres  (Fig-  6.7).  The  second  cleavage  division 
passes  through  animal  and  vegetal  poles  at  right  angles  to  the  first  and 
divides  the  two  cells  into  four.  The  third  cleavage  division  is  horizontal. 
Its  plane  is  at  right  angles  to  the  planes  of  the  first  two  divisions,  and 
the  embryo  is  split  into  four  cells  above  and  four  below  this  line  of 
cleavage.  Further  divisions  result  in  embryos  containing  16,  32,  64,  128 
cells  and  so  on  until  a  hollow  ball  of  cells,  the  blastula,  results.  The 
wall  of  the  blastula  consists  of  a  single  layer  of  cells  and  the  cavity  in 
the  center  of  the  sphere,  filled  with  fluid,  is  called  the  blastocoele.  Each 
of  the  cells  in  the  blastula  is  small,  and  the  total  mass  of  the  blastula 
is  less  than  that  of  the  original  fertilized  egg,  for  some  of  the  stored 
food  was  used  up  in  the  cleavage  process. 

The  single-layered  blastula  is  soon  converted  into  a  double-layered 
sphere,  a  gastrula,  by  the  process  of  gastrulation.  In  isolecithal  eggs, 
gastrulation  occurs  by  the  pushing  in  (invagination)  of  a  section  of  one 
wall  of  the  blastula  (Fig.  6.8).  This  pushed-in  wall  eventually  meets  the 
opposite  wall  and  the  original  blastocoele  is  obliterated.  The  new 
cavity  of  the  gastrula  is  the  archenteron  (primitive  gut),  the  rudiment  of 
the  digestive  system.  The  opening  of  the  archenteron  to  the  outside  is 
the  blastopore,   which  marks   the  site   of   the  invagination  which   pro- 


REPRODUCTION 


127 


AMPHIOXUS 


AMPHIBIAN 


Figure  6  7  Stages  in  cleavage  and  early  gastrulation  in  eggs  of  chordates.  A,  Am- 
phioxus  (holoblastic  cleavage,  isolecithal  egg  with  little  yolk).  B,  Frog  (holoblastic 
cleavage  moderately  telolecithal  egg  with  much  yolk).  C,  Bird  (meroblastic  discoidal 
cleavage,  telolecithal  egg  with  much  yolk).  D,  Mammal  (holoblastic  cleavage,  isolecithal 
egg  wi^h  essentially  no  yolk).  (From  Storer  and  Usinger:  General  Zoology,  3rd  Ed.  Copy- 
right 1957  by  McGraw-Hill  Book  Co.) 


128  GENERAL   CONCEPTS 

Ectode-rxn-i 


Entoderm 


Ectoderm 


Archente-ron 


Archenteron- 


Caudal 


Entoderm/'        ""Bla-stopore 
•Weural  plate-       rCoe.lomic  pouch. 


^Ectode-rni' 
^Entoderm'' 


Keural  oroo\^e 


Nofco chord 


Gut 


Archenteron^ 

Neural  tube 


Somite 


Somatopleure 
Myotome 
iplanclinopleure 


Figure  6.8.     Stages  in  gastrulation  and  mesoderm  formation  in  Amphioxus.  Note 
that  the  mesoderm  forms  by  the  budding  of  pouches  from  the  archenteron. 

duced  gastrulation.  The  outer  of  the  two  walls  of  the  gastrula  is  the 
ectoderm,  which  eventually  forms  the  skin  and  nervous  system.  The 
inner  layer,  lining  the  archenteron,  is  the  endoderm,  which  will  form  the 
digestive  tract,  liver,  pancreas  and  lungs. 

Cleavage  and  gastrulation  are  markedly  modified  in  telolecithal 
eggs  by  the  presence  of  the  large  amount  of  yolk.  In  the  frog  egg,  which 
may  be  called  moderately  telolecithal,  the  cleavage  divisions  in  the 
lower  part  of  the  egg  are  slowed  by  the  presence  of  the  inert  yolk.  The 
resulting  blastula  consists  of  many  small  cells  at  the  animal  pole  and  a 
few  large  cells  at  the  vegetal  pole.  The  lower  wall  of  the  blastula  is 
much  thicker  than  the  upper  one  and  the  blastocoele  is  flattened  and 
displaced  upward.  Only  the  small  disc  of  cytoplasm  at  the  animal  pole 
of  the  hen's  egg  undergoes  cleavage  divisions;  the  lower,  yolk-filled  part 
of  the  egg  never  cleaves.  As  a  result,  the  blastocoele  is  simply  a  shallow 
cavity  under  the  dividing  cells.  Gastrulation  occurs  in  both  frog  and 
chick  egg,  and  an  archenteron  is  formed,  but  the  process  is  greatly 
modified  by  the  presence  of  the  yolk.  Gastrulation  in  the  frog  involves  an 


REPRODUCTION 


129 


invagination  of  the  yolk-filled  cells  of  the  vegetal  pole,  a  turning  in  of 
cells  at  the  dorsal  lip  of  the  blastopore  (involution),  and  a  growth  of 
ectoderm  down  and  over  the  cells  of  the  vegetal  pole  (epiboly)    (Fig.  6.9). 

In  all  multicellular  animals,  except  sponges  and  coelenterates, 
which  never  develop  beyond  the  gastrula  stage,  a  third  layer  of  cells,  the 
mesoderm,  develops  between  ectoderm  and  endoderm.  In  annelids, 
molluscs  and  certain  other  invertebrates,  the  mesoderm  develops  from 
special  cells  which  are  differentiated  early  in  cleavage  (p.  237).  These 
migrate  to  the  interior  and  come  to  lie  bet^veen  the  ectoderm  and  endo- 
derm. They  then  multiply  to  form  two  longitudinal  cords  of  cells  which 
develop  into  sheets  of  mesoderm  between  the  ectoderm  and  endoderm. 
The  coelomic  cavity  originates  by  the  splitting  of  the  sheets  to  form 
pockets,  and  hence  is  called  a  schizocoeie. 

In  primitive  chordates  the  mesoderm  arises  as  a  series  of  bilateral 
pouches  from  the  endoderm  (Fig.  6.8).  These  lose  their  connection  with 
the  gut  and  fuse  one  with  another  to  form  a  connected  layer.  The 
cavity  of  the  pouches  is  retained  as  the  coelom,  which  is  called  an 
enterocoeie  because  it  is  derived  indirectly  from  the  archenteron.  The 
mesoderm  in  amphibia  is  formed  in  part  from  the  endoderm  of  the 
roof  of  the  archenteron  and  in  part  from  the  ectoderm  and  endoderm 
at  the  dorsal  lip  of  the  blastopore  (Fig.  6.9).  In  birds  and  mammals  the 
primitive  streak  which  develops  on  the  surface  of  the  developing  embryo 
is  homologous  to  the  dorsal  lip  of  the  blastopore  of  lower  forms.  It  is  a 
thickened  band  of  ectoderm  and  endoderm  cells  which  marks  the  lon- 
gitudinal axis  of  the  embryo.  At  the  primitive  streak  cells  migrate  in 
from  the  surface,  proliferate,  and  form  a  sheet  of  mesoderm  between 
ectoderm  and  endoderm. 


■Bla-stocoele." 


Archenteroii- 


■DT^^i. .... ^*-"'j"^-»'  I  Involution 

^    Blastopore -^j^^^^.^^^.^^    ^    ^YolKplug        ^ 


Notochord- 

Ectod(z.rm- 
Me-soderrtr 


N^ural  plate 
N<Lural  fold 


Entoderm 


E 

Figure   6.9.     A-D,   Successive   stages   in   gastrulation    and   mesoderm   formation   in 
Amphibia.  E,  Transverse  section  of  an  early  neurula  stage. 


130 


GENERAL   CONCEPTS 


However  the  mesoderm  may  originate,  it  typically  forms  two  sheets 
which  grow  laterally  and  anteriorly  between  the  ectoderm  and  endo- 
derm;  one  sheet  becomes  attached  to  the  inner  endoderm  and  the  other 
to  the  outer  ectoderm.  The  cavity  between  the  two  becomes  the  coelom, 
or  body  cavity.  The  layer  ot  mesoderm  associated  with  the  endoderm 
forms  the  muscles  of  the  digestive  tract. 

The  primitive  skeleton  of  the  chordates  is  the  notochord,  a  flexible, 
unsegmented,  longitudinal  rod  which  occurs  in  the  dorsal  midline  of 
all  chordate  embryos.  It  is  formed  at  the  same  time  and  in  a  similar 
way  as  the  mesoderm— as  an  outgrowth  of  the  roof  of  the  archenteron, 
from  the  dorsal  lip  of  the  blastopore,  or  from  the  primitive  streak.  Later 
in  the  development  of  vertebrates  the  notochord  is  replaced  by  the  ver- 
tebral column,  derived  from  part  of  the  mesoderm. 

The  nervous  system  of  chordates  is  derived  from  the  ectoderm  over- 
lying the  notochord.  This  first  forms  a  thickened  plate  of  cells,  the  neural 
plate;  the  center  of  the  plate  becomes  depressed  while  the  lateral  edges 
rise  as  two  longitudinal  neural  folds.  The  folds  eventually  meet  dorsally 
and  form  a  hollow  neural  tube.  The  cavity  of  the  tube  becomes  the  cen- 
tral canal  of  the  spinal  cord  and  the  ventricles  of  the  brain. 

The  sheets  of  mesoderm  grow  ventrally  and  the  ones  from  either 
side  meet  in  the  ventral  midline;  the  coelomic  cavities  on  the  two  sides 
then  fuse  into  one.  The  mesoderm  grows  dorsally  along  each  side  of  the 
notochord  and  neural  tube  and  becomes  differentiated  into  segmental 
blocks  of  tissue,  the  somites,  from  which  the  main  muscles  of  the  trunk 
develop.  Other  mesodermal  cells  become  detached  from  the  inner  border 
of  the  somites,  migrate  inward,  surround  the  notochord  and  neural  tube, 
and  develop  into  the  vertebrae.  The  kidneys  and  their  ducts,  and  the 
gonads  and  their  ducts,  are  derived  from  the  mesoderm  originally  located 
between  the  somites  and  the  coelom. 

The  contributions  of  each  germ  layer  to  the  development  of  a  typical 
mammal  are  summarized  in  the  following  table. 


ECTODERM 

Epidermis  of  the  skin 
Hair  and  nails 
Sweat  glands 

Brain,  spinal  cord,  ganglia, 

nerves 
Receptor     cells     of    sense 

organs 
Lens  of  the  eye 

Lining  of  mouth,  nostrils 

and  anus 
Enamel  of  teeth 


ENDODERM 

Lining  of  gut 

Lining  of  trachea,  bronchi 

and  lungs 
Liver 

Pancreas 

Lining  of  gallbladder 

Thyroid,  parathyroid  and 

thymus  glands 
Urinary  bladder 

Lining  of  urethra 


MESODERM 

Muscles — ^smooth,    skeletal 

and  cardiac 
Dermis  of  skin 

Connective     tissue,     bone 

and  cartilage 
Dentin  of  teeth 

Blood  and  blood  vessels 

Mesenteries 

Kidneys 

Testes  and  ovaries 


The  details  of  vertebrate  development  will  be  given  in  Chapter  31. 


REPRODUCTION  131 

38.        Protection  of  the  Embryo 

The  egg  and  the  developing  embryo  are  in  general  very  susceptible 
to  unfavorable  environmental  conditions  and  a  variety  of  adaptations 
have  evolved  in  invertebrates  and  vertebrates  to  tide  the  embryo  over 
this  critical  period. 

The  eggs  of  many  parasitic  worms  are  covered  with  shells  which 
enable  them  to  survive  exposure  to  heat,  cold,  desiccation  and  digestive 
juices.  The  skate  egg  is  covered  by  a  tough,  leathery  case  that  protects 
the  developing  embryo  within.  The  eggs  of  most  fish  and  amphibia  are 
surrounded  by  a  jelly  coat  which  is  of  some  value  in  protecting  against 
mechanical  shock.  The  eggs  of  reptiles  and  birds  are  protected  by  tough 
leathery  or  calcareous  shells.  The  developing  chick  embryo  "breathes," 
takes  in  oxygen  and  gives  off  carbon  dioxide,  through  its  shell. 

The  eggs  of  fish  and  amphibia  are  fairly  large  and  contain  yolk 
which  supplies  the  nutrients  for  the  developing  embryo.  These  eggs  are 
laid  and  typically  develop  in  water,  whence  the  oxygen,  salts  and  water 
required  for  development  are  obtained.  The  embryos  develop  a  pouch- 
like outgrowth  of  the  digestive  tract,  the  yolk  sac,  which  grows  around 
the  yolk,  elaborates  enzymes  to  digest  it,  and  transports  the  products  in 
its  blood  vessels  to  the  rest  of  the  embryo. 

The  eggs  of  reptiles  and  birds  develop  on  land  rather  than  in  water, 
and  further  adaptations  were  required  to  permit  development  in  the 
absence  of  the  large  body  of  water.  These  forms  have  three  additional 
membranes,  the  amnion,  chorion  and  allantois,  which  are  sheets  of  liv- 
ing tissue  that  grow  out  of  the  embryo  itself.  The  amnion  and  chorion 
develop  as  folds  of  the  body  wall  and  surround  the  embryo;  the  allantois 
grows  out  of  the  digestive  tract  and  functions  along  with  the  yolk  sac 
in  nutrition,  excretion  and  respiration.  Each  of  these  membranes  is  com- 
posed of  two  germ  layers  in  close  apposition  (Fig.  6.10). 

The  formation  of  the  amnion  is  a  complex  process  and  its  details 
differ  in  different  animals.  A  bilateral,  double-walled  outfolding  of  the 
body  wall  of  the  embryo  grows  upward  and  medially  to  surround  the 
embryo  and  fuse  above  it,  enclosing  a  space,  the  amniotic  cavity,  be- 
tween itself  and  the  embryo.  This  is  filled  with  a  clear,  watery  fluid 
secreted  in  part  by  the  embryo  and  in  part  by  the  amnion.  The  amnion 
develops  from  the  inner  part  of  the  original  fold;  the  outer  part  be- 
comes the  second  fetal  membrane,  the  chorion,  which  lies  outside  of 
and  surrounds  the  amnion.  The  chorionic  cavity,  also  known  as  the 
extraembryonic  coelom  (for  the  space  is  continuous  with  the  coelomic 
cavity  within  the  embryo),  is  the  space  between  the  amnion  and  chorion. 
The  embryos  of  reptiles,  birds  and  mammals  develop  in  the  liquid-filled 
amniotic  cavity,  their  own  private  pond  within  the  shell  or  uterus.  This 
arrangement  permits  the  embryo  to  move  around  to  some  extent  but 
protects  it  from  bumps  and  shocks.  The  chorion  of  reptiles  and  birds 
comes  to  lie  next  to  the  shell  and  that  of  mammals  is  in  contact  with 
the  maternal  tissues  of  the  uterus.  The  allantois  is  an  outgrowth  of  the 
digestive  tract  which  grows  between  the  amnion  and  chorion  and  largely 
fills  the  chorionic  cavity.  The  allantois  of  the  bird  and  reptile  typically 


132 


GENERAL   CONCEPTS 


B-mh-r^jonic 
a.rea. 


Ectoderm 
Enloderm 


Amniotic 
folds 


Amniotic  cavity 


D 


Chorion 
Ante-rior  end 

Amnion 
YolK  sac 


Primitive 
di^e.  stive 
tract 

Posterior  end 


Alla.ntois 


Figure  6.1 0.  A-E,  Steps  in  the  formation  of  the  extraembryonic  membranes— amnion, 
chorion,  yolk  sac  and  allantois— in  a  typical  mammal  such  as  a  pig.  Arrows  indicate 
direction  of  growth  and  folding. 

fuses  with  the  chorion  to  form  a  compound  membrane,  equipped  with 
many  blood  vessels,  by  means  of  which  the  embryo  takes  in  oxygen,  gives 
off  carbon  dioxide  and  excretes  certain  wastes. 

The  mammalian  allantois  is  usually  small  and  has  no  function  as  a 
membrane,  but  supplies  blood  vessels  to  the  placenta,  the  organ  formed 
from  chorion,  allantois  and  maternal  tissue.  Finger-like  projections,  or 
villi,  of  the  chorion  grow  into  and  become  embedded  in  the  lining  of  the 
uterus.  These  villi,  their  blood  vessels,  and  the  uterine  tissues  with  which 
they  are  in  contact,  are  called  the  placenta.  This  organ,  in  which  the 
fetal  blood  vessels  come  in  close  contact  with  the  maternal  blood  vessels, 
provides  the  developing  mammalian  fetus  with  nutrients  and  oxygen 
from  the  maternal  blood,  and  eliminates  carbon  dioxide  and  waste 
products  into  the  mother's  blood.  The  two  blood  streams  do  not  mix  at 


REPRODUCTION  133 

all;  they  are  always  separated  by  one  or  more  tissues.  However,  sub- 
stances can  diffuse,  or  be  transported  by  some  active  process,  from  mother 
to  fetus  or  the  reverse. 

The  form  of  the  placenta,  and  the  intimacy  of  the  connection  be- 
tween maternal  and  fetal  tissues,  varies  from  one  mammal  to  another. 
The  placenta  of  the  pig  or  cow  has  scattered  villi  over  the  chorionic 
surface  and  is  said  to  be  diffuse.  The  chorionic  villi  of  the  placenta  of 
carnivores  occur  in  a  cylindrical  band  around  the  chorion;  this  is  known 
as  a  zonary  placenta.  The  primate  and  rodent  placenta  is  disc-shaped 
and  is  called  a  discoidal  placenta.  The  number  of  layers  of  tissues  that 
intervene  between  maternal  and  fetal  blood  vessels  varies  from  2  in  the 
man  and  rat  to  6  in  the  sheep. 

The  giowth  of  the  embryo  and  of  the  amnion  brings  the  edges  of 
the  amniotic  folds  together  to  form  a  tube  which  encloses  the  yolk  sac 
(which  is  usually  small  or  vestigial),  the  allantois,  the  two  umbilical 
arteries  and  the  umbilical  vein  which  pass  to  the  placenta.  This  tube, 
the  umbilical  cord,  is  composed  of  a  peculiar,  jelly-like  material  which 
is  unique  to  the  cord. 

The  anmion,  chorion  and  allantois,  together  with  the  egg  shell  or 
placenta,  are  adaptations  which  permit  the  embryos  of  the  higher  verte- 
brates to  develop  on  land;  they  are  a  substitute  for  the  pond  or  sea  water 
in  which  the  embryos  of  the  lower  vertebrates  develop. 

39.       The  Control  of  Development 

Biologists  have  been  interested  for  many  years  in  the  nature  of  the 
factors  which  regulate  the  complex,  orderly  processes  leading  to  the 
production  of  a  new  adult  from  a  fertilized  egg.  How  can  a  single  cell 
give  rise  to  many  different  types  of  cells,  which  differ  widely  in  their 
morphologic,  functional  and  chemical  properties? 

Early  embryologists  believed  that  the  egg  or  the  sperm  contained  a 
completely  formed  but  minute  germ  which  simply  grew  and  expanded 
to  give  the  adult.  This  preformation  theory  explained  development  by 
denying  that  it  occurred!  An  extension  of  this  theory  postulated  that 
each  germ  contained  within  it  the  germs  for  all  succeeding  generations, 
each  within  the  next.  Some  microscopists  reported  seeing  this  germ 
within  the  sperm  or  egg  and  described  the  "homunculus,"  a  fully  formed 
little  man  inside  the  egg  or  sperm!  Others  calculated  the  number  of 
germs  that  were  present  in  the  ovaries  of  Eve,  the  mother  of  the  human 
race,  and  suggested  that  when  all  of  these  were  used  up  the  human  race 
would  end. 

The  contrasting  theory  of  epigenesis,  first  advanced  by  Wolff  in  1759, 
stated  that  the  unfertilized  egg  is  not  organized  and  that  development 
involves  progressive  differentiation  which  is  controlled  by  some  outside 
force.  VV^e  now  know  that  development  is  not  simply  epigenetic,  for  there 
are  certain  potentialities  localized  in  particular  regions  of  the  egg  and 
the  early  embryo.  The  embryos  of  certain  species,  when  separated  into 
parts  at  an  early  stage,  will  develop  normally;  each  part  forms  a  com- 
plete, normal,  though  small,  embryo.  The  embryos  of  other  species  show 


134  GENERAL   CONCEPTS 

Epidermis-]      pMearal 


Liiie  of 
inva^inatic 


EctoderiTT.- 


Mesoderm. 
Endoderni 


rNeui^al  area. 
■Wotocliord 

mesodenn 


A 


dorsal  lip 
blastopore- 


B 


Somite 
mesoderm 

iTiesoderm 


Figure  6.11.  Embryo  maps.  A,  Lateral  view  of  a  frog  gastrula  showing  the  pre- 
sumpti\e  fates  of  its  se\eral  regions.  H,  Top  \iew  of  a  chick  embryo  showing  location  in 
the  primitive  streak  stage  of  the  cells  which  will  form  particular  structures  of  the  adult. 

that  certain  potentialities  are  localized  at  an  early  stage,  for  neither  part 
can  develop  into  a  whole  embryo.  Each  halt  develops  only  those  struc- 
tures it  would  have  formed  normally  as  part  of  the  whole  embryo.  This 
localization  of  potentialities  eventually  occurs  in  the  development  of  all 
eggs;  it  simply  occurs  at  an  earlier  stage  in  some  species  than  in  others. 
It  has  been  possible  by  experimental  techniques  to  map  out  the  areas  of 
potentialities  in  the  early  amphibian  gastrula  and  in  the  primitive  streak 
stage  of  the  chick  (Fig.  6.11). 

In  the  past,  biologists  have  speculated  that  differentiation  might 
occur  (1)  by  some  sort  of  segregation  of  properties  during  mitosis,  (2) 
by  the  establishment  of  chemical  gradients  within  the  developing  em- 
bryo, (3)  by  somatic  mutations,  or  (4)  by  the  action  of  chemical  organ- 
izers. Recently  the  induction  of  adaptive  enzymes  in  bacteria  has  been 
used  as  a  model  system  to  provide  another  explanation  for  embryonic 
differentiation.  Experiments  have  shown  that  bacteria  (and,  to  some 
extent,  animals  as  well)  can  respond  to  the  presence  of  some  new  sub- 
strate molecule  by  forming  enzymes  which  will  metabolize  it.  Jacques 
Monod,  of  the  University  of  Paris,  has  suggested  that  in  an  analogous 
fashion,  extracellular  or  intracellular  influences  may  initiate  or  suppress 
the  synthesis  of  specific  enzymes,  thus  affecting  the  chemical  constitution 
of  the  cell  and  leading  to  differentiation.  The  enzyme  complement  of  a 
cell  is,  to  some  extent,  plastic,  and  can  be  changed  by  extra-  or  intra- 
cellular influences.  As  an  embryo  develops,  the  gradients  established  as  a 
result  of  growth  and  cell  multiplication  could  result  in  quantitative  and 
even  qualitative  differences  in  enzymes.  As  a  result  of  the  stimulation  or 
inhibition  of  one  enzyme,  a  chemical  product  could  accumulate  which 
would  induce  the  synthesis  of  a  new  enzyme  and  thus  confer  a  new 
functional  activity  on  these  cells. 

Morphogenesis  is  probably  too  complicated  a  phenomenon  to  be 
explained  in  terms  of  a  single  phenomenon  such  as  enzyme  induction. 
Enzymes  can  indeed  be  induced  in  an  embryo  by  the  injection  of  a  suit- 
able substance.  Adenosine  deaminase,  for  example,  has  been  induced  by 


'  REPRODUCTION  135 

the  injection  of  adenosine  into  a  chick  egg,  but  to  date  no  enzyme  has 
been  induced  which  is  not  normally  present  to  some  extent  in  the  em- 
bryo. Adult  tissues  show  marked  differences  in  their  enzymatic  activities, 
differences  which  might  be  the  result  of  "adaptations"  comparable  to 
those  seen  in  bacteria.  Adaptive  changes  in  enzymes,  however,  are  tem- 
porary and  reversible,  whereas  differentiation  is  a  permanent,  irreversible 
process.  Cells  may  lose  some  of  their  morphologic  characteristics  but  they 
retain  all  of  their  biochemical  specificities. 

Some  interesting  data  bearing  on  the  problem  of  morphogenesis  have 
been  obtained  recently  by  Briggs  and  King  of  the  Lankenau  Institute. 
They  have  been  able  to  transplant  a  nucleus  from  one  of  the  cells  of  an 
early  blastula  of  a  frog  into  an  enucleated  egg.  This  egg  will  subse- 
quently cleave,  gastrulate,  and  develop  normally.  However,  if  a  nucleus 
is  taken  from  a  cell  of  the  late  gastrula— from  a  chorda-mesoderm  or 
midgut  cell— and  transplanted  into  an  enucleated  egg,  abnormal  develop- 
ment results.  Development  is  arrested  in  the  blastula  or  gastrula  stage. 
Transplanted  chorda-mesoderm  nuclei  result  in  embryos  with  deficient 
or  absent  nervous  systems  and  transplanted  midgut  nuclei  form  embryos 
with  thin  or  absent  epidermis  and  no  nervous  system  (Fig.  6.12).  These 
experiments  indicate  some  change  in  the  intrinsic  differentiative  proper- 
ties of  the  nuclei  as  cleavage  and  development  proceed.  Nuclei  taken 
from  even  later  stages  in  development  cannot  function  in  cleavage;  an 
enucleated  egg  receiving  such  a  nucleus  does  not  develop  at  all.  The 
nature  of  this  nuclear  specialization  is  unknown,  but  the  loss  of  differ- 
entiative potentialities  bears  some  relationship  to  the  site  of  the  embryo 
from  which  the  nucleus  was  derived. 

Evidence  of  a  different  type  of  differentiation  mechanism  has  been 
obtained  from  experiments  in  which  microsurgical  instruments  are  used 
to  cut  out  a  bit  of  tissue  from  one  embryo  and  transplant  it  to  another. 
For  example,  when  a  piece  of  the  dorsal  lip  of  the  blastopore  of  a  frog 
gastrula  is  implanted  beneath  the  ectoderm  of  a  second  gastrula,  the 
tissue  heals  in  place  and  causes  the  development  of  a  second  brain, 
spinal  cord  and  other  parts  at  the  site,  so  that  a  double  embryo  or  closely 
joined  Siamese  twins  results  (Fig.  6.13).  Many  tissues  show  similar  abili- 
ties to  organize  the  development  of  an  adjoining  structure.  The  eyecup 
will  initiate  the  formation  of  a  lens  from  overlying  ectoderm  even  if  it  is 
transplanted  to  the  belly  region,  where  the  cells  would  normally  form 
belly  epidermis.  Such  experiments  indicate  that  development  is  a  co- 
ordinated series  of  chemical  stimuli  and  responses,  each  step  regularly 
determining  the  succeeding  one.  The  term  "organizer"  is  applied  to  the 
region  of  the  embryo  with  this  property  and  also  to  the  chemical  sub- 
stance given  off  by  that  region  which  passes  to  the  adjoining  tissue  and 
directs  its  development.  There  is  evidence  which  suggests  that  organizers 
are  nucleoproteins. 

It  had  been  widely  accepted  that  organizers  can  transmit  their 
inductive  stimuli  only  when  in  direct  physical  contact  with  the  reactive 
cells.  However,  evidence  from  experiments  by  Victor  Twitty  of  Stanford 
indicates  that  induction  can  occur  by  diffusible  substances  which  are 
capable  of  affecting  the  induction  of  a  second  tissue  at  a  distance.  Twitty 


136 


GENERAL  CONCEPTS 


Nuclcu-S  removed  a.nd- 
implantcd  via.  micropipebtc 


Blastulsi- 


Enuclealed  egg 


Nucleus  of 
chorda_me.sod.crm 
cell  impla.nte.d 


La.te  oa-st-rula. 


En.uclea.ted  e^^ 


Normal  embryo 


Development  s:opS 
in  blaslixla.  cr 
ga-strula.  sta-ge; 
abnoT-mal  embryo 
U/ith.  def  iciei^t 
nervouLS   syslem. 


Deuelop'm.cn.t  stops 
in  blastula.  ot 
ga-st-rula.   stage-, 
abnoTr^rrLaJ-  cmbrj^O 
with.  deficie.n.t 
<^pidermis  and 
nervous  system. 

Figure  6.12.     Diagram  of  experiments  with  the  transplantation  of  nuclei  to  enu- 
cleated eggs.  See  text  for  discussion. 


grew  small  groups  oi  frog  ectoderm,  mesoderm  and  endoderm  cells  in 
tissue  culture  and  found  that  ectoderm  alone  would  never  differentiate 
into  nerve  tissue.  Ectoderm  cells  placed  in  a  medium  in  which  mesoderm 
cells  had  been  grown  for  the  previous  week,  did  differentiate  into 
chroma tophores  and  nerve  fibers.  No  comparable  differentiation  occurred 
when  the  ectoderm  cells  were  placed  in  comparable  cultures  of  endoderm 
cells.  Twitty  concluded  that  inductor  tissues,  such  as  chorda-mesoderm, 
contain  and  release  diffusible  substances  which  are  capable  of  operating 
at  a  distance  and  inducing  the  differentiation  of  ectoderm.  This  sub- 
stance has  been  tentatively  identified  as  nucleoprotein. 

Evidence  that  steroids,  as  well  as  nucleoproteins,  may  play  a  morpho- 
genetic  role  in  development  has  been  obtained  by  Dorothy  Price,  of  the 


REPRODUCTION 


137 


University  of  Chicago.  When  the  reproductive  tract  of  a  fetal  rat  is 
dissected  out  and  grown  in  tissue  culture,  development  occurs  normally 
if  the  testis  or  ovary  is  left  in  place.  If  both  testes  are  removed,  there  is 
no  development  and  differentiation  of  the  accessory  organs— vas  deferens, 
seminal  vesicles  and  prostate  gland.  However,  if  both  testes  are  removed 
and  a  pellet  of  testosterone,  the  male  sex  hormone,  is  implanted,  de- 
velopment proceeds  normally.  This  shows  that  testosterone  is  a  morpho- 
genetic  substance  which  can  diffuse  across  a  limited  space  and  induce 
the  development  of  male  characters. 

Evidence  that  chemical  differentiation  precedes  morphologic  differ- 
entiation of  a  tissue  has  come  from  research  using  serologic  and  bio- 
chemical methods.  The  specific  protein  of  the  lens  of  the  eye  can  be 
detected  serologically  in  the  chick  embryo  before  the  lens  vesicle  closes 
and  before  there  is  any  evidence  of  morphologic  differentiation.  Cholin- 
esterase  is  the  enzyme  which  hydrolyzes  acetylcholine  and  is  believed  to 
play  an  important  role  in  the  transmission  of  the  nerve  impulse.  Edgar 
Boell  of  Yale  has  shown  by  microchemical  methods  that  the  neural  folds 
of  the  frog  embryo,  the  parts  which  will  form  the  central  nervous  system, 
have  much  more  cholinesterase  than  the  epidermis  does.  ^Vhen  epidermis 
is  stimulated  to  form  nervous  system,  by  grafting  a  piece  of  chorda- 
mesoderm  beneath  it,  the  tissue  becomes  rich  in  cholinesterase. 

In  view  of  the  extreme  complexity  of  the  developmental  process  it 
is  indeed  remarkable  that  it  occurs  so  regularly  and  that  so  few  mal- 
formations occur.  About  one  child  in  one  hundred  is  born  with  some 
major  defect,  a  cleft  palate,  club  foot,  sjjina  bifida  or  the  like.  Some  of 
these  are  inherited  and  others  result  from  environmental  factors.  Experi- 
ments with  fruit  flies,  frogs  and  mice  have  shown  that  x-rays,  ultraviolet 
rays,  temperature  changes  and  a  variety  of  chemical  substances  will 
induce  alterations  in  development.  The  kind  of  defect  produced  is  a 
function  of  the  time  in  the  course  of  development  at  which  the  environ- 
mental agent  is  applied,  and  does  not  depend  to  any  great  extent  on  the 
kind  of  agent  used.  For  example,  x-rays,  the  administration  of  cortisone 


Neural  folds 
Primary  embryo 


NcuraJ. 
Polds 


Neural  -fcuic- 
Optic  ves^el- 


Ta.ii  (Znd — ' 


O  to  cyst 


-Scconda-iy 

zmbr yo 


Somites  of 
secondcLry  <Z-3xihryo- 

D 


A  B  C 

Figure  6.13.  The  induction  of  a  second  frog  embryo  by  the  implantation  of  the 
dorsal  lip  of  the  blastopore  from  embryo  A  onto  the  belly  region  of  embryo  B.  Embryo  B 
then  develops  through  stage  C  to  a  double  embryo,  D. 


138  GENERAL   CONCEPTS 

and  the  lack  ol  oxygen  will  all  produce  similar  defects  in  mice  if  applied 
at  comjKirable  times  in  development.  Such  observations  have  led  to  the 
concept  of  critical  periods  in  development,  periods  in  which  the  develop- 
ment of  a  certain  (Mgan  or  organ  system  is  occurring  rapidly  and  hence 
is  most  susceptible  to  interference. 

The  property  of  reproduction,  which  we  regard  as  one  of  the  out- 
standing characteristics  of  living  things,  involves  a  great  many  complex 
and  interdependent  processes:  the  elaboration  of  hormones  which  regu- 
late the  development  of  gonads,  secondary  sex  structures  and  the  pro- 
duction of  gametes  in  the  parents;  behavior  patterns  which  bring  the 
parents  together  and  have  them  release  their  gametes  at  such  a  time  and 
in  such  a  place  as  to  make  their  fusion  probable;  the  union  of  male 
and  female  pronuclei  followed  by  cleavage,  gastrulation  and  morpho- 
genesis; and  devices  for  the  care  and  protection  of  the  developing  young. 
Our  descriptive  knowledge  of  these  phenomena  is  extensive  but  our 
understanding  of  the  fundamental  mechanisms  involved  in  each  of  these 
processes  is  rudimentary.  This  is  a  fertile  field  for  further  investigation. 

Questions 

1.  What  are  the  advantages  and  disadvantages  of  asexual  and  sexual  reproduction  in 
animals? 

2.  What  is  accomplished  by  the  process  of  meiosis? 

3.  Compare  mitosis  and  meiosis. 

4.  Contrast  spermatogenesis  and  oogenesis. 

5.  What  is  meant  by  the  terms  haploid,  diploid,  gamete,  zygote,  synapsis  and  tetrad? 

6.  Define  and  give  an  example  of  (a)  hermaphroditism  and  (b)  parthenogenesis. 

7.  What  is  accomplished  by  the  process  of  fertilization?  Contrast  external  fertilization 
with  internal  fertilization.  What  are  the  advantages  of  each? 

8.  Define  and  give  an  example  of  (a)  oviparous  and  (b)  viviparous  animals. 

9.  Discuss  the  effect  of  the  amount  and  distribution  of  the  yolk  on  (a)  the  cleavage 
pattern  of  the  egg  and  (b)  gastrulation  in  the  embryo. 

10.  Distinguish  between  an  enterocoele  and  a  schizocoele.  In  what  animals  are  these 
found? 

11.  What  is  the  evidence  that  the  primitive  streak  of  the  chick  is  homologous  to  the  dorsal 
lip  of  the  blastopore  of  the  frog? 

12.  Compare  the  adaptations  for  the  protection  and  nourishment  of  the  embryo  during 
development  in  the  shark,  frog,  chick  and  man. 

13.  Compare  the  current  theories  as  to  the  factors  which  regulate  development. 

14.  Define:  organizer,  chorda-mesoderm,  differentiation,  adaptive  enzyme. 

Supplementary  Reading 

An  interesting  account  of  mating  instincts  in  animals  is  presented  in  W.  P.  Pycraft's 
The  Courtship  of  Animals.  Research  on  the  dynamics  of  development  is  summarized  in 
Roberts  Rugh's  Experimental  Embryology  and  in  L.  G.  Earth's  Embryology.  Detailed 
accounts  of  vertebrate  development  are  found  in  B.  M.  Patten's  Embryology  of  the  Chick 
and  Embryology  of  the  Pig.  Comparative  vertebrate  embryology  is  well  presented  and 
illustrated  in  Emil  Witschi's  Development  of  Vertebrates. 


Part  II 


THE    ANIMAL    KINGDOM 


CHAPTER  7 


The  Principles  of  Taxonomy 


At  present  nearly  one  million  species  of  animals  have  been  identified. 
Probably  several  million  more  (mostly  very  small  organisms)  remain  to 
be  named.  Such  a  variety  makes  it  necessary  to  have  a  systematic  method 
for  naming  and  recording  what  is  already  known,  lest  species  be  named 
several  times  over,  or  the  same  name  be  assigned  to  different  species. 
The  accumulation  of  knowledge  recorded  in  an  unambiguous  fashion  is 
essential  to  scientific  progress.  Research  upon  an  organism  loses  value 
just  as  surely  if  the  author  fails  to  record  exactly  what  kind  of  animal 
he  studied,  as  if  he  fails  to  describe  adequately  his  experimental  methods 
or  results. 

40.        The  Science  of  Taxonomy 

The  proper  naming  of  organisms  (plant  and  animal)  is  the  province 
of  the  science  of  taxonomy.  Biological  literature  is  so  extensive  today 
that  only  an  expert  on  a  particular  group  of  species  can  hope  to  be 
informed  of  its  taxonomy.  If,  for  example,  a  zoologist  should  find  a 
population  of  lizards  that  were  new  to  him,  he  might  first  search  the 
literature  to  see  whether  or  not  his  lizards  were  already  named.  In  order 
to  do  this  he  would  have  to  be  familiar  with  the  details  of  the  many 
aspects  of  their  structure,  and  with  the  usage  of  the  descriptive  ter- 
minology employed,  so  that  he  could  compare  them  with  the  published 
descriptions.  If  he  failed  to  locate  any  description  that  fitted  his  lizards, 

139 


140  JH^   ANIMAL   KINGDOM 

he  might  describe  them  and  give  them  a  name.  This  description  would 
have  to  be  carelul  and  precise,  so  that  others  could  use  it.  If  this  zoologist 
were  not  a  specialist  on  li/ards,  he  probably  would  be  unable  to  make 
either  an  adequate  search  of  the  literature  or  a  proper  description  of  his 
new  animals,  and  would  turn  the  job  over  to  an  appropriate  specialist. 

41.        The  Binomial  System 

Although  the  beginning  student  cannot,  perhaps,  appreciate  the 
extreme  exactness  required  in  a  proper  description,  he  can  understand 
that  the  naming  of  a  new  species  must  follow  a  set  of  rules.  Scientific 
names  are  made  of  two  words,  the  name  of  the  genus,  a  group  containing 
several  closely  similar  kinds  of  organisms,  and  the  name  of  the  species, 
the  particular  kind  in  that  genus.  This  binomial  system  performs  a 
function  similar  to  that  of  naming  people,  in  which  the  use  of  both 
a  surname  and  a  given  name  facilitates  the  recording  and  cataloguing 
of  a  population.  The  generic  name  is  always  a  noun,  such  as  Canis  (dog, 
Perca  (perch)  or  Hymenolepis  (a  genus  of  tapeworms),  and  is  always 
capitalized.  The  specific  name  is  (in  zoology)  never  capitalized.  It  may 
be  an  adjective  (such  as  Wolf  gray),  a  noun  in  apposition  (such  as  Cat 
lion),  a  noun  in  the  genitive  (such  as  Mouse  of  California),  or  any  of 
several  other  possibilities,  always  of  course  in  Latin.  The  name  of  the 
species  serves  only  to  identify  the  particular  species  within  its  genus. 
Hence,  the  same  specific  name  may  appear  many  times  in  the  animal 
kingdom,  providing  each  time  it  is  in  a  different  genus  (Cylichna  alba, 
a  white  snail,  Fredericia  alba,  a  white  worm,  etc.). 

The  generic  name  may  be  used  only  once  in  the  entire  animal 
kingdom,  and  duplication  between  the  plant  and  animal  kingdoms  is 
discouraged.  To  facilitate  the  discovery  and  elimination  of  duplication, 
international  lists  of  genera  are  maintained.  When  an  instance  of  dupli- 
cation is  discovered,  the  earlier  usage  takes  precedence.  The  author  of 
the  second  usage  is  allowed  time  to  rename  his  genus,  but  if  he  fails  to 
do  this,  any  other  person  may  rename  it.  The  same  procedure  applies 
when  two  species  within  the  same  genus  receive  the  same  specific  name. 

When  a  single  species  has  been  named  more  than  once,  again  the 
earlier  publication  takes  precedence.  The  person  discovering  the  error 
establishes  the  first  published  name  as  the  valid  name,  and  places  the 
second  name  as  a  synonym  having  no  validity.  Synonyms  are  a  nuisance, 
since  papers  may  have  been  published  in  which  they  were  used  as  identi- 
fication. They  cannot  be  discarded,  nor  can  they  be  used  later  to  name 
new  species. 

These  are  just  a  few  of  the  rules  that  govern  the  system  of  naming. 
They  are  spelled  out  in  36  articles  in  the  International  Rules  of  Zoo- 
logical Nomenclature,  a  document  accepted  in  1901  by  the  Fifth  Inter- 
national Zoological  Congress.  The  system  is  administered  by  the  Inter- 
national Commission  on  Zoological  Nomenclature,  which  arbitrates 
disputes  that  arise  and  may  offer  interpretations  or  recommend  modi- 
fication of  the  rules  to  the  congress.  Adherence  to  the  system  is  entirely 
voluntary,  but  the  need  for  clarity  and  uniformity  is  so  obvious  that  no 


THE   PRINCIPLES   Of   MXONOAiY  141 

responsible  editor  in  any  country  of  the  world  would  knowingly  publish 
material  that  tailed  to  follow  these  rules. 

42.        Higher  Categories 

The  procedures  of  naming,  and  the  grouping  of  species  into  genera, 
are  but  a  part  of  the  subject  of  taxonomy  (literally,  the  law  of  arrange- 
ment). The  number  of  genera  is  large,  and  for  a  number  of  reasons 
which  will  become  apparent  it  is  advantageous  to  arrange  genera  into 
higher  groups,  and  these  into  still  higher  groups,  etc.  The  full  hierarchy 
of  groups  proceeds  from  the  kingdom,  the  largest  group,  through 
phylum,  class,  order  and  family  to  the  genus  and  species.  As  an  example, 
the  classification  (naming  of  all  the  grouping  levels)  for  the  tiger  is  as 
follows: 

Kingdom:  Animalin  (including  all  animals). 

Phylum:    Cliordata   (including   all   vertebrates,  sea  squirts,   Amphi- 
oxus,  etc.). 

Class:  Mammalia  (including  animals  that  give  milk). 

Order:   Carnivora  (including  bears,  dogs,  cats,  weasels,  otters,  seals, 
etc.). 

Family:  Felidae  (including  cats,  leopard,  lion,  jaguar,  etc.). 

Genus:  Patithera  (including  leopard,  lion  and  tiger). 

Species:  tigris  (the  tiger). 

Additional  categories  may  be  interpolated  by  the  use  of  prefixes. 
Thus,  the  phylum  Chordata  may  be  divided  first  into  several  subphyla, 
of  which  one  is  the  subphylum  Vertebrata,  and  the  family  Felidae  may 
be  combined  with  two  other  families  in  a  superfamily,  the  superfamily 
Feloidea. 

This  hierarchy  not  only  facilitates  reference  work  in  taxonomy,  but 
greatly  reduces  the  volume  of  descriptive  material  necessary  in  a  cat- 
alogue of  animals.  As  each  major  group  is  introduced,  all  the  characters 
common  to  the  members  of  the  group  can  be  stated  once;  they  need  not 
be  repeated  over  and  over  for  each  species.  The  characters  that  separate 
the  animals  from  the  plants  can  be  listed  under  the  kingdom  Animalia, 
and  apply  automatically  to  the  million  known  species  in  that  kingdom. 
Similarly,  chordate  characters  can  be  defined  once  for  60,000  species, 
mammalian  characters  for  7,000  species,  and  so  on.  At  each  lower  level 
of  the  hierarchy  only  those  additional  characters  common  to  all  the 
members  of  that  level  need  be  discussed.  When  the  species  level  is 
reached,  it  is  only  necessary  in  a  catalogue  to  give  the  distinguishing 
characters  of  each  species  in  the  genus.  The  catalogue  may  also  include 
for  each  species  a  reference  to  its  original  description,  which  would  con- 
tain additional  descriptive  information  and  which  may  indicate  why  the 
author  places  his  species  in  a  particular  genus  and  family. 

43.        Uses  of  Taxonomy 

A  good  taxonomic  system  has  several  uses.  It  serves  as  a  catalogue 
of  the  information   known    to   date,   it   makes   this  information   more 


142  ^WE   ANIMAL   KINGDOM 

readily  available,  and  it  provides  for  economy  in  the  length  of  descrip- 
tions. Hiese  are  practical  considerations,  and  were  the  motivating  forces 
behind  the  establishment  of  our  present  taxonomic  system,  which  de- 
veloped mostly  during  the  18th  and  19th  centuries. 

Since  the  middle  of  the  19th  century,  however,  taxonomy  has  had 
an  additional  and  equally  significant  role.  The  grouping  of  animals  is 
used  not  only  as  a  matter  of  convenience,  but  also  in  an  attempt  to 
indicate  the  degree  of  evolutionary  relationship  present.  Thus,  the 
species  of  one  genus  are  considered  to  be  more  closely  related  to  one 
another  than  to  the  species  of  other  genera,  and  to  have  evolved  from 
a  single  original  species.  Similarly,  the  genera  of  a  family  are  considered 
to  form  an  evolutionary  unit  as  well  as  a  taxonomic  unit,  and  so  on. 
Taxonomy  can  never  indicate  evolution  exactly,  since  of  necessity  the 
taxonomic  boundaries  between  groups  must  be  sharp  whereas  evolu- 
tionary relationships  form  something  closer  to  a  continuum.  Furthermore 
taxonomy  cannot  describe  the  time  dimension  involved  in  any  discussion 
of  evolutionary  paths.  Nonetheless  the  system  has  been  revised  continu- 
ally to  serve  as  well  as  possible  as  a  framework  from  which  evolutionary 
relationships  can  be  discussed.  The  analysis  of  evolutionary  relationships 
among  organisms  with  taxonomy  as  the  basic  tool  is  the  science  of 
systematics. 

Although  the  usages  of  genus  and  species  are  standardized  by  inter- 
national rules  and  official  lists  of  genera  are  maintained,  the  higher 
taxonomic  categories  are  less  well  regulated.  There  is  no  universal  agree- 
ment about  either  the  number  or  the  names  of  higher  categories.  Authors 
have  different  opinions,  depending  upon  their  conclusions  regarding 
evolutionary  relationship.  The  chordates,  for  example,  are  a  group  of 
animals  (including  the  vertebrates)  having  a  notochord  and  other  char- 
acters in  common,  and  are  a  basic  group  having  the  rank  of  a  phylum. 
In  some  organisms,  e.g.,  the  acorn  worms,  however,  the  existence  of  a 
notochord  is  debatable.  Students  who  believe  it  is  absent  place  such 
animals  in  a  separate  phylum  (Hemichordata  or  Enteropneusta)  from 
the  others  (Chordata)  while  those  who  believe  it  is  present  arrange  these 
animals  in  one  phylum  (Chordata).  Such  differences  of  opinion  persist, 
and  are  not  arbitrated  by  the  International  Commission. 

A  partial  classification  of  the  animal  kingdom  is  given  in  the  Ap- 
pendix. All  phyla  and  most  of  the  classes  are  included  (arranged  accord- 
ing to  the  views  of  these  authors).  Many  of  the  orders  of  common 
animals  are  given,  with  a  few  examples  of  each. 

44.        Definitions 

A  discussion  of  taxonomy  would  be  incomplete  without  definitions 
of  the  different  grouping  levels.  In  a  formal  sense  the  species  is  defined 
as  a  group  of  individuals  capable  of  interbreeding  under  natural  condi- 
tions and  reproductively  isolated  from  other  such  groups.  In  practice 
all  of  the  necessary  information  is  seldom  available,  and  the  species  is 
considered  to  be  a  group  of  individuals  that  could  fit  this  definition  and 
which  is  recognizable  as  a  distinct  group  by  some  dependable  criterion 


THE   PRINCIPLES    OF   TAXONOMY  143 

(usually  morphologic).  Conceptually  the  species  is  an  evolutionary  unit, 
regardless  of  the  method  by  which  actual  species  have  been  sorted  out. 

The  genus  is  defined  as  a  group  of  closely  related  species.  This  is 
not  as  satisfactory  a  definition  as  the  formal  definition  of  a  species,  since 
the  word  "closely"  involves  opinion.  Actually,  however,  this  does  not 
appear  to  be  a  serious  problem.  A  survey  of  the  genera  in  many  different 
taxonomic  groups  reveals  that  most  taxonomists  require  about  the  same 
degree  of  closeness  for  the  species  of  one  genus. 

The  family  is  composed  of  related  genera,  the  order  of  related 
families,  and  so  on.  Since  at  each  level  the  degree  of  closeness  must  be 
evaluated,  the  definitions  become  less  and  less  objective.  It  is  apparent 
from  comparisons  that  what  is  an  order  in  one  phylum  may  be  com- 
parable with  a  class  or  a  family  in  another. 

The  classification  system  becomes  more  objective  at  the  level  of  the 
phylum.  This  level  is  reached  more  directly,  of  course,  by  dividing  the 
animal  kingdom  into  a  number  of  basic  types.  The  phylum  has  been 
defined  as  an  assemblage  of  organisms  showing  some  degree  of  relation- 
ship among  themselves  and  expressing  as  a  whole  a  plan  of  existence  that 
is  unique,  fundamentally  different  from  that  of  all  other  organisms. 
Some  people  regard  the  phyla  as  unrelated,  and  therefore  as  objective 
a  category  as  the  species.  In  practice,  ho\sever,  many  of  the  phyla  show 
some  similarity  to  one  another,  and  a  value  judgment  is  still  involved, 
this  time  of  distance  rather  than  of  closeness.  Objectivity  at  the  phyletic 
level  should  be  about  the  same  as  that  at  the  generic  level. 

45.        The  History  of  Taxonomy 

The  development  of  our  taxonomic  system  is  one  of  the  more  excit- 
ing chapters  of  biological  history.  Taxonomy  was  started  by  the  Greeks 
and  Romans,  most  notably  by  Aristotle,  but  developed  very  little  for 
two  thousand  years,  until  the  end  of  the  17th  century. 

The  first  major  break  from  this  long  era  of  stagnation  is  found  in 
the  works  of  John  Ray.  Although  zoology  was  only  one  of  his  several 
interests,  between  1676  and  his  death  in  1705  he  produced  books  on 
birds,  fishes,  quadrupeds  and  insects.  Ray  introduced  a  more  complex 
grouping  system  than  had  been  used  before  and  improved  greatly  on 
the  language  of  description.  He  rejected  entirely  the  whole  mass  of 
superstition  and  medicinal  folklore  that  had  burdened  earlier  works. 
Ray  developed  the  key  by  which  students  can  identify  a  given  animal, 
using  only  a  few  distinguishing  characters.  He  also  promoted  the  con- 
cept of  the  genus  as  a  group  of  closely  similar  species  (without  the  added 
concept  of  evolutionary  relationship). 

The  work  of  John  Ray  opened  a  new  era.  Many  students  of  history 
give  him  major  credit  for  the  development  of  a  modern  system  of 
taxonomy.  It  remained  for  another,  however,  to  bring  the  new  approach 
into  sharp  focus  and  to  initiate  popular,  world-wide  activity  in  taxon- 
omy. Linnaeus  (Fig.  7.1)  was  the  first  taxonomist,  in  the  sense  that  tax- 
onomy was  his  career,  his  primary  activity.  Since  Linnaeus  made  notable 
contributions  to  the  taxonomic  system  and  since  his  work  is  enormous, 


144  '■"f   ANIMAL   KINGDOM 


Figure  7.1.  Karl  Linnaeus  (1707-1778),  the  father  of  modern  taxonomy.  In  his  day 
even  the  author's  name  was  published  in  Latin,  so  that  his  name  is  more  frequently  seen 
as  Carolus  Linnaeus.  His  father  was  born  before  surnames  were  common,  and  adopted 
Linnaeus  for  himself  and  his  family.  Karl  was  establishing  binomial  nomenclature  for 
the  natural  world  at  the  same  time  that  surnames  were  being  required  by  law  in  Europe. 

dwarfing  that  of  all  of  his  predecessors,  Linnaeus,  not  Ray,  is  usually 
called  the  "father"  of  taxonomy. 

The  unique  aspect  of  Linnaeus  was  his  motivation.  He  wished  to 
name  and  catalogue  all  the  objects  of  nature,  not  as  a  tool  for  other 
studies,  not  as  a  means  of  compiling  information,  but  for  the  sake  of 
the  process  itself.  He  enjoyed  taxonomy.  His  methods  of  classification, 
his  system  of  naming  and  the  keys  he  developed  were  even  more  simple 
to  use  than  those  of  Ray.  Others  discovered  that  they  could  use  his  sys- 
tem and  identify  organisms  themselves.  Furthermore,  his  enthusiasm 
was  infectious.  Linnaeus'  first  classification  of  nature  (minerals,  plants 
and  animals)  appeared  in  1735  and  was  an  immediate  success.  At  the  age 
of  28  Linnaeus  had  an  international  reputation,  and  within  a  short 
time  he  established  at  the  University  of  Upsala  in  Sweden  a  center  of 
taxonomic  work  to  which  students  came  from  all  over  the  world.  His 
classification,  the  Systemn  Naturae,  was  revised  and  enlarged  several 
times,  and  published  in  several  countries.  It  was  in  its  13th  edition  when 
Linnaeus  died  in  1778,  and  was  carried  through  several  more  editions 
during  the  next  fifty  years  by  his  students. 

The  system  used  by  Linnaeus  was  modified  from  edition  to  edition. 
He  began  by  following  Ray  in  the  use  of  the  genus  followed  by  the  name 
of  the  species,  the  latter  being  one  or  more  descriptive  words  epitomizing 
the  species.  In  successive  editions  more  and  more  species  were  named, 
and  in  the  interest  of  brevity  the  specific  names  became  shorter  and 
shorter.  By  the  10th  edition,  published  in  1758,  Linnaeus  adopted  a 
uniform  system  in  which  the  genus  and  the  species  were  each  a  single 


THE   PRINCIPLES   OF   TAXONOMY  145 

word.  Since  ihe  specific  name  could  no  longer  epitomize  the  species, 
Linnaeus  suggested  that  it  was  sufficient  if  it  merely  identified  the  species 
among  those  oi  the  genus.  Thus,  he  established  the  binomial  system  of 

nomenclature. 

Linnaeus  also  gave  names  to  the  groups  higher  than  the  genus.  The 
largest  groups  (similar  to  those  established  by  Aristotle)  he  called  classes, 
and  each  class  was  divided  into  orders,  which  in  turn  were  divided  into 
genera  and  species.  Before  1800,  other  workers  introduced  the  family 
as  a  category  between  the  order  and  the  genus,  and  soon  thereafter 
classes  were  grouped  into  higher  categories,  the  phyla. 

Since  the  10th  edition  of  the  Systema  Naturae  is  the  first  publication 
to  adhere  strictly  to  binomial  nomenclature,  one  of  the  International 
Rules  states  that  no  name  published  prior  to  this  is  valid.  Hence,  the 
4236  descriptions  in  this  book  include  the  earliest  official  species  names. 
The  effect  of  Linnaeus  on  biology  is  difficult  to  measure.  As  with 
most  giants,  the  world  was  ready  for  hmi,  and  without  him  someone  else 
would  certainly  have  done  the  work.  But  it  is  likely  that  taxonomy 
would  never  have  enjoyed  the  popularity  it  had  without  the  force  and 
personality  of  Linnaeus  behind  it.  Classification  became  an  amateur  as 
well  as  a  professional  "sport,"  which  still  persists  in  the  activities  of  the 
many  bird  watchers  and  bug  collectors. 

Taxonomy,  and  the  study  of  nature  that  taxonomic  work  stimu- 
lated, had  between  1750  and  1850  an  enormous  influence  upon  the  arts. 
To  be  sure,  the  attention  that  man  turned  toward  nature  was  but  one 
facet  of  his  growing  objectivity  and  curiosity,  dwarfed  beside  the  eco- 
nomic and  political  reforms  of  the  period.  Nonetheless  nature  was  a 
prominent  feature  of  literature,  music  and  painting.  The  "new  orderli- 
ness" of  taxonomy  gave  nature  a  pleasing  aspect.  The  fact  that  organisms 
could  be  neatly  placed  in  groups  and  identified  with  labels  lent  a  sense 
of  security.  Problems  of  grouping  led  to  thought  about  their  patterns, 
and  this  in  turn  developed  into  a  search  for  harmony  in  nature.  The 
foreboding,  secret  aspect  of  nature,  intimately  bound  with  medicine  and 
magic  and  the  devil,  disappeared.  The  direct  familiarity  with  nature 
initiated  by  the  popularization  of  collecting  and  classifying  organisms 
brought  nature  into  the  intellectual  circles  of  the  late  18th  and  early 
19th  centuries.  The  philosopher  Johann  Wolfgang  von  Goethe,  who  was 
a  poet  and  a  biologist  among  other  things,  more  than  any  other  person 
developed  this  emphasis  upon  harmony,  upon  the  inherent  goodness  of 
nature.  He  established  "nature-philosophy"  as  one  approach  to  the 
understanding  of  life.  In  all  the  art  forms,  the  works  of  this  period  are 
touched  with  his  approach.  Together  with  the  new  social  philosophies 
and  the  rise  of  the  common  man  they  characterize  the  period  known  as 
19th  century  romanticism. 

If  the  highest  external  achievement  of  this  generation  of  amateur 
naturalists  following  Linnaeus  is  echoed  in  the  poetry  of  Keats  and 
Shelley,  it  must  be  admitted  that  within  the  field  of  biology  the  "won- 
derful" era  came  to  a  less  satisfactory  end.  The  crescendo  of  taxonomic 
w^ork  rose  to  a  maximum  in  the  early  19th  century.  At  the  same  time  that 
Goethe  was  court  philosopher  and  biologist  in  the  German  city-state  of 


146  ^^^   ANIMAL   KINGDOM 

Weimar,  Georges  Cuvier  was  court  biologist  in  France,  surviving  both 
the  French  Rcvohition  and  Napoleon.  Cuvier  extended  classification 
into  the  more  complex  area  ol  comparative  anatomy,  a  field  which  he 
established  almost  single-handed.  He  showed  that  reconstructions  could 
be  made  Irom  lossil  bones,  and  that  they  often  represented  animals  no 
longer  living.  He  began  to  give  names  to  these  extinct  species,  of  which 
almost  100,000  have  been  identified  since  his  time. 

During  this  period  the  diversity  among  the  lower  animals  was  dis- 
covered, and  the  taxonomic  system  was  expanded  to  provide  for  more 
and  more  phyla  of  invertebrates,  while  the  several  classes  of  vertebrates 
were  joined  together  in  a  single  phylum. 

By  1830,  however,  the  museums  and  laboratories  of  the  world  sagged 
under  their  collections,  and  the  task  of  naming  all  the  species  appeared 
less  complete  than  ever.  Furthermore,  many  of  the  known  species  were 
discovered  to  vary  in  their  characteristics  from  one  region  to  another 
and  species  formerly  considered  distinct  were  found  to  have  intergrades. 
Since  the  concept  of  evolution  was  not  yet  popular,  and  was  denied 
vigorously  by  such  authorities  as  Cuvier,  all  species  were  believed  to 
have  been  created  just  as  they  were.  The  growing  confusion  over  the 
boundaries  between  species  and  the  apparent  endlessness  of  the  job  of 
naming  were  discouraging  indeed.  Both  Goethe  and  Cuvier  died  in  1832, 
at  which  time  interest  in  taxonomy  began  to  decline.  The  original  goals 
set  by  Linnaeus  have  not  yet  been  realized. 

It  was  partly  because  taxonomy  was  already  in  difficulty  that  evolu- 
tion was  accepted  so  readily  when  Darwin  presented  his  arguments  in 
1858.  From  that  moment  on,  taxonomy  was  no  longer  an  end  in  itself, 
and  the  taxonomic  system  was  adjusted  to  serve  the  interest  in  evolution. 
The  races  and  intergrades  of  species  that  had  been  taxonomic  obstacles 
became  interesting  problems,  evidence  of  evolution  in  action.  Relation- 
ships among  species  became  more  important  than  ever,  and  a  new  ques- 
tion, the  "why"  of  a  species,  could  be  asked.  Finally,  the  definition  of  a 
species  became  more  complete,  establishing  the  species  as  an  evolution- 
ary unit  as  well  as  a  taxonomic  category. 

A  student  of  zoology  in  the  time  of  Linnaeus  had  only  the  following 
groups  of  animals  to  learn:  mammals,  birds,  reptiles,  amphibians,  fishes, 
insects  and  worms.  Today,  however,  the  beginning  student  is  bewildered 
and  perhaps  dismayed  to  discover  that  Linnaeus'  category,  "worms," 
includes  most  of  the  phyla  (21  out  of  23  in  this  text).  To  facilitate  mat- 
ters the  23  phyla  have  been  divided  into  10  major  and  13  minor  phyla. 
The  major  phyla,  ones  containing  many  species,  are  described  in  10 
separate  chapters  and  accoimt  for  most  of  the  material  to  be  learned. 
The  minor  phyla  are  to  some  extent  interspersed  among  the  major 
groups,  if  it  is  especially  convenient  to  do  so,  but  most  of  them  are  con- 
sidered together  in  Chapter  18. 

The  insistence  that  all  animals,  including  the  "lower"  animals, 
should  be  studied  was  first  expressed  by  Aristotle.  In  his  treatise.  Of  the 
Parts  of  Animals,  as  he  begins  an  analysis  of  animal  structures  he  argues 
(from  the  translation  by  A.  L.  Peck): 


THE   PRINCIPLES   OF   TAXONOMY  \4J 

So  far  as  in  us  lies,  we  will  not  leave  out  any  of  them,  be  it  ever  so  mean;  for  though 
there  are  animals  that  have  no  attractiveness  for  the  senses,  yet  for  the  eye  of  science,  for 
the  student  who  is  naturally  of  a  philosophic  spirit  and  can  discern  the  causes  of  things, 
Nature  which  fashioned  them  provides  joys  that  cannot  be  measured.  If  we  study  mere 
likenesses  of  these  things  and  take  pleasure  in  so  doing,  because  then  we  are  contemplat- 
ing the  painter's  or  the  carver's  Art  that  fashioned  them,  and  yet  fail  to  delight  much 
more  in  studying  the  works  of  nature  themselves,  though  we  have  the  ability  to  discern 
the  actual  causes— that  would  be  a  strange  absurdity  indeed.  Wherefore  we  must  not 
betake  ourselves  to  the  consideration  of  the  meaner  animals  with  a  bad  grace,  as  though 
we  were  children;  since  in  all  natural  things  there  is  somewhat  the  marvelous. 

Questions 

1 .  List  the  grouping  levels  used  in  taxonomy  between  the  species  and  the  kingdom. 

2.  Give  six  of  the  international  rules  of  nomenclature. 

3.  What  did  Ray  and  Linnaeus  contribute  to  taxonomy? 

4.  What  is  a  species? 

5.  What  is  taxonomy?  What  does  it  offer  to  zoology? 

Supplementary  Reading 

Chapter  28  (What's  in  a  Name?)  of  The  Growth  of  Scientific  Ideas  by  Wightraan 
presents  an  excellent  and  readable  account  of  the  growth  of  taxonomy.  The  lives  of  early 
taxonomists  and  naturalists  are  presented  in  a  romantic  and  informative  style  by  Peattie 
in  Green  Laurels.  Critica  Botanica  by  Linnaeus  includes  a  thorough  discussion  of  the 
principles  and  methods  of  classification,  revealing  both  the  humor  and  the  incisiveness  of 
the  author. 


CHAPTER  8 


The  Phylum  Protozoa 


46.        Introduction 

The  single-celled  animals  remained  unknown  until  Antony  van 
Leeuwenhoek,  a  Dutch  lens-maker  oi  the  17th  century,  examined  water 
droplets  with  a  primitive  microscope  and  discovered  that  diverse,  very 
small  lorms  ol  lite  existed.  His  work  received  international  attention 
and  his  organisms  were  studied  extensively.  At  that  time,  however,  the 
cellular  nature  ot  organisms  was  not  known  and  van  Leeuwenhoek's 
"little  animals"  were  considered  by  most  investigators  to  be  merely  small 
varieties  of  worms  or  wormlike  animals.  It  was  not  until  1845  that  the 
unicellular  nature  oi  many  ot  these  microscopic  animals  was  appreciated, 
and  the  phylum  Protozoa  was  established  to  include  them.  Today  this 
phylum  includes  all  ot  the  one-celled  animals.  Since  all  other  animals 
are  multicellular,  the  animal  kingdom  is  otten  divided  into  the  subking- 
dom  Protozoa,  including  only  the  phylum  Protozoa,  and  the  subkingdom 
Metazoa,  including  all  the  other  phyla. 

The  typical  protozoan  has  a  single  nucleus  and  leads  an  independent 
existence.  In  some,  however,  the  cell  is  multinuclear,  while  in  others  the 
individual  cells  are  attached  and  form  colonies.  Most  of  the  individuals 
in  a  protozoan  population  are  produced  by  simple  cell  division  of  the 
parent,  although  sexual  reproduction  is  by  no  means  rare. 

Protozoa  are  primarily  aquatic,  living  in  bodies  of  water  of  all  kinds, 
fresh  and  salt,  from  puddles  to  oceans.  Some  live  in  damp  soils,  crawling 
in  the  thin  film  of  water  surrounding  dirt  particles.  Others  are  parasitic 
and  live  in  the  fluids  of  animals  and  plants.  Whatever  their  habitats, 
the  surfaces  of  active  protozoa  must  remain  wet,  for  they  cannot  survive 
desiccation. 

Variation  in  form  is  enormous.  Some  protozoans  are  shapeless 
"blobs"  while  others  are  as  elaborate  and  as  geometrically  patterned  as 
snowHakes.  In  some  groups  the  cells  may  have  internal  skeletons,  external 
skeletons,  or  protective  houses  cemented  together  from  sand  and  other 
particles.  Those  with  hard  parts  can  be  fossilized.  Only  two  thirds  of  the 
25,000  described  species  of  protozoa  are  living.  The  others  are  known 
from  their  remains  found  in  rocks. 

The  phylum  is  divided  into  five  classes  (Fig.  8.1):  (1)  Flagellata,  the 
flagellates,  having  one  or  more  long,  whiplike  flagella;  (2)  Sarcodina,  in 
which  pseudopods  are  formed  for  locomotion  and  feeding;  (3)  Ciliata, 
the  ciliates,  characterized  by  the  presence  of  many  short  cilia  for  locomo- 

148 


THE   PHYLUM   PROTOZOA  140 


Tl^gzll 


Mouth 


Nucleu-S 


Tentacles 


D  E 

Figure  8.1.     Classes  of  the  phylum  Protozoa.  A,  Flagellata.  B,  Sarcodina.  C,  Ciliata. 
D,  Suctoria.  E,  Sporozoa. 

tion  or  feeding;  (4)  Suctoria,  in  which  the  young  have  ciha  but  the  aduUs 
have  tentacles;  and  (5)  Sporozoa,  parasitic  forms  that  reproduce  by 
multiple  fission  (division  into  more  than  two  daughter  cells)  and  in 
which  adult  forms  lack  obvious  locomotor  structures. 


47.        Organelles 

Each  protozoan  cell  must  carry  on  all  the  life  processes  of  its  species. 
These  include  the  cellular  activities  described  in  Chapter  4,  and  also 
the  physiologic  activities  described  in  Chapter  5.  The  metazoa  have 
capitalized  upon  a  division  of  labor  among  cells:  some  are  nutritive, 
others  excretory,  and  still  others  muscular.  In  the  protozoa  these  various 
activities  are  accomplished  by  specialized  structures  within  the  single 
cell.  Such  structures,  whose  functions  are  comparable  with  those  of  the 
organs  of  higher  animals,  are  called  organelles.  A  few  of  these  will  be 
described  as  examples  of  intracellular  differentiation. 

Cilia  and  Flagella.  Obvious  in  many  protozoa  are  the  locomotor 
organelles.  Many  ciliates  are  rapid  swimmers,  propelling  themselves  by 
the  concerted  action  of  their  many  cilia.  Flagellates  may  also  move 
rapidly,  pulling  themselves  forward  by  lashing  the  anteriorly  located 
flagella.  Each  flagellum  (Fig.  8.2)  is  a  long,  supple  filament  containing 
an  axial  fibril.  Although  this  fibril  resembles  the  contractile  fibrils  of 


150 


THE   ANIMAL   KINGDOM 


muscle  cells  in  many  ways,  it  is  not  known  for  certain  whether  the 
flagellar  fibril  is  made  ot  actomyosin,  the  contractile  protein  of  verte- 
brate muscles.  In  a  typical  swimming  movement,  the  flagellum  lashes 
stiffly  to  one  side  from  an  extended  position  and  returns  relaxed  and 
bent.  The  flagellum  may  also  undulate,  with  waves  passing  from  tip  to 
base,  thus  pulling  the  animal  forward.  Cilia  are  structurally  similar  to, 
but  much  smaller  than,  flagella. 

Some  protozoa  can  creep  on  the  bottom  with  wormlike  movements. 
These  animals  have  just  beneath  their  surfaces  a  layer  of  contractile 
fibrils  which  form  an  organelle  comparable  to  the  muscular  body  wall 
of  worms.  Other  protozoa  "slide,"  moving  along  slowly  with  no  apparent 
means  of  propulsion.  These  also  have  a  surface  layer  of  contractile  fibrils, 
and  it  has  been  presumed  (with  no  direct  evidence)  that  they  move  by 
passing  minute  waves  of  contraction  along  the  fibrils,  after  the  fashion 
of  a  snail's  locomotion.  Finally  there  is  locomotion  by  ameboid  move- 
ment, described  in  Chapter  5.  This  kind  of  movement  apparently  lacks 
a  specific  functional  organelle. 

Conductlle  Organelles.  Other  organelles  are  conductile,  their  func- 
tions being  comparable  to  those  of  the  nervous  systems  of  higher 
animals.  The  flagellates  have,  at  the  base  of  each  flagellum,  a  basal  body 
(Fig.  8.2).  If  flagellum  and  basal  body  are  removed  intact  from  the  animal, 
flagellar  activity  continues,  but  as  soon  as  the  two  are  separated  move- 
ment usually  stops.  The  basal  body  apparently  stimulates  and  controls 


Piemen 
spot 


Figure  8.2.     The  flagellum.  A,  Details  of  flagellar  structure  in  the  flagellate, 
B,  Successive  positions  of  flagellum  in  a  typical  stroke. 


4 

Euglena, 


THE   PHYLUM    PROTOZOA 


151 


V-CilicL 


Cuticlfz- 

Cecil  Surface) 


Connecting 
fibril         ^ 


Basal  body      ^--Trichocyst 


Figure  8.3.     Cilia.  A  small  bit  of  the  surface  of  a  ciliate,  showing  the  cuticle,  pro- 
jecting cilia,  and  underlying  structures.  The  trichocysts  are  discussed  later  in  the  text. 

the  movement  of  the  flagellum.  The  flagellum  may  follow  any  of  several 
different  patterns  of  movement  at  different  times,  but  nothing  is  known 
of  the  way  in  which  this  is  controlled  by  the  cell.  The  basal  body  is 
often  joined  by  a  filament  to  the  centriole,  from  which  it  may  be  pro- 
duced during  development.  It  is  considered  to  be  a  modified  centriole, 
controlling  the  activity  of  the  ffagellum  just  as  the  centriole  of  the  sperm 
dtiring  spermiogenesis  (p.  119)  gives  rise  to  the  axial  filament  of  the 
sperm  tail  and  presumably  is  important  in  controlling  its  movement. 

The  ciliates  also  have  a  basal  body  (Fig.  8.'^)  at  the  base  of  each 
ciliimi.  These  have  no  connection  with  the  centriole,  but  are  connected 
with  each  other  by  a  network  of  slender  fibrils,  made  visible  with  a  silver 
stain,  the  kind  of  stain  used  to  demonstrate  nerve  fibers  in  the  metazoa. 
Each  basal  body  activates  its  cilium,  and  coordination  among  the  cilia 
is  accomplished  through  the  fibrillar  network.  The  basal  bodies  near 
the  mouth  initiate  a  wave  of  activity  that  passes  over  the  body  of  the 
animal.  Wave  follows  upon  wave,  as  shown  in  the  ciliary  motion  of 
the  Paramecium  (Fig.  8.12),  producing  a  smooth,  rapid  motion.  Micro- 
surgical incisions  that  cut  across  the  connecting  fibrils  produce  a  local 
asynchrony  among  the  cilia,  and  may  seriously  disrupt  locomotion. 

In  many  Hagellates  a  visual  organelle  (Fig.  8.2)  is  associated  with 
the  conductile  and  locomotor  organelles.  The  visual  organelle  ot  Eiiglenn 
has  two  parts,  a  patch  of  red  pigment  in  the  protoplasm  beside  the 
Hagellum,  and  a  tiny,  light-sensitive  photoreceptor  on  the  flagellar  base. 
The  shading  of  the  photoreceptor  by  the  pigment  spot  enables  the 
animal  to  determine  the  direction  from  which  the  light  comes.  In  other 
species  the  photoreceptor  is  seated  in  a  pigment  cup  with  the  opening 
anterior.  In  a  few  cases  the  thin  cuticle  covering  the  animal  is  swollen 
over  the  cup  to  produce  an  optic  lens. 

Confracf//e  Vacuoles.  A  prominent  structure  in  many  protozoans 
is  an  excretory  organelle,  the  contractile  vacuole  (p.  93).  It  is  found  in 
all  fresh-water  forms  and  in  many  marine  species,  but  is  uncommon 
among  the  parasites.  A  fresh-water  environment  is  hypotonic  to  the 
protozoan  and  a  method  of  removing  water  that  enters  through  osmosis 
is  needed.  In  marine  forms  (which  are  always  isotonic  with  sea  water) 
the  contractile  vacuole  is  used  to  excrete  the  water  that  accumulates  in 


152  '■"f   ANIMAL   KINGDOM 

feeding.  Many  marine  protozoa  and  most  parasitic  species  do  not  ingest 
food,  hence  they  do  not  tend  to  accumulate  water,  and  have  little  use 
for  a  contractile  vacuole.  1  he  exact  mechanism  by  which  the  contractde 
vacuole  fills  with  water  is  unknown,  but  it  is  emptied  by  the  contraction 
of  the  surrounding  cytoplasm  which  shifts  from  a  sol  to  a  gel  as  it  con- 
tracts and  forces  the  bubble  to  burst  to  the  outside. 

48.        Class  Flagellata 

Flagellates  are  spherical  to  elongate  protozoa  with  a  simple,  cen- 
trally located  nucleus  and  from  one  to  several  flagella  at  one  end.  The 
group  is  large,  including  half  the  known  living  protozoan  species.  Most 
of  these  are  small  and  difficult  to  study  but  a  few,  such  as  members  of 
the  genus  Euglena  (Fig.  8.4),  are  large  and  easily  obtained.  A  study  of 
Eugleua  will  introduce  the  class. 

Euglena.  Euglenas  (£.  vindis  and  E.  gracilis  are  common  species) 
are  elongate  flagellates  50  to  100  or  more  micra  long.  At  the  anterior 
end  a  deep  depression  forms  the  gullet.  Although  euglenas  have  never 
been  observed  to  feed,  members  of  the  genus  Peranema  of  the  same 
family  use  the  gullet  for  swallowing  prey.  The  body  is  covered  with  a 
delicate  pellicle  showing  spiral  thickenings.  Beneath  the  pellicle,  in- 
visible without  special  stains,  is  a  layer  of  contractile  fibrils  with  which 
the  organism  can  change  shape.  Euglenas  often  creep  upon  the  bottom 
in  a  wormlike  fashion.  A  single  contractile  vacuole  lies  next  to  the 
gullet  and  empties  into  its  base.  The  large  nucleus  is  in  the  posterior 
third  of  the  body. 

Scattered  in  the  cytoplasm  are  chloroplasts  and  paramylum  bodies. 
Chloroplasts  are  bright  green  with  their  contained  chlorophyll;  in  the 
light  they  are  able  to  carry  on  photosynthesis,  like  the  chloroplasts  of 
plants.  The  arrangement  of  chloroplasts  is  used  in  the  identification 
of  species.  In  E.  viridis  they  are  large  and  form  a  rosette  (Fig.  8.4).  In 
E.  gyacilis  and  in  several  other  common  species  they  are  small  and 
numerous,  obscuring  all  other  internal  structures  except  the  red  pigment 
spot.  The  transparent,  colorless  paramylum  bodies  are  a  form  of  poly- 
saccharide unique  to  the  euglenas,  difl:erent  from  both  the  glycogen  of 
other  animals  and  the  starch  of  plants.  The  arrangement  of  these  also 
varies  among  the  species.  They  are  formed  during  photosynthesis,  and 
if  they  are  so  numeious  as  to  obscure  other  structures  their  numbers 
can  be  reduced  by  keeping  the  euglenas  in  the  dark  a  day  or  two.  A 
single  long  flagellum  which  protrudes  from  the  gullet  is  used  for  swim- 
ming. It  is  formed  by  the  fusion  of  two  flagella  that  arise  from  two  basal 
granules  in  the  base  of  the  gullet.  At  the  point  of  fusion  (Fig.  8.4)  is  a 
transparent  swelling,  the  photoreceptor.  Next  to  this  in  the  wall  of  the 
gullet  is  a  red  pigment  spot. 

Swimming  is  a  complex  movement.  The  sideways  lashing  of  a  single 
flagellum  is  like  one-armed  swimming;  the  body  is  thrown  forward  bui 
also  to  one  side  at  each  stroke.  In  Euglena  the  flagellum  usually  bends 
toward  the  side  bearing  the  pigment  spot,  and  if  this  stroke  were  merely 
repeated  over  and  over  the  organism  would  move  in  a  circle  with  the 


THE  PHYLUM    PROTOZOA 


153 


pigment  spot  facing  outward.  Each  stroke  is  not  a  simple  backward  lash, 
however,  but  is  directed  obliquely  to  the  long  axis  of  the  organism  so 
that  the  body  not  only  turns  to  one  side  but  also  rotates  a  little  (Fig. 
8.4).  Successive  lashes  thus  produce  a  spiral  path  in  which  the  organism 
moves  forward  with  the  pigment  spot  continually  facing  the  outside  of 
the  spiral. 

This  swimming  pattern  makes  optimal  use  of  the  visual  organelle. 
As  Euglena  swims  forward  the  pigment  spot  shades  the  photoreceptor 
from  behind  and  from  one  side.  W^hen  Euglena  is  swimming  at  right 
angles  to  the  direction  of  light  the  photoreceptor  is  shaded  once  during 
each  spiral  loop.  If  the  organism  is  seeking  light  it  turns  to  one  side  more 
than  usual  at  the  moment  that  the  photoreceptor  is  shaded,  gradually 
turning  the  spiral  path  toward  the  light  until  the  photoreceptor  is  never 
shaded.  If  it  is  avoiding  light,  it  turns  sideways  more  than  usual  during 
that  part  of  the  spiral  in  which  the  photoreceptor  is  illuminated,  grad- 
ually turning  the  spiral   path   away  from  the   light  until   the  photore- 


Flagellw 


igme^nt-  spot — ■ 


l:3/>-— Phot  oreceptor 


-Contractile 


J/(  y^ '  vaciule 


Paramylum 
bodies 


Chloroplasts^ 


Nuclexi^ 


Figure  8.4.  pAigleua.  A.  A  lateral  \icw  of  Euglena  viridi<i.  B.  A  diagram  showing 
successive  positions  in  the  spiral  swimming  pattern  of  Euglena.  The  position  of  the  pig- 
ment spot  shows  the  rotation  that  occurs. 


154 


THE   ANIfAAL   KINGDOM 


ceptor  is  continually  shaded.  In  general,  euglenas  swim  toward  moderate 
light  but  avoid  intense  light.  During  the  day  they  usually  swim  to  the 
surface  ot  a  pond  where  they  lorm  a  green  scum,  exposing  their  chloro- 
plasts  to  the  light. 

No  Euglena  is  completely  autotrophic.  Healthy  cultures  can  be 
maintained  in  light  only  il  some  organic  substances,  especially  amino 
acids,  are  present.  Growth  is  more  rapid  if  a  considerable  variety  of 
organic  substances  is  present.  In  the  absence  of  light,  of  course,  the 
culture  medium  must  be  rich  in  all  the  basic  foods.  Some  species,  if 
cultured  in  the  dark,  gradually  lose  their  chlorophyll.  When  the  loss 
is  complete,  the  organisms  become  obligatory  saprophytes,  for  once 
chlorophyll  is  lost  it  cannot  be  regained.  A  few  natural  species  of  Eu- 
glena lack  chlorophyll,  which  suggests  that  nature  may  have  performed 
the  same  experiment  in  the  past. 

Euglenas  belong  to  the  order  Euglenida,  characterized  by  the  gidlet 
and  pigment  spot.  Some  members  of  the  order  are  nearly  autotrophic, 
some  are  saprophytic,  and  some  are  holozoic.  Of  the  remaining  15  to  25 
flagellate  orders,  three  will  be  mentioned  here. 

Dinoflagellates.  The  order  Dinoflagellata  is  characterized  by  the 
presence  of  two  flagella  in  grooves,  one  trailing  posteriorly  and  the  other 
wrapped  around  the  "waist"  (Fig.  8.5).  Usually  the  body  is  covered  by  a 
cellulose  shell  divided  into  upper  and  lower  halves.  Many  species  pos- 
sess photosynthetic  pigment  and  are  able  to  synthesize  some  of  their 
organic  needs.  None  has  been  successfully  cultured  on  a  completely  in- 
organic medium.  Thus,  like  the  euglenas,  the  dinoflagellates  are  not 
completely  autotrophic.  Some  species  can  be  cultured  without  light  if 
all  the  necessary  foods  are  supplied.  Dinoflagellates  have  also  been  ob- 
served capturing  other  organisms  and  engulfing  them  by  pseudopod 
formation  from  the  ventral  furrov*A.  It  is  now  believed  that  most  of  the 
species  combine  autotrophic  and  holozoic  nutrition.  Some  species  lack 
photosynthetic  pigment  and  live  entirely  by  holozoic  nutrition. 

Dinoflagellates  are  abundant  in  the  plankton  of  both  marine  and 


Celloxloge 
pla.te 

Transverse 
f  IcLgellum. 


■Ventral 
£urrov/ 

Posterior 
f  la-del  lum. 

Figure  8.5.     A  typical  dinoflagellate.  The  organism  propels  itself  forward  with  un- 
dulations of  the  posterior  flagellum,  and  rotates  by  activity  of  the  transverse  flagellum. 


THE   PHYLUM    PROTOZOA 


155 


fresh  waters.  They  tend  to  occur  as  "blooms,"  becoming  extremely 
abundant  for  a  short  time  and  then  disappearing.  Although  most  species 
are  harmless  and  form  an  important  source  of  food  for  other  organisms, 
a  few  produce  deadly  toxins.  Most  spectacvdar  are  the  small  reddish 
forms  that  color  the  water  when  they  become  abundant  and  produce 
"red  tides."  Such  water  is  lethal  to  fish,  killing  them  rapidly  as  they 
enter  the  poisoned  region.  The  dead  animals  decay  and  enrich  the  sup- 
ply of  nutrients  so  that  a  red  tide,  once  started,  tends  to  be  self-per- 
petuating until  ^vater  currents  or  storms  break  it  up. 

Many  dinoflagellates  are  parasitic.  Although  the  adult  forms  seldom 
resemble  dinoflagellates  they  can  be  identified  by  their  young,  which 
have  the  typical  grooves  and  flagella. 

Phytomonads.  An  order  of  more  plantlike  flagellates  are  the  phy- 
tomonads  (order  Phytomonadina).  They  are  all  autotrophic,  but  have 
visual  organelles  and  swim  about  in  a  most  animal-like  fashion.  Within 
the  group  is  a  series  of  colonial  species,  ranging  in  complexity  from 
the  one-celled  Clihiinydomonas  to  the  highly  integrated,  spherical 
colony,  J'olvox  (Fig.  8.6).  Volvox  represents  the  peak  of  protozoan 
colony  formation.  Although  each  individual  of  the  colony  feeds  for 
itself,  they  are  not  all  alike.  The  pigment  spots  are  largest  in  the  cells 
at  one  pole  of  the  sphere,  which  is  always  the  anterior  pole  in  loco- 
motion, and  decrease  steadily  in  size  around  to  the  posterior  pole.  Re- 
production is  limited  to  the  equatorial  and  posterior  cells.  All  cells  are 


Gon.iura 


Volvox: 


lorina 


Figure  8.6.  Examples  of  the  order  Phytomonadina  showing  solitary  form  {Chlamy- 
domonas),  simple  colonies  (Goniitm,  Eudorina).  and  a  complex  colony  {Volvox).  Colonial 
forms  are  embedded  in  a  matrix  of  transparent  jelly. 


156 


THE   ANIMAL   KINGDOM 


connected  with  each  other  through  cytoplasmic  bridges,  by  which  cell 
activity  can  be  synchronized.  The  superficial  resemblance  of  Volvox  to 
the  embryonic  blastula  of  metazoans  has  given  this  organism  a  prom- 
inent place  in  zoology. 

Most  botanists  believe  that  the  higher  plants  evolved  from  the 
phytomonads.  Many  of  the  noncolonial  species,  able  to  swim  with 
ilagella,  can  also  grow  upon  the  bottom  as  round  cells  without  flagella, 
in  which  case  they  take  on  a  colonial  appearance  and  resemble  plants. 
Further  kinship  with  the  higher  plants  is  suggested  by  certain  simi- 
larities in  their  chemical  structure. 

Choanoflagellates.  Most  of  the  strictly  animal  flagellates  are  small, 
uncommon,  or  are  inhabitants  of  foul  water  and  thus  are  unpleasant 
to  study.  One  order,  prominent  because  of  their  resemblance  to  the 
sponges,  are  the  choanoflagellates  (order  Choanoflagellata,  Fig.  8.7). 
These  are  sedentary  flagellates,  attached  to  the  bottom  by  a  posterior 
stalk.  The  single  flagellum  is  surrounded  by  a  delicate  protoplasmic 
collar.  The  flagellum  produces  a  water  current  over   the  animal,   and 


Desmare^llsL 


Sphaerocca, 


PhalarLstcrium. 


Figure  8.7.  Examples  of  the  order  Choanoflagellata  showing  solitary  and  colonial 
forms.  The  matrix  holding  colonial  individuals  together  is  more  pronounced  than  in  the 
Phvtomonadina. 


THE   PHYLUM    PROTOZOA 


157 


small  food  particles  brushing  against  the  collar  stick  tightly  and  are 
carried  to  the  collar  base.  Periodically,  at  the  base,  small  pseudopods 
erupt  and  engulf  the  food  in  a  food  vacuole,  where  it  is  digested.  This 
group  includes  a  variety  of  colonial  forms.  Individuals  tend  to  secrete 
a  gelatinous  material  around  their  bases,  which  in  colonial  species  may 
form  a  bulky  structure.  The  more  complex  colonies  (Fig.  8.7)  include 
ones  with  branching  patterns  and  one  free-swimming  spherical  species, 
Sphaeroeca  volvox. 

These  are  but  a  few  of  the  flagellate  groups.  Many  flagellates  are 
parasitic;  the  most  important  of  these  are  the  trypanosomes  that  pro- 
duce sleeping  sickness.  These  and  other  parasitic  protozoa  will  be  dis- 
cussed in  Chapter  39. 

49.        Class  Sarcodina 

Unlike  other  protozoa,  the  Sarcodina  have  no  definite  body  shape. 
Because  of  their  ameboid  movement  the  shape  changes  from  moment  to 
moment.  Nonetheless  shape  can  be  helpful  in  identifying  species.  Some 
species  of  amebas,  for  example,  form  several  long  narrow  pseudopods 
at  one  time,  while  others  form  only  one  or  two,  or  the  pseudopods  may 
be  short  and  blunt  (Fig.  8.8).  It  is  possible  to  describe  an  "average 
shape"  for  a  given  species.  The  internal  structures  occuj^y  no  particular 
position.  Nucleus,  contractile  vacuole  and  food  vacuoles  shift  about  as 
the  animal  moves. 

Amebas.  Amebas  (order  Amoebozoa)  are  common  in  all  waters, 
move  slowly,  and  are  easily  studied  under  the  microscope.  The  ecto- 
plasm (cytoplasm  near  the  cell  surface)  is  clear,  while  the  endoplasm 
or  inner  cytoplasm  is  granular.  From  the  behavior  of  the  granules,  it  is 
apparent  that  the  outer  part  of  the  endoplasm  is  in  the  gel  state  while 


Figure  8.8.     An  ameba.  The  animal  is  flowing  to  the  right. 


158  ^"^  ANIMAL  KINGDOM 


Fiqure  8  9  An  ameba  capturing  a  large  flagellate  The  flagellate  hits  the  side  of  the 
ameba  and  slips  into  the  crevasse  at  the  base  of  a  pseudopod.  The  ectoplasm  of  this 
region  erupts  and  rapidly  engulfs  the  prey.  Stages  shown  are  at  about  one  second 
intervals. 

much  of  the  inner  endoplasm  is  a  sol.  Where  pseudopods  are  forming 
the  outer  endoplasm  is  also  a  sol,  becoming  a  gel  along  the  sides  of  the 

advancing  lobes. 

Amebas  eat  a  wide  variety  of  materials.  They  crawl  slowly  about, 
engulfing  inactive  food  such  as  plant  cells  and  debris  by  flowing  slowly 
around  them  and  enclosing  them  in  food  vacuoles.  They  may  also 
capture  active  prey  such  as  flagellates  (Fig.  8.9)  in  a  somewhat  different 
manner.  When  a  swimming  flagellate  bumps  into  an  ameba,  it  not  only 
tends  to  slide  into  a  crevasse  between  pseudopods  but  also  stimulates  the 
ameba  to  flow  rapidly  in  its  direction.  If  the  anterior  end  of  the  flagellate 
becomes  wedged,  the  ameba's  protoplasm  engulfs  the  entire  prey  in  a 
second  or  two.  If  the  flagellate  is  not  wedged  it  is  simply  pushed  away 
by  the  advancing  protoplasm.  Sometimes  the  flagellate  appears  to  be- 
come attracted  to  the  ameba  and  returns  again  and  again,  so  that  the 
ameba  may  have  several  opportunities  to  be  successful.  Certain  species 
of  ameba  are  quite  particular  about  their  food,  and  eat  only  flagellates 
or  only  plant  cells. 

The  fate  of  food  vacuoles  has  been  studied  closely.  At  first  they 
become  acid,  owing  to  the  secretion  by  the  protoplasm  of  inorganic 
acids  (such  as  hydrochloric  acid,  secreted  in  our  own  stomachs).  This 
kills  the  prey  and  initiates  digestion.  Later  the  vacuole  becomes  alka- 
line, enzymes  are  secreted  into  it,  and  digestion  continues.  Enzymes  for 
the  hydrolysis  of  proteins,  fats  and  carbohydrates  have  been  found  in 
food  vacuoles.  The  food  particles  swell,  become  indistinct,  and  the 
vacuole  enlarges.  As  digestion  is  completed,  both  nutrients  and  water 
are  absorbed  by  the  protoplasm  and  the  vacuole  shrinks  to  a  very  small 
size.  As  the  ameba  continues  its  slow  locomotion,  the  indigestible  rem- 
nants are  expelled  and  left  behind.  This  digestive  process  in  the  food 
vacuole  has  been  observed  in  all  of  the  major  groups  of  protozoa. 

A  few  amebas,  such  as  the  genus  Diffiugia  (Fig.  8.10),  cement  sand 
particles  together  to  make  a  protective  case.  Other  species  secrete  a 
membranous  covering.  Pseudopods  project  through  a  lower  opening, 
and  by  means  of  these  the  animals  move  about. 

He/iozoa.  Related  to  the  amebas  is  the  order  Heliozoa,  a  group  of 
fresh-water  sarcodinids  with  numerous  delicate  pseudopods  projecting 
from  a  bubbly  center  (Fig.  8.10).  They  float  in  the  water  and  capture 


THE   PHYLUM    PROTOZOA 


159 


Difflugia- 


Ac  t  inophry  S 


Globiderina. 


Figure  8.10.  Other  sarcodinids.  Difflugia  is  an  ameba  with  a  shell  cemented  from 
sand  particles.  Actinophrys  is  a  member  of  the  fresh-water  order  Heliozoa  and  Globi- 
gerina  belongs  to  the  marine  order  Foraminifera. 

small  organisms  that  touch  their  pseudopods,  first  engulfing  them  in 
food  vacuoles  and  then  drawing  them  into  the  central  mass.  Although 
the  prey  are  obviously  paralyzed  upon  touching  the  pseudopods,  the 
method  of  "stinging"  is  unknown. 

Rad'iolaria  and  Foraminifera.  Members  of  the  two  marine  orders 
Radiolaria  and  Foraminifera  are  adapted  to  floating.  Radiolarians  re- 
semble heliozoans  but  they  possess  an  internal  skeleton  made  of  silica. 
These  glassy  frameworks  combine  porous  spheres  with  radiating  spines 
to  produce  intricate  and  beautiful  patterns  (Fig.  8.11).  Silica  is  durable, 
and  many  deep-water  marine  sediments  are  composed  largely  of  radio- 
larian  fossils.  Similar  fossils  are  found  in  rocks  that  date  from  pre-Cam- 
brian  times,  before  fossils  of  metazoa  occur. 

Foraminiferans  secrete  an  external  porous  capsule  of  calcium  car- 
bonate through  which  pseudopods  project  into  the  water.  As  the  animal 
grows,  it  expands  its  home  by  adding  new  chambers.  Many  genera,  such 
as  Globigerina  (Fig.  8.10),  are  abundant.  Their  skeletons  rain  contin- 
ually upon  the  ocean  floor,  and  in  the  more  shallow  seas  where  they  are 
not  dissolved  they  may  form  large  deposits,  such  as  those  which  now 
comprise  the  chalk  cliffs  of  Dover.  While  species  of  the  genus  Globi- 
gerina add  new  chambers  in  a  somewhat  irregular  fashion,  most  species 
add  them  in  a  systematic  pattern,  often  in  a  coiled  sequence  like  a 
snail  shell.  The  shape  and  arrangement  of  the  chambers  serves  to 
identify  the  species.  .Although  the  Foraminifera  are  prominent  mem- 
bers of  the  marine  plankton,  most  of  the  species,  especially  those  with 
heavy  shells,  live  on  or  near  the  bottom  in  relatively  shallow  water. 

During  the  coal  age  (later  part  of  the  Paleozoic  era),  when  the 
major  coal  and  oil  deposits  were  laid  down,  a  family  of  foraminiferans, 
the  Nummulitidae,  flourished  and  died.  In  that  brief  space  of  geologic 
time  (75  million  years)  thousands  of  species  of  nummulites  developed, 
most  of  which  lived  a  very  short  time  before  becoming  extinct.  These 
were  immense  protozoa,  up  to  an  inch  in  diameter,  that  lay  upon  the 
bottoms  of  the  shallow  seas.  Their  fossils  are  now  found  in  the  deposits 
that  contain  oil.  As  an  oil  well  is  drilled  down  into  the  rock,  it  passes. 


160 


THE   ANIMAL  KINGDOM 


Figure  8.11.     The  internal,  siliceous  skeleton  of  a  radiolarian.  (E.  Giltsch,  Jena.) 

in  rapid  succession,  these  species  of  nummulites.  From  these  the  driller 
can  estimate  just  how  far  into  the  paleozoic  deposit  he  has  drilled.  This 
is  one  of  the  few  instances  where  an  industry  uses  the  services  of  a 
taxonomist— in  this  case  a  specialist  on  the  classification  of  one  family 
of  extinct  protozoa. 


50.        Class  Ciliata 

Ciliates  can  be  distinguished  from  the  flagellates  and  rhizopods  not 
only  by  their  cilia,  but  also  by  their  nuclei.  Each  ciliate  has  two  nuclei, 
a  large  macronucleus  which  governs  the  ordinary  activities  of  the  cell, 
and  a  small  micronucleus  which  functions  during  sexual  reproduction. 
Both  nuclei  divide  at  each  mitosis,  but  at  sexual  reproduction  the 
macronucleus   disintegrates,    and   the   micronucleus   gives    rise    to    both 


THE  PHYLUM    PROTOZOA 


161 


nuclei   of   the   offspring.  The   details  of  this  process   will  be  described 
later. 

Paramecia.  The  best  known  genus  of  ciliates  is  Paramecium  (Fig. 
8.12).  Several  of  the  species  (such  as  P.  caudatum)  are  large  and  easily 
cultured.  They  present  many  interesting  biological  problems  and  are 
widely  used  in  experimental  work.  Paramecia  measure  from  0.1  to  0.3 
mm.  in  length.  The  body  is  blunt  anteriorly,  widest  just  behind  the 
middle,  and  tapered  posteriorly.  They  are  rapid  swimmers,  revolving 
as  they  move  forward  in  a  spiral  path.  The  gullet  is  located  to  one  side, 


Figure  8.12.     Paramecium.  Generalized  drawing  combining  features  of  several  species. 


162  ^"^   ANIMAL   KINGDOM 

at  the  base  of  an  oral  depression,  and  is  usually  kept  to  the  inside  of 
the  spiral  path.  Food,  which  inchides  microscopic  particles  such  as 
bacteria,  yeast  and  algae,  is  swept  into  the  gullet  by  ciliary  action  and 
is  cUgested  in  food  vacuoles.  The  cytoplasm  circulates  slowly  in  the 
body,  so  that  each  vacuole  moves  in  a  circle.  The  digestive  processes 
are  like  those  in  the  ameba,  and  the  indigestible  remnants  are  ejected 
through  the  anus,  an  organelle  posterior  to  the  mouth.  Many  of  the 
experimental  strains  of  this  genus  have  been  cultured  for  years  on 
Aerobacter  aerogenes,  a  bacterium  cultured  easily  on  boiled  hay  or  al- 
falfa. 

The  movements  of  paramecia  indicate  highly  coordinated  activity. 
When  a  paramecium  strikes  a  solid  object,  the  waves  of  ciliary  beating 
reverse  and  the  animal  backs  up  a  short  distance.  Then  it  turns  slightly 
and  goes  ahead  again.  The  rapidity  with  which  the  animal  changes 
direction  is  astonishing.  Paramecia  have  no  visible  photoreceptors,  but 
they  do  move  toward  or  away  from  a  light  source  under  certain  cir- 
cumstances. 

In  addition  to  a  network  of  neurofibrils  (Fig.  8.3)  beneath  the  cilia, 
paramecia  have  a  layer  of  trichocysts  (Figs.  8.12  and  8.3),  spindle-shaped 
structures  located  between  the  basal  bodies,  that  can  be  discharged  to 
produce  long  threads  projecting  from  the  body  surface.  All  paramecia 
and  many  other  protozoa  have  them  in  abundance.  They  may  be  used 
for  anchorage,  for  the  capture  of  prey,  or  for  the  formation  of  protective 
cyst  walls. 

Mating  Types.  Paramecia  may  have  more  than  two  sexes,  a  con- 
dition found  only  in  the  ciliates.  All  the  sexes  look  alike,  but  an  indi- 
vidual of  a  given  sex  will  mate  only  with  an  individual  of  some  other 
sex.  As  many  as  eight  sexes  exist  for  a  given  species.  Since  "male"  and 
"female"  are  inadequate  terms,  the  sexes  of  paramecia  are  called  mating 
types,  numbered  from  I  to  VIII  in  the  order  in  which  they  were  dis- 
covered. 

The  study  of  paramecia  is  further  complicated  by  the  existence  of 
varieties,  groups  of  mating  types  that  interbreed  among  themselves  but 
which  do  not  mate  with  other  mating  types  that  are  morphologically 
similar.  Thus,  in  Paramecium  bursaria  (Table  2),  the  sixteen  mating 
types  found  in  the  United  States  fall  into  three  breeding  groups  of 
4,  8  and  4,  respectively.  From  a  genetic  point  of  view  these  three  vari- 
eties are  distinct  species,  since  they  do  not  interbreed.  They  can  seldom 
be  distinguished,  however,  except  by  breeding  experiments,  and  for 
convenience  the  various  morphologically  similar  varieties  are  given  but 
one  species  name.  Most  of  the  species  of  paramecia  and  of  many  other 
ciliates  are  now  known  to  be  composed  of  several  varieties. 

The  Killer  Trait.  T.  M.  Sonneborn,  the  protozoologist  at 
Indiana  University  who  discovered  mating  types,  has  found  that  the 
inheritance  of  sex  is  determined  partly  by  the  nucleus  and  partly  by 
the  cytoplasm,  suggesting  that  in  these  animals  the  nucleus  is  not  the 
sole  agent  for  transmitting  inherited  characters.  The  best  known  ex- 
ample of  cytoplasmic  inheritance  concerns  the  killer  trait.  In  some 
strains  of  Paramecium  aurelia,  certain  individuals  are  able  to  produce 


THE  PHYLUM   PROTOZOA  153 

Table  2.     BR.EEDIXG  RELATIONS  IN  PARAMECIUM  BURSARIA 


VARIETY 

MATING 
TYPE 

I 

II 

Ill 

I 

II 

Ill 

IV 

I 

II 

Ill 

IV 

V 

VI 

VII 

VIII 

I 

II  III 

IV 

I 



+ 

+ 

+ 

— 

— 

— 

— 



— 

— 









1 

II 
III 

IV 

+ 
+ 
+ 

+ 
+ 

+ 
+ 

+ 
+ 

— 

— 

— 

— 

— 

— 

— 

— 

— 

—      



I 

— 

— 

— 

— 

— 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

— 

—       — 

— 

II 

— 

— 

— 

— 

+ 

— 

+ 

+ 

+ 

+ 

+ 

+ 

— 

—     — 

— 

III 

— 

— 

— 

— 

+ 

+ 

— 

+ 

+ 

+ 

+ 

+ 

— 

—     — 

— 

II 

IV 

— 

— 

— 

— 

+ 

+ 

+ 

— 

+ 

+ 

+ 

+ 

— 

—    — 

— 

V 

— 

— 

— 

— 

+ 

+ 

4- 

+ 

— 

+ 

+ 

+ 

— 

—    — 

— 

VI 

— 

— 

— 

— 

+ 

+ 

+ 

+ 

+ 

— 

+ 

+ 

— 

—     — 

— 

VII 

— 

— 

— 

— 

+ 

+ 

+ 

+ 

+ 

+ 

— 

+ 

— 

—     — 

— 

VIII 

— 

— 

— 

— 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

— 

— 

—     — 

— 

I 









— 







— 

— 



— 

— 

+  + 

+ 

III 

II 
III 

IV 

— 

— 

— 

— 

— 

— 

— 

— 

— 

— 

— 

— 

+ 
+ 
+ 

-  + 
+  - 
+  + 

+ 
+ 

A  plus  sign  indicates  the  possibility  of  mating,  and  a  minus  sign  indicates  that  mating 
does  not  occur.  No  mating  type  will  mate  with  another  individual  of  its  own  type,  but  it 
will  mate  with  any  other  mating  type  of  its  variety.  Three  varieties  are  found  in  the 
United  States,  and  no  mating  type  of  one  variety  will  mate  with  any  mating  type  of  any 
other  variety.  (From  Sonneborn,  1947.) 


and  secrete  into  the  medium  small  killer  particles  which,  if  they  come 
in  contact  with  a  "sensitive"  individual,  cause  death.  All  individuals  that 
are  unable  to  produce  such  particles  are  sensitive,  whereas  all  the  in- 
dividuals that  do  produce  killer  particles  are  resistant  to  their  effect  and 
are  not  killed.  These  particles  are  manufactured  in  the  cytoplasm  from 
granules  called  kappa  particles,  present,  of  course,  in  killer  animals 
but  absent  in  sensitive  ones.  A  killer  may  contain  some  800  kappa 
particles,  and  secrete  one  killer  particle  every  five  hours.  Kappa  par- 
ticles are  self-reproducing,  multiplying  in  the  cytoplasm  independently 
of  the  division  of  the  cell.  When  the  paramecium  divides,  the  kappa 
particles  are  divided  randomly  between  the  daughter  cells.  So  long  as 
each  daughter  cell  receives  at  least  one  kappa  particle,  it  remains  a 
killer.  Under  certain  culture  conditions  the  paramecia  divide  more 
rapidly  than  the  kappa  particles,  and  the  number  of  particles  per  cell 
slowly  decreases;  ultimately,  some  cells  are  produced  that  lack  particles. 
Such  animals  become  sensitive  to  killer  particles,  and  are  no  longer 
able  to  produce  either  kappa  or  killer  particles.  Thus,  an  inherited 
characteristic  may  be  lost.  Occasionally,  however,  a  sensitive  animal  may 
mate  with  a  killer  before  it  is  killed  through  chance  encounter  with  a 
killer  particle,  and  during  the  mating  process  kappa  particles  may  be 
transferred  into  the  sensitive  cell.  These  will  subsequently  survive  and 
transform   the  cell  into  a  killer.  Furthermore,  this  trait  will  be  trans- 


164 


THE   ANIMAL   KINGDOM 


mitted  to  the  offspring  so  long  as  reproduction  of  the  particles  keeps 
pace  with  tiiat  ol  the  paramecia.  Thus,  a  trait  may  be  acquired  and 
transmitted  to  the  progeny.  The  ability  to  acquire  the  trait  is  absent  in 
some  strains,  and  Sonneborn  has  shown  that  this  depends  upon  the 
presence  in  tlie  nucleus  ol  appropriate  genes,  indicating  that  this  kind 
ol  cytoplasmic  inheritance  is  ultimately  controlled  by  the  nucleus. 

Tetrahymena.  Related  to  Faraniecimn  is  a  genus  of  smaller  ciliates, 
Tetra/tyinena.  These  are  similar  in  many  respects,  especially  in  the 
complexity  of  varieties  and  mating  types  that  occurs.  One  species,  Tetra- 
JiynieJKi  pyriforrnis,  is  of  special  interest  because  it  can  be  cultured  on  a 
li(juid  medium  in  which  the  exact  amounts  and  kinds  of  all  the  dis- 
solved chemicals  are  known.  By  varying  the  chemical  nature  of  some 
of  the  ingredients,  scientists  are  discovering  not  only  what  materials  are 
essential  for  growth  and  maintenance,  but  to  what  extent  they  may  be 
converted  into  other  materials,  and  something  of  the  steps  involved  in 
these  transformations.  This  may  seem  to  be  extravagant  detail  in  the 
study  of  a  mere  protozoan,  but  it  is  now  clear  that  the  metabolic  path- 
ways in  all  organisms  are  essentially  alike,  and  the  study  of  this  animal, 
in  which  information  can  be  obtained  rapidly  and  with  relative  ease, 
is  shedding  light  on  similar  problems  for  all  organisms,  including  man. 

Other  Ciliates.  Paramecium  and  Tetrahymena  belong  to  the  order 
Holotricha,  including  ciliates  completely  or  partially  covered  with 
simple  cilia.  In  many  the  cilia  of  the  gullet  or  near  the  mouth  are  fused 
together  to  form  small,  flaplike  membranelles.  In  the  order  Spirotricha 
the  membranelles  are  large  and  are  arranged  in  a  clockwise  spiral  lead- 
ing to  the  mouth.  Examples  of  this  order  are  the  common  hypotrichs 
(Fig.  8.13)  which  lack  cilia  on  the  upper  surface.  On  the  lower  surface, 
in  addition  to  the  adoral  membranelles,  are  patches  of  cilia  fused  into 


"Ventral 
cirrus 


Hypotrich  Pcritrich 

Figure  8.13.  Other  ciliates.  The  hypotrichs  (order  Heterotricha)  run  rapidly  on  the 
ventral  cirri,  formed  by  the  fusion  of  cilia.  The  peritrichs  (order  Peritricha)  are  mostly 
sessile  and  feed  by  sweeping  food  toward  the  mouth. 


THE   PHYLUM    PROTOZOA  \Q^ 

cirri  with  which  the  animal  scrambles  over  surfaces.  A  third  order  is 
tiie  Peritricha,  in  which  the  cilia  are  limited  to  a  counter-clockwise 
spiral  leading  to  the  mouth.  Tlie  cilia  are  usually  not  fused  to  form 
membranelles.  Most  of  the  peritrichs  are  attached  by  stalks,  and  many 
species  are  colonial  (Fig.  8.13). 

51.  Class  Suctoria 

The  suctorians  (Fig.  8.1)  are  an  offshoot  of  the  ciliates  which  retain 
both  macronucleus  and  micronucleus.  The  sedentary  adults  have  no 
cilia  but  usually  have  stalks.  The  body  bears  a  group  of  tenl'acles  that 
are  used  for  feeding.  When  prey  such  as  other  protozoa  happen  to  strike 
the  end  of  a  tentacle,  they  adhere  and  are  paralyzed  by  a  toxic  secre- 
tion. The  contents  of  the  prey  are  then  sucked  through  canals  in  the 
tentacles  and  drawn  into  the  bodv  of  the  suctorian. 

Although  adult  suctorians  lack  cilia  they  do  possess  the  basal  bodies 
of  cilia.  During  asexual  reproduction  the  suctorian  forms  a  bud  in 
which  the  basal  bodies  multiply,  become  arranged  in  rows,  and  develop 
cilia  similar  to  those  of  a  holotrich.  After  nuclear  division  the  bud 
separates  and  swims  away.  It  later  attaches  to  the  bottom,  the  cilia 
disappear,  and  tentacles  develop. 

In  view  of  the  obvious  ciliate  affinities  the  suctorians  are  often 
considered  to  be  an  order  in  the  class  Ciliata.  In  recognition  of  the  re- 
semblance of  the  larvae  to  holotrichs  the  group  is  sometimes  placed  as  a 
suborder  in  the  order  Holotricha.  Another  way  to  group  the  suctorians 
and  ciliates  is  to  place  them  in  a  subphylum,  the  Ciliophora,  separate 
from  the  other  protozoan  classes. 

52.  Class  Sporozoa 

The  Sporozoans  are  a  large  group  of  parasitic  protozoa,  some  of 
which  cause  such  serious  diseases  as  coccidiosis  in  poultry  and  malaria 
(Fig.  6.1)  in  man.  Neither  locomotor  organelles  nor  contractile  vacuoles 
are  present.  Nutrition  is  saprozoic,  nutrients  from  the  host  being  ab- 
sorbed directly  through  the  cell  wall.  Most  sporozoans  live  as  intracellu- 
lar parasites  within  the  host  cells  during  the  growth  phase  of  their  life 
cycle. 

The  cycle  of  cell  division  indicated  in  Chapter  6  for  Plasmodium 
is  common  in  the  class.  The  infecti\e  spore  matures  as  a  feeding  animal 
or  trophozoite.  It  then  divides  by  multiple  fission  into  a  number  of 
young  that  infect  new  cells  of  the  same  host  and  mature  as  more  tro- 
phozoites. Eventually,  however,  some  trophozoites  fail  to  divide  and 
instead  undergo  metamorphosis  to  sexual  forms.  The  females  become 
eggs,  while  the  males  divide  by  multiple  fission  into  many  sperm.  In 
some  sporozoans  the  females  also  divide  to  form  a  number  of  eggs.  After 
fertilization  the  new  individuals  grow  and  divide  by  multiple  fission 
into  a  number  of  spores— individuals  able  to  infect  new  hosts.  The 
spores  of  most  sporozoans  are  encapsulated  to  withstand  the  dryness  of 
the  external  world.  In  blood  parasites,  however,  such  as  plasmodia,  the 


166 


THE   ANIMAL   KINGDOM 


spores  ;ire  naked  and  nuisi  be  transniitted  directly  into  the  blood  stream 
ol  the  new  host. 

Often,  as  in  tlie  malaria  organisms,  the  formation  of  eggs  and 
sperm,  fertiU/ation,  and  the  formation  of  infective  spores  take  place  in 
a  different  kind  of  host  (e.g.,  a  mos(piilo)  from  that  in  which  trophozoite 
stages  are  found.  Such  two-host  systems  and  other  phenomena  associ- 
ated with  parasitism  are  discussed  in  Chapter  39. 

53.        Reproduction  in  the  Protozoa 

Asexual  Reproduction.  Asexual  reproduction  is  found  in  all  of 
the  protozoa.  The  nucleus  divides  mitotically,  and  the  animal  separates 
into  two  complete  organisms.  The  origin  of  the  adchtional  set  of  or- 
ganelles differs  from  group  to  group.  In  Euglena  (Fig.  8.14),  the  cen- 
triole  is  the  first  to  divide,  then  each  centriole  gives  rise  to  a  new  basal 
body.  In  the  meantime  the  old  pair  of  basal  bodies  move  farther  apart, 
and  the  new  pair  come  between  them.  The  old  fiagella  separate,  and 
each  new  flagellum  growing  out  from  the  new  basal  bodies  fuses  with  an 
old  flagellum.  The  nucleus,  which  has  gone  through  prophase  and 
metaphase,  divides  next.  Separation  into  two  individuals  (Fig.  8.15) 
begins  anteriorly,  and  ends  at  the  posterior  tip. 

In  Paramecium  the  division  is  transverse  (Fig.  8.15).  The  old  gullet 
disappears  and  is  replaced  by  two  new  gullets   (in  most  other  ciliates 


■Gullet 


Ba-Scd 
bodies 


tCcnlrioles-k 


Nuclei 


Is;-  ^ 


D 


Figure  8.14.  Details  of  asexual  reproduction  in  Euglena.  In  A  the  centriole  has 
already  divided.  R.  Each  centriole  produces  a  new  basal  body  and  flagellum.  The  nucleus 
is  in  prophase  and  the  contractile  vacuole  is  double.  C,  The  old  pair  of  flagellar  roots 
separate  and  fuse  with  the  new  roots.  D,  Mitosis  proceeds  and  the  gullet  begins  to  divide. 


THE   PHYLUM   PROTOZOA 


167 


Pa-ra-nicc  i  uiTi 


Figure  8.15.     Asexual  reproduction  in  several  protozoa.  For  explanations  see  text. 


the  old  gullet  becomes  the  gullet  of  the  anterior  daughter).  The  two 
contractile  vacuoles  become  the  posterior  vacuoles  of  the  daughters  and 
two  new  anterior  vacuoles  are  formed.  New  cilia  and  basal  bodies  appear 
among  the  old.  The  micronucleus  divides  by  mitosis.  The  macronucleus 
is  apparently  a  compound  structure  formed  by  the  amalgamation  of 
several  sets  of  chromosomes  and  merely  pulls  in  half  during  asexual 
reproduction  with  no  evidence  of  mitosis. 

The  ameba  divides  very  simply  by  mitosis;  the  cytoplasm  separates 
into  approximately  equal  halves.  The  contractile  vacuole  passes  to  one 
daughter  and  a  new  one  is  formed  in  the  other. 

Sexual  Reproduction.  Sexual  reproduction  in  free-living  protozoa 
is  known  in  detail  for  only  a  few  groups:  the  phytomonads,  the  foramin- 
ifera  and  the  ciliates.  In  numerous  other  groups  fertilization  (i.e.,  the 
fusion  of  two  gametes)  has  been  observed,  but  the  details  of  the  cycle 
are  not  known.  \V^herever  meiosis  has  been  well  studied,  the  process 
has  been  found  to  be  the  same  as  that  described  in  Chapter  6,  involving 
tetrad  formation  and  two  divisions.  Variations  between  the  groups  con- 
cern the  time  relations  of  mitosis,  meiosis  and  fertilization. 

Phytomonads  are  haploid  organisms,  each  possessing  a  single  set  of 
chromosomes.  The  zygote  never  divides  by  mitosis  to  produce  new  cells. 


163  THE   ANIMAL   KINGDOM 

Successive  mitosis 

■^  ^  ^ 


Ferlilixeilion       Encystm^nt  MeiosiS 


V 


^' 


Succi^ssive.  mitosis  in  haploid  stage 

Figure  8.16.  Sexual  cycle  in  the  Phytomonadina.  Ordinary  individuals  are  haploid 
(outsides  of  diagram)  and  reproduce  asexually.  Under  certain  conditions  they  unite  in 
pairs  (left)  to  form  a  zygote  that  encysts.  Within  the  cyst  meiosis  occurs,  so  that  when 
the  individuals  emerge  (right)  they  are  haploid  again. 


Pairing      1st  meiotiC  Sndmeiotic    Haploid        Mutual  Nuclear  fusion 

division.      division     mitosis  f ertilixalion. 

Figure  8.17.  Sexual  cycle  in  Paramecium.  Two  individuals  with  diploid  micro- 
nuclei  unite  in  conjugation  (left).  After  meiosis  (second  and  third  figures)  three  of  the 
products  degenerate  and  the  fourth  divides  by  mitosis  (fourth  figure).  Mutual  fertiliza- 
tion is  followed  by  fusion  of  the  haploid  nuclei  to  form  a  new  diploid  nucleus  (last 
figure).  The  old  macronuclei  disappear.  The  new  diploid  nuclei  divide  several  times  by 
mitosis,  and  eventually  establish  both  the  new  macronuclei  and  the  new  micronuclei. 

but  immediately  undergoes  meiosis  to  produce  active  individuals  (Fig. 
8.16).  These  may  divide  mitotically  to  produce  large  populations  of 
individuals.  In  the  simplest  case,  at  the  time  of  sexual  reproduction 
two  individuals  of  opposite  sex  fuse  together  to  form  the  zygote.  In 
some  species,  especially  in  colonial  forms  like  I'olvox,  individual  cells 
undergo  metamorphosis  before  functioning  as  gametes.  In  one  sex  the 
metamorphosing  cell  becomes  large  and  egglike,  while  in  the  other  sex 
the  metamorphosing  cell  divides  rapidly  to  produce  a  number  of  small 
spermlike  gametes.  In  these  species  the  sexes  can  be  designated  as  male 
and  female.  Only  the  haploid  stages  are  sexual,  however;  the  zygotes 
are  indeterminate  as  to  sex,  and  in  a  given  species  the  meiotic  process 
is  identical  for  all  zygotes  whether  the  ultimate  gametes  be  eggs  or 
sperm. 


THE   PHYLUM    PROTOZOA  159 

Ciliates  are  diploid  organisms,  each  possessing  a  double  set  of 
chromosomes.  Each  zygote  becomes  an  ordinary  individual  that  may 
give  rise  to  a  whole  population  by  mitosis.  At  sexual  reproduction,  two 
individuals  of  different  sexes  conjugate  (Fig.  8.f7),  pressing  together 
their  oral  surfaces.  In  each  individual,  the  micronucleus  undergoes 
meiosis.  Three  of  the  four  meiotic  products  degenerate  (notice  that  this 
is  comparable  to  polar  body  formation  in  oogenesis),  leaving  only  one 
viable  haploid  nucleus.  This  divides  once  by  mitosis,  producing  two 
identical  haploid  nuclei.  One  of  these  from  each  cell  crosses  over 
through  the  oral  region  into  the  other  individual  and  fuses  with  the 
haploid  nucleus  remaining  in  that  cell.  Thus,  two  fertilizations  result 
from  each  conjugation,  and  the  two  new  diploid  nuclei  are  identical. 
The  old  macronucleus  disintegrates  and  the  individuals  separate.  The 
diploid  nucleus  then  divides  several  times  and  eventually  gives  rise  to 
a  new  macronucleus  and  a  new  micronucleus. 

Thus,  mitosis  in  the  phytomonads  is  limited  to  the  haploid  phase, 
whereas  in  the  ciliates  only  a  single  mitosis  occurs  in  this  phase. 

In  the  foraminifera  each  generation  of  haploid  animals  is  fol- 
lowed by  a  generation  of  diploid  animals.  After  fertilization,  the  zygote 
develops  into  a  typical  foraminiferan,  adding  chambers  as  it  grows. 
Throughout  this  period  the  nucleus  divides  by  mitosis  repeatedly,  pro- 
ducing diploid,  multinuclear  adults.  All  of  the  nuclei  subsequently  go 
through  meiosis,  and  the  cytoplasm  is  divided  up  among  the  many 
haploid  nuclei.  These  abandon  the  parent  shell  and  begin  life  anew 
as  the  haploid  generation,  growing  and  adding  chambers  in  much  the 
same  manner  as  the  previous  generation,  except  that  they  remain 
mononuclear.  At  maturity,  haploid  individuals  of  opposite  sex  come 
together  in  pairs  and  secrete  a  membrane  around  themselves.  They  then 
divide  rapidly  by  mitosis,  producing  large  numbers  of  gametes.  The 
gametes  of  one  individual  unite  with  those  of  the  other  to  form  zygotes 
that  break  free  from  the  membrane  and  begin  the  diploid  generation. 
Thus,  in  this  group  mitosis  occurs  in  both  the  diploid  and  haploid 
phases. 

Sexual  phenomena  are  virtually  unknown  in  such  familiar  pro- 
tozoa as  the  ameba  and  the  euglenas.  Apparently  some  parasitic  flagel- 
lates are  diploid,  and  both  haploid  and  diploid  sporozoans  have  been 
described.  At  the  present  time  our  knowledge  of  the  place  of  meiosis  in 
the  various  cycles  is  insufficient  to  warrant  general  conclusions. 

54.        Relationships  among  the  Protozoa 

The  flagellates  are  usually  considered  to  be  a  basic  stock  of  or- 
ganisms from  which  the  other  protozoa  arose.  They  are  thought  by 
some  to  be  the  source  of  higher  animals  and  higher  plants  as  well.  As 
a  group  they  are  difficult  to  exclude  from  either  the  plant  or  the  animal 
kingdom,  a  problem  that  has  prompted  some  biologists  to  erect  a  third 
kingdom.  Botanists  usually  claim  all  of  the  flagellates  in  which  a  photo- 
synthetic  pigment  occurs,  including  closely  related  forms  such  as  some  of 
the  euglenas  and  dinoflagellates  that  have   lost  the  pigment.   They  do 


170  ''WE   ANIMAL   KINGDOM 

not  include  tlie  larger,  pigmentless  groups  such  as  the  choanoflage!- 
lates  and  many  ol  the  jxnasitic  groups.  Zoologists  generally  claim  all  of 
the  flagellates,  even  the  groui:)s  that  are  completely  autotrophic.  In- 
clusion ot  the  latter,  with  the  Phytomonadina  as  an  example,  is  probably 
not  defensible  but  persists  through  custom.  A  good  argument  for  keep- 
ing all  of  the  flagellates  together  is  that  the  transition  from  autotrophic 
to  holozoic  nutrition  appears  to  have  occurred  independently  in  differ- 
ent groups. 

Sarcodinids  are  related  to  the  flagellates  through  several  genera  of 
ameboid  organisms  that  have  flagella  and  through  several  forms  that 
resemble  typical  flagellates  in  open  water  but  which  lose  their  flagella 
and  creep  like  amebas  when  they  are  next  to  solid  surfaces.  In  fact,  the 
existence  of  so  many  intergrades  suggests  that  sarcodinids  may  have 
evolved  several  different  times  from  the  flagellates.  A  further  tie  relating 
the  groups  is  found  in  the  gametes  of  foraminiferans,  each  of  which  has 
two  tiny  flagella. 

The  ciliates  are  a  distinct  group  and  probably  arose  only  once. 
Cilia  are  structurally  like  flagella  and  are  considered  to  have  evolved 
from  them  by  extensive  duplication  and  diminution.  A  significant  step 
is  the  independence  of  the  basal  granules  from  the  centriole.  The  evo- 
lutionary origin  of  the  macronucleus  is  unknown.  During  the  conjuga- 
tion of  most  ciliates  a  bit  of  protoplasm  is  transferred  along  with  the 
migrating  nuclei.  In  one  species,  the  heterotrich  Cycloposthiiim,  each 
migrating  nucleus  and  its  bit  of  protoplasm  separates  in  the  mouth  cavity 
as  a  distinct  gamete  with  a  long  tail.  The  two  gametes  then  move  past 
each  other  to  the  opposite  side.  It  has  been  suggested  that  this  is  similar 
to  sperm  formation  in  other  organisms,  and  that  it  may  reflect  a  flagellate 
ancestry.  Suctorians  are  easily  derived  from  the  ciliates  by  a  modification 
of  the  adult  stage. 

The  sporozoa  are  probably  a  composite  group.  Some  species  show 
affinities  with  the  flagellates  while  others  more  nearly  resemble  sar- 
codinids. Multiple  fission  may  be  regarded  as  an  adaptation  to  para- 
sitism and  may  well  have  developed  independently  in  several  groups  of 
flagellates  and  sarcodinids. 

It  is,  of  course,  a  challenge  to  the  systematist  that  the  group  di- 
visions are  not  sharp  and  clear,  either  between  plant  and  animal  flagel- 
lates, between  flagellates  and  sarcodinids,  or  between  both  of  these  and 
the  sporozoans.  Actually  the  number  of  evolutionary  changes  necessary 
to  develop  one  group  from  another  is  not  great,  and  it  is  likely  that 
the  course  of  evolution  is  obscured  as  much  by  repetition  as  by  the  loss 
of  intermediate  forms. 

Questions 

1 .  Name  the  five  classes  of  protozoans  and  make  a  sketch  of  an  example  from  each. 

2.  Compare  organs  and  organelles. 

3.  What  is  a  basal  body? 

4.  Compare  movement,  nutrition  and  asexual  reproduction  in  Euglena,  Paramecium, 
and  the  ameba. 


THE   PHYLUM    PROTOZOA  171 

5.  Describe  sexual  reproduction  in  Paramecium. 

6.  What  is  cytoplasmic  inheritance? 

7.  Describe  a  typical  sporozoan  life  cycle. 

Supplementary  Reading 

General  discussions  of  many  topics  and  a  thorough  description  of  protozoa  are  found 
in  volume  I  of  The  hmertebrates  by  L.  Hyman.  Short  essays  of  interest  include  Protozoa 
as  Material  for  Biological  Research  by  D.  H.  Wenrich  and  especially  Paramecium  in 
Modern  Biology  by  T.  M.  Sonneborn. 


CHAPTER  9 


The  Phylum  Porifera 


55.  Introduction 

The  Porifera,  the  phyhim  of  animals  commonly  called  sponges,  have 
porous  body  walls  and  internal  cavities  lined  with  choanocytes.  The 
bulk  of  the  body  is  composed  of  a  jelly-like  matrix  that  usually  con- 
tains a  protein,  calcareous  or  siliceous  skeleton.  A  nervous  system  ap- 
pears to  be  lacking.  Organization  among  the  cells  is  best  described  as 
"loose,"  since  cell  relations  can  be  disrupted  without  permanent  dam- 
age to  the  organism. 

Sponges  are  sedentary  organisms  ranging  in  size  from  half  an  inch 
to  six  feet  in  height  and  varying  in  shape  from  flat,  encrusting  growths 
to  balls,  cups,  fans  and  vases.  Most  sponges  are  marine;  only  the  family 
SpongilUdae  occurs  in  fresh  water. 

The  surface  of  the  sponge  is  perforated  with  numerous  small  in- 
current  pores  and  a  few  large  excurrent  pores  called  oscula.  These  open- 
ings are  connected  internally  by  a  system  of  canals  that  includes  the 
cavities  lined  with  choanocytes.  Sponges  circulate  water  through  this 
system  and  filter  out  microscopic  food  particles.  In  the  more  complex 
sponges,  which  appear  to  have  a  more  efficient  pumping  mechanism, 
an  amount  of  water  equal  to  the  volume  of  the  sponge  is  pumped 
through  the  animal  each  minute! 

56.  General  Characteristics 

The  choanocytes  (Fig.  9.1)  are  remarkably  similar  to  the  choanoflagel- 
lates  (p.  156).  Each  cell  has  a  single  flagellum  surrounded  by  a  delicate, 
protoplasmic  collar.  As  in  the  choanoflagellates,  undulations  of  the 
flagellum  propel  water  away  from  the  cell  and  occasionally  bring  food 
particles  against  the  outside  of  the  collar.  Such  particles  are  engulfed 
in  food  vacuoles  and  moved  to  the  base  of  the  cell.  The  layer  of  cho- 
anocytes forms  the  sponge  gastrodermis. 

In  the  extracellular  matrix  that  forms  the  bulk  of  the  sponge  are 
numerous,  wandering,  ameboid  cells,  the  amebocytes.  The  amebocyte 
is  a  jack-of-all-trades,  secreting  the  gelatinous  material,  constructing  the 
skeleton,  and  gathering  up  debris  and  waste  material.  Some  become 
epidermal  cells  and  form  a  delicate  membrane  over  the  outer  surface  of 
the  sponge  or  line  the  channels  not  already  lined  with  choanocytes. 
Others  become  muscle  cells  arranged  around  the  oscula  and  other 
172 


THE  PHYLUM    PORIFERA 


173 


Captured.- 

food 

pcLTticle 


Food  particle 

being 

digested. 


Figure  9.1.  Choanocytes  from  a  sponge.  The  choanocyte  at  the  left  has  just  captured 
a  food  particle.  Adjacent  cells  show  the  movement  of  the  food  vacuole  to  the  cell  base 
and  its  eventual  transfer  to  an  amebocyte. 


openings  to  regulate  their  size.  Amebocytes  accept  food  vacuoles  (Fig. 
9.1)  from  the  choanocytes,  and  appear  to  play  a  dominant  role  in  diges- 
tion. As  they  crawl  around,  the  nutrients  are  distributed  throughout  the 
sponge. 

Sponges  appear  to  have  just  two  kinds  of  cells,  the  choanocytes  and 
the  amebocytes.  Some  investigators  describe  as  a  third  type  persistent 
embryonic  cells  that  can  become  choanocytes,  amebocytes  or  sex  cells. 
It  is  also  possible  that  sex  cells  arise  from  amebocytes  or  choanocytes. 

Structural  Types.  The  arrangement  and  complexity  of  the  internal 
channels  vary  considerably  in  different  sponges.  For  convenience  sponges 
have  been  grouped  in  three  structural  types:  (1)  the  asconoid  sponges, 
having  the  simplest  organization,  exemplified  by  the  genus  Ascon;  (2) 
the  syconoid  sponges,  resembling  in  structure  the  genus  Sycon;  and  (3) 
the  leuconoid  sponges,  having  the  most  complex  organization,  named 
after  the  genus  Leuconia  (Fig.  9.2). 


Osculum 

Incurnznt 
/  pores 


Internal 
pore 


Fla^ellaied. 
cha.Tntier 


E  recurrent 
channels 


Radia-l 
canal 


Choa.nociyteS 


Pros  opy  les 


Apopyles 


Le-u-Conoid- 


AsconoidL  Syconoid 

Figure  9.2.     The  three  structural  types  of  sponges.  In  each  the  choanocytes  are  shown 
in  black. 


174  ''WE   ANIMAL   KINGDOM 

Ascoiioid  sponges  have  a  single  large  chamber,  the  spongocoel, 
lined  with  choanocytes.  The  incurrent  pores  and  osculum  lead  directly 
to  and  from  this  chamber.  Incurrent  pores  develop  as  holes  through 
tube-shaped  cells,  the  porocytes.  1  liese  cells  develop  from  amebocytes, 
and  may  degenerate,  leaving  simple,  small  holes  in  the  body  wall. 

The  body  wall  of  syconoid  sponges  resembles  a  folded  version  of 
the  asconoid  wall.  At  least  some  syconoid  sponges  actually  develop  from 
asconoid-like  juvenile  forms.  During  development  the  body  wall  pushes 
out  to  form  numerous  finger-like  projections,  carrying  the  choanocytes 
in  their  internal  cavities.  Where  the  outer  sides  of  the  projections 
touch,  they  usually  fuse.  The  arrangement  is  such  that  any  four  pro- 
jections will  enclose  a  space,  the  incurrent  canal.  The  former  incurrent 
pores  are  now  internal,  and  are  called  prosopyles.  Whether  or  not  the 
prosopyles  are  formed  in  porocytes  is  debatable,  but  if  they  are,  the 
porocytes  soon  disappear,  for  all  prosopyles  in  the  adult  are  simple 
holes  in  the  body  wall.  At  the  outer  surface  of  the  body  new  incurrent 
pores  open  into  the  incurrent  canals.  The  cavity  of  each  finger-like 
projection  is  the  radial  canal,  which  opens  into  the  spongocoel  by  an 
internal  pore.  All  of  the  choanocytes  retreat  into  the  radial  canals,  and 
a  simple  epidermis  develops  as  the  lining  of  the  spongocoel.  The  ex- 
current  pore  is  an  osculum  similar  to  the  asconoid  osculum. 

The  leuconoid  type  represents  a  further  folding  of  the  wall.  The 
gastrodermis  pushes  out  from  the  radial  canals  into  the  body  wall  to 
form  a  series  of  spherical  flagellated  chambers.  Each  chamber  has  a 
single  inlet  from  the  incurrent  canal,  the  prosopyle,  and  a  single  outlet 
to  the  radial  canal,  the  apopyle.  Incurrent  pores,  incurrent  canals  and 
radial  canals  remain  as  in  the  syconoid  type.  The  increased  bulk  given 
the  body  wall  by  this  additional  folding  results  in  a  shrinkage  of  the 
original  main  cavity.  In  leuconoid  sponges  the  spongocoel  is  divided 
into  confluent  excurrent  channels  leading  to  the  osculum.  All  of  the 
choanocytes  are  in  the  flagellated  chambers,  and  the  radial  canals  are 
lined  with  epidermis.  The  fresh-water  sponges  and  most  of  the  marine 
sponges  are  leuconoid. 

Although  the  leuconoid  structure  is  understandable  as  a  modifica- 
tion of  the  syconoid  type,  it  should  be  noted  that  many  of  the  leuconoid 
sponges  develop  directly  to  the  leuconoid  tyj^e  without  passing  through 
asconoid  or  syconoid  stages. 

The  efficiency  of  the  sponge  as  a  pump  is  related  to  its  structural 
plan.  The  only  source  of  power  is  the  beating  of  the  choanocyte  flagella, 
which  is  not  coordinated  in  any  one  chamber.  Choanocytes  surrounding 
the  incurrent  pores  or  prosopyles  propel  water  toward  the  interior,  and 
the  water  escapes  through  the  osculum.  In  the  asconoid  sponges  the 
flagellated  chamber  is  large  and  the  force  produced  by  the  flagella  is 
directed  toward  the  middle,  so  that  flow  out  of  the  osculum  is  passive. 
In  the  leuconoid  type  the  flagellated  chambers  are  small  and  the  cho- 
anocytes are  located  so  that  they  propel  water  toward  the  excurrent 
opening.  Thus  they  not  only  draw  water  in  through  the  prosopyles  but 
also  actively  direct  it  outward  to  the  osculum. 


THE   PHYLUM    PORIFERA 


175 


57.        The  Classes  of  Sponges 

The  arrangement  of  channels  in  the  sponge  provides  for  a  convenient 
structural  classification,  but  this  has  not  proved  to  be  useful  in  sep- 
arating the  classes  of  the  phylum.  Instead,  the  classes  are  distinguished 
on  the  basis  of  the  skeleton  present:  (1)  Calcarea,  with  a  skeleton  made 
of  calcium  carbonate  spicules;  (2)  Hexactinellida,  with  a  skeleton  made 
of  siliceous  spicules,  in  which  the  basic  spicule  has  six  rays  (Fig.  9.3); 
and  (3)  Demospongia,  with  a  skeleton  made  either  of  siliceous  spicules 
(never  six-rayed),  or  spongin  fibers,  or  both. 

Calcareous  Sponges.  The  calcareous  sponges  are  marine,  shallow- 
water  forms  of  small  size,  including  all  of  the  asconoid  and  syconoid 
and  some  leuconoid  forms.  The  spicules  have  one,  three  or  four  rays 
(Fig.  9.3,  A,  B  and  C).  Spicules  with  three  or  four  rays  are  interlaced 
in  the  body  wall,  forming  a  relatively  rigid  framework.  The  one-rayed 
spicules  project  from  the  body  surface,  especially  around  the  osculum, 
and  serve  to  keep  other  organisms  away.  The  choanocytes  are  consider- 
ably larger  than  those  of  other  sponges. 

Hexacf/ne//ic/  Sponges.  Hexactinellid  sponges  are  marine,  deep- 
water  forms.  Tlie  six-rayed  spicules  are  usually  cemented  together  to 
form  rigid  girders  (Fig.  9.3,  D  and  E).  Since  the  skeleton  remains  in  one 
piece  after  the  Hesh  lias  been  removed,  these  glass  sponges  are  often 
used  as  decorations.  Even  in  the  living  glass  sponge  the  tissue  is  scanty. 
Body  structure  is  intermediate  between  syconoid  and  leuconoid,  but 
the  epidermis  is  lacking.  Euplectella  (Fig.  9.1)  has  a  large  spongocoel, 
and  an  osculum  covered  by  a  sieve  plate  that  keeps  out  large  objects. 
Other  glass  sponges  are  Hattened,  fan-shaped  structures,  one  side  of 
which  represents  the  spongocoel.  These  forms  have  no  osculum  but  they 


Figure  9.3.  Sponge  spicules.  A,  Monaxon.  B,  Triaxon.  C,  Tetraxon.  D,  E,  Hexaxons. 
A,  B  and  C,  made  of  calcium  carbonate,  are  found  in  the  class  Calcarea.  The  same  shapes, 
made  of  silicate,  are  found  in  the  class  Demospongia.  Hexaxons,  made  of  silicate,  occur 
in  the  class  Hexactinellida. 


176 


THE   ANIMAL  KINGDOM 


Figure  9.4.  Photograph  of  the  skeleton  of  the  glass  sponge,  Eiiplectella.  The  hex- 
axons  are  fused  to  form  intersecting  girders.  (Courtesy  of  the  American  Museum  of 
Natural  History.) 


are  so  placed  in  the  deep  ocean  currents  that  the  water  flows  through 
them  continually. 

Hexactinellids  are  especially  coinmon  in  the  deej)  water  off  Japan, 
where  large  numbers  of  Venus's  flower  basket  may  be  gathered.  Several 
species  of  shrimps  live  within  the  large  spongocoel,  entering  through 
the  sieve  plate  while  young  and  unable  to  leave  after  they  have  grown. 
For  some  mysterious  reason  they  are  almost  always  found  in  pairs,  one 
of  each  sex.  A  glass  sponge  with  its  imprisoned  pair  of  shriinps  is  used 
as  a  wedding  gift  in  Japan,  and  symbolizes  a  marriage  lasting  until 
death. 

Demospongia.  The  Demospongia  include  a  family  of  fresh-water 
sponges  and  a  large  variety  of  marine  forms  found  at  all  depths.  The 
spicules  have  one,  three  or  four  rays.  Spongin  fibers  are  a  protein  secre- 
tion of  the  ameboid  cells  which  form  an  anastomosing  network  in  the 
body  wall.  They  are  resistant  to  digestion  and  decay,  resembling  hair  and 
silk  in  these  respects.  All  the  members  of  this  class  have  the  leuconoid 
body  plan. 

Those  that  lack  siliceous  spicules  have  a  soft,  pliable  skeleton.  The 


THE  PHYLUM    PORIFERA  177 

bath  sponges  (Fig.  9.5),  whose  skeletons  are  familiar  objects,  are  found 
in  warm  shallow  waters  with  a  rocky  bottom,  including  the  Mediter- 
ranean Sea,  the  Gulf  of  Mexico  and  the  Caribbean.  They  are  hooked 
from  the  ocean  bottom  by  poles  having  a  pronged  fork  at  the  end.  A 
short  stay  on  shipboard  is  enough  to  kill  them,  after  which  they  are 
left  lying  in  shallow  water  until  the  flesh  is  decayed.  Then  they  are 
beaten,  washed  and  finally  bleached  in  the  sun.  All  that  remains  is  the 
spongin  network,  whose  many  tiny  interstices  permit  it  to  soak  up  a 
large  amount  of  water.  The  sponge  fishery  is  limited  by  the  rate  of 
reproduction  and  growth  of  the  sponges.  Many  of  the  grounds  have 
been  overfished,  and  the  fishermen  are  beginning  to  experiment  with 
the  cultivation  of  sponges.  Sponges  are  cut  into  many  small  pieces  that 
are  fastened  to  cement  blocks  and  set  out  in  the  sea.  They  take  many 
years  to  reach  marketable  size. 

Some  of  the  Demospongia  live  only  upon  other  organisms.  The 
boring  sponges  settle  as  larvae  onto  the  shells  of  oysters  or  clams,  into 
which  the  young  sponge  bores  by  dissolving  the  shell.  It  does  not  harm 
the  host  directly,  but  as  the  shell  becomes  honeycombed  and  weakened 
it  eventually  falls  apart,  and  the  host  is  rapidly  consumed  by  predators. 
Another  group,  the  hermit  crab  sponges,  settle  on  snail  shells  inhabited 
by  hermit  crabs.  They  grow  to  a  considerable  size,  eventually  completely 
covering  the  shell.  As  time  passes  the  shell  dissolves,  leaving  a  snail- 
shaped  cavity  in  the  sponge,  still  occupied  by  the  hermit  crab.  Because 
it  is  carried  around  the  sponge  is  never  buried  by  silt  (always  a  danger 


Figure  9.5.     The  common  bath  sponge.  Only  the  spongin  skeleton  remains.  (Courtesy 
of  the  American  Museum  of  Natural  History.) 


178 


THE   ANIMAL   KINGDOM 


to  attached  organisms),  and  the  crab  is  protected  from  predation  by  the 
disagreeable  flavor  ol   the  sponge. 

Some  ol  the  spider  crabs  and  other  slow-moving  crabs  break  off 
pieces  of  living  sponge  and  hold  them  or  glue  them  on  their  backs, 
where  they  may  become  permanently  attached  and  grow.  Such  crabs 
also  plant  other  attached  organisms  on  their  backs,  and  walk  about  like 
animated  "gardens."  They  must  repeat  this  operation  each  time  they 
shed  their  shell. 

Most  sponges  apparently  have  an  unpleasant  taste  to  most  animals, 
for  only  a  few  snails  eat  them.  Fish  avoid  sponges,  and  hence  many 
smaller  organisms  seek  refuge  inside  them.  Any  sizable  sponge,  selected 
at  random,  will  be  found  to  be  sheltering  a  number  of  animals  in  its 
canals. 

58.        Reproduction 

Sexual  reproduction  in  the  sponges,  as  in  the  protozoa,  has  been 
studied  in  too  few  species  to  permit  generalizations.  All  sponges  studied 
appear  to  be  diploid,  and  to  have  the  usual  metazoan  processes  of 
oogenesis  and  spermatogenesis  as  described  in  Chapter  6.  Fertilization  is 
internal.  The  eggs  are  retained  just  beneath  the  choanocytes  where  they 
are  fertilized  by  sperm  brought  in  with  the  current. 

The  best  studies  of  early  development  are  in  the  genera  Syco7i  and 
Grantia  of  the  class  Calcarea.  In  these  the  egg  cleaves  to  form  a  blastula- 
like  structure  (Fig.  9.6,  A)  that  is  inside  out  when  compared  with  the 
blastula  stages  of  other  animals.  The  nuclei  lie  toward  the  inner  ends 


Materncd 
cTi  oa.no  C3rte-S 


Figure  9.6.  Development  in  the  sponge,  Sycon.  A,  The  embryo  lies  embedded  be- 
neath the  choanocytes  of  the  parent.  B,  Eversion.  C,  Free-swimining  amphiblastula. 
D,  Attachment  and  invagination.  (A  and  B  after  Dubosq  and  Tuzet;  C  and  D  redrawn 
from  Hyman.) 


THE  PHYLUM   POR/FERA  179 

of  the  cells  rather  than  the  outer  ends,  and  the  flagella  that  appear  on 
the  cells  toward  the  animal  pole  project  inward  instead  of  outward. 
The  embryo  is  also  peculiar  in  having  a  mouth  at  the  vegetal  pole 
through  which  food  is  taken  from  the  parent.  The  food  is  utilized  by 
the  cells  and  in  this  way  the  embryo  grows.  ^Vhen  fully  developed  the 
embryo  turns  inside  out  (Fig.  9.6,  B)  through  its  mouth  and  then  pene- 
trates through  the  maternal  choanocyte  layer  to  escape  into  the  chan- 
nels of  the  parent  sponge.  The  flagellated  cells,  whose  flagella  now 
project  outward,  form  the  anterior  half  of  the  larva  and  the  nonflagel- 
lated  cells  make  up  the  posterior  portion.  This  free-swimming  stage  is 
the  amphiblastula  (Fig.  9.6,  C)  and  is  similar  in  appearance  to  the 
blastulae  of  a  few  other  animals.  The  amphiblastula  swims  away  and 
attaches  to  the  bottom  by  its  anterior  end.  As  it  becomes  attached,  the 
anterior,  flagellated  half  invaginates  into  the  posterior  half  to  form  a 
two-layered  structure  (Fig.  9.6,  D).  The  flagellated  cells  become  the  cho- 
anocytes  while  the  outer  layer  forms  all  the  rest  of  the  sponge. 

The  presence  of  flagella  that  project  inward  and  the  later  inversion 
of  the  embryo  through  its  mouth  are  unique  to  the  sponges  as  features 
of  sexual  reproduction.  A  similar  process  is  found  in  the  colonial  flagel- 
late, Volvox,  but  is  associated  only  with  asexual  reproduction. 

The  development  of  other  sponges  is  less  well  known  but  they 
follow  different  developmental  patterns.  Free-swimming  larvae  of  many 
species  have  been  found,  and  in  some  of  these  a  process  similar  to 
gastrulation  in  other  animals  takes  place.  An  outer  flagellated  layer 
completely  or  partially  surrounds  an  inner  cell  mass  (Fig.  9.7).  When 
such  larvae  attach  and  develop,  the  inner  cell  mass  produces  the  bulk 
of  the  sponge.  In  some  forms  the  flagellated  cells  migrate  inward  to 
become  the  choanocytes,  while  in  others  they  are  destroyed  and  the 
choanocytes  develop  from  the  inner  mass. 

Most  sponges  also  reproduce  asexually.  Pieces  of  some  sponges  fall 
off,  attach  to  a  new  substrate,  and  grow.  In  others,  flagellated  embryos 
are  produced  that  resemble  the  sexually  produced  larvae.  These  swim 
away  and  attach.  In  still  others,  including  fresh-water  sponges,  balls  of 
cells  embedded  in  the  body  are  surrounded  with  a  capsule.  After  the 
sponge  dies  (during  the  winter  in  fresh-water  forms)  and  the  body  de- 
cays, these  gemmules   are  released.   Many  of   them  are  equipped   with 


A  B 

Figure  9.7.     Other  sponge  larvae.  A,  From  the  class  Calcarea.  B,  From   the  class 
Demospongia. 


1^0  THE   ANIMAL   KINGDOM 

hooks  that  serve  to  anchor  them  to  the  bottom.  When  the  environment 
is  suitable  (in  the  spring)  the  gemmules  sprout  into  young  sponges. 

Sponges  are  simple  animals,  poorly  coordinated,  and  it  is  not  sur- 
prising that  they  can  easily  regenerate  lost  parts.  Indeed,  if  the  more 
complex  sponges  with  several  oscula  are  cut  in  half,  there  are  no  lost 
parts.  The  ability  of  sponge  cells  to  reorganize  was  demonstrated  by 
E.  V.  W^ilson  in  1907.  Sponges  squeezed  into  a  dish  through  fine  silk 
cloth  are  disaggregated  into  minute  cell  clumps.  The  choanocytes  swim 
about  on  the  bottom  by  their  flagella,  and  the  amebocytes  crawl.  When- 
ever cells  come  in  contact,  they  remain  together.  The  bottom  of  the 
dish  is  soon  covered  with  balls  of  cells,  each  of  which  develops  into  a 
tiny  sponge  if  it  includes  both  choanocytes  and  amebocytes.  If  the  mass 
is  very  small,  the  choanocytes  congregate  on  the  outside  and  the  organism 
resembles  a  colonial  choanoflagellate.  If  the  mass  is  large  enough  the 
choanocytes  form  chambers  covered  by  the  amebocytes. 

Questions 

1.  Diagram  the  three  structural  types  of  sponges. 

2.  Which  sponges  are  found  in  fresh  water? 

3.  How  do  the  Demospongia  differ  from  the  Calcarea? 

4.  Discuss  gastrulation  in  the  sponges. 

Supplementary  Reading 

A  general  and  thorough  account  of  the  phylum  is  given  in  The  Invertebrates,  volume 
I,  by  L.  Hyman. 


CHAPTER  10 


The  Phyla  Coelenterata  and 

Ctenophora 


59.  Introduction 

In  addition  to  such  animals  as  fishes  and  whales  which  swim  actively 
through  considerable  distances,  open  water  contains  many  organisms 
that  are  passive  and  float  aimlessly  with  the  water  currents.  They  may 
swim,  but  not  strongly  enough  to  travel  appreciably  in  a  horizontal 
direction  or  to  stay  in  one  place  against  a  current.  This  assemblage  of 
organisms  is  the  plankton,  and  their  passive,  floating  way  of  life  is  called 
planktonic.  The  radiohiria  and  foraminifera  described  in  Chapter  8  are 
planktonic  protozoans,  belonging  to  the  marine  [jhmkton.  The  largest 
and  most  familiar  of  the  plankton  are  jellyfish,  often  seen  from  ship- 
board as  vast  swarms  in  the  upper  few  feet  of  water. 

The  common  name,  jellyfish,  is  applied  to  a  heterogeneous  group 
of  organisms  having  a  jelly-like  consistency,  members  of  the  phylum 
Coelenterata  and  the  phylum  Ctenophora  (Fig.  10.1).  The  coelenterate 
jellyfish  usually  have  numerous  tentacles  with  stinging  cells  and  swim 
by  muscular  contractions  of  an  umbrella-shaped  body.  The  ctenophores 
usually  have  two  tentacles  with  adhesive  cells,  and  move  by  the  beating 
of  numerous  combs,  each  of  which  is  a  row  of  fused  cilia.  In  both  phyla 
a  simple  epithelium,  the  epidermis,  covers  the  body,  another  simple 
epithelium,  the  gastrodermis,  lines  a  branched  gut,  and  a  jelly-like 
mesoglea  between  the  epithelia  forms  the  bulk  of  the  body.  Both  groups 
are  primarily  carnivorous,  catching  other  animals  of  appropriate  size. 
Small  jellyfish  feed  upon  small  worms,  tiny  shrimplike  crustaceans  and 
larval  fish;  larger  ones  catch  larger  fish  and  sometimes  other  jellyfish.  A 
single  pelagic  coelenterate  is  called  a  medusa;  the  ctenophore  is  called 
a  comb  jelly.  The  coelenterate  phylum  also  includes  a  number  of 
bottom-living  forms  such  as  hydras,  sea  anemones  and  corals,  and 
floating  colonies  such  as  the  Portuguese  man-of-war.  A  few  species  of 
ctenophores  creep  on  the  bottom.  As  an  introduction  to  these  phyla  we 
will  first  describe  one  of  the  medusae. 

60.  Conionemus:  General  Behavior 

Gonionemus  (Fig.  10.2)  is  a  genus  of  small  medusae  about  2  centime- 
ters in  diameter  when  fully  grown.  G.  murbachi  is  a  common  species  in 

181 


182 


THE  ANIMAL  KINGDOM 


Fiaure  10  1  Tellyfish.  The  upper  three  are  medusae,  members  of  the  phylum  Coel- 
enterau  The  lower  two  are  comb  jellies,  in  the  phylum  Ctenophora.  (Medusae  redrawn 
from  Mayer;  Mnemiopsis  from  Hyman;  Hormiphora  from  Chun.) 

Long  Island  Sound  and  G.  vertens  is  found  in  Puget  Sound.  Like  most 
medusae  it  does  not  merely  float  in  the  water,  but  moves  rhythmically 
up  and  down  through  a  span  of  several  feet.  The  upward  movement  is 
active,  produced  by  repeated,  slow,  graceful  contractions  of  the  body. 
The  contracting  muscle  fibers  are  arranged  circularly  just  beneath  the 
epidermis  of  the  lower  or  subumbrellar  surface  of  the  umbrella,  and 
also  in  the  velum,  a  delicate  membrane  extending  inward  from  the 
lower  edge  of  the  umbrella.  Contraction  closes  the  umbrella,  contraction 
of  the  velum  reduces  the  size  of  the  opening  beneath  the  umbrella,  and 
the  downward  jet  of  water  produced  pushes  the  animal  upward.  Be- 
tween contractions  elasticity  of  the  body  reopens  the  umbrella.  Through- 
out the  pulsing  ascent  the  sixty  to  eighty  tentacles  on  the  umbrellar  rim 


THE  PHYLA   COBLiNTBRATA   AND  CTENOPHORA 


183 


are  usually  shortened  by  the  contraction  ot  muscle  fibers  running 
lengthwise  through  them. 

Downward  movement  is  passive,  for  the  jellyfish  is  slightly  heavier 
than  sea  water  and  sinks  slowly  if  it  does  not  swim.  As  the  velar  and 
subumbrellar  muscles  relax  completely  the  medusa  opens  wide.  The 
muscle  fibers  in  the  tentacles  relax  and  the  tentacles  slowly  elongate. 
Probably  as  a  result  of  its  shape,  the  jellyfish  usually  turns  over  as  it 
falls.  The  tentacles  may  trail  behind,  or  be  held  out  to  the  sides.  By 
swimming  up  and  drifting  down  in  this  way  Gonionemus  "nets"  for 
food. 

The  medusa  needs  an  orienting  mechanism  if  it  is  to  swim  upward, 
rather  than  at  random.  The  statocysts  (Fig.  10.3)  are  sense  structures 
that  determine  the  direction  of  giavity.  Each  is  a  small  concretion  of 
calcium  carbonate  suspended  on  a  flexible  stalk  in  a  cavity.  The  pressure 
of  the  stone  against  the  cells  in  the  wall  of  the  cavity  apparently  pro- 
vides the  basis  for  orientation.  Many  statocysts  are  embedded  in  the 
margin  of  the  medusa  between  the  bases  of  the  tentacles. 

Although  Gonionemus  lacks  eyes  and  does  not  orient  its  body  to 
light,  it  sinks  when  the  light  is  strong  and  rises  when  it  is  weak.  Other 
species  of  medusae  have  eyespots,  some  of  which  provide  directional 
information  so  that  the  jellyfish  can  swim  toward  or  away  from  the  light. 


•Tentacle 

G  astro  dermis 


Stomach' 


Circular  canal 

Exumbrellar 
V  epidermis"^ 

-Mesoglea 

'  Radial  canzd 


Nerve  ring- 


Velum 


Mouth  V 
•  Subumbrelleir  epidermis 


Figure  10.2.  Gonionemus.  Above,  side  view  of  whole  animal,  with  many  of  the 
tentacles  incompletely  drawn.  (Redrawn  from  Mayer.)  Below,  diagrammatic  hemisection 
showing  tissue  layers;  tentacles  and  gonads  omitted. 


284  THE  ANIMAL  KINGDOM 


stalk 


Statocysfc 
csLvity 

Limestone 
Secretion 

Figure  10.3.     Detail  of  a  statocyst,  located  in  the  margin  of  the  bell  between  the 
circular  canal  on  the  left  and  the  epidermis  on  the  right.  (After  Hyman.) 

The  temperature  and  pH  of  the  water  may  also  influence  the  average 
depth  at  which  the  jellyfish  stays.  If  the  temperature  or  pH  increases  the 
medusae  move  to  greater  depths,  while  if  the  temperature  or  pH  de- 
creases they  rise  toward  the  surface.  At  night  the  light  is  greatly  decreased 
and  the  pH  of  the  water  falls  slightly  so  that  the  medusae  are  found 
closer  to  the  surface  than  in  the  daytime.  Similar  diurnal  migrations  are 
performed  by  many  pelagic  organisms,  and  are  especially  marked  in 
certain  crustaceans  that  migrate  several  hundred  feet  vertically. 

Sense  receptors  for  temperature  and  pH  are  probably  scattered 
diffusely  around  the  margin  of  the  umbrella,  a  region  known  to  be  sen- 
sitive to  certain  chemicals.  The  activity  of  Gonioneinus  increases  mark- 
edly when  the  juice  of  food  organisms  is  added  to  the  water.  It  swims 
horizontally  as  well  as  upward,  keeping  its  tentacles  extended  in  a  ran- 
dom search  for  the  prey. 

Many  other  medusae  have  behavior  patterns  like  that  of  Gonio- 
nemus.  Gonionemus  and  its  close  relatives  can  attach  themselves  to 
marine  plants  by  means  of  adhesive  pads  on  the  tentacles  (Fig.  10.2). 
They  live  primarily  in  shallow  water  where  rooted  vegetation  is  abun- 
dant, and  are  often  found  in  the  daytime  attached  by  a  few  of  their 
tentacles  with  the  rest  outstretched  in  the  netting  position.  This  adapta- 
tion to  shallow  water  is  exceptional,  and  most  medusae  remain  afloat 
all  of  the  time. 


61.        Gonionemus:  Feeding  and  Digestion 

Nematocysts.  When  a  small  organism  brushes  against  an  out- 
stretched tentacle  it  is  stung,  and  in  its  violent  reaction  to  being  stung 
it  may  throw  itself  against  more  tentacles.  Further  stinging  paralyzes 
the  prey,  which  is  tightly  held  by  the  tentacles.  Each  tentacle  has  numer- 
ous rings  of  projecting  stinging  cells  visible  under  the  microscope  (Fig. 
10.4/i).  Within  each  of  these  is  a  shiny  oval  body,  the  nematocyst  (Fig. 
10.5),  shaped  like  a  tiny  balloon  with  a  very  long  tubular  neck,  the 
nematocyst  thread.  As  the  nematocyst  develops  within  the  stinging  cell 


THE  PHYLA    COELENTERATA    AND    CTENOPHORA 


185 


A  B 

Figure  10.4.  A,  A  portion  of  a  tentacle  from  Gonionemiis,  showing  rings  of  nemato- 
cysts.  (After  Hyman.)  B,  Gonionemus  veilens  actively  swimming.  (Courtesy  Douglas  P. 
Wilson.) 


A.  Undischarged  B.  Discharged 

Figure   10.5.     Diagrammatic  view  of  a  nematocyst.  A,  Before  discharge.  B,  Right, 
Everted. 

the  thread  appears  in  an  inverted  position,  like  a  glove  finger  pulled 
inside  out.  To  accommodate  its  length  the  thread  is  tightly  coiled.  On 
the  outer  surface  of  each  nematocyst  is  a  tiny  projecting  trigger.  Upon 
suitable  stimulation,  which  appears  to  include  taste  in  addition  to  a 
touch  on  the  trigger,  the  nematocyst  fires. 

Firing  is  explosive.  The  nematocyst  absorbs  water,  which  increases 
the  internal  pressure  and  everts  the  thread,  just  as  a  pulled-in  glove 
finger  can  be  everted  by  blowing  into  the  glove.  The  diameter  of  the 
thread  is  so  small  and  its  eversion  so  fast  that  it  easily  penetrates  the  tis- 
sue of  the  prey.  After  discharge  (Fig.  10.5)  the  everted  thread  is  seen  to 
bear  recurved  hooks  on  a  swollen  base  and  to  be  open  at  the  tip.  The 
hooks  hold  the  prey  fast  while  the  poisonous  contents  of  the  nematocyst 
are  discharged  through  the  thread  into  its  body. 

Gonioyiemiis  has  just  one  kind  of  nematocyst.  Other  coelenterates 
have  several  distinct  kinds,  with  marked  differences  in  the  details  of 
hooks  and  thread. 

/ngesf/on.     Having  caught  its  prey  the  medusa  shortens  its  tentacles 


185  J""^   ANIMAL   KINGDOM 

(sometimes  only  those  holding  the  prey)  and  bends  them  toward  the  mid- 
dle ot  the  sLibumbrellar  surface.  The  side  of  the  umbrella  holding  the 
prey  shrinks  and  bends  inward.  This  movement  brings  the  prey  toward 
the  mouth,  an  opening  at  the  end  of  a  short  tube,  the  manubrium,  that 
hangs  down  from  the  middle  of  the  subumbrellar  surface.  As  tfie  prey 
is  brought  toward  the  mouth  the  manubrium  extends  and  bends  toward 
the  prey.  This  synchronized  activity  involves  the  longitudinal  muscle 
fibers  of  the  tentacles,  radial  muscle  fibers  beneath  the  subumbrellar  epi- 
dermis, and  both  circular  and  longitudinal  fibers  in  the  manubrium. 
Swimming  muscles  are  not  involved. 

Surrounding  the  mouth  are  four  lips,  each  folded  longitudinally. 
The  surface  on  the  inner  side  of  the  fold,  toward  the  mouth,  is  ciliated. 
Mucus  is  secreted  on  this  surface  and  the  ciliary  activity  moves  the 
mucous  sheet  steadily  into  the  mouth.  As  soon  as  tfie  lips,  which  are 
weakly  muscular,  have  folded  over  the  prey,  the  tentacles  release  the 
bases  of  the  discharged  nematocysts  and  the  medusa  resumes  its  nor- 
mal shape. 

Digestion.  The  mouth  opens  into  a  large  stomach  in  the  middle 
of  the  medusa.  When  the  prey  has  been  swallowed,  the  mouth  closes 
tightly  and  some  of  the  gastrodermis  cells  secrete  a  digestive  juice  con- 
taining proteases.  These  enzymes  initiate  the  breakdown  of  protein  and 
reduce  the  prey  to  a  broth. 

Close  to  the  subumbrellar  surface  the  stomach  extends  laterally  as 
four  radial  canals  (Fig.  10.2),  which  are  continuous  at  the  margin  with 
a  circular  canal,  from  which  small  branches  extend  into  the  tentacles. 
Both  the  stomach  and  the  canals  are  lined  with  tracts  of  ciliated  gastro- 
dermis cells  which  set  up  currents  to  circulate  the  broth  throughout  the 
system.  Other  gastrodermis  cells  absorb  dissolved  nutrients  and  ingest 
the  remaining  small  food  particles.  Ingestion  is  the  same  as  in  many 
protozoans.  Particles  are  taken  up  in  food  vacuoles  where  digestion  of 
fats  and  carbohydrates,  and  further  digestion  of  proteins,  take  place.  As 
among  protozoans,  the  vacuoles  become  acid  and  then  alkaline  during 
digestion. 

Since  the  stomach  and  canals  perform  both  circulatory  and  digestive 
functions,  digesting  the  food  and  distributing  it  to  all  parts  of  the  body, 
they  are  properly  called  a  gastrovascular  system.  Indigestible  residues 
are  eliminated  through  the  mouth,  which  thus  functions  as  both  mouth 
and  anus. 

62.        Gonionemus:  Diffusion 

The  jelly-like  mesoglea  is  present  everywhere  between  the  gastro- 
dermis and  epidermis.  In  the  tentacles  it  is  very  thin  but  in  the  um- 
brella it  is  thick,  providing  bulk  and  determining  the  shape  of  the 
relaxed  animal.  In  Gonionemus  the  mesoglea  lacks  cells  and  is  nonliving. 
Since  it  is  about  96  per  cent  water,  dissolved  materials  diffuse  readily  in 
all  directions.  Diffusion  is  an  adequate  mechanism  in  jellyfish  for  the 
distribution  of  nutrients  from  gastrodermis  to  epidermis,  and  for  respira- 
tion and  excretion  (cf.  Chapter  5). 


THE   PHYLA    COELENTERATA    AND   CTENOPHORA 


187 


When  a  jellyfish  is  not  digesting  food  the  mouth  usually  remains 
open  and  fresh  sea  water  is  circulated  through  the  gastrovascular  system 
by  the  ciliated  tracts.  Hence,  most  of  the  time  all  of  the  tissues  are  in 
direct  contact  with  sea  water,  facilitating  a  direct  exchange  of  gases  and 
waste  products  by  diffusion.  In  all  probability  the  water  in  the  gastro- 
vascular system  contains  enough  oxygen  to  supply  the  gastrodermis  cells 
while  the  mouth  is  closed  during  digestion. 

63.        Gonionemus:  Nervous  System 

Classically  the  nervous  system  of  the  coelenterate  is  described  as  a 
nerve  net,  a  diffuse  network  of  neurons  each  with  several  processes  that 
synapse  with  those  of  other  neurons.  The  system  is  distinguished  from 
those  of  higher  organisms  by  the  transmission  of  impulses  across  synapses 
in  either  direction,  rather  than  in  one  direction  only.  The  concept  of  a 
generalized  nerve  net,  however,  does  not  adequately  explain  the  specific 
coordinated  behavior  of  the  medusa.  Detailed  work  has  shown  that  the 
system  is  not  this  simple.  On  the  upper  or  exumbrellar  surface  of 
the  medusa  the  neurons  are  sparse  and  their  arrangement  is  indeed  that 
of  a  simple  net.  At  the  margin,  however,  the  nerve  cells  are  concentrated 
to  form  circular  fiber  tracts,  the  nerve  ring.  On  the  subumbrellar  surface 
the  nerve  fibers  are  arranged  radially,  extending  from  the  margin  to- 
ward the  center  with  few  if  any  circular  fibers. 

The  nerve  ring  of  Gonione?7iiis  is  double  (Fig.  10.6),  with  rings 
above  and  below  the  line  where  the  velum  is  attached.  The  lower  ring 
is  primarily  motor  in  function  and  sends  fibers  to  the  muscles.  The  upper 
ring  is  primarily  sensory  and  integrates  the  information  coming  in  from 


Edc  umbr  ellctr 
epithelium: 


Sensor"  37 
epithe-liiim. 


Uppers 
nerve  iri-n.^ 

Loureip 
nerve  rin^ 


G  a.strodermiS 


Subuiritrellar 
epit  Inel  i  um. 


Figure  10.6.  Section  through  the  margin  of  the  bell  in  Gonionemus,  showing  the 
nerve  ring.  It  lies  embedded  in  the  double  layer  of  epiderrnis  at  the  base  of  the  velum, 
(After  Hyman.) 


1^8  THE   ANIMAL   KINGDOM 

the  several  senses  scattered  around  the  umbrella  margin.  Notable  excep- 
tions are  tlie  nerves  from  the  statocysts,  which  go  to  the  motor  ring. 
While  the  other  senses  influence  die  activity  of  the  medusa,  and  may 
even  reverse  its  direction  of  movement,  the  direction  itself  is  related 
only  to  gravity.  The  intimate  association  of  the  gravity  sense  with  the 
motor  ring  is  therefore  significant. 

Locomotion  is  efficient  only  when  the  muscle  fibers  of  the  umbrella 
contract  synchronously.  Coordination  is  effected  by  the  circular  fibers  of 
the  nerve  ring.  If  the  ring  is  cut,  coordination  is  lost  and  the  medusa 
swims  erratically  with  lopsided  beats. 

Feeding  behavior  requires  coordination  oriented  radially  rather 
than  circularly.  The  shortening  and  bending  of  the  tentacles,  the  bend- 
ing of  the  umbrella,  and  the  lateral  bending  of  the  manubrium  must 
be  in  the  right  direction  if  the  prey  is  to  be  successfully  transferred  to 
the  mouth.  The  nerve  ring  is  not  important  in  this  behavior  and  may 
be  cut  without  serious  eftect  if  the  cut  is  not  exactly  on  the  radius 
involved.  The  radial  nerve  fibers  of  the  subumbrellar  surface  are  in- 
volved, for  this  coordination  disappears  if  they  are  cut.  The  manubrium 
may  extend  and  bend,  but  fails  to  bend  in  the  right  direction. 

64.       Gonionemus:  Reproduction 

Both  male  and  female  Gonionemus  have  four  gonads  that  develop 
in  the  epidermis  of  the  subumbrellar  surface  and  hang  downward  as 
ruffles  parallel  to  the  four  radial  canals  (Fig.  10.2).  Since  the  canals  are 
close  to  the  subumbrellar  surface  the  gonads  are  close  to  a  nutrient 
source.  Eggs  and  sperm  are  shed  into  the  surrounding  water  where  fer- 
tilization takes  place. 

The  fertilized  egg  develops  rapidly  into  a  small  ciliated  larva,  the 
planula  (Fig.  10.7,  A).  The  planula  is  a  swimming  gastrula  composed  of 
a  layer  of  ectoderm  enclosing  a  solid  core  of  large  endoderm  cells. 
Planulae  are  found  in  all  of  the  classes  of  coelenterates.  The  planula  of 
Gonionemus  does  not  develop  directly  into  a  medusa,  but  attaches  to 
some  solid  object  and  becomes  a  polyp  (Fig.  10.7,  B). 

The  polyp  is  tube-shaped  with  an  outer  epidermis  and  inner  gas- 
trodermis  separated  by  a  very  thin  mesoglea.  The  tube  is  closed  at  the 
attached  end,  forming  a  foot,  and  the  open  free  end  is  the  mouth.  The 
simple  cylindrical  cavity  is  the  stomach.  Surrounding  the  mouth  is  a 
ring  of  tentacles  bearing  nematocysts.  Like  the  medusa,  the  polyp  feeds 
by  snaring  prey  Avith  its  outstretched  tentacles  and  transferring  it  to  the 
extensible  mouth. 

Structurally  the  polyp  is  simpler  than  the  medusa.  Circular  and 
longitudinal  muscle  fibers  are  sparse  and  not  arranged  in  layers  or  sheets 
as  in  the  medusa.  The  nervous  system  lacks  a  nerve  ring,  and  throughout 
its  structure  suggests  a  nerve  net  with  neurons  somewhat  more  numerous 
around  the  mouth.  In  many  respects  the  polyp  is  a  juvenile  stage,  inter- 
mediate between  the  planula  and  the  medusa. 

The  polyp  of  Gonionemus,  only  1  mm.  in  diameter,  is  unusually 
small  and  squat.  As  it  grows,  the  polyp  reproduces  asexually  by  budding. 


THE  PHYLA    COELENTERATA    AND    CTENOPHORA 


189 


B.  Polyp 


A.  Planula. 
Figure   10.7.     Reproduction  in   Gommiemiis.  A,  Planula   larva   that  develops  from 
the  egg.  B,   1  he  polyp,  showing  mouth  and  four  tentacles.  A  frustule  is  forming  on  the 
side  and  is  also  shown  in  successive  stages  as  it  later  creeps  away.  {A  after  Hyman; 
B  modihed  from  Hyman  after  Joseph.) 

One  side  ot  the  bod)  thickens,  becomes  constricted  as  a  separate  tube, 
and  very  slowly  creeps  away.  This  bud  or  frustule  has  no  mouth, 
tentacles  or  stomach  cavity.  Over  the  span  of  several  days  the  frustule 
may  move  several  inches,  after  which  it  settles  do^\•n  with  one  end  at- 
tached and  develops  into  a  typical  polyp. 

Asexual  reproduction  by  budding  is  connnon  among  the  coelen- 
terates.  Most  j^olyp  stages  are  able  to  reproduce  this  way  but  only  a  few 
kinds  of  medusae  show  the  phenomenon.  We  have  observed  that  sponges 
reproduce  by  asexual  buds,  and  as  we  shall  see  later  many  other  animal 
groups  do  also.  Coelenterates  may  pass  through  many  generations  of 
asexual  budding  before  developing  sexually  mature  individuals. 

In  the  summer  Gouiotieiniis  polyps  produce  spherical  buds  that 
develop  into  medusae.  A  well  fed  polyp  may  produce  several  such  buds 
but  a  small  or  starved  individual  may  produce  only  one.  In  the  latter 
case  the  entire  polyp  may  transform  into  a  medusa. 

While  still  attached,  the  medusa  bud  develops  a  velum,  manubrium 
with  mouth,  and  eight  tentacles.  It  begins  to  pulsate  and  eventually 
breaks  free  by  its  own  ac  tivity.  As  it  grows,  increasing  its  diameter  from 
1  mm.  to  2  cm.,  new  tentacles  grow  out  between  those  already  present 
on  the  umbrellar  margin. 

65.        Classes  of  the  Phylum  Coelenterata 

Differences  in  structure  and  life  history  are  the  criteria  for  grouping 
coelenterates  in  three  classes.  Gonionemiis  belongs  to  the  class  Hydrozoa, 
in  which  the  medusa  has  a  velum  and  the  polyp  has  a  simple,  unpar- 


190  '■"^  ANIMAL  KINGDOM 

titioned  gut.  Medusa  buds  arise  Irom  the  side  of  the  polyp.  The  class 
Scyphozoa  includes  most  ol  the  larger  jellyfish.  The  scyphozoan  medusa 
lacks  a  velum,  the  stomach  cavity  ol  the  polyp  is  subdivided  by  four 
longitudinal  partitions,  and  medusae  are  formed  by  transformation  of 
the  end  of  the  polyp  so  that  the  polyp  mouth  becomes  the  medusa 
mouth.  The  class  Anthozoa  includes  sea  anemones  and  corals.  The 
polyps  have  a  stomach  cavity  subdivided  by  6,  8  or  more  partitions  and 
become  sexually  mature  without  transformation  into  a  free-swimming 
stage.  Medusae  are  lacking. 

66.        Class  Hydrozoa 

The  typical  hydrozoan  life  history  includes  a  juvenile  polyp  stage 
that  reproduces  asexually  and  an  adult  medusa  stage  that  reproduces 
sexually.  A  full  range  of  variations  occurs,  however,  from  species  that 
lack  medusae  to  species  that  lack  polyps.  Hydrozoans  lacking  polyps 
live  in  the  open  ocean  where  an  attached  stage  is  impractical;  the 
planula  develops  directly  into  a  medusa.  Polyps  that  lack  medusae  live 
near  the  marine  shores  or  in  fresh  water.  The  gonads  develop  on  the 
sides  of  the  polyps,  and  a  whole  series  of  forms  with  various  degrees  of 
suppression  of  the  medusa  stage  indicates  that  these  gonads  represent 
the  last  vestige  of  the  medusa,  appearing  where  medusa  buds  would 
otherwise  develop. 

Commonly  the  polyp  is  larger  and  longer-lived  than  the  medusa.  In 
many  hydrozoans  most  of  the  asexual  buds  of  the  polyp  remain  attached 
to  the  parent  to  produce  a  colony  of  many  polyps.  The  few  buds  that 
creep  off  as  frustules  establish  new  colonies.  Division  of  labor  is  frequent 
in  the  colonial  forms.  Some  polyps  catch  and  eat  food  while  others  are 
specialized  for  the  production  of  medusae  (Fig.  10.8).  In  a  few  species 
additional  polyps  are  modified  into  long  clubs  covered  with  nematocysts 
which  serve  to  protect  the  colony. 

The  genus  Obelia  (Fig.  10.8)  is  representative  of  hydrozoans  with 
colonial  polyps.  The  branching  stalk  and  terminal  polyps  are  covered 
with  a  delicate  horny  sheath,  the  perisarc,  secreted  by  the  epidermis.  It 
is  annulated  in  many  places  to  provide  flexibility  as  well  as  support  for 
the  colony.  The  feeding  polyps  are  typical.  Polyps  that  produce  medusae 
have  neither  mouth  nor  tentacles  ancl  develop  many  medusa  buds  along 
their  sides.  The  medusae  are  about  the  size  of  polyps  and  do  not  grow 
after  they  become  free-swimming.  In  related  genera  the  medusae  never 
become  free  of  the  polyp,  but  mature  sexually  and  shed  their  gametes 
while  still  attached. 

In  the  order  Siphonophora  of  the  class  Hydrozoa  the  organisms  are 
remarkably  complex.  The  planula  does  not  become  attached,  but  de- 
velops into  a  polyp  while  swimming.  The  basal  end  of  the  polyp  com- 
monly develops  an  air  sac  to  serve  as  a  float.  From  this  polyp  a  complex 
colony  of  polyps  and  medusae  develops  by  budding.  Certain  medusae 
become  permanent  air  floats  while  others  are  specialized  for  swimming. 
Some  of  the  polyps  have  no  mouths,  but  are  equipped  with  very  long 
tentacles  covered  with  powerful  nematocysts.  Other  polyps  have  mouths 


THE   PHYLA    COELENTERATA    AND   CTENOPHORA 


191 


but  no  tentacles,  and  are  used  only  for  feeding.  Still  others  develop  as 
simple  stalks  that  bear  a  third  kind  of  medusa  bud  along  their  sides. 
These  buds  produce  eggs  and  sperm,  and  are  the  only  sexually  repro- 
ductive individuals  in  the  colony.  Entire  floating  colonies  of  siphono- 
phores  may  remain  intact,  or  pieces  including  all  the  kinds  of  individuals 
may  break  loose  and  lead  independent  lives. 

A  famous  siphonophore  is  PhysaUa  (Fig.  10.9),  the  dreaded  Portu- 
guese man-of-war.  It  has  a  large  purple  air  float  up  to  12  cm.  long  that 
rides  high  out  of  the  water  and  is  carried  by  the  wind  across  the  oceans. 
Swimming  medusae  are  absent.  Tentacles  of  the  stinging  polyps  may 
trail  out  40  feet  into  the  water,  and  their  nematocysts  easily  penetrate 
the  skin  of  man.  The  intense  pain  and  occasional  paralysis  caused 
by  many  stings  can  result  in  drowning. 

The  three  kinds  of  polyps  in  PhysaUa  occur  in  groups,  one  of  which 
is  shown  in  Figure  10.10.  Although  the  mouth  of  the  feeding  polyp  can 
open  very  wide,  the  polyp  is  unable  to  swallow  prey  unless  it  is  compara- 
tively small.  Larger  prey  are  consumed  in  an  ingenious  fashion.  Many 
feeding  polyps  become  attached  to  the  prey,  each  spreading  its  mouth 


F(zecLin0 
polyp 


Meciu-sa.  bzid 


Reproductive 
polyp 


Figure  10.8.     A  hydroid,  Obelia,  showing  a  small  portion  of  the  branching  colony. 
One  polyp  is  shown  in  longitudinal  section.  (After  Parker  and  Haswell.) 


192 


THE   ANIMAL   KINGDOM 


p(z,z,ding 
polyps 


polyps 


-Reproductive 
polyps 

-Fishing 
polyp 


Fig.  10.9  Fig.  10.10 

Figure  10.9.     Pliysalia,  the  Portuguese  man-of-war.  (Courtesy  New  York  Zoological 

Society.) 

Figure  10.10.     A  cluster  of  polyps  from  Pliysalia,  showing  the  various  modifications. 

(After  Hyman.) 

as  widely  as  possible  over  the  prey.  The  edges  of  adjacent  mouths  meet 
and  enclose  the  prey  completely.  Then  digestive  juices  are  regurgitated 
and  the  prey  is  disintegrated  and  swallowed. 

Of  the  several  thousand  species  of  hydrozoans  none  is  of  economic 
importance.  Usually  the  medusae  are  too  small  to  be  a  nuisance  to  swim- 
mers. A  few  kinds  of  polyps  secrete  limestone  around  the  colonies  and 
thus  contribute  slightly  to  the  building  of  coral  reefs. 


67.        Class  Scyphozoa 

The  medusa,  which  may  be  as  much  as  a  meter  in  diameter,  is  the 
dominant  stage  in  the  class  Scyphozoa.  A  common  genus  is  Aurelia  (Fig. 
10.11),  abundant  in  Atlantic  and  Pacific  waters.  Xematocysts  of  many  of 
the  larger  forms  can  penetrate  the  hinnan  skin  and  produce  intense  pain. 

The  mesoglea  of  scyphozoan  medusae  contains  numerous  scattered 
ameboid  cells  of  unknown  fimction  and  distinct  fibers  that  stiffen  the 
jelly-like  matrix.  The  stomach  is  subdivided  into  a  central  chamber  and 
four   gastric   pouches,    each   containing    internal    endodermal    tentacles 


THE   PHYLA    COELENTERATA    AND    CTENOPHORA 


193 


armed  with  nematocysts  that  can  be  used  to  reparalyze  prey  should  it 
recover  after  being  swallowed.  The  radial  canals  are  much  branched. 

The  sensory  areas  ol  the  bell  margin  in  this  class  are  concentrated 
to  form  complex  sense  structures,  the  rhopalia  (Fig.  10.12).  These  re- 
spond to  gravity,  light,  and  chemicals  in  the  water.  Without  them  spon- 
taneous activity  of  the  medusa  ceases. 

The   gonads   of   scyphozoans    develop    in    the   gastrodermis   of    the 


R  hop  a.1  iu.Tn. 
Mouth  arra 


Gona.d  in 
ga.stric 

Ra.dial 


Figure  10.11.  A,  Ventral  view  of  Aurelia.  Compare  with  Pelagia  on  Figure  10.1, 
which  difters  primarily  in  having  larger  tentacles.  (After  Hyman.)  B,  Left,  Ephyra  larva 
of  Aurelia.  Right,  Young  Aurelia  bell  contracting  (lateral  view).  (Courtesy  Douglas  P. 
Wilson.) 


194  THE   ANIMAL   KINGDOM 


Mesoglea- 


,  Hood- 


Ga.st  r  o  d.e.r  mi  S 


Pi^me-nt  Cup 
visual  SLTCci: 


.^mcnt  spot 
sued.  a.rea 


Figure    10.12.     Section    through   a   rhopalium   showing   the   hood   and   the   various 
sensory  areas.  (Modified  from  Hyman,  after  Schewiakoff.) 


-\ 


J± 


h 


i 

€ 


Figure  10.13.  Reproduction  in  the  Scyphozoa.  A,  The  polyp.  A  frustule  forming 
on  its  right  will  creep  off  and  become  another  polyp.  B.  Stroljila.  Starting  at  the  upper 
end,  successively  lower  portions  of  the  polyp  transform  into  medusae.  {A  modified  from 
Hyman  after  Perez;  B  after  Hyman.) 


THE   PHYLA    COELENTERATA    AND   CTENOPHORA 


195 


gastric  pouches.  Gametes  are  shed  first  into  the  pouches  and  then  to  the 
outside  through  the  mouth. 

In  other  respects  the  medusa  o£  this  class  resembles  that  of  the 
Hydrozoa.  The  scyphozoan  polyp  is  an  inconspicuous  part  of  the  life 
cycle.  Asexual  reproduction  by  frustule  formation  (Fig.  10.13,  A)  is 
common. 

Medusae  form  by  the  direct  transformation  of  the  polyp  head,  rather 
than  by  lateral  budding  as  in  the  Hydrozoa.  In  some  species  the  entire 
polyp  transforms  into  a  single  medusa.  In  others  a  series  of  medusae 
may  be  produced.  Successive  medusae  may  overlap  in  development,  so 
that  new  medusae  begin  to  form  beneath  older  ones  that  have  not  yet 
broken  free.  The  result  is  a  pile  of  partially  formed  medusae  resembling 
a  stack  of  plates.  This  stage,  which  is  shown  by  Aurelia  (Fig.  10.13,  B), 
is  called  a  strobila,  and  the  process  is  called  strobilization. 

Most  of  the  200  species  of  scyphozoans  are  similar  and  adhere  to 
the  simple  jellyfish  plan.  They  are  of  little  interest  to  man  except  as 
nuisances  to  swimmers. 

68.        Class  Anthozoa 

In  the  third  class,  the  Anthozoa,  medusae  are  lacking  and  the  polyps 
become  sexually  mature.  The  polyps  are  usually  short  and  stout  (Fig. 
10.14)  with  a  large  mouth  and  numerous  internal  partitions.  The  meso- 


omadeuni 


Figure  10.14.     Metridium.  Diagrammatic  view  of  half  a  polyp.  Several  internal  struc- 
tures have  been  omitted. 


196 


THE   ANIMAL   KINGDOM 


Figure  10.15.     ^>  Lace  coral.  B,  West  Indian  coral.  C,  Star  coral.  (Courtesy  of  the 
American  Museum  of  Natural  History.) 


THE   PHYLA    COELENTERATA    AND    CTENOPHORA 


197 


glea  is  packed  with  supportive  elastic  fibers.  The  gonads  develop  in  the 
endoderm  along  the  iree  edges  ot  the  partitions.  In  general,  the  structure 
o£  these  polyps  is  closer  to  that  oi  the  Scyphozoa  than  that  of  the  Hy- 
drozoa.  The  anthozoan  polyp  also  has  a  stomodeum,  an  inturned  mouth 
lined  with  ectoderm. 

The  larger  members  ot  this  class  are  the  solitary  sea  anemones, 
which  are  liower-like  in  appearance  when  their  tentacles  are  spread  in 
search  of  prey.  Many  of  them  are  brightly  colored  and  feed  voraciously 
on  fish. 

The  Anthozoa  include  the  colonial  true  corals  which  contribute 
greatly  to  the  bulk  of  coral  reefs  along  some  tropical  shores.  The  in- 
dividual polyp  is  only  half  an  inch  across,  but  the  colony  secretes  an 
external  supporting  framework  of  calcium  carbonate  that  may  be  of 
considerable  size.  This  skeleton  may  be  encrusting,  arborescent  or  mas- 
sive (Fig.  10.15). 

Coral  reefs  are  formed  by  large  populations  of  many  species  of  coral 
and  other  limestone-secreting  organisms.  They  develop  only  in  warm 
shallow  water  exposed  to  the  ocean  waves.  At  present  the  two  major 
regions  that  offer  these  conditions  and  support  reefs  are:  (1)  the  Carib- 
bean area,  including  Florida,  Bermuda,  the  Bahamas  and  the  West 
Indies,  and  (2)  the  Indo-Pacific  area  known  as  the  Coral  Sea,  extending 
from    Australia    to    Hawaii    and    the    IMiilippines. 

The  precious  coral  of  commerce  is  not  a  true  coral  but  a  member 
of  the  third  order  of  Anthozoa,  the  alcyonarians.  These  have  an  internal 
skeleton  formed  by  the  secretion  of  calcium  carbonate  and  protein  horny 
material  into  the  mesoglea.  In  some  species  these  secretions  fuse  into  a 
rigid  framework  hard  enough  to  resist  wear.  Precious  corals  form  irregu- 
lar branching  colonies  in  the  Mediterranean  Sea  and  near  Japan. 
Wooden  frames  with  rope  tangle  mops  are  dragged  over  the  sea  bottom 


Figure  10.16.     Sea  fan.  (Courtesy  of  the  American  Museum  of  Natural  History.) 


198 


THE   ANIMAL   KINGDOM 


to  break  the  brittle  skeleton  and  gather  the  branches.  Other  alcyonarians 
are  the  yellow,  red  or  purple  sea  fans  (Fig.  10.16)  of  tropical  waters. 
These  colonies  develop  as  llattened  networks  with  a  few  main  branches 
and  numerous  cross  connections. 


69.        Fresh-Water  Coelenterates:  Hydra 

Only  a  few  species  of  coelenterates,  all  members  of  the  class  Hy- 
drozoa,  occur  in  fresh  water.  The  fresh-water  forms  include  a  colonial 
polyp  found  in  a  few  eastern  rivers  of  the  United  States,  a  jellyfish  very 
similar  to  Gonionemus  found  sporadically  in  ponds  and  streams  all  over 
the  world,  and  a  number  of  species  of  solitary  polyps,  the  hydras. 
Only  the  last  are  easily  obtained  in  most  bodies  of  fresh  water. 

The  hydra  {¥ig.  10.17,  left)  is  an  unusual  hydrozoan.  Medusae  are 
lacking  altogether,  unless  the  gonads  are  considered  to  be  their  vestiges. 
As  in  Gonionemus  the  sexes  are  separate  (Fig.  10.18).  The  testes  shed 
sperm  into  the  water  and  each  ovary  produces  one  egg  at  a  time  which 
is  retained  and  fertilized  in  the  ovary.  The  egg  develops  to  the  planula 


Figure  10.17.  Left,  Hydra.  The  tentacles  hang  in  the  water  like  a  net,  waiting  for 
prey.  (After  Hyman.)  Right,  Spine  of  a  crustacean  that  has  brushed  against  the  tentacles 
of  a  hydra.  Two  of  the  nematocysts  shown  are  similar  to  those  of  Gonionemus.  The 
others  are  of  the  coiling  type.  (After  Hyman.) 


THE   PHYLA    COELENTERATA    AND    CTENOPHORA 


199 


f 


Yoxxxig 


Mai;  lire 


"^ 


Male, 


nt 


F^'inal* 


,  .  ,^\XNN\N\\NNNNV^V^-SN'^\NNNVXNXXNNNNN\\NNNNNNNNV\V.NV<XV 


Figure  10.18.     Reproduction  in  Hydra.  Sperm  are  shed  from  the  testes  of  the  male, 
and  swim  to  the  females  where  they  fertilize  the  mature  eggs. 

Stage  while  still  attached  to  the  parent.  The  plantila  lacks  cilia,  and 
secretes  a  surrounding  shell  which  later  falls  off.  It  hatches  later  as  a 
young  polyp. 

The  polyp  feeds  in  typical  fashion  on  small  aquatic  animals.  It  is 
no  more  complex  than  the  polyp  of  Gonionemus,  except  that  it  is  con- 
siderably larger  and  has  four  kinds  of  nematocysts.  One  kind  is  radically 
different  (Fig.  10.17,  right),  having  no  spines,  poison  or  opening  at  the 
tip  of  the  thread.  Instead  the  thread  coils  tightly  after  eversion,  often 
encircling  minute  spines  or  hairs  on  the  prey  and  holding  it  fast.  The 
structural  simplicity  of  hydra  has  suggested  to  some  investigators  that  it 
is  a  juvenile  form  that  becomes  sexually  mature  without  metamorphosis. 

Asexual  buds  do  not  become  frustules  but  develop  mouths  and 
tentacles  while  still  attached  to  the  parent.  Later  the  base  constricts  and 
the  offspring  creeps  away.  A  parent  may  have  several  such  buds  and 
temporarily  resemble  a  colonial  hydrozoan  (Fig.  10.17,  left). 

Unlike  most  hydrozoan  polyps,  which  are  permanently  attached  to 
the  bottom,  hydra  moves  from  time  to  time.  The  polyp  may  slide  slowly 
on  its  base  at  the  rate  of  a  few  inches  a  day,  or  it  may  somersault  at  a 
more  rapid  rate  by  alternately  attaching  tentacles  and  base. 

70.        The  Phylum  Ctenophora 

Comb  jellies  have  a  spherical  or  vertically  elongate  body  plan  in 
contrast  to  the  umbrella  shape  of  medusae.  Familiar  representatives  are 


200 


THE   ANIMAL   KINGDOM 


lie 


Figure  10.19.     Pleurobrachia.  A,  Lateral  view  of  whole  animal.  B,  Detail  showing 
two  combs,  each  formed  by  the  fusion  of  a  row  of  cilia.  (After  Hynian.) 

the  sea  gooseberry,  genus  Pleurobrachia,  and  the  sea  wahiut,  genus 
Mnemiopsis  (Fig.  10.1).  The  mouth  is  at  one  end,  so  that  oral,  aboral 
and  lateral  surfaces  can  be  identified.  Each  ctenophore  swims  with  eight 
columns  of  combs  that  radiate  from  the  center  of  the  aboral  surface 
over  the  sides  to  the  oral  surface.  Each  comb  is  a  row  of  fused  cilia  (Fig. 
10.19).  Just  beneath  the  epidermis  along  each  comb  column  is  a  tract 
of  nerve  fibers  that  coordinate  the  beating  of  the  cilia.  In  the  resting 
position  each  comb  points  toward  the  oral  end.  When  the  comb  bends 
vigorously  toward  the  aboral  end  the  comb  jelly  moves  through  the 
water  mouth  first.  The  combs  beat  in  waves  passing  along  the  columns 
from  aboral  to  oral  ends.  Synchronized  action  of  the  eight  comb  columns 
produces  a  smooth  gliding  locomotion  that  may  be  as  fast  as  two  feet 
per  minute.  Ctenophores  usually  swim  up  and  down  through  a  few 
feet  of  water  with  the  mouth  always  forward. 

A  comb  jelly  has  but  two  tentacles.  These  are  branched  and  can  be 
retracted  into  tentacle  sheaths.  Each  tentacle  has  an  outer  layer  of  epi- 
dermis surrounding  a  core  of  mesoglea.  In  the  epidermis  are  numerous 
colloblasts,  each  one  a  modified  epidermal  cell  containing  a  peripheral 
hemisphere  of  adhesive  mucus  and  a  basal  coiled  spring.  The  spring 
ejects  the  mucus  against  prey  and  anchors  it  to  the  tentacle. 


THE   PHYLA    COELENTERATA    AND    CTENOPHORA  201 

The  processes  of  feeding  and  digestion  are  similar  to  those  found 
in  the  coelenterates. 

The  sensory  region  of  the  comb  jelly  is  concentrated  at  the  aboral 
end  where  a  statocyst  is  the  primary  sense  organ.  As  in  medusae,  the 
statocyst  is  associated  directly  with  motor  nerves  which,  in  the  cteno- 
phore,  are  the  eight  radiating  nerves  underlying  the  comb  columns.  If 
one  of  these  nerves  is  cut,  the  corresponding  column  is  no  longer  co- 
ordinated with  the  others.  If  all  the  neives  are  cut,  coordination  disap- 
pears and  the  comb  jelly  is  unable  to  control  its  locomotion. 

Like  most  jellyfish  the  ctenophores  are  transparent.  The  combs, 
however,  reflect  light  and  produce  iridescent  patterns.  This  shimmering 
color  passing  in  waves  from  aboral  to  oral  ends  shows  the  waves  of  beat- 
ing of  the  combs  and  is  useful  in  studies  of  coordination. 

At  night  many  of  the  comb  jellies  are  brightly  luminescent  when 
disturbed.  The  light  is  produced  close  to  the  nerve  tracts  beneath  the 
comb  columns.  Like  luminescence  in  most  animals  the  light  is  blue-green 
in  color.  In  ctenophores  the  luminescence  is  especially  striking  since  it 
becomes  irridescent  as  it  is  reflected  from  the  combs,  flickering  like 
colored  fire  up  and  down  the  comb  columns. 

The  eggs  and  sperm  are  shed  into  the  water  where  the  embryos 
develop  directly  into  the  comb  jelly  form.  Early  divisions  of  the  eggs 
follow  an  exact,  rigid  path  of  development.  The  first  three  divisions  are 
vertical  and  produce  a  curved  plate  of  eight  cells.  The  fourth  is  hori- 
zontal and  separates  eight  small  upper  cells  from  eight  large  lower  cells. 
Later  cleavages  continue  to  be  constant  in  all  individuals,  and  each 
upper  and  lower  cell  of  the  16-cell  stage  becomes  the  corresponding 
eighth  of  the  ctenophore.  Associated  with  this  rigid  pattern  is  an  early 
chemical  differentiation.  The  opposite  is  true  of  most  coelenterates,  in 
which  early  development  appears  to  be  unspecialized,  with  cell  division 
preceding  chemical  differentiation. 

71.        The  Regulation  of  Form 

A  fascinating  and  challenging  area  of  biology  is  concerned  with  two 
problems  associated  with  development:  the  extent  to  which  an  organism 
can  repair  injuries,  and  the  extent  to  which  it  can  correct  disarrange- 
ments. Some  of  the  coelenterates  and  ctenophores  have  remarkable 
abilities  in  these  respects.  If  parts  of  the  body  are  removed,  they  are 
usually  replaced.  If  individuals  are  cut  in  half,  each  half  may  regenerate 
the  missing  half.  Sometimes  quarters  or  even  smaller  pieces  of  animals 
will  regenerate  into  Avhole  organisms. 

Very  often  a  remarkable  regenerative  ability  is  associated  with 
natural  reproduction  by  budding.  In  many  sea  anemones,  for  example, 
pieces  of  the  base  may  break  off  spontaneously  and  develop  into  new 
individuals.  Hence,  when  pieces  are  cut  off,  they  regenerate  well.  Again, 
in  the  one  genus  of  sea  anemones  in  which  a  single,  experimentally  re- 
moved tentacle  can  regenerate  all  the  missing  parts,  it  is  found  that 
tentacles  spontaneously  do  the  same  thing  as  a  form  of  asexual  repro- 
duction. 


902  ^^^   ANIMAL   KINGDOM 

The  ability  to  correct  disarrangements  is  usually  not  as  marked  as 
the  regenerative  ability.  If  an  oral  end  ol  one  hydra  is  grafted  onto  the 
side  of  another,  the  animal  will  eventually  divide  to  form  two  normal 
individuals.  If,  however,  the  cut  surfaces  of  two  oral  ends  are  placed 
together,  they  heal  to  form  a  single  individual  with  two  mouths  and  no 
base.  Such  monsters  remain  thus,  apparently  unable  to  achieve  the 
normal  form.  If  such  a  creature  is  then  cut  in  half,  each  half  may  re- 
generate a  base. 

In  the  ctenophores,  pieces  put  together  in  the  original  orientation 
with  oral  and  aboral  ends  aligned  will  usually  regulate  into  normal 
individuals,  while  opposed  pieces  such  as  two  aboral  ends  with  their 
cut  surfaces  placed  together  will  not. 

A  most  remarkable  example  of  successful  rearrangement  is  found 
in  hydra.  As  in  most  polyps  the  mouth  can  open  very  wide,  and  it  is 
possible  to  turn  a  hydra  inside  out  through  its  mouth  without  tearing 
any  of  the  tissues!  Such  an  individual,  with  the  epidermis  inside  and 
gastrodermis  outside,  is  unable  to  turn  inside  out  again  to  recover  its 
normal  form.  It  does  regain  its  normal  form,  however,  by  a  direct  mi- 
gration of  the  individual  cells  across  the  thin  mesogleal  layer  to  their 
former  location. 

These  abilities  to  repair  or  replace  parts  and  to  rearrange  dis- 
arrangements are  two  aspects  of  form  regulation,  the  processes  by  which 
individual  organisms  come  to  have  the  morphology  of  their  kind.  Most 
coelenterates  are  able  to  regulate  throughout  their  life.  Injuries  or 
disarrangements  are  as  easily  corrected  by  embryos  as  by  adults.  Among 
the  ctenophores,  however,  the  embryo  has  much  less  regulative  ability 
than  the  adult.  If  the  two-cell  stage  is  divided  into  two  separate  cells, 
each  becomes  only  half  a  ctenophore.  One  cell  from  the  four-cell  stage 
becomes  one  quarter  of  a  ctenophore  having  only  two  comb  columns. 
Later  in  life,  however,  those  that  survive  will  spontaneously  regenerate 
the  missing  parts  to  become  normal. 

Not  all  coelenterates  have  good  regulative  abilities.  The  siphono- 
phores,  for  example,  usually  fail  to  replace  lost  parts,  and  wounds  are 
healed  by  a  simple  closure  of  the  hole.  In  this  group  regulative  ability 
is  good  in  the  embryos  and  larvae  and  becomes  poor  in  the  adults. 

Many  other  animal  groups,  including  the  sponges  already  discussed 
and  such  complex  animals  as  crabs,  starfishes  and  salamanders,  have  a 
considerable  ability  to  regulate  form.  The  coelenterates  and  ctenophores 
are  especially  suitable  for  experimentation  because  the  body  plan,  while 
relatively  simple,  is  geometrically  exact  and  provides  an  excellent  frame 
of  reference.  Survival  after  operations  is  not  difficult  to  achieve.  The 
phenomena  associated  with  form  regulation  are  considered  to  be  similar 
to  those  of  embryologic  development. 

Questions 

1.  Define  "planktonic"  and  "plankton." 

2.  Describe  mesoglea. 

3.  Draw  a  vertical  section  through  a  jellyfish  and  a  sea  anemone. 


THE   PHYLA    COELENTERATA    AND   CTENOPHORA  203 

4.  How  does  a  nematocyst  work? 

5.  Describe  the  role  of  diffusion  in  the  physiology  of  coelenterates. 

6.  What  is  a  strobila? 

7.  Distinguish  between  medusae  and  comb  jellies. 

8.  What  is  "regulation  of  form"? 

Supplementary  Reading 

The  Invertebrates,  volume  I,  by  L.  Hyman  includes  both  the  radiate  phyla,  with 
numerous  drawings  and  an  abundance  of  information.  The  photographs  for  these  phyla 
in  Animals  without  Backbones  by  Buchsbaum  are  especially  good. 


CHAPTER   11 


The  Phylum  Platyhelminthes 


The  flatworms  or  Platyhelminthes  are  wormlike  animals  with  a  single 
major  opening  to  the  gut,  which  tunctions  as  both  mouth  and  anus.  Be- 
tween the  gastrodermis  and  epidermis  the  body  is  filled  with  tissues, 
including  layers  of  muscle,  connective  tissues  and  reproductive  organs. 
Neither  a  body  cavity  (such  as  will  be  described  in  later  chapters)  nor  a 
circulatory  system  is  present. 

Included  in  the  flatworms  are  two  major  groups  of  animal  parasites, 
the  flukes  (class  Trematoda)  and  the  tapeworms  (class  Cestoda),  which 
will  be  discussed  further  in  Chapter  39.  The  free-living  forms  (class  Tur- 
bellaria)  range  in  size  from  0.1  to  600  mm.  and  are  found  in  fresh  water, 
in  salt  water,  and  on  land. 

The  phylum  is  best  approached  by  a  study  of  its  free-living  mem- 
bers. We  will  begin  with  an  example  of  a  turbellarian  which  is  inter- 
mediate in  size  and  complexity. 

72.  Dugesia:  Habitat  and  Appearance 

The  most  familiar  free-living  flatworms  are  the  planarians,  abun- 
dant in  ponds  and  streams  all  over  the  world.  This  common  name  is 
used  for  an  entire  order,  but  only  one  genus  has  the  scientific  name 
Planaria.  In  the  United  States  the  more  common  planarians  belong  to 
the  related  genus,  Dugesia.  The  species  D.  dorotocephala  occurs  in  ponds 
and  streams  and  is  available  from  biological  supply  houses. 

Dugesia  is  about  half  an  inch  long,  with  a  distinct  head  having  what 
appear  to  be  crossed  eyes  and  pointed  ears  (Fig.  11.1).  The  surface  of 
the  body  is  a  single  layer  of  cuboidal  cells  (Fig.  11.4),  the  epidermis. 
Planarians  glide  about  on  the  ciliated  ventral  side  of  this  surface.  Slime 
glands  among  the  ventral  epidermis  cells  (Fig.  11.4)  secrete  a  lubricating 
slime  that  smooths  the  path. 

73.  Dugesia:  Feeding  and  Digestion 

As  the  animal  glides  along  hunting  for  food,  the  anterior  end  is 
usually  slightly  elevated  (Fig.  11.2).  Should  a  small  organism  come  close, 
the  head  turns  quickly  toward  it.  Adhesive  glands  along  the  edges  of 
the  body,  and  especially  prominent  in  the  head  region,  secrete  a  glue 
to  which  the  passing  organism  adheres  tightly,  and  the  head  of  the 
204 


THE   PHYLUM   PLATYHELMINTHES  205 


N^ 


"Brain 


7^  Eye 
Y  Sensory  lobe 


'  Ventrolatera.1 


\  Intestine 


■ Pharynx 

'  Mouth  cavity 

■  Mouth 


Figure  11.1.     Dugesia.  Dorsal  view,  showing  the  digestive  and  nervous  systems.  The 
mouth  opens  ventrally.  (After  Hyman.) 


s^K 


s^^sS^kml■mmmkmkmm>.,.,mmgg 


.»>.;..^v^uw.^>.v;  WM^  m;w^!J)jj;)s 


xsxs-s^.kk-.u-m'.'.'W'ji.'mki! 


.^e^-,^ 


<SS»ww^U.xT?;v.wi.i„Mwi.'.' 


.v,-.^VA'A- 


Figure  11.2.     Hunting  and  feeding  in  Dugesia.  A  small  crustacean  (Daphnia)  is  cap- 
tured and  eaten,  its  tough  exoskeleton  remaining  as  an  empty  shell. 


206  ^^^   ANIMAL   KINGDOM 

Pharynx  rctra-cbe-d 


MoTxth  cavity 


Mouth- 


'■■'mm 


PhsLrynx 

e-jctended" 


Figure  1 1 .3.     Diagrammatic  side  view  of  the  pharynx  of  Dugesia  retracted  (left)  and 


extended  through  the  mouth  (right) 

Vertical  inuscle 


"Muscle  layers 


Secondary 
intestinal  branches 
Mesenchyme 


,    Venlro-laleral  nerve 

Adhesive  glands 

Pharynx 

Moath  cavity 

Figure  1 1 .4.     Cross  section  of  Dugesia  at  level  of  the  pharynx. 


Ciliated 
epidermis 


Primary  branch 
of  intestine 


planarian  folds  over  the  prey.  After  sliding  around  the  prey  once  or 
twice,  binding  it  tightly  in  slime  and  glue,  the  planarian  comes  to  rest 
with  the  anterior  half  of  its  body  on  the  bottom  and  the  posterior  half 
doubled  over  the  prey. 

The  mouth  of  Dugesia  is  midventral  (Fig.  11.1).  The  pharynx  is  a 
long  extensible  tube  which  can  pass  through  the  mouth.  When  not  in 
use,  the  pharynx  is  withdrawn  into  a  mouth  cavity  lined  with  ectoderm 
(Fig.  11.3).  The  pharynx  itself  is  covered  with  ectoderm,  and  its  wall  is 
composed  of  several  layers  of  muscle  and  connective  tissue  (Fig.  11.4). 
When  withdrawn  the  pharynx  is  short  and  stout,  but  by  contraction  of 
the  circular  muscle  fibers  it  can  be  elongated  greatly  (Figs.  11.2  and 
11.3).  When  feeding  it  is  extended  and  used  as  a  probe  to  search  the 
prey  for  a  tender  spot.  It  then  bores  into  the  prey  by  strong  sucking 
movements  and  tears  the  soft  parts  to  bits  to  be  swallowed. 

After  a  meal  the  planarian  crawls  off  a  short  distance  and  rests, 
with  the  body  rounded  up  and  firmly  attached  to  the  bottom  by  glue 
from  the  marginal  adhesive  glands. 

The  pharynx  opens  into  a  branched  intestine  (Fig.  11.1),  one 
primary  branch  extending  into  the  head,  and  two  more  extending 
toward  the  tail.  All  have  side  branches  so  that  in  a  cross  section  of  the 
worm  the  intestine  may  be  cut  across  several  times.  As  in  the  coelen- 
terates  and  ctenophores,  the  digestive  organ  is  a  simple  gastrodermis  of 
endoderm  cells.  In  planarians  these  cells  are  very  large,  making  the 
intestine  the  bulkiest  structure  in  the  body.  Although  no  digestive 
enzymes  have  been  found  in  the  lumen  of  the  intestine,  it  is  obvious 


THE   PHYLUM   PLATYHELMINTHES 


207 


from  the  disintegration  of  large  food  particles  that  at  least  some  pro- 
teolytic enzymes  are  secreted.  Most  of  the  digestion,  however,  is  intra- 
cellular. The  gastrodermis  cells  gather  up  food  particles  in  food  vacu- 
oles. The  food  vacuoles  have  not  been  observed  to  become  first  acid 
and  then  basic,  as  they  do  in  other  phyla. 

Indigestible  remains  of  food  vacuoles  are  released  back  into  the 
intestine  where,  together  with  fragments  that  could  not  be  taken  up  in 
food  vacuoles,  they  are  compressed  into  solid  masses  and  eventually 
ejected  through  the  mouth. 


74.        Dugesia:  Sensation  and  Movement 

Dugesia  is  well  supplied  with  sense  organs.  The  "nose,"  by  which 
the  animal  explores  the  physical  nature  of  the  bottom  on  which  it  is 
crawling,  contains  numerous  tactile  nerve  endings.  Chemoreceptors 
(taste-smell)  are  located  in  other  nerve  endings  scattered  over  the  body, 
but  are  localized  especially  on  the  "ears."  Each  of  these  is  held  in  a 
cupped  position  (Fig.  11.2).  Cilia  lining  the  cup  beat  more  vigorously 
than  elsewhere,  drawing  water  from  in  front  of  the  animal  for  analysis 
by  the  chemoreceptors. 

Planarians  capture  small  prey  and  are  also  quick  to  locate  and 
feed  upon  dead  organisms.  When  an  individual  first  tastes  such  food, 
it  raises  its  head  and  turns  from  side  to  side.  The  two  projections  at 
the  sides  enable  the  worm  to  locate  the  food,  and  the  worm  shortly 
lowers  its  head  and  slides  off  in  the  appropriate  direction.  At  frequent 
intervals  it  will  stop  and  raise  its  head  again  to  get  new  bearings. 

Dugesia  is  also  sensitive  to  light,  generally  retreating  from  it.  Each 
eye  (Fig.  11.5)  has  a  pigment  cup  facing  laterally,  in  the  hollow  of  which 
are  rodlike  extensions  of  visual  cells.  These  rods  are  arranged  radially, 
and  are  believed  to  be  stimulated  maximally  by  light  tra\eling  along 
their  length,  since  the  direction  of  a  light  source  is  very  accurately 
perceived.  The  bodies  of  the  visual  cells,  containing  the  nuclei,  lie  out- 
side the  cup,  and  from  them  a  bundle  of  nerve  fibers  proceeds  to  the 
brain.  Such  an  eye,  in  which  light  must  first  traverse  nerve  fibers  and 
visual  cell  bodies  before  reaching  the  sensitive  rods,  is  an  inverted  eye 
(the  human  eye  is  also  inverted).  Eyes  of  planarians  do  not  form  images. 


^Pigment  cup 

Li^hl:  sens  i-live  portion^ 
/    of  photoreceptor  cell^ 

f- Nuclei  oi  photoreceptor 
"^'-  ^  cell^ 

■Nerve  to  brain 

Figure   11.5.     Diagrammatic  section  through  the  eye  of  a  planarian.  Light  reaches 
the  sensiti\e  elements  from  the  right. 


208  ^Wf   ANIMAL   KINGDOM 

but  can  detect  roughly  the  amount  of  light  and  the  general  direction 
from  which  it  comes. 

In  a  water  current  planarians  usually  face  or  crawl  upstream.  Cur- 
rent direction  is  recognized  by  tactile  fibers,  scattered  along  the  sides 
of  the  animal,  which  are  bent  by  the  force  of  the  water. 

All  of  this  sensory  information,  especially  that  from  the  head  re- 
gion, is  relayed  by  nerve  fibers  to  the  brain,  a  bilobed  white  structure 
between  the  eyes  (Fig.  11.1).  Nerves  branch  out  in  all  directions  from  the 
brain;  the  primary  pair  are  the  ventro-lateral  nerve  cords  (Fig.  11.4). 
ff  the  brain  is  removed  coordination  is  seriously  impaired,  and  almost 
all  the  relation  of  sensory  information  to  locomotion  is  lost. 

The  brain  controls  both  ciliary  and  muscular  action.  If  a  planarian 
is  bumped,  ciliary  locomotion  ceases  at  once  and  the  body  contracts, 
withdrawing  the  end  that  was  touched.  If  touched  repeatedly  on  the 
tail,  Dugesia  will  hasten  forward  by  a  series  of  wormlike  body  con- 
tractions. If  touched  repeatedly  on  the  head,  it  will  back  up,  turn  to 
one  side,  and  go  forward  again. 

Muscles  for  these  movements  lie  beneath  the  epidermis  (Fig.  11.4). 
Outer  circular  fibers  can  constrict  and  lengthen  the  body,  while  deeper 
longitudinal  fibers  can  shorten  it.  Other  fibers  are  oblique  and  still 
others  are  vertical.  The  latter  can  flatten  the  body.  Coordination  among 
these  fibers  is  such  that  the  planarian  can  accomplish  a  number  of  ma- 
neuvers, turning,  folding  or  stretching  in  all  directions.  When  the 
organism  is  gliding  smoothly  along  the  bottom,  successive  waves  of 
contraction  of  the  longitudinal  fibers  may  pass  from  the  posterior  end 
to  the  front,  considerably  increasing  the  rate  of  locomotion. 

75.        Dugesia:  Water  Balance  and  Excretion 

The  remainder  of  the  flatworm  body,  the  space  between  muscles  and 
intestine,  is  filled  with  loosely  organized  mesodermal  cells,  the  mesen- 
chyme. Some  of  these  cells  are  pigmented,  giving  the  worm  its  char- 
acteristic brown  or  gray  color.  The  mesenchyme  forms  a  loose  mesh 
containing  a  considerable  amount  of  intercellular  fluid  that  flows  back 
and  forth  as  the  worm  changes  shape.  The  movement  of  this  fluid  prob- 
ably aids  in  the  distribution  of  nutrients  from  the  intestine  to  other 
parts  of  the  body. 

Excess  water  from  the  body  cells  diffuses  into  the  intercellular 
fluid  and  is  picked  up  by  excretory  cells.  These  are  the  protonephridia 
or  flame  cells  (Fig.  11.6,  A)  scattered  throughout  the  body.  Each  flame 
cell  surrounds  a  blind  tubule  into  which  the  water  is  excreted.  A  tuft  of 
cilia  in  the  blind  end  beats  vigorously,  propelling  the  fluid  down  the 
lumen.  These  tubules  from  the  protonephridia  empty  into  larger  tubules 
that  form  an  anastomosing  system  along  each  side  of  the  body  (Fig. 
11.6,  B).  These  open  to  the  surface  through  numerous  small  pores. 

The  number  of  flame  cells  in  the  body  is  adjusted  to  the  salinity 
of  the  environment.  Planarians  grown  in  slightly  salty  water  develop 
few  flame  cells,  but  quickly  increase  the  number  if  the  amount  of  salt 
is  later  reduced. 


THE   PHYLUM   PLATYHELMINTHES 


209 


Nucleus 

Ciliary 
"flame" 


-Tubule 


'^/      •  M  tubule 

fxWm 


-plaraz, 
cell 


A  B 

Figure  1 1 .6.     Excretory  system  of  Dugesia.  A,  Detail  showing  flame  cells  and  tubules. 
B,  The  tubule  network. 

Metabolic  wastes  other  than   water  are  believed  to  pass  from  the 
body  simj)ly  by  diffusion. 


76.        Dugesf'a:  Reproduction 

Throughout  most  of  the  year  no  reproductive  organs  are  evident 
in  Dugesia.  If  an  individual  is  well  fed,  it  grows  and  reproduces  asex- 
ually  by  pulling  itself  into  two  pieces.  The  body  becomes  elongated 
posterior  to  the  pharynx,  then  this  region  becomes  stretched,  attenuated, 
and  finally  ruptures.  The  anterior  end  moves  off  and  in  about  one  day  a 
new  tail  begins  to  form.  If  it  continues  to  be  well  fed,  the  process  can 
be  repeated.  The  posterior  end  rounds  up  and  becomes  quiescent.  In 
a  few  days  it  will  grow  a  head  and  pharynx.  At  first  it  is  very  small, 
but  with  feeding  it  soon  becomes  full  size  and  may  itself  reproduce 
asexually. 

In  the  spring  a  reproductive  system  develops  from  the  mesenchyme 
in  most  populations.  Each  individual  is  hermaphroditic,  having  com- 
plete sets  of  male  and  female  organs  for  the  production,  storage  and 
transfer  of  the  sex  cells.  When  sexually  mature,  pairs  copulate  fre- 
quently. The  initial  step  or  "courtship"  involves  a  series  of  repeated 
head  and  body  contacts,  obviously  different  from  the  casual  way  in 
which  sexually  undeveloped  individuals  pass  by  each  other.  The  two 
individuals   gradually   assume   a    copulatory   position,   facing   somewhat 


210 


THE   ANIMAL   KINGDOM 


Figure  11.7.     Dugesia  copulating.  (After  Hyman.) 


Testes 


Sperm 
du-ct 


Ova.T'y 
i       Seminal  Ttc<z,p\eL.c[z. 

IPV" Vitelline  glands 

Ovovitelline  dxzct 

"Copulatoi-y  sac 


Penis  bixli) 


Sperm  duct  ■ 
Ovovitelline  duct-" 


Penis 


Atrium      Geriitalpore 


B 


Figure  11.8.  Reproductive  system  of  Dugesia.  A,  A  dorsal  view  showing  the  male 
organs  on  the  left  and  the  female  organs  on  the  right.  B,  A  side  view  of  the  copulatory 
organs.  (After  Hyman.) 

away  from  each  other  with  the  posterior  regions  elevated  and  their 
ventral  surfaces  pressed  together  (Fig.  11.7).  On  the  ventral  surface, 
posterior  to  the  mouth,  is  the  genital  pore.  Each  individual  protrudes 
a  muscular  penis  through  its  pore  and  through  the  pore  of  its  mate  into 
a  copulatory  sac  (Fig.  11.8).  Sperm  that  have  been  produced  in  the 
many  testes  and  stored  in  the  sperm  ducts  leading  from  the  testes  to 
the  penis  bulb  now  pass  into  the  bulb  where  they  are  mixed  with  se- 
cretions of  the  bulb,  and  are  then  forced  by  muscular  contractions 
through  the  penis  into  the  sac  of  the  mate.  Secretions  of  the  penis  bulb 


THE   PHYLUM    PLATYHELMINTHES  211 

at  the  time  of  ejaculation  activate  the  sperm,  which  begin  to  undulate. 
The  mating  process  takes  only  a  few  minutes. 

After  mating  the  active  sperm  migrate  from  the  copulatory  sac 
through  the  ovovitelline  ducts  to  the  seminal  receptacles,  a  pair  of 
cavities  next  to  the  pair  of  ovaries.  Mature  eggs  cross  through  a  par- 
tition between  the  ovary  and  the  receptacle,  are  fertilized,  and  then 
pass  down  the  ovovitelline  duct  together  with  a  group  of  yolk-packed 
cells  from  the  vitelline  glands.  Several  eggs  are  produced  at  one  time. 
They  gather  with  the  yolk  cells  in  the  reproductive  atrium,  where  se- 
cretions from  the  yolk  cells  form  a  membranous  capsule  surrounding 
them.  As  the  capsule  is  released  through  the  genital  pore,  it  is  covered 
with  an  adhesive  secretion  from  cement  glands.  A  portion  of  this  secre- 
tion is  drawn  out  into  a  stalk,  which  attaches  the  capsules  to  the  under 
side  of  stones  and  other  objects.  The  eggs  develop  into  embryos  that 
consume  the  yolk  cells  in  the  capsule,  and  emerge  in  two  or  three  weeks 
as  miniature  flatworms  similar  to  adults. 

77.        Dugesia:  Regeneration  and  Polarity 

Many  flatworms  (but  not  all)  have  marked  powers  of  regeneration. 
These  are  especially  good  in  Dugesia  and  in  other  genera  that  repro- 
duce asexually.  Cutting  a  Dugesia  in  two  is,  after  all,  little  different 
from  its  natural  form  of  division.  If  the  worm  is  cut  across,  both  pieces 
will  survive  and  can  regenerate  a  complete  worm  providing  the  cut  falls 
somewhere  between  a  line  behind  the  brain  and  a  line  a  similar  dis- 
tance from  the  posterior  end.  In  fact,  any  piece  of  the  worm  that  is 
about  the  size  of  the  head  can  regenerate  a  complete  worm.  Successful 
regeneration  depends  upon  the  regeneration  of  a  head;  if  this  fails  to 
appear,  the  rest  of  the  body  also  fails  to  develop  normal  proportions  and 
spatial  arrangements. 

A  particular  aspect  of  flatworm  regeneration  that  has  been  studied 
extensively  is  polarity.  Polarity  is  a  general  phenomenon  in  organisms 
whereby  the  axes  of  symmetry  tend  to  be  established  and  maintained. 
The  flatworms  are  used  here  as  a  convenient  example  in  which  a  con- 
siderable amount  of  work  has  been  done.  Most  of  the  experiments  are 
concerned   with   the   anteroposterior   axis.   C.   M.   Child,  working  with 
Dugesia  (Fig.  11.9),  found  that,  in  general,  pieces  taken  from  the  middle 
of  a  worm  regenerate  heads  at  the  original  anterior  ends  and  tails  at  the 
original  posterior  ends.  A  more  subtle  expression  of  polarity  is  found 
in   the  ease  with  which   the  ends  regenerate.   Pieces  from   the   forward 
part  of  the  body  regenerate  heads  rapidly,  those  from  the  middle  por- 
tion of  the  body  more  slowly,  and  those  from  the  posterior  region  very 
slowly  or  not  at  all.   The  readiness   with  which  appropriate  ends   are 
formed  is   also  seen  in  occasional  errors.  A  head,  if  severed   from  the 
body,   may  regenerate  a  second  head  instead  of  a  tail  at  its  posterior 
end.  Similarly,  the  tail  end  may  sometimes  produce  a  tail  instead  of  a 
head  at  its  anterior  end.  All  of   the  evidence  suggests  that  there  is  a 
gradient  in  the  worm,  the  head-forming  tendency  being  strongest  at  the 
anterior  end  and  weakest  at  the  posterior  end,  with  a  reverse  gradient 


212  TWE   ANIMAL   KINGDOM 


Figure  1 1 .9.  Polarity  and  regeneration  in  Dugesia.  Top  left,  each  of  five  pieces  re- 
generates, but  the  rapidity  with  which  the  head  develops  depends  upon  the  level  of  the 
piece.  Lower  left,  occasional  errors  that  occur,  and  an  example  of  changed  polarity. 
Lower  right,  preservation  of  polarity  depends  upon  whether  or  not  the  piece  bends. 
Upper  right,  a  two-headed  form  produced  by  repeated  splitting  of  the  anterior  end. 

for  the  tendency  to  torm  a  tail.  Such  gradients  predict  that  any  piece 
of  the  worm  will  regenerate  so  as  to  retain  its  original  polarity. 

Polarity  can  be  altered.  If  a  triangular  piece  is  cut  from  the  side 
of  the  body  (Fig.  11.9),  it  usually  regenerates  a  head  at  the  inner  end, 
forming  a  tail  from  the  lateral  edge.  A  strip  cut  from  the  side  of  a 
worm  will  regenerate  normally  if  it  remains  straight,  but  if  it  bends 
the  head  appears  on  the  inner  side. 

Monsters  can  also  be  produced.  If  a  worm  is  partially  split  (Fig. 
1 1.9),  and  the  split  is  kept  open  by  continual  recutting,  the  worm  will 
eventually  regenerate  so  as  to  produce  some  double  structures.  Many  of 
these  monsters  eventually  solve  their  problems  by  splitting  up  and  de- 
veloping into  several  worms.  If  a  two-headed  worm  is  produced,  for 
example,  the  split  gradually  deepens  until  the  worms  separate  as  two 
complete  individuals. 


78.        Class  Turbellaria 

The  platyhelminthes  are  divided  into  the  three  classes  given  at  the 
beginning  of  the  chapter.  The  Turbellaria  are  characterized  by  the  pres- 
ence of  a  ciliated  epiclermis,  which  is  not  found  in  any  adults  of  the  other 
two  classes. 

Turbellarians  are  divided  into  a  number  of  orders  (Fig.  11.10), 
according  to  the  branching  of  the  intestine.   Planarians  belong  to  the 


THE   PHYLUM   PLATYHELMINTHES  213 


-Mouth-/ M     m       ^^    i^i^f 


fit 


Acoela.  Rhabdocoela.  Polycladida 

Figure    11.10.     Other  orders   of   the   class   Turbellaria.    An   example   of   the   order 
Tricladida  is  shown  in  Figure  11.1. 

order  Tricladida,  in  which  the  intestine  has  three  primary  branches. 
This  is  mainly  a  tresh-water  group,  but  it  also  includes  a  few  marine 
and  terrestrial  flatworms.  In  the  order  Polycladida  the  intestine  has 
many  primary  branches.  These  worms  are  all  marine.  The  Rhabdocoela 
have  an  anterior  mouth  and  a  simple,  straight  intestine,  while  the 
Acoela  have  no  intestine  at  all.  Rhabdocoels  are  common  in  all  waters 
and  include  a  large  number  ol  small  species.  The  acoels  are  marine  and 
minute.  Most  of  them  are  very  sluggish.  They  have  a  ventral  mouth 
that  opens  directly  into  a  mixture  of  mesenchyme  and  endoderm  cells. 
Bits  of  food  are  swallowed  and  phagocytized  by  the  endoderm  cells. 


79.        Class  Trematoda 

Trematodes  are  parasitic  flatworms  that  attach  to  the  host  by  means 
of  suckers  (Fig.  11.11),  and  in  which  the  entire  adult  epidermis  has  been 
replaced  by  a  cuticle  (Fig.  11.12).  The  digestive,  excretory,  muscular  and 
reproductive  systems  are  similar  to  those  of  the  Turbellaria.  The  class 
is  divided  into  two  primary  groups,  the  Monogenea,  having  a  life  cycle 
involving  only  a  single  host,  and  the  Digenea,  having  a  life  cycle  in- 
volving two  or  more  kinds  of  host. 

The  Monogenea  are  mostly  ectoparasitic,  living  on  the  external 
surface  of  the  host.  They  have  one  or  more  adhesive  organs  next  to 
the  mouth  and  one  or  more  posterior  suckers,  with  which  they  creep 
about  like  inchworms  (Fig.  11.11).  This  group  includes  the  gill  flukes, 
common  on  the  gills  of  marine  and  fresh-water  fishes.  Following  copu- 
lation, which  is  much  like  that  in  Dugesia,  the  hermaphroditic  adults 
lay  eggs,  one  to  a  capsule,  at  the  rate  of  several  to  150  per  day.  These 
have  a  thread  on  one  end  by  which  they  become  entangled  on  the  sur- 
face of  the  host  or  in  the  vegetation.  They  hatch  in  a  week  to  a  month 
into  small  larvae  that  resemble  the  parent  except  that  they  are  clothed 
in  a  ciliated  epidermis  and  have  less  elaborate  attachment  organs.  By 
means  of  the  cilia  the  larvae  swim  to  the  appropriate  host.  Maturation 


214  ^^^^   ANIMAL   KINGDOM 


Adhesive 
orOan. 


Mouth 


Midvcntral 
SucKer 


Posterior 
SucKe-r 


Ante-rior 
SucKer 


A.  Mono^&nea.  B.  Digc-nea. 

Figure  11.11.     The  two  major  groups  of  flukes,  class  Trematoda. 


Cuticle 

Circular  muscle 
Longitudinal  mascle 
Diagonal  inuscle 


Mesenchyme 
ce-Us 

Mesenchyme 


Figure   11.12.     Part  of  the  body  wall  of  a  trematode.  Note  that  an  epidermis  is 
missing,  and  that  the  covering  cuticle  lies  directly  on  tissue  of  mesodermal  origin. 

involves  modification  of  the  suckers  and  replacement  of  the  epidermis 
by  a  hard  cuticle,  apparently  secreted  by  the  underlying  mesodermal 
tissue.  They  feed  on  the  slime,  on  epithelial  cells,  and  on  blood  extrud- 
ing from  wounds  they  make  in  the  skin  of  the  host. 

The  Digenea  include  a  number  of  medically  important  parasites, 
such  as  the  liver  flukes,  lung  flukes  and  blood  flukes.  In  some  regions 
of  the  world,  especially  in  Asia  and  the  Southwest  Pacific,  whole  pop- 
ulations of  people  are  kept  in  constant  poor  health  by  a  single  species 
of  digenetic  fluke.  These  are  endoparasitic  worms,  living  inside  the 
body  of  the  host.  They  have  an  anterior  sucker  surrounding  the  mouth 
(Fig.  11.11)  and  a  large  midventral  sucker.  Adults  usually  mate,  but 
if  one  individual  is  alone  in  its  host  it  can  undergo  self-fertilization  by 
autocopulation.  In  one  group,  the  blood  flukes,  the  sexes  are  separate. 
These  live  in  the  circulatory  system  in  pairs,  the  more  slender  but 
longer  female  nestled  in  a  ventral  groove  of  the  male.  As  in  the  Mono- 
genea,  digenetic  eggs  are  laid  one  to  a  capsule,  but  the  capsules  are 
often  retained  in  the  parent  until  they  are  ready  to  hatch. 

The  life  cycles  of  this  group  are  quite  complex.  The  egg  hatches 
into  a  ciliated  larva,  the  miracidium  (Fig.  11.13),  which  invades  the  first 
host,  usually  a  snail.  The  miracidium  has  a  well  developed  brain  and  a 


THE   PHYLUM   PLATYHELMINTHES 


215 


pair  of  eyes.  It  apparently  has  no  digestive  tract  at  all  although  it  has  a 
typical  set  oi  flame  cells.  The  anterior  rostrum  lacks  cilia  and  is  equipped 
with  an  apical  gland  that  secretes  corrosive  juices  for  penetrating  the 
tissues  of  the  host.  The  body  is  filled  with  reproductive  tissue.  The 
miracidium  darts  about  rapidly  in  the  \vater,  and  if  it  fails  to  find  the 
proper  species  of  snail  in  a  few  hours  it  will  die. 

As  the  miracidium  penetrates  the  snail  it  sheds  its  ciliated  epi- 
dermis and  rounds  up  as  a  sporocyst  (Fig-  11.13)  covered  with  a  thin 
cuticle.  All  miracidial  structures  disappear  except  some  subepithelial 
muscle  fibers  and  the  flame  cells,  while  the  reproductive  tissue  develops 
into  a  variable  number  of  embryos.  Nutrients  are  absorbed  from  the 
host  directly  through  the  cuticular  wall. 

Each  embryo  develops  into  the  next  stage,  usually  a  redia  (Fig. 
11.13).  This  escapes  from  the  sporocyst  and  begins  to  feed  upon  the 
tissues  of  the  host.  The  redia  has  an  anterior  mouth,  a  muscular  pharynx 
by  which  host  tissue  is  sucked  up,  and  a  short,  saclike  intestine.  The 
body  wall  is  made  up  of  a  cuticle,  muscle  and  mesenchyme.  A  brain 
with  nerve  cords  and  a  flame  cell  system  are  also  present.  The  rest  of 
the  body  is  fdled,  as  in  the  miracidium,  with  reproductive  tissue.  Again, 
this  tissue  develops  into  a  number  of  embryos. 


Figure  11.13.  Life  cycle  of  a  digenetic  fluke.  The  stages  shown  here  belong  to  vari- 
ous species.  The  arrows  indicate  whether  one  stage  becomes  the  next  or  whether  it  pro- 
duces the  next  by  reproduction.  (After  Hyman.) 


216  ^HE   ANIMAL   KINGDOM 

Each  embryo  within  the  redia  may  develop  into  another  redia,  or 
into  the  next  stage,  a  cercaria  (Fig.  11.13),  which  escapes  from  the  redia 
through  a  birth  pore.  Each  cercaria  is  a  miniature  fluke  with  a  tail.  At 
the  front  end  it  has  a  penetration  stylet  equipped  with  an  apical  gland. 
The  cercaria  leaves  the  snail  and  swims  through  the  water  by  lashing 
its  tail,  searching  randomly  for  the  next  host,  which  varies  considerably 
(crayfish,  clam,  fish,  etc.),  according  to  the  species  of  fluke.  The  cercaria 
bores  into  the  new  host,  sheds  its  tail,  and  becomes  surrounded  by  a 
cyst. 

Within  the  cyst  the  stylet  and  apical  glands  disappear,  and  the 
other  structures  develop  further  toward  the  adult  pattern.  This  stage, 
the  metacercaria  (Fig.  11.13),  must  be  eaten  by  the  final  host  in  order 
to  mature.  Thus,  the  fluke  does  not  feed  upon  this  second  host,  in 
which  the  cercaria  becomes  a  metacercaria.  The  second  host  serves  as 
a  means  of  gaining  entry  into  the  final  host,  which  is  usually  some  kind 
of  vertebrate  carnivore  (fish,  frog,  cat,  man,  etc.).  In  some  species  the 
cercariae  encyst  and  become  metacercariae  on  aquatic  vegetation,  and 
are  thus  able  to  parasitize  an  herbivore  (sheep,  cow,  etc.)  as  the  final 
host. 

When  the  metacercaria  is  eaten  by  the  appropriate  final  host,  the 
cyst  wall  dissolves  in  the  latter's  intestine,  and  the  young  fluke  emerges. 
It  then  migrates  through  the  body  to  its  final  site  (lungs,  liver,  etc.), 
feeding  and  growing  as  it  goes,  and  finally  maturing  in  a  few  days  to 
several  weeks. 

The  details  of  reproduction  in  the  sporocyst  and  redia  have  been 
difficult  to  interpret,  and  hence  are  subjects  of  considerable  controversy. 
If  the  reproductive  tissues  of  sporocyts  and  rediae  produce  eggs  that 
become  rediae  and  cercariae,  then  the  life  cycle  involves  three  genera- 
tions of  organisms.  If,  on  the  other  hand,  the  reproductive  tissues  are 
simply  persistent  embryonic  tissue  that  divides  to  form  many  indi- 
viduals, reproduction  in  the  sporocyst  and  redia  is  similar  to  that  of 
the  coelenterate  polyp,  and  the  entire  cycle  is  a  single  generation  with 
asexual  reproduction  in  larval  stages.  The  distinction  rests  upon  whether 
or  not  meiosis  occurs  during  larval  reproduction,  an  issue  that  has  not 
yet  been  settled. 

80.        Class  Cestoda 

Tapeworms  are  endoparasitic  flatworms  without  epidermis,  mouth 
or  digestive  tract.  The  front  end  of  the  body  is  a  knoblike  scolex,  armed 
with  hooks  or  suckers  by  which  the  animal  attaches  to  the  host  (Fig. 
11.14).  Behind  the  scolex  is  a  narrow  neck,  followed  by  a  long  chain  of 
proglottids.  Proglottids  are  produced  by  segmentation  in  the  neck  re- 
gion, where  rapid  longitudinal  growth  takes  place  constantly.  As  each 
proglottid  ages,  it  is  found  farther  and  farther  back  along  the  length 
of  the  worm.  It  widens  and  lengthens,  and  eventually  becomes  mature. 
Each  proglottid  has  a  complete  set  of  reproductive  organs,  similar  to 
those  of  the  Turbellaria  except  that  the  genital  opening  is  lateral.  As 


THE   PHYLUM    PLATYHELMINTHES 


217 


each  proglottid  becomes  filled  with  eggs  it  breaks  off  and  passes  out  of 

the  host. 

Most  tapeworms  live  in  the  intestine  of  vertebrates  with  the  scolex 
buried  in  the  intestinal  wall.  They  do  not  feed  upon  the  host  itself,  but 
soak  up  nutrients,  competing  with  the  host  for  food  that  the  latter  has 
digested. 

The  scolex  contains  a  brain  from  which  two  lateral  nerves  extend 
posteriorly  through  all  of  the  proglottids.  Excretory  tubules  also  extend 
the  length  of  the  body,  opening  posteriorly  where  the  last  proglottid 
dropped  off.  Flame  cells  connected  with  these  tubules  occur  throughout 
the  body.  The  body  wall  includes  a  cuticle  and  muscular  tissue,  with 
which  the  tapeworm  can  make  slow  writhing  movements. 

When  a  proglottid  becomes  sexually  mature  it  usually  mates  with 
itself  by  autocopulation,  but  mating  between  proglottids,  either  of  the 
same  or  of  different  worms,  has  been  observed.  As  in  the  trematodes, 
each  egg  is  covered  with  a  separate  capsule.  The  eggs  are  retained  in  the 
proglottid,  which  eventually  becomes  full,  breaks  off  and  bursts. 

Most  cestodes  have  more  than  one  kind  of  host.  The  larva  hatches 
from  its  capsule  only  after  it  is  eaten  by  the  appropriate  first  host,  usu- 
ally an  arthropod,  in  whose  digestive  tract  the  capsular  membrane  is 
digested  away.  The  first  stage  is  the  oncosphere  (Fig.  11.15),  little  more 
than  a  ball  of  cells  containing  a  few  hooks.  It  bores  through  the  in- 
testinal wall  and  develops  in  various  organs  of  the  host.  In  some  tape- 
worms it  is  covered  witli  a  ciliated  epidermis  while  in  others  it  is 
covered  with  a  cuticle.   It  also  has  a  pair  of  flame  cells. 


Scolex 


Sucker 


Scolex  in 

wall  of 

intestine 


Genltol 
pore 


Figure   11.14.     The  pork  tapeworm,   Taenia  solium.  Insets  show  the  head,  an  im- 
mature and  a  mature  section  of  the  body.  (Villee:  Biology.) 


218 


THE   ANIMAL   KINGDOM 


ADULT 


Procercoid 


Ccrcoid 

Figure  11.15.     Life  cycle  of  a  tapeworm.  All  of  the  stages  are  those  of  the  fish  tape- 
worm, Diphyllobotlniiiin  latum.  (Modified  from  Hyman,  after  Rosen.) 

In  tape^V'Orms  with  a  three-host  cycle  the  oncosphere  develops  into 
a  procercoid  (Fig.  11.15).  The  body  elongates  and  the  hooks  become 
located  in  a  posterior  tail.  Anteriorly  a  rostrum  with  very  large  apical 
glands  develops.  When  the  arthropod  is  eaten  by  the  appropriate  second 
host  (fish,  or  other  vertebrate),  the  procercoid  sheds  its  tail,  bores  into 
the  tissue  of  the  new  host,  and  develops  into  a  cercoid,  which  varies  in 
appearance  in  different  tapeworms,  but  in  general  has  a  scolex  and 
somewhat  resembles  a  miniature  tapeworm  without  proglottids.  In 
tapeworms  with  a  two-host  cycle  the  oncosphere  develops  directly  into 
the  cercoid  stage. 

\Vhen  the  host  with  its  cercoid  larva  is  eaten  by  the  appropriate 
final  host  (usually  a  carnivorous  fish,  amphibian  or  mammal),  the 
larva  attaches  to  the  intestinal  wall  by  the  scolex  and  matures  into  a 
tapeworm.  Thus,  the  tapeworm  cycle  depends  at  each  transition  upon 
being  eaten  by  the  next  host.  In  some  sj^ecies  the  cercoid  stage  is  cap- 
able of  asexual  multiplication,  but  in  general  each  tapeworm  e^'g  pro- 
duces a  single  adult  worm.  The  number  of  eggs  produced  is  tremendous. 
For  example  Taenia  saginata,  a  tapeworm  that  can  infect  man,  sheds 
8  or  9  proglottids  daily,  and  each  proglottid  contains  80,000  eggs.  The 
infective  larvae  of  this  tapeworm  occur  in  beef. 


Questions 

1.  What  kinds  of  organs  are  found  anteriorly  in  flatworms? 

2.  What  influences  the  number  of  flame  cells  in  the  flatworm  body? 

3.  Describe  the  path  of  sperm  from  the  testis  to  fertilization  in  Dugesia. 

4.  What  are  the  theories  regarding  the  nature  of  polarity? 

5.  Characterize  the  classes  of  the  phylum  Platyhelminthes. 

6.  Describe  the  life  cycle  of  a  digenetic  trematode. 


THE   PHYLUM    PLATYHELMINTHES  219 

7.  Where  does  the  life  cycle  of  a  tapeworm  differ  markedly  from  that  of  a  digenetic 
trematode? 

8.  Define  sporocyst,  oncosphere  and  redia. 

Supplementary  Reading 

The  phylum  is  thoroughly  described  and  discussed  in  The  Invertebrates,  volume  II, 
by  L.  Hyman.  Problems  of  regeneration  and  polarity  in  many  organisms  are  discussed  in 
Analysis  of  Development  by  Willier,  Weiss  and  Hamburger.  Many  life  cycles  of  the 
parasitic  forms  can  be  found  in  parasitology  texts,  including  Chandler,  Introduction  to 
Parasitology. 


CHAPTER  12 


The  Phyla  Aschelminthes 
and  Nemertea 


All  of  the  animals  that  remain  to  be  considered  have  a  body  cavity, 
or  a  circulatory  system,  or  both.  A  circulatory  system  can  be  defined  as  a 
system  of  channels  containing  a  fluid  that  is  moved  around  by  muscular 
activity.  The  walls  of  the  channels  are  derived  from  mesoderm.  Two 
kinds  of  body  cavities  can  be  distinguished.  Both  are  fluid-filled  spaces 
that  permit  the  internal  organs  freedom  of  movement,  unhampered 
by  extensive  connection  with  the  body  wall.  If  the  space  lies  between 
the  gastrodermis  and  tissues  of  mesodermal  origin  (i.e.,  if  it  surrounds 
a  gut  made  only  of  endoderm),  it  is  a  pseudocoelom.  If  the  space  lies 
ivithin  tissues  of  mesodermal  origin  (if  it  surrounds  a  gut  composed 
of  gastrodermis  covered  with  mesodermal  tissues),  it  is  a  eucoelom  or, 
simply,  coelom.  A  coelom  is  lined  with  a  simple  epithelium  of  meso- 
dermal origin,  the  peritoneum.  A  pseudocoelom  lacks  an  epithelium. 
None  of  the  pseudocoelomates  has  a  circulatory  system. 

The  phylum  Aschelminthes  includes  the  pseudocoelomates  whose 
bodies  are  largely  covered  with  cuticle.  They  have  an  anterior  mouth 
and  a  posterior  anus.  The  phylum  is  large  and  includes  groups  of  di- 
verse appearance. 

The  Nemertea  are  acoelomate  (have  no  body  cavity)  but  have  a 
circulatory  system.  The  mouth  is  anterior  and  the  anus  posterior,  and 
in  front  of  the  mouth  is  an  eversible  proboscis.  The  phylum  is  small 
and  will  be  considered  at  the  end  of  this  chapter. 

Most  of  the  remaining  phyla,  to  be  considered  in  later  chapters, 
have  both  a  circulatory  system  and  a  eucoelom. 

81.        Classification  of  the  Aschelminthes 

The  groups  to  be  considered  here  have  always  been  troublesome  to 
taxonomists.  They  have  been  arranged  in  one,  two,  three,  and  even  six 
different  phyla.  In  the  face  of  so  many  diverse  opinions  any  one  position 
is  necessarily  arbitrary.  It  is  largely  for  convenience,  therefore,  that  the 
groups  will  be  treated  as  six  classes  in  one  phylum.  The  classes  are 
(Fig.   12.1): 

I.   Rotifera.  Aquatic  microscopic  animals  with  internal   jaws   and 

an  anterior  ciliated  wheel-organ. 
220 


THE   PHYLA    ASCHELMINTHES   AND   NEMERTEA 


221 


II.  Gastrotricha.  Aquatic  microscopic  animals  with  ventral  por- 
tions ot  the  epidermis  ciliated,  with  posterior  adhesive  tubes 
and  with  a  nematode-like  pharynx. 

III.  Kinorhyncha.  Marine  microscopic  animals  with  a  segmented 
cuticle  and  a  spiny  head  that  can  be  withdrawn  into  the  body. 

IV.  Nematoda.  Tapered  cylindrical  worms  with  a  triradiate  pharynx, 
a  modified  excretory  system,  and  a  very  heavy  cuticle  covering 
the  body. 


^   Rotifera.        GastrotricHa    KinorhynctiSt 
(f^ioating  ±yp^)  (head wit: 


0or'd.iaveea- 


Nexna.i>o<ia. 


Aca.TitKocepliala 
.{head  evertre<^ 

Figure  12.1.     Classes   of   the   phylum    Aschelminthes.   These   are   considereid    to 
separate  phyla  by  many  authors.  Redrawn  from  Hyman. 


be 


222  ^"^   ANIMAL   KINGDOM 

V.  Gordiacea.    Long,   slender,   cylindrical    worms   widi    a   reduced 
digestive  system  and  no  excretory  system.  Parasitic  as  juveniles. 
VI.  AcanJhocephala.  Parasitic  worms  that  (like  the  tapeworms)  lack 
a  digestive  system.  They  have  a  retractile  spiny  head. 

82.  Class  Rotifera 

Rotifers,  which  are  about  the  size  of  paramecia,  are  among  the  most 
abundant  microorganisms  in  ponds,  lakes  and  streams.  Some  fifteen 
hundred  species  are  known.  A  few  of  these  live  in  moss  or  wet  sand, 
others  live  in  the  oceans,  but  the  majority  live  in  fresh  water.  Some 
rotifers  float  in  the  water,  others  are  attached  to  the  bottom  or  to  other 
animals,  and  still  others  creep  about  with  leechlike  movements.  Most 
of  the  familiar  rotifers,  common  in  temporary  ponds  and  puddles,  are 
of  the  creeping  variety. 

A  characteristic  structure  of  the  rotifers  is  the  wheel-organ,  a 
circlet  of  cilia  extending  around  the  front  end  of  the  head  from  the 
antero-ventral  mouth  (Fig.  12.2).  It  may  be  a  simple  circle,  or  it  may  be 
elaborated  by  outfoldings  from  the  body.  A  common  plan  is  that  of  a 
double  circle  (Fig.  12.2).  When  observed  under  the  microscope  the 
wheel-organ  appears  to  rotate,  an  illusion  so  convincing  that  Leeuwen- 
hoek  believed  rotifers  possessed  wheels.  The  illusion  is  the  result  of  a 
coordinated  rhythm  of  the  ciliary  beating.  The  cilia  beat  in  waves, 
which  pass  circularly  around  the  rim.  At  any  given  moment  some  cilia 
are  relaxed  while  the  adjacent  ones  are  bending,  producing  a  momentary 
aggregation  of  cilia.  It  is  these  aggregations  of  cilia  moving  with  the 
waves  around  the  circle  that  are  seen,  and  not  the  motion  of  the  indi- 
vidual cilia. 

Most  rotifers  are  transparent  and  the  internal  jaws  are  easily  seen, 
especially  since  they  are  usually  in  motion.  In  spite  of  their  small  size 
the  jaws  are  elaborate,  being  composed  of  seven  pieces  of  varying  shape. 
In  different  species,  they  may  be  used  for  grinding,  biting  or  piercing. 
When  used  for  biting  or  piercing  the  jaws  are  everted  through  the 
mouth. 

The  body,  which  is  clothed  in  a  thin  cuticle,  usually  ends  poste- 
riorly in  a  foot  (Fig.  12.2).  The  foot  is  equipped  with  pedal  glands 
that  secrete  adhesive  mucus,  by  which  the  rotifer  can  attach  to  objects 
temporarily  or  permanently.  The  foot  is  missing  in  many  of  the  plank- 
tonic  species. 

83.  Philodina 

The  genus  Philodina  includes  a  number  of  common  species  of 
creeping  rotifers,  of  which  P.  roseola  (Fig.  12.2)  is  representative.  Its 
wheel-organ  is  divided  into  two  whorls,  with  the  funnel-shaped  mouth 
located  midventrally  between  them.  Philodina  is  usually  attached  by  its 
foot  and  creates  water  currents  with  the  wheel-organ  that  bring  minute 
food  particles  (algae,  bacteria,  etc.)   to  the  mouth. 

The  mouth  leads  to  a  muscular  pharynx  containing  hard  cuticular 


THE   PHYLA    ASCHELMINTHES   AND   NEMERTEA 


223 


jaws.  The  jaws  of  Philodlna  are  stout  and  ridged  for  grinding  the  food 
particles  into  a  soft  pulp.  They  chew  constantly  while  feeding.  The 
pharynx  leads  to  a  large  stomach  by  way  of  a  short  esophagus  sur- 
rounded by  digestive  glands.  These  glands  have  been  observed  to  se- 
crete into  the  stomach  material  that  is  assumed  to  be  enzymatic.  Di- 
gestion takes  place  rapidly  in  the  stomach  cavity  and  the  nutrients  are 
quickly  absorbed  into  the  gastrodermis  cells.  The  stomach  opens  into 
a  short  intestine,  which  leads  to  the  bladder. 

A  pair  of  nephridial  tubules  opening  into  the  bladder  drain  a  series 
of  flame  cells  that  extend  forward  in  the  body.  The  bladder  fills  and 


^^-^^ Brain 

Moulh 


Whzcl 
or^an 


Flame  cell 


Pharynx 
Jaws 


Digestive 
■''  and 


Stomach 


Ovary 


Nephridial 
tubule 


Intestine 

Bladder 
Anas 


Pedal 
lands 


Figure  12.2.     Ventral   view  of  a   rotifer,  Philodina   roseola,  showing  many  of  the 
internal  structures.  Redrawn  from  Hyman. 


224 


THE   ANIMAL   KINGDOM 


Figure  12.3.     Dorsal  view  of  the  rotifer,  Rotaria,  showing  external  structures  and 
many  of  the  muscle  strands  in  the  body  wall.  Similar  structures  occur  in  Philodina. 


empties  every  few  minutes,  suggesting  that  the  primary  function  of  the 
rotifer  excretory  system  is  water  balance.  A  pair  of  ovaries  lateral  to 
the  stomach  also  open  into  the  bladder  by  paired  oviducts.  The  bladder 
opens  dorsally  at  the  base  of  the  foot. 

Philodiyia  may  detach  itself  and  swim  away.  The  action  of  the 
wheel-organ,  which  pulls  water  toward  the  animal  in  feeding,  is  equally 
suitable  for  locomotion.  Rotifers  often  swim  off  in  this  fashion  when  they 
are  disturbed. 

When  a  rotifer  creeps  on  the  bottom  or  on  vegetation,  its  entire 
wheel-organ  is  retracted  into  the  body  by  its  retractor  muscles  (Fig. 
12.3).  The  rostrum  is  everted  and  forms  a  new  anterior  end  to  the  body, 
dorsal  to  the  wheel-organ.  At  its  tip  are  cilia,  spines  and  plates  by  which 
it  can  attach.  Like  a  leech  or  bloodsucker  (Chapter  15)  with  its  anterior 
and  posterior  suckers,  Philodina  creeps  by  alternately  attaching  rostrum 
and  foot.  When  it  finds  a  place  suitable  for  feeding,  the  rostrum  is  re- 
tracted by  means  of  the  rostral  retractor  muscles  as  the  wheel-organ  is 
everted.  In  a  sense  Philodina  has  two  anterior  ends  which  it  can  use 
alternately.  The  retractor  muscles  are  part  of  the  body  wall  musculature, 
which  also  includes  a  number  of  circular  and  longitudinal  strands  (Fig. 
12.3).  The  remainder  of  the  body  wall  is  made  of  a  simple  ectodermal 
epithelium  covered  by  the  cuticle.  In  the  creeping  rotifers  this  cuticle 
is  segmented  to  facilitate  movement. 

The  nervous  system  of  rotifers,  like  that  of  the  flatworms,  includes 
a  bilobed  brain  dorsal  to  the  pharynx,  and  several  pairs  of  nerves,  of 
which  the  ventro-lateral  pair  are  the  largest.  Additional  nerve  cell 
ganglia  are  located  on  the  pharynx,  bladder  and  foot. 

Sense  organs  include  bristles  for  touch  and  chemoreception  on  the 
body,  especially  around  the  wheel-organ  and  rostrum.  Most  rotifers  have 


THE   PHYLA    ASCHELMINTHES    AND   NEMERTEA  225 

a  dorsal  antenno  (Fig.  12.3),  a  short  projection  rich  in  sensory  endings. 
Eyespots  are  light-sensitive  cells  containing  pigment  that  screens  out  the 
light  except  from  one  direction.  These  are  found  in  many  rotifers  em- 
bedded in  the  brain,  on  the  wheel-organ  or  on  the  rostrum. 

In  addition  to  all  of  the  complex  structures  found  in  these  tiny 
animals,  they  have  a  cavity  between  the  body  wall  and  the  digestive 
tract.  The  pharynx  and  bladder,  which  have  muscles,  are  formed  as 
ectodermal  invaginations  during  development.  The  rest  of  the  digestive 
tract  is  a  simple  gastiodermis,  without  mesoderm,  so  that  the  body 
cavity  is  a  true  pseudocoel,  lying  between  endoderm  and  mesoderm. 

84.  Reproduction  in  Rotifers 

The  creeping  rotifers  (including  Philodina)  are  parthenogenetic; 
young  are  produced  from  eggs  that  have  not  been  fertilized  by  sperm. 
In  oogenesis,  the  meiotic  process  is  much  modified,  with  the  result  that 
the  eggs  remain  diploid.  Such  eggs  hatch  in  a  day  or  two  and  mature 
within  a  week  into  adults,  all  of  which  are  female.  Each  adult  produces 
only  from  10  to  50  eggs. 

Males  are  occasionally  found  in  the  other  gioups  of  rotifers,  but 
much  of  the  reproduction  is  exclusively  by  parthenogenesis.  Under  cer- 
tain environmental  conditions  the  new  generation  of  females  matures 
as  somewhat  different  organisms.  The  eggs  they  produce  are  smaller, 
and  follow  through  the  normal  meiotic  process  to  become  haploid.  The 
first  of  these  eggs  are  laid  and  hatch  quickly  as  males.  The  males  are 
haploid,  often  remain  very  small,  and  mature  rapidly.  They  mate  only 
with  members  of  their  mother  generation,  the  females  that  are  producing 
small  haploid  eggs.  The  small  eggs  that  are  fertilized,  restoring  dip- 
loidy,  are  retained  until  they  become  very  large,  when  they  are  laid  in  a 
heavy  shell,  usually  as  a  resting  egg  for  overwintering.  These  resting 
eggs  later  hatch  into  females  that  produce  only  female  offspring,  com- 
pleting the  reproductive  cycle. 

85.  Cell  Constancy 

Associated  with  rotifers  are  several  interesting  phenomena,  one  of 
which  is  cell  constancy.  In  a  given  species,  each  part  of  the  body  is 
made  of  a  precise  number  of  cells  arranged  in  a  fixed  pattern.  Many  of 
the  body  parts  are  syncytial  (cell  boundaries  disappear),  but  it  is  evident 
from  the  number  and  positions  of  the  nuclei  that  cell  constancy  is  main- 
tained. The  total  number  of  nuclei  in  the  rotifers  studied  ranges  from 
900  to  1000.  The  exact  number  in  each  organ  has  been  counted  for  sev- 
eral species.  These  numbers  are  fixed  during  embryologic  development, 
and  mitosis  then  stops  completely.  Even  the  eggs  that  the  female  will 
produce  after  maturity  are  all  present  early  in  development. 

It  has  been  impossible  to  induce  mitosis  in  adult  rotifers  experi- 
mentally. If  a  piece  of  the  body  containing  nuclei  is  removed,  no  re- 
generation takes  place.  Often  the  wound  does  not  heal  over  and  the 
individual  dies.  Young  rotifers  are   able  to  replace   bits  of  cytoplasm, 


226  '■^f   ANIMAL   KINGDOM 

and  sometimes  even  replace  a  piece  containing  nuclei,  but  the  replace- 
ment lacks  nuclei.  Thus,  rotiters  are  extremely  specialized  at  the  cel- 
lular level,  to  the  extent  that  further  growth  and  repair  are  impossible. 
One  oi  the  challenging  unsolved  problems  of  biology  concerns  the 
possible  differences  which  may  distinguish  such  nondividing  cells  from 
those  of  other  animals. 


86.  Senescence 

An  individual  rotifer  lives  an  active  life  for  only  a  few  days,  and  yet 
toward  the  end  of  this  period  it  shows  several  of  the  characteristic  fea- 
tures of  old  age.  Egg  production  ceases,  the  animal  becomes  sluggish, 
and  portions  of  the  body  begin  to  degenerate.  Lansing  has  found  that 
during  these  few  days  the  amount  of  calcium  in  the  body  increases, 
just  as  it  increases  with  age  much  more  slowly  in  the  bodies  of  man 
and  other  animals.  He  also  found  that  if  the  calcium  was  removed 
every  day  by  immersing  the  rotifers  in  sodium  citrate  for  one  minute, 
the  average  life  span  was  considerably  lengthened.  If  the  aging  of  rotifers 
is  found  to  be  similar  to  that  of  man,  they  will  be  used  widely  in  re- 
search, for  mere  days  rather  than  years  are  required  for  the  completion 
of  experiments  with  rotifers. 

A  decline  in  vigor  in  successive  parthenogenetic  generations  has 
been  reported  in  some  rotifers.  In  certain  species  the  appearance  of 
males  is  not  related  to  external  factors  but  seems  to  be  inherent.  After 
a  certain  number  of  female-producing  generations,  the  male-producers 
appear,  resting  eggs  are  produced,  and  the  population  disappears  for 
the  season.  In  some,  there  is  a  continual  decrease  in  activity  and  lon- 
gevity from  generation  to  generation  of  parthenogenetic  females  be- 
fore the  sexual  phase  appears.  Little  is  known  of  the  mechanism  by 
which  an  aging  factor  can  be  transmitted  or  accumulated  through  suc- 
cessive generations.  Many  rotifer  populations  do  not  show  this  kind 
of  aging,  and  can  be  kept  as  parthenogenetic  strains  indefinitely. 

87.  Resistance  to  Desiccation 

Perhaps  the  most  interesting  aspect  of  rotifer  physiology  is  the  abil- 
ity of  some  species,  especially  those  that  live  in  temporary  puddles  or 
moss,  to  resist  adverse  circumstances.  A  dry,  tarred  roof  in  a  hot  summer 
sun,  when  the  tar  is  bubbling  hot,  is  an  unlikely  place  to  find  delicate 
animals,  yet  if  a  bit  of  dried  scum  is  taken  from  a  spot  where  the  last  rain 
puddle  dried  up,  and  placed  in  some  fresh  water,  the  dish  may  be 
swarming  with  rotifers  within  minutes.  These  are  not  newly  hatched, 
but  are  full  grown  adults.  They  are  visible  in  the  dry  scum  as  rotifer 
mummies  (Fig.  12.4),  shrunken  bodies  with  retracted  wheel-organs. 
When  water  is  added  they  simply  swell,  stretch  out,  and  begin  to  move. 
To  see  them  open  out  the  delicate  wheel-organ  only  minutes  after  baking 
in  the  sun  is  truly  astonishing. 

If  well  fed  rotifers  are  dried  slowly  they  may  survive  several  years 
of  desiccation.  The  longest  known  record  is  59  years.  In  the  dry  state 
they  can  survive  extremes  of  temperature,  from  well  above  the  l)oiling 


THE   PHYLA   ASCHELMINTHES   AND   NEMERTEA 


227 


Figure  12.4.     A  desiccated  rotifer  from  a  dried-up  puddle.  It  can  absorb  water  and 
become  active  again  in  a  few  minutes. 

point  of  water  to  well  below  zero.  They  have  even  survived  eight  hours 
in  liquid  helium,  where  the  temperature  is  — 272°C.,  just  one  degree 
above  absolute  zero  where  molecular  activity  ceases.  Rotifers,  then,  can 
achieve  a  state  of  suspended  animation  by  the  mere  loss  of  water. 
While  a  few  other  groups  of  animals  can  also  survive  desiccation  in 
this  way,  such  a  water  loss  is  usually  lethal.  The  properties  of  rotifer 
protoplasm  that  enable  it  to  survive  desiccation  are  entirely  unknown. 


88. 


Class  Nematoda 


The  Nematoda  are  mostly  cylindrical  worms  tapered  toward  both 
ends,  and  are  commonly  called  roundworms.  The  class  includes  many 
parasites  and  a  very  large  number  of  small  free-living  species.  They  are 
common  wherever  there  is  water,  even  though  it  be  but  a  thin  film.  The 
nematode  body  is  covered  by  a  thick  cuticle  (Fig.   12.5)  that  is  elastic 


Dorsal  nczrve  cord- 


Pharynx 


Pha-rynx 
lumo^n. 


Pse-udocoel 


Cuticle 


Epidermis 


Lateral  line 


Longitudinal 
muscle  fibers 


-Ventral  nerve  cord. 
Figure  12.5.     Cross  section  through  the  pharynx  of  a  nematode,  showing  the  body 
wall  and  the  peculiar  cell  structure  of  the  pharyngeal  wall. 


228 


THE   ANIMAL   KINGDOM 


and  tends  to  hold  tlie  body  straight  if  all  the  muscles  are  relaxed.  Be- 
neath the  cuticle  is  a  simple  ectodermal  epithelium,  and  beneath  this  a 
single  layer  oi  longitudinal  muscle  fibers.  Roundworms  are  unique 
among  all  animals  in  having  longitudinal  muscles  but  no  circular  mus- 
cles. The  only  motion  possible  is  bending  of  the  body,  which  may  re- 
sult in  simple  curvature  or  in  sinuous  movements.  Roundworms  crawl 
easily  like  snakes,  but  swim  very  poorly  despite  an  extremely  vigorous 
thrashing  of  the  body. 

The  anterior  mouth  (Fig.  12.6)  leads  through  a  mouth  cavity  to  a 
muscular  pharynx  of  unusual  structure.  During  development  the 
pharynx  arises  as  an  ectodermal  invagination  together  with  surrounding 
mesoderm  cells,  which  form  epithelial  cells  and  muscle  fibers  respec- 
tively. In  the  completed  pharynx,  however,  the  two  elements  are  inter- 
mingled to  form  a  single  layer  of  tissue.  The  result  is  a  triradiate 
pharynx,  named  for  the  shape  of  its  lumen,  which  is  due  to  the  uneven 
thickness  of  the  wall.  It  is  surrounded  by  a  membrane  and  lined  with  a 
continuation  of  the  external  cuticle.  Between  these  are  epithelial  cells 
and  interspersed  radial  muscle  fibers  (Fig.  12.5).  When  the  muscle  fibers 
contract,  the  lumen  is  enlarged,  producing  a  sucking  action  at  the  mouth. 

89.        The  Vinegar  Eel,  Turbatrix  Aceti 

A  free-living  nematode  common  in  older  vinegar  is  the  vinegar  eel, 
Turbatrix  aceti  (Fig.   12.6),   about  2  mm.  long.  Under  the  microscope 


-Mouth  cavity 


Pharynx 

Nerve-  rin 

PViarynx 
bulb 


Intestine. 


Protractor 
muscle 


Ovary 


Copula.tory 
spicule 
and  cLnus 

Seminal 
vesicle 

Sperm  ciuct 
Testis 


Uterus  ^™--ss!^- 

Vagina 
FEMALE  MALE 

Figure  12.6.     Lateral  view  of  the  vinegar  eel,  Turbatrix  aceti.   (After  De  Man,  1910.) 


THE   PHYLA    ASCHELMINTHES    AND    NEMERTEA  229 

many  of  its  general  anatomic  features  are  visible.  The  pharynx  ends  in 
a  posterior  enlargement,  the  bulb,  which  leads  directly  into  a  long  sim- 
ple  intestine.   The   intestine   ends  in   a   short   rectum    that  opens  at  a 
postero-ventral  anus.  The  vinegar  eel,  like  many  free-living  nematodes, 
feeds  primarily  on  bacteria. 

A  nerve  ring  is  the  only  visible  part  of  the  nervous  system;  it  sur- 
rounds the  pharynx  just  in  front  of  the  bulb. 

The  sexes  are  separate.  Males  have  a  single  thin  testis  that  passes 
forward  from  just  in  front  of  the  anus,  then  doubles  back  on  itself  and 
continues  as  a  sperm  duct  to  a  storage  expansion,  the  seminal  vesicle, 
that  opens  into  the  rectum.  Instead  of  a  penis  the  male  has  a  pair  of 
copulatory  spines  mounted  in  the  dorsal  wall  of  the  rectum.  These  can 
be  protruded  through  the  anus  and  into  the  vagina  of  the  female  by  the 
contraction  of  protractor  muscles. 

A  single  ovary  lies  in  the  middle  third  of  the  female  (Fig.  12.6). 
From  its  anterior  end  an  oviduct,  widened  to  form  a  uterus,  leads  back 
to  the  vagina,  just  posterior  to  the  middle  of  the  body.  The  vagina 
opens  ventrally.  Posterior  to  the  uterus  a  diverticulum  serves  as  a 
seminal  receptacle  for  receiving  sperm  at  copulation.  Eggs  produced  in 
the  ovary  are  fertilized  as  they  pass  into  the  uterus  by  sperm  that  mi- 
grate forward  from  the  receptacle.  Eggs  are  retained  in  the  uterus  until 
they  hatch,  and  the  young  worms  escape  through  the  vagina.  Thus,  the 
vinegar  eel  is  ovoviviparous. 

90.        The  Pig  Roundworm,  Ascaris  Lumbricoides 

Further  details  of  nematode  anatomy  are  more  easily  seen  in  the 
few  large  species,  all  of  which  are  parasitic.  Ascaris  lumbricoides  is  a 
foot  long,  and  may  be  obtained  from  pig  intestines  at  slaughterhouses. 
This  species  differs  from  the  free-living  species  primarily  in  having  more 
prominent  reproductive  organs.  The  mouth  and  pharynx  are  somewhat 
reduced. 

As  Ascaris  is  cut  open,  the  large  pseudocoelom  (Fig.  12.7)  is  evi- 
dent. In  it  the  long  intestine  and  much-folded  reproductive  organs  lie 
loosely.  On  the  wall  are  lateral,  dorsal  and  ventral  lines,  and  the  inner 
surface  is  covered  with  small  transparent  sacs.  These  represent  some  of 
the  more  bizarre  cell  structures  in  nematodes. 

The  lateral  lines  are  internal  ridges,  each  containing  an  excretory 
canal  that  runs  the  length  of  the  worm.  The  two  canals  join  beneath 
the  pharynx  and  a  short  common  tube  runs  forward  to  open  just  be- 
hind the  mouth  as  an  excretory  pore.  The  entire  excretory  system,  often 
a  foot  long,  is  made  from  a  single  cell,  whose  nucleus  is  located  where 
the  two  tubes  join  together.  At  their  inner  ends  the  tubes  are  closed. 
Flame  cells  are  lacking,  and  little  is  known  of  the  physiology  of  this 
system. 

The  dorsal  and  ventral  lines  are  the  nerve  cords  that  extend  back 
from  the  nerve  ring  around  the  pharynx.  The  brain  in  most  nematodes 
is  located  in  the  swollen  sides  of  this  ring,  connected  above  and  below 


230 


THE   ANIMAL   KINGDOM 


Cuticle' 
G  astro  derm 

Testis 
Sperm  duct 


VeiT-tral  nerve 


Dorsal  nerve 

Cuticle 

Epidarmis 

Pseudocoelom 

Lateral  line 

Excretory 
canal 

Cell  tody  of 
muscle  cell 

Conti^actile  portion 

Conductile  prooess 

Figure  12.7.     Cross  section  through  the  inicklle  region  of  a  male  A.sraiis  lunibricoides. 
The  testes  are  sectioned  several  times  because  they  lie  folded  in  the  body. 

the  pharynx  by  many  nerve  fibers.  In  Ascaris  the  brain  is  scattered  out  as 
several  pairs  of  ganglia  associated  with  the  ring. 

The  small  transparent  sacs  lining  the  body  wall,  easily  visible  to  the 
naked  eye,  are  the  cell  bodies  ol  the  muscle  fibers.  Each  fiber  extends 
longitudinally  one  quarter  to  one  halt  inch  beneath  the  epidermis.  At 
its  middle  is  the  sac  hanging  into  the  pseudocoeloin.  The  cell  body 
contains  the  nucleus  and  is  not  contractile.  The  muscle  cells  of  nema- 
todes are  not  innervated  by  nerve  fibers  coming  from  the  nerve  cords  as 
in  most  animals.  Instead  each  muscle  cell  sends  a  conductile  process  to 
the  nerve  cord  (Fig.  12.7).  Thus  each  muscle  cell  has  three  portions,  and 
is  structurally  unique  in  the  animal  kingdom. 

The  life  cycle  of  Ascaris  involves  only  a  single  host.  The  pig  round- 
worm may  lay  as  many  as  200,000  eggs  per  day.  These  pass  out  of  the 
pig  in  its  feces,  where  the  egg  develops  into  a  small  worm  within  its 
shell.  If  these  contaminate  the  food  of  pigs  and  are  eaten,  they  hatch 
in  the  intestines.  They  then  go  through  a  seemingly  unnecessary  cycle. 
The  young  worms  burrow  through  the  intestinal  wall  into  the  blood 
stream,  whence  they  are  carried  through  the  heart  to  the  lungs.  Here 
they  burrow  into  the  air  spaces,  crawl  up  the  trachea  to  the  pharynx, 
and  are  swallowed  again.  They  finally  mature  in  the  intestines.  During 
the  burrowing  phase,  if  large  numbers  are  involved,  hemorrhage,  infec- 
tion or  pneumonia  may  result. 

Ascaris  hunbricoides  is  a  species  complex  of  a  number  of  morpho- 
logically indistinguishable  strains  that  variously  infect  pigs,  sheep,  squir- 
rels, apes  and  man.  Each  strain  can  temporarily  infect  other  hosts,  but 
can  mature  and  reproduce  only  in  its  own  host. 


THE   PHYLA    ASCHELMINTHES    AND    NEMERTEA  231 

Roundworms  also  show  the  phenomenon  of  cell  constancy  in  most 
of  their  organs.  Mitotic  divisions  continue  throughout  life  only  in  the 
epidermis,  gastrodermis  and  gonads.  In  large  nematodes,  the  organs  with 
cell  constancy  increase  in  size  entirely  by  the  growth  of  the  cells,  and  not 
by  an  increase  in  their  numbers.  This  explains  why  the  individual 
cell  bodies  of  Ascaris  muscle  fibers  are  so  easily  visible.  In  many  parts 
of  the  body  the  tissues  tend  to  become  syncytial.  As  in  the  rotifers,  the 
ability  to  regenerate  is  very  poor. 

91.  Molting 

The  growth  of  the  young  nematode  into  an  adult,  when  contained 
in  a  heavy  cuticle,  presents  a  problem  that  is  solved  by  periodically 
shedding  the  cuticle  and  expanding  rapidly  before  the  new  cuticle 
hardens.  This  process  is  called  molting.  Each  nematode  molts  four  times 
in  becoming  adult.  When  the  external  cuticle  is  shed,  the  cuticle  lining 
the  mouth  cavity,  pharynx  and  rectum  is  also  shed.  This  indicates  that 
these  structures  are  also  of  ectodermal  origin. 

92.  Parasitism 

The  roundworms  have  exploited  endoparasitism  more  fully  than 
any  other  metazoan  group.  Practically  all  metazoa  have  roundworm 
parasites  that  produce  a  wide  variety  of  diseases.  These  include  such 
human  diseases  as  pinworm,  hookworm  and  elephantiasis.  The  subject 
will  be  treated  more  fully  in  Chapter  39. 

93.  Class  Gastrotricha 

The  microscopic  gastrotrichs  (Fig.  12.1)  are  common,  but  seldom 
abundant,  in  quiet  fresh  and  salt  water.  A  few  can  be  found  in  almost 
any  sample  of  pond  debris.  Gastrotrichs  are  very  active,  darting  about 
on  the  two  longitudinal  bands  of  cilia  on  their  ventral  surface,  clamber- 
ing rapidly  over  vegetation  and  debris.  They  feed  on  bacteria  and  algae, 
sucking  them  into  the  anterior  mouth  with  a  triradiate  pharynx  very 
similar  to  that  of  the  nematodes.  Cell  constancy  is  as  rigid  in  this  class 
as  in  the  rotifers.  In  some  gastrotrichs  all  growth  is  limited  to  the  em- 
bryonic stage;  the  parent  produces  enormous  eggs,  one  at  a  time,  that 
later  hatch  into  full  grown  individuals.  The  fresh-water  species  have 
only  females  which  reproduce  parthenogenetically. 

94.  Class  Kinorhyncha 

These  small  marine  worms,  less  than  5  mm.  long,  (Fig.  12.1),  are 
seldom  found.  They  live  in  soft  sand  and  mud  at  the  bottom  of  shallow 
or  deep  seas.  Kinorhynchs  resemble  nematodes  in  two  ways.  They  grow 
by  molting,  and  they  have  a  pharynx  similar  to  the  nematodes  except 
that  the  muscle  and  epithelial  layers  remain  distinct.  The  muscle  fibers 
are  radially    arranged,    however,    and  produce   suction   by   contraction. 


232  ^^^   ANIMAL   KINGDOM 

Kinorhynchs  have  a  body  musculature  reduced  to  separate  strands  as  in 
the  rotilers.  Ihe  cuticle  is  segmented  into  l'^  or  14  joints,  and  internal 
structures  such  as  the  muscles  and  nerve  cells  are  segmentally  arranged. 

95.  Class  Gordiacea 

The  Gordiacea  are  the  hairworms  (Fig.  12.1)  that  often  appear  in 
spring  water.  The  body  is  extremely  long  and  slender  and  tapers  little 
if  at  all  at  either  end.  Hairworms  are  parasitic  as  juveniles,  free-living 
as  adults.  The  adults  live  near  or  in  water,  in  which  they  lay  long  strings 
of  eggs.  These  hatch  into  short  fat  larvae  that  infect  grasshoppers, 
crickets  and  other  insects.  They  bore  through  the  digestive  tract  into  the 
body  cavity  where  they  grow  to  adult  size,  following  a  single  molt.  After 
the  adult  leaves  the  host  it  apparently  does  not  feed  and  its  digestive 
tract  may  become  closed  and  degenerate.  The  adult  is  often  much 
tangled  with  itself,  suggesting  a  Gordian  knot. 

96.  Class  Acanthocephala 

Adult  spiny-headed  worms  (Fig.  12.1)  live  in  the  digestive  tracts  of 
vertebrates.  The  head  is  retractile,  and  may  be  withdrawn  as  the  worm 
crawls  about,  or  everted  and  thrust  into  the  intestinal  wall  as  an  anchor. 
The  head  bears  rows  of  recurved  spines  and  the  wounds  produced  by 
them  may  become  serious  if  infected.  Large  numbers  of  eggs,  usually  well 
advanced  in  development,  pass  out  in  the  host  feces,  and  hatch  only  if 
they  are  eaten  by  an  arthropod.  The  young  larva  bores  through  the 
digestive  tract  of  this  first  host  into  the  body  cavity,  where  it  develops 
into  a  miniature  adult.  If  the  arthropod  host  is  eaten  by  the  vertebrate 
host,  the  worm  matures  in  the  intestine  of  the  latter.  Most  species  are 
small,  not  more  than  an  inch  long.  The  spiny-headed  worm  of  the  pig, 
however,  which  parasitizes  beetle  grubs  as  the  arthropod  host,  grows  to  a 
length  of  25  inches. 

The  pseudocoelom  of  this  group  is  not  well  developed,  and  biol- 
ogists are  not  agreed  that  the  Acanthocephala  belong  in  the  Aschel- 
minthes.  The  total  absence  of  a  digestive  tract  in  both  larva  and  adult 
and  the  many  other  specializations  for  parasitism  make  comparisons 
difficult.  Unlike  the  nematoda,  the  Acanthocephala  have  circular  mus- 
cles in  the  body  wall  and  ciliated  excretory  organs. 

97.  Phylum  Nemertea 

The  nemerteans  are  a  small  group  numbering  550  species,  most  of 
which  are  marine.  They  are  predacious  but  sluggish,  creeping  slowly  or 
burrowing  deep  into  mud  in  search  of  prey  by  the  contraction  of  muscles 
and  by  the  beating  of  the  cilia  on  the  surface. 

In  several  respects  nemerteans  resemble  the  turbellarians:  they  lack 
a  body  cavity,  they  tend  to  be  flattened,  the  epidermis  is  ciliated,  the 
excretory  system  includes  flame  cells,  and  the  nervous  system  and  sense 


THE   PHYLA    ASCHELMINTHES   AND   NEMERTEA 


233 


Figure  12.8.     A  ribbon  worm,  member  of  the  phylum  Nemertea.  Modified  from  Coe, 
1905. 

organs  such  as  eyes  and  chemoreceptors  are  similar  in  construction. 
Nemerteans  also  tliffer  irom  the  turbellarians  in  several  respects:  the 
mouth  and  anus  are  separate  openings,  a  proboscis  may  be  everted 
through  a  pore  just  above  the  mouth,  a  circulatory  system  is  present, 
and  the  reproductive  organs  are  simple.  Because  nemerteans  tend  to  be 
flattened  and  long,  they  are  called  ribbon  worms  (Fig.  12.8). 

Although  the  circulatory  system  and  separate  anus  are  important 
characteristics  for  locating  the  Nemertea  among  the  other  phyla,  the 
eversible  proboscis  is  their  most  characteristic  feature,  for  nothing  quite 
like  it  is  found  elsewhere  in  the  animal  kingdom.  It  consists  of  a  pro- 
boscis pore  (tig.  12.9),  vestibule,  proboscis,  proboscis  cavity  and  pro- 
boscis sheath.  When  the  muscular  sheath  constricts  it  exerts  pressure  on 
the  iluid  in  the  cavity,  forcing  the  hollow  proboscis  to  turn  inside  out 
through  the  \estibule  and  pore.  The  proboscis  never  everts  all  the  way 
because  its  inner  end  is  anchored  to  the  sheath  by  a  band  of  muscle.  This 
muscle  is  the  proboscis  retractor.  Its  contraction  helps  pull  the  proboscis 
back  inside  the  sheath.  The  proboscis  is  usually  longer  than  the  body, 
and  lies  somewhat  folded  within  the  proboscis  cavity.  When  everted,  the 
outer  surface  is  sticky,  and  it  coils  tightly  around  the  prey,  drawing  it 
to  the  mouth. 

In  its  simplest  form  the  circulatory  system  (Fig.  12.9)  consists  of  two 
lateral  vessels  connected  anteriorly  by  an  anterior  lacuna  above  the 
proboscis  vestibule,  and  posteriorly  by  a  posterior  lacuna  below  the  pos- 
terior end  of  the  gut.  Each  lacuna  is  an  enlarged  space.  Additions  that 
are  found  in  some  nemerteans  include  a  middorsal  vessel  and  numerous 
circular  connections.  The  longitudinal  vessels  are  contractile,  keeping 
the  colorless  blood  and  many  corpuscles  in  constant  motion.  In  some 
species  the  corpuscles  contain  respiratory  pigment. 

The  nemertean  circulatory  system  lacks  capillaries,  and  while  it 
does  not  form  an  intimate  association  with  many  of  the  body  tissues  it 
probably  aids  in  the  distribution  of  nutrients.  Even  in  species  with 
respiratory  pigment  it  is  doubtful  whether  the  system  has  much  to  do 
with  ordinary  respiration  when  oxygen  is  available  in  the  environment. 
It  is  more  likely  that  the  blood  serves  as  an  oxygen  reservoir  for  use 
when  the  worm  burrows  into  anoxic  mud. 


234 


THE   ANIMAL   KINGDOM 


"Proboscis  pore 
"Vestibule. 


■Mouth 


-Proboscis 
csLvity 

•Proboscis 
sheath 

"Proboscis 


"Proboscis 
retir'a.ctor 


-Anus 


AntGrior  lacuncL 
Brain 

■Mouth 

-Venfcro-lafce-ral  nerve 
"Late-ra-1  bloodvessel 
'Digestive,  tract 


Posterior  lacuna. 


Anixs 


A  B 

Figure  12.9.  Diagrammatic  views  of  nemertean  structures.  A,  Lateral  view  of  the 
dieestive  tract  and  the  proboscis.  B,  Dorsal  view  of  the  digestive,  circulatory  and  nervous 
systems. 

The  reproductive  organs  are  simple,  saclike  structures  scattered 
along  each  side  of  the  body.  The  sexes  are  usually  separate  and  fer- 
tilization is  external.  Eggs  and  sperm  may  be  shed  through  short  tubes 
that  develop  from  each  gonad  to  the  body  surface,  or  they  may  merely 
burst  through  the  body  wall.  Most  nemerteans  have  excellent  powers  of 
regeneration,  and  some  of  them  reproduce  asexually  by  fragmenting  into 
a  number  of  pieces,  each  of  which  becomes  a  whole  worm. 


Questions 

1.  Give  examples  for  the  six  classes  of  the  phylum  Aschelminthes. 

2.  Explain  the  illusion  of  rotation  in  rotifers. 

3.  What  do  PInlodina  and  Turbatrix  eat? 

4.  W'hat  is  cell  constancy  and  what  is  its  apparent  relation  to  regenerative  capacity? 

5.  How  do  roundworms  move? 


THE   PHYLA    ASCHELMtNTHES    AND    NEMERTEA  235 

6.  Compare  the  excretory  systems  of  rotifers  and  roundworms. 

7.  Describe  the  life  cycle  of  Ascaris. 

Supplementary  Reading 

All  of  the  pseudocoelomates  are  included  in  The  Itn/ertebrates,  volume  III,  by 
L.  Hyman,  with  many  descriptions  and  life  cycles.  Helminthology,  the  study  of  parasitic 
worms  (trematodes,  cestodes,  acanthocephalans  and  nematodes),  is  the  subject  of  several 
texts.  The  nemerteans  are  found  in  volume  II  of  the  same  series  by  Hyman. 


CHAPTER  13 


Introduction  to  the 
Higher  Invertebrates 


The  preceding  chapters  describe  those  animals  usually  referred  to  as  the 
"lower"  invertebrates.  These  are  lower  in  the  sense  that  they  lack  some 
of  the  structural  complexity  of  the  remaining  or  "higher"  invertebrates, 
and  lower  also  in  the  sense  that  they  are  often  thought  to  represent  the 
lower  limbs  of  the  evolutionary  "tree."  The  metazoa  are  believed  to  have 
evolved  from  the  Protozoa,  and  the  sponges,  jellyfish  and  flatworms  are 
considered  to  be  living  representatives  of  groups  that  appeared  early. 
The  roundworms  and  ribbon  worms  represent  groups  that  arose  some- 
what later,  possibly  from  the  flatworms,  and  the  higher  invertebrates  are 
usually  considered  to  have  evolved  still  later. 


98.        Evolutionary  Relationships  of  the  Sponges 

The  sponges  (phylum  Porifera)  are  almost  universally  regarded  as 
the  lowest  group  of  metazoa,  since  they  lack  a  nervous  system  as  well  as 
excretory  organs  and  a  circulatory  system,  and  in  their  organization 
show  a  degree  of  independence  among  the  cells  that  is  not  matched  in 
other  metazoa.  Since  gastrulation  in  these  animals  is  not  readily  com- 
parable with  that  of  other  metazoa,  and  since  their  gastrodermis  is  also 
not  comparable,  many  authors  conclude  that  the  sponges  may  have 
arisen  separately  from  the  Protozoa,  or  that  they  separated  at  a  very 
early  time  from  other  metazoa.  To  express  this  view  the  sponges  are 
often  taken  out  of  the  metazoa  and  placed  by  themselves  in  the  Parazoa. 
That  there  is  some  degree  of  relationship  between  sponges  and  other 
metazoa  is  suggested  by  the  fact  that  their  cleavages  and  processes  of 
gastrulation  are  to  some  extent  comparable,  and  by  the  fact  that  the 
sponges  are,  like  other  metazoa,  diploid  organisms  with  identical  meiotic 
processes  in  oogenesis  and  spermatogenesis. 

The  similarity  between  sponge  choanocytes  and  protozoan  choano- 
flagellates  suggests  that  the  sponges  may  have  evolved  from  a  choano- 
flagellate-like  ancestor.  This  is  supported  further  by  the  tendencies  in 
living  choanofiagellates  to  be  colonial,  to  secrete  "jelly,"  and  to  be  en- 
tirely holozoic. 
236 


INTRODUCTION   TO   THE   HIGHER   INVERTEBRATES  237 

99.        Evolutionary  Relationships  of  the  Coelenterates 

Soon  after  the  concept  of  evolution  was  accepted,  it  was  recognized 
that  the  gastrula  stage  of  higher  animals  may  indicate  or  hark  back  to 
an  ancestral  form,  whose  basic  body  plan  was  similar  to  that  found  in 
adult  coelenterates  today.  It  is  primarily  for  this  reason  that  the  coel- 
enterates were  placed  close  to  the  base  of  the  stock  that  gave  rise  to 
higher  animals.  Continuing  this  approach,  it  was  further  recognized  that 
the  blastula  may  represent  a  still  earlier  form,  an  organism  that  was 
essentially  a  hollow  ball  of  cells.  Volvox  (Fig.  8.6)  is  a  protozoan  colony 
with  such  a  form,  and  Volvox  is  often  used  as  an  example  to  represent 
the  form  of  the  metazoan  ancestor.  The  various  degrees  of  colony  forma- 
tion in  the  Phytomonadina  {Chlamydomonas,  Gonium,  etc.)  can  be  used 
to  show  how  the  blastula-like  stage  evolved  from  simple  protozoa,  and  it 
then  remains  only  to  make  a  gastrula  out  of  a  blastula. 

Although  the  various  phytomonads  form  an  excellent  series  to  show 
how  colonial  forms  such  as  Volvox  may  have  evolved,  it  can  hardly  be 
concluded  that  this  group  of  flagellates  is  particularly  close  to  the  stock 
that  actually  gave  rise  to  the  metazoa.  The  phytomonads  are  strictly 
autotrophic  and  they  are  also  haploid  (as  described  in  Chapter  8).  Fur- 
thermore, all  of  the  present  colonial  phytomonads  are  fresh-water  organ- 
isms, and  it  seems  likely  that  the  metazoa  have  had  a  primarily  marine 
origin  and  evolution.  Hence,  the  use  of  these  organisms  as  an  example 
of  a  possible  series  of  stages  in  the  origin  of  the  metazoa  should  not  be 
confused  with  the  proposal  that  they  represent  living  descendants  of  the 
actual  ancestors. 

The  coelenterates  and  ctenophores  are  usually  considered  to  be 
the  simplest  metazoa  other  than  sponges.  They  lack  mesodermal  tissues 
and  excretory  organs.  To  a  considerable  extent  they  can  be  regarded  as 
organisms  made  of  two  layers,  folded  and  warped  in  various  ways.  Most 
students  believe  that  these  groups  are  primitively  simple,  but  a  few  prefer 
the  possibility  that  they  once  had  a  bulky  mesoderm  that  has  been  lost. 

100.        The  Evolution  of  Three  Germ  Layers 

The  flatworms  appear  to  lie  close  to  the  stock  that  produced  all  of 
the  remaining  phyla.  They  are  relatively  simple  in  the  sense  that  the  gut 
lacks  a  separate  mouth  and  anus  and  neither  a  body  cavity  nor  a  cir- 
culatory system  is  present.  They  are  more  complex  than  the  preceding, 
however,  in  that  all  three  germ  layers  (ectoderm,  mesoderm,  endoderm) 
are  present  and  well  defined.  They  have  protonephridia,  and  they  have 
muscle  layers  added  to  the  body  wall.  These  latter  characters  link  the 
remaining  phyla,  suggesting  strongly  that  from  the  flatworms  up,  at 
least,  all  metazoa  have  a  common  origin. 

The  nemertean  body  plan  can  be  derived  from  the  flatworm  type 
by  the  addition  of  the  proboscis  and  a  circulatory  system,  a  separation 
of  the  mouth  and  anus,  and  minor  elaborations  of  other  structures.  The 
resemblance  of  the  nemertean  epidermis  and  sense  organs  to  those  of 
the  flatworms  is  very  striking. 


238  '■"f   ANIMAL   KINGDOM 

The  asdielminthes  are  difficult  to  relate  to  any  of  the  other  groups. 
The  degree  to  vvhicii  their  cells  are  specialized  gives  them  a  ditferent 
appearance.  Some  zoologists  believe  they  are  derived  from  a  fiatworm 
type  by  the  separation  of  mouth  and  anus  and  the  addition  of  a  pseudo- 
coeloni.  Although  the  aschelminthes  are  simpler  than  the  higher  in- 
vertebrates in  tne  sense  that  they  lack  both  a  circulatory  system  and 
muscles  around  the  gut,  they  are  complex  from  the  point  of  view  of  such 
features  as  cell  constancy  and  cellular  differentiation. 

101 .        The  Evolution  of  the  Coelom 

The  major  groups  of  higher  invertebrates,  including  the  molluscs, 
annelids,  arthropods,  echinoderms  and  chordates,  all  have  a  separate 
mouth  and  anus,  a  muscular  gut,  a  true  coelom  and  a  well  developed 
circulatory  system.  In  some  of  the  minor  groups  one  or  another  char- 
acter is  absent,  but  such  cases  are  believed  to  represent  losses  during 
their  evolution  from  ancestors  in  which  the  characters  were  present. 

The  distinctive  characteristic  of  these  animals  is  the  coelom  (or 
eucoelom),  a  cavity  within  the  mesoderm  lined  with  a  delicate  epi- 
thelium, the  peritoneum.  These  phyla  are  often  grouped  together  as  the 
Eucoelomata. 

The  coelom  may  appear  during  development  by  either  of  two 
methods,  depending  on  the  species.  The  mesoderm  may  form  first  as 
solid  masses  and  the  coelom  later  by  cavitation  within  the  mesoderm 
(Fig.  13.1).  Such  a  coelom  is  a  schizocoelom  (cavity  by  splitting).  In 
other  eucoelomates  the  mesoderm  and  coelom  are  formed  together  as 
pouches  from  the  original  gut  cavity  of  the  gastrula  (Fig.  13.1);  the  wall 
becomes  the  mesoderm  and  the  separated  cavity  persists  as  the  coelom. 
Such  a  coelom  is  an  enterocoelom  (cavity  from  the  gut  or  enteron).  In 
both  methods  the  coelom  usually  appears  first  as  one  or  more  pairs  of 
cavities  beside  the  digestive  tract.  The  result  is  similar,  regardless  of 
method  of  origin.  The  paired  cavities  are  usually  enlarged  until  they 
meet  above  and  below  the  gut,  where  the  two  lining  epithelia  come  to- 
gether and  often  persist  as  a  supporting  membrane,   a  mesentery. 

In  general  the  molluscs,  annelids  and  arthropods  are  schizocoelous, 
whereas  the  echinoderms,  hemichordates  and  lower  chordates  are  entero- 
coeious.  Many  students  believe  that  these  two  methods  of  coelom  forma- 
tion are  basically  different,  and  that  the  eucoelomates  should  be  divided 
into  two  groups,  the  Schizocoelomata  and  the  Enterocoelomata,  im- 
plying that  they  arose  independently  from  noncoelomate  ancestors.  At 
the  present  time,  however,  a  number  of  exceptions  are  known  (the 
arthropod  housefly  and  tardigrades  are  enterocoelous,  higher  chordates 
are  schizocoelous,  both  kinds  of  development  are  found  in  the  small 
phylum  Brachiopoda,  etc.)  which  suggest  that  the  difference  is  not 
really  basic,  and  that  one  kind  of  development  may  easily  have  evolved 
from  the  other.  This  is  consistent  with  the  observation  that,  aside  from 
the  method  of  origin,  the  schizocoelom  and  enterocoelom  cannot  be 
distinguished. 

The    basic    excretory    organ    of    the    lower    invertebrates    is    the 


INTRODUCTION   TO    THE   HIGHER   INVERTEBRATES  239 


Mescntcrj/ 
Coelom 


r^ores 


(Z.rve.  co: 


rd 


r^Fla^dlafced 

protonephridiam 


Funnel 


Glandular 
reOion. 


Mesoderm 
band 


Schizocoeloin 
Mesode-rm  pouch 


Entcrocoe-Lom 


C  D 

Figure  13.1.  Coelom  formation  in  the  Eucoelomata.  A,  A  diagrammatic  cross  section 
showing  the  fully  developed  coelom  and  mesenteries.  C,  The  schizocoelom.  On  the  left 
side  is  shown  a  solid  band  of  mesoderm.  A  later  stage  is  shown  on  the  right  (C),  in  which 
a  cavity  has  appeared.  D,  The  enterocoelom.  The  mesoderm  and  pouch  are  shown  form- 
ing on  the  left  side.  \  later  stage  is  shown  on  the  right  after  complete  separation  from 
the  gut.  B,  N'ephridia  found  in  the  eucoelomates.  Protonephridia  with  one  flagellum  are 
common  in  larvae,  while  adults  often  have  the  metanephridium,  which  opens  into  the 
coelom  (lower  part  of  B). 

protonephridium,  described  in  the  chapter  on  flatworms.  In  the  eu- 
coelomates a  different  kind  of  excretory  organ  is  common,  the  meta- 
nephridium. This  is  a  tubide  open  at  both  ends  (Fig.  13.1,  B),  the 
outer  end  opening  as  a  nephridiopore  and  the  inner  end  opening  into 
the  coelom.  The  inner  opening  is  a  ciliated  funnel  that  sweeps  coelomic 
fluid  into  the  tubule.  Within  the  tubule  useful  components  of  this  fluid 
are  reabsorbed  by  a  glandular  region  of  the  tube  wall  while  the  waste  is 
left  and  eventually  ejected.  In  some  forms  additional  waste  may  be  ex- 
creted by  the  glandular  region,  and  if  the  metanephridium  is  intimately 
associated  with  the  circulatory  system  the  funnel  may  be  absent. 

Although  metanephridia  are  the  common  adult  excretory  organs  of 
eucoelomates,  many  larval  eucoelomates  have  protonephridia,  usually 
with  the  tuft  of  cilia  replaced  by  a  single  long  flagellum  (Fig.  13.1). 
This  supports  the  idea  that  the  higher  invertebrates  arose  from  the 
lower. 


102.        Spiral  Cleavage  and  Its  Evolutionary  Importance 

The  eucoelomates  are  also  related  to  the  lower  invertebrates  through 
an  embryonic  process  called  spiral  cleavage.  In  the  Platyhelminthes,  Ne- 
mertea,  Mollusca,  Annelida,  and  several  of  the  minor  phyla,  a  pattern 


240 


THE   ANIMAL   KINGDOM 


One  cell 


Two  cells 


Pour  ceils 


Ei^ht  ceils 


Sixteen  cells 


Thirty-two  cells 


General  ectoderm 


■Stomodeum 
(and  proctodeum, 

if  present) 

Endocierm,a.nd,in  one 
rfaadrant,mesoderm. 

Figure  13.2.  Spiral  cleavage.  One  quadrant  (the  progeny  of  one  cell  of  the  four-cell 
stage)  is  shaded.  Lines  indicate  the  axes  of  the  preceding  mitoses.  The  lower  diagram 
shows  the  fates  of  the  cells  of  one  quadrant.  Numbers  indicate  the  three  quartettes  or  their 
progeny. 

of  egg  cleavage  occurs  that  is  essentially  identical  throughout.  In  any 
given  species  it  is  a  fixed  pattern,  so  that  all  of  the  steps  can  be  identi- 
fied. The  following  description  refers  to  a  common,  basic  pattern  that 
is  found  in  several  of  the  phyla. 

As  in  most  eggs,  the  first  and  second  cleavages,  forming  the  two-cell 
and  then  the  four-cell  stages,  are  meridional  and  at  right  angles  to  each 
other,  dividing  the  egg  from  animal  to  vegetal  pole  into  four  quarters 
(Fig.  13.2).  . 

The  third  cleavage  is  not  transverse  as  in  many  other  eggs,  but  is 


INTRODUCTION   TO    THE   HIGHER   INVERTEBRATES  241 

oblique.  As  a  result,  the  upper  tier  of  four  cells  do  not  lie  on  top  of 
the  lower  tier,  but  are  displaced  circularly  so  that  each  upper  cell 
straddles  two  lower  cells.  The  third  cleavage  is  unequal  and  separates 
four  upper,  small  micromeres  from  four  lower,  large  macromeres.  The 
four  micromeres  are  called  the  first  quartette. 

The  fourth  cleavage  is  also  oblique,  but  always  in  the  opposite 
direction  from  the  third  (Fig.  13.2).  The  first  quartette  divides  to  form 
eight  cells.  The  macromeres  divide  unequally,  producing  an  upper  tier 
of  four  micromeres,  the  second  quartette,  and  a  lower  tier  of  four 
macromeres. 

The  fifth  cleavage  continues  the  pattern.  It  is  oblique  in  the  direc- 
tion of  the  third  cleavage  (Fig.  13.2).  The  cells  of  the  first  quartette  now 
number  sixteen.  The  second  quartette  divides  to  form  eight  cells.  The 
macromeres  again  divide  unequally  to  produce  an  upper  set  of  four 
micromeres,  the  third  quartette,  and  a  lower  tier  of  four  macromeres. 
As  shown  in  the  figure,  the  third  quartette  does  not  completely  dis- 
place the  second  quartette.  Thus,  the  macromeres  are  ringed  by  eight 
cells,  the  third  quartette  and  the  lower  four  cells  of  the  second  quartette. 

Divisions  continue  to  be  oblique,  alternating  in  clockwise  and 
counterclockwise  directions  (often  the  first  oblique  division  is  counter- 
clockwise, in  which  case  the  subsequent  divisions  are  also  reversed). 
Ultimately,  the  cells  formed  from  each  cell  of  the  four-cell  stage  lie  ap- 
proximately in  the  corresponding  quadrant  of  the  blastula.  The  animal 
pole  is  occupied  exclusively  by  the  first  quartette,  the  vegetal  pole  by 
the  macromeres,  with  the  second  and  third  quartettes  somewhat  inter- 
digitated  around  the  equator. 

Usually  gastrulation  begins  after  the  sixth  or  seventh  cleavage. 
Without  exception,  the  macromeres  of  the  32-cell  stage  or  all  their 
progeny  pass  into  the  interior.  Usually  all  of  the  mesoderm  develops 
from  one  of  the  macromeres,  which  is  often  slightly  larger  than  the 
others,  while  the  other  three  macromeres  become  endoderm.  The  three 
quartettes  form  all  of  the  ectoderm. 

Variations  from  the  pattern  given  occur  in  different  species  in  all 
of  the  phyla,  but  they  are  usually  minor  in  degree,  and  the  differences 
between  phyla  are  of  no  greater  magnitude  than  those  within  phyla. 
The  presence  of  this  pattern  of  cleavage  in  a  series  of  phyla  accounts  for 
the  concept  of  a  "main  line"  of  evolution,  proceeding  from  the  flatworms 
to  the  nemerteans,  molluscs  and  annelids.  In  relation  to  this  concept 
some  phyla,  such  as  the  Aschelminthes,  show  further  specialization  and 
modification,  while  others,  such  as  the  chordates,  show  a  loss  or  regression 
to  a  simpler  cleavage  pattern. 

103.        The  Schizocoelomata  and  Enterocoelomata 

Looking  forward  to  the  remaining  chapters,  the  student  will  find 
that  the  eucoelomates  are  presented  in  two  series,  one  including  the 
molluscs,  annelids  and  arthropods,  and  the  other  including  the  echino- 
derms,  hemichordates  and  chordates.  If  the  eucoelomates  are  divided 
into   taxonomic   groups,   these   two  series  are   the  Schizocoelomata   and 


242  ^"^   ANIMAL   KINGDOM 

Arthropoda 
Onychophora 

Annelida 
Mollusca 


Other 


Eucoelomata 
Nemertea  —  /  ^->Aschelminthes 


Platyhelminthes 


Porifera 


►  Chordata 


Hemichordata 


Echinodermata 


Entoprocta 

Coelenterata 
Ctenophora 


Protozoa 
Figure  13.3.     A  suggested  phylogeny  of  the  Metazoa,  showing  all  of  the  major  phyla 
and  some  of  the  minor  phyla.  The  Entoprocta  and  other  minor  phyla  will  be  discussed  in 
Chapter  18. 

Enterocoelomata  respectively.  It  will  be  clear  from  information  that  will 
be  presented  in  the  following  chapters  that  these  are  distinct  groups. 
Between  the  chapters  that  deal  with  these  two  series  lies  a  chapter  (18) 
on  minor  phyla,  some  of  which  can  easily  be  allied  with  one  of  the 
series  and  some  of  which  cannot.  At  this  introductory  level  it  will  be 
sufficient  to  recognize  that  there  are  two  major  groups  of  eucoelomates 
and  that  a  number  of  smaller  phyla  are  left  over. 

This  discussion  of  possible  evolutionary  relationships  among  phyla 
can  be  summarized  in  a  phylogenetic  tree  (Fig.  13.3).  This  is,  of  course, 
nothing  more  than  a  guess  based  on  the  evidence  now  available.  A 
variety  of  other  schemes  are  just  as  plausible.  A  common  variant  is  one 
in  which  the  two  series  of  eucoelomates  are  derived  separately  from  a 
flatworm  stock. 


Questions 

1.  In  what  ways  do  sponges  differ  from  all  other  metazoa? 

2.  List  the  characteristics  of  flatworms  that  are  not  found  in  coelenterates. 

3.  Compare  the  pseudocoelom  and  the  eucoelom. 


inthoduction  to  the  higher  invertebrates        243 

4.  Distinguish  an  enterocoelom  from  a  schizocoelom. 

5.  What  is  a  mesentery? 

Supplementary  Reading 

Volumes  I  and  II  of  The  Invertebrates  by  L.  Hyman  contain  an  excellent  discussion 
of  the  relations  among  the  major  taxonomic  groups  and  other  discussion  on  the  origins  of 
the  metazoa.  Buchsbaum  has  a  superb  collection  of  photographs  together  with  a  lucid 
account  of  the  invertebrates  in  Animals  without  Backbones.  A  similar  account  of  marine 
organisms,  with  many  photographs  in  color,  is  that  of  Vonge,  The  Sea  Shore.  Other 
zoology  texts  should  be  consulted  for  other  opinions  on  phylogeny. 


CHAPTER  14 


The  Phylum  Mollusca 


104.        General  Features  of  the  Molluscs 

The  Mollusca,  which  inchides  snails,  clams,  squids,  and  others,  are  a 
group  of  sott-bodied  animals  that  usually  secrete  external  protective 
shells.  The  ventral  portion  of  the  body  is  elaborated  as  a  muscular 
organ,  called  the  foot,  used  in  locomotion.  Many  of  the  molluscan  groups 
have  in  the  mouth  a  unique  rasping  organ,  the  radula.  These  structures 
will  be  described  later. 

Molluscs  have  both  a  eucoelom  and  a  circulatory  system.  The 
coelom  is  small,  and  is  associated  with  the  heart,  gonads  and  excretory 
organs.  The  portion  surrounding  the  heart,  the  pericardial  cavity,  is 
the  most  obvious. 

The  circulatory  system  is  well  developed.  It  is  modified  variously 
in  the  different  groups,  but  typically  includes  a  single  dorsal  heart  (Fig. 
14.1)  composed  of  one  anterior  ventricle  and  a  pair  of  posterior  auricles. 
Tire  auricles  receive  blood  from  veins  and  pump  it  into  the  ventricle 
while  the  latter  is  relaxed.  Then  the  ventricle,  a  heavily  muscled  organ, 
pumps  the  blood  out  through  arteries  to  all  parts  of  the  body.  The 
blood  may  pass  through  capillaries  to  the  veins,  but  usually  passes  into 


Ventricle  Auriclcz. 


Figure  14.1.     A  diagram  showing  the  principal  features  of  the  molluscan  circulatory 
system.  Auricles,  gills  and  nephridia  are  usually  paired. 

244 


THE   PHYLUM    MOLLUSCA 


245 


venous  sinuses,  spaces  among  the  various  organs  that  are  difficult  to 
observe.  Most  of  tlie  returning  blood  passes  through  the  excretory  organs 
and  then  either  directly  to  the  auricles  or  through  the  gills  to  the 
auricles.  The  amount  passing  through  the  gills  determines  the  amount 
of  freshly  oxygenated  blood  returning  to  the  heart,  and  varies  from 
mollusc  to  mollusc  according  to  its  level  of  activity. 

The  excretory  organs  are  a  single  pair  of  nephridia,  intimately  as- 
sociated with  the  circulatory  system.  In  each  a  large  glandular  region  is 
bathed  in  blood,  and  from  this  a  tubule  opens  externally  at  a  nephridio- 
pore.  In  many  molluscs  the  inner  ends  of  these  tubules  open  into  the 
pericardial  cavity. 

The  phylum  includes  three  large  classes  having  species  of  economic 
importance  and  two  additional  small  classes  (Fig.  14.2).  The  classes  are: 


~^s 


Amphmcura 


Felecy  po<i^% 

Figure  14.2.  Classes  of  the  phylum  Mollusca.  Letters  indicate  structures  that  are 
part  of  the  shell  (S),  mantle  (M),  and  foot  (F).  (Gastropoda  after  KHne  in  Curtis  and 
Guthrie,  1938;  Scaphopoda,  original;  others  after  Lankester,  1906.) 


246  ^"f   ANIMAL   KINGDOM 

I.  Amphineura.  A  small  group  with  a  shell,  if  any,  located  dor- 
sally  and  made  of  many  spicules  or  of  a  longitudinal  series  of 
plates.   Includes   the  chitons. 

II.  Gastropoda.  A  large  group  with  a  single  dorsal  shell,  if  pres- 
ent, that  is  usually  spiral  in  shape.  Includes  snails,  slugs,  whelks 
and  abalones. 

III.  Pelecypoda.  A  large  group  with  a  pair  of  lateral  shells,  hinged 
dorsally.  Includes  the  bivalves,  such  as  clams,  oysters  and  scal- 
lops. 

IV.  Scaphopoda.  A  small  group  with  a  conical  shell  open  at  both 
ends.  These  are  called  the  tooth  shells. 

V.  Cephalopoda.  A  large  group  in  which  part  of  the  foot  forms 
arms  or  tentacles  surrounding  the  mouth.  Includes  the  squids 
and  octopuses. 

105.        Class  Amphineura 

The  chitons,  which  are  common  on  the  west  coast  of  the  United 
States,  are  the  most  primitive  class  in  the  phylum,  and  illustrate  the  gen- 
eralized moUuscan  plan.  They  are  found  only  in  the  oceans,  where  they 
creep  slowly  over  the  rocks.  Many  live  between  high  and  low  tide  lines. 
Some  species  remain  in  one  place  all  the  time,  where  they  gradually 
wear  a  depression  in  the  rock.  These  feed  upon  the  debris  that  settles 
into  their  hole.  Often,  after  the  hole  has  become  deep,  encrusting 
growths  may  obstruct  the  opening  to  such  an  extent  that  the  chiton 
can  no  longer  get  out. 

Chitons  creep  upon  a  broad  foot,  moving  by  a  succession  of  small 
contraction  waves  that  pass  forward  from  the  posterior  end.  The  broad 
surface  with  its  slimy  secretions  enables  chitons  to  cling  tenaciously. 
The  mantle  extends  out  over  the  foot  on  all  sides,  enclosing  a  circular 
mantle  cavity  below.  Dorsally  the  mantle  secretes  a  shell  made  of  eight 
segments.  Because  of  the  segmental  structure  of  the  shell  a  chiton  is 
able  to  roll  into  a  ball,  shielding  the  vulnerable  ventral  surface,  if  it  is 
torn  loose  from  the  bottom  (Fig.   14.2). 

In  the  anterior  part  of  the  mantle  cavity  is  the  head,  no  more  than 
a  tubular  extension  of  the  body  bearing  a  mouth  at  its  end.  Well  de- 
veloped sense  organs  such  as  eyes  or  tentacles  are  lacking. 

In  the  floor  of  the  mouth  cavity  lies  the  radula,  with  which  the 
chiton  scrapes  up  its  food.  The  radula  is  a  thin  flexible  strip  of  toothed 
skin  that  can  be  pulled  around  the  end  of  a  stiff  tongue.  In  a  typical 
scraping  movement  (Fig.  14.3),  the  tongue  is  pushed  out  of  the  mouth 
with  the  radula  on  its  anterior  and  lower  surface.  The  radula  is  then 
pulled  around  the  end  of  the  tongue  onto  the  upper  surface,  scraping 
whatever  the  mouth  is  pressed  against.  Finally  the  tongue  is  withdrawn 
and  the  debris  on  the  radula  is  swallowed. 

The  mouth  leads  to  a  long  coiled  intestine  that  ends  posteriorly  at 
a  short  rectum  and  anus  opening  into  the  posterior  part  of  the  mantle 
cavity.  Anteriorly  the  intestine  receives  ducts  from  a  pair  of  digestive 
glands,  presumed  to  secrete  digestive  enzymes. 


THE   PHYLUM   MOLLUSCA 


247 


Mantle,  edi^e 
Moulh 


B. 


Mantle  cavity 
Radula  sheath 
I      >  I'Salivary^land 

DiO<^Stive 

eland 
O 

g^  /  /   Intestine 

'^'"/  /  Reprodaclive  pov<z 
^^-^^^ Excretory  pore 

Alius 

Figure  1 4.3.  A,  Ventral  view  of  a  chiton,  with  the  digestive  tract  indicated.  B,  Longi- 
tudinal section  through  the  mouth  showing  the  radula  extending  forward  over  the  end 
of  the  stitr,  cartilaginous  tongue.  C,  The  tongue  is  pushed  out  and  the  radula  is  pulled 
as  far  as  possible  onto  its  lower  surface.  D,  The  radula  is  pulled  posteriorly  while  the 
tongue  is  pressed  against  the  food.  After  this  maneuver  both  tongue  and  radula  are  with- 
drawn into  the  mouth. 

The  sides  of  the  mantle  cavity  have  several  pairs  of  small  gills  that 
hang  freely  in  the  water  of  the  cavity.  Beneath  the  edge  of  the  projecting 
mantle  this  water  is  continuous  with  the  environment. 

The  amphineuran  nervous  system  is  poorly  developed.  In  most 
molluscs  the  central  nervous  system  consists  of  a  brain  and  several  pairs 
of  ganglia  connected  by  nerve  cords.  In  the  chiton  the  nerve  cells  are 
spread  out  along  cords  forming  a  diffuse  system.  Such  poor  centraliza- 
tion of  the  nerve  cells  only  reflects  the  sluggish  habit  of  these  animals, 
and  does  not  necessarily  indicate  the  ancestral  pattern  of  the  central 
nervous  system  in  the  phylum. 

106.       Class  Gastropoda:  General  Features 

Snails  are  the  only  class  of  molluscs  found  on  land.  They  also  occur 
in  fresh  water  and  in  the  oceans.  Both  herbivorous  and  carnivorous 
species  are  found,  with  appropriate  modifications  of  the  radular  teeth. 
Most  snails  creep  like  chitons  upon  a  broad  muscular  foot,  but  a  few  use 
the  foot  as  a  lever  for  jumping  while  others  use  it  as  a  fin  for  swimming. 

The  basic  feature  that  distinguishes  gastropods  from  other  mol- 
luscs is  the  result  of  an  embryologic  event,  known  as  torsion  (Fig.  14.4). 
The  gastropod  embryo  develops  to  a  stage  known  as  the  veliger.  This 
early  embryo  is  symmetrical,  with  an  anterior  mouth  and  a  posterior 
anus,  but  at  a  particular  point  in  its  development  parts  of  the  body 
twist  or  rotate  as  much  as  180  degrees,  bringing  the  anus  around  (usu- 
ally to  the  right)  to  lie  over  the  mouth.  This  twist  is  abrupt  and  per- 


248 


THE   ANIMAL   KINGDOM 


Manllei 


Shell 


Anus 


rCiliated 
crown 


Mouth. 


-Anu$ 


Foot  Atlu.s-'       ^Mouth  Mouth-" 

A  B  C 

Figure  14.4.  Torsion  in  the  gastropod  Acmaea  (a  limpet).  A,  Young  larva,  showing 
beginning  of  shell  and  foot.  B,  Just  before  torsion,  with  a  U-shaped  digestive  tract.  C, 
Just  after  torsion  (arrow  indicates  movement  that  has  occurred).  All  of  these  stages  swim 
with  the  ciliated  crown  uppermost  in  the  water.  They  are  shown  here  in  positions  com- 
parable with  that  of  the  adult  snail.   (After  Boutan,  1899.) 

manent.  Following  this,  development  is  asymmetrical,  with  the  struc- 
tures of  one  side  often  suppressed.  The  body  elongates  dorsally,  grow- 
ing up  in  a  spiral  pattern.  The  enclosing  spiral  shell  forms  a  structure 
characteristic  for  the  class. 


107.        Bus/con 

The  familiar  large  whelks  of  the  eastern  seaboard  belong  to  the 
genus,  Busycon,  of  which  B.  canaliculatum  (Fig.  14.5),  about  eight  inches 
long,  is  the  most  common.  Busycon  lives  on  sand  and  mud,  where  it  can 
plow  about  with  its  large  powerful  foot  searching  for  small  clams  and 
other  prey.  The  mouth  is  borne  on  a  long,  retractile  proboscis  which  is 
usually  withdrawn  into  the  head,  but  may  be  shot  out  quickly  to  capture 
food.  The  teeth  of  the  radula  are  long,  sharp  and  recurved  so  that 
Busycon  can  not  only  pierce  the  flesh  of  its  prey,  but  draw  it  into  the 
mouth. 

Food  is  swallowed  through  a  long  esophagus  (Fig.  14.5)  to  a  curved 
stomach  lying  in  the  lower  whorl  of  the  body.  From  the  stomach  an 
intestine  bends  dorsally  and  down  the  anterior  surface  of  the  whorl  to  a 
short  wide  rectum  that  opens  at  an  anus  in  the  mantle  cavity  over  the 
head.  A  pair  of  salivary  glands  beside  the  esophagus  secrete  juices 
(probably  containing  enzymes  for  digesting  carbohydrates)  into  the  an- 
terior end  of  the  esophagus.  The  stomach  lies  between  a  pair  of  large 
digestive  glands  that  occupy  most  of  the  space  in  the  upper  body 
whorls.  Ducts  from  these  glands  open  into  the  stomach.  They  are  not 
known  to  secrete  digestive  juices,  but  do  take  up  food  particles  from 
the  fluid  that  flows  up  the  ducts  into  the  glands,  and  digest  them  in  food 
vacuoles. 

The  mantle  cavity  formed  by  the  fleshy  mantle  that  lines  the  inner 
surface  of  the  shell  surrounds  the  anterior  part  of  the  body.  On  the 
left  side  both  shell  and  mantle  are  drawn  out  into  a  long  siphon,  a 
tubular  fold  through  which  water  is  drawn  into  the  mantle  cavity.  A 
large  chemoreceptor  at  the  base  of  the  siphon  samples  the  incoming 
water  before  it  passes  over  the  single  gill,  a  flat,  oblong,  feathery  struc- 


THE   PHYLUM    MOLLUSCA 


249 


ture  richly  supplied  with  blood  vessels.  Along  the  upper  edge  of  the 
gill  numerous  glands  secrete  mucus  that  passes  over  and  cleanses  the 
gill.  Water  leaves  the  mantle  cavity  through  the  slit  between  the  ante- 
rior edge  of  the  mantle  and  the  head. 

Blood  passing  through  the  capillaries  of  the  gill  is  collected  in  a 
large  vein  that  empties  into  the  single  auricle.  The  heart,  which  lies  close 
to  the  intestine,  is  reversed  during  torsion  so  that  the  ventricle  lies 
posterior  to  the  auricle.  In  Busycon  canaliculatum,  which  twists  to  the 
right  during  development,  only  the  left  gill  and  the  left  auricle  develop. 

The  single  left  nephridium  lies  over  the  heart,  opening  dorsally  into 
the  mantle  cavity.  Most  of  the  blood  passing  through  the  nephridium 
goes  through  the  gill,  but  a  small  portion  goes  directly  to  the  auricle, 
so  that  the  final  mixture  is  not  completely  oxygenated. 

Most  of  the  nervous  system  is  centralized  anteriorly.  Except  for 
one  pair  on  the  intestine,  the  ganglia  are  located  close  to  the  brain, 
forming  an  irregular  ring  around  the  esophagus  (Fig.  14.5).  The  several 
elements  can  be  distinguished  by  careful  dissection.  The  visceral  gan- 
glia on  the  intestine  were  reversed  during  torsion,  so  that  their  con- 
nectives with  the  rest  of  the  system  are  crossed,  a  persistent  feature 
characteristic  of  many  snails. 

The  head  bears  a  pair  of  sensitive  tentacles,  and  halfway  out  on 


Mantle. ; 


Mucous  ^land— ^ 


ChcTDOTXce-ptor,^ 

Siphon- 

Tentacle''  , 
Proboscis  — \- 


•Nephridium  Anus 

N^  pDigestive  \, 


PVoboscis- 


rRectum.    /"Intestine 


Esophagus 


Brain. 


^Auricle 

Y'-'-.ricardial 
cavity 
I  Ventricle 

Digestive 
gland 

"Stomach 


Mouth" 


Nephridium" 
•    / 
Dibestive"; 
gland     I 

Ovary  — ^ 

Stomach 


Operculum  Lj^adula. 

-Shell  ^land  ^x~ 

-VVagina  /      /' 

(      \  /l)igestivA 


Nephridium 

;Posterior  linrjib  of 
,     >v  mantle  cavity 

VVSperm 
y     \    groove 


Retractor 
muscle 


Pe-nis" 


"Stomach 


Testis 


Figure  1 4.5.  Anatomy  of  Busycon  canaliculatum  (shell  removed).  A,  Left  side,  show- 
ing external  organs  and  internal  organs  visible  through  the  integument.  B,  Same  view 
with  digestive,  respiratory,  circulatorv  and  nervous  systems  indicated.  C,  Female,  show- 
ing portion  of  the  right  side.  D,  Male,  portion  of  the  right  side  with  mantle  and  retractor 
muscle  cut  short.  In  C  and  D  the  proboscis  is  withdrawn.  Adult  with  shell  is  shown  in 
Figure  14.2. 


250 


THE  ANIMAL  KINGDOM 


Lens 


■Retina 


Oplic  n^-rve. 
Figure  14.6.     Section  through  the  eye  of  a  whelk.   (After  Helger.) 

each  tentacle  is  a  small  eye.  The  eye  is  well  developed  and  probably 
forms  images.  It  is  spherical  (Fig.  14.6),  lined  with  a  retina,  a  layer  of 
light-sensitive  cells,  and  contains  a  large  globular  lens.  The  outer  sur- 
face, except  where  the  light  comes  through,  is  pigmented  to  screen  out 
extraneous  light. 

In  Busycon  sexes  are  separate.  A  single  gonad  lies  in  an  upper 
whorl  of  the  body,  between  the  digestive  glands.  From  the  ovary  an 
oviduct  passes  down  through  the  mantle,  opening  near  its  anterior  edge 
to  the  right  of  the  anus.  Near  its  end  the  oviduct  is  surrounded  by  a 
yellow  shell  gland.  A  sperm  duct  from  the  testis  opens  on  the  right 
side  into  the  posterior  limit  of  the  mantle  cavity.  From  there  a  ciliated 
sperm  groove  leads  across  the  body  (Fig.  14.5)  to  the  base  of  a  large 
penis  just  behind  the  right  tentacle.  The  groove  continues  along  the 
penis  to  its  tip.  Fertilization  is  internal. 

The  eggs  are  laid  in  cases  secreted  by  the  shell  gland,  a  dozen  eggs 
to  the  case.  The  cases  are  arranged  in  a  row  along  a  connecting  strand 
(Fig.  14.7)  that  is  attached  to  the  bottom.  The  young  pass  through  all  of 
their  larval  stages  within  the  cases,  emerging  as  small  whelks.  The  cases 
are  tough,   lasting  long  after  the  young  have  emerged,   and  are  often 


Mi 


/  *^I*^9^K 


Figure  14.7.     The  egg  case  of  Busycon.  (Photo  by  Hugh  Spencer.) 


THE   PHYLUM    MOLLUSCA 


251 


washed  ashore  where   they   are   found   and  are  known  as   "mermaids' 
necklaces." 


108.        Other  Gastropods 

Prosobranchia.  Snails  are  divided  into  three  large  orders.  Busycon 
belongs  to  the  order  Prosobranchia,  in  which  torsion  brings  the  origi- 
nally posterior  gills,  anus,  etc.,  around  to  the  anterior  side.  Although 
Busycon  has  only  the  left  nephridium,  gill  and  auricle,  other  members 
of  the  group,  such  as  the  prized  abalone  of  the  west  coast,  have  these 
organs  in  pairs.  It  is  reasonable  to  suppose  that  the  abalone  represents 
the  primitive  condition,  and  that  the  loss  of  organs  in  such  snails  as  the 
whelk  is  an  adaptation  to  the  twisted  shape  of  the  body.  Most  of  the 
Prosobranchia  are  marine,  although  a  number  of  small  forms  are  found 
in  fresh  water.  On  isolated  tropical  south  Pacific  islands  some  have  be- 
come  terrestrial. 

Op/sfhobranc/i/o.  In  a  second  large  order,  torsion  as  an  embryo- 
logic  event  is  less  extreme  and  may  not  occur  at  all.  The  gills  (if  present) 
remain  posterior,  or  at  most  are  moved  to  the  right  side,  giving  the 
order  its  name,  the  Opisthobranchia.  Since  all  members  of  this  group 
have  a  single  nephridium,  gill  and  auricle,  their  incomplete  torsion  is 
believed  to  be  secondary.  These  are  almost  entirely  marine,  and  include 
some  strange  forms. 

One  group  has  left  the  bottom  and  swims  as  plankton  in  the 
upper  water  of  the  open  oceans.  Each  side  of  the  foot  is  expanded  as 
a  muscular  flap  (Fig.  14.8  A)  suggesting  wings.  Hence  their  name,  the 
pteropods.  Pteropods  hang  shell  down  in  the  water,  swimming  upward 
by  flapping  the  "wings,"  and  falling  more  gently  while  gathering  food 
from  the  water.  They  sometimes  form  immense  swarms  and  serve  as 
food  for  whales. 

A  second  group  of  opisthobranchs  have  lost  the  shell.  With  it  the 
mantle  cavity  and  original  gill  have  also  disappeared,  to  be  replaced  by 
new  gills  on  the  back.  These  are  the  nudibranchs  or  sea  slugs  (Fig. 
14.8  B).  Many  of  them,  particularly  those  on  the  west  coast,  are 
brightly  colored,  crawling  with  great  agility  over  the  hydroids  and 
algae  upon  which  they  feed.  Nudibranchs  that  feed  on  hydroids  do  so 


ABC  D 

Figure  14.8.     Opisthobranchia:  A,  a  pteropod;  B,  a  nudibranch.  Pulmonata:   C,  a 
slug;  D,  a  garden  snail.  {A  and  C  after  Parker  and  Haswell.  B  and  D  after  Lankester.) 


252  '^"^    ANIMAL    KINGDOM 

without  iliMli.iigiiii;  ilu>  tuMn.iUx  \s(n.  In  the  Ntoniadi.  ilir  iuMna(«H\sts  arc 
^lly^c^lcHl   liiT.  aiul  alinwaul   air  pitkotl   up   b\    anichnui   <<1K  ami   (ai 
iinl  to  the  onulnuus.  whoic  tlu\  piotot  the  sea  slug  m  imu  h  the  same 
wax    that    tlu-\    were  Nupposcii   to   protect   ihc-   ludroid. 

Pulmonata.  I  he  thuil  laige  order  ol  gastropods  is  the  Pulmonata, 
or  air  hieathers.  This  iiuludes  most  ol  the  teiiestrial  ami  liesh  waiei 
speties.  1  he\  show  lull  toisiou  and  ha\e  a  single  nei)hridium  and 
autiile.  No  gill  is  pieseni.  Aii  is  tudilaied  through  the  mantle  eavity, 
whith  is  lineil  with  a  ii(hly  vascular  epidermis  that  serves  as  the  respira 
ior\  surlaie.  Iheve  aie  the  lamiliar  gardei\  snails  (Kig.  11.8  /)).  In 
iludeil  also  are  the  slugs  ^ig.  I  l.S  C),  in  which  the  shell  is  reduced 
to  Iragmeius  hmieil  m  the  n\antle  oi  is  (ompleteK  absent.  The  mantle  is 
still  piesent.  |)H)\itling  a  taxiiy  loi    Kspivaticm. 

Most  ol  the  prosobranchs  ha\e  opercula,  horny  lids  botne  on  the 
upper  surlace  ol  the  posterioi  jiaii  o[  the  loot,  by  whidi  the  opening 
to  the  shell  is  tight Iv  closcil  when  the  annual  is  withdr.iwn  inside  the 
shell.  In  the  pulmonates.  whith  are  the  most  susieptilile  to  ilesiccaiion. 
the  operculum  is  latkmg.  When  the  envitotuneni  becomes  dry.  i)id- 
tuonates  bui\  themsehes  iti  the  soil  ai\cl  seciete  a  thick  mucus  in  the 
shell  opening  that  hardens  to  lorm  .m  etlcctixe  seal.  When  rain  leiurns 
moisture  to  the  soil  the  sc.il  soltens  and  the  snails  become  active  again. 

Most  piosobtaiuhs  have  sei)arate  sexes,  but  most  ot  the  opistho- 
btamhs  and  pidmcMiates  are  henn.iphroditic.  Cross  lertili/alion  is  the 
rule,  l-.ithei  imlixiduals  are  temjioiaidx  active  as  males  or  as  females, 
or  simultaneous  cross  lertili/ation.  as  iji  tlie  llatworms,  will  occur.  vSclt- 
lertili/alion  is  known  to  occur  in  .i  lew  species. 

109.        Class  Pelecypoda:  General  Features 

The  pelecypods  include  all  ol  the  bixalves.  In  these  forms  the  foot  is 
compres-sed  to  form  a  muscular  spade  lor  digging  and  the  head  is 
greatly  reduced,  lying  xvell  within  the  mantle  cavity.  In  most  species  the 
>hell.  composed  of  two  valves  hinged  together  cUnsallv,  can  completelv  en- 
dose  the  boch.  .Strong  muscles,  the  adductors  (lig.  1  1. 10"),  can  hold  the 
shells  liglulv  shut  against  enemies. 

The  mantle  cavity  and  gills  are  elaborated  to  serve  both  respira- 
tion and  feeding.  Typically  the  edges  of  the  mantle  around  the  free 
margins  of  the  valves  are  kept  together,  forming  a  closed  cavity,  except 
postericMh  where  ihev  separate  to  Unin  twc>  opei^ingN.  a  \entral  Incur- 
rent  siphon  and  a  dorsal  excurrent  siphon.  Ihese  openings  mav  be  c\ 
tended  .is  .i  Umg  double  tube  which  cm  be  projected  up  into  the  water 
while  the  dam  lies  buried  in  the  sand.  1  he  mantle  ca\it\  extends  all 
aroutul  the  bc>d\.  Ihe  two  gills  im  each  side  are  large  and  are  attacheci 
to  the  body  along  the  whole  length  oi  each  side  and  around  the  poste- 
rior end.  The  inner  gill  of  each  side  extends  mediallv  against  the  lcxit. 
and  each  outer  gill  extends  laterally  against  the  mantle,  thus  dixiding 
the  mantle  cavity  into  upper  and  lower  chambers  that  connect  with 
the  excmrent  and  incinrent  siphons  respectively.  The  lining  of  the 
cavity  and  the  surfaces  of  the  gills  are  ciliated.   The  beating  of  the  cilia 


TH£    PHYLUM    MOLLUSCA 


253 


Upper  branchial 
chambers 


^      t 


\ 


^      j^Lowcr 
branchial 
chambers 
B  C 


Figure  14.9.  Diagrammatic  cross  sections  showing  difTcrcnl  gill  types  in  the  I'clcry- 
po(ia.  ./.  Oiilci  I'liitdbiancliiala.  (>ills  slioil  and  siinpli'.  It.  Oiiici  lilihranc  liiata.  (>ills 
long  and  loldcd  l>a(k.  (..  Ordci  l  idatncllilnancliiata.  I  ike  /•'.  bill  witli  tlic  folds  fused 
with  Mian\  biidn's.  1).  Order  Septibranchiata.  dills  mo<lilied  in  Innn  lioi  i/iniial  parti- 
tions. (.After  I  ang.) 

creates  water  (itrreiits  inward  tluongli  ilic  iikiiikiu  siphon,  iijnvaid 
throiigli  many  small  sliis  in  the  gills,  and  oiiiuaid  ihioiigli  the  excur- 
rent  siphon.  Ilic  walci  brings  oxygen  lor  resjjiraiion  at  the  gill  sm- 
faces,  and  many  small  lootl  |jarti{les  (algae  and  haderia)  that  are  lia])|)C(l 
on  a  miitons  sheath  setreted  on  the  gills.  Special  tracts  ol  cilia  move 
this  miK  us  toward  the  month,  where  it  is  eventually  swallowed.  I  hits, 
most  pelecypods  leetl  by  hltering  water,  and  have  little  need  loi  loco- 
motion. I  he  head  is  lediued  to  a  mouth  between  a  pair  ol  palps,  long 
folds  ol  (iliated  skin  dial  colkct  lood  lioni  ilie  anterior  edges  ol  the 
gills  and  liaiislci  ii  lo  the  mouth.  In  this  class  ol  moIliis(s  the  ladiila 
is  lacking. 

The  i'elecyijoda  are  divided  into  orders  (Fig.  I  !.'.>)  according  to  the 
detailed  structuie  ol  the  gilK.  In  the  most  primitive  gioup  the  gills  are 
plumes  in  the  |)osterioi  pan  ol  the  mantle  cavity  resembling  those  ol 
the  (hitons  and  snails.  I  lie  |)alps  are  correspondingly  enlarged  to  serve 
in  leeding.  In  othei  gion|)s  the  gills  are  laige  and  lamellated  with 
numerous  slits  as  desnibed  above.  Still  other  modihcations  occur,  but 
most  ol  the  lamiliar  bivalves,  such  as  mussels,  clams,  oysters  and  scallops, 
ha\e  the  lamellatetl  type. 


no. 


Venus  mercenarla 


Ouahotf,  liaidsliell  tlam,  littleneck  and  chcrrvstone  are  common 
names  lor  I'cini.s  incrcenaria  (Fig.  1  1.10),  a  heart-shaped  bivalve  found  on 
the  east  coast  of  the  United  States.  The  young  (cherrystones)  are  eaten 
alive  on  liic  hall-shell  and  the  adults  make  excellent  chowder.  I'enxis 
lives  buiied  head  down  in  the  sand  anywhere  Irom  low  tide  to  depths 
of   100  leet,  with  the  slujrt  sijilioirs  projecting  to  filter  water. 

The  shell  valves  are  thick  and  strong.  The  hinge  ligament,  which 
opens  the  valves,  is  antero-dorsal,  next  to  the  umbo,  a  prominent  swell- 
ing on  each  valve.  .Anterior  to  the  ligament  are  several  prominent 
teeth  and  several  more  apj^ear  below  the  ligament  as  long  ridges.  These 
dilfer  on  the  two  valves  to  form  a  rigid  interlocking  mechanism.   The 


254  ^'^f   ANIMAL   KINGDOM 

Umbo 


Teeth 

A\  tachmanl  of 
ant  euMuctor 


Ligament 

Allachmenl  oF 


Palp 


post,  adductor 

Ant.  addiictor 
Attachme-nt 
oF  mantle 


sl.adductor 

Siphons 

Mantle 


Foot 


B 


■Ventricle 


Stomach 

Esophagi 
Digestive  gland' 
Palp  (lifted  to 
show  mouth) 


Intesti 
Crystcilline  style.' 


icle 
testine 
rogznita.!  papilla 
Upper  branchial  chamber 

•Anus 

•E:>cCarrent:  siphon 

InCurrcnt  Siphon 


inner^ill  ia 
branchial  chamber 


Gonad 


Figure  14.10.  Anatomy  of  Venus  merceimria.  A,  Interior  of  the  right  valve.  B,  Left 
side  with  shell  and  mantle  removed.  C,  Partial  dissection,  showing  some  of  the  internal 
organs. 

valves  can  be  shut  by  the  adductor  muscles  so  completely  that  Venus 
will  live  for  days  out  of  water,  a  convenience  for  shipping  them  inland. 
Live,  healthy  specimens  can  be  purchased  in  almost  any  seafood  market. 

The  foot  can  be  extended  some  distance  out  of  the  shell,  and  is 
usually  thrust  anteriorly  (downward)  into  the  sand  as  an  anchor.  Venus 
can  also  move  slowly  by  movements  of  the  foot. 

As  in  many  clams,  each  of  the  four  gills  is  attached  along  the  dorsal 
limit  of  the  mantle  cavity,  hangs  down  toward  the  ventral  limit,  and 
then  folds  back  dorsally  (Fig.  14.9  C).  Each  inner  gill  is  attached  to  the 
base  of  the  foot,  while  each  outer  gill  attaches  to  the  mantle,  forming  a 
total  of  four  longitudinal  upper  gill  chambers  that  come  together 
posteriorly  at  the  excurrent  siphon.  Bridges  of  tissue  between  the  folds 
of  each  gill  keep  the  folds  slightly  apart  and  create  channels  leading 
from  the  many  tiny  slits  in  each  fokl  up  to  the  upper  gill  chambers. 
W^hile  I'enus  is  feeding  the  shells  are  slightly  open  and  the  siphons  ex- 
tended. Water  passes  in  the  lower  siphon,  through  the  several  gill  folds 
into  the  upper  branchial  chambers,  and  out  through  the  upper  siphon. 
On  the  free  surface  of  each  gill  fold  the  mucous  sheath  is  swept  ven- 
trally  to  the  lower  edge  of  the  fold,  and  then  forward  to  the  palps.  If 
accepted,  the  mucus  and  food  are  carried  up  the  folds  of  the  palps  into 
the  mouth.  Sometimes,  however,  dirt  or  distasteful  material  may  enter 
the  mantle  cavity,  collect  on  the  mucous  sheath,  and  arrive  at  the  palps 
as  though  it  were  food.  This  is  rejected,  and  transferred  to  the  mantle 
where  the  cilia  move  it  ventrally  and  then  posteriorly  to  accumulate 
just  below  the  incurrent  siphon.  Periodically  it  is  ejected  through  the 


THE   PHYLUM    MOLLUSCA  255 

incurrent  siphon  as  the  clam  suddenly  closes  the  valves,  squirting  water 
out  of  both  siphons. 

From  the  mouth  a  short  esophagus  leads  to  a  small  stomach.  A 
long  looped  intestine  eventually  turns  dorsally  and  runs  posteriorly 
straight  through  the  ventricle  of  the  heart,  around  the  posterior  adduc- 
tor muscle,  and  ends  at  an  anus  over  the  excurrent  siphon.  The  stomach 
is  buried  in  a  digestive  gland  that  opens  into  it,  and  the  intestine  is 
buried  in  the  gonads.  All  lour  organs  are  bound  tightly  into  a  rounded 
visceral  mass  at  the  base  ot  the  toot. 

The  first  portion  of  the  intestine  is  divided  longitudinally  to  form 
right  and  left  channels.  The  right  channel  functions  as  the  intestine 
and  is  continuous  with  the  rest  of  the  intestine.  The  left  channel  forms 
a  tubular,  blind  sac  that  contains  the  crystalline  style,  a  structure 
unique  to  the  pelecypods  and  a  few  gastropods.  In  many  pelecypods 
the  sac  is  completely  separate  from  the  intestine.  The  crystalline  style 
is  a  gelatinous  rod  secreted  by  the  wall  of  the  diverticulum  which  moves 
slowly  into  the  stomach  where  the  end  wears  away.  Its  function  is 
similar  to  that  of  the  salivary  glands  in  snails,  since  it  contains  enzymes 
for  the  digestion  of  carbohydrates.  Little  is  known  of  the  function  of 
the  digestive  gland.  Although  its  cells  may  secrete  digestive  enzymes  into 
the  stomach,  it  is  more  likely  that  they  function  as  in  the  snail,  phago- 
cytizing  small  food  particles. 

The  paired  gonads  open  through  small  ducts  ending  on  urogenital 
papillae,  one  on  each  side  of  the  posterior  part  of  the  foot.  Sexes  are 
separate.  Eggs  and  sperm  are  released  throughout  the  summer  into  the 
sea  where  fertilization  takes  place.  The  embryo  develops  into  a  larva 
known  as  a  trochophore  that  settles  to  the  bottom  by  autumn  as  a  tiny 
clam.  It  matures  in  about  three  years. 

The  circulatory  and  excretory  systems  are  similar  to  those  of  the 
snails,  except  that  two  auricles  and  two  nephridia  are  present.  The 
pericardial  cavity  surrounds  not  only  the  heart  but  also  a  small  part  of 
the  intestine.  The  tubular  portion  of  each  nephridium  opens  internally 
into  the  pericardial  cavity  as  well  as  externally  on  the  urogenital  papilla. 

The  heart  of  Venus  is  used  extensively  in  physiological  research.  If 
one  valve  is  removed  from  a  live  specimen,  the  beating  heart  can  be 
seen  in  the  dorsal  part  of  the  body.  The  only  further  dissection  neces- 
sary is  the  removal  of  a  portion  of  the  mantle  and  one  wall  of  the 
pericardial  cavity.  A  small  hook  can  then  be  inserted  into  the  ventricle 
and  attached  by  a  string  to  a  lever,  so  that  both  strength  and  frequency 
of  the  beat  may  be  recorded.  After  a  "normal"  record  is  obtained,  vari- 
ous drugs  are  dripped  onto  the  heart  and  the  results  observed.  Since 
the  moUuscan  heart  has  been  found  to  respond  to  the  same  kinds  of 
drugs  that  affect  the  human  heart,  the  heart  of  J'eniis  is  used  in  some 
laboratories  as  a  means  of  measuring  the  strength  of  various  drug  ex- 
tracts. The  response  is  very  closely  related  to  the  concentration  of  the 
drug  administered. 

The  nervous  system  follows  the  typical  molluscan  plan.  The  brain 
and  some  ganglia  are  located  over  the  esophagus.  A  pair  of  large  vis- 
ceral ganglia  can  be  easily  distinguished  on  the  anterior  surface  of  the 


256  '■"^   ANIMAL   KINGDOM 

posterior  adductor  muscle,  below  the  intestine.  An  additional  pair  of 
pedal  ganglia  (which  in  the  gastropods  have  moved  forward  to  join 
the  brain)  are  deeply  embedded  in  the  loot  of  pelecypods.  Nerve  cords 
connect  these  various  components.  Sense  organs  are  limited  to  scattered 
chemoreceptors  on  the  palps  and  siphons.  Touch  and  temperature 
sense  endings  are  probably  present  along  the  mantle  edges.  A  tew  pele- 
cypods have  eyes  but  they  are  at  the  mantle  edge,  never  on  the  head. 

111.        Other  Pelecypoda 

Many  clams,  including  the  steaming  clam,  Mya  arenaria  (Fig.  14.11), 
live  buried  in  the  sand  and  mud  like  Venus.  Others,  such  as  the  cockle, 
Cardium  edule  (Fig.  14.2),  jump  over  the  bottom  with  quick  movements 
of  the  foot.  Mussels  and  oysters  are  attached  to  rocks  and  pilings.  The 
common  mussel,  Mytilus  eduUs,  attaches  by  a  cluster  of  strong  threads 
secreted  by  a  gland  at  the  base  of  the  foot.  Oysters  cement  one  valve  to 
the  bottom. 

The  edible  oyster  (several  species  of  the  genus  Ostrea)  is  harvested 
by  the  most  intensive  and  thoroughly  regulated  fisheries  in  the  world. 
Along  the  eastern  seaboard  of  the  United  States,  for  example,  wherever 
the  bottom  is  especially  suitable  for  oysters  it  has  been  surveyed  and 
rented  to  various  fishermen  by  the  states.  Once  a  fisherman  rents  a  given 
area,  he  is  entitled  to  rent  it  for  the  rest  of  his  life,  and  to  pass  on  the 
privilege  to  his  heirs.  Each  oysterman  manages  his  own  "land"  to  pro- 
duce as  many  oysters  as  possible.  Every  year  boatloads  of  old  shells  are 
scattered  about  to  serve  as  possible  sites  for  the  attachment  of  larval 
oysters.  Once  larvae  are  attached  to  these  loose  shells,  they  may  be 
moved  about  several  times,  inshore  each  summer  for  maximum  growth, 
offshore  in  winter  for  protection,  and  finally  to  premium  spots  where 
they  develop  the  best  flavor  for  marketing.  Oysters  mature  in  three  to 
five  years.  Curiously,  most  of  the  young  are  not  produced  by  the  older 
oysters  of  the  fishery,  but  come  from  scattered  populations  along  the 
rocky  shores  and  especially  in  the  mouths  of  rivers  where  the  water  is  a 
little  less  salty.  These  "wild"  oysters  produce  enormous  numbers  of 
young  that  drift  offshore  and  eventually  settle  to  the  bottom. 

Oysters  are  hermaphroditic;  an  individual  may  be  a  male  for  a  few 
years,  and  then  become  a  female,  but  it  is  never  both  sexes  at  once. 
The  American  oysters  shed  both  eggs  and  sperm  into  the  water  where 
fertilization  is  left  to  chance.  The  gametes  shed  by  one  individual  enter 
the  siphons  of  other  oysters,  causing  them  to  release  their  gametes  also, 
and  soon  the  entire  bed  has  been  triggered. 

The  pearl  oysters  (species  of  the  genus  Meleagrina)  are  found  in 
warm  seas,  especially  around  Japan  (Fig.  14.11).  Theoretically  any 
pelecypod  can  produce  pearls,  and  many  species  such  as  the  common 
mussels  and  oysters  often  do,  but  only  the  pearl  oysters  produce  pearls 
of  consistent  high  quality.  The  formation  of  a  pearl  is  a  reaction  of 
self-defense.  If  a  small  foreign  body  should  become  lodged  between  the 
mantle  and  the  shell,  a  layer  of  shell  is  secreted  around  it  to  seal  it  off. 
If  the  foreign  body  should  be  buried  in  the  flesh  of  the  mantle,  shell  is 


THE   PHYLUM   MOLLUSCA 


257 


secreted  all  the  way  around  it  in  concentric  layers.  The  edge  of  the 
mantle,  which  makes  the  growing  edge  of  the  shell,  secretes  a  chalky 
kind  of  shell,  but  the  inner  part  of  the  mantle  that  thickens  the  shell 
secretes  a  harder,  pearly  material.  The  quality  of  the  pearl  depends  upon 
the  quality  of  the  shell  lining  normally  produced.  The  common  mussel 
produces  a  lustrous,  irridescent  shell  lining,  and  is  sometimes  infested 
with  parasites  around  which  pearls  are  secreted.  Although  there  are 
often  dozens  in  every  mussel,  none  of  them  becomes  larger  than  a  tiny 
sand  grain. 

The  Japanese  have  mastered  the  technique  of  culturing  pearls. 
Pearl  oysters  are  collected,  small  particles  are  introduced  into  the  mantle, 
and  then  they  are  put  out  to  sea  in  cages  for  several  years.  When  the 
pearls  have  had  time  to  reach  a  suitable  size,  the  oysters  are  taken  in 
and  opened. 

The  large  fresh-water  bivalves  are  a  group  of  mussels  that  no 
longer  attach,  but  live  buried  in  the  sand  like  clams.  They  are  adapted 


Pecl^en. 


O^trca. 


Mele-stdrixia, 


Figure  14.11.  Some  common  pelecypods.  Pecten,  the  scallop  (after  Johnson  and 
Snook).  Mya,  the  steaming  clam  (after  Verrill,  1873).  Ostrea,  the  oyster  (drawing  by  Hair- 
ston).  Meleagrina,  the  pearl  oyster  (after  Fischer,  1887),  showing  pearly  "warts"  in  a  shell. 


258 


THE   ANIMAL   KINGDOM 


Figure  14.12.     Glochidium,  the  larva  of  a  fresh-water  mussel. 

for  life  in  lakes  and  rivers  where  floating  larvae  would  be  swept  away. 
Eggs  are  retained  in  the  adult  until  they  become  small  bivalves  called 
glochidia  (Fig.  14.12),  mostly  shell  and  adductor  muscle  with  very  little 
else.  These  clamp  tightly  onto  the  fins  or  gills  of  fish,  where  they  grad- 
ually become  buried  and  actually  receive  nourishment.  In  this  way  they 
are  carried  about,  upstream  as  well  as  down.  In  a  few  weeks  the  glochid- 
ium assumes  an  adult  form  and  ends  its  parasitic  phase  by  dropping  off 
and  burrowing  into  the  bottom. 

The  fresh-water  mussels  of  the  Mississippi  River  system  support  a 
pearl  button  industry.  The  buttons  are  cut  from  the  inner,  pearly 
layers  of  the  shells.  At  present  the  industry  is  considerably  reduced  in 
size,  both  because  many  of  the  fisheries  have  been  depleted  and  because 
competition  with  substitutes  has  driven  the  pearl  button  into  a  semi- 
luxury  category.  So  far  plastics  have  failed  to  imitate  the  unique  luster 
of  pearl,  which  results  from  the  structure  of  its  crystals,  and  not  from 
the  material  of  which  it  is  made. 

A  few  pelecypods  lie  loosely  on  the  ocean  bottom  and  are  able  to 
swim  by  flapping  the  shells.  An  example  is  the  scallop,  a  species  of  the 
genus  Pecten  (Fig.  14.11).  The  familiar  scalloped  shells  are  closed  by  an 
enormous  adductor  muscle,  the  only  part  of  the  scallop  that  is  eaten. 
The  free  edges  of  the  mantle  are  set  with  bright  blue  eyes.  Scallops  are 
easily  frightened,  and  violently  clap  their  shells  as  they  swim  away  on 
erratic  courses. 


1 1 2.        Class  Scaphopoda 

The  tooth  shells  are  a  small  group  of  marine  molluscs  that  burrow 
in  mud  and  sand.  They  have  a  funnel-shaped  shell  open  at  both  ends 
(Fig.  14.2).  The  foot  is  conical  and  used  for  digging.  Around  the  head 
are  a  number  of  prehensile  filaments  that  are  presumed  to  be  used  to 
bring  food  particles  to  the  mouth.  A  radula  is  present.  The  smaller  open- 


THE  PHYLUM   MOLLUSCA 


259 


ing  of  the  shell  remains  above  the  mud  and  is  used  for  water  circula- 
tion. Gills  are  absent;  the  mantle  lining  is  sufficient  for  respiration. 
Strings  of  tooth  shells,  which  are  two  or  three  inches  long,  were  formerly 
used  by  west  coast  Indians  as  money. 

113.        Class  Cephalopoda:  General  Features 

Cephalopods  are  active,  fast-moving  molluscs.  The  chambered 
nautilus  (Fig.  14.13)  is  the  most  primitive  of  living  species,  with  relatives 
that  are  abundant  as  fossils  dating  all  the  way  back  to  the  beginning  of 
the  known  fossil  record.  The  nautilus  floats  by  secreting  gas  (resembling 
air,  but  with  less  oxygen)  into  its  shell.  The  shell  is  chambered,  and  the 
animal  lives  only  in  the  most  recently  added  chamber.  A  stalk,  which 
secretes  the  gas,  extends  back  through  the  other  chambers.  The  shell 
covers  the  animal  dorsally,  and  is  secreted  by  a  mantle  as  in  other  mol- 
luscs. 

The  nautilus  has  modified  the  foot  for  both  feeding  and  locomo- 
tion. The  anterior  part  grows  forward  on  each  side  of  the  head  in  a 
series  of  lateral  lobes,  at  the  edges  of  which  are  numerous  tentacles. 
The  tentacles,  annularly  ridged,  are  able  to  grasp  objects  tightly.  With 
these  the  nautilus  may  attach  to  rocks  while  resting  or  may  grasp  prey 
and  carry  it  to  the  mouth.  The  posterior  part  of  the  foot  is  folded 
longitudinally  to  form  a  large  funnel.  The  posterior  end  fits  against  the 
opening  into  the  mantle  cavity,  while  the  smaller  anterior  end  is  sup- 
plied with  a  flaplike  valve.  ^Vhen  the  funnel  enlarges,  water  enters  be- 
tween mantle  and  foot,  but  when  the  funnel  constricts,  the  posterior  edge 
of  the  foot  closes  against  the  mantle  and  the  water  is  squirted  out  the 
anterior  end.  By  this  form  of  jet  propulsion  the  nautilus  is  able  to 
swim.  Two  pairs  of  gills  lie  in  the  mantle  cavity  where  they  are  con- 
tinually flushed  with  water. 

A  stout  pair  of  horny  jaws  assist  the  radula  in  tearing  prey  to  bits. 


Tenlacle-S 


Funnel 


Mantle,  cavity 


Figure  14.13.  Lateral  view  of  Nautilus.  A  diagrammatic  section  of  the  chambered 
shell  is  sliown.  The  mantle  of  the  left  side  is  cut  away  to  show  the  mantle  cavity  and  two 
of  the  gills.  When  the  animal  retracts,  the  leathery  hood  protects  the  shell  opening.  Com- 
bined from  several  sources. 


250  ^HE   ANIMAL   KINGDOM 

The  lower  jaw  closes  outside  the  upper  jaw,  resembling  the  reverse  of  a 
parrot's  beak.  The  nautilus  also  has  a  pair  of  large,  protruding  eyes 
that  form  images.  Each  eye  is  a  simple  cavity  (Fig.  14.16)  with  a  small 
hole  opening  to  the  exterior.  Water  is  free  to  enter  the  cavity,  which  is 
lined  with  a  retina  differentiated  from  the  ectoderm.  Images  are  formed 
on  the  principle  of  the  pinhole  camera,  a  lensless  system  that  requires 
only  a  very  small  opening  to  a  dark  chamber  with  a  light-sensitive  back 

surface. 

The  nautilus  and  its  relatives,  including  the  extinct  ammonites,- 
dominated  the  seas  for  many  millions  of  years,  dwindling  to  near  ex 
tinction  at  the  end  of  the  Mesozoic  era.  Throughout  this  time  various 
cephalopod  groups  showed  tendencies  to  reduce  the  size  of  the  shell  and 
make  it  an  internal  structure.  Today  most  of  the  living  members  belong 
to  such  groups.  The  squids  (Fig.  14.14)  are  the  most  highly  developed 
in  the  direction  of  an  active,  swimming  predator.  The  tentacles  are 
fewer  in  number,  longer,  and  bear  numerous  suckers.  The  funnel  is 
closed  into  a  complete  tube,  while  the  mantle,  no  longer  confined 
within  the  shell,  has  become  a  muscular  pump  that  draws  water  in 
around  its  free  edge  and  expels  it  through  the  siphon.  The  squids  can 
match  fish  in  speed  and  agility.  The  octopuses  (Fig.  14.17)  have  gone 
back  to  the  ocean  bottom  where  they  crawl  rapidly  over  the  rocks  and 
swim  only  when  chased. 

114.        Loligo 

The  common  squids,  Loligo  pealei  of  the  east  coast  and  L.  opalescens 
of  the  west,  grow  to  8  to  12  inches  in  length  and  are  frequently  netted 
in  large  numbers  by  fishermen  and  sold  at  market.  A  glance  at  their 
streamlined  shape  and  compact  organs  (Figs.  14.14  and  14.15)  suggests 
that  the  changes  initiated  by  the  nautiloids  have  been  carried  much 
further  in  the  squid. 

The  squid  is  elongated  like  the  gastropods,  but  in  this  case  the 
elongation  remains  straight.  The  body  is  covered  by  a  thick  muscular 
mantle  that  tapers  to  a  point.  The  shell  is  reduced  to  a  pen,  buried  in 
the  upper  portion  of  the  mantle.  The  anterior  part  of  the  foot  is  com- 
pletely disassociated  from  the  rest  and  is  intimately  fused  with  the 
head,  forming  a  complete  ring  of  eight  tapered  arms  and  two  elongate 
tentacles.  The  posterior  part  of  the  foot,  much  smaller  than  in  the 
nautilus,  is  fused  into  a  tubular  funnel  attached  to  the  lower  side  of  the 
head.  The  head  is  carried  on  a  slender  neck  and  fits  snugly  into  the 
opening  of  the  mantle.  In  life  it  is  locked  in  place  by  three  articula- 


Figure  14.14.     Lateral  view  of  Loligo. 


THE  PHYLUM   MOLLUSCA 


261 


Cuticular  rim 


Flexible  bottom 


Cranial 
cartilage 

Neck 

cartilage- 
Esophagus - 


Digestive 
gland.— 


Stomach- 


-Ante.rior  artery 
"Anterior  ve-in 


'znis 

infi^omgill 

in  to  gill 

11  heart 

rmatophoric 
an 

SpermatophoriC 
sac 


lerm 


duct 


Upper 

surface 


Funnel 

"Testis  (behind 
othar  viscera) 


Figure  14  15      Anatomy  of  Loligo.  A,  Lateral  view  with  body  wall  removed, 
digesthe  and  nervous  systems.  B,  Section  through  a  sucker.  C,  Ventral  view  of  part  of  t 
circulatory  system  and  of  the  male  reproductive  system  (drawn  to  the  same  scale  as  A, 
so  that  it  can  be  turned  on  edge  and  fitted  into  A).  (After  Williams.) 


262  ^"^   ANIMAL   KINGDOM 

tions:  the  end  of  the  pen  fits  into  a  cartilaginous  groove  on  the  upper 
side  of  the  head,  and  two  cartihiginous  rods  at  the  mantle  rim  fit  into 
corresponding  grooves  on  the  funnel. 

By  changing  the  direction  of  its  funnel  the  squid  can  swim  forward 
or  backward  in  the  water.  When  the  mantle  cavity  enlarges,  the  funnel 
valve  shuts  and  water  is  sucked  in  along  the  sides  of  the  head.  When 
the  mantle  constricts,  flaps  of  skin  close  all  openings  between  mantle 
and  head,  and  the  water  is  forced  out  the  funnel.  The  funnel  is  flexible 
and  is  turned  backward  when  the  squid  wishes  to  swim  head  foremost. 
For  the  most  rapid  locomotion,  however,  the  funnel  is  held  straight, 
pointing  forward,  and  the  squid  shoots  away  with  its  head  trailing.  For 
slow  movement  jet  propulsion  may  be  assisted  or  replaced  by  the  un- 
dulations of  a  pair  of  lateral  fins  near  the  apex  of  the  mantle.  These  can 
undulate  in  either  direction  and  in  rapid  movement  are  used  for  steer- 
ing. 

A  spectacular  feature  of  the  living  squid  is  its  changing  color.  Just 
beneath  the  skin  are  numerous  chromatophores,  cells  packed  with  pig- 
ment that  may  be  black,  yellow,  or  red.  WHien  a  chromatophore  is 
spherical  and  contracted  it  is  barely  visible  to  the  naked  eye,  but  at- 
tached around  its  sides  are  numerous  muscle  fibers  that  can  stretch  it 
out  into  a  flat  disc  as  much  as  3  mm.  in  diameter.  These  muscle  fibers 
are  controlled  by  the  nervous  system,  and  can  act  rapidly.  A  squid  can 
change  color  in  less  than  a  second,  or  pass  waves  of  color  along  its 
body  by  expanding  differently  colored  sets  of  chromatophores. 

The  arms  are  covered  along  their  oral  surfaces  with  numerous 
stalked  suckers  (Fig.  14.15,  A).  The  arms  are  relatively  short  and  taper- 
ing, with  the  suckers  arranged  in  two  longitudinal  rows.  The  tentacles 
are  long,  with  cylindrical  bases  and  expanded  ends  having  four  rows  of 
suckers.  If  the  arms  are  counted  from  the  upper  surface,  the  tentacles 
lie  between  the  third  and  fourth  pairs,  and  can  be  retracted  into 
pouches  formed  by  fleshy  webs  between  these  two  pairs  of  arms.  The 
tentacles  are  shot  out  suddenly  to  capture  prey.  The  arms  serve  primarily 
to  hold  and  manipulate  the  food  after  it  is  caught. 

Each  sucker  (Fig.  14.15  B)  is  a  rigid  cup  with  a  finely  toothed  rim 
and  a  flexible  bottom  attached  to  a  slender  stalk.  When  the  tentacle  is 
pressed  against  a  surface  the  cup  is  pushed  back  upon  its  stalk,  obliter- 
ating the  cavity  beneath.  When  the  tentacle  pulls,  the  force  is  trans- 
mitted through  the  stalk  to  the  middle  of  the  flexible  bottom  of  the 
cup,  creating  suction  that  holds  the  cup  tight.  The  squid  can  release  a 
sucker  by  contracting  small  muscles  between  cup  and  stalk,  pulling  in 
the  bottom  to  eliminate  the  suction.  Thus,  the  suckers  are  attached 
automatically,  and  can  be  released  only  by  positive  action  of  the  squid 
unless  sufficient  external  force  is  applied  to  overcome  the  suction. 

Food  is  shredded  by  a  pair  of  jaws  and  a  radula  similar  to  those  of 
the  nautilus.  The  slender  esophagus  (Fig.  14.15  A)  traverses  the  neck 
to  a  muscular  stomach  in  the  body.  Next  to  the  esophagus,  at  the  an- 
terior end  of  the  stomach,  an  intestine  leads  forward  to  an  anus  just 
behind  the  inner  end  of  the  funnel.  A  very  large  delicate  sac,  the 
caecum,  opens  into  the  stomach.  Salivary  glands  open  into  the  esoph- 


THE   PHYLUM   MOLLUSCA  963 

agus  and  digestive  glands  into  the  stomach.  In  the  cephalopods  both  of 
these  secrete  enzymes  and  the  absorption  of  food  appears  to  be  Hmited 
largely  to  the  caecum.  Enzymes  rapidly  liquefy  the  meat  that  is  eaten, 
and  it  is  only  the  liquid  hydrolysate  that  passes  into  the  caecum. 

The  ink  sac  opens  just  behind  the  anus  into  the  end  of  the  in- 
testine. The  glandular  lining  of  this  sac  secretes  a  black  liquid  that  is 
expelled  when  the  squid  is  alarmed.  The  defensive  action  of  this  ink  has 
been  much  debated.  It  is  commonly  thought  to  act  as  a  "smoke  screen" 
behind  which  the  squid  can  swim  rapidly  away.  It  may  also  serve  as  a 
distracting  dark  object  that  momentarily  holds  the  attention  of  the 
pursuer.  The  ink  of  deep  sea  squids  is  luminescent,  producing  a  bright 
splotch  in  the  otherwise  black  water.  The  ink  of  the  octopus  is  known 
to  have  an  additional  function.  MacGinitie  has  shown  that  if  a  pur- 
suer swims  into  the  ink  its  sense  of  smell  is  paralyzed  for  as  much  as 
two  hours.  During  that  time  it  will  continue  to  hunt  for  the  octopus, 
but  even  if  it  touches  it  the  pursuer  seldom  recognizes  that  the  octopus 
is  there.  We  do  not  know  whether  squid  ink  has  a  similar  effect. 

A  single  pair  of  gills  hangs  in  the  lower  part  of  the  mantle  cavity. 
Associated  with  these  are  a  pair  of  auricles,  nephridia  and  a  single  ven- 
tricle, as  in  most  molluscs.  The  circulatory  system  is  closed,  however, 
unlike  that  of  other  molluscs.  Arteries  end  in  networks  of  capillaries  all 
over  the  body  that  come  together  in  veins  leading  back  to  the  nephridia. 
Furthermore,  all  of  the  blood  passing  through  the  nephridia  goes  on 
through  the  gills.  Between  each  nephridium  and  gill  is  an  auxiliary 
gill  heart  that  pumps  blood  through  the  capillary  network  of  the  gill 
to  the  auricle  (Fig.  14.15  C). 

Most  of  the  central  nervous  system  is  grouped  into  a  large  ring 
around  the  esophagus.  This  structure,  the  fused  brain  and  ganglia,  is  as 
large  as  the  brain  of  a  fish  of  similar  size.  It  is  also  encased  in  a  kind 
of  "skull,"  formed  by  several  cranial  cartilages.  Many  nerves  run  from 
this  central  mass  to  all  parts  of  the  body.  The  only  large  ganglia  out- 
side of  this  center  are  the  star-shaped  stellate  ganglia  on  the  inner  side 
of  the  mantle. 

The  large  lateral  eyes  appear  during  development  as  simple  pits 
that  resemble  the  pin-hole  eyes  of  the  nautilus.  Later,  however,  a  lens, 
iris,  cornea  and  focusing  mechanism  develop,  producing  an  eye  re- 
markably like  that  of  the  vertebrates  (Fig.  14.16).  The  lens  is  supported 
on  a  flexible  membrane  between  the  inner  and  outer  chambers.  Con- 
traction of  the  muscles  around  the  inner  chamber  squeezes  it  and  forces 
the  lens  outward  for  near  vision.  The  squid  eye  is  "direct"  since  light 
reaches  the  retina  without  having  to  traverse  nerves  and  cell  bodies.  The 
retina  is  ectodermal  in  origin,  and  is  reached  from  behind  by  nerves 
from  the  optic  ganglia,  large  lateral  outgrowths  from  the  brain.  As 
shown  in  the  figure,  the  lens  is  composed  of  two  pieces.  A  unique  feature 
of  the  squid  eye  is  that  these  two  pieces  form  at  different  times  during 
development.  The  inner  half  develops  along  with  the  retina,  while  the 
outer  half  forms  later  along  with  the  iris. 

The  apex  of  the  body  is  occupied  by  the  gonad.  In  the  female,  eggs 
are  released  into  a  part  of  the  coelom  surrounding  the  ovary  and  col- 
lected in  a  ciliated  funnel  to  be  stored  in  the  oviduct.  This  loops  back 


264 


THE   ANIMAL   KINGDOM 


Pi^ merit  layer 


Retina: 


Optic 


'•Optic  nerves 


NAUTILUS 


gangli 


ion 


Iris 


Lens 


ornea. 


Inner 
chamber 

Outer 
cha.mber 


LOLIGO 


Figure  14.16.  Cephalopod  eyes.  Left,  the  pin-hole  camera  type  in  Nautilus.  (After 
Borradaile,  et  al.)  Right,  the  lens  type  complete  with  shutter  (iris)  in  Loligo.  (After 
Williams.) 

and  forth  and  ends  to  the  left  of  the  anus.  Near  the  end  a  glandular 
region  of  the  oviduct  secretes  a  capsule  around  each  individual  egg,  and 
at  the  end  a  pair  of  large  glands  secretes  a  gelatinous  matrix  around  the 
entire  mass  of  eggs. 

In  the  male  (Fig.  14.15),  the  sperm  are  released  into  the  coelom  and 
collected  by  a  funnel.  The  sperm  duct  is  convoluted,  and  passes  forward 
on  the  left  side  of  the  body  to  an  expanded,  coiled  portion,  the  sper- 
matophoric  organ,  which  wraps  the  sperm  into  packets  called  spermato- 
phores.  The  duct  then  continues  backward  as  a  straight,  narrow  tube 
and  turns  forward  again  as  a  large  sac,  the  spermatophoric  sac,  where 
the  spermatophores  are  stored.  The  sac  opens  anteriorly  on  a  penis-like 
projection  to  the  left  of  the  anus. 

The  left  fourth  arm  of  the  male  is  modified  to  serve  as  a  copu- 
latory  organ.  A  short  distance  from  its  tip  the  sucker  cups  are  very 
small  or  absent,  but  the  stalks  are  enlarged.  During  courtship,  the  male 
moves  excitedly  around  the  female,  holds  the  copulatory  arm  in  his 
mantle  cavity  against  the  male  opening,  and  the  stored  spermatophores 
are  ejaculated  onto  the  specialized  region.  He  then  thrusts  this  arm 
toward  the  female,  and  either  inserts  it  into  her  mantle  cavity  or  presses 
it  against  the  sperm  receptacle,  a  horseshoe-shaped  depression  on  the 
posterior  side  of  the  mouth.  In  either  case  the  spermatophores  are  glued 
to  the  female. 

The  eggs  are  laid  soon  after.  They  are  fertilized  either  in  the  mantle 
cavity  or  as  they  cross  the  sperm  receptacle.  The  whole  mass  is  gathered 
by  the  female  in  her  arms,  and  after  all  the  eggs  are  laid  she  finds  a 
suitable  place  for  attachment.  The  gelatinous  matrix  hardens  slowly  to 


THE   PHYLUM    MOLLUSCA 


265 


form  a  protective  coat,  and  young  squids  hatch  in  two  or  three  weeks. 
Development  in  the  cephalopods  is  direct,  the  yolky  eggs  producing 
young  that  resemble  the  aduks. 

115.        Other  Cephalopods 

The  nautikises,  with  tour  gills,  belong  to  the  order  Tetrabranchiata, 
which  is  presumed  to  include  most  of  the  fossil  cephalopods.  All  other 
living  cephalopods  have  two  gills,  and  belong  to  the  order  Dibranchiata. 
In  addition  to  the  common  squids  and  octopuses,  the  group  includes  the 
cuttlefish  (Fig.  14.2),  whose  internal  shell  is  used  as  a  source  of  lime  for 
canaries,  and  the  deep-sea  giant  squids.  The  latter  are  the  largest  living 
invertebrates,  having  bodies  at  least  20  feet  long  with  tentacles  more 
than  35  feet  long.  They  were  first  known  from  the  marks  of  their  suckers 
on  the  skin  of  the  sperm  whale,  which  were  j^often  over  an  inch  in 
diameter,  and  from  their  jaws  in  the  whale's  stomach.  These  squids  are 
the  major  food  of  the  sperm  whale,  which  dives  to  great  depths  to  hunt 
them.  Rarely  a  dying  giant  squid  comes  to  the  surface  or  is  washed 
ashore  (Fig.   14.17). 

Octopuses  (Fig.  14.18)  lack  the  tentacles  present  in  squids  and  cut- 
tlefish. They  also  differ  from  the  other  Dibranchiata  in  having  suckers 
that  lack  stalks  and  teeth,  and  in  having  no  shell  whatsoever. 

Small  octopuses  survive  well  in  aquariums  where  observers  are  dis- 
covering that  they  liave  a  surprisingly  high  order  of  intelligence.  They 
are  able  to  make  associations  among  stimuli  and  in  general  show  an 
adaptability  of  behavior  that  more  closely  resembles  that  of  the  verte- 
brates than   the  more  stereotyped  patterns  of  other   invertebrates.  Oc- 


Figure  14.17.  A  "small"  relative  of  the  giant  squid,  the  oceanic  squid,  Ommastrephes 
caroli.  This  remarkably  intact  specimen  was  stranded.  A  meter  ruler  gives  the  scale. 
(Courtesy  Douglas  P.  Wilson.) 


266 


THE   ANIMAL   KINGDOM 


Figure    14.18.     Octopus  pursuing  a   crab,    (iiuz   Guio-Couitesy   LIFE   Magazine. 
Copr.  1955  Time  Inc.) 

topuses  feed  on  crabs  and  other  arthropods.  They  catch  their  prey  and 
first  kill  it  by  a  poisonous  secretion  from  the  salivary  glands.  Then  all 
of  the  flesh  is  delicately  picked  out,  leaving  the  hard  parts  uneaten. 
Octopuses  live  among  rocks,  seeking  shelter  in  small  caves  that  they  may 
partially  excavate.  The  motion  of  octopuses  is  incredibly  fluid,  with  no 
suggestion  of  the  strength  that  lies  in  the  eight  arms.  Their  ferocity, 
however,  has  been  overrated.  Octopuses  hide  during  the  day  and  come 
out  in  the  evening.  They  are  by  nature  timid,  and  flee  from  animals  as 
large  as  man.  The  largest  individuals,  which  may  have  arms  12  feet  long, 
are  certainly  to  be  respected  from  a  distance,  but  these  are  rare.  Most 
octopuses  have  arms  less  than  a  foot  long. 

Questions 

1.  Distinguish  among  the  five  classes  of  molluscs. 

2.  Compare  the  chiton  with  a  generalized  mollusc. 

3.  Describe  the  radula. 

4.  What  is  torsion? 

5.  Give  examples  of  gastropods  that  (a)  swim,  (b)  have  no  shell,  (c)  breathe  air. 

6.  How  does  Verms  feed? 

7.  Describe  sexual  phenomena  in  the  oyster  and  the  squid. 

8.  Why  are  the  tentacles  and  siphon  of  the  squid  considered  to  be  parts  of  the  foot? 

9.  How  does  the  squid  sucker  work? 


Supplementary  Reading 

Many  manuals  are  available  for  the  identification  and  study  of  shells  (conchology). 
MacGinitie  and  MacGinitie,  Natural  History  of  Marine  Animals,  include  interesting  in- 
formation on  the  activities  of  many  animals  and  are  especially  good  on  the  molluscs.  The 
paperbound  Seashores  by  Zim  and  Ingle  contains  many  colored  drawings  of  seashore  life, 
especially  of  the  molluscs. 


CHAPTER  15 


Phylum  Annelida 


1 1 6.        General  Features  of  the  Annelid  Worms 

The  Annelida  are  segmented  worms,  the  body  wall  and  coelom  of 
which  are  divided  into  a  longitudinal  series  of  rings  or  segments.  The 
epidermis,  circular  muscle,  longitudinal  muscle,  coelom  and  peritoneum 
are  all  arranged  in  segments. 

Some  of  the  phyla  considered  previously  have  structures  that  look 
like  segments.  The  tapeworms,  for  example,  might  be  said  to  be  seg- 
mented, with  new  segments  forming  in  the  scolex  and  the  older  segments 
moving  to  the  posterior  end  as  proglottids.  Each  segment  of  the  tape- 
worm is  eventually  shed,  however,  and  is  only  a  temporary  part  of  the 
body.  Many  rotifers  and  a  few  nematodes  have  a  superficial  segmenta- 
tion, which  involves  only  the  cuticle  and  a  part  of  the  musculature.  Most 
of  the  musculature  of  the  kinorhynchs  is  segmented  and  their  cuticle  is 
deeply  segmented.  Young  kinorhynchs  have  few  segments,  and  add  new 
ones  at  the  posterior  end  as  they  grow.  Most  zoologists  do  not  consider 
these  animals  to  be  truly  segmented  as  are  the  annelids,  arthropods  and 
chordates. 

True  segmented  animals  exhibit  metamerism,  a  repetition  of  a 
structure  or  organ  from  segment  to  segment.  The  annelid  body  is  made 
of  a  series  of  metameres  or  segments,  each  of  which  has  the  same  funda- 
mental structures  as  all  the  others.  The  nervous,  circulatory,  excretory 
and  reproductive  systems  of  the  annelids  are  metameric  in  structure.  In 
fact,  only  the  digestive  tract  of  annelids  shows  little  or  no  metamerism. 
Thus,  segmentation  is  much  more  fully  developed  in  the  Annelida  than 
in  any  of  the  other  groups  that  have  been  considered.  Young  annelids 
usually  have  few  segments,  and  add  new  segments  as  they  grow  by  sub- 
dividing the  terminal  segment. 

In  annelids  the  mouth  lies  between  the  first  and  second  segments, 
forming  one  preoral  segment  or  prostomium.  The  brain  originates  in  the 
prostomium,  and  develops  a  pair  of  circumpharyngeal  commissures  that 
reach  around  the  pharynx  to  join  the  ventral  cord,  which  appears  as  a 
chain  of  ganglia,  one  pair  in  each  segment.  The  first  segment  behind 
the  mouth  is  often  different  from  the  rest,  and  is  called  the  peristomium. 
In  counting  segments,  the  prostomium  is  ignored,  and  the  peristomium 
is  counted  as  segment  one. 

Annelids  are  covered  with  a  thin  cuticle  secreted  by  a  simple  epi- 
dermis. Each  segment  has  a  ring  of  circular  muscle  fibers  that  can  con- 

267 


268 


THE  ANIMAL   KINGDOM 


Strict  and  thereby  elongate  the  segment,  and  beneath  this  are  several 
bands  ot  longitudinal  muscles  that  can  produce  shortening  and  thick- 
ening. Various  oblique  fibers  may  also  be  present.  Between  the  body 
wall  and  the  digestive  tract  is  a  spacious  coelom,  divided  by  thin  mus- 
cular septa  between  segments  into  a  series  of  annular  (ring-like)  cavities. 
Each  of  these  originates  as  a  pair  of  lateral  cavities  lined  with  a  delicate 
mesodermal  peritoneum.  The  cavities  become  enlarged  until  they  fill 
the  segment,  but  the  two  peritoneal  sacs  remain  intact,  lining  (1)  the 
body  wall  on  each  side,  (2)  the  septa  before  and  behind,  and  (3)  the  di- 
gestive tract  between  them.  Above  and  below  the  digestive  tract  the  two 
membranes  meet  to  form  the  dorsal  and  ventral  mesenteries.  These  may 
persist  in  the  adult,  but  in  most  species  one  or  both  later  disappear. 

1 1 7.        Classification  of  the  Phylum 

Po/ychaefes.  Most  of  the  marine  annelids  have  eyes,  tentacles  and 
palps  on  the  prostomium,  and  lateral  appendages  on  the  body  segments. 
The  latter  are  flaps  of  the  body  wall,  the  parapodia,  bearing  tufts  of 
many  bristles,  the  chaetae.  These  annelids  are  placed  in  the  class  Poly- 
chaeta  (Fig.   15.1). 

Most  polychaetes  live  near  the  shore  and  on  the  bottom  of  shallow 


Figure  15.1.  Classes  of  the  phylum  Annelida.  Polychaeta:  Nereis  virens,  the  clam- 
worm.  Oligochaeta:  Lumbricus  terrestris,  the  earthworm.  (After  Lawson,  et  al.)  Hiru- 
dinea:  Hirudo  medicinalis,  the  medicinal  leech.  (After  Hegner.) 


PHYLUM    ANNELIDA  969 

seas.  A  few  species  live  in  brackish  or  fresh  water.  They  are  extremely 
diverse  in  their  habits.  Some  live  in  tubes  and  filter  water  for  microscopic 
food  while  others  scrape  up  the  thin  film  of  organic  debris  that  settles 
on  the  bottom.  Most  members  of  the  class  are  predaceous  and  have  stout 
jaws  or  denticles  on  an  eversible  pharynx  that  can  be  used  to  grasp  prey. 

The  sexes  are  separate  and  fertilization  is  external.  Both  eggs  and 
sperm  are  shed  through  tubules  that  connect  each  coelomic  cavity  with 
the  outside.  In  some  species  the  segments  producing  gametes  merely 
burst  to  release  them.  Typically  the  eggs  develop  into  planktonic  larvae 
called  trochophores  that  swim  about  and  feed,  eventually  metamorphos- 
ing into  worms  and  sinking  to  the  bottom.  In  some,  however,  the  eggs 
are  heavily  yolked  and  hatch  directly  into  small  worms. 

O/igochaefes.  Most  of  the  fresh-water  and  terrestrial  annelids  be- 
long to  one  of  two  other  classes.  Those  that  are  wormlike  and  usually 
lack  eyes  or  appendages  on  the  prostomium  belong  to  the  class  Oligo- 
chaeta  (Fig.  15.1).  Parapodia  are  also  absent,  but  each  segment  bears 
small  tufts  of  a  few  chaetae. 

The  oligochaetes  include  the  large  earthworms  and  smaller  aquatic 
worms.  Earthworms  burrow  in  soil  or  leaf-mold,  eating  their  way 
through  the  world,  or  they  live  in  temporary  burrows  from  which  they 
emerge  at  night  to  feed  on  the  surface  of  the  ground.  Aquatic  worms 
burrow  in  mud  or  clamber  on  the  vegetation,  eating  whatever  debris 
they  can  find. 

All  oligochaetes  are  hermaphroditic.  The  testes  are  located  in  a  few 
anterior  segments,  with  the  ovaries  in  a  few  following  segments.  Pairs 
copulate  and  the  eggs  are  fertilized  while  they  are  on  the  outer  surface 
of  the  parent.  Development  is  direct. 

Hirudinea.  The  other  class  of  fresh-water  and  terrestrial  annelids, 
the  Hirudinea  (Fig.  15.1),  includes  the  leeches  or  bloodsuckers,  which 
have  one  large  sucker  surrounding  the  mouth  and  another  at  the  pos- 
terior end  of  the  body.  Leeches  share  many  characteristics  with  the 
oligochaetes,  especially  in  their  reproductive  systems,  but  have  no  ap- 
pendages or  chaetae.  A  few  oligochaetes  are  ectoparasitic  and  have  a 
posterior  sucker  for  attachment  to  the  host.  It  is  generally  believed  that 
the  leeches  evolved  from  the  oligochaetes  through  such  transitional 
forms. 

Archiannelida.  A  few  marine  annelids  are  very  small  and  reduced 
in  their  complexity,  sometimes  with  no  external  segmentation,  or  no 
chaetae,  or  with  the  body  surface  covered  with  cilia  instead  of  a  cuticle. 
These  were  formerly  thought  to  be  primitive  forms  indicating  that  the 
annelids  evolved  either  from  the  flatworms  or  from  trochophore-like 
ancestors,  and  they  were  placed  in  a  fourth  class,  the  Archiannelida. 
These  worms  are  of  particular  interest  as  examples  of  simplification  from 
a  more  complex  ancestor.  Of  the  several  genera  the  most  markedly  sim- 
plified is  Dinophilus  (Fig.  15.2)  which  has  only  five  or  six  segments  and 
no  chaetae  or  parapodia.  Its  general  structure  resembles  that  of  some 
young  polychaete  larvae,  and  it  is  generally  concluded  that  the  archi- 
annelids  are  "reduced"  polychaetes.  It  is  probable  that  the  group  in- 
cludes genera  that  evolved  independently  from  the  polychaetes.  At  the 


270 


THE  ANIMAL   KINGDOM 


-Stomach 
•Moulh  rNepKridium 


Prostomium 


Ciliated  tstnds 


Figure  15.2.  An  example  of  the  class  Archiannelida.  Dinophilus,  a  diminutive  (0.5 
to  2.0  mm.  long)  annelid  that  lacks  external  segmentation  but  has  a  metanieric  arrange- 
ment of  body  organs  typical  of  the  phylum.  Most  of  the  ventral  surface  is  ciliated,  and 
the  animal  has  a  superficial  resemblance  to  a  flatworm.  (After  Meyer.) 

present  time  the  class  is  maintained  as  a  matter  of  convenience  and  not 
because  it  is  thought  to  have  evolutionary  significance  in  the  origin  of 
annelids. 

1 1 8.        Nereis  and  Lumbricus:  Habitat  and  Habit 

Several  species  of  the  polychaete  genus  Nereis  are  called  clamworms 
(Fig.  15.1).  The  common  east  coast  form,  N.  virens,  is  about  one  foot 
long  and  has  a  metallic  green  sheen  on  the  body.  On  the  west  coast  the 
common  species  is  the  somewhat  smaller,  metallic  blue-green  or  brown 
N.  vexillosa.  They  live  in  sand  and  gravel,  constructing  mucus-lined, 
semi-permanent  tunnels  from  which  they  forage  at  dusk.  Nereids  are 
omnivorous,  gobbling  down  plant  and  animal  debris  and  whatever  ani- 
mals they  can  capture.  The  single  pair  of  large  jaws  in  the  eversible 
pharynx  are  adapted  for  capture  but  not  for  chewing.  Food  is  swallowed 
whole. 

The  many  species  of  earthworms  are  difficult  to  distinguish.  The 
common  European  earthworm,  Lumbricus  terrestris  (Fig.  15.1),  is  now 
common  in  the  United  States  also,  and  is  the  favorite  species  for  study. 
It  remains  in  its  burrow  by  day,  coming  out  on  damp  nights  when  it 
can  be  collected  easily.  It  is  largely  herbivorous,  but  acts  as  a  scavenger, 
eating  whatever  organic  debris  is  available. 

These  two  representatives  of  the  polychaetes  and  oligochaetes  will 
be  treated  comparatively.  In  their  gross  appearance  they  are  more  similar 
than  most  polychaetes  and  oligochaetes,  but  in  their  detailed  anatomy 
each  is  a  good  example  of  its  class. 


119.        Nereis  and  Lumbricus:  External  Morphology 

The  prostomium  of  Nereis  (Figs.  15.1  and  15.3)  bears  a  pair  of  small 
tactile  tentacles  and  a  pair  of  stout  palps.  The  palps  are  used  for  ex- 
ploratory probing  and  their  tips  are  very  sensitive  to  touch  and  chemi- 
cals. On  the  dorsal  surface  of  the  prostomium  are  two  pairs  of  black  eyes 


PHYLUM   ANNELIDA 


271 


Figure  15.3.     Lateral  view  of  the  head  of  Nereis  with  the  pharynx  withdrawn  (left) 
and  everted  (right). 

lying  directly  over  the  brain.  Each  eye  is  a  cup  of  modified  epidermal 
cells,  the  ends  of  which  extend  through  a  black  pigment  layer  to  form  a 
retinal  lining  of  light-sensitive  rods.  The  cavity  is  filled  with  a  lens, 
protruding  from  the  cup  as  a  spherical  swelling  covered  by  a  transparent 
layer  of  skin,  the  cornea.  The  eyes  are  directed  upward  and  outward, 
and  are  probably  defensive  in  function,  warning  Xereis  when  a  fish  or 
other  large  predator  approaches  from  above.  Behind  the  eyes  are  a  pair 
of  small  ciliated  pits  believed  to  function  as  chemoreceptors. 

The  prostomium  of  Lumbricus  (Figs.  15.1  and  15.4)  lacks  special 
sense  organs  and  appendages.  It  is  richly  supplied  with  nerve  endings  for 
touch  and  chemoreception,  and  is  used  as  a  muscular  probe  in  burrow- 
ing. Althougli  Lumbricus  lacks  eyes  it  responds  to  light,  generally  moving 
away  from  it.  Certain  large  epidermal  cells  scattered  over  the  back  and 
sides  of  the  body  have  been  shown  to  be  sensitive  to  light. 

The  peristomium  of  Nereis  (Fig.  15.3)  is  actually  two  segments 
fused  together.  Four  pairs  of  tentacular  cirri,  used  as  tactile  organs,  are 
located  at  its  anterior  margin.  The  uppermost  are  the  longest,  and  they 
are  longer  in  males  than  in  females.  The  peristomium  of  Lumbricus 
lacks  appendages. 

The  body  may  be  divided  into  as  many  as  200  segments  in  Nereis, 


,    T      ,  Recepla.ck  por<z.S 

Clifcellu:m       30       Lateral  chae-t^c  w         Prostoipium 

J I  /  I  \\  I 


'  Peristomium. 

F-iCf-retoru-         Ventral  chaeta^  /  -r'\        i    _^^„ 

■porS   ■  Maleioore^   Female  pore 


Figure  15.4.  Lateral  view  of  the  anterior  40  segments  of  Lumbricus.  Reproductive 
openings  are  found  on  segments  9,  10,  14  and  15.  On  each  segment  the  excretory  pore  is 
either  ventral,  near  the  ventral  chaetae,  or  lateral,  above  the  lateral  chaetae,  with  much 
variability  between  worms. 


272 


THE   ANIMAL   KINGDOM 


NEREIS 
Doi\=;al  blood,  vessel 


Epidermis 
Circular  muscle 
Lon^.  rnuscle 

Sept  una 


Aciculum 
Chaetac 

BloodL vessel 5-  (Labzral)nVentral)'  ^Nerve  cord 

Epidermi 
Circular  muscl 
Lon6iludinal  mu-Scle 
Sc-ptum 

Chaeta  prolractor. 
Chacta-  retracl;oi 


topodium 


Ncuropodium. 
Vcnlral  cirrus 
Opening  in-to  next  Segment 


Blood  vessels :  (Venlral) 


orsal  bloodvessel 
Chlora^en  cells 
Typhlosole 
Nephridium 
Iritcstine 
•Opening  irrbo  next  Serine nl 

ly^Crztory  pore, 
.    ,  x^c-jeral  nerve 

^^       Nerve  coi'd 
LUMBRICUS 

Figure  15.5.  Cross  sections  of  Nereis  (above)  and  Lumbricus  (below).  Each  is  a  seg- 
ment viewed  from  in  front,  with  the  septmn  behind.  In  Nereis,  on  the  left  side  the  body 
wall  has  been  cut  back  to  show  the  internal  structure  of  the  parapodium.  In  Lumbricus 
the  body  wall  is  cut  at  the  le\el  of  the  excretory  pore  on  the  right  side,  and  further  back 
at  the  level  of  the  chaetae  on  the  left  side. 

180  in  Lumbricus.  Young  worms  have  fewer  segments  and  apparently 
new  ones  are  added  posteriorly  throughout  life.  A  mid-dorsal  Hne  indi- 
cates the  underlying  dorsal  blood  vessel,  and  a  mid-ventral  line  indicates 
the  position  of  the  ventral  nerve  cord.  These  lines  are  faint  in  Lum- 
bricus. Both  species  are  more  heavily  pigmented  above  than  below. 

Every  body  segment  of  Nereis  except  the  peristomium  bears  a  pair 
of  parapodia  (Fig.  15.5),  each  of  which  is  divisible  into  a  dorsal  noto- 
podium  and  ventral  neuropodium.  Each  portion  has  several  lobes  and 
bears  a  tuft  of  many  chaetae.  A  slender,  tactile  dorsal  cirrus  projects  up 
from  the  notopodium,  and  a  ventral  cirrus  extends  down  from  the  base 
of  the  neuropodium.  The  upper  lobes  of  the  notopodia  are  large  and 
richly  vascularized,  serving  as  gills.  Internally,  each  tuft  of  chaetae  clusters 


PHYIUM   ANNELIDA  273 

around  a  single  stout  aciculum  to  which  numerous  small  muscles  are 
attached.  The  chaetae  and  acicula  are  made  of  chitin,  which  resembles 
the  material  that  forms  the  exoskeletons  of  arthropods. 

For  walking  each  parapodium  is  extended  forward  and  downward, 
moved  backward,  withdrawn,  then  moved  upward  and  forward  again. 
In  walking,  the  movements  of  the  parapodia  of  each  segment  are  slightly 
ahead  of  those  on  the  next  anterior  segment,  producing  the  appearance 
of  waves  of  motion  that  pass  forward  along  the  sides. 

In  Lumbricus  every  body  segment  except  the  peristomium  bears 
four  pairs  of  chaetae  (Fig.  15.5),  each  of  which  has  small  muscles  that 
can  move  it  out  or  in,  and  slant  it  forward  or  backward.  The  location 
of  pairs  corresponds  with  the  location  of  the  notopodia  and  neuropodia 
of  the  polychaete.  These  chaetae  are  used  for  gripping  the  sides  of  the 
burrow,  to  assist  locomotion.  They  can  be  slanted  forward  or  backward 
to  help  the  worm  resist  being  pulled  from  the  burrow. 

The  anus  is  located  on  the  terminal  segment,  which  always  remains 
the  terminal  segment  as  new  segments  are  formed  from  its  anterior  edge. 
In  Nereis  the  parapodia  of  this  segment  are  reduced  to  a  pair  of  ventral 
cirri  (Fig.  15.1)  which  are  longer  than  those  of  other  segments  and  func- 
tion as  a  pair  of  posterior  tentacles. 

120.        Nereis  and  Lumbricus:  Body  Wall 

The  body  wall  is  made  of  the  same  layers  in  both  species  (Fig.  15.5). 
The  epidermis  of  Lumbricus  has  more  sensory  cells  than  that  of  Nereis, 
a  reflection,  perhaps,  of  the  lack  of  sense  organs.  The  musculature  is 
better  developed  in  Lumbricus.  In  Nereis  the  circular  layer  thins  out 
dorsally  and  ventrally,  while  in  Lumbricus  it  remains  relatively  thick. 
The  longitudinal  muscles  in  Nereis  are  restricted  to  four  bands,  whereas 
in  Lumbricus  they  form  a  nearly  continuous  layer.  The  two  musculatures 
are,  however,  very  similar  in  general  plan. 

The  muscles  are  used  differently  in  the  two  species.  Nereis  walks 
with  its  parapodia,  but  often  assists  them  with  lateral  undulations 
of  the  body  that  pass  as  waves  forward  along  the  body.  Nereis  can 
also  swim,  and  then  these  undulations  simply  become  more  vigorous.  In 
its  burrow  Nereis  circulates  water  by  vertical  undulations  of  the  body, 
the  waves  passing  backward  along  the  body  to  draw  water  in  from  the 
front.  All  of  these  sinuous  movements  involve  the  longitudinal  muscles, 
which  act  alternately  within  a  given  segment,  contracting  first  on  one 
side  and  then  on  the  other.  The  circular  muscles  are  used  to  increase 
the  length  of  the  body,  and  are  used  with  the  other  muscles  in  digging. 

Lumbricus  crawls  forward  by  extending  the  body,  gripping  the 
surface  with  its  chaetae,  and  then  shortening  the  body.  As  it  moves, 
coordinated  waves  of  extension  and  contraction  pass  posteriorly  along 
the  body.  The  pattern  can  be  reversed  so  that  the  waves  pass  forward, 
in  which  case  Lumbricus  crawls  backward.  Movement  in  the  burrow  is 
similar  but  more  efficient,  since  the  entire  circumference  of  the  worm 
can  be  used  for  gripping.  In  all  of  these  movements  the  muscles  of  a 


274  '^W^   ANIMAL   KINGDOM 

given  segment  act  together.  All  of  the  longitudinal  muscles,  or  all  of  the 
circular  muscles,  contract  at  a  given  moment.  Independent  movement  of 
the  muscles  on  one  side  occurs  only  as  the  worm  turns. 

121.        Nereis  and  Lumbrkus:  Nervous  System 

The  large  bilobed  brain  is  in  the  prostomium  of  Nereis,  but  mi- 
grates posteriorly  in  the  Lumbricus  embryo  to  lie  in  the  third  segment. 
Many  small  nerves  extend  to  all  parts  of  the  anterior  end  of  the  body. 
Paired  circumpharyngeal  commissures  pass  down  around  the  anterior 
end  of  the  pharynx  to  join  the  subpharyngeal  ganglion.  This  is  also 
bilobed;  it  is  formed  in  Nereis  by  the  ventral  ganglia  of  the  peristomium 
(two  fused  segments),  and  in  Lumbricus  by  a  fusion  of  the  ganglia  of 
the  first  three  segments.  The  whole  ventral  nervous  system  arises  as  a 
pair  of  longitudinal  cords,  but  these  fuse  together  to  make  an  apparently 
unpaired  ventral  cord.  In  each  segment  behind  the  peristomium  the 
cord  thickens  to  form  a  ganglion,  from  which  nerves  emerge  to  supply 
that  segment.  In  most  segments  an  additional  pair  of  nerves  passes  for- 
ward to  the  body  wall  of  the  next  anterior  segment. 

Locomotor  activity,  indeed  all  activities  that  pass  in  waves  along 
the  body,  are  coordinated  locally  by  the  ventral  ganglia.  A  series  of 
reflexes  coordinate  movements  so  that  what  happens  in  one  segment 
will  occur  a  moment  later  in  the  next.  This  coordination  is  achieved 
both  by  direct  neural  connections  and  by  the  tensions  produced  in  one 
segment  by  movement  in  the  adjoining  one.  The  entire  system  is  so 
constructed  that  an  activity  beginning  at  one  end  of  the  body  will  pass 
automatically  along  its  length.  Hence  adding  more  segments  does  not 
noticeably  increase  the  complexity  of  movement. 

Annelids  may  respond  to  an  alarm  with  a  sudden  violent  shortening 
of  the  entire  body.  Both  Nereis  and  Lumbricus  keep  the  posterior  end  of 
the  body  in  their  burrows  as  they  forage,  and  this  sudden  shortening  is 
sufficient  to  pull  the  entire  body  back  into  the  hole.  Such  a  response 
cannot  be  handled  by  the  usual  ventral  nervous  system  with  its  numerous 
ganglia  and  many  synapses  along  the  length  of  the  body.  Conduction  is 
very  slow  in  this  system;  an  impulse  requires  as  much  as  10  seconds  to 
travel  the  length  of  a  worm  10  inches  long.  For  the  alarm  response 
annelids  have  giant  axons,  nerve  fibers  of  large  diameter  that  run  the 
length  of  the  ventral  cord.  Nereis  has  three  central  fibers  and  a  pair  of 
larger  lateral  fibers;  Lumbricus  has  one  very  large  central  fiber  and  a 
pair  of  smaller  laterals.  The  speed  of  conduction  along  a  nerve  fiber  has 
been  found  to  depend  upon  its  diameter.  These  fibers  are  not  only  large, 
but  some  of  them  extend  the  full  length  of  the  body  without  synapses. 
Conduction  along  the  giant  fibers  requires  only  a  hundredth  of  a  second 
to  travel  10  inches.  In  the  earthworm,  T.  H.  Bullock  has  fovmd  that  the 
median  fiber,  which  is  the  fastest,  is  activated  by  sensory  information 
from  the  first  40  segments  of  the  body,  whereas  the  lateral  fibers  respond 
to  sensations  from  segments  posterior  to  this. 

Giant  fibers  are  excellent  material  for  physiological  research,  and 
have  been  used  extensively  in  studies  of  the  nerve  impulse.  They  are 


PHYLUM   ANNELIDA 


275 


found  in  the  mantle  of  the  squid  and  in  arthropods  and  certain  other 
animals  as  well  as  in  the  annelids. 

The  brain  and  subpharyngeal  ganglion  govern  the  nervous  system, 
initiating  and  controlling  bodily  activities.  If  the  brain  is  removed  the 
worm  becomes  more  active  than  before,  and  moves  about  ceaselessly. 
This  indicates  that  the  brain  functions  in  part  as  an  inhibitory  center.  If 
the  subpharyngeal  ganglion  is  destroyed,  all  spontaneous  activity  stops, 
and  the  worm  moves  momentarily  only  if  it  is  touched.  This  ganglion 
originates  the  impulses  responsible  for  such  activity.  Separation  of  in- 
hibitory and  stimulatory  centers  in  the  central  nervous  system  is  known 
only  in  the  annelids,  arthropods  and  chordates. 


122.        Nereis  and  Lumbrkus:  Digestive  System 

The  mouth  opens  into  a  muscular  pharynx  which  occupies  several 
segments.  In  Nereis  muscles  extending  from  the  prostomium  to  the  back 
of  the  pharynx  can  pull  it  forward,  everting  it  through  the  mouth  (Figs. 
15.3  and  15.6).  Muscles  from  the  gut  to  the  body  wall  several  seg- 
ments back  can  pull  it  in  again.  In  the  middle  of  the  nereid  pharynx 
are  numerous  small  denticles  and  one  pair  of  large  jaws.  The  jaws  lie 
open  at  the  anterior  limit  of  the  everted  pharynx.  To  attack  prey  the 
pharynx  is  everted  by  its  muscles  and  by  a  constriction  of  the  body  until 


Dorsal  Relraclor  Pharynx- 

bloodvessel-!    muscles     -tsophagus 


Brain 


'-Proslomlum 


Cord.-*  Di6< 


Mouth 
^  _/^Circumph  aryn^eal 
commissure 


Ventral  bl.v.-^  Nerve  Cord-"  Digestive  pouch        Satpharyngcal  gauglion. 

NEREIS 


Intestine' 


"Dorsal  blood  ves, 
Hearts 

^Esopba^as 


Ventral        IMerve. 
blood  ves.     cord 


CalcifcrouS 
glands 


Brain 
S'xrProstomiuTn 

'^W.^^Mouth 

Circumpbaryngeal 
CoramiSSure, 


'Digestive  Suhphsryn. 
pouch        oan^lion 

LUMBRICUS 

Figure  15.6.     Lateral  views  of  Nereis  (above)  and  Lumbricus  (below)  with  the  right 
body  wall  removed.  The  digestive,  circulatory  and  nervous  systems  are  shown. 


276  '■"^   ANIMAL   KINGDOM 

the  jaws  open.  As  the  pharynx  is  retracted  the  jaws  close  scissorswise 
and  the  denticles  grip  the  prey,  dragging  it  back  into  the  middle  of  the 
pharynx  by  the  time  it  is  luUy  withdrawn. 

The  pharynx  in  Liunbricus  (Fig.  15.6)  is  more  bulbous  and  is  at- 
tached to  the  body  wall  by  numerous  radiating  muscles.  When  these 
muscles  contract,  the  cavity  ot  the  pharynx  is  suddenly  enlarged,  pro- 
ducmg  suction  at  the  mouth. 

The  pharynx  leads  to  a  tubular  esophagus  into  which  a  pair  of 
glandular  digestive  pouches  open.  These  pouches  apparently  secrete 
digestive  enzymes.  In  Lumbricus  two  pairs  of  calciferous  glands  open 
behind  the  pouches.  Their  function  is  not  definitely  known. 

The  rest  of  the  digestive  system  in  Nereis  is  a  simple  long  intestine 
ending  at  a  short  rectum  in  front  of  the  anus.  The  diameter  of  the  intes- 
tine is  smallest  in  the  middle  of  each  segment,  and  sharply  expanded  at 
each  septum.  The  moderately  muscular  walls  have  a  lining  of  simple 
gastrodermis,  a  layer  of  circular  muscles,  longitudinal  muscles,  and  a 
covering  peritoneum.  During  digestion  food  is  moved  posteriorly  by 
peristaltic  waves  of  contraction  in  the  two  muscle  layers.  The  intestine 
is  suspended  in  the  coelom  at  each  septum.  The  mesenteries  have  largely 
disappeared,  remaining  as  bands  of  delicate  muscle  fibers  dorsally  and 
ventrally  in  the  posterior  part  of  each  segment. 

In  Lumbricus  the  esophagus  ends  in  an  expanded  storage  chamber, 
the  crop.  Behind  the  crop  a  muscular  gizzard  mills  the  food  to  a  fine 
pulp  before  it  is  passed  onto  the  intestine  where  it  is  digested,  lire 
intestinal  wall  has  the  same  layers  as  that  of  Nereis,  but  with  much 
thinner  musculature.  The  intestinal  diameter  is  greatest  in  the  middle 
of  each  segment,  with  moderate  constrictions  at  each  septum.  The  intes- 
tine is  infolded  dorsally,  forming  externally  a  groove  and  internally  a 
ridge,  the  typhlosole  (Fig-  15.5),  which  increases  the  absorptive  surface. 
The  intestine  terminates  in  a  short  rectum  and  anus.  The  peritoneum 
surrounding  the  intestine  in  Lumbricus  is  modified  to  form  a  glandular 
layer,  the  chloragen  cells.  These  extract  wastes  from  the  blood,  and  later 
become  detached  and  float  in  the  coelom.  Ultimately  much  of  their  sub- 
stance is  engulfed  by  ameboid  cells  and  carried  to  the  skin  where  it  is 
deposited  as  pigment. 

As  in  most  animals,  the  mouth,  pharynx  and  rectum  are  lined  with 
an  epidermis  of  ectodermal  origin.  In  the  annelids  this  epidermis  se- 
cretes a  cuticle  which  is  continuous  with  that  covering  the  body. 

123.        Nereis  and  Lumbricus:  Circulatory  System 

The  annelid  circulatory  system  is  well  developed.  A  system  of  large 
vessels  pumps  the  blood  through  capillary  beds  that  invade  all  of  the 
tissues.  The  blood  is  collected  into  a  longitudinal  dorsal  vessel  (Fig. 
15.6)  and  distributed  from  a  longitudinal  ventral  vessel.  At  the  anterior 
end  several  pairs  of  commissures  around  the  pharynx  and  esophagus 
connect  the  two  vessels.  Waves  of  contraction  force  the  blood  forward 
through  the  dorsal  vessel,  down  the  commissures,  and  posteriorly  through 
the  ventral  vessel.  In  Nereis  the  dorsal  vessel  is  the  most  powerful  pump, 


PHYLUM   ANNELIDA  277 

while  in  Lumbricus  the  commissures  are  enlarged  and  muscular,  func- 
tioning as  "hearts."  Beneath  the  ventral  vessel  small  longitudinal  vessels 
parallel  the  nerve  cord,  and  carry  blood  posteriorly. 

In  each  body  segment,  the  ventral  vessel  gives  off  paired  branches 
to  the  body  wall  and  median  unpaired  branches  to  the  intestine.  The 
dorsal  vessel  receives  similar  branches.  Some  of  the  blood  passes  close  to 
the  intestinal  gastrodermis  where  it  picks  up  nutrients,  some  passes  be- 
neath the  skin  (and  in  Nereis  through  the  parapodia)  where  it  is 
oxygenated,  and  a  little  passes  to  the  nephridia,  giving  up  wastes.  All  of 
this  blood  is  mixed  together  dorsally  in  each  segment. 

In  addition,  a  pair  of  commissures  in  each  segment  carry  some  blood 
directly  from  ventral  to  dorsal  vessels.  In  the  middle  region  of  the  body 
these  are  small  and  tortuous,  forcing  most  of  the  blood  through  the 
capillaries,  but  posteriorly  they  are  more  prominent,  permitting  a  fairly 
free  flow  of  blood  around  and  around  the  whole  circulatory  system. 

The  blood  contains  dissolved  hemoglobin  which  greatly  facilitates 
the  transport  of  oxygen  and  carbon  dioxide  (p.  90). 

The  major  advance  of  the  annelid  system  over  that  of  the  nemer- 
teans  is  the  addition  of  the  capillary  networks,  a  much  more  finely 
branched  system  which  is  an  efficient  mechanism  for  distribution. 

1 24.  Nereis  and  Lumbricus:  Excretory  System 

Each  segment  except  the  first  and  last  contains  a  pair  of  metaneph- 
ridia,  convoluted  tubules  lying  in  a  vascularized,  glandular  mass  of 
tissue.  The  mass  lies  at  the  base  of  each  neuropodium  in  Nereis  and 
against  the  anterior  sejJtum  in  Lumbricus.  From  each  nephridium  the 
tubule  extends  forward  through  the  septum  to  open  as  a  ciliated  funnel 
in  the  coelom  of  the  next  anterior  segment.  The  other  end  of  the  tubule 
opens  to  the  exterior  at  the  minute  excretory  pore. 

The  funnel  collects  coelomic  fluid,  including  some  debris  from  the 
chloragen  cells,  and  passes  it  down  the  tubule.  Along  the  way  the  fluid 
is  modified  so  that  only  waste  remains  in  the  portion  excreted.  In  Lum- 
bricus a  terminal  expansion  of  the  tubule  forms  a  bladder. 

125.  Nereis  and  Lumbricus:  Reproduction 

The  reproductive  systems  of  polychaetes  and  oligochaetes  are  very 
different.  Gonads  appear  in  Nereis  only  during  the  breeding  season, 
developing  from  the  peritoneum  lining  the  ventral  body  wall  in  many 
of  the  segments.  Eggs  or  sperm  accumulate  in  the  coelomic  cavities  and 
are  eventually  shed  through  temporary  ruptures  of  the  body  wall.  Fer- 
tilization is  left  to  chance  in  the  open  sea  water. 

In  some  species  of  Nereis,  and  in  many  other  polychaetes,  the  gonads 
appear  in  the  posterior  half  of  the  body,  which  becomes  considerably 
modified  as  the  gametes  accumulate.  The  parapodia  develop  foliaceous 
outgrowths  and  the  chaetae  become  larger  and  often  flattened.  The  eyes 
may  become  temporarily  enlarged.  On  the  night  of  breeding  the  individ- 
uals leave  their  burrows  and  swim  to  the  surface,  the  enlarged  parapodia 


278 


THE  ANIMAL   KINGDOM 


Seminal  vesicle-S 


Nepliridiurrr'  FunneP 
Sperm  ducV 


Ovary 
Oviduct 


Sperm  receptacles 
Spe-rm  reservoirs 


Figure  15.7.  Lateral  view  of  Lumbricus  (see  Fig.  15.6)  with  many  of  the  viscera  re- 
moved. Reproductive  and  excretory  systems  are  shown.  The  testes  lie  inside  the  sperm 
reservoirs.  Compare  with  Figure  15.4  for  the  external  openings. 

serving  not  only  as  better  oars  but  as  better  gills  for  increased  activity. 
After  the  body  wall  ruptures  and  the  gametes  are  shed,  the  worms  settle 
to  the  bottom  again  and  recover  their  former  morphology  and  habits. 

Reproduction  in  Lumbricus  is  considerably  more  complex  (Figs. 
15.4  and  15.7).  Segments  10  and  11  each  contain  a  pair  of  testes  in 
isolated  median  cavities  of  the  coelom,  the  sperm  reservoirs.  These  two 
reservoirs  have  three  pairs  of  prominent  lateral  pouches,  the  seminal 
vesicles,  that  extend  into  the  9th,  10th  and  11th  segments.  Sperm  elab- 
orated in  the  testes  are  shed  into  the  reservoirs  and  vesicles  where  they 
are  stored  in  large  numbers.  From  the  reservoirs  two  pairs  of  sperm 
funnels  collect  sperm  and  pass  them  posteriorly  through  a  pair  of  sperm 
ducts  to  the  male  pores  on  the  ventral  side  of  the  1 5th  segment. 

The  single  pair  of  minute  ovaries  are  in  the  13th  segment,  where 
eggs  are  shed  into  the  coelomic  cavity.  At  oviposition  the  eggs  are  col- 
lected by  a  pair  of  egg  funnels  and  passed  through  short  oviducts  to 
the  ventral  female  pores  on  the  14th  segment.  Two  pairs  of  seminal 
receptacles  in  the  9th  and  10th  segments  open  laterally  at  the  posterior 
septa.  Sperm  received  during  copulation  are  stored  here. 

The  female  system  also  includes  a  clitellum,  a  swollen  glandular 
region  of  the  epidermis  (segments  32-37).  During  copulation  two  worms 
facing  in  opposite  directions  press  their  ventral  surfaces  together  so  that 
the  clitellum  of  one  is  opposite  segment  10  of  the  other  (Fig.  15.8).  The 
chaetae  of  one  may  pierce  the  body  wall  of  the  other,  and  they  are  also 
glued  together  by  thick  mucous  secretions  of  the  clitellum  and  skin. 
These  secretions  form  grooves  between  the  worms  so  that  sperm  extruded 
on  the  15th  segment  pass  posteriorly  along  the  mucus  to  the  clitellum 
where  they  enter  the  seminal  receptacles  of  the  other  worm. 

Soon  after  copulation  the  clitellum  secretes  a  membranous  cocoon 
and  beneath  this  an  albuminous  secretion.  The  worm  may  then  lay 
several  eggs  that  pass  back  into  the  cocoon,  or  the  cocoon  may  slip  for- 
ward along  the  body  so  that  the  eggs  are  laid  directly  into  it  as  it  passes. 
The  cocoon  is  then  moved  forward  and  the  eggs  are  fertilized  as  they 
pass  the  sperm  receptacles.  Finally  the  cocoon  is  slipped  off  the  head, 


PHYLUM   ANNELIDA  279 


Figure  15.8.     Two  earthworms  copulating.    (Photograph  of  living  animals  made  at 
night,  courtesy  General  Biological  Supply.  Chicago,  111.) 


and  the  openings   in   it  constrict  to  produce  a  spindle-shaped  capsule. 
The  eggs  develop  into  tiny  worms  which  later  emerge  from  the  cocoon. 

126.        Reproductive  Periodicity  and  Palolo  Worms 

External  fertilization  like  that  of  the  polychaetes  is  Usually  accom- 
panied by  a  coordinating  behavioral  mechanism  that  will  ensure  fer- 
tilization. Many  such  organisms  respond  to  rhythms  in  the  environment 
to  achieve  this  coordination.  In  the  oceans  three  such  rhythms  are  domi- 
nant. Seasonal  cycles  produce  variations  in  temperature,  length  of  day 
and  food.  Lunar  cycles  produce  variations  in  the  height  of  tides,  strength 
of  currents,  the  relation  bet^\'een  tide  and  the  hour  of  the  day,  and  the 
amount  of  night  light.  Diurnal  cycles  produce  the  obvious  great  varia- 
tion in  light  from  day  to  night.  Several  species  of  Nereis  use  all  three 
of  these  rhythms  to  achieve  reproductive  periodicity. 

In  a  common  Atlantic  nereid  (Platynereis)  the  adults  become  sex- 
ually mature  only  in  the  summer  months,  some  individuals  breeding 
several  times  in  one  season.  During  this  season  they  reach  sexual  ma- 
turity only  during  the  second  and  third  weeks  after  the  new  moon, 
possibly  because  during  this  time  the  moon  is  bright  and  shines  much 
of  the  night,  providing  the  dim  light  in  which  nereids  will  feed.  The 
actual  moment  of  breeding  depends  upon  the  diurnal  cycle.  They  will 
breed  only  after  dark,  but  only  if  the  moon  is  not  yet  risen.  Thus,  worms 


280  ^"^   ANIMAL   KINGDOM 

reaching  maturity  during  the  second  week  will  not  breed  unless  the 
night  is  cloudy,  and  usually  are  forced  to  wait  some  time.  In  the  third 
week,  alter  the  full  moon,  a  period  of  darkness  separates  sunset  and 
moonrise,  and  nightly  during  this  period  of  darkness  large  numbers  of 
nereids  swarm  to  the  surface  to  breed.  By  compressing  the  shedding 
of  gametes  into  this  hour  or  so  in  the  third  week  of  each  lunar  month, 
enough  worms  breed  at  the  same  time  to  guarantee  fertilization  of  the 
eggs.  Other  nereids  have  different  lunar  cycles. 

Other  worms  may  use  the  same  external  rhythms,  but  respond  differ- 
ently to  them  and  thus  have  different  behavioral  rhythms.  A  remarkable 
example  of  periodic  reproduction  is  found  in  the  Palolo  worms,  a  species 
of  polychaete  living  on  coral  reefs  in  the  south  Pacific.  Over  90  per 
cent  of  the  population  breeds  within  a  single  two-hour  period  of  the 
entire  year.  The  seasonal  rhythm  limits  the  reproductive  period  to  about 
a  month,  the  lunar  rhythm  to  a  day,  and  the  diurnal  rhythm  to  a  couple 
of  hours  after  complete  darkness.  The  major  swarm  occurs  in  November 
during  the  last  quarter  of  the  moon  when  the  low  tide  is  unusually  low. 
This  is  the  spring  rainy  season  in  this  region.  A  smaller  swarm  usually 
occurs  four  weeks  earlier,  at  the  previous  neap  tide,  and  a  different 
species  of  annelid  always  swarms  the  night  before  the  Palolo. 

The  posterior  half  of  the  Palolo  worm  not  only  becomes  different 
from  the  anterior  half,  but  actually  breaks  off.  On  the  night  of  breeding 
individuals  back  out  of  their  holes  and  the  posterior  half  twists  counter- 
clockwise until  it  breaks  free.  It  then  swims  backward  to  the  surface. 
Each  segment  has  a  pair  of  eyes  beneath  the  parapodia,  so  that  broken 
pieces  will  still  swim  appropriately.  After  swimming  at  the  surface  for  a 
few  minutes  they  burst,  shedding  eggs  or  sperm  and  leaving  a  rapidly 
disintegrating  body. 

These  posterior  halves  packed  with  gametes  are  frantically  collected 
in  dip  nets  by  the  island  natives  during  the  brief  period  when  they  are 
available.  They  are  made  into  a  thick  soup  said  to  taste  like  spinach. 
The  natives  have  learned  to  predict  when  the  Palolo  will  swarm  and 
lookouts  camp  on  the  shores  at  the  right  season  to  watch  the  water  daily. 
When  the  water  is  suddenly  full  of  spume  and  debris,  apparently  be- 
cause extreme  tides  produce  severe  wave  action  on  the  reefs,  swarming 
will  follow  in  two  days. 

Reproductive  periodicities  are  found  in  many  other  animals.  The 
oysters  described  earlier  are  also  coordinated  by  the  integration  of  sea- 
sonal and  lunar  rhythms,  and  several  arthropods  and  fishes  follow  tidal 
cycles  in  their  behavior. 

127.        Earthworms  and  the  Soil 

Although  earthworms  usually  forage  on  the  surface  from  temporary 
burrows,  they  also  dig  extensively,  as  much  as  one  or  two  feet  beneath 
the  surface.  Much  of  the  dirt  is  eaten  and  later  deposited  on  the  surface 
as  castings.  They  also  pick  up  debris  while  foraging  and  carry  it  below 
the  ground,  and  at  dawn  may  pull  sticks  and  leaves  into  their  burrows 
for  concealment. 


PHYLUM    ANNELIDA  281 

Darwin  noted  the  abundance  of  earthworms  in  fields  and  estimated 
that  there  are  some  64,000  earthworms  per  acre.  He  then  speculated  on 
the  effect  that  so  many  worms  would  have,  and  concluded  that  they  are 
possibly  the  most  important  organism  influencing  the  soil.  According  to 
his  calculations  earthworms  will  bring  to  the  surface  two  inches  of  dirt 
every  ten  years.  This  not  only  mixes  the  soil,  but  slowly  buries  rocks  and 
other  large  objects.  While  such  claims  are  now  challenged,  it  cannot  be 
doubted  that  earthworms  are  an  important  agent  in  the  conditioning  of 
soil.  Their  burrows  help  to  aerate  the  soil  and  permit  water  to  enter 
easily  during  rain.  The  constant  mixing  of  soil  and  organic  debris  con- 
tributes to  the  development  of  good  humus. 

1 28.  Other  Annelid  Worms 

One  of  the  largest  annelids  (15  or  more  inches  long)  is  the  lugworm, 
a  polychaete  that  burrows  in  muddy  sand  at  the  level  of  low  tide.  The 
pharynx  is  everted  into  the  sand  and  then  withdrawn  with  its  load. 
Organic  debris  in  the  sand  serves  as  food  which  is  removed  as  the  sand 
passes  through  the  digestive  tract.  Although  the  body  is  long  and  thick, 
it  is  composed  of  relatively  few  segments.  The  parapodia  are  variously 
modified,  and  they  are  missing  from  the  first  two  and  the  last  several 
segments.  The  notopodia  and  neuropodia  are  separated  widely.  The  last 
several  notopodia  bear  feathery  gills. 

The  small  polychaete  Hydroides  builds  twisted  calcareous  tubes  on 
shells  and  rocks.  The  prostomium  bears  a  pair  of  large  ciliated  feathery 
"gills"  that  are  not  only  respiratory,  but  also  serve  as  a  device  for  catch- 
ing food  particles. 

Some  fresh-water  oligochaetes  have  more  chaetae  than  the  earth- 
worm, but  otherwise  they  tend  to  have  simplified  organ  systems.  Tubifex 
is  a  small  red  worm  that  lives  in  the  mud  beneath  standing  or  running 
water.  Large  numbers  often  form  red  patches.  Each  worm  lives  head 
down,  foraging  deep  for  food,  while  the  posterior  end  is  waved  cease- 
lessly above  the  mud  for  respiration.  The  amount  of  worm  projecting 
from  the  mud  reflects  inversely  the  amount  of  oxygen  dissolved  in  the 
water. 

Aeolosoma  is  a  microscopic  oligochaete  1  to  5  mm.  long.  The  body 
wall  contains  numerous  red,  yellow  and  green  globules  that  give  it  a 
clownlike  appearance.  It  clambers  about  on  fresh-water  vegetation,  gath- 
ering minute  debris  with  its  ciliated  prostomium. 

A  number  of  worms  can  reproduce  asexually  like  the  planarians. 
New  individuals  are  budded  posteriorly,  usually  forming  the  head  before 
detachment.  The  polychaete  Autolytus  may  have  several  offspring  bud- 
ding at  one  time.  Many  of  the  fresh-water  oligochaetes,  including 
Aeolosoma,  reproduce  in  this  way. 

129.  Class  Hirudinea 

Bloodsuckers  are  annelids  modified  for  an  ectoparasitic  existence. 
The  body  is  stout  and  bears  a  large,  powerful  sucker  on  each  end  for 


282  ^^^   ANIMAL   KINGDOM 

attachment  to  the  host.  They  creep  by  moving  the  posterior  sucker  up 
close  to  the  anterior  one,  and  then  stretching  the  anterior  sucker  for- 
ward. They  also  swim  well  by  vertical  undulations  of  the  flattened  body. 
Their  powerful  suction  is  known  to  anyone  who  has  tried  to  pull  a  leech 
off  his  skin.  Most  leeches  live  in  fresh  water,  feeding  on  fish,  amphibians 
and  other  animals.  In  the  absence  of  blooded  prey  most  leeches  can 
subsist  indefinitely  on  small  worms  and  arthropods  which  they  capture 
and  swallow  whole.  Once  leeches  find  blood,  however,  they  take  enough 
to  last  for  weeks. 

The  suckers  are  not  used  for  sucking  blood,  but  only  for  attach- 
ment. In  the  mouth  are  three  cutting  teeth  that  make  a  Y-shaped 
incision  in  the  skin.  Numerous  small  salivary  glands  around  the  mouth 
secrete  a  substance  that  prevents  the  coagulation  of  blood.  This 
substance,  hirudin,  is  commercially  extracted  from  leeches  and  used 
medicinally  when  anticoagulants  are  indicated.  Once  assured  of  a  con- 
tinuing flow  of  blood,  the  leech  sucks  with  a  powerful  pharynx  built  like 
that  of  the  earthworm  with  radiating  muscles  to  the  body  wall.  The 
esophagus,  which  in  the  earthworm  forms  a  modest  crop,  in  the  leeches 
is  expanded  into  an  enormous,  branched  crop  that  fills  much  of  the  body 
and  which  can  be  greatly  distended.  Blood  is  stored  here  during  feeding, 
and  over  the  following  weeks  trickles  slowly  into  the  small  stomach  and 
on  into  the  intestine  that  ends  in  a  short  rectum  and  anus. 

The  other  organ  systems  are  similar  to  those  already  described  for 
Nereis  and  Lumbricus,  except  that  the  coelom  is  secondarily  reduced  by 
the  invasion  of  loose  connective  tissue  to  a  series  of  sinuses  that  become 
connected  with  the  circulatory  system.  The  circulatory  system  includes 
longitudinal  vessels  and  networks  of  capillaries,  but  the  capillaries  of  the 
skin,  containing  oxygenated  blood,  drain  into  the  sinuses.  These  sinuses 
parallel  the  digestive  tract  and  the  ventral  nerve  cord. 

The  body  is  composed  of  a  fixed  number  of  segments  (36  in  the  large 
medicinal  leech)  each  of  which  is  superficially  subdivided  into  several 
rings,  giving  the  external  appearance  of  many  more  segments. 

The  male  reproductive  system,  comparable  to  that  of  the  oligo- 
chaetes  terminates  at  a  single  median  duct  that  opens  on  the  11th  seg- 
ment through  a  curved,  muscular,  eversible  penis.  Seminal  receptacles 
are  absent  from  the  female  system.  The  oviducts  terminate  at  a  single 
median  duct  that  opens  on  the  12th  segment  as  a  vagina.  Mutual  cross 
fertilization  is  followed  by  the  secretion  of  a  cocoon  (by  the  9th  to  11th 
segments)  into  which  eggs,  sperm  and  albuminous  fluid  are  placed.  The 
cocoon  is  slipped  off  the  head  and  attached  to  a  rock.  The  fertilized 
eggs  develop  into  tiny  leeches  which  eventually  hatch  from  the  cocoon. 
Some  of  the  larger  leeches  attach  the  cocoons  to  the  ventral  surface  of 
the  body,  and  after  the  young  emerge  they  remain  attached  to  the  parent 
for  some  time. 

In  moist  tropical  forests  leeches  are  terrestrial.  They  climb  up  the 
vegetation  and  stand  with  the  posterior  sucker  attached,  and  the  anterior 
end  held  over  a  pathway,  waiting  for  some  mammal  to  go  by.  They 
sometimes  occur  in  stich  numbers  as  to  pose  a  serious  threat  to  animals 
because  of  the  amount  of  blood  they  can  remove  in  a  short  time. 


PHYLUM    ANNELIDA 


283 


130.        The  Relationships  of  Annelids,  Molluscs  and  Arthropods 

Adult  annelids  and  moUviscs  differ  markedly  in  appearance  (com- 
pare Figs.  14.2  and  15.1).  Even  if  diagrammatic  representations  of  the 
phyla  are  compared  (Fig.  15.9  A  and  C),  they  have  little  in  common. 
The  annelid  coelom  is  spacious  (Fig.  15.9  B)  whereas  that  of  the  molluscs 
is  small.  The  annelid  "heart"  is  not  a  distinct  organ;  it  includes  the 
dorsal  blood  vessel  and  often  other  vessels,  whereas  the  molluscan  heart 
is  compact.  The  annelid  circulatory  system  is  closed;  that  of  the  molluscs 
includes  extensive  sinuses.  Their  nephridia,  though  basically  similar,  are 
as  different  from  each  other  as  from  the  nephridia  of  many  other 
coelomate  groups.  The  dorsal  shell  and  ventral  foot  of  the  molluscs  have 
no  counterpart  in  the  elongate,  annulated  annelid.  In  short,  a  compara- 


Shell  Uea^-rt     ^' 


^0   -«?SS^.\o\^^\„.5^' 


—  C  oeiom.  ~— 
Parapodium. 

MefcanephridiuiTt- 
Foot 


Figure  15.9.     Diagrammatic  representations  of  the  Mollusca  (A),  Annelida  (C), 
Arthropoda  (D),  including  a  cross  section  of  an  annelid  (B). 


and 


284  ^^^   ANIMAL   KINGDOM 

live  Study  of  adult  structures  yields  little  evidence  that  these  phyla  are 
at  all  related. 

A  comparison  of  annelid  and  arthropod  morphology  yields  quite 
different  results.  Although  the  arthropods  will  be  described  in  the  next 
chapter  it  is  convenient  to  indicate  some  of  their  general  features  here. 
An  extremely  diagrammatic  representation  of  an  arthropod  (Fig.  15.9  D) 
shows  many  structures  in  common  with  the  annelids.  In  both  phyla  the 
body  is  segmented,  and  each  segment  usually  has  a  single  pair  of  ap- 
pendages. Many  arthropods  have  a  long,  tubular  dorsal  heart  that  is 
more  like  that  of  the  annelids  than  is  the  molluscan  heart.  Annelids 
and  arthropods  both  have  a  ventral  chain  of  nerve  ganglia  with  meta- 
meric,  lateral  nerves  to  the  body  segments.  Several  basic  differences  also 
exist,  of  course.  Arthropods  have  a  chitinous  exoskeleton  and  jointed 
appendages,  their  circulatory  system  is  completely  open,  and  the  body 
cavity  is  a  hemocoel  rather  than  a  coelom.  The  similarities  are  such, 
however,  as  to  suggest  a  close  relationship  between  the  two  phyla. 

If  the  early  development  of  these  three  phyla  is  compared,  it  is 
found  that  both  the  annelids  and  the  molluscs  have  spiral  cleavage, 
whereas  the  arthropods  (almost  all  of  which  have  heavily  yolked  eggs) 
do  not.  Gastrulation  is  similar  in  the  annelids  and  molluscs,  and  fur- 
ther development  in  many  species  of  both  phyla  results  in  a  free-swim- 
ming larva,  the  trochophore  (Fig.  15.10,  A).  Although  the  structure  of  the 
trochophore  varies  considerably  from  species  to  species  in  both  phyla, 
no  characteristic  will  completely  separate  those  of  the  Annelida  from 
those  of  the  Mollusca.  Hence,  development  from  the  egg  through  the 
trochophore  is  strikingly  similar  in  these  two  phyla.  Arthropods  do  not 
have  larvae  of  this  type;  all  arthropod  larvae,  even  in  their  youngest 
stages,  have  jointed  legs  and  other  characteristics  that  readily  identify 
them  as  arthropods. 

The  later  development  of  the  annelids  and  molluscs  is  quite  differ- 
ent. Molluscan  trochophores  develop  a  foot  and  a  shell  gland  and  be- 
come veligers  (Fig.  14.4).  By  further  metamorphosis  the  veliger  is 
transformed  gradually  into  the  adult  form.  The  general  relation  be- 
tween the  trochophore  anatomy  and  that  of  the  adult  is  indicated  by 
diagrams  (Fig.  15.10  B)  that  for  the  sake  of  clarity  do  not  indicate  the 
actual  course  of  development  for  a  mollusc,  but  do  indicate  general 
body  relationships.  Annelid  trochophores  develop  directly  into  the 
adult  form  (Fig.  15.10  C).  In  both  phyla  the  upper  half  of  the  trocho- 
phore becomes  only  the  extreme  anterior  end  of  the  body,  and  most  of 
the  adult  body  develops  from  the  lower  half.  In  both  phyla  the  brain 
develops  by  ingrowths  of  ectoderm  from  the  upper  half  of  the  trocho- 
phore, and  the  other  ganglia  develop  from  ventral  ectoderm.  Many 
molluscs  do  not  hatch  until  they  have  developed  to  the  veliger  stage, 
and  others  hatch  with  the  adult  morphology.  Trochophores  occur, 
however,  in  all  of  the  classes  except  the  Cephalopoda.  Many  annelids  do 
not  hatch  until  later  stages  of  development,  and  then  emerge  as  small 
worms.  Trochophores  are  found  only  in  marine  annelids,  the  Poly- 
chaeta  and  the  Archiannelida. 

A  comparison  of  later  development  in  the  annelids  and  arthropods 


PHYLUM   ANNELIDA 


285 


indicates  that  the  similarities  of  adult  structure  are  associated  with 
similarities  in  development.  In  the  annelids  (Fig.  15.10  D)  the  meso- 
derm, which  remains  as  a  pair  of  bands  in  the  trochophore,  elongates 
and  becomes  divided  into  pairs  of  somites.  Within  each  somite  a 
coelomic  cavity  appears.  The  somites  of  each  pair  expand  dorsally  and 
ventrally  around  the  gut,  eventually  forming  a  ring  with  dorsal  and 
ventral  mesenteries.  To  complete  the  process  of  segmentation  the  body 
wall  constricts  between  adjacent  rings.  The  body  elongates  during  this 
process,  and  segmentation  begins  at  the  anterior  end.  In  arthropods 
the  mesoderm  follows  a  similar  pattern  of  development,  starting  as  a 
pair  of  longitudinal  bands  that  become  divided  into  somites,  with 
coelomic  cavities  appearing  in  each  somite.  Later  the  cavities  disap- 
pear, but  the  somites  correspond  with  the  segments  of  the  adult  body. 
In  both  phyla  the  ventral  nerve  cord  arises  from  the  midventral  line  as 


Esophagus 

Mouth 

Protonephridiuin 
Mesoderm  band.' 


Apical  organ 
Brain  rudiment 


Stomach 
^[_f  Profcot  roch 


Anas 


le/— Anus 


Figure  15.10.  Development  in  annelids  and  molluscs.  A,  A  typical  trochophore.  B, 
Diagrammatic  representations  of  the  development  of  a  mollusc  from  a  trochophore.  C, 
The  same  for  an  annelid.  D,  Ventral  views  of  the  gut  and  mesoderm  bands  of  an  annelid 
from  the  trochophore  stage  (left)  through  the  formation  of  a  few  anterior  segments. 


285  ''^f   ANIMAL   KINGDOM 

a  pair  of  longitudinal  cords  that  later  become  metameric.  In  both  phyla 
tlie  jKiired  nature  ol  the  ventral  cord  often  disappears  by  fusion,  pro- 
ducing a  single  atlult  nerve  cord. 

Thus,  the  early  development  of  these  forms  indicates  a  close  re- 
lation between  annelids  and  molluscs,  whereas  later  development  and 
adult  morphology  indicates  a  close  relation  between  annelids  and 
arthropods.  Hence,  the  three  phyla  are  considered  to  form  a  natural 
group  within  the  eucoelomates. 

131.        The  Trochophore  Larva 

The  trochophore  larva  has  been  the  subject  of  a  considerable  amount 
of  embryological  research.  In  a  given  species  the  cleavage  pattern  from 
egg  to  trochophore  tends  to  follow  an  exact  pattern  (which  is  somewhat 
less  exact  in  those  with  much  yolk).  This  pattern  is  termed  a  cell  lineage. 
The  cell  lineages  of  some  of  the  aschelminthes  have  been  described 
previously  (p.  240).  These  patterns  differ  in  detail  from  species  to  species 
but  are  similar  in  many  general  features.  A  comparison  of  cell  lineages 
in  annelids  and  molluscs  reveals  that  the  patterns  of  development  are 
as  similar  as  the  results,  i.e.,  the  trochophores  not  only  look  alike,  but 
develop  in  similar  ways. 

The  trochophore  (Fig.  15.10  A)  is  biconical,  with  a  ring  of  cilia, 
the  prototroch,  around  the  equator.  At  the  upper  apex  there  is  usually 
a  sensory  apical  organ  bearing  a  tuft  of  cilia.  Brain  rudiments  are  usu- 
ally evident  beneath  the  apical  organ.  The  mouth  is  just  beneath  the 
prototroch  and  the  anus  is  near  the  lower  apex.  Often  (especially  if 
yolk  is  plentiful)  the  digestive  tract  is  less  well  developed  than  shown 
here;  an  intestine  and  anus  may  be  lacking  at  this  stage  of  development. 
The  mesoderm  is  a  pair  of  undifferentiated  masses  in  the  lower  cone, 
lying  beside  a  pair  of  protonephridia  that  develop  from  the  ectoderm. 
At  this  early  stage  of  development  the  trochophore  lacks  a  coelom;  its 
body  is  composed  primarily  of  an  outer  ectoderm  with  ectodermal  de- 
rivatives such  as  nervous  tissue  and  scattered  ectomesodermal  elements, 
and  an  inner  endoderm  forming  a  gut. 

If  cell  lineage  is  followed  from  the  16-cell  stage  to  the  trochophore 
(Fig.  15.11)  in  a  number  of  species,  it  is  found  that  in  general  the 
upper  cone  and  prototroch  develop  from  the  first  quartette  (upper  eight 
cells).  Of  these  the  upper  four  cells  become  the  apical  organ  and  most 
of  the  cone  surface,  and  the  low^er  four  cells  become  the  prototroch  and 
the  lower  part  of  the  upper  cone  surface.  Most  of  the  surface  of  the 
lower  cone  is  derived  from  the  second  quartette  (middle  four  cells).  The 
four  large  cells  become  a  part  of  the  ectoderm  between  the  mouth  and 
anus  (this  portion  is  formed  by  the  cells  of  the  third  quartette,  which 
separate  from  the  large  cells  at  the  next  division),  and  all  of  the  meso- 
derm and  endoderm.  The  mesoderm  develops  from  one  of  these  cells 
while  the  endoderm  comes  mostly  from  the  other  three.  This  general 
pattern  of  development  is  found  in  both  the  annelids  and  the  molluscs. 

An  interesting  problem  in  embryology  is  whether  or  not  particular 
cells  are  able  to  develop  into  structures  other  than  those  they  become 


PHYLUM    ANNELIDA 


287 


in  norynal  development.  You  will  recall  (p.  2U2)  that  isolated  coelen- 
terate  embryo  parts  usually  become  whole  organisms,  whereas  isolated 
parts  ot  the  ctenophore  embryo  become  only  portions  of  adults.  The 
annelid-mollusc  trochophore  is  a  classic  example  of  the  second  type,  in 
which  development  is  a  mosaic.  Each  piece  is  able  when  isolated  to 
produce  only  those  structures  that  it  produces  under  normal  conditions. 
E.  B.  Wilson,  a  pioneer  in  expernnental  embryology,  separated  the 
cells  of  a  cleaving  mollusc  egg  in  1904,  and  found  that  each  cell  gave 
rise  to  only  a  portion  of  a  trochophore.  In  1945  D.  P.  Costello  did  the 
same  with  an  annelid  egg  (Fig.  15.11,  right).  In  his  experiments  Costello 
separated  the  cells  of  the  two-cell  stage  as  soon  as  they  formed,  and 
continued  to  separate  cells  as  cleavage  occurred  until  he  had  16  cells  in 
16  separate  dishes.  Thus,  none  of  the  cells  had  any  opportunity  to  in- 
fluence any  of  the  others.  The  16  cells  were  then  allowed  to  develop, 
without  further  separation  of  cells.  Sixteen  groups  of  cells,  four  of  each  of 
the  varieties  shown,  resulted.  Four  dishes  each  had  a  cluster  of  small 
cells  some  of  which  had  cilia  similar  to  those  of  the  apical  organ.  An- 
other four  dishes  each  had  a  cluster  of  four  large  cells,  three  of  which 
had  cilia  like  those  of  the  prototroch.  In  the  trochophore  of  the  species 
Costello  studied  the  prototroch  is  formed  by  a  circle  of  twelve  large 
cells,  and  just  above  the  prototroch  are  four  more  large  cells.  Thus  it 
appears  that  the  isolated  cells  formed  exactly  the  number  and  kinds  of 
cells  they  form  in  the  normal  larva.  Another  four  dishes  each  contained 
a  cluster  of  small  cells  which  were  identified  as  the  progeny  of  the 
second  quartette.  The  four  large  cells  of  the  sixteen  cell  stage  each  be- 
came a  single  large  cell  with  a  cluster  of  small  cells.  In  each  case  the 
small  cells  were  spread  out  over  the  surface  of  the  large  cell,  suggesting 
the  only  attempts  at  gastrulation  found  in  the  16  isolates.  From  this 
observation  Costello  concluded   that  the   macromeres  are  necessary  for 


First 
''quartette 


Second 
cJuai'bctte. 

Third 
c[ua.i'tetbe 

Mesendoderm. 


Figure  15.11.  Development  of  the  trochophore.  The  contribution  of  each  tier  of 
four  cells  in  the  16-cell  stage  (center)  to  the  trochophore  (left)  is  shown.  The  wavy 
boundary  between  the  ectoderm  of  the  second  and  third  quartettes  is  intended  to  show 
interdigitation  between  these  components  and  a  degree  of  variability.  \Vhen  the  cells  of 
the  16-cell  stage  are  isolated,  each  produces  a  structure  of  the  kind  shown  at  the  right 
(four  of  each  kind,  sixteen  in  all).  (Figures  on  the  right  are  after  Costello.) 


288  THE   ANIMAL   KINGDOM 

gastrulation,  and  that  none  of  the  other  cells  are  able  to  produce  meso- 
derm or  endoderm. 

The  work  oi  VV^ilson,  Costello  and  many  others  leads  to  the  same 
general  conclusions:  In  the  early  development  of  annelids  and  molluscs 
the  abilities  of  the  parts  of  the  embryos  are  limited  to  the  functions 
they  serve  in  normal  development  (with  a  few  exceptions  in  which  some 
portions  are  able  to  form  a  lew  additional  structures).  A  second  and 
equally  significant  conclusion  is  that  in  some  cases  these  abilities  can 
be  realized  in  isolation,  without  interaction  among  the  parts.  Examples 
are  the  cilia  of  the  apical  organ  and  of  the  prototroch  that  developed 
in  Costello's  isolates.  It  should  be  added  that  the  development  of  other 
structures  appears  to  require  the  integrity  of  the  embryo,  since  the 
macromeres  in  Costello's  experiments  showed  no  tendencies  to  form 
mesoderm  bands  or  digestive  tract,  and  none  of  the  ectomesodermal 
structures  appeared  in  any  of  his  isolates. 

Questions 

1.  Discuss  segmentation  in  the  animal  kingdom. 

2.  Draw  cross  sections  of  a  polychaete  and  an  oligochaete. 

3.  Compare  the  sense  organs  of  Nereis  and  Lumbricus. 

4.  Describe  a  parapodium. 

5.  Discuss  the  role  of  giant  fibers  in  annelids. 

6.  How  can  a  population  achieve  reproductive  coordination  so  that  all  individuals  breed 
at  one  time  in  the  year? 

7.  How  do  leeches  feed? 

8.  Compare  reproduction  in  Nereis,  Lumbricus  and  a  leech. 

9.  Draw  and  label  a  trochophore. 

Supplementary  Reading 

The  photographs  and  stereodiagrams  of  annelids  in  Buchsbaum,  Animals  Without 
Backbones,  are  especially  good.  The  colored  photographs  and  life  studies  of  annelids  in 
Vonge,  The  Sea  Shore,  are  excellent.  The  development  of  the  trochophore  and  of  other 
larvae  are  discussed  in  Willier,  Weiss  and  Hamburger,  The  Analysis  of  Development. 


CHAPTER  16 


Phylum  Arthropoda 


Arthropods  are  segmented  animals  whose  epidermis  secretes  an  exo- 
skeleton  of  stout  rings  corresponding  with  the  segments;  the  rings  are 
connected  by  flexible  membranes  that  act  as  joints.  Many  of  the  seg- 
ments bear  paired  lateral  appendages,  each  of  which  has  a  similar 
chitinous  skeleton  of  jointed  rings.  The  phylum  takes  its  name  from 
these  jointed  appendages  (Gr.  arthros  joint  +  podos  foot).  The  exo- 
skeleton  is  a  chemical  complex  which  includes  chitin,  a  nitrogenous 
polysaccharide  made  of  sugar,  ammonia  and  acetic  acid.  The  body  in- 
cludes a  head,  thorax  and  abdomen,  each  composed  of  several  seg- 
ments which  may  be  fused  in  various  ways.  The  body  musculature  is 
made  up  of  numerous  small  muscles  extending  across  joints  to  form 
an  intricate  mechanism  capable  of  precise  complex  movements. 

The  evolutionary  potentialities  of  such  a  structural  system  would 
appear  to  be  tremendous.  The  exoskeleton  not  only  forms  a  protective 
cover  that  has  been  successful  in  all  of  the  habitats  of  the  world,  but 
its  division  into  numerous  parts  makes  possible  many  different  mor- 
phologic adaptations  to  particular  habitats.  For  example,  the  mouth 
parts  of  an  insect  may  be  modified  for  biting,  chewing,  scraping  or 
sucking.  The  specialization  of  the  skeletal  parts  of  many  arthropods 
has  adapted  them  beautifully  for  some  particular  habitat;  they  are  so 
precisely  adapted,  in  fact,  that  they  are  severely  limited  in  their  ecologic 
distribution.  This  may  explain  the  enormous  number  of  species  of 
arthropods,  for  many  species  can  coexist  in  the  same  geographic  region 
if  each  has  different  ecologic  requirements.  At  the  present  time  the 
known  species  of  all  other  phyla  add  up  to  about  130,000,  while  those 
of  the  arthropods  alone  add  up  to  870,000!  The  majority  (800,000)  of 
these  are  insects,  most  of  which  are  terrestrial. 

132.        Classification  of  the  Phylum 

Arthropods  can  be  divided  into  four  subphyla  according  to  the  struc- 
tures of  the  appendages  of  the  first  six  segments.  In  all  arthropods  the 
first  segment,  believed  to  correspond  with  the  annelid  prostomium,  ap- 
pears in  the  embryo  but  is  never  distinct  in  the  adult.  It  never  has  ap- 
pendages. In  most  arthropods  the  mouth  opens  ventrally  between  the 
third  and  fourth  segments. 

The  four  subphyla  (Fig.    16.1)   are   the   Trilobito,  Arachnomorphc, 

289 


290  ^"^   ANIMAL   KINGDOM 


Sz-^rnLcnt 


1 


z 


3 


4 


Ti^ilobita 


? 


Ante-nnSL 


Arachnomorpha. 


Chelicera. 


L<^g 


Crusta-c<z.eL 


First 
eLnte-TLTia. 


Second 
ant<z.nn.a. 


Mandible 


First 
ma3<:illa. 


Second 
maxilla. 


Labi  at  cL 


Antenna. 


Ma-ndibl<2. 


Ma>cilla 


Labium,  (pair) 


Figure  16.1.  Appendages  of  the  first  six  segments  in  the  four  subphyla  of  the  Ar- 
thropoda.  Except  for  the  labium  (lower  right)  only  one  member  of  a  pair  is  shown.  The 
chelicera  illustrates  a  chelate  appendage,  in  which  the  next  to  last  segment  is  prolonged 
as  a  hand  against  which  the  last  segment  closes  as  a  thumb. 

Crustacea  and  Labiata.  The  first  includes  only  one  class,  Trilobita,  now 
extinct.  The  trilobites  (Fig.  16.2)  were  marine,  bottom  scavengers  with 
the  skeleton  extended  laterally  to  form  a  three-lobed  shield.  The  second 
segment  bore  a  pair  of  antennae  and  all  remaining  segments  bore 
biramous  (two-branched)  limbs.  The  inner  branch  or  ramus  of  each 
limb  was  used  for  walking  while  the  outer  ramus  apparently  served  as 
a  gill.  The  single  base  of  each  limb  was  enlarged  medially  as  a  toothed 
jaw  or  gnathobase.  Debris  was  chewed  by  this  long  row  of  gnathobases 
as  it  was  passed  forward  to  the  mouth.  The  abundance  of  their  fossils 
suggests  that  trilobites  were  dominant  organisms  of  the  Cambrian 
period,  over  500  million  years  ago.  During  the  rest  of  the  Paleozoic  era 
they  were  gradually  replaced  by  the  Crustacea  and  became  extinct  225 
million  years  ago. 

The  subphylum  Arachnomorpha  includes  a  variety  of  both  living 
and  extinct  groups  such  as  king  crabs,  eurypterids,  scorpions,  spiders  and 
mites.  In  these  forms  the  second  segment  has  no  appendages.  Those  of 
the  third  are  chelate  (tipped  with  pincers)  (Fig.  16.1).  This  particular 
pair  of  chelate  appendages  are  small,  located  in  front  of  the  mouth,  and 


PHYLUM    ARTHROPODA 


291 


called  the  chelicerae.  The  first  three  pairs  of  appendages  behind  the 
mouth  usually  serve  together  with  others  as  walking  legs,  but  they  are 
sometimes  modified  as  grasping  or  tactile  limbs.  While  the  posterior 
limbs  are  usually  biramous,  the  anterior  limbs  are  always  uniramous. 
Most  of  the  living  species  are  carnivores,  although  king  crabs  are 
scavengers  and  many  mites  are  herbivorous. 

The  subphylum  Crustacea  includes  the  single  class  Crustacea,  the 
dominant  living  aquatic  arthropods  (Fig-  16.2).  The  second  and  third 
segments  each  have  a  pair  of  antennae.  The  first  pair  of  postoral  ap- 
pendages are  short,  stout  mandibles,  or  jaws.  The  appendages  of  the 
fifth  and  sixth  segments  are  maxillae,  modified  to  aid  the  jaws  by 
holding  and  manipulating  the  food.  Many  of  these  appendages  are 
biramous.  Crustaceans  ha\e  invaded  a  variety  of  acjuatic  habitats;  some 
crawl  over  the  bottom  while  others  swim  or  drift  with  the  current.  Many 
of  the  species  are  extremely  abiuulant.  Probably  more  protoplasm  is 
embodied  in  crustaceans  as  a  whole  than  in  any  other  class  of  animals. 

The    fourth   subphylum,    Labiata,    includes    millipedes,    centipedes 


^^■^•^ 


I 
I 

I 


-Antenna 


Chelicera."^ 


TRIIOBITA 


MACMIQMORPHA 


Anbz.nnz. 


'sm':'mmmkW.-.:'-:'Mi!Ms:m.^'ssssA 


Figure  16.2.     Representatives  of  the  four  subphyla  of  the  Arthropoda.  (After  Parker  and 

Haswell.) 


292  ^^^   ANIMAL   KINGDOM 

and  insects.  Their  second  segment  has  antennae  and  the  third  segment 
lacks  appendages.  The  fourth  has  mandibles,  the  fifth  has  maxillae,  and 
on  the  sixth  appendages  comparable  to  maxillae  are  fused  together  to 
form  a  lower  lip,  the  labium,  from  which  the  subphylum  takes  its 
name.  All  of  the  appendages  are  uniramous.  This  group  apparently 
arose  on  land,  although  so  many  insects  have  developed  aquatic  young 
that  labiates  now  challenge  the  supremacy  of  crustaceans  among  fresh- 
water arthropods. 

A  fragmentary  record  of  the  appearance  and  spread  of  these  sub- 
phyla  is  shown  by  their  fossils.  At  the  beginning  of  the  fossil  record, 
585  million  years  ago,  Cambrian  seas  already  contained  numerous 
species  of  trilobites.  Within  60  million  years,  before  the  end  of  the 
Cambrian  period,  the  seas  also  contained  arachnomorphs  and  crus- 
taceans. Trilobites  never  left  the  ocean,  but  arachnomorphs  appeared  in 
fresh  water  by  the  Ordovician  period  (505  million  years  ago),  and  crus- 
taceans followed  by  the  Devonian  (375  million  years  ago).  Certain 
arachnomorphs  (scorpions)  became  terrestrial  by  the  Silurian  (425  mil- 
lion years  ago),  leaving  for  us  the  oldest  known  terrestrial  fossils.  The 
labiates  appeared  as  a  terrestrial  group  during  the  coal  age  (Pennsyl- 
vanian  period,  275  million  years  ago).  Among  the  earliest  of  these  are 
winged  insects,  indicating  that  the  air  had  already  been  conquered  50 
million  years  before  flying  reptiles  and  110  million  years  before  birds 
appeared.  Terrestrial  crustaceans  exist  today,  but  all  of  their  known 
fossils  are  of  recent  origin.  Thus  terrestrialism  developed  independently 
at  least  three  times  within  the  phylum.  The  insects  now  form  a  dominant 
terrestrial  group,  their  myriad  species  scattered  from  the  arctic  to  the 
equator,  from  the  swamps  to  the  deserts. 

The  phylum  can  be  subdivided  in  other  ways.  Subphyla  may  be 
omitted,  and  the  phylum  is  then  divided  into  seven  or  more  classes.  The 
trilobites  and  arachnomorphs  may  be  placed  in  one  subphylum.  The 
trilobites  have  also  been  grouped  with  the  crustaceans,  and  it  is  not  un- 
common to  find  all  of  the  antennate  groups  in  one  subphylum.  The 
arrangement  used  here  is  a  combination  of  views  current  in  zoology 
and  paleontology. 

1 33.        Class  Crustacea 

Crustaceans  have  two  distinguishing  features,  the  two  pairs  of  an- 
tennae already  described,  and  a  nauplius  larva  (Fig.  16.3).  This  larva 
has  an  externally  unsegmented  body,  a  simple,  median  eye,  and  only 
three  pairs  of  appendages,  the  first  pair  uniramous  and  the  other  two 
biramous.  Its  mouth  is  ventral  between  the  second  and  third  pairs  of 
limbs  and  the  anus  is  terminal.  This  minute  creature  floats  in  the  water 
feeding  upon  microscopic  plants  and  debris.  As  the  larva  grows  and 
undergoes  several  molts,  additional  limbs  appear  on  segments  added 
in  front  of  the  anus,  and  the  organism  gradually  assumes  its  adult 
shape.  The  uniramous  limbs  of  the  larva  become  the  first  antennae  of 
the  adult,  the  first  biramous  limbs  become  the  second  antennae,  and 
the  third  pair  of  limbs  become  the  adult  mandibles.  Since  the  additional 


PHYLUM   ARTHROPODA  293 


Figure  16.3.     Nauplius  larva.    (After  Dietrich.) 

limbs  are  usually  biiamous  when  they  first  appear,  the  basic  limb  plan 
in  crustaceans  is  similar  to  that  oi  tiie  trilobites:  one  pair  oi  uniranious 
antennae  followed  by  a  series  of  biramous  limbs.  The  nauplius  larva 
is  found  in  all  of  the  major  groups  of  crustaceans,  which  suggests  that 
an  animal  resembling  it  may  have  been  the  common  ancestor  of  the 
class. 

Crustaceans  are  traditionally  divided  into  the  large  and  the  small. 
Large  members  form  a  natural  subclass,  the  Malacostraca.  In  this  group 
the  order  Decapoda  (ten  walking  legs)  includes  the  familiar  shrimps, 
crayfish,  lobsters  and  crabs.  The  crayfish  will  be  described  as  an  ex- 
ample of  the  subclass.  Similarities  among  malacostracans  are  close 
enough  so  that  knowledge  of  one  form  is  a  key  to  the  understanding  of 
others.  Small  crustaceans  are  grouped  in  several  orders  that  form 
several  subclasses.  Of  these  the  water-llea  will  be  described  as  an  ex- 
ample. Unfortunately  the  orders  of  small  crustaceans  are  so  diverse 
that  one  example  is  not  an  adequate  introduction  to  the  others. 

134.  Astacus,  a  Crayfish 

Crayfish  of  the  genus  Astacus  are  common  in  this  country  west  of  the 
Rockies.  To  the  east  the  slightly  dilferent  genus,  Cambarus,  is  abundant. 
Crayfish  are  found  in  or  near  ponds,  lakes  and  streams.  Those  in  the 
water  excavate  holes  beneath  logs  and  stones  to  serve  as  temporary 
shelters,  while  those  on  the  banks  may  dig  deep  burrows.  They  are 
most  active  at  dusk  and  after  dark,  scavenging  the  neighborhood  for 
plant  or  animal  debris  and  occasionally  capturing  unwary  insects,  tad- 
poles and  fish. 

1 35.  External  Morphology  of  the  Crayfish 

The  crayfish  body  (Fig.  16.4)  is  divided  into  a  solid  cephalothorax 
and  a  jointed  abdomen.  If  we  include  the  embryonic  first  segment,  the 
cephalothorax  represents  the  fusion  of  six  cephalic  and  eight  thoracic 
seginents.  All  except  the  first  have  appendages.  The  back  extends 
laterally  as  a  pair  of  skeletal  folds   that  bend  down  over  the  sides  of 


THE   ANIMAL   KINGDOM 

Rostruni     /"Cotripoundeye 


) wimracrdslf]  Tels on 

Uropod 
Figure  16.4.     Lateral  view  of  a  crayfish.   (After  Howes.) 

the  body  iorming  the  carapace.  The  same  skeleton  extends  forward 
over  the  head  of  a  rostrum.  The  tapered  abdomen  is  composed  of_seveii 
segments,  of  which  the  first  six  have  appendages.  The  last,  called  the 
telson,  is  often  not  counted  as  a  segment.  The  anus  is  located  on  its 
ventral  side  but  it  lacks  appendages.  The  abdomen  is  flattened  and  has 
broad  dorsal  and  ventral  surfaces.  The  rigid  portion  of  the  ventral 
skeleton  is  reduced  to  narrow  transverse  rings  joined  together  with 
broad  areas  of  flexible  chitin.  This  enables  the  abdomen  to  flex  sharply 
beneath  the  body. 

The  appendages  are  modified  in  a  variety  of  ways  (Fig.  16.5).  In 
many  of  them  a  base  (protopodite),  an  inner  ramus  (endopodite)  and 
an  outer  ramus  (exopodite)  can  be  recognized. 

The  last  appendages  (on  the  20th  segment)  are  extremely  flattened 
uropods.  \V^hen  extended,  the  exopodites,  endopodites  and  the  telson 
between  form  a  tail  fan.  The  crayfish  spreads  this  fan  and  flexes  the 
abdomen  rapidly,  pulling  itself  backward  with  startling  speed. 

The  other  abdominal  appendages  are  the  much  more  delicately 
built  swimmerets  or  pleopods,  with  bristly  endopodites  and  exopodites. 
The  continual  gentle  beating  of  these  limbs  produces  a  water  current 
backward  beneath  the  animal,  probably  of  use  beneath  rocks  or  in 
burrows  where  the  water  would  become  devoid  of  ox^gen  if  not  cir- 
culated.  In  the  male  the  first  pleopods  (15th  segment)  are  modified  as 
copulatory  organs  (Fig.  16.5).  The  female  deposits  her  eggs  on  the 
pleopods,  to  which  they  are  glued  by  secretions  from  the  limbs.  Con- 
stant motion  then  keeps  the  eggs  ^vell  aerated.  If  the  pleopods  beat 
vigorously  the  current  produced  helps  the  crayfish  to  walk  forward,  and 
in  small  individuals  may  actually  produce  a  gentle  forward  swimming. 

The  last  five  pairs  of  appendages  on  the  cephalothorax  (segments 
10  to  14)  are  the  large  walking  legs  or  pereiopods.  These  are  uni- 
ramous  in  the  adult.  Each  is  formed  of  seven  segments,  of  -ivhich  the 
first  t^\•o  represent  the  protopodite  and  the  last  five  the  endopodite. 
Each  joint  (Fig.  16.6)  can  move  in  a  single  plane,  but  the  planes  of 
succeeding  joints  are  rotated  so  that  the  limb  as  a  whole  can  move 
with  considerable  flexibility.  The  first  three  pairs  of  pereiopods  are 
chelate,  and  the  first  pair  have  large  pincers.  The  jaws  of  the  pincers 
are  made  of  the  two  distal  segments  of  the  leg.  ^\'hich  are  hinged  one 
upon  the  other.  The  pincers  are  used  for  fighting  and  for  occasional 
food  capture,  and  may  assist  in  walking  over  rough  terrain.  They  are 


PHYLUM    ARTHROPODA 


295 


.....m.iit-.Ki  ■,■,.. mm. 

First  anlrenna- 


Exo. 


Second-  raayilliped- 


First  mcocilla. 


First  ma.x'illipcd. 


Fourth  pgreiopod 

End- 
Third  S^A7immgret 


Ezxro.  ^Prot 


First  sv\7immer(z.t: 
in  Taa.le 


Uropod 


Figure  16.5.  Appendages  of  the  crayfish.  Prot.  =  protopodite.  end.  =  endopodite, 
exo.  =  exopodite.  Those  on  the  left  are  drawn  to  a  larger  scale  than  those  on  the  right. 
(After  Howes.) 

also  used  as  plows  for  digging.  The  other  chelate  legs  are  used  for 
picking  up  bits  of  food  ancl  handing  them  to  the  mouth  parts.  The  last 
four  pairs  of  pereiopods  are  the  primary  walking  legs.  Crayfishes  cannot 
run,  but  they  use  the  tail  fan  for  swift  escape. 

The  anterior  three  pairs  of  thoracic  appendages  (segments  7  to  9) 
and  the  posterior  three  pairs  of  cephalic  appendages  (4  to  6)  form  the 
mouth  parts.  These  overlap  each  other  so  that  the  most  posterior  pair 
covers  those  in  front.  The  thoracic  legs  are  three  pairs  of  maxillipeds, 
with  endopodites  modified  as  small  arms  to  hold,  manipulate  and  tear 
the  food,   and  exopodites  modified  as  tactile  palps.  The  two  pairs  of 


Hinge 


jcccnsor 


~Flz.Dcors 

Figure  16.6.  Dissected  pereiopod  of  Astacus  showing  muscle  arrangement.  The 
terminal  joint  moves  up  and  down,  the  next  joint  fore  and  aft,  and  the  third  joint  up  and 
down.  (.After  Parker  and  Haswell.) 


296  ^^^  ANIMAL   KINGDOM 


Corneal  le.ns      Tactile  bristles 
Epidermis 


Chemoreceptor^ 

B 


Opening 


Statocyst 


Pigment 


Retinula  cell 
Nerve  fiber 


Nerve 


^  Statocyst 
bristle 


Figure  16.7.  Sense  organs  of  the  crayfish.  A,  Two  ommatidia  from  the  compound 
eye.  In  each,  light  passing  through  the  two  lenses  is  focused  on  the  outer  end  of  the 
rhabdome,  which  is  made  of  seven  fused  rods  or  rhabdomeres,  striated  thickenings  along 
the  inner  edges  of  seven  retinula  cells.  Pigment  screens  out  stray  light.  B,  Sensory  bristles 
on  the  antenna.  A  chemoreceptor  is  enlarged  at  the  right,  viewed  from  two  directions. 
C,  The  statocyst  (above)  and  a  still  greater  enlargement  of  one  of  the  sensory  bristles 
inside  the  statocyst  (below).  (B  and  C  from  Huxley,  1880.) 


cephalic  maxillae  have  much  flattened  protopodites  expanded  medially 
to  serve  as  plates  for  holding  food  against  the  jaws.  Endopodites  are 
similarly  flattened.  The  first  maxillae  lack  exopodites,  but  those  of  the 
second  are  expanded  laterally  with  part  of  the  protopodites  to  form 
large  flaps,  the  gill  bailers.  The  mandibles  (segment  4)  are  deeply 
seated  under  the  mouth.  Each  is  a  stout  protopodite  expanded  medi- 
ally to  form  teeth,  which  bears  a  small  tactile  endopodite,  the  man- 
dibular palp.  These  jaws  chew  the  food  which  is  brought  by  chelate 
pereiopods,  shredded  by  maxillipeds,  and  held  against  the  mouth  by 
maxillae.  The  simultaneous  activity  of  all  these  pieces  is  bewildering 
to  the  observer! 

Anterior  to  the  jaws  are  two  pairs  of  antennae.  The  second  pair 
each  have  a  very  long,  many  jointed  endopodite,  the  flagellum,  and  a 
flat  exopodite,  the  scale.  \VhiIe  the  crayfish  is  scooting  backward  the 
scales  are  held  outward  to  serve  as  rudders.  Each  first  antenna  has  a 
base  with  two  flagella,  producing  an  apparent  biramous  condition  in 
what  is  embryologically  a  uniramous  limb.  The  flagella  of  both  pairs 


PHYLUM    ARTHROPODA  297 

of  antennae  are  used  for  exploration  of  the  environment.  Those  of  the 
second  antennae  are  primarily  tactile,  while  the  others  have  many  small 
chemoreceptors. 

In  addition  to  appendages  the  crayfish  has  several  sense  organs. 
Compound  eyes  are  borne  on  stalks  at  the  front  of  the  cephalothorax. 
Each  is  a  cluster  of  10,000  or  more  ommatidia  (Fig.  16.7  A)  arranged 
radially,  with  the  outer  facets  forming  the  eye  surface.  Each  omma- 
tidium  functions  as  a  complete  eye  looking  out  at  a  restricted  part  of 
the  world.  The  visual  fields  of  adjacent  ommatidia  overlap  consider- 
ably, but  all  together  provide  a  kind  of  mosaic  view  of  the  world. 

Chemoreceptors  are  small  blunt  bristles  (Fig.  16.7  B)  usually 
found  in  groups  of  three  or  four.  They  are  especially  abundant  on  the 
first  antennae  and  on  the  mouth  parts.  Tactile  bristles  (Fig.  16.7  B)  are 
small  bristles  jointed  to  the  body  surface  and  supplied  with  nerve  cells 
at  the  base.  These  are  scattered  all  over  the  body  and  are  especially 
abundant  on  the  second  antennae. 

The  basal  segment  of  each  first  antenna  contains  an  ingenious 
statocyst  (Fig.  16.7  C).  During  development  the  dorsal  surface  in- 
vaginates  to  form  a  sac  lined  with  numerous  tactile  bristles.  The  open- 
ing remains  as  a  slit  concealed  by  a  tuft  of  surface  hairs.  The  crayfish 
pushes  its  head  into  the  sand  until  each  sac  contains  a  gioup  of  sand 
grains,  which  then  provide  stimuli  for  the  sense  of  balance  by  the  way 
they  lie  against  the  sensory  bristles. 

The  sides  of  the  carapace,  arching  over  the  body,  enclose  a  pair  of 
gill  chambers.  Numerous  gills  (20  in  Astacus)  lie  in  each  chamber,  pro- 
jecting ujiward  from  their  origins  on  the  limbs  and  body  wall  (Fig. 
16.9).  Each  gill  resembles  a  bottle  brush,  having  a  central  axis  and 
numerous  radiating  filaments.  On  each  side  six  gills  (podobranchiae) 
arise  from  the  basal  segments  of  the  second  and  third  maxillipeds  and 
first  four  pairs  of  pereiopods.  Eleven  more  (arthrobranchiae)  emerge 
from  the  joint  membrane  between  these  legs  and  the  body.  Three  addi- 
tional pairs  of  gills  (pleurobranchiae)  originate  on  the  sides  of  the 
body  above  the  last  three  pairs  of  pereiopods.  Gills  of  adjacent  body 
segments  are  separated  by  flattened  plates,  the  epipodites,  attached  to 
the  bases  of  the  legs  (Fig.  16.9). 

The  carapace  fits  snugly  against  the  bases  of  the  legs,  leaving  sizable 
openings  only  at  the  posterolateral  edge  and  anteriorly  beside  the 
mouth  parts.  The  gill  bailers  of  the  second  maxillae  (Fig.  16.5)  extend 
back  over  the  gills  and  undulate  to  produce  a  water  current.  Most  of 
the  time  water  is  drawn  in  posteriorly  and  expelled  anteriorly,  but 
occasionally  the  direction  is  reversed  to  flush  out  debris  that  may  have 
collected  on  the  gills. 

136.        Internal  Anatomy  of  the  Crayfish 

Muscles  extend  between  various  parts  of  the  body,  but  are  prominent 
only  in  the  abdomen  and  legs.  The  abdomen  is  nearly  filled  with  mus- 
cle, including  straplike  dorsal  extensors  (Fig.  16.8  C)  and  very  stout 
complex  ventral  flexors.  Obviously  flexion  is  a  much  more  powerful 
movement  than  extension.   In   the  floor  of  the   thorax  muscles  to  the 


298 


THE   ANIMAL   KINGDOM 


To  digestive ^lanc 

Eye  stalk"!   Ant.  arteries 
Rostrum 


A 

Greenglaiid 

Mandible-' 


Pericardial 

"Sinus 


rlnte.stine 
|^_ri3„st.  artery 


rAnus 


f;       V<  /,  V'entral  art 


Mandibular 
1  (-.1  n      muscle 

btomacn 


-tery 


Cardiac; 
chamber 


-Lat.  tootli 
rMed.  tooth 
J_::rc~iix  Cecum 


E^tc 


iisoT  mus 


clcs 


G  astro-/ 
lilh 


orus 


)^     '^Valves 
E  s  oph  a0us 


Flexor 
miisdeS 


Obhque 
muscles 


Figure  16.8.  Internal  anatomy  of  the  crayfish.  A,  Digestive,  circulatory,  reproductive 
and  nervous  systems.  B,  Stomach  (enlarged).  C,  Musculature  of  the  abdomen.  (After 
Howes.) 

pereiopods  are  attached  to  infolded  lamina  of  the  skeleton  which  form 
an  internal  framework.  In  the  limbs  each  joint  is  crossed  by  a  pair  of 
antagonistic  muscles  (Fig.  16.6).  These  attach  to  the  side  wall  of  one 
segment  and  insert  at  the  base  of  the  next,  which  may  be  extended  in- 
ternally to  form  a  lever.  The  muscles  betAveen  the  "hand"  and  "thumb" 
of  the  pincers  claw  fill  the  large  hand.  The  extensor  is  relatively  small, 
but  the  flexor  that  closes  the  pincers  is  enormous,  inserting  on  a  large 
flat  plate  that  extends  into  the  hand  from  the  inner  side  of  the  base 
of  the  thumb.  Little  force  is  needed  to  hold  the  pincers  shut  against 
the  effort  of  the  crayfish,  but  great  effort  is  required  to  hold  it  open. 

The  digestive  system  (Fig.  16.8  A)  includes  an  ectodermal  foregut 
and  hindgut  lined  with  chitin,  and  an  endodermal  midgut.  The  fore- 
gut  includes  a  short  ascending  esophagus  and  a  large  stomach  over 
the  mouth.  The  stomach  is  divisible  into  anterior  cardiac  and  posterior 
pyloric  portions.  The  cardiac  stomach  contains  a  gastric  mill,  including 
one  dorsal  and  two  lateral  teeth  operated  by  some  13  sets  of  muscles 
(Fig.  16.8  B).  The  pyloric  stomach  contains  several  filters  formed  by 
bristles  that  permit  only  liquids  and  very  small  food  particles  to  pass 
through.  The  anterior  wall  of  the  cardiac  stomach  may  have  a  pair  of 
large  calcareous  discs,  the  gastroliths.  These  appear  and  disappear  as 
they  play  a  role  in  the  molting  process  (p.  328). 

The  midgut  and  hindgut  form  a  straight  narrow  intestine  from 
stomach  to  anus.  The  midgut  portion,  lying  in  the  thorax,  has  a  short 
dorsal  caecum  extending  forward  over  the  stomach  and  a  pair  of  large, 
yellowish-green  digestive  glands  that  open  into  it  by  large  lateral  ducts. 
As  in  the  molluscs  these  glands  not  only  secrete  digestive  enzymes  but 
also  serve  as  regions  of  absorption. 

As  the  mill  grinds  food  to  a  pulp,  juices  from  the  digestive  glands 


PHYLUM    ARTHROPODA 


299 


are  passed  forward  through  the  pyloric  stomach  so  that  chemical  as  well 
as  mechanical  digestion  takes  place  in  the  cardiac  stomach.  Particles  too 
large  to  pass  through  the  pyloric  filters  are  regurgitated  through  the 
mouth,  while  the  rest  filters  through  into  the  midgut.  Absorption  occurs 
through  the  linings  ot  the  midgut,  dorsal  caecum  and  digestive  glands. 

The  nervous  system  (Fig.  16.8  A)  is  similar  to  that  of  the  annelids, 
except  that  the  original  brain  and  the  following  two  ganglia  are  fused 
together  to  form  the  arthropod  brain.  During  development  it  arises  as 
three  pairs  of  ganglia,  and  in  the  nauplius  the  third  pair  are  postoral. 
They  later  move  around  the  mouth  and  the  three  pairs  fuse.  Circum- 
esophageal  connectives  join  the  brain  with  the  subgastric  ganglion, 
fcrmed  by  the  fusion  of  the  six  pairs  of  ganglia  associated  with  the 
mouth  parts.  Beginning  with  segment  10,  bearing  the  large  pincers, 
each  body  segment  has  a  bilobed  ventral  ganglion  joined  with  that  in 
front  by  nerves  to  form  a  ventral  cord.  .\s  in  many  annelids  the  cord  is 
paired  in  the  embryo  and  fused  in  the  adult.  This  ventral  cord  has 
four  giant  fibers.  Stimulation  of  these  fibers  produces  rapid  strong  ab- 
dominal Hexures.  Hence,  as  in  the  annelids,  the  giant  fibers  are  asso- 
ciated with  the  escape  mechanism. 

The  circulatory  system  of  arthropods  is  unique.  The  coelom, 
which  arises  early  in  development  as  paired  pouches  like  those  of  the 
annelids,  later  regresses.  It  is  replaced  by  a  system  of  blood  sinuses 
that  appears  around  the  ventral  nerve  cord  and  spreads  into  the  space 
formerly  occupied  by  the  coelom.  Eventually  the  sinuses  extend  through- 
out the  body,  even  into  the  limbs  and  sides  of  the  carapace,  forming  a 
hemocoel. 


Arlhrobranchiac 


Inte.stine 


Podobranchia.- 


R  ud  i  ment  a  ry 
pleurobranclna" 


Epipodite- 


Gill  chamber ■ 


Base  of  l<z.g 


Pericardial 
sinus 


rHeart 


■Te.stis 


Sperm 
duct 


Digestive 
ola-nd. 


— Artery 
-Nerve  trunk 


Vein  from 
gills 

Vein  to  gills 


Figure  16.9.     Cross  section   through  a  crayfish  just  behind   the   third  pereiopods. 
(After  Howes.) 


300  ^WE   ANIMAL  KINGDOM 

In  the  crayfish  a  dorsal  part  of  this  cavity  is  separated  off  by  a 
partition,  the  pericardial  membrane,  to  form  a  pericardial  sinus  around 
the  heart.  When  the  heart  contracts  blood  is  pumped  anteriorly,  pos- 
teriorly and  ventrally  through  arteries  that  branch  out  to  all  parts  of 
the  body.  Eventually  the  arteries  end,  and  the  blood  is  poured  into  the 
hemocoel.  It  then  drains  ventrally  into  the  perineural  sinus  from  which 
veins  carry  it  to  the  gills.  After  passing  through  capillaries  in  the  gills 
the  blood  continues  in  veins  toward  the  heart,  and  is  emptied  into 
the  pericardial  sinus.  It  enters  the  heart  during  relaxation  through 
slitlike  valves  in  its  sides.  The  blood  is  nearly  colorless,  but  becomes 
bluish  when  exposed  to  air  because  of  the  presence  of  the  oxygen- 
carrying  pigment  hemocyanin,  a  copper-containing  protein.  Hemo- 
cyanin  is  also  found  in  some  arachnids  and  molluscs. 

The  excretory  system  of  the  crayfish  is  the  green  glands  at  the 
base  of  the  second  antennae.  Each  consists  of  a  ventral  green  glandular 
part  bathed  in  blood  and  a  dorsal  bladder.  Wastes  removed  from  the 
blood  in  the  glandular  part  pass  through  ducts  and  are  stored  in  the 
bladder.  A  duct  from  the  bladder  opens  on  the  ventral  surface  of  the 
basal  antennal  segment. 

Paired  gonads  lie  beside  the  midgut  and  fuse  together  over  it 
(Fig.  16.8  A).  In  the  female  a  straight  oviduct  passes  ventrally  on  each 
side  to  open  on  the  basal  segment  of  the  middle  pereiopods  (segment 
12).  In  the  male  a  pair  of  sperm  ducts  follow  a  similar  but  convoluted 
course,  opening  on  the  basal  segment  of  the  last  pereiopods  (segment  14). 
The  sperm  are  peculiar  in  lacking  flagella  and  are  gathered  into  bun- 
dles or  spermatophores  by  secretions  of  the  ducts. 

At  copulation  the  male  turns  the  female  on  her  back,  holding  her 
with  pincers  and  other  chelate  pereiopods.  The  first  pleopods,  which 
otherwise  lie  forward  against  the  body  between  the  bases  of  the  pereio- 
pods, are  then  depressed  against  the  female.  Spermatophores  issuing 
on  the  last  pereiopods  pass  down  grooves  on  the  modified  pleopods  to 
the  female,  where  they  adhere  tightly  between  the  bases  of  the  posterior 
pereiopods.  In  the  lobster  and  in  some  crayfishes  the  females  have  a 
small  hollow,  the  seminal  receptacle,  between  the  bases  of  the  fourth 
and  fifth  pereiopods  where  spermatophores  are  fastened. 

Some  days  or  weeks  later  the  eggs  are  laid.  The  female  lies  on  her 
back  with  the  abdomen  folded  tightly  against  the  thorax.  As  the  eggs 
emerge  they  are  fertilized  and  glued  to  the  pleopods.  They  hatch  after 
several  weeks  into  miniature  crayfish  that  remain  attached  for  a  while 
to  the  mother. 


1 37.        Daphnia,  the  Water-Flea 

The  crayfish  is  a  good  example  of  a  large  crustacean,  but  many  of 
this  class  are  small  and  reduced  in  their  complexity.  The  water-fleas  (or- 
der Cladocera),  1  to  3  mm.  long  (Fig.  16.10),  are  described  here  as  an  ex- 
ample of  small  crustaceans  because  they  are  transparent  and  can  be 
studied  easily  without  dissection.  They  live  primarily  in  open  fresh 
water  as  part  of  the  plankton.  The  genus  Daphnia  is  represented  all 


PHYLUM   ARTHROPODA 


301 


over  the  world  by  numerous  species.  A  large  species,  D.  magna,  can 
often  be  obtained  from  fish  hatcheries  or  from  tropical  fish  stores  where 
they  are  raised  as  fish  food. 

The  head  of  Dapfinia  (Fig.  16.10)  bears  minute  first  antennae  brist- 
ling with  chemoreceptors,  and  very  large  biramous  second  antennae, 
which  are  locomotor  organs.  On  the  very  rapid  downstroke  the  an- 
tennae are  extended  laterally,  while  on  the  slower  upstroke  the  joints 
bend,  curving  them  close  to  the  body.  Behind  the  head  and  continuous 
with  it,  the  carapace  extends  posteriorly  and  ventrally  to  enclose  the 
rest  of  the  body. 

Within  the  carapace  are  all  the  mouth  parts  and  trunk  limbs.  Small, 
blunt  mandibles  are  followed  by  two  pairs  of  minute  maxillae  and  five 
pairs  of  flattened  biramous  legs.  The  legs  are  used  both  for  respiration 
and  for  filtering  microscopic  food  from  the  water.  The  last  four  body 
segments  bend  ventrally  and  lack  appendages.  The  body  is  made  of  six 
head  segments  and  nine  trunk  segments  in  all. 

By  the  beating  of  the  trunk  limbs  and  an  intricate  arrangement  of 
bristles,  food  filtered  from  the  water  is  passed  forward  along  the  limbs 
and  pressed  against  the  body  behind  the  mouth.  The  mandibles  chew 
the  front  end  of  the  food  mass,  pushing  pieces  of  it  into  the  mouth.  A 
short  esophagus  extends  dorsally  to  open  into  the  midgut,  a  long  tube 
that  curves  through  the  length  of  the  body  to  a  short  rectum  (hindgut) 
and  anus  on  the  terminal  segment.  From  the  anterior  end  of  the  mid- 
gut a  pair  of  curved  digestive  pouches,  comparable  with  the  digestive 
glands  of  the  crayfish,  extend  into  the  head. 

A  spacious  hemocoel  fills  the  body  and  limbs.   Dorsally  a  portion 

^,.,._~^    |-P^a.-atenna. 
Dig^estive 
pouch. 


Esophagus 


Brood 
pouch' 


Anu-S 


Midgut 


Figure  16.10.  Daphnia,  the  water  flea.  Side  view  (left)  with  one  side  of  the  carapace 
removed  to  show  enclosed  body  and  organs  (modified  from  Lockhead).  Ventral  view 
(right)  with  trunk  appendages  omitted. 


302 


THE   ANIMAL   KINGDOM 


Compound 
eye 


-Lenses 


Figure  16.11.  Part  of  the  head  of  Daphnia  showing  compound  eye  with  protruding 
lenses  and  muscles  (M)  of  the  right  side  attached  to  the  side  of  the  head  (at  A).  Also  shown 
are  the  optic  nerves  {ON),  optic  ganglion  (G)  and  brain.  The  nauplius  eye  (A'^)  is  de- 
scribed in  the  text. 


is  separated  off,  as  in  the  crayfish,  to  form  a  pericardial  sinus  containing 
the  heart.  Daphnia  lacks  arteries  and  veins.  The  heart  pumps  blood 
forward,  where  it  streams  among  the  head  organs,  curves  ventrally,  and 
flows  posteriorly  through  the  body  organs.  As  in  the  crayfish  the  hemo- 
coel  also  extends  into  the  carapace.  A  coiled  tubule  on  the  antero- 
ventral  part  of  each  side  of  the  carapace  is  the  shell  gland,  believed 
to  be  an  organ  of  excretion. 

Compound  eyes  arise  embryologically  as  paired  structures  that  later 
fuse  to  form  a  single  eye  (Fig.  16.11).  As  it  develops  it  sinks  into  the 
head  and  is  covered  over  by  the  exoskeleton,  enclosing  a  cavity.  Three 
pairs  of  muscles  from  the  sides  of  the  head  to  the  rim  of  the  eye  can 
turn  it  in  various  directions.  These  muscles  also  keep  the  eye  in  con- 
stant motion,  jiggling  it  several  times  a  second.  Since  the  eye  is  com- 
posed of  only  a  few  ommatidia,  each  of  which  gathers  light  from  a 
relatively  wide  area,  this  jiggling  may  improve  vision  (the  human  eye 
has  a  microscopic  jiggle,  and  our  visual  acuity  is  better  than  the  struc- 
ture of  the  eye  alone  would  predict).  Ominatidial  lenses  are  large  and 
protruding.  From  the  eye  a  bundle  of  optic  nerves  passes  to  a  large 
optic  ganglion  connected  with  a  still  larger  brain.  The  circumesoph- 
ageal  connectives,  subesophageal  ganglion  and  the  few  ventral  ganglia 
are  seldom  visible. 

Attached  to  the  antero-ventral  margin  of  the  brain  is  another  un- 
paired median  eye,  the  nauplius  eye  (Fig.  16.11).  This  eye  is  found  as 
the  only  eye  in  nauplii,  where  it  typically  has  a  central  pigment  mass 
with  one  anterior  and  two  lateral  groups  of  visual  cells.  It  frequently 
persists  in  adidt  crustaceans.  In  Daphnia  the  anterior  group  is  reduced 
and  divided  into  a  single  anterior  cell  and  two  ventral  cells.  Each 
lateral  group  is  reduced  to  a  single  postero-lateral  cell.  This  eye  is 
suspended  in  the  blood,  its  cells  anchored  by  delicate  fibers.  The  outer 
ends  of  the  cells  turn  back  as  nerves  to  the  brain.  This  is  the  only  in- 


PHYLUM   ARTHROPODA 


303 


verted  eye  found  in  the  phylum  Arthropoda,  and  is  another  distinguish- 
ing feature  of  the  class  Crustacea. 

Most  daphnias  are  females  which  reproduce  parthenogenetically. 
Paired  ovaries  lie  beside  the  midgut.  Eggs  are  laid  through  ducts  that 
open  dorsally  into  a  brood  pouch,  an  enlarged  cavity  between  the  back 
of  the  body  and  the  carapace.  The  eggs  remain  here  until  they  develop 
into  small  daphnias  resembling  their  parents.  When  the  environment 
becomes  unfavorable  (too  cold,  no  food,  etc.)  some  of  the  young  mature 
as  males  while  the  females  produce  "resthig  eggs."  These  are  fertilized 
and  shed  to  the  bottom  where  they  may  last  for  years  without  hatching. 
The  same  females  produce  both  parthenogenetic  and  resting  eggs,  de- 
pending upon  whether  the  environment  is  favorable  or  unfavorable. 

138.        Other  Crustaceans 

Small  crustaceans  are  usually  considered  to  be  the  more  primitive 
Crustacea.  Of  these  a  natural  group  is  formed  by  the  orders  Anostraca 
(brine  shrimps  and  fairy  shrimps),  Notostraca,  Conchostraca  and  Clado- 
cera  (water-fleas)  (Fig.  16.12),  in  which  the  trunk  limbs  are  biramous, 
flattened,  and  used  for  both  respiration  and  feednig.  These  orders  form 
the  subclass  Branchiopoda.  They  are  mostly  fresh-water  organisms,  and 
are  especially  abundant  in  temporary  ponds. 

Other  small  crustaceans  include  the  orders  Ostracoda,  Copepoda 
and  Cirripedia  (Fig.  16.18).  The  first  two  are  common  in  both  fresh 
and  salt  water.  The  last  are  the  barnacles,  found  only  in  the  seas.  Cope- 
pods  are  the  most  abundant  of  all  crustaceans,  forming  dominant  or- 
ganisms of  salt  and  fresh-water  plankton.  The  evolutionary  relations  of 
these  groups  to  each  other,  to  the  Branchiopoda,  and  to  the  Malacos- 
traca  are  somewhat  obscure. 


^  >^N  v'*"'^  V  ^^ 


</' 


Anostrstca. 


^^^^M-" 


"CoTicbostra.C3u 


Motostra-c 


Figure  1 6.1 2.     Other  members  of  the  subclass  Branchiopoda.  (After  Borradaile  et  al.) 


304  ^W£   ANIMAL   KINGDOM 

Nauplius  eye 

1st  a-iitcnna- 
2nd  a.nt;enn.a.--~| 

Mandiblcz.  and 
1st  m.a>cillsL 

2-nd  max  ill  a.- 
and  max  II 1  ipcd 


1st  a.ntenna 
2nd  antenna: 


•Trun>[- 
limbs 


1st  a.Tit(Z.nna 


Figure  16.13.  Additional  orders  of  small  Crustacea.  A,  Order  Copepoda.  B,  Order 
Ostracoda,  with  a  hinged  carapace  enclosing  head  and  body.  C,  Order  Cirripedia,  the 
barnacles,  attached  by  an  enormous  first  antenna,  with  the  body  enclosed  in  calcareous 
plates.  (From  various  sources.) 


Mysidacea.. 


•^Decapod- 


Is  opod-a. 


Figure  16.14.     Some  of  the  orders  of  the  subclass  Malacostraca.  (The  first  three  are 
after  Borradaile,  et  al.,  the  fourth  after  Parker  and  Haswell.) 


The  Malacostraca  are  divided  into  nine  orders,  of  which  five  will 
be  mentioned  here  (see  the  appendix  for  all  of  them).  The  Mysidacea 
(Fig.  16.14)  are  abundant,  delicate,  shrimp-like  animals  living  near  the 
bottoms  of  shallow  seas  and  arctic  fresh  water.  They  usually  rise  into 
the  upper  water  as  plankton  at  night.  The  Euphausiacea  are  similar, 
living  deep  in  the  open  ocean  by  clay  and  coming  near  the  surface  at 


PHYLUM   ARTHROPODA 


305 


night.  They  are  remarkable  for  their  Hght  organs  and  for  the  amplitude 
of  their  daily  migration.  Schools  of  them  are  a  food  for  the  filtering 
whales. 

The  Isopoda  are  dorsoventrally  flattened  crustaceans  without  cara- 
paces. They  are  found  in  both  salt  and  fresh  water.  This  order  also  in- 
cludes the  only  truly  terrestrial  crustaceans,  the  pill-bugs  and  sow-bugs 
(Fig.  16.14).  The  Amphipoda  (shown  on  Fig.  16.2)  also  lack  carapaces, 
but  they  are  compressed  laterally  rather  than  dorsoventrally.  They  are 
common  in  all  waters,  forming  an  important  fish  food.  Finally,  the  order 
Decapoda  includes  a  variety  of  familiar  forms  such  as  shrimps,  crabs  and 
lobsters. 


139.        The  Subphylum  Labiata 

All  labiates  have  a  distinct  head  enclosed  in  a  head  capsule,  which 
usually  bears  eyes,  a  pair  of  many-jointed  antennae,  mandibles,  maxil- 
lae, and  a  labium  formed  by  the  embryonic  fusion  of  the  secontl  max- 
illae. Trunk  appendages  are  uniramous  and  usually  seven  jointed, 
ending  in  terminal  claws.  The  subphylum  can  be  divided  into  two 
superclasses,  the  Myriapoda  in  which  most  of  the  body  segments  have 
walking  legs,  and  the  Hexapoda  in  which  only  the  first  three  body 
segments  have  walking  legs. 

Myriapods  are  simpler  and  less  specialized.  They  lack  compound 
eyes,  having  instead  aggregates  of  ommatidia  clustered  on  the  sides  of 
the  head.  The  body  segments  are  similar  to  one  another  like  those  of 
the  annelids.  Behavior  patterns  are  simjjle. 

Of  the  myriapods,  centipedes  and  millipedes  are  the  only  familiar 
groups.  Centipedes,  class  Chilopoda  (Fig.  16.15  A),  are  predaceous  ani- 
mals hunting  down  insects  and  killing  them  with  their  poison  claws, 
which  are  the  modified  legs  of  the  first  body  segment.  Each  of  the  re- 
maining body  segments  except  the  last  has  a  pair  of  long  walking  legs. 
The  total  number  of  legs  ranges  from  15  to  173  pairs  in  different  species. 


B 


Figure  16.15.     Examples  of  the  Myriapoda.  A,  Order  Chilopoda,  the  centipedes.  B, 
Order  Diplopoda,  the  millipedes.   (Villee:  Biology.) 


306  ''Wf   ANIMAL   KINGDOM 


Antenna.- 


Coinpound  eye 


'Thoraci  J      /^^V ' 

legs  f=^lr- 


AbdoTninal  linits — , 


Figure  16.16.     Primitive  wingless  insects  (Apterygota),  showing  a  silverfish  (left)  and 
a  springtail  (right).  (.After  Lubbock  (left)  and  Carpenter  and  Folsom  (right).) 


Centipedes  can  run  rapidly,  the  legs  moving  in  waves  from  rear  to  front. 
Coordination  follows  the  annelid  pattern  with  reflex  pathways  between 
adjacent  segments. 

Millipedes,  class  Diplopoda  (Fig.  16.15  B),  are  herbivorous  scaven- 
gers, feeding  primarily  on  decayed  and  living  plant  material.  The  first 
maxillae  appear  in  the  embryo  but  later  disappear.  The  labium  is 
well  developed,  and  its  segment  is  fused  ventrally  with  the  first  body 
segment.  The  next  three  body  segments  remain  single,  but  beginning 
with  the  fifth  and  sixth  every  two  segments  fuse  together  during  de- 
velopment. Since  each  embryonic  segment  has  a  pair  of  legs,  most  of 
the  apparent  segments  of  the  adult  body  bear  two  pairs  of  legs,  giving 
the  order  its  name.  Millipedes  may  have  from  13  to  nearly  200  pairs  of 
legs,  manipulated  like  those  of  the  centipedes.  The  legs  are  short,  and 
millipedes  cannot  move  fast. 

The  superclass  Hexapoda  includes  only  the  class  Insecta,  although 
there  is  a  growing  tendency  to  separate  the  primitive  wingless  insects 
such  as  the  silverfish  and  springtails  (Fig.  16.16)  from  the  winged 
groups.  These  wingless  forms  have  small  appendages  on  the  abdominal 
segments,  suggesting  a  relationship  with  the  myriapods.  Silverfish  do, 
however,  have  compound  eyes  like  the  winged  insects. 

The  insects  proper  are  the  winged  forms,  including  all  hexapods 
lacking  abdominal  appendages  except  those  at  the  posterior  end  used 
in  reproduction.  Typically  they  have  two  pairs  of  membranous  wings, 
on  the  second  and  third  thoracic  segments,  ft  is  beyond  the  scope  of 
this  book  to  represent  adequately  an  invertebrate  class  that  is  divided 
into  25  orders.  The  cockroach  will  be  presented  as  a  generalized  insect 


PHYLUM    ARTHROPODA 


307 


and  some  distinguishing  features  of  the  larger  orders  will  be  described 
later.  Finally,  the  honeybee  will  be  described  as  an  example  of  a  spe- 
cialized insect. 

140.        Periplaneta  amerkana,  a  Cockroach 

Cockroaches  are  the  only  order  of  living  insects  that  have  a  fossil 
record  extending  back  into  the  Pennsylvanian  period,  250  million  years 
ago.  Other  orders  of  insects  existing  then  have  either  become  extinct 
or  evolved  sufficiently  to  warrant  separation  into  new  orders.  Cock- 
roaches have  also  been  conservative  in  their  habits,  shifting  only  from 
the  steaming  swamps  of  the  coal  age  to  the  steaming  jungles  and  steam- 
Compound  eye 

Prothora-cic  notian 
Labrum 

Antenna 


Co:x:aL 

TrochsLnter 
Fcnn-ur 


Win^ 


Tib 


la.' 


Ta-rsiLS 


M(Z.tctlhora.c  let 
sternu-m. 


Abdominal 
sternum 


—  Cla.v/ 
Pulvillus 


10^^  abdominal 
notum. 


Figure  16.17.     \'entral  \  iew  of  the  cockroach.  (After  Comstock.) 

heated  buildings  of  today.  They  require  both  moisture  and  warmth  for 
survival. 

The  large  native  cockroach  P.  aniericana  (Fig.  16.17)  is  found  in 
greenhouses  and  institutional  buildings.  Adults  are  a  dark  reddish  brown 
color,  25  to  35  mm.  long.  Like  all  cockroaches  these  have  flattened 
bodies  with  long  legs  on  which  they  can  run  rapidly  and  escape  into 
narrow  crevices. 


1 41 .        External  Morphology  of  the  Cockroach 

The  head  (Fig.  16.18)  has  dorso-lateral  compound  eyes,  anterior  an- 
tennae and  ventral  mouth  parts.  The  head  is  usually  bent  beneath  the 
body  so  that  the  eyes  actually  look  anteriorly.  The  front  of  the  head 
extends  down  as  a  movable  upper  lip  or  labrum  behind  which  are 
mandibles,    maxillae  and   labium   (Fig.    16.18)   which   are   suited   to  an 


308 


THE   ANIMAL   KINGDOM 


'^mm<///////////////////////////^^^^ 


folded  BacK' 
\o  sKov\r 
iaibiurn) 


y/V''///////v//////,//////////////////////////^^^^^ 


Figure  16.18.     Head  and  mouth  parts  of  the  cockroach  (mouth  parts  viewed  from 
behind.  (Combined  from  Comstock  and  Parker  and  Haswell.) 


omnivorous  habit.  Each  mandible  is  a  single  segment  with  sharp  cutting 
and  grinding  teeth  along  the  medial  edge.  Each  maxilla  has  seven  seg- 
ments ol  which  the  last  five  lorm  a  tactile  palp.  The  second  segment  is 
large  and  bears  two  processes.  The  labium  is  similarly  constructed,  ex- 
cept that  the  two  basal  segments  are  fused  and  the  palps  are  four- 
jointed.  The  processes  on  maxillae  and  labium,  together  with  the 
labrum,  manipulate  and  hold  food  for  the  mandibles. 

A  short  neck  joins  the  head  to  the  thorax.  The  latter  is  formed  of 
three  fused  segments,  the  prothorax,  mesothorax  and  metathorax.  The 
back  or  notum  of  the  prothorax  is  expanded  as  a  shield  partially  cov- 
ering the  head  and  mesothorax.  The  nota  of  the  other  two  segments 
are  covered  by  the  wings.  On  the  ventral  side  oblique  lateral  plates  or 
pleura  join  the  three  nota  to  the  sterna,  three  triangular  plates  in  the 
midline.  Each  sternum  bears  a  pair  of  legs,  while  the  nota  and  pleura 
of  the  last  two  segments  articulate  with  the  wings. 

Each  leg  (Fig.  16.17)  is  composed  of  a  large  flattened  coxa,  small 
trochanter,  long,  stout  femur,  long,  slender  tibia,  and  five  small  seg- 
ments collectively  called  the  tarsus.  Many  of  these  segments  are  beset 
with  spines.  Each  tarsal  segment  ends  ventrally  in  a  small  adhesive 
pad.  The  last,  called  the  pulvillus,  is  the  largest  and  is  flanked  by  a 
pair  of  tarsal  claws.  Joints  between  coxae  and  body  permit  only  a 
slight  movement,  and  the  trochanters  are  fused  immovably  onto  the 
femurs.  Most  of  the  locomotion  is  derived  from  movements  between 
coxae  and  trochanters,  and  between  femurs  and  tibias.  The  claws  and 
pulvilli  provide  for  a  grip  on  any  kind  of  surface,  and  the  several  small 


PHYLUfA   ARTHROPODA 


309 


Figure  16.19.     Wings  of  the  cockroach,  showing  the  numerous  veins  characteristic 
of  the  more  primitive  insects. 

tarsal  joints  allow  freedom  between  the  position  at  which  a  grip  is  best 
maintained  and  the  direction  of  the  tibia. 

The  anterior  wings  (Fig.  16.19)  at  rest  are  folded  over  the  body, 
covering  the  posterior  wings.  They  are  slender  and  leathery,  protecting 
the  hind  wings  when  the  animal  passes  beneath  objects.  The  posterior 
wings  are  pleated,  and  fold  fanwise  when  not  in  use.  In  flight  all  four 
wings  are  held  out  to  the  sides  and  flapped  dorso-ventrally.  Cockroaches 
seldom  fly,  and  do  so  primarily  in  search  of  new  habitats.  Each  wing  is 
strengthened  by  a  number  of  hollow  veins  which  are  continuous  with 
the  hemocoel  of  the  body.  Their  arrangement  or  venation  is  a  prom- 
inent characteristic  in  insect  classification. 

The  abdomen  (Fig.  16.17)  is  made  of  ten  segments,  each  slightly 
overlapping  the  segment  behind  and  divisible  into  a  dorsal  notum  and 
ventral  sternum.  Nota  of  the  eighth  and  ninth  segments  are  telescoped 
completely  out  of  sight  beneath  that  of  the  seventh,  and  the  tenth  ex- 
tends posteriorly  as  a  notched  plate.  From  the  sides  of  the  tenth  seg- 
ment emerge  a  pair  of  cerci,  antenna-like  structures  sensitive  to  air 
currents  and  low  frequency  sounds.  The  anus  opens  posteriorly  on  the 
tenth  segment,  with  the  reproductive  openings  beneath  it. 

Between  the  prothorax  and  eighth  abdominal  segments  are  ten  pairs 
of  spiracles,  openings  to  the  respiratory  system,  between  adjacent  seg- 
ments just  beneath  the  nota. 


1 42.        Internal  Anatomy  of  the  Cockroach 

The  digestive  tract  (Fig.  16.20)  includes  fore-,  mid-  and  hindguts  as  in 
the  Crustacea.  The  mouth  opens  into  a  mouth  cavity  that  receives  ducts 
from  a  pair  of  large,  bilobed  salivary  glands  in  the  mesothorax.  Their 
secretion  digests  starches.  The  mouth  cavity  continues  as  a  long  narrow 
esophagus  to  a  posterior  enlargement,  the  crop.  The  crop  opens  into  a 
small  muscular  gizzard  containing  six  strong  teeth  and  numerous  bris- 
tles. All  of  these  organs  are  part  of  the  foregut  and  are  lined  with 
chitin. 

The  gizzard  opens  into  the  midgut,  a  narrow  stomach.  Anteriorly 


310 


THE   ANIMAL   KINGDOM 


Testis  and  Spe-rm  duct 

Hea-T-tn 
<KRcctixm 


Malpi^hiantiibul^s  Sali^^ary  reservoir 


Salivary  glands 


soph ados 
rBrain. 


EjaLCulat-ory 


haT3?rT)C 

plna^e^ 

Mou.th.-J    ganglion. 


Seminal  vesicles 
Ne-rve  ganglion- 
Figure  16.20.     Internal  anatomy  of  the  cockroach  (male).  (After  Metcalf,  Flint  and 
Metcalf.) 

the  stomach  has  eight  digestive  pouches.  The  stomach  curves  around 
to  the  anterior  end  of  the  abdomen  where  it  joins  the  hindgut.  This 
includes  a  long  intestine  and  a  short  rectum,  lined  with  chitin.  The 
stomach  is  lined  with  a  simple  gastrodermis,  surrounded  by  thin  cir- 
cular and  longitudinal  muscle  layers.  The  gizzard  projects  into  the 
stomach,  and  the  posterior  cells  of  the  foregut  secrete  chitin  contin- 
uously, forming  a  tubular  peritrophic  membrane  that  surrounds  the 
food  as  it  passes  through  the  stomach  and  intestine.  This  remarkable 
structure  is  found  in  many  insects. 

Digestion  occurs  primarily  in  the  crop.  Secretions  from  the  digestive 
pouches  are  passed  forward  as  in  the  crayfish.  Mechanical  breakdown 
is  aided  by  the  gizzard,  and  the  finely  pulverized  and  digested  food  is 
then  passed  into  the  stomach.  Water  and  dissolved  nutrients  diffuse 
through  the  peritrophic  membrane  to  be  absorbed  by  the  lining  of  the 
stomach  and  digestive  pouches.  The  remaining  water  is  absorbed  in 
the  intestine,  leaving  dry  fecal  pellets  to  be  eliminated  through  the  anus. 

At  its  anterior  end  the  intestine  receives  six  groups  of  delicate 
Malpighian  tubules.  These  are  blind  tubules  lying  in  the  hemocoel. 
They  pick  up  waste  from  the  blood  and  excrete  it  into  the  intestine. 
Nitrogenous  wastes  are  excreted  as  uric  acid,  an  adaptation  which  con- 
serves body  water  (p.  95).  Each  tubule  has  a  muscular  coat  and  its 
slow  writhing  aids  the  passage  of  wastes  down  its  lumen.  These  are  the 
excretory  organs  of  all  labiates. 

The  brain  (Fig.  16.20),  formed  from  three  parts  as  in  the  Crustacea,  is 
a  bilobed  structure  lying  over  the  esophagus.  The  subesophageal  gang- 
lion is  formed  by  fusion  of  the  remaining  three  pairs  of  head  ganglia 
and  lies  beneath  the  esophagus.  These  are  connected  by  stout  circum- 
esophageal  connectives  forming  a  nerve  ring  around  the  esophagus. 
The  ventral  cord  continues  posteriorly  with  three  thoracic  and  six  ab- 
dominal pairs  of  ganglia.  The  last  pair  supplies  all  of  the  remaining 
abdominal  segments. 

The  compound  eyes  of  insects  are  remarkably  like  those  of  crusta- 
ceans. Each  ommatidium  of  the  cockroach  has  the  same  general  parts, 


PHYLUM   ARTHROPODA 


311 


all  of  ectodermal  origin  except  the  optic  nerve  itself.  Compound  eyes 
are  widespread  in  the  arthropods,  being  found  in  the  trilobites,  crus- 
taceans, king  crabs  and  insects.  They  are  lacking  in  the  other  groups  of 
living  arachnomorphs  and  labiates. 

Most  insects  also  have  ocelli,  small  eyes  on  the  top  of  the  head. 
Typically  three  of  these  are  arranged  in  a  triangle.  Each  ocellus  (Fig. 
16.21)  is  a  group  of  retinuli,  comparable  to  the  lower  portions  of  om- 
matidia,  underlying  a  single  large  lens.  In  most  insects  the  retinuli  lie 
too  close  to  the  lens  for  an  image  to  be  formed.  The  function  of  these 
eyes  is  not  understood.  They  are  believed  to  monitor  light  intensity  and 
to  influence  the  insect's  general  level  of  activity  rather  than  to  provide 
spatial  information  on  light  distribution.  In  Periplaneta  the  ocelli  are 
degenerate. 

Organs  of  touch  are  special  tactile  bristles  scattered  over  the  body 
and  especially  prominent  on  the  antennae,  palps  and  cerci.  On  the  cerci 
they  vibrate  in  response  to  wind  or  low  sounds.  Smell  and  taste  are 
mediated  by  chemoreceptors  clustered  on  these  same  organs.  The  chemo- 
receptors  are  projecting  cones  with  a  very  thin  exoskeleton  kept  moist 
by  glandular  secretions.  Those  on  the  antennae  and  cerci  are  olfactory, 
those  on  the  palp  are  gustatory.  The  distinction  between  smell  and  taste 
depends  upon  whether  the  chemical  sensed  is  airborne  or  dissolved  in 
liquid. 

The  tracheal  tubes  found  in  all  labiates  are  a  respiratory  system  of 
air  ducts  leading  in  from  the  spiracles  to  all  the  tissues  of  the  body  (Fig. 
16.22).  The  larger  tubes  anastomose,  forming  a  network  from  which 
smaller  tubes  ramify.  Each  is  a  cylinder  of  epidermal  tissue  lined  with  a 
thin  layer  of  chitin  thickened  spirally  to  provide  strength.  The  smallest 
branches  end  blindly  in  tracheoles  (Fig.  16.22),  minute  branching  tun- 
nels within  the  cytoplasm  of  end  cells.  End  cells  are  applied  closely  to 
the  surfaces  of  other  cells.  The  cockroach  flushes  air  in  and  out  of  the 
system  by  respiratory  movements,  or  breathing,  in  which  the  abdomen  is 
alternately  flattened  and  relaxed  by  the  contraction  and  relaxation  of 
stout  vertical  muscles  within  it. 


eal  lens 

Epidermis 
Rhabdome. 


Pi6m<zjrrt  cell 
R<ztinula_cell 


Nerve  fibers 


Figure  16.21.     Diagrammatic  section  through  an  insect  ocellus.  (After  Comstock.) 


312  THE   ANIMAL  KINGDOM 


^o  win^S 


Spiracles 


TracheoleS 


B 


Tra.ch.ea.' 


Epiderinis 


Spirally 
thickeried  chitin' 

Figure  16.22.  Tracheal  system  of  the  cockroach.  A,  The  major  tracheal  trunks. 
(After  Parker  and  Haswell.)  B,  Diagrammatic  view  of  the  tracheoles  of  a  single  cell. 
C,  Detailed  structure  of  a  trachea.   {B  and  C  adapted  from  Wigglesworth.) 


The  hemocoel  of  insects  is  a  single,  large,  branched  space  without 
a  separate  pericardial  sinus.  In  the  cockroach  the  heart  is  a  long  dorsal 
tube,  expanded  in  each  segment  of  the  thorax  and  abdomen.  In  each 
segment  a  pair  of  valves  admits  blood  from  the  hemocoel.  Anteriorly 
the  heart  continues  as  a  short  artery  that  ends  behind  the  brain.  Con- 
traction usually  proceeds  forward  along  the  heart  and  can  be  seen 
through  the  body  wall  of  an  uninjured  roach.  Relieved  of  respiratory 
duties  by  the  tracheal  system,  the  blood  in  labiates  serves  primarily  to 
distribute  nutrients  to  the  body  and  to  transport  wastes  to  the  Mal- 
pighian  tubules. 

The  male  cockroach  has  a  terminal  complex  of  copulatory  organs 
(Fig.  16.20)  formed  from  the  sternum  of  the  last  segment  and  the  much 
modified  appendages  of  the  eighth  and  ninth  abdominal  segments.  Ex- 
cept for  a  pair  of  ventral  styles  on  the  ninth  segment  these  organs  are 
usually  retracted  into  the  body.  Small  testes  lie  dorsally  in  the  fourth 
and  fifth  abdominal  segments  from  which  a  pair  of  sperm  ducts  lead  to 
seminal  vesicles,  clusters  of  delicate  tubules  in  the  sixth  and  seventh 
segments  where  the  sperm  are  stored.  At  copulation  sperm  are  passed 
through  a  single  stout  ejaculatory  duct  that  opens  among  the  copulatory 
organs. 

The  female  has  a  pair  of  large  ovaries,  each  composed  of  eight  lobes 
in  segments  4  to  6.  Within  each  lobe  the  smallest  eggs  are  anterior,  the 
larger  and  more  mature  eggs  posterior,  giving  it  a  beaded  appearance. 
Paired  oviducts  from  the  ovaries  join  to  open  ventrally  on  the  eighth 
segment.   The   ninth  segment  has  a  ventral  opening  to  a  seminal   re- 


PHYLUM    ARTHROPODA 


313 


ceptacle  where  sperm  are  received.  The  last  sternites  and  appendages 
are  greatly  modified  to  aid  in  copulation  and  in  carrying  the  eggs.  As 
the  eggs  are  laid  and  fertilized  they  are  covered  with  secretions  from  a 
pair  of  accessory  glands.  The  two  glands  secrete  dissimilar  materials 
that  react  in  the  presence  of  air  to  produce  a  tanned  protein  cover. 
The  case  thus  formed  is  carried  about  until  the  eggs  hatch.  Young 
cockroaches  resemble  adults  but  lack  wings;  they  mature  in  seven  molts. 


I 


143.        Classification  of  the  Insecta 

Insects  are  divided  into  the  wingless  subclass,  Apterygota,  and  a 
winged  subclass,  Pterygota.  The  former  includes  the  silverfish  (order 
Thysanura)  and  springtails  (order  Collembola)  (Fig.  16.16). 

The  Pterygota  are  divided  by  paleontologists  into  the  Paleoptera, 
in  which  the  wings  are  held  permanently  at  right  angles  to  the  body, 
and  the  Neoptera,  in  which  the  wings  are  folded  back  over  the  body 
when  not  in  use.  Paleopterans  were  abundant  in  ancient  times,  and 
included  many  orders  now  extinct.  Surviving  are  the  dragonflies  and 
damselfiies  (order  Odonata)  and  mayflies  (order  Ephemerida)  (Fig.  16.23), 
groups  that  have  aquatic  young.  The  adults  are  forced  to  stay  out  in  the 
open  to  avoid  breaking  their  wings,  and  have  flight  as  the  only  means 
of  escape.  Neopterans,  with  hinged  wings,  not  only  can  escape  by  flight 
but  also  may  run  fast  or  hide  in  crevices.  It  is  interesting  in  this  respect 
that,  although  wingless  species  are  found  in  all  of  the  neopteran  orders, 
none  of  the  living  or  extinct  paleoptera  are  wingless. 


Figure  16.23.     Living  Paleoptera.  Orders  Odonata   (left)  and  Ephemerida  (right). 
Adults  above,  and  nymphs  below.  (After  Borror  and  DeLong.) 


314  ^^^^   ANIMAL   KINGDOM 


MULT 


ADULT 


Figure  16.24.  Metamorphosis  in  the  insects,  showing  a  comparison  of  an  exoptery- 
gote  (grasshopper)  and  an  endopterygote  (cecropia  moth).  (Turner:  General  Endocrinol- 
ogy-) 


PHYLUM   ARTHROPODA 


315 


per).  li,  Hcmiptera  (Icafhopper).  Hemiptcrans  have  sucking  mouth  parts  (C).  Wingless 
forms  in  each  order  inchide  tlie  camel  cricket  (£)  and  the  bedbug  (D).  Other  orders  in- 
clude the  Blattaria  (cockroach,  Fig.  16.17)  and  the  Isoptera  (termite,  Fig.  17.18). 


Figure  16.26.  The  major  orders  of  the  Endopterygota.  The  coleoptera  (beetles)  have 
thick,  rigid  forewings.  1  he  Lepidoptera  (butterflies  and  moths)  have  scales  on  the  wings 
and  sucking  mouth  parts.  The  Hymenoptera  (bees,  ants,  etc.)  have  membranous  wings 
with  few  veins.  The  Diptera  (flies)  have  two  wings,  the  hindwings  being  reduced  to  bal- 
ancing organs. 

Neopterans  are  divided  into  the  Exopterygota  and  Endopterygota. 
In  the  former,  as  in  the  Paleoptera,  the  wings  appear  in  juvenile  forms 
as  external  wing  buds  (Fig.  16.24)  that  become  larger  at  each  molt, 
finally  becoming  full-sized  wings.  Such  development  is  part  of  a  pattern 
called  incomplete  metamorphosis  and  the  young  are  called  nymphs.  The 


316  TH^   ANIMAL   KINGDOM 

group  includes  many  orders,  such  as  the  Orthoptera  (grasshoppers, 
crickets,  mantids  and  roaches),  Isoptera  (termites)  and  Hemiptera  (the 
true  bugs).  Representatives  are  shown  in  Figure  16.25.  The  Endop- 
terygota  are  the  so-called  "higher"  insects.  The  young  have  internal 
wing  buds  that  later  evert  suddenly  in  a  resting  stage,  the  pupa  (Fig. 
16.24),  and  become  full-sized  wings  on  the  following  molt.  This  is  asso- 
ciated with  marked  changes  in  appearance,  so  that  the  young  seldom 
resemble  the  adults.  Such  development  is  called  complete  metamorpho- 
sis and  the  young  are  called  larvae.  The  Endopterygota  also  includes 
many  orders  (Fig.  16.26)  such  as  the  Lepidoptera  (butterflies  and  moths), 
Coleoptera  (beetles),  Hymenoptera  (bees,  ants,  wasps),  and  the  Diptera 
(flies,  mosquitoes).  Most  of  the  species  of  insects  are  included  in  these 
four  orders,  which  are  further  described  with  the  illustrations.  Although 
the  butterflies  and  some  moths  cannot  fold  the  wings  flat  upon  the 
body,  the  wing  articulations  and  muscles  indicate  that  this  represents  an 
evolutionary  loss,  and  that  these  insects  are  properly  grouped  with  the 
Neoptera. 

144.        Metamorphosis 

A  change  in  the  shape  or  relative  size  of  body  parts  during  growth 
is  called  metamorphosis.  In  organisms  such  as  man  and  other  mammals 
the  young  resemble  adults  and  little  metamorphosis  takes  place.  In  other 
organisms  metamorphosis  may  be  marked.  We  have  already  described  a 
number  of  examples,  such  as  the  coelenterate  polyp  and  medusa,  and 
the  larval  and  adult  tapeworms,  flukes,  molluscs  and  annelids. 

The  apterygote  insects  show  very  little  metamorphosis.  Young  hatch 
as  miniatures  of  their  parents,  easily  recognizable  as  to  species.  In  the 
living  Paleoptera  the  young  are  aquatic  and  often  have  a  very  different 
appearance  from  their  parents  (Fig.  16.23).  They  not  only  lack  wings, 
but  have  a  different  body  shape  so  that  the  species  cannot  be  identified 
unless  they  are  reared  to  maturity.  Although  the  young  differ  from  the 
adults,  their  bodies  are  complete  with  jointed  legs  and  compound  eyes. 
Metamorphosis  in  the  Exopterygota  is  similar,  except  that  since  both 
young  and  adults  are  terrestrial,  they  do  not  differ  so  much  in  appear- 
ance. Young  grasshoppers,  for  example,  are  easily  recognized  as  grass- 
hoppers. 

In  the  Endopterygota  the  young  not  only  show  little  resemblance  to 
the  adults,  but  often  lack  such  structures  as  compound  eyes,  jointed  legs, 
and  wings.  Some  larvae  have  no  appendages  at  all.  As  the  larva  grows, 
wing  buds  develop  inside  the  body,  but  are  not  evident  externally. 
Finally,  in  a  single  molt  the  appearance  changes  markedly  as  the  animal 
pupates.  The  pupa  is  a  nonfeeding  stage  (Fig.  16.24)  in  which  all  of  the 
adult  appendages  are  visible  as  external  buds.  Internally,  whole  organ 
systems  may  be  dissolved  and  replaced  as  the  adult  form  is  developed. 
The  pupa  molts  to  become  a  full-grown  adult. 

Metamorphosis  is  considered  to  involve  the  same  phenomena  that 
appear  in  the  formation  and  development  of  embryos.  Gastrulation,  the 
formation  of  limbs  and  development  of  organ  systems  in  the  embryo  are 


PHYLUM    ARTHROPODA 


317 


actually  forms  of  embryonic  metamorphosis.  Similarly  the  metamorphosis 
of  young  into  adults  is  a  kind  of  delayed  embryonic  development.  As 
yet  very  little  is  known  of  the  causes  and  forces  involved  in  metamorph- 
osis. The  role  of  hormones  in  insect  metamorphosis  will  be  discussed  in 
the  next  chapter. 

145.        Apis  mellifera,  the  Honeybee 

As  an  example  of  a  highly  specialized  insect  the  honeybee  offers 
interesting  contrasts  to  the  cockroach.  Sense  organs,  mouth  parts,  wings, 
legs  and  many  internal  organs  are  more  diversified  and  specialized  than 
in  the  cockroach.  The  worker  bee  (Fig.  16.27),  a  sterile  female,  shows  most 
of  these  specializations. 

The  most  striking  modifications  on  the  head  concern  the  mouth 
parts.  Labial  palps  and  maxillae  are  fused  into  a  sucking  tube  containing 
a  tongue  formed  from  the  middle  portion  of  the  labium.  When  this 
tube  is  folded  back  against  the  body  the  short  mandibles  can  still  be 
used  as  jaws,  and  the  bee  is  thus  one  of  the  few  insects  that  can  both 
suck  and  chew. 

The  wings  are  small  in  relation  to  body  size  and  have  a  much  modi- 
fied and  reduced  venation.  The  rear  wing  bears  a  row  of  minute  hooks 
that  fasten  to  the  front  wing,  forming  a  single  flight  blade.  The  round 
and  compact  thorax  houses  powerful  (light  muscles. 

The  legs  have  numerous  modifications.  The  first  tarsal  segment  of 
each  leg  has  a  patch  of  bristles  on  its  inner  surface.  Those  of  the  first 
and  second  pairs  of  legs  are  pollen  brushes.  The  bristles  on  the  tarsi 
of  the  third  pair  of  legs  are  arranged  in  regular  rows  forming  pollen 
combs.  The  tibia  of  the  third  pair  of  legs  have  a  concave  surface  fringed 
with  curved  hairs  which  forms  a  pair  of  pollen  baskets.  The  lower  inner 
edge  of  each  tibia  has  a  row  of  stout  bristles,  the  pecten,  beneath  which 
the  upper  end  of  the  first  tarsal  segment  is  expanded  and  fiattened  to 
form  an  auricle. 

As  the  bee  visits  flowers  pollen  sticks  to  its  hairy  body.  This  pollen 


I 


ne 
Pecten-^  ^Auricle    ^Pollen  brush        ^^^^^^  j 

Figure  16.27.     The  worker  honeybee.  (Adapted  from  Casteel.) 


318  '■Wf   ANIMAL   KINGDOM 

is  a  major  source  of  protein  in  the  bee  diet  and  must  be  collected  care- 
fully. The  anterior  pollen  brushes  collect  pollen  from  the  head,  the 
middle  brushes  gather  it  from  the  thorax  and  the  anterior  brushes,  while 
the  combs  collect  it  from  the  abdomen  and  the  second  pair  of  brushes. 
Each  pair  of  legs  is  drawn  between  those  behind  to  effect  transfer. 
Finally  the  pollen  on  one  comb  is  scraped  off  by  the  pecten  of  the  op- 
posite leg  and  it  falls  onto  the  auricle.  The  tarsus  is  then  bent  so  as  to 
force  the  pollen  up  the  outer  surface  of  the  tibia  into  the  pollen  basket. 
The  pollen  adheres  through  its  own  moisture  and  may  become  a  sizable 
mass.  Although  this  sounds  like  a  very  complex  process,  the  bee  actually 
does  it  all  in  midfiight  with  very  little  loss  of  pollen. 

The  base  of  the  first  tarsal  segment  of  each  front  leg  has  a  bristled 
notch  overlapped  by  a  movable  spine  at  the  end  of  the  tibia.  This  is  the 
antenna  cleaner.  The  base  of  the  antenna  is  fitted  into  the  notch  and 
locked  in  place  by  the  spine,  ft  is  then  drawn  through  the  bristly  hole. 
Above  the  spine  each  anterior  tibia  has  a  row  of  short,  evenly  spaced 
bristles,  the  eyebrush,  used  for  brushing  off  the  compound  eyes.  Each 
middle  tibia  has  a  terminal  wax  spur  for  removing  plates  of  wax  se- 
creted on  the  abdomen. 

The  abdomen  shows  two  specializations.  Paired,  ventral  wax  glands 
secrete  wax  as  plates  that  are  used  for  building  the  honeycomb.  The 
reproductive  apparatus  is  modified  at  the  posterior  end  to  form  a  stinger 
(Fig.  16.27).  The  tube  is  formed  of  a  dorsal  sheath  and  two  ventral  darts 
that  slide  on  ridges  of  the  sheath.  The  tips  of  all  three  are  barbed.  The 
sheath  initiates  a  puncture,  after  which  a  seesawing  movement  of  the 
darts  drives  the  stinger  deep  into  the  flesh.  Two  secretions  are  mixed  as 
they  are  extruded  through  the  central  canal.  That  from  a  pair  of  acid 
glands  is  stored  in  a  poison  sac,  and  during  extrusion  the  secretion  of 
a  single  alkaline  gland  is  added.  The  mixture  is  more  poisonous  than 
either  secretion  alone.  When  the  worker  bee  stings  a  mammal  and  then 
flies  away,  the  stinger  with  its  glands  and  muscles  is  pulled  from  the 
insect's  body.  The  bee  later  dies,  but  the  stinger  remains  in  the  mam- 
mal's flesh  with  all  of  its  parts  still  working,  the  darts  driving  it  deeper 
and  the  glands  pumping  in  their  poison. 

Connected  with  the  esophagus  are  large  salivary  glands  which  for 
the  first  ten  days  of  adult  life  secrete  "royal  jelly,"  the  food  of  young  bee 
larvae.  After  ten  days,  however,  these  glands  secrete  ordinary  saliva  con- 
taining enzymes  to  digest  starch.  The  crop  serves  as  a  honey-stomach 
where  nectar  is  temporarily  stored,  as  the  bee  collects  it.  Salivary  enzymes 
convert  the  disaccharide,  sucrose,  of  the  nectar  into  the  monosaccharides 
glucose  and  fructose.  In  the  hive  the  nectar  is  regurgitated,  concentrated 
by  evaporation  in  the  cells  of  the  honeycomb,  and  thus  converted  to 
honey. 

The  life  history  of  a  worker  reveals  additional  specializations.  Life 
begins  as  a  fertilized  egg  laid  by  the  queen  in  a  comb  cell  (Fig.  16.28). 
For  the  first  two  days  after  hatching  the  grublike  larva  is  fed  royal  jelly 
by  young  adult  workers,  and  for  the  next  four  days  it  receives  beebread, 
a  kneaded  mixture  of  pollen  and  honey.  The  larva  molts  several  times 
and  then  spins  a  delicate  cocoon  within  which  it  pupates.  Adult  workers 


PHYLUM   ARTHROPODA 


319 


cover  the  cell  with  a  thin  wax  cap.  After  twelve  days  (three  weeks  from 
the  day  the  egg  was  laid)  the  pupa  molts  to  form  a  full-grown  adult  that 
cuts  off  the  cap  and  emerges. 

First  the  new  bee  busies  herself  cleaning  out  newly  vacated  cells  to 
prepare  them  for  a  new  generation  of  larvae.  After  a  few  days  the  salivary 
glands  begin  to  secrete  royal  jelly  and  the  major  duty  of  the  bee  is  to 
feed  larvae.  Young  adult  Avorkers  feed  heavily  upon  protein-rich  pollen 
to  produce  this  jelly.  The  worker  also  "weans"  the  two-day  old  larvae, 
feeding  them  the  beebread  that  she  has  chewed  thoroughly.  Groups  of 
young  workers  care  for  a  whole  brood  of  yotmg,  feeding  each  of  them 
two  or  three  thousand  times  during  the  six  days  of  their  larval  life. 
Calculations  show  that  one  worker  working  full  time  can  take  care  of 
the  needs  of  only  two  or  three  larvae! 

Toward  the  end  of  this  period  of  caring  for  the  larvae  the  young 
worker  begins  to  fly  short  distances  from  the  hive.  After  the  tenth  day 
the  secretion  of  royal  jelly  stops  and  the  wax  glands  begin  to  function. 
The  worker  then  becomes  a  builder  of  new  honeycomb.  In  addition  she 
receives  nectar  and  pollen  brought  to  the  hive.  Pollen  is  stored  in  cells 
next  to  the  brood  cells,  while  nectar  is  placed  peripherally.  Many  of  the 
bees  sit  over  the  nectar  cells  fanning  the  air  with  their  wings  to  increase 
the  rate  of  evaporation.  When  cells  are  filled  with  honey  or  pollen  they 
are  capped  with  wax. 

At  this  age  the  worker  also  carries  debris  and  dead  bees  out  of  the 
hive,  taking  them  off  some  distance  and  dropping  them.  Toward  the  end 
of  this  period  a  certain  number  of  wax-secreting  bees  guard  the  entrance 
of  the  nest,  inspecting  all  incomers  to  be  sure  that  they  are  bees  of  their 
colony  (which  they  recognize  by  smell).  Raiding  bees,  wasps,  beetles  and 
flies  are  stung  mercilessly  by  these  guards.  Curiously,  the  stinger  does  not 


Egg    J^   3d.a.ys 


Larva- 


■  G  days 


Papa 


30  dayst 


Fora^ 


er 


'^   ^''^^^BuUder 


Waoc  secretion 


-Roya.1  jelly  secretion 

Nurse  bee 
Figure  1 6.28.     Life  cycle  of  the  worker  honeybee.  The  first  21  days  are  spent  in  a  cell 
of  the  comb.  All  growth  takes  place  during  the  six  days  of  larval  life.  Adults  are  drawn 
in  diagrammatic  section  to  show  the  glandular  activities.    (Combined  from  Curtis  and 
Guthrie,  and  von  Frisch.) 


320  ^"^   ANIMAL   KINGDOM 

pull  oft  alter  stinging  such  brittle-skinned  enemies,  so  that  the  guards 
live  to  sting  again.  They  also  fly  out  to  stnig  large  animals  that  approach 
too  closely. 

After  three  weeks  of  adult  life  the  wax  glands  cease  to  function  and 
the  bee  becomes  a  forager.  For  the  rest  of  her  life  her  primary  function 
is  to  collect  nectar  and  pollen.  On  the  average  workers  will  live  four  or 
five  weeks  after  reaching  this  stage. 

1  he  queen  bee  has  functional  ovaries  and  uses  the  reproductive 
apparatus  both  for  oviposition  and  stinging.  Her  iegs  lack  the  pollen- 
collecting  apparatus.  Other  characteristics  of  the  queen  and  the  drones 
will  be  discussed  in  the  next  chapter  where  insect  societies  are  con- 
sidered. 

1 46.        The  Subphylum  Arachnomorpha 

Arachnomorphs  have  a  long  and  varied  evolutionary  history.  They 
appeared  first  in  the  ocean,  then  in  fresh  water,  and  finally  on  land.  Of 
the  five  or  more  classes  only  three  will  be  mentioned  here.  The  Xipho- 
sura  (king  crabs)  are  marine,  the  Eurypterida  are  believed  to  have  lived 
in  fresh  water,  and  the  Arachnida  (scorpions,  spiders,  etc.)  are  terrestrial. 

Xiphosura.  The  class  Xiphosura  was  common,  although  never 
abundant,  during  the  Paleozoic  era.  It  survives  today  as  a  single  genus, 
Limulus,  shown  in  Fig.  16.2.  The  only  American  species  is  L.  polyphemus 
found  on  the  east  coast.  The  superficial  resemblance  between  king  crabs 
and  trilobites  is  striking.  The  body  is  flattened  with  anterior  segments 
fused  dorsally  to  form  a  shield.  In  trilobites  this  prosoma  bore  dorsally 
a  pair  of  compound  eyes  and  ventrally  one  pair  of  antennae  and  four  to 
six  pairs  of  legs.  The  king  crabs  are  generally  similar  but  lack  antennae. 
The  prosomal  legs  of  king  crabs  lack  exopodites,  which  were  the  gills 
of  trilobites.  The  remaining  body  segments  of  trilobites  were  free  and 
each  bore  limbs  like  those  of  the  prosoma.  In  king  crabs  the  remaining 
segments  are  fused  into  an  opisthosoma  and  have  much  modified  ap- 
pendages. 

The  anterior  appendages  of  king  crabs  are  the  chelicerae  (segment 
3)  hanging  in  front  of  the  mouth  in  the  typical  arachnomorph  position. 
The  next  four  pairs  of  walking  legs  are  also  chelate.  The  last  legs  end 
in  several  stout  spines  and  are  used  for  pushing  in  sand.  On  the  opistho- 
soma the  limbs  are  biramous  and  fused  medially  to  form  flat  plates.  The 
first  plate  is  an  operculum  which  overlaps  and  protects  the  others.  Each 
of  the  remaining  five  plates  is  delicate  and  bears  a  pair  of  book  gills 
formed  of  many  thin  lamellae.  The  telson  remains  as  a  free  terminal 
segment,  projecting  as  a  long  movable  spine. 

Eurypterida.  Eurypterids  were  abundant  in  Paleozoic  times,  and 
included  a  few  species  as  much  as  nine  feet  long.  They  had  a  prosoma 
with  dorsal  compound  eyes  and  six  pairs  of  ventral  appendages  (Fig. 
16.29).  The  first  appendages  were  chelicerae,  the  next  four  pairs  were 
walking  legs,  and  the  sixth  were  large  paddles  for  swimming. 

The  remaining  segments  of  eurypterids  were  unfused  and  divisible 
into  two  regions,  a  middle  mesosoma  and  a  posterior  metasoma.  The 


PHYLUM   ARTHROPODA 


321 


mesosoma  was  of  six  segments  and  bore  ventrally  an  operculum  on  the 
first,  and  five  pairs  of  flattened  plates,  believed  to  have  been  gills,  on 
the  others.  The  rnetasoma  of  seven  segments  lacked  appendages,  and 
ended  in  a  telson  spine. 

Although  eurypterids  are  believed  to  have  been  primarily  a  fresh- 
water group,  the  evidence  for  this  is  not  conclusive.  The  best  deposits 
of  fresh-water  organisms  are  usually  found  where  they  have  been  washed 
into  the  sea  at  the  mouths  of  rivers.  Although  many  of  the  fossils  in  such 
deposits  are  obviously  of  fresh-water  origin,  others  are  just  as  clearly 
marine. 

Arachnida.  In  the  class  Arachnida  the  most  primitive  order,  Scor- 
pionida,  shows  many  similarities  with  the  preceding  classes.  The  scorpion 
(Fig.  16.29)  has  a  prosoma  with  six  pairs  of  appendages,  the  first  of  which 
are  the  chelicerae.  The  second  pair  are  large  and  chelate,  forming  pincers 
comparable  with  those  of  the  crayfish.  The  remaining  four  pairs  are 
walking  legs. 

The  scorpion  mesosoma  has  six  segments,  of  which  the  first  has  a 
small  bilobate  appendage  now  part  of  the  reproductive  apparatus  and 
thought  to  be  a  vestigial  operculum.  The  second  segment  bears  a  pair 
of  combs,  modified  tactile  limbs.  The  third  to  sixth  segments  each  bears 
a  pair  of  ventral  slits  that  open  into  air  chambers  containing  book  lungs 
formed  of  many  delicate  lamellae.  Embryological  evidence  suggests  that 
these  lungs  are  borne  on  limb  vestiges  that  have  sunk  into  the  body, 
protecting  the  lamellar  respiratory  organs  from  desiccation. 

The  scorpion  metasoma  is  made  of  one  tapered  segment  and  five 


SSSiSSiSSSiSSiSiSSiSS&~!SiSSS?SSiSS!S^^ 


SSSSSSSSS-SSSSSSSSiSSSSSiSSSSSSSSS^^ 


ARACHNIDA 


Figure  16.29.  Representative  classes  of  the  subphylum  Arachnomorpha.  Two  of  the 
arachnid  orders  are  shown.  A  third  class  (Xiphosura)  is  shown  on  Figure  16.2.  (Combined 
from  various  sources.) 


322  ^'^^   ANIMAL   KINGDOM 

narrow  segments  forming  a  long  tail.  These  lack  appendages.  The  telson 
is  modified  as  a  powertul  sting. 

Although  no  arachnids  have  compound  eyes,  most  of  them  have 
ocelli  resembling  those  of  the  insects  (Fig.  16.21).  Ocelli  are  also  found 
in  king  crabs  and  eurypterids. 

Ihe  class  Arachnida  is  divided  into  eleven  or  more  orders.  Only 
two  forms  are  discussed  here,  the  scorpions,  above,  and  the  spiders,  order 
Araneae,  below.  The  other  scorpion-like  and  spider-like  orders  are  listed 
in  the  appendix. 

1 47.        Argiope,  an  Orb  Spider 

Of  the  many  species  of  spiders  only  a  few  build  geometrical  webs. 
These  are  the  orb  spiders,  about  one  inch  long.  Both  the  golden  (Argiope 
surantia)  and  the  banded  (A.  trifaciata)  orb  spiders  are  common  in  gar- 
dens and  marshes  (Fig.  16.29). 

The  prosoma  bears  four  anterior  ocelli  that  look  forward,  upward 
and  to  the  side.  Below  them  are  the  chelicerae,  no  longer  chelate  but 
modified  as  poison  fangs.  The  second  pair  of  appendages  are  small 
pedipalps,  tactile  in  function  and  used  to  manipulate  prey.  The  remain- 
ing four  pairs  are  typical  walking  legs,  each  composed  of  seven  segments. 
The  prosoma  is  joined  to  an  opisthosoma  by  a  slender  waist.  The 
opisthosoma  is  a  large  soft  bag  formed  embryologically  by  the  fusion  of 
ten  segments.  A  pair  of  ventral  slits  opens  to  the  one  pair  of  book  lungs 
and  posteriorly  are  three  pairs  of  spinnerets  and  one  pair  of  small  anal 
papillae.  The  anus  is  terminal,  just  anterior  to  the  spinnerets  is  a 
single  median  opening,  the  spiracle. 

The  mouth,  just  behind  the  chelicerae,  opens  into  a  narrow  esoph- 
agus that  leads  to  a  sucking  stomach  (Fig.  16.30).  This  is  followed  by  a 
midgut.  After  traversing  the  waist  the  midgut  expands  dorsally  and 
continues  posteriorly  to  join  the  hindgut,  a  short  rectum  with  a  dorsal 
storage  sac  leading  to  the  anus.  The  midgut  has  the  usual  pouches.  The 
first  pair  extend  forward  in  the  prosoma  and  send  branches  into  the 
bases  of  the  legs.  In  the  opisthosoma  are  several  more  highly  branched 
pouches.  As  in  other  arthropods  these  not  only  secrete  enzymes  but  also 
absorb  nutrients. 

A  pair  of  Malpighian  tubules  are  located  at  the  junction  of  the  mid- 
gut and  hindgut.  The  dorsal  heart  lies  in  a  separate  part  of  the  hemocoel 
as  in  the  crustaceans.  The  nervous  system  is  condensed  into  a  brain  and  a 
large  subesophageal  ganglion  with  connectives  and  nerves  to  all  parts  of 
the  body. 

Argiope  has  two  respiratory  systems.  The  book  lungs  are  continu- 
ally flushed  with  blood  that  is  oxygenated  as  it  passes  through.  Since  the 
blood  is  not  known  to  contain  any  respiratory  pigment,  however,  not 
much  oxygen  can  be  carried.  The  single  spiracle  opens  into  a  tracheal 
system.  The  tubes  are  small  and  do  not  branch  as  much  as  in  the  insects, 
but  they  are  structurally  identical  and  have  the  same  spirally  thickened, 
chitinous  lining. 

A  pair  of  poison  glands  fills  the  dorsal  part  of  the  prosoma,  opening 


PHYLUM    ARTHROPODA 


323 


Malpighian  tubule 
Storage  sa.c- 


Rectum 


Midgut - 


Anus 


Opanintfs  to  opisthosomal  gastric  poxichzs 

rHeart 

■ProsoTnad  gastric  pouch 

-Brain      r'^oison  gla.-na. 

<§-Ocdlus 


Cheliccra. 
Fang 


Spiracle- 
with  trachaa 


SilH  gland 

Ovar_y 


-Mouth 
"Esophagus 
-Subesophagealganglion 
SucKin^  Stomach 

Figure  16.30.  Internal  anatomy  of  the  orb-spider,  Argiope.  The  body  wall,  ap- 
pendages, and  some  of  the  internal  organs  of  the  right  side  have  been  removed.  The  much 
branched  gastric  pouches  in  the  opisthosoma  are  removed,  leaving  their  openings  into 
the  midgut.  Only  a  few  of  the  silk  glands  are  shown.  (After  Buck  and  Keister.) 


on  the  tips  of  the  chelicerae  by  way  of  slender  ckicts.  Prey  is  first  killed 
with  poison  from  these  glands,  then  wrapped  tightly  in  silk  from  the 
spinnerets.  The  spider  applies  its  mouth  to  the  prey  and  secretions  con- 
taining proteolytic  en/ymes  from  glands  behind  the  mouth  begin  diges- 
tion. The  resulting  broth  is  sucked  into  the  midgut  where  digestion  is 
completed  and  the  nutrients  are  absorbed. 

The  ovary  lies  in  the  opisthosoma  beneath  the  midgut,  opening 
antero-ventrally  by  way  of  an  oviduct  through  a  genital  pore  between 
the  openings  to  the  book  lungs.  Associated  with  the  oviduct  is  a  seminal 
receptacle  where  sperm  are  received.  The  male  is  much  smaller  than  the 
female,  Avith  testes  and  a  sperm  duct  in  the  opisthosoma.  Before  copula- 
tion the  male  transfers  the  sperm  to  specialized  cavities  in  the  tips  of  his 
pedipalps.  At  copulation  the  pedipalps  are  thrust  into  the  female  open- 
ing and  the  sperm  are  expelled  into  the  receptacle.  The  whole  maneuver 
is  remarkably  like  that  of  the  cephalopod  molluscs.  The  eggs  are  fer- 
tilized as  they  are  laid  and  are  put  in  a  cocoon  spun  by  the  spinnerets. 
They  hatch  later  into  miniature  spiders. 

Associated  with  the  three  pairs  of  spinnerets  are  five  kinds  of  silk 
glands  in  the  ventral  part  of  the  opisthosoma.  The  different  secretions 
yield  different  kinds  of  silk,  including  the  nonsticky  radial  fibers  of 
the  web,  the  sticky  circular  fibers,  and  the  brownish  fibers  of  the  cocoon. 
Silk  is  emitted  as  a  fluid  that  instantly  hardens  into  tough  protein 
threads. 


1 48.        The  Phylum  Onycophora 

The  Onycophora  are  about  seventy  species  of  wormlike,  segmented, 
terrestrial  animals  with  metameric  legs.  All  the  living  species,  found  in 
very  damp  regions  of  the  tropics,  belong  to  one  family,  and  possibly  to 
one  genus,  Peripatus  (Fig.  16.31). 

During  development,  segmentation  appears  in  Peripatus  in  a  man- 


324 


THE  ANIMAL   KINGDOM 


Figure  16.31.     Peripatus,  a  member  of  the  Onychophora,  a  "missing  link"  between 
the  AnneUda  and  the  Arthropoda.  (Courtesy  Ward's  Natural  Science  Establishment.) 


ner  very  similar  to  that  in  the  arthropods  and  annehds  (especially  in  the 
heavily  yolked  eggs  of  the  latter).  The  coelomic  cavities  neither  form 
the  main  body  cavity  as  in  the  annelids  nor  disappear  as  in  the  arthro- 
pods, but  persist  as  small  cavities  associated  with  annelid-like,  metameric 
nephridia.  The  adult  body  cavity  is  a  hemocoel  like  that  of  the  arthro- 
pods. The  embryonic  first  segment  persists  and  bears  a  pair  of  prean- 
tennae.  The  mouth  opens  on  the  second  segment  whose  appendages 
become  jaws.  The  appendages  of  the  third  segment  lie  beside  the  mouth 
as  oral  papillae  which  can  shoot  out  slime  to  entangle  an  enemy.  The 
rest  of  the  paired  appendages  are  short  un jointed  legs  ending  in  terminal 
claws.  The  body  covering  is  a  thin,  soft  cuticle  like  that  of  the  annelids, 
but  it  is  beset  with  numerous  small  spiracles  from  which  delicate 
tracheal  tubes  branch  into  the  body. 

These  characteristics  are  sufficient  to  differentiate  the  Onycophora 
from  both  the  Annelida  and  the  Arthropoda  as  a  separate  phylum.  Al- 
though the  group  is  often  used  as  a  possible  ancestral  type  for  the 
Arthropoda,  linking  them  to  the  Annelida,  a  second  look  shows  that 
Peripatus,  a  terrestrial  organism  itself,  forms  a  rather  awkward  tie  be- 
tween trilobites  and  annelids.  A  view  which  is  now  winning  acceptance 
is  that  the  first  segmented,  pre-annelid,  pre-onycophoran,  pre-arthropod 
animals  probably  radiated  into  a  number  of  groups,  of  which  three  exist 
today.  There  is  some  speculation  that  the  Onycophora  may  once  have 
been  more  widespread  and  may  have  included  a  wider  variety  of  forms. 
This  view  is  supported  by  the  discovery  in  1930  of  Aysheaia,  a  Cambrian 
fossil.  The  particular  rocks  in  which  eleven  specimens  were  found  are 
remarkable  for  the  perfection  of  their  fossils,  and  contain  clear  prints 


PHYLUM    ARTHROPODA  325 

of  many  soft-bodied  animals  otherwise  unknown  from  that  ancient 
period.  Aysheaia  appears  to  have  been  a  marine,  peripatus-like  animal. 
If  this  current  view  proves  correct,  then  although  the  Onycophora 
share  characteristics  with  both  the  annelids  and  the  arthropods,  and  are 
often  intermediate  structurally,  they  would  not  be  considered  an  evolu- 
tionary link  but  a  third  surviving  branch  of  an  ancient  and  possibly 
much  diversified  group  of  segmented  organisms. 

Questions 

1.  Compare  diagrammatic  body  segments  of  an  arthropod  and  a  polychaete  annelid. 

2.  Which  Hmbs  put  food  into  the  mouth  in  each  of  the  four  arthropod  subphyla? 

3.  When  did  each  of  the  subphyla  develop  terrestrial  forms? 

4.  Describe  the  nauplius  larva. 

5.  W'hat  is  a  pleopod? 

6.  List  the  sense  organs  of  a  crayfish  and  give  their  locations. 

7.  Describe  what  happens  to  food  from  the  time  it  is  captured  until  its  nutrients  are 
absorbed  in  the  crayfish. 

8.  Compare  the  circulatory  systems  of  arthropods  and  annelids. 

9.  What  are  the  locomotor  organs  of  Daphnia? 

10.  Contrast  centipedes,  millipedes  and  insects. 

11.  W'hat  is  a  pulvillus? 

12.  Compare  the  excretory  systems  of  Cambarus  and  Periplaneta. 

13.  What  is  the  functional  significance  of  the  characteristic  that  separates  the  Neoptera 
from  the  Paleoptera? 

14.  Describe  complete  metamorphosis. 

15.  What  is  a  pollen  basket? 

16.  Give  the  life  cycle  of  the  honeybee. 

17.  What  animal  has  two  respiratory  systems? 

18.  Discuss  the  relation  of  the  Onycophora  to  the  arthropods. 

Supplementary  Reading 

Insect  Natural  History  by  Imms  is  a  readable  account  by  one  of  our  foremost  ento- 
mologists. 1  he  paperbound  Maeterlinck,  The  Life  of  the  Bee,  and  Crompton,  The  Life 
of  the  Spider,  are  excellent  life  studies.  Of  the  many  manuals  for  insect  identification,  the 
paperbound  Insects  by  Zim  and  Cottam  is  adequate  for  the  beginner,  containing  pictures 
of  many  common  species. 


CHAPTER  17 


Physiology  and 
Behavior  of  the  Arthropoda 


In  this  large  and  varied  phylum  a  number  of  physiologic  problems  have 
been  studied  extensively.  Some  ot  these  have  revealed  mechanisms  radi- 
cally different  from  their  analogues  in  the  vertebrates.  Some  of  the  more 
unique  and  characteristic  ones— molting,  hormones,  innervation  patterns, 
flight,  compound  eye  vision,  behavior  and  social  mechanisms-will  be 
discussed  in  this  chapter. 

149.        Molting 

All  arthropods  periodically  shed  their  chitinous  exoskeleton  as  a 
part  of  growth  and  metamorphosis.  The  actual  shedding  of  the  old  and 
hardening  of  the  new  skeleton,  which  may  take  a  few  seconds  (daphnia) 
or  several  hours  (lobster),  is  only  the  obvious  culmination  of  the  elab- 
orate process  of  molting.  Before  shedding  occurs  the  new  skeleton  is 
preformed  and  materials  of  the  old  skeleton  are  salvaged. 

The  exoskeleton  is  formed  of  three  layers  (Fig.  17.1).  The  outermost 
is  a  thin,  flexible,  colorless  epicuticle  composed  of  wax  and  cuticulin, 
a  lipoprotein  containing  a  large  amount  of  fatty  material.  The  middle 
layer  is  the  primary  chitinous  layer  composed  of  chitin  and  cuticulin, 
sometimes  impregnated  with  calcium  carbonate  or  other  salts.  The  inner 
secondary  chitinous  layer  is  made  almost  entirely  of  chitin  and  protein. 
The  epidermis  lies  beneath  this  as  a  single  layer  of  cells  with  numerous 
filamentous  extensions  into  the  two  chitinous  layers. 

The  first  step  toward  a  molt  (Fig.  17.1)  is  a  separation  of  the  epi- 
dermis from  the  old  skeleton  by  the  secretion  of  a  molting  fluid.  Glandu- 
lar cells  in  the  epidermis  add  enzymes  to  the  fluid  capable  ot  digesting 
protein  and  chitin  but  not  cuticulin.  While  the  epidermis  lays  down  a 
new  epicuticle  the  molting  fluid  begins  to  erode  the  old  secondary 
chitinous  layer. 

Formation  of  a  new  skeleton  and  salvage  of  the  old  go  on  simul- 
taneously. All  of  the  secondary  chitinous  layer  and  some  of  the  primary 
layer  are  ultimately  digested,  although  the  amount  of  cuticulin  in  the 
latter  may  prevent  its  digestion.  It  growth  is  to  take  place  at  the  next 
molt,  the  epidermis  with  its  new  epicuticle  grows  and  becomes  wrinkled 
in  the  confines  of  the  old  skeleton.  It  begins  to  secrete  a  soft,  pliable, 
primary  chitinous  layer. 

326 


PHYSIOLOGY   AND   BEHAVIOR   OF   THE   ARTHROPODA 


327 


At  the  time  of  molting  the  new  epicuticle  and  primary  chitinous 
layer  are  complete,  although  they  are  still  soft  and  flexible.  The  molting 
fluid  with  its  digested  products  is  completely  absorbed  into  the  body. 
The  old  epicuticle  and  much  of  the  primary  chitinous  layer  remain  as 
a  loose  covering.  At  various  places,  especially  along  the  back,  the  old 
primary  layer  is  thin  (Fig.  17.1),  so  that  after  the  secondary  layer  is 
digested  away  a  line  appears  along  which  the  old  skeleton  will  break. 

The  arthropod  must  then  swell  up  to  burst  the  old  exoskeleton.  It 
may  contract  the  abdomen,  forcing  blood  into  the  head  and  thorax,  or 
it  may  swallow  water  or  air.  Once  the  old  exoskeleton  has  been  split  open 
the  organism  extricates  itself,  shedding  not  only  the  covermg  of  the  body 
and  legs  but  also  the  lining  of  the  foregut,  hindgut,  and  in  the  labiates 
the  linnig  of  the  tracheal  system.  If  the  arthropod  grows  during  the  molt 
it  must  swell  rapidly  to  stretch  the  wrinkled  new  exoskeleton  out  to  its 
full  size.  Most  arthropods  swallow  water  or  air  to  do  this,  and  may 
increase  their  volume  100  per  cent.  Even  if  the  organism  is  not  growing 
at  some  particular  molt,  it  may  be  necessary  to  compress  some  parts  of 
the  body  in  order  to  force  blood  into  others  to  achieve  whatever 
metamorphosis  is  taking  place.  A  newly  emerged  adult  moth,  for  ex- 


oltinO 
line-]  /EpicuticlcN 


Primary 


\ 


— chitinous — :" 
~  layizr 

■Secondary 


0 


chitinous    Molting 
layer        "X  ' 

•bpidermis — ^ 


Moiling 
fluid 

epicuticle 

New- 
primary 
chitinous 

layer 

C  Hypodc-rmis 

Figure  17.1.  Molting  in  an  arthropod.  A,  The  fully  formed  exoskeleton  and  under- 
lying epidermis  between  molts.  B,  Separation  of  the  epidermis  and  secretion  of  molting 
fluid  and  the  new  epicuticle.  C,  Digestion  of  the  old  secondary  chitinous  layer  and  secre- 
tion of  the  new  primary  chitinous  layer.  D,  Just  before  molting.  (Modified  from  Wiggles- 
worth.) 


328  ^H£  ANIMAL  KINGDOM 

ample,  contracts  the  abdomen  to  force  blood  into  the  wrinkled  wings 
and  expand  them  to  full  size.  After  the  skeleton  is  adjusted  to  its  new 
size  and  shape,  the  epidermis  secretes  enzymes  which  oxidize  and  harden 
the  epicuticle  and  primary  chitinous  layers.  Usually  the  primary  layer, 
which  is  pale  at  first,  darkens  during  this  process.  In  the  crayfish  and 
many  other  hard-shelled  forms,  calcium  carbonate  is  deposited  as  an 
additional  stiffening  agent.  The  crayfish  had  previously  absorbed  much 
of  this  lime  from  the  old  skeleton  and  stored  it  on  the  sides  of  the 
stomach  between  epidermis  and  chitinous  lining  as  the  gastroliths  (p. 
298).  After  the  molt  these  concretions  are  exposed  to  the  digestive  fluids 
and  dissolve  rapidly,  providing  an  immediate  supply  for  the  new  skele- 
ton. 

The  final  event  of  molting  occurs  later.  The  epidermis  secretes  the 
secondary  chitinous  layer  as  a  permanently  elastic  portion  of  the  exo- 
skeleton.  The  desired  flexibility  of  any  part  of  the  exoskeleton  is 
achieved  to  a  considerable  extent  by  the  thicknesses  of  the  two  chitinous 
layers.  Where  rigidity  is  required  the  outer  layer  is  thick.  Where  a  tough 
but  flexible  skeleton  is  required  the  inner  layer  is  thick,  and  where 
great  flexibility  is  wanted  both  layers  are  thin. 

1 50.       Arthropod  Hormones 

The  molting  process  has  been  extensively  studied  in  the  crustaceans 
and  insects,  and  in  both  cases  has  been  found  to  be  under  endocrine 
control.  Arthropods  have  also  been  shown  to  elaborate  other  hormones, 
related  to  metabolism,  reproduction  and  pigment  changes.  As  the  glands 
secreting  these  hormones  are  discovered  and  studied,  it  is  becoming 
apparent  that  arthropods  have  an  endocrine  system  similar  in  many 
respects  to  that  of  tire  vertebrates.  Both  are  intimately  related  to  the 
brain.  In  both  kinds  of  animals  antagonistic  hormones  are  known,  and 
in  both  some  of  the  glands  have  reciprocal  actions  on  each  other  to  pro- 
duce a  controlled  check-and-balance  system.  All  evidence  suggests,  how- 
ever, that  the  arthropod  and  vertebrate  endocrine  systems  evolved 
independently. 

Probably  the  most  important  contribution  made  by  arthropod 
physiologists  to  the  field  of  physiology  is  the  discovery  of  neurosecretion, 
the  secretion  of  physiologically  active  substances  by  nerve  cells.  In  a 
narrower  and  more  usual  sense  neurosecretion  refers  to  the  production 
of  hormonal  materials  in  the  cell  body  of  a  neuron  which  then  travel 
the  length  of  the  axon  to  be  stored  and  ultimately  released  at  the  tip 
(Fig.  17.2). 

The  primary  endocrine  organs  of  the  crustaceans,  for  example,  were 
once  believed  to  be  the  sinus  glands  on  the  optic  ganglia  of  the  eyestalks 
(Fig.  17.3).  Extracts  of  these  glands  have  been  shown  to  contain  a  variety 
of  hormones,  including  one  that  affects  pigment  distribution  in  the 
compound  eyes,  two  more  that  control  pigmentation  of  the  body,  one 
that  induces  molting,  and  several  others  influencing  metabolism  and 
reproduction.  In  contrast  to  this  complexity  of  their  secretions,  the  sinus 
glands  present  a  puzzling  anatomic  simplicity.  Each  one  appears  to  be 


PHYSIOLOGY   AND   BEHAVIOR   OF   THE  ARTHROPODA 


329 


Figure  17.2.  Neurosecretion.  The  neuron  produces  secretion  granules  in  the  cell 
body  (left)  that  are  stored  in  the  expanded  tip  of  the  axon,  where  they  may  also  be  re- 
leased (right).  If  the  axon  is  cut,  material  accumulates  at  the  cut  (below). 

a  homogeneous  group  of  pale  blue  cells.  Actually,  however,  these  are 
not  gland  cells  at  all,  but  the  expanded  tips  of  bundles  of  axons.  The 
cell  bodies  of  these  axons  lie  some  distance  back  on  the  proximal  sides 
of  the  optic  ganglia  (Fig.  17.3)  in  the  x-organ.  Cells  of  the  x-organ 
resemble  ordinary  secretory  cells,  with  a  granular  cytoplasm  of  variable 
appearance  and  large  nuclei.  The  secretions  appear  as  bluish  granules 
that  move  very  slowly  through  the  axons  to  their  tips.  Thus  the  sinus 
glands  are  merely  storage  areas  for  the  hormones  (or  their  precursors) 
which  are  produced  in  the  x-organs.  Some  axons  from  the  x-organs 
extend,  not  to  the  sinus  glands  but  to  other  structures  in  the  region.  The 
nature  and  function  of  these  smaller  structures  was  previously  misun- 
derstood but  they  are  now  known  to  be  expanded  axons  with  endocrine 
activity.  Presumably  this  structural  arrangement  permits  hormones  to  be 
elaborated  under  one  set  of  local  conditions  and  to  be  released  at  a 
distance  where  conditions  are  different. 


-Rctinulae  of 
compound,  eye 


Sinus  oland 


Conn.<2.ctiorL 
with  brain  - 


-Portions  of  _ 
optic  (SajaOlion. 


-X-  OTgZLn 


Figure  17.3.     Eyestalk  of  the  crab  with  the  skeleton  removed,  showing  sinus  gland 
and  x-organ.  (After  Passano.) 


330 


THE  ANIMAL  KINGDOM 


Figure  17.4.  Endocrine  glands  of  the  cockroach.  The  upper  group  He  above  the 
esophagus  in  the  head.  The  lower  gland  is  ventral  in  the  prothorax,  strung  among  the 
muscle  cells.  (After  Bodenstein.) 

In  the  insects  several  endocrine  glands  are  known,  and  a  little  is 
known  of  their  interactions.  The  hormonal  control  of  molting  is  now 
well  understood.  A  molt  is  initiated  by  the  intercerebral  gland  (Fig. 
17.4),  a  part  of  the  brain.  The  neurosecretory  cells  in  this  gland  pass 
their  hormones  along  the  axons  to  their  expanded  tips  in  the  corpus 
cardiacum,  just  behind  the  brain.  The  release  of  a  prothoracicotropic 
hormone  by  this  gland  sets  in  motion  an  irreversible  series  of  events. 

The  hormone  is  carried  in  the  hemocoel  to  the  prothoracic  glands, 
a  pair  of  ectodermal  glands  in  the  ventral  part  of  the  prothorax,  which 
it  stimulates  to  liberate  a  molt  and  metamorphosis  hormone  (m  &  m 
hormone).  This  hormone  acts  directly  on  the  epidermis,  causing  it  to 
secrete  molting  fluid  and  to  start  a  new  exoskeleton.  In  addition,  the 
m  &  m  hormone  causes  the  epidermis  to  assume  the  adult  morphology. 

In  young  insects,  however,  the  m  &  m  hormone  does  not  act  alone. 
Another  pair  of  glands,  the  corpora  allata,  closely  associated  with  the 
corpus  cardiacum,  secrete  a  juvenilizing  hormone.  This  hormone  does 
not  prevent  a  molt  but  it  does  prevent  metamorphosis,  thereby  preserv- 
ing the  juvenile  morphology.  The  corpora  allata  are  very  active  in 
young  insects  but  they  gradually  become  less  active,  and  finally  lose  their 
power  to  preserve  immaturity.  A  hormone  analogous  to  this,  which 
preserves  youthfulness,  is  unknown  in  vertebrates. 

Interference  with  the  molting  honnones  has  produced  interesting 
results.  Fukuda,  working  with  the  silkworm,  removed  the  corpora  allata 
from  young  caterpillars  (Fig.  17.5).  They  pupated  on  the  next  molt  and 
emerged  later  as  miniature  adults.  They  were  mature  functionally  as 
well  as  structurally,  and  even  reproduced.  The  converse  of  this  experi- 
ment was  done  by  Wigglesworth  on  a   blood-sucking  bug,   Rhodnius. 


PHYSIOLOGY   AND   BEHAVIOR   OF   THE   ARTHROPODA  ^$\ 


Figure  17.5.  Effect  of  the  removal  of  the  corpora  allata  in  the  silkworm.  Moth  at 
left  is  normal.  Moth  at  the  right  developed  from  a  young  caterpillar  whose  corpora  allata 
were  removed.  It  pupated  at  the  next  molt  after  the  operation.  (After  Bodenstein  from 
Fukuda.) 

He  implanted  corpora  allata  from  small  nymphs  into  the  hemocoel  of 
large  nymphs  that  ordinarily  would  mature  on  the  next  molt.  The  result 
was  another  nymphal  stage  the  size  of  an  adult  (Fig.  17.6).  These  nymphs 
eventually  molted  again  to  produce  oversized  adults. 

Further  interactions  among  the  endocrine  organs  are  evident  in  the 
control  of  diapause  in  moths.  Diapause  is  a  state  of  arrested  develop- 
ment which  occurs  in  many  eggs,  insect  pupae  and  plant  seeds.  The  large 
moth,  Platysamia  cecropia  (Fig.  16.24),  overwinters  as  a  pupa  formed  in 
the  middle  or  late  summer.  If  newly  formed  pupae  are  kept  at  75°  F. 
they  remain  inactive  for  five  or  six  months.  Eventually,  however,  de- 
velopment does  proceed  and  the  moths  emerge  four  weeks  later.  If  new 
pupae  are  chilled  to  40°  for  six  weeks  and  subsequently  placed  at  75°, 
development  proceeds  at  once.  Hence,  chilling  leads  to  an  end  of 
diapause  and  shortens  the  period  of  pupal  life.  Carroll  Williams,  finding 
that  if  a  chilled  and  an  unchilled  pupa  are  grafted  together  (Fig.  17.7) 
both  will  develop,  suggested  that  diapause  is  under  hormonal  control.  He 
found  also  that  the  brain  of  a  chilled  pupa  implanted  in  an  unchilled 


Figure  17.6.  The  effect  of  adding  corpora  allata  in  Rhodnius.  The  last  stage  nymph 
(left)  normally  molts  to  form  an  adult  (center).  When  corpora  allata  from  young  nymphs 
are  added  to  a  last  stage  nymph,  it  molts  to  form  an  oversized  nymph  (right).  This  later 
molts  again  to  become  an  oversized  adult.  (After  Wigglesworth.) 


332  ^"^  ANIMAL   KINGDOM 


Ddvzlops 
A     DIAPAUSE :  75° F.  No  dcveIopme.nt  75T 


Months 


B    DIAPAUSE    Develops 

Chilkdal40T    75"F 


EmcrOe-nce- 


Chilkd 


Figure  1 7.7.  Diapause  in  the  pupae  of  the  cecropia  moth.  A,  Normal  development  at 
75°  F.  B,  Normal  development  with  diapause  broken  by  six  weeks  (or  more)  of  chilling 
at  40°  F.  C,  When  chilled  and  unchilled  pupae  are  joined,  diapause  is  broken  in  both 
individuals.  D,  The  brain  of  a  chilled  pupa,  implanted  in  an  unchilled  pupa,  induces 
immediate  development  in  the  latter. 

pupa  will  end  its  diapause.  By  combining  chilled  and  unchilled  organs  in 
various  ways,  he  showed  that  only  the  chilling  of  the  brain  is  important. 
Upon  being  chilled  the  brain  releases  the  prothoracicotropic  hormone  to 
which  the  prothoracic  glands  respond  whether  they  were  chilled  or  not, 
and  the  released  m  &  m  hormone  ends  the  diapause.  Once  the  brain  acts 
upon  the  prothoracic  glands  its  continued  presence  is  not  needed  to 
produce  the  molt.  In  one  experiment  Williams  implanted  a  chilled  brain 
in  an  unchilled  pupa,  then  moved  it  into  a  second  tnichilled  pupa.  Both 
pupae  ended  diapause  promptly  and  developed  into  adults. 

151.        Patterns  of  Muscular  Innervation 

The  individual  motor  axons  of  vertebrates  each  innervate  a  few 
muscle  fibers  of  a  single  muscle,  forming  a  motor  unit  (p.  101).  The 
strength  of  muscular  contraction  varies  according  to  the  number  of  units 
active,  and  its  duration  is  controlled  by  the  duration  of  stimuli  from 
the  nerves. 

In  arthropods  the  anatomic  relations  of  nerve  and  muscle  fibers  are 
different.  A  single  axon  not  only  innervates  all  the  fibers  of  one  muscle, 
but  may  innervate  those  of  another  muscle  as  well.  Furthermore,  most 
muscles  receive  two  or  more  axons,  each  of  which  has  a  different  effect 
upon  contraction.  Usually  in  a  three-axon  system  one  axon  produces  a 
strong,  brief  contraction,  another  a  weak,  sustained  contraction,  and  the 
third  inhibits  the  action  of  the  other  two.  By  varying  the  frequency  of 
stimulation  among  the  axons  the  muscular  contraction  can  be  varied 
considerably. 

A  study  of  the  innervation  pattern  for  several  muscles  shows  how 


PHYSIOLOGY   AND   BEHAVIOR   Of   THE   ARTHROPODA 


333 


precision  can  be  achieved,  even  when  one  axon  goes  to  more  than  one 
muscle.  The  four  muscles  of  the  hand  and  claw  of  the  crayfish  are  rep- 
resented diagrammatically  in  Figure  17.8.  The  claw  opener  and  hand 
extensor  share  a  single  excitor  axon,  but  have  different  inhibitory  axons. 
The  hand  extensor  muscle  shares  its  inhibitor  with  the  claw  closer.  The 
claw  closer  has  two  excitor  axons,  one  for  rapid,  strong  contraction  and 
one  for  slow,  sustained  contraction.  The  hand  flexor  has  three  axons,  one 
of  each  type,  unshared  by  other  muscles  being  considered. 

In  the  behavior  of  the  pincers  there  is  only  one  activity  that  re- 
quires the  instantaneous  activity  of  two  muscles,  and  that  is  a  sudden 
thrust  or  reach  toward  an  adversary.  Analysis  of  the  nerve  pattern  shows 
that  only  one  axon  need  be  active  to  produce  this  response;  it  stimulates 
both  the  extension  of  the  hand  and  the  opening  of  the  claw  in  a  single 
operation.  At  the  end  of  the  thrust  the  claw  can  be  clamped  shut  by 
stimuli  in  the  rapid  excitor  axon  of  the  claw,  whether  or  not  the  claw 
opener  is  inhibited,  since  the  closer  is  a  much  more  powerful  muscle. 
Hence,  the  whole  maneuver  of  thrust  and  grab  can  be  accomplished  by 
activity  in  two  axons. 

In  more  gentle  manipulatory  movements  an  opener  inhibitor  is 
probably  useful,  to  permit  gentle  and  sustained  activity  in  the  claw 
closer.  Obviously,  if  during  manipulation  the  claw  is  to  be  opened,  the 
much  stronger  claw  closer  muscle  must  be  relaxed.  Interestingly  enough, 
the  inhibitor  of  the  claw  closer  also  inhibits  the  extensor,  so  that  open- 
ing of  the  claw  can  be  accomplished  as  a  simple  unhampered  motion 
by  activity  in  two  axons. 

This  pattern  of  connections  between  nerves  and  muscles,  which  is 
comparatively  simple  anatomically  by  vertebrate  standards,  permits  re- 
markably fine  control  and  rapid  activity.  It  appears  likely  that  this 
pattern  occurs  generally  in  arthropods  and  comparable  studies  in  other 
forms  will  further  our  understanding  of  arthropod  activity.  It  may  even 


Opener 


■>Ex^<z,nsor 

I 
I 
I 
I 


Closer 


Fle>cor' 


Figure  17.8.  Nerve  supply  to  the  last  four  muscles  of  the  crayhsh  pincers.  Each 
vertical  line  represents  a  single  axon,  which  supplies  all  the  fibers  of  the  indicated 
muscle(s).  Dotted  lines  show  inhibitory  axons,  heavy  lines  show  rapid,  strong  excitors, 
and  thin  lines  show  slow,  sustained  excitors.  Arrows  on  the  pincers  indicate  the  directions 
of  movement. 


334 


THE  ANIMAL  KINGDOM 


provide  us  with  explanations  to  offer  the  puzzled  student  who  says  as  he 
looks  in  dismay  at  the  crayfish:  "It  will  never  work,  it  has  too  many 
moving  parts." 

1 52.        The  Flight  Mechanism  in  Insects 

Unlike  birds  and  bats,  most  insects  do  not  have  large  flight  muscles 
attached  to  the  wings.  Instead,  the  wings  are  articulated  with  the  thorax 
in  such  a  way  that  very  slight  changes  in  the  shape  of  the  thorax  cause 
the  wings  to  beat  up  and  down.  The  flight  muscles  are  located  entirely 
within  the  thorax  and  are  not  attached  to  the  object  moved. 

The  thorax  can  be  compared  to  a  box  (Fig.  17.9)  having  an  under- 
sized cover.  The  inner  end  of  each  wing  is  attached  by  a  movable  joint 
to  the  upper  edge  of  the  sides  of  the  box.  When  the  vertical  muscles 
of  the  thorax  contract,  the  notum  (box  cover)  is  depressed  and  the  wings 
flip  upward.  When  longitudinal  muscles  contract,  the  notum  arches 
upward  and  the  wings  flip  down.  The  flight  muscles  are  very  stout  and 
change  little  in  length  during  contraction.  The  two  sets  of  muscles  are 
opposed  and  pull  alternately  against  each  other. 

In  many  insects  (all  the  Exopterygota  and  Lepidoptera,  and  most 
Coleoptera),  the  frequency  of  the  wing  beats  correlates  closely  with  the 
nerve  impulses  to  the  muscles.  These  impulses  are  evenly  spaced  in 
time  and  staggered  in  the  nerves  to  the  two  sets  of  muscles,  so  that 
rhythmic  up  and  down  movements  of  the  wings  result.  The  rate  varies 
with  the  rate  of  the  nerve  impulses,  from  8  wing  beats  per  second  in 
large  moths  to  75  or  more  in  the  smaller  insects. 

In  many  of  the  Diptera,  Hymenoptera,  and  possibly  a  few  Coleop- 
tera, however,  the  wing  beats  are  not  correlated  with  the  frequency 
of  the  nerve  impulses.  Low  frequency  nerve  impulses  have  little  or  no 
effect,  but  when  the  frequency  rises  above  100  or  so  per  second  the  flight 
muscles  begin  to  contract  rapidly  but  at  a  higher  frequency.  Pringle 
has  studied  this  phenomenon  and  finds  that  not  only  are  the  nerve 
impulses  not  correlated  with  the  wing  beats,  but  their  frequency  is  ir- 
regular and  not  staggered  in  nerves  to  opposing  muscles.  Hence,  al- 
though the  frequency  of  the  nerve  impulses  must  exceed  a  certain 
threshold,  once   this   is  exceeded  the   rhythm  of   muscular  contraction 


■Notum  curves  down  fore  and  aft 
NotuTn  of  thorax: 


rWin^ 


LoDoitudinal  Tnusclz 
Vertical  xnusclc 


-Stamum 

Figure  1 7.9.  Diagram  of  the  primary  flight  muscles.  Vertical  muscles  extend  between 
the  notum  and  the  sternum.  Longitudinal  muscles  extend  between  the  downturned  ends 
of  the  notum. 


PHYSIOLOGY   AND   BEHAVIOR   OF   THE   ARTHROPODA 


335 


originates  within  the  muscles.  Such  myogenic  rhythms,  as  opposed  to 
the  neurogenic  rhythms  in  other  insects,  produce  wing  beat  frequencies 
that  may  reach  300  or  400  per  second. 

A  critical  feature  of  such  myogenic  rhythms  is  the  tension  in  the 
system.  If  one  set  of  flight  muscles  is  cut,  the  other  will  not  develop  its 
rhythm.  The  two  must  act  together,  each  alternately  stretching  the 
other.  The  frequency  of  contraction  depends  upon  the  tension  in  the 
flight  muscles  and  this  tension  not  only  is  caused  by  the  opposing  flight 
muscles,  but  can  be  increased  by  other  smaller  muscles  in  the  thorax.  If 
these  smaller  muscles  contract  steadily,  they  increase  the  tension  and 
raise  the  frequency  of  wing  beat.  The  hum  of  a  mosquito,  fly  or  bee 
can  be  used  as  an  accurate  indicator  of  this  frequency,  since  changes 
in  tone  indicate  changes  in  frequency. 

Pringle  found  further  that  a  single  set  of  muscles  could  produce 
its  rhythm  if  opposed  by  powerful  springs  as  a  substitute  for  the  op- 
posing set  of  muscles.  As  one  might  expect,  the  frequency  of  contraction 
was  a  function  of  the  stretch  of  the  springs. 

In  the  evolution  of  these  small,  fast-flying  higher  insects  the  co- 
ordination of  the  flight  muscles  has  been  taken  from  the  nervous  system 
and  built  into  the  muscles  themselves.  The  ability  to  contract  repeatedly 
under  tension  is  apparently  a  result  of  adjustment  in  the  internal  physi- 
ology of  the  muscle  cells. 

In  many  insects  with  slow  neurogenic  rhythms  the  path  traced  by 
the  wing  tip  as  it  moves  up  and  down  shows  that  the  wing  moves  evenly 
from  the  up  or  down  position  to  the  horizontal,  and  then  "clicks"  sud- 
denly the  rest  of  the  way  down  or  up.  Until  very  recently  the  "click" 
mechanism  was  not  understood.  It  has  turned  out  to  be  a  marvel  of  sim- 
plicity: Some  of  the  small  muscles  of  the  thorax  attach  to  the  inner 
upper  edge  of  the  "box,"  just  belo^v'  the  point  Avhere  the  wings  articu- 
late (Fig.  17.10).  The  steady  contraction  of  these  muscles  tends  to  pull 
the  sides  of  the  box  together.  A  study  of  the  figure  shows  that  the  dis- 
tance between  the  upper  edges  of  the  box  is  least  when  the  wings  are 


= — Notutn 


Pleurumn. 
PI  e-ural  muscle 


Figure  17.10.  The  "click"  mechanism.  A  comparison  of  the  three  figures,  using  the 
dotted  hne  as  a  reference  Une,  shows  that  the  sides  of  the  thorax  are  pushed  out  when 
the  wings  are  horizontal,  and  closer  together  when  the  wings  are  up  or  down.  Oblique 
muscles  to  the  sides  of  the  thorax  produce  tension  to  enhance  the  "click,"  which  is  a  rapid 
conclusion  of  the  wing  beat  upward  or  downward. 


336  '^^  ANIMAL   KINGDOM 


Forward  flight, 
showing  air  f  lov\7 


^7 


Hoverind  Ba.cKin^  Turning 

Figure  17.11.  Flight  maneuvers  in  the  honeybee.  The  figure  eight  in  the  first  three 
diagrams  traces  the  path  of  the  left  wingtip.  The  lines  on  the  last  diagram  show  the  posi- 
tions of  the  two  wings  during  a  full  beat.  (After  Stellwaag.) 

up  or  down  and  greatest  when  the  wings  are  horizontal.  Hence,  as  the 
wings  begin  to  move  they  do  so  agaijist  the  torce  of  these  small  muscles 
until  they  reach  the  half-way  point,  when  they  move  with  the  force  and 
are  suddenly  accelerated.  This  guarantees  full  amplitude  to  the  wing 
beat  and  a  sudden,  rapid  stretching  of  the  relaxing  set  of  muscles  just 
before  their  next  contraction.  Such  stretching  is  known  to  improve  the 
strength  of  contraction  in  many  kinds  of  muscles,  including  those  of  the 
vertebrates. 

In  all  winged  insects,  smaller  muscles  in  the  thorax  attached  to  the 
sides  and  wing  bases  are  used  to  alter  the  posture  of  the  wings  as  they 
move  up  and  down.  Suitable  contraction  of  these  muscles  enables  the 
insect  to  turn,  hover  or  back  up  (Fig.  17.11).  While  the  details  of  these 
processes  are  too  intricate  to  present  here,  the  general  pattern  of  ordi- 
nary flight  is  such  that  the  wings  act  as  propeller  blades,  drawing  air 
from  above,  in  front,  and  to  the  sides,  and  propelling  it  posteriorly  as 
a  sharply  driven  column  of  air.  The  details  are  modified  endlessly  in 
the  various  groups  of  insects. 

153.        Vision 

The  functioning  of  the  compound  eyes  is  a  most  intriguing  physi- 
ologic problem.  It  was  recognized  early  that  images  formed  by  such 
eyes  must  be  very  different  from  those  formed  in  our  eyes.  Their  struc- 
ture (Figs.  16.7,  17.12)  suggests  that  each  ommatidium  records  the 
amount  of  light  received  from  a  particular  direction,  and  that  all  of 
them  together  provide  a  mosaic  impression  of  the  world.  This  theory 
received  considerable  support  when  Exner,  in  1891,  sliced  off  the  com- 
pound eye  of  a  firefly  and  used  it  as  a  lens  for  making  a  photograph. 
The  film  image  was  a  single  large  one  and  was  erect  rather  than  in- 
verted as  in  our  eye. 


PHYSIOLOGY   AND    BEHAVIOR   OF   THE   ARTHROPODA 


337 


More  recent  work  shows  that  in  addition  to  receiving  the  Hght  from 
directly  in  front,  each  ommatidium  transmits  Hght  less  and  less  effec- 
tively as  the  incident  light  arrives  more  and  more  obliquely.  Eyes  with 
few  ommatidia  gather  light  from  wide  angles.  All  of  the  light  trans- 
mitted through  the  cornea  and  cone  is  brought  to  a  point  at  the  in- 
ternal tip  of  the  cone,  where  it  enters  the  ends  of  the  seven  to  fifteen 
rhabdomes  (Fig.  17.12)  which  form  a  single  retinula.  \\^hether  each 
retinula  records  a  single  light  impression  or  whether  its  constituent 
rhabdomes  respond  to  different  properties  of  the  light,  such  as  its  ex- 
ternal direction  or  its  color,  is  at  present  an  unsolved  problem. 

'  The  curtains  of  pigment  separating  adjacent  ommatidia  vary  from 
arthropod  to  arthropod  and  in  many  species  from  day  to  night.  Diurnal 
species  usually  have  a  complete  curtain  formed  by  two  sets  of  pigment 
cells  (Fig.  17.12)  so  that  each  retinula  can  receive  light  only  from  its 
own  lens  system.  In  nocturnal  species,  however,  the  pigment  is  re- 
stricted to  the  outer  layers  and  the  retinulae  are  separated  some  dis- 
tance from  the  inner  ends  of  the  cones.  In  such  eyes  light  from  a  distant 
point  can  pass  through  several  adjacent  lenses  to  be  superimposed  on  a 
single  underlying  retinula. 

In  both  kinds  of  eyes  the  pigment  may  migrate  according  to  the 
light  intensity.  The  pigment  of  the  nocturnal  eye  (Fig.  17.12)  spreads 
inward  under  bright  light,  reducing  the  nimrber  of  facets  that  can 
superimpose  an  image.  In  this  way  the  total  light  reaching  the  light- 
sensitive  regions  is  reduced  to  avoid  glare. 

The   visual  acuity  of   arthropods   has   been  studied   extensively.   A 


-Corne-al  l<znS 
Cone  lens 


Pigment  cells 


DarK  condition."^ 


Bright  condition 


Rhabd-onze 


RctinuLa  cells- 


"Nerve  f^- 

Figure  17.12.  A,  Insect  ommatidia,  showing  a  diurnal  type  (left)  and  a  nocturnal 
type  (center).  In  the  nocturnal  type,  the  pigment  is  shown  in  two  positions,  adapted  for 
very  dark  conditions  on  the  left  side,  and  for  relatively  bright  conditions  on  the  right.  B, 
Nocturnal  type  of  eye  adapted  for  dark  conditions,  showing  how  light  can  be  concentrated 
upon  one  rhabdome  from  several  lenses.  If  the  pigment  moved  downward,  light  from 
peripheral  lenses  would  be  screened  out. 


338 


THE   ANIMAL   KINGDOM 


Figure  17.13.  A  device  for  estimating  the  visual  acuity  of  an  arthropod.  A  drum 
with  internal  vertical  stripes  is  rotated  slowly  around  a  circular  glass  dish.  If  an  arthopod 
inside  can  distinguish  the  stripes,  it  tends  to  move  with  them  and  maintain  a  fixed  rela- 
tion with  the  surroundings. 


common  method  is  to  take  advantage  of  a  "status  quo"  reflex  with  which 
many  animals  attempt  to  maintain  a  constant  relation  to  the  environ- 
ment. The  animal  is  placed  in  a  circular,  glass-walled  container   (Fig. 

17.13)  around  which  is  rotated  a  drum  with  internal,  vertical,  black  and 
white  stripes.  If  the  animal  sees  only  a  mixed  gray  it  remains  quiet.  If, 
however,  it  can  distinguish  the  stripes  the  rotational  impression  is  very 
strong  and  the  animal  turns  or  walks  in  circles  to  stay  with  the  drum. 
By  varying  the  stripe  width  the  discriminative  limit  can  be  tested. 

Two  general  conclusions  can  be  derived  from  such  studies:  (1) 
Visual  acuity  varies  according  to  the  excellence  of  the  lens  systems  in 
the  ommatidia,  which  admit  light  through  wider  incident  angles  in 
some  arthropods  than  in  others.  (2)  Acuity  also  varies  inversely  with  the 
number  of  ommatidia.  The  best  arthropods  have  an  acuity  about  %(,  as 
good  as  that  of  man.  Most  of  them  are  much  poorer  than  this. 

Von  Frisch  has  extended  his  study  of  vision  to  an  investigation  of 
the  honeybee's  ability  to  discriminate  among  various  shapes.  If  a  group 
of  white  cards  is  placed  on  the  ground  with  a  glass  dish  on  each  (Fig. 

17.14)  and  syrup  is  placed  in  only  one  dish,  bees  discovering  the  syrup 
will  load  up,  return  to  the  hive,  and  come  back  for  more.  Others  come 
too,  and  soon  many  may  be  coming  and  going.  The  bees  are  marked 
with  paint  as  they  feed  so  that  they  can  be  recognized  when  they  return. 
If  all  the  cards  look  alike  to  the  bees  they  alight  on  all  of  the  dishes. 
If,  however,  the  card  with  the  syrup  is  recognizably  different,  once  each 
bee  has  found  it  she  will  return  only  to  that  dish. 

By  using  cards  marked  in  various  ways  von  Frisch  found  that  bees 
did  not  discriminate  among  squares,  circles  or  triangles  (Fig.  17.15),  nor 
did  they  distinguish  two  lines  from  a  cross.  They  did,  however,  dis- 
tinguish between  solid  and  open  figures,  and  between  one  line  and  two 
lines.  While  they  did  not  distinguish  between  a  bar  and  a  solid  square. 


PHYSIOLOGY   AND   BEHAVIOR   OF   THE   ARTHROPODA 


339 


Figure  17.14.  Design  for  studying  discrimination  in  bees.  All  16  cards  had  dishes, 
but  only  a  few  dishes  had  syrup.  By  varying  background  (as  on  one  card  above),  the 
ability  of  the  bees  to  discriminate  can  be  observed.  This  design  was  also  used  to  study 
color  vision.  (After  von  Frisch.) 

they  easily  distinguished  one  bar  from  two  that  occupied  less  space  than 
the  square.  Hence,  the  observed  failures  cannot  be  attributed  to  poor 
acuity.  Apparently  shape  as  such  is  not  recognized  by  bees  when  feed- 
ing, but  discontinuity  is.  All  the  pairs  of  objects  that  the  bees  can  tell 
apart  differ  in  discontinuity.  As  the  bee  flies  over  the  targets,  with  its 
compound  eyes  fixed  rigidly  on  its  body,  a  solid  square,  circle  or  bar 
produces  a  single  wave  of  darkening  across  the  ommatidia,  whereas  an 
open  figure  or  two  lines  or  an  X  produce  two  waves,  at  least  in  some 
regions  of  the  eye.  The  bees  appear  to  be  counting  interruptions  not  ob- 
serving form. 

These  results  indicate  the  risk  involved  in  drawing  negative  con- 
clusions from  experiments  with  behavior.  If  only  solid  figures  had  been 
used  von  Frisch  might  well  have  concluded  that  bees  discriminate  very 
poorly  if  at  all.  Actually,  however,  the  choices  presented  to  the  bees 
simply  would  not  have  provided  stimuli  appropriate  for  the  response 
being  studied. 

A  more  dramatic  case  of  this  kind  occurred  earlier  in  experiments 
with  color  vision.  Men  have  long  wondered  whether  other  animals  per- 
ceive   color,    and   many   early   experiments    were    negative.    Again,    the 


■  ▲/ 

XDAli 


Figure  17.15.  In  feeding  experiments,  bees  did  not  distinguish  among  the  figures 
of  the  top  row,  or  among  those  of  the  bottom  row.  They  did  distinguish  between  the 
members  of  any  pair  including  one  upper  and  one  lower  figure.  (After  von  Frisch.) 


340  ^"^   ANIMAL   KINGDOM 

critical  factor  has  turned  out  to  be  whether  or  not  the  stimulus  used  was 
an  appropriate  cue  for  the  situation.  Kupelweiser  stated  in  1913  that 
bees  were  colorblind.  Discovering  that  captured  bees  released  in  a  dark 
room  invariably  flew  to  windows,  he  performed  a  variety  of  excellent 
experiments  in  which  bees  could  choose  between  two  windows  of  vary- 
ing brightness  and  color.  He  showed  without  a  doubt  that  only  bright- 
ness is  involved  in  the  choice.  The  same  year,  however,  von  Frisch  did 
his  classic  experiments  on  color  vision  in  bees,  using  groups  of  colored 
cards  some  of  which  had  syrup.  He  found  proof  of  good  color  vision,  in 
which  what  we  call  orange,  yellow  and  green  were  seen  as  one  color, 
blue-green  another,  blue  and  violet  a  third,  and  ultraviolet  a  fourth. 

These  two  sets  of  experiments  are  not  contradictory.  Both  have 
been  repeated  successfully.  They  illustrate  that  in  its  escape  reactions  the 
bee  uses  only  brightness  cues,  whereas  in  jeeding  it  uses  color  cues. 
Man  is  handicapped  to  the  extent  that  he  cannot  ignore  color  in  an 
attempt  to  evaluate  brightness.  Ordinarily  man  does  very  poorly  in 
judging  the  relative  brightness  of  dissimilar  colors. 

Color  vision  has  now  been  demonstrated  in  a  wide  variety  of  in- 
sects and  crustaceans.  Even  the  tiny  daphnia  with  a  single  compound 
eye  distinguishes  between  orange-yellow-green  and  blue-green-blue- 
violet.  Probably  most  compoinid  eyes  distinguish  color.  Butterflies,  as  a 
final  example,  are  easily  trained  to  feed  at  blue  or  yellow  cards  among 
other  colors  and  grays,  but  cannot  be  trained  to  visit  green.  The  con- 
clusion that  they  cannot  distinguish  green  is  shown  to  be  false  by  the 
demonstration  that,  when  laying  eggs,  they  visit  only  green  cards. 

1 54.        Behavior 

The  activities  of  arthropods  are  a  source  of  endless  fascination.  Be- 
cause many  of  their  responses  are  inherited  as  patterns  that  follow  auto- 
matically upon  the  presentation  of  appropriate  stimuli,  the  behavior 
of  arthropods  has  been  analyzed  more  successfully  than  that  of  most 
organisms.  To  be  sure  most  of  their  activities  are  only  partially  pre- 
dictable, but  nevertheless  a  variety  of  basic  patterns  have  been  recog- 
nized. 

The  simplest  effect  a  stimulus  can  have  upon  an  organism  is 
kinesis,  an  increase  in  activity.  Light,  for  example,  has  a  kinetic  effect 
on  many  diurnal  animals.  Povdtrymen  use  this  as  a  means  of  increasing 
egg  production.  Many  arthropods  become  inactive  in  the  dark  (i.e., 
"go  to  sleep"),  moving  about  only  in  the  light  and  at  rates  related  to 
the  intensity  of  the  light.  The  evidence  of  this  relationship  reaches  a 
dramatic  level  in  some  butterflies  and  diurnal  moths.  The  hummingbird 
moth  flies  only  in  light,  and  if  flying  in  a  room  imder  artificial  light 
will  fall  instantly  to  the  floor  when  the  light  is  turned  off.  Temperature 
and  humidity  are  other  stimuli  that  may  influence  kinesis.  Pill  bugs 
(terrestrial  isopods)  cannot  survive  low  humidities,  and  respond  to  dry 
air  by  restless  movement.  They  come  out  to  feed  at  night,  spending  the 
day  beneath  objects.  If  they  should  happen  to  crawl  beneath  a  stone 
where  it  is  dry  they  are  unable  to  rest,  and  eventually  crawl  out  even 


PHYSIOLOGY   AND   BEHAVIOR   Of   THE   ARTHROPODA 


341 


Figure  17.16.  A,  A  walking  stick  (Orthoptera)  that  has  been  balanced  on  its  head 
while  feigning  death.  (After  Schmidt.)  B,  A  katydid  (Orthoptera)  on  a  branch.  By 
"freezing"  whenever  frightened  or  disturbed,  this  katydid  easily  passes  as  a  leaf,  com- 
plete with  veins  and  blotched  as  if  with  blight.  Thus,  immobilization  is  combined  with 
camouflage.   (Alfred  Eisenstaedt-Courtesy  LIFE  Magazine.  Copr.  1955  Time,  Inc.) 


342  ^"^   ANIMAL   KINGDOM 

into  the  light  in  search  of  another  shelter.  Kinesis  does  not  result  in  any 
particular  response  or  produce  movement  in  any  particular  direction, 
but  is  expressed  only  as  an  increase  in  the  rate  of  general  activity. 

The  opposite  of  kinesis  is  immobilization.  Night-flying  moths,  for 
example,  are  "put  to  sleep"  by  light.  Many  organisms  settle  down  to 
rest  if  their  body  is  touching  several  surfaces,  but  not  if  contact  is  pre- 
vented. A  sudden  and  complete  immobilization  is  called  "death  feign- 
ing." Many  insects  fall  immobile  to  the  ground  if  the  leaves  on  which 
they  are  sitting  are  jarred.  In  this  immobile  state  they  can  sometimes  be 
picked  up  and  squeezed  without  producing  resistance  or  spontaneous 
movement.  This  plastic  immobility  superficially  resembles  human  cata- 
lepsy. The  walking  stick  (Fig.  17.16  A),  a  wingless  orthopteran,  will  feign 
death  if  rubbed  in  the  presence  of  light.  Its  legs  can  then  be  moved  in 
various  ways,  and  the  walking  stick  will  hold  such  postures  for  several 
minutes.  Death  feigning  has  an  obvious  advantage  as  a  defense  against 
those  predators  that  recognize  their  prey  by  its  motion. 

A  stimulus  may  produce  an  orientation  response,  the  turning  or 
locomotion  of  the  animal  toward  or  away  from  the  direction  of  the 
stimulus.  Commonly  effective  stimuli  include  light,  gravity,  wind  and 
water  currents,  odors,  sound  and  radiant  heat.  Maggots,  for  example, 
have  a  negative  reaction  to  light;  if  they  are  placed  on  a  paper  beside  a 
light  they  crawl  rapidly  away  from  it.  Many  other  such  orientations 
are  known.  A  fly  on  a  flat  surface  will  turn  to  face  the  wind,  male  moths 
will  fly  toward  the  odor  of  the  female,  crayfish  are  attracted  to  dead 
flesh,  and  daphnia  turns  its  back  to  a  side  light. 

Orientation  to  a  given  stimulus  is  seldom  the  same  under  all  con- 
ditions. It  may  be  altered  or  even  reversed  if  certain  other  stimuli  are 
present.  The  maggots  can  be  made  to  seek  light,  if  they  are  grown  in 
the  presence  of  ammonia  or  other  harmful  chemicals.  Also,  a  maggot 
about  to  pupate  will  spontaneously  go  toward  the  light.  Such  response 
patterns  are  adjusted  to  suit  the  survival  of  the  organism.  Maggots  live 
in  manure  and  rotting  flesh  and  require  moisture.  Eggs  are  laid  on  the 
surfaces  of  such  food,  and  a  strong  negative  reaction  to  light  ensures 
that  the  maggots  will  burrow  in.  If  the  material  contains  harmful 
chemicals  the  maggots  would  do  better  to  leave  and  take  their  chances  on 
finding  another  food  source.  Although  the  maggot  requires  moisture, 
the  pupa  would  mold  in  a  moist  environment.  Hence,  crawling  toward 
the  light  just  before  pupation  places  the  pupa  in  a  drier  situation. 

Many  orientations  are  easily  reversed.  Caterpillars  tend  to  go  toward 
light  if  hungry,  but  crawl  away  from  it  if  they  are  full.  Some  orienta- 
tions are  sensitive  to  a  variety  of  other  stimuli.  One  of  the  best  known 
of  these  is  the  light  orientation  of  daphnia,  studied  for  over  100  years. 
Daphnias  will  swim  toward  or  away  from  a  light  depending  upon  the 
brightness  of  the  light,  the  color  of  the  light,  the  temperature  of  the 
water,  the  amount  of  carbon  dioxide  present,  and  their  state  of  hunger. 
Bright  light,  short  wavelengths,  high  temperatures,  low  carbon  dioxide 
and  hunger  all  favor  a  negative  response.  Other  modifying  stimuli,  too 
complex  to  be  presented  here,  also  influence  the  orientation  of  daphnia. 

A  somewhat  more  complicated  orientation   to  the  direction  of  a 


PHYSIOLOGY   AND   BEHAVIOR   OF   THE   ARTHROPODA  343 

Stimulus  is  best  described  as  navigation,  locomotion  at  some  fixed  angle 
with  respect  to  the  stimulus,  which  acts  as  a  landmark.  Many  swimming, 
walking,  and  especially  Hying  organisms  use  landmarks  as  a  means  of 
staying  on  some  particular  course.  An  ant  will  use  a  tall  tree  or  a  house 
as  a  prominent  distant  object  for  navigation.  If  while  walking  away 
from  the  nest  the  tree  was  on  the  left  and  a  little  in  front,  it  follows 
that  the  proper  route  home  is  one  that  places  the  tree  on  the  right  and 
a  little  behind.  Navigation  is  obviously  useful  to  organisms  that  have 
homes  from  which  they  make  journeys,  and  to  organisms  that  migrate, 
but  it  is  also  useful  in  many  other  situations.  An  insect  flying  a  random 
path  will  retrace  its  path  over  and  over,  covering  little  new  territory, 
while  an  insect  able  to  fly  in  straight  lines  will  cover  much  more  ground 
in  its  search  for  food. 

A  landmark  is  useful  in  proportion  to  its  distance.  The  relation 
between  a  moving  organism  and  a  nearby  object  changes  rapidly,  render- 
ing such  objects  useless  as  guides.  Even  with  more  distant  landmarks, 
the  relation  changes  slowly.  This  is  particularly  true  in  the  case  of 
flying  insects  that  may  move  several  miles.  The  most  distant  objects  are 
the  most  useful  guides  and  the  most  distant  of  all  are  the  sun  and  moon, 
so  far  away  that  their  relationship  to  a  moving  organism  remains  es- 
sentially constant  except  for  the  rotation  of  the  earth.  Perhaps  for  this 
reason,  the  sun  or  moon  is  used  preferentially  as  a  landmark  if  it  is 
visible.  Night-flying  moths  use  the  moon  for  navigation,  particularly  to 
cross  open  stretches,  and  bees  foraging  for  nectar  and  pollen  several 
miles  from  the  hive  use  the  sun. 

During  the  day,  of  course,  the  sun  moves  across  the  sky,  and  the 
bees  must  continually  make  allowance  for  its  movement.  Experiments 
have  shown  that  bees  captured  in  the  field  and  imprisoned  for  one  or 
two  hours  do  not  correct  for  this  movement,  but  when  released  fly  off 
using  the  original  bearing  with  the  sun  and  consequently  miss  the  hive 
by  some  distance.  They  do  get  close  enough  to  recognize  the  surround- 
ings, however.  Apparently  no  adjustments  are  made  in  the  field,  but 
back  at  the  hive,  in  more  familiar  surroundings,  the  bees  do  allow  for 
movement  of  the  sun.  They  take  a  new  bearing  each  time  they  leave  the 
hive. 

Civilization  has  added  an  ecologic  artifact  to  the  world  of  night, 
tricking  many  night  flyers  into  suicidal  behavior.  The  moon  is  no  longer 
the  only  light.  All  too  often  the  light  that  comes  into  the  view  of  a 
flying  insect  is  a  street  light  or,  to  use  a  more  poetic  example,  a  candle 
flame.  Immediately  it  uses  the  light  as  a  landmark,  locating  it,  for  ex- 
ample ahead  and  somewhat  to  the  left  (Fig.  17.17).  As  the  insect  con- 
tinues to  fly  it  must  turn  repeatedly  in  order  to  maintain  the  bearing, 
and  thus  follows  a  spiral  course  which  will  take  it  inevitably  to  the 
light.  Many  nocturnal  insects  are  unable  to  cope  with  such  artifacts. 
Their  instinctive  responses,  which  never  failed  with  the  moon,  are  so 
strong  that  they  are  unable  to  substitute  another  landmark  in  place  of 
light.  For  the  moth  the  candle  flame  can  be  the  fatal  end.  In  the  case 
of  the  more  prosaic  street  light  the  moth  will  eventually  settle  beneath 


344 


THE  ANIMAL   KINGDOM 


Figure  "17.17.  A  nocturnal  insect,  flying  a  straight  path,  uses  the  lamp  (L)  as  a  land- 
mark when  it  comes  into  view  (position  A).  Since  the  lamp  is  not  far  away  the  insect  must 
repeatedly  turn  to  maintain  a  fixed  bearing.  (After  Buddenbrock.) 

it  and  go  to  sleep,   unless  eaten  by  a  bat  or  frightened  away.   Fright 
appears  to  be  the  only  stimulus  that  can  break  up  the  impasse. 

These  are  but  a  lew  oi  the  relatively  simple  responses  that  can  be 
found  abundantly  in  arthropods.  There  are,  in  addition,  many  complex 
and  less  well  understood  patterns.  The  most  elaborate  of  these  are 
social,  in  which  the  stimuli  include  others  of  the  same  species.  Social 
responses  are  well  developed  in  the  social  insects,  those  that  live  to- 
gether in  colonies. 


155.        Social  Mechanisms  in  Insects 

The  development  of  integrated  colonies  is  limited  largely  to  insects, 
and  a  division  of  labor  among  the  members  of  the  colony  is  found  only 
in  the  termites,  ants  and  bees.  These  colonies  are  marked  by  a  restriction 
of  the  function  of  reproduction  to  a  limited  portion  of  the  population 
and  by  a  separation  of  duties  among  the  nonreproductives. 

Termites.  The  termite  colony  (Fig.  17.18)  begins  when  a  pair  of 
winged  primary  reproductives  shed  their  wings  and  set  up  housekeeping. 
The  young  which  they  raise  are  sterile  wingless  workers,  who  do  all 


PHYSIOLOGY   AND    BEHAVIOR   OF   THE   ARTHROPODA 


345 


the  work  from  then  on,  including  nest  building  and  feeding  the  pri- 
maries. The  latter  continually  secrete  juices  in  return  for  being  fed,  and 
these  are  licked  up  eagerly  by  the  workers.  It  is  thought  that  this  secre- 
tion reward  is  the  basic  mechanism  that  integrates  the  colony,  and  the 
phenomenon  is  called  trophallaxis.  As  the  colony  grows  in  size  a  few 
of  the  offspring  develop  wing  buds  and  become  sexually  mature;  these 
are  called  the  secondary  reproductives.  Some  of  the  young  develop  into 
sterile,  wingless  soldiers  which  must  also  be  fed. 

All  of  the  termites  produce  secretions  to  some  extent,  and  these 
are  continually  licked  up  by  the  workers  and  by  the  young  nymphs.  In 
this  way  the  colony  as  a  whole  develops  a  blended  odor  distinct  from 
that  of  any  other  colony,  a  distinction  used  by  the  members  of  a  colony 
in  recognizing  strangers.  Recently  it  has  been  suggested  that  trophal- 
laxis is  a  mechanism  for  distributing  substances  that  act  as  hormones, 
and  that  these  determine  the  kind  of  individual  a  young  nymph  will 
become.  According  to  this  interpretation  the  reproductives  secrete  an 
"anti-reproductive  substance"  that  prevents  sexual  maturity  in  the 
young.  Similarly  the  soldiers  secrete  an  "anti-soldier  substance."  If  the 
number  of  either  of  these  groups  falls  below  a  critical  level,  not  enough 
of  their  "anti"  substance  will  be  circulating  in  the  colony  and  some  of 
the  young  develop  in  that  direction.  This  explains  the  appearance  of 
secondary  reproductives  after  the  original  colony  has  become  large,  and 
also  explains  the  immediate  replacement  of  any  group  after  it  has  been 
removed  experimentally. 

Seasonally    large   numbers   of  primary   reproductives  are  produced 


.^^si 


« 


:.^ 


Figure  17.18.     Castes  of  termites.  A,  Male  (king),  before  shedding  wings.  B,  Female 


(queen),  after  shedding  wings.  C,  \Vorker.  D,  Soldier.  Workers  and  soldiers  are  sterile  in- 
dividuals of  either  sex. 


346 


THE  ANIMAL  KINGDOM 


Figure  17.19.  Castes  of  ants.  A,  Female  (queen),  after  shedding  wings.  B,  Male.  Note 
large  eyes  and  long  antennae.  C,  Soldier.  D,  Worker  (stunted  workers  may  be  still  smaller). 
E,  Honey-ant,  which  hangs  motionless  in  the  nest,  is  fed  excessively  when  food  is  plentiful, 
and  serves  as  a  food  source  when  food  is  scarce.  The  last  three  categories  are  all  sterile 
females. 


and  they  leave  in  a  mass  flight  as  soon  as  they  are  mature.  The  mechan- 
ism by  which  they  are  produced  is  not  known. 

Ants.  The  ant  colony  is  founded  by  a  winged  queen,  the  only 
reproductive  of  the  colony,  after  she  has  mated  with  a  winged  male 
upon  leaving  the  parent  colony.  She  sheds  her  wings  and  uses  the 
nourishment  of  stored  fat  and  degenerating  wing  muscles  to  produce 
the  first  group  of  workers,  which  are  small,  stunted  individuals.  They 
take  care  of  subsequent  young  and  do  all  the  work  of  the  colony.  They 
leave  the  nest  and  gather  food,  nourishment  becomes  plentiful,  and 
the  later  offspring  are  of  normal  size.  Most  of  the  larvae  mature  as 
workers,  but  in  various  species  other  kinds  (called  castes),  such  as  sol- 


PHYSIOLOGY   AND    BEHAVIOR   OF   THE   ARTHROPODA  347 

diers  and  honey-ants  (Fig.  17.19),  appear.  Trophallaxis  is  again  the 
prime  integrating  force  of  the  colony.  As  in  the  termites,  great  numbers 
of  winged  reproductives  are  produced  seasonally  by  some  unknown 
mechanism. 

Honeybees.  The  queen  honeybee  is  unable  to  found  a  colony  or  to 
survive  at  all  without  workers.  She  appears  in  an  established  colony, 
flies  out  of  the  hive  with  males  in  chase  on  the  nuptial  flight,  is  fer- 
tilized, and  then  returns  to  the  hive  to  begin  producing  eggs.  Most  of 
the  eggs  develop  into  workers,  and  as  in  the  other  societies  the  integrat- 
ing mechanism  assuring  that  all  members  will  be  fed  is  trophallaxis. 
Although  there  is  only  one  caste  of  sterile  female  bees,  these  workers 
are  ditterentiated  by  age  into  three  physiologically  different  groups  as 
described  in  section  319.  If  all  the  individuals  of  one  of  these  age  groups 
are  removed  from  the  colony,  the  time  scale  of  development  in  the 
others  is  altered,  and  sometimes  a  given  group  will  revert  to  a  younger 
stage.  This  suggests  that  trophallaxis  not  only  assures  feeding  and  de- 
velops a  hive  odor  but  by  the  distribution  of  hormonal  substances  it 
keeps  the  colony  structure  balanced. 

W^hen  the  colony  becomes  large  the  nurse  bees  set  aside  a  few  eggs 
to  be  raised  as  queens.  Adjacent  cell  walls  are  torn  out  to  make  larger 
chambers,  and  there  these  few  larvae  are  fed  exclusively  on  royal  jelly 
for  the  whole  six  days  of  their  larval  life.  When  they  pupate,  the  colony 
begins  to  split  up.  About  half  of  the  workers  induce  the  old  queen  to 
leave,  and  they  fly  off  with  her  to  begin  a  new  colony.  The  other  half 
remains.  W'hen  the  first  new  queen  emerges  from  her  cell,  she  may  also 
be  induced  to  leave  with  another  group  of  workers  if  the  colony  has 
become  very  large.  Either  she  or  the  next  queen,  however,  remains  as 
the  new  resident  queen.  As  soon  as  one  or  the  other  is  established  the 
few  other  queens  that  may  be  hatching  are  destroyed. 

It  had  been  thought  that  the  determining  factor  in  queen  produc- 
tion was  nutritional,  but  recent  work  suggests  that  the  royal  jelly  con- 
tains a  hormone;  if  a  larva  receives  enough  of  this  it  will  mature  as  a 
queen. 

Sex  is  determined  by  the  usual  chromosomal  mechanism  (p.  660) 
in  the  termites,  half  of  which  are  male  and  half  female  in  all  of  the 
castes.  Most  ants  and  bees,  however,  are  female.  Males  are  produced 
only  from  unfertilized  eggs  and  are  haploid.  They  appear  only  with  the 
female  reproductives  in  the  ant  colony,  as  part  of  the  seasonal  swarming. 
Male  bees  are  produced  sporadically.  They  hang  around  the  hive  as 
drones,  doing  no  work  and  feeding  themselves  when  hungry.  Their 
only  function  is  to  be  there  when  new  queens  emerge.  They  chase  after 
her  on  the  nuptial  flight,  and  high  in  the  air  one  of  them  mates  with 
her.  In  the  fall,  as  the  colony  prepares  for  winter  they  are  expelled  from 
the  hive. 


156.        Bee  Language 

Bees  have  an  additional  social  mechanism  that  greatly  increases  the 
efficiency   of    the   colony,   an   elaborate   and    remarkable   language.    Bee 


348 


THE  ANIMAL   KINGDOM 


o 


Figure  17.20.     The  round-dance,  an  alternation  of  circling  first  one  way  and  then 
the  other.  (After  von  Frisch.) 

language  is  oriented  entirely  toward  economy  in  the  effort  of  gather- 
ing nectar  and  pollen.  It  is  a  device  by  which  a  bee,  having  found  a 
honey  or  pollen  source,  is  able  to  communicate  to  other  workers  the 
necessary  information  about  its  location.  Communication  falls  into  two 
categories,  that  for  sources  nearby,  and  that  for  sources  some  distance 
away. 

A  bee  returning  with  a  load  of  nectar  or  pollen  from  a  good  source 
within  100  yards  of  the  hive  unloads  and  then  begins  a  round-dance, 
turning  to  the  right  and  left  in  small  circles  (Fig.  17.20).  This  excites 
other  foragers  near  her,  who  keep  their  antennae  on  her  and  chase  be- 
hind her  in  the  dance  (in  the  darkness  of  the  hive  antennal  contact  is 
used  because  vision  is  useless).  Chemoreceptors  on  the  antennae  pick 
up  the  scent  of  the  food,  identifying  the  kind  of  pollen  or  nectar.  Then 
the   dancing   bee   abruptly  moves   elsewhere   in    the   hive   and   repeats, 

A  B  CD 


t    ^ 


V 

; 


Food 

Hive.  D 

Hive  n-go--;-^ 

/OCX 

VFood. 

\ 

\ 

(I) 

\ 

0    * 

Da-n.ce 

Sun. 

Daaice                 Sujt. 

^Food 

Food 

\ 
\ 

\ 
\ 

X D 

Hive 

n  Hive 

Suii 


'# 


)un 


Danccz-  Suii.      Dance 

Figure  17.21.  The  wagging  dance.  A-D,  the  four  successive  steps  of  the  dance.  The 
following  four  figures  demonstrate  the  relation  of  the  straight  rush  to  the  direction  of  the 
food,  in  which  upward  (toward  top  of  page  here)  on  the  vertical  surface  inside  the  hive 
is  substituted  for  sunward  outdoors.  (After  von  Frisch.) 


PHYSIOLOGY   AND   BEHAVIOR   Of   THE   ARTHROPODA 


349 


while  the  excited  bees  fly  out  of  the  hive  and  circle  the  neighborhood 
until  they  locate  the  same  scent.  When  they  return  they,  too,  will  round- 
dance.  All  dancing  bees  repeat  the  j^rocess  only  a  few  times  before  re- 
turning to  the  source  for  another  load.  As  long  as  the  source  holds  out, 
the  returning  bees  dance.  When  the  supply  dwindles  and  becomes 
harder  to  get  the  bees  will  no  longer  dance,  but  they  will  continue  to 
return  to  the  source  until  nothing  at  all  is  left. 

This  procedure  is  not  adequate,  however,  to  locate  sources  that 
are  farther  away.  A  bee  returning  from  a  distance  performs  a  wagging 
dance  (Fig.  17.21).  She  walks  in  a  semicircle,  then  rushes  straight  back 
to  the  starting  point,  walks  around  the  other  way,  rushes,  and  repeats. 
On  the  straight  rush  she  waggles  the  abdomen  vigorously.  Neighboring 
foragers  become  excited  by  this  dance  also,  following  closely  with  their 
antennae.  An  astonishing  amount  of  information  is  thus  transmitted. 
The  followers  not  only  become  excited  and  pick  up  the  scent  of  the 
source,  but  they  also  learn  how  far  away  it  is  and  in  what  direction! 

Distance  is  indicated  by  the  tempo  of  the  dance,  by  the  speed  with 
which  the  cycle  is  completed  (the  closer  the  source,  the  more  rapid  the 
dance  (Fig.  17.22)).  For  long  distances  the  rate  may  be  as  slow  as  four 
cycles  per  minute.  Von  Frisch,  who  worked  out  this  interpretation  of 
bee  language,  found  that  most  of  the  "listening"  bees  learned  the  dis- 
tance to  within  25  per  cent.  Detailed  studies,  with  winds  in  various 
directions,  showed  that  distance  was  actually  given  as  the  amount  of 
time  required  to  fly  to  the  source.  Most  remarkable  is  that  bees  flying 
home  with  the  wind  make  a  correction  for  this  and  signal  a  distance 
appropriate  for  the  time  it  w-ould  take  to  get  there  against  the  wind. 

The  direction  to  the  source  is  indicated  by  the  direction  of  the 
straight  rush.  This  is  the  most  ingenious  part  of  the  vocabulary  since  it 
necessarily  involves  a  translation  of  marks.  Outside  the  hive  the  bee 
uses    the   sun    as    a    landmark.    Inside,   on    the   vertical   surfaces   of    the 

10 


8 


VI 

C 
0 

^    7 
\n 

in  6 


m 
C 

o 

;-. 

E 


1 

t 

1 

1 

1 

1 

\ 

- 

\ 

- 

\ 

- 

( 

1 

1 

1 

1 

1    ( 

i     3      ^ 

t      5      € 

i       7       i 

I     9 

IC 

Kilometers  between  source  a.-ndhive 

Figure  17.22.  The  relation  between  the  distance  of  the  food  source  from  the  hive 
and  the  number  of  straight  rushes  per  fifteen  seconds  in  the  wagging  dance.  As  the  dis- 
tance becomes  shorter  the  dance  merges  into  the  round-dance.  (After  von  Frisch.) 


350  ^^^   ANIMAL   KINGDOM 


nHive 


Hioh  ridge. 


Figure  17.23.  The  contour  lines  indicate  a  high  ridge  separating  the  hive  from  a 
syrup  source.  Bees  returned  to  the  hive  along  the  dotted  line,  and  in  their  wagging  dance 
indicated  the  true  direction  (heavy  arrows),  but  149  meters  distance.  (After  von  Frisch.) 

combs,  Upward  is  substituted  for  sunward  (Fig.  17.21).  Thus,  if  the 
source  lies  toward  and  a  little  to  the  right  of  the  sun,  the  straight  rush 
is  a  little  to  the  right  of  straight  up.  Von  Frisch  found  that  most 
"listening"  bees  learned  the  direction  of  the  source  within  a  few  de- 
grees of  its  true  position. 

The  most  spectacular  of  all  experiments  by  von  Frisch  involved 
placing  the  hive  and  sugar  source  on  opposite  sides  of  a  towering  rock 
ridge  (Fig.  17.23).  Bees  discovering  the  source  filled  up  and  flew  around 
the  end  of  the  ridge  to  get  home,  more  than  twice  the  distance  straight 
through  the  ridge.  Once  there,  they  communicated  to  others  the  flight 
distance,  but  signalled  the  true  direction!  Excited  bees  then  flew  out 
of  the  hive  straight  at  the  cliff.  Meeting  this  obstacle  they  turned  and 
went  around  the  end,  flew  back  along  the  outer  side  to  the  proper  point, 
and  then  circled  to  locate  the  source.  It  is  evident,  therefore,  that  the 
bee  language  can  be  adapted  to  specific  problems. 

In  another  experiment  von  Frisch  placed  the  hive  at  the  base  of  a 
radio  tower,  and  the  source  on  top  of  it.  Bees  taken  to  the  top  filled  up, 
went  home,  and  round-danced.  Many  others  went  out  searching,  but 
none  of  them  went  high  enough  to  find  the  honey.  When  the  hive  was 
placed  some  distance  away  from  the  tower,  the  returning  bees  wag- 
danced.  Others  flew  out  in  the  right  direction,  for  the  right  distance, 
and  were  seen  circling  around  the  base  of  the  tower,  but  none  of  them 
found  the  source  at  the  top.  Bees,  apparently,  have  no  word  for  "up." 

Von  Frisch's  discoveries  have  greatly  broadened  the  field  of  animal 
behavior.  If  bees  can  "talk,"  what  can  other  invertebrates  do? 

Questions 

1.  How  does  an  arthropod  escape  from  its  old  exoskeleton? 

2.  What  are  gastroliths? 

3.  Describe  the  role  of  the  sinus  glands  in  the  crustacean  endocrine  system. 

4.  Define  neurosecretion. 

5.  How  was  the  role  of  the  corpora  allata  in  insects  detennined? 


PHYSIOLOGY   AND   BEHAVIOR   OF   THE   ARTHROPODA  351 

6.  Describe  the  general  features  of  the  innervation  of  muscles  in  arthropods. 

7.  What  is  a  myogenic  rhythm? 

8.  Draw  a  diagram  of  an  ommatidium  in  a  day-flying  insect. 

9.  How  did  von  Frisch  discover  color  vision  in  bees? 

10.  Describe  kinesis,  simple  orientation  and  navigation. 

1 1.  What  is  trophallaxis? 

12.  With  diagrams  in  which  up  is  toward  the  top  of  the  page,  show  how  a  bee  would 
indicate  to  others  the  direction  of  a  food  source  a  mile  northeast  of  the  hive  (a)  just 
after  dawn,  (b)  at  noon,  and  (c)  just  before  sunset. 

Supplementary  Reading 

General  sources  of  information  include  Prosser  et  al.,  Comparative  Animal  Physi- 
ology, Wigglesworth,  The  Principles  of  Insect  Physiology,  and  Wheeler,  The  Social  In- 
sects. Part  of  the  work  on  insect  hormones  by  Williams  appeared  in  LIFE  Magazine 
(1952).  Von  Frisch,  Dancing  Bees,  is  a  popular  and  informative  account  of  bee  behavior, 
based  mostly  on  the  work  of  the  author. 


CHAPTER  18 


Minor  Phyla 


The  major  phyla,  the  ones  composed  of  many  and  diverse  kinds  of 
animals,  are  each  discussed  in  separate  chapters.  The  animal  kingdom 
contains  in  addition  a  number  of  forms  which  are  not  related  closely 
enough  to  any  of  these  major  phyla  to  be  a  class  within  one  of  them  but 
are  classified  as  separate  phyla.  This  emphasizes  that  a  phylum  is  not  a 
large  assemblage  of  organisms  but  a  group  of  organisms  which  are  so 
unique  in  structure  and  function  that  they  are  not  closely  related  to 
any  other  group.  Some  of  these  minor  phyla  (Ctenophora,  Nemertea, 
Onychophora  and  Hemichordata)  are  treated  elsewhere;  the  remaining 
ones  are  described  briefly  here. 


1 57.        Mesozoa 

The  Mesozoa  (Fig.  18.1  A)  are  minute  parasites  found  in  the  body 
cavities  of  certain  invertebrates;  one  is  found  in  the  kidney  of  the 
octopus.  The  body  structure  is  the  simplest  of  any  multicellular  animal. 

Anas 


Mouth- 
Subgastric; 


Sta-Ik 


A 

Megozoa. 


Stomach 


B 


Figure  18.1.     Mesozoa  (A),  showing  the  ciliated  epithelium  surrounding  an  inner 
mass  of  reproductive  cells,  and  Entoprocta  (jB),  showing  some  of  the  internal  organs. 

352 


MINOR   PHYLA  353 

Each  is  composed  of  a  ciliated  outer  cell  layer  and  an  inner  mass  of 
reproductive  cells.  W'hile  the  group  itself  is  well  defined,  it  cannot  be 
related  easily  to  other  animals.  Two  views  on  the  origin  of  these  animals 
are  current:  that  they  arose  directly  from  the  Protozoa,  and  that  they 
represent  extremely  degenerate  flatworms.  They  have  complex  life 
cycles  with  asexual  as  well  as  sexual  reproduction. 

158.  Entoprocta 

The  Entoprocta  (Fig.  18.1  B)  are  small,  sedentary,  stalked  animals 
with  a  complete  digestive  tract  and  a  pair  of  protonephridia.  Although 
the  larva  has  a  brain,  this  is  lost  in  metamorphosis  and  the  central  nervous 
system  of  the  adult  consists  of  a  subesophageal  (actually  subgastric  in 
position)  ganglion.  The  gut  is  U-shaped,  and  both  mouth  and  anus  are 
surrounded  by  a  circle  of  ciliated  tentacles.  W^ater  is  swept  upward 
through  the  tentacles,  and  food  particles  are  passed  from  the  tentacle 
sides  around  to  the  upper  surfaces  where  short  cilia  carry  them  down 
to  the  mouth.  The  digestive  tract  is  a  simple  gastrodermis  without  mus- 
culature except  on  the  stomodeal  and  proctodeal  portions.  Between  the 
gut  and  the  body  wall  is  a  space  filled  with  a  few  scattered  cells  and  a 
viscous  fluid.  This  structure  resembles  closely  the  pseudocoelom  of  the 
Aschelminthes. 

Entoprocts  are  primarily  marine,  with  one  family  occurring  in  fresh 
water.  Many  of  the  species  form  branching  colonies  by  asexual  budding 
from  the  stalk.  In  several  species  the  upper  portions  or  calyces  of  the 
individuals  die  during  the  winter  or  other  adverse  circumstances,  but 
the  stalks  remain  alive  and  regenerate  new  calyces  in  the  spring  or  when 
suitable  conditions  return.  In  sexual  development  the  egg  follows  a 
modified  spiral  cleavage  to  produce  a  ciliated  free-swimming  larva.  The 
larva  attaches  by  its  ventral  surface,  but  the  organs  rotate  180  degrees 
so  that  in  the  adult  the  "ventral"  surface  is  directed  upward. 

The  taxonomic  position  of  this  phylum  is  uncertain.  The  body 
structure  is  that  of  a  pseudocoelomate,  and  much  of  the  body  is  clothed 
in  cuticle  as  in  the  Aschelminthes.  But  the  entoprocts  adhere  more 
closely  to  typical  spiral  cleavage  than  the  Aschelminthes,  and  at  the 
cellular  level  they  show  none  of  the  extreme  specializations  of  the 
Aschelminthes.  The  entoprocts  have  good  powers  of  regeneration.  Asex- 
ual budding  is  common  in  the  entoprocts  but  is  unknown  in  the 
Aschelminthes.  It  seems  reasonable,  therefore,  to  suppose  that  this  group 
evolved  from  a  flatworm  stock  independently  from  the  Aschelminthes, 
but  has  reached  a  comparable  degree  of  structural  complexity. 

159.  Sipuncuioids  and  Echiuroids 

The  Sipunculoidea  and  Echiuroidea  (Fig.  18.2)  are  annelid  relatives 
in  which  segmentation  has  been  lost.  In  both  phyla  the  egg  follows  spiral 
cleavage  to  produce  a  trochophore  larva.  In  the  further  development  of 
the  trochophore  segmentation  begins  to  appear  (three  pair  of  somites 
in  sipuncuioids,  15  in  echiuroids),  but  then  disappears.  Both  groups  are 
marine. 

Echiuroids  are  sausage-shaped  worms  that  move  about  very  little, 


354 


THE   ANIMAL   KINGDOM 


Moull 
Retractor 

TYiUScle 


Re-lraclilc 
on 


Ne-pliridiam 


Iritesiiiie 

Ventral 

nerve  cord" 


appendage 


Sipu"nculoidga 


Priapuloidea 


Figure  18.2.  Three  groups  of  marine  worms.  Sipunculoidea,  cut  open  to  show  some 
of  the  internal  organs.  (After  Brown.)  Echiuroidea  (after  Parker  and  Haswell)  and  Pri- 
apuloidea  (after  Theel)  shown  in  side  view. 

and  lie  buried  in  the  mud  or  widiin  cavities  of  shells  with  a  greatly 
developed,  mucus-covered  prostomium  projecting.  The  prostomium  is 
ciliated,  and  is  used  for  gathering  detritus  from  the  bottom  surface  and 
passing  it  to  the  mouth.  Neither  a  distinct  brain  nor  sense  organs  are 
present;  the  esophagus  is  surrounded  by  a  nerve  ring  continuous  with 
the  ventral  nerve  cord. 

In  the  genus  BonneUa,  which  has  a  very  long,  forked  prostomium, 
an  interesting  case  of  sexual  dimorphism  is  found.  Each  larva  can  de- 
velop into  either  sex.  If  it  settles  by  itself  on  the  bottom  it  becomes  a 
female,  which  is  a  sizable,  fully  developed  worm.  If  the  larva  lands  on  a 
female,  however,  it  becomes  a  male,  which  remains  microscopic  in  size 
and  simplified  in  morphology,  and  lives  in  the  mouth  or  nephridia  of 
the  female. 

Sipunculoids  are  elongate,  flexible  worms  with  a  retractile  an- 
terior end  used  for  burrowing  in  sand.  They  swallow  the  sand  and 
digest  the  debris  and  small  organisms  it  contains.  The  mouth  is  sur- 
rounded by  a  ciliated,  tentacled  disc.  The  digestive  tract  includes  a 
long  intestine  that  doubles  back  from  the  posterior  end  to  a  dorsal  anus 
well  forward  on  the  body.  The  nervous  system  is  well  developed  and  is 
similar  to  that  of  the  annelids,  but  the  circulatory  system  is  reduced  and 
restricted  to  the  anterior  end  of  the  body.  The  coelom  is  large  and  un- 
divided. 

160.        The  Priapuloids 

The  Priapuloidea  (Fig-  18.2)  is  another  phylum  of  sizable  marine 
worms  that  lack  segmentation.  The  anterior  end  is  retractile  and  carries  a 


MINOR   PHYLA 


355 


large  mouth  that  opens  into  a  muscular  pharynx  lined  with  teeth.  Pri- 
apuloids  plow  through  mud  and  swallow  whole  whatever  prey  they  can 
seize.  The  nervous  system  resembles  that  of  the  echiuroids  and  a  cir- 
culatory system  is  lacking.  Young  priapuloids  have  a  sheath  of  cuticular 
plates  enclosing  the  posterior,  nonretractile  portion  of  the  body.  Their 
early  development  is  unknown. 

For  many  years  these  animals  have  been  allied  with  the  sipunculoids 
and  echiuroids.  Recently,  however,  notice  was  taken  of  the  fact  that 
although  the  body  cavity  is  lined  with  a  membrane  it  is  not  cellular, 
and  the  possibility  arises  that  the  cavity  is  a  pseudocoelom.  Comparisons 
have  been  made  between  the  young  and  some  of  the  rotifers  (although 
they  differ  greatly  in  size).  The  poorly  developed  nervous  system  and  ab- 
sence of  a  circulatory  system  are  further  evidence  for  grouping  the 
priapuloids  with  the  Aschelminthes.  The  digestive  tract,  however,  is 
completely  muscularized,  and  the  large  size  and  general  appearance  of 
these  worms  do  not  suggest  a  pseudocoelomate  affinity.  Until  their  early 
development  is  learned  the  true  relations  of  this  group  probably  will  not 
be  known,  and  for  the  present  they  will  be  left  as  a  group  related  to  the 
annelids. 


161.        The  Phoronlds  and  Brachiopods 

The  Phoronida  and  Brachiopoda  (Fig.  18.3)  include  medium-sized 
sessile  marine  animals  with  a  lophophore,  a  circle  of  ciliated  tentacles  sur- 
rounding the  mouth.  Typically  the  lophophore  in  these  phyla  is  drawn 


Pho 


i''onii 


Bra-chiopoda- 
-Tcntacles  icuit)  C 


'Terrt  a.clcB 
"Lophopliore 

Figure  1  8.3.  Phoronida:  A,  Whole  animal  in  side  view.  B,  Half  of  anterior  end  show- 
ing internal  organs.  (After  Parker  and  Haswell.)  Brachiopoda:  C,  Side  view  of  shell. 
D,  Side  view  showing  internal  structure.   (After  Borradaile,  et  al.) 


356  ^W£   ANIMAL   KINGDOM 

out  to  each  side  into  whorls.  The  cilia  draw  water  toward  the  animal 
and  food  particles  are  passed  down  a  ciliated  tract  at  the  tentacle  bases 
to  the  mouth.  In  both  phyla  the  eucoelom,  circulatory  system  and  meta- 
nephridia  are  well  developed,  so  that  they  are  obviously  eucoelomates. 
Each  group  contains  few  living  species. 

Phoronids  live  in  membranous  tubes  in  the  sand  or  cemented  to 
rocks.  They  have  a  long  body  with  a  U-shaped  gut,  the  anus  opening 
just  behind  the  lophophore.  During  feeding,  the  lophophore  is  extended 
from  the  tube  into  the  open  water.  Brachiopods  are  encased  in  a  bi- 
valved  shell,  the  ventral  shell  being  slightly  larger  than  the  dorsal  shell. 
They  have  a  superficial  resemblance  to  the  bivalved  molluscs.  In  some 
species  a  stalk  projects  through  the  hinge  to  attach  the  animal  to  rocks; 
in  others,  the  stalk  is  absent  and  the  animals  lie  free  on  the  bottom. 
Either  the  anus  opens  to  one  side  of  the  lophophore  or  the  gut  ends 
blindly  without  an  anus.  During  feeding  the  shells  are  opened  slightly 
and  water  is  drawn  in. 

Although  the  Brachiopoda  are  a  minor  phylum  today,  they  were  a 
major  group  in  the  past.  Throughout  the  Paleozoic  era  they  were 
abundant,  with  thousands  of  species  in  all  the  oceans  of  the  world. 
Most  of  the  fossil  shells  that  can  be  found  today  in  shale  and  slate  de- 
posits are  not  those  of  clams,  but  of  brachiopods. 

The  relation  of  these  phyla  to  other  eucoelomates  is  obscure.  Their 
early  development  is  variable,  but  in  all  cases  shows  a  wide  departure 
from  the  spiral  cleavage-trochophore  pattern.  Cleavage  follows  a  sim- 
pler pattern.  Some  species  are  schizocoelous  while  others  are  entero- 
coelous.  In  some  the  mouth  forms  from  the  blastopore  (characteristic  of 
the  mollusc-arthropod  series)  while  in  others  it  is  a  new  opening  (char- 
acteristic of  the  echinoderm-chordate  series).  In  the  light  of  these  varia- 
tions the  two  phyla  are  sometimes  considered  to  represent  survivors  of 
an  intermediate  group  between  the  two  major  series,  a  group  that  pos- 
sibly was  involved  in  the  evolution  of  the  echinoderm-chordate  series 
from  the  "main  line"  with  its  spiral  cleavage. 

162.        The  Bryozoa 

The  Bryozoa  are  minute  colonial  animals  (Fig.  18.4)  that  also  have 
a  lophophore.  They  are  common  in  both  salt  and  fresh  water.  They  have 
a  long  fossil  record,  but  apparently  were  never  a  dominant  group.  Al- 
though they  have  no  circulatory  system  or  excretory  organs,  they  have 
a  well  developed  eucoelom.  The  absence  of  some  structures  is  probably 
an  adaptation  to  small  size.  The  lophophore  is  circular  or  U-shaped, 
and  the  cilia  draw  water  toward  the  animal.  Food  particles  are  swirled 
into  the  mouth.  The  tentacles  bend  actively  and  are  somewhat  selective, 
knocking  large  debris  to  one  side  and  sometimes  hitting  smaller  particles 
toward  the  mouth.  The  colonies  are  formed  by  asexual  budding,  and 
often  a  particular  individual  in  the  colony  will  degenerate,  to  be  re- 
placed by  the  development  of  a  surviving  bud  of  tissue. 

The  position  of  bryozoans  in  the  animal  kingdom  is  debatable. 
They  are  usually  grouped  with  the  brachiopods  and  phoronids  to  form 


MINOR   PHYLA  357 


Operculum 
closed. 


Tentacles 


Mouth 

Ope.rculura 
opened. 


Area,  of 
tuddind 

~Anae 


Stomach 


Retractor 
muscle 

Figure  1  8.4.     Diagrammatic  view  of  two  individuals  in  a  colony  of  Bryozoa.  The  up- 
per individual  is  retracted.  (After  Twenhofel  and  Schrock.) 

an  assemblage  of  animals  with  lophophores.  Bryo/oans  also  show  some 
similarities  with  the  pterobranchs,  a  class  in  the  phylum  Hemithordata 
(Chapter  19).  It  seems  best  to  leave  this  phylum  in  an  indefinite  position 
between  the  two  major  series  ot  eucoelomates. 

163.        The  Chaetognatha 

The  Chaetognatha  (Fig.  18.5)  or  arrowworms  are  a  phylum  of  a  few 
species  that  may  be  extremely  abundant  in  the  marine  plankton.  These 
small  worms  prey  voraciously  on  other  small  animals,  grasping  them 
with  the  anterior  spines  and  s^vallowing  them  whole.  They  float  mo- 
tionless in  the  water  and  move  in  sudden  jerks  by  flips  of  the  body. 
Arrowworms  are  transparent,  revealing  much  of  their  internal  anatomy 
without  dissection.  They  lack  both  circulatory  and  excretory  systems, 
but  have  a  spacious  coelom  divided  into  a  head  cavity,  a  pair  of  trunk 
cavities,  and  a  pair  of  postanal  tail  cavities.  The  paired  cavities  are 
separated  by  vertical  mesenteries.  The  worms  are  hermaphroditic,  with 
ovaries  in  the  trunk  cavities  and  testes  in  the  tail  cavities.  The  nervous 
system  is  composed  of  a  well  developed  brain  and  a  single  large  ventral 
ganglion. 

Development  is  direct.  The  egg  undergoes  simple  cleavage  and  the 
coelom  is  enterocoelous.  The  mouth  forms  as  a  new  opening  consider- 
ably in  front  of  the  blastopore.  In  its  early  embryology,  therefore,  the 
arrowworm  resembles  the  echinoderm-chordate  series.  In  other  respects, 
however,  they  show  no  resemblance  whatsoever.  The  phylum  is  usually 
grouped  with  the  echinoderm-chordate  series,  but  it  seems  preferable  to 
place  a  gap  between  them.  It  is  possible  that  this  phylum  evolved  from 
the  same  stock  that  produced  the  chordates,  but  diverged  early  and  then 
followed  a  somewhat  parallel  course  of  evolution. 

The  minor  phyla  include  animals  of  interest  primarily  to  the  zoolo- 


358  ^^^   ANIMAL   KINGDOM 


Mouth- 


Esopha^us" 


Laleral 


.,/— Ovary 


-Anixs 


"Testis 


"Tail  fin. 


Figure  18.5.     Chaetognatha.  A  ventral  view  of  a  mature  specimen.  (Modified  from 
Parker  and  Haswell.) 


gist,  who  is  interested  in  any  animal  that  presents  a  unique  way  of  life. 
These  phyla  represent  life  forms  that  have  failed  to  dominate  the  scene, 
and  this  in  itself  is  a  challenging  problem.  They  are  of  special  interest 
to  the  student  of  phylogeny,  for  among  them  may  be  found  intermediate 
stages  that  will  reveal  how  the  major  groups  arose.  It  is  evident  from 
the  foregoing  (and  from  the  discussions  of  minor  phyla  in  other  chapters) 
that  in  some  cases  a  study  of  minor  groups  has  helped  our  understanding 
of  phylogeny.  In  other  cases  new  and  interesting  situations  are  revealed 
that  are  of  little  use  in  the  understanding  of  other  groups,  and  in  some 
instances  the  result  is  more  confusion  rather  than  less.  Although  they 
offer  no  simple  solution  to  the  problem  of  phylogeny,  the  minor  phyla 
enrich  the  subject  considerably. 


A1/N0R   PHYLA  359 

Questions 

1.  List  the  ten  major  phyla. 

2.  Compare  an  entoproct  and  a  bryozoan. 

3.  Compare  sipunculoids  and  echiuroids  with  the  annelids. 

4.  What  is  a  lophophore? 

5.  Sketch  and  label  a  chaetognath. 

Supplementary  Reading 

In  addition  to  the  suggestions  in  Chapter  9,  Parker  and  Haswell,  A  Text-Book  of 
Zoology,  volume  1,  and  Borradaile,  Easthani,  Potts,  and  Saunders.  The  Invertebrata,  con- 
tain excellent  descriptions  of  the  various  phyla  and  numerous  brief  discussions  of  their 
relationships.  Schrock  and  Twenhofel,  Prmciples  of  Invertebrate  Paleontology,  not  only 
summarize  the  fossil  record  for  all  of  the  groups  but  also  offer  a  surprising  amount  of  in- 
formation on  functional  anatomy  and  embr)ology. 


CHAPTER  19 


The  Phyla 
Hemichordata  and  Echinodermata 


Hemichordates  and  echinoderms  are  sedentary  or  slow-moving  inhab- 
itants of  the  ocean  floor.  Most  of  them  feed  on  debris  and  microscopic 
organisms,  although  a  few  echinoderms  are  predaceous.  Both  phyla  are 
entirely  marine.  They  range  from  the  shoreline  to  the  ocean  depths, 
and  from  the  tropics  to  the  poles.  Echinoderms  are  conspicuous  and 
common  everywhere,  but  the  hemichordates  are  seldom  noticed,  al- 
though they  may  be  locally  abundant  in  the  sand  and  mud.  Echinoderms 
have  a  predominantly  radial  symmetry  which  is  not  as  well  developed, 
however,  as  that  of  the  coelenterates.  The  hemichordates  are  of  special 
interest  to  the  zoologist  because  they  show  affinities  with  both  the 
echinoderms  and  the  chordates. 


164.        The  Phylum  Hemichordata 

Hemichordates  are  bilaterally  symmetrical  animals  with  a  body  di- 
vided into  three  regions  (Fig.  19.1):  the  proboscis,  the  collar  and  the 
trunk.  The  proboscis  contains  an  anterior  projection  of  the  gut,  the 
stomochord.  The  collar  has  a  well  developed  dorsal  collar  nerve,  and  nu- 
merous gill  slits  open  into  the  pharynx  along  the  sides  of  the  trunk. 

Each  body  region  contains  a  separate  portion  of  the  eucoelom.  The 
portion  in  the  proboscis  (coelomj)  opens  to  the  outside  through  one 
or  two  dorsal  pores.  The  muscular  proboscis  can  expand  or  contract, 
flushing  sea  water  in  and  out  of  its  cavity.  The  portion  in  the  collar 
(coelomo)  opens  to  the  outside  through  a  pair  of  lateral  pores;  it  can 
also  be  filled  and  emptied  with  seawater.  The  third  portion  (coelomg) 
forms  a  typical  body  cavity  in  the  trunk,  lying  between  the  viscera  and 
the  body  wall. 

The  phylum  is  divided  into  two  classes.  The  larger  class  is  the 
Enteropneusta  which  includes  the  wormlike  form  used  in  Figure  19.1  C. 
Its  sixty  species  vary  in  length  from  one  to  one  hundred  inches.  The 
smaller  class,  Pterobranchia,  includes  a  few  minute  species,  some  of 
which  are  colonial.  In  the  pterobranchs  the  collar  with  its  coelomic 
cavity  is  expanded  dorso-laterally  (Fig.  19.2)  as  a  pair  of  branched 
tentacles  used  for  gathering  food.  Tlie  trunk  is  folded  so  that  the  anus 
lies  just  behind  the  mouth.  Despite  their  small  size   the  pterobranchs 

360 


MoutK 


THE   PHYLA    HEMICHORDATA    AND    ECHINODERMATA  36  J 

Anus  Body  cavity  (coelomJ 

SKeLetal  pla.fce 


Radial  nerve 
Nerue  rind 


Mou-fch 


Tube  f  oo-fc 


Radicii  canai(coeLom5) 


Proboscis 
pore 


Collar  neruZ' 


Heart 


Mouth 
Stomochord. 


c. 


Figure  19.1.  Diagrammatic  representations  of  the  Echinodermata  and  the  Hemi- 
chordata.  A,  Ventral  view  of  a  starfish  (echinoderm).  B,  Vertical  section  through  a  starfish 
at  the  position  of  the  arrows  in  A.  C,  Lateral  view  of  an  acorn  worm  (hemichordate) 
showing  a  few  internal  structures. 


Proboscis 
pore 


Coelom, 
Stomochord 

Pnaryroc 
Storaa-ch 


A  B 

Figure  19.2.  Class  Pterobranchia  (genus  Rhabdopleura).  A,  Lateral  view  of  one 
animal  in  its  case  (lower  portion  of  case  and  stalk  omitted),  showing  external  features.  B, 
Diagrammatic  section  showing  some  of  the  internal  organs. 

show  all  of  the  hemichordate  characteristics  except  the  gill  slits,  which 
are  reduced  to  a  single  pair  in  some  species  and  are  absent  altogether 
in  others.  They  live  mostly  at  considerable  depths  and  have  seldom 
been  studied  alive. 

Saccog/ossus.     Enteropneusts,  many  of  which  live  in  shallow  water, 
have  been  studied  extensively.  A  familiar  species  is  Saccoglossus  kowal- 


362 


THE  ANIMAL   KINGDOM 


Proboscis 


Mouth- 
Collar 


Anterior  region: 
oF  trunk 

Gill  slits 


Coelomj 

Glomeralus 

Stomochorci 

Proboscis  pore. 
Collar  ne.rv(Z. 


Coelonrz, 


Coelom: 


"La-terad  fold 


Figure  1 9.3.  Class  Enteropneusta  (genus  Saccoglossus).  Left,  external  view  showing 
external  features  (after  Bateson).  Right,  a  diagrammatic  section  through  the  anterior  part 
of  the  body  showing  some  of  the  internal  organs.  A  lateral  fold  subdivides  the  pharynx 
into  a  ventral  channel  along  which  the  sand  passes  and  a  dorsal  channel  containing  the 
gill  slits. 


evski  (Fig.  19.3)  of  the  Atlantic  coast.  These  burrow  in  sandflats  near  the 
low  tide  line,  living  in  semipermanent  tunnels  lined  with  a  mucous 
secretion.  The  mouth,  which  apparently  cannot  be  closed,  lies  ventrally 
between  the  proboscis  and  the  collar.  As  the  worm  burrows,  much  of 
the  sand  is  swallowed.  In  the  pharynx  excess  water  passes  out  through 
the  gill  slits  and  the  sand  passes  down  a  long  intestine.  All  of  the  nourish- 
ment of  Saccoglossus  comes  from  organic  debris  in  the  sand.  Eventually 
the  sand  is  eliminated  through  a  terminal  anus,  often  piling  up  in  long 
coils  aroimd  openings  to  the  burrows. 

The  yellowish  pink  proboscis  of  Saccoglossus  is  longer  than  that  of 
most  enteropneusts.  The  junction  of  proboscis  to  collar  is  a  narrow 
stalk.  The  proboscis  pore  that  opens  into  the  coelomic  cavity  of  the 
proboscis  is  located  dorsally  at  the  posterior  margin  of  the  proboscis. 
The  reddish  collar  overlaps  the  stalk  in  front  and  the  trunk  behind.  Its 
coelomic  cavity  opens  on  the  sides  through  a  pair  of  ducts  that  end  at 
the  first  pair  of  gill  slits  in  the  trunk.  Saccoglossus  burrows  by  inflat- 
ing the  collar  against  the  tunnel  wall,  pushing  the  deflated  proboscis 
forward,  inflating  the  proboscis,  deflating  the  collar  and  pulling  the  body 
forward. 

The  trunk  is  divisible  into  three  regions.  In  the  anterior  part  nu- 


THE   PHYLA    HEMICHORDATA    AND    ECHINODERMATA 


363 


merous  pairs  of  gill  slits  open  externally  near  the  mid-dorsal  line.  The 
middle  part  of  the  trunk  contains  the  gonads,  which  are  gray  in  the 
female  and  yellow  in  the  male.  The  posterior  region  contains  only 
the  posterior  part  of  the  intestine  and  tapers  gradually  to  the  anus. 

Although  each  gill  slit  first  appears  as  a  simple  slit,  later  in  de- 
velopment the  internal  aperture,  the  opening  into  the  pharynx,  becomes 
U-shaped  (Fig.  19.4).  The  fleshy  tongue  bar  that  grows  down  from  the 
dorsal  margin  is  primarily  a  respiratory  organ,  and  contains  a  capillary 
network  in  which  blood  is  oxygenated  as  it  passes  from  the  ventral  blood 
vessel  to  the  dorsal  blood  vessel. 

Blood  is  carried  forward  in  a  dorsal  vessel  of  the  trunk  and  collar 
to  the  heart,  which  lies  in  the  proboscis  (Fig.  19.1).  It  is  then  pumped 
through  the  glomerulus  (Fig.  19.3),  a  tortuous  knot  of  vessels  projecting 
into  the  coelomic  cavity  of  the  proboscis,  and  passes  posteriorly  through 
a  ventral  vessel.  The  coelomic  epithelium  covering  the  glomerulus  is 
glandular,  and  waste  products  are  believed  to  be  removed  from  the  blood 
at  this  point.  The  waste  is  excreted  into  the  coelom  and  flushed  out 
with  the  sea  water  as  the  cavity  is  filled  and  emptied.  Branches  from 
the  ventral  vessel  in  the  trunk  lead  not  only  to  the  gills,  but  also  to  the 
gonads,  intestine  and  body  wall.  Collecting  vessels  from  these  organs 
return  all  blood  to  the  dorsal  vessel  where  it  is  mixed  as  it  passes  forward 
again.  All  of  the  major  vessels  are  contractile.  The  stomochord  (Fig. 
19.3)  is  an  outgrowth  of  the  pharynx  that  extends  into  the  proboscis. 
The  cells  of  this  diverticulum  are  large  and  vacuolated,  resembling  the 
cells  of  the  chordate  notochord.  On  the  ventral  surface  of  the  stomo- 
chord the  mesoderm  secretes  a  chitinous  plate  which  together  with  the 
stomochord  supports  and  stiffens  the  proboscis.  A  notochord-like  tissue 
is  also  found  along  the  ventral  margin  of  the  intestine  in  some  enterop- 
neusts. 

The  nervous  system  is  very  poorly  centralized  and  is  more  primitive 
in  most  respects  than  that  of  the  flatworms.  The  proboscis  is  underlaid 
with  a  thin,  continuous  layer  of  neural  tissue.  Most  of  the  collar  lacks 
this  layer,  but  dorsally  a  longitudinal  strip  of  ectoderm  constricts  off  to 
form  a  tubular  collar  nerve.  The  trunk  has  a  layer  of  neural  tissue 
similar  to  that  of  the  proboscis,  and  in  addition  the  nerve  fibers  tend 
to  concentrate  dorsally  and  ventrally  to  form  longitudinal  nerves.  The 
collar  nerve  appears  to  function  primarily  as  a  pathway  for  nerve  fibers 


Openings  to  cxt(z,rior 


ue 


TonO^ 


■Openings  intopharyrix- 

Figure  19.4.  Diagram  showing  how  simple  gill  slits  (left)  become  U-shaped  (right) 
by  the  doungrowth  of  tongue-bars  from  the  roof  of  each  slit.  The  external  opening 
remains  simple. 


364 


THE   ANIMAL   KINGDOM 


between  the  proboscis  and  trunk,  and  cannot  be  considered  to  be  the 
central  nervous  system.  Except  for  the  collar  nerve  the  entire  system  is 
at  the  body  surface  and  is  covered  only  with  epidermis. 

165.        Classification  of  the  Phylum  Echinodermata 

Living  echinoderms  are  divided  into  five  classes  (Fig.  19.5):  (1) 
Crinoidea,  the  sea  lilies  and  feathers  stars,  (2)  Holothuroidea,  the  sea 
cucumbers,  (3)  Echinoidea,  the  sea  urchins  and  sand  dollars,  (4)  Aster- 
oidea,   the  starfish,   and   (5)  Ophiuroidea,   the  brittle  stars  and  basket 


Figure  19.5.  The  five  living  classes  of  the  Echinodermata.  A,  Ophiuroidea,  brittle 
stars.  B,  Asteroidea,  starfishes.  C,  Echinoidea,  sea  urchins.  D,  Holothuroidea,  sea  cucum- 
bers. E,  Crinoidea,  sea  lilies.  (C  after  Hunter  and  Hunter,  others  after  Hyman.) 

Stars.  In  addition  a  number  of  extinct  echinoderms  have  been  iden- 
tified that  are  placed  in  some  five  additional  classes.  Most  echino- 
derms are  large  and  have  skeletons,  and  many  of  the  species  are  or  have 
been  abundant.  This  phylum  has  a  rich  fossil  record,  probably  the  best 
known  of  any  phylum,  that  reaches  back  to  the  early  part  of  the  Paleo- 
zoic Era.  The  number  of  known  extinct  species  greatly  outnumbers  the 
number  of  known  living  species. 

The  five  living  classes  are  so  different  in  their  structural  features 
that  space  does  not  permit  an  adequate  description  of  each  one.  The 
general  aspects  of  the  classes  will  be  given  following  a  detailed  descrip- 
tion of  a  member  of  the  Asteroidea. 

166.        Asterias  forbesif  a  Typical  Five-rayed  Starfish 

Asterias  lives  on  rocky  or  shell-covered  bottoms  where  it  preys 
extensively  on  shellfish.  The  common  species  of  the  east  coast,  A.  forbesi 


THE   PHYLA    HEMICHORDATA    AND   ECHINODERMATA 


365 


(Fig.  19.6),  is  at  times  abundant  and  may  seriously  deplete  whole  popula- 
tions of  oysters.  Because  ot  its  economic  importance  this  starfish  has  been 
studied  extensively. 

Its  color  is  variable,  including  shades  of  brown,  yellow,  orange,  pink 
and  purple.  The  five  arms  or  rays  are  joined  at  the  center  to  form  a 
disc.  On  its  upper  surface  the  disc  bears  a  bright  orange  or  yellow 
madreporite,  a  fine-meshed  sieve  that  opens  into  a  part  of  the  coelom. 
The  eccentric  location  of  the  madreporite  is  the  only  obvious  departure 
from  radial  symmetry  in  the  starfish. 

Asterias  is  protected  from  predators  by  a  spiny  skeleton  in  the  meso- 
derm just  beneath  the  epidermis.  A  layer  of  calcareous  plates  (Fig. 
19.6  D)  comiected  by  short  bands  of  connective  tissue  and  muscle  forms 
a  tough  barrier.  In  addition  many  of  the  plates  bear  tubercles  and 
spines.  The  former  are  mere  bumps  whereas  the  latter  are  jointed  at 
the  base  and  supplied  with  muscles  so  that  they  can  be  pointed  in  vari- 
ous directions.  Spines  bordering  the  ambulacral  grooves  are  especially 
long  and  numerous,  and  can  be  closed  over  the  grooves  to  protect  them 
if  the  starfish  is  torn  loose  from  the  bottom.  Each  skeletal  piece  is  se- 
creted as  a  single  crystal  of  calcium  carbonate.  Although  all  of  the 
skeleton  is  originally  covered  with  epidermis,  that  on  the  spines  is 
often  worn  off. 

The  settling  mud  and  the  larvae  of  various  organisms  seeking  places 
to  attach  are  threats  to  a  slowly  moving  creature.  In  echinoderms  the 


Ambulacrsd 
plate 

Ampulla 


Tube 


Madreporite' 
Anus 
Rectal  sac — ' 


-"Dermalgill 


Spines 

Figure  19.6.  Asterias  viewed  from  above  with  the  arms  in  various  stages  of  dissec- 
tion. A,  Arm  turned  to  show  lower  side.  B,  Upper  body  wall  removed.  C,  Upper  body 
wall  and  digestive  glands  removed,  with  a  magnified  detail  of  the  ampullae  and  ambu- 
lacral plates.  D,  All  internal  organs  removed  except  the  retractor  muscles,  showing  the 
inner  surface  of  the  lower  body  wall,  E,  Upper  surface,  with  a  magnified  detail  showing 
surface  features. 


366  ^W^  ANIMAL   KINGDOM 


Figure  19.7.  A,  The  common  starfish,  Asterias,  eating  a  fish.  Transparent  lobes  of 
the  cardiac  stomach  can  be  seen  surrounding  the  body  of  the  fish.  A  number  of  tube  feet 
are  being  used  to  hold  the  starfish  to  the  side  of  the  aquarium.  (Courtesy  Robert  S. 
Bailey.)  B,  A  Caribbean  brittle  star,  shown  in  repetitive  flash  photographs,  pulls  itself 
along  with  its  two  anterior  arms  and  shoves  with  the  other  three.  It  is  far  more  agile 
and  flexible  than  its  sluggish,  stiff-armed  cousin,  the  common  starfish.  (Fritz  Goro— 
Courtesy  LIFE  Magazine.  Copr.  1955  Time  Inc.) 


THE   PHYLA    HEMICHORDATA    AND   ECHINODERMATA 


367 


epidermis  is  ciliated  and  the  ciliary  currents  continually  sweep  the  fine 
debris  that  settles  on  the  starfish  out  to  the  sides  where  it  falls  off.  The 
larvae  of  molluscs,  barnacles,  bryozoans  and  others  that  might  attach 
to  the  naked  spines  are  discouraged  by  the  pedicellariae  (Figs.  19.6  E 
and  19.8  A),  each  a  microscopic  pincers.  These  snap  vigorously  when 
stimulated  and  any  pedicellaria  that  catches  anything  remains  shut  for 
several  days.  These  are  scattered  over  the  body  surface  and  clustered  in 
rosettes  at  the  bases  of  spines.  Occasionally  the  tissue  around  the  base 
of  a  spine  contracts,  lifting  the  rosette  so  that  the  pedicellariae  reach  to 
the  tip  of  the  spine,  snapping  all  the  way  up  and  down  to  clean  its 
surface.  In  this  way  the  starfish  does  not  become  a  travefing  home  for 
attached  organisms. 

Asterias  creeps  slowly  on  a  multitude  of  tube  feet,  delicate  pro- 
jections ending  in  suckers.  These  project  from  deep  ambulacral  grooves 
radiating  from  the  disc  along  the  lower  surface  of  each  ray.  The  tube 
feet  are  arranged  in  two  longitudinal  rows,  each  of  which  is  staggered 
so  as  to  look  like  a  double  row.  Their  epidermis  is  not  ciliated. 

Each  tube  foot  operates  as  an  independent  hydraulic  mechanism 
(Fig.  19.8  B).  Its  cavity,  which  is  a  part  of  the  coelom,  extends  inward 
through  the  body  wall  and  expands  inside  the  body  as  a  bulb  or  am- 
pulla. W^hen  the  muscular  coat  of  the  ampulla  contracts,  the  fluid  in 
the  cavity  is  forced  into  the  foot.  Since  the  wall  of  the  foot  contains 
connective  tissue  rings  that  prevent  expansion  of  the  tube  diameter,  the 
foot  elongates  as  it  fills.  The  end  ot  the  foot  forms  a  suction  cup,  and 


'^Ampulla. 


"Jaw 


Closing  muscle 
Opening  iiiuscle 

Tube, 
foot 


Te.nta.cle 


RadiaJ  canal 


Lza.-tei'a.l  Ccoial 

itudinal  miLScle 

Circular  connective 
tissue,  fibers 

Muscle  fibers 
(increase  Su-Ction) 

Connective,  tissue 
spray 


Nerve  tissue 


•Eye  spots 

Figure  19.8.  A,  One  of  the  several  varieties  of  pedicellariae.  B,  The  tube  foot  and 
associated  apparatus.  C,  Section  through  a  terminal  tentacle  (suckerless  tube  foot)  showing 
the  eyespots  at  its  base.  (All  figures  diagrammatic,  modified  from  Hyman.) 


368  '■Wf   ANIMAL   KINGDOM 

once  it  is  pressed  against  a  smooth  surface  it  will  stick  tightly.  Suction 
is  improved  by  a  sticky  secretion  from  the  end  of  the  foot,  and  it  can 
be  increased  further  by  the  contraction  of  small  muscles  attached  to  a 
connective  tissue  "spray"  that  pulls  on  the  middle  of  the  suction  cup. 
To  release  the  foot,  longitudinal  muscle  fibers  in  the  tube  contract  and 
lift  the  edges  of  the  sucker.  When  these  longitudinal  fibers  contract 
completely,  the  tube  foot  is  drawn  up  close  to  the  body  and  its  fluid  is 
forced  into  the  ampulla.  In  creeping  the  tube  feet  work  asynchronously. 
Each  foot  elongates  in  the  direction  of  motion,  attaches  to  the  bottom, 
and  then  is  swung  beneath  the  body  so  as  to  propel  the  body  forward. 

A  small  lateral  canal  joins  each  tube  foot  with  a  radial  canal  (Fig. 
19.8  B).  The  lateral  canal  is  valved  so  that  it  can  be  closed  or  opened  to 
adjust  the  amount  of  fluid  in  the  tube  foot  and  ampulla.  The  five  radial 
canals  join  a  circular  ring  canal  in  the  lower  part  of  the  disc,  and  from 
this  a  single  stone  canal  leads  upward  to  the  madreporite.  All  of  these 
cavities  together  are  the  water  vascular  system  which  is  a  unique 
echinoderm  feature,  derived  from  a  portion  of  the  coelom. 

The  tube  feet  at  the  tips  of  the  rays  are  long  and  slender,  acting  as 
tentacles  to  explore  the  bottom  as  the  starfish  moves.  No  one  or  two  rays 
are  permanently  anterior,  but  at  any  moment  if  a  starfish  is  cut  like  a 
pie  into  five  pieces  the  rays  that  were  anterior  will  creep  with  the  ray 
tips  forward  for  a  few  minutes,  whereas  the  rays  that  were  posterior  will 
creep  with  their  bases  forward.  Hence,  temporary  anteroposterior  axes 
are  established  in  the  starfish. 

The  only  sense  organs  in  addition  to  the  tactile  tube  feet  are  small 
eyespots  at  the  tips  of  the  rays  (Fig.  19.8  C).  Each  eyespot  is  composed 
of  about  one  hundred  pigment  cups,  each  lined  with  a  layer  of  retinal 
cells.  The  starfish,  however,  shows  no  evidence  of  form  vision,  and  has 
only  general  movements  toward  or  away  from  light.  The  tips  of  the  arms 
are  curved  so  that  the  eyespots  face  outward  or  upward. 

Scattered  throughout  the  epidermis  are  numerous  cells  that  act  as 
chemoreceptors.  Starfish  have  been  observed  to  move  toward  dead  fish 
and  are  often  caught  in  baited  traps  such  as  those  used  for  crabs  and 
lobsters. 

The  mouth  is  in  the  center  of  the  lower  surface  surrounded  by  a 
membranous  area,  the  peristome.  The  mouth  opens  directly  into  a  large 
cardiac  stomach,  which  in  turn  opens  upward  into  a  smaller  pyloric 
stomach.  The  digestive  tract  continues  upward  as  a  small  intestine  that 
ends  at  an  anus  in  the  middle  of  the  upper  surface  of  the  disc.  Five 
large,  hollow,  branched  digestive  glands  that  extend  out  to  the  tips  of 
the  rays  open  into  the  pyloric  stomach.  The  intestine  has  a  lobulated 
diverticulum,  the  rectal  sac,  of  unknown  function.  In  feeding,  the 
cardiac  stomach  everts  through  the  mouth  and  spreads  over  the  food 
(Fig.  19.7  A).  A  copious  fluid  containing  powerful  enzymes  is  secreted  by 
the  digestive  glancls  and  poured  over  the  food,  rapidly  reducing  it  to  a 
broth.  The  digested  material  is  then  swallowed,  and  the  nutrients  are 
absorbed  by  the  gastrodermis  lining  the  pyloric  stomach  and  digestive 
glands.  Five  pairs  of  retractor  muscles  (Fig-    19.6  D)  from  the  cardiac 


THE   PHYLA    HEMICHORDATA    AND    ECHINODERMATA  369 

Stomach  to  the  ventral  body  wall  of  the  rays  contract  to  pull  the  cardiac 
stomach  back  inside  the  disc. 

Aster ias  feeds  mostly  on  live  bivalves  whose  shells  close  tightly.  The 
starfish  can  open  these  easily  in  a  few  minutes,  although  the  mechanism 
is  not  entirely  understood.  Apparently  the  starfish  grips  the  two  shells 
with  its  many  tube  feet  and  pulls  slowly  and  steadily.  As  soon  as  the 
bivalve  opens  the  least  bit  the  cardiac  stomach  is  slipped  inside  and 
digestion  begins.  Some  of  the  larger  starfish,  such  as  the  genus  Pisaster 
of  the  west  coast,  are  so  powerful  that  they  will  break  the  shells  of 
bivalves  that  are  wired  shut. 

The  nervous  system  of  Asterias  is  composed  of  a  nerve  ring  en- 
circling the  mouth  and  five  radial  nerves  adjacent  to  the  lower  epidermis 
(Fig.  19.1).  Other  fibers  have  been  identified  in  the  walls  of  the  digestive 
tract  and  inside  the  upper  body  wall.  The  separated  rays  of  a  starfish 
each  with  a  pie-shaped  piece  of  the  disc  will  continue  to  creep  for  a  few 
minutes  in  the  same  direction  as  they  were  creeping  before  being  sep- 
arated. After  a  few  minutes,  however,  all  of  the  rays  will  creep  with 
the  tip  forward  as  though  all  of  them  were  acting  as  anterior  rays.  These 
pieces  retain  a  part  of  the  nerve  ring  and  its  junction  with  the  radial 
nerve.  If  the  radial  nerve  is  severed  at  its  junction  with  the  nerve  ring, 
then  the  ray  will  creep  with  its  base  forward,  as  though  it  were  a  pos- 
terior ray.  This  suggests  that  the  part  of  the  nerve  ring  near  each  radial 
nerve  is  a  center  from  which  stimuli  pass  along  the  arm  and  cause  it  to 
advance  with  the  tip  forward.  In  the  intact  animal  the  centers  on  one 
side  temporarily  inhibit  those  on  the  other,  permitting  the  animal  to 
move  in  a  coordinated  manner  in  one  direction. 

The  large  body  cavity  of  Asterias  surrounds  all  the  digestive  organs 
and  extends  to  the  tips  of  the  rays.  Although  the  coelom  appears  in  the 
embryo  as  a  pair  of  lateral  cavities,  these  migrate  and  come  to  lie  one 
above  the  other  after  metamorphosis.  In  Asterias  the  horizontal  mesen- 
tery dividing  this  pair  of  cavities  disappears  except  for  the  five  pairs  of 
retractor  muscles  of  the  cardiac  stomach. 

All  over  the  top  and  sides  of  the  starfish  the  body  cavity  projects 
through  the  body  wall  as  numerous  tiny  papillae  covered  with  epidermis. 
The  ciliated  epithelium  lining  the  body  cavity  circulates  the  coelomic 
fluid  rapidly  in  and  out  of  these  papillae;  they  probably  function  in 
respiration.  The  coelomic  fluid  contains  numerous  wandering  cells  that 
gather  up  waste.  When  carmine  particles  are  injected  into  the  body 
cavity  they  are  picked  up  by  these  cells.  After  a  few  minutes  the  cells  can 
be  seen  in  the  papillae,  and  many  of  them  leave  the  body  cavity  by 
crau'ling  through  the  wall  to  the  outside,  thus  removing  the  carmine 
from  the  body.  Whether  this  is  a  usual  or  major  method  of  eliminating 
wastes  is  not  known. 

The  circulatory  system  of  Asterias  is  composed  of  circular  and 
radial  vessels  filled  with  a  fluid  similar  to  that  of  the  body  cavity,  which 
in  turn  is  not  very  different  from  sea  water.  The  vessels  lie  above  the 
nervous  system  enclosed  in  a  body  cavity  of  their  own,  derived  em- 
bryologically  from  a  part  of  the  coelom.  Contractions  have  been  ob- 
served in  some  of  the  vessels. 


370 


THE  ANIMAL   KINGDOM 


A  pair  of  gonads  (Fig.  19.6  C)  are  located  one  on  each  side  of  the 
gastric  gland  in  the  base  of  each  ray.  They  hang  free  in  the  body  cavity 
except  where  each  is  attached  by  a  short  duct  to  a  reproductive  pore 
opening  externally  between  the  bases  of  adjacent  rays.  In  the  spring  the 
gray  testes  or  orange  ovaries  are  prominent,  and  large  numbers  of 
gametes  are  released  in  June.  Fertilization  is  external. 

1 67.        Class  Asteroidea,  the  Starfish 

Asterias  forbesi  is  a  member  of  this  class.  Most  starfish  have  five  rays 
and  a  relatively  small  disc.  In  some,  however,  the  disc  is  large  relative  to 
the  rays  and  the  body  is  pentagonal  rather  than  star-shaped  (Fig.  19.9). 
In  one  genus,  Leptasterias  (Fig.  19.9),  the  animals  have  six  rays.  In  other 
starfishes  the  number  of  rays  may  be  as  high  as  25  or  50.  Usually  the 


SgSSSSSSSSSiSSSiSSiSSSiSSSSSiimSSSSSSSSSSSi^^ 


Figure  19.9.  Other  members  of  the  Asteroidea.  A  pentagonal  starfish,  Culcita  (left). 
A  starfish  with  six  rays,  Leptasterias  (center).  A  starfish  with  12  or  more  rays,  Crossaster 
(right).  (After  Hyman.) 

number  of  rays  in  a  species  is  variable  if  it  is  greater  than  seven.  In  all 
cases  studied  where  the  number  exceeds  five  the  embryo  first  develops 
five  rays  and  adds  the  others  later. 

Many  starfish  live  and  feed  like  Asterias.  Some  eat  only  small  bi- 
valves and  other  organisms  which  are  swallowed  whole  into  the  cardiac 
stomach.  Many  of  the  large  species  one  to  three  feet  in  diameter  feed 
primarily  upon  other  echinoderms. 


1 68.        Class  Crinoidea,  the  Sea  Lilies 

Sea  lilies  are  echinoderms  attached  to  the  bottom  by  a  stalk  (Fig. 
19.5  E).  The  mouth  is  directed  upward,  with  the  anus  located  to  one  side 
on  a  small  projection.  The  five  rays  are  usually  branched  to  form  a 
graceful  pattern.  Ciliated  grooves  in  the  epidermis  extend  out  from  the 
mouth  along  the  upper  surfaces  of  all  the  rays  and  branches.  Each  groove 
is  flanked  on  both  sides  by  tube  feet,  but  these  lack  suckers  and  are 
covered  with  numerous  tiny  sensory  papillae.  The  movement  of  the  tube 
feet  pushes  tiny  organisms  and  food  particles  against  the  ciliated  groove, 
which  is  covered  with  a  mucous  secretion  that  is  continually  swept 
toward  the  mouth.  In  this  way  food  is  trapped  and  swallowed.  Move- 


THE   PHYLA    HEMICHORDATA    AND   ECHINODERMATA 


371 


ment  in  sea  lilies  is  limited  to  postural  changes  of  the  body  and  the 
spreading  or  folding  together  of  the  branches.  Although  5000  extinct 
species  have  been  described,  only  80  living  species  of  attached  crinoids 
are  known. 

The  sea  lilies  were  first  known  as  fossils.  In  the  19th  century,  shortly 
after  evolution  became  an  accepted  theory,  a  number  of  scientists  sug- 
gested that  living  representatives  of  extinct  groups  might  still  be  found 
in  the  ocean  depths.  W^hen  the  first  dredging  explorations  into  these 
depths  yielded  living  sea  lilies,  there  was  much  excitement  and  hope 
that  other  "living  fossils"  would  be  found.  The  failure  to  find  animals 
such  as  trilobites  was  a  disappointment;  although  a  number  of  survivors 
of  groups  that  are  mostly  extinct  have  been  found  in  deep  water,  the 
number  is  not  much  greater  than  that  found  in  other  regions. 

Another  550  living  species  of  crinoids  occur  in  a  recently  evolved 
family  that  are  free-living  as  adults.  These  are  the  feather  stars  (Fig. 
19.10)  in  the  family  Comatulidae.  They  attach  as  larvae  and  grow  a 
short  stalk  like  that  of  the  sea  lilies,  but  later  break  loose.  Their  general 
anatomy  and  method  of  feeding  are  unchanged.  Feather  stars  differ  from 
the   sea   lilies   primarily   in    locomotion.    They   can   crawl   through   the 


Figure  19.10.     The  feather  star,  a  crinoid  that  lacks  a  stalk  as  an  adult.  (Austin  H. 
Clark:  in  Smithsonian  Misc.  Coll.,  Vol.  72,  No.  7.) 


372 


THE   ANIMAL   KINGDOM 


vegetation  using  the  rays  as  prehensile  organs  and  they  can  swim.  In 
swimming  the  ten  arms  are  raised  and  lowered  as  fast  as  100  times  a 
minute.  The  two  primary  branches  of  each  of  the  five  rays  alternate,  so 
that  while  arms  1,  3,  5,  7  and  9  are  moving  up,  arms  2,  4,  6,  8  and  10  are 
moving  down.  The  many  tiny  branches  are  folded  against  the  arm  as  it 
is  raised,  and  are  spread  out  during  the  down  swing.  A  feather  star  may 
swim  15  feet  in  one  minute. 


169.       Class  Holothuroidea,  the  Sea  Cucumbers 

Sea  cucumbers  (Fig.  19.5  D)  creep  or  burrow  in  the  sand  and  mud. 
The  calcareous  plates  beneath  their  epidermis  are  microscopically  small 
and  the  body  wall  is  soft  and  flexible.  The  body  is  elongated  between 
mouth  and  anus  and  usually  one  side  becomes  the  permanent  lower 
side  so  that  the  radial  symmetry  is  imperfect.  Five  rows  of  tube  feet 
extend  from  mouth  to  anus  indicating  the  five  ambulacral  areas.  Often 
only  the  tube  feet  of  the  three  lower  rows  have  suckers  and  are  used  for 
creeping.  The  tube  feet  surrounding  the  mouth  are  modified  to  form  a 
circle  of  branched  tentacles.  The  tentacles  in  most  species  are  covered 
with  mucus  and  extended  into  the  water  as  a  trap  for  small  organisms. 
Periodically  each  tentacle  is  bent  into  the  mouth  and  wiped  clean.  A  few 
species  use  the  tentacles  for  shovelling  mud  and  debris  into  the  mouth. 

In  many  sea  cucumbers  (Fig.  19.11)  the  rectum  has  a  pair  of  large, 
much  branched  diverticula  that  extend  into  the  body  cavity.  These  are 
the  respiratory  trees.  Rhythmically  the  anus  opens  and  water  is  drawn 
into  the  rectum.  Then  the  anus  closes  and  the  rectum  contracts,  forcing 
water  into  the  trees.  This  may  be  repeated  several  times,  filling  the  trees 
more  and  more.  Finally  the  anus  opens  and  the  whole  body  contracts, 
expelling  all  the  water. 


Intestine 


•Ri-n^  vesicle 
■Rin^  canal 

Stomach 


Mesentery 


Anus — i 
Reclui 


An-iiulacrum 

Respi  ralory 
trz.e- 


Madreporite. 


Tizntacle 
•Aristotks 
lantern. 

•Lante.-m 
muscle 


^^iS;5t^?j5JS07D7' 


Figure  19.11.  The  sea  cucumber,  Thy  one  briareus,  cut  open  along  one  side.  The 
digestive  tract  has  been  moved  to  one  side  to  show  the  respiratory  trees,  retractor  muscles 
of  the  anterior  end,  and  the  internal  surface  of  the  body  wall  with  its  five  ambulacra.  In 
holothurians  the  madreporite  lies  in  the  body  cavity,  so  that  the  water  vascular  system 
is  not  filled  with  sea  water,  but  with  coelomic  fluid. 


THE   PHYLA    HEMICHORDATA    AND    ECHINODERMATA  373 

Holothurians  are  remarkable  for  the  ease  with  which  they  will 
throw  away  their  viscera.  Whenever  conditions  are  unfavorable,  whether 
this  be  clue  to  lack  of  oxygen,  high  temperatures  or  excessive  irritation, 
the  sea  cucumbers  contracts  violently  and  ejects  the  entire  digestive 
tract.  In  different  species  it  may  be  thrown  out  through  the  mouth, 
through  the  anus,  or  rupture  through  the  side  of  the  body.  Later  a  new 
digestive  tract  is  regenerated.  Spontaneous  evisceration  has  been  sug- 
gested to  be  a  device  by  which  the  sea  cucumbers  offers  a  morsel  to  a 
potential  predator  on  the  chance  that  the  less  succulent  body  wall  will 
be  left  unharmed.  When  evisceration  is  found  in  nature,  however,  it  is 
usually  associated  with  unfavorable  environmental  conditions.  Possibly 


Figure  19.12.     Larvae  of  the  sea  cucumber,  Cucumaria  frotidosa.  Left,  "sitting,"  and 
right,  "walking."  (After  Runnstroin  and  Runnstrom.) 

by  throwing  out  the  viscera  sea  cucumbers  can  close  up  tightly  and  live 
at  a  reduced  metabolic  level  until  favorable  conditions  return. 

When  larval  sea  cucumbers  settle  to  the  bottom,  the  first  tube  feet 
to  develop  are  five  around  the  mouth  and  a  single  pair  near  the  anus 
on  the  lower  side.  These  tiny  holothurians  clamber  about  actively,  often 
walking  on  the  pair  of  posterior  tube  feet  (Fig.  19.12).  This  is  the  only 
example  of  bipedal  locomotion  known  in  the  invertebrates. 

170.        Class  Echinoidea,  the  Sea  Urchins,  Heart  Urchins  and 
Sand  Dollars 

The  body  skeleton  of  the  echinoids  forms  a  rigid  box  (Fig.  19.5  C). 
Five  ambulacral  grooves  with  tube  feet  radiate  from  the  mouth  up 
around  the  sides  to  end  near  the  anus.  The  tube  feet  on  the  lower 
surface  usually  have  suckers  and  are  used  in  locomotion  whereas  the 
lateral  and  upper  tube  feet  are  often  long  and  filamentous,  apparently 
used  for  respiration. 

Sea  urchins  have  numerous  long  spines,  some  of  which  aid  the  tube 
feet  in  walking.  The  urchins  creep  slowly  about,  using  their  five  sharp 
teeth  to  scrape  and  chew  whatever  they  pass  over.  The  skeleton  and 
musculature  associated  with  the  teeth  form  a  distinctive  structure  known 
as  Aristotle's  lantern  (Fig.  19.13). 


374 


THE  ANIMAL  KINGDOM 


Madreporit 


Anu5 


Storie  ca.nal 
Esophagus 
SiphoiT 

Bo(iy  xv7aJl 


Gonad. 

l-n-tdsbinz 

Bociy  cavity 
Stomadh. 


Aristotle.^  lan-bern. 


Me-mbrane 

around  mouth -^         Teeth 

Figure  19.13.  A  sea  urchin,  Arbacia  punctulata,  with  one  side  of  the  body  wall  re- 
moved and  only  some  of  the  structures  shown.  The  teeth  protrude  from  Aristotle's 
lantern,  of  which  only  the  outer  structures  are  indicated.  The  digestive  tract  circles  twice 
around  the  body,  once  in  each  direction.  A  second  tube,  the  siphon,  by-passes  the  esoph- 
agus and  stomach.  Each  of  the  five  gonads  opens  above  near  the  anus. 


l^-)s»ss*!SJisssjsssisssss^sssg3fcsiS!!^^ 


ia?    ff^ 


Figure  19.14.  Ventral  and  lateral  views  of  heart  urchins  {A,  B)  and  sand  dollars 
(C,  D),  showing  modifications  for  burrowing  and  for  creeping  through  sand.  (After 
Hyman.) 


Some  urchins  live  on  coral  reefs  where  waves  are  continually  break- 
ing over  them.  Their  spines  are  as  thick  as  pencils  and  are  used  as  props 
to  hold  the  urchins  tightly  in  shallow  crevices.  They  remain  for  long 
periods  in  one  place  and  often  carve  out  a  depression  in  which  they  sit, 
feeding  upon  the  debris  brought  to  them  by  the  waves. 

Some  urchins  are  ovoid  and  have  lost  much  of  their  radial  sym- 
metry. These,  known  as  heart  urchins  (Fig.  19.14),  plow  through  the 
sand  just  beneath  the  surface.  Ciliated  grooves  along  the  ambulacral 
areas  collect  fine  debris  which  is  eaten,  and  the  long  upper  tube  feet 
project  above  the  sand  for  respiration. 

A  group  of  much-flattened  echinoids  are  the  sand  dollars  (Fig. 
19.14).  These  creep  almost  entirely  by  the  action  of  numerous  short 
spines.  They  usually  move  slowly  just  under  the  surface  of  the  sand,  and 
use  the  upper  spines  to  keep  a  thin  layer  of  sand  moving  over  the  top 
as  they  creep  on  their  lower  spines.  The  sand  dollars  also  have  ciliated 
grooves  that  collect  fine  debris  for  food. 


THE   PHYLA    HEMICHORDATA    AND    ECHINODERMATA 


375 


Figure  1 9.1  5.     The  basket  star,  Gorgonocephalus,  showing  the  branched  arms.  (Cour- 
tesy of  the  American  Museum  of  Natural  History.) 

1 71 .  Class  Ophiuroidea,  the  Brittle  Stars 

Brittle  stars  have  slender  rays  attached  to  a  circular  disc  (Fig.  19.5  A). 
Each  ray  is  composed  in  part  of  a  row  of  large  cylindrical  skeletal  pieces 
called  vertebrae  joined  together  with  short  but  powerful  muscles.  Each 
ray  as  a  whole  is  very  supple,  and  brittle  stars  move  by  pushing  and 
pulling  on  surrounding  objects,  "slithering"  like  a  snake  (Fig.  19.7  B). 
The  tube  feet  are  poorly  developed  and  lack  suckers.  They  function 
primarily  as  tactile  sense  organs.  The  delicate  ciliated  epidermis  that 
covers  most  of  the  skeleton  in  other  echinoderms  is  replaced  in  this  class 
by  a  tough  cuticle.  Pieces  of  the  rays  are  easily  broken  off,  but  are  easily 
legenerated. 

Some  ophiuroids,  such  as  Gorgonocephalus  (Fig.  19.15)  of  the  west 
coast,  are  called  basket  stars  because  the  arms  branch  and  intertwine 
repeatedly.  One  wonders  how  basket  stars  are  able  to  keep  track  of  all 
the  branches  as  they  clamber  through  vegetation,  and  indeed  they  have 
been  observed  to  leave  behind  pieces  that  are  hopelessly  entangled. 

Most  ophiuroids  feed  on  debris  and  mud.  Some  capture  prey  with 
their  prehensile  rays  and  bring  it  to  the  mouth.  The  mouth  opens  into 
a  simple  saclike  stomach  where  food  is  digested  and  adsorbed.  Indi- 
gestible remains  must  be  eliminated  through  the  mouth,  for  no  other 
digestive  organs  are  present. 

172.  Relationships  among  Echinoderm  Classes 

The  most  primitive  echinoderms  of  which  we  have  any  record  are 
believed  to  be  members  of  the  extinct  class   Heterostelea   (Fig.   19.16), 


376  ^^^   ANIMAL   KINGDOM 


^^s^^^■•^W^ii^^^«^^W^^^NSN^v.\s;W^s^NV,^^\\^^  l^■^•^^\^x^^^^^^^^^^^^xxxv^\^^x-^^N<^^Ns^c^^^ 


Figure  19.16.     An  extinct  class,  Heterostelea,  view  of  upper  side.  This  is  one  of  sev- 
eral bilaterally  symmetrical  genera  of  early  Paleozoic  echinoderms.  (After  Bather.) 

bilaterally  symmetrical  echinoderms  of  the  early  Paleozoic.  These  were 
attached  by  a  stalk  like  the  crinoids,  but  apparently  held  the  body  in  a 
horizontal  position.  All  the  other  echinoderm  classes  have  radial  sym- 
metry. 

Three  more  of  the  extinct  classes  were  attached  by  stalks  like  the 
crinoids.  All  of  these  attached  forms  (including  the  feather  stars)  are 
placed  together  in  the  subphylum  Pelmatozoa.  Within  this  subphylum 
the  bilateral  symmetry  of  the  Heterostelea  gave  way  to  the  radial  sym- 
metry of  the  other  classes,  presumably  as  an  adaptation  to  an  attached 
existence.  Except  for  the  feather  stars,  which  are  attached  only  when 
young,  the  Pelmatozoa  appear  to  be  on  the  verge  of  extinction. 

The  unattached  echinoderms,  which  include  the  Holothuroidea, 
Echinoidea,  Asteroidea,  Ophiuroidea  and  one  extinct  class,  are  placed  in 
the  subphylum  Eleutherozoa.  In  a  few  species  of  starfish  the  larva  at- 
taches to  the  bottom  briefly  during  its  metamorphosis  into  the  adult 
form,  but  in  most  asteroids  and  in  all  other  eleutherozoans  that  have 
been  studied,  the  individuals  are  never  attached.  While  it  is  generally 
concluded  that  the  Eleutherozoa  evolved  from  the  Pelmatozoa,  it  is  not 
known  whether  they  evolved  once  or  whether  some  of  the  classes  arose 
separately  from  attached  forms.  The  Eleutherozoa  apparently  did  not 
evolve  from  the  Crinoidea,  but  arose  from  some  extinct  and  possibly 
unknown  pelmatozoan. 

Evolutionary  relations  among  the  four  living  classes  of  the  Eleuth- 
erozoa are  obscure.  A  comparison  of  the  adult  anatomy  suggests  that  the 
Asteroidea  and  Ophiuroidea  are  the  most  closely  related,  and  that  the 
Echinoidea  and  Holothuroidea  form  two  distantly  related  groups. 
The  fossil  record  supports  this  arrangement.  The  Holothuroidea,  Echi- 
noidea and  Asteroidea  are  found  as  fossils  in  the  early  part  of  the 
Paleozoic  Era,  350  million  years  ago.  The  Ophiuroidea  begin  as  fossils 
only  275  million  years  ago.  During  the  75  million  years  between  the  first 
asteroids  and  the  first  ophiuroids  there  were  a  number  of  species  inter- 
mediate in  morphology  between  these  two  classes.  They  can  be  arranged 
in  a  series  suggesting  many  steps  in  the  evolution  of  the  Ophiuroidea 
from  asteroid-like  ancestors,  particularly  in  the  skeletal  modification  of 
the  rays. 

Fossil  evidence  as  convincing  as  this  for  the  origin  of  a  class  is  rare, 
and  should  be  conclusive.  Other  evidence,  however,  contradicts  the  con- 
clusion that  ophiuroids  are  close  to  the  asteroids.  Ophiuroid  larvae  are 
different  from  those  of   the  Asteroidea,   but  resemble  echinoid  larvae 


THE   PHYLA    HEMICHORDATA    AND   ECHINODERMATA 


377 


closely  (Fig.  19.19).  In  two  different  chemical  analyses,  one  on  sterols  and 
one  on  phosphagens,  the  ophiuroids  differed  from  the  asteroids  and 
were  identical  with  the  echinoids  whereas  the  asteroids  resembled  the 
holothurians  and  crinoids.  These  embryologic  and  chemical  studies  sug- 
gest that  the  ophiuroids  are  more  closely  related  to  the  echinoids  than 
to  the  asteroids. 

Obviously  both  theories  cannot  be  correct.  The  echinoids  existed 
long  before  the  ophiuroids,  so  that  the  ophiuroids  cannot  be  both  closely 
related  to  the  echinoids  and  yet  descendants  of  the  asteroids.  Other 
chemical  studies,  which  are  still  in  progress,  tend  to  negate  the  evidence 
from  the  sterol  and  phosphagen  analyses. 

1 73.        Relationships  among  the  Hemichordata,  Echinodermata, 

and  Other  Phyla 

From  the  study  of  adult  structure  given  in  this  chapter  there  is  little 
evidence  for  relating  the  hemichordates  and  echinoderms.  Both  have  a 
poorly  developed  nervous  system  with  few  sense  organs,  a  negative  char- 
acteristic that  can  also  be  found  in  other  animal  groups.  In  both  groups 
a  portion  of  the  coelom  opens  to  the  outside,  filling  that  portion  with 
sea  water  to  serve  as  a  hydraulic  mechanism.  1  his  is  most  remarkable, 
since  such  a  device  is  not  found  in  other  animals.  The  similarity,  how- 
ever, appears  to  be  functional  rather  than  structural,  for  the  adult 
morphologies  of  the  two  mechanisms  are  very  different. 


-Anus 


E  F 

Figure  19.17.  Diagrammatic  representation  of  early  development  in  the  hemichor- 
dates and  echinoderms.  The  lower  figures  are  sections  through  the  embryo  indicated  by 
arrows  in  B,  and  show  two  different  methods  of  coelom  formation,  both  of  which  are 
found  in  both  phyla. 


378 


THE   ANIMAL   KINGDOM 


Cilialedtracl-\  fA 

C  oelom,' 

-C  oel.  pore-    ^^%r^  J 
Gill  slits 

Coe-loTTi'^ 
Coe-lom — = '~^' 


illL^Ciliatedtracl^    |^| 


Coelom, 

Coe-1 .  pore 
Coelom 


oe 


-Proboscis^ 
Stomochorci' 
Colleur 


Ciliated  rirzd — 


'reoral  lobe. 


lorn. 


'  i^^iVi,**' 


Moalh 
Co  elom. 

H 


Figure  19.18.  Diagrammatic  side  views  of  larvae  of  the  Hemichordata  (A  to  D)  and 
of  the  Echiiiodermata  (£  to  H).  Although  the  early  stages  {A  and  E)  are  similar,  after 
different  patterns  of  metamorphosis  (B,  C,  and  F,  G),  the  end  results  are  strikingly 
different  (D  and  H).  The  ciliated  tract  has  been  omitted  from  G,  where  it  is  somewhat 
more  lobulated  than  in  f .  ft  has  disappeared  in  C,  D  and  H. 

The  close  relationship  of  these  phyla  is  suggested  by  their  develop- 
ment. Not  only  are  the  early  stages  oi  some  hemichordates  and  some 
echinoderms  very  similar,  but  the  development  of  the  hydraulic  mech- 
anisms shows  that  they  are  essentially  homologous. 

In  both  phyla  the  eggs  usually  divide  in  simple  fashion  into  2,  4,  8, 
etc.,  cells  with  no  evidence  of  a  specialized  pattern  such  as  spiral  cleav- 
age. Gastrulation  (Fig.  19.17)  is  accomplished  by  simple  invagination,  fol- 
lowed by  a  concentric  ingrowth  of  the  blastopore  rim.  In  both  phyla  the 
blastopore  becomes  the  anus,  and  the  mouth  forms  as  a  new  opening  some 
distance  away.  On  this  account  these  phyla  are  called  deuterostomous 
("second  mouth"). 

In  both  phyla  the  coelom  usually  forms  as  pouches  from  the  archen- 
teron  of  the  gastrula.  The  coelom  (Fig.  19.17)  typically  has  three  por- 
tions, one  which  is  usually  unpaired  and  two  that  are  paired.  The  un- 
paired portion  (coelomj)  forms  as  a  pouch  from  the  anterior  end  of  the 
archenteron.  The  paired  portions  (coelom^  and  coelom^)  may  form  as 
posterior  growths  from  coelom^  or  as  separate  pouches  from  the  sides  of 
the  archenteron.  Other  variations  also  occur.  Since  several  variations  are 
found  in  both  phyla  it  is  concluded  that  the  final  result  (three  portions) 
is  of  more  significance  in  a  study  of  relationship  than  is  the  particular 
way  in  which  they  are  formed. 


THE   PHYLA    HEMICHORDATA    AND    ECHINODERMATA  379 

In  both  phyla  the  embryos  develop  into  larvae  (Fig.  19.18)  that  have 
a  tuft  of  sensory  cilia  at  the  anterior  end,  a  tract  of  locomotor  cilia  on 
the  body,  a  ventral  mouth  and  a  posterior  anus.  The  anterior  coelomic 
cavity  opens  dorsally  through  a  pore  (in  a  very  few  instances  both  the 
cavity  and  the  pore  are  paired).  In  the  echinoderms  the  first  and  second 
portions  of  the  coelom  are  connected. 

Only  a  few  hemichordates  become  free-swimming  at  this  stage  of 
development.  They  develop  rapidly  to  the  next  stage,  which  has  a  pos- 
terior ring  of  stout  locomotor  cilia.  The  ciliated  tract  becomes  more 
elaborate  and  extends  anteriorly  on  both  sides,  finally  meeting  to  form 
two  tracts:  a  preoral  circle  between  the  mouth  and  the  sensory  tuft,  and 
a  postoral  circle  behind  the  mouth  and  dorsal  to  the  tuft.  The  rudiments 
of  several  gill  slits  appear  in  the  walls  of  the  pharynx.  This  is  the 
tornaria  larva.  In  some  species  the  ciliated  tracts  are  greatly  folded  over 
the  surface  of  the  larva. 

Some  of  the  echinoderms  become  free-swimming  before  gastrulation, 
whereas  others  emerge  at  various  times  from  gastrulation  to  metamorph- 
osis. Development  varies  enormously  in  this  phylum,  depending  in  part 
upon  the  amount  of  yolk  in  the  eggs,  and  in  part  upon  the  taxonomic 
group.  Those  with  little  yolk  usually  pass  through  larval  stages  com- 
parable with  those  of  the  hemichordates.  As  in  the  hemichordates,  the 
ciliated  tract  usually  extends  anteriorly  and  fuses  to  form  two  loops. 
Meanwhile  the  body  becomes  concave  ventrally  and  the  ciliated  tract 
becomes  lobulated,  extending  out  trom  the  body  surface  on  body  folds. 
A  larva  of  this  sort,  the  auricularia,  is  found  in  many  holothurians.  In 
the  asteroids  the  lobulation  of  the  ciliated  tracts  becomes  much  more 
pronounced,  while  in  the  echinoids  and  ophiuroids  the  lobes  become  ex- 
tremely long  and  slender  with  skeletal  supporting  rods  inside  (Fig.  19.19). 

The  tornaria  metamorphoses  by  a  straightforward  fashion  into  an 
adult  hemichordate.  The  ciliated  tracts  disappear  and  a  constriction 
separates  the  proboscis  from  the  rest  of  the  body.  The  pharynx  elongates 
while  the  middle  coelomic  pouches  move  anteriorly.  A  second  constric- 
tion separates  the  collar  from  the  trunk,  and  the  gill  slits  in  the  pharynx 
open  through  the  sides  of  the  trunk.  The  anterior  coelomic  pouch  be- 
comes the  proboscis  cavity  with  its  dorsal  pore.  The  pair  of  middle 
cavities  become  right  and  left  collar  cavities,  with  dorsal  and  ventral 
mesenteries  separating  them  above  and  below  the  pharynx.  Each  half 
later  develops  an  opening  into  the  first  gill  slit.  The  posterior  coelomic 
pouches  form  the  right  and  left  cavities  in  the  trunk,  also  with  dorsal 
and  ventral  mesenteries  between  them.  The  ciliated  ring  disappears,  and 
the  swimming  tornaria  becomes  a  burrowing  worm.  (See  Fig.  19.18.) 

Metamorphosis  in  the  echinoderms  involves  not  only  a  drastic 
alteration  of  body  parts,  but  also  a  change  in  symmetry.  The  details 
vary  considerably,  and  only  a  simplified  course  of  events  in  some  of  the 
starfish  will  be  followed  here.  One  of  the  first  events  of  metamorphosis 
is  the  migration  of  the  mouth  around  to  the  left  side  of  the  body,  where 
the  left  middle  coelomic  pouch  surrounds  it  to  form  the  ring  of  the  water 
vascular  system.  At  the  same  time  the  anus  begins  to  migiate  to  the 
right  side.    The   region   anterior    to   the   mouth    becomes   a   prominent 


380  THE  ANIMAL  KINGDOM 


Figure  19.19.  Echinoderm  larvae.  Upper  right,  ventral  view  of  an  auricularia  of 
the  sea  cucumber,  Labidoplax  digitata  (compare  with  Fig.  19.18  F).  Upper  left,  pluteus 
larva  of  the  sea  urchin,  Psamniechinus  miliaris.  Lower,  pluteus  larva  of  the  brittle  star, 
Ophiothrix  fragilis.  (Courtesy  Douglas  P.  Wilson.) 

preoral  lobe,  and  may  be  used  for  temporary  attachment  to  the  ocean 
bottom. 

The  ring  canal  develops  five  branches  which  will  become  the  radial 
canals,  and  the  body  around  the  mouth  begins  to  grow  out  in  the  five- 
part  radial  symmetry  of  the  adult.  The  pore  of  the  anterior  coelomic 
pouch  migrates  to  the  original  right  side  and  the  anterior  pouch  becomes 
constricted  to  form  two  portions.  The  preoral  lobe  with  its  sensory  tuft 
and  coelomic  cavity  degenerates  and  is  absorbed,  while  the  pore  and  a 
portion  of  the  anterior  pouch  become  the  madreporite  and  stone  canal 
(and  associated  structures)  of  the  adult.  Thus,  the  lower  side  of  a  starfish 
develops  from  the  left  side  of  the  larva,  while  the  upper  side  develops 
from  the  larval  right  side.  The  third  pair  of  coelomic  pouches,  which 
lie  left  and  right  in  the  larva,  become  upper  and  lower  in  the  adult,  with 
a  horizontal  mesentery  between  them.  All  that  remains  of  this  mesentery 
in  the  adult  is  the  five  pairs  of  retractors  of  the  cardiac  stomach. 

The  steps  in  this  metamorphosis  that  are  general  for  echinoderms 
include:  (1)  the  development  of  an  adult  oral  surface  from  the  larval  left 
side,  and  an  aboral  surface  from  the  larval  right  side;  (2)  the  develop- 
ment of  most  of  the  water  vascular  system  from  the  left  middle  coelomic 


THE   PHYLA    HEMICHORDATA    AND    ECHINODERMATA  3gl 

pouch;  (3)  the  development  of  the  adult  madreporite  from  the  pore  of 
the  anterior  pouch;  (4)  the  loss  of  the  preoral  region;  and  (5)  the  develop- 
ment of  lower  and  upper  body  cavities  from  the  left  and  right  posterior 
pouches. 

Thus,  if  a  comparison  between  the  hemichordates  and  echinoderms 
is  valid,  the  proboscis  pore  is  homologous  with  the  madreporite,  the 
collar  cavity  with  the  ring  canal  and  radial  canals,  and  the  trunk  cavity 
with  the  echinoderm  body  cavity. 

Relationships  of  the  hemichordates  and  echinoderms  with  phyla 
previously  considered  are  difficult  to  establish.  Like  many  of  the  phyla 
they  have  bottom-living  adults  and  planktonic  larvae  but  the  larvae 
may  have  evolved  independently  in  several  different  lines  to  serve  as  a 
dispersing  mechanism. 

In  all  of  the  preceding  phyla  with  a  separate  mouth  and  anus  the 
blastopore  of  the  gastrula  tends  to  become  a  ventral  portion  of  the  adult. 
Usually  it  becomes  elongated  as  it  closes,  and  forms  an  antero-ventral 
mouth  at  its  anterior  end.  Typically  an  anus  forms  from  its  posterior  end. 
Numerous  exceptions  occur  in  which  mouth,  anus  or  both  form  from 
tissue  beyond  the  ends  of  the  elongated  blastopore.  Those  phyla  in  which 
the  mouth  is  clearly  a  part  of  the  blastopore  are  called  protostomous. 
In  the  hemichordates  and  echinoderms  (and  possibly  in  some  of  the 
minor  phyla  previously  discussed)  the  fate  of  the  blastopore  is  decidedly 
different.  The  blastopore  is  posterior  and  closes  without  elongation  to 
form  the  anus,  while  the  mouth  forms  a  considerable  distance  away.  It 
is  possible  that  this  difference  is  so  fundamental  that  the  evolutionary 
relationships  between  the  protostomes  and  the  deuterostomes  will  never 
be  discovered. 

In  both  the  hemichordates  and  echinoderms  the  coelom  is  entero- 
coelous,  whereas  in  many  of  the  previously  considered  groups  it  is 
schizocoelous.  In  some,  however,  an  enterocoelous  origin  is  the  rule,  so 
that  a  relationship  between  the  hemichordates  and  such  groups  as  the 
Chaetognatha  and  some  of  the  Brachiopoda  may  be  indicated.  On  the 
other  hand,  exceptions  for  the  origin  of  the  coelom  are  becoming  so 
numerous  as  more  species  in  each  phylum  are  studied  that  whether  it  is 
enterocoelous  or  schizocoelous  may  not  prove  to  be  a  very  useful  char- 
acteristic. 

At  the  present  time  the  Chaetognatha,  which  are  enterocoelous  and 
in  which  the  blastopore  forms  the  anus  and  extends  only  halfway  up 
the  ventral  surface,  are  generally  considered  to  be  the  closest  of  the 
previously  considered  phyla  to  the  Hemichordata  and  Echinodermata. 

The  chordates,  like  the  hemichordates  and  echinoderms,  are  clearly 
deuterostomous.  The  fate  of  the  chordate  blastopore  is  not  like  that  of 
the  hemichordates  and  echinoderms.  As  the  chordate  blastopore  closes  the 
lips  are  drawn  together  dorsally  and  elongated,  with  the  anus  forming 
from  the  posterior  end.  Such  a  pattern,  while  different  from  that  of  the 
hemichordates  and  echinoderms,  is  more  easily  related  to  theirs  than  to 
the  protostomous  pattern. 

Chordates  are  related  to  the  hemichordates  through  a  comparison 
of  the  adults.  Both  groups  have  gill  slits,  and  in  some  members  of  both 


382  '""^   ANIMAL   KINGDOM 

groups  the  slits  become  U-shaped  through  the  development  of  tongue- 
bars.  The  details  of  structure  in  the  tongue-bars  of  hemichordates  and 
chordates  are  so  similar  that  they  become  the  strongest  evidence  for 
relating  the  groups.  Formerly  much  stress  was  placed  on  the  possible 
homology  of  the  stomochord  of  hemichordates  and  the  notochord  of 
chordates,  and  on  the  hemichordate  collar  nerve  and  chordate  nerve 
cord,  but  as  more  is  learned  of  the  details  of  structure  and  function  in 
these  organs,  more  doubt  is  cast  on  the  validity  of  their  homology.  Even 
so,  such  structures  represent  similar  experiments  in  evolution,  and  as 
such  do  not  argue  against  a  relationship  of  the  two  groups. 

The  hemichordates,  echinoderms  and  chordates  are  reminiscent  of 
the  annelids,  molluscs  and  arthropods,  in  which  two  of  the  phyla  can  be 
related  through  similarities  of  development,  while  another  two  are  re- 
lated through  a  comparison  of  adult  structure.  These  relationships  are 
much  more  obvious  in  the  annelids,  molluscs  and  arthropods. 

Questions 

1.  What  characteristics  link  the  hemichordates  and  echinoderms?  The  hemichordates  and 
chordates?  The  chordates  and  echinoderms? 

2.  Characterize  the  fi\  e  living  classes  of  echinoderms. 

3.  How  do  tube  feet  function? 

4.  Describe  the  skeleton  of  Asterias. 

5.  Compare  feeding  in  sea  lilies  and  sea  cucumbers. 

6.  What  is  Aristotle's  lantern? 

7.  What  was  unique  about  the  Heterostelea? 

8.  Discuss  conflicting  evidence  concerning  evolutionary  relationships  among  the  Echi- 
noidea,  Asteroidea  and  Ophiuroidea. 

9.  What  is  a  tornaria? 

Supplementary  Reading 

The  Invertebrates,  vol.  W,  by  L.  Hyman  is  devoted  to  the  Echinodermata,  and  con- 
tains the  same  comprehensive  and  richly  illustrated  treatment  found  in  her  other  vol- 
umes. The  phylum  is  treated  thoroughly  in  Schrock  and  Twenhofel,  Principles  of  Inverte- 
brate Paleontology,  where  the  extent  of  the  fossil  record  is  revealed  and  problems  of 
evolutionary  relationship  are  discussed. 


CHAPTER  20 


The  Chordates 


1 74.        Chordate  Characteristics 

The  chordates  are  perhaps  more  familiar  than  the  invertebrates  de- 
scribed in  the  preceding  chapters;  the  phykim  includes  the  back-boned 
animals  or  vertebrates— man  and  his  domestic  creatures,  birds,  frogs, 
fishes,  and  the  like.  The  Vertebrata,  however,  is  but  one  subphylum  of 
the  phylum  Chordata.  Two  others,  the  Urochordata  and  Cephalochor- 
data,  contain  less  conspicuous,  soft-bodied,  marine  species  often  col- 
lectively called  the  lower  chordates.  The  urochordates  are  represented 
by  the  sea  squirts  (Molgtila),  and  the  cephalochordates  by  the  lancelet 
(Amphioxus,  Fig.  20.1  C).  One  may  well  ask,  what  do  such  diverse 
groups  have  in  common  that  all  are  placed  in  the  same  phylum?  Cer- 
tainly the  adults  do  not  look  alike,  but  at  some  stage  in  their  life 
history  these  animals  share  three  unique  features. 

First,  a  dorsal,  longitudinal  rod  known  as  the  notochord  is  present 
in  the  embryos  of  all  and  sometimes  in  the  adults.  It  is  composed  of  a 


Incurrent 
-  siphon 

Excurrent 
siphon. 


Oral-, 
hood 


Myomere — i 


Gelalinou-S 
ma.trix 


Ca-udal 
fin 


Gonad 
'-Meta.pleura.l  fold 


Anus 


Ventral  fin 
Atriopore- 


Figure  20.1 .  A  group  of  lower  chordates.  A,  The  tunicate  Molgula,  partly  buried  in 
sand;  B,  a  portion  of  the  colonial  tunicate  Botryllus,  viewed  from  above;  C,  a  lateral 
view  of  Amphioxus.  Molgula  is  natural  size;  the  others  are  enlarged. 

383 


384  ^WE  ANIMAL  KINGDOM 

fibrous  sheath  encasing  many  vacuolated  cells,  whose  turgidity  makes  it 
firm  yet  flexible.  It  is  generally  assumed  that  the  notochord  provides 
support  for  the  body,  but  it  can  be  argued  that  the  small,  marine 
chordates,  which  first  acquired  a  notochord,  did  not  need  this  extra 
support.  A  more  plausible  suggestion  is  that  it  prevents  the  body  from 
shortening  in  the  manner  of  an  earthworm  when  the  longitudinal 
muscle  fibers  in  the  body  wall  contract.  Since  telescoping  is  prevented, 
the  contraction  of  muscle  fibers  first  on  one  side  and  then  on  the  other 
causes  the  animal  to  bend  from  side  to  side  and  move  through  the 
water  with  fishlike,  lateral  undulations.  Undulatory  movements  are 
possible  without  a  rod  of  this  type— certain  marine  worms,  for  example, 
swim  in  this  fashion— but  the  notochord  may  increase  the  efficiency  and 
precision  of  this  type  of  locomotion. 

Secondly,  a  longitudinal  nerve  cord  lies  dorsal  to  the  notochord. 
It  differs  from  the  ventral  nerve  cord  of  certain  nonchordates,  both  in 
position  and  in  structure,  for  it  is  a  single  rather  than  a  double  cord, 
and  is  tubular  rather  than  solid. 

Finally,  chordates  differ  from  most  nonchordates  in  having  pha- 
ryngeal pouches  that  extend  laterally  from  the  anterior  part  of  the 
digestive  tract  toward  the  sides  of  the  body,  often  breaking  through  as 
gill  slits.  All  chordates  have  gill  slits,  or  at  least  pharyngeal  gill 
pouches,  at  some  stage  of  their  life  cycle.  Certain  hemichordates  (p.  360) 
also  have  gill  slits.  This  arrangement  appears  to  have  served  originally  as 
a  means  of  letting  the  water  taken  into  the  mouth  escape  from  the 
digestive  tract,  thereby  concentrating  the  small  food  particles  that  were 
in  the  water.  The  lower  chordates  and  the  larvae  of  the  most  primitive 
vertebrates  are  food-sifters,  or  filter-feeders,  and  live  upon  minute  or- 
ganic matter  gathered  in  this  way. 

In  addition  to  these  diagnostic  features,  chordates  share  many 
other  characters  with  certain  of  the  more  advanced,  nonchordate  groups. 
They  are  bilaterally  symmetrical;  they  are  triploblastic;  their  general 
plan  of  body  organization  is  a  tube  within  a  tube,  for  in  most  chordates 
a  coelom  separates  the  digestive  tract  from  the  body  wall;  the  gut  tube 
is  complete,  i.e.,  there  is  a  separate  mouth  and  anus.  Diffusion  is  ade- 
quate for  gas  exchange  and  excretion  in  the  simpler  chordates,  but 
special  respiratory  and  excretory  organs  are  present  in  the  vertebrates. 
The  vertebrates  are  active  animals,  with  a  high  degree  of  cephalization 
(accumulation  of  nerves  and  sense  organs  in  the  head)  and  segmental 
muscular  and  related  systems. 

175.        Subphylum  Urochordata 

The  first  chordate  subphylum,  the  Urochordata,  includes  the  marine 
tunicates  and  their  allies.  Most  urochordates  belong  to  the  class  Ascidi- 
acea,  and  are  sessile  organisms  that  are  frequently  seen  attached  to  sub- 
merged rocks  and  wharf  pilings,  or  are  found  partially  buried  in  sand 
and  mud  in  coastal  waters.  They  may  be  either  solitary  or  colonial 
(Fig.  20.1).  Molgula  is  a  familiar  example  of  the  former  type  occurring 


THE   CHORD ATES  385 


IncurrerA 


Oral  -tenta-dc 


rNeural  alamd 


ExCurrcnt 
siphon. 


"Atrium. 


Intestine 
Gon.ci.cL 

Heart 


.Genital  duct 
Esopha-^uS 

Digestive 

gizLn-d. 


S'bomsLCti. 


Otolith- 

Tncurrent 
openin6- 


Sznsory  vesicle 
I— Ocellus 

Ex-currcnt  opening 


r- Nerve  cord 


Adhesive 
pa.pilla. 

'Pharynx — 
Endostyle 


Heart 

■Gill  slii 
"Atrium. 


'"Not  o  chord 
-Strand  of  enioAerm. 
'^Stomach. 

B 


Figure  20  7.     Diagrammatic  lateral  views  of  an  adult,  -^    and  a  l-^^l'/^'^^f;,^^^^^^^ 

are  embedded  in  the  mantle. 

along  the  Atlantic  coast.  Other  classes  of  tunicates,  the   Larvacea  and 

the  Thaliacea,  are  pelagic.  enclosed 

A  solitary  adult  ascidian  is  a  sac-shaped  creature   that    ^  en^losea 

in  a  leather/ tunic,  which  has  been  secreted  by   the   -d- Ytng  ^^^^ 
wall,  or  mantle   (Fig.  20.2,  A).  A   considerable  -^^^^J'^l^^^^^^^ 
complex  carbohydrate  characteristic  of  the  cell  walls  of  plants  but  rarely 


386  ''"^   ANIMAL   KINGDOM 

found  in  animals,  is  present  in  the  tunic.  The  animal  is  attached  to 
the  substrate  by  its  base,  and  two  tubular  openings  are  present  near  the 
upper  surface.  The  uppermost  one,  or  incurrent  siphon,  leads  into  a 
large,  barrel-shaped  pharynx,  which  occupies  most  of  the  space  within 
the  body.  The  gill  slits  in  the  pharyngeal  wall  do  not  open  directly  to 
the  body  surface,  but  into  a  specialized,  ectodermally  lined  chamber, 
called  the  atrium,  lying  on  each  side  of  the  pharynx  and  along  its  dorsal 
edge.  The  atrium  opens  at  the  surface  through  an  excurrent  siphon. 
Ciliated  cells  in  the  pharynx  maintain  a  flow  of  water  into  the  incur- 
rent and  out  of  the  excurrent  siphon. 

Gas  exchange  occurs  between  the  water  passing  through  the 
pharynx  and  blood  channels  in  the  pharyngeal  wall,  but  the  pharynx 
is  also  a  food-gathering  mechanism.  Mucus  produced  in  the  endostyle 
(a  longitudinal  groove  in  the  floor  of  the  pharynx)  is  moved  across  the 
lateral  walls  of  the  pharynx  to  its  dorsal  surface.  Minute  food  particles 
are  entrapped  in  this  sheet  of  mucus,  which  is  then  carried  along  a 
dorsal  band  into  the  more  posterior  parts  of  the  digestive  tract.  The 
intestine  finally  opens  into  the  atrium. 

A  tube-shaped,  muscular  heart  is  enclosed  in  a  reduced  coelom,  and 
a  vessel  leads  out  from  each  of  its  ends  into  open  channels  in  the  wall 
of  the  pharynx  and  other  organs.  Capillaries  are  absent.  The  beating 
of  the  heart  is  unique  in  that  waves  of  contraction  move  from  one  end 
of  the  heart  to  the  other  for  a  while,  and  then  the  beat  reverses  and 
the  contractions  move  in  the  opposite  direction.  The  heart  and  blood 
vessels  have  no  valves. 

A  solid  nerve  ganglion,  from  which  nerves  extend  to  various  parts 
of  the  body,  lies  in  the  mantle  between  the  siphons,  and  a  peculiar 
neural  gland  lies  beside  the  ganglion.  The  latter  opens  into  the  pharynx 
by  means  of  a  short  ciliated  duct.  Its  function  is  uncertain,  but  some 
investigators  consider  it  to  be  an  endocrine  gland  and  have  compared 
it  to  the   pituitary   gland  of  vertebrates. 

Ascidians  are  hermaphroditic,  part  of  the  gonad  being  ovary  and 
part  testis.  One  or  more  ducts  lead  from  the  gonad  to  the  atrium.  Cer- 
tain ascidians  are  self-fertilizing,  that  is,  the  eggs  of  one  individual  can 
be  fertilized  by  sperm  from  the  same  individual,  but  in  others  the 
sperm  must  come  from  a  different  individual.  Asexual  reproduction  by 
budding  also  occurs. 

Pharyngeal  gill  slits  are  well  developed  in  the  adult,  but  one  must 
examine  a  tunicate  larva  to  find  the  other  chordate  characteristics  (Fig. 
20.2,  B).  The  larva  is  tadpole-shaped  with  an  expanded  body  and  a 
long  mobile  tail  equipped  with  longitudinal  muscle  fibers.  A  notochord 
supports  the  tail  (whence  the  term  urochordate)  and  a  distinct  tubular 
nerve  cord  lies  dorsal  to  it.  The  anterior  end  of  the  nerve  cord  expands 
to  form  a  brainlike  sensory  vesicle  containing  a  light-sensitive  ocellus 
and  an  otolith  concerned  with  equilibrium.  The  pharynx  and  other 
digestive  organs  develop  within  the  body,  but  do  not  function  in  most 
larvae.  A  pair  of  dorsal,  ectodermal  invaginations,  which  eventually  ac- 
quire a  common  external  opening,  grow  down  beside  the  pharynx  to 
form  the  atrium.   Within  a  day  or  two   the  tadpole   finds  a  favorable 


THE   CHORDATES 


387 


substrate  to  which  it  attaches  by  its  anterior  adhesive  glands.  It  loses  its 
tail  and  is  transformed  into  an  adult.  Notochord  and  nerve  cord  are 
resorbed,  only  the  ganglion  and  neural  gland  remaining  as  traces  of 
the  latter. 

176.        Subphylum  Cephalochordata 

Amphioxus  and  a  related  genus  of  small,  superficially  fish-shaped 
chordates  constitute  the  subphylum  Cephalochordata.  Species  occur  in 
the  United  States  in  coastal  waters  south  from  Chesapeake  {Amphioxus 
virginiae)  and  Monterey  {A.  calijorniense*)  Bays.  They  usually  lie  buried 
in  sand  with  only  their  anterior  end  protruding,  but  they  can  also  swim 
fairly  well. 

The  body  of  Amphioxus  (Fig.  20.1,  C)  is  elongate,  tapers  at  each 
end,  and  is  compressed  from  side  to  side.  A  dorsal,  a  caudal  and  a 
ventral  fin  lie  in  the  median  plane  of  the  body,  and  a  pair  of  long 
finlike  metapleural  folds  are  present  ventro-laterally.  Dorsal  and  ventral 
fins  are  supported  by  blocks  of  connective  tissue,  but  these  fins  and 
folds  are  apparently  not  large  or  strong  enough  to  keep  the  animal  on 


Figure  20.3.  A  diagrammatic  lateral  view  of  Amphioxus.  White  arrows  represent 
the  course  of  the  current  of  water;  black  arrows  that  of  the  food. 

an  even  keel,  for  Aynphioxus  spirals  as  it  swims.  Swimming  is  accom- 
plished by  the  contraction  of  longitudinal  muscle  fibers  in  the  body 
wall  that  are  arranged  in  segmental,  <-shaped  muscle  blocks,  or  myo- 
meres. These  can  easily  be  seen  through  the  thin  skin.  Successive  myo- 
meres are  separated  by  connective  tissue  septa  to  which  the  muscle 
fibers  are  attached.  Shortening  of  the  body  is  prevented  by  an  unusually 
long  notochord  (Fig.  20.3)  that  extends  farther  anteriorly  than  in  any 
other  chordate,  an  attribute  after  which  the  subphylum  is  named. 

Water  and  minute  food  particles  are  taken  in  through  the  oral 
hood,  whose  edges  bear  a  series  of  delicate  projections,  the  cirri,  that 
act  as  a  strainer  to  exclude  larger  particles.  The  inside  of  the  oral  hood 
is  lined  with  bands  of  cilia  called  the  wheel  organ,  which,  together  with 
cilia  in  the  pharynx,  produce  a  current  of  water  that  enters  the  mouth. 
The  mouth  proper  lies  deep  within  the  oral  hood  and  is  surrounded  by 
twelve  velar  tentacles. 

*  Branchiostoma  Costa,  1834  has  priority  over  Amphioxus  Yarrell,  1836  as  the  generic 
name  for  these  animals,  but  there  is  some  question  as  to  the  adequacy  of  Costa's  descrip- 
tion and  hence  as  to  the  validity  of  his  name. 


388  ^"^   ANIMAL   KINGDOM 

Food  is  entrapped  within  the  pharynx  in  mucus  secreted  by  an 
endostyle  just  as  it  was  in  urochordates.  Water  in  the  pharynx  escapes 
into  an  ectodermally  lined  atrium  through  nearly  two  hundred  gill 
slits.  Gill  bars,  supported  by  delicate  skeletal  rods,  lie  between  the 
slits.  At  one  stage  in  development  the  gill  slits  are  U-shaped,  and  re- 
semble those  of  hemichordates  (Fig.  19.4),  a  detail  that  may  point  to  an 
affinity  between  these  animals,  but  in  Amphioxus  the  tonguelike  process 
that  causes  the  slit  to  be  U-shaped  subsequently  continues  its  down- 
ward growth  and  completely  subdivides  the  slit.  Some  gas  exchange 
occurs  in  the  pharynx,  but  the  skin  is  the  main  respiratory  surface. 
The  pharynx,  therefore,  is  primarily  a  food-gathering  device. 

After  leaving  the  pharynx,  the  food  enters  a  short  esophagus,  a 
midgut,  and  finally  an  intestine,  which  opens  at  the  surface  through  an 
anus.  The  intestine  terminates  before  the  end  of  the  body,  so  there  is  a 
postanal  tail  as  in  vertebrates.  A  prominent  midgut  caecum,  which 
produces  digestive  enzymes,  extends  from  the  floor  of  the  midgut  for- 
ward along  the  right  side  of  the  pharynx. 

Absorbed  food  and  other  substances  are  distributed  by  a  circulatory 
system.  A  series  of  veins  returns  blood  from  the  various  parts  of  the 
body  to  a  sinus  which  is  located  ventral  to  the  posterior  part  of  the 
pharynx,  and  may  be  comparable  to  the  posterior  part  of  the  vertebrate 
heart.  A  muscular  heart,  however,  is  not  present,  and  the  blood  is 
propelled  by  the  contraction  of  the  arteries.  A  ventral  aorta  extends  from 
the  sinus  forward  beneath  the  pharynx,  and  leads  into  branchial  ar- 
teries that  travel  dorsally  through  the  gill  bars  into  a  pair  of  dorsal 
aortas.  The  dorsal  aortas,  in  turn,  carry  the  blood  posteriorly  to  spaces 
within  the  tissues.  True  capillaries  are  absent,  but  the  general  direction 
of  blood  flow,  i.e.,  anteriorly  in  the  ventral  part  of  the  body  and  pos- 
teriorly in  the  dorsal  part,  is  similar  to  that  of  a  vertebrate  and  different 
from  that  of  other  lower  chordates. 

The  excretory  organs  are  segmentally  arranged,  ciliated  proto- 
nephridia  (p.  208)  that  lie  dorsal  to  certain  gill  bars  and  open  into  the 
atrium. 

The  nervous  system  of  Amphioxus  consists  of  a  tubular  nerve  cord 
located  dorsal  to  the  notochord.  Its  anterior  end  is  differentiated  slightly, 
but  does  not  expand  to  form  a  brain.  Paired,  segmental  nerves,  con- 
sisting of  dorsal  and  ventral  roots,  extend  into  the  tissues.  The  roots 
remain  separate  and  do  not  unite.  The  ventral  roots  go  directly  into 
the  myomeres,  and  the  dorsal  roots  pass  between  myomeres  to  supply 
the  skin,  gut  wall  and  ventral  parts  of  the  body.  Amphioxus  is  sensitive 
to  light,  and  to  chemical  and  tactile  stimuli,  but  elaborate  sense  organs 
are  not  present.  The  cirri  on  the  oral  hood  and  a  flagellated  pit  in 
the  skin  near  the  front  of  the  nerve  cord  appear  to  be  chemoreceptors. 
Photoreceptive  cells,  which  are  partly  masked  with  pigment,  lie  in  the 
nerve  cord.  The  prominent  pigment  spot  at  the  anterior  end  of  the 
cord  apparently  does  not  function  in  light  reception. 

Numerous  gonads,  which  are  either  all  testes  or  all  ovaries,  for  the 
sexes  are  separate  in  Amphioxus,  bulge  into  the  atrial  cavity.  Actually, 
they  lie  within  a  portion  of  a  highly  modified  coelom  (Fig.  20.4).  The 


THE   CHORDATES 


389 


Dorsal  root  of  nerve 


Fin  ra-y 
Skin. y 

Nerve  cord  /^^ 

Ventral  root LI 

of  ne-Tve 

Dorsal  a.orta. 

Protonephriciium 

Epipharyngeal 
6roove 

Coelom 
Middut  caecum 
Ovary 

Gill  slit 

Gill  bar 


Branchial  arteri/ 

Endostyle 

Coelom 


omcre 


NotocKord 


Ectodevmai 
epith-elitLm. 

PtiaJ^yrvjc 


Transverse 
mtL^cie 


'LynrpK  spa.CC 

Ventral  ^      \vLetapL^.^  ^.  x  ^.^ 
aorta      \  J 

Atrium. 

Figure  20.4.  A  diagrammatic  cross  section  through  the  posterior  part  of  the 
pharynx  of  Ainpliioxiis.  Branchial  arteries  extend  from  the  ventral  aorta  through  the 
gill  bars  to  the  dorsal  aortas.  The  portion  of  the  coelom  ventral  to  the  endostyle  is 
connected  through  alternate  gill  bars  with  the  pair  of  coelomic  canals  lying  dorsal 
to  the  atrium.  Other  parts  of  the  coelom  are  associated  with  the  midgut  caecum  and 
gonads. 

gametes  are  discharged  into  the  atrium  upon  the  rupture  of  the  gonad 
walls.  Fertilization  and  development  are  external. 


177.        Subphylum  Vertebrata 

The  Vertebrata  is  by  far  the  largest  and  most  important  of  the  chor- 
date  subphyla,  for  all  but  about  2000  of  the  approximately  35,000  living 
species  of  chordates  are  vertebrates.  The  subphylum  in  turn,  is  divided 
into  eight  classes.  The  oldest  and  most  primitive  vertebrates,  which 
lack  jaws  and  paired  appendages,  are  placed  in  the  class  Agnatha. 
Most  of  these  are  extinct,  but  the  lamprey  is  a  living  representative  of  this 
group.  The  Agnatha  gave  rise  to  the  class  Placodermi,  a  group  of  prim- 
itive jawed  fishes,  all  of  which  are  extinct.  Placoderms,  in  turn,  gave 
rise  to  the  large  groups  of  living  fishes— the  class  Chondrichthyes  and 
the  class  Osteichthyes.  The  Chondrichthyes  are  the  fishes  with  carti- 
laginous skeletons  such  as  the  sharks  and  rays;  the  Osteichthyes  are  the 
more  familiar  fishes  with  bony  skeletons  such  as  salmon,  minnows  and 
perch.  The  first  terrestrial  vertebrates  evolved  from  certain  of  the  bony 


390 


THE  ANIMAL  KINGDOM 


Vertebra- 


Notochord-, 
Gonadi 


1 


rOviduct 


Myomere - 


Spinal  cord 


Spleen 


i>p 


-Ercretory 
duct 


£2 


Mouth-* 


"Olfactory 
organ 


Gill  alii? 


Bsopha^^ 


'Its 


Heart 
■Phsxyroc 


Gall- 
bladder 


Pancreas 

•-Stomach 
Intestme- 


Cloaca 
Urinary  bladder 
Kidney 

•-Coelora 


Figure  20.5.     A  diagrammatic  sagittal  section  through  a  generalized  vertebrate  to 
show  the  characteristics  of  vertebrates  and  the  arrangement  of  the  major  organs. 


fishes  and  are  placed  in  the  class  Amphibia.  Adult  frogs,  salamanders 
and  other  amphibians  are  more  or  less  terrestrial,  but  they  generally 
return  to  the  water  to  reproduce.  Amphibians  gave  rise  to  the  class 
Reptilia,  a  group  that  includes  turtles,  alligators,  lizards  and  snakes. 
Reptiles  are  better  adapted  to  the  terrestrial  environment  and  reproduce 
on  land,  but  they  resemble  all  of  the  lower  vertebrates  in  being  cold- 
blooded. The  remaining  two  classes,  the  birds  (class  Aves)  and  mam- 
mals (class  Mammalia),  evolved  from  the  reptiles,  and  the  members  of 
both  groups  have  become  active  and  warm-blooded.  Birds  are  clothed 
with  feathers  and  lay  eggs;  most  mammals  are  covered  with  hair  and 
give  birth  to  living  young  which  are  nourished  by  milk  secreted  by  the 
mammary  glands. 

Vertebrates  share  with  the  lower  chordates  the  three  diagnostic 
characteristics  of  the  phylum.  The  latter  are  clearly  represented  at  some 
stage  in  the  life  history  of  the  various  groups.  A  dorsal,  tubular  nerve 
cord,  which  has  differentiated  into  a  brain  and  spinal  cord,  is  present 
in  the  embryos  and  adults  of  all  (Fig.  20.5).  Embryonic  vertebrates 
have  a  notochord  lying  ventral  to  the  nerve  cord  and  extending  from 
the  middle  of  the  brain  nearly  to  the  posterior  end  of  the  body,  but  a 
vertebral  column  replaces  the  notochord  in  most  adults.  All  embryonic 
vertebrates  have  a  series  of  pharyngeal  pouches  that  grow  out  from 
the  lateral  walls  of  the  pharynx,  but  these  pouches  break  through  the 
body  surface  to  form  gill  slits  only  in  fishes  and  larval  amphibians. 

Vertebrates  differ  from  the  lower  chordates  most  obviously  in  having 
at  least  traces  of  a  vertebral  column,  and  in  having  a  better  developed 
head  containing  an  aggregation  of  sense  organs  and  an  enlarged  brain 
enclosed  in  a  brain  case  or  cranium.  An  alternate  name  for  the  sub- 
phylum,  the  Craniata,  emphasizes  this  last  point.  In  addition,  the 
superficial  layer  of  vertebrate  skin  is  a  stratified  epithelium  rather  than 
a  simple  epithelium.  A  liver,  serving  as  a  site  for  food  storage  and  con- 
version, is  present  as  a  ventral  outgrowth  from  the  digestive  tract.  In 


THE   CHORDATES  391 

the  more  primitive  vertebrates  the  digestive  tract  and  the  urinary  and 
genital  ducts  terminate  in  a  common  cavity,  the  cloaca.  This  opens  to 
the  surface  by  an  anus  located  somewhat  anterior  to  the  end  of  the 
body,  and  there  is  a  distinct  postanal  tall.  The  respiratory  organs  are 
either  gills,  which  lie  within  the  gill  slits,  or  lungs— paired,  saccular 
outgrowths  from  the  floor  of  the  pharynx.  The  circulatory  system  is 
closed,  for  capillaries  connect  arteries  and  veins.  Blood  is  propelled  by 
the  action  of  a  muscular  heart  lying  ventral  to  the  digestive  tract  in  an 
anterior  division  of  the  coelom.  The  excretory  organs  are  kidneys  com- 
posed of  numerous  kidney  tubules  (Fig.  5.6  D)  that  remove  both  water 
and  excretory  products  from  the  blood.  In  most  vertebrates  much  oi 
the  water  is  later  reabsorbed  into  the  blood. 

1 78.        The  Origin  of  Chordates 

Ever  since  the  general  acceptance  of  the  theory  of  organic  evolution, 
man  has  been  interested  in  the  origin  of  the  chordates.  But  this  prob- 
lem does  not  have  an  easy  solution,  for  chordates  are  a  distinctive 
group  separated  by  a  wide  morphologic  gap  from  other  phyla. 

The  segmentation  of  cephalochordates  and  vertebrates  early  drew 
attention  to  a  possible  evolutionary  relationship  between  chordates  and 
the  annelid-arthropod  stock.  Annelids  and  arthropods  are  segmented, 
but  they  differ  from  chordates  in  so  many  basic  characters  that  this  view 
has  been  abandoned.  Their  nerve  cord,  for  example,  is  not  a  single, 
dorsal,  tubular  cord,  but  a  solid,  essentially  double  cord,  lying  ventral 
to  the  digestive  tract.  It  would  be  necessary  to  turn  an  annelid  or 
arthropod  upside  down,  evolve  a  completely  new  nerve  cord,  and  make 
many  other  radical  transformations  in  order  to  derive  a  chordate  from 
these  animals.  Intermediate  stages  in  such  a  transformation  are  difficult 
to  visualize.  Moreover,  the  urochordates,  generally  considered  to  be  the 
most  primitive  chordates,  are  not  segmented  and  are  a  source  of  em- 
barrassment to  those  who  would  derive  chordates  from  segmented  an- 
cestors. 

Other  evidence  indicates  that  the  lower  chordates  may  have  evolved 
from  the  echinoderm-hemichordate  stock.  The  presence  in  certain  hemi- 
chordates  and  chordates  of  pharyngeal  gill  slits  and  the  unusual  tongue- 
bar  that  causes  the  slits  to  become  U-shaped  suggest  an  evolutionary 
relationship  between  these  groups.  Indeed,  some  authors  include  the 
hemichordates  as  a  subphylum  of  the  chordates.  Some  have  concluded 
that  the  radial  symmetry  of  echinoderms  negates  a  relationship  with  the 
bilaterally  symmetrical  hemichordates  and  chordates,  but,  as  we  have 
learned  (p.  376),  the  radial  symmetry  of  the  adult,  present-day  echino- 
derms has  been  secondarily  superimposed  upon  a  basically  bilateral  or- 
ganization. Both  the  primitive,  extinct  echinoderms  and  the  early 
echinoderm  larvae  are  bilaterally  symmetrical.  Many  features  of  the  early 
development  (cleavage,  origin  of  mesoderm  and  coelom,  fate  of  the 
blastopore)  of  echinoderms,  hemichordates  and  chordates  are  similar  and 
suggest  an  evolutionary  relationship  (see  section  173).  Moreover,  there  is 
a  closer  similarity  between  the  body  fluid  proteins  of  the  chordates, 
hemichordates  and  echinoderms  than  between  those  of  chordates  and 


392  ^"^   ANIMAL   KINGDOM 

annelids  or  arthropods,  and  the  degree  of  resemblance  of  the  proteins  of 
live  animals  has  been  shown  to  be  a  good  measure  of  their  evolutionary 
relationship.  The  serological  technique  by  which  the  degree  of  protein 
similarity  is  determined  is  described  in  Chapter  35. 

Professor  Berrill  of  McGill  University  has  recently  proposed  that 
primitive  chordates  were  sessile,  filter  feeding,  marine  organisms  not 
unlike  present-day  ascidians.  Gill  slits  presumably  evolved  in  this  group 
as  a  means  of  concentrating  food;  a  respiratory  function  for  gill  slits  was 
a  secondary  development.  The  tadpole-type  larva,  with  its  sensory  ves- 
icle and  mobile  tail  supported  by  a  notochord,  evolved  as  a  means  of 
selecting  a  suitable  habitat  for  permanent  settlement.  Berrill  postulates 
that  at  a  later  time,  and  as  an  adaptation  for  exploiting  the  rich  pas- 
ture of  oceanic  surface  waters,  certain  of  these  larvae  became  neotenic. 
That  is,  they  matured  sexually  but  retained  the  other  larval  features  and 
failed  to  undergo  metamorphosis.  Contemporary,  pelagic  tunicates  of  the 
class  Larvacea  have  unquestionably  evolved  through  neoteny,  so  this  is  a 
reasonable  proposal.  Certain  of  these  neotenic  tadpoles  came  to  exploit 
the  rich  detritus  at  river  mouths.  An  increase  in  size  and  in  powers  of 
locomotion,  particularly  the  evolution  of  a  segmented  muscular  system, 
would  have  enabled  them  to  overcome  the  current  and  ascend  the 
rivers.  The  segmentation  of  certain  chordates  and  of  annelids  and  arth- 
ropods is  therefore  attributed  to  the  independent  evolution  of  increased 
activity  in  unrelated  lines  of  descent.  Berrill  believes  that  vertebrates 
gradually  evolved  in  this  way  as  a  fresh-water  adaptation  of  neotenic 
tunicate  larvae.  He  considers  At?iphioxiis  to  be  a  relic  of  a  phase  in  which 
chordates  were  becoming  more  active  and  entering  fresh  water,  but  that 
it  has  subsequently  readapted  to  the  life  of  a  marine  filter-feeder. 

It  seems  likely  that  vertebrates  evolved  from  soft-bodied  forms,  and 
the  ancestral  fossils  may  never  be  found.  Thus  direct  paleontologic  evi- 
dence bearing  on  Berrill's  and  other  theories  of  the  origin  of  chordates 
may  never  be  available. 

Questions 

1.  How  does  the  nerve  cord  of  chordates  differ  from  that  of  nonchordates? 

2.  What  is  the  function  of  the  notochord? 

3.  What  was  the  primitive  function  of  the  gill  slits? 

4.  Briefly  characterize  each  of  the  chordate  subphyla. 

5.  Compare  the  method  of  feeding  of  Molgula  and  Amphioxus. 

6.  List  the  eight  classes  of  vertebrates  and  give  an  example  of  an  animal  that  belongs  to 
each  one. 

7.  Make  a  diagram  of  a  generalized  vertebrate  showing  the  arrangement  of  the  major 
organs  and  the  features  that  distinguish  it  from  other  chordates. 

8.  To  which  group  of  nonchordates  are  chordates  most  closely  related?  What  is  the 
evidence? 

Supplementary  Reading 

Excellent  accounts  of  the  lower  chordates  can  be  found  in  Parker  and  Haswell, 
Text-Book  of  Zoology,  and  in  Young,  Life  of  Vertebrates.  An  older,  yet  very  valuable 
reference  is  Delage  and  Herouard,  Traite  de  Zoologie  Concrete,  Vol.  8,  Les  Protocordes. 
The  urochordates  are  discussed  thoroughly  and  interestingly  by  Berrill  in  his  books.  The 
Tunicata  and  The  Origin  of  Vertebrates. 


Part  III 

VERTEBRATE    LIFE 
AND   ORGANIZATION 


CHAPTER  21 


The  Frog — A  Representative  Vertebrate 


The  vertebrates  will  be  considered  more  fully  than  any  other  gioup  of 
animals  because  a  knowledge  of  their  biology  is  particularly  important 
for  an  appreciation  of  human  form  and  function.  The  frog  is  selected  as 
an  example  of  the  vertebrates  because  of  its  availability,  ease  of  study 
and  importance  in  zoological  research.  It  is  not  the  most  representative 
of  vertebrates;  indeed  no  single  type  can  be  truly  representative  of  so 
diverse  a  subphylum.  As  a  member  of  the  class  Amphibia,  it  occupies  an 
evolutionary  position  between  the  primitive,  ancestral  fishes  and  the 
advanced,  terrestrial  mammals.  A  frog  retains  certain  of  the  primitive 
features  of  fishes,  yet  it  has  also  evolved  certain  of  the  features  charac- 
teristic of  the  more  advanced  terrestrial  vertebrates. 


1 79.        Frogs  and  Other  Amphibians 

Amphibians  live  both  in  water  and  in  moist  places  on  land.  The 
eggs  and  immature  individuals  are  normally  aquatic,  and  the  adults 
never  get  far  from  the  water,  for  their  ability  to  prevent  excessive  loss 
of  body  water  in  a  terrestrial  environment  is  rather  rudimentary.  The 
adults  are  found  on  the  land  close  to  ponds,  streams  and  other  bodies 
of  fresh  water  to  which  they  can  retreat,  or  in  other  moist  places  such  as 
beneath  stones  and  logs  in  damp  woods.  The  most  terrestrial  of  the 

393 


394 


VERTEBRATE   LIFE   AND    ORGANIZATION 


Figure  21.1.     The  leopard  frog,  Rana  pipiens. 

amphibians,  the  toads,  are  particularly  active  at  night  when  the  hu- 
midity is  relatively  high. 

Contemporary  members  of  the  class  are  grouped  into  three  orders. 
The  frogs  and  toads  are  placed  in  the  order  Anura.  The  other  orders 
consist  of  the  lizard-shaped,  scaleless  salamanders  (order  Urodela),  and 
the  legless,  wormlike  caecilians  of  tropical  continents  (order  Apoda).  The 
several  orders  of  extinct  amphibians  are  discussed  in  a  subsequent 
chapter  (23). 

Anurans  differ  from  the  others  in  having  powerful  hind  legs  for 
jumping  on  land  and  swimming  in  the  water.  Their  short  trunk,  the 
absence  of  a  tail,  and  the  enlarged  hind  legs  with  webbed  feet  are  among 
the  many  features  which  adapt  them  for  their  mode  of  life. 

Approximately  100  species  of  frogs  and  toads  occur  in  the  United 
States  and  Canada.  The  most  widespread  is  the  leopai-d  frog,  Rana 
pipiens  (Fig.  21.1).  This  species  is  found  throughout  North  America 
except  for  the  more  northern  parts  and  the  west  coast  of  the  continent. 
The  following  description  applies  specifically  to  Rana  pipiens,  but  most 
of  what  follows  applies  to  other  anurans  as  well. 


180.        External  Features 

The  body  of  most  terrestrial  vertebrates  can  be  divided  into  four 
regions:  a  head  containing  the  mouth,  brain  and  organs  of  special  sense; 
a  somewhat  narrower  neck  connecting  the  head  with  the  trunk;  and  a 
tail  located  posterior  to  the  anus,  or  termination  of  the  digestive  tract.  Of 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE  395 

these  only  the  head  and  trunk  are  present  in  the  frog.  A  neck  region  is 
not  evident  and  the  tail  is  lost  during  embryonic  development. 

A  large  mouth  is  located  at  the  anterior  end  of  the  head  and  a  pair 
of  external  nostrils,  or  external  nares,  is  dorsal  to  the  front  of  the 
mouth.  The  large  and  protruding  eyes  are  protected  by  eyelids.  The 
upper  one  is  a  simple  skin  fold;  the  lower  one  is  a  translucent  mem- 
brane. When  the  eyeball  is  retracted  into  the  eye  socket,  the  lower  lid 
spreads  over  its  surface.  Between  and  in  front  of  the  eyes  on  the  top  of 
the  head  is  a  light-colored  spot  about  the  size  of  a  small  pinhead.  It  is 
known  as  the  brow  spot,  and  is  a  vestige  of  the  median  eye  of  very 
primitive  vertebrates.  A  round  eardrum,  or  tympanic  membrane,  lies 
posterior  to  each  eye.  It  is  noticeably  larger  in  the  males  than  in  the 
females  of  some  common  frogs  such  as  the  green  frog  {R.  clamitans)  and 
bullfrog  {R.  catesbiana),  but  not  in  R.  pipiens. 

The  forelegs  {pectoral  appendages)  are  much  shorter  than  the  hind 
legs  (pelvic  appendages)  and  do  little  more  than  hold  up  the  front  of 
the  body;  the  powerful  hind  legs  are  the  main  organs  of  locomotion. 
Comparisons  can  easily  be  made  between  the  frog's  appendages  and  our 
own,  for  they  consist  of  the  same  parts,  but  several  differences  in  details 
will  be  observed.  Only  four  fingers  (digits)  are  present  on  the  hand  of 
the  frog,  for  the  first  digit,  i.e.,  the  thumb,  is  missing.  The  most  medial 
digit  (which  phylogenetically  is  the  second)  is  stouter  in  the  males  than 
in  the  females  of  many  species  of  frogs,  especially  during  the  breeding 
season,  and  helps  the  male  to  grasp  the  female.  Five  digits  are  present 
in  the  foot,  the  most  medial  being  the  first,  the  equivalent  of  our  own 
big  toe.  A  membranous  web  extends  between  the  toes.  A  small  spurlike 
digit  known  as  the  prehallux  is  located  medial  to  the  base  of  the  first 
typical  toe.  Two  of  the  ankle  bones  are  elongated,  so  the  foot  is  very 
long.  This  increases  the  le\erage  for  jumping  and  swimming. 

An  anus,  or  cloacal  aperture,  is  located  at  the  posterior  end  of 
the  trunk.  This  opening  is  best  called  a  cloacal  aperture  in  the  frog,  for 
a  cloaca  (a  chamber  receiving  the  products  of  the  digestive,  excretory 
and  genital  tracts)  is  present.  Strictly  speaking,  the  anus  is  the  posterior 
opening  of  the  digestive  tract  only. 

181.        Skin  and  Coloration 

The  soft,  smooth,  moist  skin,  or  integument,  is  more  complex  than 
one  might  suspect.  It  serves  for  protection,  sensory  reception  and  for  gas 
exchange  between  the  organism  and  its  environment.  The  integument 
consists  of  two  layers  of  tissue— a  superficial  epidermis  and  a  deeper  and 
much  thicker  dermis  (Fig.  21.2).  The  epidermis  is  composed  of  stratified 
squamous  epithelium,  whose  basal  cells  are  columnar  in  shape.  These 
cells  proliferate  actively  by  mitosis  and  this  portion  of  the  epidermis  is 
known  as  the  stratum  germinativum.  Newly  formed  cells  move  outward, 
are  flattened  through  various  pressures,  accumulate  some  horny  material 
(only  a  small  amount  in  frogs),  eventually  die,  and  are  finally  sloughed 
off  in  large  sheets.  The  outer,  somewhat  horny  layer  of  the  epidermis  is 
known  as  the  stratum  corneum. 


396 


VERTEBRATE   LIFE   AND    ORGANIZATION 


Strat  u.  ni 

corneu.m. 
Slra-tum 


Oermina.tivuin 
Chrotnatophora 

Mucous  ^lancL 


Blood  ve-ssal 


-Epidermis 


>- Dermis 


■  Nerve.  1 


•   .^ 


Figure  21.2.     A  photomicrograph  of  a  vertical  section  through  the  skin  of  a  frog. 

The  dermis  consists  of  fibrous  connective  tissue.  The  fibers  in  the 
deep  portion  are  more  regularly  arranged  and  more  tightly  packed  than 
those  immediately  beneath  the  epidermis.  The  deeper  layer  of  the 
dermis,  which  commonly  contains  a  few  smooth  muscle  fibers,  constitutes 
the  stratum  compactum,  whereas  the  more  superficial  layer  is  known 
as  the  stratum  spongiosum.  Blood  vessels,  nerves  and  simple  sense  organs 
are  found  throughout  the  dermis.  They  come  close  to  the  epidermis,  but 
only  a  few  naked  nerve  processes  actually  enter  this  layer. 

The  stratum  spongiosum  contains  many  alveolar  glands,  which 
consist  of  simple,  round  sacs  of  cells  that  have  pushed  into  the  dermis 
from  the  epidermis.  They  have  an  epithelial  wall  and  a  cavity  or  lumen 
which  remains  connected  to  the  surface  by  a  duct.  The  most  numerous 
glands  are  mucous  glands,  whose  secretion  is  a  slimy  mucus  that  is 
discharged  over  the  surface  of  the  body  where  it  helps  to  protect  the  frog 
against  desiccation  and  excessive  water  entrance.  A  few  poison  glands 
are  found  in  certain  areas  of  the  skin,  notably  in  the  dorsolateral  folds 
in  Rana  pipiens.  These  are  larger  and  produce  a  watery  secretion  that 
is  presumed  to  be  distasteful  and  irritating  to  certain  of  the  frog's 
predators. 

Frog  skin  is  richly  colored.  In  Raiia  pipiens,  the  general  greenish 
tone  blends  with  the  surroundings,  while  the  darker  spots  and  blotches 
tend  to  obscure  the  form  of  the  animal.  This  concealing  coloration 
presumably  helps  the  frog  elude  its  predators  and  stalk  its  prey. 

Most  of  the  pigment  and  refractive  granules  responsible  for  the 
coloration  are  contained  within  stellate  cells  known  as  chromatophores, 
which  are  concentrated  just  beneath  the  epidermis.  Some  chromato- 
phores (melanophores)  contain  a  brown  to  blackish  pigment,  some 
(lipophores)  a  yellowish  to  reddish  pigment,  and  some  (guanophores) 
refractive  granules  of  guanine.  There  is  no  green  pigment  in  frog  skin. 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE  397 

The  lipophores  reflect  yellow  light  back  through  the  epidermis.  The  rest 
of  the  light  penetrates  to  the  guanophores,  is  dispersed,  and  blue  light  is 
reflected  back.  The  remaining  light  rays  are  absorbed  by  the  melano- 
phores.  Yellow  and  blue  light  reflected  back  together  result  in  a  greenish 
color. 

Changes  in  the  general  color  tone  of  the  skin  are  effected  by  the 
migration  of  pigment  within  the  melanophores.  W'hen  the  skin  darkens, 
pigment  streams  out  into  the  processes  of  these  cells,  some  of  which 
mask  the  guanophores;  when  it  becomes  paler,  the  pigment  concentrates 
near  the  center  of  the  melanophores.  It  is  the  pigment  that  migrates; 
the  processes  of  the  melanophores  remain  extended.  The  movement 
of  the  pigment  is  controlled  in  part  by  the  hormone  intermedin  secreted 
by  the  pituitary  gland  (page  622). 

182.        Skeleton 

The  skeleton  of  vertebrates  forms  the  supporting  framework  of  the 
body,  provides  a  point  of  attachment  for  most  of  the  muscles,  and  encases 
and  protects  much  of  the  delicate  nervous  system. 

The  somatic  skeleton  is  the  skeleton  of  the  "outer  tube"  of  the 
body  and  is  located  in  the  body  wall  and  appendages.  It  includes  an 
axial  portion  lying  in  the  longitudinal  axis  of  the  body  (vertebral  col- 
umn, sternum  and  most  of  the  skull),  and  an  appendicular  portion 
supporting  the  paired  appendages  (girdles  and  limbs).  The  visceral 
skeleton  is  the  skeleton  of  the  "inner  tube"  of  the  body,  and  is  associated 
with  the  anterior  part  of  the  digestive  tract.  It  is  prominent  in  fish  where 
it  supports  the  gills  and  helps  to  form  and  sujiport  the  jaws.  In  ter- 
restrial vertebrates  it  is  reduced,  but  parts  of  it  remain  associated  with 
the  jaws,  and  parts  become  associated  with  the  ear,  tongue  and  larynx. 

Skull  and  Hyoid.  The  anterior  end  of  the  axial  skeleton,  together 
with  certain  parts  of  the  visceral  skeleton,  forms  the  skull,  a  complex  of 
bone  and  cartilage  encasing  the  brain  and  major  sense  organs,  and 
forming  the  jaws.  The  central  portion  of  the  skull  surrounding  the  brain 
is  known  as  the  cranium;  its  more  peripheral  parts  constitute  the  facial 
skeleton  (Figs.  21.3  and  21.4).  The  nasal  cavities  are  situated  near  the 
front  of  the  skull;  a  pair  of  large  openings  for  the  eyes,  orbits,  lie  lateral 
to  the  middle  of  the  cranium;  and  the  inner  part  of  the  ears,  containing 
the  receptive  cells,  lie  in  posterolateral  extensions  of  the  cranium  known 
as  the  otic  capsules.  A  slender  bony  rod,  the  stapes,  extends  laterally 
from  each  otic  capsule.  It  is  a  part  of  the  visceral  skeleton  which  has 
become  modified  to  transmit  vibrations  from  the  tympanic  membrane 
to  the  inner  ear.  The  spinal  cord  passes  through  a  large  hole,  foramen 
magnum,  at  the  posterior  end  of  the  cranium.  A  pair  of  rounded  bumps, 
occipital  condyles,  lie  ventrolateral  to  the  foramen  and  articulate  the 
skull  with  the  vertebral  column. 

The  upper  jaw  bears  teeth  along  its  margin  and  two  patches  of 
vomerine  teeth  are  borne  by  the  vomer  bones  in  the  roof  of  the  mouth, 
but  the  lower  jaw  lacks  teeth.  The  jaw  joint  lies  between  a  quadrate 
cartilage  of  the   upper  jaw  and  Meckel's   cartilage   of  the   lower  jaw; 


398 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  21.3.  A  dorsal  view  of  the  frog's  skeleton.  Major  cartilaginous  areas  are 
stippled  in  this  and  in  other  drawings  of  the  skeleton.  Roman  numerals  refer  to  digit 
numbers.  (After  Parker  and  Haswell.) 

both  are  parts  of  the  visceral  skeleton.  Most  other  skull  bones  are  of 
axial  origin.  The  names  of  major  ones  are  shown  in  Figures  21.3 
and  21.4. 

The  greater  part  of  the  visceral  skeleton  is  incorporated  in  the 
hyoid  apparatus— a  plate  of  cartilage  and  bone  that  supports  the  floor 
of  the  mouth  and  the  base  of  the  tongue. 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE 


399 


Vertebral  Column.  The  vertebral  column,  which  forms  a  firm  yet 
movable  support  for  the  trunk,  is  a  part  of  the  axial  skeleton.  It  is 
unusually  short  in  frogs,  consisting  in  most  species  of  only  nine  verte- 
brae, jilus  an  elongate  terminal  piece  known  as  the  urostyle  (Fig.  21.3). 
The  urostyle  represents  an  uncertain  number  of  caudal  vertebrae  fused 
together  and  specialized  for  the  attachment  of  powerful  pelvic  muscles. 
The  short,  compact  vertebral  column  is  adapted  for  the  frog's  jumping 
mode  of  progression. 

A  representative  vertebra  consists  of  a  ventral,  spool-shaped  centrum, 
and  a  dorsal  neural  arch  enclosing  the  neural  canal,  in  which  the  spinal 
cord  lies.  The  neural  arch  bears  a  pair  of  prominent,  broad,  lateral 
extensions  called  transverse  processes,  a  small  mid-dorsal  neural  spine, 
and  an  articular  process,  or  zygapophysis,  on  each  dorsal  corner.  The 


mwMmmmMMMMMMmmmm. 

Figure  21.4.     A,  A  ventral  view  of  the  frog's  skull;  B,  a  ventral  view  of  the  lower 
jaw  and  hyoid  apparatus.  (After  Gaupp.) 


400 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  21.5.     Girdles  of  the  frog.  A,  A  ventral  view  of  the  sternum  and  pectoral 
girdle;  B,  a  lateral  view  of  the  pelvic  girdle.  (Modified  after  Gaupp.) 


transverse  process  represents  a  true  vertebral  process  fused  with  a  short, 
rudimentary  rib.  Free  ribs  articulating  movably  with  the  vertebrae  are 
absent  in  the  adults  of  all  but  a  few  very  primitive  species  of  frogs. 
Foramina  for  the  passage  of  the  spinal  nerves  are  found  laterally  between 
successive  vertebrae. 

The  most  anterior  vertebra,  known  as  the  atlas,  is  modified  for 
articulating  with  the  skull  and  lacks  transverse  processes.  The  vertebra 
preceding  the  urostyle,  the  sacral  vertebra,  is  also  modified  with  unusu- 
ally large  transverse  processes,  for  supporting  the  pelvic  girdle. 

Appendicular  Skeleton.  The  sternimi,  though  a  part  of  the  axial 
skeleton,  is  intimately  associated  with  the  pectoral  girdle.  Sternum  and 
girdle  together  form  an  arch  of  bone  and  cartilage  that  nearly  encircles 
the  front  of  the  trunk  and  supports  the  pectoral  appendages.  Each  half 
of  the  pectoral  girdle  (Fig.  21.5)  consists  of  two  bones  extending  lat- 
erally from  the  midventral  line,  an  anterior  clavicle  and  a  posterior 
coracoid.  Clavicle  and  coracoid  of  opposite  sides  are  connected  by  a 
narrow  strip  of  cartilage.  Another  bone,  the  scapula,  extends  dorsally 
from  the  lateral  end  of  these.  The  concavity  where  these  three  meet, 
known  as  the  glenoid  fossa,  articulates  with  the  humerus,  the  bone  of 
the  upper  arm.  A  partly  ossified  suprascapula  lies  dorsal  to  the  scapula 
and  folds  over  the  back  of  the  animal.  Only  muscles  bind  the  pectoral 


THE  FROG — A    REPRESENTATIVE   VERTEBRATE  401 

girdle  to  the  trunk,  for  there  is  no  direct  connection  between  the  girdle 
and  the  vertebral  column. 

The  sternum  is  divided  into  four  midventral  pieces,  two  of  which 
extend  anteriorly  from  the  clavicles  and  two  posteriorly  from  the  cora- 
coids.  The  terminal  piece  at  each  end  is  unossified. 

The  forelimb  is  composed  of  a  humerus  extending  from  the  shoulder 
to  the  elbow  joint;  a  radio-ulna  (fusion  of  a  radius  and  ulna)  continuing 
to  the  wrist  joint;  a  series  of  small  wrist  bones,  the  carpals,  lying  in  the 
proximal  part  of  the  hand;  four  long  metacarpals  in  the  region  of 
the  palm;  and  a  series  of  small  segments  known  as  phalanges  in  each 
of  the  four  digits.  Although  the  first  finger  is  not  apparent  in  an  entire 
frog,  its  vestigial  metacarpal  can  often  be  seen  in  the  skeleton. 

The  pelvic  girdle  is  attached  to  the  sacral  vertebra  and  provides  a 
solid  support  for  the  pelvic  appendages  (Fig.  21.5).  Each  side  of  the 
girdle  consists  of  a  long  ilium  extending  posteriorly  from  the  sacrum  to 
the  ischium  and  pubis.  The  ventral  pubis  is  unossified.  A  concavity,  the 
acetabulum,  is  situated  where  the  three  join,  and  serves  for  the  articula- 
tion of  the  hind  limb. 

The  femur  extends  from  the  acetabulum  to  the  knee,  and  a  fused 
tibio-fibula  from  the  knee  to  the  ankle  joint.  Ankle  bones,  the  tarsals, 
form  the  proximal  part  of  the  foot.  These  are  followed  by  five  meta- 
tarsals in  the  region  of  the  sole,  and  a  series  of  phalanges  in  each  digit. 
The  frog  foot  is  unusual  in  that  the  two  proximal  tarsals  are  elongated 
and  form,  in  effect,  an  extra  segment  to  the  limb.  These  elongated 
tarsals  are  followed  distally  by  two  small  and  inconspicuous  ones.  A 
bone  called  the  calcar  supports  the  prehallux.  The  fusion  of  the  radius 
and  ulna  and  of  the  tibia  and  fibula,  and  the  extra  leverage  provided 
by  the  elongation  of  the  tarsals,  are  adaptions  for  jumping. 

183.        Muscular  System 

Smooth  muscles  are  found  in  the  walls  of  many  visceral  organs, 
cardiac  muscles  in  the  wall  of  the  heart  and  striated  muscles  attach  to 
the  skeleton.  The  striated  muscles,  which  are  generally  under  voluntary 
control,  form  the  bulk  of  the  muscular  system.  Most  of  these  are  attached 
to  bones  by  tendons.  The  origin  of  the  muscle  is  its  fixed  end;  the 
insertion  is  the  end  attached  to  the  structure  that  moves  when  the  muscle 
contracts.  The  origin  is  generally  the  end  nearer  the  longitudinal  axis  of 
the  body,  or,  in  the  case  of  longitudinal  muscles,  the  more  anterior 
end;  the  insertion  is  the  peripheral  or  posterior  end  (Fig.  21.6). 

Muscles  can  induce  movement  only  by  contracting  or  shortening, 
hence  the  muscles  of  the  body  are  grouped  into  antagonistic  sets.  One 
set  of  muscles  is  responsible  for  moving  a  part  in  one  direction,  whereas 
movement  in  the  opposite  direction  entails  the  relaxation  of  the  first 
set  of  muscles  and  the  contraction  of  an  antagonistic  set  on  the  opposite 
side  of  the  part.  Various  terms  are  used  to  describe  movement  in  dif- 
ferent directions.  For  example,  flexion  is  the  bending  of  a  joint  with  a 
consequent  diminishing  of  the  angle  between  the  bones,  as  occurs  at  the 
knee  or  elbow;  extension  is  the  opposite  movement,  i.e.,  a  straightening. 


402  VERTEBRATE   LIFE   AND   ORGANIZATION 

'Femixr 


Gastrocnemius 
ori 


Tibiofibula 


Tibialis 
a.niicus 
lon^us 


*^  Tibiofibtrla.-iarsai 
■joint  =  Fulcrum,  (a) 


Fulcrum. 

Point  of  cLction 
of  muscles 


Point  of 
exertion  of 
force 


Gastrocnemius^ 
insertionCb) 

Meta-taj-sals 


Distance  aJp  x  pull  of  muscles 
=  Distance  a.c  x  force  exerted 


.Point  of  exertion 
of  force  (c) 


Figure  21.6.  A,  A  diagram  to  illustrate  the  antagonistic  action  of  muscles  on  the 
frog's  right  hind  foot.  The  gastrocnemius  moves  the  foot  in  the  direction  of  the  solid 
arrow;  the  tibialis  amicus  in  the  direction  of  the  shaded  arrow.  B,  A  comparable  lever 
system. 

The  forward  movement  of  the  entire  limb  at  the  hip  or  shoulder  is  some- 
times also  called  flexion,  but  protraction  is  a  more  appropriate  term. 
Retraction  is  the  opposite  movement.  Adduction  is  a  movement  that 
brings  the  distal  end  of  an  appendage  toward  the  midventral  line  of 
the  body;  abduction,  away  from  the  midventral  line. 

Most  of  the  muscles  are  attached  to  the  bones  in  such  a  way  that  the 
fulcrum  is  at  one  end  of  the  lever,  and  the  muscle  attachment  is  nearer 
the  fulcrum  than  the  point  at  which  the  lever  exerts  its  force  (Fig.  21.6). 
Such  levers  are  mechanically  inefficient,  but  this  arrangement  provides 
for  compactness  and  speed  of  movement. 

The  superficial  skeletal  muscles  of  the  frog  are  shown  in  Figure  21.7. 


1 84.        Body  Cavity  and  Mesenteries 

The  internal  organs  of  the  frog  protrude  into  the  body  cavity,  or 
coelom,  which  contains  a  small  amount  of  watery  coelomic  fluid.  The 
space  and  fluid  facilitate  the  expansion,  contraction,  and  slight  move- 
ment of  the  organs  in  relation  to  each  other.  The  coelom  is  divided  into 
an   anterior    pericardial    cavity    containing   the    heart,    and    a    posterior 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE 


403 


Fle^coT 
digitorura 
brevis 


Pyrifo 
Seraime-mtr  a.Tio  sus 


eltoideiAS 

Forearm 
flexors 

and 
xte,nsors 
Triceps 
Pectora.liS 
orsalis  scapulae 
atissixnus  d.orsa.p. 
n6issimu.s  dorsa.e 
Oblic[u.us  e-Dcternus 

tuS  aixiominis 

ctu-S  a.nticu.S 

stu.^  intcrriTx 
Vastus  e.xtcrnu.s 

Sartor 

lUofitu., 

Qracil 
minor 

Gastrocnemius 
Tibialis  a.nt 
Peroncus    j^g^ais 

Abductor  brevis 


^^Adductor 

ma^nuS 

Adductor  lon^uS 

racilis  major 

ibialis  posticus 


Tarsalis 
posticus 


Figure  21.7.     Superficial  skeletal  muscles  of  the  frog  in  a  dorsal  (left  side  of  figure) 
and  a  ventral  (right  side)  view. 

pleuroperitoneal  cavity  containing  the  other  visceral  organs.  The  coelom 
is  hned  with  a  thin  layer  ot  epithelium.  The  internal  organs  have  pushed 
into  the  coelom  (Fig.  21.8)  and  are  covered  by  a  layer  of  coelomic 
epithelium  called  the  visceral  peritoneum.  The  visceral  peritoneum  is 
continuous  with  the  parietal  peritoneum  lining  the  body  wall  by  way 
of  thin,  double-layered  mesenteries  which  support  the  internal  organs. 
Blood  vessels  and  nerves  pass  through  the  mesenteries  in  gomg  from 
the  body  wall  to  the  visceral  organs.  Relations  in  the  pericarc^ial  cavity 
are  much  the  same,  but  mesenteries  are  absent  in  the  adult.  1  he 
coelomic  epithelium  here  is  called  the  visceral  and  parietal  pericardium. 


404 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Subcutaneous 
lymph  Sa.C 

Dorsal  a-orta 

Wolffian 
duct 

Kidney 

StomacK 
Pancreas 


S.  common 
bile  duct 

Liver 
Visceral  peritone- 


Dorsolateral  sktrLfold 
Spinal  cord 

3ubverfcebral 

lymph  Sac 

Post.  vena_cava. 
Testis 


Mesenteries 

Small 
inttsbine 

Duodenum 

Parietal 

peritoneum. 


Plearoperitoneal  Cavity 


Ventral  a.bdominaLvein. 


Figure  21.8.  A  diagrammatic  cross  section  through  the  trunk  of  a  frog  viewed  from 
behind.  At  a  more  anterior  level,  a  mesentery  would  pass  to  the  stomach  rather  than 
to  the  intestine. 


1 85.        Digestive  System 

Adult  frogs  are  carnivorous  and  feed  upon  any  animal  small  enough 
for  them  to  catch  and  swallow— worms,  crustaceans,  insects  and  the  like. 
Many  of  these  are  captured  by  a  flick  of  the  tongue,  which  is  covered  by 
a  sticky  secretion.  In  this  process  the  back  of  the  tongue  vaults  over  the 
front,  for  the  tongue  is  attached  anteriorly  (Fig.  21.9).  The  tongue  is 
protruded  by  muscular  action  and  by  a  sudden  filling  of  a  lymph  sac 
at  its  base  Food  is  held  in  the  mouth  by  the  teeth  and  then  swallowed 
whole.  A  lubricating  mucous  secretion,  the  tongue,  the  beating  of  micro- 
scopic cilia  on  the  cells  lining  the  mouth  cavity,  and  an  inward  move- 
ment of  the  eyes  all  aid  in  swallowing. 

From  the  mouth  cavity  proper  the  food  passes  through  the  pharynx 
(back  of  the  apparent  mouth  cavity  where  food  and  air  passages  cross) 
into  a  narrow  esophagus.  The  esophagus  is  a  short  tube  leading  to  the 
stomach,  where  food  is  temporarily  stored  and  its  digestion  initiated. 
The  stomach  terminates  in  a  muscular  valve,  the  pyloric  sphincter. 
From  the  stomach  a  segment  of  the  small  intestine  known  as  the  duo- 
denum passes  anteriorly,  receiving  secretions  from  the  liver  and  pancreas 
by  way  of  a  common  bile  duct.  The  remainder  of  the  small  intestine 
continues  posteriorly  in  a  number  of  convolutions,  finally  emptying  into 
the  large  intestine,  or  colon.  Digestion  is  completed  in  the  intestine 
and  the  food  is  absorbed  into  the  circulatory  system.  The  large  intestine 
narrows  posteriorly  before  entering  the  cloaca— a  chamber  receiving  the 
products  of  the  digestive,  excretory  and  genital  systems.  The  cloaca 
opens  on  the  body  surface  through  the  cloacal  aperture. 

The  basic  histology  of  the  alimentary  canal  can  be  seen  to  advan- 
tage in  a  cross  section  through  the  anterior  part  of  the  stomach  (Fig. 
21.10).  Progressing  from  the  coelom  toward  the  lumen  there  is  (1)  the 
visceral  peritoneum,  or  serosa,  consisting  of  a  single  layer  of  squamous 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE 


405 


epithelium  supported  by  fibrous  connective  tissue;  (2)  two  layers  of 
smooth  muscle— a  much  reduced  (in  the  stomach)  outer,  longitudinal 
layer  in  which  the  fibers  more  or  less  parallel  the  long  axis  of  the  gut, 
and  a  thick,  inner  circular  layer  with  fibers  nearly  at  right  angles  to  the 
preceding;  (3)  a  layer  of  highly  vascular,  fibrous  connective  tissue  known 
as  the  submucosa;  and  finally  (4)  the  mucosa,  or  mucous  membrane. 
Movement  of  the  food  within  the  stomach  and  along  the  intestine  is 
accomplished  by  rhythmic  waves  of  contraction  of  the  muscle  layers, 
which  are  known  as  peristalsis. 

The  mucosa  consists  of  thin  layers  of  longitudinal  and  circular 
muscles  (muscularis  mucosae)  next  to  the  submucosa,  plus  connective 
tissue  and  the  simple  columnar  epithelium  lining  the  lumen.  The  epi- 
thelium contains  numerous  mucus-secreting  goblet  cells,  and  is  invagi- 
nated  to  form  many  gastric  pits.  From  the  base  of  each  pit  one  or  two 


Vomerine  teeth 


Small  intestine 
Mesentery 


Al/ 


Laryngotrdch^dl  chaabtT 
Eustachidn , 

Esophagus 


Hepatic  duct 
Pancreatic  duct 

Stomach 
Common  bile  duct 

Duodenum 
Pyloric  sphincter 

Urinary  bladder 


aperture  (dorsal) 


Figure  21.9.  A  \ential  view  of  the  frog's  digestive  system.  Ihe  liver  lobes  have 
been  turned  forward  to  show  the  gallbladder.  Tongue  action  is  shown  in  the  insert. 
(Insert  after  Gadow.) 


406 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Mtise-ntiiry 


Serosa 


LoTT^itudiiial 
muscle 


Bloodvessel 

Mxiscu]a.ris 
mucoSa.c 


Mucosa.- 
iSubmitc  osa." 


Gaslricoland- 


Muscularis 


'Circular 
muscle. 


Sabmucosa. 


"Miicosa 


B 


Figure  21.10.  Diagrams  of  cross  sections  through  the  frog's  stomach.  A,  Low  mag- 
nification; B,  an  enlargement  of  tlie  segment  of  the  preceding  lying  between  the  dotted 
lines. 

narrow,  tubular  gastric  glands  continue  toward  the  muscularis  mucosae. 
These  glands  contain  several  large,  clear,  mucus-producing  cells  and 
other  secretory  cells  filled  with  granules.  The  protein-splitting  enzyme 
pepsin  is  secreted  by  these  glands  in  the  anterior  part  of  the  stomach  and 
the  adjacent  portion  of  the  esophagus,  but  hydrochloric  acid,  needed  to 
activate  pepsin,  is  secreted  by  glands  in  more  posterior  parts  of  the 
stomach. 

Multicellular  glands  are  absent  from  the  mucosa  of  the  frog's  intes- 
tine. The  intestine  receives  digestive  juices  from  the  liver  and  pancreas, 
and  into  a  laryngotracFieal  chamber  (comparable  to  the  larynx  and 
The  intestinal  mucosa  is  thrown  into  many  longitudinal  and  transverse 
folds  which  slow  up  the  passage  of  the  food  and  increase  the  digestive 
and  absorptive  surface. 

The  pancreas  and  liver  are  large  glands  that  develop  embryonically 
as  outgrowths  from  the  intestine.  The  pancreas  produces  a  variety  of 
enzymes  that  are  discharged  through  a  pancreatic  duct  into  the  common 
bile  duct.  Certain  of  its  cells  also  produce  the  hormones  insulin  and 
glucagon  (p.  615).  The  liver's  secretion,  known  as  bile,  leaves  the  liver 
through  hepatic  ducts,  is  stored  temporarily  in  the  gall  bladder,  then 
is  discharged  into  the  intestine  through  the  cystic  and  common  bile 
ducts.  Bile  contains  no  digestive  enzymes,  but  its  bile  salts  emulsify  fats 
and  aid  in  their  absorption.  In  addition,  the  liver  has  an  important  role 
in  determining  the  concentration  of  certain  constituents  of  the  blood. 


1 86.       Respiratory  System 

The  respiratory  system  of  the  frog  includes  the  skin  and  the  mucous 
membranes  lining  the  mouth  and  pharynx  as  well  as  the  lungs.  AD  of 
these  are  moist,  vascular,  semipermeable  membranes  exposed  to  the  en- 
vironment, through  which  gases  can  diffuse  in  both  directions  between 
the  blood  and  the  environment.  Lungs  are  the  characteristic  organs  of 


THE  FROG — A    REPRESENTATIVE   VERTEBRATE 


407 


terrestrial  vertebrates  for  gas  exchange;  the  delicate  respiratory  mem- 
brane in  the  lungs  is  protected  against  excessive  desiccation,  yet  is  acces- 
sible to  the  external  environment. 

Air  is  taken  into  the  body  through  the  paired  external  nares, 
traverses  the  short  nasal  cavities,  and  enters  the  front  of  the  mouth 
through  the  paired  internal  nares  (Fig.  21.11).  It  then  passes 
through  the  glottis,  a  slit-shaped  opening  in  the  floor  of  the  pharynx, 
and  into  a  laryngotracheal  chamber  (comparable  to  the  larynx  and 
trachea  of  higher  vertebrates).  Small  cartilages,  which  are  homologous 
with  parts  of  the  visceral  skeleton  of  fish,  support  this  chamber,  and  a 
pair  of  bronchi  lead  from  its  posterior  corners  to  the  lungs.  The  lungs 
of  frogs  are  simple,  ovoid  sacs  in  external  shape,  but  their  internal  sur- 
face is  increased  by  numerous  pocket-shaped  folds  that  give  them  a 
honeycomb  appearance. 

Air  is  moved  into  the  lungs  by  the  pumping  action  of  the  floor  of 
the  mouth.  During  inspiration  the  floor  of  the  mouth  is  lowered  and  air 
is  drawn  into  the  mouth  and  pharynx  through  the  nasal  cavities.  The 
external  nares  are  then  closed  by  a  push  of  the  lower  jaw  on  the  movable 
premaxillary  bones.  Small  valves  present  in  the  nares  apparently  move 
passively.  The  floor  of  the  mouth  is  then  raised,  and  air  is  forced  through 
the  open  glottis  into  the  lungs.  Since  the  lungs  contain  elastic  connective 
tissue,  they  increase  considerably  in  size  as  they  fill.  Expiration  results 
from  the  elastic  recoil  of  the  lungs  and  the  contraction  of  the  abdominal 
muscles  which  compresses  the  internal  viscera  and  lungs.  Air  does  not 
enter  the  lungs  with  each  set  of  throat  movements,  however;  the  glottis 


.lusta.ch.icLn  tube 


Esoph.a.^u.$ 


Externa.1  na-ris 
'^  /Premavilla 
^  ''^    Internal 


ncLTlS 


o^wer  law 


Glottis 
Vocal  cord 

L  a.ryn^ot  ra.cheal 

chamber 
ronchus 


Figure  21.11. 
the  frog. 


A  diagrammatic   longitudinal  section  of  the   respiratory   system  of 


408  VERTEBRATE   LIFE   AND   ORGANIZATION 

is  closed  much  ot  the  time,  and  the  throat  movements  then  simply  move 
air  in  and  out  ot  the  mouth  and  pharynx  where  gas  exchange  also  occurs. 

Voice.  A  mechanism  for  sound  production  is  closely  associated 
with  the  respiratory  system.  Two  longitudinal,  elastic  bands,  the  vocal 
cords,  are  situated  in  the  laryngotracheal  chamber  near  the  glottis  (Fig. 
21.11).  Air  forced  from  the  lungs  sets  the  free  edges  of  these  cords  in 
vibration,  and  they  in  turn  vibrate  the  column  of  air  in  the  pharynx 
and  mouth.  The  pitch  of  the  sound  is  controlled  by  muscular  tension 
on  the  vocal  cords.  Some  of  the  expelled  air  inflates  the  vocal  sacs, 
which  serve  as  resonating  sacs  and  considerably  increase  the  volume  of 
the  sound.  The  vocal  sacs  may  be  paired  evaginations  from  the  lateral 
walls  of  the  pharynx,  or  there  may  be  a  single  median  vocal  sac  ventral 
to  the  floor  of  the  pharynx.  Contraction  of  muscle  fibers  in  the  wall  of 
the  vocal  sac  returns  the  air  to  the  lungs,  and  the  same  air  can  be  used 
repeatedly.  Some  frogs,  such  as  the  bullfrog,  can  even  call  from  beneath 
water. 

The  vocal  cords  are  more  prominent  in  males  than  in  females,  and 
only  the  males  have  vocal  sacs.  The  males  gather  first  in  the  breeding 
ponds  during  the  spring,  and  their  familiar  croaking  attracts  the  females 
of  the  appropriate  species.  The  females  recognize  the  voice  of  the 
males  of  their  own  species  and  come  to  them. 

187.        Circulatory  System 

The  circulatory  system  is  the  transport  system  of  the  body.  It  con- 
sists of  the  circulating  fluids,  chiefly  blood,  and  of  the  heart  and  a  series 
of  vessels  that  carry  the  fluids.  As  explained  in  Chapter  3,  blood  is  com- 
posed of  a  liquid  plasma,  in  which  red  cells,  white  cells  and  throm- 
bocytes are  suspended.  The  thrombocytes  are  spindle-shaped  cells  con- 
cerned with  blood  clotting.  The  exchange  of  materials  between  the 
blood  and  the  tissues  occurs  in  the  microscopic,  thin-walled  capillaries 
situated  between  the  arteries  and  veins.  Food,  oxygen  and  water  leave 
the  capillaries,  and  carbon  dioxide  and  other  wastes  enter  them  to  be 
removed  by  the  veins.  A  volume  of  water  nearly  equal  to  the  amount 
that  left  the  capillaries  also  reenters  them.  Some  liquid  remains  in  the 
tissues  and  is  returned  by  lymph  vessels,  which  usually  parallel  the  veins 
and  eventually  empty  into  them.  Before  connecting  with  the  veins,  some 
of  these  vessels  lead  into  lymph  sacs.  Unusually  large  lymph  sacs  lie 
ventral  to  the  vertebral  column  and  beneath  the  skin,  separating  it  from 
most  of  the  underlying  musculature  (Fig.  21.8). 

Arteries.  The  pattern  of  the  major  blood  vessels  of  the  frog  is 
shown  in  Figure  21.12.  Many,  though  not  all,  of  these  vessels  are  also 
present  in  the  higher  vertebrates,  including  man.  A  pair  of  arteries,  each 
known  as  the  truncus  arteriosus,  leave  the  front  of  the  heart.  Each  soon 
divides  into  three  vessels— carotid  arch,  aortic  arch  and  pulmocutaneous 
arch.  Each  carotid  arch  extends  anteriorly  and  divides  into  an  external 
carotid  supplying  the  tongue  and  adjacent  parts,  and  an  internal  carotid 
supplying  the  upper  parts  of  the  head  and  the  brain.  A  swelling  at  the 
base  of  the  internal  carotid,  the  carotid  gland,  is  believed  to  equalize 


THE  FROG — A    REPRESENTATIVE   VERTEBRATE 


409 


the  flow  in  the  internal  carotid.  It  contains  a  spongy  network  which  re- 
sists blood  flow  and  becomes  somewhat  distended  when  the  heart  con- 
tracts. When  the  heart  is  relaxed,  it  contracts  and  aids  the  flow. 

Each  aortic  arch  curves  dorsally  and  posteriorly,  giving  off  an  artery 
to  the  back  (the  occipitovertebral)  and  one  to  the  arms  (the  subclavian). 
The  left  and  right  arches  then  unite  to  form  a  median  dorsal  aorta  that 
continues  posteriorly,  ventral  to  the  vertebral  column.  The  aorta  sup- 
plies the  abdominal  viscera  (except  for  the  lungs),  trunk  and  hind  legs 
(Fig.  21.12).  Among  the  structures  supplied  is  the  spleen,  an  organ  in 
which  blood  cells  are  produced,  stored  and  destroyed. 

The  pulmocutaneous  arch  carries  blood  to  organs  where  gas  ex- 
change with  the  external  environment  occurs.  Each  vessel  soon  divides 
into  a  pulmonary  artery  to  the  lungs  and  a  cutaneous  artery.  The  latter 
supplies  not  only  the  skin,  but  also  much  of  the  lining  of  the  mouth 
and  pharynx. 

Veins.  The  veins  returning  blood  to  the  heart  have  a  more  com- 
plex pattern.  The  digestive  tract  and  associated  organs  are  drained  by 


Carotid  arcK" 


xternal  ca-rolid  artery 
•Internsd  carolid  artery 

Aortic  arcH 

PulmocutaileoS  s^ch. 


Anterior 
vena  Cava' 


Subclavian. ' 
vein.' 

Brachial  vein" 

Hcpati 

Musculocutarjeous  vein 

Dorsal  aorta 

Coeliacoinese-r!t«,ric  artery 

Dorsolumbar  vein 

Posterior  vena  cava' 

Ren  2*1  artery  and  vein. 

Renal  portal  vein 

Ventral  abdominalvcin 

Pelvic  vein 

Femoral  artery 
and  vein 


Occipito-.-ertebral  aortziy 

PrjUmanaxy  artery  &  vein 

Subcla-vian  arttry 

Cutaneous 

arterjr 

Hepatic  portal . 
•^         •"       vein 

Hepatic  artery 


Gastric  artery 
and  vein 

Mesenteric  arteiy  i  vein 
Splenic  cirtery  &  vein 
>^x\v//  Iliac  artery 

^Posterior  mesenteric 

ajrtery 

Sciatic  artery 
and  vein. 


Figure  21.12.  A  ventral  view  of  the  major  arteries  and  veins  of  the  frog.  Veins  are 
shown  in  black;*  arteries  are  white.  Certain  of  the  anterior  veins  have  been  omitted 
from  the  right  side  of  the  drawing  and  certain  of  the  anterior  arteries  from  the  left  side. 


410 


VERTEBRATE  LIFE  AND   ORGANIZATION 


Tr-uncuS 

a-rfceriostxs 


PolTnocutaneouS 


-Aortic 


Spiral 
va-lve 


A^rio- 

ventr-icu-liT 

value 


ABC 

Figure  21.13.  The  frog's  heart.  A,  Dorsal  view  of  the  surface  of  the  heart;  B, 
ventral  view  of  the  surface;  C,  ventral  view  of  a  dissection  of  the  heart.  L.A.,  left  atrium; 
P.V.,  pulmonary  vein;  R.A.,  right  atrium.  In  C,  blood  entering  the  ventricle  from  the 
right  atrium  is  more  darkly  shaded  than  the  blood  entering  from  the  left  atrium,  and 
the  classic  hypothesis  of  the  separation  of  the  blood  within  the  single  ventricle  is  shown. 


the  hepatic  portal  system.  Various  tributaries  from  the  viscera  unite  to 
form  a  large  hepatic  portal  vein,  which  enters  the  hver  and  breaks  up 
into  many  capillary-like  spaces  among  the  liver  cells.  Absorbed  materials, 
therefore,  pass  directly  from  the  gut  to  the  liver,  which,  as  explained  in 
Chapter  5,  has  an  important  role  in  the  metabolism  of  food.  The  liver 
receives  blood  from  the  aorta  by  the  hepatic  artery  and  is  drained  by 
hepatic  veins,  which  empty  into  the  large  posterior  vena  cava. 

Much  of  the  blood  from  the  hind  legs  and  the  back  enters  a  pair 
of  renal  portal  veins  leading  to  capillaries  within  the  kidneys.  The  kid- 
neys are  drained  by  renal  veins  which  enter  the  posterior  vena  cava.  As 
the  vena  cava  continues  forward  it  also  receives  veins  from  the  repro- 
ductive organs  and  the  liver.  Some  blood  from  the  legs  passes  through 
pelvic  veins  to  the  ventral  abdominal  vein.  This  vessel  continues  for- 
ward, draining  the  urinary  bladder  and  ventral  body  wall,  and  finally 
joins  the  hepatic  portal  as  the  latter  enters  the  liver. 

Blood  from  the  head,  shoulders  and  arms  returns  to  the  heart 
through  a  pair  of  anterior  venae  cavae  (Fig.  21.12).  Certain  of  the  tribu- 
taries of  the  anterior  venae  cavae,  e.g.,  the  musculocutaneous  vein  from 
the  skin,  and  tributaries  of  the  jugulars  from  the  mouth  lining,  come 
from  respiratory  membranes  and  carry  blood  with  a  relatively  high 
oxygen  content. 

The  lungs  are  drained  by  a  separate  pair  of  vessels,  the  pulmonary 
veins,  which  unite  and  enter  the  heart  independently  of  the  anterior 
venae  cavae. 

Heart.  The  frog's  heart  consists  of  a  series  of  chambers  having 
muscular  walls  to  force  the  blood  along  and  valves  to  prevent  its  back- 


THE  FROG — A    REPRESENTATIVE   VERTEBRATE  411 

flow  (Fig.  21.13).  A  thin-walled  sinus  venosus  receives  blood  from  the 
posterior  and  anterior  venae  cavae,  and  passes  it  into  the  right  atrium. 
The  right  atrium  receives  blood  low  in  oxygen  content  from  the  body, 
and  blood  high  in  oxygen  content  from  the  skin  and  lining  of  the 
mouth.  The  pulmonary  veins  bring  additional  oxygen-rich  blood  from 
the  lungs  to  the  left  atrium.  Both  atria  lead  into  a  single  ventricle  having 
a  thick,  muscular  wall.  The  ventricle  forces  the  blood  through  a  final 
chamber,  the  conus  arteriosus,  and  into  each  truncus  arteriosus.  A 
peculiar  spiral  valve  is  found  in  the  conus. 

Some  mixing  of  blood  from  the  two  atria  takes  place  in  the  ven- 
tricles, but  how  much  is  uncertain.  According  to  the  classic  view,  a 
slight  difference  in  the  time  of  entrance  of  the  blood  from  the  two  atria, 
the  spongy  ventricular  wall,  and  the  deflective  effect  of  the  spiral  valve 
in  the  conus  result  in  most  of  the  blood  from  the  left  atrium  passing 
into  the  carotid  and  aortic  arches,  while  most  of  the  blood  from  the 
right  atrium  passes  into  the  pulmocutaneous  arch.  More  recent  studies, 
in  which  opaque  materials  were  injected  into  the  blood  and  photo- 
graphed with  x-rays,  indicate  that  there  generally  is  little  separation  of 
the  blood  streams  in  the  ventricle,  but  that  sometimes  the  postulated 
separation  takes  place.  The  blood  in  the  right  atrium  is  partially  oxy- 
genated, for  some  of  it  has  returned  from  vessels  in  the  skin.  The 
problem  is  simply  to  what  extent  this  is  mixed  with  blood  from  the  lungs 
containing  even  more  oxygen. 

1 88.        Excretory  System 

The  skin  and  the  lungs  remove  some  waste  products  of  metabolism, 
but  the  kidneys  are  the  major  excretory  organs  and  remove  most  of  the 
nitrogenous  wastes.  They  also  help  to  maintain  the  constancy  of  the  in- 
ternal environment  by  removing  from  the  blood  substances  in  excess  and 
by  conserving  those  in  short  supply. 

The  frog's  kidneys  are  a  pair  of  elongate  organs  lying  in  the  sub- 
vertebral  lymph  sac  dorsal  to  the  pleuroperitoneal  cavity  (Figs.  21.8, 
21.14,  21.15).  They  are  composed  of  a  great  many  microscopic  kidney 
tubules  that  are  intimately  related  to  blood  entering  the  kidneys  in  the 
renal  arteries  and  renal  portal  veins.  These  tubules  are  described  more 
fully  in  Chapter  28;  briefly,  they  remove  certain  products  from  the  blood 
and  carry  them  as  urine  to  the  Wolffian  ducts.  A  Wolffian  duct,  which  is 
functionally  but  not  structurally  comparable  to  the  ureter  of  higher 
vertebrates  (section  238),  extends  along  the  lateral  border  of  each  kidney 
and  continues  to  the  dorsal  surface  of  the  cloaca.  The  urine  may  be 
discharged  directly  through  the  cloaca,  or  it  may  cross  and  enter  the 
urinary  bladder  attached  to  the  ventral  surface  of  the  cloaca.  Urine  may 
be  stored  temporarily  here  and  (especially  in  the  terrestrial  toads)  some 
water  may  be  reabsorbed. 

The  adrenal  glands  are  endocrine  glands  that  appear  as  a  pair  of 
irregular,  light-colored  bands,  one  on  the  ventral  surface  of  each  kidney. 
They  produce  a  variety  of  hormones  which  will  be  considered  in  the 
chapter  on  endocrine  glands. 


412 


VERTEBRATE   LIFE  AND   ORGANIZATION 


Fa.4;  body 


Posterior  ~i 
vena  cava. 


OvcLry 


Ovarian 
mesentery 


Entrance  oB 
oviduct 

Cloaca. 


Oviduct 


Kidney 
Adrenal 

^land 


y— Rena.1  vein 
^  Wolffian  duct 


Ovisac 
Urinary  bladder 


Entrance  of 


Wolffian  duct 


Figure  21.14.     Ventral   view  of  the  urogenital  system   of  a   female  frog.  The  left 
ovary  has  been  removed. 


1 89.        Reproductive  System 

The  reproductive  system  includes  the  gonads  which  produce  the 
gametes  (eggs  and  sperm)  and  the  reproductive  ducts  which  transfer 
the  gametes  to  the  exterior  (Figs.  21.14  and  21.15).  A  pair  of  gonads, 
testes  in  the  male  and  ovaries  in  the  female,  are  suspended  by  mesen- 
teries from  the  kidneys,  and  a  fingerlike  fat  body  is  attached  to  the 
anterior  end  of  each  gonad.  Since  the  fat  bodies  are  largest  in  the  fall 
prior  to  hibernation,  and  smallest  in  the  spring  after  mating,  it  would 
seem  that  they  serve  as  a  reserve  supply  of  food  for  the  animal,  and  in 
particular  for  the  development  of  the  gametes. 

In  the  breeding  season,  the  ripe  eggs  are  forced  out  of  the  ovary  by 
the  contraction  of  smooth  muscles  in  the  wall  of  a  saclike  follicle  which 
surrounds  each  egg  within  the  ovary.  These  muscle  fibers  are  stimulated 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE 


413 


by  a  hormone  from  the  pituitary  gland  (section  192).  This  mechanism 
for  discharging  eggs  from  the  ovary  is  quite  different  from  that  in  mam- 
mals, in  which  an  accumulation  of  liquid  within  the  follicle  causes  it 
to  rupture.  The  eggs  pass  into  the  pleuroperitoneal  cavity,  and  are 
carried  anteriorly  by  the  action  of  peritoneal  cilia  (present  only  in 
females)  toward  the  openings  (ostia)  of  the  paired  oviducts.  As  the  eggs 
are  carried  down  these  highly  coiled  tubes  by  the  beating  of  cilia  within 
the  ducts,  they  are  covered  with  several  layers  of  a  jelly-like  albumin 
secreted  by  certain  oviducal  cells.  Just  before  entering  the  cloaca,  each 
oviduct  expands  to  form  a  thin-walled  ovisac  where  the  eggs  are  stored 
for  a  short  time  until  mating  takes  place. 

Sperm  are  produced  in  numerous,  microscopic  seminiferous  tubules 
within  the  testes.  During  the  breeding  season,  under  the  stimulus  of  a 
pituitary  hormone,  the  mature  sperm  leave  the  testis  through  minute 


1  f-H^ 


f  t,  <  \  ?fs 


« 


Fai  jbody 


Testis~r 

Adrenal 

^land 

Renal  vein 

Wolffian  duct  ^ 


Entrajice  of 
Wolffiajiduct'l 


Cloaca-^ 


""'§»l  Posterior 

vena.  Cava. 

Testis 
mesentery 

*f-  Kidney 
4 
-^^Vestigial 

oviduct 


Urinary  bladder 

%r  Seminal  vesicle 

Entrance  of 

'■'%.  ve^sti^ial 

oviduct 


Figure  21.15.     Ventral  view  of  the  urogenital  system  of  a  male  frog.  The  vestigial 
oviduct  shown  in  this  figure  is  not  always  present. 


414  VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  21.16.     Leopard  frogs  in  amplexus. 

ducts,  the  vasa  efferentia,  which  cross  to  the  anterior  portion  of  the 
kidney  in  the  mesentery  supporting  the  testis.  The  vasa  efferentia  con- 
nect with  certain  of  the  kidney  tubules  through  which  the  sperm  pass 
to  the  Wolffian  duct.  The  sperm  may  be  stored  briefly  until  mating  in  a 
slight  enlargement  of  the  Wolffian  duct  known  as  the  seminal  vesicle. 
Certain  of  the  kidney  tubules  and  the  Wolffian  duct  thus  have  a  dual 
function  in  the  male— the  production  and  transport  of  urine,  and  the 
transport  of  sperm. 

Male  frogs  often  have  vestigial  oviducts  lying  beside  the  Wolffian 
ducts.  These  are  remnants  of  a  sexually  indifferent  stage  of  the  embryo 
when  rudiments  for  both  male  and  female  systems  are  present. 

During  mating,  the  male  grasps  the  female  about  her  trunk  with 
his  forelimbs,  an  embrace  termed  amplexus  (Fig.  21.16).  Then,  as  the 
female  discharges  eggs  into  the  water,  the  male  sheds  sperm.  Fertiliza- 
tion is  external.  As  the  eggs  are  laid,  the  protective  layers  of  jelly  imbibe 
water  and  swell. 

190.        Sense  Organs 

The  survival  of  an  organism  requires  that  it  respond  stiitably  to 
changes  in  the  environment.  This  entails  the  perception  of  changes  in 
the  internal  and  external  environments,  the  integration  of  this  in- 
formation, and  the  stimulation  and  coordination  of  appropriate  effectors 
—muscles,  glands,  cilia  and  chromatophores.  Although  some  sensations, 
such  as  pain,  are  detected  by  free  nerve  endings,  most  stimuli  are  re- 
ceived by  special  cells  or  groups  of  cells,  called  sense  organs,  or  re- 
ceptors. 

Receptors  for  touch,  pressure,  temperature  changes,  and  the  like 
are  widely  scattered,  but  those  for  smell,  taste,  light,  sound  and  equi- 
librium are  usually  aggregated.  The  receptors  for  smell  are  collected  in 
a  special  olfactory  epithelium   lining  part  of  the  nasal  cavities.  Those 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE 


415 


^rantum. 


AcotLstic 
nerue 

Medulla 
oblon^a.^a. 


Inner  ea-r 
(meirLbraxious  labyrinth) 

Otic  Capsule 
Tympanic  membrane 
Fenestra,  ovalis 

■pta-pes 

Middle  ear 
cavity 


Eustachian 
tu.bc 


Figure  21.17.     A  diagrammatic  cross  section  through  the  head  of  a   frog  to  show 
the  ear  and  its  relation  to  surrounding  parts. 

for  taste  are  gathered  in  taste  buds  located  on  the  tongue,  and  in  other 
parts  of  the  lining  of  the  mouth  and  pharynx. 

The  eyes  of  frogs  are  very  similar  in  basic  structure  to  those  of 
mammals,  which  are  described  in  Chapter  29,  but  the  method  of  ac- 
commodation is  different.  A  frog  focuses  on  near  objects  by  moving  the 
lens  of  the  eye  forward,  thereby  increasing  tlie  distance  between  the 
lens,  which  is  located  near  the  front  of  the  eye,  and  the  light-sensitive 
retina  located  at  the  back  of  the  eye.  The  same  thing  is  done  to  focus 
a  camera  on  near  objects.  In  the  mammalian  eye,  the  shape  of  the  lens 
is  changed  in  focusing. 

The  ears  receive  sound  vibrations  which  set  a  tympanic  membrane 
in  vibration.  The  vibrations  are  transmitted  across  a  middle  ear  cavity 
by  a  rod-shaped  bone  known  as  the  stapes  (Fig.  21.17).  This  cavity  is 
comparable  to  a  gill  pouch  of  a  fish,  which  is  connected  to  the  pharynx, 
so  it  is  not  surprising  that  it  is  connected  to  the  pharynx  by  a  Eustachian 
tube.  The  inner  end  of  the  stapes  fits  into  an  opening  in  the  otic 
capsule  known  as  the  oval  window  (fenestra  ovalis).  An  inner  ear, 
consisting  of  a  series  of  liquid-filled  canals  and  sacs,  lies  within  the 
otic  capsule.  Vibrations  of  the  stapes  are  transmitted  to  a  specific  group 
of  cells  within  the  inner  ear,  which  are  stimulated  and  initiate  impulses 
in  the  acoustic  nerve.  By  this  means  the  vibrations  are  perceived  as 
sounds.  Other  cells  in  the  inner  ear  are  stimulated  by  the  motion  of  the 
liquid  in  the  canals  and  sacs  that  is  brought  about  by  changes  in  the 
position  of  the  body.  Thus  the  inner  ear  is  concerned  with  equilibrium 
as  well  as  sound  detection. 


191.        Nervous  System 

The  various   parts   of  the  nervous  system   are   commonly  grouped 
into  a   central    nervous   system,   which   includes   the   brain   and   spinal 


416 


VERTEBRATE   LIFE   AND   ORGANIZATION 


I  Olfactory  n. 
Olfactory  bulb 


tuzmiBphjire ' 
E  Optic  nr— 

Diencephalon 

Optic  lob^0 
Pituitary  ^land- 


M^ulla. 
oblongata - 


Ciioroid  pkxus 
Spixial  <2ord~— 


Optic  tra.<| 

Mtmdilji 

1 

1 

-IIan4X« 


■First  ^pi 


Figure  21.18.     A,  A  dorsal  and  B,  a  ventral  view  of  the  brain  of  the  frog.  (Modified 
after  Gaupp.) 

cord,  and  a  peripheral  nervous  system,  which  includes  the  nerves  con- 
necting the  brain  and  cord  with  the  receptors  and  effectors  of  the  body. 
Both  the  brain  and  the  spinal  cord,  which  lie  respectively  within  the 
cranium  and  the  neural  canal  of  the  vertebral  column,  are  hollow.  A 
single,  dorsal,  tubular  nerve  cord,  you  will  remember,  is  a  diagnostic 
characteristic  of  chordates.  Within  the  brain,  parts  of  the  central  cavity 
are  expanded  to  form  large  chambers  known  as  ventricles.  All  parts  of 
the  nervous  system  are  composed  largely  of  specialized,  elongate  cells, 
the  neurons,  described  earlier  (Chapter  3). 

The  structure  of  the  brain  is  shown  in  Figure  21.18.  It  can  be 
divided  into  five  major  regions:  (1)  An  anterior  telencephalon  bears 
the  paired  olfactory  bulbs  and  rather  small  cerebral  hemispheres,  the 
latter  containing  the  first  and  second  ventricles.  (2)  An  indented  region, 
the  diencephalon,  lies  posterior  to  the  cerebral  hemispheres.  Its  lateral 
walls  constitute  the  thalamus.  The  pituitary  gland  is  attached  to  a 
part  of  the  floor  of  the  diencephalon  known  as  the  infundibulum.  An 
inconspicuous  pineal  body  extends  from  the  roof  of  the  diencephalon 
to  the  brow  spot.  Most  of  the  roof  of  the  diencephalon  is  thin  and 
vascularized,  forming  a  choroid  plexus  which  dips  into  the  third  ven- 
tricle. This  region  is  followed  by  (3)  the  mesencephalon  bearing  the 
paired  optic  lobes  containing  optic  ventricles;  (4)  the  metencephalon 
with  a  small,  dorsal,  transverse  ridge  known  as  the  cerebellum;  and 
(5)    the    myelencephalon    consisting   of    the    medulla    oblongata.    The 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE  417 

medulla  also  has  a  thin  roof  which  forms  a  choroid  plexus  dipping 
into  the  large  fourth  ventricle.  The  choroid  plexuses  secrete  a  cerebro- 
spinal fluid  which  fills  the  ventricles  and  central  canal.  Some  of  this 
fluid  escapes  via  pores  in  the  roof  of  the  medulla  to  circulate  between 
the  brain  and  cord  and  certain  of  their  meninges,  or  connective  tissue 
sheaths.  It  forms  a  protective  liquid  cushion  and  helps  nourish  the 
central  nervous  tissue. 

Ten  pairs  of  cranial  nerves  extend  from  the  brain  to  various  parts 
of  the  body.  The  first  pair  are  the  olfactory  nerves  (I),  which  bring 
impulses  from  the  olfactory  epithelium  to  the  olfactory  bulbs  and 
cerebral  hemispheres.  Fibers  in  the  optic  nerves  (11)  come  from  the 
retina,  cross  to  the  opposite  side  of  the  brain,  forming  an  optic  chiasma 
on  the  ventral  surface  of  the  diencephalon,  then  continue  as  optic 
tracts  to  end  chiefly  in  the  optic  lobes.  The  oculomotor  (HI),  trochlear 
(IV)  and  abducens  (VI)  nerves  contain  motor  fibers  to  the  muscles  that 
move  the  eyeball.  The  third  also  includes  motor  fibers  to  muscles 
within  the  eye  that  move  the  lens.  The  trigeminal  nerve  (V)  brings  in 
sensory  impulses  from  the  skin  of  the  head,  and  carries  motor  impulses 
to  the  jaw  muscles.  The  facial  nerve  (VII)  is  also  mixed,  supplying 
motor  fibers  to  certain  of  the  throat  muscles  and  to  the  tear  glands,  and 
sensory  fibers  to  the  mouth  and  pharynx.  Many  of  the  latter  innervate 
taste  buds.  The  acoustic  nerve  (VIII)  brings  impulses  from  the  inner 
ear  to  the  anterior  portion  of  the  medulla.  The  glossopharyngeal 
nerve  (IX),  like  the  facial,  conducts  sensory  impulses  from  the  mouth 
and  pharynx,  and  carries  motor  impulses  to  a  few  throat  muscles.  The 
last  of  the  frog's  cranial  nerves,  the  vagus  (X),  is  attached  to  the  side 
of  the  medulla  in  common  with  the  glossopharyngeal  nerve.  It  supplies 
motor  and  sensory  fibers  to  the  posterior  part  of  the  pharynx,  certain 
of  the  shoulder  muscles  and  most  of  the  abdominal  viscera  (heart,  lungs, 
digestive  tract). 

Like  the  entire  trunk  region  of  the  frog,  the  spinal  cord  is  short, 
and  the  number  of  spinal  nerves  is  reduced  to  ten  pairs.  Each  of  the 
spinal  nerves  is  attached  to  the  cord  by  a  dorsal  and  a  ventral  root 
(Fig.  29.10).  The  former  contains  sensory  fibers  and  an  enlargement,  the 
dorsal  root  ganglion,  in  which  the  cell  bodies  of  these  neurons  are 
located;  the  latter,  motor  fibers.  The  roots  join  peripherally  and  the 
spinal  nerves  are  mixed.  As  the  spinal  nerves  emerge  from  the  vertebral 
column,  they  are  surrounded  by  calcareous  bodies  of  uncertain  signifi- 
cance. They  are  then  distributed  to  the  trunk  and  limbs  in  the  manner 
illustrated  in  Figure  21.19.  The  first  spinal  nerve  supplies  the  tongue 
muscles.  This  nerve  is  actually  comparable  to  the  second  spinal  nerve 
of  other  vertebrates,  for  a  more  anterior  spinal  nerve  is  lost  during 
embryonic  development.  Each  spinal  nerve  has  a  ventral  branch,  the 
ramus  communicans,  which  passes  to  a  ganglionic  enlargement  on  the 
sympathetic  cord-a  pair  of  longitudinal  nerve  tracts  lying  on  each  side 
of  the  dorsal  aorta.  A  pair  of  splanchnic  nerves  extends  from  the 
sympathetic  cords  along  the  coeliacomesenteric  artery  to  the  abdominal 
viscera.    The    motor    fibers    in    the    sympathetic    cords    and    splanchnic 


418 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Glossopharyngeal 
andvagLts 


■Aortic  arches 


commun.ica.ns 


Splanchnic  nerve 

/ 

Coeliaco  mesenteric 
artery 


/ 


Dorsal  aorta 
/ 
Sympathetic  cord 

Urostyle 


Figure  21.19.     A  ventral  view  of  the  spinal  nerves  and  sympathetic  cord  lying  on 
the  right  side  of  the  vertebral  column.  (Modified  after  Gaupp.) 


nerves,  together  with  certain  of  the  motor  fibers  in  several  of  the  cranial 
nerves,  constitute  a  special  part  of  the  peripheral  nervous  system  known 
as  the  autonomic  nervous  system.  The  autonomic  system,  which  in- 
nervates visceral  organs,  blood  vessels  and  glands,  will  be  considered 
more  fully  later. 

The  nervous  system  receives  impulses  from  the  sense  organs,  in- 
tegrates them,  and  sends  out  impulses  to  appropriate  effectors.  This  is 
often  accomplished  by  simple  reflexes— stereotyped,  subconscious  re- 
sponses to  specific  stimuli.  For  example,  when  one  pinches  the  toe  of  a 
frog,  a  sensory  neuron  carries  the  impulse  into  the  spinal  cord.  Here  it 
is  transferred  by  a  short  connector  neuron  (or  perhaps  directly  by  the 
sensory  neuron)  to  a  motor  neuron  that  transmits  it  to  the  leg  muscles, 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE  419 

and  the  frog  retracts  its  leg.  The  response  to  this  kind  of  a  stimulus  is 
always  the  same,  it  happens  very  rapidly,  and  need  not  involve  much, 
or  any,  passage  of  impulses  up  and  down  the  central  nervous  system.  A 
great  deal  of  the  integration  of  the  body  is  achieved  by  reflexes  occurring 
subconsciously  either  in  the  cord  or  in  the  brain. 

Some  regions  of  the  brain  have  evolved  as  integration  centers  for 
impulses  coming  in  from  major  sense  organs.  The  telencephalon  and 
diencephalon  of  frogs  are  concerned  primarily  with  the  integration  of 
olfactory  impulses.  When  these  regions  are  destroyed,  the  frog  does  not 
move  spontaneously,  presumably  because  it  cannot  respond  to  olfactory 
or  visual  stimuli.  (Generally  optic  tracts  also  are  destroyed  in  this 
operation.)  However,  the  frog  does  maintain  its  posture,  and  can  feed, 
jump  and  swim  upon  proper  stimulation.  The  optic  lobes  integrate 
impulses  of  sight,  but  some  other  sensory  impulses  are  projected  to  the 
optic  lobes  in  frogs  and  other  lower  vertebrates,  so  this  region  has,  to 
a  limited  extent,  the  over-all  integrative  function  assumed  by  the 
cerebral  hemispheres  in  higher  vertebrates.  Electrical  stimulation  of  the 
area  can,  for  example,  induce  movement  of  the  limbs.  Its  destruction 
prevents  response  to  optic  impulses,  and  also  removes  a  dampening  or 
inhibiting  effect  upon  spinal  reflexes.  The  cerebellum  and  medulla 
receive  impulses  from  the  ear,  and  also  sensory  impulses  from  most 
muscles  which  indicate  their  present  state  of  activity.  In  addition, 
respiratory  movements  and  some  other  vital  activities  are  controlled 
reflexly  in  the  medulla.  When  these  regions  are  destroyed  the  frog  loses 
its  ability  to  maintain  its  posture,  or  right  itself  when  turned  over. 
Muscular  coordination  is  impaired,  though  not  as  much  as  in  birds  and 
other  vertebrates  with  a  larger  cerebellum.  Feeding  is  impossible  and 
respiratory  movements  stop.  Spinal  reflexes  continue  for  a  while,  but 
the  animal  eventually  dies. 

192.        Endocrine  Glands 

Some  of  the  integration  of  metabolic  processes  and  other  vital 
activities  is  controlled  by  the  secretions  (hormones)  of  the  endocrine 
glands.  Although  chemical  integration  in  the  vertebrates  is  discussed 
more  fully  later  (Chapter  30),  two  major  endocrine  glands  of  the  frog 
may  be  brieffy  considered. 

The  pituitary  gland,  which  is  attached  to  the  floor  of  the  brain,  is 
often  regarded  as  the  master  endocrine  gland,  for  it  produces  a  variety 
of  hormones  including  some  that  regulate  ^the  activity  of  many  other 
endocrine  glands.  Among  its  hormones  are  intermedin,  which  helps 
control  skin  coloration;  a  gonad-stimulating  hormone,  which  stim- 
ulates amplexus  and  the  release  of  the  gametes;  and  a  growth-stimulat- 
ing  hormone,  which  controls  growth  of  the  larvae. 

The  thyroid  gland  is  paired  in  frogs,  and  located  on  each  side  of 
the  posterior  part  of  the  hyoid  apparatus.  Its  hormone,  thyroxin,  is 
necessary  for  metamorphosis  from  larva  to  adult,  and  for  an  adequate 
level  of  metabolism  in   the  adult. 


420  VERTEBRATE   LIFE   AND   ORGANIZATION 

193.        Life  Cycle 

A  look  at  certain  aspects  of  the  frog's  development  is  no  less  im- 
portant than  studying  the  adidt,  for  the  continuation  of  the  species  re- 
quires its  reproduction,  and  the  development  of  a  reasonable  propor- 
tion of  the  fertilized  eggs  into  adults  of  the  next  generation.  Great 
numbers  of  eggs  must  be  laid  by  frogs  and  other  animals  that  do  not 
care  for  them  because  the  mortality  of  such  eggs  and  young  is  very  high. 
The  leopard  frog  lays  from  2000  to  3000  eggs,  and  the  bullfrog  can  lay 
up  to  20,000  per  year. 

Eggs  are  laid  in  the  spring,  often  in  rather  cold  water.  However, 
development  can  proceed,  for  the  pigmentation  of  the  upper  hemisphere 
of  each  egg  absorbs  some  heat,  metabolic  activity  produces  more,  and 
the  jelly  coats  provide  some  insulation.  The  fertilized  egg,  or  zygote, 
cleaves  systematically  into  progressively  smaller  cells  during  the  early 
stages  of  development,  finally  attaining  the  blastula  stage,  at  which 
time  the  embryo  is  a  hollow  sphere  of  cells  (Fig.  21.20).  Since  its  lower 
cells  contain  more  yolk  and  are  larger,  the  cavity  of  the  blastula,  the 
blastocoele,  is  excentric  in  position. 

This  stage  is  followed  by  gastrulation,  a  dynamic  process  during 
which  the  cells  of  the  blastula  that  are  destined  to  form  the  major 
organs  of  the  body  are  moved  to  appropriate  regions  of  the  embryo. 
This  involves  the  inward  movement  of  many  cells  (p.  128),  the  elimina- 
tion of  the  former  blastocoele,  and  the  formation  of  the  primitive  gut 
cavity,  the  archenteron.  The  last  temporarily  opens  to  the  surface 
through  the  blastopore,  an  opening  which  is  occluded  to  some  extent 
in  frogs  by  a  plug  of  yolk-laden  cells,  the  yolk  plug. 

Shortly  after  this,  the  embryo  begins  to  elongate.  A  pair  of  lon- 
gitudinal neural  folds,  destined  to  meet  dorsally  and  close  to  form  the 
tubular  nervous  system,  appears  along  its  back,  and  the  embryo  begins 
to  acquire  a  distinct  head,  trunk  and  tail.  A  pair  of  oral  suckers,  for 
later  attachment,  and  primordia  for  the  eyes  and  gills  are  evident  upon 
the  head.  Embryonic  muscle  segments  (myotomes)  form  along  the  trunk 
and  tail,  and  the  heart  begins  to  beat. 

About  this  time  the  embryo  wriggles  out  of  its  jelly  capsule  and 
hatches  into  a  free-swimming  larva,  or  tadpole.  Nasal  cavities,  finger- 
like external  gills,  and  mouth  and  cloacal  openings  soon  appear,  and 
the  larva  can  take  care  of  itself.  Most  frog  tadpoles  feed  upon  minute 
plant  material,  scraping  it  up  with  horny  teeth.  The  younger  tadpoles 
attach  onto  the  plants  on  which  they  are  feeding  by  means  of  their 
oral  suckers.  Plant  inaterial  is  more  difficult  to  digest  than  animal  matter, 
and  plant-eating  vertebrates  generally  have  longer  intestines,  which 
provides  more  digestive  and  absorptive  surface,  than  their  carnivorous 
relatives.  The  intestine  of  a  tadpole  is  many  times  the  length  of  the 
body  and  is  coiled  like  a  watch  spring. 

Later  in  larval  life,  the  external  gills  become  covered  by  the  growth 
of  a  fold  known  as  the  operculum,  and  gill  slits  develop  that  lead  from 
the  pharynx  to  the  opercular  chamber.  About  this  time  the  external 
gills  are  lost  and  the  larvae  respire  by  internal  gills  that  develop  within 


THE  FROG — A    REPRESENTATIVE   VERTEBRATE 


421 


D 
Blastocoelei  YolK  plug-i     Archenterom        Neural  fold — i 

/        N  ^J;««^*Blastopor«  ^  -    •K 


3^ 


r^ 


\ 


^, 


G  H  I 


Eye  pcimoirtJiiun  /-Gill  primordium  Brain  rSpina 

I     /  -— .,      Optic      L^-,  .^,  ,  [    ^ 


nal cord 


I  Nasal  t, 

f  primordiuTrr  ytf^v 

Oredi  primorniixiin 

K 


Myotomefi- 


'^lail 


bud      Pharynx - 

Liver  diverticulum 


"OradsucKcr 
Moulh-\      I — -K>- 


^Notochord 
Cloaca 


YolK 


"^mm^  (^ 


External  gills 


yk>u.lh. 


H  p 


Figure  21.20.  The  development  of  the  frog.  A,  Zygote;  B-E,  cleavage;  F-G, 
blastula:  H-I,  late  gastrula:  J,  neural  folds:  K-L,  late  embryo;  M,  early  larva;  A',  opercu- 
lar folds;  O,  late  larva;  P,  metamorphosis.  (A-H,  J-K,  M-O,  after  Shumway;  G,  1,  L, 
from  Rugh  after  Huettner;  P,  after  Rugh.) 


422  VERTEBRATE   LIFE   AND    ORGANIZATION 

the  gill  slits.  Water  containing  oxygen  enters  the  mouth  and  pharynx, 
and  crosses  the  internal  gills  on  its  way  out  of  the  gill  slits  into  the 
opercular  chamber.  It  leaves  this  chamber  through  a  small  opening  on 
the  left  side,  the  spiracle.  Late  tadpoles  also  have  lungs,  and  may  be 
seen  surfacing  to  gulp  air.  Hind  limbs  appear  at  the  base  of  the  tail, 
and  forelimbs  develop  within  the  opercular  chamber. 

After  two  and  one  half  to  three  months,  leopard  frogs  undergo  a 
metamorphosis,  a  period  of  rapid  differentiation  during  which  larval 
features  are  lost  and  those  of  the  adult  are  acquired.  The  front  legs 
burst  through  the  operculum,  the  left  one  first,  gills  and  gill  slits  are 
lost  and  the  tail  is  resorbed.  The  mouth  widens,  the  horny  teeth  are 
lost,  a  tongue  develops  and  the  digestive  tract  shortens.  A  tympanic 
membrane  and  eyelids  appear,  and  even  the  shape  of  the  lens  changes 
to  provide  for  good  vision  in  air,  which  has  a  different  refractive  index. 
Finally  gonads  develop  and  differentiate  into  testes  or  ovaries. 


Questions 

1.  How  can  the  sexes  of  frogs  be  distinguished  externally? 

2.  What  makes  a  frog's  skin  appear  green?  What  is  the  advantage  to  the  frog  of  its  green- 
ish color  and  dark  spots? 

3.  What  parts  of  the  frog's  skeleton  are  classified  as  visceral  skeleton,  axial  skeleton, 
appendicular  skeleton? 

4.  Describe  the  parts  of  a  vertebra. 

5.  In  what  ways  is  the  frog's  skeleton  well  adapted  for  jumping? 

6.  Distinguish  between  the  following:  the  origin  and  insertion  of  a  muscle,  flexion  and 
extension,  protraction  and  retraction,  adduction  and  abduction. 

7.  Make  a  diagram  of  a  cross  section  of  a  frog  showing  the  relationship  of  the  internal 
organs  to  the  coelom,  peritoneum  and  mesenteries. 

8.  How  do  frogs  catch  their  food? 

9.  List  in  correct  sequence  the  parts  of  the  digestive  tract  of  a  frog. 

10.  Describe  the  route  that  air  takes  in  going  to  the  lungs  of  a  frog.  How  is  the  air 
moved  in  and  out  of  the  lungs? 

11.  How  do  frogs  produce  sound?  What  is  the  purpose  of  their  croaking? 

12.  List  in  correct  sequence  the  vessels  through  which  a  drop  of  blood  would  pass  in 

traveling  from  the  intestine  of  a  frog  forward  through  the  heart  and  back  to  the 
intestine. 

13.  Where  does  the  blood  of  a  frog  become  aerated?  Does  oxygen-rich  blood  mix  with 
oxygen-poor  blood  in  the  heart? 

14.  Trace  the  route  of  sperm  from  the  testis  of  the  frog  to  the  outside.  Do  eggs  have  a 
comparable  route? 

15.  List  the  five  regions  of  the  frog's  brain  and  the  major  structures  that  are  present  in 
each. 

16.  Are  the  cerebral  hemispheres  important  integration  centers  in  the  frog?  What  hap- 
pens if  they  are  destroyed? 

17.  Briefly  describe  the  reproduction  of  the  frog. 

18.  What  is  the  value  of  the  jelly  layers  that  surround  frogs'  eggs?  Where  are  these  layers 
added  to  the  egg? 

19.  Briefly  describe  the  main  features  of  frog  development. 


THE   FROG — A    REPRESENTATIVE   VERTEBRATE  423 

Supplementary  Reading 

An  old,  yet  ven-  valuable  reference  on  the  anatomy  of  the  frog  is  Gaupp's  Anatomic 
des  Frosches.  A  more  readily  available,  though  less  detailed  account  is  Holmes'  Biology 
of  the  Frog.  Rugh's  The  Frog,  Its  Reproduction  and  Development  is  an  excellent,  though 
advanced,  book  on  the  embryology  of  the  frog.  Those  interested  in  the  taxonomy  and 
natural  history  of  frogs  will  hnd  that  the  Wrights'  Handbook  of  Frogs  and  Toads  of  the 
United  States  and  Canada,  is  a  rich  source  book.  Many  of  the  references  cited  at  the  end 
of  Chapters  22  and  23  also  contain  much  information  on  the  frog. 


426 


VERTEBRATE   LIFE   AND   ORGANIZATION 


A    HISTORY   OF   VERTEBRATES:   FISHES  427 

Records  of  these  types,   together  with  footprints  and  other  indications 
of  the  activity  of  organisms,  are  known  as  fossils. 

From  the  fortuitous  ways  in  which  fossils  are  formed,  uncovered 
(and  sometime  destroyed)  by  erosion,  and  finally  discovered,  it  follows 
that  the  fossil  record  is  far  from  complete.  It  is  also  a  somewhat  biased 
sample  of  the  life  of  the  past  because  organisms  living  in  or  near  water, 
or  on  plains  where  their  remains  can  be  covered  by  wind-blown  sand, 
are  more  likely  to  be  fossilized  and  preserved.  Forest-dwelling  species 
in  particular  leave  few  fossils,  for  decay  is  very  rapid  on  the  forest 
floor.  Nevertheless,  the  study  of  fossils  (the  science  of  paleontology)  and 
earth  history  (geology)  can  provide  us  with  much  information  con- 
cerning the  history  of  organisms.  Earth  history  will  be  considered  more 
fully  in  Chapter  35.  For  our  present  purposes  it  is  sufficient  to  realize 
that  geology  can  tell  us  the  sequence  of  the  fossils,  can  give  us  estimates 
of  their  age,  and  can  help  to  tell  us  something  of  the  environment  in 
which  the  organisms  were  living.  Geologists  clivide  earth  history  into 
eras,  periods  and  sometimes  smaller  units  of  time.  Those  that  pertain  to 
a  history  of  vertebrates,  together  with  an  indication  of  their  age,  are 
shown  in  the  diagram  of  vertebrate  evolution  (Fig.  22.2). 

195.        Vertebrate  Beginnings 

Although  vertebrate  origins  are  obscure,  we  have  a  reasonably  com- 
plete fossil  record  of  their  subsequent  evolution.  The  most  primitive 
are  jawless  types  placed  in  the  class  Agnatha.  This  group  flourished 
during  the  middle  Paleozoic  era,  when  it  was  represented  by  several 
orders  collectively  known  as  the  ostracoderms.  These  ancestral  verte- 
brates were  small,  fresh-water,  bottom-feeding  animals  that  were  fishlike 
in  general  proportions,  but  somewhat  flattened  dorso-ventrally,  espe- 
cially near  the  front  of  the  body  (Fig.  22.3).  They  had  an  extensive 
armor  of  thick,  bony  plates  and  scales  develo^^ed  for  the  most  part  in 
the  dermis  of  the  skin. 

The  ostracoderms  had  median  fins  but  (with  the  possible  exception 
of  pectoral  flaps  in  a  few  genera)  not  paired  fins  equivalent  to  the 
paired  appendages  of  other  vertebrates.  The  upper  portion  of  the  caudal 
fin  was  larger  and  more  rigid  than  the  lower  because  it  included  an 
extension  of  the  body  axis.  This  heterocercai  tail  is  characteristic  of  both 
fossil  and  living  primitive  fishes  (Fig.  22.6). 

The  significance  of  this  type  of  tail  was  demonstrated  in  1936  by 
Professor  Harris  of  Cambridge,  who  studied  the  role  of  the  fins  of 
fishes  by  a  series  of  amputation  experiments  and  by  measuring  the 
forces  at  work  on  models  placed  in  a  wind-tunnel.  He  pointed  out  that 
primitive  fishes  lack  lungs  or  swim  bladders  and  hence  their  bodies 
have  a  relatively  high  specific  gravity.  Such  a  fish  tends  to  sink  to  the 
bottom,  but  a  head  flattened  on  the  ventral  surface,  or  large  pectoral 
appendages  such  as  those  found  in  many  sharks,  tend  to  raise  the 
anterior  part  of  the  body  off  the  bottom  when  the  fish  moves  forward 
through  the  water.  The  lateral  motion  of  the  trunk,  and  the  hetero- 
cercai tail  with  its  rigid  upper  portion,  give  a  compensatory  lift  to  the 


428 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Se-nsory 
f  ie-ids 


Lateral  ey<2- 


nostril 

Pineal 
eye 


Figure  22.3.     Heinicyclaspis,  a  representative  ostracoderm  of  the  early  Devonian  pe- 
riod.  This  fish  was  about  eight  inches  long.  (Modified  after  Stensio.) 

rear  end.  More  advanced  fishes  have  lungs  or  air-filled  swim  bladders 
and  hence  are  more  buoyant.  They  do  not  have  flattened  heads  or 
large  pectoral  appendages,  and  they  have  evolved  symmetrical  tail  fins 
(Fig.  22.9). 

The  ostracoderm  head  was  rather  unusual.  A  single  median  nostril 
was  present  on  the  top  of  the  head  in  the  best  known  species.  There 
was  a  pair  of  lateral  eyes  and  a  single,  median  pineal  eye  on  the  top 
of  the  head  posterior  to  the  nostril.  Professor  Young  of  University 
College,  Lonclon,  has  removed  the  pineal  eye  from  primitive  living 
fishes  (lampreys)  and  finds  that  they  no  longer  undergo  the  rhythmic 
color  changes  (light  at  night  and  dark  in  the  day)  observed  in  the 
usual  diurnal  cycle.  These  color  changes  are  known  to  be  controlled 
by  the  hypothalamic  portion  of  the  brain  and  by  the  pituitary  gland, 
hence  the  pineal  eye  must  affect  these  organs.  The  hypothalamus  and 
pituitary  gland  also  control  many  other  physiologic  activities,  so  it  is 
possible,  as  Young  has  postulated,  that  the  pineal  is  an  organ  that  in 
these  animals  adjusts  the  rate  of  activity  to  changing  conditions  of 
illumination.  Three  regions  of  the  ostracoderm  head,  one  dorso-medial 
and  a  pair  of  dorso-lateral  areas,  contained  small  plates  beneath  which 
were  enlarged  cranial  nerves.  These  peculiar  areas  may  have  been  sen- 
sory fields,  or  areas  beneath  which  lay  muscles  modified  for  the  produc- 
tion of  electric  shocks.  Much  of  the  ventral  surface  of  the  head  was 
covered  with  small  plates  forming  a  flexible  floor  to  the  gill  region,  or 
pharynx.  Movement  of  this  floor  presumably  drew  water  and  minute 
particles  of  food  into  the  jawless  mouth.  The  water  then  left  the 
pharynx  through  as  many  as  ten  pairs  of  small  gill  slits,  but  the  food 


A    HISTORY   OF    VERTEBRATES:   FISHES 


429 


particles  were  somehow  trapped  in  the  pharynx.  It  seems  likely  that  the 
ancestral  vertebrates,  like  the  lower  chordates  of  the  present  day,  were 
filter-feeders. 

196.        Living  Jawless  Vertebrates 

Ostracoderms  became  extinct  by  the  end  of  the  Devonian,  and  the 
living  lampreys  and  hagfishes  of  the  order  Cyclostomata  are  a  specialized 
remnant  of  the  class  Agnatha  (Figs.  22.4  and  22.5).  They  are  jawless, 


Rudimentary  vertebrae- 
Spinal  cord. 


Dorsa.1  aorta — i 
Esophagus  — 
Respiratory  tube 
Internal  gill  slit  — 
Sinus  venosus 


Cardinal  veiii' 
Intestine 
Liver 
Pleuroperitoncal 
cavity 
Pericardial 
cavity 
Ventricle 
Ventral  aorta- 

Inferior  

jugular  vein 


Olfactory  Sac 
Pineal  eye 
Cranial  cartilaoes 
Brain 
Myomere 


Median  nostril 
Sinus 
Mouth 


tai-tila^e  of  branchial /^     Vzlu.m-' 
basket  /^hypophyseal 
lingual  cartilage —       pouch.  SinuS"" 

and  muscles 


p" '-Horny 
teeth 

Buccal  funnel 


"Tongue 
"Bucced  cavity 


Figure  22.4.     A  diagrammatic  representation  of  the  more  important  organs  found 
in  the  anterior  part  of  the  lamprey. 

have  more  gill  slits  than  other  living  fishes,  lack  paired  appendages, 
retain  a  pineal  eye,  and  have  a  single  median  nostril.  Besides  leading 
to  an  olfactory  sac,  this  nostril  opens  into  an  hypophyseal  sac  that 
passes  beneath  the  front  of  the  brain.  Much  of  the  pituitary  gland  of 
higher  vertebrates  is  derived  from  an  embryonic  hypophysis.  Cyclostomes 
differ  from  ostracoderms  in  several  respects:  they  have  an  eel-like  shape 
and  a  slimy,  scaleless  skin,  and  they  are  predators  or  scavengers.  Many 
lampreys,  like  the  ostracoderms,  live  in  fresh  water,  but  some  spend 
their  adult  life  in  the  ocean  and  rettirn  to  fresh  water  only  to  reproduce. 
The  hagfishes  are  exclusively  marine. 

A  familiar  example  of  the  group  is  the  sea  lamprey,  Petrojiiyzon 
marinus.  The  chief  axial  support  for  the  body  is  a  notochord  which 
persists  throughout  life  and  is  never  replaced  by  vertebrae.  Rudimentary 
vertebrae  are  present,  however,  on  each  side  of  the  notochord  and 
spinal  cord.  The  brain  is  encased  by  a  cartilaginous  cranium,  and  the 
gills  are  supported  by  a  complex,  cartilaginous  lattice-work  known  as 
the  branchial  basket,  which  appears  to  be  hoijiologous  to  the  visceral 
skeleton  of  other  fishes. 

The  mouth  lies  deep  within  a  buccal  Tunnel,  a  suction-cup  mech- 
anism with  which  the  lamprey  attaches  to  other  fishes  (Fig.  22.5).  The 
mobile  tongue  armed  with  horny  "teeth"  rasps  away  at  the  prey's  flesh, 
and  the  lamprey  sucks  in  the  blood  and  bits  of  tissue.  It  has  special 
oral   glands   that  secrete  an  anticoagulant  which  enables  the  blood  to 


428 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Later  a.1  eye 


nostril 


Sensory 
f  ie-lds 


Pineal" 
eye 


Figure  22.3.     HemJcyclaspis,  a  representative  ostracodenn  of  the  early  Devonian  pe- 
riod. This  fish  was  about  eight  inches  long.  (Modified  after  Stensio.) 

rear  end.  iMore  advanced  fishes  have  lungs  or  air-filled  swim  bladders 
and  hence  are  more  buoyant.  They  do  not  have  flattened  heads  or 
large  pectoral  appendages,  and  they  have  evolved  symmetrical  tail  fins 
(Fig.  22.9). 

The  ostracoderm  head  was  rather  unusual.  A  single  median  nostril 
was  present  on  the  top  ot  the  head  in  the  best  known  species.  There 
was  a  pair  of  lateral  eyes  and  a  single,  median  pineal  eye  on  the  top 
of  the  head  posterior  to  the  nostril.  Professor  Young  of  University 
College,  London,  has  removed  the  pineal  eye  from  primitive  living 
fishes  (lampreys)  and  finds  that  they  no  longer  undergo  the  rhythmic 
color  changes  (light  at  night  and  dark  in  the  day)  observed  in  the 
usual  diurnal  cycle.  These  color  changes  are  known  to  be  controlled 
by  the  hypothalamic  portion  of  the  brain  and  by  the  pituitary  gland, 
hence  the  pineal  eye  must  affect  these  organs.  The  hypothalamus  and 
pituitary  gland  also  control  many  other  physiologic  activities,  so  it  is 
possible,  as  Young  has  postulated,  that  the  pineal  is  an  organ  that  in 
these  animals  adjusts  the  rate  of  activity  to  changing  conditions  of 
illumination.  Three  regions  of  the  ostracoderm  head,  one  dorso-medial 
and  a  pair  of  dorsolateral  areas,  contained  small  plates  beneath  which 
were  enlarged  cranial  nerves.  These  peculiar  areas  may  have  been  sen- 
sory fields,  or  areas  beneath  which  lay  muscles  modified  for  the  produc- 
tion of  electric  shocks.  Much  of  the  ventral  surface  of  the  head  was 
covered  with  small  plates  forming  a  flexible  floor  to  the  gill  region,  or 
pharynx.  Movement  of  this  floor  presumably  drew  water  and  minute 
particles  of  food  into  the  jawless  mouth.  The  water  then  left  the 
pharynx  through  as  many  as  ten  pairs  of  small  gill  slits,  but  the  food 


A    HISTORY   OF   VERTEBRATES:   FISHES 


429 


particles  were  somehow  trapped  in  the  pharynx.  It  seems  hkely  that  the 
ancestral  vertebrates,  like  the  lower  chordates  of  the  present  day,  were 
filter  feeders. 

196.        Living  Jawless  Vertebrates 

Ostracoderms  became  extinct  by  the  end  of  the  Devonian,  and  the 
living  lampreys  and  hagfishes  of  the  order  Cyclostomata  are  a  specialized 
remnant  of  the  class  Agnatha  (Figs.  22.4   and  22.5).   They  are  jawless, 


Dorsal  aorba — i 
Esophagus  — 
Respiratory  tube.—] 
Internal  gill  slit 
Sin  as  venosus 


Olfactory  Sac 

Pineal  eye. 
Cranial  cartilaoeS" 
Brain 


Median  riostril 
rSinuS 

Mouth 


Horny 
tce-th 

cal  funnel 


of  br^x2ichia] /, 

basket  /Hypophyseal" 
igual  cartilage —       pouch 
and  muscles 


Tongue 
Buccal  cavity 


Sinus 


Rudimentary  vertebrae- 
Spinal  cord. 
Notochord 
Cardinal  veiiv 
Intestine 
Liver 
Pleuroperitoncal 
cavity 
Pericardial 
cavity 
Ventricle 
Vcntrcil  aorta 

InFerior  

jugular  vein. 

Figure  22.4.     A  diagrammatic  representation  of  the  more  important  organs  found 
in  the  anterior  part  of  the  lamprey. 

have  more  gill  slits  than  other  living  fishes,  lack  paired  appendages, 
retain  a  pineal  eye,  and  have  a  single  median  nostril.  liesides  leading 
to  an  olfactory  sac,  this  nostril  opens  into  an  hypophyseal  sac  that 
passes  beneath  the  front  of  the  brain.  Much  of  the  pituitary  gland  of 
higher  vertebrates  is  derived  from  an  embryonic  hypophysis.  Cyclostomes 
differ  from  ostracoderms  in  several  respects:  they  have  an  eel-like  shape 
and  a  slimy,  scaleless  skin,  and  they  are  predators  or  scavengers.  Many 
lampreys,  like  the  ostracoderms,  live  in  fresh  water,  but  some  spend 
their  adult  life  in  the  ocean  and  return  to  fresh  water  only  to  reproduce. 
The  hagfishes  are  exclusively  marine. 

A  familiar  example  of  the  group  is  the  sea  lamprey,  Petromyzon 
marinus.  The  chief  axial  support  for  the  body  is  a  notochord  which 
persists  throughotit  life  and  is  never  replaced  by  vertebrae.  Rudimentary 
vertebrae  are  present,  however,  on  each  side  of  the  notochord  and 
spinal  cord.  The  brain  is  encased  by  a  cartilaginous  cranium,  and  the 
gills  are  supported  by  a  complex,  cartilaginous  lattice-work  known  as 
the  branchial  basket,  which  appears  to  be  homologous  to  the  visceral 
skeleton  of  other  fishes. 

The  mouth  lies  deep  within  a  buccal  funnel,  a  suction-cup  mech- 
anism with  which  the  lamprey  attaches  to  other  fishes  (Fig.  22.5).  The 
mobile  tongue  armed  with  horny  "teeth"  rasps  away  at  the  prey's  flesh, 
and  the  lamprey  sucks  in  the  blood  and  bits  of  tissue.  It  has  special 
oral   glands   that  secrete  an  anticoagulant  which  enables  the  blood  to 


430  VERTEBRATE   LIFE   AND   ORGANIZATION 

.,■2  TO  20  MONTHS  . 


Figure  22.5.  The  life  cycle  of  the  sea  lamprey  in  the  Great  Lakes.  The  lamprey 
spends  all  but  a  year  or  two  of  its  six  and  one-half  to  seven  and  one-half  years  of  life 
as  a  larva.  (From  Applegate  and  Moffett:  Scientific  American,  April  1955.) 


flow  freely.  From  the  mouth  cavity,  the  food  enters  a  specialized 
esophagus  that  by-passes  the  pharynx  to  lead  into  a  straight  intestine. 
There  is  no  stomach  or  spleen.  A  liver  is  present,  but  the  adult  has  no 
bile  duct.  The  intestine  does  not  receive  bile  from  the  liver,  but  the 
liver  functions  as  a  site  for  the  storage  and  conversion  of  much  of  the 
absorbed  food  brought  to  it  by  the  circulatory  system.  A  separate  pan- 
creas is  not  present,  but  pancreatic  tissue  is  embedded  in  the  wall  of 
the  intestine  and  liver. 

Since  a  lamprey  is  often  attached  to  another  fish  by  its  buccal 
funnel,  water  cannot  pass  into  the  mouth  and  out  of  the  gill  slits  in 
respiration,  as  it  does  in  most  fishes.  Instead,  a  pumping  action  of  the 
pharyngeal  region  moves  water  both  in  and  out  of  the  seven  gill 
pouches  through  as  many  external  gill  slits.  Each  pouch  is  lined  with 
highly  vascular  gills  and  connected  with  the  pharynx  through  an 
internal  gill  slit.  The  mixing  of  food  and  water  is  prevented,  however, 
by    the   separation    of    the   pharynx    from   the    digestive    passages.    The 


A    HISTORY   Of   VERTEBRATES:   FISHES  4$\ 

pharynx  is  a  blind  sac  posteriorly  and  is  separated  anteriorly  from  the 
mouth  cavity  by  a  small  flap  of  tissue.  Because  of  its  isolation,  the 
pharynx  is  often  called  a  respiratory  tube. 

The  kidneys,  as  in  the  frog,  are  drained  by  Wolffian  ducts.  These 
ducts  carry  only  urine,  for  sperm  or  eggs  pass  from  the  large  median 
(embryonically  paired)  testis  or  ovary  into  the  coelom.  A  pair  of  genital 
pores  leads  from  the  coelom  into  a  urogenital  sinus,  formed  by  the 
fused  posterior  ends  of  the  Wolffian  ducts,  and  thence  to  the  cloaca  and 
outside.  The  absence  of  genital  ducts  may  be  a  very  primitive  feature. 
The  sexes  are  separate  in  the  adult  lamprey,  though  sexual  differentia- 
tion occurs  rather  late  in  development,  and  the  gonads  of  young  indi- 
viduals may  contain  both  developing  sperm  and  eggs. 

The  eggs  are  laid  on  the  bottom  of  streams  in  a  shallow  nest, 
which  the  lampreys  make  by  removing  the  larger  stones  with  their 
buccal  funnels  (Fig.  22.5).  During  mating  the  female  attaches  to  a 
stone  on  the  upstream  side  of  the  nest,  and  the  male  to  the  female,  each 
by  its  buccal  funnel.  As  the  eggs  are  laid,  the  sperm  is  discharged  over 
them.   The  adults  die   after  spawning. 

Developing  sea  lampreys  pass  through  a  larval  stage  that  lasts  five  to 
six  years.  The  larva  is  so  different  in  appearance  from  adult  lampreys 
that  it  was  originally  believed  to  be  a  different  kind  of  animal,  and 
was  named  Ammocoetes.  The  ammocoetes  larva  is  eel-shaped,  but  lacks 
the  specialized  feeding  mechanism  of  the  adult.  It  lies  within  burrows 
in  the  mud  at  the  bottom  of  streams,  and  sifts  minute  food  particles 
from  water  passing  through  the  pharynx.  Like  the  lower  chordates,  it 
lias  a  mucus-producing  endostyle  to  aid  in  trapping  the  food. 

Adult  lamj^reys  injure  and  kill  many  other  fishes.  In  recent  years 
the  sea  lamprey  has  passed  the  Niagara  barrier,  presumably  through 
the  Welland  Canal,  and  extended  its  range  from  Lake  Ontario  into 
the  other  Great  Lakes.  The  lake  fishing  industry  has  been  harmed 
greatly.  For  example,  the  lake  trout  catch  in  Lake  Michigan  was 
6,860,000  pounds  in  1943.  It  began  to  decrease  markedly  in  1945  and 
was  a  mere  3,000  pounds  in  1952.  In  terms  of  1950  prices,  the  1943 
catch  was  worth  $3,430,000;  the   1952  catch,  $1,500! 

The  hagfishes  resemble  the  lampreys  in  major  respects,  though 
differing,  of  course,  in  certain  details.  Hags  are  believed  to  be  primarily 
scavengers  feeding  upon  dead  fish  along  the  ocean  bottom,  but  they  also 
attack  disabled  fish  of  any  sort,  including  those  hooked  or  netted.  They 
burrow  into  the  fish  and  eat  out  the  inside,  leaving  little  but  a  bag  of 
skin  and  bone.  They  are  a  commercial  nuisance,  but  their  over-all 
damage  is  not  great,  since  they  are  abundant  in  only  a  few  localities. 

197.        Jaws  and  Paired  Appendages 

During  the  Silurian  and  Devonian  periods,  certain  descendants  of 
the  sluggish  ostracoderms  acquired  paired  appendages  and  jaws,  and 
became  more  active  and  predaceous.  The  earliest  fishes  of  this  type  are 
placed  in  the  class  Placodermi,  and  the  earliest  of  these,  like  their 
ostracoderm  ancestors,   were  fresh-water  forms. 


432 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Branchial  arches 


Jaws: 

mandibular^ 

arch 


Gill  slits- 

Figure  22.6.  A,  The  "spiny  shark."  Climatius,  was  among  the  first  jawed  verte- 
brates, riiis  fish  was  about  three  inches  long.  After  removal  of  the  gill  covering  and 
superficial  bony  scales  and  plates  on  the  head  of  a  related  placoderm  {Acanthodes,  B), 
it  can  be  seen  that  the  jaws  are  modified  gill  arches.  {A,  After  Watson;  B,  modified 
after  Watson.) 

A  well  known  placoderm  is  the  "spiny  shark,"  Climatius  (Fig. 
22.6  A).  It  was  a  streamlined  fish  which  retained  the  primitive  hetero- 
cercal  tail  and  a  covering  of  thick,  bony  scales,  but  the  scales  were 
smaller,  so  that  a  greater  freedom  of  movement  of  the  trunk  would  have 
been  possible.  Stabilizing  paired  appendages  were  present,  but  instead 
of  a  pair  of  pectoral  and  pelvic  fins  there  was  a  long  series  of  paired 
ventrolateral  spines.  Each  may  have  supported  a  web  of  flesh.  Fishes 
with  these  features  were  probably  able  to  move  throvigh  the  water  with 
fair  rapidity. 

A  major  advance  was  the  development  of  jaws,  which  evolved  as 
a  modification  of  a  gill  arch  (Fig.  22.6  B).  The  numerous  gill  arches  of 
ostracoderms  appear  to  have  been  firmly  united  with  the  bony  plates 
covering  the  head.  In  other  fishes  each  gill  arch  is  movable  and  more 
or  less  > -shaped,  with  the  apex  of  the  >  hinged  and  pointing  poste- 
riorly. During  the  evolution  of  jaws,  certain  of  the  anterior  gill  arches 
were  lost,  but  the  most  anterior  remaining  arch  became  enlarged  and, 
together  with  bone  developed  in  the  skin  adjacent  to  it,  formed  the 
jaws.  This  arch  is  known  as  the  mandibular  arch.  The  arch  next  pos- 
terior is  the  hyoid  arch,  and  the  remaining  are  typical  gill  or  branchial 
arches.  In  higher  fishes  the  hyoid  and  mandibular  arch  are  very  close 
together,  the  hyoid  often  helping  to  support  the  jaws,  and  the  gill  slit 


A    HISTORY   OF   VERTEBRATES:   FISHES  433 

that  one  might  expect  to  find  between  these  arches  is  either  absent  or 
vestigial.  It  is  of  interest  that  the  mandibuhir  and  hyoid  arches  of 
placoderms  were  not  so  close  together,  and  there  was  a  complete  gill 
slit  between  them.  Here  we  actually  see  an  evolutionary  stage  that  we 
would  theoretically  expect.  The  development  of  jaws  was  an  important 
step  in  the  evolution  of  vertebrates,  for  the  presence  of  jaws  enabled 
vertebrates  to  adapt  to  many  more  modes  of  life.  The  success  of  the 
jawed  vertebrates  doubtless  led  to  the  extinction  of  ostracoderms,  and  to 
the   limitation  of  cyclostomes   to  rather  specialized   ecologic  niches. 

198.        Characteristics  of  Cartilaginous  Fishes 

Although  a  few  ostracoderms  and  some  of  the  later  placoderms  en- 
tered the  sea,  most  of  these  early  vertebrates  were  fresh-water  animals. 
The  first  fishes  to  achieve  lasting  success  in  the  ocean  were  the  sharks, 
skates  and  their  relatives  of  the  class  Chondrichthyes.  The  earliest  mem- 
bers of  this  class  appeared  during  the  Devonian  period.  They  were 
marine  and  the  group  has  remained  marine  except  for  a  few  species 
that  have  secondarily  entered  fresh  water. 

Although  they  originated  from  some  early  placoderm  stock,  quite 
possibly  the  "spiny  sharks,"  the  cartilaginous  fishes  differ  from  placo- 
derms in  many  ways  (Fig.  22.7).  In  general  these  fishes  are  highly  stream- 
lined, yet  they  retain  the  primitive  heterocercal  tail.  Only  pectoral  and 
pelvic  paired  fins  are  present,  and  these  are  fan-shaped  structures  sup- 
ported internally  by  a  A\ell  developed  appendicular  skeleton  rather  than 
simply  by  anterior  spines.  In  early  members  of  the  group  each  fin  had  a 
broad  attachment  to  the  body,  but  in  recent  sharks  the  base  of  the  fin 
is  rather  narrow.  The  latter  type  of  fin  is  more  mobile  and  hence  more 
effective  in  stabilizing  and  steering.  The  medial  part  of  the  pelvic  fin 
of  the  male  is  modified  as  a  clasper,  or  copulatory  organ. 

A  lateral  line  sensory  system  is  well  developed.  It  consists  of  a 
canal  in  the  skin  that  extends  along  the  side  of  the  tail  and  trunk,  and 
ramifies  over  the  head.  This  canal  may  be  an  open  groove,  as  in  the 
chimaeras  (Fig.  22.7  D),  or  it  may  be  closed,  opening  to  the  surface  only 
through  small  pores.  It  contains  minute  sensory  organs  that  enable  the 
fish  to  detect  low  frequency  vibrations,  movements,  and  perhaps  pres- 
sure changes  in  the  water.  Such  a  system  is  found  in  all  fishes  but  is 
rather  inconspicuous  in   cyclostomes. 

Shark  skin  feels  like  sand  paper  because  of  the  minute  placoid 
scales  embedded  in  it.  These  scales  are  the  evolutionary  remnant  of  the 
extensive  covering  of  thick,  dermal  scales  of  primitive  fishes.  The  rest 
of  the  skeleton  is  cartilaginous,  not  bony.  A  cartilaginous  cranium,  ver- 
tebral column,  appendicular  skeleton  and  visceral  skeleton  are  present. 
Calcium  salts  may  be  deposited  in  the  cartilage  and  may  strengthen  it, 
but  there  is  never  any  ossification.  A  skeleton  coinposed  of  cartilage  is 
believed  to  represent  the  retention  in  the  adult  of  the  embryonic 
skeletal  material.  It  is  not  regarded  as  the  primitive  adult  condition, 
for  these  parts  of   the  skeleton  were   at   least   partly  ossified  in  earlier 


434  VERTEBRATE   LIFE   AND   ORGANIZATION 


F.gure  22.7  A  group  of  cartilaginous  fishes.  A,  A  male  dogfish,  Sqiialus  acanthias; 
B  the  sawfish  Pristis:  C,  the  sting  ray,  Dasyatis:  D,  the  ratfish,  Chimaera.  (A,  modified 
alter  liigelow  and  Schroeder;  C,  courtesy  of  Marine  Studios;  D,  from  Romer  after  Dean  ) 


fishes.  The  nature  of  the  skeleton  gives  the  name  to  the  class;  Chon- 
drichthyes  means  cartilaginous  fishes. 

The  upper  and  lower  jaws  are  formed  from  the  mandibular  arch 
and  are  provided  with  a  great  many  sharp,  triangular  teeth  which 
evolved  as  a  modification  of  bony  scales.  The  resemblance  between  a 


A   HIiTORY  OF  VERTeBkAteS:  FISHES 


435 


placoid  scale  and  a  tooth  is  very  close.  Placoderms,  in  contrast  to  the 
Chondrichthyes,  had  no  teeth,  or  only  a  few  on  the  lower  jaw.  The 
dorsal  part  of  the  hyoid  arch,  the  hyomandibuiar,  extends  as  a  prop 
from  the  otic  capsule  of  the  cranium  to  the  angle  of  the  jaw,  and  the 
ventral  part  of  the  arch  continues  into  the  floor  of  the  mouth.  The 
postmandibular  gill  slit,  which  was  complete  in  placoderms,  is  reduced 
to  a  dorsal  spiracle  or  is  lost.  Five  branchial  arches  lie  behind  the 
hyoid  in  most  species,  and  five  typical  gill  slits  open  independently  to 
the  surface. 

The  visceral  organs  of  the  dogfish,  Squalus  acanthias  (Fig.  22.8), 
are  in  many  ways  more  characteristic  of  primitive  fishes  than  are  those 
of  the  specialized  cyclostomes.  The  mouth  cavity  is  continuous  poste- 
riorly with  the  pharynx.  A  spiracle,  containing  a  vestigial  gill,  and  the 
gill  slits,  containing  functional  gills,  open  from  the  pharynx  to  the 
body  surface.  A  wide  esophagus  leads  from  the  back  of  the  pharynx 
to  a  J-shaped  stomach.  A  short,  straight  valvular  intestine  continues 
back  to  the  cloaca.  The  valvular  intestine  receives  secretions  from  the 
liver  and  pancreas,  and  contains  an  elaborate  spiral  fold  known  as  the 
spiral  valve.  This  helical  fold  serves  both  to  slow  the  passage  of  food 
and  to  increase  the  digestive  and  absorptive  surface  of  the  intestine. 

The  heart  consists  of  a  series  of  chambers  arranged  in  linear  se- 
quence. Blood  from  the  veins,  low  in  oxygen  content,  enters  the  pos- 
terior end  of  the  heart  and  is  pumped  out  the  anterior  end  into  an 
artery  that  leads  to  capillary  beds  in  the  gills.  Aerated  blood  from  the 
gills  is  collected  by  a  dorsal  aorta  and  carried  to  the  body  wall  and 
visceral  organs.  Such  a  circulatory  system  is  a  sluggish,  low-pressure 
system,  for  the  pressure  built  up  by  the  beating  of  the  heart  is  im- 
mediately reduced  by  friction  in  the  gill  capillaries. 

The  kidneys  are  elongate  organs  drained,  as  in  the  frog,  by  Wolffian 
ducts.  They  play  a  major  role  in  water  balance  and  excretion;  however, 
the  gills  help  eliminate  much  of  the  nitrogenous  waste.  The  Wolffian 
ducts  also  carry  sperm  in  males. 

At  the  time  of  reproduction,  eggs  are  discharged  from  the  ovary, 
pass  through  a  part  of  the  coelom,  and  enter  the  oviducts.  The  male 
cartilaginous  fish  uses  a  clasper  to  deposit  sperm  in  the  oviducts  and 
fertilization  is  internal.  A  horny  protective  capsule  is  secreted  around 
the  fertilized  eggs  by  certain  oviducal  cells,  and,  in  all  of  the  skates,  the 


Wolffian  duct 
Rectal  gland- 


Gill  slits 
^Si^sx..,.  r— Spira-cle 


Mou.lh 

'Cloaca.  "-Pancr-e-as       Heart      ^Pericardial  cavity 

Figure  22.8.     The  visceral  organs  of  the  dogfish. 


436  VERTEBRATE    LIFE    AND    ORGANIZATION 

eggs  are  laid  and  develop  externally.  Skates  are  oviparous,  but  there  is 
no  free  larval  stage,  as  there  is  in  Irogs  and  many  other  oviparous  am 
mals.  The  eggs  are  very  heavily  laden  with  yolk  and  the  embryos  develop 
within  the  protective  capsule.  A  lew  sharks  are  also  oviparous,  but  mosf 
have  clei>artc(l   Ironi  this  primitive  egg-laying  habit.  In  the  dogfish,  tor 
example,  the  lertilized  eggs  are  retained  in  a  modified  portion  of  the 
oviduct  known  as  the  uterus.  Each  embryo  derives  its  food  in  part  from 
yolk  within   the  egg  and   in   part   from  the  mother's  blood  stream   by 
means  of  a  primitive  yolk   sac   placenta  in  which  blood  vessels  in  the 
wall  of  the  embryo's   )i>lk  sac    are   in  direct   contact   with  vascularized 
flaps  of   the    uterine   lining.   Food,   gases  and   possibly   other  materials 
diffuse  between  the  mother  and  embryo,  but  no  blood  exchange  occurs 
between  them.  This  is  an  example  of  viviparous  reproduction,  for  the 
embryo  derives   a  large  jjart  of  its   nutrients   from   the  mother's  blood 
stream,  and    the  young  fish   is   born   in  an  advanced  stage  of  develop- 
ment as  a  miniature  adult.  Still  other  sharks  are  ovoviviparous;  the  egg 
is  retained  within  the  mother's  reproductive  tract  but  most  of  the  em- 
bryo's nutrients  come  from  yolk  stored  within  the  egg.  It  is  frequently 
difficult  to  make  a  sharp   distinction   between   viviparous  and  ovovivi- 
parous reproduction,  for  the  young  are  born  at  an  advanced  stage  of 
development  in  both  cases. 

199.        Evolution  of  Cartilaginous  Fishes 

The  ancestral  Chondrichthyes  were  essentially  shark-like,  but 
in  their  subsequent  evolution  the  cartilaginous  fishes  have  diverged 
widely,  and  have  become  adapted  to  many  modes  of  life  within  the 
aquatic  environment.  One  line  of  evolution  (subclass  Holocephali)  has 
led  to  our  present-day,  rather  rare,  deep-water  ratfish  (Chimaera)  (Fig. 
22.7  D).  In  these  fishes,  the  gill  slits  are  covered  by  an  operculum  so 
there  is  a  common  external  orifice,  and  the  tail  is  long  and  ratlike.  The 
other  line  of  evolution  (subclass  Elasmobranchii)  is  distinguished  by 
having  separate  external  openings  for  each  gill  slit.  Elasmobranchs  have 
been  far  more  successful,  and  have  diverged  into  two  contemporary 
orders— Selachii  (sharks  and  dogfish)  and  Batoidea  (skates  and  rays). 

Sharks.  Most  selachians  are  active  fishes  that  feed  voraciously  with 
their  sharp,  triangular-shaped  teeth  upon  other  fishes,  crustaceans  and 
certain  molluscs.  Although  there  are  many  records  of  sharks  attacking 
and  killing  man  in  the  warmer  seas,  most  species  will  not  do  so,  and 
there  is  little  danger  to  swimmers  in  temperate  waters.  The  largest 
sharks,  such  as  the  whale  shark  {RJiincodon)  which  may  reach  a  length 
of  about  50  feet,  have  minute  teeth,  and  feed  entirely  upon  small  crus- 
taceans and  other  organisms  that  form  the  drifting  plankton  of  the 
surface  layers  of  the  ocean.  They  gulp  mouthfuls  of  water,  and  as  the 
water  passes  out  of  the  gill  slits,  the  food  is  kept  in  their  pharynx  by 
a  branchial  sieve.  Whale  sharks  are  the  largest  living  fishes. 

Skates  and  Rays.  Skates  and  rays  are  bottom-dwelling  fishes  that 
are  flattened  dorsoventrally,  and  have  enormous  pectoral  fins  whose 
undulations   propel    the    fish    along    the    bottom    (Fig.    22.7    C).    Their 


A   HISTORY   Of   VERTEBRATES:   FISHES 


437 


mouth  is  often  buried  in  the  sand  or  mud,  and  water  for  respiration 
enters  the  pharynx  via  the  pair  of  enlarged  spiracles.  A  spiracular 
valve  in  each  one  is  then  closed,  and  the  water  is  forced  out  the  typical 
gill  slits.  Most  skates  and  rays  have  crushing-type  teeth  and  feed  upon 
shellfish,  but  others  are  adapted  for  other  methods  of  feeding.  The 
Sawfish  (Pristis)  has  an  elongated,  blade-shaped  snout  armed  with 
toothlike  scales.  By  thrashing  about  in  a  shoal  of  small  fishes,  it  can 
disable  many  and  eat  them  at  leisure.  As  in  the  sharks,  the  largest  mem- 
bers of  the  group  (the  devilfish,  Manta)  have  reduced  teeth,  and  are 
plankton  feeders.  Some  devilfish  have  a  "wing  spread"  of  20  feet  and 
can  easily  upset  small  boats.  Harpooning  these  is  an  exciting  sport! 

200.        Lungs  and  Swim  Bladders 

While  early  sharks  were  becoming  dominant  in  the  ocean,  another 
offshoot  of  the  placoderms,  the  bony  fishes  of  the  class  Osteichthyes, 
became  dominant  in  fresh  water.  They  subsequently  entered  the  ocean 
and  became  the  most  successful  group  there  as  well.  Most  of  the  familiar 
present-day  fishes  (gar,  herring,  minnows,  perch,  cod,  lungfish)  belong 
in  this  group. 

The  Osteichthyes  resemble  the  (^hondrichthyes  in  being  evolution- 
ally  advanced  fishes  with  efficient  paired  appendages  and  jaws  (Fig.  22.9). 


Ye.llov\r  perch 


iP^lteSSS^S' 


Austrsd.ia.-a  lundf'is'h 


Figure  22.9.  Representative  bony  fishes.  A,  The  yellow  perch,  Perca  flavescens,  is 
a  member  of  the  ray-finned  group  of  bony  fishes;  B,  the  Australian  lungfish,  Epicemto- 
dus,  belongs  to  the  fleshy-finned  group.  {A,  After  Hubbs  and  Lagler;  B,  after  Norman.) 


438 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Finray- 
Myomeres 


Lateral 
line 


Fin  spine 


Kidney 


Swim  bladder 


ctres 


Haemal- 
arches 


Urogenital  tract 
Wolffian  duct 


Spleen^ 
Fat  body - 
Intestine 
Pelvic  fms 


Heart 
Pericardia]  cavity 


Liver 
_,  .,  V  Gall  bladder  and 

Pleuropcrjtoncal  tile  duct 

cavitv 


Pyloric  caeca 


Figure  22.10.     The  visceral  organs  of  the  perch. 


An  obvious  way  in  which  they  differ  from  the  cartilaginous  fishes  is  in 
having  an  ossified  internal  skeleton  and  in  retaining  more  of  the  primi- 
tive bony  scales  and  plates.  The  internal  skeleton  consists  of  cartilage 
replacement  bone  that  has  developed  embryologically  in  association 
with  cartilaginous  rudiments,  which  it  gradually  replaces.  The  bone  in 
the  scales  and  plates,  although  histologically  similar  to  the  preceding 
type,  is  dermal  bone.  It  develops  in  the  dermis  of  the  skin  and  is  not 
preceded  by  cartilage.  The  deeper  portions  of  the  dermal  plates  in  the 
head  and  shoulder  region  become  intimately  associated  with  the  internal 
skeleton;  thus  the  skull  and  pectoral  girdle  of  these  fishes,  and  of  the 
terrestrial  vertebrates  which  have  descended  from  them,  contain  both 
types  of  bone. 

The  jaws  are  formed  partly  by  the  ossified  mandibular  arch  of  the 
visceral  skeleton  (cartilage  replacement  bone),  and  partly  by  dermal  bone 
encasing  this  arch.  The  hyoid  arch  lies  close  behind  the  mandibular, 
and  its  hyomandibular  may  take  part  in  the  suspension  of  the  jaws. 
There  is  no  room  for  a  postmandibular  gill  slit,  and  even  the  spiracle, 
when  present,  does  not  open  to  the  surface.  Typical  branchial  arches 
lie  behind  the  hyoid,  but  the  gill  region  is  covered  by  a  flap  containing 
dermal  bone  (the  operculum),  so  the  gill  slits  have  a  common  opening 
just  anterior  to  the  pectoral  fin. 

The  soft  parts  of  most  bony  fishes,  the  perch  for  example  (Fig. 
22.10),  show  a  peculiar  mixture  of  primitive  and  highly  specialized 
characters.  Most  need  not  concern  us,  but  one  of  great  interest  is  the 
swim  bladder.  In  the  perch,  this  is  a  median,  membranous  sac  lying 
in  the  dorsal  portion  of  the  coelom.  The  bladder  is  filled  with  gases 
similar  to  those  dissolved  m  the  water  (nitrogen,  oxygen,  carbon  diox- 
ide). It  functions  primarily  as  a  hydrostatic  organ,  adjusting  the  specific 
gravity  of  the  body  so  that  the  fish  can  stay  at  various  depths  with  a 
minimum  of  effort.  Gases  may  be  secreted  into  the  bladder  or  absorbed 
from  it,  as  conditions  warrant,   through  specialized   capillary   networks 


A   HISTORY   Of   VERTEBRATES:   FISHES 


439 


in  its  wall.  Under  conditions  of  oxygen  deficiency,  the  fish  can  utilize 
the  oxygen  in  the  bladder,  so  the  organ  also  functions  as  a  temporary 
storage  site  for  this  gas. 

In  some  bony  fishes,  the  swim  bladder  is  connected  to  the  pharynx 
by  a  pneumatic  duct,  and  in  a  few,  functional  lungs  are  present  instead. 
This  led  many  to  postulate  that  the  swim  bladder  was  the  precursor  of 
lungs.  At  present,  however,  the  lungs  are  considered  to  be  the  precursor 
of  the  swim  bladder,  for  the  organ  is  most  lunglike  in  the  most  primi- 
tive bony  fishes. 

It  is  believed  that  the  ancestral  bony  fishes  had  lungs  similar  to 
those  of  the  living  African  lungfish  (Protopterus).  In  the  lungfish  (Fig. 
22.11)  a  pair  of  saclike  lungs  develop  as  a  ventral  outgrowth  from  the 
posterior  part  of  the  pharynx.  The  lungs  enable  the  fish  to  survive 
conditions  of  stagnant  water  and  drought.  The  rivers  in  which  the 
African  lungfish  live  may  completely  dry  up,  but  the  fish  can  survive 
curled  up  within  a  mucous  cocoon  that  it  secretes  around  itself  in  the 
dried  mud.  A  small  opening  from  the  cocoon  to  the  surface  of  the  mud 
enables  the  fish  to  breathe  air  during  this  period.  The  African  lungfish 
has  become  so  dependent  upon  its  lungs  that  it  will  die  if  it  cannot 
occasionally  reach  the  surface  to  gulp  air. 

Air  breathing  probably  evolved  in  fishes  as  a  supplement  to  gill 
respiration.  Presumably  early  bony  fishes,  or  perhaps  their  placoderm 
ancestors,  evolved  lungs  as  an  adaptation  to  the  unreliable  fresh-water 
conditions  of  the  Devonian  period.  Geologic  evidence  indicates  that  the 
Devonian  was  a  period  ol  Irecjuent  seasonal  drought.  Bodies  of  fresh 
water  undoubtedly  either  became  stagnant  swamps  with  a  low  oxygen 
content,  or  dried  up  completely.  Only  fishes  with  such  an  adaptation 
could  survive  these  conditions.  The  others  became  extinct  or  migrated 
to  the  sea,  as  did  many  later  placoderms  and  the  cartilaginous  fishes. 


^:0    Lixng  of 

land  vertebrates 


Teleost 

Swrim  blad-der 


Primitive 
fish  lang 


TrcLnsitional  type. 


Figure  22.11.     A  diagram  to  illustrate  the  evolution  of  lungs  and  the  swim  blad- 
der. (.After  Dean.) 


440  VERTEBRATE   LIfE   AND   ORGANIZATION 

Groups  of  bony  fishes  that  have  remained  in  fresh  water  throughout 
their  history  tendetl  to  retain  kmglike  organs,  but  those  that  went  to 
sea  no  longer  needed  lungs,  for  ocean  waters  are  rich  in  oxygen.  Their 
useless  lungs  evolved  into  useful  hydrostatic  organs.  What  are  presumed 
to  be  intermediate  stages  in  this  shift  can  still  be  seen  in  certain  species. 
Later,  when  conditions  were  more  favorable,  many  salt-water  bony  fishes 
reentered  fresh  water,  but  retained  their  swim  bladders.  The  fresh-water 
perch  has  had  such  a  history.  Its  ancestors  first  evolved  lungs  in  a  fresh- 
water environment,  then  went  into  the  ocean  where  the  lungs  changed 
into  swim  bladders;  later  the  fish  reentered  fresh  water  and  retained 
the  swim  bladders. 


201 .        Evolution  of  Bony  Fishes 

Bony  fishes  have  enough  features  in  common  to  indicate  their  evolu- 
tion from  a  common  ancestral  stock,  but  they  early  diverged  into  two 
separate  lines— the  subclasses  Actinopterygii  and  Sarcopterygii.  The 
actinopterygians  are  the  ray-finned  fishes  like  the  perch  (Fig.  22.9  A). 
Their  paired  appendages  are  fan-shaped  and  are  supported  by  numerous 
dermal  rays  derived  from  bony  scales.  Their  paired  olfactory  sacs  connect 
only  with  the  outside.  The  sarcopterygians  are  the  fleshy-finned  fishes 
such  as  our  present  day  lungfishes  (Fig.  22.9  B).  Their  paired  appendages 
are  typically  elongate  and  lobe-shaped,  supported  internally  by  an  axis 
of  flesh  and  bone,  fn  many  species,  each  of  the  olfactory  sacs  connects  to 
the  body  surface  through  an  external  nostril  and  to  the  front  part  of  the 
roof  of  the  mouth  cavity  through  an  internal  nostril. 

Ray-Finned  Fishes.  Actinopterygian  evolution  presents  a  good  ex- 
ample of  a  succession  in  which  early  dominant  groups  became  replaced 
by  more  successful  types.  Three  superorders  are  recognized,  and  each  in 
turn  had  its  day  (Fig.  22.2).  Currently  the  superorder  Chondrostei  have 
dwindled  to  a  few  species  of  which  the  Nile  bichir  (Pulypterus)  and  the 
sturgeon  {Scaphirhynclnis)  are  examples  (Fig.  22.12).  The  superorder 
Holostei  have  also  dwindled  and  are  represented  today  by  such  relict 
species  as  the  gar  (Lepisosteiis)  and  bowfin  (Amia).  The  superorder 
Teleostei,  in  contrast,  have  been  continuously  expanding  since  their 
origin  near  the  middle  of  the  Mesozoic  era.  It  is  to  this  group  that  the 
perch  and  most  fishes  belong. 

Various  evolutionary  tendencies  can  be  traced  through  this  suc- 
cession. The  functional  lungs  of  early  actinopterygians  (still  retained  in 
Polypterus)  became  transformed  into  swim  bladders  with  little  respira- 
tory function.  Correlated  with  increased  buoyancy  and  better  streamlin- 
ing, we  find  that  the  primitive  heterocercal  tail  of  most  chondrosteans 
(Polypterus  is  an  exception)  became  superficially  symmetrical  in  teleosts, 
but  the  caudal  skeleton  still  shows  indications  of  the  upward  tilt  of  the 
vertebral  column.  Such  a  tail  is  said  to  be  homocercal  (Fig.  22.9  A). 
Holosteans  have  an  intermediate  abbreviated  heterocercal  tail.  Early 
actinopterygians  were  clothed  with  thick,  bony  scales  characterized  by 
having  many  layers  of  enamel-like  ganoin  covering  the  surface.  During 
subsequent  evolution  the  superficial  layers  were  lost,  and  the  bone  was 


A    HISTORY   Of   VERTEBRATES:   FISHES 


441 


Figure  22.12.  A  group  of  primitive  ray-finned  fishes  that  have  survived  to  the  pres- 
ent day.  A,  The  Nile  bichir,  Polypterus;  B,  the  shovel-nosed  sturgeon,  Scaphirhynchus 
platorhynchus;  C,  the  longnose  gar,  Lepisosteus  oseiis;  D,  the  bowfin,  Ainia  calva.  (A, 
After  Dean;  B,  C  and  D,  courtesy  American  Museum  of  Natural  History.) 

reduced  to  a  thin  disc.  Such  a  scale  is  termed  cycloid  if  its  surface  is 
smooth,  or  ctenoid  if  the  exposed  part  of  its  surface  bears  minute 
processes  resembling  the  teeth  of  a  comb. 

Adaptive  Radiation  of  the  Teleosts.     The  more  primitive   tele- 
osts,  such  as  the  herring  and  tarpon  (Fig.  22.13  A),  are  active,  predaceous, 


442 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Streamlined  fishes  of  the  open  waters.  But  there  have  been  many  inter- 
esting departures  trom  these  generahzed  types.  The  20,000  or  more 
species  of  teleosts  have  spread  out  into  all  parts  of  the  aquatic  environ- 
ment, and  have  become  adapted  to  nearly  every  conceivable  ecologic 
niche.  This  phenomenon  of  adaptive  radiation  is  seen  in  all  large 
groui:)s.  Apparently  the  resources  of  the  environment  can  be  utilized 
more  fully  if  subgroups  become  specialized  for  certain  parts  of  the  en- 
vironment than  if  all  try  to  compete  with  each  other  in  the  total  environ- 
ment. 

Tlie   halibut,   soles   and   flounders  have    become   specialized   for    a 


Figure  22.13.  Adaptive  radiation  among  the  teleosts.  A,  A  tarpon,  Tarpon,  one  of 
the  more  primitive  types  of  teleosts;  B,  a  halibut,  Hippoglossus,  one  of  the  flatfish  that 
feeds  along  the  bottom;  C,  a  male  sea  horse,  Hippocampus,  with  the  brood  pouch  in 
which  the  female  deposits  her  eggs,  young  shown  at  right;  D,  the  sargassum  fish,  Histrio, 
appears  very  bizarre  out  of  its  natural  environment,  but  is  well  concealed  among  sea- 
weed; E,  the  sharksucker,  Remora;  F,  the  moray  eel,  GymnoUwrax,  normally  lurks  within 
interstices  of  coral  reefs.  {A,  C,  D,  E,  F,  courtesy  of  Marine  Studios;  B,  courtesy  of  the 
American  Museum  of  Natural  History.) 


A   HISTORY  Of   VERTEBRATES:   FISHES  443 

bottom-dwelling  life.  Like  the  skates  and  rays,  they  are  flattened  and 
glide  along  the  bottom  with  up  and  down  undulatory  movements.  But 
instead  of  being  flattened  dorso-ventrally,  they  are  greatly  compressed 
from  side  to  side,  and  swim  turned  over  on  one  side  (Fig.  22.13  B). 
During  larval  development,  the  eye  that  would  be  on  the  "ventral"  side 
migrates  to  the  top  surface,  but  the  mouth  does  not  change  position.  The 
skates  and  flounders  present  a  good  example  of  convergent  evolution, 
by  which  animals  that  are  widely  separated  in  the  evolutionary  scale 
independently  adapt  to  similar  modes  of  life.  They  acquire  similar 
adaptive  features,  in  this  case  a  flattened  body  shape,  though  in  diffier- 
ent  ways. 

Other  teleosts  have  adapted  to  a  life  among  seaweeds  and  in  coral 
reefs.  The  sea  horses  with  their  monkey-like,  prehensile  tails;  the  sar- 
gassum  fish  with  its  camouflaging  color  and  weedlike  protuberances; 
and  the  elongate,  snakelike  moray  eels  are  examples  (Fig.  22.13). 

A  few  teleosts  have  adapted  to  life  in  the  ocean  depths.  Such  fish 
often  have  light-producing  knninescent  organs,  presumably  for  species 
recognition,  and  large  mouths  and  greatly  distensible  stomachs  to  take 
full  advantage  of  the  occasional  meal  that  may  come  their  way. 

Some  teleosts  live  in  intimate  association  with  other  fishes.  The 
remora  has  an  anterior  dorsal  fin  that  is  modified  as  a  suction  cup  and 
is  used  to  attach  to  sharks.  It  feeds  upon  crinnbs  of  the  larger  fish's 
meals,  or  obtains  free  rides  to  favorable  feeding  grounds.  Relationships 
of  this  type,  in  which  one  organism  benefits  and  the  other  receives 
neither  benefit  nor  harm,  are  known  as  commensalism    (Fig.  22.13). 

A  few  teleosts  have  become  amphibious.  The  Australian  mudskip- 
per  frequently  hops  about  on  the  mud  flats  of  mangrove  swamps  at  low 
tide  in  search  of  food,  and  may  even  bask  in  the  sun.  It  has  unusually 
muscular  pectoral  fins  to  help  pull  itself  along  the  land,  and  it  can  close 
its  opercular  chamber  and  extract  oxygen  from  the  air  with  its  gills. 

Many  other  fascinating  adaptations  are  found  among  these  fishes, 
but  we  must  not  dwell  upon  them  for  the  teleosts  are  only  a  side  issue 
in  the  total  picture  of  vertebrate  evolution.  The  main  branch  toward 
the  higher  vertebrates  passed  through  the  less  spectacular  Sarcopterygians 
of  ancient  Devonian  swamps. 

Fleshy-Finned  Fishes.  Sarcopterygian  evolution  diverged  at  an 
early  time  into  two  lines— the  lungfishes  (order  Dipnoi)  and  the  crossop- 
terygians  (order  Crossopterygii).  The  primitive  crossopterygians  were 
the  less  specialized,  having  a  well  ossified  internal  skeleton  and  small 
conical  teeth  suited  for  seizing  prey.  It  is  from  this  group  that  the  am- 
phibians arose.  Lungfishes  early  in  their  evolution  developed  specialized 
crushing  tooth  plates,  and  showed  tendencies  toward  reduction  of  the 
internal  skeleton  and  paired  appendages.  In  certain  other  features  lung- 
fishes and  crossopterygians  have  paralleled  actinopterygian  evolution. 
They  evolved  symmetrical  tails,  though  of  a  type  that  is  symmetrical 
internally  as  well  as  externally  (diphycercal),  and  their  primitive,  thick, 
bony  scales,  which  were  characterized  by  having  a  thick  layer  of  dentin- 
like  cosmin,  have  tended  to  thin  to  the  cycloid  type. 


444  VERTEBRATE   LIFE   AND    ORGANIZATION 


v*^ 


Figure  22.14.     Laliineria,  a  living  coelacanth  found  off  the  coast  of  the  Comoro 
Islands.  (From  Millet.) 

Both  crossopterygians  and  lungfishes  were  successful  in  the  fresh 
waters  of  the  Devonian,  but  have  dwindled  to  a  few  relict  species  today. 
Lungfishes  retained  their  lungs  and  have  survived  in  the  unstable  fresh- 
water environments  of  tropical  South  America,  Africa  and  Australia.  It 
was  long  believed  that  all  crossopterygians  had  become  extinct,  as  indeed 
the  primitive  fresh-water  ones  have.  However,  a  few  specimens  of  a 
soinewhat  specialized  side  branch  (the  coelacanths)  have  been  found  in 
recent  years  near  the  Comoro  Islands  between  Africa  and  Madagascar 
(Fig.  22.14).  Their  internal  anatomy  is  being  studied  carefully  by  Pro- 
fessor Millot  of  Paris,  who  has  several  well  preserved  specimens  at  his 
disposal,  and  we  should  soon  know  more  about  the  structure  of  these 
interesting  creatures. 


Questions 

1.  How  do  homologous  organs  differ  from  analogous  organs?  Can  it  be  assumed  that 
organisms  having  homologous  organs  are  closely  related? 

2.  What  factors  prevent  the  fossil  record  from  giving  us  a  complete  and  unbiased  picture 
of  the  life  of  the  past? 

3.  Briefly  describe   the  general  nature  and   mode  of  life  of  the  ostracoderms.  What 
living  vertebrates  are  most  closely  related  to  them? 

4.  What  were  the  major  evolutionary  advances  of  the  placoderms? 

5.  How  do  members  of  the  class  Chondrichthyes  differ  from   members  of  the  class 
Ostcichthyes? 

6.  How  have  a  typical  shark,  a  whale  shark,  a  skate  and  a  sawfish  diverged  in  their 
method  of  feeding? 

7.  Under  what  conditions  did  lungs  probably  evolve?  Is  the  swim  bladder  more  primi- 
tive than  lungs? 

8.  How  do  actinopterygians  differ  from  sarcopterygian  fishes? 

9.  What  morphologic  changes  occurred  during  actinopterygian  evolution? 

10.  From  which  group  of  fishes  did  tetrapods  evolve? 

11.  Define  and  give  an  example  of  adaptive  radiation. 

12.  Define  and  give  an  example  of  convergent  evolution. 


A    HISTORY   Of   VERTEBRATES:   FISHES  445 

Supplementary  Reading 

Parker  and  Haswell,  Text-Book  of  Zoology,  and  Young,  Life  of  Vertebrates,  contain 
very  good  accounts  of  the  major  groups  of  vertebrates.  Living  species  are  emphasized. 
Romer's  Man  the  Vertebrates  and  Colbert's  Evolution  of  the  Vertebrates  are  very  read- 
able and  fascinating  accounts  of  the  evolution  of  back-boned  animals.  More  technical 
details  can  be  found  in  Romer,  Vertebrate  Paleontology,  or  Gregory,  Evolution  Emerging. 

The  adaptations  of  fishes  and  other  aspects  of  their  biology  are  interestingly  dis- 
cussed by  Norman  in  A  History  of  Fishes.  The  damage  caused  by  the  sea  lamprey  in  the 
Great  Lakes,  its  life  history  and  possible  means  of  control  are  considered  by  Applegate 
and  Moffett  in  an  article,  Sea  Lamprey  and  Lake  Trout,  published  in  Flanagan's  Twen- 
tieth Century  Bestiaiy.  Those  interested  in  the  taxonomy  and  natural  history  of  marine 
fishes  of  the  Atlantic  Coast  should  consult  Breder,  Field  Book  of  Marine  Fishes.  More 
technical  details  of  this  group  of  fishes  are  a\ailable  in  a  monograph.  Fishes  of  the  West- 
ern North  Atlantic,  being  prepared  by  the  Sears  Foundation  for  Marine  Research.  Two 
volumes  on  cyclostomes  and  cartilaginous  fishes,  written  by  Bigelow  and  Schroeder,  have 
been  published. 


CHAPTER  23 


A  History  of  Vertebrates: 
Amphibians  and  Reptiles 


202.       The  Transition  from  Water  to  Land 

The  transition  tiom  fresh  water  to  land  was  a  momentous  step  in 
vertebrate  evolution  that  opened  up  vast  new  areas  for  exploitation.  It 
was  an  extremely  difficult  step  because  the  physical  conditions  on  land 
are  so  very  different  from  those  in  water.  Air  neither  affords  as  much 
support,  nor  offers  as  much  resistance  as  water.  The  terrestrial  environ- 
ment provides  little  of  the  essential  body  water  and  salts.  Oxygen  is  more 
abundant  in  the  air  than  in  water,  but  it  must  be  extracted  from  a 
different  medium.  The  ambient  temperature  fluctuates  much  more  on 
the  land  than  in  the  water.  Air  and  water  have  different  refractive 
indices. 

Successful  adaptation  to  the  terrestrial  environment  necessitated 
changes  throughout  the  body.  Stronger  skeletal  support  and  different 
methods  of  locomotion  evolved.  Changes  occurred  in  the  equipment  for 
sensory  perception  and  changes  in  the  nervous  system  were  a  natural 
corollary  of  the  more  complex  muscular  system  and  altered  sense  organs. 
An  efficient  method  of  obtaining  oxygen  from  the  air  evolved,  as  did 
adaptations  to  prevent  desiccation.  The  delicate,  free-swimming,  aquatic 
larval  stage  was  suppressed,  and  reproduction  upon  land  became  pos- 
sible. Finally,  the  ability  to  maintain  a  fairly  constant  and  high  body 
temperature  was  achieved,  and  terrestrial  vertebrates  could  then  be 
active  under  a  wide  range  of  external  temperatures. 

In  view  of  the  magnitude  of  these  changes,  it  is  not  surprising  that 
the  transition  from  water  to  land  was  not  abrupt,  but  took  millions  of 
years,  and  involved  the  participation  of  many  groups.  Indeed  the  main 
theme  in  the  evolution  of  the  terrestrial  vertebrates,  or  tetrapods,  has 
been  a  continual  improvement  in  their  adjustment  to  terrestrial  condi- 
tions. 

The  crossopterygians  unwittingly  made  the  first  steps  in  this  transi- 
tion. Their  lungs,  as  we  have  seen,  were  probably  an  adaptation  to 
survive  conditions  of  stagnant  water  or  temporary  drought.  Their  rela- 
tively strong,  lobate,  paired  fins  enabled  them  to  squirm  from  one  drying 
and  overcrowded  swamp  to  another  more  favorable  one.  Crossopterygians 
were  not  trying  to  get  onto  the  land,  but,  in  adapting  to  their  own 
environment,  they  evolved  features  that  made  them  viable  in  a  new  and 

446 


A    HISTORY   Of   VERTEBRATES:   AMPHIBIANS    AND    REPTILES 


447 


Figure  23.1.  A  restoration  of  life  in  a  Carboniferous  swamp  250  million  years  ago. 
The  labyrinthodont  amphibians  were  the  first  terrestrial  vertebrates.  (Courtesy  of  the 
American  Museum  of  Natural  History.) 

different  environment.  That  is,  they  became  preadapted  to  certain 
terrestrial  conditions.  Given  this  preadaptation,  an  abundance  of  food 
(various  invertebrates,  stranded  fishes,  plants)  upon  the  land  or  the 
shores  of  swamps,  little  competition  upon  the  land,  and  overcrowding 
and  intense  population  pressure  in  the  swamps,  it  is  not  hard  to  imagine 
some  of  the  crossopterygians  making  the  adaptive  shift  from  water  to 
land  and  becoming  the  amphibians  (Fig.  23.1).  No  one  knows  how  long 
the  transition  from  crossopterygians  to  amphibians  took,  but  the  first 
amphibian  fossils  are  found  in  strata  that  were  formed  nearly  50  million 
years  later  than  those  containing  the  first  crossopterygians. 

Amphibians,  in  turn,  acquired  additional  terrestrial  features,  and 
reptiles  still  more.  But  the  pinnacle  of  terrestrial  adaptation  is  achieved 
only  by  the  reptiles'  descendants— the  birds  and  mammals. 


203.        Evolution  and  Characteristics  of  Amphibians 

The  ancestral  amphibians,  which  are  known  as  the  labyrinthodonts, 
finally  diverged  from  the  crossopterygians  during  the  late  Devonian 
period  (Fig.  22.2).  An  interesting  detail  they  shared  with  the  crossop- 
terygians was  a  peculiar,  labyrinthine  infolding  of  the  enamel  in  their 
teeth.  The  name  labyrinthodont  is  derived  from  this  feature.  All  were 
fairly  clumsy,  salamander-shaped  creatures  with  rudimentary  necks  and 
heavy,  muscular  tails  inherited  from  their  piscine  ancestors  (Fig.  23.1). 
Their  rather  heavy  limbs  were  sprawled  out  at  right  angles  to  the  body, 
and  probably  served  only  as  aids  to  fishlike,  lateral  undulations  of  the 
trunk  and  tail  in  progressing  along  the  land.  All  became  extinct  during 


448  VERTEBKATE   LIFE   AND   ORGANIZATION 

the  Triassic.  However,  the  group  is  important  for  it  included  not 
only  the  ancestors  ol  modern  amphibians,  but  also  those  of  reptiles  and 
hence  ol  all  higher  tetrapods.  Studies  of  the  details  of  vertebral  develop- 
ment suggest  that  frogs  and  toads  (order  Anura)  are  fairly  dnect  de- 
scendants of  labvrinthodonts,  whereas  the  salamanders  (order  Urodela) 
and  ilic  legless,  burrowing  caecilians  of  the  tropics  (order  Apoda)  appear 
to  have  followed  a  different  course  of  evolution  from  some  early  lab- 
yrinthodont  stock. 

In  the  course  of  evolution,  amphibians  lost  many  fishlike  character- 
istics, such  as  bony  scales,  the  lateral  line  sensory  system  (present  in 
larval  amphibians  but  not  the  adults),  and  gills.  The  loss  of  gills  made 
possible  a  more  efficient  circulatory  system,  for  blood  returning  to  the 
heart  from  the  lungs  can  be  distributed  directly  to  the  tissues  of  the  body 
without  the  loss  of  pressure  entailed  in  passing  through  gill  capillaries. 
Ami)hibians  also  evolved  such  terrestrial  features  as  the  five-toed, 
tetrapod  ajjpendage,  a  vertebral  column  with  interlocking  vertebrae  that 
provides  greater  support  for  the  body,  a  tongue  with  which  food  is 
manipulated  within  the  mouth,  eyelids  and  tear  glands  that  protect  and 
cleanse  the  eye,  and  a  mechanism  with  which  ground  or  air-borne  vibra- 
tions can  be  detected. 

However,  the  terrestrial  adaptation  of  amphibians  is  deficient  in 
several  respects.  First,  most  are  unable  to  prevent  a  large  loss  of  body 
water  when  on  land  and  must  stay  close  to  fresh  water.  Second,  all  are 
cold-blooded,  or  poikilothermic,  as  are  fishes;  their  body  temperature 
is  close  to  that  of  the  environment  and  fluctuates  with  it.  They  cannot 
maintain  a  constant  and  rather  high  body  temperature.  Since  the  rate 
of  metabolic  processes  fluctuates  with  temperature  changes,  they  cannot 
be  active  at  low  temperatures.  The  terrestrial  poikilotherms  living  in 
temperate  regions  must  move  during  the  winter  to  areas  that  do  not 
freeze,  and  enter  a  dormant  state  known  as  hibernation.  Amphibians 
bury  themselves  in  the  mud  at  the  bottom  of  ponds,  or  burrow  into  soft 
ground  below  the  frost  line.  During  hibernation  metabolic  activities  are 
at  a  minimum.  The  only  food  utilized  is  that  stored  within  the  body; 
respiration  and  circulation  are  very  slow.  Some  tropical  amphibians 
during  the  hottest  and  driest  parts  of  the  year  go  into  a  comparable 
dormant  state  known  as  aestivation. 

Finally,  amphibians  are  unable  to  reproduce  under  truly  terrestrial 
conditions.  Like  the  common  leopard  frog  (Rana  pipiens),  most  of  them 
must  return  to  the  water  to  lay  their  eggs.  Even  the  terrestrial  toad 
returns  to  this  medium,  for  it  has  no  means  of  internal  fertilization  and 
sperm  cannot  be  sprayed  over  eggs  upon  the  land.  Neither  has  it  sup- 
pressed the  free  larval  stage  in  development,  and  these  larvae  cannot 
withstand  the  rigors  of  the  terrestrial  environment. 

204.       Amphibian  Adaptations 

Sofamanders.  Salamanders  are  not  such  familiar  amphibians  as 
frogs  and  toads,  for  most  have  secretive  habits.  They  may  be  found 
beneath  stones  and  logs  in  damp  woods  or  beneath  stones  along  the  side 


A    HISTORY   OF   VERTEBRATES:   AMPHIBIANS    AND    REPTILES  449 

of  Streams,  and  some  are  entirely  aquatic.  A  rather  generalized  type  is 
Jefferson's  salamander,  Amby stoma  jeffersonianum  (Fig.  23.2),  of  the 
eastern  United  States.  This  species  is  terrestrial  as  an  adult,  but  returns 
to  the  water  in  early  spring  to  reproduce.  Breeding  is  sometimes  pre- 
ceded by  a  nuptial  dance  in  which  many  individuals  writhe  about  in 
the  water,  rubbing  and  nosing  one  another.  The  males  deposit  sperm 
in  clumps  called  spermatophores  on  sticks  and  leaves  in  the  water.  Later 
the  females  pick  these  up  with  their  cloacal  lips.  Fertilization  is  internal, 
and  the  fertilized  eggs  are  deposited  in  masses  attached  to  sticks  in  the 
water.  The  larvae  of  salamanders  differ  from  those  of  frogs  and  toads  in 
retaining  external  gills  throughout  their  larval  life,  and  in  having  true 
rather  than  horny  teeth. 

There  have  been  many  special  adaptations  among  salamanders.  The 
most  abundant  of  our  American  species  are  woodland  types  like  the 
red-backed  salamander  {Plethodon  cinereus),  which  belongs  to  the  family 
Plethodontidae.  A  particularly  interesting  feature  of  plethodonts  is  their 
complete  loss  of  lungs;  gas  exchange  occurs  entirely  across  the  moist 
membranes  lining  the  mouth  and  pharynx  and  the  skin.  The  skin  is  a 
more  effective  respiratory  organ  than  in  frogs  because  the  epidermis  is 
very  thin  and  capillaries  come  close  to  the  surface.  Loss  of  lungs  may 
seem  to  be  a  curious  adaptation  for  a  terrestrial  vertebrate,  but  it  has 
been  postulated  that  early  in  their  evolution  plethodonts  became 
adapted  for  life  in  rapid  mountain  streams.  Air  in  the  lungs  would  be 
disadvantageous  under  these  conditions,  for  the  animals  would  float  and 
be  washed  away.  Lungs  may  have  been  lost  in  adapting  to  this  habitat. 
Subsequently  jjlethodonts  may  have  entered  different  environments,  but 
never  regained  the  lost  lungs. 


Figure  23.2.  Jefferson's  salamander,  Ambystoma  jeffersonianum,  reproduces  in  the 
water.  The  four  white  structures  attached  to  sticks  are  spermatophores.  A  clump  of 
eggs  can  be  seen  in  the  upper  righthand  corner. 


450  VERTEBRATE    LIFE   AND   ORGANIZATION 

^onnwr^r--  tWiiilllllllllHI—HHIIWIII  '  'I ^ '' ' ''  H "  IH  H^n 


Figure  23.3.  Neotenic  salamanders.  4,  The  mudpuppy,  Necturus  maculosus,  is  a 
permanent  larva.  B,  The  tiger  salamander,  Anibystoi/ia  tigritiuin,  metamorphoses  in 
most  environments,  but  fails  to  do  so  in  certain  mountain  lakes.  C,  The  axolotl,  or  neo- 
tenic form  of  Ambystoma  tigrinum.  {A,  Courtesy  of  Shedd  Aquarium,  Chicago;  B-C, 
courtesy  of  the  Philadelphia  Zoological  Society.) 

Several  groups  of  salamanders,  including  the  mudpuppy  {Necturus 
maculosus,  Fig.  23.3  A),  have  become  entirely  aquatic.  The  development 
of  the  reproductive  organs  has  been  speeded  up  in  relation  to  develop- 
ment of  other  parts  of  the  body.  Sexual  maturity  is  achieved  in  the 
larval  stage  and  metamorphosis  is  never  completed.  This  is  another 
example  of  neoteny,  a  phenomenon  encountered  earlier  in  the  lower 
chordates.  The  hormone  of  the  thyroid  gland,  thyroxin,  is  necessary  for 
metamorphosis.  The  failure  of  Necturus  to  metamorphose  appears  to 
result  from  the  inability  of  the  tissues  to  respond  to  thyroxin  rather  than 
from  an  absence  of  this  hormone.  Thyroxin  is  produced,  for  the  thyroid 
of  Necturus  hastens  metamorphosis  when  transplanted  to  frog  tadpoles. 


A    HISTORY   Of    VERTEBRATES:    AMPHIBIANS    AND    REPTILES 


451 


In  some  other  neotenic  salamanders,  the  failure  to  metamorphose  may 
result  from  an  inhibition  of  the  mechanism  that  releases  thyroxin.  The 
tiger  salamander,  Ambystoma  tigrinum  (Fig.  23.3  B),  metamorphoses 
under  most  conditions,  but  those  living  at  high  altitudes  in  the  Rocky 
Mountains  fail  to  do  so  and  remain  permanent  larvae  known  as  ax- 
olotls.  Apparently  cold  inhibits  the  release  of  thyroxin,  for  when  axolotls 
are  fed  thyroxin  or  when  they  are  brought  to  warmer  climates,  meta- 
morphosis is  normal. 

frogs  and  Toads.  Most  anurans  are  amphibious  as  adults,  living 
near  water  to  which  they  frequently  go  to  feed  or  escape  danger,  but 
some  are  more  terrestrial  in  habits,  and  others  have  become  adapted  to 
an  arboreal  life.  The  terms  frog,  toad,  and  tree  frog  or  tree  toad  or- 
dinarily imply  amphibious,  terrestrial  and  arboreal  modes  of  life,  not 
natural  evolutionary  groups.  Members  of  several  distinct  families  of 
anurans,  for  example,  have  become  adapted  independently  to  an  ar- 
boreal life. 

Toads  have  adjusted  to  a  terrestrial  life  by  evolving  structures  and 
patterns  of  behavior  that  reduce  water  loss.  The  epidermis  of  their  skin 
is  more  horny  and  less  pervious  to  water  than  that  of  frogs.  A  thick, 
dry  skin  reduces  cutaneous  respiration,  but  this  is  compensated  for  by 
an  increase  in  the  respiratory  surface  of  the  lungs.  The  lining  of  toad 
lungs  is  more  complexly  folded  than  that  of  frogs.  Much  of  the  water 
lost  through  the  kidneys  is  reabsorbed  in  the  urinary  bladder.  Toads 
are  crepuscular  in  habits;  they  burrow  or  take  shelter  by  day,  and  come 
out  in  the  moist  evening  to  feed  upon  insects. 


Figure  23.4.     The  tree  frog.  Hyla  versicolor,  clings  to  trees  by  means  of  its  expanded 
digital  pads.  (Courtesy  of  the  New  York  Zoological  Society.) 


452  VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  23.5.  Adaptations  of  frogs  that  protect  the  larvae  from  aquatic  predators. 
A,  The  imid  craters  of  the  BraziHan  tree  frog,  Hyla  faber;  B,  the  brood  pouch  of  the 
marsupial  frog,  Gastrotheca,  cut  open  to  show  the  eggs;  C,  the  modified  embi7o  of 
Eleuthewdactylus.  {A,  After  Barbour;  B,  after  Noble;  C,  after  Lynn.) 


The  chief  adaptation  to  arboreal  life  has  been  the  evolution  of 
digital  pads  upon  the  tips  of  the  toes  (Fig.  23.4).  The  surface  epithelium 
of  the  pads  is  rough  and  grips  the  substratum  by  friction.  The  gripping 
action  is  enhanced  by  the  discharge  of  a  sticky  mucus  from  numerous 
glands  within  the  pads. 

A  particularly  fascinating  aspect  of  anuran  biology  is  the  evolution 
of  methods  by  which  development  can  proceed  elsewhere  than  in  the 
open  water.  This  has  occurred  primarily  among  tropical  frogs  and  prob- 
ably as  a  protection  against  varying  aquatic  conditions  and  numerous 
acjuatic  enemies  such  as  predaceous  insect  larvae.  A  Brazilian  tree  frog 
(Hyla  faber)  protects  its  young  by  laying  its  eggs  in  mud  craters  which 
it  has  built  in  the  water  (Fig.  23.5  A).  A  more  striking  means  of  protec- 
tion is  seen  in  a  small  Chilean  frog,  Rhinoderma  darwinii.  The  male  of 
this  species  stuffs  the  fertilized  eggs  into  his  vocal  sacs  where  they  remain 
iMitil  metamorphosis  is  complete.  Both  of  these  frogs  have  fairly  typical 
anuran  larvae  that  hatch  from  the  egg  and  develop  in  a  shelterecl  en- 
vironment. 

In  certain  species  the  vulnerable  larval  stage  is  omitted,  and  the 
embryo  develops  directly  into  a  miniature  adult.  Anurans  with  direct 
development  include  the  marsupial  frog,  Gastrotheca,  and  Eleuthero- 
dactyl us— both  of  the  New  World  tropics  (Fig.  23.5  B).  The  former 
carries  her  eggs  in  a  dorsal  brood  pouch;  the  latter  lays  eggs  in  protected 
damp  places  such  as  beneath  stones  or  in  the  axil  of  leaves.  The  jelly 
layers  about  the  egg  of  Eleutherodactylus  help  prevent  desiccation  (Fig. 
23.5  C);  sufficient  yolk  is  stored  within  the  egg  for  the  nutritive  require- 
ments of  the  embryo;  such  larval  features  as  horny  teeth,  gills  and 
opercular  fold  are  vestigial  or  absent;  the  fins  of  the  larval  tail  are  ex- 


A    HISTORY   OF    VERTEBRATES:   AMPHIBIANS    AND   REPTILES 


453 


panded,  become  highly  vascular  and  form  an  organ  for  gas  exchange; 
and  the  period  of  development  is  accelerated. 

Something  similar  to  what  has  taken  place  among  these  frogs  today 
may  have  occurred  among  the  amphibians  which  were  ancestral  to 
reptiles.  Ancient  amphibians  were  certainly  no  more  consciously  trying 
to  improve  upon  their  terrestrial  adaptation  than  crossopterygians  were 
trying  to  get  onto  the  land.  U  the  aquatic  larvae  of  the  ancestral  am- 
phibians were  subjected  to  a  very  high  predation,  any  variation  that 
tended  toward  the  suppression  of  the  defenseless  larval  stage  and  toward 
the  direct  development  of  their  embryos  in  a  less  vulnerable  environ- 
ment would  have  a  selective  advantage.  It  is  possible  that  the  amphibians 
that  gave  rise  to  the  reptiles  developed  means  of  terrestrial  reproduction 
before  the  adults  completely  left  the  water, 

205.        Characteristics  of  Reptiles 

The  adjustment  of  most  amphibians  to  terrestrial  conditions  is 
deficient  in  three  respects:  (1)  poor  means  of  conserving  body  water,  (2) 
inability  of  most  to  reproduce  on  the  land  and  (3)  inability  to  maintain 
their  body  temperature  and  metabolic  processes  at  a  fairly  constant  level. 
Reptiles,  as  a  group,  evolved  adequate  solutions  to  the  first  two  of  these 
problems,  and  certain  extinct  reptiles  probably  achieved  some  measine 
of  control  over  their  body  temperature.  Reptiles  also  improved  upon 
the  means  of  locomotion  and  gas  exchange,  and  other  terrestrial  at- 
tributes of  their  amphibian  ancestors. 

A  lizard,  such  as  the  collared  lizard  (Crotapliytits  coUaris  Fig.  23.6) 
of  the  southwestern  United  States,  is  a  typical  reptile,  llie  surface  of 
the  skin  is  covered  with  dry,  horny  scales  that  prevent  water  loss  by  this 
route.  These  scales  develop  through  the  deposition  of  considerable 
keratin  (a  very  insoluble  and  hence  waterproofing  protein)  in  the  super- 
ficial layers  of  the  epidermis. 


Figure  23.6.     The  collared  lizard,  Crotaphytus  coUaris.  (Courtesy  of  the  New  York 
Zoological  Society.) 


454  VERTEBRATE   LIFE   AND    ORGANIZATION 

The  kidney  tubules  of  reptiles  are  modified  in  such  a  way  that  less 
water  is  initially  removed  from  the  blood  than  in  amphibians,  and  much 
of  the  water  that  is  removed  is  later  reabsorbed  by  other  parts  of  the 
kidney  tubule  and  by  the  urinary  bladder.  In  some  reptiles  the  nitrog- 
enous waste  products  are  excreted  as  uric  acid,  which  is  much  less 
soluble  and  less  toxic  than  ammonia  or  urea.  Urea  and  ammonia  are 
characteristic  excretory  products  of  fresh-water  vertebrates.  The  urine 
of  animals  excreting  uric  acid  typically  has  a  pastelike  consistency.  The 
reptilian  kidney  also  differs  from  that  of  lower  vertebrates  in  being 
drained  by  a  duct  called  the  ureter  instead  of  by  the  Wolffian  duct.  The 
latter  becomes  a  genital  duct  in  males,  and  is  lost  in  females. 

The  reptilian  body  shape  is  better  adapted  to  land  life  than  the 
amphibian.  The  neck  is  longer  and  the  first  two  cervical  vertebrae  are 
specialized  to  permit  the  head  to  move  independently  of  the  rest  of  the 
body  as  the  animal  feeds.  The  tail  is  more  slender  than  in  the  lab- 
yrinthodonts  and  salamanders.  This  reflects  the  decreasing  importance 
of  fishlike  lateral  undulations  of  the  trunk  and  tail  in  locomotion,  and 
the  increasing  importance  of  the  limbs.  Well  formed  claws,  which  are 
basically  modified  horny  scales,  are  borne  upon  the  toes.  The  more 
powerful  hind  legs  require  a  pelvic  girdle  that  is  attached  more  firmly 
onto  the  vertebral  column.  Reptiles  typically  have  two  sacral  vertebrae 
whereas  amphibians  have  only  one. 

Improved  locomotion  and  increased  agility  also  involve  a  more 
elaborate  muscular  system,  nervous  system  and  sense  organs.  The  delicate 
tympanic  membrane  is  protected  by  lying  deep  within  a  canal,  the 
external  auditory  meatus,  and  the  eye  is  further  protected  through 
the  evolution  of  a  third,  transparent  eyelid  known  as  the  nictitating 
membrane. 

The  dry,  horny  skin  of  reptiles  reduces  cutaneous  respiration  to  a 
negligible  amount,  but  an  increase  in  the  respiratory  surface  of  the  lungs 
not  only  compensates  for  this,  but  also  provides  for  the  increased  volume 
of  gas  exchange  necessitated  by  a  general  increase  in  activity.  Mech- 
anisms for  moving  air  into  and  out  of  the  lungs  are  also  more  efficient. 
Instead  of  pumping  air  into  the  lungs  by  froglike  throat  movements, 
reptiles  decrease  the  pressure  within  their  body  cavity,  and  atmospheric 
pressure  drives  in  air.  A  subatmospheric  pressure  is  created  around  the 
lungs  during  inspiration  by  the  forward  movement  of  the  ribs  and 
the  concomitant  increase  in  size  of  the  body  cavity.  The  contraction  of 
abdominal  muscles  and  the  elastic  recoil  of  the  lungs  force  out  air. 
Circulatory  changes,  discussed  in  a  later  chapter,  further  separate  the 
oxygenated  and  unoxygenated  blood  leaving  the  heart,  and  make  the 
oxygen  siipjily  to  the  tissues  more  effective. 

Major  changes  have  come  about  in  the  method  of  reproduction. 
Male  reptiles  have  evolved  copulatory  organs  which  introduce  the  sperm 
directly  into  the  female  reproductive  tract.  Fertilization  is  internal,  and 
the  delicate  sperm  are  not  exposed  to  the  external  environment.  A  large 
quantity  of  nutritive  yolk  is  stored  within  the  egg  while  it  is  still  in  the 
ovary.  As  the  eggs  j)ass  down  the  oviduct  after  ovulation,  they  are 
fertilized,   and  additional  substances   and   a  shell    are   secreted   around 


A    HISTORY   Of   VERTEBRATES:   AMPHIBIANS   AND   REPTILES 


455 


each  one  by  certain  oviducal  cells.  Albumin  and  similar  materials 
around  the  egg  provide  additional  food,  ions  and  water.  The  leathery  or 
calcareous  shell  serves  tor  protection  against  mechanical  injury  and 
desiccation,  yet  it  is  porous  enough  to  permit  gas  exchange.  Such  an  egg, 
which  contains  or  has  the  means  of  providing  all  substances  necessary 
for  the  complete  development  of  the  embryo  to  a  miniature  adult,  is 
called  a  cleidoic  egg.  Reptiles  lay  fewer  eggs  than  lower  vertebrates,  but 
the  eggs  are  larger,  better  equipped  and  laid  in  sheltered  situations, 
so  the  mortality  is  low.  A  collared  lizard  lays  only  four  to  twenty-four 
eggs  in  contrast  to  the  two  or  three  thousand  of  the  leopard  frog. 

As  the  embryo  develops,  it  separates  from  the  yolk,  which  becomes 
suspended  in  a  yolk  sac  (Fig.  23.7).  Protective  layers  of  tissue  fold  over 
the  embryo.  The  outermost  of  these  is  the  chorion.  An  amnion  lies 
beneath  it  and  forms  around  the  embryo  a  fluid-filled  chamber,  which 
serves  as  a  protective  water  cushion  and  provides  an  aquatic  environ- 
ment in  which  the  embryo  develops.  These  two  membranes  appear  to  be 
derived  phylogenetically  from  something  similar  to  the  superficial  layers 
covering  the  yolk  sac  of  certain  large  yolked  fish  embryos.  Another  mem- 
brane, the  allantois,  is  a  saclike  outgrowth  from  the  embryo's  hindgut. 
It  is  homologous  to  the  urinary  bladder  of  the  frog,  but  extends  beyond 
the  body  wall,  passing  between  the  amnion  and  the  chorion.  Its  highly 
vascular  wall  unites  with  the  chorion,  and  gas  exchange  with  the  ex- 
ternal environment  occurs  there.  Nitrogenous  excretory  products,  largely 
in  the  form  of  crystals  of  uric  acid,  accumulate  in  the  cavity  of  the 
allantois. 

Yolk  sac,  chorion,  amnion  and  allantois  are  collectively  called  the 
extraembryonic  membranes.  These  adaptations  for  terrestrial  reproduc- 
tion are  found  in  the  embryos  of  all  reptiles,  birds  and  mammals.  These 
groups  of  vertebrates  are  often  called  amniotes,  after  one  of  these  mem- 
branes. In  contrast,  the  various  fish  groups  and  amphibians  are  called 
the  anamniotes. 


''EmhryoTiic  iut  Chorioa.mn.otic        Chorion" 

Coelom    .^---~~/~^       ^  .,        .  ;r^ —^  ^  /"  fold 

^— '-  ^     ^—laminar        ^  /^^ .X  r-v 

Amnion- 


Allanlois 


-  ilaminco- 
A'-  ' "  °H^''%*  •  ';'°iv*^ — yolk  sac 
■'^^t^iis:'^     — Ectoderm         -^— -..'■ 
^  Mesoderm 
A  — Endoderm.  B  *-• 

Figure  23.7.  Sections  of  vertebrate  embryos  to  show  the  extraembryonic  mem- 
branes. A,  The  trilaminar  yolk  sac  of  a  large  yolk  fish  embryo  consists  of  all  three  germ 
layers.  B,  The  chorioamniotic  folds  of  an  early  embryo  of  a  reptile  appear  to  have 
evolved  from  the  ectoderm  and  part  of  the  mesoderm  of  a  trilaminar  yolk  sac.  C,  A 
later  reptile  embryo  in  which  the  extraembryonic  membranes  are  complete.  Notice  that 
the  yolk  sac  is  bilaminar.  The  albumin  and  shell,  which  surround  the  reptile  embryo 
and  extraembryonic  membranes,  have  not  been  shown. 


456  VERTEBRATE   LIFE   AND   ORGANIZATION 

206.        Evolution  and  Adaptations  of  Reptiles 

Stem  Reptiles.  Having  solved  the  essential  problems  of  terrestrial 
life  at  a  time  when  there  were  few  competitors  upon  the  land,  the  rep- 
tiles multiplied  rapidly,  spread  into  all  of  the  ecologic  niches  available 
to  them,  and  became  specialized  accordingly.  The  earliest  reptiles,  which 
separated  from  the  labyrinthodonts  during  the  late  Carboniferous 
period,  were  the  cotylosaurs  (order  Cotylosauria).  This  stem  group  was 
soon  replaced  by  other  lines  of  reptilian  evolution  that  arose  directly  or 

indirectlv  from  it. 

Turtles.  Turtles  (order  Chelonia)  are  believed  to  be  direct  de- 
scendants of  cotylosaurs  (Fig.  22.2),  but  they  are  specialized  by  being 
encased  in  a  protective  shell  composed  of  bony  plates  overlaid  by  horny 
scales.  The  bony  plates  have  ossified  in  the  dermis  of  the  skin,  but  they 
have  also  fused  with  the  ribs  and  some  other  deeper  parts  of  the  skele- 
ton. The  portion  of  the  shell  covering  the  back  is  known  as  the  cara- 
pace; the  ventral  portion,  the  plastron. 

Ancestral  turtles  were  stiff-necked  creatures,  unable  to  retract  their 
heads,  but  modern  species  can  withdraw  theirs  into  the  shell.  This  is 
accomplished  by  bending  the  neck  in  an  S-shaped  loop  in  either  the 
vertical  plane  (North  American  species  such  as  the  red-eared  turtle, 
Pseudemys  scripta  elegans)  or  in  the  horizontal  plane  (Australian  side- 
necked  turtle,  Chelodma  longicollis,  Fig.  23.8  C).  Sea  turtles  belong  to 
the  former  group.  They  have  also  adapted  to  an  aquatic  mode  of  life, 
swimming  about  by  means  of  oarlike  flippers.  They  come  ashore  only 
to  lay  their  cleidoic  eggs  in  holes  which  they  dig  on  the  beaches. 

Niarine  Blind  Alleys.  Sea  turtles  are  not  the  only  reptiles  that  have 
returned  to  the  ocean.  In  the  Mesozoic,  two  lines  of  reptilian  evolution 
adapted  to  marine  conditions.  Plesiosaurs  (order  Sauropterygia,  Fig. 
23.9)  were  superficially  turtle-shaped  (though  they  lacked  the  shell), 
with  squat,  heavy  bodies  and  long  necks.  Some  species  reached  a  length 
of  40  feet.  They  propelled  themselves  by  means  of  large  paddle-shaped 
appendages.  Members  of  the  other  line,  the  ichthyosaurs  (order  Ichthy- 
osauria,  Fig.  23.9),  were  porpoise-like  in  size  and  probably  in  habits. 
They  moved  with  fishlike  undulations  of  the  trunk. 

Plesiosaurs  could  probably  get  onto  the  beaches  to  lay  their  eggs, 
but  the  extreme  aquatic  adaptation  of  the  ichthyosaurs  would  preclude 
their  doing  so.  How  then  did  they  reproduce,  for  cleidoic  eggs  cannot 
develop  submerged  in  water?  In  an  unusual  fossil,  several  small  ichthy- 
osaurs are  lodged  in  the  posterior  part  of  the  mother's  abdominal  cavity, 
and  one  individual  is  part  way  out  the  cloaca.  These  must  have  been 
offspring  about  to  be  born,  for  the  skeletons  of  specimens  that  had  been 
eaten  would  not  remain  intact  during  a  passage  through  the  digestive 
tract.  Apparently  these  reptiles,  like  some  modern  lizards  and  snakes, 
were  viviparous,  the  eggs  being  retained  in  the  oviduct  until  embryonic 
development  was  complete. 

These  marine  reptiles  flourished  during  the  Mesozoic,  competing 
with  the  more  primitive  kinds  of  fishes.  Just  why  they  became  extinct 
near  the  close  of  this  era   is  uncertain,   but   their  extinction   coincides 


A   HISTORY   Of   VERTEBRATES:   AMPHIBIANS    AND    REPTILES 


457 


Figure  23  8  A,  Copulating  loggerhead  sea  turtles,  Caretta  caretta;  B,  sea  turtle 
laying  eggs;  C,  the  Australian  side-necked  turtle.  Chelodina  longicollis.  (A,  photograph 
by  Frank  Essapian,  courtesy  Marine  Studios;  B,  Life  photo  by  Fritz  Goro.  ©  Time,  Inc.; 
Cj  courtesy  of  the  New  York  Zoological  Society.) 


458  VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  23.9.  Aquatic  reptiles  of  the  Mesozoic  era.  Plesiosaurs  on  the  left;  ichthyo- 
saurs  on  the  right.  Plesiosaurs  reached  a  length  of  forty  feet:  ichthyosaurs,  a  length  of 
about  ten  feet.  (Courtesy  of  the  Chicago  Museum  of  Natural  History.) 

with  the  evolution  and  increase  of  the  teleosts  (Fig.  22.2).  Possibly  they 
could  not  compete  successfully  with  these  fishes. 

Lizard-Like  Reptiles.  The  most  abundant  of  our  present-day  rep- 
tiles are  the  lizard-like  ones  of  which  lizards  and  snakes  are  the  most 
familiar  examples.  The  most  primitive  living  member  of  this  group  is 
the  tuatara  (Sphenodon,  Fig.  23.10)— the  only  surviving  representative  of 
the  order  Rhynchocephalia.  Rhynchocephalians  are  lizard-like  in  general 
appearance,  but  have  a  more  primitive  skidl  structure  than  any  true 
lizard.  At  one  time  the  group  was  very  widespread,  but  now  it  is 
limited  to  a  few  small  islands  off  the  coast  of  New  Zealand.  Sphenodon 
is  a  surviving  "fossil,"  for  it  has  not  changed  greatly  from  species  that 
were  living  150  million  years  ago. 


Figure   23.10.     The   tuatara.   Sphenodon,   is   one   of   the  most  primitive  of   living 
reptiles.  (Courtesy  of  the  New  York  Zoological  Society.) 


A    HISTORY   OF   VERTEBRATES:   AMPHIBIANS    AND    REPTILES 


459 


Lizards  and  snakes,  though  superficially  different  from  each  other, 
are  similar  enough  in  basic  structure  to  be  placed  in  the  single  order 
Squamata.  Li/ards  (suborder  Lacertilia)  are  the  older  and  more  primi- 
tive. They  doubtless  evolved  from  some  rhynchocephalian-like  ancestor 
early  in  the  Mesozoic  era.  For  the  most  part  lizards  are  diurnal,  ter- 
restrial quadrupeds,  but,  like  other  successful  groups,  they  have  under- 
gone an  extensive  adaptive  radiation   (Fig.  23.11). 

Several  groups  have  become  arboreal  and  evolved  interesting 
adaptations  for  climbing.  The  true  chameleon  of  Africa  (not  to  be 
confused  with  the  circus  chameleon  of  our  Southeast)  has  a  prehensile 
tail,  and  an  odd  foot  structure  in  which  the  toes  of  each  foot  are  fused 
together  into  two  groups  that  oppose  each  other  like  the  jaws  of  a  pair 
of  pliers.  Geckos,  in  contrast,  cling  to  trees  by  means  of  expanded 
digital  pads.  Numerous  fine  ridges  on  the  under  surface  of  the  pads 
increase  the  friction. 

Many  lizards,  including  the  horned  toads  of  our  Southwest  {Phryn- 


Figure  23. 11.  Adaptive  radiation  among  lizards.  A,  The  Old  World  chameleon  has 
grasping  feet  and  a  prehensile  tail  with  which  to  climb  about  the  trees.  B,  The  gecko 
climbs  by  means  of  digital  pads.  C,  The  horned-toad,  Phrynosoma,  is  a  ground-dwelling 
species  that  often  burrows.  D,  The  glass  snake,  Ophisaurus,  also  burrows.  E,  The  Gila 
monster,  Heloderma,  and  a  related  Mexican  species  are  the  only  poisonous  lizards  in 
the  world.  (Courtesy  of  the  New  York  Zoological  Society.) 


460 


VERTEBRATE   LIFE   AND   ORGANIZATION 


'""iTifei*.         ■'■        ■■    '  '  '-      ■'-•- 

Figure  23.12.     A  gopher  snake  eating  a  rat.  (Courtesy  of  the  New  York  Zoological 
Society.) 


osoma),  burrow  to  some  extent  for  protection,  and  some  have  taken  to 
a  burrowing  mode  of  life.  Appendages  are  lost  in  many  burrowing 
lizards,  though  vestiges  of  girdles  are  present.  The  eyes  may  be  reduced, 
and  the  body  form  becomes  wormlike.  The  glass  snake  (Ophisaurus), 
although  it  burrows  only  part  of  the  time,  is  a  lizard  of  this  type.  The 
glass  snake  derives  its  name  from  its  ability  to  break  off  its  tail  when 
seized.  The  tail,  which  constitutes  about  two  thirds  of  the  animal's 
length,  fragments  into  many  pieces  that  writhe  about,  attracting  atten- 
tion while  the  lizard  moves  quietly  away.  Other  lizards  also  have  this 
ability,  though  developed  to  a  less  spectacular  degree.  Lost  tails  are 
regenerated,  but  the  new  tails  are  supported  by  a  cartilaginous  rod 
rather  than  by  vertebrae. 

The  only  poisonous  lizards  are  the  beaded  lizards,  such  as  the  Gila 
monster  (Heloderma)  of  the  Southwestern  United  States.  Modified 
glands  in  the  floor  of  the  mouth  discharge  a  neurotoxic  poison,  which 
is  injected  into  the  victim  by  means  of  grooved  teeth.  This  is  a  rela- 
tively inefficient  method,  so  the  bite  is  not  as  dangerous  as  the  bite  of 
most  poisonous  snakes.  Charles  Bogert  of  the  American  Museum 
of  Natural  History  reports  that  8  of  34  bites  that  have  come  to  his 
attention  were  fatal  and  he  believes  that  the  majority  of  minor  bites 
are  never  reported.  It  is  probable  that  the  poison  is  used  for  defense 
rather  than  for  killing  prey,  for  the  Gila  monster  crushes  its  food  with 
its  powerful  jaws. 

Snakes  (suborder  Ophidia)  differ  from  lizards  most  notably  in 
being  able  to  swallow  animals  several  times  their  own  diameter  (Fig. 
23.12).  This  is  made  possible  by  an  unusually  flexible  jaw  mechanism. 
The  posterior  ends  of  the  lower  jaw  of  a  lizard  are  movably  articulated 
with  the  quadrate  bones  of  the  upper  jaw,  and  the  two  halves  of  the 
lower  jaw  are  firmly  united  with  each  other  at  the  chin.  In  snakes  there 
is  a  movable  joint  between  each  half  of  the  lower  jaw  at  the  chin,  and 
another  on  each  side  midway  between  the  chin  and  the  quadrate.  Then 


A    HISTORY   OF   VERTEBRATES:   AMPHIBIANS   AND    REPTILES 


461 


there  is  the  usual  joint  between  the  lower  jaw  and  the  quadrate,  and 
finally  one  between  the  quadrate  and  the  rest  oi  the  skull.  Other 
features  which  characterize  snakes  are  the  absence  of  movable  eyelids, 
of  a  tympanic  membrane  and  middle  ear  cavity,  and  of  legs.  There  are 
exceptions  to  these  generalizations,  for  geckos  do  not  have  movable 
eyelids,  glass  "snakes"  lack  legs,  and  some  of  the  more  primitive  snakes, 
such  as  the  python,  have  vestigial  hind  legs. 

Snakes  doubtless  evolved  from  some  primitive  lizard  group,  and 
very  probably  from  burrowing  members  of  that  group.  The  most  primi- 
tive living  snakes  are  burrowing  species,  and  many  details  of  ophidean 
anatomy  suggest  a  iossorial  ancestry.  The  structure  of  their  eyes,  for 
example,  indicates  that  the  eyes  redeveloped  from  eyes  that  had  under- 
gone marked  retrogressive  changes.  Their  forked  tongue,  which  is  often 
seen  darting  from  the  mouth  (Fig.  23.13  A),  is  an  organ  concerned  with 
touch  and  smelling.  Odorous  particles  adhere  to  it,  the  tongue  is  with- 
drawn into  the  mouth,  and  the  tip  is  projected  into  a  specialized  part 
of  the  nasal  cavity  (jacobson's  organ).  The  great  elaboration  of  such  a 
device  would  seem  to  be  an  adaptation  to  a  burrowing  mode  of  life  in 
which  other  senses  woidd  be  less  useiul. 

In  their  subsequent  evolution  some  snakes  gave  up  the  burrowing 
habit  and  developed  a  method  of  locomotion  that  depended  upon 
squirming  and  the  movement  of  their  ventral  scales.  Their  adjustments 
to  epigean  life  and  their  unique  feeding  mechanism  enabled  them  to  be 
a  successful  group  and  to  undergo  an  extensive  adaptive  radiation. 

Among  the  more  interesting  adaptations  has  been  the  evolution, 
in  several  distinct  lines,  of  a  poison  mechanism  that  involves  specialized 
oral  glands  associated  with  grooved  or  hollow,  hypodermic-like  teeth— 
the  fangs.  Most  of  our  poisonous  North  American  snakes  (rattlesnakes, 
copperhead,  cottonmouth,  \\ater  moccasion)  are  pit  vipers.  They  have 
a  pair  of  large,  hollow  fangs  at  the  front  of  the  mouth  that  are  articu- 


Figure  23.13.  A,  A  coachwhip  snake  protruding  its  tongue;  B,  "milking"  a  rattle- 
snake to  get  poison  for  the  production  of  antivenom.  The  tongue  is  a  tactile  and  ol- 
factory organ  that  is  perfectly  harmless;  it  should  not  be  confused  with  fangs,  which 
are  specialized  teeth.  (A,  Courtesy  of  the  New  York  Zoological  Society;  B,  courtesy  of 
Ross  Allen's  Reptile  Institute.) 


462  VERTEBRATE   LIFE   AND    ORGANIZATION 

lated  to  bones  of  the  upper  jaw  and  palate  in  such  a  way  that  they  are 
loldetl  against  the  rool  ol  the  mouth  when  the  mouth  is  closed,  and 
automatically  brought  forward  when  the  mouth  is  opened  (Fig. 
23.13  B).  The  poison  ol  these  snakes  is  hemolytic,  and  causes  a  break- 
down ol  the  red  blood  cells  in  the  animal  bitten.  Coral  snakes  belong 
to  a  group  related  to  the  Old  World  cobras.  Their  poison  is  neurotoxic, 
and  their  fangs  are  a  pair  of  hollow,  short,  stationary  teeth  attached  to 
the  front  of  the  upper  jaw.  The  poison  of  snakes  is  used  to  immobilize 
and  kill  their  prey,  which  they  swallow  whole.  In  addition,  the  poison 
of  some  snakes  contains  digestive  enzymes  that  are  carried  by  the  vic- 
tim's blood  stream  throughout  its  body  before  its  death. 

Dinosaurs  and  Their  Allies.  Lizards  and  snakes  are  the  successful 
reptiles  today,  but  during  the  Mesozoic  era  the  land  was  dominated  by 
another  offshoot  of  primitive  rhynchocephalian-like  reptiles.  These 
"ruling  reptiles"  were  the  archosaurs— an  assemblage  of  several  orders 
that  shared  many  features,  including  a  tendency  to  evolve  a  two-legged 
gait.  Reduced  pectoral  appendages,  enlarged  pelvic  appendages,  and  a 
heavy  tail  that  could  act  as  a  counterbalance  for  the  trvmk  were  cor- 
related with  this  mode  of  life. 

Saurischian  dinosaurs  (order  Saurischia)  evolved  from  ancestors 
that  were  only  three  or  four  feet  long,  but  later  saurischians  became 
giants  of  the  land  and  swamps.  Tyrannosaiirus  (Fig.  23.14  A)  was  the 
largest  terrestrial  carnivore  that  the  world  has  ever  seen.  It  stood  about 
20  feet  high,  and  had  large  jaws  armed  with  dagger-like  teeth  six  inches 
long— a  truly  formidable  creature!  Other  saurischian  dinosaurs  were 
herbivorous  swamp-dwellers  that  reverted  to  a  quadruped  gait,  but  the 
bipedal  gait  of  their  ancestors  was  reflected  in  their  long  hind  legs. 
The  buoyancy  of  the  water  permitted  some  to  grow  to  enormous  size. 
Brontosaurus  (Fig.  23.14  B)  and  certain  of  its  allies  attained  lengths  of 
80  feet  and  weights  of  50  tons.  Only  certain  modern  whales  have  ex- 
ceeded them  in  size. 

Many  dinosaurs  in  another  group  (order  Ornithischia)  became  ter- 
restrial, rather  than  swamp  herbivores.  These  also  reverted  to  a  quad- 
ruped gait  and  increased  in  size,  though  none  was  as  large  as  the 
saurischians.  These  animals  undoubtedly  formed  much  of  the  diet  of 
carnivores  such  as  Tyrannosaurus,  and  many  evolved  protective  devices 
such  as  spiked  tails,  bony  plates  on  the  body  and  horned  skulls.  Stego- 
saurus  and  Triceratops  (Fig.  23.14  C  and  D)  are  examples  of  this  group. 

The  reasons  for  the  evolution  of  large  size  are  not  entirely  clear. 
Within  limits,  large  size  has  a  protective  value,  but  it  may  also  have 
been  a  way  of  achieving  a  more  nearly  constant  body  temperature. 
Reptiles,  being  poikilothermic,  derive  a  great  deal  of  their  body  heat 
during  warm  weather  from  the  external  environment.  As  mass  in- 
creases, the  relative  amount  of  body  surface  available  for  the  absorption 
of  heat  decreases,  and  body  temperature  would  fluctuate  less.  An  adap- 
tation of  this  type  may  have  been  particularly  important  for  animals  that 
lived  in  a  warm  climate  and  were  too  big  to  shelter  by  burrowing  or 
hiding  beneath  debris,  for  it  would  help  prevent  body  temperature  from 


A   HISTORY   OF   VERTEBRATES:   AMPHIBIANS    AND    REPTILES  463 


y;„  •.«»*'*  *• 


Figure  23.14.  Representatives  of  the  main  groups  of  dinosaurs  that  flourished  dur- 
ing the  late  Mesozoic  era.  A,  Tyrannosaurus,  a  carnivorous  saurischian;  B,  Brontosaurus, 
an  herbivorous  saurischian;  C,  Stegosaurus,  an  ornithischian;  D,  Triceratops,  another 
ornithischian.  Brontosaurus  was  the  largest  and  reached  a  length  of  about  eighty  feet. 
(Courtesy  of  the  Chicago  Museum  of  Natural  History.) 


464 


VERTEBRATE   LIFE  AND   ORGANIZATION 


Figure  23.15.     Pteratwdon,  one  of  the  flying  reptiles,  or  pterosaurs,  that  lived  dur- 
ing tlie  late  Mesozoic  era.  (Courtesy  of  the  American  Museum  of  Natural  History.) 

reaching  a  lethal  point.  As  explained  earlier,  prolonged  high  tempera- 
ture destroys  most  enzyme  systeins. 

A  bii)e(lal  gait  naturally  Ireed  the  front  legs  from  use  in  terrestrial 
locomotion.  The  front  legs  became  reduced  in  many  dinosaurs,  but  in 
one  group  of  archosaurs  they  were  converted  to  wings.  The  wings  of  the 
flying  reptiles  (order  Pterosauria)  consisted  of  a  membrane  of  skin  sup- 
ported by  a  greatly  elongated  fourth  finger  (Fig.  23.15).  The  fifth  finger 
was  lost,  and  the  others  probably  were  used  for  clinging  to  cliffs.  The 
hind  legs  were  very  feeble,  and  the  animal  must  have  been  helpless  on 
the  ground.  Certain  pterosaurs  became  very  large,  one  having  a  wing 
spread  of  25  feet. 

Most  of  the  archosaurs  became  extinct  toward  the  end  of  the  Meso- 
zoic, but  the  reason  for  this  is  not  entirely  clear.  Perhaps  the  pterosaurs 
succumbed  in  competition  with  birds,  which  also  evolved  from  primitive. 


A    HISTORY   OF   VERTEBRATES:   AMPHIBIANS   AND    REPTILES 


465 


Figure  23.16.  The  alligators  and  ciooHiilcs  are  the  only  surviving  members  of  the 
archosaurian  reptiles,  a  group  to  which  the  dinosaurs  belonged.  A,  The  American  alli- 
gator; B,  the  American  crocodile.  The  southern  part  of  the  Florida  Everglades  is  the 
only  place  in  the  United  States  where  crocodiles  can  be  found  in  the  wild.  (Courtesy  of 
Ross  Allen's  Reptile  Institute.) 

bipedal  archosaurs.  The  extinction  of  the  dinosaurs  may  have  resulted 
from  climatic  changes.  .\n  inability  of  the  specialized  herbivores  to  adapt 
to  the  drying  up  of  the  large  swamps  and  to  changes  in  vegetation  would 
have  led  to  their  death.  Their  disappearance,  in  turn,  would  deprive  the 
huge  carnivores  of  most  of  their  food  supply,  so  their  days  would  be 
numbered  too. 

Only  one  group  of  archosaurs  survived  this  wholesale  extinction— 
the  alligators  and  crocodiles  (order  Crocodilia).  Crocodiles  have  reverted 
to  a  quadruped  gait  (though  their  hind  legs  are  much  longer  than  the 
front)  and  an  amphibious  mode  of  life.  Only  two  species  occur  in  the 
United  States-the  American  alligator,  which  can  be  distinguished  by  its 
rounded  snout,  and  the  American  crocodile,  which  has  a  much  more 
pointed  snout  (Fig.  23.16). 

Mammal-like  Reptiles.  Another  line  of  evolution,  which  was  des- 
tined to  lead  to  mammals,  diverged  from  the  cotylosaurs  millions  of  years 
before  the  advent  of  lizard-like  reptiles  or  archosaurs.  Early  mammal-like 
reptiles  (order  Pelycosauria)  were  very  similar  to  cotylosaurs.  These  were 
medium-sized,  somewhat  clumsy,,  terrestrial  quadrupeds  with  limbs 
sprawled  out  at  right  angles  to  the  body.  Their  jaws  contained  numerous 
conical  teeth  and  were  composed  of  many  dermal  bones  covering  the 
mandibular  arch.  The  jaw  joint  lay  between  the  ossified  posterior  ends 
of  the  mandibular  arch,  i.e.,  between  the  quadrate  bone  of  the  upper 
jaw  and  the  articular  bone  of  the  lower  jaw.  This  is  where  the  jaw  joint 
is  located  in  the  frog  (Fig.  21.4)  and  other  low^er  vertebrates.  The  stapes 
(a  derivative  of  the  hyoid  arch  of  fishes)  transmitted  vibrations  to  the 
inner  ear.  It,  in  turn,  may  have  received  air-borne  vibrations  from  the 
external  environment  by  means  of  a  tympanic  membrane  as  it  does  in 
frogs  (Fig.  21.17),  or  ground-borne  vibrations  may  have  been  picked  up 


466 


VERTEBRATE   LIFE   AND    ORGANIZATION 


Figure  23.17.  Two  mammal-like  reptiles.  A,  Dimetrodon,  an  early  member  of  the 
group;  B.  I.yaienops,  a  later  mammal-like  reptile  similar  to  those  that  gave  rise  to 
mammals.  (Courtesy  American  Museum  of  Natural  History.) 


by  the  lower  jaw  and  transmitted  via  the  articular  and  quadrate  to  the 
stapes.  This  point  is  uncertain,  for  a  soft  part  such  as  a  tympanic  mem- 
brane would  not  be  preserved  in  the  fossils.  It  is  known  that  a  process 
of  the  stapes  did  connect  with  the  quadrate. 

Some  pelycosaurs,  such  as  Dimetrodon  (Fig.  23.17  A)  were  quite 
active,  and  may  have  been  warm-blooded.  Dimetrodon  had  a  peculiar 
sail  along  its  back  supported  by  long  neural  spines  of  the  vertebrae.  This 
sail  may  have  been  a  device  to  radiate  heat,  for  it  considerably  increased 
the  body  surface  relative  to  mass.  Active  animals  produce  a  large  amount 
of  heat  as  a  by-product  of  their  metabolic  activity,  and  need  special 
means  of  dissipating  it. 

Later  mammal-like  reptiles  (Lycaenops,  order  Therapsida,  Fig. 
23.17  B)  came  to  resemble  mammals  more  closely.  Their  limbs  were  be- 
neath the  body  where  they  could  provide  better  support  and  move  more 
rapidly  back  and  forth.  Their  teeth  were  specialized,  like  those  of  mam- 
mals, into  ones  suited  for  cropping,  stabbing,  cutting  and  grinding.  The 
major  osteologic  character  that  separated  them  from  mammals  was 
the  reptilian  nature  of  the  jaw  joint  and  the  sound-transmitting  ap- 
paratus. The  mammalian  jaw  joint  is  between  two  dermal  bones  (the 
dentary  of  the  lower  jaw  and  squamosal  of  the  upper  jaw)  that  lie  just 
anterior  to  the  quadrate  and  articular.  The  mammalian  homologues  of 
the  quadrate  and  articular  (the  incus  and  malleus  respectively)  are 
covered  by  a  tympanic  membrane,  and  form  with  the  stapes  a  chain  of 
three  delicate  auditory  ossicles  that  transmit  air-borne  vibrations  from 
the  tympanic  membrane  to  the  inner  ear  (Fig.  29.6).  This  character  had 
not  been  achieved  by  the  late  therapsids,  but  the  dentary  and  squamosal 
were  very  close  together,  and  the  quadrate  and  articular  were  small.  The 
change  to  the  mammalian  condition  was  made  by  the  middle  of  the 
Mesozoic;    shortly  afterward   the   mammal-like  reptiles   became   extinct. 


A    HISTORY   OF    VERTEBRATES:    AMPHIBIANS    AND    REPTILES  467 

Their  mammalian  descendants   remained  a  rather   inconspicuous   part 
of  the  fauna  until  the  disappearance  of  the  dinosaurs. 


Questions 

1.  List  five  differences  in  physical  conditions  between  the  aquatic  and  terrestrial  en- 
vironments to  which  terrestrial  vertebrates  had  to  adapt. 

2.  In  what  ways  were  crossopterygians  preadapted  to  a  terrestrial  life?  What  other  con- 
ditions favored  crossopterygians  in  making  the  adaptive  shift  from  water  to  land? 

3.  In  what  ways  have  aijiphibians  successfully  adapted  to  the  terrestrial  environment? 
In  what  ways  are  they  poorly  adapted  for  life  on  land? 

4.  List  and  give  the  distinguishing  characters  of  the  orders  of  living  amphibians. 

5.  Which  salamanders  do  not  have  lungs?  How  do  these  salamanders  respire? 

6.  What  features  of  toads  enable  them  to  li\e  in  drier  environments  than  frogs? 

7.  In  what  ways  are  the  embryos  modified  in  frogs  that  have  a  direct  development? 

8.  Under  what  environmental  conditions  have  the  larval  stages  of  frogs  been  suppressed? 
Is  it  possible  that  similar  conditions  played  a  role  in  the  evolution  of  terrestrial  re- 
production in  the  ancestors  of  reptiles? 

9.  In  what  ways  are  reptiles  better  adapted  for  terrestrial  life  than  amphibians? 

10.  List  the  extraembryonic  membranes  of  a  reptile  einbryo  and  briefly  state  the  func- 
tion of  each. 

11.  Define  a  cleidoic  egg. 

12.  Which  groups  of  vertebrates  are  amniotes;  which  are  anamniotes? 

13.  What  were  the  earliest  reptiles? 

14.  Name  two  groups  of  Mesozoic  reptiles  that  returned  to  the  sea  and  l)ecame  very 
well  adapted  to  the  marine  environment. 

15.  How  can  one  distinguish  a  legless  lizard  from  a  snake?  How  can  one  distinguish  a 
lizard  from  a  salamander? 

16.  Which  group  of  living  reptiles  is  most  closely  related  to  the  dinosaurs? 

17.  Which  group  of  reptiles  gave  rise  to  the  mammals? 

Supplementary  Reading 

A  great  deal  of  interesting  information  on  amphibians  and  reptiles  can  be  found  in 
the  general  references  for  vertebrates  cited  at  the  end  of  Chapter  22.  An  invaluable  addi- 
tional reference  on  the  classification,  anatomy,  physiology  and  habits  of  amphibians  is 
Noble's  Biology  of  the  Amphibia.  The  many  fascinating  adaptations  of  amphibians  and 
reptiles  have  been  discussed  by  Barbour,  Reptiles  and  Amphibians.  There  are  many  good 
accounts  of  the  United  States  species  of  amphibians  and  reptiles  and  their  natural  his- 
tory. Among  those  of  value  to  specialist  and  general  reader  alike  are:  Wright  and  Wright, 
Handbook  of  Frogs:  Bishop,  Handbook  of  Salamanders;  Carr,  Handbook  of  Turtles; 
Smith,  Handbook  of  Lizards;  Schmidt  and  Davis,  Field  Book  of  Snakes. 


CHAPTER  24 


A  History  of  Vertebrates: 
Birds  and  Mammals 


The  reptiles  made  two  significant  improvements  upon  the  terrestrial 
adaptations  of  amphibians:  the  evohition  of  the  cleidoic  egg  and  the 
development  of  a  means  of  conserving  body  water.  Birds  (class  Aves)  and 
mammals  (class  Mammalia)  evolved  from  reptiles,  and  both  groups  have 
further  improved  upon  the  adaptations  of  reptiles  by  developing  mech- 
anisms for  the  maintenance  of  fairly  high  and  constant  body  tempera- 
tures. They  are  said  to  be  homoiothermic,  or  warm-blooded,  animals. 
Their  metabolic  processes  can  proceed  at  an  optimal  rate  despite  the 
wide  range  in  external  temperatures  common  in  the  terrestrial  environ- 
ment, and  they  are  typically  very  active  creatures. 

Higher  metabolic  rates  require  higher  rates  of  exchange  of  materials 
with  the  environment  and  rapid  distribution  of  these  materials  within 
the  body.  Birds  and  mammals  have  met  these  requirements  in  somewhat 
similar  ways;  their  adaptations  for  increased  activity  provide  interesting 
examples  of  convergent  evolution,  although  in  other  respects  they  are 
quite  different.  Birds  evolved  from  early  bipedal  archosaurs  (Fig.  22.2) 
and  have  undergone  specializations  for  flight;  mammals  evolved  from 
a  stock  of  mammal-like  reptiles  and  have  become  specialized  for  ter- 
restrial life. 


207.        Principles  of  Flight 

A  group  of  extinct  reptiles,  the  pterosaurs,  and  a  group  of  mammals, 
the  bats,  have  evolved  true  flight,  but  neither  group  has  been  as  suc- 
cessful fliers  as  have  birds.  Bird  wings  are  modified  pectoral  appendages 
and  the  flying  surfaces  are  covered  with  feathers.  A  bird's  wings  must, 
of  course,  provide  a  lift  force  at  least  equal  to  the  weight  of  the  bird. 
The  wing  is  shaped  so  that  it  is  slightly  concave  on  the  under  surface 
and  convex  on  the  upper  surface,  and  its  angle  of  attack  is  such  that  its 
anterior  edge  is  slightly  higher  than  the  posterior  edge  (Fig.  24.1).  As 
the  airstream  flows  across  the  wing,  it  is  deflected  in  such  a  way  that  it 
reduces  the  pressure  above  the  wing  and  increases  the  pressure  on  the 
lower  surface.  These  two  forces,  especially  the  reduced  pressure  on 
the  upper  surface,  provide  the  lift.  The  lift  force  can  be  increased  by  an 
increase  in  the  speed  of  the  airstream  across  the  wing,  and  by  raising 

468 


A    HISTORY   Of   VERTEBRATES:   BIRDS   AND   MAMMALS 


469 


the  anterior  edge,  i.e.,  increasing  its  angle  of  attack.  However,  increas- 
ing the  angle  of  attack  also  disturbs  the  airstream  in  such  a  way  that  it 
causes  the  formation  of  eddies  above  the  wing.  This  turbulence  produces 
a  drag  that  tends  to  reduce  lift;  however,  this  can  be  minimized  if  the 
front  of  the  wing  is  thick  and  stiff  and  the  posterior  margin  is  thin  and 
trailing.  Providing  wing  slots  by  spreading  the  posterior  feathers  apart 
slightly,  or  elevating  a  group  of  feathers  at  the  anterior  edge  of  the 
wrist  (the  alula),  also  smooths  the  airstream  and  reduces  turbulence. 
When  birds  are  flying  rapidly,  the  speed  of  the  airstream  provides  suffi- 
cient lift  and  the  wing  need  not  be  tilted  greatly.  But  during  takeoff 
or  landing,  when  speeds  are  necessarily  low,  the  angle  of  attack  of  the 
wing  must  be  increased  and  slots  must  be  formed  to  give  increased  lift. 
Some  birds  obtain  additional  lift  on  landing  by  fanning  out  the  tail 
feathers  and  bending  them  down.  The  tail,  then,  acts  both  as  a  brake 
and  as  high-lift,  low-speed  airfoil. 

The  wings  not  only  provide  the  lift,  but  they  are  also  the  pro- 
pellers. In  the  familiar  flapping  flight  (Fig.  24.2),  the  up  and  down 
movement  of  the  wings  relative  to  the  body  of  the  bird  is  responsible 
for  the  forward  movement,  but  the  wings  do  not  simply  push  back 
against  the  air  as  a  swimmer  would  push  back  against  the  water.  On  the 
downstroke,  they  move  down  and  forward;  on  the  upstroke,  up  and 
back.  As  a  wing  moves  down,  the  air  pushes  up  against  it  and  the  more 
flexible  posterior  margin  of  the  distal  part  of  the  wing  is  twisted  up. 
The  distal  portion  of  the  wing  twists  the  opposite  way  on  the  up- 
stroke. The  twisting  of  the  distal  portion  of  the  wing  gives  it  a  pitch 
comparable  to  that  of  a  propeller  and  this,  together  with  the  movements 
of  this  part  of  the  wing,  is  responsible  for  the  forward  motion.  In  soaring 
flight,    the  wings   are   held   still   and    the   bird   skillfully   makes   use  of 


Figure  24.1.  The  effect  of  wings  on  the  airstream.  In  A,  the  wing  is  held  at  such 
an  angle  that  the  airstream  flows  smoothly  across  it.  The  air  flows  more  rapidly  over 
the  upper  surface  than  across  the  under  surface.  This  creates  a  low-pressure  area  above 
the  wing  that  provides  a  lift  force.  In  B,  the  wing  is  held  at  such  an  angle  that  lift- 
reducing  turbulence  and  eddies  form  above  it.  (Modified  after  Young.) 


470  VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  24.2.  Birds  in  flight.  A,  A  photograph  showing  the  flapping  flight  of  pigeons; 
B,  an  osprev  soaring.  (A,  U.  S.  Army  photograph;  B,  photograph  by  Allan  D.  Cruick- 
shank,  National  Audubon  Society.) 


ascending  thermal  currents  and  differences  in  wind  velocity  to  maintain 
its  forward  movement.  In  each  type  of  flight,  the  tail  helps  to  support 
and  balance  the  body  and  is  used  as  a  rudder. 

Wing  shape  and  area  vary  considerably  with  the  species,  depending 
upon  the  size  of  the  bird  and  the  speed  and  type  of  flight.  Since  weight 
increases  as  the  cube  of  the  linear  dimension  and  surface  as  the  square, 
large  birds  need  relatively  larger  wing  areas  than  small  birds.  However, 
the  body  and  wings  cannot  increase  in  size  indefinitely,  for  there  is  a 
limit  to  the  strength  of  the  flight  muscles.  Birds  that  fly  fast  can  and  do 
have  a  smaller  wing  area   than  others,  for  increased  speed  of  the  air- 


A    HISTORY   OF    VERTEBRATES:    BIRDS    AND   MAMMALS  471 

Stream  across  the  wing  results  in  greater  lift.  Soaring  birds,  in  contrast, 
need  larger  wing  areas. 

Most  of  the  features  of  bird  wings  also  apply  to  the  wings  of  air- 
planes. But  a  bird's  wings  and  tail  have  one  great  advantage  over  those 
of  an  airplane  in  that  they  can  be  varied  considerably  to  adjust  to  dif- 
ferent speeds  and  types  of  flight,  for  different  angles  of  attack,  and  for 
many  other  variables.  An  individual  bird  is  much  more  versatile  than  a 
single  type  of  airplane. 

208.        Structure  of  Birds 

There  are  few  features  of  the  anatomy  of  birds  that  are  not  directly 
or  indirectly  related  to  flight.  They  are  adapted  structurally  and  func- 
tionally to  provide  a  high  energy  output  in  a  body  of  low  weight. 

Scales  and  Feathers.  Birds  have  retained  the  horny  scales  of  rep- 
tiles on  parts  of  their  legs,  on  their  feet,  and,  in  modified  form,  as  a 
covering  for  their  beaks,  but  the  scales  that  cover  the  rest  of  the  reptilian 
body  have  been  transformed  into  feathers.  Feathers,  like  horny  scales, 
are  epidermal  outgrowths  whose  cells  have  accumulated  large  amounts 
of  keratin  and  are  no  longer  living.  Pigment  deposited  in  these  cells 
during  the  development  of  the  feather,  together  with  surface  modifica- 
tions that  reflect  certain  light  rays,  is  responsible  for  the  brilliant  colors 
of  birds.  Feathers,  more  than  any  other  single  feature,  characterize  birds, 
for  they  are  found  only  in  members  of  this  class.  They  overlap,  entrap 
air  and  form  an  insulating  layer  that  reduces  loss  of  body  heat  and  helps 
to  make  a  high  body  temperature  possible.  Those  on  the  tail  and  wings 
form  the  primary  flying  surfaces. 

The  contour  feathers  that  cover  the  body  or  provide  the  flying 
surface  consist  of  a  stilt,  central  shaft  bearing  numerous  parallel  side 
branches,  the  barbs,  which  collectively  form  the  vane  (Fig.  24.3).  Each 
barb  bears  minute  hooked  branches,  barbules,  along  its  side,  which 
interlock  with  the  barbules  of  adjacent  barbs  to  hold  the  barbs  together. 
If  the  barbs  separate,  the  bird  can  preen  the  feather  with  its  bill  until 
they  hook  together  again;  thus  the  vane  is  a  strong  and  easily  repaired 
surface  ideal'for  flight.  In  birds  that  have  lost  the  power  of  flight,  such 
as  the  ostrich,  booklets  are  not  present  upon  the  barbules,  and  the 
feather  is  very  fluffy.  The  proximal  end  of  the  shaft  does  not  have  barbs 
and  is  known  as  the  quill;  much  of  it  is  lodged  within  an  epidermal 
follicle  in  the  skin.  The  quill  is  hollow  and  blood  vessels  enter  it  during 
the  development  of  the  feather.  A  small  aftershaft,  bearing  a  few  barbs, 
may  arise  from  the  distal  end  of  the  quill. 

Other  types  of  feathers  include  the  hairlike  flloplumes,  sometimes 
visible  on  plucked  fowl,  and  down  feathers.  Down  covers  young  birds, 
and  is  found  under  the  contour  feathers  in  the  adults  of  certain  species, 
particularly  aquatic  ones.  It  is  unusually  good  insulation  for  it  has  a 
reduced  shaft  and  long,  fluffy  barbs  arising  directly  from  the  distal  end 
of  the  quill. 

Birds  molt  periodically.  They  lose  their  worn  feathers  and  new 
ones  grow  out  from  germinal  tissue  at  the  base  of  the  follicles.  Molting 


472  VERTEBRATE   LIFE   AND   ORGANIZATION 


Shaft 


Aftershaft 


Shaft 


Figure  24.3.  Types  of  feathers.  A,  Contour  feather;  B,  enlargement  of  two  barbs 
to  show  interlocking  barbules;  C,  filoplume;  D,  down  feather.  (A-C,  modified  after 
Young;  D,  after  Thomson.) 


is  commonly  a  gradual  process  occurring  after  the  breeding  season. 
Ducks,  however,  molt  abruptly;  most  of  the  flight  feathers  are  lost  simul- 
taneously and  ducks  cannot  fly  until  new  ones  develop. 

Scales  and  feathers  are  the  major  derivatives  of  the  skin  in  birds, 
but  there  is  one  conspicuous  skin  gland  (the  uropygial  gland),  located 
on  the  back  at  the  base  of  the  tail.  It  produces  an  oily  secretion  that 
some  birds  spread  over  their  feathers  with  their  beak  during  preening. 
The  gland  is  particularly  well  developed  in  water  fowl  and  its  secretions 
are  important  in  waterproofing  the  feathers.  Its  secretions  may  also  have 
other  functions,  including  the  maintenance  of  the  horny  covering  of 
the  beak. 

Skeleton.  Many  adaptations  for  flight  are  apparent  in  the  skeleton 
of  birds  (Fig.  24.4).  The  bones  are  very  light  in  weight,  for  they  are 
hollow  and  remarkably  thin.  Extensions  from  the  air  sacs  enter  the  limb 
bones  in  many  species.  Robert  Cushman  Murphy  of  the  American  Mu- 
seum of  Natural  History  has  reported  that  the  skeleton  of  a  frigate  bird 
having  a  wingspread  of  seven  feet  weighed  only  four  ounces,  which  was 
less  than  the  weight  of  its  feathers!  This  is  an  extreme  example,  but  the 
skeleton   of  all  birds  weighs  less   relative   to   their  body  weight   than 


A    HISTORY   OF   VERTEBRATES:   BIRDS    AND   MAMMALS 


473 


the  skeleton  of  mammals.  The  bones  are  very  strong  because  most  of  the 
bone  substance  is  located  at  the  periphery  of  the  bone  where  it  gives 
better  structural  support.  A  bird  bone  may  be  compared  to  a  metal  tube, 
which  is  more  resistant  to  certain  types  of  stress  than  a  metal  rod  of 
equal  weight.  The  rod  would  be  much  narrower  and  could  be  bent  more 
easily  than  the  tube.  Many  bird  bones  are  further  strengthened  by  in- 
ternal struts  of  bone  arranged  in  a  manner  similar  to  the  struts  inside 
the  wing  of  an  airplane. 

The  skull  is  notable  for  the  large  size  of  the  cranial  region,  the  large 
orbits  and  the  toothless  beak.  The  neck  region  is  very  long,  and  the 
cervical  vertebrae  are  articulated  in  such  a  way  that  the  head  and  neck 
are  very  mobile.  Since  the  bird's  bill  is  used  for  feeding,  preening,  nest 
building,  defense  and  the  like,  freedom  of  movement  of  the  head  is  very 
important.  The  trunk  region,  in  contrast,  is  shortened  and  the  trunk 
vertebrae  are  firmly  united  to  form  a  strong  fulcrum  for  the  action  of 
the  wings  and  a  strong  point  of  attachment  for  the  pelvic  girdle  and 
hind  legs.  The  hind  legs  bear  the  entire  weight  of  the  body  when  the 
bird  is  on  the  ground.  In  the  pigeon,  thirteen  of  the  more  posterior 
vertebrae  (some  of  the  trunk,  all  of  the  sacral  and  some  of  the  caudal  ver- 
tebrae) are  fused  together  to  form  a  synsacrum  with  which  the  pelvic 
girdle  is  fused.  Several  free  caudal  vertebrae,  which  permit  movement 


CavpometaczLrpus — 


Cranium 
i-Oi'bit 


PKala.n§cs 
Digib  B 
Digit  C 


Cei'vica.1 
vertebra 


Pygosbyle^. 


External 
ares 


Beak 


Free 
caudal 
verte.br  a 

IschiuHT 
Pubis 
Famui 


Phalanges 


Figure  24.4.     Skeleton  of  a  pigeon.  The  distal  part  of  the  right  wing  has  been 
tted.  (Modified  after  Heilmann.) 


474  VERTEBRATE   LIFE    AND    ORGANIZATION 

of  the  tail,  lollow  the  synsacrum.  The  terminal  caudal  vertebrae  are 
fused  together  as  a  pygostyle  and  support  the  large  tail  feathers. 

The  last  two  cervical  vertebrae  of  the  pigeon  and  the  thoracic 
vertebrae  bear  distinct  ribs.  The  thoracic  basket  is  very  firm,  for  most 
of  the  ribs  have  posteriorly  projecting  processes  that  overlap  the  next 
posterior  rib,  and  the  thoracic  ribs  articulate  with  the  expanded  breast- 
bone, or  sternum.  The  sternum  has  a  large  midventral  keel  which  in- 
creases the  area  available  for  the  attachment  of  the  flight  muscles. 

llie  bones  of  the  wing  are  homologous  to  those  of  the  pectoral  ap- 
pendage of  the  frog  and  other  tetrapods.  A  humerus,  radius  and  ulna 
can  be  recognized  easily,  but  the  bones  of  the  hand  have  been  greatly 
modified.  Two  free  carpels  are  present  and  a  carpometacarpus  (a  safety- 
pin-shaped  complex  of  bone  representing  the  fused  carpals  and  meta- 
carpals of  three  fingers)  lies  distal  to  them.  The  end  of  the  most  anterior 
finger  is  represented  by  a  spur-shaped  phalanx  articulated  to  the 
proximal  end  of  the  carpometacarpus.  The  main  axis  of  the  hand  passes 
through  the  next  finger,  and  it  has  two  distinct  phalanges  articulated 
to  the  distal  end  of  the  carpometacarpus.  Another  small,  spur-shaped 
jjhalanx  at  the  distal  end  of  the  carpometacarpus  represents  the  end  of 
the  last  finger.  There  is  some  doubt  whether  the  fingers  are  homologous 
to  the  first  three  or  to  the  second,  third  and  fourth  fingers.  The  pectoral 
girdle,  which  supports  the  wing,  consists  of  a  narrow,  dorsal  scapula,  a 
stout  coracoid  extending  as  a  prop  from  the  shoulder  joint  to  the 
sternum,  and  a  delicate  clavicle,  which  unites  distally  with  its  mate  of 
the  opposite  side  to  form  the  wishbone. 

The  legs  of  birds  resemble  the  hind  legs  of  bipedal  archosaurs.  The 
femur  articulates  distally  with  a  reduced  fibula  and  a  large  tibiotarsus 
(fusion  of  the  tibia  with  certain  tarsals).  The  remaining  tarsals  and  the 
elongated  metatarsals  have  fused  to  form  a  tarsometatarsus.  The  fifth 
toe  has  been  lost  in  all  birds  and  the  fourth  in  some  species.  The  first 
toe  is  turned  posteriorly  in  the  pigeon  and  many  other  birds.  It  serves 
as  a  prop  and  increases  the  grasping  action  of  the  foot  when  the  bird 
perches.  The  action  of  the  leg  as  a  lever  in  running  on  the  ground  and 
jumping  on  the  take-off  is  increased  by  the  elongation  of  the  metatarsals, 
and  by  the  elevation  of  the  heel  off  the  ground.  The  various  fusions  of 
the  limb  bones  reduce  the  chance  of  dislocation  and  injury,  for  birds' 
legs  must  act  as  shock  absorbers  when  they  land.  The  pelvic  girdle  is 
equally  sturdy;  the  ilium,  ischium  and  pubis  of  each  side  are  firmly 
united  with  each  other  and  with  the  vertebral  column.  The  pubes  and 
ischia  of  the  two  sides  do  not  unite  to  form  a  midventral  pelvic  sym- 
physis as  they  do  in  other  tetrapods.  This  permits  a  more  posterior 
displacement  of  the  viscera,  which,  together  with  the  shortened  trunk, 
shifts  the  center  of  gravity  of  the  body  nearer  to  the  hind  legs.  The  ab- 
sence of  a  symphysis  also  makes  possible  the  laying  of  large  eggs  with 
calcareous  shells. 

Muscles.  The  intricate  movements  of  the  neck  and  the  support  of 
the  body  by  a  single  pair  of  legs  entail  numerous  modifications  of  the 
muscular  system,  but  the  muscles  concerned  with  flight  are  of  particular 
interest.  A  large  pectoralis,  which  originates  on  the  sternum  and  inserts 


A    HISTORY   OF   VERTEBRATES:   BIRDS   AND   MAMMALS 


475 


on  the  ventral  surface  of  the  humerus,  is  responsible  for  the  powerful 
downstroke  of  the  wings.  In  some  species  this  muscle  accounts  for  one- 
fifth  of  the  body  weight.  One  might  expect  that  dorsally  placed  muscles 
would  be  responsible  for  the  recovery  stroke,  but  instead,  another  ventral 
muscle,  the  supracoracoideus  (pectoralis  minor  of  some  authors),  is  re- 
sponsible for  the  upstroke  by  virtue  of  a  peculiar  pulley-like  arrange- 
ment of  its  tendon  of  insertion.  The  origin  of  the  supracoracoideus  is 
on  the  sternum  dorsal  to  the  pectoralis.  Its  tendon  passes  through  a 
canal  in  the  pectoral  girdle  near  the  shoulder  joint  and  inserts  on  the 
dorsal  surface  of  the  humerus.  Muscles  within  the  wing  are  responsible 
for  its  folding  and  unfolding  and  the  regulation  of  its  shape  and  angles 
during  flight.  Other  muscles  attach  to  the  follicles  of  the  large  flight 
feathers  of  the  wings  and  tail  and  control  their  positions. 

Major  Features  of  the  Visceral  Organs.  Less  obvious  but  no  less 
important  adaptations  for  increased  activity  and  flight  are  present  in 
many  of  the  internal  organs.  Increased  activity  and  a  high  metabolic 
rate  necessitate  a  large  intake  of  food.  The  digestive  system  (Fig.  24.5) 
is  compact,  but  it  is  so  effective  that,  in  some  of  the  smaller  birds,  an 
amount  of  food  equivalent  to  30  per  cent  of  the  body  weight  can  be 
processed  each  day!  Moreover,  most  of  the  food  that  is  selected  has  a 
high  caloric  value.  Birds  eat  a  variety  of  insects  and  other  animals  and 
such  plant  food  as  fruit  and  seeds.  They  do  not  attempt  to  eat  such 
bulky,  low  caloric  foods  as  leaves  and  grass.  Food  taken  into  the  mouth 
is  mixed  with  a  lubricating  saliva  and  passes  through  the  pharynx  and 
down  the  esophagus  without  further  treatment,  for  birds  have  no  teeth. 
In  grain-eating  species,  such  as  the  pigeon,  the  lower  end  of  the  esoph- 
agus is  modified  to  form  a  crop  in  which  the  seeds  are  temporarily  stored 


Eustachian,  tube-- 


Uretcr 
Uropygca-l 
bland 


^ — Esophagus 


External  na.ris 


Tongixe 
Larynx 


Main  bronchas 


Lar^e 
intestine 
Pancreas 
Duodtnum 


Pectoral  mu.scles 


Bile  ducts- 
Figure  24.5.     A  lateral  dissection  of  a  pigeon  to  show  the  major  visceral  organs. 


476  VERTEBRATE   LIFE   AND   ORGANIZATION 


Glottis   //"t  \}  r\\  .  Larynx 


Anterior 

thoracic; 

Sac 


Lung 


Posterior — I 
thoracic       V 
sac 


Abdlominal 
sac 


^=^— CervicaL  sac 

Interclavicalar  sac 


Diverticula  into 
bones  of  the 
pectoral  girdle  and 
appendage 


Main 
bronchus 

Parabronchi 
and-  ail" 
capillaries 


Recurrent 
bi'onchuS 


Figure  24.6.  The  respiratory  organs  of  a  bird  as  seen  in  a  dorsal  view.  The  course 
of  the  main  bronchus  through  the  huig  to  the  air  sacs,  the  major  branches  of  the  main 
bronchus,  and  the  recurrent  bronchi  are  shown  on  the  right  side.  Minute  parabronchi 
and  air  capillaries,  a  few  of  which  are  shown,  interconnect  the  recurrent  bronchi  and 
the  branches  of  the  main  bronchus. 


and  softened  by  the  uptake  of  water.  Food  is  mixed  with  peptic  enzymes 
in  the  proventriculus,  or  first  part  of  the  stomach,  and  then  passes  into 
the  gizzard,  the  highly  modified  posterior  part  of  the  stomach  charac- 
terized by  thick  muscular  walls  and  modified  glands  that  secrete  a  horny 
lining.  Small  stones  that  have  been  swallowed  are  usually  found  in  the 
gizzard  and  aid  in  grinding  the  food  to  a  pulp  and  mixing  it  with 
the  gastric  juices.  The  intestinal  region  is  relatively  short  compared 
to  the  intestine  of  mammals,  and  is  lined  with  microscopic,  finger-like 
projections,  the  villi,  that  greatly  increase  the  surface  area.  Digestion  is 
completed  in  this  region  with  the  aid  of  enzymes  from  the  liver,  pan- 
creas and  intestinal  glands,  and  the  digested  food  is  absorbed. 

The  anterior  parts  of  the  respiratory  system  are  similar  to  those  of 
lower  tetrapods,  except  that  birds  have  a  longer  neck  and  hence  a  longer 
windpipe,  or  trachea.  The  lungs  themselves  are  relatively  small  and 
compact  organs,  but  they  are  subdivided  internally  into  many  passages 
that  greatly  increase  the  respiratory  surface.  They  are  unusual  in  that 
the  two  main  bronchi  which  lead  from  the  trachea  not  only  communi- 
cate ultimately  with  minute,  vascularized  air  capillaries  in  the  lung, 
where  gas  exchange  occurs,  but  also  continue  through  the  lung  into  a 


A    HISTORY   Of   VERTEBRATES:   BIRDS   AND   MAMMALS  477 

series  of  air  sacs  that  extend  into  the  abdomen,  thorax,  and  even  up  into 
the  neck  and  into  many  of  the  bones  (Fig.  24.6).  Most  of  the  air  sacs  are 
connected  through  recurrent  bronchi  with  the  same  air  capillaries  that 
diverge  from  the  main  bronchus.  Since  the  air  capillaries  are  connected 
to  bronchi  at  each  end,  it  is  possible  that  air  flows  directly  through  them 
rather  than  ebbing  and  flowing.  The  exact  path  that  the  air  takes,  how- 
ever, is  uncertain.  Some  investigators  have  postulated  that  during  in- 
spiration most  of  the  air  passes  through  the  main  bronchi  into  the  air 
sacs,  especially  the  posterior  sacs,  and  that  during  expiration  it  returns 
to  the  lungs  through  the  recurrent  bronchi  and  then  passes  through  the 
air  capillaries  to  the  main  bronchi.  Whatever  the  path  taken,  the  absence 
of  blind  passages  would  indicate  that  little  stale  air  is  held  in  the  lung. 
The  composition  of  the  air  in  the  air  capillaries  must  be  very  similar 
to  the  external  air. 

The  How  of  air  through  the  lungs  is  brought  about  by  the  contrac- 
tion of  muscles  in  the  thoracic  and  abdominal  walls,  probably  aided  by 
the  action  of  the  wings  during  liight.  The  thoracic  and  abdominal  cavi- 
ties and  their  contained  air  sacs  are  alternately  expanded  and  contracted, 
but  the  lungs  themselves  are  relatively  inelastic  and  do  not  change 
greatly  in  volume. 

A  mechanism  for  the  production  of  sounds  is  associated  with  the 
air  passages.  Membranes  are  set  vibrating  by  the  movement  of  air;  how- 
ever, the  vibratory  membranes  are  not  in  the  larynx  at  the  anterior  end 
of  the  trachea,  but  in  a  syrinx  at  its  posterior  end  (Fig.  24.6).  Muscles 
associated  with  the  syrinx  vary  the  pitch  of  the  notes. 

The  bird  heart  is  completely  divided  internally.  Venous  blood  re- 
turning from  the  body  and  going  to  the  lungs  passes  through  a  right 
atrium  and  right  ventricle  as  it  does  in  mammals  (Fig.  27.3),  whereas 
the  arterial  blood  returning  from  the  lungs  en  route  to  the  body  flows 
through  the  left  atrium  and  left  ventricle.  The  sinus  venosus  of  primi- 
tive vertebrate  hearts  has  been  absorbed  into  the  right  atrium,  and  the 
conus  arteriosus  contributes  to  the  arterial  trunks  leading  to  the  lungs 
and  body.  An  interesting  feature  of  the  vessels  supplying  the  body  is  the 
unusually  large  size  of  those  going  to  the  powerful  flight  muscles.  The 
complete  separation  of  venous  and  arterial  blood  within  the  heart, 
the  rapid  heart  beat  (400  to  500  times  per  minute  in  a  small  bird  such 
as  a  sparrow  when  it  is  at  rest),  and  an  increase  in  blood  pressure  make 
for  a  very  rapid  and  efficient  circulation.  This  is  of  the  utmost  im- 
portance in  a  homoiotherm,  for  the  tissues  need  a  large  supply  of  food 
and  oxygen,  and  waste  products  of  metabolism  must  be  removed  quickly. 
Nitrogenous  wastes  are  removed  from  the  blood  by  a  pair  of  kid- 
neys, drained  by  ureters,  and  basically  similar  to  those  of  reptiles  (Fig. 
24.5)'  A  large  volume  of  excretory  products  can  be  removed,  chiefly  as 
uric  acid  which  can  be  eliminated  with  little  loss  of  water.  Birds  have 
lost  the  urinary  bladder  and  the  right  ovary  and  oviduct  of  more  primi- 
tive tetrapods,  possibly  as  one  means  of  reducing  body  weight. 

The  sense  of  smell  is  less  important  in  vertebrates  that  spend  a 
considerable  part  of  their  life  off  the  ground  than  it  is  in  terrestrial 
species,  so  it  is  not  surprising  to  find  that  the  olfactory  organ  and  olfac- 
tory portions  of  the  brain  are  reduced  in  birds.  Sight,  on  the  other  hand. 


478 


VERTEBRATE   LIFE   AND    ORGANIZATION 


is  very  important,  and  the  eyes  and  optic  regions  of  the  brain  are 
unusually  well  developed.  The  eyes  oi  birds  occupy  a  large  portion  of 
the  head  and  both  eyes  together  are  often  heavier  than  the  brain.  The 
visual  acuity  of  birds,  that  is,  their  ability  to  distinguish  objects  as  they 
become  smaller  and  closer  together,  is  several  times  as  great  as  that  of 
man.  The  ability  to  accommodate  rapidly  is  also  well  developed  in  birds' 
eyes,  lor  birds  must  change  quickly  from  distant  to  near  vision  as  they 
maneuver  among  the  branches  of  a  tree  or  swoop  down  to  the  ground 
from  a  considerable  height.  Muscular  coordination  is  also  very  important 
in  the  bird  way  of  life,  and  the  cerebellum  is  correspondingly  well 
developed.  The  cerebral  hemispheres  are  large  and  are  an  important 
association  center. 


209.        The  Origin  and  Evolution  of  Birds 

One  might  infer  simply  from  the  structure  of  modern  birds  that 
they  have  evolved  from  archosaurian  reptiles,  but  we  need  not  stretch 
our  inferences,  for  two  specimens  of  a  fossil  bird  are  known  that  are 
clearly  intermediate  between  archosaurs  and  modern  birds.  The  fossils 
are  preserved  with  remarkable  detail  in  a  fine-grained,  lithographic  lime- 
stone from  Jurassic  deposits  in  Bavaria. 

Arcliaeopteryx  lithographica  (Fig.  24.7  A)  was  about  the  size  of  a 
crow.  Its  skeleton  is  reptilian  in  having  toothed  jaws,  no  fusion  of  trunk 
or  sacral  vertebrae,  a  long  tail,  and  a  poorly  developed  sternum.  Birdlike 
tendencies  are  evident  in  the  enlarged  orbits,  some  expansion  of  the 
brain  case,  and  in  the  winglike  structure  of  the  hand.  As  in  modern 
birds,   the   "hand"    is   elongated  and  only   three   "fingers"   are   present; 


Figure  24.7.  Extinct  birds.  A,  A  restoration  of  Archaeopteryx,  the  earliest  known 
bird;  B,  a  restoration  of  Hesperornis,  a  large  diving  bird  of  the  Cretaceous.  (A,  Heilmann; 
B,  courtesy  of  the  American  Museum  of  Natural  History.) 


A    HISTORY   OF   VERTEBRATES:   BIRDS   AND   MAMMALS 


479 


however,  there  is  Httle  fusion  of  bones  and  each  finger  bears  a  claw.  If 
the  skeleton  alone  were  known,  the  creature  would  probably  have  been 
regarded  as  a  peculiar  archosaur,  but  it  is  evident  that  this  was  a  primi- 
tive bird,  and  not  a  reptile,  for  there  are  clear  impressions  of  feathers 
(Fig.  35.1).  The  feathers  would  suggest  that  Archaeopteiyx  was  active 
and  warm-blooded.  The  ratio  of  its  wing  surface  to  its  body  size,  together 
with  the  poorly  developed  sternum,  indicates  that  it  was  not  a  strong 
flier.  These  most  primitive  birds  are  placed  in  the  subclass  Archaeor- 

nithes. 

The  next  group  of  fossil  birds  lived  in  the  Cretaceous.  These  birds 
had  lost  the  long  reptilian  tail,  had  evolved  a  well  developed  sternum, 
and  were  modern  in  many  other  ways.  A  true  pygostyle  had  not  yet 
evolved,  and  teeth  were  present  in  at  least  certain  species.  There  are 
clear  indications  of  teeth  in  fossils  of  Hesperornis  (Fig.  22.7  B),  a  large 
diving  species  with  powerful  hind  legs  and  vestigial  wings.  The  nature 


B  -     _  _ 

Figure  24.8.     Representative  paleognathous  birds.  A,  Ostriches;  B,  a  kiwi  with  its 
relatively  huge  egg.  (Courtesy  of  the  American  Museum  of  Natural  History.) 


480 


VERTEBRATE   LIFE  AND   ORGANIZATION 


ffliWW»«J'WPSJWl  W  V*"'™'  W  WM^^I^ 


Figure  24.9.  A  group  of  neognathous  birds.  A,  Penguins  use  their  modified  wings 
as  flippers;  li,  courtship  of  albatrosses;  C,  the  young  cormorant  has  to  reach  into  the 
throat  of  its  parent  to  get  its  food;  D,  an  American  egret,  or  heron,  a  wading  bird;  E, 
bhicbird  perching  on  a  hmb;  F,  noddy  terns  on  nest.  (A,  Smithsonian  Institute;  B, 
courtesy  of  Lt.  Col.  N.  Rankin;  C,  photo  by  L.  W.  Walker  from  National  Audubon 
Society;  D,  E,  F,  courtesy  American  Museum  of  Natural  History.) 


A    HISTORY   Of   VERTEBRATES:   BIRDS   AND   MAMMALS  481 

of  the  jaws  of  Ichthyornis,  which  was  a  tern-sized,  flying  species,  is  uncer- 
tain. Marsh,  who  described  it  in  1880,  considered  that  a  toothed  lower 
jaw,  which  was  found  in  close  association  with  the  rest  of  the  fossil, 
belonged  to  it.  More  recently,  Gregory  has  identified  the  lower  jaw  as 
belonging  to  a  small  marine  lizard.  Although  they  are  placed  in  the 
subclass  Neornithes  along  with  modern  birds,  the  more  primitive  nature 
of  these  Cretaceous  species  is  recognized  by  placing  them  in  a  distinct 
superorder— the  Odontognathae. 

All  later  birds  have  lost  the  reptilian  teeth,  but  a  few  (superorder 
Palaeognathae)  retain  a  somewhat  reptilian  palate,  whereas  others  (sub- 
order Neognathae)  have  a  more  specialized  palatal  structure.  Living 
paleognathous  birds  are  for  the  most  part  ground-dwelling,  flightless 
species,  such  as  the  ostriches  of  Africa,  the  rheas  of  South  America,  the 
cassowaries  of  Australia  and  the  peculiar  kiwi  of  New  Zealand  (Fig. 
24.8).  The  legs  are  well  developed  and  powerful,  the  wings  vestigial,  and 
the  feathers  do  not  have  booklets.  Presumably  these  birds  evolved  from 
flying  ancestors,  but  readapted  to  a  terrestrial  mode  of  life  in  areas  where 
there  was  an  abundant  food  supply  upon  the  ground  and  few  com- 
petitors or  enemies.  The  ancestry  of  certain  of  them  can  be  traced  back 
to  the  early  Cenozoic  era.  A  number  of  large,  ground-dwelling  neognath- 
ous  birds  also  lived  then,  which  suggests  that  there  might  have  been  a 
competition  at  this  time  between  birds  and  early  mammals  for  the 
conquest  of  the  land  surface,  which  had  recently,  geologically  speaking, 
been  vacated  by  the  large  reptiles.  Mammals  won,  and  only  a  few 
ground-dwelling  birds  survived. 

All  other  birds,  including  the  vast  majority  of  living  species,  are 
neognathous  types.  They  have  been  very  successful  and  have  adapted  to 
numerous  habitats  and  modes  of  life  (Fig.  24.9).  Some,  including  the 
loons,  ducks  and  gulls,  are  aquatic  as  well  as  good  fliers.  Other  aquatic 
species,  such  as  the  penguins,  have  lost  their  ability  to  fly,  and  their 
wings  are  modified  as  paddles  for  swimming  under  water.  The  herons, 
cranes  and  coots  have  become  specialized  for  a  wading,  marsh-dwelling 
mode  of  life.  Hawks,  eagles  and  owls  are  birds  of  prey.  The  grouse, 
pheasants  and  fowl  are  predominantly  terrestrial  forms,  though  they  can 
fly  short  distances,  and  the  song  and  perching  birds  are  well  adapted  for 
life  in  the  trees.  Twenty-three  orders  of  neognathous  birds  are  recog- 
nized (cf.  appendix).  The  song  birds  are  members  of  the  order  Pas- 
seriformes. 

210.        The  Bird  Way  of  Life 

Man  has  learned  more  about  the  habits  of  birds  and  their  way  of 
life  than  about  most  members  of  other  classes,  for  he  has  long  been 
fascinated  by  these  colorful  creatures  that  lead  such  intense  and  active 
lives.  A  few  of  their  more  interesting  features  are  considered  below. 

Food  Geii'ing.  Birds  have  a  high  rate  of  metabolism  and  must 
obtain  large  quantities  of  food  to  support  it.  During  most  of  their 
waking  hours  they  are  on  the  look-out  for  seeds,  insects,  worms,  or  what- 
ever makes  up  their  diet.  Crows  and  some  other  birds  eat  a  variety  of 


482  VERTEBRATE   LIFE   AND   ORGANIZATION 


Sparrow  (perching 


Jacana, 

(walKin^  on  floatm^  plants) 


"S.vs<v 


Woodpedicer 
(graspmg) 


Duclc        ^^ 


Pheasant 

(walking, 
scratching)/ 


Figure  24.10.     Adaptations  of  the  feet  of  birds. 

food,  both  plant  and  animal,  but  many  birds  have  become  specialists, 
and  have  evolved  adaptations  for  utilizing  particular  types  of  food. 
Their  bills  are  modified  accordingly,  and  one  can  often  tell  from  this 
alone  the  nature  of  their  food  and  how  they  get  it  (Fig.  38.1).  Finches 
have  short,  heavy  bills  well  suited  for  picking  up  and  breaking  open 
seeds.  The  hooked  beak  of  hawks  is  ideal  for  tearing  apart  small  animals, 
which  they  have  seized  with  their  powerful  talons.  Herons  use  their  long, 
sharp  bills  for  spearing  fish  and  frogs,  which  they  deftly  flip  into  their 
mouths  (Fig.  24.9).  The  length  and  shape  of  hummingbirds'  bills  are 
correlated  with  the  structure  of  the  flowers  from  which  they  extract 
nectar.  Whippoorwills  and  swallows  fly  about  in  the  evening  catching 
insects  with  their  gaping  mouths.  Bristle-like  feathers  at  the  base  of  the 
bill  help  them  to  catch  their  prey.  When  feeding,  the  skimmer  flies  just 
above  the  ocean  with  its  elongated  lower  jaw  skimming  the  surface.  Any 
fish  or  other  organisms  that  are  hit  are  flicked  into  its  open  mouth.  The 
woodcock's  long  and  sensitive  bill  is  adapted  for  probing  for  worms  in 
the  soft  ground.  The  woodcock  can  open  the  tip  of  its  bill  slightly  to 
grasp  a  worm  without  opening  the  rest  of  the  mouth! 

Support  and  Locomotion.  Flight  in  its  various  forms  is,  of  course, 
an  important  means  of  bird  locomotion.  When  not  flying,  most  birds 
support  themselves  and  move  about  on  their  hind  legs.  The  foot  has 
undergone  a  variety  of  modifications  as  the  bird  has  become  adapted  to 
special  modes  of  life  (Fig.  24.10).  The  foot  and  toes  become  particularly 
sturdy  in  ground-dwelling  species,  and  the  power  of  grasping  is  espe- 
cially well  developed  in  such  perching  specialists  as  our  song  birds.  In 


A    HISTORY   OF   VERTEBRATES:    BIRDS   AND   MAMMALS  483 

perching  birds,  the  tendons  of  the  foot  are  so  arranged  that  the  weight 
of  the  body  automatically  causes  the  toes  to  flex  and  grasp  the  perch 
when  the  bird  alights  upon  a  branch.  The  woodpeckers  have  sharp 
claws  and  the  fourth  toe  is  turned  backward  with  the  first  to  form  a  foot 
ideally  suited  for  clinging  onto  the  sides  of  trees.  Swimming  birds  have 
a  web  stretching  between  certain  of  their  toes— the  three  anterior  toes 
in  loons,  albatrosses,  ducks,  gulls  and  many  others;  all  four  toes  in 
pelicans,  cormorants  and  their  relatives.  The  marsh-dwelling  jacana  of 
the  tropics  has  a  foot  with  exceedingly  long  toes  and  claws  that  enable 
it  to  scamper  across  lily  pads  and  other  floating  vegetation.  Swifts  and 
hummingbirds  have  very  small  feet  barely  strong  enough  to  grasp  a 
perch.  These  birds  spend  most  of  their  time  on  the  wing  and  almost 
never  alight  on  the  ground. 

Reproduction.  Birds  have  developed  elaborate  behavioral  patterns 
and  structural  modifications  associated  with  reproduction,  many  of 
which  have  been  carefully  studied.  For  example,  in  most  of  the  common 
species  each  male  bird  stakes  out  for  himself  a  well  delineated  nesting 
territory  which  he  vigorously  defends  against  all  rivals  and  into  which  he 
hopes  to  attract  a  female.  The  distinctive  song  of  the  male  during  the 
breeding  season  advertises  the  territory  to  females  of  the  appropriate 
species  and  warns  rival  males  to  stay  away.  The  brilliant  plumage  of  the 
male  birds  plays  a  similar  role,  serving  both  to  warn  rivals  and  attract 
and  stimulate  females.  An  advantage  of  this  territorial  organization  of 
breeding  birds  is  that  it  ensures  a  reasonably  uniform  distribution  of 
mating  pairs  in  the  inhabitable  area.  This  facilitates  finding  food  with- 
out going  far  afield,  and  helps  to  get  and  to  keep  the  parent  birds  to- 
gether. 

Once  a  female  has  been  attracted  to  the  territory,  courtship  begins. 
Sometimes  it  is  accompanied  by  elaborate  display  rituals  which  appar- 
ently serve  as  a  sexual  stimulant  leading  to  nest-building  and  copulation 
(Fig.  24.9  B).  A  brief  cloacal  apposition  is  sufficient  to  transfer  sperm  to 
the  female  reproductive  tract;  only  a  few  male  birds,  chiefly  primitive 
species,  retain  the  reptilian  copulatory  organ.  Further  courtship  and 
copulation  may  occur  after  the  eggs  have  been  fertilized  and  laid.  Pre- 
sumably this  aids  in  keeping  the  parents  together  for  the  tasks  of  incu- 
bating the  eggs  and  caring  for  the  young.  Voung  chickens  and  some  other 
birds  are  precocial.  They  are  covered  with  down,  and  can  run  about 
and  feed  for  themselves  when  hatched.  But  most  of  our  song  birds  are 
altriclal,  and  are  naked  and  helpless  when  they  first  emerge  from  the 
eggs.  Such  birds  need  close  parental  care  to  supply  food  and  warmth 
during  the  critical  period  of  their  infancy.  Either  or  both  parents  care 
for  the  young.  As  Professor  Young,  of  University  College,  London,  has 
so  aptly  put  it,  "In  birds,  as  in  man,  the  'procreation  of  children'  is  not 
accomplished  by  a  single  act  of  fertilization." 

Migration.  The  capture  of  sufficient  food  and  the  reproductive 
process  are  the  motivations  responsible  for  most  of  birds'  activities.  Some 
birds  are  able  to  fill  all  of  their  needs  in  the  general  area  in  which  they 
were  hatched,  but  others  have  taken  full  advantage  of  their  power  of 
flight  and  go  considerable  distances  in  their  search  for  favorable  nesting 


484  VERTEBRATE   LIFE   AND   ORGANIZATION 

sites  and  Iccdiiig  areas.  During  much  of  the  year  the  food  supply  in  a 
given  area  is  ade(juate  to  sustain  a  population  of  reasonable  size.  The 
food  available  may  not  suffice,  however,  during  the  breeding  season 
when  the  increased  activity  of  the  birds  increases  their  food  requirement 
and  when  the  population  is  more  than  doubling.  Spreading  out  into 
new  areas  at  this  time  has  some  advantage,  and  permits  a  larger  bird 
population.  .Mthough  the  reasons  for  the  evolution  of  migration  are 
luuertain,  the  search  for  food  may  have  been  a  factor  in  the  tendency 
for  many  birds  to  migrate  north  in  the  summer.  There  is  a  large  land 
mass  in  the  north,  and  during  the  summer,  at  least,  this  area  is  rich  in 
food.  The  tendency  to  return  south  as  the  weather  becomes  inclement 
in  tlie  autumn  might  be  correlated  with  the  reduction  of  the  food 
supply;  it  apparently  is  in  the  case  of  certain  insect-eating  species,  but 
otliers  migrate  before  there  is  any  food  shortage.  The  glaciation  of  large 
parts  of  the  Northern  Hemisphere  during  the  great  Ice  Ages  may  have 
been  an  additional  factor  in  the  evolution  of  the  migratory  habit. 

The  pattern  of  migration  is  regular  for  each  species;  it  begins  at 
very  nearly  the  same  time  each  year.  Apparently  the  stimulus  is  a  change 
in  day  length— its  increase  in  spring  and  reduction  in  fall— for  this  is 
the  only  environmental  factor  that  varies  in  a  manner  regular  enough 
to  serve  as  a  consistent  timetable.  Rowan  and  others  have  shown  that 
day  length  operates  by  affecting  the  activity  of  the  bird  which  in  turn 
influences  the  size  of  the  gonads.  The  gonads  increase  in  size  as  day 
length  increases  and  the  birds  become  more  active,  and  decrease  in  size 
as  day  length  decreases.  Artificial  illumination  or  darkness,  and  forced 
activity  or  inactivity,  have  comparable  effects. 

Most  birds  migrate  at  night,  stopping  to  feed  and  rest  during  the 
day.  Some  may  fly  several  hundred  miles  during  a  single  night,  but  then 
may  rest  for  several  days.  The  northward  advance  of  these  birds  in  the 
spring  averages  about  20  to  25  miles  per  day.  Many  species  tend  to  follow 
the  advance  of  certain  temperature  lines,  or  isotherms  (Fig.  24.11).  The 
length  of  migration  and  the  route  taken  are  very  consistent  for  each  type 
of  bird,  but  vary  with  the  species.  The  Canada  goose  winters  in  the 
United  States  from  the  Great  Lakes  south,  breeds  in  Canada  as  far  north 
as  the  Arctic  coast,  and  migrates  along  a  broad  front  between  the  two 
areas.  The  scarlet  tanager  winters  in  parts  of  South  America  and  breeds 
in  the  area  from  Nova  Scotia,  southern  Quebec  and  southern  Manitoba 
south  to  South  Carolina,  northern  Georgia,  northern  Alabama  and 
Kansas.  In  contrast  to  the  Canada  goose,  it  has  a  narrow  migration  route, 
which  extends  through  southern  Central  America  and  then  across  the 
center  of  the  Gulf  of  Mexico,  passing  between  Yucatan  and  Cuba.  The 
longest  migration  is  that  of  the  Arctic  tern;  some  of  these  birds  travel 
25,000  miles  in  a  year.  This  species  breeds  in  the  Arctic,  then  follows 
the  coast  line  of  Europe  and  Africa  to  its  winter  quarters  in  the  South 
Atlantic. 

The  season,  speed  and  routes  of  migration  have  been  carefully  de- 
scribed for  most  species  of  birds,  but  how  birds  navigate  and  find  their 
way  during   their  migrations  remains  one  of  the   intriguing,   unsolved 


A    HISTORY   Of   VERTEBRATES.-   BIRDS   AND   MAMMALS 


485 


problems  of  animal  behavior.  Obviously  the  birds  must  know  where  they 
are  going;  there  must  be  some  feature  of  the  environment  that  is  related 
to  the  goal  of  the  bird,  and  the  bird  must  have  some  way  of  perceiving 
this  feature.  Theories  of  navigation  based  on  magnetic  fields  of  the  earth, 
visual  landmarks,  celestial  points  of  reference  and  other  aspects  of  the 
environment  have  been  proposed,  but  no  single  one  explains  all  of  the 
facts.  The  magnetic  field  theory  is  weakened  by  our  inability  to  demon- 
strate that  birds  are  sensitive  to  magnetic  fields.  Visual  landmarks  are 
certainly  used  in  some  cases,  but  apparently  not  in  all.  Dr.  Griffin  of 
Harvard  University,  in  a  study  of  the  related  problem  of  homing,  re- 
leased sea  birds  (gannets)  in  unknown  territory  one  hinidred  miles  or 
more  inland  from  their  nests  and  followed  their  return  from  an  air- 
plane. The  birds  did  not  head  straight  for  home,  but  Hew  in  widening 
circles  over  large  areas,  apparently  in  an  exploratory  fashion,  until  they 
came  into  familiar  territory,  and  then  they  headed  directly  home.  Dr. 
Matthews  of  Cambridge  University  questions  the  significance  of  explora- 
tory flight  and  has  suggested  that  birds  use  the  position  of  the  sun,  a 
sense  of  time,  and  a  knowledge  of  the  position  of  the  smi  at  different 
times  in  their  home  territory  to  determine  their  position  and  to  find 
their  way  home.  This  would  be  analogous  to  a  mariner  who  uses  a 
sextant,  a  chronometer  and  a  knowledge  of  the  latitude  and  longitude 
of  his  destination  to  find  his  way.  The  fact  that  birds  released  in  un- 
familiar territory  find  their  way  home  better  on  sunny  days  than  on  over- 
cast days  lends  support  to  his  hypothesis.  Both  landmarks  and  the  sun 
may  be  used  in  homing,  but  it  is  still  difficult  to  explain  many  of  the 


ru 


Figure  24.11.     The   northward  migration   of   the  Canada   goose   keeps  pace  with 
spring,  following  the  isotherm  of  35°  F.  (Modified  after  Lincoln.) 


486  VERTEBRATE   LIFE   AND   ORGANIZATION 

phenomena  of  migration,  particularly  such  things  as  the  ability  of  young 
birds  to  reach  their  destination  on  their  first  migration  even  though 
unaccompanied  by  adults.  More  observations  and  experiments  are  neces- 
sary before  the  riddle  of  bird  migration  will  be  solved. 

211.        Characteristics  of  Mammals 

Mammals  are  the  familiar  haired  creatures,  such  as  cats,  mice,  pigs 
and  men.  They  are  the  group  of  organisms  to  which  the  term  "animal" 
is  often  restricted  by  laymen,  though  zoologists  object  to  such  a  usage. 
A  jaw  joint  between  the  dentary  and  squamosal  bones,  and  the  presence 
of  three  auditory  ossicles  within  the  middle  ear,  are  convenient  osteo- 
logic  features  for  distinguishing  between  mammals  and  the  extinct  mam- 
mal-like reptiles  from  which  they  evolved.  Osteologic  criteria  are 
necessary  in  dealing  with  fossil  material,  but  contemporary  reptiles  and 
mammals  can  be  distinguished  in  many  other  ways.  The  presence  of  hair 
and  mammary  glands  is  the  most  obvious  diagnostic  feature  of  mam- 
mals, but  these  are  only  two  reflections  of  more  fundamental  changes- 
increased  activity  and  greater  care  of  the  young. 

Increased  Activity.  Birds  are  the  most  active  of  all  vertebrates,  but 
mammals  are  a  close  second,  and  they  are  certainly  the  most  active  of 
the  primarily  terrestrial  vertebrates.  Their  appendages  extend  directly 
down  to  the  ground  in  the  vertical  plane,  instead  of  out  from  the  body 
in  the  horizontal  plane  as  the  proximal  segment  of  the  limb  does  in 
amphibians  and  most  reptiles.  This  improves  the  effectiveness  of  the 
limbs  in  support,  and  permits  them  to  move  rapidly.  A  firmer  support 
is  also  provided  for  the  pelvic  girdle  and  hind  limbs,  because  most 
mammals  have  three  sacral  vertebrae  in  contrast  to  the  two  of  most 
reptiles.  Arboreal  species  use  the  tail  for  balancing,  and  it  plays  a  major 
role  in  the  propulsion  of  aquatic  mammals  such  as  the  whales,  but  in 
most  mammals  it  has  lost  its  primitive  role  in  locomotion  and  is  fre- 
quently reduced  in  size.  Further  details  of  the  mammalian  skeleton,  and 
of  other  organ  systems,  will  be  emphasized  in  succeeding  chapters,  but, 
in  short,  the  whole  skeleton  reflects  the  increased  activity  and  agxlity. 

The  increased  speed  of  locomotion  also  entailed  changes  in  the 
neuromuscular  apparatus.  Shifts  in  many  of  the  muscles  concerned  with 
support  and  locomotion  are  correlated  with  the  new  limb  posture. 
Moreover,  the  muscular  system  of  mammals  is  considerably  more  elab- 
orate than  that  of  reptiles,  for  many  primitive  muscles  have  been  sub- 
divided. This,  together  with  a  more  highly  developed  nervous  system, 
permits  more  varied  responses  and  adjustments  to  environmental  con- 
ditions. 

A  consistently  active  life  naturally  requires  a  high  and  constant 
rate  of  metabolism,  and  mammals  have  had  the  same  problems  to  solve 
in  this  respect  as  their  avian  relatives.  Mammals  are  also  homoiothermic, 
but  there  are  differences  in  the  way  temperature  regulation  is  achieved. 
Hair,  rather  than  feathers,  entraps  air  and  forms  an  insulating  layer 
over  the  body  surface  that  reduces  heat  loss.  Heat  is  dissipated,  when 
necessary,   by   an   increased    blood   flow    through   the    skin   and    by   the 


A    HISTORY   OF   VERTEBRATES:   BIRDS   AND   MAMMALS 


487 


evaporation  of  water.  Many  mammals  lose  water  from  the  body  surface 
in  the  form  of  sweat,  secreted  by  sweat  glands,  but  mammals  such  as 
dogs,  that  have  few  sweat  glands,  pant  vigorously  and  lose  water  from 
the  mouth  and  respiratory  passages.  Birds  can  cool  themselves  by  the 
evaporation  of  water  from  the  respiratory  tract,  but  none  have  evolved 
sweat  glands. 

The  dentition  of  mammals  is  adapted  for  the  purpose  of  obtaining 
and  handling  a  wide  variety  of  foods.  Their  teeth  are  not  all  the  same 
shape,  as  is  generally  the  case  in  reptiles,  but  are  differentiated  into 
various  types  (Fig.  24.12).  Chisel-shaped  incisors  are  present  at  the  front 
of  each  jaw  and  are  used  for  nipping  and  cropping.  Next  is  a  single 
canine  tooth,  which  is  primitively  a  long,  sharp  tooth,  useful  in  attack- 
ing and  stabbing  the  prey,  or  in  defense.  A  series  of  premolars  and 
molars  follow  the  canine.  These  teeth  tear,  crush  and  grind  up  the  food. 
In  primitive  mammals,  the  premolars  are  sharper  than  the  molars  and 
have  more  of  a  tearing  function.  Most  mammals  do  not  swallow  their 
food  whole,  but  break  it  up  mechanically  with  their  teeth  and  mix  it 
with  saliva,  which,  in  addition  to  lubricating  the  food,  usually  contains 
an  enzyme  that  begins  the  digestion  of  carbohydrates.  Digestion  is  com- 
pleted in  the  stomach  and  intestinal  region.  Numerous  microscopic 
villi  line  the  small  intestine,  as  they  do  in  birds,  and  increase  the  surface 
area  available  for  absorption. 

A  greater  exchange  of  oxygen  and  carbon  dioxide  is  made  possible 
by  a  many-fold  increase  in  the  respiratory  surface  of  the  lungs  and  by 


Figure  24.12.  Teeth  of  mammals.  A,  The  relatively  unspecialized  teeth  of  a  prim- 
itive insectivore;  B  and  C,  lateral  and  crown  views  of  the  left  upper  and  lower  molars 
of  an  insectivore  to  show  their  occlusion;  D,  the  stabbing  and  cutting  teeth  of  a  cat; 
E  and  F,  a  crown  view  and  a  vertical  section  through  the  left  upper  molar  of  a  horse 
to  show  its  adaptation  for  crushing  and  grinding. 


488  VERTEBRATE   LIFE   AND   ORGANIZATION 


Diaphragm 

(— Plcu.ra.1  cavity 
"Lung 


Esophagus 
I^harynx 

Eustachi  an  tobc 


Brain. 

-N/^IntarnsLl  naris 

Nasal 
cavity 


External 
naris 


— Mouth  cavity 
Secondary  palate. 


Larynx 
"-Trachea 
■Heart 
-Alveoli  (greatly  enlcirged) 


Figure  24.13.     A  sagittal  section  of  the  head  of  a  pig  showing  the  relationship  be- 
tween the  digestive  and  respiratory  systems.  The  route  of  air  is  shown  by  arrows. 


improved  methods  of  ventilation  (Fig.  24.13).  The  increase  in  surface  is 
accomplished  by  a  subdivision  of  the  air  passages  within  the  lung  so 
that  all  end  in  clusters  of  thin-walled  sacs  (alveoli)  whose  walls  contain 
a  dense  capillary  network.  It  has  been  estimated  that  the  respiratory 
surface  of  the  human  lungs  is  between  50  and  100  square  meters,  or  25 
to  50  times  the  surface  area  of  the  body.  Birds  also  have  a  large  respira- 
tory surface,  but  their  lungs  are  more  compact  organs  and  the  respiratory 
surface  may  not  be  relatively  as  great  as  in  mammals.  Birds  and  mam- 
mals differ  in  the  method  of  ventilation.  Air  must  be  moved  in  and  out 
of  blind  sacs  in  mammalian  lungs,  whereas  there  can  be  a  through  draft 
in  avian  lungs.  The  lungs  of  birds  are  more  efficient  as  gas  exchangers, 
for  the  air  in  the  air  capillaries  contains  relatively  more  oxygen  than 
the  somewhat  stale  air  in  the  alveoli,  but  the  more  thorough  ventilation 
of  avian  lungs  probably  results  in  a  greater  loss  of  body  water  via  this 
route. 

The  mechanics  for  the  ventilation  of  mammalian  lungs  are  more 
efficient  than  those  of  amphibians  and  reptiles.  One  important  factor  in 
improved  ventilation  has  been  the  evolution  of  a  muscular  diaphragm 
whose  contractions,  together  with  a  forward  movement  of  the  ribs, 
expand  the  chest  cavity  and  draw  air  into  the  lungs.  Another  factor  has 
been  the  evolution  of  a  secondary  palate,  a  horizontal  partition  of  bone 
and  flesh  in  the  roof  of  the  mouth  that  separates  the  air  and  food  pas- 
sages in  this  region.  In  lower  tetrapods,  the  nasal  cavities  lead  directly 
into  the  front  of  the  mouth,  but  in  mammals  they  open  more  posteriorly 
into    the   pharynx.    The    secondary    palate    permits    nearly    continuous 


A   HISTORY   OF   VERTEBRATES:   BIRDS   AND   MAMMALS  489 

breathing,  which  is  certainly  a  desirable  attribute  for  organisms  with 
a  high  rate  of  metabolism.  Mammals  can  manipulate  food  in  their 
mouth  and  breathing  need  be  interrupted  only  momentarily  when  the 
food  is  swallowed,  and  in  some  species  not  even  then  (young  of  opos- 
sum, p.  492). 

Mammals,  like  birds,  have  evolved  an  efficient  system  of  internal 
transport  of  materials  between  sites  of  intake,  utilization  and  excretion. 
Their  heart  is  completely  divided  internally  so  there  is  no  mixing  of 
venous  and  arterial  blood.  Venous  blood  coming  from  the  body  and 
going  to  the  lungs  passes  through  the  right  atrium  and  right  ventricle, 
while  arterial  blood  coming  from  the  lungs  and  going  to  the  body  passes 
through  the  left  atrium  and  ventricle.  Increased  blood  pressure  also 
makes  for  a  more  rapid  and  efficient  circulation. 

Nitrogenous  wastes  from  the  breakdown  of  proteins  and  nucleic 
acids  must  be  eliminated  without  an  excessive  loss  of  body  water.  In 
mammals,  most  of  the  nitrogenous  wastes  are  eliminated  in  the  form  of 
urea,  which  is  more  soluble  and  requires  more  water  for  its  removal  than 
does  the  uric  acid  excreted  by  some  reptiles  and  birds.  Approximately 
99  per  cent  of  the  water  that  starts  down  the  kidney  tubules  is  reab- 
sorbed in  special  regions  of  the  tubules,  and  the  net  loss  of  water  is 
minimal.  The  generally  high  metabolic  rate  of  mammals  results  in  the 
formation  of  a  large  amount  of  wastes  to  be  eliminated.  An  increase 
in  blood  pressure,  and  hence  in  blood  flow  through  the  kidney,  and  an 
increase  in  the  number  of  kidney  tubules  have  enabled  mammals  to  in- 
crease the  rate  of  excretion. 

Care  of  the  Young.  The  evolution  by  reptiles  of  the  cleidoic  egg 
was  a  successful  adjustment  to  terrestrial  reproduction  so  long  as  verte- 
brates were  cold-blooded.  However,  embryos  that  are  to  develop  into 
homoiothermic  adults  must  apparently  have  a  warm,  constant  tempera- 
ture to  develop  normally,  so  birds  and  mammals  cannot  lay  eggs  and 
then  ignore  them.  Birds  lay  cleidoic  eggs,  but  incubate  them  by  sitting 
on  them,  and  one  group  of  primitive  mammals,  which  includes  the 
duckbilled  platypus  of  Australia,  does  the  same.  All  other  mammals  are 
viviparous.  The  eggs  are  retained  within  a  specialized  region  of  the 
female  reproductive  tract,  the  uterus,  and  the  young  are  born  as  minia- 
ture adults. 

All  of  the  extraembryonic  membranes  characteristic  of  reptiles  are 
present  in  viviparous  mammals,  but  albuminous  materials  are  not 
ordinarily  secreted  about  the  egg.  The  allantois,  or  in  a  few  species 
the  yolk  sac,  unites  with  the  chorion,  thereby  carrying  the  fetal  blood 
vessels  over  to  this  outermost  membrane.  The  vascularized  chorion  unites 
in  varying  degrees  with  the  uterine  lining  to  form  a  placenta,  in  which 
fetal  and  maternal  blood  streams  come  close  together,  though  they 
remain  separated  by  some  layers  of  tissue  (Fig.  28.7).  The  embryo  de- 
rives its  food  and  oxygen,  and  eliminates  its  carbon  dioxide  and  nitrog- 
enous wastes  across  these  membranes. 

Care  of  the  young  does  not  stop  at  birth,  for  all  female  mammals 
have  specialized  mammary  glands,  which  secrete  a  nutrient  milk  on 
which  the  young  feed.  In  such  primitive  mammals  as  the  platypus  (Fig. 


490  VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  24.14.  Monotremes  and 
marsupials.  A,  The  duckbilled  platypus; 
B,  the  spiny  anteater;  C,  opossum  and 
young;  D,  koala  bear;  E,  kangaroo.  The 
platypus  and  anteater  are  monotremes; 
the  others  are  marsupials.  (A  and  B, 
courtesy  of  the  New  York  Zoological 
Society;  C  and  D,  courtesy  of  American 
Museum  of  Natural  History;  E,  Aus- 
tralian News  and  Information  Bureau.) 


A    HISTORY   Of   VERTEBRATES:   BIRDS    AND   MAMMALS  491 

24.14  A),  the  milk  is  discharged  onto  the  hairs  and  the  young  lap  it  up, 
but  in  other  mammals,  nipples  or  teats  are  associated  with  the  glands 
and  the  young  are  suckled.  When  the  young  finally  leave  their  mother, 
they  are  at  a  relatively  advanced  stage  of  development,  and  are  equipped 
to  care  for  themselves. 


212.        Primitive  Mammals 

Monofremes.  The  most  primitive  mammals  are  the  platypus 
(Ornithorhynchus),  and  its  close  relative,  the  spiny  anteater  (Tachyglos- 
sus)  (Fig.  24.14  A  and  B).  In  addition  to  the  egg-laying  habit,  these 
mammals  retain  many  other  reptilian  characteristics,  including  a  cloaca. 
The  ordinal  name  for  the  group,  Monotremata,  refers  to  the  presence  of 
a  single  opening  for  the  discharge  of  feces,  excretory  and  genital  prod- 
ucts. In  other  mammals,  the  cloaca  has  become  divided,  and  the  opening 
of  the  intestine,  the  anus,  is  separate  from  that  of  the  urogenital  ducts. 

Monotremes  are  curious  animals  that  have  survived  to  the  present 
only  because  they  have  been  isolated  from  serious  competition  in  the 
Australian  region.  The  platypus  is  a  semiaquatic  species  with  webbed 
feet,  short  hairs  and  a  bill  like  a  duck's  used  in  grubbing  in  the  mud  for 
food.  Spiny  anteaters  have  large  claws  and  a  long  beak  adapted  for 
feeding  upon  ants  and  termites.  The  animal  can  burrow  very  effectively 
with  these  claws,  completely  burying  itself  in  fairly  hard  ground  in  a 
few  minutes.  Many  of  its  hairs  are  modified  as  quills. 

A\4ien  the  first  skins  of  the  platypus  were  shipped  to  Europe  in  the 
late  18th  century,  many  zoologists  viewed  them  as  skillful  fakes  such 
as  the  then  current  Chinese  mermaids  (the  forepart  of  a  monkey  sewn 
onto  the  tail  of  a  fish).  After  the  authenticity  of  the  platypus  was  estab- 
lished, a  long  controversy  ensued  as  to  whether  to  consider  it  a  reptile 
or  a  mammal.  Monotremes  were  finally  regarded  as  mammals,  but  as 
such  primitive  and  unusual  ones  that  they  are  placed  in  a  separate  sub- 
class—the Prototheria.  Many  investigators  now  believe  that  monotremes 
evolved  from  mammal-like  reptiles  earlier  than,  and  independently  of, 
the  other  mammals.  If  this  is  true,  mammals  have  had  a  polyphyletic 
rather  than  a  common  evolutionary  origin  (Fig.  22.2).  A  corollary  of  such 
a  view  is  that  hair  and  mammary  glands  either  evolved  independently 
in  monotremes  and  other  mammals,  or  were  attributes  of  the  mammal- 
like reptiles. 

hAarsup'ials.  All  other  mammals  are  believed  to  have  had  a  com- 
mon origin,  and  are  placed  in  the  subclass  Theria.  Therian  mammals 
were  present  in  the  last  half  of  the  Mesozoic  era,  but  they  did  not 
become  abundant  until  the  extinction  of  the  ruling  reptiles.  During  the 
Cenozoic,  they  increased  rapidly,  radiated  widely,  and  became  the  domi- 
nant terrestrial  vertebrates. 

Contemporary  therians  fall  into  two  infraclasses— (1)  the  Meta- 
theria,  which  includes  the  opossum,  kangaroo  and  other  pouched  mam- 
mals of  the  order  Marsupialia  (Fig.  24.14);  and  (2)  the  Eutheria,  or  true 
placental  mammals.  Both  groups  are  viviparous,  though  the  placental 
arrangement  of  marsupials  is  less  effective  than  that  of  eutherians.  In 


492  VERTEBRATE   LIFE   AND   ORGANIZATION 

most  marsupials  the  extraembryonic  membranes,  and  chiefly  the  yolk 
sac,  simply  absorb  a  "uterine  milk"  secreted  by  the  mother.  There  is  no 
intimate  union  between  the  extraembryonic  membranes  and  the  uterine 
lining  as  there  is  in  most  eutherians. 

Marsupials  are  born  in  what  we  would  regard  as  a  very  premature 
stage.  Their  front  legs,  however,  are  well  developed  at  birth,  and  the 
young  jnill  themselves  into  a  marsupium,  or  pouch  on  the  belly  of 
the  mother,  attach  to  a  nipjjle,  and  there  complete  their  development. 
It  has  long  been  believed  that  they  are  too  immature  to  suck,  and  that 
milk  is  squirted  from  the  mammary  glands  into  their  mouths.  But  in  his 
recent  book  on  "  Tossums,"  Hartman  relegates  this  notion  to  the  limbo 
of  false  myths  and  cites  careful  experiments  and  observations  proving 
that  the  young  do  indeed  suck.  A  forward  extension  of  the  tubular 
epiglottis  dorsal  to  the  secondary  palate  completely  separates  the  diges- 
tive and  respiratory  tracts,  and  breathing  and  feeding  can  take  place 
concurrently. 

Marsupials  were  world-wide  during  the  early  Cenozoic,  but  as  eu- 
therians began  to  spread  out,  marsupials  became  restricted.  They  have 
been  most  successful  in  those  parts  of  the  world  where  they  have  been 
isolated  from  competition  with  eutherians.  They  are  the  dominant  type 
of  mammal  in  Australia,  and  have  undergone  an  adaptive  radiation  and 
have  become  specialized  for  many  modes  of  life.  There  are  carnivorous 
marsupials  such  as  the  Tasmanian  wolf,  ant-eating  types,  molelike  types, 
semiarboreal  phalangers  and  koala  bears  (the  original  "Teddy-bear"), 
plains-dwelling  kangaroos  and  rabbit-like  bandicoots.  In  contrast,  the 
only  marsupial  present  in  North  America  is  the  semiarboreal  opossum. 

213.        Adaptive  Radiation  of  Eutherians 

/nsecf/vores.  The  eutherians,  or  placental  mammals,  as  they  are 
frequently  called,  are  the  most  successful  mammals  in  all  the  parts  of 
the  world  that  they  have  reached.  They  have  radiated  widely  and 
adapted  to  nearly  every  conceivable  ecologic  niche  upon  the  land.  Others 
have  rcadapted  successfully  to  an  aquatic  mode  of  life,  and  some  have 
evolved  true  flight. 

The  most  primitive  eutherians,  that  is,  the  stem  group  from  which 
the  other  lines  of  descent  evolved,  were  rather  generalized,  semiarboreal, 
insect-eating  types  of  the  order  Insectivora.  Modern  shrews  and  moles 
(Fig.  24.15)  are  specialized  insectivores. 

Flying  Mammals.  Bats,  order  Chiroptera,  are  closely  related  to  this 
stem  group,  and  are  sometimes  characterized  as  flying  insectivores.  As  in 
other  flying  vertebrates,  the  pectoral  appendages  have  been  transformed 
into  wings.  Bat  wings  are  structurally  closer  to  those  of  pterosaurs  than 
to  birds'  wings,  for  the  flying  surface  is  a  leathery  membrane,  but  the 
wing  of  a  bat  is  supported  by  four  elongated  fingers  (the  second  to  fifth) 
rather  than  by  a  single  one  as  in  the  pterosaur.  The  wing  membrane 
attaches  onto  the  hind  legs,  and  in  some  bats  the  tail  is  included  in  the 
membrane.  The  first  finger  is  free  of  the  wing,  bears  a  small  claw  and 
is  used  for  grasping  and  clinging.  The  hind  legs  are  small  and  are  of 


A    HISTORY   OF   VERTEBRATES:   BIRDS   AND   MAMMALS 


493 


little  use  upon  the  ground,  but  they,  too,  are  effective  grasping  organs, 
and  are  used  for  cUnging  to  a  perch  from  wiiich  the  bat  liangs  upside 
down  when  at  rest. 

Most  bats  are  crepuscular  in  habits,  flying  about  at  dusk  in  search 
of  insect  prey.  As  Galambos  and  Griffin  have  shown,  most  of  them  rely 
upon  a  biologic  sonar  system  for  finding  their  ^\ay  and  avoiding  objects, 
rather  than  upon  their  eyes,  which  are  small  and  weak.  As  they  fly,  they 
emit  ultrasonic  clicks  that  bounce  off  objects  and  are  reflected  back  to 
their  sensitive  ears.  A  bat  which  has  been  blinded  can  successfully  navi- 
gate in  a  room  full  of  obstacles,  but  bumps  into  objects  if  its  ears  are 
plugged  or  its  mouth  covered. 

Bats  are  the  only  mammals  to  have  evolved  true  flight,  but  some 
other  mammals  can  stretch  a  loose  skin  fold  between  their  front  and 
hind  legs  and  glide  from  tree  to  tree.  The  flying  squirrel  (Fig.  24.20  A) 
of  the  order  Rodentia  is  one.  Another  is  the  "flying  lemur"  of  the  East 
Indies.  This  animal  is  not  a  lemur,  which  is  a  primitive  primate,  but 
belongs  to  an  order  of  its  own— the  Dermoptera. 

Toothless  Mammals.  Since  primitive  mammals  were  insectivorous, 
it  is  not  surprising  that  certain  ones  became  specialized  to  feed  upon 
ants   and   termites,   which   are  very   abundant  in   certain   regions.   The 


Figure  24.15.     A,  A  mole  in  its  burrow;  B,  a  bat  with  its  baby;  C,  the  giant  ant- 
eater.  (Courtesy  of  the  American  Museum  of  Natural  History.) 


494  VERTEBRATE   LIFE   AND    ORGANIZATION 

South  American  anteater,  order  Edentata  (Fig.  24.15  C),  is  representative 
ol  tliis  mode  ol  lile.  Its  large  claws  enable  it  to  open  ant  hills,  and  then 
it  laps  up  the  insects  with  its  long,  sticky  tongue.  In  contrast  to  a  primi- 
tive insectivore,  which  crushes  its  insect  food  with  its  teeth,  an  anteater 
swallows  whole  the  insects  that  it  eats.  Its  teeth  were  not  needed  for 
survival  and  have  been  lost. 

The  tree  sloth  and  armadillo  belong  to  this  same  order,  though 
they  retain  vestiges  of  teeth.  The  pangolins  of  Africa  and  Asia  (order 
Pholidota)  and  the  aardvark  of  South  Africa  (order  Tubulidentata)  are 
superficially  similar,  but  this  is  a  result  of  adaptation  to  a  similar  mode 
of  life.  The  acquisition  by  distantly  related  or  unrelated  groups  of 
similar  features  as  a  result  of  adaptation  to  a  common  environment  is 
known  as  convergent  evolution.  When  closely  related  groups  evolve 
similarly  the  phenomenon  is  known  as  parallel  evolution. 

Primates.  Members  of  the  order  Primates,  the  group  to  which 
monkeys  and  man  belong,  are  also  closely  related  to  the  primitive  in- 
sectivorous stock.  Indeed,  one  member  of  the  order,  the  Oriental  tree 
shrew  (Tupnia,  Fig.  24.16  A),  has  at  times  been  considered  to  be  an 
insectivore.  Primates  evolved  from  primitive,  semiarboreal  insectivores, 
and  underwent  further  specializations  for  life  in  the  trees.  Even  those 
that  have  secondarily  reverted  to  a  terrestrial  life  bear  the  stamp  of  this 
prior  arboreal  adaptation.  Our  flexible  limbs  and  grasping  hands  are 
fundamentally  adaptations  for  life  in  the  trees.  Claws  were  transformed 
into  finger-  and  toenails  when  grasping  hands  and  feet  evolved.  The 
reduction  of  the  olfactory  organ  and  olfactory  portion  of  the  brain,  and 
the  development  of  stereoscopic,  or  binocular,  vision,  represent  other 
adaptations  of  our  ancestors  to  arboreal  life.  Keen  vision  and  the 
ability  to  appreciate  depth  are  very  important  for  animals  moving 
through  trees,  whereas  smell  is  less  important  for  organisms  living  some 
distance  from  the  ground  than  it  is  for  terrestrial  species.  Muscular 
coordination  is  also  very  important,  and  the  cerebellum  of  primates  is 
unusually  well  developed.  The  evolution  of  stereoscopic  vision,  increased 
agility,  and  particularly  the  influx  of  a  new  sort  of  sensory  information 
gained  by  the  handling  of  objects  with  a  grasping  hand,  was  accom- 
panied by  an  extraordinary  development  of  the  cerebral  hemispheres. 
The  cerebrum  is  the  chief  center  for  the  integration  of  sensory  informa- 
tion and  the  initiation  of  appropriate  motor  responses  in  all  mammals, 
but  it  is  particularly  prominent  in  primates.  It  is  believed  that  higher 
mental  functions  such  as  conceptual  thought  could  only  have  evolved 
in  organisms  with  a  grasping  hand.  In  a  very  real  sense,  we  are  a  product 
of  the  trees. 

Three  levels  of  primate  organization  are  commonly  recognized  by 
dividing  the  order  Primates  into  three  suborders.  The  first,  suborder 
Lemuroidea,  includes  the  tree  shrew,  lemurs,  lorises,  and  the  peculiar 
aye-aye.  Although  fossils  of  lemurs  are  found  in  North  America,  lemur- 
like primates  are  now  confined  to  the  Old  World  tropics;  Madagascar 
has  a  particularly  rich  fauna  of  lemurs.  All  are  rather  primitive  creatures, 
in  which  such  primate  specializations  as  grasping  feet  and  toenails  have 
begun  to  appear.  However,  most  lemurs  retain  a  rather  long  snout,  for 


Figure  24.16.  A  group  of  primates.  A,  tree  shrew;  B,  lemur;  C  and  D,  tarsier;  E, 
chimpanzee;  F,  orang-utang.  (A,  C,  D,  E,  F,  Courtesy  of  the  American  Museum  of  Natu- 
ral History;  B,  courtesy  of  the  San  Diego  Zoo.) 

495 


496 


VERTEBRATE   LIFE   AND   ORGANIZATION 


the  nasal  region  has  not  been  greatly  reduced.  The  suborder  Tarsioidea 
in<  hides  a  single  living  genus,  Tarsius,  oi  the  East  Indies  and  Philip- 
pines. Tarsius  is  a  rai-sized  animal  with  large  eyes  suited  lor  nocturnal 
vision,  antl  elongated  tarsals  and  digital  pads  to  aid  in  hopping  thiough 
the  tree  tops.  It,  and  the  known  iossil  tarsioids,  are  too  specialized  to 
be  tlie  ancestors  oi  other  primates,  but  its  flattened  face  and  forward 
turned  eyes  are  the  sort  of  advances  over  lemurs  that  we  would  expect 
to  find  in  the  ancestors  of  the  highest  primate  suborder,  the  Anthro- 
poidea.  Anilnopoids  include  the  monkeys,  great  apes  and  man.  All  have 
a  relatively  flat  face,  stereoscopic  vision,  the  capacity  to  sit  on  their 
haunches  and  examine  objects  with  their  hands,  and  an  unusually  large 


Figure  24.17.  Representative  carnivores  and  cetaceans.  A,  Raccoon;  B,  walrus;  C, 
the  birth  of  a  porpo.sc;  D.  the  whalebone  phites  of  a  toothless  whale  han^  down  from 
the  roof  of  the  mo.uh,  K,  weasels  in  summer  pelage.  The  porpoise  ami  whale  are 
of  xrZ'i  u'  "'^'''^'^  carnivores.  (A,  B,  D,  E,  courtesy  of  the  American  Museum 
ot  .Natural  History;  C,  courtesy  of  Marine  Studios.) 


A    HISTORY   OF   VERTEBRATES:   BIRDS   AND   MAMMALS  497 

brain.  Primates  will  be  considered  more  fully  in  connection  with  the 
evolution  of  man  (Chapter  36). 

Carnivores  and  Whales.  As  mammals  increased  in  number  and 
diversity,  the  opportunity  arose  for  them  to  feed  upon  one  another. 
Certain  ones  became  specialized  for  a  carnivorous  mode  of  life.  The 
living  members  of  the  order  Carnivora  are  the  weasels,  dogs,  raccoons, 
bears  and  cats  (Fig.  24.17).  The  shift  from  an  insectivorous  to  a  flesh- 
eating  diet  was  not  difficult.  An  improvement  in  the  stabbing  and 
shearing  action  of  the  teeth,  and  the  evolution  of  a  foot  structure  that 
enabled  them  to  run  fast  enough  to  catch  their  prey,  was  about  all  that 
was  necessary.  Speed  has  been  increased  in  most  by  the  development  of 
a  longer  foot,  and  by  standing  upon  their  toes  (though  not  their  toe  tips) 
with  the  rest  of  the  foot  raised  off  the  ground  in  the  manner  of  a 
sprinter.  This  digitigrade  foot  posture  (Fig.  24.18)  is  in  contrast  to  the 
primitive  plantigrade  posture,  in  which  the  entire  foot  is  placed  squarely 
upon  the  ground  or  tree  branch. 

Most  carnivores  are  semiarboreal  or  terrestrial,  but  one  branch  of 
the  order,  which  includes  the  seals,  sea  lions  and  walruses,  early  spe- 
cialized for  exploiting  the  resources  of  the  sea.  In  addition  to  their 
adaptations  as  carnivores,  which  include  the  large  canine  tusks  of  the 
walrus  used  in  gathering  shell  fish,  these  species  evolved  flippers  and 
other  aquatic  modifications.  \Vhen  they  swim,  the  large  pehic  flippers 
are  turned  posteriorly  and  are  mo\ed  from  side  to  side  like  the  tail  of 
a  fish. 

Whales,  dolphins  and  porpoises,  of  the  order  Cetacea,  are  more 
highly  specialized  marine  mammals  that  also  may  have  evolved  from 
primitive,  terrestrial  carnivores.  They  have  a  fish-shaped  body,  pectoral 
flippers  for  steering  and  balancing,  no  pelvic  flippers,  and  horizontal 
flukes  on  a  powerful  tail  that  is  moved  up  and  down  to  propel  the 
animal  through  the  water.  Some  species  have  even  reevolved  a  dorsal  fin. 


Figure  24.18.  Lateral  and  anterior  views  of  the  skeleton  of  the  left  hind  foot  of 
representative  mammals.  A,  The  primitive  plantigrade  foot  of  a  lemur:  B,  the  digiti- 
grade foot  of  a  cat;  C  and  D,  the  unguligrade  foot  of  a  pig,  an  even-toed  ungulate;  E, 
the  unguligrade  foot  of  a  horse,  an  odd-toed  ungulate.  The  digits  are  indicated  by 
Roman  numerals,  the  metatarsals  are  black  and  the  tarsals  are  stippled. 


498  VERTEBRATE   LIFE  AND   ORGANIZATION 

Desjjite  these  fish-like  attributes,  cetaceans  are  air-breathing,  viviparous 
and  suckle  their  young  (Fig.  24.17). 

Most  cetaceans  have  a  good  complement  of  conical  teeth  well  suited 
for  feetling  upon  fish,  but  the  largest  whales  have  lost  their  teeth  and 
feed  upon  plankton.  With  fringed,  horny  plates  (the  whalebone)  that 
hang  down  from  the  palate,  a  toothless  whale  strains  these  minute  organ- 
isms from  water  passing  through  its  mouth.  The  richness  of  the  plankton 
together  with  the  buoyancy  of  the  water  has  enabled  these  whales  to 
attain  enormous  size.  The  blue  whale,  which  reaches  a  length  of  100 
feet  and  a  weight  of  150  tons,  is  the  largest  animal  that  has  ever  existed. 

Ungulates.  Horses,  cows  and  similar  mammals  have  become  spe- 
cialized lor  a  plant  diet.  This  has  entailed  a  considerable  change  in  their 
dentition,  for  plant  food  must  be  thoroughly  ground  by  the  teeth  before 
it  can  be  acted  upon  by  the  digestive  enzymes.  The  molars  of  plant- 
eating  mammals  (and  those  of  omnivorous  species  sucli  as  man)  have 
become  square,  as  seen  in  a  surface  view.  Those  of  the  upper  and  lower 
jaws  no  longer  slide  vertically  across  each  other  to  give  some  cutting 
action,  as  do  the  triangular  molars  of  more  primitive  mammals,  but  meet 
and  crush  the  food  between  them  (Fig.  24.12).  A  simple  squaring  of  the 
molars,  and  to  some  extent  of  the  premolars,  is  sufficient  for  herbivorous 
mammals  that  browse  upon  soft  vegetation.  But  those  that  feed  upon 
grass  and  other  hard  and  gritty  fare,  as  do  the  grazing  species,  are  con- 
fronted with  the  additional  problem  of  the  wearing  away  of  the  teeth. 
Two  adaptations  have  occurred:  the  height  of  the  cusps  of  the  teeth  has 
increased,  and  cement  (a  hard  material  previously  found  only  on  the 
roots  of  the  teeth)  has  grown  up  over  the  surface  of  the  tooth  and  into 
the  "valleys"  between  the  elongated  cusps.  More  tooth  is  provided  to 
wear  away,  and  the  tooth  is  more  resistant  to  wear.  Teeth  of  this  type 
are  referred  to  as  high-crowned  in  contrast  to  the  more  primitive  low- 
crowned  type. 

Herbivores  constitute  the  primary  food  supply  of  carnivores,  and 
protect  themselves  primarily  by  the  simple  expedient  of  running  away. 
Speed  has  been  increased  by  the  evolution  of  an  unguligrade  foot  pos- 
ture, i.e.,  lengthening  the  foot  and  standing  on  the  toe  tips  (Fig.  24.18). 
Those  toes  that  no  longer  reach  the  ground  became  vestigial,  or  disap- 
peared, and  the  primitive  claw  on  the  remaining  ones  was  transformed 
into  a  hoof— a  characteristic  that  gives  the  name  ungulate  to  these 
mammals. 

The  numerous  and  varied  contemporary  ungulates  are  grouped  into 
two  orders  that  can  be  separated  on  the  basis  of  the  type  of  toe  reduc- 
tion. In  the  order  Perissodactyla,  the  axis  of  the  foot  passes  through  the 
third  toe,  and  this  is  always  the  largest.  Ancestral  perissodactyls,  includ- 
ing the  primitive  forest-dwelling  horses  of  the  early  Tertiary,  had  three 
well  developed  toes  (the  second,  third  and  fourth)  and  sometimes  a  trace 
of  a  fourth  toe  (the  fifth).  The  tapir  and  rhinoceros,  which  still  walk 
upon  soft  ground,  retain  the  middle  three  toes  as  functional  toes,  but 
only  the  third  is  left  in  modern,  plains-dwelling  horses.  Perissodactyls 
are  characterized  by  having  an  odd  number  of  toes. 

In  the  order  Artiodactyla,  the  axis  of  the  foot  passes  between  the 
third  and  fourth  toes,  which  are  equal  in  size  and  importance.  Ancestral 


A   HISTORY  OF  VERTEBRATES:   BIRDS   AND  MAMMALS 


499 


artiodactyls  had  four  toes  (the  second,  third,  fourth  and  fifth).  Pigs  and 
their  alUes,  which  live  in  a  soft  ground  habitat,  retain  tliese  four  toes, 
though  the  second  and  fifth  are  reduced  in  size.  Vestiges  of  the  second 
and  fifth  toes,  the  dew  claws,  are  present  in  some  deer,  but  camels, 
giraffes,  antelope,  sheep  and  cattle  retain  only  the  third  and  fourth 
toes.  Artiodactyls,  then,  are  even-toed  ungulates.  It  is  probable  that  these 
two  orders  have  had  a  separate  evolutionary  origin,  and  owe  their  points 
of  similarity  to  parallel  evolution. 

Subungufafes.  Subungulates  are  a  group  of  plant-eating  mammals 
that  have  certain  incipient  ungulate  tendencies.  Elephants  (order  Pro- 
boscidea,  Fig.  24.19  A),  for  example,  have  five  toes,  each  ending  in  a 
hooflike  nail.  They  also  walk  to  some  extent  upon  their  toe  tips,  but 
a  pad  of  elastic  tissue  posterior  to  the  digits  supports  most  of  the  body 
weight.  Elephants  are  noted  for  their  enormous  size,  which  must  ap- 
proach the  maximum  for  a  completely  terrestrial  animal.  Though  large 
mammals  have  a  relatively  lower  metabolic  rate  than  small  mammals, 
the  huge  mass  of  elephants  necessitates  their  obtaining  large  quantities 
of  food.  The  trunk,  which  rei^resents  the  drawn  out  upper  lip  and  nose, 
is  an  effective  food-gathering  organ.  Elephants  have  a  unique  dentition 
in  which  all  of  the  front  teeth  are  lost  except  for  one  pair  of  incisors, 
which  are  modified  as  tusks.  Their  premolars,  which  have  come  to  re- 
semble molars,  and  their  molars  are  very  effective  organs  for  grinding  up 
large  quantities  of  rather  coarse  plant  food.  They  are  high-crowned  and 
so  large  that  there  is  room  for  only  one  in  each  side  of  the  upper 
and  lower  jaws  at  a  time.  When  it  is  worn  down,  a  new  one  replaces  it. 
Mammals,  unlike  reptiles  and  other  lower  vertebrates  in  which  there  is  a 
continuous  replacement  of  worn-out  teeth,  have  a  limited  replacement 
of  teeth.  Deciduous  incisors,  canines  and  premolars  are  present  in  young 
individuals  and  these  are  replaced  later  in  life  by  permanent  ones.  The 
molars,  which  do  not  develop  until  after  infancy,  are  not  replaced. 
Elephants,  by  using  up  their  premolars  and  molars  one  at  a  time,  have 
evolved  an  interesting  way  of  prolonging  total  tooth  life. 

Living  elephants  are  restricted  to  Africa  and  tropical  Asia,  and  are 
only   a  small  remnant  of   a  once  world-wide  and  varied  proboscidean 


Figure  24.19.  Subungulates.  The  elephant,  A,  and  the  manatee,  B,  are  believed 
to  have  had  a  common  ancestry.  (Courtesy  of  the  American  Museum  of  Natural  His- 
tory.) 


500  VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  24.20.  Rodcnts  and  lagomorphs.  A,  A  flying  squirrel;  B,  the  pika;  C,  a  chip- 
munk shelling  a  nut;  D,  a  group  of  beavers  (Courtesy  of  American  Museum  of  Natural 
History.) 

population.  During  the  Pleistocene,  or  Ice  Age,  mastodons,  mammoths 
and  other  proboscideans  were  abundant  in  North  America. 

The  conies  of  the  Middle  East  (order  Hyracoidea),  though  super- 
ficially rabbit-like  animals,  show  an  affinity  to  the  elephants  in  their  foot 
structure,  and  in  certain  features  of  their  dentition. 

A  final  group  of  contemporary  subungulates  are  the  sea  cows  or 
manatees  (order  Sirenia).  These  animals  live  in  warm  coastal  waters  and 
feed  upon  seaweed,  grinding  it  up  with  molars  that  are  replaced  from 
behind  in  elephant-like  fashion.  Sea  cows  have  a  powerful,  horizontally 
flattened  tail,  and  well  developed  pectoral  flippers.  These  features,  to- 
gether with  a  very  mobile  and  expressive  snout  and  a  single  pair  of 
pectoral  mammary  glands,  led  mariners  of  long  ago  to  regard  them  as 
mermaids. 

Rodents  and  Lagomorphs.  Other  herbivorous  mammals  gnaw,  and, 
in  addition  to  high-crowned,  grinding  molars,  have  an  upper  and  lower 
pair  of  enlarged,  chisel-like  incisor  teeth  that  grow  out  from  the  base 
as  fast  as  they  wear  away  at  the  tip.  This  has  been  a  very  successful 
mode  of  life;  in  fact,  there  are  more  species,  and  possibly  more  individ- 
uals, of  gnawing  mammals,  or  rodents  (order  Rodentia),  than  of  all  other 
mammals  combined.  Rodents  have  undergone  their  own  adaptive  radia- 
tion and  have  evolved  specializations  for  a  variety  of  ecologic  niches. 
Rats,  mice  and  chipmunks  live  on  the  ground,  gophers  and  woodchucks 
burrow,  squirrels  and  porcupines  are  adept  at  climbing  trees,  and  musk- 
rats  and  beavers  are  semiaquatic  (Fig.  24.20). 

Rabbits  and  the  related  pika  of  our  Western  mountains  are  super- 
ficially similar  to  rodents,  and  were  at  one  time  placed  in  this  order. 
True  rodents,   however,  have  only   one   pair  of   incisors   in  each  jaw, 


A    HISTORY   OF   VERTEBRATES:   BIRDS    AND   MAMMALS  501 

whereas  rabbits  have  a  reduced  second  pair  hidden  behind  the  large  pair 
of  upper  incisors.  It  is  now  believed  that  rabbits  and  the  pika  belong 
to  a  separate  order,  the  Lagomorpha,  and  that  their  resemblance  to 
rodents  is  a  result  of  parallel  evolution. 

Questions 

1.  Contrast  homoiothermic  and  poikilothermic  vertebrates. 

2.  How  do  wings  support  and  propel  a  bird? 

3.  Describe  a  typical  feather.  In  what  ways  is  it  adapted  for  flight? 

4.  Compare  the  structure  of  the  wings  of  a  pterosaur,  bird  and  bat. 

5.  In  what  ways  are  the  internal  organs  of  birds  adapted  for  flight? 

6.  How  does  Archaeopteryx  differ  from  modern  birds? 

7.  List  some  modifications  of  birds'  bills  and  feet.  How  are  these  correlated  with  meth- 
ods of  feeding  and  locomotion? 

8.  What  are  the  advantages  to  birds  of  nesting  territories? 

9.  What  factors  may  have  been  involved  in  the  evolution  of  the  migratory  habit? 

10.  Distinguish  between  matnmals  and  reptiles. 

11.  What  are  the  major  anatomic  features  of  mammals  that  are  correlated  with  their 
increased  activity? 

12.  What  is  the  importance  of  a  placenta?  What  structures  form  it? 

13.  List  three  ways  in  which  monotremes  are  more  primitive  than  other  mammals. 

14.  Why  are  marsupials  particularly  abundant  in  Australia?  Give  an  example  of  a  North 
American  marsupial. 

15.  What  is  the  most  primitive  group  of  eutherian  mammals? 

16.  W'hat  features  of  man  are  a  direct  or  indirect  result  of  the  arboreal  adaptations  of 
man's  primate  ancestors? 

17.  Distinguish  between  plantigrade,  digitigrade  and  unguligrade  foot  postures.  Give  an 
example  of  a  mammal  with  each  type. 

18.  How  have  the  molar  teeth  of  ungulates  been  adapted  for  the  animals'  herbivorous 
diet? 

19.  How  do  perissodactyls  differ  from  artiodactyls? 

Supplementary  Reading 

Your  attention  is  again  called  to  the  general  references  on  vertebrates  cited  at  the 
end  of  Chapter  22.  Those  interested  in  the  adaptation  and  habits  of  birds  are  referred 
to  Allen,  Birds  and  Their  Attributes,  Thomson,  Biology  of  Birds,  and  to  a  series  of 
fascinating  articles  written  by  Deevey,  Griffin,  Lack,  Storer  and  Welty  on  various  aspects 
of  avian  biology  and  reprinted  in  Flanagan's  Twentieth-century  Bestiary.  Further  in- 
formation on  bird  flight  and  superb  illustrations  can  be  found  in  Storer,  The  Flight  of 
Birds.  The  possible  methods  by  which  birds  find  their  way  in  homing  and  on  long  dis- 
tance migrations  are  explored  and  carefully  analyzed  by  Matthews  in  Bird  Navigation. 
Those  interested  in  learning  to  recognize  the  various  kinds  of  birds  should  try  using 
Peterson's  admirable  Field  Guides  to  Birds. 

The  habits  and  natural  history  of  mammals  are  considered  in  Hamilton,  American 
Mammals;  and  Bourliere,  The  Natural  History  of  Mammals:  those  of  the  opossum,  to- 
gether with  the  fascinating  folklore  of  this  unusual  creature,  in  Hartman,  'Possums. 
Howell's  Aquatic  Mammals  deals  with  the  interesting  adaptations  of  whales  and  other 
mammals  that  have  reverted  to  an  aquatic  mode  of  life.  The  primitive  horses  and  camels, 
the  giant  mastodons  and  mammoths,  and  other  fascinating  mammals  that  roamed  our 
continent  in  ages  past  are  described  in  Scott,  A  History  of  the  Land  Mammals  of  the 
Western  Hemisphere.  Burt  and  Grossenheider,  A  Field  Guide  to  the  Mammals,  and 
Hamilton,  Mammals  of  Eastern  United  States,  are  useful  guides  for  identifying  the  vari- 
ous kinds  of  mammals. 


CHAPTER  25 


Protection,  Support  and  Movement 


The  preceding  chapters  traced  the  main  currents  of  vertebrate  evolution 
and  discussed  the  major  changes  made  by  the  various  groups  of  verte- 
brates as  they  became  adapted  to  the  changing  environment.  With  this 
as  a  background,  the  succeeding  chapters  will  present  the  morphologic 
and  physiologic  aspects  of  each  of  the  organ  systems  in  turn.  In  these 
the  major  emphasis  will  be  placed  on  the  mammalian  condition  and  on 
those  transformations  that  have  occurred  in  the  line  of  evolution  that 
leads  to  mammals. 


214.       The  Integument 

The  skin,  or  integument,  is  the  outermost  layer  of  the  body  and 
separates  the  organism  from  its  external  environment.  It  helps  to  main- 
tain a  constant  internal  environment  and  protects  the  body  against  a 
variety  of  mechanical  and  chemical  injuries.  Yet  the  skin  does  not  com- 
pletely isolate  the  organism  from  its  environment,  for  many  sensory 
stimuli  are  received  by  the  skin  and  some  exchange  of  gases,  water  and 
excretory  products  may  occur  through  it.  In  addition,  a  variety  of  bony 
plates,  scales,  feathers,  hair,  pigment  cells  and  glands  develop  from  the 
skin  and  serve  a  variety  of  purposes.  The  skin  is  truly  a  "jack-of-all 
trades." 

In  general  it  may  be  said  that  the  greater  the  difference  between  the 
internal  and  external  environments,  the  greater  is  the  importance  of  this 
organ  in  protecting  the  underlying  tissues,  and  the  more  elaborate  is  its 
structure.  Lower  chordates,  for  example,  whose  internal  environment  is 
very  similar  to  the  sea  water  in  which  they  are  living,  have  a  very 
delicate  skin  consisting  of  a  single  layer  of  columnar  epithelium  sup- 
ported by  a  few  connective  tissue  fibers.  In  all  vertebrates,  the  skin  is 
more  highly  developed  and  is  made  up  of  an  outer  stratified  epithelium 
(the  epidermis)  and  a  deeper,  rather  thick  layer  of  dense  connective 
tissue  (the  dermis). 

The  epidermis  of  fishes  and  amphibians  contains  relatively  little 
horny  material,  but  a  large  amount  of  horny  keratin  is  deposited  in  the 
outer  cells  of  the  epidermis  of  the  higher  terrestrial  vertebrates.  These 
flattened,  cornified  cells  are  dead,  and  in  mammals  form  a  thick,  water- 
proofing stratum  corneum  that  is  clearly  demarcated  from  the  deeper, 
proliferating  layers  of  the  epidermis  known  as  the  stratum  germinativum 

502 


PROTECTION,   SUPPORT   AND   MOVEMENT 


503 


(Fig.  25.1).  Intermediate  layers  can  also  be  recognized  where  the  epi- 
dermis is  especially  thick,  as  on  the  palm  of  the  hand  and  the  sole  of 
the  foot.  As  new  cells  are  produced  and  differentiate,  the  outer  cells 
of  the  stratum  corneum  are  lost.  Groups  of  such  cells  are  continually 
being  shed  in  mammals;  dandruff  is  a  familiar  example. 

The  dermis  is  composed  of  fibrous  connective  tissue;  bone  may 
develop  in  it  in  certain  regions.  The  dermis  is  richly  supplied  with  blood 
vessels,  some  of  which  lie  close  to  the  surface  and  enter  papilla-like  pro- 
jections of  the  dermis  that  extend  into  the  base  of  the  epidermis.  In 
addition  to  their  nutritive  function,  these  vessels  in  mammals  play  an 
important  role  in  thermoregulation.  Xerves  and  microscopic  sense  organs 
that  receive  stimuli  of  touch,  pressure  and  temperature  are  abundant 
in  the  dermis,  but  only  a  few  naked  nerve  endings,  which  are  believed  to 
initiate  pain  impulses,  penetrate  the  epidermis.  Fat  may  accumulate  in 
the  deeper  parts  of  the  dermis  and  in  the  subcutaneous  tissue.  The  fat 
serves  as  a  reserve  supply  of  food,  as  a  thermal  insulator,  and  as  a  cushion 
against  mechanical  injury.  The  blubber  of  Avhales  serves  as  a  good  insula- 
tion in  the  aquatic  environment.  Hair  is  not  an  efficient  insulator  in 


Stratum 

corneum 

Epidermis 
Stratum 

oermmativum 
o J 


Sense  or^an 


Bloodvessel 


ermis 


Hair  follicle. 


Dermal  papilla. 


Figure  25.1.     Diagrammatic  section  through  the  skin  of  a  mammal. 


504  VERTEBRATE   LIFE  AND   ORGANIZATION 


Epiclermis 
Dermis 


Epidermis 
Dermis 


B 


-"Horny  scale. 


Epid-ermis 
Dermis 


"Bony  plate       Q 


Figure  25.2.  Vertical  sections  through  the  skin  of  vertebrates  to  show  the  rela- 
tionship of  the  various  types  of  scales.  A,  Bony  scales  of  a  fish;  B,  horny  scales  of  a 
snake;  C,  horny  scales  and  bony  plates  as  in  the  skin  of  certain  lizards,  crocodiles  and 
turtles. 

aquatic  animals,  for  its  thermal  insulation  depends  on  air  trapped  within 
it,  and  it  has  been  lost  on  most  of  the  body  surface  of  adult  whales. 

Though  the  skin  itself  is  relatively  simple,  its  derivatives  are  nu- 
merous and  complex.  These  may  be  grouped  into  bony  structures,  horny 
structures,  glands  and  pigment.  The  bony  structures  develop  within  the 
dermis,  though  parts  of  them  may  become  exposed  if  the  overlying 
epidermis  wears  off.  Thick  bony  scales  and  plates  were  prominent  in 
ancestral  vertebrates,  and  have  been  retained  in  reduced  form  in  most 
groups  of  living  fishes  (Fig.  25.2  A).  Certain  of  the  dermal  plates  in  the 
head  region  early  in  evolution  became  associated  with  the  skull  and 
pectoral  girdle,  and  these  have  been  retained  by  later  vertebrates  as 
integral  parts  of  the  skeleton.  Most  of  the  primitive  bony  scales  have 
been  lost  in  tetrapods,  but  the  dermis  retains  the  ability  to  form  bone 
and  becomes  heavily  ossified  in  certain  species.  The  shell  of  a  turtle  is 
composed  of  dermal  plates  covered  by  large  horny  laminae;  a  compar- 
able condition  is  found  in  the  skin  of  certain  lizards  and  crocodiles  and 
in  the  shell  of  the  armadillo.  The  antlers  of  deer  (Fig.  25.3)  are  also 
composed  of  derm.al  bone.  During  its  development,  the  antler  is  covered 
by  skin,  the  velvet,  but  this  sloughs  off  when  the  antler  is  fully  formed. 
Antlers  branch,  are  shed  annually  and,  with  the  exception  of  the  rein- 
deer and  caribou,  are  found  only  on  males.  The  horns  of  sheep  and 


PROTECTION,   SUPPORT  AND   MOVEMENT  505 


Velvet 
slou6hinb  off 


Fronted  bone 
Antler  of  deer 


Core  of  bone 


Horn,  of  co^^ 


Male  and.  female 


Figure  25.3.  A  diagram  to  show  the  differences  between  antlers  (deer)  and  horns 
(cow).  Antlers  are  annual  growths  that  are  shed  in  the  winter;  horns  are  permanent 
outgrowths. 


cattle,  in  contrast,  do  not  branch,  are  not  shed  and  occur  in  both  sexes. 
These  horns  have  a  core  of  bone  covered  by  a  highly  cornified  skin. 

Horny  skin  derivatives  develop  by  the  accumulation  of  keratin  in 
the  cells  of  the  epidermis.  Reptiles  have  a  covering  of  horny  scales  that 
reduce  water  loss  through  the  skin.  As  the  animal  increases  in  size,  the 
horny  scales  are  periodically  shed  and  newly  formed  ones  are  exposed 
beneath  them.  Bony  scales,  in  contrast,  are  not  shed  but  increase  in  size 
by  the  addition  of  new  bone.  Except  for  their  retention  in  such  regions 
as  the  feet  of  birds  and  the  tails  of  certain  mammals,  horny  scales  are 
not  present  in  most  birds  and  mammals,  though  a  prominent  stratum 
corneiun  persists. 

Feathers  are  believed  to  be  modified  horny  scales,  but  the  hairs  of 
mammals  are  regarded  as  a  different  kind  of  horny  skin  derivative.  A 
hair  lies  within  a  hair  follicle  (Fig.  25.1),  which  is  composed  of  a  tubular 
invagination  of  the  epidermis  supported  by  surrounding  fibers  of  the 
dermis.  A  hair  papilla,  containing  blood  vessels  and  nerves,  protrudes 
into  the  base  of  the  follicle  and  nourishes  the  adjacent  epithelial  cells. 
These  proliferate  rapidly  and  add  to  the  base  of  the  hair,  which  extends 
up  through  the  follicle  as  a  column  of  keratinized  cells.  A  small  smooth 
muscle,  the  arrector  pili,  is  associated  with  each  follicle.  These  muscles 
contract  when  temperatures  fall  and  pull  the  hair  follicles  and  hairs 
into  a  more  erect  position,  thereby  increasing  the  depth  of  the  hair  layer 


506  VERTEBRATE   LIFE   AND   ORGANIZATION 

and  its  effectiveness  in  insulation.  They  also  depress  the  skin  between 
the  hairs,  leaving  little  hillocks  where  the  hairs  emerge.  We  are  familiar 
with  this  as  "goose  flesh."  There  have  been  many  modifications  of  hair, 
e.g.,  the  tactile  whiskers  of  a  cat  and  the  quills  of  a  porcupine.  Even 
the  "horn"  of  a  rhinoceros,  which  lacks  a  core  of  bone,  appears  to  be  a 
clinnp  of  specialized  hairs. 

Other  horny  derivatives  of  the  integument  include  claws,  which 
first  appear  in  reptiles  and  may  be  modified  as  nails  or  hoofs  in  certain 
mammals,  the  whalebone  plates  of  toothless  whales,  and  the  covering 
of  the  horns  of  sheep  and  cattle. 

Individual  mucus-secreting  cells  are  common  in  the  epidermis  of 
fishes,  and  multicellular  mucous  glands  are  abundant  in  amphibian 
skin.  Fishes  and  amphibians  also  have  a  few  cutaneous  poison  glands. 
Reptiles  have  lost  the  mucous  and  poison  glands,  and  only  a  few  glands, 
chiefly  scent  glands,  are  present  in  their  dry,  horny  skin.  This  paucity 
persists  in  birds,  but  glands  have  again  become  abundant  in  mammalian 
skin.  Alveolar-shaped  sebaceous  glands,  epithelial  outgrowths  from  the 
hair  follicles  (Fig.  25.1),  discharge  their  oily  secretions  onto  the  hairs. 
Coiled,  tubular  sweat  glands  are  also  abundant  in  many  areas  of  mam- 
malian skin.  A  little  urea  and  some  salts  are  eliminated  in  the  sweat, 
but  sweat  glands  are  particularly  important  in  secreting  water  whose 
evaporation  cools  the  body  surface.  The  vascular  supply  to  the  skin,  the 
hairs  and  their  muscles,  and  the  sweat  glands  all  play  a  role  in  regulating 
body  temperature.  Though  the  nature  and  function  of  their  secretion 
is  entirely  different,  mammary  glands  are  regarded  as  modified  sweat 
glands.  Musk  and  other  scent  glands,  serving  for  sexual  recognition,  are 
also  common  in  many  mammals,  although  they  do  not  occur  in  man. 

In  lower  vertebrates,  e.g.,  in  the  frog,  pigments  are  contained  within 
chromatophores  located  beneath  the  epidermis,  and  skin  color  can 
change  by  the  concentration  or  dispersion  of  pigment  within  these 
stellate  cells.  Chromatophores  are  rare  in  mammals,  but  the  brownish 
pigment  melanin  is  present  within  and  between  the  cells  of  the  epi- 
dermis. Some  melanin  is  present  in  the  skin  of  all  men  (except  albinos, 
p.  683)  but  it  is  especially  abundant  in  the  skin  of  Negroes.  Skin  color  is 
determined  not  only  by  the  pigment  present  but  by  the  vascularity  of 
the  dermis  and  by  the  presence  of  refractive  substances  such  as  guanine. 

215.        The  Skeleton 

Nature  and  Parts  of  the  Vertebrate  Skeleton.  Organisms  must  re- 
main small  and  slow  moving  unless  they  have  a  skeleton  for  support 
and  to  serve  as  levers  on  which  muscles  can  act.  All  vertebrates  have  a 
skeleton  that  provides  for  this,  and  encloses  and  protects  some  of  the 
more  delicate  internal  organs.  The  central  cavities  of  the  bones  of 
higher  vertebrates,  which  contain  red  bone  marrow,  are  the  sites  of  the 
formation  of  red  blood  cells  and  certain  of  the  white  cells.  The  verte- 
brate skeleton  is  basically  an  internal  skeleton,  for  it  develops  within  the 
skin  or  in  deeper  body  tissues.  None  of  it  is  a  secretion  on  the  body  sur- 
face, as  is  the  exoskeleton  of  certain  invertebrates,  although  such  struc- 


PROTECT/ON,   SUPPORT  AND   MOVEMENT 


507 


tures  as  horny  scales,  feathers  and  hair  are  sometimes  classified  as  an 
exoskeleton. 

The  skeleton  is  subdivided  into  a  dermal  skeleton  consisting  of  the 
bony  scales  and  plates  mentioned  earlier  in  this  chapter,  and  an  endo- 
skeleton  situated  beneath  the  skin.  During  early  embryonic  development 
the  endoskeleton  is  composed  of  the  notochord  and  cartilage,  but  the 
notochord  is  ephemeral  in  most  vertebrates  and  cartilage  is  replaced  by 
bone  in  most  adults.  This  bone  is  called  cartilage  replacement  bone  to 
distinguish  it  from  the  dermal  bone  that  develops  in  more  superficial 
parts  of  the  body  without  any  cartilaginous  precursor.  These  types  of 
bone  differ  only  in  their  mode  of  development;  they  are  the  same  his- 
tologically. 

The  endoskeleton  and  its  associated  dermal  bones  can  be  fiuther 
subdivided  into  somatic  and  visceral  skeletons: 

Somatic  skeleton  (skeleton  of  the  body  wall) 

Axial  skeleton  (vertebral  column,  ribs,  sternum  and  most  of  the  skull) 

Appendicular  skeleton  (girdles  and  limb  bones) 
Visceral  skeleton  (skeleton  of  the  pharyngeal  wall,  primitively  associated  with  the 
gills) 

The  Fish  Skeleton.  The  parts  of  the  skeleton  can  be  seen  more 
clearly  in  a  dogfish  (Fig.  25.4)  than  in  terrestrial  vertebrates.  The  dogfish 
skeleton  is  typical  of  the  skeleton  of  primitive  vertebrates,  except  that 
the  skeleton  is  entirely  cartilaginous.  It  will  be  recalled  that  the  failure 
of  the  dogfish's  skeleton  to  ossify  is  believed  to  represent  the  retention  of 
an  embryonic  condition  rather  than  a  primitive  adult  condition.  The 


Centrum-i 
Neureil  arch— i 


Anterior  dorsal  f  in- 

Trunh  verl^bra. 
Spinc- 


Hyomamdlbular- 

Otic  Capsule 1 

Chondrocranium 


Naisal  capsule 
PalaVoquadrate 
(Lower  jaw) 


Pdlvic  girdle 


Caudal  vertebra 


-Pelvic  fin 
Figure  25.4,     A  lateral  view  of  the  skeleton  of  a  dogfish, 


508  VERTEBRATE   LIFE   AND   ORGANIZATION 

vertebral  column  is  composed  ol  vertebrae,  each  of  which  has  a  bicon- 
cave centrum,  which  develops  around  and  hirgely  rephices  the  notochord. 
Dorsal  to  each  centrum  is  a  neural  arch  encasing  the  spinal  cord.  Short 
ribs  attach  to  the  vertebrae.  A  sternum  is  absent.  The  individual  verte- 
brae are  rather  loosely  held  together.  A  strong  vertebral  support  is  not 
necessary  in  the  aquatic  environment. 

Most  ol  the  skull  oi  the  dogfish  is  an  odd-shaped  box  of  cartilage 
encasing  the  brain  antl  major  sense  organs.  This  belongs  to  the  axial 
skeleton  and  is  known  as  the  chondrocranium.  It  forms  the  core  of  the 
skull  of  all  vertebrates.  Other  basic  components  of  a  vertebrate  skull 
include  the  anterior  arches  of  the  visceral  skeleton  and  dermal  bones 
that  encase  the  chondrocranium  and  anterior  visceral  arches.  These 
dermal  bones  have  been  lost  during  the  evolution  of  cartilaginous  fishes, 
but  they  were  present  in  the  fishes  ancestral  to  tetrapods. 

The  visceral  skeleton  consists  of  seven  pairs  of  > -shaped  visceral 
arches.  The  arches  are  hinged  at  the  apex  of  the  >;  they  are  intercon- 
nected ventrally,  but  are  free  dorsally.  Each  arch  lies  in  the  wall  of  the 
pharynx  and  supports  gills  in  primitive  vertebrates.  In  jawed  vertebrates 
the  first  or  mandibular  arch  becomes  enlargeci  and,  together  with  asso- 
ciated dermal  bones,  forms  the  upper  and  lower  jaws.  It  forms  all  of 
the  jaws  in  the  dogfish,  for  there  are  no  surrounding  dermal  bones.  The 
second  or  hyoid  arch  has  moved  forward  in  the  dogfish  and  helps  to 
support  the  jaws.  Its  dorsal  portion  extends  as  a  prop  from  the  otic 
capsule  (the  part  of  the  chondrocranium  housing  the  inner  ear)  to  the 
angle  of  the  jaw.  The  gill  slit  that  in  primitive  fish  lay  between  the  man- 
dibular and  hyoid  arches  is  reduced  to  a  spiracle.  The  third  to  seventh 
visceral  arches  are  known  as  branchial  arches;  they  support  the  gills  and 
complete  gill  slits  lie  between  them. 

The  appendicular  skeleton  is  very  simple  in  the  dogfish.  A 
U-shaped  bar  of  cartilage,  the  pectoral  girdle,  lies  in  the  body  wall 
posterior  to  the  gill  region  and  supports  the  pectoral  fins.  The  pelvic 
girdle  is  a  transverse  bar  of  cartilage  in  the  ventral  body  wall  anterior 
to  the  cloaca.  It  supports  the  pelvic  fins  but  is  not  connected  with  the 
vertebral  column. 

The  Mammalian  Skeleton.  Many  changes  in  the  skeleton  have 
taken  place  during  evolution  of  the  skeleton  from  primitive  fishes  to 
mammals  (Fig.  25.5).  The  vertebral  column  must  support  the  weight 
of  the  body  in  all  tetrapods  and  it  has  become  much  stronger.  It  is 
thoroughly  ossified,  and  the  individual  vertebrae  are  strongly  united 
by  overlapping  articular  processes  (zygapophyses)  borne  on  the  neural 
arches.  Correlated  with  changes  in  the  methods  of  locomotion  and 
the  independent  movement  of  various  parts  of  the  body,  we  find 
that  there  is  more  regional  differentiation  of  the  vertebral  column. 
Man  has  seven  cervical  vertebrae,  twelve  thoracic  vertebrae,  five  lumbar 
vertebrae,  five  sacral  vertebrae  fused  together  to  form  a  sacrum  that 
articulates  with  the  pelvic  girdle,  and  three  to  five  reduced  caudal 
vertebrae  generally  fused  into  a  single  piece,  the  coccyx.  Only  the 
thoracic  vertebrae  bear  distinct  ribs,  most  of  which  connect,  via  the 
costal  cartilages,  with   the  ventral   breast   bone,  or  sternum.   Rudimen- 


PROTECTION,   SUPPORT   AND   MOVEMENT 


509 


tary  ribs,  which  are  present  in  the  other  regions  during  embryonic 
development,  fuse  onto  the  transverse  processes.  The  first  two  cer- 
vical vertebrae  are  modified  to  permit  a  free  movement  of  the  head. 
The  first,  known  as  the  atlas,  has  a  pair  of  facets  for  articulating  with 
the  pair  of  occipital  condyles,  the  rounded  bumps  on  the  base  of  the 
skull  on  each  side  of  the  foramen  magnum.  The  head  can  rock  back 
and  forth  at  this  point.  Tinning  motion  occurs  at  a  unique  joint  be- 
tween the  atlas  and  the  second  cervical  vertebra,  the  axis.  All  tetra- 
pods  have  an  atlas,  but  an  axis  does  not  appear   in  the  evolutionary 


Zygomatic 


Cla.vicle 

Cnra-r-oid.      /.(^ 
process      M7^\ 

Scapula 
Humerus     t 


OrBit 
Exte.rna.1  narcs 

Cervical 

vertebra. 


Sternura 

•Thora.cic 
vertebra. 

Lumbar 
vertebra. 

Ilium. 

Sa.crum 


S^^J^'.S     W\    Coccy3c 


Cajrpals 

Met  a 


Pubis 
Ischium, 


Tarsals 

■Metatarsals 
Phalanges 

Figure  25.5.     A  ventral  view  of  the  human  skeleton. 


510 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Alisphenoid 
(I) 


MeSe-thmoid 


Occipital- 


y D  e  ntary 

— ■^       (Mandible) 

'-Meckel's  carlila^^Cl) 
'HyoidCH,™;) 


Ca.rtila0e  of  larynx 
(Ei:,T,  3Zr,3ZIl) 


.Tractieal  car-'bila.gzs 


Figure  25.6.  Components  of  the  human  skull,  hyoid  and  larynx.  Dermal  bones  have 
been  left  plain,  chondrocranial  derivatives  are  hatched,  those  parts  of  the  embryonic 
visceral  skeleton  that  disappear  are  stippled,  parts  of  the  visceral  arches  that  persist 
are  shown  in  black.  Roman  numerals  refer  to  visceral  arches  and  their  derivatives. 
(Modified  after  Neal  and  Rand.) 

sequence  until  reptiles.  The  number  of  sacral  vertebrae  has  increased 
as  the  tetrapods  have  evolved  more  effective  terrestrial  locomotion. 
Typically  amphibians  have  one,  reptiles  two  and  mammals  three.  The 
greater  number  in  man  is  probably  correlated  with  the  additional  prob- 
lems oi  support  inherent  in  a  bipedal  gait. 

The  mammalian  skull  has  many  of  the  features  found  in  the  frog's 
skull.  The  expanded  portion  housing  the  brain  is  the  cranium;  the  jaws 
and  the  bones  surrounding  the  eyes  and  supporting  the  nose  constitute 
the  facial  skeleton.  The  eyes  are  lodged  in  orbits,  the  nasal  cavities 
open  on  the  surface  through  external  nares,  an  external  auditory 
meatus  leads  into  the  middle  ear  cavity,  the  spinal  cord  emerges 
through  the  foramen  magnum,  and  there  are  many  smaller  foramina 
for  blood  vessels  and  nerves.  A  temporal  fossa,  in  which  jaw  muscles 
are  lodged,  lies  posterior  to  the  orbit.  It  is  bounded  laterally  by  a 
handle-like  bar  of  bone,  the  zygomatic  arch.  A  bony  hard  palate 
separates  the  mouth  and  nasal  cavities  and  the  internal  nares  lie  at 
the  posterior  end  of  this. 

The  skull  is  a  hodgepodge  of  cartilage  replacement  and  dermal 
bones  that  can  be  understood  only  when  considered  from  an  evolution- 


PROTECT/ON,   SUPPORT   AND   MOVEMENT 


511 


ary  point  of  view.  As  the  brain  grew  larger  during  the  course  of  evolu- 
tion, the  cartilage  replacement  bones  of  the  chondrocranium  could  no 
longer  completely  encase  it.  They  form  a  ring  of  bone  around  the  fora- 
men magnum  (the  occipital  bone),  encase  the  inner  ear  (part  of  the 
temporal  bone),  and  form  the  floor  of  the  cranium.  The  sides  and  roof 
of  the  cranium  are  completed  by  dermal  bones  such  as  the  frontal  and 
parietals,  and  by  a  portion  of  the  mandibular  arch  known  as  the  alis- 
phenoid  (Fig.  25.6).  The  last  is  a  cartilage  replacement  bone. 

Although  the  mandibular  arch  is  associated  with  the  jaws  in  most 
vertebrates,  at  least  to  the  extent  of  forming  the  jaw  joint,  the  jaws  of 
mammals  are  formed  entirely  of  certain  of  the  dermal  bones  that  encased 
the  mandibular  arch  in  primitive  vertebrates.  The  mammalian  jaw  joint 
lies  between  two  of  these— the  dentary  and  squamosal  (part  of  the 
temporal).  The  posterior  end  of  the  mandibular  arch,  which  forms 
the  jaw  joint  in  more  primitive  vertebrates,  has  become  the  incus  and 
malleus— two  of  the  three  small  auditory  ossicles  that  transmit  vibra- 
tions across  the  middle  ear  cavity.  Our  ancestral  jaw  joint  is  now  part 
of  our  hearing  mechanism,  and  earlier  it  was  part  of  a  gill  arch  and 
concerned  with  respiration!  The  third  auditory  ossicle,  the  stapes, 
evolved  from  the  dorsal  part  of  the  hyoid  arch.  It  is  of  interest  to  observe 
that  the  auditory  ossicles  have  the  same  relationship  to  each  other  as 
their  homologues  in  fish.  The  ventral  part  of  the  hyoid  arch,  together 
with  the  remains  of  the  third  visceral  arch,  form  the  hyoid  bone  (a 
sling  for  the  support  of  the  tongue),  and  the  styloid  process  of  the  skull 
to  which  the  hyoid  is  connected  by  a  ligament.  With  the  loss  of  gills  in 
tetrapods,  the  remaining  visceral  arches  have  become  greatly  reduced, 
but  parts  of  them  form  the  cartilages  of  the  larynx. 


.  <8Z'-4;  -^i  :.1ff<?if.//'!..%: 


-Supcra.cl«.if;hrum 

■Sc«.puIocor«x.cid 

-Humerus 
-Radius 


Clcithrum 


ScatpulocoratcoM* 


Phalztnges 


Figure  25.7.  Lateral  views  of  the  appendicular  skeleton  of  a  crossopterygian,  A, 
and  labyrinthodont,  B,  to  show  the  changes  that  occurred  in  the  transition  from  water 
to  land.  Dermal  bones  have  been  left  plain,  cartilage  replacement  bones  are  stippled. 
(A,  Modified  after  Gregory;  B,  after  Romer.) 


512  VERTEBRATE   LIFE   AND    ORGANIZATION 

Aliliough  ilic  appenclicuhn  skeleton  oi  the  dogfish  is  quite  different 
from  that  ol  terrestrial  vertebrates,  there  is  a  close  resemblance  between 
the  appendicular  skeleton  of  crossopterygian  fishes  and  tetrapods  (Fig. 
25.7).  The  humerus  oi  our  arm,  or  the  femur  of  our  leg,  represents  the 
single  proximal  bone  of  the  crossopterygian  fin;  the  radius  and  ulna, 
or  tibia  and  fibula,  the  next  two  l)ones.  The  carpals  or  tarsals,  meta- 
tarsals or  metacarpals,  and  phalanges  of  the  hand  or  foot  are  homol- 
ogous with  the  more  peripheral  elements  of  the  crossopterygian  fin.  We 
tetrapods  have  a  single  bone  in  the  proximal  part  of  the  appendage 
followed  by  two  bones  in  the  second  part  because  this  pattern  was  es- 
tablished by  our  piscine  ancestors. 

The  girdles  of  tetrapods  are  necessarily  stronger  than  those  of  fish. 
The  pectoral  girdle  is  bound  onto  the  body  by  muscles,  but  the  pelvic 
girdle  extends  dorsally  and  is  firmly  attached  to  the  vertebral  column. 
A  pubis,  ischium  and  ilium  are  present  on  each  side  of  our  pelvic  girdle, 
though  all  have  fused  together  in  the  adult.  Our  pectoral  girdle  includes 
a  scapula,  a  coracoid  process,  which  is  a  distinct  bone  in  most  lower 
tetrapods,  and  a  clavicle.  The  clavicle  is  the  only  remnant  of  a  series  of 
dermal  bones  that  are  primitively  associated  with  the  girdle.  All  other 
girdle  bones  are  cartilage  replacement  bones. 

216.        Muscles 

The  movement  of  the  vertebrate  body  and  its  parts,  and  the  posture 
of  the  vertebrate  body,  depend  upon  the  contraction  of  muscles.  The 
nature  of  muscle  contraction  and  the  source  of  the  energy  required  have 
been  considered  earlier.  At  this  time  we  will  be  concerned  with  certain 
aspects  of  the  evolution  of  the  muscular  system,  and  the  relation  of  these 
to  changes  in  methods  of  locomotion. 

Histologically,  muscles  may  be  classified  as  smooth,  cardiac  and 
skeletal.  In  tracing  their  evolution  it  is  more  convenient  to  divide  them 
into  somatic  muscles  associated  with  the  body  wall  and  appendages, 
and  visceral  muscles  associated  with  the  pharynx  and  other  parts  of  the 
gut  tube.  This  grouping  parallels  the  major  subdivisions  of  the  skeletal 
system.  Somatic  muscles  are  striated  and  under  voluntary  control.  Most 
of  the  visceral  muscles  are  smooth  and  involuntary;  however,  the  visceral 
muscles  associated  with  the  visceral  arches,  called  branchial  muscles,  are 
striated  and  under  voluntary  control. 

Most  of  the  somatic  musculature  of  fishes  consists  of  segmented  myo- 
meres (Fig.  25.8).  This  is  an  effective  arrangement  for  bringing  about 
the  lateral  undulations  of  the  trunk  and  tail  that  are  responsible  for 
locomotion.  The  muscles  of  the  paired  fins  are  very  simple,  and  consist 
of  little  more  than  a  single  dorsal  extensor  that  pulls  the  fin  up  and 
caudally,  and  a  ventral  flexor  that  pulls  the  fin  down  and  anteriorly. 

The  transition  from  water  to  land  entailed  major  changes  in  the 
somatic  muscles.  The  appendages  became  increasingly  important  in  loco- 
motion, and  movements  of  the  trunk  and  tail  less  important.  The 
primitive  single  fin  extensor  and  flexor  became  divided  into  many  com- 
ponents,   and    these    became    larger    and    more    powerful.    Despite    the 


PROTECTION,   SUPPORT   AND   MOVEMENT  513 


Iretpezi-us 


"Myomere 


Posterior  branchialTamT 
Hyoid-  mm. 


Mandibular  m 


Gill  si  it — ' 


Fin  eoctensor  m. 

Figure  25.8.     A  lateral  view  of  the  anterior  muscles  of  a  dogfish.  (Modified  after 
Howell.) 


empora.. 


lis 


Serra.tus  aj"jterior 


Recbus 
ai?dominis 


StcrnocleidoTnastoid 
Trapezius 

Pe-ctoralis 
major 

DeltoicL 

Biceps 
.Triceps 


Figure  25.9.     An  anterior  view  of  certain  of  the  superficial  muscles  of  man. 


514  VERTEBRATE   LIFE   AND    ORGANIZATION 

complexiLy  of  tetrapod  appendiculai  muscles,  it  is  possible  to  divide 
tliem  into  a  dorsal  group  that  evolved  from  the  fish  extensor  and  a 
ventral  group  derived  from  the  flexor.  Our  latissimus  dorsi  and  triceps 
(Fig.  25.9),  for  examjjle,  are  dorsal  appendicular  muscles,  whereas  the 
pectoralis  and  biceps  are  ventral  appendicular  muscles.  Segmentation 
is  lost  for  the  most  part  as  one  ascends  the  evolutionary  scale,  though 
traces  of  segmentation  remain  in  the  mammalian  rectus  abdominis.  The 
muscle  layers  on  the  flank  became  relatively  thin,  and  some  trunk  mus- 
cles, the  serratus  anterior,  for  example,  became  associated  with  the 
pectoral  girdle. 

Branchial  muscles  are  well  developed  in  fishes,  and  are  grouped 
according  to  the  visceral  arches  with  which  they  are  associated  (Fig. 
25.8).  Branchial  muscles  obviously  become  less  important  in  tetrapods, 
for  the  gills  are  lost  and  the  visceral  arches  are  reduced.  Nevertheless, 
certain  ones  are  retained.  Those  of  the  mandibular  arch  remain  as  the 
temporalis,  masseter  and  other  jaw  muscles  (Fig.  25.9).  Most  of  those 
of  the  hyoid  arch  move  to  a  superficial  position  and  become  the  facial 
muscles  that  are  responsible  for  smiling  and  other  facial  expressions. 
Those  of  the  remaining  arches  are  associated  with  the  pharynx  and 
larynx  and  some,  e.g.,  the  sternocleidomastoid  and  trapezius,  even 
acquire  attachments  onto  the  pectoral  girdle. 

Questions 

1.  Of  what  value  is  the  accumulation  of  keratin  in  the  skin  of  tetrapods? 

2.  How  would  the  structures  in  the  skin  that  are  concerned  with  thermoregulation  in- 
teract to  reduce  the  body  temperature  of  a  mammal? 

3.  Give  an  example  of  a  bone  in  the  human  skull  that  is  derived  from  each  of  the  three 
basic  components  of  the  skull. 

4.  What  changes  are  encountered  in  the  visceral  skeleton  as  one  ascends  the  evolutionary 
scale  from  fish  to  mammal?  With  what  are  these  changes  correlated? 

5.  What  changes  in  the  muscular  system  are  correlated  with  the  changes  in  the  method 
of  locomotion  encountered  between  fish  and  mammals? 

Supplementary  Reading 

Romer's  The  Vertebrate  Body  is  an  excellent  reference  for  those  wishing  to  pursue 
further  the  morphologic  aspects  of  the  evolution  of  the  organ  systems  described  in  this 
and  subsequent  chapters.  This  book  is  also  available  in  a  condensed  edition  entitled  A 
Shorter  Version  of  the  Vertebrate  Body.  Comparable  references  to  summaries  of  animal 
physiology  are  difficult  to  find,  but  Prosser,  Brown,  Bishop,  Jahn  and  Wulff,  Comparative 
Ammul  Physiology,  is  an  extremely  valuable  source  book.  Mammalian  physiology  is 
considered  in  detail  in  such  medical  texts  as  Fulton's  A  Textbook  of  Physiology  or  Guy- 
ton's  Textbook  of  Medical  Physiology.  Less  detailed  and  very  readable  accounts  are  to 
be  found  in  Carlson  and  Johnson,  The  Machinery  of  the  Body,  and  in  Cannon  The 
lUsdom  of  the  Body.  An  excellent  analysis  of  the  role  of  the  muscles  in  the  various  types 
of  vertebrate  locomotion  can  be  found  in  Gray's  little  book.  How  Animals  Move. 


CHAPTER  26 


Digestion  and  Respiration 


A  FUNDAMENTAL  characteristic  of  living  organisms  is  their  ability  to  take 
in  materials  quite  unlike  themselves  and  to  synthesize  their  own  unique 
protoplasm  from  these  materials.  Grass  becomes  beef  and  beef  becomes 
human  flesh  by  the  alchemy  of  living  organisms.  Animals  must  take  into 
their  bodies  a  wide  variety  of  substances  to  provide  the  raw  materials 
and  energy  necessary  for  the  synthesis  and  maintenance  of  protoplasm, 
for  reproduction  and  for  the  various  activities  of  the  body.  These  sub- 
stances include  energy-rich  organic  foods,  vitamins,  oxygen,  water  and 
mineral  salts.  The  organic  foods  (carbohydrates,  fats  and  proteins)  and 
vitamins  are  synthesized  by  plants  and  other  animals. 

In  vertebrates  oxygen  enters  through  the  respiratory  system— gills 
or  lungs— and  through  the  skin  in  certain  animals;  the  other  materials 
enter  through  the  digestive  system.  These  are  the  intake  systems  of  the 
body,  but  they  also  serve  to  some  extent  in  the  removal  of  waste 
products.  Some  toxins  are  removed  by  the  digestive  system,  and  most 
of  the  carbon  dioxide  produced  in  celkdar  respiration  is  eliminated  by 
the  respiratory  system  along  with  some  water  and,  in  fishes  at  least,  some 
nitrogenous  wastes  from  the  metabolism  of  proteins  and  nucleic  acids. 

The  vertebrate  digestive  tract  is  a  tube  enclosing  part  of  the  external 
environment  and  passing  through  the  body  with  openings  at  either  end. 
Food  is  taken  into  this  tract,  where  most  of  it  is  digested  and  absorbed. 
The  undigested  and  unabsorbed  residues  are  eliminated  as  feces  from  the 
posterior  end  of  the  tract.  The  process  of  elimination,  known  as  defeca- 
tion, should  not  be  confused  with  excretion,  which  is  the  discharge  of 
the  by-products  of  metabolism.  Excretion  is  primarily  a  function  of  the 
excretory  and  respiratory  systems  and  the  skin.  Most  of  the  material  in 
the  feces  has  in  fact  neither  entered  the  tissues  of  the  body  nor  taken 
part  in  metabolism. 

217.        The  Mouth 

The  basic  pattern  of  the  vertebrate  digestive  system  is  similar  in  all 
vertebrates  to  that  of  the  frog  described  in  Chapter  21.  In  very  primitive 
vertebrates  the  mouth  is  unsupported  by  jaws  but  most  vertebrates  have 
jaws  and  a  good  complement  of  teeth  to  aid  in  food-getting. 

Teeth  are  similar  in  structure  to  the  placoid  scales  of  sharks,  and 
are  believed  to  have  evolved  from  body  scales.  A  representative  mam- 

515 


516 


VERTEBRATE  LIFE  AND  ORGANIZATION 


Ena.m.e-1 


Cro'wn. 


Root    < 


Bone  of  ja-W 


Ceinent 


Figure  26.1.     Diagram  of  a  section  through  a  human  molar  tooth.  (Modified  after 
Maxiniow  and  Bloom.) 

malian  tooth  (Fig.  26.1)  consists  of  a  crown  projecting  above  the  gum 
and  one  or  more  roots  embedded  in  sockets  in  the  jaws.  The  crown  is 
covered  by  a  layer  ol  enamel.  Enamel  is  the  hardest  substance  in  the 
body  and  consists  almost  entirely  of  crystals  of  calcium  salts.  Calcium, 
phosphate  and  fluoride  are  important  constituents  of  enamel  and  all 
must  be  present  in  the  diet  in  suitable  amounts  for  proper  tooth  de- 
velopment and  maintenance.  The  rest  of  the  tooth  is  composed  of  dentin, 
a  substance  very  similar  to  bone.  In  the  center  of  the  tooth  is  a  pulp 
cavity  containing  blood  vessels  and  nerves.  A  layer  of  cement  covers 
much  of  the  root  and  holds  the  tooth  firmly  in  place  in  the  jaw. 

The  teeth  of  most  vertebrates  are  cone-shaped  structures  used  pri- 
marily for  seizing  and  holding  the  prey.  In  mammals,  the  teeth  are 
differentiated  into  several  types  that  are  used  not  only  for  seizing  food 
but  also  for  its  mechanical  breakdown.  Mammalian  teeth,  unlike  those 
of  lower  vertebrates,  are  not  continually  replaced.  Man,  for  example, 
first  has  a  set  of  deciduous,  or  milk  teeth— two  incisors,  one  canine  and 
two  premolars  on  each  side  of  each  jaw.  These  are  later  replaced  by 
permanent  teeth;  in  addition,  three  molars  develop  on  each  side  of  each 
jaw  behind  the  premolars.  The  molars  last  throughout  life  and  are  not 
replaced. 

Once  the  food  is  in  the  mouth,  a  fish  easily  manipulates  and 
swallows  it,  for  the  flow  of  water  aids  in  carrying  it  back  into  the 
pharynx.  Oral  glands  and  a  tongue  are  poorly  developed  in  fishes.  The 
evolution  of  these  structures  accompanied  the  transition  from  water  to 
land  and  they  became  more  elaborate  in  the  higher  tetrapods.  In  addi- 
tion to  a  liberal  sprinkling  of  simple  glands  in  the  lining  of  the  mouth 
cavity,  mammals  have  evolved  three  pairs  of  conspicuous  salivary  glands 
that  are  connected  to  the  mouth  by  ducts.  The  location  of  the  parotid, 
submaxillary  and  sublingual  glands  of  man  is  shown  in  Figure  26.2. 
Originally  oral  glands   simply   secreted  a   mucous  and  watery  fluid    to 


DIGESTION   AND   RESPIRATION 


517 


lubricate  the  food,  and  this  is  still  the  major  function  of  our  saliva. 
The  saHva  of  most  mammals  and  of  a  few  other  tetrapods  contains 
digestive  enzymes  and  the  chemical  breakdown  of  food  begins  in  the 
mouth.  Ptyalin,  which  must  be  activated  by  chloride  ions  present  in 
the  saliva,  is  an  amylase  that  hydrolyzes  starch  to  the  double  sugar  mal- 
tose. 1  he  small  amount  of  maltose  present  splits  some  of  the  maltose, 
yielding  the  single  sugar  glucose.  The  poison  glands  of  reptiles  and 
the  glands  of  vampire  bats  that  secrete  an  anticoagulant  are  other  spe- 
cialized oral  glands. 

The  tongue  of  frogs  and  anteaters  is  specialized  as  a  food  gathering 
device,  and  that  of  snakes  is  part  of  the  olfactory  mechanism  (p.  46), 
but  its  chief  function  in  most  vertebrates  is  to  manipulate  food  in  the 
mouth  and  to  aid  in  swallowing.  The  tongue  pushes  the  food  between 
the  teeth,  so  that  the  food  is  thoroughly  masticated  and  mixed  with 
saliva.  Then  the  food  is  shaped  into  a  ball,  a  bolus,  and  moved  by  the 
tongue  into  the  pharynx.  1  he  tongue  bears  numerous  microscopic  taste 
buds,  and  the  human  tongue  is  of  great  importance  in  speech. 


Nasal  cavity 


E^ct.  naris 

Se-c.  palate 
Ton. 6 lie  ~ 
Su.blingix.al  gland 
SiLbma>cillary  gland 
Vocal  cord 

Trachea' 


Gall  bladd&ir 

Liv^r  (lif  tcdzxp) 
Common  bile  duct 

Duodenixm. 

Transverse  colon 

Ascendind  Colon 


Ca.e.cum 
Appendioc 


Parotid  gland 

Soft  palate 
Pharynx 


Epiglottis 
Esophad  U-S 


Storaach 

Spleen 

Pancreas 


■Jejunum. 

Descending  colon 
Ileum. 

Rectum. 


Figure  26.2     The  digestive  system  of  man. 


518 


VERTEBRATE   LIFE   AND   ORGANIZATION 


f 


First  s-waJlo-wing 

Chews  CTxd. 


S<z-cond  swallo-winO 
Esop"ha.^uS 
Reticulum 


Diaphragra 


To  intestine 

Figure  26.3.     Course  of  food  through  the  "stomach"  of  a  cow.  Only  the  abomasum 
represents   the   true   stomach.   The   other  chambers   are   derived    from    the   esophagus. 

218.        The  Pharynx  and  Esophagus 

Part  of  the  pharynx  ot  man  lies  above  the  soft  palate  (Fig.  26.2) 
and  receives  the  internal  nares  and  the  openings  ol  the  pan-  ot  Eu- 
stachian tubes  from  the  middle  ear  cavities.  Another  part  lies  beneath 
the  soft  palate  and  is  in  communication  with  the  mouth  cavity.  The 
rest  of  the  pharynx  lies  posterior  to  these  parts  and  leads  to  the  esoph- 
agus and  larynx.  Passage  of  the  food  into  the  pharynx  initiates  a  series 
of  reflexes:  The  muscular  soft  palate  rises  to  prevent  food  from  entering 
the  nasal  cavities,  breathing  momentarily  stops,  the  larynx  is  elevated  and 
the  epiglottis  swings  over  the  glottis  to  prevent  food  from  entering  the 
larynx,  the  tongue  ^^revents  food  from  returning  to  the  mouth,  and  mus- 
cular contractions  of  the  pharynx  move  the  bolus  into  the  esophagus. 

The  pharynx  of  tetrapods  is  a  rather  short  region  in  which  the 
food  and  air  passages  cross,  but  in  fishes  it  is  a  more  extensive  area 
associated  with  the  gill  slits.  Gill  pouches  are  present  in  the  embryos  of 
mammals,  and  some  of  them  give  rise  to  glandular  structures  such  as 
the  thymus  and  parathyroids,  but  only  the  first  two  remain  as  rudiments 
in  adults.  The  middle  ear  cavity  and  the  Eustachian  tube  develop  from 
the  first  pouch  (the  spiracle  of  fishes),  and  part  of  the  second  forms  the 
fossa  in  which  the  palatine  tonsil  lies.  The  thyroid  gland  and  the  lungs 
are  outgrowths  from  the  floor  of  the  pharynx.  Glands  derived  from  the 
pharynx  are  endocrine  in  nature  and  will  be  considered  in  Chapter  30. 

Successive  waves  of  contraction  and  relaxation  of  the  muscles,  known 
as  peristalsis,  propel  the  bolus  down  the  esophagus  to  the  stomach.  The 
muscles  relax  in  front  of  the  food  and  contract  behind  it.  When  the 
food  reaches  the  end  of  the  esophagus  the  cardiac  sphincter,  which  closes 
off  the  entrance  to  the  stomach,  relaxes  and  allows  it  to  enter. 

The  esophagus  is  generally  a  simple  conducting  tube  but  in  some 
animals  its  structure  has  been  modified  for  storage.  The  crop  of  the 
pigeon  and  the  three  anterior  chambers  of  the  cow's  "stomach,"  for 
example,  are  modified  parts  of  the  esophagus.  They  are  lined  with  the 
stratified  squamous  epithelium  characteristic  of  the  anterior  parts  of 
the  digestive  tract,  whereas  a  true  stomach  and  the  intestine  are  lined 
with  a  simple  columnar  epithelium.  The  "stomach"  of  the  cow  and  other 
ruminants  consists  of  a  series  of  four  chambers  (Fig.  26.3).  Food  passes 


DIGESTION   AND    RESPIRATION  519 

first  into  the  rumen,  where  it  is  temporarily  stored,  and  the  cellulose  it 
contains  is  acted  upon  by  the  enzyme  cellulose,  produced  there  by  bac- 
teria. The  food  then  passes  into  the  reticulum,  is  afterwards  regurgitated 
and  the  animal  ruminates,  or  chews  its  cud.  The  thoroughly  masticated 
and  partly  digested  food  next  passes  to  the  omasum  and  finally  into  the 
true  stomach,  or  abomasum.  It  has  been  postulated  that  this  complex 
mechanism  evolved  in  plains-dwelling  animals  to  permit  them  to  feed 
hastily  when  exposed  to  predators  and  to  chew  their  food  later  and  more 
leisurely  in  shelter.  The  mechanism  also  facilitates  the  digestion  of 
celkdose.  Vertebrates  cannot  digest  this  carbohydrate  without  the  aid 
of  micro-organisms  living  in  the  stomach  or  intestine  since  none  of  them 
can  synthesize  the  necessary  enzyme,  cellulase. 

219.        The  Stomach 

The  stomach  is  a  J-shaped  pouch  whose  chief  functions  are  the 
storage  and  mechanical  churning  of  food,  and  the  initiation  of  the 
chemical  breakdown  of  proteins.  Lampreys,  lungfishes  and  some  other 
primitive  fishes  do  not  have  a  stomach,  and  the  absence  of  this  organ  is 
thought  to  have  been  a  characteristic  of  the  ancestral  vertebrates.  The 
early  vertebrates,  like  the  lower  chordates,  were  probably  filter-feeders 
that  fed  more  or  less  continuously  on  minute  food  particles  that  could 
be  digested  by  the  intestine  alone.  Presumably  the  evolution  of  jaws  and 
the  habit  of  feeding  less  frequently  and  on  larger  pieces  of  food  required 
an  organ  for  the  storage  and  initial  conversion  of  this  food  into  a  state 
which  could  be  digested  further  by  the  intestine.  In  most  vertebrates 
both  mechanical  and  chemical  digestion  begins  in  the  stomach. 

After  food  enters  the  stomach,  the  cardiac  sphincter  at  the  anterior 
end  of  the  stomach  and  the  pyloric  sphincter  at  the  posterior  end  close. 
Muscular  contractions  of  the  stomach  churn  the  food,  breaking  it  up 
mechanically  and  mixing  it  with  the  gastric  juice.  This  juice  is  very  acid 
and  eventually  stops  the  action  of  the  salivary  enzymes,  but  it  may  take 
40  minutes  or  more  before  there  has  been  sufficient  mixing  to  accom- 
plish this.  During  this  period  the  salivary  enzymes  continue  to  function 
and  it  has  been  estimated  that  they  will  break  down  about  40  per  cent 
of  the  starch  into  maltose.  Gastric  juice  is  secreted  by  tubular-shaped 
gastric  glands,  which,  in  mammals,  contain  two  types  of  secretory  cells. 
The  chief  cells  secrete  the  enzyme  precursor  pepsinogen.  The  parietal 
cells  secrete  hydrochloric  acid,  which  is  required  for  the  conversion  of 
pepsinogen  into  the  active  enzyme,  pepsin.  It  also  makes  the  stomach 
contents  acid.  Pepsin,  with  a  very  acid  pH  optimum  (about  pH  2.0), 
hydrolyzes  proteins  to  large  polypeptides  such  as  proteoses  and  pep- 
tones. Other  proteolytic  enzymes  secreted  by  the  pancreas  also  attack 
intact  protein  molecules  but  preferentially  split  peptide  bonds  adjacent 
to  certain  amino  acids. 

Pepsin  is  the  most  important  enzyme  in  the  gastric  juice,  but  not 
the  only  one  present.  Rennin  is  particularly  abundant  in  the  stomach 
of  young  mammals,  and  causes  the  milk  protein  casein  to  coagulate  so 
that  it  will  remain  in  the  stomach  long  enough  to  be  digested  by  pepsin. 


520  VERTEBRATE   LIFE   AND   ORGANIZATION 

Reiinin  has  been  extracted  tor  centuries  irom  the  stoinachs  of  calves  and 
used  to  cmdle  milk;  this  is  an  important  step  in  the  manufacture  of 
cheese. 

In  view  of  the  strong  proteolytic  action  of  pepsin,  one  might  wonder 
why  it  does  not  digest  the  wall  of  the  stomach.  A  major  factor  preventing 
sucli  autodigestion  is  the  secretion  of  copious  amounts  of  mucus  by  other 
multicellular  glands  in  the  stomach  and  by  scattered  cells  throughout 
the  stomach  lining.  The  mucus  forms  a  coating  which  protects  the 
stomach  walls  from  the  action  of  pepsin.  Furthermore,  the  amounts  of 
pepsin  and  acid  in  the  stomach  are  very  small  except  when  food  is 
present  to  be  digested.  Sometimes,  however,  these  safeguards  break  down, 
pepsin  digests  away  part  of  the  stomach  lining,  and  a  peptic  ulcer  results. 

When  the  food  is  reduced  to  a  creamy  consistency  and  most  of  the 
micro-organisms  that  entered  the  stomach  with  it  have  been  killed  by 
the  action  of  the  gastric  juices,  the  pyloric  sphincter  opens  and  the  food 
passes  into  the  small  intestine.  The  most  fluid  food  passes  first.  Indeed, 
upon  entering  the  stomach,  water  passes  almost  immediately  into  the 
intestine.  The  food  enters  the  intestine  in  spurts  and  is  quickly  neu- 
tralized by  the  alkalinity  of  secretions  flowing  into  the  intestine  from 
the  liver  and  pancreas. 

220.        The  Liver  and  Pancreas 

The  liver  and  pancreas  are  large  glandular  outgrowths  from  the 
anterior  part  of  the  intestine.  The  liver,  in  fact,  is  the  largest  organ  of 
the  body.  Its  cells  continually  secrete  bile,  which  passes  through  hepatic 
ducts  into  the  common  bile  duct  and  then  up  the  cystic  duct  into  the 
gall  bladder.  Bile  does  not  enter  the  intestine  immediately,  for  a 
sphincter  at  the  end  of  the  bile  duct  is  closed  until  food  enters  the  intes- 
tine. Contraction  of  the  wall  of  the  gall  bladder  forces  the  bile  out.  The 
bile  that  is  finally  poured  into  the  intestine  is  concentrated,  for  a  con- 
siderable amount  of  water  is  absorbed  from  the  bile  in  the  gall  bladder. 

Although  bile  contains  no  digestive  enzymes  it  nevertheless  has  a 
twofold  digestive  role.  Its  alkalinity,  along  with  that  of  the  pancreatic 
secretions,  neutralizes  the  acid  food  entering  the  intestine  and  creates  a 
pH  favorable  for  the  action  of  pancreatic  and  intestinal  enzymes.  Its 
bile  salts  emulsify  fats,  breaking  them  up  into  smaller  globules  and 
thereby  providing  more  surfaces  on  which  fat-splitting  enzymes  can  act. 
These  salts  are  also  essential  for  the  absorption  of  fats  and  fat-soluble 
vitamins  (A,  D,  K).  Most  of  the  bile  salts  are  not  eliminated  with  the 
feces,  but  are  absorbed  in  the  intestine  along  with  the  fats  and  are  car- 
ried back  to  the  liver  by  the  blood  stream  to  be  used  again. 

The  color  of  bile  (green,  yellow,  orange  or  red  in  different  species) 
is  due  to  the  presence  of  bile  pigments,  excretory  products  derived  from 
the  breakdown  of  hemoglobin  in  the  liver.  The  bile  pigments  undergo 
further  chemical  reactions  by  the  intestinal  bacteria  and  are  converted 
to  the  brown  pigments  responsible  for  the  color  of  the  feces.  If  their 
excretion  is  prevented  by  a  gall  stone  or  some  other  obstruction  of  the 
bile  duct,  they  are  reabsorbed  by  the  liver  and  gall  bladder,  the  feces 


DIGESTION   AND    RESPIRATION  59} 

are  pale  and  the  skin  assumes  the  yellowish  tinge  characteristic  of 
jaundice. 

All  of  the  blood  returning  from  the  intestine,  where  it  has  absorbed 
a  variety  of  materials,  passes  through  the  liver  before  entering  the 
general  circulation  of  the  body.  In  the  minute  capillary-like  spaces  of 
the  liver  the  blood  comes  into  intimate  contact  with  the  hepatic  cells, 
which  take  up,  store,  interconvert,  and  alter  in  many  ways  the  absorbed 
food  molecules.  The  liver  cells  also  detoxify  certain  poisonous  substances 
and  excrete  some  of  them  in  the  bile. 

The  pancreas  is  an  important  digestive  gland,  producing  quantities 
of  enzymes  that  act  upon  carbohydrates,  proteins  and  fats.  These  enzymes 
enter  the  intestine  by  way  of  a  pancreatic  duct  that  joins  the  common 
bile  duct.  An  accessory  pancreatic  duct  may  be  present  and  empty 
directly  into  the  intestine.  The  pancreas  contains  patches  of  endocrine 
tissue,  the  islets  of  Langerhans,  which  will  be  considered  in  Chapter  30. 

221.        The  Intestine 

Most  digestion,  and  virtually  all  of  the  absorption  of  the  usual  end 
products  of  digestion,  occur  in  the  intestine.  Most  of  the  digestive 
enzymes  foiuid  in  tlie  intestine  of  vertebrates  come  from  the  pancreas, 
but  distinct  intestinal  glands  are  also  present  in  the  wall  of  the  intes- 
tine of  birds  and  mannnals.  Adequate  siuface  area  tor  absorption  is 
made  available  by  the  length  of  the  intestine,  and  by  outgrowths  and 
internal  foldings  of  various  sorts. 

The  structural  details  of  the  intestine  vary  considerably  among 
vertebrates.  Primitive  fishes  have  a  short,  straight  valvular  intestine 
extending  from  the  stomach  to  the  cloaca.  Its  internal  surface  is  in- 
creased by  a  spiral  valve.  Tetrapods  have  lost  the  spiral  valve  and  make 
up  for  this  by  an  increase  in  the  length  of  the  intestine,  which  becomes 
more  or  less  coiled.  The  tetrapod  intestine  has  become  further  differ- 
entiated into  an  anterior  small  intestine  and  a  posterior  large  intestine. 
The  first  part  of  the  small  intestine  is  known  as  the  duodenum,  and,  in 
mammals,  the  two  succeeding  parts  are  the  jejunum  and  ileum.  Most 
of  the  large  intestine  is  known  as  the  colon,  but  in  mammals  the  caudal 
end,  which  has  evolved  from  part  of  the  cloaca  of  more  primitive 
vertebrates,  constitutes  the  rectum.  The  rectum  opens  on  the  body 
surface  through  the  anus.  A  blind  pouch  called  the  caecum  is  present 
at  the  junction  of  small  and  large  intestines.  This  is  very  long  in  such 
herbivores  as  the  rabbit  and  horse  and  contains  a  colony  of  bacteria 
that  digest  cellulose.  Man  has  a  small  caecum  with  a  vestigial  vermi- 
form appendix  on  its  end.  An  ileocaecal  valve  is  located  at  the  end  of 
the  small  intestine  and  prevents  bacteria  in  the  colon  from  backing  up 
into  this  region. 

A  transverse  section  of  the  small  intestine  of  a  mammal  illustrates 
the  microscopic  structure  of  the  digestive  tract  (Fig.  26.4).  As  in  the 
frog's  stomach  (section  185),  there  is  an  outer  covering  of  visceral 
peritoneum,  a  layer  of  smooth  muscle,  a  layer  of  vascular  connective 
tissue,   the  submucosa,   and   finally   the  innermost   layer,   the  mucosa. 


522 


VERTEBRATE   LIFE   AND   ORGANIZATION 


sceral  peritoneum 
Lon^itu-dinaL  muscle 

Circular  muscle 
Submucosa. 

Mucos 

Villus 


Villus 


Mucous 

secreting 
ola.nd. 

Capillaries 


Lyrnpha-tic- 
ve-ssel 


Musculcuns 
m.ucosa.e 

V&irb 
Lymphatics'^  ^ 

Arle-rj/  — 


Circular 

muscl  e 

LonOitudinal- 
muscle 


"Intestinal 
^lan-d 


•Enzyme 
seci'etind 
Cells 


Sabmucosao 


"Mcrve-s 


Visceral  pcritoncunx- 

Figure  26.4.  A,  A  cross  section  of  the  small  intestine  of  a  mammal  to  show  its 
constituent  layers;  B,  a  further  enlargement  of  a  block  of  tissue  from  the  wall  of  the 
small  intestine. 


The  stomach  and  intestine  lie  in  the  peritoneal  cavity— the  largest 
division  of  the  coelom— and  are  covered  by  the  visceral  peritoneum. 
Mesenteries,  which  support  the  internal  organs  and  provide  a  route  for 
blood  vessels  and  nerves,  extend  from  the  viscera  to  the  body  wall.  The 
outer  fibers  of  the  muscular  coat  are  usually  described  as  longitudinal; 
the  inner  as  circular.  Actually  both  layers  are  spiral;  the  outer  is  an 
open  spiral  and  the  inner  a  tight  spiral.  The  relaxations  and  contrac- 
tions of  these  layers  are  responsible  for  the  peristaltic  and  churning 
movements.  The  mucosa  consists  of  a  layer  of  smooth  muscle,  con- 
nective tissue,  and  finally  the  simple  columnar  epithelium  next  to  the 
lumen.  In  the  small  intestine  of  mammals  and  birds,  the  mucosa  bears 


DIGESTION   AND    RESPIRATION 


523 


numerous  minute,  finger-shaped  villi  containing  blood  capillaries  and 
small  lymphatic  vessels.  The  villi  protrude  into  the  lumen  and  increase 
the  intestinal  surface  manyfold.  Intestinal  glands  lie  at  their  base. 
Many  mucus-producing  goblet  cells  are  present  in  the  lining  epithelium 
and  their  secretion  helps  to  lubricate  the  food  and  to  protect  the  lining 
of  the  intestine. 

When  food  enters  the  duodenum  the  liver  and  pancreas  pour  their 
secretions  into  the  gut  and  a  series  of  reactions  begins.  Bile  salts  emul- 
sify the  fats,  and  lipase,  produced  by  the  pancreas  and  intestinal  glands, 
hydrolyzes  them  into  fatty  acids  and  glycerol.  The  pancreas  also  se- 
cretes trypsinogen  \vhich,  in  the  presence  of  enterokinase  secreted  by 
the  intestine,  is  converted  into  trypsin.  Chymotrypsinogen  secreted 
by  the  pancreas  is  changed  into  chymotrypsin  in  the  presence  of  trypsin. 
Trypsin  and  chymotrypsin  split  proteins  and  large  polypeptides  (pro- 
teoses and  peptones)  into  smaller  groups  of  amino  acids  known  as 
peptides.  Peptides  are  further  split  to  individual  amino  acids  by  vari- 
ous peptidases  secreted  by  the  pancreas  and  intestinal  glands.  Amylase 
secreted   by   the  pancreas,  and   to   a  lesser  extent  by  intestinal  glands, 


Table   3.     DIGESTIVE   ENZYMES 


PRODUCED  BY 

ENZYME 

SUBSTRATE   ACTED  UPON 

PRODUCT 

Salivary  glands 

Ptyalin 

Starch 

Maltose  (double 
sugar) 

(Maltase) 

Maltose 

Glucose  (single  sugar) 

Gastric  glands 

Pepsinogen,  converted 

Proteins 

Proteoses  and  pep- 

to pepsin 

tones 

Rennin 

Casein 

Precipitates  casein 

(Lipase?) 

Fats 

Fatty  acids  and 
glycerol 

Pancreas 

Amylase 

Starch 

Maltose 

Trypsinogen,  con- 

Proteins, proteoses 

Peptides 

verted  to  trypsin 

and  peptones 

Chymotrypsinogen 

Chymyotrypsin 

Ciiymotrypsinogen , 

Proteins,  proteoses 

Peptides 

converted  to 

and  peptones 

chymotrypsin 

Peptidases 

Peptides 

Amino  acids 

Lipase 

Emulsified  fat 

Fatty  acids  and 
glycerol 

Intestinal  glands 

(Amylase) 

Starch 

Maltose 

Maltase 

Maltose 

Glucose 

Sucrase 

Sucrose 

(double  sugar) 

Glucose  and  fructose 

Lactase 

Lactose 

Glucose  and  gal- 

(double sugar) 

actose 

Enterokinase 

Trypsinogen 

Trypsin 

Peptidases 

Peptides 

Amino  acids 

(Lipase) 

Emulsified  fats 

Fatty  acids  and 
glycerol 

The  less  important  enzymes  of  a  given  region  have  been  put  in  parentheses. 


524  VERTEBRATE   LIFE   AND   ORGANIZATION 

digests  starches  to  maltose  (malt  sugar).  Maltose,  and  the  double  sugars 
sucrose  and  lactose  (cane  and  milk  sugar)  that  may  be  in  the  ingested 
food,  are  finally  cleaved  to  single  sugars  by  the  intestinal  enzymes 
maltase,  sucrose  and  lactase,  respectively.  Glucose  is  the  most  im- 
portant single  sugar,  though  lesser  amounts  ol  fructose  and  galactose 
are  derived  Irom  the  breakdown  of  sucrose  and  lactose.  Most  of  the 
final  hydrolysis  probably  occurs  in  the  lumen  of  the  intestine,  though 
some  double  sugars  may  enter  the  mucosal  cells  and  be  digested  intra- 
cellularly.  The  digestive  enzymes  of  man  are  summarized  in  Table  3. 

Digestion  is  completed  in  the  small  intestine  and  the  products  of 
digestion  are  absorbed.  Absorption  results  partly  from  the  simple  dif- 
fusion of  molecules  from  the  lumen  of  the  intestine  through  the  mucosa 
and  into  the  blood  and  lymph  vessels,  and  partly  from  the  active 
uptake  of  molecules  by  the  mucosal  cells.  That  the  mucosal  cells  play 
an  active  role  is  indicated  by  the  fact  that  poisons  which  interfere  with 
their  metabolism  greatly  reduce  the  rate  and  amount  of  absorption. 
Most  of  the  products  of  digestion  are  in  solution  and  can  be  absorbed 
easily,  but  the  absorption  of  the  fats  and  fatty  acids  presents  a  special 
problem  that  is  not  completely  understood.  Apparently  their  uptake 
is  facilitated  by  combining  with  bile  salts,  for  this  makes  a  soluble 
complex.  Once  they  have  passed  through  the  cells  lining  the  intestine, 
the  fatty  acids  recombine  with  glycerol  to  form  fat  and  the  bile  salts 
are  freed.  Most  of  the  absorbed  fats  enter  the  lymph  vessels,  but  the 
sugars,  amino  acids  and  other  absorbed  materials  enter  the  capillaries 
of  the  blood  vessels. 

The  material  left  in  the  small  intestine,  which  is  still  very  fluid, 
passes  into  the  large  intestine.  Water  and  many  of  the  salts  are  absorbed 
as  the  residue  passes  through  the  colon.  If  too  much  water  is  absorbed, 
the  feces  become  very  dry  and  hard  and  constipation  may  result.  Many 
bacteria  reside  in  the  colon  and  synthesize  a  variety  of  vitamins  which 
are  absorbed  from  the  colon.  The  bacteria  reproduce  very  rapidly,  and 
many  are  eliminated.  As  much  as  25  per  cent  of  the  feces  may  consist 
of  bacteria. 

222.       The  Control  of  Digestive  Secretions 

Each  of  the  various  enzymes  is  secreted  at  an  appropriate  time: 
We  salivate  when  we  eat,  and  gastric  jince  is  produced  when  food 
reaches  the  stomach.  The  control  of  these  digestive  secretions  is  partly 
nervous  and  partly  endocrine.  The  smell  of  food  or  its  presence  in 
the  mouth  stimulates  sensory  nerves  that  carry  impulses  to  a  salivating 
center  in  the  medulla  of  the  brain.  From  there  the  impulses  are  relayed 
along  motor  nerves  to  the  salivary  glands,  which  then  secrete. 

The  control  of  gastric  secretion  is  more  complex.  Years  ago  the 
famous  Russian  physiologist,  Pavlov,  performed  an  experiment  in 
which  he  brought  the  esophagus  of  a  dog  to  the  surface  of  the  neck 
and  severed  it.  When  the  dog  ate  the  food  did  not  reach  the  stomach, 
yet  some  gastric  juice  was  secreted  provided  that  the  vagus  nerve, 
which  carries  motor  fibers   to  the  stomach  and  other  internal  organs, 


DIGESTION   AND    RESPIRATION  525 

was  left  intact.  If  the  vagus  nerve  was  cut,  this  secretion  did  not  occur. 
This  experiment  proved  that  the  control  of  gastric  secretion  was  at 
least  partly  nervous.  Subsequently  it  was  discovered  that  if  the  vagus 
was  cut  but  food  was  permitted  to  reach  the  stomach,  a  considerable 
flow  of  gastric  juice  was  produced.  Obviously  the  vagus  nerve  is  not  the 
only  means  of  stimulating  the  gastric  glands.  Further  investigation 
revealed  that  when  partly  digested  food  reaches  the  pyloric  region  of 
the  stomach,  certain  of  the  mucosal  cells  produce  the  hormone  gastrin, 
which  is  absorbed  into  the  blood  through  the  stomach  wall  and  ulti- 
mately reaches  the  gastric  glands,  stimulating  them  to  secrete.  When 
food,  especially  fats,  enters  the  duodenum,  the  duodenal  mucosa  pro- 
duces the  hormone  enterogastrone  which,  on  reaching  the  stomach, 
inhibits  the  secretion  of  the  gastric  glands  and  slows  down  the  churn- 
ing action  of  the  stomach.  The  rate  of  digestion  in  the  stomach  is 
reduced  or  stopped.  This  not  only  helps  to  prevent  the  stomach  from 
digesting  its  own  lining,  but  also  enables  fatty  foods  to  stay  for  a  longer 
period  in  the  duodenum  where  they  can  be  acted  on  by  bile  salts  and 
lipase. 

The  first  hormone  to  be  discovered  was  secretin,  which  initiates 
pancreatic  secretion.  In  1902  Bayliss  and  Starling  were  investigating  the 
current  belief  that  the  secretion  of  pancreatic  juice  was  under  nervous 
control.  They  found  that  the  pancreas  secreted  its  juice  when  acid  food 
entered  the  small  intestine  even  though  the  nerves  to  and  from  the  in- 
testine were  cut.  A  stimulant  of  some  sort  apparently  traveled  in  the 
blood.  The  injection  of  acids  into  the  blood  stream  had  no  effect,  so 
they  reasoned  that  some  stimulating  principle  must  be  produced  by  the 
intestinal  mucosa  upon  exposure  to  acid  foods.  When  they  injected  ex- 
tracts of  such  a  mucosa  into  the  circulatory  system  the  pancreas  secreted. 

Secretin  has  a  side  effect  on  the  liver  for  it  increases  slightly  the 
rate  of  bile  secretion.  However,  another  hormone,  cholecystokinin, 
which  is  also  produced  by  the  duodenal  mucosa  when  acid  food  is  in 
the  duodenum,  is  largely  responsible  for  causing  the  gall  bladder  to 
contract  and  release  the  bile.  Vagal  stimulation  also  plays  a  role  in 
the  release  of  bile. 

223.        Use  of  Absorbed  Materials 

The  absorbed  products  may  be  used  as  raw  materials  for  the  syn- 
thesis of  the  components  of  protoplasm  and  as  a  source  of  energy  to 
stoke  the  cellular  fires,  or  they  may  be  stored  for  later  use.  The  energy 
requirements  of  a  young  adult  man  vary  from  1600  to  6000  or  more 
Calories  a  day,  depending  on  whether  he  is  at  complete  rest,  not  even 
digesting  foods,  or  doing  heavy  physical  work.  A  person  leading  a 
rather  sedentary  life  requires  2500  to  3000  Calories  a  day.  All  kinds  of 
food  yield  energy,  when  metabolized,  but  not  to  the  same  extent.  When 
burned  completely  in  a  calorimeter,  one  gram  of  carbohydrate  or 
protein  yields  about  4  Calories,  and  one  gram  of  fat  9.5  Calories. 
Though  carbohydrates  do  not  contain  as  many  Calories  per  gram  as 
fats,    they   constitute   the   major  body   fuel  for  most  people.   Normally 


526  VERTEBRATE   LIFE   AND   ORGANIZATION 

our   diet   contains   more  carbohydrates   than   fats  or   proteins,   and   the 
carbohychates  are  the  prime  source  of  energy  for  the  cells. 

The  various  kinds  of  single  sugars  that  are  absorbed  are  carried  to 
the  li\er  where  most  of  them  are  converted  to  glycogen  (animal  starch) 
for  storage.  When  needed,  liver  glycogen  is  broken  down  and  released 
into  the  blood  stream  as  glucose.  The  role  of  the  liver  in  maintaining 
a  constant  level  of  glucose  in  the  blood  was  discussed  in  section  26. 
The  glucose  molecules  are  carried  to  all  the  cells  of  the  body  where 
they  are  oxidized  via  the  Krebs  citric  acid  cycle  to  carbon  dioxide  and 
water  and  their  energy  is  released.  If  sugars  are  absorbed  in  great 
excess,  not  all  are  converted  to  glycogen  and  released  as  glucose.  Some 
are  converted  by  the  liver  and  other  cells  to  fat  and  then  transported 
to  the  subcutaneous  connective  tissue  and  other  sites  for  storage.  It  is  a 
common  observation  that  an  excessive  intake  of  carbohydrates  or  pro- 
teins is  just  as  fattening  as  an  excessive  intake  of  fats. 

Absorbed  fats  may  be  metabolized  in  the  citric  acid  cycle  to  yield 
energy  for  cellular  activities,  but  fats  are  also  an  important  raw  ma- 
terial in  the  synthesis  of  components  of  protoplasm.  The  plasma  and 
nuclear  membranes  and  the  membranes  around  mitochondria  contain 
many  lipids. 

Most  of  the  amino  acids  are  used  as  raw  materials  for  the  synthesis 
of  proteins— the  major  constituents  of  protoplasm.  A  small  amount  of 
amino  acids  may  be  stored  as  such  in  the  liver  and  other  organs  but 
most  of  those  not  used  as  raw  materials  undergo  various  conversions.  If 
the  amino  group  is  stripped  off  (deamination),  the  rest  of  the  molecule 
can  enter  the  citric  acid  cycle  to  be  used  immediately  as  a  source  of 
energy,  or  it  can  be  converted  to  glycogen  or  fat.  Deamination  occurs 
principally  in  the  cells  of  the  liver,  but  it  can  take  place  in  any  of  the 
cells  of  the  body.  After  deamination,  the  amino  group  is  converted  to 
ammonia,  a  toxic  substance  that  would  be  injurious  if  it  accumulated 
in  the  cells.  In  mammals,  ammonia  is  combined  with  carbon  dioxide  to 
form  the  less  toxic  urea,  which  is  excreted  by  the  kidney.  Urea  syn- 
thesis takes  place  in  the  liver  and  kidney  cells  and  involves  a  number 
of  intermediate  steps,  including  the  temporary  combination  of  am- 
monia and  carbon  dioxide  with  ornithine  and  the  eventual  release  of 
ornithine.  This  series  of  reactions  is  known  as  the  urea  cycle. 

Other  absorbed  materials  include  minerals,  steroids,  nucleotides, 
water  and  vitamins.  Most  of  these  substances  are  involved  in  the  syn- 
thesis of  protoplasm,  and  have  been  discussed  in  Chapter  2.  However, 
a  bit  more  should  be  said  concerning  the  vitamins  at  this  time.  By 
definition,  the  vitamins  are  organic  substances  that  an  animal  needs  in 
minute  amounts  and  must  obtain  from  its  environment,  for  they  can- 
not be  synthesized  by  the  animal  in  question,  at  least  not  in  adequate 
quantity.  In  so  far  as  their  specific  role  in  metabolism  is  understood, 
they  are  constituents  of  coenzymes.  If  they  are  lacking  in  the  diet,  the 
reservoir  of  vitamins  that  can  be  stored  in  the  body  cells  (chiefly  liver 
cells)  is  used  up,  metabolic  processes  dependent  on  these  coenzymes 
are  impaired,  and  deficiency  diseases  result.  A  list  of  the  more  common 


DIGESTION   AND    RESPIKATION 


527 


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528 


VERTEBRATE    LIFE    AND    ORGANIZATION 


Figure   26.5.     A   child    with    rickets.   (Cooper,   Uarber   and   Mitchell:    Nutrition   in 
Hcahli  and  Disease  for  Nurses,  J.  B.  Lippincott  Co.) 

vitamins    needed   by    man    and    their    characteristics    is    presented    in 

Table  4. 

Certain  vitamin  deficiencies  are  the  cause  of  diseases  that  have  long 
plagued  man.  Beriberi  has  been  common  tor  centuries  among  Orientals 
and  other  peoples  who  subsist  largely  on  polished  rice.  Rice  husks, 
which  contain  thiamine,  prevent  the  disease  when  added  to  the  diet. 
Pellagra  used  to  be  common  in  our  southern  states,  tor  corn  meal, 
which  tormerly  made  up  such  a  large  part  ot  the  diet,  is  very  low  in 
niacin.  Scurvy  was  long  the  scourge  ot  sailors,  explorers  and  others  who 
could  not  get  tresh  truits  and  vegetables  and  the  ascorbic  acid  they 
contain.  Many  Civil  War  prisoners  such  as  the  ones  in  Andersonville 
prison  were  victims  of  this  disease.  Captain  James  Cook  was  among 
tlie  first  to  notice  that  feeding  his  crew  such  unusual  foods  (to  sailors 
at  least)  as  sauerkraut  reduced  the  incidence  of  scurvy.  He  reported 
his  findings  to  the  Royal  Society  in  1776,  and  about  two  decades  later, 
when  more  was  known  about  the  disease,  the  British  Navy  periodically 
enforced  a  ration  ot  lime  juice  on  members  of  all  crews.  British  sailors 
have  been  called  "limeys"  ever  since.  Rickets  is  a  disease  ot  children 
who  do  not  receive  sufficient  vitamin  D;  it  is  characterized  by  marked 
malformation  of  the  skeleton  (Fig.  26.5). 


224.        Respiratory  Membranes 

Cellular  respiration   is  an  oxidative  process  in  which  most  of  the 
energy  in  the  absorbed  food  molecules  is  released  and  made  available 


DIGESTION   AND    RESPIRATION  529 

for  the  various  cellular  activities.  To  maintain  it,  oxygen  must  be 
continuously  supplied  and  the  by-products,  carbon  dioxide  and  water, 
must  be  continuously  removed.  In  vertebrates  this  involves  the  uptake 
of  oxygen  and  the  release  of  carbon  dioxide  in  the  respiratory  organ, 
the  transportation  of  these  gases  by  the  blood  and  their  exchange  be- 
tween the  blood  and  cells.  These  processes  were  fully  considered  in 
Chapter  5.  Here  we  are  concerned  with  the  structure  and  function  of 
the  vertebrate  respiratory  organs,  in  which  gas  exchange  with  the  en- 
vironment occurs. 

All  respiratory  surfaces,  whether  in  a  worm,  a  fish  or  a  man,  con- 
sist of  a  moist,  semipermeable,  vascular  membrane  exposed  to  the 
external  environment  so  that  gas  exchange  by  diffusion  can  take  place 
between  the  blood  and  the  environment.  The  entire  body  surface  of 
primitive  organisms  may  serve  as  a  respiratory  membrane,  but  the 
respiratory  surface  in  the  higher  animals  is  generally  confined  to  a 
limited  region  and  protected  in  various  ways.  This  reduces  the  chance 
of  mechanical  injury  and  the  amount  of  body  water  lost  or  gained  by 
osmosis  via  this  route,  but  restricting  the  extent  of  this  membrane  poses 
the  problem  of  providing  adequate  surface  for  gas  exchange.  Each 
kind  of  vertebrate  has  had  to  solve  the  dilemma  of  how  to  expose  these 
delicate  membranes  to  the  environment  yet  protect  them  from  it  to 
some  extent. 

225.        The  Respiratory  System  of  Fishes 

Superficially,  there  is  little  resemblance  between  the  respiratory 
system  of  mammals  and  that  of  most  fishes.  The  respiratory  organs  of 
fishes  are  gills  located  in  the  gill  slits  and  attached  to  the  visceral 
arches.  A  fish  respires  (Fig.  26.6)  by  expanding  its  pharynx  and  taking 
water  in  through  the  mouth.  Then  the  mouth  is  closed,  or  in  certain 
species  oral  valves  close,  the  pharynx  is  contracted,  and  water  is  forced 
out  through  the  gill  slits.  Water  cannot  go  down  the  esophagus,  for 
this  is  collapsed  except  when  swallowing.  Gill  rakers  act  as  a  strainer 
to  prevent  food  from  clogging  the  gills.  In  sharks,  each  gill  slit  opens 
independently  at  the  body  surface.  In  bony  fishes,  all  of  the  slits  empty 
into  an  opercular  chamber  which  is  closed  when  water  is  taken  in,  but 
opens  when  water  is  expelled. 

The  gills  themselves  consist  of  numerous,  thin-walled  lamellae,  or 
filaments,  containing  a  rich  network  of  capillaries.  They  are  protected 
in  the  gill  slits,  they  have  a  large  surface  area,  the  blood  and  external 
environment  are  in  close  proximity,  and  gas  exchange  occurs  readily 
as  water  passes  over  them.  In  addition,  the  body  gains  or  loses  water 
through  the  gills,  and  some  nitrogenous  wastes  are  excreted  here.  The 
salt-water  teleosts  also  excrete  salts  through  the  gills.  These  fishes  live 
in  an  environment  in  which  the  salt  concentration  is  greater  than  that 
in  their  bodies  so  they  tend  to  lose  water  by  osmosis.  They  must  drink 
large  amounts  of  salt  water  and  then  excrete  the  salts  by  specialized 
cells  in  their  gills. 

A  number  of  fishes  live  in  water  which  has  a  low  oxygen  content, 


530 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Opercalar 
-chamber 


Visceral  arch- 


Gill  raKer 

Oxygenated  blood  leaving 

gills 

Unoxytfenated  Hood 

^        .^  gills 


Gill  rakers 


Moulh 
cavity 


Gill  slit— y^    '^^     M^   ^  , 

P .  „  _Jm  Ph  ary nx»\^  Opercular 


Gill 


r,  1       i  '^^  HscpK-  ^-^chamber 


rj^''T//\jVBody  wall 
^Coeloru 

c 


^-Qralvalve. 


D 


Figure  26.6.  External  respiration  in  fishes.  A,  The  operculum  has  been  cut  away 
to  slidw  tlie  gills  in  the  gill  chamber.  C  and  D,  frontal  sections  through  the  mouth 
and  pliaiynx  in  the  plane  of  Une  a-b  in  the  preceding  figure.  Water  is  entering  the 
pharynx  in  C;  leaving  in  D.  B,  An  enlargement  of  one  of  the  gill  sections  shown  in 
C  and  D.  (Modified  after  Storer.) 

and  they  supplement  gill  respiration  by  occasionally  gulping  air.  There 
is  more  oxygen  in  the  air  than  in  water  and  it  can  be  extracted  from 
the  air  by  gills  so  long  as  they  remain  moist.  Closing  the  opercular 
chamber  enables  the  mudskipper  to  keep  its  gills  moist  for  a  while,  and 
even  to  come  out  onto  the  land.  The  European  loach  swallows  air 
and  extracts  the  oxygen  in  a  special  chamber  of  its  intestine!  Other 
fishes  have  vascular  outgrowths  from  various  parts  of  the  pharynx  or 
opercular  chamber  that  serve  as  accessory  respiratory  organs.  Seem- 
ingly the  development  of  lungs,  which  are  ventral  outgrowths  from 
the  pharynx,  by  early  bony  fishes  was  jtist  one  of  many  adaptations 
which  have  evolved  to  supplement  aquatic  respiration. 

Extracting  oxygen  from  swamp  water,  which  is  probably  the  en- 
vironment in  which  lungs  evolved,  poses  the  problem  of  saturating 
the  blood  with  oxygen  in  an  environment  with  a  low  oxygen  and  high 
carbon  dioxide  content.  As  we  explained  in  Chapter  5,  the  presence 
of  carbon  dioxide  reduces  the  oxygen-carrying  capacity  of  hemoglobin. 
The  chemical  properties  of  the  hemoglobin  of  contemporary  swamp 
fish  have  changed  in  such  a  way  that  it  can  take  up  more  oxygen  in  the 
presence  of  a  given  amount  of  carbon  dioxide.  This  change  must  also 
have  occurred  dtiring  the  evolution  of  terrestrial  vertebrates,  for  the 
carbon  dioxide  content  of  the  lungs  is  always  higher  than  that  of 
the  external  environment,  though,  of  course,  not  as  high  as  that  in  the 
tissues. 


DIGESTION   AND    RESPIRATION 


531 


226.        The  Respiratory  System  of  Terrestrial  Vertebrates 

The  lungs  of  early  bony  fishes  evolved  into  hydrostatic  swim  blad- 
ders in  most  of  their  descendants,  but  they  were  retained  in  some  that 
remained  in  fresh  water,  and  it  is  from  certain  of  these  fishes  that  tetra- 
pods  evolved.  Gills,  which  dry  out  on  exposure  to  the  air,  have  been 
lost  by  adult  amphibians,  but  are  retained  by  their  aquatic  larvae. 
Many  larval  amphibians,  however,  have  external  gills  protruding  from 
the  surface  of  the  neck  rather  than  ones  within  the  gill  slits.  Adult 
amphibians  breathe  by  simple,  saccular  lungs,  supplemented  by  a  moist 
skin  and  other  mucous  membranes.  The  somewhat  awkward  mouth 
pump  for  moving  air  in  and  out  of  the  lungs,  and  the  need  for  aux- 
iliary respiratory  membranes,  are  among  the  factors  that  prevent  am- 
phibians from  fully  exploiting  the  terrestrial  environment.  The  internal 
surfaces  of  the  lungs  of  higher  letrapods  have  become  greatly  sub- 
divided and  have  increased  in  area  enough  to  dispense  with  respiration 
in  the  skin.  These  organisms  have  also  developed  more  efficient  means 
of  ventilating  the  limgs. 

In  mammals  (Fig.  2(").7),  air  is  drawn  into  the  paired  nasal  cavities 
through  the  external  nares.  These  cavities  are  separated  from  the 
mouth  cavity  by  a  bony  palate,  and  the  animal  can  breathe  while  food 
is  in  its  mouth.  The  surface  area  of  the  cavities  is  increased  by  a  series 
of  ridges  known  as  conchae,  and  the  nasal  mucosa  (in  addition  to 
having  receptors  for  smell)  is  vascular,  ciliated  and  contains  many 
mucous  glands.  In  the  nasal  cavities  the  air  is  warmed,  moistened  and 


Concha 
PaUte 


Epi^glottis 
Tracheal rin^ 


Eusta.cKla.n  tabg 

Nasopharynx 

Larynx. 
Esopha-gus 

Pleura^, 


Bronchus 


Bronchiole 


Alveolar 
duct  and  sa.cs 


Lung 
(upper  lobe) 

'erLcardiam. 

Lung 
(lovv'erlobe) 


Diaphi-a.om. 


Figure   26.7.     Respiratory  system  of  man.  Details  of  the  alveolar  sacs,  here  drawn 
from  above.  A,  normal  position  of  vocal  cords;  B,  position  of  cords  during  speech. 


532  VERTEBRATE  LIFE  AND   ORGANIZATION 


Epiglottis 


-Vocal  cordS' 
Glottis- 
"Laxynizal  cartilages ' 

A  B 

Figure  26.8.     A   laryngoscopic   view   of   the  vocal  cords,   looking  into   the   larynx 
from  above.  A,  normal  position  of  vocal  cords;  B,  position  of  cords  during  speech. 

minute  foreign  particles  are  entrapped  in  a  sheet  of  mucus,  wliich  is 
carried  by  ciliary  action  into  the  pharynx  where  it  is  swallowed  or 
expectorated.  Inspired  air  is  moistened  in  primitive  tetrapods  such 
as  the  frog,  but  cold-blooded  tetrapods  in  general  do  not  need  as  much 
conditioning  of  the  air  as  birds  and  mammals. 

Air  continues  through  the  internal  nares,  passes  through  the 
pharynx,  and  enters  the  larynx,  which  is  open  except  when  food  is 
swallowed.  The  raising  of  the  larynx  during  swallowing  can  be  demon- 
strated by  placing  your  hand  on  the  Adam's  apple,  the  external 
protrusion  of  the  larynx.  The  epiglotrls  Hips  back  over  the  entrance 
of  the  larynx  when  it  is  raised. 

The  larynx  is  composed  of  cartilages  derived  from  certain  of  the 
visceral  arches,  and  serves  both  to  guard  the  entrance  to  the  windpipe, 
or  trachea,  and  to  house  the  vocal  cords  (Fig.  26.8).  The  vocal  cords 
are  a  pair  of  folds  in  the  lateral  walls  of  the  larynx.  They  can  be 
brought  close  together,  or  be  moved  apart,  by  the  pivoting  of  laryngeal 
cartilages  connected  to  their  dorsal  ends.  When  we  speak,  they  are 
moved  toward  each  other  and  the  current  of  air  expelled  from  the 
lungs  sets  them  vibrating.  They  in  turn  vibrate  the  column  of  air  in 
the  larynx,  pharynx  and  mouth,  just  as  the  reed  in  an  organ  pipe 
vibrates  the  column  of  air  in  the  pipe.  Muscle  fibers  extending  between 
the  various  cartilages  of  the  larynx  control  the  tension  of  the  cords 
and  the  pitch  of  the  sound.  The  shape  of  the  pharynx,  mouth,  tongue 
and  lips  affects  the  final  quality  of  the  sound.  The  glottis  is  the  opening 
into  the  larynx  between  the  vocal  cords. 

The  trachea  extends  down  the  neck  and  finally  divides  into 
bronchi  that  lead  to  the  pair  of  lungs.  Unlike  the  esophagus,  which 
is  collapsed  except  when  a  ball  of  food  is  passing  through,  the  trachea  is 
held  open  by  C-shaped  cartilaginous  rings  and  air  can  move  freely  back 
and  forth.   Its  mucosa  continues  to  condition  the  air. 

The  lungs  of  amphibians  lie  in  the  anterodorsal  part  of  the  pleuro- 
peritoneal  cavity,  which  is  the  larger  part  of  the  coelom.  (The  peri- 
cardial cavity  is  the  other  part.)  In  most  higher  vertebrates,  the  pleuro- 
peritoneal  cavity  is  subdivided  into  a  pleural  cavity  around  each  lung 
and  a  peritoneal  cavity  housing  the  abdominal  viscera.  The  pleural 
cavities  of  mammals  lie  within  the  chest,  or  thorax,  and  are  separated 
from  the  peritoneal  cavity  by  a  muscular  diaphragm.  A  coelomic 
epithelium,  the  pleura,  lines  the  pleural  cavities  and  covers  the  lungs. 
Each  bronchus  enters  a  lung,  accompanied  by  blood  vessels  and  nerves, 
in  a  mesentery-like  fold  of  pleura  (Fig.  26.7). 


DIGESTION   AND    RESPIRATION  533 


Bronchiole. 


Surf 


ace  viev/ 


Ve-nul 


Alveolar  duct 


Alveola.r  sac 


Alveoli 


Arte^riole- 
Capillaries 


Figure  26.9.  A,  Termination  of  the  respiratory  passages  in  the  mammalian  lung; 
B,  a  further  enlargement  to  show  the  dense  capillary  network  covering  a  single 
alveolus;   C,  an  alveolus  in  section.  Alveoli  have  a  diameter  of  0.2  to  0.3  mm. 

The  bronchi  branch  profusely  within  the  lungs  and  the  walls  of 
the  respiratory  passages  become  progressively  thinner  (Fig.  26.7).  Each 
passage  eventually  terminates  in  an  alveolar  sac  whose  walls  are  so 
puckered  by  pocket-shaped  alveoli  that  it  resembles  a  cluster  of  grapes 
(Fig.  26.9).  A  dense  network  of  capillaries  is  intimately  associated  with 
the  wall  of  the  alveoli.  Whether  the  capillaries  themselves  form  the 
wall  of  the  alveoli,  and  hence  are  in  direct  contact  with  the  alveolar 
air,  or  whether  they  are  separated  from  the  alveolar  lumen  by  a  thin 
layer  of  epithelium,  has  long  been  a  controversial  problem.  In  recent 
years  Low  and  others  have  studied  the  structure  of  the  lung  with  the 
electron  microscope,  and  they  find  that  the  alveoli  do  have  a  very  thin 
epithelial  wall  of  their  own  separating  the  lumen  from  the  capillaries. 
The  plexus  of  capillaries  covering  the  alveoli  is  so  dense  that  little 
space  is  left  between  the  individual  vessels.  All  this  provides  a  huge 
protected  area  for  the  exchange  of  gases.  A  large  surface  is,  of  course, 
essential  in  a  homoiothermic  animal.  A  frog's  lung  is  a  hollow  sac  with 
a  few  pockets  in  its  wall,  but  the  mammalian  lung  is  greatly  subdivided 
internally   and   is  like  a  fine-grained  sponge. 


227.       The  Mechanics  and  Control  of  Breathing 

Mammalian  lungs  are  ventilated  by  changing  the  dimensions  of 
the  thorax  and  consequently  the  pressure  within  the  lungs.  During 
normal,  quiet  inspiration,   the  size  of  the   thorax  is  increased  slightly, 


534 


VERTEBRATE   LIFE  AND   ORGANIZATION 


Brain 

Sternum 

Position  of  Rit3 
during  inspiration 

expiration 

Position  of  Rib  4 
during  inspiration 

expiration 

Diaphragm  during  expiration 

inspiration 

Position  of  abdominal 
muscles  during  inspiration 


Spinal 
cord 


l^exp: 


iration 


Neuron  circuit  pTom 
inspiratory  to 
expiratory  center 

Inhibiting  neuron 
from  lung 

Lund 

Neuron  from 
inspiratory  center 

Diaphragm 

'Abdominal  muscles 

Neuron  From 
expiratory  center 


Figure  26.10.  Mechanics  and  control  of  breathing.  A,  The  elevation  of  the  ribs 
and  depression  of  the  diaphragm  during  inspiration  increases  the  size  of  the  chest 
cavity,  indicated  by  tlie  black  area.  B,  A  diagram  of  the  nervous  mechanism  for 
controlling  the  rhythm  of  breathing.  See  text  for  explanation. 

intrapulmonary  pressure  falls  to  about  3  mm.  of  mercury  below  at- 
mospheric pressure,  and  air  passes  into  the  lungs  until  intrapulmonary 
and  atmospheric  pressures  are  the  same.  During  normal  expiration,  the 
size  of  the  thorax  is  decreased,  intrapulmonary  pressure  is  raised  to 
about  3  mm.  of  mercury  above  atmospheric  pressure,  and  air  is  driven 
out  of  the  lungs  until  equilibrium  is  again  reached.  During  inspiration, 
the  thorax  is  enlarged  by  the  contraction  of  the  dome-shaped  diaphragm 
and  the  external  intercostal  muscles.  The  diaphragm  pushes  the  ab- 
dominal viscera  posteriorly  and  increases  the  length  of  the  chest  cavity 
(Fig.  26.10  A);  the  external  intercostals  raise  the  sternal  ends  of  the 
ribs  and  expand  the  dorsoventral  diameter  of  the  chest.  During  expira- 
tion, the  relaxation  of  these  muscles  and  the  contraction  of  antagonistic 
muscles  decrease  the  size  of  the  thorax.  Contraction  of  abdominal 
muscles  forces  the  abdominal  viscera  against  the  diaphragm  and  pushes 
it  forward;  internal  intercostals  pvdl  the  sternal  ends  of  the  ribs  pos- 
teriorly. The  elastic  recoil  of  the  lungs,  which  are  stretched  during 
inspiration,  is  also  important  in  expelling  air. 

The  lungs  of  an  adult  man  can  hold  about  6  liters  of  air,  but  in 
quiet  breathing  they  contain  only  about  half  this  amount,  of  which  0.5 
liter  is  exchanged  in  any  one  cycle  of  inspiration  and  expiration.  This 
half  liter  of  tidal  air  is  mixed  with  the  2.5  liters  of  air  already  in  the 
lungs.  Vigorous  respiratory  movements  can  lower  and  raise  the  intra- 
pulmonary pressure  60  mm.  of  mercury  below  and  above  atmospheric 
pressure,  and  under  these  conditions  4  to  5  liters  of  air  can  be  exchanged. 
There  is  always,  however,  at  least  a  liter  of  residual  air  left  in  the  lungs 
to  mix  with  the  tidal  air,  for  the  strongest  respiratory  movements  cannot 


DIGESTION   AND    RESPIRATION  535 

collapse  all  of  the  alveoli  and  respiratory  passages.  Since  the  inspired 
air  always  mixes  with  a  certain  amount  oi  stale  air  already  in  the  lungs, 
alveolar  air  always  has  a  lower  oxygen  content  and  a  higher  content  of 
carbon  dioxide  than  atmospheric  air.  Alveolar  air  is  also  saturated  with 
water  vapor. 

Respiratory  movements  are  cyclic  and  are  controlled  by  inspiratory 
and  expiratory  centers  (collectively  called  the  respiratory  center)  in  the 
medulla  of  the  brain.  The  inspiratory  center  sends  out  impulses  along 
the  nerves  to  the  inspiratory  muscles  (neuron  ^1,  Fig.  26.10  B),  and  we 
breathe  in.  The  alveoli  fill  with  air,  become  stretched,  and  the  resultant 
sensory  impulses  traveling  to  the  respiratory  center  inhibit  inspiration 
{#~}-  At  the  same  time,  impulses  that  were  initiated  in  the  inspiratory 
center  and  took  a  rather  circuitous  route  within  the  brain  reach  the 
expiratory  center  {#S),  and  stimulate  it  to  send  impulses  out  to  the  ex- 
piratory muscles  (^4).  We  breathe  out,  another  volley  of  impulses  leaves 
the  inspiratory  center,  and  the  breathing  cycle  begins  again.  The  in- 
spiratory center  tends  to  be  active  all  the  time,  ceasing  to  send  out 
impulses  only  when  it  is  momentarily  inhibited. 

This,  in  brief,  is  the  basis  for  our  regular  breathing,  but  many  other 
factors  can  affect  the  rate  and  dejjth  of  respiration.  Increased  metab- 
olism during  exercise,  for  example,  results  in  an  increased  carbon  diox- 
ide content  of  the  blood.  This  stimulates  the  respiratory  center,  and  we 
automatically  breathe  faster  and  deeper.  The  same  thing  happens  when 
we  voluntarily  hold  our  breath.  Since  the  lungs  are  not  being  ventilated, 
carbon  dioxide  accumulates  in  the  alveolar  air  and  blood,  and  eventually 
reaches  a  level  that  stimulates  the  respiratory  center  and  we  breathe 
again  involinitarily.  One  cannot  suffocate  by  holding  one's  breath. 

By  expiring  vigorously  and  frequently,  we  can  reduce  the  carbon 
dioxide  content  of  the  alveolar  air  and  blood  below  normal  limits,  and 
breathing  stops  until  carbon  dioxide  accumulates  again.  The  accumula- 
tion of  carbon  dioxide  in  the  blood  is  responsible  for  initiating  breathing 
in  a  newborn  baby. 

Receptors  in  the  larynx  and  trachea  can  also  affect  respiration.  If 
food  inadvertently  enters  these  passages,  these  receptors  are  stimulated 
and  a  very  vigorous  expiration,  i.e.,  a  cough,  results.  The  cough  reflex 
is  one  of  many  safeguards  in  the  body  that  are  activated  if  something 
goes  wrong  with  the  primary  control  mechanism,  in  this  case  the  swal- 
lowing reflex. 

Questions 

1.  How  do  the  teeth  of  mammals  differ  from  those  of  lower  vertebrates? 

2.  \V'hat  normally  pre\ents  food  from  going  down  "the  wrong  way'"  when  we  swallow? 
What  happens  if  it  does  start  down  the  larynx? 

3.  \Vhat  reasons  can  yon  give  for  the  absence  of  a  stomach  in  ancestral  vertebrates? 

4.  ^Vonld  you  expect  rennin  to  be  present  in  the  stomach  of  the  young  of  non-mam- 
malian vertebrates? 

5.  What  prevents  the  wall  of  the  digestive  tract  from  being  digested? 

6.  How  is  it  possible  for  herbivorous  vertebrates  to  digest  cellulose? 

7.  If  one  were  to  eat  a  ham  sandwich,  where  and  by  what  would  its  various  components 


536  VERTEBRATE   LIFE   AND    ORGANIZATION 

l,c  (ligisttci?  Wliat  controls  the  secretion  of  the  digestive  enzymes  required?  What 
would  happc-n  to  the  products  of  digestion? 

8.  What  are  the  functions  of  the  large  intestine? 

9.  How  tioes  defecation  differ  from  excretion?  What  excretory  products  may  be  present 
in  the  feces? 

10.  List  the  functions  of  the  liver. 

11.  How  do  the  gills  of  fishes  fulfill  the  requirements  of  respiratory  membranes?  How  is 
water  circulated  across  them? 

12.  What  exchanges  between  the  body  and  the  environment  occur  in  the  gills  of  fishes? 

13.  In  what  group  of  vertebrates,  and  under  what  environmental  conditions,  did  lungs 
first  evolve? 

14.  In  what  respects  is  the  external  respiration  of  amphibians  poorly  adapted  to  the 
terrestrial  environment?  How  has  this  been  improved  in  higher  tetrapods? 

15.  How  is  inspired  air  conditioned  in  mammals?  Why  is  this  more  important  in  a  mam- 
mal than  in  a  frog? 

16.  \Vhy  is  it  that  alveolar  air  differs  in  composition  from  atmospheric  air?  Of  what  sig- 
nificance is  this? 

17.  What  causes  the  increase  in  the  rate  and  depth  of  breathing  during  exercise?  Why  is 
such  an  increase  necessary? 

Supplementary  Reading 

The  references  cited  at  the  end  of  Chapter  25  also  apply  to  this  chapter.  Interesting 
accounts  of  the  discovery  of  the  digestive  and  respiratory  processes  can  be  found  in  Fulton, 
Selected  Readings  in  the  History  of  Physiology.  Guyton's  Textbook  of  Medical  Physiology 
has  an  excellent  chapter  on  the  fascinating  problems  of  respiration  in  deep-sea  diving 
and  in  aviation.  Respiratory  adaptations  to  deserts  and  mountains  are  considered  in  Dill's 
Life,  Heat,  and  Altitude. 


CHAPTER  27 


Blood  and  Circulation 


All  animals,  from  the  simplest  protozoa  to  the  most  complex  verte- 
brates, must  have  some  arrangement  for  transporting  a  wide  variety  of 
materials  throughout  their  bodies.  As  we  pointed  out  in  Chapter  5,  the 
simple  diffusion  of  molecules  always  plays  an  important  part  in  trans- 
portation and  this  is  adequate  in  itself  in  the  smaller  and  less  active 
organisms.  But  the  vertebrates  and  many  of  the  higher  invertebrates  are 
so  large  and  active  that  diffusion  alone  cannot  suffice.  Complex  cir- 
culatory systems  are  necessary  for  the  rapid  transport  of  digested  food 
from  the  alimentary  tract,  and  of  oxygen  from  the  lungs,  to  all  the 
tissues,  and  for  carrying  carbon  dioxide  and  other  metabolic  wastes  to 
the  sites  where  they  are  discharged  from  the  body. 

The  vertebrate  circulatory  system  not  only  transports  gases,  foods 
and  waste  products,  but  has  other  important  functions  as  well.  By  con- 
veying hormones  it  supplements  the  nervous  system  in  the  integration 
of  body  activities.  It  plays  an  important  role  in  maintaining  the  con- 
stancy of  the  internal  environment.  The  blood  carries  away  excess  water 
from  the  tissues  and  su})plies  water  when  necessary.  It  helps  to  regulate 
the  pH  of  the  body  fluids.  The  rate  of  its  circulation  through  the  skin 
is  a  factor  in  the  control  of  body  temperature  in  birds  and  mammals. 
Special  cells  in  the  blood  function  in  wound  healing  and  in  protecting 
the  body  from  the  invasion  of  viruses  and  bacteria. 

The  circulatory  system  includes  not  only  the  complex  system  of 
vessels  but  also  the  fluids  within  them.  There  are  about  15  liters  of 
extracellular  fluid  in  the  body  of  an  adult  man,  and  about  one-third 
of  this  is  blood.  The  remainder  includes  the  tissue  fluid  that  lies  between 
and  bathes  the  cells  of  the  body,  the  lymph  that  moves  slowly  in  the 
lymph  vessels,  the  cerebrospinal  fluid  in  the  cavities  of  the  central 
nervous  system,  the  aqueous  and  vitreous  humors  of  the  eye,  and  the 
fluids  in  the  coelom.  The  chief  difference  between  blood  and  tissue  fluid 
or  lymph  is  the  presence  of  red  blood  cells  and  abundant  soluble  pro- 
teins in  the  blood. 

The  fundamental  pattern  of  the  vessels  in  a  mammal  is  shown  in 
Figure  27.1.  A  muscular  heart  propels  blood  through  arteries  to  capil- 
laries in  the  tissues.  Exchanges  between  the  circulatory  system  and  the 
cells  of  the  body  can  occur  only  through  the  walls  of  the  capillaries. 
Molecules  of  nutrients,  wastes,  oxygen,  carbon  dioxide  and  water,  but 
not    the   large   protein  molecules   or    the   red    blood   cells,   pass   readily 

537 


538  VERTEBRATE   LIFE   AND    ORGANIZATION 

He.a.rt 


Vein. 


Lymph 


-A-rt  ery 


Cell 


pilla.ry 
'issue  fluid 


Figure  27.1.     The    fundamental    structure  of   the   mammalian    circulatory    system. 
Arrows  indicate  the  direction  of  blood  flow. 

through  the  capillary  walls.  The  tissues  are  drained  by  the  veins,  which 
return  blood  to  the  heart,  and  by  a  separate  system  of  lymph  capillaries. 
Lymph  capillaries  lead  to  lymph  vessels,  which  pass  through  lymph 
nodes,  and  finally  empty  into  the  veins  where  the  venous  pressure  is 
lowest,  a  short  distance  from  the  heart.  The  lymph  nodes  are  an  important 
link  in  the  body's  system  of  defense  mechanisms.  They  produce  one  kind 
of  white  blood  cell  (lymphocytes),  and  contain  cells  that  engulf  foreign 
particles. 

228.        Blood  Plasma 

Blood  is  one  of  the  tissues  of  the  body.  It  consists  of  a  liquid  com- 
ponent, the  plasma,  and  several  types  of  formed  elements— red  blood 
cells,  white  blood  cells  and  platelets  (Fig.  3.14)— which  flow  along  in  it. 
The  plasma  is  a  complex  liquid  that  is  in  a  dynamic  equilibrium  with 
the  tissue  fluid  and  the  fluid  within  the  cells.  It  is  constantly  gaining 
and  losing  substances,  yet  its  composition  is  essentially  constant.  We  have 
seen,  for  example,  how  the  liver  maintains  a  constant  glucose  level  in 
the  blood  despite  the  heavy  intake  of  glucose  from  the  digestive  tract 
after  a  meal,  and  the  constant  release  of  glucose  to  the  tissue  fluid  and 
cells.  Plasma  is  about  90  per  cent  water,  7  to  8  per  cent  soluble  proteins, 
1  per  cent  salts,  and  the  remaining  1  to  2  per  cent  is  made  up  of  a  variety 
of  small  organic  molecules— urea,  amino  acids,  glucose,  lipids,  and  hor- 
mones. 

The  chief  plasma  proteins  are  fibrinogen,  albumins  and  globulins. 
Other  components  of  the  plasma  can  pass  through  the  semipermeable 
capillary  walls,  but  the  proteins  are  rather  large  molecules  and  remain 


BLOOD   AND   CIRCULATION  539 

in  the  blood  in  the  capillary  bed.  They  exert  an  osmotic  pressure  that  is 
responsible  for  the  return  of  water  from  the  tissue  fluids.  Hydrostatic 
pressure,  i.e.,  blood  pressure,  forces  the  water  out  of  the  capillaries  into 
the  tissue  fluid.  These  two  forces  normally  just  balance  and  keep  the 
blood  volume  constant. 

The  plasma  proteins,  together  with  the  hemoglobin  in  the  red 
blood  cells,  are  also  important  buffers.  A  buffer  is  a  mixture  of  a  weak 
acid  and  its  salt,  or  of  a  weak  base  and  its  salt.  A  buffer  tends  to  prevent 
a  change  in  the  pH  of  a  solution  when  an  acid  or  base  is  addded.  Com- 
plex animals  such  as  mammals  cannot  tolerate  wide  fluctuations  in  pH, 
and  the  pH  of  the  blood  is  held  remarkably  constant,  at  about  7.4. 
Buffers  combine  reversibly  with  the  hydrogen  ions  (H  +  )  released  by  the 
dissociation  of  acids  into  their  constituent  ions.  Acidic  substances  are 
constantly  produced  as  by-products  of  cell  metabolism  and  enter  the 
blood.  Carbon  dioxide,  for  example,  is  produced  in  cellular  respiration 
and  tends  to  increase  the  acidity  of  the  blood  for  it  combines  with  water 
to  form  carbonic  acid,  H0CO3.  Basic  substances,  which  release  hydroxyl 
ions  (OH^),  are  much  less  common  by-products  of  metabolism.  Buffers 
neutralize  their  effects  by  releasing  hydrogen  ions,  which  combine  with 
the  hydroxyl  ions  to  form  water  (H^O).  Eventually  the  acidic  or  basic 
substances  are  removed  from  the  body,  carbon  dioxide  by  the  lungs  and 
the  others  by  the  kidneys.  Inorganic  buffers  such  as  carbonic  acid- 
bicarbonate  are  present  in  the  blood,  but  the  blood  proteins,  especially 
hemoglobin,  are  extremely  important  and  abundant  buffers. 

229.       Red  Blood  Cells 

The  red  blood  cells,  or  erythrocytes,  are  the  most  numerous  of  the 
formed  elements  of  the  blood,  there  being  about  5,000,000  of  them  in 
each  cubic  millimeter  of  blood  in  an  adult  human.  Those  of  mam- 
mals lose  their  nuclei  as  they  develop,  and  mature  mammalian  red  cells 
are  biconcave  discs.  Such  a  shape  provides  more  surface  area  than  a 
sphere  of  equal  volume,  and  the  increased  surface  area  in  turn  facilitates 
the  passage  of  materials  through  the  plasma  membrane. 

Erythrocytes  contain  the  respiratory  pigment  hemoglobin,  which 
acts  as  a  buffer  and  is  essential  for  the  transport  of  oxygen  and  carbon 
dioxide.  As  we  explained  in  section  28,  hemoglobin  (Hb)  combines 
with  oxygen  in  the  capillaries  of  the  lungs,  where  the  oxygen  tension  is 
high,  to  form  oxyhemoglobin  (HbOo),  and  oxyhemoglobin  releases 
oxygen  in  the  tissue  capillaries,  where  the  oxygen  tension  is  low.  It  has 
been  estimated  that  we  would  need  a  volume  of  blood  35  times  as  great 
or  the  blood  would  have  to  circulate  35  times  as  fast  as  it  does  if  all  of 
the  oxygen  were  carried  in  physical  solution  instead  of  in  combination 
with  hemoglobin. 

Carbon  dioxide  diffuses  into  the  blood  from  the  tissues  of  the  body. 
Some  is  carried  in  physical  solution  in  the  plasma,  but  most  of  it  (about 
95  per  cent)  enters  the  erythrocytes.  Some  of  this  combines  with  certain 
amino  groups  on  the  hemoglobin  molecule  to  form  carbaminohemo- 
globin,  but  most  of  it  combines  with  water  to  form  carbonic  acid.  This 


540 


VERTEBRATE   LIFE   AND   ORGANIZATION 


reaction  occurs  much  more  rapidly  in  the  erythrocytes  than  in  the 
plasma  because  they  contain  the  enzyme  carbonic  anhydrase,  which 
speeds  up  the  reaction  1500  times.  Carbonic  acid  in  turn  dissociates  into 
hydrogen  and  bicarbonate  ions,  and  many  of  the  bicarbonate  ions  diffuse 
out  ot  the  erythrocytes  into  the  plasma.  A  great  deal  of  carbon  dioxide, 
then,  is  carried  as  bicarbonate  ions.  The  hydrogen  ions  combine  with 
oxyhemoglobin  and  this  facilitates  the  dissociation  of  oxyhemoglobin  and 
the  release  of  oxygen  to  the  tissues.  Oxyhemoglobin  is  a  stronger  acid  than 
hemoglobin  and  the  conversion  of  oxyhemoglobin  (HbOo)  to  hemoglobin 
(Hb)  would  tend  to  raise  the  pH  within  the  red  cell  (make  it  more 
alkaline).  The  formation  and  dissociation  of  carbonic  acid  would  tend 
to  lower  the  pH  within  the  red  cell  (make  it  more  acid).  These  two 
opposing  phenomena  tend  to  balance  each  other  and  the  pH  of  the 
erythrocyte  is  maintained  essentially  unchanged.  The  potassium  ions 
(K  +  )  previously  neutralized  by  the  oxyhemoglobin  are  now  neutralized 
by  the  bicarbonate  ions: 


CO2 


Tissues 


Plasma 


carbonic 


anhydrase 
H+  +  KHb02 


->  H2CO3 


^H+  +  HCO3 


HCOy  +  K+ 


->K+  +  HHb  +  O2 
>  KHCO3 


Red 


Cell 


In  the  lung  capillaries,  where  the  tension  of  carbon  dioxide  is  lower 
than  in  the  venous  blood,  the  reactions  described  above  move  in  the 
opposite  direction  and  carbon  dioxide  diffuses  out  of  the  blood.  Oxygen 
entering  the  blood  from  the  lungs  combines  with  hemoglobin  and  facili- 
tates its  giving  up  hydrogen  ions  and  hence  the  release  of  carbon  dioxide 
by  the  blood.  Tfiis  set  of  reactions  may  be  expressed  as  follows: 


Lungs 


Plasma 


KHCO3 

>02  +  HHb  +  K  + 


■^  K+  +  Hcor 


H+  +  HCOJ 


-^H2C03- 


^KHbOa  +  H  + 

carbonic 


anhydrase 


>  H2O  +  CO2 


Red 


Cell 


These  reactions  enable  the  blood  to  carry  a  great  deal  more  oxygen 
and   carbon   dioxide   than   it    could   in   simple    physical   solution,    they 


BLOOD   AND   CIRCULATION  541 

prevent  the  pH  of  the  blood  from  changing  greatly,  and  they  facilitate 
the  release  of  oxygen  in  the  tissues  of  the  body  and  the  release  of  carbon 
dioxide  in  the  lungs. 

Mature  mammalian  erythrocytes  have  lost  their  nuclei  and  do  not 
survive  indefinitely.  Experiments  which  involve  tagging  them  with 
radioactive  iron  show  that  they  have  a  life  span  of  about  127  days.  Red 
cells  are  eventually  destroyed  in  the  spleen  and  liver.  Cells  lining  the 
blood  spaces  of  the  spleen  and  liver  engulf  or  phagocytize  the  red  cells 
and  digest  them.  The  iron  of  the  hemoglobin  is  salvaged  by  the  liver  and 
is  reused,  but  the  rest  of  the  molecule  is  excreted  as  bile  pigment.  To 
replace  those  destroyed,  new  red  cells  are  constantly  produced  in  un- 
specialized  connective  tissues  whose  cells  retain  their  embryonic  poten- 
cies. The  kidney,  spleen  and  liver  of  lower  vertebrates  contain  tissue  of 
this  type.  These  sites  are  of  most  importance  during  the  embryonic 
development  of  mammals,  but  the  red  bone  marrow  is  the  primary 
source  of  erythrocytes  in  the  adult. 

Erythrocyte  destruction  and  production  are  surprisingly  rapid.  From 
the  total  number  of  red  cells  in  the  body  and  their  average  life  span, 
one  can  calculate  that  about  10,000,000  are  made  and  destroyed  each 
second  of  the  day  and  night.  If  the  rate  of  production  of  cells  or  of 
hemoglobin  decreases,  some  type  of  anemia  results.  Anemia  is  charac- 
terized by  a  decrease  in  the  number  of  red  cells  per  cubic  millimeter 
of  blood,  by  a  decrease  in  the  amount  of  hemoglobin  per  red  cell, 
or  both.  In  pernicious  anemia  the  number  of  erythrocytes  steadily 
decreases.  Eating  large  quantities  of  liver  increases  the  rate  of  red  cell 
formation,  for  liver  is  rich  in  vitamin  B^^  which  is  necessary  for  normal 
erythrocyte  development.  A  person  with  pernicious  anemia  cannot  ab- 
sorb enough  Bj.  even  though  the  requisite  amount  may  be  present  in 
the  diet,  for  the  lining  of  his  stomach  does  not  secrete  enough  "intrinsic 
factor,"  necessary  for  the  absorption  of  Bjo.  If  an  excess  is  made  available 
by  giving  foods  especially  rich  in  Bjo,  enough  can  be  absorbed. 

230.        Platelets  and  Blood  Clotting 

Platelets  are  non-nucleated  blobs  of  cytoplasm  that  bud  off  from 
giant  cells  in  the  bone  marrow.  They,  and  the  thrombocytes  of  lower 
vertebrates,  are  responsible  for  initiating  blood  clotting,  for  they  break 
down  at  the  site  of  injury  and  release  the  enzyme  thromboplastin.  This 
initiates  a  complex,  and  as  yet  incompletely  understood,  series  of  reac- 
tions that  leads  to  the  formation  of  a  blood  clot.  Apparently  a  plasma 
globulin  known  as  prothrombin,  in  the  presence  of  thromboplastin  and 
calcium  ions,  is  changed  into  thrombin.  Thrombin  in  turn  acts  as  an 
enzyme  and  mediates  the  change  of  the  soluble  protein  fibrinogen  into 
an  insoluble  one  known  as  fibrin.  Fibrin  forms  a  mesh  of  delicate  fibers 
that  entraps  the  blood  cells,  and  the  clot  forms.  Blood  plasma  without 
its  fibrin  is  known  as  serum,  and  of  course  will  not  clot.  Vitamin  K  does 
not  enter  into  this  series  of  reactions  directly,  but  is  essential  for  the 
production  of  prothrombin  in  the  liver. 

Clotting  rarely  occurs  within  blood  vessels,  since  the  process  must 


542  VERTEBRATE   LIFE   AND    ORGANIZATION 

be  triggered  by  the  breakdown  ol  platelets  on  exposure  to  rough  and 
injured  tissue.  A  clot  within  a  vessel  is  known  as  a  thrombus,  and  it 
can  be  very  serious  ii  it  plugs  a  vessel  that  supplies  a  vital  area.  In  the 
hereditary  disease  hemophilia  the  platelets  do  not  readily  break  down, 
clots  do  not  form  and  die  slightest  scratch  may  lead  to  fatal  bleeding. 
This  disease  attracted  special  attention  because  it  appeared  in  several 
different  European  royal  families  and  was  apparently  inherited  from 
Queen  Victoria  of  England. 

231.  White  Blood  Cells 

Five  types  of  white  blood  cells,  or  leukocytes,  can  be  recognized- 
lymphocytes,  monocytes,  neutrophils,  eosinophils  and  basophils  (Fig. 
3.14).  They  differ  in  the  size  and  shape  of  the  nucleus,  and  in  the 
amount  and  granulation  of  the  cytoplasm.  Collectively  they  are  not  as 
numerous  as  erythrocytes,  for  there  are  only  about  7000  per  cubic  milli- 
meter in  human  blood,  and  their  life  span  is  much  shorter.  They  are 
produced  in  the  lymph  nodes,  the  spleen  and  red  bone  marrow,  and 
live  from  one  to  four  days.  Although  they  are  passively  carried  by  the 
blood,  most  leukocytes  can  also  creep  about  by  sending  out  cytoplasmic 
processes  in  ameboid  fashion.  This  enables  them  to  squeeze  between 
the  cells  of  the  capillary  walls,  and  many  are  lost  from  the  body  by 
escaping  through  the  capillaries  in  the  lungs,  digestive  tract  and  kidneys. 

Their  primary  function  is  that  of  protecting  the  body  against  dis- 
ease organisms.  They  are  apparently  attracted  by  chemicals  released  by 
invading  bacteria,  move  to  the  site  of  the  injury,  and  phagocytize  the 
foreign  microorganisms.  Frequently,  the  leukocytes  are  themselves  de- 
stroyed, and  the  products  of  their  breakdown  contribute  to  the  forma- 
tion of  pus  in  an  infected  wound. 

232.  Immunity 

Leukocytes  also  protect  the  body  by  producing  substances  known  as 
antibodies,  which  can  neutralize  or  destroy  foreign  proteins  (antigens) 
that  may  enter  the  body.  Many  of  the  plasma  globulins  are  antibodies 
synthesized  by  leukocytes,  plasma  cells  or  the  liver.  Although  any  foreign 
protein  may  act  as  an  antigen,  the  antigens  with  which  we  are  perhaps 
most  familiar  are  the  microorganisms  that  cause  infectious  diseases. 
Viruses,  bacteria  and  the  toxins  that  they  produce  are  all  antigenic.  The 
body  responds  to  their  presence  by  forming  antibodies  which  combat 
the  antigens  in  one  of  several  ways.  The  antibodies  may  combine  with 
the  antigens  and  neutralize  them;  they  may  cause  the  invading  micro- 
organisms to  clump,  or  agglutinate,  thereby  effectively  preventing  a 
further  penetration  of  the  body;  they  may  attack  the  invading  micro- 
organisms and  cause  them  to  break  up  and  dissolve  (a  phenomenon 
known  as  lysis);  or  they  may  make  the  invaders  more  susceptible  to 
phagocytosis. 

The  antigen-antibody  reaction  is  generally  very  specific.  Antibodies 


BLOOD   AND   CIRCULATION  543 

that  have  developed  in  response  to  mumps  viruses,  for  example,  will  not 
combine  with  other  antigens.  It  is  believed  that  the  specific  configuration 
of  the  antigen  and  antibody  molecules  resembles  a  lock  and  key.  Only 
antibodies  that  have  developed  in  response  to  a  given  antigen  can  fit 
on  the  surface  of  the  antigen  and  react  with  it. 

The  production  of  antibodies  by  certain  of  the  body. cells  continues, 
perhaps  for  many  years,  after  the  patient  has  recovered.  If  a  subsequent 
invasion  of  the  same  type  of  antigen  occurs  during  this  period,  anti- 
bodies specific  for  it  will  already  be  present.  The  infected  person  does 
not  contract  the  disease  and  is  said  to  be  immune.  The  immunity  that 
is  acquired  as  a  result  of  having  once  had  mumps,  smallpox  and  certain 
other  infectious  diseases  lasts  a  very  long  time,  generally  for  life.  The 
immunity  to  certain  other  diseases  lasts  for  a  much  shorter  time,  and 
after  it  is  lost,  one  can  get  the  disease  again. 

One  need  not,  however,  get  sick  in  order  to  develop  an  immunity 
to  many  diseases.  During  the  late  eighteenth  century,  Edward  Jenner 
observed  that  milkmaids  and  others  who  handled  the  udders  of  cows 
infected  with  cowpox  never  got  smallpox.  In  1796,  he  took  a  bit  of  the 
material  from  the  pustules  of  an  infected  cow  and  scratched  it  into 
the  skin  of  a  person.  Individuals  so  treated  acquired  a  mild  disease  but 
thereafter  were  immune  to  smallpox.  Cowpox  is  caused  by  a  virus  known 
as  the  vaccinia  virus;  smallpox  by  a  different  but  related  one  known  as 
the  variola  virus.  Vaccinia  is  not  a  serious  disease  in  man,  but  it  is  similar 
enough  to  variola  so  that  antibodies  that  develop  in  response  to  it  are 
effective  in  combating  variola.  Jenner's  experiments  were  the  beginning 
of  the  vaccination  technique.  Since  then,  many  kinds  of  vaccination 
have  been  developed.  Usually  a  related  and  less  virulent  microorganism, 
which  could  serve  as  the  basis  of  a  vaccine,  is  not  available,  but  vaccines 
can  be  produced  by  taking  the  actual  disease  organisms,  rendering  them 
harmless  by  appropriate  treatment,  and  injecting  them.  Although  the 
organisms  are  incapable  of  causing  the  disease,  they  are  still  capable  of 
inducing  antibody  formation.  One  of  man's  most  recent  triumphs  over 
disease  has  been  the  development  by  this  method  of  a  vaccine  for  polio- 
myelitis. 

Immunities  may  be  natural,  be  actively  acquired,  or  be  passively 
acquired.  All  of  us  have  a  natural  immunity  to  certain  infectious  diseases 
that  affect  other  organisms.  Thus  the  virus  for  distemper,  which  is  often 
fatal  to  dogs,  has  no  effect  on  man.  It  is  probable  that  some  of  our 
naturally  occurring  plasma  proteins  react  with  these  invading  antigens 
before  they  can  cause  any  trouble.  Immunity  that  is  acquired  by  an 
exposure  to  the  antigen,  either  by  contracting  the  disease  or  by  vac- 
cination, is  said  to  be  active  immunity,  for  the  person  exposed  actively 
produces  the  antibodies.  A  passive  immunity  can  be  acquired  by  inject- 
ing serum  containing  antibodies  that  have  been  produced  by  another 
individual  or  organism  (antisera).  A  passive  immunity  lasts  for  only 
a  few  weeks,  so  injections  of  antisera  are  used  to  help  combat  antigens 
that  have  already  invaded  a  patient  rather  than  as  a  long-term  pre- 
ventive measure. 


544 


VERTEBRATE   LIFE   AND   ORGANIZATION 


233.       Blood  Groups 

When  the  practice  of  transfusing  blood  from  one  person  to  another 
was  begun,  it  was  found  that  the  transfusions  were  sometimes  successful, 
but  more  often  they  were  not  and  the  erythrocytes  in  the  blood  of  the 
recipient  would  clump  (agglutinate)  with  fatal  results.  Careful  analysis 
by  Landsteiner  at  the  begmning  of  this  century  showed  that  specific 
antigenic  proteins,  called  A  and  B,  might  be  present  within  the  erythro- 
cytes. These  antigens  are  called  agglutinogens  since  they  may  cause 
agglutination  of  the  red  cells.  Some  individuals  have  protein  A,  some 
B,  some  both  A  and  B,  and  some  neither.  Antibodies  (agglutinins) 
specific  for  these  agglutinogens,  and  designated  a  and  b,  may  be  present 
in   the  plasma.   If  an   individual   whose  plasma   contains   agglutinin   a 


Table 

5.     HUMAN  BLOOD  GROUPS 

BLOOD  GROUP 

AGGLUTINOGEN 
IN  ERYTHROCYTES 

AGGLUTININS 
IN  PLASMA 

O  (Universal  Donor) 

None 

a  and  b 

A 

A 

b 

B 

B 

a 

AB  (Universal  Recipient) 

AandB 

None 

should  receive  blood  from  another  whose  erythrocytes  contain  agglutino- 
gen A,  an  antigen-antibody  reaction  occurs,  and  the  erythrocytes  ag- 
glutinate. 

Four  main  groups  of  persons  can  be  recognized,  according  to  the 
presence  or  absence  of  these  agglutinogens  and  agglutinins  (Table  5). 
Blood  containing  a  certain  agglutinogen  does  not,  of  course,  contain 
the  agglutinin  specific  for  it.  It  it  did,  it  would  agglutinate  itself.  Trans- 
fusions between  members  of  the  same  group  are  perfectly  safe,  and 
transfusions  between  different  groups  are  also  safe  provided  that  the 
donor's  erythrocytes  do  not  contain  an  agglutinogen  that  will  react  with 
the  recipient's  agglutinins.  The  agglutinins  in  the  donor's  plasma  be- 
come so  diluted  in  the  recipient  that  they  have  no  effect  and  they  may 
be  disregarded  unless  an  unusually  large  transfusion  is  given.  Members 
of  Group  O,  who  have  neither  of  the  agglutinogens,  can  give  blood  to 
members  of  any  group  and  are  "universal  donors."  But  since  their 
plasma  contains  both  of  the  agglutinins,  they  can  receive  blood  only 
from  members  of  their  own  group.  Members  of  Group  AB,  in  contrast, 
have  neither  agglutinin,  and  can  receive  blood  from  members  of  any 
group.  Since  they  have  both  agglutinogens,  they  can  give  blood  only  to 
members  of  their  own  group.  They  are  "viniversal  recipients."  Members 
of  Group  A  and  B  can  give  blood  to  members  of  Group  AB  and  receive 
from  members  of  Group  O.  The  inheritance  of  these  blood  groups  is 
considered  in  section  282. 


BLOOD   AND    CIRCULATION  545 

234.        The  Rh  Factor 

A  number  of  other  inherited  antigenic  proteins  may  be  present  in 
the  blood.  Most  are  rare  and  not  apt  to  be  involved  in  transfusions,  but 
one  that  is  common  is  the  Rh  factor,  so  called  because  it  was  first  dis- 
covered in   the  rhesus  monkey.  About  87  per  cent  of  North  American 
whites  have  this  factor  in  their  red  cells  and  are  said  to  be  Rh  positive. 
The  remaining  13  per  cent  do  not  have  it,  hence  are  Rh  negative.  If  a 
mother  is  Rh  negative  and  the  father  Rh  positive,  the  fetus  may  inherit 
the  factor  from  the  father.  In  theory  none  of  the  fetal  blood  crosses  the 
placenta  to  enter  the  mother's  blood,  but  there  are  usually  small  breaks 
in  the  placenta  that  permit  some  mixing.  Rh  positive  blood  of  the  fetus, 
on  entering  the  mother,  induces  the  formation  of  antibodies.  This  is  a 
slow  process,  and  not  enough  are  likely  to  be  formed  to  cause  trouble 
in  the  first  pregnancy.   If  a  second  fetus  is  also  Rh  positive,  more  Rh 
positive  blood   enters  the  mother  and   more  antibodies  are   formed   in 
the  mother's  blood.  Some  of  these  get  back  into  the  Rh  positive  blood 
of  the  fetus   and  cause  agglutination   and  hemolysis  of  the   red   blood 
cells.  This  condition,  erythroblastosis  fetalis,  may  be  fatal,  or  may  result 
in  injury  to  the  brain  from  the  bile  pigment  (bilirubin)  formed  from 
the  hemoglobin  released  by  the  hemolysis  of  the  red  cells.  A  newborn 
infant  showing  symptoms  of  it  can  be  saved  by  extensive  transfusions. 
Ordinarily  not  enough  Rh   positive   blood  enters  the  mother  to  cause 
any  harm,  but  her  blood  contains  the  antibodies,  and  if  she  subsequently 
needs  a  transfusion  for  any  reason,  Rh  negative  blood  must  be  used. 


235.        Patterns  of  Circulation 

Heart,  arteries,  capillaries  and  veins  constitute  the  cardiovascular 
system;  the  lymphatic  vessels  and  nodes  comprise  the  lymphatic  system. 
Most  vertebrates  have  both,  but  primitive  vertebrates  such  as  cyclostomes 
and  cartilaginous  fishes  have  no  lymphatic  system.  These  groups  have 
a  lower  blood  pressure  than  other  vertebrates,  and  their  veins  provide 
adequate  drainage  for  the  tissues.  A  lymphatic  system  apparently  evolved 
as  blood  pressures  became  higher,  and  the  veins  could  no  longer  drain  a 
sufficient  amount  of  liquid  from  the  tissues.  Lymphatic  vessels  arise  as 
outgrowths  from  the  veins,  and,  in  general,  they  tend  to  parallel  the 
veins,  and  ultimately  empty  into  them.  Most  are  inconspicuous  and  are 
seen  only  in  special  preparations. 

Primitive  Fishes.  The  cardiovascular  system  has  undergone  some 
striking  changes  during  the  evolution  of  vertebrates.  Most  of  these  are 
correlated  with  the  shift  in  the  site  of  external  respiration  that  occurred 
during  the  transition  from  water  to  land,  and  with  the  development 
of  the  efficient,  high  pressure  circulatory  system  necessary  for  an  active 
terrestrial  vertebrate. 

In  a  primitive,  lungless  fish  (Fig.  27.2),  all  of  the  blood  entering 
the  heart  from  the  veins  has  a  low  oxygen  and  a  high  carbon  dioxide 
content,  i.e.,  it  is  venous  blood.  The  heart  consists  of  a  series  of  chambers 
(a  sinus  venosus,  a  single  atrium,  a  single  ventricle  and  a  conus  arterio- 


546 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Anterior 
cardinaJ  vaini 


Ce-phal 
capi" ' 


Dorsal 
r  aorta. 


•Posberior  cardinal  vein. 
Coelia-c  a-rberj/ 

■Renal  artery 


■Renal  portal 
vein 


Gill 


capillar! 


-Ventral 
aorta. 


Chambers  of  the  heart 


a.rter5f 
Liver  capillaries 


portal 
vein. 


Tail 
capillaries" 


Figure  27.2.  The  major  parts  oi  the  cardiovascular  system  of  a  primitive  fish. 
1,  sinus  vcnosus;  2,  atrium;  3,  ventricle;  4,  conns  arteriosus  of  the  heart.  The  aortic 
arches  are  numbered  with  Roman  numerals.  Only  traces  of  the  first  aortic  arch  remain 
in  the  adults  of  most  fishes. 

sus)  arranged  in  linear  sequence.  The  heart  increases  the  blood  pressure, 
which  is  very  low  in  the  veins,  and  sends  the  blood  out  through  an 
artery,  the  ventral  aorta,  to  five  or  six  pairs  ot  aortic  arches  that  extend 
dorsally  through  capillaries  in  the  gills  to  the  dorsal  aorta.  Carbon 
dioxide  is  removed  and  oxygen  is  added  as  the  blood  flows  through 
the  gills,  i.e.,  it  changes  to  arterial  blood.  The  dorsal  aorta  distributes 
this  through  its  various  branches  to  all  parts  of  the  body. 

Blood  pressure  decreases  as  blood  flows  along  because  of  the  friction 
between  the  blood  and  the  lining  of  the  vessels.  Blood  pressure  is 
reduced  considerably  as  the  blood  passes  through  the  capillaries  of  the 
gills,  for  friction  is  greatest  in  vessels  of  small  diameter.  The  mean 
blood  pressure  in  the  ventral  aorta  of  a  dogfish,  for  example,  is  28 
mm.  fig;  that  in  the  dorsal  aorta  is  15  mm.  Hg.  Thus  the  IdIoocI  dis- 
tributed by  the  dorsal  aorta  is  under  relatively  low  pressure,  and  this 
will  be  much  lower  by  the  time  it  reaches  the  capillaries  in  the  tissues. 
Circulation  in  primitive  fishes  is  rather  sluggish,  and  not  conducive  to 
great  activity. 

Veins  drain  the  capillaries  of  the  body  (where  blood  pressure  is 
further  reduced)  and  lead  to  the  heart,  but  not  all  veins  go  directly 
to  the  heart.  In  primitive  fish,  blood  returning  from  the  tail  first  passes 
through  capillaries  in  the  kidneys  before  entering  veins  leading  to  the 
heart.  Veins  that  drain  one  capillary  bed  and  lead  to  another  are  called 
portal  veins,  and  these  particular  veins  are  known  as  the  renal  portal 
system.  Another  group,  known  as  the  hepatic  portal  system,  drain  the 
digestive  tract  and  lead  to  capillaries  in  the  liver.  Since  much  of 
the  blood  returning  to  the  heart  has  passed  through  one  or  the  other 
of  these  portal  systems  in  addition  to  the  capillaries  in  the  gills  and 
tissues,  its  pressure  is  quite  low. 

It  is  not  difficult  to  appreciate  the  significance  of  an  hepatic  portal 
system,  since  the  liver  plays  such  an  important  role  in  the  metabolism 
of  foods,  but  the  adaptive  significance  of  a  renal  portal  system  in  primi- 
tive  vertebrates  is  less  clear.  One   might  postulate   that   it  ensures  an 


BLOOD    AND   CIRCULATION 


547 


adequate   blood   supply    to    the   kidneys,   for   the   low    pressure   arterial 
system  alone  might  not  deliver  enough  blood  to  these  vital  organs. 

Primitive  Tetrapods.  When  the  shift  was  made  from  gills  to  lungs, 
many  changes  occurred  in  the  heart  and  aortic  arches  (Fig.  27.3).  The 
aortic  arches  were  reduced  in  number,  the  first  two  and  the  fifth  being 
lost.  Those  that  remain  are  no  longer  interrupted  by  gill  capillaries.  In 
a  primitive  tetrapod,  such  as  the  frog,  the  third  pair  of  aortic  arches 
forms  part  of  the  internal  carotid  arteries  supplying  the  head;  the  fourth, 
the  aortic  arches  proper  leading  to  the  dorsal  aorta;  and  the  sixth,  the 


Lefb 

Internal  ca-Trotid: 


Dorsal  a.orta.- 


Atriuni 


Common, 
cardinal- 


ET    Carotid.  arcVi 


Conus  a-rteriosaS 


Veintricle 

Subcla.'via.n 

artery 

Rioht  a-trium. 
Sinus  venosus 


Ve-nae  ca.va-e 
Hepa±ic  vein. 


External  carotid 


Aortic  a-rch. 

Trun-Cus 
arteriosus 

Pulmocutaxieous 
a-rcK 


"Left  cLtrium 
Pulmonary  vein 


DorSa-1  n  nrta. 


Sinus 


venosus 


PRI MITIVE  FISH  AMPHIBIAN  (Fro^) 

-  External  CcLTotid.- 
Internal  Carotid.- 


Common,  cajrotid. 
—  Su.bcIa3/iajrL- 

Pulmonary  artery 
Forrner  conus 


ormer  sinu.s  \ 

venosus 


Venae  ca.vajz. 


Venaz  cavajs 
Pulmonary  vein, 
rsal  a_o-rta- 


Subclavian 
Arch  of  aorta 

Embryonic 

ductus 

ai"teriosu.S 


Pulmonary  vzin. 
Dorsal  aorta. 


REPTILE  MAMMAL 

Figure  27.3.  Diagrams  of  the  heart  and  aortic  arches  to  show  the  changes  that 
occurred  in  tlie  evolution  from  primitive  fishes  to  mammals.  All  are  ventral  views. 
The  heart  tube  has  been  straightened  so  that  -the  atrium  lies  posterior  to  the 
ventricle. 


548 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Deep  cepVia.lic 

Ca.pilla.ries 

Internal  Ccixotid 
Iiitei-nalju^wli 
Subcla-via-Ti  aj-tery 
Anbcrior  veiiacava. 

Artn 
ca.pillaries 


Supe,rf  iciaJ. 
ccphaJic  ca.pillaji'ics 

E^cternal  Ca-rotid 

External  j  ugular  V. 

Com.m.on  carotid 
Brachiocephalic  veia 

Palnionarv/  a.rtery 
PuliTLonai-y  vein. 

Lun^ 
capillaries 


E^.-temal  ilac 
a.rtertj  arid  vein. 


Posterior  vena 
cava. 

Hepatic  veiiT- 
Hepa-tic  portal  vein- 
Liver  capillai'ies 

Aiiterior 
mesenteric  ai-tery 

Intestine., 

CapiUa.i-ics 
Fbstei-i-oi- 
mesenteric 
a.rtery  


'  Dorsal 
a-orta. 
CeJiajC  a-rtcry 
Spleen  capillaries 

Ston\acH 

capillaries 
Rcual  aj'teri/  and 
vein.. 

Kidney 

capillaries 

-  Co mrTLoru  i li ac 
artery  and  vein 

Internal  iliaC 
ai'tery  and  vein 
Pelvic 
capillaries 


Lzg  capillaries 


Figure  27.4.     The   major  parts   of   the   cardiovascular  system   of  man   as   seen   in 
an  anterior  view. 

pulmocutaneous  arches  leading  to  the  lungs  and  skin.  New  veins,  the 
pulmonary  veins,  return  aerated  blood  from  the  lungs  to  the  heart. 
The  heart  now  receives  blood  Irom  both  the  body  and  lungs.  Though 
blood  streams  from  the  body  and  lungs  are  separated  in  the  frog  by  a 
divided  atrium,  they  can,  and  probably  do,  mix  to  a  considerable  extent 
in  the  single  ventricle.  Ihis  mixing  is  not  detrimental  to  amphibians 
for  some  of  the  blood  from  the  body  is  returning  from  the  skin  where 
it  has  been  aerated. 

These  changes  result  in  a  much  higher  blood  pressure  in  the  arteries 


BLOOD   AND    CIRCULATION  549 

of  a  primitive  tetrapod  than  in  a  fish.  Blood  in  tlie  dorsal  aorta  of  a 
frog  has  a  mean  pressure  of  30  mm.  Hg.,  twice  that  of  the  dogfish.  This 
makes  for  a  greater  efficiency  of  circulation,  but  this  benefit  is  somewhat 
offset  by  the  fact  that  the  blood  delivered  to  the  tissues  is  mixed  to  some 
extent,  and  does  not  contain  relatively  as  much  oxygen  as  it  did  in  a  fish. 

Mammals.  Higher  tetrapods  depend  upon  their  lungs  for  external 
respiration.  Since  no  respiration  occurs  in  the  skin,  there  is  no  mixing 
of  aerated  blood  from  the  skin  with  blood  from  the  body.  The  mixing  in 
the  heart  of  arterial  blood  from  the  lungs  with  venous  blood  from  the 
body  is  lessened  in  reptiles  by  a  partial  division  of  the  ventricle  and  by  a 
complex,  tripartite  division  of  the  conus  (Fig.  27.3).  In  birds  and  mam- 
mals, there  is  no  mixing  at  all,  for  the  ventricle  is  completely  divided. 
Venous  blood  from  the  body  enters  the  right  atrium,  into  which  the 
primitive  sinus  venosus  has  become  incorporated.  Arterial  blood  from 
the  lungs  enters  the  left  atrium.  The  atria  pass  the  blood  on  to  the 
right  and  left  venJricles  resjjectively.  The  ventricles  have  more  muscular 
walls  than  in  lower  vertebrates,  and  so  can  increase  the  blood  pressure 
considerably.  The  primitive  conus  arteriosus  has  become  completely 
divided,  part  contributing  to  the  pulmonary  artery  leading  from  the 
right  ventricle  to  the  lungs  and  the  rest  to  the  arch  of  the  aorta  leading 
from  the  left  ventricle  to  the  body. 

The  sixth  pair  of  aortic  arches  form  the  major  part  of  the  mam- 
malian pulmonary  arteries,  and  the  third  pair  contribute  to  the  internal 
carotid  arteries.  But  it  will  be  observed  in  Figure  27.3  that  only  the  left 
side  of  the  fourth  arch,  known  as  the  arch  of  the  aorta,  leads  to  the 
dorsal  aorta.  The  right  fourth  arch  contributes  to  the  right  subclavian 
artery  to  the  shoulder  and  arm,  but  does  not  connect  with  the  aorta.  In 
])irds  it  is  the  right  fourth  arch  that  leads  to  the  dorsal  aorta  and  the 
left  fourth  arch  contributes  to  the  left  subclavian  artery. 

The  major  change  in  the  veins  is  the  complete  loss  of  a  renal  portal 
system.  Blood  from  the  tail  and  posterior  appendages  enters  a  posterior 
vena  cava,  which  continues  forward  to  the  heart.  It  receives  blood  from 
the  kidneys  but  does  not  carry  blood  to  them.  An  anterior  vena  cava 
drains  the  head  and  arms.  The  hepatic  portal  system  is  still  present.  The 
pattern  of  the  major  arteries  and  veins  of  man  is  shown  in  Figure  27.4. 

These  evolutionary  changes  have  resulted  in  a  very  efficient  mam- 
malian circulatory  systern.  Mammals  have  relatively  more  blood  than 
lower  vertebrates,  it  is  distributed  under  gieater  pressure,  and  there  is  no 
mixing  of  arterial  and  venous  blood.  Man,  for  example,  has  7.6  ml.  of 
blood  per  100  gm.  of  body  weight  compared  with  2  ml.  per  100  gm.  in  a 
fish.  The  mean  pressure  in  the  dorsal  aorta  of  man  is  about  100  mm.  of 
mercury. 

236.        The  Fetal  Circulation 

The  placenta  of  the  mammalian  fetus,  rather  than  the  digestive 
tract,  lungs  and  kidneys,  is  the  site  for  exchange  of  materials.  This, 
together  with  the  fact  that  the  vessels  in  the  lungs  of  the  fetus  are  not 
developed  enough  to  handle  the  total  volume  of  blood  that  is  circulating 


550 


VERTEBRATE   LIFE   AND   ORGANIZATION 


1  Arm. 
Duclrus  a.rteriosuS 

PulTnonai'y  cLrlery 

Pulmonajry  vein. 
Fors-merL  ovale 


DorsaJ  aorta. 
^    Kidney 


I —  Posterior  vena-Ca-v^a. 


ommon  iha.c 


a.rtery 


Umbil  ic  aJ  ar  tc  ly 


Figure  27.5.  Circulation  in  a  fetal  mammal.  The  shading  gives  some  indication 
of  the  mixing  of  the  blood,  though  there  is  more  mixing  than  can  be  indicated 
diagrannnatically.  The  lightest  shading  represents  blood  with  the  highest  oxygen 
content;  the  darkest  shading,  blood  with  the  lowest  oxygen  content.  (Modified  after 
Patten.) 

through  the  body,  requires  certain  differences  in  the  fetal  circulation 
(Fig.  27.5).  Blood  rich  in  oxygen  returns  from  the  placenta  in  an  um- 
bilical vein  that  enters  the  posterior  vena  cava,  where  it  is  mixed  with 
blood  returning  from  the  posterior  half  of  the  fetus.  The  posterior  vena 
cava  empties  into  the  right  atrium,  which  also  receives  venous  blood 
from  the  head  by  way  of  the  anterior  vena  cava. 

The  lungs  cannot  handle  all  of  this  blood  and  are  largely  by-passed 
in  one  of  two  ways.  The  entrance  of  the  posterior  vena  cava  is  directed 
toward  an  oj^ening,  the  foramen  ovale,  in  the  partition  separating  the 
two  atria.  Most  of  the  blood  from  the  posterior  vena  cava  tends  to  go 
through  this  into  the  left  atrium,  thence  to  the  left  ventricle  and  out 
to  the  body  through  the  arch  of  the  aorta.  The  rest  of  the  blood  from 
the  posterior  vena  cava  enters  the  right  ventricle  along  with  the  blood 
from  the  anterior  vena  cava,  and  starts  out  the  pulmonary  artery  toward 
the  lungs.  However,  only  a  fraction  of  this  blood  passes  through  the 


BtOOD   AND   CIRCULATION  55  } 

lungs  to  return  to  the  left  atrium  and  mix  with  blood  from  the  posterior 
vena  cava.  Most  of  the  blood  in  the  pulmonary  artery  goes  through 
another  by-pass,  the  ductus  arteriosus,  to  the  dorsal  aorta.  The  ductus 
arteriosus  represents  the  dorsal  part  of  the  left  sixth  aortic  arch  (Fig. 
27.3).  Since  the  ductus  arteriosus  enters  the  aorta  after  the  arteries  to  the 
head  have  been  given  off,  the  head  receives  the  blood  with  the  highest 
oxygen  content.  After  the  entrance  of  the  ductus  arteriosus,  the  blood 
in  the  aorta  is  highly  mixed.  This  is  the  blood  that  is  distributed  to 
the  rest  of  the  body  and,  by  way  of  umbilical  arteries,  to  the  placenta. 

As  the  lungs  develop  during  fetal  life,  more  and  more  blood  is  sent 
through  their  capillary  bed,  because  the  foramen  ovale  becomes  rela- 
tively smaller  and  less  blood  by-passes  the  lungs  via  this  route.  The 
return  of  blood  from  the  lungs  to  the  left  atrium  is  consequently  grad- 
ually increased,  which  increases  the  blood  pressure  in  the  left  atrium. 
The  increased  pressure  in  the  left  atrium  keeps  the  flap  guarding  the 
foramen  ovale  closed  a  greater  fraction  of  the  time  and  decreases 
the  amount  of  blood  entering  from  the  right  atrium.  These  changes 
insure  a  normal  development  of  the  pulmonary  circulation  and  make 
the  transition  from  the  fetal  to  the  adult  pattern  less  abrupt.  At  birth,  the 
placenta  is  expelled,  carbon  dioxide  accumulates  in  the  blood  and  stim- 
ulates the  respiratory  center.  Concurrently,  the  ductus  arteriosus  con- 
tracts. More  blood  goes  through  the  now  functioning  lungs,  pressure 
increases  further  in  the  left  atrium,  and  the  flap  in  the  foramen  ovale 
is  held  shut.  The  adult  pattern  is  now  established.  As  time  goes  on,  the 
flap  in  the  foramen  ovale  grows  against  the  interatrial  wall,  the  lumen 
of  the  ductus  arteriosus  is  occluded  by  the  rapid  proliferation  of  its 
lining  cells,  and  most  portions  of  the  umbilical  vessels  within  the  infant 
atrophy.  The  failure  of  any  of  these  changes  to  occur  at  birth  results 
in  poor  oxygenation  of  the  blood,  producing  a  condition  known  as 
"blue  baby." 

237.        Flow  of  Blood  and  Lymph 

The  Heart.  The  heart  (Fig.  27.6)  is  the  pump  that  builds  up  the 
pressure  gradient  necessary  for  the  blood  and  lymph  to  flow.  It  lies 
within  a  division  of  the  coelom,  the  pericardial  cavity,  which  contains 
some  tissue  fluid  that  lubricates  it  and  facilitates  its  movements.  It  is 
covered  with  a  smooth  coelomic  epithelium,  the  visceral  pericardium, 
and  is  lined  by  the  simple  squamous  epithelium,  the  endothelium,  which 
lines  all  parts  of  the  circulatory  system.  The  rest  of  its  wall  is  composed 
of  cardiac  muscle,  which  is  unique  in  that  its  fibers  branch  and  anas- 
tomose profusely  without  cell  membranes  at  their  ends  (Fig.  3.13).  The 
musculature  of  the  atria  is  separate  from  that  of  the  ventricles,  but  each 
may  be  regarded  as  a  syncytium,  that  is,  a  single  multinucleated  cell. 
Each  responds  as  a  unit.  Any  stimulus  that  is  strong  enough  to  elicit  a 
response  will  elicit  a  total  response.  Thus  the  atria  and  ventricles  follow 
the  "all-or-none"  law  that  applies  to  individual  motor  units  of  skeletal 
muscle. 

During  a  heart  cycle,  the  atria  and  ventricles  contract  and  relax  in 


552 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Arch,  of 


-AcLult  rcTnn.a.rLt  of 
embryonic  ductus 


An^  (trior 
ve-na  cavaL" 


Semilunar  valve, 
Rioht  atrium 


CoronsLry 

Posterior 
ve-na.  ca-va. 

Tricuspidv 


mona-ry  vein 
a.triu.m. 
cuspid,  va-lve 
mi  lunar  va-lve. 
inous  cord 

ventricle 
art  muscle 


Sirio -a.tr  ia.1 
n.ode/ 


Atrioventricular 
node 


^— At  riove  n.t  rlcalaj;' 
bundle 


Figure  27.6.  The  adult  mammalian  heart.  Upper,  Course  of  blood  through  the 
heart;  lower,  distribution  of  the  specialized  cardiac  muscle  that  forms  the  conducting 
system  of  the  heart. 

succession.  Contraction  of  these  chambers  is  known  as  systole;  relaxation, 
as  diastole.  Ventricular  systole  is  very  powerful  and  drives  the  blood 
out  into  the  pulmonary  artery  and  arch  of  the  aorta  under  high  pres- 
sure. Since  the  muscle  fibers  of  the  ventricles  are  arranged  in  a  spiral, 
the  blood  is  not  just  pushed  out,  but  is  virtually  wrinig  out  of  them. 
When  the  ventricles  relax,  their  elastic  recoil  reduces  the  pressure 
within  them  and  blood  enters  from  the  atria.  Atrial  contraction  does 
not  occur  until  the  ventricles  are  nearly  filled  with  blood;  indeed,  the 
filling  of  the  ventricles  is  nearly  normal  in  cases  where  disease  has 
destroyed  the  ability  of  the  atria  to  contract.  The  atria  are  primarily 
antechambers  that  accumulate  blood  during  ventricular  systole. 

Blood  being  pumped  by  the  heart  is  prevented  from  moving  back- 


BLOOD   AND   CIRCULATION  553 

ward  by  the  closure  of  a  system  of  valves.  One  with  three  cusps,  known 
as  the  tricuspid  valve,  lies  between  the  right  atrium  and  ventricle;  one 
with  two  cusps,  the  bicuspid  valve,  between  the  left  chambers.  These 
valves  operate  automatically  as  pressures  change,  opening  when  atrial 
pressure  is  greater  than  ventricular,  closing  when  ventricular  pressure 
is  greater.  Tendinous  cords  extend  from  the  free  margins  of  the  cusps  to 
the  ventricular  wall,  and  prevent  them  from  turning  into  the  atria 
during  the  powerful  ventricular  contractions.  When  the  ventricles  relax, 
blood  in  the  pulmonary  artery  and  aorta,  which  is  under  pressure,  tends 
to  back  up  into  them.  This  closes  the  pocket-shaped  semilunar  valves  at 
the  base  of  these  vessels  which  prevent  blood  from  returning  to  the 
ventricles.  Abnormalities  in  the  structure  of  the  valves  produced  con- 
genitally  or  by  disease  organisms  may  prevent  their  closing  properly. 
Blood  then  leaks  back  during  diastole;  the  leaking  blood  is  heard  as  a 
heart  "murmur." 

Cardiac  muscle  has  an  inherent  capacity  for  beating,  and  the  hearts 
of  vertebrates,  if  properly  cultured,  will  continue  to  beat  rhythmically 
when  excised  from  the  body.  Each  contraction  is  initiated  in  the  sino- 
atrial node,  or  "pacemaker"-a  node  of  specialized  cardiac  muscle 
(Purkinje  fibers)  located  in  that  part  of  the  wall  of  the  right  atrium  into 
which  the  primitive  sinus  venosus  is  incorporated.  1  he  impulse  spreads 
through  a  network  of  Purkinje  fibers  to  all  parts  of  the  atria,  and  to  an 
atrioventricular  node  from  which  the  impulse  continues  to  all  parts  of 
the  ventricles.  The  factors  that  stimulate  the  sinoatrial  node  and  cause 
it  to  send  out  impulses  to  other  parts  of  the  heart  are  not  completely 
understood.  Apparently  the  leakage  of  positively  charged  sodium  ions, 
which  are  abundant  outside  the  cells,  through  the  plasma  membrane 
and  into  the  cells  of  the  node,  and  a  temporary  reversal  of  the  electrical 
polarity  of  their  membranes,  is  involved.  A  similar  phenomenon  occurs, 
as  we  shall  see  later,  in  the  initiation  and  transmission  of  the  nerve 
impulse.  The  sinoatrial  node  has  a  shorter  refractory  period  than  other 
cardiac  muscle;  thus  it  recovers  more  rapidly  after  each  beat  and  is 
ready  to  act  again  before  the  rest  of  the  heart  has  recovered. 

Though  the  heart  has  an  inherent  rhythm,  its  rate  of  contraction 
and  the  volume  of  blood  pumped  per  stroke  can  be  regulated  by  a 
number  of  extrinsic  factors  so  as  to  adjust  the  heart  output  to  body 
requirements.  Nervous  pathways  are  present  for  many  cardiac  reflexes. 
Motor  nerves  that  increase  or  decrease  the  heart  rate  go  to  the  heart 
from  centers  in  the  brain,  and  sensory  impulses  from  many  parts  of  the 
body  reach  these  centers.  For  example,  sensory  fibers  in  the  right  atrium 
are  stimulated  by  the  increase  in  the  pressure  of  the  venous  blood  re- 
turning to  the  heart  which  occurs  during  exercise.  They  initiate  a  reflex 
that  increases  the  heart  rate.  If  the  arterial  pressure  becomes  too  high, 
sensory  fibers  from  the  arch  of  the  aorta  reflexly  reduce  the  heart  rate. 
The  increased  pressure  and  more  rapid  return  of  venous  blood 
during  exercise  stretches  the  heart  musculature.  This  causes  it  to  contract 
with  greater  force,  and  to  send  out  the  greater  volume  of  blood  received 
during  each  period  of  atrial  diastole.  Within  physiologic  limits,  the 
greater  the  tension  on  cardiac  (or  any  other)  muscle,  the  more  powerful 


554  VERTEBRATE   LIFE   AND   ORGANIZATION 

will  be  its  contraction.  This  capacity  of  the  heart  to  adjust  its  output 
per  stroke  to  the  volume  of  blood  delivered  to  it  is  known  as  Starling's 

"law  of  the  heart." 

The  heart  of  a  normal  adult  man,  who  is  not  exercising,  sends 
about  70  ml.  of  blood  per  beat  out  into  the  aorta.  At  the  normal  rate 
of  72  beats  per  minute,  this  is  a  total  output  of  5  liters  per  minute, 
which  is  approximately  equivalent  to  the  total  amount  of  blood  in  the 
body.  A  similar  observation  made  in  1628  by  William  Harvey  helped 
to  lead  him  to  the  conclusion  that  the  blood  recirculates.  Until  that 
time  it  was  believed  that  blood  was  continually  produced  in  the  liver, 
pumped  to  the  tissues,  and  consumed.  Harvey's  calculations  showed 
that  the  amount  of  blood  pumped  by  the  heart  each  hour  was  much 
more  than  could  possibly  be  produced  and  consumed.  He  made  the  cor- 
rect inference  that  the  blood  must  recirculate,  even  though  he  could  not 
see  the  microscopic  capillaries  that  connect  arteries  and  veuis. 

Although  a  large  volume  of  blood  flows  through  the  cavities  of  the 
heart,  this  blood  does  not  provide  for  the  metabolic  needs  of  the  heart 
musculature.  A  pair  of  coronary  arteries  arise  from  the  base  of  the  arch 
of  the  aorta  and  supply  capillaries  in  the  heart  wall.  This  capillary  bed 
is  drained  ultimately  by  a  coronary  vein  that  empties  into  the  right 
atrium.  Obviously  any  damage  to  the  coronary  vessels,  the  plugging  of 
one  of  the  larger  arteries  by  a  thrombus,  for  example,  could  have  serious 
consequences,  for  the  heart  muscles  cannot  function  without  a  continu- 
ing supply  of  oxygen  and  food. 

The  Arteries.  Arteries  are  lined  with  endothelium  and  have  a  rela- 
tively thick  wall  containing  elastic  connective  tissue  and  smooth  muscles. 
The  walls  of  the  larger  arteries  are  richly  supplied  with  elastic  tis- 
sue. The  force  of  each  ventricular  systole  forces  blood  into  the  arteries 
and  stretches  them  to  accommodate  it.  During  diastole,  the  elastic  recoil 
of  the  arterial  walls  keeps  the  blood  moving,  ff  they  were  rigid  pipes, 
the  arteries  would  deliver  blood  to  the  tissues  in  spurts  that  coincided 
with  ventricular  systole.  The  blood  would  pound  like  steam  rushing 
into  empty  radiator  pipes.  The  elasticity  of  the  large  arteries  transforms 
what  would  otherwise  be  an  intermittent  ffow  into  a  steady  flow. 

The  smaller  arteries,  and  especially  the  arterioles  preceding  the 
capillaries,  contain  a  relatively  large  amount  of  smooth  muscle,  and 
they  are  concerned  with  regulating  the  supply  of  blood  to  the  various 
organs.  Vasodilator  and  vasoconstrictor  nerves  supply  these  muscles, 
causing  them  to  relax  or  contract.  If  a  region  of  the  body  becomes  very 
active,  its  small  arteries  enlarge  and  the  blood  flow  through  them  is 
increased.  If  an  area  is  not  particularly  active,  its  small  arteries  constrict 
and  blood  flow  is  reduced.  In  this  way  a  maximum  use  is  made  of  the 
volume  of  blood  available. 

As  the  arteries  extend  to  the  tissues,  they  branch  and  rebranch. 
Each  time  their  lumen  becomes  smaller  but  the  total  cross  sectional  area 
of  all  of  the  branches  increases  greatly.  The  velocity  of  blood  flow,  there- 
fore, decreases,  for  the  blood,  like  a  river  widening  out  and  flowing  into 
a  lake,  is  moving  into  an  area  that  grows  larger  and  larger.  The  mean 
blood  pressure  is  also  decreased  continually  because  of  the  friction  of 


BLOOD   AND    CIRCULATION 


555 


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Figure  27.7.  Variation  in  blood  velocity  and  pressure  in  different  parts  of  the 
cardio\asciilar  system.  The  velocity  does  not  return  to  its  original  value  for  the 
cross-sectional  area  of  the  veins  is  greater  than  the  cross-sectional  area  of  the  arteries. 
The  blood  pressure  in  the  veins  near  the  heart  is  less  than  atmospheric  pressure 
because  of  the  negative  pressure  within  the  thorax. 

the  blood  moving  in  the  vessels  (Fig.  27.7).  Blood  pressure  continues  to 
decrease  as  the  blood  flows  through  the  capillaries  and  veins.  The  rate 
of  flow,  however,  increases  as  the  blood  passes  from  the  capillaries  to 
the  venules,  and  these  smaller  veins  lead  into  fewer  larger  ones.  The 
blood  is  now  moving  into  a  smaller  and  smaller  area,  and,  like  water 
flowing  out  of  a  lake  into  a  narrowing  river,  moves  faster  and  faster. 

Capillary  Exchange.  Capillaries  are  small  and  exceedingly  thin- 
walled  vessels.  Their  diameter  is  about  that  of  the  blood  cells,  and  their 
walls  consist  of  little  more  than  an  endothelial  lining,  which  is  con- 
tinuous with  that  of  the  larger  vessels.  The  capillary  wall  is  a  semi- 
permeable membrane,  and  molecules  that  are  small  enough  can  easily 
pass  back  and  forth  between  the  blood  and  the  surrounding  tissue  fluid 
(Fig.  27.8).  Most  substances  are  exchanged  by  simple  diffusion  following 
concentration  gradients.  There  is  more  glucose  and  oxygen  in  the  blood 
than  in  the  tissue  fluid,  so  their  net  movement  is  out  of  the  capillaries. 
There  are  more  wastes  and  carbon  dioxide  in  the  tissue  fluid,  so  their 
net  movement  is  into  the  capillaries. 

The  exchange  of  water  is  more  complicated  than  the  exchange  of 
solutes,  for  its  movement  depends  upon  two  opposing  forces.  The  blood 
pressure  tends  to  force  water  out  of  the  capillaries,  whereas  the  osmotic 
pressure  exerted  by  the  plasma  protein  molecules  tends  to  draw  water 


556  VERTEBRATE   LIFE   AND    ORGANIZATION 

Tissue  fluid \         .-— -5rrr~->.^^'^ Capillary 


Lymph 
capillary —  ■^ 


Erythrocyte 
Cell 


eus  oF 
(Z.ndotheIial  cell 


Arteriole 


Venule 


eukocyte 

—  Bacterium. 
(bein6  ingested  by  leuKocyte) 

Figure  27.8.  Exchange  of  materials  in  a  capillary  bed.  Solutes  enter  and  leave 
all  parts  of  a  capillary.  Most  of  the  water  leaves  at  the  arterial  end  and  reenters  at 
the  venous  end.  Less  than  one  per  cent  of  the  water  that  leaves  the  capillaries  is 
returned  by  the  lymphatic  system. 

back  in.  The  osmotic  pressure  remains  constant  from  the  arterial  to  the 
venous  ends  of  the  capillary  bed,  but  blood  pressure  continues  to  de- 
crease. At  the  arterial  end  of  the  capillary  bed,  blood  pressure  is  greater 
than  osmotic  pressure  and  water  is  driven  out  of  the  capillaries.  At  the 
venous  end,  osmotic  pressure  is  greater  than  blood  pressure,  and  water 
is  drawn  back  into  the  capillaries.  Any  residual  liquid  is  drained  by  the 
lymphatics. 

Venous  and  Lymphatic  Return.  The  structure  of  the  veins  is  funda- 
mentally the  same  as  that  of  arteries,  though  a  vein  is  larger  and  has  a 
much  thinner  and  more  flaccid  wall  than  its  companion  artery.  Since 
they  are  larger,  the  veins  hold  more  blood  than  the  arteries,  and  are  an 
important  reservoir  for  blood.  Lymphatic  vessels  have  even  thinner  walls. 
Valves  present  in  both  veins  and  lymphatics  permit  the  blood  and  lymph 
to  flow  only  toward  the  heart.  It  is  sometimes  easy  to  demonstrate  the 
valves  in  the  veins  on  the  back  of  your  hand.  Push  your  finger  on  a  vein 
at  the  point  where  several  join  on  the  back  of  your  wrist  and  move  your 
finger  distally  along  the  vessel.  This  will  force  the  blood  out  of  the  vein, 
and  you  will  notice  that  blood  does  not  reenter  this  vein  from  the  others 
at  the  wrist  for  valves  prevent  it  from  doing  so.  Remove  your  finger  from 
the  vein  and  it  immediately  fills  with  blood  from  the  periphery. 

Though  blood  pressure  is  low  in  the  veins,  and  lowest  in  the  large 
veins  near  the  heart,  it  is  still  the  major  factor  in  the  return  of  blood. 
Two  other  factors  assist  it.  One  is  the  fact  that  the  elastic  lungs  are 
always  stretched  to  some  extent  and  tend  to  contract  and  pull  away  from 
the  walls  of  the  pleural  cavities.  This  creates  a  slight  subatmospheric  or 
negative  pressure  within  the  thoracic  cavity,  which  is  greatest  during 
inspiration.  The  larger  veins,  of  course,  pass  through  the  thorax,  and 
the  reduction  of  pressure   around   them  decreases   the  pressure   within 


BLOOD   AND    CIRCULATION  557 

them  and  increases  the  pressure  gradient.  The  other  factor  is  that  the 
contraction  and  relaxation  of  body  muscles  exert  a  "milking"  action  on 
the  veins.  When  the  muscles  contract,  their  bulging  squeezes  the  veins 
and  forces  the  blood  toward  the  heart,  for  the  valves  in  the  veins  prevent 
the  blood  from  moving  in  any  other  direction.  All  of  these  factors 
increase  during  exercise,  Avhich  makes  for  a  more  rapid  return  of  blood, 
and  an  increased  cardiac  output. 

The  return  of  lymph  is  dependent  upon  similar  forces.  The  tissue 
fluid  itself  has  a  certain  pressure  derived  from  the  flow  of  liquid  out  of 
the  capillaries.  This  establishes  a  pressure  gradient  in  the  lymphatics 
that  is  made  steeper  by  the  negative  intrathoracic  pressure.  The  "milk- 
ing" action  of  surrounding  muscles,  and,  for  lymphatics  returning  from 
the  intestine,  the  contraction  of  the  villi,  help  considerably.  Some  lower 
vertebrates  have  lymph  "hearts"— specialized,  pulsating  segments  of 
lymphatic  vessels. 

Questions 

1.  How  does  the  blood  maintain  a  relatively  constant  pH  despite  its  uptake  of  acid  sub- 
stances in  the  tissues? 

2.  Describe  the  current  theory  of  the  mechanism  of  blood  clotting. 

3.  One  of  the  adaptations  to  high  altitude  is  an  increase  in  the  number  of  erythrocytes. 
Of  what  advantage  to  the  organism  is  this? 

4.  Describe  two  ways  that  leukocytes  protect  the  body  from  microorganisms. 

5.  What  factors  would  have  to  be  taken  into  consideration  in  giving  a  blood  transfusion 
to  an  Rh  negative  woman  who  has  had  several  Rh  positive  children? 

6.  How  did  the  transition  from  water  to  land  alfect  the  pattern  of  the  blood  vessels  and 
the  structure  of  the  heart?  \\  hat  further  changes  have  occurred  during  the  evolution 
to  mammals? 

7.  Define  and  give  an  example  of  a  portal  system. 

8.  How  does  the  circulation  through  the  heart  of  a  mammalian  fetus  differ  from  that  in 
an  adult? 

9.  ^Vhat  prevents  blood  from  flowing  the  wrong  way  in  the  heart? 

10.  How  does  the  heart  adjust  its  rate  and  output  per  beat  to  the  increased  venous  return 
that  occurs  during  increased  body  activity? 

11.  Describe  two  functions  of  arteries  in  addition  to  their  function  of  transportation. 

12.  What  forces  are  involved  in  the  exchange  of  water  and  solutes  between  the  capillaries 
and  tissue  fluid? 

13.  What  factors  supplement  blood  pressure  in  the  return  of  venous  blood? 

14.  List  the  functions  of  the  lymphatic  system.  Do  all  vertebrates  have  this  system? 

Supplementary  Reading 

The  reader  is  referred  again  to  the  general  references  on  anatomy  and  physiology 
cited  at  the  end  of  Chapter  25.  Wiener's  Blood  Groups  and  Transfusions  contains  inter- 
esting accounts  of  the  disco\er)'  of  the  blood  groups  and  their  applications  to  problems  of 
transfusion,  anthropology-,  disputed  paternity  and  forensic  medicine.  Harvey's  Anatomical 
Studies  on  the  Motion  of'the  Heart  and  Blood,  originally  published  in  1628,  was  translated 
and  reprinted  in  1931.  His  classic  experiments  established  the  circulation  of  the  blood 
and  introduced  the  experimental  method  into  biologic  research.  Krogh's  Silliman  lectures 
on  The  Anatomy  and  Physiology  of  Capillaries  contain  excellent  accounts  of  that  portion 


558  VERTEBRATE   LIFE   AND   ORGANIZATION 

of  the  circulatory  svsten.  where  the  actual  exchanges  occur.  Barclay  F'^-nkHn  and 
PrichLd  s  7/t  Foelal  Circulation  describes  the  brilliant  experimental  work  that  led 
o  our  under  anding  of  the  fetal  circulation.  A  fine  discussion  of  the  activity  of  he  heart 
.nd  the  facto  s  of  safety  that  enable  it  to  continue  operating,  even  though  partially  im- 
paired b>clnary  diseLe,  can  be  found  in  an  article  by  Wiggers,  Tke  Heart,  published 
in  the  Scientific  American. 


CHAPTER  28 


The  Urogenital  System — 
Excretion  and  Reproduction 


Functionally  the  kidneys  have  nothing  in  common  with  the  repro- 
ductive organs.  They  are  concerned  with  excretion  of  wastes  and  regu- 
lation oi  body  fluids;  the  reproductive  organs  only  with  the  perpetuation 
of  the  species.  But  the  t^vo  systems  are  morphologically  interrelated  in 
vertebrates  because  certain  excretory  ducts  are  used  for  discharging 
gametes,  and  it  is  convenient  to  treat  them  together  as  the  urogenital 
system.  First  we  shall  consider  the  excretory  portion  of  the  system,  and 
then  relate  the  reproductive  organs  to  it. 

Although  the  kidneys  come  to  mind  when  one  thinks  of  excretion 
in  vertebrates,  they  do  not  have  a  monopoly  on  the  removal  of  the 
waste  products  of  metabolism.  The  gills  and  lungs,  the  skin,  and  to  some 
extent  the  digestive  tract  play  a  role  in  excretion.  Gills  eliminate  carbon 
dioxide  and  some  nitrogenous  wastes;  lungs,  carbon  dioxide;  the  skin 
(especially  in  amphibians),  a  certain  amount  of  carbon  dioxide  and 
traces  of  salts  and  nitrogenous  wastes;  the  digestive  tract,  bile  pigments 
and  certain  metal  ions.  The  kidneys  remove  most  of  the  nitrogenous 
wastes  in  the  higher  vertebrates,  but  this  is  not  their  only  function.  By 
removing,  or  conserving,  water,  salts,  acids,  bases  and  various  organic 
substances,  they  play  a  vital  role  in  regulating  the  composition  of  the 
blood  and  the  internal  environment  of  the  body. 

238.        Evolution  of  the  Kidneys  and  Their  Ducts 

The  kidneys  of  vertebrates  are  paired  organs  that  lie  dorsal  to  the 
coelom  on  each  side  of  the  dorsal  aorta.  All  vertebrate  kidneys  are  com- 
posed of  units  called  kidney  tubules,  or  nephrons,  which  remove  ma- 
terials from  the  blood,  but  the  number  and  arrangement  of  the  nephrons 
differ  in  the  various  groups  of  vertebrates.  Comparative  studies  have 
led  to  the  conclusion  that  each  kidney  in  ancestral  vertebrates  contained 
one  nephron  for  each  of  those  body  segments  that  lay  between  the  an- 
terior and  posterior  ends  of  the  coelom  (Fig.  28.1,  A).  These  nephrons 
drained  into  a  Wolffian  duct  which  continued  posteriorly  to  the  cloaca. 
Such  a  kidney  may  be  regarded  as  a  complete  kidney,  or  holonephros, 
for  it  extends  the  entire  length  of  the  coelom.  A  holonephros  is  found 
today  in  the  larvae  of  certain  cyclostomes,  but  not  in  any  adult  verte- 
brate. 

559 


560 


VERTEBRATE   LIFE   AND    ORGANIZATION 


Testis n 
Nephron. — 
Pe.ricaL-rdial 
Cavity 


-Holonephros 

rlntestine 


''»//.,„.... 


-Opisthonephros 

'M////m 


^CloBcca. 


Pleuroperitonealca.vity-'  "-Wolffia-n  duct 

A  B 

,,//,"r'n„  rWolffia.n  duct 


•WolfFia.ixdu.ct 


Remnant  oF 
opisthonephroS 


Meta.nephros 
~- — Ureter 


Pronephros 


Perica.rdia.1  cavity 


D 


»^./^  Urinary  ■ 
bladder 

c 

Mesonephros 
1%    (developing) 

Gonei-d 

Wolffian, 
duct 

Intestine. 

'^^Urinary 

blajlder 

Cloaca. 

'-Allantois 


Gonad 
Oviduct- 


Bladder 

Urethra 

Genital  tubercle- 


Pronephros 

(degenerating) 

MesoneplTros 

Wolffian 

duct 

■Metaneplii'os 
(developing) 


P^^Ureter 

■Remnant  of  Mesonephros 

Wolffian  duct 
Metanephros 

Ureter 

■Rectum 


Cloac 


Figure  28.1.  A  comparison  of  the  evolution  and  embryonic  development  of  the 
kidney  and  its  ducts.  A,  B,  C:  the  evolutionary  sequence  of  kidneys.  A,  Hypothetical 
ancestral  vertebrate  with  a  holonephros;  B,  a  fish  with  an  opisthonephros;  C,  a  reptile 
with  a  metanephros.  D,  E,  The  developmental  sequence  of  kidneys  in  a  reptile.  F,  A 
mammalian  embryo  in  which  the  cloaca  is  becoming  divided  by  the  growth  of  the 
fold  indicated  by  the  arrow.  The  ventral  part  of  the  cloaca  contributes  to  the  urethra 
in  tlie  male.  It  becomes  further  subdivided  in  the  female  and  contributes  to  both 
urethra  and  vagina.  In   both  sexes,   the  dorsal  part  of  the   cloaca   forms   the   rectum. 

In  the  kidney  ot  adult  fishes  and  amphibians  (Fig.  28.1  B),  the 
most  anterior  tubules  have  been  lost,  some  of  the  middle  tubules  are 
associated  with  the  testis,  and  there  is  a  concentration  and  multiplication 
of  tubules  posteriorly.  Such  a  kidney  is  known  as  a  posterior  kidney  or 
opisthonephros. 

Reptiles,  birds  and  mammals  (Fig.  28.1  C)  have  lost  all  of  the  mid- 
dle tubules  not  associated  with  the  testis,  and  have  an  even  greater 
multiplication  and  posterior  concentration  of  tubules.  The  number  of 
nephrons  is  particularly  large  in  birds  and  mammals;  their  high  rate 
of  metabolism  yields   a   large   amount  of  wastes   to  be  removed.    It  is 


THE   UROGENITAL   SYSTEM — EXCRETION   AND    REPRODUCTION  551 

estimated  that  man  has  1,000,000  or  more  nephrons  per  kidney  whereas 
certain  salamanders  have  less  than  100.  The  tubules  concerned  with 
urine  production  drain  into  a  ureter,  which  evolved  as  an  outgrowth 
from  the  Wolffian  duct.  The  W^olffian  duct  itself  has  been  taken  over 
completely  by  the  male  genital  system.  The  kidney  of  the  higher  verte- 
brates is  known  as  a  metanephros. 

The  evolutionary  sequence  of  kidneys  is  holonephros,  opistho- 
nephros  and  metanephros.  In  the  development  of  vertebrates,  we  find 
a  slightly  different  sequence,  but  one  that  also  involves  a  posterior 
concentration  of  kidney  functions  (Fig.  28.1  D,  E).  In  an  early  embryo 
of  a  reptile,  for  example,  segmentally  arranged  tubules  appear  dorsal  to 
the  anterior  end  of  the  coelom,  form  the  Wolffian  duct,  and  disappear. 
Ihese  transitory  tubules  constitute  a  pronephros.  Then  a  middle 
group  of  tubules,  known  as  the  mesonephros,  appear  and  connect  with 
the  Wolffian  duct  (Fig.  28.3).  These  function  during  much  of  embryonic 
life,  but  when  the  metanephric  tubules  develop,  all  of  the  mesonephric 
tubules  are  lost  except  for  those  associated  with  the  testes.  The  em- 
bryonic sequence  of  kidneys  in  the  development  of  a  higher  vertebrate 
is  pronephros,  mesonephros  and  metanephros. 

A  urinary  bladder,  for  the  temporary  accumulation  of  urine,  is 
associated  with  the  excretory  ducts  of  many  vertebrates.  Most  tetrapods 
have  a  bladder,  which  develops  as  a  ventral  outgrowth  from  the  cloaca. 
Generally  the  excretory  ducts  from  the  kidneys  lead  to  the  dorsal  part 
of  the  cloaca,  and  urine  must  flow  across  it  to  enter  the  bladder,  but  in 
mammals  (Fig.  28.1  F)  the  ureters  lead  directly  to  the  bladder,  and  the 
bladder  opens  to  the  body  surface  through  a  short  tube,  the  urethra. 
The  cloaca  becomes  divided  and  disappears  as  such  in  all  but  the  most 
primitive  mammals.  The  dorsal  part  of  the  cloaca  forms  the  rectum 
and  the  ventral  part  contributes  to  the  urethra  of  higher  mammals 
(Fig.  28.1  F). 

Urine  is  produced  continually  by  the  kidneys,  and  is  carried  down 
the  ureters  by  peristaltic  contractions.  It  accumulates  in  the  bladder, 
for  a  smooth  muscle  sphincter  at  the  entrance  of  the  urethra  and  a 
striated  muscle  sphincter  located  more  distally  along  the  urethra  are 
closed.  Uruie  is  prevented  from  backing  up  into  the  ureters  by  valvelike 
folds  of  mucous  membrane  within  the  bladder.  When  the  bladder 
becomes  filled,  stretch  receptors  are  stimulated  and  a  reflex  is  initiated 
which  leads  to  the  contraction  of  the  smooth  muscles  in  the  bladder 
wall  and  the  relaxation  of  the  smooth  muscle  sphincter.  Relaxation  of 
the  striated  muscle  sphincter  is  a  voluntary  act. 

239.        The  Nephron  and  Its  Function 

Nephron  Structure.  The  excretory  ducts  and  the  urinary  bladder 
are  important  adjuncts  to  the  kidneys,  but  the  essential  work  of  the 
system,  the  selective  removal  of  materials  from  the  blood,  is  performed 
by  the  individual  kidney  tubules.  The  general  nature  and  function  of 
these  tubules  was  described  in  Chapter  5.  The  mammalian  nephron 
may  be  taken  as  an  example.  The  proximal  end  of  each  nephron  (Fig. 


562 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Efferent 
arberi'ole 


Afferent 
arteriole. 


Bowman's 

capsule - 


Proximsd 
convoluted  tubule 


Intralobular 
pZirt  of 
renal  artaxy 


Distal 

convoluted 

tubule 


Collecting 
tubule 


Intralobular  pJii^t 
of  renal  vein. 


Henle's  loop 


Figure  28.2.     A  diagram  of  the  mammalian  nephron.  (^From  Campbell.) 

28.2)  is  known  as  Bowman's  capsule.  It  is  a  hollow  ball  of  squamous 
epithelial  cells,  one  end  ot  which  has  been  pushed  in  by  a  knot  of 
capillaries  called  a  glomerulus.  Bowman's  capsule  and  the  glomerulus 
constitute  a  renal  corpuscle.  The  rest  of  the  nephron  is  a  tubule  largely 
composed  of  cuboidal  epithelial  cells,  and  subdivided  in  mammals  into 
a  proximal  convoluted  tubule,  a  loop  of  Henle  and  a  distal  convoluted 
tubule.  A  collecting  tubule  receives  the  drainage  of  several  nephrons 
and  leads  to  the  renal  pelvis,  an  expansion  within  the  kidney  of  the 
proximal  end  of  the  ureter  (Fig.  28.3).  The  collecting  tubules  and  the  loops 
of  Henle  lie  toward  the  center,  in  the  medulla  of  the  kidney;  the  other 
parts  of  the  nej)hron  occur  in  the  outer  part,  or  cortex,  of  the  kidney. 

Glomerular  Filtration.  The  wall  of  Bowman's  capsule  is  a  semi- 
permeable membrane,  and  small  molecules  in  the  glomerular  capil- 
laries should  pass  through  it  readily.  By  carefully  inserting  a  micro- 
pipette  into  a  Bowman's  capsule  in  a  frog's  kidney,  and  drawing  off 
and  analyzing  a  sample  of  the  contents  (the  glomerular  filtrate),  Dr.  A. 
N.  Richards  of  the  University  of  Pennsylvania  demonstrated  that  this 
is  indeed  the  case.  Only  the  blood  cells,  fats  and  plasma  proteins  are 
held  back  in  the  capillaries.  The  other  plasma  components  are  found 
in  the  glomerular  filtrate  in  nearly  the  same  proportion  as  in  the 
plasma.  Other  experiments  have  shown  that  this  is  true  for  glomerular 
filtration   in  mammalian   nephrons  too. 


THE   UROGENITAL   SYSTEM — EXCRETION   AND    REPRODUCTION 


563 


Materials  leave  the  blood  in  the  glomeruli  as  they  do  in  other 
capillary  beds,  but  the  arrangement  ot  the  blood  vessels  is  such  that  a 
large  volume  of  material  is  forced  out.  An  afferent  arteriole  leads  from 
a  branch  of  the  renal  artery  to  each  glomerulus,  and  an  efferent 
arteriole  from  the  glomerular  capillaries  to  a  second  capillary  network 
distributed  over  the  rest  of  the  tubule.  These  capillaries  are  drained 
by  branches  of  the  renal  vein.  A  glomerulus  thus  lies  between  two 
arterioles.  The  efferent  arteriole  is  smaller  than  the  afferent  one;  this 
insures  a  high  blood  pressiue  in  the  glomerular  capillaries,  and  hence 
a  high  filtration  pressure  to  drive  fluids  from  the  blood.  The  filtration 
pressure  in  a  glomerulus  is  normally  about  twice  as  great  as  that  at  the 
arterial  end  of  an  ordinary  capillary  bed.  It  has  been  estimated  that 
some  184  liters  of  glomerular  filtrate  are  normally  produced  by  a  man 
in  one  day! 

Tubular  Reabsorption  and  Augmentation.  Glomerular  filtration 
is  not  a  selecti\e  process.  Glucose,  amino  acids,  inorganic  ions  and 
many  other  usefid  materials  leave  the  blood  along  with  urea,  other 
wastes,  and  enough  water  to  dehydrate  a  terrestrial  vertebrate  in  a  few 
hours.  Fortunately  the  glomerular  filtrate  tmdergoes  further  treatment 
as  it  passes  down  the  tubule.  Virtually  all  of  the  glucose  and  amino 
acids  and  most  of  the  water  and  inorganic  ions  are  reabsorbed,  entering 
the  capillaries  around  the  tubule.  The  various  parts  of  the  tubule 
reabsorb  different  materials.  In  mammals,  most  of  the  glucose,  amino 
acids  and   water  reenter  the   blood  from   the  proximal  convoluted  tu- 


Pronephros 
(deo  e  ne-r  all  ng) 

Oviduct 
MesonephroS ' 

Wolffian  duct 
Metanephros 


Ostium 


Rcte  cords 


Indifferent 
onad. 


Renal  pelvis 
Ureter 


Urinary 
bladder 

Ventral 
pa-rt  of 
cloeLca 

Figure  28.3.     A  ventral  view  of  the  urogenital  organs  of  the  sexually  indifferent 
stage  of  the  embryo. 


564  VERTEBRATE   LIFE   AND   ORGANIZATION 

bule;  additional  water  molecules  are  reabsorbed  from  Henle's  loop  and 
the  distal  tubule.  Inorganic  ions  are  reabsorbed  from  both  the  proximal 
and  distal  tubule.  Reabsorption  involves  both  the  passive  diffusion  of 
materials  back  into  the  capillaries  surrounding  the  tubule,  and  the 
active  uptake  of  materials  by  the  tubular  cells  and  their  secretion  into 
the  blood  against  a  concentration  gradient.  About  85  per  cent  of  the 
water,  in  the  glomerular  filtrate,  for  example,  diffuses  back  into  the 
blood  simply  because  the  blood  contains  more  osmotically  active  solutes 
than  the  filtrate,  but  the  balance  of  the  water  is  actively  reabsorbed 
and  involves  work  on  the  part  of  the  tubular  cells.  Passive  reabsorption  of 
water  occurs  in  the  proximal  tubule,  and  the  active  reabsorption  takes 
place  in  the  loop  of  Henle  and  distal  tubule. 

In  addition  to  a  selective  reabsorption  of  materials,  certain  of  the 
tubular  cells  can  secrete  wastes  into  the  tubule— a  process  known  as 
augmentation.  In  certain  teleost  fishes,  which  have  lost  the  renal  cor- 
puscles, this  is  an  important  way  of  eliminating  waste  products,  but 
relatively  little  is  added  to  the  filtrate  by  augmentation  in  mammals. 
Creatinine,  ammonia,  hydrogen  ions  and  various  drugs  (penicillin)  are 
among  the  few  substances  eliminated  in  this  way. 

The  fluid  that  reaches  the  end  of  the  tubules  is  known  as  urine. 
In  man,  the  volume  of  urine  is  only  about  1  per  cent  of  the  volume  of 
the  glomerular  filtrate,  and  its  composition  is  quite  different  from  that 
of  the  filtrate,  for  a  great  many  substances  have  been  reabsorbed  and 
others  have  been  added.  As  a  result  of  these  processes  the  waste  products 
are  concentrated  in  the  urine.  The  most  important  nitrogenous  wastes 
in  the  urine  are  urea,  ammonia,  uric  acid  and  creatinine.  The  yellowish 
color  of  the  urine  is  due  to  the  presence  of  urochrome,  a  pigment 
derived  from  the  breakdown  of  hemoglobin  and  hence  related  to  the 
bile  pigments. 

Kidney  Regulation  of  Body  Fluids.  Although  some  of  the  urea 
present  in  the  filtrate  diffuses  back  into  the  blood,  most  of  the  wastes 
present  in  the  glomerular  filtrate  are  excreted  by  the  kidneys,  for  these 
substances  are  not  actively  reabsorbed.  Those  materials  that  can  be 
actively  reabsorbed  are  taken  back  in  varying  amounts  depending 
upon  their  concentration  in  the  blood.  If  the  concentration  of  one  of 
these  materials  in  the  blood  and  glomerular  filtrate  rises  above  a  certain 
level,  known  as  the  renal  threshold,  not  all  of  it  will  be  reabsorbed 
into  the  blood  from  the  tubule,  and  the  amount  present  in  excess  of 
the  renal  threshold  is  excreted.  The  quantitative  value  of  the  renal 
threshold  differs  for  different  substances.  In  diabetes  mellitus,  for  ex- 
ample, in  which  impaired  cellular  utilization  of  glucose  leads  to  a 
high  concentration  of  glucose  in  the  blood,  the  renal  threshold  for 
glucose  (about  150  mg.  of  glucose  per  100  ml.  blood)  is  exceeded  and 
the  sugar  appears  in  tlie  urine  in  large  amounts.  The  osmotic  pressure 
of  the  body  fluids  is  controlled  by  the  amount  of  salts,  and  the  pH  by 
the  amount  of  hydrogen  ions,  that  are  taken  back  into  the  blood 
from  the  glomerular  filtrate. 

The  volume  of  the  body  fluids  is  also  regulated  by  the  kidneys.  If 
an  excess  of  water  is  present  in  the  body  fluids,  the  blood  volume  and 


THE   UROGENITAL   SYSTEM — EXCRETION   AND    REPRODUCTION  555 

pressure  increase.  This  raises  the  glomerular  filtration  pressure  and 
more  filtrate  is  produced.  An  increase  in  the  amount  of  water  in  the 
tissue  fluid  inhibits  the  release  of  an  antidiuretic  hormone  produced 
by  the  posterior  lobe  of  the  pituitary  (p.  622).  Since  this  hormone  is 
necessary  for  the  active  reabsorption  of  water,  a  reduction  of  the 
amount  present  in  the  blood  decreases  the  amount  of  water  taken  back 
from  the  tubule.  Increased  production  of  filtrate  and  decreased  reab- 
sorption of  water  rapidly  bring  the  volume  of  body  fluids  down  to 
normal.  If  the  volume  of  body  fluids  falls  below  normal,  as  in  a  severe 
hemorrhage,  these  factors  work  in  the  opposite  direction:  Less  glo- 
merular filtrate  is  produced,  more  water  is  reabsorbed,  and  the  volume 
of  body  fluid  is  soon  raised  to  normal.  The  osmotic  pressure  of  the 
tubular  contents  also  affects  the  amount  of  water  removed.  If  a  large 
amount  of  salts  or  sugars  is  being  eliminated,  the  osmotic  pressure  of 
the  tubular  contents  is  increased  and  less  water  can  be  reabsorbed.  The 
urine  volume  is  greater  when  there  is  a  large  amount  of  osmotically 
active  substances  in  the  urine,  as  after  a  large  intake  of  salt,  or  in 
diabetes  mellitus. 

Nephron  Evolution.     The  nephrons  of  other  vertebrates  are  essen- 
tially similar  in  structure  and  function  to  these  mammalian  nephrons, 
although  there  are  differences  in  detail.  In  addition  to  being  associated 
with  the  glomerulus,  some  of  the  nephrons  of  primitive  vertebrates  are 
connected  with   the   coelom   via   a  nephrostome,    and  can  remove  ma- 
terials from   the  coelomic  fluid.  This  is  analogous  to  the  nephridia  of 
the  earthworm  (p.  239).  It  may  have  been  the  primitive  condition  in 
vertebrates,   for   in   the   tubules  of  still  more  primitive   vertebrates  the 
glomerulus   protrudes   into    the  coelom,   instead  of  into   the  beginning 
of  the  tubule,  and  the  glomerular  filtrate  is  discharged  into  the  coelom. 
The  size  of  the  renal   corpuscles   and   the  presence   or  absence  of 
water-reabsorbing   segments   vary   with   the   environment   in   which   the 
animal    lives.   Primitive   fresh-water   fishes   have    large  renal   corpuscles, 
which  produce  copious   amounts  of   filtrate,   and   do  not  have  special 
water-reabsorbing   segments.    The    concentration    of    salts   within    their 
bodies   is    greater   than    that    in    the    surrounding   medium    and    water 
moves  by  osmosis  into  their  bodies.  Their  problem  is  to  pump  out  the 
excess  water,  yet  retain  the  needed  salts.  The  type  of  tubule  found  in 
fresh-water  fishes   is  well  adapted   for  this,  and   the  primitive  function 
of  the  tubule  may  have  been  water  regulation.  Nitrogenous  wastes  can 
be  eliminated  through  the  gills,  and  their  removal  by  the  kidneys  may 
have  been  secondary,  but  when  vertebrates  became  terrestrial  and  lost 
their   gills,   the   kidneys   became   the    main   organs   for   removing   these 
wastes.    Amphibians    retain    the    primitive   fresh-water    type    of    tubule, 
and  have  little  control  over  the  loss  of  water.  Frogs  can  lose  in  the  urine 
an  amount  of  water,  equivalent  to  one  third  of  their  body  weight  each 
day.  The  need  to  soak   up  water  and   to  keep  the  skin  moist  for  gas 
exchange  is  a  factor  that  compels  frogs   to  stay  near  water.  Water  is 
conserved  in  reptiles  by  the  small  size  of  their  glomeruli.  Less  water 
is  removed  from  the  blood  by  these  glomeruli  than  by  the  large  ones  of 
primitive   fishes  and  amphibians.   Birds  and  mammals   have  glomeruli 


566  VERTEBRATE   LIFE   AND    ORGANIZATION 

of  moderate  size,  but  have  evolved  segments  of  the  tubule  that  take 
back  into  the  blood  most  of  the  water  that  is  removed  by  the  glomeruli. 
Some  terrestrial  vertebrates  (toads  and  many  reptiles)  also  reabsorb 
water  from  the  urinary  bladder,  although  ordinarily  urine  is  not 
changed  after  it  leaves  the  nephron. 

.\s  we  pointed  out  in  Chapter  5,  animals  can  also  save  water  by 
converting  ammonia  into  nitrogenous  wastes  that  require  less  water 
for  their  removal.  Ammonia,  which  is  produced  by  the  deamination  of 
amino  acids,  is  a  very  toxic  compound,  but  it  is  highly  soluble  in  water 
and  can  be  excreted  rapidly  if  ample  water  is  available  to  carry  it  away. 
If  an  animal  converts  its  ammonia  to  urea,  some  water  can  be  con- 
served, for  each  molecule  of  urea  is  formed  from  two  molecules  of  am- 
monia. If  ammonia  is  converted  to  uric  acid,  more  water  can  be  saved, 
for  uric  acid  has  a  low  toxicity,  is  relatively  insoluble  and  can  be  ex- 
creted as  an  insoluble  paste.  Ammonia  is  the  primary  nitrogenous  waste 
of  fresh-water  fishes  whereas  urea  and  uric  acid  are  excreted  by  ter- 
restrial vertebrates. 


240.        The  Gonads 

From  a  biological  point  of  view,  all  of  the  structures  and  processes 
that  permit  a  species  to  survive  are  of  no  avail  unless  the  species  can 
reproduce  its  kind.  The  general  aspects  of  reproduction,  including  the 
production  of  gametes  in  the  gonads,  fertilization,  and  the  early  de- 
velopment of  the  embryo,  were  considered  in  Chapter  6.  At  this  time 
we  shall  be  concerned  more  specifically  with  the  reproductive  organs 
of  vertebrates  and  their  role  in  reproduction. 

Reproduction  is  sexual  in  vertebrates,  and  the  sexes  are  separate. 
The  testes  are  paired  organs  of  modest  size,  each  consisting  of  numer- 
ous, highly  coiled  seminiferous  tubules  (Fig.  28.4),  whose  total  length 
in  man  has  been  estimated  at  250  meters!  This  provides  an  area  large 
enough  for  the  production  of  billions  of  sperm.  As  the  sperm  mature, 
they  enter  the  lumen  of  the  tubule  and  move  toward  the  genital  ducts. 
The  ovaries  are  more  variable  in  size.  They  fill  much  of  the  body  cavity 
in  primitive  vertebrates  that  produce  millions  of  eggs,  but  are  much 
smaller  in  higher  vertebrates  that  produce  fewer  eggs  and  give  more 
care  to  those  produced.  The  human  ovary  is  little  more  than  an  inch 
long  (Fig.  28.5).  The  eggs  are  not  free  within  the  ovary  for  each  one  is 
surrounded  by  a  follicle  of  epithelial  and  connective  tissue  cells.  When 
the  egg  is  ripe  the  follicle  bursts  and  the  egg  is  discharged  into  the 
coelom,  a  process  know  as  ovulation  (Fig.  28.6).  The  accumulation  of 
fluid  within  the  follicle  causes  it  to  burst  in  mammals,  although,  as  we 
have  seen,  muscular  contraction  produces  ovulation  in  frogs. 

In  the  frog  and  most  other  vertebrates,  the  gonads  are  suspended 
by  mesenteries  in  the  abdominal  cavity,  and  they  remain  there  through- 
out life.  But  in  the  males  of  most  mammals  the  testes  undergo  a 
posterior  migration,  or  descent,  and  move  out  of  the  main  part  of  the 
abdominal  cavity  into  a  sac  of  skin  known  as  the  scrotum  (Fig.  28.4). 
As  they  move  into  the  scrotum,  they  carry  a  coelomic  sac,   the  tunica 


THE  UROGENITAL  SYSTEM — EXCRETION  AND   REPRODUCTION 


567 


vaginalis,  down  with  them,  so,  despite   their  superficial  position,   they 
still  protrude  into  a  portion  of  the  coelom. 

In  man  and  other  mammals  in  which  spermatogenesis  goes  on 
continually  throughout  adult  life,  the  testes  remain  permanently  de- 
scended. But  in  most  wild  mammals  spermatogenesis  occurs  only  dur- 
ing specific  breeding  seasons,  and  the  testes  descend  only  at  this  time.  At 


Urete-rsn 


Urinary  bla.d.de-r 


PeritoneaJ.- 
cavity 

Se.mirLa.1 
v<Z/Sicle 

Prosta.te 
Re-ctum. 

Prostatic 
utricle 


Cowpers  gland 


Vas  deferens        &^^\^ 

Retc  cords— ^'^^^'" 

Epididymis—^  /*^ 

Seminiferous-^ 
tubulcS 


■Vks  dz-f  erons 


Pobis 


Cavernous 
bodies 

Ui^cttira. 


GlaxiS  penis 

-•otum 
■  Tunica  vaginalis 


•-Testis 


Figure  28.4.  A  diagrammatic  sagittal  section  through  the  pelvic  region  of  a  man 
to  show  the  genital  organs.  The  prostatic  utricle  is  a  vestige  of  the  oviduct  that  is 
present   in    the   sexually    indifferent   stage    of    the   embryo.   (Modified    after   Turner.) 


-Ovary 


Uterus 


CcrvL 


■Fallopian  tube 


Peritoneal 
cavity 

Urinary  bladder 


Pubis 


Rectum 


Clitoris 
-Labium  majus 
Labium  minus 


Vadina — 

Urethra     ' 

Figure  28.5.     A    diagrammatic    sagittal    section    through    the    pelvic    region    of   a 
woman  to  show  the  genital  organs.  (Modified  after  Turner.) 


568 


VERTEBRATE   LIFE  AND    ORGANIZATION 


Fertiliza-tion 


Ovula-tion 

FedlopicLn  toLc 


Figure  28.6.  A  diagram  to  show  the  path  of  an  egg  from  the  ovary  to  the  uterus, 
and  the  clianges  that  occur  en  route.  The  last  stage  is  about  a  week  and  one  half  old. 
(Modified  after  Dickinson.) 

Other  times,  they  are  withdrawn  into  the  abdominal  cavity.  Spermato- 
genesis, like  other  vital  processes,  can  only  occur  within  a  limited 
temperature  range.  Apparently  this  range  is  exceeded  by  the  tempera- 
ture in  the  abdominal  cavity,  but  not  by  the  temperature  in  the  scro- 
tum, which  is  approximately  4°  C.  lower.  In  order  to  test  this  hypoth- 
esis. Dr.  Carl  R.  Moore  of  the  University  of  Chicago  confined  the  testes 
of  rats  to  the  abdominal  cavity  and  found  that  spermatogenesis  did 
not  occur.  Indeed,  the  seminiferous  tubules  underwent  regression.  He 
also  insulated  the  scrotum  of  a  ram  in  which  the  testes  were  descended. 
This  raised  the  temperature,  and  again  spermatogenesis  did  not  oc- 
cur. Apparently  during  the  evolution  of  homoiothermism  in  mammals 
spermatogenesis  did  not  become  adapted  to  the  higher  body  temperatures. 

241.        Reproductive  Passages 

Once  the  sperm  and  eggs  have  been  produced,  they  must  be  re- 
moved from  the  body  and  be  brought  together  to  form  a  zygote.  This 
is  a  simple  procedure  in  primitive  vertebrates  such  as  cyclostomes.  No 
reproductive  ducts  are  present,  and  both  eggs  and  sperm  simply  break 
out  of  the  gonad  into  the  coelom.  Ciliary  currents  carry  them  to  the 
posterior  end  of  the  coelom  where  they  are  discharged  through  a  pore 
into  the  cloaca.  Fertilization  and  development  are  external. 

Embryonic  Formation  of  Reproductive  Ducts.  Other  vertebrates 
have  a  system  of  ducts  for  the  removal  of  the  gametes,  and  some  of 
them  are  intimately  related  to  the  excretory  system.  In  order  to  under- 
stand this  relationship,  it  is  necessary   to  go  back  to  a  period  in  em- 


THE   UROGENITAL   SYSTEM — EXCRETION   AND    REPRODUCTION  569 

bryonic  development  when  the  embryo  is  sexually  indifferent  (Fig. 
28.3).  Its  sex  is  determined  genetically  at  the  time  of  fertilization  (p. 
660),  but  early  in  development  the  embryo  has  the  potentiality  of 
differentiating  into  either  a  male  or  a  female,  for  the  primordia  of  both 
male  and  female  duct  systems  are  present.  A  pair  of  oviducts  are  pres- 
ent, each  one  opening  anteriorly  into  the  coelom  through  a  funnel- 
shaped  ostium  and  connecting  posteriorly  with  the  cloaca.  The  de- 
veloping gonad,  which  is  not  recognizable  as  an  ovary  or  a  testis  at  first, 
is  adjacent  to  each  mesonephros,  and  rete  cords  develop  to  connect 
the  gonad  with  some  of  the  mesonephric  tubules.  Gametes  can  thus 
pass  through  the  rete  cords,  the  mesonephric  tubules  and  the  Wolffian 
duct.  If  the  embryo  differentiates  into  a  male,  the  route  through  the 
mesonephros  materializes,  and  the  embryonic  oviduct  disappears,  leav- 
ing at  most  a  few  traces.  If  the  embryo  differentiates  into  a  female, 
the  route  through  the  coelom  and  oviducts  is  used,  the  oviducts  de- 
velop further,  and  those  parts  of  the  male  system  not  concerned  with 
excretion  largely  disappear. 

Male  Vertebrates.  In  male  frogs  (Fig.  21.15)  and  other  lower  ver- 
tebrates the  rete  cords  become  the  vasa  efferentia,  which  carry  sperm 
from  the  seminiferous  tubules  in  the  testis  to  the  anterior  part  of  the 
kidney.  The  frog's  kidney  is  an  opisthonephros,  but  its  anterior  portion 
develops  from  the  embryonic  mesonephros.  Sperm  pass  through  kidney 
tubules  into  the  Wolffian  duct,  which  carries  both  sperm  and  urine  to 
the  cloaca,  though  not  at  the  same  time. 

Higher  vertebrates,  such  as  man  (Fig.  28.4),  have  metanephric 
kidneys,  and  sperm  pass  from  each  testis  to  an  epididymis,  thence  out  a 
vas  deferens  to  the  urethra.  This,  seemingly,  is  a  different  pattern,  but 
it  is  not  as  different  as  it  first  appears.  Rete  cords  connect  the  semi- 
niferous tubules  with  the  epididymis  and  the  epididymis  represents 
that  part  of  the  mesonephros  that  was  associated  embryonically  with 
the  testis,  together  with  a  highly  convoluted  portion  of  the  Wolffian 
duct.  The  vas  deferens  represents  the  rest  of  the  Wolffian  duct,  and 
most  of  the  urethra  represents  the  ventral  part  of  a  divided  cloaca. 
Man  thus  utilizes  passages  homologous  to  those  of  a  frog. 

Other  differences  between  the  male  reproductive  organs  of  lower 
and  higher  vertebrates  are  correlated  with  differences  in  mode  of  re- 
production. Frogs  mate  in  the  water  and  spray  the  sperm  over  the  eggs 
as  they  are  discharged.  Fertilization  is  external.  This  mating  procedure 
is  perfectly  satisfactory  for  species  that  mate  in  water,  but  the  gametes 
are  too  delicate  for  external  fertilization  in  the  terrestrial  environment. 
To  accomplish  internal  fertilization,  male  mammals  have  a  penis  with 
which  to  deposit  the  sperm  in  the  female  reproductive  tract,  and  a 
series  of  accessory  sex  glands  that  secrete  a  fluid  in  which  the  sperm 
are  carried.  The  penis  develops  around  the  urethra,  and  contains  three 
cavernous  bodies  composed  of  spongy  erectile  tissue.  Venous  spaces 
within  the  erectile  tissue  become  filled  with  blood  during  sexual  excite- 
ment, making  the  penis  turgid  and  effective  as  a  copulatory  organ.  The 
accessory  sex  glands  are  a  pair  of  seminal  vesicles,  which  connect  with 


570  VEKTEBRATE   LIFE   AND   ORGANIZATION 

the  distal  end  of  the  vasa  deferentia;  a  prostate  gland  surrounding  the 
urethra  at  the  point  of  entrance  of  the  vasa  deferentia;  and  a  pair  of 
Cowper's   glands   located  more  distally  along  the  urethra. 

Female  Vertebrates.  Eggs  are  removed  from  the  coelom  in  most 
female  vertebrates  by  a  pair  of  oviducts,  but  the  oviducts  are  modified 
for  various  modes  of  reproduction.  Lower  vertebrates  reproduce  in  the 
water.  Most  are  oviparous,  fertilization  is  external,  and  the  eggs  de- 
velop into  larvae  that  can  care  for  themselves.  In  the  frog  (Fig.  21.14), 
each  oviduct  is  a  simple  tube  that  extends  from  the  anterior  end  of 
the  coelom  to  the  cloaca.  The  oviducts  may  contain  glandular  cells  that 
secrete  layers  of  jelly  about  the  eggs,  and  their  lower  ends  may  be 
expanded  for  temporary  storage  of  the  eggs,  but  they  are  not  other- 
wise specialized. 

Fertilization  is  internal  in  vertebrates  that  reproduce  on  the  land, 
and  the  free  larval  stage  has  been  replaced  by  the  evolution  of  a 
cleidoic  egg.  Most  reptiles  and  all  birds  are  oviparous  and  the  eggs 
develop  externally.  The  oviducal  glands,  which  secrete  the  albumm 
and  a  shell  around  the  egg,  are  more  numerous  in  the  oviducts  of 
reptiles  than  in  those  of  amphibians  and  most  fishes,  but  in  other 
respects  the  oviducts  of  reptiles  have  not  changed  greatly.  Birds  have 
lost  the  right  oviduct  along  with  the  right  ovary,  but  the  remaining  left 
oviduct  is  essentially  similar  to  the  reptilian  oviduct. 

Most  mammals  and  a  few  fishes  and  reptiles  have  become  vivip- 
arous; they  retain  the  fertilized  egg  within  the  reproductive  tract  until 
embryonic  development  is  complete.  The  oviducts  are  modified  ac- 
cordingly. In  the  human  female  (Figs.  28.5  and  28.6),  the  ostium  lies 
adjacent  to  the  ovary  and  may  even  partially  surround  it.  When  ovula- 
tion occurs,  the  discharged  eggs  are  close  enough  to  the  ostium  to  be 
easily  carried  into  it  by  ciliary  currents.  The  anterior  portion  of  each 
oviduct  is  a  narrow  tube  known  as  the  Fallopian  tube,  and  eggs  are 
carried  down  it  by  ciliary  action  and  muscular  contractions.  The  re- 
mainder of  the  primitive  oviducts  have  fused  with  each  other  to  form 
a  thick-walled,  muscular  uterus  and  part  of  the  vagina.  The  terminal 
portions  of  the  vagina  and  urethra  develop  from  a  further  subdivision 
of  the  ventral  part  of  the  cloaca.  The  vagina  is  a  tube  specialized  for 
the  reception  of  the  penis.  It  is  separated  from  the  main  body  of  the 
uterus,  in  which  the  embryo  develops,  by  a  sphincter-like  neck  of 
the  uterus  known  as  the  cervix.  The  orifices  of  the  vagina  and  urethra 
are  flanked  by  paired  folds  of  skin,  the  labia  minora  and  labia  majora. 
A  small  bundle  of  sensitive  erectile  tissue,  the  clitoris,  lies  just  in  front 
of  the  labia  minora.  Structures  comparable  to  these  are  present  in  the 
sexually  indifferent  stage  of  the  embryo,  and  develop  into  more  con- 
spicuous organs  in  the  male.  The  labia  majora  are  comparable  to  the 
scrotum;  the  labia  minora  and  clitoris,  to  the  penis.  A  pair  of  glands, 
homologous  to  Cowper's  glands  in  the  male,  discharge  a  mucous  secre- 
tion near  the  orifice  of  the  vagina.  A  fold  of  skin,  the  hymen,  partially 
occludes  the  opening  of  the  vagina,  but  is  usually  ruptured  during 
the  first  intercourse. 


THE   UROGENITAL   SYSTEM — EXCRETION   AND    REPRODUCTION  ^J  \ 

242.        Mammalian  Reproduction 

Fertilization.  During  copulation,  the  sperm  that  have  been  stored 
in  the  epididymis  and  vas  deferens  are  ejaculated  by  peristaltic  contrac- 
tions of  the  male  ducts,  and  the  accessory  sex  glands  concurrently 
discharge  their  secretions.  The  seminal  fluid  that  is  deposited  in  the 
upper  part  of  the  vagina  may  contain  as  many  as  400,000,000  sperm.  It 
also  contains  glucose  and  fructose  from  which  the  sperm  derive  energy, 
mucus  that  serves  as  a  conveyance,  and  alkaline  materials  that  neu- 
tralize the  acids  produced  by  sperm  metabolism  and  those  normally 
present  in  the  vagina.  Sperm  are  quickly  killed  in  an  acid  environment. 

Sperm  move  from  the  vagina  through  the  uterus  and  up  the  Fal- 
lopian tube  in  a  matter  of  a  few  hours  or  less.  How  they  do  this  is  not 
entirely  understood.  They  can  swim,  tadpole  fashion,  by  the  beating  of 
the  tail,  but  muscular  contractions  of  the  uterus  and  Fallopian  tubes 
must  help  considerably.  Fertilization  occurs  in  the  upper  part  of  the 
Fallopian  tube  (Fig.  28.6),  but  the  arrival  of  an  egg  and  the  sperm  in 
this  region  need  not  coincide  exactly.  Sperm  retain  their  fertilizing 
powers  for  a  day  or  two,  and  the  egg  moves  slowly  down  the  oviduct, 
retaining  its  ability  to  be  fertilized  for  about  a  day.  The  chance  of 
fertilization  is  further  increased  in  many  species  of  mammals  (but  not 
in  human  beings)  by  the  female  coming  into  "heat"  and  receiving  the 
male  only  near  the  time  of  ovulation.  Ovulation,  heat,  and  changes  in 
the  uterine  lining  in  preparation  for  the  reception  of  a  fertilized  egg 
are  controlled  by  an  intricate  endocrine  mechanism  that  will  be  con- 
sidered in  Chapter  30. 

Only  one  sperm  fertilizes  each  egg,  yet  unless  millions  are  dis- 
charged, fertilization  does  not  occur.  One  reason  for  this  is  that  only 
a  fraction  of  the  sperm  deposited  in  the  vagina  reach  the  upper  part  of 
the  Fallopian  tube.  The  others  are  lost  or  destroyed  along  the  way. 
Another  reason  is  that  when  the  egg  enters  the  Fallopian  tube,  it  is 
still  surrounded  by  a  few  of  the  follicle  cells  that  encased  the  egg  within 
the  ovary  (Fig.  3.16),  and  a  sperm  cannot  penetrate  the  egg  until  these  are 
dispersed.  This  requires  an  enzyme,  hyaluronidase,  which  can  break  down 
hyaluronic  acid,  a  component  of  the  intercellular  cement.  Hyaluroni- 
dase is  believed  to  be  produced  by  the  sperm  themselves,  and  large 
numbers  are  apparently  necessary  to  produce  enough  of  it. 

Establishment  of  the  Embryo  in  the  Uterine  Lining.  The  fertilized 
egg  passes  down  the  Fallopian  tube  into  the  uterus,  undergoing  cleav- 
age along  the  way.  Energy  for  early  development  is  supplied  by  the 
small  amount  of  food  within  the  egg  (mammalian  eggs  are  isolecithal), 
and  by  secretions  from  glands  in  the  uterine  lining.  About  a  week 
after  fertilization  the  embryo  of  most  mammals  penetrates  the  uterine 
lining,  apparently  by  secreting  digestive  enzymes,  and  the  lining  folds 
over  it.  The  extraembryonic  membranes  that  form  the  placenta  de- 
velop very  rapidly  in  mammalian  embryos.  A  functional  placenta  is 
established  in  the  human  embryo  about  three  weeks  after  fertilization, 
and  this  provides  for  the  metabolic  requirements  of  the  embryo  during 
the  rest  of  embryonic  life  (Fig.  28.7).  The  development  of  the  embryo 
itself  is  described  in  Chapter  31. 


572  VERTEBRATE   LIFE   AND    ORGANIZATION 


Uterine, 
muscle 


Pla.cen.ta-' 

Chorionic 
villi 


Ubcrine 


lining 


Umbilica.1  cord 


Uterine  linind 


Chorion 


mnion 


ervix 


YolK  sa.c 


Figure  28.7.  A  young  human  embryo  surrounded  by  its  extraembryonic  mem- 
branes and  lying  within  the  uterus.  Notice  that  the  whole  complex  of  embryo  and 
membranes  is  embedded  in  the  uterine  lining.  Villi  are  present  all  over  the  surface 
of  the  chorion  at  this  stage,  but  only  those  on  the  side  toward  the  uterine  wall  enlarge 
and  contribute  to  the  delinitive  placenta.  (Modified  after  Patten.) 


Birth.  As  the  embryo  develops,  the  uterus  enlarges  considerably 
to  accommodate  it.  At  the  time  of  conception,  the  human  uterus  does 
not  protrude  lar  above  the  pubic  symphysis  (Fig.  28.5),  but  nine 
months  later,  when  embryonic  development  has  been  completed,  it 
extends  up  in  the  abdominal  cavity  nearly  to  the  level  of  the  breasts. 
During  this  enlargement,  the  individual  muscle  fibers  in  its  wall 
increase  in  size,  and  additional  muscle  develops  from  undifferentiated 
cells  in  the  uterine  wall.  The  uterus  becomes  a  powerful  muscular 
organ  ready  to  assume  its  role  in  childbirth,  or  parturition. 

The  factors  that  initiate  birth  are  uncertain,  but  hormones  pro- 
duced by  the  pituitary,  ovary  and  the  placenta  itself  have  prepared 
the  mother's  body  for  the  birth.  The  mammary  glands  have  enlarged 
and  are  ready  for  milk  production,  the  uterine  musculature  has  in- 
creased, and  the  pubic  and  other  pelvic  ligaments  have  relaxed  so  that 
the  pelvic  canal  can  enlarge  slightly.  Birth  begins  by  a  series  of  involun- 
tary uterine  contractions,  "labor,"  that  gradually  increase  in  intensity 
and  push  the  fetus,  generally  head  first,  against  the  cervix.  The  cervix 
gradually  dilates,  but  in  human  begins  as  much  as  18  hours  or  more 
may  be  required  to  completely  open  the  cervical  canal  at  the  first  birth. 
The  sac  of  amniotic  fluid  that  surrounds  the  fetus  acts  as  a  wedge  and 
also  helps  to  open  the  cervix.  The  amnion  normally  ruptures  during 
this  process,  and  the  amniotic  fluid  is  discharged.  \Vhen  the  head 
begins  to  move  down  the  vagina,  particularly  strong  uterine  contrac- 
tions set  in,  and  the  baby  is  born  within  a  few  minutes.  A  few  more 
contractions    of    the    uterus    force    most   of    the    fetal    blood    from    the 


THE   UROGENITAL   SYSTEM — EXCRETION   AND    REPRODUCTION  573 

placenta  to  the  baby,  and  the  umbilical  cord  can  be  cut  and  tied,  al- 
though tying  is  unnecessary  for  contraction  of  the  uterine  vessels  would 
prevent  excessive  bleeding  of  the  infant.  Other  mammals  simply  bite 
through  the  cord.  W^ithin  a  week,  the  stump  of  the  cord  shrivels,  drops 
off,  and  leaves  a  scar  known  as  the  navel. 

Uterine  contractions  continue  for  a  while  after  birth,  and  the 
placenta  and  remaining  extraembryonic  membranes  are  expelled  as 
the  "after-birth."  Much  of  the  uterine  lining  is  lost  at  birth,  for  the 
human  placenta  is  an  intimate  union  of  fetal  membranes  and  maternal 
tissue.  Uterine  contractions  prevent  excessive  bleeding  at  this  time. 
Following  the  birth  the  uterine  lining  is  gradually  reconstituted,  and 
the  uterus  decreases  in  size,  though  it  does  not  become  as  small  as  it 
was  originally. 

Questions 

1.  Describe  the  evolutionary  sequence  of  kidneys.  Compare  this  with  the  embryonic 
sequence. 

2.  What  are  the  mammalian  homologues  of  the  cloaca  of  more  primitive  vertebrates? 

3.  Describe  a  mammalian  nephron  and  its  blood  supply,  citing  the  functions  of  the 
various  parts. 

4.  Define  renal  threshold.  Of  what  significance  is  this  in  maintaining  the  constancy  of 
the  internal  environment? 

5.  What  changes  have  occurred  in  nephron  structure  and  in  the  products  of  excretion 
during  the  evolution  of  terrestrial  vertebrates? 

6.  With  what  is  the  descent  of  the  mammalian  testes  correlated? 

7.  How  are  the  male  genital  ducts  related  to  the  kidney  and  excretory  ducts? 

8.  How  are  the  reproductive  organs  of  male  and  female  mammals  adapted  for  reproduc- 
tion in  a  terrestrial  environment? 

9.  Why  are  millions  of  sperm  necessary  to  insure  fertilization  in  mammals? 
10.  Describe  the  birth  process  in  man. 

Supplementary  Reading 

Baldwin's  Comparative  Biochemistry  contains  an  interesting  account  of  the  osmotic 
and  excretory  problems  that  confronted  the  ancestors  of  vertebrates  in  moving  from  a 
marine  to  a  fresh-water  environment,  and  of  the  problems  vertebrates  subsequently  en- 
countered when  they  entered  other  environments.  H.  W.  Smith,  a  leading  student  of  the 
vertebrate  kidney,  deals  with  these  same  problems,  as  well  as  with  kidney  structure  and 
function,  in  a  very  readable  book  entitled  From  Fish  to  Philosopher,  and  in  an  article  in 
the  Scientific  American  entitled  The  Kidney.  He  considers  renal  functions  more  thor- 
oughly in  his  book,  Principles  of  Renal  Physiology.  There  is  a  very  good  account  of  the 
biology  of  sex  and  reproduction  in  Turner's  General  Endocrinology.  .Asdell's  Patterns 
of  Mammalian  Reproduction  is  an  important  source  book  on  differences  in  reproduction 
and  reproductive  cycles  that  occur  in  the  various  kind  of  mammals  from  the  aardvark 
to  the  zebu. 


CHAPTER  29 


Sense  Organs 
and  Nervous  Coordination 


If  an  organism  is  to  be  successful  and  survive  in  the  complex  world  in 
which  it  lives,  the  activities  of  all  of  its  organs  must  be  integrated  so 
that  the  organism  will  function  and  will  make  appropriate  responses 
to  its  external  and  internal  environment.  In  the  higher  animals,  in- 
tegration is  accomplished  by  special  receptors,  or  sense  organs,  which 
detect  changes  in  the  environment,  and  by  the  nervous  system,  which 
conveys  the  impulses  initiated  by  the  sense  organs  to  appropriate  ef- 
fectors (muscles,  glands),  whose  activity  brings  about  the  appropriate 
response.  Many  vertebrate  and  invertebrate  effectors  are  regulated  in  part 
by  hormones  that  are  secreted  by  endocrine  glands  and  transported  in  the 
blood  stream.  It  will  be  shown  in  the  next  chapter  that  endocrine  integra- 
tion tends  to  be  general  rather  than  specific  in  its  action;  that  is  to  say, 
one  hormone  may  affect  more  than  one  organ.  Endocrine  integration  is 
generally  slower  but  longer  lasting  than  nervous  integration;  it  is  espe- 
cially effective  in  controlling  continuing  processes  such  as  metabolism  and 
growth.  In  a  few  instances,  e.g.,  in  the  control  of  pancreatic  secretion, 
endocrine  integration  is  specific  and  rapid,  but  most  of  the  specific  and 
rapid  adjustments  are  achieved  by  the  sense  organs  and  the  nervous 
system.  Nervous  integration  is  highly  specific;  the  neurons  carry  im- 
pulses from  specific  receptors  to  the  spinal  cord  or  brain,  from  which 
impulses  go  out  through  other  neurons  to  specific  effectors.  It  is  rapid 
because  the  nerve  impulse  can  travel  very  fast— as  fast  as  140  meters  per 
second  in  the  larger,  myelinated  mammalian  neurons— and  a  second 
impulse  can  follow  after  a  brief  recovery  period  that  lasts  at  most  only 
several   milliseconds. 

It  will  be  recalled  from  Chapter  5  that  our  ability  to  perceive 
different  kinds  of  stimuli  (touch,  light,  sound,  etc.)  is  a  function  of 
the  specificity  of  the  receptors,  which  are  attuned  to  specific  stimuli, 
and  of  their  specific  connections  within  the  nervous  system.  The  nerve 
impulse  that  is  initiated  is  not  specific  and  is  fundamentally  the  same 
regardless  of  where  it  comes  from.  Awareness  of  the  sensation  depends 
on  the  precise  part  of  the  brain  the  impulse  reaches.  This  can  be  dem- 
onstrated by  by-passing  the  receptor  and  stimulating  its  neurons  di- 
rectly. The  subject  then  feels  the  same  sort  of  sensation  as  if  the 
receptors  themselves  had  been  stimulated.  People  who  have  had  ampu- 

574 


SENSE   ORGANS   AND   NERVOUS    COORDINATION  575 

tations  sometimes  experience  "phantom  limbs,"  i.e.,  sensations  that 
appear  to  come  from  the  missing  part,  for  nerves  in  the  stump  that 
were  formerly  connected  with  the  missing  part  may  be  stimulated  by 
pressure  or  other  factors. 

Vertebrates  have  many  kinds  of  receptors,  more  than  the  usual 
"five  senses."  There  are  chemoreceptors  in  the  nose  and  mouth  that 
provide  for  smell  and  taste;  various  kinds  of  mechanoreceptors  in 
many  parts  of  the  body  that  detect  touch,  pressure,  muscle  stretch, 
vibrations  and  balance;  photoreceptors  for  light;  thermoreceptors  in 
the  skin  and  mouth  for  heat  and  cold;  and  free  nerve  endings  in  the 
skin  and  internal  organs,  whose  stimulation  results  in  sensations  of 
pain.  Most  of  these  receptors  are  microscopic,  consisting  of  only  a  iew 


Encapsulating 
connective  tissue 


Myelin  sheath. 


Nerve 
fibers 


xon 


B 


Conneccive 

tissue 

Shea-th 


Muscle 
fibers 


TaLste  bud 


^ — Pore 


Surface  of 
tongue 


■Ne-rve 
fibers 


^ Surface  fluid 

Supporting  ce.ll 

Olfactory  cell 

-  Fibers  to 
olfactory  bulb 


Figure  29.1.  A  group  of  mammalian  receptors.  A,  Meissner's  corpuscle  found 
beneath  the  epidermis,  assumed  to  be  sensitive  to  touch;  B,  Pacinian  corpuscle  found  in 
the  dermis  and  many  internal  organs,  sensitive  to  pressure;  C,  neuromuscle  spindle, 
sensitive  to  muscle  tension  (proprioception);  D,  taste  buds  between  papillae  on  the 
surface  of  the  tongue;  E,  olfactory  cells  in  the  nasal  mucosa.  The  olfactory  cells  are 
known  as  neurosensory  cells  for  they  are  both  receptive  and  transmitting  cells.  (A, 
Modified   after   Ranson;    B    and  E,  after   Gardner;    C,   after    Maximow    and    Bloom.) 


576  VERTEBRATE    LIFE   AND    ORGANIZATION 

receptive  cells  embecUled  in  the  skin,  on  the  tongue,  or  in  other  parts 
ol  the  body  (I'ig.  29.1).  In  the  late  nineteenth  century,  von  Frey  cor- 
related specific  receptors  with  specific  sensations.  There  is  no  doubt 
that  we  discriminate  between  modalities  of  sensation,  and  most  biolo- 
gists have  accepted  von  Frey's  conclusions,  but  recently  (1954)  Weddell 
and  others  have  cpiestioned  von  Frey's  specific  correlations,  at  least  as 
regards  the  modalities  oi  cutaneous  sensation.  Other  sense  organs,  such 
as  the  eye  and  ear,  are  complex  aggregations  of  receptor  cells  and 
associated   tissues. 


243.        The  Eye 

Ancestral  vertebrates  had  eyes  of  two  types— a  median  eye  on  the 
top  of  the  head,  which  probably  distinguished  only  between  light  and 
dark,  and  a  pair  of  image-forming  eyes  on  the  sides  of  the  head. 
Cyclostomes  and  a  few  reptiles  retain  a  functional  median  eye,  but  in 
most  groups  it  has  become  a  small  vestigial  organ,  the  pineal  body, 
attached  to  the  top  of  the  brain.  The  mammalian  pineal  body  is  a 
small,  glandlike  organ  that  has  been  suspected  of  being  an  endocrine 
organ,  largely  because  it  has  no  other  known  function.  There  is,  how- 
ever, no  clear  evidence  of  this.  Neither  its  removal  nor  the  injection 
of  extracts  of  pineal  glands  has  a  reproducible  effect  on  experimental 
animals. 

Structure  of  the  Mammalian  Eye.  Although  the  lateral,  image- 
forming  eyes  of  different  groups  of  vertebrates  vary  in  their  adaptation 
for  seeing  beneath  water,  in  the  air,  and  under  varying  light  intensities, 
all  are  alike  in  their  major  features.  Those  of  mammals  may  be  taken 
as  an  example.  Each  eyeball  is  an  oval-shaped  organ  constructed  on  the 
principles  of  a  simple  camera  (Fig.  29.2  A).  It  has  a  small  opening  at 
the  front,  the  pupil,  through  which  light  enters,  a  lens,  which  brings 
the  images  of  objects  into  sharp  focus,  and  a  light-sensitive  retina, 
which  is  analogous  to  the  film. 

The  wall  of  the  eyeball  is  composed  of  three  layers  of  tissue.  The 
outermost  one  is  a  dense,  fibrous  connective  tissue  that  gives  strength 
to  the  wall.  Most  of  this  layer  is  opaque  and  is  known  as  the  sclera, 
but  its  anterior  portion,  through  which  light  passes,  is  clear  and  is 
called  the  cornea.  The  surface  of  the  cornea  is  covered  with  a  layer  of 
stratified  epithelium,  the  conjunctiva,  which  is  continuous  with  the 
epidermis. 

The  next  layer  of  the  eyeball  wall  is  a  darkly  pigmented  and 
very  vascular  choroid  coat.  Its  pigmentation  absorbs  light  rays,  thereby 
reducing  internal  reflections  that  might  blur  the  image,  and  its 
vessels  nourish  the  retina.  The  anterior  portion  of  the  choroid  coat, 
together  with  a  nonsensitive  portion  of  the  retina,  extends  in  front 
of  the  lens  and  forms  the  iris-an  opaque  disc  with  the  pupil  in  its 
center.  The  iris  prevents  the  light  from  entering  the  eye  except  through 
the  center  of  the  lens,  which  is  optically  the  most  efficient  part.  The 
amount  of  light  entering  the  eye  is  controlled  by  circularly  and  radially 
arranged  smooth  muscles  in  the  iris  that  constrict  or  dilate  the  pupil.  In 


SENSE   ORGANS   AND   NERVOUS    COORDINATION  577 


Cornea.' 


•Iris 


Anlerior  chamber 
Posterior'  cTiamter 


Ciliary  body 


Ciliary 
process 


Retina.- 

Choroid 
coat 


Conjunctiva. 

Canal  op  Schlemm 
Ciliary  muscle 


Extrinsic 
muscle 


fL  Fibers  oF 
Coptic  nerve 

QanSlion 
cells 


Bipolar 
cell 


-Blind  spot 
Optic  nerve  and  sheath 
A 

Figure  29.2.  The  mammalian  eye.  A,  Diagram  of  a  section  through  the  eye;  B, 
diagram  of  the  layer  of  the  retina  containing  the  receptor  cells,  rods  and  cones,  and 
the  neurons.  (Modified  after  Walls.) 

this  respect  the  iris  is  analogous  to  the  iris  diaphragm  of  a  camera  or 
microscope.  The  thickened  portion  of  the  choroid  around  the  base  of  the 
iris  is  the  ciliary  body.  A  number  of  zonule  fibers  extend  from  it  to  the 
lens  and  help  to  hold  it  in  place.  Muscles  within  the  ciliary  body  are 
concerned  with  focusing  the  eye. 

The  retina  is  the  innermost  layer  of  the  eyeball.  It  consists  of  a 
pigmented  layer,  intimately  associated  with  the  choroid,  and  a  nerv- 
ous layer,  which  contains  millions  of  receptor  cells,  the  rods  and 
cones,  and  afferent  neurons  that  continue  through  the  optic  nerve 
to  the  brain.  The  rods  and  cones  lie  in  the  surface  of  the  nervous 
layer  that  faces  the  choroid,  and  light  must  pass  through  most  of  the 
retina  before  it  can  stimulate  them.  This  apparently  illogical  arrange- 
ment is  explained  by  the  mode  of  development  of  the  eye.  The  retina 
develops  from  an  outgrowth  of  the  brain,  which  in  turn  develops 
from  an  infolding  of  the  surface  ectoderm  (Fig.  29.3).  What  was  the 
outer  surface  of  the  ectodermal  cells  becomes  the  inner  surface  of  the 
nervous  layer  of  the  retina.  The  polarity  of  the  cells  is  retained  dur- 
ing their  various  developmental  gymnastics.  The  fact  that  the  retina 
and  optic  nerves  are  developmentally  parts  of  the  brain  also  explains 
why  at  least  two  afferent  neurons  (bipolar  and  ganglion  cells)  are 
involved  in  transmitting  impulses  from  the  rods  and  cones.  Chains 
of  neurons  are  common  in  brain  tracts,  but  in  most  nerves  only  one 
neuron  extends  from  a  receptor  cell  to  the  brain  or  spinal  cord. 

Rods  respond  to  light  of  much  lower  intensity  than  cones  and 
are  particularly  efficient  in  dim  light.  It  is  not  surprising,  therefore, 
that  they  are  abundant  in  the  eyes  of  nocturnal  animals.  Cones  are 
more  efficient  in  brighter  light,  and  they  also  distinguish  between 
colors  in  some  way  not  yet  understood.  One  theory  is  that  there  are 
three  types  of  cones,  each  type  sensitive  to  light  of  the  wavelength  of 
one  of  the  three  primary  colors.  Each  cone  typically  activates  a  single 


578 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Neural  -Bold 


Optic  ve-sicle 


Surfa.ce 
ctoderm- 


Lens 
placode- 


Developin^  brain- 


B 


Lens 
vesicle 


Surfa.ce — ' 
ectoderm 


—Pigment  layer  of  retina. 
pNervous  laye-r  of  retina 


Cornea- 


I— Brain 

ectoderm 


■  Optic  stalK 
^  Optic  cup 

C 


Sclera 

Choroid 
Retina 


D 


Figure  29.3.  The  development  of  the  eye.  A,  Cross  section  through  an  embryo 
in  which  the  anterior  portions  of  the  neural  folds  are  closing  to  form  the  brain; 
B,  the  optic  vesicles  evaginate  from  the  sides  of  the  forebrain;  C,  an  optic  cup  develops 
from  each  optic  vesicle  and  the  lens  forms  from  adjacent  surface  ectoderm;  D,  the 
choroid,  sclera  and  part  of  the  cornea  develop  from  surrounding  mesoderm.  Arrows 
indicate  the  original  polarity  of  the  ectoderm  cells.  (D,  From  Romer.) 

neuron  chain  that  extends  to  the  brain;  a  number  of  rods,  on  the  other 
hand,  usually  converge  upon  a  single  neuron  (Fig.  29.2  B).  Thus, 
light  that  falls  upon  the  cones  is  translated  into  a  sharper  image  in 
the  brain  than  that  falling  upon  the  rods.  Cones  are  concentrated 
near  the  center  of  the  retina,  and  are  particularly  abundant  in  an 
area  known  as  the  fovea,  which  is  the  region  of  keenest  vision  in 
bright  light.  However,  if  one  wishes  to  see  in  dim  light  one  must 
look  somewhat  to  the  side  so  the  image  of  the  object  will  fall  on 
the  periphery  of  the  retina  where  there  are  more  rods.  Neither  rods 
nor  cones  are  present  in  the  part  of  the  retina  through  which  the 
optic  nerve  passes,  hence  this  region  is  called  the  blind  spot. 

The  cavities  within  the  eye  are  filled  with  liquid.  A  gelatinous 
vitreous  humor  occupies  the  large  chamber  that  lies  between  the 
lens  and  the  retina,  and  helps  to  hold  the  lens  in  place.  A  watery 
aqueous  humor  fills  the  posterior  chamber  between  the  iris  and 
the  lens,  and  the  anterior  chamber  between  the  iris  and  the  cornea. 
The  aqueous  humor  is  secreted  continually  by  the  ciliary  body  and 
drained  through  the  canal  of  Schlemm  at  the  base  of  the  cornea. 
By  maintaining  the  intraocular  pressure,  the  aqueous  humor  helps 
to  maintain  the   turgidity  and  shape  of   the   eyeball.   Blockage  of   the 


SENSE   ORGANS   AND   NERVOUS   COORDINATION 


579 


canal  of  Schlemm  leads  to  increased  intraocular  pressure  and  the  disease 
glaucoma,  in  which  the  pressure  flattens  and  eventually  injures  the  retina. 

The  eyeball  lies  in  the  orbit  of  the  skull,  and  six  extrinsic  ocular 
muscles,  which  move  the  entire  eyeball,  extend  from  it  to  the  walls 
of  the  orbit.  A  pair  of  movable  eyelids  cover  the  eyeball  and  the 
cornea  is  kept  moist,  cleansed  and  possibly  nourished,  by  the  secre- 
tion of  tears  from  several  tear  glands.  Tears  are  drained  from  the 
median  corner  of  the  eye  by  a  lacrimal  duct  which  leads  into  the 
nasal  cavity.  Pigs,  cats  and  many  other  mammals  have  a  third  lid, 
known  as  the  nictitating  membrane,  located  in  the  median  corner  of 
the  eye.  It  is  moved  passively  over  the  cornea  when  the  eyeball  is 
retracted  slightly,  and  aids  in  cleaning  and  protecting  the  eye.  This 
membrane  is  reduced  to  a  vestigial  semilunar  fold  in  man. 

Vision.  Light  that  enters  the  eye  is  bent  toward  the  optic  axis 
in  such  a  way  that  it  forms  a  sharp,  though  inverted  image  upon  the 
retina  (Fig.  29.4  A).  The  lens  is  important  in  bending  the  light  rays 
but  the  cornea,  humors  and  the  retina  itself  are  also  involved.  The 
cornea  is  the  major  refractive  agent  in  terrestrial  vertebrates,  for 
the  difference  between  the  refractive  index  of  air  and  the  cornea  is 
greater  than  that  between  any  of  the  other  refractive  media.  The 
action  of  the  cornea  places  the  image  approximately  on  the  retina; 
the   lens   brings   it   into   sharp   focus. 

When  the  eye  is  at  rest,  distant  objects  are  in  focus.  The  re- 
fractive power  of  the  eye  must  be  increased  in  viewing  a  near  ob- 
ject, or  its  image  would  be  blurred,  for  the  image  would  come  into 
sharp  focus  theoretically  at  a  point  behind  the  retina.  Accommoda- 
tion   for    near    vision    is   accomplished    by    the    contraction   of   muscles 


Lioht  rays 


Object  — 


Optic  a:xis 


A 


Convex  lens 


Concave  lens 
4^ 


D  E 

Figure  29.4.  Image  formation  by  the  eye.  A,  Normal  eye;  B,  far-sighted  eye; 
C,  far-sighted  eye  corrected  by  a  convex  lens;  D,  near-sighted  eye;  E,  near-sighted  eye 
corrected  by  a  concave  lens. 


580  VERTEBRATE   LIFE   AND   ORGANIZATION 

within  the  ciliary  body.  This  brings  the  point  of  origin  of  the  zonule 
fibers  a  bit  closer  to  the  lens  and  releases  the  tension  of  these  fibers. 
The  front  of  the  elastic  lens  bulges  out  slightly,  and  its  refractive 
powers  are  increased  accordingly.  When  the  ciliary  muscles  are  re- 
laxed, intraocular  pressure  pushes  the  wall  of  the  eyeball  outward, 
increases  the  tension  of  the  zonule  fibers,  and  the  lens  is  flattened 
a  bit.  The  lens  becomes  less  elastic  with  age,  and  our  ability  to 
focus   on    near   objects   decreases. 

The  refractive  parts  of  the  eye  form  a  sharp  image  of  an  object 
on  the  retina  only  in  an  eyeball  of  appropriate  length.  If  the  eyeball 
is  shorter  than  normal,  as  it  is  in  far-sighted  people,  the  image  of  an 
object  in  theory  falls  behind  the  retina.  Accommodation  is  necessary 
to  bring  the  image  into  focus,  and  the  power  of  accommodation 
may  not  be  great  enough  to  locus  on  a  near  object.  This  can  be 
corrected  by  placing  a  convex  lens  in  front  of  the  eye  (Fig.  29.4  B 
and  C).  Near-sighted  people  have  eyeballs  that  are  longer  than  normal 
and  the  image  falls  short  of  the  retina.  This  can  be  corrected  by  a  con- 
cave lens  (Fig.  29.4  D  and  E). 

Light  that  strikes  the  rods  and  cones  activates  them  and  they  in  turn 
initiate  nerve  impulses.  Recent  studies  have  given  us  an  indication  of 
some  of  the  steps  in  this  process.  Each  rod  contains  a  light-sensitive  pig- 
ment known  as  rhodopsin  (visual  purple).  Exposure  to  light  causes  this 
to  split  into  its  components,  a  protein  (opsin)  and  retinene  (visual  yel- 
low), and  the  rod  is  activated  in  the  process.  Recovery  involves  the  re- 
synthesis  of  visual  purple  from  its  components.  Since  retinene  is  an 
aldehyde  of  vitamin  A,  a  person  suffering  from  a  severe  vitamin  A  de- 
ficiency does  not  have  as  much  visual  purple  as  a  normal  person  and 
cannot  see  as  well  in  dim  light.  A  cycle  of  breakdown  and  reconstitution 
of  rhodopsin  goes  on  continually  if  the  eyes  are  exposed  to  any  light. 
The  cycle,  however,  is  influenced  by  the  amount  of  light,  for  visual 
purple  breaks  down  faster  in  bright  light,  and  is  reconstituted  faster  in 
the  dark.  To  see  well  in  dim  light  one  should  stay  in  a  dark  room  for  a 
while  so  that  a  maximum  amount  of  visual  purple  is  reconstituted. 

The  cones  contain  a  light-sensitive  pigment  known  as  visual  violet 
(iodopsin),  which  is  composed  of  retinene  and  a  protein  different  from 
that  in  visual  purple.  The  action  and  biochemistry  of  visual  violet  are 
less  well  understood. 

Eyes  o^  Other  Vertebrates.  The  eyes  of  all  vertebrates  are  essen- 
tially alike,  but  those  of  primitive  vertebrates  differ  from  mammalian 
eyes  in  several  important  respects,  for  the  problems  associated  with  sight 
beneath  water  are  not  identical  with  those  in  the  air.  For  one  thing, 
the  water  itself  cleans  and  moistens  the  eye,  and  fishes  have  not  evolved 
movable  eyelids  or  tear  glands.  Secondly,  the  refractive  index  of  water 
is  nearly  the  same  as  that  of  the  cornea,  so  the  cornea  of  a  fish's  eye 
does  not  bend  light  rays.  Most  refraction  is  accomplished  by  the  lens, 
which  is  nearly  spherical  and  hence  has  a  greater  refractive  power  than 
the  oval  lens  of  tetrapods.  It  is  interesting  in  this  connection  that  the 
lens  of  a  frog's  eye  flattens  a  bit  during  metamorphosis,  when  a  change 
in  environment  occurs.  Finally,  the  method  of  accommodation  differs. 


SENSE  ORGANS   AND   NERVOUS   COORDJNATJON 


581 


for  the  lens  is  moved  back  and  forth  in  camera  fashion  in  fishes  and 
amphibians  and  does  not  change  shape. 

244.        The  Lateral  Line  and  Ear 

Equilibrium.  AH  vertebrates  have  the  ability  to  perceive  differences 
in  the  orientation  of  their  bodies  with  respect  to  their  surroundings  and 
to  maintain  their  equilibrium.  Although  vision  and  proprioceptive  im- 
pulses from  the  muscles  play  a  part,  this  ability  is  primarily  a  function 
of  the  inner  ear.  The  inner  ear  is  embedded  within  the  otic  capsule  of 
the  skull  and  consists  of  a  complex  of  membranous  walled  sacs  and 
canals,  the  membranous  labyrinth,  which  are  filled  with  a  liquid  endo- 
lymph  and  surrounded  by  a  protective  liquid  cushion,  the  perilymph 
(Fig.  29.5).  The  dorsal  part  of  the  membranous  labyrinth  consists  of 
three  semicircular  canals,  each  of  which  is  perpendicular  to  the  other 
two.  Two  lie  in  the  vertical  plane,  but  at  right  angles  to  each  other, 
and  one  is  in  the  horizontal  plane  at  right  angles  to  the  other  two.  Each 
has  a  round  swelling,  an  ampulla,  at  one  of  its  ends  in  which  there  is  a 
patch  of  hair  cells— receptor  cells  bearing  hairlike  processes.  The  three 
semicircular  canals  connect  with  a  chamber  known  as  a  utriculus,  and 
this  in  turn  connects  with  a  more  ventral  chamber  known  as  a  sacculus. 
Both  of  these  chambers  contain  patches  of  hair  cells.  Calcareous  otoliths 
are  in  contact  with  these  cells.  Different  parts  of  the  membranous  labyrinth 
are  concerned  with  different  aspects  of  equilibrium— static  equilibrium, 
linear  acceleration  and  angular  acceleration.  Differences  in  the  position  of 
the  head  and  body  (static  equilibrium)  affect  the  way  in  which  gravity  pulls 
the  otoliths  uik)ii  the  underlying  hair  cells.  Rapid  forward  movement 
(linear  acceleration)  cause  the  otoliths,  which  have  more  inertia  than 
the  surrounding  endolymph  and  hence  lag,  to  push  back  upon  certain 


Utriculus 


Ante-rior  vert  iced 
Semicircula-r  csLnal 


Ampiilla.' 

Horizontal 
Scrnicirculair  canal 


EndolyiTLphatic  sac 


Perilymph 

^-Posterior  ve.rtical 
.'se-micircular  canal 


Patches  of  hair  cells 
Sa.cculus 


ena 


Figure  29  5  The  left  ear  of  a  fish  seen  in  a  lateral  view.  Only  an  inner  ear  is 
present,  embedded  within  spaces  in  the  otic  capsule  of  the  skull.  (Modified  after 
Kingsley.) 


582  VBRTEBHATE   LIFE   AND   ORGANIZATION 

hair  cells.  Sudden  turns  of  the  head  in  various  planes  (angular  accelera- 
tion) induce  movements  ol  the  endolymph  within  the  semicircular 
canals,  which   in  turn  stimulate  hair  cells  in  the  ampullae. 

Phonoreception  in  Fishes.  The  part  of  the  ear  concerned  with 
equilibrium  is  essentially  the  same  in  all  vertebrates,  but  the  part  con- 
cerned with  phonoreception  or  hearing,  that  is,  the  detection  of  sound 
vibrations,  differs  considerably  among  vertebrates.  Mammals,  birds  and 
some  reptiles  have  a  cochlear  duct,  an  elongated  cul-de-sac  extending 
from  the  sacculus  which  is  clearly  concerned  with  phonoreception. 
Fishes  have  a  homologous  but  very  small  diverticulum  known  as  the 
lagena.  The  rudimentary  nature  of  this  structure,  together  with  early 
experiments  in  which  fishes  were  shown  to  be  unresponsive  to  sounds 
made  in  the  air,  led  to  the  conclusion  that  they  could  not  hear.  Later 
this  conclusion  was  questioned  when  it  was  realized  that  most  air-borne 
sound  waves  are  reflected  by  the  air-water  surface,  and  when  it  was  dis- 
covered that  there  are  a  great  many  sounds  produced  in  the  water  by 
aquatic  organisms.  Dr.  Moulton  of  Bowdoin  College  has  been  able  to 
induce,  or  to  suppress,  the  staccato  calls  of  the  sea  robin  (Prionotus)  by 
appropriate  underwater  noises!  Sound  waves  travel  rapidly  in  the  water 
and  pass  without  interruption  through  the  flesh  of  a  fish;  tissues  have  a 
high  content  of  water.  More  recent  experiments  by  Dr.  von  Frisch  of 
the  University  of  Munich  and  Dr.  Griffin  of  Harvard  University  have 
shown  that  many  fishes  respond  to  underwater  sounds  of  a  wide  range  of 
frequencies  provided  the  sacculus  and  lagena  are  intact.  Catfishes  and 
some  other  fishes  that  are  particularly  sensitive  to  sounds  apparently 
use  their  swim  bladder  as  a  hydrophone.  This  picks  up  vibrations  pass- 
ing through  a  large  part  of  the  body,  and  transmits  them  via  a  chain 
of  small  bones  derived  from  the  vertebrae  (Weberian  ossicles)  to  the 
sacculus  and  lagena. 

Clearly,  fishes  can  detect  underwater  sounds  by  means  of  a  part  of 
the  membranous  labyrinth.  In  addition,  fishes  have  a  lateral  line  system 
that  is  sensitive  to  currents,  to  changes  in  pressure  and  to  vibrations  of 
low  frequency.  It  consists  of  a  longitudinal  canal  extending  the  length 
of  the  trunk  and  tail,  and  of  a  series  of  canals  that  ramify  over  the  head. 
These  canals  are  embedded  in  the  skin  and  connect  with  the  surface 
through  pores.  Water  enters  these  canals  and  stimulates  hair  cells  in  the 
lining  similar  to  those  in  the  ear.  Neurons  from  these  receptors  enter 
an  acoustico-lateralis  area  of  the  brain  along  with  neurons  from  the 
ear,  which  suggests  that  there  is  a  close  relationship  between  the  ear 
and  lateral  line.  The  inner  ear  develops  embryonically  in  close  asso- 
ciation with  certain  lateral  line  canals,  and  it  may  have  evolved  in  the 
same  way.  Larval  amphibians  have  a  lateral  line  system,  but  it  is  lost 
during  metamorphosis.  Higher  vertebrates  never  have  this  system  at  all. 

Phonorecepffon  in  Tetrapods.  In  all  tetrapods,  a  part  of  the  mem- 
branous labyrinth,  generally  the  lagena  or  cochlear  duct,  is  specialized 
for  phonoreception,  and  various  devices  have  evolved  which  transmit 
either  ground  or  air-borne  vibrations  to  it.  Frogs  have  an  external  tym- 
panic membrane  (Fig.  21.17)  which  responds  to  vibrations  in  the  air, 
and    a    stapes,    which    transmits    the   vibrations    across    the   middle   ear 


SENSE   ORGANS   AND   NERVOUS    COORDINATION 


583 


cavity  to  a  fenestra  ovalis  in  the  otic  capsule.  The  fenestra  ovalis  com- 
municates with  the  inner  ear. 

The  hearing  apparatus  of  mammals  is  basically  similar  but  much 
more  elaborate  (Fig.  29.6  A).  Most  mammals  have  a  well  developed 
external  ear  consisting  of  a  canal,  the  external  auditory  meatus,  and 
an  external  flap,  the  pinna,  which  in  some  species  helps  funnel  sound 
waves  into  the  meatus.  The  delicate  tympanic  membrane  hes  at  the 
internal  end  of  the  meatus  where  it  is  protected  against  injury.  The 
three  auditory  ossicles  (the  hammer-shaped  malleus,  the  anvil-shaped 
incus  and  the  stirrup-shaped  stapes,  arranged  in  sequence)  transmit 
vibrations  across  the  middle  ear  cavity  to  the  fenestra  ovalis,  or  oval 
window.  The  stapes  evolved  from  a  part  of  the  hyoid  arch  of  fishes, 
and  the  malleus  and  incus  were  derived  from  the  posterior  part  of  the 
mandibular  arch  when  a  new  jaw  joint  evolved  in  mammals  anterior 
to  the  former  one.  These  three  ossicles  form  a  system  of  levers  that 
reduces  the  amplitude,  but  increases  the  force  of  the  sound  waves.  The 
movement  of  the  loot  plate  of  the  stapes  against  the  membrane  within 
the  oval  window  is  only  about  one  half  as  extensive  as  the  movement 


-Membrekxious  labyrinth 

■Perilymph 

-Middle  S3cr  cavity 

"Inciis 

rPinria. 

-Malleus 


Craniad 
cavity 


Canal  for 
acoustic  nerve 


Cochlea 


External  auditory  meatus 
■Tympanic  membrane 
Stapes  in  oved -window 
Rourid  v7indo-w 
■Eustachian  tube 


Cochlear 
duct 


Basilar 
meiTibrane 


Scalatympani      Round  window-' 


B  C 

Figure  29.6.  The  mammalian  ear.  A,  Schematic  drawing  of  the  outer,  middle 
inner  ear  of  a  human.  B,  Diagram  of  the  cochlea  as  though  it  were  uncoiled.  C, 
enlarged  cross  section  through  the  cochlea.  The  cochlear  duct  and  other  parts  of 
membranous  labyrinth  are  filled  with  endolymph. 


and 
An 
the 


584  VERTEBRATE   LIFE   AND   ORGANIZATION 

of  the  tympanic  membrane,  but  the  force  of  the  movement  is  two  or 
three  times  as  great.  The  increased  pressure  provides  for  keener  hearing 
because  the  sound  waves  must  be  converted  to  waves  in  the  Hquid  of 
the  inner  ear,  antl  liquid  is  much  less  compressible.  The  fact  that  the 
tympanic  membrane  has  nearly  ten  times  the  surface  area  of  the  mem- 
brane in  the  oval  window  also  increases  the  pressure  of  waves  in  the 
endolymph.  Virtually  all  of  the  force  that  impinges  on  the  tympanic 
membrane  reaches  the  membrane  in  the  oval  window,  and,  since  this 
membrane  is  smaller,  the  force  per  square  millimeter  is  increased. 

The  middle  ear  cavity,  in  which  the  ossicles  lie,  evolved  from  the 
first  gill  slit,  homologous  to  the  spiracle  of  many  fishes.  It  connects  with  the 
jjharynx  via  the  Eustachian  tube  and  hence  indirectly  with  the  outside 
of  the  body.  The  pharyngeal  opening  of  the  Eustachian  tube  is  nor- 
mally closed,  but  if  pressures  become  unequal  on  the  two  sides  of  the 
tympanic  membrane,  swallowing  is  stimulated  by  a  reflex,  the  Eu- 
stachian tube  opens  and  the  pressures  are  equalized. 

A  long,  cochlear  duct  has  evolved  from  the  lagena  of  fishes,  and  it 
contains  the  actual  receptive  structure,  the  organ  of  Corti  (Fig.  29.6  B 
and  C).  The  cochlear  duct  is  filled  with  endolymph  and  is  a  part  of 
the  membranous  labyrinth.  Vibrations  reach  the  cochlear  duct  via 
specialized  perilymphatic  channels.  A  scala  vestibuli  begins  at  the  oval 
window,  extends  along  the  cochlear  duct,  curves  around  its  apex,  and 
returns  as  the  scala  tympani  to  a  fenestra  rotunda,  or  round  window, 
that  is  separated  by  a  delicate  membrane  from  the  middle  ear  cavity. 
The  round  window  permits  the  escape  of  the  vibrations  of  the  peri- 
lymph induced  by  the  vibrations  of  the  ossicles  against  the  oval  win- 
dow. Since  liquids  are  incompressible  the  liquids  in  the  inner  ear  could 
not  vibrate  unless  there  were  some  mechanism  similar  to  this.  The 
scala  vestibuli  and  scala  tympani  have  a  different  origin  than  the  coch- 
lear duct,  but  all  three  are  in  intimate  association  and  collectively 
form  the  spiral-shaped  cochlea. 

Vibrations  or  pressure  waves  induced  by  the  stapes  at  the  oval 
window  pass  through  the  scala  vestibuli,  cross  the  cochlear  duct,  travel 
back  through  the  scala  tympani,  and  escape  at  the  round  window.  The 
basilar  membrane,  which  supports  the  organ  of  Corti,  is  set  in  vibra- 
tion and  rubs  the  hair  cells  of  this  organ  against  an  overlying  tectorial 
membrane.  Sensory  neurons  of  the  acoustic  nerve  extend  from  the 
hair  cells  to  the  brain.  Cochlear  mechanisms  are  very  complex,  and 
just  how  the  basilar  membrane  is  activated  is  uncertain.  It  is  well  estab- 
lished that  tones  of  different  frequency  are  detected  in  different  regions 
of  the  cochlea— low  notes  near  the  apex  and  high  notes  near  the  base. 
Presumably  a  loud  sound  of  a  certam  frequency  is  distinguished  from 
a  soft  sound  of  the  same  frequency  because  the  loud  sound  sets  up 
stronger  vibrations  that  stimulate  the  hair  cells  more  vigorously,  and 
they  initiate  more  nerve  impulses. 

In  an  organ  as  elaborate  as  the  ear,  many  things  can  go  wrong. 
Infections  may  enter  the  middle  ear  via  the  Eustachian  tube  and  affect 
the  auditory  ossicles.  The  stapes  may  become  locked  in  the  oval  window 
by  an   abnormal  growth  of  bone,   or   the   individual  ossicles  may  fuse 


SENSE   ORGANS   AND    NERVOUS    COORDINATION  585 

together.  Conduction  deafness  of  these  types  can  be  corrected  by  a 
hearing  aid  that  amplifies  vibrations  enough  to  be  transmitted  directly 
through  the  skull  bones  to  the  cochlea.  More  rarely  the  acoustic 
nerve  or  the  cochlea  may  be  damaged.  Deafness  of  this  sort  cannot  be 
corrected.  If  only  a  part  of  the  cochlea  is  injured,  one  may  become  deaf 
only  to  sounds  of  certain  frequencies.  The  continuing,  loud,  high- 
pitched  noises  to  which  boilermakers  are  subjected  sometimes  destroy  a 
part  of  the  cochlea,  and  they  become  deaf  to  sounds  of  this  frequency. 
Observations  of  this  type,  and  similar  experiments  performed  on  vari- 
ous mammals,  established  the  fact  that  sounds  of  different  frequency 
are  detected  by  different  parts  of  the  cochlea. 

245.        Organization  of  the  Nervous  System 

Neurons  and  the  Nerve  Impulse.  The  nervous  system  provides 
for  the  coordination  and  integration  of  the  body's  many  activities  by 
conducting  impulses  from  the  receptors  to  the  appropriate  effectors. 
It  is  composed  of  nerve  cells  or  neurons,  which  conduct  the  impulses, 
and  of  supporting  cells  known  as  neuroglia.  We  previously  considered 
the  morphology  and  many  aspects  of  the  physiology  of  these  cells,  but 
it  is  appropriate  at  this  time  to  examine  the  nature  of  the  nerve  im- 
pulse more  thoroughly. 

The  biochemical  processes  that  are  responsible  for  a  nerve  im- 
pulse are  not  completely  imderstood,  but  the  impulse  itself  is  a  wave 
of  "depolarization"  that  spreads  along  the  plasma  membrane  of  the 
neuron  (Fig.  29.7).  An  electric  potential  exists  across  the  membrane 
of  a  resting  netnon,  for  there  are  more  positively  charged  ions  on  the 
outside  of  the  membrane  than  on  the  inside;  sodium  ions  (Na+)  in 
particular  are  abundant  on  the  outside.  The  membrane  is  said  to  be 
polarized.  Stimulating  the  neuron  at  any  point  increases  the  perme- 
ability of  its  plasma  membrane.  Ions  that  were  held  apart  now  can 
and  do  move  from  one  side  to  the  other.  The  outside  of  the  mem- 
brane at  the  point  of  stimulation  loses  positive  ions,  and  therefore 
becomes  negative  relative  to  other  parts  of  the  surface.  The  opposite 
condition  is  found  on  the  inside.  Although  the  membrane  is  said 
to  be  "depolarized,"  the  polarity  of  the  membrane  is  actually  re- 
versed at  the  point  of  stimulation.  This  reversed  polarity  increases 
the  permeability  of  adjacent  parts  of  the  plasma  membrane,  ions 
move  freely  through,  the  polarity  of  these  regions  becomes  reversed,  and 
this  reversal  in  turn  affects  the  next  adjacent  parts  of  the  membrane.  The 
impulse  continues  in  this  way  along  the  neuron  in  both  directions  from 
the  point  of  stimulation. 

The  electrical  changes  that  accompany  the  nerve  impulse  are  known 
as  the  acHon  potential.  These  changes  can  be  measured,  and  it  has  been 
found  that  impulses  travel  along  mammalian  neurons  at  speeds  ranging 
from  0.5  to  140  meters  per  second.  Myelinated  fibers  and  fibers  with 
relatively  large  diameters  transmit  impulses  faster  than  nonmyelinated 
and  small  fibers.  Even  140  meters  per  second  is  very  slow  compared  to 
the  speed  of  an  electric  current  flowing  through  copper  wire.  The  elec- 


586 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  29.7.     The  transmission  of  a  nerve  impulse.  (Modified  after  Guyton.) 

trie  current  is  a  flow  of  electrons;  the  nerve  impulse  a  wave  of  depolari- 
zation involving  changes  in  the  permeability  of  the  plasma  membrane 
and  the  movement  of  ions.  The  electric  current  derives  its  energy  from 
the  difference  in  potential  at  the  opposite  ends  of  the  wire;  the  nerve 
impulse,  from  chemical  changes  that  take  place  in  each  part  of  the 
neuron.  In  this  respect,  the  transmission  of  a  nerve  impulse  is  analogous 
to  the  burning  of  a  fuse:  the  powder  in  a  given  part  of  the  fuse  provides 
the  energy  for  the  burning  of  the  fuse  in  that  region. 

As  the  nerve  impulse  passes  along  the  neuron,  the  membrane  be- 
comes repolarized  at  the  site  of  stimulation,  and  a  wave  of  repolariza- 
tion spreads  along  the  neuron.  However,  it  takes  a  measurable  period 
of  time  for  the  chemical  changes  responsible  for  repolarization  to  occur. 
During  the  period  that  the  membrane  is  depolarized,  a  second  impulse 
cannot  be  transmitted  along  the  neuron,  but  as  soon  as  the  membrane 
is  repolarized,  another  impulse  can  proceed.  The  very  brief  period,  one 
or  two  milliseconds,  during  which  the  neuron  is  recovering  is  known  as 
the  refractory  period.  The  presence  of  such  a  period  means,  of  course, 
that  a  neuron  can  transmit  only  a  limited  number  of  impulses  per  unit 


SENSE   ORGANS    AND   NERVOUS    COORDINATION  587 

of  time;  about  1,000  per  second  in  the  case  of  certain  neurons  with  very 
short  refractory  periods. 

When  a  neuron  is  artificially  stimulated  at  some  point  near  the 
middle,  the  impulse  spreads  along  the  neuron  in  both  directions.  Im- 
pulses can  travel  in  either  direction,  but  under  normal  conditions 
neurons  are  stimulated  only  at  their  dendritic  ends  and  impulses  travel 
only  toward  the  axonal  ends.  A  neuron  is  normally  stimulated  at  its 
dendritic  end  because  this  is  the  end  that  is  related  to  the  sense  organs, 
and  because  the  nerve  impulse  can  travel  in  only  one  direction  across 
a  synapse— from  the  axon  of  one  neuron  to  the  dendrites  or  cell  body  of 
another. 

The  initiation  of  an  impulse  in  a  neuron,  either  by  a  sense  organ 
or  by  the  transmission  of  an  impulse  across  a  synapse  from  an  adjacent 
neuron,  is  a  complex  phenomenon.  A  neuron  will  not  initiate  an  impulse 
unless  the  stimulus  that  it  receives  from  the  sense  organ  or  presynaptic 
neuron  is  strong  enough  to  cause  the  chemical  changes  that  underlie  an 
impulse  to  reach  a  certain  threshold  level.  In  this  way,  too,  the  neuron  is 
analogous  to  a  fuse  which  does  not  burn  until  the  temperature  (stimulus) 
reaches  a  certain  threshold  level.  The  threshold  levels  of  neurons  vary. 
Some  postsynaptic  neurons  with  a  low  threshold  will  fire  if  a  single  im- 
pulse reaches  them.  But  most  postsynaptic  neurons  have  a  higher  thresh- 
old, and  a  single  impulse  reaching  them  is  insufficient  to  initiate  an 
impulse  in  them.  Such  neurons  will  not  fire  unless  several  impulses  reach 
them  simultaneously  from  several  presynaptic  neurons,  or  in  rapid  succes- 
sion from  one.  It  must  not  be  thought,  however,  that  because  a  single  pre- 
synaptic impulse  is  subthreshold  it  has  no  effect  upon  the  postsynaptic 
neuron.  It  initiates  certain  changes  leading  toward  the  firing  of  the 
neuron,  and  if  enough  subthreshold  stimuli  reach  the  neuron  at  the 
same  time,  or  before  the  effects  of  the  first  are  worn  off,  their  effects  are 
added  to  those  of  the  first  and  the  threshold  of  stimulation  may  be 
reached. 

The  impulses  in  some  presynaptic  axons  have  an  inhibitory  rather 
than  an  excitatory  effect  upon  the  postsynaptic  neuron.  Whether  or 
not  the  threshold  of  stimulation  is  reached  and  a  neuron  fires  is  a 
product  of  the  interaction  of  all  of  the  inhibitory  and  excitatory  in- 
fluences that  reach  it  at  any  given  time.  If  synaptic  transmission  were 
simply  an  electrical  phenomenon,  excitation  and  inhibition  would  be 
difficult  to  understand.  The  consensus  at  present  is  that  synaptic  trans- 
mission involves  the  secretion  by  the  presynaptic  ending  of  hormone-like 
substances.  Some  endings  may  produce  an  excitatory  substance  (possibly 
acetylcholine),  and  others  an  inhibitory  hormone.  It  is  known  that  this 
happens  at  the  junction  between  neuron  and  muscle  (myoneural  junc- 
tion) in  the  autonomic  system;  some  of  the  autonomic  neurons  are  ex- 
citatory and  others  inhibitory  (p.  596).  This  theory  of  synaptic  trans- 
mission is  consistent  with  the  observed  delay  in  the  transmission  of  an 
impulse  across  a  synapse  and  with  one-way  transmission  across  a  syn- 
apse. One-way  transmission  across  a  synapse  is  a  very  important  inte- 
grating factor  for  it  enables  the  presynaptic  neuron  to  modify  the 
activity  of   the  postsynaptic   neuron   without  being  affected   itself. 


588  VERTEBRATE   LIFE   AND   ORGANIZATION 

The  iminilse  that  is  initiated  in  a  neuron  when  the  threshold  is 
reached  is  qualitatively  the  same  regardless  of  what  sort  of  a  stimulus 
initiated  the  impulse,  or  whether  the  stimulus  was  just  at  or  far  above 
the  threshold.  In  other  words,  the  nerve  impulse  is  an  all-or-none 
phcMiomenon.  Nerves  do  not  conduct  "strong"  impulses  or  "weak"  im- 
pulses correlated  with  the  strength  of  the  stimulus,  yet  we  can  dis- 
tinguish between  stimuli  that  are  just  at  threshold  and  those  that  are 
strong.  The  intensity  of  a  stimulus  does  not  afEect  the  quality  of  the 
impulse,  but  it  does  affect  the  frequency  of  the  impulse.  A  threshold 
stimulus  may  generate  one  or  two  impulses  per  second,  but  as  the 
stimulus  increases,  the  frequency  of  impulses  increases  up  to  a  maximum 
which  cannot  be  exceeded  no  matter  how  much  the  stimulus  is  in- 
creased. Neurons  differ  markedly  in  the  number  of  impulses  initiated 
in  response  to  a  stimulus  of  a  given  strength  and  in  the  maximum  fre- 
quency of  impulses  that  can  be  generated. 

Neuron  Interrelations.  The  neurons  in  the  body  are  so  arranged 
that  it  is  possible  to  divide  the  nervous  system  grossly  into  a  central 
nervous  system  consisting  of  the  brain  and  spinal  cord,  and  a  peripheral 
nervous  system  which  includes  the  nerves  that  extend  between  the 
central  nervous  system  and  the  receptors  and  effectors.  The  neurons 
themselves  can  be  grouped  into  three  broad  categories— (1)  sensory  or 
afferent  neurons,  which  carry  impulses  from  the  sense  organs  through 
the  nerves  to  the  brain  or  cord;  (2)  motor  or  efferent  neurons,  which 
carry  impulses  from  the  brain  or  cord  through  the  nerves  to  the  muscles 
and  other  effectors  of  the  body;  and  (3)  connector  or  internuncio!  neu- 
rons, which  lie  entirely  within  the  central  nervous  system  and  are 
interposed  between  the  other  two.  When  you  touch  a  hot  stove,  for 
example  (Fig.  29.8),  a  receptor  in  the  skin  is  stimulated  and  it  initiates 
an  impulse  in  an  afferent  neuron.  This  neuron  is  part  of  a  spinal  nerve 
and  extends  into  the  spinal  cord,  where  it  ends  in  a  synapse  with  one 
or  more  internuncial  neurons.  An  internuncial  neuron,  in  turn,  carries 
the  impulse  to  an  appropriate  efferent  neuron,  which  extends  from  the 
cord  and  carries  the  impulse  back  through  the  spinal  nerve  to  a  group  of 
extensor  muscle  fibers  of  the  hand.  Their  contraction  withdraws  your 
hand  from  the  stove.  For  the  movement  to  be  effective,  however,  the 
antagonistic  flexor  muscles  should  relax,  and  this  relaxation  would  in- 
volve the  inhibition  of  impulses  going  to  these  muscles.  Normally  some 
impulses  go  out  to  all  of  the  muscles  of  the  body  continually,  and 
cause  a  partial  contraction,  a  condition  called  muscle  tonus.  Inhibition 
might  be  accomplished  by  impulses  in  another  branch  of  the  inter- 
nuncial neuron  in  question,  or  in  another  internuncial  neuron,  passing 
to  the  efferent  neurons  that  innervate  the  flexor  muscles. 

The  stimulus  and  response  just  described  is  a  simple  spinal  reflex, 
and  the  neuronal  pathway  along  which  the  impulse  travels  is  called  a 
reflex  arc.  Reflexes  are  fixed  patterns  of  response  to  stimuli  and  they 
need  not  involve  an  awareness  of  the  stimulus.  The  impulse  need  not 
pass  through  any  of  the  higher  centers  in  the  brain  in  order  that  the 
response  occur.  An  impulse  may  be  carried  to  the  cerebral  cortex  of 
the   brain   by   other  connector  neurons,   afferent   internuncial   neurons. 


SENSE   ORGANS    AND   NERVOUS   COORDINATION 


589 


You  then  become  aware  of  the  stimulus  and  may  voluntarily  decide  to 
do  something  about  it,  perhaps  withdraw  your  whole  arm  or  turn  off 
the  stove.  If  so,  impulses  will  pass  out  from  the  brain  along  efferent 
internuncial  neurons  to  the  appropriate  efferent  neurons. 

Many  other  kinds  of  reflexes  occur  in  the  spinal  cord  and  in  parts 
of  the  brain  in  addition  to  the  three-neuron  reflex  discussed  above.  The 
familiar  knee  jerk  is  a  two-neuron  reflex;  the  afferent  neuron  synapses 
directly  with  the  efferent  neuron,  and  no  internuncial  neurons  are  in- 
volved. Reflexes  often  involve  several  regions  of  the  body.  If  a  drop  of 
acid  is  placed  on  the  flank  skin  of  a  frog,  both  hind  legs  will  converge 
on  this  spot  and  alternately  flex  and  extend  in  an  attempt  to  scrape  off 
the  acid.  This  will  happen  even  if  the  entire  brain  has  been  destroyed. 
Complex,  coordinated  reflexes  of  this  type  are  possible  because  inter- 
nuncial neurons  extend  from  the  afferent  neurons  through  the  cord  to 
many  different  efferent  neurons. 

Reflexes  of  the  types  described  are  present  in  all  individuals  as  soon 
as  the  neuronal  pathways  have  developed.  These  are  inherited  or  inborn 
reflexes,  and  they  are  not  dependent  upon  the  training  that  the  indi- 
vidual receives.  Other  reflexes,  known  as  conditioned  reflexes,  develop 
as  a  result  of  specific  training.  Conditioned  reflexes  were  first  demon- 
strated by  Pavlov,  the  Russian  physiologist  who  also  performed  experi- 
ments on  the  control  of  gastric  secretion.  In  a  classic  experiment,  Pavlov 
fed  a  dog  and  simultaneously  rang  a  bell.  The  bell,  of  course,  had 
nothing  to  do  with  salivation,  and  at  the  beginning  of  the  experiment 


"Internuncial 
neuron 


Cerebral  corbejc 

0^  (  C^:^  internuncial 
Jt,.^^  V^T^  neuron. 

Th.cda.mus 


Cerebellum 


Afferent  neuron 

Temperature 
receptor 
(skin) 

Hand  muscle- 


Afferent  internuncied 
neurons 


"Efferent  neurons 
-Arm  muscle 

Figure  29.8.  The  types  of  neurons  that  make  up  the  nervous  system.  An  afferent 
neuron,  an  internuncial  neuron,  and  an  efferent  neuron  are  involved  in  the  spinal 
reflex  described  in  the  text. 


590  VERTEBRATE   LIFE  AND   ORGANIZATION 


Figure  29.9.  Diagrams  of  important  types  of  neuronal  interrelationships.  A,  A 
divergent  pathway;  B,  a  convergent  pathway;  C,  a  multiple  chain  circuit;  D,  a  closed 
chain  circuit. 

would  not  induce  salivation  by  itself.  Salivation  was  reflexly  stimulated 
by  the  sight  or  smell  of  food.  The  bell  was  rung  each  time  the  dog  was 
fed  and  the  dog  gradually  learned  to  associate  the  bell  with  food. 
Eventually  ringing  the  bell  without  presenting  food  would  initiate 
salivation  and  a  conditioned  reflex  had  been  established.  Many  of  our 
responses,  including  subconscious  responses  to  stimuli  when  driving  a 
car,  are  conditioned  reflexes  that  have  developed  as  a  result  of  our  spe- 
cific training.  An  inexperienced  driver  must  consciously  think  of  what 
to  do  if  his  car  starts  to  skid,  but  an  experienced  driver  reflexly  re- 
sponds to  the  feel  of  a  car  that  is  beginning  to  skid. 

Reflexes  in  the  spinal  cord  and  brain  form  the  basis  of  a  great  many 
of  our  responses,  but  there  are  other  neuronal  interrelations  that  are 
important  for  an  understanding  of  the  activities  of  the  nervous  system. 
Most  pathways  within  the  nervous  system  involve  many  neurons,  not 
just  two  or  three  as  in  the  simpler  reflexes,  and  this  permits  a  variety  of 
complex  interrelations.  A  great  many  pathways  are  divergent  (Fig. 
29.9  A).  The  axon  of  a  neuron  may  branch  many  times,  synapse  with  a 
number  of  different  neurons,  and  these  in  turn  may  branch  further. 
Such  an  arrangement  permits  a  single  impulse  to  exert  an  effect  over 
a  wide  area;  a  single  impulse  may  ultimately  activate  a  thousand  or 
more  neurons.  Many  other  pathways  are  convergent  (Fig.  29.9  B); 
neurons  coming  from  many  different  areas  converge  upon  a  single 
neuron  or  group  of  neurons.  The  convergence  of  neurons  upon  centers 
in  the  brain  and  upon  the  cell  bodies  of  efferent  neurons  are  examples 
of  this  type  of  pathway.  It  has  been  estimated  that  the  efferent  neurons 
receive  impulses  that  originate  from  fifteen  or  twenty  different  sources. 
The  response  of  the  last  neuron  in  a  convergent  pathway  is  the  result 
of  the  interaction  of  a  variety  of  excitatory  and  inhibitory  influences. 
Convergent  pathways  are  important  in  forming  the  structural  basis  for 
the  integrative  activity  of  the  nervous  system. 

Many  neuronal  circuits,  including  those  diagrammed  in  Figure 
29.9  A  and  B,  involve  the  passage  of  impulses  only  as  long  as  the  first 


SENSE   ORGANS   AND    NERVOUS    COORDINATION 


591 


neuron  continues  to  be  stimulated.  When  the  stimulation  stops,  the 
passage  of  impulses  stops.  There  are  other  arrangements  in  the  nervous 
system  that  ensure  the  continuation  of  the  impulse  for  a  period  of  time 
after  the  stimulus  has  stopped.  One  of  these  is  tlie  multiple  chain  circuit 
(Fig.  29.9,  C).  The  first  neuron  is  stimulated  momentarily,  an  impulse 
travels  rapidly  to  the  terminal  neuron,  and  also  via  a  branch  to  a  second 
neuron.  The  second  neuron  is  stimidated  and  a  moment  later  sends  a 
second  impulse  to  the  same  terminal  neuron  and  also  via  a  branch  to  a 
third  neuron,  which  is  stimulated  and  sends  yet  a  third  impulse  to  the 
same  terminal  neuron.  If  a  great  many  neurons  are  involved,  the  ter- 
minal neuron  will  receive  a  whole  series  of  impulses,  and  receive  them 
for  some  time  after  the  initial  stimulus  has  stopped.  In  another  arrange- 
ment, the  closed  chain  circuit  (Fig.  29.9,  D),  one  or  more  branches  of  the 
neurons  in  the  circuit  feed  back  to  a  point  near  the  beginning  of  the 
circuit.  Once  such  a  circuit  is  activated,  impulses  could  continue  in- 
definitely unless  the  neurons  became  fatigued  or  were  inhibited.  Presum- 
ably such  circuits  form  the  basis  for  the  spontaneous  activity  of  the 
inspiratory  center  and  similar  centers  in  the  brain. 

246.        Peripheral  Nervous  System 

Spinal  Nerves.  The  vertebrate  body  is  segmented  (although  seg- 
mentation is  obscure  in  the  head  region)  and  there  is  a  pair  of  peripheral 
nerves  for  each  body  segment:  those  arising  from  the  spinal  cord  are 
known  as  spinal  nerves;  those  from  the  brain,  as  cranial  nerves.  Afferent 
and  efferent  neurons  lie  together  in  most  of  a  spinal  nerve,  but  near 
the  cord  the  nerve  splits  into  a  dorsal  and  a  ventral  root,  and  the  neurons 
are  segregated  (Fig.  29.10).  The  dorsal  root  contains  the  afferent  neurons, 
and   bears   an   enlargement,    the   dorsal   root  ganglion,    which   contains 


White  mafbcr 
Dorseil  root 


Central 
-canal 


Dorsal  root 
Oan^lion 


Dorsal 
ramus 


Vent  rail 
ramus 


VentraJ. 
root 


Postganglionic  fiber 
■fco  visceral  effector 


White  ramus 
^^^communicans 

^  I  AV^Fromskin 

Sympathetic    \      receptor 
ganglion  \^         ,     .    ^    , 

lo  skeletal 
Postganglion-/|r        muscles 
Fiber  To  sweat  gland 

or  cutaneous 
From,  visceral  blood  vessel 


receptor 

Figure  29.10.  A  diagrammatic  cross  section  through  the  spinal  cord  and  a  spinal 
nerve.  Each  spinal  nerve  is  formed  by  the  union  of  dorsal  and  ventral  roots,  and 
divides  laterally  into  several  branches  (rami)  going  to  different  parts  of  the  body.  The 
dorsal  ramus  contains  the  same  types  of  neurons  as  the  ventral  ramus. 


592 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Table  6.     CRANIAL  NERVES  OF  MAN 


NERVE 

ORIGIN  OF  AFFERENT  NEURONS 

DISTRIBUTION  OF  EFFERENT 
NEURONS 

1,  Olfactory 

Olfactory  portion  of  nasal 
mucosa  (smell). 

II,  Optic 

Retina  (sight). 

III,  Oculomotor 

A  few  fibers  from  propriocep- 
tors in  extrinsic  muscles  of 
eyeball  (muscle  sense). 

Most  fibers  to  extrinsic  muscles 
of  eyeball,  a  few  to  muscles  in 
ciliary  body  and  pupil. 

IV,  Trochlear 

Proprioceptors  in  extrinsic 
muscles  of  eyeball. 

Other  extrinsic  muscles  of  eye- 
ball. 

V,  Trigeminal 

Teeth,  and  skin  receptors  of  the 
head    (touch,   pressure,   tem- 
perature,  pain);  propriocep- 
tors in  jaw  muscles. 

Muscles  derived  from  muscula- 
ture of  first  visceral  arch,  i.e., 
jaw  muscles. 

VI,  Abducens 

Proprioceptors  in  extrinsic  mus- 
cles of  eyeball. 

Still   other   extrinsic    muscles   of 
eyeball. 

VII,  Facial 

Taste  buds  of  anterior  two- 
thirds  of  tongue  (taste). 

Muscles  derived  from  muscula- 
ture of  second   visceral   arch, 
i.e.,  facial  muscles;  salivary 
glands;  tear  glands. 

VIII,  Acoustic 

Semicircular  canals,   utriculus, 
sacculus    (sense  of  balance); 
cochlea  (hearing). 

IX,  Glossopharyngeal 

Taste  buds  of  posterior  third  of 
tongue;  lining  of  pharynx. 

Muscles  derived  from   muscula- 
ture of  third  visceral  arch,  i.e., 
pharyngeal  muscles  concerned 
in  swallowing;  salivary  glands. 

X,  Vagus 

Receptors  in  many  internal  or- 
gans:   larynx,    lungs,    heart, 
aorta,  stomach. 

Musculature  derived  from  mus- 
culature of  remaining  visceral 
arches  (excepting  those  of  pec- 
toral girdle),   i.e.,   muscles  of 
pharynx  (swallowing)  and 
larynx  (speech);  muscles  of 
gut,  heart;  gastric  glands. 

XI,  Spinal  Accessory 

Proprioceptors  in  certain  shoul- 
der muscles. 

Visceral  arch  muscles  associated 
with  pectoral  girdle,  i.e., 
sternocleidomastoid 
and  trapezius. 

XII,  Hypoglossal 

Proprioceptors  in  tongue. 

Muscles  of  tongue. 

SENSE  ORGANS   AND   NERVOUS   COORD/NAHON  593 

their  cell  bodies.  The  cell  bodies  of  afferent  neurons  are  nearly  always 
located  in  ganglia  on  both  spinal  and  cranial  nerves.  The  afferent  neu- 
rons enter  the  spinal  cord,  and  generally  terminate  in  synapses  with  the 
dendrites  or  cell  bodies  of  internuncial  neurons.  These  cell  bodies  are 
located  in  the  dorsal  portion  of  the  gray  matter  of  the  cord.  The  ventral 
root  contains  the  efferent  neurons,  and  their  cell  bodies  nearly  always 
lie  in  the  ventral  portion  of  the  gray  matter  of  the  cord. 

The  spinal  nerves  of  most  vertebrates  are  essentially  alike,  although 
in  the  most  primitive  vertebrates  the  roots  do  not  unite  peripherally,  and 
the  segregation  of  afferent  and  efferent  neurons  within  the  roots  is  not 
as  clear-cut.  In  most  vertebrates,  the  roots  unite  to  form  a  spinal  nerve 
that  divides  into  a  dorsal  branch,  or  dorsal  ramus,  which  supplies  the 
skin  and  muscles  in  the  dorsal  part  of  the  body,  a  ventral  ramus,  which 
innervates  the  lateroventral  parts  of  the  body,  and  frequently  one  or 
more  communicating  rami  to  the  visceral  organs.  Afferent  and  efferent 
neurons  occur  in  each  ramus.  Man  has  31  pairs  of  spinal  nerves.  Those 
supplying  the  receptors  and  effectors  of  the  limbs  are  larger  than  the 
others,  and  their  ventral  rami  are  interlaced  to  form  a  complex  network, 
or  plexus,  from  which  nerves  extend  to  the  limbs. 

Cranial  Nerves.  The  nerves  from  the  nose,  the  eyes  and  the  ear 
contain  only  afferent  neurons,  and  have  evolved  along  with  the  organs 
of  special  sense.  The  other  cranial  nerves  are  mixed,  and  they  are  con- 
sidered to  be  serially  homologous  with  the  separate  roots  of  the  spinal 
nerves  of  primitive  vertebrates.  Some  of  them  are  essentially  the  cephalic 
counterparts  of  dorsal  roots;  others,  the  counterparts  of  ventral  roots. 
The  location  of  the  cell  bodies  of  the  neurons  of  cranial  nerves,  and 
of  their  endings  within  the  brain,  follows  the  pattern  described  for  spinal 
neurons. 

Reptiles,  birds  and  mammals  have  twelve  pairs  of  cranial  nerves, 
if  we  omit  the  minute  and  poorly  understood  nervus  terminalis.  Though 
distributed  to  the  nasal  mucosa,  this  nerve  is  not  olfactory.  The  other 
cranial  nerves  and  their  distribution  are  shown  in  Table  6,  and  their 
stumps  can  be  seen  in  a  figure  of  the  brain  (Fig.  29.11). 

Fishes  and  amphibians  lack  discrete  spinal  accessory  and  hypoglossal 
nerves.  The  homologues  of  neurons  that  are  segregated  in  the  spinal 
accessory  of  higher  vertebrates  are  included  in  the  vagus  of  fishes  and 
amphibians,  and  the  homologues  of  neurons  in  the  hypoglossal  are  in- 
cluded in  several  minute  nerves  emerging  from  the  occipital  region  of 
the  skull.  The  trigeminal,  facial,  glossopharyngeal  and  vagus  nerves  of 
fishes  are  primarily  associated  with  the  muscles  of  the  visceral  arches, 
and,  as  shown  in  Table  6,  they  supply  the  derivatives  of  this  musculature 
in  the  higher  vertebrates.  Muscles  change  in  shape  and  function  during 
the  course  of  evolution,  but  their  innervation  remains  remarkably  con- 
stant. 

Autonomic  Nervous  System.  Most  of  the  efferent  fibers  in  the 
spinal  and  cranial  nerves  supply  somatic  muscles  of  the  body  and  visceral 
muscles  associated  with  the  gill  region.  But  in  addition  to  these,  certain 
of  the  cranial  and  spinal  nerves  contain  other  efferent  fibers  going  to 
muscles  in  the  walls  of  the  gut,  heart,  blood  vessels  and  other  internal 


594 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Lortgitadinal 
fissui^e  oF 
Cerebrum 


Optic  chiasma- 


Rhinal  fissure- 

La.tei'a.1. 
f  issui'e 


Optic  tract 


Hypothcdamus 

Meszncephalon^ 

Tri^i 


eminal 

ncx-ve" 


Abducens  nerve    k:;> 
Acoustic  nerve- 

GlossopHaryn^eal 
nervz' 

VaOuS  nerve 
Hypoglossal  nerve 

Ventral  median -Fissure 


actory  bulb 
Optic  nerve. 


Cerebral 
hemisphere 

Pituitary 
Stalk 

Oculomotor 
nci've 

Trochlear 
nerve 

Pons 


:iW''-' 


Y/l 


Facial  nerve 
cIluiTi. 


O^ 


■Medulla  oblongata 


^j — Spinal  eiccessory  nerve. 


Figure  29.11.  A  ventral  view  of  the  brain  of  a  sheep.  The  stumps  of  all  but  the 
first  pair  of  cranial  nerves  are  visible.  The  olfactory  nerves  consist  of  the  processes  of 
olfactory  cells  (cf.  Fig.  29.1  £),  which  enter  the  olfactory  bulbs  in  many  small  groups 
that  cannot  be  seen  with  the  unaided  eye.  The  rhinal  fissure  separates  the  ventral, 
olfactory  portion  of  each  cerebral  hemisphere  from  the  rest  of  the  hemisphere.  The 
paths  of  the  optic  fibers  in  the  optic  chiasma  have  been  indicated   by   broken  lines. 

organs;  to  the  small  muscles  associated  with  the  hairs;  to  the  ciliary  and 
iris  muscles  in  the  eye;  and  to  many  of  the  glands  of  the  body  (Fig. 
29.12).  These  efferent  fibers  constitute  the  autonomic  nervous  system. 
The  organs  supplied  by  these  fibers  function  automatically,  requiring 
no  thought  on  our  part.  Indeed,  they  cannot  be  controlled  voluntarily. 
It  should  be  emphasized  that  the  autonomic  nervous  system  is  by  defini- 
tion a  motor  system,  and  the  afferent  fibers  that  return  from  internal 
organs  are  not  a  part  of  this  system,  even  though  they  may  be  in  nerves 
composed  largely  of  autonomic  fibers. 

The  autonomic  nervous  system  is  morphologically  unique  in  that 
the  autonomic  neurons  that  emerge  from  the  central  nervous  system  do 
not  extend  all  the  way  to  the  effectors,  as  do  other  efferent  neurons. 
They  go  only  to  a  peripheral  ganglion  in  which  there  is  a  relay,  and  a 
second  set  of  autonomic  fibers  continues  from  the  ganglion  to  the  organ. 
Autonomic  fibers  having  their  cell  bodies  in  the  central  nervous  system 
and  extending  to  a  peripheral  ganglion  are  known  as  preganglionic 
fibers;  those  having  their  cell  bodies  in  the  ganglia  and  extending  to  the 
organs  are  the  postganglionic  fibers. 

The  autonomic  nervous  system  is  subdivided  into  sympathetic  and 
parasympathetic  systems.  Most  organs  innervated  by  the  autonomic 
nervous  system  receive  fibers  of  both  types.  The  preganglionic  sympa- 


SENSE  ORGANS   AND   NERVOUS   COORDINATION 


595 


thetic  fibers  leave  the  central  nervous  system  through  the  ventral  roots 
of  spinal  nerves  in  the  thoracic  and  anterior  lumbar  regions  (Figs.  29.10 
and  29.12),  and  pass  through  the  ramus  communicans  to  a  sympathetic 
cord,  one  of  which  lies  on  each  side  of  the  vertebral  column.  These 
fibers  may  synapse  with  the  postganglionic  fibers  in  the  sympathetic 
ganglia  in  the  sympathetic  cord,  or  they  may  continue  from  the  sympa- 
thetic cord  through  splanchnic  nerves  to  collateral  ganglia  located  at 
the  base  of  the  coeliac  and  mesenteric  arteries.  Postganglionic  sympa- 
thetic fibers  continue  from  the  ganglia  to  the  organs  they  supply.  Those 
to  the  skin  reenter  the  spinal  nerves,  but  the  others  tend  to  follow  along 
the  arteries  to  the  organs.  Preganglionic  parasympathetic  fibers  are  dis- 
tributed to  the  organs  through  the  oculomotor,  facial,  glossopharyngeal 
and  vagus  nerves,  and  through  a  pelvic  nerve  derived  from  certain  spinal 


■Midbrain 


iary  ^anolion— 


snopalatine 
ganglion- 


,j^^ >rj;^Lacrimal  dl 

■  4^^ZZ^^^^~  Subrnaxillary  gland 
77^^ — Sublingual  ol^ind 


Spinal 
cord 


First 
thoracic 
segment 


"--.^  Otic  OanOlion^  -^      i>uJ3lin0u< 


nOual  oland 
ti" 
gla.nd 

-Heart 


^^^::y-Thyroid 
^  -'—Stomach. 


First 
lumbair 
segment 


Second 

sacral 

Segment 


land 

mucosa. 


Liver 

Pancreas 

■Adrenal 

medulla 


Small 
intestine 


Figure  29.12.  The  human  autonomic  nervous  system.  Sympathetic  fibers  are  drawn 
in  solid  Unes;  parasympathetic  fibers  in  broken  lines.  The  sympathetic  fibers  that  go 
to  the  skin  are  not  shown.  (After  Howell.) 


596  VeRTEBRATE   LIFE   AND    ORGANIZATION 

nerves  in  the  sacral  region.  Preganglionic:  parasympathetic  fibers  are 
longer  than  those  of  the  sympathetic  system  tor  they  end  in  ganglia  that 
are  very  near  the  organs  they  supply,  or  are  in  the  walls  oi  the  organs. 
Relatively  short  postganglionic  parasympathetic  fibers  continue  to  the 
muscle  and  gland  cells. 

Sympathetic  and  parasympathetic  systems  have  opposite  effects  upon 
the  organs  innervated.  Sympathetic  stimulation  speeds  up  the  rate  and 
increases  the  force  of  the  heart  beat,  causes  arteries  to  constrict,  thereby 
increasing  the  blood  pressure,  increases  the  glucose  content  of  the  blood, 
and  in  general  has  effects  that  enable  the  body  to  adjust  to  conditions 
of  stress.  It  inhibits  the  secretion  of  the  salivary  glands  and  the  activity 
of  the  digestive  tract  generally.  Parasympathetic  stimulation,  on  the 
other  hand,  speeds  up  salivary  secretion,  peristalsis  of  the  digestive  tract 
and  similar  vegetative  processes,  but  it  slows  down  the  heart  and  de- 
creases blood  pressure. 

Ingenious  experiments  performed  by  Loewi  in  I92I  demonstrated 
the  cause  of  the  opposite  effects  of  sympathetic  and  parasympathetic 
fibers.  He  removed  the  heart  of  a  frog,  leaving  only  its  nerve  supply 
intact,  then  perfused  a  salt  solution  through  it  and  into  another  com- 
pletely isolated  heart.  Both  hearts  continued  to  beat.  When  the  vagus 
nerve  (parasympathetic  fibers)  going  to  the  first  heart  was  stimulated, 
the  rate  of  both  hearts  slowed  down;  when  the  sympathetic  fibers  were 
stimulated,  the  rate  of  both  hearts  increased.  Apparently  some  substance 
secreted  by  the  nerves  going  to  the  first  heart  entered  the  salt  solution  and 
reached  the  second  heart.  Further  work  revealed  that  two  neurohumors 
are  produced.  Acetylcholine  is  secreted  by  the  parasympathetic  and 
sympathin  by  the  sympathetic  fibers.  Acetylcholine  may  also  be  involved 
in  the  transmission  of  the  nerve  impulse  across  the  synapses  in  other  parts 
of  the  nervous  system,  and  across  the  junction  between  neuron  and 
muscle.  It  may  also  play  a  role  in  the  transmission  of  the  nerve  impulse 
along  the  neuron.  Sympathin  has  been  found  only  in  connection  with 
postganglionic  sympathetic  fibers,  but  it  is  closely  related  to  epinephrine, 
secreted  by  the  mecluUary  cells  of  the  adrenal  gland.  There  is  fairly  clear 
evidence  that  these  cells  are  themselves  modified  postganglionic  sym- 
pathetic fibers. 

247.        Central  Nervous  System 

Spinal  Cord.  A  small  central  canal  (Fig.  29.10)  extends  through 
the  center  of  the  spinal  cord,  gray  matter  surrounds  the  central  canal, 
and  white  matter  lies  peripheral  to  the  gray.  The  gray  matter  is  dark  in 
color,  for  it  is  composed  of  the  cell  bodies  of  neurons  and  of  unmy- 
elinated fibers;  the  white  matter  is  light,  because  it  is  composed  of  fibers 
surrounded  by  fatty  myelin  sheaths.  The  gray  matter  forms  continuous 
longitudinal  columns,  which  are  H-shaped  in  cross  section.  There  is  a 
pair  of  dorsal  columns,  a  pair  of  ventral  columns,  and  a  gray  commis- 
sure connecting  the  columns  of  opposite  sides.  The  dorsal  column  con- 
tains the  dendrites  and  cell  bodies  of  afferent  internuncial  neurons,  with 
which  many  afferent  neurons  synapse.  The  ventral  column  contains  the 


SENSE   ORGANS    AND   NERVOUS    COORDINATION  597 

dendrites  and  cell  bodies  of  the  efferent  neurons.  The  gray  commissure 
is  composed  of  fibers  crossing  from  one  side  of  the  spinal  cord  to  the 
other.  The  gray  matter  lying  dorsal  to  the  central  canal  is  concerned 
with  relaying  sensory  impulses  that  enter  the  cord,  and  the  part  lateral 
and  ventral  to  the  central  canal  relays  motor  impulses  that  leave  the 
cord  in  the  efferent  neurons. 

Much  of  the  white  matter  consists  of  the  fibers  of  afferent  neurons, 
some  of  which  extend  some  distance  in  the  central  nervous  system  before 
entering  the  gray  matter,  and  of  afferent  internuncial  neurons  which 
end  in  the  brain.  The  rest  of  the  white  matter  consists  of  the  processes 
of  efferent  internuncial  neurons  coming  from  the  brain  to  the  efferent 
neurons.  All  afferent  impulses  that  enter  the  spinal  cord  cross  to  the 
opposite  side  before  they  reach  the  brain,  and  efferent  impulses  coming 
from  the  brain  cross  within  the  brain.  Thus  afferent  impulses  initiated 
on  the  left  side  of  the  body  reach  the  right  side  of  the  brain,  and  efferent 
impulses  initiated  in  the  right  side  of  the  brain  reach  the  left  side  of 
the  body. 

Though  all  of  the  white  matter  looks  the  same,  careful  experimenta- 
tion has  enabled  neuroanatomists  to  localize  the  various  groups  of  fibers 
that  comprise  it.  Impulses  initiated  by  temperature  receptors  on  the  left 
side  of  the  body,  for  example,  are  carried  to  the  brain  by  fibers  located 
in  the  lateral  portion  of  the  white  matter  on  the  right  side  of  the  cord 
(Fig.  29.8).  A  lesion  in  this  part  of  the  cord  would  prevent  one  from 
being  conscious  of  temperature  changes  on  the  opposite  side  of  the  body 
posterior  to  the  lesion,  though  one  would  still  respond  reflexly  to  such 
changes. 

The  Brain.  Major  Parts  of  the  Brain.  A  brief  consideration  of 
the  embryonic  development  of  the  brain  makes  it  easier  to  understand 
its  major  divisions  and  parts.  The  brain  develops  as  a  series  of  enlarge- 
ments of  the  anterior  portion  of  the  embryonic  neural  tube  (Fig.  29.13). 
In  an  early  embryo,  there  are  only  three  swellings  (a  forebrain,  mid- 
brain and  hindbrain),  but  the  forebrain  and  hindbrain  are  later  sub- 
divided, so  five  regions  are  present  in  an  adult.  The  forebrain  divides 
into  a  telencephalon  and  a  diencephalon.  The  telencephalon  differen- 
tiates into  a  pair  of  olfactory  bulbs,  which  receive  the  endings  of  olfactory 
cells,  and  a  pair  of  cerebral  hemispheres.  The  lateral  walls  of  the 
diencephalon  become  the  thalamus,  its  roof  the  epithalamus,  and  its  floor 
the  hypothalamus.  Fibers  in  the  optic  nerves  cross  below  the  hypothalamus 
and  form  an  optic  chiasma  (Fig.  29.11).  All  of  the  optic  fibers  cross  and 
go  to  the  opposite  side  of  the  brain  in  most  vertebrates,  but  only  half 
of  them  cross  in  mammals.  The  pituitary  gland  is  attached  to  the  hy- 
pothalamus just  posterior  to  the  chiasma,  and  the  pineal  body  is  attached 
to  the  epithalamus.  No  further  division  occurs  in  the  midbrain,  or 
mesencephalon,  but  its  roof  differentiates  into  a  pair  of  optic  lobes  in  all 
vertebrates.  In  addition  to  the  optic  lobes,  or  superior  colliculi,  the 
mesencephalic  roof  of  mammals  bears  a  pair  of  interior  colliculi.  The 
hindbrain  divides  into  a  metencephalon,  the  dorsal  portion  of  which 
forms  the  cerebellum,  and  a  myelencephalon,  which  becomes  the  me- 
dulla oblongata. 


598 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Oplic 
vesicle- 


Mebencephalon 

Mesencephalon 1   ,-~-^^^^ 

rForebrain 

-Midbrain 

rHindbrain 

Dien- 
ccphalon 


My  elen  cephEjon 


Telencephalon 


— { — Spinal 


Heart 


Spinal  cord 


—Diencephalic 
outline 


Cerebral  hemispherc" 
(Tclencephcxlon) 


Olfactory  bulb 
(.Telencephalon) 


B 


cuius  iMesencephsJic 


ferior 
lliculus 


roof 


Cerebellum. 
(Metencephalon) 

Pons 
(Metencephalon) 

Medulla  oblongata 
(Myelencephalon) 


Spinal  cord 


Figure  29.13.  Three  stages  in  the  development  of  the  human  brain.  A,  The  three 
primary  brain  regions  can  be  recognized  in  an  embryo  that  is  about  three  and  one 
half  weeks  old.  B,  All  five  brain  regions  are  evident  in  an  embryo  seven  weeks  old. 
C,  The  various  structures  foimd  in  a  fully  de\eloped  brain  are  beginning  to  differentiate 
in  an  embryo  eleven  weeks  old.  (After  Patten.) 


The  central  canal  of  the  spinal  cord  extends  into  the  brain,  and  is 
continuous  with  several  large,  interconnected  chambers  known  as  ven- 
tricles (Fig.  29.14).  A  lateral  ventricle  lies  in  each  cerebral  hemisphere 
and  each  is  connected  with  the  third  ventricle  in  the  diencephalon  by  a 
foramen  of  Monro.  The  aqueduct  of  Sylvius  extends  from  the  third 
ventricle  through  the  mesencephalon  to  a  fourth  ventricle  in  the  meten- 
cephalon and  medulla  oblongata.  All  of  these  passages  are  filled  with  a 
lymphlike  cerebrospinal  fluid,  which  is  produced  by  vascular  choroid 
plexuses.  Choroid  plexuses  develop  in  the  thin  roof  of  the  diencephalon 
and  medulla  and  are  also  present  in  the  lateral  ventricles  of  mammals. 
Cerebrospinal  fluid  escapes  from  the  brain  through  foramina  in  the  roof 
of  the  medulla,  and  slowly  circulates  in  the  spaces  between  the  layers  of 


SENSE   ORGANS   AND   NERVOUS    COORDINATION 


599 


connective  tissue,  the  meninges,  that  encase  the  brain  and  spinal  cord. 
The  innermost  meninx,  the  pia  mater,  is  a  very  vascular  membrane  that 
is  closely  applied  to  the  surface  ot  the  brain  and  spinal  cord.  Certain 
parts  of  it  help  to  form  the  choroid  plexuses.  A  delicate  arachnoid  mem- 
brane lies  peripheral  to  the  pia,  and  a  very  tough  dura  mater  forms  a 
protective  envelope  around  the  entire  central  nervous  system.  The  cere- 
brospinal fluid  lies  in  the  space  between  the  arachnoid  and  pia.  It  is 
produced  continuously  and  reenters  the  circulatory  system  by  filtering 
into  certain  venous  sinuses  located  in  the  dura  mater  covering  the  brain. 
The  cerebrospinal  fluid  forms  a  protective  liquid  cushion  about  the 
brain  and  spinal  cord,  and  also  helps  to  nourish  the  tissue  of  the  central 
nervous  system. 

Medulla  Oblongata.  Brain  functions  are  exceedingly  complex, 
and  far  from  completely  understood.  The  medulla  oblongata  (Fig.  29.14) 
lies  between  the  spinal  cord  and  the  rest  of  the  brain  and  is  funda- 
mentally the  same  in  all  vertebrates.  The  gray  columns  of  the  spinal 
cord  extend  into  the  medulla,  but  within  the  brain  they  become  dis- 
continuous, breaking  up  into  discrete  islands  of  cell  bodies  known  as 
nuclei.  The  dorsal  nuclei  receive  the  afferent  neurons  from  cranial 
nerves  that  are  attached  to  this  region,  and  contain  the  cell  bodies  of 


Venous  sinus 
in  dura,  maler 

Cerebral  veins 


Sulcus  of  Rolando 
rFornix 


Corpus 
callosu 


Lateral 
ventricL 


Foramen 
of  Monro 

Ventricle  EL 


i^racTnnoid  villus 
ater 

"Arachnoid. 

embrane 


Dura 
mater 

Epilhalcunus 

Pinecd 
body 

Colliculi 


Cerebellum 
Ventricle  ]Z 

Cerebral  aqueduct 

Medulla  oblongata 

Central  canal- 
Spinal  cord 

Figure  29.14.  A  sagittal  section  of  the  human  brain  and  its  surrounding  meninges. 
Cerebrospinal  fluid  is  produced  by  the  choroid  plexuses,  circulates  as  indicated  by  the 
arrows,  and  finally  enters  a  venous  sinus  in  the  dura  mater.  (Modified  after  Rasmussen.) 


goo  VERTEBRATE   LIFE   AhlD   ORGANIZATION 

afferent  internuncial  neurons.  These  are  sensory  nuclei,  just  as  the  dorsal 
columns  of  the  cord  are  sensory  columns.  The  ventral  nuclei  contain 
the  cell  bodies  of  the  efferent  neurons  of  the  cranial  nerves,  and  hence 
are  motor  nuclei.  In  mammals,  reflexes  that  regulate  the  rate  of  heart 
beat,  the  diameter  of  arterioles,  respiratory  movements,  salivary  secre- 
tion, swallowing  and  many  other  processes  are  mediated  by  these  nuclei. 
Afferent  impulses  come  into  the  sensory  nuclei,  are  relayed  by  the  inter- 
nuncial neurons  to  the  motor  nuclei,  and  efferent  impulses  go  out  to 

the  effectors. 

Cerebellum  and  Pons.  Motor  and  sensory  nuclei  associated  with 
cranial  nerves  are  also  found  in  the  metencephalon  and  mesencephalon 
and  other  reflex  arcs  involve  these  regions.  All  vertebrates  have  a  cere- 
bellum, which  develops  in  the  dorsal  part  of  the  metencephalon,  and  is 
a  center  for  balance  and  motor  coordination.  Impulses  from  the  parts 
of  the  ear  concerned  with  equilibrium,  from  the  lateral  line  (if  present), 
and  from  the  proprioceptors  in  the  muscles  of  the  body  enter  it.  It  is 
small  in  many  of  the  lower  vertebrates  such  as  the  frog  (Fig.  21.18),  in 
which  muscular  movements  are  not  complex,  but  it  is  very  large  in  birds 
and  mammals.  The  mammalian  cerebellum  has  neuronal  connections 
with  the  cerebral  hemispheres,  and  many  motor  impulses  initiated  in 
the  cerebral  hemispheres  pass  through  the  cerebellum  for  final  integra- 
tion with  respect  to  the  position  of  the  body  and  degree  of  contraction 
of  the  muscles  before  going  to  the  motor  nuclei  and  columns.  Much  of 
the  gray  matter  of  the  mammalian  cerebellum  lies  on  the  surface,  where 
there  is  more  room  for  the  increased  number  of  cell  bodies.  The  surface 
is  also  complexly  folded,  which  further  increases  the  surface  area  avail- 
able for  cell  bodies. 

The  floor  of  the  metencephalon  is  unspecialized  in  lower  vertebrates, 
but  this  region  differentiates  into  a  pons  in  mammals  (Figs.  29.11  and 
29.14).  Evolution  of  the  pons  is  correlated  with  the  elaboration  of  the 
cerebellum.  It  contains  nuclei  that  relay  cerebral  impulses  into  the  cere- 
bellum, and  transverse  fibers  that  interconnect  the  two  sides  of  the 
cerebellum. 

Optic  Lobes.  In  fishes  and  amphibians,  the  optic  lobes  (Fig.  21.18) 
receive  impulses  not  only  from  the  eyes,  but  also  from  many  of  the  other 
sense  organs.  This  sensory  information  is  integrated,  and  motor  impulses 
are  sent  to  the  appropriate  efferent  neurons.  The  optic  lobes  are  the 
master  integrating  center  of  the  brain,  in  so  far  as  these  vertebrates  have 
such  a  center.  The  cerebral  hemispheres  of  the  lower  vertebrates  are 
concerned  almost  exclusively  with  integrating  olfactory  impulses.  In 
reptiles,  other  sensory  data  are  sent  to  the  cerebral  hemispheres,  and 
they  begin  to  assume  some  of  the  functions  of  the  optic  lobes.  Still  more 
sensory  information  is  sent  to  the  cerebral  hemispheres  of  birds  and 
mammals,  and  the  hemispheres  of  mammals  have  taken  over  most  of 
the  functions  of  the  optic  lobes.  The  optic  lobes  (superior  colliculi) 
of  mammals  (Fig.  29.14)  remain  as  relatively  small  centers  for  pupillary 
and  other  optic  reflexes.  A  pair  of  inferior  colliculi  are  present  posterior 
to  them,  and  they  are  a  center  for  certain  auditory  reflexes. 

Thalamus  and  Hypothalamus.     The  thalamus  is  a  relay  center  to 


SENSE  ORGANS   AND   NERVOUS   COORDINATION 


601 


and  from  the  cerebral  hemispheres,  and  it  has  become  enlarged  during 
the  course  of  evolution  as  the  cerebral  hemispheres  have  assumed  a 
dominant  role  in  integrating  the  activities  of  the  body.  All  of  the  sen- 
sory impulses  that  go  to  the  cerebrum,  except  those  from  the  olfactory 
organ,  are  relayed  in  the  thalamus.  Many  motor  impulses  descending 
from  the  cerebrum  go  directly  to  the  motor  nuclei  and  columns,  but 
some  of  these  are  also  relayed  in  the  thalamus.  Other  parts  of  the  dien- 
cephalon  have  not  changed  very  much  during  vertebrate  evolution.  The 
hypothalamus  is  an  important  center  for  the  control  of  many  autonomic 
functions.  Body  temperature,  water  balance,  appetite,  carbohydrate  and 
fat  metabolism  and  sleep  are  among  the  processes  regulated  by  the 
hypothalamus  in  mammals.  The  hypothalamus  exerts  its  control  by 
neuronal  connections  with  the  motor  nuclei  and  columns,  and  also  by 
neuronal  connections  with  the  posterior  lobe  of  the  pituitary  gland. 
Damage  to  it  is  often  fatal,  for  so  many  vital  processes  are  disturbed. 

Cerebral  Hemispheres.  As  the  cerebral  hemispheres  assumed  the 
dominant  role  in  nervous  integration  during  the  course  of  evolution, 
they  enlarged  and  grew  posteriorly  over  the  diencephalon  and  mesen- 
cephalon (Fig.  29.14).  A  layer  of  gray  matter  has  developed  on  the  sur- 
face of  the  cerebrum  and  has  formed  a  gray  cortex  which  provides  more 
area  for  the  increased  number  of  cell  bodies.  Billions  of  cells  are  present 
in  the  cerebrum  of  man.  Complex  folds  of  the  cortex  increase  further 
the  area  of  the  cortex.  Ridges  (gyri)  are  present  with  furrows  (sulci) 
between  them.  Parts  of  the  cerebral  hemispheres  are  still  concerned  with 
their  primitive  function  of  olfactory  integration,  but  their  great  enlarge- 
ment is  correlated  with  the  evolution  of  other  integration  centers  (Fig. 
29.15).  Afferent  impulses  from  the  eyes,  ears,  skin  and  many  other  parts 
of  the  body  are  carried  to  the  cerebral  cortex  by  afferent  internuncial 
neurons,  after  being  relayed  in  the  thalamus  as  shown  in  Figure  29.8. 


Sulcu-S  of 
Rolando 


SKin  sensations 


Motor  coi'teoc  -^ 
Frontal  lobe.  ^ 


Fissxirc  of 
Sylvius 

Smell" 

Hearixig 

Temporail  lobe' 


Pons- 
Medulla.- 


Parietal 
lobe. 


•Vision 


Occipital 
lobe 


Cerebellum. 


Figure  29.15.     Cortical  areas  of  the  human  brain  as  seen  in  a  lateral  view.  The 
association  areas  of  the  cortex  have  not  been  hatched, 


5Q9  VERTEBRATE   LIFE   AND   ORGANIZATION 

The  impulses  terminate  in  specific  parts  of  the  cerebral  cortex  which 
have  been  determined  by  correlating  brain  injuries  with  loss  of  sensa- 
tion, and  by  electrical  stimulation  during  brain  operations.  Many  human 
brain  operations  can  be  performed  under  local  anesthesia,  and  the  pa- 
tient can  describe  the  sensations  that  are  felt  when  particular  regions 
are  stimulated.  Impulses  from  the  skin  terminate  in  the  gyrus  that  is 
located  just  posterior  to  the  central  sulcus  of  Rolando,  a  promnient 
sulcus  extending  down  the  side  of  each  hemisphere  and  dividing  the 
hemisphere  into  an  anterior  frontal  and  a  posterior  parietal  lobe.  The 
sensory  areas  of  the  skin  are  projected  upside  down.  Impulses  from 
the  head  are  conducted  to  the  lower  part  of  the  gyrus  whereas  those 
from  the  feet  reach  the  upper  part.  The  extent  of  the  area  receiving 
impulses  from  any  part  of  the  body  is  proportional  to  the  number  of 
sense  organs  in  that  part  of  the  body.  Thus  the  area  receiving  impulses 
from  the  fingers   is  more  extensive   than  that  receiving  impulses  from 

the  trunk. 

Impulses  from  the  ear  are  carried  to  the  temporal  lobe,  which  is 
separated  from  the  frontal  and  parietal  lobes  by  the  lateral  fissure  of 
Sylvius.  Impulses  from  the  eye  are  received  in  the  occipital  lobe,  which 
lies  just  posterior  to  the  parietal  lobe.  The  path  of  the  optic  fibers  of 
mammals  is  an  exception  to  the  generalization  that  afferent  impulses 
cross  at  some  point  during  their  ascent  to  the  brain.  Half  of  the  fibers 
in  each  optic  nerve  cross  in  the  optic  chiasma  and  end  up  on  the  opposite 
side  of  the  brain,  but  the  other  half  do  not.  Thus,  destruction  of  one 
occipital  lobe  results  in  inability  to  perceive  images  that  fall  on  half  of 
each  retina  rather  than  complete  loss  of  vision  in  one  eye  (Fig.  29.11). 

Appropriate  motor  impulses  to  the  striated  muscles  are  initiated  in 
response  to  all  of  the  sensory  data  that  enters  the  cerebrum.  The  cell 
bodies  of  the  efferent  internuncial  neurons  are  contained  in  the  motor 
cortex,  which  lies  just  anterior  to  the  sulcus  of  Rolando.  The  motor 
cortex  is  subdivided,  in  the  manner  of  the  adjacent  sensory  cortex,  into 
areas  associated  with  the  different  parts  of  the  body.  Fibers  to  the  hand 
occupy  a  large  portion  of  it,  tor  the  muscles  that  control  finger  move- 
ments contain  more  motor  units  than  do  most  muscles.  This  is  correlated 
with  the  intricacy  of  our  finger  movements.  Most  efferent  internuncial 
neurons  pass  directly  to  the  motor  nuclei  of  the  brain  and  to  the  motor 
columns  of  the  spinal  cord,  crossing  to  the  opposite  side  along  the  way 
(Fig.  29.8).  Some  are  relayed  in  a  mass  of  gray  matter,  the  corpus  striatum, 
situated  deep  within  each  cerebral  hemisphere;  others  are  relayed  in 
the  thalamus,  or  at  other  points. 

Many  association  neurons  interconnect  the  sensory  and  motor  areas 
of  each  cerebral  cortex  and  commissural  fibers  extend  from  one  hemi- 
sphere to  the  other.  A  particularly  large  commissure,  the  corpus  callo- 
sum,  can  be  seen  in  a  sagittal  section  of  the  brain  (Fig.  29.14).  Such 
interconnections  jDcrmit  the  integration  of  the  many  different  sorts  of 
impulses  that  reach  the  cerebrum  and  enable  mammals  to  make  mean- 
ingful responses  to  a  combination  of  sensory  stimuli. 

The  cerebral  cortex  of  most  mammals  is  composed  almost  entirely 
of  the  specific  sensory  and  motor  areas  just  described,  but  in  man,  large 


SENSE  ORGANS   AND   NERVOUS   COORD/NAT/ON  503 

association  areas  lie  between  the  sensory  and  motor  regions  (Fig.  29.15). 
Presumably  such  complex  mental  processes  as  learning,  memory,  thought 
and  imagination  occur  here.  It  these  areas  are  destroyed,  one  loses  the 
ability  to  comprehend  symbols  and  formulate  expressions,  a  condition 
known  as  aphasia.  In  one  type  of  aphasia,  words  are  heard,  but  they 
might  as  well  be  in  an  unknown  language  for  they  cannot  be  recog- 
nized. The  ability  to  learn  and  understand  is  not  localized  in  any 
particular  association  area;  instead,  the  cerebral  cortex  appears  to 
function  as  a  whole  in  the  higher  mental  processes.  In  many  injuries 
the  nature  of  the  lost  ability  is  correlated  more  with  the  amount  of 
cortex  destroyed  than  with  the  specific  part  destroyed.  Biologists  and 
psychologists  are  just  beginning  to  understand  the  functioning  of  the 
human  brain,  and  many  of  its  aspects  are  beyond  our  comprehension 
at  present. 

Questions 

1.  Describe  what  happens  to  a  ray  of  dim  light  that  enters  the  eye  from  a  point  near  the 
observer.  Through  what  structures  does  it  pass;  what,  if  any,  adjustments  are  neces- 
sary to  make  it  fall  upon  the  retina;  and  how  does  it  activate  a  receptor  cell? 

2.  What  effect  did  the  transition  from  water  to  land  during  the  course  of  vertebrate 
evolution  have  upon  the  eyeball  and  surrounding  structures? 

3.  Describe  how  we  become  aware  of  a  loud  sound  of  low  frequency. 

4.  How  has  the  ear  changed  during  evolution  from  fish  to  mammal?  What  part  of  the 
ear  has  changed  very  little? 

5.  Describe  the  electrical  changes  that  occur  in  a  neuron  during  the  transmission  of  a 
nerve  impulse. 

6.  List  the  major  categories  of  neurons  that  make  up  the  nervous  system.  Which  ones 
are  involved  in  a  spinal  reflex? 

7.  Distinguish  between  the  roots  and  rami  of  a  spinal  nerve. 

8.  \Vhat  are  the  major  differences  between  the  cranial  nerves  of  mammals  and  fishes? 

9.  Define  the  autonomic  nervous  system.  How  does  autonomic  innervation  differ  from 
the  innervation  of  other  organs? 

10.  How  do  the  dorsal  and  ventral  columns  of  the  spinal  cord  differ? 

11.  List  the  five  divisions  of  the  brain  and  the  major  brain  structures  that  develop  in 
each. 

12.  In  what  ways  has  the  structure  and  function  of  the  cerebral  hemispheres  changed  in 
the  evolution  from  fish  to  mammals. 

13.  Briefly  state  the  function  of  each  of  the  following:  medulla,  cerebellum,  thalamus, 
hypothalamus. 

14.  What  is  believed  to  be  the  function  of  the  association  areas  of  the  cerebral  hemi- 
spheres? What  happens  if  they  are  destroyed? 

Supplementary  Reading 

The  structure  and  physiology  of  all  of  the  sense  organs  of  man  are  considered  care- 
fully by  Geldard  in  The  Human  Senses.  The  fascinating  story  of  the  evolution  of  the 
vertebrate  eye  and  its  adaptation  to  all  environments  in  which  vertebrates  live  are  con- 
sidered in  Walls'  monograph,  The  Vertebrate  Eye.  Stevens  and  Davis.  Hearing,  Its  Psy- 
chology and  Pliysiology,  is  a  valuable  reference  work  on  the  ear.  Recent  investigations  on 
the  nerve  impulse  are  summarized  in  an  article  by  Katz,  The  Nerve  Impulse,  in  Flan- 
agan's The  Physics  and  Chemistry  of  Life.  Gardner's  Fundamentals  of  Neurology  is  a 
good,  concise  account  of  the  morphology  and  physiology  of  the  human  nervous  system. 


504  VERTEBRATE   LIFE   AND   ORGANIZATION 

Additional  anatomical  details  can  be  found  in  such  standard  texts  as  Ranson  and  Clark, 
The  Anatomy  of  the  Nervous  Systein,  or  Rasmussen.  The  Principal  Nervous  Pathways. 
Sherrington's  Integrative  Action  of  the  Nemous  System  is  a  very  good  account  of  the 
functioning  of  this  complex  system.  Walter  describes  the  main  features  of  the  evolution 
of  the  brain,  its  elaboration  in  man,  and  such  problems  as  learning  and  memory  in  an 
authoritative  and  very  interesting  manner  in  The  Living  Brain.  Similar  problems  are 
considered  in  a  less  technical  style  by  Pfeiffer  in  The  Human  Brain. 


CHAPTER  30 


The  Endocrine  System 


The  integration  of  the  activities  of  tlie  several  parts  of  tlie  higher, 
more  complex  animals  has  been  achieved  by  the  evolution  of  two  major 
coordinating  systems,  the  nervous  system,  discussed  in  the  previous 
chapter,  and  the  endocrine  system.  The  nerves  and  sense  organs  enable 
an  animal  to  adapt  very  rapidly— with  responses  measured  in  millisec- 
onds—to changes  in  the  environment.  The  swift  responses  of  muscles 
and  glands  are  typically  under  nervous  control.  The  glands  of  the 
endocrine  system  secrete  substances  called  hormones  which  diffuse  or 
are  transported  by  the  blood  stream  to  other  parts  of  the  body  and 
coordinate  their  activities.  The  responses  under  endocrine  control  are 
generally  somewhat  slower— measured  in  minutes,  hours  or  weeks— but 
longer  lasting  than  those  under  nervous  control.  The  long-range  ad- 
justments of  metabolism,  growth  and  reproduction  are  typically  under 
endocrine  control. 

Endocrine  glands  secrete  their  products  into  the  blood  stream, 
rather  than  into  a  duct  leading  to  the  exterior  of  the  body  or  to  one 
of  the  internal  organs  as  do  exocrine  glands,  and  hence  are  called  duct- 
less glands  or  glands  of  internal  secretion.  The  pancreas  is  an  example 
of  a  gland  with  both  endocrine  and  exocrine  functions,  for  it  secretes 
enzymes  which  pass  via  the  pancreatic  duct  to  the  duodenum  and  it 
secretes  hormones  which  are  transported  to  other  parts  of  the  body  in 
the  blood  stream.  In  the  toadfish  the  two  parts  of  the  pancreas  are 
anatomically  separate. 

The  term  "hormone"  was  originated  in  1905  by  the  British  physi- 
ologist E.  H.  Starling,  who  was  studying  the  control  of  the  exocrine 
function  of  the  pancreas  by  secretin,  a  substance  produced  in  the  duo- 
denal mucosa.  Starling  defined  a  hormone  as  "any  substance  normally 
produced  in  the  cells  in  some  part  of  the  body  and  carried  by  the 
blood  stream  to  distant  parts,  which  it  affects  for  the  good  of  the  body 
as  a  whole."  Our  rapidly  increasing  knowledge  of  the  many  different 
hormones  produced  by  both  vertebrate  and  invertebrate  animals  and 
by  plants  has  led  to  the  generalization  that  these  are  special  chemical 
substances,  produced  by  some  restricted  region  of  an  organism,  which 
diffuse,  or  are  transported  by  the  blood  stream,  to  another  region  of 
the  organism,  where  they  are  effective  in  very  low  concentrations  in 
regulating  and  coordinating  the  activities  of  the  cells. 

The  hormones  isolated  and  characterized  to  date  have  proved   to 

605 


506  VERTEBRATE   LIFE   AND   ORGANIZATION 

be  proteins,  amino  acids  or  steroids;  thus,  we  cannot  deRne  a  hormone 
as  a  member  of  some  particular  chiss  ol  organic  compound.  All  of  the 
hormones  are  required  for  normal  body  function  and  they  must  be 
present  in  certain  optimal  amounts.  Either  a  hyposecretion  (deficiency) 
or  hypersecretion  (excess)  of  any  one  may  result  in  a  characteristic 
pathologic  condition. 

Some  practical  knowledge  of  endocrinology,  such  as  the  results  of 
the  castration  of  men  and  animals,  has  existed  for  several  thousand 
years.  However,  it  was  not  until  1849  that  Berthold,  from  clear-cut 
experiments  in  which  testes  were  transplanted  from  one  bird  to  an- 
other, postulated  that  these  male  sex  glands  secrete  some  blood-borne 
substance  which  is  essential  for  the  differentiation  of  the  male  sec- 
ondary sex  characters.  In  1855  the  British  physician,  Thomas  Addison, 
describetl  the  signs  and  symptoms  of  the  human  disease  which  now 
bears  his  name,  "and  realized  that  this  was  associated  with  the  deteri- 
oration of  the  cortex  of  the  adrenal.  The  first  attempt  at  endocrine 
therapy  was  made  in  1889,  when  the  French  physiologist,  Brown- 
Sequard,  injected  himself  with  testicular  extracts  and  claimed  that  they 
had  a  rejuvenating  effect.  Epinephrine  was  the  first  hormone  to  be 
isolated  and  chemically  identified  (1902).  Many  of  our  theoretical  con- 
cepts regarding  endocrines  stem  from  the  classic  work  of  Starling  and 
of  Bayliss  with  secretin  during  the  first  two  decades  of  this  century. 

The  basic  problem  of  just  how  a  hormone  may  act  upon  a  tissue  to 
regulate  its  activities  remains  to  be  solved.  It  would  appear  that  hor- 
mones are  not  essential  for  the  survival  of  individual  cells,  for  many 
kinds  of  cells  can  be  grown  in  tissue  culture  indefinitely  without  added 
hormones.  It  has  been  postulated  that  hormones  produce  their  effects 
by  directly  stimulating  or  inhibiting  one  or  more  of  the  intracellular 
enzyme  systems,  or  by  modifying  in  some  way  the  permeability  of  the 
cell  membrane  so  that  substances  can  enter  more  readily  to  be  me- 
tabolized. The  tissues  in  various  parts  of  the  body  differ  greatly  in  their 
sensitivity  to  particular  hormones,  but  the  explanation  for  this  phe- 
nomenon is  lacking.  It  is  not  clear  at  present  whether  a  hormone  is 
used  up  in  the  process  of  regulating  metabolism  in  a  target  cell.  Hor- 
mones are  gradually  inactivated  and  eliminated  from  the  blood  stream, 
and  hence  must  be  continually  replaced  by  the  appropriate  endocrine 
gland.  Both  the  synthesis  and  the  inactivation  and  degradation  of  hor- 
mone molecules  are  enzymatic  processes. 

248.        Methods  of  Investigating  Endocrines 

The  complete  understanding  of  the  role  of  an  endocrine  gland 
requires  information  about  (1)  the  number  and  kinds  of  hormones  it 
secretes,  (2)  what  chemical  and  physical  properties  each  of  these  hor- 
mones has,  (3)  where  and  how  they  are  made  within  the  endocrine 
organ,  (4)  what  factors  control  their  production,  (5)  what  stimulates 
their  secretion  by  the  gland,  (6)  how  they  are  transported  to  the  target 
organ,  (7)  how  they  act  to  alter  the  metabolism  of  the  target  organ,  (8) 
how  they  are  broken  down  and  eliminated  from  the  body,  (9)  how  they 


THE   ENDOCRINE   SYSTEM  507 

may  be  produced  synthetically  and  (10)  what  use  they  may  have  in 
the  treatment  of  disease.  The  assembling  of  all  of  this  information 
requires  the  efforts  of  anatomists,  histologists,  physiologists,  biochem- 
ists, pharmacologists  and  clinicians. 

The  fact  that  a  certain  gland  has  endocrine  function  is  frequently 
first  learned  as  a  result  of  its  accidental  or  deliberate  removal.  The 
deprivation  of  the  organism  of  its  normal  source  of  the  hormone 
usually  results  in  readily  observable  abnormalities.  As  we  shall  see,  the 
normal  functioning  of  any  given  organ  is  usually  the  result  of  the  ef- 
fects of  a  number  of  different  hormones,  some  of  which  work  together 
(act  synergistically)  while  others  oppose  the  action  of  the  first  (act 
antagonistically).  It  may  be  incorrect  to  attribute  the  effects  of  the 
surgical  removal  of  one  gland  to  the  simple  lack  of  its  hormone;  they 
may  result  from  the  unopposed  action  of  hormones  secreted  by  other 
glands.  It  may  require  a  complex  experimental  design,  including  the 
removal  of  several  endocrine  glands  and  the  replacement  of  their  secre- 
tions by  injected  pure  hormones,  to  elucidate  the  role  of  each. 

Further  information  about  endocrine  function  is  obtained  by  re- 
placing the  surgically  extirpated  gland  by  transplanting  a  gland  from 
another  animal,  by  feeding  dried  glands,  or  by  injecting  an  extract  or 
a  purified  compound  obtained  from  the  gland.  The  administration 
of  one  hormone  frequently  suppresses  or  stimulates  the  secretion  of 
hormones  by  other  glands.  By  proper  experimental  design,  one  can 
distinguish  between  the  primary  effect  of  the  injected  hormone  and 
its  possible  secondary  effects  via  the  stimulation  or  inhibition  of  other 
endocrines. 

Another  experimental  approach  to  the  endocrine  problem  is  the 
extraction  and  purification  of  the  hormone  by  chemical  and  physical 
procedures  from  the  gland  itself  or  from  the  blood  or  urine  of  the 
organism.  Only  an  extremely  small  amount  of  hormone  is  required  to 
produce  its  normal  effects,  and  the  amount  present  in  the  endocrine 
gland,  or  in  the  blood  and  urine,  is  usually  quite  small.  The  isolation 
of  a  pure  hormone  is  a  difficult  procedure;  more  than  two  tons  of  pig 
ovaries  had  to  be  extracted  to  yield  a  few  milligrams  of  estradiol,  the 
female  sex  hormone,  and  to  get  15  mg.  of  androsterone,  a  male  sex 
hormone,  it  was  necessary  to  extract  over  5000  gallons  of  urine! 

Much  has  been  learned  about  endocrine  function  by  careful  ob- 
servation of  the  symptoms  of  human  diseases  resulting  from  the  hypo- 
or  hypersecretion  of  hormones.  Further  information  has  been  derived 
from  the  careful  study  of  strains  of  rats,  mice  and  other  animals  with 
particular   endocrine    abnormalities-dwarf   mice,    obese   mice,    diabetic 

mice,  and  so  on. 

The  location  of  the  human  endocrine  glands  is  shown  in  Figure 
30.1.  Their  relative  position  in  the  body  is  much  the  same  in  all  the 
vertebrates.  The  source  and  physiologic  effects  of  the  principal  hor- 
mones are  listed  in  Table  7.  It  must  be  kept  in  mind  that  hormones 
are  not  found  solely  in  vertebrates,  but  occur  as  well  in  such  inverte- 
brates as  insects,  crustaceans,  annelids  and  molluscs. 


608 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Pitixifcajry 


Thymus      ""       ■  -"Y^ 


roi 


ds 


R.  Kidney 


Ovaries  (^Female)  — 


Stomach* 

Pa.ncreas 

Intestine* 


-Xv 


XK 


Te-stes  Cmale) 


Figure  30.1.     The  human  body  showing  the  location  of  the  endocrine  glands.  The 
starred  organs,  though  not  primarily  endocrine  glands,  do  secrete  one  or  more  hormones. 

249.       The  Thyroid 

All  vertebrates  have  a  pair  of  thyroid  glands  located  in  the  neck. 
In  mammals  the  two  glands  are  located  on  either  side  of  the  larynx 
and  are  joined  by  a  narrow  isthmus  of  tissue  which  passes  across  the 
ventral  surface  of  the  trachea  near  its  junction  with  the  larynx.  The 
thyroid  has  an  exceptionally  rich  blood  supply,  which  reflects  its  func- 
tion as  an  endocrine  gland.  The  thyroids  develop  as  a  ventral  outgrowth 
of  the  floor  of  the  pharynx  but  the  connection  with  the  pharynx  is 
usually  lost  early  in  development.  In  a  microscopic  section  the  thyroid 
is  seen  to  consist  of  many  hollow  spheres,  called  follicles.  Each  follicle  is 
composed  of  a  single  layer  of  cuboidal  epithelial  cells  surroiniding  a 
cavity  filled  with  a  gelatinous  material  called  colloid,  secreted  by  the 
follicle  cells  (Fig.  30.2). 

The  follicle  cells  have  a  remarkable  ability  to  accumulate  iodide 
from  the  blood.  This  is  used  in  the  synthesis  of  the  protein  thyro- 
globulin  which  is  secreted  into  the  colloid  and  stored.  Thyroglobulin 
is  a  large  molecule  and  not  readily  diffusible  into  the  blood  stream, 
but  proteolytic  enzymes  in  the  colloid  hydrolyze  thyroglobulin  to  its 
constituent  amino  acids,  one  of  which  is  thyroxin,  a  derivative  of  the 
amino  acid  tyrosine  containing  65  per  cent  iodine.  Thyroxin  passes  into 
the  blood  stream  where  it  is  transported  loosely  bound  to  certain 
plasma   proteins.    In    tissues    thyroxin,    which    contains    four    atoms   of 


THE   ENDOCRINE   SYSTEM 


609 


iodine,  may  be  converted  to  triiodothyronine,  which  contains  one  less 
atom  of  iodine  and  is  several  times  more  active  than  thyroxin.  It  is 
not  yet  clear  whether  the  hormone  active  at  the  cellular  level  is  thy- 
roxin itself,  triiodothyronine,  or  some  closely  related  derivative. 

The  first  clues  as  to  thyroid  function  came  from  observations  on 
human  disease  in  1874  by  the  British  physician,  Sir  William  Gull,  who 
noted  the  association  of  spontaneous  decreased  function  of  the  thyroid 
and   puffy,  dry   skin,   dry,  brittle   hair,   and   mental  and  physical   lassi- 

Table  7.     HORMONES  AND  THEIR  EFFECTS 


HORMONE 


SOURCE 


PHYSIOLOGIC  EFFECT 


Thyroxin 
Parathormone 

^    Insulin 


Glucagon 

Epinephrine 
Norepinephrine 
Hydrocortisone 

Aldosterone 

Adrenosterone 

Growth  hormone 

Thyrotropin 

Adrenocorticotropin 

(ACTH) 
Follicle-stimulating 
hormone 
(FSH) 
Luteinizing  hormone 
(LH) 

Prolactin 

Oxytocin 

Vasopressin 

Intermedin 

Testosterone 

Estradiol 

Progesterone 

Chorionic  gonadotropin 

Relaxin 


Thyroid  gland 
Parathyroid  glands 

Beta  cells  of  islets  in 
pancreas 

Alpha  cells  of  islets  in 

pancreas 
.Adrenal  medulla 
Adrenal  medulla 
Adrenal  cortex 

Adrenal  cortex 

Adrenal  cortex 

Anterior  lobe  of 

pituitary 
Anterior  pituitary 

Anterior  pituitary 

Anterior  pituitary 

Anterior  pituitary 

Anterior  pituitary 

Hypothalamus,  via 

posterior  pituitary 
Hypothalamus,  via 

posterior  pituitary 
Intermediate  lobe  of 

pituitary 
Interstitial  cells  of 

testis 
Follicle  of  ovary 

Corpus  luteum  of 

ovary 
Placenta 

Ovary  and  placenta 


Increases  basal  metabolic  rate 

Regulates  calcium  and  phosphorus 
metabolism 

Decreases  blood  sugar  concentration,  in- 
creases glycogen  storage  and  metabo- 
lism of  glucose 

Stimulates  conversion  of  liver  glycogen  to 
blood  glucose 

Reinforces  action  of  sympathetic  nerves 

Constricts  blood  vessels 

Stimulates  conversion  of  proteins  to  carbo- 
hydrates 

Regulates  metabolism  of  sodium  and  po- 
tassium 

Androgen,  stimulates  development  of  male 
characters 

Controls  bone  growth  and  general  body 
growth 

Stimulates  growth  and  functional  activity 
of  the  thyroid 

Stimulates  adrenal  cortex  to  produce  cor- 
tical hormones 

Stimulates  growth  of  graafian  follicles  in 
female  and  of  seminiferous  tubules  in 
male 

Controls  production  and  release  of  estro- 
gens and  progesterone  by  ovary  and  of 
testosterone  by  testis 

Stimulates  secretion  of  milk  by  breast, 
controls  maternal  instinct 

Stimulates  contraction  of  uterine  muscles 

Stimulates  contraction  of  smooth  muscles; 

has  antidiuretic  action  on  kidney  tubules 
Stimulates  dispersal  of  pigment  in  chro- 

matophores 
Androgen;  stimulates  development  and 

maintenance  of  male  sex  characters 
Estrogen;  stimulates  development  and 

maintenance  of  female  sex  characters 
Acts  with  estradiol  to  regulate  the  estrous 

and  menstrual  cycles 
Acts,  along  with  other  hormones,  in  the 

maintenance  of  pregnancy 
Relaxes  pelvic  ligaments 


510  VERTEBRATE   LIFE   AND   ORGANIZATION 


tfCKCrOHy   CPITHCLUJU 


tLooo  yesscL 


IMTtKfaUICUlAK  CONNCCriy£  TISSUC 


Figure  30.2.  Upper,  Cells  of  the  normal  thyroid  gland  of  the  rat.  Lower  left, 
Thyroid  from  a  normal  rat  which  had  received  ten  daily  injections  of  thyrotropin. 
Lower  right.  Thyroid  from  a  rat  six  months  after  complete  removal  of  the  pituitary 
gland.  (Turner:  General  Endocrinology.) 

tude.  The  Swiss  surgeon  Kocher  removed  the  thyroids  from  a  series  of 
patients  and  then  noted  that  they  developed  the  same  symptoms  as 
Gull's  patients.  In  1895,  using  a  newly  devised  calorimeter  to  measure 
the  rate  of  metabolism  in  patients  by  the  amount  of  heat  they  pro- 
duced, Magnus-Levy  found  that  persons  with  myxedema  (Gull's  dis- 
ease) had  notably  lower  than  normal  metabolic  rates.  VV^hen  these 
patients  were  fed  thyroid  tissue,  their  metabolic  rate  was  raised  toward 
normal.  This  led  to  the  idea  that  the  thyroid  secretes  a  hormone  which 
regulates  the  metabolic  rate  of  all  body  cells.  It  was  found  in  1896  that 
the  thyroid  hormone  contains  iodine.  Thyroglobulin  was  first  isolated 
in  1897  and  thyroxin  in  1914.  Its  chemical  formula  was  determined  in 
1926  and  it  was  first  synthesized  in   1927. 

The  role  of  thyroid  hormone  in  all  vertebrates  is  to  increase  the 
rate  of  a  certain  series  of  enzyme  reactions  which  lead  to  the  release  of 
biologically  available  energy.  The  amount  of  energy  released  by  an 
organism  under  standard  conditions  at  rest,  measured  in  a  calorimeter 


THE   ENDOCRINE   SYSTEM 


611 


by  the  amount  of  heat  given  off,  or  calculated  from  the  amount  of 
oxygen  consumed,  is  decreased  in  thyroid  deficiency  and  increased 
when  thyroid  is  administered  or  when  the  gland  is  overactive.  Com- 
plete removal  of  the  thyroid  glands  from  a  mammal  reduces  its  meta- 
bolic rate  to  half  of  the  normal  vakie,  and  the  body  temperature 
decreases  slightly.  Since  foods  are  metabolized  at  a  lower  rate,  they 
tend  to  be  stored  and  the  animal  becomes  obese.  Not  only  is  the  meta- 
bolic rate  of  the  intact  animal  decreased  by  thyroid  deficiency,  but 
individual  bits  of  tissue  removed  from  the  animal  and  incubated  in 
vitro  show  a  decreased  metabolic  rate— decreased  oxygen  consumption 
and  decreased  utilization  of  substrate  molecules.  The  metabolism  of 
carbohydrates,  fats,  proteins,  water  and  salts  is  affected,  probably  sec- 
ondarily, by  the  amount  of  thyroid  hormone  present. 

Thyroid  hormone,  by  its  action  on  metabolic  processes,  has  a 
marked  influence  on  growth  and  differentiation.  Extirpating  the  thy- 
roid of  young  animals  causes  decreased  body  growth,  retarded  mental 
development,  and  delayed  or  decreased  differentiation  of  gonads  and 
external  genitalia.  All  of  these  changes  are  reversed  by  the  administra- 
tion of  thyroxin.  The  metamorphosis  of  frog  and  salamander  tadpoles 
into  adults  is  controlled  by  the  thyroid.  Removal  of  the  larval  thyroid 


Figure  30  3  The  effect  of  thyroid  feeding  upon  the  tadpoles  of  Rana  catesbtana. 
A  is  the  untreated  control,  which  was  killed  at  the  end  of  the  experiment.  The 
metamorphosed  animal  at  the  lower  right  (G)  was  killed  two  weeks  after  starting 
the  feeding  of  thyroid  gland.  The  remaining  animals  {B  to  F)  were  removed  from  the 
experiment  at  intervals  during  this  period.  Note  the  effect  of  thyroid  substances  on 
the  metamorphosis  of  the  mouth,  tail  and  paired  appendages.  (Turner:  General 
Endocrinology.) 


512  VERTEBRATE   LIFE   AND   ORGANIZATION 

completely  prevents  metamori^hosis  and  administering  thyroxin  to  tad- 
poles causes  them  to  metamorphose  prematurely  into  miniature  adults 
(Fig.  30.3).  1  he  effect  of  thyroxin  on  amphibian  metamorphosis  ap- 
pears not  to  be  simply  a  secondary  result  ot  its  effect  on  metabolism,  for 
tadpole  metabolism  can  be  increased  by  dinitrophenol  but  premature 
metamorphosis  does  not  occur.  Some  specific  effect  of  thyroxin  on 
metamorphosis  appears  to  be  involved. 

Thyroxin  stimulates  the  oxidative,  energy-releasing  processes  in 
all  tissues  of  the  body.  Our  current  biochemical  concept  is  that  it 
uncouples  the  phosphorylation  process  from  oxidative  processes  so  that 
the  latter  occur  rapidly,  yet  energy,  as  energy-rich  phosphate  bonds 
(p.  67),  is  less  available. 

The  production  and  discharge  of  thyroxin  is  not  regulated  by  the 
nervous  system,  but  by  the  hormone  thyrotropin  secreted  by  the  an- 
terior lobe  of  the  pituitary  gland.  In  1916,  P.  E.  Smith  found  that  the 
removal  of  the  pituitary  of  frog  tadpoles  produced  deterioration  of 
the  thyroid  and  prevented  metamorphosis.  The  same  pituitary  control 
of  thyroid  function  has  been  found  in  rats,  man  and  other  mammals. 
The  secretion  of  thyrotropin  by  the  pituitary  is  regulated  in  part  by  the 
amount  of  thyroxin  in  the  blood.  Thus,  a  decreased  production  of 
thyroxin  by  the  thyroid  leads  to  less  thyroxin  in  the  blood  stream  and 
this  stimulates  the  pituitary  to  release  thyrotropin,  which  passes  to  the 
thyroid  gland  and  raises  its  output  of  thyroxin.  When  the  blood  level 
of  thyroxin  is  brought  back  to  normal,  the  release  of  thyrotropin  is 
decreased.  By  this  "feed-back"  mechanism  the  output  of  thyroxin  is 
kept  relatively  constant  and  the  basal  metabolic  rate  is  kept  within  the 
normal  range.  Since  iodine  is  an  essential  atom  in  thyroxin,  a  deficiency 
of  this  element  leads  to  decreased  synthesis  of  thyroxin.  Iodine  de- 
ficiency stimulates  the  thyroid  follicle  cells  to  enlarge  and  to  increase 
in  number.  The  enlargement  of  the  thyroid  is  known  as  a  goiter. 
Thiouracil  and  related  compounds  are  goitrogenic.  They  inhibit  the 
production  of  thyroid  hormone  by  blocking  the  reactions  by  which 
iodide  is  oxidized  and  fixed  onto  the  tyrosine  molecule.  The  deficiency 
of  thyroid  hormone  stimulates  the  pituitary  to  release  more  thyro- 
tropin, which  in  turn  stimulates  the  thyroid  cells  to  enlarge  and  pro- 
duce a  goiter.  Thiouracil  is  used  clinically  to  decrease  thyroxin  pro- 
duction by  hyperactive  thyroids. 

The  chief  human  diseases  of  the  thyroid  are  cretinism,  myxedema, 
simple  goiter  and  exophthalmic  goiter.  Thyroid  deficiency  in  infancy 
produces  a  dwarfed,  mentally  retarded  child  known  as  a  cretin  (Fig. 
30.4  A).  A  cretin  has  an  enlarged  tongue,  coarse  features,  malformed 
bones,  distended  belly  and  wrinkled,  cold  skin.  If  thyroid  therapy  is 
begun  early  enough,  normal  development  of  the  brain  and  body  can 
be  induced.  Thyroid  deficiency  in  adults  results  in  myxedema,  char- 
acterized by  decreased  metabolic  rate,  mental  deterioration,  obesity, 
loss  of  hair  and  cold  rough  skin.  Simple  goiter,  or  enlarged  thyroid, 
results  usually  from  a  deficiency  of  iodine,  with  a  secondary  increase 
in  the  size  of  the  thyroid  due   to  its  stimulation  by   thyrotropin  (Fig. 


THE   ENDOCRINE   SYSTEM 


618 


30.4  B).  The  increased  size  of  the  thyroid  presumably  permits  maximal 
use  of  the  small  amount  of  iodine  available.  Iodine  is  deficient  in  the 
soil  and  water  of  certain  parts  of  the  world,  and  hence  deficient  in 
plants  grown  there  and  in  the  animals  eating  these  plants.  The  preval- 
ence of  human  goiter  has  been  greatly  decreased  by  the  practice  of 
adding  iodide  to  table  salt,  and  by  better  distribution  of  food. 

The    overproduction    of    thyroid    hormone    produces    a    condition 


B 


Figure  30  4  A,  A  cretin.  B,  Simple  goiter.  C,  Exophthalmic  goiter.  (A  and  B 
from  Selye-  Textbook  of  Endocrinology',  published  by  Acta  Endocrinologia,  Inc.; 
C  from    Houssay:    Human   Physiology,   published    by    McGraw-Hill    Book    Company.) 


514  VERTEBRATE   LIFE   AND    ORGANIZATION 

known  as  Graves's  disease,  or  exophthalmic  goiter  (Fig.  30.4  C).  The 
thyroid  may  be  enhirged,  or  may  be  of  nearly  normal  size,  but  it  pro- 
duces excessive  amounts  oi  hormone,  with  a  resulting  increased  basal 
metabolic  rate,  increased  production  of  heat,  loss  of  weight,  increased 
heart  rate  and  blood  pressure,  nervousness,  and  exophthalmos,  or  pro- 
trusion of  the  eyeballs.  Hyperthyroidism  can  be  treated  by  surgical 
removal  of  part  of  the  thyroid,  or  by  its  destruction  with  x-rays  or  with 
radioactive  iodine. 

250.        The  Parathyroid  Glands 

Embedded  in  or  attached  to  the  thyroid  glands  are  small  masses  of 
tissue  called  the  parathyroid  glands.  There  are  usually  two  pairs 
of  jKirathyroids  which  develop  embryologically  as  outgrowths  of  the 
third  and  fourth  pairs  of  pharyngeal  pouches.  Each  gland  consists  of 
solid  masses  and  cords  of  epithelial  cells,  rather  than  of  spherical 
follicles  as  in  the  thyroid.  The  hormone  secreted  by  the  parathyroids, 
called  parathormone,  is  a  protein,  and  was  first  extracted  from  para- 
thyroid glands  by  Collip  in  1925.  It  regulates,  by  mechanisms  which 
are  not  yet  clear,  the  levels  of  calcium  and  phosphorus  in  the  blood 
and  body  fluids,  and  is  essential  for  life.  The  complete  removal  of  the 
parathyroids  results  in  death  in  a  few  days.  Parathyroidectomy  produces 
a  decreased  concentration  of  calcium  in  the  serum,  a  decreased  excre- 
tion of  phosphorus,  and  a  resulting  increase  in  the  amount  of  phos- 
phorus in  the  serum.  The  animal  is  subject  to  muscular  tremors,  cramps 
and  convulsions,  a  condition  known  as  tetany,  which  results  from  the 
low  level  of  calcium  in  the  body  fluids.  An  injection  of  a  solution  of 
calcium  stops  the  tetanic  convulsions  and  further  convulsions  can  be 
prevented  by  repeated  administration  of  calcium. 

Recent  experiments  indicate  that  there  are  two  hormones  secreted 
by  the  parathyroid,  both  of  which  regulate  calcium  and  phosphorus 
concentrations  in  body  fluids  but  by  different  mechanisms.  One  hor- 
mone acts  primarily  on  the  kidney  and  leads  to  an  increased  excretion 
of  phosphorus;  the  other  acts  primarily  on  the  cells  within  the  bone 
and  regulates  the  deposition  and  dissolution  of  the  bone  salts. 

Parathyroid  deficiencies  are  rare,  occurring  occasionally  when  the 
glands  are  removed  inadvertently  during  an  operation  on  the  thyroid, 
or  when  degeneration  results  from  an  infection.  The  administration  of 
parathormone  cannot  be  used  for  the  long-term  treatment  of  parathy- 
roid deficiencies,  for  the  patient  becomes  refractory  to  repeated  injec- 
tions of  the  extract.  The  deficiency  can  be  treated  successfully  by  a  diet 
rich  in  calcium  and  vitamin  D  and  low  in  phosphorus. 

Hyperfunction  of  the  parathyroid,  induced  by  a  tumor  of  the 
gland,  is  characterized  by  high  calcium  and  low  phosphorus  content  of 
the  blood  and  by  increased  urinary  excretion  of  both  calcium  and 
phosphorus.  The  calcium  comes  at  least  in  part  from  the  bones  and 
soft,  easily  broken  bones  result.  The  increased  level  of  calcium  in  the 
body  fluids  eventually  leads  to  deposits  of  calcium  in  abnormal  places 
—the  kidney,  intestinal  wall,  heart  and  lungs. 


THE   ENDOCRINE   SYSTEM  Q\^ 

251.        The  Islet  Cells  of  the  Pancreas 

The  pancreas  is  known  to  secrete  two  hormones,  insuHn  and  glu- 
cagon, in  addition  to  a  number  of  digestive  enzymes.  Scattered  among 
the  acinar  cells  which  secrete  the  digestive  enzymes  are  clusters  of 
hormone-secreting  cells,  called  islets  of  Langerhans,  which  are  quite 
different  in  appearance  and  staining  properties.  They  have  a  richer 
supply  of  blood  vessels  than  the  acinar  cells  and  have  no  associated 
ducts.  The  islet  cells  can  be  differentiated  into  two  or  more  types  by 
the  staining  reactions  of  their  cytoplasmic  granules.  The  pancreas  de- 
velops as  two  otugrowths  from  the  duodenum  which  grow  together  and 
fuse  in  most  vertebrates.  The  islet  cells  develop  as  buds  from  the  pan- 
creatic ducts  and  eventually  lose  all  connection  with  the  ducts.  In 
some  bony  fishes  the  acinar  and  islet  tissues  form  spatially  separate 
organs.  The  pancreas  of  the  cyclostomes  is  ductless  and  located  in  the 
wall  of  the  duodenum  or  in  the  liver. 

The  human  disease  diabetes  had  been  recognized  for  many  cen- 
turies but  its  cause  and  cure  were  equally  unknown.  A  similar  condition 
was  produced  experimentally  in  dogs  by  von  Mering  and  Minkowski 
in  1889  when  they  surgically  removed  the  pancreas  while  studying  its 
role  in  digestion.  Many  attempts  were  subsequently  made  to  feed  pan- 
creas or  to  prepare  an  extract  for  injection  into  diabetics,  but  all  were 
unsuccessful  because  the  proteolytic  enzymes  made  by  the  pancreas  de- 
stroyed the  protein  hormone  before  it  could  be  extracted.  Finally,  in 
1922,  Banting  and  Best  prepared  an  extract  of  fetal  pancreas  which  had 
antidiabetic  potency.  The  endocrine  cells  of  the  pancreas  become  active 
before  the  exocrine  ones  do.  The  first  preparation  of  pure  crystalline 
insulin  was  made  in  1927  by  Abel.  The  present  commercial  insulin  is 
extracted  from  beef,  sheep  or  hog  pancieas  by  an  acid  alcohol  method 
which  rapidly  inactivates  the  proteolytic  enzymes.  Insulin  is  a  protein 
with  a  molecular  weight  of  12,000.  From  the  brilliant  work  of  F.  Sanger 
in  England  the  exact  sequence  of  the  amino  acids  in  each  of  the  two 
peptide  chains  making  up  the  insulin  molecule  is  now  known.  One 
chain  contains  21  amino  acids  and  the  other  contains  30. 

Most  commercial  preparations  of  insulin  were  found  to  contain  a 
second  hormone,  which  increases  blood  sugar  concentration  instead 
of  decreasing  it  as  insulin  does.  This  hormone,  now  christened  glu- 
cagon, has  been  separated  from  insulin,  crystallized,  and  found  to  be 
a  protein.  Glucagon  is  secreted  by  the  alpha  cells  of  the  islets  and 
insulin  by  the  beta  cells. 

Insulin  and  glucagon  both  take  part  in  the  regulation  of  carbohy- 
drate metabolism,  along  with  certain  hormones  secreted  by  the  pitui- 
tary, adrenal  medulla  and  adrenal  cortex.  Glucagon  activates  the 
enzyme  phosphorylase,  which  is  involved  in  the  conversion  of  liver 
glycogen  to  blood  glucose,  and  thus  raises  the  concentration  of  glucose 
in  the  blood.  Insulin  increases  the  rate  of  conversion  of  blood  glucose 
to  intracellular  glucose-phosphate,  thereby  decreasing  the  blood  glucose 
level,  increasing  the  storage  of  glycogen  in  liver  and  muscle,  and  in- 
creasing the  metabolism  of  glucose  to  carbon  dioxide  and  water.  A 
deficiency  of  insulin  decreases  the  utilization  of  sugar  and  the  resulting 


516  VERTEBRATE   LIFE   AND    ORGANIZATION 

ujxsets  in  carbohydrate  metabolism  secondarily  produce  many  other 
changes  in  tlie  metabolism  ol  proteins,  tats  and  other  substances. 

The  surgical  removal  of  the  pancreas,  or  its  hypofunction  in  dia- 
betes mellitus,  produces  impaired  glucose  utilization,  which  results  in 
high  concentration  of  glucose  in  the  blood  (hyperglycemia)  and  the 
excretion  of  large  amounts  of  glucose  in  the  mine  (glycosuria)  because 
the  concentration  of  sugar  in  the  blood  exceeds  the  renal  threshold 
(p.  564).  Extra  water  is  required  to  excrete  this  sugar,  the  urine  volume 
increases,  and  the  patient  tends  to  become  dehydrated  and  thirsty.  Be- 
cause the  tissues  are  unable  to  get  enough  glucose  from  the  blood,  they 
break  down  protein  and  convert  the  carbon  chains  of  the  amino  acids 
into  glucose.  Much  of  this  is  excreted  and  there  is  a  steady  loss  of 
weight.  The  fat  deposits  are  also  mobilized  and  broken  down,  and  the 
concentration  of  fat  in  the  blood  may  increase  to  the  point  where 
the  blood  has  a  milky  appearance.  The  fatty  acids  are  not  metabolized 
completely  but  tend  to  accumulate  as  partially  oxidized  ketone  bodies 
such  as  acetoacetic  acid.  These  acidic  substances  accumulate  in  the 
blood  and  are  excreted  in  the  urine,  causing  an  acidosis  (loss  of  base) 
which  finally  results  in  coma  and  death.  The  injection  of  insulin  al- 
leviates all  of  these  symptoms;  with  the  utilization  of  glucose  made 
normal  by  insulin  all  of  the  other  metabolic  conditions  return  to 
normal. 

The  effect  of  an  injection  of  insulin  lasts  for  only  a  short  time,  a 
day  at  most,  for  the  insulin  is  gradually  destroyed  in  the  tissues.  A 
person  with  diabetes  must  receive  daily  injections  of  insulin  to  main- 
tain good  health.  Long-lasting  insulins,  such  as  protamine  zinc  insulin 
and  globin  insulin,  have  been  discovered  which  reduce  the  number  of 
injections  to  one  a  day  for  most  diabetics. 

The  administration  of  a  large  dose  of  insulin  to  a  normal  or  a 
diabetic  person  causes  a  marked  decrease  in  the  blood  sugar  level.  The 
nerve  cells,  which  require  a  certain  amount  of  glucose  for  normal 
function,  become  hyperirritable  and  then  fail  to  respond  as  the  glucose 
level  decreases.  The  patient  becomes  bewildered,  incoherent,  and  coma- 
tose and  may  die  unless  some  glucose  is  administered.  There  are  rare 
cases  of  pancreatic  tumors  which  by  hypersecretion  of  insulin  cause 
recurring  attacks  of  convulsions  and  unconsciousness  by  reducing  the 
blood  glucose  level. 

The  secretion  of  insulin  is  controlled  by  the  level  of  glucose  in 
the  blood.  When  the  blood  glucose  level  rises,  e.g.,  after  a  meal,  the 
secretion  of  insulin  is  stimulated  and  it  acts  to  restore  the  glucose  level 
to  normal.  When  the  glucose  concentration  has  been  lowered,  the 
stimulus  for  insulin  secretion  is  removed,  and  it  decreases  or  stops. 
The  long-continued  injection  of  insulin  into  a  nondiabetic  animal  or 
person  will  render  it  diabetic. 

252.        The  Adrenal  Glands 

The  small,  paired  adrenal  glands  of  mammals  are  located  at  the 
anterior  end  of  each  kidney.   The  two  human  glands  weigh  less  than 


THE   ENDOCRINE   SYSTEM 


617 


half  an  ounce,  but  have  a  richer  supply  of  blood  vessels  per  mass  of 
tissue  than  any  other  organ  of  the  body.  Each  adrenal  consists  of  two 
parts,  an  outer,  pale,  yellowish-pink  cortex  and  a  dark,  reddish-brown, 
inner  medulla.  In  cyclostomes  and  fishes  the  two  parts  are  spatially 
separate;  in  amphibians,  reptiles  and  birds  their  anatomic  relations  are 
quite  variable  and  the  two  parts  are  interspersed.  Cortical  tissue  de- 
velops from  coelomic  mesoderm  near  the  mesonephric  kidneys,  whereas 
the  medullary  tissue  is  ectodermal,  derived  from  the  neural  crest  cells 
which  also  form  the  sympathetic  ganglia. 

The  cells  of  the  medulla  are  arranged  in  irregular  cords  and  masses 
around  the  blood  vessels  (Fig.  30.5).  The  medulla  secretes  two  closely 
related  hormones,  epinephrine  (also  called  adrenin  and  adrenaline)  and 
norepinephrine.  These  are  comparatively  simple  chemicals  derived 
from  the  amino  acid  tyrosine.  Epinephrine  produces  an  increase  in  heart 
rate,  a  rise  in  blood  pressure,  a  decrease  in  liver  glycogen  and  an  increase 
in  blood  glucose.  It  causes  dilation  of  the  pupils  of  the  eye,  gooseflesh 
and  dilation  of  most  blood  vessels  but  constriction  of  those  of  the  skin, 
so  that  the  skin  becomes  pale.  Norepinephrine  has  much  weaker  effects 
on  blood  sugar  and  heart  rate  but  is  a  more  powerful  vasoconstrictor. 


cotnxt 


ton* 
CLOJueKULOtA 


ZONA 
FASCICULATA 


iOMA 
RCTICt/LARtS  " 


ycetra 


Mteuua 


Figure  30.5.     Sections    through    the   adrenal   cortex   and    medulla   of   normal    (A) 
and  hypophysectomized  (B)  rats.  (Turner:  General  Endocrinology.) 


518  VERTEBRATE   LIFE   AND   ORGANIZATION 

The  adrenal  medulla,  in  contrast  to  most  other  endocrine  glands, 
is  not  essential  for  life;  its  removal  does  not  cause  a  deficiency  disease. 
This  gland  is  believed  to  secrete  a  small  amount  of  epinephrine  and 
norepinephrine  continuously;  the  rate  of  secretion  is  under  nervous 
control. 

It  is  widely  believed  that  the  secretion  of  the  adrenal  medulla 
functions  during  emergencies  to  reinforce  and  prolong  the  action  of 
the  sympathetic  nervous  system.  There  is  good  evidence  that  epineph- 
rine secretion  is  greatly  increased  by  stresses  such  as  cold,  pain,  trauma, 
emotional  states,  and  certain  drugs.  The  changes  resulting  from  the 
action  of  the  sympathetic  nerves  and  epinephrine  would  prepare  an 
animal  to  attack  its  prey,  defend  itself  against  enemies,  or  run  away. 
These  include  the  following:  (1)  the  efficiency  of  the  circulatory  system 
is  increased  by  increased  blood  pressure,  heart  rate,  and  the  dilation 
of  the  large  blood  vessels;  (2)  the  increase  in  the  ability  of  blood  to  coag- 
ulate and  the  constriction  of  the  vessels  in  the  skin  tend  to  minimize 
the  loss  of  blood  if  the  animal  is  wounded;  (3)  the  intake  of  oxygen  is 
increased  by  increased  rate  of  breathing  and  dilation  of  the  respiratory 
passages;  (4)  the  mobilization  of  the  glycogen  stores  of  the  liver  and 
muscle  makes  glucose  available  for  energy;  and  (5)  the  release  of  ACTH 
from  the  pituitary  is  stimulated  (p.  625).  The  ACTH  in  turn  stimu- 
lates the  release  of  glucocorticoids  from  the  adrenal  cortex  which  in- 
crease the  breakdown  of  protein  and  make  further  carbohydrate 
available. 

Epinephrine  is  widely  used  clinically  in  treating  asthma  (it  dilates 
respiratory  passages),  in  increasing  blood  pressure,  and  in  stimulating  a 
heart  that  has  stopped  beating. 

The  adrenal  cortex  is  more  complex  than  the  medulla  both  histologi- 
cally, for  it  is  composed  of  three  layers  of  cells,  and  functionally,  for  it  se- 
cretes a  number  of  hormones  with  different  types  of  activity.  The  cortex 
is  composed  of  three  zones:  an  outer  glomerulosa,  a  middle  fasciculata 
and  an  inner  reticularis  (Fig.  30.5).  Cells  are  formed  by  mitosis  in  the 
outer  layer  and  are  pushed  inward  to  the  reticularis,  where  they  degen- 
erate and  disappear.  The  cells  of  the  fasciculata  are  believed  to  be  most 
active  in  hormone  production.  The  embryos  of  man  and  other  mammals 
have  very  large  adrenals— as  large  as  the  kidneys— which  result  from  the 
presence  of  a  large  mass  of  cells,  the  fetal  zone,  interposed  between  the 
cortex  and  medulla.  The  fetal  zone  regresses  and  disappears  after  birth. 

Some  thirty  different  hormones  have  been  extracted  from  the  adrenal 
cortex  of  various  species;  all  belong  to  the  class  of  chemicals  called 
steroids,  to  which  the  male  and  female  sex  hormones  also  belong.  No 
single  one  of  these  hormones  is  the  physiologic  equivalent  of  whole 
adrenal  extract,  and  a  mixture  of  at  least  two  of  them  must  be  injected 
if  the  glands  have  been  removed.  The  cortical  hormones  have  been 
grouped  into  three  categories,  although  there  is  some  overlapping.  These 
are:  (1)  glucocorticoids,  which  stimulate  the  conversion  of  proteins  to 
carbohydrates,  (2)  mineralocorticoids,  which  regulate  sodium  and  po- 
tassium metabolism,  and  (3)  androgens,  which  have  male  sex  hormone 
activity.  The  most  potent  glucocorticoid  is  hydrocortisone   (Compound 


THE   ENDOCRINE   SYSTEM 


619 


F).  The  most  potent  mineralocorticoid  is  aldosterone,  discovered  in 
1953;  desoxycorticosterone  is  an  effective  regulator  of  salt  and  water 
metabolism  and  is  widely  used  clinically.  Adrenosterone  and  dehydro- 
epiandrosterone    are    typical   adrenal   androgens. 

Experiments  on  the  biosynthesis  of  steroids  have  shown  that  they 
are  made  by  the  union  of  two-carbon  acetyl  coenzyme  A  units  to  form 
cholesterol.  The  cholesterol  content  of  the  adrenal  cortex  exceeds  that 
of  any  other  organ;  as  much  as  5  per  cent  of  the  wet  weight  of  the 
gland  may  be  cholesterol.  Steroids  are  synthesized  from  cholesterol  not 
only  in  the  adrenal  cortex  but  in  the  testis,  ovary  and  placenta  as  well. 
The  synthetic  pathways  of  these  compounds  are  interrelated;  proges- 
terone, for  example,  appears  to  be  the  precursor  of  both  aldosterone 
and  hydrocortisone  in  the  adrenal  cortex  and  of  testosterone  and  es- 
tradiol as  well.  The  hormones  produced  by  each  of  these  organs  are 
summarized  in  Table  8. 

The  complete  removal  of  the  adrenal  cortex,  or  its  hypofunction 
in  Addison's  disease,  results  in  an  increased  excretion  of  sodium  in 
the  urine  and  a  corresponding  excretion  of  chloride,  bicarbonate  and 
water.  The  loss  of  sodium  produces  an  acidosis,  and  the  loss  of  body 
fluid  leads  to  lowered  blood  pressure  and  a  decreased  rate  of  blood 
flow.  The  concentration  of  potassium  in  the  blood  increases.  There  is  a 
marked  decline  in  blood  sugar  concentration  and  in  the  glycogen  con- 
tent of  liver,  muscle  and  other  tissues.  It  is  clear  from  experimental 
evidence  that  the  animal's  ability  to  produce  carbohydrates  from  pro- 
teins is  greatly  impaired. 

The  appetite  for  food  and  water  decreases  and  there  is  loss  of 
weight.  There  are  marked  upsets  in  the  digestive  tract,  with  diarrhea, 
vomiting  and  pain.  Muscles  are  more  readily  fatigued,  and  less  able 
to  do  work.  The  basal  metabolic  rate  decreases  and  the  animal  is  less 
able  to  withstand  exposure  to  cold  and  other  stresses.  Death  ensues 
within  a  few  days  after  complete  adrenalectomy.  The  skin  of  a  patient 
with  Addison's  disease  develops  a  peculiar  bronzing  in  patches,  owing 
to  the  deposition  of  melanin. 

Hydrocortisone  and  cortisone  have  marked  effects  in  inhibiting 
hypersensitivity,  allergies,  and  inflammation  in  tissues,  presumably  by 
modifying  the  reactivity  of  mesenchymal  tissue.  They  also  inhibit  the 
proliferation  of  tissues  in  the  joints  of  persons  suffering  from  rheuma- 
toid arthritis.  The  two  hormones  are  widely  used  clinically  in  the  treat- 
ment of  these  conditions. 

Enlargement  of  the  adrenal  cortex  and  hypersecretion  of  adrenal 


Toble  8.     STEROID  HORMONES 


ADRENAL  CORTEX 


OVARV 


Hydrocortisone 

Desoxycorticosterone 

Aldosterone 

Androstereone 

Dehydroepiandrosterone 

Estradiol 


Estradiol 

Progesterone 

Androgens 


TESTIS 


Testosterone 

Androstenedione 

Estradiol 

Estrone 

Corticoids 


PLACENTA 


Progesterone 
Estradiol 
Androgens?? 
Corticoids 


520  VERTEBRATE   LIFE   AND   ORGANIZATION 

hormones  is  known  as  Cushing's  syndrome.  All  three  types  of  corti- 
coids  are  produced  in  excess,  and  salt,  water  and  carbohydrate  metab- 
olism is  deranged.  Females  with  this  disease  develop  a  pattern  ol  body 
hair  like  the  male,  and  have  an  enlarged  clitoris.  Fat  is  deposited  in 
the  trunk,  but  not  the  legs,  muscles  are  weak  and  tend  to  waste  away, 
bones  are  weakened  and  fracture  easily,  and  the  excess  ot  glucocorti- 
coids produces  a  metabolic  condition  very  similar  to  diabetes  mellitus. 
This  can  be  cured  by  surgical  removal  of  the  adrenal.  A  different  dis- 
ease, called  adrenogenital  syndrome,  results  from  the  hyperactivity  of 
the  adrenal  cortex  from  birth.  This  disease  is  characterized  by  in- 
creased production  of  adrenal  androgens,  which  leads  to  precocious 
sexual  maturity  in  males  and  to  masculinization  of  females. 

We  can  summarize  the  major  roles  of  the  several  adrenal  hormones 
as  follows:  they  regulate  the  concentration  of  sodium,  potassium  and 
water  in  the  body  fluids  and  tissues,  they  participate  in  the  control  of 
carbohydrate  metabolism,  accelerating  the  conversion  of  proteins  to 
carbohydrates,  and  they  supplement  the  actions  of  the  sex  hormones. 

253.        The  Pituitary  Gland 

The  pituitary  gland,  or  hypophysis  cerebri,  is  an  unpaired  endo- 
crine gland  which  lies  in  a  small  depression  on  the  floor  of  the  skull, 
just  below  the  hypothalamus  of  the  brain,  to  which  it  is  attached  by  a 
narrow  stalk.  Its  only  known  function  is  the  secretion  of  hormones.  The 
pituitary  has  a  double  origin:  a  dorsal  outgrowth  (Rathke's  pouch)  from 
the  roof  of  the  mouth  grows  up  and  surrounds  a  ventral  evagination 
(the  infundibulum)  from  the  hypothalamus  (Fig.  30.6).  Both  parts  are 
of  ectodermal  origin.  Rathke's  pouch  soon  loses  its  connection  to  the 
mouth,  but  the  connection  to  the  brain,  the  infundibular  stalk,  remains. 
The  hypophysis  has  three  lobes:  anterior  and  intermediate  lobes  de- 
rived from  Rathke's  pouch  and  a  posterior  lobe  from  the  infundibulum. 
The  pituitary,  like  the  adrenal,  is  a  double  gland  whose  parts  have 
quite  different  functions.  The  anterior  lobe  has  no  nerve  fibers  and  is 
stimulated  to  release  its  hormones  by  hormonal  factors  reaching  it 
through  its  blood  vessels.  The  anterior  lobe  receives  a  double  blood 
supply,  arterial  and  portal.  Some  branches  of  the  internal  carotid  artery 
pass  directly  to  the  pituitary;  others  serve  a  capillary  bed  around  the 
infundibular  stalk  and  the  median  eminence  of  the  hypothalamus  (Fig. 
30.7).  Portal  veins  from  these  capillaries  then  pass  down  the  infundib- 
ular stalk  and  empty  into  the  capillaries  surrounding  the  secretory  cells 
of  the  anterior  lobe.  The  posterior  lobe  has  a  separate  blood  supply, 
via  the  inferior  hypophysial  arteries.  There  is  thus  a  direct  route  for 
substances  to  pass  from  the  hypothalamus  to  the  anterior  lobe  by  way 
of  these  portal  vessels.  Axons  are  known  to  release  active  neurohumors 
(e.g.,  acetylcholine  or  sympathin)  at  their  tips  and  this  portal  system 
provides  a  means  by  which  substances  released  by  the  tips  of  axons  end- 
ing in  the  median  eminence  may  be  carried  to  the  anterior  lobe  and  in- 
fluence its  secretory  rate. 

The   anterior  lobe  is  composed   of   irregular  cords   and  masses   of 


THE   ENDOCRINE   SYSTEM 


621 


Brain 


Rathkc's 
poucVi 


InFund  i  biilum 


RathKe's 
pozich 


B 


Third  ventricle 
Pars  tuber  all  s 


Anl.  lobe 


Post,  lobe 


Centred  cavity 


Intermediate  lobe 


Figure  30.6.  The  development  of  the  pituitary  gland.  A,  Sagittal  section  through 
head  of  young  embryo.  B-F,  Sagittal  sections  of  successive  stages  of  developing  pituitary 
gland. 

Hypothalamic -hypophyseal 
■n<z-rve  tracts 


Supraoptic  nucleus 
Optic  chiasraa.- 


Internal  carotid 
artery 

J 


Infundibular  stalk. 

erior  cerebral 
'tery 


Figure  30.7.     Blood  supply  of  the  pituitary  gland. 


522  VERTEBRATE   LIFE   AND   ORGANIZATION 

epithelial  cells  surrounding  blood  vessels.  Three  kinds  of  cells  can  be 
distinguished  by  the  shape  and  staining  properties  of  their  granules: 
acidophils,  basophils  and  chromophobes.  The  intermediate  lobe  con- 
tains basophil  cells  smaller  than  those  of  the  anterior  lobe,  some  with 
and  some  without  granules.  The  posterior  lobe  is  composed  of  many 
nonmyelinated  nerve  fibers  and  branching  cells  (pituicytes)  which  con- 
tain brownish  cytoplasmic  granules. 

The  posterior  lobe  contains  two  hormones,  oxytocin  and  vaso- 
pressin. The  latter  is  also  known  as  antidiuretic  hormone,  or  ADH.  The 
brilliant  work  of  Vincent  du  Vigneaud,  for  which  he  was  awarded  the 
Nobel  Prize  in  1955,  has  led  to  the  isolation  of  these  two  hormones, 
the  determination  of  their  molecular  structure,  and  their  synthesis.  Each 
is  a  peptide  containing  nine  amino  acids,  seven  of  which  are  identical  in 
the  two.  It  is  of  considerable  interest  that  these  two  substances,  with 
cjuite  different  physiologic  properties,  differ  only  in  two  amino  acids. 
Oxytocin  stimulates  the  contraction  of  the  uterine  muscles  and  is  some- 
times injected  after  childbirth  to  contract  the  uterus.  Vasopressin  causes 
a  contraction  of  smooth  muscles;  its  contraction  of  the  muscles  in  the 
wall  of  arterioles  causes  a  general  increase  in  blood  pressure.  It  also 
regulates  the  reabsorption  of  water  by  the  cells  of  the  distal  convoluted 
tubules  and  Henle's  loop  in  the  kidney.  Most  investigators  agree  that 
these  two  hormones  are  not  produced  in  the  posterior  lobe,  but  are  se- 
creted by  neurosecretory  cells  in  the  supraoptic  and  paraventricular 
nuclei  of  the  brain.  They  then  pass  along  the  axons  of  the  hypothalamic- 
hypophysial  tract,  and  are  stored  and  released  by  the  posterior  lobe. 
An  injury  of  these  brain  nuclei,  of  the  posterior  lobe,  or  of  the  con- 
necting nerve  tracts  may  lead  to  a  deficiency  of  ADH  and  the  condi- 
tion known  as  diabetes  insipidus.  In  this  disease  the  patient's  kidneys 
have  a  lessened  ability  to  reabsorb  water  and  his  urine  volume  in- 
creases from  the  normal  one  or  two  liters  to  10  to  25  liters  per  day. 
He  suffers  from  excessive  thirst  and  drinks  copiously.  A  comparable 
condition  can  be  produced  in  experimental  animals  by  severing  the 
hypothalamic-hypophysial  tract  by  electrolytic  lesions  accurately  placed 
with  a  microelectrode.  Injection  of  ADH  relieves  all  of  the  symptoms 
but  the  injections  must  be  repeated  every  few  days. 

The  intermediate  lobe  of  the  pituitary  secretes  a  hormone,  Inter- 
medin, which  darkens  the  skin  of  fishes,  amphibians  and  reptiles  by 
dispersing  the  pigment  granules  in  the  chromatophores.  The  skin  of  a 
frog  becomes  darkened  in  a  cool,  dark  environment  and  light-colored 
in  a  warm,  light  place  (Fig.  30.8).  Hypophysectomy  produces  a  perma- 
nent blanching  of  the  skin,  and  injection  of  intermedin  causes  darken- 
ing. The  location  of  the  pigment  in  the  chromatophore  is  controlled 
directly  by  the  amount  of  intermedin  present,  not  by  nerves.  The 
pituitaries  of  birds  and  mammals  are  rich  in  intermedin  but  there  is  no 
known  function  for  this  hormone  in  these  animals;  it  does  not  affect 
their  pigmentation. 

The  anterior  lobe  of  the  pituitary  secretes  the  following  hormones, 
all  of  which  are  proteins:  growth  hormone  (somatotropin),  thyro- 
tropin, adrenocorticotropin  (ACTH),  follicle-stimulating  hormone  (FSH), 


THE   ENDOCRINE  SYSTEM 


623 


luteinizing  hormone  (LH)  and  prolactin  (lactogenic  hormone).  A  num- 
ber of  other  hormones  have  been  postulated  to  be  products  of  the 
anterior  lobe  but  their  existence  has  not  generally  been  confirmed.  The 
importance  of  these  hormones  is  demonstrated  by  the  marked  abnormal- 
ities which  follow  hypophysectomy:  cessation  of  growth  in  young  ani- 
mals, regression  of  gonads  and  reproductive  organs,  and  atrophy  of  the 
thyroid  and  adrenal  cortex  (Fig.  30.9). 

Growth  hormone  was  the  first  pituitary  hormone  to  be  described. 


Figure  30  8  Integumentary  adaptations  in  normal  frogs  {Rana  pipiens).  A,  Light- 
adapted  animal:  B,  dark-adapted  animal.  (Turner:  General  Endocrinology'.)  C  A 
chromatophore,  greatly  magnified,  showing  the  pigment.  D,  A  section  of  skm  of  frog 
adapted  to  a  warm,  light  environment.  E,  Skin  adapted  to  a  cool,  dark  environment. 


624 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  30.9.  The  effects  of  hypophysectomy  in  the  rat.  A,  Normal  littermate 
control;  B,  littermate  hypophysectomized  when  36  days  of  age.  Ai,  A2  and  A^  are  thyroids, 
adrenals  and  ovaries  from  normal  animal;  Bi,  B2  and  Bs  are  thyroids,  adrenals  and 
ovaries  from  hypophysectomized  animal.  Note  marked  differences  in  size.  ( 1  urner: 
General  Endocrinology.) 

As  early  as  1860  it  was  recognized  that  gigantism  was  correlated  with  an 
enlargement  ol  the  pituitary.  A  growth-promoting  extract  of  beef  pitiii- 
taries  was  prepared  by  Evans  and  Long  in  1921  and  pure  growth  hor- 
mone was  isolated  in  1944.  This  controls  general  body  growth  and  bone 
growth  and  leads  to  an  increase  in  the  amount  of  cellular  protein  (Fig. 
30.10).  Overactivity  of  the  pituitary  during  the  growth  period  leads  to 
very  tall,  but  well-proportioned  persons,  and  imderactivity  leads  to  small 
persons  of  normal  body  proportions,  called  midgets.  After  normal  growth 
has  been  completed,  hypersecretion  of  growth  hormone  produces  acro- 
megaly, characterized  by  the  thickening  of  the  skin,  tongue,  lips,  nose 
and  ears  and  by  growth  of  the  bones  of  the  hands,  feet,  jaw  and  skull. 
Other  bones  have  lost  their  ability  to  respond  to  growth  hormone.  A 
race  of  hereditarily  dwarf  mice  is  known  whose  pituitaries  apparently 
lack  the  type  of  cell  which  secretes  growth  hormone.  These  animals  can 
be  induced  to  grow  to  normal  size  by  implanting  a  pituitary  from  a 
normal  mouse.  Growth  hormones  from  different  species  have  been  found 
to  differ  slightly  in  their  amino  acid  composition  and  in  their  effective- 
ness. Thus  beef  growth  hormone  will  cause  growth  in  rats  but  not  in 
man  or  monkeys.  Growth  hormone  prepared  from  human  or  monkey 
pituitaries  will  stimulate  growth  in  man  and  monkeys. 


THE  ENDOCRINE  SYSTEM 


625 


Chemical  analysis  of  the  adrenocorticotropic  hormone,   ACTH,   has 

shown  that  the  active  fraction  is  a  peptide  containing  39  amino  acids. 
The  sequence  of  these  amino  acids  is  now  known.  In  recent  years  ACTH 
has  become  famous  because  of  the  remarkable  results  it  sometimes  gives 
in  the  treatment  of  allergies  and  arthritis.  However,  the  prime,  and 
perhaps  the  only  physiologic  function  of  ACTH  is  to  stimulate  the 
adrenal  cortex  to  grow  and  to  release  cortical  steroids.  The  injection  of 
ACTH  reduces  the  amount  of  cholesterol  and  ascorbic  acid  in  the 
adrenal  cortex,  presumably  because  they  are  used  in  the  synthesis  of 
steroids.  The  injection  of  ACTH  stimulates,  within  a  few  minutes,  a 
marked  increase  in  the  amount  of  hydrocortisone  in  the  blood.  The 
adrenal  cortex  undergoes  a  prompt  atrophy  after  the  removal  of  the 
pituitary  and  can  be  returned  to  normal  by  the  injection  of  ACTH. 

The  extirpation  of  the  pituitary  also  causes  atrophy  of  the  thyroid. 
The  gland  decreases  in  size  and  the  follicle  cells  become  flattened.  The 
thyroid  is  returned  to  normal  by  the  implantation  of  a  pituitary  gland  or 
by  the  administration  of  an  extract  containing  thyrotropin.  The  injection 
of  thyrotropin  in  a  normal  animal  causes  growth  of  the  thyroid  and 
thickening  of  the  follicle  cells  so  that  they  become  columnar  rather  than 
cuboidal  epithelium  (Fig.  30.2). 

The  ovaries  or  testes  of  a  hypophysectomized  young  animal  never 
become  mature;   they  neither  produce  gametes  nor  secrete  enough  sex 


Figure  30.10.  The  effect  of  growth  hormone  on  the  dachshund.  Top,  normal  dog. 
Bottom,  dog  injected  with  growth  extract  for  a  period  of  six  months.  (From  Evans, 
Simpson,  Meyer  and  Reichert.) 


595  VERTEBRATE   LIFE   AND    ORGANIZATION 

hormones  to  develop  the  secondary  sex  characters.  Hypophysectomy  of 
an  aduU  results  in  involution  and  atrophy  of  the  gonads.  It  is  now  clear 
that  there  are  two  gonadotropins,  called  foiiicle-stimulating  hormone 
(FSH)  and  luteinizing  hormone  (LH),  and  that  both  are  necessary  for 
achieving  sexual  maturity  and  for  the  regulation  of  the  estrous  cycle. 
The  effect  of  follicle-stimulating  hormone  is  primarily  on  the  develop- 
ment of  graafian  follicles  in  the  ovaries:  it  does  not  produce  any  signifi- 
cant release  of  estrogen.  Luteinizing  hormone  controls  the  release  of  ripe 
eggs  from  the  follicle,  the  formation  of  corpora  lutea,  and  the  produc- 
tion and  release  of  estrogens  and  progesterone.  Prolactin,  or  lactogenic 
hormone,  maintains  the  secretion  of  estrogens  and  progesterone,  and 
stimulates  the  secretion  of  milk  by  the  breast.  It  is  effective,  however, 
only  after  the  breast  has  been  stimulated  by  the  proper  amounts  of 
estrogen  and  progesterone.  Prolactin  induces  behavior  patterns  leading 
to  the  care  of  the  young  (the  "maternal  instinct")  in  mammals  and  in 
other  vertebrates  as  well.  Roosters  treated  with  prolactin  will  take  care 
of  chicks,  taking  them  to  food  and  water,  sheltering  them  under  their 
wings,  and  protecting  them  from  predators.  The  cyclic  release  of  FSH 
and  LH  is  involved  in  the  control  of  the  estrous  cycles  of  lower  mam- 
mals and  the  menstrual  cycles  of  primates.  The  simultaneous  adminis- 
tration of  FSH  and  LH  produces  much  greater  effects  on  ovarian  growth 
than  either  one  alone;  similar  instances  of  hormonal  synergism  have 
been  observed  with  certain  other  pairs  of  hormones. 

The  development  and  functioning  of  the  testis  is  also  controlled  by 
FSH  and  LH.  FSH  increases  the  size  of  the  seminiferous  tubules  and 
both  FSH  and  LH  are  needed  for  normal  spermatogenesis.  LH,  but  not 
FSH,  stimulates  the  interstitial  cells  of  the  testis  to  produce  male  sex 
hormone. 

Extracts  of  the  pituitary  liave  been  prepared  which  have  other 
effects  when  injected,  and  it  has  been  postulated  that  the  gland  secretes 
other  hormones  in  addition  to  these  six.  Despite  repeated  attempts,  it  has 
not  been  possible  to  separate  and  purify  the  agents  of  these  other  activ- 
ities and  many  investigators  now  regard  them  as  side  effects  of  one  of 
the  known  hormones.  The  insulin  antagonist  effect  of  the  pituitary,  the 
"diabetogenic  hormone,"  is  now  believed  to  be  a  property  of  the  growth 
hormone. 

The  control  of  pituitary  function,  which  ensures  that  the  proper 
amount  of  each  of  these  hormones  will  be  released  at  the  proper  mo- 
ment in  response  to  the  demands  of  the  organism,  is  indeed  complex. 
Recent  research  has  revealed  that  the  release  of  each  tropic  hormone 
is  controlled  in  part  by  the  level  of  the  target  hormone  in  the  circulat- 
ing blood.  The  release  of  ACTH  is  inhibited  by  hydrocortisone,  the 
release  of  thyrotropin  is  inhibited  by  thyroxin,  estrogens  decrease  the 
output  of  FSH  and  progesterone  decreases  the  secretion  of  LH.  This 
provides  for  a  cut-off  mechanism  so  that  in  a  normal  animal  the  secre- 
tions of  the  pituitary  and  its  target  organs  are  kept  in  balance. 

The  release  of  ACTH  is  also  stimulated  by  epinephrine.  This  is 
apparently  a  direct  effect,  for  it  is  observed  when  the  pituitary  is  re- 
moved from  its  normal  site  and  transplanted  to  the  eye.  Epinephrine 


THE   ENDOCRINE   SYSTEM  627 

is  not  indispensable  for  ACTH  release;   the  latter  can  occur  normally 
after  removal  of  the  adrenal  medulla. 

The  hypothalamus  provides  a  third,  and  very  important,  control  of 
pituitary  fvmction.  It  is  cvirrently  believed  that  axons  from  certain 
centers  in  the  hypothalamus  end  in  the  median  eminence  (Fig.  30.7). 
The  tips  of  these  axons  secrete  some  neurohumor  which  is  carried  by 
the  portal  veins  to  the  hypophysis,  where  it  stimulates  the  release  of 
ACTH.  Evidence  of  ACTH  secretion  is  obtained  when  the  median 
eminence  is  stimulated  electrically,  but  not  when  the  stimulus  is  ap- 
plied to  the  supraoptic  nuclei  whose  axons  pass  to  the  posterior  lobe 
of  the  pituitary.  The  electrical  stimulus  is  ineffective  if  the  blood  ves- 
sels between  the  hypothalamus  and  pituitary  are  cut.  If  the  nerve 
fibers  to  the  median  eminence  are  destroyed,  ACTH  is  no  longer  re- 
leased in  response  to  stresses.  Some  investigators  believe  that  the  re- 
lease of  other  pituitary  hormones— growth  hormone,  thyrotropin  and 
the  gonadotropins— is  also   under  hypothalamic  control. 

All  the  living  vertebrates  have  pituitaries  which  are  basically 
similar,  and  they  all  appear  to  secrete  the  same  battery  of  hormones. 
The  intermediate  lobes  of  birds  and  mammals  secrete  intermedin,  al- 
though these  forms  have  no  chromatophores;  birds  secrete  luteinizing 
hormone  but  have  no  corpora  lutea;  and  all  vertebrates  secrete  prolactin, 
but  only  mannnals  have  its  target  organ,  the  mammary  glands. 

254.        The  Testis 

In  between  the  seminiferous  tubules  of  the  testes  are  hormone- 
secreting  cells,  the  interstitial  cells  of  Leydig.  Although  Berthold  con- 
cluded in  1849  that  the  testis  produces  a  blood-borne  substance  needed 
for  the  development  of  male  sex  characters,  no  effective  testicular  ex- 
tract was  prepared  until  1927.  Extracts  of  human  urine  with  androgenic 
activity  were  made  in  1929,  and  by  1934  two  hormones,  androsterone 
and  dehydroepiandrosterone,  had  been  isolated  from  urine  and  identi- 
fied. A  new  androgen,  testosterone,  six  times  more  potent  than  andros- 
terone, was  extracted  from  testicular  tissue  in  1935.  All  of  these 
androgens  are  steroids.  It  has  recently  been  demonstrated  that  the  testis 
will  synthesize  carbon^*  labeled  testosterone  if  provided  with  C^^  labeled 
acetate.  The  testis  also  produces  estrogenic  substances. 

Testosterone  has  a  general  effect  on  metabolism,  inducing  growth 
by  stimulating  the  formation  of  cell  proteins.  The  administration  of 
androgens  leads  to  an  increase  in  body  weight  due  to  the  synthesis  of 
protein  in  muscle  and  to  a  lesser  extent  in  liver  and  kidney. 

Testosterone  and  other  androgens  stimulate  the  development  and 
maintenance  of  the  secondary  male  characters:  the  enlargement  of  the 
external  genitals,  the  growth  of  the  accessory  glands  such  as  the  prostate 
and  seminal  vesicles,  the  growth  of  the  beard  and  of  body  hair,  and  the 
deepening  of  the  voice.  The  secondary  sex  characters  of  other  animals, 
the  antlers  of  deer  and  the  combs,  wattles  and  plumage  of  birds,  are 
controlled  by  androgens.  Male  sex  hormones  are  responsible  in  part  for 
the  development  of  mating  behavior. 


528  VERTEBRATE   LIFE   AND   ORGANIZATION 

The  removal  of  the  testis  (castration)  of  an  immature  male  pre- 
vents the  development  of  the  secondary  sex  characters.  A  castrated  man, 
a  eunuch,  has  a  high-pitched  voice,  beardless  face,  and  small  genitals 
and  accessory  glands.  Castration  was  practiced  in  the  past  to  provide 
guardians  for  the  harem  and  sopranos  for  choirs.  Many  kinds  of  do- 
mestic animals  are  castrated  to  make  them  more  placid.  The  injection 
of  testosterone  into  a  castrated  animal  restores  all  of  the  sex  characters 
to  normal.  The  anal  fin  of  the  male  mosquito  fish,  Gambusia,  is  dif- 
ferentiated into  a  penis-like  organ  used  to  transfer  sperm  to  the 
female.  This  fails  to  develop  if  the  fish  is  castrated  but  appears  if  the 
castrate  male  or  the   female  is   treated  with   testosterone. 

It  should  be  emphasized  that  males  produce  female  sex  hormones 
(estrogens)  and  that  females  produce  androgens  in  considerable  amounts. 
The  normal  diflferentiation  of  the  sex  characters  is  a  function  of  a  balance 
between  the  two. 

The  failure  of  the  testes  to  descend  normally  from  the  body  cavity 
to  the  scrotal  sac,  called  cryptorchidism,  produces  sterility  but  has  little 
or  no  effect  on  the  production  of  testosterone.  Microscopic  examination 
of  an  undescended  testis  shows  that  the  cells  in  the  seminiferous  tubules 
regress,  but  the  interstitial  cells  are  normal.  The  cells  of  the  seminiferous 
tubules  are  particularly  susceptible  to  heat,  and  the  temperature  of  the 
body  cavity,  3  or  4  degrees  higher  than  that  of  the  scrotal  sac,  destroys 
them.  It  is  probable  that  the  elevated  temperature  during  a  prolonged 
fever  makes  a  man  sterile  for  some  time.  In  many  wild  animals  the 
testes  remain  in  the  body  cavity  except  during  the  breeding  season,  when 
they  descend  into  the  scrotal  sac. 

The  removal  of  the  pituitary  causes  regression  of  both  the  inter- 
stitial cells  and  the  seminiferous  tubules  of  the  testis.  Androgen  secretion 
is  decreased  and  the  secondary  sex  characters  regress.  Normal  develop- 
ment and  spermatogenesis  of  the  cells  of  the  seminiferous  tubules  ap- 
parently requires  the  combined  action  of  FSH,  LH  and  testosterone. 
The  administration  of  excessive  amounts  of  testosterone  or  estrogen  may 
produce  regression  of  the  testes,  presumably  by  inhibiting  the  release 
of  FSH  and  LH  from  the  pituitary. 

The  cyclic  growth  and  regression  of  the  testis  in  animals  with 
periodic  breeding  seasons  appears  to  be  mediated  via  the  pituitary.  Such 
animals  have  very  low  amounts  of  gonadotropin  in  the  nonbreeding 
season.  Changes  in  the  temperature  or  in  the  amount  of  daily  illumina- 
tion produce  stimuli  which  are  mediated  by  the  brain  and  hypothalamus 
to  induce  gonadotropin  secretion  by  the  pituitary  and  consequent 
growth  and  functional  state  of  the  testis  and  secondary  sex  characters. 

255.        The  Ovaries 

The  ovaries  of  vertebrates  are  endocrine  organs  as  well  as  the 
source  of  eggs;  they  produce  the  steroid  hormones  estradiol  and  pro- 
gesterone. Some  mammalian  ovaries  produce  a  third  hormone,  the  pro- 
tein relaxin. 


THE   ENDOCRINE   SYSTEM 


629 


Both  ovaries  and  testes  develop  from  mesoderm,  from  the  genital 
ridge  on  the  ventral  side  of  the  mesonephros  (Fig.  30.11).  It  consists  of 
closely  packed  cells  covered  by  a  thickened  mesothelium  called  the 
germinal  epithelium.  During  embryonic  development  certain  cells  of 
the  germinal  epithelium  enlarge,  push  into  the  mass  of  cells  below, 
and  become  primordial  germ  cells.  According  to  one  view,  these  cells 
are  not  derived  from  the  mesothelium,  but  originate  in  the  epithelium 


MesONEPHROS 

.GENITAL  RIDGE 


RETE  PORTION 


WOLFFIAN 
DUC 


CLOACA 


GERMINAL  EPITHELIUM 


■SEX  CORD 
PORTION 


PRIMARY  .^ , 

SEX  CORDS — j^^\ 

RETE  CORDS — (^ —   - 


POSrSEXUAL  PORTION 


GENITAL  RIDGE 
MESONEPHROS, 


MULLERIAN  DUCT 
WOLFFIAN  DUCT 

CLOACA 


■APPENDIX  TESTIS 


TESTIS    ^ 
(PRIMARY  SEX^ 
CORDS) 


GERMINAL  EPITHELIUM 
TUNICA  ALBUGINEA 
RETE  TESTIS 

TUNICA  ALBUGINEA 


Oi^ARlAN 
MEDULLA 

(primary  sex 

CORDS) 

^       OVARIAN  CORTEX 
(SECONDARY  SEX  CORDS) 


GERMINAL  EPITHELIUM 


SEMINAL 
VESICLE 


URINARY  BLADDER 


PROSTATIC  UTRICLE 
PROSTATE  GLAND 


GARTNER'S 
DUCT 


PAROOPHORON 


'RETHRA 


Figure  30.11.     The  development   of  the   genital   system.   A,   Section   through   the 
dorsal  region  of  an  early  embryo.  B,  The  Wolffian  body  and  genital  ridge  in  frontal 
section.  C,  The  indifferent  stage.  D,  Differentiation  of  the  male  genitalia.  E,  Differen 
tiation  of  the  female  genitalia.  (Modified  from  Turner.) 


630 


VERTEBRATE   LIFE   AND   ORGANIZATION 


Figure  30.12.  Stages  in  the  development  of  an  egg,  follicle  and  corpus  luteum 
in  a  nianinialian  ovary.  Successive  stages  are  depicted  clockwise,  beginning  at  the 
mesentery.  Insets  show  the  cellular  structure  of  the  successive  stages.  (Villee:  Biology.) 

of  the  yolk  sac  and  migrate  to  their  final  position  in  the  gonad.  Other 
investigators  maintain  that  the  functional  eggs  do  not  come  from  these 
primordial  germ  cells  visible  in  the  ovary  at  birth,  but  arise  by  new 
proliferations  from  the  germinal  epithelium   in   the  adult. 

As  each  oocyte  develops,  it  becomes  surrounded  by  other  cells  de- 
rived from  the  germinal  epithelium  which  form  a  spherical  follicle 
about  it  (Fig.  30.12).  These  cells  proliferate  and  form  a  thick  layer, 
called  the  stratum  granulosum,  around  the  egg.  A  cavity,  the  antrum, 
filled  with  liquid  appears  in  the  mass  of  follicle  cells.  The  connective 
tissue  of  the  ovary  forms  a  sheath,  the  theca,  around  the  follicle.  As 
the  follicle  enlarges  and  its  antrum  becomes  dilated  with  follicular 
fluid,  it  is  pushed  near  the  surface  of  the  ovary.  It  finally  bursts  and 
releases  the  egg  into  the  peritoneal  cavity,  whence  it  passes  into  the 
oviduct.  The  release  of  the  egg  is  known  as  ovulation.  If  the  egg  is  fer- 
tilized in  the  oviduct  it  will  subsequently  become  embedded  in  the 
lining  of  the  uterus  and  begin  development. 

The  follicular  cells  remaining  after  the  rupture  of  the  follicle 
multiply  and  increase  in  size,  filling  the  cavity  left  by  the  follicle.  Cells 
from  the  theca  grow  in  along  with  the  granulosa  cells  and  the  two  form 
the  corpus  luteum.  This  yellowish  structure,  a  solid  mass  of  cells  about 
the  size  of  a  pea,  projects  from  the  surface  of  the  ovary.  If  the  egg  is 
fertilized  the  human  corpus  luteum  persists  for  months,  but  if  no  fer- 


THE   ENDOCRINE   SYSTEM  631 

tilization  takes  place  it  regresses  after  about  two  weeks  to  a  small  patch 
of  whitish  scar  tissue,  the  corpus  albicans. 

Histochemical  evidence  indicates  that  the  thecal  cells  are  the  source 
of  estrogen  and  that  these  plus  the  granulosa  cells  of  the  corpus  luteum 
are   the  source  of  progesterone.   The  primary  estrogen  is  probably  es- 
tradiol; other  estrogens  such  as  estrone  and  estriol  may  be  metabolites 
of  estradiol.  Estradiol  stimulates  the  changes  which  occur  at  sexual  ma- 
turity:  the  growth  of  the  accessory  sex  organs,  uterus  and  vagina,   the 
development  of   the  breasts,   changes   in  skeletal  structure  such   as  the 
broadening  of   the  pelvis,   the  change   in  voice  quality,  the  growth  of 
pubic  hair  and  the  onset  of  the  menstrual  cycle.  Progesterone  together 
with  estradiol  is  required  for  the  growth  of  the  uterine  lining  in  each 
menstrual  cycle  to  the  stage  at  which  implantation  of  the  fertilized  egg 
is  possible.  It  is  also  necessary  for  the  maintenance  of  the  developing 
embryo  in  the  uterus.  Progesterone  along  with  estradiol  causes  develop- 
ment of  the  breasts  during  pregnancy. 

Progesterone  is  related  chemically  to  the  adrenal  cortical  hormones 
and  is  believed  to  be  an  intermediate  in  their  synthesis,  as  well  as  an 
intermediate  in  the  synthesis  of  estradiol  and  testosterone. 

256.        Estrous  and  Menstrual  Cycles 

The  females  of  most  mammalian  species  show  cyclic  periods  of  the 
sex  urge  and  will  permit  copulation  only  at  certain  times,  known  as 
periods  of  estrus  or  "heat,"  when  conditions  are  optimal  for  the  union  of 
egg  and  sperm.  Most  wild  animals  have  one  estrous  period  a  year,  the 
dog  and  cat  have  two,  and  rats  and  mice  have  estrous  periods  every  five 
days.  Estrus  is  characterized  by  heightened  sex  urge,  ovulation,  and 
changes  in  the  lining  of  the  uterus  and  vagina.  The  uterine  lining 
thickens,  and  its  glands  and  blood  vessels  develop  to  provide  optimal 
conditions  for  implantation. 

The  menstrual  cycle  of  the  primates  is  characterized  not  by  periods 
of  mating  urge,  but  by  periods  of  bleeding  caused  by  the  degeneration 
and  sloughing  of  the  uterine  lining.  Ovulation  occurs  about  midway 
between  two  successive  menstruations,  or  periods  of  bleeding.  Primates, 
unlike  other  mammals,  permit  copulation  at  any  time  in  the  menstrual 

cycle. 

The  menstrual  cycle  is  controlled  by  the  interaction  of  ovarian  and 
pituitary  hormones,  and  includes  events  in  the  ovary,  uterus  and  vagina. 
One  menstrual  cycle,  from  the  beginning  of  one  period  of  bleeding  to 
the  next,  lasts  28  to  30  days  in  the  human  female  (Fig.  30.13). 

The  lining  of  the  uterus  is  almost  completely  sloughed  off  at  each 
menstruation  and  thus  is  thinnest  just  after  the  menstrual  flow.  At  that 
time,  under  the  influence  of  FSH  from  the  pituitary,  one  or  more  of 
the  follicles  in  the  ovary  begin  to  giow  rapidly.  The  follicular  cells 
produce  estradiol,  which  stimulates  the  growth  of  the  uterme  Immg 
(the  endometrium),  and  some  growth  of  the  uterine  glands  and  blood 
vessels.  The  rupture  of  the  follicle  in  ovulation  does  not  occur  auto- 
matically when  a  certain  size  is  reached,  but  is  induced  by  the  proper 


632 


VERTEBRATE   LIFE   AND    ORGANIZATION 


Figure  30.13.  The  menstrual  cycle  in  the  human  female.  The  solid  lines  indicate 
the  course  of  events  if  the  egg  is  not  fertilized;  the  dotted  lines  indicate  the  course 
of  events  when  pregnancy  occurs.  The  actions  of  the  hormones  of  the  pituitary  and 
ovary  in  regulating  the  cycle  are  indicated  by  arrows.  (Villee:   Biology.) 


mixture  of  FSH  and  LH  from  the  pituitary.  Ovulation  occurs  about 
fifteen  days  after  tfie  beginning  of  the  previous  period  of  menstruation. 
The  corpus  luteum  develops  and  under  the  stimulation  of  LH,  secretes 
progesterone.  Progesterone,  together  with  estradiol,  promotes  further 
growth  of  the  endometrium.  The  endometrial  glands  grow  further  and 
become  secretory  and  the  blood  vessels  become  long  and  coiled.  Pro- 
gesterone decreases  the  activity  of  the  uterine  muscles  and  brings  the 
uterus  into  a  condition  so  that  the  developing  embryo  formed  from  the 
fertilized  egg  can  become  implanted  and  develop.  Progesterone  inhibits 
the  development  of  other  follicles.  If  fertilization  and  implantation  do 
not  occur  the  corpus  luteum  begins  to  regress,  it  secretes  less  progester- 
one, and  the  endometrium,  no  longer  provided  with  sufficient  progester- 
one to  be  maintained,  begins  to  slough.  Thus  menstruation  ensues, 
completing  the  cycle. 

If  pregnancy  occurs  the  corpus  luteum  remains  and  continues  se- 
creting progesterone,  which  is  necessary  for  the  continuation  of  preg- 
nancy. Removal  of  the  ovary  or  of  the  corpus  luteum  results  in  the 
termination  of  pregnancy.  In  some  animals  the  placenta  produces  enough 
progesterone  so  that  loss  of  the  corpus  luteum  does  not  result  in  abor- 
tion. Progesterone  also  stimulates  the  growth  of  the  glands  and  ducts  of 
the  breasts  during  the  latter  months  of  pregnancy,  and  prepares  them 


THE   ENDOCRINE   SYSTEM 


633 


for  the  action  of  prolactin  secreted  by  the  pituitary,  which  stimulates 
the  flow  of  milk. 

257.        The  Hormones  of  Pregnancy 

The  placenta,  which  develops  in  part  from  the  extraembryonic 
membranes  of  the  fetus  and  in  part  from  the  lining  of  the  uterus  (p. 
639),  is  primarily  an  organ  for  the  support  and  nourishment  of  the 
fetus.  It  is  also  an  endocrine  organ,  which  produces  hormones  similar 
to  those  of  the  ovary,  the  adrenal  cortex  and  the  pituitary.  These 
placental  hormones,  together  with  those  of  the  maternal  endocrine 
glands,  control  the  many  adaptations  necessary  for  the  continuation  and 
successful  termination  of  pregnancy. 

The  placenta  secretes  a  protein  hormone,  chorionic  gonaclotropin, 
which  is  produced  by  the  cells  of  the  chorionic  villi.  Its  effects  are 
similar  to,  yet  distinct  from,  those  of  the  pituitary  gonadotropins.  It  is 
known  that  the  placenta  secretes  this,  and  does  not  merely  accumulate 
a  hormone  made  elsewhere,  for  bits  of  placenta  grown  in  tissue  culture 
produce  the  hormone.  One  of  the  earliest  signs  of  pregnancy  is  the 
appearance  of  this  hormone  in  the  blood  and  urine.  The  peak  of 
chorionic  gonadotropin  production  is  reached  in  the  second  month  of 
pregnancy,  after  which  the  amount  in  blood  and  urine  decreases  to  low 
levels  (Fig.  30.14).  Several  pregnancy  tests  involve  the  effect  of  this 
gonadotropin,  obtained  from  a  sample  of  urine  from  the  woman  to  be 
tested,  on  sperm  release  in  the  frog  or  African  toad  or  on  the  production 
of  corpora  lutea  in  rats  or  rabbits.  These  tests  are  quite  accurate  and 
make  possible  a  diagnosis  of  pregnancy  within  a  few  weeks  of  concep- 
tion. Chorionic  gonadotropin  stimulates  the  corpus  luteum  to  remain 
functional  and  not  regress  as  it  would  in  the  absence  of  pregnancy. 

The  placenta  also  secretes  estrogens  and  progesterone  which  rein- 
force the  ovarian  hormones  in  the  maintenance  of  pregnancy.  There  is 
good  evidence  that  the  placenta  actually  produces  these  hormones  and 
does  not  accumulate  them  from  the  blood.  There  is  a  considerable  body 


I 

4  600 
U 

5i 


I 

5  400 

i 


g  200 


100 
76 
90 
2S 


<r,  20 
§10 


L 


i 

1 180 

nIiso 

•.  140 


§120 

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tolOQ- 

80 
«  60 
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if  20 


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5S 


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§20 

gro 
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ESTROGEN 

(MAINLY  ESTRIOL)'-' 


^_  I »!-.- 

— ■'    > 1 — «i 1 ' 


17-OH-CORTICOIOS 


17-KETOSTEROIDS 

GONADOTROPIN 


_l_ 


10  IS  20  25  30 

iree/CS      OF      PREeitANCY 


35 


40 


Figure  30.14.     Hormone  levels  in  blood  and  urine  during  pregnancy. 


534  VERTEBRATE   LIFE   AND   ORGANIZATION 

of  evidence  that  the  placenta  may  produce  hydrocortisone,  cortisone 
and  other  adrenal  corticoids,  and  a  hormone  similar  to  ACTH. 

In  some  animals,  such  as  the  rabbit,  the  placenta  is  a  significant 
source  of  relaxin.  This  protein  hormone,  also  produced  by  the  ovary, 
functions  to  relax  the  ligaments  of  the  pelvis  to  facilitate  the  birth  of 
the  young.  Relaxin  is  effective  only  after  the  connective  tissue  of  the 
pubic  symphysis  has  been  sensitized  by  the  action  of  estradiol.  Relaxin 
also  inhibits  the  motility  of  the  uterine  muscles. 

The  production  of  estrogens  and  progesterone,  as  reflected  by  the 
amount  present  in  blood  and  urine,  increases  gradually  throughout 
pregnancy,  reaches  a  peak  just  before  or  at  the  time  of  parturition,  then 
abruptly  declines  after  birth  (Fig.  30.14).  The  factors  which  determine 
the  onset  of  labor,  the  expulsion  of  the  fetus  from  the  uterus,  remain  a 
mystery.  The  possibility  that  oxytocin  has  a  role  in  this  was  mentioned 
(p.  622).  There  are  many  hormonal  changes  which  occur  at  about  the 
time  of  parturition— decreases  in  estrogen  and  progesterone,  and  an  in- 
crease in  chorionic  gonadotropin— but  whether  these  are  causes,  effects, 
or  unrelated  phenomena  remains  to  be  determined. 

258.        Other  Endocrine  Glands 

The  thymus  and  pineal  body  may  have  endocrine  functions.  The 
thymus  lies  in  the  upper  part  of  the  chest,  just  above  the  heart.  Its  cells 
closely  resemble  lymph  tissue.  The  thymus  is  large  during  the  years  of 
rapid  growth  but  begins  to  regress  after  puberty.  It  has  been  postulated 
to  affect  growth  or  sexual  maturity,  but  extirpation  of  the  gland  or  the 
administration  of  extracts  fails  to  reveal  any  endocrine  function.  The 
pineal  body  is  a  dorsal  outgrowth  of  the  diencephalon  which  lies  on  the 
upper  surface  of  the  thalamus.  It  has  been  suspected  of  having  some  role 
in  body  growth  and  genital  development,  but  the  evidence  is  somewhat 
conflicting  and  no  endocrine  function  can  be  ascribed  to  it  with  cer- 
tainty. 

The  cells  of  certain  parts  of  the  digestive  tract  are  known  to  secrete 
hormones  in  response  to  the  presence  of  certain  kinds  of  food,  which 
stimulate  the  production  and  release  of  digestive  juices.  These  are  sum- 
marized in  Table  9. 

Table  9.     HORMONES  OF  THE  DIGESTIVE  TRACT 

STIMULUS  FOR  RESPONSE  OF 

HORMONE       SECRETED  BY  TARGET  ORGAN 

SECRETION  TARGET  ORGAN 

®°**""  Pyloric  mucosa         Presence  of  food   Mucosa  of  Secretion  of 

in  stomach  stomach  fundus     gastric  juice 

Secretin  Duodenal  mucosa   Presence  of  acid   Pancreas  Secretion  of 

food  in  duo-  pancreatic  juice 

denum 
Enterogastrone        Duodenal  mucosa    Neutral  fat  Stomach  Decreased  motility 

and  secretion  of 
HCl 
Cholecyslokinin      Duodenal  mucosa    Acid  food  Gall  bladder        Contraction  of 

gall  bladder 


THE   ENDOCRINE   SYSTEM  535 

259.        Endocrine  Interrelationships 

In  the  course  of  our  discussion  some  of  the  effects  of  one  hormone 
on  the  production  or  action  of  another  have  been  described.  It  is  now 
becoming  clear  that  eacli  gland  affects  the  functioning  of  almost  every 
other  one,  and  tliat  they  together  constitute  an  interrelated  and  inter- 
dependent system  which  coordinates  body  activities.  AVhen  the  role  of 
the  pituitary  in  regulating  the  activity  of  the  thyroid,  adrenal  and  gonads 
was  first  discovered,  the  pituitary  was  described  as  a  "master  controlling 
gland."  But  in  view  of  the  reciprocal  effects  of  the  hormones  of  these 
glands  on  the  pituitary,  and  of  the  further  control  of  the  pituitary  im- 
posed by  the  hypothalamus,  it  is  probably  unwarranted  to  regard  the 
pituitary  as  a  special  master  gland. 

The  interplay  of  estradiol,  progesterone,  FSH  and  LH  in  regulating 
the  menstrual  cycle,  and  of  estrogen,  progesterone  and  prolactin  in  pro- 
ducing the  development  and  functioning  of  the  breasts,  is  now  well 
established.  The  rate  of  cell  metabolism  and  the  relative  rates  of  utiliza- 
tion of  carbohydrates,  fats  and  proteins  are  under  the  complex  control 
of  thyroxin,  insulin,  epinephrine,  glucagon,  growth  hormone,  hydro- 
cortisone, estradiol  and  testosterone.  Normal  growth  requires  not  only 
growth  hormone  and  thyroxin  but  also  insulin,  androgens  and  others. 

Hans  Selye,  of  the  University  of  Montreal,  has  done  much  in  recent 
years  to  investigate  the  role  of  hormones  in  adapting  the  body  to  en- 
vironmental stresses.  Stresses  such  as  trauma,  burns,  cold,  starvation, 
hemorrhage,  intense  soimd  or  light  and  anoxia  provoke  a  pattern  of 
adaptation  which  tends  to  resist  damage  from  the  stress.  The  stress 
stimulates  the  release  of  epinephrine  from  the  adrenal  medulla,  which  in 
turn  leads  to  the  release  of  ACTH  by  the  anterior  lobe  of  the  pituitary. 
The  adrenal  cortical  hormones  released  by  the  action  of  the  ACTH 
produce  changes  in  mineral  and  carbohydrate  metabolism  and  in  tissue 
reactivity  which  adapt  the  animal  to  resist  the  effects  of  the  stress.  Long 
continued  stresses  eventually  overcome  the  body's  adaptive  ability  and 
produce  exhaustion  and  shock.  In  the  absence  of  either  the  hypophysis 
or  the  adrenal  cortex,  the  body's  ability  to  tolerate  stress  is  gieatly  de- 
creased. 

Questions 

1.  Contrast  the  integrative  effects  of  the  nervous  and  endocrine  systems. 

2.  Define  a  hormone.  Distinguish  between  a  hormone  and  a  vitamin;  a  hormone  and  an 
enzyme. 

3.  ^Vhat  kinds  of  experiments  might  be  used  to  determine  whether  a  newly  discovered 
gland  in  a  vertebrate  secretes  a  hormone? 

4.  Name  and  gi\e  the  functions  of  the  hormones  secreted  by  the  mammalian  thyroid, 
parathyroid  and  adrenal  medulla. 

5.  What  radioactive  substance  is  particularly  useful  in  studying  thyroid  physiolog)? 
Why? 

6.  What  hormone  dysfunctions  result  in  (a)  myxedema,  (b)  .Addison's  disease,  (c)  diabetes 
insipidus,  (d)  diabetes  mellitus.  (e)  Cushing's  syndrome  and  (f)  tetanv? 

7.  Why  can  thyroxin  be  effective  when  administered  orally  whereas  insulin  must  be 
injected  subcutaneously? 


536  VERTEBRATE   LIFE   AND   ORGANIZATION 

8.  Describe  the  feed-back  mechanism   that  regulates  the  production  of  thyroxin  and 
thyrotropin. 

9.  Describe  the  feed-back  mechanism  that  regulates  the  events  of  the  menstrual  cycle. 

10.  Contrast  the  effects  of  insulin  and  glucagon. 

11.  Compare  the  roles  of  parathormone  and  vitamin  D  in  bone  formation  and  dissolution. 

12.  Name  and  describe  the  effects  of  all   the  hormones  that  are  required  for  normal 
growth. 

13.  Name  and  describe  the  effects  of  all  the  hormones  that  are  required  for  the  normal 
completion  of  pregnancy. 

14.  Name  and  give  the  functions  of  the  main  hormones  of  the  adrenal  cortex  and  the 
anterior  lobe  of  the  pituitary. 

15.  Discuss  the  theory  that  epinephrine  has  a  special  role  in  emergencies. 

16.  Describe  the  hormonal  interrelations  which  control  the  development  and  functioning 
of  the  breasts. 

Supplementary  Reading 

A  complete  and  well  written  discussion  of  the  biologic  aspects  of  endocrinology  is 
found  in  C.  D.  Turner's  General  Endocrinology.  The  principles  of  endocrinology  and 
their  clinical  applications  are  discussed  in  the  textbooks  of  endocrinology  by  R.  H. 
Williams  and  by  Hans  Selye.  Selye's  theory  of  the  role  of  stress  in  inducing  endocrine 
imbalances  is  presented  in  his  textbook.  A  fascinating  account  of  the  role  of  hormones  in 
controlling  behavior  in  the  several  classes  of  vertebrates  is  given  in  Frank  Beach's  Hor- 
mones and  Behavior.  Endocrine  mechanisms  in  other  animals,  particularly  in  inverte- 
brates, are  described  by  Frank  Brown  in  Prosser's  Comparative  Animal  Physiology.  De- 
tailed discussions  of  the  current  status  of  particular  fields  of  endocrinology  are  found  in 
the  series  entitled  Ciba  Foundation  Colloquia  on  Endocrinology,  edited  by  G.  E.  W. 
Wolstenholme. 


CHAPTER  31 


The  Development  of  Mammals 


We  shall  conclude  our  consideration  of  the  organ  systems  of  verte- 
brates by  briefly  examining  the  embryonic  development  of  the  organs. 
The  general  features  of  vertebrate  development  were  discussed  in 
Chapter  6  and  should  be  reviewed  at  this  time.  We  shall  focus  our  atten- 
tion on  the  early  stages  in  the  development  of  mammals,  which  differ 
in  some  respects  from  those  of  other  vertebrates,  and  on  the  establish- 
ment of  the  organ  systems. 

260.        Early  Stages  of  Mammalian  Development 

Monotreme  embryos  derive  their  nutrients  in  reptilian  fashion 
from  the  large  accumulation  of  yolk  stored  in  the  cleidoic  egg,  but 
other  mammalian  embryos  develop  within  the  uterus  and  derive  their 
nutrients  from  the  mother  through  the  placenta.  These  mammals  do 
not  provide  their  eggs  with  much  yolk.  The  eggs  are  isolecithal  and  so 
small  that  they  can  barely  be  seen  with  the  unaided  eye.  Indeed  they 
are  so  small  that  the  early  stages  of  mammalian  development  remained 
a  mystery  long  after  the  early  development  of  other  vertebrates  had 
been  described.  William  Harvey,  famed  for  his  discovery  of  the  circula- 
tion of  the  blood,  searched  the  uteri  of  deer  in  vain  for  early  embryos, 
and  finally  concluded  that  the  embryo  might  somehow  be  secreted  by 
the  uterus  when  seminal  fluid  was  introduced.  In  1672,  de  Graaf  dis- 
covered early  cleavage  stages  (he  called  them  eggs)  in  the  Fallopian 
tube  of  a  rabbit,  and  concluded,  correctly,  that  the  eggs  came  from  the 
ovary.  The  first  mammalian  egg  to  be  seen,  a  dog's  egg,  was  observed 
by  von  Baer  in  1827.  Human  eggs  free  within  the  Fallopian  tube  and 
early  developmental  stages  have  been  described  only  in  recent  years. 

Cleavage  might  be  expected  to  be  a  very  regular  process  in  mam- 
malian eggs  as  it  is  in  other  isolecithal  eggs.  The  mammalian  egg  does 
cleave  completely,  and  the  first  two  or  three  cleavages  in  primates  are 
regular  and  produce  blastomeres  of  nearly  equal  size  (Fig.  31.1).  Sub- 
sequently, certain  blastomeres  divide  faster  than  others,  and  cleavage 
becomes  somewhat  irregular.  This  may  be  a  reflection  of  the  irregular 
cleavage  characteristic  of  the  reptilian  telolecithal  egg,  which  was,  of 
course,  the  type  of  egg  present  in  mammalian  ancestors. 

a'  solid  ball  of  cells,  the  morula,  is  produced,  and  as  the  cells 
continue    to    divide,    they   arrange    themselves   about   a    central    cavity. 

637 


538  VERTEBRATE   LIFE   AND   ORGANIZATION 


.<•  •'••j_.    * 


\    ,■'». 


B 


(Peripheral  cells  of  D  and 
E  are  arranging  in  a  layer, 
while  fluid  vacuoles  are 
appearing  between  the  in- 
ternal cells.  F  and  G  are 
parts  of  blastocysts  whose 
proportions  are  shown  in 
small,   outline   drawings.) 


D 


,'f^^. 


Figure  31.1.  Photomicrographs  of  cleavage  in  mammalian  eggs  developing  in  a 
tissue  culture.  A-C,  Two,  four  and  eight-celled  stages  of  the  monkey;  D,  morula  of  a 
rabbit;  E-G,  blastocysts  of  a  rabbit.  Observe  the  thick  membrane  that  surrounds  the 
early  stages.  Several  sperm  are  entrapped  in  this  in  A  and  B.  (After  Lewis,  Hartman 
and  Gregory.) 


This  stage,  known  as  the  blastocyst,  can  be  compared  to  the  blastula 
of  other  vertebrates.  However,  only  a  group  of  cells  at  one  pole  of  the 
blastocyst,  the  Inner  cell  mass,  forms  the  embryo  (Fig.  31.2).  The 
peripheral  layer  of  cells,  known  as  the  trophoblast,  comes  in  contact 
with  the  uterine  lining  and  begins  to  form  a  placenta  before  the 
embryo  itself  has  developed  to  any  great  extent.  The  value  of  the 
precocious  development  of  this  layer  in  a  yolkless  embryo  that  is  not 
free  to  forage  for  itself  is  obvious.  The  trophoblast  is  comparable  to 
the  ectoderm  of  the  chorion,  which  is  the  outermost  of  the  extra- 
embryonic membranes  of  all  amniotes.  Bushy  projections  called  villi 
develop  on  its  surface  and  penetrate  the  uterine  lining  in  most  mam- 


THE   DEVELOPMENT   OF   MAMMALS 


639 


mals.  The  parts  of  the  chorion  and  uterine  lining  that  are  intimately 
associated   constitute   the   placenta. 

In  vertebrates  such  as  the  frog  (Fig.  6.9),  gastrulation  involves  an 
inpushing  of  certain  cells  of  the  vegetal  hemisphere  (invagination),  a 
growth  of  cells  from  the  animal  hemisphere  over  the  vegetal  cells  (epi- 
boly)  and  an  inturning  of  certain  of  these  cells  (involution).  These 
complex  processes  are  largely  by-passed  in  mammalian  development  and 
gastrulation  is  greatly  abbreviated.  In  primates,  cells  on  the  lower  part 
of  the  inner  cell  mass  simply  differentiate  as  endoderm,  and  a  small 
space,  the  yolk  sac,  appears  in  their  midst  (Fig.  31.2).  The  yolk  sac  is 
an  embryonic  vestige  and  is  devoid  of  yolk.  Its  presence  is  a  hold-over 


Inner 

C(zll  m.ass~n 


Trophobla.st 


r~  Amniotic 
ca.vii;y 


Aranion 
Embryo 

i^      YolK 


r-Body 


Mesodermal  ce-lls 

Chorionic 
villu-S' 

Figure  31.2.     A  series  of  diagrams  to  illustrate  the  differentiation  of  the  inner 
mass  into  the  yolk  sac,  amnion  and  embryonic  disc,  and  to  show  the  migration  of 
mesoderm.  These  changes  occur  in  a  human  embryo  during  the  second  week.  (Modi 
after  Patten.) 


cell 
the 
fied 


540  VERTEBRATE   LIFE   AND   ORGANIZATION 

Embryonic  d.iSC 


AmnioTT.- 
(cut  edge) 


r  Primitive. 
strecLK 


Yolk  sa-cJ-J 


-Body  staJK 


A 

Ectoderra- 


Alla.ntois 


rPi^imitive 
^trea.H 


EiTLbryonic 
disc 


Endod.(2rm- 


Mesoderm. 


B 


Figure  31.3.  Mesoderm  formation.  A,  A  surface  view  of  the  embryonic  disc  of  a 
sixteen-day  human  embryo  showing  the  primitive  streak.  B,  A  cross  section  through 
the  primitive  streak.  Prospective' mesoderm,  which  originally  lies  on  the  surface  of  the 
embr)onic  disc,  moves  in  through  the  primitive  streak  and  spreads  out  between  the 
ectoderm  and  endoderm  in  the  manner  shown  by  the  arrows.  (After  Arey.) 

from  the  reptilian  stage  in  the  ancestry  of  mammals.  The  rest  of  the 
cells  of  the  inner  cell  mass  are  prospective  ectoderm  and  mesoderm.  An 
amniotic  cavity  appears  among  the  ectoderm  cells  at  about  the  same 
time  that  the  yolk  sac  develops  (Fig.  31.2).  The  double-layered  plate  of 
cells  lying  between  the  yolk  sac  and  amniotic  cavity  is  the  embryonic 
disc.  A  primitive  streak,  similar  to  that  of  reptiles  and  birds,  develops 
upon  the  upper  surface  of  the  embryonic  disc,  and  establishes  the 
longitudinal  axis  of  the  embryo  (Fig.  31.3).  Cells  destined  to  become 
mesoderm  move  inward  through,  and  perhaps  proliferate  from,  the 
primitive  streak.  They  spread  out  between  the  endoderm  of  the  yolk 
sac  and  the  ectoderm  that  forms  the  surface  of  the  embryonic  disc. 
Mesodermal  involution  through  a  primitive  streak  is  similar  to  the  in- 
volution of  prospective  mesoderm  through  the  blastopore  of  a  frog,  for 
the  primitive  streak  and  the  blastopore  are  homologous.  As  mesodermal 
cells  continue  to  spread,  they  form  a  layer  beneath  the  trophoblastic 
ectoderm,  and  this  becomes  a  fairly  typical  chorion  composed  of  ecto- 
derm and  mesoderm.  Mesodermal  cells  also  surround  the  endoderm  of 
the  yolk  sac  and  the  ectoderm  lining  the  amniotic  cavity  (Fig.  31.2).  A 
group  of  mesodermal  cells  known  as  the  body  stalk  extends  between 
the  embryonic  disc  and  the  chorion,  and  an  endodermal  evagination 
grows  into  it  from  the  posterior  part  of  the  yolk  sac  (Figs.  31.3  and 
31.5).  This  evagination  and  the  surrounding  mesoderm  constitute  the 
allantois.  The  part  of  the  yolk  sac  from  which  this  evagination  arises  is 


THE   DEVELOPMENT   Of   MAMMALS 


641 


destined  to  become  the  hindgut,  so  the  allantois  of  mammals  has  the 
same  relationship  to  the  gut  that  the  allantois  has  in  reptiles  and  birds. 

261.        Formation  of  the  Notochord  and  Neural  Tube 

All  of  the  extraembryonic  membranes  characteristic  of  amniotes 
(amnion,  chorion,  allantois  and  yolk  sac)  are  now  present,  and  the  em- 
bryo itself  is  beginning  to  take  shape.  A  notochord  develops  beneath 
the  surface  ectoderm  in  the  longitudinal  axis  as  the  primitive  streak 
shortens  and  retreats  toward  the  posterior  end  of  the  embryonic  disc. 
The  ectoderm  overlying  the  notochord  thickens  and  becomes  a  neural 
plate.  The  lateral  edges  of  the  neural  plate  are  elevated  as  a  pair  of 
neural  folds,  which  gradually  come  together  (Fig.  31.4).  The  inner 
limbs  of  the  folds  become  the  neural  tube,  which  differentiates  into  the 
spinal  cord  and  brain  as  described  in  section  247;  the  outer  limbs,  along 
with  the  rest  of  the  surface  ectoderm,  become  the  epidermis  of  the  skin. 
Ectodermal  cells  that  are  pinched  off  near  the  apex  of  each  neural  fold 
form  a  ridge,  the  neural  crest,  on  each  side  of  the  neural  tube.  The  cells 
of  the  neural  crest  become  segmentally  arranged  and  many  of  them 
differentiate  into  the  afferent  neurons  of  the  spinal  and  cranial  nerves. 
Other  neural  crest  cells  migrate  and  form  postganglionic  sympathetic 
fibers  (other  types  of  efferent  neurons  grow  out  from  the  neural  tube), 
the  medullary  cells  of  the  adrenal  gland,  the  neurilemmal  sheath  cells 
of  peripheral  neurons,  and  certain  other  structures.  Surface  ectoderm  that 


NeuraJ— 1 
pla.te 


S  ui^f a.ce  — 1 
ectoderna 


Neurad  fold 


Notochord 


Neural  groove— 


B 


Neural 
cre-st 


Figure  31.4.     A  series  of  cross  sectional  diagrams  through  the  surface  ectoderm  to 
show  the  formation  of  the  neural  tube  and  neural  crest.  (After  Arey.) 


542  VERTEBRATE   LIFE   AND   ORGANIZATION 

does  not  contribute  to  the  neural  tube  iorms  the  epidermis,  hair  and 
skin  glands. 

262.       The  Digestive  Tract  and  Its  Derivatives 

The  neural  tube  and  embryo  elongate  laster  than  the  embryonic 
disc  upon  which  the  embryo  is  developing.  As  a  result,  the  embryonic 
disc  buckles  at  each  end.  The  embryo  continues  to  elongate,  and  the 
parts  of  the  embryonic  disc  that  originally  lay  anterior  or  posterior  to 


Embryo- 


rAmnion 


-Forcgut 


Hindout 


Heart 


Body  sta-lK 
>^--\     AllarAois 


Chorion 


H<z.art 


Embryo-i    ^^       pAmnioxi. 


Hearb ■ 


'Allantois 


Live.r 
primordium 

Mouth,  pocket 
(Stomodaeum) 


Umbiliccd  cord- 


is al  pcoicrea-S 


•ArchentcT-on. 


AnalpocKet 

(p  r  o  ct  o  d.a.iz.ij.m.) 


AllantoiS 


Chorion 


Yolk 


sa.c 


c 


Figures  31.5.  A  series  of  diagrams  of  sagittal  sections  of  embryos  of  different  ages 
to  sliow  the  folding  processes  that  separate  the  embryo  from  its  extraembryonic  mem- 
branes. Sohd  lines  represent  ectoderm,  broken  lines  endoderm,  and  stippled  lines  and 
shaded  areas  mesoderm.  (Modified  after  Arey.) 


the  neural  tube  fold  underneath  the  embryo  (Fig.  31.5).  Folds  first 
separate  the  head  and  tail  from  surrounding  structures.  These  folds 
deepen  and  the  folding  process  continues  along  each  side  until  the  em- 
bryo is  more  or  less  cylindrical  in  shape  and  remains  connected  to  its 
surrounding  membranes  only  by  a  narrow  umbilical  cord.  The  folding 
process  is  somewhat  analogous  to  the  gradual  tightening  of  a  pair  of 
purse  strings. 

These  folding  processes  gradually  pinch  off  the  dorsal  part  of  the 
yolk  sac  and  convert  it  into  the  primitive  gut,  or  archenteron,  of  the 


THE   DEVELOPMENT   OF   MAMMALS  543 

embryo.  The  archenteron  remains  connected  with  the  yolk  sac  by  a 
narrow  stalk  that  extends  through  the  umbilical  cord.  The  anterior 
part  of  the  archenteron,  the  foregut,  differentiates  into  the  pharynx, 
esophagus,  stomach  and  a  small  portion  of  the  duodenum.  The  rest  of 
the  archenteron,  the  hindgut,  forms  most  of  the  intestinal  region  and 
much  of  the  embryonic  cloaca.  Only  the  linings  of  these  organs  are 
endodermal;  the  connective  tissue  and  muscles  in  their  wall  are  derived 
from  mesoderm. 

The  pharyngeal  pouches,  thyroid  gland,  trachea  and  lungs  develop 
as  outgrowths  from  the  pharynx,  as  described  in  section  218.  A  ventral 
outgrowth  from  the  posterior  end  of  the  foregut  differentiates  into  the 
liver  and  much  of  the  pancreas,  but  part  of  the  pancreas  develops  as  a 
separate  dorsal  outgrowth  (Figs.  31.5  and  31.6).  This  explains  why  the 
pancreas  has  two  ducts,  one  entering  the  intestine  with  the  bile  duct 
and  one  independently. 

The  most  anterior  and  posterior  ends  of  the  digestive  tract  develop 
from  ectodermal  pockets  that  invaginate  and  meet  the  archenteron. 
Initially,  plates  of  tissue  separate  these  pockets  from  the  archenteron, 
but  these  plates  eventually  break  down.  The  lining  of  the  mouth,  the 
enamel  of  the  teeth  and  the  secretory  cells  of  the  salivary  glands  are 
ectodermal  in  origin.  The  anterior  and  intermediate  lobes  of  the  pitui- 
tary gland  develop  as  an  ectodermal  evagination  from  the  roof  of  the 
mouth  pocket,  as  described  in  Chapter  30,  but  the  posterior  lobe  of 
the  pituitary  develops  as  an  evagination  from  the  floor  of  the  dien- 
cephalic region  of  the  brain.  Part  of  the  embryonic  cloaca  is  of  ecto- 
dermal origin.  A  cloaca  persists  in  the  adults  of  most  vertebrates,  but  is 
divided  in  most  mammals  to  form  the  rectum  and  parts  of  the  urogenital 
passages  (part  of  the  urethra  in  the  male;  part  of  the  urethra  and  vagina 
in  the  female). 

263.        DiflFerentiation  of  the  Mesoderm 

As  the  mesoderm  spreads  out  from  the  primitive  streak,  its  lateral 
portion  splits  into  two  lavers  (Fig.  31.6).  This  part  of  the  mesoderm  is 
known  as  the  lateral  pSate,  and  the  space  between  the  two  layers  is  the 
embryonic  coelom.  The  embryonic  coelom  is  continuous  with  the  large 
extraembryonic  coelom,  or  chorionic  cavity,  until  the  folding  processes 
described  above  separate  the  embryo  from  surrounding  structures.  The 
inner  layer  of  the  lateral  plate  mesoderm,  which  lies  next  to  the  archen- 
teron, forms  the  connective  tissue  and  musculature  (visceral  muscles)  of 
the  digestive  tract,  the  visceral  peritoneum  and  the  mesenteries.  The 
outer  layer  forms  the  lateral  wall  of  the  coelom,  that  is,  the  parietal 
peritoneum,  and  may  contribute  to  the  musculature  of  the  body  wall. 

Unlike  the  lateral  plate,  the  mesoderm  on  each  side  of  the  neural 
tube  and  notochord  becomes  segmented  and  forms  a  series  of  paired 
somites  Some  of  the  mesoderm  of  the  somites  spreads  out  beneath  the 
surface  ectoderm  to  form  the  dermis  of  the  skin,  some  migrates  around 
the  neural  tube  and  notochord  and  differentiates  into  the  vertebral 
column  and  much  of  the  skull,  and  the  rest  forms  the  segmented,  em- 


544  VERTEBRATE   LIFE   AND   ORGANIZATION 


Somite" 


Nephrogenic 


"Amnion- 
-Neurad  fold- 


Late  rail     y^ 


Embryonic 
Coelom. 


Extra. - 
^embryonic 
co<z.Jom 


Somite 


Nephrogenic 
ridOe 

Lateral  plate 


Dorsal  aorta." 

Dorsal 
mesentery /5f 


Myotome 
Genita.1  ridoe 


Somite:  — 
^Developin.^  dermis 
^uT  Developing 

vertebra. 
I  Myotome^ 

Kidney 
(Ne-phrodenic 
ridrfe) 

Peritoneum 
Mesentery 

Lateral  plate :  -^ 


^rr-Dorsa.1  pa-ncreas 


Livfey 


Figure  31.6.  Diagrammatic  cross  sections  through  vertebrate  embryos  of  different 
ages.  The  separation  of  the  embryo  from  the  yolk  sac,  the  differentiation  of  the  meso- 
derm, and  the  formation  of  the  Hver  and  dorsal  pancreas  are  shown.  (Modified  after 
Patten.) 

bryonic  skeletal  muscle  blocks,  or  myotomes.  The  myotomes  extend  out 
between  the  surface  ectoderm  and  the  lateral  plate  and  develop  into 
most  of  the  musculature  of  the  body  wall  and  appendages  (somatic 
muscles).  The  segmentation  of  the  muscles  is  retained  in  adult  fishes 
but  muscle  segmentation  is  largely  lost  during  the  later  development  of 
most  higher  vertebrates. 

The  resemblance  of  certain  of  the  embryonic  stages  of  the  higher 
vertebrates  to  the  adults  of  lower  vertebrates,  such  as  we  see  in  the 
segmentation  of  the  muscles,  is  regarded  as  strong  evidence  for  evolution. 
In  the  late  nineteenth  century,  Ernst  Haeckel  postulated  that  embryos 
pass  through  stages  during  their  embryonic  development  (ontogeny)  that 
their  ancestors  passed  through  during  evolution  (phylogeny).  In  other 
words,  "ontogeny  recapitulates  phylogeny,  or  the  embryo  climbs  its  own 
family  tree."  This  generalization  is  no  longer  taken  as  literally  as 
Haeckel  intended.  It  is  now  clear  that  the  embryos  of  higher  vertebrates 


THE   DEVELOPMENT   OF   MAMMALS  545 

resemble  the  embryos  and  not  necessarily  the  adults  of  lower  vertebrates. 
Early  vertebrates  evolved  a  series  of  developmental  stages  that  resulted 
in  their  characteristic  organs.  Higher  vertebrates  have  certain  differen- 
ces, but  these  develop  by  introducing  changes  in  the  later  stages  of 
development  rather  than  by  altering  the  whole  complex  and  intricately 
interrelated  development  sequence.  Development,  therefore,  tends  to  be 
conservative,  and  the  early  embryos  of  different  animals  may  bear 
marked  resemblances  to  each  other.  However,  the  early  development  of 
an  embryo  may  be  altered  and  correlated  with  special  conditions  to 
which  the  embryo  has  become  adapted.  The  extraembryonic  membranes 
of  mammals,  for  example,  develop  in  advance  of  the  main  body  of  the 
embryo,  and  the  placenta  is  formed  very  early.  This  is  an  adaptation  of 
the  embryo  to  intrauterine  life.  In  reptiles  the  extraembryonic  mem- 
branes develop  only  after  the  body  of  the  embryo  is  well  established. 

A  narrow  band  of  mesoderm,  known  as  the  nephrogenic  ridge,  lies 
between  the  somites  and  the  lateral  plate.  This  part  of  the  mesoderm 
differentiates  into  the  kidney,  as  described  in  section  238,  and  helps 
form  the  gonads. 

The  entire  circulatory  system  develops  from  the  mesoderm,  and  its 
development  is  rapid  in  all  vertebrates.  Transporting  vessels  are  neces- 
sary for  the  embryo  to  obtain  nutrients  from  the  placenta,  or  yolk,  as 
the  case  may  be.  The  blood  vessels  differentiate  by  the  hollowing  out  and 
coalescence  of  cords  and  knots  of  mesodermal  cells  that  appear  first  in 
the  mesodermal  layer  next  to  the  yolk  sac.  A  pair  of  vessels  that  are 
destined  to  become  the  heart  develop  in  the  anterior  part  of  the  em- 
bryonic disc  before  the  neural  tube  is  completely  formed.  Subsequent 
foldings  that  give  the  embryo  its  shape  carry  these  vessels  beneath  the 
front  of  the  embryo  (Fig.  31.5).  They  fuse  to  form  a  single  cardiac  tube, 
and  the  cardiac  tube  differentiates  into  the  series  of  chambers  found  in 
fish  hearts  (sinus  venosus,  atrium,  ventricle,  and  conus  arteriosus).  Since 
the  cardiac  tube  grows  in  length  faster  than  the  part  of  the  coelom  (the 
pericardial  cavity)  in  which  it  lies,  it  folds  and  forms  an  S-shaped  tube. 
The  atrium,  which  originally  lay  posterior  to  the  ventricle,  thus  comes 
to  lie  in  front  of  the  ventricle.  Gradually  the  cardiac  tube  differentiates 
into  the  adult  heart.  The  atrium  and  ventricle  become  divided  in  mam- 
mals, the  sinus  venosus  is  incorporated  into  the  right  atrium,  and  the 
conus  arteriosus  forms  part  of  the  pulmonary  artery  and  the  arch  of  the 
aorta. 

A  series  of  paired  aortic  arches,  which  are  similar  in  arrangement 
to  those  of  a  fish,  but  are  not  interrupted  by  capillaries,  carry  blood 
from  the  heart  up  through  the  pharyngeal  region  to  the  dorsal  aorta. 
Vitelline  arteries  extend  from  the  aorta  to  the  yolk,  and  umbilical 
arteries  follow  the  allantois  to  the  chorion  and  developing  placenta. 
Veins  develop  in  a  similar  manner,  and  return  blood  to  the  heart  from 
the  yolk  sac,  chorionic  villi  and  the  embryo  itself.  In  the  early  mam- 
malian embryo,  the  pattern  of  the  veins  resembles  the  pattern  seen  in 
fishes.  Cardinal  veins  are  present  and  the  venae  cavae  do  not  develop 
until  later.  The  pattern  of  the  circulation  in  a  late  fetus,  and  the  changes 
that  occur  at  birth,  were  considered  in  section  236. 


^aVILLARV    PROCESS 


OTIC   VESICLE  3HO    PHAnVNGEAL   ABCH 


PERICAROIAL 
SWELLING 


HIND   LIM8 


K». 


1TK    PHAHVJOEAL 
ARCH 


t^'iPER^CAROlAL 
HiDGfC 


■  roHE  LIMO 


Figure  31.7.  Upper,  Side  view  of  a  human  embryo  about  four  weeks  old;  its  crown- 
rump  length  is  5  mm.  Lower,  A  human  fetus  about  eight  weeks  old;  its  crown-rump 
length  is  30  mm.  (From  Hamilton,  Boyd  and  Mossman:  Human  Embryology,  Williams 
and  Wilkins  Co.) 

646 


THE   DEVELOPMENT  OF   MAMMALS  547 

264.        Growth  of  the  Embryo 

The  main  morphologic  changes  in  embryonic  development  take 
place  surprisingly  fast.  A  human  embryo  four  weeks  old  is  only  5  mm. 
long,  but  it  has  already  developed  enough  to  be  recognized  as  some  sort 
of  a  vertebrate  embryo  (Fig.  31.7  A).  The  development  of  all  of  the 
organ  systems  is  well  under  way,  the  heart  has  begun  to  beat,  limb  buds 
that  will  differentiate  into  arms  and  legs  are  protruding  from  the  surface, 
and  a  small  tail  is  present.  Pregnancy  may  only  be  suspected  at  this 
time.  At  eight  weeks  (Fig.  31.7  B),  the  embryo  can  be  recognized  as 
human.  The  face  and  distinct  fingers  and  toes  have  developed.  The  organ 
systems  are  approaching  their  adult  condition.  Some  of  the  bones  are 
beginning  to  ossify  and  taste  buds  are  developing  on  the  tongue.  The 
embryo  is  arbitrarily  called  a  fetus  from  this  age  on. 

Only  relatively  small  changes  occur  in  the  organ  systems  during  the 
remaining  seven  months  of  pregnancy,  but  a  great  increase  in  size  takes 
place.  An  eight-week  fetus  has  a  crown-rump  length  of  30  mm.  At  term, 
its  crown-rump  length  is  about  35  cm.  Among  the  morphologic  changes 
that  occur  during  this  period  are  differentiation  of  the  external  genitalia, 
development  of  body  hair,  muscularization  of  the  digestive  tract,  and 
myelinization  of  the  neurons.  Though  the  infant  is  well  developed  at  the 
time  of  birth,  development  does  not  cease.  Changes  in  the  organ  systems 
and  in  the  relative  size  of  body  parts  continue  throughout  infancy,  child- 
hood and  adolescence.  Human  development  is  not  really  completed  until 
the  late  teens. 


265.        Twinning 

Many  offspring  are  born  at  the  same  time  in  pigs,  rats  and  a  number 
of  other  mammals.  The  number  in  a  pig  litter,  for  example,  ranges 
from  7  to  23.  But  many  other  mammals,  including  man  and  the  other 
higher  primates,  whales  and  horses,  normally  have  only  one  offspring 
at  a  time.  Occasionally  multiple  births  occur  in  these  mammals.  Twins 
are  produced  about  once  in  every  88  human  births.  Approximately  three- 
fourths  of  these  are  dizygotic,  or  fraternal  twins.  Two  eggs  have  been 
ovulated  and  fertilized  at  about  the  same  time.  Such  twins  do  not 
resemble  each  other  any  more  closely  than  brothers  or  sisters  born  at 
different  times,  for  they  have  somewhat  different  genetic  constitutions. 
Fraternal  twins  occur  more  frequently  in  some  families  than  in  others, 
so  it  is  possible  that  there  are  certain  hereditary  tendencies  for  the 
maturation  and  ovulation  of  more  than  one  ovum  during  a  single 
menstrual  cycle. 

More  rarely,  monozygotic  or  identical  twins  are  formed.  Only  one 
egg  is  fertilized,  but  two  embryos  develop  from  it.  Identical  twins  are 
always  of  the  same  sex  and  resemble  each  other  closely  for  they  have 
identical  genetic  constitutions.  Monozygotic  twinning  may  occur  in  one 
of  several  ways.  The  two  blastomeres  produced  by  the  first  cleavage  may 
separate  and  each  become  an  embryo,  the  inner  cell  mass  may  subdivide, 
or   two   primitive  streaks   may  develop   upon  a   single  embryonic  disc. 


548  VERTEBRATE   LIFE   AND   ORGANIZATION 

Twins  have  been  produced  experimentally  by  the  first  method  in  lower 
vertebrates.  This  method  is  a  possibility  in  mammals,  but  it  is  not  as 
likely  to  occur  as  the  others,  for  the  mammalian  egg  and  cleavage  stages 
are  surrounded  by  a  strong  membrane,  the  zona  pellucida,  that  should 
prevent  the  blastomeres  from  separating  (Fig.  31.1). 

A  particularly  interesting  case  of  twinning  is  seen  in  the  armadillo. 
This  animal  always  has  quadruplets  and  the  four  individuals  are  always 
of  the  same  sex.  The  fact  that  only  one  corpus  luteum  is  found  in  the 
ovary,  which  means  that  only  one  follicle  and  egg  matured  and  ovu- 
lated, indicates  that  all  are  identical  twins.  When  the  blastocyst  is  ex- 
amined, it  is  discovered  that  the  inner  cell  mass  has  subdivided  into 
foia-  parts. 

If  identical  twins  are  produced  by  subdivision  of  the  inner  cell  mass, 
or  by  the  formation  of  two  primitive  streaks,  one  would  expect  to  find 
occasional  cases  in  which  the  separation  is  incomplete.  Though  fortu- 
nately rare,  conjoined  twins  are  born  from  time  to  time.  All  degrees  of 
union  have  been  found.  Usually  such  individuals  die  in  infancy,  but  the 
most  famous  pair,  Chang  and  Eng,  lived  to  be  63.  Though  Chinese, 
Chang  and  Eng  were  born  in  Siam.  They  worked  for  a  circus,  married 
and  fathered  22  children!  Their  fame  led  to  the  popular  term  "Siamese 
twins"  for  such  conjoined  twins. 

Questions 

1.  How  has  the  early  development  of  mammals  been  modified  by  the  retention  of  the 
embryo  in  a  uterus? 

2.  Compare  endoderm  and  mesoderm  formation  in  a  mammal  and  a  frog. 

3.  To  what  extent  does  ontogeny  recapitulate  phylogeny? 

4.  How  does  it  happen  that  certain  parts  of  the  digestive  tract  are  of  ectodermal  origin? 

5.  What  structures  develop  from  the  somites,  the  nephrogenic  ridge  and  the  lateral  plate 
mesoderm? 

6.  What  sort  of  changes  occur  in  the  human  fetus  after  the  second  month? 

7.  Distinguish  between  fraternal  and  identical  twins.  How  may  identical  twins  be  formed? 

Supplementary  Reading 

Corner  has  written  a  very  interesting  essay  on  human  development  entitled  Ourselves 
Unborn.  Further  information  on  the  development  of  man  and  other  vertebrates  can  be 
found  in  such  standard  texts  as  Arey,  Developmental  Anatomy,  or  Witschi,  Develop- 
ment of  Vertebrates.  Those  interested  in  experimental  embryology  will  find  Willier,  Weiss 
and  Hamburger,  Analysis  of  Development,  an  invaluable  source.  The  relationship  of 
embryology  to  evolution  is  carefully  discussed  by  DeBeer  in  Embryos  and  Ancestors.  The 
biology  of  twinning  and  the  differences  and  similarities  between  fraternal  and  identical 
twins  are  thoroughly  considered  by  Newman,  Freeman  and  Holzinger,  Twins. 


Part  IV 


GENETICS    AND    EVOLUTION 


CHAPTER  32 


Principles  of  Heredity 


266.        History  of  Genetics 

It  must  have  been  thousands  of  years  ago  when  man  first  made  one 
of  the  fundamental  observations  of  heredity— that  "like  tends  to  beget 
like."  But  his  curiosity  as  to  why  this  is  true  and  how  it  is  brought  about 
remained  unsatisfied  until  the  beginning  of  the  present  century.  A  num- 
ber of  breeders,  such  as  Kolreuter  who  worked  with  tobacco  plants  about 
1770,  crossed  different  varieties  of  plants  and  produced  hybrids.  Kol- 
reuter recognized  that  parental  characters  were  transmitted  by  both  the 
pollen  and  the  ovule.  Mendel's  careful  work  with  peas  revealed  the 
fundamental  principles  of  heredity,  but  the  report  of  his  work,  published 
in  1866,  was  far  ahead  of  his  time.  It  is  clear  that  his  work  was  known 
to  a  number  of  the  leading  biologists  of  the  time,  such  as  the  botanist, 
Nageli,  but  in  the  absence  of  our  present  knowledge  of  chromosomes 
and  their  behavior,  its  significance  was  unappreciated. 

The  chromosomal  details  of  mitosis  were  described  by  Eduard  Stras- 
burger  in  1876.  Eduard  van  Beneden  (1887)  discovered  the  process  of 
meiosis  and  understood  its  significance.  Earlier  that  same  year  Weis- 
mann  had  pointed  out,  simply  from  theoretical  considerations,  that  the 
chromosome  number  in  gametes  must  be  half  of  that  in  somatic  cells.  It 
is  conceivable  that  some  brilliant  theoretical  biologist  with  these  facts  at 
hand  might  have  postulated  that,  if  hereditary  factors  were  units  located 
in  the  chromosomes,  the  mating  of  different  parental  types  would  yield 
offspring  in  predictable  ratios.  However,  no  such  mental  synthesis  was 
made,  and  the  existence  of  these  definite  ratios  of  the  types  of  offspring 

649 


550  GENETICS   AND   EVOLUTION 

resulting  from  a  given  mating  remained  to  be  demonstrated  experi- 
mentally. 

In  1900,  three  different  biologists,  working  independently-de  Vries 
in  Holland,  Correns  in  Germany  and  von  Tschermak  in  Austria-redis- 
covered the  phenomenon  of  regular,  predictable  ratios  of  the  types  of 
offspring  produced  by  mating  pure-bred  parents.  They  then  found  Men- 
del's published  report  and,  realizing  his  priority  in  these  discoveries, 
gave  him  credit  for  his  work  by  naming  two  of  the  fundamental  prin- 
ciples of  heredity  Mendel's  Laws. 

With  the  genetic  and  cytologic  facts  at  hand,  W.  S.  Sutton  and  C. 
E.  McClung  independently  came  to  the  conclusion  (1902)  that  the 
hereditary  factors  are  located  in  the  chromosomes.  They  also  pointed  out 
that  since  there  is  a  much  greater  number  of  hereditary  factors  than  of 
chromosomes,  there  must  be  more  than  one  hereditary  factor  per  chrom- 
osome. By  1911,  T.  H.  Morgan  was  able  to  postulate,  from  the  regularity 
with  which  certain  characters  tended  to  be  inherited  together,  that  the 
hereditary  factors  (which  he  named  "genes")  were  located  in  the  chro- 
mosomes in  linear  order,  "like  the  beads  on  a  string." 

267.        Mendel's  Discoveries 

Gregor  Johann  Mendel  (1822-1884)  was  an  Austrian  abbot  who 
spent  some  eight  years  breeding  peas  in  the  garden  of  his  monastery  at 
Briinn,  now  part  of  Czechoslovakia.  He  succeeded  in  reaching  an  under- 
standing of  the  basic  principles  of  heredity  because  (1)  he  studied  the 
inheritance  of  single  contrasting  characters  (such  as  green  versus  yellow 
seed  color,  wrinkled  versus  smooth  seed  coat),  instead  of  attempting  to 
study  the  complete  inheritance  of  each  organism;  (2)  his  studies  were 
quantitative;  he  counted  the  number  of  each  type  of  offspring  and  kept 
accurate  records  of  his  crosses  and  results;  and  (3),  by  design  or  by  good 
fortune,  he  chose  a  plant,  and  particular  characters  of  that  plant,  that 
gave  him  clear  ratios.  If  he  had  worked  with  other  plants,  or  with 
certain  other  characters  of  peas,  he  would  have  been  unable  to  get  these 
ratios.  Now  that  the  principles  of  heredity  have  been  established,  the 
explanation  for  these  more  complicated  types  of  inheritance  is  clear. 

Mendel  established  pure-breeding  strains  of  peas  with  contrasting 
characters— yellow  seed  coat  vs.  green  seed  coat,  round  seeds  vs.  wrinkled 
ones— and  then  made  crosses  of  the  contrasting  varieties.  He  found  that 
the  offspring  of  a  cross  of  yellow  and  green  all  had  yellow  seed  coats;  the 
result  was  the  same  whether  the  male  or  the  female  parent  had  been 
the  yellow  one.  Thus,  the  character  of  one  parent  can  "dominate"  over 
that  of  the  other,  but  which  of  the  contrasting  characters  is  dominant 
depends  upon  the  specific  trait  involved,  not  upon  which  parent  con- 
tributes it.  This  observation,  repeated  for  several  different  strains  of 
peas,  led  Mendel  to  the  generalization,  the  "Law  of  Dominance,"  that 
when  two  factors  for  the  alternative  expression  of  a  character  are  brought 
together  in  one  individual,  one  may  be  expressed  completely  and  the 
other  not  at  all.  The  character  which  appears  in  the  first  generation  is 
said  to  be  dominant;  the  contrasting  character  is  said  to  be  recessive. 


PRINCIPLES   OF   HEREDITY  Q^\ 

Mendel  then  took  the  seeds  produced  by  this  first  generation  of  the 
cross  (called  the  first  filial  generation,  abbreviated  Fi),  planted  them  and 
had  the  resulting  plants  fertilize  themselves  to  produce  the  second  filial 
generation,  the  Fo.  He  found  that  both  the  dominant  and  the  recessive 
characters  appeared  in  this  generation,  and  upon  counting  the  number 
of  each  type  (Table  10)  he  found  that,  whatever  set  of  characters  he 
used,  the  ratio  of  plants  with  the  dominant  character  to  those  with  the 
recessive  character  was  very  close  to  3  :  1.  From  such  experiments  Mendel 
concluded  that  (1)  there  must  be  discrete  unit  factors  which  determine 
the  inherited  characters,  (2)  these  unit  factors  must  exist  in  pairs,  and 
(3)  in  the  formation  of  gametes  the  members  of  these  pairs  separate  from 
each  other,  with  the  result  that  each  gamete  receives  only  one  member 
of  the  pair.  The  unit  factor  for  green  seed  color  is  not  affected  by  exist- 
ing for  a  generation  within  a  yellow  seeded  plant  (e.g.,  the  Fj  individ- 
uals). The  two  separate  during  gamete  formation  and,  if  a  gamete 
bearing  this  factor  for  green  seed  coat  fertilizes  another  gamete  with 
this  factor,  the  resulting  seed  has  a  green  color.  The  generalization 
known  as  Mendel's  First  Law,  the  Law  of  Segregation,  may  now  be 
stated  as:  Genes  exist  in  pairs  in  individuals,  and  in  the  formation 
of  gametes  each  gene  separates  or  segregates  from  the  other  member  of 
the  pair  and  passes  into  a  different  gamete,  so  that  each  gamete  has  one, 
and  only  one,  of  each  kind  of  gene. 

In  other  experiments  Mendel  observed  the  inheritance  of  two  pairs 
of  contrasting  characters  in  a  single  cross.  He  mated  a  pure-breeding 
strain  with  round,  yellow  seeds  and  one  with  wrinkled,  green  seeds.  The 
first  filial  generation  all  had  round,  yellow  seeds,  but  when  these  were 
self-fertilized  he  found  in  the  Fo  generation  all  four  possible  combina- 
tions of  seed  color  and  shape.  When  he  counted  these  he  found  315 
round,  yellow  seeds,  108  round,  green  seeds,  101  wrinkled,  yellow  seeds, 
and  32  wrinkled,  green  seeds.  There  is  a  close  approximation  of  a  3  :  1 
ratio  for  seed  color  (416  yellow  to  140  green)  and  for  seed  shape  (423 
round  to  133  wrinkled).  Thus  the  inheritance  of  seed  color  is  inde- 
pendent of  the  inheritance  of  seed  shape;  neither  one  affects  the  other. 
When  the  two  types  of  traits  are  considered  together,  it  is  clear  that 
there  is  a  ratio  of  9  with  two  dominant  traits  (yellow  and  round):  3  with 
one  dominant  and  one  recessive  (green  and  round):  3  with  the  other 
dominant  and  recessive  (yellow  and  wrinkled):  1  with  the  two  recessive 
traits  (green   and  wrinkled).  Mendel's  Second  Law,   the   Law  of   Inde- 

Table  10.     AN  ABSTRACT  OF  THE  DATA  OBIAINED  BY  MENDEL  FROM 
HIS  BREEDING  EXPERIMENTS  WITH  GARDEN  PEAS 

FIRST 
PARENTAL  CHARACTERS  SECOND  GENERATION  RATIOS 

GENERATION 


Yellow  seeds  X  green  seeds  all  yellow  6022  yellow: 2001  green  3.01 

Round  seeds  X  wrinkled  seeds  all  round  5474  round:  1850  wrinkled  2.96 

Green  pods  X  yellow  pods  all  green  428  green:  152  yellow  2.82 

Long  stems  X  short  stems  all  long  787  long: 277  short  2.84 

Axial  flowers  X  terminal  flowers  all  axial  651  axial: 207  terminal  3.14 

Inflated  pods  X  constricted  pods  all  inflated  882  inflated :  299  constricted  2.95 

Red  flowers  X  white  flowers  all  red  705  red: 224  white  3.15 


652  GENETICS   AND    EVOLUTION 

pendent  Assortment,  may  now  be  given  as:  the  distribution  of  each  pair 
oi  genes  into  gametes  is  independent  of  the  distribution  of  any  other 
pair. 

268.        Chromosomal  Basis  of  the  Laws  of  Heredity 

Each  cell  of  every  organism  of  a  given  species  of  animal  or  plant 
contains  a  definite  number  of  chromosomes;  the  constancy  of  the  chrom- 
osome number  is  assured  by  the  precise  and  regular  events  of  mitotic 
division  (p.  39).  Many  widely  different  species  of  animals  and  plants 
have  the  same  number  of  chromosomes.  It  is  not  the  number  of  chromo- 
somes, but  the  nature  of  the  hereditary  factors  within  them,  that  dif- 
ferentiates species. 

The  constancy  of  the  chromosome  number  in  successive  generations 
of  the  same  species  is  assured  by  the  precise  separation  of  the  members  of 
the  pairs  of  homologous  chromosomes  in  the  meiotic  divisions  leading 
to  the  formation  of  gametes.  The  normal  number  of  chromosomes  for 
somatic  cells  is  reconstituted  in  fertilization  when  the  egg  and  sperm 
nuclei  fuse. 

The  laws  of  heredity  follow  directly  from  the  behavior  of  the 
chromosomes  in  mitosis,  meiosis  and  fertilization.  W^ithin  each  chromo- 
some are  numerous  hereditary  factors,  called  genes,  each  of  which  con- 
trols the  inheritance  of  one  or  more  characteristics.  Each  gene  is  located 
at  a  particular  point,  called  a  locus  (plural,  loci),  along  the  chromosome. 
Since  the  genes  are  located  in  the  chromosomes,  and  each  cell  has  two 
of  each  kind  of  chromosome,  it  follows  that  each  cell  has  two  of  each 
kind  of  gene.  The  chromosomes  separate  in  meiosis  and  recombine  in 
fertilization  and  so,  of  course,  do  the  genes  within  them.  We  currently 
believe  that  the  genes  are  arranged  in  a  linear  order  within  the  chromo- 
some; the  homologous  chromosomes  have  similar  genes  arranged  in  a 
similar  order.  When  the  chromosomes  undergo  synapsis  during  meiosis 
(p.  117)  the  homologous  chromosomes  become  attached  point  by  point 
and,  presumably,  gene  by  gene. 

269.        Allelomorphs 

Studies  of  inheritance  are  possible  only  when  there  are  two  alter- 
nate, contrasting  conditions,  such  as  Mendel's  yellow  and  green  peas  or 
round  and  wrinkled  ones,  which  are  called  allelomorphs,  or  alleles.  A 
pair  of  alleles  are  two  contrasting  traits  inherited  in  such  a  way  that  an 
individual  may  have  one  or  the  other  but  not  both.  Thus,  curly  hair 
and  straight  hair  are  alleles,  for  a  person's  hair  is  one  or  the  other,  but 
curly  and  blond  are  not  alleles,  for  hair  may  be  both  blond  and  curly. 

Brown  and  black  coat  color  are  allelomorphic  traits  in  guinea  pigs. 
Each  body  cell  of  the  guinea  pig  has  a  pair  of  chromosomes  which  con- 
tain genes  for  coat  color;  since  there  are  two  chromosomes,  there  are 
two  genes  per  cell.  A  "pure"  black  guinea  pig  (one  of  a  pedigreed  strain 
of  black  guinea  pigs)  has  two  genes  for  black  coat,  one  in  each  chromo- 
some, and  a  "pure"  brown  guinea  pig  has  two  genes  for  brown  coat.  The 


PRINCIPLES   OF   HEREDITY 


653 


genes  themselves  have  no  color;  they  are  neither  brown  nor  black. 
The  brown  gene  controls  certain  chemical  reactions  which  lead  to  the 
formation  of  a  brown  pigment  in  the  hair  cells,  whereas  the  black  gene 
directs  the  chemical  reactions  toward  the  formation  of  black  pigment 
in  the  hair  cells.  In  working  genetic  problems,  letters  are  conventionally 
used  as  symbols  for  the  genes.  A  pair  of  genes  for  black  pigment  is 
represented  as  BB,  and  a  pair  of  genes  for  brown  pigment  by  bb.  A 
capital  letter  is  used  for  one  gene  and  the  corresponding  lower  case  letter 
is  used  to  represent  the  gene  for  the  contrasting  trait,  the  allele. 

270.       A  Monohybrid  Cross 

The  events  of  a  hypothetical  mating  of  a  pure-bred  brown  male 
guinea  pig  (bb)  with  a  pure  black  female  (BB)  are  given  in  Figure  32.1. 
During  meiosis  in  the  male  the  two  bb  genes  separate  and  each  sperm 


Brown 


Black 


Parents 


§)     Gometes    (W) 


Block 


Black 


Black 


Brown 


1  v.*/^.«j?" 


Figure  32.1.     An  example  of  a  monohybrid  cross:   the  mating  of  a  brown  with  a 
black  guinea  pig.  (Villee:  Biology.) 


554  GENETICS   AND   EVOLUTION 

receives  only  one  b  gene.  Similarly,  during  meiosis  in  the  female,  the 
BB  genes  separate  and  each  egg  receives  only  one  B  gene.  There  is  only 
one  type  of  sperm,  those  containing  a  b  gene,  and  one  type  of  egg,  those 
with  a  B  gene,  and  their  union  leads  to  a  single  type  of  individual,  Bb. 
Thus,  all  the  offspring,  the  Fj  generation,  are  similar.  Since  these  in- 
dividuals have  one  gene  for  black  color  and  one  gene  for  brown  color, 
you  might  guess  that  the  offspring  would  be  dark  brown,  or  gray,  or 
perhaps  spotted.  However,  all  the  Fi  individuals  are  just  as  black  as  the 
mother.  The  black  gene  is  dominant  to  the  brown  one  and  produces 
black  coat  color  even  in  the  presence  of  the  other  gene.  The  brown  gene 
is  said  to  be  recessive  to  the  black  one.  By  convention,  the  dominant 
gene  is  symbolized  by  a  capital  letter  and  the  recessive  gene  by  the  cor- 
responding lower  case  letter.  The  phenomenon  of  dominance  supplies 
part  of  the  explanation  as  to  how  it  is  that  an  offspring  may  resemble 
one  of  its  parents  much  more  than  the  other,  despite  the  fact  that  both 
parents  make  equal  contributions  to  its  genetic  constitution. 

An  animal  or  plant  with  two  genes  exactly  alike,  two  blacks  (BB)  or 
two  browns  (bb),  is  said  to  be  homozygous  or  "pure"  for  the  character. 
An  organism  with  one  dominant  and  one  recessive  gene  (Bb)  is  said  to 
be  heterozygous  or  "hybrid."  Thus,  in  the  mating  under  consideration 
the  black  and  brown  parents  were  homozygous,  BB  and  bb,  respectively, 
and  the  offspring  in  the  F^  were  all  heterozygous,  Bb.  Recessive  genes 
are  those  which  will  produce  their  effect  only  when  homozygous;  a 
dominant  gene  is  one  which  will  produce  its  effect  whether  it  is  ho- 
mozygous or  heterozygous. 

In  the  process  of  gamete  formation  in  these  heterozygous  black  Fi 
guinea  pigs,  the  chromosome  containing  the  B  gene  undergoes  synapsis 
with,  and  then  separates  from,  the  homologous  chromosome  containing 
the  b  gene,  so  that  each  sperm  or  egg  has  a  B  gene  or  a  b  gene.  No  sperm 
or  egg  is  without  one  or  the  other  and  none  has  both.  Since  there  are 
two  kinds  of  eggs  and  two  kinds  of  sperm,  the  mating  of  two  of  these 
heterozygous  black  guinea  pigs  permits  four  different  combinations  of 
eggs  and  sperm.  To  see  these  possible  combinations  of  eggs  and  sperm 
it  is  conventional  to  arrange  them  in  a  Punnett  square  (Fig.  32.1),  de- 
vised by  the  English  geneticist,  R.  C.  Punnett.  Gametes  containing  B 
genes  and  ones  containing  b  genes  are  formed  in  equal  numbers.  There 
is  no  special  attraction  or  repulsion  between  an  egg  and  a  sperm  con- 
taining similar  genes;  an  egg  containing  a  B  gene  is  just  as  likely  to  be 
fertilized  by  a  B  sperm  as  by  a  b  sperm.  The  four  possible  combinations 
occur  with  equal  frequency. 

The  possible  types  of  eggs  are  written  across  the  top  of  the  Punnett 
square  and  the  possible  types  of  sperm  are  arranged  down  its  left  side, 
then  the  squares  are  filled  in  with  the  resulting  zygote  combinations 
(Fig.  32.1).  Three-fourths  of  the  offspring  are  either  BB  or  Bb,  and  con- 
sequently have  a  black  coat  color,  and  one-fourth  are  bb,  with  a  brown 
coat  color.  This  three  to  one  ratio  is  characteristically  obtained  in  the 
second  generation  of  a  monohybrid  cross,  i.e.,  a  mating  of  two  individ- 
uals which  differ  in  a  single  trait  governed  by  a  single  pair  of  genes.  The 


PRINCIPLES   OF   HEREDITY  655 

genetic  mechanism  responsible  for  the  3:1  ratios  obtained  by  Mendel  in 
his  pea  breeding  experiments  is  now  evident. 

The  appearance  ot  an  individual  with  respect  to  a  certain  trait,  the 
end  result  ot  the  action  ot  the  gene,  is  known  as  its  phenotype;  the 
individual's  genetic  constitution  is  called  its  genotype.  In  the  Fo  gen- 
eration ot  the  guinea  pig  mating,  the  phenotypic  ratio  is  3  black  :  1 
brown;  the  genotypic  ratio  is  1  BB  :  2  Bb  :  1  bb.  Guinea  pigs  which  are 
BB  and  Bb  have  similar  phenotypes— both  have  black  coat  color— but 
they  have  different  genotypes  which  could  be  distinguished  only  by 
further  breeding  tests.  It  is  also  possible,  as  we  shall  see  later,  for  in- 
dividuals to  have  similar  genotypes  but  different  phenotypes. 

271 .  Laws  of  Probability 

It  is  important  to  realize  that  all  genetic  ratios  are  expressions  of 
probability,  based  on  the  laws  of  chance  or  probability;  they  do  not 
express  certainties.  If  two  heterozygous  black  guinea  pigs  are  mated  and 
have  exactly  four  offspring  there  is  no  guarantee  that  there  will  be 
exactly  three  black  ones  and  one  brown  one.  All  might  be  black,  or  all 
might  be  brown,  though  this  would  occur  only  rarely  (one  can  calculate 
from  the  laws  of  probability  that  there  is  one  chance  in  256  of  having 
four  brown  guinea  pigs  in  such  a  mating).  Any  of  the  combinations  of 
3  black  :  1  brown,  2  black  :  2  brown,  or  1  black  :  3  brown  might  appear. 
But  if  enough  similar  matings  are  made  to  produce  a  total  of  400 
otfspring,  the  ratio  of  black  to  brown  among  the  offspring  will  be  very 
close  to  300  to  100.  The  theoretical  3  :  1  ratio  is  approximated  more 
and  more  exactly  as  the  total  number  of  individuals  increases;  this  is 
predicted  by  the  laws  of  probability  and  actually  found  when  genetic 
tests  are  made.  One  can  state,  perhaps  more  exactly,  that  in  the  mating 
of  two  individuals  heterozygous  for  a  given  trait  there  are  three  chances 
out  of  four  that  any  particular  offspring  will  show  the  dominant  trait 
and  one  chance  out  of  four  that  it  will  show  the  recessive  one.  Each 
mating,  each  union  of  an  egg  and  a  sperm,  is  an  independent  event 
which  is  not  influenced  by  the  results  of  previous  matings.  No  matter 
how  many  black-coated  offspring  have  been  produced  by  the  mating  of 
two  heterozygous  black  ones,  the  probability  that  the  next  offspring  to 
be  born  will  have  a  brown  coat  is  one  chance  in  four,  and  the  prob- 
ability that  it  will  have  a  black  coat  is  three  chances  in  four. 

272.  Test  Crosses 

In  the  ¥-2  generation  of  a  monohybrid  cross,  one-third  of  the  in- 
dividuals with  the  dominant  phenotype  are  homozygous  and  two-thirds 
are  heterozygous.  In  the  guinea  pig  mating  (Fig.  32.1)  the  black-coated 
individuals  in  the  F2  generation  include  some  with  the  genotype  BB  and 
some  with  the  genotype  Bb.  These  can  be  distinguished  by  a  test  cross, 
in  which  the  black-coated  guinea  pig  is  mated  with  a  brown-coated  one 
(genotype  bb).  If  all  of  the  offspring  are  black,  the  parent  is  probably 


556  GENETICS   AND    EVOLUTION 

homozygous  (BB),  but  if  any  of  the  offspring  are  brown  the  black  parent 
is  heterozygous  (Bb). 

Test  crosses  are  of  obvious  importance  to  the  commercial  breeder 
of  animals  or  plants  who  is  trying  to  establish  a  strain  which  will  "breed 
true"  for  a  certain  trait.  Formerly,  farmers  and  commercial  breeders 
could  select  plants  to  be  used  lor  seed,  or  animals  to  be  used  as  breeding 
stock,  only  by  their  phenotypes.  Without  some  means  of  differentiating 
homozygous  and  heterozygous  individuals  this  method  is  unsatisfactory, 
for  the  heterozygous  individuals  would  bear  some  offspring  with  the 
recessive  trait. 

In  the  more  modern  method,  the  breeder  tests  the  genotypes  of  his 
breeding  stock  by  observing  the  qualities  of  their  offspring.  If  the  off- 
spring have  the  traits  desired,  then  these  same  parents  are  used  for 
further  breeding.  Two  bulls,  for  example,  may  look  equally  healthy  and 
vigorous,  yet  one  may  have  daughters  with  qualities  of  milk  production 
which  are  distinctly  superior  to  the  daughters  of  the  other  bull.  By  this 
method,  called  progeny  selection,  the  desirable  qualities  of  a  strain  of 
animals  can  be  increased  rapidly.  One  geneticist,  for  example,  by  progeny 
selection  over  a  period  of  eight  years  increased  the  average  annual  egg 
production  of  a  flock  of  hens  from  114  to  200. 

273.  Incomplete  Dominance 

In  many  different  species  and  for  a  variety  of  traits  it  has  been 
found  that  one  gene  is  not  completely  dominant  to  the  other.  Heter- 
ozygous individuals  have  a  phenotype  which  can  be  distinguished  from 
that  of  the  homozygous  dominant;  it  may  be  intermediate  between  the 
phenotypes  of  the  two  parental  strains.  The  mating  of  red  shorthorn 
cattle  with  white  ones  yields  offspring  which  have  an  intermediate, 
roan-colored  coat.  The  mating  of  two  roan-colored  cattle  yields  offspring 
in  the  ratio  of  1  red  :  2  roan  :  1  white;  thus  the  genotypic  and  pheno- 
typic  ratios  are  the  same;  each  genotype  has  a  recognizably  different 
phenotype.  This  phenomenon,  called  incomplete  dominance,  is  found 
with  a  number  of  traits  in  different  animals  and  with  some  human 
characteristics.  Studies  of  a  number  of  human  diseases  inherited  by  reces- 
sive genes— sickle  cell  anemia,  Mediterranean  anemia,  gout,  epilepsy  and 
many  others— have  shown  that  the  individuals  who  are  heterozygous  for 
the  trait  have  slight  but  detectable  differences  from  the  homozygous 
normal  individual. 

274.  A  Dihybrid  Cross 

The  mating  of  individuals  that  differ  in  two  traits,  called  a  dihybrid 
cross,  follows  the  same  principles  as  those  of  the  simpler  monohybrid 
cross,  but  since  there  is  a  greater  number  of  types  of  gametes,  the  num- 
ber of  different  types  of  zygotes  is  correspondingly  larger. 

If  two  pairs  of  genes  are  located  in  different  (nonhomologous) 
chromosomes,  each  pair  is  inherited  independently  of  the  other;  each 
pair  separates  during  meiosis  independently  of  the  other.  Another  pair 


PRINCIPLES   OF   HEREDITY 


657 


of  genes  in  the  guinea  pig  governs  the  length  of  the  hair  in  the  coat;  the 
gene  for  short  hair  (S)  is  dominant  to  the  gene  for  long  hair  (s).  The 
genes  for  hair  color  and  hair  length  are  located  in  different  chromosomes. 
Each  guinea  pig  has  two  of  each  kind  of  gene;  thus  the  genotype  of  a 
homozygous  black,  short-haired  animal  is  BBSS  and  the  genotype  of 
a  homozygous  brown,  long-haired  animal  is  bbss.  The  black,  short-haired 
animal  produces  only  one  kind  of  gamete,  for  all  of  them  are  BS.  Simi- 
larly, the  brown,  long-haired  animals  produce  only  bs  eggs  or  sperm. 

The  mating  of  a  black,  short-haired  animal   with   a  brown,  long- 
haired one  produces  offspring  all  of  which  have  short,  black  hair;  they 


Block,  Short-holred 


Brown,  Long-hoired 


3   Brown,  Short-hoired 

1  Brown,  Long -haired 

Figure  32  2  An  example  of  a  dihybrid  cross:  the  mating  of  a  black,  short-haired 
guinea  pig  and  a  brown,  long-haired  one,  illustrating  independent  assortment.  (Villee: 
Biology.) 


558  GENETICS    AND   EVOLUTION 

are  heterozygous  for  both  hair  length  and  hair  color  genes  and  have  the 
genotype  BlaSs.  Each  ol  the  h\  individuals  will  produce  four  kinds  of 
gametes,  BS,  Bs,  bS  and  bs,  and  there  will  be  equal  numbers  of  each 
type.  When  two  of  these  Fi  individuals  are  mated,  there  will  be  sixteen 
possible  combinations  in  the  Fo  (Fig.  32.2),  with  a  phenotypic  ratio  of 
y  black,  short  :  3  black,  long  :  3  brown,  short  :  1  brown,  long.  This 
9:3:3:1  ratio  is  characteristic  of  the  second  generation  of  a  cross  of 
individuals  differing  in  two  traits  whose  genes  are  located  in  non- 
homologous chromosomes.  This  is,  of  course,  a  probability  ratio,  which 
means  that  there  are  nine  chances  out  of  sixteen  that  any  particular 
offspring  will  have  black,  short  hair,  three  chances  out  of  sixteen  that  it 
will  have  black,  long  hair,  three  chances  out  of  sixteen  that  it  will  have 
brown,  short  hair,  and  one  chance  in  sixteen  that  it  will  have  brown, 
long  hair.  The  genetic  mechanism  underlying  Mendel's  Second  Law, 
the  Law  of  Independent  Assortment,  should  now  be  clear. 

The  results  of  crosses  with  three  or  more  different  pairs  of  genes 
may  be  predicted  by  similar  reasoning.  The  Fi  individuals  of  a  trihybrid 
cross  will  produce  eight  different  kinds  of  gametes  in  equal  numbers, 
and  the  random  union  of  eight  types  of  sperm  and  eight  types  of  eggs 
gives  64  different  combinations  of  genes  in  the  Fo  generation.  In  the  pea 
plant  studied  by  Mendel,  the  crossing  of  a  plant  with  round,  yellow 
seeds  and  long  stems  (YYRRLL)  and  a  plant  with  wrinkled,  green  seeds 
and  short  stems  (yyrrll)  yields  Fi  individuals  with  the  genotype  YyRrLI, 
all  with  round,  yellow  seeds  and  long  stems.  When  these  plants  are  self- 
fertilized,  offspring  are  produced  in  the  ratio  of  27  yellow,  round, 
long  :  9  yellow,  round,  short  :  9  yellow,  wrinkled,  long  :  9  green, 
round,  long  :  3  yellow,  wrinkled,  short  :  3  green,  round,  short  :  3  green, 
wrinkled,  long  :  and  1  green,  wrinkled  short. 

Set  up  a  Punnett  square  with  the  eight  types  of  eggs  across  the  top 
and  the  eight  types  of  sperm  down  the  sides.  Fill  in  the  64  squares  with 
the  appropriate  Fo  genotypes  and  add  up  the  phenotypes.  Compare  the 
phenotypic  ratio  you  obtain  with  the  one  given  here. 

275.        Problem  Solving 

The  science  of  genetics  resembles  mathematics  in  that  when  one 
has  a  firm  grasp  of  the  few  basic  principles  involved  he  can  solve  a  wide 
variety  of  problems.  These  basic  principles  include:  (1)  Inheritance  is 
biparental;  both  parents  contribute  to  the  genetic  constitution  of  the 
offspring.  (2)  Genes  are  not  altered  by  existing  together  in  a  hetero- 
zygote.  (3)  Each  individvial  has  two  of  each  kind  of  gene,  but  each  gamete 
has  only  one  of  each  kind.  (4)  Two  pairs  of  genes  located  in  different 
chromosomes  are  inherited  independently.  (5)  Gametes  unite  at  random; 
there  is  neither  attraction  nor  repulsion  between  an  egg  and  a  sperm 
containing  identical  genes. 

In  working  genetics  problems,  it  is  helpful  to  use  the  following  pro- 
cedure: 

1.  Write  down  the  symbols  used  for  each  gene. 

2.  Determine   the   genotypes  of   the  parents,   deducing   them  from 


PRINCIPLES   OF   HEREOnY 


659 


the  phenotypes  o£  the  parents  and,  if  necessary,  from  the  phenotypes 
of  the  offspring. 

3.  Derive  all  of  the  possible  types  of  gametes  each  parent  would 
produce. 

4.  Prepare  the  appropriate  Punnett  square  and  write  the  possible 
types  of  eggs  across  its  top  and  the  types  of  sperm  along  its  side. 

5.  Fill  in  the  squares  with  the  appropriate  genotypes  and  read  off 
the  genotypic  and  phenotypic  ratios  of  the  offspring. 

As  an  example  of  the  method  of  solving  a  problem  in  genetics,  let 
us  consider  the  following:  The  length  of  fur  in  cats  is  an  inherited 
trait;  the  gene  for  long  hair  (I),  as  in  Persian  cats,  is  recessive  to  the  gene 
for  short  hair  (L)  of  the  common  tabby  cat.  Let  us  suppose  that  a  short- 
haired  male  is  bred  to  three  different  females,  two  of  which,  A  and  C, 
are  short-haired  and  one,  B,  is  long-haired  (Fig.  32.3).  Cat  A  gives  birth 
to  a  short-haired  kitten,  but  cats  B  and  C  each  produce  a  long-haired 
kitten.  What  offsj^ring  could  be  expected  from  further  mating  of  this 
male  with  these  three  females? 

Since  the  longhaired  trait  is  recessive  we  know  that  all  the  long- 


00 

Cat  A,  short-haired 


Short -haired  Kitten 


Short-haired  mr  le  caJb 


00 

Cats,  long-h.cLxrzci 


m 

Long-haired  Kitten 


00 

Ca-tC,  short-haired 


00 

Lorjg-haired  kitten. 


Figure  32.3.     An  example  of  problem-solving  in  genetics:  deducing  parental  geno- 
types from  the  phenotypes  of  the  ofEspring.  See  text  for  discussion. 


560  GENETICS   AND   EVOLUTION 

haired  cats  imisi  be  homozygous.  We  can  deduce,  then,  that  cat  B  and 
the  kittens  produced  by  cats  B  and  C  have  the  genotype  II.  All  the  short- 
haired  cats  have  at  least  one  L  gene.  The  fact  that  any  of  the  offspring 
of  the  male  cat  has  long  hair  proves  that  he  is  heterozygous,  with  the 
genotype  LI,  The  kitten  produced  by  cat  B  received  one  I  gene  from 
its  mother  but  must  have  received  the  other  from  its  father.  The  fact 
that  cat  C  gave  birth  to  a  long-haired  kitten  proves  that  she,  too,  is 
heterozygous,  and  has  the  genotype  LI.  It  is  impossible  to  decide,  from 
the  data  at  hand,  whether  the  short-haired  cat  A  is  homozygous  LL  or 
heterozygous  LI.  A  test  cross  with  a  long-haired  male  would  be  helpful 
in  deciding  this.  Further  mating  of  the  short-haired  male  with  cat  B 
would  give  half  long-haired  ancf  half  short-haired  kittens,  whereas  fur- 
ther mating  of  the  short-haired  male  with  cat  C  would  give  three  times 
as  many  short-haired  kittens  as  long-haired  ones. 

276.        The  Genetic  Determination  of  Sex 

The  sex  of  an  organism  is  a  genetically  determined  trait.  There  is 
an  exception  to  the  general  rule  that  all  homologous  pairs  of  chromo- 
somes are  identical  ni  size  and  shape:  the  so-called  sex  chromosomes. 
In  one  sex  of  each  species  of  animals  there  is  either  an  unpaired  chromo- 
some or  an  odd  pair  of  chromosomes,  the  two  members  of  which  differ 
in  size  and  shape.  In  most  species  the  females  have  two  identical  chromo- 
somes, called  X  chromosomes,  and  males  have  either  a  single  X  chro- 
mosome or  one  X  plus  a  generally  somewhat  smaller  one  called  the  Y 
chromosome.  The  existence  of  these  unpaired  chromosomes  was  discov- 
ered by  C.  E.  McClung  in  1902,  when  he  was  studying  the  process  of 
meiosis  in  the  testes  of  grasshoppers.  He  made  the  shrewd  guess  that 
these  might  play  some  role  in  sex  determination.  In  a  few  animals,  the 
butterflies  and  birds,  the  system  is  reversed  and  the  male  has  two  X 
chromosomes  and  the  female  one  X  and  one  Y.  The  Y  chromosome 
usually  contains  few  or  no  genes  and  in  most  species  the  X  and  Y 
chromosomes  are  distinguished  by  their  different  size  and  shape.  Yet  in 
meiosis  the  X  and  Y  chromosomes  act  like  homologous  chromosomes; 
they  undergo  synapsis,  separate,  pass  to  opposite  poles,  and  become  in- 
corporated into  different  gametes  (Fig.  32.4).  Human  beings  have  23 
pairs  of  chromosomes;  males  have  22  pairs  of  ordinary  chromosomes, 
called  autosomes,  one  X  and  one  Y  chromosome,  whereas  females  have 
22  pairs  of  autosomes  and  two  X  chromosomes. 

It  is  not  the  presence  of  the  Y  chromosome,  however,  which  deter- 
mines maleness,  for  in  a  number  of  species  the  male  has  no  Y  chromo- 
some at  all,  just  a  single  X  chromosome.  Whether  an  individual  is  male 
or  female  is  determined  by  the  presence  of  one  or  two  X  chromosomes. 

The  experiments  of  C.  B.  Bridges  revealed  that  the  sex  of  fruit 
flies,  Drosophila,  is  determined  by  the  ratio  of  the  number  of  X  chromo- 
somes to  the  number  of  haploid  sets  of  autosomes.  Males  have  one  X 
and  two  haploid  sets  of  autosomes,  a  ratio  of  1  :  2,  or  0.5.  Females  have 
two  X  and  two  haploid  sets  of  autosomes,  a  ratio  of  2  :  2,  or  1.0.  By 
genetic  techniques  possible  in  fruit  flies,  Bridges  established  abnormal 


PRINCIPLES  OF  HEREDITY 


661 


flies  with  one  X  and  three  sets  of  autosomes.  These  flies,  with  a  ratio  of 
0.33,  had  all  their  male  characteristics  exaggerated;  Bridges  called  them 
"supermales."  Other  abnormal  individuals,  with  three  X  and  two  sets 
of  autosomes  were  "superfemales,"  with  all  the  female  characteristics 
exaggerated.  Individuals  with  two  X  chromosomes  and  three  sets  of 
autosomes,  a  ratio  of  0.67,  were  intersexes,  with  characters  intermediate 
between  those  of  normal  males  and  normal  females.  All  of  these  unusual 
flies,  supermales,  superfemales  and  intersexes,  were  sterile. 

All  of  the  eggs  produced  by  XX  females  have  one  X  chromosome. 
Half  of  the  sperm  produced  by  XY  males  contain  an  X  chromosome 
and  half  contain  a  Y  chromosome.  The  fertilization  of  an  X-bearing 


Diploid  cells 
of  parents 


Synapsis 


Anaphase  of 
meiotic 
division 


Egg: 

one  type 


Sperm.: 
two  types 


OFFspririg 

equal 

numbers 

of  males 

and 

females 


Figure  32.4.     Diagram  illustrating  the  transmission  of  the  sex  chromosomes  of  the 
fruit  fly. 


662  GENETICS   AND   EVOLUTION 

egg  by  an  X-bearing  sperm  results  in  an  XX,  female,  zygote,  and  the 
feriili/ation  ol  an  X-bearing  egg  by  a  Y-bearing  sperm  results  in  an  XY, 
male,  zygote.  Since  there  are  equal  numbers  ot  X-  and  Y-bearing  sperm, 
there  are  equal  numbers  of  male  and  female  offspring.  In  human  beings, 
there  are  api)roximately  107  males  born  for  every  100  females,  and  the 
ratio  at  conception  is  said  to  be  even  higher,  about   114  males  to  100 
females.  One  possible  explanation  of  the  numerical  discrepancy  is  that 
the  \   chromosome  is  smaller  than  the  X  chromosome,  and  a  sperm  con- 
taining a  Y  chromosome,  being  a  little  lighter  and  perhaps  able  to  swim 
a  little  faster  than  a  sperm  containing  an  X  chromosome,  would  win  the 
race  to  the  egg  slightly  more  than  half  of  the  time.  Both  during  the 
period  of  intrauterine  development  and  after  birth,  the  death  rate  among 
males  is  slightly  greater  than  that  among  females,  so  that  by  the  age  of 
ten  or  twelve  there  are  equal  numbers  of  males  and  females.  In  later 
life  there  are  more  females  than  males  in  each  age  group. 

277.        Sex-Linked  Characteristics 

The  X  chromosome  contains  many  genes,  and  the  traits  controlled 
by  these  genes  are  said  to  be  sex-linked,  because  their  inheritance  is 
linked  with  the  inheritance  of  sex.  The  Y  chromosome  contains  very 
few  genes,  so  that  the  somatic  cells  of  an  XY  male  contain  only  one  of 
each  kind  of  gene  in  the  X  chromosome  instead  of  two  of  each  kind  as 
in  XX  females.  A  male  receives  his  single  X  chromosome,  and  thus  all 
of  his  genes  for  sex-linked  traits,  from  his  mother.  Females  receive  one 
X  from  the  mother  and  one  from  the  father.  In  writing  the  genotype 
of  a  sex-linked  trait  it  is  customary  to  write  that  of  the  male  with  the 
letter  for  the  gene  in  the  X  chromosome  plus  the  letter  Y  for  the  Y 
chromosome.  Thus  AY  would  represent  the  genotype  of  a  male  with  a 
dominant  gene  for  trait  "A"  in  his  X  chromosome. 

The  phenomenon  of  sex-linked  traits  was  discovered  by  T.  H. 
Morgan  and  C.  B.  Bridges  in  the  fruit  fly,  Drosophila.  These  flies  nor- 
mally have  eyes  with  a  dark  red  color,  but  Morgan  and  Bridges  dis- 
covered a  strain  with  white  eyes.  The  gene  for  white  eye,  w,  proved  to  be 
recessive  to  the  gene  for  red  eye,  W,  but  in  certain  types  of  crosses  the 
male  offspring  had  eyes  of  one  color  and  the  female  offspring  had  eyes 
of  the  other  color.  Morgan  reasoned  that  the  peculiarities  of  inheritance 
could  be  explained  if  the  genes  for  eye  color  were  located  in  the  X 
chromosome;  later  work  has  proven  the  correctness  of  this  guess.  Cross- 
ing a  homozygous,  red-eyed  female  with  a  white-eyed  male  (WW  X  wY) 
produces  offspring  all  of  which  have  red  eyes  (Ww  females  and  WY  males). 
But  crossing  a  iiomozygous  white-eyed  female  with  a  red-eyed  male 
(ww  X  WY)  yields  red-eyed  females  and  white-eyed  males  (Ww  and  wY) 
(Fig.  :^2.5). 

In  man,  hemophilia  (bleeder's  disease)  and  color-blindness  are  sex- 
linked  traits.  About  4  men  in  every  hundred  are  color-blind,  but  some- 
what less  than  one  per  cent  of  all  women  are  color-blind.  Only  one  gene 
for  color-blindness  produces  the  trait  in  males,  but  two  such  genes  (the 
trait  is  recessive)  are  necessary  to  produce  a  color-blind  female. 


PRINCIPLES   Of   HEREDITY 


663 


Pa-rents 

Female 
white -eyed 


GaineteS 


Offspring 

Female 
red- eyed 


Male, 
red- eyed. 


Male 
\A?hite-eyed 


Female 
red- eyed. 


Male 
red-  eyed 


Female  Female  Male  Male 

red-eyed       red-eyed       red- eyed   v/hiteeyed 


Figure   32.5.     Diagram    illustrating  sex-linked   inheritance,   the  inheritance  of  red 
vs.  white  eye  color  in  fruit  flies.  See  text  for  discussion. 

Not  all  the  characters  which  differ  in  the  two  sexes  are  sex-linked. 
Some,  the  sex-influenced  traits,  are  inherited  by  genes  located  in  auto- 
somes rather  than  X  chromosomes,  but  the  expression  of  the  trait,  the 
action  of  the  gene  which  produces  the  phenotype,  is  altered  by  the  sex 
of  the  animal,  presumably  by  the  action  of  one  of  the  sex  hormones.  The 
presence  or  absence  of  horns  in  sheep,  mahogany-and-white  spotted 
coat  vs.  red-and-white  spotted  coat  in  Ayrshire  cattle,  and  pattern  bald- 
ness in  man  are  examples  of  such  sex-infiuenced  traits. 


278.        Linkage  and  Crossing  Over 

In  the  discussion  of  Mendel's  Law  of  Independent  Assortment,  we 
stressed  the  fact  that  this  law  is  valid  only  for  two  pairs  of  genes  located 
in  different,  nonhomologous  chromosomes.  The  ratio  of  9:3:3:1  is  ob- 
tained in  the  F..  generation  of  a  dihybrid  cross  only  if  the  pairs  of  genes 
are  located  in  different  chromosomes.  Since  there  are  many  hundreds  of 
inherited  traits  and  a  very  limited  number  of  pairs  of  chromosomes 
(23  in  man,  4  in  the  fruit  fly),  it  is  obvious  that  each  chromosome  must 
contain  many  genes.  All  of  the  genes  located  in  the  same  chromosome 
tend  to  be  inherited  as  a  group  and  are  said  to  be  linked.  In  meiosis 


564  GENETICS    AND   EVOLUTION 

the  members  of  the  pairs  of  homologous  chromosomes  separate  as  units 
and  go  to  opposite  poles.  Hence,  all  of  the  genes  lying  in  one  chromo- 
some go  to  one  pole  and  become  incorporated  into  one  gamete,  and  all 
of  the  genes  in  the  other  member  of  the  homologous  pair  go  to  the 
opposite  pole  and  become  incorjjorated  in  another  gamete. 

The  linkage  between  the  genes  in  a  given  chromosome  is  usually 
not  complete.  During  the  process  of  synapsis,  when  the  homologous 
chromosomes  are  twisted  around  one  another  and  attached  point  by 
point,  they  frequently  exchange  whole  segments  of  chromosomal  ma- 
terial together  with  the  genes  located  within  that  part  of  the  chromo- 
some. The  exact  mechanism  of  this  exchange  is  still  unknown,  but  it 
appears  to  occur  at  random  along  the  length  of  the  chromosome.  The 
chance  that  an  exchange  of  segments  will  occur  between  the  loci  of 
any  two  genes  in  a  chromosome  depends  on  the  distance  between  the 
loci:  the  greater  the  distance,  the  greater  the  opportunity  for  exchange. 
The  exchange  of  segments  between  homologous  chromosomes,  called 
crossing   over,   makes  possible  new  combinations  of   linked  genes. 

1  he  genes  for  plant  size  and  fruit  shape  in  tomatoes  are  located 
in  the  same  chromosome  and  therefore  are  linked;  they  tend  to  be  in- 
herited together.  The  gene  for  tall  plants  (T)  is  dominant  to  dwarf  (t) 
and  the  gene  for  spherical  fruit  (S)  is  dominant  to  the  one  for  pear- 
shaped  fruit  (s).  The  mating  of  a  homozygous  TTSS  plant  with  a  homo- 
zygous ttss  plant  yields  an  Fi  generation  all  of  which  are  TtSs,  tall 
plants  with  spherical  fruit  (Fig.  32.6).  So  far,  there  appears  to  be  no 
difference  from  the  ordinary  dihybrid  cross  in  which  the  genes  are 
located  in  different  chromosomes.  The  difference  becomes  apparent, 
however,  when  one  of  these  TtSs  plants  is  crossed  to  a  homozygous  re- 
cessive one,  ttss.  If  the  two  pairs  of  genes  were  located  in  different 
chromosomes,  the  four  classes  of  offspring— tall,  spherical;  dwarf,  spher- 
ical; tall,  pear;  and  dwarf,  pear— would  be  found  in  equal  numbers. 
If  the  genes  were  completely  linked,  that  is,  if  no  crossing  over  occurred 
between  them,  only  two  classes,  tall  plants  with  spherical  fruit  and 
dwarf  plants  with  pear-shaped  fruit,  would  be  found  and  these  two 
classes  would  occur  in  equal  numbers.  When  the  cross  is  actually  made, 
most  of  the  offspring  are  either  tall  plants  with  spherical  fruit  or  dwarf 
plants  with  pear-shaped  fruit  (the  non-crossovers)  and  only  a  few  are 
either  tall  plants  with  pear-shaped  fruit  or  dwarf  plants  with  spherical 
fruit  (the  crossovers).  Crossing  over  between  these  two  pairs  of  genes 
occurs  in  20  per  cent  of  the  chromosomes;  the  offspring  are  found  in 
the  ratio  of  40  tall  plants  with  spherical  fruit:  40  dwarf  plants  with 
pear-shaped  fruit:  10  tall  plants  with  pear-shaped  fruit:  10  dwarf  plants 
with  spherical  fruit.  The  distance  between  two  genes  in  a  chromosome 
is  measured  in  units  of  the  percentage  of  crossing  over  that  occurs 
between  them;  thus  T  and  S  are  said  to  be  20  units  apart  on  the 
chromosome. 

The  facts  of  crossing  over  provide  proof  that  the  genes  lie  in  a 
linear  order  in  the  chromosomes.  If  three  genes.  A,  B  and  C,  lie  in  the 
same  chromosome  and  tests  show  that  crossing  over  between  A  and  B 
occurs  5  per  cent  of  the  time  (A  and  B  are  5  units  apart)  and  cros.sing 


PRINCIPLES   Of  HEREDITY  QQP, 

over  between  B  and  C  occurs  3  per  cent  of  the  time  (B  and  C  are 
three  units  apart),  the  percentage  of  crossing  over  between  A  and  C 
is  found  to  be  either  8  per  cent  or  2  per  cent.  If  it  is  8  per  cent,  C  lies 

8 

to  the  right  of  B  and  the  order  is:  A     B^.  If  A  and  C  are  two  units 


apart,  then  C  lies  between  A  and  B  and  the  order  is:  A     C     B.  In  all 

2  3 

such  tests,   the  percentage  of  crossing  over  between  the  first  and  third 
genes  is  either  the  sum  or  the  difference  between  the  percentages  of 


p.  Tall  Tomoto  Plant 

Spherlcol    Fruit 


Dwarf  Tomato  Plant 
Pear-shaped   Fruit 


All  Toll   ond  I  Spherical  Dwarf,  Pear-shaped 


GAMETES 


t 


Non-crossovers    Crossovers 


Toll 
Spherical 

40 


Dwarf 
Peor-shoped 

40 


Tall 
Pear-shoped 

10 


Dwarf 
Spherical 

10 


Figure  32.6.  Diagram  of  a  cross  involving  linkage  and  crossing  over.  The  genes 
for  tall  vs.  dwarf  plants,  and  spherical  vs.  pear-shaped  fruits  in  tomatoes  are  linked; 
they  are  located  in  the  same  chromosome.  (Villee:   Biology.) 


566  GENETICS   AND   EVOLUTION 

crossing  over  ol  the  first  and  second,  and  the  second  and  third.  These 
facts  are  best  exphiined  by  the  assumption  that  the  genes  lie  in  a 
linear  order  in  the  chromosome. 


279.        Chromosome  Maps 

All  the  genes  in  a  particular  chromosome  constitute  a  linkage 
group.  In  all  the  species  tested  the  number  of  linkage  groups  deter- 
mined by  genetic  tests  and  the  number  of  pairs  of  chromosomes  ob- 
served under  the  microscope  are  the  same.  This  is  another  bit  of 
evidence  that  the  genes  are  located  in  the  chromosomes  and  not  else- 
where within  the  cell.  The  genes  which  make  up  a  linkage  group  re- 
main constant  from  generation  to  generation  and  are  altered  only  by 
some  major  change  in  chromosome  morphology  such  as  a  translocation 
(p.  685),  in  which  a  piece  of  one  chromosome  breaks  off  and  becomes 
attached  to  a  different,  nonhomologous  chromosome.  The  linkage  be- 
tween two  particular  genes,  such  as  the  linkage  between  tall  and 
spherical  in  tomatoes,  is  called  a  specific  linkage.  The  specific  linkage 


lOi 


r\ 


Ga.nxeT;iz-S : 


Single  crossovers 


Gametes ' 


DouLle  crossovers 
Figure    32.7.     Diagram    illustrating    crossing   over,    the    exchange    of    segments    of 


chromosomes  during  synapsis.  See  text   for  discussion. 


PRINCIPLES   Of   HEREDITY 


667 


between  two  genes  is  changed  by  crossing  over,  e.g.,  tall  becomes 
linked  to  pear-shaped,  and  then  those  two  particular  genes  tend  to  be 
inherited  together  until  in  some  subsequent  generation  another  crossing 
over  occurs. 

In  the  species  whose  inheritance  has  been  studied  most  extensively, 
fruit  flies,  corn  and  mice,  the  data  on  crossing  over  have  been  assem- 
bled,   and    chromosome    maps,    showing    the    relative    location    of    the 


13 


14 


15   i 


16 


17 


0.0 
0.0 
0.0 
0.1 

8.1 

1.5 
1.7 

3.0 
4.6 
5.5 
6.9 
7.5 

no 

137 
15.0 

178 
182 

20.0 
21.0 

23.0 
243 


27.5 
27.7 


32.8 
33.0 


36.1 
36.2 

38.7 
40.7 
43.0 


9 

ty 

44.4 
44.5 
45.2 

-pi 

47.9 

sd 

50.5 

mc 
un 
r 

54.1 
54.4 
54.5 

f 

56.7 

B 

57.0 

Bx 
fu 

59.4 
59.5 

cor 

-Mn 
■sw 
bb 

625 
62.7 
64.0 
66.0 

18 


sp-f  66.0 

Figure  32  8  Diagram  of  the  X  chromosome  of  a  fruit  fly  as  seen  in  a  cell  of  the 
salivary  gland  together  with  a  map  of  the  loci  of  the  genes  located  n.  the  >-.  chromo- 
some, with  the  distances  between  them  as  determined  by  frequency  of  crossu.g  over. 
(Hunter  and  Hunter:  College  Zoolog) .) 


568  GENETICS    AND    EVOLUTION 

genes  within  a  given  chromosome,  have  been  made  (Figs.  32.7  and  32.8). 
The  only  human  chromosome  which  has  been  even  partially  mapped  is 
the  sex  chromosome. 


Questions 

1.  Define  in  your  own  words:  dominant,  recessive,  homozygous,  heterozygous,  genotype, 
phcnotype,  gene,  allele,  locus  and  back-cross. 

2.  Discuss  Mendel's  studies  of  heredity  as  an  example  of  the  scientific  method. 

3.  Give  briefly  the  implications  of  Mendel's  two  laws  of  heredity. 

4.  In  peas,  the  gene  for  smooth  seed  coat  is  dominant  to  the  one  for  wrinkled  seeds.  What 
would  be  the  result  of  the  following  matings:  heterozygous  smooth  x  heterozygous 
smooth?  Heterozygous  smooth  x  wrinkled?  Heterozygous  smooth  x  homozygous 
smooth?  Wrinkled  x  wrinkled? 

5.  In  peas,  the  gene  for  red  flowers  is  dominant  to  the  one  for  white  flowers.  What  would 
be  the  result  of  mating  heterozygous  red-flowered,  smooth-seeded  plants  with  white- 
flowered,  wrinkled-seeded  plants? 

6.  The  mating  of  two  black,  short-haired  guinea  pigs  produced  a  litter  which  included 
some  black,  long-haired  and  some  white,  short-haired  offspring.  What  are  the  geno- 
types of  the  parents  and  what  is  the  probability  of  their  having  black,  short-haired 
offspring  in  subsequent  matings? 

7.  Human  color-blindness  is  a  sex-Hnked,  recessive  trait.  What  is  the  probability  that  a 
woman  with  normal  vision  whose  husband  is  color-blind  will  have  a  color-blind  son? 
a  color-blind  daughter?  What  is  the  probability  that  a  woman  with  normal  vision 
whose  father  was  color-blind  but  whose  husband  has  normal  vision  will  have  a  color- 
blind son?  a  color-blind  daughter? 

8.  The  gene  for  white  eye  color  (w)  in  fruit  flies  is  sex-linked  and  recessive  to  normal  red 
eye  color  (W).  Give  the  results  of  mating  (a)  a  heterozygous,  red-eyed  female  with  a 
red-eyed  male,  (b)  a  white-eyed  female  with  a  red-eyed  male  and  (c)  a  heterozygous, 
red-eyed  female  with  a  white-eyed  male. 

9.  A  blue-eyed  man,  both  of  whose  parents  were  brown-eyed,  marries  a  brown-eyed 
woman  whose  father  was  blue-eyed  and  whose  mother  was  brown-eyed.  Their  first 
child  has  blue  eyes.  Give  the  genotypes  of  all  the  individuals  mentioned  and  give  the 
probability  that  the  second  child  will  also  have  blue  eyes. 

10.  Outline  a  breeding  procedure  whereby  a  true-breeding  strain  of  red  cattle  could 
be  established  from  a  roan  bull  and  a  white  cow. 

11.  Suppose  you  learned  that  shmoos  may  have  long,  oval  or  round  bodies  and  that  mat- 
ings of  shmoos  gave  the  following  results: 

long  X  oval  gave  ,'j2  long  and  48  oval 
long  X  round  gave  99  oval 
oval  X  round  gave  51  oval  and  50  round 
oval  X  oval  gave  24  long,  53  oval  and  27  round. 
What  hypothesis  about  the  inheritance  of  shmoo  shape  would  be  consistent  with 
these  results? 

Supplementary  Reading 

Ihere  are  several  good  elementary  textbooks  of  genetics  which  provide  further  read- 
ing for  those  interested  in  the  subject:  L.  H.  Snyder's  The  Principles  of  Heredity,  E.  O. 
Dodson's  Genetics,  R.  B.  Goldschmidt's  Understanding  Heredity  and  Srb  and  Owen's 
General  Genetics.  Ynii  and  Heredity,  by  A.  Scheinfeld,  is  a  popular  account  of  the  inheri- 
tance of  human  characters.  Curt  Stern's  Principles  of  Human  Genetics  is  a  clear,  well- 
written  text  of  general  genetics  with  special  emphasis  on  human  inheritance. 


CHAPTER  33 


Genetics 


The  genetic  principles  basic  to  the  simpler  types  o£  inheritance  dis- 
cussed in  the  previous  chapter  have  been  understood  for  half  a  century 
or  more.  In  the  intervening  years  research  in  genetics  has  been  pursued 
enthusiastically  and  a  great  many  complicating  factors  have  been  dis- 
covered and  analyzed.  In  each  case  it  has  been  found  that  the  distribu- 
tion of  traits  among  the  successive  generations  is  a  reproducible 
phenomenon  and  that  it  can  be  explained  as  some  variation  of  Mendel- 
ian  genetics. 

The  relationship  between  the  genes  discussed  in  the  previous  chapter 
and  their  traits  is  simple  and  clear:  each  gene  produces  a  single  trait. 
Genetic  research  with  many  different  kinds  of  animals  and  plants  has 
revealed  that  the  relationship  between  gene  and  trait  may  be  quite  com- 
plex. Several  pairs  of  genes  may  interact  to  affect  the  production  of  a 
single  trait;  one  pair  of  genes  may  inhibit  or  reverse  the  effect  of  an- 
other pair;  or  a  given  gene  may  produce  different  effects  when  the  en- 
vironment is  altered  in  some  way.  The  genes  are  inherited  as  units,  but 
may  interact  with  one  another  in  some  complex  fashion  to  produce  the 
trait.  The  relation  between  gene  and  trait,  the  mode  of  action  of  the 
gene  in  producing  the  recognizably  altered  characteristic,  has  fascinated 
geneticists  for  many  years.  This  general  field,  called  physiological 
genetics  or  biochemical  genetics,  is  being  investigated  very  actively  at 
present. 

280.        The  Interactions  of  Genes 

Two  or  more  independent  pairs  of  genes  may  interact  in  any  one 
of  several  ways  as  they  affect  the  phenotypic  expression  of  a  given  trait. 
The  total  number  of  genes  which  must  be  present  and  interact  prop- 
erly for  the  normal  development  of  a  given  trait  is  quite  large;  several 
dozen  different  genes  affect  the  coat  color  of  mammals  such  as  rats, 
rabbits  or  guinea  pigs  and  nearly  100  different  genes  affect  the  size, 
shape  and  color  of  the  eves  of  the  fruit  fly. 

Complementary  Genes.  Two  independent  pairs  of  genes  may  be 
interrelated  in  such  a  way  that  neither  dominant  can  produce  its  effect 
unless  the  other  is  present  too.  The  presence  of  at  least  one  dominant 
gene  from  each  pair  produces  one  character;  the  alternate  condition 
results  from  the  absence  of  either  dominant  or  of  both  dominants. 

669 


570  GENETICS    AND    EVOLUTION 


P, 


Gomefes 


White 


Gometes 


Purple 
00110 


Purple 

|cl[ili@ 


CCEE 

Purple 

CCEe 

Purple 

CcEE 

Purple 

CcEe 

Purple 

CCEe 

Purple 

CCee 

White 

CcEe 

Purple 

Ccee 

White 

CcEE 

Purple 

CcEe 

Purple 

ccEE 

White 

ccEe 

White 

CcEe 

Purple 

Ccee 

White 

ccEe 

White 

ccee 

White 

>?.  ?<oJe<Ajyjt    '¥» 


Phenotypes!  9  purple  =  7  white 

Figure  33.1.  Diagram  of  a  cross  illustrating  the  action  of  complementary  genes, 
the  two  pairs  of  genes  which  regulate  flower  color  in  sweet  peas.  At  least  one  C  gene 
and  one  E  gene  must  be  present  to  produce  a  colored  flower.  The  absence  of  either 
one  or  both  results  in  a  white  flower.  (Villee:   Biology.) 

In  the  course  of  breeding  experiments  with  varieties  of  cultivated 
sweet  peas,  Bateson  and  Piinnett  tound  that  purple  flower  color  was 
dominant  to  white.  Several  different  varieties  with  white  flowers  are 
known  and  the  mating  of  most  white-flowered  plants  produces  only 
white-flowered  offspring.  However,  when  plants  from  two  particular 
white-flowered  varieties  were  crossed,  all  the  offspring  had  purple 
flowers!  When  two  of  these  purple  Fi  plants  were  crossed,  or  when  they 
were  self-fertilized,   an    t\,  generation   was  produced   in   the   ratio  of   9 


GENETICS  671 

purple  to  7  white  (Fig.  33.1).  Subsequent  analysis  has  shown  that  two 
pairs  of  genes  are  involved,  one  of  which  (C)  regulates  some  essential 
step  in  the  production  of  a  raw  material  and  the  other  (E)  controls  the 
formation  of  an  enzyme  which  converts  the  raw  material  into  purple 
pigment.  The  homozygous  recessive  cc  is  unable  to  synthesize  the  raw 
material  and  the  homozygous  recessive  ee  lacks  the  enzyme  to  convert 
the  raw  material  into  purple  pigment.  One  of  the  white-flowered  varie- 
ties was  genotypically  ccEE— lacked  the  gene  for  the  synthesis  of  raw 
material— and  the  other  was  CCee,  without  the  gene  for  the  enzyme  re- 
quired for  pigment  synthesis.  Crossing  CCee  and  ccEE  produces  an  Fi 
generation  all  of  which  are  CcEe  and  have  purple  flowers  because  they 
have  both  raw  material  and  enzyme  for  the  synthesis  of  the  pigment. 
The  C  and  E  genes  are  located  in  different  chromosomes,  hence  their 
inheritance  follows  Mendel's  Law  of  Independent  Assortment.  There 
are  nine  chances  out  of  sixteen  that  any  one  of  the  Fo  generation  from 
the  mating  of  two  F^  plants  will  have  at  least  one  C  gene  and  one  E 
gene  and  therefore  have  purple  flowers,  and  seven  chances  out  of  sixteen 
that  it  will  lack  either  a  C  gene  or  an  E  gene  or  both  and  hence  have 
white  flowers.  Two  independent  pairs  of  genes  which  interact  to  produce 
a  trait  in  such  a  way  that  neither  dominant  will  produce  its  effect  unless 
the  other  dominant  is  also  present  are  called  complementary  genes;  the 
action  of  each  one  "complements"  the  action  of  the  other  in  the  pro- 
duction of  the  phenotype.  This  9:7  ratio  is  characteristic  of  the  Fg 
generation  of  a  cross  involving  two  complementary  genes.  A  pure-breed- 
ing variety  of  purple-flowered  sweet  peas  could  be  established  by  self- 
fertilization  of  a  plant  with  the  genotype  CCEE. 

Supplementary  Genes.  The  term  supplementary  genes  is  applied 
to  two  independent  pairs  of  genes  which  interact  in  the  production  of  a 
trait  in  such  a  way  that  one  dominant  will  produce  its  effect  whether 
or  not  the  second  is  present,  but  the  second  gene  can  produce  its  effect 
only  in  the  presence  of  the  first.  The  inheritance  of  coat  color  in  guinea 
pigs,  studied  by  Sewall  Wright  of  the  University  of  Chicago,  provides  a 
classic  example  of  supplementary  genes.  In  addition  to  the  pair  of  genes 
for  black  vs.  brown  coat  color  (B  and  b)  the  gene  C  controls  the  produc- 
tion of  an  enzyme  which  converts  a  colorless  precursor  into  the  pigment, 
melanin,  and  hence  is  required  for  the  production  of  any  pigment  at  all 
in  the  coat.  The  homozygous  recessive,  cc,  lacks  the  enzyme,  no  melanin 
is  produced  and  the  animal  is  a  white-coated,  pink-eyed  albino,  no  mat- 
ter what  combination  of  B  and  b  genes  may  be  present.  The  eyes  have 
no  pigment  in  the  iris  and  the  pink  color  results  from  the  color  of  the 
blood  in  the  tissues  of  the  eye.  The  mating  of  an  albino,  ccBB,  with  a 
brown  guinea  pig,  CCbb,  produces  offspring  all  of  which  are  geno- 
typically CcBb  and  have  black-colored  coats!  When  two  of  these  Fi 
black  guinea  pigs  are  mated,  offspring  appear  in  the  Fo  in  the  ratio  of 
9  black  :  3  brown  :  4  albino.  Make  a  Punnett  square  to  prove  this. 

Some  combination  of  complementary  and  supplementary  genes  may 
be  involved  in  the  inheritance  of  a  single  trait.  The  dominant  genes 
C  and  R  are  both  necessary  for  the  production  of  red  kernels  in  maize, 
and   the   absence  of  either  dominant  results   in  white-colored   kernels. 


672 


GENETICS   AND    EVOLUTION 


Pi- 


Gametes 


.Eggs 


3perm. 

R 


ffl 


00 


R 


0  m  m  m 


RRSS 

Red 

RRSs 

Red 

RrSS 
Red 

RrSs 
Red 

RRSs 
Red 

RRss 

Sandy 

RrSs 
Red 

Rrss 
Sandy 

RrSS 
Red 

RrSs 
Red 

rrSS 
Sandt/ 

rr  Ss 
Sandy 

RrSs 
Red 

Rrss 

Sandy 

rr  Ss 

Sandy 

rrss 
White 

l-iA/hite 

Figure  33.2.     Diagram  of  the  mode  of  inheritance  of  coat  color  in   Duroc-Jersey 
pigs,  illustrating  inheritance  by  "mutually  supplementary"  genes. 


GENEHCS 


673 


There  is,  in  addition,  a  P  gene  which  produces  purple-colored  kernels 
if  both  C  and  R  genes  are  present.  The  P  gene  is  supplementary  to  the 
other  two  pairs  of  genes  and  C  and  R  are  complementary. 

The  coat  color  of  Duroc-Jersey  pigs  represents  a  slightly  different 
type  of  gene  interaction.  Two  independent  pairs  of  genes  (R-r  and  S-s) 
regulate  coat  color;  at  least  one  dominant  of  each  pair  must  be  present 
to  give  the  full,  red-colored  coat.  Partial  color,  sandy,  results  when  only 
one  type  of  dominant  is  present  and  an  animal  which  is  homozygous  for 
both  recessives  (rrss)  has  a  white-colored  coat.  The  mating  of  two 
different  strains  of  sandy-colored  pigs,  RRss  X  "SS,  yields  offspring  all  of 
which  are  red,  and  the  mating  of  two  of  these  red  F^  individuals  pro- 
duces an  F2  generation  in  the  ratio  of  9  red  :  6  sandy  :  1  white  (Fig.  33.2). 


Rose  coml) 
RRpp 


Pea.  comb 
rrPP 


Fi 


All  walnut  comb 
RrPp 


RP 


rP 


rp 


RP        Rp 


rP 


rp 


RRPP 
Walnut 

RRPp 
Walnut 

R^-PP 
Walnut 

RrPp 
Walnut 

RRPp 

WaJnut 

RRpp 
Rose 

RrPp 
Walnut 

Rrpp 
Rose, 

RrPF 
Walnut 

RrPp 
Walnut 

rrPP 

P^2L 

rrPp 
Pea. 

RrPp 
Walnut 

Rrpp 
Rose 

rrPp 
Pea 

rrpp 

Single 

9  walnut 


3  rose 


3  pea. 


i  single. 
Figure  33.3.     Diagram  of  the  inheritance  of  comb  types  in  chickens.  See  text  for 


discussion. 


574  GENETICS    AND   EVOLUTION 

Genes  which  interact  in  this  fashion  have  been  termed  "mutually  sup- 
plementary." 

The  inheritance  of  comb  type  in  poultry  provides  an  interesting 
example  of  genie  interaction.  Leghorns  have  single  combs,  Wyandottes 
have  rose  combs  and  Brahmas  have  pea  combs  (Fig.  33.3).  Each  of  these 
types  is  true  breeding.  Suitable  crosses  demonstrate  that  the  gene  for 
rose  comb  (R)  is  dominant  to  single  (r)  and  that  the  gene  for  pea  comb 
(P)  is  also  dominant  to  its  allele  (p)  for  single  comb.  However,  when  a 
pea-combed  fowl  is  mated  with  a  rose-combed  one,  all  of  the  offspring 
have  a  different  type  of  comb,  resembling  half  of  a  shelled  walnut  and 
called  walnut.  When  two  of  these  walnut-combed  Fi  individuals  are 
mated,  offspring  appear  in  the  ratio  of  9  walnut  :  3  pea  :  3  rose  :  1 
single.  We  can  deduce  from  this  that  the  genotype  of  a  single-combed 
fowl  must  be  rrpp;  a  pea-combed  fowl  is  either  PPrr  or  Pprr;  a  rose- 
combed  fowl  is  either  ppRR  or  ppRr,  and  a  walnut  comb  develops  in 
animals  with  at  least  one  P  and  one  R  gene.  Thus  the  genotypes  PPRR, 
PpRR,  PPRr  and  PpRr  all  yield  walnut  combs.  Certain  Malay  varieties  of 
chicken  have  walnut  combs. 

It  is  clear  that  there  is  nothing  unusual  about  the  method  of  in- 
heritance of  any  of  these  genes;  the  phenotypic  ratios  observed  are  simply 
the  result  of  some  variation  in  the  interaction  of  the  genes  in  the  pro- 
duction of  the  phenotype. 

281.        Multiple  Factors 

Many  human  characteristics,  height,  body  form,  intelligence  and 
skin  color,  and  many  commercially  important  characters  such  as  milk 
production  in  cows,  egg  production  in  hens,  the  size  of  fruits,  and  the 
like,  are  not  separable  into  distinct  alternate  classes,  and  are  not  in- 
herited by  single  pairs  of  genes.  However,  these  traits  are  nonetheless 
governed  by  genetic  factors;  there  are  several,  perhaps  many,  different 
pairs  of  genes  which  affect  the  same  characteristic.  The  term  multiple 
factors  (or  cumulative  factors)  is  applied  to  two  or  more  independent 
pairs  of  genes  which  affect  the  same  character  in  the  same  way  and  in 
an  additive  fashion.  When  two  varieties  which  differ  in  some  trait  con- 
trolled by  multiple  factors  are  crossed,  the  Fj  are  very  similar  to  one 
another  and  are  usually  intermediate  in  the  expression  of  this  character 
between  the  two  parental  types.  Crossing  two  F^  individuals  yields  a 
widely  variable  Fo  generation,  with  a  few  members  resembling  one 
grandparent,  a  few  resembling  the  other  grandparent,  and  the  rest  show- 
ing a  range  of  conditions  intermediate  between  the  two. 

The  inheritance  of  human  skin  color  was  carefully  investigated  by 
C.  B.  Davenport  in  Jamaica.  He  concluded  that  the  inheritance  of  skin 
color  in  man  is  controlled  by  two  pairs  of  genes,  A-a  and  B-b,  in- 
herited independently.  The  genes  for  dark  pigmentation,  A  and  B,  are 
incompletely  dominant,  and  the  darkness  of  the  skin  color  is  propor- 
tional to  the  sum  of  the  dominant  genes  present.  Thus,  a  full  Negro  has 
four  dominant  genes,  AABB,  and  a  white  person  has  four  recessive  genes, 
aabb.  The  Fi  offspring  of  a  mating  of  white  and  Negro  are  all  AaBb, 


GENETICS  575 

with  two  dominant  genes  and  a  skin  color  (mulatto)  intermediate  be- 
tween white  and  Negro.  The  mating  of  two  such  mulattoes  produces 
offspring  with  skin  colors  ranging  from  full  Negro  to  white  (Table  11). 
A  mulatto  with  the  genotype  AaBb  produces  four  kinds  of  eggs  or  sperm 
with  respect  to  the  genes  for  skin  color:  AB,  aS,  Ab  and  ab.  From  a 
Punnett  square  for  the  mating  of  two  doubly  heterozygous  mulattoes 
(AaBb)  it  will  be  evident  that  there  are  16  possible  zygote  combinations: 
one  with  four  dominants  (black),  four  with  three  dominants  (dark 
brown  skin),  six  with  two  dominants  (mulatto),  four  with  one  dominant 
(light  brown  skin)  and  one  with  no  dominants  (white  skin).  The  genes 
A  and  B  produce  about  the  same  amount  of  pigmentation  and  the  geno- 
types AaBb,  AAbb  and  aaBB  produce  the  same  phenotype,  mulatto  skin 
color. 

This  example  of  multiple  factor  inheritance  is  fairly  simple,  for 
only  two  pairs  of  genes  appear  to  be  involved.  With  a  larger  number 
of  pairs  of  genes,  perhaps  ten  or  more,  there  are  so  many  classes,  and 
the  differences  between  them  are  so  slight,  that  the  classes  are  not  dis- 
tinguishable. A  continuous  series  is  obtained.  The  inheritance  of  human 
stature  is  governed  by  a  large  number  of  pairs  of  multiple  factors,  with 
shortness  dominant  to  tallness.  Since  height  is  affected  not  only  by  these 
multiple  factors  but  also  by  a  variety  of  environmental  agents,  there  are 
adults  of  every  height  from  perhaps  55  inches  up  to  84  inches.  If  we 
measure  the  height  of  1000  adult  men  selected  at  random  and  draw  a 
graph  of  the  number  having  each  height,  we  will  obtain  a  bell-shaped 
normal  curve,  or  curve  of  normal  distribution  (Fig.  33.4).  It  is  evident 
that  there  are  few  extremely  tall  or  extremely  short  men,  but  many  of 
intermediate  height.  This  resembles  the  Fg  of  the  simpler  situation  with 
skin  color,  for  there  were  few  individuals  with  black  or  white  skin  but 
many  with  mulatto  skin. 

All  living  things  show  comparable  variations  in  certain  of  their 
characteristics.  If  one  were  to  measure  the  length  of  1000  shells  from  the 
same  species  of  clam,  or  the  weight  of  1000  hen's  eggs,  or  the  amount  of 
milk  produced  per  year  by  1000  dairy  cows,  or  the  intelligence  quotient 
(I.Q.)  of  1000  grade  school  children,  and  make  graphs  of  the  number  of 
individuals  in  each  subclass,  one  would  obtain  a  normal  curve  of  dis- 
tribution in  each  instance.  The  variation  is  due  in  part  to  the  action 
of  multiple  factors  and  in  part  to  the  effects  of  a  variety  of  environ- 
mental agents.  In  a  few  species  it  has  been  possible  to  establish  strains 
which   are   genetically   identical-all    the   individuals    have    exactly    the 

Table   11.     MULTIPLE  FACTOR  INHERITANCE  OF  SKIN  COLOR  IN   MAN 


Parents AaBb  AaBb 

(Mulatto)  (Mulatto) 

Gametes AB  Ab  aB  ab     AB  Ab  aB  ab 

Offspring: 

1  with  4  dominants— AABB— phenotypically  Negro 

4  with  3  dominants— 2  AaBB  and  2  AABb— phenotypically  "dark" 

6  with  2  dominants— 4  AaBb,  1  AAbb,  1  aaBB— phenotypically  mulatto 

4  with  1  dominant— 2  Aabb,  2  aaBb— phenotypically  "light" 

1  with  no  dominants— aabb— phenotypically  white 


576  GENETICS   AND    EVOLUTION 

I8O1 


Number 


Height  in  inches  of  1083  adult  men 

Figure  33.4.  An  example  of  a  "normal  curve,"  or  curve  of  normal  distribution: 
the  heights  of  1083  adult  white  males.  The  blocks  indicate  the  actual  number  of  men 
whose  heights  were  within  the  unit  range.  For  example,  there  were  163  men  between 
67  and  68  inches  in  height.  The  smooth  curve  is  a  normal  curve  based  on  the  mean 
and  standard  deviation  of  the  data.  (Villee:   Biology.) 

same  genetic  constitution.  Human  identical  twins  (p.  646)  have  identical 
sets  of  genes.  The  individuals  of  these  strains,  and  human  identical 
twins,  are  not  identical  in  all  of  their  characters,  however,  for  the  varia- 
tions due  to  environmental  influences  remain.  One  method  of  estimating 
the  relative  importance  of  genetic  and  environmental  factors  on  a  given 
character  is  to  compare  the  variability  of  that  character  in  a  genetically 
heterogeneous  group  and  in  a  genetically  homogeneous  one. 

When  a  commercial  breeder  attempts  to  establish  a  new  strain  of 
hens  that  will  lay  more  eggs  per  year,  or  a  strain  of  turkeys  with  more 
breast  meat,  or  a  strain  of  sheep  with  longer,  finer  wool,  he  selects  indi- 
viduals which  show  the  desired  trait  in  greatest  amount  for  further  breed- 
ing. There  is  a  limit,  of  course,  to  the  effectiveness  of  selective  breeding 
in  increasing  some  desirable  trait  or  in  decreasing  some  undesirable  one. 
When  the  strain  becomes  homozygous  for  all  the  genetic  factors  in- 
volved, further  selective  breeding  will  be  ineffective. 


GENETICS  577 

The  inheritance  of  certain  traits  depends  not  only  on  a  single  pair 
of  genes  which  determines  the  presence  or  absence  of  the  trait  but  also 
on  a  number  of  multiple  factors  which  determine  the  extent  of  the 
trait.  For  example,  the  presence  or  absence  of  spots  in  the  coat  of  most 
mammals  is  determined  by  a  single  pair  of  genes;  the  gene  for  the  pres- 
ence of  spots  (s)  is  recessive  to  the  gene  for  solid  color  (S).  The  size  and 
distribution  of  the  spots,  however,  are  determined  by  a  series  of  multiple 
factors,  and  can  be  varied  by  selective  breeding.  Crossing  two  different 
strains  produces  an  F^  generation  intermediate  between  the  two  parental 
types  and  with  little  variability,  and  an  Fo  generation  which  is  widely 
variable,  with  some  individuals  having  as  many  spots  as  the  one  grand- 
parent and  other  individuals  with  as  few  spots  as  the  other  grandparent. 
The  term  modifying  factors  has  been  suggested  for  multiple  factors 
which  affect  the  degree  of  expression  of  another  gene. 

282.        Multiple  Alleles 

In  all  of  the  types  of  inheritance  discussed  so  far,  there  have  been 
only  two  possible  alleles,  one  dominant  and  one  recessive  gene,  which 
could  be  represented  by  capital  and  lower  case  letters  respectively.  In 
addition  to  a  dominant  and  a  recessive  gene,  there  may  be  one  or  more 
additional  kintls  of  gene  found  at  that  same  location  in  the  chromo- 
some that  affect  the  same  trait  in  an  alternate  fashion.  The  term  multiple 
alleles  is  applied  to  the  type  of  inheritance  in  which  there  are  three  or 
more  different  kinds  of  gene,  three  or  more  alternate  conditions  at  a 
single  locus  in  the  chromosome,  each  of  which  produces  a  distinctive 
phenotype.  Among  the  members  of  the  species,  of  course,  the  alleles  are 
inherited  in  such  a  way  that  each  individual  has  any  two,  and  no  more 
than  two,  of  the  possible  types  of  alleles.  The  members  of  an  allelic 
series  are  indicated  by  the  same  letter,  with  suitable  distinguishing  super- 
scripts. 

One  series  of  multiple  alleles  which  affects  coat  color  in  rabbits  in- 
cludes the  dominant  gene  C  for  normal  coat  color,  the  recessive  gene  c 
which  produces  albino  coat  color  when  homozygous,  and  two  other 
alleles,  c''  and  c'"''.  The  gene  c'',  when  homozygous,  produces  the  "Hima- 
layan" pattern  of  white  coat  over  the  body  but  with  a  dark  color  on  the 
tips  of  the  ears,  nose,  tail  and  legs.  The  gene  c'^'',  when  homozygous, 
produces  the  "Chinchilla"  pattern  of  light  gray  fur  all  over  the  body. 
These  alleles  may  be  arranged  in  the  series  C,  c'''',  c^  and  c,  in  which 
each  gene  is  dominant  to  the  succeeding  genes  but  recessive  to  the  pre- 
ceding ones.  In  other  series  of  multiple  alleles  the  genes  may  be  incom- 
pletely dominant  so  that  the  heterozygote  has  a  phenotype  intermediate 
between  those  of  its  two  parents,  or  one  which  is  some  combination  of 
the  two  parental  phenotypes. 

Multiple  alleles  govern  the  inheritance  of  the  human  blood  groups 
O,  A,  B  and  AB  (p.  544).  The  three  alleles  of  the  series,  a*^,  a^  and  a, 
regulate  the  kind  of  agglutinogen  in  the  red  blood  cells  (Table  12). 
Gene  a-^  produces  agglutinogen  A,  gene  a^  produces  agglutinogen  B 
and  gene  a  produces  no  agglutinogens.  Gene  a  is  recessive  to  the  other 


578  GENETICS   AND   EVOLUTION 

Table  12.     THE  INHERITANCE  OF  THE  HUMAN  BLOOD  GROUPS 


BLOOD 
GROUP 

GENOTYPES 

AGGLUTINOGEN 
IN  RED  CELLS 

AGGLUTININ 
IN  PLASMA 

CAN  GIVE 

BLOOD 
TO  GROUPS 

CAN  RECEIVE 

BLOOD 
FROM  GROUPS 

O 
A 
B 

AB 

aa 

a*aA,  a*a 

a"a",  a"a 

a*aB 

none 

A 

B 

A  and  B 

a  and  b 

b 

a 

none 

O,  A,  B,  AB 

A,  AB 

B,  AB 
AB 

O 

0,A 
0,B 

O,  A,  B,  and  AB 

two,  but  neither  a^  nor  a^  is  dominant  to  the  other;  each  produces  its 
characteristic  agglutinogen  independently  of  the  other.  Transfusions  of 
blood  from  one  person  to  another  are  successful  only  when  the  two 
bloods  are  compatible,  when  the  agglutinins  in  the  plasma  of  the  re- 
cipient do  not  react  with  the  agglutinogens  in  the  red  cells  of  the  donor 
to  cause  agglutination,  clumpmg  of  the  red  cells.  People  with  type  O 
blood  (no  agglutinogens  in  their  red  cells)  are  known  as  "universal 
donors";  their  blood  can  be  transfused  into  the  veins  of  persons  with 
any  of  these  blood  groups.  People  with  type  AB  blood  are  called  "uni- 
versal recipients";  they  have  no  agglutinins  in  the  plasma  and  hence 
their  plasma  will  not  cause  agglutination  of  the  red  cells  from  any  per- 
son. 

Since  blood  types  are  inherited,  and  do  not  change  in  a  person's 
lifetime,  they  are  useful  indicators  of  parentage.  In  cases  of  disputed 
parentage,  genetic  evidence  can  show  only  that  a  certain  man  or  woman 
could  be  the  parent  of  a  particular  child,  and  never  that  he  is  the  parent. 
In  certain  circumstances,  however,  the  genetic  evidence  can  definitely 
exclude  a  particular  man  or  woman  as  the  parent  of  a  given  child.  Thus, 
if  a  child  of  blood  group  A  is  born  to  a  type  O  woman,  no  man  with 
type  O  or  type  B  blood  could  be  its  father  (Table  13). 

There  are  now  eleven  different  sets  of  blood  groups,  inherited  by 
different  pairs  of  genes,  all  of  which  are  helpful  in  establishing  paternity. 
The  most  important  of  these  are  the  Rh  alleles  which  determine  the 


Table   13.     EXCLUSION  OF  PATERNITY   BASED  ON   BLOOD  TYPES 


CHILD 

MOTHER 

FATHER  MUST  BE  OF 
TYPE 

FATHER  CANNOT  BE  OF 
TYPE 

o 

o 

O,  A,  or  B 

AB 

o 

A 

O,  A,  or  B 

AB 

o 

B 

O,  A,  or  B 

AB 

A 

O 

A  or  AB 

O  or  B 

A 

A 
B 
B 

A,  B,  AB,  or  O 

A,  B,  AB,  or  O 

A  or  AB 

B 

A 

O  orB 

B 

A 

B  or  AB 

O  or  A 

B 

O 

B  or  AB 

Oor  A 

AB 

A 

B  or  AB 

O  or  A 

AB 

B 

A  or  AB 

OorB 

AB 

AB 

A,  B,  or  AB 

O 

GENETICS  679 

presence  or  absence  of  a  different  agglutinogen,  the  Rh  factor,  first 
found  in  the  blood  of  rhesus  monkeys.  There  are  actually  several  alleles 
at  the  rh  locus,  but  to  simplify  matters  we  shall  consider  just  two:  Rh, 
which  produces  the  rh  positive  antigen,  and  the  recessive  rh,  which  does 
not  produce  the  antigen.  Genotypes  RhRh  and  Rhrh  are  phenotypically 
rh  positive  and  genotype  rhrh  is  phenotypically  rh  negative.  An  rh 
negative  woman  married  to  an  rh  positive  man  may  have  an  rh  positive 
child.  If  some  blood  manages  to  pass  across  the  placenta  from  the  fetus 
to  the  mother  it  will  stimulate  the  formation,  in  her  blood,  of  antibodies 
to  the  rh  factor.  Then,  in  a  subsequent  pregnancy,  some  of  these  rh 
antibodies  may  pass  through  the  placenta  to  the  child's  blood,  and  react 
with  the  rh  antigen  in  the  child's  red  cells.  The  red  cells  are  agglutinated 
and  destroyed  and  a  serious,  often  fatal,  anemia,  called  erythroblastosis 
fetalis,  ensues.  This  is  now  treated  by  massive  blood  transfusions,  so  that 
essentially  all  of  the  blood  of  the  newborn  is  replaced. 

Extensive  surveys  have  shown  that  41  per  cent  of  native  white 
Americans  are  type  O,  45  per  cent  are  type  A,  10  per  cent  are  type  B 
and  4  per  cent  are  type  AB.  The  frequency  of  the  blood  groups  in  other 
races  may  be  quite  different;  American  Indians,  for  example,  have  a 
low  frequency  of  group  A  and  a  high  frequency  of  group  B.  No  one 
blood  type  is  characteristic  of  a  single  race;  the  racial  differences  lie  in 
the  relative  frequency  of  the  several  blood  types.  Studies  of  the  relative 
frequencies  of  the  blood  groups  found  in  different  races  living  today  and 
in  mummies  and  skeletons  have  provided  valuable  evidence  as  to  the 
relationships  of  the  present  races  of  man. 

283.        Lethal  Genes 

Certain  genes  produce  such  a  tremendous  deviation  from  normal 
development  that  the  organism  is  unable  to  survive.  Many  such  genes 
will  escape  detection,  for  their  action  is  usually  evident  only  in  special 
circumstances  in  which  the  usual  genetic  ratios  are  altered  because  one 
of  the  expected  classes  is  completely  missing.  One  of  the  first  such  lethal 
genes  to  be  discovered  was  found  when  the  inheritance  of  yellow  coat 
color  in  mice  was  investigated.  It  proved  impossible  to  establish  a  true- 
breeding  strain  of  vellow  mice;  breeding  two  yellow  mice  resulted  ni  off- 
spring in  the  ratio  of  two  yellow  to  one  nonyellow  (gray,  black  or  brown). 
Breeding  yellow  mice  with  nonvellow  ones  produced  equal  numbers  of 
yellow  and  nonyellow  offspring.  This  indicated  that  the  yellow  mice 
were  heterozygous,  Yy,  and  that  in  the  mating  of  two  yellow  mice,  Yy  x 
Yy  the  ratio  of  2  yellow  to  1  nonyellow  (rather  than  the  expected  5 
yellow  to  1  nonyellow)  was  obtained  because  the  homozygous  yellow  YY 
animals  died.  Investigators  then  noticed  that  the  number  of  offspring 
produced  by  a  yellow  X  yellow  mating  was  indeed  smaller,  only  about 
three-quarters  as  large  as  the  average  mouse  litter.  Later  research  showed 
that  these  homozygous  yellow  mice  do  begin  development  but  die  and 
are  resorbed.  If  the  uterus  of  the  mother  is  dissected  open  early  in  preg- 
nancy, the  abnormal  embryos  are  visible. 

The   "creeper"  gene   in  chickens  provides  an   exactly  comparable 


580  GENETICS   AND   EVOLUTION 

case.  Creeper  fowl  have  wings  and  legs  which  are  shorter  than  normal. 
When  two  creeper  towl  are  bred,  the  ratio  of  creepers  to  normal  in  the 
offspring  is  2:1.  One  quarter  of  the  eggs,  those  homozygous  for  the 
creeper  factor,  have  marked  abnormalities  of  the  whole  skeletal  system, 
especially  of  the  vertebrae,  and  die  without  hatching. 

These  lethal  genes  produce  a  visible  phenotypic  expression  when 
heterozygous  and  thus  are  dominant  to  the  normal  allele.  Many— perhaps 
most-lethal  genes  have  no  effect  when  heterozygous,  but  result  in  the 
death  of  the  organism  when  homozygous.  These  recessive  lethals  can  be 
detected  only  by  special  genetic  techniques.  When  such  techniques  have 
been  applied  to  wild  populations  of  the  fruit  fly,  Drosophila,  the  pres- 
ence of  many  recessive  lethals  has  been  revealed  and  it  is  believed  that 
similar  lethals  occur  in  most  wild  populations. 

284.  Penetrance  and  Expressivity  of  Genes 

Genetic  research  on  the  mode  of  inheritance  of  certain  traits  is  com- 
plicated by  the  fact  that  these  genes  do  not  always  produce  the  expected 
phenotype.  In  the  examples  presented  so  far,  recessive  genes  always  pro- 
duce their  phenotype  when  homozygous  and  dominant  genes  always 
produce  their  phenotype  when  homo-  or  heterozygous.  Such  genes  are 
said  to  have  complete  or  100  per  cent  penetrance.  With  certain  other 
genes  only  a  fraction  of  the  inclividuals  homozygous  for  a  recessive  gene 
actually  show  the  expected  phenotype.  Such  genes  are  said  to  show  in- 
complete penetrance;  the  percentage  of  penetrance  is  calculated  from 
the  number  of  individuals  that  actually  show  the  phenotype  in  every 
hundred  individuals  that  would  be  expected  to  show  it.  Penetrance  is 
essentially  a  statistical  concept  of  the  regularity  with  which  a  gene  pro- 
duces its  effect  when  present  in  the  requisite  homozygous  (or  hetero- 
zygous) state.  The  percentage  penetrance  of  many  genes  may  be  altered 
by  changing  the  environmental  conditions— temperature,  nutrition, 
moisture,  etc.— under  which  the  organism  develops. 

Certain  inbred  stocks  homozygous  for  a  particular  gene  show  wide 
variations  in  the  phenotype.  For  example,  fruit  flies  of  a  stock  homo- 
zygous for  a  gene  which  produces  shortening  and  scalloping  of  the  wings 
may  show  wide  variations  in  the  degree  of  shortening  and  scallopmg 
in  the  wings  of  any  individual  fly.  Such  differences  are  known  as  varia- 
tions in  the  expression  or  expressivity  of  the  gene;  they  may  also  be 
altered  by  changing  the  environmental  conditions  during  the  organism's 
development. 

285.  Inbreeding  and  Outbreeding 

It  is  commonly  believed  that  the  mating  of  two  closely  related  indi- 
viduals—brother and  sister  or  father  and  daughter— is  harmful  and  leads 
to  the  production  of  monstrosities.  Even  the  marriage  of  first  cousins  is 
forbidden  by  law  in  some  states.  Carefully  controlled  experiments,  car- 
ried out  over  many  generations  and  with  many  different  kinds  of  plants 
and  animals,  have  shown  that  there  is  nothing  harmful  in  the  process 


G£N£r/CS  581 

of  inbreeding  itself.  It  is,  in  fact,  one  of  the  standard  procedures  used 
by  commercial  breeders  to  improve  strains  of  cattle,  corn,  cats  and  canta- 
loupes. It  is  not  necessarily  a  bad  practice  in  the  human  species.  In  all 
anmials  or  plants  it  simply  tends  to  make  the  strain  homozygous.  All 
natural  populations  of  individuals  are  heterozygous  for  many  traits; 
some  of  the  hidden  recessive  genes  are  for  desirable  traits,  others  are 
for  undesirable  ones,  inbreeding  will  simply  permit  these  genes  to  be- 
come homozygous  and  lead  to  the  unmasking  of  the  good  or  bad  traits. 
If  a  stock  is  good,  inbreeding  will  improve  it;  but  if  a  stock  has  many 
undesirable  recessive  traits,  inbreeding  will  lead  to  their  phenotypic 
expression. 

The  crossing  of  two  completely  unrelated  strains,  called  outbreed- 
ing, is  another  widely  used  genetic  maneuver.  It  is  frequently  found 
that  the  offspring  of  such  a  mating  are  much  larger,  stronger  and 
healthier  than  either  parent.  Much  of  the  corn  grown  in  the  United 
States  is  a  special  hybrid  variety  developed  by  the  United  States  Depart- 
ment of  Agriculture  from  a  mating  of  four  different  inbred  strains. 
Each  year,  the  seed  to  grow  this  uniformly  fine  hybrid  corn  is  obtained 
by  mating  the  original  inbred  lines.  If  the  hybrid  corn  were  used  in 
mating  it  would  give  rise  to  many  different  kinds  of  corn,  since  it  is 
heterozygous  for  many  different  traits.  The  mule,  the  hybrid  offspring  of 
the  mating  of  a  horse  and  donkey,  is  a  strong,  sturdy  animal,  better 
adapted  for  many  kinds  of  work  than  either  of  its  parents.  This  phe- 
nomenon of  hybrid  vigor,  or  heterosis,  does  not  result  from  the  act  of 
outbreeding  itself,  but  from  the  heterozygous  nature  of  the  F^  organisms 
which  result  from  outbreeding.  Each  of  the  parental  strains  is  homo- 
zygous for  certain  undesirable  recessive  traits,  but  the  two  strains  are 
homozygous  for  different  traits,  and  each  one  has  dominant  genes  to 
mask  the  undesirable  recessive  genes  of  the  other.  As  a  concrete  ex- 
ample, let  us  suppose  that  there  are  four  pairs  of  genes.  A,  B,  C  and  D; 
the  capital  letters  represent  the  dominant  gene  for  some  desirable  trait 
and  the  lower  case  letters  represent  the  recessive  gene  for  its  unde- 
sirable allele.  If  one  parental  strain  is  then  AAbbCCdd  and  the  other 
aaBBccDD  the  offspring  will  all  be  AaBbCcDd  and  have  all  of  the  de- 
sirable and  none  of  the  undesirable  traits.  The  actual  situation  in  any 
given  cross  is  undoubtedly  much  more  complex  and  involves  many 
pairs  of  genes. 

286.        Population  Genetics 

A  question  that  appears  to  trouble  many  new  students  of  genetics 
is  why,  if  the  gene  for  brown  eyes  is  dominant  to  the  gene  for  blue 
eyes,  are  there  any  blue  eye  genes  left?  The  answer  lies  partly  in  the 
fact  that  a  recessive  gene,  such  as  the  one  for  blue  eyes,  is  not  altered 
in  any  w^ay  by  existing  for  a  generation  in  a  heterozygote  next  to  a 
brown  eye  gene.  The  rest  of  the  explanation  follows  from  the  fact  that 
as  long  as  there  is  no  selection  for  either  eye  color,  as  long  as  people 
with  blue  eyes  are  just  as  likely  to  marry  and  to  have  as  many  children 


682 


GENETICS    AND   EVOLUTION 


Table   14.     THE  OFFSPRING  OF  THE  RANDOM   MATING  OF  A  POPULATION 
COMPOSED  OF  14   AA,  1/2  Aa  AND  14   aa  INDIVIDUALS 


MATING 

FREQUENCY 

OFFSPRING 

MALE       FEMALE 

AA  X  AA 

K  X  K 

1/16  AA 

AA  X  Aa 

M  X  M 

1/16  AA 

+ 

1/16  Aa 

AA  X  aa 

H  X  H 

1/16  Aa 

Aa    X  AA 

VzXH 

1/16  AA 

+ 

1/16  Aa 

Aa    X  Aa 

K  X  M 

1/16  AA 

+ 

1/8     Aa 

+ 

1/16  aa 

Aa    X  aa 

H  X  M 

1/16  Aa 

+ 

1/16  aa 

aa     X  AA 

H  X  H 

1/16  Aa 

aa      X  Aa 

K  X  M 

1/16  Aa 

+ 

1/16  aa 

aa     X  aa 

M  X  M 

1/16  aa 

Sum:  4/16  AA 

+ 

8/16  Aa 

+ 

4/16  aa 

as  people  with  brown  eyes,  successive  generations  will  have  the  same 
proportion  of  blue-  and  brown-eyed  people  as  the  present  one. 

A  brief  excursion  in  mathematics  is  needed  to  illustrate  this  point. 
If  we  consider  the  distribution  of  a  single  pair  of  genes,  A  and  a,  in  a 
population  (of  men,  animals  or  plants),  any  member  of  the  population 
will  have  one  of  these  three  genotypes:  AA,  Aa  or  aa.  Let  us  suppose 
that  these  genotypes  are  present  in  the  population  in  the  ratio  of 
^AA:%Aa:^aa.  (The  point  of  the  argument,  that  there  is  no  change 
in  the  proportion  in  successive  generations,  will  be  the  same  no  matter 
what  particular  initial  ratio  we  assume.)  If  all  the  members  of  the 
population  select  their  mates  at  random,  without  regard  as  to  whether 
they  are  genotypically  AA,  Aa  or  aa,  and  if  all  the  pairs  produce  com- 
parable numbers  of  offspring,  the  succeeding  generation  will  also  have 
genotypes  in  the  ratio  of  Y^AA-.Y^^o-Vioa.  This  can  be  demonstrated 
by  setting  down  all  the  possible  types  of  matings,  the  frequency  of  their 
random  occurrence,  and  the  kinds  and  proportions  of  offspring  which 
result  from  each  type  of  mating,  and  finally  adding  up  all  the  kinds  of 
offspring  (Table  14). 

Hardy,  a  mathematician,  and  Weinberg,  a  physician,  independently 
concluded  in  1908  that  the  frequencies  of  the  members  of  a  pair  of  allelic 
genes  are  described  by  the  expansion  of  a  binomial  equation.  The  gen- 
eral relationship  can  be  stated  if  we  let  p  be  the  proportion  of  A  genes 
in  the  population  and  let  q  be  the  proportion  of  a  genes  in  the  popula- 
tion. Since  any  gene  must  be  either  A  or  a  (there  is,  by  definition,  no 
other  possibility),  then  p  -f  q  =  1.  Thus,  if  we  know  either  p  or  q  we 
can  calculate  the  other. 

When  we  consider  all  the  possible  matings  of  any  generation,  a  p 
number  of  A-containing  eggs  and  a  q  number  of  a-containing  eggs  are 
fertilized  by  a  p  number  of  A-containing  sperm  and  a  q  number  of 
a-containing  sperm:  (pA  +  qa)  X  (pA  -f  qa),  or  (pA  +  qaF.  The 
proportion  of  the  types  of  offspring  of  all  of  the  possible  matings  is 
described  by  the  algebraic  product:  p^AA  +  2pqAa  +  q-aa.  This 
formulation,  and  its  implication  of  genetic  stability  in  a  population  in 
the  absence  of  selection,  is  known  as  the  Hardy-Weinberg  Law. 

In  studies  of  human  genetics,  in  which  test  matings  are  impossible 


GENETICS  683 

and  the  number  of  offspring  is  rather  small,  statistical  methods  based 
on  this  law  have  enabled  investigators  to  determine  the  method  of  in- 
heritance of  many  traits  and  to  predict  the  proportion  of  types  of  off- 
spring. For  example,  albinism,  the  complete  lack  of  pigment  which 
results  in  white  skin  and  hair  and  pink  eyes,  is  a  rare  condition  in 
man  that  is  inherited  by  a  single  pair  of  genes.  The  gene  a  for  albinism 
is  recessive  to  the  gene  A  for  normal  pigmentation.  Surveys  have  shown 
that  albinos  (genetically  aa)  occur  in  the  population  with  a  frequency 
of  about  1  in  20,000.  Substituting  this  number,  1/20,000,  for  q^  in  the 
Hardy-Weinberg  equation,  we  can  calculate  that  q,  the  square  root  of 
1/20,000,  equals  1/141.  Since  p  +  q  =  1,  then  p  =  1  _  q  or  1  —  1/141, 
or  140/141.  The  frequency  of  heterozygous  individuals,  Aa,  in  the  pop- 
ulation is  equal  to  2  pq,  or  2  X  140/141  X  1/141,  which  equals  1/70. 
Thus,  about  1  person  in  70  is  heterozygous  for  albinism— is  a  "carrier" 
of  the  gene  for  albinism.  It  is  surprising,  perhaps,  to  find  that  there  are 
so  many  carriers  for  such  a  rare  trait.  H.  J.  Muller  has  calculated  that 
each  of  us  is,  on  the  average,  heterozygous  for  about  eight  undesirable 
genes. 

287.        Biochemical  Genetics 

Since  1911,  when  the  gene  theory  was  formulated  by  T.  H.  Morgan, 
biologists  have  accepted  the  idea  that  genes  are  the  fundamental  units 
of  heredity,  located  in  a  linear  order  on  the  chromosomes,  and  that  these 
units  govern  the  development  of  all  the  characters  of  the  body.  Re- 
search in  the  field  of  biochemical  genetics  has  been  directed  toward 
providing  an  explanation  of  (1)  the  chemical  and  physical  nature  of  the 
gene  and  (2)  the  mechanisms  by  which  the  genes  may  control  the  devel- 
opment and  maintenance  of  the  individual  organism. 

Many  attempts  have  been  made  to  observe  the  genes  within  the 
chromosomes  but  not  even  electron  microscopy  has  been  able  to  reveal 
them.  By  a  fortunate  coincidence,  one  of  the  organisms  which  has  been 
most  extensively  used  in  genetic  experiments,  the  fruit  fly  Drosophila, 
has  greatly  enlarged,  giant  chromosomes  in  the  cells  of  its  salivary 
glands.  Each  of  the  four  giant  chromosomes  has  a  distinctive  pattern  of 
cross  bands  by  which  it  can  be  recognized.  The  detailed  pattern  of 
bands  is  repeated  with  extreme  fidelity  in  all  the  animals  of  a  given 
strain.  C.  B.  Bridges  and  others  have  mapped  the  pattern  of  stripes  on 
each  chromosome  and  then  compared  these  cytologic  maps  with  the 
genetic  maps  calculated  from  crossover  values.  From  such  studies  it  has 
been  possible  to  conclude  that  the  gene  for  a  particular  character  is 
located  in  (or  is  associated  with)  a  particular  band  of  the  chromosome. 
It  appears,  however,  that  the  band  itself  is  not  the  gene;  some  bands  con- 
tain several  genes. 

Chemical  Nature  of  the  Gene.  It  has  been  possible,  by  special 
techniques,  to  isolate  chromosomes  from  ground-up  cells  and  to  show 
by  direct  chemical  analysis  that  they  contain  proteins  and  nucleic  acids. 
One  of  the  two  kinds  of  nucleic  acid,  desoxyribonucleic  acid  (abbrevi- 
ated DNA),  is  found  only  within  the  chromosomes,  nowhere  else  in  the 
cell.  This  fact,  plus  the  parallelism  between  the  number  of  genes  and 


584  GENETICS    AND    EVOLUTION 

the  amount  of  DNA  per  nucleus,  has  led  to  the  conclusion  that  DNA 
is  an  integral  part  oi  the  gene.  Microchemical  analyses  have  shown  that 
the  amount  ot  DNA,  like  the  number  ot  genes,  is  the  same  in  all  of 
the  somatic  cells  of  a  given  species,  and  that  there  is  only  half  as  much 
DNA  in  an  egg  or  sperm  as  there  is  in  a  somatic  cell  of  the  same 
species.  There  is  other  evidence  that  DNA  is  responsible  for  the  trans- 
mission of  genetic  information  from  one  generation  to  the  next.  "Trans- 
forming agents"  can  be  isolated  from  certain  strains  of  bacteria,  such 
as  the  one  causing  pneumonia,  which  will  transform  one  strain  of  bac- 
teria into  another.  These  agents,  with  j^roperties  quite  similar  to  those 
of  genes,  are  composed  solely  of  DNA.  DNA  is  the  carrier  of  genetic 
information  in  bacterial  viruses  (bacteriophages).  W^hen  a  bacteriophage 
enters  a  bacterium,  its  protein  coat  remains  outside;  only  the  core  of 
nucleic  acid  enters.  This  nucleic  acid  core  produces  many  additional 
bacteriophage  particles,  both  their  nucleic  acid  cores  and  their  specific 
protein  coats.  When  the  infected  bacterial  cell  finally  bursts,  many  bac- 
teriophage particles,  complete  with  protein  coats,  are  released. 

The  separation  of  the  nucleic  acid  part  from  the  protein  part  of  a 
plant  virus  has  been  achieved  by  W.  M.  Stanley.  The  nucleic  acid  part, 
but  not  the  protein  part,  has  some  weak  viral  activity  alone;  viral  activity 
returns  to  normal  when  the  two  parts  are  recombined.  Stanley  then 
added  nucleic  acid  isolated  from  one  virus  to  protein  obtained  from  an- 
other kind  and  found  that  the  new  "hybrid"  virus  had  the  genetic  prop- 
erties only  of  the  strain  which  contributed  the  nucleic  acid,  and  did 
not  resemble  the  strain  which  had  contributed  the  protein.  He  believes 
that  the  nucleic  acid  determines  the  biologic  properties  of  the  virus  and 
the  protein  forms  a  protective  coat  which  stabilizes  the  nucleic  acid. 
Evidence  from  the  experimental  production  of  gene  mutations  also 
favors  the  concept  that  DNA  is  an  essential  component  of  the  gene, 
for  the  physicochemical  properties  of  the  substance  which  mutates  and 
those  of  DNA  are  very  similar. 

Estimates  of  the  Number  and  Size  of  Genes.  We  have  fairly  reli- 
able estimates  of  the  number  of  genes  per  unit  length  of  chromosome 
in  organisms  such  as  corn  and  fruit  Hies.  If  we  assume  that  the  number 
of  genes  per  chromosome  in  man  is  comparable,  then  man  has  about 
25,000  pairs  of  genes  in  the  nucleus  of  each  cell.  The  error  in  this 
estimate  is  probably  no  more  than  five-fold,  and  the  true  number  of 
genes  lies  between  5,000  and   125,000. 

Early  estimates  of  the  size  of  a  gene  suggested  that  it  was  a  very 
large  particle,  with  a  molecular  weight  in  the  range  of  40,000,000  to 
60,000,000.  Hemoglobin,  an  average-sized  protein,  has  a  molecular  weight 
of  68,000.  More  recently,  estimates  of  gene  size  have  been  revised  down- 
ward to  perhaps  half  of  the  original  value.  At  one  time  it  was  believed 
that  a  gene  was  a  true,  indivisible  unit,  and  it  was  enthusiastically 
hailed  as  the  "basic  unit  of  life."  It  was  believed  that  the  unit  of  cross- 
ing over  in  the  chromosome,  the  unit  which  undergoes  mutation  to  form 
new  types  of  genes,  and  the  functional  unit  which  regulates  the  pheno- 
typic  appearance  of  the  character  are  all  the  same  unit,  the  gene.  It  is 
now  clear,  however,  that  these  units  have  quite  different  sizes,  the  muta- 


GENETICS  685 

tion  unit  being  much  smaller  and  the  functional  unit  perhaps  larger 
than  the  unit  of  crossing  over.  Our  concept  of  the  intimate  nature  of 
the  gene  is  being  revised  constantly  as  new  experimental  evidence  ap- 
pears. 

288.  Changes  in  Genes:  Mutations 

Although  genes  are  remarkably  stable  and  are  transmitted  to  suc- 
ceeding generations  with  gieat  fidelity,  they  do,  from  time  to  time, 
undergo  changes,  called  mutations.  After  a  gene  has  mutated  to  a  new 
form,  this  new  form  is  stable  and  usually  has  no  greater  tendency  to 
change  again  than  the  original  gene. 

Two  types  of  mutation  are  distinguished.  Some,  called  chromo- 
somal mutations,  are  accompanied  by  some  visible  change  in  the  struc- 
ture of  the  chromosome— the  deletion  or  duplication  of  a  small  segment 
of  the  chromosome,  the  translocation  of  a  segment  of  chromosome  to  a 
new  position  in  a  different  chromosome,  or  the  inversion,  turning  end 
for  end,  of  a  segment  of  chromosome.  Others,  called  point  mutations, 
have  no  visible  change  in  chromosome  structure  and  we  assume  that 
these  involve  such  small  alterations  at  the  molecular  level  that  they  are 
not  visible.  From  our  current  theory  that  genes  are  complex  nucleic 
acid  molecules,  we  can  guess  that  mutations  involve  some  change  in  the 
order  or  arrangement  of  the  nucleotide  units  of  the  DNA. 

Gene  mutations  can  be  induced  by  exposing  the  cell  to  radiation; 
x-rays,  gamma  rays,  cosmic  rays,  ultraviolet  rays  and  all  the  types  of 
radiation  which  are  by-products  of  atomic  power  are  effective  mutation 
agents.  Mutations  do  occur  spontaneously  at  low  but  measurable  rates 
which  are  characteristic  of  the  species  and  of  the  gene;  some  genes  are 
much  more  "mutable"  than  others.  Natural  radiations  such  as  cosmic 
rays  probably  play  some  role  in  causing  spontaneous  mutations,  but 
there  are  undoubtedly  other  important  factors.  The  rates  of  spontaneous 
mutation  of  different  human  genes  range  from  1  X  l^-^  to  1  X  10~^ 
mutations  per  gene  per  generation.  Since  man  has  a  total  of  some  2.5  X 
10^  genes,  this  means  that  the  total  mutation  rate  is  on  the  order  of  one 
mutation  per  person  per  generation.  Each  one  of  us,  in  other  words,  has 
some  mutant  gene  that  neither  of  our  parents  had. 

289.  Gene  Action 

There  is  a  tremendous  amplification  of  effect  in  the  train  of  events 
from  single  pairs  of  ionizations,  produced  by  the  passage  of  x-rays 
through  a  tissue,  to  a  gene  mutation  which  in  turn  produces  the  altered 
phenotypic  expression.  To  explain  this,  genes  are  believed  to  act  as 
catalysts  for  the  production  of  enzymes.  Enzymes  are  believed  to  owe 
their  specificity  to  the  specific  configuration  of  the  surface  of  the  molecule 
(p.  69).  Only  those  substances  whose  molecules  have  the  proper  shape 
can  fit  on  the  surface  of  the  enzyme,  make  contact  at  a  number  of  points, 
and  form  an  enzyme-substrate  complex.  According  to  our  present  theory, 
the  surface  of  the  gene  has  a  comparable  specific  conformation,  and  this 


586  GENETICS   AND    EVOLUTION 

A 


Su.bsti'a-'bc 


Enzyme-  substi'-ate,  complex 

i 


This  -molecule  is 
not  a.  Substra.tc.'" 
It  doc5  not  fit  on 


yjl TTEJ 


_r  the  e.nzyrae 


-  Surf  a.ce. 
J 


A  :»     B 

Split  prod-ucts 


B 

Qbtus 


G&n& 


Gene  molecule  -wibK 
specific  sui^fa-ce 


Ge-ne-  inolccule  produces 
a.  templa-te. 


/ 


Te.rapia.tc  molecule  produC£.S 
e.n-z,ymz  xnoleculcS  -with  specific  surf  a-ce 

Figure  33.5.  Diagram  comparing  the  theory  of  the  production  of  an  enzyme  mole- 
cule by  a  gene  via  a  template  with  the  theory  of  the  formation  of  an  enzyme-substrate 
complex. 

specific  conformation  is  transferred  either  directly  or  via  an  intermediate 
template  to  the  enzyme  (Fig.  33.5).  This  theory  requires  that  there  be  a 
separate  gene  for  each  type  of  enzyme,  and  there  is  quite  a  bit  of  experi- 
mental evidence  which  indicates  that  this  is  true. 

Our  current  idea  of  gene  function  may  be  summarized  as  follows: 
The  materials  transferred  from  one  generation  to  the  next  in  the  nucleus 
of  the  egg  and  sperm,  the  genes,  are  templates  composed  of  DNA  and 
protein.  These  templates  are  duplicated  and  are  distributed  during  cell 
division  to  all  the  daughter  cells  that  make  up  the  animal  or  plant 
body.  In  each  cell,  the  DNA,  either  by  itself  or  in  combination  with 
protein,  produces  an  intermediate  template  made  of  ribonucleic  acid 
and  protein.  This  intermediate  template  passes  from  the  nucleus  to  the 


GENETICS  587 

cytoplasm  of  the  cell,  where  it  in  turn  impresses  this  specific  surface  con- 
formation onto  a  protein  molecule  as  it  is  synthesized  and  converts  it 
into  the  specific  enzyme. 

If  we  assume  that  a  specific  gene  may  indeed  produce  a  specific 
enzyme  by  this  or  some  other  method,  we  must  next  inquire  how  the 
presence  or  absence  of  this  specific  enzyme  may  affect  the  development 
of  the  zygote.  The  expression  of  any  trait  is  the  result  of  a  number  of 
chemical  reactions  which  occur  in  series,  with  the  product  of  each  reac- 
tion serving  as  the  substrate  for  the  next:  A  -^  B  -^  C  -^  D.  The  dark 
color  of  most  mammalian  skin  and  fur  is  due  to  the  pigment  melanin 
(D),  produced  from  dihydroxyphenylalanine  (dopa)  (C),  produced  in 
turn  from  tyrosine  (B)  and  phenylalanine  (A).  Each  of  these  reactions  is 
mediated  by  a  particular  enzyme;  the  conversion  of  dopa  to  melanin, 
for  example,  is  controlled  by  the  enzyme  dopa  oxidase.  The  condition 
known  as  albinism,  characterized  by  the  absence  of  melanin,  results  from 
the  absence  of  dopa  oxidase.  The  gene  for  albinism,  a,  does  not  pro- 
duce the  enzyme  dopa  oxidase,  but  its  normal  allele,  A,  does. 

In  most  animals  and  plants  it  is  difficult  to  investigate  the  stepwise 
control  of  the  expression  of  a  character  except  those  in  which  some 
colored  product  is  formed.  This  difficulty  was  overcome  when  George 
Beadle  and  Edward  Tatum  conceived  the  idea  of  irradiating  the  bread 
mold,  Xeurospora,  and  looking  for  mutations  which  interfered  in  some 
way  with  the  normal  reactions  by  which  the  chemicals  essential  for  its 
growth  are  produced.  The  normal  bread  mold  requires  as  raw  materials 
only  sugar,  salts,  inorganic  nitrogen  and  biotin,  the  so-called  "minimal" 
medium  (Fig.  33.6).  By  exposing  the  mold  to  x-rays  or  ultraviolet  rays, 
a  great  many  mutations  were  produced.  After  irradiation  the  mold  was 
supplied  with  "complete"  medium,  an  extract  of  yeast  which  contains 
all  the  known  amino  acids,  vitamins,  and  so  on.  Any  nutritional  mutant 
produced  by  the  irradiation  will  thus  be  enabled  to  survive  and  repro- 
duce to  be  tested  subsequently. 

A  bit  of  the  irradiated  mold  is  then  placed  on  minimal  medium.  If 
it  is  unable  to  grow  we  know  that  a  mutant  has  been  produced  which 
interferes  with  the  production  of  some  compound  essential  for  growth. 
Then,  by  trial  and  error,  by  adding  substances  to  the  minimal  medium 
in  groups  or  singly,  the  nature  of  this  missing  substance  is  determined.  In 
each  instance  genetic  tests  show  that  the  mutant  strain  produced  by 
irradiation  differs  from  the  normal  wild  mold  by  a  single  gene,  and 
chemical  tests  show  that  if  a  single  chemical  substance  is  added  to  the 
minimal  medium  the  mutant  strain  can  grow  normally.  The  inference 
is  that  each  gene  produces  a  single  enzyme  which  regulates  one  step  in 
the  biologic  synthesis  of  this  chemical  substance.  It  has  been  possible 
in  some  instances  to  show  that  the  particular  enzyme  cannot  be  extracted 
from  cells  of  the  mutant  strain  but  can  be  extracted  from  those  of 
normal  Xeurospora.  The  synthesis  of  each  of  these  substances  includes  a 
number  of  separate  steps,  each  mediated  by  a  gene-controlled  enzyme. 
An  estimate  of  the  minimal  number  of  steps  involved  can  be  obtained 
from  the  number  of  different  mutants  which  interfere  with  its  pro- 
duction. 


688 


GENETICS   AND   EVOLUTION 


Trra.dia.te  to 
produce.  mutaJbionS 


"Wild  type'neurosporcL 


No  growth 
Min  im  al'ine  dium. 


Figure   33.6.     The  method  of  producing  and   testing  for  biochemical  mutants  in 
the  mold,  Neurospoia.  See   text  for  discussion. 

Similar  one-to-one  relationships  of  gene,  enzyme  and  biochemical 
reaction  in  man  were  first  described  by  the  English  physician  A.  E. 
Garrod,  in  1909.  Alcaptonuria  is  a  trait,  inherited  by  a  recessive  gene,  in 
which  the  patient's  mine  turns  black  on  exposure  to  the  air.  The  urine 
contains  homogentisic  acid;  the  tissues  ot  normal  people  have  an  enzyme 
which  oxidizes  homogentisic  acid  so  that  it  is  excreted  as  carbon  dioxide 
and  water.  Alcaptonurics  lack  this  enzyme  because  they  lack  the  gene 
which  produces  it.  As  a  result,  homogentisic  acid  accumulates  in  the 
tissues  and  blood  and  spills  over  into  the  urine.  Garrod  used  the  term 
"inborn  errors  ol  metabolism"  to  describe  alcaptonuria  and  comparable 
conditions  such  as  phenylketonuria  and  albinism. 

It  has  recently  been  found  that  when  a  wing  bud  from  a  creeper 
chick  is  transplanted  onto  normal  chick  blastoderm  it  will  develop  into 
a  normal  wing,  not  a  creeper  wing.  Evidently  the  creeper  gene  interferes 
with  the  production  of  some  substance  reqviired  for  normal  wing  de- 
velopment, a  substance  which  can  be  supplied  by  the  enzyme  systems 
of  the  normal  tissue.  If  this  missing  substance  could  be  identified  and 
supplied  to  a  fertilized  creeper  egg  in  suitable  amount,  the  egg  might 
develop  into  a  normal  rather  than  a  creeper  chick. 

The  identification  of  the  chemical  and  biologic  mechanisms  which 
underlie  differentiation  remains  one  of  the  major  unsolved  problems  of 


GENETICS  689 

this  field.  The  regularity  of  the  mitotic  process  appears  to  assure  every 
cell  of  the  body  the  same  number  and  kinds  of  genes  as  every  other  cell, 
yet  the  tissues  of  the  body  have  marked  differences  in  their  chemical, 
physical  and  biologic  properties.  These  differences  apparently  result 
from  the  different  metabolic  effects  of  similar  genes  working  in  different 
cytoplasmic  environments. 

One  of  the  clearest  demonstrations  that  the  same  genes  working  in 
dissimilar  environments  do  have  different  effects  was  provided  by  experi- 
ments with  three  races  of  frogs  found  naturally  in  Florida,  Pennsylvania 
and  Vermont.  Each  of  these  races  normally  develops  at  a  speed  which 
is  adapted  to  the  length  of  the  spring  and  summer  season  in  its  normal 
environment.  Southern  frogs  develop  slowly  and  Northern  frogs  develop 
more  rapidly.  Eggs  of  Northern  frogs  raised  under  Southern  conditions 
are  overaccelerated  in  development  whereas  eggs  of  Southern  frogs  raised 
under  Northern  conditions  are  overretarded.  By  fertilizing  an  egg  with 
sperm  of  a  different  race,  and  then  removing  the  egg  nucleus  before  the 
sperm  nucleus  unites  with  it,  it  is  possible  to  establish  a  situation  in 
which  Northern  genes  are  operating  in  Southern  cytoplasm  or  vice  versa. 
Northern  genes  for  rapid  development  in  Southern,  slow  developing 
cytoplasm  resulted  in  poorly  regulated  development;  the  animal's  head 
grew  more  rapidly  than  the  posterior  region  and  was  disproportionately 
large.  When  Southern  genes  were  introduced  into  Northern  cytoplasm 
theie  was  poorly  regulated  development  but  the  head,  rather  than  the 
posterior  region,  was  retarded  in  development.  Genes  from  the  Pennsyl- 
vania race  acted  as  "Northern"  with  Florida  cytoplasm,  but  as  "South- 
ern" with  Vermont  cytoplasm.  Thus,  exactly  the  same  set  of  genes  pro- 
duced opposite  morphologic  effects  when  acting  upon,  and  interacting 
with,  different  cytoplasmic  environments. 

290.        Cytoplasmic  Inheritance 

The  gene  theory  of  inheritance  is  well  established;  there  is  no  doubt 
that  the  genes  within  the  chromosomes  afford  the  physical  basis  for  the 
transmission  of  traits  from  one  generation  to  the  next.  The  question  as 
to  whether  the  genes  are  the  sole  means  of  inheritance,  or  whether  some 
characters  may  be  transmitted  by  other  means,  has  been  hotly  debated. 
No  definite  answer  can  be  made  at  present,  other  than  that  if  instances 
of  nongenic  inheritance  do  occur  they  are  quite  rare.  Some  of  the  ex- 
perimental evidence  which  at  first  was  interpreted  as  proof  of  cytoplasmic 
inheritance  has  since  been  shown  to  be  explainable  in  terms  of  the  usual 
genie  mechanism. 

The  contributions  of  egg  and  sperm  to  the  nucleus  of  the  zygote 
are  equal,  but  their  cytoplasmic  contributions  are  not.  If  any  trait  were 
inherited  by  factors  located  in  the  cytoplasm  and  independent  of  the 
genes,  the  offspring  would  resemble  the  mother  and  not  the  father.  ^Vith 
almost  all  traits  tested  in  a  wide  variety  of  plants  and  animals,  the  char- 
acters of  the  offspring  are  the  same  whether  the  cross  is  made  female 
AA  X  male  aa  or  female  aa  X  male  AA.  This  indicates  that  the  con- 


590  GENETICS    AND    EVOLUTION 

tributions  ot  the  male  and  female  gametes  are  equivalent.  Since  the 
nuclear  components  ot  egg  and  sperm  are  equivalent,  but  the  cytoplasmic 
components  are  not  (the  sperm  contributes  essentially  no  cytoplasm  to 
the  fertilized  egg),  this  is  a  strong  argument  that  the  cytoplasm  plays  at 
most  only  a  minor  role  in  the  transmission  of  hereditary  traits. 

The  direction  of  coiling  in  the  shell  of  the  snail  Limnaea  peregra 
is  inherited,  with  right-handed  coil  dominant  to  left-handed  coil.  The 
direction  of  coiling  in  an  individual  snail  is  governed,  however,  not  by 
its  own  genes,  but  by  those  of  its  mother.  The  results  of  reciprocal  crosses 
at  first  suggested  that  the  direction  of  coiling  was  inherited  by  some 
factor  transmitted  in  the  cytoplasm.  The  direction  of  coiling  is  deter- 
mined by  the  orientation  of  the  mitotic  spindle  apparatus  in  the  first 
two  cleavage  divisions.  The  orientation  of  the  spindle  is  in  turn  deter- 
mined by  some  action  of  the  maternal  genes  on  the  unfertilized  egg, 
during  its  maturation  within  the  ovary.  This  is  clearly  not  an  example 
of  cytoplasmic  inheritance,  but  of  normal  genie  inheritance. 

Some  of  the  clearest  evidence  for  cytoplasmic  inheritance  comes 
from  experiments  in  which  an  egg  is  fertilized,  then  deprived  of  the 
female  pronucleus  before  it  fuses  with  the  sperm.  The  embryo  which 
subsequently  develops  receives  all  of  its  nuclear  material  from  the  male 
parent  and  all  of  its  cytoplasmic  material  from  the  female  parent.  Such 
embryos  do  not  develop  very  far  and  usually  cease  development  in  the 
late  blastula  stage.  The  German  embryologist,  Hadorn,  fertilized  an  egg 
from  one  species  of  salamander  (Triton  palmatus)  with  sperm  from 
another  species  (T.  cristatus),  removed  the  egg  nucleus,  grew  the  embryo 
to  the  blastula  stage,  and  then  grafted  some  of  the  presumptive  epidermis 
of  the  blastula  onto  a  normal  larva  of  a  third  species,  T.  alpestris.  The 
transplanted  epidermis  was  able  to  survive  and  developed  into  adult 
skin  characteristic  of  the  species  palmatus,  which  had  contributed  its 
cytoplasm.  It  appears  that  the  cytoplasm,  not  the  nuclear  genes,  controls 
the  development  of  this  trait  in  these  animals. 

The  investigations  of  T.  M.  Sonneborn  of  the  inheritance  of  the 
"killer"  trait  in  paramecia  (p.  162)  provide  another  example  of  a  char- 
acter transmitted  in  the  cytoplasm  to  succeeding  generations.  Only  a  very 
few  other  traits  are  known  in  which  there  is  fairly  clear  evidence  that 
cytoplasmic  inheritance  occurs:  the  susceptibility  of  fruit  flies  to  carbon 
dioxide  poisoning,  and  the  inheritance  of  certain  respiratory  enzymes 
in  yeast. 

291.        Inheritance  of  Acquired  Characters 

It  was  generally  believed  at  one  time  that  traits  acquired  by  an 
individual  during  his  lifetime  by  some  effect  of  the  environment— by 
use,  training  or  accident— might  be  passed  on  to  his  offspring.  It  was  an 
important  part  of  certain  theories  as  to  how  evolution  occurs  (p.  698). 
The  development  of  the  science  of  genetics  has  shown  that  this  is  theo- 
retically improbable,  if  not  impossible,  and  no  experimental  evidence  to 
support  this  concept  has  ever  been  found.  Weismann,  whose  theory  of  the 


GENETICS  591 

continuity  of  the  germplasm  rules  out  the  inheritance  of  acquired  char- 
acters, cut  off  the  tails  of  generation  after  generation  of  mice.  The  tails 
of  the  nineteenth  generation  of  mice,  however,  were  just  as  long  as 
those  of  the  first.  In  an  even  more  searching  experiment,  Zeleny  raised 
250  successive  generations  of  fruit  flies  in  total  darkness,  yet  the  charac- 
ter of  the  eyes  remained  unaltered. 

292.        Human  Inheritance 

The  results  of  many  studies  have  shown  that  the  inheritance  of 
human  traits  follows  the  same  laws  as  those  of  other  animals  and  plants. 
Human  traits  may  be  controlled  by  multiple  factors,  multiple  alleles, 
sex-linked  genes,  and  so  on.  The  study  of  human  inheritance  offers 
special  difficulties,  not  only  because  test  crosses  cannot  be  made,  but  also 
because  human  beings  have  so  few  offspring  per  generation,  because 
the  large  number  of  years  between  successive  generations  means  that 
records  for  only  a  few  generations  are  available,  and  because  human 
beings  are  heterozygous  for  many  traits.  Careful  examination  of  pedigree 
records  has  revealed  that  the  inheritance  of  several  hundred  human 
traits  is  determined  by  single  dominant  or  recessive  genes. 

In  the  last  forty  years  statistical  methods  have  been  developed  which 
enable  investigators  to  pool  the  results  of  similar  matings  and  calculate 
the  relative  frequency  of  dominant  and  recessive  alleles.  Many  of  the 
methods  of  such  investigations  of  population  genetics  are  derived  from 
the  Hardy-Weinberg  Law.  The  study  of  L.  H.  Snyder  of  the  inheritance 
of  the  abdity  to  taste  phenylthiocarbamide  provides  an  example  of  the 
application  of  these  principles.  Snyder  tested  3643  people  and  found  that 
70.2  per  cent  reported  that  this  substance  has  a  bitter  taste  and  29.8 
per  cent  found  it  to  be  completely  tasteless.  If  this  trait  is  inherited  by  a 
single  pair  of  genes,  with  "tasting"  dominant  to  "nontasting,"  the  meth- 
ods of  population  genetics  permit  one  to  calculate  that  12.4  per  cent  of 
the  children  of  marriages  of  tasters  with  tasters  will  be  nontasters  and 
35.4  per  cent  of  the  children  of  marriages  of  tasters  with  nontasters  will 
not  be  able  to  taste  phenylthiocarbamide.  In  Snyder's  survey  the  per- 
centages actually  found  were  12.3  per  cent  and  33.6  per  cent,  respectively; 
the  close  agreement  of  the  theoretical  and  observed  values  indicates 
that  the  original  assumption  is  correct,  and  that  tasting  and  nontasting 
are  inherited  by  a  single  pair  of  genes. 

It  is  important  to  realize  that  not  all  of  the  characters  present  at 
birth  are  inherited  and  that,  conversely,  not  all  inherited  traits  are  evi- 
dent at  birth.  A  condition  present  at  birth  is  said  to  be  congenital;  some 
congenital  traits  are  inherited,  others  are  the  result  of  environmental 
influences  acting  during  development.  For  example,  if  a  woman  has 
German  measles  during  the  first  three  months  of  pregnancy  she  is  very 
likely  to  give  birth  to  a  blind,  deaf  or  deformed  child.  Many  inherited 
traits  are  not  evident  at  birth,  but  develop  some  time  later.  Amaurotic 
idiocy  becomes  expressed  during  childhood  and  Huntington's  chorea 
may  not  develop  until  a  person  is  40  years  old. 


592  GENETICS   AND    EVOLUTION 

293.        Heredity  and  Environment 

At  one  time  a  bitter  argument  raged  as  to  whether  heredity  or  en- 
vironment is  more  important  in  determining  human  traits.  It  is  now 
clear  that  the  two  are  interdependent  and  interact  in  many  ways  in  the 
development  oi  physical  and  mental  traits.  Some  genes,  tor  example 
the  ones  controlling  the  inheritance  of  the  blood  groups,  produce  their 
effects  regardless  of  the  environment.  The  expression  of  other  genes  may 
be  greatly  altered,  even  overcome  or  reversed,  by  environmental  inffu- 
ences.  Our  increasing  knowledge  of  biochemical  genetics  suggests  that  the 
greater  the  number  of  biochemical  reactions  there  are  interposed  be- 
tween a  gene  and  its  trait,  the  greater  will  be  the  opportunity  for  en- 
vironmental influences  to  produce  evident  changes  in  the  trait. 

It  is  sometimes  stated,  quite  incorrectly,  that  if  a  trait  has  a  genetic 
basis,  it  cannot  be  affected  by  altering  the  environment;  that  is,  inherited 
diseases  cannot  be  alleviated  or  cured  by  medical  treatment.  During 
World  War  II  experimenters  reported  that  feeding  large  doses  of  vitamin 
A  would  cure  color-blindness.  Vitamin  A  is  a  constituent  of  the  light- 
sensitive  pigment  of  the  cones,  visual  violet,  and  it  was  not  unreasonable 
that  administering  vitamin  A  might  cure  color-blindness.  The  gene  for 
color-blindness  might  in  some  way  alter  the  cones  so  that  a  higher  level 
of  vitamin  A  is  required  to  achieve  normal  vision.  The  experiments 
were  repeated  by  other  investigators,  none  of  whom  could  demonstrate 
any  effect  of  vitamin  A  on  color  vision.  The  original  authors  had  stated 
that  "since  color-blindness  is  curable  it  is  not  the  simple  mendelian 
trait  popular  theories  assume  it  to  be."  The  critics,  who  found  negative 
results,  argued  that  the  disease  is  inherited  and  therefore  incurable.  Both 
of  these  arguments  are  incorrect,  because  inherited  diseases  can  be  al- 
leviated. It  now  seems  clear  that  vitamin  A  in  the  doses  used  will  not 
improve  color  vision,  but  the  fact  that  color-blindness  is  inherited  does 
not  preclude  the  possibility  of  finding  some  way  to  enable  such  people 
to  see  color.  If,  for  example,  the  color-blind  gene  blocks  some  step  in  the 
synthesis  of  visual  violet,  supplying  the  substance  normally  made  by  this 
step  should  "cure"  color-blindness. 

Careful  studies  of  monozygotic  (identical)  twins  provide  an  estimate 
of  the  relative  importance  of  genetic  and  environmental  factors  in  the 
development  of  any  particular  trait.  Identical  twins,  which  develop  from 
a  single  fertilized  egg,  have  identical  genes;  any  differences  between  them 
are  due  to  environmental  factors.  Fraternal  twins,  which  develop  from 
separate  fertilized  eggs,  are  no  more  alike  than  ordinary  brothers  and 
sisters  born  separately.  Identical  twins  are  much  more  similar  in  intelli- 
gence, as  well  as  in  a  host  of  physical  traits,  than  are  fraternal  twins; 
indeed,  identical  twins  reared  apart  are  more  similar  in  intelligence  than 
fraternal  twins  reared  together.  Children  reared  together  in  an  orphan- 
age, where  the  environment  is  fairly  constant,  show  just  as  wide  vari- 
ability in  intelligence  as  children  reared  separately  in  their  own  homes. 
Even  when  children  are  adopted  early  in  infancy,  there  is  a  much  greater 
correlation  between  the  intelligence  of  the  child  and  its  true  parents 
than  between  the  child  and  its  foster  parents. 


GENETICS  693 

The  upper  limit  of  a  person's  mental  ability  is  determined  geneti- 
cally, but  training,  experience  and  other  environmental  influences  play 
a  role  in  determining  how  fully  the  inherited  abilities  are  developed. 
Since  the  coordinated  action  of  many  pairs  of  genes  is  involved  in  the 
inheritance  of  intelligence,  the  fortuitous  combination  of  genes  which 
produced  high  intelligence  in  one  or  both  parents  may  be  separated  so 
that  the  offspring  are  less  intelligent  than  either  parent.  Conversely,  the 
chance  combination  of  favorable  genes  may  produce  a  brilliant  child 
from  parents  of  average  intelligence;  however,  geniuses  are  never  pro- 
duced by  two  feeble-minded  parents, 

294.        Medical  Genetics 

Within  the  past  two  decades  rapid  progress  has  been  made  in  the 
analysis  of  human  genetics,  and  there  are  now  several  good  texts  of  medi- 
cal genetics  and  a  number  of  medical  schools  have  established  depart- 
ments or  courses  of  instruction  in  the  subject.  It  is  proving  possible  to 
detect  genetic  carriers  of  disease,  i.e.,  individuals  heterozygous  for  a 
recessive  trait  such  as  sickle  cell  anemia,  and  thus  to  provide  more  ac- 
curate estimates  of  the  probability  that  the  potential  offspring  of  a 
particular  couple  will  have  some  particular  inherited  trait.  The  proper 
use  of  our  knowledge  of  medical  genetics  permits  the  physician  to  iden- 
tify certain  diseases  more  accurately  and  at  an  earlier  stage  in  their 
development,  and  thus  to  begin  treatment  or  preventive  measures.  As 
the  infectious  diseases  are  gradually  conquered,  the  chronic  diseases, 
many  of  which  are  inherited,  become  more  important  in  medical  prac- 
tice. Inherited  conditions  cause  about  half  of  all  cases  of  blindness  and 
deafness,  and  they  play  a  role  in  diabetes,  epilepsy,  certain  heart  dis- 
eases, mental  disease,  cerebral  palsy,  arthritis  and  many  metabolic  dis- 
eases. A  knowledge  of  medical  genetics  is  useful  in  certain  medico-legal 
cases,  such  as  in  disputed  parentage,  which  was  discussed  previously. 

Questions 

1.  Distinguish  between  complementary  genes  and  supplementary  genes;  multiple  factors 
and  multiple  alleles;  penetrance  and  expressivity;  an  inherited  trait  and  a  congenital 
trait. 

2.  A  mating  of  an  albino  guinea  pig  and  a  black  one  gave  6  white  (albino),  3  black  and 
3  brown  offspring.  What  are  the  genotypes  of  the  parents?  What  kinds  of  offspring, 
and  in  what  proportions,  would  result  from  the  mating  of  the  black  parent  with 
another  animal  that  has  exactly  the  same  genotype  as  it  has? 

3.  Mating  a  red  Duroc-Jersey  hog  to  sow  A  (white)  gave  pigs  in  the  ratio  of  1  red  :  2 
sandy!  1  white.  Mating  this  same  hog  to  sow  B  (sandy)  gave  3  red  :  4  sandy  :  1  white 
pigs.  When  this  hog  was  mated  to  sow  C  (sandy)  the  litter  had  equal  numbers  of  red 
and  sandy  piglets.  Give  the  genotypes  of  the  hog  and  the  three  sows. 

4.  A  walnut-combed  rooster  is  mated  to  three  hens.  Hen  A  (walniU-combed)  has  off- 
spring in  the  ratio  of  3  walnut  :  1  rose.  Hen  B  (pea-combed)  has  offspring  in  the  ratio 
of  3  vvalnut  :  3  pea  :  1  rose  :  1  single.  Hen  C  (walnut-combed)  has  only  walnut-combed 
offspring.  What  are  the  genotypes  of  the  rooster  and  the  three  hens? 

5    The  size  of  egg  laid  by  one  variety  of  hens  is  determined  by  three  pairs  of  genes;  hens 

■  with  the  genotype  AABBCC  lay  eggs  weighing  90  grams  and  hens  with  the  genotype 

Tabbcc  lay  eggJ  weighing  30  grams.  Each  dominant  gene  adds  10  grams  to  the  weight 


594  GENETICS   AND    EVOLUTION 

of  the  egg.  Wlicn  a  hen  from  the  90  gram  strain  is  mated  with  a  rooster  from  the  30 
gram  strain,  the  hens  in  the  F-,  generation  lay  eggs  weighing  60  grams.  If  a  hen  and 
rooster  from  this  h\  generation  are  mated,  what  will  be  the  weights  of  the  eggs  laid 
by  the  hens  of  the  ¥2  generation? 

6.  Mrs.  Doe  and  Mrs.  Roe  had  babies  at  the  same  hospital  and  at  the  same  time.  Mrs. 
Doe  took  home  a  girl  and  named  her  Nancy.  Mrs.  Roe  received  a  boy  and  named  him 
Harry.  However,  she  was  sure  that  she  had  had  a  girl  and  brought  suit  against  the 
hospital.  Blood  tests  showed  that  Mr.  Roe  was  type  O,  Mrs.  Roe  was  type  AB,  Mr.  and 
Mrs.  Doe  were  both  type  B,  Nancy  was  type  A  and  Harry  was  type  O.  Had  an  ex- 
change occurred? 

7.  A  woman  who  is  type  O  and  rh  negative  is  married  to  a  man  who  is  type  AB  and  rh 
positive.  The  man's  father  was  type  AB  and  rh  negative.  What  are  the  genotypes  of 
the  man  and  woman  and  what  blood  types  may  occur  among  their  offspring?  Is  there 
any  danger  that  any  of  their  offspring  may  have  erythroblastosis  fetalis? 

8.  What  are  the  advantages  and  disadvantages  of  inbreeding? 

9.  A  certain  recessive  trait  occurs  in  a  human  population  with  a  frequency  of  about  1 
in  10,000.  What  proportion  of  the  population  are  heterozygous  for  this  gene? 

10.  Discuss  the  evidence  that  desoxyribonucleic  acid  is  an  integral  part  of  the  gene. 

11.  Differentiate  between  chromosomal  and  point  mutations.  How  would  you  define  a 
mutation? 

12.  Discuss  the  evidence  which  has  led  to  the  abandonment  of  the  theory  of  the  inheri- 
tance of  acquired  characters. 

Supplementary  Reading 

Two  more  advanced  texts  of  genetics  are  Sturtevant  and  Beadle,  An  Introduction  to 
Genetics,  and  Sinnott,  Dunn  and  Dobzhansky,  Principles  of  Genetics.  Genetics  in  the 
Twentieth  Century,  edited  by  L.  C.  Dunn,  is  a  collection  of  papers  presented  at  the 
Golden  Jubilee  of  Genetics  on  the  50th  anniversary  of  the  rediscovery  of  Mendel's  work. 
Some  articles  of  special  interest  are  those  on  the  history  of  genetics  by  H.  litis,  C.  Zirkle, 
W.  E.  Castle  and  H.  J.  Muller,  one  by  R.  B.  Goldschmidt  on  the  relations  of  genetics  to 
other  sciences,  one  by  G.  W.  Beadle  on  chemical  genetics,  and  those  on  practical  applica- 
tions of  genetic  knowledge  by  L.  H.  Snyder,  J.  W.  Gowen,  C.  C.  Little,  A.  Miintzing, 
J.  L.  Lush,  J.  C.  Walker  and  P.  C.  Mangelsdorf.  The  Chemical  Basis  of  Heredity,  edited 
by  W.  D.  McElroy  and  B.  Glass,  presents  the  discussions  of  a  symposium  held  in  June 
1956.  Although  some  of  the  discussions  are  rather  advanced,  there  is  much  of  general 
interest  in  the  book. 


CHAPTER  34 


The  Concept  of  Evolution 


The  preceding  chapters  have  served  as  an  introduction  to  the  immense 
variety  of  forms  of  life  which  inhabit  every  conceivable  place  on  land 
and  in  the  water,  and  exhibit  tremendous  variations  in  size,  shape,  de- 
gree of  complexity,  and  methods  of  obtaining  food,  of  evading  predators 
and  of  reproducing  their  kind.  How  all  these  species  came  into  exist- 
ence, how  they  came  to  have  the  particular  adaptations  which  make 
them  peculiarly  fitted  for  survival  in  a  particular  environment,  and  why 
there  are  orderly  degrees  of  resemblance  between  forms  which  permit 
their  classification  in  genera,  orders,  classes  and  phyla,  are  fundamental 
problems  of  zoology.  From  the  detailed  comparison  of  the  structures  of 
living  and  fossil  forms,  from  the  sequence  of  the  appearance  and  extinc- 
tion of  species  in  times  past,  from  the  physiologic  and  biochemical 
similarities  and  differences  between  species,  and  from  the  analyses  of 
heredity  and  variation  in  many  different  animals  and  plants  has  come 
one  of  the  great  unifying  concepts  of  biology,  that  of  evolution.  Evolu- 
tion is  not  a  new  topic  at  this  point,  for  it  has  been  fundamental,  both 
implicitly  and  explicitly,  to  many  of  the  subjects  discussed  previously. 

295.        The  Principle  of  Organic  Evolution 

The  term  evolution  means  an  unfolding,  or  unrolling,  a  gradual, 
orderly  change  from  one  state  to  the  next.  The  planets  and  stars,  the 
topography  of  the  earth,  and  the  chemical  compounds  of  the  universe 
have  undergone  gradual,  orderly  changes  sometimes  called  inorganic 
evolution.  The  principle  of  organic  evolution,  now  universally  accepted 
by  biologists,  simply  applies  this  concept  to  living  things:  all  the  various 
plants  and  animals  living  today  have  descended  from  simpler  organisms 
by  gradual  modifications  which  have  accumulated  in  successive  genera- 
tions. 

Evolution  is  continuing  to  occur;  indeed,  it  is  occurring  more 
rapidly  today  than  in  many  of  the  past  ages.  In  the  last  few  hundred 
thousand  years,  hundreds  of  species  of  animals  and  plants  have  become 
extinct  and  other  hundreds  have  arisen.  The  process  is  usually  too 
gradual  to  be  observed,  but  there  are  some  remarkable  examples  of 
evolutionary  changes  which  have  taken  place  within  historic  times.  For 
example,  some  rabbits  were  released  early  in  the  fifteenth  century  on  a 
small   island   near   Madeira   called  Porto   Santo.   There   were  no   other 

695 


596  GENETICS   AND    EVOLUTION 

rabbits  and  no  carnivorous  enemies  on  the  island  and  the  rabbits  mul- 
tipHed  at  an  amazing  rate.  In  400  years  they  became  quite  different  from 
the  ancestral  European  stock;  they  were  only  half  as  large,  had  a  different 
color  pattern,  and  were  more  nocturnal  animals.  Most  miportant,  they 
could  not  produce  offspring  when  bred  with  members  of  the  European 
species.  They  were,  in  fact,  a  new  species  of  rabbit. 

296.        Development  of  Ideas  about  Evolution 

The  idea  that  the  present  forms  of  life  have  arisen  from  earlier, 
simpler  ones  was  far  from  new  when  Charles  Darwin  published  The 
Origin  of  Species  in  1859.  The  oldest  speculations  about  evolution  are 
found  in  the  writings  of  certain  Greek  philosophers,  Thales  (624-548 
B.C.),  Anaximander  (588-524  b.c),  Empedocles  (495-435  b.c.)  and  Epi- 
curus (341-270  B.C.).  The  spirit  of  this  age  of  Greek  philosophy  was 
somewhat  similar  to  that  of  our  own  age,  for  simple,  natural  causes  were 
sought  to  explain  all  phenomena.  Since  they  knew  very  little  biology, 
however,  their  ideas  about  evolution  were  extremely  vague  and  can 
scarcely  be  said  to  foreshadow  our  present  theory  of  organic  evolution. 
Aristotle  (384-322  b.c),  who  was  a  great  biologist  as  well  as  a  philosopher, 
knew  a  great  deal  about  animals  and  plants  and  wrote  detailed,  accurate 
descriptions  of  many  of  them.  He  observed  that  organisms  could  be 
arranged  in  graded  series  from  lower  to  higher,  and  drew  the  correct 
inference  that  one  evolved  from  the  other.  However,  he  had  the  meta- 
physical belief  that  the  gradual  evolution  of  living  things  occurred  be- 
cause nature  strives  to  change  from  the  simple  and  imperfect  to  the  more 
complex  and  perfect.  An  evolutionary  explanation  of  the  origin  of  plants 
and  animals  was  given  by  the  Roman  poet  Lucretius  (99-55  b.c.)  in  his 
poem  De  Rerum  Natura. 

With  the  Renaissance,  interest  in  the  natural  sciences  quickened 
and  the  increasing  knowledge  of  the  many  kinds  of  animals  led  more 
and  more  scientists  to  consider  the  concept  of  evolution  favorably. 
Among  these  were  Hooke  (1635-1703),  Ray  (1627-1705),  Buffon  (1707- 
1788),  Erasmus  Darwin  (1731-1802)  and  Lamarck  (1744-1829).  Even  be- 
fore the  Renaissance  men  had  discovered  shells,  teeth,  bones  and  other 
parts  of  animals  buried  in  the  ground.  Some  of  these  corresponded  to 
parts  of  familiar,  living  animals,  but  others  were  strangely  unlike  any 
known  form.  Many  of  the  objects  found  in  rocks  high  in  the  mountains, 
far  from  the  sea,  resembled  parts  of  marine  animals.  In  the  fifteenth 
century,  the  versatile  artist  and  scientist,  Leonardo  da  Vinci,  gave  the 
correct  explanation  of  these  curious  finds,  and  gradually  his  conclusion, 
that  they  were  the  remains  of  animals  that  had  existed  at  one  time  but 
had  become  extinct,  was  accepted.  This  evidence  of  former  life  suggested 
to  some  people  the  theory  of  catastrophism— the  idea  that  a  succession  of 
catastrophes,  fires  and  floods,  have  periodically  destroyed  all  living 
things,  followed  each  time  by  the  origin  of  new  and  higher  types  by  acts 
of  special  creation. 

Three  Englishmen  in  the  eighteenth  and  early  nineteenth  centuries 
laid  the  foundations  of  modern  geology,  and  by  their  careful,  cogent 


THE  CONCEPT  OF   EVOLUTION  597 

arguments  advanced  the  theory  of  uniformitarianism  to  replace  the  con- 
cept of  catastrophism.  In  1785  James  Hutton  developed  the  concept  that 
the  geologic  forces  at  work  in  the  past  were  the  same  as  those  operating 
now.  He  arrived  at  this  conclusion  after  a  careful  study  of  the  erosion  of 
valleys  by  rivers,  and  the  formation  of  sedimentary  deposits  at  the 
mouths  of  rivers.  He  demonstrated  that  the  processes  of  erosion,  sedi- 
mentation, disruption  and  uplift,  carried  on  over  long  periods  of  time, 
could  account  for  the  formation  of  fossil-bearing  rock  strata.  The  pub- 
lication of  John  Playfair's  Illustrations  of  the  Huttonian  Theofy  of  the 
Earth  in  1802  gave  further  explanation  and  examples  of  the  idea  of 
uniformitarianism  in  geologic  processes.  Sir  Charles  Lyell,  one  of  the 
most  influential  geologists  of  his  time,  finally  converted  most  of  the  con- 
temporary geologists  to  the  theory  of  uniformitarianism  by  the  publica- 
tion of  his  Principles  of  Geology  (1832).  A  necessary  corollary  of  the  idea 
that  slowly  acting  geologic  forces  have  worn  away  mountains  and  filled 
up  seas  is  that  geologic  time  has  been  immensely  long.  This  idea,  com- 
pletely revolutionary  at  the  time,  paved  the  way  for  the  acceptance  of 
the  theory  of  organic  evolution,  for  the  process  of  evolution  requires 
an  extremely  long  time. 

The  earliest  theory  of  organic  evolution  to  be  logically  developed 
was  that  of  Jean  Baptiste  de  Lamarck,  the  great  French  zoologist  whose 
Philosophie  Zoologique  was  published  in  1809.  Lamarck,  like  most  biol- 
ogists of  his  time,  was  a  vitalist,  and  believed  that  all  living  things  are 
endowed  with  a  vital  force  that  controls  the  development  and  function- 
ing of  their  parts  and  enables  them  to  overcome  handicaps  in  the  en- 
vironment. He  believed  that  any  trait  acquired  by  an  organism  during 
its  lifetime  was  passed  on  to  succeeding  generations— that  acquired  char- 
acters are  inherited.  Developing  the  notion  that  new  organs  arise  in 
response  to  the  demands  of  the  environment,  he  postulated  that  the  size 
of  the  organ  is  proportional  to  its  use  or  disuse.  The  changes  produced 
by  the  use  or  disuse  of  an  organ  are  transmitted  to  the  offspring  and 
this  process,  repeated  for  many  generations,  would  result  in  marked 
alterations  of  form  and  function.  One  of  the  classic  illustrations  proposed 
by  Lamarck  is  the  evolution  of  the  long  neck  of  the  giraffe.  Lamarck 
suggested  that  the  short-necked  ancestor  of  the  giraffe  took  to  browsing 
on  the  leaves  of  trees,  instead  of  on  grass,  and  that,  in  reaching  up,  it 
stretched  and  elongated  its  neck.  The  offspring,  inheriting  the  longer 
neck,  stretched  still  farther,  and  the  process  was  repeated  untd  the 
present  long  neck  was  achieved. 

Both  Buffon  and  Erasmus  Darwin  had  similar  ideas  about  the  role 
in  evolution  of  the  direct  response  of  the  organism  to  its  environment, 
but  had  not  expressed  them  so  clearly.  This  theory,  called  Lamarckism, 
provides  a  fine  explanation  for  the  remarkable  adaptation  of  many 
plants  and  animals  to  their  environment,  but  is  completely  unacceptable 
because  of  the  overAvhelming  genetic  evidence  that  acquned  character- 
istics cannot  be  inherited.  The  theoretical  distinction  between  somato- 
plasm and  germ  plasm  made  by  Weismann  (1887)  refuted  all  theories 
of  evolution  based  on  the  inheritance  of  acquired  characters.  Acquired 
characters  are  present  only  in  the  body  cells  (somatoplasm)  and  not  in 


598  GENETICS    AND    EVOLUTION 

the  germ  cells  (germ  plasm),  and  only  traits  present  in  the  germ  plasm 
are  transmitted  to  the  next  generation. 

297.        Background  for  The  Origin  of  Species 

Charles  Darwin  made  two  great  contributions  to  the  body  of  scien- 
tific knowledge:  he  presented  a  wealth  of  detailed  evidence  and  cogent 
arguments  to  show  that  organic  evolution  had  occurred,  and  he  formu- 
lated a  theory,  that  of  natural  selection,  to  explain  the  mechanism  of 
evolution. 

Darwin  was  born  in  1809  and  was  sent  at  the  age  of  15  to  study 
medicine  at  the  University  of  Edinburgh.  Finding  the  lectures  intoler- 
ably dull,  he  transferred,  after  two  years,  to  Christ's  College,  Cambridge 
University,  to  study  theology.  Many  of  Darwin's  friends  at  Edinburgh 
were  interested  in  geology  and  zoology,  and  at  Cambridge  he  joined  a 
circle  of  friends  interested  in  collecting  beetles.  Through  them  he  came 
to  know  Professor  Henslow,  the  naturalist.  Shortly  after  leaving  college, 
and  upon  the  recommendation  of  Professor  Henslow,  Darwin  was  ap- 
pointed naturalist  on  the  ship  Beagle,  which  was  to  make  a  five-year 
cruise  around  the  world  preparing  navigation  charts  for  the  British 
Navy.  The  Beagle  left  Plymouth  in  1831  and  cruised  slowly  down  the 
east  coast  and  up  the  west  coast  of  South  America.  While  the  rest  of 
the  company  mapped  the  coasts  and  harbors,  Darwin  studied  the  ani- 
mals, plants  and  geologic  formations  of  both  coastal  and  inland  regions. 
He  made  extensive  collections  of  specimens  and  copious  notes  of  his 
observations.  The  Beagle  then  spent  some  time  at  the  Galapagos  Islands, 
west  of  Ecuador,  where  Darwin  continued  his  observations  of  the  flora 
and  fauna,  comparing  them  to  those  on  the  South  American  mainland. 
These  observations  convinced  Darwin  that  the  theory  of  special  creation 
was  inadequate  and  set  him  to  thinking  about  alternate  explanations. 

Upon  his  return  to  England  in  1836,  Darwin  spent  his  time  assem- 
bling the  notes  of  his  observations  for  publication  and  searching  for 
some  reasonable  explanation  for  the  diversity  of  organisms  and  the 
peculiarities  of  their  distribution.  As  Darwin  wrote  in  his  notebook: 

"On  my  return  home  in  the  autumn  of  1836  I  immediately  began  to  prepare  my 
journal  for  publication,  and  then  saw  how  many  facts  indicated  the  common  descent  of 
species.  ...  In  July  (1837)  I  opened  my  first  notebook  for  facts  in  relation  to  the  origin 
of  species,  about  which  I  had  long  reflected,  and  never  ceased  working  for  the  next  twenty 
years.  .  .  .  Had  been  greatly  struck  from  about  the  month  of  March  on  character  of  South 
American  fossils,  and  species  on  Galapagos  Archipelago.  These  facts  (especially  latter) 
origin  of  all  my  views.  .  .  . 

"In  October  (1838),  that  is  fifteen  months  after  I  had  begun  my  systematic  inquiry,  I 
happened  to  read  for  amusement  Maltlius  on  Population,  and  being  well  prepared  to 
appreciate  the  struggle  for  existence  which  everywhere  goes  on,  from  long-continued 
observation  of  the  habits  of  animals  and  plants,  it  at  once  struck  me  that  under  these 
circumstances  favorable  variations  would  tend  to  be  preserved,  and  unfavorable  ones  to 
be  destroyed.  The  result  of  this  would  be  the  origin  of  new  species.  Here  then  I  had  at 
last  got  a  theory  by  which  to  work." 

Darwin  spent  the  next  twenty  years  accimiulating  data  from  many 
fields  of  biology,  examining  it  critically,  and  building  up  a  tremendous 


THE   CONCEPT  OF   EVOLUTION  599 

body  of  facts  that  demonstrated  that  evolution  had  occurred,  and  formu- 
lating his  arguinents  for  natural  selection.  In  1857  he  submitted  a  draft 
of  his  theory  to  a  number  of  scientific  friends  for  comment  and  criticism. 
Alfred  Russell  Wallace,  a  naturalist  and  explorer  who  was  studying  the 
flora  and  fauna  of  Malaya  and  the  East  Indies,  was  similarly  struck  by 
the  diversity  of  living  things  and  the  peculiarities  of  their  distribution. 
Like  Darwin,  he  happened  to  read  Malthus'  treatise  and  came  inde- 
pendently to  the  same  conclusion,  that  evolution  occurred  by  natural 
selection.  In  1858  Wallace  sent  a  manuscript  to  Darwin,  and  asked  him, 
if  he  thought  it  of  sufficient  interest,  to  present  it  to  the  Linnaean 
Society.  Darwin's  friends  persuaded  him  to  present  an  abstract  of  his  own 
work  along  with  Wallace's  paper  and  this  was  done  at  a  meeting  of  the 
Linnaean  Society  in  July,  1858.  Darwin's  monumental  On  the  Origin  of 
Species  by  Means  of  Natural  Selection  was  published  in  November,  1859. 
The  time  was  ripe  for  the  formulation  and  acceptance  of  the  theory 
of  organic  evolution.  The  publication  of  Lyell's  Principles  of  Geology^ 
and  the  subsequent  acceptance  of  the  idea  of  geologic  evolution,  the 
publication  of  Malthus'  ideas  on  population  growth  and  pressure  and 
the  struggle  for  existence,  together  with  the  vast  accumulation  of  in- 
formation about  the  distribution  of  living  and  fossil  forms  of  life,  and 
studies  of  comparative  anatomy  and  embryology,  all  showed  the  inade- 
quacy of  the  theory  of  special  creation.  Because  the  time  was  ripe,  Dar- 
win's theory  rapidly  gained  acceptance. 

298.        The  Theory  of  Natural  Selection 

Darwin's  explanation  of  the  way  in  which  evolution  occurs  can  be 
summarized  as  follows: 

1.  Variation  is  characteristic  of  every  group  of  animals  and  plants, 
and  there  are  many  ways  in  which  organisms  may  differ.  (Darwin  did 
not  understand  the  cause  of  variation,  and  assumed  it  was  one  of  the 
innate  properties  of  living  things.  VV^e  now  know  that  inherited  varia- 
tions are  caused  by  mutations.) 

2.  More  organisms  of  each  kind  are  born  than  can  possibly  obtain 
food  and  survive.  Since  the  number  of  each  species  remains  fairly  con- 
stant under  natural  conditions,  it  must  be  assumed  that  most  of  the 
offspring  in  each  generation  perish.  If  all  the  offspring  of  any  species 
remained  alive  and  reproduced,  they  would  soon  crowd  all  other  species 

from  the  earth. 

3.  Since  more  individuals  are  born  than  can  survive,  there  is  a 
struggle  for  survival,  a  competition  for  food  and  space.  This  contest 
may  be  an  active  kill-or-be-kiUed  struggle,  or  one  less  immediately 
apparent  but  no  less  real,  such  as  the  struggle  of  plants  or  animals  to 
survive  drought  or  cold.  This  idea  of  competition  for  survival  in  an 
overpopulated  world  was  derived  from  Malthus. 

4  Some  of  the  variations  exhibited  by  living  things  make  it  easier 
for  them  to  survive;  others  are  handicaps  which  bring  about  the  ehmina- 
tion  of  their  possessors.  This  idea  of  "the  survival  of  the  fittest  is  the 
core  of  the  theory  of  natural  selection. 


700  GENETICS    AND    EVOLUTION 

5.  The  surviving  individuals  will  give  rise  to  the  next  generation, 
and  in  this  way  tlie  "successful"  variations  are  transmitted  to  the  suc- 
ceeding generations.  The  less  fit  will  tend  to  be  eliminated  before  they 
have  reproduced. 

Successive  generations  in  this  way  tend  to  become  better  adapted  to 
their  environment;  as  the  environment  changes,  further  adaptations 
occur.  The  operation  of  natural  selection  over  many  generations  may 
produce  descendants  which  are  quite  different  from  their  ancestors, 
different  enough  to  be  separate  species.  Furthermore,  certain  members 
of  a  population  with  one  group  of  variations  may  become  adapted  to 
the  environment  in  one  way,  while  others,  with  a  different  set  of  varia- 
tions, become  adapted  in  a  different  way,  or  become  adapted  to  a  differ- 
ent environment.  In  this  way  two  or  more  species  may  arise  from  a  single 
ancestral  stock. 

Animals  and  plants  exhibit  many  variations  which  are  neither  a 
help  nor  a  hindrance  to  them  in  their  struggle  for  survival.  These  are 
not  affected  directly  by  natural  selection  but  are  transmitted  to  suc- 
ceeding generations. 

Darwin's  theory  of  natural  selection  was  so  reasonable  and  well 
documented  that  most  biologists  soon  accepted  it.  One  of  the  early,  seri- 
ous objections  to  the  theory  was  that  it  did  not  explain  the  appearance 
of  many  apparently  useless  structures  in  an  organism.  We  now  know 
that  many  of  the  visible  differences  between  species  are  not  important 
for  survival,  but  are  simply  incidental  effects  of  genes  that  have  other 
physiologic  effects  of  great  survival  value.  Other  nonadaptive  differences 
may  be  controlled  by  genes  that  are  closely  linked  in  the  chromosomes 
to  genes  for  traits  which  are  important  for  survival. 

Another  of  the  early  objections  to  the  theory  was  that  new  variations 
would  be  lost  by  "dilution"  as  the  individuals  possessing  them  bred  with 
others  without  them.  We  now  know  that  although  the  phenotypic  ex- 
pression of  a  gene  may  be  altered  when  the  gene  exists  in  combination 
with  certain  other  genes,  the  gene  itself  is  not  altered  and  is  transmitted 
unchanged  to  succeeding  generations. 

299.        Modern  Changes  in  the  Theory  of  Natural  Selection 

The  rediscovery  of  Mendel's  laws  in  1900  made  necessary  two  major 
corrections  to  the  theory  of  natural  selection:  (1)  only  inherited  varia- 
tions can  provide  the  raw  material  for  natural  selection,  and  (2)  in- 
cipient species  must  be  separated  by  some  sort  of  geographic  or  ecologic 
isolation  to  prevent  interbreeding. 

Modifications  and  Mutations.  Darwin  did  not  clearly  distinguish 
between  variations  resulting  from  some  chemical  or  physical  action  of 
the  environment  on  the  developing  individual,  and  variations  resulting 
from  some  alteration  of  the  hereditary  materials,  the  genes  and  chromo- 
somes. The  first  type  of  variations,  called  modifications,  are  not  in- 
heritable and  play  no  role  in  evolution,  but  variations  arising  from 
changes  in  the  genes  or  chromosomes,  called  mutations,  are  the  raw 
materials  for  evolution  by  natural  selection.  Evolution,  clearly,  cannot 


THE   CONCEPT   Of   EVOLUTION 


701 


take  place  without  mutation,  and  although  natural  selection  does  not 
create  new  characteristics  it  plays  an  important  part  in  determining 
which  of  them  shall  survive. 

Isolation.  The  differentiation  of  a  new  group  of  organisms  requires 
that  they  be  prevented  from  breeding  with  their  relatives  and  in 
this  way  passing  to  them  whatever  new  genes  have  appeared.  Interbreed- 
ing must  be  prevented  by  some  sort  of  isolation. 

Perhaps  the  commonest  type  of  isolation  is  geographic,  whereby 
groups  of  related  organisms  become  separated  by  some  physical  barrier, 
a  sea,  mountain,  desert,  glacier  or  river  (Fig.  34.1).  In  mountainous 
regions  the  individual  ranges  provide  effective  barriers  between  the 
valleys,  and  there  are  usually  a  greater  number  of  different  species  in  a 
given  area  than  in  a  comparable  area  of  the  plains.  For  example, 
twenty-three  species  and  subspecies  of  rabbits  are  known  in  the  moun- 
tains of  the  western  United  States  but  only  eight  species  are  found  in 
the  larger  plains  area  of  the  Midwest  and  East.  Valleys  only  a  short  dis- 
tance apart,  but  separated  by  ridges  perpetually  covered  with  snow,  may 
each  have  species  of  plants  and  animals  not  found  in  the  other.  One  of 
the  most  striking  examples  of  geographic  isolation  is  provided  by  the 
area  divided  by  the  Isthmus  of  Panama.  On  either  side  of  the  Isthmus 
the  phyla  and  classes  of  marine  invertebrates  are  made  up  of  different 


Nissan 


Bougainville 


Malaita. 


Sa.n  Cristobal 

Figure  34  1  The  distribution  of  the  subspecies  o£  the  golden  whistler  (Pachy- 
cephala  pectoralis)  in  the  Solomon  Islands.  A  major  factor  in  the  evolution  of  these 
subspecies  has  been  their  geographic  isolation  on  separate  islands.  Green-colored 
plumage  is  indicated  by  cross-hatching;  yellow  by  light  gray  tone.  (Modified  from 
Dobzhansky.) 


702  GENETICS    AND    EVOLUTION 

but  closely  related  species.  For  some  16,000,000  years  during  the  Tertiary 
period  there  was  no  connection  between  North  and  South  America  and 
animals  could  migrate  freely  between  what  is  now  the  Gulf  of  Mexico 
and  the  Pacific  Ocean.  When  the  Isthmus  reemerged  the  closely  related 
groups  of  animals  were  separated,  and  the  differences  between  the  two 
fauna  today  represent  the  mutations  which  have  accumulated  since. 

Geographic  isolation  is  usually  not  permanent  and  two  closely  re- 
lated but  previously  isolated  groups  may  come  into  contact  and  inter- 
breed unless  genetic  isolation,  or  intersterility,  has  arisen.  The  several 
races  of  man  arose  by  geographic  isolation  and  the  accumulation  of 
chance  mutations,  but  since  interracial  sterility  has  not  developed  the 
differences  begin  to  disappear  rapidly  when  geographic  isolation  breaks 
down  and  interbreeding  occurs. 

Genetic  isolation  results  from  one  or  more  mutations  which  occur 
by  chance,  independently  of  other  mutations.  The  mutations  for  inter- 
sterility may  arise  only  after  a  long  period  of  geographic  isolation  has 
produced  marked  differences  between  two  groups  of  organisms,  or  they 
may  arise  within  a  single,  otherwise  homogeneous  group.  An  example 
of  the  latter  is  found  in  the  fruit  fly,  DrosopJiila  pseudoobscura,  which 
consists  of  two  varieties,  externally  indistinguishable,  yet  completely 
sterile  when  crossed.  The  two  groups  are  isolated  as  effectively  as  if  they 
lived  on  different  continents.  In  succeeding  generations,  as  mutations 
accumulate  in  the  two  groups  by  chance  and  selection,  they  will  un- 
doubtedly become  visibly  different.  Biologists  do  not  usually  consider 
two  closely  related  but  different  groups  of  organisms  to  be  separate 
species  unless  genetic  isolation  has  occurred. 

Two  groups  of  closely  related  animals  living  in  the  same  geographic 
area  may  nevertheless  be  effectively  isolated  if  they  occupy  different 
habitats.  This  is  called  ecologic  isolation;  marine  animals,  for  example, 
that  live  in  the  intertidal  zone  are  effectively  isolated  from  others  living 
only  a  few  feet  away  below  the  low-tide  mark.  Two  groups  of  animals 
that  breed  at  different  times  of  the  year  are  also  effectively  prevented 
from  interbreeding;  this  might  be  called  physiologic  isolation. 

Ro/e  of  Natural  Selection.  Darwin  assumed  that  the  variation  in  a 
particular  character  would  continue  to  occur,  so  that  natural  selection 
would  operate  indefinitely.  From  the  facts  of  heredity  in  the  previous 
chapters  it  should  be  clear  that  selection  can  operate  only  until  the 
population  becomes  homozygous  for  all  the  genes  for  that  trait— for  large 
body  size,  for  example.  After  that  condition  has  been  reached,  neither 
artificial  nor  natural  selection  can  affect  the  trait  further  until  additional 
mutations  for  large  body  size  have  occurred. 

It  must  be  emphasized  that  natural  selection  can  operate  only  upon 
the  organism  as  a  whole,  not  upon  its  individual  traits.  One  organism 
may  survive  despite  certain  obviously  disadvantageous  characters,  while 
another  may  be  eliminated  despite  traits  which  appear  to  be  very  helpful 
for  getting  along  in  life.  The  animals  and  plants  that  win  the  struggle 
for  existence  are  usually  not  perfectly  adapted  to  their  environment,  but 
have  qualities  the  sum  of  which  renders  them  a  little  more  capable  of 
surviving  and  reproducing  than  their  competitors.  Since  the  environment 


THE   CONCEPT   Of   EVOLUTION  703 

itself  may  change  as  time  passes,  a  trait  which  has  adaptive  value  at  one 
time  may  be  useless  or  even  detrimental  at  another. 

It  has  only  recently  been  appreciated  that  chance,  as  well  as  natural 
selection,  may  play  a  role  in  evolution.  Chance  may  play  a  role  in  de- 
termining whether  a  new  mutation  will  be  passed  from  the  individual 
originally  possessing  it  to  succeeding  generations.  During  the  process  of 
meiosis  the  new  mutant  gene  may  or  may  not  be  included  in  the  gametes 
which  produce  the  zygotes;  even  if  it  is  included  in  one  or  a  few  of  these 
zygotes,  a  series  of  unlucky  accidents  may  eliminate  these  organisms 
despite  the  high  survival  value  of  the  new  trait. 

300.  Genetic  Drift 

Professor  Sewall  Wright  of  the  University  of  Chicago  has  described 
the  role  of  chance  in  the  phenomenon  of  "genetic  drift."  This  is  the 
tendency  within  small  interbreeding  populations  for  those  gene  pairs 
which  are  heterozygous  to  become  homozygous  for  one  allele  or  the 
other  by  chance,  even  though  neither  gene  is  particularly  advantageous 
or  disadvantageous.  Each  small,  interbreeding  group  thus  tends  to  be- 
come homozygous,  genetically  stable.  A  group  which  becomes  stabilized 
by  chance  rather  than  by  selection  is  likely  to  have  certain  disadvanta- 
geous characteristics  and  therefore  to  be  eliminated  subsequently.  The 
effects  of  genetic  drift  are  counterbalanced  in  some  groups  by  the  effects 
of  mutation,  selection,  and  occasional  matings  with  members  of  another 
group.  Investigations  have  shown  that  in  nature  most  populations  of 
animals  are  indeed  subdivided  into  several  or  many  subgroups  which 
may  be  small  enough  to  be  affected  by  genetic  drift. 

The  phenomenon  of  genetic  drift,  the  tendency  of  small  populations 
to  become  homozygous,  is  an  exception  to  the  Hardy-Weinberg  Law 
(p.  682),  the  tendency  for  populations  to  maintain  their  proportions  of 
homozygous  and  heterozygous  individuals.  Since  the  Hardy-VV^einberg 
Law  is  based  on  statistical  events,  it,  like  all  statistical  laws,  does  not  hold 
true  for  small  numbers.  The  phenomenon  of  genetic  drift  becomes  im- 
portant in  evolution  whenever  the  effective  breeding  population  of  a 
species  becomes  small,  as  the  result,  perhaps,  of  extreme  cold,  drought,  a 
severe  storm  or  the  migration  of  a  small  group  to  a  new  territory.  Genetic 
drift  may  help  explain  the  common  observation  that  similar  and  closely 
related  species  in  different  parts  of  the  world  frequently  differ  in  curious 
ways  which  have  no  apparent  adaptive  value. 

301 .  Preadaptation 

One  of  the  more  recent  modifications  of  the  theory  of  natural  se- 
lection is  called  the  theory  of  preadaptation.  Mutations  occur  completely 
at  random,  and  some  result  in  characters  which  are  either  unimportant 
or  disadvantageous  to  the  organism  in  its  usual  environment.  However, 
if  the  environment  changes,  or  if  the  organisms  migrate  to  a  new  en- 
vironment, these  same  traits  may  be  of  marked  value  for  survival.  In 
effect,  an  animal  or  plant  may  by  chance  be  adapted  to  an  environment 


704  GENETICS    AND    EVOLUTION 

before  being  exposed  to  it.  Let  us  suppose  that  a  mutation  occurs 
wliich  causes  both  eyes  of  a  fish  to  develop  on  the  same  side  of  the 
skull.  If  the  fish  continues  in  its  old  habits  this  will  be  a  definite 
handicap.  But  if  it  changes  its  mode  of  life  and  lies  on  its  side  at  the 
bottom  of  the  sea  and  grubs  in  the  mud  for  food,  the  new  arrangement 
will  be  advantageous.  This  mutation  actually  has  occurred  in  the 
flounder  and  sole. 

The  theory  of  preadaptation  provides  a  reasonable  explanation  for 
occurrences  such  as  the  evolution  of  land  forms.  For  example,  in  a  species 
of  fish  inhabiting  a  lake  or  river  of  the  Devonian  Period,  some  350,- 
0()(),()0()  years  ago,  mutations  may  have  occurred  for  the  formation  of 
primitive  lungs  and  for  changing  the  fan-shaped  fins  to  sturdier,  limblike 
fins  with  a  fleshy  lobe  at  the  base.  These  changes  would  have  had  no 
survival  value  for  the  fish  as  long  as  it  lived  in  a  lake  or  stream.  Indeed, 
the  loss  of  the  fan-shaped  fins  might  have  been  deleterious,  by  interfering 
with  its  ability  to  swim  rapidly.  The  Devonian  Period  was  one  of  violent 
climatic  changes,  with  seasons  of  drought  alternating  with  rainy  seasons. 
As  the  streams  dried  up  during  one  of  the  periods  of  drought,  the  water 
became  stagnant  and  lacked  enough  oxygen  for  the  gills  to  function 
properly  in  respiration.  The  fish  with  lungs,  however,  could  come  to 
the  surface,  take  a  gulp  of  air,  and  obtain  oxygen  by  diffusion  across  the 
membrane  lining  the  lungs.  When  the  pond  or  stream  dried  up  com- 
pletely, he  could  use  his  sturdy,  lobe-shaped  fins  to  help  squirm  across 
the  intervening  land  to  some  other  stream.  Some  process  such  as  this 
probably  began  the  conquest  of  the  land  by  vertebrates.  Certainly  the 
first  vertebrates  to  venture  out  of  the  water  onto  land  were  not  seeking 
air,  for  they  and  their  ancestors  had  lungs  and  they  could  get  air  by 
coming  to  the  surface  of  the  water.  It  is  unlikely  that  they  were  fleeing 
from  predators,  for  they  were  among  the  largest  animals  of  the  time. 
Since  they  ate  other  fish,  and  the  only  food  on  land  consisted  of  plants 
and  insects,  it  can  hardly  be  supposed  that  they  were  looking  for  food. 
We  are  led  to  the  somewhat  paradoxical  conclusion  that  the  first  verte- 
brates to  come  out  on  land  may  have  been  looking  for  water,  for  their 
own  stream  had  just  dried  up! 

302.        Mutations,  the  Raw  Material  of  Evolution 

The  Dutch  botanist  Hugo  de  Vries,  one  of  the  three  rediscoverers 
of  Mendel's  laws,  was  the  first  to  emphasize  the  importance  in  evolution 
of  sudden,  large  changes  rather  than  the  gradual  accumulation  of  many 
small  changes  postulated  by  Darwin.  In  his  experiments  with  plants, 
such  as  the  evening  primrose,  de  Vries  found  that  many  unusual  forms, 
which  differed  markedly  from  the  ancestral  wild  plant,  appeared  and 
bred  true  thereafter.  He  applied  the  term  mutations  to  these  sudden 
changes  in  the  characteristics  of  an  organism  (earlier  breeders  had 
called  them  "sports").  Darwin  had  observed  such  changes,  but  thought 
they  occurred  too  rarely  to  be  of  importance  in  evolution.  Darwin  be- 
lieved that  these  sudden  changes  would  upset  the  harmonious  relations 
between   the  various  parts  of   an   organism  and   its  adaptation    to   the 


THE   CONCEPT   Of   EVOLUTION 


705 


environment.  Thousands  of  breeding  experiments  with  plants  and  ani- 
mals since  the  turn  of  the  century  have  shown  that  such  mutations  do 
occur  constantly  and  that  their  effects  may  be  of  adaptive  value.  With 
the  development  of  the  gene  theory,  the  term  mutation  has  come  to  refer 
to  sudden,  discontinuous,  random  changes  in  the  genes  and  chromo- 
somes, although  it  is  still  used  to  some  extent  to  refer  to  the  new  type 
of  plant  or  animal. 

In  the  plants  and  animals  most  widely  used  in  breeding  experiments 
—corn  and  fruit  flies— some  400  to  600  mutations,  respectively,  have  been 
detected.  The  fruit  fly  mutations  are  tremendously  varied,  and  in- 
clude all  shades  of  body  color  from  yellow  through  brown  and  gray  to 
black;  red,  white,  brown  or  purple  eyes;  crumj^led,  curled,  shortened, 
and  peculiarly  shaped  wings— even  the  complete  absence  of  wings;  oddly 
shaped  legs  and  bristles;  and  such  extraordinary  changes  as  a  pair  of 
legs  growing  on  the  forehead  in  place  of  the  antennae  (Fig.  34.2).  Mu- 
tations are  found  in  domestic  animals;  the  six-toed  cats  of  Cape  Cod  and 
the  short-legged  Ancon  sheep  are  two  of  many  examples  of  the  per- 
sistence of  a  single  mutation. 

Early  in  the  present  century  there  was  a  heated  discussion  as  to 


FORKED 


DICHAHE 


RUDIMENTARY 


VESTIGIAL 


CURLED 


STUBBLE  MINIATURE  SCUTE  CROSSVEINLESS  CUT 

Figure  34.2.     Some  wing  and  bristle  mutants  in  the  fruit  fly,  Drosophila  melano- 
gaster.  (Drawn  by  E.  M.  Wallace;  Sturtevant  and  Beadle:  An  Introduction  to  Genetics.) 


706  GENETICS   AND    EVOLUTION 

whether  evolution  was  the  result  of  natural  selection  or  of  mutation. 
As  more  was  learned  about  heredity,  it  became  clear  that  natural  selec- 
tion can  operate  only  when  there  is  something  to  be  selected,  that  is, 
when  mutations  present  alternate  ways  of  coping  with  the  environment. 
The  evolution  of  new  species,  then,  involves  both  mutation  and  natural 

selection. 

One  of  the  current  controversies   in  evolutionary  theory  concerns 
the   possible  role  of  small   and  large  mutations   in   the   origin   of  new 
species.  The  Neo-Darwinists  argue  that  new  species  (and  all  the  higher 
categories)  evolve  by   the   gradual  accumulation   of  many  small   muta- 
tions;   thus   there   should  exist   many   forms   intermediate   between   the 
original   species   and   the   new  one.   Other   biologists   believe    that  new 
species  and  genera  arise  in  a  single  step  by  a  macromutation,  a  major 
change  in  the  genetic  system  which  produces  a  major  change  in  the  pat- 
tern of  development.  This  results  in  an  adult  which  is  morphologically 
and  physiologically  quite   different   from  its   parents.  The  macromuta- 
tionists  would  hold  that  one  should  not  expect  to  find  forms  which  are 
intermediate    between    the    original    species    and    the    new    one.    Many 
macromutations  result   only   in   "monsters"   which  would  be   unable   to 
survive.  (The  term  monster  simply  means  any  form  which  is  markedly 
different  from  the  usual   type  of   the  species,  and  does  not  necessarily 
imply   that  it   is   ugly.)  Other  macromutations   may   give   rise    to  what 
Richard   Goldschmidt   of    the    University   of   California    calls    "hopeful 
monsters,"   organisms  which  are  enabled  by   their  mutation   to   occupy 
some   new   environment.  The  evolution   of   the  extinct  ancestral   bird, 
Archaeopteiyx,  into  modern  birds,  he  believes,  may  have  occurred  by  a 
macromutation.  Archaeopteryx   (Fig.  24.7)   had  a  long,  reptile-like   tail 
covered  with  feathers;  a  macromutation  which  altered  development  so 
that  the  tail  was  gieatly  shortened  would  result  in  a  "hopeful  monster" 
with  the  fan-shaped  arrangement  of  tail  feathers  seen  in  modern  birds. 
This  new  shape  of  the  tail,  which  is  better  suited  for  flying  than  the 
long    tail    of   Archaeopteryx,   gave    its    possessors    an    advantage    in    the 
struggle  for  existence.  There  is,  of  course,  no  proof  that  modern  birds 
evolved  in  this  way,  but  there  is  ample  evidence  that  similar  marked 
skeletal  changes  may  result  from  a  single  mutation.  The  stubby  tail  of 
the  Manx  cat  is  the  result  of  a  mutation  which  causes  the  tail  vertebrae 
to  shorten  and  fuse.   Professor  Goldschmidt  does  not  deny  that  small 
mutations  may  occur  and  accumulate,  but  holds  that  they  can  lead  only 
to   varieties  or   geographic   races,   and    not   to   species,   genera   and   the 
higher   taxonomic  divisions. 

The  causes  of  natural  or  spontaneous  mutations  are  unknown.  Both 
gene  and  chromosome  mutations  can  be  produced  artificially  by  a  vari- 
ety of  agents:  x-rays,  alpha,  beta  and  gamma  rays  emitted  by  radio- 
active elements,  neutrons,  ultraviolet  rays,  chemicals  such  as  the  war 
gas  known  as  nitrogen  mustard,  even  heat  and  cold  are  slightly  effec- 
tive. Cosmic  rays  and  other  particles  bombarding  the  earth  may  account 
for  some  of  the  spontaneous  mutations,  but  since  genes  are  exceedingly 
complex  molecules  it  is  quite  likely  that  metabolic  processes  in  the  cell 


THE   CONCEPT   OF   EVOLUTION  707 

may  bring  about  some  spontaneous  mutations  without  the  intervention 
of  external  agents. 

Both  spontaneous  and  artificially  induced  mutations  occur  at  ran- 
dom; the  appearance  of  a  mutation  bears  no  relationship  to  the  kind  of 
inducing  agent  or  to  the  particular  need  of  the  organism  at  that  time. 
There  is  no  way  of  producing  to  order  a  particular  kind  of  mutation— 
a  particular  kind  of  biochemical  mutant  in  Neurospora,  for  example. 
An  investigator  who  wants  to  use  some  particular  mutant  has  no  choice 
but  to  irradiate  many  organisms,  produce  hundreds  or  even  thousands 
of  mutations,  and  then  select  the  one  he  particularly  wants. 

Whatever  the  causes  of  mutations  may  be,  their  central  role  in 
evolution  as  the  raw  material  for  natural  selection  is  now  generally  ac- 
cepted. Some  evolutionists  have  in  the  past  objected  that  the  spontaneous 
or  induced  mutations  observed  in  the  laboratory  could  not  be  the  basis 
for  evolution  for  almost  all  of  them  are  deleterious,  and  because  the 
differences  between  species  are  usually  slight  variations,  affecting  many 
different  parts  of  the  organism  and  inherited  by  means  of  multiple 
factors,  whereas  the  mutations  observed  in  the  laboratory  are  usually 
large  variations,  involving  a  single  organ  and  inherited  by  single  gene 
differences.  Studies  in  the  genetics  of  wild  populations  have  shown  that 
mutations  that  occur  in  the  wild,  like  the  ones  observed  in  the  labora- 
tory, are  usually  for  detrimental  traits.  We  must  keep  clearly  in  mind 
that  the  animals  and  plants  living  today  are  the  result  of  a  long  and 
rigorous  process  of  natural  selection.  In  the  course  of  their  evolution, 
most  of  the  possible  mutations  have  occurred,  and  the  beneficial  ones 
have  been  selected  and  preserved.  The  organisms  are  well  adapted  to 
their  surroundings  and  further  mutations  are  much  more  likely  to  be 
harmful  than  helpful.  However,  a  few  of  the  mutations  seen  in  the 
laboratory  and  in  wild  populations  are  beneficial  and  have  survival 
value.  Mutations  may  produce  traits  which  are  deleterious  in  one  en- 
vironment but  advantageous  in  another.  Sickle  cell  anemia,  for  example, 
is  generally  disadvantageous  but  its  resistance  to  malaria  is  advantageous 
in  regions  such  as  Central  Africa  where  malaria  is  very  widespread. 

Closer  study  of  populations  has  shown  that  the  sort  of  variations 
which  differentiate  a  species  do  appear  in  stocks  bred  in  thfe  laboratory. 
However,  being  somewhat  more  difficult  to  detect  and  study,  they  were 
missed  in  some  of  the  earlier  work.  More  recent  experiments  indicate 
that  such  mutations  occur  at  an  even  greater  rate  than  the  larger,  more 
obvious  ones. 

303.        Straight-Line  Evolution 

Many  of  the  earlier  paleontologists  and  other  students  of  evolution 
were  led  to  the  conclusion  that  there  are  trends  in  evolution,  that 
evolution  tends  to  progress  in  a  straight  hne.  The  term  orfhogenesis 
was  coined  to  refer  to  straight-line  evolution;  some  investigators  had  the 
somewhat  mystical  belief  that  organisms  have  an  inherent  tendency  to 
evolve  in  a  predetermined  direction.  More  recent,  fuller  exanunations  of 
the  accumulating  fossil  data,  however,   have  shown   that  many  of  the 


708  GENETICS   AND   EVOLUTION 

EOCENE     OLIGOCENE  MIOCENE 


PLIOCENE     PLEISTOCENE-RECENT 


Eohippas      Miohippus       Mzrychippus  Pliohippus  Ecfuus 

Figure  34.3.  Stages  in  the  evolution  of  the  horse,  illustrating  (top)  the  changes 
in  size  and  shape  (the  numbers  indicate  the  shoulder  height  in  inches),  (second  row) 
the  bones  of  the  fore  and  hind  feet,  (third  row)  the  skull,  and  (bottom)  the  grinding 
surfaces  of  the  second  upper  molar  tooth.  Eohippus  is  a  synonym  of  Hyracotherium. 

instances  often  quoted  as  examples  of  orthogenesis  are  not  truly  evolu- 
tion in  a  straight  line.  The  horse  is  often  said  to  have  evolved  in  a 
straight  line  from  the  primitive  Hyracotherium  (a  small  animal,  the 
size  of  a  fox,  with  four  toes  on  the  front  feet  and  three  toes  on  the  hind 
feet)  to  the  modern  Equus,  but  the  complete  fossil  record  shows  that 
there  were  many  side  branches  in  horse  evolution  (Fig.  34.3).  The  evo- 
lution of  the  present-day  horse  is  not  at  all  the  simple  progression  along 
a  single,  straight  line  of  evolution  that  it  was  once  thought  to  be.  The 
evolution  of  the  horse  was  said  to  show  the  following  "trends":  an 
increase  in  size,  a  lengthening  of  the  legs,  enlargement  of  the  third 
digit  and  reduction  of  the  others,  an  increase  in  the  size  of  the  molar 
teeth  and  in  the  complexity  of  the  patterns  of  ridges  on  their  crowns, 
and  increases  in  the  size  of  the  lower  jaw  and  the  skull.  More  recent 
work  has  shown  that  there  are  so  many  exceptions  to  each  of  these  that 
the  concept  of  a  straight-line  evolution  of  the  horse  has  been  abandoned. 
The  term  orthogenesis  is  sometimes  applied  to  the  evolutionary  over- 
development of  some  characteristic.  The  classic  example  of  this  is  the 
development  of  the  antlers  of  the  extinct  Irish  deer.  In  successive  genera- 
tions the  antlers  became  larger,  and  although  this  may  have  been  of 


THE   CONCEPT   OF   EVOLUTION  709 

adaptive  significance  at  first,  the  antlers  eventually  became  so  big,  with  a 
total  spread  o£  11  feet,  that  the  deer  could  not  support  them  and  the 
species  became  extinct. 

Our  increasing  knowledge  of  how  genes  act  in  controlling  develop- 
ment has  enabled  us  to  explain  whatever  straight-line  trends  in  evolu- 
tion may  be  real  in  terms  of  conventional  evolution  by  mutation  and 
selection.  Many  different  types  of  developmental  patterns  may  arise  by 
random  mutation,  yet  most  of  them  will  result  in  unharmonious  proc- 
esses, ones  which  will  not  interdigitate  properly  and  will  lead  to  the 
death  of  the  organism.  Others,  with  no  particular  value  for  survival, 
will  remain  or  be  eliminated  by  chance.  The  ones  most  likely  to  survive, 
perhaps,  are  those  which  provide  for  further  improvement  in  some  pe- 
culiar adaptive  structure  already  present.  Thus,  orthogenetic  series  can 
be  explained  as  the  result  of  random  mutation  and  selection  occurring 
along  one  of  the  few  possible  lines  of  development.  An  explanation  for 
the  overdevelopment  of  parts  is  now  possible  as  well:  genes  do  not  func- 
tion independently,  but  must  operate  against  the  background  of  many 
other  genes  also  present.  Those  controlling  larger  horns,  for  example, 
might  cause  the  horns  to  be  proportionately  larger  than  the  rest  of  the 
body,  and  if  other  genes  cause  an  increase  in  total  body  size,  the  horns 
may  become  unmanageably  large  and  finally  lethal  to  their  possessors. 

304.        The  Origin  of  Species  by  Hybridization 

The  crossing  of  two  different  varieties  or  species,  called  hybridiza- 
tion, provides  another  way  in  which  new  species  may  originate.  The  new 
species  may  combine  the  best  characters  of  each  of  the  parental  species, 
thereby  becoming  better  able  to  survive  than  either  of  its  parents. 
Hybridization  is  used  routinely  by  animal  and  plant  breeders  to  estab- 
lish new  combinations  of  desirable  characters. 

When  two  different  species  are  crossed,  and  especially  ones  with 
different  chromosome  numbers,  the  offspring  are  usually  sterile.  The 
unlike  chromosomes  cannot  pair  properly,  cannot  undergo  synapsis  m 
the  process  of  meiosis,  and  the  resulting  eggs  and  sperm  do  not  receive 
the  proper  assortment  of  chromosomes.  However,  if  one  of  these  inter- 
specific hybrids  undergoes  a  chromosome  mutation  which  results  in  the 
doubling  of  the  chromosome  number,  meiosis  can  then  occur  normally 
and  fertile  eggs  and  sperm  are  produced.  The  hybrid  will  breed  true 
thereafter  ancf  will  generally  not  produce  fertile  offspring  when  bred 
with  either  of  the  parental  species.  It  is  widely  believed  that  this  process 
has  been  quite  important  in  the  evolution  of  the  higher  plants;  more 
than  half  of  the  higher  plants  appear  to  be  polyploids.  There  are  species 
of  wheat  with  14,  28  and  42  chromosomes,  species  of  roses  with  14,  28, 
42  and  56  chromosomes,  and  species  of  violets  with  every  multiple  of 
six  from  12  to  54.  The  fact  that  similar  series  of  plants  with  related 
numbers  of  chromosomes  can  be  established  by  experimental  breedmg 
lends  credence  to  the  idea  that  these  natural  series  arose  by  successive 
hybridization  and  chromosome  doubling. 

One  of  the  more  famous  experimental  hybrids  was  the  radish  x  cab- 


7  1  0  GENETICS   AND   EVOLUTION 

bage  cross  made  by  Karpechenko.  Although  radishes  and  cabbages  be- 
long to  different  genera,  each  has  18  chromosomes.  The  resulting  hybrid 
also  had  18  chromosomes,  9  from  the  radish  parent  and  9  from  the  cab- 
bage parent.  The  radish  and  cabbage  chromosomes  were  not  sufficiently 
alike  to  permit  synapsis  to  occur  normally  and  the  hybrid  was  almost 
completely  sterile.  The  chance  distribution  of  the  chromosomes  led  to 
the  formation  of  a  few  eggs  and  sperm  that  had  18  chromosomes  each, 
and  the  union  of  such  eggs  and  sperm  resulted  in  a  plant  with  36 
chromosomes.  This  new  plant  was  fertile;  in  meiosis  the  homologous 
radish  chromosomes  paired  with  each  other  and  the  homologous  cab- 
bage chromosomes  paired  with  each  other.  The  new  hybrid  had  some 
of  the  characteristics  of  each  of  its  parents  and  bred  true  for  them.  It 
was  not  valuable  commercially,  however,  for  it  had  roots  like  a  cabbage 
and  a  top  like  a  radish.  Since  this  hybrid  could  not  be  crossed  readily 
with  either  of  its  parental  species,  Raphanus  sativus,  the  radish,  or 
Brassica  oleracea,  the  cabbage,  Karpechenko  named  this  new,  experi- 
mentally produced  genus  Raphanobrassica. 

There  are  many  other  examples  of  species  of  plants  produced  by 
hybridization  and  chromosome  doubling,  but  this  process  appears  to 
have  played  a  negligible  role  in  the  evolution  of  animals.  Two  explana- 
tions of  this  have  been  advanced:  the  gametes  of  animals  are  more 
sensitive  to  imbalances  of  chromosomes  and  are  nonviable  unless  a  nor- 
mal haploid  set  is  present;  since  the  sexes  are  separate  in  most  animals, 
the  random  segregation  of  several  pairs  of  sex  chromosomes  in  a  poly- 
ploid animal  might  lead  to  the  formation  of  sterile  combinations. 

305.        The  Origin  of  Life 

The  modern  theories  of  mutation,  natural  selection  and  popula- 
tion dynamics  provide  us  with  a  satisfactory  explanation  of  how  the 
present-day  animals  and  plants  evolved  from  previous  forms  by  descent 
with  modification.  The  question  of  the  ultimate  origin  of  life  on  this 
planet  has  been  given  serious  consideration  by  many  different  biologists. 
Some  have  postulated  that  some  kind  of  spores  or  germs  may  have  been 
carried  through  space  from  another  planet  to  this  one.  This  is  unsatis- 
factory, not  only  because  it  begs  the  question  of  the  ultimate  source  of 
these  spores,  but  because  it  is  extremely  unlikely  that  any  sort  of  living 
thing  could  survive  the  extreme  cold  and  intense  irradiation  of  inter- 
planetary travel. 

The  concept  that  the  first  living  things  did  evolve  from  nonliving 
things  has  been  put  forward  by  Pfliiger,  J.  B.  S.  Haldane,  R.  Beutner, 
and  especially  by  the  Russian  biochemist,  A.  I.  Oparin,  in  his  book, 
The  Origin  of  Life  (1938).  The  earth  originated  some  2.5  billion  to  4.5 
billion  years  ago,  either  as  a  part  broken  off  from  the  sun  or  by  the 
gradual  condensation  of  interstellar  dust.  Most  authorities  seem  agreed 
that  the  earth  at  first  was  very  hot  and  molten,  and  that  conditions 
consistent  with  life  arose  only  one  billion  or  perhaps  a  billion  and  a 
half  years  ago.  At  that  time  the  earth's  atmosphere  contained  essentially 
no  free  oxygen;  all  the  oxygen  was  combined  as  water  and  as  oxides. 


THE   CONCEPT   Of   EVOLUTION  711 

A  number  of  reactions  by  which  organic  substances  can  be  made 
from  inorganic  ones  are  known.  It  is  believed  that  originally  much  of 
the   earth's   carbon  was   in   the   form   of  metallic  carbides;   these   could 
react  with  water  to  form  acetylene  which  would  subsequently  polymerize 
to  form  compounds  containing  long  chains  of  carbon  atoms.  It  has  been 
shown  experimentally  that  high  energy  radiation,  such  as  that  of  cosmic 
rays,  can  produce  organic  compounds.  This  has  been  demonstrated  by 
M.  Calvin,  who  irradiated  solutions  of  carbon  dioxide  and  water  in  a 
cyclotron  and  obtained  formic,  oxalic  and  succinic  acids,  which  contain 
one,    two   and   four   carbons   respectively.    These    are    important   inter- 
mediates in  the  metabolism  of  living  organisms.  Irradiation  of  solutions 
wath  ultraviolet  light,  or  with  electric  charges  to  simulate  lightning,  also 
produces  organic  compounds.  Harold  Urey  and  Stanley  Miller,  at  the 
University  of  Chicago,  showed  in  1953  that  amino  acids  such  as  glycine 
and  alanine,  and  even  more  complex  organic  substances,  can  be  formed 
in  vitro  by  exposing  a  mixture  of  water  vapor,  methane,  ammonia  and 
hydrogen  gases  to  electric  discharges  for  a  mere  week.  All  of  these  gases 
are  believed  to  have  been  present  in  adequate  amounts  in  the  earth's 
atmosphere  in  prebiotic  times. 

The  spontaneous  origin  of  living  things  at  the  present  time  is  be- 
lieved to  be  extremely  improbable,  yet  that  this  same  event  occurred  in 
the  past  is  quite  probable.  The  difference  lies  in  the  conditions  existing 
on  the  earth:  the  accumulation  of  organic  molecules  was  possible  before 
there  were  living  things  because  there  were  no  molds,  no  bacteria,  no 
living  things  of  any  kind  to  bring  about  their  decay.  Furthermore,  there 
was  little  or  no  oxygen  in  the  atmosphere  to  bring  about  their  spon- 
taneous oxidation. 

The  details  of  the  chemical  reactions  which  could  give  rise,  without 
the  intervention  of  living  things,  to  carbohydrates,  fats  and  amino  acids 
have  been  worked  out  by  Oparin  and  extended  by  Calvin  and  others. 
Most  of  the  reactions  by  which  the  more  complex  organic  substances 
were  formed  probably  occurred  in  the  sea,  in  which  were  dissolved  and 
mixed  the  organic  molecules  formed.  The  sea,  we  may  postulate,  be- 
came a  sort  of  dilute  broth  in  which  these  molecules  collided,  reacted, 
and  aggregated  to  form  new  molecules  of  increasing  size  and  complexity. 
The  known  forces  of  intermolecular  attraction,  and  the  tendency  for 
certain  molecules  to  form  liquid  crystals,  provide  us  with  means  by 
which  large,  complex,  specific  molecules  can  form  spontaneously.  Oparm 
suggested  that  natural  selection  can  operate  at  the  level  of  these  com- 
plex molecules,  before  anything  recognizable  as  life  is  present.  As  the 
molecules  came  together  to  form  colloidal  aggregates,  these  aggregates 
began  to  compete  with  one  another  for  raw  materials.  Some  of  the 
aggregates,  which  had  some  particularly  favorable  internal  arrangemeirt 
would  acquire  new  molecules  more  rapidly  than  others  and  would 
eventually  become  the  dominant  types. 

Once  some  protein  molecules  had  formed  and  had  achieved  the 
ability  to  catalyze  reactions,  the  rate  of  formation  of  additional  molecules 
would  be  greatly  stepped  up.  Next,  these  complex  protein  molecules  ac- 
quired the  ability  to  catalyze  the  synthesis  of  molecules  like  themselves; 


7  1  9  GENETICS   AND    EVOLUTION 

they  became  autocatalytic.  This  hypothetical,  autocatalytic  particle 
would  have  some  oi  the  properties  of  a  virus,  or  perhaps  of  a  free-livhig 
gene.  Ihe  next  step  in  the  development  of  a  living  thing  is  the  addition 
of  the  ability  of  the  autocatalytic  particle  to  undergo  inherited  changes 
-to  mutate.  Then,  if  a  number  of  these  free  genes  had  joined  to  form 
a  single  larger  unit,  the  resulting  organism  would  have  been  similar  to 
certain  present-day  viruses.  All  the  known  viruses  are  parasites  that  can 
live  only  within  the  cells  of  higher  animals  and  plants.  However,  a 
little  reflection  will  suggest  that  free-living  viruses,  ones  which  do  not 
produce  a  disease,  would  be  very  difficult  to  detect;  such  organisms  may 
intleed  exist. 

The  first  living  organisms,  having  arisen  in  a  sea  of  organic  mole- 
cules and  in  contact  with  an  atmosphere  free  of  oxygen,  presumably 
obtained  energy  by  the  fermentation  of  certain  of  these  organic  sub- 
stances. These  heterotrophs  could  survive  only  as  long  as  the  supply  of 
organic  molecules  in  the  sea  broth,  accumulated  from  the  past,  lasted. 
Before  the  supply  was  exhausted,  however,  the  heterotrophs  evolved 
further  and  became  autotrophs,  able  to  make  their  own  organic  mole- 
cules by  chemosynthesis  or  photosynthesis.  One  of  the  by-products  of 
photosynthesis  is  gaseous  oxygen,  and  it  is  likely  that  all  the  oxygen  in 
the  atmosphere  was  produced  and  is  still  produced  in  this  way.  It  is 
estimated  that  all  the  oxygen  of  our  atmosphere  is  renewed  by  photo- 
synthesis every  2000  years  and  all  the  carbon  dioxide  molecules  pass 
through  the  photosynthetic  process  every  300  years.  All  the  oxygen  and 
carbon  dioxide  in  the  earth's  atmosphere  are  the  products  of  living  or- 
ganisms and  have  passed  through  living  organisms  over  and  over  again  in 
times  past. 

The  explanation  of  how  an  autotroph  may  have  evolved  from  one 
of  these  primitive,  fermenting  heterotrophs  was  presented  by  N.  H. 
Horowitz  in  1945.  According  to  Horowitz'  hypothesis,  an  organism 
might  acquire,  by  successive  mutations,  the  enzymes  needed  to  synthesize 
complex  from  simple  substances,  in  the  reverse  order  to  the  sequence 
in  which  they  are  used  in  normal  metabolism.  Let  us  suppose  that  our 
first  primitive  heterotroph  required  organic  compound  A  for  its 
growth.  Substance  A,  and  a  variety  of  other  organic  compounds,  B,  C, 
D,  etc.,  were  present  in  the  organic  sea  broth  which  was  the  environ- 
ment of  this  heterotroph.  They  had  been  synthesized  previously  by  the 
action  of  nonliving  factors  of  the  environment.  The  heterotroph  would 
survive  nicely  as  long  as  the  supply  of  compound  A  lasted.  If  a  mutation 
occurred  which  enabled  the  heterotroph  to  synthesize  substance  A  from 
substance  B,  the  strain  of  heterotroph  with  this  mutation  would  be 
able  to  survive  when  the  supply  of  substance  A  was  exhausted.  A  second 
mutation,  which  established  an  enzyme  catalyzing  a  reaction  by  which 
substance  B  could  be  made  from  the  simpler  substance  C,  would  again 
have  great  survival  value  when  the  supply  of  B  was  exhausted.  Similar 
mutations,  setting  up  enzymes  enabling  the  organism  to  use  success- 
ively simpler  substances,  D,  E,  F,  .  .  .  and  finally  some  inorganic  sub- 
stance, Z,  would  eventually  result  in  an  organism  able  to  make  sub- 
stance A,  which  it  needs  for  growth,  out  of  substance  Z  by  way  of  all 


THE   CONCEPT   OF   EVOLUTION  J\$ 

the  intermediate  compounds.  W^hen,  by  other  series  of  mutations,  the 
organism  was  able  to  synthesize  all  of  its  requirements  from  simple 
inorganic  compounds,  as  the  green  plants  can,  it  would  have  become 
an  autotroph.  Once  the  first  simple  autotrophs  had  evolved,  the  way 
was  clear  for  the  further  evolution  of  the  vast  variety  of  green  plants, 
bacteria,  molds,   and  animals  that  inhabit  the  world   today. 

These  considerations  lead  us  to  the  conclusion  that  the  origin  of 
life,  as  an  orderly  natural  event  on  this  planet,  was  not  only  possible, 
it  was  almost  inevitable.  Furthermore,  with  the  vast  number  of  planets 
in  all  the  known  galaxies  of  the  universe,  many  of  them  must  have 
conditions  which  permit  the  origin  of  life.  It  is  probable,  then,  that 
there  are  many  other  planets  on  which  life  as  we  know  it  exists. 
W^herever  life  is  possible,  it  should,  if  given  enough  time,  appear  and 
ramify  into  a  wide  variety  of  types.  Some  of  these  may  be  quite  dis- 
similar from  the  ones  on  this  planet,  but  others  may  be  quite  like 
those  found  here;   some  may,   perhaps,  be   like  ourselves. 

It  seems  unlikely  that  we  will  ever  know  how  life  originated, 
whether  it  happened  only  once  or  many  times,  or  whether  it  might 
happen  again.  The  theory  (1)  that  organic  substances  were  formed  from 
inorganic  substances  by  the  action  of  physical  factors  in  the  environ- 
ment; (2)  that  they  interacted  to  form  more  and  more  complex  sub- 
stances, finally  enzymes,  and  then  self-reproducing  enzyme  systems 
("free  genes");  (3)  that  these  "free  genes"  diversified  and  united  to  form 
a  primitive,  j^erhaps  virus-like  heterotroph;  and  (4)  that  autotrophs  then 
evolved  from  these  heterotrophs,  has  the  virtue  of  being  quite  plausible. 
Many  of  the  parts  of  this  theory  have  been  subjected  to  experimental 
verification  and  have  been  shown  to  be  feasible. 

306.        Principles  of  Evolution 

However  much  students  of  evolution  may  disagree  as  to  the  nature 
of  mutations,  the  kind  of  mutations  involved  in  evolution,  and  the  de- 
gree to  which  such  factors  as  natural  selection,  isolation,  genetic  re- 
combination and  population  dynamics  may  affect  the  evolution  of  some 
particular  organism,  there  are  several  fundamental  principles  upon 
which  they  are  agreed:  changes  within  the  genes  and  chromosomes  are 
the  raw  material  of  evolution,  some  sort  of  isolation  is  necessary  for 
the  establishment  of  a  new  species,  and  natural  selection  is  involved  in 
the  survival  of  some,  but  not  all,  of  the  mutations  which  occur.  In  addi- 
tion, there  are  five  principles  of  evolution  to  which  nearly  all  biologists 

would  subscribe. 

1.  Evolution  occurs  more  rapidly  at  some  times  than  at  others.  At 
the  present  time  it  is  occurring  rapidly,  with  many  new  forms  appearing 
and  many  old  ones  becoming  extinct. 

2  Evolution  does  not  proceed  at  the  same  rate  among  different  types 
of  organisms.  At  one  extreme  are  the  lampshells  or  brachiopods,  some 
species  of  which  have  been  exactly  the  same  for  the  last  500,000,000 
years  at  least,  for  fossil  shells  found  in  rocks  deposited  at  that  time  are 
identical  with  those  of  animals  living  today.  In  contrast,  several  species 


714  GENETICS    AND    EVOLUTION 

of  man  have  appeared  and  become  extinct  in  the  past  few  hundred 
tliousand  years.  In  general,  evolution  tends  to  occur  rapidly  when  a  new 
species  first  appears,  and  then  gradually  slows  down  as  the  group  becomes 
established  and  adapted  to  its  particular  environment. 

3.  New  species  do  not  evolve  from  the  most  advanced  and  specialized 
forms  already  living,  but  from  relatively  simple,  unspecialized  forms. 
The  mammals,  for  example,  did  not  evolve  from  the  large,  specialized 
dinosaurs,  but  from  a  group  of  rather  small  and  unspecialized  reptiles. 

4.  Evolution  is  not  always  from  the  simple  to  the  complex.  There  are 
many  examples  of  "regressive"  evolution,  in  which  a  complex  organism 
has  given  rise  to  simpler  ones.  Most  parasites  have  evolved  from  free- 
living  ancestors  which  were  more  complex  than  they;  wingless  birds, 
such  as  the  cassowary  and  emu,  have  descended  from  birds  that  could 
fly;  many  wingless  insects  have  evolved  from  winged  ones;  the  legless 
snakes  came  from  reptiles  with  appendages;  the  whale,  which  has  no 
hind  legs,  evolved  from  a  mammal  that  had  the  customary  two  pairs  of 
legs  and  so  on.  These  are  all  reflections  of  the  fact  that  mutations  occur 
at  random,  and  not  necessarily  from  the  simple  to  the  complex  or  from 
the  imperfect  to  the  perfect.  If  there  is  some  advantage  to  a  species  in 
having  a  simpler  structure,  or  in  doing  without  some  structure  alto- 
gether, any  mutations  which  happen  to  occur  for  such  conditions  will 
tend  to  accumulate  by  natural  selection. 

5.  Evolution  occurs  by  populations,  not  by  individuals;  evolutionary 
processes  are  brought  about  by  the  processes  of  mutation,  natural  selec- 
tion and  genetic  drift. 

Questions 

1.  What  were  Darwin's  chief  contributions  to  the  theory  of  evolution? 

2.  Discuss  the  essential  points  of  Lamarckism.  What  has  led  to  the  rejection  of  this 
theory? 

3.  Discuss  the  advances  in  the  science  of  geology  that  paved  the  way  for  the  theory  of 
evolution. 

4.  Describe  in  your  own  words  what  Darwin  meant  by  natural  selection. 

5.  What  changes  in  the  theory  of  natural  selection  have  been  made  necessary  by  dis- 
coveries since  Darwin's  time? 

6.  What  contributions  to  the  principle  of  evolution  were  made  by  Erasmus  Darwin, 
Alfred  Russell  Wallace,  Thomas  Huxley,  Thomas  Malthus  and  Hugo  de  Vries? 

7.  Discuss  the  role  of  isolation  in  the  origin  of  species. 

8.  What  is  meant  by  "genetic  drift"?  Under  what  circumstances  is  it  important  in  evolu- 
tion? 

9.  Discuss  the  theory  of  preadaptation.  Describe  two  examples  of  preadaptation  other 
than  the  ones  given  in  the  text. 

10.  Compare  the  Neo-Darwinian  and  the  macromutation  theories  of  the  origin  of  species. 

11.  Distinguish  between  the  several  types  of  mutations.  What  physical  and  chemical 
agents  are  known  to  produce  mutations  in  the  laboratory?  What  agents  may  produce 
spontaneous  mutations  in  natural  populations? 

12.  What  explanation  may  be  given  for  the  observation  that  most  spontaneous  and  in- 
duced mutations  produce  phenotypes  which  are  less  well  adapted  for  survival  than 
the  original  form? 

13.  Describe  the  steps  by  which  simple  inorganic  substances  may  have  undergone  chem- 


THE   CONCEPT  OF   EVOLUTION  7^5 

ical  evolution  to  yield  the  complex  system  of  organic  chemicals  we  recognize  as  "living 
protoplasm."  Which  of  these  has  been  duplicated  experimentally? 

14.  List  the  general  principles  of  evolution.  Are  there  any  you  think  should  be  deleted 
or  added? 


Supplementary  Reading 

Darwin's  classic.  The  Origin  of  Species,  is  available  in  a  number  of  modern  editions 
and  is  well  worth  sampling  for  its  clear,  logical  arguments  and  wealth  of  examples.  The 
impact  of  the  theory  of  evolution  on  Victorian  England,  and  a  vivid  portrayal  of  Thomas 
Huxley's  championing  of  Darwin's  theory,  are  presented  in  William  Irvine's  Apes,  Angels 
and  Victorians.  Henry  Fairfield  Osborn's  From  the  Greeks  to  Darwin  is  an  interesting 
history  of  ideas  on  evolution.  A  good,  nontechnical  presentation  of  our  current  ideas  on 
evolution  is  found  in  G.  G.  Simpson,  The  Meaning  of  Evolution.  Technical  books  on 
special  phases  of  evolution  are  Carter's  Animal  Evolution:  A  Study  of  Recent  Views  on  its 
Causes,  Stebbins'  Variation  and  Evolution  in  Plants,  Simpson's  The  Major  Features  of 
Evolution,  which  discusses  the  paleontologic  and  genetic  aspects  of  evolution,  Dobzhan- 
sky's  Genetics  and  the  Origin  of  Species,  which  presents  the  Neo-Darwinian  viewpoint  of 
the  importance  of  natural  selection,  and  Goldschmidt's  The  Material  Basis  of  Evolution, 
which  gives  the  detailed  argument  for  the  importance  of  large  mutations  in  evolution. 
Theories  of  the  origin  of  life  are  discussed  in  Oparin's  The  Origin  of  Life  and  in  Blum's 
Time's  Arrow  and  Evolution.  Two  very  readable,  short  discussions  of  the  origin  of  life 
are  given  by  Melvin  Calvin  in  Cliemical  Evolution  and  the  Origin  of  Life,  and  by  George 
Wald  in  The  Origin  of  Life. 


CHAPTER  35 


The  Evidence  for  Evolution 


The  evidence  that  organic  evolution  has  occurred  is  so  overwhelming 
that  no  one  who  is  acquainted  with  it  has  any  doubt  that  new  species  are 
derived  from  previously  existing  ones  by  descent  with  modification. 
The  fossil  record  provides  direct  evidence  of  organic  evolution  and  gives 
the  details  of  the  evolutionary  relationships  of  many  lines  of  descent. 
In  addition,  there  are  vast  quantities  of  facts  from  all  of  the  subdivisions 
of  biological  science  which  acquire  significance,  and  make  sense,  only 
when  viewed  against  the  background  of  evolution. 

307.       The  Fossil  Evidence 

The  evidence  of  life  in  former  times  is  now  both  abundant  and 
diverse.  The  science  of  paleontology,  which  deals  with  the  finding,  cata- 
loguing and  interpretation  of  fossils,  has  aided  immensely  in  our  under- 
standing of  the  lines  of  descent  of  many  vertebrate  and  invertebrate 
stocks.  The  term  "fossil"  (Latin  jossiliiim,  something  dug  up)  refers  not 
only  to  the  bones,  shells,  teeth  and  other  hard  parts  of  an  animal's  body 
which  may  survive,  but  to  any  impression  or  trace  left  by  previous  or- 
ganisms. Footprints  or  trails  made  in  soft  mud,  which  subsequently 
hardened,  are  a  common  type  of  fossil.  For  example,  the  tracks  of  an 
amphibian  from  the  Pennsylvanian  period,  discovered  in  1948  near  Pitts- 
burgh, revealed  that  the  animal  moved  by  hopping  rather  than  by  walk- 
ing, for  the  footprints  lay  opposite  each  other  in  pairs. 

The  commonest  vertebrate  fossils  are  skeletal  parts.  From  the 
shape  of  bones,  and  the  position  of  the  bone  scars  which  indicate  points 
of  muscle  attachment,  paleontologists  can  make  inferences  about  an 
animal's  posture  and  style  of  walking,  the  position  and  size  of  its  mus- 
cles, and  hence  the  contours  of  its  body.  Careful  study  of  fossil  remains 
has  enabled  paleontologists  to  make  reconstructions  of  what  the  animal 
must  have  looked  like  in  life  (Fig.  35.1  and  Fig.  24.7). 

In  some  fossils,  the  original  hard  parts,  or  more  rarely  the  soft 
tissues  of  the  body,  have  been  replaced  by  minerals,  a  process  called 
petrifaction.  Iron  pyrites,  silica  and  calcium  carbonate  are  some  of  the 
common  petrifying  minerals.  The  petrified  muscle  of  a  shark  more  than 
300,000,000  years  old  was  so  well  preserved  by  petrifaction  that  not  only 
individual  muscle  fibers,  but  even  their  cross  striations,  could  be  ob- 
served in  thin  sections  under  the  microscope.  A  famous  example  of  the 
process  of  petrifaction  is  the  Petrified  Forest  in  Arizona. 

716 


THE  EVIOENCE  FOR  EVOLUTION  'J  {J 


Figure  35.1.  An  example  of  a  fossil,  the  remains  of  Archaeopteryx,  a  tailed, 
toothed  bird  from  the  Jurassic  Period.  (Courtesy  of  the  American  Museum  of  Natural 
History.) 

Molds  and  casts  are  superficially  similar  to  petrified  fossils  but  are 
produced  in  a  different  way.  Molds  are  formed  by  the  hardening  of  the 
material  surrounding  a  buried  organism,  followed  by  the  decay  and  re- 
moval of  the  body  of  the  organism.  The  mold  may  subsequently  be 
filled  by  minerals  which  harden  to  form  casts  which  are  exact  replicas 
of  the  original  structures.  Some  animal  remains  have  been  exceptionally 
well  preserved  by  being  embedded  in  tar,  amber,  ice  or  volcanic  ash. 
The  remains  of  woolly  mammoths,  deep  frozen  in  Siberian  ice  for  more 
than  25,000  years,  were  so  well  preserved  that  the  meat  was  edible! 

308.        The  Geologic  Time  Table 

Studies  of  the  earth's  crust  have  shown  that  it  consists  of  sheets  of 
rock  lying  one  on  top  of  the  next.  There  are  five  major  rock  strata  and 
each  of  these  is  subdivided  into  minor  strata.  These  layers  were  gen- 
erally formed  by  the  accumulation  of  sediment— sand  or  mud— at  the 
bottom  of  oceans,  seas  or  lakes.  Each  rock  stratum  contains  certain 
characteristic  kinds  of  fossils  which  can  now  be  used  to  identify  deposits 
made  at  the  same  time  in  different  parts  of  the  world.  Geologic  time 
has  been  divided,  according  to  the  succession  of  these  rock  strata,  into 
eras,  periods  and  epochs  (Table  15).  The  duration  of  each  period  or 
epoch  can  be  estimated  from  the  thickness  of  the  sedimentary  deposits, 
although,  of  course,  the  rate  of  deposition  was  not  exactly  the  same  in 
different  places  and  at  different  times. 


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THE   EVIDENCE   FOR   EVOLUTION 


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720  GENETICS   AND    EVOLUTION 

Tlie  layers  of  sedimentary  rock  should  occur  in  the  sequence  of 
tiieir  deposition,  with  the  newer  strata  on  top  of  the  older  ones,  but 
subsequent  geologic  events  may  have  changed  the  relationship  of  the 
layers.  Not  all  of  the  expected  strata  may  occur  in  some  particular  re- 
gion, for  that  land  may  have  been  exposed  rather  than  submerged 
timing  one  or  more  geologic  ages.  In  some  regions  the  strata  formed 
previously  have  subsequently  emerged,  been  washed  away,  and  then 
relatively  recent  strata  have  been  deposited  directly  on  very  ancient 
ones.  Certain  sections  of  the  earth's  crust,  in  addition,  have  undergone 
massive  foldings  and  splittings,  so  that  early  layers  come  to  lie  on  top  of 
later  ones. 

Rock  deposits  are  now  dated  largely  by  taking  advantage  of  the  fact 
that  certain  radioactive  elements  are  transformed  into  other  elements 
at  rates  which  are  slow  and  essentially  unaffected  by  the  pressures  and 
temperatures  to  which  the  rock  has  been  subjected.  Half  of  a  given 
sample  of  uranium  will  be  converted  to  a  special  isotope  of  lead  in  4.5 
billion  years.  Hence,  by  measuring  the  proportion  of  uranium  and  lead 
in  a  bit  of  crystalline  rock,  its  age  can  be  measured.  In  this  way  the 
oldest  rocks  of  the  earliest  geologic  period  are  calculated  to  be  about 
3,500,000,000  years  old  and  the  latest  Cambrian  rocks  to  be  500,000,000 
years  old.  Events  in  more  recent  times  can  be  dated  quite  accurately  by 
the  decay  of  carbon^^,  which  has  a  half  life  of  5568  years.* 

Between  the  major  eras  there  were  widespread  geologic  disturbances, 
called  revolutions,  which  raised  or  lowered  vast  regions  of  the  earth's 
surface  and  created  or  eliminated  shallow  inland  seas.  These  revolutions 
produced  great  changes  in  the  distribution  of  sea  and  land  organisms 
and  wiped  out  many  of  the  previous  forms  of  life.  The  Paleozoic  era 
ended  with  the  revolution  that  raised  the  Appalachian  mountains  and, 
it  is  believed,  killed  all  but  3  per  cent  of  the  forms  of  life  existing  then. 
The  Rocky  Mountain  revolution  (which  raised  the  Andes,  Alps  and  Hi- 
malayas as  well  as  the  Rockies)  annihilated  most  reptiles  of  the  Mesozoic. 

309.       The  Geologic  Eras 

Archeozoic  Era.  The  rocks  of  the  oldest  geologic  era  are  very 
deeply  buried  in  most  parts  of  the  world,  but  are  exposed  at  the  bottom 
of  the  Grand  Canyon  and  along  the  shores  of  Lake  Superior.  The  oldest 
geologic  era,  the  Archeozoic,  begins  not  with  the  origin  of  the  earth 
but  with  the  formation  of  the  earth's  crust,  when  rocks  and  mountains 
were  in  existence  and  the  processes  of  erosion  and  sedimentation  had 
begun.  The  Archeozoic  era  lasted  about  two  billion  years,  about  as  long 
as  all  the  succeeding  eras  combined.  It  was  characterized  by  widespread 
volcanic  activity  and  large  upheavals  which  resulted  in  the  raising  of 
mountains.  The  heat,  pressure  and  churning  associated  with  the  move- 
ments of  the  earth's  crust  probably  destroyed  most  of  whatever  fossil 
remains  there  may  have  been,  but  a  few  traces  of  life  remain.  Scattered 
through  the  Archeozoic  rocks  are  flakes  of  graphite,  pure  carbon,  which 

*  Organic  carbon  is  derived  by  CO2  fixation  from  atmospheric  CO2  and  the  ratio  of 
C12  to  Ci*  in  living  organisms  is  the  same  as  that  in  the  atmosphere.  No  exchange  of 
carbon  atoms  with  the  atmosphere  occurs  after  death  and  the  Ci*  in  the  body  is  slowly 
transformed  into  Ni*.  The  age  of  organic  remains  can  be  estimated  from  their  Ci^/Ci^ 
ratio  and  the  half  life  of  Ci*. 


' 


THE   EVIDENCE   FOR   EVOLUTION  721 

are  probably  the  transformed  remains  of  plants  and  animal  bodies.  Al- 
though graphite  can  originate  inorganically,  its  distribution  in  the  rocks 
suggests  that  it  was  formed  organically.  If  the  amount  of  graphite  in  these 
rocks  can  be  taken  as  a  measure  of  the  amount  of  living  things  in  the 
Archeozoic,  and  there  are  reasons  for  believing  that  this  is  justified,  then 
life  must  have  been  abundant  in  the  Archeozoic  seas,  for  there  is  more 
carbon  in  these  rocks  than  in  the  coal  beds  of  the  Appalachians. 

Proferozoic  Era.  The  second  geologic  era,  which  lasted  about  one 
billion  years,  was  characterized  by  the  deposition  of  large  quantities  of 
sediment,  and  by  at  least  one  great  period  of  glaciation  during  which  ice 
sheets  stretched  to  within  20  degrees  of  the  equator.  There  was  less  vol- 
canic activity  in  this  than  in  the  preceding  era  and  the  rocks  are  better 
preserved.  Only  a  few  fossils  have  been  found  in  Proterozoic  rocks  but 
they  show  not  only  that  life  was  present  but  that  evolution  had  pro- 
ceeded quite  far  before  the  end  of  the  era.  Plants  and  animals  were 
differentiated,  multicellular  forms  had  evolved  from  unicellular  ones 
and  some  of  the  major  groups  of  plants  and  animals  had  appeared. 
Sponge  spicules,  jellyfish,  and  the  remains  of  fungi,  algae,  brachiopods 
and  annelid  worm  tubes  have  been  found  in  Proterozoic  rocks. 

Paleozoic  Era.  A  second  great  revolution  ended  the  Proterozoic 
era.  During  the  ensuing  360,000,000  years  of  the  Paleozoic  every 
phylum  and  class  of  animals  except  birds  and  mammals  appeared.  Some 
of  these  animals  appeared  and  became  extinct  in  a  short  time  (geo- 
logically speaking)  and  their  fossils  provide  convenient  markers  by 
which  rocks  of  the  same  era  in  different  localities  can  be  correlated. 

The  fossil  deposits  of  the  first  three  periods  of  the  Paleozoic  era, 
the  Cambrian,  Ordovician  and  Silurian,  were  mostly  laid  down  in  the 
seas.  Large  shallow  seas  covered  most  of  the  continents  during  these  three 
periods  and  they  teemed  with  life.  Many  of  these  forms  had  hard  skele- 
tons or  armor  coverings  which  left  a  good  fossil  record.  The  organisms 
living  in  the  Cambrian  were  so  varied  and  complex  that  they  must  have 
evolved  from  ancestors  dating  back  to  the  Proterozoic  era.  Appar- 
ently both  plants  and  animals  lived  in  the  sea,  and  the  land  was  a  curi- 
ous lifeless  waste  until  the  Ordovician,  when  plants  became  established 
on  land.  The  Cambrian  seas  contained  small,  floating  plants  and  animals 
that  were  eaten  by  primitive,  shrimplike  crustaceans  and  swimming 
annelid  worms.  The  sea  floor  was  covered  with  simple  sponges,  corals, 
echinoderms  growing  on  stalks,  snails,  pelecypods  and  primitive  cephalo- 
pods.  An  exceptionally  well  preserved  collection  of  Cambrian  fossils 
was  found  in  the  mountains  of  British  Columbia;  it  included  annelids, 
crustaceans,  and  a  connecting  link  similar  to  peripatus.  The  most  numer- 
ous animals  were  brachiopods  and  trilobites.  Brachiopods,  sessile,  bi- 
valved  plankton  feeders,  flourished  in  the  Cambrian  and  the  rest  of 
the  Paleozoic.  One  of  the  present  day  brachiopods,  Lingula,  is  the 
oldest  known  genus  of  animals  and  is  almost  identical  with  its  Cam- 
brian ancestors.  The  trilobites  (Fig.  16.2)  were  primitive  arthropods, 
with  flattened,  elongated  bodies  covered  dorsally  by  a  hard  shell.  The 
shell  had  two  longitudinal  grooves  that  divided  the  body  into  three 
lobes.  On  the  ventral  side  of  the  body  was  a  pair  of  legs  on  each  somite 
but  the  last,  and  each  leg  was  biramous,  had  an  outer  gill  branch  and 


722  GENETICS   AND    EVOLUTION 


IL- 


Figure  35.2.  Texas  in  the  Permian  Period,  about  230,000,000  years  ago.  Various 
pelycosaurs  are  shown.  Some  had  large  fins,  others  were  essentially  like  lizards.  In  the 
lower  illustration  is  a  salamander-like  amphibian  with  a  flat,  triangular  skull.  (Copyright, 
Chicago  Natural  History  Museum,  from  the  painting  by  Charles  R.  Knight.) 


THE   EVIDENCE  FOR   EVOLUTION  793 

an  inner  walking  or  swimming  branch.  Most  trilobites  were  only  two 
or  three  inches  long  but  the  largest  was  about  two  feet.  They  reached 
their  peak  of  importance  in  the  late  Cambrian  and  then  dwindled  and 
became  extinct  in  the  Permian. 

Evolution  since  the  Cambrian  has  been  characterized  by  the  elab- 
oration and  ramification  of  the  lines  already  present,  rather  than  by 
the  establishment  of  entirely  new  forms.  The  original,  primitive  mem- 
bers of  most  lines  were  replaced  by  more  complex,  better  adapted  ones. 
The  Ordovician  seas  contained,  among  other  forms,  giant  cephalopods, 
squidlike  animals  with  straight  shells  15  to  20  feet  long  ancl  a  foot  in 
diameter.  The  Ordovician  seas  were  apparently  quite  warm,  for  corals, 
which  grow  only  in  warm  waters,  lived  as  far  north  as  Ontario  and 
Greenland.  The  first  vertebrates,  the  jawless,  limbless,  armored,  bottom- 
dwelling  fishes  called  ostracoderms,  appeared  in  the  Ordovician.  These 
lived  in  fresh  water  and  their  bony  armor  may  have  served  as  a  defense 
against  their  chief  predator,  the  carnivorous,  giant  arachnids  called 
eurypterids.  Two  important  events  of  the  Silurian  were  the  evolution  of 
land  plants  and  of  the  first  air-breathing  animals,  primitive  scorpions. 

The  evolution  of  the  vertebrates,  from  ostracoderms  to  placoderms, 
cartilaginous  and  bony  fishes,  amphibians,  reptiles,  birds  and  mammals 
has  been  traced  in  Chapters  22  to  24.  The  Devonian  seas  contained 
corals,  sea  lilies  and  brachiopods  in  addition  to  a  great  variety  of  fishes. 
Trilobites  were  still  present  but  were  declining  in  numbers  and  im- 
portance. The  first  land  vertebrates,  the  amphibians  called  labyrintho- 
donts,  appeared  in  the  latter  part  of  the  Devonian;  this  period  also 
saw  the  first  true  forests  of  ferns,  "seed  ferns,"  club  mosses  and  horsetails 
and  the  first  wingless  insects  and  millipedes. 

The  Mississippian  and  Pennsylvanian  periods  are  frequently 
grouped  together  as  the  Carboniferous,  for  during  this  time  there  flour- 
ishecl  the  great  swamp  forests  whose  remains  gave  rise  to  the  major  coal 
deposits  of  the  world.  The  earliest  stem  reptiles  appeared  in  the  Penn- 
sylvanian and  from  these  there  evolved  in  the  succeeding  Permian  period 
a  group  of  early,  mammal-like  reptiles,  the  pelycosaurs,  from  which  the 
mammals  eventually  evolved  (Fig.  35.2). 

The  Permian  period  was  characterized  by  widespread  changes  in 
topography  and  climate.  The  land  began  to  rise  early  in  the  period,  so 
that  the  swamps  and  shallow  seas  were  drained,  and  the  Appalachian 
Revolution  that  ended  the  period,  together  with  widespread  glaciation, 
killed  off  a  great  many  kinds  of  animals.  The  trilobites  finally  dis- 
appeared and  the  brachiopods,  stalked  echinoderms,  cephalopods,  and 
many  other  kinds  of  invertebrates  were  reduced  to  small,  unimportant, 
relict  groups. 

Alesozoic  Era.  The  Mesozoic  era,  which  began  some  225,000,000 
years  ago  and  lasted  about  150,000,000  years,  is  subdivided  into  the 
Triassic,  Jurassic  and  Cretaceous  periods.  During  the  Triassic  and  Ju- 
rassic most  of  the  continental  area  was  above  water,  warm  and  fairly 
dry.  During  the  Cretaceous  the  Gulf  of  Mexico  expanded  into  Texas 
and  New  Mexico,  and  the  sea  once  again  overspread  large  parts  of  the 
continents.  There  were  great  swamps  from  Colorado  to  British  Columbia 


724 


GENETICS   AND   EVOLUTION 


Figure  35.3.  Western  Canada  in  the  Cretaceous  period,  about  110,000,000  years 
ago.  ']  lie  land  was  low,  well  watered,  and  co\ered  Avith  numerous  swamps.  Most  of  the 
dinosaurs  were  harmless,  plant-eating  Ornithischians,  reptiles  with  bird-like  pelvic 
bones.  Two  types  of  duck-billed  dinosaurs  can  be  seen— three  large,  uncrested  ones 
in  upper  portion,  and  two  kinds  of  crested  ones  in  the  lower  portion.  In  the  upper 
right  foreground  is  a  heavily  armored,  four-footed  dinosaur  covered  with  bony  plates 
and  spines.  In  the  upper  right  and  lower  left  background  are  ostrich  dinosaurs-tall 
slender  animals,  with  the  general  proportions  of  an  ostrich,  but  with  short  forelegs 
and  a  long,  slender  tail.  (Copyright,  Chicago  Natural  History  Museum,  from  the 
painting  by  Charles  R.  Knight.) 


THE   EVIDENCE   FOR   EVOLUTION  725 

(Fig.  35.3).  In  the  latter  part  of  the  Cretaceous  the  interior  of  the 
North  American  continent  was  further  submerged  and  cut  into  two  by 
the  union  of  a  bay  from  the  Gulf  of  Mexico  and  one  from  the  Arctic 
Sea.  The  Rocky  Mountain  revolution  ended  the  Cretaceous  with  the 
upheaval  of  the  Rockies,  Alps,  Himalayas  and  Andes  mountains.  The 
Mesozoic  is  characterized  by  the  tremendous  evolution,  diversification 
and  specialization  of  the  reptiles,  and  is  commonly  called  the  Age  of 
Reptiles.  Mammals  originated  in  the  Triassic  and  birds  in  the  Jurassic. 
Most  of  the  modern  orders  of  insects  appeared  in  the  Triassic,  and  snails, 
bivalve  molluscs  and  sea  urchins  underwent  important  evolutionary  ad- 
vances. 

At  the  end  of  the  Cretaceous  a  great  many  reptiles  became  extinct; 
they  were  apparently  unable  to  adapt  to  the  marked  changes  brought 
about  by  the  Rocky  Mountain  revolution.  As  the  climate  became  colder 
and  drier  many  of  the  plants  which  served  as  food  for  the  herbivorous 
reptiles  disappeared.  Some  of  the  herbivorous  reptiles  were  too  large 
to  walk  about  on  land  when  the  swamps  dried  up.  The  smaller,  warm- 
blooded mammals  Avhich  appeared  were  better  able  to  compete  for  food, 
and  many  of  these  ate  reptilian  eggs.  The  demise  of  the  many  kinds  of 
reptiles  was  probably  the  result  of  a  combination  of  a  whole  host  of 
factors,  rather  than  any  single  one. 

The  Cenozo/c  Era.  The  Cenozoic  era,  extending  from  the  Rocky 
Mountain  revolution  to  the  present,  is  subdivided  into  the  earlier  Ter- 
tiary period,  which  lasted  some  74,0()0,()()0  years,  and  the  present  Quarter- 
nary  period,  which  includes  the  last  million  or  million  and  one-half 
years. 

The  Tertiary  is  subdivided  into  five  epochs,  the  Paleocene,  Eocene, 
Oligocene,  Miocene  and  Pliocene.  The  Rockies,  formed  at  the  begin- 
ning of  the  Tertiary,  were  considerably  eroded  by  the  Oligocene,  and 
the  North  American  continent  had  a  gently  rolling  topography.  Another 
series  of  uplifts  in  the  Miocene  raised  the  Sierra  Nevadas  and  a  new  set 
of  Rockies,  and  resulted  in  the  formation  of  the  western  deserts.  The 
climate  of  the  Oligocene  was  rather  mild,  and  palm  trees  grew  as  far 
north  as  Wyoming.  The  uplifts  of  the  Miocene  and  Pliocene,  and  the 
successive  ice  ages  of  the  Pleistocene,  killed  off  many  of  the  mammals 
that  had   evolved. 

The  last  elevation  of  the  Colorado  Plateau,  which  initiated  the 
cutting  of  the  Grand  Canyon,  occurred  almost  entirely  in  the  short 
Pleistocene  and  Recent  epochs,  the  two  subdivisions  of  the  Quaternary 
period.  Four  periods  of  glaciation  occurred  in  the  Pleistocene,  between 
which  the  sheets  of  ice  retreated.  At  their  greatest  extent,  these  ice 
sheets  extended  as  far  south  as  the  Missouri  and  Ohio  rivers  and  cov- 
ered 4,000,000  square  miles  of  North  America.  The  Great  Lakes,  which 
were  carved  out  by  the  advancing  glaciers,  changed  their  outlines  and 
connections  several  times.  It  is  estimated  that  at  one  time,  when  the 
Mississippi  river  drained  lakes  as  far  west  as  Duluth  and  as  far  east  as 
Buffalo,  its  volume  was  more  than  60  times  as  great  as  at  present. 
During  the  Pleistocene  glaciations  enough  water  was  removed  from 
the  oceans  and  locked  in  the  vast  sheets  of  ice  to  lower  the  water  level 


726  GENETICS    AND    EVOLUTION 

from  200  to  300  feet.  1  his  created  land  connections,  highways  for 
the  cUspersal  of  many  hind  forms,  l^etween  Siberia  and  Alaska  at  Bering 
Strait,  and  between  England  and  the  continent  of  Europe.  Many  mam- 
mals, including  the  saber-toothed  tiger,  the  mammoth  and  the  giant 
ground  sloth,  became  extinct  in  the  Pleistocene  after  primitive  man  had 
apjjeared. 

The  fossil  record  available  today  makes  it  impossible  to  doubt 
that  the  present  species  arose  from  previously  existing,  different  ones. 
For  many  lines  of  evolution  the  individual  steps  are  well  known;  other 
lines  have  some  gaps  which  remain  to  be  filled  by  futaue  paleontolo- 
gists. 

Even  if  there  were  no  fossil  record  at  all,  the  results  of  the  detailed 
studies  of  the  morphology,  physiology  and  biochemistry  of  present-day 
animals  and  plants,  of  their  mode  of  development,  of  the  transmission 
of  inherited  characteristics,  and  of  their  distribution  over  the  earth's 
surface  would  be  sufficient  to  prove  organic  evolution. 

310.        The  Evidence  from  Taxonomy 

The  science  of  naming,  describing  and  classifying  organisms, 
taxonomy,  was  discussed  in  Chapter  7.  The  science  of  taxonomy  began 
long  before  the  doctrine  of  evolution  was  accepted;  indeed  the  founders 
of  scientific  taxonomy,  Ray  and  Linnaeus,  were  firm  believers  in  the 
fixity,  the  unchangingness,  of  species.  Present-day  taxonomists  are  con- 
cerned with  the  naming  and  describing  of  species  primarily  as  a  means 
of  discovering  evolutionary  relationships,  based  upon  the  assumption 
that  the  degree  of  resemblance  in  homologous  structures  is  a  measure  of 
the  degree  of  relationship.  The  fact  that  the  characteristics  of  living 
things  are  such  that  they  can  be  fitted  into  a  hierarchical  scheme  of 
categories,  each  more  inclusive  than  the  previous  one— species,  genera, 
families,  orders,  classes  and  phyla,  can  best  be  interpreted  as  proof  of 
evolutionary  relationship.  If  the  kinds  of  animals  and  plants  vvere  not 
related  by  evolutionary  descent,  their  characters  would  be  present  in  a 
confused,  random  pattern  and  no  such  hierarchy  of  forms  could  be 
established. 

The  basic  unit  of  taxonomy  is  the  species,  a  population  of  closely 
similar  individuals,  which  are  alike  in  their  morphologic,  embryologic 
and  physiologic  characters,  which  in  nature  breed  only  with  each  other, 
and  which  have  a  common  ancestry.  It  is  difficult  to  give  a  definition  of 
species  that  is  universally  applicable.  The  definition  must  be  modified 
slightly  to  include  species  wliose  life  cycle  includes  two  or  more  quite 
different  forms  (many  coelenterates,  parasitic  worms,  larval  and  adult 
insects  and  amphibians,  for  example).  A  species  which  is  spread  over  a 
wide  territory  may  show  local  or  regional  differences  which  may  be 
called  subspecies.  Many  instances  are  known  in  which  a  species  is  broken 
up  into  a  chain  of  subspecies,  each  of  which  differs  slightly  from  its 
neighbors  but  interbreeds  with  them.  The  subspecies  at  the  two  ends 
of  the  chain,  however,  may  be  so  different  that  they  cannot  interbreed. 


THE  EVIDENCE  FOR   EVOLUTION 


727 


Such  a  series  o£  geographically  distributed  subspecies  is  called  a  Rassen- 
kreis  (German,  race-circle). 

The  classification  of  living  organisms  into  well  defined  groups  is 
possible  because  most  of  the  intermediate  forms  have  become  extinct. 
If  representatives  of  every  type  of  animal  and  plant  that  have  ever  lived 
were  still  living  today,  there  would  be  many  series  of  intergrading  forms 
and  the  division  of  these  into  neat  taxonomic  categories  would  be  diffi- 
cult indeed.  The  present-day  species  have  been  compared  to  the  terminal 
twigs  of  a  tree  whose  main  branches  and  trunk  have  disappeared.  The 
fascinating  puzzle  for  the  taxonomist  is  to  reconstruct  the  missing 
branches  and  put  each  twig  on  the  proper  branch. 

311.        The  Evidence  from  Anatomy 

Comparisons  of  the  anatomy  of  different  animals  have  been  made 
throughout  this  text.  In  each  instance  it  was  found  that  if  we  study  the 
details  of  the  structure  of  any  particular  organ  system  in  the  diverse 
members  of  a  given  phylum,  it  is  clear  that  there  is  a  basic  similarity 
of  form  which  is  varied  to  some  extent  from  one  class  to  another.  The 
skeletal,  muscular,  circulatory  and  excretory  systems  of  the  vertebrates 
provide  especially  clear  illustrations  of  this  principle,  but  this  is  gen- 
erally true  of  all  systems  in  all  phyla.  You  w'lW  recall  that  not  all  simi- 
larities can  be  used  in  classification,  but  only  those  based  on  homologous 
organs  (p.  424),  ones  which  are  basically  similar  in  their  structure,  in 
their  relationship  to  adjacent  structures,  in  their  embryonic  development, 
and  in  their  ner\'e  and  blood  supply.  A  seal's  front  flipper,  a  bat's  wing, 
a  cat's  paw,  a  horse's  front  leg  and  a  human  hand,  though  superficially 
dissimilar  and  adapted  for  quite  dilierent  functions,  nevertheless  are 
homologous  organs.  Each  consists  of  almost  the  same  number  of  bones, 
muscles,  nerves  and  blood  vessels  arranged  in  the  same  pattern,  and  their 
mode  of  development  is  very  similar.  The  existence  of  such  homologous 
organs  implies  a  common  evolutionary  origin. 


Wisdom  tooth 


•Plica 
semilunaris 


NippUe  in 
maJe,l 


Pyratnidalis  nasi  muscles 


Ji  ■^•|Lnf':^;^ndinous  J 

1'  iwl  p .-.W inscraptionsU     ^l 
l|it!')''"i\  p'*rof  Rectus   (/ 
V     \i'<  M  abdominis! 
\\M   ' ij      muscle       \ 

mml        muscle. 


Coccygeal 
vertebrae 


Figure   35.4.     Diagrams  of  some  of  the  vestigial  organs  of  the  human  body. 


728  GENETICS    AND    EVOLUTION 

Many  species  of  animals  have  organs  or  parts  of  organs  wfricfi  are 
useless  and  often  small  or  lacking  some  essential  part;  in  related  organ- 
isms, the  organ  is  full-sized,  complete  and  functional.  There  are  more 
than  100  such  vestigial  organs  in  the  human  body,  including  the  ap- 
pendix, the  coccyx  (fused  tail  vertebrae),  the  wisdom  teeth,  the  nictitat- 
ing membrane  of  the  eye,  body  hair,  and  the  muscles  that  move  the  ears 
(Fig.  .H5.-1).  Such  organs  are  the  remnants  of  ones  which  were  functional 
in  the  ancestral  forms,  but  when  some  change  in  the  environment  ren- 
dered the  organ  no  longer  necessary  for  survival  it  gradually  became 
reduced  to  a  vestige.  This  appears  at  first  glance  to  be  an  application  of 
Lamarck's  idea  of  the  role  of  "use  and  disuse"  of  an  organ  in  evolution, 
but  the  underlying  mechanism  is  quite  different.  Mutations  for  the 
decrease  in  the  size  and  functional  importance  of  an  organ  are  occurring 
constantly;  as  long  as  the  organ  is  necessary  for  survival,  such  mutations 
are  lethal  and  eliminate  their  possessors.  But  if  the  organ  is  no  longer 
needed  for  survival,  such  mutations  will  not  be  lethal  and  they  may 
accumulate  and  lead  to  the  reduction  of  the  organ. 

312.        Evidence  from  Comparative  Physiology  and  Biochemistry 

The  study  of  the  physiologic  and  biochemical  traits  of  organisms 
generally  requires  complex  apparatus  and  is  more  difficult  than  the 
direct  observation  of  morphologic  characters.  Yet,  as  such  studies  have 
been  made  using  a  wide  variety  of  animal  types,  it  has  become  clear  that 
there  are  functional  similarities  and  differences  which  parallel  closely 
the  morphologic  ones.  Indeed,  if  one  were  to  establish  taxonomic  rela- 
tionships based  on  physiologic  and  biochemical  characters  instead  of  on 
the  usual  structural  ones,  the  end  result  would  be  much  the  same. 

The  fundamental  similarity  of  the  chemical  constituents  and  pat- 
terns of  enzymes  present  in  cells  of  different  animals  was  presented  in 
Chapter  4.  There  are,  however,  certain  chemical  constituents,  certain 
enzymes  and  certain  hormones  that  are  found  in  some  animals  and  not 
in  others.  The  distribution  of  these  biochemical  characters  strongly 
parallels  the  evolutionary  relationships  inferred  from  other  evidence. 

The  blood  serum  of  each  species  of  animal  contains  certain  specific 
proteins.  The  degree  of  similarity  of  these  serum  proteins  can  be  deter- 
mined by  antigen-antibody  reactions.  To  perform  the  test,  an  experi- 
mental animal,  usually  a  rabbit,  is  injected  with  a  small  amount  of  the 
serum,  as,  for  example,  a  sample  of  human  serum.  The  proteins  of 
the  injected  serum  are  foreign  to  the  rabbit's  blood  and  hence  act  as 
antigens,  stimulating  the  production  of  antibodies  which  are  specific  for 
human  serum  antigens.  These  antibodies  are  then  obtained  by  with- 
drawing blood  from  the  rabbit  and  allowing  it  to  clot;  the  antibodies 
are  in  the  serum.  \\^hen  a  dilute  sample  of  this  serum  is  mixed  with  a 
drop  of  human  serum,  the  antibody  for  human  serum  reacts  with  the 
human  serum  antigen  and  produces  a  visible  precipitation.  The  strength 
of  the  reaction  can  be  measured  by  making  successive  dilutions  of  the 
human  serum,  mixing  each  dilution  with  a  fresh  sample  of  the  antibody 
solution  (the  rabbit  serum),  and  observing  at  what  point  the  precipita- 


THE  EVIDENCE  FOR   EVOLUTION  729 

tion  no  longer  occurs.  When  serum  from  an  animal  other  than  man  is 
mixed  with  rabbit  serum  containing  antibodies  for  human  serum  pro- 
teins, there  is  either  no  precipitation  at  all,  or  else  a  precipitation  occurs 
only  with  concentrated  antigen  solutions.  By  testing  in  turn  the  sera  of  a 
variety  of  animals  with  rabbit  serum  containing  antibodies  for  human 
serum  proteins,  the  degree  of  similarity  between  the  proteins  can  be 
determined.  If  the  serum  of  another  animal  contains  proteins  which  are 
similar  to  those  of  man,  a  precipitation  will  occur.  In  this  way,  man's 
closest  "blood  relations"  have  been  found  to  be  the  great  apes,  and  then, 
in  order,  the  Old  World  monkeys,  the  New  World  monkeys,  and  finally 
the  tarsioids.  The  serum  of  the  lemur  gives  the  smallest  amount  of  pre- 
cipitation when  mixed  with  antibodies  specific  for  human  serum. 

The  biochemical  relationships  of  a  variety  of  forms,  tested  in  this 
way,  correlate  with  and  complement  the  relationships  determined  by 
other  means.  Cats,  dogs  and  bears  are  closely  related,  as  determined  by 
this  test;  cows,  sheep,  goats,  deer  and  antelopes  constitute  another  closely 
related  group.  This  test  reveals  that  there  is  a  closer  relationship  among 
the  modern  birds  than  among  the  mammals,  for  all  of  the  several 
hundred  species  of  birds  tested  give  strong  and  immediate  reactions  with 
serum  containing  antibodies  for  chicken  serum.  From  other  tests  it  was 
concluded  that  birds  are  more  closely  related  to  the  crocodile  line  of 
reptiles  than  to  the  snake-lizard  line,  which  corroborates  the  paleonto- 
logic  evidence.  Similar  tests  of  the  sera  of  crustaceans,  insects  and  mol- 
luscs have  shown  that  those  forms  regarded  as  being  closely  related  from 
morphologic  or  paleontologic  evidence  also  show  similarities  in  their 
serum  proteins. 

It  might  seem  unlikely  that  an  analysis  of  the  urinary  wastes  of 
different  species  would  provide  evidence  of  evolutionary  relationship, 
yet  this  is  true.  The  kind  of  waste  excreted  depends  upon  the  particular 
kinds  of  enzymes  present,  and  the  enzymes  are  determined  by  genes 
which  have  been  selected  in  the  course  of  evolution.  The  waste  products 
of  the  metabolism  of  purines  (one  of  the  constituents  of  nucleic  acids) 
are  excreted  by  man  and  other  primates  as  uric  acid,  by  other  mammals 
as  allantoin,  by  amphibians  and  most  fishes  as  urea,  and  by  most  inverte- 
brates as  ammonia.  Vertebrate  evolution  has  been  marked  by  the  suc- 
cessive loss  of  enzymes  required  for  the  stepwise  degradation  of  uric 
acid.  Joseph  Needham  made  the  interesting  observation  that  the  chick 
embryo  in  the  early  stages  of  development  excretes  ammonia,  later  it 
excretes  urea,  and  finally  it  excretes  uric  acid.  The  enzyme  uricase,  which 
catalyzes  the  first  step  in  the  degradation  of  uric  acid,  is  present  in  the 
early  chick  embryo  but  disappears  in  the  later  stages  of  development. 
The  adult  frog  excretes  urea  but  the  larval  form  excretes  ammonia. 
These  are  biochemical  examples  of  the  principle  of  recapitulation. 

313.        Evidence  from  Embryology 

The  importance  of  the  embryologic  evidence  for  evolution  was  em- 
phasized by  Darwin  and  brought  into  even  greater  prominence  by  Ernst 
Haeckel  in  1866  when  he  developed  his  Biogenetic  Law,  that  embryos. 


730 


GENETICS   AND   EVOLUTION 


Fish  Salamander  Turtle     Chicken. 


Pi^ 


Cow      Rabbit       Man 


Figure  35.5.     Comparison  of  early  and  later  stages  in  the  development  of  verte- 
brate embryos.  Note  the  similarity  of  the  earliest  stages  of  each. 


in  the  course  of  development,  repeat  the  evolutionary  history  of  their 
ancestors  in  some  abbreviated  form.  This  idea,  succinctly  stated  as 
"Ontogeny  recapitulates  phylogeny,"  stimulated  research  in  embryology 
and  focused  attention  on  the  general  resemblance  between  embryonic 
development  and  the  evolutionary  process,  but  it  now  seems  clear  that 
the  embryos  of  the  higher  animals  resemble  the  embryos  of  lower  forms, 
not  the  adults,  as  Haeckel  had  believed.  1  he  early  stages  of  all  vertebrate 
embryos,  for  example,  are  remarkably  similar,  and  it  is  not  easy  to  dif- 
ferentiate a  human  embryo  from  the  embryo  of  a  fish,  frog,  chick  or  pig 
(Fig.  35.5).  In  recapitulating  its  evolutionary  history  in  a  few  days,  weeks 
or  months  the  embryo  must  eliminate  some  steps,  and  alter  and  distort 
others.  In  addition,  some  new  characters  have  evolved  which  are  adaptive 
and  enable  the  embryo  to  survive  to  later  stages.  For  example,  mam- 
malian embryos,  which  have  many  early  characteristics  in  common  with 
those  of  fish,  amphibia  and  reptiles,  have  other  structures  which  enable 
them  to  survive  and  develop  within  the  mother's  uterus  rather  than 
within  an  egg  shell.  Such  secondary  traits  may  alter  the  original  char- 
acters common  to  high  and  low  forms  so  that  the  basic  resemblances  are 
blurred.  The  concept  of  recapitulation  must  be  used  with  caution,  rather 
than  rigorously,  but  it  does  provide  an  explanation  for  many  otherwise 
inexplicable  events  in  development. 

Studies  of  the  embryonic  forms  may  provide   the  only  means  for 
identifying  the  relationships  of  certain  organisms.  Sacculina,  for  example, 


THE  EVIDENCE  FOR   EVOLUTION 


731 


is  an  extremely  aberrant  barnacle  which  parasitizes  crabs.  The  adult 
form  is  a  saclike  structure  which  sends  processes  into  the  tissues  of  the 
host  to  absorb  nourishment.  It  resembles  no  other  organism  and  its  rela- 
tionship became  clear  only  when  it  was  found  that  its  larva  is  like  that 
of  other  barnacles  until  it  becomes  attached  to  the  abdomen  of  the  host. 
Then  it  loses  its  appendages  and  other  structures  and  becomes  the  adult, 
saclike  creature. 

The  concept  of  recapitulation  is  very  helpful  in  understanding  the 
curious  and  complex  development  of  the  vertebrate  circulatory  and 
excretory  systems.  It  is  also  useful,  when  not  taken  too  literally,  in  get- 
ting a  broad  picture  of  the  whole  of  development.  Thus  the  fertilized 
egg  can  be  compared  to  the  putative  single-celled  flagellate  ancestor  of 
all  animals,  and  the  blastula  can  be  compared  to  a  colonial  protozoan 
or  to  some  hypothetical  blastula-like  animal  which  has  been  postulated 
to  be  the  ancestor  of  all  Metazoa.  Haeckel  believed  that  the  ancestor  of 
coelenterates  and  all  the  higher  animals  was  a  gastrula-like  organism 
with  two  layers  of  cells  and  a  central  cavity  connected  by  a  blastopore 
to  the  outside.  After  gastrulation,  development  follows  one  of  two  main 
lines.  In  the  echinoderms  and  chordates  the  blastopore  becomes  the  anus, 
or  comes  to  lie  near  the  anus.  In  the  annelid-mollusc-arthropod  line  the 
blastopore  becomes  the  mouth  or  comes  to  lie  near  the  mouth.  In  both 
lines  the  mesoderm  develops  between  the  ectoderm  and  endoderm.  In 
the  chordate-echinoderm  line  the  mesoderm  develops,  at  least  in  part,  as 
pouches  from  the  primitive  digestive  tract,  whereas  in  the  annelid-mol- 
lusc line  the  mesoderm  usually  originates  from  special  cells  differentiated 
early  in  development. 

All  chordate  embryos  develop,  shortly  after  the  mesoderm  begins  to 
appear,  a  dorsal  hollow  nerve  cord,  a  notochord  and  pharyngeal  pouches. 


AorHc 


-"Ventricle.- 
Atrium 

Ca.rdinal 
veins 

Fiqure  35  6  Ventral  views  of  the  heart  and  aortic  arches  of  a  hurnan  embryo 
/ri^htf  and  an  adult  shark  (left).  Both  have  a  single  atrium  and  single  ventricle, 
sefral  aortic  arche!!  Tnd  anterior  and  posterior  cardinal  veins  emptying  into  the 
heart. 


732  GENETICS   AND    EVOLUTION 

The  early  human  embryo  at  this  stage  resembles  a  fish  embryo,  with  gill 
pouches,  pairs  ol  aortic  arches,  a  fishlike  heart  with  a  single  atrium  and 
ventricle,  a  primitive  fish  kidney,  and  a  well  differentiated  tail  complete 
with  muscles  for  wagging  it  (Fig.  35.6).  At  a  slightly  later  stage  the 
human  embryo  resembles  a  reptdian  embryo.  Its  gill  pouches  regress; 
the  bones  which  make  up  each  vertebra,  and  which  had  been  separate 
as  in  the  most  primitive  fishes,  fuse;  a  new  kidney,  the  mesonephros, 
forms  and  the  pronephros  disappears  or  becomes  incorporated  into  other 
structures;  and  the  atrium  becomes  divided  into  right  and  left  chambers. 
Still  later  in  development  the  human  embryo  develops  a  mammalian, 
four-chambered  heart  and  a  third  kidney,  the  metanephros.  During  the 
seventh  month  of  intrauterine  development  the  human  embryo,  with 
its  coat  of  embryonic  hair  and  in  the  relative  size  of  body  and  limbs, 
resembles  a  baby  ape  more  than  it  resembles  an  adult  human. 

Our  increasing  understanding  of  physiological  genetics  provides  us 
with  an  explanation  of  the  phenomenon  of  recapitulation.  All  chordates 
have  in  common  a  certain  number  of  genes  which  regulate  the  processes 
of  early  development.  But,  as  our  ancestors  evolved  from  fish,  through 
amphibian  and  reptilian  stages,  they  accumulated  mutations  for  new 
characteristics  but  kept  some  of  the  original  "fish"  genes,  which  still 
control  early  development.  Later  in  development  the  genes  which  the 
human  shares  with  amphibians  influence  the  course  of  development  so 
that  the  embryo  resembles  a  frog  embryo.  Subsequently  some  of  the  genes 
which  we  have  in  common  with  reptiles  come  into  control.  Only  after 
this  do  most  of  the  peculiarly  mammalian  genes  exert  their  influence, 
and  these  are  followed  by  the  action  of  genes  we  have  in  common  with 
other  primates.  The  anthropoid  apes,  which  have  the  most  immediate 
ancestors  in  common  with  us,  have  the  most  genes  in  common  with  us 
and  their  development  is  identical  with  ours  except  for  some  fine  details. 
A  pig  or  rat,  whose  ancestors  are  the  same  as  ours  only  up  to  the  stage  of 
the  primitive  placental  mammals,  has  fewer  genes  in  common  and  has 
developmental  processes  that  diverge  at  an  earlier  time.  In  general  dur- 
ing development,  the  general  characters  that  distinguish  phyla  and 
classes  appear  before  the  special  characters  that  distinguish  genera  and 
finally  species.  Within  each  phylum,  the  higher  forms  pass  through  a 
sequence  of  developmental  stages  which  are  similar  to  those  of  lower 
forms,  but  achieve  a  different  final  form  by  adding  changes  at  the  end 
of  the  original  sequence  and  by  altering  certain  of  the  earlier  embryonic 
stages  they  share  with  the  lower  forms. 

314.        Evidence  from  Genetics  and  Cytology 

For  the  past  several  thousand  years  man  has  been  selecting  and 
breeding  animals  and  plants  for  his  own  uses,  and  a  great  many  varieties, 
adapted  for  different  purposes,  have  been  established.  These  results  of 
artificial  selection  provide  striking  models  of  what  may  be  accomplished 
by  natural  selection.  All  of  our  breeds  of  dogs  have  descended  from  one, 
or  perhaps  a  very  few,  species  of  wild  dog  or  wolf,  yet  they  vary  so  much 
in  color,  size  and  body  proportions   that  if  they  occurred  in  the  wild 


THE   EVIDENCE   FOR   EVOLUTION  733 

they  would  undoubtedly  be  considered  separate  species.  They  are  all 
inierfertile  and  are  known  to  come  from  common  ancestors,  so  they  are 
regarded  as  varieties  of  a  single  species.  A  comparable  range  of  varieties 
has  been  produced  by  artificial  selection  in  cats,  chickens,  sheep,  cattle 
and  horses.  Plant  breeders  have  established  by  selective  breeding  a  tre- 
mendous variety  of  plants.  From  the  cliff  cabbage,  which  still  grows  wild 
in  Europe,  have  come  cultivated  cabbage,  cauliflower,  kohlrabi,  Brussels 
sprouts,  broccoli  and  kale. 

Geneticists  have  been  able  to  trace  the  ancestry  of  certain  modern 
plants  by  a  combination  of  cytologic  techniques,  in  which  the  morphol- 
ogy of  the  chromosomes  is  compared,  and  breeding  techniques  which 
compare  the  kinds  of  genes  and  their  order  in  particular  chromosomes 
in  a  series  of  plants.  In  this  way  the  present  cultivated  tobacco  plant, 
Nicotiana  tabaciun,  was  shown  to  have  arisen  from  two  species  of  wild 
tobacco,  and  corn  was  traced  to  teosinte,  a  grasslike  plant  which  grows 
wild  in  the  Andes  and  Mexico.  The  details  of  the  structure  of  the  giant 
chromosomes  of  the  salivary  glands  of  fruit  flies  have  been  of  prime 
importance  in  unraveling  the  evolutionary  history  of  the  many  species 
of  Drosopliila. 

31  5.        Evidence  from  the  Geographic  Distribution  of  Organisms 

In  the  course  of  the  voyage  of  the  Beagle,  Darwin  was  greatly  im- 
pressed by  his  observations  that  the  plants  and  animals  of  South  America 
and  the  Galapagos  Islands  were  not  found  everywhere  that  they  could 
exist  if  climate  and  topography  were  the  only  factors  determining  their 
distribution.  The  facts  of  biogeography,  the  geographic  distribution  of 
plants  and  animals,  were  of  prime  importance  in  leading  both  Darwin 
and  Alfred  Russell  Wallace  to  the  conclusion  that  organic  evolution  had 
occurred  by  natural  selection.  The  present  distribution  of  organisms, 
and  the  sites  at  which  their  fossil  remains  are  found,  are  understandable 
only  on  the  basis  of  the  evolutionary  history  of  each  species. 

The  range  of  each  species  is  that  particular  portion  of  the  earth  in 
which  it  is  found.  The  range  of  a  species  may  be  restricted  to  a  few 
square  miles  or  less,  or,  as  with  man,  may  include  almost  the  entire 
earth.  In  general,  the  ranges  of  closely  related  species  or  subspecies  are 
not  identical,  nor  are  they  widely  separated,  but  are  adjacent  and  sep- 
arated by  a  barrier  of  some  sort.  This  generalization  was  stated  by  David 
Starr  Jordan  and  is  known  as  Jordan's  rule.  The  explanation  for  this 
should  be  clear  from  the  discussion  of  the  role  of  isolation  in  species 
formation.  A  single  species  cannot  be  subdivided  as  long  as  interbreeding 
can  occur  throughout  the  whole  population.  But  when  some  barrier  is 
interposed  between  two  parts  of  the  population  so  that  interbreeding 
is  prevented,  the  two  populations  will,  in  the  subsequent  course  of  time, 
accumulate  different  gene  mutations.  ,      .      u  u 

One  of  the  fundamental  assumptions  of  biogeography  is  that  eacfi 
species  of  animal  or  plant  originated  only  once.  The  place  where  this 
occurred  is  known  as  its  center  of  origin.  The  center  of  origin  is  not  a 
single  point,  but  the  range  of  the  population  when  the  new  species  was 


734  GENETICS    AND    EVOLUTION 

formed.  From  this  center  ot  origin  each  species  spreads  out,  under  the 
pressure  of  an  increasing  population,  until  it  is  halted  by  a  barrier  of 
some  kind:  a  physical  one  such  as  an  ocean,  mountain  or  desert,  an 
environmental  one  such  as  unfavorable  climate,  or  a  biologic  barrier 
such  as  the  absence  of  food  or  the  presence  of  other  species  which  prey 
upon  it  or  compete  with  it  for  food  or  shelter. 

As  one  might  expect,  regions  which  have  been  separated  from  the 
rest  of  the  world  for  a  long  time,  such  as  South  America  and  Australia, 
have  a  unique  assemblage  of  animals  and  plants.  Australia  has  a  mam- 
malian population  of  monotremes  and  marsupials  that  is  found  nowhere 
else.  Australia  became  separated  from  Malaya  during  the  Mesozoic,  before 
placental  mammals  evolved,  and  its  primitive  mammals  were  not  elim- 
inated, as  were  the  monotremes  and  most  of  the  marsupials  in  the  other 
parts  of  the  world,  by  the  competition  of  the  better  adapted  placental 
mammals.  The  Australian  marsupials  evolved  into  a  wide  variety  of 
forms,  each  adapted  to  some  particular  combination  of  environmental 
factors. 

The  kinds  of  animals  and  plants  found  on  oceanic  islands  are  in- 
structive. They  resemble,  in  general,  those  of  the  nearest  mainland,  yet 
they  are  made  up  to  some  extent  of  species  found  nowhere  else.  Darwin 
studied  the  flora  and  fauna  of  the  Cape  Verde  Islands,  some  400  miles 
west  of  Dakar  in  Africa,  and  of  the  Galapagos  Islands,  a  comparable 
distance  west  of  Ecuador.  On  each  archipelago  the  plants  and  the  non- 
flying  animals  were  indigenous,  but  those  of  Cape  Verde  resembled 
African  species  and  those  of  the  Galapagos  resembled  South  American 
ones.  It  is  clear  that  species  from  the  neighboring  continent  migrated  or 
were  carried  to  the  island  and  that  by  subsequent  evolution  they  became 
differentiated  from  their  ancestral  forms.  The  animals  and  plants  found 
on  oceanic  islands  are  only  those  that  could  survive  the  trip  there.  There 
are,  for  example,  no  frogs  or  toads  on  the  Galapagos,  and  no  terrestrial 
mammals,  even  though  conditions  would  favor  their  survival. 

There  are  many  facts  of  the  present-day  distribution  of  animals  and 
plants  which  can  be  explained  only  by  knowledge  of  their  history.  Alli- 
gators, for  example,  are  found  only  in  the  rivers  of  southeastern  United 
States  and  in  the  Yangtse  River  in  China.  Sassafras,  tulip  trees  and  mag- 
nolias are  found  only  in  the  eastern  United  States,  Japan,  and  eastern 
China.  The  explanation  for  these  curious  patterns  of  distribution  lies 
in  the  fact  that  early  in  the  Cenozoic  era  the  northern  hemisphere  was 
much  flatter  than  at  present  and  the  North  American  continent  was 
connected  with  eastern  Asia  by  a  land  bridge  at  what  is  now  Bering 
strait.  The  climate  of  the  whole  region  was  much  warmer  than  at  present 
and  fossil  evidence  shows  that  alligators,  magnolia  trees  and  sassafras 
were  distributed  over  the  entire  region.  Later  in  the  Cenozoic,  as  the 
Rockies  increased  in  height,  the  western  part  of  North  America  became 
much  colder  and  drier.  During  the  Pleistocene  the  ice  sheets  moving 
down  from  the  north  met  the  desert  and  mountain  regions  of  western 
North  America,  and  the  animals  and  plants  that  had  lived  in  that  region 
either  became  extinct  or  migrated.  In  southeastern  United  States  and  in 


THE  EVIDENCE  FOR  EVOLUTION 


735 


eastern  China  were  regions  untouched  by  the  glaciations  and  here  the 
alHgators  and  magnolia  trees  survived.  Because  the  alligators  and  mag- 
nolias of  the  two  regions  have  been  separated  for  several  million  years, 
they  have  had  the  opportunity  to  accumulate  different  random  muta- 
tions. They  are  thus  slightly  different  but  closely  related  species  of  the 
same  genera. 

316.        The  Biogeographic  Realms 

Careful  studies  of  the  distribution  of  plants  and  animals  over  the 
earth  have  revealed  the  existence  of  six  major  biogeographic  realms,  each 
characterized  by  the  presence  of  certain  unique  organisms.  These  realms 
were  originally  defined  on  the  basis  of  the  distribution  of  mammals,  but 
they  have  proved  to  be  valid  for  many  other  kinds  of  animals  and  plants 
as  well.  The  various  parts  of  each  realm  may  be  widely  separated  and 
have  quite  different  conditions  of  climate  and  topography,  but  it  has 


Figure  35.7.     A  polar  projection  map  of  the  world  showing  the  biogeographic  realms. 
(After  Matthew.) 


736  GENETICS    AND    EVOLUTION 

been  possible,  during  most  geologic  eras,  for  organisms  to  pass  more  or 
less  Ireely  from  one  part  to  another.  In  contrast,  the  six  realms  are  sep- 
arated from  each  other  by  major  barriers  of  sea,  desert  or  mountains 

(Fig.  35.7). 

The  Palaearctic  realm  includes  Europe,  Africa  north  of  the  Sahara 
desert,  and  Asia  north  of  the  Himalaya  and  Nan-Ling  mountains,  plus 
Japan,  Iceland  and  the  Azores  and  Cape  Verde  Islands.  The  animals 
indigenous  to  the  Palaearctic  are  moles,  deer,  oxen,  sheep,  goats,  robins 
and  magpies. 

The  Nearctic  realm  includes  Greenland  and  North  America  north 
of  the  northern  plateau  of  Mexico.  This  contains  many  of  the  same 
animals  as  the  Palaearctic,  plus  species  of  mountain  goats,  prairie  dogs, 
opossums,  skunks,  raccoons,  bluejays,  turkey  buzzards  and  wren-tits  found 
nowhere  else.  The  land  bridge  connecting  North  America  and  Asia  at 
Bering  Strait  in  former  geologic  times  permitted  the  migration  back  and 
forth  of  many  kinds  of  animals  and  plants.  The  flora  and  fauna  of  the 
Palearctic  and  Nearctic  realms  are  similar  in  many  respects  and  the  two 
are  sometimes  combined  as  the  Holarctic  region. 

The  Neotropical  realm  consists  of  South  America,  Central  America, 
southern  Mexico  and  the  islands  of  the  West  Indies.  Its  fauna  is  quite 
distinctive,  including  alpacas,  llamas,  prehensile-tailed  monkeys,  blood- 
sucking bats,  sloths,  tapirs,  anteaters,  and  a  host  of  bird  species— toucans, 
puff  birds,  tinamous  and  others— found  nowhere  else  in  the  world. 

The  part  of  Africa  south  of  the  Sahara,  plus  the  island  of  Mada- 
gascar, comprises  the  Ethiopian  realm.  The  gorilla,  chimpanzee,  zebra, 
rhinoceros,  hippopotamus,  giraffe,  aardvark,  and  many  birds,  reptiles 
and  fishes  live  only  in  this  realm. 

The  Oriental  realm  includes  India,  Ceylon,  Indo-China,  southern 
China,  the  Malay  peninsula  and  some  of  the  islands  of  the  East  Indies— 
the  Philippines,  Borneo,  Java  and  Bali.  Some  of  the  animals  peculiar  to 
it  are  the  orang-utan,  black  panther,  Indian  elephant,  gibbon  and 
tarsier. 

Australia,  New  Zealand,  New  Guinea,  and  the  remaining  islands  of 
the  East  Indies,  those  east  of  Celebes  and  Lombok,  make  up  the  Aus- 
tralian realm.  The  line  separating  the  Oriental  and  Australian  realms, 
known  as  Wallace's  Line,  separates  Bali  and  Lombok,  goes  through  the 
straits  of  Macassar  between  Borneo  and  Celebes,  and  passes  east  of 
the  Philippines.  Although  the  islands  of  Bali  and  Lombok  are  separated 
by  a  channel  only  20  miles  wide,  their  respective  animals  and  plants  are 
more  unlike  than  are  those  of  England  and  Japan,  almost  on  the  op- 
posite sides  of  the  world  from  each  other.  Native  to  the  Australian  realm 
are  the  duck-billed  platypus,  echidna,  kangaroo,  wombat,  koala  bear, 
and  other  marsupials.  Its  assortment  of  curious  birds  includes  the  cas- 
sowary and  emu,  the  lyre-bird,  cockatoo  and  bird-of-paradise. 

Why  certain  animals  appear  in  one  region  yet  are  excluded  from 
another  in  which  they  are  well  adapted  to  survive  (and  in  which  they 
flourish  when  introduced  by  man)  can  be  explained  only  by  their  evolu- 
tionary history. 


THE   EVIDENCE   FOR   EVOLUTION  737 

Questions 

1.  What  methods  are  used  for  estimating  the  age  of  rocks? 

2.  What  is  a  geological  revolution?  What  effects  have  such  revolutions  on  the  course  of 
evolution? 

3.  Describe  the  life  of  the  Cambrian  period.  What  are  the  biggest  differences  between 
the  animal  life  of  that  time  and  the  present? 

4.  Discuss  the  thesis  that  the  hierarchical  scheme  of  animal  classification  is  evidence  for 
organic  evolution. 

5.  How  would  you  define  a  species?  What  difficulties  might  be  encountered  in  trying  to 
decide  whether  two  populations  of  animals  are  one  or  two  species? 

6.  Define:  homologous  organs,  vestigial  organs,  Rassenkreis,  petrifaction. 

7.  Describe  the  method  used  to  determine  evolutionary  relationship  by  the  nature  of 
serum  proteins. 

8.  Discuss  the  implications  of  the  phrase  "Ontogeny  recapitulates  phylogeny."  What 
changes  in  Haeckel's  theory  have  been  made  necessary  by  subsequent  research? 

9.  Discuss  the  genetic  explanation  for  the  phenomenon  of  recapitulation. 

10.  What  is  Jordan's  rule? 

11.  Define  the  terms  "range"  and  "center  of  origin." 

12.  If,  in  tracing  evolutionary  relationships,  anatomic  evidence  pointed  one  way  and 
biochemical  evidence  another,  which  do  you  think  would  be  the  more  reliable?  Why? 

Supplementary  Reading 

The  fossil  evidence  for  evolution  is  summarized  in  Dodson's  Textbook  of  Evolution. 
R.  S.  Lull's  Organic  Evolution  and  W.  K.  Gregory's  Evolution  Emerging  provide  more 
advanced  discussions  of  paleontology.  The  more  important  fossil  vertebrates  are  described 
in  A.  S.  Romer's  Man  and  the  Vertebrates,  P.  E.  Raymond's  Prehistoric  Life,  and  Colbert's 
Evolution  of  the  Vertebrates.  Two  excellent  recent  books  on  the  evolution  of  the  inverte- 
brates are  Principles  of  Invertebrate  Paleontology  by  Shrock  and  Twenhofel  and  Inverte- 
brate Fossils  by  Moore,  Lalicker  and  Fischer. 

An  Introduction  to  Comparative  Biochemistry,  by  Ernest  Baldwin,  provides  an  inter- 
esting account  of  some  of  the  biochemical  similarities  in  different  animals  which  point 
to  evolutionary  relationships.  A  detailed  but  readable  discussion  of  the  biochemical  facts 
bearing  on  evolutionary  theories  is  Marcel  Florkin's  Biochemical  Evolution.  A  brief  dis- 
cussion of  this  topic  is  found  in  George  Walds  Biochemical  Evolution,  in  Trends  in 
Physiology  and  Biochemistry,  edited  by  E.  S.  G.  Barron. 


CHAPTER  36 


The  Evolution  of  Man 


317.        Primate  Evolution 

The  line  of  evolution  that  led  from  the  ostracoderms  to  the  primates 
was  traced  in  Chapters  22  to  24.  Although  the  fossil  records  of  horses, 
elephants,  camels,  and  many  other  mammals  are  quite  good,  those  of 
the  primates  are  regrettably  fragmentary.  Most  of  our  primate  ancestors 
lived  in  tropical  forests,  where  fossils  are  not  likely  to  be  preserved. 
However,  there  are  representatives  of  several  primitive  groups  of  pri- 
mates alive  today  from  which  we  can  get  some  idea  of  what  our  ancestral 
primates  might  have  looked  like.  The  earliest  placental  mammals  were 
small,  tree-dwelling,  insect-eating  animals;  from  these  insectivores  have 
evolved  all  the  kinds  of  placental  mammals  alive  today.  The  primates 
remained  mostly  arboreal  and  are  relatively  unspecialized. 

There  are  three  groups  (suborders)  of  the  primates:  the  lemuroids, 
which  includes  the  tree  shrews,  lemurs  and  lorises;  the  tarsioids,  the 
tarsier;  and  the  anthropoids,  monkeys,  apes  and  man.  The  primates  are, 
in  general,  rather  unspecialized  mammals;  the  specializations  they  do 
have  are  adaptations  for  arboreal  life;  grasping  hands  and  feet  (with 
opposable  thumbs  and  great  toes);  some  or  all  of  the  fingers  and  toes 
with  flattened  nails;  very  flexible,  mobile  arms  and  legs;  well  developed 
brains  (especially  the  cerebrum);  and  binocular  vision. 

The  primate  line  appears  to  have  begun  with  the  tree  shrews,  which 
are  intermediate  between  the  primitive  insectivores  and  the  primates. 
There  are  fossil  tree  shrews  known  from  the  Oligocene,  and  some  tree 
shrews,  such  as  Tupaia  (Fig.  24.16),  which  still  survive  in  the  forests  of 
Malaya  and  the  Philippines.  The  tree  shrew  looks  a  bit  like  a  squirrel 
with  a  long  snout  and  tail,  but  has  opposable  first  toes.  During  most  of 
primate  evolution,  the  trend  was  toward  greater  adaptation  for  an 
arboreal  life.  Only  in  some  of  the  larger  apes  and  man  has  this  trend 
been  reversed. 

318.        The  Lemurs 

The  lemurs  are  believed  to  represent  the  next  stage  in  the  evolution 
of  the  primates.  These  are  small  nocturnal,  arboreal  animals,  with  long 
tails,  long,  flexible  limbs,  and  grasping  hands  and  feet  (Fig.  36.1).  Lemurs 
are  found  today  in  the  tropics  of  Africa  and  Asia,  but  especially  on  the 
island  of  Madagascar.  Fossil  lemurs  have  been  found  in  deposits  from 

738 


Figure  36.1.  The  varied 
lemur,  Lemur  variegatus. 
(Courtesy  of  the  American 
Museum  of  Natural  History.) 


Figure  36.2.  The  tarsier, 
Tarsius,  found  in  the  East  In- 
dies. Note  the  large,  forward- 
directed  eyes  and  the  adhesive 
pads  on  the  tips  of  the  digits 
which  facihtate  its  clinging  to 
the  branches  of  trees.  (Cour- 
tesy of  the  American  Museum 
of  Natural  History.) 


the  Paleocene  and  Eocene  of  Eu- 
Yope  and  North  America.  A  com- 
plete skeleton  oi  the  Eocene  lemur, 
NotJiarctiis,  shows  that  it  was  quite 
similar  to  the  modern  forms  such 
as  Lemur. 

319.        The  Tarsioids 

The  tarsioids  are  represented 
today  by  a  single  genus,  Tarsius, 
found  in  the  East  Indies.  Tarsiers 
are  also  small,  nocturnal  and  arbo- 
real; they  have  large  ears  and  dis- 
tinctive, enormous  eyes,  set  close 
together  and  directed  forward 
(Fig.  36.2).  The  hind  legs  are  long 
and  specialized  for  hopping;  Tar- 
sius is  noted  for  its  ability  to  leap 
great  distances  through  the  tree 
tops.  Its  toes  are  long,  slender,  and 
supplied  with  adhesive  pads  for 
grasping.  Fossil  tarsioids  have  been 
found  in  Eocene  deposits  from 
both  North  America  and  Europe. 
These  primitive  tarsioids  are  inter- 
mediate in  many  respects  between 
lemurs  and  the  anthropoids  and 
the  latter  probably  evolved  from 
some  early   tarsioid  group. 

739 


740 


GENETICS  AND   EVOLUTION 


320.        The  Anthropoids 

Monkeys,  apes  and  man,  which  have  many  characteristics  in  com- 
mon, are  grouped  in  the  suborder  Anthropoidea.  The  anthropoids  have 
larger,  more  complicated  brains,  and  large,  forward-directed  eyes  en- 
closed in  complete  bony  sockets.  Most  of  the  anthropoids  walk  on  all 
lour  legs,  but  tend  to  sit  upright,  so  that  the  hands  are  free  to  manipu- 
late objects.  The  opposability  of  the  thumb  and  great  toe  is  highly  de- 
veloped. 

The  anthropoids  are  subdivided  into  two  groups,  the  more  primitive 
platyrrhine  or  broad-nosed  monkeys  of  South  and  Central  America,  and 
the  catarrhine  or  narrow-nosed  forms,  which  include  the  Old  World 
monkeys,  apes  and  man. 

The  platyrrhines,  which  have  widely  separated  nostrils  directed  for- 
ward and  sideward,  are  a  group  of  primates  which  became  isolated  in 
South  America  during  the  Tertiary  and  evolved  independently  of  the 
other  anthropoids.  They  include  the  marmosets,  which  are  primitive  and 
resemble  lemurs  in  general  body  form,  and  the  capuchin,  squirrel  and 
spider  monkeys,  most  of  which  have  strongly  prehensile  tails  which  serve 

as  "fifth  hands"  in  climbing  (Fig.  36.3). 
The  catarrhines  have  a  much  nar- 
rower nose,  with  nostrils  set  close 
together  and  directed  downward.  They 
all  have  the  same  dental  formula,  a 
large  brain,  flattened  nails  on  all  digits, 
and  a  tail  which  may  be  long,  short  or 
absent,  but  is  never  prehensile  (Fig. 
36.4). 

The  oldest  fossil  catarrhine  is 
Parapithecus,  whose  remains  have 
been  found  in  the  lower  Oligocene  in 
Egypt.  It  was  a  small  monkey  and  is 
believed  to  represent  the  common 
ancestor  of  today's  Old  World  mon- 
keys, apes  and  man.  The  present-day 
Old  World  monkeys  are  a  large  group, 
which  includes  the  macaque,  guenon, 
mandrill,  mangabey,  baboon,  langur 
and  others.  They  all  tend  to  sit  up- 
right and  have  buttocks  with  bare, 
hardened  sitting  pads,  called  ischial 
callosities,  which  are  frequently  a 
brilliant  red  or  blue.  The  mandrills 
and  baboons  have  taken  to  living  on 
the  ground  and  walking  on  all  fours. 
They  have  an  elongated  snout  and 
large  canine  teeth.  Baboons  are  intel- 

.J''^,T  ^*;^-    ^Pi'^'^''  monkey,  a  New  jj  animals   that    travel    in   troops 

World  monkey  with  a  strong  prehensile  ^  •         i       •     •         r       i         j 

tail,  used  in  swinging  from  tree  to  tree,  ^nd  cooperate  m  obtammg  food  and 

(Courtesy  of  the  San  Diego  Zoo.)  protecting  the  females  and  young. 


THE   EVOLUTION   OF  MAN 


741 


Figure    36.4.     Old   ^Vorld   monkeys   (Nilgiri    langur).    (Courtesy   of   the   American 
Museum  of  Natural  History.) 

In  the  same  Oligocene  deposits  in  which  Parapithecus  was  found 
occur  fossils  of  the  first  anthropoid  ape,  Propliopithecus.  This  small, 
gibbon-like  animal  probably  descended  from  Parapithecus  and  is  widely 
believed  to  be  close  to  the  common  ancestor  of  all  the  anthropoid  apes 
and  man.  In  the  evolution  of  the  apes  there  has  been  a  trend  toward  a 
general  increase  in  body  size  and  an  increase  in  the  brain  and  skull.  Most 
apes  move  by  swinging  from  one  branch  to  the  next,  and  have  developed 
long  arms  and  fingers.  The  hind  legs  are  rather  short. 

Apes  were  widely  distributed  throughout  Europe,  Asia  and  Africa 
during  the  middle  and  later  Cenozoic.  Fossils  of  Limnopithecus,  believed 
to  be  ancestral  to  the  gibbons,  and  Proconsul,  on  the  line  of  evolution  of 
the  other  apes,  have  been  found  in  lower  Miocene  deposits  in  Africa. 
Paleositnia,  apparently  the  ancestor  of  the  orang-utan,  is  known  from 
Miocene  deposits  in  India.  The  genus  Dryopithecus  includes  anthropoid 
apes  that  flourished  in  Europe  and  Asia  during  the  Miocene  and  Pli- 
ocene; they  were  probably  the  ancestors  of  modern  gorillas,  chimpan- 
zees and  man. 

321.       The  Modern  Great  Apes 

The  family  Pongidae  includes  the  four  living  great  apes,  the  gibbon, 
orang-utan,  chimpanzee  and  gorilla.  The  gibbon  is  smaller  than  man 
but  the  other  three  are  as  large  as  or  larger  than  we  are.  They  all  have 
extremely  rudimentary  tails,  arms  that  are  longer  than  their  legs,  op- 
posable thumbs  and  great  toes,  a  semierect  posture,  and  chests  which 
are   broad    and    flat    like   man's   rather   than    thin    and   deep   like    the 

monkey's.  „  j         i. 

The  gibbon,  found  in  Malaya,  is  the  smallest  and  perhaps  most 
primitive  of  tlie  great  apes.  It  has  extraordinarily  long  arms,  which  reach 


742 


GENETICS    AND    EVOLUTION 


Figure   36.5.     'Hie    wliite-banded   gibbon.    Ihcse    anthropoid    apes    use    their   long 
arras  to  swing  from  tree  to  tree  with  great  agiHty.  (Courtesy  of  the  San  Diego  Zoo.) 

to  the  ground  when  it  stands  erect  (Fig.  36.5).  Its  slender,  graceiul  body 
is  covered  with  iur.  Gibbons  are  the  most  skiUiul  "brachiators,"  swing- 
ing gracetully  and  surely  Irom  branch  to  branch,  clearing  20  to  40  ieet 
at  each  swing  and  using  the  arms  alternately.  The  spectacular  aerial 
acrobatics  of  the  gibbon  requires  great  agility,  coordination,  keen  eye- 
sight, and  the  ability  to  make  rapid  judgments  of  distance  and  possible 
landing  sites. 

The  orang-utan,  a  native  of  Borneo  and  Sumatra,  is  a  bulky  and 
powerful  animal  covered  with  long,  reddish-brown  hair.  Although  it  is 
short-legged  and  scarcely  five  feet  tall,  it  may  weigh  as  much  as  160 
pounds.  Orang-utans  have  enormously  long  arms,  with  a  span  of  7  or 
8  feet,  and  long,  slender  hands  and  feet.  They  are  successful  arboreal 
animals,  but  because  of  their  considerable  weight  they  move  more  de- 
liberately than  the  gibbons  do.  Orangs  eat  fruit  and  leaves  and  build 
nests  in  trees  on  which  to  sleep. 

Chimpanzees  and  gorillas  both  live  in  Africa,  are  closely  related, 
and  have  many  characteristics  in  common.  Both  are  more  terrestrial  and 
less  arboreal  than  the  other  apes,  and  have  relatively  shorter  arms  and 
longer,  stronger  legs  than  gibbons  and  orangs.  Both  are  large,  powerful 
animals;  a  male  chimpanzee  is  about  5  feet  tall  and  weighs  150  pounds 
and  a  male  gorilla  may  be  over  6  feet  tall  and  weigh  as  much  as  500 
pounds.  Chimpanzees  are  primarily  tree-dwellers  but  are  quite  at  home 
on  the  ground  and  walk  in  a  semierect  position.  The  hands  and  feet  of 
the  chimpanzee  are  long  and  narrow,  with  small  thumbs  and  great  toes, 
but  those  of  the  gorilla  are  shorter  and  broader,  more  closely  resembling 


THE   EVOLUTION   Of   MAN  743 

those  of  man.  The  gorilla  has  a  massive  head,  with  large  bony  crests  on 
top  of  the  skull  for  the  attachment  of  the  neck  and  jaw  muscles  and 
with  prominent  bony  ridges  over  the  eyes.  The  gorilla  walks,  like  man, 
on  the  soles  of  his  feet  with  the  toes  extended,  rather  than  on  the  outer 
edge  of  the  foot  with  the  toes  curled  underneath  as  do  other  apes.  Both 
chimpanzees  and  gorillas  may  build  nests  in  low  trees. 

Psychologic  studies  of  chimpanzees  and  gorillas  have  shown  that 
they  are  curious,  perceptive,  able  to  reason,  and  have  strong  emotions 
and  social  instincts. 

Man  is  more  nearly  similar  to  the  chimpanzee  and  gorilla  than  to 
any  other  primate,  yet  differs  in  enough  characters  to  be  placed  in  a 
separate  family,  the  Hominidae.  The  anatomic  differences  between  the 
great  apes  and  ourselves  are  rather  small,  and  are  generally  differences 
in  proportion  of  parts  correlated  with  our  adaptation  to  terrestrial  life. 
Some  of  the  characters  which  distinguish  man  from  the  other  primates 
are:  (1)  man's  posture  is  fully  erect;  (2)  his  legs  are  longer  than  his  arms; 
(3)  his  great  toe  is  not  opposable,  but  is  in  line  with  the  others  and 
adapted  for  walking;  (4)  the  human  foot  is  adapted  for  bearing  weight 
by  the  presence  of  lengthwise  and  transverse  arches;  (5)  man's  brain  is 
large— two  to  three  times  larger  than  the  gorilla's;  (6)  the  human  nose 
has  a  prominent  bridge  and  a  peculiar,  elongated  tip;  (7)  the  upper  lip 
has  a  median  furrow,  and  both  lips  are  rolled  outward  so  that  the  mu- 
cous membrane  is  visible;  (8)  man  has  a  jutting  chin;  (9)  his  canine  teeth 
project  slightly,  if  at  all,  beyond  the  level  of  the  others,  and  (10)  man 
is  relatively  hairless. 

There  is  no  single  ape  that  resembles  man  in  all  respects  more  than 
the  other  apes.  The  hands,  feet  and  pelvis  of  the  gorilla  most  closely 
resemble  man's,  but  the  skull  and  hair  color  of  the  chimpanzee  are 
nearest  to  the  human.  The  orang  is  the  only  ape  to  have  the  same  num- 
ber of  ribs  we  have,  and  the  posture  and  gait  of  the  gibbon  is  most 
nearly  human.  With  respect  to  any  structure  or  proportion  of  parts, 
however,  the  difference  between  man  and  any  of  the  great  apes  is  less 
than  between  any  of  these  and  the  monkeys. 

322.        The  Man  Apes 

From  Pleistocene  cave  deposits  in  South  Africa  have  come  the  re- 
mains of  fossil  anthropoids  that  almost  bridge  the  gap  from  ape  to  man. 
These  man  apes  probably  existed  too  recently  to  be  man's  ancestors,  but 
they  show  the  kind  of  changes  by  which  the  transition  from  ape  to  man 
was  made.  They  are  no^v  regarded  as  "progressive  apes,"  adapted  tor 
walking  upright  on  the  ground,  which  evolved  independently  of  the 
human  line  from  common  dryopithecine  ancestors  m  the  Miocene. 

The  first  of  these  fossils,  the  skull  of  a  baby  man  ape,  was  found 
in  the  Transvaal  by  Dart  in  1925  and  named  Australopithecus  (Fig 
36.6).  Subsequentlv,  Dart  and  Broom  found  adult  skulls  and  parts  of 
skeletons,  and  although  these  were  given  separate  names,  Plesianthropus 
and  Paranthropus,  they  probably  represent  animals  very  closely  related 
to,  if  not  identical  with,  the  original  Australopithecus  These  australo- 
pithecines  have  an  interesting  mixture  of  apelike  and  human  character- 
istics  The  head  was  apelike,  with  a  low-vaulted  skull,  protruding  muzzle 


744  GENETICS   AND   EVOLUTION 


Figure  36.6.     Reconstruction  of  the  skull  of  the  man-ape  Australopithecus.  (Clark: 
The  History  of  the  Primates.) 

and  heavy  jaws,  but  the  brain  capacity  was  large,  650  ml.,  greater  than 
that  of  any  known  ape  and  almost  as  large  as  that  of  the  earliest  ape 
man.  The  cheekbone,  jaw  hinge  and  teeth  were  very  similar  to  man's; 
the  small  canine  teeth  and  molars  resemble  ours.  These  man  apes  lived 
in  caves,  hunted  animals,  and  may  have  learned  how  to  use  fire.  From 
the  structure  of  the  pelvis  and  leg  bones,  and  from  the  fact  that  the 
foramen  magnum  (the  hole  in  the  skull  through  which  the  spinal  cord 
emerges)  is  located  far  under  the  skull,  we  conclude  that  these  man  apes 
had  a  fairly  erect  posture.  The  largest  of  the  australopithecines,  the 
Swartkrans  man  ape  found  in  1949,  appears  to  have  been  a  veritable 
giant,  larger  and  heavier  than  the  largest  gorillas. 


323.       Fossil  Ape  Men 

The  human  stock  appears  to  have  diverged  from  the  great  apes 
some  time  after  the  Miocene,  and  the  remains  of  a  number  of  creatures 
with  characters  intermediate  between  the  fossil  apes  and  living  man 
have  been  found  in  Pliocene  and  Pleistocene  deposits  in  widely  scattered 
parts  of  Europe,  Asia  and  Africa  (Fig.  36.7).  The  evidence  from  these 
fossils  indicates  that  the  characteristics  which  distinguish  man  from  the 
apes  did  not  appear  simultaneously  in  a  single  form,  for  these  ape  men 
show  a  mixture  of  apelike  and  human  traits.  Whether  these  are  apes 
or  men  is,  perhaps,  a  matter  of  definition,  but  they  were  large-brained 
anthropoids  who  walked  erect,  had  well  formed  hands,  and  made  and 
used  tools.  We  have  a  fairly  clear  idea  of  what  these  ape  men  looked 
like  from  their  fossil  remains,  and  we  also  know  quite  a  bit  about  how 
they  lived  from  the  tools,  weapons,  ornaments,  and  other  cultural  re- 
mains that  have  been  discovered. 

One  of  the  most  primitive  ape  men  was  PitJiecanthropus  erectus, 


THE   EVOLUTION   OF  MAN 


745 


the  Java  man,  whose  remains  were  found  in  1891  in  Pleistocene  de- 
posits on  the  banks  of  the  Solo  River  in  eastern  Java  (Fig.  36.10).  Several 
other  skulls  and  leg  bones  found  since  give  us  a  good  idea  of  what  Java 
man  looked  like.  He  was  of  stocky  build,  about  5  feet  8  inches  tall, 
weighed  154  pounds  and  walked  erect.  His  face  was  rather  apelike,  with 
massive,  protruding,  chinless  jaws  equipped  with  a  set  of  huge  teeth 
(although  the  canine  teeth  were  not  enlarged  tusks  as  in  the  apes).  The 
nose  was  broad  and  low-bridged  and  there  was  a  heavy,  bony,  protruding 
ridge  over  the  eyes.  The  skull  had  a  cranial  capacity  of  about  900  ml., 
intermediate  between  the  1500  ml.  which  is  average  for  modern  man, 
and  the  600  ml.  of  the  gorilla  and  australopithecines.  By  studying  casts 
of  the  interior  of  the  skull,  the  contours  and  relative  proportions  of  the 
various  parts  of  the  brain  can  be  determined.  Pithecanthropus  appears 
to  have  had  the  part  of  the  brain  which  controls  speech,  though  we 
have  no  way  of  knowing  whether  he  could  speak.  The  frontal  lobes  of 
the  brain,  which  were  the  last  parts  to  appear  in  evolution,  were  smaller 
in  the  Java  man  than  in  modern  man,  but  larger  than  in  any  living  ape. 
Java  man's  brain  was  more  human  than  simian,  larger  and  more  con- 
voluted than  that  of  any  of  the  primitive  or  present  apes. 

Australoids  Modern  Man 

I  ^^ 

I       Neandertlial    Cro-Magnon 
Keilor  ^  I 

1       Heidelberg     Galley  Hill 
Solo       #'  1 

Peking        Swanscombe 


V 


Java                     A 
Meganthropus          m 

r 

Gorillaw 

\\^ 

^^^^  Rhodesian  Man 
^^g^mt  Australopithecus 
^^^^  Paranthropus 

Chimpanzee^ 

"^JP 

^^^mm  Dryopithecus 

Orangutan^ 

'^^— ^  T^^^ 

Gibbon  a 

^■^S^^^W^^H 

^soH  ^ 

^^^^  Propliopithecus 

Old  World  Monkey  « 

"-«^*?Hr 

^^m^m  Parapithecus 

New  World  Monl<ey  ^ 

■"^NH 

^^^^»  TorsioidS 

Lemuroids^ 

^             ^^H  .^ 

^^ 

TREE 
INSECTIVORES 

Fiqure  36  7  An  evolutionary  tree  of  the  primates,  beginning  with  the  primitive 
tree  insectivores.  The  forms  known  only  as  fossils  are  indicated  in  itahcs.  (Villee: 
Biology.) 


746 


GENETICS   AND   EVOLUTION 


Figure  36.8.  Front  and  side  views  of  a  reconstructed  skull  of  Peking  man, 
Sinanlhiol)us  pekinensis.  Note  the  massive  bony  ridges  over  the  eyes,  the  low,  retreat- 
ing forehead,  the  protruding  jaws  and  the  absence  of  a  chin.  (Courtesy  of  the  Amer- 
ican Museum  of  Natural  History,  New  York.) 

Other  remains  found  in  limestone  caves  near  Peking,  China,  are 
those  of  a  primitive  ape  man  of  the  middle  Pleistocene,  some  half 
million  years  ago.  Their  discoverer,  Davidson  Black,  named  them  Sinan- 
thropus  pekineusis.  The  skeletons  of  more  than  forty  individuals  have 
now  been  found  and  it  is  possible  to  make  fairly  complete  reconstruc- 


THE   EVOLUTION   OF   MAN  747 

tions  of  their  form.  Peking  man  had  a  skull  very  similar  to  that  of  Java 
man,  with  heavy  bony  ridges  over  the  eyes,  a  low,  slanting  forehead,  a 
broad  flat  nose  and  a  massive,  chinless  jaw  (Fig.  36.8).  The  remains  fall 
into  two  groups,  one  considerably  larger  than  the  other,  which  suggests 
that  the  difference  between  the  size  of  males  and  females  was  greater  than 
at  present.  The  cranial  capacity  of  Peking  man  was  about  1075  ml.,  dis- 
tinctly larger  than  that  of  Java  man.  The  fact  that  many  of  them  are 
found  with  their  bases  broken  open  suggests  that  Peking  man  was  a  can- 
nibal with  a  taste  for  brains. 

As  more  specimens  of  Java  and  Peking  man  have  been  found,  it 
has  become  clear  that  the  two  are  really  quite  similar,  and  represent 
two  races  or  subspecies  of  the  same  species,  rather  than  separate  genera. 
The  anthropologist  who  has  studied  them  most  intensively,  Franz 
W^eidenreich,  found  that  Java  and  Peking  man  are  identical  in  57  out 
of  74  characters  of  the  skull,  and  that  there  are  clear  differences  in  only 
four  characters,  one  of  which  is  the  difference  in  size.  He  has  suggested 
that  they  be  named  Homo  erectus  erectus  and  Homo  erectus  pekinensis, 
respectively. 

Traces  of  other  ape  men,  much  larger  than  Java  and  Peking  man, 
have  also  been  found  in  southern  Asia.  The  lower  Pleistocene  deposits 
of  Java  have  yielded  a  large  lower  jaw  with  molar  teeth  that  appears 
to  have  belonged  to  an  ape  man  as  big  as  a  gorilla.  Probably  this  Jav- 
anese giant,  named  Meganthropus,  was  exceeded  in  size  by  another 
giant,  named  Gigantopithecus,  known  only  from  some  extremely  large, 
human-like  fossil  teeth  found  in  a  Hong  Kong  drugstore!  These  were 
traced  back  to  cave  deposits  from  the  lower  Pleistocene  in  southern 
China.  The  largest  molar  found  is  some  six  times  larger  than  a  human 
molar  and  must  have  belonged  to  an  exceptionally  large  ape  man. 
VV^iether  these  giants  represent  ancestors  of  modern  man  or  side  branches 
of  anthropoid  evolution  cannot  be  decided  at  present. 

The  fossils  of  primitive  man  found  in  Europe,  Asia  and  Africa  are 
slightly  different,  but  similar  enough  to  be  grouped  together  as  the 
Neanderthaloids.  The  Neanderthaloids,  which  include  Heidelberg  man, 
Neanderthal  man,  Solo  man  and  Rhodesian  man,  probably  are  descended 
from  the  pithecanthropoids,  Java  and  Peking  man. 

Heidelberg  man  (Homo  heidelbergensis)  is  known  only  from  a  mas- 
sive lower  jaw  found  buried  under  80  feet  of  sand  in  a  pit  near  Heidel- 
berg, Germany.  The  jaw  is  large  and  heavy  and  lacks  a  chin,  but  the 
teeth  are  of  moderate  size  and  generally  like  modern  man's.  Since  it 
resembles  the  jaw  of  Neanderthal  man  in  many  respects,  Heidelberg 
man,  who  lived  more  than  500,000  years  ago,  may  have  been  an  ancestor 
of  Neanderthal  man. 

The  first  human  fossils  to  be  discovered,  a  skull  and  some  bones, 
were  found  in  the  Neander  valley  near  Dusseldorf,  Germany,  in  1856. 
Similar  skulls  and  skeletons  have  been  found  in  widely  separated  parts 
of  Europe,  Asia  Minor,  North  Africa,  Siberia  and  the  islands  of  the 
Mediterranean.  Neanderthal  remains  are  associated  with  a  particular 
Stone  Age  culture  known  as  the  Mousterian  (named  after  le  Moustier 
cave   on   the   bank  of   the  Vezere  River  in  France).   Neanderthal  man 


748 


GENETICS   AND    EVOLUTION 


Figure  36.9.  An  artist's  reconstruction  of  a  Neanderthal  family  living  in  a  cave 
in  the  Rock  of  Gibraltar.  (Courtesy  of  the  Chicago  Natural  History  Museum.  Frederick 
Blaschke,  sculptor;  Charles  A.  Corwin,  artist.) 

(Homo  neanderthalensis)  lived  in  Europe  for  thousands  of  years  during 
and  after  the  third  interglacial  period,  about  150,000  years  ago,  and 
became  extinct  only  about  25,000  years  ago.  A  typical  Neanderthal  man 
was  short,  stocky,  and  powerfully  built,  about  five  feet  tall,  with  stooped 
shoulders,  and  bent  knees  (Fig.  36.9).  The  head  jutted  forward  from  a 
short  thick  neck  and  massive  shoulders.  The  massive  skull  had  a  thick 
bony  ridge  over  the  eyes  and  a  receding  forehead.  The  nose  was  broad 
and  short  and  the  jaws  were  large  and  strong  with  very  little  chin.  De- 
spite these  primitive  features.  Neanderthal  man's  cranial  capacity  was 
as  large  as  or  larger  than  modern  man's,  averaging  1550  ml.,  and  he 
was  probably  quite  intelligent.  He  lived  primarily  in  caves,  used  fire, 
made  beautiful  chipped  stone  tools  and  weapons,  and  buried  his  dead 
reverently  with  food  and  ornaments. 

Human  fossils  found  in  caves  in  Mount  Carmel  in  Galilee  include 
some  that  are  typically  Neanderthaloid  and  others  that  have  characters 
more  like  those  of  modern  man— greater  height,  smaller  face,  less  re- 
ceding forehead,  and  so  on.  It  is  clear  that  these  were  all  contemporane- 
ous, but  whether  they  represent  the  emergence  of  Homo  sapiens  from 
Neanderthal  man,  or  hybridization  between  two  separate  stocks,  is  un- 
known. 

Remains  of  another  primitive  man,  quite  similar  to  the  Neander- 
thalers,  have  been  found  on  the  banks  of  the  Solo  River  in  Java,  only 
a  few  miles  from  the  spot  where  Java  man  was  found.  Eleven  skulls  of 
Solo  man  have  been  found  since  1936,  all  with  their  bases  bashed  in, 
suggesting  that  Solo  man  inherited  a  taste  for  human  brains  along  with 
other  traits  from  his  ancestral  Javan  man.  These  Solo  skulls  resemble 
the  Neanderthal  ones  in  general  characteristics,  with  heavy  brow  ridges 
and  a  sloping  forehead,  but  the  head  is  somewhat  rounder  and  more  like 


THE   EVOLUTION   OF  MAU  749 

modern  man's  in  shape.  The  Australian  bushmen  are  believed  to  be 
descendants  of  Solo  man,  a  conclusion  strengthened  by  the  finding  in 
1940  of  two  Pleistocene  skulls  at  Keilor,  near  Melbourne,  Australia, 
which  were  intermediate  in  character  between  Solo  man  and  the  present 
aboriginal  Australians. 

Another  primitive  skull,  to  which  the  name  Rhodesian  man  has  been 
given,  was  found  in  1921  in  a  limestone  cave  at  Broken  Hill,  Rhodesia. 
The  skull  was  well  preserved  and  has  thick  bones,  very  large  eyebrow 
ridges  and  a  low,  receding  forehead  but  a  cranial  capacity  of  about  1300 
ml.  The  teeth  are  large,  but  human  rather  than  apelike  and  are  badly 
decayed,  an  unusual  condition  in  apes  and  primitive  man.  The  rela- 
tions of  this  finding  to  other  primitive  man  are  obscure. 

324.        Modern  Man  (Homo  sapiens) 

The  species  Homo  sapiens  includes  all  the  living  races  of  man  and 
some  extinct  ones  such  as  the  Cro-Magnons.  The  idea  that  this  species 
appeared  relatively  recently  in  the  late  Pleistocene,  when  the  Neander- 
thalers  were  vanishing,  is  no  longer  valid,  for  the  Swanscombe  man  is 
now  known  to  have  existed  in  the  middle  Pleistocene.  This  skull,  essen- 
tially modern  in  shape  and  size,  though  having  somewhat  thicker  bones, 
was  found  in  1935  in  the  Thames  Valley  at  Swanscombe  in  a  gravel 
deposit  from  the  Middle  Pleistocene.  Its  antiquity  was  confirmed  in 
1949  by  the  fluorine  test,  which  depends  on  the  fact  that  buried  bones 
and  teeth  gradually  accumulate  fluorine.  The  age  of  a  fossil  can  be 
estimated  from  its  fluorine  content.  Other  remains  of  Homo  sapiens 
which  bridge  the  long  gap  between  Swanscombe  man  and  the  Cro- 
Magnon  races  have  been  found  in  central  France  in  1948  and  in  northern 
Iran  in  1951. 

More  than  100  fossils  of  Hotno  sapiens  have  been  found  from  the 
period  between  15,000  and  60,000  or  so  years  ago.  The  first  of  these  were 
found  in  the  Cro-Magnon  rock  shelters  in  the  Vezere  valley  in  south 
central  France,  and  these  are  all  referred  to  as  Cro-Magnon  men,  even 
though  they  fall  into  several  different  groups.  The  Cro-Magnons  were 
tall  and  large-boned,  with  massive,  long  skulls,  a  high  forehead,  prom- 
inent chin,  and  no  eyebrow  ridges  (Fig.  36.10).  They  lived  in  rock 
shelters  and  caves  and  drew  superb  pictures  of  the  contemporary  animals 
on  the  walls  of  these  caves  (Fig.  l.I).  Cro-Magnon  man  was  a  contem- 
porary of  the  Neanderthalers  and  may  have  displaced  and  exterminated 
him. 

The  center  of  origin  of  modern  man  appears  to  have  been  in  Asia, 
in  the  general  region  of  the  Caspian  Sea.  The  white  races  spread  west- 
ward around  both  shores  of  the  Mediterranean  to  Europe,  Southwestern 
Asia  and  North  Africa,  displacing  the  Cro-Magnons  who  had  in  turn 
displaced  the  earlier  Neanderthalers.  Some  of  the  inhabitants  of  Ireland 
and  Scandinavia,  and  the  Basques  of  southern  France  and  northern 
Spain,  show  marked  similarities  to  Cro-Magnons  and  may  represent 
their  descendants  who  were  pushed  westward  by  the  migrating  Neolithic 
man. 


750 


GENETICS    AND    EVOLUTION 


Figure  36.10.  Restorations  by  Dr.  J.  H.  McGregor  of  what  prehistoric  men  prob- 
ably looked  like.  From  left  to  right,  the  Java  ape-man.  Neanderthal  man  and  Cro- 
Magnon  man.  (Courtesy  of  Dr.  J.  H.  McGregor  and  the  American  Museum  of  Natural 
History,  New  York.) 

The  Negroid  races  spread  south  on  both  sides  of  the  Indian  Ocean 
to  Africa  and  Melanesia.  It  appears  that  they,  too,  displaced  more  prim- 
itive races  and  pushed  the  Bushmen  to  the  tip  of  South  Africa  and  the 
Australoids  into  Australia. 

The  Mongoloids  spread  east  and  north,  occupying  Siberia  and 
China.  About  20,000  years  ago  they  crossed  the  Bering  Straits  to  occupy 
North  and  South  America. 

There  are  four  basic  stocks  of  modern  man,  all  of  which  belong  to 
the  species  Homo  sapiens.  The  Australian  aborigines  appear  to  be  the 
most  primitive  and  perhaps  have  a  slightly  different  line  of  descent 
from  the  others.  The  other  three,  the  whites,  the  negroids,  and  the  mon- 
goloids, are  each  subdivided  into  a  number  of  races.  A  race,  whether 
of  human  beings,  or  some  other  animal  or  plant,  may  be  defined  genet- 
ically as  a  population  which  differs  significantly  from  other  populations 
with  respect  to  the  frequency  of  one  or  more  of  the  genes  it  possesses. 
Or  it  may  be  defined  phenotypically  as  a  population  whose  members, 
though  varying  individually,  are  distinguished  as  a  group  by  a  certain 
combination  of  morphologic  and  physiologic  characteristics  which  they 
share  because  of  their  common  descent. 

In  the  course  of  his  evolution  from  the  ape  men,  man  has  increased 
slightly  in  height  but  his  frame  has  become  much  less  massive.  He  now 
stands  completely  erect  and  his  head  is  balanced  on  a  relatively  slender 
neck,  instead  of  jutting  forward  from  the  shoulders  and  being  held  in 
place  by  massive  neck  muscles.  His  cranial  capacity  has  increased,  the 
frontal  lobes  of  the  brain  have  enlarged  and  the  skull  is  more  rounded, 
the  forehead  is  more  vertical  and  the  bony  ridges  over  the  eyes  have 
become  smaller.  The  face  and  jaws  have  become  smaller  and  the  re- 


THE  EVOLUTION  Of  MAN  751 

duction  in  jaw  size  is  correlated  with  a  reduction  in  the  size  and  com- 
plexity of  the  teeth.  There  is  a  strong  tendency  for  the  third  molars, 
the  wisdom  teeth,  to  become  vestigial.  These  changes  probably  follow, 
directly  or  indirectly,  from  the  evolutionary  trend  towards  larger  brains 
and  greater  intelligence.  These  more  intelligent  descendants  were  less 
dependent  upon  sheer  physical  strength  for  getting  food  and  fighting 
enemies,  animals  and  other  men.  Speech  was  invented,  tools  and  weap- 
ons were  made,  man  began  to  live  in  clans  and  tribes  and  progressed 
beyond  his  former  state  of  being  a  tree-dwelling  primate  to  that  of  a 
ground-dwelling,  civilized  animal. 

325.        Cultural  Evolution 

Corroborative  evidence  for  the  relationships  and  temporal  order 
of  these  primitive  and  modern  men  comes  from  the  objects  they  made 
and  used,  called  artifacts,  which  were  deposited  along  with  the  fossils. 
The  science  of  archeology  is  concerned  with  the  finding,  identifying 
and  interpreting  of  the  tools,  weapons,  cooking  utensils,  ornaments  and 
other  objects  made  by  man. 

Although  early  man  must  have  learned  to  pick  up  and  use  stones 
of  a  convenient  size  and  shape,  it  was  not  until  the  middle  Pleistocene, 
apparently,  that  he  learned  how  to  chip  pieces  of  flint  to  make  hand 
axes.  The  culture  characterized  by  these  chipped  stone  tools  is  called 
the  Lower  Paleolithic,  and  was  the  culture  of  Java  and  Peking  man. 
These  men  lived  in  caves  and  were  hunters  and  food  gatherers  who  had 
learned  how  to  use  fire.  The  association  of  certain  kinds  of  axes  and 
scraping  tools  with  the  Java  and  Peking  men  provides  clues  for  the 
study  of  their  distribution,  for  similar  artifacts  without  skeletal  remains 
have  been  found  in  India  and  Burma.  More  advanced  tools  from  the 
third  interglacial  and  the  last  glacial  periods  represent  the  Middle  Paleo- 
lithic culture.  Neanderthal  man  is  associated  with  the  Mousterian  cul- 
ture, a  Middle  Paleolithic  one.  Each  of  these  cultures  is  recognized  by 
the  style  of  tools  and  weapons  made.  The  Mousterian  implements  were 
made  by  chipping  flakes  from  a  piece  of  flint  and  then  sharpening  the 
edges  by  removing  more  flakes  with  a  bone  tool.  The  common  weapon 
of  this  time  was  a  triangular  piece  of  stone,  the  forerunner  of  both  the 
spear  and  arrowhead. 

Later,  in  the  Upper  Paleolithic  culture,  an  improved  method  of  tool 
making  was  discovered,  in  which  the  flakes  were  removed  from  the 
piece  of  flint  by  means  of  steadily  and  carefully  applied  pressure,  rather 
than  by  blows.  This  produced  long,  slender,  knifelike  blades,  many  of 
which  were  elaborately  and  skillfully  carved,  and  were  true  works  of 
art.  These  Upper  Paleolithic  men,  Cro-Magnons  and  others,  were 
painters  as  well  as  skilled  craftsmen;  their  cave  paintings,  found  in 
France  and  Spain,  show  a  remarkable  grasp  of  the  principles  of  design. 
These  men  of  the  Upper  Paleolithic  introduced  bone  needles  and  other 
tools  and  probably  invented  the  bow  and  arrow. 

The  Mesolithic,  or  Middle  Stone  age,  shows  no  important  advance 
over   the   Paleolithic   cultures.   Mesolithic  man   was  still  a   hunter  and 


752  GENETICS   AND   EVOLUTION 

food  gatherer,  living  in  small,  isolated  breeding  groups,  which  would 
favor  the  occurrence  of  genetic  drift,  and  lead  to  the  formation  of  di- 
vergent groups. 

The  Neolithic  or  New  Stone  Age  culture  originated  in  the  Near 
East,  between  Egypt  and  Iran.  This  culture  is  marked  not  only  by  tools 
which  were  carefully  ground  and  polished,  but  by  the  beginnings  of 
agriculture  and  animal  husbandry.  Man  gradually  changed  from  a 
wandering  hunter  and  food  gatherer  to  a  settled  food  producer,  raising 
grain,  making  pottery  and  cloth,  and  living  in  villages.  The  increase  in 
the  food  supply  led  to  an  increase  in  population,  breeding  groups  be- 
came larger  and  interbred  with  neighboring  ones,  and  the  tendency 
toward  genetic  drift  was  greatly  decreased.  The  evolution  of  social 
organization  from  the  family  groups  and  clans  of  the  Old  Stone  Age 
to  the  present-day  large  nations,  which  is  dependent  upon  man's  social 
behavior,  his  ability  to  cooperate  with  others  and  to  restrain  his  own 
behavior,  has  been  an  important  factor  in  the  evolutionary  success  of 
Hotno  sapiens. 

Questions 

1.  List  and  discuss  the  characters  of  the  human  body  which  are  remnants  of  our  former 
adaptation  for  living  in  the  trees. 

2.  Indicate  the  current  belief  as  to  the  course  of  evolution  from  primitive  insectivores 
to  man. 

3.  Distinguish  between  platyrrhine  and  catarrhine  anthropoids. 

4.  List  the  characters  which  distinguish  man  from  the  great  apes. 

5.  Compare  the  structures  and  functions  of  gibbons,  orangs  and  gorillas.  Which  shows 
the  best  adaptation  to  arboreal  life? 

6.  Do  you  consider  any  of  the  ape  men  or  man  apes  to  be  the  "missing  link"  in  human 
evolution? 

7.  Compare  the  appearance  of  Neanderthal  and  Cro-Magnon  men.  What  became  of 
each? 

8.  Why  is  the  structure  of  the  human  body  said  to  be  "relatively  unspecialized?" 

9.  Why  is  it  incorrect  to  say  that  man  came  from  monkeys?  What  is  the  correct  state- 
ment? 

10.  What  characters  distinguish  the  present  races  of  man? 

11.  What  is  an  archeological  artifact?  Of  what  use  are  they  in  tracing  human  evolution? 

12.  In  what  ways  do  the  Upper  Paleolithic  and  Neolithic  cultures  differ? 

13.  Why  is  genetic  drift  less  important  in  human  evolution  at  present  than  it  was  10,000 
or  more  years  ago? 

Supplementary  Reading 

A.  S.  Romer's  Man  and  the  Vertebrates,  H.  F.  Osborn's  Men  of  the  Old  Stone  Age, 
Howell's  Mankind  So  Far,  and  W.  E.  L.  Clark's  History  of  the  Primates  give  fine  descrip- 
tions of  prehistoric  men.  E.  A.  Hooton  gives  an  amusing  and  informative  discussion  of 
the  primates,  of  human  evolution  and  of  the  present  races  of  man  in  Up  from  the  Ape. 
Read  Weidenreich's  Apes,  Giants  and  Man  for  a  fascinating  account  of  the  ape  men  by 
one  of  the  major  researchers  in  the  field.  The  Races  of  Europe,  by  C.  S.  Coon,  is  an  excel- 
lent treatise  of  the  many  subdivisions  of  the  white  race.  An  interesting  recent  discussion 
of  human  heredity  and  evolution  is  found  in  Dobzhansky's  Evolution,  Genetics  and  Man. 


Part  V 

ANIMALS    AND 
THEIR    ENVIRONMENT 


CHAPTER  37 


Ecology 


The  animals  and  plants  living  today  are  related  not  only  by  evolutionary 
descent,  as  described  in  the  preceding  three  chapters,  but  also  by  their 
relations  to  each  other  and  to  the  physical  environment.  One  form  may 
provide  food  or  shelter  for  another;  it  may  produce  some  substance 
beneficial  or  harmful  to  the  second;  or  the  two  may  compete  for  food 
and  shelter.  The  study  of  the  interrelationships  between  living  things— 
both  within  species  and  between  species— and  their  physical  environ- 
ment is  known  as  ecology.  Each  organism,  by  the  process  of  evolution, 
has  become  adapted  to  survive  in  some  particular  kind  of  environment, 
has  developed  a  tolerance  for  a  certain  range  of  moisture,  light,  tem- 
perature, wind  and  so  on,  and  has  developed  certain  relationships  with 
other  living  organisms  in  its  immediate  vicinity.  Since  the  study  of 
ecology,  and  an  appreciation  of  its  prime  importance  in  zoology,  re- 
quire a  good  background  knowledge  of  the  anatomy  and  physiology  of 
a  wide  variety  of  animals,  the  discussion  of  this  topic  has  been  reserved 
for  these  concluding  chapters. 

326.        Ecosystems 

When  any  species  of  animal  is  carefully  studied  in  the  wild,  it  be- 
comes clear  that  it  is  not  independent  of  other  living  things,  but  is  one 
of  a  system  of  interacting  and  interdependent  parts  which  form  a 
larger  unit.  Ecologists  use  the  term  ecosystem  to  indicate  a  natural  unit 
of  living  and  nonliving  parts  that  interact  to  form  a  stable  system  in 
which  the  exchange  of  materials  between  living  and  nonliving  parts 
follows  a  circular  path.  Ecosystems  may  be  as  large  as  a  lake  or  forest, 
or  one  of  the  cycles  of  the  elements  (p.  755),  or  as  small  as  an  aquarium 
jar  containing  tropical  fish,  green  plants  and  snails. 

A  small  lake  or  pond  is  a  classic  example  of  an  ecosystem  small 
enough  to  be  investigated  easily  (Fig.  37.1).  The  nonliving  parts  of  the 

753 


754 


ANIMALS   AND    THEIR   ENVIRONMENT 


(Cornivoret) 

Stcondary   consumars 


Zooplonkfon 

(HerbivofM) 

Primory  contuintr* 


Bocleria  ond  fungi 
(Reducers) 


Gottom  forms  (HerblvorcO 
Primary  consumers 


Figure  37.1.  A  small  fresh-water  pond  as  an  example  of  an  ecosystem.  The  pro- 
ducer, consumer  and  decomposer  (reducer)  organisms  plus  the  nonliving  parts  are 
indicated.  (Villee:    Biology.) 

lake  include  the  water,  dissolved  oxygen,  carbon  dioxide,  inorganic 
salts  such  as  phosphates  and  chlorides  ol  sodium,  potassium  and  calcium, 
and  a  host  of  organic  compounds.  The  living  organisms  may  be  sub- 
divided into  producers,  consuiners  and  decomposers,  according  to  their 
role  in  keeping  the  ecosystem  operating  as  a  stable,  interacting  whole. 
The  producer  organisms  are  the  green  plants  that  manufacture  organic 
compounds  from  simple,  inorganic  substances.  There  are  two  kinds  of 
producer  organisms  in  a  typical  small  lake:  the  larger  plants  growing 
along  the  shore  or  floating  in  shallow  water,  and  the  microscopic  float- 
ing plants,  mostly  algae,  distributed  throughout  the  water,  as  far  down 
as  light  will  penetrate.  Such  small  plants  are  collectively  known  as 
phytoplankton;  they  are  usually  invisible  unless  present  in  great  abun- 
dance, when  they  give  the  water  a  greenish  tinge.  The  phytoplankton 
are  usually  much  more  important  as  food  producers  for  the  lake  than 
are  the  larger  plants. 

The  consumer  organisms  include  insects  and  insect  larvae,  Crus- 
tacea, fish,  and  perhaps  some  fresh-water  clams.  The  plant  eaters  are 
called  primary  consumers,  the  carnivores  that  eat  the  primary  consumers 
are  called  secondary  consumers,  and  so  on.  The  ecosystem  is  completed 


ECOtOGY         755 

by  decomposer  organisms,  bacteria  and  fungi,  which  break  down  the 
organic  compounds  of  dead  protoplasm  from  producer  and  consumer 
organisms  into  inorganic  substances  that  can  be  used  as  raw  materials  by 
green  plants. 

No  matter  how  large  and  complex  an  ecosystem  may  be,  it  can  be 
shown  to  consist  of  these  same  major  parts— producer,  consumer  and 
decomposer  organisms,  and  nonliving  components. 

327.  Habitat  and  Ecologic  Niche 

Two  important  concepts  which  are  basic  to  the  description  of  the 
ecologic  relations  of  organisms  are  the  habitat  and  the  ecologic  niche. 
The  habitat  of  an  organism  is  the  place  where  it  lives— a  physical  area, 
some  specific  part  of  the  earth's  surface,  air,  soil  or  water.  It  may  be  as 
large  as  the  ocean  or  a  prairie,  or  as  small  as  the  underside  of  a  rotten 
log  or  the  intestine  of  a  termite,  but  it  is  always  a  tangible,  physically 
demarcated  region.  More  than  one  animal  or  plant  may  live  in  a  single 
habitat. 

The  ecologic  niche  is  the  status  of  an  organism  within  the  com- 
munity or  ecosystem  and  depends  upon  the  organism's  structural  adap- 
tations, physiologic  responses  and  behavior.  E.  P.  Odum  has  made  the 
analogy  that  the  habitat  is  an  organism's  "address"  and  the  ecologic 
niche  is  its  "profession,"  biologically  speaking.  The  ecologic  niche  is  an 
abstraction  that  includes  all  the  physical,  chemical,  physiologic  and 
biotic  factors  that  an  organism  requires  to  live.  To  describe  an  organ- 
ism's ecologic  niche,  one  must  know  what  it  eats,  what  eats  it,  its  range 
of  movement,  and  its  effects  on  other  organisms  and  on  the  nonliving 
parts  of  the  surroundings. 

The  difference  between  these  two  concepts  may  be  made  clearer  by 
an  example.  In  the  shallow  waters  at  the  edge  of  a  lake  one  could  find 
many  different  kinds  of  water  bugs,  all  of  which  have  the  same  habitat. 
Some  of  these,  such  as  the  backswimmer,  Notonecta,  are  predators,  catch- 
ing and  eating  other  animals  of  about  its  size,  while  others,  such  as 
Corixa,  feed  on  dead  and  decaying  organisms.  Each  has  quite  a  different 
role  in  the  biologic  economy  of  the  lake  and  thus  each  occupies  an  en- 
tirely different  ecologic  niche. 

328.  The  Cyclic  Use  of  Matter 

The  total  mass  of  the  organisms  that  have  lived  in  the  past  billion 
or  so  years  is  much  greater  than  the  mass  of  the  entire  planet.  The  Law 
of  the  Conservation  of  Matter,  which  is  firmly  established,  assures  us 
that  matter  is  neither  created  nor  destroyed;  obviously,  then,  matter 
must  have  been  used  over  and  over  again  in  the  formation  of  new  gen- 
erations of  animals  and  plants.  The  earth  neither  receives  any  great 
amount  of  matter  from  other  parts  of  the  universe  nor  does  it  lose 
significant  amounts  of  matter  to  outer  space.  Each  element-carbon, 
hydrogen,  oxygen,  nitrogen,  phosphorus,  sulfur,  and  the  rest-is  taken 
from  the  environment,  made  a  part  of  living  material  and  finally,  per- 
haps by  a  quite  circuitous  route  involving  a  number  of  other  organisms, 
is  returned  to  the  environment  to  be  used  again.  An  appreciation  of  the 


756 


ANIMALS   AND   THEIR   ENVIRONMENT 


roles  of  animals,  green  plants  and  bacteria  in  this  cyclic  use  of  the  ele- 
ments can  be  gained  from  a  consideration  of  the  details  of  the  more 
important  cycles. 

329.        The  Carbon  Cycle 

There  are  about  six  tons  of  carbon  (in  the  form  of  carbon  dioxide) 
in  the  atmosphere  over  each  acre  of  the  earth's  surface.  Yet  each  year 
an  acre  of  luxurious  plant  growth,  such  as  sugar  cane,  will  extract  as 
much  as  twenty  tons  of  carbon  from  the  atmosphere  and  incorporate 
it  into  plant  protoplasm.  According  to  one  estimate,  the  green  plants 
would  use  up  the  entire  supply  of  atmospheric  carbon  dioxide  in  about 
35  years.  Carbon  dioxide  fixation  by  bacteria  and  animals  is  another, 
but  quantitatively  minor,  drain  on  the  supply  of  carbon  dioxide.  Carbon 
dioxide  is  returned  to  the  atmosphere  by  respiration.  Plants  carry  on 
respiration  continuously  and  green  plant  tissues  are  eaten  by  animals 
who,  by  respiration,  return  more  carbon  dioxide  to  the  air.  But  respira- 
tion alone  would  be  unable  to  return  enough  carbon  dioxide  to  the  air 
to  balance  that  withdrawn  by  photosynthesis;  vast  amounts  of  carbon 
would  accumulate  in  the  dead  bodies  of  plants  and  animals.  The  carbon 
cycle  is  balanced  by  the  decay  bacteria  and  fungi  which  break  down 
the  carbon  compounds  of  dead  plants  and  animals  and  convert  the  car- 
bon to  carbon  dioxide  (Fig.  37.2). 

When  the  bodies  of  plants  are  compressed  under  water  they  are  not 
destroyed  by  bacteria,  but  undergo  a  series  of  chemical  changes  to  form 
peat,  then  brown  coal  or  lignite,  and  finally  coal.  The  bodies  of  certain 
marine  plants  and  animals  may  undergo  somewhat  similar  changes  to 
form  petroleum.  These  processes  remove  some  carbon  from  the  cycle 


in  Air  or 
Ived  in  Water 


imals 

Compounds 

of 

ol  Bodies 


Decay  Bacteria 
and  Fungi 


Dead  Organisms 

Corbon  Compounds  of 
Dead  Plant*  and  Animals 


^^/>/, 


^O/s 


Proteins,  Fats  and 
Other  Carbon  Compoundt 


/ 


Plant  Porosites 

Carbon  Compounds 
of  tlie  Bodies 
of  Parasites 


o  » jifuiL^  yt 


Figure  37.2.     The  carbon  cycle  in  nature.  See  text  for  discussion. 


ECOLOGr 


757 


temporarily,  but  eventually  geologic  changes  or  man's  mining  and  drill- 
ing bring  the  coal  and  oil  to  the  surface  to  be  burned  to  carbon  dioxide 
and  restored  to  the  cycle. 

Much  of  the  earth's  carbon  is  present  in  rocks  as  carbonates— lime- 
stone and  marble.  These  rocks  are  gradually  worn  down  and  the  car- 
bonates are  in  time  added  to  the  carbon  cycle,  but  other  rocks  are 
forming  at  the  bottom  of  the  sea  from  the  sediments  of  dead  animals 
and  plants,  so  that  the  amount  of  carbon  in  the  carbon  cycle  remains 
about  the  same. 

330.        The  Nitrogen  Cycle 

The  nitrates  of  the  soil  and  water  are  taken  up  by  plants  and  are 
the  source  of  nitrogen  for  the  synthesis  of  amino  acids  and  proteins. 
The  plants  may  then  be  eaten  by  animals  that  in  turn  use  the  amino 
acids  from  the  plant  proteins  in  synthesizing  their  own  amino  acids, 
proteins,  nucleic  acids,  and  other  nitrogenous  compounds.  \V'hen  animals 
and  plants  die,  the  decay  bacteria  convert  these  nitrogenous  compounds 
into   ammonia.   Animals  excrete  several  kinds  of   nitrogenous   wastes- 


Nitrogen  Fixing 
Bacteria 


Figure  37.3. 


The  nitrogen  cycle  in  nature.  See  text  for  discussion. 


758  ANIMALS   AND   THEIK   ENVIRONMENT 

urea,  uric  acid,  creatinine  and  ammonia-and  decay  bacteria  convert 
these  into  ammonia.  Most  of  the  ammonia  is  converted  by  nitrite  bac- 
teria to  nitrites  and  this  in  turn  is  converted  by  nitrate  bacteria  into 
nitrates,  thus  completing  the  cycle  (Fig.  37.3).  Denitrifying  bacteria 
convert  some  of  the  ammonia  to  atmospheric  nitrogen.  Atmospheric 
nitrogen  can  be  converted  to  amino  acids  and  other  organic  nitrogen 
compounds  by  some  algae  (Nostoc)  and  by  the  soil  bacteria  Azotobacter 
and  Clostridium.  Other  bacteria  of  the  genus  Rhizobimn,  though  un- 
able to  fix  atmospheric  nitrogen  by  themselves,  can  carry  out  this  process 
when  in  combination  with  cells  from  the  roots  of  legumes  such  as  peas 
and  beans.  The  bacteria  invade  the  roots  and  stimulate  the  formation 
of  root  nodules,  a  sort  of  benign  tumor.  The  combination  of  legume 
cell  and  bacteria  is  able  to  fix  nitrogen,  something  neither  can  do  alone. 
For  this  reason  legumes  are  often  planted  to  restore  soil  fertility  by 
increasing  the  content  of  fixed  nitrogen.  Nodule  bacteria  may  fix  as 
much  as  5  pounds  of  nitrogen  per  acre  per  year  and  soil  bacteria  as 
much  as  6  pounds  per  acre  per  year.  Atmospheric  nitrogen  can  also  be 
fixed  by  electrical  energy,  either  by  lightning  or  by  man-made  electricity. 
Although  80  per  cent  of  the  gases  in  the  atmosphere  is  nitrogen,  no 
animals  and  only  these  few  plants  can  utilize  it  in  this  form.  When 
the  bodies  of  the  nitrogen-fixing  bacteria  are  decayed,  the  amino  acids 
are  metabolized  to  ammonia  and  this  in  turn  is  converted  by  the  nitrite 
and  nitrate  bacteria  to  complete  the  cycle. 

331.  The  Water  Cycle 

The  seas  are  the  world's  great  reservoir  of  water.  The  sun's  heat 
vaporizes  water,  forms  clouds,  and  these  are  blown  over  the  land,  where 
they  are  cooled  enough  to  precipitate  the  water  as  rain  or  snow.  Some 
of  the  precipitated  water  soaks  into  the  ground,  some  runs  off  the 
surface  into  streams  and  goes  directly  back  to  the  sea.  The  ground  water 
is  returned  to  the  surface  by  springs,  by  pumps,  and  by  the  activities  of 
the  roots  and  stems  of  plants.  Water  inevitably  ends  up  in  the  sea,  but 
it  may  become  incorporated  into  the  bodies  of  several  successive  organ- 
isms en  route.  The  energy  to  run  the  cycle— the  heat  needed  to  evaporate 
water— comes  from  sunlight. 

332.  Mineral  Cycles 

As  water  runs  over  rocks  it  gradually  wears  away  the  surface  and 
carries  off  a  variety  of  minerals,  some  in  solution  and  some  in  suspen- 
sion. Some  of  these  minerals,  such  as  the  phosphates,  sulfates,  and 
other  salts  of  calcium,  magnesium,  sodium  and  potassium,  are  essential 
for  the  growth  of  plants  and  animals.  Phosphorus,  an  essential  com- 
ponent of  many  of  the  compounds  found  in  protoplasm,  enters  plants 
as  inorganic  phosphate  and  is  converted  to  a  variety  of  organic  phos- 
phates. Animals  obtain  their  phosphorus  as  inorganic  phosphate  in  the 
water  they  drink  or  as  inorganic  and  organic  phosphates  in  the  food 
they  eat.  The  phosphorus  cycle  is  not  completely  balanced,  for  phos- 


ECOLOGY  759 

phates  are  being  carried  into  the  sediments  at  the  bottom  of  the  sea 
taster  than  they  are  benig  returned  by  the  actions  of  fish  and  marine 
birds.  Sea  birds  play  an  important  role  in  returning  phosphorus  to  the 
cycle  by  depositing  phosphate-rich  guano  on  land.  Man  and  other  ani- 
mals, by  catching  hsh,  also  recover  some  phosphorus  from  the  sea.  Min- 
erals are  recovered  from  the  sea  botton  and  made  available  for  use  once 
more  when  geologic  upheavals  bring  some  of  the  sea  bottom  back  to  the 
surface  and  raise  new  mountains. 

333.  The  Energy  Cycle 

The  cycles  of  matter  are  closed:  the  atoms  are  used  over  and  over 
again.  Keeping  the  cycles  going  does  not  require  new  matter  but  it  does 
require  energy,  for  the  energy  cycle  is  not  a  closed  one.  Although  energy 
IS  neither  created  nor  destroyed,  but  converted  from  one  form  to  an- 
other (First  Law  of  1  hermodynamics),  there  is  a  decrease  in  the  amount 
of  useful  energy  whenever  one  of  these  transformations  occurs;  some 
energy  is  degraded  into  heat  and  dissipated  (Second  Law  of  Thermo- 
dynamics). 

Only  a  small  fraction  of  the  light  energy  reaching  the  earth  is 
trapped;  considerable  areas  of  the  earth  have  no  plants,  and  plants  can 
utilize  in  photosynthesis  only  about  3  per  cent  of  the  incident  energy. 
This  is  converted  into  the  chemical  energy  of  the  bonds  of  the  organic 
substances  made  by  the  plant.  When  an  animal  eats  the  plant,  or  when 
bacteria  decompose  the  plant  material,  and  these  organic  substances  are 
oxidized,  the  energy  liberated  is  just  equal  to  the  amount  used  in 
synthesizing  the  substances  (First  Law  of  1  hermodynamics)  but  some  of 
the  energy  is  heat  and  is  not  useful  energy  (Second  Law  of  Thermo- 
dynamics). If  the  animal's  llesh  is  eaten  by  another  animal,  a  further 
decrease  in  useful  energy  occurs  as  the  second  animal  oxidizes  the  or- 
ganic substances  of  the  first  to  liberate  energy  to  synthesize  its  own 
protoplasm. 

Eventually,  all  the  energy  originally  trapped  by  plants  in  photo- 
synthesis is  converted  to  heat  and  dissipated  to  outer  space  and  all  the 
carbon  of  the  organic  compounds  ends  up  as  carbon  dioxide.  The  only 
important  source  of  energy  on  earth  is  sunlight-energy  derived  from 
atomic  disintegrations  occurring  at  extremely  high  temperatures  in  the 
interior  of  the  sun.  W'hen  this  energy  is  exhausted  and  the  radiant 
energy  of  the  sun  can  no  longer  support  photosynthesis,  the  carbon  cycle 
will  stop,  all  plants  and  animals  will  die  and  organic  carbon  will  be 
converted   to  carbon  dioxide. 

334.  Physical  Factors  in  the  Environment 

No  species  of  animal  or  plant  is  found  everywhere  in  the  world; 
some  parts  of  the  earth  are  too  hot,  too  cold,  too  wet,  too  dry,  or  too 
something  else  for  the  organism  to  survive  there.  The  environment  may 
kill  the  animal  or  plant  directly,  or  it  may  keep  the  species  from  be- 
coming established  by  preventing  its  reproduction  or  by  killing  off  the 


760  ANIMALS   AND    THEIR   ENVIRONMENT 

egg,  embryo,  or  some  other  j^eculiarly  sensitive  stage  in  the  life  cycle. 
Most  species  of  organisms  are  not  even  found  in  all  the  regions  of  the 
world  where  they  could  survive.  The  existence  of  barriers  prevents 
their  further  migration  and  enables  us  to  distinguish  the  major  bio- 
geographic  realms  (p.  735),  characterized  by  certain  assemblages  of 
plants  and  animals. 

Biologists  were  aware  more  than  a  century  ago  that  each  kind  of 
animal  requires  certain  materials  for  growth  and  reproduction,  and  is 
unable  to  survive  if  the  environment  does  not  provide  a  certain  mini- 
mum of  each  of  the  materials  required.  V.  E.  Shelford  pointed  out  in 
1913  that  too  much  of  a  certain  factor  would  act  as  a  limiting  factor 
just  as  effectively  as  too  little  of  it.  Thus,  the  distribution  of  each  species 
is  determined  by  its  range  of  tolerance  to  variations  in  each  of  the  en- 
vironmental factors.  Much  ecologic  research  has  been  done  to  define 
the  limits  of  tolerance,  the  limits  within  which  species  can  exist,  and 
the  results  have  been  very  helpful  in  understanding  the  pattern  of 
distribution  of  animals  and  plants.  One  stage  in  the  life  cycle— perhaps 
the  larvae  or  eggs— is  usually  more  sensitive  to  some  environmental 
factor  and  is  effective  in  limiting  the  distribution  of  the  species.  The 
adult  blue  crab,  for  example,  can  survive  in  water  of  low  salt  content, 
and  can  migrate  for  some  distance  up  river  from  the  sea,  but  the  larvae 
cannot  survive  low  salinity  and  the  species  cannot  become  permanently 
established   there. 

Some  organisms  have  very  narrow  ranges  of  tolerance  to  environ- 
mental changes;  others  can  survive  within  much  broader  limits.  Any 
particular  species,  of  course,  may  have  narrow  limits  of  tolerance  for  one 
factor  and  wide  limits  for  another.  Ecologists  use  the  prefixes  steno- 
and  eury-  to  refer  to  species  with  narrow  and  wide,  respectively,  ranges 
of  tolerance  to  a  given  factor.  A  stenothermic  organism  is  one  which  will 
tolerate  only  narrow  variations  in  temperature.  The  housefly,  in  con- 
trast, is  eurythermic,  tolerating  temperatures  ranging  from  43  to  113°  F. 

Temperature.  Temperature  is  an  important  limiting  factor,  as  the 
relative  sparseness  of  life  in  the  desert  and  arctic  testifies.  Even  birds 
and  mammals  with  temperatures  kept  relatively  constant  by  physiologic 
thermostats  and  body  insulation  may  be  limited  by  extremes  of  tempera- 
ture. Extreme  heat  or  cold  may  limit  their  food  supplies  or  act  in  some 
other  indirect  fashion  to  prevent  their  survival.  Most  of  the  animals 
found  in  the  desert  have  adapted  to  the  rigors  of  the  environment  by 
living  in  burrows  during  the  day  and  foraging  only  at  night.  Many 
animals  escape  the  bitter  cold  of  the  northern  winter  by  migrating  south- 
ward or  by  burrowing  beneath  the  snow.  Measurements  made  in  Alaska 
show  that  when  the  surface  temperature  is  —68°  F.  the  temperature 
two  feet  under  the  snow,  at  the  surface  of  the  soil,  is  -|-20°  F.  Animals 
such  as  deer  and  elk  that  spend  the  summer  in  the  high  mountains 
migrate  to  lower  levels  during  the  winter.  Certain  bats,  rodents  and 
shrews  survive  the  winter  in  a  state  of  markedly  reduced  metabolism, 
known  as  hibernation  (p.  448).  The  body  temperature  falls  to  just  a 
degree  or  two  above  that  of  the  surrounding  air,  metabolism  is  greatly 
decreased,   and   the  heart  beat   and  respiration  become   very  slow.    No 


ECOLOGY  761 

food  is  eaten  and  the  metabolic  demands  of  the  body  are  met  from  the 
stores  of  body  fat.  Crocodiles,  certain  frogs  and  fishes  survive  periods 
of  high  temperature  and  dryness  by  undergoing  aestivation,  a  torpid, 
inactive  state  comparable  to  hibernation. 

Birds  and  mammals  have  physiologic  mechanisms  which  keep  body 
temperature  constant  despite  wide  fluctuations  in  the  environmental 
temperature  (p.  486).  These  thermostated  animals  are  said  to  be  homoio- 
thermlc  ("warm-blooded"  is  not  quite  the  proper  synonym;  they  are 
really  "constant  temperature-blooded").  Reptiles,  amphibia,  fish  and 
all  invertebrates  are  poikilothermic;  their  body  temperature  fluctuates 
with  that  of  the  environment.  "Cold-blooded"  is  not  properly  descrip- 
tive, for  a  lizard  sitting  in  the  sun  may  have  warmer  blood  than  ours. 
All  of  the  metaboHc  processes  in  poikilotherms  are  directly  influenced 
by  the  environmental  temperature.  Such  animals  move,  feed  and  grow 
in  warm  weather  and  become  inactive  in  cold  weather.  Many  marine 
organisms  have  seasonal  north-south  migrations  to  find  water  with  the 
optimal  temperature. 

Light.  The  amount  of  Ught  is  an  important  factor  in  determining 
the  distribution  and  behavior  of  both  plants  and  animals.  Light  is,  of 
course,  the  ultimate  source  of  energy  for  life  on  this  planet,  yet  pro- 
longed direct  exposure  of  protoplasm  to  light  is  fatal.  The  amount  of 
daylight  per  day,  known  as  the  photoperiod,  has  been  found  to  have  a 
marked  influence  on  the  time  of  flowering  of  plants,  the  time  of  migra- 
tion of  birds,  the  time  of  spawning  of  fish,  and  the  seasonal  change  of 
color  of  certain  birds  and  mammals.  The  effects  of  the  photoperiod  on 
the  vertebrates  appear  to  occur  via  some  hormonal  mechanism  involving 
the  pituitary.  Knowledge  of  photoperiod  phenomena  has  proven  to  be 
of  considerable  economic  importance.  Chicken  farmers  have  found  that 
artificial  illumination  in  the  hen  house,  by  extending  the  photoperiod, 
stimulates  the  hens  to  lay  more  eggs. 

Water.  Water  is  a  physiologic  necessity  for  all  protoplasm,  but  is 
a  limiting  factor  primarily  for  land  organisms.  The  total  amount  of 
rainfall,  its  seasonal  distribution,  the  humidity,  and  the  ground  supply 
of  water  are  some  of  the  factors  limiting  distribution  of  animals  and 
plants.  Some  lakes  and  streams,  especially  in  the  western  and  south- 
western United  States,  periodically  become  dry  or  almost  dry  and  the 
fish  and  other  aquatic  animals  are  killed.  During  periods  of  low  water, 
the  water  temperature  may  rise  sufficiently  to  kill  off  the  aquatic  forms. 
Many  of  the  protozoa  form  thick-walled  cysts  which  enable  them  to  sur- 
vive the  drying  of  the  puddles  in  which  they  normally  live.  Some  desert 
animals  have  adapted  to  desert  conditions  by  digging  and  living  in 
burrows  where  the  temperature  is  lower  and  the  humidity  is  higher  than 
at  the  surface.  Measurements  have  shown  that  the  burrow  of  a  kangaroo 
rat  two  feet  underground  may  have^a  temperature  of  only  60  F.  when 
the  surface  temperature  is  over  100°  F.  ,  u  r 

An  excess  of  water  is  fatal  to  some  animals;  earthworms,  for  ex- 
ample may  be  driven  from  their  burrows  by  heavy  rainfall  because 
oxygen  is  only  sparingly  soluble  in  water  and  they  are  unable  to  get 


752  ANIMALS   AND   THEIR   ENVIRONMENT 

enough  oxygen  when  immersed.  Knowledge  of  the  limits  of  water  toler- 
ance is  helpful  in  attacking  insect  and  other  pests.  Wire  worms  have 
rather  narrow  limits  of  tolerance  to  water  and  are  most  sensitive  as 
larvae  and  pupae.  They  can  be  killed  by  flooding  the  infested  fields  or 
by  planting  alfalfa  or  wheat  to  dry  out  the  soil  below  the  limit  of  tol- 
erance of  the  wire  worm  larvae. 

Other  Factors.  The  supply  of  oxygen  and  carbon  dioxide  is  usu- 
ally not  limiting  for  land  organisms  except  for  animals  living  deep  in 
the  soil,  on  the  tops  of  mountains,  or  within  the  bodies  of  other  animals. 
Animals  living  in  aquatic  environments  may  be  limited  by  the  amount 
of  dissolved  oxygen  present;  the  oxygen  tension  in  stagnant  ponds  or  in 
streams  fouled  by  industrial  wastes  may  become  so  low  as  to  be  incom- 
patible with  many  forms  of  life.  Some  parasites  have  adapted  to  the 
low  oxygen  tension  within  the  host's  body  by  evolving  special  metabolic 
pathways  by  which  energy  can  be  released  from  foodstuffs  without  the 
utilization  of  free  oxygen. 

The  trace  elements  necessary  for  plant  and  animal  life  are  limiting 
factors  in  certain  parts  of  the  world.  The  soil  in  certain  parts  of  Aus- 
tralia, for  example,  is  extremely  deficient  in  copper  and  cobalt  and  is 
unsuitable  for  raising  cattle  or  sheep.  Other  trace  elements  which  may 
be  a  limiting  factor  are  manganese,  zinc,  iron,  sulfur  and  boron. 

The  amount  of  carbon  dioxide  in  the  air  is  remarkably  constant, 
but  the  amount  dissolved  in  water  varies  widely.  An  excess  of  carbon 
dioxide  may  be  a  limiting  factor  for  fish  and  insect  larvae.  The  hydrogen 
ion  concentration,  pH,  of  water  is  related  physicochemically  to  the 
carbon  dioxide  concentration  and  it,  too,  may  be  an  important  limiting 
factor  in   aquatic  environments. 

Water  currents  are  limiting  for  a  number  of  kinds  of  animals  and 
plants;  the  fauna  and  flora  of  a  still  pond  and  of  a  rapidly  flowing  stream 
are  quite  diffierent.  Winds  may  have  a  comparable  limiting  effect  upon 
land  organisms. 

The  type  of  soil,  the  amount  of  topsoil,  its  pH,  porosity,  slope, 
water-retaining  properties,  and  so  on,  are  limiting  factors  for  a  variety 
of  plants,  and  hence  indirectly  for  animals.  The  ability  of  many  animals 
to  survive  in  a  given  region  depends  upon  the  presence  of  certain  plants 
to  provide  shelter  and  cover,  as  well  as  food.  Grasses,  shrubs  and  trees 
on  land  each  provide  shelter  for  certain  kinds  of  animals,  and  seaweeds 
and  fresh-water  aquatic  plants  have  a  similar  role  for  aquatic  animals. 
Some  animals  require  special  shelter  for  breeding  places  and  the  care  of 
the  young.  In  many  different  kinds  of  birds,  mammals,  crustaceans  and 
other  animals,  each  animal  or  pair  establishes  a  territory,  a  region  which 
supplies  food  and  shelter  for  it  and  its  offspring,  and  which  it  defends 
vigorously  against  invasion  by  other  members  of  the  same  species. 

In  summary,  whether  an  animal  can  become  established  in  a  given 
region  is  the  result  of  a  complex  interplay  of  such  physical  factors  as 
temperature,  light,  water,  winds  and  salts,  and  biotic  factors  such  as 
the  plants  and  other  animals  in  that  region  which  serve  as  food,  com- 
pete for  food  or  space,  or  act  as  predators  or  disease  organisms. 


EcotoGy       753 

335.  Types  of  Interactions  between  Species 

The  members  of  two  different  species  may  affect  each  other  in  any 
one  of  several  different  ways.  If  neither  population  is  affected  by  the 
presence  of  the  other,  so  that  there  is  no  interaction,  the  situation  is 
termed  neutralism.  If  each  population  is  adversely  affected  by  the  other 
in  its  search  for  food,  space,  shelter,  or  some  other  fundamental  require- 
ment for  life,  the  interaction  is  one  of  competition.  If  each  population  is 
benefited  by  the  presence  of  the  other,  but  can  survive  in  its  absence, 
the  relationship  is  termed  protocooperation.  But  if  each  population  is 
benefited  in  some  way  by  the  other,  and  cannot  survive  in  nature  without 
it,  the  relationship  is  termed  mutualism.  Commensalism  refers  to  the 
relationship  in  which  one  species  is  benefited  and  the  second  is  not 
affected  by  existing  together,  and  amensalism  to  the  relationship  where 
one  species  is  inhibited  by  the  second  but  the  second  is  unaffected  by 
the  first.  Where  one  species  affects  the  second  adversely  but  cannot  live 
without  it,  the  relationship  is  one  of  parasitism  or  predation;  parasitism 
if  one  species  lives  in  or  on  the  body  of  the  second  and  predation  if 
the  first  species  catches,  kills  and  feeds  upon  the  second.  The  older  term 
symbiosis,  "living  together,"  is  used  by  some  authors  as  a  synonym  of 
mutualism  and  by  others  in  a  wider  sense  as  a  term  including  mutualism, 
commensalism  and  even  parasitism. 

336.  Competition 

Two  species  may  compete  for  the  same  space,  food,  light,  or  in 
escaping  from  predators  or  disease;  these  may  be  summarized  as  com- 
petition for  the  same  ecologic  niche.  Competition  results  in  one  species 
dying  off,  or  being  forced  to  move  to  a  different  space  or  use  a  different 
food.  Careful  ecologic  studies  usually  reveal  that  there  is  only  one  species 
in  an  ecologic  niche  (Cause's  rule).  One  of  the  clearest  examples  of 
competition  was  provided  by  the  classic  experiments  of  Cause  with  pop- 
ulations of  paramecia.  When  either  of  two  closely  related  species,  Para- 
meciutn  caudatuin  or  Paramecium  aurelia,  was  cultured  separately  on  a 
fixed  amount  of  bacteria  as  food,  it  multiplied  and  finally  reached  a  con- 
stant level  (Fig.  37.4).  But  when  both  species  were  placed  in  the  same  cul- 
ture vessel  with  a  limited  amount  of  food,  only  Paramecium  aurelia  was 
left  at  the  end  of  sixteen  days  (Fig.  37.4).  The  Paramecium  aurelia  had 
not  attacked  the  other  species,  or  secreted  any  harmful  substance;  it 
simply  had  been  more  successful  in  competing  for  the  limited  food  sup- 
ply. Studies  in  the  field  generally  corroborate  Cause's  rule.  Two  fish-eat- 
ing, cliff-nesting  birds,  the  cormorant  and  the  shag,  which  seemed  at  first 
glance  to  have  survived  despite  occupying  the  same  ecologic  niche,  were 
found  upon  analysis  to  have  slightly  different  niches.  The  cormorant 
feeds  on  bottom-dwelling  fish  and  shrimps  whereas  the  shag  hunts  fish 
and  eels  in  the  upper  levels  of  the  sea.  Further  study  showed  that 
these  birds  typically  have  slightly  different  nesting  sites  on  the  cliffs 
as  well. 


764 


ANIMALS   AND    THEIR   ENVIRONMENT 


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Figure  37.4.  An  experiment  to  demonstrate  the  competition  between  two  closely 
related  species  of  paramecia  which  have  identical  niches.  When  grown  separately  in 
controlled  cultures  with  a  fixed  supply  of  food  (bacteria),  both  ParaineciiDii  caudatiim 
and  P.  aurelia  show  normal  S-shaped  growth  curves  (solid  lines).  When  grown 
together,  P.  caiidatum  is  eliminated  (dotted  lines).  (After  Cause,  from  Allee,  et  al.: 
Principles  of  Animal  Ecology.) 


337.        Commensalism 

Commensalism,  the  living  together  of  two  species,  one  of  which 
(the  commensal)  derives  benefit  from  the  association  whereas  the  other  is 
unharmed  by  it,  is  especially  common  in  the  sea  (Fig.  22.13£).  Practically 
every  worm  burrow  and  shellfish  contains  some  uninvited  guests  that 
take  advantage  of  the  shelter,  and  possibly  of  the  abundant  food,  pro- 
vided by  the  host  organism  but  do  it  neither  good  nor  harm.  Certain 
flatworms  live  attached  to  the  gills  of  the  horseshoe  crab  and  get  their 
food  from  the  scraps  of  the  crab's  meals.  They  obtain  shelter  and  trans- 
portation from  the  host  but  apparently  do  it  no  harm.  Many  oysters 
and  other  bivalves  have  small  crabs  living  in  their  mantle  cavity,  and 
there  is  a  species  of  small  fish  that  lives  in  the  posterior  end  of  the 
digestive  tract  of  the  sea  cucumber! 


338.        Protocooperation 

If  both  species  gain  from  an  association,  but  are  able  to  survive 
without  it,  the  association  is  termed  protocooperation.  A  number  of 
crabs  put  coelenterates  of  one  sort  or  another  on  top  of  their  shells, 


ECOLOGY  755 

presumably  as  camouflage.  The  coelenterates  benefit  from  the  associa- 
tion by  getting  bits  of  food  when  the  crab  captures  and  eats  an  animal. 
Neither  crab  nor  coelenterate  is  absolutely  dependent  upon  the  other. 

339.  Mutualism 

When  both  species  gain  from  an  association  and  are  unable  to  sur- 
vive separately,  the  association  is  termed  mutualism.  It  is  probable  that 
associations  begin  as  commensalism  and  then  evolve  through  a  stage  of 
protocooperation  to  one  of  mutualism.  A  striking  example  of  mutualism 
is  provided  by  the  relationship  of  termites  and  their  intestinal  flagel- 
lates. Termites  have  no  enzymes  for  the  digestion  of  wood,  yet  that  is 
their  staple  diet.  Certain  flagellate  protozoa  that  live  only  in  their  in- 
testines do  have  the  enzymes  to  digest  the  cellulose  of  wood  to  sugars. 
Although  the  flagellates  require  some  of  this  sugar  for  their  own  metab- 
olism, there  is  enough  left  over  for  the  termite.  Termites  are  unable  to 
survive  without  their  intestinal  inhabitants;  newly  hatched  termites  in- 
stinctively lick  the  anus  of  another  termite  to  get  a  supply  of  flagellates. 
Since  a  termite  loses  all  of  its  flagellates  along  with  most  of  its  gut 
lining  at  each  molt,  termites  must  live  in  colonies  so  that  a  newly 
molted  individual  will  be  able  to  get  flagellates  from  a  neighbor.  The 
flagellates  are  provided  with  plenty  of  food  in  a  well  protected,  rela- 
tively constant  environment;  they  can,  in  fact,  survive  only  in  the  in- 
testines of  termites. 

340.  Amensalism 

Commensalism,  protocooperation  and  mutualism  are  types  of  posi- 
tive interactions,  ones  in  which  one  or  both  members  of  the  associated 
pair  derive  benefit  from  the  association  yet  neither  is  harmed  by  it. 
Negative  interactions  between  species— amensalism,  parasitism  and  pre- 
dation— are  those  in  which  one  species  is  harmed  by  the  association.  If  the 
second  species  is  unaff^ected,  the  relationship  between  the  two  is  termed 
amensalism.  Organisms  that  produce  antibiotics  and  the  species  in- 
hibited by  the  antibiotic  are  examples  of  amensalism.  The  mold  Pe}ii- 
cillium  produces  the  antibiotic  penicillin  which  inhibits  the  growth  of 
a  variety  of  bacteria,  but  the  mold  is  unaffected  by  the  bacteria.  The 
clinical  use  of  these  bacteria-inhibiting  agents  has  had  the  unexpected 
effect  of  increasing  the  incidence  of  fungus-induced  diseases  in  man 
which  are  normally  kept  in  check  by  the  presence  of  the  bacteria.  When 
the  bacteria  are  killed  off  by  the  antibiotics,  the  pathogenic  fungi  have  a 
golden  opportunity. 

341.  Parasitism  and  Predation 

It  is  incorrect  to  assume  that  the  host-parasite  and  predator-prey 
relationships  are  invariably  harmful  to  the  host  or  prey  as  a  species. 
This  is  usually  true  when  such  relationships  are  first  established,  but 
the  forces  of  natural  selection  tend,  in  time,  to  decrease  the  detrimental 


766  ANIMALS   AND    THEIR   ENVIRONMENT  I 

effects.  II  this  did  not  occur  the  parasite  would  eventually  exterminate  1 

the  host  species  and,  unless  it  lound  a  new  species  to  parasitize,  would  die 

itself. 

Studies  of  many  examples  of  parasite-host  and  predator-prey  asso- 
ciations show  that  in  general,  when  the  associations  have  been  estab- 
lished for  a  long  time,  evolutionarily  speaking,  the  long-term  effect  on 
the  host  or  prey  species  is  not  very  detrimental.  Conversely,  newly  ac- 
quired predators  or  parasites  are  usually  quite  damaging.  The  plant 
parasites  and  insect  pests  that  are  most  troublesome  to  man  and  his 
crops  are  usually  those  which  have  recently  been  introduced  into  some 
new  area  and  thus  have  a  new  group  of  organisms  to  attack. 

The  role  of  the  predator-prey  relationship  in  maintaining  a  balance 
between  the  number  of  predators  and  of  prey  is  beautifully  illustrated 
by  the  story  of  the  Kaibab  deer.  The  Kaibab  plateau  is  located  on  the 
north  side  of  the  Grand  Canyon  of  the  Colorado  river.  In  1907  there 
were  some  4000  deer  living  on  the  plateau,  together  with  a  considerable 
population  of  predators,  mountain  lions  and  wolves.  When  a  concerted 
effort  was  made  to  "protect"  the  deer  by  killing  off  the  predators,  the 
deer  population  increased  tremendously  and  by  1925  some  100,000  deer 
roamed  the  plateau,  far  more  than  the  supply  of  vegetation  could  sup- 
port. The  deer  ate  everything  in  reach— grass,  tree  seedlings  and  shrubs— 
and  there  was  marked  damage  to  the  vegetation.  Over  the  next  two 
winters  large  numbers  of  the  deer  died  of  starvation  and  the  size  of  the 
herd  fell  to  about  10,000.  In  the  wild,  the  size  of  the  predator  popula- 
tion varies  with  the  size  of  the  population  of  the  species  which  is  preyed 
upon,  with  the  swings  in  the  size  of  the  predator  population  lagging 
somewhat  behind  those  of  the  prey. 

342.        Intraspecific  Relations 

In  addition  to  the  associations  between  the  members  of  two  different 
species  just  described,  aggregations  of  animals  or  plants  of  a  single  species 
frequently  occur.  Some  of  these  aggregations  are  temporary,  for  breed- 
ing; others  are  more  permanent.  Despite  the  fact  that  the  crowding 
which  accompanies  dense  aggregations  of  animals  is  ecologically  un- 
desirable and  deleterious,  both  laboratory  experiments  and  field  obser- 
vations show  that  such  aggiegations  of  individuals  are  able  to  survive 
when  a  single  individual  of  the  same  species  placed  in  the  same  environ- 
ment dies.  A  herd  of  deer,  with  many  noses  and  pairs  of  eyes,  is  less 
likely  to  be  surprised  by  a  predator  than  is  a  single  one.  A  pack  of 
wolves  hunting  together  are  more  likely  to  make  a  kill  than  is  a  lone 
wolf.  The  survival  value  of  aggregations  is  less  obvious,  but  nonetheless 
real,  in  some  of  the  lower  animals.  It  can  be  shown  experimentally  that 
a  group  of  insects  is  less  likely  to  dry  up  and  die  in  a  dry  environment 
than  is  a  single  insect,  and  a  group  of  planaria  is  less  likely  to  be  killed 
by  a  given  dose  of  ultraviolet  light  than  is  a  single  flatworm.  When  a 
dozen  goldfish  are  placed  in  one  bowl  and  a  single  one  in  a  second 
bowl,  and  the  same  amount  of  a  toxic  agent  such  as  colloidal  silver  is 
added  to  each  bowl,  the  single  fish  will  die  but  the  group  survives.  The 


ECOLOGY  'JQ'J 

explanation  for  this  proved  to  be  that  the  sHme  secreted  by  the  group 
of  fish  was  enough  to  precipitate  much  of  the  colloidal  silver  and  ren- 
der it  nontoxic,  whereas  the  amount  secreted  by  a  single  fish  was  not. 

Such  animal  aggregations  do  have  survival  value  for  the  species. 
Allee  has  called  this  "unconscious  cooperation."  When  genes  governing 
a  tendency  toward  aggregation  arise  in  a  species  and  prove  to  have 
survival  value,  natural  selection  will  tend  to  preserve  this  inherited 
behavior  pattern.  The  occurrence  of  many  fish  in  schools,  of  birds  in 
flocks,  and  so  on,  are  examples  of  this  "unconscious  cooperation"  which 
occurs  very  widely  in  the  animal  kingdom. 

From  such  simple  animal  aggregations  there  may  evolve  complex 
animal  societies,  composed  of  specialized  types  of  individuals,  such  as 
the  colonies  of  bees,  ants  and  termites  (section  155).  Man  is  another 
example  of  a  social  animal. 

343.        Food  Chains 

The  ultimate  source  of  all  the  energy  used  by  living  things  is  sun- 
light, the  energy  of  which  is  converted  to  a  biologically  useful  form  by 
the  process  of  photosynthesis  carried  on  by  green  plants.  Only  a  small 
fraction,  about  3  per  cent,  of  the  light  energy  striking  the  leaves  of  a 
green  plant  is  transformed  by  photosynthesis  into  the  potential  energy 
of  a  food  substance;  the  rest  escapes  as  heat.  This  loss  is  not  the  result 
of  inefficiency  of  the  biochemical  processes  involved,  but  of  the  opera- 
tion of  the  laws  of  thermodynamics.  The  Second  Law  of  Thermody- 
namics may  be  stated  as  "whenever  energy  is  transformed  from  one 
form  into  another  there  is  a  decrease  in  the  amount  of  useful  energy; 
some  energy  is  degraded  into  heat  and  dissipated."  In  other  words,  no 
transformation  of  energy  can  be  100  per  cent  efficient. 

When  an  animal  eats  a  plant,  much  of  the  energy  is  again  dissi- 
pated as  heat  and  only  a  fraction  is  used  to  synthesize  the  animal's 
protoplasm.  When  a  second  animal  eats  the  first,  there  is  a  further  loss 
of  energy  as  heat,  and  so  on.  The  transfer  of  food  energy  from  its  ulti- 
mate source  in  plants  through  a  series  of  organisms,  each  of  which  eats 
the  preceding  and  is  eaten  by  the  following,  is  known  as  a  food  chain. 
The  number  of  steps  in  a  food  chain  is  limited  to  perhaps  four  or  five 
because  of  the  great  decrease  in  available  energy  at  each  step.  The  per- 
centage of  the  food  energy  consumed  that  is  converted  to  new  proto- 
plasm, and  thus  is  available  as  food  energy  for  the  next  organism  m 
the  food  chain,  is  known  as  the  efficiency  of  energy  transfer. 

The  first  step  in  any  food  chain,  the  capture  of  light  energy  by 
photosynthesis  and  the  production  of  energy-containing  foods  by  plants, 
is  relatively  inefficient;  only  about  0.2  per  cent  of  the  mcident  light 
energy  is  stored  as  food.  The  efficiency  of  energy  transfer  when  one 
animal  eats  a  plant  or  another  animal  is  higher,  ranging  from  5  to  20 
per  cent.  Some  animals  eat  but  one  kind  of  food  and  therefore  are 
members  of  a  single  food  chain.  Other  animals  eat  many  different  kinds 
of  food  and  are  not  only  members  of  different  food  chains,  but  may 
occupy  'different  positions  in  different  food  chains.  An  animal  may  be 


758  ANIMALS   AND   THEIR   ENVIRONMENT 

a  primary  consumer  in  one  chain,  eating  green  plants,  but  a  secondary 
or  tertiary  consumer  in  other  chains,  eating  herbivorous  animals  or 
other  carnivores.  Man  is  the  end  of  a  number  of  food  chains.  For  ex- 
ample, man  eats  a  fish  such  as  a  black  bass,  which  ate  smaller  fish,  which 
in  "turn  ate  small  crustacea,  which  in  turn  ate  algae.  The  ultimate  size 
of  the  human  population,  or  of  the  population  of  any  animal,  is  limited 
(1)  by  the  length  of  the  food  chain,  (2)  by  the  percentage  efficiency  of 
energy  transfer  at  each  step  in  the  chain,  and  (3)  by  the  amount  of  light 
energy  falling  on  the  earth.  Since  man  can  do  nothing  about  increasing 
the  amount  of  incident  sunlight,  and  very  little  about  the  percentage 
efficiency  of  energy  transfer,  he  can  increase  his  supply  of  food  energy 
only  by  shortening  his  food  chain,  i.e.,  by  eating  the  primary  producers, 
plants,  rather  than  animals.  In  overcrowded  countries  such  as  India 
and  China,  men  are  largely  vegetarians  because  this  food  chain  is 
shortest  and  a  given  area  of  land  can  in  this  way  support  the  greatest 
number  of  people.  Steak  is  a  luxury  ecologically  as  well  as  economically! 

In  addition  to  predator  food  chains,  such  as  the  man-black  bass- 
minnow-crustacean  one,  there  are  parasite  food  chains  and  saprophyte 
food  chains.  The  ingestion  of  organic  nutrients  derived  from  decom- 
posing animal  or  plant  bodies  or  by-products  directly  through  the  body 
wall,  a  mode  of  nutrition  known  as  saprophytic  or  saprozoic,  is  not  very 
common  in  the  animal  kingdom,  being  restricted  generally  to  certain 
protozoa.  Parasite  food  chains  are  common  and  may  be  quite  complex. 
For  example,  mammals  and  birds  are  parasitized  by  fleas;  in  the  fleas 
live  protozoa  which  are  in  turn  the  hosts  of  bacteria.  Since  the  bacteria 
might  be  parasitized  by  viruses,  there  could  be  a  five-step  parasite  food 
chain.  It  is  obvious  that  in  general  the  organisms  in  a  parasite  food  chain 
are  smaller  than  their  hosts  whereas  the  organisms  in  a  predator 
chain  are  larger  than  their  prey. 

Since,  in  any  food  chain,  there  is  a  loss  of  energy  at  each  step,  it 
follows  that  there  is  usually  a  smaller  amount  of  protoplasm  in  each 
successive  step.  H.  T.  Odum  has  calculated  that  17,850  pounds  of  alfalfa 
plants  are  required  to  provide  the  food  for  2250  pounds  of  calves,  which 
provide  enough  food  to  keep  one  twelve-year-old  boy  alive  for  one  year. 
Although  boys  eat  many  things  other  than  veal,  and  calves  other  things 
besides  alfalfa,  these  numbers  illustrate  the  principle  of  a  food  chain. 
A  food  chain  may  be  visualized  as  a  pyramid;  each  step  in  the  pyramid 
is  much  smaller  than  the  one  on  which  it  feeds.  Since  the  predators  are 
usually  larger  than  the  ones  on  which  they  prey,  the  pyramid  of  num- 
bers of  individuals  in  each  step  of  the  chain  is  even  more  striking  than 
the  pyramid  of  the  mass  of  protoplasm  of  the  individuals  in  successive 
steps:  one  boy  requires  4.5  calves,  which  require  20,000,000  alfalfa 
plants. 

344.        Communities  and  Populations 

Each  region  of  the  earth— sea,  lake,  forest,  prairie,  tundra,  desert- 
is  inhabited  by  a  characteristic  assemblage  of  animals  and  plants  which 
are  interrelated  in  many  and  diverse  ways  as  competitors,  commensals. 


ECOLOGY 


769 


Figure    37.5.     Diagram    of    a    food    chain    in    an    Illinois    deciduous   forest.    (After 
Shelford.)  (Courtesy  of  Dr.  V.  E.  Shelford.) 

predators,  and  so  on.  The  members  of  each  assemblage  are  not  deter- 
mined by  chance  but  by  the  total  effect  of  the  many  interacting  physical 
and  biotic  factors  of  the  environment.  The  ecologist  refers  to  the  or- 
ganisms living  in  a  given  area  as  a  biotic  community;  this  is  composed 
of  smaller  groups,  the  members  of  which  are  more  intimately  associated, 
known  as  populations.  There  is  no  sharp  distinction  between  a  popu- 
lation and  a  community. 

The  intermeshings  of  the  food  chains  in  any  biotic  community  are 
very  complicated  and  are  sometimes  called  a  food  web,  or  "web  of  life." 
Some  of  the  interrelated  food  chains  of  a  deciduous  forest  in  eastern 
North  America  are  indicated  in  Figure  37.5.  The  basic  principles  of  the 
ecologic  relations  of  biotic  communities  have  been  elucidated  by  the 
study  of  somewhat  simpler  communities  such  as  the  arctic  tundra  or 
desert.  The  producer  organisms  of  the  tundra  are  lichens,  mosses  and 
grasses.  Reindeer  and  caribou  feed  on  the  lichens  and  are  preyed  upon 
by  wolves  and  man.  Grasses  are  eaten  by  the  arctic  hare  and  the  lem- 
ming, which  are  eaten  by  the  snowy  owl  and  the  arctic  fox,  which  is 
preyed  upon  by  man  for  its  fur.  During  the  brief  arctic  summer  the 
food  web  is  enlarged  by  many  insects  and  by  migratory  birds  which 
feed  upon  them. 


345.        Populations  and  Their  Characteristics 

A  population  may  be  defined  as  a  group  of  organisms  of  the  same 
or  similar  species  which  occupy  a  given  area.  It  has  characteristics 
which  are  a  function  of  the  whole  group  and  not  of  the  individual 
members;  these  are  population  density,  birth  rate,  death  rate,  age 
distribution,  biotic  potential,  rate  of  dispersion  and  growth  form.  Al- 
though individuals  are  born  and  die,  individuals  do  not  have  birth 
rates  or  death  rates;  these  are  characteristics  of  the  population  as  a 
whole  Modern  ecology  deals  especially  with  the  community  and  popu- 
lation aspects  of  the  science,  and  the  study  of  group  organization  is  the 
most  unique  part  of  the  science  of  ecology.  Population  and  community 


770  ANIMALS   AND   THEIR   ENVIRONMENT 

relationships  are  often  more  important  in  determining  the  occurrence 
and  survival  of  organisms  in  nature  than  are  the  direct  eftects  of  physi- 
cal and  chemical  factors  in  the  environment. 

One  important  attribute  of  a  population  is  its  density— the  number 
of  individuals  per  unit  area  or  volume,  e.g.,  the  number  of  animals  per 
square  mile,  of  trees  per  acre  in  a  forest,  or  millions  of  diatoms  per 
cubic  meter  of  sea  water.  This  is  a  measure  of  the  population's  success 
in  a  given  region.  Frequently  in  ecologic  studies  it  is  important  to  know 
not  only  the  population  density  but  whether  it  is  changing  and,  if  so, 
what  the  rate  of  change  is.  Population  density  is  often  difficult  to 
measure  in  terms  of  individuals,  but  estimates  such  as  the  number  of 
insects  caught  per  hour  in  a  standard  trap,  the  number  of  sea  urchins 
caught  in  a  standard  "sea  mop,"  or  the  number  of  birds  seen  or  heard 
per  hour,  are  usable  substitutes.  A  method  that  will  give  good  results 
when  used  with  the  proper  precautions  is  that  of  capturing,  let  us  say, 
lOU  animals,  tagging  them  in  some  way,  and  then  releasing  them.  On 
some  subsequent  day,  another  100  animals  are  trapped  and  the  pro- 
portion of  tagged  animals  is  determined.  This  assumes  that  animals 
caught  once  are  neither  more  nor  less  likely  to  be  caught  again,  and 
that  both  sets  of  trapped  animals  are  random  samples  of  the  popula- 
tion. If  the  100  animals  caught  on  the  second  day  include  20  tagged 
ones,  the  total  population  of  tagged  and  untagged  animals  in  the  area 
of  the  traps  is  500;  x/100  =   100/20,  hence  x  =  500. 

For  many  kinds  of  ecologic  investigations,  an  estimate  of  the  num- 
ber of  individuals  per  total  area  or  volume,  known  as  the  "crude  den- 
sity," is  not  exact  enough.  Only  a  fraction  of  that  total  area  may  be  a 
habitat  suitable  for  the  population,  and  the  size  of  the  individual  mem- 
bers of  a  population  may  vary   tremendously.  Ecologists   therefore   cal- 
culate an  ecologic  density,  defined  as  the  number,  or  more  exactly  as 
the  mass,  of  individuals  per  area  or  volume  of  habitable  space.  Trap- 
ping and  tagging  experiments  might  give  an  estimate  of  500  rabbits  per 
square  mile,  but  if  only  half  of   that  square  mile  actually   consists  of 
areas  suitable  for  rabbits   to  inhabit,   then    the   ecologic  density  would 
be  1000  rabbits  per  square  mile  of  rabbit  habitat.  With  species  whose 
individuals  vary  greatly  in  size,  such  as  fish,  live  weight  or  some  other 
estimate   of  the   total   mass  of  living  fish  is   a  much   more  satisfactory 
estimate  of  density  than  simply  the  total  number  of  individuals  present. 
A   graph   in  which  the  number  of  organisms,   or  its   logarithm,   is 
plotted  against  time  is  a  population  growth  curve  (Fig.  37.6).  Since  such 
curves  are  characteristic  of  populations,  rather  than  of  a  single  species, 
they  are  amazingly  similar  for  populations  of  almost  all  organisms  from 
bacteria  to  man.  From  a  study  of  the  human  population  growth  curve 
to  date,  and  by  comparing  this  curve  to  a  general  one,  Raymond  Pearl 
estimated  that  the  human  population,  about  2.2  billion  in   1936,  would 
reach  2.65  billion  in  the  year  2100  and  would  remain  stable  thereafter 
unless   there  was  some  change  in   the  ability  of   the  earth   to   support 
human  life.  Subsequent  scientific  discoveries  may  change  somewhat  the 
estimated  upper  limit  of  the  human  population,  but  the  principle  that 


ECOtOGY 


771 


POSITIVE 

ACCELERATION 

PHASE 


Logarithm 
of  number 

of 
Individuals 

in 
population 


Time 


Figure  37.6.  A  typical  growth  curve  of  a  population,  one  in  which  the  logarithm 
of  the  total  number  of  individuals  is  plotted  against  the  time.  The  absolute  units  of 
time  and  the  total  number  in  the  population  would  vary  from  one  species  to  another, 
but  the  shape  of  the  growth  curve  would  be  similar  for  all  populations. 

there  is  an  upper  limit  to  the  number  of  men  that  can  be  supported 
on  the  earth  is  perfectly  sound. 

The  birth  rate,  or  natality,  of  a  population  is  simply  the  number 
of  new  individuals  produced  per  unit  time.  The  maximum  birth  rate 
is  the  largest  number  of  individuals  that  could  be  produced  per  unit 
time  under  ideal  conditions,  wlien  there  are  no  limiting  factors.  This 
is  a  constant  for  a  species  and  is  determined  by  physiologic  factors  such 
as  the  nimiber  of  eggs  produced  per  female  per  unit  time,  the  propor- 
tion of  females  in  the  species,  and  so  on.  The  actual  birth  rate  is  usually 
considerably  less  than  this,  for  not  all  of  the  eggs  laid  are  able  to 
hatch,  not  all  the  larvae  or  young  survive,  and  so  on.  The  size  and 
composition  of  the  population  and  a  variety  of  environmental  condi- 
tions affect  the  actual  birth  rate.  It  is  difficult  to  determine  the  maxi- 
mum natality,  for  it  is  difficult  to  be  sure  that  all  limiting  factors  have 
been  removed.  However,  under  experimental  conditions,  or  by  careful 
field  studies,  one  can  get  an  estimate  of  this  value  which  is  useful  in 
predicting  the  rate  of  increase  of  the  population  and  in  providing  a 
yardstick  for  comparison  with  the  actual  birth  rate. 

The  mortality  rate  of  a  population  refers  to  the  number  of  indi- 
viduals dying  per  unit  time.  There  is  a  theoretical  minimum  mortality, 
somewhat  analogous  to  the  maximum  birth  rate,  which  is  the  number 
of  deaths  that  would  occur  under  ideal  conditions-deaths  due  simply 
to  the  physiologic  changes  of  old  age.  This  minimum  mortality  rate  is 
also  a  constant  for  a  given  population.  The  actual  mortality  rate  will, 
of  course,  depend  upon  physical  factors  and  upon  the  size  and  compo- 
sition of  the  population.  By  plotting  the  number  of  survivors  in  a 
population  against  time,  one  gets  a  survival  curve  (Fig.  67./).  U  the 
units  of   the   time  axis  are  the  percentage  of  total  life  span,  one  can 


772 


ANIMALS   AND   THEIR   ENVIRONMENT 


compare  the  survival  curves  for  organisms  with  very  cHfferent  total  life 
spans.  Civilized  man  has  improved  his  average  life  expectancy  greatly 
by  modern  medical  practices,  and  the  curve  for  human  survival  ap- 
proaches the  curve  for  minimum  mortality.  From  such  curves  one  can 
determine  at  what  stage  in  the  life  cycle  a  particular  species  is  most 
vulnerable.  Reducing  or  increasing  the  mortality  in  this  vulnerable 
period  will  have  the  greatest  effect  on  the  future  size  of  the  population. 
Since  the  death  rate  is  more  variable  and  more  affected  by  environ- 
mental factors  than  the  birth  rate,  it  has  a  primary  role  in  population 
control. 

It  is  quite  obvious  that  populations  that  differ  in  the  relative  num- 
bers of  young  and  old  will  have  quite  different  characteristics,  different 
birth  and  death  rates,  and  different  prospects.  Death  rates  typically  vary 
with  age,  and  birth  rates  are  usually  proportional  to  the  number  of  indi- 
viduals able  to  reproduce.  Three  ages  can  be  distinguished  in  a  popula- 
tion in  this  respect:  prereproductive,  reproductive  and  postreproductive. 
A.  }.  Lotka  has  shown  from  theoretical  considerations  that  a  population 
will  tend  to  become  stable  and  have  a  constant  proportion  of  individuals 
of  these  three  ages.  Censuses  of  the  ages  of  plant  or  animal  populations 
thus  are  valuable  in  predicting  population  trends.  Rapidly  growing 
populations  have  a  high  proportion  of  young  forms.  The  age  of  fishes 
can  be  estimated  from  the  growth  rings  on  their  scales,  and  studies  of 


1000 


800 


Number 

of 
survivors 

per 
thousand 


600- 


400- 


200- 


Percenf  of  total  life  span 

Figure  37.7.  Survival  curves  of  four  different  animals,  plotted  as  number  of  sur- 
vivors left  at  each  fraction  of  the  total  life  span  of  the  species.  The  total  life  span  for 
man  is  about  100  years;  the  solid  curve  indicates  that  about  10  per  cent  of  the  babies 
born  die  during  the  first  few  years  of  life.  Only  a  small  fraction  of  the  human  popu- 
lation dies  between  ages  5  and  45  but  after  45  the  number  of  survivors  decreases 
rapidly.  Starved  fruit  flies  live  only  about  five  days,  but  almost  the  entire  population 
lives  the  same  length  of  time  and  dies  at  once.  The  vast  majority  of  oyster  larvae  die 
but  the  few  that  become  attached  to  the  proper  sort  of  rock  or  to  an  old  oyster  shell 
survive.  The  survival  curve  of  hydras  is  one  typical  of  most  animals  and  plants,  in 
which  a  relatively  constant  fraction  of  the  population  dies  off  in  each  successive  time 
period.  (Villee:   Biology.) 


ECOIOGY         773 

the  age  ratios  of  commercial  fish  catches  are  of  great  use  in  predicting 
future  catches  and  in  preventing  overfishing  of  a  region. 

The  term  biotic  potential,  or  reproductive  potential,  refers  to  the 
inherent  power  of  a  population  to  increase  in  numbers,  when  the  age 
ratio  is  stable  and  all  environmental  conditions  are  optimal.  The  biotic 
potential  is  defined  mathematically  as  the  slope  of  the  population  growth 
curve  during  the  logarithmic  phase  of  growth  (Fig.  37.6).  When  environ- 
mental conditions  are  less  than  optimal,  the  rate  of  population  growth 
is  less.  The  difference  between  the  potential  ability  of  a  population  to 
increase  and  the  actual  change  in  the  size  of  the  population  is  a  meas- 
ure of  environmental   resistance.  Even  when   a  population  is   growing 
rapidly  in  numbers,  each  individual  organism  of  the  reproductive  age 
carries  on  reproduction  at  the  same  rate  as  at  any  other  time;   the  in- 
crease in  numbers  is  due  to  increased  survival.  At  a  conservative  esti- 
mate, one  man  and  one  woman,  with  the  cooperation  of  their  children 
and  grandchildren,  could  produce  200,000  progeny  within  a  century,  and 
a  pair  of  fruit  flies  could  increase  to  3368  X  10^"  individuals  in  a  year. 
Since  optimal  conditions  are  not  maintained,  such  biologic  catastrophes 
do  not  occur,  but  the  situations  in  India  and  China  indicate  the  tragedy 
implicit  in  the  tendency  toward  overpopulation. 

The  sum  of  the  physical  and  biologic  factors  which  prevent  a  species 
from  reproducing  at  its  maximum  rate  is  termed  the  environmental 
resistance.  Environmental  resistance  is  often  low  when  a  species  is  first 
introduced  into  a  new  territory,  and  the  species  increases  in  number 
at  a  fantastic  rate.  The  introduction  of  the  rabbit  into  Australia,  and 
the  English  sparrow  or  Japanese  beetle  into  the  United  States,  are  ex- 
amples of  these.  As  a  species  increases  in  numbers  the  environmental 
resistance  to  it  also  increases  in  the  form  of  organisms  which  prey  upon 
it  or  parasitize  it,  and  the  competition  between  the  members  of  the 
species  for  food  and  living  space. 

When  a  few  individuals  enter  a  previously  unoccupied  area,  the 
increase  in  numbers  is  slow  at  first  (called  the  positive  acceleration 
phase),  then  becomes  rapid  and  exponential  (the  logarithmic  phase), 
slows  down  as  environmental  resistance  increases  (the  negative  accelera- 
tion phase)  and  finally  reaches  an  equilibrium  or  saturation  level  (Fig. 
37.6). 

346.        Population  Cycles 

Once  a  population  becomes  established  in  a  certain  region,  and 
has  reached  its  equilibrium  level,  the  numbers  will  vary  up  and  down 
from  year  to  year,  depending  on  variations  in  environmental  resistance 
or  on  factors  intrinsic  to  the  population.  Some  of  these  population 
variations  are  completely  irregular,  but  others  are  regular  and  cyclic. 
One  of  the  best  known  of  these  is  the  regular  9  to  10  year  cycle  of  abun- 
dance and  scarcity  of  the  snowshoe  hare  and  the  lynx  in  Canada  which 
is  based  on  the  records  of  the  number  of  pelts  received  by  the  Hudson 
Bay  Company.  The  peak  of  the  hare  population  occurs  about  a  year 
before  thi  peak  of  the  lynx  population  (Fig.  37.8).  Since  the  lynx  feeds 


774 


ANIMALS   AND   TH£IR   ENVIRONMENT 


on  the  hare,  it  is  obvious  that  the  lynx  cycle  is  related  to  the  hare  cycle. 

A  three  to  lour  year  cycle  ot  abundance  is  shown  by  lemmings  and 
voles,  small  mouselike  animals  living  in  the  northern  tundra  region. 
Every  three  or  lour  years  there  is  a  great  increase  in  the  number  of 
lemmings;  they  eat  all  the  available  food  in  the  tundra  and  then  migrate 
in  vast  numbers  looking  for  food.  They  invade  villages  in  hordes  and 
finally  many  reach  the  sea  and  drown.  The  numbers  of  arctic  foxes  and 
snowy  owls,  which  feed  on  lemmings,  increase  similarly  and  when  the 
lemming  population  decreases,  the  foxes  starve  and  the  owls  migrate 
south— there  is  an  invasion  of  snowy  owls  in  the  United  States  every 
three  or  four  years. 

Although  some  cycles  recur  with  great  regularity,  others  do  not.  For 
example,  in  the  carefully  managed  forests  of  Germany  the  numbers  of 
four  species  of  moths  whose  caterpillars  feed  on  pine  needles  were  esti- 
mated from  censuses  made  each  year  for  the  period  from  1880  to  1940. 
The  numbers  varied  from  less  than  one  to  more  than  10,000  per  thousand 
square  meters.  The  cycles  of  maxima  and  minima  of  the  four  species 
were  quite  independent  and  were  irregular  in  their  frequency  and  dura- 
tion. 

Attempts  to  explain  these  vast  oscillations  in  the  numbers  of  a 
species  on  the  basis  of  climatic  changes  have  been  unsuccessful.  At  one 
time  it  was  believed  that  the  cycles  were  caused  by  sunspots,  and  the 
sunspot  and  lynx  cycles  do  appear  to  correspond  during  the  early  part 
of  the  nineteenth  century.  However,  the  cycles  are  of  slightly  different 
lengths  and  by  1920  were  completely  out  of  phase,  with  sunspot  maxima 
corresponding  to  lynx  minima.  Attempts  to  correlate  these  cycles  with 
other  periodic  weather  changes  or  with  cycles  of  disease  organisms  have 
been  unsuccessful. 

The  snowshoe  hares,  for  example,  die  off  cyclically  even  in  the  ab- 
sence of  predators  and  in  the  absence  of  known  disease  organisms  or 
parasites.  The  animals  apparently  die  of  "shock,"  characterized  by  low 
blood   sugar,  exhaustion,   convulsions   and   death,   symptoms   which   re- 


160 


HARE 

LYNX 


1675 
TIME 


1685  1895 

IN      YEARS 


l«05 


1915 


1935 


Figure  37.8.  Changes  in  the  abundance  of  the  lynx  and  snowshoe  hare,  as  in- 
dicated by  the  number  of  pelts  received  by  the  Hudson's  Bay  Company.  This  is  a 
classic  example  of  cyclic  oscillation  in  population  density.  (Redrawn  from  MacLulich, 
1937.) 


ECOLOGY  775 

semble  the  "alarm  response"   induced   in  laboratory  animals  subjected 
to  physiologic  stress.  This  similarity  led  J.  J.  Christian  in   1950  to  pro- 
pose that  their  death,  like  the  alarm  response,  is  the  result  of  an  upset 
in    the    adrenal-pituitary   system.    As   the   population   density    increases, 
there  is  increasing  physiologic  stress  on  individual  hares  due  to  crowd- 
ing and  competition  for  food.  Some  individuals  are  forced  into  poorer 
habitats,  where  the  food  is  less  abundant  and  predators  more  abundant. 
The    physiologic    stresses    stimulate    the    adrenal    medulla    to    secrete 
epinephrine    which    stimulates    the    pituitary    to    secrete    more    ACTH 
(adrenocorticotropic  hormone).  This  in  turn  stimulates  the  adrenal  cor- 
tex to  produce  corticoids,  an  excess  or  imbalance  of  which  produces  the 
alarm  response  or  physiologic  shock.  In  the  latter  part  of  the  winter  of 
a  peak  year,  with  the  stress  of  cold  weather,  lack  of  food  and  the  onset 
of    the   new   reproductive   season    putting    additional   demands   on    the 
pituitary  to  secrete  gonadotropins,   the  adrenal-pituitary  system  breaks 
down,  carbohydrate  metabolism  (normally  under  its  control)  is  upset, 
and  low  blood  sugar,  convulsions  and  death  ensue.  This  is  an  attractive 
theory  but  the  appropriate  experiments  and  observations  in  the  wild 
to  test  it  have  not  yet  been  made. 

347.  Population  Dispersal 

Populations  have  a  tendency  to  disperse,  or  spread  out  in  all  direc- 
tions until  some  barrier  is  reached.  Within  the  area,  the  members  of 
the  population  may  occur  at  random  (this  is  rarely  found),  they  may  be 
distributed  more  or  less  uniformly  throughout  the  area  (this  occurs 
when  there  is  competition  or  antagonism  to  keep  them  apart),  or,  most 
commonly,  they  may  occur  in  small  groups  or  clumps.  Aggregation  in 
clumps  may  increase  the  competition  between  the  members  of  the  group 
for  food  or  space,  but  this  is  more  than  counterbalanced  by  the  greater 
survival  power  of  the  group  during  unfavorable  periods.  Aggregation 
may  be  caused  by  local  differences  in  habitat,  by  weather  changes,  re- 
productive urges  or  social  attractions.  Certain  animals  regularly  are 
found  spaced  apart;  they  establish  and  defend  certain  territories.  Many 
species  of  birds,  some  mammals,  reptiles,  fish,  crabs  and  insects  establish 
such  territories,  either  as  regions  for  gathering  food,  or  as  nesting  areas. 

348.  Biotic  Communities 

A  biotic  community  is  an  assemblage  of  populations  living  in  a  de- 
fined area  or  habitat;  it  can  be  either  large  or  small.  The  concept  that 
animals  and  plants  live  together  in  an  orderly  maner,  not  strewn  hap- 
hazardly over  the  surface  of  the  earth,  is  one  of  the  important  principles 
of  ecology.  Sometimes  adjacent  communities  are  sharply  defined  and 
separated  from  each  other;  more  frequently  they  blend  imperceptibly 
together  The  unraveling  of  why  certain  plants  and  animals  comprise 
a  given  community,  how  they  affect  each  other,  and  how  man  can  con- 
trol them  to  his  advantage  are  some  of  the  major  problems  of  ecologic 
research    In  trying  to  control  some  particular  species,  it  has  frequently 


776  ANIMALS   AND    THEIR   ENVIRONMENT 

been  found  more  effective  to  modify  the  community  than  to  attempt 
direct  control  of  the  species  itself.  For  example,  the  most  effective  way 
to  increase  the  quail  population  is  not  to  raise  and  release  birds  (arti- 
ficially "stocking"  the  area)  or  to  kill  off  predators,  but  to  develop  and 
maintain  the  particular  biotic  community  in  which  quail  are  most  suc- 
cessful. 

Although  each  community  may  contain  hundreds  or  thousands  of 
species  of  plants  and  animals,  most  of  these  are  relatively  unimportant 
and  only  a  few,  by  their  size,  numbers  or  activities,  exert  a  major  control 
of  the  community.  In  land  communities  these  major  species  are  usually 
plants,  for  they  both  produce  food  and  provide  shelter  for  many  other 
species,  and  many  land  communities  are  named  for  their  dominant 
plants— sagebrush,  oak-hickory,  pine,  and  so  on.  Aquatic  communities, 
with  no  conspicuous  large  plants,  are  usually  named  for  some  physical 
characteristic— stream  rapids  community,  mud  flat  community  and  sandy 
beach  community. 

In  ecologic  investigations  it  is  unnecessary  (in  fact  it  is  usually  im- 
possible) to  consider  all  of  the  species  present  in  a  commvmity.  Usually 
a  study  of  the  major  plants  which  control  the  community,  the  larger 
populations  of  animals,  and  the  fundamental  energy  relations— food 
chains— of  the  ecosystem  will  define  the  ecologic  relations  within  the 
commimity.  For  example,  in  studying  a  lake  one  would  first  investigate 
the  kinds,  distribution  and  abundance  of  the  important  producer 
plants,  and  the  physical  and  chemical  factors  which  might  be  limiting. 
Then  the  reproductive  rate,  mortality  rate,  age  distribution  and  other 
important  population  characteristics  of  the  important  game  fish  would 
be  determined.  A  study  of  the  kinds,  distribution  and  abundance  of  the 
primary  and  perhaps  secondary  consumers  of  the  lake  which  constitute 
the  food  of  the  game  fish,  and  the  nature  of  other  organisms  which 
compete  for  food  with  these  fish,  would  elucidate  the  basic  food  chains 
in  the  lake.  Quantitative  studies  of  these  would  reveal  the  basic  energy 
relationships  of  the  whole  ecosystem  and  show  how  efficiently  the  inci- 
dent energy  is  being  converted  into  the  desired  end  product,  the  flesh 
of  game  fish.  On  the  basis  of  this  knowledge,  the  lake  could  intelligently 
be  managed  to  increase  the  production  of  game  fish. 

Most  of  the  studies  of  biotic  communities  made  to  date  have  been 
of  regions  in  the  arctic  or  desert,  where  there  are  fewer  organisms,  and 
their  relatively  simpler  interrelations  are  more  easily  analyzed  and 
understood.  A  thorough  ecologic  investigation  of  a  particular  region 
requires  that  it  be  studied  throughout  the  year  for  a  period  of  several 
years.  The  physical,  chemical,  climatic  and  other  factors  of  the  region 
are  carefully  evaluated  and  an  intensive  study  is  made  of  a  number  of 
carefully  delimited  areas  which  are  large  enough  to  be  representative 
of  the  region  but  small  enough  to  be  studied  quantitatively.  The  num- 
ber and  kinds  of  plants  and  animals  in  these  "study  areas"  are  estimated 
by  suitable  sampling  techniques.  Estimates  are  made  periodically 
throughout  the  year  to  learn  not  only  the  components  of  the  com- 
munity at  any  one  time  but  also  their  seasonal  and  annual  variations. 


ECOLOGY  777 

Finally  the  biologic  and  physical  data  are  correlated,  the  major  and 
minor  communities  of  the  region  are  identified,  and  the  food  chains  and 
other  important  ecologic  relations  of  the  communities  and  the  particular 
adaptations  of  the  animals  and  plants  for  their  role  in  the  community 
are  studied. 

349.        Community  Succession 

Any  given  area  tends  to  have  an  orderly  sequence  of  communities 
with  time,  which  change  together  with  the  physical  conditions  and  lead 
eventually  to  a  stable  mature  community  or  climax  community.  The 
entire  series  of  communities  is  known  as  a  sere,  and  the  individual 
transition  communities  as  serai  stages  or  serai  communities.  These 
series  are  so  regular  in  many  parts  of  the  world  that  an  ecologist,  rec- 
ognizing the  particular  serai  community  present  in  a  given  area,  can 
predict  the  sequence  of  future  changes.  The  ultimate  causes  of  these 
successions  are  not  clear.  Climate  and  other  physical  factors  play  some 
role,  but  the  succession  is  directed  in  part  by  the  nature  of  the  com- 
munity itself,  for  the  action  of  each  serai  community  is  to  make  the  area 
less  favorable  for  itself  and  more  favorable  for  other  species  until  the 
stable,  climax  commimity  is  reached. 

One  of  the  classic  studies  of  ecologic  succession  was  made  on  the 
shores  of  Lake  Michigan  (Fig.  37.9).  As  the  lake  has  become  smaller 
it  has  left  successively  younger  sand  dunes,  and  one  can  study  the  stages 
in  ecologic  succession  as  one  goes  away  from  the  lake.  The  youngest 
dunes,  nearest  the  lake,  have  only  grasses  and  insects;  the  next  older 
ones  have  shrubs  such  as  cottonwoods,  then  evergreens,  and  finally  a 
beech-maple  climax  community  with  a  rich  soil  full  of  earthworms  and 
snails.  As  the  lake  retreated  it  also  left  a  series  of  ponds.  The  youngest  of 
these  contain  little  rooted  vegetation  and  lots  of  bass  and  bluegills. 
Later  the  ponds  become  choked  with  vegetation  and  smaller  in  size  as 
the  basins  fill.  Finally  the  ponds  become  marshes  and  then  dry  ground, 
invaded  by  shrubs  and  ending  in  the  beech-maple  climax  forest.  Man- 
made  ponds,  such  as  those  impounded  by  dams,  similarly  tend  to  be- 
come filled  up,  becoming  first  marshes,  then  dry  land. 

Ecologic  succession  can  be  demonstrated  in  the  laboratory.  If  a 
few  pieces  of  dry  hay  are  placed  in  some  pond  water,  a  population  of 
bacteria  will  appear  in  a  few  days.  Next,  flagellates  appear  and  eat  the 
bacteria,  then  ciliated  protozoa  such  as  paramecia  followed  by  predator 
protozoa  such  as  Didinium  emerge.  The  protozoa,  present  as  spores  or 
cysts  in  the  pond  water  or  attached  to  the  hay,  emerge  in  a  definite  suc- 
cession of  protozoan  communities. 

Biotic  communities  typically  show  a  marked  vertical  stratification, 
determined  in  large  part  by  vertical  differences  in  physical  factors  such 
as  temperature,  light  and  oxygen.  The  operation  of  such  physical  factors 
in  determining  vertical  stratification  in  lakes  and  the  ocean  is  quite 
evident  In  a  forest  there  is  a  vertical  stratification  of  plant  life  from 
mosses  and  herbs  on  the  ground,  then  shrubs,  low  trees  and  tall  trees. 


778 


ANIMALS   AND   THEIR   ENVIRONMENT 


IF  WE  WERE  TO   SIT  ON   THE   MIDDLE   BEACH  OF  TODAY... 


AS  THE  YEARS  GO  BY,  THE  PREVAILING 
WINDS   WOULD  PILE   OP   THE  SAND, 
WHICH   WOULD  BE  CAPTURED  BY 

GRASS... 


AS  THE  HUMUS   INCREASED  WE  WOULD 
FIND  OURSELVES    SUCCESSIVELY  AMONG 

THE  COTTONWOODS,  THE  PINES. 

THE  OAKS...  . 


AFTER  A  FEW   THOUSAND  YEARS  WE 
WOULD  BE  SURROUNDED   BY  A   BEECH 
AND   MAPLE  FOREST 


3AND-AT  THE  TIME  WE 
FIRST  SAT  ON    THE 
MIDDLE  BEACH. 


SAND-WASHED  UP  BY  THE 
WAVES  AND  BLOWN  BY  THE 
WIND,  SINCE  WE  FIRST 
SAT  ON   THE  BEACH. 


M 


HUMUS-ADDED  BY 
PLANTS    &  ANIMALS. 


Figure  37.9.     Diagram  of  the  succession  of  communities  with  time  along  the  shores 
of  Lake  Michigan  in  northern  Indiana.  (Allee,  et  al.:   Principles  of  Animal  Ecology.) 


ECOLOGY  779 

Each  of  these  strata  has  a  distinctive  animal  population.  Even  such 
highly  motile  animals  as  birds  have  been  found  to  be  restricted  to  cer- 
tam  layers.  Some  birds  are  found  only  in  shrubs,  others  only  in  the  tops 
of  tall  trees.  There  are  daily  and  seasonal  changes  m  the  populations 
found  in  each  stratum  and  many  animals  are  found  first  in  one  layer 
and  then  in  another  as  they  pass  through  their  life  history.  These  strata 
are  strongly  interdependent  and  most  ecologists  consider  them  to  be 
subdivisions  of  one  large  community  rather  than  separate  communities. 
Vertical  stratification,  by  increasing  the  number  of  ecologic  niches  in  a 
given  surface  area,  reduces  competition  between  species  and  enables 
more  species  to  exist  in  a  given  area. 

350.        The  Dynamic  Balance  of  Nature 

The  concept  of  the  dynamic  state  of  the  body  constituents  was  dis- 
cussed in  Chapter  4,  and  we  learned  that  the  protein,  fat,  carbohydrate, 
and  other  constituents  of  both  animal  and  plant  bodies  are  constantly 
being  broken  down  and  resynthesized.  Biotic  communities  are  con- 
stantly undergoing  an  analogous  reshuffling  and  the  concept  of  the 
dynamic  state  of  communities  is  an  important  ecologic  principle.  Not 
only  are  plant  and  animal  populations  constantly  subject  to  changes  in 
their  physical  and  biotic  environment  to  which  they  must  adapt  or  die, 
but  communities  undergo  a  number  of  rhythmic  changes— daily,  lunar, 
seasonal,  tidal,  etc.— in  the  activities  or  movements  of  their  component 
organisms  which  result  in  periodic  changes  in  the  composition  of  the 
community  as  a  whole.  A  population  may  vary  in  size,  but  if  it  outruns 
its  food  supply,  like  the  Kaibab  deer  or  the  lemmings,  equilibrium  is 
quickly  restored.  Communities  of  organisms  are  comparable  in  many 
ways  to  a  many-celled  organism,  and  exhibit  growth,  specialization  and 
interdependence  of  parts,  characteristic  form,  and  even  development 
from  immaturity  to  maturity,  old  age  and  death. 


Questions 

1.  Define  an  ecosystem.  Discuss  an  aquarium  of  tropical  fish  as  an  example  of  an  eco- 
system. 

2.  Differentiate  clearly  between  a  habitat  and  an  ecologic  niche. 

3.  Discuss  the  various  pathways  of  the  nitrogen  cycle.  What  can  man  do  to  increase  the 
supply  of  nitrates? 

4.  Define:    range  of  tolerance,   hibernation,  photoperiod,  biologic  potential,  environ- 
mental resistance. 

5.  Define  and  give  examples  of  commensalism,  mutualism  and  parasitism. 

6.  What  is  meant  by  a  food  chain?  Why  is  the  number  of  steps  in  a  food  chain  limited? 
Describe  a  food  chain  ending  in  a  bird  hawk. 

7.  AVhat  is  meant  by  a  survi^al  curve?  Discuss  the  importance  of  such  curves  to  a  life 
insurance  company. 

8.  Discuss  the  factors  that  tend  to  keep  relatively  constant  the  size  of  a  population  of 
animals  in  the  wild. 

9.  \Vhat  factors  tend  to  cause  cyclic  variations  in  the  size  of  a  population  of  anmials  in 

the  wild? 


780  ANIMALS   AND   THEIR   ENVIRONMENT 

10.  Define  and  give  an  example  of  a  biotic  community.  \Vhat  information  is  required  to 
define  a  particular  biotic  community? 

11.  Explain  why  there  is  a  tendency  for  there  to  be  an  orderly  sequence  of  communities 
leading  to  a  climax  community.  What  is  the  climax  community  in  your  region? 

Supplementary  Reading 

The  principles  of  ecology  are  clearly  and  interestingly  presented  by  E.  P.  Odum  in 
his  Fundamentals  of  Ecology.  A  standard  reference  work  in  animal  ecology  is  the  treatise 
by  Allee,  Emerson,  Park.  Park  and  Schmidt,  Principles  of  Animal  Ecology. 


CHAPTER  38 


The  Adaptation  of  Animals 
to  the  Environment 


A  COMPLETE  discussion  of  the  many  ways  in  which  living  things  have 
become  adapted  to  overcome  or  neutrahze  deleterious  aspects  of  the 
environment  or  to  take  advantage  of  favorable  factors  would  fill  a  large 
library.  In  this  chapter  we  shall  describe  and  give  examples  of  some  of 
the  general  types  of  adaptations  developed  by  animals  to  the  physical 
environment  and  to  other  living  things. 

Careful  study  of  any  group  of  animals  shows  that  some  have  gen- 
eralized structures  which  can  be  used  to  survive  in  a  wide  range  of  en- 
vironments. Others  animals  are  highly  specialized  for  some  particular 
mode  of  life.  Many  insects,  for  example,  have  become  adapted  to  living 
in  one  region  and  feeding  on  one  sort  of  material— one  or  a  few  kinds  of 
plants.  The  mouth  parts  of  certain  insects  are  adapted  for  sucking  nectar 
from  certain  kinds  of  plants;  others  are  specialized  for  sucking  blood, 
for  biting,  or  for  chewing  vegetation.  The  bills  of  various  kinds  of  birds 
and  the  teeth  of  various  kinds  of  mammals  may  be  highly  adapted  for 
particular  kinds  of  food  (Fig.  38.1).  Animals  that  are  highly  specialized, 
adapted  for  a  very  narrow  ecologic  niche,  will  have  some  advantage  as 


Finch 
(Seeds) 


^         Hawk 
(Predatory) 


Whip-poor-vy^ill 
(Catches  flying  insects) 


SKiramer 


(SHims  over  su.rf ace 
of  water) 


K       Woodcock 
(Probi-ng) 


Figure  38.1.     Diagrams  of  the  bills  of  a  variety  of  birds,  illustrating  their  adapta- 


tion to  the  type  of  food  eaten. 


781 


782 


ANIMALS   AND   THEIR   ENVIRONMENT 


long  as  that  environment  is  present,  but  are  at  a  great  evolutionary  dis- 
advantage when  the  environment  changes.  In  the  course  of  time,  organ- 
isms have  had  to  become  readapted  many  times  as  their  environment 
changed  or  as  they  migrated  to  a  new  environment.  As  a  result,  many 
animals  today  have  structures  or  physiologic  mechanisms  that  are  useless, 
or  even  somewhat  deleterious,  but  which  were  useiul  lor  survival  in 
earlier  times  when  the  organism  was  adapted  for  a  rather  different  en- 
vironment. 

351.        Adaptive  Radiation 

The  competition  for  food  and  living  space  tends  to  make  each  group 
of  organisms  spread  out  and  occupy  as  many  different  habitats  as  they 
can  reach  and  which  will  support  them.  The  evolution  from  a  single 
ancestral  group  of  a  variety  of  forms  which  occupy  different  habitats  is 
called  adaptive  radiation.  In  this  way  organisms  tap  new  sources  of 
food  and  escape  from  some  of  their  enemies.  The  placental  mammals 
provide  a  classic  example  of  adaptive  radiation,  for  from  a  primitive, 
insect-eating,  five-toed,  short-legged  creature  that  walked  with  the  soles 
of  its  feet  flat  on  the  ground  have  evolved  all  of  the  present-day  types. 


Figure  38.2.  Adaptive  radiation.  All  the  various  mammals  have  evolved  from  a 
common  ancestral  insectivore.  As  they  have  evolved  they  have  become  adapted  to  a  wide 
variety  of  environments.  The  insectivores  also  underwent  evolution,  resulting  in  a  number 
of  specialized  forms  such  as  the  mole  shown  in  the  center.  (Villee:  Biology.) 


THE   ADAPTATION   OF   ANIMALS   TO   THE   ENVIRONMENT  783 

There  are  dogs  and  deer,  adapted  for  terrestrial  life  in  which  running 
rapidly  is  important  for  survival;  squirrels  and  primates,  adapted  for 
life  in  the  trees;  bats,  equipped  for  flying;  beavers,  otters  and  seals  that 
maintain  an  amphibious  existence;  the  completely  aquatic  whales,  por- 
poises and  sea  cows;  and  the  burrowing  animals,  moles,  gophers  and 
shrews  (Fig.  38.2).  The  number  and  shape  of  the  teeth,  the  length  and 
number  of  leg  bones,  the  number  and  sites  of  attachment  of  muscles, 
the  thickness  and  color  of  the  fur,  and  the  nails,  claws  or  hoofs  at  the 
tips  of  the  toes  are  some  of  the  structures  which  are  involved  in  adapta- 
tion. In  Australia,  where  there  were  no  placental  mammals  until  very 
recently  when  they  were  introduced  by  man,  the  marsupials  underwent 
a  comparable  adaptive  radiation  to  fill  the  different  habitats  there.  With 
a  little  study  the  many  unusual  animals  of  Australia  can  be  recognized 
as  the  ecologic  equivalents  of  the  more  familiar  animals  native  to  the 
United  States. 


352.        Convergent  Evolution 

The  animals  living  in  the  same  type  of  habitat  tend  to  develop 
structures  which  make  them  superficially  alike,  even  though  they  may 
be  but  distantly  related.  This  evolution  of  similar  structures  by  animals 
as  they  become  adapted  to  similar  environments  is  known  as  convergent 
evolution,  or  adaptive  convergence.  The  dolphins  and  porpoises  (which 
are  mammals),  the  extinct  ichthyosaurs  (which  were  reptiles)  and  both 
bony  and  cartilaginous  fishes  have  evolved  streamlined  shapes,  dorsal 
fins,  tail  fins  and  flipper-like  fore  and  hind  limbs  which  make  them  look 
very  much  alike  (Fig.  38.3).  Seals  and  penguins  have  streamlined  shapes 
and  flipper-like  limbs  but  lack  the  dorsal  and  tail  fins  of  the  other  aquatic 
animals.  Moles  and  gophers,  in  adapting  to  a  burrowing  life,  have 
evolved  similar  fore  and  hind  leg  structures  adapted  for  digging,  but  the 
mole  is  an  insectivore  and  the  gopher  is  a  rodent. 


Figure  38  3  Convergent  evolution.  All  of  these  aquatic  \ertebrates  have  a  marked 
superficial  similarity  despite  their  distant  relationship,  because  of  their  adaptations  to 
similar  environments. 


784  ANIMALS    AND    THEIR   ENVIRONMENT 

353.  Structural  Adaptations 

Animals  become  adapted  tor  a  particular  mode  of  life  in  a  par- 
ticular environment  by  specializations  of  structure,  function,  color, 
chemical  composition  or  behavior.  Structural  adaptations  are,  perhaps, 
the  most  easily  recognized;  changes  in  the  size,  shape,  relative  propor- 
tion and  so  on  of  the  bones  and  muscles  of  the  body  which  adapt  for 
running,  jumping,  climbing,  gliding,  flying,  burrowing  or  swimming 
are,  in  general,  readily  evident.  The  adaptive  nature  of  some  other 
structural  modifications  only  becomes  clear  when  an  animal  is  studied 
in  its  environment. 

In  many  animals,  the  specialized  adaptation  to  a  certain  way  of  life 
now  evident  is  simply  the  latest  stage  in  a  series  of  adaptations.  For 
example,  both  man  and  the  baboon,  whose  immediate  ancestors  were 
tree-dwellers,  have  returned  to  the  ground  and  have  become  readapted 
for  walking  rather  than  climbing  trees.  The  process  of  readaptation  may 
be  quite  complicated.  The  contemporary  Australian  tree-climbing  kan- 
garoos are  the  descendants  of  an  original  ground-dwelling  marsupial. 
From  these  ground-dwellers  evolved  forms  which,  in  the  course  of  adap- 
tive radiation,  took  to  the  trees  and  developed  limbs  adapted  to  tree 
climbing  (or  perhaps  the  sequence  of  events  was  the  reverse— first  the 
evolution  of  specialized  limbs  and  then  the  adoption  of  an  arboreal 
life).  Some  of  these  tree-dwellers  eventually  left  the  trees  and  became 
readapted  for  ground  life,  accumulating,  by  mutation  and  selection, 
genes  for  hind  legs  which  were  longer,  stronger,  and  adapted  for  leap- 
ing. Some  of  these  readapted  ground-dwellers  then  returned  to  the  trees 
in  the  course  of  further  evolution,  but  their  legs  were  so  highly  special- 
ized for  leaping  that  they  could  not  be  used  for  grasping  a  tree  trunk. 
In  consequence,  the  present-day  tree  kangaroos  must  climb  like  bears, 
by  bracing  their  feet  against  the  tree  trunk.  A  comparison  of  the  feet  of 
the  existing  Australian  marsupials  reveals  all  the  stages  in  this  compli- 
cated, shifting  process  of  adaptation. 

354.  Physiologic  and  Chemical  Adaptations 

Since  one  of  the  major  struggles  among  organisms  stems  from  the 
competition  for  food,  any  mutation  which  enables  an  animal  to  utilize 
a  new  type  of  food  will  be  extremely  advantageous.  This  might  in- 
volve the  evolution  of  a  new  digestive  enzyme  or  of  a  new  energy-liberat- 
ing enzyme  system.  The  evolution  of  a  new  enzyme  system  enables  the 
sulfur  bacteria  to  obtain  biologically  useful  energy  from  hydrogen  sul- 
fide, a  substance  which  is  poisonous  to  almost  all  other  organisms.  The 
evolution  of  a  special  enzyme  for  reducing  disulfide  bridges  gives  the 
clothes  moth  its  unique  ability  to  digest  wool,  the  protein  molecules  of 
which  are  held  together  by  such  disulfide  bridges. 

A  mutation  that  decreases  the  growing  season  of  a  plant  or  the 
total  length  of  time  required  for  an  insect  or  other  animal  to  complete 
development  will  enable  it  to  survive  farther  from  the  equator,  thus 
opening  up  new  areas  of  living  space  and  new  sources  of  food  for  the 


THE  ADAPTATION   OF   ANIMALS   TO    THE   ENVIRONMENT  785 

new  organism.  Any  mutation  that  increases  the  Hmits  of  temperature 
tolerance  of  a  species— makes  it  more  eurythermic— may  enable  it  to  in- 
habit a  new  part  of  the  earth,  at  a  higher  latitude  or  higher  altitude. 

Marine  fish  are  usually  adapted  to  survive  within  a  certain  range 
of  pressures  and  thus  are  found  at  certain  depths.  Animals  adapted  to 
live  near  the  surface  are  crushed  by  the  terrific  pressures  of  the  deep, 
and  deep  sea  animals  usually  burst  when  brought  to  the  surface.  The 
whale  has  a  remarkable  ability  to  withstand  changes  in  pressure,  and 
can  dive  to  depths  of  2500  feet  without  injury.  Presumably  its  lung 
alveoli  collapse  when  the  pressure  on  the  body  reaches  a  certain  point 
and  then  gases  are  no  longer  absorbed  into  the  blood.  A  man  can  sur- 
vive pressures  as  great  as  six  atmospheres  if  the  pressure  is  increased  and 
subsequently  decreased  slowly.  The  increase  in  pressure  increases  the 
amoiuit  of  gas  dissolved  in  the  blood,  in  body  fluids  and  within  the 
cells.  If  the  pressure  is  decreased  suddenly,  the  gases  come  out  of  solu- 
tion and  form  bubbles  throughout  the  body.  Those  in  the  blood  impede 
circidation  and  bring  about  the  symptoms  of  diver's  disease,  or  "the 
bends."  The  pilot  of  a  jet  plane  may  gain  altitude  so  quickly  that  the 
atmospheric  pressure  is  reduced  rapidly  enough  to  bring  bubbles  of  gas 
out  of  solution  in  his  blood  and  produce  a  type  of  the  bends. 

355.       Color  Adaptations 

Adaptations  are  evident  in  the  color  and  pattern  of  animals  and 
plants  as  well  as  in  their  structure  and  physiologic  processes.  Ecologists 
recognize  three  types  of  color  adaptation:  concealing  or  protective  color- 
ation, which  enables  the  organism  to  blend  with  its  background  and 
be  less  visible  to  predators;  warning  coloration,  which  consists  of  bright, 
conspicuous  colors  and  is  assumed  by  poisonous  or  unpalatable  animals 
to  warn  ott  potential  predators;  and  mimicry,  in  which  the  organism 
resembles  some  other  living  or  nonliving  object— a  twig,  leaf,  stone,  or 
perhaps  some  other  animal  which,  being  poisonous,  has  warning  colora- 
tion. 

Concealing  coloration  may  serve  to  hide  an  animal  which  wants  to 
escape  the  notice  of  a  potential  predator  or  it  may  hide  a  predator  from 
his  intended  prey.  Examples  of  such  coloration  are  legion-the  white 
coats  of  arctic  animals,  and  the  stripes  and  spots  of  tigers,  leopards, 
zebras  and  giraffes  which,  though  conspicuous  in  a  zoo,  blend  impercep- 
tibly with  the  moving  pattern  of  light  and  dark  typical  of  their  native 
savanna.  Some  animals-frogs,  flounders,  chameleons,  crabs  and  others- 
can  change  color  and  pattern  as  they  move  from  a  dark  to  a  light  back- 
ground or  from  one  that  is  uniform  to  one  that  is  mottled  (Fig.  38.4). 

To  demonstrate  experimentally  that  concealing  coloration  does 
have  survival  value-that  what  looks  to  a  man  like  a  good  match  be- 
tween animal  and  background  will  also  fool  the  animal's  predators- 
investigators  fastened  grasshoppers  with  different  body  colors  to  plots  of 
different  colored  soils-light,  dark,  grassy  or  sandy.  After  these  plots  had 
been  exposed  to  the  predatory  activities  of  chickens  or  wild  birds  for  a 
given  length  of  time,  the  survivors  were   tabulated.   It  was  found  that 


786 


ANIMALS   AND    THEIR   ENVIRONMENT 


•    •* 


Figure   38.4.     An   experiment   to   sliow   the  remarkable   ability   of  the   floinider   to 

change  its  color  and  pattern   to  conform   with  its  backgromicl.   Left,  a   tiomider  on  a 

uniform,  light  backgroimd;  right,  the  same  fish  after  being  placed  on  a  spotted,  darker 
backgromul.  (\'illee:    Biology.) 

there  was  a  significantly  higher  percentage  of  survivors  among  those 
grasshoppers  which  matched  their  background. 

When  an  animal  is  protected  by  poison  iangs,  stinging  mechanisms, 
or  some  chemical  which  gives  it  a  noxious  taste,  it  is  to  its  advantage  to 
have  this  lact  widely  advertised.  In  fact,  many  animals  with  such  pro- 
tective adaptations  do  have  warning  colors.  A  European  species  of  toad, 
lor  example,  has  skin  glands  which  secrete  an  unpleasant,  unpalatable 
substance.  Its  belly  is  bright  scarlet,  and  whenever  a  potential  predator, 
such  as  a  stork,  swoops  over  a  congregation  of  these  toads,  they  flop  on 
their  backs,  exposing  their  scarlet  bellies  as  a  warning.  The  storks  and 
other  birds  apparently  become  conditioned  to  the  association  of  the  red 
color  and  the  bad  taste,  and  do  not  try  to  eat  the  toads. 

Other  animals  survive  by  mimicking  one  of  these  protectively  col- 
ored animals.  Some  harmless,  defenseless  and  palatable  animals  have 
evolved  to  be  almost  identical  in  shape  and  color  with  a  poisonous  or 
noxious  animal  of  quite  a  different  family  or  order,  and,  being  mis- 
taken for  it  by  predators,  are  left  alone.  Examples  of  mimicry  are  par- 
ticularly common  among  tropical  insects.  This  type  of  adaptation  is 
successful  only  where  there  are  many  more  genuinely  disagreeable  or 
dangerous  organisms  than  forms  which  mimic  them.  Obviously  if  a 
predator  finds  that  any  considerable  percentage  of  the  animals  with  a 
particular  shape  and  color  are  palatable,  he  will  not  be  conditioned  to 
avoid  them. 

The  reality  of  the  selective  advantage  of  color  adaptations  has  been 
much  debated.  It  has  been  argued  that  animal  vision  may  be  quite 
different  from  human  vision;  that  animals  may  be  color-blind,  or  per- 
haps able  to  see  light  in  the  ultraviolet  or  infra-red  part  of  the  spectrum, 
and  therefore  that  an  animal  which  appears  to  be  protectively  colored 
to  human  eyes  may  be  readily  evident  to  its  natural  predators.  How- 
ever, many  experimental  studies,  such  as  the  grasshopper  experiment 
cited  previously,  have  shown  that  protective  coloration  does  has  survival 
value. 

Color  and  patterns  may  serve  to  attract  other  organisms  when  such 
attraction  is  necessary  for  survival.  The  red  and  blue  ischial  callosities  of 
monkeys,  and  the  extravagantly  colored  plumage  of  many  birds,  appar- 
ently have  an  attraction  for  the  members  of  the  opposite  sex.  The  vivid 
colors  of  flowers  appear  to  attract  the  birds  or  insects  whose  activities 


THE  ADAPTATION   OF   ANIMALS    TO   THE   ENVIRONMENT  787 

are  needed  to  insure  the  pollination  of  the  plant  or  the  dispersal  ol  its 
seed. 

356.        Adaptations  of  Species  to  Species 

The  evolutionary  adaptation  of  each  species  has  not  occurred  in  a 
biologic  vacuum,  independently  of  other  organisms.  On   the  contrary, 
the  adaptation   of  each  species   has   been  influenced  markedly   by   the 
concurrent  adaptations  of  other  species.  As  a  result  of  this,  many  types  of 
interdependencies  between  species  have  arisen,  some  of  the  clearest  and 
best  understood  of  which  involve  insects.  Insects  are  necessary  for  the 
pollination  of  a  great  many  plants;  the  plants  are  so  dependent  on  these 
insects   that  they  are   unable   to  become  established  in  a  given  region 
unless  those  particular  insects  are  present.  The  Smyrna  fig,  for  example, 
could  not  be  grown  in  California,  even  though  all  climatic  conditions 
were   favorable,   until   the    fig  insect,    which   pollinates   the   plant,   was 
introduced.   Birds,  bats,  and  even   snails  serve   as  pollinators  for   some 
plants  but  insects  are  the  prime  animals  with  this  function.  Flowering 
plants  have  evolved  bright  colors  and  strong  fragrances,  presumably  to 
attract  insects  and  birds  and  ensure  pollination.  There  has  been  some 
doubt  as  to  whether  insects  can  detect  these  colors  and  odors,  but  the 
experiments  of  Karl  von  Frisch    (p.  338)  show  that  honeybees,  at  least, 
can  differentiate  colors,  shapes  and  scents  and  are  guided  in  their  visits 
to  flowers  by   these   stimuli. 

Some  of  the  species  to  species  adaptations  are  so  exact  that  neither 
one  can  exist  without  the  other.  The  yucca  plant  and  the  yucca  moth 
have  evolved  to  a  state  of  complete  interdependence.  The  yucca  moth, 
by  a  series  of  instinctive  acts,  goes  to  a  yucca  flower,  collects  some  pollen 
and  takes  it  to  a  second  flower.  There  it  pushes  its  ovipositor  through 
the  wall  of  the  ovary  of  the  flower  and  lays  an  egg.  It  then  carefully 
places  some  pollen  on  the  stigma.  The  yucca  plant  is  fertilized  and 
produces  seeds  on  a  few  of  which  the  larva  of  the  yucca  moth  feeds. 
The  plant  produces  a  large  number  of  seeds  and  can  easily  spare  the 
ones  eaten  by  the  moth  larva. 

357.        The  Distribution  of  Animals 

Three  major  habitats  can  be  distinguished,  marine,  fresh-water  and 
terrestrial.  No  animal  is  found  in  all  three  major  habitats  and,  mdeed, 
no  animal  is  found  everywhere  within  any  one  of  these.  Every  species 
of  animal  and  plant  tends  to  produce  more  offspring  than  can  survive 
within  the  normal  range  of  the  organism.  There  is  a  strong  population 
pressure  tending  to  force  the  individuals  to  spread  out  and  become  es- 
tablished in  new  territories.  Competing  species,  predators,  lack  of  food, 
adverse  climate  and  the  unsuitability  of  the  adjacent  regions,  perhaps 
due  to  lack  of  some  requisite  physical  or  chemical  factor  act  to  counter- 
balance the  population  pressure  and  prevent  the  spread  of  the  species. 
Since  all  of  these  factors  are  subject  to  change,  the  range  of  a  species 
may  change  suddenly.  The  range  of  a  species  tends  to  be  dynamic  rather 
than  stati?   The  spread  of  a  species  is  prevented  by  geographic  barriers 


788 


ANIMALS   AND    THEIR   ENVIRONMENT 


THE   ADAPTATION   Of   ANIMALS    TO   THE   ENVIRONMENT  ygQ 

such  as  oceans,  mountains,  deserts  and  large  rivers  and  tacilitated  by 
"highways"  such  as  land  connections  between  continents.  The  present 
distribution  of  the  species  of  animals  is  determined  by  the  barriers  and 
highways  that  exist  and  have  existed  in  the  geologic  past.  The  biogeo- 
graphic  realms,  discussed  on  page  735,  are  regions  made  up  of  whole 
continents,  or  of  large  parts  of  a  continent,  separated  by  major  geo- 
graphic barriers,  and  characterized  by  the  presence  of  certain  unique 
animals  and  plants.  Within  these  biogeographic  realms,  and  established 
by  a  complex  interaction  of  climate,  other  physical  factors  and  biotic 
factors,  are  large,  distinct,  easily  differentiated  community  units  called 
biomes.  In  each  biome  the  kind  of  climax  vegetation  is  uniform— grasses, 
conifers,  deciduous  trees— but  the  particular  species  of  plant  may  vary 
in  different  parts  of  the  biome.  The  kind  of  climax  vegetation  depends 
upon  the  physical  environment  and  the  two  together  determine  the 
kinds  of  animals  present.  The  definition  of  a  biome  includes  not  only  the 
actual  climax  community  of  a  region  but  also  the  several  intermediate 
serai  communities  that  precede  the  climax  community. 

358.        Terrestrial  Life  Zones 

Some  of  the  biomes  recognized  by  ecologists  are  tundra,  coniferous 
forest,  deciduous  forest,  broad-leaved  evergreen  subtropical  forest, 
grassland,  desert,  chaparral  and  tropical  rain  forest.  These  biomes  are 
distributed,  though  somewhat  irregularly,  as  belts  around  the  earth 
(Fig.  38.5),  and  as  one  travels  from  the  equator  to  the  pole  he  may  tra- 
verse tropical  rain  forest,  grassland,  desert,  deciduous  forest,  coniferous 
forest,  and  finally  reach  the  tundra  in  Northern  Canada,  Alaska  or 
Siberia.  Since  climatic  conditions  at  higher  altitudes  are  in  many  ways 
similar  to  those  at  higher  latitudes,  there  is  a  similar  succession  of  biomes 
on  the  slopes  of  high  mountains  (Fig.  38.6).  As  one  ascends  from  the 


Snow  t  ICC 

Vf-fJ^T—  S«OW     LINC 

MOSSES    t    tlCMCNS 
>^^^^\  LOW      HERBACEOUS      VE&CTATIOM 

-IREC    U~t 


Figure  38.6.     The  correspondence  of  the  life  zones  at  high  altitudes  and  at  high 
latitudes.  (Allee  et  al.:  Principles  of  Animal  Ecology.) 


790  ANIMALS    AND    THEIR   ENVIRONMENT 


jlipiMi 


*■. 


i»4". 


-  ^         't       "Li.**        ^  *■ 


?21^> 


wi; 


:..!».«^'.li_*.*-»t^3 


'.^Sj-x 


■iig.riiii 


Figure  38.7.  The  tundra  bionie.  Above,  View  of  the  low  tundra  near  Churchill, 
Manitoba,  in  July.  Note  the  numerous  ponds.  Below,  View  of  tundra  vegetation  show- 
ing "hmipy"  nature  of  low  tundra  and  a  characteristic  tundra  bird,  the  willow  ptar- 
migan. (Lower  photo  by  C.  Lynn  Haywood.) 


San  Joaquin  Valley  of  California  into  the  Sierras,  one  passes  from  desert 
and  chapparal  through  deciduous  forest  and  coniferous  forest  to,  above 
timberline,  a  region  resembling  the  tundra  of  the  Arctic. 

Tundra.  The  tundra  biome  (Fig.  38.7),  foimd  in  northern  North 
America,  northern  Europe  and  Siberia,  is  characterized  by  low  tempera- 
tures and  a  very  short  growing  season.  The  plants  are  lichens,  mosses, 
grasses  and  a  few  low  shrubs.  The  chief  animals  are  the  caribou  of 
North  America  and  the  reindeer  of  Europe  and  Siberia,  the  musk  ox, 
arctic  hare,  arctic  fox,  lemming,  snowy  owl  and  ptarmigan.  These  are 
joined  during  the  short  summer  by  many  migiatory  birds  and  by  large 
numbers  of  insects,  especially  mosquitoes  and  black  flies. 


THE   ADAPTATION   OF   ANIMALS    TO   THE   ENVIRONMENT 


791 


Northern  Coniferous  Forest.  This  biome,  stretching  across  both 
North  America  and  Eurasia  just  south  of  the  tundra,  has  long,  cold 
winters,  cool  summers,  and  moderate  amounts  of  rainfall  or  snow.  The 
forest  is  made  up  of  spruce,  pine,  fir  and  cedar  trees  which  grow  very 
densely,  shading  the  ground  so  that  herbs  and  shrubs  do  not  grow  well. 
The  forest  floor  is  typically  covered  with  a  thick  layer  of  needles  from 
the  evergreen  trees.  The  snowshoe  hare,  lynx,  wolf,  moose,  marten, 
fisher,  wolverine,  some  small  rodents,  grouse,  jays  and  a  few  reptiles  and 
amphibians  are  found  in  the  forest  or  in  the  occasional  patches  of  open 
grassland  interspersed  in  the  forest. 

Temperate  Deciduous  Forest.  The  areas  with  abundant,  evenly 
distributed  rainfall  and  moderate  temperatures  and  with  distinct  summer 
and  winter  seasons,  e.g.,  eastern  North  America,  Europe,  eastern  China, 
Japan  and  the  east  coast  of  Australia,  were  originally  covered  with  ex- 
tensive forests  of  beech,  maple,  oak,  hickory  and  chestnut  trees.  Most 
of  these  forests  have  now  been  replaced  by  cultivated  fields.  The  animals 
that  live  in  the  temperate  deciduous  forests  of  North  America  include 
Virginia  deer,  bears,  squirrels,  foxes,  bobcats,  wild  turkeys,  woodpeckers 
and  thrushes  and  many  snakes  and  amphibians. 

Broad-leaved  Evergreen  Subtropical  Forest.  In  regions  of  fairly 
high  rainfall,  but  where  the  temperatures  are  generally  higher  and  the 
differences  between  winter  and  summer  are  less  marked,  as  in  Florida, 
the  characteristic  trees  are  live  oaks,  magnolias,  tamarinds  and  palms, 
with  many  vines  and  epiphytes  such  as  orchids  and  Spanish  moss.  A 
rich  fauna  of  insects  and  arachnids,  many  amphibia  and  reptiles  such 
as  the  alligator  and  coral  snake  are  found  in  this  biome. 

Grasslands.     This  biome  (Fig.  38.8)  occurs  where  rainfall  is  about 


Fiaure  38  8      The  grassland  biome;  chavaccnstic  animals  ..1  the  African  grasslands 
zebra  'and    wfldebeest.'  Kruger    National    Park,    Transvaal.    (Photograph    by    Herbert 
Lang.) 


792  ANIMALS   AND   THEIR   ENVIRONMENT 


^'*         ^^    ji..: 


Figure  38.9.  Two  types  of  desert  in  western  North  America,  a  "cool"  desert  in 
Idaho  dominated  by  sagebrush  (above)  and  (below)  a  rather  luxuriant  "hot"  desert  in 
Arizona,  with  giant  cactus  (Saguaro)  and  palo  verde  trees,  in  addition  to  creosote 
bushes  and  other  desert  shrubs.  In  extensive  areas  of  desert  country  the  desert  shrubs 
alone  dot  the  landscape.  (Upper  photograph  by  U.  S.  Forest  Service,  lower  by  U.  S. 
Soil  Conservation  Service.) 


THE   ADAPTATION   Of   ANIMALS   TO    THE   ENVIRONMENT 


793 


10  to   30  inches  per   year,   insufficient  to  support  a  forest,   yet  greater 
than  that  ot  a  true  desert.  Grasslands  are  usually  found  in  the  interiors 
of  continents— the  prairies  ot  western  United  States  and  those  of  Argen- 
tina, Australia,  southern  Russia  and  Siberia.  It  appears  that  early  human 
civilizations  developed  in  this  grasslands  region,  where  early  man  raised 
grazing  animals  and  cultivated  and  selected  the  grasses  to  produce  his 
prime  food  plants,   the  cereals  such  as  wheat  and  rye.  The  animals  of 
the  grasslands  are  either  grazing  or  burrowing  mammals— bison,   ante- 
lope,  zebras,   rabbits,   ground  squirrels,   prairie  dogs   and  gophers— and 
birds  such  as  prairie  chickens,  meadow  larks  and  rodent  hawks.  There 
is  a  broad  band  of  tropical  grassland  or  savanna  in  Africa  lying  be- 
tween the  Sahara  desert  and  the  tropical  rain  forest  of  the  Congo  basin. 
Although  the  annual  rainfall  is  high,  as  much  as  50  inches,  there  is  a 
distinct  dry  season  from  June  to  August  which  prevents  the  develop- 
ment of  forests.  There  are  great  numbers  and  many  different  kinds  of 
grazing  animals  in   this  region,  together  with  predators  such   as  lions, 
rhis  is  the  storied  "big  game  country"  of  Africa.  Kangaroos  and  walla- 
bies are  the  grazing  animals  of  the  Australian  grasslands  that  are  eco- 
logically comparable  to  the  antelope  and  zebras  of  the  African  savanna. 
Deserts.     In  regions  with  less  than   10  inches  of  rainfall  per  year 
vegetation  is  sparse  and  consists  of  greasewood,  sagebrush  or  cactus  inter- 
spersed with   sparse   grasses   (Fig.    38.9).    In   the   brief  rainy  season   the 


m 


A^.-s:  / 


.^n:>: 


•^^-:r-, 


Fiaure  38  10  The  rain  forest  biome:  border  of  a  u.auny  ...  the  Ituri  Forest  of 
Nala^BelgLn  Congo.  (Photograph  by  Herbert  Lang;  courtesy  of  The  Arr^encan 
Museum  of  Natural  History.) 


794 


AJWW-t^j    ivr    '-: 


Cafifonua  desert   i  eted  idth  an  amaiinjg:  variety  of  wild 

flowas  and  girasjej..  nsomst  of  wfaidi  com|deie  tbeir  life  cyde  foom  seeii 
u>  seed  in  a  tew  weeLs.  The  animah  {Mresmt  are  Tq>tiks— litaids  and 
snakes—^  insects  and  bunovdi^  rodents  such  as  die  kangaroo  lat  and 
pocket  mDose^  bodi  of  which  are  aUe  to  live  widmut  drinking  water. 
br  esiractiii^  water  firam  seeds  and  succulent  cacti.  The  maiumals 
for^ie  at  n%ht  and  remain  in  dieir  buirows  durii^  the  day  to  niiniimie 
water  loss. 

Tfopjcvrf  f  €■■  Fofesf.  Low-King;  regions  near  the  equator,  with  an- 
nual rainEaJk  of  90  incht»  cv  more,  are  characterized  by  thick  rain 
Iwesis,  widi  an  enonnoos  variety  of  plants  and  animals  (Fig.  38.10). 
No  sii^iie  ^ecies  is  |Mresent  in  lai^  enou^  ninnbers  to  be  dominant 
The  valleys  of  the  Amazm^  Orinoco^  Om^P  and  Zambesi  rivers;,  and 
parts  of  Central  America^  Madagascar.  Malaya  and  New  Guinea,  are 
covered  with  trt^cal  rain  fnests.  The  vegetation  is  very  thid^  and 
vertically  stratified.  Tall  treess  shrubs,  vines  and  epiphyses  such  as  or- 
chids crowd  together,  and  many  animals,  are  arlxweal.  Uving  in  the  upper 
layers  of  the  vegetatkHO.  McMokeis^  lemurs^  marmosets,  sloths,  anteaters, 
many  reptiles,  a  wealth  of  Imlliantly  col<xed  birds,  butterflies,  beetles, 
WTtniws  and  other  insects  coffloprise  the  rich  fauna  of  the  rain  forest. 


Marine  Life  Zones 


~  cover  about  70  per  cent  of  the  earth's  surface 

..21V  rich  fauna  and  flc»a.  The  mass  of  organisms 

~  rreds  the  mass  of  terrestrial  animals  and  plants. 

'n  even  the  greatest  depths  of  the  ocean.  The 

ranges  froai  about  28°  F.  in  the  polar  seas 


359. 

and  have 
living  in 
Ljvii^  s.: 
tempera: 

to  90*^  F.  or  move  3pics,  but  the  aimual  range  of  variation  in 

any  locality  is  usuaiiy  not  mcnre  than  10  degrees.  The  oceans  are  in 
OHitinuous  circidation  Ixoug^  about  by  the  trade  winds  and  the  rota- 
ticMi  of  the  earth.  These  currents,  such  as  the  famous  Gulf  Scream^  }^P^^ 
current  and  Humboldt  current,  not  only  play  a  major  role  in  the  ecolog^ 
of  the  oceans  but  also  have  marked  effects  on  the  climate  and  other  eco- 
logic  factors  of  the  adjacent  land  ma<!<p<  The  major  cinrents  circulate 
in  a  clockwise  fastiion  in  the  northon  hemisphere  and  in  a  counter- 
clockwise direction  in  the  southern  hemisphere.  The  combination  of 
these  cnrroits  and  die  prevailing  winds  tends  to  cause  upwellings  of 
cool  water  laden  with  nutrients  frcmi  the  depths  to  the  surface  on  the 
west  coasts  of  the  continents.  These  upwellings  on  the  coasts  of  Cali- 
fornia, Peru  and  Portugal  support  large  populations  of  sardines,  tuna 
and  other  fish. 

.All  the  phyla  except  the  Onychophora,  and  all  the  classes  except  the 
amphibians,  centipedes,  millipedes  and  insects,  are  well  represented  in 
the  oceans;  ctenofrfiores,  brachiopods,  ecfainoderms,  chaetognaths,  and 
a  few  lesser  phyla  are  foimd  only  in  the  oceans.  The  ocean  has  clearly 
demarcated  regions  characterized  by  di£Eerent  physical  conditions,  and 
consequendy  inhabited  by  di£^ent  kinds  of  animals  and  plants.  Four 
main  regions  are  recognized:  (1)  the  Hdof  zone,  the  beach  between  the 


rMf    AOAPTATION    Of    ANI^AAli    TO    JHl    INVIHONMINl  795 

liigli   .111(1   low    II. Ic  111.11  ks;  CJ)  ihc  shallow  ioa,  ilic  ic|.>,i<)ii    lynif.;  ovci    ilic 
COlilmciil.il  shell    .iiul  cMciidiiifj;  oiil    (o  ;i  dcplli  oj    .iIxmM   .')()()  led;  (:i)   ili< 

pelagic    zono,    iIk     <)|>c cm    <\i(  ndni);    .lowii    ;i.s    I. it    .is   Miiili^lil    i.iii 

|)c'iu'li.ilr  (M.mc   '.(Id  (..    KlOO  ic.i);   m.!   (j)   ij,,'  ahyttal  zono,   ilic  (.<(.iii 
beyond   the  ( onliiiciii.d   slicll    imd   bciicidi    iIk    ikI.i^k    /one. 

I  he  111. nine  ()i).;;inisnis  ;ii<'  (hissed  ec  oIo^^k  .illy  ;is  plankton,  <»i).;.in 
isnis  lli.ii  llo.il  .111(1  ;iie  moved  passivfly  hy  die  (iiiieiils.  wind,  .md  w;ives, 
nekton,  .1111111. iK  di.ii  swim  .k  lively;  :iiid  benthot,  ilir  Ixdiom  dwell«-is 
lh;it  (i;ivvl  over,  hiiiiow  iiilo.  01  ,im-  . hi. k  lied  lo  die  1)0110111.  I  lie  id. ink 
loii  are  generally  very  small -pioio/oa,  alga*-,  small  laiv.d  l<»iiii'.  ol  .1 
variety  ol  animals,  and  a  lew  woims.  The  ncklon  iiuliide  llie  )ellylisli, 
squid,  lisli,  (miles,  seals  and  whales.  .Some  ol  die  heiidiic  animals,  <ial>s, 
snails,  slaifisli  and  some  worms,  crawl  over  die  siibsitate;  (lams  and 
worms  hiiiiow  iiilo  llie  sand,  mud  01  io(k  ol  die  se.i  holloiii,  .iiid  .1  lliitd 
group,  including  sponges,  sea  .iiieinoiies,  (r»i;i|s,  htyo/oans,  (iiiioidH, 
oysters,   barnacles  and    luni(ales,  aic  alladied    lo   rlie   subsUale. 

llie  tidal  /one  is  one  ol  the  most  lavoiablc  ol  all  llie  liabilali  ol 
the  woild,  with  an  abundaiue  ol  light,  oxygen,  (aibon  dM>xi<|e  and 
minerals  to  foster  a  rich  growth  ol  plants,  and  die  |>laiiis,  providing 
food  and  shelter,  make  it  an  ex(ellenl  habitat  \t>t  animals.  I  lie  pi. mi 
life  is  laigely  composetl  ol  algae,  wiih  only  a  lew  grawen  in  addiiK^n, 
There  is  keen  competition  atnong  the  plants  lot  space  and  am(;ng  the 
animals  for  space  and  lood,  so  the  lojms  living  lictc  have  had  to  evolve 
special  adaptatic>ns  to  survive. 

The  iniertidal  zone  is  exposed  Kj  tlie  air  twicer  daily  and  its  in- 
habitants have  had  to  develop  some  sort  ol  protection  against  desicca- 
tion. .Some  animals  avoid  this  by  burrowing  luio  the  cl;imp  sand  ot  r'xkh 
until  the  tide  returns;  others  liave  evolved  shells  wfiich  can  be  r  |f>sec| 
to  retain  a  supply  of  water  within  them.  .Many  plants  contain  jelly  like 
substances  such  as  agar  which  absorb  and  retain  large  cjuantitieo  of 
water.  One  of  the  outstanding  characteristics  c^l  this  region,  ol  (ourse, 
is  the  ever-present  action  of  the  waves,  and  the  organisms  in  adapting 
to  life  here  have  evolved  ways  of  resisting  wave  action.  Ifie  many  sea 
weeds  have  tough  pliable  bcjdies,  able  to  fjend  witfi  the  waves  without 
breaking,  while  the  animals  are  either  encased  in  hard  calc,areou<>  sheIN, 
such  as  those  of  molluscs,  bryozoa,  starfish,  Ijarnatles  and  (.r'd\)S,  or  a/* 
covered  by  a  strong  leathery  skin  that  can  fxrnd  without  fjreaking,  sucft 
as  that  of  the  sea  anemone  and  octopus. 

The  shallow  sea  region,  just  h>eyond  the  intertidal  /one,  is  alw 
thicklv  populated,  for  it  has  plenty  of  Hght  and  an  abuncJance  of  rrtin- 
erals  and  other  nutrients  for  plant  growth.  The  absence  of  the  j^ericxJjc 
exposure  to  air  and  the  lesser  wave  action  permit  many  plant*  and  ani- 
mals to  live  here  which  could  not  survive  in  the  interticbl  /one. 

The  pelagic  region,  distinguished  by  the  presence  of  sunligfit  and 
the  absence  of"  a  substrate,  is  populated  by  plankton  and  neku..n.  There 
are  no  large  seaweeds,  except  occasional  pieces  torn  from  their  anchw- 
age  in  die  shallow  sea,  and  fewer  microscopic  algae,  generally,  than  m 
the  shallow  sea.  There  are  protozoa  such  as  foraminifera  and  radio- 
laria,  small  Crustacea  and  many  larval  forms.  The  larger  amniaU  in- 


796 


ANIMALS   AND    THEIR   ENVIRONMENT 


Figure  38.11.  Sexual  parasitism  in  the  deep-sea  angler  fish,  Photocorynus  spi- 
niceps,  in  which  the  difficulty  of  one  sex  finding  the  other  is  met  by  permanent 
attachment  of  the  much  smaller  male  to  the  female.  The  union  is  so  complete  that 
the  male  has  no  independent  existence  at  all,  being  nourished  by  the  blood  of  the 
female  to  which  he  is  attached.  (After  Norman,  from  Allee  et  al.:  Principles  of 
Animal  Ecology.) 


elude  the  Portuguese  man-of-war,  jellyfish,  squid,  fishes  and  whales. 
Some  whales  are  equipped  with  strainers  and  feed  upon  the  microscopic 
plankton;  others  have  teeth  and  prey  upon  fish,  squid  and  other  whales. 

The  abyssal  region,  lying  below  the  pelagic,  is  characterized  by  the 
absence  of  light  and  the  consequent  absence  of  living  green  plants.  The 
waters  are  quiet  and  very  cold  and  the  pressure  is  stupendous.  The  ani- 
mals that  live  here  feed  upon  each  other  and  upon  the  bodies  of  dead 
plants  and  animals  that  are  constantly  settling  down  from  above.  Most 
of  the  fish  of  the  abyssal  region  are  small  and  peculiarly  shaped;  many 
are  equipped  with  luminescent  organs,  which  may  serve  as  lures  for  their 
prey.  The  majority  of  the  deep-sea  creatures  are  related  to  shallow-sea 
forms  and  are  believed  to  have  migrated  to  their  present  habitat  rela- 
tively recently  (by  geologic  standards),  for  none  is  older  than  the 
Mesozoic. 

Since  the  number  of  members  of  any  one  species  in  these  vast,  dark 
depths  is  small,  reproduction  is  more  of  a  problem  than  in  any  other 
region,  and  some  fish  have  evolved  a  curious  adaptation  to  ensure  that 
the  two  sexes  will  be  in  proximity  to  reproduce.  At  an  early  age  the 
male  becomes  attached  to  the  head  of  the  female  and  fuses  with  it.  There 
he  continues  to  live  as  a  small  (inch-long)  parasite  (Fig.  38.11).  In  due 
course  he  becomes  sexually  mature  and  when  the  female  lays  her  eggs, 
he  releases  his  sperm  into  the  water  to  fertilize  them. 

The  bottom  of  the  sea  is  a  soft  ooze,  composed  of  the  organic  re- 
mains and  shells  of  foraminifera,  radiolaria,  and  other  animals  and 
plants.  Many  invertebrates  live  at  great  depths  on  the  ocean  floor,  and 
characteristically  have  thin,  almost  transparent  shells,  whereas  the  related 


THE   ADAPTATION   Of   ANIMALS   TO   THE   ENVIRONMENT  797 

shallow-sea  forms,  exposed  to  wave  action,  have  hard,  thicker  shells.  Even 
the  greatest  depths  are  inhabited,  for  tube-dwelling  worms  have  been 
dredged  from  depths  of  24,000  feet,  and  sea  urchins,  starfish,  bryozoa  and 
brachiopods  have  been  found  at  depths  of  18,000  feet. 

360.        Fresh-Water  Life  Zones 

Fresh-water  habitats  may  be  divided  into  standing  water— lakes, 
ponds  and  swamps— and  running  water— springs,  creeks  and  rivers— 
though  of  course  each  intergrades  with  the  other.  The  biologic  communi- 
ties of  fresh-water  habitats  are  in  general  more  familiar  than  the  marine 
ones  and  many  of  the  animals  used  as  specimens  in  zoology  classes  are 
from  fresh  water— amebas  and  other  protozoa,  hydras,  planarians,  cray- 
fish and  frogs. 

A  lake  or  other  large  body  of  standing  water  can  be  subdivided, 
much  as  the  zones  of  the  ocean  are  distinguished,  into  the  shallow  water 
near  the  shore— the  littoral  zone— the  surface  waters  away  from  the  shore 
—the  limnetic  zone— and  the  deep  waters  under  the  limnetic  zone.  Some 
aspects  of  the  ecology  of  a  fresh-water  lake  were  discussed  in  section 
326.  The  ecologic  factors  which  may  be  limiting  a  fresh  water  habitat 
are  temperature,  turbidity  of  the  water,  the  amount  of  the  current  and 
the  concentration  of  oxygen,  carbon  dioxide  and  salts,  especially  phos- 
phates and  nitrates.  The  organisms  of  the  fresh-water  community  may 
also  be  subdivided  into  plankton,  nekton  and  benthos.  The  most  im- 
portant animal  members  of  the  community  are  fish,  insects  and  Crustacea 
and  the  plant  members  are  algae  and  aquatic  seed  plants. 

Fresh-water  habitats  change  much  more  rapidly  than  other  life 
zones;  ponds  may  become  swamps  and  swamps  become  filled  in  and  form 
dry  land  in  a  few  hundred  years.  Streams  are  constantly  eroding  their 
banks  and  changing  their  course.  Consequently  the  kinds  of  plants  and 
animals  present  may  change  markedly  and  show  ecologic  successions 
analogous  to  those  on  land.  The  large  lakes,  such  as  the  Great  Lakes, 
are  relatively  stable  habitats  and  their  populations  of  animals  and  plants 
change  much  less  rapidly.  A  large,  deep  lake  will  show  vertical  stratifica- 
tion with  marked  differences  in  temperature,  dissolved  gases,  light  and 
other  factors.  Particular  species  of  fish  and  other  animals  are  more  or 
less  restricted  to  a  certain  range  of  depths.  The  deeper  waters  of  many 
lakes  become  almost  depleted  of  oxygen  during  the  summer  In  the 
summer  the  top  layer  becomes  much  warmer  than  the  water  below  and 
the  circulation  of  water  is  essentially  restricted  to  the  warm  upper  layer. 
The  increased  activity  of  decomposer  organisms  in  the  lake  depths  ex- 
hausts the  supply  of  oxygen  and  the  lack  of  circulation  prevents  its 
renewal  by  the  algae  and  other  plants  in  the  upper  layers. 

The  ecologic  factors  which  are  most  important  in  limiting  the  dis- 
tribution of  animals  in  running  water  are  the  speed  of  the  current  the 
degree  to  which  basic  nutrients  can  be  obtained  from  the  adjacent  land 
or  from  connected  lakes,  and  the  amount  of  oxygen  present.  Running 
streams  are  in  general  well  oxygenated  and  the  --^.1^;-"^  J^  - 
usually  have  a  very  low  tolerance  to  reduced  oxygen  tension.  The  pollu- 


798  ANIMALS    AND   THEIR   ENVIRONMENT 

tion  of  Streams  by  sewage  or  industrial  wastes  may  kill  the  fauna  either 
by  direct  toxic  ellect  of  one  of  the  chemicals  or  indirectly  by  encouraging 
the  growth  of  decomposer  organisms  which  reduce  the  oxygen  tension 
in  the  water. 

The  adaptations  made  by  animals  for  survival  in  streams  are  con- 
cerned primarily  with  ways  of  maintaining  their  position  in  the  current. 
Some  have  developed  permanent  connections  with  the  substrate  by 
evolving  hooks,  suckers  or  glands  for  the  secretion  of  threads  or  sticky 
masses  with  which  to  attach  to  the  substrate.  Others  have  evolved  stream- 
lined, flattened  bodies  and  behavior  patterns  by  which  they  normally 
orient  themselves  so  as  to  head  upstream  and  swim  against  the  current. 

In  studying  any  animal  it  is  important  to  consider  whether  it  is  a 
generalized  or  specialized  representative  of  its  group,  what  adaptations 
it  has  made  for  survival  in  its  habitat,  and  what  its  ecologic  role  is  in 
the  population,  community,  biome  or  ecosystem  of  which  it  is  a  member. 

Questions 

1.  Define  and  give  an  example  of  adaptive  radiation. 

2.  Define  the  term  con\ergent  evolution.  Discuss  convergent  evolution  of  flying  animals 
and  of  burrowing  animals. 

3.  Differentiate  between  protective  coloration  and  mimicry.  Give  examples  of  each. 

4.  What  experiments  could  you  devise  to  determine  whether  color  adaptations  have  a 
selective  advantage? 

5.  Discuss  the  subdivisions  of  the  marine  habitat  and  give  examples  of  animals  found 
typically  in  each. 

6.  What  is  a  biome?  How  does  it  differ  from  a  biotic  community? 

7.  What  adaptations  are  needed  for  survival  in  the  intertidal  zone? 

8.  Differentiate  between  plankton  and  nekton.  Give  examples  of  each. 

9.  Why  are  similar  biomes  found  at  high  latitudes  and  high  altitudes?  Would  you  expect 
to  find  exactly  the  same  species  of  plants  and  animals  in  the  tundra  region  of  Alaska 
and  in  the  tundra  region  of  the  Andes?  Why? 

10.  Describe  briefly  the  characteristics  of  the  temperate  deciduous  forest  biome;  of  the 
desert  biome. 

Supplementary  Reading 

A  wonderfully  illustrated  account  of  animal  camouflage  is  to  be  found  in  H.  B.  Cott's 
Adaptive  Coloration  in  Animals. 


CHAPTER  39 


Parasitism 


The  relationship  between  two  species  of  organisms  in  which  one  species 
lives  in  or  on  the  body  of  the  second  and  at  its  expense  is  termed  para- 
sitism. The  species  that  derives  benefit  from  the  relationship,  and 
usually  cannot  survive  otherwise,  is  called  the  parasite,  and  the  species 
which  is  injured  or  affected  adversely  in  some  way  is  called  the  host.  This 
relationship  is  distinguished  from  mutualism  (p.  766),  in  which  both 
species  derive  some  benefit  from  the  association  and  cannot  survive  in 
nature  without  it.  The  term  symbiosis  has  been  used  with  several  dif- 
ferent meanings  in  the  past,  but  it  is  now  widely  used  as  a  general  term 
to  indicate  a  persistent  physical  association  between  two  different  species 
of  animals,  plants  or  micro-organisms  without  special  connotation  of 
harm  or  of  benefit  to  the  host  species. 

Green  plants,  fungi,  bacteria  and  viruses,  as  well  as  animals  of  many 
different  phyla,  may  be  parasites.  There  are  animals  parasitic  on  plants, 
and  plants  which  are  parasites  of  animals. 

361 .        Origin  of  Parasitism 

The  ecologic  relationship  of  parasitism  may  arise  by  any  of  several 
evolutionary  paths.  Predation,  commensalism  or  competition  between 
species  for  food  may  develop  into  parasitism.  Animals  which  are  saprozoic 
or  bacterial  feeders  are  to  some  extent  adapted  beforehand  to  living  in 
the  digestive  tract  and  can  become  parasites  directly  on  their  first  contact 
with  the  host  species. 

Predation  and  Parasitism.  ^Vhen  predation  evolves  into  para- 
sitism, the  diet  is  usually  changed  from  small  prey  to  a  large  host  species. 
The  mites,  for  example,'  which  are  small  relatives  of  the  spiders  include 
many  predators  that  hunt  down  and  kill  small  arthropods,  sucking  out 
their  body  juices.  Some  of  these  attack  large  prey  and,  in  the  process  of 
removing  a  full  meal,  do  not  kill  the  prey.  These  have  taken  the  first 
step  toward  parasitism.  Still  other  mites  not  only  do  not  remove  enough 
juice  to  kill  the  host  at  one  meal,  but  remain  on  the  host  between  meals 
so  that  much  of  their  life  is  spent  there.  These  are  fully  evolved  parasites. 
The  predaceous  mites  generally  attack  small  arthropods;  the  parasitic 
mites  attack  larger  arthropods  and  vertebrates. 

Leeches  show  a  similar  progression  from  predation  to  parasitism. 

799 


^00  ANIMALS    AND    THEIR   ENVIRONMENT 

Some  leeclies  feed  prinuuily  on  small  arthropods,  snails  and  worms. 
Others  feed  upon  vertebrates  when  they  are  available,  removing  a  meal 
of  blood  and  then  falling  off.  A  few  species  are  completely  parasitic  in 
the  sense  that  they  do  not  kill  the  host  but  live  in  continuous  association 
with  it.  Again,  predation  is  associated  with  small  invertebrates,  para- 
sitism with  vertebrates. 

Bats  provide  a  third  example  of  this,  but  only  the  first  step  toward 
parasitism  has  been  taken.  Most  bats  are  insectivorous  and  feed  upon 
insects  which  they  capture  in  Hight.  Certain  South  American  bats 
have  changed  their  food  source  to  large  mammals,  and  instead  of  killing 
and  consuming  their  prey  they  draw  blood  from  the  neck.  Vampire  bats 
feed  like  parasites,  but  in  their  failure  to  remain  with  the  host  and  their 
hunting  activities  they  are  still  predators. 

Commensalism  and  Parasitism.  Commensalism  and  parasitism  are 
easily  distinguished  in  theory,  but  in  practice  we  know  so  little  about 
many  organisms  that  we  cannot  be  sure  whether  an  association  that 
appears  to  be  commensalism  may  not  in  fact  be  parasitism.  We  can  only 
say,  for  example,  that  peritrich  ciliates  appear  to  be  commensals  on 
hydras,  feeding  upon  stray  bits  of  debris  without  harming  the  host.  The 
same  is  true  for  many  of  the  associations  found  in  the  sea.  In  some 
cases,  however,  the  innocence  of  the  commensal  is  dubious.  Certain 
marine  annelids  live  on  echinoderms,  especially  in  the  ambulacral 
grooves  of  starfishes.  In  general  these  are  commensals,  seeking  shelter  on 
the  host  and  feeding  on  "leftovers"  at  mealtime.  At  least  one  species, 
however,  has  been  observed  to  feed  on  more  than  leftovers,  poking  its 
head  into  the  host's  stomach  in  its  enthusiasm  to  share  the  meal.  The 
evolutionary  path  from  shelter-seeking  commensalism  to  food-robbing 
parasitism  is  not  rare. 

In  another  type  of  commensalism,  the  commensal  feeds  upon  ma- 
terials shed  and  no  longer  wanted  by  the  host.  This  may  develop  into 
parasitism  if  the  commensals  become  more  aggressive,  feeding  first  upon 
the  materials  before  they  are  shed  and  finally  feeding  on  living  tissues. 
Certain  kinds  of  mites  are  common  in  the  nests  of  birds  and  mammals 
and  feed  upon  the  shed  hair,  feathers  and  flakes  of  skin.  This  is  a  loose 
type  of  commensalism,  since  the  mites  do  not  live  directly  on  the  hosts. 
Other  mites  do  live  directly  on  the  hosts;  those  feeding  mostly  on  flaked 
skin  do  little  if  any  harm,  but  those  feeding  on  feathers  or  hair  may 
impair  the  plumage  or  fur.  These  might  be  called  commensals  with 
parasitic  tendencies.  Some  mites  have  extended  their  diet  to  include  the 
living  tissues  of  the  host  and  thus  are  completely  parasitic. 

Food  Competition  and  Parasitism.  The  development  of  parasitism 
from  food  competition  has  occurred  many  times  in  the  nematodes.  Both 
free-living  and  parasitic  nematodes  are  covered  by  a  thick  cuticle  which 
undoubtedly  has  facilitated  their  evolution  as  intestinal  parasites. 
Many  species  feed  on  fruits  and  vegetables  in  competition  with  other 
herbivores.  Related  to  these  are  intestinal  parasites  still  feeding  on  food 
bits,  but  from  the  security  of  the  host's  digestive  tract.  They  may  have 
evolved  from  free-living  forms  that  were  inadvertently  eaten. 


PARASITISM 


801 


Saprozo/c  Animals  and  Bacterial  Feeders.  Saprozoic  animals  may 
become  parasitic  if  they  can  withstand  the  digestive  enzymes  of  the  host 
and  the  low  oxygen  tension  in  its  digestive  tract.  Many  of  the  free- 
living  saprozoic  flagellates  have  parasitic  relatives  which  are  specialized 
so  that  they  can  grow  only  within  the  digestive  tract  of  particular  hosts. 
Other  relatives  are  intracellular  parasites,  especially  of  other  pro- 
tozoa. Other  saprozoic  parasites  such  as  cestodes  and  acanthocephalans 
apparently  became  saprozoic  after  they  became  parasites,  for  they  do 
not  have  free-living  saprozoic  relatives. 

The  bacterial  feeders  that  can  withstand  digestive  enzymes  and  low 
oxygen  tension  may  become  intestinal  commensals  and  feed  on  the  bac- 
terial population  of  the  large  intestine  which  otherwise  becomes  a  part 
of  the  feces.  Such  commensals  are  found  among  flagellates,  ciliates, 
amebas  and  roundworms.  Many  of  these  groups  have  close  relatives  that 
either  have  become  saprozoic  and  rob  the  host  of  digested  nutrients  or 
directly  attack  the  host  tissues.  The  most  striking  case  of  this  kind  is 
found  in  the  ameban  genus,  Entamoeba  (Fig.  39.1).  E.  coli  lives  in  the 
large  intestine  of  man  and  feeds  upon  bacteria.  Although  it  is  abundant 
in  the  tropics  and  by  no  means  rare  in  temperate  regions,  it  appears  to 
be  harmless.  It  has  a  close  relative,  E.  histolytica,  which  also  appears 
to  be  a  bacterial  feeder  normally  but  which  at  times  destroys  the  lining 
of  the  large  intestine  and  feeds  on  red  blood  corpuscles.  An  acute  at- 
tack by  these  parasites  can  produce  severe  dysentery  and  riddle  the 
entire  large  intestine  with  deep  ulcers  and  abscesses. 

Parasites  may  begin  as  ectoparasites  on  the  host  surface  or  as  endo- 
parasites  in  the  digestive  tract.  From  either  of  these  initial  positions 
the  parasites  may  become  endoparasitic  among  the  tissues  and  organs 
of  the  body,  or  even  become  intracellular,  living  within  the  host  cells. 


E.  Histolytica. 


Red  blood,  c&lls 


E.CoH 
Ba-cteria. 


Cyst 


Cyst 


Fiqure  39  1  A  parasite.  Entamoeba  histolytica  (left),  and  a  commensal  £  coU 
(rigMrof  the'humrn  large  intestine.  Active  amebas  above,  cysts  below  that  are 
passed  in  the  feces  and  can  infect  new  individuals. 


802  ANIMALS   AND    THEIR   ENVIRONMENT 


ChicKe-n  lotxse-  Cattle  lozise 

Figure  39.2.  Mallophaga.  Ventral  views  showing  biting  mandibles.  Most  species 
infect  birds  and  have  two  claws  on  each  foot  (left).  The  few  that  infect  mammals 
have  single  claws  (right)  resembling  those  of  the  Anoplura.  (After  Borror  and  DeLong.) 


362.        Ectoparasites 

Parasites  that  feed  at  the  surface  of  the  host  fall  into  three  major 
categories:  those  that  eat  dead  material  such  as  hair,  feathers,  flakes  of 
skin;  those  that  suck  blood;  and  those  that  feed  on  living  tissue. 

Parasites  Feeding  on  Dead  Material.  The  largest  group  of  ecto- 
parasites that  feed  on  dead  surface  material  is  an  order  of  insects,  the 
Mallophaga  (Fig.  39.2).  These  are  known  as  bird  lice,  since  most  of  them 
are  found  on  birds,  or  biting  lice,  because  they  have  jaws  for  biting  and 
chewing.  A  few  species  are  found  on  mammals.  They  do  not  directly 
injure  the  host  but  the  constant  irritation  of  their  presence  as  they  feed 
on  feathers  or  fur  can  produce  restlessness  and  insomnia  with  loss  of 
vigor  and  weight.  A  few  of  the  species  chew  down  into  the  shafts  of  the 
feathers  until  they  reach  live  tissues  and  draw  blood. 

Bloodsuckers.  The  list  of  animals  that  suck  blood  but  do  not  re- 
main with  the  host  between  meals  is  long:  leeches,  mites,  ticks,  lice, 
fleas,  bedbugs,  mosquitoes,  sandflies,  midges,  blackflies,  horseflies,  tsetse 
flies  and  vampire  bats.  The  true  parasites  that  remain  with  the  host  are  a 
much  smaller  group,  including  a  few  of  the  leeches,  a  few  ticks  and  mites, 
bedbugs,  the  sucking  lice  and  fleas.  The  two  major  groups  are  the  suck- 
ing lice  (order  Anoplura)  and  the  fleas  (order  Siphonaptera)  in  the  class 
Insecta. 

Sucking  lice  spend  their  entire  life  cycle  on  the  same  host  and  are 
transferred  to  new  host  individuals  through  body  contact  or  by  migra- 
tion from  hosts  that  die.  All  of  the  species  parasitize  mammals.  The 
head  louse,  the  body  louse  and  the  pubic  louse  or  "crab"  parasitize 
man  (Fig.  39.3).  Fleas  (Fig.  39.4)  are  free-living  as  larvae.  The  eggs  are 
dropped,  usually  in  the  nest  or  sleeping  place  of  the  host,  where  they 
hatch  into  small  worms  that  feed  on  debris.  After  pupation  they  emerge 
as  full-grown  adults  that  seek  the  proper  host.  Although  a  few  species 
parasitize  birds,  most  fleas  are  found  on  mammals. 

Bloodsuckers  are  not  only  harmful  as  parasites  but  are  dangerous 
as  carriers  of  disease  organisms.  During  the  fourteenth  century  about 
25  million  people,  one  fourth  of  the  population  of  Europe,  died  of 
bubonic  and  pneumonic  plague.  This  disease  is  caused  by  a  bacterium 
that  can  be   carried   in   rats   and   other   rodents   where   it  is  relatively 


PARASITISM 


803 


harmless.  It  is  transmitted  from  individual  to  individual  by  rat  fleas. 
Unfortunately  rat  fleas  occasionally  bite  man,  and  in  this  way  transmit 
the  disease  to  a  host  in  which  its  effects  are  devastating.  Fleas  can 
transmit  typhus  fever,  tularemia,  undulant  fever  and  other  diseases  as 
well  as  the  plague.  The  human  louse  will  transmit  typhus,  but  the  dis- 
ease kills  both  the  humans  and  the  lice.  In  regions  where  lice  are 
abundant  the  spread  of  typhus  can  reach  epidemic  proportions.  During 
World  War  I  louse-borne  typhus  killed  at  least  3,000,000  men.  The 
common  tick  Dermacentor  andersoni  (Fig.  39.5)  carries  more  pathogens 
than   any  other  parasite,   including  those   that  produce   spotted   fever, 


S^«J«SSJS>SS55SS«>KX«S!!SiS!SiS^^ 


1  %.^J  ,        v-^         . 


;^^X^"^^SSSSx^'?S:S§SSiii:f$iSS5S$SSSSs!^^ 


Head  louse 


Pubic  lou-Se 


Body  louse 


Figure  39.3.  Anoplura.  The  three  varieties  o£  human  lice.  The  head  louse, 
Pediciilus  hunianus  var.  capitis,  and  body  louse,  P.  h.  var.  corporis,  are  interfertile 
varieties  of  one  species  that  rarely  interbreed  because  one  lives  on  the  head,  laymg 
eggs  on  the  hairs,  while  the  other  hves  on  the  clothed  portion  of  the  body,  laying 
eggs  in  the  clothing.  The  pubic  louse,  Phthirus  pubis,  lives  in  the  pubic  region  and 
occasionally  in  the  armpits.  (After  Patton  and  Evans.) 


Figure  39  4.  Siphonaptera.  Life  cycle  of  the  rat  flea,  Xenopsylla  cheopts^  ^g?^ 
fall  o  the  ground  and  hatch  into  free-living  larvae.  These  feed  on  debris,  eventually 
pupate  ani  emerge  as  adults  that  seek  out  the  proper  host.  (Adult  after  Chandler; 
others  after  Patton  and  Evans.) 


304  ANIMALS    AND    THEIR   ENVIRONMENT 


n^: 


X'-'*^^   s  ^ 


,>J*.>.N       •   •  SVC 


Male 


Fetnstle 


Eirxgor^d  female 


Figure  39.5.  The  common  tick,  Dermacentor  andersoni.  Eggs  laid  on  the  ground 
hatch  into  six-legged  larvae  that  feed  on  small  mammals.  These  drop  off,  molt 
into  eight-legged  nymphs  that  return  to  small  mammals.  ,'\fter  molting  on  the  ground 
again  the  adults  attack  large  mammals.  The  females  become  enormous  after  mating 
and  eventually  fall  to  the  ground  to  lay  a  thousand  or  more  eggs.  (After  Chandler.) 

Colorado  tick  fever,  Q  fever,  tularemia,  undulant  fever  and  several 
forms  of  virus  encephalitis. 

Bloodsuckers  may  serve  as  alternate  hosts  for  the  pathogens  they 
carry.  The  role  of  the  mosquito  in  malaria  has  already  been  described. 
Dog  tapeworms  use  the  dog  flea  as  an  intermediate  host,  and  a  few  of 
the  nematodes  pass  parts  of  their  life  cycles  in  blackflies  and  horse- 
flies. African  sleeping  sickness,  a  disease  caused  by  protozoan  parasites, 
includes  the  tsetse  fly  as  an  alternate  host,  and  leishmaniasis,  a  related 
disease,  involves  sandflies. 

Paras/fes  feeding  on  Living  Tissues.  Ectoparasites  that  feed  di- 
rectly on  living  flesh  include  trematodes,  crustaceans,  mites  and  fly 
maggots.  Many  of  these  feed  on  blood  as  well  as  flesh.  Certain  trema- 
todes parasitize  the  gills  of  fishes,  crustaceans  parasitize  a  variety  of 
animals  including  other  crustaceans,  annelids,  molluscs,  echinoderms 
and  fishes,  and  the  mites  and  flies  parasitize  terrestrial  vertebrates.  Man 
may  be  infested  with  the  mange  or  itch  mites  (Fig.  39.6)  that  burrow 
in  the  skin,  or  with  chiggers,  a  mite  that  secretes  enzymes  which  dis- 
solve small  holes  in  the  host's  skin  for  feeding. 

The  maggots  of  several  kinds  of  flies  burrow  in  the  skin  of  mam- 


PARASITISM 


805 


mals.  One  of  the  common  and  curious  species  is  the  skin  botfly,  Derina- 
tobia  hominis  (Fig.  39.7).  The  maggots  burrow  into  the  skin  and  feed 
on  dissolved  flesh  and  blood.  In  Central  and  South  America  it  may 
be  so  abundant  that  the  hides  of  cows  are  riddled.  The  flies  burrow  in 
man  as  easily  as  in  other  mammals.  When  the  maggots  are  mature  they 
drop  to  the  ground  and  pupate.  The  female  fly  lays  her  eggs  not  on 
the  mammalian  host  but  on  the  lower  side  of  a  bloodsucking  arthro- 


Adalt  female. 


Man6e  mite  burrowing  in  skin 


Figure  39.6.  The  mange  mite.  Sarcoptes  scabiei.  These  pass  their  entire  Hfe 
cycle  on  the  host.  Eggs  laic!  in  the  bnrrows  hatch  into  young  mites  that  begin  burrows 
of  their  own.  Note  the  suckers  on  the  anterior  legs.  (After  Craig  and  Faust.) 


Fioure  39  7  The  skin  botfly,  Dermatobia  hominis.  The  adult  {A)  lays  its  eggs  on 
bloodsucking  arthropods  (B).  When  this  carrier  feeds  ^'^  J^^^^  ^^^  J^J.'^^J"^ 
burrow  into  the  skin  (C).  After  feeding  and  growing  beneath  the  skur  the  full-grown 
larva  (£>)  drops  to  the  ground,  pupates,  and  emerges  later  as  an  adult. 


806  ANIMALS   AND   THEIR   ENVIRONMENT 

pod,  usually  a  mosquito.  The  eggs  are  ready  to  hatch  in  eight  or  ten 
days.  When  the  mosquito  feeds,  the  warmth  of  the  mammal  stimulates 
the  maggots  to  emerge  and  drop  onto  the  host. 

Some  of  the  parasitic  copepods  (class  Crustacea)  are  attached  to  the 
host  by  their  antennae  while  they  feed  upon  the  host  with  the  mouth- 
parts.  In  other  species  the  antennae  grow  into  the  host  to  serve  as  an 
anchor,  and  in  still  others  this  anchor  serves  as  a  nutritive  organ  and 
soaks  up  nourishment  from  the  host.  Finally,  in  several  groups  of  para- 
sitic copepods  the  mouth  parts  are  degenerate  and  the  antennae  form 
a  root  system  that  spreads  throughout  the  host.  In  the  barnacles  this 
type  of  parasitism  has  developed  directly  from  nonparasitic  forms. 
Barnacles  usually  attach  to  inanimate  objects,  but  a  few  species  attach 
to  other  organisms.  In  some  of  these  the  attachment  organ,  the  antenna 
of  the  larva,  extends  into  the  host  as  an  anchor,  and  in  other  species 
it  becomes  a  nutritive  organ.  In  some  species  of  both  groups  the  root 
system  becomes  much  developed  while  the  body  left  outside  degen- 
erates completely,  giving  rise  to  endoparasitism. 

363.        Parasites  of  the  Digestive  Tract 

These  can  be  divided  into  several  categories:  those  that  eat  the 
host's  food,  those  that  are  saprozoic,  soaking  up  food  the  host  has 
digested,  those  that  feed  on  the  digestive  tissues  and  those  that  suck 
blood.  Intestinal  organisms  feeding  on  bacteria  are  usually  commensals 
and  do  little  or  no  harm  to  the  host. 

Intestinal  parasites  that  compete  with  the  host  for  food  may  cause 
malnourishment.  Nematodes  are  the  most  numerous  of  these  parasites. 
As  far  as  we  know,  all  nematodes  swallow  food,  and  many  species  live 
in  the  small  intestine  eating  partially  digested  material  supplied  by  the 
host.  They  are  often  harmless  in  the  sense  that  the  host  can  usually  eat 
enough  for  everybody,  but  if  they  become  too  numerous  or  if  the  host  is 
starved  the  host  suffers.  Ascaris  lumbricoides  is  so  prevalent  throughout 
the  world  that  Chandler  has  described  it  as  "one  of  man's  most  faithful 
and  constant  companions  from  time  immemorial."  Most  mammals  have 
their  species  of  ascaris-like  roundworms  and  it  is  unusual  to  open  a 
mammalian  intestine  and  not  find  them. 

Saprozoic  intestinal  parasites  live  in  the  small  intestine  where 
food  is  digested  by  the  host.  The  tapeworms  (class  Cestoda)  and  spiny- 
headed  worms  (phylum  Acanthocephala)  are  the  two  large  groups  of 
such  parasites.  A  number  of  flagellates  are  also  saprozoic.  In  man  the 
flagellate  Giardia  lavibUn  (Fig.  39.8)  applies  its  concave  ventral  surface 
to  an  intestinal  cell  and  attaches  by  suction.  It  feeds  by  absorbing 
nutrients  from  fluid  that  is  swept  past  by  the  flagella.  If  this  species 
is  so  abundant  as  to  carpet  the  gut  wall,  absorption  by  the  host  may 
be  impaired.  Tapeworms  attach  by  suckers  or  hooks  and  spiny-headed 
worms  bury  the  head  in  the  intestinal  wall.  Both  groups  lack  digestive 
tracts  and  soak  up  nutrients  through  the  integument.  Their  major  harm 
is  in  the  injuries  caused  by  attachment,  which  may  become  infected  and 


PARASITISM 


807 


ulcerated.   They   may   also   produce   systemic   disorders   such  as   allergy 
and  anemia. 

Those  intestinal  parasites  that  feed  on  the  intestinal  wall  include 
protozoa,  the  intestinal  flukes,  a  lew  roundworms  and  a  few  fly  larvae. 
Man  is  attacked  by  an  ameba,  a  flagellate  and  a  ciliate,  all  of  which 
live  in  the  large  intestine.  The  ameba,  Ent(unoeba  histolytica,  is  the 
most  harmful  and  has  already  been  described.  The  flagellate,  Triclio- 
mojias  honinis,  is  the  least  harmful.  It  feeds  primarily  on  bacteria  and 
debris  and  only  occasionally  produces  diarrhea  or  other  signs  of  dis- 
tress. At  such  times  it  is  suspected  of  feeding  on  the  intestinal  lining. 
The  ciliate,  Balantidium  coli,  is  injurious  but  uncommon.  It  digests  the 
intestinal  mucosa,  produces  ulcers  like  those  of  the  ameba,  and  can 
cause  death. 

Several  families  of  flukes  live  in  the  intestine  and  its  associated 
passages  (bile  ducts,  etc.).  Like  their  ectoparasitic  relatives  on  the  gills 
of  fishes,  these  trematodes  attach  by  the  ventral  or  posterior  sucker  and 
feed  through  the  oral  sucker,  scraping  oft  the  superficial  layer  of  cells. 
Their  damage  is  slight  unless  they  become  numerous. 

The  most  injurious  group  of  intestinal  parasites  is  the  bloodsuck- 
ing hookworms,  a  group  of  nematodes.  Their  effect  is  seldom  sudden 
or  catastrophic  but  is  chronic  and  insidious,  sapping  the  vitality  of  the 


Fiaure  39  8  Giardia  lamblia.  A,  Ventral  view  showing  two  nuclei.  B,  Lateral 
view  showing  attachment  to  host  intestinal  cell.  C,  Cyst  passed  in  the  feces,  capable 
of  infecting  a  new  host.  (After  Chandler.) 


808 


ANIMALS   AND    THEIR   ENVIRONMENT 


m^,,/m///////m/////////////////////////////////////////////////^^^^^ 


Figure  39.9.  Hookworm.  A,  Longitudinal  section  through  head  of  adult  showing 
mouthful  of  intestinal  wall  being  sucked.  Eggs  («)  pass  out  in  the  host  feces,  hatch 
in  the  soil  (C)  and  grow  to  the  infective  stage  (D).  These  penetrate  the  host  skin 
and  migrate  by  way  of  the  blood,  lungs,  and  throat  to  the  small  intestine.  {A  after 
Ash  and  Spitz;  others  after  Chandler.) 

host  and  undermining  his  health  year  after  year.  Two  species  are  com- 
mon in  the  small  intestine  of  man,  Ancylostoyna  duodenale  and  Necator 
americanus  (Fig.  39.9).  The  adult  gathers  a  bit  of  intestinal  lining  in 
its  mouth  and  sucks  blood  from  the  capillaries.  These  are  one-host 
parasites  with  a  free-living  larva.  Eggs  pass  out  in  the  feces  and  hatch 
in  the  soil,  where  the  larvae  develop  to  the  infective  stage.  Once  on 
the  host  they  bore  through  the  skin  into  the  blood,  are  swept  through 
the  circulatory  system  to  the  lungs,  where  they  burrow  into  the  air 
cavities,  crawl  up  the  bronchial  tubes,  and  are  swallowed.  In  warm, 
moist  climates  where  people  are  often  barefoot,  hookworms  are  common 
and  contribute  greatly  to  the  lethargy,  indifterence  and  poverty  of  man. 
In  recent  years  the  prevalence  of  hookworm  in  southeastern  United 
States  has  been  greatly  decreased  through  improved  health  habits  and 
economic  status. 


364.        Parasites  in  Body  Tissues 

Parasites  that  live  within  the  tissues  of  the  host  may  enter  through 
the  skin  or  from  the  digestive  tract.  Some  of  these  feed  upon  the  tissues; 
others  lie  among  the  cells  and  are  saprozoic.  The  two  largest  and  most 
important  groups  are  the  trypanosomes  (class  Flagellata)  and  the  blood 


PARASITISM 


809 


flukes  (class  Trematoda),  both  of  which  live  in  the  blood  stream.  Para- 
sites that  burrow  extensively  in  body  organs  include  some  trematodes, 
nematodes,  and  a  few  fly  maggots. 

Trypanosomes.  Trypanosomes  live  in  the  blood  of  all  kinds  of 
vertebrates  and  usually  are  transmitted  by  blood-sucking  arthropods  in 
which  a  part  of  the  life  cycle  is  passed.  Most  of  them  do  little  harm 
to  their  hosts  and  those  that  are  dangerous  are  believed  to  represent 
instances  in  which  the  trypanosomes  have  invaded  new  hosts.  Such  may 
be  the  case  with  African  sleeping  sickness,  a  disease  of  man  caused  by 
two  species  of  the  genus  Trypanosoma  (Fig.  39.10).  The  ancestral 
species,  T.  brucei,  is  common  in  many  African  wild  mammals  where  it 
is  harmless.  It  is  virulent  in  domestic  animals  such  as  horses  and  camels 
but  is  unable  to  attack  man.  Early  in  this  century  in  Rhodesia,  how- 
ever, the  population  of  native  mammals  was  greatly  reduced  and  the 
tsetse  flies  that  carry  T.  brucei  were  forced  to  feed  more  frequently  on 
humans.  In  1909  a  case  of  human  sleeping  sickness  caused  by  a 
trypanosome  very  similar  to  T.  brucei  was  discovered.  Since  then  there 
have  been  numerous  instances  of  human  infection  by  this  strain  of 
protozoa  called  T.  rliodesiense  although  it  is  probably  only  a  variety 
of  T.  brucei.  Trypansoma  gambiense  has  had  a  longer  association  with 
man  and  also  is  found  in  monkeys,  antelopes,  and  pigs.  It  originally 
was  found  in  central  Africa  where  it  produces  a  serious  but  not  devas- 
tating disease  of  man.  Late  in  the  nineteenth  century,  apparently  as  a 
result  of  exploration  by  whites,  the  organisms  were  carried  north  into 
Uganda  and  the  lake  region  where  the  human  population  had  not 
previously  been  exposed  to  the  disease  and  where  tsetse  flies  were  abun- 


y,,,,,,,,,,,,,y,,,,,.y.,y,,,,,y^///^///////////////.Y////////////////^^^^^ 


Fiaure  39  10  A,  African  sleeping  sickness.  Active  trypanosomes  in  the  blood  (B) 
are  sucked  up  by  the  tsetse  fly  (C).  The  protozoa  reproduce  nr  the  d.gestne  tract. 
mTgrate  to  the^salivary  glands  where  they  attach  to  the  walls  and  finally  become  infective, 
(D),  passing  into  a  new  host  during  salivary  secretion. 


810 


ANIMALS   AND    THEIR   ENVIRONMENT 


dant.  The  result  was  a  terrible  epidemic  ol  sleeping  sickness  that  killed 
two  thirds  ot  the  population  and  rendered  hirge  areas  ol  land  unin- 
habitable. Today  a  major  activity  ol  the  Uganda  government  is  the 
gradual  reclamation  ol  its  land  by  systematically  killing  off  all  ol 
tiie  large  mammals  that  carry  the  disease  and  iniect  the  tsetse  Hies. 

Alrican  sleeping  sickness  begins  with  lever  and  headache,  loUowed 
by  weakness  and  anemia.  The  patient  may  then  recover  partially  or 
completely.  Olten,  however,  the  trypanosomes  reach  the  central  nervous 
system  and  then  the  host  becomes  progressively  less  active,  repeatedly 
lalling  asleep  and  abhorring  exertion.  Emaciation,  coma  and  death 
ioUow  alter  several  weeks.  In  South  America  trypanosomes  cause  a  dis- 
ease involving  lever,  anemia  and  mental  disturbances.  The  parasites 
are  normally  lound  in  small  mammals  and  are  transmitted  to  man  by 
a  bloodsucking  bug  (order  Hemiptera). 

Blood  Flukes.  Blood  flukes  belong  to  the  lamily  Schistosomatidae 
and  iniect  birds  and  mammals.  Two  characteristics  distinguish  them 
from  other  trematodes:  the  sexes  are  separate,  and  the  cercariae  pene- 
trate directly  through  the  skin  ol  the  final  host  rather  than  being  eaten. 
Man  may  be  inlected  by  three  species  ol  the  genus  Schistosoma  (Fig. 
39.11).  Two  species  live  in  blood  vessels  near  the  digestive  tract  and 
their  eggs  appear  in  the  feces;  the  third  lives  in  vessels  near  the  bladder 


IN  FECES  •• 
S.  TT-iansonl  S.  japonicum> 


Eggs 
in  -water 


Miracidia 


Sna.ll  hosts 
IfCercaria  -^ 


IN  URINE: 
S.  "haematobium 


in.  I -water 


Miracidium. 
i 


Snaiil  host 


Figure  39.11.  Three  species  of  Schistosotna  that  infect  man.  In  severe  cases  (top 
figure)  the  body  is  emaciated  and  the  feet  edematous  while  the  spleen  is  greatly 
enlarged.  Eggs  hatch  on  contact  with  water  and  each  species  enters  its  own  par- 
ticular kind  of  snail  host.  Emerging  cercariae  penetrate  directly  into  the  human  skin. 
In  regions  where  these  parasites  are  prevalent,  children  usually  become  infected  as 
soon  as  they  start  playing  in  water. 


PARAsnisfA       811 

and  its  eggs  appear  in  the  urine.  They  are  frequently  found  in  pairs, 
the  broad  male  folded  around  the  long  slender  female. 

Infection  is  widespread  in  Africa,  the  Near  East  and  the  Orient, 
where  more  than  90  per  cent  of  the  human  population  may  carry  the 
worms.  The  disease  usually  passes  through  several  stages  of  fever,  pain 
and  diarrhea  without  serious  harm  and  then  continvies  for  years  as  an 
insidious  drain  on  body  vigor.  Occasionally,  however,  infection  may 
become  acute,  with  internal  bleeding,  secondary  bacterial  infection  and 
death.  The  Egyptian  government  considers  this  disease  to  be  a  major 
obstacle  in  the  path  of  the  country's  economic  progress.  At  the  request 
of  the  governments  concerned  the  W^orld  Health  Organization  has  major 
research  programs  aimed  at  the  control  of  this  disease  in  Egypt  and  in 
the  Philijjpines. 

Blood  flukes  infecting  birds  and  mammals  are  common  everywhere. 
Several  species  in  North  America  are  able  to  penetrate  the  skin  of  man 
should  he  enter  the  water  where  the  cercariae  occur.  They  burrow 
in  the  skin,  producing  "swimmer's  itch,"  but  are  unable  to  develop 
properly  and  soon  perish. 

Filariae.  Of  parasites  that  live  in  tissues  other  than  blood  the 
most  harmful  group  are  the  filarial  roundworms,  slender  nematodes 
several  centimeters  long  and  no  thicker  than  a  coarse  thread.  Adults 
burrow  beneath  the  skin  or  live  in  the  lymph  nodes  and  connective 
tissue,  releasing  minute  larvae  into  the  blood  stream.  The  larvae  may 
be  picked  up  by  some  bloodsucking  arthropod  and  thus  be  transmitted 
to  a  new  host.  A  common  but  relatively  harmless  example  is  the  African 
eye  worm,  Loa  loa  (Fig.  39.12),  which  burrows  beneath  the  skin  near 
the  eyes  and  often  can  be  seen  coiled  in  the  white  of  the  eye. 

The  filarial  genus  Wiichereria,  especially  W.  bancrojti  (Fig.  39.13), 
can  produce  a  serious  disease.  These  live  in  the  lymph  nodes,  lymph 
ducts,  and  in  the  connective  tissue  associated  with  various  glands.  They 
may  produce  little  effect,  but  interaction  of  parasite  and  host  often 
results  in  repeated  inflammation  of  the  lymphatic  ducts.  If  the  ducts 
become  obstructed  the  tissues  begin  to  swell,  producing  a  progressive 
enlargement   known   as   elephantiasis.    The    disorder   is   commonly   lo- 


Figore  39.12.     Adult  of  the  African  eye  worm,  Loa   loa,  visible  in   the  white  of 
the  eye.  (After  Fiilleborn.) 


812 


ANIMALS   AND    THEIR   ENVIRONMENT 


Human  Ho$t 


Mosc[uito  Host 


Figure  39.13.  Wuchereria  bancrofti.  Adult  worms  in  human  lymphatic  tissue  (A) 
release  microscopic  larvae  into  the  blood  (B).  If  these  are  taken  up  by  a  mosquito 
(C)  they  migrate  to  the  thoracic  muscle  where  they  metamorphose  and  grow  (D,  E,  F). 
The  infective  stage,  F,  migrates  to  the  proboscis  where  it  can  penetrate  into  man 
while  the  insect  is  feeding. 


Figure  39.14.  Trichinella  spiralis.  Larvae  encysted  in  muscle  (A)  mature  into  in- 
testinal worms  when  eaten  (D).  These  give  birth  to  larvae  that  burrow  into  the  host, 
encysting  in  muscle.  The  natural  reservoir  is  rodents  (C)  and  similar  animals  which 
eat  their  dead  (D).  Pigs  (£)  will  also  eat  dead  rodents.  Furthermore,  killed  rodents 
and  pig  scraps  are  fed  to  them  in  garbage  (F).  Man  can  become  infected  by  eating 
insufficiently  cooked  meat  containing  larvae. 


PARASITISM  813 

calized  in  a  lower  part  of  the  body  such  as  a  leg  or  the  scrotum  which 
may  become  tremendously  enlarged. 

Trichinella.     Another   kind   of  nematode,   Trichinella  spiralis,  bur- 
rows in  the  host  body  during  a  portion  of  its  life  cycle  (Fig.  39.14).  The 
adult  is  a  small  intestinal  parasite,  females  3  to  4  mm.  long,  males  1.5 
mm.    long.    They    are   ovoviviparous,   and   the    female   usually    burrows 
slightly  into  the  intestinal  wall  so  that  the  young  are  released  mto  the 
tissues.  These  larvae  (0.1  mm.  long)  are  distributed  throughout  the  body 
by  the  circulatory  system  and  eventually  burrow  into  striated  muscles. 
Within  the  muscle  they  grow  rapidly  to  a  length  of   1  mm.  and  then 
roll  into  a  spiral  form  embedded  in  cysts  between  the  muscle  cells.  This 
is  a  waiting  stage,  for  the  worms  will  develop  no  further  unless  the  meat 
is  eaten  by  another  host.  They  will  survive  in  this  condition  for  periods 
ranging  from  several  months  to  several  years.  If  the  meat  is  eaten  by  an 
appropriate  host  (man,  swine,  rodents,  cats,  sometimes  other  mammals) 
the  worms  are  digested  free  of  the  cyst  and  mature  in  about  four  days 
in  the  new  host's  intestine.  The  disease  trichinosis  is  caused  by  a  sudden 
heavy    infestation   and    is    manifested    in    two   stages.    While    the   adult 
females   are  burrowing  into  the   intestinal   wall  various  intestinal  and 
systemic  disorders,  including  diarrhea,  pain  and  fever,  may  result.  The 
second  stage  is  caused  by  the  activities  of  the  larvae  as  they  penetrate 
the  muscles,  and  is  accompanied  by  intense  muscular  pain,  disturbances 
of  muscular  activity,  and  sometimes  death.   Unlike  most  parasites   Tri- 
chinella is  most  abundant  in  temperate  climates.  Although  its  natural 
reservoir  is  probably  in  rodents,  wild  pigs  and  carnivorous  mammals, 
it  is  common  only  where  it  has  found  especially  suitable  conditions  on 
swine  farms  where  pigs  are  fed  raw  garbage,  including  pig  scraps  and 
dead  rodents.  It  is  more  abundant  in  this  country  than  elsewhere. 

Botflies.  Maggots  of  many  botflies  burrow  throughout  the  body. 
The  skin  botfly  described  previously  stays  beneath  the  skin  but  others, 
such  as  cattle  bots,  burrow  deep  into  the  body  and  wander  at  will. 
Eventually  they  migrate  to  the  skin  of  the  back  and  produce  blisters 
or  warbles.  When  full  grown  they  drop  off  and  pupate  in  the  ground. 
Head  bots  of  sheep  and  goats  penetrate  the  lining  of  the  nose  and 
burrow  in  the  face,  sometimes  destroying  an  eye. 

365.        Intracellular  Parasites 

Only  the  protozoa  and  nematodes  have  given  rise  to  intracellular 
parasites.  Probablv  the  first  parasites  were  ones  living  within  the  cells  of 
other  protozoa,  possibly  forms  like  some  of  the  dinoflagellates  that  are 
endoparasites  of  ciliates.  Among  the  intracellular  parasites  of  metazoans 
are  a  genus  of  flagellates  related  to  trypanosomes,  Leishmama,  and  the 

entire  class  of  sporozoans. 

Trypanosomes  themselves  are  to  some  extent  mtracellular,  espe- 
cially in  the  arthropod  host  where  they  may  grow  and  reproduc^  ni  the 
cells  lining  the  intestine.  One  species  {T.  cruzi)  is  intracellular  m 
the  vertebrate  host,  but  several  species  are  completely  extracellular  in 
both  hosts.  In  the  related  genus,  Leishmama,  the  parasites  are  entirely 


gl4  ANIMALS   AND   THEIR   ENVIRONMENT 

intracellular  in  the  vertebrate  host.  These  are  responsible  for  a  variety 
ot  tropical  sores  and  ulcers  where  the  skin  and  underlying  tissue  have 
been  destroyed.  One  species,  L.  donovani,  invades  the  inner  body  tis- 
sues, especially  the  spleen,  producing  a  disease  known  as  kala-azar. 
Fever,  jxan  and  anemia  are  lol lowed  by  progressive  emaciation  ot  the 
body  while  the  spleen  becomes  enlarged.  Untreated  cases  are  95  per 
cent  fatal.  Within  the  last  twenty  years,  however,  drugs  have  been 
found  which  reduce  the  mortality  rate  to  5  per  cent  or  less. 

Sporozoans  are  common  parasites  ot  the  intestinal  tract  of  arthro- 
pods, infecting  the  individual  cells  of  the  lining.  Other  species  infect 
the  intestinal  cells  of  vertebrates,  including  all  the  domestic  mammals 
and  birds.  The  most  important  of  these  belong  to  the  order  Coccidia 
and  produce  a  disease  called  coccidiosis.  fn  wild  animals  they  are  not  a 
serious  problem  because  the  spores  are  shed  in  the  feces  and  must  be 
eaten  to  cause  reinfection.  Domestication  often  forces  animals  into  a 
closer  association  with  tlieir  excrement  than  is  natural,  and  the  con- 
tamination of  food  by  feces  is  common.  Chickens  particularly  suffer 
from  the  conditions  imposed  upon  them.  If  too  many  of  the  intestinal 
cells  are  destroyed  at  once  the  animal  suffers  weakness,  diarrhea,  bloody 
feces,  loss  ot  appetite,  and  often  death. 

Another  group  of  sporozoans,  the  order  Haemosporidia,  pass  a 
part  of  their  lite  cycle  as  intracellular  parasites  ot  blood  cells  and  an- 
other part  in  an  arthropod  bloodsucker.  The  malarial  parasites  of  man, 
described  earlier  (Fig.  6.1),  belong  to  this  group,  in  regions  where 
malaria  is  common  it  is  typically  a  chronic  disease.  Those  infected 
suffer  periodic  relapses  of  fever,  weakness,  and  a  general  decrease  in 
resistance  to  other  diseases.  The  fever  produced  wlien  malarial  parasites 
burst  from  one  set  of  blood  cells  and  infect  a  new  set  is  liigh  enough 
to  be  deleterious  to  other  parasites,  notably  the  bacterial  spirochete 
producing  syphilis.  In  tact,  several  tropical  tribes  liave  been  found  in 
which  all  the  individuals  have  both  syphilis  and  malaria.  The  people 
have  some  resistance  to  malaria  so  that  it  is  not  a  serious  illness,  and 
suffer  very  little  from  syphilis  because  the  malarial  fevers  keep  it  under 
control.  When  some  of  these  individuals  were  cured  of  malaria  their 
syphilis  immediately  became  worse.  Before  the  discovery  of  penicillin 
a  mild  form  of  malaria  was  used  in  American  hospitals  as  one  means  of 
controlling  advanced   cases   of   syphilis. 

Intracellular  nematodes  are  common  and  sometimes  serious  para- 
sites of  plants. 

The  insidious  parasitic  diseases  of  man  which  have  a  widespread 
distribution  are  preponderantly  blood  diseases.  Malaria,  caused  by  an 
intracellular  parasite  of  red  blood  cells,  has  been  the  most  serious 
world-wide  parasitic  disease  but  modern  medicine  has  somewhat  re- 
duced its  importance.  Schistosomiasis,  caused  by  trematodes  which  live 
in  blood  vessels  and  eat  blood,  remains  a  medical  challenge.  The  extent 
of  its  damage  in  regions  where  most  people  are  infected  is  vuiknown. 
Hookworm  disease,  caused  by  bloodsucking  parasites  in  the  intestine, 
and  amebiasis,  caused  by  Entamoeba  histolytica  eroding  the  intestine 
and   eating    red    blood   cells,    are    both   extremely    widespread    diseases. 


PARASITISM  815 


Fmure   39  15      The   world   distribution   of   malaria,   hookworm,    blood    flukes  and 
Figure   39.15.      i  "c    v^u  jnHirates  extreme  prevalence,  shaded  areas 

sleeping  sickness.  For  malana,  solid  black  indicates  exireiuc  pic  a 

show  moderate  or  occasional  presence. 


^1(3  ANIMALS    AND    THEIR   ENVIRONMENT 

Both  undoubtedly  weaken  the  host  but  their  actual  damage  is  difficult 
to  estimate.  Both  can  be  controlled.  Sleeping  sickness,  caused  by  a 
blood  sajMozoite,  is  the  scourge  ot  much  of  Africa.  These  five  diseases 
are  probably  the  most  important,  if  both  the  seriousness  of  the  disease 
and  the  nimiber  of  people  affected  are  taken  into  consideration.  The 
extent  to  which  tour  of  these  parasites  are  distributed  in  the  world  is 
shown  on  the  accompanying  maps  (Fig.  39.15).  Entamoeba  histolytica 
is  virtually  world-wide,  but  is  a  serious  problem  only  in  the  tropics. 

366.        Adaptations  to  Parasitism 

Adaptations  that  are  common  among  parasitic  animals  include  the 
development  of  devices  for  attachment  and  of  methods  of  transmission, 
and  simplification  or  loss  of  sensory,  locomotor  and  digestive  structures. 
These  adaptations  are  found  in  other  organisms,  of  course,  and  none 
of  them  is  found  in  all  parasites. 

Means  for  Attachment.  Devices  for  attachment  are  especially 
common  among  ectoparasites  and  intestinal  parasites.  The  suckers  of 
trematodes  (Fig.  11.11)  and  leeches  (Fig.  15.1)  are  obvious  examples. 
The  burrowing  habit  of  some  of  the  skin  mites  is  a  less  obvious  way  of 
solving  the  attachment  problem.  Most  of  the  fleas  and  lice  have  legs 
and  claws  adapted  for  gripping  hair  or  feathers.  In  the  human  crab 
louse,  for  example  (Fig.  39.3),  the  second  and  third  pairs  of  legs  are 
chelate  in  such  a  way  that  when  the  claw  closes  against  the  "hand" 
a  hole  is  left  that  is  slightly  smaller  than  the  diameter  of  a  pubic  hair. 
This  enables  the  louse  to  grip  pubic  hairs  tightly  without  cutting  them 
through.  These  lice  are  limited  to  the  pubic  region  primarily  because 
the  head  and  body  hair  is  too  fine  to  be  gripped,  but  men  with  lux- 
uriant coarse  body  hair  can  be  infested  from  head  to  toe.  The  head 
and  body  lice  (Fig.  39.3)  have  more  delicate  claws. 

The  ventral  sucker  of  intestinal  trematodes  is  used  for  attachment 
inside  the  body  just  as  the  posterior  sucker  of  their  ectoparasitic  rela- 
tives is  used  on  the  outside.  The  suckers  or  hooks  of  tapeworms,  the 
spiny  heads  of  acanthocephalans,  and  the  ventral  concavity  of  Giardia 
have  already  been  described.  Hookworms  are  securely  attached  by  the 
mouthful  of  intestinal  wall  through  which  they  suck  blood  (Fig.  39.9). 
Prominent  among  intestinal  parasites  that  are  not  attached  are  Ascaris 
and  its  relatives.  These  continually  crawl  "upstream"  as  a  means  of 
staying  in  the  host  (they  occasionally  crawl  too  far  and  come  out  the 
mouth  or  nose). 

Means  for  Iransmhs'ion.  Two  problems  are  involved  in  the  trans- 
fer of  the  parasite  from  one  host  to  another:  the  development  of  stages 
in  the  life  cycle  that  can  survive  crossing  the  ecologic  desert  that  lies 
between  hosts,  and  the  production  of  sufficient  numbers  of  such  stages 
to  enhance  the  chance  of  locating  a  new  host.  The  first  problem  is 
associated  with  the  survival  of  the  individual,  the  second  with  the  sur- 
vival of  the  species. 

Organisms  that  are  only  partially  modified  as  parasites,  for  ex- 
ample leeches  and  mosquitoes,  have  no  difficulty  getting  from  host  to 


PARASITISM  817 

host.  Fleas  and  lice  that  are  wingless  have  a  greater  problem.  Fleas  are 
free-living  as  larvae  and  have  powerful  jumping  legs  as  adults  so  that 
they  can  move  rapidly  through  a  considerable  distance.  Lice  cannot 
move  fast  and  will  perish  in  a  short  time  if  removed  from  the  host. 
They  seldom  attempt  to  cross  voids  between  hosts,  and  rely  on  body 
contacts  between  hosts  as  a  means  of  transmission. 

Most  ectoparasites  have  no  serious  problem  in  transmission.  In- 
ternal parasites,  however,  are  adapted  to  an  environment  very  different 
from  that  outside  the  host,  and  must  produce  stages  in  the  life  cycle 
able  to  withstand  external  conditions  if  they  are  to  infect  new  hosts. 
Most  intestinal  parasites  produce  resistant  spores,  cysts  or  eggs,  which 
pass  out  in  the  feces  of  the  host.  These  stages  may  survive  long  periods  of 
exposure  and  are  infective  when  eaten  by  the  next  host.  Others  require 
an  alternate  host  that  frequently  is  part  of  the  food  chain  of  the  final 
host.  Thus,  some  tapeworm  eggs  hatch  when  eaten  by  an  arthropod 
host  and  develop  to  the  next  stage,  which  continues  to  develop  only 
when  the  arthropod  is  eaten  by  a  vertebrate  host.  In  a  sense  the  arthro- 
pod is  used  as  a  means  of  transmission  from  the  vertebrate's  feces  to 
its  mouth.  Some  of  the  intestinal  parasites  take  an  active  role  in  trans- 
mission. The  resistant  stages  expelled  in  the  feces  by  hookworms  and 
certain  trematodes  develop  into  active  stages  that  seek  out  the  next 
host  and  penetrate  through  its  skin  rather  than  waiting  to  be  eaten. 

Parasites  of  body  tissues  use  two  routes  of  dispersal.  Some,  such  as 
blood  flukes,  release  stages  which  make  their  way  into  the  intestine  and 
pass  out  with  the  feces.  Their  subsequent  problems  of  transmission 
are  the  same  as  those  of  intestinal  parasites.  Other  release  stages  into  the 
blood  that  will  survive  passage  through  arthropod  bloodsuckers.  They 
usually  develop  through  several  stages  of  the  life  cycle  in  these  arthro- 
pods. The  use  of  this  route  by  malarial  parasites,  trypanosomes  and 
filariae  has  been  described.  These  parasites  avoid  the  problems  of  the 
outside  world  by  remaining  inside  hosts  throughout  the  life  cycle. 

Filarial  nematodes  release  their  larvae  into  the  blood  stream  only 
during  those  hours  of  the  day  in  which  the  arthropod  vectors  are  active. 
In  regions  where  the  insects  bite  in  the  daytime  the  larvae  are  found  in 
the  blood  only  in  the  daytime.  Strains  of  Wuchereria  occur  on  dif- 
ferent islands  in  the  South  Pacific,  some  of  which  have  diurnal,  others 
nocturnal,  insects.  The  strains  of  parasites  found  on  the  different  islands 
have  evolved  to  conform  with  these  patterns. 

If  the  transmission  stages  are  passive,  or  if  the  sojourn  between 
hosts  is  at  all  protracted,  the  odds  that  an  individual  parasite  released 
from  one  host  will  successfully  arrive  at  another  host  are  small.  To 
balance  these  odds  many  parasites  produce  tremendous  numbers  of 
such  stages.  Tapeworms,  roundworms,  acanthocephalans  and  internal 
trematodes  all  produce  millions  of  eggs.  Protozoan  parasites  such  as 
Giardia  and  Entamoeba  produce  "showers"  of  encysted  stages.  The 
number  of  filarial  larvae  in  the  blood  at  the  appropriate  time  of  day 
can  be  enormous.  These  adaptations  not  only  assure  the  survival  of  the 
species,    but    if   environmental    conditions    are    such    that    transmission 


gig  ANIMALS    AND    THEIR   ENVIRONMENT 

becomes  more  probable,  such  parasites  can  rapidly  produce  extremely 
high  infection  rates. 

In  many  cases  where  the  parasite  is  found  in  more  than  one  kind 
of  host,  reproduction  takes  place  in  all  hosts.  The  mosquito  that  picks 
up  a  few  infective  malarial  parasites  from  one  person  shortly  has 
enough  parasites  to  infect  all  of  the  people  it  may  bite  after  that.  Simi- 
larly,\he  trematode  miracidium  lucky  enough  to  get  from  its  vertebrate 
host  to  a  snail  reproduces  so  as  to  produce  many  cercariae,  not  just 
one.  The  eggs  of  many  of  the  insect  parasites  of  other  insects  go  through 
a  process  called  polyembryony  to  produce  a  number  of  larvae  from 
each  egg  that  successfully  reaches  a  new  host. 

The  remarkable  rate  of  reproduction,  often  at  more  than  one  point 
in  the  life  cycle,  makes  it  difficult  to  control  parasites.  Although  all  but 
a  few  parasites  may  be  eliminated  by  intensive  medical  treatment,  those 
few  can  shortly  replace  the  entire  population. 

Selective  Modificafion  of  Organs.  The  intimate  association  of  a 
parasite  with  its  host  may  eliminate  the  usefulness  of  certain  of  its 
organs.  The  selective  disadvantage  of  some  structures  is  obvious,  such 
as  the  cumbersome  wings  of  ectoparasitic  insects  that  crawl  through 
feathers  or  fur.  Useless  structures  tend  to  become  reduced  or  absent  in 
parasites  because  there  is  no  longer  any  positive  selection  in  their  favor 
and  the  gene  complexes  responsible  for  their  existence  gradually  are 
dispersed.  Such  is  presumed  to  have  been  the  fate  of  the  digestive  tracts 
of  tapeworms  and  acanthocephalans.  A  mouth,  gut  and  digestive  glands 
are  not  required  for  the  survival  of  an  organism  living  in  the  host's 
digestive  tract,  where  saprozoic  nutrition  is  possible. 

Locomotor  organs  may  also  be  useless.  Most  adult  tapeworms  do 
not  move  again  once  the  head  is  attached,  and  these  tapeworms  have 
such  poor  musculature  that  they  cannot  crawl  effectively.  Although 
larval  parasitic  copepods  and  barnacles  have  typical  larval  legs  in  the 
free-swimming  stage,  in  many  species  the  legs  rapidly  disintegrate  as 
soon  as  the  individual  attaches  to  its  host.  Even  the  protozoan  Sporozoa 
have  lost  their  original  locomotor  organelles.  Most  of  the  ectoparasites, 
however,  have  fair  to  good  locomotor  organs,  and  the  insects  that  have 
lost  their  wings  still  have  well  developed  legs. 

Sense  organs  become  somewhat  less  useful  as  the  locomotor  organs 
of  ectoparasites  decrease  in  size.  Fleas,  which  have  strong  jumping  legs, 
have  well  developed  eyes  and  an  excellent  sense  of  the  warmth  of 
mammals  at  a  distance.  The  latter  is  shown  by  waving  first  a  cold 
object  and  then  the  hand  past  fleas  on  the  floor.  The  fleas  show  little 
response  to  the  cold  object,  but  as  the  hand  approaches  they  all  turn 
to  face  it  and  then  jump  upon  it  at  the  appropriate  moment.  Lice  have 
weak  legs  and  most  of  them  are  blind,  but  they  retain  a  good  chemical 
sense  for  use  as  they  crawl  over  the  host. 

Internal  parasites  have  even  less  use  for  eyes,  ears  and  other  sense 
organs.  The  only  sense  found  in  many  internal  parasites  is  a  little 
understood  ability  to  migrate  to  a  specific  portion  of  the  body,  which 
is  presumably  a  form  of  chemical  sense.  Internal  parasites  with  complex 
life  cycles   including  a   free-living  stage,   such  as   the  miracidium    and 


PARASITISM  819 

cercaria  of  trematodes,  many  have  various  organs  including  eyes  in  the 
free-living  stage  but  these  are  absent  from  the  parasitic  stage. 

The  evolutionary  reduction  of  organs  is  called  simplification  or 
degeneration.  It  would  be  a  mistake,  however,  to  consider  that  parasitic 
organisms  are  "degenerate"  because  they  have  some  degenerate  organs. 
Degenerate  organisms  are  ineffective,  inefficient  individuals.  Parasites 
are  both  efficient  and  effective.  The  degeneration— simplification  or 
loss— of  some  of  their  organs  is  balanced  by  other  adaptations  with  which 
they  exploit  the  parasitic  way  of  life.  Sucking  lice,  for  example,  are 
abundant  wherever  mammals  are  found,  and  in  spite  of  their  weak 
bodies  and  near  or  total  blindness  they  live  with  remarkable  security, 
having  longer  lives  and  requiring  fewer  offspring  for  perpetuation  of 
the  species  than  many  free-living  insects  of  their  size. 

367.        Host  Specificity 

Many  parasites  can  infect  a  variety  of  animals.  The  common  tick 
(Fig.  39.5)  will  feed  on  almost  any  mammal,  and  a  single  acantho- 
cephalan  species  may  be  found  in  the  intestines  of  birds  belonging  to 
several  different  orders.  Most  parasites,  however,  are  more  restricted 
and  infect  only  a  group  of  species  that  are  closely  related.  One  genus 
of  tapeworms  is  foiuid  only  in  carnivores,  another  only  in  rodents  and 
a  third  only  in  marsupials.  Some  parasites  are  still  more  restricted  and 
can  infect  only  one  host  species  or  possibly  a  few  species  of  the  same 
genus.  This  extreme  host  specificity  is  common  in  malarial  parasites 
(those  of  man  will  not  infect  any  other  animal),  sucking  lice  (the  crab 
louse  can  live  on  the  gorilla  but  the  head  and  body  lice  live  only  on 
man),  and  nematodes  (the  human  Ascaris  can  live  in  other  mammals 
but  will  not  reproduce  there),  and  it  is  not  rare  in  other  groups  such  as 
fleas  and  tapeworms. 

Where  host  specificity  is  extreme  the  parasites  may  have  been  as- 
sociated with  their  hosts  for  a  considerable  period  of  geologic  history, 
and  as  the  host  evolved  into  a  number  of  species  the  parasites  evolved 
with  them.  In  such  cases  the  taxonomic  arrangement  of  the  hosts  and 
the  parasites  often  shows  similar  or  identical  patterns.  This  phe- 
nomenon has  been  used  as  a  means  of  settling  certain  taxonomic 
problems.  In  the  last  century,  for  example,  it  was  observed  that  the 
llamas  of  the  South  American  Andes  were  similar  to  the  camels  of 
northern  Africa  and  central  Asia,  but  the  geographic  distance  between 
the  two  groups  was  considered  to  be  a  barrier  to  placing  them  in  the 
same  family.  When  their  lice  were  studied  it  was  discovered  that  they 
also  were  similar  to  each  other  and  different  from  other  lice.  On  the 
strength  of  this  concordance  the  llamas  and  camels  were  grouped  in 
the  family  Camelidae  and  the  lice  were  grouped  in  the  genus  Micro- 
thoracius.  This  decision  was  shown  to  be  correct  later  when  an 
abundance  of  fossil  camels  was  found  in  North  America.  Today,  in 
fact,  it  is  believed  that  the  group  together  with  its  lice  arose  in  North 
America  and  spread  to  both  South  America  and  Asia  before  becoming 
extinct  on   this  continent. 


g2U  ANIMALS   AND   THEIR   ENVIRONMENT 

The  extent  to  which  the  taxonomic  schemes  of  parasites  and  their 
hosts  agree  can  be  used  as  an  indication  of  the  age  of  the  association 
between  parasites  and  hosts.  The  conclusion  that  ancestral  camels  were 
infested  with  ancestors  of  Microthoracius,  together  with  the  age  of 
camel  fossils,  indicates  that  this  association  has  existed  for  at  least  30 
million  years.  The  Australian  fauna  was  isolated  about  75  million 
years  ago,  and  the  Australian  marsupials  were  separated  from  their 
relatives  in  America.  The  tapeworms  of  these  two  marsupial  groups 
are  similar,  suggesting  that  tapeworms  were  parasitizing  them  before 
their  separation.  On  the  other  hand,  their  internal  trematodes  are  not 
similar  and  it  is  concluded  that  these  parasites  have  infected  marsupials 
for  less  than  75  million  years.  The  same  conclusion  is  reached  for  the 
sucking  lice,  which  are  found  on  all  the  American  marsupials  but 
which   are   entirely  absent  from   Australia. 

Concordance  in  the  evolution  of  parasites  and  their  hosts  is  often 
marred,  however,  by  occasional  "jumps"  to  new  hosts.  Most  of  the 
species  of  sucking  lice  in  the  genus  Linognathus  are  found  on  ungulates, 
and  the  genus  is  believed  to  have  evolved  with  this  mammalian  group. 
One  species,  however,  is  found  on  the  fox  and  dog.  This  does  not  sug- 
gest that  the  latter  evolved  from  ungulates,  but  rather  that  the  lice 
established  a  new  beach-head  on  the  predators  of  their  usual  hosts. 
These  changes  are  often  associated  with  ecologic  relationships.  One 
species  of  a  genus  of  rabbit  fleas  is  a  parasite  of  birds  that  nest  in 
rabbit  holes.  The  relationship  is  less  obvious  in  the  case  of  malarial 
parasites  of  the  genus  Plosinodium.  Some  species  are  foiuid  in  man  and 
a  few  other  primates,  while  other  species  are  found  in  several  different 
groups  of  birds.  All  of  these  parasites  use  mosquitoes  as  the  alternate 
host,  and  it  is  the  mosquitoes  that  provide  the  ecologic  link,  sucking 
the  blood  of  warm-blooded  birds  as  well  as  that  of  mammals.  Since 
jumps  to  new  hosts  of  parasite  groups  with  extreme  host  specificity 
are  known  to  occur  occasionally,  agreement  of  taxonomic  relationship 
among  parasites  and  their  hosts  can  never  be  used  as  absolute  proof 
for  the   course  of  evolution   implied  in   the   taxonomy. 

368.       Social  Parasites 

Animal  societies  may  be  subjected  to  a  kind  of  parasitism  in  which 
the  parasite  does  not  feed  on  individuals  but  intrudes  itself  into  the 
social  economy.  The  American  cowbird  and  European  cuckoo  are 
examples  of  this.  These  birds  lay  their  eggs  in  the  nests  of  other 
species  where  the  involuntary  foster  parents  obligingly  feed  and  care  for 
the  young.  The  rightful  nestlings  are  often  smaller  and  less  vigorous 
than  the  social  parasites  and  may  be  crowded  out  of  the  nest.  These 
parasites  successfully  invade  the  social  family  life  of  the  host  birds. 

Insect  societies  are  invaded  by  a  variety  of  beetles  and  wasps  that 
in  one  way  or  another  become  accepted  as  a  part  of  the  colony.  Some 
of  these  parasites  are  food-robbers,  masquerading  as  colony  members 
while  they  actually  do  nothing  but  steal  food  when  hungry.  Others 
enter  into  the  trophallaxis  of  the  colony,  offering  secretions  in  return 


PARASITISM  821 

for  being  fed  so  that  the  hosts  appear  content  with  their  presence. 
They  are  worse  than  commensals  since  they  use  up  some  of  the  food 
supply  of  the  colony.  Other  insects  that  actually  eat  larvae  are  toler- 
ated and  to  some  extent  protected  by  the  colony.  This  is  predation 
against  the  larvae,  but  in  relation  to  the  whole  colony  may  be  regarded 
as  a  form  of  parasitism  since  the  invaders  remain  with  the  colony  and 
do  not  kill  it. 


Questions 

1.  Give  examples  of  ectoparasitism,  intestinal  parasitism,  blood  parasitism  and  intracel- 
lular parasitism. 

2.  Describe  the  evolutionary  pathways  by  which  an  animal  may  become  a  parasite. 

3.  Name  an  ectoparasite  and  an  endoparasite  which  eat  the  flesh  of  man  and  describe  the 
life  history  of  each. 

4.  Discuss  three  adaptations  common  in  parasites. 

5.  Distinguish  between  biting  and  sucking  lice  according  to  both  their  ta.vonomy  and 
their  hosts. 

6.  Describe  the  life  cycle  of  the  common  tick. 

7.  Where  are  hookworms  prevalent?  What  counter  measures  are  effective  against  hook- 
worms? 


Supplementary  Reading 

Chandler,  Introduction  to  Parasitology,  is  a  standard  text  of  the  subject.  Ecologic 
aspects  are  discussed  and  many  interesting  examples  are  given  in  Ecology  of  Ani)nal  Para- 
sites by  Baer.  Many  books  are  devoted  entirely  to  the  medical  and  clinical  aspects  of 
human  parasites.  Rats,  Lice  and  History  by  Zinsser  is  a  popular  and  authoritative  account 
of  typhus  down  through  the  ages.  .\n  excellent  source  book  for  tropical  parasites  is  the 
Manual  of  Tropical  Diseases  by  Mackie,  Hunter,  and  Worth.  An  excellent  semipopular 
account  of  parasitism  is  that  of  Rothschild  and  Clay,  Fleas,  Flukes,  and  Cuckoos. 


CHAPTER  40 


Conservation 


There  are  many  ways  in  which  a  knowledge  ot  the  principles  of 
ecology  can  be  used  to  further  human  society,  one  of  the  most  im- 
portant of  which  is  the  rational  conservation  of  our  natural  resources. 
Conservation  does  not  mean  simply  hoarding— not  using  the  resources 
at  all— nor  does  it  imply  a  simple  rationing  of  our  supplies  so  that  some 
will  be  left  for  the  future.  True  conservation  implies  taking  full  advan- 
tage of  our  knowledge  of  ecology  and  managing  our  ecosystems  so  as 
to  establish  a  balance  of  harvest  and  renewal,  thus  ensuring  a  continu- 
ous yield  of  useful  plants,  animals  and  materials.  In  general,  man  is 
still  acting  as  though  he  had  not  yet  learned  that  he  is  part  of  a  com- 
plex environment  which  must  be  studied  and  treated  as  a  whole,  and 
not  in  terms  of  isolated  "projects,"  for  in  attempting  to  carry  out  one 
project  he  may  nullify  or  completely  overcome  the  results  of  another 
one. 

The  record  of  man's  past  squandering  of  natural  resources  is 
indeed  a  dark  one— the  slaughter  of  the  bison  that  once  roamed  the 
western  plains,  the  decimation  of  the  whales,  the  depletion  of  our  sup- 
plies of  many  kinds  of  fresh-water  and  marine  fishes,  the  extinction  of 
birds  such  as  the  passenger  pigeon,  the  razing  of  thousands  of  square 
miles  of  forests  and  the  burning  of  more  by  careless  use  of  fire,  the  pollu- 
tion of  streams  with  sewage  and  industrial  wastes,  the  careless  cultivation 
of  land  which  has  resulted  in  the  complete  ruin  of  many  square  miles  of 
land  and  the  silting  of  streams  are  some  of  the  more  flagrant  examples  of 
natural  resources  wasted  beyond  hope  of  regaining.  State  and  federal 
departments  of  conservation  and  professional  ecologists  have  been 
aware  of  the  problem  for  many  years  and  have  begun  counter-measures, 
but  the  chief  task  at  present  is  to  make  the  population  at  large  realize 
the  urgency  and  the  magnitude  of  the  job  to  be  done  and  to  get  general 
support  for  the  measures  which  must  be  taken.  For  many  aspects  of  the 
conservation  problem,  additional  basic  ecologic  research  is  needed  to 
determine  the  possible  effects  of  some  proposed  conservation  measure  on 
the  whole  ecology  of  the  region. 

369.        Agriculture 

After  decades  of  the  destructive  exploitation  of  farm  lands  by 
planting  one  crop  such  as  corn  or  cotton  year  after  year,  the  soil  con- 
servation program  sponsored  jointly  by   federal   and   local    agencies  is 

822 


CONSERVATION 


823 


effective  because  it  is  based  on  sound  ecologic  principles.  The  rotation 
of  crops,  contour  farming,  the  establishment  of  wind  breaks  to  prevent 
soil  erosion  by  winds,  and  the  use  of  proper  fertilizers  to  renew  the 
soil  are  all  measures  which  are  effective  in  maintaining  a  balanced 
ecosystem.  Successful  farming  must  follow  the  principles  of  good  land 
use.  It  is  not  conservation  to  reclaim  marginal  land  for  agricultural 
purposes  or  to  build  expensive  dams  and  canals  to  irrigate  land  unless 
the  land  can  produce  crops  which  will  make  the  irrigation  worth  while. 
If  the  grasslands  of  regions  with  slight  rainfall  are  plowed  and  planted 
with  wheat,  a  "dust  bowl"  will  inevitably  develop,  but  if  the  land  is 
kept  as  grassland  and  grazed  in  moderation  the  soil  will  be  kept  in 
place,  no  dust  bowl  will  develop,  and  the  land  can  be  used  economically 
year  after  year.  Overgrazing,  by  destroying  the  grass  covering  the  soil, 
can  lead  to  destructive  erosion  just  as  plowing  does.  Overgrazing  also 
leads  to  the  invasion  of  the  grassland  by  undesirable  weeds  and  desert 
shrubs.  These  are  difficult  to  eradicate  so  that  grass  may  grow  again. 
It  is  now  evident  that  poor  land  use  affects  not  only  the  unwise  farmer 
but  the  whole  population  which  is  eventually  taxed  to  pay  for  rehabili- 
tation. 

The  ecologists  specializing  in  the  management  of  land  have  classi- 
fied land  on  the  basis  of  its  slope,  kind  of  soil  and  natural  biotic  com- 
munities, into  eight  categories,  from  Class  I,  which  is  excellent  for 
farming  and  can  be  cultivated  continuously,  through  three  classes  which 
can  be  used  for  farming  only  with  special  care  and  another  three  classes, 
which  are  suitable  only  for  permanent  pasture  or  forest,  to  Type  VIII, 
suitable  only  to  be  left  as  it  is  for  game  (Fig.  40.1). 


Figure  40.1.  Classification  of  land  according  to  its  usefulness.  Types  I  and  II 
may  be  cultivated  continuously;  types  III  and  IV  are  subject  to  erosion  and  must  be 
cultivated  with  great  care;  types  V,  VI  and  VII  are  suitable  for  pasture  or  forests 
but  not  for  cultivation;  type  VIII  is  productive  only  as  a  habitat  for  game.  (U.  S.  Soil 
Conservation  Service.) 


g94  ANIMALS   AND    THEIR   ENVIRONMENT 

The  control  of  insect  pests  by  chemicals  such  as  DDT  must  be 
carried  out  cautiously,  with  possible  ecologic  upsets  in  mind.  Spraying 
orchards,  forests  and  marshes  may  destroy  not  only  the  pests  but  also 
useful  insects  such  as  honeybees  which  pollinate  many  kinds  of  fruit 
trees  and  crops,  and  useful  insect  parasites.  In  some  cases  the  insect  pests 
have  actually  increased  after  the  use  of  DDT  because  the  chemical 
killed  off  greater  numbers  of  insect  enemies  of  the  pest  than  of  the 
pests  themselves.  A  number  of  strains  of  insects  resistant  to  DDT  have 
developed. 

DDT  and  related  chemicals  kill  other  animals  in  addition  to  in- 
sects; amphibians  and  reptiles  are  the  most  vulnerable  vertebrates.  The 
vertebrates  are  less  sensitive  than  insects,  and  DDT  applied  at  a  level  of 
about  1  j)ound  per  acre  is  effective  in  insect  control  without  endanger- 
ing the  vertebrates.  However,  when  applied  at  a  level  of  5  to  10  pounds 
per  acre  some  of  the  useful  animals  are  killed  along  with  the  insects. 
Some  of  the  newer,  stronger  insecticides  have  been  used  without  ade- 
quate testing  of  their  effects  on  other  animals. 

370.  Forestry 

The  management  of  our  forests  is  an  important  aspect  of  applied 
ecology.  Careful  forest  management  has  been  carried  on  in  Europe  for 
many  decades  but  is  only  beginning  in  this  country.  Proper  timber 
management  in  our  national  and  state  forests  has  been  important  in 
demonstrating  to  the  owners  of  private  forests  the  results  which  can  be 
obtained  in  this  way.  Since  in  some  regions  the  desirable  timber  trees 
are  members  of  the  climax  community,  the  ecologic  problem  is  simply 
to  find  the  best  way  to  speed  the  return  of  the  climax  community  after 
the  trees  have  been  cut.  In  other  regions  the  desirable  trees  are  earlier 
serai  stages  of  the  ecologic  succession,  and  forest  management  involves 
establishing  means  of  preventing  the  succession  from  proceeding  to  the 
climax  community.  This  is  also  true  of  many  kinds  of  animals;  most 
game  birds  and  many  of  the  most  valuable  game  fish  are  members  of, 
and  thrive  best  in,  an  early  serai  stage  of  their  community. 

371.  Wildlife 

The  management  of  our  fish  and  wildlife  resources  is  a  field  of 
applied  ecology  which  is  supported  by  wide  public  interest,  especially 
by  sportsmen's  clubs  and  associations.  "Wildlife"  used  in  this  connec- 
tion usually  means  game  and  fur-bearing  animals.  Since  the  various 
types  of  wildlife  are  adapted  to  different  stages  of  ecologic  succession, 
their  management  requires  a  knowledge  of  and  the  proper  use  of  these 
stages.  As  the  Middle  West  became  more  and  more  intensively  farmed, 
and  the  original  forests  and  prairies  were  reduced  to  small  patches,  the 
prairie  chickens  and  ruffed  grouse  which  were  adapted  to  these  habitats 
were  greatly  decreased  in  numbers.  However,  this  region  has  been  par- 
tially  restocked   with   game   birds   by   introducing  pheasants    and   par- 


CONSERVATION  825 

tridges,  which  had  become  adapted  to  the  intensively  farmed  regions  of 
Europe. 

Of  the  three  general  methods  used  to  increase  the  population  of 
game  animals— laws  restricting  the  number  killed,  artificial  stocking  and 
the  improvement  of  the  habitat— the  latter  is  the  most  effective.  If  the 
game  habitats  are  destroyed  or  drastically  altered,  protective  laws  and 
artificial  stocking  are  useless.  Protective  laws  must  operate  to  prevent  a 
population  from  getting  too  large  as  well  as  too  small.  Deer  populations, 
in  the  absence  of  natural  predators  but  subject  to  a  constant,  moderate 
amount  of  hunting,  may  increase  to  a  point  where  they  actually  ruin  the 
vegetation  of  the  forest.  Hunting  should  be  restricted,  of  course,  when 
populations  are  small  and  increased  when  they  are  larger.  This  requires 
accurate  annual  estimates  of  the  population  density  of  the  game  species. 

Stocking  a  region  artificially  with  game  animals  is  effective  only  if 
they  are  being  introduced  into  a  new  region  or  into  one  from  which 
they  had  been  killed  off.  Beavers,  for  example,  had  been  trapped  to  ex- 
termination in  Pennsylvania,  but  restocking  with  Canadian  beavers 
has  been  very  successfid  and  it  is  estimated  that  there  are  some  fifteen 
to  twenty  thousand  beavers  busy  building  dams  in  Pennsylvania.  These 
are  now  an  important  factor  in  flood  control  in  that  region.  The  prin- 
ciples of  population  growth  make  it  clear  that  if  game  animals  of  a 
certain  species  are  already  present,  artificially  stocking  that  region  with 
additional  members  of  the  species  will  be  futile.  Stocking  a  region  with 
a  completely  new  species  must  be  done  cautiously,  or  the  species  may 
succeed  so  well  as  to  become  a  pest  and  upset  the  biotic  community, 
as  has  happened  with  rabbits  in  Australia  and  the  English  sparrow  in 
the  United  States. 

The  management  of  the  fish  in  a  pond  may  be  directed  toward 
providing  sport  for  hook  and  line  fishermen  or  toward  raising  a  crop  of 
food  fish  and  draining  the  pond  at  regular  intervals  to  harvest  the  crop. 
To  provide  the  best  sport  fishing  it  has  been  found  that  a  lake  or  pond 
should  be  stocked  with  a  combination  of  the  sport  fish  and  its  natural 
prey;  stocking  a  pond  with  large-mouth  bass  plus  bluegills  gives  seven 
to  ten  times  more  bass  in  three  years  than  does  stocking  with  bass  alone. 
Stocking  with  fish  must  be  done  with  care,  for  if  a  lake  that  already  has 
about  as  many  fish  occupying  a  certain  ecologic  niche  as  possible  is 
stocked  with  more  of  the  same  kind,  there  will  be  a  decrease  in  the  rate 
of  growth  and  the  average  size  of  the  fish.  It  has  been  found  that  sport 
fishing  with  hook  and  line  is  not  likely  to  overfish  a  lake;  the  lake  is 
more  likely  to  be  underfished  and  the  resulting  crowding  leads  to  a 
decrease  in  the  average  size  of  the  fish  population. 

The  building  of  dams  raises  intricate  ecologic  problems,  for  dams 
may  be  intended  for  power,  for  flood  control,  for  the  prevention  of  soil 
erosion,  for  irrigation  or  for  the  creation  of  recreational  areas.  Since 
no  one  dam  can  satisfactorily  accomplish  all  of  these  objectives,  the  pri- 
mary objective  must  be  clearly  delineated  and  the  secondary  results 
must  be  understood.  A  contrast  of  two  proposals  for  dealing  with  the 
same   watershed   (Table    16)    shows   that   the    multiple   dam   plan   costs 


826 


ANIMALS    AND   THEIR   ENVIRONMENT 


Table   16.     A  COMPARISON  OF  A  SINGLE  MAIN  RIVER  RESERVOIR  PLAN 
\VI  I  n  A  PLAN  FOR  MULTIPLE  SMALLER  HEADWATERS  RESERVOIRS 


MAIN 

STREAM 

RESERVOIR 


MULTIPLE 

HEADWATERS 

RESERVOIRS 


Number  of  reservoirs 

Drainage  area,  square  miles 

Flood  storage,  acre  feet 

Surface  water  area  for  recreation,  acres 

Flood  pool,  acres 

Bottom  farm  land  inundated,  acres 

Bottom  farm  land  protected,  acres 

Total  cost: 

From  E.  P.  Odum:  Fundamentals  of  Ecology. 


1 

34 

195 

190 

52,000 

59,100 

1,950 

2,100 

3,650 

5,100 

1,850 

1,600 

3,371 

8,080 

$6,000,000 

51,983,000 

less,  destroys  a  smaller  amount  ot  productive  farm  land,  impounds  more 
water  and  is  more  effective  in  controlling  floods  and  soil  erosion.  The 
management  of  the  fish  population  in  the  lakes  created  by  large  dams 
is  more  difficult  than  the  management  of  a  pond.  Sport  fishing  is  usually 
very  good  when  a  dam  has  first  been  built,  but  gradually  the  silting  up 
of  the  reservoir  and  the  decrease  in  productivity  change  the  nature  of 
the  fish  community  from  game  fish  to  less  desirable  catfish  and  shiners. 
The  three  chief  sources  of  stream  pollution  are  industrial  materials 
which  are  either  directly  toxic  themselves  or  which  reduce  the  oxygen 
supply  in  the  water,  sewage  and  other  materials  which  decrease  the 
oxygen  content  of  the  water  and  introduce  bacteria  and  other  septic 
organisms  (Fig.  40.2),  and  turbidity  due  to  soil  erosion  in  the  watershed. 
As  the  silt  settles  out  downstream  it  may  cover  up  the  spawning  grounds 
of  fish  and  have  other  direct  deleterious  effects.  Erosion  can  be  pre- 
vented by  proper  soil  management,  industrial  wastes  can  be  prevented 
by  suitable  design  of  the  manufacturing  process,  and  properly  treated 
sewage  can  be  emptied  into  a  stream  without  deranging  its  ecologic 
relations. 


372.        Marine  Fisheries 

The  primary  productivity  of  the  sea,  as  measured  by  the  pounds  of 
organic  carbon  produced  per  year  per  acre  of  surface,  is  very  high.  The 
productivity  of  the  western  Atlantic  off  the  coast  of  North  America  is 
2.5  to  3.5  tons  of  organic  carbon  per  acre  and  that  of  Long  Island 
Sound  is  2.5  to  4.5  tons  per  acre.  The  productivity  of  the  average  forest 
is  about  one  ton  per  acre,  most  cultivated  land  fixes  only  about  three- 
quarters  of  a  ton  of  organic  carbon  per  acre,  and  only  the  rich,  in- 
tensively cultivated  cornfields  of  Ohio  produce  as  much  as  4  tons  per 
acre.  Despite  this  high  productivity,  man's  actual  harvest  from  the 
ocean,  in  terms  of  pounds  of  fish  caught  per  acre  of  surface,  is  very  low. 
Only  the  rich  fishing  grounds  of  the  North  Sea  produce  as  much  as  15 
pounds  of  fish  per  acre.  The  ecologic  reasons  for  this  are  clear:  the  fish 
are  secondary  or  tertiary  consumers  and  are  on  top  of  a  vast  "pyramid 
of  protoplasm."   There   are    many   organisms    competing    for   the    food 


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828  ANIMALS   AND   THEIR   ENVIRONMENT 

energy  fixed  by  the  algae  in  addition  to  tlie  edible  fish  and  Crustacea 
harvested  from  the  sea. 

Man  could  undoubtedly  recover  for  his  use  much  more  of  the  bio- 
logic productivity  of  the  sea.  Although  he  might  be  reluctant  to  eat 
marine  algae  himself,  they  might  be  filtered  from  sea  water  and  proc- 
essed so  as  to  be  suitable  as  feed  for  cattle  or  some  other  gastronomically 
acceptable  animal.  Careful  studies  by  the  U.  S.  Fish  and  Wildlife  Service 
of  the  fish  population  of  George's  Bank  and  other  commercially  fished 
areas  have  led  to  recommendations  about  the  rate  of  fishing  and  the  size 
of  nets  used  which  ensure  that  the  fish  are  harvested  at  an  optimal  size 
for  greatest  yield  at  present  and  in  the  fvuure.  These  areas,  which  had 
been  fished  so  extensively  that  some  of  the  most  desirable  species  were 
reduced  greatly  in  numbers,  are  now  beginning  to  revive  under  careful 
management. 

The  shellfish— oysters,  clams,  shrimps  and  lobsters— present  some- 
what different  and  more  difficult  problems,  for  their  habitat  is  more 
limited  than  that  of  commercial  fish  and  they  are  more  affected  by  ad- 
verse environmental  changes.  Oysters,  whose  food  consists  of  algae  or 
detritus  of  a  certain  size  filtered  from  the  sea  water  by  their  gills,  are 
unable  to  use  algae  of  a  different  size.  Oysters  were  unable  to  survive 
in  certain  bays  of  Long  Island  Sound  when  commercial  duck-raising 
was  carried  out  on  the  adjacent  shore.  The  wastes  from  the  duck  farms 
were  washed  into  the  bays  and  the  addition  of  this  organic  matter 
changed  the  community  ecology  in  such  a  way  that  the  normal  food  of 
the  oyster,  diatoms,  were  replaced  by  other  algae  which  could  not  be 
used  by  the  oysters.  Once  an  oyster  bed  has  been  seriously  depleted  it 
may  fail  to  recover  even  if  seeded  with  oyster  larvae,  because  the  larvae 
require  a  favorable  surface  for  attachment  and  the  most  favorable  is 
the  shell  of  an  old  oyster.  In  commercial  oyster  farms  the  larvae  are 
provided  with  artificial  sites  for  attachment.  Once  they  have  become 
attached  they  may  be  moved  to  other  waters,  even  from  one  ocean  to 
another,  to  complete  their  growth  in  waters  that  are  favorable  for  feed- 
ing although  not  favorable  for  the  reproduction  of  the  species. 

373.       Public  Health 

Many  aspects  of  the  field  of  public  health  require  the  application 
of  ecologic  principles;  the  prevention  of  the  spread  of  diseases  carried 
by  animals  is  an  ecologic  as  well  as  a  medical  problem.  The  most 
effective  way  of  eliminating  malaria,  for  example,  is  to  eliminate  the 
particular  species  of  mosquito  which  is  the  vector  of  the  malaria  para- 
site, yet  this  must  be  done  without  destroying  the  useful  insects  of  the 
region.  The  mosquitoes  which  transmit  malaria  in  different  parts  of 
the  world  have  quite  different  ecologic  niches,  and  therefore  measures 
that  may  be  effective  in  mosquito  control  in  one  region  may  be  quite 
ineffective  in  another.  The  malaria  of  the  southeastern  United  States  is 
transmitted  by  mosquitoes  living  in  marshes,  Italian  malarial  mosquitoes 
live  in  cool  running  water  in  the  uplands,  and  Puerto  Rican  malarial 
mosquitoes  live  in  brackish  (slightly  salty)  water.  Careful  ecologic  sur- 


CONSERVATION  g29 

veys  of  each  region  are  necessary  to  formulate  the  proper  measures  to 
control  the  insects. 

The  size  of  the  populations  of  rats,  mice  and  many  insect  pests  in- 
creases with  the  size  of  cities  and  the  correlated  tendency  toward  the 
development  of  slums  in  the  older  parts  of  the  town.  A  survey  in  Eng- 
land in  1953  reported  that  only  0.1  per  cent  of  the  houses  in  towns 
with  less  than  25,000  houses  were  infested  with  bedbugs,  but  over  1.0 
per  cent  were  infested  in  towns  with  more  than  100,000  houses!  Careful 
ecologic  studies  in  Baltimore  showed  that  although  professional  crews 
of  rat  trappers  might  catch  as  much  as  half  of  the  rat  population,  it 
quickly  returned  to  its  former  level.  Cats  proved  to  be  much  overrated 
as  rat  predators  and  were  not  effective  in  controlling  the  rat  popula- 
tion. However,  by  changing  the  essential  elements  of  the  rats'  habitat, 
by  improving  sanitation,  thus  decreasing  the  garbage  on  which  the  rats 
fed  and  the  wastes  in  which  they  hid,  the  rat  population  was  reduced 
to  about  10  per  cent  of  its  former  size.  It  remained  at  this  lower  level 
because  that  was  the  total  number  of  rats  which  could  survive  in  the 
altered  environment. 

374.        Human  Ecology 

No  great  amount  of  thought  is  required  to  realize  that  the  ecologic 
principles  discussed  in  these  pages  apply  to  human  populations  as  well 
as  to  animals  and  plants.  Human  ecology  deals  not  only  with  the  dy- 
namics of  human  populations  but  also  with  the  relationship  of  man  to 
the  many  physical  and  biotic  factors  which  impinge  upon  him.  By  re- 
alizing that  human  populations  are  a  part  of  larger  units— of  biotic 
communities  and  ecosystems— man  can  deal  with  his  own  special  prob- 
lems more  intelligently.  Man  has  a  great  deal  of  control  over  his  environ- 
ment and  has  modified  the  communities  and  ecosystems  of  which  he  is 
a  part.  However,  this  control  is  far  from  complete,  and  man  must,  like 
other  animals,  adapt  to  those  situations  which  he  cannot  change.  By 
understanding  and  cooperating  with  the  various  cycles  of  nature,  man 
has  a  better  chance  of  surviving  in  the  future  than  if  he  blindly  attempts 
to  change  and  control  them. 

There  is  a  lively  controversy  at  present  as  to  whether  the  human 
population  is  in  danger  of  multiplying  beyond  the  ability  of  the  earth 
to  support  it.  In  the  past  several  centuries  the  population  of  the  world 
has  increased  tremendously  as  new  territories  have  been  opened  for  ex- 
ploitation and  as  methods  of  food  production  have  become  more  effi- 
cient. Part  of  the  disagreement  involves  the  question  of  whether 
comparable  increases  in  the  "carrying  capacity"  of  the  earth  may  be 
expected  in  the  future.  There  are  many  biologists  and  social  scientists 
who  believe  that  the  danger  of  overpopidation  is  both  great  and  im- 
minent, and  others  who  hold  the  opposite  view.  It  has  been  amply  shown 
that  the  Malthusian  principle  that  populations  have  an  inherent 
ability  to  grow  exponentially  is  true  for  organisms  generally,  and  the 
growth  of  the  human  population  in  the  past  three  hundred  years  does 
follow  an  exponential  curve.  W^hether  other  factors  will  come  into  play 


^30  ANIMALS    AND    THEIR   ENVIRONMENT 

to  prevent  the  biologic  catastrophe  of  human  overpopulation  remains  to 
be  seen.  At  present  we  lack  some  of  the  basic  information  needed  for 
sound  predictions  in  this  field.  Much  more  study  of  man's  relationships 
with  his  physical  and  biologic  environment  is  needed. 

Questions 

1.  What  is  meant  by  conservation?  What  conservation  measures  are  being  taken  in  your 
state? 

2.  What  methods  may  be  used  to  increase  the  number  of  game  fish  in  a  large  lake?  the 
number  of  game  birds  in  a  forest? 

3.  What  ecologic  problems  may  be  raised  by  the  damming  of  a  river,  by  mining  opera- 
tions and  by  the  establishment  of  a  large  chemical  factory? 

4.  Discuss  the  ecologic  principles  involved  in  the  operation  of  an  oyster  farm. 

5.  In  what  ways  are  ecology  and  public  health  related? 

6.  What  is  meant  by  human  ecology?  How  is  it  related  to  sociology? 

Supplementary  Reading 

The  problem  of  the  conservation  of  our  natural  resources  is  considered  in  Fairfield 
Osborn's  Out  Plundered  Planet.  Paul  Sears'  Deserts  on  the  March  and  William  Vogt's 
Road  to  Suniival  present  the  urgent  need  for  the  adoption  of  proper  conservation  meas- 
ures. The  Challenge  of  Man's  Future,  by  Harrison  Brown,  is  an  able  and  fascinating  dis- 
cussion of  some  important  aspects  of  human  ecology. 


APPENDIX 
A  Synopsis  of  the  Animal  Kingdom 


This  synopsis  is  primarily  for  reference  purposes.  All  of  the  major  groups  of 
animals  have  been  included.  In  many  cases  the  classification  is  carried  down 
to  orders,  with  one  or  more  genera  cited  as  examples.  Some  of  the  extinct  groups 
in  the  molluscs,  arthropods,  echinoderms  and  chordates  have  been  included,  all 
others  are  omitted.  Those  included  are  preceded  by  a  dagger  (f). 

The  phyla  appear  in  the  order  given  on  the  following  key. 

Unicellular:  Phylum  1.  Protozoa. 
Uncertain  status:  Phylum  2.  Mesozoa. 
Multicellular:  The  subkingdom  Metazoa. 
No  nervous  system:  Phylum  3.  Porijera. 
Nervous  system:  All  remaining  phyla. 

Little  or  no  mesoderm  and  radial  symmetry:  the  radiate  phyla. 
Ciliary  locomotion,  colloblasts:  Phylum  4.  Clenophora. 
Muscular  locomotion,  nematocysts:  Phylum  5.  Coelenterata. 
Well  developed  mesoderm  with  nephridia. 

Mouth-anus  as  a  single  opening:  Phylum  6.  Platyhelminthes. 
Mouth  and  anus  separate  openings. 

Pseudocoelom,  no  circulatory  system. 

No  asexual  budding,  poor  powers  of  regeneration,  never  with 

ciliated  tentacles:  Phylum  7.  Aschelminthes. 
Asexual    budding,    good    powers    of    regeneration,    with    a    ring    of 
ciliated  tentacles:  Phylum  8.  Entoprocto. 
No  coelom  circulatory  system  present:  Phylum  9.  Nemertea. 
Eucoelom,  circulatory  system  usually  present:  Superphylum  Eucoelo- 
m.ata. 
Protostomous  and  primarily  schizocoelous. 

Reduced  coelom  and  no  segmentation:  Phylum  10.  Mollusca. 
Well  developed  coelom,  usually  segmented  at  least  as  larvae: 
Phylum  11.  Annelida  and  the  related  phyla  12.  Echi- 
uroidea,  13.  Sipunculoidea,  and  14.  Priapuloidea. 
Reduced  coelom  and  segmented:  Phyla  15.  Onycophora  and 
16.  Arthropoda. 
Minor  phyla  that  are  protostomous  but  not  closely  related  to  the 
preceding:  some  are  enterocoelous:  The  lophophore-bear- 
ing  phyla  17.  Phoronida,  18.  Brachiopoda,  and  19.  Bryozoa, 
and  the  phylum  20.  Chaetognatha. 
Deuterostomous  and  primarily  enterocoelous. 

Hydraulic  coelom  and  secondary  radial  symmetry:  Phylum  21. 

Echinodermata. 
Hydraulic  coelom  and  gill  slits:  Phylum  22,  Hemichordata. 
Gill  pouches  and  notochord:  Phylum  23.  Chordata. 

831 


332  APPENDIX 

PHYLUM    1.   PROTOZOA.      Ihc  protozoans.  Unicellular  animal-s,  sometimes  colonial. 

CLASS   1.  FLAGELLATA    (or    MASTIGOPHORA).     Flagellates.    With    one    to    many 
flagella  as  locomotor  organelles.  15  to  25  orders,  including: 

Order   1.   Dinoflagellata.     One    transverse    and    one    longitudinal    flagel- 
luin. 

Order  2.  Euglenida.     Two  flagella  arise  in  a  gullet.  Euglena,  PeraJiema. 

Order  3.   Phytomonadina    (or    Volvacoles).     Chlamydomonas,    Volvox,  etc. 

Order  4.  Choonoflagellota.     Sphneroeca,  Codosiga,  etc. 

Order  5.  Trypanosomida.      Trypanosoma,  Leishmania. 

Order  6.  Disfomata.     Two  nuclei.   Giardia. 

Order  7.  Trichomonodino.     Trichomonas. 
CLASS  2.  SARCODINA.     Rhi/opods.  With  pseudopodia  for  locomotion. 

Order  1.  Amoebozoa.     Di[jlugia,  Endamoeba,  and  other  amebas. 

Order  2.   Foraminifero.     Globigerina,  nummulites,  etc. 

Order  3.  Heliozoa.     No  skeleton,  many  radiating  pseudopods. 

Order  4.   Rodiolaria.     Many  radiating  pseudopods  and  internal  skeleton. 
CLASS  3.  CI  LI  ATA.     The  ciliates.  With  cilia  as  locomotor  organelles. 

Order  1.  Holotrlcha.     Paramecium,  Tetrahymena,  Balantidium. 

Order  2.  Spirotricho.     Hypotrichs,  etc. 

Order  3.   Peritricha.      Vorticella,  etc. 
CLASS  4.  SUCTORIA.     Cilia  in  young,  adults  with  tentacles.  One  order. 
CLASS  5.  SPOROZOA.     Parasitic.    Reproduction    by    multiple   fission.    Eight    or 
ten  orders,  including: 

Order  1.  Gregorinida.     Gregarines,  invertebrate  hosts  only. 

Order  2.  Coccidia.     All  kinds  of  hosts,  produce  coccidiosis. 

Order  3.  Haerriosporidla.     Vertebrate  hosts.  Plasmodixim. 

PHYLUM  2.  MESOZOA.  Parasitic.  A  single  layer  of  outer  cells  surrounds  a  few  repro- 
ductive cells.  Two  orders.  Uncertain  whether  they  arose  from  the 
Protozoa  or  by  simplification  from  the  Platyhelminthes. 

PHYLUM  3.   PORIFERA.     Sponges.  Body  with  many  small  incurrent  pores  and  a  few  large 

excurrent  openings  connected  by  chambers  lined  with  choanocytes. 

CLASS  1.  CALCAREA.     Calcareous   spicules    with    1,    3    or   4   rays.    Two    orders. 

Ascon,  Sycon,  Leuconia,  etc. 
CLASS  2.  HEXACTINELLIDA.     Glass  sponges.  Siliceous  spicules  with  6  rays  often 

united  in  networks.  Two  orders.  Euplectella. 
CLASS  3.  DEMOSPONGIA.     Skeleton  various,  not  as  above. 
Subclass  1.  Tetractinellida.     No  spongin,  siliceous  spicules  4-rayed.  Three 

orders. 
Subclass  2.  Monaxonida.     Siliceous  spicules  1 -rayed,  spongin  sometimes  pre- 
sent. Four  orders.  Includes  fresh-water  sponges,  Spongillidae. 
Subclass  3.  Keratosa.     Spongin  only,  no  spicules.  One  order.  Includes  the 
bath  sponges,  Spongiidae. 

PHYLUM  4.   CTENOPHORA.     Comb  jellies.  Radiata  with  eight  rows  of  ciliary  combs  and 

colloblasts. 
CLASS   1.  TENTACULATA.     W'ith   one   pair    of   branched    tentacles.   Four   orders. 

Pleurobrachia,  Mnemiopsis,  Coeloplana. 
CLASS  2.  NUDA.     No  tentacles.  One  order. 


APPENDIX  833 

PHYLUM    5.   COELENTERATA.     Polyps   and    medusae.   Radiata    without   combs    and   with 
nematocysts. 
CLASS  1.  HYDROZOA.     Medusae  with  a  vehmi,  polyps  with  simple  gut. 
Order   1.  Trachylina.     Gotiioneiims,   fresh-water  jellyfishes. 
Order  2.   Hydroidea.     Obelia,  hydras. 
Order  3.   Siphonophora.     Physalia. 

Orders  4  and  5.  Milleporina   and   Stylasterina.     Colonial   polyps    that   se- 
crete massi\e  limestone  exoskeletons. 
CLASS  2.  SCYPHOZOA.     Medusae  without  a   velum,  polyps  with   four  internal 

partitions.  Five  orders.  Aurelia. 
CLASS  3.  ANTHOZOA.     Polyp    with     6,    8,    or    more    internal     partitions.    No 
medusa. 
Subclass  1.  Alcyonaria.     Eight  feathery  tentacles  and  8  internal  partitions. 
Often   with    an   internal   skeleton.  Six  orders.   Sea    fans,    precious 
coral. 
Subclass  2.  Zoantharla.     Internal   partitions   6   or   more,    tentacles   simple. 
Five  orders,  including: 
Order   1.  Actinaria.     Sea  anemones. 
Order  2.  Madrepororio.     True  corals. 

PHYLUM   6.   PLATYHELMINTHES.     Flatworms.   Well  developed   mesoderm  with   nephridia, 
a  single  opening  foi   mouili  and  anus. 
CLASS   1.  TURBELLARIA.     Free-living,  epidermis  ciliated   in   adult. 
Order   1.  Acoela.      No  gut  cavity. 
Order  2.   Rhabdocoala.     Simple   tubular  gut. 
Order  3.  Alloeocoela.     Cut    has    one    main    branch    with    small    side 

branches. 
Order  4.  Tricladida.     Cut  has  three  branches.  Planaria,  Diigesia. 
Order  5.  Polycladida.     Gut  has  many  main  branches. 
CLASS   2.  TREMATODA.     Fhik.es.    Parasitic,   willi   o\d\  sucker,  epidermis   lacking. 
Order   1.  Monogenea.     Fctoparasitic  with  a  one-host  life  cycle. 
Order   2.   Aspidobofhria.     Fndoparasitic  with  a  one-host  cycle. 
Order  3.  Digenea.     Fndoparasitic     with     at     least     a     two-host     cycle. 
Srhi^tosotna. 
CLASS  3.  CESTODA.     Tapeworms.  Endoparasites   with   no  epidermis,  no  gut. 
Subclass   1.  Cestodarl^.     Body  not  segmented.  Two  orders. 
SuBCi^ss  2.  Eucestoda.     Body   segmented   into   proglottids.    Nine   orders,   of 
which  the  follovving  two  are  foiuid  in  mammals: 
Order  1.  Bothriocephaloidea.     Fish  tapeworms   (fish  carry  cercoid  stage). 
Order   2.  Taenioidea.     Pig  and  beef  tapeworms,  etc. 

PHYLUM   7.   ASCHELMINTHES.     Pseudocoelomates  with   tendencies  toward  extreme  cellu- 
lar differentiation  and  loss  of  regenerative  powers.  Body  covered 
by  a  cuticle. 
CLASS   1.  ROTIFERA.     Rotifers.  Wheel  organ  around  mouth  and  jaws  in  phar- 

vnx.  Three  orders.  Philodina,  Rotaria. 
CLASS  2.  GASTROTRICHA.     Gastrotrichs.     Cilia    on     ventral     surface,     pharynx 

nematode-like.  Two  orders. 
CLASS  3.  KINORHYNCHA.     Body    segmented    with    eversible    spiny    head.    One 

order. 
CLASS  4.  NEMATODA.     Roundworms.  Triradiate  pharynx  and  modified  nephri- 
dia. About  17  orders,  including: 
Order   1.   Rhabditoidea.  Vinegar  eel. 
Order  2.  Ascaroidea.  Ascaris,  other  large  intestinal  roundworms. 


g34  APPENDDt 

Order  3.   Oxyuroidea.      PinworniS. 

Order  4.  Strongyloidea.     Hookworms.    Ancylostoma,  Necatof. 
Order  5.   Filarioidea.     Loa,  Wuchereria. 
Order  6.  Trichuroidea.      Trichinella  and  whipworms. 
CLASS  5.  GORDIACEA     (or     NEMATOMORPHA).     Hairworms.     Reduced     digestive 

tract  and  no  nephridia.  Two  orders. 
CLASS  6.  ACANTHOCEPHALA.     Spiny-headed  worms.  Endoparasitic  aschelminths 
with  no  mouth  or  digestive  tract.  Three  orders. 

PHYLUM   8.    ENTOPROCTA.     Pseudocoelomates    with    a    circle    of    ciliated    tentacles    sur-  ± 

rounding  both  mouth  and  anus.  One  order  in  one  class.  " 

PHYLUM  9.  NEMERTEA.  Ribbon  worms.  With  a  circulatory  system  but  no  body  cavity. 
An  e%ersible  proboscis  lies  in  a  special  cavity  in  front  of  the 
mouth.  Two  subclasses  and  four  orders. 

PHYLUM    10.   MOLLUSCA.     With  a  ventral  foot  and  dorsal  shell.  Coelom  reduced,  circu-  i 

latory  system  with  extensive  sinuses.  I 

CLASS   1.  AMPHINEURA.     Foot  flattened,  shell   in  more   than    two   pieces.  | 

Order  1.  Polyplacophora.     Chitons.  Shell  a  dorsal  row  of  eight  plates. 
Order  2.  Aplacophora.     Shell   reduced   to  buried  spicules,   body  worm- 
like. 
CLASS  2.  GASTROPODA.     Snails.  Foot  broad  and  flat,  shell  single  and  usually 
coiled. 
Order   1.   Prosobranchia.     Abalone,  Busycon. 
Order  2.  Opisfhobranchia.     Pteropods,  nudibranchs. 
Order  3.  Pulmonata.     Garden  snails,  slugs. 
CLASS  3.  SCAPHOPODA.     Tooth  shells.  Foot  conical,  shell   tubular.  One  order. 
CLASS  4.  PELECYPODA.     Foot  spadelike,  shell  hinged  dorsally  with  two  lateral 
valves. 
Order   1.   Protobranchiafa.     Gills  plumose,  palps  large. 
Order  2.   Filibranchiafa.     Marine  mussels  and  scallops. 
Order  3.   Eulamellibranchiata.     Clams,  oysters,   fresh-water  mussels. 
Order  4.  Septibranchiata.     Gills    form    horizontal    partitions   in   mantle 
cavity. 
CLASS  5.  CEPHALOPODA.     Foot  forms  tentacles  and  siphon. 
Subclass  1.   Ietr-^branchiata.     Four    gills,    chambered    external    shell,    no 
suckers  on  tentacles. 
Order  1.  Nautiloidea.     The  chambered   nautilus, 
t  Order  2.  Ammonoidea.     Ammonites.   Partitions   in   shell  wrinkled. 
Subclass  2.  Dibr.\nchl\ta.     Two   gills,   shell  internal   or  absent,   arms  with 
suckers, 
t  Order  1.  Belemnoidea.     Belemnites.  Shell  straight,  slender,  heavy. 
Order  2.  Sepioidea.     Cuttlefish. 
Order  3.  Teuthoidea.     l.oligo:  deep-sea  squids. 
Order  4.  Octopoda.     Octopuses. 

PHYLUM    11.   ANNELIDA.     Segmented  worms  with  a  large  coelom  and  a  closed  circulatory 
system.  Protostomous. 
CLASS   1.  POLYCHAETA.     With  parapodia  and  numerous  chaetae. 
Order   1.   Errantia.     Nereis,  Autolytus,  Palolo  worm. 
Order  2.  Sedenfaria.     Hydroides,  lugworm. 
CLASS  2.  ARCHIANNELIDA.     Small  marine  annelids  with  simplified  body.  Once 
thought  to  be  ancestral  to  other  annelids,  now  believed   to  have 
come  from  the  polychaetes.  One  order. 


APPENDIX  835 

CLASS  3.  OLIGOCHAETA.     Parapodia    absent,    chaetae    few    per    segment.    One 

order.  Lumbricus,   Tubifex,  Aeolosoma. 
CLASS  4.  HIRUDINEA.     Leeches.  Parapodia  and  cheetae  absent.  With  suckers. 

Order  1.  Rhynchobdellida.     No  jaws,  pharynx  eversible,  blood  colorless. 

Order  2.  Gnathobdellida.     Three  jaws,  blood  red.  Hirudo. 

PHYLUM  12.  ECHIUROIDEA.  Adults  not  segmented,  larvae  with  up  to  15  segments.  One 
pair  of  ventral  chaetae.  One  order.  Bonnelia.  Often  considered  to 
be  a  class  of  the  Annelida. 

PHYLUM  13.  SIPUNCULOIDEA.  Adults  not  segmented,  larvae  with  up  to  3  segments.  No 
chaetae,  anus  dorsal,  head  retractile.  One  order.  Often  considered 
to  be  a  class  of  the  .Annelida. 

PHYLUM  14.  PRIAPULOIDEA.  Adults  not  segmented,  larvae  unknown.  No  chaetae,  anus 
posterior,  head  retractile.  .No  circulatory  system.  One  order.  Often 
considered  to  be  a  class  of  the  Annelida  and  sometimes  a  class  of 
the  Aschelminthes. 

PHYLUM  15.  ONYCOPHORA.  Segmented,  with  a  hemocoel,  one  pair  of  unjointed  limbs 
per  segment.  One  order.  Peripatus.  Often  considered  to  be  a  class 
of  the  .\rthropoda. 

PHYLUM   16.  ARTHROPODA.     Segmented  protostomous  eucoelomates  with  a  hemocoel  and 

jointed   legs, 
t  Subphylum    1.   Trilobita.     Antennae  on  second  segment,  biramous  limbs  on  all  succeed- 
ing segments.  One  class  with  five  orders. 
Subphylum   2.   Arachnomorpha.     Chelicerae  on  third  segment,  no  antennae. 

t  CLASS   1.  AGLASPIDA.     Limbs  on  opisthosoma  small  but  leglike.  One  order. 
CLASS  2.  XIPHOSURA.     Kingcrabs.     One    order.     Gill    books    on     opisthosoma. 
Limulus. 
t  CLASS  3.  EURYPTERDA.     Opisthosoma   divided    into    mesosoma    and    metasoma. 
.\pj)ciuiages  of  mesosoma  gill  like.  One  order. 
CLASS  4.  PYCNOGONIDA.     Sea     spiders.    Body     greatly    reduced,     opisthosoma 

rudimentary.  One  order. 
CLASS  5.  ARACHNIDA.     Respiration    by   book    lungs   or    trachea    or    both.    Ap- 
pendages of  the  fourth  segment  often  specialized  as  pedipalps. 
Subclass  1.  Laticasfra.     Mesosoma  broadly  joined  to  prosoma. 

Order  1.  Scorpiones.     Scorpions.     Poison     sting    on     telson,    pedipalps 

chelate. 
Order  2.  Pseudoscorpiones.     Like  scorpions  but  very  small,  no  sting. 
Order  3.  Opiliones.     Daddy-longlegs   or  harvestmen.   Pedipalps   tactile, 

legs  verv'  long,  opisthosoma  very  short. 
Order  4.  Acari.     Mites  and   ticks.  Sarcoptes,  Dermacentor. 
Additional  orders  of  uncertain  taxonomic  affinities: 
Order  5.  Myzostomida.     Parasites   with   a   much   simplified   adult   mor- 
phology, usually  considered  to  have  evolved  from   the  mites. 
Order  6.  Tardigrada.     The  water  bears.  Small  aquatic  or  semiterrestrial 
arthropods  with  a  simplified  morphology.  Usually  considered   to 
have  evolved  from  the  mites. 
Subclass  2.  Cauligastra.     Constriction   between   mesosoma   and   prosoma. 

Order  1.  Palpigradi.     Minute,  legs  long,  metasoma  long  and  threadlike. 
Orders   2,  3.  Schizomida   and    Thelyphonida.     Whip   scorpions.   Pedipalps 
large  and  chelate,  metasoma  long  and  whiplike. 


836  APPENDIX 

Order  4.  Phrynichida.     Pedipalps    large    but    not    chelate,    opisthosoma 

rounded. 
Order  5.  Araneae.     Spiders.  Poison  sting  in  chelicerae.  Argiope. 
Order  6.  Ricinulei.     Rare  tropical  spider-like  forms. 
Order  7.  Solifugae.     Chelicerae  short  but  very  stout,  pedipalps  leglike. 
Sobphylum  3.   Crustacea.     Antennae    on    second    and    third    segments.    One    class.    If 
another  class  is  included  (see  doubtful  groups  at  the  end  of  this 
phylum)  the  class  of  Crustacea  would  be  defined  further  as  having 
mandibles  on  the  fourth  segment. 
SuBCL.\ss  1.  Branchiopoda.     Thoracic  limbs  leaf  like,  respiratory. 
Order  1.  Anostraca.     Brine  shrimps  and  fairy  shrimps. 
Order  2.   Notosfroca.     Tadpole   shrimps,  Apus. 
Order  3.  Conchostraca.     Clam  shrimps. 
Order  4.  Cladocera.     Water   fleas.   Daphnia. 
Subclass  2.  Ostracoda.     Body  without  segmentation   and  entirely   enclosed 
in  a  bivalved  carapace.  Five  orders,  including: 
Order  1.  Podocopa.     Includes  most   of  the  fresh-water  species. 
Order  2.  Myodocopo.     Includes  several  common  marine  species. 
Subclass  3.  Cirripedia.     Sedentary,   compound  eyes  lacking,  carapace  forms 
a  mantle  covering  body  and  often  secreting  a  shell. 
Order  1.  Thorocica.     Acorn  and  gooseneck  barnacles. 
Order  2.  Acrothoracica.     Barnacles  commensal  on  mollusc  shells. 
Order  3.  Ascothoracica.     Parasites  of  corals  with  enlarged   mantle. 
Order  4.  Apoda.     Parasites  of  barnacles,  mantle  and  limbs  lacking. 
Order  5.  Rhizocephalo.     Parasites    of    crabs,    shrimps,    etc.,    largely    in- 
ternal. 
Subclass  4.  Copepoda.     Small,  one   pair  of  maxillipeds,  no  abdominal   ap- 
pendages. 
Order  1.  Bronchiura.     Fish  lice.  Ectoparasites,  with  compound  eyes. 
Order  2.  Eucopepodo.     No  compound  eyes.  The  copepods. 
Subclass  5.  Cephalocarida.     Small,  intermediate  between  the  Copepoda  and 

Malacostraca,  possibly  ancestral  to  both.  One  order. 
Subclass  6.  MYSTAcocARmA.     Similar   to    copepods    but    with   different   seg- 
mentation. By  broadening  the  definition  of  subclass  4,  subclasses 
5  and  6  can  be  included  as  orders  equal  in  rank  to  the  fish  lice 
and  true  copepods. 
Subclass  7.  Malacostraca,  the  large  crustaceans.  Thorax  of  8  segments. 

Superorder  1.  Leptostraca.     Abdomen    of    8   segments    including    telson 

(all  others  have  seven).  One  order.  Nebalia. 
Superorder  2.  Peracarida.     Incomplete   carapace,   abdomen   narrow. 
Order   1.  Mysidacea.     Mysid  shrimps.  Short  carapace  present. 
Order  2.  Cumocea.     Mud-inhabiting  relatives  of  the  mysids. 
Order  3.  Amphipoda.     No  carapace.  Beach  fleas,  scuds. 
Order  4.  Isopoda.     No  carapace.  Cribbles,  sowbugs,   pillbugs. 
Superorder  3.  Hoplocarida.     Short  carapace,  abdomen  wider  than  cepha- 

lothorax.  One  order.  The  mantis  shrimps. 
Superorder  4.  Eucarida.     Carapace  covers  entire  thorax. 
Order   1.  Euphausiacea.     Krill. 

Order  2.  Decopoda.     Shrimps,     lobsters,     crabs,     crayfish      (Cainbarus, 
Astacus). 
Subphylum  4.   Labiata.     Antennae  on  second  segment,  nothing  on  third,  mandibles  on 
fourth.  Second  maxillae  form  lower  lip. 
SUPERCLASS  1.  MYRL\PODA.     Adults  with  more  than  three  pairs  of  legs. 
CLASS   1.  CHILOPODA.     Centipedes.  First  legs  are  poison  fangs.  Five  orders. 


APPENDIX  837 

CLASS  2.  DIPLOPODA.     Millipedes.  Every  other  body  segment  reduced,  especially 

dorsally.  About  eight  orders. 
CLASS  3.  PAUROPODA.     Similar    to    millipedes.    Small,    eyeless,    with    branched 

antennae.  Two  orders. 
CLASS  4.  SYMPHYLA.     Small,  eyeless.  Mouth  parts  and  legs  similar  to  those  of 
insects.  One  order. 
SUPERCLASS  2.  HEXAPODA.     Adults  with  three  pairs  of  legs.  One  class,  Insecta. 
GROUP  1.  APTERYGOTA.     Primitively    wingless,    very    little    metamorphosis. 
Orders  1  and  2  are  often  placed  in  a  separate  class. 
Order   1.  Thysanura.     Silverfish,  firebrats. 

Order  2.  Entotrophi.     Similar  to  the  Thysanura  but  lack  scales  on  body. 
Order  3.  Profura.     Lack   both  eyes  and  antennae.  Often  considered   to 

be  a  class. 
Order  4.  Collembola.     Springtails,  snowfleas.  Often  considered  to   be  a 
class,  sometimes  a  superclass. 
GROUP  2.  PTERYGOTA.     With   wings,   although   numerous  species   have  sec- 
ondarily lost  the  wings.  When  the  above  groups  are  separated  as 
three    classes,    this   group   forms    a    fourth   and    is    usually    called 
Insecta. 
Subclass  1.  Paleoptera.     Wings   held  stiffly  out   at   the  sides.   Five  extinct 
orders  and: 
Order  1.  Odonata.     Dragon  flies  and  damsel  flies. 
Order  2.   Ephemeroptera.     .Mayflies. 
Subclass  2.  Neoptera.     Wings  fold  back  when  at  rest. 

Superorder  1.  Exopterygota.     \\'ingbuds    external,     metamorphosis     in- 
complete. Five  extinct  orders  and: 
Order   1.   Plecoptera.     Stoneflies. 
Order  2.  Orfhopfera.     Praying    mantids,    walking    sticks,    grasshoppers, 

crickets  and  katydids. 
Order  3.  Blattaria.     Cockroaches. 
Order  4.   Isoptera.     Termites. 
Order  5.   Dermaptera.     Earwigs. 

Order  6.  Embioptera.     Somewhat  like  termites  and  earwigs. 
Order  7.   Thysanoptera.     Thrips. 
Order  8.   Psocoptera.     Book  lice. 
Order  9.  Mallophaga.     Birdlice  or  biting  lice. 
Order   10.  Anoplura.     Sucking  lice.  Pediculus,  Phthirus,  etc. 
Order  11.  Hemipfera.     True  bugs,  plant  lice,  cicadas,  Rhodnins. 
Superorder  2.  Endopterygota.     Wingbuds  internal,  metamorphosis  com- 
plete. 
Order   1.  Neuropfera.     Lacewings,  ant  lions,  etc. 
Order  2.  Mecoptera.     Scorpion  flies. 
Order  3.  Trichoptera.     Caddis  flies. 
Order  4.  Lepidoptera.     Butterflies  and  moths. 
Order  5.  Coleoptera.     Beetles. 

Order  6.  Sfrepsiptera.     Small,  with  vestigial  anterior  wings. 
Order  7.  Hymenoptera.     Sawflies,  ants,  bees  (Apis),  wasps,  etc. 
Order  8.  Diptera.     True  flies,  gnats,  mosquitos,  Dermatobia,  etc. 
Order  9.  Siphonaptera.     Fleas.  Xenopsylla. 

Of  several  extinct  arthropod  groups  of  imcertain  affinities,  the  Archaeostraca  (four 
orders)  are  probably  a  subclass  of  the  class  Crustacea.  The  Homopoda  (four  orders), 
with  two  pairs  of  antennae  followed  by  biramous  limbs,  can  be  considered  a  separate 
class  in  the  subphylum  that  includes  the  Crustacea.  The  Xenopoda   (one  order)   are 


g38  APPENDIX 

intermediate  between  iiilobites  and  the  Arachnomorpha.  Since  they  have  antennae 
they  should  probably  be  placed  as  a  class  in  the  subphylum  containing  the  class  Tnlo- 
bita. 

PHYLUM   17.   PHORONIDA.       VVith  a  lophophore.  No  skeleton.  One  order. 

PHYLUM   18.   BRACHIOPODA.     With  a  lophophore.  Dorsal  and  ventral  shells. 

CLASS   1.  INARTICULATA.     Shells    without    hinge,    anus    present.    Two    orders. 

Lingula. 
CLASS  2.  ARTICULATA.     Shells  hinged,  anus  absent.  Two  or  three  orders.  Lamp- 
shells. 

PHYLUM    19.   BRYOZOA.     Moss   animals.   With    a   lophophore   surrounding   the    mouth. 
Circulatory  system  and  nephridia  absent.  Two  orders.  Bugula. 

PHYLUM   20.   CHAETOGNATHA.     Arrow    worms.    Enterocoelous,    with    lateral    fins.    One 
order. 


I 


PHYLUM   21.   ECHINODERMATA.     Deuterostomes    with     subepidermal     calcareous    plates 
and  usually  radial  symmetry  on  a  plan  of  five. 

Subphylum   1.   Pelmatozoa.      Attached  in  youth  or  throughout  life  by  an  aboral  stem, 
t  CLASS  1.  HETEROSTELEA.     Bilaterally   symmetrical,  possibly  ancestral   to  others, 
t  CLASSES   2-4.  CYSTIDEA,    BLASTOIDEA,   EDRIOASTEROIDEA.     Radial   symmetry,   no 
arms. 
CLASS  5.  CRINOIDEA.     Sea  lilies  and  sea  feathers.  Well  developed  arms,  anus  on 
oral  surface.  One  living  and  three  extinct  orders. 
Subphylum   2.   Eleutherozoa.     Stemless  unattached  echinoderms. 

CLASS   1.  HOLOTHURO.DEA.     Sea  cucumbers.  Armless,  elongate,  with  secondary 
bilateral    symmetry,    skeleton    reduced    to    microscopic    spicules. 
Five  orders,  all  living. 
CLASS  2.  ECHINOIDEA.     Sea    urchins,   sand  dollars.  Armless,   skeleton   well   de- 
veloped and   usually  rigid  with   numerous  mobile  spines.   About 
three  living  and  five  more  extinct  orders. 
CLASS  3.  ASTEROIDEA.     Starfishes.  With  arms,  skeleton  well  developed  but  flex- 
il:)le,  locomotion  by  tube  feet.  Three  living  and  two  more  extinct 
orders.  Asterias,  Leptasterias. 
CLASS  4.  OPHIUROIDEA.     Brittle   stars.  With  arms  and   flexible  skeleton,   loco- 
motion by  prehension.  Two  orders,  both  living, 
t  CLASS  5.  OPHIOCISTIOIDEA.     Armless.  Body  heavily  armored,  with  a  few  pairs 
of  very   large  scaly   tube  feet.  One  extinct  order. 

PHYLUM   22.   HEMICHORDATA.     Deuterostomes  with  bilateral  symmetry,  stomochord,  and 
usually  with  gill  slits. 
CLASS  1.  ENTEROPNEUSTA.     .\corn   worms.    Burrowing,  wormlike  animals   with 

numerous  gill  slits.  One  order.  Snccoglossiis. 
CLASS  2.  PTEROBRANCHIA.     Sedentary   animals  with   a   dorsal   anus,   collar  ex- 
panded as  a  lophophore  around  mouth. 
Order   1.   Rhabdopleuridea.     No  gill  slits,   lophophore  of  two  branching 

arms. 
Order  2.  Cephalodiscoidea.     One   pair  of  gill  slits,    lophophore  of  sev- 
eral branching  arms. 


APPENDIX  839 

PHYLUM   23.   CHORDATA.     1  he  chordates.  Deuterostomes  having  at  some  stage  of  their 
life  a  notochord;  pharyngeal  gill  pouches;  a  single,  dorsal,  tubular 
nerve  cord. 
Subphylum    1 .   Urochordata.     1  he  sea  squirts.  Notochord  and  nerve  cord  found  only  in 
the  larva. 
CLASS   1.  ASCIDIACEA.     Sessile  sea  squirts,  solitary  or  colonial.  Molgula. 
CLASS  2.  THALIACEA.     Pelagic  sea  squirts  propelled  by  jets  of  water  ejaculated 

by  the  contractions  of  the  body  wall.  Salpa. 
CLASS  3.  LARVACEA.     Pelagic,  neotenic  sea  squirts  retaining   the  larval  tail   as 
a  propulsive  organ.  Apfjendicularia. 
Subphylum   2.    Cephalochordata.      The  lancelets,  fusiform  chordates  in  which  the  noto- 
chord extends  the  length  of  the  bodv.  Ainphioxus. 
Subphylum   3.   Vertebrate.     The    vertebrates,    chordates    with    a    cranium    encasing   a 
brain;   notochord  generally  replaced   in   adult   by  vertebrae. 
CLASS   1.  AGNATHA.     Primitive,  jawless  vertebrates. 

t  Ostracoderms.     A  collective   name   for  four  orders  of  ancient,   heavily 
armored  fishes.  Hemicyclaspis. 
Order  5.  Cyclostomata.     Living,  jawless  fishes.  The  lampreys  and   hag- 
fishes.    The  sea  lamprey,  Petromyzon. 
t  CLASS  2.  PLACODERMI.     Six    orders    of    early    jawed    Hshes.    The    spiny    shark, 
Climatius. 
CLASS  3.  CHONDRICHTHYES.     Fishes   with    cartilaginous   skeletons. 

Subclass   1.  Ei.ASMoiiRANcini.     Cartilaginous    fishes    in    which    the    gill    slits 
open   independently  at  the  body  surface, 
t  Order  1.  Cladoselachii.     Primitive     sharks     with      broad      based      fins. 
Cladoselache. 
Order  2.  Selachii.     Modern  sharks.  The  dogfish,  Squalus:  whale  shark, 

Rluucadon. 
Order  3.  Batoidea.     Skates  and   rays.  The  common  skate,  Raja;  devil- 
fish, Manta;  sawfish,  Pristis. 
Subclass  2.   Hoi  ocfpii  all     Abberant   cartilaginous   fishes. 
Order   1.   Chimaerae.      The  ratHsh,  Cliiuiaera. 
CLASS  4.     OSTEiCHTHYES.     Fishes  with   at  least  partly  ossified  skeletons;   lungs 
or  swim  bladder  generally  present. 
Subclass   1.  Actinoptfr^  cii.     Bony  fishes  with  rav  fins. 

Superorder  I.  Cliotidrustei.     Four  orders  of  primitive  ray-finned   fishes. 
The    bichir,    Polypterus;    sturgeon,    Scaphirhynchus:    paddlefish, 
Polyodon. 
Superorder  2.  Hulostei.     Five   orders  of  intermediate    ray-finned    fishes. 

The  garpike,  Lepisosteus;  bowfin,  Anna. 
Superorder  3.  Teleostei.     Advanced  ray-finned  fishes. 

Order   1.  Isospondyii.     Primitive    teleosts.    The    tarpon.    Tarpon;    her- 
ring. Cltipea:  salmon  and  trout,  Sahno. 
Order  2.  Ostariophysi.     Most  fresh-water  teleosts.  such  as  the  carp,  cat- 
fish, suckers  and  true  minnows.  The  bullhead,  Ameiurus. 
Order  3.  Apodes.     The  eels.  The  .American  eel,  Anguilla. 
Order  4.  Heteromi.     Certain    deep-sea    fishes. 
Order  5.  Mesichthyes.     Intermediate   teleosts.  The  pike,  Esox:  killifish, 

Fundulus;  stickleback,  Gasterosteus:  sea  horse.  Hippocampus. 
Order  6.  Acanthopterygii.     Teleosts    having    spines    in    their    fins.    The 
perch,   Perca:   simfish,   Lepomis;    bass,   Micropterus;   cod,   Gadus; 
halibut,  Hippoglossus;  and  most  other  teleosts. 
Subclass  2.  Sarcopterygii.     Bony  fishes  with  fleshy  fins  and  often  internal 
nostrils. 


840  APPENDIX 

Order  1.  Dipnoi.     The   lungfishes.  Epiceratodus   of   Australia,   Protop- 

terus  of  Africa,  Lepidosiren  of  South  America. 
Order  2.  Crossopferygii.     Crossopterygians. 

•f- Suborder  1.  Rliipidistia.     Fresh-water   ancestors   of  amphibians. 
Suborder  2.  Coelacanthini.     More  specialized  freshwater  and  marine 
crossopterygians.  Latimeria. 
CLASS  5.  AMPHIBIA.     The    amphibians.    Larvae   generally    aquatic;    adults    ter- 
restrial. 
Subclass  1.  Aspidospondyli.    Vertebrae   develop   embryonically  from   cartil- 
aginous rudiments, 
t  Labyrinthodonfia.     A  collective  name  for   five  orders  of  ancestral  am- 
phibians. 
Order  6.  Anura.     The  frogs  and  toads.  The  leopard  frog,  Rana;   tree 
frog,  Hyla;  American  toad,  Bufo. 
Subclass  2.  Lepospondvli.     Vertebrae   develop    without    cartilaginous    rudi- 
ment, 
t  Order   1.  Microsauria.     Ancestral   lepospondyls. 
Order  2.  Urodela.     The  salamanders.  The  spotted  salamander,  Amby- 

stoiiui:  redbacked  salamander,  Plethodoii;  mudpuppy,  Necturus. 
Order  3.  Apoda.     The  legless  caecilians. 
CLASS  6.  REPTILIA.     The  reptiles:   tetrapods  that  are  covered  with  horny  scales 
and  reproduce  on  the  land. 
Subclass  1.  Anapsida.     Primitive  reptiles. 

t  Order  1.  Cotylosauria.     The  cotylosaurs,  the  ancestral  reptiles. 
Order  2.  Chelonia.     The    turtles.    Red-eared    turtle,    Pseudemys;   green 
sea  turtle,  Chelonia;  side-necked  turtle,  Chelodina. 
f  Subclass  2.  Eurvapsida.     Ancient  marine  reptiles  that  propelled  themselves 
with   paddles, 
t  Order   1.  Sauropterygla.     The  plesiosaurs. 
t  Subclass  3.  Ichthyoptervgia.     Ancient,  marine,  fishlike  reptiles, 
t  Order   1.   Ichythyosauria.     The  ichthyosaurs. 
Subclass  4.  DLVPsmA.     The  most  abundant  reptiles. 
Superorder  1.  Lepidosauria.     Lizard-like  reptiles, 
t  Order  1.  Eosuchia.     Ancestral  lepidosaurians. 
Order  2.  Rhynchocephalia.     Primitive   lizard-like   reptiles.   The   tuatara, 
Splteuodon. 

Order  3.  Squamata.     Lizards  and  snakes. 

Suborder  1.  Lacertilia.     Lizards.   The  collared   lizard,   Crotaphytus; 

horned  toad,  PInynosoma:  Gila  monster,  Heloderma. 
Suborder  2.  Ophidia.     Snakes.     The     Garter    snake,     Thamnophis; 
water  snake,  Natrix;  rattlesnake,  Crotalus. 
Superorder  2.  Archosauria.     The  ruling  reptiles, 
t  Order  1.  Thecodonfia.     Ancestral  archosaurs. 

t  Order  2.  Saurischia.     Saurischian    dinosaurs:     Tyrannosaurus,    Bronto- 
saurus. 

t  Order  3.  Ornithischia.     Ornithischian   dinosaurs:    Stegosaurus,    Tricera- 
tops. 

t  Order  4.   Pterosauria.     The   flying  reptiles:    Pteranodon. 
Order  5.  Crocodilia.     Alligators  and  crocodiles:  the  American  alligator, 
Alligator;  American  crocodile,  Crocodilus. 
t  Subclass  5.  Svnapsida.     The  mammal-like  reptiles. 

t  Order  1.  Pelycosauria.     Early   mammal-like  reptiles:   Dimetrodon. 
t  Order  2.  Therapsida.     Later  mammal-like  reptiles:   Lycaenops. 


APPENDIX  841 

r,A«  7    AVES      The  birds;  warm-blooded  tetrapods  covered  with  ^athers^ 
t  SuBcLsf  1    AKCHAEOKMTHES.     Ancestral  birds  with  a  long  senes  of  caudal 
vertebrae.  Archaeopteryx. 
SUBCASS  2.  NEORMTHEs.     Bhds  wuh  a  reduced  number  of  -"^^alj-^;^- 
^Superorder  1.  Odontognathae.     Cretaceous  birds,  some,  at  least,  retain 
ing  teeth.  Hesperornis,  Ichthyorms. 
Superorder  2.  Palaeognathae.     Modern    birds   with    a   primitive    palate. 
Most  are  flightless.  The  ostrich,  Struthio;  cassowaries.  Casuarius, 

kiwi,  Abteiyx.  .  ..     . 

Superorder  3    Neognathae.     Modern    birds    with    a     more    specialized 

OrderrSpheniscifor.es.     Penguins.    The    emperor    penguin.    Apterro- 

dytes.  . 

Order  2.  Gaviiformes.     Loons.  The  common  loon.  Gavm. 
Order  3    Colymbiformes.     Grebes.   Eared   grebe.   Colymbns. 
oZ  A.  Proce..oriifor.es.     Albatrosses,    shearwaters,    fulmars,    petrels. 

tropic  birds.  The  petrel,  Oceanodroma. 
Order  5    Peleconifor.es.     Pelicans,    gannets.    cormorants,    water-turkey. 

man-o'-war  bird.  The  pelican,  Pelecanus. 
Order  6.  Ciconiiformes.     Herons,  bitterns,  storks,  ibises,  flamingos.  Great 

blue  heron,  Ardea. 

,    ,        .,  niicks    eeese  and  swans.   The   mallard.  Anas, 

Order  7.  Anseriformes.     UUCKS,    gecsc 

white-fronted  goose.  Auser.  r-^^r.^r\ 

Order  8.  Foiconiformes.     Vultures,  kites,  hawks,  falcons,  eagles.  Coopers 

hawk,  Accipiter;  duck  hawk,  Falco. 
Order  9.  Gollifor.es.     Grouse,    quails,    partridges,    pheasants,    turkeys, 

chickens.  The  chicken.  Callus. 
Order  10.  Groiformes.     Cranes,   rails,   gallinules,   coot,.   The    whooping 

crane,  Grits.  ,      _, 

t  Order  11.  Dio.rymifor.es.     Large  flightless  birds  of  the  early  Cenozoic. 

Diatryma.  ,    .  .»:i,e 

Order  12      Chorodriifor.es.     Plovers,  woodcock,  snipe,  -^dpipe^  salt  , 
phalaropes,   gulls,    terns,    skimmers,   auks,   puffins.    The    killdeer, 

Charadrius.  ,  .         .   ^^^ 

Order  13.  Columbiformes.     Pigeons   and    doves.   The    domestic    pigeon, 

Columba. 
rs  A       ^A    Pcittaciformes      Parrots.  Carolina  parakeet,  Conurus. 
oZ  \t  c"::;!::;.      cuckoos,  .oad.ru„ners    The  cucUoo.  CuCus. 

°-'  .6.  S...i.o™...     °^'^:^^^  °I  pprrwiUs.    The    .hip- 
Order   17.  Coprimulgiformes.     .MgntnawKS,    wi.ipp 

noorwill,  Caprimulgus. 
oJ,  ^S.  «i;™p;.Ho,.l    Swifs  and    hummmgWrds.  The  Cheney 

swift,  Chaetura. 
r>.^or  19    Coliiformes      The  colies  of  Africa. 

or  O".  Tro     oTorLs.     Trogons.  The  coppery-tailed  trogon,  Trogon. 

Order  21.  Corociifor.es.     Kingfishers.     The    belted    kingfisher.    Mega- 

Ord^?/' Picifor.es.     Woodpeckers  and  toucans.  The  Aic^er.  Co|.p.e. 

Order  23  Posserifor.es.  The  perching  and  song  birds.  The  largest 
"rder  of  birds,  it  includes  the  flycatchers,  larks,  swallows,  crows, 
lays,  chickadees,  nuthatches,  creepers,  wrens,  dippers,  thrashers, 
thrushes,  robins,  bluebirds,  kinglets,  pipits,  waxwings.  shrikes, 
starlings,  vireos,  wood  warblers,  weaver  finches,  blackbirds,  orioles, 
tanagers,  finches,  sparrows,  etc.  The  English  sparrow,  Passer. 


842  APPENDIX 

CLASS  8.  MAMMALIA.     The   mammals;    warm-blooded   tetrapods   generally   cov- 
ered with  hair. 
Subclass  1.  Prototheria.     Egg-laying  mammals. 

Order   1.  Monotremata.     The  monotremes.  The  platypus,   Orjiithorhyn- 
clnis;  spiny  anteater,  Tachyglossus. 
Subclass  2.  Theria.     Viviparous  mammals, 
f  iNFRACLASs  1.  pANTOTHERiA.     Auccstral  thcrians. 
infraclass  2.  METATHERiA.     Pouclied  mammals. 

Order  1.  Marsupialia.     The    marsupials.    The   opossum,   Didelphis. 
infraclass  3.  EUTHERiA.     The  placental  mammals. 

Order  1.  Insectivora.     The  insectivores,   the  most  primitive  placentals. 
The  order  includes  the  moles,  shrews  and  hedgehogs.  The  com- 
mon shrew,  Sorex. 
Order  2.  Dermoptera.     The  flying  lemur,  Galeopithecus. 
Order  3.  Chiroptera.     Bats.  The  little  brown  bat,  Myotis. 
Order  4.  Edentata.     New  world  edentates:  sloths,  anteaters,  armadillos. 

The  armadillo,  Dasypus. 
Order  5.   Pholidota.     The  pangolin,  Manis. 
Order  6.   Primates.     The  primates. 

Suborder  1.  Lemuroidea.     Tree  shrews,  lemurs,  lorises,  aye-aye.  The 

lemur,  Lejnur. 
Suborder  2.  Tarsioidea.     The  tarsier,  Tarsius. 

Suborder  3.  Anthropoidea.     The    monkeys,    apes    and    men.    Man, 
Homo. 
Order  7.  Cetacea.     The  whales. 
Suborder  1.  Odontoceti.     Toothed  whales.  The  bottlenosed  dolphin, 

Tursiops. 
Suborder  2.  Mysticeti.     Whalebone  whales.  The   blue  whale,  Bala- 
eyioptera. 
Order  8.  Carnivora.     Carnivores. 
Suborder  1.  Fissipedia.     Modern  terrestrial  carnivores  including  the 
dogs,    wolves,    foxes,    raccoons,    pandas,    bears,    weasels,    marten, 
wolverines,  badgers,  skunks,  mink,  otters,  cats,  lions,  tigers,  mon- 
goose, hyenas.  The  domestic  cat,  Felis. 
Suborder  2.  Pinnipedia.     Marine  carnivores.  The  seals,  sea  lions  and 
walruses.  The  harbor  seal,  Phoca. 
Order  9.  Tubulidentata.     The  aardvark,  Orycteropus,  of  South  Africa. 
Order  10.  Proboscidea.     Mastodons    and    elephants,    .\frican    elephant, 

Loxodonta:  Indian  elephant,  Elephas. 
Order  11.   Hyracoidea.     The  conies,  Hyrax,  of  the  Middle  East. 
Order  12.  Sirenia.     Sea  cows.  The  manatee,  Manatus. 
Order  13.  Perissodactyla.     Odd-toed  ungulates:  tapirs,  rhinoceroses  and 

horses.  The  horse,  Equus. 
Order  14.  Artiodacfyla.     Even-toed    ungulates:    pigs,    peccaries,    hippo- 
potamuses, camels,  llamas,  chevrotains,  deer,  giraffes,  pronghorns, 
antelopes,  cattle,  sheep  and  goats.  The  pig,  Sns. 
Order  15.  Rodentia.     Rodents.   The    largest  order  of  mammals,   it   in- 
cludes the  squirrels,  chipmunks,  marmots,  gophers,  beavers,  rats, 
mice,   muskrats,   lemmings,   voles,   porcupines,  guinea    pigs,   capy- 
baras,  and  chinchillas..  The  woodchuck,  Marmota. 
Order  16.  Lagomorpha.     Hares,   rabbits  and   pikas.    The  rabbit,  I.epits. 


4 


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■OSIC/Q- 


<^> 


INDEX 

This  index  is  intended  to  serve  as  a  Glossary  as  well.  The 
page  on  which  a  term  is  defined  is  indicated  in  boldface 
type. 


Aardvark,  494 

Abalone,   251 

Abducens  nerve,  417 

Abduction,  402 

Abomasum,   519 

Absorption,   524 

Abyssal  zone,  795 

Acanthocephala.  232,  834 

Acanthodes,  432 

Acanthopterygii,  839 

Acara,  835 

Accessory  respiratory  organs,  530 

Accipiter,  841 

Accommodation,  579 

Acetabularia,  35 

Acetabulum,  401 

Acetoacetic  acid,  616 

Acetylcholine,  106,  587,  596 

Acetyl  coenzyme  A,  72,  619 

Aciculum,  273 

Acid.  23 

Acoela,  213,  833 

Acorn  worms,  142 

Acoustic  nerve,  417 

Acoustico-lateralis  area,  582 

Acquired  characters,  inheritance  of,  690 

Acromegaly,  624 

Acrothoracica,  836 

ACTH,  625 

Actin,  44 

Actinaria,  833 

Actinopterygii,  440,  839 

Action  potential,  101,  585 

Active  reabsorption,  564 

Active  transport,  45 

Actomyosin,  98,  99 

Adaptation(s).   16,  781-797 

color,  785 

interspecific,  787 


Adaptation(s),    physiologic   and    chemical, 
784 

structural,  784 
Adaptive  con\ergence,  783 
Adaptive  cn/ymes,    134 
Adaptive  radiation,  442,  782 
Addison.  1  homas,  606 
Addisons  disease.  619 
Adduction,  402 

Adenosine  triphosphate,  67,  71 
Adhesive  glands,  204 
Adrenal  cortex,  617 
Adrenal  glands.  411,  616 
Adrenal  medulla,  617 
Adrenocorticotropic  hormone,  625 
Adrenogenital  syndrome,  620 
Aeolosoma,  281.  835 
Aestivation,  448,  762 
Afferent  neurons.  588 
African  eye  worm,  811 
African  sleeping  sickness,  809 
After-birth.  573 
Aftershaft,  471 
Age     of     rocks,     method     oi     measuring, 

720 
Agglutination,  542 
Agglutinins,  544 
Agglutinogens,  544 
Aggregations,  animal,  767 
Aglaspida,  835 
Agnatha,  389,  427,  839 
Agriculture,  822 
Air  sacs,  472,  477 
Airstream,  468 
Alarm  response,  274,  775 
Albatrosses,  480 
Albinism,  683 
Albumin,  455,  538 
Alcaptonuria,  688 
Alcyonarians,  197,  833 
Aldosterone,  619 

849 


850 


INDEX 


Alisphenoid  bone,  511 
AUantois,  131,  ^Sf),  640 
Alleles.  652 

multiple,  677 
Allelomorphs.  652 
Alligators,  465,  840 
Alloecoela,  833 
All  or  none  effect,  101 
Alplia-totopherol,  527 
Altricial,  483 
Alula,  469 
Aheolar  glands,  396 
Alveolar  sac,  533 
Alveoli,  488,   533 
Ambulacra!  grooves,  367 
Ambystoma,  449,  840 
Amehn,   157 
Amebocytes,  172 
Amensalism,  765 
Amia,  440,  839 
Amino  acids,  28,  523 

essential,  29 
Ammocoetes,  431 
Ammonia,  95,  454 
Ammonoidea,  854 
Amnion,   131,  455 
Amniotes,  455 
Amniotic  cavity,  131,  640 
Amniotic  fluid,  572 
Amoebozoa,  832 
Amphibia,  390,  393,  448,  840 
adaptations  of,  448 
characteristics  of,  447 
evolution  of,  447 

Amphiblastula,  179 

Amphineura,  246,  834 

Amphioxus,  383,  387,  392,  839 

Amphipoda,  305,  836 

Amplexus,  414 

Ampulla,  367,   581 

Amylase,  81,  523 

Anabolism,  15 

Analogous  structures,  424 

Anamniotes,   455 

Anaphase,  43 

Anapsida,  840 

Anatomic  evidence  for  evolution,  727 

Ancylostoma  duodenale,  808 

Androgens,  618,  620,  627 

Androsterone,  607,  627 

Anemia,  541 

Animal  pole,  126 

Anions,  20 

Annelida,  267-288,  834 
classes  of,  268 
reproduction  of,  277 

Anomalops,  76 

Anopheles,  116 

Anoplura,  802,  837 

Anostraca,  303,  836 


Anser,  841 

Antagonism,  607 

Antagonistic  muscles,  100 

Anteater,  494 

Antelope,  499 

Anthozoa,   190,  195,  833 

Anthropoidea,  496,  842 

Anthropoids,  738,  740 

Antibodies,  542 

Antidiuretic  hormone,  565,  622 

Antigen,  542 

Antigen-antibody  reaction,  542,  728 

Anti-reproductive  substance,  345 

Antisera.   543 

Antlers,  504 

Antrum,  630 

Ants,  346 

Anura,  394,  448,  840 

Anus,  521 

Aorta,  546 

Aortic  arch,  408,  546,  547,  645 

Ape  men,  744 

Aphasia,  603 

Apical  organ,  286 

Apis  mellifera,  317 

Aplacophora,  834 

Apoda  (Amphibia),  394,  448,  840 

Apoda  (Crustacea),  836 

Apodeme,  97 

Apodes,  839 

Apopyle,   174 

Appalachian  Revolution,  723 

Appendages,  486 

Appendicular  skeleton,  400,  508,  512 

Appendicularia,  839 

Aptenodytes,  841 

Apterygota,  313,  837 

Apteryx,  841 

Aqueduct  of  Sylvius,  598 

Aqueous  humor,  578 

Arachnida,  320,  321,  835 

Arachnoid  membrane,  599 

Arachnomorpha.  290,  320-323,  835 

Aragonite,  57 

Araneae,  322,  836 

Arbacia  punctulata,  374 

Archaeopteryx,  478,  706,  717,  841 

Archaeornithes,  479,  841 

Archenteron,  126,  420,  642 

Archeology,  751 

Archeozoic  era,  720 

Archiannelida,  269,  834 

Archosaurs,  462,  840 

Arctic  foxes,  774 

Arctic  tern,  484 

Argiope,  322,  836 

Aristotle,  8,  146,  696 

Aristotle's  lantern,  373 

Armadillo,  494 

Arms,  260 


♦ 


INDEX 


851 


Arrector  pili,  505 
Arrowworms,  357 
Arteries,  86,  537,  5.54 
Arterioles,  554 
Arthropoda,  289-350,  835 

behavior  of,  340 

circulatory  system  of,  299 

classification  of,  289 

physiology  of,  326-350 

visual  acuity  of,  337 
Articular  bone,  465 
Articulata,  838 
Artifacts,  751 
Artificial  selection.  732 
Artificial  stocking,  825 
Artiodactyla,  498,  842 
Ascaris,  833 

life  cycle  of,  230 
Ascaris  lumbricoides,  120.  229,  806 
Ascaroidea,  833 
Aschelminthes,  220,  833 

classification  of.  220 
Ascidiacea,  384,  839 
Ascon,  832 

Asconoid  sponges,  173 
Ascorbic  acid,  527,  528 
Ascothoracica,  836 
Aspidobothria,  833 
Aspidospondyli,  840 
Association  areas,  603 
Association  neurons,  602 
Astacus,  293,  836 
Asterias,  364-370,  838 
Asteroidea,  370,  838 
Asthma,  618 
Atlas,  400,  509 
Atom,  19 

Atrioventricular  node,  553 
Atrium,  86,  386,  388,  411,  545,  549 
Augmentation.  564 
Amelia,  192,  193,  833 
Auricle,  249 
Auricularia  larva,  379 
Australian  bushmen.  749 
Australian  realm,  736 
Australian  sidenecked  turtle,  456 
Australopithecus,  743 
Autocatalytic  particles,  712 
Autolytus,  281,   834 
Autosomes,  660 
Autotrophic  nutrition,  78 
Autotrophs,  evolution  of,  712 
Aves,  390,  468,  840 
Axial  filament,  61 
Axis,  509 
Axolotls,  451 
Axon,  60,  332 

giant.  106,  274 
Aye-aye,  494 
Aysheaia,  325 


B 


Baboons,  740 

Bacon,   Roger,  9 

Bacteriophages,  684 

Balaenoptera,  842 

Balantidium  coli,  807,  832 

Bandicoots,  492 

Barbules,  471 

Barriers,  787 

Basal  body,  150,  151,  165 

Basal  metabolic  rate,  525 

Base,  23 

Basement  membrane,  54 

Basilar  membrane,  584 

Basket  stars.  375 

Basophils.  59,  542 

Bath  sponges,  177 

Batoidea,  436,  839 

Bats,  492 

Bayliss,  William,  11,  525 

Beadle,  George,  687 

Beagle,  voyage  of,  698 

Bears,  497 

Beavers,  500,  825 

Bee  language,  347 

Beebread,  318 

Bees,  vision  of,  338 

Belemnoidea,  834 

Bell,  Charles,  10 

Benthos,  795 

Beriberi,  5,  83,  527,  528 

Bernard,  Claude,  10 

Bestiaries.  9 

Beutner,  R.,  710 

Biceps,  101,  514 

Bicuspid  \al\e,  553 

Bile,  406,  520 

Bile  pigment,  520,  541 

Bile  salts,  520 

Bills,  bird.  482,  781 

Binomial    system    of    nomenclature,    140, 

145 
Biochemical  genetics,  683 
Biochemical  recapitulation,  729 
Biogenetic  Law,  729 
Biogeographic  realms,  735 
Biogeography,  733 
Bioluminescence,  75 
Biome,  789 

Biotic  comminiity,  769,  77.5-777 
Biotic  potential,  773 
Biramous  appendages,  289 
Bird  lice,  802 
Birds,  468 

behavior  of,  481 

bills,  482,  781 

circulatory  system  of,  477 

digestive  system  of,  475 

evolution  of,  478 


852 


INDEX 


Birds,  excretory  system  of,  477 

feet,  482 

flight,  468 

migration  of,  483 

muscles,    174 

na\igation,  484 

origin,  478 

reproduction,  483 

respiratory  system  of,  476 

sense  organs,  477 

skeleton,  472 

structure  of,  471 

wings,  468 
lUrtii,  ,'')72 
Birth  rate,  771 
Biting  lice,  802 
Blastocoele,  126,  128,  420 
Blastocyst,  638 
Blastoidea,   838 
Blastomeres,  1 26,  637 
Blastopore,  126,  381,  420 
Blastula,  126,  420 
Blattaria,  837 
Blind  spot,  578 
Blood,  58,  537,  538 
Blood  cells,  84 
Blood  dotting,  541 
Blood  flukes,  214,  810 
Blood  groups,  544 

inheritance  of,  678 
Blood  pressure,  546,  618 

dogfish,  546 

frog,  549 

man,  555 
Blood  velocity,  man,  555 
Blood  vessels,  84 
Bloodsuckers,  269,  802 
Blubber,  503 
Blue  baby,   551 
Bluebird,  480 
Blue  whale,  498 

Body  fluids,  regulation  of,  559,  564 
Body  folds,  642 
Body  stalk,  640 
Bolus,  517 
Bone,  57 
Bonnelia,  354 
Bony  fishes,  437 
Bony  scales,  448,  504 
Book  gills,  320 
Hook  lungs,  321,  322 
Botflies,  813 
Bothriocephaloidea,  833 
Bouditch,  Henry,  10 
Bowfin,  440 

Bowman's  capsule,  93,  562 
Brachet,  Jean,  35 
Brachiopoda,       3.55,       356,       381 
838 


Brain,  inhibitory  center.  275 

parts  of,  597 

stimulatory  center,  275 
Branchial  arches,  432,  508 
Branchial  muscles,  512 
Branchiopoda,  303,  836 
Brnnrluostonut,  387 
Branchiura,  836 
Breathing,  311,  533 

in  frog,  407 
Breeding  habits,  125 
Bridges,  C.  B.,  660 
Brittle  stars,  375 
Broad-leaved  evergreen  subtropical  forest 

biome,  791 
Bronchus,  407,  476,  532 
Brontosaurus,  462,  840 
Brood  pouch,  303 
Brow  spot,  395 
Brownian  movement,  49 
Bryozoa,  356,  838 
Bubonic  plague,  802 
Buccal  funnel,  429 
Biichner,  Edward,  66 
Budding,   116,   189 
Buffer,  539 
Bufo,  840 

Bullock,  T.  H.,  274 
Busy  con,  248-251,  834 
egg  case  of,  250 


Caecilians,  448 
Caecum,   82,   521 
Calcar,  401 
Calcarea,  175,  832 
Calciferol,  527 
Calciferous  glands,  276 
Calcium,  23,  614 
Calcium  carbonate,  328 
Caloric  requirements,  525 
Calorie,  65,  525 
Calorimeter,  610 
Cam  barns,  293,  836 
Cambrian  period,  72 1 
Camels,  499 

body  lice  of,  819 
Canada  goose,  484 
Canal  of  Schlemm,  573 
Canine,  516 
Canine  tooth,  487 
Cape  Verde  Islands,  734 
Capillaries,  86,  537 
Capillary  exchange,  555 
Caprimulgiformes,  841 
Caprimulgus,  841 
721,       Carapace.  294,  456 

Carbaminohemoglobin,  539 


INDEX 


853 


Carbohydrates,  25 
Carbon,  22 
Carbon  cycle,  756 
Carbon  dioxide,  539 

fixation,  756 

transport  of,  90,  540 
Carbonic  acid,  539 
Carbonic  anhydrase,  540 
Carboniferous  period,  723 
Cardiac  sphincter,  519 
Cardiac  stomach,  368 
Cardiovascular  system,  545 
Caidiuin  edule,  256 
Caribou,  790 
Carni\ora,  497 
Carnivores,  79,  842 
Carotid  arch,  408 
Carotid  arteries,  549 
Carotid  gland,  408 
Carpals,  401,   512 
Carpometacarpus,  474 
Cartilage,  56,  438 
Cartilage  replacement  bone,  507 
Cassowaries,   481 
Casts,  425,  717 
Casuarins,  841 
Catabolism,   15 
Catalase,  67 
Catalepsv,  342 
Catalysis,  65 
Catalyst,  65 

Catarrhine  monkevs,  740 
Catastrophism,  696 
Caterpillars,  342 
Cations,   19 
Cats,  497 
Cattle,  499 
Cauligastra,  835 
Cave  paintings,   7 
Cavernous  bodies,  569 
Cavity,  516 
Cell,  14,  33 
Cell  constancy,  225 
Cell  constituents,  dynamic  state  of,  22 
Cell  lineage,  286 
Cell  theory,  1 1 ,  34 
Cellidar  energy,  71 
Cellular  respiration,  87 
Cellulase,  519 
Cellulose,  385 
Cement,  516 
Cenozoic  era,  725 
Center  of  origin,  733 
Centipede,  305 
Centriole,  38,  42 
Centrolecithal  egg,  126 
Centrum,  399,  508 
Cephalization,  384 
Cephalochordata,  387,  839 
Cephalogarida,  836 


Cephalodiscoidea,  838 

Cephalopoda,  259 

Cephalothorax,  293 

Cercaria,   215 

Cerci,  309 

Cercoid  larva,  21 8 

Cerebellum,  4I6,  478,  597,  600 

Cerebral  cortex,  602 

Cerebral  hemispheres,  416,  478,  597,  601 

Cerebrosides,  27 

Cerebrospinal  fluid,  417,  537,  598 

Cervix,  570 

Cestoda.  216,  833 

Cestodaria,  833 

Cetacea,  497,  842 

Chaetae,  268,  273 

Chaetognatha,  357,  381,  838 

Chaetura,  841 

Chambered  nautilus,  259 

Chameleon,  459 

Charadriiformes,  841 

Charadrius,  841 

Chelate  appendages,  290 

Chelicerae,  291,  320,  322 

Chelodina,  456,  840 

Chelonia,  456,  840 

Chemical  compounds,  21 

Chemical  ditterentiation,  137 

Chemoreceptor.  107,   108,  248,  297,  575 

Cherrystone  clam,  253 

Chief  cells,  519 

Child,  C.  M.,  211 

Chilopoda,  305,  836 

Chimaera,  434,  436 

Chimaerae.  8.39 

Chimpanzee,  495,  741,  742 

Chipmunk,  500 

Chiroptera,  492,  842 

Chitin,  273,  289 

Chlamydomonas,  155 

Chloragen  cells,  276,  277 

Chlorophyll,  152 

Choanocytes,  172,  236 

Choanoflagellates,  156,  236,  832 

Cholecystokinin,  525 

Cholesterol,  27,  619 

Cholinesterase.  106,  137 

Chondrichthyes.  389,  433,  839 

Chondrocraniimi,  508 

Chondrostei,  440,  839 

Chorda-mesoderm,  136 

Chordates,  142,  383-392,  839 

characteristics  of,  383 

origin  of,  391 

subphyla  of,  383 
Chorion,  131,  455,  640 
Chorionic  cavity,  131 
Chorionic  gonadotropin,  633 
Chorionic  villi,  132 
Choroid  coat,  576 


854 


INDEX 


C:horoid  plexus,  416,  r)98 

Ciliiomatiii,   37 

Chroma tophores.   112.  262,  396,  506,  622 

Chronionieres,  39 

Cliiomonema,  39 

Chromosome  maps,  666 

Chromosomes,  37,  39 

homologous,  117 
Chymotrypsin,  81,  523 
Ciconiiformes,  841 
Cilia,  15,  99,  149,  151 
Ciliary  body,  577 
Ciliata,  148,   160,  832 
Ciliophora,  165 
Circuit,  multiple  chain,  .591 
Circulation,  84,  537-557 

fetal,  549 

patterns  of,  545 
Circulatory  system,  birds,  477 

closed,  84 

development  of,  645 

dogfish.  435 

fetal  mammal,  550 

man,  548 

open,  86 

primitive  fish,  546 
Circumesophageal  connectives.  310 
Circinnpharyngeal  commissures,  267 
Cirri.  387 

Cirripedia.  303,  836 
Cladocera,  300.  303,  836 
Cladoselache,  839 
Cladoselachii,  839 
Clam,  steaming.  256 
Clamworm,  270 
Clasper,  433 
Class.   141 
Clavicle,  400,  512 
Claws,  454,  506 
Cleavage,  126,  637 
Click  mechanism,  335 
Climatius,  432,  839 
Climax  community,  777 
Clitellum,  278 
Clitoris,  570 

Cloaca.  391.  404,  435,  49!,  562 
Clothes  moth,  784 
Clupea,  839 
Coachwhip  snake,  461 
Coal,  756 
Coat  color,  inheritance  of,  673 

rabbit.  677 
Cobras,  462 
Coccidia,  832 
Coccidiosis,  165,  814 
Coccyx.  508 
Cochlea.  584 
Cochlear  duct,  582,  584 
Cockle,  256 
Cockroach,  307-313 


Cocoon.  278.  318 
Coelacanth,  444 
Coelenterata.  181,  833 

evolutionary  relationships  of,  237 
Coelom,  220,  268 

evolution  of,  238 
Coelomic  fluid,  402 
Coeloplana,  832 
Coenzyme,  68,  83 
Coiling,  direction  of,  690 
Colaptes,  841 

Cold-blooded  animals,  448 
Coleoptera,  316,  837 
Coliiformes,  841 
Collagen,  56 
Collar  cells,  80 
Collar  nerve.  363 
Collared  lizard,  453 
Collecting  tubule,  562 
Collembola,  313,  837 
Colloblasts.  200 
Colloid,  30,  608 
Colon,  404,  521 
Colon  bacteria,  524 
Colonial  insects,  344 
Color-blindness,  662,  692 
Columba,  841 
Columbiformes.  841 
Colymibiformes,  841 
Colymibus,  841 
Comatulidae.  371 
Comb  jellies.  181,  199 
Comb  types,  inheritance  of,  673 
Commensalism.  442,  764 
Commissural  fibers.  602 
Common  bile  duct,  520 
Communities,  dynamic  state  of,  779 
Community  succession,  777 
Comparative  anatomy,  146 
Competition.  763 
Complementary  genes.  669 
Compound  eyes,  302 
Conceptural  scheme,  4 
Conchae,  531 
Conchostraca,  300,  836 
Conductile  process,  230 
Congenital  traits,  691 
Conies,  500 

Coniferous  forest  biome,  791 
Conjoined  twins.  648 
Conjugation.  169 
Conjunctiva.  576 
Connector  neuron.  418 
Conservation.  822-830 
Conservation  of  Energy,  Law  of.  48 
Conservation  of  Matter,  Law  of,  65 
Constipation,  524 
Consumer  organisms,  754 
Contour  feathers,  471 
Contractile  fibrils,  150 


INDEX 


855 


Contractile  vacuole,  53,  93,  151 

Contraction  period,  101 

Control  group,  6 

Conurus,  841 

Conus  arteriosus,  411,  545,  549 

Convergent  evolution,  442,  494,  783 

Coordination,  111 

Copepoda,  303,  836 

Copulatory  sac,  210 

Coraciiformes,  841 

Coracoid,  400,  474 

Coral,  196 

precious,  197 
Coral  snakes,  462 
Corals,  true,  197 
Corixa,  755 
Cormorant,  480,  763 
Cornea,  576 
Coronary  arteries,  554 
Coronary  vein,  554 
Corpus  allatum.  111.  330 
Corpus  callosum,  602 
Corpus  cardiacum.  330 
Corpus  luteum,  630 
Corpus  striatum,  602 
Cortex,  562 
Cosmic  rays,  706 
Cosmin,  443 
Costello,  D.   P..   287 
Cotylosaurs,   456,   840 
Cough  reflex,  535 
Coupled    reactions,    67 
Cow,  stomach  of,  518 
Cowbird,  820 
Cowpers  glands,  123.  570 
Cowpox,  543 
Coxa,   308 

Cranial   nerves,   591,   592,  593 
Cranium,  390,   397,  429,  510 
Crayfish,  293-300 

muscle   innervation   in,   333 
Creeper  fowl,  680 
Cretaceous  period,   723 
Cretin,  612 
Crinoidea,   370,   838 
Critical   periods,    138 
Croaking,  408 
Crocodiles,  465 
Crocodilia,  465,  840 
Crocodilus,  840 
Cro-Magnon  men,  749 
Crop,  81,  276,  309,  475,  518 
Cross,  dihybrid,    656 

monohybrid,  653 

test,   655 
Crossing  over,  664 
Crossopterygii,  442,  446,  840 
Crotalus.  840 
Crotaphytus,  453,  840 
Crown,  of  tooth,  51 6 


Crustacea,  291,  292-305,  836 

endocrine  organs  of,  328 
Cryptorchidism,  628 
Crystalline  style.  255 
Ctenoid   scale,   441 
Ctenophora,  199,  832 
Cuckoo,  820 
Cuculiformes,    841 
Cuculus,  841 

Cucumaria  frondosn,  373 
Cumacea,  836 
Curare,   lOl 

Curve  of  normal  distribution,  675 
Cushings    syndrome,    620 
Cutaneous  artery.  409 
Cuticle,  55,  227,  267 
Cuticulin,    326 
Cuttlefish,   265 
Cuviei,  Georges.   11.  146 
Cyanide,  53 
Cycles,  diurnal.  279 

lunar.  279 

metabolic,  22 

seasonal,  279 
Cycloid  scale,  441 
Cycloposthiuni,    170 
Cyclostomata.  429.  839 
Cyclotron.  20 
Cystic  duct.  520 
Cystidea,  838 
Cytoplasm,   14 
Cytoplasmic  bridges.  1  56 
Cytoplasmic  inheritance,  689 


Dams,  825 

Daphnia,  300,  342 

Darwin,  Charles,  281,  698 

Dasyatis,  434 

Dasypus,  842 

Da  Vinci,  Leonardo.  9 

Deamination,  29,  82.  526 

"Death  feigning".   342 

Decapoda,  293,  305,  836 

Decarboxylation,  74 

Deciduous  forest  biome,  791 

Decomposer  organisms,   755 

Deep-sea  animals,  796 

Deer,  499 

Defecation,   515 

De  Humani  corporis   fabrica,  9 

Dehydroepiandrosterone,  619,  627 

Deletion,  685 

Demospongia,  176,  832 

Dendrite,  60 

Dentary  bone,  466,  511 

Denticles,    275 

Dentin,  516 

Depolarization,   585 


856 


INDEX 


Dermacentor,  803.  835 
Uermal   bone,   438,   507 
Dermal  skeleton,  507 
Derinaptera,  837 
Deruialobia  hominis,  805 
Dermis,  395,  502,  503 
Dermoptera,  493,  842 
Descartes,  Rene,   10 
Desert  biome,  793 
Desiccation,  resistance  to,  226 
Design  of  experiments,  6 
Desmarella,  156 
Desoxycorticosterone,  619 
Desoxyribonucleic  acid,  29,  683 
Deuterostomous   animals,   378,    381 
Development,  control  of,  133 

direct,  452 
Devilfish,  437 
de  Vries,  Hugo,  704 
Diabetes  insipidus,  622 
Diabetes  mellitus,  564,  615,  620 
Diabetogenic  hormone,  626 
Dialysis,  51 
Diapause,  331 
Diaphragm,  488,  532,  534 
Diapsida,  840 
Diastole,   552 
Diatrynia,  841 
Diatryniiformes,  841 
Dibranchiata,  265,  834 
Didelphis,  842 
Diencephalon,  416,  597 
Differentiation,   135,  287 
Difflugia,  158,  159,  832 
Diffusion,  45,  49 
Digenea,  213,  833 
Digestion,  81,  515 
extracellular,  81 
intracellular,  81 
Digestive  glands,  246,   368 
Digestive  pouches,  276,  310 
Digestive  secretions,   control  of,   524 

Digestive  system,  bird,  475 
dogfish,  435 
lamprey,  429 
man,  517 
vertebrates,  515 

Digestive  tract,  81 
development  of,  642 
hormones  of,  634 

Digital  pads.  452 

Digitigrade,  497 

Dihybrid  cross,  656 

Dimetrodon,  466,  840 

Dinofiagellates,  154,  832 

Dinophilus,  269 

Dinosaurs,  462 

Diphycercal  tail,  443 

Diploid  number,   117,    169 

Diplopoda,  306,  837 


Dipnoi,  442,  840 

Diptera,  316,  837 

Distal  convoluted  tubule,  562 

Distomata,  832 

Diurnal  migrations,  184 

DNA,  29 

Dog,  497 

Dogfish,  434,  436 

blood  pressure  of,  546 

circulatory   system   of,  435 

digestive  system  of,  435 

excretory  system  of,  435 

muscles,  513 

reproduction  of,  435 

skeleton,  507 
Dolphin,  497 

Dominance,  incomplete,   656 
Dominant  gene,  654 
Dorsal  aorta,  409,  546 
Dorsal  cirrus,  272 
Dorsal  pores,  360 
Dorsal  root  ganglion,  417,  591 
Down   feathers,  471 
Drones,   347 
Dryopithecus,  741 
Ducks,  481,  483 
Ductus  arteriosus,  551 
Dugesia,  204-211 

feeding  in,  205 

reproduction  in,  209 

sense  organs  of,  207 
Duodenum,  404,  521 
Duplication,  685 
Dura  mater,  599 
Dutrochet,  Rene,  11 
du  Vigneaud,  Vincent,  622 
Dynamic  balance  of  nature,  779 


Ear,  581 

Ear  ossicles,  584 

Earthworm,  269,  270 

Echinoderm-hemichordate 
377 

Echinoderms,  391 

classification  of,  364,  838 
metamorphosis  in,  379 
relationships  among,  375 

Echinoidea,  373,   838 

Echiuroidea,  353,   835 

Ecologic  density,  770 

Ecologic  isolation,  702 

Ecologic  niche,  755 

Ecology,  753-830 

Ecosystem,  753 

Ectoderm,  128 

Ectoparasites,  802 

Ectoplasm,    1  57 

Edentata,  494,  842 


relationships. 


INDEX 


857 


Effectors,  103,  574 
Efferent   neurons,   588 
Egg,   114,  566 

cleidoic,  455 

mammalian,   637 
Egg  cells,  61 
Egg  shell,  454 
Elasmobranchii,  436,  839 
Elastic  fibers,  56 
Electric  organ,   103 
Electrolytes,  23 
Electron,    1 9 
Electron  microscope,   17 
Electron  transmitting  enzymes,  72 
Elements,   19 

radioactive,  21 
Elephant,  499 
Elephantiasis,   811 
Elephas,  842 
Eleutherodactylus,  452 
Eleutherozoa,  376,  838 
Embioptera,  837 
Embryo,  protection  of,  131 
Embryologic   evidence    for   evolution,    729 
Embryonic  development,   126 
Embryonic  disc,  640 
Enamel,  516 

Endocrine  interrelationships,  635 
Endocrine  systems.   111,  605-636 
Endoderm,   128,  639 
Endolyraph,  581 
Endometrium,  631 
Endoplasm,   157 
Endoplasmic   reticulum,    17 
Endopodite,  294 
Endopterygota,  315,  837 
Endoskeleton,  96,  507 
Endostyle,  386,  388,  431 
Endothelium,   551 
Energy,  47 
Energy  cycle,  759 

"Energy-rich"   phosphate    compounds,   71 
Energy   transfer,  efficiency  of,  767 
Entamoeba,  801,  832 
Enterocoele,   1 29 
Enterocoelom,  238 
Enterocoelomata,  241 
Enterocoelous,  381 
Enterogastrone,   525 
Enterokinase,  523 
Enteropneusta,    360,   838 
Entoprocta,  353,  834 
Entotrophi,  837 
Environmental   resistance,    773 
Enzyme(s),   27,  65,  66 

digestive,  523 

properties  of,  66 
Enzyme  activity,   factors  affecting,  69 
Enzyme  denaturation,  69 


Enzyme  inhibitors,  70 

Enzyme-substrate  complex,  69 

Enzyme  synthesis,  686 

Enzyme  systems,  evolution  of,  784 

Eocene,  725 

Eosinophils,  59,  542 

Eosuchia,  840 

Ephemerida,    313 

Ephemeroptera,  837 

Epiboly,   129 

Epiceratodm,  437,  840 

Epidermis.   181,  395,  502 

Epididymis,   123,  569 

Epigenesis,    133 

Epiglottis,  532 

Epinephrine,  606,  617,  626 

Epitheliinn,  ciliated,   55 

columnar,  54 

cuboidal,  54 

squamous,   54 
Equilibrium,    109,  581 
Equus,  708,  842 
Erectile  tissue,   1 23,   569 
Ergosterol,  527 
Erioasteroidea,  838 
Errantia,   834 

Erythroblastosis  fetalis,   545,  679 
Erythrocytes,  539 
Esophagus,  430,  518 
Esox,  839 

Estradiol,   607,  628.   632 
Estrous  cycle,  631 

control  of,  626 
Ethiopian  realm,   736 
Eucarida,  836 
Eucestoda,  833 
Eucoelom,  220,  244 
Eucoelomata,  238 
Eucopepoda,  836 
Eudorina,  155 
Euglena,  150,  152 

cell  division  of,  166 
Euglenida,   154,   832 
Eulamellibranchiata,   834 
Eunuch,   628 
Euphausiacea,  304,  836 
Euplectella,  175,  832 
Euryapsida,  840 
Eurypterida,  320,  723,  835 
Eurythermic,  760 
Eustachian   tubes,  415,  518,  584 
Eutheria,  491,  842 

adaptive  radiation  of,  492 
Evolution,   695-753 

cultural,   751 

evidence  for,  716-736 

history  of,  696 

of  land  vertebrates,  704 

principles  of,  713 


858 


INDEX 


Evolution,  straight-line,  707 

F.volutionary    relationships   of   the   higher 

in\ertcbrates,  283 
Excitation,  587 
Excretion,  515.  559 
Excretory  system,  birds,  477 

dogfish,  435 

lamprey.  431 

mammals.  489 

reptiles,  454 
Excurrent  siphon,  252,  386 
Exophthalmic  goiter,  614 
Exopodite,   294 
Exopterygota,  315,  837 
Exoskeleton,   96,  327 
Expiration,  534 
Expressivity,   680 
Extension,  401 
Extensor,  512 

External  auditory  meatus,  454,  510,  583 
External  carotid,  408 
External  gills,  420 
External  gill  slits,  430 
External  nares,  395,  510,  531 
Exteroceptors,    1 07 
Extracellular  fluid,  537 
Extraembryonic  coelom,   131,   643 
Extraembryonic  membranes,  455,  571 
Exumbrellar  surface,   187 
Eye,  250,  478 

camera,  110 

chambers  of,  578 

mosaic,   110 

vertebrate,  576,  580 
Eyebrush,  318 
Eyelids,  395,  448,  579 
Eyespots,  225 


Facial  muscles.  514 

Facial  nerve,  417 

Facilitation,   106 

Falco,  841 

Falconiiformes,  841 

Fallopian  tube,  570,  571 

Family,  141,  143 

Fangs,  461 

Farsightedness,  580 

Fasciculata,  618 

Fatigue,   103 

Fats,  26 

Fatty  acids,  26,  523 

Feather  stars,  371 

Feathers,  471,  505 

Feces,  515 

Feed-back  mechanism,  612 

Felis,  842 

Femur,  308,  401,  512 

Fenestra  ovalis,  415 


Fermentation,   71 
Fertilization,   123,    124,  571 
external,   123 
internal,    124 
Fertilization  membrane,  124 
Fertilizin,    124 
Fetal  zone  of  adrenal,  618 
Fetus,  646 
Fibrin,  541 
Fibrinogen,   538,    541 
Fibrous  connective  tissue,  55 
Fibula,  512 
Fig  insect,  787 
Filarial  roundworms,  811 
Filarioidea,  834 
Filibranchiata,  834 
Filoplumes,   471 
Filtration  pressure,  563 
Finches,  482 
Fischer,  Emil,  69 
Fishes,  424-444 

characteristics  of,  433 
evolution  of,  436,  440 
respiratory  system,  529 
Fission,  115 
Fissipedia,  842 
Fissure  of  Sylvius,  602 
Flagella,   15,  99,   149 
Flagellata,   148,   152,  832 
Flame  cells,  93,  208,  223 
Fleas,  803 

Fleshy-finned  fishes,  442 
Flexion,  401 
Flexor,  512 
Flight,  97 

principles  of,  468 
Flight  muscles,  334 
Flounders,  442 

Flukes,  213,  807 

Flying  lemur,  493 

Flying  reptiles,  464 

Flying  squirrel,  493 

Folic  acid,  527 

Follicle,   566,   608 

Follicle-stimulating  hormone,  626 

Food,  515 

Food  chains,   767 

Food  vacuole,  79,   158,   186 

Foramen  magnum,  397,  509,  510 

Foramen  of  Monro,  598 

Foramen  ovale,   550 

Foraminifera,  159,  832 

Foregut,  643 

Forestry,  824 

Form,  regulation   of,   201 

Fossil,  424,   716 

Fovea,  578 

Fowl,  481 

Fraternal   twins,   646,   692 

Fresh-water  habitats,  797 


INDEX 


859 


Frog,  393-422,  448 
blood  pressure,  549 
circulatory   system   of,   408 
development  of,  421 
digestive  system  of,  404 
endocrine  glands  of,  419 
excretory  system  of,  411 
external   features  of,  394 
heart,  mixing  of  blood  in.  411 
life  cycle  of,  420 
muscles  of,  401 
nervous  system  of,  415 
reproduction  in,   452 
reproductive  system  of,  412 
respiratory  system  of,  406 
skeleton  of,  397 
skin  of,  395 

Frontal  section,  62 

Fructose,  25,  524 

Fruit  flies,  705 

Frustule,   l  89 

Fundulus,  839 

Funnel,  259,  260 


Galactose,  524 

Galapagos  Islands,  733 

Galen,   8 

Galeopithecus,  842 

Gall  bladder,  406,  520 

Galliformes,   841 

Gambusia,  628 

Game  birds,  824 

Game  habitats.  825 

Gamete,  1 1 4 

Ganglion,  61,  107 

Ganoin,  440 

Gar,  440 

Garden  snails,  252 

Gas  tension,  88 

Gasterosteus,  839 

Gastric  glands.  406.  519 

Gastric  pits,  405 

Gastric   secretion,  control  of,  524 

Gastrin,   525 

Gastrodermis.   172,    181 

Gastroliths,  298,  328 

Gastropoda,    834 

general   features  of,   247 
Gastrotheca,  452 
Gastrotricha,  231.  833 
Gastrovascular  system,  81,   186 
Gastrula,  126 
Gastrulation,  128,  241.  420 
Gause's   rule.   763 
Gai'ia,  841 
Gaviiformes,   841 
Geckos,   461 
Geiger  counter,  21 


Gel,   17,  31 

Gemmule,   1 79 

Gene,  chemical  nature  of.  683 

Gene-environment  interrelations,  689 

Gene  mutation,  685 

Gene  syinbols,  653 

Genes,  40,  652 

action  of,  685 

interactions  of.  669 

lethal,  679 

linear  order  of,  664 

number  of,  684 

size  of,  684 
Genetic  drift,  703 
Genetic  isolation.  702 
Genetics,   649-693 

history  of,  649 
Genital  pores,  431 
Genital  ridge,  629 
Genotype.  655 
Genus.   140,    143 

Geographic  distribution  of  organisms,  733 
Geographic   isolation,   701 
Geologic  time  table,  717 
Geology,  427 

Germ   layers,  evolution  of,  237 
German  measles,  691 
Giant  axons,   106 
Giant  scjuids,  265 
Giardia,  832 
Giardia  lamb  Ha,  806 
Gibbon,  741 
Gigantism,   624 
Gigantopithecus,  lAl 
Gila  monster,  459,  460 
Gill,  87,  247,  248.  263,  448,  529,  559 
Gill  arches,  432 
Gill  bailer,  296 
Gill  bar,  388 
Gill  heart.   263 
Gill  pouches.  430,  518 
Gill  slits,  362,  384,  420,  428,  529 
Giraffes,  499 

Gizzard,  81,  276,  309,  476 
Glaciation,  725 
Glands,  tvpes  of,  506 
Glass  snake,  459,  460 
Glass   sponge,    175,    176 
Glaucoma,  579 
Glenoid  fossa,   400 
Globigerina,  159,  832 
Globulins,   538 
Glochidia,   258 
Glomerular  filtration,   562 
Glomerulosa,   618 
Glomerulus,  363,  562 
Glossopharyngeal  nerve,  417 
Glottis,  407,  532 
Glucagon,  615 
Glucocorticoids,  618 


860 


INDEX 


Glucose.  22,  25,  517,  524 

Glucose  phosphate,  72 

Glycerol.  26,  523 

Glycogen,  26,   83,  526 

Glycolytic  cycle,  71 

(Glycosuria.   616 

Gnathobasc.    290 

Gnathobdellida,  835 

(ioblet  cells.  405 

Goethe,  Johanii   Wolfgang  von,   145 

Goiter,  612 

Goldschmidt.  Richard,  706 

Golgi  apparatus,  17 

Golgi  bodies,  38 

Gonadotropin,   628 

Gonads,   119,  566 

Gonionemus,  181,  833 

reproduction,  188 
Goniuin,  155 
Gooseflesh,  506 
Gophers,  500 
Gordiacea,  232,  834 
Gorgonocephalus,  375 
Gorilla,  741,  742 
Grantia,  178 
Grassland  biome,  791 
Graves's  disease,  614 
Gray  commissure,  597 
Gray  cortex,  601 
Gray  matter,  596 
Great  apes,  496 
Great  Lakes,  797 
Green  glands,  93,  300 
Gregaridina,  832 
Growth,   15 
Growth  hormone,  623 
Grus,  841 
Guano,  759 
Guanophore,  396 
Gymnothorax,  443 
Gyrus,  601 

H 

Habitat.  755 

Haeckel,    Ernst,    729 

Haemosporidia,  814,  832 

Hair,  486,  505 

Hair  cells,  581 

Hair  follicle,  505 

Hairworms,  232 

Haldane,  J.  B.  S.,  710 

Halibut,  442 

Haltere,   109 

Haploid  number,  117,  167,  169 

Hardy-Weinberg  Law,  682 

Harrison,  Ross,  34 

Harvey,  William,  9 

Haversian  canals,  57 


Hawks,  481,  482 

Hearing,  109 

Heart,  84,  410.  477,  547,  557 

earthworm,  277 

fish,  545 

mammal,  551 
Heart  murmur,  553 
Heart  urchins,  374 
Heat,   631 

Heidelberg  man,  747 
Heliozoa,  158,  832 
Heloderma,  459.  840 
Hemichordata,  360-364,  391,  838 
Hemichordate-echinoderm     relationships, 

377 
Hemicyclaspis,  428,  839 
Hemiptera,  316,  837 
Hemocoel,  86,  299,  312 
Hemocyanin,    84,   300 
Hemoglobin,    59,    84,   90,    530.    539 
Hemophilia.   542,   662 
Hepatic  ducts,  406,  520 
Hepatic  portal  system,  410,  546,  549 
Herbivores,  79 
Heredity,   chromosomal  basis  of,   652 

and  environment,  692 
Hermaphroditism,    122,   209 
Heron,  480,  481,  482 
Hesperornis,  479,  841 
Heterocercal  tail,  440 
Heteromi,  839 
Heterosis,  681 
Heterostelea,  375,  838 
Heterotrophs,  78,  712 
Heterozygous,  654 
Hexactinellida.  175,  832 
Hexapoda.  305,  306,  837 
Hibernation.  448,  760 
Hindgut.  643 
Hippocampus,  443,  839 
HippogJossus,   443,   839 
Hirudin.  282 
Hirudinea,  269,  281,  835 
Hirudo,  835 
Histochemistry,  45 
Histology,  53 
Historia  animaliiim,  8 
Histrio,  443 
Holocephali,  436,  839 
Holonephros,  559 
Holostei,  440,  839 
Holothuroidea.  372,  838 
Holotricha,    164,   832 
Homing,  485 
Homo,  842 
Homo  sapiens,  749 
Homocercal  tail,  440 
Homoiothermic,  468,   761 
Homologous  chromosomes,   652 


INDEX 


861 


Homologous  organs,  727 
Homologous  structures,  424 
Homozygous,  654 
Honey-ants,   347 
Honeybee,   317,  347 
Honey-stomach,  318 
Hoof,  498,  506 
Hooke,  Robert,  10 
Hookworm,  807 
Hopeful  monster.  706 
Hoplocarida,  836 
Hormones,   111,   328,  605 

arthopod,  328 

effects  of,  609 

purification  of.  607 
Horned  toad,  459 
Horns,  504 

Horny  scales.  453,  471,  505 
Horses,  498 

Human  bodv.  composition  of,  21 
Human  ecology,  829 
Human   inheritance,  691 
Hinncrus,  401 ,  512 
Humidity,   340 
Hummingbird,  482,  483 
Hummingl)ird  moth,  340 
Hunter,  John,   11 
Hutton,   James,  697 
Hyaluronidase,  571 
Hybrid  vigor,  681 

Hybridization,  origin  of  species  by,  709 
Hydra,   198 

reproduction  in,  199 
Hydrocortisone,  618 
Hydroides,  281,  834 
Hydrolysis.  81 
Hydrozoa,   189,   190,  833 
Hyla,  451,  840 
Hymen,   570 
Hymenoptera,  316,  837 
Hyoid.  510 
Hyoid   apparatus,   398 
Hyoid  arch,  432,  508 
Hyoid  bone,  511 
Hyomandibular.   435 
Hyperglycemia,  616 
Hypersecretion,  606 
Hypersensitivity,  619 
Hypertonic  solution.   52 
Hypophyseal  sac.  429 
Hypophysis,  620 
Hyposecretion,   606 

Hypothalamus,  428,  597,  601.  620,  627 
Hypothesis,  3 
Hypotonic  solution,  53 
Hypotrichs,  164,  832 
Hyracoidea,  500,   842 
Hyracotherium,   708 
Hyrax,  842 


I 


Ichthyornis,  481,  841 

Ichthyosaurs,  456,  840 

Identical  twins,  115,  646,  692 

Ileocaecal  valve,  521 

Ileum,  521 

lUum,  401,  512 

Immobilization,  342 

Immunitv.   542 

Implantation  of  fertilized  egg,  571 

Impulse,  initiation  of,  587 

Inarticulata,   838 

Inborn  errors  of  metabolism,  688 

Inbreeding,   681 

Incisors,  487,  516 

Incurrent  pores,  172 

Incurrent  siphon,  252,  386 

Incus,  466,  511,  583 

Inductor  tissues,    136 

Infectious  diseases.   543 

Infundibulum.  416,  620 

Inheritance  of  acquired  characters,  697 

Inhibition,  587 

Ink  sac,  263 

Inner  cell  mass.  638 

Innervation  of  arthropod  muscles,  332 

Inorganic  compounds  in  cell,  23 

Insect  development,  control  of.  Ill 

Insect  pests,  control  of,  824 

Insecta,  306 

classification  of,  313 
Insectivores,   492,   842 
Insects,  flight  mechanism  in,  334 
metamorphosis  in.  314 
social  mechanisms  in,  344 
vision  in.  336 
Insertion.  401 
Inspiration,  533 
Insulin,  28,  615,  616 
Integrated  centers.  419 
Integration,  1 1 1 
nervous,  574 

Integument,  395,  502 

Interactions   between   species,  763 

Intercerebral  gland,  330 

Intercostal  muscles,  534 

Intermedin,  419,  622 

Internal  carotid,  408 

Internal   gill.  420 

Internal  gill  slit.  430 

Internal  nares,  510,  532 

Internuncial  neurons,  588 

Interoceptors,   108 

Intersexes,   661 

Intestinal  glands,  521 

Intestine,  430,  521 
cross  section  of,  521 

Intracellular  differentiation,   149 


862 


INDEX 


Invagination.  126 

Inversion,  685 

Invertebrates,  higher,  236-359 

lower,  148-235 
Inverted  eye,  207 
Involution,   129 
Iodine,  53 
lodopsin,  580 
lonone,  108 
Ions,   19 
Iris,  576 

Irritability,   14,    103 
Ischial  callosities,   740 
Ischium,  401,  512 
Islets  of  Langerhans,  521,  615 
Tsolecithal   egg,   126 
Isopoda,  305,  836 
Isoptera,  316,  837 
Isospondyli,  839 
Isotonic  solution,   52 
Isotope,  20 
Isthmus  of  Panama,  701 


Jacana,  483 

Jaundice,  521 

Java  man,  745 

Jaw  joint,   mammals,  466,  511 

reptiles,  465 
Jaws,  222,  432 
Jeffersons  salamander,  449 
Jejunum,  521 
felly  coat,  131 
Jellyfish,    181 
Joint,  97 
Jordan's  rule,  733 
Jugular  vein,  410 
Jurassic  period,  723 
Juvenilizing  hormone,  330 

K 

Kala-azar,  814 

Kangaroos,  tree-climbing,  784 

Kappa  particles,  163 

Keel,  474 

Keratin,  96,  453,  502 

Keratosa,  832 

Ketone  bodies,  616 

Key,  taxonomic,  143 

Kidney  tubule,  93,  411,  454,  563 

Kidneys,  92,  477,  562 

evolution  of,  559 
Killer  trait,   162,  690 
Kinesis,  340 
Kinetic  energy,  47 
King  crab,  320 
Kingdom,   141 
Kinorhyncha,  231,  833 


Kiwi,   481 

Koala  bear,  492 

Krebs  citric  acid  cycle,  72 


Labia  majora,   570 

Labia  minora,  570 

Labiata,  291,   305-320,  836 

Labium,   292 

Labor,  572 

Laboratories,  marine  biological,   1 1 

Labyrinthodonts,  447,  723,  840 

Lacertilia,   459,   840 

Lacrimal  duct,  579 

Lactase,  81 

Lactose,  26,  524 

Lagena,   582 

Lagomorpha,  501,  842 

Lamarck,  Jean  Baptiste  de,  697 

Lamarckism,  697 

Lamprey,  commercial   damage   by,  431 

reproduction,  430-431 

respiratory  system,  430 

structure,  429 
Land,  classification  of,  823 
Landsteiner,   K.,   544 
Large  intestine,  521 
Larvacea,  385,  392,  839 
Laryngotracheal  chamber,  407 
Larynx,  510,  532 
Latent  period,  101 
Lateral  line  sensory  system,  433,  448,  581, 

582 
Lateral  plate,  643 
Latigastra,  835 
Latimeria,  444,  840 
Latissimus  dorsi,  514 
Law  of   Independent   Assortment,   652 
Law  of  Segregation,  651 
Leeches,  269,   282,  799 
Leeuwenhoek,   Antony   van,    148 
Leishmania.  832 
Lemmings,  774 
Lemur,  842 
Lemuroidea,  494,  842 
Lemurs,  494,  738 
Lens,  250,  311,  576 
Lepidoptera,  316,  837 
Lepidosauria,  840 
Lepidosiren,  840 
Lepisosteus,  440,  839 
Lepornis,  839 
Lepospondyli,  840 
Leptasterias,  370,  838 
Leptostraca,  836 
Lepus,  842 
Lethal  genes,  679 
Leuconia,  832 
Leuconoid    sponges,    1 73 


INDEX 


863 


Leukocytes,  542 

Liebig,  J.,  66 

Life,  origin  of,  710 

Life   zones,   fresh-water,   797-798 

marine,  794,  797 

terrestrial,  789-794 
Ligament,    56 

Light,  effect  on  animal  distribution,  761 
Lignite,  756 
Limnetic  zone,  797 
Limnopitliecus,  741 
Limulus,  320,  835 
lAngtda,  838 
Linkage,   663 
Linkage  group.   666 
Linnaeus,   Karl,    10,    143 
Lipase,  67,  81,  523 
Lipophores,  396 
Littleneck   clams,    253 
Littoral  zone,  797 
Liver,  520 
Liver  flukes.  214 
Living  fossil,  371 

Living   things,   characteristics  of,    14 
Lizards,  459 
Loa,  834 

Lobes  of  brain,  602 
Locomotion  of  worms,  273 
Locomotor  cilia,  379 
Locus,  652 
Loligo,  260,  834 
Loons,  483 
Loop   of   Henle.  562 
Lophophore,  355,  356 
Lorises,   494 
Louse,  802 

Lower  Paleolithic  culture,  751 
Loxodonta,  842 
Luciferase,   76 
Luciferin,   76 
Lugworm,   281 
Lumbricus,  270-279,  835 
Lungfish,  437,  442 
Lung  fluke,  214 
Lungs,  87,  518,  559 

birds,  476 

evolution  of,  476 

fishes,  439 

frog,  406,  451 

mammals,  488 

toad,   451 

vertebrates,  532 
Luteinizing  hormone,  626 
Lycaenops,  466,  840 
Lyell,  Sir  Charles,  697 
Lymph,   537,  538 
Lymph    capillaries,    538 
Lymph    nodes.    538 
Lymph  sacs.  408 
Lymph  vessels,   408,   538 


Lymphatic   system,   545 
Lymphatic  vessels,  556,  557 
Lymphocytes,   59,   542 
Lynx,  773 
Lysis,   542 

M 

Macromeres,    241 
Macromutation,  706 
Macronucleus,    160 
Madreporaria,   833 
Madreporite,   365,  381 
Magendie,  Francois,   10 
Maggots,  342,  804 
Magnesium,   23,  68 
Magnus,  Albertus,  9 
Malacostraca,  293,  836 
Malaria,   1 16,  165,  814 
Male  sex  hormones,  627 
Malformations,   137 
Malleus,  466,  511,  583 
Mallophaga,  802.  837 
Malpighi,  Marcello,  10 
Malpighian   tubules.  93,  310,  322 
Makase,  81,  517 
Maltose,  26,  517,  524 
Mammal-like  reptiles,  465 
Mammals,   390,  468,  842 

characteristics  of,  486 

development  of,  637 

excretory  system,  489 

eye,  structure  of,   576 

heart  of,  551 

reproduction  in,  489 

respiratory  system,  488 

skeleton,  508 

teeth  of,  487 
Mammary  glands,  489,  506,  572 
Mammoths,  500 
Man,  496 

blood  pressure,  555 

blood  velocity,  555 

circulatory   system,  548 

digestive  system,  517 

evolution  of,  738-752 

muscles,  513 

respiratory  system,  531 

skeleton  of,  509 
Man  apes,   743 
Manatees,  500 
Manatus,  842 
Mandible,   291,   296 
Mandibular  arch,  432,  508 
Mandrills,  740 
Manis,  842 
Manta,  437,  839 
Mantle,  246,  385 
Mantle  cavity,  254 
Manubrium,    \  8^ 


864 


INDEX 


Marine  fisheries,  826 

Mariuota,  842 

Marsupial  frog,  452 

Marsupials,  491.  734,  842 

Marsupium,  492 

Mass  spectrometer,  21 

Mastigophora.  832 

Mastodons,  500 

Maternal  instinct,  626 

Mating   behavior,   124 

Mating  types,    116,   162 

Matrix,  55 

Matter,  cyclic  use  of,  755 

Maxillae,  291,  296 

Maxillipeds,  295 

McC.lung,  C.   E.,  650 

Mechanistic  theory  of  life,  13 

Mechanoreceptors,   107,    108,  575 

Meckel's  cartilage,  397 

Mecoptera,  837 

Median  eminence,  620 

Medical  genetics,  693 

Medulla  oblongata,  416,  562,  597,  599 

Medusa,  181 

Megaceiyle,  841 

Mega  n  t  h  rop  us,  747 

Meiosis,    1 1 6 

Melanin,   506 

Melanophores,  396 

Meleagrina,  256 

Membrane  potential,  104 

Membranelles,  164 

Membranous  labyrinth,  581 

Mendel,  Gregor  Johann,  650 

Mendel's  Laws,  650 

Meninges,  417,  599 

Menstrual  cycle,  631 

Mesencephalon,  416,  597 

Mesenchyme,  208 

Mesenteries,  238,  403,  522 

Mesichthyes,  839 

Mesoderm,  129,  640 

differentation  of,  643 
Mesoglea,   181 
Mesonephros,  561,  732 
Mesosoma,  320 
Mesothorax,  308 
Mesozoa,  352,  832 
Mesozoic  era,  723 
Metabolic  tracers,  20 
Metabohsm,  15,  64 

carbohydrate,  526 

fat,  526 

protein,  526 

special  types  of,  75 
Metacarpals,  401,  512 
Metacercaria,  216 
Metameres,  267 
Metamerism,  267 
Metamorphosis,  330,  380,  422,  611 


Metamorphosis  of  insects,  316 

of  tornaria,  379 
Metancphridia,  239,  277 
Metanephros,  561,  732 
Metaphase,  42 
Metapleural  folds,  387 
Metasoma,  320 
Metatarsals,  401,  512 
Metatheria,  491,  842 
Metathorax,  308 
Metazoa,   148 

phylogeny  of,  242 
Metencephalon,  416,  597 
Method  of  agreement,  5 
Method  of  concomitant  variation,  6 
Method  of  difference,  5 
Metridium,  195 
Michaelis,  Leonor,  69 
Micromeres,   241 
Micronucleus,  160 
Micropodiformes,  841 
Micropterus,  839 
Microsauria,  840 
Microsomes,  17,  38,  69 
Middle  ear  cavity,  518 
Middle  Paleolithic  culture,  751 
Middle  piece,  1 1 9 
Midgets,  624 
Migration,  483 

bird,  483 
Milleporina,  833 
Millipede,  305 
Mimicry,  785 
Mineral  cycles,  758 
Mineralocorticoids,  618 
Minerals,  83 
Miocene  epoch,  725 
Miracidium,  214 
Mississippian  period,  723 
Mitochondria,   17,  38,  69 
Mitosis,  39 

regulation  of,  43 
Mixture,  22 
Mneiuiopsis,  200,  832 
Modifications,  700 
Modifying  factors,  677 
Molars,  487,  516 
Molds,  717 
Molecular  motion,  48 
Molecule,  21 
Moles,  492 
Molgula,  383,  384 
Molluscs,  244-258,  834 

classes  of,  246 

general  features  of,  244 
Molt  and  metamorphosis  hormone,  330 
Molting,  96,  231,  326,  471 
Molting  fluid,  326 
Monaxonida,  832 
Mongoloids,  750 


INDEX 


865 


Monkeys,  496 
Monocytes,  59,  542 
Monod,  Jacques,  134 
Monogenea,  213,  833 
Monohybrid  cross,  654 
Monotremes,  491,  734,  842 
Monsters,  212 
Moore,  Carl  R.,  568 
Moray  eels,  442 
Morgan,  T.  H.,  650 
Morphogenesis,  134 
Morphogenetic  substances,  120 
Mortality  rate,  771 
Morula,  637 
Mosaic  vision.  336 
Motion,  98 

ameboid,  98 
Motor  neuron,  418 
Motor  unit,   101 
Mount  Carmel  fossils,  748 
Mouth,  515 
Movement,  15 
Mucosa,  405,  521 
Mucous  glands,  396,  506 
Mudpuppy,  450 
Mudskipper,  442 
Miiller,  Johannes,  10 
Multiple  alleles,  677 
Multiple  factors,  674 
Multiple  fission,  165 
Muscle,  belly  of,  101 

cardiac,  57 

insertion  of.  101 

origin  of,  101 

skeletal,  57 

smooth,  57 
Muscles,  99 

bird,  474 

dogfish,  513 

fishes,  512 

groups  of,  512 

man,  513 

vertebrate,  512 
Muscular      contraction,      mechanism      of, 

101 
Muscular  coordination,  419 
Muscular  system  of  frog,  401 
Muscularis  mucosae,  405 
Musculocutaneous  vein,  410 
Muskrats,  500 
Mussel,  256,  257 
Mutagenic  agents,  706 
Mutants,  biochemical,  687 
Mutations,  685,  700,  704 
Mutualism,  765 
Alya  arena) ia.  256 
Myelencephalon,  416,  597 
Myelin  sheath,  27,  61 
Myodocopa,  836 
Myofibrils,  57 


Myogenic  rhythms,  335 
Myomere,  387,  512 
Myosin,  44 
Myotis,  842 
Myotomes,  420,  644 
Myriapoda,  305,  836 
Mysidacea,  304,  836 
Mystacocarida,  836 
Mysticeti,  842 
Myxedema,  610 
Myzostomida,  835 

N 

Nails,  506 

Nares,  531 

Nasal  cavities,  531 

Natural  selection,  698,  702,  732 

theory  of,  699 
Nature-philosophy,  145 
Nauplius  eye,  302 
Nauplius  larva,  292 
Nautiloidea,  834 
Navel,  573 
Navigation,  bird.  485 

insect,  343 
Neanderthal  man,  747 
Nearctic  realm.  736 
Nearsightedness,  580 
Necator  americanus,  808 
Necturus,  450.  840 
Need  ham,  Joseph,  729 
Negroid  races,  750 
Nekton.  795 

Nematocvsl.  184,  185,  199 
Nematoda.  227,  833 

reproduction  in.  229 
Nematomorpha,  834 
Nemertea,  220,  232,  834 

circulatory  system  of,  233 
Neodarwinism.  700 
Neognathae,  481,  841 
Neolithic  culture.  752 
Neoptera.  313,  837 
Neornithes,  481,  841 
Neoteny,  392,  450 
Neotropical  realm,  736 
Nephridia,  93,  245,  249 
Nephridial  tubules,  223 
Nephridiopore,  245 
Nephrogenic  ridge,  645 
Nephron,  559,  562 

evolution  of,  565 
Nephrostome,  565 
Nereis,  270-279,  834 
Nerve  cord,  384,  390 
Nerve  impulse,  104,  585 
Nerve  net,  104,  187 
Nerve  nuclei,  599 
Nerve  ring,  187,  369 


866 


INDEX 


Nerve  transmission,  membrane  theory  of, 

104 
Nervous  system,  574 

autonomic,  418,  593 

central,  596 

divisions  of,  588 

organization  of,  585 

peripheral.  591 
Neural  arch.  399,  508 
Neural  canal.  399 
Neural  crest.  641 
Neural  folds,  420 
Neural  gland,  386 
Neural  pathways,  590 
Neural  spine,  399 
Neural  tube,  130,  641 
Neurilemma,  61 
Neurofibrils,  104.  162 
Neurogenic  rhythms,  335 
Neuroglia,  61,  585 
Neurohormones,  111 
Neurohumors,  106,  596,  620 
Neuromuscular  junction,  101 
Neurons,  60,  585 
Neuropodium,  272 
Neuroptera,  837 
Neurosecretion,  328 
Neurospora,  687,  707 
Neurotoxic  poison,  460 
Neurulation,  641 
Neutralism,  763 
Neutrophils,  59,  542 
Niacin,  527,  528 

Nictitating  membrane,  454,  579 
Nile  bichir,  440 

Nitrogen  cycle,  757 

Nitrogen  fixation,  758 

Nitrogenous  wastes,  559 

Noddy  terns,  480 

Nodes  of  Ranvier,  61 

Nonelectrolytes,  23 

Norepinephrine.  617 

Notochord,  130,  382,  383,  387,  390,  429,  641 

Notonecta,  755 

Notopodium,  272 

Notostmca,  303,  836 

Notum,  308 

Nuclear  reactor,  20 

Nucleic  acids.  29 

Nucleolus,  37 

Nucleoplasm.  14 

Nucleotide,  29 

Nucleus,  14,  35 
role  of,  35 

Nuda,  832 

Nudibranchs,  251,  834 

Nummulitidae,  159 

Nutrition,  holozoic,  78 

types  of,  78 
Nymphs,  315 


Obelia,  190.  191.  833 
Occipital  condyles.  397,  509 
Oceanic  islands,  fauna  of,  734 
Oceanodroma,  841 
Ocellus.  311,  322,  386 
Octopoda,  834 
Octopus,  263 

psychologic  studies  of,  265 
Oculomotor  nerve,  417 
Odonata,  313,  837 
Odontoceti,  842 
Odontognathae,  481,  841 
Olfactory  bulbs,  416,  597 
Olfactory  epithelium,  108,  414 
Olfactory  nerves,  417 
Oligocene  epoch,  725 
Oligochaeta,  269,  834 
Omasum,  519 
Ommatidia,  110,  297,  336 
Omnivores,  79 
Oncosphere,  217 
Ontogeny,  644,  730 
Onycophora,  323,  835 
Oocytes,  primary,  120 
Oogenesis,  1 20 
Oogonia,  120 
Ootid,  122 
Oparin,  A.  I.,  710 
Opercular  chamber,  529 
Operculum,  252,  320,  420.  438 
Ophidia.  460,  840 
Ophiocistioidea,  838 
Ophisaurus,  459 
Ophiuroidea,  375,  838 
Opiliones,  835 
Opisthobranchia.  251,  834 
Opisthonephros.  560 
Opisthosoma,  320,  322 
Opossum,  492 
Optic  chiasma,  417,  597 
Optic  lobes,  416,  597,  600 
Optic  nerves,  417.  577 
Optic  tracts,  417 
Oral  groove,  80 
Oral  hood,  387 
Oral  suckers,  420 
Oral  valves,  529 
Orang-utan.  495.  741,  742 
Orbits,  397,  510 
Order,  141,  143 
Ordovician  period,  721 
Organ,  34 

Organ  of  Corti,  584 
Organ  systems,  34 
Organelle,  149 

conductile,  150 
Organic  compounds,  24 
synthesis  in  vitro,  711 


INDEX 


867 


Organic  evolution,  695ff. 

Organizer,  135 

Oriental  realm,  736 

Orientation  response,  342 

Origin,  401 

Ornithischia,  462,  724,  840 

Ornithorhynchus,  491,  842 

Orthogenesis,  707 

Orthoptera,  316,  837 

Orycteropus,  842 

Osculum,  172 

Osmosis,  51 

Osmotic  pressure,  52,  556 

Osprey,  470 

Ostariophysi,  839 

Osteichthyes.  389,  437,  839 

Osteoblasts,  57 

Ostium,  570 

Ostracoda.  303,  836 

Ostracoderms,  427,  723,  839 

Ostriches,  481 

Otic  capsules,  397,  508 

Otolith,  109,  386,  581 

Outbreeding,  681 

Ovary,  119,  412,  566 

Overpopulation,  829 

Overwintering,  225 

Oviduct,  123,  413,  435 

Oviparous,  125,  436 

Ovisac,  413 

Ovoviviparous,  125,  436 

Ovulation,  566,  630 

Ovum,  122 

Owen,  Richard,  11 

Oxygen,  89 

Oxygen  debt,  103 

Oxygen  dissociation  curves,  91 

Oxygen  transport,  90,  539,  540 

Oxyhemoglobin,  539 

Oxytocin,  622,  634 

Oxyuroidea,  834 

Oysters,  256 


Pacemaker,  553 
Paired  appendages,  432 
Pair-feeding,  6 
Palaearctic  realm,  736 
Palaeognathae,  481,  841 
Palate,  488 
Paleocene  epoch,  725 
Paleontology,  427,  716 
Paleoptera,  313,  837 
Paleosimia,  741 
Paleozoic  era,  721 
Palolo  worms,  280 
Palpigradi,  835 
Palps,  253,  270,  295 


Pancreas,  521 

islet  cells  of,  615 
Pancreatic  duct,  406,  521 
Pangolins,  494 
Panotheria,  842 
Paper,  scientific,  2 
Parallel  evolution,  494 
Paramecium,  161,  763,  832 

sex  cycle  in.  168 
Paramyluin  bodies,  1  52 
Paranthropus,  743 
Paiapithecus,  741 
Parapodia,  268 
Parasites,  attachment  of,  816 

evolutionary  loss  of  organs  in,  818 

host  specificity  of,  819 

intestinal.  806 

intracellular,  813 

transmission  of,  816 
Parasitism.  79.  231.  765,  799-821 

adaptations  to,  816 

origin  of,  799 
Parasympathetic  system.  594 
Parathormone,  614 
Parathyroid  glands,  518,  614 
Parazoa.  236 
Parentage  tests,  678 
Parietal  bones,  51 1 
Parietal  cells,  519 
Parietal  pericardium.  403 
Parietal  peritoneum,  403 
Parotid  glands,  516 
Parthenogenesis.  1 24,  225 
Partial  pressure  of  gas,  87 
Parturition,  634 
Passer,  841 

Passeriformes,  481,  841 
Pasteur,  Louis,  66 
Pauropoda,  837 
Pavlov,  Ivan,  524,  589 
Pearl  button,  258 
Pearl,  cultured,  257 
Pearl  oysters,  256 
Peat,  756 
Pecten,  258,  317 
Pectoral  fins.  508 
Pectoral  girdle,  400,  474,  508,  512 
Pectoralis  muscle,  514 
Pedal  ganglia,  256 
Pedal  glands,  222 
Pedicellariae,   367 
Pediculus,  837 
Pediculus  humanus,  803 
Pedipalps,  322 
Peking  man,  746 
Pelagic  zone,  795 
Pelecaniformes,  841 
Pelecanus,  841 
Pelecypoda,  252,  256,  834 


868 


INDEX 


Pellagra.  83,  527,  528 
I'cUiclc,  95 
I'elmatozoa,  376,  838 
Pelvic  girdle,  474,  508,  512 
Pelytosaurs,  465,  722,  723,  840 
Penetrance,  680 
Penguins,  480,  481 
Penis,   123,  210,  569 
Pennsylvanian  period,  723 
Pepsin,  81,  406,  519 
Pepsinogen,  519 
Peptidases,  81 
Peptide  bonds.  28 
Peptones,   519 
Peracarida,  836 
Perca,  437,  839 
Perch,  437 

structure  of,  438 
Pereiopods,  294 

Pericardial  cavity,  86,  244,  402,  551 
Pericardial  sinus,  86 
Pericardimn,  86,  551 
Perilymph,   581 
Periosteum,  57 
Peripatus,  323,  835 
Peripheral  ganglion,  594 
Periplaneta,  307 
Perisarc,  190 
Perissodactyla,  498,  842 
Peristalsis,  405,  518 

Peristome,  368 

Peristomium,  267,  271 

Peritoneal  cavity,  522 

Peritoneum,  220 

Peritricha,  165,  832 

Peritrophic  membrane,  310 

Permeability,  51 

Permian  period,  723 

Peroxidase,   67 

Petrifaction,  425,  716 

Petroleum,  756 

Petromyzon,  839 

pH,  23,  539 
optimum,  70 

Phagocytosis,  541,  542 

Phalangers,  492 

Phalanges,  401,  512 

Phalansterium,  156 

Pharyngeal  pouches,  384,  390,  643 

Pharynx,  518,  532 

Phase  contrast  lenses,  44 

Pheasant,  482 

Phenotype,  655 

Phenylthiocarbamide,  691 

Philodina,  222,  833 

Phoca,  842 

Pholidota,  494,  842 

Phonoreception,  582 

Phoronida,  355,  838 

Phospholipids,  27 


Phosphorus,  614,  759 
Phosphorylase,  615 
Photocorynus,  796 
Photoperiod,  761 
Photoreceptor,  107,  109,  152,  575 
Photosynthesis,  78,  756 
Phrynichida,  836 
Phrynosoma,  459,  840 
Phthiriis,  837 
Phylogeny,  358,  644,  730 
Phylum,  141,   143 
Physalia,  191,  833 
Physiologic  isolation.  702 
Physiological  genetics,  732 
Phytomonadina,  168,  832 
Phytomonads,  155 

sex  cycle  in,  168 
Phytoplankton,  754 
Pia  mater,  599 
Piciformes,  841 
Pigeons,  470 
Pigment  cup,  207 
Pigmentation,  112 
Pigs,  499 
Pika,  500 

Pineal  body,  416,  576,  597,  634 
Pineal  eye,  428 
Pinna,  583 
Pinnipedia,  842 
Pit  vipers,  461 

Pithecanthropus  erectus,  744 
Pituicytes,  622 

Pituitary  function,  control  of,  626 
Pituitary  gland,  419,  428,  597,  620 

blood  supply,  621 

development  of,  621 

hormones  of,  622 
Placebos,  7 

Placenta,  132,  489,  491,  549,  571,  633,  639 
Placentation,   133 
Placodermi,  389,  431,  839 
Placoid  scales,  433 
Planarians,  204 
Plankton,  181,  795 
Plantigrade,  497 
Planula,  188 
Plasma,  58,  84,  538 
Plasma  membrane,  14,  34 
Plasma  proteins,  538 
Plasmodium,  115,  116,  832 
Plastron,  456 
Platelets,  58,  60,  541 
Platyhelminthes,  204-219,  833 
Platypus,  489,  491 
Platyrrhine  monkeys,  740 
Platysamia  cecropia,  331 
Plecoptera,   837 
Pleistocene  epoch,  725 
Pleopods,  294 
Plesianthropus,  743 


INDEX 


869 


Plesiosaurs,  456 

Plethodon,  449,  840 

Pleura,  308,  532 

Pleural  cavity,  532 

Pleurobrachia,  200,  832 

Pleuroperitoneal  cavity,  403 

Pliny,  8 

Pliocene  epoch,  725 

Podocopa,  836 

Poikilothermic,  448,  761 

Point  mutation,  685 

Poison,  460,  462 

Poison  claws,  305 

Poison  glands,  322,  396.  506,  517 

Poison  sac,  318 

Polar  body,  122 

Polarity,  211 

Poliomyelitis,  543 

Pollen  brushes.  317 

Polocyte,  1  22 

Polychaeta.  268.  834 

Polycladida,  213,  833 

Polyneuritis.  5 

Polyodon,  839 

Polyp.  188,  190 

Polyplacophora,  834 

Polypterus,  44U,  839 

Polyspermy,  124 

Pongidae.  741 

Pons.  600 

Population  cycles,  773 

Population  density,  770 

Population  dispersal,  775 

Population  genetics.  681,  691 

Population  growth  curve,  770 

Population  pressure.  787 

Populations.  769 

ecologic   characteristics   of,  770 
Porcupines.  500 
Porifcra,  172-180,  236,  832 
Porocytes,  1 74 
Porpoises,  497 
Portal  veins,  620 
Porto  Santo  rabbits.  695 
Portuguese  man-of-war,  191,  192 
Posterior  vena  cava,  410,  549 
Postganglionic  fibers,  594 
Postoral  circle.  379 
Potassium,  23 
Potential  energy,  47 
Preadaptation.  447,  703 
Preantennae,  324 
Precocial.  483 

Predator-prey  relationship,  766 
Preformation  theory.  133 
Preganglionic  fibers,  594 
Pregnancy.   632,  633 

tests  for,  633 
Prehallux,  395 
Premolars,  487,  516 


Preoral  circle,  379 

Priapuloidea,  354,  835 

Price,  Dorothy,  136 

Primates,  494,  842 

Primitive  streak,  1 29,  640 

Primordial  germ  cells,  629 

Pristis,  434,  437,  839 

Probability,  laws  of,  655 

Proboscidea,  499,  842 

Proboscis,  220,  233,  248,  360 

Proboscis  pore,  362,  381 

Proboscis  worms,  232 

Procellariiformes,  841 

Procercoid  larva.  218 

Proconsul,  741 

Producer  organisms.  754 

Progeny  selection,  656 

Progesterone,  619,  628,  632 

Proglottids,  216 

Prolactin,  626 

Pronephros,  561,  732 

Prophase,  41 

Propliopithecus,  741 

Proprioceptors,  107 

Prosobranchia.  251.  834 

Prosopyle,  174 

Prostate  glands,  123,  570 

Prostomium,  267 

Protamine  zinc  insulin,  616 

Protection,  95 

Protective  coloration,  785 

Protein.  27 

Proteose,  519 

Proterozoic  era,  721 

Prothoracic  glands,  330 

Prothoracicotropic  hormone,  330 

Prothorax,  308 

Prothrombin,  541 

Protocooperation,  764 

Protobranchiata,  834 

Protonephridia,  208,  239,  286,  353.  388 

Protoplasm.  13,  16-33 

chemical  composition  of,  19 

dynamic  state  of,  74 

physical   characteristics  of,   30 
Protopodite,  294 
Protopterus,  439,  840 
Protostomous  animals,  381 
Prototheria,  491,  842 
Prototroch,  286 
Protozoa.  33,  148-171,  832 

reproduction  of,  166 
Protraction,  402 
Protura,  837 
Proventriculus,  476 
Proximal  convoluted  tubule,  562 
Pseudemys,  456,  840 
Pseudocoelom,  220,  229,  355 
Pseudopod,  79,  99 
Pseudoscorpiones,  835 


870 


INDEX 


Psittacifonnes,  841 
Psocoptcra,  837 
PtemmnUm,  464,  840 
Plerubrancliia.  360,  838 
Pleropods,  251,   834 
Pterosauiia.  464,  840 
Pterygota,  313,  837 
Ptyalin,  68,  81,  517 
Pubis,  401,  51^ 
Public  health,  828 
Pulniocutaneous  arch,  408 
Pulmonary  artery,  409,  549 
Pulmonary  circulation,  548 
Pulmonary  veins,  410 
Pulmonata,  252,  834 
Pulp,  516 
Pulvillus,  308 
Punnett,  R.  C,  6.54 
Pupa,  316 
Pupil,  576 
Purkinjc,  S3 
Purkinje  fibers,  553 
Pvcnogonida,  835 
Pygostyle,  474 
Pyloric  sphincter,  404,  519 
Pyloric  stomach,  368 
Pyruvic  acid,  72 


Quadrate  bone,  465 
Quadrate  cartilage  397 
Quahog,  253 
Quarternary  period,  725 
Queen  ant,  346 
Quill,  471,  491,  506 


Rabbits,  500 
Raccoons,  497 
Radial  canal,  174 
Radiolaria,   159,  832 
Radius,  512 
Radula,  244,  246 
Railroad  worm,  77 

Raja,  839 

Ramus  communicans,  417 

Rana,  840 

Rana  pipiens,  394 

Range,  733 

Range  of  tolerance,  760 

Raphanobrassica,  710 

Rassenkreis,  727 

Rat  fish,  436 

Rathke's  pouch,  620 

Rats,  500 

Rattlesnake,  461 

Ray,  John,  10,  143 


Ray-finned   fishes,  440 
Rays,  365,  436,  437 
Reactions,  chemical,  64 
Recapitulation  theory,  644,  730 
Recent  epoch,  725 
Receptors,  103.  574 
Recessive  genes,  654 
Recovery  period,  101 
Rectum,  521 
Rectus  abdominis,  514 
Recurrent  bronchi,  477 
Red -backed  salamander,  449 
Red  bone  marrow,  506,  541 
Red  cells,  58,  539 
life  span  of,  .541 
rate  of  production  of,  541 
Redi,  Francesco,  114 
Redia.  215 
Reflex,  418,  600 
conditioned,  589 
inborn,  589 
spinal,  588 
Reflex  arc,  588 
Refractory  period,  586 
Regeneration,  116,  202,  211 

lizard  tails,  460 
Reindeer,  790 
Relaxation  period,  101 
Relaxin,  628,  634 
Remora,  442,  443 
Renal  corpuscle,  562 
Renal  pelvis,  .562 
Renal  portal  system,  546 
Renal  portal  veins,  410 
Renal  threshold,  564 
Rennin,  519 
Replacement  bone,  438 
Reproduction,  16,  114,  139 
asexual,  114,  115 
bird,  483 
dogfish,  435 
frog,  452 
lamprey,  430-431 
mammalian,  489,  571 
reptile,  454 
sexual,  114,  116 
vertebrate,  566 
Reproductive  ducts.  568 
Reproductive  periodicity,  279 
Reproductive  systems,  122 
Reptiles,  390,  840 
adaptations  of,  456 
characteristics  of,  453 
evolution  of,  456 
excretion,  454 
reproduction,  454 
respiration,  454 
skin  of,  453 
Residual  air,  534 


INDEX 


871 


Respiration,  71,  87 

direct,  88 

external,  89 

indirect,  88 

internal,  88 
Respiratory  center,  535 
Respiratory  movements,  535 

control  of,  535 
Respiratory  pigment,  539 
Respiratory  surfaces,  86,  529 
Respiratory  system,  birds,  476 

lamprey,  430 

mammals,  488 

man,  531 

reptiles,  454 
Respiratory  trees,  372 
Respiratory  tube,  431 
Reie  cords,  569 
Reticular  fibers,  55 
Reticularis,  618 
Reticulum,  519 
Retina,  250,  415.  576,  577 
Retinula,  337 
Retinuli,  311 
Retraction,  402 

Reversibility  of  chemical  reactions,  64 
Resolutions,  geologic,  720 
Rh  factor,  545,  679 
Rhabditoidea.  833 
Rhabdocoeia.  833 
Rhabdocoela.  213 
Rhabdomes.  337 
Rhabdopleura.  361 
Rhabdopleuridea.  838 
Rheas,  481 
Rhincodon,  436,  839 
Rhinoceros,  498 
Rhinoderma,  452 
Rhipidistea,  840 
Rhizocephala.  836 
Rhizopods.  832 
Rhodesian  man.  749 
Rhodnius,  330,  837 
Rhodopsin,  580 
Rhopalia,  193 
Rhynchobdellida.    835 
Rhynchocephalia.  458,  840 
Ribbon  worms,  233 
Riboflavin,  527 
Ribose  nucleic  acid,  29 
Ribs.  474,  508 
Richards,  A.  N.,  562 
Ricinulei.  836 
Rickets,  83,  527,  528 
Ring  canal,  368 
RNA,  29 

Rocky  Mountain  revolution,  725 
Rodentia,  493,  500,  842 
Rods,  577 
Root  nodules,  758 


Rostral  retractor,  224 
Rostrum,  224,  294 
Rotaria,  833 
Rotifera,  220,  222,  833 
reproduction  in,  225 
Round-dance,  348 
Roundworms,  227,  806 
Royal  jelly,  347 
Rumen,  82,  519 


Saccoglossus.  361 
SaccultJia,  730 
Sacculus.  581 
Sacrum,  508 
Sagittal  section,  62 
Salamanders,  394,  448 
Salivary  glands,  516 
Salivation,  control  of,  524 
Salt,  23 

Salts,  concentration  of,  24 
Sand  dollars,  374 
Saprozoic  animals,  79,  801 
Sarcodimi,  148,   157.  832 
Sarcopterygii,  440,  839 
Sarcoptes,  835 
Sarcoptes  scabiei,  C05 
Sargassum  fish,  442 
Saurischia,  462,  840 
Sauropterygia,  456,  840 
Savanna,  793 
Sawfish.  434,  437 
Scala  tympani.  584 
Scala  vestibuli,  584 
Scallop,  258 

Scaphirhynclnis,  440,  839 
Scaphopoda,  258,  834 
Scapula,  400,  512 
Scarlet  tanager.  484 
Scent  glands.  506 
Schistosoma,  810,  833 
Schizocoele,   1 29 
Schizocoelom,  238 
Schizocoelomata,  241 
Schizocoelous,  381 
Schizomida,  835 
Schleiden,  M.  J..  11 
Schoenheimer,  Rudolf,  75 
Schwann,  Theodor,  11 
Scientific  literature,  2 
Scientific  method,  3 
Sclera.  576 
Scolex,  2 1 6 
Scorpiones.   835 
Scorpionida.  321 
Scrotum.  566 
Scurvy,  83,  527,  528 
Scyphozoa,  190,  192,  833 
reproduction  of,  194 


872 


INDEX 


Sea  anemones,  197 

Sea  cows,  500 

Sea  cucumbers,  372 

Sea  fan,  197 

Sea  horses,  442 

Sea  lilies,  370 

Sea  lions,  497 

Sea  slugs,  251 

Sea  turtles.  456 

Sea  urchins,  373 

Seals,  497 

Sebaceous  glands,  506 

Secondary  oocyte,  120 

Secondary  spermatocytes,   119 

Secretin,  525,  605 

Sedentaria,  834 

Segmentation,  285,  514 

Segments,  267 

Selachii.  436,  839 

Selective  accumulation,  53 

Selye,  Hans,  635 

Semicircular  canals,  581 

Semilunar  fold,  579 

Semilunar  valves,  553 

Seminal  fluid,  123 

Seminal  receptacle,  300 

Seminal  vesicle,  123,  414,  569 

Seminiferous  tubules,  413,  566,  02/ 

Semipermeable  membrane,  51 

Senescence,  226 

Sense  organs,  107,  414 
birds,  477 

Sensory  cilia,  379 

Sensory  fields,  428 

Sensory  neuron,  418 

Sensory  vesicle,  386 

Sepioidea,  834 

Septa,  268 

.Septibranchiata,  834 

Sere,  777 

Serosa,  404 

Serratus  anterior,  514 

Serum,  541 

Serum  proteins,  evolution  of,  729 

Sex  chromosomes,  660 

Sex,  genetic  determination   of,  660 

Sex-influenced  trait,  663 

Sex-linked   characteristic,  662 

Sexual  reproduction,   116,  566-573 

Shaft,  471 

Shag,  763 

Shallow  sea,  795 

Sham  operation,  35 

Sharks,  436 

Sheep,  499 

Shell  gland,  250,  302 

Shellfrsh,  828 

Shrews,  492 

Siamese  twins,  135,  648 

Sickle  cell  anemia,  693 


Silk   glands,  323 

Silkworm,   111 

Silurian  period,  721 

Sinaiillnopus  pekinensis,  746 

Single  twitch,  loi 

Sinoatrial  node,  553 

Sinus  gland,  112,  328 

Sinus  venosus,  411,   545,  549,  553 

Siphon,  248 

Siphonaptera,  802,  837 

Siphonophora,   190,  833 

Sipunculoidea,  353,  835 

Sireiiia,  500,  842 

Skates,  436 

Skeleton,  96 
bird,  472 
fish,  507 
frog,  397 
mammalian,  508 
man,  509 

subdivisions  of  vertebrate,  507 
vertebrate,  506 

Skimmer,  482 

Skin,  95,  449,  502,  559 
reptiles,  453 

Skin  color,  inheritance  of,  674 

Skull,  397,  473,  510 

Slime  glands,   204 

Slugs,  252 

Small  intestine,  521 

Smallpox,  543 

Snakes,  460 

evolution  of,  461 
feeding  of,  460 
tongue   of,   461 
Snowshoe  hare,  773 
Snowy  owls,  774 
Social  parasites,  820 
Sodium,  23 

Soil,  effect  of  worms  on,  280 
Sol,    17,   31 
Soles,  442 
Solifugae,  836 
Solo  man,  748 
Solute,  30 
Solution,  30 
Solvent,  30 
Somatic  skeleton,  397 
Somites,   130,  285,  643 
Sonar,  493 
Song  birds,  481 
Sonneborn,  T.  M.,  116,  162 
Sparrow,  482 

Species,  140,   142,  146,   726 
Species  specificity,  28 
Specific  linkage,  666 
Sperm,  61,  114,  119,  566 
Sperm  reservoirs,  278 
Spermatids,  119 
Spermatocytes,  primary,  119 


INDEX 


873 


Spermatogenesis,  1 1 9 
Spermatogonia,  119 
Spermatophore,  1 24,  264,  300,  449 
Spermatozoan,   119 
Spliaeroeca,  832 
Sphaeroeca  volvox,  156,  157 
Sphenisciformes,  841 
Sphenodon,  458,  840 
Spicules,  sponge,   175 
Spinal  cord,  596 
Spinal   ner\es,  591 

rami  of,  593 

roots  of,  593 
Spinal  reflexes,  419 
Spindle,  42 
Spindle  fibers,  38 
Spinnerets,  322 
Spiny  anteater,  491 
Spiny-headed  worms,  232,  806 
Spiny  shark,  432 

Spiracle,   89,   309,   311,  422,  435,  437,  518 
Spiral  cleavage,  239-241 
Spiral  vahe,  435 
Spirotricha,    164,   832 
Splanchnic  nerves,  417,  595 
Spleen,  409 
Sponges,  172 

evolutionary  relationships  of,  236 

reproduction  of,  178 

types  of,  173 
Sponiiiidae,  832 
Spongillidae,   832 
Spongin,    176 
Spongocoel,   l  74 
Spontaneous  evisceration,  373 
Spontaneous  generation.  114 
Spontaneous  mutation.   706 
Spontaneous   origin   of   living    things,    711 
Spores,  116 
Sporocyst,  215 
Sporozoa,  149,  832 
Squalus  acanthias,  434 
Squamata,  459,  840 
Squamosal,  466,  511 
Squid,  260 

anatomy  of,  262 

reproduction  of.  264 
Squirrels,  500 
Stanley,  W.  M.,  684 
Stapes,  397,  415,  511,  565,  583 
Starfish,  364 

Starling,  Ernest  H.,  11,  525,  605 
Starling's  "law  of  the  heart."   554 
Statistical  analysis,  6 
Statocyst,  109,  183,  201,  297 
Stegosaurus,  462,  840 
Stem  reptiles,  456 
Stenothermic,  761 
Sternum,  308,  401,  474,  508 
Steroid  hormones,  619 


Steroids,  27,  618 

biosynthesis  of,   619 
Stimulus,  intensity  of,  588 
Stinger,  318 
Stinging  cells,   184 
Stomach,   519 

cow,  518 
Stomochord,   360,  363,  382 
Stomodeum,  197 
Stone  ages,  751 
Stone  canal,  368 
Stratum  compactum,  396 
Stratum  corneimi.  395,  502 
Stratum   germinativum,   395,  502 
Stratum  granulosum,  630 
Stratum  spongiosum,  396 
Stream  pollution,  826 
Strepsiptera,  837 
Stress,  635 
Strigiformes,  841 
Strix,  841 
Strobila,  194,    195 
Strongyloidea,  834 
Structural  formula,  25 
Struggle  for  survival,  699 
Struthio,  841 
Sturgeon,  440 
Stylasterina,  833 
Styloid  process,  511 
Subclavian  artery,  549 
Subesophageal  ganglion,  310 
Sublingual  glands,   516 
Submaxillary  glands,  516 
Submucosa,  405,   521 
Subphylum,    141 
Subumbrellar  surface,  182 
Subungulates,   499 
Sucrase,  81 
Sucrose,  26,  524 
Suctoria,  149.  165,  832 
Sugars,  25 
Sulcus,  601 

Sulcus  of  Rolando,  602 
Superfemale,   661 
Superior  colliculi,  597 
Supermale,  661 
Supplementary   genes,  671 
Supracoracoid,  475 
Suprascapula,  400 
Survival  curve,  771 
Survival  of  the  fittest,  699 
Sus,  842 
Suspension,  30 
Sutton,  W.  S.,  650 
Swallowing,  518 
Swallows,  482 
Swammerdam,  Jan,  10 
Swanscombe  man,  749 
Swartkrans  man  ape,  744 
Sweat  glands,  487,  506 


874 


INDEX 


Swifts,  483 
Swim  bladder,  438 
Swimmers  itch,  81 1 
Sycon,  178,  832 
Syconoid  sponges,  173 
Symbiosis,  763,  799 
Symmetry,  62 

bilateral,  62 

radial,  62 

spherical,  62 
Sympathetic  cord,  417 
Sympathetic  system,    594 
Sympathin,    106,  596 
Symphyla,  837 
Synapse,  60 

transmission   across,    106 
Synapsida,  840 
Synapsis,  1 1 7 
Synaptic  transmission,  587 
Synergism,  607 
Synsacrum,  473 
Syrinx,  477 
Systema  Naturae,  144 
Systematics,  142 
Systole,  552 


Tachyglossus,  491,  842 

Tactile  bristles,  311 

Tadpole,  420 

Taenia,  217 

Taenioidea,  833 

Tail  fan,  294 

Tail,  heterocercal,  427 

Tapeworm,  216,  806 

Tapir,  498 

Tardigrada,  835 

Tarpon,  443,  839 

Tarsal  claws,  308 

Tarsals,  4OI,  512 

Tarsioids,  496,  738,  739 

Tursius,  496,  842 

Tarsometatarsus,  474 

Tarsus.  308 

Tasmanian  wolf,  492 

Taste  buds,  108,  415,  517 

Tasting,  inheritance  of,  691 

Tatum,  Edward,  687 

Taxonomic  evidence  for  evolution,  726 

Taxonomy,  categories  in,  141 

history  of,    143 

principles  of.  137-147 
Tear  glands,  448,  579 
Tectorial  membrane,  584 
Teeth,  487,  498,  515,  516 
Telencephalon,  41 6,  597 
Teleostei,  440,  839 

adaptive  radiation  of,  441 
Telolecithal  eggs,  126 


Telophase,  43 
Telson,   294 
Temperature,  340 

effect  on  animal   distribution,  760 

regulation  of,  506 
Template,  686 
Temporal  fossa,  510 
Tendinous  cords,  553 
Tendons,  56 

Tentacles,  200,  249,  259,  260,  372 
Tentacular  cirri,  271 
Tentaculata,  832 
Tergum,  97 
Termites,  344 
Terrestrial  vertebrates,  evolution  of,  446 

respiratory  system  of,  531 
Territoriality,  483,   762 
Territory,  483 
Tertiary  period,  725 
Testis,  119,  412,  566 
Testosterone,  627 
Tetanus,  102 
Tetany,  614 

Tetrabranchiata,  265,  834 
Tetractinellida,  832 
Tetrad,  1 1 7 
Tetrahymena,   164,  832 
Tetrapod,  446 
Teuthoidea,   834 
Thalamus,   416,    597,  600 
Thaliacea,  385,  839 
Thamnophis,  840 
Theca,  630 
Thecodontia,  840 
Thelyphonida,   835 
Theory.  4 

Therapsida,  466,  840 
Theria,  491,   842 
Thermodynamics,  laws  of,  759 
Thermoreceptors,  107,   110,  575 
Thiamine,  5,  527,  528 
Thiouracil,  612 
Thoracica,  836 
Thorax,  532 
Thrombin,  541 
Thrombocytes,  541 
Thromboplastin,  541 
Thrombus,  542 
Thymus,  518,  634 
Thyone,  372 
Thyroglobulin,  608 
Thyroid  gland.  419,  450,  518,  608 
Thyrotropin,  612,  625 
Thyroxin,  450,  608 
Thysanoptera,  837 
Thysanura,  313,  837 
Tibia,   308,   512 
Tibiotarsus,  474 
Tidal  air,  534 
Tidal  zone,  794 


INDEX 


875 


Tiger  salamander,  451 
Tissue(s),  34,  53 
adipose,  56 
connective,  55 
epithelial,  54 
muscular,  57 
nervous,  60 
reproductive,  61 
vascular,  58 
Tissue  culture,  44 
Tissue  Huid,  537 
Toads,  394,  448,  451 
Tongue,  448,  517 
Tongue  bar.  363.  382 
Tonus,    102,  588 
Tornaria  larva,  379 
Torsion,  247 
Toxin,  542 

Trace  elements.  69,  762 
Trachea,  476,   532 
Tracheae.  89 
Tracheal  system,  322 
Tracheal  tubes,  311,  324 
Tracheoles,  311 
Trachylina.  833 
Trait,  dominant.  650 

recessive,   650 
Translocation,   666,   685 
Transverse  section,  62 
Tree  frog,  451,  452 
Tree  shrew,  494,  738 
Tree  sloth,  494 
Trematoda,  213,  833 
Triassic  period,  723 
Triceps,  101,  514 
Triceratops.  462,  840 

Trichinella,   834 

Trichinella  spiralis,  813 
Trichinosis,  813 

Trichocyst,   151,    162 

Trichomonadina,  832 

Trichomonas,  832 

Trichomonas   hominis,  807 

Trichoptera,  837 

Trichuroidea,  834 

Tricladida,  213,  833 

Tricuspid  valve,   553 

Trigeminal  ner\e.  417 

Triiodothyronine,   609 

Trilobita,  290,  721,  835 

Triose  phosphates,  72 

Trochanter,  308 

Trochlear  nerve,  417 

Trochophore  larva,  255,  269,  284,  286,  353 

Trogon,  841 

Trogoniformes,  841 

Trophallaxis,  345,  349 

Trophoblast,  638 

Trophozoite,  165 

Tropical  rain  forest  biome,  794 


Truncus  arteriosus,  408 

1  runk,  499 

Trypanosomes,  808,  813,  832 

Trypsin,  81,  523 

Trypsinogen.  523 

Tsetse  flies,  809 

Tuatara,  458 

Tube  feet,  367 

Tubercles,  365 

Tubifex,  281,  835 

Tubular  reabsorption,  563 

Tubule,  kidney,  93,  411,  454,  563 

Tubulidentata,  494 

Tundra  biome.  790 

Tunica  vaginalis,  566 

Tupaia,  494 

Turhatrix  aceti,  228 

Turbellaria.  212,  833 

Turnover  number,  67 

Tursiops,  842 

Turtles,  456 

Tusks.  499 

Twinning.  646 

Twins,  dizygotic.  646 

monozygotic,  646 
Twitty,  Victor,   135 
Tympanic  membrane,  395,  415,  583 
Typhlosole,  276 
Typhus,  803 
Tyrannosaurus.  462,  840 

u 

Ulna,   512 

Ultrasonic  clicks,  493 
Umbilical  arteries,  645 
Umbilical  cord,  133,  642 
Umbilical  vein,  550 
Umbo,  253 

Unconscious   cooperation,    767 
Ungulates.  498 
Unguligrade,  498 
Uniformitarianism,  697 
Uniramous  limbs,  292 
Universal  donors.  544 
Universal  recipients,  544 
Upper  paleolithic  culture,   751 
Urea,  23.  82.  95.  454 
Urea  cycle,  526 
Urease,  67 
Ureter,  454,  561 
Urethra,   123,  562 
Uric  acid,  95,  4.54.  477 
Urinary  bladder,  411,  455,  561 
Urine,  564 

Urochordata,   384,  839 
Urochrome,   564 
Urodela,  394,  448,  840 
Urogenital  sinus,  431 
Urogenital  system,   123,  559 


876 


INDEX 


Uropod,   294 
Uropygial   }^l;m(l.  472 
Urostylc,   399 
I'terinc  contractions,  572 
Uterus,   123.   4,'5G,  489,  570 
Utriculus,  581 


Vaccination,  543 
Vaccinia  virus,  543 
Vacuoles,  38 

contractile,  39 

food,  39 
Vagina,  123,  570 
Vagus  nerve,  417,  524 
Valves,  252 

Valvular  intestine,  435,  521 
Vanadium,  53 

van   Leeuwenhoek,  Antony,  10 
Vane,  471 
Variola  virus,  543 
Vas  deferens,  123,  569 
Vas  efferens,  123,  414,  569 
Vasoconstrictor  nerves,  554 
Vasodilator  nerves,    554 
Vasopressin,  622 
Vegetal  pole,   1 26 
Veins,  86,  556 
Veliger,  247,  284 
Velum,  182 
Vena  cava,  anterior,  410,  549 

posterior,  410,  549 
Ventral  abdominal  vein,  410 
Ventral  aorta,  546 
Ventral  cirrus,  272 
Ventricle,  86,  411,  545,  549,  598 
Ventrolateral  nerve  cords,  208 
Venus  mercenaria,  253 
Venus's  flower  basket,  176 
Vermiform  appendix,  521 
Vertebrae,   130,  399,  448,   473,   508 

bird,  473 
Vertebral  column,  390,  399 
Vertebrates,   389-648,  839 

beginnings  of,  427 

characteristics,  389-391 

classes,   389-390 

organization,  393  fl". 
Vertical   stratification,  777 
Vesalius,  Andreas,  9 
Vestigial  organs,  728 
Villi,  476,  523 

placental,  638 
Vinegar  eel,  228 
Virchow,  34 
Viruses,  712 

Visceral  arches,  508,  510 
Visceral  ganglia,  249,  255 
Visceral  mass,  255 


Visceral  peritoneum,  403 
Visceral  skeleton,   397,  508 
Vision,  579 
color,  339 
Visual   organelle,   151 
Vitalism,  13 
Vitamin  A,  527 
Vitamin  B12,  527,  541 
Vitamin   D,   528 
Vitamin   K,  527,  541 
Vitamins,  68,  83.  526,  527 
Vitelline  glands,  211 
Vitreous  humor.  578 
Viviparous,   125,  436,  456 
Vocal  cords,  408,   532 
Vocal  sacs,  408 
Volvox,  155 

von  Baer.  Karl  Ernst,  10 
von  Frisch.  Karl,  338 
Vorticella,  832 

w 

Wagging  dance,  349 
Walking  stick,  342 
Wallace,  Alfred  Russell,  699 
Wallace's  line,  736 
Walruses,  497 
Warbles,  813 

Warm-blooded  animals,  468 
Warning  coloration,  785 
Wastes,  elimination  of,  92 

nitrogenous,  92 
Water,  21,  83 

eff^ect  on  animal  distribution,  761 
Water  cycle,  758 
Water  fleas,  300 
Water  vascular  system.  368 
Wave  of  depolarization,  105 
Wax  glands,  318 
Wax  spur.  318 
Waxes,  27 
Weasels,  497 
Weberian  ossicles,  582 
Weidenreich,  Franz,  747 
Weismann,  A.,  34 
Whale  shark,  436 
Whalebone,  498 

plates,  506 
Whales,  497 
Wheel  animals.  220 
Wheel  organ,  222,  387 
Whippoorwills,  482 
White  blood  cells,  58,  542 
White  matter,  596 
White  races,  749 
Wildlife  resources,  824 
Williams,  Carroll,  331 
Wilson,  E.  B.,  287 
Wilson,  E.  v.,  180 


INDEX 


877 


Wing,  468,  492 

Wing  beats,  frequency  of,  334 

Wing  buds,  315 

Wing  slots,  469 

Wishbone,  474 

Wolff,  Kasper,  10,  133 

Wolffian  ducts,  411,  431,  435,  559 

Woodchucks,  500 

Woodcocks,  482 

Woodpeckers,  483 

Worker   termites,    344 

\Vorm,  spiny-headed,  232 

Wuchereria,  811,  834 

X 

X  chromosomes,  660 
X  organ,  329 
Xenopsylla  cheopis,  803 
Xiphosura,  320,  835 


Y  chromosomes,  660 
Yolk,  120,  454 
Yolk  plug,  420 
Yolk  sac,  131,  455,  639 
Yolk  sac  placenta,  436 
Yucca  moth,  787 


Zoantharia,  833 

Zoological   Nomenclature,   commission  on, 

140 
Zoolog)',  applications  of,   12 

histor)'  of.  7 

subdivisions  of,  1 
Zygapophysis,  399,  508 
Zygomatic  arch,  510 
Zygote,  114,  168,  420 


i