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GIFT   OF 
MICHAEL  REESE 


BIOLOGY 
UBffARY 

G 


LESSONS 


IN 


ELEMENTARY    BIOLOGY 


LESSONS 


IN 


ELEMENTARY   BIOLOGY 


BY 

T.  JEFFERY   PARKER,  D.Sc,  F.R.S. 

PROFESSOR   OF    BIOLOGY   IN   THE   UNIVERSITY   OF   OTAGO,    DUNEDIN,    NEW   ZEALAND 


WITH    EIGHTY-EIGHT    ILLUSTRATIONS 


Eonton 
MACMILLAN     AND     CO. 

AND    NEW    YORK 
1893 

The  Right  of  Translation  and  Reproduction  is  Reserved 


BiOLOGY 

LIBRARY 

G 


RICHARD  CLAY  AND  SONS,  LIMITED. 

LONDON   AND   BUNGAY. 

First  Edition,  1891. 
Second  Edition  Revised,  1893. 


PREFACE    TO  THE  FIRST  EDITION 

IN  his  preface  to  the  new  edition  of  the  well-known  Practical 
Biology ',  Professor  Huxley  gives' his  reasons  for  beginning  the 
study  of  organized  nature  with  the  higher  forms  of  animal 
life,  to  the  abandonment  of  his  earlier  method  of  working 
from  the  simpler  to  the  more  complex  organisms.  He  says 
in  effect  that  experience  has  taught  him  the  unwisdom  of 
taking  the  beginner  at  once  into  the  new  and  strange  region 
of  microscopic  life,  and  the  advantage  of  making  him  com- 
mence his  studies  with  a  subject  of  which  he  is  bound  to 
know  something — the  elementary  anatomy  and  physiology 
of  a  vertebrate  animal. 

Most  teachers  will  probably  agree  with  the  general  truth 
of  this  opinion.  The  first  few  weeks  of  the  beginner  in 
natural  science  are  so  fully  occupied  in  mastering  an  un- 
familiar and  difficult  terminology  and  in  acquiring  the  art 
of  using  his  eyes  and  fingers,  that  he  is  simply  incapable  for 
a  time  of  grasping  any  of  the  principles  of  the  science ;  and, 
this  being  the  case,  the  more  completely  his  new  work  can 


vi  PREFACE 

be  connected  with  any  knowledge  of  the  subject,  however 
vague,  he  may  already  possess,  the  better  for  his  progress. 

On  the  other  hand,  the  advantage  to  logical  treatment  of 
proceeding  from  the  simple  to  the  complex— of  working 
upwards  from  protists  to  the  higher  plants  and  animals — is 
so  immense  that  it  is  not  to  be  abandoned  without  very 
good  and  sufficient  reasons. 

In  my  own  experience  I  have  found  that  the  difficulty 
may  be  largely  met  by  a  compromise,  namely,  by  beginning 
the  work  of  the  class  by  a  comparative  study  of  one  of  the 
higher  plants  (flowering  plant  or  fern)  and  of  one  of  the 
higher  animals  (rabbit,  frog,  or  crayfish).  If  there  were  no 
limitations  as  to  time,  and  if  it  were  possible  to  avoid  alto- 
gether the  valley  of  the  shadow  of  the  coming  examination, 
this  preliminary  work  might  be  extended  with  advantage,  and 
made  to  include  a  fairly  complete  although  elementary  study 
of  animal  physiology,  with  a  minimum  of  anatomical  detail, 
and  a  somewhat  extensive  study  of  flowering  plants  with 
special  reference  to  their  physiology  and  to  their  relations 
to  the  rest  of  nature. 

In  any  case  by  the  time  this  introductory  work  is  over, 
the  student  of  average  intelligence  has  overcome  pre- 
liminary difficulties,  and  is  ready  to  profit  by  the  second 
and  more  systematic  part  of  the  course  in  which  organisms 
are  studied  in  the  order  of  increasing  complexity. 

It  is  such  a  course  of  general  elementary  biology  which 
I  have  attempted  to  give  in  the  following  Lessons,  my  aim 
having  been  to  provide  a  book  which  may  supply  in  the 


PREFACE  vii 

study  the  place  occupied  in  the  laboratory  by  "  Huxley  and 
Martin,"  by  giving  the  connected  narrative  which  would  be 
out  of  place  in  a  practical  handbook.  I  also  venture  to 
hope  that  the  work  may  be  of  some  use  to  students  who 
have  studied  zoology  and  botany  as  separate  subjects,  as 
well  as  to  that  large  class  of  workers  whose  services  to 
English  science  often  receive  but  scant  recognition — I 
mean  amateur  microscopists. 

As  to  the  general  treatment  of  the  subject  I  have  been 
guided  by  three  principles.  Firstly,  that  the  main  object  of 
teaching  biology  as  part  of  a  liberal  education  is  to  familiarize 
the  student  not  so  much  with  the  facts  as  with  the  ideas  of 
science.  Secondly,  that  such  ideas  are  best  understood,  at 
least  by  beginners,  when  studied  in  connection  with  concrete 
types  of  animals  and  plants.  And,  thirdly,  that  the  types 
chosen  should  illustrate  without  unnecessary  complication 
the  particular  grade  of  organization  they  are  intended  to 
typify,  and  that  exceptional  cases  are  out  of  place  in  an 
elementary  course. 

The  types  have  therefore  been  selected  with  a  view  of 
illustrating  all  the  more  important  modifications  of  structure 
and  the  chief  physiological  processes  in  plants  and  animals  ; 
and,  by  the  occasional  introduction  of  special  lessons  on 
such  subjects  as  biogenesis,  evolution,  &c.,  the  entire  work 
is  so  arranged  as  to  give  a  fairly  connected  account  of  the 
general  principles  of  biology.  It  is  in  obedience  to  the  last 
of  the  principles  just  enunciated  that  I  have  described  so 
many  of  the  Protozoa,  omitted  all  but  a  brief  reference  to 


viii  PREFACE 

the  development  of  Hydra  and  to  the  so-called  sexual  pro- 
cess in  Penicillium,  and  described  Nitella  instead  of  Chara, 
and  Polygordius  instead  of  the  earthworm.  The  last-named 
substitution  is  of  course  only  made  possible  by  the  book 
being  intended  for  the  study  and  not  for  the  laboratory,  but 
I  feel  convinced  that  the  student  who  masters  the  structure 
of  Polygordius,  even  from  figures  and  descriptions  alone, 
will  be  in  a  far  better  position  to  profit  by  a  practical  study 
of  one  of  the  higher  worms. 

Lessons  XXVII.  and  XXX.  are  mere  summaries,  and  can 
only  be  read  profitably  by  those  who  have  studied  the 
organisms  described,  or  allied  forms,  in  some  detail.  Such 
abstracts  were  however  necessary  to  the  plan  of  the  book,  in 
order  to  show  how  all  the  higher  animals  and  plants  may  be 
described,  so  to  speak,  in  terms  of  Polygordius  and  of  the  fern. 

For  many  years  I  have  been  convinced  of  the  urgent  need 
for  a  simplification  of  nomenclature  in  biology,  and  have  now 
attempted  to  carry  out  a  consistent  scheme,  as  will  be  seen 
by  referring  to  the  definitions  in  the  glossary.  Many  of 
Mr.  Harvey  Gibson's  suggestions  are  adopted  and  three  new 
words  are  introduced — phyllula,  gamobium,  and  agamo- 
bium.  I  expect  and  perhaps  deserve  to  be  criticised,  or, 
what  is  worse,  let  alone,  for  the  somewhat  extreme  step  of 
using  the  word  ovary  in  its  zoological  sense  throughout  the 
vegetable  kingdom ;  and  for  describing  as  the  venter  of  the 
pistil  the  so-called  ovary  of  Angiosperms.  I  would  only 
beg  my  critics  before  finally  pronouncing  judgment  to  try 
and  look  at  the  book,  from  the  point  of  view  of  the  begin- 


PREFACE  ix 

ner,  as  a  graduated  course  of  instruction,  and  to  consider 
the  effect  upon  the  entire  scheme  of  using  a  term  of  funda- 
mental importance  in  two  utterly  different  senses. 

A  large  proportion  of  the  figures  are  copied  either  from 
original  sources  or  from  my  own  drawings — the  latter  when 
no  authority  is  mentioned.  The  majority,  even  of  those 
which  have  previously  appeared  in  text-books,  have  been 
specially  engraved  for  the  work,  the  draughtsman  being 
my  brother,  Mr.  M.  P.  Parker.  In  order  to  facilitate 
reference  the  illustrations  referring  to  each  subject  have,  as 
far  as  possible,  been  grouped  together,  so  that  the  actual  is 
considerably  larger  than  the  nominal  number  of  figures. 
Full  descriptions  are  given  instead  of  mere  lists  of  reference- 
letters  :  these  will,  I  hope,  be  found  useful  as  abstracts  of 
the  subjects  illustrated. 

I  have  to  thank  my  friends  Mr.  A.  Dillon  Bell  and  Pro- 
fessor J.  H.  Scott,  M.D.,  for  constant  and  valuable  help  in 
criticising  the  manuscript.  To  Dr.  Paul  Meyer,  of  the 
Zoological  Station,  Naples,  I  am  indebted  for  specimens 
of  Polygordius ;  and  to  Professer  Sale,  of  this  University, 
Professor  Haswell,  of  Sydney,  Professor  Thomas,  of  Auck- 
land, and  Professors  Howes  and  D.  H.  Scott,  of  South 
Kensington,  for  important  information  and  criticism  on 
special  points.  My  brother,  Professor  W.  Newton  Parker, 
has  kindly  promised  to  undertake  a  final  revision  for  the 
press. 

DUNEDIN,  N.2., 

890. 


PREFACE  TO  THE   SECOND  EDITION 

IN  addition  to  a  thorough  revision,  Lessons  VI.  and 
XXIV.  have  been  largely  re-written.  Figs.  9,  10,  52,  60, 
64,  and  66  are  new,  and  Figs.  9,  10,  n,  64,  66,  and  67  of 
the  first  edition  have  been  withdrawn. 

I  have  received  valuable  help  from  Professors  W.  N. 
Parker  and  G.  B.  Howes,  Miss  M.  Greenwood,  and 
Mr.  J.  E.  S.  Moore.  Much  of  the  proof-correcting  has, 
as  before,  fallen  upon  my  brother. 

March  1893. 


TABLE  OF  CONTENTS 


PREFACE  TO  THE  FIRST  EDITION 
PREFACE  TO  THE  SECOND  EDITION    . 
LIST  OF  ILLUSTRATIONS 


AMCEBA  .... 


H^MATOCOCCUS 


HETEROMITA 


EUGLENA 


PROTOMYXA        .     . 
THE    MYCETOZOA 


LESSON  I. 


LESSON  II. 


LESSON  III. 


LESSON  IV. 


LESSON  V. 


PAGE 
V 


44 


49 

52 


xiv  TABLE  OF  CONTENTS 

LESSON  VI. 

PAGE 

A  COMPARISON  OF  THE  FOREGOING  ORGANISMS  WITH  CERTAIN 
CONSTITUENT  PARTS  OF  THE  HIGHER  ANIMALS  AND 
PLANTS  56 

ANIMAL  AND  PLANT  CELLS 56 

MINUTE    STRUCTURE  AND   DIVISION    OF    CELLS    AND 

NUCLEI 62 

OVA  OF  ANIMALS  AND   PLANTS 68 

LESSON  VII. 

SACCHAROMYCES 71 

LESSON  VIII. 

BACTERIA 82 

LESSON  IX. 

BIOGENESIS  AND  ABIOGENESIS 95 

HOMOGENESIS  AND  HETEROGENESIS IO2 

LESSON  X. 

PARAMCECIUM    j IO6 

STYLONYCHIA  /. Il6 

OXYTRICHA 120 

LESSON  XI. 

OPALINA 121 

LESSON  XII. 

VORTICELLA 126 

ZOOTHAMNIUM 135 


TABLE  OF  CONTENTS  xv 

LESSON  XIII. 

PAGE 

SPECIES  AND  THEIR   ORIGIN  :   THE   PRINCIPLES   OF   CLASSIFICA- 
TION            137 

LESSON  XIV. 

THE  FORAMINIFERA 148 

THE  RADIOLARIA . 152 

THE  DIATOMACE/E 155 

LESSON  XV. 

MUCOR 158 

LESSON  XVI. 

VAUCHERIA 169 

CAULERPA 175 

LESSON  XVII. 

THE  DISTINCTIVE  CHARACTERS  OF  ANIMALS  AND  PLANTS  ...         176 

LESSON  XVIII. 

PENICILLIUM „ 184 

AGARICUS igi 

LESSON  XIX. 

SPIROGYRA 194 

LESSON  XX. 

MONOSTROMA 2OI 

ULVA 203 

LAMINARIA,  &C 2O3 


xvi  TABLE  OF  CONTENTS 

LESSON  XXI. 

I'AGE 

NITELLA     . 206 

LESSON  XXII. 

HYDRA ; 221 

LESSON  XXIII. 

HYDROID  POLYPES 237 

BOUGAINVILLEA,  &C 237 

DIPHYES 250 

PORPITA 253 

^LESSON  XXIV. 

SPERMATOGENESIS  AND  OOGENESIS 255 

THE  MATURATION  AND  IMPREGNATION  OF  THE  OVUM     ....  259 
THE  CONNECTION  BETWEEN   UNICELLULAR  AND   DIPLOBLASTIC 

ANIMALS 264 

LESSON  XXV. 

POLYGORDIUS 271 

LESSON  XXVI. 
POLYGORDIUS  (continued} 293 

LESSON  XXVII. 

THE  GENERAL  CHARACTERS  OF  THE  HIGHER  ANIMALS     ....  307 

THE  STARFISH 309 

THE  CRAYFISH         314 

THE  FRESH-WATER  MUSSEL 32O 

THE  DOGFISH 324 


TABLE  OF  CONTENTS  xvii 
LESSON  XXVIII. 

PACK 

MOSSES 332 

LESSON  XXIX. 

FERNS 344 

LESSON  XXX. 

THE  GENERAL  CHARACTERS  OF  THE  HIGHER  PLANTS  .....  363 

EQUISETUM ' 366 

SALVINIA 368 

SELAGINELLA 371 

GYMNOSPERMS        373 

ANGIOSPERMS 378 

SYNOPSIS 385 

INDEX  AND  GLOSSARY 395 


LIST   OF    ILLUSTRATIONS 


FIG.  PAGE 

1.  Amoeba,  various  species 2 

2.  Protamceba  primitiva   .     , 9 

3.  Hfzmatococcits  pluvialis  and  H.  lacnstris 24 

4.  Heteromita  rostrata 38 

5.  Euglena  viridis 45 

6.  Protomyxa  atirantiaca 5° 

7.  Badhamia  and  Chondrioderma       53 

8.  Typical  animal  and  vegetable  cells 57 

9.  Animal  and  plant  cells,  detailed  structure 62 

10.  Stages  in  the  binary  fission  of  a  cell 64 

11.  Ova  of  Carmarina  and  Gymnadenia 69 

1 2.  Saccharomyces  cerevisice 72 

13.  Bacterium  termo .  83 

14.  Bacteririm  termo,  showing  flagella 84 

15.  Micrococcus 86 

16.  Bacillus  subtilis 87 

1 7.  Vibrio  serpens,  Spirillum  tenue,  and  S.  volutans 88 

1 8.  Bacillus  anthracii, 90 

19.  Beaker  with  culture- tubes 100 


xx  LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

20.  Faramoecium  aurelia    . 108 

21.  Paramcecium  aurelia,  conjugation 115 

22.  Stylonychia  mytiltis 117 

23.  Oxytricha  fava 120 

24.  Opalina  ranarum 122 

25.  Vorticella 127 

26.  Zoothamnium  arbuscida 134 

27.  Zoothamnium,  various  species 138 

28.  Diagram  illustrating  the   Origin   of  the  Species  of  Zootham- 

nium by  Creation 142 

29.  Diagram  illustrating  the  Origin  of  the  Species  of  Zootham- 

nium  by  Evolution 144 

30.  Rotalia 149 

31.  Diagrams  of  Foraminifera 150 

32.  Alveolina  quoii  , I51 

33.  Lithocircus  annularis 152 

34.  Actinomma  asteracanthion 153 

35.  Diagrams  of  a  Diatom  and  shells  of  Navicula  and  Attlaco- 

di setts 156 

36.  Mucor  mucedo  and  M.  stolonifer 159 

37.  Moist  Chamber 163 

38.  Vaucheria 170 

39.  Caulerpa  scalpelliformis 174 

40.  Penicillinm  glaucum 186 

41.  Agarictts  campestris 192 

42.  Spirogyra .  195 

43.  Monostroma  Imllosum  and  M.  laceratum 202 

44.  Laminaria  claustoni  and  Lessonia  frtsccscens  .    .    .   •     ...  204 

45.  Nitella,  general  structure 207 

46.  Nitella,  terminal  bud 212 

47.  Nitella,  spermary      215 

48.  Nitella,  ovary 217 

49.  Chara,  pro-embryo 219 

50.  Hydra  viridis  and  H.  fusca,  external  form 222 

51.  Hydra,  minute  structure  .    .    .    .  ' 226 


LIST  OF  ILLUSTRATIONS  xxi 

FIG.  PAGE 

52.  Hydra,  nematocyst  and  nerve-cell 228 

53.  Hydra  viridis,  ovum 235 

54.  Bougainvillca  ramosa -. 238 

55.  Diagrams  illustrating  derivation  of  Medusa  from  Hydrautli  .  242 

56.  Eucopella  campanularia,  muscle  fibres  and  nei've-cells.  .    .    .  245 
57-  Laomedta  flexuosa  and  Eudendrimn  ramosum,  development  .  249 

58.     DipJiycs  campanulata 252 

59-     Porpita  pacifica  and  P.  mediterranea 253 

60.  Spermatogenesis  in  the  Mole-Cricket 256 

61.  Ovum  of  Toxopneustes  lividus 259 

62.  Maturation  and  impregnation  of  the  animal  ovum 260 

63.  The  gastrula 265 

64.  Pandorina  morum 266 

65.  Volvox  globator 268 

66.  Volvox  globator 269 

67.  Polygordius  neapolitanus,  external  form 272 

68.  Polygordius  neapolitanus,  anatomy 274 

69.  Polygordius  neapolitanus,  nephridium 285 

70.  Polygordius,  diagram  illustrating  the  relations  of  the  nervous- 

system    287 

71.  Polygordius  neapolitanus,  reproductive  organs 294 

72.  Polygordius  neapolitanus,  larva  in  the  trochosphere  stage  .    .  296 
73-  Diagram  illustrating  the  origin  of  the  trochosphere  from  the 

gastrula 298 

74.  Polygordius  neapolitamis,  advanced  trochosphere 300 

75.  Polygordius  neapolitanus,  larva  in  a  stage  intermediate   be- 

tween the  trochosphere  and  the  adult 303 

76.  Starfish,  diagrammatic  sections 310 

77.  Crayfish,  diagrammatic  sections 316 

78.  Mussel,  diagrammatic  sections 321 

79.  Dogfish,  diagrammatic  sections 326 

80.  Mosses,  various  genera,  anatomy  and  histology 333 

81.  Funaria,  reproduction  and  development  .    . 338 

82.  Pteris  and  Aspidium,  anatomy  and  histology 346 

83.  Ferns,  various  genera,  reproduction  and  development    .    .    .  356 


xxii  LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

84.  Equisetum,  reproduction  and  development 367 

85.  Salvinia,  reproduction  and  development  . 369 

86.  Selaginella,  reproduction  and  development 372 

87.  Gymnosperms,  reproduction  and  development 374 

88.  Angiosperms,  reproduction  and  development 379 


LESSONS 


IX 


ELEMENTARY    BIOLOGY 


;    UNIT? 

\   c^ 


LESSONS 

IN 

ELEMENTARY    BIOLOGY 

LESSON  I 

AMCEBA 

IT  is  hardly  possible  to  make  a  better  beginning  of  the 
systematic  study  of  Biology  than  by  a  detailed  examination 
of  a  microscopic  animalcule  often  found  adhering  to  weeds 
and  other  submerged  objects  in  stagnant  water,  and  known 
to  naturalists  as  Anuzba. 

Amoebae  are  mostly  invisible  to  the  naked  eye,  rarely 
exceeding  one-fourth  of  a  millimetre  (^-^  inch)  in  dia- 
meter, so  that  it  is  necessary  to  examine  them  entirely  by 
the  aid  of  the  microscope.  They  can  be  seen  and  re- 
cognized under  the  low  power  of  an  ordinary  student's 
microscope  which  magnifies  from  twenty-five  to  fifty  dia- 
meters ;  but  for  accurate  examination  it  is  necessary  to 
employ  a  far  higher  power,  one  in  fact  which  magnifies 
about  300  diameters. 

Seen  under  this  power,  an  Amoeba  appears  like  a  little 

B 


AMCEBA 


LESS. 


FIG  i.— A.  Amasba  quarta,   a  living    specimen,    showing    granular 
endosarc  surrounded  by  clear  ectosarc,  and  several  pseudopods  (fist*), 


i  GENERAL  CHARACTERS  3 

some  formed  of  ectosarc  only,   others  containing  a  core  ot  endosarc. 
The  larger  bodies  in  the  endosarc  are  mostly  food-particles  (  x  300). 1 

B.  The  same  species,  killed  and  stained  with  carmine  to  show  the 
numerous  nuclei  (mi)  ( x  300). 

C.  Amceba  proteus,    a   living    specimen,     showing    large    irregular 
pseudopods,   nucleus   (mi),   contractile  vacuole    (c.vac),   and  two  food 
vacuoles  (f.vac),  each  containing  a  small  infusor  (see  Lesson  X.)  which 
has  been  ingested  as  food.     The  letter  a  to  the  right  of  the  figure  in- 
dicates the  place  where  two  pseudopods  have  united  to  inclose  the  food 
vacuole.     The  contractile  vacuole  in  this  figure  is  supposed  to  be  seen 
through  a  layer  of  granular   protoplasm,    whereas   in  the  succeeding 
figures  (D,  E,  and  G)  it  is  seen  in  optical  section,  and  therefore  appears 
clear. 

D.  An  encysted  Amoeba,  showing  cell-wall  or  cyst  (cy),  nucleus  (mi), 
clear  contractile  vacuole  (c.vac),  and  three  diatoms  (see  Lesson  XIV.) 
ingested  as  food. 

E.  Amoeba  proteus,  a  living  specimen,  showing  several  large  pseudo- 
pods  (psd).  single  nucleus  (nu),  and  contractile  vacuole  (c.vac},   and 
numerous  food-particles  embedded  in  the  granular  endosarc  (  x  330). 

F.  Nucleus  of  the  same  after  staining,  showing  a  ground  substance  or 
achromatin,    containing    deeply-stained   granules    of    chromatin,    and 
surrounded  by  a  distinct  membrane  (  x   1010). 

G.  Amceba  verrucosa,   living    specimen,  showing    wrinkled    surface, 
nucleus  (mi),    large   contractile   vacuole   (c.vac)  and   several  ingested 
organisms  (  x  330). 

H.  Nucleus  of  the  same,  stained,  showing  the  chromatin  aggregated 
in  the  centre  to  form  a  nucleolus  (  x  1010). 

I.  Amceba  proleus,  in  the  act  of  multiplying  by  binary  fission 
(  x  500). 

(A,  B,  E,  F,  G,  and  H  after  Gruber  ;  c  and  I  after  Leidy  ;  D  after. 
Howes. ) 

shapeless  blob  of  jelly,   nearly  or  quite  colourless.     The 
central  part  of  it  (Fig.  i,  A,  c,  and  E)  is  granular  and  semi-  \ 
transparent — something  like  ground  glass — while  surround-  \ 
ing  this  inner  mass  is  a  border  of  perfectly  transparent  and 
colourless  substance.     So  clear,  indeed,  is  this  outer  layer 
that  it  is  easily  overlooked  by  the  beginner,  who  is  apt  to  take 
the  granular  internal  substance  for  the  whole  Amceba.     If 
in  any  way  the  creature  can  be  made  to  turn  over,  or  if  a 
number   of   specimens   are  examined  in  various  positions, 
these  two  constitutents  will  always  be  found  to   have  the 

1  A  number  preceded  by  the  sign  of  multiplication  indicates  the 
number  of  diameters  to  which  the  object  is  magnified. 

13    2 


4  AMCEBA  LESS. 

same  relations,  whence  we  conclude  that  an  Amoeba  con- 
sists of  a  granular  substance  the  endosarc,  completely 
surrounded  by  a  clear  transparent  layer  or  ectosarc. 

One  very  noticeable  thing  about  Amoeba  is  that  it  is  never 
of  quite  the  same  shape  for  long  together.  Often  the 
changes  of  form  are  so  slow  as  to  be  almost  imperceptible, 
like  the  movements  of  the  hour-hand  of  a  watch,  but  by 
examining  it  at  successive  intervals  the  alteration  becomes 
perfectly  obvious,  and  at  the  end  of  half  an  hour  it  will 
probably  have  altered  so  much  as  to  be  hardly  like  the 
same  thing. 

In  an  active  specimen  the  way  in  which  the  changes  of 
form  are  brought  about  is  easily  seen.  At  a  particular 
point  the  ectosarc  is  pushed  out  in  the  form  of  a  small 
pimple-like  elevation  (Fig.  i,  A,  left  side)  :  this  increases  in 
size,  still  consisting  of  ectosarc  only,  until  at  last  granules 
from  the  endosarc  stream  into  it,  and  the  projection  or 
pseudopod  (A,  c,  E,  psd)  comes  to  have  the  same  structure 
as  the  rest  of  the  Amoeba.  It  must  not  be  forgotten  that 
the  animal  does  not  alter  perceptibly  in  volume  during 
|  the  process,  every  pseudopod  thus  protruded  from  one  part 
of  the  body  necessitating  the  withdrawal  of  an  equal  volume 
from  some  other  part. 

This  peculiar  mode  of  movement  may  be  illustrated  by 
taking  an  irregular  lump  of  clay 'or  putty  and  squeezing  it 
between  the  fingers.  As  it  is  compressed  in  one  direction 
it  will  elongate  in  another,  and  the  squeezing  process  may 
be  regulated  so  as  to  cause  the  protrusion  of  comparatively 
narrow  portions  from  the  solid  lump,  when  the  resemblance 
to  the  movements  described  in  the  preceding  paragraph  will 
be  fairly  close.  Only  it  must  be  borne  in  mind  that  in 
Amoeba  there  is  no  external  compression,  the  "  squeezing  " 
being  done  by  the  animalcule  itself. 


i  COMPOSITION  OF  PROTOPLASM  5 

The  occurrence  of  these  movements  is  alone  sufficient  to 
show  that  Amoeba  is  an  organism  or  living  thing,  and  no 
mere  mass  of  dead  matter. 

The  jelly-like_substance_  o£ .which  Amoeba  is  composed 
is  called  protoplasm.  It  is  shown  by  chemical  analysis x 
to  consist  mainly  of  certain  substances  known  %s>  protcids, 
bodies  of  extreme  complexity  in  chemical  constitution,  the 
most  familiar  example  of  which  is  white  of  egg  or  albumen. 
They  are  compounds  of  carbon,  hydrogen,  oxygen,  nitrogen, 
and  sulphur,  the  five  elements  being  combined  in  the 
following  proportions  : — 

Carbon  .     .  from  51*5  to  54*5  per  cent. 
Hydrogen  .      „       6-9  „     7-3    „      „ 
Oxygen       .      „     20-9  „   23-5    „      „ 

Nitrogen     .  „     15-2  „   17-0    „      „ 

Sulphur      .  „       0-3  „     2-0    „      „ 

Besides  proteids,  protoplasm  contains  small  proportions 
of  mineral  matters,  especially  phosphates  and  sulphates  of 
potassium,  calcium,  and  magnesium.  It  also  contains  a 
considerable  quantity  of  water  which,  being  as  essential  a 
constituent  of  it  as  the  proteids  and  the  mineral  salts,  is 
called  water  of  organization. 

Protoplasm  is  dissolved  by  prolonged  treatment  with  weak 
acids  or  alkalies.  Strong  alcofiol  coagulates  it,  f.e.j  causes  it 
to  shrink  by  withdrawal  of  water  and  become  comparatively 
hard  and  opaque.  Coagulation  is  also  produced  by  raising 
the  temperature  to  about  40°  C. ;  the  reader  will  remember 
how  the  familiar  proteid  white  of  egg  is  coagulated  and 
rendered  hard  and  opaque  by  heat. 

1  Accurate  analyses  of  the  protoplasm  of  Amoeba  have  not  been 
made,  but  the  various  micro-chemical  tests  which  can  be  applied  to  it 
leave  no  doubt  that  it  agrees  in  all  essential  respects  with  the  protoplasm 
of  other  organisms,  the  composition  of  which  is  known  (see  p.  7). 


6  AMCEBA  LESS. 

There  is  another  important  property  of  proteids  which  is 
tested  by  the  instrument  called  a  dialyser.  This  consists 
essentially  of  a  shallow  vessel,  the  bottom  of  which  is  made 
of  bladder,  or  vegetable  parchment^jat,  some  other  organic 
(animal  or  vegetable)  membrane.  If  a  solution  of  sugar  or  of 
salt  is  placed  in  a  dialyser  and  the  instrument  floated  in  a 
larger  vessel  of  distilled  water,  it  will  be  found  after  a  time  that 
some  of  the  sugar  or  salt  has  passed  from  the  dialyser  into 
the  outer  vessel  through  the  membrane.  On  the  other  hand, 
if  a  solution  of  white  of  egg  is  placed  in  the  dialyser  no 
such  transference  to  the  outer  vessel  will  take  place. 

The  dialyser  thus  allows  us  to  divide  substances  into 
two  classes  :  crystalloids — so  called  because  most  of  them, 
like  salt  and  sugar,  are  capable  of  existing  in  the  form  of 
crystals — which,  in  the  state  of  solution,  will  diffuse  through 
an  organic  membrane  ;  and  colloids  or  glue-like  substances 
which  will  not  diffuse.  Protoplasm,  like  the  proteids  of 
which  it  is  largely  composed,  is  a  colloid,  that  is,  is  non- 
diffusible. 

Another  character  of  proteids  is  their  instability.  A 
lump  of  salt  or  of  sugar,  a  piece  of  wood  or  of  chalk,  may 
be  preserved  unaltered  for  any  length  of  time,  but  a  proteid 
if  left  to  itself  very  soon  begins  to  decompose ;  it  acquires 
an  offensive  odour,  and  breaks  up  into  simpler  and  simpler 
compounds,  the  most  important  of  which  are  water  (H2O), 
carbon  dioxide  or  carbonic  acid  (CO2),  ammonia  (NH3), 
and  sulphuretted  hydrogen  (H^S)1.  In  this  character  of 
instability  or  readiness  to  decompose  protoplasm  notoriously 
agrees  with  its  constituent  proteids  ;  any  dead  organism  will, 

1  For  a  more  detailed  account  of  the  phenomena  of  putrefaction  see 
Lesson  VIII.,  in  which  it  will  be  seen  that  the  above  statement  as  to 
the  instability  of  (dead)  proteids  requires  qualification  ;  as  a  matter  of 
fact  they  onty  decompose  in  the  presence  of  living  Bacteria. 


T  CHARACTERS  OF  THE  NUCLEUS  7* 

unless  special  means  are  taken  to  preserve  it,  undergo  more 
or  less  speedy  decomposition. 

Many  of  these  properties  of  protoplasm  can  hardly  be 
verified  in  the  case  of  Amoeba,  owing  to  its  minute  size 
and  the  difficulty  of  isolating  it  from  other  organisms  (water- 
weeds,  &c.)  with  which  it  is  always  associated ;  but  there 
are  some  tests  which  can  be  readily  applied  to  it  while 
under  observation  beneath  the  microscope. 

One  of  the  most  striking  of  these  micro-chemical  tests 
depends  upon  the  avidity  with  which  protoplasm  takes  up 
certain  colouring  matters.  If  a  drop  of  a  neutral  or  slightly 
alkaline  solution  of  carmine  or  logwood,  or  of  some  aniline 
dye,  or  a  weak  solution  of  iodine,  is  added  to  the  water  con- 
taining Amoeba,  the  animalcule  is  killed,  and  at  the  same 
time  becomes  more  or  less  deeply  stained.  The  theory  is 
that  protoplasm  has  a  slightly  acid  reaction,  and  thus  pro- 
duces precipitation  of  the  colouring  matter  from  the  neutral 
or  alkaline  solution. 

The  staining  is,  however,  not  uniform.  The  endosarc, 
owing  to  the  granules  it  contains,  appears  darker  than  the 
ectosarc,  and  there  is  usually  to  be  seen,  in  the  endosarc,  a 
rounded  spot  more  brightly  stained  than  the  rest.  This 
structure,  which  can  sometimes  be  seen  in  the  living  Amoeba 
(Fig.  i,  c,  E,  and  G,  nu),  while  frequently  its  presence  is  re- 
vealed only  by  staining  (comp.  A  and  B),  is  called  the  nucleus. 

But  when  viewed  under  a  sufficiently  high  power,  the 
nucleus  itself  is  seen  to  be  unequally  stained.  It  has  lately 
been  shown,  in  many  Amoebae,  to  be  a  globular  body,  en- 
closed in  a  very  delicate  membrane,  and  made  up  of  two 
constituents,  one  of  which  is  deeply  stained  by  colouring 
matters,  and  is  hence  called  chromatin,  while  the  other,  the 
nuclear  matrix  or  achromatin,  takes  a  lighter  tint  (Fig.  i,  F). 
The  relative  arrangement  of  chromatin  and  matrix  varies 


8  AMCEBA  LESS. 

in  different  Amoebae  :  sometimes  there  are  granules  of 
chromatin  in  an  achromatic  ground  substance  (F)  ;  some- 
times the  chromatin  is  collected  towards  the  surface  or 
periphery  of  the  nucleus ;  sometimes,  again,  it  becomes 
aggregated  in  the  centre  (G,  H).  In  the  latter  case  the 
nucleus  is  seen  to  have  a  deeply-stained  central  'portion, 
which  is  then  distinguished  as  the  nucleates. 

When  it  is  said  that  Amoebae  sometimes  have  one  kind  of 
nucleus  and  sometimes  another,  it  must  not  be  inferred  that 
the  same  animalcule  varies  in  this  respect.  What  is  meant 
is  that  there  are  found  in  stagnant  water  many  kinds  or 
species  of  Amoeba  which  are  distinguished  from  one  another, 
amongst  other  things,  by  the  character  of  their  nuclei, 
just  as  the  various  species  of  Felis — the  cat,  lion,  tiger, 
lynx,  &c. — are  distinguished  from  one  another,  amongst 
other  things,  by  the  colour  and  markings  of  their  fur. 
According  to  the  method  of  binomial  nomenclature  intro- 
duced into  biology  by  Linnaeus,  the  same  generic  name 
is  applied  to  all  such  closely  allied  species,  while  each  is 
specially  distinguished  by  a  second  or  specific  name  of  its 
own.  Thus  under  the  genus  Amoeba  are  included  Amceba 
proteus  (Fig.  I,  c,  E,  and  F),  with  long  lobed  pseudopods  and 
a  nucleus  containing  evenly-disposed  granules  of  chromatin  ; 
A.  quarta  (A  and  B),  with  short  pseudopods  and  numerous 
nuclei ;  A.  verrucosa  (G  and  H)  with  crumpled  or  folded 
surface,  no  well-marked  pseudopods,  and  a  nucleus  with  a 
central  aggregation  of  chromatin,  or  nucleolus  ;  and  many 
others. 

Besides  the  nucleus,  there  is  another  structure  frequently 
visible  in  the  living  Amoeba.  This  is  a  clear,  rounded  space 
in  the  ectosarc  (c,  E,  and  G,  c.  vac},  which  periodically  dis- 
appears with  a  sudden  contraction  and  then  slowly  re-appears, 
its  movements  reminding  one  of  the  beating  of  a  minute 


i  MORPHOLOGY  AND  PHYSIOLOGY  9 

colourless  heart.  It  is  called  the  contractile  vacuole^  and 
consists  of  a  cavity  in  the  ectosarc  containing  a  watery 
fluid. 

Occasionally  Amoebae—  or  more  strictly  Amoeba-like 
organisms  —  are  met  with  which  have  neither  nucleus  J  nor 
contractile  vacuole,  and  are  therefore  placed  in  the  separate 
genus  Protamceba  (Fig.  2).  They  may  be  looked  upon  as 
the  simplest  of  living  things. 

The  preceding  paragraphs  may  be  summed  up  by  saying 
that  Amoeba  is  a  mass  of  protoplasm  produced  into  tempo- 
rary processes  or  pseudopods,  divisible  into  ectosarc  and 

'    A  B  C  D          ^HP^F 

FrHv.^2  —  Protam&ba  primitive*,  ;  A,  B,  the  same  specimen  drawn  at 
short  intervals  of  time,  showing  changes  of  form. 

C  —  E.   Three  stages  in  the  process  of  binary  fission.   (After  Haeckel.  ) 

endosarc,  and  containing  a  nucleus  and  a  contractile  vacuole  : 
that  the  nucleus  consists  of  two  substances,  chromatin  and 
achromatin,  enclosed  in  a  distinct  membrane  :  and  that  the 
contractile  vacuole  is  a  mere  cavity  in  the  protoplasm  con- 
taining fluid.  All  these  facts  come  under  the  head  of 
Morphology,  the  division  of  biology  which  treats  of  form 
and  structure  :  we  must  now  study  the  Physiology  of  our 
animalcule  —  that  is,  consider  the  actions  or  functions  it  is 
capable  of  performing. 

1  Judging  from  the  analogy  of  the  Infusoria  it  seems  very  probable 
that  such  apparently  non-nucleate  forms  as  Protamoeba  contain  chroma- 
tin  diffused  in  the  form  of  minute  granules  throughout  their  substance 
(see  end  of  Lesson  X.,  p.  118),  or  that  they  are  forms  which  have  lost 
their  nuclei. 


- 


io  AMOEBA  LESS. 

First  of  all,  as  we  have  already  seen,  it  moves,  the  move- 
ment consisting  in  the  slow  protrusion  and  withdrawal  of 
pseudopods.  This  may  be  expressed  generally  by  saying 
that  Amoeba  is  contractile,  or  that  it  exhibits  contractility. 
But  here  it  must  be  borne  in  mind  that  contraction  does 
not  mean  the  same  thing  in  biology  as  in  physics.  When 
it  is  said  that  a  red-hot  bar  of  iron  contracts  on  cooling, 
what  is  meant  is  that  there  is  an  actual  reduction  in 
volume,  the  bar  becoming  smaller  in  all  dimensions.  But 
when  it  is  said  that  an  Amoeba  contracts,  what  is  meant  is 
that  it  diminishes  in  one  dimension  while  increasing  in 
another,  no  perceptible  alteration  in  volume  taking  place  : 
each  time  a  pseudopod  is  protruded  an  equivalent  volume 
of  protoplasm  is  withdrawn  from  some  other  part  of  the 
body. 

We  may  say  then  that  contractility  is  a  function  of  the 
protoplasm  of  Amoeba — that  is,  that  it  is  one  of  the  actions 
which  the  protoplasm  is  capable  of  performing. 

A  contraction  may  arise  in  one  or  other  of  two  ways.  In 
most  cases  the  movements  of  an  Amoeba  take  place  without 
any  obvious  external  cause  ;  they  are  what  would  be  called 
in  the  higher  animals  voluntary  movements — movements 
dictated  by  the  will  and  not  necessarily  in  response  to  any 
external  stimulus.  Such  movements  are  called  automatic. 
On  the  other  hand,  movements  may  be  induced  in  Amoeba 
by  external  stimuli,  by  a  sudden  shock,  or  by  coming  into 
contact  with  an  object  suitable  for  food  :  such  movements 
are  the  result  of  irritability  of  the  protoplasm,  which  is 
thus  both  automatic  and  irritable — that  is,  its  contractility 
may  be  set  in  action  either  by  internal  or  by  external 
stimuli. 

Under  certain  circumstances  an  Amoeba  temporarily  loses 
its  power  of  movement,  draws  in  its  pseudopods,  and 


I  MODE  OF  FEEDING  11 

becomes  a  globular  mass  around  which  is  formed  a  thick, 
shell-like  coat,  called  the  cyst  or  cell-wall  (Fig.  i,  D,  cy). 
The  composition  of  this  is  not  known ;  it  is  certainly  not 
protoplasmic,  and  very  probably  consists  of  some  nitrogenous 
substance  allied  in  composition  to  horn  and  to  the  chitin 
which  forms  the  external  shell  of  Crustacea,  insects,  &c. 
After  remaining  in  this  encysted  condition  for  a  time,  the 
Amoeba  escapes  by  the  rupture  of  its  cell-wall,  and  resumes 
its  active  life.  * 

Very  often  an  Amoeba  in  the  course  of  its  wanderings 
comes  in  contact  with  a  still  smaller  organism,  such  as  a 
diatom  (see  Lesson  XIV.,  Fig.  35)  or  a  small  infusor  (see 
Lessons  X. — XII.).  When  this  happens  the  Amoeba  may 
be  seen  to  send  out  pseudopods  which  gradually  creep 
round  the  prey,  and  finally  unite  on  the  far  side  of  it,  as  in 
Fig.  i,  c,  a.  The  diatom  or  other  organism  becomes  in  this 
way  completely  enclosed  in  a  cavity  or  food-vacuole  (f. 
vac),  which  also  contains  a  small  quantity  of  water  neces- 
sarily included  with  the  prey.  The  latter  is  taken  in  by  the 
Amoeba  as  food  :  so  that  another  function  performed  by  the 
animalcule  is  the  reception  of  food,  the  first  step  in  the 
process  of  nutrition.  It  is  to  be  noted  that  the  reception 
of  food  takes  place  in  a  particular  way,  viz.  by  ingestion — 
i.e.  it  is  enclosed  raw  and  entire  in  the  living  protoplasm.  It 
has  been  noticed  that  Amoeba  usually  ingests  at  its  hinder 
end — that  is,  the  end  directed  backwards  in  progression. 

Having  thus  ingested  its  prey,  the  Amoeba  continues  its 
course,  when,  if  carefully  watched,  the  swallowed  organism 
will  be  seen  to  undergo  certain  changes.  Its  protoplasm 
is  slowly  dissolved  ;  if  it  contains  chlorophyll — the  green 
colouring  matter  of  plants — this  is  gradually  turned  to  brown  ; 
and  finally  nothing  is  left  but  the  case  or  cell-wall  in  which 
many  minute  organisms,  such  as  diatoms,  are  enclosed. 


12  AMCEBA  LESS. 

Finally,  the  Amoeba  as  it  creeps  slowly  on  leaves  this  empty 
cell-wall  behind,  and  thus  gets  rid  of  what  it  has  no  further 
use  for.  It  is  thus  able  to  ingest  living  organisms  as  food  ; 
to  dissolve  or  digest  their  protoplasm  ;  and  to  egest  or  get 
rid  of  any  insoluble  materials  they  may  contain.  Note 
that  all  this  is  done  without  either  ingestive  aperture  (mouth), 
digestive  cavity  (stomach),  or  egestive  aperture  (anus) ;  the 
food  is  simply  taken  in  by  the  flowing  round  it  of  pseudopods, 
digested  as  it  lies  enclosed  in  the  protoplasm,  and  got  rid  of 
by  the  Amoeba  flowing  away  from  it. 

It  has  just  been  said  that  the  protoplasm  of  the  prey  is 
dissolved  or  digested  :  we  must  now  consider  more  particu- 
larly what  this  means. 

The  stomachs  of  the  higher  animals — ourselves,  for 
instance — produce  in  their  interior  a  fluid  called  gastric 
mice.  When  this  fluid  is  brought  into  contact  with  albumen 
or  any  other  proteid  a  remarkable  change  takes  place.  The 
proteid  is  dissolved  and  at  the  same  time  rendered  diffusible, 
so  as  to  be  capable,  like  a  solution  of  salt  or  sugar,  of  passing 
through  an  organic  membrane  (see  p.  6).  The  diffusible 
proteids  thus  formed  by  the  action  of  gastric  juice  upon 
ordinary  proteids  are  called  peptones :  the  transformation  is 
effected  through  the  agency  of  a  constituent  of  the  gastric 
juice  called  pepsin. 

There  can  be  little  doubt  that  the  protoplasm  of  Amoeba 
is  able  to  convert  that  of  its  prey  into  a  soluble  and  diffusible 
form,  possibly  by  the  agency  of  some  substance  analogous 
to  pepsin,  and  that  the  dissolved  matters  diffuse  through  the 
body  of  the  Amoeba  until  the  latter -is,  as  it  were,  soaked 
through  and  through  with  them.  Under  these  circumstances 
the  Amoeba  may  be  compared  to  a  sponge  which  is  allowed 
to  absorb  water,  the  sponge  itself  representing  the  living 
protoplasm,  the  water  the  solution  of  proteids  which  per- 


i  GROWTH  13 

meates  it.  It  has  been  proved  by  experiment  that  proteids 
are  the  only  class  of  food  which  Amoeba  can  make  use  of : 
it  is  unable  to  digest  either  starch  or  fat — two  very  important 
constituents  of  the  food  of  the  higher  animals.  Mineral 
matters  must,  however,  be  taken  with  the  food  in  the  form 
of  a  weak  watery  solution,  since  the  water  in  which  the 
animalcule  lives  is  never  absolutely  pure. 

The  Amoeba  being  thus  permeated,  as  it  were,  with  a 
nutrient  solution,  a  very  important  process  takes  place.  The 
elements  of  the  solution,  hitherto  arranged  in  the  form  of 
peptones,  mineral  salts,  and  water,  become  re-arranged  in 
such  a  way  as  to  form  ne.w  particles  of  living  protoplasm, 
which  are  deposited  among  the  pre-existing  particles.  In  a 
word,  the  food  is  assimilated  or  converted  into  the  actual 
living  substance  of  the  Amoeba. 

One  effect  of  this  formation  of  new  protoplasm  is  obvious  : 
if  nothing  happens  to  counteract  it,  the  Amoeba  must  grow, 
the  increase  in  size  being  brought  about  in  much  the  same 
way  as  that  of  a  heap  of  stones  would  be  by  continually 
thrusting  new  pebbles  into  the  interior  of  the  heap.  This 
mode  of  growth — by  the  interposition  of  new  particles  among 
old  ones — is  called  growth  by  intussusception,  and  is  very 
characteristic  of  the  growth  of  protoplasm.  It  is  neces- 
sary to  distinguish  it,  because  there  is  another  mode  of 
growth  which  is  characteristic  of  minerals  and  occurs  also 
in  some  organized  structures.  A  crystal  of  alum,  for 
instance,  suspended  in  a  strong  solution  of  the  same 
substance  grows,  but  the  increase  is  due  to  the  deposition 
of  successive  layers  on  the  surface  of  the  original  crystal, 
in  much  the  same  way  as  a  candle  might  be  made  to  grow 
by  repeatedly  dipping  it  into  melted  grease.  This  can  be 
proved  by  colouring  the  crystal  with  logwood  or  some  other 
dye  before  suspending  it,  when  a  gradually-increasing  colour- 


14  AMCEBA  -  LESS. 

less  layer  will  be  deposited  round  the  coloured  crystal :  if 
growth  took  place  by  intussusception  we  should  have  a 
gradual  weakening  of  the  tint  as  the  crystal  increased  in  size. 
This  mode  of  growth — by  the  deposition  of  successive  layers 
— is  called  growth  by  accretion. 

It  is  probable  that  the  cyst  of  Amoeba  referred  to  above 
(p.  n)  grows  by  accretion.  Judging  fron  the  analogy  of 
other  organisms  it  would  seem  that,  after  rounding  itself  off, 
the  surface  of  the  sphere  of  protoplasm  undergoes  a 
chemical  change  resulting  in  the  formation  of  a  thin  super- 
ficial layer  of  non-protoplasmic  substance.  The  process  is 
repeated,  new  layers  being  continually  deposited  within  the 
old  ones  until  the  cell-wall  attains  its  full  thickness.  The 
cyst  is  therefore  a  substance  separated  or  secreted  from 
the  protoplasm  ;  it  is  the  first  instance  we  have  met  with  of 
a  product  of  secretion. 

From  the  fact  that  Amoeba  rarely  attains  a  greater  dia- 
meter than  \  mm.,  it  follows  that  something  must  happen  to 
counteract  the  constant  tendency  to  grow,  which  is  one  of 
the  results  of  assimilation.  We  all  know  what  happens  in 
our  own  case  :  if  we  take  a  certain  amount  of  exercise — 
walk  ten  miles  or  lift  a  series  of  heavy  weights — we  undergo 
a  loss  of  substance  manifested  by  a  diminution  in  weight 
and  by  the  sensation  of  hunger.  Our  bodies  have  done  a 
certain  amount  of  work,  and  have  undergone  a  proportional- 
amount  of  waste,  just  as  a  fire  every  time  it  blazes  up 
consumes  a  certain  weight  of  coal. 

Precisely  the  same  thing  happens  on  a  small  scale  with 
Amoeba.  Every  time  it  thrusts  out  or  withdraws  a  pseudo- 
pod,  every  time  it  contracts  its  vacuole,  it  does  a  certain 
amount  of  work — moves  a  definite  weight  of  protoplasm 
through  a  given  space.  And  every  movement,  however 
slight,  is  accompanied  by  a  proportional  waste  of  substance, 


I  POTENTIAL  AND  KINETIC  ENERGY  15 

a  certain  fraction  of  the  protoplasm  becoming  oxidized,  or 
in  other  words  undergoing  a  process  of  low  temperature 
combustion. 

When  we  say  that  any  combustible  body  is  burnt  what  we 
usually  mean  is  that  it  has  combined  with  oxygen,  forming 
certain  products  of  combustion  due  to  the  chemical  union 
of  the  oxygen  with  the  substance  burnt.  For  instance,  when 
carbon  is  burnt  the  product  of  combustion  is  carbon  dioxide 
or  carbonic  acid  (C  +  O2  =  CO2)  :  when  hydrogen  is  burnt, 
water  (H2  +  O  =  H2O).  The  products  of  the  slow  com- 
bustion which  our  own  bodies  are  constantly  undergoing 
are  these  same  two  bodies — carbon  dioxide  given  off  mainly 
in  the  air  breathed  out,  and  water  given  off  mainly  in  the 
form  of  perspiration  and  urine — together  with  two  com- 
pounds containing  nitrogen,  urea  (CH4N2O)  and  uric  acid 
C5H4N4O3),  both  occurring  mainly  in  the  urine.  In  some 
animals  urea  and  uric  acid  are  replaced  by  other  com- 
pounds such  as  guanin  (C5H5N5O),  but  it  may  be  taken  as 
proved  that  in  all  living  things  the  product  of  combustion 
are  carbon  dioxide,  water,  and  some  nitrogenous  substance 
of  simpler  constitution  than  proteids,  and  allied  to  the  three 
just  mentioned. 

With  this  breaking  down  of  proteids  the  vital  activity  of 
all  organisms  are  invariably  connected.  Just  as  useful 
mechanical  work  may  be  done  by  the  fall  of  a  weight  from 
a  given  height  to  the  level  of  the  ground,  so  the  work  done 
by  the  organism  is  a  result  of  its  complex  proteids  falling^ 
so  to  speak,  to  the  level  of  simpler  substances.  In  both 
instances  potential  energy  or  energy  of  position  is  converted 
into  kinetic  or  actual  energy. 

In  the  particular  case  under  consideration  we  have  to  rely 
upon  analogy  and  not  upon  direct  experiment.  We  may, 
however,  be  quite  sure  that  the  products  of  combustion 


16  AMCEBA  LESS. 

or  waste  matters  of  Amoeba  include  carbon  dioxide,  water 
and  some  comparatively  simple  (as  compared  with  proteids) 
compound  of  nitrogen. 

These  waste  matters  or  excretory  products  are  given 
off  partly  from  the  general  surface  of  the  body,  but  partly, 
it  would  seem,  through  the  agency  of  the  contractile  vacuole. 
It  appears  that  the  water  taken  in  with  the  food,  together  in 
all  probability  with  some  of  that  formed  Jby  oxidation  of 
the  protoplasm,  makes  its  way  to  the  vacuole,  and  is  ex- 
pelled by  its  contraction.  We  have  here  another  function 
performed  by  Amoeba,  that  of  excretion^  or  the  getting  rid 
of  waste  matters. 

In  this  connection  the  reader  must  be  warned  against  a 
possible  misunderstanding  arising  from  the  fact  that  the 
word  excretion  is  often  used  in  two  senses.  We  often  hear, 
for  instance,  of  solid  and  liquid  "  excreta."  In  Amoeba 
the  solid  excreta,  or  more  correctly  faces,  consist  of  such 
things  as  the  indigestible  cell-walls,  starch-grains,  &c.,  of  the 
Organisms  upon  which  it  feeds ;  but  the  rejection  of  these 
is  no  more  a  process  of  excretion  than  the  spitting  out  of 
a  cherry-stone,  since  they  are  simply  parts  of  the  food 
which  have  never  been  assimilated — never  formed  part  and 
parcel  of  the  organism.  True  excreta,  on  the  other  hand, 
are  invariably  products  of  the  waste  or  decomposition  of 
protoplasm. 

The  statement  just  made  that  the  protoplasm  of  Amoeba 
constantly  undergoes  oxidation  presupposes  a  constant  sup- 
ply of  oxygen.  The  water  in  which  the  animalcule  lives 
invariably  contains  that  gas  in  solution  :  on  the  other  hand, 
as  we  have  seen,  the  protoplasm  is  continually  forming 
carbon  dioxide.  Now  when  two  gases  are  separated  from 
one  another  by  a  porous  partition,  an  interchange  takes  place 
between  them,  each  diffusing  into  the  space  occupied  by  the 


i  METABOLISM  17 

other.  The  same  process  of  gaseous  diffusion  is  continually 
going  on  between  the  carbon  dioxide  in  the  interior  of 
Amoeba  and  the  oxygen  in  the  surrounding  water,  the  proto- 
plasm acting  as  the  porous  partition.  In  this  way  the  carbon 
dioxide  is  got  rid  of,  and  at  the  same  time  a  supply  of 
oxygen  is  obtained  for  further  combustion. 

The  taking  in  of  oxygen  might  be  looked  upon  as  a  kind 
of  feeding  process,  the  food  being  gaseous  instead  of  solid 
or  liquid,  just  as  we  might  speak  of  "feeding"  a  fire  both 
with  coals  and  with  air.  Moreover,  as  we  have  seen,  the 
giving  out  of  carbon  dioxide  is  a  process  of  excretion.  It 
is,  however,  usual  and  convenient  to  speak  of  this  process 
of  exchange  of  gases  as  respiration  or  breathing,  which 
'is  therefore  another  function  performed  by  the  protoplasm  of 
Amoeba. 

The  oxidation  of  protoplasm  in  the  body  of  an  organism, 
like  the  combustion  of  wood  or  coal  in  a  fire,  is  accompanied 
by  an  evolution  of  heat.  That  this  occurs  in  Amoeba  can- 
not be  doubted,  although  it  has  never  been  proved.  The 
heat  thus  generated  is,  however,  constantly  being  lost  to  the 
surrounding  water,  so  that  the  temperature  of  Amoeba,  if  we 
could  but  measure  it,  would  probably  be  found,  like  that  of 
a  frog  or  a  fish,  to  be  very  little  if  at  all  above  that  of  the 
medium  in  which  it  lives. 

We  thus  see  that  a  very  elaborate  series  of  chemical  pro- 
cesses is  constantly  going  on  in  the  interior  of  Amoeba. 
These  processes  are  divisible  into  two  sets  :  those  which 
begin  with  the  digestion  of  food  and  end  with  the  manufac- 
ture of  living  protoplasm,  and  those  which  have  to  do  with 
the  destruction  of  protoplasm  and  end  with  excretion. 

The  whole  series  of  processes  are  spoken  of  collectively 
as  metabolism.  We  have,  first  of  all,  digested  food  diffused 
through  the  protoplasm  and  finally  converted  into  fresh 

c 


1 8  AMOEBA  LESS. 

living  protoplasm  :  these  are  processes  of  constructive  meta- 
bolism or  anabolism.  Next  we  have  the  protoplasm  gradually 
breaking  down  and  undergoing  conversion  into  excretory 
products  :  this  is  the  process  of  destructive  metabolism  or 
katabolism.  There  can  be  little  doubt  that  both  are  pro- 
cesses of  extreme  complexity :  it  seems  probable  that 
after  the  food  is  once  dissolved  there  ensues  the  successive 
formation  of  numerous  bodies  of  gradually  increasing 
complexity  (anabolic  mesostates  or  anastates),  culminating 
in  protoplasm ;  and  that  the  protoplasm,  when  once  formed, 
is  decomposed  into  a  series  of  substances  of  gradually 
diminishing  complexity  (katabolic  mesostates  or  katastates], 
the  end  of  the  series  being  formed  by  the  comparatively 
simple  products  of  excretion.  The  granules  in  the  endosarc 
are  probably  to  be  looked  upon  as  various  mesostates 
imbedded  in  the  protoplasm  proper. 

Living  protoplasm  is  thus  the  most  unstable  of  substances  ; 
it  is  never  precisely  the  same  thing  for  two  consecutive 
seconds:  it  "decomposes  but  to  recompose,"  and  recom- 
poses  but  to  decompose ;  its  existence,  like  that  of  a  water- 
fall or  a  fountain,  depends  upon  the  constant  flow  of  matter 
into  it  and  away  from  it. 

It  follows  from  what  has  been  said  that  if  the  income  of 
an  Amoeba,  *.*.,  the  total  weight  of  substances  taken  in  (food 
plus  oxygen  plus  water)  is  greater  than  its  expenditure  or 
the  total  weight  of  substances  given  out  (feces  plus  excreta 
proper  plus  carbon  dioxide)  the  animalcule  will  grow  :  if 
less  it  will  dwindle  away  :  if  the  two  are  equal  it  will 
remain  of  the  same  weight  or  in  a  state  of  physiological 
equilibrium. 

We  see  then  that  the  fundamental  condition  of  existence 
of  the  individual  Amoeba  is  that  it  should  be  able  to  form 
new  protoplasm  out  of  the  food  supplied  to  it.  But  some- 


i  REPRODUCTION  19 

thing  more  than  this  is  necessary.  Amoebae  are  subject  to 
all  sorts  of  casualties ;  they  may  be  eaten  by  other  organ- 
isms or  the  pool  in  which  they  live  may  be  dried  up  ;  in  one 
way  or  another  they  are  constantly  coming  to  an  end. 
From  which  it  follows  that  if  the  race  of  Amoebae  is  to  be 
preserved  there  must  be  some  provision  by  which  the 
individuals  composing  it  are  enabled  to  produce  new  in- 
dividuals. In  other  words  Amoeba  must,  in  addition  to  its 
otjier  functions,  perform  that  of  reproduction. 

An  Amoeba  reproduces  itself  in  a  very  simple  way.  The 
nucleus  first  divides  into  two  :  then  the  whole  organism 
elongates,  the  two  nuclei  at  the  same  time  travelling  away 
from  one  another  :  next  a  furrow  appears  across  the  middle 
of  the  drawn-out  body  between  the  nuclei  (Fig.  i,  I;  fig.  2, 
C,  D)  :  the  furrow  deepens  until  finally  the  animalcule  sepa- 
rates into  two  separate  Amoebae  (Fig.  2,  E),  which  hence- 
forward lead  an  independent  existence. 

This,  the  simplest  method  of  reproduction  known,  is  called 
simple  or  binary  fission.  Notice  how  strikingly  different  it 
is  from  the  mode  of  multiplication  with  which  we  are 
familiar  in  the  higher  animals.  A  fowl,  for  instance,  multi- 
plies by  laying  eggs  at  certain  intervals,  in  each  of  which, 
under  favourable  circumstances,  and  after  a  definite  lapse  of 
time,  a  chick  is  developed  :  moreover,  the  parent  bird,  after 
continuing  to  produce  eggs  for  a  longer  or  shorter  time,  dies. 
An  Amoeba,  on  the  other  hand,  simply  divides  into  two 
Amoebae,  each  exactly  like  itself,  and  in  doing  so  ceases  to 
exist  as  a  distinct  individual.  Instead  of  the  successive 
production  of  offspring  from  an  ultimately  dying  parent,  we 
have  the  simultaneous  production  of  offspring  by  the  divi- 
sion of  the  parent,  which  does  not  die,  but  becomes  simply 
merged  in  its  progeny.  There  can  be  no  better  instance  of 
the  fact  that  reproduction  is  discontinuous  growth. 

C    2 


20  AMCEBA  LESS. 

From  this  it  seems  that  an  Amoeba,  unless  suffering  a 
violent  death,  is  practically  immortal,  since  it  divides  into 
two  completely  organized  individuals,  each  of  which  begins 
life  with  half  of  the  entire  body  of  its  parent,  there  being 
therefore  nothing  left  of  the  latter  to  die.  It  would  appear, 
however,  judging  from  the  analogy  of  the  Infusoria  (see 
Lesson  X.)  that  such  organisms  as  Amoeba  cannot  go  on 
multiplying  indefinitely  by  simple  fission,  and  that  occasion- 
ally two  individuals  come  into  contact  and  undergo  complete 
fusion.  A  conjugation  of  this  kind  has  been  observed  in 
Amoeba,  but  has  been  more  thoroughly  studied  in  other 
forms  (see  Lessons  III.  and  X.).  Whether  it  is  a  necessary 
condition  of  continued  existence  in  our  animalcule  or  not, 
it  appears  certain  that  "death  has  no  place  as  a  natural" 
recurrent  phenomenon  "  in  that  organism. 

If  an  Amoeba  does  happen  to  be  killed  and  to  escape 
being  eaten  it  will  undergo  gradual  decomposition,  becoming 
converted  into  various  simple  substances  of  which  carbon 
dioxide,  water,  and  ammonia  are  the  chief.  (See  p.  90.) 

In  conclusion,  a  few  facts  may  be  mentioned  as  to  the 
conditions  of  life  of  Amoeba — the  circumstances  under 
which  it  will  live  or  die,  flourish  or  otherwise. 

In  the  first  place,  it  will  live  only  within  certain  limits  of 
temperature.  In  moderately  warm  weather  the  temperature 
to  which  it  is  exposed  may  be  taken  as  about  15°  C.  If 
gradually  warmed  beyond  this  point  the  movements  at  first 
show  an  increased  activity,  then  become  more  and  more 
sluggish,  and  at  about  30° — 35°  C.  cease  altogether,  re- 
commencing, however,  when  the  temperature  is  lowered. 
If  the  heating  is  continued  up  to  about  40°  C.  the  animalcule 
is  killed  by  the  coagulation  of  its  protoplasm  (see  p.  5) :  it 
is  then  said  to  suffer  heat-rigor  or  death-stiffening  pro- 


i  CONDITIONS  OF  LIFE  21 

duced  by  heat.  Similarly  when  it  is  cooled  below  the 
ordinary  temperature  the  movements  become  slower  and 
slower,  and  at  the  freezing  point  (o°  C.)  cease  entirely. 
But  freezing,  unlike  over-heating,  does  not  kill  the  pro- 
toplasm, but  only  renders  it  temporarily  inert ;  on  thawing, 
the  movements  recommence.  We  may  therefore  distin- 
guish an  optimum  temperature  at  which  the  vital  actions 
are  carried  on  with  the  greatest  activity ;  maximum  and 
minimum  temperatures  above  and  below  which  respect- 
ively they  cease ;  and  an  ultra-maximum  temperature  at 
which  death  ensues.  There  is  no  definite  ultra-minimum 
temperature  known  in  the  case  of  Amoeba. 

The  quantity  of  water  present  in  the  protoplasm — as  water 
of  organization  (see  p.  5) — is  another  matter  of  importance. 
The  water  in  which  Amoeba  lives,  although  fresh,  always 
contains  a  certain  percentage  of  salts  in  solution,  and  the 
protoplasm  is  affected  by  any  alteration  in  the  density  of  the 
surrounding  medium ;  for  instance,  by  replacing  it  by  dis- 
tilled water  and  so  reducing  the  density,  or  by  adding 
salt  and  so  increasing  it.  The  addition  of  common  salt, 
(sodium  chloride)  to  the  amount  of  2  per  cent,  causes 
Amoeba  to  withdraw  its  pseudopods  and  undergo  a  certain 
amount  of  shrinkage  :  it  is  then  said  to  pass  into  a  con- 
dition of  dry-rigor.  Under  these  circumstances  it  may 
be  restored  to  its  normal  condition  by  adding  a  sufficient 
proportion  of  water  to  bring  back  the  fluid  to  its  original 
density. 

In  this  connection  it  is  interesting  to  notice  that  the  dele- 
terious effects  of  an  excess  of  salt  are  produced  only  when 
the  salt  is  added  suddenly.  By  the  very  gradual  addition  of 
sodium  chloride  Amoebae  have  been  brought  to  live  in  a  4 
per  cent,  solution,  i.e.,  one  twice  as  strong  as  would,  if  added 
suddenly,  produce  dry-rigor. 


22  AMCEBA 


From  what  has  been  said  above  on  the  subject  of  respira- 
tion (p.  17)  it  follows  that  free  oxygen  is  necessary  for  the 
existenoe  of  Amoeba.  Light,  on  the  other  hand,  appears  to 
be  unnecessary,  amoeboid  movements  having  been  shown"  to 
go  on  actively  in  darkness. 


LESSON  II 

H.EMATOCOCCUS 

THE  rain-water  which  collects  in  puddles,  open  gutters, 
&c.,  is  frequently  found  to  have  a  green  colour.  This  colour 
is  due  to  the  presence  of  various  organisms — plants  or 
animals — one  of  the  commonest  of  which  is  called  H<zma~ 
tococcus  (or  as  it  is  sometimes  called  Protococcus  or  Sphcerella) 
pluvialis. 

Like  Amoeba,  Haematococcus  is  so  small  as  to  require  a 
high  power  for  its  examination.  Magnified  three  or  four 
hundred  diameters  it  has  the  appearance  (Fig.  3,  A)  of  an 
ovoidal  body,  somewhat  pointed  at  one  end,  and  of  a  bright 
green  colour,  more  or  less  flecked  with  equally  bright  red. 

Like  Amoeba,  moreover,  it  is  in  constant  movement,  but 
4he  character  of  the  movement  is  very  different  in  the  two 
cases.  An  active  Haematococcus  is  seen  to  swim  about 
the  field  of  the  microscope  in  all  directions  and  with 
considerable  apparent  rapidity.  We  say  apparent  rapidity 
because  the  rate  of  progression  is  magnified  to  the  same 
extent  as  the  organism  itself,  and  what  appears  a  racing 
speed  under  the  microscope  is  actually  a  very  slow  crawl 
when  divided  by  300.  It  has  been  found  that  such 
organisms  as  HDematococcus  travel  at  the  rate  of  one  foot 
in  from  a  quarter  of  an  hour  to  an  hour  :  or,  to  express 


24 


H^MATOCOCCUS 


LESS. 


the  fact  in  another  and  fairer  way,  that  they  travel  a  distance 
equal  to  2\  times  their  own  diameter  in  one  second.  In 
swimming  the  pointed  end  is  always  directed  forwards  and 


FIG.  3. — A.  Hamatococcus  pluvialis,  motile  phase.  Living  speci- 
men, showing  protoplasm  with  chromatophores  (chr}  and  pyrenoids 
(pyr),  cell-wall  (c.w]  connected  to  cell-body  by  protoplasmic  filaments, 
and  flagella  fl.  The  scale  to  the  left  applies  to  Figs.  A' — D. 

B.  Resting  stage  of  the  same,  showing  nucleus  (nu)  with  nucleolus 
(•nu'})  and  thick  cell-wall  (c.vv)  in  contact  with  protoplasm. 

C.  The  same,  showing  division  of  the  cell-body  in  the  resting  stage 
into  four  daughter-cells. 

D.  The  same,  showing  the  development  of  flagella  and  detached  cell- 
wall  by  the  daughter-cells  before  their  liberation  from  the  inclosing 
mother-cell-wall. 

E.  Hannalococcus    laciistris,    showing    nucleus    (>*&}»     single    large 
pyrenoid  (pyr}>  and  contractile  vacuole  (c.vac), 

F.  Diagram  illustrating  the  movement  of  a  flagellum  :  ab,  its  base  ; 
c,  c',  c",  different  positions  assumed  by  its  apex.     (E,  after  Biitschli. ) 


IT  FLAGELLA  25 

the  forward  movement  is  accompanied  by  a  rotation  of  the 
organism  upon  its  longer  axis. 

Careful  watching  shows  that  the  outline  of  a  swimming 
Haematococcus  does  not  change,  so  that  there  is  evidently 
no  protrusion  of  pseudopods,  and  at  first  the  cause  of 
the  movement  appears  rather  mysterious.  Sooner  or  later, 
however,  the  little  creature  is  sure  to  come  to  rest,  and  there 
can  then  be  seen  projecting  from  the  pointed  end  two  exces- 
sively delicate  colourless  threads  (Fig.  3,  A,y7),  each  about 
half  as  long  again  as  the  animalcule  itself  :  these  are  called 
flagella  or  sometimes  cilia}-  In  a  Haematococcus  which 
has  come  to  rest  these  can  often  be  seen  gently  waving 
from  side  to  side  :  when  this  slow  movement  is  exchanged 
for  a  rapid  one  the  whole  organism  is  propelled  through 
the  water,  the  flagella  acting  like  a  pair  of  extremely  fine 
and  flexible  fins  or  paddles.  Thus  the  movement  of 
Haematococcus  is  not  amceboid,  i.e.,  produced  by  the  pro- 
trusion and  withdrawal  of  pseudopods,  but  is  ciliary,  i.e., 
due  to  the  rapid  vibration  of  cilia  or  flagella. 

The  flagella  are  still  more  clearly  seen  by  adding  a  drop 
of  iodine  solution  to  the  water  :  this  immediately  kills  and 
stains  the  organism,  and  the  flagella  are  seen  to  take  on  a 
distinct  yellow  tint.  By  this  and  other  tests  it  is  shown  that 
Haematococcus,  like  Amoeba,  consists  of  protoplasm,  and 
that  the  flagella  are  simply  filamentous  processes  of  the 
protoplasm. 

It  was  mentioned  above  that  in  swimming  the  pointed  end 

1  The  word  cilium  is  sometimes  used  as  a  general  term  to  include 
any  delicate  vibratile  process  of  protoplasm  :  often,  however,  it  is  used 
in  a  restricted  sense  for  a  rhythmically  vibrating  thread,  of  which  each 
cell  bears  a  considerable  number  (see  Fig.  8,  E,  and  Fig.  21)  ;  a  flagel- 
lum  is  a  cilium  having  a  whip-lash-like  movement,  and  each  cell 
bearing  only  a  limited  number  —  one  or  two,  or  occasionally  as  many 
as  four. 


^ 


26  .  HdEMATOCOCCUS  LESS. 

with  the  flagella  goes  first ;  this  may  therefore  be  distin- 
guished as  the  ant£ja<jf3j^tremity,  the  opposite  or  blunt 
end  being  posterior.  So  that,  as  compared  with  Amoeba, 
Hsematococcus  exhibits  a  differentiation  of  structure  :  an 
anterior  and  a  posterior  end  can  be  distinguished,  and  a 
part  of  the  protoplasm  is  differentiated  or  set  apart  as 
flagella. 

The  green  colour  of  the  body  is  due  to  the  presence  of 
a  special  pigment  called  chlorophyll,  the  substance  to  which 
the  colour  of  leaves  is  due.  That  this  is  something  quite 
distinct  from  the  protoplasm  may  be  seen  by  treatment  with 
alcohol,  which  simply  kills  and  coagulates  the  protoplasm, 
but  completely  dissolves  out  the  chlorophyll,  producing  a 
clear  green  solution.  The  solution,  although  green  by  trans- 
mitted light,  is  red  under  a  strong  reflected  light,  and  is 
hence  fluorescent :  when  examined  through  the  spectro- 
scope it  has  the  effect  of  absorbing  the  whole  of  the  blue 
and  violet  end  of  the  spectrum  as  well  as  a  part  of  the  red. 
The  red  colour  which  occurs  in  so  many  individuals,  some- 
times almost  replacing  the  green,  is  due  to  a  colouring 
matter  closely  allied  in  its  properties  to  chlorophyll  and 
called  hcematochrome. 

At  first  sight  the  chlorophyll  appears  to  be  evenly  distri- 
buted over  the  whole  body,  but  accurate  examination  under 
a  high  power  shows  it  to  be  lodged  in  a  variable  number 
of  irregular  structures  called  chromatophores  (Fig.  3,  A, 
chr.\  which  together  form  a  layer  immediately  beneath  the 
surface.  Each  chromatophore  consists  of  a  protoplasmic 
substance  impregnated  with  chlorophyll. 

After  solution  of  the  chlorophyll  with  alcohol  a  nucleus 
(B,  nu.)  can  be  made  out ;  like  the  nucleus  of  Amoeba  it  is 
stained  by  iodine,  magenta,  &c.  Other  bodies  which  might 
easily  be  mistaken  for  nuclei  are  also  visible  in  the  living 


ii  CELL-WALL  27 

organism.  These  are  small  ovoidal  structures  (A,  pyr.}, 
with  clearly  defined  outlines  occurring  in  varying  numbers 
in  the  chromatophores.  When  treated  with  iodine  they 
assume  a  deep,  apparently  black  but  really  dark  blue, 
colour.  The  assumption  of  a  blue  colour  with  iodine  is  the 
characteristic  test  of  the  well-known  substance  starch,  as 
can  be  seen  by  letting  a  few  drops  •  of  a  weak  solution  of 
iodine  fall  upon  some  ordinary  washing  starch.  The  bodies 
in  question  have  been  found  to  consist  of  a  proteid  substance 
covered  with  a  layer  of  starch,  and  are  called  pyrenoids. 
Starch  itself  is  a  definite  chemical  compound  belonging 
to  the  group  of  carbo-hydrates,  i.e.,  bodies  containing  the 
elements  carbon,  hydrogen,  and  oxygen  :  its  formula  is 

C6  H10  °5' 

In  Haematococcus  pluvialis  there  is  no  c^ntra^ti^vaciiokv^ 
but  in  another  species,  H.   lacustris,   this  structure  is  pre- 
sent as  a  minute  space  near  the  anterior  or  pointed  end 
(Fig.  3,  E,  c.  vac.). 

There  is  still  another  characteristic  structure  to  which  no 
reference  has  yet  been  made.  This  appears  at  the  first  view 
something  like  a  delicate  haze  around  the  green  body,  but 
by  careful  focusing  is  seen  to  be  really  an  extremely  thin 
globular  shell  (A,  c.w.)  composed  of  some  colourless  trans- 
parent material  and  separated  by  a  space  containing  water 
from  the  body  to  which  it  is  connected  by  very  delicate 
radiating  strands  of  protoplasm.  It  is  perforated  by  two 
extremely  minute  apertures  for  the  passage  of  the  flagella. 
Obviously  we  may  consider  this  shell  as  a  cyst  or  cell- 
wall  differing  from  that  of  an  encysted  Amoeba  (Fig.  i,  D)  in 
not  being  in  close  contact  with  the  protoplasm. 

A  more  important  difference,  however,  lies  in  its  chemical 
composition.  The  cyst  or  cell-wall  of  Amoeba,  as  stated  in 
the  preceding  lesson  (p.  n)  is  very  probably  nitrogenous  : 


28  H^EMATOCOCCUS  LESS. 

that  of  Haematococcus,  on  the  other  hand,  is  formed  of  a 
carbohydrate  called  cellulose,  allied  in  composition  to 
starch,  sugar,  and  gum,  and  having  the  formula  C6  H10  O5. 
Many  vegetable  substances,  such  as  cotton,  consist  of 
cellulose,  and  wood  is  a  modification  of  the  same  com- 
pound. Cellulose  is  stained  yellow  by  iodine,  but  iodine 
and  sulphuric  acid  together  turn  it  blue,  and  a  similar 
colour  is  produced  by  a  solution  of  iodine  and  potassium 
iodide  in  zinc  chloride  known  as  Schulze's  solution.  These 
tests  are  quite  easily  applied  to  Hsematococcus  :  the  proto- 
plasm stains  a  deep  yellowish-brown,  around  which  is  seen 
a  sort  of  blue  cloud  due  to  the  stained  and  partly-dissolved 
cell-wall. 

It  has  been  stated  that  in  stagnant  water  in  which  it  has 
been  cultivated  for  a  length  of  time  Haematococcus  some- 
times assumes  an  amoeboid  form.  In  any  case,  after  leading 
an  active  existence  for  a  longer  or  shorter  time  it  comes  to 
rest,  loses  its  flagella,  and  throws  around  itself  a  thick  cell- 
wall  of  cellulose  (Fig.  3,  B),  thus  becoming  encysted.  So 
that,  as  in  Amoeba,  there  is  an  alternation  of  an  active 
or  motile  with  a  stationary  or  resting  condition. 

In  the  matter  of  nutrition  the  differences  between  Haema- 
tococcus  and  Amceba  are  very  marked  and  indeed  funda- 
mental. As  we  have  seen,  Haematococcus  has  no  pseudopods, 
and  therefore  cannot  take  in  solid  food  after  the  manner 
of  Amoeba  :  moreover,  even  in  its  active  condition  it  is 
usually  surrounded  by  an  imperforate  cell-wall,  which  of 
course  quite  precludes  the  possibility  of  ingestion.  As  a 
matter  of  observation,  also,  however  long  it  is  watched  it  is 
never  seen  \.Q  feed  in  the  ordinary  sense  of  the  word.  Never- 
theless it  must  take  in  food  in  some  way  or  other,  or  the  de- 
composition of  its  protoplasm  would  soon  bring  it  to  an  end. 


TT  DECOMPOSITION  OF  CARBON  DIOXIDE  29 

Hsematococcus  lives  in  rain-water.  This  is  never  pure 
water,  but  always  contains  certain  mineral  salts  in  solution, 
especially  nitrates,  ammonia  salts,  and  often  sodium  chloride 
or  common  table  salt.  These  salts,  being  crystalloids,  can 
and  do  diffuse  into  the  water  of  organization  of  the  ani- 
malcule, so  that  we  may  consider  its  protoplasm  to  be  con- 
stantly  permeated  by  a  very  weak  saline  solution,  the  most 
important  elements  contained  in  which  are  oxygen,  hydro- 
gen, nitrogen,  potassium,  sodium,  calcium,  sulphur,  and 
phosphorus. 

If  water  containing  a  large  quantity  of  Hsematococcus 
is  exposed  to  sunlight,  minute  bubbles  are  found  to  appear 
in  it,  and  these  bubbles,  if  collected  and  properly  tested, 
are  found  to  consist  largely  of  oxygen.  Accurate  chemical 
analysis  has  shown  that  this  oxygen  is  produced  by  the  de- 
composition of  the  carbon  dioxide  contained  in  solution  in 
rain-water,  and  indeed  in  all  water  exposed  to  the  air,  the 
gas,  which  is  always  present  in  small  quantities  in  the 
atmosphere,  being  very  soluble  in  water. 

As  the  carbon  dioxide  is  decomposed  in  this  way,,  its 
oxygen  being  given  off,  it  is  evident  that  its  carbon  must  be 
retained.  As  a  matter  of  fact  it  is  retained  by  the  organism 
but  not  in  the  form  of  carbon  •  in  all  probability  a  double 
decomposition  takes  place  between  the  carbon  dioxide  ab- 
sorbed and  the  water  of  organization,  the  result  being  the 
liberation  of  oxygen  in  the  form  of  gas  and  the  simultaneous 
production  of  some  extremely  simple  form  of  carbohydrate, 
i.e.,  some  compound  of  carbon,  hydrogen,  and  oxygen,  with 
a  comparatively  small  number  of  atoms  to  the  molecule. 

The  next  step  seems  to  be  that  the  carbohydrate  thus 
formed  unites  with  the  ammonia  salts  or  the  nitrates  absorbed 
from  the  surrounding  water,  the  result  being  the  formation 
of  some  comparatively  simple  nitrogenous  compound,  prob- 


30  H^EMATOCOCCUS  LESS. 

ably  belonging  to  the  class  of  amides,  one  of  the  best 
known  of  which — asparagin— has  the  formula  C4  H8  N2  O3. 
Then  further  combinations  take  place,  substances  of  greater 
and  greater  complexity  are  produced,  sulphur  from  the  ab- 
sorbed sulphates  enters  into  combination,  and  proteids  are 
formed.  From  these,  finally,  fresh  living  protoplasm  arises. 

From  the  foregoing  account,  which  only  aims  at  giving 
the  very  briefest  outline  of  a  subject  as  yet  imperfectly  un- 
derstood, it  will  be  seen  that,  as  in  Amoeba,  the  final  result 
of  the  nutritive  process  is  the  manufacture  of  protoplasm, 
and  that  this  result  is  attained  by  the  formation  of  various 
substances  of  increasing  complexity  or  anastates  (see  p.  18). 
But  it  must  be  noted  that  the  steps  in  this  process  of  con- 
structive metabolism  are  widely  different  in  the  two  cases. 
In  Amoeba  we  start  with  living  protoplasm — that  of  the  prey 
— which  is  killed  and  broken  up  into  diffusible  proteids, 
these  being  afterwards  re-combined  to  form  new  molecules 
of  the  living  protoplasm  of  Amoeba.  So  that  the  food  of 
Amoeba  is,  to  begin  with,  as  complex  as  itself,  and  is  first 
broken  down  by  digestion  into  simpler  compounds,  these 
being  afterwards  re-combined  into  more  complex  ones.  In 
Hsematococcus,  on  the  other  hand,  we  start  with  extremely 
simple  compounds,  such  as  carbon  dioxide,  water,  nitrates, 
sulphates,  &c.  Nothing  which  can  be  properly  called  diges- 
tion, i.e.,  a  breaking  up  and  dissolving  of  the  food,  takes 
place,  but  its  various  constituents  are  combined  into  sub- 
stances of  gradually  increasing  complexity,  protoplasm,  as 
before,  being  the  final  result. 

To  express  the  matter  in  another  way  :  Amoeba  can  only 
make  protoplasm  out  of  proteids  already  formed  by  some 
other  organism  :  Haematococcus  can  form  it  out  of  simple 
liquid  and  gaseous  inorganic  materials. 

Speaking  generally,  it  may  be  said  that  these  two  methods 


ii  DESTRUCTIVE  METABOLISM  31 

of  nutrition  are  respectively  characteristic  of  the  two  great 
groups  of  living  things.  Animals  require  solid  food  con- 
taining ready-made  proteids,  and  cannot  build  up  their  pro- 
toplasm out  of  simpler  compounds.  Green  plants,  /.<?.,  all 
the  ordinary  trees,  shrubs,  weeds,  &c.,  take  only  liquid  and 
gaseous  food,  and  built  up  their  protoplasm  out  of  carbon 
dioxide,  water,  and  mineral  salts.  The  first  of  these  methods 
of  nutrition  is  conveniently  distinguished  as  holozoic,  or 
wholly-animal,  the  second  as  holophytic,  or  wholly-vegetal. 

It  is  important  to  note  that  only  those  plants  or  parts  of 
plants  in  which  chlorophyll  is  present  are  capable  of  holo- 
phytic  nutrition.  Whatever  may  be  the  precise  way  in  which 
the  process  is  effected,  it  is  certain  that  the  decomposition 
of  carbon  dioxide  which  characterizes  this  form  of  nutrition 
is  a  function  of  chlorophyll,  or  to  speak  more  accurately,  of 
chromatophores,  since  there  is  reason  for  thinking  that 
it  is  the  protoplasm  of  these  and  not  the  actual  green 
pigment  which  is  the  active  agent  in  the  process. 

Moreover,  it  must  not  be  forgotten  that  the  decomposition 
of  carbon  dioxide  is  carried  on  only  during  daylight,  so  that 
organisms  in  which  holophytic  nutrition  obtains  are  depend- 
ent upon  the  sun  for  their  very  existence.  While  Amoeba 
derives  its  energy  from  the  breaking  down  of  the  proteids 
in  its  food  (see  p.  12),  the  food  of  Haematococcus  is  too 
simple  to  serve  as  a  source  of  energy,  and  it  is  only  by  the 
help  of  sunlight  that  the  work  of  constructive  metabolism 
can  be  carried  on.  This  may  be  expressed  by  saying  that 
Haematococcus,  in  common  with  other  organisms,  contain- 
ing chlorophyll,  is  supplied  with  kinetic  energy  (in  the  form 
of  light  or  radiant  energy)  directly  by  the  sun. 

As  in  Amoeba,  destructive  metabolism  is  constantly  going 
on  side  by  side  with  constructive.  The  protoplasm  becomes 
oxidized,  water,  carbon  dioxide,  and  nitrogenous  waste 


32  H^EMATOCOCCUS  LESS. 

matters  being  formed  and  finally  got  rid  of.  Obviously, 
then,  absorption  of  oxygen  must  take  place,  or  in  other 
words,  respiration  must  be  one  of  the  functions  of  the  pro- 
toplasm of  Haematococcus  as  of  that  of  Amoeba.  In  many 
green,  i.e.,  chlorophyll-containing,  plants,  this  has  been  proved 
to  be  the  case ;  respiration,  i.e.,  the  taking  in  of  oxygen  and 
giving  out  of  carbon  dioxide,  is  constantly  going  on,  but 
during  daylight  is  obscured  by  the  converse  process — the 
taking  in  of  carbon  dioxide  for  nutritive  purposes  and  the 
giving  out  of  the  oxygen  liberated  by  its  decomposition.  In 
darkness,  when  this  latter  process  is  in  abeyance,  the 
occurrence  of  respiration  is  more  readily  ascertained. 

Owing  to  the  constant  decomposition,  during  sunlight,  of 
carbon  dioxide,  a  larger  volume  of  oxygen  than  of  carbon 
dioxide  is  evolved;  and  if  an  analysis  were  made  of  all 
the  ingesta  of  the  organism  (carbon  dioxide  plus  mineral 
salts  plus  respiratory  oxygen)  they  would  be  found  to  con- 
tain less  oxygen  than  the  egesta  (oxygen  from  decomposition 
of  carbon  dioxide  plus  water,  excreted  carbon  dioxide  and 
nitrogenous  waste) ;  so  that  the  nutritive  process  in  Haema- 
tococcus  is,  as  a  whole,  a  process  of  deoxidation.  In 
Amoeba,  on  the  other  hand,  the  ingesta  (food  plus  respi- 
ratory oxygen)  contain  more  oxygen  than  the  egesta  (faeces 
plus  carbon  dioxide,  water,  and  nitrogenous  excreta),  the 
nutritive  process  being  therefore  on  the  whole  one  of 
oxidation.  This  difference  is,  speaking  broadly,  character- 
istic of  plants  and  animals  generally;  animals,  as  a  rule, 
take  in  more  free  oxygen  than  they  give  out,  while  green 
plants  always  give  out  more  than  they  take  in. 

But  destructive  metabolism  is  manifested  not  only  in  the 
formation  of  waste  products,  but  in  that  of  substances 
simpler  than  protoplasm  which  remain  an  integral  part  of 
the  organism,  viz.,  cellulose  and  starch.  The  cell-wall  is 


ii  CILIARY  MOVEMENT  33 

probably  formed  by  the  conversion  of  a  thin  superficial 
layer  of  protoplasm  into  cellulose,  the  cyst  attaining  its  final 
thickness  by  frequent  repetition  of  the  process  (see  p.  14). 
The  starch  of  the  pyrenoids  is  apparently  formed  by  a  similar 
process  of  decomposition  or  destructive  metabolism  of  pro- 
toplasm, growth  taking  place  in  both  instances  by  accretion 
and  not  by  intussusception. 

We  see  then  that  destructive  metabolism  may  result  in  the 
formation  of  (a)  waste  products  and  (fr)  plastic  products, 
the  former  being  got  rid  of  as  of  no  further  use,  while 
the  latter  remain  an  integral  part  of  the  organism. 

Let  us  now  turn  once  more  to  the  movements  of  Hsemato- 
coccus,  and  consider  in  some  detail  the  manner  of  their 
performance. 

Each  flagellum  (Fig.  3,  \,fl)  is  a  thread  of  protoplasm  of 
uniform  diameter  except  at  its  distal  or  free  end  where  it 
tapers  to  a  point.  The  lashing  movements  are  brought 
about  by  the  flagellum  bending  successively  in  different 
directions ;  for  instance,  if  in  Fig.  3  F,  abc  represents  it  in 
the  position  of  rest,  abc '  will  show  the  form  assumed  when 
it  is  deflected  to  the  left,  and  abc "  when  the  bending  is 
towards  the  right.  rln  the  position  abc  the  two  sides  ab,  ac 
are  obviously  equal  to  one  another,  but  in  the  flexed 
positions  it  is  equally  obvious  that  the  concave  sides  ac' ,  be" 
are  shorter  than  the  convex  sides  be ',  ac"  ;  in  other  words,  as 
the  flagellum  bends  to  the  left  side  ac  becomes  shortened, 
as  it  bends  to  the  right  the  side  be. 

This  may  be  otherwise  expressed  by  saying  that,  in  bend- 
ing to  the  left  the  side  ^contracts  (see  p.  10),  in  bending  to 
the  right  the  side  be,  or  that  the  movement  is  performed 
by  the  alternate  contraction  of  opposite  sides,  of  the 
flagellum. 

D 


34  H^EMATOCOCCUS  LESS. 

Thus  the  ciliary  movement  of  Haematococcus,  like  the 
amoeboid  movement  of  Amoeba,  is  a  phenomenon  of  con- 
tractility. Imagine  an  Amoeba  to  draw  in  all  its  pseudo- 
pods  but  two,  and  to  protrude  these  two  until  they  became 
mere  threads ;  imagine  further  these  threads  to  contract 
regularly  and  rapidly  instead  of  irregularly  and  slowly  ;  the 
result  would  be  the  substitution  of  pseudopods  by  flagella, 
i.e.,  of  temporary  slow-moving  processes  of  protoplasm 
by  permanent  rapidly-moving  ones. 

To  put  the  matter  in  another  way  :  in  Amceba  the 
function  of  contractility  is  performed  by  the  whole  organism  ; 
in  Haematococcus  it  is  discharged  by  a  small  part  only,  viz., 
the  flagella,  the  rest  of  the  protoplasm  being  incapable  of 
movement.  We  have  therefore  in  Haematococcus  a  dif- 
ferentiation .of  structure  accompanied  by  a  differentiation  of 
function  or  division  of  physiological  labour. 

The  expression  "division  of  physiological  labour"  was 
invented  by  the  great  French  physiologist,  Henri  Milne- 
Edwards,  to  express  the  fact  that  a  sort  of  rough  correspond- 
ence exists  between  lowly  and  highly  organized  animals 
and  plants  on  the  one  hand,  and  lowly  and  highly  organized 
human  societies  on  the  other.  In  primitive  communities 
there  is  little  or  no  division  of  labour  :  every  man  is  his 
own  butcher,  baker,  soldier,  doctor,  &c.,  there  is  no  distinc- 
tion between  "  classes  "  and  "  masses,"  and  each  individual 
is  to  a  great  extent  independent  of  all  the  rest.  Whereas  in 
complex  civilized  communities  society  is  differentiated  into 
politicians,  soldiers,  professional  men,  mechanics,  labourers, 
and  so  on,  each  class  being  to  a  great  extent  dependent  on 
every  other.  This  comparison  of  an  advanced  society  with 
a  high  organism  is  at  least  as  old  as  JEsop,  who  gives 
expression  to  it  in  the  well-known  fable  of  "  the  Belly  and 
Members." 


ii  DIMORPHISM  35 

We  see  the  very  first  step  towards  a  division  of  labour  in 
the  minute  organism  now  under  consideration.  If  we  could 
cut  off  a  pseudopod  of  Amoeba  the  creature  would  be  little 
or  none  the  worse,  since  every  part  would  be  capable  of 
sending  off  similar  processes,  and  so  movement  would  be  in 
no  way  hindered.  But  if  we  could  amputate  the  flagella  of 
Hsematococcus  its  movements  would  be  absolutely  stopped. 

Haematococcus  multiplies  only  in  the  resting  condition 
(p.  28,  and  Fig.  3,  B)  ;  as  in  Amoeba  its  protoplasm  undergoes 
simple  or  binary  fission,  but  with  the  peculiarity  that  the 
process  is  immediately  repeated,  so  that  four  daughter-cells 
are  produced  within  the  single  mother-cell-wall  (Fig.  3  c). 
By  the  rupture  of  the  latter  the  daufhter-cells  are  set 
free  as  the  ordinary  motile  form  ;  sometimes  they  acquire 
their  flagella  and  detached  cell-wall  before  making  their 
escape  (D). 

Under  certain  circumstances  the  resting  form  divides  into 
eight  instead  of  four  daughter-cells,  and  these  when  liberated 
are  found  to  be  smaller" than  the  ordinary  motile  form,  and 
to  have  no  cell-wall.  Hsematococcus  is  therefore  dimorphic, 
i.e.,  occurs,  in  the  motile  condition,  under  two  distinct 
forms  :  the  larger  or  ordinary  form  with  detached  cell-wall 
is  called  a  megazooid,  the  smaller  form  without  a  cell-wall  a 
microzooid. 


D    2 


• 


LESSON   III 

HETEROMITA 

WHEN  animal  or  vegetable  matter  is  placed  in  water  and 
allowed  to  stand  at  the  ordinary  temperature,  the  well-known 
process  called  decomposition  sooner  or  later  sets  in,  the 
water  becoming  turbid  and  acquiring  a  bad  smell.  A  drop 
of  it  examined  under  the  microscope  is  then  found  to  teem 
with  minute  organisms.  To  one  of  these,  called  "the 
Springing  Monad,"  or  in  the  language  of  zoology,  Hetero- 
mita rostrata,  we  must  now  direct  our  attention ;  it  is 
found  in  infusion  of  cod's  head  which  has  been  allowed  to 
stand  for  two  or  three  months. 

Heteromita  (Fig.  4,  A)  is  considerably  smaller  than  either 
Amoeba  or  Hsematococcus,  being  only  T^  mm.  (^oV  o  mcn) 
in  average  length.  It  has  a  certain  resemblance  in  general 
form  to  Hsematococcus,  being  somewhat  ovoidal  and  pointed 
at  one  end.  Like  Haematococcus  also  it  has  two  flagella, 
but  only  one  of  these  (fl.  i)  proceeds  from  its  beak-like 
anterior  end  and  is  directed  forwards  as  the  creature  swims  ; 
the  other  (fl.  2)  springs  a  short  distance  from  the  beak,  and 
in  the  ordinary  swimming  position  is  trailed  after  the 
organism  as  in  A2  and  F4.  Thus  in  Heteromita,  besides  an 
anterior  and  a  posterior  end,  we  may  distinguish  a  ventral 


LESS,  in  NUTRITION  37 

surface  which  is  directed  downwards  in  the  ordinary 
position,  and  bears  the  second  or  trailing  flagellum,  and  an 
opposite  or  dorsal  surface  directed  upwards. 

Often  instead  of  swimming  freely  in  the  fluid  a  Hetero- 
mita  is  found  anchored  as  it  were  to  a  bit  of  the  decompos- 
ing substance  by  its  ventral  flagellum  as  in  A1.  Under 
these  circumstances  it  is  in  constant  movement,  springing 
backwards  and  forwards  by  alternately  coiling  and  uncoiling 
the  attached  ventral  flagellum.  The  general  character  of 
the  movement  will  be  readily  understood  from  the  figure,  in 
which  A1  shows  the  monad  with  coiled  flagellum,  A2  after  it 
has  sprung  forward  to  the  full  extent  of  the  flagellum,  It 
is  from  this  curious  habit  that  the  name  "  springing  monad  " 
is  derived. 

Towards  the  posterior  end  of  the  body  is  a  nucleus  (nu\ 
and  at  the  anterior  end  a  contractile  vacuole  (c.  vac).  There 
is  no  trace  of  an  investing  membrane  or  cell-wall,  and  the 
protoplasm  is  colourless.  Also,  as  is  invariably  the  case 
with  organisms  devoid  of  chlorophyll,  there  is  no  starch. 

T 

In  considering  the  nutrition  of  Heteromita  it  is  necessary, 
first  of  all,  to  take  into  consideration  the  precise  nature  of 
its  surroundings.  It  lives,  as  already  stated,  in  decomposing 
infusions  of  animal  matter.  Such  infusions  contain  proteids 
in  solution,  in  part  split  up  by  the  process  of  decomposition 
into  simpler  compounds  some  of  which  are  diffusible ;  this 
process  is  due,  as  we  shall  see  hereafter  (Lesson  VIII.),  to 
the  action  of  the  minute  organisms  known  as  Bacteria, 
which  are  always  present  in  vast  numbers  in  putrescent 
substances. 

As  Heteromita  contains  no  chlorophyll  its  nutrition  is 
obviously  not  holophytic.  Observation  seems  to  show 
pretty  conclusively  that  it  is  not  holozoic ;  apart  from  the 


nu  c.vac 


FIG.  4. — Heteromita  rostrata. 
A1,  the  living  organism,  showing  nucleus  (nu),  contractile   vacuole 


LESS,  in  NUTRITION  39 

(c.  vac),  anterior  flagellum  (ft.  i),  and  coiled  ventral  flagellum  (ft.  2) 
by  which  the  organism  is  anchored ;  A?  shows  the  position  at  the 
forward  limit  of  the  spring,  the  ventral  flagellum  being  fully  extended. 

B1 — B3,  three  stages  in  the  longitudinal  fission  of  the  anchored  form. 

c1 — c3.  Three  stages  in  the  transverse  fission  of  the  same  :  ft.  I1, 
rudiment  of  newly  formed  anterior  flagellum. 

D1 — D3,  three  stages  in  the  fission  of  the  free-swimming  form  :  ft.  21, 
rudiment  of  the  newly-formed  ventral  flagella. 

E1,  free-swimming  and  anchored  forms  about  to  conjugate  :  E2,  com- 
mencement of  conjugation  :  E3,  E4,  two  stages  in  the  development  of 
the  zygote :  E5,  the  fully  formed  zygote  :  EK,  dehiscence  of  the  zygote 
and  emission  of  spores. 

F1 — F4,  four  stages  in  the  development  of  the  spores. 

(After  Dallinger. ) 

fact  that  it  possesses  neither  mouth  nor  pseudopods,  examples 
have  been  kept  under  observation  for  hours  together  by 
trained  microscopists,  and  have  never  been  observed  to 
ingest  the  bacteria  or  other  particles,  dead  or  alive,  contained 
in  the  fluid.  There  remains  only  one  way  in  which 
nutrition  can  take  place,  namely,  by  absorption  of  the 
proteids  and  other  nutrient  substances  in  the  solution,  i.e., 
by  these  substances  diffusing  into  the  water  of  organization 
of  the  monad.  Whether  the  proteids  are  rendered  diffusible 
by  the  process  of  decomposition  alone,  i.e.,  by  the  action 
of  bacteria  (see  p.  91),  or  whether  a  kind  of  surface 
digestion  takes  place,  the  protoplasm  of  Heteromita  con- 
verting the  proteids  in  immediate  contact  with  it  into  pep- 
tones or  allied  compounds,  is  not  certain. 

Thus  Heteromita  feeds  neither  by  taking  solid  pro- 
teinaceous  food  into  its  interior  (holozoic  nutrition)  nor  by 
decomposing  carbon  dioxide  and  combining  the  carbon  with 
water  and  mineral  salts  (holophytic  nutrition),  but  by  absorb- 
ing decomposing  proteids  and  other  nutrient  substances  in 
the  liquid  form ;  this  is  the  saprophytic  mode  of  nutrition. 
It  will  be  seen  that  the  main  difference  between  saprophytic 
and  holozoic  nutrition  is  that  in  the  former  digestion,  i.e., 
the  process  of  rendering  food-stuffs  soluble  and  diffusible, 


40  HETEROMITA  LESS. 

takes  place  outside  the  body  so  that  constructive  meta- 
bolism can  begin  at  once. 

It  is  worthy  of  notice  that  while  the  process  of  feeding  is 
strictly  intermittent  in  Amoeba,  which  only  takes  in  food  at 
intervals,  and  largely  intermittent  in  Haematococcus,  in  which 
the  decomposition  of  carbon  dioxide  takes  place  only  during 
daylight,  in  Heteromita  it  is  continuous,  the  organism  living 
in  a  solution  of  putrefying  proteids  which  it  is  constantly 
absorbing.  It  may  be  said  to  live  immersed  in  an  immense 
cauldron  of  broth  which  it  is  for  ever  imbibing,  not  by  its 
mouth,  for  it  has  none,  but  by  the  whole  surface  of  its 
body. 

Respiration  and  excretion  probably  take  place  in  the  same 
manner  as  in  Amoeba.  It  has  been  shown  that  the  optimum 
temperature  for  saprophytic  monads  is  about  18°  C.,  the 
ultra-maximum  or  thermal  death-point  about  60°  C.  But 
it  is  an  interesting  fact  that  by  very  slowly  increasing  the 
temperature,  Dr.  Dallinger  was  able  in  the  course  of  several 
months  to  accustom  some  of  these  forms — not  Heteromita 
itself  but  closely  allied  genera — to  live  at  a  temperature 
exceeding  68°  C. 

The  ordinary  method  of  reproduction  is  by  simple  fission, 
the  process  affecting  not  only  the  body  but  the  flagella 
as  well.  In  Fig  4,  B1,  the  commencement  of  fission  is 
shown ;  the  anterior  flagellum  has  undergone  complete 
longitudinal  division,  while  the  split  has  only  extended  about 
a  third  of  the  length  of  the  body  and  ventral  flagellum.  In 
B2  the  process  has  gone  further,  and  in  BS  the  products  of 
division  are  on  the  point  of  separating. 

More  frequently,  however,  fission  instead  of  being  longitudinal,  i.e., 
in  the  direction  of  the  long  axis  of  the  monad,  is  transverse,  i.e.,  at 
right  angles  to  the  long  axis.  This  process  is  shown  in  c1 — C3,  and  is 
seen  to  differ  from  that  described  in  the  preceding  paragraph  in  the  cir- 


TII  CONJUGATION  41 

cumstance  that  the  anterior  flagellum  of  the  parent  form  is  unaffected,  and 
becomes  without  alteration  the  anterior  flagellum  of  one  of  the  daughter- 
forms — that  to  the  right  in  the  figures.  The  anterior  flagellum  of  the 
other  product  of  division — that  to  the  left — is  a  new  structure  formed  as 
an  outgrowth  from  the  body  :  its  commencement  is  shown  in  C1,^?.  i'. 

These  two  modes  of  fission — longitudinal  and  transverse — both  occur 
in  the  anchored  form  of  Heteromita,  i.e.,  in  individuals  attached  by 
the  ventral  flagellum.  The  free-swimming  form  presents  a  third 
variety  of  the  process.  It  comes  to  rest,  loses  its  regular  outline  (D1), 
becoming  almost  amoeboid  in  form  and  finally  (D2)  globular.  Division 
then  takes  place  :  the  flagella  of  the  parent  become  each  the  anterior 
flagellum  of  one  of  the  daughter-cells  (compare  D1,  D2,  and  D3),  while 
their  ventral  flagella  are  formed  by  the  splitting  of  a  little  outgrowth  of 
the  dividing  body  (D2,  fl.  2'}. 

As  in  Amoeba  fission  is  invariably  preceded  by  division 
of  the  nucleus. 

But  in  Heteromita  fission  is  not  the  only  mode  of  repro- 
duction. Under  certain  circumstances  a  free-swimming  form 
approaches  an  anchored  form,  and  applies  itself  to  it  in  such 
a  way  that  the  posterior  ends  of  the  two  are  in  contact  (E1). 
The  two  individuals  then  fuse  with  one  another  as  completely 
as  two  drops  of  gum  on  a  plate  unite  when  brought  into 
contact.  Fusion  of  the  nuclei  also  takes  place,  and  there  is 
formed  an  irregular  body  (E2)  with  a  single  nucleus  and 
with  two  flagella  at  each  end.  This  swims  about  freely,  and 
as  it  does  so  the  last  trace  of  distinction  between  the  two 
monads  of  which  it  is  formed  is  lost,  and  a  triangular  form 
is  assumed  (ES),  the  two  pairs  of  cilia  being  situated  at  two 
of  the  angles.  Still  later  the  protoplasm  of  this  triangular 
body  loses  all  trace  of  nucleus,  granules,  &c.,  and  becomes 
perfectly  clear  (E4)  :  then  it  comes  to  rest  and  loses  its 
flagella,  appearing  as  a  clear,  homogeneous,  three-cornered 
sac  with  slightly  convex  sides  (E5).  This  body,  formed  by 
the  conjugation  of  the  two  monads,  is  called  a  zygote,  the 
two  conjugating  individuals  being  distinguished  as  gametes. 


42  HETEROMITA  LESS. 

The  zygote  remains  quiescent  for  some  time,  and  then,  after 
undergoing  wave-like  movements  of  its  surface,  bursts  at  its 
three  angles  (EG),  its  contents  escaping  in  the  form  of  granules 
called  spores,  so  minute  as  to  be  barely  visible  even  under 
the  highest  powers  of  the  best  modern  microscopes.  They 
are  formed  by  the  protoplasm,  of  the  zygote  dividing  into  an 
immense  number  of  separate  masses,  a  process  known  as 
multiple  fission. 

Carefully  watched,  these  almost  ultra-microscopic  particles 
(p1)  are  found  to  grow  into  clear  visibility  and  to  take  on  a 
distinctly  oval  shape  (r2).  Still  increasing  in  size  they 
develop  a  ventral  flagellum  (r3)  which  is  at  first  quite 
quiescent :  finally,  the  pointed  end  sends  out  a  process  which 
becomes  an  anterior  flagellum  (r4).  The  spore  has  now 
become  a  Heteromita  resembling  the  parent  form  in  all  but 
size. 

It  will  be  seen  that  this  remarkable  mode  of  multiplication 
by  conjugation  differs  from  multiplication  by  fission  in  the 
fact  that  it  requires  the  co-operation  of  two  individuals  which 
undergo  complete  fusion.  As  we  shall  see  more  plainly 
later  on  (Lessons  XV.  and  XVI.)  conjugation  is  the  simplest 
case  of  sexual  reproduction,  differing  from  the  sexual  repro- 
duction of  the  higher  organisms  in  that  the  two  conjugating 
bodies  or  gametes  are  each  an  entire  individual,  and  in  the 
further  circumstance  that  the  gametes  resemble  one  another 
in  form  and  size,  so  that  there  is  no  distinction  of  sex,1  but 
each  takes  an  equal  and  similar  share  in  the  production  of 
the  zygote.  Binary  fission,  on  the  other  hand,  is  an  example 
of  asexual  reproduction. 

1  It  might  perhaps  be  allowable  to  consider  the  active,  free- 
swimming  monad  which  seeks  and  attaches  itself  to  the  anchored  form 
as  a  male,  and  the  passive  anchored  form  as  a  female  gamete 
(see  Lesson  XII.). 


m  LIFE  HISTORY  43 

Notice  also  another  important  fact.  The  spores  when 
first  emitted  from  the  ruptured  zygote  are  mere  granules  of 
protoplasm,  approaching  as  nearly  as  anything  in  nature  to 
the  mathematical  definition  of  a  point,  "  without  parts  and 
without  magnitude."  And  during  its  growth  a  spore  increases 
not  only  in  size  but  also  in  complexity,  in  other  words 
undergoes  a  progressive  differentiation  or  development. 
This  is  an  instance  of  the  principle  known  as  Von  Baer's 
law,  according  to  which  "  development  is  a  progress  from 
the  simple  to  the  complex,  from  the  general  to  the  special, 
from  the  homogeneous  to  the  heterogeneous."  In  Heteromita, 
then,  we  have  our  first  instance  of  development,  since  in 
simple  fission  there  is  no  development,  each  product  of 
division  being  from  the  first  similar  to  the  parent  in  all  but 
size. 

Lastly,  Heteromita  is  the  first  instance  we  have  had  of 
an  organism  with  a  definite  life-history.  It  multiplies 
asexually  by  simple  fission  producing  free-swimming  and 
anchored  forms  :  these  conjugate  in  pairs  forming  a  zygote, 
in  which,  by  multiple  fission,  numerous  spores  are  formed  : 
the  spores  develop  into  the  adult  form,  asexual  multiplica- 
tion begins  once  more,  and  so  the  cycle  of  existence  is 
completed. 

It  must  be  borne  in  mind  that  further  researches  may 
reveal  the  occurrence  of  a  true  sexual  process  in  Amoeba 
and  Haematococcus. 


LESSON  IV 

EUGLENA 

THE  rain-water  collected  in  puddles  by  the  road-side,  on 
roofs,  &c.,  is  often  found  to  have  a  bright  green  colour  : 
this  is  sometimes  due  to  the  presence  of  delicate  water 
weeds  visible  to  the  naked  eye  (Lesson  XVI.),  but  frequently 
the  water  when  held  up  to  the  light  in  a  glass  vessel  appears 
uniformly  green,  no  suspended  matter  being  visible  to  the 
unaided  sight.  Under  these  circumstances  the  green  colour 
is  frequently  due  to  the  presence  of  vast  numbers  of  an 
organism  known  as  Euglena  viridis. 

Although  microscopic,  Euglena  is  considerably  larger  than 
either  Haematococcus  or  Heteromita,  its  length  varying  from 
•ff  mm.  to  J  mm.  The  body  is  spindle-shaped,  wide  in  the 
middle  and  narrow  at  both  ends  (Fig.  5,  A — E)  :  one 
extremity  is  blunter  than  the  other,  and  from  it  proceeds 
a  single  long  flagellum  (fl)  by  the  action  of  which  the 
organism  swims  with  great  rapidity,  the  flagellum  being, 
as  in  Haematococcus,  directed  forwards.  Besides  its  rapid 
swimming  movements  Euglena  frequently  performs  slow 
movements  of  contraction  and  expansion,  something  like 
those  of  a  short  worm,  the  body  becoming  broadened  out 
first  at  the  anterior  end,  then  in  the  middle,  then  at  the 


GENERAL  CHARACTERS 


45 


posterior  end,  twisting  to  the  right  and  left,  and  so  on  (Fig. 
5,  A — D).  These  movements  are  so  characteristic  of  the 
genus  that  the  name  euglenoid  is  applied  to  them. 


FIG.  5. — Euglena  viridis. 

A— u,  four  views  of  the  living  organism,  showing  the  changes  of  form 
produced  by  the  characteristic  euglenoid  movements. 

E,  enlarged  view,   showing  the  nucleus  (nu},   reservoir  of  the  con- 
tractile vacuole  (c.vac},  with  adjacent  pigment  spot,  and  gullet  with  a 
single  flagellum  springing  from  it. 

F,  enlarged  view  of   the  anterior  end  of   E,  showing  pigment-spot 
(t>g)  and  reservoir    (c.  vac\   mouth  (m),    gullet    (ce.  s),    and  origin  of 
flagellum  (/). 

G,  resting  form  after  binary  fission,  showing  cyst  or  cell- wall  (n/), 
and  the  nuclei  (nu)  and  reservoirs  (c.  vac}  of  the  daughter-cells. 

H,  active  form  showing  contractile  vacuole  (c.  vac],  reservoir  (r), 
and  paramylum-bodies  (p}. 

(A— G,  after  Saville  Kent :  H,  from  Biitschli  after  Klebs.) 

The  body  consists  of  protoplasm  covered  with  a  very 
delicate  skin  or  cuticle  which  is  often  finely  striated,  and 
is  to  be  looked  upon  as  a  superficial  hardening  of  the 
protoplasm.  The  green  colour  is  due  to  the  presence  of 


46  EUGLENA  .  LESS. 

chlorophyll,  which  tinges  all  the  central  part  of  the  body, 
the  two  ends  being  colourless.  It  is  difficult  to  make  out 
whether  the  chlorophyll  is  lodged  in  one  chromatophore  or 
in  several. 

In  Haematococcus  we  saw  that  chlorophyll  was  asso- 
ciated with  starch  (p.  27).  In  Euglena  there  are,  near  the 
middle  of  the  body,  a  number  of  grains  of  paramylum 
(H,  p\  a  carbohydrate  of  the  same  composition  as  starch 
(C6H10O5),  but  differing  from  it  in  remaining  uncoloured 
by  iodine. 

Water  containing  Euglena  gives  off  bubbles  of  oxygen  in 
sunlight :  as  in  Hsematococcus  the  carbon  dioxide  in  solution 
in  the  water  is  decomposed  in  the  presence  of  chlorophyll, 
its  oxygen  evolved,  and  its  carbon  combined  with  the 
elements  of  water  and  used  in  nutrition.  For  a  long  time 
Euglena  was  thought  to  be  nourished  entirely  in  this  way, 
but  there  is  a  good  deal  of  reason  for  thinking  that  this  is 
not  the  case. 

When  the  anterior  end  of  a  Euglena  is  very  highly 
magnified  it  is  found  to  have  the  form  shown  in  Fig.  5,  F. 
It  is  produced  into  a  blunt  snout-like  extremity  at  the  base 
of  which  is  a  conical  depression  (<x.  s)  leading  into  the  soft 
internal  protoplasm  : — just  the  sort  of  depression  one  could 
make  in  a  clay  model  of  Euglena  by  thrusting  one's  finger  or 
the  end  of  a  pencil  into  the  clay.  From  the  bottom  of  this 
tube  the  flagellum  arises,  and  by  its  continual  movement 
gives  rise  to  a  sort  of  whirlpool  in  the  neighbourhood.  By 
the  current  thus  produced  minute  solid  food-particles  are 
swept  down  the  tube  and  forced  into  the  soft  internal 
protoplasm,  where  they  doubtless  become  digested  in  the 
same  way  as  the  substances  ingested  by  an  Amoeba.  That 
solid  particles  are  so  ingested  by  Euglena  has  been  proved 
by  diffusing  finely  powdered  carmine  in  the  water,  when  the 


iv  MOUTH  AND  GULLET  47 

coloured  particles  were  seen  to  be  swallowed  in  the  way 
described. 

The  depression  in  question  is  therefore  a  gullet,  and  its 
external  aperture  or  margin  (m)  is  a  mouth.  Euglena, 
like  Amoeba,  takes  in  solid  food,  but  instead  of  ingesting  it 
at  almost  any  part  of  the  body,  it  can  do  so  only  at  one 
particular  point  where  there  is  a  special  ingestive  aperture 
or  mouth.  This  is  clearly  a  case  of  specialization  or 
differentiation  of  structure  :  in  virtue  of  the  possession  of  a 
mouth  and  gullet  Euglena  is  more  highly  organized  than 
Amoeba. 

It  thus  appears  that  in  Euglena  nutrition  is  both  holozoic 
and  holophytic  :  very  probably  it  is  mainly  holophytic  during 
daylight  and  holozoic  in  darkness. 

Near  the  centre  of  the  body  or  somewhat  towards  the 
posterior  end  is  a  nucleus  (E,  nu)  with  a  well-marked 
nucleolus,  and  at  the  anterior  end  is  a  clear  space  (c.  vac) 
looking  very  like  a  contractile  vacuole.  It  has  been  shown, 
however,  that  this  space  is  in  reality  a  non-contractile  cavity 
or  reservoir  (H,  r)  into  which  the  true  contractile  vacuole 
(c.  vac}  opens,  and  which  itself  discharges  into  the  gullet. 

In  close  relation  with  the  reservoir  is  found  a  little  bright 
red  speck  (pg)  called  the  pigment  spot  or  stigma.  It  con- 
sists of  haematochrome  (see  p.  26)  and  is  curiously  like  an 
eye  in  appearance,  so  much  so  that  it  is  sometimes  known 
as  the  eye-spot.  There  seems,  however,  to  be  no  reason  for 
assigning  a  visual  function  to  it :  indeed  it  has  been  shown 
that  the  greatest  sensitiveness  to  light  is  manifested  by  the 
colourless  anterior  end  of  the  body. 

As  in  Haematococcus  a  resting  condition  alternates  with 
the  motile  phase:  the  organism  loses  its  flagellum  and 


48  EUGLENA  LESS,  iv 

surrounds  itself  with  a  cyst  of  cellulose  (Fig.  5,  G,  cy\  from 
which,  after  a  period  of  rest,  it  emerges  to  resume  active 

life. 

Reproduction  takes  place  by  simple  fission  of  the  resting 
form,  the  plane  of  division  being  always  longitudinal  (G). 
Sometimes  each  product  of  division  or  daughter-cell  divides 
again  :  finally  the  two,  or  four,  or  sometimes  even  eight 
daughter-cells  emerge  from  the  cyst  as  active  Euglenae. 
A  process  of  multiple  fission  (p.  42)  has  also  been  de- 
scribed, numerous  minute  active  spores  being  produced 
which  gradually  assume  the  ordinary  form  and  size. 


LESSON  V 

PROTOMYXA  AND  THE  MYCETOZOA 

WHEN  Professor  Haeckel  was  investigating  the  zoology  of 
the  Canary  Islands  more  than  twenty  years  ago  he  discovered 
a  very  remarkable  organism  which  he  named  Protomyxa 
aurantiaca.  It  was  found  in  sea-water  attached  to  a  shell 
called  Spirula^  and  was  at  once  noticeable  from  the  bright 
orange  colour  which  suggested  its  specific  name.  Appar- 
ently no  one  has  since  been  fortunate  enough  to  find  it. 

In  its  fully  developed  stage  Protomyxa  is  the  largest  of  all 
the  organisms  we  have  yet  studied,  being  fully  imm.  (—inch) 
in  diameter,  and  therefore  visible  to  the  naked  eye  as  a 
small  orange  speck.  In  general  appearance  (Fig.  6,  A)  it  is 
not  unlike  an  immense  Amoeba,  the  chief  difference  lying 
in  the  fact  that  the  pseudopods  (psd)  instead  of  being  short, 
blunt  processes,  few  in  number  (comp.  Fig.  i,  p.  2)  are  very 
numerous,  slender,  branching  threads  which  often  unite  with 
one  another  so  as  to  form  networks.  No  nucleus  was  ob- 
served 1  and  no  contractile  vacuole,  but  it  is  quite  possible 
that  a  renewed  examination  might  prove  the  presence  of  one 
or  both  of  these  structures. 

The  figure  (A)  is  enough  to  show  that  nutrition  is  holozoic, 

1  See  p.  9,  note. 


psd 


FIG    6  — Frotomyxa  aurantiaca. 

A,  the  living  organism  (plasmodium),  showing  fine  branched  pseudo- 
pods  (psd]  and  several  ingested  organisms. 
B   the  same,  encysted  :  cy  the  cell-wall. 

c   the  protoplasm  of  the  encysted  form  breaking  up  into  spores. 
D   dehiscence  of  the  cyst  and  emergence  of 

E,  flagellulse  which  afterwards  become  converted  into 

F,  amcebulae.  ___      ,    ,  » 

G,  amcebulse  uniting  to  form  a  plasmodium.     (After  Haeckel. ) 


LESS,  v  LIFE-HISTORY  51 

the  specimen  has  ingested  several  minute  organisms  and  is 
in  the  act  of  capturing  another. 

But  the  main  interest  of  Protomyxa  lies  in  its  very  curious 
and  complicated  life-history.  After  crawling  over  the  Spirula 
shell  for  a  longer  or  shorter  time  it  draws  in  its  pseudopods, 
comes  to  rest,  and  surrounds  itself  with  a  cyst  (B,  cy).  The 
composition  of  the  cyst  is  not  known,  but  it  is  apparently  not 
cellulose,  since  it  is  not  coloured  by  iodine  and  sulphuric 
acid  (p.  28). 

Next,  the  encysted  protoplasm  undergoes  multiple  fission, 
dividing  into  a  number  of  spores  (c)  :  soon  the  cyst  bursts 
and  its  contents  emerge  (D)  as  bodies  which  differ  utterly  in 
appearance  from  the  amoeboid  form  from  which  we  started. 
Each  spore  has  in  fact  become  a  little  ovoid  body  of  an 
orange  colour,  provided  with  a  single  flagellum  (E,  fi]  by  the 
lashing  of  which  it  swims  through  the  water  after  the  manner 
of  a  monad. 

It  is  convenient  to  have  a  name  by  which  to  distinguish 
these  flagellate  bodies,  just  as  we  have  special  names  for 
the  young  of  the  higher  animals,  such  as  tadpoles  or  kittens. 
From  the  fact  of  their  distinguishing  character  being  the 
possession  of  a  flagellum  they  are  called  flagellula ;  the 
same  name  will  be  applied  to  the  flagellate  young  of  various 
other  organisms  which  we  shall  study  hereafter. 

After  swimming  about  actively 'for  a  time  each  flagellula 
settles  down  on  some  convenient  substratum  and  undergoes 
a  remarkable  change  :  its  movements  become  sluggish,  its 
outline  irregular,  and  its  flagellum  short  and  thick,  until  it 
finally  takes  on  the  form  of  a  little  Amoeba  (F).  For  this 
stage  also  a  name  is  required  :  it  is  not  an  Amoeba  but  an 
amoeboid  phase  in  the  life-history  of  a  totally  different 
organism  :  it  is  called  an  amcebula. 

The   process  just  described  may  be  taken  as  a  practical 

K    2 


52  PROTOMYXA   AND   THE   MYCETOZOA  LESS. 

proof  of  the  statement  made  in  a  previous  Lesson  (p.  34) 
that  a  flagellum  is  nothing  more  than  a  delicate  and  rela- 
tively permanent  pseudopod.  In  Protomyxa  we  have  a 
flagellula  directly  converted  into  an  amoebula,  the  flagellum 
of  the  former  becoming  one  of  the  pseudopods  of  the 
latter. 

The  amcebulse  thus  formed  may  simply  increase  in  size 
and  send  out  numerous  delicate  pseudopods,  thus  becoming 
converted  into  the  ordinary  Protomyxa-form.  Frequently, 
however,  they  attain  this  form  by  a  very  curious  process : 
they  come  together  in  twos  and  threes  until  they  are  in 
actual  contact  with  one  another,  when  they  undergo  complete 
and  permanent  fusion  (G).  In  this  case  the  Protomyxa-form 
is  produced  not  by  the  development  of  a  single  amcebula 
but  by  the  conjugation  or  fusion  of  a  variable  number  of 
amcebulae.  A  body  formed  in  this  way  by  the  fusion  of 
amoebulae  is  called  a  plasmodium,  so  that  in  the  life-history 
of  Protomyxa  we  can  distinguish  an  encysted,  a  ciliated  or 
flagellate,  an  amoeboid,  and  a  plasmodial  phase. 

The  nature  of  a  plasmodium  will  be  made  clearer  by  a 
short  consideration  of -the  strange  group  of  organisms  known 
as  Mycetozoa  or  sometimes  "  slime-fungi."  They  occur 
as  gelatinous  masses  on  the  bark  of  trees,  on  the  surface  of 
tan-pits,  and  sometimes  in  water.  It  must  be  remembered 
that  Mycetozoa  is  the  name  not  of  a  genus  but  of  a  class 
in  which  are  included  several  genera,  such  as  Badhamia, 
Chondrioderma,  &c.  (see  Fig.  7) :  a  general  account  of  the 
class  is  all  that  is  necessary  for  our  present  purpose. 

The  Mycetozoa  consists  of  sheets  or  networks  of  proto- 
plasm which  may  be  as  much  as  30  cm.  (ift.)  in  diameter, 
and  throughout  the  substance  of  which  are  found  numerous 
nuclei.  In  this  condition  they  creep  about  over  bark  or  some 


THE   PLASMODIUM   OF    BADHAMIA 


H 


FIG.    7. — A,   part  of  the  plasmodium  of  Badhamia  (X   3^) ',   l> 
short  pseudopod  enclosing  a  bit  of  mushroom  stem. 

B,  spore  of  Chondrioderma. 

C,  the  same,  undergoing  dehiscence. 

D,  flagellulse  liberated  from  spores  of  the  same. 

K,  amcebulse  formed  by  metamorphosis  of  flagellulse. 

F,  two  amcebulse  about  to  fuse  :  F',  the  same  after  complete  union. 

G,  G',  two  stages  in  the  formation  of  a  three-celled  plasmodium. 
H,  a  small  plasmodium. 

(A,  after  Lister  :  B— H,  from  Sachs  after  Cienkowski.) 


54  PROTOMYXA   AND   THE   MYCETOZOA  LESS. 

other  substance :  and  in  doing  so  ingest  solid  food 
(Fig.  7,  A).  It  has  been  proved  that  they  digest  protoplasm  : 
and  in  one  genus  pepsin — the  constituent  of  our  own  gastric 
juice  by  which  the  digestion  of  proteids  is  effected  (see  p.  12) 
—has  been  found.  They  can  also  digest  starch  which 
has  been  swollen  by  a  moderate  heat — as  in  our  own  bread 
and  rice-puddings — but  are  unable  to  make  use  of  raw 
starch. 

After  living  in  this  free  condition,  like  a  gigantic  terrestrial 
Amoeba,  for  a  longer  or  shorter  time,  either  a  part  or  the 
whole  of  the  protoplasm  becomes  encysted 1  and  breaks  up 
into  spores.  These  (B)  consist  of  a  globular  mass  of  proto- 
plasm covered  with  a  wall  of  cellulose  :  the  cysts  are  also 
formed  of  cellulose. 

By  the  rupture  of  the  cell-wall  of  the  spore  (c)  the  proto- 
plasm is  liberated  as  a  flagellula  (D)  provided  with  a  nucleus 
and  a  contractile  vacuole,  and  frequently  exhibiting  amoeboid 
as  well  as  ciliary  movements.  After  a  time  the  flagellulse 
lose  their  cilia  and  pass  into  the  condition  of  amoebulse  (E), 
which  finally  fuse  to  form  the  plasmodium  with  which 
we  started  (F — H).  In  the  young  plasmodia  (c1)  the 
nuclei  of  the  constituent  amcebulse  are  clearly  visible,  and 
from  them  the  nuclei  of  the  fully  developed  plasmodia  are 
probably  derived.  It  would  seem,  therefore,  that  in  the 
fusion  of  amcebulae  to  form  the  plasmodium  of  Mycetozoa  the 
cell-bodies  (protoplasm)  alone  coalesce,  not  the  nuclei. 

There  is  a  suggestive  analogy  between  this  process  of 
plasmodium-formation  and  that  of  conjugation  as  seen  in 
Heteromita.  Two  Heteromitae  fuse  and  form  a  zygote  the 

1  The  process  of  formation  of  the  cyst  or  sporangium  is  a  compli- 
cated one,  and  will  not  be  described  here.  See  De  Bary,  Fun%i, 
Mycetozoa,  and  Bacteria  (Oxford,  1887). 


v     PLASMODIUM— FORMATION  AND   CONJUGATION     55 

protoplasm  of  which  divides  into  spores.  In  Protomyxa  and 
the  Mycetozoa  not  two  but  several  amoebulae  unite  to  form 
a  plasmodium  which  after  a  time  becomes  encysted  and 
breaks  up  into  spores.  So  that  we  might  look  upon  the 
conjugation  of  Heteromita  as  an  extremely  simple  plasmo- 
dial  phase  in  its  life-history,  or  upon  the  formation  of  a 
plasmodium  by  Protomyxa  and  the  Mycetozoa  as  a  process 
of  multiple  conjugation. 

There  is,  however,  an  important  difference  between  the 
two  cases  by  reason  of  which  the  analogy  is  far  from  complete. 
In  Heteromita  the  nuclei  of  the  two  gametes  are  no  longer 
visible  (p.  41)  :  they  coalesce  during  conjugation,  and 
the  product  of  their  union  subsequently,  in  all  probability, 
breaks  up  to  form  the  nuclei  of  the  spores.  In  the  Myce- 
tozoa neither  fusion  nor  apparent  disappearance  of  the 
nuclei  of  the  amcebulae  has  been  observed. 


LESSON   VI 

A  COMPARISON  OF  THE  FOREGOING  ORGANISMS  WITH  CER- 
TAIN CONSTITUENT  PARTS  OF  THE  HIGHER  ANIMALS 
AND  PLANTS 

WHEN  a  drop  of  the  blood  of  a  crayfish,  lobster,  or  crab  is 
examined  under  a  high  power,  it  is  found  to  consist  of  a 
nearly  colourless  fluid,  the  plasma,  in  which  float  a  number 
of  minute  solid  bodies,  the  blood-corpuscles  or  leucocytes. 
Each  of  these  (Fig.  8,  A)  is  a  colourless  mass  of  proto- 
plasm, reminding  one  at  once  of  an  Amoeba,  and  on 
careful  watching  the  resemblance  becomes  closer  still,  for 
the  corpuscle  is  seen  to  put  out  and  withdraw  pseudopods 
(A1 — A4)  and  so  gradually  to  alter  its  form  completely. 
Moreover  the  addition  of  iodine,  logwood,  or  any  other 
suitable  colouring  matter  reveals  the  presence  of  a  large 
nucleus  (A5,  A6,  nu)  :  so  that,  save  for  the  absence  of  a  con- 
tractile vacuole  in  the  leucocyte,  the  description  of  Amoeba 
in  Lesson  I.  would  apply  almost  equally  well  to  it. 

The  blood  of  a  fish,  a  frog  (B1),  a  reptile,  or  a  bird  contains 
quite  similar  leucocytes,  but  in  addition  there  are  found  in 
the  blood  of  these  red-blooded  animal  bodies  called  red 
corpuscles.  They  are  flat  oval  discs  of  protoplasm  (B5,  u°) 


FIG.   8. — Typical  Animal  and  Vegetable  Cells. 

A1 — A4,  living  leucocyte  (blood  corpuscle)  of  a  crayfish  showing 
amoeboid  movements  :  A5,  A6,  the  same,  killed  and  stained,  showing 
the  nucleus  (nu). 

B1,  leucocyte  of  the  frog,  nu  the  nucleus;  B2,  two  leucocytes 
beginning  to  conjugate  :  B3,  the  same  after  conjugation,  a  binucleate 
plasmodium  being  formed  :  B4,  a  leucocyte  undergoing  binary  fission  : 
B5,  surface  view  and  Be,  edge  view  of  a  red  corpuscle  of  the  same, 
nu,  the  nucleus. 

C1,  C-,  leucocytes  of  the  newt  ;  in  C1  particles  of  vermilion,  repre- 
sented by  black  dots,  have  been  ingested. 

c3,  surface  view  and  c4,  edge  view  of  a  red  corpuscle  of  man. 

D1,  columnar   epithelial  cells  from   intestine  of  frog  :  D2,  a  similar 


58  EPITHELIAL  CELLS  LESS. 

cell  showing  striated  distal  border  from  which  in  D3  pseudopods  are 
protruded. 

E1,  ciliated  epithelial  cell  from  mouth  of  frog  :  E2,  E3,  similar  cells 
from  windpipe  of  dog. 

F1,  parenchyma  cell  from  root  of  lily,  showing  nucleus  (mi),  vacuoles 
(vac],  and  cell-wall :  F2,  a  similar  cell  from  leaf  of  bean,  showing 
nucleus,  vacuoles,  cell-wall  and  chromatophores  (chr}. 

(B,  D1,  and  E1,  after  Howes  :  C,  E2,  and  E3,  after  Klein  and  Noble 
Smith  :  D8,  D3,  after  Wiedersheim  :  F1,  after  Sachs  :  F2,  after  Behrens. ) 

coloured  by  a  pigment  called  haemoglobin,  and  provided 
each  with  a  large  nucleus  (nu)  which,  when  the  corpuscle  is 
seen  from  the  edge,  produces  a  bulging  of  its  central  part. 
These  bodies  may  be  compared  to  Amoebae  which  have 
drawn  in  their  pseudopods,  assumed  a  flattened  form,  and 
become  coloured  with  haemoglobin. 

In  the  blood  of  mammals,  such  as  the  rabbit,  dog,  or  man, 
similar  leucocytes  occur,  but  their  red  blood  corpuscles  (c3,c4) 
have  the  form  of  biconcave  discs,  and  are  devoid  of  nuclei. 

In  many  animals  the  leucocytes  have  been  observed  to 
ingest  solid  particles  (c1),  to  multiply  by  simple  fission  (B4) 
and  to  coalesce  with  one  another  forming  plasmodia  (B2) 

(p.  53). 

The  stomach  and  intestines  of  animals  are  lined  with  a 
sort  of  soft  slimy  skin  called  mucous  membrane.  If  a 
bit  of  the  surface  of  this  membrane — in  a  frog  or  rabbit  for 
instance — is  snipped  off  and  "teased  out,"  i.e.,  torn  apart 
with  needles,  it  is  found  when  examined  under  a  high  power 
to  be  made  up  of  an  immense  number  of  microscopic 
bodies  called  epithelial  cells,  which  in  the  living  animal,  lie 
close  to  one  another  in  the  inner  layer  of  mucous  mem- 
brane in  something  the  same  way  as  the  blocks  of  a  wood 
pavement  lie  on  the  surface  of  a  road.  An  epithelial  cell 
(D1,  D2)  consists  of  a  rod-like  mass  of  protoplasm,  contain- 
ing a  large  nucleus,  and  is  therefore  comparable  to  an 


vi  PARENCHYMA   CELLS  59 

elongated  Amoeba  without  pseudopods.  In  some  animals 
the  resemblance  is  still  closer  :  the  epithelial  cells  have  been 
observed  to  throw  out  pseudopods  from  their  free  surfaces 
(o3),  that  is,  from  the  only  part  where  any  such  movement 
is  possible,  since  they  are  elsewhere  in  close  contact  with 
their  fellow  cells. 

The  mouth  of  the  frog  and  the  trachea  or  windpipe  of  air- 
breathing  vertebrates  such  as  reptiles,  birds,  and  mammals, 
are  also  lined  with  mucous  membrane,  but  the  epithelial 
cells  which  constitute  its  inner  layer  differ  in  one  important 
respect  from  those  of  the  stomach  and  intestine.  If  ex- 
amined quite  fresh  each  is  found  to  bear  on  its  free  surface, 
i.e.,  the  surface  which  bounds  the  cavity  of  the  mouth  or 
windpipe,  a  number  of  delicate  protoplasmic  threads  or 
cilia  (ET — E3)  which  are  in  constant  vibratory  movement.  In 
the  process  of  teasing  out  the  mucous  membrane  some  of 
the  cells  are  pretty  sure  to  become  detached,  and  are  then 
seen  to  swim  about  in  the  containing  fluid  by  the  action 
of  their  cilia.  These  ciliated  epithelial  cells  remind  one 
strongly  of  Heteromita,  except  for  the  fact  that  they  bear 
numerous  cilia  in  constant  rhythmical  movement  instead  of 
two  only — in  this  case  distinguished  as  flagella — presenting 
an  irregular  lashing  movement. 

Similar  ciliated  epithelial  cells  are  found  on  the  gills  of 
oysters,  mussels,  &c.,  and  in  many  other  situations. 

The  stem  or  root  of  an  ordinary  herbaceous  plant,  such 
as  a  geranium  or  sweet-pea,  is  found  when  cut  across  to 
consist  of  a  central  mass  of  pith,  around  which  is  a  circle 
of  woody  substance,  and  around  this  again  a  soft  greenish 
material  called  the  cortex.  A  thin  section  shows  the  latter 
to  be  made  up  of^^^i^^le .  polyhedral  bodies  called 

'  UNIVERSITY 

V      ^, 


60  PARENCHYMA   CELLS  LESS. 

parenchyma  cells,  fitting  closely  to  one  another  like  the 
bricks  in  a  wall. 

A  parenchyma  cell  examined  in  detail  (F1)  is  seen  to 
consist  of  protoplasm  hollowed  out  internally  into  one  or 
more  cavities  or  vacuoles  (vac)  containing  a  clear  fluid. 
These  vacuoles  differ  from  those  of  Amoeba,  Heteromita,  or 
Euglena  in  being  non-contractile ;  they  are  in  fact  mere 
cavities  in  the  protoplasm  containing  a  watery  fluid  :  the 
layer  of  protoplasm  immediately  surrounding  them  is  denser 
than  the  rest.  Sometimes  there  is  only  one  such  space 
occupying  the  whole  interior  of  the  cell,  sometimes,  as  in 
the  example  figured,  there  are  several,  separated  from  one 
another  by  delicate  bands  or  sheets  of  protoplasm.  The 
cell  contains  a  large  nucleus  (nu)  and  is  completely  enclosed 
in  a  moderately  thick  cell-wall  composed  of  cellulose. 

The  above  description  applies  to  the  cells  composing  the 
deeper  layers  of  the  cortex,  i.e.,  those  nearest  the  woody 
layer  :  in  the  more  superficial  cells,  as  well  as  in  the  internal 
cells  of  a  leaf,  there  is  something  else  to  notice.  Imbedded 
in  the  protoplasm,  just  within  the  cell  wall,  are  a  number  of 
minute  ovoid  bodies  of  a  bright  green  colour  (F2,  chr). 
These  are  chromatophores  or  chlorophyll  corpuscles ;  they 
consist  of  protoplasm  coloured  with  chlorophyll  which  can 
be  proved  experimentally  to  have  the  same  properties  as 
the  chlorophyll  of  Hsematococcus  and  Euglena. 

Such  a  green  parenchyma  cell  is  clearly  comparable  with 
an  encysted  Hsematococcus  or  Euglena,  the  main  differences 
being  that  in  the  plant  cell  the  form  is  polyhedral  owing  to 
the  pressure  of  neighbouring  cells  and  that  the  chromato- 
phores are  relatively  small  and  numerous.  Similarly  a 
colourless  parenchyma  cell  resembles  an  encysted  Amoeba. 

The  pith,  the  epidermis  or  thin  skin  which  forms  the 
outer  surface  of  herbaceous  plants,  the  greater  part  of  the 


vi  MINUTE   STRUCTURE   OF   CELLS  61 

leaves  and  other  portions  of  the  plant  may  be  shown  to 
consist  of  an  aggregation  of  cells  agreeing  in  essential 
respects  with  the  above  description. 

We  come  therefore  to  a  very  remarkable  result.  The 
higher  animals  and  plants  are  built  up — in  part  at  least — of 
elements  which  resemble  in  their  essential  features  the 
minute  and  lowly  organisms  studied  in  previous  lessons. 
Those  elements  are  called  by  the  general  name  of  cells . 
hence  the  higher  organisms,  whether  plants  or  animals,  are 
multicellular  or  are  to  be  considered  as  cell-aggregates, 
while  in  the  case  of  such  beings  as  Amoeba,  Haematococ- 
cus,  Heteromita,  or  Euglena,  the  entire  organism  is  a 
single  cell,  or  is  unicellular. 

Note  further  that  the  cells  of  the  higher  animals  and 
plants,  like  entire  unicellular  organisms,  may  occur  in  either 
the  amoeboid  (Fig.  8,  A,  B1  c1,)  the  ciliated  (E),  or  the 
encysted  (F)  condition,  and  that  a  plasmodial  phase  (B2)  is 
sometimes  produced  by  the  union  of  two  or  more  amceboid 
cells. 

One  of  the  most  characteristic  features  in  the  unicellullar 
organisms  described  in  the  preceding  lessons  is  the  con- 
stancy of  the  occurrence  of  binary  fission  as  a  mode  of ' 
multiplication.  The  analogy  between  these  organisms  and 
the  cells  of  the  higher  animals  and  plants  becomes  still 
closer  when  we  find  that  in  the  latter  also  simple  fission  is 
the  normal  mode  of  multiplication,  the  increase  in  size  of 
growing  parts  being  brought  about  by  the  continual  division 
of  their  constituent  cells. 

The  process  of  division  in  animal  and  vegetable  cells 
is  frequently  accompanied  by  certain  very  characteristic  and 
complicated  changes  in  the  nucleus  to  which  we  must  now 


62 


MINUTE  STRUCTURE  OF  CELLS 


LESS. 


direct  our  attention.  First  of  all,  however,  it  will  be  neces- 
sary to  describe  the  exact  microscopic  structure  of  cells  and 
their  nuclei  as  far  as  it  is  known  at  present. 


chr 


nu.m 


-c.b 


FIG.  9. — A,  Cell  from  the  genital  ridge  of  a  young  salamander, 
showing  cell-membrane  (c.  m),  protoplasm  or  cell-body  (c.  b)  with 
directive  sphere  (s)  and  central  particle  (<:),  and  nucleus  with  membrane 
(nu.  m)  and  irregular  network  of  chromatin  (chr). 

B.  Cell  from  the  immature  stamen  of  a  lily,  showing  cell-wall  (c.  7t»), 
protoplasm  with  two  directive  spheres  (s),  and  nucleus  as  in  A. 

Both  figures  very  highly  magnified. 

(A,  from  a  drawing  by  Mr.  J.  E.  S.  Moore  :  B,  after  Guignard.) 

There  seems  to  be  a  good  deal  01  variation  in  the  precise 
structure  of  various  animal  and  plant  cells,  but  the  more 
recent  researches  show  that  in  the  cell-body  or  protoplasm 
(Fig.  9,  c.  b)  two  constituents  may  be  distinguished,  a  clear 
semi-fluid  substance,  traversed  by  a  delicate  sponge-work. 
Now  under  the  microscope  the  whole  cell  is  not  seen  at 
once  but  only  an  optical  section  of  it,  that  is  all  the 
parts  which  are  in  focus  at  one  time  :  by  altering  the 
focus  we  view  the  object  at  successive  depths,  each  view 
being  practically  a  slice  parallel  to  the  lenses  of  the 
instrument.  This  being  the  case,  protoplasm  presents  the 
microscopic  appearance  of  a  clear  or  slightly  granular 


vi        .  MINUTE  STRUCTURE  OF  NUCLEI  63 

« 

matrix  traversed  by  a  delicate  network.  In  the  epithe- 
lial cells  of  animals  the  protoplasm  is  bounded  exter- 
nally by  a  cell-membrane  (Fig.  9,  A,  c.  m)  of  extreme 
tenuity,  in  plants  by  a  cell-wall  (B,  c.  w)  of  cellulose  :  in 
amoeboid  cells  the  ectosarc  or  transparent  non-granular 
portion  of  the  cell  consists  of  clear  protoplasnJjjJj^  the 
granular  endosarc  alone  possessing  the  sponge  ^Vork.  In 
the  majority  of  full-grown  plant  cells  (Fig.  8,  F)  and  in 
some  animal  cells  the  protoplasm  is  more  or  less  exten- 
sively vacuolated,  but  in  the  young  growing  parts  as  well 
as  in  the  ordinary  cells  of  animals  the  foregoing  description 
holds  good.  It  is  quite  possible  that  the  reticular  character 
of  the  cell  may  be  merely  the  optical  expression  of  an 
extensive  but  minute  vacuolation,  or  may  be  due  to  the 
presence  of  innumerable  minute  granules  developed  in  the 
protoplasm  as  products  of  metabolism. 

The  nucleus  is  usually  spherical  in  form  :  it  is  enclosed 
in  a  delicate  nuclear  membrane  (n.m)  and  contains,  as  in 
Amoeba  (p.  7)  two  constituents,  the  nuclear  matrix  and  the 
chroinatin  which  exhibit  far  more  striking  differences  than 
the  two  constituents  of  the  cell-body.  The  nuclear  matrix 
is  a  homogeneous  semi-fluid  substance  which  forms  the 
ground-work  of  the  nucleus  :  it  resembles  the  clear  cell- 
protoplasm  in  its  general  characters,  amongst  other  things 
in  being  unaffected  by  dyes.  The  chromatin  (chr)  takes  the 
form  of  a  network  or  sponge-work  of  very  variable  form, 
and  is  distinguished  from  all  other  constituents  of  the  cell 
by  its  strong  affinity  for  aniline  and  other  dyes.  Frequently 
one  or  more  minute  globular  structures,  the  nudeoli  (B,  nu'\ 
occur  in  the  nucleus  either  connected  with  the  network  or 
lying  freely  in  its  meshes  :  they  also  have  a  strong  affinity 
for  dyes  although  they  often  differ  considerably  from  the 
chromatin  in  their  micro-chemical  reactions. 


f     A 


D 


B 


x  E 


H 


FIG.  10. — Diagrams  illustrating  the  process  of  indirect  cell  division 
or  karyokinesis. 

A,  The  resting  cell  :  the  nucleus  shows  a  nuclear  membrane  («#./#), 
chromatin  (chr]  arranged  in  loops  united  into  a  network  (the  latter 
shown  on  the  right  side  only),  and  two  nucleoli  (nu1)  :  near  the  nucleus 
is  a  directive  sphere  (s),  containing  a  centrosome  (c )  and  surrounded  by 
radiating  protoplasmic  filaments. 

B,  The  chromatin  has  resolved  itself  into  distinct  loops  or  chromo- 
somes (chr]  which  have  divided  longitudinally  :  the  nuclear  membrane 
has  begun  to  disappear  :  there  are  two  directive  spheres  and  between 
them  is  seen  the  commencement  of  the  nuclear  spindle  (sp). 

C,  The  nuclear  membrane  has  disappeared  :  the  chromosomes  are 


vi  CELL   DIVISION  65 

arranged  irregularly  :  the  spindle  has  increased  in  size  and  is  situated 
definitely  within  the  nuclear  area. 

D,  The  chromosomes  are  arranged  round  the  equator  of  the  fully 
formed  nuclear  spindle. 

E,  The  daughter-loops  of  the  chromosomes  are  passing  in  opposite 
directions  towards  the  poles  of  the  spindle,  each  having  a  spindle-fibre 
attached  to  it. 

F,  Later  stage  of  the  same  process,     fc^ 

G,  The  chromosomes  are  now  arranged  in  two  distinct  groups  one  at 
each  pole  of  the  spindle. 

H,  The  daughter-cells  are  partly  separated  by  constriction  and  the 
chromosomes  of  each  group  are  uniting  to  form  the  network  of  the 
daughter-nucleus. 

i,  Shows  the  division  of  a  plant  cell  by  the  formation  of  a  cell-plate 
{c.  pl] :  the  daughter  nuclei  are  fully  formed!  • 

(Altered  from  Flemming,  Rabl,  &c. ) 


In  the  body  of  some  cells  and  possibly  of  all  there  is 
found  a  globular  body,  surrounded  by  a  radiating  arrange- 
ment of  the  protoplasm  and  called  the  directive  sphere  (s)  : 
it  lies  close  to  the  nucleus,  and  contains  a  minute  granule 
known  as  the  central  particle  or  centrosome  (c).  In  many 
plant  cells  two  directive  spheres  have  been  found  in  each 
cell  (B,  s). 

The  precise  changes  which  take  place  during  the  fission 
of  a  cell  are,  like  the  structure  of  the  cell  itself,  subject 
to  considerable  variation.  We  will  consider  what  may 
probably  be  taken  as  a  typical  case  (Fig.  10). 

First  of  all,  the  directive  sphere  divides  (B,  s)  and  the 
products  of  its  division  gradually  separate  from  one  another 
(c),  ultimately  passing  to  opposite  poles  of  the  nucleus  (D). 
At  the  same  time  the  network  of  chromatin  divides  into  a 
number  of  separate  filaments  called  chroinoAOines  (B,  chr),  the 
number  of  which  appears  to  be  constant  in  any  given 
species  of  animal  or  plant,  although  it  may  vary  in  different 
species  from  two  to  twenty-four.  Soon  after  this  the  nuclear 
membrane  and  the  free  nucleoli  disappear  (B,  c)  and  the 


T?  T?  C  \  T  Y 


66  MINUTE  STRUCTURE  OF  CELLS  LESS. 

nucleus  is  seen  to  contain  a  spindle-shaped  body  (sp)  formed 
of  excessively  delicate  fibres  which  converge  at  each  pole 
to  the  corresponding  directive  sphere.  The  precise  origin 
of  this  nuclear  spindle  is  uncertain  :  it  may  arise  either 
from  the  nuclear  matrix  or,  more  probably,  from  the 
protoplasm  of  the  cell  :  it  is  not  affected  by  colouring 
matters. 

At  the  same  time  each  chromosome  splits,  sometimes 
transversely,  but  usually  along  its  whole  length  so  as  to 
form  two  parallel  rods  or  loops  in  close  contact  with  one 
another  (B)  :  in  this  way  the  number  of  chromosomes  is 
doubled,  each  one  being  now  represented  by  a  pair. 

The  divided  chromosomes  now  pass  to  the  equator  of  the 
spindle  (D)  and  assume  the  form  either  of  V-  shaped  loops, 
or  of  short  rods,  which  arrange  themselves  in  a  radiating 
manner  so  as  to  present  a  star-like  figure  when  the  cell  is 
viewed  in  the  direction  of  the  long  axis  of  the  spindle. 
Everything  is  now  ready  for  division  to  which  all  the  fore- 
going processes  are  preparatory. 

The  two  chromosomes  of  each  pair  now  gradually  pass 
to  opposite  poles  of  the  spindle  (E,  F),  two  distinct  groups 
being  thus  produced  (G)  and  each  chromosome  of  each 
group  being  the  twin  of  one  in  the  other  group.  Probably 
the  fibres  of  the  spindle  are  the  active  agents  in  this 
process,  the  chromosomes  being  dragged  in  opposite 
directions  by  their  contraction. 

After  reaching  the  poles  of  the  spindle  the  chromosomes 
of  each  group  unite  with  one  another  to  form  a  network  (H) 
around  which  a  nuclear  membrane  finally  makes  its  appear- 
ance (i).  In  this  way  two  nuclei  are  produced  within  a 
single  cell,  the  chromosomes  of  the  daughter-nuclei,  as  well 
as  their  attendant  directive  spheres,  being  formed  by  the 
binary  fission  of  those  of  the  mother-nucleus. 


vi  CELL-DIVISION  67 

But  pan  passu  with  this  process  of  nuclear  division, 
fission  of  the  cell-body  is  also  going  on.  This  may  take 
place  by  a  simple  process  of  constriction  (H) — in  much  the 
same  way  as  a  lump  of  clay  or  dough  would  divide  if  a  loop 
of  string  were  tied  round  its  middle  and  then  tightened — or 
by  the  formation  of  what  is  known  as  a  cell-plate.  This 
arises  as  a  row  of  granules  formed  from  the  equatorial  part 
of  the  nuclear  spindle  (i)  :  the  granules  extend  until  they 
form  a  complete  equatorial  plate  dividing  the  cell-body  into 
two  halves  :  fission  then  takes  place  by  the  cell-plate  split- 
ting into  two  along  a  plane  parallel  with  its  flat  surfaces.1 
In  plants  the  cell-plate  gives  rise  to  a  partition  wall  of 
cellulose  which  divides  the  two  daughter-cells  from  one 
another. 

In  some  cases  the  dividing  nucleus  instead  of  going 
through  the  complicated  processes  just  described  divides 
by  simple  constriction.  We  have  therefore  to  distinguish 
between  direct  and  indirect  nuclear  division.  To  the  latter 
very  elaborate  method  the  name  karyokinesis  is  often 
applied. 

In  this  connection  the  reader  will  not  fail  to  note  the 
extreme  complexity  of  structure  revealed  in  cells  and  their 
nuclei  by  the  highest  powers  of  the  microscope.  When  the 
constituent  cells  of  the  higher  animals  and  plants  were 
discovered,  during  the  early  years  of  the  present  century,  by 
Schleiden  a.nd  Schwann,  they  were  looked  upon  as  the  ultima 
Thule  of  microscopic  analysis.  Now  the  demonstration  of 
the  cells  themselves  is  an  easy  matter,  the  problem  is  to 
make  out  their  ultimate  constitution.  What  would  be  the 

1  It  must  not  be  forgotten  that  the  cells  which  are  necessarily  repre- 
sented in  such  diagrams  as  Fig.  10  as  planes  are  really  solid  bodies, 
and  that  consequently  the  cell-plate  represented  in  the  figures  as  a  line 
is  actually  a  plane  at  right  angles  to  the  plane  of  the  paper. 

!•'    2 


68  COMPLEXITY   OF   CELL   STRUCTURE  LESS. 

result  if  we  could  get  microscopes  as  superior  to  those  of 
to-day  as  those  of  to-day  are  to  the  primitive  instruments  of 
eighty  or  ninety  years  ago,  it  is  impossible  even  to  conjecture. 
But  of  one  thing  we  may  feel  confident — of  the  enormous 
strides  which  our  knowledge  of  the  constitution  of  living 
things  is  destined  to  make  during  the  next  half  century. 

The  striking  general  resemblance  between  the  cells  of  the 
higher  animals  and  plants  and  entire  unicellular  organisms 
has  been  commented  on  as  a  very  remarkable  fact :  there  is 
another  equally  significant  circumstance  to  which  we  must 
now  advert. 

All  the  higher  animals  begin  life  as  an  egg,  which  is  either 
passed  out  of  the  body  of  the  parent  as  such,  as  in  most 
fishes,  frogs,  birds,  £c.,  or  undergoes  the  first  stages  of  its 
development  within  the  body  of  the  parent,  as  in  sharks, 
some  reptiles,  and  nearly  all  mammals. 

The  structure  of  the  egg  is,  in  essential  respects,  the  same 
in  all  animals  from  the  highest  to  the  lowest.  In  a  jelly-fish, 
for  instance,  it  consists  (Fig.  n,  A)  of  a  globular  mass  of 
protoplasm  (gd),  in  which  are  deposited  granules  of  a  pro- 
teinaceous  substance  known  as  yolk-spherules.  Within  the 
protoplasm  is  a  large  clear  nucleus  (g.v.\  the  chromatin  of 
which  is  aggregated  into  a  central  mass  or  nucleolus  (g.vi.}- 
An  investing  membrane  may  or  may  not  be  present.  In 
other  words  the  egg  is  a  cell :  it  is  convenient,  for  reasons 
which  will  appear  immediately,  to  speak  of  it  as  the  ovum 
or  egg-cell. 

The  young  or  immature  ova  ot  all  animals  present  this 
structure,  but  in  many  cases  certain  modifications  are  under- 
gone before  the  egg  is  mature,  i.e.,  capable  of  development 
into  a  new  individual.  For  instance,  the  protoplasm  may 
throw  out  pseudopods,  the  egg  becoming  amoeboid  (see 


YI  STRUCTURE   OF   THE   EGG  69 

Fig.  53) ;  or  the  surface  of  the  protoplasm  may  secrete  a  thick 
cell-wall  (see  Fig.  6 1).  The  most  extraordinary  modification 
takes  place  in  some  Vertebrata,  such  as  birds.  In  a  hen's 
egg,  for  instance,  the  yolk-spherules  increase  immensely, 
swelling  out  the  microscopic  ovum  until  it  becomes  what  we 
know  as  the  "  yolk  "  of  the  egg :  around  this  layers  of 
albumen  or  "  white  "  are  deposited,  and  finally  the  shell 
membrane  and  the  shell.  Hence  we  have  to  distinguish 
carefully  in  eggs  of  this  character  between  the  entire  "  egg  " 
in  the  ordinary  acceptation  of  the  term,  and  the  ovum  or 
egg-cell. 

But  complexities  of  this  sort  do  not  alter  the  fundamental 


FIG.  ii. — A,  ovum  of  an  animal  {Car marina  hastata,  one  of  the 
jelly  fishes),  showing  protoplasm  (gd),  nucleus  (gv),  andnucleolus  (,^-w). 

B,  ovum  of  a  plant  (Gymnadcnia  conofsca,  one  of  the  orchids), 
showing  protoplasm  (pis  in),  nucleus  (nit),  and  nucleolus  (mi). 

(A,  from  Balfour  after  Ilaeckel  :  B,  after  Marshall  Ward.) 


fact  that  all  the  higher  animals  begin  life  as  a  single  cell,  or 
in  other  words  that  multicellular  animals,  however  large  and 
complex  they  may  be  in  their  adult  condition,  originate  as 
unicellular  bodies  of  microscopic  size. 

The  same  is  the  case  with  rdl  the  higher  plants.  The 
pistil  or  seed-vessel  of  an  ordinary  flower  contains  one  or 
more  little  ovoidal  bodies,  the  so-called  "  ovules  "  (more 
accurately  megasporangia  (see  Lesson  XXX.,  and  Fig.  89), 
which,  when  the  flower  withers,  develop  into  the  seeds.  A 
section  of  an  ovule  shows  it  to  contain  a  large  cavity,  the 


70  THE   PLANT  OVUM  LESS,  vi 

embryo-sac  or  megaspore  (see  Fig.  89,  D),  at  one  end  of 
which  is  a  microscopic  cell  (0v9  and  Fig.  1 2  B),  consisting  as 
usual  of  protoplasm  (plsm],  nucleus  (««),  and  nucleolus 
(nu'\  This  is  the  ovum  or  egg-cell  of  the  plant  :  from  it 
the  new  plant,  which  springs  from  the  germinating  seed, 
arises.  Thus  the  higher  plants,  like  the  higher  animals,  are, 
in  their  earliest  stage  of  existence,  microscopic  and 
unicellular. 


LESSON    VII 

SACCHAROMYCES 

EVERY  one  is  familiar  with  the  appearance  of  the  ordinary 
brewer's  yeast — the  light-brown,  muddy,  frothing  substance 
which  is  formed  on  the  surface  of  the  fermenting  vats  in 
breweries  and  is  used  in  the  manufacture  of  bread  to  make 
the  dough  "  rise." 

Examined  under  the  microscope  yeast  is  seen  to  consist 
of  a  fluid  in  which  are  suspended  immense  numbers  of 
minute  particles,  the  presence  of  which  produces  the  mud- 
diness  of  the  yeast.  Each  of  these  bodies  is  a  unicellular 
organism,  the  yeast-plant,  or  in  botanical  language  Sac- 
char  omyces  cerevisice. 

Saccharomyces  consists  of  a  globular  or  ellipsoidal  mass 
of  protoplasm  (Fig.  12),  about  TJy-  mm.  in  diameter,  and 
surrounded  with  a  delicate  cell-wall  of  cellulose  (c,  c.w.). 
In  the  protoplasm  are  one  or  more  non-contractile  vacuoles 
(vac) — mere  spaces  rilled  with  fluid  and  varying  according  to 
the  state  of  nutrition  of  the  cell.  Granules  also  occur  in 
the  protoplasm  which  are  products  of  metabolism,  some 
of  them  being  of  a  proteid  material,  others  fat  globules. 
Under  ordinary  circumstances  no  nucleus  is  to  be  seen  : 
but  recently,  by  the  employment  of  a  special  mode  of 


72  SACCHAROMYCES  LESS. 

staining,  a  small  rounded  nucleus  has  been  shown  to  exist 
near  the  centre  of  the  cell. 

The  cell-wall  is  so  thin  that  it  is  difficult  to  be  sure  of 
its  presence  unless  very  high  powers  are  employed.  It 
can  however  be  easily  demonstrated  by  staining  yeast  with 


~bcl' 


FIG.  12. — Sacckaromyces  cerevisia. 

A,  a  group  of  cells  under  a  moderately  high  power.     The  scale  to 
the  left  applies  to  this  figure  only. 

B,  several  cells  more    highly  magnified,  showing  various  stages  ot 
budding,  vac,  the  vacuole. 

C,  a  single    cell    with    two  buds    (bd,    bd'}  still    more    highly  mag- 
nified :  c.w,  cell -wall  :  vac,  vacuole. 

D,  cells,   crushed  by  pressure  :  c.iv,   the  ruptured  cell-walls  :  plsm, 
the  squeezed  out  protoplasm. 

E,  E',  starved  cells,  showing  large  vacuoles  and  fat  globules  (/). 

F,  F',  formation  of  spores  by  fission  of  the  protoplasm  of  a  starved 
cell  :  in  F  the  spores  are  still  enclosed  in  the  mother-cell-wall,    in  F' 
they  are  free. 

magenta,  and  then  applying  pressure  to  the  cover-glass  so  as 
to  crush  the  cells.  Under  this  treatment  the  cell-walls  are 
burst  and  appear  as  crumpled  sacs,  split  in  various  ways  and 
unstained  by  the  magenta  (D,  c.w),  while  the  squeezed-out 
protoplasm  is  seen  in  the  form  of  irregular  masses  (plsm) 
stained  pink  by  the  dye. 


vii  GEMMATION  73 

The  mode  of  multiplication  of  Saccharomyces  is  readily 
made  out  in  actively  fermenting  yeast,  and  is  seen  to  differ 
from  anything  we  have  met  with  hitherto.  A  small  pimple- 
like  elevation  (c,  bd)  appears  on  the  surface  of  a  cell  and 
gradually  increases  in  size  :  examined  under  a  high  power 
this  bud  is  found  to  consist  of  an  offshoot  of  the  protoplasm 
of  the  parent  cell  covered  with  a  very  thin  layer  of  cellulose  : 
it  is  formed  by  the  protoplasm  growing  out  into  an  offshoot 
• — like  a  small  pseudopod — which  pushes  the  cell-wall  before 
it.  The  bud  increases  in  size  (bdf )  until  it  forms  a  little 
globular  body  touching  the  parent  cell  at  one  pole  :  then  a 
process  of  fission  takes  place  along  the  plane  of  junction, 
the  protoplasm  of  the  bud  or  daughter-cell  becoming  sepa- 
rated from  that  of  the  mother-cell  and  a  cellulose  partition 
being  secreted  between  the  two.  Finally  the  bud  becomes 
completely  detached  as  a  separate  yeast-cell. 

It  frequently  happens  that  a  Saccharomyces  buds  in 
several  places  and  each  of  its  daughter-cells  buds  again, 
before  detachment  of  the  buds  takes  place.  In  this  way 
chains  or  groups  of  cells  are  produced  (B),  such  cell- 
colonies  consisting  of  two  or  more  generations  of  cells,  the 
central  one  standing  in  relation  of  parent,  grandparent,  or 
great-grandparent  to  the  others. 

It  must  be  observed  that  this  process  of  budding  or 
gemmation  is  after  all  only  a  modification  of  simple 
fission.  In  the  latter  the  two  daughter-cells  are  of  equal  size 
and  are  both  smaller  than  the  parent-cell,  while  in  gemma- 
tion one — the  mother-cell — is  much  larger  than  the  other — 
the  daughter-cell  or  bud — and  is  of  the  same  size  as,  indeed  is 
practically  identical  with,  the  original  dividing-cell.  Hence 
in  budding,  the  parent  form  does  not,  as  in  simple  fission, 
lose  its  individuality,  becoming  wholly  merged  in  its  twin 
offspring,  but  merely  undergoes  separation  of  a  small  portion 


74  SACCHAROMVCES  LESS. 

of  its  substance  in  the  form  of  a  bud,  which  by  assimilation 
of  nutriment  gradually  grows  to  the  size  of  its  parent, 
the  latter  thus  retaining  its  individuality  and  continuing  to 
produce  fresh  buds  as  long  as  it  lives. 

Multiplication  by  budding  goes  on  only  while  the  Sac- 
charomyces  is  well  supplied  with  food  :  if  the  supply  of 
nutriment  fails,  a  different  mode  of  reproduction  obtains. 
Yeast  can  be  effectually  starved  by  spreading  out  a  thin 
layer  of  it  on  a  slab  of  plaster-of-Paris  kept  moist  under 
a  bell-jar :  under  these  circumstances  the  yeast  is  of  course 
supplied  with  nothing  but  water. 

In  a  few  days  the  yeast-cells  thus  circumstanced  are  found 
to  have  altered  in  appearance  :  large  vacuoles  appear  in 
them  (Fig.  1 2. E;'E')  and  numerous  fat-globules  (/)  are  formed. 
The  protoplasm  has  been  undergoing  destructive  meta- 
bolism, and,  there  being  nothing  to  supply  new  material,  has 
diminished  in  quantity,  and  at  the  same  time  been  partly 
converted  into  fat-  Both  in  plants  and  in  animals  it  is  found 
that  fatty  degeneration,  or  the  conversion  of  protoplasm 
into  fat  by  destructive  metabolism,  is  a  constant  phenomenon 
of  starvation. 

After  a  time  the  protoplasm  collects  towards  the  centre  01 
the  cell  and  divides  simultaneously  into  four  masses  arranged 
like  a  pyramid  of  four  billiard  balls,  three  at  the  base  and 
one  above  (F).  Each  of  these  surrounds  itself  with  a  thick 
cellulose  coat  and  becomes  a  spore,  the  four  spores  being 
sooner  or  later  liberated  by  the  rupture  of  the  mother-cell 
wall  (F'). 

The  spores  being  protected  by  their  thick  cell-walls  are 
able  to  withstand  starvation  and  drought  for  a  long  time ; 
when  placed  under  favourable  circumstances  they  develop 
into  the  ordinary  form  of  Saccharomyces.  So  that  repro- 


vii  ALCOHOLIC   FERMENTATION  75 

duction  by  multiple  fission  appears  to  be,  in  the  yeast-plant, 
a  last  effort  of  the  organism  to  withstand  extinction. 

The  physiology  of  nutrition  of  Saccharomyces  has  been 
studied  with  great  care  by  several  men  of  science  and 
notably  by  Pasteur,  and  is  in  consequence  better  knownthan 
that  of  any  other  low  organism.  For  this  reason  it  will  be 
advisable  to  consider  it  somewhat  in  detail. 

The  first  process  in  the  manufacture  of  beer  is  the  pre- 
paration of  a  solution  of  malt  called  "  sweet-wort."  Malt 
is  barley  which  has  been  allowed  to  germinate  or  sprout,  i.e., 
the  young  plant  is  allowed  to  grow  to  a  certain  extent  from 
the  seed.  During  germination  the  starch  which  forms  so 
large  a  portion  of  the  grain  of  barley  is  partly  converted  into 
sugar  :  barley  also  contains  soluble  proteids  and  mineral 
salts,  so  that  when  malt  is  infused  in  hot  water  the  sweet- 
wort  formed  may  be  looked  upon  as  a  solution  of  sugar, 
proteid,  and  salts. 

Into  this  wort  a  quantity  of  yeast  is  placed.  Very  soon 
the  liquid  begins  to  froth,  the  quantity  of  yeast  increasing 
enormously :  this  means  of  course  that  the  yeast-cells  are 
budding  actively,  as  can  be  readily  made  out  by  microscopic 
examination.  If  while  the  frothing  is  going  on  a  lighted 
candle  is  lowered  into  the  vat  the  flame  will  be  immediately 
extinguished  :  if  an  animal  were  placed  in  the  same  position 
it  would  be  suffocated. 

Chemical  examination  shows  that  the  extinction  of  the 
candle's  flame  or  of  the  animal's  life  is  caused  by  a  rapid 
evolution  of  carbon  dioxide  from  the  fermenting  wort,  the 
frothing  being  due  to  the  escape  of  the  gas  from  the  liquid. 

After  a  time  the  evolution  of  gas  ceases,  and  the  liquid 
is  then  found  to  be  no  longer  sweet  but  to  have  acquired 
what  we  know  as  an  alcoholic  or  spirituous  flavour.  Analysis 


76  SACCHAROMYCES 

shows  that  the  sugar  has  nearly  or  quite  disappeared,  while 
a  new  substance,  alcohol,  has  made  its  appearance.  The 
sweet-wort  has,  in  fact,  been  converted  into  beer. 

Expressed  in  the  form  of  a  chemical  equation  what  has 
happened  is  this  :— 

C,.H12O(;  «  2(C2H6O)  +  2(CO2) 
Grape  sugar.         Alcohol.     Carbon  dioxide. 

One  molecule  of  sugar  has,  by  the  action  of  yeast,  been 
split  up  into  two  molecules  of  alcohol  which  remain  in  the 
fluid,  and  two  of  carbon  dioxide  which  are  given  off  as  gas. 
This  is  the  process  known  as  alcoholic  fermentation. 

It  has  been  shown  by  accurate  analysis  that  only  about  95 
per  cent,  of  the  sugar  is  thus  converted  into  alcohol  and 
carbon  dioxide  :  4  per  cent,  is  decomposed,  with  the  for- 
mation of  glycerine,  succinic  acid,  and  carbon  dioxide,  and 
i  per  cent,  is  used  as  nutriment  by  the  yeast  cells. 

For  the  accurate  study  of  fermentation  the  sweet-wort  of 
the  brewer  is  unsuitable,  being  a  fluid  of  complex  and  un- 
certain composition,  and  the  nature  of  the  process,  as  well 
as  the  part  played  in  it  by  Saccharomyces,  becomes  much 
clearer  if  we  substitute  the  artificial  wort  invented  by 
M.  Pasteur,  and  called  after  him  Pasteur's  solution.  It 
is  made  of  the  following  ingredients  : — 

Water,  H2O        8376  per  cent. 

Cane  sugar,  C12R22OU iS'°°    »      » 

Ammonium  tartrate  (NH4)2C4H4O0  .  i-oo  ,,  ,, 
Potassium  phosphate,  KoPO4.  .  .  .  0-20  ,,  ,, 
Calcium  phosphate,  Ca3(PO4)2  .  .  .  0-02  „  ,, 
Magnesium  sulphate,  MgSO4  .  .  .  o'02  ,,  ,, 

lOO'OO 


VII 


EXPERIMENTS    IN    NUTRITION 


77 


The  composition  of  this  fluid  is  not  a  matter  ot  guess- 
work, but  is  the  result  of  careful  experiments,  and  is  deter- 
mined by  the  following  considerations. 

It  is  obvious  that  if  we  are  to  study  alcoholic  fermentation 
sugar  must  be  present,1  since  the  essence  of  the  process  is 
the  formation  of  alcohol  from  sugar. 

Then  nitrogen  in  some  form  as  well  as  carbon,  oxygen, 
and  hydrogen  must  be  present,  since  these  four  elements 
enter  into  the  composition  of  protoplasm,  and  all  but  the 
first-named  (nitrogen)  into  that  of  cellulose,  and  they  are 
thus  required  in  order  that  the  yeast  should  live  and 
multiply.  The  form  in  which  nitrogen  can  best  be  assimi- 
lated was  found  out  by  experiment.  We  saw  that  in  the 
manufacture  of  beer  the  yeast  cells  obtain  their  nitrogen 
largely  in  the  form  of  soluble  proteids  :  green  plants  obtain 
theirs  largely  in  the  simple  form  of  nitrates.  It  was  found 
that  while  proteids  are,  so  to  say,  an  unnecessarily  complex 
food  for  Saccharomyces,  nitrates  are  not  complex  enough, 
and  an  ammonia  compound- is  necessary,  ammonium  tartrate 
being  the  most  suitable.  Thus  while  Saccharomyces  can 
build  up  the  molecule  of  protoplasm  from  less  complex  food- 
stuffs than  are  required  by  Amoeba,  it  cannot  make  use  of 
such  comparatively  simple  compounds  as  suffice  for  Hsema- 
tococcus  :  moreover  it  appears  to  be  indifferent  whether  its 
nitrogen  is  supplied  to  it  in  the  form  of  ammonium  ^tartrate 
or  in  the  higher  form  of  proteids. 

Then  as  to  the  remaining  ingredients  of  the  fluid — 
potassium  and  calcium  phosphate  and  magnesium  sulphate. 
If  a  quantity  of  yeast  is  burnt,  precisely  the  same  thing 
happens  as  when  one  of  the  higher  animals  or  plants  is 
subjected  to  the  same  process.  It  first  chars  by  the  libera- 

1  It  is  a  matter  of  indifference  whether  cane-sugar  or  grape-su^ar  is 
used. 


78  SACCHAROMYCES  LESS. 

tion  of  carbon,  then  as  the  heat  is  continued  the  carbon 
is  completely  consumed,  going  off  by  combination  with  the 
oxygen  of  the  air  in  the  form  of  carbon  dioxide ;  at  the 
same  time  the  nitrogen  is  given  off  mostly  as  nitrogen  gas, 
the  hydrogen  by  union  with  atmospheric  oxygen  as  water- 
vapour,  and  the  sulphur  as  sulphurous  acid  or  sulphur 
dioxide.  (SO,,).  Finally,  nothing  is  left  but  a  small  quantity 
of  white  ash  which  is  found  by  analysis  to  contain  phos- 
phoric acid,  potash,  lime,  and  magnesia ;  i.e.,  precisely  the 
ingredients  of  the  three  mineral  constituents  of  Pasteur's  solu- 
tion with  the  exception  of  sulphur,  which,  as  already  stated, 
is  given  off  during  the  process  of  burning  as  sulphur  dioxide: 

Thus  the  principle  of  construction  of  an  artificial  nutrient 
solution  such  as  Pasteur's  is  that  it  should  contain  all  the 
elements  existing  in  the  organism  it  is  designed  to  support ; 
or  in  other  words,  the  substances  by  the  combination  of 
which  the  waste  of  the  organism  due  to  destructive  meta- 
bolism may  be  made  good. 

That  Pasteur's  solution  exactly  fulfils  these  requirements 
may  be  proved  by  omitting  one  or  other  of  the  constituents 
from  it,  and  finding  out  how  the  omission  affects  the  well- 
being  of  Saccharomyces. 

If  the  sugar  is  left  out  the  yeast-cells  grow  and  multiply, 
but  with  great  slowness.  This  shows  that  sugar  is  not 
necessary  to  the  life  of  the  organism,  but  only  to  that  active 
condition  which  accompanies  fermentation.  A  glance  at 
the  composition  of  Pasteur's  solution  will  show  that  all  the 
necessary  elements  are  supplied  without  sugar. 

Omission  of  ammonium  tartrate  is  fatal  :  without  it  the 
cells  neither  grow  nor  multiply.  This,  of  course,  is  just 
what  one  would  expect  since,  apart  from  ammonium  tartrate, 
the  fluid  contains  no  nitrogen  without  which  the  molecules  of 
protoplasm  cannot  be  built  up. 


vii  EXPERIMENTS   IN   NUTRITION  79 

It  is  somewhat  curious  to  find  that  potassium  and  calcium 
phosphates  are  equally  necessary;  although  occurring  in 
such  minute  quantities  they  are  absolutely  essential  to  the 
well-being  of  the  yeast-cells,  and  without  them  the  organism, 
although  supplied  with  abundance  of  sugar  and  ammonium 
tartrate,  will  not  live.  This  may  be  taken  as  proving  that 
phosphorus,  calcium,  and  magnesium  form  an  integral  part 
of  the  protoplasm  of  Saccharomyces,  although  existing  in 
almost  infinitesimal  proportions. 

Lastly,  magnesium  sulphate  must  not  be  omitted  if  the 
organism  is  to  flourish  :  unlike  the  other  two  mineral 
constituents  it  is  not  absolutely  essential  to  life,  but  without 
it  the  vital  processes  are  sluggish. 

Thus  by  growing  yeast  in  a  fluid  of  known  composition 
it  can  be  ascertained  exactly  what  elements  and  combina- 
tions of  elements  are  necessary  to  life,  what  advantageous 
though  not  absolutely  essential,  and  what  unnecessary. 

The  precise  effect  of  the  growth  and  multiplication  of 
yeast  upon  a  saccharine  fluid,  or  in  other  words  the  nature 
of  alcoholic  fermentation,  can  be  readily  ascertained  by  a 
simple  experiment  with  Pasteur's  solution.  A  quantity  of 
the  solution  with  a  little  yeast  is  placed  in  a  flask  the  neck 
of  which  is  fitted  with  a  bent  tube  leading  into  a  vessel  of 
lime-water  or  solution  of  calcium  oxide.  When  the  usual 
disengagement  of  carbon  dioxide  (see  p.  75)  takes  place  the 
gas  passes  through  the  tube  into  the  lime-water  and  causes 
an  immediate  precipitation  of  calcium  carbonate  as  a  white 
powder  which  effervesces  with  acids.  This  proves  the  gas 
evolved  during  fermentation  to  be  carbon  dioxide  since  no 
other  converts  lime  into  carbonate.  When  fermentation  is 
complete  the  presence  of  alcohol  may  be  proved  by  distil- 
lation :  a  colourless,  mobile,  pungent,  and  inflammable 
liquid  being  obtained. 


8o  SACCHAROMYCE8  LESS. 

By  experimenting  with  several  flasks  of  this  kind  it  can 
be  proved  that  fermentation  goes  on  as  well  in  darkness  as 
in  light,  and  that  it  is  quite  independent  of  free  oxygen. 
Indeed  the  process  does  not  go  on  if  free  oxygen — i.e., 
oxygen  in  the  form  of  dissolved  gas — is  present  in  the  fluid  ; 
from  which  it  would  seem  that  Saccharomyces  must  be  able 
to  obtain  the  oxygen,  which  like  all  other  organisms  it 
requires  for  its  metabolic  processes,  from  the  food  supplied 
to  it. 

The  process  of  fermentation  goes  on  most  actively, 
between  28°  and  34°C  :  at  low  temperatures  it  is  com- 
paratively slow,  and  at  38°C.  multiplication  ceases. 

If  a  small  portion  of  yeast  is  boiled  so  as  to  kill  the 
cells,  and  then  added  to  a  flask  of  Pasteur's  solution,  no 
fermentation  takes  place,  from  which  it  is  proved  that  the  de- 
composition of  sugar  is  effected  by  the  living  yeast-cells  only. 
There  seems  to  be  no  doubt  that  the  property  of  exciting 
alcoholic  fermentation  is  a  function  of  the  living  protoplasm 
of  Saccharomyces.  The  yeast-plant  is  therefore  known  as 
an  organized  ferment :  when  growing  in  a  saccharine  solu- 
tion it  not  only  performs  the  ordinary  metabolic  processes 
necessary  for  its  own  existence,  but  induces  decomposition 
of  the  sugar  present,  this  decomposition  being  unaccom- 
panied by  any  corresponding  change  in  the  yeast- plant 
itself. 

It  is  necessary  to  mention  in  this  connection  that  there  is 
an  important  group  of  not-living  bodies  which  produce 
striking  chemical  changes  in  various  substances  with- 
out themselves  undergoing  any  change  :  these  are  distin- 
guished as  unorganized  ferments.  A  well-known  example  is 
pepsin,  which  is  found  in  the  gastric  juice  of  the  higher 
animals,  and  has  the  function  of  converting  proteids  into 
peptones  (see  p.  12)  :  its  presence  har  been  proved  in 


vii  FERMENTS  81 

the  Mycetozoa  (p.  52),  and  probably  it  or  some  similar  pep- 
tonizing  or  proteolytic  ferment  effects  this  change  in  all 
organisms  which  have  the  power  of  digesting  proteids. 
Another  instance  is  furnished  by  diastase,  which  effects  the 
conversion  of  starch  into  grape  sugar :  it  is  present  in  ger- 
minating barley  (see  p.  73),  and  an  infinitesimal  quantity 
of  it  can  convert  immense  quantities  of  starch.  The  ptyalin 
of  our  own  saliva  has  a  like  action,  and  probably  some 
similar  diastatic  or  amylolytic  ferment  is  present  in  the 
Mycetozoa  which,  as  we  saw  (p.  52),  are  able  to  digest 
cooked  starch. 


*%: 

[v»-  r 

UNIVERSITY 


LESSON    VIII 

BACTERIA 

IT  is  a  matter  of  common  observation  that  if  certain  moist 
organic  substances,  such  as  meat,  soup,  milk,  &c.,  are  allowed 
to  stand  at  a  moderate  temperature  for  a  few  days — more  or 
fewer  according  as  the  weather  is  hot  or  cold — they  "  go 
bad  "  or  putrefy ;  i.e.  they  acquire  an  offensive  smell,  a  taste 
which  few  are  willing  to  ascertain  by  direct  experiment,  and 
often  a  greatly  altered  appearance. 

One  of  the  most  convenient  substances  for  studying  the 
phenomena  of  putrefaction  is  an  infusion  of  hay,  made  by 
pouring  hot  water  on  a  handful  of  hay  and  straining  the 
resultant  brown  fluid  through  blotting  paper.  Pasteur's 
solution  may  also  be  used,  or  mutton-broth  well  boiled 
and  filtered,  or  indeed  almost  any  vegetable  or  animal 
infusion. 

If  some  such  fluid  is  placed  in  a  glass  vessel,  covered  with 
a  sheet  of  glass  or  paper  to  prevent  the  access  of  dust,  the 
naked-eye  appearances  of  putrefaction  will  be  found  to 
manifest  themselves  with  great  regularity.  The  fluid,  at  first 
quite  clear  and  limpid,  becomes  gradually  dull  and  turbid. 
The  opacity  increases  and  a  scum  forms  on  the  surface  : 
at  the  same  time  the  odour  of  putrefaction  arises,  and 


LESS.  VIII 


BACTERIUM   TERMO 


especially  in  the  case  of  animal  infusions,  quickly  becomes 
very  strong  and  disagreeable. 

The  scum  after  attaining  a  perceptible  thickness  breaks  up 
and  falls  to  the  bottom,  and  after  this  the  fluid  slowly  clears 
again,  becoming  once  more  quite  transparent  and  losing  its 
bad  smell.  If  exposed  to  the  light  patches  of  green  appear 
in  it  sooner  01  later,  due  to  the  presence  of  microscopic 
organisms  containing  chlorophyll.  The  fluid  has  acquired, 
in  fact,  the  characteristics  of  an  ordinary  stagnant  pond,  and 
is  quite  incapable  of  further  putrefaction.  The  whole  series 
of  changes  may  occupy  many  months. 

Microscopic  examination  shows  that  the  freshly-prepared 
fluid  is  free  from  organisms,  and  indeed,  if  properly  filtered, 


1 

FIG.  13. — Bacterium  termo.  A,  motile  stage  :  B,  resting  stage  or 
zooglaea.  (From  Klein.) 

from  particles  of  any  sort.  But  the  case  is  very  different 
when  a  drop  of  infusion  in  which  turbidity  has  set  in  is 
placed  under  a  high  power.  The  fluid  is  then  seen  to  be 
crowded  with  incalculable  millions  of  minute  specks,  only 
just  visible  under  a  power  of  300  or  400  diameters,  and  all 
in  active  movement.  These  specks  are  Bacteria,  or  as 
they  are  sometimes  called,  microbes  or  micro-organisms  ; 
they  belong  to  the  particular  genus  and  species  called 
Bacterium  ter7no. 

Seen  under  the  high  power  01  an  ordinary  student's 
microscope  Bacterium  termo  has  the  appearance  shown  in 
Fig.  13,  A  :  it  is  like  a  minute  finger-biscuit,  i.e.  has  the  form 

G    2 


84  BACTERIA  LESS. 

of  a  rod  constricted  in  the  middle.  But  it  is  only  by  using 
the  very  highest  powers  of  the  microscope  that  its  precise 
form  and  structure  can  be  satisfactorily  made  out.  It  is  then 
seen  (Fig.  14)  to  consist  of  a  little  double  spindle,  showing 
neither  nucleus,  vacuole,  nor  other  internal  structure.  It 
stains  very  deeply  with  aniline  dyes,  and  from  this  and  other 
circumstances  there  is  reason  for  thinking  that  the  whole 
cell  consists  of  chromatin  covered  with  a  membrane  of 
extreme  tenuity  formed  of  cellulose.  It  may  therefore  be 
considered  as  a  cell  consisting  of  cell-wall  and  nucleus  only, 
the  cell-body  being  absent.  At  each  end  is  attached  a 
flagellum  about  as  long  as  the  cell  itself. 

Bacterium  termo  is  much  smaller  than  any  organism  we 
have  yet  considered,  so  small  in  fact  that,  as  it  is  always 


FIG.    14. — Bacterium-  termo  (  x  4000),  showing  the  terminal  flagella. 
(After  Dallinger.) 

easier  to  deal  with  whole  numbers  than  with  fractions,  its 
size  is  best  expressed  by  taking  as  a  standard  the  one- 
thousandth  of  a  millimetre,  called  a  micromillimetre  and 
expressed  by  the  symbol  /,t.  The  entire  length  of  the 
organism  under  consideration  is  from  i  '5  to  2  /x,  i.e.  about 
the  TJo  mm.  or  the  T^TTTTT  mc^-  In  otner  words,  its  entire 
length  is  not  more  than  one-fourth  the  diameter  of  a  yeast- 
cell  or  of  a  human  blood-corpuscle.  The  diameter  of  the 
flagellum  has  been  estimated  by  Dallinger  to  be  about  \  //. 
or  -^-L-^jj  inch,  a  smallness  of  which  it  is  as  difficult  to  form 
any  clear  conception  as  of  the  distances  of  the  fixed  stars. 

Some  slight  notion  of  these  almost  infinitely  small  dimen- 
sions may,  however,  be  obtained  in  the  following  way.     Fig. 


vni  BACILLUS  85 

14  shows  a  Bacterium  termo  magnified  4000  diameters,  the 
scale  above  the  figure  representing  T^-  mm.  magnified  to  the 
same  amount.  The  height  of  this  book  is  a  little  over  18  cm.  ; 
this  multiplied  by  4,000  gives  7  2,000  cm.  =  720  metres  =  2362 
feet.  We  therefore  get  the  proportion — as  2362  feet,  or 
nearly  six  times  the  height  of  St.  Paul's,  is  to  the  height  of 
the  present  volume,  so  the  length  of  Fig.  14  is  to  that  of 
Bacterium  termo. 

It  was  mentioned  above  that  at  a  certain  stage  of  putre- 
faction a  scum  forms  on  the  surface  of  the  fluid.  This  film 
consists  of  innumerable  motionless  Bacteria  imbedded 
in  a  transparent  gelatinous  substance  formed  of  a  proteid 
material  (Fig.  13,  B).  After  continuing  in  the  active  con- 
dition for  a  time  the  Bacteria  rise  to  the  surface,  lose  their 
flagella,  and  throw  out  this  gelatinous  substance  in  which 
they  lie  imbedded.  The  bacterial  jelly  thus  formed  is  called 
a  zooglaa.  Thus  in  Bacterium  termo,  as  in  so  many  of  the 
organisms  we  have  studied,  there  is  an  alternation  of  an 
active  with  a  resting  condition. 

During  the  earlier  stages  of  putrefaction  Bacterium  termo 
is  usually  the  only  organism  found  in  the  fluid,  but  later  on 
other  microbes  make  their  appearance.  Of  these  the  com- 
monest are  distinguished  by  the  generic  names  Micrococcus, 
Bacillus,  Vibrio,  and  Spirillum. 

Micrococcus  (Fig.  15)  is  a  minute  form,  the  cells  of  which 
are  about  2/x  (j-J-^-  mm.)  in  diameter.  It  differs  from 
Bacterium  in  being  globular  instead  of  spindle-shaped  and 
in  having  no  motile  phase.  Like  Bacterium  it  assumes  the 
zooglsea  condition  (Fig.  15,  4). 

Bacillus  is  commonly  found  in  putrescent  infusions  in 
which  the  process  of  decay  has  gone  on  for  some  days  :  as 


86  BACTERIA  LESS. 

its  numbers  increase  those  of  Bacterium  termo  diminish, 
until  Bacillus  becomes  the  dominant  form.  Its  cells  (Fig. 
1 6)  are  rod-shaped  and  about  6/x  (Ty<)  mm.)  in  length  in  the 
commonest  species.  Both  motionless  and  active  forms  are 
found,  the  latter  having  a  flagellum  at  each  end.  The 
zooglaea  condition  ?.s  often  assumed,  and  the  rods  are  fre- 
quently found  united  end  to  end  so  as  to  form  filaments. 

Vibrio  resembles  Bacillus,  but  the  rod-like  cells  (Fig.  1 7,  A) 
are  wavy  instead  of  straight.  They  are  actively  motile  and 
when  highly  magnified  are  found  to  be  provided  with  a 


•"**: 


FIG.  15.  —  Micrococcus.  I,  single  and  double  (dumb-bell  shaped) 
forms  :  2  and  3,  chain-forms  :  4,  a  zooglaea. 

flagellum  at  each  end.      Vibriones  vary  from  8/x  to  25^,  in 
length. 

Spirillum  is  at  once  distinguished  by  its  spiral  form,  the 
cells  resembling  minute  corkscrews  (Fig.  17,  B  &:  c)  and 
being  provided  with  a  flagellum  at  each  end  (c).  The 
smaller  species,  such  as  S.  tenue  (B)  are  from  2  to  5  /x  in 
length,  but  the  larger  forms,  such  as  S.  volutans  (c)  attain  a 
length  of  from  25  to  3o/x.  In  swimming  Spirillum  appears 
on  a  superficial  examination  to  undulate  like  a  worm  or  a 
serpent,  but  this  is  an  optical  illusion  :  the  spiral  is  really  a 
permanent  one,  but  during  progression  it  rotates  upon  its 


VIII 


BINARY    FISSION 


long  axis,  like  Haematococcus  (p.  25),  and  this  double  move- 
ment produces  the  appearance  of  undulation. 

Most  Bacteria  are  colourless,  but  three  species  (Bacterium 
viride,  J3.  chlorinum,  and  Bacillus  virens]  contain  chlorophyll, 
and  several  others  form  pigments  of  varying  tints  and  often 
of  great  intensity.  For  instance,  there  are  red,  yellow, 
brown,  blue,  and  violet  species  of  Micrococcus  which  grow 


FIG.   16. — Bacilhis  subtilis,  showing  various  stages  between  single 
orms  and  long  filaments  (Leptothrix). 

on  slices  of  boiled  potato,  hard-boiled  egg,  &c.,  forming 
brilliantly  coloured  patches  ;  and  the  yellow  colour  often 
assumed  by  milk  after  it  has  been  allowed  to  stand  for  a 
considerable  time  is  due  to  the  presence  of  Bacterium 
xanthinum. 

All  Bacteria  multiply  by  simple  transverse  fission,  the 
process  taking  place  sometimes  during  the  motile,  sometimes 
during  the  resting  condition.  Frequently  the  daughter-cells 
do  not  separate  completely  from  one  another  but  remain 


88 


BACTERIA 


loosely  attached,  forming  chains.     These  are  very  common 
in  some  species  of  micrococcus  (see  Fig.  15). 

Bacillus  when  undergoing  fission  behaves  something  like 
Heteromita  :  the  mother-cell  divides  transversely  across  the 
middle,  and  the  two  halves  gradually  wriggle  away  from  one 
another,  but  remain  connected  for  a  time  by  a  very  fine  thread 


FIG.   17. — A,    Vibrio. 
(From  Klein.) 


B,  Spirilhtm  tenue.      c,  S-birihum  voiutans. 


of  protoplasm  which  extends  between  their  adjacent  ends. 
This  is  drawn  out  by  the  gradual  separation  of  the  two  cells 
until  it  attains  twice  the  length  of  a  flagellum  when  it  snaps 
in  the  middle,  thus  providing  each  daughter-cell  with  a  ne\v 
flagellum.  Bacillus  may,  however,  divide  while  in  the 
resting  condition  and,  under  certain  circumstances,  the 
process  is  repeated  again  and  again,  and  the  daughter-cells 


vni  NATURE   OF   GENERIC   FORMS  89 

remaining  in  contact  form  a  long  wavy  or  twisted  filament 
called  Leptothrix  (Fig.  16)  the  separate  elements  of  which 
are  usually  only  visible  after  staining. 

Bacillus  also  multiplies  by  a  peculiar  process  ot  spore- 
formation  which  may  take  place  either  in  the  ordinary  resting 
form  or  in  a  leptothrix  filament.  A  bright  dot  appears  at 
one  place  in  the  protoplasm  (Fig.  18)  :  this  increases  in  size, 
the  greater  part  of  the  protoplasm  being  used  up  in  its 
formation,  and  finally  takes  on  the  form  of  a  clear  oval 
spore  which  remains  for  some  time  enclosed  in  the  cell-wall 
of  the  Bacillus,  by  the  rupture  of  which  it  is  finally  liberated. 
Spores  of  this  kind  are  termed  endospores.  In  other  Bacteria 
spores  are  formed  directly  from  the  ordinary  cells,  which 
become  thick  walled  (a?'throspores).  The  spores  differ  from 
the  Bacilli  in  being  unstained  by  aniline  dyes. 

After  a  period  of  rest  the  spores,  under  favourable  cir- 
cumstances, germinate  by  growing  out  at  one  end  so  as  to 
become  rod-like,  and  thus  finally  assuming  the  form  of 
ordinary  Bacilli. 

There  are  other  genera  often  included  among  Bacteria  for 
the  description  of  which  the  student  is  referred  to  the  more 
special  treatises.1  One  remark  must,  however,  be  made  in 
concluding  the  present  brief  account  of  the  morphology  of 
the  group.  There  is  a  great  deal  of  evidence  to  show  that 
what  have  been  spoken  of  as  genera  (Bacterium,  Bacillus, 
Spirillum,  &c. )  may  merge  into  one  another  and  are  therefore 
to  be  looked  upon  as  phases  in  the  life-history  of  various 
microbes  rather  than  as  true  and  distinct  genera.  But  this 
is  a  point  which  cannot  at  present  be  considered  as  settled. 

The  conditions  of  life  of  Bacteria  ai'e  very  various.  Some 
live  in  water,  such  as  that  of  stagnant  ponds,  and  of  these 

1  See  especially  De  Bary,  Fungi,  Mycetozoa,  and  Bacteria  (Oxford, 
1887),  and  Klein,  Micro-organisms  and  Disease  (London,  1 886). 


BACTERIA 


LESS. 


three  species,  as  already  stated  (p.  85),  contain  chlorophyll. 
The  nutrition  of  such  forms  must  obviously  be  holophytic, 
and  in  the  case  of  Bacterium  chlorinum  the  giving  off  of 
oxygen  in  sunlight  has  actually  been  proved. 

But  this  mode  of  nutrition  is  rare  among  the  Bacteria  : 
nearly  all  of  those  to  which  reference  has  been  made  are 


FlG.    18. — Spore-formation  in  Bacillus.     (From  Klein.) 

saprophytes,  that  is,  live  upon  decomposing  animal  and 
vegetable  matters.  They  are,  in  fact,  nourished  in  precisely 
the  same  way  as  Heteromita  (see  p.  37).  Many  of  these 
forms  such  as  Bacterium  termo,  and  species  of  Bacillus, 
Vibrio,  &c.,  will,  however,  flourish  in  Pasteur's  solution,  in 
which  they  obtain  their  nitrogen  in  the  form  of  ammonium 


vin  BACTERIA   AS   FERMENTS  91 

tartrate  instead  of  decomposing  proteid.  It  has  also  been 
shown  that  some  Bacteria  can  go  further  and  make  use  of 
nitrates  as  a  source  of  nitrogen,  and  of  a  carbonate  or  even 
of  carbon  dioxide  as  a  source  of  carbon  :  in  other  words, 
they  are  able  to  live  upon  purely  inorganic  matter  in  spite 
of  the  fact  that  they  contain  no  chlorophyll.  Some  species 
may  even  multiply  to  a  considerable  extent  in  distilled  water. 

But  pari  passu  with  their  ordinary  nutritive  processes, 
many  Bacteria  exert  an  action  on  the  fluids  on  which 
they  live  comparable  to  that  exerted  on  a  saccharine 
solution  by  the  yeast-plant.  Such  microbes  are,  in  fact, 
organized  ferments. 

Every  one  is  familiar  with  the  turning  sour  of  milk.  This 
change  is  due  to  the  conversion  of  the  milk-sugar  into 
lactic  acid. 

C6H1206  =  2(C8H608), 
Sugar.  Lactic  Acid. 

The  transformation  is  brought  about  by  the  agency  of 
Bacterium  lactis,  a  microbe  closely  resembling  B.  termo. 

Beer  and  wine  are  two  other  fluids  which  frequently  turn 
sour,  there  being  in  this  case  a  conversion  of  alcohol  into 
acetic  acid,  represented  by  the  equation — 

C2H6O  +  O2  =  H2O  +  C2H4O2, 

Alcohol.   Oxygen.       Water.     Acetic  Acid. 

The  ferment  in  this  instance  is  Bacterium  aceti,  often 
called  My  coder  ma  aceti,  or  the  "  vinegar  plant."  It  will 
be  noticed  that  fti  this  case  oxygen  enters  into  the  reaction  : 
it  is  a  case  of  fermentation  by  oxidation. 

Putrefaction  itself  is  another  instance  01  fermentation 
induced  by  a  microbe.  Bacterium  termo — the  putrefactive 
ferment — causes  the  decomposition  of  proteids  into  simpler 
compounds,  amongst  which  are  such  gases  as  ammonia 


92  BACTERIA  LESS. 

(NH3),  sulphuretted  hydrogen  (H^S),  and  ammonium 
sulphide  (  (NH4)2S),  the  evolution  of  which  produces  the 
characteristic  odour  of  putrefaction. 

The  final  stage  in  putrefaction  is  the  formation  of  nitrates 
and  nitrites.  The  process  is  a  double  one,  both  stages 
being  due  to  special  forms  of  Bacteria.  In  the  first  place, 
by  the  agency  of  the  nitrous  ferment,  ammonia  is  converted 
into  nitrous  acid — 

NH3  +  3O  =  H2O   +  HNO2 
Ammonia.    Oxygen.   Water.       Nitrous  Acid. 

The  nitric  ferment  then  comes  into  action,  converting  the 
nitrous  into  nitric  acid — 

NHO2  +  O  =  HNO.> 

Nitrous  Acid.   Oxygen.    Nitric  Acid. 

This  process  is  one  of  vast  importance,  since  by  its  agency 
the  soil  is  constantly  receiving  fresh  supplies  of  nitric  acid 
which  is  one  of  the  most  important  substances  used  as 
food  by  plants. 

Besides  holophytes  and  saprophytes  there  are  included 
among  Bacteria  many  parasites,  that  is,  species  which  feed 
not  on  decomposing  but  on  living  organisms.  Many  of  the 
most  deadly  infectious  diseases,  such  as  tuberculosis,  diph- 
theria, typhoid  fever,  and  cholera,  are  due  to  the  presence 
in  the  tissues  or  fluids  of  the  body  of  particular  species  of 
microbes,  which  feed  upon  the  parts  affected  and  give  rise 
to  the  morbid  symptoms  characteristic  of  the  disease. 

Some  Bacteria,  like  the  majority  of  the  organisms  pre- 
viously studied,  require  free  oxygen  for  their  existence,  but 
others,  like  Saccharomyces  during  active  fermentation  (see 
p.  78),  are  quite  independent  of  free  oxygen  and  must  there- 
fore be  able  to  take  the  oxygen,  without  which  their  metabolic 


vin  CONDITIONS   OF   LIFE  93 

processes  could  not  go  on,  from  some  of  the  compounds 
contained  in  the  fluid  in  which  they  live.  Bacteria  are  for 
this  reason  divided  into  aerobic  species  which  require  free 
oxygen,  and  anaerobic  species  which  do  not. 

As  to  temperature,  common  observation  tells  us  that 
Bacteria  flourish  only  within  certain  limits.  We  know  for 
instance  that  organic  substances  can  be  preserved  from 
putrefaction  by  being  kept  either  at  the  freezing-point,  or  at 
or  near  the  boiling-point.  One  important  branch  of  modern 
industry,  the  trade  in  frozen  meat,  depends  upon  the  fact  that 
the  putrefactive  Bacteria,  like  other  organisms,  are  rendered 
inactive  by  freezing,  and  every  housekeeper  knows  how  easily 
putrefaction  can  be  staved  off  by  roasting  or  boiling.  Simi- 
arly  it  is  a  matter  of  common  observation  that  a  moderately 
igh  temperature  is  advantageous  to  these  organisms,  the 
heat  of  summer  or  of  the  tropics  being  notoriously  favourable 
to  putrefaction.  In  the  case  of  Bacterium  termo,  it  has  been 
found  that  the  optimum  temperature  is  from  30°  to  35°  C., 
but  that  the  microbe  will  flourish  between  5°  and  40°  C. 

Although  fully-formed  Bacteria,  like  other  organisms >  are 
usually  killed  by  exposure  to  heat  several  degrees  below 
boiling-point,  yet  the  spores  of  some  species  will  withstand, 
at  any  rate  for  a  limited  time,  a  much  higher  temperature — 
even  one  as  high  as  130°  C.  On  the  other  hand,  putrefactive 
Bacteria  retain  their  power  of  development  after  being 
exposed  to  a  temperature  of  -m°C.,  although  during  the 
time  of  exposure  all  vital  activity  is  of  course  suspended. 

'Bacteria  also  resemble  other  organisms  in  being  unable 
to  carry  on  active  life  without  a  due  supply  of  water  :  no 
perfectly  dry  substance  ever  putrefies.  The  preservation  for 
ages  of  the  dried  bodies  of  animals  in  such  countries  as 
Egypt  and  Peru  depends  at  least  as  much  upon  the  moisture- 
less  air  as  upon  the  antiseptics  used  in  embalming. 


94  BACTERIA  LESS,  vin 

For  the  most  part  Bacteria  are  unaffected  by  light,  since 
they  grow  equally  well  in  darkness  and  in  ordinary  daylight. 
Many  of  them,  however,  will  not  bear  prolonged  exposure  to 
direct  sunlight,  and  it  has  been  found  possible  to  arrest  the 
putrefaction  of  an  organic  infusion  by  insolation,  or  exposure 
to  the  direct  action  of  the  sun's  rays.  It  has  also  been 
proved  that  it  is  the  light-rays  and  not  the  heat-rays  which 
are  thus  prejudicial  to  the  life  of  micro-organisms. 


LESSON    IX 

BIOGENESIS    AND    ABIOGENESIS  :  HOMOGENESIS  AND  HETERO 
GENESIS 

THE  study  of  the  foregoing  living  things  and  especially  ot 
Bacteria,  the  smallest  and  probably  the  simplest  of  all  known 
organisms,  naturally  leads  us  to  the  consideration  of  one  of 
the  most  important  problems  of  biology — the  problem  of 
the  origin  of  life. 

In  all  the  higher  organisms  we  know  that  each  individual 
arises  in  some  way  or  other  from  a  pre-existing  individual : 
no  one  doubts  that  every  bird  now  living  arose  by  a  process 
of  development  from  an  egg  formed,  in  the  body  of  a 
parent  bird,  and  that  every  tree  now  growing  took  its  origin 
either  from  a  seed  or  from  a  bud  produced  by  a  parent  plant. 
But  there  have  always — until  quite  recently,  at  any  rate — 
been  upholders  of  the  view  that  the  lower  forms  of  life, 
bacteria,  monads,  and  the  like,  may  under  certain  circum- 
stances originate  independently  of  pre-existing  organisms  : 
that,  for  instance,  in  a  flask  of  hay-infusion  or  mutton-broth, 
boiled  so  as  to  kill  any  living  things  present  in  it,  fresh 
forms  of  life  may  arise  de  novo,  may  in  fact  be  created 
then  and  there. 

We  have  therefore  two  theories  of  the  lower  organisms, 


96  BIOGENESIS   AND    HOMOGENESIS  LESS. 

the  theory  of  Biogenesis,  according  to  which  each  living 
thing,  however  simple,  arises  by  a  natural  process  of  bud- 
ding, fission,  spore-formation,  or  what  not,  from  a  parent 
organism  :  and  the  theory  of  Abiogenesis,  or  as  it  is  some- 
times called  Spontaneous  or  Equivocal  Generation,  accord- 
ing to  which  fully  formed  living  organisms  sometimes 
arise  from  not-living  matter. 

In  former  times  the  occurrence  of  abiogenesis  was  uni- 
versally believed  in.  The  expression  that  a  piece  of  meat 
has  "  bred  maggots  "  ;  the  opinion  that  parasites  such  as  the 
gall-insects  of  plants  or  the  tape-worms  in  the  intestines  of 
animals  originate  where  they  are  found  ;  the  belief  still  held 
in  some  rural  districts  in  the  occurrence  of  showers  of  frogs, 
or  in  the  transformation  of  horse-hairs  kept  in  water  into 
eels ;  all  indicate  a  survival  of  this  belief. 

Aristotle,  one  of  the  greatest  men  of  science  of  antiquity, 
explicitly  teaches  abiogenesis.  He  states  that  some  animals 
"spring  from  putrid  matter,"  that  certain  insects  "spring 
from  the  dew  which  falls  upon  plants,"  that  thread-worms 
"originate  in  the  mud  of  wells  and  running  waters,"  that 
fleas  "originate  in  very  small  portions  of  corrupted  matter," 
and  that  "  bugs  proceed  from  the  moisture  which  collect:* 
on  the  bodies  of  animals,  lice  from  the  flesh  of  other 
creatures." 

Little  more  than  200  years  ago  one  Alexander  Ross, 
commenting  on  Sir  Thomas  Browne's  doubt  as  to  "  whether 
mice  may  be  bred  by  putrefaction,"  says,  "so  may  he  doubt 
whether  in  cheese  and  timber  worms  are  generated  ;  or  if 
beetles  and  wasps  in  cow's  clung ;  or  if  butterflies,  locusts, 
grasshoppers,  shell-fish,  snails,  eels,  and  such  like,  be  pro- 
created of  putrefied  matter,  which  is  apt  to  receive  the  form 
of  that  creature  to  which  it  is  by  formative  power  disposed. 
To  question  this  is  to  question  reason,  sense,  and  experience. 


ix        PROBLEM  LIMITED  TO  MICROSCOPIC  FORMS      97 

If  he  doubts  of  this  let  him  go  to  Egypt,  and  there  he  will 
find  the  fields  swarming  with  mice,  begot  of  the  mud  of 
Nylus,  to  the  great  calamity  of  the  inhabitants." 

As  accurate  inquiries  into  these  matters  were  made,  the 
number  of  cases  in  which  equivocal  generation  was  sup- 
posed to  occur  was  rapidly  diminished.  It  was  a  simple 
matter— when  once  thought  of — to  prove,  as  Redi  did  in 
1638,  that  no  maggots  were  ever  "bred  "  in  meat  on  which 
flies  were  prevented  by  wire  screens  from  laying  their  eggs. 
Far  more  difficult  was  the  task,  also  begun  in  the  seventeenth 
century,  of  proving  that  parasites,  such  as  tape-worms,  arise 
from  eggs  taken  in  with  the  food  ;  but  gradually  this  pro- 
position was  firmly  established,  so  that  no  one  of  any 
scientific  culture  continued  to  believe  in  the  abiogenetic 
origin  of  the  more  highly  organized  animals  any  more  than 
in  showers  of  frogs,  or  in  the  origin  of  geese  from 
barnacles. 

But  a  new  phase  of  the  question  was  opened  with  the  in- 
vention of  the  microscope.  In  1683,  Anthony  van  Leeuwen- 
hoek  discovered  Bacteria,  and  it  was  soon  found  that  however 
carefully  meat  might  be  protected  by  screens,  or  infusions  by 
being  placed  in  well-corked  or  stoppered  bottles,  putrefaction 
always  set  in  sooner  or  later,  and  was  invariably  accom- 
panied by  the  development  of  myriads  of  bacteria,  monads, 
and  other  low  organisms.  It  was  not  surprising,  considering 
the  rapidity  with  which  these  were  found  to  make  their 
appearance,  that  many  men  of  science  imagined  them  to  be 
produced  abiogenetically. 

Let  us  consider  exactly  what  this  implies.  Suppose  we 
have  a  vessel  of  hay-infusion,  and  in  it  a  single  Bacterium. 
The  microbe  will  absorb  the  nutrient  fluid  and  convert  it 
into  fresh  protoplasm  :  it  will  divide  repeatedly,  and,  its 
progeny  repeating  the  process,  the  vessel  will  soon  con- 

H 


98  BIOGENESIS    AND   HOMOGENESIS  LESS. 

tain  millions  of  Bacteria  instead  of  one.  This  means,  of 
course,  that  a  certain  amount  of  fresh  living  protoplasm  has 
been  formed  out  of  the  constituents  of  the  hay-infusion, 
through  the  agency  in  the  first  instance  of  a  single  living 
Bacterium.  The  question  naturally  arises— Why  may  not 
the  formation  of  protoplasm  take  place  independently  of 
this  insignificant  speck  of  living  matter  ? 

It  must  not  be  thought  that  this  question  is  in  any  way 
a  vain  or  absurd  one.  That  living  protoplasm  has  at  some 
period  of  the  world's  history  originated  from  not-living 
matter  seems  a  necessary  corollary  of  the  doctrine  of 
evolution,  and  is  obviously  the  very  essence  of  the  doctrine 
of  special  creation  ;  and  there  is  no  a  priori  reason  why  it 
should  be  impossible  to  imitate  the  unknown  conditions 
under  which  this  took  place.  At  present,  however,  we  have 
absolutely  no  data  towards  the  solution  of  this  fundamental 
problem. 

But  however  insoluble  may  be  the  question  as  to  how  life 
first  dawned  upon  our  planet,  the  origin  of  living  things  at 
the  present  day  is  capable  of  investigation  in  the  ordinary 
way  of  observation  and  experiment.  The  problem  may  be 
stated  as  follows  : — any  putrescible  infusion, — i.e.  any  fluid 
capable  of  putrefaction — will  be  found  after  a  longer  or 
shorter  exposure  to  swarm  with  bacteria  and  monads  :  do 
these  organisms  or  the  spores  from  which  they  first  arise 
reach  the  infusion  from  without,  or  are  they  generated  within 
it?  And  the  general  lines  upon  which  an  investigation 
into  the  problem  must  be  conducted  are  simple  :  given  a 
vessel  of  any  putrescible  infusion ;  let  this  be  subjected  to 
some  process  which,  without  rendering  it  incapable  of  sup- 
porting life,  shall  kill  any  living  things  contained  in  it ;  let 
it  then  be  placed  under  such  circumstances  that  no  living 
particles,  however  small,  can  reach  it  from  without.  If, 


ix  EXPERIMENTS    ON    BIOGENESIS  99 

after  these  two  conditions  have  been  rigorously  complied 
with,  living  organisms  appear  in  the  fluid,  such  organisms 
must  have  originated  abiogenetically. 

To  kill  any  microbes  contained  in  the  fluid  it  is  usually 
quite  sufficient  to  boil  it  thoroughly.  As  we  have'  seen, 
protoplasm  enters  into  heat-rigor  at  a  temperature  consider- 
ably below  the  boiling-point  of  water,  so  that,  with  an 
exception  which  will  be  referred  to  presently,  a  few  minutes' 
boiling  suffices  to  sterilize  all  ordinary  infusions,  i.e.,  to  kill 
any  organisms  they  may  contain. 

Then  as  to  preventing  the  entrance  of  organisms  or  their 
spores  from  without.  This  may  be  done  in  various  ways. 
One  way  is  to  take  a  flask  with  the  neck  drawn  out  into 
a  very  slender  tube,  to  boil  the  fluid  in  it  for  a  sufficient 
time,  and  then,  while  ebullition  is  going  on,  to  close  the 
end  of  the  tube  by  melting  the  glass  in  the  flame  of  a 
Bunsen-burner  or  spirit-lamp,  thus  hermetically  sealing  the 
flask. 

By  this  method  not  only  organisms  and  their  spores  are 
excluded  from  the  flask  but  also  air.  But  this  is  obviously 
unnecessary :  it  is  evident  that  air  may  be  admitted  to  the 
fluid  with  perfect  impunity  if  only  it  can  be  filtered,  that  is, 
passed  through  some  substance  which  shall  retain  all  solid 
particles  however  small,  and  therefore  of  course  bacteria, 
monads,  and  their  spores. 

A  perfectly  efficient  filter  for  this  purpose  is  furnished  by 
cotton-wool.  A  flask  or  test-tube  is  partly  filled  with  the 
infusion  :  the  latter  is  boiled,  and  during  ebullition  cotton- 
wool is  pushed  into  the  mouth  of  the  vessel  until  a  long  and 
firm  plug  is  formed  (Fig  19).  When  the  source  of  heat  is 
removed,  and,  by  the  cooling  of  the  fluid,  the  steam  which 
filled  the  upper  part  of  the  tube  condenses,  air  passes  in  to 
supply  its  place,  but  as  it  does  so  it  is  filtered  of  even  the 

H    2 


ioo  BIOGENESIS  AND  HOMOGENESIS  LESS. 

smallest  solid  particles  by  having  to  pass  through  the  close 
meshes  of  the  cotton-wool. 

Experiments  of  this  sort  conducted  with  proper  care  have 
been  known  for  many  years  to  give  negative  results  in  the 
great '  majority  of  cases  :  the  fluids  remain  perfectly  sterile 
for  any  length  of  time.  But  in  certain  instances,  in  spite  of 
the  most  careful  precautions,  bacteria  were  found  to  appear 


FIG.  19. — A  Beaker  with  a  number  of  test-tubes  containing  putu 
cible  infusions  and  plugged  with  cotton-wool.     (From  Klein. ) 


in  such  fluids,  and  for  years  a  fierce  controversy  raged 
between  the  biogenists  and  the  abiogenists,  the  latter  in- 
sisting that  the  experiments  in  question  proved  the  occurrence 
of  spontaneous  generation,  while  the  biogenists  considered 
that  all  such  cases  were  due  to  defective  methods — either  to 
imperfect  sterilization  of  the  fluid  or  to  imperfect  exclusion 
of  germ-containing  atmospheric  dust. 

The  matter  was  finally  set  at  rest,  and  the  biogenists 


IX  EXPERIMENTS   ON    BIOGENESIS  101 

proved  to  be  in  the  right,  by  the  important  discovery  that 
the  spores  of  bacteria  and  monads  are  not  killed  by  a  tem- 
perature many  degrees  higher  than  is  sufficient  to  destroy  the 
adult  forms  :  that  in  fact  while  the  fully  developed  organisms 
are  killed  by  a  few  minutes'  exposure  to  a  temperature  of 
70°  C.  the  spores  are  frequently  able  to  survive  several 
hours'  boiling,  and  must  be  heated  to  130° — 150°  C.  in 
order  that  their  destruction  may  be  assured.  It  was  also 
shown  that  the  more  thoroughly  the  spores  are  dried  the 
more  difficult  they  are  to  kill,  just  as  well-dried  peas  are 
hardly  affected  by  an  amount  of  boiling  sufficient  to  reduce 
fresh  ones  to  a  pulp. 

This  discovery  of  the  high  thermal  death-point  or  ultra- 
maximum  temperature  of  the  spores  of  these  organisms  has 
necessitated  certain  additional  precautions  in  experiments 
with  putrescible  infusions.  In  the  first  place  the  flask  and 
the  cotton-wool  should  both  be  heated  in  an  oven  to  a 
temperature  of  150°  C.,  and  thus  effectually  sterilized.  The 
flask  being  filled  and  plugged  with  cotton-wool  is  well  boiled 
and  then  kept  for  some  hours  at  a  temperature  of  32° — 38°C., 
the  optimum  temperature  for  bacteria.  The  object  of  this 
is  to  allow  any  spores  which  have  not  been  killed  by  boiling 
to  germinate,  in  other  words  to  pass  into  the  adult  con- 
dition in  which  the  temperature  of  boiling  water  is  fatal. 
The  infusion  is  then  boiled  again,  so  as  to  destroy  any  such 
freshly  germinated  forms  it  may  contain.  The  same  process 
is  repeated  once  or  twice,  the  final  result  being  that  the 
very  driest  and  most  indurated  spores  are  induced  to  ger- 
minate, and  are  thereupon  slain.  It  must  not  be  forgotten 
that  repeated  boiling  does  not  render  the  fluid  incapable  of 
supporting  life,  as  may  be  seen  by  removing  the  cotton-wool 
plug,  when  it  will  in  a  short  time  swarm  with  microbes. 

Experiments  conducted  with  these  precautions  all  tell  the 


102  BIOGENESIS  AND    HOMOGENESIS  LESS. 

same  tale  :  they  prove  conclusively  that  in  properly  sterilized 
putrescible  infusions,  adequately  protected  from  the  entrance 
of  atmospheric  germs,  no  micro-organisms  ever  make  their 
appearance.  So  that  the  last  argument  for  abiogenesis  has 
been  proved  to  be  fallacious,  and  the  doctrine  of  biogenesis 
shown,  as  conclusively  as  observation  and  experiment  can 
show  it,  to  be  of  universal  application  as  far  as  existing 
conditions  known  to  us  are  concerned. 

It  is  also  necessary  to  add  that  the  presence  of  microbes 
in  considerable  quantities  in  our  atmosphere  has  been 
proved  experimentally.  By  drawing  air  through  tubes 
lined  with  a  solid  nutrient  material  Prof.  Percy  Frankland 
showed  that  the  air  of  South  Kensington  contained  about 
thirty-five  micro-organisms  in  every  ten  litres,  and  by  ex- 
posing circular  discs  coated  with  the  same  substance  he  was 
further  able  to  prove  that  in  the  same  locality  279  micro- 
organisms fall  upon  one  square  foot  of  surface  in  one 
minute. 

There  is  another  question  intimately  connected  with  that 
of  Biogenesis,  although  strictly  speaking  quite  independent 
of  it.  It  is  a  matter  of  common  observation  that,  in  both 
animals  and  plants,  like  produces  like  :  that  a  cutting  from 
a  willow  will  never  give  rise  to  an  oak,  nor  a  snake  emerge 
from  a  hen's  egg.  In  other  words,  ordinary  observation 
teaches  the  general  truth  of  the  doctrine  of  Homogenesis. 

But  there  has  always  been  a  residuum  of  belief  in  the 
opposite  doctrine  of  HeUrogcnesis,  according  to  which  the 
offspring  of  a  given  animal  or  plant  may  be  something 
utterly  different  from  itself,  a  plant  giving  rise  to  an  animal 
or  vice  versa,  a  lowly  to  a  highly  organized  plant  or  animal 
and  so  on.  Perhaps  the  most  extreme  case  in  which  hetero- 
genesis  was  once  seriously  believed  to  occur  is  that  of 


ix  HETEROGENESIS  103 

the  "  barnacle-geese."  Buds  of  a  particular  tree  growing 
near  the  sea  were  said  to  produce  barnacles,  and  these 
falling  into  the  water  to  develop  into  geese.  This  sounds 
absurd  enough,  but  within  the  last  twenty  years  two  or  three 
men  of  science  have  described,  as  the  result  of  repeated 
observations,  the  occurrence  of  quite  similar  cases  among 
microscopic  organisms.  For  instance,  the  blood-corpuscles 
of  the  silkworm  have  been  said  to  give  rise  to  fungi,  the 
protoplasm  of  the  green  weed  Nitella  (see  Fig.  45)  to 
Amoeba  and  Infusoria  (see  p.  107),  Euglense  to  thread- 
worms, and  so  on. 

It  is  proverbially  difficult  to  prove  a  negative,  and  it  might 
not  be  easy  to  demonstrate,  what  all  competent  naturalists 
must  be  firmly  convinced  of,  that  every  one  of  these  sup- 
posed cases  of  heterogenesis  is  founded  either  upon  errors 
of  observation  or  upon  faulty  inductions  from  correct 
observations. 

Let  us  take  a  particular  case  by  way  of  example.  Many 
years  ago  Dr.  Dallinger  observed  among  a  number  of  Vorti- 
cellse  or  bell-animalcules  (Fig.  26)  one  which  appeared  to 
have  become  encysted  upon  its  stalk.  After  watching  it  for 
some  time,  there  was  seen  to  emerge  from  the  cyst  a  free- 
swimming  ciliated  Infusor  called  Amphileptus,  not  unlike  a 
long-necked  Paramcecium  (Fig.  20,  p.  108).  Many  ob- 
servers would  have  put  this  down  as  a  clear  case  of  hetero- 
genesis :  Dallinger  simply  recorded  the  observation  and 
waited.  Two  years  later  the  occurrence  was  explained  :  he 
found  the  same  two  species  in  a  pond,  and  watched  an 
Amphileptus  seize  and  devour  a  Vorticella,  and,  after  finish- 
ing its  meal,  become  encysted  upon  the  stalk  of  its  victim. 

It  is  obvious  that  the  only  way  in  which  a  case  of  hetero- 
genesis could  be  proved  would  be  by  actually  watching  the 
transformation,  and  this  no  heterogenist  has  ever  done ;  at 


104  BIOGENESIS   AND    HOMOGENESIS  LESS. 

the  most,  certain  supposed  intermediate  stages  between  the 
extreme  forms  have  been  observed — say,  between  a  Euglena 
and  a  thread-worm — and  the  rest  of  the  process  inferred. 
On  the  other  hand,  innumerable  observations  have  been 
made  on  these  and  other  organisms,  the  result  being  that 
each  species  investigated  has  been  found  to  go  through  a 
definite  series  of  changes  in  the  course  of  its  development, 
the  ultimate  result  being  invariably  an  organism  resembling 
in  all  essential  respects  that  which  formed  the  starting-point 
of  the  observations  :  Euglense  always  giving  rise  to  Euglense 
and  nothing  else,  Bacteria  to  Bacteria  and  nothing  else,  and 
so  on. 

There  are  many  cases  which  imperfect  knowledge  might 
class  under  heterogenesis,  such  as  the  origin  of  frogs  from 
tadpoles  or  of  jelly-fishes  from  polypes  (Lesson  XXIII.  Fig. 
53),  but  in  these  and  many  other  cases  the  apparently 
anomalous  transformations  have  been  found  to  be  part  of 
the  normal  and  invariable  cycle  of  changes  undergone  by 
the  organism  in  the  course  of  its  development ;  the  frog 
always  gives  rise  ultimately  to  a  frog,  the  jelly-fish  to  a  jelly- 
fish. If  a  frog  at  one  time  produced  a  tadpole,  at  another  a 
trout,  at  another  a  worm  :  if  jelly-fishes  gave  rise  sometimes 
to  polypes,  sometimes  to  infusoria,  sometimes  to  cuttle- 
fishes, and  all  without  any  regular  sequence — that  would  be 
heterogenesis. 

It  is  perhaps  hardly  necessary  to  caution  the  reader  against 
the  error  that  there  is  any  connection  between  the  theory  of 
heterogenesis  and  that  of  organic  evolution.  It  might  be 
said — if,  as  naturalists  tell  us,  dogs  are  descended  from 
wolves  and  jackals  and  birds  from  reptiles,  why  should  not, 
for  instance,  thread-worms  spring  from  Euglenae  or  Infusoria 
from  Bacteria  ?  To  this  it  is  sufficient  to  answer  that  the 
evolution  of  one  form  from  another  takes  place  by  a  series 


ix  HETEROGENESIS  105 

of  slow,  orderly,  progressive  changes  going  on  through  a 
long  series  of  generations  (see  Lesson  XIII.);  whereas 
heterogenesis  presupposes  the  casual  occurrence  of  sudden 
transformations  in  any  direction — i.e.,  leading  to  either  a  less 
or  a  more  highly  organized  form — and  in  the  course  of  a 
single  generation. 


LESSON  X 

PARAMGECIUM,    STYLONYCHIA,    AND    OXYTRICHA 

IT  will  have  been  noticed  with  regard  to  the  simple  uni- 
cellular organisms  hitherto  considered  that  all  are  not  equally 
simple :  that  Protamceba  (Fig.  2,  p.  9)  and  Micrococcus 
(Fig.  15,  p.  86)  may  be  considered  as  the  lowest  of  all, 
and  that  the  others  are  raised  above  these  forms  in  the  scale 
of  being  in  virtue  of  the  possession  of  nucleus  or  contractile 
vacuole,  or  of  flagella,  or  even,  as  in  the  case  of  Euglena 
(Fig.  5,  p.  45),  of  a  mouth  or  gullet. 

Thus  we  may  speak  of  any  of  the  organisms  already 
studied  as  relatively  "  high  "  or  "  low  "  with  regard  to  the 
rest :  the  lowest  or  least  differentiated  forms  being  those 
which  approach  most  nearly  to  the  simplest  conception  of  a 
living  thing — a  mere  lump  of  protoplasm  :  the  highest  or 
most  differentiated  those  in  which  the  greatest  complication 
of  structure  has  been  attained.  It  must  be  remembered, 
too,  that  this  increase  in  structural  complexity  is  always 
accompanied  by  some  degree  of  division  of  physiological 
labour,  or,  in  other  words,  that  morphological  and  physio- 
logical differentiation  go  hand  in  hand. 

We  have  now  to  consider  certain  organisms  in  which  this 
differentiation  has  gone  much  further ;  which  have,  in  fact, 


LESS,    x  GENERAL   CHARACTERS  107 

acquired  many  of  the  characteristics  of  the  higher  animals 
and  plants  while  remaining  unicellular.  The  study  of  several 
of  these  more  or  less  highly  differentiated  though  unicellular 
forms  will  occupy  the  next  seven  Lessons. 

It  was  mentioned  above  that,  in  the  earlier  stages  of  the 
putrefaction  of  an  organic  infusion,  bacteria  only  were 
found,  and  that  later,  monads  made  their  appearance.  Still 
later  organisms  much  larger  than  monads  are  seen,  generally 
of  an  ovoidai  form,  moving  about  very  quickly,  and  seen  by 
the  use  of  a  high  power  to  be  covered  with  innumerable  fine 
cilia.  These  are  called  ciliate  Infusoria,  in  contradistinction 
to  monads,  which  are  often  known  as  flagellate  Infusoria : 
many  kinds  are  common  in  putrefying  infusions,  some  occur 
in  the  intestines  of  the  higher  animals,  while  others  are 
among  the  commonest  inhabitants  of  both  fresh  and  salt 
water.  Five  genera  of  these  Infusoria  will  form  the  subjects 
of  this  and  the  four  following  Lessons. 

A  very  common  ciliate  infusor  is  the  beautiful  "  slipper 
animalcule,"  Paramczcium  aurelia,  which  from  its  compara- 
tively large  size  and  from  the  ease  with  which  all  essential 
points  of  its  organization  can  be  made  out  is  a  very  con- 
venient and  interesting  object  of  study. 

Compared  with  the  majority  of  the  organisms  which  have 
come  under  our  notice  it  may  fairly  be  considered  as  gigantic, 
being  no  less  than  \ — J  mm.  (200 — 26o/x)  in  length  :  in 
fact  it  is  just  visible  to  the  naked  eye  as  a  minute  whitish 
speck. 

Its  form  (Fig.  20  A)  can  be  fairly  well  imitated  by  making 
out  of  clay  or  stiff  dough  an  elongated  cylinder  rounded  at 
one  end  and  bluntly  pointed  at  the  other ;  then  giving  the 
broader  end  a  slight  twist  ;  and  finally  making  on  the  side 


:B 


FIG.  20. — Paramcecium  aurelia. 

A,  the  living  animal  from  the  ventral  aspect,  showing  the  covering  of 
cilia,  the  buccal  groove  (to  the  right)  ending  posteriorly  in  the  mouth 


LESS,  x  MOVEMENTS  109 

(mth}  and  gullet  (gut]  ;  several  food  vacuoles  (/.  vac),  an'd  the  two 
contractile  vacuoles  (c.  vac}. 

B,  the  same  in  optical  section,  showing  cuticle  (at},  cortex  (cort],  and 
medulla  (med) ;  buccal  groove  (buc.  gr},  mouth,  and  gullet  (gul)  ; 
numerous  food  vacuoles  (f.  vac)  circulating  in  the  direction  indicated 
by  the  arrows,  and  containing  particles  of  indigo,  which  are  finally 
ejected  at  an  anal  spot ;  meganucleus  (nu),  micronucleus  (pa.  nu),  and 
trichocysts,  some  of  which  (trch}  are  shown  with  their  threads  ejected. 

The  scale  to  the  right  of  this  figure  applies  to  A  and  B. 

c,  a  specimen  killed  with  osmic  acid,  showing  the  ejection  of  tricho- 
cyst-threads,  which  project  considerably; beyond  the  cilia. 

D,  diagram  of  binary  fission  :  the  micronucleus  (pa.  nu}  has  already 
divided,  the  nucleus  (mi)  is  in  the  act  of  dividing. 

(D  after  Lankester.) 

rendered  somewhat  concave  by  the  twist  a  wide  shallow 
groove  beginning  at  the  broad  end  and  gradually  narrowing 
to  about  the  middle  of  the  body,  where  it  ends  in  a  tolerably 
deep  depression. 

The  grove  is  called  the  buccal  groove  (Fig.  20,  A  &  B, 
buc.  gr)  :  at  the  narrow  end  is  a  small  aperture  the  mouth 
(mth\  which,  like  the  mouth  of  Euglena  (Fig.  5),  leads  into 
the  soft  internal  protoplasm  of  the  body.  The  surface  of 
the  creature  on  which  the  groove  is  placed  is  distinguished 
as  the  ventral  surface,  the  opposite  surface  being  upper  or 
dorsal ;  the  broad  end  is  anterior,  the  narrow  end  posterior, 
the  former  being  directed  forwards  as  the  animalcule  swims. 
These  descriptive  terms  being  decided  upon,  it  will  be  seen 
from  Fig.  20  A,  that  the  buccal  groove  begins  on  the  left  side 
of  the  body,  and  gradually  curves  over  to  the  middle  of  the 
ventral  surface. 

As  the  animal  swims  its  form  is  seen  to  be  permanent, 
exhibiting  no  contractions  of  either  an  amoeboid  or  a 
euglenoid  nature.  It  is  however  distinctly  flexible,  often 
being  bent  in  one  or  other  direction  when  passing  between 
obstacles  such  as  entangled  masses  of  weed.  This  perma- 
nence of  contour  is  due  to  the  presence  of  a  tolerably  firm 
though  delicate  cuticle  (cu)  which  invests  the  whole  surface. 


i  io     PARAMCECIUM,  STYLONYCHIA,  OXYTRICHA     LESS, 

The  protoplasm  thus  enclosed  by  the  cuticle  is  distinctly 
divisible  into  two  portions— an  external  somewhat  dense  layer, 
the  cortical  layer  or  cortex  (cort\  and  an  internal  more  fluid 
material,  the  medullary  substance  or  medulla  (med}.  It  will  be 
remembered  that  a  somewhat  similar  distinction  of  the 
protoplasm  into  two  layers  is  exhibited  by  Amoeba  (p.  3),  the 
ectosarc  being  distinguished  from  the  endosarc  simply  by 
the  absence  of  granules.  In  Paramoecium  the  distinction  is 
a  far  more  fundamental  one  :  the  cortex  is  radially  striated 
and  is  comparatively  firm  and  dense,  while  the  medulla  is 
granular  and  semi-fluid,  as  may  be  seen  from  the  fact  that 
food  particles  (/.  vac,  see  below,  p.  112,)  move  freely  in  it, 
whereas  they  never  pass  into  the  cortex.  It  has  recently  been 
found  that  the  medulla  has  a  reticular  structure  similar  to 
that  of  the  protoplasm  of  the  ordinary  animal  cell  (Fig.  9, 
p.  62),  consisting  of  a  delicate  granular  network  the  meshes 
of  which  are  filled  with  a  transparent  material.  In  the 
cortex  the  meshes  of  the  network  are  closer,  and  so  form  a 
comparatively  dense  substance.  The  cortex  also  exhibits 
a  superficial  oblique  striation,  forming  what  is  called  the 
myophan  layer. 

The  mouth  (mtti)  leads  into  a  short  funnel-like  tube,  the 
gullet  (gul),  which  is  lined  by  cuticle  and  passes  through  the 
cortex  to  end  in  the  soft  medulla,  thus  making  a  free  com- 
munication between  the  latter  and  the  external  water. 

The  cilia  with  which  the  body  is  covered  are  of  approxi- 
mately equal  size,  quite  short  in  relation  to  the  entire 
animal,  and  arranged  in  longitudinal  rows  over  the  whole 
outer  surface.  They  consist  of  prolongations  of  the  cortex, 
and  each  passes  through  a  minute  perforation  in  the  cuticle, 
They  are  in  constant  rhythmical  movement,  and  are  thereby 
distinguished  from  the  flagella  of  Hsematococcus,  Euglena, 
&c.,  which  exhibit  more  or  less  intermittent  lashing  move- 


x  CONTRACTILE  VACUOLES  in 

ments  (see  p.  25,  note,  and  p.  59).  Their  rapid  motion  and 
minute  size  make  them  somewhat  difficult  to  see  while  the 
Paramoecium  is  alive  and  active,  but  after  death  they  are 
very  obvious,  and  look  quite  like  a  thick  covering  of  fine 
silky  hairs. 

Near  the  middle  of  the  body,  in  the  cortex,  is  a  large  oval 
nucleus  (B,  nu],  which  is  peculiar  in  taking  on  a  uniform  tint 
when  stained,  showing  none  of  the  distinction  into  chroma- 
tin  and  nuclear  matrix  which  is  so  marked  a  feature  in  many 
of  the  nuclei  we  have  studied  (see  especially  Fig.  i,  p.  2,  and 
Fig.  9,  p.  62).  It  has  also  a  further  peculiarity  :  against  one 
side  of  it  is  a  small  oval  structure  (pa.  nu)  which  is  also  deeply 
stained  by  magenta  or  carmine.  This  is  the  micronudeus  :  it 
is  to  be  considered  as  a  second,  smaller  nucleus,  the  larger 
body  being  distinguished  as  the  meganudeus. 

There  are  two  contractile  vacuoles  (c.  vac),  one  situated  at 
about  a  third  of  the  entire  length  from  the  anterior  end  of  the 
body,  the  other  at  about  the  same  distance  from  the  posterior 
end  :  they  occur  in  the  cortex. 

The  action  of  the  contractile  vacuoles  is  very  beautifully 
seen  in  a  Paramoecium  at  rest  :  it  is  particularly  striking  in  a 
specimen  subjected  to  slight  pressure  under  a  cover  glass, 
but  is  perfectly  visible  in  one  which  has  merely  temporarily 
suspended  its  active  swimming  movements.  It  is  then  seen 
that  during  the  diastole,  or  phase  of  expansion  of  each  vacuole, 
a  number — about  six  to  ten — of  delicate  radiating,  spindle- 
shaped  spaces  filled  with  fluid  appear  round  it,  like  the  rays 
of  a-star  (upper  vacuole  in  A  &  B)  :  the  vacuole  itself  contracts 
or  performs  its  systole,  completely  disappearing  from  view, 
and  immediately  afterwards  the  radiating  canals  flow  together 
and  re-fill  it,  becoming  themselves  emptied  and  therefore 
invisible  for  an  instant  (lower  vacuole  in  A  &  B)  but  rapidly 
appearing  once  more.  There  seems  to  be  no  doubt  that  the 


H2     PARAMCECIUM,  STYLONYCHlA,  OXYTRICHA     LESS. 

water  taken  in  with  the  food  is  collected  into  these  canals, 
emptied  into  the  vacuole,  and  finally  discharged  into  the 
surrounding  medium. 

The  process  of  feeding  can  be  very  conveniently  studied 
in  Paramcecium  by  placing  in  the  water  some  finely-divided 
carmine   or   indigo.     When   the   creature   comes  into  the 
neighbourhood  of  the  coloured  particles,  the  latter  are  swept 
about  in  various  directions  by  the  action  of  the  cilia  :  some 
of  these  are  however  certain  to  be  swept  into  the  neighbour- 
hood of  the  buccal  groove  and  gullet,  the  cilia  of  which  all 
work  downwards,  i.e.  towards  the  inner  end  of  the  gullet. 
The  grains  of  carmine  are  thus  carried  into  the  gullet,  where 
for  an  instant  they  lie  surrounded  by  the  water  of  which  it  is 
full  :  then,  instantaneously,  probably  by  the  contraction  of 
the  tube  itself,  the  animalcule  performs  a  sort  of  gulp,  and 
the  grains  with  an  enveloping  globule  of  water  or  food-vacuole 
are  forced  into  the  medullary  protoplasm.     This  process  is 
repeated  again  and  again,  so  that   in  any  well-nourished 
Paramcecium  there  are  to  be  seen  numerous  globular  spaces 
filled  with  water  and  containing  particles  of  food — or  in  the 
present  instance  of  carmine  or  indigo.     At  every  gulp  the 
newly  formed  food-vacuole  pushes,  as  it  were,  its  predecessor 
before  it  :  contraction  of  the  medullary  protoplasm  also  takes 
place  in  a  definite  direction,  and  thus  a  circulation  of  food- 
vacuoles  is  produced,  as  indicated  in  Fig.  20,  B,  by  arrows. 
After  circulating  in  this  way  for  some  time  the  water  of  the 
food-vacuoles  is  gradually  absorbed,  being  ultimately  excreted 
by  the  contractile  vacuoles,  so  that  the  contained  particles 
come  to  lie  in  the  medulla  itself  (refer  to  figure).     The  circu- 
lation still  continues,  until  finally  the  particles  are  brought  to 
a  spot  situated  about  half-way  between  -  the  mouth  and  the 
posterior  end  of  the  body :  here  if  carefully  watched  they 
are  seen  to  approach  the  surface  and  then  to  be  suddenly 


x  TRICHOCYSTS  113 

ejected.  The  spot  in  question  is  therefore  to  be  looked 
upon  as  a  potential  anus,  or  aperture  for  the  egestion  of 
faeces  or  undigested  food-matters.  It  is  a  potential  and  not 
an  actual  anus,  because  it  is  not  a  true  aperture  but  only  a 
soft  place  in  the  cortex  through  which  by  the  contractions 
of  the  medulla  solid  particles  are  easily  forced. 

Of  course  when  Paramcecium  ingests,  as  it  usually  does, 
not  carmine  but  minute  living  organisms,  the  latter  are 
digested  as  they  circulate  through  the  medullary  protoplasm, 
and  only  the  non-nutritious  parts  cast  out  at  the  anal  spot. 
It  has  been  found  by  experiment  that  this  infusor  can 
digest  not  only  proteids  but  also  starch  and  perhaps  fats. 
The  starch  is  probably  converted  into  dextrin,  a  carbo- 
hydrate having  the  same  formula  (C6H10O5)  but  soluble 
and  diffusible.  Oils  or  fats  seem  to  be  partly  converted 
into  fatty  acids  and  glycerine.  The  nutrition  of  Paramcecium 
is  therefore  characteristically  holozoic. 

It  was  mentioned  above  (p.  108)  that  the  cortex  is  ra- 
dially striated  in  optical  section.  Careful  examination  with 
a  very  high  power  shows  that  this  appearance  is  due  to  the 
presence  in  the  cortex  of  minute  spindle-shaped  bodies  (A 
and  B,  trcJi)  closely  arranged  in  a  single  layer  and  perpen- 
dicular to  the  surface.  These  are  called  trichocysts. 

When  a  Paramoecium  is  killed,  either  by  the  addition  of 
osmic  acid  or  some  other  poisonous  reagent  or  by  simple 
pressure  of  the  cover  glass,  it  frequently  assumes  a  remark- 
able appearance.  Long  delicate  threads  suddenly  appear, 
projecting  from  its  surface  in  all  directions  (c)  and  looking 
very  much  as  if  the  cilia  had  suddenly  protruded  to  many 
times  their  original  length.  But  these  filaments  have  really 
nothing  to  do  with  the  cilia  ;  they  are  contained  under  or- 
dinary circumstances  in  the  trichocysts,  probably  coiled  up ; 
and  by  the  contraction  of  the  cortex  consequent  upon  any 

I 


ii4    PARAMCECIUM,  STYLONYCHIA,  OXYTRICHA  LESS.X 

sudden  irritation  they  are  projected  in  the  way  indicated. 
In  Fig.  20  B,  a  few  trichocysts  (trcJi)  are  shown  in  the  ex- 
ploded condition,  i.e.  with  the  threads  protruded.  Most 
likely  these  bodies  are  weapons  of  offence  like  the  very 
similar  structures  (nematocysts)  found  in  polypes  (see  Lesson 
XXII.  Fig  51). 

Paramoecium  multiplies  by  simple  fission,  the  division  of 
the  body  being  always  preceded  by  the  elongation  and 
subsequent  division  of  the  mega-  and  micronucleus  (Fig. 
20,  D).  Division  of  the  meganucleus  is  direct,  that  of  the 
micronucleus  indirect,  i.e.  takes  place  by  karyokinesis. 

Conjugation  also  occurs,  usually  after  multiplication  by 
fission  has  gone  on  for  some  time,  but  the  details  and  the 
results  of  the  process  are  very  different  from  what  are  found 
to  obtain  in  Heteromita  (p.  62).  Two  Paramcecia  come 
into  contact  by  their  ventral  faces  (Fig.  2 1,  A)  and  the  mega- 
nucleus  (mg.  nu)  of  each  gradually  breaks  up  into  minute 
fragments  (D — G)  which  are  either  absorbed  into  the  proto- 
plasm or  ejected.  At  the  same  time  the  micronucleus 
(mi.  nu)  divides,  by  karyokinesis,  and  the  process  is  repeated, 
the  result  being  that  each  gamete  contains  four  micro- 
nuclei  (B).  Two  of  these  become  absorbed  and  disappear, 
(c  mi.  nu',  mi.  nu")  of  the  remaining  two  one  is  now  distin- 
guished as  the  active  pronudeus,  the  other  as  the  stationary 
pronudeus.  Next,  the  active  pronucleus  of  each  gamete 
passes  into  the  body  of  the  other  (c)  and  fuses  with  its 
stationary  pronucleus  (D):  in  this  way  each  gamete  con- 
tains a  single  nuclear  body,  the  conjugation-nucleus  (E), 
formed  by  the  union  of  two  similar  pronuclei  one  of 
which  is  derived  from  another  individual.  It  is  this 
fusion  of  two  nuclear  bodies,  one  from  each  of  the  con- 
jugating cells,  which  is  the  essential  part  of  the  whole 


Mg.nu. 


FlG.  21. — Stages  in  the  Conjugation  of  Paramcecium. 

A,  Commencement  of  conjugation  :  the  meganuclei  (mg.  mi)  of  the 
two  gametes  are  almost  unaltered  :  the  micronuclei  (mi.  nu)  are  in  an 
early  stage  of  karyokinesis. 

B,  The  micronuclei  have  divided  twice,  each  gamete  now  containing 
four. 

C,  Two   of  the  micronuclei    (mi.   nu',   mi.  nu")of  each  gamete   are 
degenerating  :   of  the  remaining   two   one — the  active   pronucleus — is 
passing  into  the  other  gamete. 

D,  The  active  pronucleus  of  each  gamete  has  passed  into  the  other 
gamete  and  is  conjugating  with  its  stationary  pronucleus.     The  mega- 
nucleus  (mg.  mi)  has  begun  to  break  up. 

E,  Each  gamete  contains  a  single  conjugation-nucleus  formed  by  the 
union  of  its  own  stationary  pronucleus  with  the  active  pronucleus  of 
the  other  gamete.   On  the  right  side  the  conjugation-nucleus  is  beginning 
to  divide. 

F,  Conjugation  is  over  and  only  one  of  the  separated  gametes  is  shown. 
It  contains  the  fragments  of  the  meganucleus  (dotted)  and  four  nuclear 
bodies  (mi.  nu)  produced  by  the  division  and  re-division  of  the  con- 
jugation-nuc'eus. 

G,  Two  of  the  products  of  division  of  the  conjugation-nucleus  (Mg.  nu) 
are  enlarging  to  form  mega-nuclei,  the  other  two  (Mi.nu)  are  taking  on 
the  characters  of  micronuclei. 

(After  H'ortwig.) 

I    2 


ii6     PARAMGECTUM,  STYLONYCHIA,  OXYTRTCHA     LESS. 

process.  Soon  after  this  the  gametes  separate  from  one 
another  and  begin  once  more  to  lead  an  independent 
existence;  the  conjugation  nucleus  of  each  undergoing 
a  twice  repeated  process  of  division,  the  infusor  thus 
acquiring  four  small  nuclei  (F).  Two  of  these  enlarge 
and  take  on  the  character  of  meganuclei  (G,  Mg.  nu\  the 
other  two  remaining  unaltered  and  having  the  character  of 
micronuclei  (Mi.  nu).  Thus  shortly  after  the  completion 
of  conjugation  each  individual  contains  two  mega-  and 
two  micronuclei  all  derived  from  the  conjugation-nucleus. 
Ordinary  transverse  fission  now  takes  place,  as  described 
in  the  preceding  paragraph,  each  of  the  two  daughter  cells 
having  one  mega-  and  one  micronucleus,  and  thus  the 
normal  form  of  the  species  is  re-acquired. 

It  will  be  noticed  that,  in  the  present  instance,  conjuga- 
tion is  not  a  process  of  multiplication  :  it  has  been 
ascertained  that  during  the  time  two  infusors  are  conju- 
gating each  might  have  produced  several  thousand  offspring 
by  continuing  to  undergo  fission  at  the  usual  rate.  The 
importance  of  the  process  lies  in  the  exchange  of  nuclear 
material  between  the  two  conjugating  individuals  :  without 
such  exchange  these  organisms  have  been  shown  to  undergo 
a  gradual  process  of  senile  decay  characterized  by  diminution 
in  size  and  degeneration  in  structure. 

Another  ciliated  infusor  common  in  stagnant  water  and 
organic  infusions  is  Stylonychia  mytilus,  an  animalcule  vary- 
ing from  y^rnm.  to  |mm. 

Like  Paramcecium  it  is  often  to  be  seen  swimming  rapidly 
in  the  fluid,  but  unlike  that  genus  it  frequently  creeps  about, 
almost  like  a  wood-louse  or  a  caterpillar,  on  the  surface 
of  the  plants  or  other  solid  objects  among  which  it  lives. 
In  correspondence  with  this,  instead  of  being  nearly 


CILIA   OF   STYLONYCHIA 


117 


cylindrical,  it  is  flattened  on  one — the  ventral — side, 
and  is  thus  irregularly  plano-convex  in  transverse  section 
(Fig.  22,  c). 

It  resembles  Paramcecium  in  general  structure  (compare 


FIG.  22. — A,  Stylonychia  mytihis,  ventral  aspect,  showing  the  buccal 
groove  (buc.  gr.}  and  mouth  (mt/i),  two  nuclei  (mi,  nu),  contractile 
vacuole  (c.vac),  and  cilia  differentiated  into  hook-like  (h.  «'),  bristle- 
like  (b.  ci),  plate-like  ( p.  ci),  and  fan-like  (in.  ci)  organs. 

B,  one  of  the  plate-like  cilia  of  the  same  (/.  ci  in  A),  showing  its 
frayed  extremity. 

C,  transverse  section  of  Gastrostyla,  a  form  allied  to  Stylonychia, 
showing  buccal  groove  (btic.  gr.),  small  dorsal  cilia  (d.  ci},  hook-like 
cilium  (h.  ci),  and  the  various  cilia  of  the  buccal  groove,  including  an 
expanded  fan-like  organ  (m.  ci).     A  and  B  after  Claparede  and  Lach- 
mann  :  c  after  Sterki. 

Fig.  22,  A,  with  Fig.  20,  A)  ;  but  owing  to  the  absence  of 
trichocysts  the  distinction  between  cortex  and  medulla  is 
less  obvious  :  moreover,  it  has  two  nuclei  («//,  nu)  and  only 
one  contractile  vacuole  (c.  vac]. 


ii8     PARAMCECIUM,  STYLONYCHIA,  OXYTRICHA      LESS. 

But  it  is  in  the  character  of  its  cilia  that  Stylonychia 
is  most  markedly  distinguished  from  Paramoecium  :  these 
structures,  instead  of  being  all  alike  both  in  form  and  size, 
are  modified  in  a  very  extraordinary  way. 

On  the  dorsal  surface  the  cilia  are  represented  only  by 
very  minute  processes  of  the  cortex  (c,  d.  ci.)  set  in  longi- 
tudinal grooves  and  exhibiting  little  movement.  It  seems 
probable  that  these  are  to  be  looked  upon  as  vestigial  or 
rudimentary  cilia,  i.e.,  as  the  representatives  of  cilia  which 
were  of  the  ordinary  character  in  the  ancestors  of  Stylo- 
nychia, but  which  have  undergone  partial  atrophy,  or 
diminution  beyond  the  limits  of  usefulness,  in  correspond- 
ence with  the  needs  of  an  animalcule  which  has  taken  to 
creeping  on  its  ventral  surface,  instead  of  swimming  freely 
and  so  using  all  its  cilia  equally. 

On  the  other  hand,  the  cilia  on  the  ventral  surface  have 
undergone  a  corresponding  enlargement  or  hypertrophy. 
Near  the  anterior  and  posterior  ends  and  about  the  middle  are 
three  groups  of  cilia  of  comparatively  immense  size,  shaped 
either  like  hooks  (h.  ci.),  or  like  flattened  rods  frayed  at 
their  ends  (p.  a,  and  B).  All  these  structures  neither  vibrate 
rhythmically  like  ordinary  cilia  nor  perform  lashing  move- 
ments like  flagella,  but  move  at  the  base  only  like  one- 
jointed  legs.  The  movement  is  under  the  animal's  control, 
so  that  it  is  able  to  creep  about  by  the  aid  of  these  hooks 
and  plates  in  much  the  same  way  as  a  caterpillar  by  means 
of  its  legs. 

Notice  that  we  have  here  a  third  form  of  contractility :  in 
amoeboid  movement  there  is  an  irregular  flowing  of  the  pro- 
toplasm (pp.  4  and  10) ;  in  ciliary  movement  a  flexion  of 
a  protoplasmic  filament  from  side  to  side  (p.  33) ;  while 
in  the  present  case  we  have  sudden  contractions  taking  place 
at  irregular  intervals.  The  movements  of  these  locomotor 
hooks  and  plates  are  therefore  very  similar  to  the  muscular 


x  DIFFERENTIATION   OF   CILIA  119 

contractions  to  which  the  movements  of  the  higher  animals 
are  due  :  it  cannot  be  said  that  definite  muscles  are  present 
in  Stylonychia,  but  the  protoplasm  in  certain  regions  of  the 
unicellular  body  is  so  modified  as  to  be  able  to  perform  a 
sudden  contraction  in  a  definite  direction.  The  nature  of 
muscular  contraction  will  be  further  discussed  in  the  next 
Lesson  (see  p.  130). 

The  remainder  of  the  ventral  surface,  with  the  exception 
of  the  buccal  groove,  is  bare,  but  along  each  side  of  the 
margin  is  a  row  of  large  vibratile  cilia,  of  which  three  at 
the  posterior  end  are  modified  into  long,  stiff,  bristle-like 
processes  (A,  b.  a). 

There  is  also  a  special  differentiation  of  the  cilia  of  the 
buccal  groove  (buc.gr.'].  On  its  left  side  is  a  single  row  of 
very  large  and  powerful  cilia  (A  and  c,  m.  cf]  which  are  the 
chief  organs  for  causing  the  food-current  as  well  as  the 
main  swimming-organs  :  each  has  the  form  of  a  triangular 
fan-like  plate  (c,  m.  a).  On  the  right  side  of  the  buccal 
groove  is  a  row  of  smaller  but  still  large  cilia  of  the  ordinary 
form,  and  in  the  interior  of  the  gullet  a  row  of  extremely 
delicate  cilia  which  aid  in  forcing  particles  of  food  down  the 
gullet  into  the  medulla. 

In  Stylonychia  and  allied  genera  intermediate  forms  are 
found  between  these  peculiar  hooks,  plates,  bristles,  and 
fans,  and  ordinary  cilia ;  from  which  we  may  conclude  that 
these  diverse  appendages  are  to  be  looked  upon  as  highly 
modified  or  differentiated  cilia.  Probably  they  have  been 
evolved  in  the  course  of  time  from  ordinary  cilia,  and  on 
the  principle  that  the  more  complicated  or  specialized 
organisms  are  descended  from  simpler  or  more  generalized 
forms  (see  Lesson  XIII.),  we  may  consider  Stylonychia  as 
the  highly-specialized  descendant  of  some  uniformly-ciliated 
progenitor. 


120     PARAMCECIUM,  STYLONYCHIA,  OXYTRICHA  LESS.X 

A  third  genus  of  ciliated  Infusoria  must  be  just  referred 
to  in  concluding  the  present  Lesson.  We  have  seen  how 
the  nucleus  of  a  Paramoecium  which  has  just  conjugated 
breaks  up  and  apparently  disappears  (Fig.  21,  K — o). 
In  Oxytricha,  a  genus  closely  resembling  Stylonychia,  the 
two  nuclei  have  been  found  to  break  up  into  a  large  number 
of  minute  granules  (Fig.  23),  which  can  be  seen  only  after 


FIG.    23. — Oxytricha  flava,   killed  and  stained,   showing    the  frag- 
mentation of  the  nuclei.     (After  Gruber. ) 

careful  staining  and  by  the  use  of  high  magnifying  powers. 
This  process  is  called  fragmentation  of  the  nucleus;  in 
other  cases  it  goes  even  further,  and  the  nucleus  is  reduced 
to  an  almost  infinite  number  of  chromatin  granules  only  just 
visible  under  the  highest  powers.  From  this  it  seems  very 
probable  that  organisms  which,  like  Protamceba  (p.  9)  and 
Protomyxa  (p.  49),  appear  non-nucleate,  are  actually  pro- 
vided with  a  nucleus  in  this  pulverized  condition,  and  that 
a  nucleus  in  some  form  or  other  is  an  essential  constituent 
of  the  cell. 


UNIVERSITY 


LESSON  XI 

OPALINA 

THE  large  intestine  of  the  common  frog  often  contains 
numbers  of  ciliate  Infusoria  belonging  to  two  or  three 
genera.  One  of  these  parasitic  animalcules,  called  Opalina 
ranarum,  will  now  be  described.  It  is  easily  obtained  by 
killing  a  frog,  opening  the  body,  making  an  incision  in  the 
rectum,  and  spreading  out  a  little  of  its  blackish  contents  in 
a  drop  of  water  on  a  slide. 

Opalina  has  a  flattened  body  with  an  oval  outline  (Fig. 
24,  A,  B),  and  full-sized  specimens  may  be  as  much  as  one 
millimetre  in  length.  The  protoplasm  is  divided  into  cortex 
and  medulla,  and  is  covered  with  a  cuticle,  and  the  cilia  are 
equal-sized  and  uniformly  arranged  in  longitudinal  rows  over 
the  whole  surface  (A). 

On  a  first  examination  no  nucleus  is  apparent,  but  after 
staining  a  large  number  of  nuclei  can  be  seen  (B,  ;/#),  each 
being  a  globular  body  (c,  i),  consisting  of  a  nuclear  matrix 
surrounded  by  a  membrane  and  containing  a  coil  or  net- 
work of  chromatin.  These  nuclei  multiply  within  the  body 
of  the  infusor,  and  in  so  doing  pass  through  the  various 
changes  characteristic  of  karyokinesis  or  indirect  nuclear 


122  OPALINA  LESS. 

division    (compare    Fig.    10,    p.    64,    with    Fig.    23)  :    the 


FlG.  24. — Opalina  ranarum. 

A,  living  specimen,  surface  view,  showing  longitudinal  rows  of  cilia. 

B,  the  same,  stained,  showing  numerous  nuclei  (nu)  in  various  stages 
of  division. 

c,   I — 6,  stages  in  nuclear  division. 

D,  longitudinal  fission. 

E,  transverse  fission. 

F,  the  same  in  a  specimen  reduced  in  size  by  repeated  division. 

G,  final  product  of  successive  divisions. 
H,  encysted  form. 

i,  uninucleate  form  produced  fi'om  cyst. 

K,  the  same  after  multiplication  of  the  nucleus  has  begun. 

(A — C,  after  Pfitzner  ;  D — K,  from  Saville  Kent  after  Zeller.) 

chromatin  breaks  up  (c,  2),  a  spindle  is  formed  with  the 
chromosomes  across  its  equator  (3),  the  chromosomes  pass 


XJ  PARASITISM  123 

to  the  poles  of  the  spindle  (4,  5),  and  the  nucleus  becomes 
constricted  (5),  and  finally  divides  into  two  (6). 

The  presence  of  numerous  nuclei  in  Opalina  is  a  fact 
worthy  of  special  notice.  The  majority  of  the  organisms 
we  have  studied  are  uninucleate  as  well  as  unicellular  :  the 
higher  animals  and  plants  we  found  (Lesson  VI.)  to  consist 
of  numerous  cells  each  with  a  nucleus,  so  that  they  are 
multicellular  and  multinucleate :  Opalina,  on  the  other 
hand,  is  multinucleate  but  unicellular.  An  approach  to 
this  condition  of  things  is  furnished  by  Stylonychia,  which  is 
unicellular  and  binucleate  (Fig.  24,  A),  but  the  only  organisms 
we  have  yet  studied  in  which  numerous  nuclei  of  the  ordi- 
nary character  occur  in  an  undivided  mass  of  protoplasm  are 
the  Mycetozoa  (p.  52),  and  in  them  the  multinucleate  con- 
dition of  the  plasmodium  is  largely  due  to  its  being  formed 
by  the  fusion  of  separate  cells,  while  in  Opalina  it  is  due,  as 
we  shall  see,  to  the  repeated  binary  fission  of  an  originally 
single  nucleus. 

There  is  no  contractile  vacuole,  and  no  trace  of  either 
mouth  or  gullet,  so  that  the  ingestion  of  solid  food  is  impos- 
sible. The  creature  lives,  as  already  stated,  in  the  intestine 
of  the  frog  :  it  is  therefore  an  internal  parasite,  or  endo- 
parasite,  having  the  frog  as  its  host.  The  intestine  contains 
the  partially-digested  food  of  the  frog,  and  it  is  by  the  ab- 
sorption of  this  that  the  Opalina  is  nourished.  Having  no 
mouth,  it  feeds  solely  by  imbibition  :  whether  it  performs 
any  kind  of  digestive  process  itself  is  not  certainly  known, 
but  the  analogy  of  other  mouthless  parasites  leads  us  to 
expect  that  it  simply  absorbs  food  ready  digested  by  its  host, 
upon  which  it  is  dependent  for  a  constant  supply  of  soluble 
and  diffusible  nutriment. 

Thus  Opalina,  in  virtue  of  its  parasitic  mode  of  life,  is 
saved  the  performance  of  certain  work — the  work  of  diges- 


124  OPALINA  LESS. 

tion,  that  work  being  done  for  it  by  its  host.  This  is  the 
essence  of  internal  parasitism  :  an  organism  exchanges  a  free 
life,  burdened  with  the  necessity  of  finding  food  for  itself,  for 
existence  in  the  interior  of  another  organism,  on  which,  in 
one  way  or  another,  it  levies  blackmail. 

Note  the  close  analogy  between  the  nutrition  of  an  internal 
parasite  like  Opalina  and  the  saprophytic  nutrition  of  a 
monad  (p.  39).  In  both  the  organism  absorbs  proteids 
rendered  soluble  and  diffusible,  in  the  one  case  by  the 
digestive  juices  of  the  host,  in  the  other  by  the  action  of 
putrefactive  bacteria. 

The  reproduction  of  Opalina  presents  certain  points  of 
interest,  largely  connected  with  its  peculiar  mode  of  life.  It 
is  obvious  that  if  the  Opalinae  simply  went  on  multiplying, 
by  fission  or  otherwise,  in  the  frog's  intestine,  the  population 
would  soon  outgrow  the  means  of  subsistence  :  moreover, 
when  the  frog  died  there  would  be  an  end  of  the  parasites. 
What  is  wanted  in  this  as  in  other  internal  parasites  is  some 
mode  of  multiplication  which  shall  serve  as  a  means  of  dis- 
persal^ or  in  other  words,  enable  the  progeny  of  the  parasite 
to  find  their  way  into  the  bodies  of  other  hosts,  and  so  start 
new  colonies  instead  of  remaining  to  impoverish  the  mother 
country. 

Opalina  multiplies  by  a  somewhat  peculiar  process  of 
binary  fission  :  an  animalcule  divides  in  an  oblique  direction 
(Fig.  24,  D),  and  then  each  half,  instead  of  growing  to  the 
size  of  the  parent  cell,  divides  again  transversely  (E).  The 
process  is  repeated  again  and  again  (F),  the  plane  of  division 
being  alternately  oblique  and  transverse,  until  finally  small 
bodies  are  produced  (G),  about  •%•§—$$  mm-  in  length,  and 
containing  two  to  four  nuclei. 

If  the  parent  cell  had  divided  simultaneously  into  a  num- 


xi  DEVELOPMENT  125 

her  of  these  little  bodies  the  process  would  have  been  one  of 
multiple  fission  :  as  it  is  it  forms  an  interesting  link  between 
simple  and  multiple  fission. 

Opalina  ranarum  multiplies  in  this  way  in  the  spring — i.e. 
during  the  frog's  breeding  season.  Each  of  the  small  pro- 
ducts of  division  (G)  becomes  encysted  (H),  and  in  this 
passive  condition  is  passed  out  with  the  frog's  excrement, 
probably  falling  on  to  a  water-weed  or  other  aquatic  object. 
Nothing  further  takes  place  unless  the  cyst  is  swallowed  by 
a  tadpole,  as  must  frequently  happen  when  these  creatures, 
produced  in  immense  numbers  from  the  frogs'  eggs,  browse 
upon  the  water-weeds  which  form  their  chief  food. 

Taken  into  the  tadpole's  intestine,  the  cyst  is  burst  or 
dissolved,  and  its  contents  emerge  as  a  lanceolate  mass  of 
protoplasm  (i),  containing  a  single  nucleus  and  covered  with 
cilia.  This,  as  it  absorbs  the  digested  food  in  the  intestine 
of  its  host,  grows,  and  at  the  same  time  its  nucleus  divides 
repeatedly  (K)  in  the  way  already  described,  until  by  the  time 
the  animalcule  has  attained  the  maximum  size  it  has  also 
acquired  the  large  number  of  nuclei  characteristic  of  the 
genus. 

Here,  then,  we  have  another  interesting  case  of  develop- 
ment (see  p.  43)  :  the  organism  begins  life  as  a  very  small 
uninucleate  mass  of  protoplasm,  and  as  it  increases  in  size 
increases  also  in  complexity  by  the  repeated  binary  fission 
of  its  nucleus. 


LESSON  XII 

VORTICELLA    AND    ZOOTHAMNIUM 

THE  next  organism  we  have  to  consider  is  a  ciliated  infusor 
even  commoner  than  those  described  in  the  two  previous 
lessons.  It  is  hardly  possible  to  examine  the  water  of  a 
pond  with  any  care  without  finding  in  it,  sometimes  attached 
to  weeds,  sometimes  to  the  legs  of  water-fleas,  sometimes  to 
the  sticks  and  stones  of  the  bottom,  numbers  of  exquisitely 
beautiful  little  creatures,  each  like  an  inverted  bell  with  a 
very  long  handle,  or  a  wine-glass  with  a  very  long  stem. 
These  are  the  well-known  "bell-animalcules;"  the  com- 
monest among  them  belong  to  various  species  of  the  genus 
Vorticflla. 

The  first  thing  that  strikes  one  about  Vorticella 
(Fig.  25,  A)  is  the  fact  that  it  is  permanently  fixed, 
like  a  plant,  the  proximal  or  near  end  of  the  stalk 
being  always  firmly  fixed  to  some  aquatic  object,  while  to 
the  distal  or  far  end  the  body  proper  of  the  animalcule  is 
attached. 

But  in  spite  of  its  peculiar  form  it  presents  certain  very 
obvious  points  of  resemblance  to  Paramoecium,  Stylonychia, 
and  Opalina.  The  protoplasm  is  divided  into  cortex  (  Fig. 
25,  c,  corf)  and  medulla  (med),  and  is  invested  with  a 


FlG.  25.  —  Vorticella. 

A,  living  specimen  fully  expanded,  showing  stalk  (st)  with  axial  fibre 
(ax.  f.),   peristome  (per),  disc  (d),    mouth    (mth},   gullet    (gull],  and 
contractile  vacuole. 

B,  the  same,  bent  on  its  stalk  and  with  the  disc  turned  away  from 
the  observer. 

c,  optical  section  of  the  same,  showing  cuticle  (cu\  cortex  (corf), 
medulla  (med),  nucleus  (»«),  gullet  (gull),  several  food-vacuoles,  and 
anus  (an),  as  well  as  the  structures  shown  in  A. 

D1,  a  half-retracted  and  D2  a  fully-retracted  specimen,  showing  the 
coiling  of  the  stalk  and  overlapping  of  the  disc  by  the  peristome. 


128  VORTICELLA   AND   ZOOTHAMNIUM  LESS. 

E1,  commencement  of  binary  fission  ;  E2,  completion  of  the  process  ; 
E3,  the  barrel-shaped  product  of  division  swimming  freely  in  the 
direction  indicated  by  the  arrow. 

F1,  a  specimen  dividing  into  a  megazooid  and  several  microzooids  (m)  ; 
F'2,  division  into  one  mega-  and  one  microzooid. 

G1,  G2,  two  stages  in  conjugation  showing  the  gradual  absorption  of 
the  microgamete  (m)  into  the  megagamete. 

H1,  multiple  fission  of  encysted  form,  the  nucleus  dividing  into  nume- 
rous masses  :  H2,  spore  formed  by  multiple  fission  ;  H3 — H7,  development 
of  the  spore  ;  H4  is  undergoing  binary  fission. 

(E — H  after  Saville  Kent.) 


delicate  cuticle  (cu).  There  is  a  single  contractile  vacuole 
(c.  vac]  the  movements  of  which  are  very  readily  made  out 
owing  to  the  ease  with  which  the  attached  organism  is  kept 
under  observation.  There  is  a  meganucleus  (nu)  remarkable 
for  its  elongated  band-like  form,  and  having  in  its  neighbour- 
hood a  small  rounded  micronucleus.  Cilia  are  also  present, 
but  the  way  in  which  they  are  disposed  is  very  peculiar  and 
characteristic.  To  understand  it  we  must  study  the  form 
of  the  body  a  little  more  closely. 

The  conical  body  is  attached  by  its  apex  or  proximal  end 
to  the  stalk  :  its  base  or  distal  end  is  expanded  so  as  to  form 
a  thickened  rim,  the  peristome  (per),  within  which  is  a  plate- 
like  body  elevated  on  one  side,  called  the  disc  (d),  and 
looking  like  the  partly  raised  lid  of  a  chalice.  Between  the 
raised  side  of  the  disc  and  the  peristome  is  a  depression,  the 
mouth  (mtti),  leading  into  a  conical  gullet  (gull). 

There  is  reason  for  thinking  that  the  whole  proximal 
region  of  Vorticella  answers  to  the  ventral  surface  of  Para- 
mcecium,  and  its  distal  surface  with  the  peristome  and 
disc  to  the  dorsal  surface  of  the  free-swimming  genus  :  the 
mouth  is  to  the  left  in  both. 

A  single  row  of  cilia  is  disposed  round  the  inner  border 
of  the  peristome,  and  continued  on  the  one  hand  down  the 
gullet,  and  on  the  other  round  the  elevated  portion  of  the 


xii  AXIAL   FIBRE  129 

disc ;  the  whole  row  of  cilia  thus  takes  a  spiral  direction. 
The  rest  of  the  body  is  completely  bare  of  cilia. 

The  movements  of  the  cilia  produce  a  very  curious 
optical  illusion  :  as  one  watches  a  fully-expanded  specimen 
it  is  hardly  possible  to  believe  that  the  peristome  and  disc 
are  not  actually  revolving — a  state  of  things  which  would 
imply  that  they  were  discontinuous  from  the  rest  of  the 
body.  As  a  matter  of  fact  the  appearance  is  due  to  the 
successive  contraction  of  all  the  cilia  in  the  same  direction, 
and  is  analogous  to  that  produced  by  a  strong  wind  on  a 
field  of  corn  or  long  grass.  The  bending  down  of  suc- 
cessive blades  of  grass  produces  a  series  of  waves  travelling 
across  the  field  in  the  direction  of  the  wind.  If  instead  of 
a  field  we  had  a  large  circle  of  grass,  and  if  this  were  acted 
upon  by  a  cyclone,  the  wave  would  travel  round  the  circle, 
which  would  then  appear  to  revolve. 

Naturally  the  movement  of  the  circlet  of  cilia  produces  a 
small  whirlpool  in  the  neighbourhood  of  the  Vorticella,  as 
can  be  seen  by  introducing  finely-powdered  carmine  into 
the  water.  It  is  through  the  agency  of  this  whirlpool  that 
food  particles  are  swept  into  the  mouth,  surrounded,  as  in 
Paramcecium,  by  a  globule  of  water :  the  food-vacuoles 
(/  vac)  thus  constituted  circulate  in  the  medullary  proto- 
plasm, and  the  non-nutritive  parts  are  finally  egested  at  an 
anal  spot  (an)  situated  near  the  base  of  the  gullet. 

The  stalk  (st)  consists  of  a  very  delicate,  transparent, 
outer  substance,  which  is  continuous  with  the  cuticle  of  the 
body  and  contains  a  delicate  axial  fibre  (ax.f.)  running 
along  it  from  end  to  end  in  a  somewhat  spiral  direction. 
This  fibre  is  a  prolongation  of  the  cortex  of  the  body 
(c,  ax.f.)  :  under  a  very  high  power  it  appears  granular  or 
delicately  striated,  the  striae  being  continued  into  the  cortex 
of  the  proximal  part  of  the  body. 


130  VORTICELLA   AND   ZOOTHAMNIUM  LESS. 

A  striking  characteristic  of  Vorticella  is  its  extreme 
irritability,  i.e.,  the  readiness  with  which  it  responds  to  any 
external  stimulus  (see  p.  10).  The  slightest  jar  of  the 
microscope,  the  contact  of  some  other  organism,  or  even  a 
current  of  water  produced  by  some  free-swimming  form  like 
Paramcecium,  is  felt  directly  by  the  bell-animalcule  and  is 
followed  by  an  instantaneous  change  in  the  relative  position 
of  its  parts.  The  stalk  becomes  coiled  into  a  close  spiral 
(D1,  D2)  so  as  to  have  a  mere  fraction  of  its  original  length, 
and  the  body  from  being  bell-shaped  becomes  globular,  the 
disc  being  withdrawn  and  the  peristome  closed  over  it 
(Di,  D*).  ! 

The  coiling  of  the  stalk  leads  us  to  the  consideration  of 
the  particular  form  of  contractility  called  muscular,  which 
we  have  already  met  with  in  Stylonychia  (p.  116).  It  was 
mentioned  above  that  while  the  stalk  in  its  fully  expanded 
condition  is  straight,  the  axial  fibre  is  not  straight,  but  forms 
a  very  open  spiral,  i.e.,  it  does  not  lie  in  the  centre  of 
the  stalk  but  at  any  transverse  section  is  nearer  the  surface 
at  one  spot  than  elsewhere,  and  this  point  as  we  ascend  the 
stalk  is  directed  successively  to  all  points  of  the  compass. 

Now  suppose  that  the  axial  fibre  undergoes  a  sudden  con- 
traction, that  is  to  say,  a  decrease  in  length  accompanied  by 
an  increase  in  diameter,  since  as  we  have  already  seen 
(p.  10)  there  is  no  decrease  in  volume  in  protoplasmic 
contraction.  There  will  naturally  follow  a  corresponding 
shortening  of  the  elastic  cuticular  substance  which  forms  the 
outer  layer  of  the  stalk.  If  the  axial  fibre  were  entirely 
towards  one  side  of  the  stalk,  the  result  of  the  contraction 
would  be  a  flexure  of  the  stalk  towards  that  side,  but,  as  its 
direction  is  spiral,  the  stalk  is  bent  successively  in  every 
direction,  that  is,  is  thrown  into  a  close  spiral  coil. 

The  axial   fibre  is  therefore  a  portion  of  the  protoplasm 


xii  FISSION  131 

which  possesses  the  property  of  contractility  in  a  special  de- 
gree ;  in  which  moreover  contraction  takes  place  in  a  definite 
direction — the  direction  of  the  length  of  the  fibre — so  that 
its  inevitable  result  is  to  shorten  the  fibre  and  consequently 
to  bring  its  two  ends  nearer  together.  This  is  the  essential 
characteristic  of  a  muscular  contraction,  and  the  axial  fibre 
in  the  stalk  of  Vorticella  is  therefore  to  be  looked  upon  as 
the  first  instance  of  a  clearly  differentiated  muscle  which  has 
come  under  our  notice. 

There  are  some  interesting  features  in  the  reproduction  of 
Vorticella.  It  multiplies  by  binary  fission,  dividing  through 
the  long  axis  of  the  body  (Fig.  25,  E1,  E2).  Hence  it  is 
generally  said  that  fission  is  longitudinal,  not  transverse,  as 
in  Paramcecium.  But  on  the  theory  (p.  107)  that  the  peris- 
tome  and  disc  are  dorsal  and  the  attached  end  ventral, 
fission  is  really  transverse  in  this  case  also. 

It  will  be  seen  from  the  figures  that  the  process  takes  place 
by  a  cleft  appearing  at  the  distal  end  (E1),  and  gradually 
deepening  until  there  are  produced  two  complete  and  full- 
sized  individuals  upon  a  single  stalk  (E2).  This  state  of 
things  does  not  last  long  :  one  of  the  two  daughter-cells  takes 
on  a  nearly  cylindrical  form,  keeps  its  disc  and  peristome 
retracted,  and  acquires  a  new  circlet  of  cilia  near  its  proximal 
end  (ES)  :  it  then  detaches  itself  from  the  stalk,  which  it 
leaves  in  the  sole  possession  of  its  sister-cell,  and  swims  about 
freely  for  a  time  in  the  direction  indicated  by  the  arrow. 
Sooner  or  later  it  settles  down,  becomes  attached  by  its 
proximal  end,  loses  its  basal  circlet  of  cilia,  and  develops  a 
stalk,  which  ultimately  attains  the  normal  length. 

The  object  of  this  arrangement  is  obvious.  If  when  a 
Vorticella  divided,  the  plane  of  fission  extended  down  the 
stalk  until  two  ordinary  fixed  forms  were  produced  side  by 
side,  the  constant  repetition  of  the  process  would  so  increase 

K  2 


132  VORT1CELLA   AND   ZOOTHAMNIUM  LESS. 

the  numbers  of  the  species  in  a  given  spot  that  the  food- 
supply  would  inevitably  run  short.  This  is  prevented  by 
one  of  the  two  sister-cells  produced  by  fission  leading  a  free 
existence  long  enough  to  enable  it  to  emigrate  and  settle  in 
a  new  locality,  where  the  competition  with  its  fellows  will  be 
less  keen.  The  production  of  these  free-swimming  zooids 
is  therefore  a  means  of  dispersal  (see*p.  122)  :  contrivances 
having  this  object  in  view  are  a  very  general  characteristic 
of  fixed  as  of  parasitic  organisms. 

Conjugation  occasionally  takes  place,  and  presents  certain 
peculiarities.  A  Vorticella  divides  either  into  two  unequal 
halves  (r2)  or  into  two  equal  halves,  one  of  which  divides 
again  into  from  two  to  eight  daughter-cells  (r1).  There  are 
thus  produced  from  one  to  eight  microzooids  which  resemble 
the  barrel-shaped  form  (ES)  in  all  but  size,  and  like  it  become 
detached  and  swim  freely  by  means  of  a  basal  circlet  of  cilia. 
After  swimming  about  for  a  time,  one  of  these  microzooids 
comes  in  contact  with  an  ordinary  form  or  megazooid,  when 
it  attaches  itself  to  it  near  the  proximal  end  (c1),  and  under- 
goes gradual  absorption  (c2),  the  mega-  and  microzooids 
becoming  completely  and  permanently  fused.  As  in  Para- 
mcecium,  conjugation  is  followed  by  increased  activity  in 
feeding  and  dividing  (p.  113). 

Notice  that  in  this  case  the  conjugating  bodies  or  gametes 
are  not  of  equal  size  and  similar  characters,  but  one,  which 
is  conveniently  distinguished  as  the  microgamete  ( =  micro- 
zooid)  is  relatively  small  and  active,  while  the  other  or 
megagamete  ( =  megazooid,  or  ordinary  individual)  is  rela- 
tively large  and  passive.  As  we  shall  see  in  a  later  lesson, 
this  differentiation  of  the  gametes  is  precisely  what  we  get  in 
almost  all  organisms  with  two  sexes  :  the  microgamete  being 
the  male,  the  megagamete  the  female  conjugating  body  (see 
Lesson  XVI.). 


xii  METAMORPHOSIS  133 

The  result  of  conjugation  is  strikingly  different  in  the  three 
cases  already  studied  :  in  Heteromita  (p.  41)  the  two  gametes 
unite  to  form  a  zygote,  a  motionless  body  provided  with  a 
cell-wall,  the  protoplasm  of  which  divides  into  spores  :  in 
Paramcecium  (p.  113)  no  zygote  is  formed,  conjugation  being 
a  mere  temporary  union  :  in  Vorticella  the  zygote  is  an 
actively  moving  and  feeding  body,  indistinguishable  from  an 
ordinary  individual  of  the  species. 

Vorticella  sometimes  encysts  itself  (Fig.  25,  H1),  and  the 
nucleus  of  the  encysted  cell  has  been  observed  to  break  up 
into  a  number  of  separate  masses,  each  doubtless  surrounded 
by  a  layer  of  protoplasm.  After  a  time  the  cyst  bursts,  and 
a  number  of  small  bodies  or  spores  (n2)  emerge  from  it,  each 
containing  one  of  the  products  of  division  of  the  nucleus. 
These  acquire  a  circlet  of  cilia  (HS),  by  means  of  which  they 
swim  freely,  and  they  are  sometimes  found  to  multiply  by 
simple  fission  (H4).  Finally,  they  settle  down  (n5)  by  the 
end  at  which  the  cilia  are  situated,  the  attached  end  begins 
to  elongate  into  a  stalk  (HG),  this  increases  in  length,  the 
basal  circlet  of  cilia  is  lost,  and  a  ciliated  peristome  and 
disc  are  formed  at  the  free  end  (a7).  In  this  way  the 
ordinary  form  is  assumed  by  a  process  of  development 
recalling  what  we  found  to  occur  in  Heteromita  (p.  42),  but 
with  an  important  difference  :  the  free-swimming  young  of 
Vorticella  (n3),  to  which  the  spores  formed  by  division  of 
the  encysted  protoplasm  give  rise,  differ  strikingly  in  form 
and  habits  from  the  adult.  This  is  expressed  by  saying 
that  development  is  in  this  case  accompanied  by  a  meta- 
morphosis, this  word,  literally  meaning  simply  a  change,  being 
always  used  in  biology  to  express  a  striking  and  fundamental 
difference  in  form  and  habit  between  the  young  and  the 
adult ;  as,  for  instance,  between  the  tadpole  and  the  frog, 
or  between  the  caterpillar  and  the  butterfly.  It  is  obvious 


134 


VORTICELLA    AND   ZOOTHAMNtUM 


LESS. 


that  in  the  present  instance  metamorphosis  is  another  means 
of  ensuring  dispersal. 

In  Vorticella,  as  we  have  seen,  fission  results  not  in  the 


FIG.  26. — Zoothamnium  arbuscula. 

A,  entire  colony,  magnified,  showing  nutritive  (n.  z)  and  reproductive 
(r.  0)  zooids  ;  ax.  f  axial  fibre  of  the  stem. 

B,  the  same,  natural  size. 

C,  the  same,  magnified,  in  the  condition  of  retraction. 

D,  nutritive  zooid,  showing  nucleus  (««)»  contractile  vacuole  (c.  vac), 
gullet,  and  axial  fibre  (ax.f). 

E,  reproductive  zooid,  showing  nucleus  (mi}  and  contractile  vacuole 
(c.  vac),  and  absence  of  mouth  and  gullet. 

F1,   F2,    two   stages   in  the  development  of  the  reproductive  zooid. 
(After  Saville  Kent.) 

production  of  equal  and  similar  daughter-cells,  but  of  one 
stalked  and  one  free-swimming  form.  It  is  however  quite 
possible  to  conceive  of  a  Vorticella-like  organism  in  which 
the  parent  cell  divides  into  two  equal  and  similar  products, 
each  retaining  its  connection  with  the  stalk.  If  this  process 
were  repeated  again  and  again,  and  if,  further,  the  plane  of 


xn  DIMORPHISM  135 

fission  were  extended  downwards  so  as  to  include  the  distal 
end  of  the  stalk,  the  result  would  be  a  branched,  tree-like 
stem  with  a  Vorticella-like  body  at  the  end  of  every  branch. 

As  a  matter  of  fact,  this  process  takes  place  not  in  Vorti- 
cella  itself,  but  in  a  nearly  allied  infusor,  the  beautiful 
Zoothamnium,  a  common  genus  found  mostly  in  sea-water 
attached  to  weeds  and  other  objects. 

Zoothamnium  arbuscula  (Fig.  26,  A)  consists  of  a  main 
stem  attached  by  its  proximal  end  and  giving  off  at  its  distal 
end  several  branches,  on  each  of  which  numerous  shortly- 
stalked  bell-animalcules  are  borne,  like  foxgloves  or  Canter- 
bury-bells on  their  stem.  The  entire  tree  is  about  i  cm. 
high,  and  so  can  be  easily  seen  by  the  naked  eye  :  it  is  shown 
of  the  natural  size  in  Fig.  26,  B. 

We  see,  then,  that  Zoothamnium  differs  from  all  our 
previous  types  in  being  a  compound  organism.  The  entire 
"tree"  is  called  a  colony  or  stock,  and  each  separate 
bell-animalcule  borne  thereon  is  an  individual  or  zooid, 
morphologically  equivalent  to  a  single  Vorticella  or 
Paramcecium. 

As  in  Vorticella,  the  stem  consists  of  a  cuticular  sheath 
with  an  axial  muscle-fibre  (ax.  /),  which,  at  the  distal  end 
of  the  main  stem,  branches  like  the  stem  itself,  a  prolonga- 
tion of  it  being  traceable  to  each  zooid  (D).  So  that  the 
muscular  system  is  common  to  the  whole  colony,  and  any 
shock  causes  a  general  contraction,  the  tree-like  structure 
assuming  an  almost  globular  form  (c). 

It  will  be  noticed  from  the  figure  that  all  the  zooids  of 
the  colony  are  not  alike  :  the  majority  are  bell-shaped  and 
resemble  Vorticellse  (A,  n.  z,  and  D),  but  here  and  there  are 
found  larger  bodies  (A,  r.  z,  and  E)  of  a  globular  form,  with- 
out mouth,  peristome,  or  disc,  and  with  a  basal  circlet  of 
cilia.  The  characteristic  band-like  nucleus  (mi)  and  the 


136  VORTICELLA   AND   ZOOTHAMNIUM       LESS,  xn 

contractile  vacuole  (c.  vac)  are  found  in  both  the  bell-shaped 
and  the  globular  zooids. 

It  is  to  these  globular,  mouthless  zooids  that  the  functions 
of  reproducing  the  whole  colony  and  of  ensuring  dispersal 
are  assigned.  They  become  detached,  swim  about  freely 
for  a  time,  then  settle  down,  develop  a  stalk  and  mouth 
(r1,  F2),  and  finally,  by  repeated  fission,  give  rise  to  the 
adult,  tree-like  colony. 

The  Zoothamnium  colony  is  thus  dimorphic,  bearing  indi- 
viduals of  two  kinds  :  nutritive  zooids,  which  feed  and  add 
to  the  colony  by  fission  but  are  unable  to  give  rise  to  a  new 
colony,  and  reproductive  zooids,  which  do  not  feed  while 
attached,  but  are  capable,  after  a  period  of  free  existence,  of 
developing  a  mouth  and  stalk,  and  finally  producing  a  new 
colony.  Dimorphism  is  a  differentiation  of  the  individuals 
of  a  colony,  just  as  the  formation  of  axial  fibre,  gullet,  con- 
tractile vacuole,  and  cilia  are  cases  of  differentiation  of  the 
protoplasm  of  a  single  cell. 


LESSON  XIII 

SPECIES    AND    THEIR   ORIGIN THE    PRINCIPLES    OF 

CLASSIFICATION 

MORE  than  once  in  the  course  of  the  foregoing  lessons  we 
have  had  occasion  to  use  the  word  species — for  instance,  in 
Lesson  I.  (p.  8)  it  was  stated  that  there  were  different 
kinds  or  species  of  Amoebae,  distinguished  by  the  characters 
of  their  pseudopods,  the  structure  of  their  nuclei,  &c. 

We  must  now  consider  a  little  more  in  detail  what  we 
mean  by  a  species,  and,  as  in  all  matters  of  this  sort,  the 
study  of  concrete  examples  is  the  best  aid  to  the  formation 
of  clear  conceptions,  we  will  take,  by  way  of  illustration, 
some  of  the  various  species  of  Zoothamnium. 

The  kind  described  in  the  previous  lesson  is  •  called 
Zoothamnium  arbuscula.  As  Fig.  26,  A,  shows,  it  consists  of  a 
tolerably  stout  main  stem,  from  the  distal  end  of  which 
spring  a  number  of  slender  branches  diverging  in  a  brush- 
like  manner,  and  bearing  on  short  secondary  branchlets  the 
separate  individuals  of  the  colony  :  these  are  of  two  kinds, 
bell-shaped  nutritive  zooids,  and  globular  reproductive 
zooids,  so  that  the  colony  is  dimorphic. 

Zoothamnium  (or,  for  the  sake  of  brevity,  Z.)  alternans 
(Fig.  27,  A)  is  found  also  in  sea-water,  and  differs  markedly 


SPECIES   AND   THEIR   ORIGIN 


LESS. 


from  Z.  arbuscula  in  the  general  form  of  the  colony.  The 
main  stem  is  continued  to  the  extreme  distal  end  of  the 
colony  and  terminates  in  a  zooid  ;  from  it  branches  are 
given  off  right  and  left,  and  on  these  the  remaining  zooids 
are  borne.  To  use  Mr.  Saville  Kent's  comparison,  Z.  arbus- 


FIG.  27. — Species  of  Zoothamnium.  A,  Z.  alternans.  B,  Z. 
dichotonmm.  C,  Z.  simplex.  D,  Z.  affine.  E,  Z.  nntans.  (After 
Saville  Kent.) 

cula  may  be  compared  to  a  standard  fruit  tree,  Z.  alternans 
to  an  espalier.  In  this  species  also  the  colony  is  dimorphic. 

Z.  dichotomum  (Fig.  27,  B)  is  also  dimorphic  and  presents  a 
third  mode  of  branching.  The  main  stem  divides  into  two, 
and  each  of  the  secondary  branches  does  the  same,  so  that 
a  repeatedly  forking  stem  is  produced.  The  branching  of 
this  species  is  said  to  be  dichotomous,  while  that  of  Z.  alter- 
nans is  monopodial,  and  that  of  Z.  arbuscula  umbellate. 

Another  mode  of  aggregation  of  the  zooids  is  found  in  Z. 
simplex  (Fig.  27,  c)  in  which  the  stem  is  unbranched  and 


xni  GENUS   AND   SPECIES  139 

bears  at  its  distal  end  about  six  zooids  in  a  cluster.  The 
zooids  are  more  elongated  than  in  any  of  the  preceding 
species,  and  there  are  no  special  reproductive  individuals,  so 
that  the  colony  is  homomorphic. 

In  Z.  affine  (Fig.  27,  D)  the  stalk  is  dichotomous  but  is 
proportionally  thicker  than  in  the  preceding  species,  and 
bears  about  four  zooids,  all  alike.  It  is  found  in  fresh  water 
attached  to  insects  and  other  aquatic  animals. 

The  last  species  we  shall  consider  is  Z.  nutans  (Fig.  27,  E), 
which  is  the  simplest  known,  never  bearing  more  than  two 
zooids,  and  sometimes  only  one. 

A  glance  at  Figs.  26  and  27  will  show  that  these  six  species 
agree  with  one  another  in  the  general  form  of  the  zooids,  in 
the  characters  of  the  nucleus,  contractile  vacuole,  &c.,  in 
the  arrangement  of  the  cilia,  and  in  the  fact  that  they  are  all 
compound  organisms,  consisting  of  two  or  more  zooids 
attached  to  a  common  stem,  the  axial  fibre  of  which  branches 
with  it,  i.e.,  is  continuous  throughout  the  colony. 

On  account  of  their  possessing  these  important  characters 
in  common,  the  species  described  are  placed  in  the  single 
genus  Zoothamnium,  and  the  characters  summarized  in  the 
preceding  paragraph  are  called  generic  characters.  On  the 
other  hand  the  points  of  difference  between  the  various 
species,  such  as  the  forking  of  the  stem  in  Z.  dichotomum, 
the  presence  of  only  two  zooids  in  Z.  nutans,  and  so  on,  are 
called  specific  characters.  Similarly  the  name  Zoothamnium, 
which  is  common  to  all  the  species,  is  the  generic  name, 
while  those  which  are  applied  only  to  a  particular  species, 
such  as  arbuscula,  simplex,  &c.,  are  the  specific  names.  As 
was  mentioned  in  the  first  lesson  (p.  8),  this  method  of 
naming  organisms  is  known  as  the  Linnean  system  of 
binomial  nomenclature. 

It  will  be    seen    from   the  foregoing  account  that  by   a 


140  SPECIES    AND   THEIR   ORIGIN  LESS. 

species  we  understand  an  assemblage  of  individual  or- 
ganisms, whether  simple  or  compound,  which  agree  with  one 
another  in  all  but  unessential  points,  such  as  the  precise 
number  of  zooids  in  Zoothamnium,  which  may  vary  con- 
siderably in  the  same  species,  and  come,  therefore,  within 
the  limits  of  individual  variation.  Similarly,  what  we  mean 
by  a  genus  is  a  group  of  species  agreeing  with  one  another 
in  the  broad  features  of  their  organization,  but  differing  in 
detail,  the  differences  being  constant. 

A  comparison  of  the  six  species  described  brings  out 
several  interesting  relations  between  them.  For  instance,  it 
is  clear  that  Z.  arbuscula  and  Z.  alternans  are  far  more 
complex  /.<?.,  exhibit  greater  differentiation  of  the  entire 
colony,  than  Z.  simplex,  or  Z.  nutans ;  so  that,  within  the 
limits  of  the  one  genus,  we  have  comparatively  low  or 
generalized,  and  comparatively  high  or  specialized  species. 
Nevertheless,  a  little  consideration  will  show  that  we  cannot 
arrange  the  species  in  a  single  series,  beginning  with  the 
lowest  and  ending  with  the  highest,  for,  although  we  should 
have  no  hesitation  in  placing  Z.  nutans  at  the  bottom  of 
such  a  list,  it  would  be  impossible  to  say  whether  Z.  affine 
was  higher  or  lower  than  Z.  simplex,  or  Z.  arbuscula  than 
Z.  alternans. 

It  is,  however,  easy  to  arrange  the  species  into  groups 
according  to  some  definite  system.  For  instance,  if  we  take 
the  mode  of  branching  as  a  criterion,  Z.  nutans,  afrlne,  and 
dichotomum  will  all  be  placed  together  as  being  dichoto- 
mous,  and  Z.  simplex  and  arbuscula  as  being  umbellate — 
the  zooids  of  the  one  and  the  branches  of  the  other  all 
springing  together  from  the  top  of  the  main  stem  :  on  this 
system  Z.  alternans  will  stand  alone  on  account  of  its  mono- 
podial  branching.  Or,  we  may  make  two  groups,  one  of 
dimorphic  forms,  including  Z.  arbuscula,  alternans,  and 


xiir  CREATION    AND    EVOLUTION  141 

dichotomum,  and  another  of  homomorphic  species,  including 
Z.  affine,  simplex,  and  nutans.  We  have  thus  two  very 
obvious  ways  of  arranging  or  classifying  the  species  of 
Zoothamnium,  and  the  question  arises  —  which  of  these,  if 
either,  is  the  right  one  ?  Is  there  any  standard  by  which 
we  can  judge  of  the  accuracy  of  a  given  classification  of 
these  or  any  other  organisms,  or  does  the  whole  thing  depend 
upon  the  fancy  of  the  classifier,  like  the  arrangement  of 
books  in  a  library  ?  In  other  words,  are  all  possible  classi- 
fications of  living  things  more  or  less  artificial,  or  is  there 
such  a  thing  as  a  natural  classification  ? 

Suppose  we  were  to  try-  and  classify  all  the  members  of  a 
given  family  —  parents  and  grandparents,  uncles  and  aunts, 
cousins,  second  cousins,  and  so  on.  Obviously  there  are  a 
hundred  ways  in  which  it  would  be  possible  to  arrange 
them  —  into  dark  and  fair,  tall  and  short,  curly-haired  and 
straight-haired  and  so  on.  But  it  is  equally  obvious  that  all 
these  methods  would  be  purely  artificial,  and  that  the  only 
natural  way,  i.e.,  the  only  way  to  show  the  real  connection  of 
he  various  members  of  the  family  with  one  another  would 
be  to  classify  them  according  to  blood-relationship,  in  other 
words  to  let  our  classification  take  the  form  of  a  genea- 
logical tree. 

It  may  be  said  —  what  has  this  to  do  with  the  point  under 
discussion,  the  classification  of  the  species  of  Zoothamnium  ? 

There  are  two  theories  which  attempt  to  account  for  the 
existence  of  the  innumerable  species  of  living  things  which 
inhabit  our  earth  :  the  theory  of  creation  and  the  theory  of 
evolution. 

According  to  the  theory  of  creation,  all  the  individuals  of 
every  species  existing  at  the  present  day  —  the  tens  of 
thousands  of  dogs,  oak  trees,  amoebae,  and  what  not  —  are 
derived  by  a  natural  process  ofjlescent  from  a  single  indi- 


U2  SPECIES    AND   THEIR   ORIGIN  LESS. 

vidual,  or  from  a  pair  of  individuals,  in  each  case  precisely 
resembling,  in  all  essential  respects,  their  existing  descend- 
ants, which  came  into  existence  by  a  process  outside  the 
ordinary  course  of  nature  and  known  as  Creation.  On  this 
hypothesis  the  history  of  the  genus  Zoothamnium  would  be 
represented  by  the  diagram  (Fig.  28) ;  each  of  the  species 
being  derived  from  a  single  individual  which  came  into 

Existing    Individuals 

yV 

Z.arbuscu/a       Z.alternans    Z.dichotomum     Z. simplex  Z.affint  Z.nutans 


y 

Ancestral   Individuals 

FIG.   28. — Diagram   illustrating  the  origin  of  the  species  of 
Zoothamnium  by  creation. 

existence,  independently  of  the  progenitors  of  all  the  other 
species,  at  some  distant  period  of  the  earth's  history. 

Notice  that  on  this  theory  the  various  species  are  no  more 
actually  related  to  one  another  than  is  either  of  them  to 
Vorticella,*or  for  the  matter  of  that  to  Homo.  The  in- 
dividuals of  any  one  species  are  truly  related  since  they  all 
share  a  common  descent,  but  there  is  no  more  relationship 
between  the  individuals  of  any  two  independently  created 
species  than  between  any  two  independently  manufactured 


xin  EVOLUTION  143 

chairs  or  tables.  The  words  affinity,  relationship,  £c.,  as 
applied  to  different  species  are,  on  the  theory  of  Creation 
purely  metaphorical,  and  mean  nothing  more  than  that  a 
certain  likeness  or  community  of  structure  exists ;  just  as 
we  might  say  that  an  easy  chair  was  more  nearly  related  to  a 
kitchen  chair  than  either  of  them  to  a  three-legged  stool. 

We  see  therefore  that  on  the  hypothesis  of  creation  the 
varying  degrees  of  likeness  and  unlikeness  between  the 
species  receive  no  explanation,  and  that  we  get  no  absolute 
criterion  of  classification  :  we  may  arrange  our  organisms, 
as  nearly  as  our  knowledge  allows,  according  to  their  resem- 
blances and.  differences,  but  the  relative  importance  of  the 
characters  relied  on  becomes  a  purely  subjective  matter. 

According  to  the  rival  theory — that  of  Descent  or  Organic 
Evolution — every  species  existing  at  the  present  day  is 
derived  by  a  natural  process  of  descent  from  some  other 
species  which  lived  at  a  former  period  of  the  world's 
history.  If  we  could  trace  back  from  generation  to  gener- 
ation the  individuals  of  any  existing  species  we  should,  on 
this  hypothesis,  find  their  characters  gradually  change,  until 
finally  a  period  was  reached  at  which  the  differences  were  so 
considerable  as  to  necessitate  the  placing  of  the  ancestral 
forms  in  a  different  species  from  their  descendants  at  the 
present  day.  And  in  the  same  way  if  we  could  trace  back 
the  species  of  any  one  genus,  we  should  find  them  gradually 
approach  one  another  in  structure  until  they  finally  con- 
verged in  a  single  species,  differing  from  those  now  existing 
but  standing  to  all  in  a  true  parental  relation. 

Let  us  illustrate  this  by  reference  to  Zoothamnium.  As  a 
matter  of  fact  we  know  nothing  of  the  history  of  the  genus,  but 
the  comprehension  of  what  is  meant  by  the  evolution  of  species 
will  be  greatly  faciltated  by  framing  a  working  hypothesis. 

Suppose  that  at  some  distant  period  of  the  world's  history 


144 


SPECIES   AND   THEIR   ORIGIN 


there  existed  a  Vorticella-like  organism  which  we  will  call 
A  (Fig.  29),  having  the  general  characters  of  a  single 
stalked  zooid  of  Zoothamnium  (compare  Fig.  26,  F2),  and 
suppose  that,  of  the  numerous  descendants  of  this  form, 
represented  by  the  lines  diverging  from  A,  there  were  some 
in  which  both  the  zooids  formed  by  the  longitudinal  division 
of  the  body  remained  attached  to  the  stalk  instead  of  one  of 
them  swimming  off  as  in  Vorticella.  The  result — it  matters 

D          ,  •         j-  ,    .  Branching  Branching 

^  Branding  dichotomous  umbeUate  monof,dial 

5.  J>f  Z   dichotomitm  Z.  arbuscula  Z    alternans\  DIMORPHIC 

f  HOMOMORPHIC 

....V..-J 


FIG.    29. — Diagram  illustrating    the   origin   of  the   species   of 
Zoothamnium  by  evolution. 

not  for  our  present  purpose  how  it  may  have  been  caused — 
would  be  a  simple  colonial  organism  consisting  of  two  zooids 
attached  to  the  end  of  a  single  undivided  stalk.  Let  us  call 
this  form  B. 

Next  let  us  imagine  that  in  some  of  the  descendants  of  B, 
represented  as  before  by  the  diverging  lines,  the  plane  of 
division  was  continued  downwards  so  as  to  include  the 
distal  end  of  the  stalk  :  this  would  result  in  the  production 


xiii  DIVERGENCE  OF  CHARACTER  H5 

of  a  form  (c)  consisting  of  two  zooids  borne  on  a  forked 
stem  and  resembling  Z.  nutans.  If  in  some  of  the  descend- 
ants of  c  this  process  were  repeated,  each  of  the  two  zooids 
again  dividing  into  two  fixed  individuals  and  the  division 
as  before  affecting  the  stem,  we  should  get  a  species  (D)  con- 
sisting of  four  zooids  on  a  dichotomous  stem,  like  Z.  affine. 
Let  the  same  process  continue  from  generation  to  genera- 
tion, the  colony  becoming  more  and  more  complex;  we 
should  finally  arrive  at  a  species  E,  consisting  of  numerous 
zooids  on  a  complicated  dichotomously  branching  stem, 
and  therefore  resembling  Z.  dichotomum. 

Let  us  further  suppose  that,  in  some  of  the  descendants 
of  our  hypothetical  form  B,  repeated  binary  fission  took 
place  without  affecting  the  stem  :  the  result  would  be  a  new 
form  F,  consisting  of  numerous  zooids  springing  in  a  cluster 
from  the  end  of  the  undivided  stem,  after  the  manner  of 
Z.  simplex.  From  this  a  more  complicated  umbellate  form 
(G),  like  Z.  arbuscula,  may  be  supposed  to  have  originated, 
and  again  starting  from  B  with  a  different  mode  of  branch- 
ing a  monopodial  form  (H)  might  have  arisen. 

Finally,  let  it  be  assumed  that  while  some  of  the  descend- 
ants of  the  forms  c,  D,  and  F  became  modified  into  more 
and  more  complex  species,  others  survived  to  the  present 
time  with  comparatively  little  change,  forming  the  existing 
species  nutans,  affine,  and  simplex  :  and  that,  in  the  similarly 
surviving  representatives  of  E,  G,  and  H,  a  differentiation  of 
the  individual  zooids  took  place  resulting  in  the  evolution  of 
the  dimorphic  species  dichotomum,  arbuscula,  and  alternans. 

It  will  be  seen  that,  on  this  hypothesis,  the  relative  like- 
ness and  unlikeness  of  the  species  of  Zoothamnium  are 
explained  as  the  result  of  their  descent  with  greater  or  less 
modification  or  divergence  of  character  from  the  ancestral 
form  A.  And  that  we  get  an  arrangement  or  classification 

L 


H6  SPECIES    AND   THEIR   ORIGIN  LESS. 

in  the  form  of  a  genealogical  tree,  which  on  the  hypothesis 
is  a  strictly  natural  one,  since  it  shows  accurately  the 
relationship  of  the  various  species  to  one  another  and  to 
the  parent  stock.  So  that,  on  the  theory  of  evolution,  a 
natural  classification  of  any  given  group  of  allied  organisms 
is  simply  a  genealogical  tree,  or  as  it  is  usually  called,  a 
phytogeny. 

It  must  not  be  forgotten  that  the  forms  A,  B,  c,  D,  E,  F,  G, 
and  H  are  purely  hypothetical :  their  existence  has  been 
assumed  in  order  to  illustrate  the  doctrine  of  descent  by  a 
concrete  example.  The  only  way  in  which  we  could  be 
perfectly  sure  of  an  absolutely  natural  classification  of  the 
species  of  Zoothamnium  would  be  by  obtaining  specimens 
as  far  back  as  the  distant  period  when  the  genus  first  came 
into  existence  ;  and  this  is  out  of  the  question,  since  minute 
soft-bodied  organisms  like  these  have  no  chance  of  being 
preserved  in  the  fossil  state. 

It  will  be  seen  that  the  theory  of  evolution  has  the 
advantage  over  that  of  creation  of  offering  a  reasonable 
explanation  of  certain  facts.  First  of  all  the  varying  degrees 
of  likeness  and  unlikeness  of  the  species  are  explained  by 
their  having  branched  off  from  one  another  at  various 
periods  :  for  instance,  the  greater  similarity  of  structure 
between  Z.  affine  and  Z.  dichotomum  than  between  either  of 
them  and  any  other  species  is  due  to  these  two  species 
having  a  common  ancestor  in  D,  whereas  to  connect  either 
of  them,  say  with  Z.  arbuscula,  we  have  to  go  back  to  B. 
Then  again  the  fact  that  all  the  species,  however  complex  in 
their  fully  developed  state,  begin  life  as  a  simple  zooid  which 
by  repeated  branching  gradually  attains  the  adult  complexity, 
is  a  result  of  the  repetition  by  each  organism,  in  the  course 
of  its  single  life,  of  the  series  of  changes  passed  through  by 
its  ancestors  in  the  course  of  ages.  In  other  words  ontogeny, 


xni  HEREDITY  AND  VARIABILITY  147 

or  the  evolution  of  the  individual,  is,  in  its  main  features,  a 
recapitulation  of  phytogeny  or  the  evolution  of  the  race. 

One  other  matter  must  be  referred  to  in  concluding  the 
present  lesson.  It  is  obvious  that  the  evolution  of  one 
species  from  another  presupposes  the  occurrence  of  varia- 
tions in  the  ancestral  form.  As  a  matter  of  fact  such 
individual  variation  is  of  universal  occurrence  :  it  is  a  matter 
of  common  observation  that  no  two  leaves,  shells,  or  human 
beings  are  precisely  alike,  and  in  our  type  genus  Zootham- 
nium  the  number  of  zooids,  their  precise  arrangement,  the 
details  of  branching,  &c.,  are  all  variables.  This  may  be 
expressed  by  saying  that  heredity,  according  to  which  the 
offspring  tends  to  resemble  the  parent  in  essentials,  is 
modified  by  variability,  according  to  which  the  offspring 
tends  to  differ  from  the  parent  in  details.  If  from  any 
cause  an  individual  variation  is  perpetuated  there  is  produced 
what  is  known  as  a  variety  of  the  species,  and,  according  to 
the  theory  of  the  origin  of  species  by  evolution,  such  a 
variety  may  in  course  of  time  become  a  new  species.  Thus 
a  variety  is  an  incipient  species,  and  a  species  is  a  (relatively) 
permanent  variety. 

It  does  not  come  within  the  scope  of  the  present  work  to 
discuss  either  the  causes  of  variability  or  those  which  deter- 
mine the  elevation  of  a  variety  to  the  rank  of  a  species  : 
both  questions  are  far  too  complex  to  be  adequately  treated 
except  at  considerable  length,  and  anything  of  the  nature  of 
a  brief  abstract  could  only  be  misleading.  As  a  preliminary 
to  the  study  of  Darwin's  Origin  of  Species,  the  student  is 
recommended  to  read  Romanes's  Evidences  of  Organic 
Evolution,  in  which  the  doctrine  of  Descent  is  expounded 
as  briefly  as  is  consistent  with  clearness  and  accuracy. 


L  2 


LESSON  XIV 

FORAMINIFERA,  RADIOLARIA,  AND  DIATOMS 

IN  the  four  previous  lessons  we  have  learnt  how  a  uni- 
cellular organism  may  attain  very  considerable  complexity 
by  a  process  of  differentiation  of  its  protoplasm.  In  the 
present  lesson  we  shall  consider  briefly  certain  forms  of  life 
in  which,  while  the  protoplasm  of  the  unicellular  body  un- 
dergoes comparatively  little  differentiation,  an  extraordinary 
variety  and  complexity  of  form  is  produced  by  the  develop- 
ment of  a  skeleton,  either  in  the  shape  of  a  hardened  cell- 
wall  or  by  the  formation  of  hard  parts  within  the  protoplasm 
itself. 

The  name  Foraminifera  is  given  to  an  extensive  group  of 
organisms  which  are  very  common  in  the  sea,  some  living ' 
near  the  surface,  others  at  various  depths.  They  vary  in 
size  from  a  sand-grain  to  a  shilling.  They  consist  of  variously- 
shaped  masses  of  protoplasm,  containing  nuclei,  and  pro- 
duced into  numerous  pseudopods  which  are  extremely  long 
and  delicate,  and  frequently  unite  with  one  another  to  form 
networks,  as  at  x  in  Fig.  30.  The  cell-body  of  these 
organisms  is  therefore  very  simple,  and  may  be  compared 
to  that  of  a  multinucleate  Amoeba  with  fine  radiating 
pseudopods. 


LESS,  xiv  THE  SHELL  149 

But  what  gives  the  Foraminifera  their  special  character  is 
the  fact  that  around  the  protoplasm  is  developed  a  cell-wall, 
sometimes  membranous,  but  usually  impregnated  with  cal- 
cium carbonate,  and  so  forming  a  shell.  In  some  cases,  as 
in  the  genus  Rotalia  (Fig.  30),  this  is  perforated  by  nume- 
rous small  holes,  through  which  the  pseudopods  are  pro- 
truded, in  others  it  has  only  one  large  aperture  (Fig.  31), 


FIG.  30. — A  living  Foraminifer  (Rotalia} ,  showing  the  fine  radiating 
pseudopods  passing  through  apertures  in  the  chambered  shell  :  at  x 
several  of  them  have  united.  (From  Gegenbaur. ) 

through  which  the  protoplasm  protrudes,  sending  off  its 
pseudopods  and  sometimes  flowing  over  and  covering  the 
outer  surface  of  the  shell.  Thus  while  in  some  cases  the 
shell  has  just  the  relations  of  a  cell-wall  with  one  or  more 
holes  in  it,  in  others  it  becomes  an  internal  structure,  being 
covered  externally  as  well  as  rilled  internally  by  protoplasm. 
The  mode  of  growth  of  Foraminifera  is  largely  determined 
by  the  hard  and  non-distensible  character  of  the  cell-wall, 


150          FORAMINIFERA,  RADIOLARIA,  DIATOMS        LESS. 

which  when  once  formed  is  incapable  of  being  enlarged.  In 
he  young  condition  they  consist  of  a  simple  mass  of  proto- 
plasm covered  by  a  more  or  less  globular  shell,  having  at 
least  one  aperture.  But  in  most  cases  as  the  cell-body 
grows,  it  protrudes  through  the  aperture  of  the  shell  as  a 
mass  of  protoplasm  at  first  naked,  but  soon  becoming 
covered  by  the  secretion  around  it  of  a  second  compartment 
or  chamber  of  the  shell.  The  latter  now  consists  of  two 


FIG.  31. — A,  diagram  of  a  Foraminifer  in  which  new  chambers  are 
added  in  a  straight  line  :  the  smallest  first-formed  chamber  is  below, 
the  newest  and  largest  is  above  and  communicates  with  the  exterior. 

B,  diagram  of  a  Foraminifer  in  which  the  chambers  are  added  in  a 
flat  spiral  :  the  oldest  and  smallest  chamber  is  in  the  centre,  the  newest 
and  largest  as  before  communicates  with  the  exterior.  (From 
Carpenter. ) 


chambers  communicating  with  one  another  by  a  small 
aperture,  and  one  of  them — the  last  formed — communi- 
cating with  the  exterior.  This  process  may  go  on  almost 
indefinitely,  the  successive  chambers  always  remaining  in 
communication  by  small  apertures  through  which  continuity 
of  the  protoplasm  is  maintained,  while  the  last  formed 
chamber  has  a  terminal  aperture  placing  its  protoplasm  in 
free  communication  with  the  outer  world. 


xiv  COMPLEXITY  OF  SHELL  151 

The  new  chambers  may  be  added  in  a  straight  line  (Fig. 
31,  A)  or  in  a  gentle  curve,  or  in  a  flat  spiral  (Fig.  31,  B), 
.or  like  the  segments  of  a  Nautilus  shell,  or  more  or  less 
irregularly.  In  this  way  shells  of  great  variety  and  beauty 


FIG.  32. — Section  of  one  of  the  more  complicated  Foraminifera 
(Aveolina),  showing  the  numerous  chambers  containing  protoplasm 
(dotted),  separated  by  partitions  of  the  shell  (white).  x  60.  (From 
Gegenbaur  after  Carpenter. ) 


of  form  are  produced,  often  resembling  the  shells  of  Mol 
lusca,  and  sometimes  attaining  a  marvellous  degree  of  com- 
plexity (Fig.  32).  The  student  should  make  a  point  of 
examining  mounted  slides  of  some  of  the  principal  genera 
and  of  consulting  the  plates  in  Carpenter's  Introduction  to 
the  Study  of  Foraminifera  (Ray  Society,  1862),  or  in  Brady's 
Report  on  the  Foraminifera  of  the  "  Challenger"  Expedition, 
in  order  to  get  some  notion  of  the  great  amount  of  dif- 
ferentiation attained  by  the  shells  of  these  extremely  simple 
organisms. 


152          FORAMINIFERA,  RADIOLARTA,  DIATOMS         LESS. 

The  Radiolaria  form  another  group  of  marine  animal- 
cules, the  numerous  genera  of  which  are,  like  the  Foram- 
inifera,  amongst  the  most  beautiful  of  microscopic  objects. 
They  also  (Fig.  33)  consist  of  a  mass  of  protoplasm  giving 
off  numerous  delicate  pseudopods  (psd)  which  usually  have 
a  radial  direction  and  sometimes  unite  to  form  networks. 
In  the  centre  of  the  protoplasmic  cell-body  one  or  more 
nuclei  (nu)  of  unusual  size  and  complex  structure  are 
found. 


SKel. 


Int. caps,  pi- 
cent  caps 


-JExt.cqps.fr. 


FIG.  33. — Lithocircus  annularis,  one  of  the  Radiolaria,  showing 
central  capsule  (cent,  caps.},  intra-  and  extra  capsular  protoplasm  (int. 
caps.pr.,  ext. caps.pr.),  nucleus  (nu),  pseudopods  (psd),  silicious  skeleton, 
(skel\  and  symbiotic  cells  of  Zooxanthella  (z).  (After  Butschli.) 

In  the  interior  of  the  protoplasm,  surrounding  the  nucleus, 
is  a  sort  of  shell,  called  the  central  capsule  (cent,  caps.}, 
formed  of  a  membranous  material,  and  perforated  by  pores 
which  place  the  inclosed  or  intra-capsular  protoplasm  (int. 
caps,  pr.}  in  communication  with  the  surrounding  or  extra- 
capsular  protoplasm  (ext.  caps.  pr.}.  But  besides  this  simple 
membranous  shell  there  is  often  developed,  mainly  in  the 
extra-capsular  protoplasm,  a  skeleton  (skel)  formed  in  the 
majority  of  cases  of  pure  silica,  and  often  of  surpassing 


xiv  COMPLEXITY  OF  SHELL  153 

beauty  and  complexity.  One  very  exquisite  form  is  shown 
in  Fig.  34  :  it  consists  of  three  perforated  concentric  spheres 
connected  by  radiating  spicules  :  the  material  of  which  it  is 
composed  resembles  the  clearest  glass. 

The  student  should  examine  mounted  slides  of  the  silicious 
shells  of  these  organisms — sold  under  the.  name  of  Poly- 
cys tinea — and  should  consult  the  plates  of  Haeckel's  Die 


FIG.  34. — Skeleton  of  a  Radiolarian  (Actinommd),  consisting  of 
three  concentric  perforated  spheres — the  two  outer  partly  broken  away 
to  show  the  inner — connected  by  radiating  spicules.  (From  Gegenbaur 
after  Haeckel. ) 

Radiolarien :  he  cannot  fail  to  be  struck  with  the  complexity 
and  variety  attained  by  the  skeletons  of  organisms  which  are 
themselves  little  more  complex  than  Amoebae. 

Before  leaving  the  Radiolaria,  we  must  touch  upon  a 
matter  of  considerable  interest  connected  with  the  physio- 


154          FORAMINIFERA,  RADIOLARIA,  DIATOMS         LESS. 

logy  of  the  group.  Imbedded  usually  in  the  extra-capsular 
protoplasm  are  found  certain  little  rounded  bodies  of  a 
yellow  colour,  often  known  as  "  yellow  cells  "  (Fig.  33,  z). 
Each  consists  of  protoplasm  surrounded  by  a  cell-wall  of 
cellulose,  and  coloured  by  chlorophyll,  with  which  is  asso- 
ciated a  yellow  pigment  of  similar  character  called  diatomin. 

For  a  long  time  these  bodies  were  a  complete  puzzle  to 
biologists,  but  it  has  now  been  conclusively  proved  that  they 
are  independent  organisms  resembling  the  resting  condition 
of  Haematococcus,  and  called  Zooxanthella  nutricola. 

Thus  an  ordinary  Radiolarian,  such  as  Lithocircus  (Fig. 
33),  consists  of  two  quite  distinct  things,  the  Lithocircus  in 
the  strict  sense  of  the  word  plus  large  numbers  of  Zooxan- 
thellse  associated  with  it.  The  two  organisms  multiply  quite 
independently  of  one  another  :  indeed  Zooxanthella  has 
been  observed  to  multiply  by  fission  after  the  death  of  the 
associated  Radiolarian. 

This  living  together  of  two  organisms  is  known  as  Sym- 
biosis. It  differs  essentially  from  parasitism  (see  p.  121),  in 
which  one  organism  preys  upon  another,  the  host  deriving 
no  benefit  but  only  harm  from  the  presence  of  the  parasite. 
In  symbiosis,  on  the  contrary,  the  two  organisms  are  in  a 
condition  of  mutually  beneficial  partnership.  The  carbon 
dioxide  and  nitrogenous  waste  given  off  by  the  Radiolarian 
serve  as  a  constant  food-supply  to  the  Zooxanthella  :  at  the 
same  time  the  latter  by  decomposing  the  carbon  dioxide 
provides  the  Radiolarian  with  a  constant  supply  of  oxygen, 
and  at  the  same  time  with  two  important  food-stuffs — starch 
andproteids,  which,  after  solution,  diffuse  from  the  protoplasm 
of  the  Zooxanthella  into  that  of  the  Radiolarian.  The 
Radiolarian  may  therefore  be  said  to  keep  the  Zooxanthellae 
constantly  manured,  while  the  Zooxanthellae  in  return  supply 
the  Radiolarian  with  abundance  of  oxygen  and  of  ready- 


xiv  STRUCTURE  OF  CELL-WALL  155 

digested  food.  It  is  as  if  a  Haematococcus  ingested  by  an 
Amoeba  retained  its  vitality  instead  of  being  digested  :  it 
would  under  these  circumstances  make  use  of  the  carbon 
dioxide  and  nitrogenous  waste  formed  as  products  of  kata- 
bolism  by  the  Amoeba,  at  the  same  time  giving  off  oxygen 
and  forming  starch  and  proteids.  The  oxygen  evolved  would 
give  an  additional  supply  of  this  necessary  gas  to  the  Amoeba, 
and  the  starch  after  conversion  into  sugar  and  the  proteids 
after  being  rendered  diffusible  would  in  part  diffuse  through 
the  cell-wall  of  the  Hsematococcus  into  the  surrounding 
protoplasm  of  the  Amoeba,  to  which  they  would  be  a 
valuable  food. 

Thus,  as  it  has  been  said,  the  relation  between  a  Radio- 
larian  and  its  associated  yellow-cells  are  precisely  those 
which  obtain  between  the  animal  and  vegetable  kingdoms 
generally. 

The  Diatomacetz,  or  Diatoms,  as  they  are  often  called  for 
the  sake  of  brevity,  are  a  group  of  minute  organisms,  in- 
cluded under  a  very  large  number  of  genera  and  species,  and 
so  common  that  there  is  hardly  a  pond  or  stream  in  which 
they  do  not  occur  in  millions. 

Diatoms  vary  almost  indefinitely  in  form  :  they  may  be  rod- 
shaped,  triangular,  circular,  and  so  on.  Their  essential 
structure  is,  however,  very  uniform  :  the  cell-body  contains  a 
nucleus  (Fig.  35,  A,  nu)  and  vacuoles  (vac\  as  well  as  two 
large  chromatophores  (chr]  of  a  brown  or  yellow  colour  ; 
these  are  found  to  contain  chlorophyll,  the  characteristic 
green  tint  of  which  is  veiled,  as  in  Zooxanthella,  by  diatomin. 
The  cell  is  motile,  executing  curious,  slow,  jerky  or  gliding 
movements,  the  cause  of  which  is  still  obscure. 

The  most  interesting  feature  in  the  organization  of  diatoms 
is  however  the  structure  of  the  cell-wall  :  it  consists  of  two 


156         FORAMINIFERA,   RAD1OLARIA,  DIATOMS        LESS. 

parts  or  valves  (B,  c,  c.  w,  c.  w'\  each  provided  with  a  rim  or 
girdle,  and  so  disposed  that  in  the  entire  cell  the  girdle  of 
one  valve  (c.  w)  fits  over  that  of  the  other  (c.  w')  like  the 


FIG.  35. — A,  semi-diagrammatic  view  of  a  diatom  from  its  flat  face, 
showing  cell- wall  (c.  zu)  and  protoplasm  with  nucleus  («#),  two  vacuoles 
(vac),  and  two  chromatophores  (chr}. 

B,  diagram  of  the  shell  of  a  diatom  from  the  side,  i.e.,  turned  on  its 
long  axis  at  right  angles  to  A,  showing  the  two  valves  (c.  w,  c.  w')  with 
their  overlapping  girdles. 

c,  the  same  in  transverse  section.  x 

D,  surface  view  of  the  silicious  shell  of  Navicnla  truncata. 

E,  surface  view  of  the  silicious  shell  of  Aulacodiscus  sollittianus. 
(D,  after  Donkin  ;  E,  after  Norman.)  0 

lid  of  a  pill-box.  The  cell-wall  is  impregnated  with  silica, 
so  that  diatoms  can  be  boiled  in  strong  acid  or  exposed  to 
the  heat  of  a  flame  without  losing  their  form  :  the  protoplasm 


xiv  MARKINGS  OF  DIATOMS  157 

is  of  course  destroyed,  but  the  flinty  cell -wall  remains 
uninjured. 

Moreover,  the  cell-walls  of  diatoms  are  remarkable  for  the 
beauty  and  complexity  of  their  markings,  which  are  in  some 
cases  so  delicate  that  even  now  microscopists  are  not  agreed 
as  to  the  precise  interpretation  of  the  appearances  shown 
by  the  highest  powers  of  the  microscope.  Two  species  are 
shown  in  Fig.  35,  D  and  E,  but,  in  order  to  form  some  con- 
ception of  the  extraordinary  variety  in  form  and  ornamenta- 
tion, specimens  of  the  mounted  cell-walls  should  be  ex- 
amined and  the  plates  of  some  illustrated  work  consulted. 
See  especially  Schmidt's  Atlas  fur  Diatomaceenkunde  and 
the  earlier  volumes  of  the  Quarterly  Journal  of  Micro- 
scopical Science. 

We  see  then  that  while  Diatoms  are  in  their  essential 
structure  as  simple  as  Haematococcus,  they  have  the  power 
of  extracting  silica  from  the  surrounding  water,  and  of 
forming  from  it  structures  which  rival  in  beauty  of  form  and 
intricacy  of  pattern  the  best  work  of  the  metal-worker  or 
the  ivory-carver. 


LESSON  XV 

MUCOR 

THE  five  preceding  lessons  have  shown  us  how  complex  a 
cell  may  become  either  by  internal  differentiation  of  its 
protoplasm,  or  by  differentiation  of  its  cell-wall.  In  this 
and  the  following  lesson  we  shall  see  how  a  considerable 
degree  of  specialization  may  be  attained  by  the  elongation  of 
cells  into  filaments. 

Mucor  is  the  scientific  name  of  the  common  white  or  grey 
mould  which  every  one  is  familiar  with  in  the  form  of  a 
cottony  deposit  on  damp  organic  substances,  such  as  leather, 
bread,  jam,  &c.  For  examination  it  is  readily  obtained  by 
placing  a  piece  of  damp  bread  or  some  fresh  horse-dung 
under  an  inverted  tumbler  or  bell-jar  so  as  to  prevent  evapo- 
ration and  consequent  drying.  In  the  course  of  two  or 
three  days  a  number  of  delicate  white  filaments  will  be  seen 
shooting  out  in  all  directions  from  the  bread  or  manure  ;  these 
are  filaments  of  Mucor.  The  species  which  grows  on  bread 
is  called  Mucor  stolonifer,  that  on  horse-dung,  M.  mucedo. 

The  general  structure  and  mode  of  growth  of  the  mould 
can  be  readily  made  out  with  the  naked  eye.  It  first 
appears,  as  already  stated,  in  the  form  of  very  fine  white 
threads  projecting  from  the  surface  of  them  ouldy  substance  ; 
and  these  free  filaments  (Fig.  36,  A,  a.  hy)  can  be  easily 


Q 

*-mf^     f 


FIG.  36. — Mucor. 

A,  portion  of  mycelium  of  M.  mucedo  (my}  with  two  aerial  hyphse 
(#.  hy\  each  ending  in  a  sporangium  (spg). 

B,  small  portion  of  an  aerial  hypha,  highly  magnified,  showing  pro- 
toplasm (plsm)and  cell-wall  (c  w).     The  scale  above  applies  to  this 
figure  only. 

c1,  immature  sporangium,  showing  septum  (sep}  and  undivided  pro- 
toplasm :  c2,  mature  sporangium  in  which  the  protoplasm  has  divided 
into  spores  ;  the  septum  (sep)  has  become  very  convex  distally,  forming 
the  columella. 

D1,  mature  sporangium  in  the  act  of  dehiscence,  showing  the  spores 
(sp)  surrounded  by  mucilage  (g) ;  D2,  small  portion  of  the  same,  more 
highly  magnified,  showing  spicules  of  calcium  oxalate  attached  to  wall. 

E,  a  columella,  left  by  complete  dehiscence  of  a  sporangium,  showing 
the  attachment  of  the  latter  as  a  black  band. 

The  scale  above  c'J  applies  to  c1  c2,  D1,  and  E. 


160  MUCOR  LESS. 

F,  spoi-es. 

G1,  G2,  G3,  three  stages  in  the  germination  of  the  spores. 
H,  a  group  of  germinating  spores  forming  a  small  mycelium. 
i1, — I6,  five  stages  in  conjugation,  showing  two  gametes  (gam)  uniting 
to  form  the  zygote  (zyg). 

K1,  K2,  development  of  ferment  cells  from  submerged  hyphae. 
(A,  C2  D,  E,  F,  G,  and  K,  after  Howes  ;  I,  after  De  Bary. ) 


ascertained  to  be  connected  with  others  (my)  which  form  a 
network  ramifying  through  the  substance  of  the  bread  or 
horse-dung.  This  network  is  called  a  mycelium  ;  the  threads 
of  which  it  is  composed  are  mycelial  hypha  ;  and  the  fila- 
ments which  grow  out  into  the  air  and  give  the  characteristic 
fluffy  appearance  to  the  growth  are  aerial  hypha. 

The  aerial  hyphae  are  somewhat  thicker  than  those  which 
form  the  mycelium,  and  are  at  first  of  even  diameter  through- 
out :  they  continue  to  grow  until  they  attain  a  length,  in  M. 
mucedo,  of  6-8  cm.  (two  or  three  inches).  As  they  grow 
their  ends  are  seen  to  become  dilated,  so  that  each  is  termi- 
nated by  a  minute  knob  (A,  spg)  :  this  increases  in  size  and 
darkens  in  tint  until  it  finally  becomes  dead  black.  In  its 
earlier  stages  the  knobs  may  be  touched  gently  without 
injury,  but  when  they  have  attained  their  full  size  the 
slightest  touch  causes  them  to  burst  and  apparently  to  dis- 
appear— their  actual  fate  being  quite  invisible  to  the  naked 
eye.  As  we  shall  see,  the  black  knobs  contain  spores,  and 
are  therefore  called  sporangia  or  spore-cases. 

Examined  under  the  microscope,  a  hypha  is  found  to  be 
a  delicate  more  or  less  branched  tube,  with  a  clear  trans- 
parent wall  (B,  c.  w)  and  slightly  granular  contents  (plsm)  : 
its  free  end  tapers  slightly  (H),  and  the  wall  is  somewhat 
thinner  at  the  extremity  than  elsewhere.  If  a  single  hypha 
could  be  obtained  whole  and  unbroken,  its  opposite  end 
would  be  found  to  have  much  the  same  structure,  and  each 
of  its  branches  would  also  be  seen  to  end  in  the  same  way. 


xv  ASEXUAL   REPRODUCTION  161 

So  that  the  mould  consists  of  an  interlacement  of  branched 
cylindrical  filaments,  each  consisting  of  a  granular  substance 
completely  covered  by  a  kind  of  thin  skin  of  some  clear 
transparent  material. 

By  the  employment  of  the  usual  reagents,  it  can  be  ascer- 
tained that  the  granular  substance  is  protoplasm,  and  the 
surrounding  membrane  cellulose.  The  protoplasm  moreover 
contains  vacuoles  at  irregular  intervals  and  numerous  small 
nuclei. 

Thus  a  hypha  of  Mucor  consists  of  precisely  the  same 
constituents  as  a  yeast-cell — protoplasm,  containing  nuclei 
and  vacuoles,  surrounded  by  cellulose.  Imagine  a  yeast 
cell  to  be  pulled  out — as  one  might  pull  out  a  sphere  of  clay 
or  putty — until  it  assumed  the  form  of  a  long  narrow  cylin- 
der, and  suppose  it  also  to  be  pulled  out  laterally  at  intervals 
so  as  to  form  branches  :  there  would  be  produced  by  such  a 
process  a  very  good  imitation  of  a  hypha  of  Mucor.  We 
may  therefore  look  upon  a  hypha  as  an  elongated  and 
branched  cell,  so  that  Mucor  is,  like  Opalina,  a  multinucleate 
but  unicellular  organism.  We  shall  see  directly  however 
that  this  is  strictly  true  of  the  mould  only  in  its  young  state. 

As  stated  above,  the  aerial  hyphae  are  at  first  of  even 
calibre,  but  gradually  swell  at  their  ends,  forming  sporangia. 
Under  the  microscope  the  distal  end  of  an  aerial  hypha  is 
found  to  dilate  (Fig.  36,  c1)  :  immediately  below  the  dilata- 
tion the  protoplasm  divides  at  right  angles  to  the  long  axis 
of  the  hypha,  the  protoplasm  in  the  dilated '  portion  thus 
becoming  separated  from  the  rest.  Between  the  two  a 
cellulose  partition  or  septum  (sep)  is  formed,  as  in  the  ordi- 
nary division  of  a  plant  cell  (Fig.  n,  p.  66).  The  portion 
thus  separated  is  the  rudiment  of  a  sporangium. 

Let  us  consider  precisely  what  this  process  implies.  Before 
it  takes  place  the  protoplasm  is  continuous  throughout  the 

M 


162  MUCOR  LESS. 

whole  organism,  which  is  therefore  comparable  to  the  un- 
divided plant-cell  shown  in  Fig.  9,  B.  As  in  that  case,  the 
protoplasm  divides  into  two  and  a  new  layer  of  cellulose  is 
formed  between  the  daughter-cells.  Only  whereas  in  the 
ordinary  vegetable  cell  the  products  of  division  are  of  equal 
size  (Fig  10,  i),  in  Mucor  they  are  very  unequal,  one  being 
the  comparatively  small  sporangium,  the  other  the  rest  of 
the  hypha. 

Thus  a  Mucor-plant  with  a  single  aerial  hypha  becomes, 
by  the  formation  of  a  sporangium,  bicellular :  if,  as  is  ordi- 
narily the  case,  it  bears  numerous  aerial  hyphae,  each  with 
its  sporangium,  it  is  multicellular. 

Under  unfavourable  conditions  of  nutrition,  septa  fre- 
quently appear  at  more  or  less  irregular  intervals  in  the 
mycelial  hyphae  :  the  organism  is  then  very  obviously  multi- 
cellular,  being  formed  of  numerous  cylindrical  cells  arranged 
end  to  end. 

The  sporangium  continues  to  grow,  and  as  it  does  so,  the 
septum  becomes  more  and  more  convex  upwards,  finally 
taking  the  form  of  a  short,  club-shaped  projection,  the  colu- 
mella,  extending  into  the  interior  of  the  sporangium  (c2) :  at 
the  same  time  the  protoplasm  of  the  sporangium  under- 
goes multiple  fission,  becoming  divided  into  numerous  ovoid 
masses  each  of  which  surrounds  itself  with  a  cellulose  coat 
and  becomes  a  spore  (D\  D2,  sp).  A  certain  amount  of  the 
protoplasm  remains  unused  in  the  formation  of  spores,  and 
is  converted  into  a  gelatinous  material  (g\  which  swells  up 
in  water. 

The  original  cell-wall  of  the  sporangium  is  left  as  an 
exceedingly  delicate,  brittle  shell  around  the  spores  :  minute 
needle-like  crystals  of  calcium  oxalate  are  deposited  in  it, 
and  give  it  the  appearance  of  being  closely  covered  with 
short  cilia  (D2). 


XV 


GERMINATION  OF  SPORES 


163 


In  the  ripe  sporangium  the  slightest  touch  suffices  to 
rupture  the  brittle  wall  and  liberate  the  spores,  which  are 
dispersed  by  the  swelling  of  the  transparent  intermediate 
substance.  The  aerial  hypha  is  then  left  terminated  by  the 
columella  (E),  around  the  base  of  which  is  seen  a  narrow 
black  ring  indicating  the  place  of  attachment  of  the 
sporangium. 

The  spores  (F)  are  clear,  bright-looking,  ovoidal  bodies 
consisting  of  protoplasm  containing  a  nucleus  and  sur- 


FlG.  37. — Moist  chamber  formed  by  cementing  a  ring  of  glass  or 
metal  (c)  on  an  ordinary  glass  slide  (A),  and  placing  over  it  a  cover-slip 
(B),  on  the  under  side  of  which  is  a  hanging  drop  of  nutrient  fluid  (p). 
The  upper  figure  shows  the  apparatus  in  perspective,  the  lower  in 
vertical  section.  (From  Klein.) 

rounded    by  a   thick  cell-wall.     A   spore   is   therefore   an 
ordinary  encysted  cell,  quite  comparable  to  a  yeast-cell. 

The  development  of  the  spores  is  a  very  instructive  process, 
and  can  be  easily  studied  in  the  following  way  :  A  glass  or 
metal  ring  (Fig.  37,  c)  is  cemented  to  an  ordinary  microscopic 
slide  (A)  so  as  to  form  a  shallow  cylindrical  chamber.  The 
top  of  the  ring  is  oiled,  and  on  it  is  placed  a  cover  glass  (B), 
with  a  drop  of  Pasteur's  solution  on  its  under  surface. 
Before  placing  the  cover-glass  in  position  a  ripe  sporangium 
of  Mucor  is  touched  with  the  point  of  a  needle,  which  is 

M   2 


1 64  MUCOR 

then  stirred  round  in  the  drop  of  Pasteur's  solution,  so  as  to 
sow  it  with  spores.  By  this  method  the  drop  of  nutrient 
fluid  is  prevented  from  evaporating,  and  the  changes  under- 
gone by  the  spores  can  be  watched  by  examination  from  time 
to  time  under  a  high  power. 

The  first  thing  that  happens  to  a  spore  under  these  con- 
ditions is  that  it  increases  in  size  by  imbibition  of  fluid,  and 
instead  of  appearing  bright  and  clear  becomes  granular  and 
develops  one  or  more  vacuoles.  Its  resemblance  to  a 
yeast-cell  is  now  more  striking  than  ever.  Next  the  spore 
becomes  bulged  out  in  one  or  more  places  (c1,  Fig.  36),  looking 
not  unlike  a  budding  Saccharomyces.  The  buds,  however, 
instead  of  becoming  detached  increase  in  length  until  they 
become  filaments  of  a  diameter  slightly  less  than  that  of  the 
spore  and  somewhat  bluntly  pointed  at  the  end  (c2).  These 
filaments  continue  to  grow,  giving  off  as  they  do  so  side 
branches  (c3)  which  interlace  with  similar  threads  from 
adjacent  spores  (H).  The  filaments  are  obviously  hyphse, 
and  the  interlacement  is  a  mycelium. 

Thus  the  statement  made  in  a  previous  paragraph  (p.  161), 
that  Mucor  was  comparable  to  a  yeast -cell  pulled  out  into  a 
filament,  is  seen  to  be  fully  justified  by  the  facts  of  develop- 
ment, which  show  that  the  branched  hyphse  constituting  the 
Mucor-plant  are  formed  by  the  growth  of  spores  each  strictly 
comparable  to  a  single  Saccharomyces. 

It  will  be  noticed  that  the  growth  of  the  mycelium  is  cen- 
trifugal :  each  spore  or  group  of  spores  serves  as  a  centre 
from  which  hyph?e  radiate  in  all  directions  (H),  continuing 
to  grow  in  a  radial  direction  until,  in  place  of  one  or  more 
spores  quite  invisible  to  the  naked  eye,  we  have  a  white 
patch  more  or  less  circular  in  outline,  and  having  the  spores 
from  which  the  growth  proceeded  in  its  centre.  Owing  to 
the  centrifugal  mode  of  growth  the  mycelium  is  always 


CONJUGATION 


165 


thicker  at  the  centre  than  towards  the  circumference,  since 
it  is  the  older  or  more  central  portions  of  the  hyphse  which 
have  had  most  time  to  branch  and  become  interlaced  with 
one  another. 

Under  certain  circumstances  a  peculiar  process  of  con- 
jugation occurs  in  Mucor.  Two  adjacent  hyphae  send  out 
short  branches  (Fig.  36,  i1),  which  come  into  contact  with 
one  another  by  their  somewhat  swollen  free  ends  (i2).  In 
each  a  septum  appears  so  as  to  shut  off  a  separate  terminal 
cell  (i3,  gam}  from  the  rest  of  the  hypha.  The  opposed 
walls  of  the  two  cells  then  become  absorbed  (i4)  and  their 
contents  mingle,  forming  a  single  mass  of  protoplasm 
(i5,  zyg),  the  cell- wall  of  which  becomes  greatly  thickened 
and  divided  into  two  layers,  an  inner  delicate  and  trans- 
parent, and  an  outer  dark  in  colour,  of  considerable  thick- 
ness, and  frequently  ornamented  with  spines. 

Obviously  the  swollen  terminal  cells  (gam)  of  the  short 
lateral  hyphse  are  gametes  or  conjugating  bodies,  and  the 
large  spore-like  structure  (zyg)  resulting  from  their  union 
is  a  zygote.  The  striking  feature  of  the  process  is  that  the 
gametes  are  non-motile,  save  in  so  far  as  their  growth 
towards  one  another  is  a  mode  of  motion.  In  Heteromita 
both  gametes  are  active  and  free-swimming  (p.  41)  :  in 
Vorticella  one  is  free-swimming,  the  other  fixed  but  still 
capable  of  active  movement  (p.  132)  ;  here  both  conjugating 
bodies  exhibit  only  the  slow  movement  in  one  direction  due 
to  growth. 

There  are  equally  important  differences  in  the  result  of 
the  process  in  the  three  cases.  In  Heteromita  the  proto- 
plasm of  the  zygote  breaks  up  almost  immediately  into 
spores  ;  in  Vorticella  the  zygote  is  active,  and  the  result  of 
conjugation  is  merely  increased  activity  in  feeding  and  fissive 


1 66  MUCOR  LESS. 

multiplication  ;  in  Mucor  the  zygote  remains  inactive  for  a 
longer  or  shorter  time,  and  then  under  favourable  conditions 
germinates  in  much  the  same  way  as  an  ordinary  spore, 
forming  a  mycelium  from  which  sporangium-bearing  aerial 
hyphae  arise.  A  resting  zygote  of  this  kind,  formed  by  the 
conjugation  of  equal-sized  gametes,  is  often  distinguished  as 
a  zygospore. 

Notice  that  differentiation  of  a  very  important  kind  is 
exhibited  by  Mucor.  In  accordance  with  its  comparatively 
large  size  the  function  of  reproduction  is  not  performed  by 
the  whole  organism,  as  in  all  previously  studied  types,  but  a 
certain  portion  of  the  protoplasm  becomes  shut  off  from  the 
rest,  and  to  it — as  spore  or  gamete — the  office  of  reproduc- 
ing the  entire  organism  is  assigned.  So  that  we  have  for 
the  first  time  true  reproductive  organs,  which  may  be  of  two 
kinds,  asexual — the  sporangia,  and  sexual — the  gametes.1 

In  describing  the  reproduction  of  Amoeba  it  was  pointed 
out  (p.  20)  that  as  the  entire  organism  divided  into  two 
daughter-cells,  each  of  which  began  an  independent  life,  an 
Amceba  could  not  be  said  ever  to  die  a  natural  death.  The 
same  thing  is  true  of  the  other  unicellular  forms  we  have 
considered  in  the  majority  of  which  the  entire  organism 
produces  by  simple  fission  two  new  individuals.2  But  in 
Mucor  the  state  of  things  is  entirely  altered.  A  compara. 

1  In    Mucor  no  distinction  can  be  drawn   between  the  conjugating 
body  (gamete)  and  the  organ  which  produces  it  (gonad).     See  the  de- 
scription of  the  sexual  process  in    Vaucheria  (Lesson    XVI.)  and  in 
Spirogyra  (Lesson  XIX.). 

2  An  exception  is  formed  by  colonial  forms  such  as  Zoothamnium,  in 
which  life  is  carried  on  from  generation  to  generation  by  the  reproduc- 
tive zooids  only.     In  all  probability  the  colony  itself,  like  an  annual 
plant,  dies  down  after  a  longer  or  shorter  time.     Moreover  the  ciliate 
infusoria  are  found,  as   already  stated  (p.  116),  to  sink  into  decrepitude 
after  multiplying  by  fission  for  a  long  series  of  generations. 


xv  NUTRITION  167 

tively  small  part  of  the  organism  is  set  apart  for  repro-- 
duction,  and  it  is  only  the  reproductive  cells  thus  formed — 
spores  or  zygote— which  carry  on  the  life  of  the  species 
the  remainder  of  the  organism,  having  exhausted  the 
available  food  supply  and  produced  the  largest  possible 
number  of  reproductive  products,  dies.  That  is,  all  vital 
manifestations  such  as  nutrition  cease,  and  decomposition 
sets  in,  the  protoplasm  becoming  converted  into  pro- 
gressively simpler  compounds,  the  final  stages  being  chiefly 
carbon  dioxide,  water,  and  ammonia. 

Mucor  is  able  to  grow  either  in  Pasteur's  or  in  some 
similar  nutrient  solution,  or  on  various  organic  matters  such 
as  bread,  jam,  manure,  &c.  In  the  latter  cases  it  appears  to 
perform  some  fermentative  action,  since  food  which  has 
become  "mouldy"  is  found  to  have  experienced  a  definite 
change  in  appearance  and  flavour  without  actual  putre- 
faction. When  growing  on  decomposing  organic  matter,  as 
it  often  does,  the  nutrition  of  Mucor  is  saprophytic,  but  in 
some  instances,  as  when  it  grows  on  bread,  it  seems  to 
approach  very  closely  to  the  holozoic  method.  M.  stolo- 
nifer  is  also  known  to  send  its  hyphae  into  the  interior  of 
ripe  fruits,  causing  them  to  rot,  and  thus  acting  as  a  para- 
site. The  parasitism  in  this  case  is,  however,  obviously  not 
quite  the  same  thing  as  that  of  Opalina  (p.  121)  :  the  Mucor 
feeds  not  upon  the  ready  digested  food  of  its  host  but  upon 
its  actual  living  substance,  which  it  digests  by  the  action  of 
its  own  ferments.  Thus  a  parasitic  fungus  such  as  Mucor, 
unlike  an  endo-parasitic  animal  such  as  Opalina  or  a  tape- 
worm, is  no  more  exempted  from  the  work  of  digestion 
than  a  dog  or  a  sheep  :  the  organism  upon  which  it  lives 
is  to  be  looked  upon  rather  as  its  prey  than  as  its  host. 

It  is  a  remarkable  circumstance  that,  under  certain  con- 


1 68  MUCOR  LESS,  xv 

ditions,  Mucor  is  capable  of  exciting  alcoholic  fermentation 
in  a  saccharine  solution.  When  the  hyphge  are  submerged 
in  such  a  fluid  they  have  been  found  to  break  up,  forming 
rounded  cells  (Fig.  36,  K1,  K2),  which  not  only  resemble 
yeast-cells  in  appearance  but  are  able  like  them  to  set  up 
alcoholic  fermentation. 

The  aerial  hyphae  of  Mucor  exhibit  in  an  interesting  way 
what  is  known  as  heliotropism,  i.e.,  a  tendency  to  turn  to- 
wards the  light.  This  is  very  marked  if  a  growth  of  the 
fungus  is  placed  in  a  room  lighted  from  one  side  :  the  long 
aerial  hyphae  all  bend  towards  the  window.  This  is  due  to 
the  fact  that  growth  is  more  rapid  on  the  side  of  each  hypha 
turned  away  from  the  light  than  on  the  more  strongly 
illuminated  aspect. 


LESSON  XVI 

VAUCHERIA    AND    CAULERPA 

STAGNANT  ponds,  puddles,  and  other  pieces  of  still,  fresh 
water  usually  contain  a  quantity  of  green  scum  which  in  the 
undisturbed  condition  shows  no  distinction  of  parts  to  the 
naked  eye,  but  appears  like  a  homogeneous  slime  full  of 
bubbles  if  exposed  to  sunlight.  If  a  little  of  the  scum 
is  spread  out  in  a  saucer  of  water,  it  is  seen  to  be  com- 
posed of  great  numbers  of  loosely  interwoven  green 
filaments. 

There  are  many  organisms  which  have  this  general  naked- 
eye  character,  all  of  them  belonging  to  the  Alga,  a  group 
of  plants  which  includes  most  of  the  smaller  fresh-water 
weeds,  and  the  vast  majority  of  sea-weeds.  One  of  these 
filamentous  Algae,  occurring  in  the  form  of  dark-green, 
thickly-matted  threads,  is  called  Vaucheria.  Besides  occur- 
ring in  water  it  is  often  found  on  the  surface  of  moist  soil, 
e.g.,  on  the  pots  in  conservatories. 

Examined  microscopically  the  organism  is  found  to  consist 
of  cylindrical  filaments  with  rounded  ends  and  occasionally 
branched  (Fig.  38,  A).  Each  filament  has  an  outer  cover- 
ing of  cellulose  (u,  c.w)  within  which  is  protoplasm  con- 
taining a  vacuole  so  large  that  the  protoplasm  has  the 


ths  mm 


FlG.   38. —  Vaucheria. 

A,  tangled  filaments  of  the  living  plant,  showing  mode  of  branching. 

B,  extremity  of  a  filament,  showing  cell -wall  (c.  w)  and  protoplasm 
with  chromatophores  (chr\  and  oil-drops  (o\     The  scale  above  applies 
to  this  figure  only. 

c1,  immature  sporangium  (spg)  separated  from  the  filament  by  a  _sep- 
tum  •  c2  mature  sporangium  with  the  spore  (sf>)  in  the  act  of  escaping  ; 
c3,  free-swimming  spore,  showing  cilia,  colourless  ectoplasm  containing 


LESS,  xvi  ASEXUAL  REPRODUCTION  171 

nuclei,  and  endoplasm  containing  the  green  chromatophores  ;  c4,  the 
same  at  the  commencement  of  germination. 

D1,  early,  and  D-,  later  stages  in  the  development  of  the  gonads,  the 
spermary  to  the  left,  the  ovary  to  the  right ;  D3,  the  fully-formed 
spermary  (spy)  and  ovary  (ovy),  each  separated  by  a  septum  (sep)  from 
the  filament. 

D4,  the  ovary  after  dehiscence,  showing  the  ovum  (ov),  with  small 
detached  portion  of  protoplasm  ;  D5,  sperms  ;  D6,  distal  end  of  ripe 
ovary,  showing  sperms  (sp)  passing  through  the  aperture  towards  the 
ovum  (ov). 

D7,  the  gonads  after  fertilization,  showing  the  oosperm  (osp)  still 
inclosed  in  the  ovary  and  the  dehisced  spermary. 

E1,  oosperm  about  to  germinate  :  E2,  further  stage  in  germination. 

(C1  and  c:},  after  Strasburger  ;  C2  and  C4,  after  Sachs  ;  D  and  E,  after 
Pringsheim.) 

character  of  a  membrane  lining  the  cellulose  coat. 
Numerous  small  nuclei  occur  in  the  protoplasm,  as  well  as 
oil-globules  (0),  and  small,  close-set,  ovoid  chromatophores 
(chr)  coloured  with  chlorophyll  and  containing  starch. 

Thus  a  Vaucheria-plant,  like  a  Mucor-plant,  is  comparable 
to  a  single  multinucleate  cell,  extended  in  one  dimension  of 
space  so  as  to  take  on  the  form  of  a  filament. 

Various  modes  of  asexual  reproduction  occur  in  different 
species  of  Vaucheria  :  of  these  we  need  only  consider  that 
which  obtains  in  V.  sessilis.  In  this  species  the  end  of  a 
branch  swells  up  (c1)  and  becomes  divided  off  by  a  septum 
(sep),  forming  a  sporangium  (spg)  in  principle  like  that  of 
Mucor,  but  differing  in  shape.  The  protoplasm  of  the 
sporangium  does  not  divide  but  separates  itself  from  the 
wall,  and  takes  on  the  form  of  a  single  naked  ovoidal  spore 
(c3),  formed  of  a  colourless  cortical  layer  containing  nume- 
rous nuclei  and  giving  off  cilia  arranged  in  pairs,  and  of  an 
inner  or  medullary  substance  containing  numerous  chroma- 
tophores. 

The  wall  of  the  sporangium  splits  at  its  distal  end  (c2), 
and  the  contained  spore  (sp)  escapes  and  swims  freely  in  the 
water  for  some  time  by  the  vibration  of  its  cilia  (c3).  After 


172  VAUCHERIA   AND   CAULERPA  LESS. 

a  short  active  life  it  comes  to  rest,  develops  a  cell-wall,  and 
germinates  (c4),  i.e.,  gives  out  one  or  more  processes  which 
extend  and  take  on  the  form  of  ordinary  Vaucheria-filaments, 
so  that  in  the  present  case,  as  in  Mucor  (p.  164),  the  de- 
velopment of  the  plant  shows  it  to  be  a  single  immensely 
elongated  multinucleate  cell. 

In  its  mode  of  sexual  reproduction  Vaucheria  differs 
strikingly  not  only  from  Mucor,  but  from  all  the  organisms 
we  have  hitherto  studied. 

The  filaments  are  often  found  to  bear  small  lateral  pro. 
cesses  arranged  in  pairs  (DT),  and  each  consisting  of  a  little 
bud  growing  from  the  filament  and  quite  continuous  with  it. 
These  are  the  rudiments  of  the  sexual  reproductive  organs 
or  gonads.  The  shorter  of  the  two  becomes  swollen  and 
rounded  (D2),  and  afterwards  bluntly  pointed  (D3,  ovy]  :  its 
protoplasm  becomes  divided  from  that  of  the  filament,  and 
a  septum  (o3,  sep')  is  formed  between  the  two  :  the  new  cell 
thus  constituted  is  the  ovary.1  The  longer  of  the  two  buds 
undergoes  further  elongation  and  becomes  bent  upon  itself 
(D2),  its  distal  portion  is  then  divided  off  by  a  septum  (DB, 
sep}  forming  a  separate  cell  (spy),  the  spermary? 

Further  changes  take  place  which  are  quite  different  in 
the  two  organs.  At  the  bluntly-pointed  distal  end  of  the 
ovary  the  cell-wall  becomes  gelatinized  and  the  protoplasm 
protrudes  through  it  as  a  small  prominence  which  divides 
off  and  is  lost  (D4).  The  remainder  of  the  protoplasm  then 
separates  from  the  wall  of  the  ovary  and  becomes  a  naked 
cell,  the  ovum*  or  egg-cell  (D4,  ov\  which,  by  the  gelatiniza. 
tion  and  subsequent  disappearance  of  a  portion  of  the 

1   Usually  called  the  oogonium. 
"  Usually  called  the  antheridiitm. 
:{  Frequently  called  oosphere. 


xvr  SEXUAL  REPRODUCTION  173 

wall  of  the  ovary,  is  in  free  contact  with  the  surrounding 
water. 

At  the  same  time  the  protoplasm  of  the  spermary  under- 
goes multiple  fission,  becoming  converted  into  numerous 
minute  green  bodies  (D5),  each  with  two  flagella,  called 
sperms.^  These  are  liberated  by  the  rupture  of  the  spermary 
(D7)  at  its  distal  end,  and  swim  freely  in  the  water. 

Some  of  the  sperms  make  their  way  to  an  ovary,  and,  as 
it  has  been  expressed,  seem  to  grope  about  for  the  aperture, 
which  they  finally  pass  through  (D°),  and  are  then  seen 
moving  actively  in  the  space  between  the  aperture  and  the 
colourless  distal  end  of  the  ovum.  One  of  them,  and  pro- 
bably only  one,  then  attaches  itself  to  the  ovum  and  be- 
comes completely  united  with  it,  forming  the  oosperm,-  a 
body  which  we  must  carefully  distinguish  from  the  ovum, 
since,  while  agreeing  with  the  latter  in  form  and  size,  it 
differs  in  having  incorporated  with  it  the  substance  of  a 
sperm. 

Almost  immediately  the  oosperm  (D",  osp)  surrounds  itself 
with  a  cellulose  wall,  and  numerous  oil-globules  are  formed 
in  its  interior.  It  becomes  detached  from  the  ovary,  and, 
after  a  period  of  rest,  germinates  (E1,  E2)  and  forms  a  new 
Vaucheria  plant. 

It  is  obvious  that  the  fusion  of  the  sperm  with  the  ovum 
is  a  process  of  conjugation  in  which  the  conjugating  bodies 
differ  strikingly  in  form  and  size,  one — the  megagamete  or 
ovum — being  large,  stationary,  and  more  or  less  amoeboid  : 
the  other — the  microgamete  or  sperm — small,  active,  and 
flagellate.  In  other  words,  we  have  a  more  obvious  case  of 
sexual  differentiation  than  was  found  to  occur  in  Vorticella, 

1  Often  called  spermatozooids  or  anthcrozooids : 

2  Often  called  oospore. 


174 


VAUCHERIA   AND   CAULERPA 


(p.  132)  :  the  large  inactive  egg-cell  which  furnishes  by  far 
the  greater  portion  of  the  material  of  the  oosperm  is  the 
female  gamete ;  the  small  active  sperm-cell,  the  function  of 
which  is  probably  (see  Lesson  XXIV.)  to  furnish  additional 
nuclear  material,  is  the  male  gamete. 

Similarly  the  oosperm  is  evidently  a  zygote,  but  a  zygote 
formed  by  the  union  of  the  highly  differentiated  gametes, 


FIG.  39. — Caulerpa  scalpelliformis  (§  nat.  size),  showing  the  stem- 
like,  root-like,  and  leaf-like  portions  of  the  unicellular  plant.  (After 
Harvey. ) 

ovum  and  sperm,  just  as  a  zygospore  (p.   164)  is  one  formed 
by  the  union  of  equal  sized  gametes. 

As  we  shall  see,  this  form  of  conjugation — often  distin- 
guished as  fertilization — occurs  in  a  large  proportion  of 
flowerless  plants,  such  as  mosses  and  ferns  (Lessons  XXVIII. 
and  XXIX.),  as  well  as  in  all  animals  but  the  very  lowest. 
From  lowly  water-weeds  up  to  ferns  and  club  mosses,  and 
from  sponges  and  polypes  up  to  man,  the  process  of  sexual 
reproduction  is  essentially  the  same,  consisting  in  the  conju- 
gation of  a  microgamete  or  sperm  with  a  megagamete  or 


xvi  CAULERPA  175 

ovum,  a  zygote,  the  oosperm  or  unicellular  embryo,  being 
produced,  which  afterwards  develops  into  an  independent 
plant  or  animal  of  the  new  generation.  It  is  a  truly  remark- 
able circumstance  that  what  we  may  consider  as  the  highest 
form  of  the  sexual  process  should  make  its  appearance  so 
low  down  in  the  scale  of  life. 

The  nutrition  of  Vaucheria  is  purely  holophytic  ;  its  food 
consists  of  a  watery  solution  of  mineral  salts  and  of  carbon 
dioxide,  the  latter  being  split  up,  by  the  action  of  the  chro- 
matophores,  into  carbon  and  oxygen. 

Mucor  and  Vaucheria  are  examples  of  unicellular  plants 
which  attain  some  complexity  by  elongation  and  branching. 
The  maximum  differentiation  attainable  in  this  way  by  a 
unicellular  plant  may  be  illustrated  by  a  brief  description  of 
a  sea-weed  belonging  to  the  genus  Caulerpa. 

Caulerpa  (Fig.  39)  is  commonly  found  in  rock-pools 
between  tide-marks,  and  has  the  form  of  a  creeping  stem 
from  which  root-like  fibres  are  given  off  downwards  and 
branched  leaf-like  organs  upwards.  These  "  leaves  "  may 
attain  a  length  of  30  cm.  (i  ft.)  or  more.  So  that,  on  a 
superficial  examination,  Caulerpa  appears  to  be  as  complex 
an  organism  as  a  moss  (compare  Fig.  39  with  Fig.  82,  A). 
But  microscopical  examination  shows  that  the  plant  consists 
of  a  single  continuous  mass  of  vacuolated  protoplasm, 
containing  numerous  nuclei  and  green  chromatophores  and 
covered  by  a  continuous  cell-wall.  Large  and  complicated 
in  form  as  it  is,  the  whole  plant  is  therefore  nothing  more 
than  a  single  branched  cell,  or,  as  it  may  be  expressed,  a 
continuous  mass  of  protoplasm  in  which  no  cellular  structure 
has  appeared. 


LESSON  XVII 

THE  DISTINCTIVE  CHARACTERS  OF  ANIMALS  AND  PLANTS 

HITHERTO  the  words  "  animal  "  and  "  plant  "  have  been 
either  avoided  altogether  or  Used  incidentally  without  any 
attempt  at  definition.  We  are  now  however  in  a  position  to 
consider  in  some  detail  the  precise  meaning  of  the  two  words, 
since  in  the  last  half-dozen  lessons  we  have  been  dealing 
with  several  organisms  which  can  be  assigned  without  hesi- 
tation to  one  or  other  of  the  two  great  groups  of  living  things. 
No  one  would  dream  of  calling  Paramcecium  and  Stylonychia 
plants,  or  Mucor  and  Vaucheria  animals,  and  we  may  there- 
fore use  these  forms  as  a  starting-point  in  an  attempt  to  form 
a  clear  conception  of  what  the  names  plant  and  animal  really 
signify,  and  how  far  it  is  possible  to  place  the  lowly  organisms 
described  in  the  earlier  lessons  in  either  the  vegetable  or  the 
animal  kingdom. 

Let  us  consider,  first  of  all,  the  chief  points  of  resemblance 
and  of  difference  between  the  indubitable  animal  Paramcecium 
on  the  one  hand,  and  the  two  indubitable  plants  Mucor  and 
Vaucheria  on  the  other. 

In  the  first  place,  the  essential  constituents  of  all  three 
organisms  is  protoplasm,  in  which  are  contained  one  or  more 
nuclei.  But  in  Paramcecium  the  protoplasm  is  invested 


LESS,  xvn          DIFFERENCES  IN  NUTRITION  177 

only  by  a  delicate  cuticle  interrupted  at  the  mouth  and  anus, 
while  in  Mucor  and  Vaucheria  the  outer  layer  is  formed  by 
a  firm,  continuous  covering  of  cellulose. 

We  thus  have  as  the  first  morphological  difference  between 
our  selected  animal  and  vegetable  organisms  the  absence  of 
a  cellulose  cell-wall  in  the  former  and  its  presence  in  the 
latter.  This  is  a  fundamental  distinction,  and  applies 
equally  well  to  the  higher  forms.  The  constituent  cells  of 
plants  are  in  nearly  all  cases  covered  with  a  cellulose  coat 
(p.  60),  while  there  is  no  case  among  the  higher  animals  of 
cells  being  so  invested. 

Next,  let  us  take  a  physiological  character.  In  all  three 
organisms  there  is  constant  waste  of  substance  which  has  to 
be  made  good  by  the  conversion  of  food  material  into  proto- 
plasm :  in  other  words,  constructive  and  destructive  meta- 
bolism are  continually  being  carried  on.  But  when  we  come 
to  the  nature  of  the  food  and  the  mode  of  its  reception,  we 
meet  at  once  with  a  very  fundamental  difference.  In  Para- 
moecium  the  food  consists  of  living  organisms  taken  whole 
into  the  interior  of  the  body,  and  the  digestion  of  this  solid 
proteinaceous  food  is  the  necessary  prelude  to  constructive 
metabolism.  In  Vaucheria  the  food  consists  of  a  watery 
solution  of  carbon  dioxide  and  mineral  salts — i.e.,  it  is  liquid 
and  inorganic,  its  nitrogen  being  in  the  form  of  nitrates  or 
of  simple  ammonia  compounds.  Mucor,  like  Paramoecium, 
contains  no  chlorophyll,  and  is  therefore  unable  to  use 
carbon  dioxide  as  a  food  :  like  Vaucheria,  it  is  prevented 
by  its  continuous  cellulose  investment  from  ingesting  solid 
food,  and  is  dependent  upon  an  aqueous  solution.  It  takes 
its  carbon  in  the  form  of  sugar  or  some  such  compound, 
while  it  can  make  use  of  nitrogen  either  in  the  simple  form 
of  a  nitrate  or  an  ammonia  salt,  or  in  the  complex  form  of 
proteids  or  peptones. 

N 


178        CHARACTERS   OF  ANIMALS    AND    PLANTS      LESS. 

In  this  case  also  our  selected  organisms  agree  with  animals 
and  plants  generally.  Animals,  with  the  exception  of  some 
internal  parasites,  ingest  solid  food,  and  they  must  all  have 
their  nitrogen  supplied  in  the  form  of  proteids,  being  unable 
to  build  up  their  protoplasm  from  simpler  compounds. 
Plants  take  their  food  in  the  form  of  a  watery  solution  ; 
those  which  possess  chlorophyll  take  their  carbon  in  the 
form  of  carbon  dioxide  and  their  nitrogen  in  that  of  a  nitrate 
or  ammonia  salt :  those  devoid  of  chlorophyll  cannot,  ex- 
cept in  the  case  of  some  bacteria,  make  use  of  carbon 
dioxide  as  a  food,  and  are  able  to  obtain  nitrogen  either 
from  simple  salts  or  from  proteids.  Chlorophyll-less  plants 
are  therefore  nourished  partly  like  green  plants,  partly  like 
animals. 

This  difference  in  the  character  of  the  food  is  connected 
with  a  morphological  difference.  Animals  have,  as  a 
rule,  an  ingestive  aperture  or  mouth,  and  some  kind  of 
digestive  cavity,  either  permanent  (stomach)  or  temporary 
(food-vacuole).  In  plants  neither  of  these  structures 
exists. 

Another  difference  which  was  referred  to  at  length  in  an 
early  lesson  (p.  32),  is  not  strictly  one  between  plants  and 
animals,  but  between  organisms  with  and  organisms  without 
chlorophyll.  It  is  that  in  green  plants  the  nutritive  processes 
result  in  deoxidation,  more  oxygen  being  given  out  than  is 
taken  in  :  while  in  animals  and  not-green  plants  the  precise 
contrary  is  the  case. 

There  is  also  a  difference  in  the  method  of  excretion.  In 
Paramoecium  there  is  a  special  structure,  the  contractile 
vacuole,  which  collects  the  superfluous  water  taken  in  with 
the  food  and  expels  it,  doubtless  along  with  nitrogenous  and 
other  waste  matters.  In  Vaucheria  and  Mucor  there  is  no 
contractile  vacuole,  and  excretion  is  simply  performed  by 


xvn  DEFINITIONS  179 

diffusion  from  the  general  surface  of  the  organism  into  the 
surrounding  medium. 

This  character  also  is  of  some  general  importance.  The 
large  majority  of  animals  possess  a  special  organ  of  excretion, 
plants  have  nothing  of  the  kind. 

Another  difference  has  to  do  with  the  general  form  of  the 
organism.  Paramoecium  has  a  certain  definite  and  constant 
shape,  and  when  once  formed  produces  no  new  parts. 
Vaucheria  and  Mucor  are  constantly  forming  new  branches, 
so  that  their  shape  is  always  changing  and  their  growth  can 
never  be  said  to  be  complete. 

Finally,  we  have  what  is  perhaps  the  most  obvious  and 
striking  distinction  of  all.  Paramoecium  possesses  in  a  con- 
spicuous degree  the  power  of  automatic  movement ;  in  both 
Mucor  and  Vaucheria  the  organism,  as  a  whole,  exhibits  no 
automatism  but  only  the  slow  movements  of  growth.  The 
spores  and  sperms  of  Vaucheria  are,  however,  actively 
motile. 

Thus,  taking  Paramoecium  as  a  type  of  animals,  and 
Mucor  and  Vaucheria  as  types  of  plants,  we  may  frame  the 
following  definitions  : — 

Animals  are  organisms  of  fixed  and  definite  form,  in  which 
the  cell-body  is  not  covered  with  a  cellulose  wall.  They 
ingest  solid  proteinaceous  food,  their  nutritive  processes 
result  in  oxidation,  they  have  a  definite  organ  of  excretion, 
and  are  capable  of  automatic  movement. 

Plants  are  organisms  of  constantly  varying  form  in  which 
the  cell-body  is  surrounded  by  a  cellulose  wall ;  they  cannot 
ingest  solid  food,  but  are  nourished  by  a  watery  solution  of 
nutrient  materials.  If  chlorophyll  is  present  the  carbon 
dioxide  of  the  air  serves  as  a  source  of  carbon,  nitrogen  is 
obtained  from  simple  salts,  and  the  nutritive  processes 

N    2 


iSo        CHARACTERS   OF   ANIMALS   AND  PLANTS     LESS. 

result  in  deoxidation ;  if  chlorophyll  is  absent  carbon  is 
obtained  from  sugar  or  some  similar  compound,  nitrogen 
either  from  simple  salts  or  from  proteids,  and  the  process  of 
nutrition  is  one  of  oxidation.  There  is  no  special  excretory 
organ,  and,  except  in  the  case  of  certain  reproductive  bodies, 
there  is  usually  no  locomotion. 

Let  us  now  apply  these  definitions  to  the  simple  forms 
described  in  the  first  eight  lessons,  and  see  how  far  they 
will  help  us  in  placing  those  organism  in  one  or  other  of  the 
two  "kingdoms"  into  which  living  things  are  divided. 

Amoeba  has  a  cell- wall,  probably  nitrogenous,  in  the 
resting  condition  :  it  ingests  solid  proteids,  its  nutrition  being 
therefore  holozoic  :  it  has  a  contractile  vacuole  :  and  it 
performs  amoeboid  movements.  It  may  therefore  be  safely 
considered  as  an  animal. 

Haematococcus  has  a  cellulose  wall  :  it  contains  chloro- 
phyll and  its  nutrition  is  purely  holophytic  :  a  contractile 
vacuole  is  present  in  H.  lacustris  but  absent  in  H.  pluvialis  : 
and  its  movements  are  ciliary. 

Euglena  has  a  cellulose  wall  in  the  encysted  state  :  in 
virtue  of  its  chlorophyll  it  is  nourished  by  the  absorption  of 
carbon  dioxide  and  mineral  salts,  but  it  can  also  ingest  solid 
food  through  a  special  mouth  and  gullet  :  it  has  a  contractile 
vacuole,  and  performs  both  euglenoid  and  ciliary  move- 
ments. 

In  both  these  organisms  we  evidently  have  conflicting 
characters  :  the  cellulose  wall  and  holophytic  nutrition 
would  place  them  both  among  plants,  while  from  the  con- 
tractile vacuole  and  active  movements  of  both  genera  and 
from  the  holozoic  nutrition  of  Euglena  we  should  group 
them  with  animals.  That  the  difficulty  is  by  no  means 


xvii  DOUBTFUL   FORMS  181 

easily  overcome  may  be  seen  from  the  fact  that  both  genera 
are  claimed  at  the  present  day  both  by  zoologists  and  by 
botanists.  For  instance,  Prof.  Huxley  considers  Haema- 
tococcus  as  a  plant,  and  expresses  doubts  about  Euglena ; 
Mr.  Saville  Kent  ranks  Hsematococcus  as  a  plant  and 
Euglena  as  an  animal ;  Prof.  Sachs  and  Mr.  Thiselton 
Dyer  place  both  genera  in  the  vegetable  kingdom  ;  while 
Profs.  Ray  Lankester  and  Biitschli  group  them  both  among 
animals. 

In  Heteromita  the  only  cell-wall  is  the  delicate  cuticle 
which  in  the  zygote  is  firm  enough  to  hold  the  spores  up  to 
the  moment  of  their  escape  :  food  is  taken  exclusively  by 
absorption  and  nutrition  is  wholly  saprophytic  :  there  is  a 
contractile  vacuole,  and  the  movements  are  ciliary. 

Here  again  the  characters  are  conflicting  :  the  probable 
absence  of  cellulose,  the  contractile  vacuole  and  the  cilia 
all  have  an  "  animal  "  look,  but  the  mode  of  nutrition  is 
that  of  a  fungus. 

In  Protomyxa  there  is  a  decided  preponderance  of  animal 
characteristics — ingestion  or  living  prey,  and  both  amoeboid 
and  ciliary  movements.  There  is  no  chlorophyll,  and  the 
composition  of  the  cell-wall  is  not  known. 

In  the  Mycetozoa,  the  life-history  of  which  so  closely 
resembles  that  of  Protomyxa,  the  cyst  in  the  resting  stage 
consists  of  cellulose,  and  so  does  the  cell-wall  of  the  spore  : 
nutrition  is  holozoic,  a  contractile  vacuole  is  present  in  the 
flagellulre,  and  both  amoeboid  and  ciliary  movements  are 
performed.  Here  again  we  have  a  puzzling  combination  of 
animal  and  vegetable  characters,  and  as  a  consequence  we 
find  these  organisms  included  among  plants — under  the 
name  of  Myxomycetes  or  "  slime- fungi  " — by  Sachs  and 
Goebel,  while  De  Bary,  Biitschli,  and  Ray  Lankester  place 
them  in  the  animal  kino-dom. 


i§2        CHARACTERS    OF   ANIMALS   AND   PLANTS     LESS. 

In  Saccharomyces  there  is  a  clear  preponderance  of 
vegetable  characters.  The  cell-wall  consists  of  cellulose, 
nutrition  takes  place  by  absorption  and  proteids  are  not  essen- 
tial, there  is  no  contractile  vacuole,  and  no  motile  phase. 

Lastly,  in  the  Bacteria  the  cell-wall  is  composed  of  cellu- 
lose, nutrition  is  usually  saprophytic,  there  is  no  contractile 
vacuole,  and  the  movements  are  ciliary.  So  that  in  all  the 
characters  named,  save  in  the  presence  of  cellulose  and  the 
absence  of  a  contractile  vacuole,  the  Bacteria  agree  with 
Heteromita,  yet  they  are  universally — except  by  Prof.  Glaus 
— placed  among  plants,  while  Heteromita  is  as  constantly 
included  among  animals. 

We  see  then  that  while  it  is  quite  easy  to  divide  the  higher 
organisms  into  the  two  distinct  groups  of  plants  and  animals, 
any  such  separation  is  by  no  means  easy  in  the  case  of  the 
lowest  forms  of  life.  It  was  in  recognition  of  this  fact  that 
Haeckel  proposed,  many  years  ago,  to  institute  a  third 
"  kingdom,"  called  Protista,  to  include  all  unicellular  organ- 
isms. Although  open  to  many  objections  in  practice,  there 
is  a  great  deal  to  be  said  for  the  proposal.  From  the  strictly 
scientific  point  of  view  it  is  quite  as  justifiable  to  make  three 
subdivisions  of  living  things  as  two  :  the  line  between  animals 
and  plants  is  quite  as  arbitrary  as  that  between  protists  and 
plants  or  between  protists  and  animals,  and  no  more  so  :  the 
chief  objection  to  the  change  is  that  it  doubles  the  difficulties 
by  making  two  artificial  boundaries  instead  of  one. 

The  important  point  for  the  student  to  recognize  is  that 
these  boundaries  are  artificial,  and  that  there  are  no  scientific 
frontiers  in  Nature.  As  in  the  liquefaction  of  gases  there  is 
a  "critical  point  "  at  which  the  substance  under  experiment 
is  neither  gaseous  nor  liquid  :  as  in  a  mountainous  country 
it  is  impossible  to  say  where  mountain  ends  and  valley 


xvir  PROTISTA  183 

begins  :  as  in  the  development  of  an  animal  it  is  futile  to 
argue  about  the  exact  period  when,  for  instance,  the  egg 
becomes  a  tadpole  or  the  tadpole  a  frog :  so  in  the  case 
under  discussion.  The  distinction  between  the  higher 
plants  and  animals  is  perfectly  sharp  and  obvious,  but  when 
the  two  groups  are  traced  downwards  they  are  found 
gradually  to  merge,  as  it  were,  into  an  assemblage  of  organ- 
isms which  partake  of  the  characters  of  both  kingdoms,  and 
cannot  without  a  certain  violence  be  either  included  in  or 
excluded  from  either.  Where  any  given  "  protist  "  has  to 
be  classified  the  case  must  be  decided  on  its  individual 
merits :  the  organism  must  be  compared  in  detail  with  all 
those  which  resemble  it  closely  in  structure,  physiology,  and 
life-history  :  and  then  a  balance  must  be  struck  and  the 
doubtful  form  placed  in  the  kingdom  with  which  it  has, 
on  the  whole,  most  points  in  common. 

It  will  no  doubt  occur  to  the  reader  that,  on  the  theory  of 
evolution,  we  may  account  for  the  fact  of  the  animal  and 
vegetable  kingdoms  being  related  to  one  another  like  two 
trees  united  at  the  root,  by  the  hypothesis  that  the  earliest 
organisms  were  protists,  and  that  from  them  animals  and 
plants  were  evolved  along  divergent  lines  of  descent.  And 
in  this  connection  the  fact  that  some  bacteria — the  simplest 
organisms  known  and  devoid  of  chlorophyll— may  flourish 
in  solutions  wholly  devoid  of  organic  matter,  is  very 
significant. 


LESSON  XVIII 

PENICILLIUM    AND    AGARICUS 

ONE  of  the  commonest  and  most  familiar  of  the  lower 
organisms  is  the  "  green  mould  "  which  so  quickly  covers 
with  a  thick  sage-green  growth  any  organic  substances  ex- 
posed to  damp,  such  as  paste,  jam,  cheese,  leather,  &c. 
This  mould  is  a  plant  belonging,  like  Mucor,  to  the  group 
of  Fungi,  and  is  called  Penicillium  glaucum. 

Examined  with  the  naked  eye  a  growth  of  Penicillium  is 
seen  to  have  a  powdery  appearance,  and  if  the  finger  is 
passed  over  it  a  quantity  of  extremely  fine  dust  of  a  sage- 
green  colour  comes  away.  This  dust  consists,  as  we  shall 
see,  of  the  spores  of  Penicillium.  The  best  way  to  study 
the  plant  is  to  sow  some  of  the  spores  in  a  saucer  of 
Pasteur's  solution  by  'drawing  a  needle  or  brush  over  a 
growth  of  the  mould  and  stirring  it  round  in  the  fluid. 

It  is  as  well  to  study  the  naked-eye  appearances  first.  If 
the  quantity  of  spores  taken  is  not  too  large  and  they  are 
sufficiently  well  diffused  through  the  fluid,  little  or  no  trace 
of  them  will  be  apparent  to  the  naked  eye.  After  a  few 
days,  however,  extremely  small  white  dots  appear  on  the 
surface  of  the  fluid ;  these  increase  in  size  and  are  seen, 
especially  by  the  aid  of  a  hand-magnifier,  to  consist  of  little 


LESS,  xvni  MYCELIUM  185 

discs,  circular  or  nearly  so  in  outline,  and  distinctly  thicker 
in  the  centre  than  towards  the  edge :  they  float  on  the  fluid 
so  that  their  upper  surfaces  are  dry.  Each  of  these  patches 
is  a  young  Penicillium-growth,  formed,  as  will  be  seen 
hereafter,  by  the  germination  of  a  group  of  spores. 

As  the  growths  are  examined  day  by  day  they  are  found 
to  increase  steadily  in  size,  and  as  they  do  so  to  become 
thicker  and  thicker  in  the  middle  :  their  growth  is  evidently 
centrifugal.  The  thicker  central  portion  acquires  a  fluffy 
appearance,  and,  by  the  time  the  growth  has  attained  a 
diameter  of  about  4  or  5  mm.,  a  further  conspicuous  change 
takes  place  :  the  centre  of  the  patch  acquires  a  pale  blue 
tint,  the  circumference  still  remaining  pure  white.  When 
the  diameter  has  increased  to  about  6-10  mm.  the  colour  of 
the  centre  gradually  changes  to  dull  sage-green  :  around  this 
is  a  ring  of  light  blue,  and  finally  an  outer  circle  of  white. 
In  all  probability  some  of  the  growths,  several  of  which  will 
most  likely  occur  in  the  saucer,  will  by  this  time  be  found 
to  have  come  together  by  their  edges  :  they  then  become 
completely  interwoven,  their  original  boundaries  remaining 
evident  for  some  time  by  their  white  tint.  Sooner  or  later, 
however,  the  white  is  replaced  by  blue  and  the  blue  by  sage- 
green,  until  the  whole  surface  of  the  fluid  is  covered  by  a 
single  growth  of  a  uniform  green  colour. 

Even  when  they  are  not  more  than  2-3  mm.  in  diameter 
the  growths  are  strong  enough  to  be  lifted  up  from  the  fluid, 
and  are  easily  seen  under  a  low  power  to  be  formed  of  a 
tough,  felt-like  substance,  the  mycelium,  Fig.  40,  A  (iny\  from 
the  upper  surface  of  which  delicate  threads,  the  aerial 
]iyph<z  (a.  hy.\  grow  vertically  upwards  into  the  air,  while 
from  its  lower  surface  similar  but  shorter  threads,  the  sub- 
merged hyphce  (s.  hy.\  hang  vertically  downwards  into  the 
fluid. 


FIG.  40. — Penicilliwn  glaticum. 

A,  Diagrammatic  vertical  section  of  a  young  growth  (  x  5),  showing 
mycelium  (my],  submerged  hyphae  (s.  hy\  and  aerial  hyphse  (a.  Ay). 

B,  group  of  spores  :  I,  before  commencement  of  germination  ;  2,  after 
imbibition  of  fluid  :  the  remaining  three  have  begun  to  germinate. 

C,  very  young  mycelium  formed  by  a  small  group    of  germinating 
spores. 


LESS,  xvin  MULTICELLULAR   HYPILE  187 

D,  more  advanced  mycelium  :  the  hyphae  have  increased  in   length 
and  begun  to  branch,  and  septa  (sep)  have  appeared. 

E,  germinating   spore   (sp)  very  highly  magnified,  sending  out  one 
short  and  one  long  hypha,  the  latter  with  a  short  lateral  branch  and 
several  septa  (sep).     Both  spore  and  hyphae  contain  vacuoles  (vac]  in 
their  protoplasm. 

F^F4,  development  of  the  spore-bearing  brushes  by  repeated  branch- 
ing of  an  aerial  hypha  :  the  short  terminal  branches  or  sterigmata  are 
already  being  constricted  to  form  spores. 

F5,  a  fully-developed  brush  with  a  row  of  spores  developed  from  each 
sterigma  (sfg). 

F6,  a  single  sterigma  (stg)  with  its  spores  (sp). 

F7,  an  over-ripe  brush  in  which  the  structure  is  obscured  by  spores 
which  have  dropped  from  the  sterigmata. 

B-D,  Fa-F5,  and  F7  x   150  :  F6  x  200  :  E  x  500. 

As  long  as  the  growths  are  white  or  blue  in  colour  no 
powder  can  be  detached  by  touching  the  aerial  hyphae, 
showing  that  the  spores  are  not  yet  fully  formed,  but  as  soon 
as  the  permanent  green  hue  is  attained  the  slightest  touch 
is  sufficient  to  detach  large  quantities  of  spores. 

A  bit  of  the  felt-like  mycelium  is  easily  teased  out  or  torn 
asunder  with  two  needles,  and  is  then  found,  like  actual  felt, 
to  be  formed  of  a  close  interlacement  of  delicate  threads  (D). 
These  are  the  mycelial  hyphce  :  they  are  regularly  cylindrical, 
about  TJy  mm.  in  diameter,  frequently  branched,  and  differ 
in  an  important  particular  from  the  somewhat  similar  hyphse 
of  Mucor  (p.  161).  The  protoplasm  is  not  continuous,  but 
is  interrupted  at  regular  intervals  by  transverse  partitions  or 
septa  (D,  E,  sep).  In  other  words,  a  hypha  of  Penicillium 
is  normally  what  a  hypha  of  Mucor  becomes  under  un- 
favourable conditions  (p.  162),  multicelhtlar^  the  septa 
dividing  it  into  separate  portions,  each  of  which  is 
morphologically  comparable  to  a  single  yeast  cell. 

Penicillium  shows  therefore  a  very  important  advance  in 
structure  over  the  organisms  hitherto  considered.  While  in 
these  latter  the  entire  organism  is  a  single  cell ;  in  Peni- 


i88  .PENICILLIUM   AND   AGARICUS  LESS. 

cillium  it  is  a  cell-aggregate — an  accumulation  of  numerous 
cells  all  in  organic  connection  with  one  another.  As  the 
cells  are  arranged  in  a  single  longitudinal  series,  Penicillium 
is  an  example  of  a  linear  aggregate. 

Each  cell  is  surrounded,  as  already  described,  by  a  wall 
of  cellulose  :  its  protoplasm  is  more  or  less  vacuolated  (E,  vac\ 
sometimes  so  much  so  as  to  form  a  mere  thin  layer  within 
the  cell-wall,  the  whole  interior  of  the  cell  being  occupied  by 
one  large  vacuole.  Recently,  by  staining  with  logwood, 
numerous  nuclei  have  been  found,  so  that  the  Penicillium 
cell,  like  an  Oxytricha  (p.  120),  or  a  filament  of  Mucor  or 
Vaucheria,  is  multinucleate. 

The  submerged  hyphae  have  the  same  structure,  but  it  is 
easier  to  find  their  actual  ends  than  those  of  the  mycelial 
hyphse.  The  free  extremity  tapers  to  a  blunt  point  where 
the  cellulose  wall  is  thinner  than  elsewhere  (see  E). 

The  aerial  hyphae  from  the  youngest  (white)  part  of  a 
growth  consist  of  unbranched  filaments,  but  taken  from  a 
part  which  is  just  beginning  to  turn  blue  they  are  found  to 
have  a  very  characteristic  appearance  (r1 — F4).  Each  sends 
off  from  its  distal  or  upper  end  a  larger  or  smaller  number  of 
branches  which  remain  short  and  grow  parallel  to  one 
another  :  the  primary  branches  (r1,  F2)  form  secondary  ones 
(FB),  and  the  secondary  tertiary  (r4),  so  that  the  hypha  finally 
assumes  the  appearance  of  a  little  brush  or  pencil,  or  more 
accurately  of  a  minute  cactus  with  thick-set  forking  branches. 
The  ultimate  or  distal  branches  are  short  cells  called  sterig- 
mata  (r5,  sfg). 

Next,  the  ends  of  the  sterigmata  become  constricted, 
exactly  as  if  a  thread  were  tied  round  them  and  gradually 
tightened  (r1,  F°),  the  result  being  to  separate  the  distal  end 
of  the  sterigma  as  a  globular  daughter-cell,  in  very  much  the 
same  way  as  a  bud  is  separated  in  Saccharomyces  (p.  72). 


GERMINATION    OF   SPORES  189 

In  this  way  a  spore  is  produced.  The  process  is  repeated, 
the  end  of  the  sterigma  is  constricted  again  and  a  new  spore 
formed,  the  old  one  being  pushed  further  onwards.  By  a 
continual  repetition  of  the  same  process  a  longitudinal  row 
of  spores  is  formed  (r5,  F6),  of  which  the  proximal  or  lower 
one  is  the  youngest,  the  distal  or  upper  one  the  oldest.  The 
spores  grow  for  some  time  after  their  formation,  and  are 
therefore  found  to  become  larger  and  larger  in  passing  from 
the  proximal  to  the  distal  end  of  the  chain  (F°).  Sooner  or 
later  they  lose  their  connection  with  each  other,  become 
detached,  and  fall,  covering  the  whole  growth  with  a  fine 
dust  which  readily  adheres  to  all  parts  owing  to  the  some- 
what sticky  character  of  the  spores.  In  this  stage  it  is  by 
no  means  easy  to  make  out  the  structure  of  the  brushes, 
since  they  are  quite  obscured  by  the  number  of  spores 
adhering  to  them  (F"). 

It  is  at  the  period  of  complete  formation  of  the  spores  that 
the  growth  turns  green.  The  colour  is  not  due  to  the  pres- 
ence of  chlorophyll.  Under  a  high  power  the  spores  appear 
quite  colourless,  whereas  a  cell  of  the  same  size  coloured 
with  chlorophyll  would  appear  bright  green. 

The  germination  of  the  spores  can  be  readily  studied  by 
sowing  them  in  a  drop  of  Pasteur's  solution  in  a  moist  chamber 
(Fig.  37,  p.  163).  The  spores,  several  of  which  usually  adhere 
together,  are  at  first  clear  and  bright  (B1)  :  soon  they  swell 
considerably,  and  the  protoplasm  becomes  granular  and 
vacuolated  (B2)  :  in  this  stage  they  are  hardly  distinguishable 
from  yeast-cells  (compare  Fig.  13,  p.  71).  Then  one  or  more 
buds  spring  from  each  and  elongate  into  hyphae  (B,  c),  just 
as  in  Mucor.  But  the  difference  between  the  tw.o  moulds  is 
soon  apparent  :  by  the  time  a  hypha  has  grown  to  a  length 
equal  to  about  six  or  eight  times  its  own  diameter,  the  pro- 
toplasm in  it  divides  transversely  and  a  cellulose  septum  is 


190  PENICILLIUM    AND   AGARICUS  LESS. 

formed  (  D,  E.  sep)  dividing  the  young  hypha  into  two  cells 
(compare  Fig.  36,  H,  p.  159).  The  distal  cell  then  elongates 
and  divides  again,  and  in  this  way  the  hyphae  are,  almost  from 
the  first,  divided  into  cells  of  approximately  equal  length. 

The  mode  of  growth  of  the  distal  or  apical  cell  of  a  hypha 
is  probably  as  follows.  The  free  end  tapers  slightly  (E)  and 
the  cellulose  wall  thins  out  as  it  approaches  the  apex.  The 
protoplasm  performing  constructive  more  rapidly  than  de- 
structive metabolism  increases  in  volume,  and  its  tendency  is 
to  grow  in  all  directions  :  as,  however,  the  cellulose  mem- 
brane surrounding  it  is  thinner  at  the  apex  than  elsewhere, 
it  naturally,  on  the  principle  of  least  resistance,  extends  in 
that  direction,  thus  increasing  the  length  of  the  cell  without 
adding  to  its  thickness.  Thus  the  growth  of  a  hypha  of 
Penicillium  is  apical,  i.e.  takes  place  only  at  the  distal  end,  the 
cells  once  formed  ceasing  to  grow.  Thus  also  the  oldest  cells 
are  those  nearest  the  original  spore  from  which  the  hypha 
sprang,  the  youngest  those  furthest  removed  from  it. 

A  process  which  has  been  described  as  sexual,  sometimes,  but  appa- 
rently very  rarely,  occurs  in  Penicillium,  and  is  said  to  consist  essentially 
in  the  conjugation  of  two  gametes  having  the  form  of  twisted  hyphge, 
and  the  subsequent  development  of  spores  in  the  resulting  branched 
zygote.  But  as  the  details  of  the  process  are  complicated  and  its  sexual 
character  is  doubtful,  it  is  considered  best  to  do  no  more  than  call 
attention  to  it.  The  student  is  referred  to  Brefeld's  original  account  of 
the  process  in  the  Quarterly  Journal  of  Microscopical  Science,  vol.  xv. , 
p.  342.  The  so-called  sexual  reproduction  of  the  closely-  allied  Eurotium 
is  described  in  Huxley  and  Martin's  Elementary  Biology  (new  edition), 
p.  419,  and  figured  in  Howes's  Atlas  of  Elementary  Biology,  pi.  xix., 
figs,  xxvi  and  xxvii. 

•  The  nutrition  of  Penicillium  is  essentially  like  that  of  Mucor 
(p.  167).  But,  as  it  has  been  remarked,  "  it  is  often  content 
with  the  poorest  food  which  would  be  too  bad  for  higher 
fungi.  It  lives  in  the  human  ear  ;  it  does  not  shun  cast-off 


xvin  PILEUS    AND    LAMELLA  191 

clothes,  damp  boots,  or  dried-up  ink.  Sometimes  it  contents 
itself  with  a  solution  of  sugar  with  a  very  little  [nitrogenous] 
organic  matter,  at  other  times  it  appears  as  if  it  preferred  the 
purest  solution  of  a  salt  with  only  a  trace  of  organic  matter. 
It  will  even  tolerate  the  hurtful  influence  of  poisonous 
solutions  of  copper  and  arsenious  acid."  It  flourishes  best 
in  a  solution  of  peptones  and  sugar. 

This  eclecticism  in  matters  of  diet  is  one  obvious  ex- 
planation of  the  universal  occurrence  of  Penicillium  ;  another 
is  the  extraordinary  vitality  of  the  spores.  They  will  ger- 
minate at  any  temperature  between  1*5°  and  43°  C.,  the 
optimum  being  about  22°  C.  They  are  not  killed  by  a  dry 
heat  of  1 08°  C.,  and  some  will  even  survive  a  temperature 
of  120°.  And  lastly,  they  will  germinate  after  being  kept 
for  two  years. 

We  have  seen  that  the  form  of  a  Penicillium  growth  is  ir- 
regular, and  is  determined  by  the  surface  on  which  it  grows. 
There  are,  however,  certain  fungi  which  are  quite  constant 
and  determinate  both  in  form  and  size,  and  are  yet  found 
on  analysis  to  be  formed  exclusively  of  interlaced  hyphae, 
that  is,  to  belong  to  the  type  of  linear  aggregates.  Among 
the  most  striking  of  these  are  the  mushrooms  and  toad- 
stools. 

A  mushroom  (Agaricus)  consists  of  a  stout  vertical  stalk 
(Fig.  41,  A,  £/),  on  the  upper  or  distal  end  of  which  is  borne 
an  umbrella-like  disc  or  pileus  (/).  The  lower  or  proximal 
end  of  the  stalk  is  in  connection  with  an  underground 
mycelium  (my),  from  which  it  springs. 

On  the  underside  of  the  pileus  are  numerous  radiating 
vertical  plates  or  lamella  (I)  extending  a  part  or  the  whole 
of  the  distance  from  the  circumference  of  the  pileus  to  the 
stalk.  In  the  common  edible  mushroom  (Agaricus  cam- 


192 


PENICILLIUM   AND    AGARICUS 


LESS. 


pestris)  these  lamellae   are   pink   in  young  specimens,  and 
afterwards  become  dark  brown. 

The  mushroom  is  too  tough  to  be  readily  teased  out  like 


FlG.  ^i.—Agaricus  campestrts. 

A,  Diagrammatic  vertical  section,   showing  the  stalk   (st)   springing 
from  a  mycelium  (my],  and  expanding  into  the  pileus  (/),  on  the  under 
side  of  which  are  the  radiating  lamella?  (/). 

B,  transverse  vertical  section  of  a  lamella,  showing  the  hypha;   (hy} 
turning  outwards  to  form  the  layer  of  club-shaped  cells  (a)  from  which 
the  sterigmata  spring. 

C,  one  of  the  club-shaped  cells  (a),  highly  magnified,  showing  its  two 
sterigmata  (^),  each  bearing  a  spore  (sp). 

(B  and  C  after  Sachs.) 

the  mycelium  of  Penicillium,  and  its  structure  is  best  in- 
vestigated by  cutting  thin  sections  of  various  parts  and 
examining  them  under  a  high  power. 


XVTII  HISTOLOGY   OF  MUSHROOM  193 

Such  sections  show  the  whole  mushroom  to  be  composed 
of  immense  numbers  of  closely  interwoven,  branched  hyphse 
(B)  divided  by  numerous  septa  into  cells.  In  the  stalk  the 
hyphae  take  a  longitudinal  direction  ;  in  the  pileus  they  turn 
outwards,  passing  from  the  centre  to  the  circumference,  and 
finally  send  branches  downwards  to  form  the  lamellae.  Fre- 
quently the  hyphae  are  so  closely  packed  as  to  be  hardly 
distinguishable  one  from  another. 

At  the  surfaces  of  the  lamellae  the  hyphae  turn  outwards, 
so  that  their  ends  are  perpendicular  to  the  free  surfaces  of 
those  plates.  Their  terminal  cells  become  dilated  or  club- 
shaped  (B,  c,  a),  and  give  off  two  small  branches  or  sterig- 
mata  (c,  stg),  the  ends  of  which  swell  up  and  become 
constricted  off  as  spores  (sp).  These  fall  on  the  ground  and 
germinate,  forming  a  mycelium  from  which  more  or  fewer 
mushrooms  are  in  due  course  .produced. 

Thus  in  point  of  structure  a  mushroom  bears  much  the 
same  relation  to  Penicillium  as  Caulerpa  (p.  175)  bears  to 
Vaucheria.  Caulerpa  shows  the  extreme  development  of 
which  a  single  branched  cell  is  capable,  the  mushroom  how 
complicated  in  structure  and  definite  in  form  a  simple  linear 
aggregate  may  become. 


LESSON  XIX 

SPIROGYRA 

AMONGST  the  numerous  weeds  which  form  a  green  scum 
in  stagnant  ponds  and  slowly-flowing  streams,  one,  called 
Spirogyra,  is  perhaps  the  commonest.  It  is  recognised  at 
once  under  a  low  power  by  the  long  delicate  green  filaments 
of  which  it  is  composed  being  marked  with  a  regular  green 
spiral  band. 

Examined  under  the  microscope  the  filaments  are  seen  to 
be,  like  the  hyphse  of  Penicillium,  linear  aggregates,  that  is, 
to  be  composed  of  a  single  row  of  cells  arranged  end  to 
end.  But  in  Penicillium  the  hyphae  are  frequently  branched, 
and  it  is  always  possible  in  an  entire  hypha  to  distinguish 
the  slightly  tapering  distal  end  from  the  proximal  end 
which  springs  either  from  another  hypha  or  from  a  spore. 
In  Spirogyra  the  filaments  do  not  branch,  and  there  is  no 
distinction  between  their  opposite  ends. 

The  cells  of  which  the  filaments  are  composed  (Fig.  42,  A) 
are  cylindrical,  covered  with  a  cellulose  cell-wall  (c.  w\  and 
separated  from  adjacent  cells  by  septa  (sep)  of  the  same 
substance.  The  protoplasmic  cell-body  presents  certain 
characteristic  peculiarities. 

It  has  been  noticed  in  more  than  one  instance  that  in  the 


FIG.  42. — Spirogyra. 

A,  small  portion  of  a  living  filament,  showing  a  single  cell,  with  cell- 
wall  (c.  w),  septa  (sep)  separating  it  from  adjacent  cells,  peripheral  layer 
of  protoplasm  (phm}  connected  by  threads  with  a  central  mass  contain- 


O  2 


196  SPIROGYRA  LESS. 

ing  the  nucleus  (nu),  two  spiral  chromatophores  (chr\   and  pyrenoids 

(pyr\ 

B1,    B2,    middle   portion   of  a   cell,    showing   two   stages   in  binary 
fission. 

C,  four  stages  in  dioecious  conjugation  :  in  c1  the  gonads  (gori^,  gon~) 
are  connected  by  short  processes  of  their  adjacent  sides  :  in  c2  the  active 
or  male  gamete  (gam1}  has  separated  from  the  wall  of  the  gonad  (gon1} 
preparatory  to  passing  across  the  connecting  bridge  to  the  stationary  or 
female  gamate  (gam1*)  which  has  not  yet  separated  from  its  containing 
gonad  (gon2) :  in  C3  the  female  gamete  (gam2)  has  undergone  separa- 
tion, and  the  male  gamete  (gam1)  is  in  the  act  of  conjugating  with  it :  in 
C4  the  two  have  united  to  form  a  zygote  (zyg)  lying  in  the  female  gonad. 

D,  two  stages  in  monoecious  conjugation  :  in  D1  the   adjacent   cells 
(gonads)  have  sent  out  conjugating  processes  (a) :  in  D2  conjugation  is 
complete,  the  male  gamete  having  passed  through  the  aperture  between 
the  conjugating  processes  and  united  with  the  female  gamete  to  form  the 
zygote  (zyg). 

E,  parthenogenetic  formation  of  zygotes. 

F,  fully  developed  zygote  (zygospore). 

G,  early  stage  in  the  germination  of  the  zygote. 

(B   after   Sachs :  C   after   Strasburger :  F   and   G   from    Sachs    after 
Pringsheim.) 


larger  cells  of  plants  the  development  of  vacuoles  is  so  ex- 
tensive that  the  protoplasm  is  reduced  to  a  thin  layer  in 
contact  with  the  cell-wall  (see  pp.  169  and  188).  This  state 
of  things  is  carried  to  excess  in  Spirogyra  :  the  central  vacuole 
is  so  large  that  the  protoplasm  (A,  plsm)  has  the  character 
of  a  mere  delicate  colourless  membrane  within  the  cell-wall : 
to  make  it  out  clearly  the  specimen  should  be  treated  with 
a  fluid  of  greater  density  than  water,  such  as  a  10  per  cent, 
solution  of  sodium  chloride,  which  by  absorbing  the  water 
in  the  vacuole  causes  the  protoplasm  to  shrink  away  from 
the  cell-wall  and  so  brings  it  clearly  into  view.  It  is  to  this 
layer  of  protoplasm  that  the  name  primordial  utricle  is 
applied  by  botanists,  but  the  student  should  remember  that 
a  primordial  utricle  is  not  a  special  constituent  of  those 
cells  in  which  it  occurs,  but  is  merely  the  protoplasm  of  a 
vegetable  cell  in  which  the  vacuole  is  inordinately  large. 
The  protoplasm  of  the  cell  of  Spirogyra  is  not,  however, 


xix  INTERSTITIAL   GROWTH  197 

confined  to  the  primordial  utricle ;  towards  the  centre  of  the 
vacuole  is  a  small  irregular  mass  of  protoplasm  connected  to 
the  peripheral  layer  by  extremely  delicate  protoplasmic 
strands.  Imbedded  in  this  central  mass  is  the  nucleus  (nu\ 
which  has  the  form  of  a  biconvex  lens  and  contains  a  distinct 
nucleolus. 

The  chromatophores  differ  from  anything  we  have  yet 
considered,  having  the  form  of  green  spiral  bands  (chr),  of 
which  each  cell  may  contain  one  (D1)  or  two  coiled  in  oppo- 
site directions  (A).  Imbedded  in  the  chromatophores  are 
numerous  pyrenoids  (pyr,  see  p.  27),  to  which  the  strands 
of  protoplasm  proceeding  from  the  central  nucleus-containing 
mass  can  be  traced. 

The  process  of  growth  in  Spirogyra  is  brought  about  by 
the  binary  fission  of  its  constituent  cells.  It  takes  place 
under  ordinary  circumstances  during  the  night  (i  i — 12  P.M.), 
but  by  keeping  the  plant  cold  all  night  may  be  delayed  until 
morning. 

The  nucleus  divides  by  the  complicated  process  (karyo- 
kinesis)  already  described  in  general  terms  (p.  67),  so  that 
two  nuclei  are  found  at  equal  distances  from  the  centre  of 
the  cell.  The  cell-body  with  its  chromatophores  then  begins 
to  divide  across  the  centre  (B1),  the  process  commencing 
near  the  cell-wall  and  gradually  proceeding  inwards  :  as  it 
goes  on  cellulose  is  secreted  between  the  halves  of  the 
dividing  protoplasm  so  that  a  ring  of  cellulose  is  formed 
lying  transversely  across  the  middle  of  the  cell,  and  in  con- 
tinuity externally  with  the  wall  (B2).  The  ring  is  at  first  very 
narrow,  but  as  the  annular  furrow  across  the  dividing  cell- 
body  deepens,  so  the  ring  increases  in  width,  until  by  the 
time  the  protoplasm  has  divided  it  has  become  a  complete 
partition  separating  the  newly-formed  daughter-cells  from 
one  another. 


198  SPIROGYRA  LESS. 

Any  of  the  cells  of  a  Spirogyra-filament  may  divide  in  this 
way,  so  that  the  filament  grows  by  the  intercalation  of  new 
cells  between  the  old  ones.  This  is  an  example  of  interstitial 
growth.  Note  its  difference  from  the  apical  growth  which 
was  found  to  take  place  in  Penicillium  (p.  190),  a  difference 
which  explains  the  fact  mentioned  above  (p.  194)  that  there  is 
no  distinction  between  the  two  ends  of  a  filament  of  Spirogyra, 
while  in  Penicillium  the  proximal  and  distal  ends  can  always 
be  distinguished  in  a  complete  hypha. 

The  sexual  reproduction  of  Spirogyra  is  interesting,  as 
being  intermediate  between  the  very  different  processes  which 
were  found  to  obtain  in  Mucor  (p.  165)  and  in  Vaucheria 
(p.  172). 

In  summer  or  autumn  adjoining  filaments  become  arranged 
parallel  to  one  another  and  the  opposite  cells  of  each  send 
out  short  rounded  processes  which  meet  (Fig.  43,  c1),  and 
finally  become  united  by  the  absorption  of  the  adjacent  walls, 
thus  forming  a  free  communication  between  the  two  connected 
cells  or  gonads  (gon1,  gon2).  As  several  pairs  of  cells  on  the 
same  two  filaments  unite  simultaneously  a  ladder-like  ap- 
pearance is  produced. 

The  protoplasmic  cell-bodies  (c2,  gam1,  gam2)  of  the  two 
gonads  become  rounded  off  and  form  gametes  or  conjugating 
bodies  (see  p.  166,  note  1)  :  it  is  observable  that  this  process 
of  separation  from  the  wall  of  the  gonad  always  takes  place 
earlier  in  one  gamete  (c2,  gam1)  than  in  the  other  (c2,  c3, 
gam2).  Then  the  gamete  which  is  ready  first  (gam1)  passes 
through  the  connecting  canal  (c3)  and  conjugates  with  the 
other  (gam2),  forming  a  zygote  (c4,  zyg)  which  soon  surrounds 
itself  with  a  thick  cell-wall.  It  has  been  ascertained  that  the 
nuclei  of  the  gametes  unite  to  form  the  single  nucleus  of  the 
zygote. 


xix  CONJUGATION  199 

Thus,  as  in  Mucor,  the  gametes  are  similar  and  equal- 
sized,  and  the  result  of  the  process  is  a  resting  zygote  or 
zygospore.  But  while  in  Mucor  each  gamete  meets  the  other 
half  way,  so  that  there  is  absolutely  no  sexual  differentiation, 
in  Spirogyra,  as  in  Vaucheria,  one  gamete  remains  passive, 
and  conjugation  is  effected  by  the  activity  of  the  other.  So 
that  we  have  here  the  very  simplest  case  of  sexual  differen- 
tiation :  the  gametes,  although  of  equal  size  and  similar  ap- 
pearance, are  divisible  into  an  active  or  male  cell,  correspond- 
ing with  the  sperm  of  Vaucheria,  and  a  passive  or  female 
cell  corresponding  with  the  ovum.  It  will  be  seen  that  in 
Spirogyra  the  whole  of  the  protoplasm  of  each  gonad  is  used 
up  in  the  formation  of  a  single  gamete,  whereas  in  Vaucheria, 
while  this  is  the  case  with  the  ovary,  numerous  gametes 
(sperms)  are  formed  from  the  protoplasm  of  the  spermary. 

In  some  forms  of  Spirogyra  conjugation  takes  place  not 
between  opposite  cells  of  distinct  filaments,  but  between 
adjacent  cells  of  the  same  filament.  Each  of  the  gonads 
sends  out  a  short  process  (D1,  a)  which  abuts  against  a 
corresponding  process  from  the  adjoining  cell :  the  two 
processes  are  placed  in  communication  with  one  another  by 
a  small  aperture  (o2)  through  which  the  male  gamete  makes 
its  way  in  order  to  conjugate  with  the  female  gamete  and 
form  a  zygote  (zyg). 

In  the  ordinary  ladder-like  method  of  conjugation  the 
conjugating  filaments  appear  to  be  of  opposite  sexes,  one 
producing  only  male,  the  other  only  female  gametes  :  the  plant 
in  this  case  is  said  to  be  dioecious,  i.e.,  has  the  sexes  lodged  in 
distinct  individuals,  and  conjugation  is  a  process  of  cross- 
fertilization.  But  in  the  method  described  in  the  preceding 
paragraph  the  individual  filaments  are  monoecious,  i.e.,  produce 
both  male  and  female  cells,  and  conjugation  is  a  process  of 
self-fertilization. 


200  SPIROGYRA  LESS,  xix 

Sometimes  filaments  are  found  in  which  the  protoplasm  of 
certain  cells  separates  from  the  wall,  and  surrounds  itself 
with  a  thick  coat  of  cellulose  forming  a  body  which  is  quite 
indistinguishable  from  a  zygote  (E).  There  seems  to  be 
some  doubt  as  to  whether  such  cells  ever  germinate,  but  they 
have  all  the  appearance  of  female  cells  which  for  some 
reason  have  developed  into  zygote -like  bodies  without  fertil- 
ization. Such  development  from  an  unfertilized  female 
gamete,  although  it  has  not  been  proved  in  Spirogyra  is 
known  to  occur  in  many  cases,  and  is  distinguished  as 
parthenogenesis. 

When  the  zygote  is  fully  developed  (F)  its  cell  wall  is 
divided  into  three  layers,  the  middle  one  undergoing  a 
peculiar  change  which  renders  it  waterproof :  at  the  same 
time  the  starch  in  its  protoplasm  is  replaced  by  oil.  In  this 
condition  it  undergoes  a  long  period  of  rest,  its  structure 
enabling  it  to  offer  great  resistance  to  drought,  frost,  &c. 
Finally  it  germinates  :  the  two  outer  coats  are  ruptured,  and 
the  protoplasm  covered  by  the  inner  coat  protrudes  as  a 
club-shaped  process  (G)  which  gradually  takes  on  the  form 
of  an  ordinary  Spirogyra  filament,  dividing  as  it  does  so  into 
numerous  cells. 

Thus  in  the  present  case,  as  in  Penicillium  and  the 
mushroom,  the  multicellular  adult  organism  is  originally 
unicellular. 

The  nutrition  of  Spirogyra  is  purely  holophytic  :  like 
Haematococcus  and  Vaucheria  it  lives  upon  the  carbon 
dioxide  and  mineral  salts  dissolved  in  the  surrounding 
water.  Like  these  organisms  also  it  decomposes  carbon 
dioxide  and  forms  starch  only  under  the  influence  of 
sunlight. 


LESSON  XX 

MONOSTROMA,  ULVA,  LAMINARIA,  &C. 

IT  was  pointed  out  in  a  previous  lesson  (p.  193)  that  the 
highest  and  most  complicated  fungi,  such  as  the  mushrooms, 
are  found  on  analysis  to  be  built  up  of  linear  aggregates  of 
cells — to  consist  of  hyphee  so  interwoven  as  to  form  struc- 
tures often  of  considerable  size  and  of  definite  and  regular 
form. 

This  is  not  the  case  with  the  Algae  or  lower  green  plants — 
the  group  to  which  Vaucheria,  Caulerpa,  Spirogyra,  the 
diatoms,  and  in  the  view  of  some  authors  Hsematococcus 
and  Euglena,  belong.  These  agree  with  fungi  in  the  fact 
that  the  lowest  among  them  (e.g.  Zooxanthella)  are  unicellu- 
lar, and  others  (e.g.  Spirogyra)  simple  linear  aggregates,  but 
the  higher  forms,  such  as  the  majority  of  sea-weeds,  have 
as  it  were  gone  beyond  the  fungi  in  point  of  structure  and 
attained  a  distinctly  higher  stage  of  morphological  differen- 
tiation. This  will  be  made  clear  by  a  study  of  three  typical 
genera. 

Amongst  the  immense  variety  of  seaweeds  found  in  rock- 
pools  between  high  and  low  water-marks  are  several  kinds 
having  the  form  of  flat  irregular  expansions,  of  a  bright  green 


202 


MONOSTROMA,   ULVA,  LAMINARIA,  &c.         LESS. 


colour  and  very  transparent.  One  of  these  is  the  genus 
Monostroma,  of  which  M.  bullosum  is  a  fresh-water  species. 
Examined  microscopically  the  plant  (Fig.  43)  is  found  to 
consist  of  a  single  layer  of  close-set,  green  cells,  the  cell- walls 
of  which  are  in  close  approximation,  so  that  the  cell-bodies 
appear  as  if  embedded  in  a  continuous  layer  of  transparent 
cellulose.  Thus  Monostroma,  like  Spirogyra,  is  only  one 
cell  thick  (B),  but  unlike  that  genus  it  is  not  one  but  many 


B 


1 

FIG.  43.  — Monostroma. 

A,  surface  view  of  M.  bullosum,  showing  the  cells  embedded  in  a 
common  layer  of  cellulose  :  many   of  them  are  in  various  stages  of 
division. 

B,  vertical  section  of  M.  laceratum,  showing  the  arrangement  of  the 
cells  in  a  single  layer. 

(A  after  Reinke  :  B  after  Cooke. ) 

cells  broad.  In  other  words,  instead  of  being  a  linear  it  is 
a  superficial  aggregate. 

To  use  a  geometrical  analogy  : — a  unicellular  organism 
like  Hsematococcus  may  be  compared  to  a  point ;  a  linear 
aggregate  like  Penicillium  or  Spirogyra  to  a  line  ;  a  superficial 
aggregate  like  Monostroma  to  a  plane. 

Growth  takes  place  by  the  binary  fission  of  the  cells  (A), 
but  here  again  there  is  a  marked  and  important  difference 
from  Spirogyra.  In  the  latter  the  plane  of  division  is  always 
at  right  angles  to  the  long  axis  of  the  filament,  so  that  growth 


xx  SOLID   AGGREGATES  203 

takes  place  in  one  dimension  of  space  only,  namely  in  length. 
In  Monostroma  the  plane  of  division  may  be  inclined  in  any 
direction  provided  it  is  perpendicular  to  the  surface  of  the 
plant,  so  that  growth  goes  on  in  two  dimensions  of  space, 
namely  in  length  and  breadth. 

Another  of  the  flat,  leaf-like,  green  sea-weeds  is  the  very 
common  genus  Ulva,  sometimes  called  "  sea-lettuce."  It 
consists  of  irregular,  more  or  less  lobed  expansions  with 
crinkled  edges,  and  under  the  microscope  closely  resembles 
Monostroma,  with  one  important  difference  :  it  is  formed 
not  of  one  but  of  two  layers  of  cells,  and  is  therefore  not  a 
superficial  but  a  solid  aggregate.  To  return  to  the  geometrical 
analogy  used  above  it  is  to  be  compared  not  to  a  plane  but 
to  a  solid  body. 

As  in  Monostroma  growth  takes  place  by  the  binary 
fission  of  the  cells.  But  these  divide  not  only  along  variously 
inclined  planes  at  right  angles  to  the  surface  of  the  plant 
but  also  along  a  plane  parallel  to  the  surface,  so  that  growth 
takes  place  in  all  three  dimensions  of  space — in  length, 
breadth,  and  thickness. 

Ulva  may  be  looked  upon  as  the  simplest  example  of  a 
solid  aggregate  :  the  largest  and  most  complicated  sea-weeds 
are  the  great  olive-brown  forms  known  as  "tangles"  or 
"kelp,"  so  common  at  low  water-mark.  They  belong  to 
various  genera,  of  which  the  commonest  British  form  is 
Laminaria. 

Laminaria  (Fig.  44,  A)  consists  of  a  cylindrical  stem, 
which  may  be  as  much  as  two  metres  (6  ft.)  in  length  and 
5  or  6  cm.  in  diameter  :  its  proximal  end  is  fastened  to  the 
rocks  by  a  branched,  root-like  structure,  while  distally  it 
expands  into  a  great,  flat,  irregularly-cleft,  leaf-like  body, 


204 


MONOSTROMA,   ULVA,  LAMINARIA,  &c.         LESS. 


which  may  be  as  much  as  2-3  metres  long  and  70-80  cm. 
wide. 

Other  genera  of  tangles  attain  even  greater  dimensions. 
A  common  New  Zealand  genus,  Lessonia  (Fig.  44,  B)  is  a 
gigantic  tree-like  weed,  the  trunk  of  which  is  sometimes 
more  than  three  metres  (9-10  ft.)  long,  and  as  thick  as  a 


FIG.  44. — A,  Laminaria  claustoni,  a  young  plant,  showing  stem  with 
branched  root-like  organ  of  attachment,  and  deeply-cleft  leaves  (about 
£th  natural  size). 

B,  Lessonia  fascescens,  showing  tree-like  form  (about  -faih  natural 
size). 

(A  after  Sachs  :  B  after  Le  Maout  and  Decaisne. ) 

man's  thigh,  while  the  graceful  Macrocystis,  another  southern 
genus,  is  believed  to  attain  a  length  of  over  200  metres 
(700  ft.),  and  is  known  to  grow  as  much  as  5  J  metres  (over 
1 8  ft.)  in  six  months. 

But  in  spite  of  their  immense  size  these  olive  sea-weeds 
are  comparatively  simple  solid  aggregates  of  cells.  Ex- 
amined with  the  naked  eye  the  difference  between  them 


xx  HISTOLOGY   OF   TANGLES  205 

and  a  tree  or  shrub  is  quite  obvious  :  when  cut  across  they 
are  seen  to  consist  of  a  nearly  homogeneous  substance  of 
the  consistency  of  soft  gristle,  neither  bark,  wood,  nor  pith 
being  distinguishable.  Under  the  microscope,  however, 
the  cells  of  which  they  are  composed  are  seen  to  vary 
considerably  in  form  and  size,  some  of  them  even  assuming 
the  characters  of  what  we  shall  learn  in  our  studies  of  the 
higher  plants  (Lesson  XXIX)  to  distinguish  as  sieve-tubes. 


Of   THE 


LESSON    XXI 

NITELLA 

IN  the  linear,  superficial,  and  solid  aggregates  discussed  in 
the  three  previous  lessons,  the  organism  was  seen  to  be 
composed  of  cells  which  in  most  cases  differed  but  little 
from  one  another,  all  complications  of  structure  being  due 
to  a  continued  repetition  of  the  process  of  cell-multiplica- 
tion accompanied,  except  in  Laminaria  and  its  allies,  by 
little  or  no  cell-differentiation.  In  the  present  lesson  we 
shall  make  a  detailed  study  of  a  solid  aggregate  in  which 
the  constituent  cells  differ  very  considerably  from  one 
another  in  form  and  size. 

Nitella  (Fig.  45,  A)  is  a  not  uncommon  fresh-water  weed, 
found  in  ponds  and  water-races,  and  distinguished  at  once 
from  such  low  Algae  as  Vaucheria  and  Spirogyra  by  its  ex- 
ternal resemblance  to  one  of  the  higher  plants,  since  it 
presents  structures  which  may  be  distinguished  as  stem, 
branches,  leaves,  &c. 

A  Nitella  plant  consists  of  a  slender  cylindrical  stem, 
some  15-20  cm.  and  upwards  in  length,  but  not  more  than 
about  J  mm.  in  diameter.  The  proximal  end  is  loosely 
rooted  to  the  mud  at  the  bottom  of  the  stream  or  pond  by 
delicate  root  filaments  or  rhizoids  (A,  rh)  :  the  distal  end  is 


FIG.  M.—Nit 
A,  the  entire  plant  (nat.  size),  showing  the  segmented  stem,  each  seg- 

1   This    and   the   following   figures  are  taken  from  a  New  Zealand 
species  closely  allied  to,  if  not  identical  with,  the  British  N.  flexilis. 


208  NITELLA.  LESS. 

ment  (seg)  consisting  of  a  proximal  internode  (int.  nd)  and  distal  node 
(nd]  :  the  leaves  (/)  arranged  in  whorls  and  ending  in  leaflets  (/') :  the 
rhizoids  (rh] :  and  two  branches  (br),  each  springing  from  the  axil  of  a 
leaf  and  ending,  like  the  main  stem,  in  a  terminal  bud  (term.  bud). 

B,  distal  end  of  a  shoot  with  gonads  attached  to  the  leaves  :  ovy,  the 
ovaries  ;  spy,  the  spermaries. 

C,  distal  end  of  a  rhizoid. 

D,  distal  end  of  a  leaf  (/)  with  two  leaflets  (/ ),  showing  the  chroma- 
tophores  and  the  while  line.     The  arrows  indicate  the  direction  of  rota- 
tion of  the  protoplasm. 

E,  distal  end  of  a  leaflet,  showing  the  general  structure  of  a  typical 
cell  of  Nitella  in  optical  section  :  c.  w,  the  cell-wall ;  plsrn^,  the  quies- 
cent outer  layer  of  protoplasm  containing  chromatophores  (chr) ;  plsm*, 
the  inner  layer,  rotating  in  the  direction  indicated  by  the  arrows,  and 
containing  nuclei  (nu)  ;  vac,  the  large  vacuole. 

F,  terminal  bud,  partly  dissected,  showing  the  nodes  (nd),  internodes 
(int.  nd),  and  leaf-whorls  (/),  numbered  from  I  to  4,  starting  from  the 
proximal  end  ;  gr.  pt,  growing  point. 

G,  distal  end  of  a  leaf  (/)  with  two  leaflets  (/' ),  at  the  base  of  which 
are  attached  a  spermary  (spy)  and  two  ovaries  (ovy). 

free.  Springing  from  it  at  intervals  are  circlets  or  whorls  of 
delicate,  pointed  leaves  (/). 

Owing  to  the  regular  arrangement  of  the  leaves  the  stem 
is  divisible  into  successive  sections  or  segments  (seg)y  each 
consisting  of  a  very  short  distal  division  or  node  (nd)  from 
which  the  leaves  spring,  and  of  an  elongated  proximal 
division  or  internode  (int.  nd\  which  bears  no  leaves. 

Throughout  the  greater  part  of  the  stem  the  whorls  ot 
leaves  are  disposed  at  approximately  equal  distances  from 
one  another,  so  that  the  internodes  are  of  equal  length,  but 
towards  the  distal  end  the  internodes  become  rapidly  shorter 
and  the  whorls  consequently  closer  together,  until,  at  the 
actual  distal  end,  a  whorl  is  found  the  leaves  of  which,  in- 
stead of  spreading  outwards  like  the  rest,  are  curled  upwards 
so  that  their  points  are  in  contact.  In  this  way  is  formed 
the  terminal  bud  (term.  bud\  by  which  the  uninjured  stem 
is  always  terminated  distally. 

The  angle  between  the  stem  and  a  leaf,  above  (distad  of) 
the  attachment  of  the  latter,  is  called  the  axil  of  the  leaf. 


xxi  HISTOLOGY  209 

There  is  frequently  found  springing  from  the  axil  of  one  of 
the  leaves  in  a  whorl  a  branch  or  shoot  (br)  which  repeats 
the  structure  of  the  main  stem,  i.e.  consists  of  an  axis  from 
which  spring  whorls  of  leaves,  the  whole  ending  in  a  ter- 
minal bud.  The  axis  or  stem  of  a  shoot  is  called  a  second- 
ary axis,  the  main  stem  of  the  plant  being  the  primary  axis. 
It  is.  important  to  notice  that  both  primary  and  secondary 
axes  always  end  in  terminal  buds,  and  thus  differ  from  the 
leaves  which  have  pointed  extremities. 

The  rhizoids  or  root-filaments  (rh)  arise,  like  the  leaves 
and  branches,  exclusively  from  nodes. 

In  the  autumn  the  more  distal  leaves  present  a  peculiar 
appearance,  owing  to  the  development  on  them  of  the  gonads 
or  sexual  reproductive  organs  (Fig.  45,  B  and  G)  :  of  these 
the  spermaries  (antheridia)  look  very  like  minute  oranges, 
being  globular  structures  (spy)  of  a  bright  orange  colour : 
the  ovaries  (oogonia)  are  flask-shaped  bodies  (ovy)  of  a 
yellowish  brown  colour  when  immature,  but  turning  black 
after  the  fertilization  of  the  ova. 

Examined  under  the  microscope  each  internode  is  found 
to  consist  of  a  single  gigantic  cell  (F,  int.  nd2)  often  as  much 
as  3  or  4  cm.  long  in  the  older  parts  of  the  plant.  A  node 
on  the  other  hand  is  composed  of  a  transverse  plate  of  small 
cells  (ndl)  separating  the  two  adjacent  internodes  from  one 
another.  The  leaves  consist  each  of  an  elongated  proximal 
cell  like  an  internode  (D,  / ;  F,  / *),  then  of  a  few  small  cells 
having  the  character  of  a  node,  and  finally  of  two  or  three 
leaflets  (D,  G,  /'),  each  consisting  usually  of  three  cells,  the 
distal  one  of  which  is  small  and  pointed. 

Thus  the  Nitella  plant  is  a  solid  aggregate  in  which  the 
cells  have  a  very  definite  and  characteristic  arrangement. 

The  details  of  structure  of  a  single  cell  are  readily  made 

p 


210  NITELLA  LESS. 

out  by  examining  a  leaflet  under  a  high  power.  The  cell  is 
surrounded  by  a  wall  of  cellulose  (E,  c.w)  of  considerable 
thickness.  Within  this  is  a  layer  of  protoplasm  (primordial 
utricle,  p.  196),  enclosing  a  large  central  vacuole  (vac),  and 
clearly  divisible  into  two  layers,  an  outer  (plsm1)  in  im- 
mediate contact  with  the  cell-wall,  and  an  inner  (plsm*) 
bounding  the  vacuole. 

In  the  outer  layer  of  protoplasm  are  the  chromatophores 
or  chlorophyll-corpuscles  (chr)  to  which  the  green  colour  of 
the  plant  is  due.  They  are  ovoidal  bodies,  about  TJ^  mm. 
long,  and  arranged  in  obliquely  longitudinal  rows  (D).  On 
opposite  sides  of  the  cylindrical  cell  are  two  narrow  oblique 
bands  devoid  of  chromatophores  and  consequently  colourless 
(D).  The  chromatophores  contain  minute  starch  grains. 

The  inner  layer  of  protoplasm  contains  no  chlorophyll 
corpuscles,  but  only  irregular,  colourless  granules,  many  of 
which  are  nuclei  (E,  nu :  see  below,  p.  213).  If  the  tem- 
perature is  not  too  low  this  layer  is  seen  to  be  in  active 
rotating  movement,  streaming  up  one  side  of  the  cell  and 
down  the  other  (E),  the  boundary  between  the  upward  and 
downward  currents  being  marked  by  the  colourless  bands 
just  mentioned,  along  which  no  movement  takes  place  (D). 
This  rotation  of  protoplasm  is  a  form  of  contractility  very 
common  in  vegetable  cells  in  which,  owing  to  the  confining 
cell-wall,  no  freer  movement  is  possible. 

The  numerous  nuclei  (E,  nu}  are  rod-like  and  often 
curved  :  they  can  be  seen  to  advantage  only  after  staining 
(Fig.  46).  Lying  as  they  do  in  the  inner  layer  of  protoplasm, 
they  are  carried  round  in  the  rotating  stream. 

In  the  general  description  of  the  plant  it  was  mentioned 
that  the  stem  ended  distally  in  a  terminal  bud  (Fig.  45,  A, 
term,  bud}  formed  of  a  whorl  of  leaves  with  their  apices 
curved  towards  one  another.  If  these  leaves  (F,  71)  are  dis- 


xxr  APICAL   GROWTH  211 

sected  away,  the  node  from  which  they  spring  (ndl)  is  found 
to  give  rise  distally  to  a  very  short  internode  (int.  nd2), 
above  which  is  a  node  (nd2)  giving  rise  to  a  whorl  of  very 
small  leaves  (/2),  also  curved  inwards  so  as  to  form  a  bud. 
Within  these  is  found  another  segment  consisting  of  a  still 
smaller  internode  (int.  nd^)  and  node,  bearing  a  whorl  of 
extremely  small  leaves  (/3),  and  within  these  again  a  segment 
so  small  that  its  parts  (int.  nd\  /4)  are  only  visible  under 
the  microscope.  The  minute  blunt  projections  (/4),  which 
are  the  leaves  of  this  whorl,  surround  a  blunt,  hemispherical 
projection  (gr.  pt),  the  actual  distal  extremity  of  the  plant — 
the  growing  point  m  punctum  vegetationis. 

The  structure  of  the  growing  point  and  the  mode  of 
growth  of  the  whole  plant  is  readily  made  out  by  examining 
vertical  sections  of  the  terminal  bud  in  numerous  specimens 
(Fig.  46). 

The  growing  point  is  formed  of  a  single  cell,  the  apical 
cell  (A,  ap.  c\  approximately  hemispherical  in  form  and  about 
-£Q  mm.  in  diameter.  Its  cell-wall  is  thick,  and  its  cell-body 
formed  of  dense  granular  protoplasm  containing  a  large 
rounded  nucleus  (nu)  but  no  vacuole. 

In  the  living  plant  the  apical  cell  is  continually  undergoing 
binary  fission.  It  divides  along  a  horizontal  plane,  i.e.,  a 
plane  parallel  to  its  base,  into  two  cells,  the  upper  (distal)  of 
which  is  the  new  apical  cell  (B,  ap.  c\  while  the  lower  is  now 
distinguished  as  the  sub-apical  or  segmental  cell  (s.  ap.  c). 
The  sub-apical  cell  divides  again  horizontally,  forming  two 
cells,  the  uppermost  of  which  (c,  nd4)  almost  immediately 
becomes  divided  by  vertical  planes  into  several  cells  (D,  nd^]\ 
the  lower  (c,  D,  int.  nd*)  remains  undivided. 

The  sub-apical  cell  is  the  rudiment  of  an  entire  segment ; 
the  uppermost  of  the  two  cells  into  which  it  divides  is  the 
rudiment  of  a  node,  the  lower  of  an  internode.  The  future 

^        -.> 

TT  XT  T  \.'  T7  T?  v.  1  T1  ^ 


int. 


FIG.  46.  —  Nitella  :  Vertical  sections  of  the  growing  point  at  four 
successive  stages.  The  nodes  (nd),  internodes  (int.  nd),  and  leaf- 
whorls  (/)  are  all  numbered  in  order  from  the  proximal  to  the  distal  end 
of  the  bud,  the  numbers  corresponding  in  all  the  figures.  The  proximal 
segment  (int.  nd\  nd1,  I1}  in  these  figures  corresponds  with  the  third 
segment  (int.  nd3,  /3)  shown  in  Fig.  46,  F. 

A,  the  apical  cell  (ap,  c)  is  succeeded  by  a  very  rudimentary  node 
(nd3)  without  leaves  :  int.  nd1  is  in  vertical  section,  showing  .the  proto- 
plasm (plsm\  vacuole  (vac),  and  two  nuclei  (nu). 

B,  the  apical  cell  has  divided  transversely,  forming  a  new  apical  cell 
(ap.  c)  and  a  sub-apical  cell   (s.   ap.   c)  :  the  leaves  (/3)  of  nd3)  have 
appeared. 

c,  the  sub-apical  cell  has  divided  transversely  into  the  proximally- 
situated  internode  (int.  nd*)  and  the  distally-situated  node  (nd*)  of  a 
new  segment  ;  in  the  node  the  nucleus  has  divided  preparatory  to  cell- 
clivision.  The  previously  formed  segments  have  increased  in  size  :  int. 
nd2  has  developed  a  vacuole  (vac),  and  its  nucleus  has  divided  (comp. 
int.  nd1  in  A)  :  int.  nd^  is  shown  in  surface  view  with  three  dividing 
nuclei  (nu). 

D.  nd*  has  divided  vertically,  forming  a  transverse  plate  of  cells,  and 
is  now  as  far  advanced  as  ndA  in  A  :  the  nucleus  of  int.  nd3  is  in  the  act 
of  dividing,  while  int.  ndz,  shown  in  surface  view,  now  contains  nume- 
rous nuclei,  some  of  them  in  the  act  of  dividing. 


LESS,  xxi        MULTIPLICATION   OF  NUCLEUS  213 

fate  of  the  two  is  shown  at  once  by  the  node  dividing  into 
a  horizontal  plate  of  cells  while  the  internode  remains 
unicellular. 

Soon  the  cells  of  the  new  node  begin  to  send  out  short 
blunt  processes  arranged  in  a  whorl :  these  increase  in  size, 
undergo  division,  and  form  leaves  (A — D,  /2,  /3). 

These  processes  are  continually  being  repeated  ;  the  apical 
cell  is  constantly  producing  new  sub-apical  cells,  the  sub- 
apical  cells  dividing  each  into  a  nodal  and  an  internodal 
cell ;  and  the  nodal  cell  dividing  into  a  horizontal  plate  of 
cells  and  giving  off  leaves,  while  the  internodal  cell  remains 
undivided. 

The  special  characters  of  the  fully-formed  parts  of  the 
plant  are  due  to  the  unequal  growth  of  the  new  cells.  The 
nodal  cells  soon  cease  to  grow  and  undergo  but  little  altera- 
tion (comp.  ndl  and  nd^\  whereas  the  internodes  increase 
immensely  in  length,  being  quite  3,000  times  as  long  when 
full-grown  as  when  first  separated  from  the  sub-apical  cell. 
The  leaves  also,  at  first  mere  blunt  projections  (A,  /2),  soon 
increase  sufficiently  in  length  to  arch  over  the  growing  point 
and  so  form  the  characteristic  terminal  bud  :  gradually  they 
open  out  and  assume  the  normal  position,  their  successors 
of  the  next  younger  whorl  having  in  the  meantime  developed 
sufficiently  to  take  their  place  as  protectors  of  the  growing 
point. 

The  multinucleate  condition  of  the  adult  internodes  is 
also  a  result  of  gradual  change.  In  its  young  condition  an 
internodal  cell  has  a  single  rounded  nucleus  (A,  int.  nd2,  int. 
ndB),  but  by  the  time  it  is  about  as  long  as  broad  the  nucleus 
has  begun  to  divide  (D,  int.  nd* ;  c,  int.  nd2),  and  when  the 
length  of  the  cell  is  equal  to  about  twice  its  breadth,  the 
nucleus  has  broken  up  into  numerous  fragments  (c,  int.  ndl, 
D,  int.  nd2),  many  of  them  still  in  active  division.  This 


214  NITELLA  LESS. 

repeated  fission  of  the  nucleus  reminds  us  of  what  was 
found  to  occur  in  Opalina  (p.  1 1 9). 

Thus  the  growth  of  Nitella  like  that  of  Penicillium  (p. 
1 88),  is  apical :  new  cells  arise  only  in  the  terminal  bud, 
and,  after  the  first  formation  of  nodes,  internodes,  and 
leaves,  the  only  change  undergone  by  these  parts  is  an  in- 
crease in  size  accompanied  by  a  limited  differentiation  of 
character. 

A  shoot  arises  by  one  of  the  cells  in  a  node  sending 
off  a  projection  distad  of  a  leaf,  te.,  in  an  axil :  the  process 
separates  from  the  parent  cell  and  takes  on  the  characters  of 
an  apical  cell  of  the  main  stem,  the  structure  of  which  is  in 
this  way  exactly  repeated  by  the  shoot. 

The  leaves,  unlike  the  branches,  are  strictly  limited  in 
growth.  At  a  very  early  period  the  apical  cell  of  a  leaf 
becomes  pointed  and  thick-walled  (Fig.  45,  E),  and  after  this 
no  increase  in  the  number  of  cells  takes  place. 

The  rhizoids  also  arise  exclusively  from  nodal  cells  :  they 
consist  of  long  filaments  (Fig.  45,  c),  not  unlike  Mucor- 
hyphse,  but  occasionally  divided  by  oblique  septa  into  linear 
aggregates  of  cells,  and  increase  in  length  by  apical  growth. 

The  structure  of  the  gonads  is  peculiar  and  somewhat 
complicated. 

As  we  have  seen,  the  spermary  (Fig.  45,  G,  spy)  is  a 
globular,  orange-coloured  body  attached  to  a  leaf  by  a  short 
stalk.  Its  wall  is  formed  of  eight  pieces  or  shields,  which 
fit  against  one  another  by  toothed  edges,  so  that  the  entire 
spermary  may  be  compared  to  an  orange  in  which  an  equa- 
torial incision  and  two  meridional  incisions  at  right  angles 
to  one  another  have  been  made  through  the  rind  dividing 
it  into  eight  triangular  pieces.  Strictly  speaking,  however, 
only  the  four  distal  shields  are  triangular  :  the  four  proximal 


XXI 


STRUCTURE   OF   SPERMARY 


215 


ones  have  each  its  lower  angle  truncated  by  the  insertion  of 
the  stalk,  so  that  they  are  actually  four-sided. 

Each  shield  (Fig.  47,  A  and  B,  sh)  is  a  single  concavo~ 
convex  cell  having  on  its  inner  surface  numerous  orange- 
coloured  chromatophores  :  owing  to  the  disposition  of  these 
on  the  inner  surface  only,  the  spermary  appears  to  have  a 


FIG.  47. — A,  diagrammatic  vertical  section  of  the  spermary  of  Nitella, 
showing  the  stalk  (stk),  four  of  the  eight  shields  (sh),  each  bearing  on 
its  inner  face  a  handle  (hn),  to  which  is  attached  a  head-cell  (hd)  :  each 
head  cell  bears  six  secondary  head-cells  (hd1),  to  each  of  which  four 
spermatic  filaments  (sp.  f.)  are  attached. 

B,  one  of  the  proximal  shields  (sh),  with  handle  (hn),  head-cell  (hd), 
secondary  head-cells  (hd1),  and  spermatic  filaments  (sp.  f.). 

C,  a  single  sperm. 

D1,  D2,  D3,  three  stages  in  the  development  of  the  spermary. 
(c,  after  Howes. ) 

colourless  transparent  outer  layer — like  an  orange  inclosed 
in  a  close-fitting  glass  case. 

Attached  to  the  middle  of  the  inner  surface  of  each  shield 
is  a  cylindrical  cell,  the  handle  (Jiri),  which  extends  towards 
the  centre  of  the  spermary,  and,  like  the  shield  itself,  con- 
tains orange  chromatophores.  Each  of  the  eight  handles 
bears  a  colourless  head-cell  (hd'\  to  which  six  secondary  head 


216  NITELLA  LESS. 

cells  (hd1)  are  attached,  and  each  of  these  latter  bears  four 
delicate  coiled  filaments  (sp.f.)  divided  by  septa  into  small 
cells  arranged  end  to  end,  and  thus  not  unlike  the  hyphae  of 
a  fungus.  There  are  therefore  nearly  two  hundred  of  these 
spermatic  filaments  in  each  spermary,  coiled  up  in  its  interior 
like  a  tangled  mass  of  white  cotton. 

The  cells  of  which  the  filaments  are  composed  have  at 
first  the  ordinary  character,  but  as  the  spermary  arrives  at 
maturity  there  is  produced  in  each  a  single  sperm  (c),  having 
the  form  of  a  spirally-coiled  thread,  thicker  at  one  end  than 
the  other,  and  bearing  at  its  thin  end  two  long  flagella.  In 
all  probability  the  sperm  proper,  i.e.,  the  spirally-coiled  body, 
is  formed  from  the  nucleus  of  the  cell,  the  flagella  from  its 
protoplasm.  As  each  of  the  200  spermatic  filaments  con- 
sists of  from  100  to  200  cells,  a  single  spermary  gives  rise 
to  between  20,000  and  40,000  sperms. 

When  the  sperms  are  formed  the  shields  separate 
from  one  another  and  the  spermatic  filaments  protrude 
between  them  like  cotton  from  a  pod  :  the  sperms  then 
escape  from  the  containing  cells  and  swim  freely  in  the 
water. 

The  ovary  (Fig.  45,  G,  ovy,  and  Fig.  48  A)  is  ovoidal  in 
form,  attached  to  the  leaf  by  a  short  stalk  (stk],  and  ter- 
minated distally  by  a  little  chimney-like  elevation  or  crown 
(cr).  It  is  marked  externally  by  spiral  grooves  which  can  be 
traced  into  the  crown,  and  in  young  specimens  its  interior  is 
readily  seen  to  be  occupied  by  a  large  opaque  mass  (ov}. 
Sections  show  that  this  central  body  is  the  ovum,  a  large  cell 
very  rich  in  starch  :  it  is  connected  with  the  unicellular  stalk 
by  a  small  cell  (nd}  from  which  spring  five  spirally-arranged 
cells  (sp.  c.)  :  these  coil  round  the  ovum  and  their  free  ends 
—each  divided  by  septa  into  two  small  cells — project  at  the 
distal  end  of  the  organ  and  form  the  crown,  enclosing  a 


XXI 


DEVELOPMENT  OF  GONADS 


217 


narrow  canal  which  places  the  distal  end  of  the  ovum  in  free 
communication  with  the  surrounding  water. 

We  saw  how  the  various  parts  of  the  fully  formed  plant — 
nodal,  and  internodal  cells,  leaves,  and  rhizoids — were  all 
formed  by  the  modification  of  similar  cells  produced  in  the 
apical  bud.  It  is  interesting  to  find  that  the  same  is  true  of 
the  diverse  parts  of  the  reproductive  organs. 

The  spermary  arises  as  a  single  stalked  globular  cell  which 


Sf.C, 


FIG.  48. — A,  vertical  section  of  the  ovary  of  Nitella,  showing  the 
stalk  (stk],  small  node  (nd)  from  which  spring  the  five  spirally-twisted 
cells  (sp.  c),  each  ending  in  one  of  the  two-celled  sections  of  the  crown 
(cr).  The  ovum  contains  starch  grains,  and  is  represented  as  trans- 
parent, the  spiral  cells  being  seen  through  it. 

B1,  surface  view,  and  B2,  section  of  a  very  young  ovary  :  B3,  later 
stage  in  vertical  section  :  B*,  still  later  stage,  surface  view,  with  the 
ovum  seen  through  the  transparent  spiral  cells.  Letters  as  in  A,  except 
x,  small  cells  formed  by  division  from  the  base  of  the  ovum.  (B2-B4 
after  Sachs.) 

becomes  divided  into  eight  octants  (Fig.  47,  D1).  Each  of 
these  then  divides  tangentially  (i.e.  parallel  to  the  surface 
of  the  sphere)  into  two  cells  (D2),  the  inner  of  which  divides 
again  (o3)  so  that  each  octant  is  now  composed  of  three  cells. 
Of  these  the  outermost  forms  the  shield,  the  middle,  the 
handle,  and  the  inner  the  head-cell :  from  the  latter  the 
secondary  head-cells  and  spermatic  filaments  are  produced 


218  NITELLA  LESS. 

by  budding.     The  entire  spermary  appears  to  be  a  modified 
leaflet. 

The  ovary  also  arises  as  a  single  cell,  but  soon  divides  and 
becomes  differentiated  into  an  axial  row  of  three  cells  (Fig. 
48,  B2,  0vt  nd,  stk]  surrounded  by  five  others  (sp.  c)  which  arise 
as  buds  from  the  middle  cell  of  the  axial  row  (nd}  and  are 
at  first  knob-like  and  upright  (B1).  The  uppermost  or  distal 
cell  of  the  axial  row  becomes  the  ovum  (BS,  B4,  ov\  the 
others  the  stalk  (stk)  and  intermediate  cells  (nd,  x)  :  the  five 
surrounding  cells  elongate,  and  as  they  do  so  acquire  a  spiral 
twist  which  becomes  closer  and  closer  as  growth  proceeds 
(compare  B1,  B4,  and  Fig.  45,  G,  ovy).  At  the  same  time  the 
distal  end  of  each  develops  two  septa  (BS)  and,  projecting 
beyond  the  level  of  the  ovum,  forms  with  its  fellows  the 
chimney  or  crown  (cr)  of  the  ovary.  There  is  every  reason 
to  believe  that  the  entire  ovary  is  a  highly-modified  shoot : 
the  stalk  representing  an  internode,  the  cell  nd  a  node,  the 
spiral  cells  leaves,  and  the  ovum  an  apical  cell. 

Thus  while  the  ciliate  Infusoria  and  Caulerpa  furnish  ex- 
amples of  cell-differentiation  without  cell-multiplication,  and 
Spirogyra  of  cell-multiplication  without  cell-differentiation, 
Nitella  is  a  simple  example  of  an  organism  in  which  com- 
plexity is  obtained  by  the  two  processes  going  on  hand  in 
hand.  It  is  a  solid  aggregate,  the  constituent  cells  of  which 
are  so  arranged  as  to  produce  a  well-defined  external  form, 
while  some  of  them  undergo  a  more  or  less  striking  differen- 
tiation according  to  the  position  they  have  to  occupy,  and 
the  function  they  have  to  perform. 

Impregnation  takes  place  in  the  same  manner  as  in 
Vaucheria  (p.  173).  A  sperm  makes  its  way  down  the 
canal  in  the  chimney-like  crown  of  cells  terminating  the 


XXI 


GERMINATION 


219 


ovary,  and  conjugates  with  the  ovum  converting  it   into  an 
oosperm. 

After  impregnation  the  ovary,  with  the  contained  oosperm, 
becomes  detached  and  falls  to  the  bottom,  where,  after  a 


ap.c 


term  Trud 


ovy 


r7t 


FIG.  49. — Pro-embryo  of  Chara,  showing  the  ovary  (ovy}  from  the 
oosperm  in  which  the  pro-embryo  has  sprung  :  the  two  nodes  (nd), 
apical  cell  (ap.  c),  rhizoids  (r/i),  and  leaves  (/)  of  the  pro-embryo  :  and 
the  rudiment  of  the  leafy  plant  ending  in  the  characteristic  terminal  bud 
(term.  bud).  (After  Howes,  slightly  altere'd.) 

period  of  rest,  it  germinates.  The  process  of  germination 
does  not  appear  to  be  known  in  Nitella,  but  has  been  followed 
in  detail  in  the  closely  allied  genus  Chara. 

The  oosperm  sends  out  a  filament  which  consists  at  first 
of  a  single  row  of  cells  (Fig.  49)  giving  out  a  root-fibre  (rh) 


220  NITELLA  LESS.  XXT 

at  its  proximal  end.  Soon  two  nodes  (nd)  are  formed  on 
the  filament,  or  pro-embryo,  from  the  lower  of  which  rhizoids 
(rti)  proceed,  while  the  upper  gives  rise  to  a  few  leaves  (/), 
not  arranged  in  a  whorl,  and  to  a  small  process  which  is  at 
first  unicellular,  but,  behaving  like  an  apical  cell  of  Nitella, 
soon  becomes  a  terminal  bud  (term,  bud)  and  grows  into  the 
ordinary  leafy  plant. 

This  is  an  instance  of  what  is  known  as  alternation  of 
generations.  The  Chara — and  presumably  the  Nitella — 
plant  gives  rise  by  a  sexual  process  to  a  pro-embryo  which  in 
turn  produces,  by  an  asexual  process  of  budding,  the  Chara 
(or  Nitella)  plant.  No  case  is  known  of  the  pro-embryo 
directly  producing  a  pro-embryo  or  the  leafy-plant  a  leafy- 
plant.  In  order  to  complete  the  cycle  of  existence  or  life- 
history  of  the  species  two  generations  which  alternate  with 
one  another  are  required  :  a  sexual  generation  orgamobium, 
which  reproduces  by  the  conjugation  of  gametes  (ovum  and 
sperm),  and  an  asexual  generation  or  agamobium,  which 
reproduces  by  budding. 


LESSON  XXII 

HYDRA 

WE  have  seen  that  with  plants,  both  Fungi  and  Algae,  the 
next  stage  of  morphological  differentiation  after  the  simple 
cell  is  the  linear  aggregate.  Among  animals  there  are  no 
forms  known  to  exist  in  this  stage,  but  coming  immediately 
above  the  highest  unicellular  animals,  such  as  the  ciliate 
Infusoria,  we  have  true  solid  aggregates.  The  characters  of 
one  of  the  simplest  of  these  and  the  fundamental  way  in 
which  it  differs  from  the  plants  described  in  the  two  previous 
lessons  will  be  made  clear  by  a  study  of  one  of  the  little 
organisms  known  as  "  fresh-water  polypes "  and  placed 
under  the  genus  Hydra. 

Although  far  from  uncommon  in  pond-water,  Hydra  is  not 
always  easy  to  find,  being  rarely  abundant  and  by  no  means 
conspicuous.  In  looking  for  it  the  best  plan  is  to  fill  either 
a  clear  glass  bottle  or  beaker  or  a  white  saucer  with  weeds 
and  water  from  a  pond  and  to  let  it  remain  undisturbed  for 
a  few  minutes.  If  the  gathering  is  successful  there  will  be 
seen  adhering  to  the  sides  of  the  glass,  the  bottom  of  the 
saucer,  or  the  weeds,  little  white,  tawny,  or  green  bodies, 
about  as  thick  as  fine  sewing  cotton,  and  2 — 6  mm.  in 
length.  They  adhere  pretty  firmly  by  one  end,  and  examin- 


FIG.  50. — Hydra. 

A,  Two  living  specimens  of  H.  viridis  attached  to  a  bit  of  weed. 
The  larger  specimen  is  fully  expanded,  and  shows  the  elongated  body 
ending  distally  in  the  hypostome  (hyp),  surrounded  by  tentacles  (/),  and 
three  buds  (bd1,  bd?,  bd3)  in  different  stages  of  development :  a  small 
water-flea  (a)  has  been  captured  by  one  tentacle.     The  smaller  specimen 
(to  the  right  and  above)  is  in  a  state  of  complete  retraction,  the  tentacles 
(t)  appearing  like  papilla;. 

B,  H.  fitsca,  showing  the  mouth  (mth)  at  the  end  of  the  hypostome 
(hyp),  the  circlet  of  tentacles  (/),  two  spermaries  (spy),  and  an  ovary 
(ovy). 

c,  a  Hydra  creeping  on  a  flat  surface  by  looping  movements. 
D,  a  specimen  crawling  on  its  tentacles, 
(c  and  D  after  W.  Marshall.) 


LESS,  xxii  MOVEMENTS  223 

ation  with  a  pocket  lens  shows  that  from  the  free  extremity 
a  number  of  very  delicate  filaments,  barely  visible  to  the 
naked  eye,  are  given  off. 

Under  the  low  power  of  a  compound  microscope,  a  Hydra 
(Fig.  50,  B)  is  seen  to  have  a  cylindrical  body  attached  by  a 
flattened  base  to  a  weed  or  other  aquatic  object,  and  bearing 
at  its  opposite  or  distal  end  a  conical  structure,  the  hypostome 
(Jiyp\  at  the  apex  of  which  is  a  circular  aperture,  the  mouth 
(mth.).  At  the  junction  of  the  hypostome  with  the  body 
proper  are  given  off  from  six  to  eight  long  delicate  ten- 
tacles (f)  arranged  in  a  circlet  or  whorl.  A  longitudinal 
section  shows  that  the  body  is  hollow,  containing  a  spacious 
cavity,  the  enteron  (Fig.  51,  A,  ent.  cav),  which  communicates 
with  the  surrounding  water  by  the  mouth.  The  tentacles  are 
also  hollow,  their  cavities  communicating  with  the  enteron. 

There  are  three  kinds  of  Hydra  commonly  found  :  one, 
H.  vulgaris,  is  colourless  or  nearly  so  ;  another,  H.  fusca,  is 
of  a  pinkish-yellow  or  brown  colour ;  the  third,  H.  viridis,  is 
bright  green.  In  the  two  latter  it  is  quite  evident,  even 
under  a  low  power,  that  the  colour  is  in  the  inner  parts  of 
the  body-wall,  the  outside  of  which  is  formed  by  a  transparent 
colourless  layer  (Fig.  50,  A,  B). 

It  is  quite  easy  to  keep  a  Hydra  under  observation  on  the 
stage  of  the  microscope  for  a  considerable  time  by  placing  it 
in  a  watch-glass  or  shallow  "  cell "  with  weeds,  &c.,  and  in 
this  way  its  habits  can  be  very  profitably  studied. 

It  will  be  noticed,  in  the  first  place,  that  its  form  is 
continually  changing.  At  one  time  (Fig.  50,  A,  left-hand 
figure)  it  extends  itself  until  its  length  is  fully  fifteen  times  its 
diameter  and  the  tentacles  appear  like  long  delicate  filaments  : 
at  another  time  (right-hand  figure)  it  contracts  itself  into  an 
almost  globular  mass,  the  tentacles  then  appearing  like  little 
blunt  knobs. 


224  HYDRA  LESS. 

Besides  these  movements  of  contraction  and  expansion, 
Hydra  is  able  to  move  slowly  from  place  to  place.  This  it 
usually  does  after  the  manner  of  a  looping  caterpillar  (Fig. 
50,  c) :  the  body  is  bent  round  until  the  distal  end  touches 
the  surface ;  then  the  base  is  detached  and  moved  nearer  the 
distal  end,  which  is  again  moved  forward,  and  so  on.  It  has 
also  been  observed  to  crawl  like  a  cuttle  fish  (D)  by  means  of 
its  tentacles,  the  body  being  kept  nearly  vertical. 

It  is  also  possible  to  watch  a  Hydra  feed.  It  is  a  very 
voracious  creature,  and  to  see  it  catch  and  devour  its  prey  is 
a  curious  and  interesting  sight.  In  the  water  in  which  it 
lives  are  always  to  be  found  numbers  of  "  water-fleas,"  minute 
animals  from  about  a  millimetre  downwards  in  length, 
belonging  to  the  class  Crustacea^  a  group  which  includes 
lobsters,  crabs,  shrimps,  &c. 

Water-fleas  swim  very  rapidly,  and  occasionally  one  may  be 
seen  to  come  in  contact  with  a  Hydra's  tentacle.  Instantly 
its  hitherto  active  movements  stop  dead,  and  it  remains 
adhering  in  an  apparently  mysterious  manner  to  the  tentacle. 
If  the  Hydra  is  not  hungry  it  usually  liberates  its  prey  after  a 
time,  and  the  water-flea  may  then  be  seen  to  drop  through 
the  water  like  a  stone  for  a  short  distance,  but  finally  to 
expand  its  limbs  and  swim  off.  If  however  the  Hydra  has 
not  eaten  recently  it  gradually  contracts  the  tentacle  until 
the  prey  is  brought  near  the  mouth,  the  other  tentacles  being 
also  used  to  aid  in  the 'process.  The  water-flea  is  thus  forced 
against  the  apex  of  the  hypostome,  the  mouth  expands 
widely  and  seizes  it,  and  it  is  finally  passed  down  into  the 
digestive  cavity.  Hydrae  can  often  be  seen  with  their  bodies 
bulged  out  in  one  or  more  places  by  recently  swallowed 
water-fleas. 

The  precise  structure  of  Hydra  is  best  made  out  by  cutting 


xxii  MINUTE   STRUCTURE  225 

it  into  a  series  of  extremely  thin  sections  and  examining 
them  under  a  high  power.  The  appearance  presented  by  a 
vertical  section  through  the  long  axis  of  the  body  is  shown 
in  Fig.  51. 

The  whole  animal  is  seen  to  be  built  up  of  cells,  each 
consisting  of  protoplasm  with  a  large  nucleus  (P,  nu\  and 
with  or  without  vacuoles.  As  in  the  case  of  most  animal 
cells,  there  is  no  cell-wall.  Hydra  is  therefore  a  solid  aggre- 
gate ;  but  the  way  in  which  its  constituent  cells  are  arranged 
is  highly  characteristic  and  distinguishes  it  at  once  from  a 
plant. 

The  essential  feature  in  the  arrangement  of  the  cells  is 
that  they  are  disposed  in  two  layers  round  the  central 
digestive  cavity  or  enteron  (A,  ent.  cav}  and  the  cavities  of 
tentacles  (ent.  cav).  So  that  the  wall  of  the  body  is  formed 
throughout  of  an  outer  layer  of  cells,  the  ectoderm  (eci),  and 
of  an  inner  layer,  the  endoderm  (end\  which  bounds  the 
enteric  cavity.  Between  the  two  layers  is  a  delicate  trans- 
parent membrane,  the  mesoglcea,  or  supporting  lamella  (msgl). 
A  transverse  section  shows  that  the  cells  in  both  layers  are 
arranged  radially  (B). 

Thus  Hydra  is  a  two-layered  or  diploblastic  animal,  and 
may  be  compared  to  a  chimney  built  of  two  layers  of  radially 
arranged  bricks  with  a  space  between  the  layers  filled  with 
mortar  or  concrete. 

Accurate  examination  of  thin  sections,  and  of  specimens 
teased  out  or  torn  into  minute  fragments  with  needles,  shows 
that  the  structure  is  really  much  more  complicated  than  the 
foregoing  brief  description  would  indicate. 

The  ectoderm  cells  are  of  two  kinds.  The  first  and  most 
obvious  (B,  ect  and  c),  are  large  cells  of  a  conical  form,  the 
bases  of  the  cones  being  external,  their  apices  internal.  Spaces 


FIG.  51.—  Hydra. 

A,  Vertical  section  of  the  entire  animal,  showing  the  body- wall  corny 
posed  of  ectoderm  (eci]  and  endoderm  (end),  enclosing  an  enteric  cavit- 


LESS,  xxn  ECTODERM  227 

(ent.  cav),  which,  as  well  as  the  two  layers,  is  continued  (ent.  cav')  into 
the  tentacles,  and  opens  externally  by  the  mouth  (mth)  at  the  apex  of 
the  hypostome  (hyp).  Between  the  ectoderm  and  endoderm  is  the 
mesogloea  (msgl),  represented  by  a  black  line.  In  the  ectoderm  are  seen 
large  (ntc)  and  small  (ntc1)  nematocysts :  some  of  the  endoderm  cells 
are  putting  out  pseudopods  (psd),  others  flagella  (/?).  Two  buds  (bd1, 
bcfi)  in  different  stages  of  development  are  shown  on  the  left  side,  and 
on  the  right  a  spermary  (spy)  and  an  ovary  (ovy)  containing  a  single 
ovum  (ov). 

B,  portion  of  a  transverse  section  more  highly  magnified,  showing  the 
large  ectoderm  cells  (ect)  and  interstitial  cells  (in(.  c)  :  two  cnidoblasts 
(cnbl)  enclosing  nematocysts  (ntc),  and  one  of  them  produced  into   a 
cnidocil  (cnc)  :  the  layer  of  muscle- processed  (m.  pr)   cut   across  just 
external   to   the   mesogloea   (msgl) :  endodeMi   cells  (end)  with   large 
vacuoles   and   nuclei   (nu),  pseudopods  (psd),\  and  flagella  (fi).     The 
endoderm  cell  to  the  right  has  ingested  a  diatom  (a),  and  all  enclose 
minute  black  granules. 

C,  two  of  the  large  ectoderm  cells,  showing  nucleus  (nu)  and  muscle- 
process  (m.  pr}. 

D,  an  endoderm  cell  of  H.  viridis,  showing  nucleus  (««),  numerous 
chromatophores  (c/ir),  and  an  ingested  nematocyst  (ntc). 

E,  one  of  the  larger  nematocysts  with  extruded  thread  barbed  at  the 
base. 

F,  one  of  the  smaller  nematocysts. 

G,  a  single  sperm. 

(D  after  Lankester  :  F  and  G  after  Howes.) 

are  necessarily  left  between  their  inner  or  narrow  ends,  and 
these  are  filled  up  with  the  second  kind  of  cells  (int.  c),  small 
rounded  bodies  which  lie  closely  packed  between  their  larger 
companions  and  are  distinguished  as  interstitial  cells. 

The  inner  ends  of  the  large  ectoderm  cells  are  continued 
into  narrow,  pointed  prolongations  (c,m.pr\  placed  at  right 
angles  to  the  cells  themselves  and  parallel  to  the  long  axis  of 
the  body.  There  is  thus  a  layer  of  these  longitudinally- 
arranged  muscle-processes  lying  immediately  external  to  the 
mesogloea  (B,  m.  pr).  They  appear  to  possess,  like  the  axial 
fibre  of  Vorticella  (p.  129),  a  high  degree  of  contractility,  the 
almost  instantaneous  shortening  of  the  body  being  due,  in 
great  measure  at  least,  to  their  rapid  and  simultaneous 
contraction.  It  is  probably  correct  to  say  that,  while  the 
ectoderm  cells  are  both  contractile  and  irritable,  a  special 

9  2 


c?ib 


FIG.  52. — Hydra. 

A,  A  nematocyst  contained  in  its  cnidoblast  (cnb),  showing  the  coiled 
filament  and  the  cnidocil  (cue]. 

B,  The  same  after  extrusion  of  the  thread,  showing  the  larger  and 
smaller  barbs   at   the   base   of  the  thread,      nu,   the  nucleus  of  the 
cnidoblast. 

c,  A  cnidoblast,  with  its  contained  nematocyst,  connected  with  one 
of  the  processes  of  a  nerve-cell  (nv.  c), 
(After  Schneider.) 


LESS,  xxn  NEMATOCYSTS  229 

degree  of  contractility  is  assigned  to  the  muscle-processes 
while  the  cells  themselves  are  eminently  irritable,  the  slightest 
stimulus  applied  to  them  being  followed  by  an  immediate 
contraction  of  the  whole  body. 

Imbedded  in  some  of  the  large  ectoderm  cells  are  found 
clear,  oval  sacs  (A  and  B,  ntc\  with  very  well  defined  walls, 
and  called  nematocysts.  Both  in  the  living  specimen  and  in 
sections  they  ordinarily  present  the  appearance  shown  in 
Fig.  51,  B,  ntc,  and  Fig.  52  A,  but  are  frequently  met  with 
in  the  condition  shown  in  Fig.  51  E,  and  Fig.  52  B,  that 
is,  with  a  short  conical  tube  protruding  from  the  mouth  of 
the  sac,  armed  near  its  distal  end  with  three  recurved 
barbs,  besides  several  similar  processes  of  smaller  size, 
^and  giving  rise  distally  to  a  long,  delicate,  flexible  fila- 
ment. 

Accurate  examination  of  the  nematocysts  shows  that  the 
structure  of  these  curious  bodies  is  as  follows  :— each  con- 
sists of  a  tough  sac  (Fig.  52,  A),  one  end  of  which  is  turned 
in  as  a  hollow  pouch  :  the  free  end  of  the  latter  is  continued 
into  a  hollow  coiled  filament,  and  from  its  inner  surface 
project  the  barbs.  The  whole  space  between  the  wall  of 
the  sac  and  the  contained  pouch  and  thread  is  tensely  filled 
with  fluid.  When  pressure  is  brought  to  bear  on  the  outside 
of  the  sac  the  whole  apparatus  goes  off  like  a  harpoon-gun 
(B),  the  compression  of  the  fluid  forcing  out  first  the  barbed 
pouch  and  then  the  filament,  until  finally  both  are  turned 
inside  out. 

It  is  by  means  of  the  nematocysts — the  resemblance  of 
which  to  the  trichocysts  of  Paramcecium  (p.  113)  should  be 
noted — that  the  Hydra  is  enabled  to  paralyze  its  prey.  Prob- 
ably some  specific  poison  is  formed  and  ejected  into  the 
wound  with  the  thread  :  in  the  larger  members  of  the  group 
to  which  Hydra  belongs,  such  as  jelly-fishes,  the  nematocysts 


230  HYDRA  LESS. 

produce  an  effect  on  the  human  skin  quite  like  the  sting  of 
a  nettle. 

The  nematocysts  are  formed  in  special  interstitial  cells 
called  cnidoblasts  (Fig.  51,  B,  <r#£/and  Fig.  52),  and  are  thus 
in  the  first  instance  at  a  distance  from  the  surface.  But  the 
cnidoblasts  migrate  outwards,  and  so  come  to  lie  quite 
superficially  either  in  or  between  the  large  ectoderm  cells. 
On  its  free  surface  the  cnidoblast  is  produced  into  a  delicate 
pointed  process,  the  cnidocil  or  "  trigger-hair  "  (cnc).  In  all 
probability  the  slightest  touch  of  the  cnidocil  causes  con- 
traction of  the  cnidoblast,  and  the  nematocyst  thus  com- 
pressed instantly  explodes. 

Nematocysts  are  found  in  the  distal  part  of  the  body,  but 
are  absent  from  the  foot  or  proximal  end,  where  also  there 
are  no  interstitial  cells.  They  are  especially  abundant  in  the 
tentacles,  on  the  knob-like  elevations  of  which — due  to  little 
heaps  of  interstitial  cells — they  are  found  in  great  numbers. 
Amongst  these  occur  small  nematocysts  with  short  threads 
and  devoid  of  barbs  (Fig.  51,  A,  ntc'  and  F). 

There  are  sometimes  found  in  connection  with  the  cnido- 
blast small  irregular  cells  with  large  nuclei :  they  are  called 
nerve-cells  (Fig.  52,  c,  nv.  c),  and  constitute  a  rudimentary 
nervous  system,  the  nature  of  which  will  be  more  con- 
veniently discussed  in  the  next  lesson  (p.  244). 

The  ectoderm  cells  of  the  foot  differ  from  those  of  the  rest 
of  the  body  in  being  very  granular  (Fig.  51  A).  The 
granules  are  probably  the  material  of  the  adhesive  substance 
by  which  the  Hydra  fixes  itself,  and  are  to  be  looked  upon  as 
products  of  destructive  metabolism  :  i.e.  as  being  formed  by 
conversion  of  the  protoplasm  in  something  the  same  way  as 
starch-granules  (p.  33).  This  process  of  formation  in  a  cell 
of  a  definite  product  which  accumulates  and  is  finally  dis- 
charged at  the  free  surface  of  the  cell  is  called  secretion, 


xxli  ENDODERM  231 

and  the  cell  performing  the  function  is  known  as  a  gland 
cell. 

The  endoderm  consists  for  the  most  part  of  large  cells 
which  exceed  in  size  those  of  the  ectoderm,  and  are  re- 
markable for  containing  one  or  more  vacuoles,  sometimes 
so  large  as  to  reduce  the  protoplasm  to  a  thin  superficial 
layer  containing  the  nucleus  (Fig.  51,  A  and  B,  end}.  Then 
again,  their  form  is  extremely  variable,  their  free  or  inner 
ends  undergoing  continual  changes  of  form.  This  can  be 
easily  made  out  by  cutting  transverse  sections  of  a  living 
Hydra,  when  the  endoderm  cells  are  seen  to  send  out  long 
blunt  pseudopods  (psd)  into  the  digestive  cavity,  and  now 
and  then  to  withdraw  the  pseudopods  and  send  out  from 
one  to  three  long  delicate  flagella  (fi).  Thus  the  endoderm 
cells  of  Hydra  illustrate  in  a  very  instructive  manner  the 
essential  similarity  of  flagella  and  pseudopods  already  re- 
ferred to  (p.  51).  In  the  hypostome  the  endoderm  is  thrown 
into  longitudinal  folds,  so  as  to  allow  of  the  dilatation  of 
the  mouth  in  swallowing. 

Amongst  the  ordinary  endoderm-cells  are  found  long 
narrow  cells  of  an  extremely  granular  character.  They  are 
specially  abundant  in  the  distal  part  of  the  body,  beneath 
the  origins  of  the  tentacles,  and  in  the  hypostome,  but  are 
absent  in  the  tentacles  and  in  the  foot.  There  is  no  doubt 
that  they  are  gland-cells,  their  secretion  being  a  fluid  used 
to  aid  in  the  digestion  of  the  food. 

In  Hydra  viridis  the  endoderm-cells  (D)  contain  chroma- 
tophores  (chr)  coloured  green  by  chlorophyll,  which  performs 
the  same  function  as  in  plants,  so  that  in  this  species  holozoic 
is  supplemented  by  holophytic  nutrition.  There  is  reason 
for  believing  that  the  chromatophores  are  to  be  regarded  as 
symbiotic  algae,  like  those  found  in  connection  with  Radio- 


232  HYDRA  LESS. 

laria  (p.  154).  In  H.  fusca  bodies  resembling  these  chromato- 
phores  are  present,  but  are  of  an  orange  or  brown  colour,  and 
devoid  of  chlorophyll.  Brown  and  black  granules  occurring 
in  the  cells  (B)  seem  to  be  due  in  part  to  the  degeneration  of 
the  chromatophores,  and  in  part  to  be  products  of  excretion. 

Muscle-processes  exist  in  connection  with  the  endoderm 
cells,  and  they  are  said  to  take  a  transverse  or  circular 
direction,  i.e.,  at  right  angles  to  the  similar  processes  of 
the  ectoderm  cells. 

When  a  water-flea  or  other  minute  organism  is  swallowed 
by  a  Hydra,  it  undergoes  a  gradual  process  of  disintegration. 
The  process  is  begun  by  a  solution  of  the  soft  parts  due  to 
the  action  of  a  digestive  fluid  secreted  by  the  gland-cells  of 
the  endoderm  :  it  is  apparently  completed  by  the  endoderm 
cells  seizing  minute  particles  with  their  pseudopods  and 
engulfing  them  quite  after  the  manner  of  Amoebae.  It  is 
often  found  that  the  protrusion  of  pseudopods  during 
digestion  results  in  the  almost  complete  obliteration  of  the 
enteric  cavity. 

It  would  seem  therefore  that  in  Hydra  the  process  of 
digestion  or  solution  of  the  food  is  to  some  extent  at  least 
infra-cellular^  i.e.,  takes  place  in  the  interior  of  the  cells 
themselves,  as  in  Amoeba  or  Paramcecium  :  it  is  however 
mainly  extra-cellular  or  enteric,  i.e.,  is  performed  in  a  special 
digestive  cavity  lined  by  cells. 

The  ectoderm  cells  do  not  take  in  food  directly,  but  are 
nourished  entirely  by  diffusion  from  the  endoderm.  Thus 
the  two  layers  have  different  functions  :  the  ectoderm  is  pro- 
tective and  sensory ;  it  forms  the  external  covering  of  the 
animal,  and  receives  impressions  from  without  :  the  endo- 
derm, removed  from  direct  communication  with  the  outer 
world,  performs  a  nutrient  function,  its  cells  alone  having 
the  power  of  digesting  food. 


xxn  INDIVIDUATION  233 

The  essential  difference  between  digestion  and  assimilation 
is  here  plainly  seen  :  all  the  cells  of  Hydra  assimilate,  all 
are  constantly  undergoing  waste,  and  all  must  therefore  form 
new  protoplasm  to  make  good  the  loss.  But  it  is  the  endo- 
derm  cells  alone  which  can  make  use  of  raw  or  undigested 
food  :  the  ectoderm  has  to  depend  upon  various  products  of 
digestion  received  by  osmosis  from  the  endoderm. 

It  will  be  evident  from  the  preceding  description  that 
Hydra  is  comparable  to  a  colony  of  Amoebae  in  which  par- 
ticular functions  are  made  over  to  particular  individuals — 
just  as  in  a  civilized  community  the  functions  of  baking  and 
butchering  are  assigned  to  certain  members  of  the  commu- 
nity, and  not  performed  by  all.  Hydra  is  therefore  an  ex- 
ample of  individuation :  morphologically  it  is  equivalent 
to  an  indefinite  number  of  unicellular  organisms :  but, 
these  acting  in  concert,  some  taking  one  duty  and  some 
another,  form,  physiologically  speaking,  not  a  colony  ot 
largely  independent  units,  but  a  single  multicellular  in- 
dividual. 

Like  so  many  of  the  organisms  which  have  come  under 
our  notice,  Hydra  has  two  distinct  methods  of  reproduction, 
asexual  and  sexual. 

Asexual  multiplication  takes  place  by  a  process  of  budding. 
A  little  knob  appears  on  the  body  (Fig.  50,  A,  bdl\  and  is 
found  by  sections  to  arise  from  a  group  of  ectoderm  cells  ; 
soon  however  it  takes  on  the  character  of  a  hollow  out- 
pushing  of  the  wall  containing  a  prolongation  of  the  enteron, 
and  made  up  of  ectoderm,  mesoglcea,  and  endoderm.  (Fig. 
51,  A,  bd1).  In  the  course  of  a  few  hours  this  prominence 
enlarges  greatly,  and  near  its  distal  end  six  or  eight  hollow 
buds  appear  arranged  in  a  whorl  (Fig.  50,  A,  bd^ ;  Fig.  51, 


234  HYDRA  LESS. 

A,  bd'2).  These  enlarge  and  take  on  the  characters  of  ten- 
tacles :  a  mouth  is  formed  at  the  distal  end  of  the  bud, 
which  thus  acquires  the  character  of  a  small  Hydra  (Fig. 
50,  A,  bdz\  Finally  the  bud  becomes  constricted  at  its  base, 
separates  from  the  parent,  and  begins  an  independent  ex- 
istence. Sometimes,  however,  several  buds  are  produced  at 
one  time,  and  each  of  these  buds  again  before  becoming 
detached  :  in  this  way  temporary  colonies  are  formed.  But 
the  buds  always  separate  sooner  or  later,  although  they 
frequently  begin  to  feed  while  still  attached. 

It  is  a  curious  circumstance  that  Hydra  can  also  be  mul- 
tiplied by  artificial  division  :  the  experiment  has  been  tried 
of  cutting  the  living  animal  into  pieces,  each  of  which  was 
found  to  grow  into  a  perfect  individual. 

As  in  Vaucheria  and  Nitella,  the  sexual  organs  or  gonads 
are  of  two  kinds,  spermaries  and  ovaries.  Both  are  found 
in  the  same  individual,  Hydra  being,  like  the  plants  just 
mentioned,  hermaphrodite  or  monoecious. 

The  spermaries  (Fig.  50,  B,  and  Fig.  51,  A,  spy)  are  white 
conical  elevations  situated  near  the  distal  end  of  the  body : 
as  a  rule  not  more  than  one  or  two  are  present  at  the  same 
time,  but  there  may  be  as  many  as  twenty.  They  are  per- 
fectly colourless,  even  in  the  green  and  brown  species,  being 
obviously  formed  of  ectoderm  alone. 

In  the  immature  condition  the  spermary  consists  of  a  little 
heap  of  interstitial  cells  covered  by  an  investment  of  some- 
what flattened  cells  formed  by  a  modification  of  the  ordinary 
large  cells  of  the  ectoderm.  When  mature  each  of  the  small 
internal  cells  becomes  converted  into  a  sperm  (Fig.  51,  G), 
consisting  of  a  small  ovoid  head  formed  from  the  nucleus  of 
the  cell,  and  of  a  long  vibratile  tail  formed  from  its  proto- 
plasm. By  the  rupture  of  the  investing  cells  or  wall  of  the 


SPERMARY   AND   OVARY 


235 


sperm  ary  the  sperms  are  liberated  and  swim  freely  in  the 
water. 

The  ovaries  (Fig.  50,  B,  and  Fig.  51,  A,  ovy)  are  found 
near  the  proximal  end  of  the  body,  and  vary  in  number  from 
one  to  eight.  When  ripe  an  ovary  is  larger  than  a  spermary, 
and  of  a  hemispherical  form.  It  begins,  like  the  spermary, 
as  an  aggregation  of  interstitial  cells,  so  that  in  their  earlier 
stages  the  sex  of  the  gonads  is  indeterminate.  But  while 


FIG.  53. — A,  Ovum  of  Hydra  viridis,  showing  pseudopods,  nucleus 
(gv),  and  numerous  chromatophores  and  yolk  spheres. 

B,  a  single  yolk  sphere.      (From  Balfour  after  Kleinenberg. ) 

in  the  spermary  each  cell  is  converted  into  a  sperm,  in  the 
ovary  one  cell  soon  begins  to  grow  faster  than  the  rest, 
becomes  amoeboid  in  form  (Fig.  51,  A,  ov,  and  Fig.  53,  A), 
sending  out  pseudopods  amongst  its  companions  and  ingest- 
ing the  fragments  into  which  they  become  broken  up,  thus 
continually  increasing  in  size  at  their  expense.  Ultimately 
the  ovary  comes  to  consist  only  of  this  single  amoeboid 
ovum,  and  of  a  layer  of  superficial  cells  forming  a  capsule 
for  it.  As  the  ovum  grows  yolk-spheres  (Fig.  53),  small 


236  HYDRA  LESS,  xxn 

rounded  masses  of  proteid  material,  are  formed  in  it,  and  in 
Hydra  viridis  it  also  acquires  green  chromatophores. 

When  the  ovary  is  ripe  the  ovum  draws  in  its  pseudopods 
and  takes  on  a  spherical  form  :  the  investing  layer  then 
bursts  so  as  to  lay  bare  the  ovum  and  allow  of  the  free  access 
to  it  of  the  sperms.  One  of  the  latter  conjugates  with  the 
ovum,  producing  an  oosperm  or  unicellular  embryo. 

The  oosperm  divides  into  a  number  of  cells,  the  outer- 
most of  which  become  changed  into  a  hard  shell  or  capsule. 
The  embryo,  thus  protected,  falls  to  the  bottom  of  the  water, 
and  after  a  period  of  rest  develops  into  a  Hydra.  As,  how- 
ever, there  are  certain  abnormal  features  about  the  develop- 
ment of  this  genus  which  cannot  well  be  understood  by  the 
beginner,  it  will  not  be  described  in  detail,  but  the  very 
important  series  of  changes  by  which  the  oosperm  of  a 
multicellular  animal  becomes  converted  into  the  adult  will 
be  considered  in  the  next  lesson. 


LESSON   XXIII 

HYDROID  POLYPES  : BOUGAINVILLEA,  DIPHYES,  AND  PORPITA 

IT  was  stated  in  the  previous  lesson  (p.  234)  that  in  a 
budding  Hydra  the  buds  do  not  always  become  detached 
at  once,  but  may  themselves  bud  while  still  in  connection 
with  the  parent,  temporary  colonies  being  thus  produced. 

Suppose  this  state  of  things  to  continue  indefinitely :  the 
result  would  be  a  tree-like  colony  or  compound  organism 
consisting  of  a  stem  with  numerous  branchlets  each  ending 
in  a  Hydra-like  zooid.  Such  a  colony  would  bear  much  the 
same  relation  to  Hydra  as  Zoothamnium  bears  to  Vorticella 
(see  p.  134). 

As  a  matter  of  fact  this  is  precisely  what  happens  in  a 
great  number  of  animals  allied  to  Hydra  and  known  by  the 
name  of  Hydroid  polypes. 

Every  one  is  familiar  with  the  common  Sertularians  of  the 
sea-coast,  often  mistaken  for  sea-weeds  :  they  are  delicate, 
much-branched,  semi-transparent  structures  of  a  horny  con- 
sistency, the  branches  beset  with  little  cups,  from  each  of 
which,  during  life,  a  Hydra-like  body  is  protruded. 

A  very  convenient  genus  for  our  purpose  is  Bougainvillea, 
a  hydroid  polype  found  as  little  tufts  a  few  centimetres  long 
attached  to  rocks  and  other  submarine  objects.  Fig.  54,  A 


FIG.  54. — Bottgainmllea  ramolsa. 

A,  a  complete  living  colony  of  the  natural  size,  showing  the  branched 
stem  and  root-like  organ  of  attachment. 

B,  a  portion  of  the  same  magnified,  showing  the  branched  stem  bear- 
ing hydranths  (hyd)  and  medusae  (mcd),  one  of  the  latter  nearly  mature, 
the  others  undeveloped  :  each  hydranth  has  a  circlet  of  tentacles    (/) 
surrounding  a  hypostome  (hyp),  and  contains  an  enteric  cavity  (ent.  cav) 
continuous  with  a  narrow  canal  (ent,  cav')  in  the  stem.     The  stem  is 
covered  by  a  cuticle  (cu). 

c,  a  medusa  after  liberation  from  the  colony,  showing  the  bell  with 
tentacles  (/),  velum  (v),  manubrium  (rnnb),  radial  (rad.  c)  and  circular 
(fir.  c)  canals,  and  eye-spots  (oc).  (After  Allman.) 


LESS,  xxiii       STRUCTURE   OF   A   HYDRA^TH  239 

shows  a  colony  of  the  natural  size,  B  a  part  of  it  magnified  : 
it  consists  of  a^  much-branched  stem  of  a  yellowish  colour 
attached  by  root-like  fibres  to  the  support.  The  branches 
terminate  .in  little  Hydra-like  bodies  called  hydranths  (B, 
hyd\  each  with  a  hypostome  (hyp)  and  circlet  of  tentacles 
(/).  Lateral  branchlets  bear  bell-shaped  structures  or 
medusae  (med) :  these  will  be  considered  presently. 

Sections  show  that  the  hydranths  have  just  the  structure 
of  a  Hydra,  consisting  of  a  double  layer  of  cells — ectoderm 
and  endoderm — separated  by  a  supporting  lamella  or 
mesoglcea  and  enclosing  a  digestive  cavity  (ent.  cav.)  which 
opens  externally  by  a  mouth  placed  at  the  summit  of  the 
hypostome. 

The  stem  is  formed  of  the  same  layers  and  contains  a 
cavity  (ent.  cav')  continuous  with  those  of  the  'hydranths, 
and  thus  the  structure  of  a  hydroid  polype  is,  so  far,  simply 
that  of  a  Hydra  in  which  the  process  of  budding  has 
gone  on  to  an  indefinite  extent  and  without  separation  of 
the  buds. 

There  is  however  an  additional  layer  added  in  the  stem 
for  protective  and  strengthening  purposes.  It  is  evident 
that  a  colony  of  the  size  shown  in  Fig.  54,  A  would,  if  formed 
only  of  soft  ectodermal  and  endodermal  cells,  be  so  weak  as 
to  be  hardly  able  to  b^ar  its  own  weight  even  in  water.  To 
remedy  this  a  layer  of  transparent,  yellowish  substance  of 
horny  consistency,  called  the  cuticle,  is  developed  outside 
the  ectoderm  of  the  stem,  extending  on  to  the  branches  and 
only  stopping  at  the  bases  of  the  hydranths  and  medusse. 
It  is  this  layer  which,  when  the  organism  dies  and  decays, 
is  left  as  a  semi-transparent  branched  structure  resembling 
the  living  colony  in  all  but  the  absence  of  hydranths  and 
medusae.  The  cuticle  is  therefore  a  supporting  organ  or 
skeleton,  not  like  our  own  bones  formed  in  the  interior  of 


24o  IIYDROID  POLYPES  LESS. 

the  body  (endo  skeleton),  but  like  the  shell  of  a  crab  or  lobster 
lying  altogether  outside  the  soft  parts  (exoskeletonj). 

As  to  the  mode  of  formation  of  the  cuticle  : — we  saw  that 
many  organisms,  such  as  Amoeba  and  Hsematococcus,  form, 
on  entering  into  the  resting  condition,  a  cyst  or  cell-wall,  by 
secreting  or  separating  from  the  surface  of  their  protoplasm 
a  succession  of  layers  either  of  cellulose  or  of  a  transparent 
horn-like  substance.  But  Amoeba  and  Haematococcus  are 
unicellular,  and  are  therefore  free  to  form  this  protective 
layer  at  all  parts  \)f  their  surface.  The  ectoderm  cells  of 
Bougainvillea  on  the  other  hand  are  in  close  contact  with 
their  neighbours  on  all  sides  and  with  the  mesogloea  at  their 
inner  ends,  so  that  it  is  not  surprising  to  find  the  secretion 
of  skeletal  substance  taking  place  only  at  their  outer  ends. 
As  the  proces^  takes  place  simultaneously  in  adjacent  cells, 
the  result  is  a  continuous  layer  common  to  the  whole 
ectoderm  instead  of  a  capsule  to  each  individual  cell.  It  is 
to  an  exoskeletal  structure  formed  in  this  way,  i.e.  by  the 
secretion  of  successive  layers  from  the  free  faces  of  adjacent 
cells,  that  the  name  cuticle  is  in  strictness  applied  in  multi- 
cellular  organisms. 

The  medusae  (B,  med.  and  c),  mentioned  above  as  occur- 
ring on  lateral  branches  of  the  colony,  are  found  in  various 
stages  of  development,  the  younger  ones  having  a  nearly 
globular  shape,  while  when  fully  formed  each  resembles  a 
bell  attached  by  its  handle  to  one  of  the  branches  of  the 
colony  and  having  a  clapper  in  its  interior.  When  quite 
mature  the  medusae  become  detached  and  swim  off  as  little 
jelly-fishes  (c). 

The  structure  of  medusa  must  now  be  described  in, 
some  detail.  The  bell  (c)  is  formed  of  a  gelatinous  sub- 
stance (Fig.  55,  D,  msgl)  covered  on  both  its  inner  and 


xxni  STRUCTURE   OF   A   MEDUSA  241 

outer  surfaces  by  a  thin  layer  of  delicate  cells  (ect]\  The 
clapper-like  organ  or  manubrium  (Fig.  54,  c  and  Fig.  55  D 
and  D',  mnb)  is  formed  of  two  layers  of  cells,  precisely 
resembling  the  ectoderm  and  endoderm  of  Hydra,  and 
separated  by  a  thin  mesoglcea ;  it  is  hollow,  its  cavity  (Fig. 
55,  D,  ent.  cav]  opening  below,  i.e.  at  its  distal  or  free  end, 
by  a  rounded  aperture,  the  mouth  (intJi),  used  by  the  medusa 
for  the  ingestion  of  food.  At  its  upper  (attached  or  proxi- 
mal) end  the  cavity  of  the  manubrium  is  continued  into  four 
narrow,  radial  canals  (Fig.  54,  c,  rod.  c,  and  Fig.  55,  D  and 
D'  rad)  which  extend  through  the  gelatinous  substance  of  the 
bell  at  equal  distances  from  one  another,  like  four  meridians, 
and  finally  open  into  a  circular  canal  (dr.  c)  which  runs 
round  the  edge  of  the  bell.  The  whole  system  of  canals  is 
lined  by  a  layer  of  cells  (Fig.  55,  D  and  D',  end]  continuous 
with  the  inner  layer  or  endoderm  of  the  manubrium  ;  and 
extending  from  one  canal  to  another  in  the  gelatinous  sub- 
stance of  the  bell,  is  a  delicate  sheet  of  cells,  the  endoderm- 
lamella  (D',  end.  la). 

From  the  edge  of  the  bell  four  pairs  of  tentacles  (Fig.  54, 
c  and  Fig.  55,  D,  /)  are  given  off,  one  pair  corresponding  to 
each  radial  canal,  and  close  to  the  base  of  each  tentacle  is 
a  little  speck  of  pigment  (Fig.  54,  <?<:),  the  ocellus  or  eye-spot. 
Lastly,  the  margin  of  the  bell  is  continued  inwards  into  a 
narrow  circular  shelf,  the  velum  (v). 

At  first  sight  there  appears  to  be  very  little  resemblance 
between  a  medusa  and  a  hydranth,  but  it  is  really  quite 
easy  to  derive  the  one  form  from  the  other. 

Suppose  a  short  hydranth  or  Hydra-like  body  with  four 
tentacles  (Fig.  55,  A,  A')  to  have  the  region  from  which  the 
tentacles  spring  pulled  out  so  as  to  form  a  hollow,  trans- 
versely extended  disc  (B).  Next,  suppose  this  disc  to  become 
bent  into  the  form  of  a  cup  with  its  concavity  towards  the 

R 


FIG.  55. — Diagrams  illustrating  the  derivation  of  the  medusa  from 
the  hydranth.  In  the  whole  series  of  figures  the  ectoderm  (ect)  is  dotted, 
the  endoderm  (end)  striated,  and  the  mesogloea  (msgl)  black. 

A,  longitudinal  section  of  a  Hydra-like  body,  showing  the  tubular  body 
with  enteric  cavity  (ent.  cav),  hypostome  (hyp)  ,  mouth  (mtti),  and 
tentacles  (/). 


LESS,  xxin     MEDUSA  DERIVED  FROM  HYDRANTH          243 

A',  transverse  section  of  the  same  through  the  plane  a  b. 

B,  the  tentacular  region  is  extended  into  a  hollow  disc. 

C,  the  tentacular  region  has  been  further  extended  and  bent  into  a 
bell-like  form,  the  enteric  cavity  being  continued  into  the  bell  (ent.  cav'} : 
the  hypostome  now  forms  a  manubrium  (mnb}. 

c',  transverse  section  of  the  same  through  the  plane  a  b,  showing  the 
continuous  cavity  (ent.  cav'}  in  the  bell. 

D,  fully   formed  medusa  :  the  cavity  in  the  bell  is  reduced  to  the 
radiating  (rad)  and  circular  (dr.  c}  canals,   the  velum   (v)  is  formed, 
and  a  double  nerve-ring  (nv,  nv'}  is  produced  from  the  ectoderm. 

D',  transverse  section  of  the  same  through  the  plane  a  b,  showing  the 
four  radiating  canals  (rad}  united  by  the  endoderm-lamella  (end.  la}, 
produced  by  partial  obliteration  of  the  continuous  cavity  ent.  '' 


hypostome,  and  to  undergo  a  great  thickening  of  its  meso- 
gloea.  A  form  would  be  produced  like  c,  i.e.  a  medusa-like 
body  with  bell  and  manubrium,  but  with  a  continuous  cavity 
(c',  ent.  cav'}  in  the  thickness  of  the  bell  instead  of  four 
radial  canals.  Finally,  suppose  the  inner  and  outer  walls 
of  this  cavity  to  grow  towards  one  another  and  meet,  thus 
obliterating  the  cavity,  except  along  four  narrow  radial  areas 
(D,  rad)  and  a  circular  area  near  the  edge  of  the  bell 
(D,  dr.  c}.  This  would  result  in  the  substitution  for  the 
continuous  cavity  of  four  radial  canals  opening  on  the  one 
hand  into  a  circular  canal '  and  on  the  other  into  the  cavity 
of  the  manubrium  (ent.  cav),  and  connected  with  one  another 
by  a  membrane — the  endoderm-lamella  (end.  la) — indi- 
cating the  former  extension  of  the  cavity. 

It  follows  from  this  that  the  inner  and  outer  layers  of  the 
manubrium  are  respectively  endoderm  and  ectoderm  :  that 
the  gelatinous  tissue  of  the  bell  is  an  immensely  thickened 
mesogloea  :  that  the  layer  of  cells  covering  both  inner  and 
outer  surfaces  of  the  bell  is  ectodermal :  and  that  the  layer 
of  cells  lining  the  system  of  canals,  together  with  the 
endoderm-lamella,  is  endodermal. 

Thus  the  medusa  and  the  hydranth  are  similarly  con- 
structed or  homologous  structures,  and  the  hydroid  colony, 

R  2 


244  HYDROID  POLYPES  LESS. 

like  Zoothamnium  (p.  136),  is  dimorphic,  bearing  zooids  of 
two  kinds. 


The  ectoderm  cells  of  the  hydranth  bear  muscle-processes 
like  those  of  Hydra  (p.  227,  Fig.  51,  c)  :  in  the  medusae 
similar  processes  are  found  on  the  inner  concave  side  of  the 
bell  and  in  the  velum.  Sometimes,  however,  the  place  of 
these  processes  is  taken  by  a  layer  of  spindle-shaped  fibres 
(Fig.  56,  A),  many  times  longer  than  broad,  and  provided 
each  with  a  nucleus.  Such  muscle-fibres  are  obviously  cells 
greatly  extended  in  length,  so  that  the  ectoderm  cell  of 
Hydra  with  its  continuous  muscle-/ra:6M  is  here  represented 
by  an  ectoderm  cell  with  an  adjacent  muscle-^//.  We 
thus  get  a  partial  intermediate  layer  of  cells  between 
the  ectoderm  and  endoderm,  in  addition  to  the  gelatinous 
mesoglcea,  and  so,  while  a  hydroid  polyp  is,  like  Hydra, 
diploblastic  (p.  225),  it  shows  a  tendency  towards  the  as- 
sumption of  a  three-layered  or  triploblastic  condition.  Both 
the  muscle-processes  and  muscle-cells  of  the  medusae  differ 
from  those  of  the  hydranths  in  exhibiting  a  delicate 
transverse  striation  (Fig.  56). 

Sooner  or  later  the  medusae  separate  from  the  hydroid 
colony  and  begin  a  free  existence.  Under  these  circum- 
stances the  rhythmical  contraction — i.e.  contraction  taking 
place  at  regular  intervals — of  the  muscles  of  the  bell  causes 
an  alternate  contraction  and  expansion  of  the  'whole  organ, 
so  that  water  is  alternately  pumped  out  of  and  drawn  into  it. 
The  obvious  result  of  this  is  that  the  medusa  is  propelled 
through  the  water  by  a  series  of  jerks. 

There  is  still  another  important  matter  in  the  structure  of 
the  medusa  which  has  not  been  referred  to.  At  the  junction 
of  the  velum  with  the  edge  of  the  bell  there  lies,  imme- 
diately beneath  the  ectoderm,  a  layer  of  peculiar  branched 


NERVOUS    SYSTEM 


245 


cells  (Fig.  56,  B,  n.  c),  containing  large  nuclei  and  produced 
into  long  fibre-like  processes.  These  nerve-cells  (see  p.  230) 
are  so  disposed  as  to  form  a  double  ring  round  the  margin 
of  the  bell,  one  ring  (Fig.  55,  D,  nv)  being  immediately 
above,  the  other  (nv')  immediately  below  the  insertion  of 
the  velum.  An  irregular  network  of  similar  cells  and  fibres 


n.c 


FIG.  56. — A,  Muscle  fibres  from  the  inner  face  of  the  bell  of  the 
medusa  of  a  hydroid  polype  (Eucopella  campannlaria],  showing  nucleus 
and  transverse  striation. 

B,  portion  of  the  nerve-ring  of  the  same,  showing  two  large  nerve- 
cells  (n.  c)  and  muscle-fibres  (m.  c)  on  either  side.  (After  von  Len- 
denfeld.) 

occurs  on  the  inner  or  concave  face  of  the  bell,  between  the 
ectoderm  and  the  layer  of  muscle-fibres.  The  whole  consti- 
tutes the  nervous  system  of  the  medusa ;  the  double  nerve-ring 
is  the  central,  the  network  the  peripheral  nervous  system. 

Some  of  the  processes  of  the  nerve-cells  are  connected 
with  ordinary  ectoderm-cells,  which  thus  as  it  were  connect 
the  nervous  system  with  the  external  world  :  others,  in  some 
instances  at  least,  are  probably  directly  connected  with 
muscle-fibres. 


246  HYDROID  POLYPES  LESS. 

We  thus  see  that  while  the  manubrium  of  a  medusa  has 
the  same  simple  structure  as  a  hydranth,  or  what  comes  to 
the  same  thing,  as  a  Hydra,  the  bell  has  undergone  a  very 
remarkable  differentiation  of  its  tissues.  Its  ordinary  ecto- 
derm cells  instead  of  being  large  and  eminently  contractile 
form  little  more  than  a  thin  cellular  skin  or  epithelium  over  the 
gelatinous  mesoglcea  :  they  have  largely  given  up  the  function 
of  contractility  to  the  muscle  processes  or  fibres,  and  have 
taken  on  the  functions  of  a  protective  and  sensitive  layer. 

Similarly  the  function  of  automatism,  possessed  by  the 
whole  body  of  Hydra,  is  made  over  to  the  group  of  specially 
modified  ectodermal  cells  which  constitute  the  central 
nervous  system.  If  a  Hydra  is  cut  into  any  number  of 
pieces,  each  of  them  is  able  to  perform  the  ordinary  move- 
ments of  expansion  and  contraction,  but  if  the  nerve-ring 
of  a  medusa  is  removed  by  cutting  away  the  edge  of  the 
bell,  the  rhythmical  swimming  movements  stop  dead  :  the 
bell  is  in  fact  permanently  paralysed. 

It  is  not,  however,  rendered  incapable  of  movement,  for 
a  sharp  pinch,  i.e.  an  external  stimulus,  causes  a  single  con- 
traction, showing  that  the  muscles  still  retain  their  irritability. 
But  no  movement  takes  place  without  such  external  stimulus, 
each  stimulus  giving  rise  infallibly  to  one  single  contraction  : 
the  power  possessed  by  the  entire  animal  of  independently 
originating  movement,  i.e.  of  supplying  its  own  stimuli,  is 
lost  with  the  central  nervous  system. 

Another  instance  of  morphological  and  physiological 
differentiation  is  furnished  by  the  pigment  spots  or  ocelli 
(Fig.  54,  c,  oc]  situated  at  the  bases  of  the  tentacles.  They 
consist  of  groups  of  ectoderm  cells  in  which  are  deposited 
granules  of  deep  red  pigment.  Their  function  is  proved  by 
the  following  experiment. 

If  a  number  of  medusae  are  placed  in  a  glass  vessel  of 


GONADS  247 

water  in  a  dark  room,  and  a  beam  of  light  from  a  lantern  is 
allowed  to  pass  through  the  water,  the  animals  are  all  found 
to  crowd  into  the  beam,  thus  being  obviously  sensitive  to  and 
attracted  by  light.  If  however  the  ocelli  are  removed  this 
is  no  longer  the  case  :  the  medusae  do  not  make  for  the 
beam  of  light,  and  are  incapable  of  distinguishing  light  from 
darkness.  The  ocelli  are  therefore  organs  of  sight. 

In  Zoothamnium  we  saw  that  the  two  forms  of  zooid  were 
respectively  nutritive  and  reproductive  in  function,  the  re- 
productive zooids, becoming  detached  and  swimming  off  to 
found  a  new  colony  elsewhere  (p.  135). 

This  is  also  the  case  with  Bougainvillea :  the  hydranths 
are  purely  nutritive  zooids,  the  medusae,  although  capable  of 
feeding,  are  specially  distinguished  as  reproductive  zooids. 
The  gonads  are  found  in  the  walls  of  the  manubrium,  between 
the  ectoderm  and  endoderm,  some  medusae  reproducing 
ovaries,  others  spermaries  only.  Thus  while  Hydra  is 
monoecious,  both  male  and  female  gonads  occurring  in  the 
same  individual,  Bougainvillea  is  dioecious,  certain  individuals 
producing  only  male,  others  only  female  products. 

In  some  Hydroids  it  has  been  found  that  the  sexual  cells 
from  which  the  ova  and  sperms  are  developed  do  not  originate 
in  the  manubrium  of  a  medusa,  but  apparently  arise  in  the 
endoderm  of  the  stem  of  the  hydroid  colony,  afterwards 
migrating,  while  still  small  and  immature,  to  their  permanent 
situation  where  they  undergo  their  final  development.1  In 
Bougainvillea,  however,  the  reproductive  products  are  said 
to  originate  in  the  manubrium. 

1  This  migration  of  the  sexual  cells  renders  the  question  of  their 
origin  in  many  cases  a  very  difficult  one.  In  some  Hydroids,  at  any 
rate,  they  arise  in  the  ectoderm,  but  migrate  into  the  endoderm  at  a 
very  early  stage. 


248  HYDROID  POLYPES  LESS. 

The  medusae,  when  mature,  become  detached  and  'swim 
away  from  the  hydroid  colony.  The  sperms  of  the  males 
are  shed  into  the  water  and  carried  to  the  ovaries  of  the 
females,  where  they  fertilize  the  ova,  converting  them,  as 
usual,  into  oosperms. 

The  changes  by  which  the  oosperm  or  unicellular  embryo 
of  a  hydroid  polype  is  converted  into  the  adult  are  very 
remarkable. 

The  process  is  begun  by  the  oosperm,  still  enclosed 
within  the  body  of  the  parent  (Fig.  57,  A),  undergoing 
binary  fission,  so  that  a  two-celled  embryo  is  formed  (B). 
Each  of  the  two  cells  again  divides  (c),  and  the  process  is 
repeated,  the  embryo  consisting  successively  of  2,  4,  8,  16, 
32,  &c.,  cells,  until  a  solid  globular  mass  of  small  cells  is 
produced  (D,  E)  by  the  repeated  division  of  the  one  large 
cell  which  forms  the  starting-point  of  the  series.  The  embryo 
in  this  stage  has  been  compared  to  a  mulberry,  and  is  called 
the  morula  or  polyplast. 

So  far  all  the  cells  of  the  polyplast  are  alike — globular 
nucleated  masses  of  protoplasm  squeezed  into  a  polyhedral 
form  by  mutual  pressure.  But  before  long  the  cells  lying 
next  the  surface  alter  their  form,  becoming  cylindrical,  with 
their  long  axes  disposed  radially  (F)  .  In  this  way  a  superficial 
layer  of  cells,  or  ectoderm,  is  differentiated  from  an  internal 
mass,  or  endoderm. 

The  embryo  now  assumes  an  elongated  form  (G)  and 
begins  to  exhibit  slow,  worm-like  movements,  finally  escaping 
from  the  parent  and  beginning  a  free  existence  (H).  The 
ectoderm  cells  are  now  found  to  be  ciliated,  and  before  long 
a  cavity  appears  in  the  previously  solid  mass  of  endoderm 
cells  :  this  is  the  first  appearance  of  the  enteron  or  digestive 
cavity.  In  this  stage  the  embryo  is  called  a  planula :  it 


xxni  DEVELOPMENT  249 

swims  slowly  through  the  water  by  means  of  its  cilia,  the 

A   _  B  c 


FIG.  57. — Stages  in  the  development  of  two  hydroid  polypes,  Lao- 
niedea  flexiwsa  (A-H)  and  Eudendrium  ramosum  (l-M). 

A,  oosperm. 

B,  two-celled,  and  C,  four-celled  stage. 
D,  E,  polyplast. 

F,  G,  formation  of  planula  by  differentiation  of  ectoderm  and 
endoderm. 

In  A-G  the  embryo  is  embedded  in  the  maternal  tissues. 

H,  free  swimming  planula,  showing  ciliated  ectoderm,  and  endoderm 
enclosing  a  narrow  enteric  cavity. 

I,  planula,  after  loss  of  its  cilia,  about  to  affix  itself. 

K,  the  same  after  fixation. 

L,  Hydra-like  stage,  still  enclosed  in  cuticle. 

M,  the  same  after  rupture  of  the  cuticle  and  liberation  of  the  tentacles. 
(After  Allman.) 

broader  end  being  directed  forwards  in  progression.    It  then 
loses  its  cilia  and  settles  down  on  a  rock,  shell,  sea-weed,  or 


250  HYDROID  POLYPES  LESS. 

other  submarine  object,  assuming  a  vertical  position  with  its 
broader  end  fixed  to  the  support  (i). 

The  attached  or  proximal  end  widens  into  a  disc  of  attach- 
ment, a  dilatation  is  formed  a  short  distance  from  the  free  or 
distal  end,  and  a  thin  cuticle  is  secreted  from  the  whole 
surface  of  the  ectoderm  (K).  From  the  dilated  portion 
short  buds  arise  in  a  circle  :  these  are  the  rudiments  of  the 
tentacles  :  the  narrow  portion  beyond  their  origin  becomes 
the  hypostome  (L).  Soon  the  cuticle  covering  the  distal  end 
is  ruptured  so  as  to  set  free  the  growing  tentacles  (M)  :  an 
aperture,  the  mouth,  is  formed  at  the  end  of  the  hypostome, 
and  the  young  hydroid  has  very  much  the  appearance  of  a 
Hydra  with  a  broad  disc  of  attachment,  and  with  a  cuticle 
covering  the  greater  part  of  the  body. 

Extensive  budding  next  takes  place,  the  result  being  the 
formation  of  the  ordinary  hydroid  colony. 

Thus  from  the  oosperm  or  impregnated  egg-cell  of  the 
medusa  the  hydroid  colony  arises,  while  the  medusa  is 
produced  by  budding  from  the  hydroid  colony.  The  analogy 
with  Nitella  (p.  219)  will  be  at  once  obvious  :  in  each  case 
there  is  an  alternation  of  generations,  the  asexual  genera- 
tions or  agamobium  (hydroid  colony,  pro-embryo  of  Nitella) 
giving  rise  by  budding  to  the  sexual  generation  or  gamobium 
(medusa,  Nitella-plant),  which  in  its  turn  produces  the 
agamobium  by  a  sexual  process,  i.e.  by  the  conjugation  of 
ovum  and  sperm. 

Two  other  Hydroids  must  be  briefly  referred  to  in  con- 
cluding the  present  lesson. 

Floating  on  the  surface  of  the  ocean  in  many  parts  of  the 
world  is  found  a  beautiful  transparent  organism  called 
Diphyes.  It  consists  of  a  long,  slender  stem  (Fig.  58,  A,  a\ 
at  one  end  of  which  are  attached  two  structures  called 


xxni  GENERAL   CHARACTERS  251 

swimming-bells  (m,  in)  in  form  something  like  the  bowl  of  a 
German  pipe,  while  all  along  the  stem  spring  at  intervals 
groups  of  structures  (e),  one  of  which  is  shown  on  an  enlarged 
scale  at  B. 

Each  group  contains,  first,  a  tubular  structure  (B,  n)  with 
an  expanded,  trumpet-like  mouth,  through  which  food  is 
taken  :  this  is  clearly  a  hydranth.  From  the  base  of  the 
hydranth  proceeds  a  single,  long,  branched  tentacle  or 
"  grappling-line  "  (/),  abundantly  provided  with  nematocysts. 
Springing  from  the  stem  near  the  base  of  the  hydranth  is  a 
body  called  a  medusoid  (g),  very  like  a  sort  of  imperfect 
medusa,  and  like  it  containing  gonads.  Lastly,  enclosing  all 
these  structures,  much  as  the  white  petaloid  bract  of  the 
common  Arum-lily  encloses  the  flower-stalk,  is  a  delicate 
folded  membranous  plate,  to  which  the  name  bract,  borrowed 
from  botany,  is  applied.  The  whole  organism  is  propelled 
through  the  water  by  the  rhythmical  contraction  of  the 
swimming-bells. 

Microscopic  examination  shows  that  the  stem  consists,  like 
that  of  Bougainvillea,  of  ectoderm,  mesoglcea,  and  endo- 
derm,  but  without  a  cuticle.  The  hydranth  has  a  similar 
structure  to  that  of  Bougainvillea,  only  differing  in  shape  and 
in  the  absence  of  tentacles  round  the  mouth  :  the  medusoids 
are  merely  simplified  medusae  :  the  swimming-bells  are  prac- 
tically medusae  in  which  the  manubrium  is  absent  :  and 
both  the  bracts  and  grappling-lines  are  shown  by  com- 
parison with  allied  forms  to  be  greatly  modified  medusa-like 
structures. 

Diphyes  is  in  fact  a  free-swimming  hydroid  colony  which, 
instead  of  being  dimorphic  like  Bougainvillea,  '^polymorphic. 
In  addition  to  nutritive  zooids  or  hydranths,  it  possesses 
locomotive  zooids  or  swimming-bells,  protective  zooids  or 
bracts,  and  tentacular  zooids  or  grappling-lines.  Morpho- 


252 


HYDROID  POLYPES 


LESS. 


FIG.  58. — Diphyes  campamilata. 

A,  the  entire  colony,  natural  size,  showing  stem  (a)  bearing  groups  01 
zooids  (e)  and  two  swimming  bells  (m,  m],  the  apertures  of  which  ai-e 
marked  o. 

B,  one  of  the  groups  of  zooids  marked  e  in  A,  showing  common  stem 
(a),  hydranth  («),  medusoid  (g),  bract  (/),    and  branched  tentacle  or 
grappling  line  (?').  (From  Gegenbaur.) 


XXIII 


INDIVIDUATION 


253 


logical   and   physiological   differentiation   are   thus  carried 
much  further  than  in  such  a  form  as  Bougainvillea. 


FIG.  59. — A,  Porpita  pacifica  (nat.  size),  from  beneath,  showing  disc- 
like  stem  surrounded  by  tentacles  (/),  a  single  functional  hydranth  (hy), 
and  numerous  mouthless  hydranths  (hyr). 

B,  vertical  section  of-  P.  mediterranea,  showing  the  relative  positions 
of  the  functional  (hy)  and  mouthless  (hy1}  hydranths,  the  tentacles, 
and  the  chambered  shell  (sli).  (A  after  Duperrey  ;  B  from  Huxley  after 
Kolliker.) 

Porpita  is  another  free-swimming  Hydroid,  presenting  at 
first  sight  no  resemblance  whatever  to  Diphyes.  It  has  much 
the  appearance  of  a  flattened  medusa  (Fig.  59),  consisting 
of  a  circular  disc,  slightly  conv( 


mvex  above  and  concave  below. 

'S»7        "" 


254  HYDROID   POLYPES  LESS,  xxm 

bearing  round  its  edge  a  number  of  close-set  tentacles,  and 
on  its  under  side  a  central  tubular  organ  (hy)  with  a  ter- 
minal mouth,  like  the  manubrium  of  a  medusa,  surrounded 
by  a  great  number  of  structures  like  hollow  tentacles  (hy')> 
The  discoid  body  is  supported  by  a  sort  of  shell  having  the 
consistency  of  cartilage  and  divided  into  chambers  which 
contain  air  (B,  sh). 

Accurate  examination  shows  that  the  manubrium-like 
body  (hy)  on  the  under  surface  is  a  hydranth,  that  the  short, 
hollow,  tentacle-like  bodies  (hy'}  surrounding  it  are  mouthless 
hydranths,  and  that  the  disc  represents  the  common  stem  of 
Diphyes  or  Bougainvillea.  So  that  Porpita  is  not  what  it 
appears  at  first  sight,  a  single  individual,  like  a  Medusa  or  a 
Hydra,  but  a  colony  in  which  the  constituent  zooids  have 
become  so  modified  in  accordance  with  an  extreme  division 
of  physiological  labour,  that  the  entire  colony  has  the  char- 
acter of  a  single  physiological  individual. 

It  was  pointed  out  in  the  previous  lesson  (p.  233)  that 
Hydra,  while  morphologically  the  equivalent  of  an  indefinite 
number  of  unicellular  organisms,  was  yet  physiologically  a 
single  individual,  its  constituent  cells  being  so  differentiated 
and  combined  as  to  form  one  whole.  A  further  stage  in  this 
same  process  of  individuation  is  seen  in  Porpita,  in  which  not 
cells  but  zooids,  each  the  morphological  equivalent  of  an 
entire  Hydra,  are  combined  and  differentiated  so  as  to  form 
a  colony  which,  from  the  physiological  point  of  view,  has 
the  characters  of  a  single  individual. 


LESSON  XXIV 

SPERMATOGENESIS  AND  OOGENESIS.  THE  MATURATION  AND 
IMPREGNATION  OF  THE  OVUM.  THE  CONNECTION  BE- 
TWEEN UNICELLULAR  AND  DIPLOBLASTIC  ANIMALS 

IN  the  preceding  lessons  it  has  more  than  once  been  stated 
that  sperms  arise  from  ordinary  undifferentiated  cells  in  the 
spermary,  and  that  ova  are  produced  by  the  enlargement 
of  similar  cells  in  the  ovary.  'Fertilization  has  also  been  de- 
scribed as  the  conjugation  or  fusion  of  ovum  and  sperm.  We 
have  now  to  consider  in  greater  detail  what  is  known  as  to 
the  precise  mode  of  development  of  sperms  (spermatogenesis) 
and  of  ova  (oogenesis\  as  well  as  the  exact  steps  of  the  pro- 
cess by  which  an  oosperm  or  unicellular  embryo  is  formed 
by  the  union  of  the  two  sexual  elements.  The  following 
description  applies  to  animals  :  recent  researches  show  that 
essentially  similar  processes  take  place  in  plants. 

Both  ovary  and  spermary  are  at  first  composed  of  cells  of 
the  ordinary  kind,  the  primitive  sex-cells,  and  it  is  only  by 
the  further  development  of  these  that  the  sex  of  the  gonad 
is  determined. 

In  the  spermary  the  sex  cells  (Fig.  60,  A)  undergo  repeated 
fission,  forming  what  are  known  as  the  sperm-mother-cells 
(B).  These  have  been  found  in  several  instances  to  be 


256  SPERMATOGENESIS   AND   OOGENESIS  LESS. 

distinguished  by  a  peculiar  condition  of  the   nucleus.     We 
saw  (p.  65)  that  the  number  of  chromosomes  is  constant  in 


B 


FIG.  60. — Spermatogenesis  in  the  Mole-Cricket  (Gryllotaipa). 

A.  Primitive  sex-cell,  just  preparatory  to  division,  showing  twelve 
chromosomes  (chr} ;  c,  the  centrosome. 

B.  Sperm-mother-cell,  formed  by  the  division  of  A,  and  containing 
twenty-four  chromosomes.     The  centrosome  has  divided  into  two. 

C.  The  sperm-mother-cell  has  divided  into  two  by  a  reducing  division, 
each  daughter  cell  containing  twelve  chromosomes. 

D.  Each  daughter  cell  has  divided   again  in  the    same    manner,  a 
group  of  four  sperm-cells  being  produced,  each  with  six  chromosomes. 

E.  A  single  sperm-cell  about  to  elongate  to  form  a  sperm. 

F.  Immature  sperm  ;   the  six  chromosomes  are  still  visible    in  the 
head. 

G.  Fully  formed  sperm. 
(After  vom  Rath.) 


xxiv  REDUCING  DIVISION  257 

any  given  animal,  though  varying  greatly  in  different  species. 
In  the  formation  of  the  sperm-mother-cells  from  the  primitive 
sex-cells  the  number  becomes  doubled  :  in  the  case  of 
the  mole-cricket,  for  instance,  shown  in  Fig.  60,  while  the 
ordinary  cells  of  the  body,  including  the  primitive  sex- 
cells,  contain  twelve  chromosomes,  the  sperm-mother-cells 
contain  twenty-four. 

The  sperm-mother-cell  now  divides  (c),  but  instead  of  its 
chromosomes  splitting  in  the  ordinary  way  (p.  66,  Fig.  10) 
half  of  their  total  number — in  the  present  instance  twelve — 
passes  into  each  daughter  cell :  in  this  way  two  cells  are 
produced  having  the  normal  number  of  chromosomes.  The 
process  of  division  is  immediately  repeated  in  the  same 
peculiar  way  (D),  the  result  being  that  each  sperm-mother- 
cell  gives  rise  to  a  group  of  four  cells  having  half  the  normal 
number  of  chromosomes — in  the  present  instance  six.  The 
four  cells  thus  produced  are  the  immature  sperms  (E)  :  in 
the  majority  of  cases  the  protoplasm  of  each  undergoes  a 
great  elongation,  being  converted  into  a  long  vibratile  thread, 
the  tail  of  the  sperm  (F,  G),  while  the  nucleus  becomes  its 
more  or  less  spindle-shaped  head. 

Thus  the  sperm  or  male  gamete  is  a  true  cell,  specially 
modified  in  most  cases  for  active  movement  :  its  head, 
representing  the  nucleus,  is  directed  forwards  in  progres- 
sion, its  long  tail,  formed  from  the  protoplasm,  backwards. 
The  direction  of  movement  is  thus  the  precise  opposite  of 
that  of  a  monad  (p.  36)  to  which  a  sperm  presents  a  certain 
resemblance.  This  actively  motile  tailed  form  is,  however, 
by  no  means  essential :  in  many  animals  the  sperms  are 
non-motile  and  in  some  they  resemble  ordinary  cells. 

The  peculiar  variety  of  karyokinesis  described  above,  by 
which  the  number  of  chromosomes  in  the  sperm-mother-cells 
is  reduced  by  one-half,  is  known  as  a  reducing  division. 


258  SPERMATOGENESIS   AND   OOGENESIS  LESS 

As  already  stated,  the  ova  arise  from  primitive  sex-cells, 
precisely  resembing  those  which  give  rise  to  sperms.  These 
divide  and  give  rise  to  the  egg-mother-cells  in  which,  as  in 
the  sperm-mother-cells,  the  number  of  chromosomes  is 
doubled.  The  egg-mother-cells  do  not  immediately  undergo 
division  but  remain  passive  and  increase,  often  enormously, 
in  size,  by  the  absorption  of  nutriment  from  surrounding 
parts :  in  this  way  each  egg-mother-cell  becomes  an  ovum. 
Sometimes  this  nutriment  is  simply  taken  in  by  osmosis, 
in  other  cases  the  growing  ovum  actually  ingests  neigh- 
bouring cells  after  the  manner  of  an  Amoeba.  Thus  in  the 
developing  egg  the  processes  of  constructive  are  vastly 
in  excess  of  those  of  destructive  metabolism. 

We  saw  in  the  second  lesson  (p.  33)  that  the  products  of 
destructive  metabolism  might  take  the  form  either  of  waste 
products  which  are  got  rid  of,  or  of  plastic  products  which 
are  stored  up  as  an  integral  part  of  the  organism.  In  the 
developing  egg,  in  addition  to  increase  in  the  bulk  of  the 
protoplasm  itself,  a  formation  of  plastic  products  usually 
goes  on  to  an  immense  extent.  In  plants  the  stored-up 
materials  may  take  the  form  of  starch,  as  in  Nitella  (p.  216), 
of  oil,  or  of  proteid  substance  :  in  animals  it  consists  of 
rounded  or  angular  grains  of  proteid  material,  known  as 
yolk-granules.  These  being  deposited,  like  plums  in  a 
pudding,  in  the  protoplasm,  have  the  effect  of  rendering  the 
fully-formed  egg  opaque,  so  that  its  structure  can  often  be 
made  out  only  in  sections.  When  the  quantity  of  yolk  is 
very  great  the  ovum  may  attain  a  comparatively  enormous 
size,  as  for  instance  in  birds,  in  which,  as  already  mentioned 
(p.  68),  the  "  yolk  "  is  simply  an  immense  egg-cell. 

When  fully  formed,  the  typical  animal  ovum  (Fig.  61) 
consists  of  a  more  or  less  globular  mass  of  protoplasm, 
generally  exhibiting  a  reticular  structure  and  enclosing  a 


xxiv  STRUCTURE  OF  THE  OVUM  259 

larger  or  smaller  quantity  of  yolk-granules.  Surrounding 
the  cell-body  is  usually  a  cell-wall  or  cuticle,  often  of  con- 
siderable thickness  and  known  as  the  vitelline  membrane. 
The  nucleus  is  large  and  has  the  usual  constituents  (p.  63) — 
nuclear  membrane,  nuclear  matrix,  and  chromatin.  As  a 
rule  there  is  a  very  definite  nucleolus,  which  is  often  known 
as  the  germinal  spot,  the  entire  nucleus  being  called  the 
germinal  vesicle. 

Such  a  fully-formed  ovum  is,  however,  incapable  of  being 
fertilized  or  of  developing  into  an  embryo  :  before  it  is  ripe  for 


FIG.  61. — Ovum  of  a  Sea-urchin  (Toxopneustes  lividus),  showing  the 
radially-striated  cell-wall  (vitelline  membrane),  the  protoplasm  contain- 
ing yolk  granules  (vitellus),  the  large  nucleus  (germinal  vesicle)  with  its 
network  of  chromatin,  and  a  large  nucleolus  (germinal  spot).  (From 
Balfour  after  Hertwig.) 

conjugation  with  a  sperm  or  able  to  undergo  the  first  stages 
of  yolk  division  it  has  to  go  through  a  process  known  as  the 
maturation  of  the  egg. 

Maturation  consists  essentially  in  a  twice-repeated  process 
of  cell-division.  The  nucleus  (Fig.  62,  A,  nu)  loses  its  mem- 
brane, travels  to  the  surface  of  the  egg,  and  takes  on  the 

S    2 


FIG    62. — The' Maturation  and  Impregnation  of  the  Animal  Ovum. 

A    portion  of  the  ovum  of  a  Round  worm  (Ascaris  megaiocephala], 

showing  the  sperm  (sp)  in  the  act  of  conjugation,  and  the  unaltered 


LESS,  xxiv  POLAR  CELLS  261 

nucleus  (««)  of  the  egg,  Ascaris  being  an  animal  in  which  the  conjuga- 
tion of  ovum  and  sperm  takes  place  before  the  maturation  of  the  former. 
In  the  nucleus,  the  nuclear  membrane  and  matrix,  and  a  band-like  mass 
of  chromatin  are  visible.  The  sperm  of  Ascaris  is  of  peculiar  form,  and 
is  non-motile. 

B,  the  same  at  the  commencement  of  maturation  :  the  nucleus  (nu) 
has  travelled  to  the  periphery  of  the  egg  and  taken  on  the  spindle  form. 
In  this  and  the  two  next  figures  the  vitelline  membrane  is  shown. 

C,  formation  of  the  first  polar  cell  (p.  c.  i). 

D,  the  entire  egg  after  the  completion  of  maturation,  showing  the  two 
polar  cells,  the  first  (p.  c.  i)  adhering  to  the  vitelline  membrane,  the 
second,  (p.  c.  2}  to  the  surface  of  the  protoplasm  :  the  female  pronucleus 
(pr.  nu.   ?  ) :  and  the  sperm  (sf>),  which  has  penetrated  into  the  cell- 
protoplasm,  but  has  not  yet   become   converted  into  the  male  pro- 
nucleus. 

E1,  E2,  two  stages  in  the  conjugation  of  the  pronuclei  in  Molluscs 
(E1,  Pterotrachea,  E2,  Phyllirhoe], 

In  E1  the  male  (pr.  mi.  <J )  and  female  (pr.  mi.  ?  )  pronuclei  are 
separated  :  in  E2  they  are  applied  by  their  flattened  adjacent  faces  :  in 
connection  with  each  the  cell-protoplasm  has  a  radiating  arrangement 
around  one  of  the  directive  spheres  ;  the  polar  cells  (p.c.i,  p.c.  2)  are 
shown. 

F^F3,  three  stages  in  the  development  of  the  nucleus  of  the  oosperm 
in  a  Sea-urchin  (Echinus  microtiiberculatns) :  in  F1  the  nucleus  contains 
nine  chromatin-fibres  (chrom.  9  )  derived  from  the  female  pronucleus, 
and  a  globular  mass  of  the  same  (chrom,  <$ )  derived  from  the  male  pro- 
nucleus  :  the  two  directive  spheres  are  now  situated  one  at  each  end  of 
the  nucleus.  In  F2  the  male  chromatin  (chrom.  $  )  has  begun  to  unwind 
itself :  in  F  there  is  no  longer  any  distinction  between  male  and  female 
elements,  the  nucleus  containing  eighteen  similar  chromatin-threads. 

G,  central  portion  of  the  egg  of  a  Hermit-Crab  (Eupagurus prideauxit), 
showing  the  conjugation  of  the  pronuclei.  The  male  and  female  chro- 
matin-networks  appear  to  be  fused  along  the  plane  of  union.  The  pro- 
nuclei  are  surrounded  by  finely-granular  protoplasm  devoid  of  yolk- 
spheres. 

(A-F  after  Boveri ;  G  after  Weismann  and  Ischikawa.) 


form  of  an  ordinary  nuclear  spindle  (B,  nu,  see  p.  65).  Next, 
the  protoplasm  grows  out  into  a  small  projection  or  bud,  into 
which  one  end  of  the  spindle  projects  (c).  The  usual  pro- 
cess of  nuclear  division  then  takes  place  (Fig.  10,  p.  64), 
one  of  the  daughter  nuclei  remaining  in  the  bud,  the  other 
in  the  ovum  itself.  Nuclear  division  is  followed  as  usual  by 
division  of  the  protoplasm,  and  the  bud  becomes  separated 


262  SfERMATOGENESIS   AND   OOGENESIS  LESS. 

as  a  small  cell  distinguished  as  the  first  polar  cell  (c — E 
p.c.  i). 

It  was  mentioned  in  a  previous  lesson  (p.  200)  that  in 
some  cases  development  from  an  unfertilized  female  gamete 
took  place,  the  process— which  is  not  uncommon  among 
insects  and  crustaceans — being  distinguished  as  partheno- 
genesis. It  has  been  proved  in  many  instances  and  may  be 
generally  true  that  in  such  cases  the  egg  begins  to  develop 
after  the  formation  of  the  first  polar  cell.  Thus  in  partheno- 
genetic  ova  it  appears  that  maturation  is  completed  by  the 
separation  of  a  single  polar  cell. 

In  the  majority  of  animals,  however,  development  takes 
place  only  after  fertilization,  and  in  such  cases  maturation 
is  not  complete  until  a  second  polar  cell  (D  and  E,  p.c.  2)  has 
been  formed  in  the  same  manner  as  the  first.  The  ovum 
has  now  lost  a  portion  of  its  protoplasm  together  with  three- 
fourths  of  its  chromatin,  half  having  passed  into  the  first 
polar  cell  and  half  of  what  remained  into  the  second  :  the 
remaining  one-fourth  of  the  chromatin  takes  on  a  rounded 
form  and  is  distinguished  as  the  female  pronncleus  (D, 
pr.  nu.  ?). 

The  formation  of  both  polar  cells  takes  place  by  a 
reducing  division,  so  that,  while  the  immature  ovum  con- 
tains double  the  number  of  chromosomes  found  in  the 
ordinary  cells  of  the  species,  the  mature  ovum,  like  the 
sperm,  contains  only  one-half  the  normal  number. 

In  some  animals  the  first  polar  body  has  been  found  to 
divide  after  separating  from  the  egg.  In  such  cases  the  egg- 
mother-cell  or  immature  ovum  gives  rise  to  a  group  of 
four  cells— the  mature  ovum  and  three  polar-cells;  just 
as  the  sperm-mother-cell  gives  rise  to  a  group  of  four  cells, 
all  of  which,  however,  become  sperms. 

Shortly    after,    or  in  some  cases   before  maturation   the 


xxiv  FUSION  OF  PRONUCLEI  263 

ovum  is  fertilized  by  the  conjugation  with  it  of  a  single 
sperm.  As  we  have  found  repeatedly,  sperms  are  produced 
in  vastly  greater  numbers  than  ova,  and  it  often  happens 
that  a  single  egg  is  seen  quite  surrounded  with  sperms,  all 
apparently  about  to  conjugate  with  it.  It  has  however  been 
found  to  be  a  general  rule  that  only  one  of  these  actually 
conjugates  :  the  others,  like  the  drones  in  a  hive,  perish 
without  fulfilling  the  one  function  they  are  fitted  to 
perform. 

The  successful  sperm  (A,  sp}  takes  up  a  position  at  right 
angles  to  the  surface  of  the  egg  and  gradually  works  its  way 
through  the  vitelline  membrane  until  its  head  lies  within  the 
egg  protoplasm  (D,  sp).  The  tail  is  then  cast  off,  and  the 
head,  penetrating  deeper  into  the  protoplasm,  takes  on  the 
form  of  a  rounded  nucleus-like  body,  the  male  pronudeus 
(E1,  pr.  nu.  $ ). 

The  two  pronuclei,  each  accompanied  by  its  directive 
sphere  and  centrosome,  approach  one  another  (E],  E2)  and 
finally  unite  to  form  the  single  nucleus  (p1 — -F3)  of  what  is 
now  not  the  ovum  but  the  oosperm — the  impregnated  egg  or 
unicellular  embryo.  The  fertilizing  process  is  thus  seen  to 
consist  of  the  union  of  two  nuclear  bodies,  one  contributed 
by  the  male  gamete  or  sperm,  the  other  by  the  female 
gamete  or  ovum.  It  follows  from  this  that  the  essential 
nuclear  matter  or  chromatin  of  the  oosperm  is  derived  in 
equal  proportions  from  each  of  the  two  parents. 

Moreover,  as  both  male  and  female  pronuclei  contain  only 
half  the  number  of  chromosomes  found  in  the  ordinary  cells 
of  the  species,  the  union  of  the  pronuclei  results  in  the 
restoration  of  the  normal  number  to  the  oosperm. 

There  is  reason  for  thinking  that  the  directive  spheres  of 
the  sperm  and  ovum  as  well  as  their  nuclei  unite  with  one 
another  :  in  this  way  the  directive  sphere  of  the  oosperm 


264  SPERMATOGENESIS   AND   OOGENESIS  LESS. 

is  derived,  like  its  nucleus,  in  equal  proportions  from  the 
two  parents. 

Fertilization  being  thus  effected,  the  process  of  segmenta- 
tion or  division  of  the  oosperm  takes  place  as  described  in 
the  preceding  lesson  (p.  248). 

In  concluding  the  present  lesson,  we  shall  consider 
briefly  a  point  which  has  probably  already  struck  the  reader. 
Among  the  plant-forms  which  have  come  under  our  notice 
there  has  been  a  very  complete  series  of  gradations  from  the 
simple  cell,  through  the  branched  cell,  linear  aggregate,  and 
superficial  aggregate,  to  the  solid  aggregate,  whilst  among 
the  animals  already  discussed  there  has  so  far  been  no 
attempt  to  fill  up  the  very  considerable  gap  between  the 
unicellular  Infusoria  and  Hydra,  which  is  not  only  a  solid 
aggregate,  but  has  its  cells  arranged  in  two  definite  layers 
enclosing  a  digestive  cavity. 

When  we  say  that  no  attempt  has  been  made  to  fill  up 
this  gap,  we  mean  as  far  as  adult  forms  are  concerned.  If 
the  reader  will  turn  to  the  account,  in  the  previous  lesson,  of 
the  development  of  hydroid  polypes  (p.  248),  he  will  see  that 
the  facts  there  described  do  as  a  matter  of  fact  help 
us  to  see  a  possible  connection  between  unicellular 
animals  and  multicellular  two-layered  forms  with  mouth 
and  digestive  cavity.  The  oosperm  of  the  hydroid  (Fig. 
57,  A)  has  the  essential  character  of  an  Amoeba,  the 
polyplast  (E)  is  practically  a  colony  of  Amoebae,  and  the 
planula  (H)  .  a  similar  colony  in  which  the  zooids  (cells) 
are  dimorphic,  being  arranged  in  two  layers  with  a  central 
digestive  cavity  which  finally  communicates  with  the  exterior 
by  a  mouth.  In  hydroids  the  mouth  is  not  formed  until 
after  the  appearance  of  the  tentacles,  but  in  a  large  propor- 
tion of  the  higher  animals  the  polyplast  stage  is  succeeded 


XXIV 


THE  GASTRULA 


265 


not  by  a  mouthless  planula  but  by  a  two-layered  embryo 
with  a  mouth  at  one  end,  called  &  gastrula  (Fig.  63).  This 
is  a  very  important  stage,  since  it  exhibits  in  the  simplest 
possible  way  the  essential  characteristic  of  a  diploblastic 
animal— a  two-layered  sac  with  mouth  (Blp)  and  stomach 
(U),  the  outer  layer  of  cells  (Ekt)  being  protective  and 
sensory,  the  inner  (Ent)  having  a  digestive  function.  The 


FIG.  63. — A  typical  animal  gastrula  in  vertical  section,  showing 
ectoderm  (Ekt),  endoderm  (Ent),  enteron  or  digestive  cavity  (£7),  and 
mouth  (Blp).  (From  Wiedersheim. ) 

planula  of  a  hydroid  may  be  looked  upon  as  a  gastrula  in 
which  the  mouth  has  not  yet  appeared. 

Another  very  important  difference  is  the  fact  that  in  uni- 
cellular organisms  reproduction  is  effected  either  asexually 
by  the  fission  of  the  entire  individual,  or,  in  the  case  of 
sexual  reproduction,  by  two  entire  individuals  undergoing 
conjugation.  In  multicellular  forms,  on  the  other  hand, 
single  cells  are  set  apart  for  sexual  reproduction. 


FiG.  64. — Panaorina  mo  rum. 

A.  The  entire  colony,  consisting  of  sixteen  flagellate  zooids,  enclosed 
in  a  gelatinous  envelope. 

B  Asexual  reproduction  ;  each  zooid  has  divided  into  sixteen,  forming 
as  many  daughter  families,  still  enclosed  within  the  original  gelatinous 

ellc?  Sexual  reproduction ;  zooids  are  being  set  free  from  the  colony, 
forming  gametes. 

D.  Conjugation  of  two  gametes. 

E.  The  same  after  complete  fusion. 

F.  The  immature  zygote. 

G.  The  fully-formed  zygote. 

H.  Protoplasm  of  zygote  escaping  from  cell-wall. 
I.  The  same  after  acquisition  of  flagella. 

K.  The  same  undergoing  division  and  forming  a  young  colony. 
(From  Goebel.) 


LESS,  xxvi  PANDORINA  267 

There  are  several  interesting  organisms  which  help  to 
bridge  this  gulf.  Two  of  the  more  accessible  and  well- 
known  forms  will  now  be  described. 

Pandorina  (Fig.  64,  A)  is  a  colony  consisting  of  sixteen 
zooids  closely  packed  in  a  gelatinous  case  of  a  globular 
form.  Each  zooid  resembles  in  general  characters  a  mo- 
tile Haematococcus  or  Euglena,  having  an  ovoid  cell-body 
coloured  green  by  chlorophyll,  a  red  pigment  spot,  and 
two  flagella,  which  protrude  through  the  gelatinous  wall  of 
the  colony,  and  by  their  action  impart  to  it  a  rotatory 
movement. 

In  asexual  reproduction  each  of  the  sixteen  zooids  divides 
and  re-divides,  forming  at  last  a  group  of  sixteen  cells.  In 
this  way  sixteen  daughter  colonies  are  produced  within  the 
gelatinous  envelope  of  the  original  mother  colony  (B).  By 
the  solution  of  the  envelope  the  daughter  colonies  are  set 
free,  and  each  begins  an  independent  existence. 

In  sexual  reproduction  the  zooids  are  set  free  singly  from 
the  colony  (c).  They  swim  about  actively,  approach  one' 
another  in  pairs,  and  conjugate  (D),  becoming  completely 
fused  together  (E)  to  form  a  zygote  (F).  This  increases  in 
size  and  develops  a  thick  cell  wall  (G).  After  a  period  of 
rest,  the  protoplasm  escapes  from  the  cell  wall  (H),  puts  out 
a  pair  of  flagella  (i),  and  swims  about.  Finally  it  settles 
down,  divides  and  re-divides,  and  so  gives  rise  to  a  new 
colony  (K). 

It  is  obvious  that  Pandorina  resembles  the  polyplast 
stage  of  an  embryo  :  moreover  it  is  produced  by  the  repeated 
fission  of  a  flagellula,  just  as  the  polyplast  is  formed  by  the 
repeated  fission  of  an  oosperm. 

The  beautiful  Volvox  (Figs.  65  and  66),  one  of  the  favourite 
studies  of  microscopists,  is  a  colony  of  Hsematococcus-like 
zooids  arranged  in  the  form  of  a  hollow  sphere  containing  a 


F  JG.  65.  —  Volvox  globator. 

A,  the  entire  colony,  surface  view,  showing  the  biflagellate  zooids  and 
several  daughter-colonies  swimming  freely  in  the  interior  ;  the  latter  are 
produced  by  the  repeated  fission  of  non-flagellate  reproductive  zooids 


B,  the   same   during  sexual  maturity,  showing  spermaries  from  the 
surface  (spy),  in  profile  (spy'}  and  after  complete  formation  of  sperms 
(spy")  :  and  ovaries  from  the  surface  (ovy,  ovy",  ovy'"}  and  in  profile 
(ovy'\ 

C,  four  zooids  in  optical  section,  showing  cell-wall,  nucleus,  contractile 
vacuole,  with  adjacent  pigment-spot,  and  flagella  (fl.) 

D1-D5,    stages  in  the  formation  of  a  colony  by  the  repeated  binary 
fission  of  an  asexual  reproductive  zooid. 

E,  a  ripe  spermary. 

F,  a  single  sperm,  showing  pigment-spot  (pg)  and  flagella  (fl). 

G,  an  ovary  containing  a  single  ovum  surrounded  by  several  sperms. 
H,  oosperm  enclosed  in  its  spinose  cell  wall. 

(A  from  Geddes  and  Thomson,  after  Kirchner  ;  B-H  after  Cohn.  ) 


LESS.  XXIV 


VOLVOX 


269 


transparent  mucilage.  Each  cell  (c)  has  a  nucleus,  a  con- 
tractile vacuole,  a  large  green  chromatophore,  a  small  red 
pigment-spot  like  that  of  Euglena  (p.  47)  and  two  flagella. 
The  cells  are  surrounded  by  thick  mucilaginous  cell  walls 
which  do  not  give  the  reaction  of  cellulose,  but  are  probably 
formed  of  an  allied  carbohydrate.  By  the  combined  move- 
ment of  all  the  flagella  a  rotating  movement  is  given  to  the 
entire  colony. 

Asexual  reproduction  takes  place  by  certain  of  the  zooids 


,$ 


,o 


FIG.  66. 

Part  of  a  Volvox-colony  showing  the  structure  in  greater  detail  than 
in  Fig.  65  :  s,  spermaries  ;  o,  ovaries.   (After  Lang.) 

which  are  not  ciliated,  undergoing  a  process  very  like  the 
segmentation  of  the  hydroid  egg  (p.  248),  dividing  into  2,  4, 
8,  1 6,  &c.  cells  (A,  a,  and  D1 — D5),  and  so  forming  a  daughter 
colony  which  becomes  detached  and  swims  freely  in  the 
interior  of  the  parent  colony  (A),  by  the  rupture  of  which  it 
is  finally  liberated.  In  sexual  reproduction  certain  cells 
enlarge  and  take  on  the  characters  of  ovaries  (B,  ovy,  ovy', 
ovy",  ovy'",  and  Fig.  66,  o)  the  protoplasm  of  each  forming 


270  SPERMATOGENESIS   AND   OOGENESIS  LESS,  xxiv 

a  single  ovum  :  the  protoplasm  of  others  divides  repeatedly 
and  forms  aggregations  of  sperms  (B,  spy,  spy',  spy",  and 
Fig.  66,  s).  By  the  conjugation  of  a  sperm  (F)  with  an 
ovum  (G)  an  oosperm  (H)  is  produced,  and  from  this  by 
continued  division  a  new  colony  arises. 

Volvox  is  clearly  comparable  to  a  hollow  polyplast,  and 
further  resembles  the  higher  or  multicellular  animals  in 
that  certain  of  its  cells  are  differentiated  to  form  true  sexual 
products. 


LESSON  XXV 

POLYGORDIUS 

POLYGORDIUS  is  a  minute  worm,  about  3  or  4  cm.  in  length, 
found  in  the  European  seas,  where  it  lives  in  sand  at  a 
depth  of  a  few  fathoms.  It  has  much  the  appearance  of  a 
tangle  of  pink  thread  with  one  end  produced  into  two  delicate 
processes  (Fig.  67,  A).  These,  which  are  the  tentacles,  mark 
the  anterior  end  of  the  animal — the  opposite  extremity? 
which  in  some  species  also  bears  a  pair  of  slender  processes, 
is  the  posterior  end.  As  the  creature  creeps  along,  one  side 
is  kept  constantly  upwards  and  is  distinguished  as  the  dorsal 
aspect  ;  the  lower  surface  is  called  ventral. 

The  anterior  end  is  narrower  than  the  rest  of  the  body, 
and  is  marked  off  behind  by  a  groove  (B  and  c) ;  this 
division  is  called  the  prostomium  (Pr.  st]  and  bears  the 
tentacles  (/)  already  mentioned  in  front  and  above  ;  and  on 
each  side  a  small  oval  depression  (c.  p)  lined  with  cilia. 
Immediately  following  the  prostomium  is  a  region  clearly 
marked  off  in  front,  but  ill-defined  posteriorly,  and  known  as 
the  peristomium  (Per.  st) ;  on  its  ventral  surface  is  a  trans- 
verse triangular  aperture  the  mouth  (MtJi).  The  rest  of 
the  body  is  more  or  less  distinctly  marked  by  annular 
grooves  (D  and  E,  gr)  into  body-segments  or  metameres 


FIG.  67. — Polygordius  neapolitanus. 

A,  the  living  animal,  dorsal  aspect,  about  five  times  natural  size. 

B,  anterior  end  of  the  worm  from  the  right  side,  more  highly  magni- 
fied, showing  the  prostomium  (Pr.  st),  peristomium  (Per.  st],  tentacles 
(/),  with  setae  (s)  and  ciliated  pit  (c.  p\ 

C,  ventral  aspect  of  the  same  :  letters  as  before  except  Mth,  mouth. 

D,  portion  of  body  showing  metameres  (Mtmr)  separated  by  grooves 

*  E,  posterior  extremity  from  the  ventral  aspect,  showing  the  last  three 
metameres  (Mtmr}  separated  by  distinct  grooves  (gr),  the  anal  seg- 
ment (An.  seg]  bearing  the  anus  (An),  and  a  circlet  of  papillae  (/). 
(After  Fraipont.) 


xxv  GENERAL  CHARACTERS  273 

(Mtmr\  the  number  of  which  varies  considerably.  Poly- 
gordius  is  thus  the  first  instance  we  have  met  with  of  a  trans- 
versely segmented  animal.  The  last  or  anal  segment 
(E,  An.  seg)  differs  from  the  others  by  its  swollen  form  and 
by  bearing  a  circlet  of  little  prominences  or  papillae  (p) ;  it 
is  separated  from  the  preceding  segment  by  a  deep  groove, 
and  bears  at  its  posterior  end  a  small  circular  aperture,  the 
anus  (An). 

Polygordius  may  therefore  be  described  as  consisting  of  a 
number  of  more  or  less  distinct  segments  which  follow  one 
another  in  longitudinal  series ;  three  of  these,  \beprostoinium, 
which  lies  altogether  in  front  of  the  mouth,  the  peristomium^ 
which  contains  the  mouth,  and  the  anal  segment,  which 
contains  the  anus,  are  constant ;  while  between  the  peri- 
stomium  and  the  anal  segment  are  intercalated  a  variable 
number  of  metameres  which  resemble  one  another  in  all 
essential  respects. 

Polygordius  feeds  in  much  the  same  way  as  an  earth- 
worm :  it  takes  in  sand,  together  with  the  various  nutrient 
matters  contained  in  it,  such  as  infusoria,  diatoms,  &c.,  by 
the  mouth,  and  after  retaining  it  for  a  longer  or  shorter  time 
in  the  body,  expels  it  by  the  anus.  It  is  obvious,  therefore, 
that  there  must  be  some  kind  of  digestive  cavity  into  which 
the  food  passes  by  the  mouth,  and  from  which  effete  matters 
are  expelled  through  the  anus.  Sections  (Fig.  68)  show 
that  this  cavity  is  not  a  mere  space  excavated  in  the  interior 
of  the  body,  but  a  definite  tube,  the  enteric  canal  (A,.  B), 
which  passes  in  a  straight  line  from  mouth  to  anus,  and  is 
separated  in  its  whole  extent  from  the  walls  of  the  body 
(A,  B.  W.}  by  a  wide  space,  the  body  cavity  or  ccelome  (ccel). 
So  that  the  general  structure  of  Polygordius  might  be  imi- 
tated by  taking  a  wide  tube,  stopping  the  ends  of  it  with 
corks,  boring  a  hole  in  each  cork,  and  then  inserting  through 

T 


xxv  GENERAL  CHARACTERS  275 

Between  the  enteric  canal  and  the  body- wall  is  the  ccelome  (Gael], 
divided  into  right  and  left  portions  by  the  dorsal  (D.  Mes}  and  ventral 
( V.  Mes)  mesenteries,  and  into  segmental  compartments  by  the  septa 
(Sept}. 

Lying  in  the  mesenteries  are  the  dorsal  (D.  V)  and  ventral  ( V.  V} 
blood-vessels,  connected  by  commissural  vessels  (Com.  V)  running  in 
the  septa  ;  from  the  latter  go  off  recurrent  vessels  (ft.  V] 

Nephridia  (Nphm}  are  shown  in  the  second  and  third  metameres, 
each  consisting  of  a  horizontal  portion  which  perforates  a  septum  and 
opens  in  the  preceding  segment  by  a  nephrostome  (Nph.  si),  and  of  a 
vertical  portion  which  perforates  the  body-wall  and  opens  externally  by 
a  nephridiopore  (Nph.  p}. 

The  brain  (Br)  lies  in  the  prostomium  and  is  connected  with  the 
ventral  nerve-cord  ( V.  Nv.  Cd}  by  a  pair  of  oesophageal  connectives 
(CEs.  Com}. 

B,  diagrammatic  longitudinal  section  showing  the  cell-layers. 

The  cuticle  is  represented  by  a  black  line,  the  ectoderm  is  dotted 
the  endoderm  radially  striated,  the  muscle-plates  evenly  shaded,  the 
coelomic  epithelium  represented  by  a  beaded  line,  and  the  nervous 
system  finely  dotted. 

The  body-wall  is  composed  of  cuticle  (Cu},  deric  epithelium  (Der> 
Epthm},  muscle-plates  (M.  PI),  and  parietal  layer  of  coelomic  epithe- 
lium (Cxi.  Epthm}. 

The  enteric  canal  is  formed  of  enteric  epithelium  (Ent.  Epthm} 
covered  by  the  visceral  layer  of  coelomic  epithelium  (CceL  Epthm'} ;  in 
the  neighbourhood  of  the  mouth  (MtJi)  and  anus  (An}  the  enteric  epithe- 
lium is  ectodermal,  elsewhere  it  is  endodermal ;  Ph,  pharynx  ;  Oes, 
oesophagus  ;  Int,  intestine  ;  Ret,  rectum. 

The  septa  (Sept}  are  formed  of  muscle  covered  on  both  sides  by  ccelomic 
epithelium. 

Four  nephridia  (Nphm}  with  nephrostome  (Nph.  st}  and  nephridiopore 
(Nph.  p}  are  shown. 

The  brain  (Br)  and  ventral  nerve  cord  ( V.  Nv.  Cd)  are  seen  to  be  in 
contact  with  the  ectoderm  :  from  the  brain  a  nerve  (nv)  passes  to  the 
tentacle. 

C,  diagrammatic  transverse  section  showing  the  cell-layers  as  in  B, 
viz  :  the  cuticle  (Cu),  deric  epithelium   (Der.  Epthm},   muscle-plates 
(M.  PI.},  and  parietal  layer  of  coelomic  epithelium  (Cat.  Epthm},  form- 
ing the   body-wall ;    and   the   enteric   epithelium    (Ent.  Epthm}  and 
visceral  layer  of  coelomic  epithelium  ( Ccel.  Epthm'},  forming  the  enteric 
canal. 

The  dorsal  (D.  Mes}  and  ventral  ( V.  Mes}  mesenteries  are  seen  to  be 
formed  of  a  double  layer  of  coelomic  epithelium,  and  to  enclose  respec- 
tively the  dorsal  (D.  V}  and  ventral  ( V.  V}  blood-vessels. 

A  nephridium  (Nphm}  is  shown  on  each  side  with  nephrostome  (Nph. 
st)  and  nephridiopore  Nph.  p}. 

The  connection  of  the  ventral  nerve-cord  with  the  ectoderm  (deric 
epithelium)  is  well  shown. 

Fig.  71,  A  (p.  294),  should  be  compared  with  this  figure,  as  it 
is  an  accurate  representation  of  the  parts  here  shown  diagram- 
matically. 

T    2 


276  POLYGORDIUS  LESS 

the  holes  a  narrow  tube  of  the  same  length  as  the  wide  one. 
The  outer  tube  would  represent  the  body-wall,  the  inner  the 
enteric  canal,  and  the  cylindrical  space  between  the  two  the 
ccelome.  The  inner  tube  would  communicate  with  the  ex- 
terior by  each  of  its  ends,  representing  respectively  mouth 
and  anus  ;  the  space  between  the  two  tubes,  on  the  other 
hand,  would  have  no  communication  with  the  outside. 

Polygordius  is  the  first  example  we  have  studied  of  a 
cculomate  animal :  one  in  which  there  is  a  definite  body- 
cavity  separating  from  one  another  -the  body-wall  and  the 
enteric  canal,  and  in  which  therefore  a  transverse  section  of 
the  body  has  the  general  character  of  two  concentric  circles 
(Fig  68,  c). 

It  will  be  remembered  that  a  transverse  section  of  Hydra 
has  the  character  of  two  concentric  circles,  formed  re- 
spectively of  ectoderm  and  endoderm  (Fig.  55,  A',  p.  242), 
the  two  layers  being,  however,  in  contact  or  only  separated 
by  the  thin  mesogloea.  At  first  sight  then,  it  seems  as  if  we 
might  compare  Polygordius  to  a  Hydra  in  which  the  ecto- 
derm and  endoderm  instead  of  being  in  contact  were 
separated  by  a  wide  interval ;  we  should  then  compare  the 
body-wall  of  Polygordius  with  the  ectoderm  of  Hydra  and 
its  enteric  canal  with  the  endoderm.  But  this  comparison 
would  only  express  part  of  the  truth. 

A  thin  transverse  section  shows  the  body-wall  of  Poly- 
gordius to  consist  of  four  distinct  layers.  Outside  is  a  thin 
transparent  cuticle  (Fig.  68,  c,  and  Fig.  71,  A,  cu)  showing 
no  structure  beyond  a  delicate  striation.  Next  comes  a 
layer  of  epithelial  cells  (Der.  Epthm\  their  long  axes  at 
right  angles  to  the  surface  of  the  body,  and  the  boundaries 
between  them  very  indistinct,  so  as  to  give  the  whole  layer 
the  character  of  a  sheet  of  protoplasm  with  regularly  dis- 
posed nuclei  :  this  is  the  deric  epithelium  or  epidermis. 
Within  it  is  a  rather  thick  layer  of  muscle-plates  (M.  PL\ 


xxv  ENTERIC  EPITHELIUM  277 

having  the  form  of  long  flat  spindles  (Fig.  70,  p.  287,  M.  PL} 
exhibiting  a  delicate  longitudinal  striation  and  covered  on 
their  free  services  with  a  fine  network  of  protoplasm  con- 
taining scattered  nuclei.  Each  plate  is  arranged  longitu- 
dinally, extending  through  several  segments,  and  with  its 
short  axis  perpendicular  to  the  surface  of  the  body  (Fig.  71, 
M.  PL}.  It  is  by  the  contraction  of  the  muscle-plates  that 
the  movements  of  the  body,  which  resemble  those  of  an 
earthworm,  are  produced.  Finally,  within  the  muscular 
layer  and  lining  the  ccelome  is  a  very  thin  layer  of  cells,  the 
ccdomic  epithelium  (CceL  Epthm\ 

A  transverse  section  of  the  enteric  canal  shows  only  two 
layers.  The  inner  consists  of  elongated  cells  (Ent.  Epthm] 
fringed  on  their  inner  or  free  surfaces  with  cilia  :  these  con- 
stitute the  enteric  epithelium.  Outside  these  is  a  very  thin 
layer  of  flattened  cells  (CceL  Epthm'}  bounding  the  coelome, 
and  hence  called,  like  the  innermost  layer  of  the  body-wall, 
coelomic  epithelium.  We  have,  therefore,  to  distinguish 
two  layers  of  coelomic  epithelium,  an  outer  or  parietal  layer 
(Ccel.  Epthm}  which  lines  the  body-wall,  and  an  inner  or  vis- 
ceral layer  (Ccel.  Epthm'}  which  invests  the  enteric  canal. 

We  are  now  in  a  better  position  to  compare  the  transverse 
section  of  Hydra  and  of  Polygordius  (Fig.  55,  A',  and  Fig. 
68,  c).  The  deric  epithelium  of  Polygordius  being  the 
outermost  cell-layer  is  to  be  compared  with  the  ectoderm  of 
Hydra,  and  its  cuticle  with  the  layer  of  the  same  name 
which,  though  absent  in  Hydra,  is  present  in  the  stem  of 
hydroid  polypes  such  as  Bougainvillea  (p.  239).  The  enteric 
epithelium  of  Polygordius,  bounding  as  it  does  the  digestive 
cavity,  is  clearly  comparable  with  the  endoderm  of  Hydra. 
So  that  we  have  the  layer  of  muscle-plates  and  the  two  layers 
of  ccelomic  epithelium  not  represented  in  Hydra,  in  which 
their  position  is  occupied  merely  by  the  mesoglcea. 


378  POLYGORDIUS  LESS. 

But  it  will  be  remembered  that  in  Medusae  there  is  some- 
times found  a  layer  of  separate  muscle-fibres  between  the 
ectoderm  and  the  mesoglcea,  and  it  was  pointed  out  (p.  244) 
that  such  fibres  represented  a  rudimentary  intermediate  cell- 
layer  or  mesoderm.  We  may  therefore  consider  the  muscular 
layer  and  the  coelomic  epithelium  of  Polygordius  as  meso- 
derm, and  we  may  say  that  in  this  animal  the  mesoderm  is 
divisible  into  an  outer  or  somatic  layer,  consisting  of  the 
muscle-plates  and  the  parietal  layer  of  ccelomic  epithelium, 
and  an  inner  or  splanchnic  layer,  consisting  of  the  visceral 
layer  of  ccelomic  epithelium.1 

The  somatic  layer  is  in  contact  with  the  ectoderm  or  deric 
epithelium,  and  with  it  forms  the  body-wall ;  the  splanchnic 
layer  is  in  contact  with  the  endoderm  or  enteric  epithelium 
and  with  it  forms  the  enteric  canal.  The  ccelome  separates 
the  somatic  and  splanchnic  layers  from  one  another,  and  is 
lined  throughout  by  ccelomic  epithelium. 

The  relation  between  the  diploblastic  polype  and  the 
triploblastic  worm  may  therefore  be  expressed  in  a  tabular 
form  as  follows — 

Hydroid.  Polygordius. 

Cuticle Cuticle. 

Ectoderm      ....     Deric  epithelium  or  epidermis. 

.    Musfele-plates. 

Somatic      \    Coelomic  epithelium 
Mesoderm     .     J        iayer        }         (parietal  layer). 
(mdimentary):  Splanchnic    f    Ccelomic  epithelium 

layer         I        (visceral  layer). 
Endoderm Enteric  epithelium. 

i  In  the  majority  of  the  higher  animals  there  is  a  layer  of  muscle 
between  the  enteric  and  ccelomic  epithelia  :  in  such  cases  the  body-wall 
and  enteric  canal  consist  of  the  same  layers  but  in  reverse  order,  the 
ccelomic  epithelium  being  internal  in  the  one,  external  in  the  other. 


xxv  GENERAL  STRUCTURE  279 

Strictly  speaking,  this  comparison  does  not  hold  good  of 
the  anterior  and  posterior  ends  of  the  worm  :  at  both  mouth 
and  anus  the  deric  passes  insensibly  into  the  enteric  epithe- 
lium, and  the  study  of  development  shows  (p.  298)  that  the 
cells  lining  both  the  anterior  and  posterior  ends  of  the  canal 
are,  as  indicated  in  the  diagram  (Fig.  68,  B),  ectodermal.  For 
this  reason  the  terms  deric  and  enteric  epithelium  are  not 
mere  synonyms  of  ectoderm  and  endoderm  respectively. 

It  is  important  that  the  student  should,  before  reading 
further,  understand  clearly  the  general  composition  of  a 
triploblastic  animal  as  typified  by  Polygordius,  which  may 
be  summarised  as  follows  :  It  consists  of  two  tubes  formed 
of  epithelial  cells,  one  within  and  parallel  to  the  other,  the 
two  being  continuous  at  either  end  of  the  body  where  the 
inner  tube  (enteric  epithelium)  is  in  free  communication 
with  the  exterior ;  the  outer  tube  (deric  epithelium)  is  lined 
by  a  layer  of  muscle-plates  within  which  is  a  thin  layer  of 
coelomic  epithelium,  the  three  together  forming  the  body- 
wall;  the  inner  tube  (enteric  epithelium)  is  covered  ex- 
ternally by  a  layer  of  coelomic  epithelium  which  forms  with 
it  the  enteric  canal ;  lastly,  the  body-wall  and  enteric  canal 
are  separated  by  a  considerable  space,  the  ccelome. 

The  enteric  canal  is  not,  as  might  be  supposed  from  the 
foregoing  description,  connected  with  the  body-wall  only  at 
the  mouth  and  anus,  but  is  supported  in  a  peculiar  and 
somewhat  complicated  way.  In  the  first  place  there  are 
thin  vertical  plates,  the  dorsal  and  ventral  mesenteries  (Fig. 
68,  A  and  c,  D.  Mes,  V.  Afes),  which  extend  longitudinally 
from  the  dorsal  and  ventral  surfaces  of  the  canal  to  the  body 
wall,  dividing  the  coelome  into  right  and  left  halves.  The 
structure  of  the  mesenteries  is  seen  in  a  transverse  section 
(Fig.  68,  c,  and  Fig.  71,  A)  which  shows  that  at  the  middle 


280  POLYGORDIUS  LESS. 

dorsal  line  the  parietal  layer  of  ccelomic  epithelium  becomes 
deflected  downwards,  forming  a  two-layered  membrane,  the 
dorsal  mesentery ;  the  two  layers  of  this  on  reaching  the 
enteric  canal  diverge  and  pass  one  on  either  side  of  it,  form- 
ing the  visceral  layer  of  coelomic  epithelium  ;  uniting  again 
below  the  canal,  they  are  continued  downwards  as  the  ventral 
mesentery,  and  on  reaching  the  body-wall  diverge  once  more 
to  join  the  parietal  layer.  Thus  the  mesenteries  are  simply 
formed  of  a  double  layer  of  coelomic  epithelium,  continuous 
on  the  one  hand  with  the  parietal  and  on  the  other  with  the 
visceral  layer  of  that  membrane. 

Besides  the  mesenteries,  the  canal  is  supported  by  trans- 
verse vertical  partitions  or  septa  (Fig.  68,  A  and  B,  Sept)  which 
extend  right  across  the  body-cavity,  each  being  perforated  by 
the  canal.  The  septa  are  regularly  arranged  and  correspond 
with  the  external  grooves  by  which  the  body  is  divided  into 
metameres.  Thus  the  transverse  or  metameric  segmen- 
tation affects  the  coelome  as  well  as  the  body-wall.  Each 
septum  is  composed  of  a  sheet  of  muscle  covered  on  both 
sides  with  ccelomic  epithelium  (B,  Sept). 

Where  the  septa  come  in  contact  with  the  enteric  canal, 
the  latter  is  more  or  less  definitely  constricted  so  as  to  pre- 
sent a  beaded  appearance  (A  and  B)  ;  thus  we  have  segmen- 
tation of  the  canal  as  well  as  of  the  body-wall  and  coelome. 

The  digestive  canal,  moreover,  is  not  a  simple  tube  of 
even  calibre  throughout,  but  is  divisible  into  four  portions. 
The  first  or  pharynx  (Ph)  is  very  short,  and  can  be  pro- 
truded during  feeding ;  the  second,  called  the  gullet  or 
ossophagus  (Oes),  is  confined  to  the  peristomium  and  is  distin- 
guished by  its  thick  walls  and  comparatively  great  diameter  ; 
the  third  or  intestine  (Int)  extends  from  the  first  metamere 
to  the  last — i.e.,  from  the  segment  immediately  following 
the  peristomium  to  that  immediately  preceding  the  anal 


xxv  DIGESTION  281 

segment;  it  is  laterally  compressed  so  as  to  have  an 
elongated  form  in  cross  section  (c,  and  Fig.  71,  A)  :  the 
fourth  portion  or  rectum  (Ret)  is  confined  to  the  anal  seg- 
ment j  it  is  somewhat  dilated  and  is  not  laterally  compressed. 
The  epithelium  of  the  intestine  is,  as  indicated  in  the 
diagram  (B),  endodermal ;  that  of  the  remaining  division  of 
the  canal  is  ectodermal.  The  large  majority  of  the  cells  in 
all  parts  of  the  canal  are  ciliated. 

The  cells  of  the  enteric  canal  and  especially  those  of  the 
gullet  are  very  granular,  and  like  the  endoderm  cells  of  the 
hypostome  of  Hydra  (p.  231)  are  to  be  considered  as  gland 
cells.  They  doubtless  secrete  a  digestive  juice  which, 
mixing  with  the  various  substances  taken  in  by  the  mouth, 
dissolves  the  proteids  and  other  digestible  parts,  so  as  to 
allow  of  their  absorption.  There  is  no  evidence  of  intra- 
cellular  digestion  such  as  occurs  in  Hydra  (p.  232),  and  it  is 
very  probable  that  the  process  is  purely  extra-cellular  or 
enteric,  the  food  being  dissolved  and  rendered  diffusible 
entirely  in  the  cavity  of  the  canal.  By  the  movements  of 
the  canal — caused  partly  by  the  general  movements  of  the 
body  and  partly  by  the  contraction  of  the  muscles  of  the 
septa — aided  by  the  action  of  the  cilia,  the  contents  are 
gradually  forced  backwards  and  the  sand  and  other  indi- 
gestible matters  are  expelled  at  the  anus. 

The  coelome  is  filled  with  a  colourless  transparent 
cwlomic  fluid  in  which  are  suspended  minute,  irregular, 
colourless  bodies,  as  well  as  oval  bodies  containing  yellow 
granules.  From  the  analogy  of  the  higher  animals  one 
would  expect  these  to  be  leucocytes  (p.  56),  but  their 
cellular  nature  has  not  been  proved. 

The  function  of  the  coelomic  fluid  is  probably  to  distribute 
the  digested  food  in  the  enteric  canal  to  all  parts  of  the 


282  POLYGORDIUS  LESS. 

body.  In  Hydra,  where  the  lining  wall  of  the  digestive 
cavity  is  in  direct  contact  with  the  simple  wall  of  the  body, 
the  products  of  digestion  can  pass  at  once  by  diffusion  from 
endoderm  to  ectoderm,  but  in  the  present  case  a  means  of 
communication  is  wanted  between  the  enteric  epithelium 
and  the  comparatively  complex  and  distant  body-wall.  The 
peptones  and  other  products  of  digestion  diffuse  through 
the  enteric  epithelium  into  the  coelomic  fluid,  and  by  the  con- 
tinual movement  of  the  latter — due  to  the  contractions  of 
the  body-wall — are  distributed  to  all  parts.  Thus  the 
external  epithelium  and  the  muscles,  as  well  as  the  nervous 
system  and  reproductive  organs,  not  yet  described,  are 
wholly  dependent  upon  the  enteric  epithelium  for  their 
supply  of  nutriment. 

We  have  now  to  deal  with  structures  which  we  find  for  the 
first  time  in  Polygordius,  namely  blood-vessels.  Lying  in 
the  thickness  of  the  dorsal  mesentery  is  a  delicate  tube  (Fig. 
68,  A  and  c,  D.  F)  passing  along  almost  the  whole  length  of 
the  body  :  this  is  the  dorsal  vessel.  A  similar  ventral  vessel 
(V.V)  is  contained  in  the  ventral  mesentery,1  and  the  two  are 
placed  in  communication  with  one  another  in  every  segment 
by  a  pair  of  commissural  vessels  (A,  Com.v)  which  spring  right 
and  left  from  the  dorsal  trunk,  pass  downwards  in  or  close 
behind  the  corresponding  septum,  following  the  contour  of 
body-wall,  and  finally  open  into  the  ventral  vessel.  Each 
commissural  vessel,  at  about  the  middle  of  its  length,  gives 
off  a  recurrent  vessel  (R.V.)  which  passes  backwards  and 

5  The  statement  that  the  dorsal  and  ventral  vessels  lie  in  the  thickness 
of  the  mesenteries  requires  qualification.  As  a  matter  of  fact,  these 
vessels  are  simply  spaces  formed  by  the  divergence  of  the  two  layers  of 
epithelium  composing  the  mesentery  (Fig.  68,  c,  and  Fig.  71,  A)  :  only 
their  anterior  ends  have  proper  walls. 


xxv  HEMOGLOBIN  283 

ends  blindly.  The  anterior  parts  of  the  commissural  vessels 
lie  in  the  peristomium  and  have  an  oblique  direction,  one  on 
each  side  of  the  gullet.  The  whole  of  these  vessels  form  a 
single,  closed  vascular  system,  there  being  no  communication 
between  them  and  any  of  the  remaining  cavities  of  the 
body. 

The  vascular  system  contains  a  fluid,  the  blood,  which 
varies  in  colour  in  the  different  species  of  Polygordius,  being 
either  colourless,  red,  green,  or  yellow.  In  one  species  cor- 
puscles (?  leucocytes)  have  been  found  in  it. 

The  function  of  the  blood  has  not  been  actually  proved 
in  Polygordius,  but  is  well  known  in  other  worms.  In  the 
common  earthworm,  for  instance,  the  blood  is  red,  the  colour 
being  due  to  the  same  pigment,  hcemoglobin,  which  occurs 
in  our  own  blood  and  in  that  of  other  vertebrate  animals. 

Haemoglobin  is  a  nitrogenous  compound,  containing,  in 
addition  to  carbon,  hydrogen,  nitrogen,  oxygen,  and  sulphur, 
a  minute  quantity  of  iron.  It  can  be  obtained  pure  in  the 
form  of  crystals  which  are  soluble  in  water.  Its  most 
striking  and  physiologically  its  most  important  property  is 
its  power  of  entering  into  a  loose  chemical  combination  with 
oxygen.  If  a  solution  of  haemoglobin  is  brought  into  contact 
with  oxygen  it  acquires  a  bright  scarlet  colour,  and  the  solu- 
tion is  then  found  to  have  a  characteristic  spectrum  distin- 
guished by  two  absorption-bands,  one  in  the  yellow,  another 
in  the  green.  Loss  of  oxygen  changes  the  colour  from  scarlet 
to  purple,  and  the  spectrum  then  presents  a  single  broad 
absorption-band  intermediate  in  position  between  the  two  of 
the  oxygenated  solution. 

This  property  is  of  use  in  the  following  way.  All  parts 
of  the  organism  are  constantly  undergoing  destructive  meta- 
bolism and  giving  off  carbon  dioxide  :  this  gas  is  absorbed 
by  the  blood,  and  at  the  same  time  the  haemoglobin  gives  up 


284  POLYGORDIUS  LESS. 

its  oxygen  to  the  tissues.  On  the  other  hand,  whenever  the 
blood  is  brought  sufficiently  near  the  external  air — or  water 
in  the  case  of  an  aquatic  animal — the  opposite  process  takes 
place,  oxygen  being  absorbed  and  carbon  dioxide  given  off. 
Haemoglobin  is  therefore  to  be  looked  upon  as  a  respiratory 
or  oxygen-carrying  pigment ;  its  function  is  to  provide  the 
various  parts  of  the  body  with  a  constant  supply  of  oxygen, 
while  the  carbon  dioxide  formed  by  their  oxidation  is  given 
up  to  the  blood.  The  particular  part  of  the  body  in  which 
the  carbon  dioxide  accumulated  in  the  blood  is  exchanged 
for  the  oxygen  of  the  surrounding  medium  is  called  a 
respiratory  organ ;  in  Polygordius,  as  in  the  earthworm  and 
many  others  of  the  lower  animals,  there  is  no  specialised 
respiratory  organ — lung  or  gill — but  the  necessary  exchange 
of  gases  is  performed  by  the  entire  surface  of  the  body. 

In  discussing  in  a  previous  lesson  the  differences  between 
plants  and  animals,  we  found  (p.  178)  that  in  the  unicellular 
organisms  previously  studied,  the  presence  of  an  excretory 
organ  in  the  form  of  a  contractile  vacuole  was  a  characteristic 
feature  of  such  undoubted  animals  as  the  ciliate  infusoria, 
but  was  absent  in  such  undoubted  plants  as  Vaucheria  and 
Mucor.  But  the  reader  will  have  noticed  that  Hydra  and  its 
allies  have  no  specialised  excretory  organ,  waste  products 
being  apparently  discharged  from  any  part  of  the  surface. 
In  Polygordius  we  meet  once  more  with  an  animal  in  which 
excretory  organs  are  present,  although,  in  correspondence 
with  the  complexity  of  the  animal  itself,  they  are  very 
different  from  the  simple  contractile  vacuoles  of  Paramce- 
cium  or  Vorticella. 

The  excretory  organs  of  Polygordius  consist  of  little  tubes 
called  nephridia,  of  which  each  metamere  possesses  a  pair, 
one  on  either  side  (Fig.  68,  A,  B,  and  c,  Nphm).  Each 


xxv  NEPHRIDIA  285 

nephridium  (Fig.  69)  is  an  extremely  delicate  tube  consisting 
of  two  divisions  bent  at  right  angles.  The  outer  division  is 
placed  vertically,  lies  in  the  thickness  of  the  body-wall,  and 
opens  externally  by  a  minute  aperture,  the  nephridiopore 
(Figs.  68  and  69,  Nph.  /).  The  inner  division  is  horizontal 
and  lies  in  the  ccelomic  epithelium  ;  passing  forward  it  pierces 
the  septum  which  bounds  the  segment  in  front  (Fig.  68, 
A  and  B),  and  then  dilates  into  a  funnel-shaped  extremity  or 
nephrostome  (Nph.  st\  which  places  its  cavity  in  free  com- 
munication with  the  coelome.  The  whole  interior  of  the 
tube  as  well  as  the  inner  face  of  the  nephrostome  is  lined 
with  cilia  which  work  outwards. 


Nph.st 


FIG.  69. — A  nephridium  of  Polygordius,  showing  the  cilia  lining  the 
tube,  the  ciliated  funnel  or  nephrostome  (Nph.  st),  and  the  external 
aperture  or  nephridiopore  {Nph.  /).'  (After  Fraipont.) 

A  nephridium  may  therefore  be  defined  as  a  ciliated  tube, 
lying  in  the  thickness  of  the  body- wall  and  opening  at  one 
end  into  the  ccelome  and  at  the  other  on  the  exterior  of 
the  body. 

In  the  higher  worms,  such  as  the  earthworm,  the  nephridia 
are  lined  in  part  by  gland-cells,  and  are  abundantly  supplied 
with  blood-vessels.  Water  and  nitrogenous  waste  from  all 
parts  of  the  body  pass  by  diffusion  into  the  blood  and  are 
conveyed  to  the  nephridia,  the  gland-cells  of  which  withdraw 
the  waste  products  and  pass  them  into  the  cavities  of  the 
tubes,  whence  they  are  finally  discharged  into  the  surround- 
ing medium.  In  all  probability  some  such  process  as  this 
takes  place  in  Polygordius. 


286  fOLYGORDIUS  LESS. 

In  discussing  the  hydroid  polypes  we  found  that  one  of 
the  most  important  points  of  difference  between  the  loco- 
motive medusa  and  the  fixed  hydranth  was  the  presence  in 
the  former  of  a  well-developed  nervous  system  (p.  244)  con- 
sisting of  an  arrangement  of  peculiarly  modified  cells,  to 
which  the  function  of  automatism  was  assigned.  It  is 
natural  to  expect  in  such  an  active  and  otherwise  highly- 
organized  animal  as  Polygordius  a  nervous  system  of  a 
considerably  higher  degree  of  complexity  than  that  of  a 
medusa. 

The  central  nervous  system  consists  of  two  parts,  the 
brain  and  the  ventral  nerve-cord.  The  brain  (Fig.  68,  A  and 
B,  JBr.}  is  a  rounded  mass  occupying  the  whole  interior  of 
the  prostomium  and  divided  by  a  transverse  groove  into  two 
lobes,  the  anterior  of  which  is  again  marked  by  a  longitu- 
dinal groove.  The  ventral  nerve-cord  ( V.  Nv.  Cd.}  is  a 
longitudinal  band  extending  along  the  whole  middle  ventral 
line  of  the  body  from  the  peristomium  to  the  anal  segment. 
The  posterior  lobe  of  the  brain  is  connected  with  the  anterior 
end  of  the  ventral  nerve-cord  by  a  pair  of  nervous  bands, 
the  ctsophageal  connectives  (CEs.  Con.}  which  pass  respectively 
right  and  left  of  the  gullet. 

It  is  to  be  noted  that  one  division  of  the  central  nervous 
system — the  brain — lies  altogether  above  and  in  front  of  the 
enteric  canal,  the  other  division — the  ventral  nerve-cord — 
altogether  beneath  it,  and  that,  in  virtue  of  the  union  of  the 
two  divisions  by  the  cesophageal  connectives,  the  enteric 
canal  perforates  the  nervous  system. 

It  is  also  important  to  notice  that  the  nervous  system  is 
throughout  in  direct  contact  with  the  epidermis  or  ectoderm, 
the  ventral  cord  appearing  in  sections  (Fig.  68,  c,  and  Fig. 
71,  A)  as  a  mere  thickening  of  the  latter. 

Both  brain  and  cord  are  composed  of  delicate  nerve-fibres 


xxv 


NERVOUS  SYSTEM 


287 


(Fig.  70,  Nv.  F.}  interspersed  with  nerve-cells  (Nv.  C).  In 
the  cord  the  fibres  are  arranged  longitudinally,  and  the 
nerve-cells  are  ventral  in  position,  forming  a  layer  in  imme- 


ter.Epthm, 

FIG.  70.  —  Diagram  illustrating  the  relations  of  the  nervous  system  oi 
Polygordius. 

The  deric  epithelium  (Dcr.  Epthni)  is  either  indirect  contact  with  the 
central  nervous  system  (lower  part  of  figure),  or  is  connected  by  afferent 
nerves  (af.  nv.)  with  the  inter-muscular  plexus  (int.. muse,  plex.} :  the 
latter  is  connected  to  the  muscle-plates  (M.  PI)  by  efferent  nerves  (Ef. 
nv). 

The  central  nervous  system  consists  of  nerve- fibres  (Nv.  F)  and 
nerve-cells  (Nv.  C) :  other  nerve-cells  (Nv.  C)  occur  at  intervals  in 
the  inter-muscular  plexus. 

The  muscle-plates  (M.  PI),  one  of  which  is  entire,  while  only  the 
middle  part  of  the  other  is  shown,  are  invested  by  a  delicate  protoplasmic 
network,  containing  nuclei  (nu),  to  which  the  efferent  nerves  can  be 
traced.  (The  details  copied  from  Fraipont. ) 

diate  contact  with  the  deric  epithelium.  In  the  posterior 
lobe  of  the  brain  the  nerve-cells  are  superficial  and  the 
central  part  of  the  organ  is  formed  of  a  finely  punctate 
substance  in  which  neither  cells  nor  fibres  can  be  made 
out. 


288  POLYGORDIUS  LESS. 

Ramifying  through  the  entire  muscular  layer  of  the  body- 
wall  is  a  network  of  delicate  nerve-fibres  (int.  muse,  plx.) 
with  nerve- cells  (Nv.  C)  at  intervals,  the  inter-muscular 
-blexus.  Some  of  the  branches  of  this  plexus  are  traceable 
to  nerve-cells  in  the  central  nervous  system,  others  (af.  nv.) 
to  epidermic  cells,  others  (Ef.  nv.)  to  the  delicate  proto- 
plasmic layer  covering  the  muscle-plates.  The  superficial 
cells  of  both  brain  and  cord  are  also,  as  has  been  said,  in 
direct  connection  with  the  overlying  epidermis,  and  from  the 
anterior  end  of  the  brain  a  bundle  of  nerve-fibres  (Fig.  68,  B, 
/.,  Nv.)  is  given  off  on  each  side  to  the  corresponding  tentacle, 
constituting  the  nerve  of  that  organ,  to  the  epidermic  cells  of 
which  its  fibres  are  distributed. 

We  see  then  that,  apart  from  the  direct  connection  of 
nerve-cells  with  the  epidermis,  the  central  nervous  system  is 
connected,  through  the  intermediation  of  nerve -fibres  (a) 
with  the  sensitive  cells  of  the  deric  epithelium  and  (b)  with 
the  contractile  muscle-plates.  And  we  can  thus  distinguish 
two  sets  of  nerve-fibres,  (a)  sensory  or  afferent  (af.  nv.) 
which  connect  the  central  nervous  system  with  the  epidermis, 
and  (b)  motor  or  efferent  (Ef.  nv.)  which  connect  it  with  the 
muscles. 

Comparing  the  nervous  system  of  Polygordius  with  that 
of  a  medusa  (p.  244)  there  are  two  chief  points  to  be  noticed. 
Firstly,  the  concentration  of  the  central  nervous  system  in 
the  higher  type,  and  the  special  concentration  at  the  anterior 
end  of  the  body  to  form  a  brain.  Secondly,  the  important 
fact  that  the  inter-muscular  plexus  is  not,  like  the  peripheral 
nervous  system  of  a  medusa  which  it  resembles,  situated 
immediately  beneath  the  epidermis  (ectoderm)  but  lies  in  the 
muscular  layer,  or,  in  other  words,  has  sunk  into  the 
mesoderm. 

It  is  obvious  that  direct  experiments  on  the  nervous  system 


xxv  FUNCTIONS   OF   NERVOUS   SYSTEM  289 

would  be  a  very  difficult  matter  in  so  small  an  animal  as 
Polygordius.  But  numerous  experiments  on  a  large  number 
of  other  animals,  both  higher  and  lower,  allow  us  to  infer 
with  considerable  confidence  the  functions  of  the  various 
parts  in  this  particular  case. 

If  a  muscle  be  laid  bare  or  removed  from  the  body  in  a 
living  animal  it  may  be  made  to  contract  by  the  application 
of  various  stimuli,  such  as  a  smart  tap  (mechanical  stimulus),  a 
drop  of  acid  or  alkali  (chemical  stimulus),  a  hot  wire  (thermal 
stimulus),  or  an  electric  current  (electric  stimulus).  If  the 
motor  nerve  of  the  muscle  is  left  intact  the  application  to  it  of 
any  of  these  stimuli  produces  the  same  effect  as  its  direct 
application  to  the  muscle,  the  stimulus  being  conducted 
along  the  eminently  irritable  but  non-contractile  nerve. 

Further,  if  the  motor  nerve  is  left  in  connection  with  the 
central  nervous  system,  i.e.,  with  one  or  more  nerve-cells, 
direct  stimulation  of  these  is  followed  by  a  contraction,  and 
not  only  so,  but  stimulation  of  a  sensory  nerve  connected 
with  such  cells  produces  a  similar  result.  And  finally, 
stimulation  of  an  ectoderm  cell  connected,  either  directly 
or  through  the  intermediatidf  of  a  sensory  nerve,  with  the 
nerve-cells,  is  also  followed  by  muscular  contraction.  An 
action  of  this  kind,  in  which  a  stimulus  applied  to  the  free 
sensitive  surface  of  the  body  is  transmitted  along  a  sensory 
nerve  to  a  nerve-cell  or  group  of  such  cells  and  is  then,  as  it 
were,  reflected  along  a  motor  nerve  to  a  muscle,  is  called  a 
reflex  action  ;  the  essence  of  the  arrangement  is  the  inter- 
position of  nerve-cells  between  sensory  or  afferent  nerves 
connected  with  sensory  cells,  and  motor  or  efferent  nerves 
connected  with  muscles. 

The  diagram  (Fig.  70)  serves  to  illustrate  this  matter. 
The  muscle-plate  (M.  PI.)  may  be  made  to  contract  by  a 
stimulus  applied  (a)  to  itself  directly,  (b)  to  the  motor  fibre 

U 


290  POLYGORDIUS  LESS. 

(Ef.  nv\  (c)  to  the  nerve-cells  (Nv.  C)  in  the  central 
nervous  system,  or  to  those  (Nv.  C'}  in  the  inter-muscular 
plexus,  (d]  to  the  sensory  fibre  (of.  nv.\  or  (e)  to  the 
epidermic  cells  (Der.  Epthm.}. 

In  all  probability  the  whole  central  nervous  system  of 
Polygordius  is  capable  of  automatic  action.  It  is  a  well- 
known  fact  that  if  the  body  of  an  earthworm  is  cut  into 
several  pieces  each  performs  independent  movements ;  in 
other  words,  the  whole  body  is  not,  as  in  the  higher  animals, 
paralysed  by  removal  of  the  brain.  There  can,  however,  be 
little  doubt  that  complete  co-ordination,  i.e.,  the  regulation 
of  the  various  movements  to  a  common  end,  is  lost  when 
the  brain  is  removed. 

The  nervous  system  is  thus  an  all-important  means  of 
communication  between  the  various  parts  of  the  organism 
and  between  the  organism  and  the  external  world.  The 
outer  or  sensory  surface  is  by  its  means  brought  into 
connection  with  the  entire  muscular  system  with  such 
perfection  that  the  slightest  touch  applied  to  one  end  of  the 
body  may  be  followed  by  the  almost  instantaneous  contrac- 
tion of  muscles  at  the  other. 

In  some  species  of  Polygordius  the  prostomium  bears  a 
pair  of  eye-specks,  but  in  the  majority  of  species  the  adult 
animal  is  eyeless,  and,  save  for  the  ciliated  pits  (Fig.  67, 
B,  c.p],  the  function  of  which  is  not  known,  the  only  definite 
organs  of  sense  are  the  tentacles,  which  have  a  tactile 
function,  their  abundant  nerve-supply  indicating  that  their 
delicacy  as  organs  of  touch  far  surpasses  that  of  the  general 
surface  of  the  body.  They  are  beset  with  short,  fine  pro- 
cesses of  the  cuticle  called  setce  (Figs.  67  and  68,  s),  which 
probably,  like  the  whiskers  of  a  cat,  serve  as  conductors  of 
external  stimuli  to  the  sensitive  epidermic  cells. 


xxv    PHYSIOLOGICAL  DIFFERENTIATION  :  ORGANS    291 

There  are  two  matters  of  general  importance  in  connec- 
tion with  the  structure  of  Polygordius  to  which  the  student's 
attention  must  be  drawn  in  concluding  the  present  lesson. 

Notice  in  the  first  place  how  in  this  type,  far  more  than  in 
any  of  those  previously  considered,  we  have  certain  definite 
parts  of  the  body  set  apart  as  organs  for  the  performance  of 
particular  functions.  There  is  a  mouth  for  the  reception  of 
food,  an  enteric  canal  for  its  digestion,  and  an  anus  for  the 
extrusion  of  faeces  :  a  coelomic  fluid  for  the  transport  of  the 
products  of  digestion  to  the  more  distant  parts  of  the  body  : 
a  system  of  blood-vessels  for  the  transport  of  oxygen  to  and 
of  carbon  dioxide  from  all  parts  :  an  epidermis  as  organ  of 
touch  and  of  respiration  :  nephridia  for  getting  rid  of  water 
and  nitrogenous  waste  :  and  a  definite  nervous  system  for 
regulating  the  movements  of  the  various  parts  and  forming 
a  means  of  communication  between  the  organism  and  the 
external  world.  It  is  clear  that  differentiation  of  structure 
and  division  of  physiological  labour  play  a  far  more  obvious 
and  important  part  than  in  any  of  the  organisms  hitherto 
studied. 

Notice  in  the  second  place  the  vastly  greater  complexity 
of  microscopic  structure  than  in  any  of  our  former  types. 
The  adult  organism  can  no  longer  be  resolved  into  more  or 
less  obvious  cells.  In  the  deric,  enteric,  and  coelomic 
epithelia  we  meet  with  nothing  new,  but  the  muscle-plates 
are  not  cells,  the  nephridia  show  no  cell-structure,  neither  do 
the  nerve-fibres  nor  the  punctate  substance  of  the  brain. 
The  body  is  thus  divisible  into  tissues  or  fabrics  each  clearly 
distinguishable  from  the  rest.  We  have  epithelial  tissue, 
cuticular  tissue,  muscular  tissue,  and  nervous  tissue  :  and 
the  blood  and  coelomic  fluid  are  to  be  looked  upon  as 
liquid  tissues.  One  result  of  this  is  that,  to  a  far  greater 
extent  that  in  the  foregoing  types,  we  can  study  the 

u  3 


292  POLYGORDIUS  LESS,  xxv 

morphology  of  Polygordius  under  two  distinct  heads  : 
anatomy,  dealing  with  the  general  structure  of  the  parts, 
and  histology,  dealing  with  their  minute  or  microscopic 
structure. 

One  point  of  importance  must  be  specially  referred  to  in 
connection  with  certain  of  the  tissues.  It  has  been  pointed 
out  (p.  276)  that  the  epidermis  has  rather  the  character  of 
a  sheet  of  protoplasm  with  regularly-arranged  nuclei  than  of 
a  layer  of  cells,  and  that  the  muscle-plates  are  covered  with 
a  layer  of  protoplasm  with  which  the  ultimate  nerve-fibres 
are  continuous  (p.  277).  Thus  certain  of  the  tissues  of 
Polygordius  exhibit  continuity  of  the  protoplasm,  a  phenomenon 
which  appears  to  be  of  wide  occurrence  both  in  animals 
and  in  plants. 


LESSON  XXVI 
POLYGORDIUS  (Continued} 

ASEXUAL  reproduction  is  unknown  in  Polygordius,  and 
the  organs  of  sexual  reproduction  are  very  simple.  The 
animal  is  dioecious,  gonads  of  one  sex  only  being  found  in 
each  individual. 

In  the  species  which  has  been  most  thoroughly  investi- 
gated (P.  neapolitanus]  the  reproductive  products  are  formed 
in  each  metamere  from  the  fourth  to  the  last.  Crossing 
these  segments  obliquely  are  narrow  bands  of  muscle  (Fig. 
71,  A,  O.M)  and  certain  of  the  cells  of  ccelomic  epithelium 
covering  these  bands  multiply  by  fission  and  form  little 
heaps  of  cells  (Spy\  each  of  which  is  to  be  looked  upon  as  a 
gonad.  There  is  thus  a  pair  of  gonads  to  each  segment  with 
the  exception  of  the  prostomium,  the  peristomium,  the  first 
three  metameres,  and  the  anal  segment,  the  reproductive 
organs  exhibiting  the  same  simple  metameric  arrangement 
as  the  digestive,  excretory,  and  circulatory  organs.  It  will 
be  noticed  that  the  primitive  sex-cells,  arising  as  they  do 
from  ccelomic  epithelium,  are  mesodermal  structures,  not 
ectodermal  as  in  hydroids  (pp.  234  and  247). 

In  the  male  the  primitive  sex-cells  divide  and  sub-divide, 
the  ultimate  products  being  converted  into  sperms  (Fig.  71, 


D.I 


C'a 


M.PI 


Sff>— 


FiG.  71. — Polygordins  neapolitanus. 

A,  transverse  section  of  a  male  specimen  to  show  the  position  of  the 
immature  gonads  (spy)  and  the  precise  form  and  arrangement  of  the 
various  layers  represented  diagrammatically  in  Fig.  68,  C. 

The  body-wall  consists  of  cuticle  (Cu),  cleric  epithelium  (Der.  Epthtn), 
muscle-plates  (M.  Pl\  and  parietal  layer  of  ccelomic  epithelium  (Ccvl. 
Epthm).  The  ventral  nerve  cord  ( V.  Nv.  Cd)  is  shown  to  be  continu- 
ous with  the  deric  epithelium. 

The  enteric  canal  consists  of  ciliated  enteric  epithelium  (Ent.  Epthm} 
covered  by  the  visceral  layer  of  ccelomic  epithelium  (Ccel.  Epthm)  : 
connecting  it  with  the  body-wall  are  the  dorsal  and  ventral  mesenteries 
formed  of  a  double  layer  of  coelomic  epithelium,  and  containing  respec- 
tively the  dorsal  (D.  V)  and  ventral  (V.  V)  blood-vessels. 

Passing  obliquely  across  the  coelome  are  the  oblique  muscles  (0.  M) 


LESS,  xxvi  DEVELOPMENT  295 

covered  with  ccelomic  epithelium  :  by  differentiation  of  groups  of  cells 
of  the  latter  the  spermaries  (Spy]  are  formed. 

B,  a  single  sperm,  showing  expanded  head  and  delicate  tail. 

c,  horizontal  section  of  a  sexually  mature  female. 

The  body- wall  (Ctt,  Der.  Epthm,  M.  PI]  has  undergone  partial 
histological  degeneration,  and  is  ruptured  in  two  places  to  allow  of  the 
escape  of  the  ova  (ov}  which  still  fill  the  ccelomic  spaces  enclosed  between 
the  body-wall,  the  enteric  canal  (Ent.  Epthm),  and  the  septa  (Se£). 
(After  Fraipont.) 

B  :  see  p.  255)  :  in  the  female  they  enlarge  immensely,  and 
take  on  the  character  of  ova  (c,  ov).  Multiplication  of  the 
sexual  products  takes  place  to  such  an  extent  that  the  whole 
ccelome  becomes  crammed  full  of  either  sperms  or  ova  (c). 

In  the  female  the  growth  of  the  eggs  takes  place  at  the 
expense  of  all  other  parts  of  the  body,  which  undergo  more 
or  less  complete  atrophy  :  the  epidermis,  for  instance,  be- 
comes liquefied  and  the  muscles  lose  their  contractility. 
Finally  rupture  of  the  body-wall  takes  place  in  each  segment 
(c),  and  through  the  slits  thus  formed  the  eggs  escape.  So 
that  Polygordius,  like  an  annual  plant,  produces  only  a 
single  brood :  death  is  the  inevitable  result  of  sexual 
maturity.  Whether  or  not  the  same  dehiscence  of  the 
body-wall  takes  place  in  the  male  is  not  certain  :  it  has 
been  stated  that  the  sperms  make  their  escape  through  the 
nephridia. 

Thus  while  there  are  no  specialized  gonaducts,  or  tubes  for 
carrying  off  the  sexual  products,  it  is  possible  that  the  ne- 
phridia may,  in  addition  to  their  ordinary  function,  serve 
the  purpose  of  male  gonaducts  or  spermiducts.  Female  gona- 
ducts or  oviducts  are  however  entirely  absent. 

The  ova  and  sperms  being  shed  into  the  surrounding  water, 
impregnation  takes  place,  and  the  resulting  oosperm  under- 
goes segmentation  or  division  (see  p.  248),  a  polyplast  being 
formed.  By  the  arrangement  of  its  cells  into  two  layers  and 


296  POLYGORDIUS  LESS. 

the  formation  of  an  enteron  or  digestive  cavity  the  polyplast 
becomes  a  gastmla  (see  p.  265)  which  by  further  develop- 
ment is  converted  into  a  curious  free-swimming  creature 
shown  in  Fig.  72,  A,  and  called  a  trochosphere. 


FIG.  72. — A,  larva  of  Polygordius  neapolitanus  in  the  trochosphere 
stage  ;  from  a  living  specimen. 

B,  diagrammatic  vertical  section  of  the  same  :  the  ectoderm  is  dotted, 
the  endoderm  radially  striated,  the  mesoderm  evenly  shaded,  and  the 
nervous  system  finely  dotted. 

C,  transverse  section  through  the  plane  ab  in  B. 

The  body -wall  consists  of  a  single  layer  of  ectoderm  cells,  which,  at 
the  apex  of  the  prostomium  (upper  hemisphere)  are  modified  to  form  the 
brain  (Br)  and  a  pair  of  ocelli  (oc). 

The  enteric  canal  consists  of  three  parts  :  the  stomodseum  (St.  dm], 
opening  externally  by  the  mouth  (Mth\  and  lined  by  ectoderm  ;  the 
enteron  (Ent)  lined  by  endoderm  ;  and  the  proctodseum  (Prc.  dm), 
opening  by  the  anus  (An)  and  lined  by  ectoderm. 

Between  the  body- wall  and  the  enteric  canal  is  the  larval  body-cavity 
or  blastocoele  (Bl.  cazl). 

The  mesoderm  is  confined  to  two  narrow  bands  of  cells  (B  and  C, 
Msd)  in  the  blastocoele,  one  on  either  side  of  the  proctodasum  ;  slender 
mesodermal  bands  (Msd')  are  also  seen  in  the  prostomium  in  A. 

The  cilia  consists  of  a  pras-oral  circlet  (Pr.  or.  ci)  above  the  mouth,  a 
post-oral  circlet  (Pt.  or.  ci)  below  the  mouth,  and  an  anal  circlet  (An. 
ci)  around  the  anus. 

(A  after  Fraipont. ) 

The  trochosphere,  or  newly-hatched  larva  of  Polygordius 
(Fig.  72,  A)  is  about  \  mm.  in  diameter,  and  has  something 
the  form  of  a  top,  consisting  of  a  dome-like  upper  portion, 
the  prostomium^  produced  into  a  projecting  horizontal  rim; 


xxvi  THE   TROCHOSPHERE  297 

of  an  intermediate  portion  or  peristomium,  having  the  form 
of  an  inverted  hemisphere  ;  and  of  a  lower  somewhat  conical 
anal  region.  Around  the  projecting  rim  is  a  double  circlet 
of  large  cilia  (Pr.  or.  a)  by  means  of  which  the  larva  is 
propelled  through  the  water. 

Beneath  the  edge  of  the  ciliated  rim  is  a  rounded  aperture, 
the  mouth  (Mth);  this  leads  by  a  short,  nearly  straight 
gullet  (St.  dm),  into  a  spacious  stomach  (Ent\  from  the 
lower  side  of  which  proceeds  a  short  slightly  curved  intestine 
(Prc.  dm),  opening  at  the  extremity  of  the  conical  inferior 
region  by  an  anus  (An).  Between  the  body-wall  and  the 
enteric  canal  is  a  space  filled  with  fluid  (Bl.  cat),  but,  as  we 
shall  see,  this  does  not  correspond  with  the  body-cavity  of 
the  adult.  The  body-wall  and  the  enteric  canal  consist  each 
of  a  single  layer  of  epithelial  cells,  all  the  tissues  included  in 
the  adult  under  the  head  of  mesoderm  (p.  278)  being  absent 
or  so  poorly  developed  that  they  may  be  neglected  for  the 
present. 

Leaving  aside  all  details,  it  will  be  seen  that  the  trocho- 
sphere of  Polygordius  is  comparable  in  the  general  features 
of  its  organization  to  a  medusa  (compare  Fig.  55,  p.  242), 
consisting  as  it  does  of  an  outer  layer  of  cells  forming  the 
external  covering  of  the  body  and  of  an  inner  layer  lining 
the  digestive  cavity.  There  are,  however,  two  important 
differences  :  the  space  between  the  two  layers  is  occupied  by 
the  mesoglcea  in  the  medusa,  while  in  the  worm  it  is  a  cavity 
filled  with  fluid  ;  and  the  digestive  cavity  of  the  trochosphere 
has  two  openings  instead  of  one. 

But  in  order  to  compare  more  accurately  the  medusa 
with  the  trochosphere,  it  is  necessary  to  fill  up,  by  the  help 
of  other  types,  an  important  gap  in  our  knowledge  of  the 
development  of  Polygordius — the  passage  from  the  gastrula 
to  the  trochosphere.  From  what  we  know  of  the  do^dbp- 


298 


POLYGORDIUS 


LESS. 


ment  of  other  worms,  the  process,  in  its  general  features, 
is  probably  as  follows  : — 

The  ectoderm  and  endoderm  of  the  gastrula  (Fig.  73,  A) 
are  not  in  close  contact  with  one  another  as  in  Fig.  63 
(p.  265),  but  are  separated  by  a  space  filled  with  fluid — the 
blastocozle  or  larval  body-cavity.  The  mouth  of  the  gastrula 
closes  (£),  the  enteron  (Eni),  being  thus  converted  into  a 
shut  sac.  At  about  the  same  time  the  ectoderm  is  tucked 


FIG.  73. — Diagram  illustrating  the  origin  of  the  trochosphere  from 
the  gastrula.  The  ectoderm  is  dotted,  the  endoderm  striated. 

A,  gastrula,  with  enteron  (Ent}  and  gastrula-mouth  (Cast.  Mi/i),  and 
with  the  ectoderm  and  endoderm  separated  by  the  larval  body-cavity  or 
blastocoele  (Bl  ccel). 

B,  the  gastrula-mouth  has  closed,  the  enteron  (Ent}  becoming  a  shut 
sac. 

c,  two  ectodermal  pouches,  the  stomodseum  (St.  dm)  and  proctodseum 
(Prc.  dm)  have  appeared. 

D,  the  stomodaeum  (St.  dm)  and  proctodseum  (Prc.  dm)  have  opened 
into  the  enteron  (£nt),  forming  a  complete  enteric  canal  with  mouth 
(Mth)  and  anus  (An). 

in  or  invaginated  at  two  places  (C),  and  the  two  little 
pouches  (St.  dm,  Prc.  dni)  thus  formed  grow  inwards  until 
they  meet  with  the  closed  enteron  and  finally  open  into  it 
(D),  so  that  a  complete  enteric  canal  is  formed — formed, 
we  must  not  fail  to  notice,  of  three  distinct  parts  :  (i)  an 
anterior  ectodermal  pouch,  opening  externally  by  the  mouth, 
and  distinguished  as  the  stomodceum  ;  (2)  the  enteron,  lined 
with  endoderm ;  and  (3)  a  posterior  ectodermal  pouch, 
opening  externally  by  the  anus,  and  called  the  proctodaum. 


xxvi  METAMORPHOSIS  299 

In  the  trochosphere  (Fig.  72)  the  gullet  is  derived  from 
the  stomodseum,  the  stomach  from  the  enteron,  and  the 
intestine  from  the  proctodaeum ;  so  that  only  the  stomach  of 
the  worm-larva  corresponds  with  the  digestive  cavity  of  a 
medusa  :  the  gullet  and  intestine  are  structures  not  repre- 
sented in  the  latter  form. 

Two  or  three  other  points  in  the  anatomy  of  the  trocho- 
sphere must  now  be  referred  to. 

At  the  apex  of  the  dome-shaped  prostomium  the  ecto- 
derm is  greatly  thickened,  forming  a  rounded  patch  of  cells 
(Figs.  72  and  74,  JBr\  the  rudiment  of  the  brain.  On  the 
surface  of  the  same  region  and  in  close  relation  with  the 
brain  is  a  pair  of  small  patches  of  black  pigment,  the 
eye-spots  or  ocelli  (Oc). 

On  either  side  of  the  intestine,  between  its  epithelium  and 
the  external  ectoderm,  is  a  row  of  cells  forming  a  band 
which  partly  blocks  up  the  blastoccele  (B  and  c,  Msd\  These 
two  bands  are  the  rudiments  of  the  whole  of  the  meso- 
dermal  tissues  of  the  adult — muscle,  coelomic  epithelium, 
&c. — and  hence  called  mesodermal  bands. 

Finally  on  either  side  of  the  lower  or  posterior  end  of  the 
stomach  is  a  delicate  tube  (Fig.  74,  A,  Nph)  opening  by  a 
small  aperture  on  to  the  exterior,  and  by  a  wide  funnel- 
shaped  extremity  into  the  blastocoele  :  it  has  all  the  relations 
of  a  nephridium,  and  is  distinguished  as  the  head-kidney. 

As  the  larva  of  Polygordius  is  so  strikingly  different  from 
the  adult,  it  is  obvious  that  development  must,  in  this,  as  in 
several  cases  which  have  come  under  our  notice,  be  accom- 
panied by  a  metamorphosis. 

The  first  obvious  change  is  the  elongation  of  the  conical 
anal  region  of  the  trochosphere  into  a  tail-like  portion  which 


3oo 


POLYGORDIUS 


LESS. 


may  be  called  the  trunk  (Fig.  74,  A).  The  stomach 
(enteron),  which  was  formerly  confined  to  the  pro-  and  peri- 
stomium,  has  now  grown  for  a  considerable  distance  into 


Br 


B 


An  c 


FIG.  74. — A,  living  specimen  of  an  advanced  trochosphere-larva  of 
Polygordius  neapolitanus,  showing  the  elongation  of  the  anal  region  to 
form  the  trunk. 

B,  diagrammatic  vertical  section  of  the  same  :  the  ectoderm  is  coarsely, 
the  nervous  system  finely,  dotted,  the  endoderm  radially  striated,  and 
the  mesoderm  evenly  shaded. 

C,  transverse  section  through  the  plane  ab  in  B. 

The  pre-oral  (Pr.  or.  ci),  post-oral  (Pt.  or.  a'.),  and  anal  (An.  ci) 
cilia,  brain  (Br},  ocelli  (Oc),  blastocoele  (BL),  mouth  (Mth\  stomo- 
daeum  (St.  dm},  proctodseum  (Prc.  dm),  and  anus  (An)  as  in  Fig.  72, 
A  :  the  enteron  (Ent)  has  extended  some  distance  into  the  trunk. 

In  A,  slender  mesodermal  bands  (Msd.  bd)  in  the  prostomium,  and  the 
branched  head-kidney  (NpK]  are  shown. 

In  B  and  C  the  mesoderm  (Msd)  is  seen  to  have  obliterated  the  blasto- 
ccele  in  the  trunk-region :  the  ectoderm  has  undergone  a  thickening, 
forming  the  ventral  nerve -cord  (  V.  Nv.  Cd). 

(A  after  Fraipont. ) 

the  trunk    (B,    enf),    so   that   the   proctodaeum   (Prc.   dm) 
occupies  only  the  portion  in  proximity  to  the  anus. 

Important  internal  changes  have  also  taken  place.  The 
deric  epithelium  or  external  ectoderm  is  for  the  most  part 
composed,  as  in  the  preceding  stage,  of  a  single  layer  of 


xxvi  DEVELOPMENT  OF   METAMERES  301 

cells  ;  but  on  that  aspect  of  the  trunk  which  lies  on  the  same 
side  as  the  mouth — i.e.,  to  the  left  in  Fig.  74,  A  and  B — this 
layer  has  undergone  a  notable  thickening,  being  now  com- 
posed of  several  layers  of  cells.  This  ectodermal  thickening 
is  the  rudiment  of  the  ventral  nerve-cord  ( V.  Nv.  Cd\  and 
the  side  of  the  trunk  on  which  it  appears  is  now  definitely 
marked  out  as  the  ventral  aspect  of  the  future  worm,  the 
opposite  aspect — that  to  the  right  in  the  figures — being 
dorsal.  At  a  later  stage  two  ectodermal  cords — the  cesopha- 
geal  connectives — are  formed,  connecting  the  anterior  end  of 
the  ventral  nerve-cord  with  the  brain.  Note  that  the  two 
divisions  of  the  central  nervous  system  are  originally  quite 
distinct. 

The  mesodermal  bands,  which  were  small  and  quite 
separate  in  the  preceding  stage  (Fig.  72,  B  and  c,  Msd\ 
have  now  increased  to  such  an  extent  as  to  surround  com- 
pletely the  enteron  and  obliterate  the  blastocoele  (Fig.  74, 
B  and  B,  Msd).  At  this  stage  therefore  there  is  no  body- 
cavity  in  the  trunk,  but  the  space  between  the  deric  and 
enteric  epithelia  is  occupied  by  a  solid  mass  of  mesoderm. 
In  a  word,  the  larva  is  at  present,  as  far  as  the  trunk  is  con- 
cerned, triploblastic  but  acoelomate. 

Development  continues,  and  the  larva  assumes  the  form 
shown  in  Fig.  75,  A.  The  trunk  has  undergone  a  great 
increase  in  length  and  at  the  same  time  has  become  divided, 
by  a  series  of  annular  grooves,  into  segments  or  metameres, 
like  those  of  the  adult  worm  but  more  distinct  (compare 
Fig.  67,  D,  p.  272).  By  following  the  growth  of  the  larva 
from  the  preceding  to  the  present  stage,  it  is  seen  that  these 
segments  are  formed  from  before  backwards,  i.e.,  the  seg- 
ment next  the  peristomium  is  the  oldest,  and  new  ones  are 
continually  being  added  between  the  last  formed  and  the 


302  POLYGORDIUS  LESS,  xxvi 

extremity  of  the  trunk,  or  what  may  now  be  called  the  anal 
segment.  By  this  process  the  larva  has  assumed  the  appear- 
ance of  a  worm  with  an  immense  head  and  a  very  slender 
trunk. 

The  original  larval  stomach  (enteron)  has  extended,  with 
the  formation  of  the  metameres,  so  as  to  form  the  greater 
portion  of  the  intestine :  the  proctodseum  (Prc.  dm)  is 
confined  to  the  anal  segment. 

Two  other  obvious  changes  are  the  appearance  of  a  pair 
of  small  slender  processes  (A,  /) — the  rudiments  of  the 
tentacles — on  the  apex  of  the  prostomium,  and  of  a  circlet 
of  cilia  (Pr.  an.  d)  round  the  posterior  end  of  the  trunk. 

The  internal  changes  undergone  during  the  assumption  of 
the  present  form  are  very  striking.  In  every  fully  formed 
metamere  the  mesoderm — solid,  it  will  be  remembered, 
in  the  previous  stage — has  become  divided  into  two  layers, 
a  somatic  layer  (B  and  c,  Msd  (soni)  )  in  contact  with  the 
ectoderm  and  a  splanchnic  layer  (Msd  (spl)  )  in  contact 
with  the  endoderm.  The  space  between  the  two  layers 
(Cxi]  is  the  permanent  body-cavity  or  ccelome,  which  is 
thus  quite  a  different  thing  from  the  larval  body-cavity 
or  blastoccele,  being  formed,  not  as  a  space  between 
ectoderm  and  endoderm,  but  by  the  splitting  of  an 
originally  solid  mesoderm. 

The  division  of  the  mesoderm  does  not  however  extend 
quite  to  the  middle  dorsal  and  middle  ventral  lines  :  in  both 
these  situations  a  layer  of  undivided  mesoderm  is  left  (c), 
and  in  this  way  the  dorsal  and  ventral  mesenteries  are 
formed.  Spaces  in  these,  apparently  the  remains  of  the 
blastocoele,  form  the  dorsal  and  ventral  blood-vessels.  More- 
over the  splitting  process  takes  place  independently  in  each 
segment  and  a  transverse  vertical  layer  of  undivided 
mesoderm  (B,  Sep)  is  left  separating  each  segment  from  the 


An.ci 


FIG.  75.  —  A,  larva  of  Polygordius  neapolitanus  in  a  condition  inter- 
mediate between  the  trochosphere  and  the  adult  worm,  the  trunk-region 
being  elongated  and  divided  into  metameres. 

B,  diagrammatic  vertical  section  of  the  same  :  the  ectoderm  is  coarsely, 
the  nervous  system  finely,  dotted,  the  endoderm  radially  striated,  and 
the  mesoderm  evenly  shaded. 

C,  transverse  section  along  the  plane  ab  in  B. 

The  pre-oral  (Pr.  or.  ci),  post-oral  (Pt.  or.  ci),  and  anal  (An.  ci) 
cilia,  the  blastocoele  (Bl.  ccel),  stomodseum  (6V.  dm],  and  proctodseum 
(Prc.  dm)  are  as  in  Fig.  72,  A  and  B  :  the  enteron  now  extends  through- 
out the  segmented  region  of  the  trunk. 

A  pair  of  tentacles  (/)  has  appeared  on  the  prostomium  near  the  ocelli 
(o),  and  a  pre-anal  circlet  of  cilia  (Pr.  an.  ci)  is  developed. 

The  mesoderm  has  divided  into  somatic  (  Msd  (sotn)  )  and  splanchnic 
(Msd(spl)  )  layers  with  the  ccelome  (Ccel)  between  :  the  septa  (Sep)  are 
formed  by  undivided  plates  of  mesoderm  separating  the  segments  of  the 
coelome  from  one  another. 

D^D3,  three  stages  in  the  development  of  the  somatic  mesoderm.  In 
D1  it  (Msd  (Som)  )  consists  of  a  single  layer  of  cells  in  contact  with  the 
deric  epithelium  (Der.  Epthm)  :  in  D2  the  cells  have  begun  to  split  up 
in  a  radial  direction  :  in  D3  each  has  divided  into  a  number  of  radially 
arranged  sections  of  muscle-plates  (M.  PI)  and  a  single  cell  of  ccelomic 
epithelium  (Ccel.  Epthm). 

(A  after  Fraipont.  ) 


304  POLYGORDIUS  LESS. 

adjacent  ones  before  and  behind  :  in  this  way  the  septa 
arise. 

The  nephridia  appear  to  have  a  double  origin,  the  super- 
ficial portion  of  each  being  formed  from  ectoderm,  the 
deep  portion,  including  the  nephrostome,  from  the  somatic 
layer  of  mesoderm. 

In  the  ventral  nerve-cord  the  cells  lying  nearest  the  outer 
surface  have  enlarged  and  formed  nerve-cells,  while  those  on 
the  dorsal  aspect  of  the  cord  have  elongated  longitudinally 
and  become  converted  into  nerve-fibres.  This  process  has 
already  begun  in  the  preceding  stage. 

But  the  most  striking  histological  changes  are  those  which 
gradually  take  place  in  the  somatic  layer  of  mesoderm.  At 
first  this  layer  consists  of  ordinary  nucleated  cells  (D\  Msd 
(Som}\  but  before  long  each  cell  splits  up  in  a  radial 
direction  (D2)  from  without  inwards — i.e.,  from  the  ectoderm 
(Der.  Epthni)  towards  the  ccelome — finally  taking  on  the 
form  of  a  book  with  four  or  more  slightly  separated  leaves 
directed  outwards  or  towards  the  surface  of  the  body,  and 
with  its  back — the  undivided  portion  of  the  cell — bounding 
the  ccelome.  The  cells  being  arranged  in  longitudinal  series, 
we  have  a  number  of  such  books  placed  end  to  end  in 
a  row  with  the  corresponding  leaves  in  contact — page  one 
of  the  first  book  being  followed  by  page  one  of  the  second, 
third,  fourth,  &c.,  page  two  by  page  two,  and  so  on  through 
one  or  more  segments  of  the  trunk.  Next,  what  we  have 
compared  with  the  leaves  of  the  books — the  divided 
portions  of  the  cells — become  separated  from  the  backs — 
the  undivided  portions  (DS) — and  each  leaf  (M~.  PI)  fuses 
with  the  corresponding  leaves  of  a  certain  number  of  books 
in  the  same  longitudinal  series.  The  final  result  is  that  the 
undivided  portions  of  the  cells  (backs  of  the  books,  Ca>l. 
Epthni)  become  the  parietal  layer  of  ccelomic  epithelium,  the 


xxvi     SIGNIFICANCE  OF  DEVELOPMENTAL  STAGES     305 

longitudinal  bands  formed  by  the  union  of  the  leaves 
(M.  PI)  becoming  the  muscle-plates,  which  are  thus  cell- 
fusions^  each  being  formed  by  the  union  of  portions  of 
a  series  of  longitudinally  arranged  cells. 

At  the  same  time  the  cells  of  the  splanchnic  layer 
of  mesoderm  thin  out  and  become  the  visceral  layer  of 
ccelomic  epithelium. 

We  see  then  that  by  the  time  the  larva  has  reached  the 
stage  shown  in  Fig.  75,  it  is  no  longer  a  mere  aggregate  of 
simple  cells  arranged  in  certain  layers.  The  cells  them- 
selves have  undergone  differentiation,  some  becoming  modi- 
fied into  nerve-fibres,  others  by  division  and  subsequent 
fusion  with  their  neighbours  forming  muscle-plates,  while 
others,  such  as  the  epithelial  cells,  remain  almost  unaltered. 

Thus,  in  the  course  of  the  development  of  Polygordius, 
cell-multiplication  and  cell-differentiation  go  hand  in  hand, 
the  result  being  the  formation  of  those  complex  tissues  the 
presence  of  which  forms  so  striking  a  difference  between  the 
worm  and  the  simpler  types  previously  studied. 

It  is  important  to  notice  that  this  comparatively  complex 
animal  is  in  one  stage  of  its  existence — the  oosperm — as 
simple  as  an  Amoeba ;  in  another — the  polyplast — it  is  com- 
parable to  a  Pandorina  or  a  Volvox;  in  a  third — the 
gastrula — it  corresponds  in  general  features  with  a  Hydra ; 
while  in  a  fourth — the  trochosphere — it  resembles  in  many 
respects  a  Medusa.  As  in  other  cases  we  have  met  with, 
the  comparatively  highly-organized  form  passes  through 
stages  in  the  course  of  its  individual  development  similar  in 
general  characters  to  those  which,  on  the  theory  of  evolution, 
its  ancestors  may  be  considered  to  have  passed  through  in 
their  gradual  ascent  from  a  lower  to  a  higher  stage  of 
organization. 

x 


306  POLYGORDIUS  LESS,  xxvi 

The  rest  of  the  development  of  Polygordius  may  be 
summarized  very  briefly.  The  trunk  grows  so  much  faster 
than  the  head  (pro-flhts  peri-stomium) — that  the  latter  under- 
goes a  relative  diminution  in  size,  finally  becoming  of  equal 
diameter  with  the  trunk,  as  in  the  adult.  The  ciliated  rings 
are  lost,  the  tentacles  grow  to  their  full  size,  the  eye-spots 
atrophy,  and  thus  the  adult  form  is  assumed. 


LESSON  XXVII1 

THE  GENERAL  CHARACTERS  OF  THE  HIGHER  ANIMALS 

THE  student  who  has  once  thoroughly  grasped  the  facts  of 
structure  of  such  typical  unicellular  animals  as  Amoeba  and 
the  Infusoria,  of  such  typical  diploblastic  animals  as  Hydra 
and  Bougainvillea,  and  of  such  a  typical  triploblastic  animal 
as  Polygordius,  ought  to  have  no  difficulty  in  understanding 
the  general  features  of  the  organization  of  any  other  members 
of  the  animal  kingdom.  When  once  the  notions  of  a  cell,  a 
cell-layer,  a  tissue,  body-wall,  enteron,  stomodaeum,  procto- 
dseum,  coelome,  somatic  and  splanchnic  mesoderm,  are  fairly 
understood,  all  other  points  of  structure  become  hardly  more 
than  matters  of  detail. 

If  we  turn  to  any  text-book  of  Zoology  we  shall  find  that 
the  animal  kingdom  is  divisible  into  seven  primary  sub- 
divisions, called  sub-kingdoms,  types,  or  phyla.  These  are 
as  follows  : — 

Protozoa.  Ccelenterata. 

Venues.  Echinodermata. 

Arthropoda.  Mollusca. 
Vertebrata 

1  Readers  who  have  not  studied  zoology,  or  at  least  examined  a  series 
of  selected  animal  types,  should  omit  this  lesson  and  go  on  to  the  next. 

X    2 


3o8        CHARACTERS  OF  THE  HIGHER  ANIMALS       LESS. 

With  a  few  exceptions,  the  discussion  of  which  would  be  out 
of  place  here,  the  vast  number  of  animals  known  to  us  may 
be  arranged  in  one  or  other  of  these  groups. 

The  Protozoa  are  the  unicellular  animals  :  they  have  been 
represented  in  previous  lessons  by  Amoeba  and  Protamoeba, 
Hsematococcus,  Heteromita,  Euglena,  the  Mycetozoa,  Para- 
mcecium,  Stylonychia,  Oxytricha,  Opalina,  Vorticella,  Zooth- 
amnium,  the  Foraminifera,  and  the  Radiolaria.  According  to 
many  authors,  Pandorina  and  Volvox  are  also  included  in 
this  group.  The  reader  will  therefore  have  no  difficulty  in 
grasping  the  general  features  of  this  phylum. 

The  Cxlenterata  are  the  diploblastic  animals,  and  have 
also  been  well  represented  in  the  foregoing  pages,  namely, 
by  Hydra,  Bougainvillea,  Diphyes,  and  Porpita.  The  sea- 
anemones,  corals,  and  sponges  also  belong  to  this  phylum. 

The  Vermes,  or  Worms,  are  a  very  heterogeneous  assem- 
blage. They  are  all  triploblastic,  but  while  some  are 
ccelomate,  others  have  no  body-cavity;  some,  again,  are 
segmented,  others  not.  Still,  if  the  structure  of  Polygordius 
is  thoroughly  understood,  there  will  be  little  difficulty  in 
understanding  that  of  a  fluke,  a  tape-worm,  a  round-worm, 
an  earthworm,  or  one  of  the  ordinary  marine  worms. 

Of  the  remaining  four  sub-kingdoms  we  have,  so  far, 
studied  no  example,  but  a  brief  description  of  a  single 
typical  form  of  each  will  show  how  they  all  conform  to  the 
general  plan  of  organization  of  Polygordius,  being  all  triplo- 
blastic and  ccelomate. 

•  Under  the  Echinodermata  are  included  the  various  kinds 
of  starfishes — sand-stars,  brittle-stars,  and  feather-stars,  as 
well  as  sea-urchins,  sea-cucumbers,  &c.  A  starfish  will  serve 
as  an  example  of  the  group. 

The  phylum  Arthropoda  includes  crayfishes,  lobsters, 
crabs,  shrimps,  prawns,  wood-lice,  and  water-fleas ;  scorpions, 


GENERAL   STRUCTURE  3°9 

spiders,  and  mites ;  centipedes  and  millipedes ;  and  all 
kinds  of  insects,  such  as  cockroaches,  beetles,  flies,  ants, 
bees,  butterflies,  and  moths.  A  crayfish  forms  a  very  fair 
type  of  the  group. 

In  the  phylum  Mollusca  are  included  the  ordinary  bi- 
valves, such  as  mussels  and  oysters ;  snails,  slugs,  and  other 
univalves  or  one-shelled  forms ;  sea-butterflies ;  and  cuttle- 
fish, squids,  and  Octopi.  An  account  of  a  fresh-water 
mussel  will  serve  to  give  a  general  notion  of  the  character 
of  this  group. 

Finally,  under  the  head  of  Vertebrata  are  included  all  the 
backboned  animals  :  the  lampreys  and  hags ;  true  fishes, 
such  as  the  shark,  skate,  sturgeon,  cod,  perch,  trout,  &c.  ; 
amphibians,  such  as  frogs,  toads,  newts,  and  salamanders  ; 
true  reptiles,  such  as  lizards,  crocodiles,  snakes,  and  tor- 
toises ;  birds  ;  and  mammals,  or  creatures  with  a  hairy  skin 
which  suckle  their  young,  such  as  the  ordinary  hairy 
quadrupeds,  whales  and  porpoises,  apes,  and  man.  The 
essential  structure  of  a  vertebrate  animal  will  be  understood 
from  a  brief  description  of  a  dog-fish. 

THE    STARFISH.1 

A  common  starfish  consists  of  a  central  disc-like  portion, 
from  which  radiate  five  arms  or  rays.  It  crawls  over  the 
rocks  with  its  ventral  surface  downwards,  its  dorsal  surface 
upwards.  It  can  move  in  any  direction,  so  that,  in  the 
ordinary  sense  of  the  words,  anterior  and  posterior  extremi- 
ties cannot  be  distinguished.  Radial  symmetry  such  as  this, 
i.e.,  the  division  of  the  body  into  similar  parts  radiating  from 
a  common  centre,  is  characteristic  of  the  Echinodermata 
generally. 

1  For  a  detailed  description  of  a  Starfish,  see  Rolleston  and  Hatchett 
Jackson,  Forms  of  Animal  Life  (Oxford,  1888),  pp.  190  and  311. 


Tire,  fa 


FIG.   76. — "Diagrammatic  sections  of  a  Starfish. 

A,  vertical  section  passing  on  the  right  through  a  radius  on  the  left 
through  an  inter-radius.     The  off  side  of  the  ambulacral  groove  with 
the  tube  feet  ( T,  F)  and  ampullae  (Amp)  are  shown  in  perspective. 

B,  transverse  section  through  an  arm. 

The  ectoderm  is  coarsely  dotted,  the  nervous  system  finely  dotted,  the 
endoderm  radially  striated,  the  mesoderm  evenly  shaded,  the  ossicles  of 
the  skeleton  black,  and  the  ccelomic  epithelium  represented  by  a  beaded 
line. 

The  body-wall  consists  of  deric  epithelium  (Der.  Eptkm),  derails 
(Derm),  and  the  parietal  layer  of  ccelomic  epithelium  (Ccel.  Epthni). 

To  the  body-wall  are  attached  pedicellariae  (Ped),  and  the  end  of  the 
arm  bears  a  tentacle  (/)  with  an  ocellus  (oc)  at  its  base. 

The  skeleton  consists  of  ossicles  (as)  imbedded  in  the  derm  :  large 
ambulacral  ossicles  (Amb.  os)  bound  the  ambulacral  grooves  on  the 
ventral  surfaces  of  the  arms. 

The  mouth  (Mth)  leads  by  a  short  gullet  into  a  stomach  (St),  which 
gives  off  a  cardiac  caecum  (Cd.  ca>)  and  a  pair  cf  pyloric  caeca  (Pyl.  ccc) 
to  each  arm,  and  passes  into  an  intestine  (hit}  which  gives  off  intestinal 
caeca  (Int.  cce)  to  the  inter-radii,  and  ends  in  the  anus  (An).  The 
pyloric  caeca  are  connected  to  the  dorsal  body- wall  by  mesenteries 
(Mes.  in  B).  The  wall  of  the  enteric  canal  consists  of  enteric  epithelium 
covered  by  the  visceral  layer  of  coelomic  epithelium  (Ccel.  Epthm'}. 

From  .the  ccelome  are  given  off  respiratory  caeca  (Resp.  CCE),  which 
project  through  the  body-wall :  the  latter  contains  peri-haemal  spaces 
(p.  h)  derived  from  the  ccelome. 


LESS,  xxvn  TUBE-FEET  311 

The  circular  blood-vessel  (C.  B.  V)  surrounds  the  gullet  and  gives 
off  radial  vessels  (Rad.  B.  V}  to  the  arms  and  an  inter-radial  plexus 
connected  with  a  pentagonal  ring  round  the  intestine. 

The  circular  ambulacral  vessel  (C.  Amb.  V}  gives  off  radial  vessels 
(Rad.  Amb.  V}  to  the  arms  connected  with  the  ampulla;  (Amp}  and 
tube-feet  (T.  F) :  it  is  also  connected  with  the  stone-canal  (St.  Q,  which 
opens  externally  by  the  madreporite  (Mdpr). 

The  nerve-ring  (Nv.  R]  gives  off  radial  nerves  (Rad.  Nv)  to  the 
arms. 

The  ovary  (Ovy]  is  inter-radial,  and  opens  by  a  dorsal  oviduct  (Ovd]. 

In  the  centre  of  the  disc  on  the  ventral  surface  is  the  large 
mouth  (Fig.  76,  A,  Mth\  and  from  it  radiate  five  grooves, 
one  along  the  ventral  surface  of  each  arm  (A  and  B).  In  the 
living  animal  numerous  delicate  semi-transparent  cylinders, 
the  ttibefeet  (T.  F),  are  protruded  from  these  grooves;  they 
are  very  extensible  and  each  ends  in  a  sucker.  It  is  by 
moving  these  structures  in  various  directions,  protruding 
some  and  withdrawing  others,  that  the  starfish  is  able  to 
move  along  either  a  horizontal  or  a  vertical  surface,  and 
even  to  turn  itself  over  when  placed  with  the  ventral  side 
upwards. 

Near  the  middle  of  the  disc,  on  the  dorsal  surface,  is  the 
very  minute  anus  (A,  An) ;  it  is  situated  on  a  line  drawn 
from  the  centre  of  the  disc  to  the  re-entering  angle  between 
two  of  the  rays,  and  is  therefore  said  to  be  inter-radial  in 
position.  Near  the  anus,  and  also  inter-radially  situated,  is 
a  circular  calcareous  plate,  the  madreporite  (Mdpr),  per- 
forated by  numerous  microscopic  apertures.  Innumerable 
other  calcareous  plates,  or  ossicles  (os),  are  embedded  in 
the  body-wall,  and  constitute  a  skeleton,  to  which  the  firm 
and  resistant  character  of  the  starfish  is  due. 

Sections  show  that  there  is  a  well-marked  ccelome,  separ- 
ating the  body-wall  from  the  enteric  canal  and  containing 
the  gonads,  blood-vessels,  &c.  The  body-wall  consists  ex- 
ternally of  a  very  thin  cuticle,  then  of  a  layer  of  deric 


312  THE   STARFISH  LESS. 

epithelium  or  epidermis  (Der.  EptJwi],  then  of  a  thick 
fibrous  layer  (Derm) — the  dermis  or  deep  layer  of  the  skin, 
then  of  a  thin  and  interrupted  layer  of  muscle,  and  finally, 
of  a  layer  of  coelomic  epithelium  (CoeL  Epthui]  bounding 
the  body  cavity. 

The  dermis  is  formed  of  connective  tissue,  a  substance  not 
met  with  in  Polygordius,  formed  by  the  elongation  of  meso- 
derm  cells  into  wavy  fibres.  The  ossicles  of  the  skeleton 
(as)  are  formed  by  deposits  of  calcium  carbonate  in  the 
dermis ;  the  skeleton  is  therefore  a  dermal  exoskeleton. 
The  large  ambulacral  ossicles  (Amb.  os),  however,  which 
bound  the  ambulacral  grooves,  lie  internal  to  the  vessels 
(Rad.  B.  V.,  Rad.  Amb.  V.)  and  have  an  endoskeletal 
character. 

The  enteric  canal  passes  vertically  from  mouth  (A,  Mth) 
to  anus  (An),  and  is  divisible  into  gullet,  stomach  (St\  and 
intestine  (Int).  The  stomach  gives  off  five  wide  pouches 
(Cd.  cce),  one  extending  into  the  base  of  each  arm,  and 
above  these  five  other  pouches  (Pyl.  cce),  each  of  which 
divides  into  two  (B,  PyL  CCE)  and  extends  to  the  extremity 
of  the  corresponding  arm.  The  intestine  gives  off  smaller 
pouches  (Int,  cce)  which  are  inter-radial  in  position.  Thus 
the  enteric  canal,  like  the  body  as  a  whole,  exhibits  radial 
symmetry.  The  canal  is  lined  by  enteric  epithelium,  mostly 
endodermal,  and  is  covered  externally  by  coelomic  epithelium 
(Ccel.  Epthm'}. 

Respiration  is  affected  by  blind,  finger-like  offshoots  of  the 
coelome,  the  respiratory  cczca  (Resp.  coe],  which  pass  between 
the  ossicles  of  the  skeleton  and  project  on  the  surface  of  the 
body,  thus  bringing  the  coelomic  fluid  into  close  relation 
with  the  surrounding  water. 

The  blood-system  consists  of  a  circular  vessel  (A,  C.  B.  V) 
round  the  gullet,  connected  with  a  pentagonal  vessel  round 


xxvii  NERVOUS    SYSTEM,  ETC.  313 

the  intestine  by  an  elongated  network  or  plexus  of  vessels. 
From  the  circular  vessel  five  radiating  trunks  (Rad.  B.  V] 
pass  to  the  arms. 

Parallel  with  and  above  the  circular  blood-vessel  is  a 
similar  but  larger  structure,  the  ambulacral  ring  ( C.  Amb.  V) 
which  also  sends  off  five  radiating  vessels  (Rad.  Amb.  V]  to 
the  arms.  These  give  off  a  branchlet  to  each  tube-foot 
(B,  T.F.\  the  branchlet  having  a  sac  or  ampulla  (Amp)  at 
its  base.  From  the  ambulacral  ring  a  tube  with  calcareous 
walls,  the  stone-canal  (St.  C}  passes  upwards  and  ends 
in  the  madreporite  (Mdpr\  by  the  apertures  in  which  the 
fluid  filling  the  whole  of  the  ambtilacral  system  of  vessels  is 
placed  in  communication  with  the  surrounding  water. 

The  function  of  the  ambulacral  system  is  mainly  locomo- 
tive. By  the  contraction  of  the  ampullae  fluid  is  forced  into 
the  tube  feet,  and  by  the  action  of  the  muscles  of  the  tube- 
feet  it  is  sent  back  into  the  ampullae,  and  in  this  way  the 
tube-feet  are  protruded  and  retracted  at  the  will  of  the 
animal.  The  system,  which  is  peculiar  to  the  Echinodermata, 
is  lined  with  epithelium,  continuous,  in  the  larva,  with  the 
ccelomic  epithelium.  It  has  been  compared  to  a  gigantic 
and  greatly  modified  nephridium. 

The  nervous  system  is  very  simple.  It  consists  of  a 
pentagonal  ring  (A,  Nv."  R]  round  the  mouth  giving  off 
five  radial  nerves  (A  and  B,  Rad.  Nv)  which  pass  along  the 
ambulacral  grooves,  below  the  blood-vessels,  to  the  ex- 
tremities of  the  arms,  where  each  is  connected  with  an  eye- 
spot.  Both  nerve-ring  and  radial  nerves  are  mere  thicken- 
ings of  the  deric  epithelium. 

The  gonads  (A,  Ovy)  are  branched  organs,  five  in  num- 
ber, which  lie  inter-radially  near  the  bases  of  the  arms,  and 
open  by  gonaducts  (Ovd)  on  the  dorsal  surface  of  the  disc. 
The  sexes  are  lodged  in  distinct  individuals. 


3H  THE   CRAYFISH  LESS. 

Both  eggs  and  sperms  are  shed  into  the  water,  and  after 
impregnation  the  oosperm  becomes  a  gastrula,  which  is  con- 
verted into  a  peculiar  free-swimming  larva  ;  this  undergoes 
metamorphosis  and  is  converted  into  the  adult  form. 


THE    CRAYFISH.1 

In  a  crayfish  or  lobster  the  body  is  bilaterally  symmetrical 
and  is  distinctly  segmented,  consisting  of  a  prostomium  and 
of  nineteen  metameres.  The  anterior  twelve  metameres  are 
united  with  one  another  and  with  the  prostomium  to  form  an 
unjointed  portion  of  the  body,  the  cephalothorax  (Fig.  77, 
A,  C.  Th.)  :  the  seven  posterior  segments  are  free  and  con- 
stitute the  abdomen  (Abd.  Seg.  i,  Abd.  Seg.  7).  It  is  very 
generally  characteristic  of  Arthropods  to  have  the  meta- 
meres limited  and  constant  in  number,  and  for  more  or 
fewer  of  them  to  undergo  concrescence. 

Another  distinctive  arthropod  character  illustrated  fcby 
the  Crayfish  is  the  possession  of  lateral  appendages  of  the 
body.  These  are  given  off  from  the  ventral  region,  two  pairs 
being  borne  by  the  prostomium  and  one  by  each  of  the 
metameres,  except  the  last.  Moreover  the  appendages 
themselves  are  segmented,  being,  divided  into  freely  arti- 
culated limb-segments  m  podomeres. 

In  the  Crayfish  there  is  a  marked  differentiation  of  the 
appendages.  Those  of  the  prostomium  are  a  pair  of  eye- 
stalks,  and  one  of  small  feelers  or  antennules  which  perform 

1  For  detailed  descriptions  of  the  Crayfish  see  Huxley,  The  Crayfish 
(London,  1880)  :  Huxley  and  Martin,  Elementary  Biology,  new  ed. 
(London,  1888),  p.  173:  Rolleston  and  Jackson,  Forms  of  Animal 
Life  (Oxford,  1888),  pp.  162  and  307  :  Marshall  and  Hurst,  Practical 
Zoology,  3rd.  ed.  (London,  1892),  p.  130:  and  Parker,  The  Skeleton  of 
the  New  Zealand  Crayfishes  (Wellington,  N.Z.,  1889). 


xxvii  STRUCTURE   OF   BODY-WALL  315 

an  olfactory  function  and  also  contain  the  organ  of  hearing.1 
The  metameres  of  the  cephalothorax  bear  one  pair  of  tactile 
appendages  or  antennae,  six  pairs  acting  as  jaws  (mandibles, 
first  and  second  maxillae,  and  first,  second,  and  third  max- 
illipedes),  and  five  pairs  of  legs,  the  first  of  which  are — in 
the  fresh-water  crayfishes  and  in  lobsters — much  larger  than 
the  rest.  The  abdomen  bears  small  fin-like  swimmerets  on 
its  first  five  metameres,  the  sixth  bearing  larger  appendages 
which,  together  with  the  seventh  segment  or  telson,  con- 
stitute the  tail-fin. 

Sections  show  the  body-wall  to  consist  of  a  layer  of  deric 
epithelium  (Der.  Epthm)  secreting  a  thick  cuticle  (Cu\  a 
layer  of  connective  tissue  forming  the  Dermis  (Derm),  and 
a  very  thick  layer  of  large  and  complicated  muscles  (M\ 
which  fill  up  a  great  part  of  the  interior  of  the  body. 

The  cuticle  (Cu)  is  of  great  thickness,  and  except  at  the 
joints  between  the  various  segments  of  the  body  and  limbs, 
is  impregnated  with  lime  salts  so  as  to  form  a  hard,  jointed 
armour.  It  thus  constitutes  a  skeleton  which,  unlike  that 
of  the  starfish  (p.  312),  is  a  cuticular  exoskeleton,  forming  a 
continuous  investment  over  the  whole  body  but  discon- 
tinuously  calcified. 

The  mouth  (MtK)  is  on  the  ventral  surface  of  the  head, 
in  the  segments  of  the  mandibles  or  first  pair  of  jaws.  It 
has  therefore,  as  compared  with  the  mouth  of  Polygordius, 
undergone  a  backward  shifting,  the  appendages  of  the  first 
metamere  (antennae)  being  altogether  in  front  of  it.  The 
enteric  canal  consists  of  a  short  gullet  (Gut),  a  large 
stomach  (St),  and  a  straight  intestine  divisible  into  a  short 
anterior  division  or  small  intestine  (6*.  Jnt)  and  a  long 
posterior  division  or  large  intestine  (L.  Int)  :  the  latter 

1  The  antennules  are  frequently  considered  as  belonging  to  the  first 
metamere,  the  number  of  segments  being  then  reckoned  as  twenty. 


LESS,  xxvii  GENERAL   CHARACTERS  317 

The  body  is  divided  into  a  head  (Hd)  and  thorax  (T/i),  together 
constituting  the  cephalothorax  (C.  Th),  and  seven  free  abdominal 
segments  (Abd.  seg.  i,  Abd.  seg.  7)  :  the  head  is  produced  in  front  into 
a  rostrum. 

The  body- wall  consists  of  cuticle  (Cu),  partly  calcified  to  form  the 
exoskeleton,  deric  epithelium  (Der.  Epthm),  dermis  (Derm},  and  a 
very  thick  layer  of  muscle  (M)  which  in  the  abdomen  is  distinctly 
segmented. 

The  mouth  (Mth)  leads  by  a  short  gullet  (Gul)  into  a  large  stomach 
(St),  from  which  a  short  small  intestine  (S.  Inf)  leads  into  a  large  in- 
testine (L.  Int\  ending  in  the  anus  (An).  Opening  into  the  small 
intestine  are  the  digestive  glands  (D.  67).  The  epithelium  of  the  small 
intestine  and  digestive  glands  is  endodermal,  that  of  the  rest  of  the  canal 
is  ectodermal  and  secretes  a  cuticle  :  the  outer  layer  throughout  is 
mesodermal  (connective  tissue  and  muscle). 

The  cavity  (B.  S)  between  the  enteric  canal  and  the  body-muscles  is 
a  blood-sinus. 

The  heart  (Ht)  is  enclosed  in  the  pericardial  sinus  (Per.  S) :  the 
chief  ventral  blood-vessel  or  sternal  artery  (St.  A)  is  shown  in  B. 

The  gills  (B,  Gill)  are  enclosed  in  a  cavity  formed  by  a  fold  of  the 
thoracic  body-wall  called  the  branchiostegite  (Brstg) :  they  are  formed 
of  the  same  layers  as  the  body-wall,  of  which  they  are  offshoots. 

The  kidneys  (A,  K)  are  situated  in  the  head. 

The  brain  (Br)  lies  in  the  prostomium  :  the  ventral  nerve-cord  ( V. 
Nv.  Cd)  consists  of  a  chain  of  ganglia  (Gn)  united  by  connectives. 

The  ovary  (ovy)  is  a  hollow  organ  opening  by  an  oviduct  (B,  ovd)  on 
the  base  of  one  of  the  legs  (Leg). 


opens  by  an  anus  (An)  on  the  ventral  surface  of  the  last 
segment.  The  study  of  development  shows  that  the  only 
part  of  the  canal  derived  from  the  enteron  of  the  embryo  is 
the  small  intestine  :  the  gullet  and  stomach  arise  from  the 
stomodseum,  the  large  intestine  from  the  proctodseum. 
Thus  the  only  portion  of  the  enteric  epithelium  which 
is  endodermal  is  that  of  the  small  intestine  :  the  epithelium 
of  gullet,  stomach,  and  large  intestine  is  ectodermal,  and 
like  the  deric  epithelium  secretes  a  cuticle.  The  outer 
layer  of  the  whole  enteric  canal  consists  of  connective 
tissue  and  muscle  :  there  is  no  ccelomic  epithelium. 

On  each  side  of  the  small  intestine  is  a  large  organ,  the 
digestive  gland  (D.  Gl)  \  it  consists  of  numberless  glove- 
finger-like  processes  or  caeca  which  open  by  a  short  tube  or 


3i8  THE    CRAYFISH  LESS. 

duct  into  the  small  intestine  (B,  D.  Gl).  Both  caeca 
and  duct  are  lined  with  epithelium  derived  from  the  endo- 
derm,  and  the  whole  digestive  gland  is  to  be  looked  upon 
as  a  branched  lateral  outgrowth  of  the  enteron.  The 
secretion  of  digestive  juice  is  performed  exclusively  by  the 
epithelium  of  the  digestive  glands. 

Between  the  enteric  canal  and  the  body-wall  are  a  series 
of  spaces  (B.S)  containing  blood  and  having  the  general 
relations  of  a  coelome,  but  very  probably  only  representing 
a  number  of  enlarged  blood-spaces  or  sinuses. 

Respiration  is  performed  by  special  organs,  the  gills 
(B,  Gill)  see  p.  317),  developed  in  the  thoracic  region  as  out- 
growths of  the  body-wall  and  containing  the  same  layers 
(cuticle,  epithelium,  and  connective  tissue)  as  the  latter. 
They  have  a  brush-like  form  and  are  protected  by  a  fold  of 
the  body-wall  (Brstg). 

The  blood-system  is  constructed  on  the  same  general 
lines  as  those  of  Polygordius,  but  is  greatly  modified.  A 
portion  of  the  dorsal  vessel  is  enlarged  to  form  a  muscular 
dilatation,  the  heart  (Ht\  and  the  rest  of  the  vessels,  now 
called  arteries  (B,  St.  A],  instead  of  forming  by  themselves 
a  closed  system,  ramify  extensively  over  the  body,  their  ulti- 
mate branches  opening  into  larger  cavities  or  sinuses  between 
the  muscles.  One  of  these  cavities — the  pericardial  sinus 
Pcd.  S) — surrounds  the  heart.  The  heart,  arteries,  and 
sinuses  together  form  a  closed  system  through  which  the 
blood  is  propelled  in  a  definite  direction  by  the  contractions 
of  the  heart. 

Renal  excretion  is  performed  by  a  pair  of  glandular 
bodies,  the  kidneys  (A,  K\  situated  in  the  front  part  of  the 
head  and  enclosed  in  spacious  sacs  which  open  by  ducts  on 
the  bases  of  the  antennae.  They  consist  of  convoluted  tubes 
lined  by  epithelium,  and  are  probably  to  be  looked  upon  as 
greatly  modified  nephridia. 


xxvn  ABSENCE  OF   CILIA  319 

The  Crayfish  is  dioecious.  The  ovaries  (ovy)  are  a  pair 
of  hollow  organs,  united  in  the  middle  line  in  some  genera, 
situated  in  the  thorax,  and  opening  by  oviducts  (B,  ovd)  on 
the  bases  of  the  third  pair  of  legs.  The  spermaries  (testes) 
are  also  frequently  united  in  the  middle  line  and  open 
by  spermiducts  (vasa  deferentia;  on  the  bases  of  the  fifth 
pair  of  legs.  There  is  some  reason  for  thinking  that  the 
gonaducts  represent  modified  nephridia,  and  the  cavities 
of  the  hollow  gonads  a  greatly  reduced  ccelome  from  the 
epithelium  of  which  the  sex-cells  are  produced. 

The  nervous  system  is  formed  on  quite  the  same  plan  as 
that  of  Polygordius,  consisting  of  a  dorsal  brain  (Br]  united 
by  cesophageal  connectives  to  a  ventral  nerve-cord  (V. 
Nv.CcT).  In  the  cord,  however,  the  nerve-cells,  instead  of 
being  evenly  distributed,  are  aggregated  into  little  enlarge- 
ments or  ganglia  (Gn],  of  which  there  is  primatively  a  pair 
to  each  metamere,  the  number  being  reduced  in  the  adult 
by  concrescence.  The  portions  of  the  ventral  nerve-cord 
between  the  ganglia  consist  of  nerve-fibres  only,  and  are 
called  connectives.  In  the  embryo  the  nervous  system  is, 
as  in  Polygordius,  in  direct  connection  with  the  epidermis, 
but  in  the  adult  it  has  sunk  inwards  so  as  to  be  entirely 
surrounded  by  mesoderm. 

A  striking  feature  in  the  histology  of  the  Crayfish,  and 
one  in  which  it  agrees  with  the  vast  majority  of  Arthropoda, 
is  the  entire  absence  of  cilia.  Another  peculiarity  —  also 
shared  by  the  greater  part  of  the  phylum  —  is  that  the  sperms 
are  non-motile. 

The  laid  eggs  become  attached  to  the  swimmerets  of  the 
mother,  and  in  this  situation  undergo  their  development.  In 
the  fresh-water  crayfish  the  young  is  hatched  in  a  condition 
closely  resembling  the  adult,  but  in  the  lobster  and  the  sea- 


crayfish  there  is  a  metamorphosis. 


320  THE   FRESH-WATER   MUSSEL  LESS. 

THE   FRESH-WATER  MussEL.1 

The  body  is  bilaterally  symmetrical,  and  is  greatly  com- 
pressed from  side  to  side.  Its  dorsal  margin  is  produced 
into  paired  flaps,  the  mantle-lobes  (Fig.  78,  A  and  B,  Mant\ 
which  pass  downwards  one  on  either  side  of  the  body, 
Closely  applied  to  the  outer  surface  of  the  mantle-lobes,  and 
formed  as  a  cuticular  secretion  of  their  deric  epithelium,  ar  j 
the  two  valves  of  the  bivalved,  strongly  calcined  shell  (B.,  S/i). 
The  ventral  region  of  the  body  is  produced  into  a  laterally 
compressed  muscular  structure,  ihefoot  (A  and  B,  Foot],  by 
the  contraction  of  which  the  animal  can  move  slowly 
through  the  sand  or  mud  in  which  it  lives  partly  buried. 

The  possession  of  a  mantle  formed  as  a  prolongation  of 
the  dorsal  region,  of  a  calcareous  shell  secreted  by  the 
mantle,  and  of  a  muscular  foot  formed  as  an  unpaired 
prolongation  of  the  ventral  region,  are  the  most  characteristic 
features  of  the  Mollusca  generally. 

Posteriorly  the  edges  of  the  mantle-lobes  are  greatly 
thickened,  and  are  connected  with  one  another  in  such  a 
way  as  to  form  two  apertures,  a  large  ventral  inhalent  (Ink. 
Ap\  and  a  small  dorsal  exhalent  aperture  (Exh.  Ap).  By 
means  of  the  cilia  of  the  gills  (see  below)  a  current  of  water 
is  produced  which  enters  at  the  inhalent  aperture,  carrying 
abundant  oxygen  and  the  minute  organisms  used  as  food, 
and  makes  its  escape  at  the  exhalent  aperture,  taking  with  it 
the  various  products  of  excretion  and  faecal  matter. 

The  mouth  (Mth)  is  anterior  and  ventral,  lying  just  in 
front  of  the  foot :  it  is  bounded  on  either  side  by  a  pair  of 

1  For  detailed  descriptions  of  the  fresh-water  Mussel  see  Rolleston 
and  Jackson,  Forms  of  Animal  Life,  pp.  124  and  285  :  Huxley  and 
Martin,  Elementary  Biology ;  p.  305  :  and  Marshall  and  Hurst,  Practical 
Zoology,  p.  80. 


Der. 


Ccel  .Epthm. 
'CceLFpthm- 
Sh 
•Intf 
•JJcr.Eptlnn 


FIG.  78. — Diagrammatic  sections  of  the  Fresh-water  Mussel. 

A,  longitudinal  section  :  the  right  mantle-lobe  (Mant)  and  gills  (/.  G, 
O.  G)  are  shown  in  perspective. 

B,  transverse  section. 

The  cuticular  shell  (Sh},  shown  only  in  B,  is  black,  the  ectoderm 
dotted,  the  nervous  system  finely  dotted,  the  endoderm  radially  striated, 
the  mesoderm  evenly  shaded,  and  the  ccelomic  epithelium  represented 
by  a  beaded  line. 

The  dorsal  region  is  produced  into  the  right  and  left  mantle-lobes 
(Afant),  attached  to  which  are  the  valves  of  the  shell  (Sh)  joined  dorsally 
by  an  elastic  ligament  (tig). 

The  mantle-lobes  are  partly  united  so  as  to  form  the  inhalent  (Ink. 
Ap)  and  exhalent  (Exh.  Ap)  apertures  at  the  posterior  end. 

The  body  is  produced  ventrally  into  the  foot  (Foot),  on  each  side  of 
which  are  the  gills,  an  inner  (/.  G)  and  an  outer  (O.  G),  each  formed 
of  an  inner  and  an  outer  lamella. 

The  body  is  covered  externally  by  cleric  epithelium  (Der.  Epthm), 
within  which  is  mesoderm  (Msd)  largely  differentiated  into  muscles,  of 
which  the  anterior  (A.  Ad)  and  posterior  (P.  Ad)  adductors  are  indi- 
cated in  A. 

The  mouth  (Mth)  leads  by  the  short  gullet  ( Gul)  into  the  stomach 
(St),  from  which  proceeds  the  coiled  intestine  (/«/),  ending  in  the  anus 


322  THE  FRESH-WATER  MUSSEL  LESS. 

(An) :  the  enteric  epithelium  is  mostly  endodermal.  The  digestive  gland 
(D.  67)  surrounds  the  stomach.  The  ccelome  (Ccel)  is  reduced  to  a 
small  dorsal  chamber  enclosing  part  of  the  intestine  and  the  heart  ;  the 
parietal  (Cal.  Epthm)  and  visceral  (Cael.  EpthnP]  layers  of  ccelomic 
epithelium  are  shown. 

The  heart  consists  of  a  median  ventricle  ( Vent],  enclosing  part  of  the 
intestine,  and  of  paired  auricles  (Aur). 

The  paired  nephridia  (Nphm)  open  by  apertures  into  the  coelome 
(Nph.  st)  and  on  the  exterior  (Nph.  p}. 

The  gonads  (Gon)  are  imbedded  in  the  solid  mesoderm,  and  open  on 
the  exterior  by  gonaducts  (Gnd). 

The  nervous  system  consists  of  a  pair  of  cerebro-pleural  ganglia 
(C.  P.  Gn)  above  the  gullet,  a  pair  of  pedal  ganglia  (Pd.  Gn)  in  the 
foot,  and  a  pair  of  visceral  ganglia  ( V.  Gn}  below  the  posterior  adductor 
muscle. 

triangular  bodies,  the  labial  palpi,  and  leads  by  a  short 
gullet  (Gut)  into  a  stomach  (St)  from  which  proceeds  a 
long,  coiled  intestine  (Int) :  this  makes  several  turns  in  the 
ventral  region  of  the  trunk,  then  passes  to  the  dorsal  region, 
and  finally  backwards  in  the  median  plane  to  open  by  an 
anus  (An)  at  the  posterior  end  of  the  body,  just  within  the 
exhalent  aperture.  The  enteric  canal  is  formed  almost 
exclusively  from  the  enteron,  the  stomodseum  and  procto- 
daeum  being  both  insignificant :  hence  the  enteric  epithelium 
is  almost  wholly  endodermal.  There  is  a  large  digestive 
gland  (D.  Gl)  surrounding  the  stomach  and  opening  into 
it  by  several  ducts. 

The  coelome  (Cod)  is  a  small  cavity  in  the  dorsal  region 
containing  a  portion  of  the  intestine  :  the  rest  of  the  enteric 
canal  is  embedded  in  solid  mesoderm. 

The  mesoderm,  as  usual,  is  largely  differentiated  into 
muscle.  There  are  numerous  muscles  connected  with  the 
foot,  and  two  very  large  ones  (A.  Ad,  P.  Ad)  pass  trans- 
versely from  valve  to  valve  of  the  shell,  one  immediately 
above  the  gullet,  the  other  immediately  below  the  anal  end 
of  the  intestine ;  these  latter  are  called  adductors,  and  serve 
to  close  the  shell. 


xxvn  GENERAL  CHARACTERS  323 

On  either  side  of  the  body,  between  the  trunk  and  the 
mantle,  are  two  gills  (/.  G,  O.  G),  each  having  the  form  of  a 
double  plate  (B)  nearly  as  long  as  the  body.  They  serve,  in 
conjunction  with  the  mantle,  as  respiratory  organs,  but  their 
main  function  is  to  produce  the  current  of  water  referred  to 
above  by  means  of  the  cilia  with  which  they  are  covered. 

There  is  an  extensive  system  of  blood-vessels.  The  heart 
lies  in  the  coelome,  and  consists  of  three  chambers,  a  median 
ventricle  ( Ve?it\  which  surrounds  the  intestine,  and  paired 
auricles  (Aur). 

Excretion  is  performed  by  a  single  pair  of  nephridia 
(Nphm)  which  open  at  one  end  (Nph.  st)  into  the  coelome 
and  at  the  other  (Nph.  p)  on  to  the  exterior. 

The  nervous  system  consists  of  three  pairs  of  ganglia,  the 
two  ganglia  of  each  pair  being  united  by  transverse  com- 
missures. The  cerebro-pleural  ganglia  ( C.  P.  Gri)  lie  above 
the  gullet,  and  represent,  in  a  general  way,  the  brain  of 
Polygordius  and  the  crayfish ;  they  are  united  by  longitu- 
dinal connectives  with  the  pedal  ganglia  (P.  Gn},  which  lie 
in  the  foot  and  may  be  taken  as  representing  the  ventral 
nerve-cord  of  worms  and  arthropods,  and  with  the  visceral 
ganglia  (V.  Gn)  which  are  placed  beneath  the  posterior 
adductor  muscle. 

The  gonads  (Goti)  are  large  irregular  organs,  very  similar 
in  appearance  in  the  two  sexes,  situated  among  the  coils  of 
the  intestine  and  opening  by  a  duct  (Gnd)  on  either  side  of 
the  trunk,  close  to  the  nephridiopore.  The  impregnated 
eggs  are  passed  into  the  cavity  of  the  outer  gill  of  the 
female,  where  they  undergo  the  early  stages  of  their  develop- 
ment. The  larva  of  the  fresh-water  mussel  is  a  peculiar 
bivalved  form,  very  unlike  the  adult,  and  called  %.glochidium  ; 
but  in  the  more  typical  molluscs  the  embryo  leaves  the  egg 
as  a  trochosphere,  closely  resembling  that  of  Polygordius. 

Y  2 


324  THE  DOG-FISH  LESS. 

THE  Doc-FiSH.1 

A  dog-fish  is  bilaterally  symmetrical,  the  nearly  cylin- 
drical body  (Fig.  79,  A)  terminating  in  front  in  a  blunt 
snout  and  behind  passing  insensibly  into  an  upturned  tail. 
Externally  there  is  no  appearance  of  segmentation. 

The  mouth  (MtJi)  is  on  the  ventral  surface  of  the  head 
or  anterior  region  of  the  body  ;  it  is  transversely  elongated, 
and  is  supported  by  jaws  which  are  respectively  anterior 
(upper)  and  posterior  (lower).  They  thus  differ  funda- 
mentally from  the  jaws  of  arthropods,  which  are  modified 
appendages  and  are  therefore  disposed  right  and  left. 

A  short  distance  behind  the  mouth  are  five  vertical  slits 
(B,  Ext.  br.  ap)  arranged  in  a  longitudinal  series,  the 
external  branchial*  apertures  or  gill-clefts.  The  vent,  or 
cloacal  aperture  (An)  is  situated  on  the  ventral  surface  a 
considerable  distance  from  the  end  of  the  tail.  That  part 
of  the  body  lying  in  front  of  the  last  gill-cleft  is  counted  as 
the  head,  all  behind  the  vent  as  the  tail,  the  intermediate 
portion  as  the  trunk. 

Appendages  are  present,  but  in  a  very  different  form  from 
those  of  the  crayfish.  They  consist  of  flat  processes  of  the 
body-wall  called  fins.  Two  of  them  (D.F^,D.  F*)  are 
situated  in  the  middle  line  of  the  back  (dorsal  fins)  :  one 
(V.F)  in  the  middle  ventral  line  behind  the  cloacal  aperture 
(ventral fin\  and  one  (C.F)  is  attached  to  the  up-turned  end 
of  the  tail  (caudal fin)  :  all  these  being  unpaired  structures  or 
median  fins.  Then  there  is  a  pair  of  pectoral  fins  situated 

1  For  a  detailed  description  of  a  dog-fish  see  Marshall  and  Hurst, 
Practical  Zoology  (London,  1892),  p.  206.  For  descriptions  of  other 
fishes,  equally  suitable  in  some  respects  as  types  of  Vertebrata,  see 
Rolleston  and  Jackson,  Forms  of  Animal  Life  (Oxford,  1888),  pp.  83 
and  273  :  and  Parker,  Zoototny  (London,  1884),  pp.  I,  27,  86. 


xxvn  GENERAL   CHARACTERS  325 

one  on  each  side  just  behind  the  last  gill-cleft,  and  a  pair  of 
pelvic  fins  placed  one  on  either  side  of  the  vent :  these  are 
the  lateral  or  paired  fins.  It  is  characteristic  of  Vertebrata 
that  the  number  of  lateral  appendages  never  exceeds  two 
pairs. 

The  skin  or  external  layer  of  the  body-wall  consists  of  an 
outer  epidermis  (Der.  Epthni)  composed  of  several  layers  of 
cells,  and  of  an  inner  connective  tissue  layer  or  dermis 
(Derm).  In  the  latter  are  found  innumerable  bony  scales 
(Derm.  Sp)  constituting  a  dermal  exoskeleton.  The  muscular 
layer  of  the  body-wall  ( M)  is  of  great  thickness,  especially 
in  the  dorsal  region,  and  is  distinctly  segmented,  indicating 
that  the  body  of  the  dog-fish,  like  that  of  Polygordius  and 
the  crayfish,  is  divisible  into  metameres,  although  there  is  no 
indication  of  them  externally. 

The  large  ccelome  (Cat)  is  confined  to  the  trunk  :  it  is 
characteristic  of  vertebrates  that  both  head  and  tail  are 
acoelomate  in  the  adult.  The  coelomic  epithelium  (Cal. 
Epthm,  Ccel.  Mpthm')  is  underlaid  by  a  distinct  layer  of 
connective  tissue,  the  two  together  forming  the  peritoneum. 

Another  important  vertebrate  character  is  that  the  dorsal 
region  of  the  body-wall  contains  a  median  longitudinal 
canal  (C.  Sp.  Cav.)  extending  from  shortly  behind  the  snout 
to  near  the  end  of  the  tail.  This  is  the  cerebro-spinal  cavity 
and  contains  the  central  nervous  system. 

Still  another  characteristic  feature  is  the  presence,  in 
addition  to  the  dermal  exoskeleton,  of  an  endoskeleton,  or 
system  of  internal  supporting  structures.  Between  the 
cerebro-spinal  cavity  above  and  the  ccelome  below  is  a 
longitudinal  series  of  biconcave  discs  or  vertebral  centra 
(V.  Cent)  :  they  are  formed  of  a  peculiar  tissue  called 
cartilage  or  gristle,  and  are  strongly  impregnated  with  lime- 
salts  :  in  the  young  condition  their  place  is  occupied  by  a 


LESS,  xxvii  GENERAL   CHARACTERS  327 

A,  longitudinal  vertical  section. 

B,  horizontal  section  through  the  pharynx  and  gills, 
c,  transverse  section  through  the  trunk. 

The  ectoderm  is  dotted,  the  nervous  system  finely  dotted,  the  endo- 
derm  radially  striated,  the  mesoderm  evenly  shaded,  the  ccelomic 
epithelium  represented  by  a  beaded  line,  and  all  skeletal  structures 
black. 

The  body  gives  origin  to  the  dorsal  (D.  F1,  D.  F\  ventral  (V.  F), 
and  caudal  (C.  F}  fins  ;  the  paired  fins  are  not  shown. 

The  body- wall  consists  of  deric  epithelium  (Der.  Epthm},  dermis 
Derm},  and  muscle  (M}  :  the  latter  is  metamerically  segmented  and  is 
very  thick,  especially  dorsally,  where  it  forms  half  the  total  vertical 
height  (c). 

The  exoskeleton  consists  of  calcified  dermal  spines  {Derm.  Sp}  in  the 
dermis,  and  of  dermal  fin-rays  (Derm.  F.  R}  in  the  fins. 

The  endoskeleton  consists  of  a  row  of  vertebral  centra  ( V.  Cent}  below 
the  spinal  cord  (Sp.  Cd},  giving  rise  to  neural  arches  (N.  A),  which  enclose 
the  cord,  and  in  the  caudal  regions  to  haemal  arches  (ff.  A.) :  a  cranium 
(Cr}  enclosing  the  brain  (Br}  :  upper  and  lower  jaws  :  branchial  arches 
(Br.  A)  and  rays  (Br.  J?,  Br.  R'),  shown  only  in  B,  supporting  the 
gills  :  shoulder  (Sh.  G)  and  pelvic  (Pelv.  G)  girdles  :  and  pterygiosphores 
(Ptgph}  supporting  the  fins. 

The  mouth  (Mth}  leads  into  the  oral  cavity  (Or.  cav},  from  which  the 
pharynx  (Ph}  and  gullet  (Gul)  lead  to  the  stomach  (St}  :  this  is  con- 
nected with  a  short  intestine  (Tnt}  opening  into  a  cloaca  (C!)  which 
communicates  with  the  exterior  by  the  vent  (An}.  The  oral  cavity  and 
cloaca  are  the  only  parts  of  the  canal  lined  by  ectoderm. 

Connected  with  the  enteric  canal  are  the  liver  (Lr)  with  the  gall- 
bladder (G,  Bl)  and  bile-duct  (B.  D),  the  pancreas  (/*«),  and  the  spleen 
(Spl).  The  mouth  is  bounded  above  and  below  by  teeth  (T). 

The  respiratory  organs  consist  of  pouches  (shown  in  B)  communicating 
with  the  pharynx  by  internal  (Int.  br.  ap}  and  with  the  exterior  by 
external  (Ext.  br.  ap}  branchial  apertures,  and  lined  by  mucous  mem- 
brane raised  into  branchial  filaments  (Br.  Fil}. 

The  heart  (Ht}  is  ventral  and  anterior,  and  is  situated  in  a  special 
compartment  of  the  ccelome  (Pcd}.  Six  of  the  most  important  blood- 
vessels, the  dorsal  vessel  (dorsal  aorta,  D.  Ao},  the  cardinal  veins 
(Card.  V),  the  lateral  vessels  (lateral  veins,  Lat.  F),  and  the  ventral 
vessel  (intra-intestinal  vein,  /.  int.  V}  are  shown  in  C. 

The  whole  ccelome  is  lined  by  epithelium,  showing  parietal  (Ccel. 
Epthm}  and  visceral  (Ccel.  Epthm'}  layers. 

The  ovaries  (Ovy}  are  connected  with  the  dorsal  body-wall :  the 
oviducts  ( Ovd}  open  anteriorly  into  the  ccelome  (ovd'}  and  posteriorly 
into  the  cloaca. 

The  kidneys  (K}  are  made  up  of  nephridia  (Nph}  and  open  by  ureters 
( Ur}  into  the  cloaca. 

The  nervous  system  is  lodged  in  the  cerebro-spinal  cavity  ( C.  Sp.  Cav} 
hollowed  out  in  the  dorsal  body- wall :  it  consists  of  brain  (Br}  and 
spinal  cord  (Sp.  Cd},  and  contains  a  continuous  cavity,  the  neuroccele 
n.  cce}. 


328  THE   DOG-FISH  LESS. 

gelatinous  rod,  the  notochord.  The  centra,  which  alternate 
with  the  muscle-segments,  are  connected  with  a  series  of 
cartilaginous  arches  (N.A),  which  extend  over  the  cerebro- 
spinal  cavity  and  with  the  centra  constitute  the  vertebral 
column.  In  the  tail  there  is  also  a  ventral  series  of  arches 
(ff.A.}  enclosing  a  space  (ff.  C)  which  indicates  a  backward 
extension  of  the  ccelome  in  the  embryo. 

Anteriorly  the  vertebral  column  is  continued  into  a 
cartilaginous  box,  the  cranium  ( Cr)  which  encloses  the  brain 
and  the  organs  of  smell  and  hearing.  The  jaws,  referred  to 
above,  are  cartilaginous  rods  which  bound  the  mouth  above 
and  below.  The  gills  are  supported  by  a  complicated 
system  of  cartilages  (Br.  A,  Br.  R.,  Br.  R'\  and  both 
median  and  paired  fins  by  parallel  rods  of  the  same 
material  (Ptgpli).  All  these  cartilages  are  strengthened 
by  a  more  or  less  extensive  superficial  deposit  of  bony 
matter. 

The  mouth  (Mtti)  leads  into  a  large  oral  cavity  (Or.  cav) 
which  passes  insensibly  into  a  wide  throat  or  pharynx  (P/i) : 
from  this  a  short  gullet  (Gul)  leads  into  a  large  U-shaped 
stomach  (St),  whence  is  continued  a  short  wide  intestine 
(Int)  opening  on  to  the  exterior  through  the  intermediation 
of  a  small  chamber,  the  cloaca  (C!).  From  the  gullet 
backwards  the  enteric  canal  is  contained  in  the  ccelome. 
The  greater  part  of  the  enteric  epithelium  is  endodermal  : 
only  the  oral  cavity  arises  from  the  stomodaeum  and  the 
cloaca  from  the  proctodaeum. 

In  the  skin  covering  the  jaws  dermal  ossicles  of  unusual 
size  are  developed  and  constitute  the  teeth  (T).  The  chief 
digestive  glands  are  two  in  number,  an  immense  liver  (Lr) 
occupying  the  whole  anterior  and  ventral  region  of  the 
coelome,  and  a  small  pancreas  (Pn),  attached  to  the  anterior 
end  of  the  intestine.  The  ducts  of  both  glands  open  into 


GILLS   AND   HEART  329 

the  intestine,  and  their  secreting  cells  are,  as  in  former  cases, 
endodermal.  Gland-cells  are  also  found  in  the  walls  of 
the  stomach  and  intestine. 

The  respiratory  organs  or  gills  (B)  consist  of  five  pairs  of 
pouches  opening  on  the  one  hand  into  the  pharynx  (Ph) 
and  on  the  other  to  the  exterior  by  the  branchial  clefts 
already  noticed  :  they  have  their  walls  raised  into  ridges, 
the  branchial  filaments  (Br.  Fit),  which  are  covered  with 
epithelium  (Resp.  Epthni]  and  are  abundantly  supplied 
with  blood-vessels.  The  gills  are  developed  as  offshoots  of 
the  pharynx,  and  the  respiratory  epithelium  is  therefore 
endodermal,  not  ectodermal  as  in  the  crayfish  and  mussel. 

The  heart  (H£)  lies  below  the  pharynx  in  a  separate 
anterior  compartment  of  the  coelome,  the  pericardial  cavity. 
It  is  composed  of  four  chambers  arranged  in  a  single  longi- 
tudinal series  (sinus  venosus,  auricle,  ventricle,  and  conus 
arteriosus),  and  is  to  be  looked  upon  as  a  muscular  dilatation 
of  a  ventral  blood-vessel.  The  blood  is  propelled  by  the 
heart  from  the  conus  arteriosus  into  a  paired  series  of 
hoop-like  vessels  (aortic  arches)  resembling  the  transverse 
commissures  of  Polygordius  (Fig.  69,  A,  p.  282),  which  take 
it  through  the  gills  and  pour  it,  in  a  purified  condition,  into 
the  dorsal  vessel  (dorsal  aorta,  D.  Ao)  whence  it  is  taken  to 
all  parts  of  the  body  to  be  finally  returned  by  thin-walled 
vessels,  called  veins,  to  the  sinus  venosus.  The  ventral 
position  of  the  heart  and  the  fact  that  the  blood  is  sent 
directly  from  the  heart  to  the  respiratory  organs  are 
characteristic  vertebrate  features  :  so  also  is  the  circumstance 
that  the  blood  from  the  stomach,  intestine,  &c.,  is  taken  by 
a  specially  modified  portion  of  the  ventral  vessel  (portal 
vein)  through  the  liver  on  its  way  to  the  heart.  The  blood 
is  red,  containing,  in  addition  to  leucocytes,  oval  corpuscles 
coloured  by  haemoglobin  (see  p.  58). 


330  THE   DOG-FISH  LESS. 

The  excretory  organs  are  a  pair  of  kidneys  (K}  situated 
at  the  posterior  end  of  the  dorsal  region  of  the  coelome,  and 
opening  by  ducts,  the  ureters  (Ur\  into  the  cloaca.  De- 
velopment shows  that  they  consist  of  an  aggregation  of 
nephridia  (Nph\  the  nephrostomes  of  which  open  in  the 
young  and  sometimes  throughout  life,  into  the  ccelome, 
while  the  nephridiopores  discharge  not  directly  on  the 
exterior,  but  into  a  common  tube. 

The  gonads  (ovaries,  Ovy,  or  spermaries)  are  situated  in 
the  anterior  part  of  the  ccelome,  attached  by  peritoneum 
to  its  dorsal  wall.  The  sex-cells  are  differentiated  from 
ccelomic  epithelium.  The  gonaducts  of  both  sexes 
(Ovd}  are  developed  from  the  nephridial  system  of 
the  embryo. 

As  already  stated,  the  central  nervous  system  is  contained 
in  a  cavity  (C.  Sp.  Cav)  of  the  dorsal  body-wall,  and  is 
therefore  far  removed  from  the  ectoderm  from  which  it 
originates.  It  consists  of  a  long  cylindrical  rod,  the  spinal 
cord(Sp.  Cd}  which  is  continued  in  front  into  a  complicated 
brain  (£r).  It  has  the  further  peculiarity  of  being  hollow, 
a  more  or  less  cylindrical  cavity,  the  neurocoele  (n.  cce)  ex- 
tending through  its  whole  length. 

The  possession  of  a  hollow  nervous  system  lying  altogether 
dorsal  to  the  enteric  canal  and  ccelome,  of  either  a  noto- 
chord  or  a  chain  of  vertebral  centra  below  the  nervous 
system,  and  of  pharyngeal  pouches  communicating  with  the 
exterior,  are  the  three  most  characteristic  features  of  the 
vertebrate  phylum. 

The  organs  of  sense  are  highly  developed,  and  consist  of 
paired  olfactory  sacs,  eyes,  and  auditory  sacs  situated  in  the 
head,  together  with  an  extensive  system  of  integumentary 
organs.  Their  sensory  cells  are  in  every  case  ectodermal. 

The  eggs  are  very  large,  and  are  impregnated  within  the. 


xxvii  DEVELOPMENT  331 

body  of  the  female.  In  the  common  Dog-fish  (Scy  Ilium) 
they  are  laid  shortly  after  impregnation,  each  enclosed  in  a 
horny  egg-shell :  in  the  Piked  Dog-fish  (Acanthias)  and  the 
Smooth  Hound  (Mustelus)  they  are  retained  in  the  oviduct 
until  the  adult  form  is  assumed. 


LESSON  XXVIII 

MOSSES. 

IN  the  four  previous  lessons  we  have  traced  the  advance 
in  organization  of  animals  from  the  simple  diploblastic 
Hydra  to  the  complicated  triploblastic  forms  which  con- 
stitute the  five  higher  phyla  of  the  animal  kingdom.  We 
have  now  to  follow  in  the  same  way  the  advance  in  structure 
of  plants.  The  last  member  of  the  vegetable  kingdom  with 
which  we  were  concerned  was  Nitella  (p.  206),  a  solid 
aggregate,  exhibiting  a  certain  differentiation  of  form  and 
structure,  but  yet  composed  of  what  were  clearly  recognizable 
as  cells,  there  being,  as  in  Hydra,  none  of  that  formation  of 
well-marked  tissues  which  is  so  noticeable  a  feature  in 
Polygordius  as  in  other  animals  above  the  Ccelenterata. 

Taking  Nitella  as  a  starting  point,  we  shall  see  that  among 
plants,  as  among  animals,  there  is  an  increasing  differentiation 
in  structure  and  in  function  as  we  ascend  the  series.  The 
first  steps  in  the  process  are  well  illustrated  by  a  considera- 
tion of  that  very  abundant  and  beautiful  group  of  plants,  the 
Mosses.  In  spite  of  the  variations  in  detail  met  with  in 
different  genera  of  the  group,  the  essential  features  of  their 
organization  are  so  constant  that  the  following  description 
will  be  found  to  apply  to  any  of  the  common  forms. 


FIG.  80. — The  Anatomy  and  Histology  of  Mosses. 

A,  Entire  plant  of  Funaria  hygrometrica,  showing  stem  (sf),  leaves 
(/),  and  rhizoids  (rh).     (x  6.) 

B,  leaf  of  the  same,  showing  midrib  (tnd.  r)  and   lateral  portions, 
(x  25.) 


334  MOSSES  LESS. 

C,  semi-diagrammatic  vertical  section  of  a  moss,  showing  the  arrange- 
ment of  the  tissues.     The  stem  is  formed  externally  of  sclerenchyma 
(set),  and  contains  an  axial  bundle  (ax.  b) :  in  some  of  the  leaves  (/) 
the  section  passes  through  the  midrib,  in  others  (/')  through  the  lateral 
portion  :  the  stem  ends  distally  in  an  apical  cell  (ap.  <r),  from  which 
segmental  cells  (seg.  c]  are  separated. 

D,  transverse  section  of  the  stem  of  Brytun  roseum,  showing  scleren- 
chyma (scl),  axial  bundle  (ax.  b},  and  rhizoids  (rh).     (  x  60.) 

E,  transverse  section  of  a  leaf  of  Funaria,  showing  the  midrib  (md.  r) 
formed  of  several  layers  of  cells,  and  the  lateral  portions  one  cell  thick, 
(x    150.) 

F,  small  portion  of  the  lateral  region  of  the  same,  showing  the  form 
of  the  cells  and  the  chromatophores  (chr).     (x   150.) 

G,  distal  end  of  the  stern  of  Fontinalis  antipyretica  in  vertical  section, 
showing  the  apical  cell  (ap.  c}  giving  rise  to  segmental  cells  (seg.  c), 
which  by  subsequent  division  form  the  segments  of  the  stem  with  the 
leaves  :  the  thick  lines  show  the  boundaries  of  the  segments. 

H,  diagram  of  the  apical  cell  of  a  moss  in  the  form  of  a  tetrahedron 
with  rounded  base  abc  and  three  flat  sides  abd,  bcd>  acd. 
(D  after  Sachs  ;  G  after  Leitgeb. ) 

The  plant  consists  of  a  short  slender  stem  (Fig.  80,  A,  st), 
from  which  are  given  off  structures  of  two  kinds,  rhizoids  or 
root-hairs  (rh),  which  pass  downwards  into  the  soil,  and  leaves 
(/),  which  are  closely  set  on  the  stem  and  its  branches.  As 
in  Nitella  (p.  208)  the  portion  of  the  stem  from  which  a  leaf 
arises  is  called  a  node,  and  the  part  intervening  between  any 
two  nodes  an  internode,  while  the  name  segment  is  applied 
to  a  node  with  the  internode  next  below  it.  At  the  upper  or 
distal  end  of  the  stem  the  leaves  are  crowded,  forming  a 
terminal  bud. 

Owing  to  the  opacity  of  the  stem,  its  structure  can  only  be 
made  out  by  the  examination  of  thin  sections  (c  and  D).  It  is 
a  solid  aggregate  of  close-set  cells  which  are  not  all  alike,  but 
exhibit  a  certain  amount  of  differentiation.  In  the  outer 
two  or  three  rows  the  cells  (scl)  are  elongated  in  the  direction 
of  the  length  of  the  stem,  so  as  to  have  a  spindle-shape,  and 
their  walls  are  greatly  thickened  and  of  a  reddish  colour. 
They  thus  form  a  protective  and  supporting  tissue,  to  which 
the  name  sclerenchyma  is  applied.  Running  longitudinally 


xxviii  TERMINAL  BUD  335 

through  the  centre  of  the  stem  is  a  mass  of  tissue  (ax.  b] 
distinguished  by  its  small,  thin-walled  cells,  and  constituting 
the  axial  bundle. 

The  leaves  (B)  are  shaped  like  a  spear-head,  pointed 
distally,  and  attached  proximally  by  a  broad  base  to  the 
stem.  The  axial  portion  (B  and  E,  md.  r.,  c.  /)  consists  of 
several  layers  of  somewhat  elongated  cells  and  is  called  the 
midrib :  the  lateral  portions  (E  and  F  :  c,  /')  are  formed  of  a 
single  layer  of  short  cells.  Thus  the  leaf  has,  for  the  most 
part,  the  character  of  a  superficial  aggregate.  The  cells 
contain  oval  chromatophores  (F,  chr). 

The  rhizoids  (c  and  D,  rh)  are  linear  aggregates,  being 
formed  of  elongated  cells,  devoid  of  chlorophyll,  arranged 
end  to  end. 

In  the  terminal  bud  the  leaves,  as  in  Nitella  (pp.  208  and 
210),  arch  over  the  growing  point  of  the  stem,  which  in  this 
case  also  is  formed  of  a  single  apical  cell  (c  and  G,  ap.  c). 
But  in  correspondence  with  the  increased  complexity  of  the 
plant,  the  apical  cell  is  not  a  hemisphere  from  which  new 
segments  are  cut  off  parallel  to  its  flat  base,  but  has  the  form 
(H)  of  an  inverted,  three-sided  pyramid  or  tetrahedron,  the 
rounded  base  of  which  (abc)  forms  the  apex  of  the  stem 
while  segments  (seg.  c)  are  cut  off  from  each  of  its  three 
triangular  sides  in  succession. 

The  best  way  to  understand  the  apical  growth  of  a  moss 
is  to  cut  a  tetrahedron  with  rounded  base  out  of  a  carrot  or 
turnip  :  this  represents  the  apical  cell  (H)  :  then  cut  off  a 
slice  parallel  to  the  side  abd,  a  second  parallel  to  bed,  and  a 
third  parallel  to  acd :  these  represent  three  successively 
formed  segments.  Now  imagine  that  after  every  division 
the  tetrahedron  grows  to  its  original  size,  and  a  very  fair 
notion  will  be  obtained  of  the  way  in  which  the  successive 
segments  of  the  moss-stem  are  formed  by  the  fission  in  three 


336  MOSSES  LESS. 

planes  of  the  apical  cell.  Each  segment  (c  and  G,  seg.  c) 
immediately  after  its  separation  divides  and  subdivides,  pro- 
ducing a  mass  of  cells  from  which  a  projection  grows  out 
forming  a  leaf,  and  in  this  way  the  stem  increases  in  length 
and  the  leaves  in  number. 

Asexual  reproduction  takes  place  in  various  ways  :  all  of 
them  are,  however,  varieties  of  budding,  and  the  buds  always 
arise  in  the  form  of  a  linear  aggregate  of  cells  called  a 
protonema :  from  this  the  moss-plant  develops  in  the  same 
way  as  from  the  protonema  arising  from  a  spore  (p.  339). 

The  gonads  are  developed  at  the  extremity  of  the  main 
stem  or  one  of  its  branches,  and  are  enclosed  in  a  tuft  of 
leaves  often  of  a  reddish  colour — the  terminal  bud  of  the 
fertile  shoot  or  so-called  "flower"  of  the  moss. 

The  spermary  (Fig.  81,  A1,  A2)  is  an  elongated  club-shaped 
body  consisting  of  a  solid  mass  of  cells,  the  outermost  of 
which  form  the  wall  of  the  organ,  while  the  inner  (AS)  become 
converted  into  sperms.  The  latter  (A4)  are  spirally  coiled 
and  provided  with  two  cilia :  they  are  liberated  by  the 
rupture  of  the  wall  of  the  spermary  at  its  distal  end  (A2). 

The  ovaries  1  (see  Preface,  p.  x,  and  p.  381)  (s1,  B2)  may 
or  may  not  occur  on  the  same  plant  as  the  spermaries,  some 
mosses  being  monoecious,  others  dioecious.  Like  the  sperm- 
aries, they  consist  at  first  of  a  solid  mass  of  cells  which 
assumes  the  form  of  a  flask,  having  a  rounded  basal  portion 
or  venter  (v)  and  a  long  neck  («).  The  outer  layer  of  cells 
in  the  neck  and  the  two  outer  layers  in  the  venter  form  the 
wall  of  the  ovary,  the  internal  cells  are  arranged  in  a  single 
axial  row  at  first  similar  to  those  of  the  wall.  As  the  ovary 
develops,  the  proximal  or  lowermost  cell  of  the  axial  row 

1  The  ovary  of  mosses,  ferns,  &c.,  is  usually  called  an  archegonium  : 
the  spermary,  as  in  the  lower  plants,  an  antheridium. 


DEVELOPMENT   OF   SPOROGONIUM  337 

takes  on  the  character  of  an  ovum  (e2,  ov] ;  the  others,  called 
canal  cells  (en.  c)  are  converted  into  mucilage,  which  by  its 
expansion  forces  open  the  mouth  of  the  flask  and  thus  makes 
a  clear  passage  from  the  exterior  to  the  ovum  (BS). 

Through  the  passage  thus  formed  a  sperm  makes  it  way 
and  conjugates  with  the  ovum,  producing  as  usual  ah 
oosperm  or  unicellular  embryo. 

The  development  of  the  embryo  is  at  first  remarkably 
like  what  we  have  found  to  take  place  in  Hydroids  (p.  248). 
The  oosperm  divided  into  two  cells  by  a  wall  at  right  angles 
to  the  long  axis  of  the  ovary  :  each  of  these  cells  divides 
again  repeatedly,  and  there  is  produced  a  solid  multicellular 
embryo  or  polyplast  (c1,  spgnm). 

Very  early,  however,  the  moss-polyplast  exhibits  a  striking 
difference  from  the  animal  polyplast  or  morula  :  one  of  its 
cells — that  nearest  the  neck  of  the  ovary — takes  on  the 
character  of  an  apical  cell,  and  begins  to  form  fresh  seg- 
ments like  the  apical  cell  of  the  stem.  Thus  the  plant 
embryo  differs  almost  from  the  first  from  the  animal  embryo. 
In  the  animal  there  is  no  apical  cell  :  all  the  cells  of  the 
polyplast  divide  and  take  their  share  in  the  formation  of  the 
permanent  tissues.  In  the  plant  one  cell  is  at  a  very  early 
period  differentiated  into  an  apical  cell,  and  from  it  all  cells 
thereafter  produced  are,  directly  or  indirectly,  derived. 

The  embryo  continues  to  grow,  forming  a  long  rod-like 
body  (c2,  spgnni)  the  base  of  which  becomes  sunk  in  the 
tissue  of  the  moss-stem,  while  its  distal  end  projects  vertically 
upwards,  covered  by  the  distended  venter  (v)  of  the  ovary. 
Gradually  it  elongates  more  and  more  and  its  distal  end 
dilates  :  the  embryo  has  now  become  a  sporogonium,  con- 
sisting of  a  slender  stalk  (c4,  sf]  bearing  a  vase-like  capsule 
or  urn  (u)  at  its  distal  end.  In  the  meantime  the  elonga- 
tion of  the  stalk  has  caused  the  rupture  of  the  enveloping 


FIG.  81. — Reproduction  and  Development  of  Mosses. 
A1,  A  spermary  of  Funaria  in  optical  section,  showing  the  wall  en- 
closing a  central  mass  of  sperm-cells  :  A2,  the  same  from  the  surface 
discharging  its  sperms.     (  x  300. ) 


LESS,  xxvni  PROTONEMA  339 

A3,  a  sperm-cell  with  enclosed  sperm  :  A4,  a  free-swimming  sperm. 
(x  800.) 

B1,  an  ovary  of  Funaria,  surface  view,  showing  venter  (v)  and  neck 
(n)  :  B-,  the  same  in  optical  section,  showing  ovum  (ov)  and  canal  cells 
(en.  c) :  B3,  the  same  after  disappearance  of  the  canal  cells  :  the  neck  is 
freely  open,  and  the  ovum  (ov}  exposed.  (  x  200. ) 

C1,  ovary  with  withered  neck  containing  an  embryo  (spgnni)  in  the 
polyplast  stage  (  x  200) :  in  C2  the  ovary,  consisting  of  swollen  venter  (v) 
and  shrivelled  neck  (n),  encloses  a  young  sporogonium  (spgnni) ;  the 
distal  end  of  the  stem  is  shown  with  bases  of  leaves  (/)  ;  in  c3  the  venter 
has  ruptured,  forming  a  proximal  portion  or  sheath  and  a  distal  portion 
or  calyptra  which  is  carried  up  by  the  growth  of  the  sporogonium. 
(x  10.) 

C4,  a  small  plant  of  Funaria  with  ripe  sporogonium  consisting  of  seta 
(st),  with  urn  («)  and  lid  (/)  covered  by  the  calyptra  (c}. 

C5,  diagrammatic  vertical  section  of  urn  («),  showing  lid  (/),  air  spaces 
(a),  and  spores  (sp). 

D1,  a  germinating  spore  of  Funaria,  showing  ruptured  outer  coat  (sp) 
and  young  protonema  (pr)  with  rhizoid  (rh).  (  x  550.) 

D2,  portion  of  protonema  of  the  same,  showing  lateral  bud  (bd),  from 
which  the  leafy  plant  arises.  (  x  90. ) 

(A  and  D  after  Sachs  ;  B,  c1,  and  C5  altered  from  Sachs.) 

venter  of  the  ovary  (c3)  :  its  proximal  part  remains  as  a  sort 
of  sheath  round  the  base  of  the  stalk,  while  its  distal  portion, 
with  the  shrivelled  remains  of  the  neck  (n),  is  carried  up  by 
the  elongation  of  the  sporogonium  and  forms  an  extinguisher- 
like  cap  or  calyptra  (c4,  c}  over  the  urn. 

As  development  goes  on,  the  distal  end  of  the  urn  be- 
comes separated  in  the  form  of  a  lid  (c4,  c5,  /),  and  certain 
of  the  cells  in  its  interior,  called  spore-mother  cells,  divide 
each  into  four  daughter  cells,  which  acquire  a  double  cell- 
wall  and  constitute  the  spores  (c5,  sp)  of  the  moss. 

When  the  spores  are  ripe  the  calyptra  falls  off  or  is  blown 
away  by  the  wind,  the  lid  separates  from  the  urn,  and  the 
spores  are  scattered. 

In  germination,  the  protoplasm  of  the  spore  covered  by 
the  inner  layer  of  the  cell-wall  protrudes  through  a  split  in 
the  outer  layer  (D1,  sp)  and  grows  into  a  long  filament,  the 
protonema  (pr.},  divided  by  oblique  septa  into  a  row  of  cells. 
The  protonema — which  it  will  be  observed  is  a  simple  linear 

z  2 


340  MOSSES  LESS. 

aggregate — branches,  and  may  form  a  closely- matted  mass 
of  filaments.  Sooner  or  later  small  lateral  buds  (o2,  bd) 
appear  at  various  places  on  the  protonema  :  each  of  these 
takes  on  the  form  of  a  three-sided  pyramidal  apical  cell, 
which  then  proceeds  to  divide  in  the  characteristic  way 
(p.  335),  forming  three  rows  of  segments  from  which  leaves 
spring.  In  this  way  each  lateral  bud  of  the  protonema  gives 
rise  to  a  moss-plant. 

Obviously  we  have  here  a  somewhat  complicated  case  of 
alternation  of  generations  (see  p.  220).  The  gamobium  or 
sexual  generation  is  represented  by  the  moss-plant,  which 
originates  by  budding  and  produces  the  sexual  organs,  while 
the  agamobium  consists  of  the  sporogonium,  developed  from 
the  oosperm  and  reproducing  by  means  of  spores.  The 
protonema,  arising  from  a  spore  and  producing  the  leafy 
plant  by  budding,  is  merely  a  stage  of  the  gamobium. 

The  nutrition  of  mosses  is  holophytic  ;  but  there  is  a 
striking  differentiation  of  function  correlated  with  terrestrial 
habits.  In  Nitella  the  entire  organism  is  submerged  in 
water  and  all  the  cells  contain  chlorophyll,  so  that  decom- 
position of  carbon  dioxide  and  absorption  of  an  aqueous 
solution  of  salts  are  performed  by  all  parts  alike,  every 
cell  being  nourished  independently  of  the  rest.  In  the 
moss,  on  the  other  hand,  the  rootlets  are  removed  from 
the  influence  of  light  and  contain  no  chlorophyll :  hence 
they  cannot  decompose  carbon  dioxide ;  but,  being  sur- 
jounded  by  moist  soil,  are  in  the  most  favourable  position 
for  absorbing  water  and  mineral  salts.  The  stem,  again,  is 
converted  into  an  organ  of  support :  the  thickness  of  its 
external  cells  prevents  absorption  and  it  contains  no 
chlorophyll.  Hence  the  function  of  decomposing  carbon 
dioxide  is  confined  to  the  leaves. 


xxvin        DISTRIBUTION   OF    FOOD-MATERIALS  341 

We  have  thus  as  an  important  fact  in  the  nutrition  of  an 
ordinary  terrestrial  plant  that  its  carbon  is  taken  in  at  one 
place,  its  water,  nitrogen,  sulphur,  potassium,  &c.,  at  another. 
But  as  all  parts  of  the  plant  require  all  these  substances  it  is 
evident  that  there  must  be  some  means  by  which  the  root 
can  obtain  a  supply  of  carbon,  and  the  leaves  a  supply  of 
elements  other  than  carbon.  In  other  words,  we  find  for 
the  first  time  in  the  ascending  series  of  plants,  just  as  we 
did  in  ascending  from  the  simple  Hydra  to  the  complex 
Polygordius  (p.  281)  the  need  for  some  contrivance  for  the 
distribution  of  food-materials. 

The  way  in  which  this  distributing  process  is  performed 
has  been  studied  chiefly  in  the  higher  plants,  but  its  essential 
features  are  probably  the  same  for  mosses. 

Water  is  continually  evaporating  from  the  surface  of  the 
leaves,  its  place  being  as  constantly  supplied  by  water — with 
salts  in  solution  — taken  in  by  the  rhizoids.  This  trans- 
piration, or  the  giving  off  of  water  from  the  leaves,  is  one 
important  factor  in  the  process  under  consideration,  since 
it  ensures  a  constant  upward  current  of  water,  or,  more 
accurately,  of  an  aqueous  solution  of  mineral  salts.  The 
withering  of  a  plucked  moss-plant  is  of  course  due  to  the 
fact  that  when  the  roots  are  not  embedded  in  moist  soil  or 
in  water,  transpiration  is  no  longer  balanced  by  absorption.1 
In  the  higher  plants  it  has  been  found  that  the  root-hairs 
have  an  absorbent  action  independent  of  transpiration,  so 
that  water  may  be  absorbed  in  the  absence  of  leaves. 

By  the  transpiration  current,  then,  the  leaves  are  kept 
constantly  supplied  with  a  solution  of  mineral  salts  derived 
from  the  soil,  and  are  thus  nourished  like  any  of  the  aquatic 
green  plants  considered  in  previous  lessons  :  by  the  double 

1  Mosses,  however,  unlike  most  higher  plants,  can  absorb  water  by 
their  leaves. 


342  MOSSES  LESS. 

decomposition  of  water  and  carbon  dioxide  a  carbo-hydrate 
is  formed  :  this,  by  further  combination  with  the  nitrogen 
of  the  absorbed  ammonium  salts  or  nitrates,  forms  simple 
nitrogenous  compounds,  and  from  these,  probably  through 
a  long  series  of  mesostates  or  intermediate  products,  proto- 
plasm is  finally  manufactured. 

In  this  way  the  food  supply  of  the  green  cells  of  the 
leaves  is  accounted  for,  but  we  have  still  to  consider  that  of 
the  colourless  cells  of  the  stem  and  rhizoids,  which,  as  we 
have  seen,  are  supplied  by  the  transpiration  current  with 
everything  they  require  except  carbon,  and  this,  owing  to 
their  possessing  no  chlorophyll,  they  are  unable  to  take  in 
in  the  form  of  carbon  dioxide. 

As  a  matter  of  fact  the  chlorophyll-containing  cells  of  the 
leaves  have  to  provide  not  only  their  own  food,  but  also 
that  of  their  not-green  fellows.  In  addition  to  making  good 
the  waste  of  their  own  protoplasm  they  produce  large 
quantities  of  plastic  products  (see  p.  33)  such  as  grape 
sugar,  and  simple  nitrogenous  compounds  like  asparagin, 
and  these  pass  by  diffusion  from  cell  to  cell  until  they  reach 
the  uttermost  parts  of  the  plant,  such  as  the  centre  of  the 
stem  and  the  extremities  of  the  rhizoids.  The  colourless 
cells  are  in  this  way  provided  not  only  with  the  salts 
contained  in  the  ascending  transpiration  current,  but  with 
carbo-hydrates  and  nitrogenous  compounds.  From  these 
they  derive  their  nutriment,  living  therefore  like  yeast-cells 
in  Pasteur's  solution,  or  like  Bacteria  in  an  organic 
infusion. 

We  see  then  that  the  colourless  cells  of  the  stem  and 
rhizoids  are  dependent  upon  the  green  cells  of  the  leaves 
for  their  supplies.  Like  other  cells  devoid  of  chlorophyll 
they  are  unable  to  make  use  of  carbon  dioxide  as  a  source 
of  carbon,  but  require  ready-made  carbo-hydrates,  the 


xxvni       DISTRIBUTION   OF   FOOD    MATERIALS  343 

manufacture  of  which  is  continually  going  on,  during 
daylight,  in  the  chlorophyll-containing  cells  of  the  leaves. 
This  striking  division  of  labour  is  the  most  important 
physiological  difference  between  mosses  and  the  more  lowly 
organized  green  plants  described  in  previous  lessons. 


LESSON  XXIX 

FERNS 

WE  saw  in  the  previous  lesson  that  in  mosses  there  is  a 
certain  though  small  amount  of  histological  differentiation, 
some  cells  being  modified  to  form  sclerenchyma,  others  to 
form  axial  bundles.  We  have  now  to  consider  a  group  of 
plants  which  may  be  considered  to  be,  in  this  respect,  on 
much  the  same  morphological  level  as  Polygordius,  the 
adult  organism  being  composed  not  of  a  mere  aggregate  of 
simple  cells,  but  of  various  well-marked  tissues. 

A  fern-plant  has  a  strong  stem  which  in  some  forms,  such 
as  the  common  Bracken  (Pteris  aquilina)  is  a  horizontal 
underground  structure,  and  is  hence  often  incorrectly  con- 
sidered as  a  root :  in  others  it  creeps  over  the  trunks  of 
trees  or  over  rocks  :  in  others  again,  such  as  the  tree-ferns, 
it  is  vertical,  and  may  attain  a  height  of  three  or  four  metres. 
From  the  stem  are  given  off  structures  of  two  kinds — the 
leaves,  which  present  an  almost  infinite  variety  of  form  in 
the  various  species,  and  the  numerous  slender  roots.  In 
some  cases,  such  as  the  tree-ferns  and  the  common  Male 
Shield-fern  (Aspidium  filix-mas\  the  plant  ends  distallyin  a 
terminal  bud,  consisting,  as  in  Nitella  and  mosses,  of  the 
growing  end  of  the  stem  over-arched  by  leaves  :  in  others 


LESS,  xxix     .          TISSUES   OF  THE  STEM  345 

such  as  Pteris,  the  stem  ends  in  a  blunt,  knob-like  extremity 
quite  uncovered  by  leaves.  On  the  proximal  portion  of  the 
stem  are  usually  found  the  withered  remains  of  the  leaves 
of  previous  seasons,  or  the  scars  left  by  their  fall.  The 
roots  are  given  off  from  the  whole  surface  of  the  stem, 
often  covering  it  with  a  closely-matted  mass  of  dark  brown 
fibres. 

When  the  stem  is  cut  across  transversely  (Fig.  82,  A)  it 
is  seen,  even  with  the  naked  eye,  to  consist  of  three  well 
marked  tissues.  The  main  mass  of  it  is  formed  of  a  whitish 
substance,  soft  and  rather  sticky  to  the  touch,  and  called 
ground-parenchyma  (par) :  this  is  covered  by  an  external 
layer  of  very  hard  tissue,  dark  brown  or  black  in  colour,  the 
hypodermis  (hyp} :  bands  of  a  similar  hard  brown  substance 
are  variously  distributed  through  the  parenchyma,  and  con- 
stitute the  sclerenchyma  (scl)  :  and  interspersed  with  these 
are  rounded  or  oval  patches  of  a  yellowish  colour  (V.B] 
harder  than  the  parenchyma,  but  not  so  hard  as  the 
sclerenchyma,  and  called  vascular  bundles. 

The  general  distribution  of  these  tissues  can  be  made  out 
by  making  longitudinal  sections  of  the  stem  in  various 
planes  or  by  cutting  away  the  hypodermis,  and  then  scraping 
the  parenchyma  from  the  vascular  bundles  and  bands  of 
sclerenchyma.  The  hypodermis  is  found  to  form  a  more  or 
less  complete  hard  sheath  or  shell  to  the  stem,  while  the 
internal  sclerenchyma  and  vascular  bundles  form  longi- 
tudinal bands  and  rods  imbedded  in  the  parenchyma,  and 
serve  as  a  sort  of  supporting  framework  or  skeleton. 

The  minute  structure  of  the  stem  can  be  made  out  by 
the  examination  either  of  very  thin  longitudinal  and  trans- 
verse sections,  or  of  a  bit  of  stem  which  has  been  reduced 
to  a  pulp  by  boiling  in  nitric  acid  with  the  addition  of  a  few 
crystals  of  potassium  chlorate  :  by  this  process  the  various 


T.S 


4CV 


cp.o 

FIG.  82. — Anatomy  and  Histology  of  Ferns. 


LESS,  xxix  GENERAL   CHARACTERS  347 

A,  Transverse  section  of  the  stem  of  Pteris  aqnilina,  showing  hypo- 
dermis  (hyp],  ground  parenchyma  (par],  sclerenchyma  (set),  and  vascular 
bundles  (  V.  B).     (x  2.) 

B,  transverse  section  of  a  vascular  bundle,   showing  bundle-sheath 
(b.  sh\  sieve-tubes  (sv.  f),  scalariform  vessels  (sc.  v),  and  spiral  vessels 
(sp.  v}.     (x   6.) 

C,  semi-diagrammatic  vertical  section  of  the  growing  point  of  the 
stem,  showing  apical  cell  (op.  c),  segmental  cells  (seg.  c),  and  apical 
meristem  (ap.  mer)  passing  into  permanent  tissue  consisting  of  epidermis 
(ep\  hypodermis  (hyp],  ground  parenchyma  (par],  sclerenchyma  (scl), 
and  vascular  bundles  in  which  the  sheath  (b.  s/i),  sieve-tubes  (sv.  /), 
scalariform  vessels  (sc.  v),  and  spiral  vessels  (sp.  v)  are  indicated. 

D,  a  single  parenchyma  cell,  showing  nucleus   (««),  and  vacuole 
(vac]. 

E,  cell  of  hypodermis. 

F,  portion  of  a  sieve-tube,  showing  sieve-plates  (sv.  pi). 

G,  portion  of  a  spiral  vessel  with  the  spiral  fibre  partly  unrolled  at  the 
lower  end. 

H,  fibre-like  cell  of  sclerenchyma. 

I,  portion  of  a  scalariform  vessel,  part  of  the  wall  being  supposed  to 
be  removed. 

K,  vertical  section  of  a  leaf  of  Pteris,  showing  upper  and  lower  epi- 
dermis (ep\  mesophyll  cells  (ms.  ph),  with  intercellular  spaces  (i.  c.  sp), 
a  stoma  (sf)  in  the  lower  epidermis,  and  hairs  (A). 

L,  surface  view  of  epidermis  of  leaf  of  Aspidium,  showing  two  stomata 
(sf)  with  their  guard-cells  (gd.  c). 

M,  vertical  section  of  the  end  of  a  root,  showing  apical  cell  (ap.  c}, 
segmental  cells  (seg.  c),  and  root-cap  (r.  cp}  with  its  youngest  cap-cells 
marked  cp.  c. 

(A,  B,  and  D-K  after  Howes  ;  M  from  Sachs,  slightly  altered.) 

tissue  elements  are  separated  from  one  another,  and  can  be 
readily  examined  under  a"  high  power. 

By  combining  these  two  methods  of  sectioning  and 
dissociation,  the  parenchyma  is  found  to  consist  of  an 
aggregate  of  polyhedral  cells  (D)  considerably  longer  than 
broad,  their  long  axes  being  parallel  with  that  of  the  stem 
itself.  The  cells  are  to  be  considered  as  right  cylinders 
which  have  been  converted  into  polyhedra  by  mutual  pres- 
sure. They  have  the  usual  structure,  and  their  protoplasm  is 
frequently  loaded  with  large  starch-grains.  They  do  not  fit 
quite  closely  together,  but  spaces  are  left  between  them, 
especially  at  the  angles,  called  intercellular  spaces. 


348  FERNS  LESS. 

The  cells  of  the  hypodermis  (E)  are  proportionally  longer 
than  those  of  the  parenchyma,  and  are  pointed  at  each  end  : 
they  contain  no  starch.  Their  walls  are  greatly  thickened, 
and  are  composed  not  of  cellulose  but  of  lignin,  a  carbo- 
hydrate allied  in  composition  to  cellulose,  but  containing  a 
larger  proportion  of  carbon  Schulze's  solution,  which,  as 
we  have  seen,  stains  cellulose  blue,  imparts  a  yellow  colour 
to  lignin. 

Outside  the  hypodermis  is  a  single  layer  of  cells  (c,  ep) 
not  distinguishable  by  the  naked  eye  and  forming  the  actual 
external  layer  of  the  stem  :  the  cells  have  slightly  thickened, 
yellowish-brown  walls,  and  constitute  the  epidermis.  From 
many  of  them  are  given  off  delicate  filamentous  processes  con- 
sisting each  of  a  single  row  of  cells  :  these  are  called  hairs. 

In  the  sclerenchyma  the  cells  (H)  are  greatly  elongated' 
and  pointed  at  both  ends,  so  as  to  have  the  character  rather 
of  fibres  than  of  cells.  Their  walls  are  immensely  thickened 
and  lignified,  and  present  at  intervals  oblique  markings  due  to 
narrow  but  deep  clefts  :  these  are  produced  by  the  deposition 
of  lignin  from  the  surface  of  the  protoplasm  (see  p.  32)  being 
interrupted  here  and  there,  instead  of  going  on  continuously 
as  in  the  case  of  a  cell-wall  of  uniform  thickness. 

The  vascular  bundles  have  in  transverse  section  (B)  the 
appearance  of  a  very  complicated  network,  with  meshes  of 
varying  diameter.  In  longitudinal  sections  (c)  and  in  dis- 
sociated specimens  they  are  found  to  be  partly  composed  of 
cells,  but  to  contain  besides  structures  which  cannot  be 
called  cells  at  all. 

In  the  centre  of  the  bundle  are  a  few  narrow  cylindrical 
tubes  (B  and  c,  sp.  v.)  characterized  at  once  by  a  spiral 
marking,  and  hence  called  spiral  vessels.  Accurate  exam- 
ination shows  that  their  walls  (G)  are  for  the  most  part  thin, 
but  are  thickened  by  a  spiral  fibre,  just  as  a  paper  tube 


xxix  XYLEM    AND    PHLOEM  349 

might  be  strengthened  by  gumming  a  spiral  strip  of  paste- 
board to  its  inner  surface.  These  vessels  are  of  considerable 
length,  and  are  open  at  both  ends  :  moreover  they  contain 
no  protoplasm,  but  are  filled  with  either  air  or  water  :  they 
have  therefore  none  of  the  characteristics  of  cells.  They 
are  shown,  by  treatment  with  Schulze's  solution,  to  be  com- 
posed of  lignin. 

Surrounding  the  group  of  spiral  vessels,  and  forming  the 
large  polygonal  meshes  so  obvious  in  a  transverse  section, 
are  wide  tubes  (B  and  c,  sc.  v)  pointed  at  both  ends  and 
fitting  against  one  another  in  longitudinal  series  by  their 
oblique  extremities.  They  have  transverse  markings  like 
the  rungs  of  a  ladder,  and  are  hence  called  scalariform 
vessels.  The  markings  (i)  are  due  to  wide  transverse  pits 
in  the  otherwise  thick  lignified  walls  :  in  the  oblique  ends 
by  which  the  vessels  fit  against  one  another  the  pits  are 
frequently  replaced  by  actual  slits,  so  that  a  longitudinal 
series  of  such  vessels  forms  a  continuous  tube  containing, 
like  the  spiral  vessels,  air  or  water,  but  no  protoplasm.  In 
most  ferns  the  terminal  walls  are  not  thus  perforated,  and 
the  elements  are  then  called  tracheides. 

The  presence  of  these  vessels — spiral  and  scalariform — 
is  the  most  important  histological  character  separating  ferns 
and  mosses.  Arhe  latter  group  and  all  plants  below  them  are 
composed  exclusively  of  cells  :  ferns  and  all  plants  above 
them  contain  vessels  in  addition,  and  are  hence  called  vas- 
cular  plants. 

The  vessels,  together  with  small  parenchyma-cells  inter- 
spersed among  them,  make  up  the  central  portion  of  the 
vascular  bundle,  called  the  wood  or  xylem.  The  peripheral 
portion  is  formed  of  several  layers  of  cells  composing  the  bast 
or  phloem,  and  surrounding  the  whole  is  a  single  layer  of 
small  cells,  the  bundle-sheath  (b.  sh). 


35°  FERNS  LESS. 

The  cells  of  the  phloem  are  for  the  most  part  parenchy- 
matous,  but  amongst  them  are  some  to  which  special 
attention  must  be  drawn.  These  (B  and  c,  sv.  /),  are  many 
times  as  long  as  they  are  broad,  and  have  on  their  walls 
irregular  patches  or  sieve-plates  (F,  sv.  pi.)  composed  of  groups 
of  minute  holes  through  which  the  protoplasm  of  the  cell  is 
continuous  with  that  of  an  adjacent  cell.  The  transverse  or 
oblique  partitions  between  the  cells  of  a  longitudinal  series 
are  also  perforated,  so  that  a  row  of  such  cells  forms  a  sieve- 
tube  in  which  the  protoplasm  is  continuous  from  end  to  end. 
We  have  here,  therefore,  as  striking  an  instance  of  proto- 
plasmic continuity  as  in  the  deric  epithelium  and  certain  other 
tissues  of  Polygordius  (see  p.  276). 

The  distal  or  growing  end  of  the  stem  terminates  in  a  blunt 
apical  cone  or  punctum  vegetationis  (c),  surrounded  by  the 
leaves  of  the  terminal  bud  in  the  case  of  vertical  stems,  or 
sunk  in  a  depression  and  protected  by  close-set  hairs  in  the 
underground  stem  of  the  bracken.  A  rough  longitudinal 
section  shows  that,  at  a  short  distance  from  the  apical  cone, 
the  various  tissues  of  the  stem — epidermis,  parenchyma, 
sclerenchyma,  and  vascular  bundles — merge  insensibly  into 
a  whitish  substance,  resembling  parenchyma  to  the  naked 
eye,  and  called  apical  merislem  (ap.  mer). 

Thin  sections  show  that  the  summit  of  the  apical  cone  is 
occupied  by  a  wedge-shaped  apical  cell  (ap.  c}  which  in 
vertical  stems  is  three-sided  like  that  of  mosses  (Fig.  80,  H, 
p.  335),  while  in  the  horizontal  stem  of  Pteris  it  is  two-sided. 
As  in  mosses,  segmental  cells  (seg.  c]  are  cut  off  from  the  three 
(or  two)  sides  of  the  apical  cell  in  succession,  and  by  further 
division  form  the  apical  meristem  (ap.  mer),  which  consists 
of  small,  close-set  cells  without  intercellular  spaces.  As  the 
base  of  the  apical  cone  is  reached,  the  meristem  is  found  to 


xxix  APICAL   GROWTH  351 

pass  insensibly  into  the  permanent  tissues,  the  cells  near  the 
surface  gradually  merging  into  epidermis  and  hypodermis, 
those  towards  the  central  region  into  sclerenchyma  and  the 
various  constituents  of  the  vascular  bundles,  and  those  of 
the  intermediate  regions  into  parenchyma. 

The  examination  of  the  growing  end  of  the  stem  shows  us 
how  the  process  of  apical  growth  is  carried  on  in  a  compli- 
cated plant  like  the  fern.  The  apical  cell  is  continually 
undergoing  fission,  forming  a  succession  of  segmental  cells ; 
these  divide  and  form  the  apical  meristem,  which  is  thus 
being  constantly  added  to  at  the  growing  end  by  the  formation 
and  subsequent  fission  of  new  segmental  cells  :  in  this  way  the 
apex  of  the  stem  is  continually  growing  upwards  or  forwards. 
But  at  the  same  time  the  meristem  cells  farthest  from  the 
apex  begin  to  differentiate  :  some  elongate  but  slightly, 
increasing  greatly  in  size,  and  become  parenchyma  cells  : 
others  by  elongation  in  the  direction  of  length  of  the  stem 
and  by  thickening  and  lignification  of  the  cell-wall  become 
sclerenchyma  cells  :  others  again  elongate  greatly,  become 
arranged  end  to  end  in  longitudinal  rows,  and,  by  the  loss 
of  their  protoplasm  and  of  the  transverse  partitions  between 
the  cells  of  each  row,  are  converted  into  vessels — spiral  or 
scalariform  according  to  the  character  of  their  walls.  Thus 
while  the  epidermis,  parenchyma,  and  sclerenchyma  are 
formed  of  cells,  the  spiral  and  scalariform  vessels  are  cell- 
fusions,  or  more  accurately  cell-wall-fusions,  being  formed  by 
the  union  in  a  longitudinal  series  of  a  greater  or  less  number 
of  cell- walls.  It  will  be  remembered  that  the  muscle-plates 
of  Polygordius  are  proved  by  the  study  of  development  to  be 
cell-fusions  (p.  305). 

We  thus  see  that  every  cell  in  the  stem  of  the  fern  was  once 
a  cell  in  the  apical  meristem,  that  every  vessel  has  arisen  by 
the  concrescence  of  a  number  of  such  cells,  and  that  the 


352  FERNS  LESS. 

meristem  cells  themselves  are  all  derived,  by  the  ordinary 
process  of  binary  fission,  from  the  apical  cell.  In  this  way 
the  concurrent  processes  of  cell-division,  cell-differentiation, 
and  cell-fusion  result  in  the  production  of  the  various  and 
complex  tissues  of  the  fully-formed  stem. 

The  leaves  vary  greatly  in  form  in  the  numerous  genera 
and  species  of  ferns  :  they  may  consist  of  an  unbranched 
stalk  bearing  a  single  expanded  green  blade:  or  the  stalk 
may  be  more  or  less  branched,  its  ramifications  bearing  the 
numerous  subdivisions  of  the  blade,  ox  pinnce. 

The  anatomy  of  the  leaf,  like  that  of  the  stem,  can  be 
readily  made  ou,t  by  a  rough  dissection.  The  leaf-stalk  and 
its  branches  have  the  same  general  structure  as  the  stem, 
consisting  of  parenchyma  coated  externally  with  epidermis 
and  strengthened  internally  by  vascular  bundles,  which  are 
continuous  with  those  of  the  stem.  But  the  blade,  or  in  the 
case  of  a  compound  leaf,  the  pinna,  has  a  different  and  quite 
peculiar  structure.  It  is  invested  by  a  layer  of  epidermis 
which  can  be  readily  stripped  off  as  an  extremely  thin,  colour- 
less membrane,  exposing  a  soft,  green  substance,  the  leaf 
parenchyma  or  mesophylL  The  leaf  is  marked  externally  by 
a  network  of  delicate  ridges,  the  veins  ;  these  are  shown  by 
dissection  to  be  due  to  the  presence  of  fine  white  threads 
which  ramify  through  the  mesophyll,  and  can  be  proved  by 
tracing  them  into  the  leaf-stalk  to  spring  from  its  vascular 
bundles,  of  which  they  are  in  effect  the  greatly  branched 
distal  ends. 

Microscopic  examination  shows  the  epidermis  of  the  leaf 
(K,  ep  and  L)  to  consist  of  flattened,  colourless  cells  of  very 
irregular  outline  and  fitting  closely  to  one  another  like  the 
parts  of  a  child's  puzzle.  Amongst  them  are  found  at 
intervals  pairs  of  sausage -shaped  cells  (gd.  c)  placed  with 


xxix  LEAVES   AND    ROOTS  353 

their  concavities  towards  one  another  so  as  to  bound  a 
narrow  slit-like  aperture  (st).  These  apertures,  which  are 
the  only  intercellular  spaces  in  the  epidermis,  are  called 
stomates :  the  cells  bounding  them  are  the  guard-cells,  and 
are  distinguished  from  the  remaining  epidermic  cells  by 
the  possession  of  a  few  chromatophores. 

The  mesophyll,  which  as  we  have  seen  occupies  the  whole 
space  between  the  upper  and  lower  epidermis,  is  formed  of 
thin-walled  cells  loaded  with  chromatophores  (K,  ms.pli)  and 
therefore  of  a  deep  green  colour.  The  cells  in  contact  with 
the  upper  epidermis  are  cylindrical,  and  are  arranged  verti- 
cally in  a  single  row  :  those  towards  the  lower  surface  are 
very  irregular  both  in  form  and  arrangement.  Large  inter- 
cellular spaces  (/.  c.  sp]  occur  between  the  mesophyll-cells 
and  communicate  with  the  outer  air  through  the  stomates. 

The  leaves  arise  as  outgrowths  of  the  distal  or  growing 
end  of  the  stem,  each  originating  from  a  single  segmental 
cell  of  the  apical  cone. 

The  fern  is  the  first  plant  we  have  yet  considered  which 
possesses  true  roots,  the  structures  so-called  differing  funda- 
mentally from  the  simple  rhizoids  of  Nitella  and  the  mosses. 
Instead  of  being  mere  linear  aggregates  of  cells,  they  agree 
in  general  structure  with  the  stem  from  which  they  spring, 
consisting  of  an  outer  layer  of  epidermis  within  which  is 
parenchyma  strengthened  by  bands  of  selerenchyma  and  by 
a  single  vascular  bundle  in  the  centre.  The  epidermic  cells 
give  rise  to  unicellular  prominences,  the  root- hairs. 

The  apex  of  the  root,  like  that  of  the  stem,  is  formed  of 
a  mass  of  meristem  in  which  a  single  wedge-shaped  apical 
cell  (Fig.  82,  M,  ap.  c]  can  be  distinguished.  But  instead 
of  the  base  of  this  cell  forming  the  actual  distal  extremity, 
as  in  the  stem  (compare  c),  it  is  covered  by  several  lavers  of  : 


t>*' 


354  FERNS  LESS. 

cells  which  constitute  the  root-cap  (r.cp).  In  fact  the  apical 
cell  of  the  root  divides  not  only  by  planes  parallel  to  its 
three  sides,  but  also  by  a  plane  parallel  to  its  base,  and  in 
this  way  produces  not  only  three  series  of  segmental  cells 
(seg.  c)  which  afterwards  subdivide  to  form  the  apical 
meristem,  but  also  a  series  of  cap-cells  (cp.  c]  which  form  a 
protective  sheath  over  the  tender  growing  end  of  the  root  as 
it  forces  its  way  through  the  soil. 

Roots  are  also  peculiar  in  their  development.  Instead  ot 
being,  like  leaves,  prominences  of  the  superficial  tissues  of 
the  stem,  they  arise  from  a  layer  of  cells  immediately  ex- 
ternal to  the  vascular  bundles,  and  in  growing  force  their 
way  through  the  superficial  portion  of  the  stem,  through 
a  fissure  from  which  they  finally  emerge.  They  are  thus  said 
to  be  endogenous  in  origin  while  leaves  are  exogenous. 

The  nutrition  of  ferns  is  carried  on  in  much  the  same 
way  as  in  mosses  (see  p.  340).  Judging  from  the  analogy  of 
flowering  plants  it  would  seem  that  the  ascending  current  of 
water  from  the  roots  passes  mainly  through  the  xylem  of  the 
vascular  bundles,  while  the  descending  current  of  nitrogenous 
and  other  nutrient  matters  for  the  supply  of  the  colourless 
cells  of  the  stem  and  roots  passes  chiefly  through  the  phloem 
and  especially  through  the  sieve-tubes.  The  absorption  of 
water  is  effected  by  the  root-hairs. 

In  the  autumn  there  are  found  on  the  under  surfaces  of 
the  leaves  brown  patches  called  sori,  differing  greatly  in 
form  and  arrangement  in  the  various  genera,  and  formed  of 
innumerable,  minute,  seed-like,  bodies,  the  sporangia  (Fig. 
83,  A),  just  visible  to  the  naked  eye.  Each  sorus  or  group 
of  sporangia  is  covered  by  a  fold  of  the  epidermis  of  the 
leaf,  called  the  indusiwn. 


xxix  REPRODUCTION  355 

A  sporangium  is  attached  to  the  leaf  by  a  multicellular 
stalk  (st),  and  consists  of  a  sac  resembling  two  watch-glasses 
placed  with  their  concave  surfaces  towards  one  another  and 
their  edges  united  by  a  thick  rim  (an).  The  sides  are 
formed  by  thin  flattened  cells  with  irregular  outlines,  the 
rim  or  annulus  of  peculiarly  shaped  cells  which  are  thin  and 
broad  at  one  edge  (to  the  left  in  A),  but  on  the  other  (to  the 
right)  are  thick,  strongly  lignified,  and  of  a  yellowish-brown 
colour.  The  whole  internal  cavity  is  filled  with  spores 
(B,  sp)  having  the  form  of  tetrahedra  with  rounded  edges, 
and'each  consisting  of  protoplasm  containing  a  nucleus,  and 
surrounded  by  a  double  wall  of  cellulose.  A  spore  is  there- 
fore, as  in  mosses,  a  single  cell. 

Each  sporangium  arises  from  a  single  epidermic  cell  of 
the  leaf.  This  divides  repeatedly  so  as  to  form  a  solid  mass 
of  cells,  of  which  the  outermost  become  the  wall  of  the 
sporangium  while  the  inner  are  the  spore-mother-cells.  The 
latter  divide  each  into  four  spores,  as  in  mosses  (p.  339). 

As  the  spores  ripen,  the  wall  of  the  sporangium  dries,  and 
as  it  does  so  the  thickened  part  of  the  annulus  straightens 
out,  tearing  the  thin  cells  and  producing  a  great  rent  through ' 
which  the  spores  escape  (B). 

When  the  spores  are  sown  on  moist  earth  they  germinate, 
by  the  protoplasm,  covered  by  the  inner  coat,  protruding 
through  the  ruptured  outer  coat  (c,  sp)  in  the  form  of  a 
short  filament.  This  divides  transversely,  forming  two  cells, 
the  proximal  of  which  sends  off  a  short  rhizoid  (rh).  The 
resemblance  of  this  stage  to  the  young  protonema  of  a  moss 
is  sufficiently  obvious  (see  Fig.  81,  D1.,  p.  338). 

Further  cell-division  takes  place,  and  before  long  the 
distal  cells  divide  longitudinally,  a  leaf-like  body  being 
produced,  which  is  called  the  prothallus  (D).  This  is  at  first 

A  A  2 


T-7l. 


FiG.  83. — Reproduction  and  Development  of  Ferns. 

A,  Sporangium   of  Pteris>  external   view,    showing  stalk    (s£)   and 
annulus  (an). 

B,  the  same,  during  dehiscence,  the  spores  (sp)  escaping. 

C,  a  germinating  spore,  showing  the  ruptured  outer  coat  (.?/),  and  a 


LESS,  xxix  THE   PROTHALLUS  357 

rhizoid  (rJi)  springing  from  the  proximal  cell  of  the  rudimentary  (two- 
celled)  prothallus. 

D,  a  young   prothallus,    showing   spore,    rhizoid   (rJi),    apical    cell 
(ap.  c),  and  segmental  cells  (seg.  c). 

E,  an   advanced  prothallus,   from  beneath,  showing  rhizoids   (rh}, 
ovaries  (ovy),  and  spermaries  (spy}. 

F,  a  mature  spermary  of  Pteris,  inverted  (i.e.   with  its  distal  end 
directed  upwards)  so  as  to  compare  with  Fig.  82,  A. 

G,  a  single  sperm,  showing  coiled  body  and  numerous  cilia. 

H,  a  mature  ovary  of  Aspidium,  inverted  so  as  to  compare  with  Fig. 
82,  B2,  showing  venter  (v),  neck  (#},  ovum  (ov),  and  canal  cells  (en.  c). 

I,  small  portion  of  a  prothallus  of  Asplenium  in  vertical  section, 
showing  the  venter  {v)  and  part  of  the  neck  (n)  of  a  single  ovary  after 
fertilization.  The  venter  contains  an  embryo  just  passing  from  the 
polyplast  into  the  phyllula  stage,  and  divided  into  four  groups  of  cells, 
the  rudiments  respectively  of  the  foot  (ft],  stem  (st),  root  (rt),  and 
cotyledon  (ct}. 

K,  vertical  section  of  a  prothallus  (prtJi)  of  Nephrolepis,  bearing 
rhizoids  (r/i),  and  a  single  ovary  with  greatly  dilated  venter  (v)  and 
withered  neck  («).  The  venter  contains  an  embryo  in  the  phyllula 
stage,  consisting  of  foot  (ft},  rudiments  of  stem  (.$•/),  and  root  (rt},  and 
cotyledon  (ct}  beginning  to  grow  upwards. 

L,  prothallus  (prth}  with  rhizoids  (r/i),  bearing  a  young  fern  plant, 
consisting  of  foot  (//),  rudiment  of  stem  (st),  first  root  (rt),  cotyledon 
(ct),  and  first  ordinary  leaf  (/).  (After  Howes.) 


only  one  layer  of  cells  thick,  but  it  gradually  increases  in 
size,  becoming  more  or  less  kidney-shaped  (E),  and  as  it  does 
so  its  cells  divide  parallel  to  the  surface,  making  it  two  and 
finally  several  cells  in  thickness.  Thus  the  prothallus  is 
at  first  a  linear,  then  a  superficial,  and  ultimately  a  solid 
aggregate.  Root-hairs  (rh)  are  produced  in  great  number 
from  its  lower  surface,  and  penetrating  into  the  soil  serve 
for  the  absorption  of  nutriment.  At  an  early  period  a  two- 
sided  apical  cell  (D,  ap.  c)  is  differentiated,  and  gives  off 
segmental  cells  (seg.  c)  in  the  usual  way  :  an  abundant  forma- 
tion of  chromatophores  also  takes  place  at  a  very  early  period 
in  the  cells  of  the  prothallus,  which  therefore  resembles  both 
in  structure  and  in  habit  some  very  simple  form  of  moss. 

On  the  lower  surface  of  the  prothallus  gonads  (E,  spy,  ovy) 
are  developed,  resembling  in  their  essential  features  those  of 


358  FERNS  LESS. 

mosses.  The  spermaries  (spy]  make  their  appearance  first, 
being  frequently  found  on  very  young  prothalli.  One  of  the 
lower  cells  forms  a  projection  which  becomes  divided  off  by 
a  septum :  further  division  takes  place,  resulting  in  the 
differentiation  (F)  of  an  outer  layer  of  cells  forming  the  wall 
of  the  spermary,  and  of  an  internal  mass  of  sperm-mother-cells 
in  each  of  which  a  sperm  is  produced.  The  sperm  (G)  is  a 
corkscrew-like  body,  probably  formed  from  the  nucleus  of  the 
cell,  bearing  at  its  narrow  end  a  number  of  cilia  which 
appear  to  originate  from  the  protoplasm.  To  the  thick  end 
is  often  attached  a  globular  body,  also  arising  from  the 
protoplasm  of  the  mother-cell ;  this  is  finally  detached. 

The  ovaries  (E  and  H,  ovy)  are  not  usually  formed  until  the 
prothallus  has  attained  a  considerable  size.  Each  arises,  like 
a  spermary,  from  a  single  cell  cut  off  by  a  septum  from  one 
of  the  lower  cells  of  the  prothallus  :  the  cell  divides  and 
forms  a  structure  resembling  in  general  characters  the  ovary 
of  a  moss  (see  Fig.  81,  B,  p.  338),  except  that  the  venter  (H, 
v)  is  sunk  in  the  prothallus,  and  is  therefore  a  less  distinct 
structure  than  in  the  lower  type.  As  in  mosses,  also,  an 
axial  row  of  cells  is  early  distinguished  from  those  forming 
the  wall  of  the  ovary  :  the  proximal  of  these  becomes  the 
ovum  (ov),  the  others  are  the  canal  cells  (en.  <:),  which  are 
converted  into  mucilage,  and  by  their  expansion  force  open 
the  neck  and  make  a  clear  passage  for  the  sperm. 

The  sperms  swarm  round  the  aperture  of  the  ovary  and 
make  their  way  down  the  canal,  one  of  them  finally  conju- 
gating with  the  ovum  and  converting  it  into  an  oosperm. 

The  early  stages  in  the  development  of  the  embryo 
remind  us,  in  their  general  features,  of  what  we  found  to 
occur  in  mosses  (p.  337).  The  oosperm  first  divides  by  a 
plane  parallel  to  the  neck  of  the  ovary,  forming  two  cells,  an 
anterior  nearest  the  growing  or  distal  end  of  the  prothallus, 


POLYPLAST   AND   PHYLLULA  359 

and  a  posterior  towards  its  proximal  end.  Each  of  these 
divides  again  by  a  plane  at  right  angles  to  the  first,  there 
being  now  an  upper  and  a  lower  anterior,  and  an  upper  and 
a  lower  posterior  cell  :  the  lower  in  each  case  being  that 
towards  the  downwardly  directed  neck  of  the  ovary.  Each 
of  the  four  cells  undergoes  fission,  the  embryo  then  consisting 
of  eight  cells,  two  upper  anterior  (right  and  left),  two  lower 
anterior,  two  upper  posterior,  and  two  lower  posterior.  We 
thus  get  a  multicellular  but  undifferentiated  stage,  the 
polyplast. 

It  will  be  remembered  that  in  mosses  the  polyplast  forms 
an  apical  cell,  and  develops  directly  into  the  sporogonium 
(P-  337)-  In  tne  fern  the  later  stages  are  more  complex. 
One  of  the  upper  anterior  cells  remains  undeveloped,  the 
other  (Fig.  83,  i  and  K,  st)  takes  on  the  form  of  a  wedge- 
shaped  apical  cell,  and,  dividing  in  the  usual  way,  forms  a 
structure  like  the  apex  of  the  fern-stem,  of  which  it  is  in  fact 
the  rudiment.  The  two  upper  posterior  cells  divide  and 
subdivide,  and  form  a  multicellular  mass  called  the/w/(//), 
which  becomes  embedded  in  the  prothailus,  and  serves  the 
growing  embryo  for  the  absorption  of  nutriment.  One  of  the 
lower  posterior  cells  remains  undeveloped,  the  other  (rt) 
takes  on  the  form  of  the  apical  cell  of  a  root,  /.<?.,  of  a  wedge- 
shaped  cell,  which  not  only  produces  three  sets  of  segmental 
cells  from  its  sides  but  also  cap-cells  from  its  base  (p.  354)  : 
division  of  this  cell  goes  on  very  rapidly,  and  a  root  is  pro- 
duced which  at  once  grows  downwards  into  the  soil.  Finally 
the  two  lower  anterior  cells  undergo  rapid  fission,  and 
develop  into  the  first  leaf  of  the  embryo  or  cotyledon  (<:/), 
which  soon  begins  to  grow  upwards  towards  the  light. 

Thus  at  a  comparatively  early  stage  of  its  development 
the  fern-embryo  has  attained  a  degree  of  differentiation  far 
beyond  anything  which  occurs  in  the  moss-embryo.  The 


360  FERNS  LESS. 

scarcely  differentiated  polyplast  has  passed  into  a  stage 
which  may  be  called  the  phyllula,  distinguished  by  the 
possession  of  those  two  characteristic  organs  of  the  higher 
plants,  the  leaf  and  root. 

Notice  how  early  in  development  the  essential  features  of 
animal  or  plant  manifest  themselves.  In  Polygordius  the 
polyplast  is  succeeded  by  a  gastrula  distinguished  by  the 
possession  of  a  digestive  cavity  :  in  the  fern  no  such  cavity 
is  formed,  but  the  polyplast  is  succeeded  by  a  stage  dis- 
tinguished by  the  possession  of  a  leaf  and  root.  In  the 
one  case  the  characteristic  organ  for  holozoic,  in  the  other 
the  characteristic  organs  for  holophytic  nutrition  make  their 
appearance,  and  so  mark  the  embryo  at  once  as  an  animal 
or  plant.  We  may  say  then  that  while  the  oosperm  and 
the  polyplast  stages  of  the  embryo  are  common  to  the 
higher  plants  and  the  higher  animals,  the  correspond- 
ence goes  no  further,  the  next  step  being  the  formation 
in  the  animal  of  an  enteron,  in  the  plant  of  a  leaf  and 
root.  In  other  words  the  phyllula  is  the  correlative  of 
the  gastrula. 

The  cotyledon  increases  rapidly  in  size,  and  emerges 
between  the  lobes  of  the  kidney-shaped  prothallus  (L)  :  the 
root  at  the  same  time  grows  to  a  considerable  length,  the 
result  being  that  the  phyllula  becomes  a  very  obvious 
structure  in  close  connection  with  the  prothallus,  and  indeed 
appearing  to  be  part  of  it.  The  two  are  actually,  however, 
quite  distinct,  their  union  depending  merely  upon  the  fact 
that  the  foot  of  the  phyllula  is  embedded  in  the  tissue  of 
the  prothallus  like  a  root  in  the  soil.  Hence  the  phyllula 
is  related  to  the  prothallus  in  precisely  the  same  way  as  the 
sporogonium  to  the  moss  plant  (compare  Fig.  83,  K,  with 
Fig.  81,  c2,  and  Fig.  83,  L,  with  Fig.  81,  c4). 

The  rudiment  of  the  stem  (L,  sf)  continues  to  grow  by  the 


xxix  GAMOBIUM   AND   AGAMOBIUM  361 

production  of  fresh  segments  from  its  apical  cell :  leaves  (/)  are 
developed  from  the  segments,  and  grow  upwards  parallel  with 
the  cotyledon.  The  leaves  first  formed  are  small  and 
simple  in  structure,  but  those  arising  later  become  succes- 
sively larger  and  more  complicated,  until  they  finally  attain 
the  size  and  complexity  of  the  ordinary  leaves  of  the  fern. 
In  the  meantime  new  roots  are  formed  ;  the  cotyledon,  the 
foot,  and  the  prothallus  wither,  and  thus  the  phyllula,  by  the 
successive  formation  of  new  parts  from  its  constantly  growing 
stem,  becomes  a  fern-plant. 

We  see  that  the  life-history  of  the  fern  resembles  in 
essentials  that  of  the  moss.  In  both,  alternation  of  genera- 
tion occurs,  a  gamobium  or  sexual  generation  giving  rise,  by 
the  conjugation  of  ovum  and  sperm,  to  an  agamobium  or 
asexual  generation,  which,  by  an  asexual  process  of  spore- 
formation,  produces  the  gamobium.  But  in  the  relative 
proportions  of  the  two  generations  the  difference  is  very  great. 
What  we  know  as  the  moss  plant  is  the  gamobium,  and  the 
agamobium  is  a  mere  spore-producing  structure,  never  getting 
beyond  the  stage  of  a  highly  differentiated  polyplast,  and 
dependent  throughout  its  existence  upon  the  gamobium,  to 
which  it  is  permanently  attached.  What  we  know  as  the 
fern  plant  is  the  agamobium,  a  large  and  complex  structure 
dependent  only  for  a  brief  period  of  its  early  life  upon  the 
small  and  insignificant  gamobium.  Thus  while  the  gamobium 
is  the  dominant  phase  in  the  life-history  of  mosses,  the 
agamobium  appearing  like  a  mere  organ,  in  ferns  the 
positions  are  more  than  reversed — the  agamobium  may 
assume  the  proportions  of  a  tree,  while  the  gamobium  is  so 
small  that  its  very  existence  is  unknown  to  a  large  propor- 
tion of  fern-collectors. 

It  follows  from  what  has  just  been  said  that  the  various 
organs  of  a  fern  do  not  severally  correspond  with  those  of  a 


362  FERNS  LESS,  xxix 

moss.  The  leaves  of  a  moss  are  not  homologous  with  those 
of  a  fern,  but  are  rather  comparable  to  lobes  of  the  pro- 
thallus  :  in  the  same  way  the  rhizoids  of  a  moss  correspond 
not  with  the  complicated  roots  of  the  fern  but  with  the 
rhizoids  of  the  prothallus. 


LESSON  XXX1 

THE  GENERAL  CHARACTERS  OF  THE  HIGHER  PLANTS 

IN  the  2yth  Lesson  (p.  307)  it  was  pointed  out  that  a 
thorough  comprehension  of  the  structure  and  development 
of  Polygordius  would  enable  the  student  to  understand  the 
main  features  of  the  organization  of  all  the  higher  animals. 

In  the  same  way  the  study  of  the  fern  paves  the  way  to 
that  of  the  higher  groups  of  plants,  all  of  which  indeed,  differ 
far  less  from  the  fern  than  do  the  various  animal  types  con- 
sidered in  Lesson  XXVII  from  Polygordius.  We  saw  that 
the  differences  between  these  included  matters  of  such  im- 
portance as  the  presence  or  absence  of  segmentation  and  of 
lateral  appendages,  the  characters  of  the  skeleton,  and  the 
structure  and  position  of  the  nervous  system.  In  the  higher 
plants,  on  the  other  hand,  the  essential  organs — root,  stem, 
and  leaves — are,  save  in  details  of  form,  size,  &c.,  practically 
the  same  in  all  :  the  tissues  always  consist  of  epidermis, 
ground-parenchyma,  and  vascular  bundles,  the  latter  being 
divisible  into  phloem  and  xylem  :  the  growing  point  both  of 
stem  and  of  root  is  formed  of  meristem,  from  which  the  per- 
manent tissues  arise ;  and  the  growing  point  of  the  root  is 

1  Readers  who  have  not  studied  botany,  or  at  least  examined  types 
of  the  chief  groups  of  plants,  will  derive  little  benefit  from  this  lesson. 


364         CHARACTERS   OF   THE   HIGHER   PLANTS      LESS. 

always  protected  by  a  root-cap,  that  of  the  stem  being  simply 
over-arched  by  leaves.  Moreover  an  alternation  of  genera- 
tions can  be  traced  in  all  cases. 

Plants  may  be  conveniently  divided  into  the  following 
chief  groups  or  phyla  : 

Alga. 

Fungi. 

Muscinea. 

Vascular  Cryptogams. 

Filicinae. 

Equisetaceae. 

Lycopodineae. 
Phanerogams. 

Gymnosperms. 

Angiosperms. 

The  Alga  are  the  lower  green  plants.  They  may  be 
unicellular,  or  may  take  the  form  of  linear,  superficial, 
or  solid  aggregates :  they  never  exhibit  more  than  a 
limited  amount  of  cell-differentiation.  This  group  has  been 
represented  in  the  foregoing  pages  by  Zooxanthella,  diatoms, 
Vaucheria,  Caulerpa,  Monostroma,  Ulva,  Laminaria,  and 
Nitella.1 

The  Fungi  are  the  lower  plants  devoid  of  chlorophyll  : 
some  are  unicellular,  others  are  linear  aggregates :  in  none 
is  there  any  cell-differentiation  worth  mentioning.  Saccharo- 
myces,  Mucor,  Penicillium,  and  the  mushroom  belong  to 
this  group. 

The  position  of  some  of  the  lower  forms  which  have  come 
under  our  notice  is  still  doubtful.  Bacteria,  for  instance, 
are  considered  by  some  authors  to  be  Fungi,  by  others  Algae, 

1  By  some  authors  Nitella  is  placed  near  the  Muscinea:. 


xxx  CHARACTERS   OF   THE   PHYLA  365 

while  others  place  them  in  a  group  apart.  Diatoms  also  are 
sometimes  placed  in  a  distinct  group.  It  must,  moreover, 
be  remembered  that  most  botanists  include  Haematococcus 
and  Volvox  among  Algae,  and  place  the  Mycetozoa  either 
among  Fungi  or  in  a  separate  group  of  chlorophyll-less 
plants  (p.  181). 

The  Mustinetz  are  the  mosses  and  liverworts,  the  former 
of  which  were  fully  described  in  Lesson  XXVIII. 

The  Vascular  Cryptogams  are  flowerless  plants  in  which 
vascular  bundles  are  present.  Together  with  the  Phanero- 
gams they  constitute  what  are  known  as  vascular  plants,  in 
contradistinction  to  the  non-vascular  Algae,  Fungi,  and 
Muscineae,  in  which  no  formation  of  vessels  takes  place.  The 
group  contains  three  subdivisions. 

The  first  division  of  Vascular  Cryptogams,  the  Filicince, 
includes  the  ferns,  an  account  of  which  has  been  given  in 
the  previous  lesson.  It  will  be  necessary,  however,  to  devote 
some  attention  to  an  aquatic  form,  called  Salvinia,  which 
differs  in  certain  important  particulars  from  the  more  familiar 
members  of  the  group. 

The  Equisetacece  include  the  common  horsetails  (genus 
Equisctum),  a  brief  account  of  which  will  be  given,  as 
they  form  an  interesting  link  in  their  reproductive  processes 
between  the  ordinary  ferns  and  Salvinia. 

The  Lycopodine&i  or  club-mosses,  are  the  highest  of  the 
Cryptogams  or  flowerless  plants.  A  short  description  of  one 
of  them,  the  genus  Selaginella,  will  illustrate  the  most 
striking  peculiarities  of  the  group. 

The  Phanerogams,  or  flowering  plants,  are  so  called  from 
the  fact  that  their  reproductive  organs  take  the  form  of 
specially  modified  shoots,  called  cones  or  flowers.  They  are 
sometimes  called  by  the  more  appropriate  name  of  Sperma- 
phytes,  or  seed-plants,  from  the  fact  that  they  alone  among 


366  EQUISETUM  LESS. 

plants,  reproduce  by  means  of  seeds,  structures  which  differ 
from  spores  in  the  fact  that  each  contains  an  embryo  plant 
in  the  phyllula  stage. 

The  Gymnosperms,  or  naked-seeded  Phanerogams,  include 
the  cone-bearing  trees,  such  as  pines,  larches,  cypresses,  &c., 
as  well  as  cycads  and  some  other  less  familiar  forms.  A 
general  account  of  this  group  will  be  given. 

The  Angiosperms,  or  covered-seeded  Phanerogams,  include 
all  the  ordinary  flowering  plants,  as  well  as  such  trees  as 
oaks,  elms,  poplars,  chestnuts,  &c.  A  brief  description  of 
the  general  features  of  this  group  will  conclude  the  lesson. 

EQUISETUM 

A  horsetail  consists  of  an  underground  creeping  stem 
from  which  vertical  shoots  are  given  off.  Some  of  these 
bear  only  leaves  and  branches,  others  are  peculiarly  modified 
and  produce  sporangia. 

A  fertile  or  sporangium-bearing  shoot  terminates  distally 
in  a  conical  body  (Fig.  84,  A),  formed  of  closely-fitting 
hexagonal  scales  (sp.pti).  Each  scale  (B,  sp.ph)  is  attached 
by  a  stalk  to  the  axis  of  the  shoot,  and  bears  on  its  inner 
surface  a  number  of  sporangia  (spg).  The  scales  are 
modified  leaves,  and  since  they  alone  produce  sporangia 
they  are  distinguished  from  the  ordinary  foliage-leaves  as 
sporophylls. 

The  spores,  which  have  the  same  general  structure  as  those 
of  ferns,  are  liberated  by  the  bursting  of  the  sporangia,  and 
germinate,  giving  rise  to  prothalli.  But  instead  of  the 
prothalli  being  all  alike  in  form  and  size  and  all  monoecious, 
some  (c)  remain  small  and  simple,  and  produce  only 
spermaries  (spy}-,  others  (D)  attain  a  complicated  form  and 
a  length  of  over  a  centimetre,  and  produced  only  ovaries 


XXX 


DIMORPHISM    OF   THE   GAMOBIUM 


367 


(ovy).  Thus  although  there  is  no  difference  in  the  spores, 
the  prothalli  produced  from  them  are  of  two  distinct  kinds, 
the  smaller  being  exclusively  male,  the  larger  female. 


FIG.  84.  —  Reproduction  and  Development  of  Equisetuin. 

A,  distal  end  of  a  fertile  shoot,  showing  two  leaf-sheaths  (/.  sh\  and 
the  cone  formed  of  hexagonal  sporophylls  (sp.  ph}.     (Nat.  size.) 

B,  diagrammatic  vertical  section  of  a  portion  of  the  cone,  showing  the 
sporophylls  (sp.  p/t)  attached  by  short  stalks  to  the  axis  of  the  cone,  and 
bearing  sporangia  (spg)  on  their  inner  surfaces. 

c,  a  male  prothallus  bearing  three  s'permaries  (spy),     (x   100.) 

D,  portion  of  a  female  prothallus  bearing  three  ovaries  (ovy},  those  to 

the  right  and  left  containing  ova,  that  in  the  middle  a  polyplast ;  rh, 

rhizoids.     (  x  30.) 

(A,  after  Le  Maout  and  Decaisne  ;  C  and  D,  after  Hofmeister. ) 

The  oosperm  develops  in  much  the  same  way  as  in 
ferns  :  it  divides  and  forms  a  polyplast,  which,  by  formation 
of  a  stem,  root,  foot,  and  two  cotyledons,  becomes  a 
phyllula  and  grows  into  the  adult  plant, 


368  SALVINIA  LESS. 

As  in  the  fern,  the  Equisetum  plant,  reproducing  as  it 
does  by  asexual  spores,  is  the  agamobium,  the  gamobium 
being  represented  by  the  prothallus.  The  peculiarity  in  the 
present  case  is  that  the  gamobium  is  sexually  dimorphic, 
some  prothalli  producing  only  male,  others  only  female 
gonads. 

SALVINIA 

Salvinia  is  a  fresh-water  plant,  consisting  of  a  long  floating 
stem  bearing  at  intervals  whorls  of  leaves.  Of  these  some 
have  the  ordinary  character  while  others  hang  downwards 
into  the  water  and  have  the  form  and  function  of  roots. 
True  roots  are  absent. 

The  sori  or  groups  of  sporangia  (Fig.  85,  A)  are  borne  on 
the  proximal  ends  of  the  submerged  leaves,  each  being  en- 
closed in  a  globular  case  corresponding  to  the  indusium  of 
ordinary  ferns.  They  differ  from  the  sori  of  the  typical 
ferns  in  being  dimorphic,  some  containing  a  comparatively 
small  number  of  large  sporangia  (mg.  spg)  others  a  much 
larger  number  of  small  ones  (mi.  spg).  The  larger  kind, 
distinguished  as  megasporangia,  contain  each  a  single  large 
spore,  or  megaspore :  the  smaller  kind,  or  microsporangia, 
contain  a  large  number  of  minute  spores,  like  those  of  an 
ordinary  fern,  and  called  microspores.  It  is  this  striking 
dimorphism  of  the  sori,  sporangia,  and  spores  which  forms 
the  chief  distinction  between  Salvinia  and  its  allies  and  the 
true  ferns. 

The  microspore  germinates  (B),  while  still  enclosed  in  its 
sporangium,  by  sending  out  a  filament,  the  end  of  which  (spy] 
becomes  separated  off  by  a  septum  and  then  divided  into 
two  cells.  The  protoplasm  of  each  of  these  divides  into 
four  sperm-mother-cells,  and  from  these,  spirally-twisted 
sperms  are  produced  in  the  usual  manner.  It  is  obvious 


XXX 


REDUCTION    OF   THE   GAMOBIUM 


369 


that  the  two  cells  in  which  the  sperms  are  developed  repre- 
sent a  greatly  simplified  sperm ary  :  the  single  proximal  cell 


mi. 


FIG.  85. — Reproduction  and  Development  of  Salvinia. 

A,  portion  of  a  submerged  leaf,  showing  three  sori  in  vertical  section, 
two  containing  microsporangia  (mi.  spg}  and  one  megasporangia  (mg. 
spg}.     (x   10.) 

B,  a  germinating  microspore  (mi.  ste\  showing  the  vestigial  prothallus 
(prth}  and  spermary  (spy}.     ( x   150.) 

c,  diagrammatic  vertical  section  of  a  germinating  megaspore,  showing 
the  outer  (mg.  sp}  and  inner  (nig.  sp'}  coats  of  the  spore,  and  its  cavity 
(c)  containing  plastic  products,  separated  by  a  septum  (d]  from  the  pro- 
thallus (prth},  in  which  two  ovaries  (ovy}  are  shown,  that  to  the  left 
containing  an  ovum,  that  to  the  right  a  polyplast.  (  x  50.) 

D,  megaspore  (mg.  sp}  with  prothallus  (prth}  and  phyllula  just  begin- 
ning to  develop  into  the  leafy  plant :  st,  stem  ;  cf,  cotyledon  ;  and  /, 
outermost  leaf  of  the  terminal  bud.  (  x  20.) 

(A  and  B,  after  Sachs  ;  D,  after  Pringsheim.) 


(prth)  of  the  filament  arising  from  the  microspore,  a  still 
more  simplified  prothallus.  Both  prothallus  and  spermary 
are  vestigial  structures  ;  the  prothallus  is  microscopic  and 
unicellular  instead  of  being  a  solid  aggregate  of  considerable 
size,  as  in  the  two  preceding  types ;  the  spermary  is  bicellular 

B  B 


370  SALVINIA  LESS. 

instead  of  being  formed  of  a  distinct  wall  and  an  internal 
mass  of  cells ;  and  the  number  of  sperms  is  reduced  to 
eight. 

The  contents  of  the  megaspore  are  divisible  into  a  com- 
paratively small  mass  of  protoplasm  at  one  end  and  of  starch 
grains,  oil-globules,  and  proteid  bodies,  which  fill  up  the  rest 
(c,  c}  of  the  spore.  The  megaspore  has  in  fact  attained  its 
large  size  by  the  accumulation  of  great  quantities  of  plastic 
products,  which  serve  as  nutriment  to  the  future  prothallus 
and  embryo. 

The  protoplasm  of  the  megaspore  (c)  divides  and  forms  a 
prothallus  (prtti}  in  the  form  of  a  three-sided  multicellular 
mass  projecting  from  the  spore,  which  it  slightly  exceeds  in 
size.  Several  ovaries  (oi>y]  are  formed  on  it,  having  much 
the  same  structure  as  in  ordinary  ferns.  Thus  the  reduction 
of  the  prothallus  produced  from  the  megaspore,  although 
obvious,  is  far  less  than  in  the  case  of  that  arising  from  the 
microspore. 

We  see  that  sexual  dimorphism  has  gone  a  step  further  in 
Salvinia  than  in  Equisetum  :  not  only  are  the  prothalli 
differentiated  into  male  and  female,  but  also  the  spores 
from  which  they  arise. 

Impregnation  takes  place  in  the  usual  way,  and  the 
oosperm  divides  to  form  a  polyplast,  which,  by  differentiation 
of  a  stem-rudiment,  a  cotyledon,  and  a  foot,  passes  into  the 
phyllula  stage  :  no  root  is  developed  in  Salvinia.  By  the 
gradual  elongation  of  the  stem  (D,  sf]  and  the  successive 

formation    of    whorls    of    leaves    (/),    the    adult    form    is 

assumed. 

Thus   the   life-history  of  Salvinia  resembles  that  of  the 

fern,  but  with  two   important  differences  :  the  spores  are 

dimorphic,  and  the  gamobium,  represented  by  the  male  and 

female  prothalli,  is  greatly  reduced. 


xxx  SELAGINELLA  371 

SELAGINELLA 

Selaginella,  one  of  the  club-mosses,  consists  of  a  long 
branching  stem  bearing  numerous  close-set  leaves.  It  thus 
resembles  in  external  appearance  a  moss,  but  the  essential 
difference  between  the  two  is  seen  from  a  study  of  their 
histology,  Selaginella  having  a  distinct  epidermis  and 
vascular  bundles  like  the  other  Vascular  Cryptogams. 

The  branches  terminate  in  cones  (Fig.  86,  A)  formed  of 
small  leaves  (sp.  p/i)  which  overlap  in  somewhat  the  same 
way  as  the  scales  of  a  pine-cone.  Each  of  these  leaves  is  a 
sporophyll,  and  bears  on  its  upper  or  distal  side,  near  the 
base,  a  globular  sporangium.  The  sporangia  are  fairly 
uniform  in  size,  but  some  are  megasporangia  (mg.  spg)  and 
contain  usually  four  megaspores,  others  are  microsporangia 
(mi.  spg)  containing  numerous  microspores. 

The  microspore  (B)  cannot  be  said  to  germinate  at  all.  Its 
protoplasm  divides,  forming  a  small  cell  (prtti),  which  repre- 
sents a  vestigial  prothallus,  and  a  large  cell,  the  representative 
of  a  spermary.  The  latter  (spy)  undergoes  further  division, 
forming  six  to  eight  cells  in  which  numerous  sperm-mother- 
cells  are  developed. 

A  similar  but  less  complete  reduction  of  the  prothallus  is  • 
seen  in  the  case  of  the  megaspore  (c).  Its  contents  are 
divided,  as  in  Salvinia,  into  a  small  mass  of  protoplasm  at 
one  end,  and  a  large  quantity  of  plastic  products  rilling  up 
the  rest  of  its  cavity.  The  protoplasm  divides  and  forms  a 
small  prothallus  ( prtti),  and  a  process  of  division  also  takes 
place  in  the  remaining  contents  (prth1)  of  the  spore,  pro- 
ducing a  large-celled  tissue,  the  secondary  prothallus. 

By  the  rupture  of  the  double  cell-wall  of  the  megaspore 
the  prothallus  is  exposed  to  the  air,  but  it  never  protrudes 
through  the  opening  thus  made,  and  is  therefore,  Uklt$hfe\  •" 

snv 


372 


SELAGINELLA 


LESS. 


corresponding  male  structure,  purely  endogenous.  A  few 
ovaries  (ovy)  are  formed  on  it,  each  consisting  of  a  short 
neck,  an  ovum,  and  two  canal-cells  afterwards  converted  into 


spar 
spsr 


FIG.  86. — Reproduction  and  Development  of  Selaginella. 

A,  diagrammatic  vertical  section  of  a  cone,  consisting  of  an  axis  bear- 
ing close-set  sporophylls  (sp.  ph},  on  the  bases  of  which  microsporangia 
(mi.  spg}  and  megasporangia  (mg.  spg)  are  borne. 

B,  section  of  a  microspore,  showing  the  outer  coat  (mi.  sp},  prothallial 
cell  (prt)i)y  and  multicellular  spermary  (spy). 

C,  vertical  section  of  a  megaspore,  the  wall  of  which  (mg.  sp}  has  been 
burst  by  the  growth  of  the  prothallus  (prtfi) :  its  cavity  (prth'}  contains 
a  large-celled  tissue,  the  secondary  prothallus :    in  the  prothallus  are 
three  ovaries  (ovy},  that  to  the  left  containing  an  ovum,  that  to  the  right 
an  embryo  (emb}  in  the  polyplast  stage,  and  that  in  the  centre  an  embryo 
in  the  phyllula  stage,  showing  stem-rudiment  (st}>  foot  (/),   and  two 
cotyledons  (ct}  :  both  embryos  are  provided  with  suspensors  (dotted) 
(spsr),  and  have  sunk  into  the  secondary  prothallus. 

(Altered  from  Sachs. ) 

mucilage  :  there  is  no  venter,  and  the  neck  consists  of  only 
two  tiers  of  cells. 

The  oosperm  divides  by  a  plane  at  right  angles  to  the 
neck  of  the  ovary,  forming  the  earliest  or  two-celled  stage  of 


LESS,  xxx  EMBRYO   AND   SUSPENSOR  373 

the  polyplast.  The  upper  cell  undergoes  further  division, 
forming  an  elongated  structure,  the  suspensor  (dotted  in  c)  : 
the  lower  or  embryo  proper  (emb)  is  forced  downwards  into 
the  secondary  prothallus  by  the  elongation  of  the  suspensor, 
and  soon  passes  into  the  phyllula  stage  by  the  differentiation 
of  a  stem-rudiment  (si),  two  cotyledons  (<:/),  a  foot,  (/)  and 
subsequently  of  a  root. 

A  further  reduction  of  the  gamobium  is  seen  in  Selagi- 
nella  :  both  male  and  female  prothalli  are  quite  vestigial, 
never  emerging  from  the  spores  :  and  the  spermary  and 
ovary  are  greatly  simplified  in  structure. 

GYMNOSPERMS 

Such  common  Gymnosperms  as  the  pines  and  larches 
have  the  character  of  forest  trees,  the  stem  being  a  strong, 
woody  trunk.  The  numerous,  close-set  branches  bear  small, 
needle-like  leaves,  and  the  root  is  large  and  extensively 
branched. 

On  the  branches  are  borne  structures  of  two  kinds,  the 
male  and  female  cones  or  flowers  (Fig.  87,  A  and  c).  Both 
ar.e  to  be  considered  as  abbreviated  shoots  consisting  of  an 
axis  bearing  numerous  sporophylls  (up.  p/i).  Frequently,  as 
in  the  pines,  several  male  cones  are  aggregated  together, 
forming  an  inflorescence,  or  group  of  flowers. 

In  the  male  cone  (A)  the  sporophylls  (stamens,  sp.  ph.  $ ) 
are  more  or  less  leaf-like  structures,  each  bearing  on.  its 
under  or  proximal  side  two  or  more  microsporangia  (pollen- 
sacs,  mi.  spg).  The  mother-cells  of  these  divide  each  into 
four  microspores  (pollen-grains),  which  are  liberated  by  the 
rupture  of  the  microsporangia  in  immense  quantities.  The 
microspore  (B)  is  at  first  an  ordinary  cell  consisting  of  proto- 
plasm with  a  nucleus  and  a  double  cell-wall,  but  upon  being 


FIG.  87. — Reproduction  and  Development  of  Gymnosperms. 
A,  diagrammatic  vertical  section  of  male  cone,  showing  axis  with  male 
sporophylls  (sp.  ph.  6  )  bearing  microsporangia  (mi.  s_/>g)  :  per,  scale-like 
leaves  forming  a  rudimentary  perianth. 


LESS,  xxx  SPOROrilYLLS  .  375 

B,  a  single  microspore,  showing  bladder-like  processes  of  outer  coat, 
and  contents  divided  into  small  prothallial  cell  (a}  and  large  cell  (/'), 
from  which  the  pollen-tube  arises. 

c,  diagrammatic  vertical  section  of  female  cone,  showing  axis  with 
female  sporophylls  (sp.  ph.  ?  )  bearing  megasporangia  (ing.  spg),  each  of 
which  contains  a  single  megaspore  (ing.  sp):  per,  the  scale-like  perianth 
leaves. 

D,  diagrammatic   vertical    section   of  a   megasporangium,    showing 
cellular  coat  (t}>  and  nucellus  (ncl},  micropyle  (inpy},  and  megaspore 
(ing.  sp}  :  the  latter  contains  the   prothallus  (prth}  in  which  are  two 
ovaries,  that  to  the  left  showing  a  large  ovum  (ov}  and  neck-cells,  while 
that  to  the  right  has  given  rise  to  an  embryo  (emb]  which  is  in  the 
phyllula  stage,  and  has  sunk  into  the  tissue  of  the  prothallus  by  the 
elongation  of  the  long  suspensor  (spsr). 

A  microspore  (mi.  sp}  is  seen  in  the  micropyle  sending  off  a  pollen - 
tube  (/.  t},  the  end  of  which  is  applied  to  the  necks  of  the  two  ovaries. 

E,  diagrammatic  vertical  section  of  a  seed,  showing  coat  (/),  micro- 
pyle (inpy},  and  endosperm  (end],  in  which  is  embedded  an  embryo  in 
the  phyllula  stage,  consisting  of  stem-rudiment  (st),  cotyledons  (ct},  and 
root  (r). 

(A  and  B,  altered  from  Strasburger  ;  D  and  E,  altered  from  Sachs. ) 

liberated  the  protoplasm  divides,  as  in  Selaginella,  into  two 
cells,  a  small  one  (a)  the  vestige  of  the  male  prothallus,  and 
a  large  one  (b)  which  does  not  develop  sperms,  but  under 
favourable  circumstances  undergoes  changes  which  will  be 
described  presently. 

In  the  female  cone  (c)  each  sporophyll  (carpel,  sp.  ph,  2  ) 
bears  on  its  upper  or  distal  side  two  megasporangia  (so-called 
ovules,  mg.  spg)  the  structure  of  which  is  peculiar.  Each 
consists  of  a  solid  mass  of  small  cells  called  the  nucellus  (v,ncl\ 
attached  by  its  proximal  end  to  the  sporophyll,  and  sur- 
rounded by  a  wall  or  integument  (/)  also  formed  of  a  small- 
celled  tissue.  The  integument  is  in  close  contact  with  the 
nucellus,  but  is  perforated  distally  by  an  aperture,  the 
micropyle  (mpy\  through  which  a  small  area  of  the  nucellus 
is  exposed. 

Each  megasporangium  contains  only  a  single  megaspore 
(embryo  sac,  c  and  D,  mg.  sp)  in  the  form  of  a  large  ovoidal 
body  embedded  in  the  tissue  of  the  nucellus.  It  has  at 


376  GYMNOSPERMS  LESS. 

first  the  characters  of  a  single  cell,  but  afterwards,  by  division 
of  its  protoplasm,  becomes  filled  with  small  cells  representing 
a  prothallus  (prth\  As  in  Vascular  Cryptogams,  single 
superficial  cells  of  the  prothallus  are  converted  into  ovaries 
which  are  extremely  simple  in  structure,  each  consisting 
of  a  large  ovum  (ov),  and  of  a  variable  number  of  neck- 
cells. 

The  pollen,  liberated  by  the  rupture  of  the  microsporangia, 
is  carried  to  considerable  distances  by  the  wind,  some  of  it 
falling  on  the  female  cones  of  the  same  or  another  tree.  In 
this  way  single  microspores  (pollen-grains)  find  their  way 
into  the  micropyle  of  a  megasporangium  (D,  mi.  sp).  This 
is  the  process  known  as  pollination,  and  is  the  necessary 
antecedent  of  fertilization. 

The  microspore  now  germinates  :  the  outer  coat  bursts, 
and  the  larger  of  the  two  cells  (B,  b)  protrudes  in  the  form 
of  a  filament  resembling  a  hypha  of  Mucor,  and  called  a 
pollen-tube  (D,  p.t).  This  forces  its  way  into  the  tissue  of 
the  nucellus,  like  a  root  making  its  way  through  the  soil, 
and  finally  reaches  the  megaspore  in  the  immediate  neigh- 
bourhood of  an  ovary.  A  process  then  grows  out  from  the 
end  of  the  tube,  passes  between  the  neck-cells,  and  comes  in 
contact  with  the  ovum. 

In  the  meantime  the  nucleus  of  the  large  cell  (b)  of  the 
microspore— that  from  which  the  pollen-tube  grows — has 
travelled  to  the  end  of  the  pollen-tube  and  divided  into  two. 
Protoplasm  collects  round  each  of  the  daughter  nuclei,  con- 
verting them  into  cells,  one  of  which  remains  undivided, 
while  the  other  divides,  and  its  substance  passes  from  the 
pollen-tube  into  the  ovum,  where  it  forms  a  cell-like  body, 
to  which  the  name  of  male  pronucleus  (see  p.  263)  has 
been  applied.  This  conjugates  with  the  nucleus  of  the 
ovum,  or  female  pronucleus,  and  thus  effects  the  process 


xxx  FORMATION    OF   THE    SEED  377 

of  fertilization,    or   the  conversion  of  the  ovum  into  the 
oosperm. 

The  mode  of  formation  of  cells  described  in  the  preceding 
paragraph  should  be  specially  noted.  Instead  of  the  ordin- 
ary process  of  fission  hitherto  met  with,  the  products  of 
division  of  a  nucleus  become  surrounded  by  protoplasm, 
cells  being  produced  which  lie  freely  in  the  interior  of  the 
mother-cell.  This  is  called  free  cell-formation. 

The  development  of  the  oosperm  is  a  very  complicated 
process,  and  results  in  the  formation  not  of  a  single  polyplast 
but  of  four,  each  at  the  end  of  a  long  suspensor  (D,  spsr\ 
in  the  form  of  a  linear  aggregate  of  cells,  which  by  its  elonga- 
tion carries  the  embryo  (emb)  down  into  the  tissue  of  the 
prothallus.  As  a  rule  only  one  of  these  embryos  comes  to 
maturity  :  it  develops  a  rudimentary  stem,  root,  and  four 
or  more  cotyledons,  and  so  becomes  a  phyllula. 

While  these  processes  are  going  on  the  female  cone 
increases  greatly  in  size  and  becomes  woody.  The  mega- 
sporangia  also  become  much  larger,  their  integuments  (E,  /), 
becoming  brown  and  hard,  and  the  megaspore  in  each 
enlarges  so  much  as  to  displace  the  nucellus  :  at  the  same 
time  the  cells  of  the  prothallus  filling  the  megaspore  develop 
large  quantities  of  plastic  products,  such  as  fat  and  albumin- 
ous substances,  to  be  used  in  the  nutrition  of  the  embryo  : 
the  tissue  thus  formed  is  the  endosperm  (end).  The  mega- 
sporangium  is  now  called  a  seed  (see  p.  365). 

Under  favourable  circumstances  the  seed  germinates. 
By  absorption  of  moisture  its  contents  swell  and  burst  the 
seed-coat,  and  the  root  of  the  phyllula  (r)  emerges,  followed 
before  long  by  the  stem  (j/)  and  cotyledons  (ct).  The 
phyllula  thus  becomes  the  seedling  plant,  and  by  further 
growth  and  the  successive  formation  of  new  parts  is  converted 
into  the  adult 


378  ANGIOSPERMS  LESS,  xxx 

In  Gymnosperms  we  s£e  an  even  more  striking  reduction 
of  the  gamobium  than  in  Selaginella.  The  female  prothallus 
is  permanently  inclosed  in  the  megaspore,  and  the  megaspore 
in  the  megasporangium  :  the  ovaries  also  are  greatly  simplified. 
The  male  prothallus  is  represented  by  the  smaller  cell  of  the 
microspore,  and  no  formation  of  sperms  takes  place,  fertiliza- 
tion being  effected  by  cells  developed  in  the  extremity  of  a 
tubular  prolongation  of  the  larger  cell  of  the  microspore,  and 
resulting  from  a  modification  of  its  nucleus. 

It  is  worthy  of  notice  that  Phanerogams  alone  among 
the  higher  organisms,  have  abandoned  the  ordinary  method 
of  fertilization  by  the  conjugation  of  ovum  and  sperm.  In 
this  respect  they  are  the  most  specialized  of  living  things, 

ANGIOSPERMS 

In  this  group  the  general  relations  of  the  main  parts  of 
the  plant — stem,  leaves,  roots,  &c. — are  the  same  as  in 
Gymnosperms. 

The  flowers,  in  which,  as  in  Gymnosperms,  the  organs  of 
reproduction  are  contained,  have  a  very  characteristic  struc- 
ture, which,  although  presenting  almost  infinite  variety  in 
detail,  is  the  same  in  its  essential  features  throughout  the 
group. 

A  typical  angiospermous  flower  (Fig.  88,  A)  is  a  greatly 
abbreviated  shoot,  consisting  of  a  short  axis  (fl.  r)  of  limited 
growth  bearing  four  whorls  of  leaves,  of  which  those  of  the 
two  distal  whorls  are  sporophylls. 

The  axis  of  the  floral  shoot  (A.  fl.  r)  is  usually  broad  and 
more  or  less  conical  in  form  and  is  called  the  floral 'receptacle. 
The  leaves  of  the  lower  or  proximal  whorl  (pcr^\  usually 
from  three  to  five  in  number,  are  small  green  bodies  which 
cover  the  other  parts  in  the  unopened  flower  :  they  are  called 
sepals  and  together  constitute  the  calyx. 


ntZ 


FIG.  88. — Reproduction  and  Development  of  Angiosperms. 
A,  diagrammatic  vertical  section  of  a  flower  consisting  of  an  abbreviated 
axis  or  floral  receptacle  (ft.  r)  bearing  a  proximal  (per1}  and  a  distal 
(per"}  whorl  of  perianth   leaves  (sepals  and  petals),  a  whorl  of  male 


380  ANGIOS  PERMS  LESS. 

sporophylls  or  stamens  (sp.  ph.  cJ ),  and  one  of  female  sporophylls  or 
carpels  (sp.  ph.  9  ). 

The  male  sporophyll  bears  microsporangia  (mi.  spg)  containing 
microspores  (mi.  sp). 

The  female  sporophyll  consists  of  a  solid  style  (st)  terminated  by  a 
stigma  (sfg),  and  of  a  hollow  venter  (v)  containing  a  megasporangium 
(nig.  spg)  in  which  is  a  single  megaspore  (nig.  sp}. 

On  the  right  side  a  microspore  is  shown  on  the  stigma,  and  has  sent 
off  a  pollen-tube  (p.  t)  through  the  tissue  of  the  style  to  the  micropyle 
of  the  megasporangium. 

B1,  diagram  of  a  female  sporophyll  from  the  distal  aspect,  and  B2,  the 
same  in  transverse  section,  showing  the  folding  in  of  its  edges  to  form 
the  cavity  or  venter  in  which  the  megasporangia  (mg.  spg)  are  enclosed  : 
m.r,  the  midrib. 

C1,  a  microspore,  showing  the  two  cells  (a  and  b)  into  which  its 
contents  divide. 

C2,  the  same,  sending  out  a  pollen-tube  (p.  t)  :  nu,  mtlt  the  two  nuclei. 

D,  diagrammatic  vertical  section  of  a  megasporangium,  showing  the 
double  integument  C/1,/2),  nucellus  (ncl),  micropyle  (m.py),  and  mega- 
spore  (mg.  sp) :  the  latter  contains  the  secondary  nucleus  (nu)  in  the 
centre,  three  antipodal  cells  (ant)  at  the  proximal  end,  and  two  syner- 
gidse  (sng)  and  an  ovum  (ov)  at  the  distal  end. 

A  pollen-tube  (p.  t)  is  shown  with  its  end  in  contact  with  the 
synergidae. 

E,  semi-diagrammatic   section  of  the  megaspore  of  a  young   seed, 
showing  an  embryo  (emb)   in  the  polyplast   stage  with  its  suspensor 
(spsr)  :  also  numerous  vacuoles  (vac)  and  nuclei  (nu). 

F,  diagrammatic  vertical  section  of  a  ripe  seed,  showing  the  seed-coat 
(/),  mieropyle  (mpy\  perisperm  (per)  derived  from  the  tissue  of  the 
nucellus,  and  endosperm  (end}  formed  in  the  megaspore  and  containing 
an  embryo  in  the  phyllula  stage  with  stem-rudiment  (st),  cotyledons  (ct), 
and  root  (r). 

(B1  after  Behrens  ;  c1,  c2,  and  E  altered  from  Howes.) 

Above  the  sepals  comes  a  whorl  of  leaves  (per*),  usually 
of  large  size  and  bright  colour,  forming  in  fact  the  most 
obvious  part  of  the  flower.  These  are  the  petals  and  together 
constitute  the  corolla.  The  calyx  and  corolla  together  are 
conveniently  called  the  perianth,  because  they  inclose  the 
sporophylls  or  essential  part  of  the  flower.  The  presence  of 
a  well-marked  perianth  is  characteristic  of  the  majority  of 
Angiosperms,  and  distinguishes  them  from  Gymnosperms,  in 
which  this  part  of  the  flower  is  quite  rudimentary  (see  Fig. 
87,  A.  and  P.,  per}. 


xxx  SPOROPHYLLS  381 

The  third  whorl  is  called  collectively  the  &ndrad*m,  and 
consists  of  a  variable  number  of  stamens  or  male  sporo- 
phylls(-y/./>/2.  $ ).  Each  stamen  is  along  narrow  leaf  bearing  at 
its  distal  end  four  microsporangia  (pollen  sacs,  mi.  spg)  united 
into  a  lobed  knob-like  body,  the  anther.  The  microspores 
(cl)  are  at  first  simple  cells  with  double  cell-walls,  but  sub- 
sequently the  protoplasm  becomes  divided  into  two  cells, 
as  in  Gymnosperms,  a  smaller  (a)  and  a  larger  (fr).  The  two 
are  not,  however,  separated  by  a  firm  septum  of  cellulose, 
and  the  smaller  cell  frequently  comes  to  lie  freely  in  the 
protoplasm  of  the  larger.  Moreover  it  appears  that  the 
nucleus  of  the  smaller  is  the  active  agent  in  fertilization,  and 
that  the  larger  must  therefore  be  considered  as  representing 
the  vestigial  prothallus. 

The  fourth  or  distal  whorl  of  the  flower  is  called  collec- 
tively the  gyncecium  or  pistil,  and  consists  of  one  or  more 
carpels  or  female  sporophylls  (sp.  ph.  $),  which  are  modified 
in  a  characteristic  manner.  In  some  cases  each  carpel  (B1, 
B2)  becomes  folded  longitudinally  along  its  midrib  (m.r),  and 
its  two  edges,  thus  brought  into  contact,  unite  so  as  to 
inclose  a  cavity.  Concrescence  only  affects  the  proximal 
part  of  the  carpel,  which  thus  becomes  a  hollow  capsule,  the 
venter  (so-called  ovary,  A,  v)  :  its  distal  portion  usually  takes 
the  form  of  a  slender  rod-like  body,  the  style  (j/),  terminated 
by  an  enlarged  extremity,  the  stigma  (stg)  which  is  covered 
with  hairs  and  is  frequently  sticky.  In  some  flowers,  on  the 
other  hand,  all  the  carpels  of  the  gynaecium  unite  with  one 
another  by  their  adjacent  edges,  so  as  to  inclose  a  cavity 
common  to  all  :  in  this  case  also  the  hollow  portion  or  venter 
is  formed  by  the  proximal  part  only  of  the  carpels,  their 
distal  portions  forming  a  simple  or  multiple  style  and 
stigma. 

The  megasporangia  (ovules,  A  and  B,  mg.  spg)  are  usually 


382          CHARACTERS   OF   THE   HIGHER    PLANTS      LESS. 

borne  on  the  edges  of  the  carpels,  and,  owing  to  the  union 
of  the  latter,  become  inclosed  in  the  cavity  of  the  venter, 
and  are  thus  completely  shut  off  from  all  direct  communica- 
tion with  the  external  world.  It  is  this  inclosure  of  the 
megasporangia  in  a  cavity  formed  by  the  sporophylls  on 
which  they  are  borne  which  constitutes  the  chief  character 
distinguishing  Angiosperms  from  Gymnosperms. 

The  megasporangia  (D)  differ  from  those  of  Gymnosperms 
chiefly  in  having  a  double  integument  :  both  coats  (tl,  /2)  as 
well  as  the  nucellus  (nd\  or  central  mass  of  tissue,  are  com- 
posed of  small  cells  :  and  the  megaspore  (embryo-sac,  mg. 
sp)  is  a  single  cell  of  great  size  embedded  in  the  nucellus. 

No  prothallus  is  formed  in  the  megaspore,  but  its  nucleus 
divides,  the  products  of  division  pass  to  opposite  ends  of  the 
spore,  and  each  divides  again  and  then  again,  so  that  four 
nuclei  are  produced  at  each  extremity.  Three  of  the  nuclei 
at  the  proximal  end — that  furthest  from  the  micropyle — 
become  surrounded  by  protoplasm  and  take  on  the  character 
of  cells  (D,  ant] :  the  fourth  remains  unchanged.  Similarly 
of  the  four  nuclei  at  the  distal  or  micropylar  end,  one 
remains  unchanged  and  three  assume  the  form  of  cells  by 
becoming  invested  with  protoplasm  (see  p.  376).  Of  these 
three,  two  lie  near  the  wall  of  the  megaspore  and  are  called 
synergidce  (sng)  :  the  third,  more  deeply  placed,  is  the  ovum 
(ov).  The  two  unaltered  nuclei  now  travel  to  the  centre  of 
the  megaspore  and  unite  with  one  another,  forming  the 
secondary  nucleus  (nu)  of  the  spore. 

There  is  thus  a  single  ovum  produced  in  each  megaspore, 
but  no  ovary  and  no  prothallus  :  the  female  portion  of  the 
gamobium  is  reduced  to  its  simplest  expression. 

Pollination  may  take  place,  as  in  Gymnosperms,  by  the 
agency  of  the  wind,  but  usually  the  microspores  are  carried 
by  insects,  which  visit  the  flowers  for  the  sake  of  obtaining 


xxx  POLLINATION   AND   FERTILIZATION  383 

nectar,  a  saccharine  fluid*  secreted  by  certain  parts.  The 
microspores  are  deposited  on  the  stigma  (A),  where  they 
germinate,  each  sending  off  a  pollen-tube  (A  and  c2,  /.  /), 
which  grows  downwards  through  the  tissue  of  the  stigma  and 
style  to  the  cavity  of  the  venter,  where  it  reaches  a  megaspo- 
rangium,  and  entering  at  the  micropyle  (D,  p.  /),  continues 
its  course  through  the  nucellus,  finally  applying  itself  to  the 
distal  end  of  the  megaspore  in  the  immediate  neighbourhood 
of  the  synergidae. 

In  the  meantime  the  nuclei  of  the  microspore  (c2,  nu, 
iiu*)  have  passed  into  the  end  of  the  pollen-tube.  The 
nucleus  of  the  larger  cell  undergoes  degeneration,  becoming 
shrivelled  and  unaffected  by  dyes ;  that  of  the  smaller  cell 
divides  by  karyokinesis.  One  of  the  two  daughter-nuclei 
thus  formed  also  degenerates,  the  other,  accompanied  by 
its  directive  spheres,  passes  through  the  softened  cell-wall  of 
the  swollen  end  of  the  pollen-tube  and  enters  the  ovum, 
uniting  with  its  nucleus  in  the  usual  way. 

The  ovum  is  thus  converted  into  an  oosperm  or  unicellu- 
lar embryo  :  it  acquires  a  cell-wall  and  almost  immediately 
divides  into  two  cells,  of  which  that  nearest  the  micropyle 
becomes  the  suspensor  (E,  spsr\  the  other,  or  embryo 
proper  (emb\  forming  a  solid  aggregate  of  cells,  the 
polyplast.  By  further  differentiation  rudiments  of  a  stem 
(F,  st),  a  root  (r)  and  either  one  or  two  cotyledons  (<:/) 
are  formed,  and  the  embryo  passes  into  the  phyllula  stage. 

While  the  early  development  of  the  embryo  is  going  on, 
the  secondary  nucleus  of  the  megaspore  divides  repeatedly, 
and  the  products  of  division  (E,  nu]  becoming  surrounded 
by  protoplasm,  a  number  of  cells  are  produced,  which,  by 
further  multiplication,  fill  up  all  that  part  of  the  megaspore 
which  is  not  occupied  by  the  embryo.  The  tissue  thus 
formed  is  called  the  endosperm  (F.  end),  and  occupies  pre- 


384  ANGIOSPERMS 


LESS. 


cisely  the  position  of  the  vestigial  prothallus  of  Gymnosperms 
(Fig,  87,  p.  374,  n,prth,  and  E,  end:  and  p.  376),  differing 
from  it  in  the  fact  that  it  is  only  formed  after  fertilization. 
We. have  here  a  case  of  retarded  development  :  the  degenera- 
tion of  the  prothallus  has  gone  so  far  that  it  arises,  by  free 
cell-formation,  long  after  the  formation  of  the  ovum  which, 
in  both  Gymnosperms  and  Vascular  Cryptogams,  is  a 
specially  modified  prothallial  cell. 

The  phyllula  continues  to  grow  and  remains  inclosed  in 
the  megasporangium,  which  undergoes  a  corresponding  in- 
crease in  size  and  becomes  the  seed.  One  or  more  seeds 
also  remain  inclosed  in  the  venter  of  the  pistil,  which  grows 
considerably  and  constitutes  \\\Q  fruit.  Finally  the  seeds  are 
liberated,  the  phyllula  protrudes  first  its  root,  and  then  its 
stem  and  cotyledons  through  the  ruptured  seed-coat,  and 
becomes  the  seedling  plant. 

We  learn  from  the  present  lesson  that  there  is  a  far  greater 
uniformity  of  organization  among  the  higher  plants  than 
among  the  higher  animals,  not  only  in  anatomical  and 
histological  structure,  but  also  in  the  fact  that  alternation  of 
generations  is  universal  from  Nitella  and  the  mosses  up  to 
the  highest  flowering  plants.  But  as  we  ascend  the  series, 
the  gamobium  sinks  from  the  position  of  a  conspicuous 
leafy  plant  to  that  of  a  small  and  insignificant  prothallus, 
becoming  finally  so  reduced  as  to  be  only  recognizable  as 
such  by  comparison  with  the  lower  forms. 


SYNOPSIS 


A.—  AN  ACCOUNT  OF  THE  STRUCTURE,  PHYSIOLOGY, 
AND  LIFE-HISTORY  OF  A  SERIES  OF  TYPICAL 
ORGANISMS  IN  THE  ORDER  OF  INCREASING 
COMPLEXITY. 


I.  —  THE  SIMPLER  UNICELLULAR  ORGANISMS. 


I'AGR 

Cell-body  amoeboid  or  encysted  :  cell-wall  nitrogenous  (?): 
nutrition  holozoic  :  reproduction  by  simple  or  binary 
fission  ......................  |!  i 

2.  Htcmatococcus. 

Cell-body  ciliated  or  encysted  :  cell-wall  of  cellulose  : 
nutrition  holophytic  :  reproduction  by  binary  fission  .  .  23 

3.  Heteromita. 

Cell-body  ciliated  :  nutrition  saprophytic  :  asexual  repro- 
duction by  binary  fission  :  sexual  reproduction  by  conju- 
gation of  equal  and  similar  gametes  followed  by  multiple 
fission  of  the  protoplasm  of  the  zygote,  forming  spores  .  36 

4.  Euglena. 

Cell-body  ciliated  or   encysted  :    cell-wall  of  cellulose 
mouth   and   gullet    present  :    nutrition    holophytic    and 
holozoic  :  reproduction  by  binary  and  multiple  fission  .  .'      44 

5.  Protomyxa. 

Cell-body  amoeboid,  ciliated  or  encysted  :  plasmodia 
formed  by  concrescence  of  amcebulse  :  cell-wall  nitro- 
genous (?)  :  nutrition  holozoic  :  reproduction  by  multiple 
fission  of  encysted  plasmodium  ..........  49 

C  C 


386  SYNOPSIS 

PAGE 

6.  Mycetozoa. 

Like  Protomyxa,  but  owing  to  the  presence  of  nuclei  the 
relation  of  the  individual  cell-bodies  to  the  plasmodium 
is  more  clearly  seen  :  cell  wall  of  cellulose 52 

o  7-  Saccharomyces. 

Cell-body  encysted :  cell-wall  of  cellulose  :  nutrition 
saprophytic  :  reproduction  by  gemmation  or  by  internal 
fission  :  acts  as  an  organized  ferment 7° 

-  8.  Bacteria. 

Cell-body  ciliated  or  encysted  :  cell-wall  of  cellulose  : 
nutrition  saprophytic  :  reproduction  by  binary  fission  or 
by  spore-formation :  act  as  organized  ferments  :  the 
simplest  and  most  abundant  of  organisms 82 


II. — UNICELLULAR  ORGANISMS  IN  WHICH  THERE  is  CONSIDERABLE 
COMPLEXITY  OF  STRUCTURE  ACCOMPANIED  BY  PHYSIOLOGICAL 
DIFFERENTIATION. 

a.   Complexity  attained  by  differentiation  of  cell-body. 

9.   Parani(Kcium . 

Medulla,  cortex,  and  cuticle  :  trichocysts  :  complex  con- 
tractile vacuoles :  nucleus  and  paranucleus  :  mouth, 
gullet,  and  anal  spot  :  conjugation  temporary,  no  zygote 
being  formed,  but  interchange  of  nuclear  material  during 
temporary  union 106 

10.  Stylonychia. 

Extreme  differentiation  or  heteromorphism  of  cilia  .    .    .     116 

1 1 .  Oxytricha. 

Fragmentation  of  nucleus 120 

1 2    Opalina. 

Multiplication  of  nuclei  ;  parasitism  and  its  results ; 
necessity  for  special  means  of  dispersal  of  an  internal 
parasite 121 

13.  Vorticella. 

A  stationary  organism :  limitation  of  cilia  to  defined 
regions :  muscle- fibre  in  stalk  :  necessity  for  means  of 
dispersal  in  a  fixed  organism  :  conjugation  between  free- 
swimming  micro-  and  fixed  mega-gamete  :  zygote  indis- 
tinguishable from  a  zooid  of  the  ordinary  kind  ....  126 

14.  Zoothamnium. 

A  compound  organism  or  colony  with  dimorphic  (nutri- 
tive and  reproductive)  zooids  :  begins  life  as  a  single 
zooid " 135 


SYNOPSIS  387 

PAGE 

/>.   Complexity  attained  by  differentiation  of  cell-wall  or  by  forma- 
tion of  skeletal  structures  in  the  protoplasm. 

1 5  •   Foraminifera . 

Calcareous  shells  (cell-walls)  of  various  and  complicated 
form 148 

1 6.  Radiolaria. 

Membranous  perforated  shell  (cell-wall)  and  external 
silicious  skeleton  often  of  great  complexity  :  symbiotic 
relations  with  Zooxanthella 152 

17.  Diatoms. 

Silicious,  two-valved,  highly-ornamented  shells    ....     155 

c.   Complexity  attained  by  simple  elongation  and  branching  of  the 
cell. 

1 8.  Mucor. 

A  branching  filamentous  fungus  :  necessity  for  special 
reproductive  organs  in  such  an  organism  :  they  may  be 
sporangia  producing  asexual  spores,  or  equal  and  similar 
gametes  producing  a  resting  zygote 158 

19.  Vaiicheria. 

A  branched  filamentous  alga :  clear  distinction  between 
the  gametes  or  conjugating  bodies  and  the  sexual  repro- 
ductive organs  or  gonads  in  which  they  are  produced  : 
gonads  differentiated  into  male  (spermary)  and  female 
(ovary) :  gametes  differentiated  into  male  (sperm)  and 
female  (ovum)  :  zygote  an  oosperm 169 

20.  Caulerpa. 

Illustrates  maximum  differentiation  ol  a  unicellular 
plant :  stem-like,  leaf-like,  and  root-like  parts 175 

III. — ORGANISMS  IN  WHICH  COMPLEXITY  is  ATTAINED  BY  CELL- 
MULTIPLICATION,  ACCOMPANIED  BY  NO  OR  BUT  LITTLE  CELL- 
DlFFERENTIATION. 

a.  Linear  aggregates. 

21.  Penicillium. 

A  multicellular,  filamentous,  branched  fungus  :  mycelial, 
submerged,  and  aerial  hyphae  :  apical  growth  :  abundant 
production  of  spores  by  constriction  of  aerial  hyphse  .  .  184 

22.  Agaricus. 

Complexity  attained  by  interweaving  of  hyphae  in  a  de- 
finite form  :  illustrates  maximum  complexity  of  a  linear 

aggregate > 

C    C    2 


388  SYNOPSIS 

PAGE 

23.  Spirogyra. 

A  multicellular  filamentous  unbranched  alga  :  interstitial 
growth  :  gonads  equal  and  similar,  but  gametes  show 
first  indication  of  sexual  differentiation 194 

/'.   Superficial  aggregate. 

24.  Moiwstroma. 

Cell-division  takes  place  in  two  dimensions 202 

c.  Solid  aggregates. 

25.  Ulva. 

Like  Monostroma,  but  cell -division  takes  place  in  three 
dimensions 203 

26.  Laminaria. 

Illustrates  maximum  size  and  complexity  of  a  solid 
aggregate  of  comparatively  slightly  differentiated  cells  .  203 


IV. — SOLID  AGGREGATES   IN    WHICH   COMPLEXITY  is  INCREASED 

BY   A   LIMITED   AMOUNT   OF   CELL-DlFFERENTIATION. 

27.  Nite.Ua. 

Segmented  axis  :  nodes  and  internodes  :  appendages — 
leaves  and  rhizoids  :  apical  growth  by  binary  fission  of 
apical  cell  accompanied  by  immediate  division  and  dif- 
ferentiation of  newly-formed  segmental  cells  :  complex 
gonads  (ovaries  and  spermaries)  :  alternation  of  genera- 
tions, a  gamobium  or  sexual  generation  (the  leafy  plant) 
alternating  with  an  agamobium  or  asexual  generation 
(the  pro-embryo) 206 

28.  Hydra. 

Example  of  a  simple  diploblastic  animal  :  cells  arranged 
in  two  layers  (ecto-  and  endoderm)  inclosing  an  enteron 
which  opens  externally  by  the  mouth  :  combination  of 
intra-cellular  with  extra-cellular  or  enteric  digestion  .  .  221 

29.  Bougainvillea. 

Example  of  a  colony  with  diploblastic  zooids  which  are 
nutritive  (hydranths)  and  reproductive  (medusae) :  differ- 
entiation of  a  rudimentary  mesoderm  producing  imper- 
fect tripoblastic  condition  :  central  and  peripheral  nervous 
system :  alternation  of  generations,  a  gamobium  (the 
medusa)  alternating  with  an  agamobium  (the  hydroid 
colony)  ;  significance  of  developmental  stages — oosperm 
(unicellular),  polyplast  (multicellular  but  undifferentiated), 
and  planula  (diploblastic) 237 


SYNOPSIS  389 

PAGE 

30.  Dipkyes. 

A  free-swimming   colony   with   polymorphic  (nutritive, 
reproductive,  protective,  and  natatory)  zooids 250 

31.  For  pita. 

Extreme  polymorphism  of  zooids  giving  the  colony  the 
character  of  a  single  physiological  individual 253 


V. — SOLID  AGGREGATES  IN  WHICH  CELL-DIFFERENTIATION,  AC- 
COMPANIED BY  CELL  FUSION,  TAKES  AN  IMPORTANT  PART  IN 
PRODUCING  GREAT  COMPLEXITY  IN  THE  ADULT  ORGANISM. 

32.  Polygordius. 

A  triploblastic,  coelomate  animal  with  metameric  seg- 
mentation :  prostomium,  peristomium,  metameres,  and 
anal  segment  :  besides  ecto-  and  endoderm  there  is  a 
well  developed  mesoderm  divided  into  somatic  and 
splanchnic  layers  separated  by  the  coelome  :  differentia- 
tion of  cells  into  fibres,  &c. :  muscle-plates  formed  as 
cell-fusions  :  necessity  for  distributing  system  for  supply 
of  food  to  parts  of  the  body  other  than  the  enteric  canal, 
and  for  the  removal  of  waste  matters  : — circulatorv,  res- 
piratory, and  excretory  systems :  high  development  of 
nervous  system — brain  and  ventral  cord,  afferent  and 
efferent  nerves  :  characteristic  developmental  stages — 
oosperm,  polyplast,  gastrula  (diploblastic),  trochosphere 
(diploblastic  with  stomodseum  and  proctodeeum),  late 
trochosphere  (triploblastic  but  accelomate) 271 

33.  Mosses. 

Cell-differentiation  very  slight,  but  the  type  necessary  to 
lead  up  to  ferns  :  sclerenchyma  and  axial  bundle  :  dis- 
tributing system  rendered  necessary  by  carbon  dioxide 
being  taken  in  by  the  leaves,  water  and  mineral  salts  by 
the  rhizoids  :  alternation  of  generations— the  leafy  plant 
is  the  gamobium,  the  agamobium  being  represented  by 
the  spore-producing  sporogonium  :  developmental  stages 
— oosperm  and  polyplast,  the  latter  becoming  highly  diffe- 
rentiated to  form  the  sporogonium 332 

34.  Ferns. 

Extensive  cell-differentiation  :  formation  of  fibres  (elon- 
gated cells)  and  vessels  (cell-fusions) ;  general  differentia- 
tion of  tissues  into  epidermis,  ground-parenchyma,  and 
vascular  bundles  :  presence  of  true  roots  :  the  leafy  plant 
is  the  amagobium  and  produces  spores  from  which  the 
gamobium,  in  the  form  of  a  small  prothallus,  arises  : 
developmental  stages — oosperm,  polyplast,  and  phyllula 
(leaf-  and  root-bearing  stage)  , 344 


390  SYNOPSIS 


VI. — BRIEF  DESCRIPTIONS  OF  TYPES  OF  THE  HIGHER  GROUPS  OF 
ANIMALS  AND  PLANTS  IN  TERMS  OF  POLYGORDIUS  AND  OF 
THE  FERN  RESPECTIVELY. 


a.  Animals. 

All  are  triploblastic  and  ccelomate. 

35.  Starfish. 

Radially  symmetrical :  discontinuous  dermal  exoskele- 
ton  :  characteristic  organs  of  locomotion  (tube  feet)  in 
connection  with  ambulacral  system  of  vessels 309 

36.  Crayfish. 

Metamerically  segmented :  segmented  lateral  append- 
ages :  differentiation  of  metameres  and  appendages  : 
continuous  cuticular  exoskeleton  discontinuously  calci- 
fied :  gills  as  paired  lateral  offshoots  of  the  body-wall : 
heart  as  muscular  dilatation  of  dorsal  vessel :  ccelome 
greatly  reduced  and  its  place  taken  by  an  extensive  series 
of  blood-spaces  :  nervous  system  sunk  in  the  mesoderm 
and  consisting  of  brain  and  ventral  nerve-cord '.  ....  314 

37.  Mussel. 

Non-segmented  :  mantle  formed  as  paired  lateral  out- 
growths of  dorsal  region  :  foot  as  unpaired  median  out- 
growth of  ventral  region  :  cuticular  exoskeleton  in  the 
form  of  a  calcified  bivalved  shel.l :  gills  as  paired  lateral 
outgrowth  of  body-wall :  heart  as  muscular  dilatation  of 
dorsal  vessel :  coslome  reduced  to  pericardium  :  nervous 
system  consists  of  three  pairs  of  ganglia  sunk  in  the 
mesoderm 320 

38.  Dogfish. 

Metamerically  segmented :  differentiated  into  head,  trunk, 
and  tail  :  trunk  alone  ccelomate  in  adult :  appendages  as 
median  (dorsal,  ventral,  and  caudal)  and  paired  (pectoral 
and  pelvic)  fins  :  discontinuous  dermal  exoskeleton  and 
extensive  endoskeleton  of  partially  calcified  cartilage, 
including  a  chain  of  vertebral  centra  below  the  nervous 
system  replacing  an  embryonic  notochord :  gills  as 
pouches  of  pharynx  opening  on  exterior :  heart  as 
muscular  dilatation  of  ventral  vessel :  hollow  dorsal 
nervous  system  not  perforated  by  enteric  canal 324 

b.   Plants. 

All  exhibit  alternation  of  generations  and  the  series 
shows  the  gradual  subordination  of  the  gamobium  to  the 
agamobium. 


SYNOPSIS  91 

PAGE 

39.  Equisetum. 

Sporangia  borne  on  sporophylls  arranged  in  cones : 
spores  homomorphic :  prothalli  dimorphic  (male  and 
female) • 366 

40.  Salvinia. 

Spores  dimorphic  ;  microspore  produces  vestigial  male 
prothallus  :  megaspore  produces  greatly  reduced  female 
prothallus • 368 

41.  Selaginella. 

Microspore  produces  unicellular  prothallus  and  multi- 
cellular  spermary,  both  endogenously :  female  prothallus 
formed  in  megaspore  and  is  almost  endogenous  :  embryo 
provided  with  suspensor 371 

42.  Gymnosperms. 

Cones  dimorphic  (male  and  female),  with  rudimentary 
perianth  :  no  sperms  formed  but  microspore  gives  rise  to 
pollen  tube,  nuclei  in  which  are  the  active  agents  in  fer- 
tilization :  single  megaspore  permanently  inclosed  in  each 
megasporangium  :  female  prothallus  purely  endogenous  : 
embryo  (phyllula)  remains  inclosed  in  megasporangium 
which  becomes  a  seed •  .  .  .  .  373 

43.  Angio sperms. 

Cone  modified  into  flower  by  differentiation  of  sporo- 
phylls and  perianth  :  female  sporophyll  forms  closed 
cavity  in  which  megasporangia  are  contained :  mega- 
spore produces  a  -single  ovary  represented  simply  by  an 
ovum  and  two  synergidse  :  formation  of  prothallus  re- 
tarded until  after  fertilization 378 

B.— SUBJECTS  OF   GENERAL  IMPORTANCE   DISCUSSED 
IN  SPECIAL  LESSONS. 

I. — CELLS  AND  NUCLEI. 

a.  The  higher  plants  and  animals  contain  cells  similar  in  struc- 

ture to  entire  unicellular  organisms,  and  like  them  exist- 
ing in  either  the  amoeboid,  ciliated,  encysted,  or  plas- 
modial  condition 56 

b.  Minute  structure  of  cells  : — cell-protoplasm,  cell-membrane, 

nuclear  membrane,  achromatin,  chromatin 62 

c.  Direct  and  indirect  nuclear  division 65 

d.  The  higher  plants  and  animals  begin  life  as  a  single  cell,  the 

ovum 68 


392  SYNOPSIS 

II. — BIOGENESIS. 

PAGE 

a.  Definition  of  biogenesis  and  abiogenesis  :    brief  history  of 

the  controversy 95 

b.  Crucial  experiment  with  putrescible  infusions  :  sterilization  : 

germ-filters  :  occurrence  of  abiogenesis  disproved  under 
known  existing  conditions 98 

III.—  HOMOGENESIS. 

Definition    of  homogenesis   and   heterogenesis  ;    truth   of    the 

former  firmly  established 102 

IV. — ORIGIN  OF  SPECIES. 

a.  Meaning  of  the  term  Species  :  the  question  illustrated  by  a 

consideration  of  certain  species  of  Zoothamnium  ....     137 

b.  Definition  of  Creation  and  Evolution  :  hypothetical  histories 

of  Zoothamnium  in  accordance  with  the  two  theories  .    .     141 

c.  The    principles    of    Classification  :    natural    and    artificial 

classifications 140 

d.  The  connection  between  ontogeny  and  phylogeny 146 

V.— PLANTS  AND  ANIMALS. 

a.  Attempt  to  define  the  words  plant  and  animal,  and  to  place 

the  previously  considered  types  in   one   or  other  king- 
dom   176 

b.  Significance  of  the  "third  kingdom,"  Protista 182 

VI. — SPERMATOGENESIS  AND  OOGENESIS. 

Origin  of  sperms  and  ova  from  primitive  sex-cells  ;   differences 

in  structure  and  development  of  the  sexual  elements    .    .    255 

VII. — MATURATION  AND  IMPREGNATION. 

a.  Formation  of  first  and   second  polar  cells  and   of  female 

pronucleus       259 

b.  Entrance  of  sperm  and  formation  of  male  pronucleus     .        .     263 

c.  Conjugation  of  pronuclei      .......        263 


SYNOPSIS  393 

VIII. — UNICELLULAR  AND  DIPLOBLASTIC  ANIMALS. 

PACK 

a.  In  plants  there  is  a  clear  transition  from  unicellular  forms  to 
solid  aggregates,  but  in  animals  the  connection  of  the 
gastrula  with  unicellular  forms  is  uncertain 264 

l>.   Hypothesis  of  the  origin  of  multicellular  forms  from  a  colony 

of  unicellular  zooids 265 


C. — Other  matters  of  general  importance,  such  as  the  composition 
and  properties  of  protoplasm,  cellulose,  chlorophyll,  starch,  &c.  :  meta- 
bolism :  holozoic,  holophytic,  and  saprophytic  nutrition :  intra-  and 
extra-cellular  digestion  :  amoeboid,  ciliary,  and  muscular  movements  : 
the  elementary  physiology  of  muscle  and  nerve  :  parasitism  and  sym- 
biosis :  asexual  and  sexual  generation  :  and  the  elements  of  embryology 
— are  discussed  under  the  various  types,  and  will  be  most  conveniently 
referred  to  by  consulting  the  Index. 


INDEX   AND    GLOSSARY 


AbiOgenesiS  (a,  not:  /3i'os,  life:   yeVeo-is, 
origin),    the  origin   of   organisms    from 
not-living  matter  :  former  belief  in,  96 
Absorption  by  root-hairs,  341 
Accre'tion  (ad,   to  :  cresco,  to  grow),  in- 
crease by  addition  of  successive  layers, 
J4 

Achrom'atin  (a,  not :  xpwjaa,  colour),  the 
constituent  of  the  nucleus  which  is  un- 
affected or  but  slightly  affected  by  dyes, 
7,  63 
Accel om'ate  (a,  not :  KotA.<o/u.a,  a  hollow), 

having  no  coelome  (q.v.)  :  301 
AddUCt'or  muscles,  Mussel,  322 
Aerob'iC  (cbjp,  air :  /3t'os,  life),  applied  to 
those  microbes  to  which  free  oxygen  is 
unnecessary,  93 

Aganiob'ium.  (a,  not  :  ya/oto?,  marriage  : 
/3ios,  life),  the  asexual  generation  in  or- 
ganisms exhibiting  alternation  of  gene- 
rations (q.v.) 

AGAB'ICUS   (mushroom)  '.—Figure,  192  : 
general     characters,     191  :     microscopic 
structure,  193  :  spore-formation,  193 
Algae  (a/ga,  sea-weed),  169 
Alternation  of  Generations,  meaning  of 
the  phrase  explained  under  Nitella,  220  : 
Bougainvillea,    250  :    Moss,    340 :  Fern, 
361  :  Equisetum,  367,  368  :  Salvinia,  370  : 
Selaginella,    373  :    Gymnosperms,    377 : 
Angiosperms,  383,  384 
Ambula'cral  (ambulacrum^     a    walking 

place)  system,  starfish,  313 
AMCEB'A   (aju.oij36s,    changing) : — Figure, 
2  :  occurrence  and  general  characters,  i  : 
movements,  4,  to  :  species  of,  8  :  resting 
condition,  10,  n  :  nutrition,  n  :  growth, 
13:  respiration,  17  :  metabolism,  17  ,  re- 
production, 19  :  immortality,  20  :  conju- 
gation, 20  :  death,  20,  21 :  conditions  of 
life,  21  :  animal  or  plant?  180 
Amoeb'oid  movements,  4 
Amceb'ula  (diminutive   of  Amoeba),   the 


amoeboid  germ  of  one  of  the  lower  or 
ganisms,  51-54 

Anab'olism  (ai>a/3oArj,  that  which  is 
thrown  up).  See  Metabolism,  construc- 
tive. 

Anaerobic  (a,  not :  a??p,  air :  /3t'os,  life), 
applied  to  those  microbes  to  which  free 
oxygen  is  unnecessary,  93 

An'al  (anus,  the  vent)  segment,  Poly- 
gordius,  273 

An'al  spot,  Paramoecium,  113 

An'astates  (avao-Taros,  from  ai/aar^i/at, 
to  rise  up,  18.  See  Mesostates,  anabolic. 

Anatomy  (ivareVi/w,  to  cut  up),  the  study 
of  the  structure  of  organisms  as  made 
out  by  dissection. 

Androe'cium  (ai/ijp,  a  male  :  OIKOS,  a 
dwelling),  the  collective  name  for  the 
male  sporophylls  in  the  flower  of  Angio- 
sperms, 381 

AN'GIOSPERMS  (iy-yeZoi/,  a  vessel: 
oWpjua,  seed)  : — Figure,  379  :  general 
characters,  378-381  :  structure  of  flower, 
378  :  reduction  of  gamobium,  381,  382  : 
pollination  and  fertilization,  382,  383 : 
formation  of  fruit  and  seed,  and  develop- 
ment of  the  leafy  plant,  383,  384 

Animal,  definition  of,  176 

Animals,  classification  of,  307 

Animals  and  Plants,  comparison  of  type 
forms,  176,  177  :  discussion  of  doubtful 
forms,  1 80 

Animals,  Protists,  and  Plants,  boun- 
daries between  artificial,  181-183 

Anther,  ^81 

Antherid'ium.     See  Spermary. 

Antherozo'id     See  Sperm. 

Antip'odal  cells,  382 

An'US  (anus,  the  vent),  the  posterior  aper- 
ture of  the  enteric  canal,  273 

Ap'ical  cell  :— Penicillium,  190:  Nitella, 
21 1  :  Moss,  335  :  stem  of  Fern  350  :  n  ot 
of  Fern,  353  :  prothallus  of  Fern.  357 

Ap'ical  cone,  Fern,  350 

A'pical  growth,  190,351 


396 


INDEX  AND  GLOSSARY 


A'pical  mer'istem,  a  mass  of  meristem 
(g.v.)  at  the  apex  of  a  stem  or  root,  350 

Appen'dages,  lateral : — crayfish,  314  :  dog- 
fish, 324 

Archegonlum  (apx»?,  beginning  :  -yoi/os, 
production),  the  name  usually  given  to 
the  ovary  of  the  higher  plants 

Aristotle,  abiogenesis  taught  by,  96 

Arteries,  in  the  crayfish,  318 

Arthropoda,  the,  308 

Arthosppre  (apOpov,  a  joint :  a-nopa,  a 
seed),  in  Bacteria,  89 

Artificial  reproduction  of  Hydra,  234 

Asexual  generation.    See  Agamobium. 

Asexual  reproduction.  See  Fission, 
Budding,  Spore. 

Asparagin,  338 

Assimila,'tion(assimi70,  to  make  like),  the 
conversion  of  food  materials  into  living 
protoplasm,  13 

At  rophy  (a,  without :  rpo^,  nourish- 
ment), a  wasting  away,  118 

Auricle.     See  Heart. 

Autom'atism  (avrdjuaTO?,  acting  of  one's 
own  will),  10,  246 

Axial  bundle,  Moss,  335 
Axial  fibre,  Vorticella,,^ 
Axil  (axilla,  the  :irm-pit),  208 
Axis,  primary  and  secondary,  209 


B 


BACIL'LUS  (Imcillum,  a  little  staff),  85  _ 
Figure,  87 

BACTERIA  (/3a/fTrjptoi/,  a  little  staff)  or 
MICROBES  (juic POS,  small :  /3t'os.  life)  :— 
occurrence.  82  :  structure  of  chief  genera, 
84-87  :  reproduction,  87-89 :  nutrition, 
90  :  ferment-action,  91  :  parasitism,  92  : 
conditions  of  life,  92-94  :  presence  in  at- 
mosphere, 101-102:  animals  or  plants? 
182 

BACTERIUM  termo  (Figures)  83,  84 

Baer,  von,  Law  of  Development,  43 

Barnacle-geese,  supposed  heterogenetic 
production  of,  103 

Bast.     See  Phloem. 

Binomial  nomenclature,  8,  139 

Biogen'esis  (jSt'os,  life  :  •yeVeais,  origin), 
the  origin  of  organisms  from  pre-existing 
organisms,  96  :  early  experiments  on,  96, 
97  :  crucial  experiment  on,  97-100 

Biol'ogy  (|3ios,  life  :  Ao-yos,  a  discussion), 
the  science  which  treats  of  living  things 

Blast'OCCele  (/SAacrro?,  a  bud  ;  /coiAo^,  a 
hollow),  the  larval  body-cavity,  298 

Blood,  Polygordius,  283 

Blood-corpuscles  :  colourless,  see  Leuco- 
cytes :  red,  56 :  Figures,  57 

Blood-vessels,  Polygordius,  282:  develop- 
ment of,  302 

Body-cavity.  See  Blastocccleand  Ccclome. 

Body-segments.     See  Metameres. 


BOUGAINVILLEA  (after  L.  A.  de  Bou- 
gainville, the  French  navigator  :— 
Figures,  238,  241  :  occurrence  and  gene- 
ral characters,  237  :  microscopic  struc 
ture,  239 :  structure  of  medusa,  240 
structure  and  functions  of  nervous  sys 
tern,  245  :  organs  of  sight,  246  :  repro 
duction  and  development,  247,  248  . 
alternation  of  generations,  250 

Bract  (bractea,  a  thin  plate),  251 

Brain: — Polygordius.  286:  trochosphere, 
299  :  Crayfish,  319  :  Dogfish,  330 

Branch,  Nitella,  209 

Branchial     (Ppdyxia.,     branchiae,      gills) 

apertures,  Dogfish,  324,  329 

Browne,  Sir  Thomas,  on  abiogenetic  origin 
of  mice,  96 

Buc'cal  (bucca,  the  cheek)  groove,  Para- 
mcecium,  109 

Bud,  budding,  Saccharomyces,  73  :  com- 
parison of  with  fission.  73  :  Hydra,  233 

Bundle-sheath,  349 


Calyp'tra  (/caAvTrrpa.  a  veil),  339 
Cal'yx  (»caAu£,  the  cup  of  a  "flower),  the 

outer  or  proximal  whorl  of  the  perianth 

in  the  flower  of  Angiosperms,  378 
Canals,  radial  and  circular,  medusa,  241 
Canal-cells  of  ovary,  337,  358 
Cap-cells  of  roots,  354 
Carbon    dioxide,    decomposition    of    by 

chlorophyll  bodies,  29 
Car'pel  (Kap-n-os,  fruit),  a  female  sporophyll. 

381 
Cartilage,  324 

Cauler'pa    (/covAd?    a    stem  :     e'p7rw,      to 

creep),  174  (Figure) 
Cell  (cella,  a  closet  or  hut,  from  the  first 

conception  of  a  cell  having  been  derived 

from  the  walled  plant-cell)  : — meaning  of 

term,  60  :  minute  structure  of  (Figure), 

62  :  varieties  of  (Figure),  57 
Cell-aggregate,  meaning  of  term,  188 
Cell-colony : — temporary,  Saccharomyces. 

73:  permanent,  Zoothamnium,  135,  136 
Cell-division,  64-67 
Cell-fusion  305,  351 
Cell-layer,  277 

Cell-membrane  or  wall,  u,  27,  28,  63 
Cell-multiplication  and  differentiation, 

218  :  Polygordius,  305  :  Fern,  351 
Cell-plate,  67 
Cell-protoplasm,  60 

Cell'ulOSe,  composition  and  properties  of, 

28 

Central  capsule,  Radiolaria,  152 
Central  particle  or  Centrosome  (KtWpor. 

centre  :  (ru>ju.a,  the  body),    65,    256  (Fig- 
ure).     See  also  Directive  sphere. 

Ceph'alothor'ax,  Crayfish,  314 
Cerebral  ganglion.    See  Brain. 
Cerebro-pleural  ganglion,  Mussel,  323 


INDEX   AND  GLOSSARY 


397 


Cerebro-spinal  cavity,  Dogfish,  325 

CHARA  (\apa,  delight),  development  and 
alternation  of  generations,  219,  220 

Chlor'ophyll  (\Au>p6?,  green  :  <}>v\\oi>,  a 
leaf),  the  green  colouring  matter  of 
plants,  properties  of,  26  :  occurrence  in 
Bacteria,  87  :  in  Hydra,  231 

Chrpm'atin  (xpw/xa,  a  colour),  the  con- 
stituent of  the  nucleus  which  is  deeply 
stained  by  dyes,  7,  63  :  male  and  female 
in  nucleus  of  oosperm,  263 

Chrom'atophore  (xpw^a,  colour :  <£e'pw, 
to  bear),  a  mass  of  proteid  material  im- 
pregnated with  chlorophyll  or  some  other 
colouring  matter,  26,  46,  197,  215,  231 

Chromosome  (^pio/na,  colour :  orco/ua, 
body),  65,  66,  257,  262,  263 

Cil'ium  (cilimn,  an  eye-lash),  defined, 
25«  :  comparisons  of  with  pseudopod,  34, 
52  :  absence  of  cilia  in  Arthropoda,  319 

Ciliary  movement,  25  :  a  form  of  con- 
tractility, 34 

Cillate  Infusoria,  107 

Classification,  natural  and  artificial,  141  : 
natural,  a  genealogical  tree,  142 

Cnid  Oblast  (Kclfy  a  nettle  :  j3A<xcn-o's,  a 
bud),  the  cell  in  which  a  nematocyst 
(q.v.)  is  developed,  230 

Cnid'OCil  (wiSy  and  c ilium),  the  "trigger- 
hair"  of  a  cnidoblast,  230 

Coelenterata,  the,  308 

Coelome  (/cot'Aw/xa,  a  hollow),  the  body- 
cavity  : — Polygordius,  273  :  Starfish,  311: 
Crayfish,  319  :  Mussel,  322 :  Dogfish, 
325  :  development  of,  Polygordius,  302 

CoBlom'ate,  provided  with  a  coelome,  276 

Ccelomic  epithelium.     See  Epithelium. 

Coelomic  fluid,  Polygordius,  281 

Colloids  (KoAAa,  glue  :  el6o?,  form),  pro- 
perties of,  6 

Colony,  Colonial  organism,  meaning  of 

term,  135,  247  :  formation  of  temporary 

colonies,  Hydra,  234 
Columel'la  (a  little  column),  162 
Com'miSSUre  (commissura,  a  band),  282 
Compound  organism.    See  Colony. 
Concres'cence  (cum,  together  :  cresco,  to 

grow),  the  union  of  parts  during  growth 
Cone,  an  axis  bearing  sporophylls  : — Equi- 

setum,  366:    Selaginella,  371  :  Gymno- 

sperms.  373 

Conjuga'tion  (conjug&tio,  a  coupling),  the 

union  of  two  cells,  in  sexual  reproduc- 
tion : — Amoeba,  20  :  Heteromita,  41,  42  : 
Paramoecium,  114—116:  Vorticella,  132  : 
Alucor,  165  :  Spirogyra,  198  :  of  ovum 
and  sperm,  263  :  monoecious  and  dioe- 
cious, 199  :  comparison  with  plasmodiiun- 
formation,  54 

Connective,  oasophageal,  286 

Connective  tissue,  312 

Contractile  vac'uole  (vacuus,  empty)  :•— 
Amoeba,  8,  9 :  Euglena,  47 :  Paramoe- 
cium,  in 


Contractility  (contracts,  a  drawing  to 
gether),  nature  of,  10,  34  :  muscular,  130 

Contraction,  physical  and  biological,  10 

Corolla  (corolla,  a  little  wreath),  the 
inner  or  distal  whorl  of  the  perianth  in 
the  flower  of  Angiosperms,  380 

Corpuscles.  See  Blood-corpuscles,  and 
Leucocytes 

Cortex,  cor'tical  layer  (cortex,  bark), 
constitution  of,  59  :  Infusoria,  no 

Cotton-WOOl  as  a  germ-fitter,  99 

Cotyle'don  (KoruArjSwv,  a  cup  or  socket), 
the  first  leaf  or  leaves  of  the  phyllula 
(q.v.)  in  vascular  plants,  359 

Cranium  (/cpdi/iW,  the  skull),  328 

CRAYFISH  : — Figure,  316: general  charac- 
ters, 314,  315  :  limited  number  and  con- 
crescence of  metameres,  314 :  append- 
dages,  314  :  exoskeleton,  311  :  enteric 
canal,  315 :  gills,  318  :  blood-system, 
318  :  kidney,  318  :  nervous  system,  319 

Creation  (creo,  to  produce),  definition  of, 
141  :  illustrated  in  connection  with  species 
of  Zoothamnium  (Diagram),  142 

Cross-fertilization :  applied  to  the  sexual 
process  when  the  gametes  spring  from 
different  individuals,  199 

Cryst'alloids  (KpuoraAAos,  crystal :  etSos. 
form),  properties  of,  6 

Cuticle  (cnticnla,  the  outer  skin),  nature 
of  in  unicellular  animals,  45,  109 :  in 
multicellular  animals,  239 

Cyst  (KVCTTIS,  a  bag),  used  for  cell-wall  in 
many  cases,  n,  54 


Dallinger,  Dr.  W.  H.,  observations  on  an. 
apparent  case  of  heterogenesis,  103 

Daughter-cells,  cells  formed  by  the  fission 
or  gemmation  of  a  mother-cell,  35,  67 

Death,  phenomena  attending,  20,  21,  166. 
167 

Decomposition,  nature  of,  6 

Dennis  (Se'p/xa,  skin),  the  deep  or  connec- 
tive tissue  layer  of  the  skin,  312 

Descent,  doctrine  of.     See  Evolution. 

Development,  meaning  of  the  term,  43. ' 
For  development  of  the  various  types 
see  under  their  names 

Dextrin,  113 

Diastase,  81 

Diast'Ole  (fitao-TeAAco,  to  separate),  the 
phase  of  dilatation  of  a  heart,  contractile 
vacuole,  &c.,  in 

DIATOMA'CE.33  (fiiare'javto,  to  cut  across, 
because  of  the  division  of  the  shell  into 
two  valves),  155  :  Figure,  156 

Diat'omin,  the  characteristic  yellow  colour- 
ing matter  of  diatoms,  155 

Dichot'omous  (SixoTo/aew,  to  cut  in  two), 
applied  to  branching  in  which  the  stem 
divides  into  two  axes  of  equal  value,  138 


398 


INDEX  AND  GLOSSARY 


Differentia'tion  (differo,  to  carry  different 
ways),  explained  and  illustrated,  34,  119 

Diges'tion  (digero,  to  arrange  or  digest), 
the  process  by  which  food  is  rendered  fit 
for  absorption,  12,  13:  intra- and  extra- 
cellular, 232 :  contrasted  with  assimila- 
tion, 233 

Digestive  gland,  317 

Dimorpb/ism,  dimorph'ic  (Si's,  twice ' 
/u.op0rj,  form),  existing  under  two  forms, 
35,  136,  243,  368,  370 

Dioe'cious  (8i's,  twice :  ol/co?,  a  dwelling), 
applied  to  organisms  in  which  the  male 
and  female  organs  occur  in  different  in- 
dividuals, 199 

DIPH'YES  (St<|>vij?,  double)  :  Figure,  252  : 
occurrence  and  general  characters,  250  : 
polymorphism,  251 

Diploblast'iC  (SiTrAoo?,  double  :  /3Aao-Tos,  a 
bud),  two-layered  :  applied  to  animals  in 
which  the  body  consists  of  ectoderm  and 
endoderm,  244  :  derivation  of  diploblas- 
tic  from  unicellular  animals  (Figures), 
266,  268,  269 

Directive  sphere,  65,  261,  263.  See  also 
Centrosome. 

DISC,  Vorticella,  128 

Dispersal,  means  of :  in  internal  parasite, 
124  :  in  h'xed  organisms,  133-136 

Distal,  the  end  furthest  from  the  point  ot 
attachment  or  organic  base,  126 

Distribution  of  food-materials  :— in  a 
complex  animal,  281  :  in  a  complex 
plant,  341 

Divergence  of  character,  145 

Division  of  physiological  labour,  34 

DOGFISH  '.—Figure,  326  :  general  charac- 
ters, 324  :  fins,  324  :  exoskeleton,  325  : 
endoskeleton,  325  :  enteric  canal,  328  t 
gills,  329  :  blood-system,  329  :  kidney, 
330  :  gonads,  330  :  nervous  system,  330 

Dry-rigor,  stiffening  of  protoplasm  due  to 
abstraction  of  water,  21 


Bchinodermata,  the,  308 

Ect'oderm  (CKTOS,  outside  :  Sep/xa,  skin), 
the  outer  cell-layer  of  diploblastic  and 
triploblastic  animals,  225-230,  278 

Ect'osarc  (CKTOS,  outside  :  <rap£,  flesh),  the 
outer  layer  of  protoplasm  in  the  lower 
unicellular  organisms,  distinguished  by 
freedom  from  granules,  4 

Egest'ion  (egero,  to  expel),  the  expulsion 
of  waste  matters,  12 

Egg-cell.     See  Ovum. 

Em'bryo  (e/ajSpvov,  an  embryo  or  foetus), 
the  young  of  an  organism  before  the 
commencement  of  free  existence. 

Em'bryo-SaC.     See  Megaspore. 

Encysta'tion,  being  enclosed   in   a  cyst 


£« 

End'oderm 


,   within  :   Sep/ita,   skin 


the  inner  cell-layer  of  diploblastic  and 
triploblastic  animals,  225,  231,  278 

End'oderm-lamella,  Medusa,  241 

Endogenous  (ZvSov,  within  :  •yiyi'oju.ai,  to 
come  into  being),  arising  from  within, 
e.g.  the  roots  of  vascular  plants,  354 

End  OSarc  (ei/Soi/,  within :  cr<xp£,  flesh), 
the  inner,  granular  protoplasm  of  the 
lower  unicellular  organisms,  4 

Endoskel'eton  (evSov, within,  and  skeleton, 
from  ovce'AAw,  to  dry),  the  internal  skele- 
ton of  animals,  325 

End'OSperm  (evSov,  within  :  o-7rep|u,a,  seed), 
nutrient  tissue  formed  in  the  megaspore 
of  Phanerogams,  377,  383 

Endospore  (evSov,  within  :  (TTropa,  a  seed), 
a  spore  formed  within  a  vegetative  cell, 
89 

Energy,  conversion  of  potential  into 
kinetic,  15  :  source  of,  in  chlorophyll- 
containing  organisms,  31 

Enteric  (ej/repov,  intestine),  Canal,  the 
entire  food-tube  from  mouth  to  anus  : — 
Polygordius,  273,  279  :  Starfish,  312  : 
Crayfish,  315  :  Mussel,  322  :  Dogfish,  328 

Ent'eron  or  Enteric  cavity,  the  simple 
digestive  chamber  of  diploblastic  ani- 
mals, 225 

Epidermis  (CTU  upon :  Sep^a,  the  skin)  : 
in  animals  synonymous  with  deric  epi- 
thelium (q.v.,  under  Epithelium)  :  in  vas- 
cular plants  a  single  external  layer  of 
cells,  348,  352,  353 

Epithelial  cells  :  columnar,  58  :  ciliated, 

Epithelium  (en-i,  upon  :  flrjA^,  the  nipple), 
a  cellular  membrane  bounding  a  free 
surface,  246 :  coelomic,  277,  304  :  deric, 
272 :  enteric,  276,  278 

Equatorial  plate,  67 

EQUISETUM  (eqnus,  a  horse :  seta,  a 
bristle  : — Figure,  367  :  general  charac- 
ters, 366 :  cone  and  sporophylls,  366  : 
male  and  female  prothalli,  366,  367  :  al- 
ternation of  generations,  367,  368 

Equiv'ocal  generation.  See  Abiogenesis. 

EUGLEN'A  ( euyAiji/o?,  bright-eyed)  :— 
Figure,  45  :  occurrence  and  general 
characters,  44  :  movements,  44 :  struc- 
ture, 45  :  nutrition,  46 :  resting  stage, 
47  :  reproduction,  48  :  animal  or  plant  ? 
178 

Euglen'oid  movements,  45 

Ev'olution  (evolvo,  to  roll  out),  organic  : 
definition,  143  :  illustration  of  in  connec- 
tion with  species  of  Zoothamnium  (Dia- 
gram), 144 

Excre'tion  (excerno,  to  separate),  the 
separation  of  waste  matters  derived  from 
the  destructive  metabolism  of  the  or- 
ganism, 16,  284 

Exogenous  (e£,  put  of:  yCyvo/j-ai.,  to  come 
into  being),  arising  from  the  exterior, 
t.g.  leaves,  354 


INDEX  AND  GLOSSARY 


399 


Exoskel'eton  (e'£w,  outside,  and  skeleton, 
Irom  ovce'AAw  to  dry),  the  external  or 
skin-skeleton  :  CUtiCUlar,  239,  277-279  : 
cuticular  and  calcified,  315,  320:  epi- 
dermal (hair  and  nails):  dermal,  312, 325 

Eye-spots  or  Ocelli :— Medusa,  241  : 
,  290,  299 


Faeces  (faex,  dregs),  solid  excrement, 
consisting  of  the  undigested  portions  of 
the  food,  16 

Ferm'ent  (fermentum,  yeast,  from,  for- 
veo,  to  boil  or  ferment),  a  substance 
which  induces  fermenta'tion,  i.e.  a 
definite  chemical  change,  in  certain  sub- 
stances with  which  it  is  brought  into 
contact,  without  itself  undergoing 
change  :  unorganized  and  organized 
ferments  80  :  alcoholic,  76-81  :  ace- 
tous, 91 :  diastatic  or  amylolytic,  81 : 
lactic,  91  :  peptonizing  or  proteolytic, 
8 1  :  putrefactive,  91  :  ferment-cells  of 
Mucor,  168 

FERNS  :— Figures,  346,  356  :  general 
characters  344,  345  :  histology  of  stem, 
leaf,  and  root,  344-354  :  nutrition,  354  : 
spore-formation,  355  :  prothallus  and 
gonads,  357-358  :  development,  359  :  al- 
ternation of  generations,  361 

Fertiliza'tion  (fertilis,  bearing  fruit)  ; 
the  process  of  conjugation  of  a  sperm  or 
sperm-nucleus  with  an  ovum,  whereby 
the  latter  is  rendered  capable  of  develop- 
ment :  a  special  case  of  conjugation 
(g.v.),  199  :  details  of  process,  263  :  in 
Vaucheria,  173  :  in  Gynmosperms,  376, 
in  Angiosperms,  383 

Filtering  air,  method  of,  99 

Fins,  Dogfish  324 

Fission  (Jissio,  a  cleaving),  Simple  or 
binary,  the  division  of  a  mother- cell 
into  two  daughter-cells  :  in  Amoeba,  19  ; 
Heteromita,  40  :  animal-  and  plant-cells 
generally,  65-67  :  Paramcecium,  114  : 
Vorticella  131 

Fission,  multiple,  the  division  of  a 
mother-cell  into  numerous  daughter- 
cells  : — in  Heteromita,  42  :  Protomyxa, 
51  :  Saccharomyces,  74 

Fission,  process  intermediate  between 
simple  and  multiple,  Opalina,  124 

Flagella.     See  Cilium. 

Flag'ellate  Infusoria,  107 

Flagell'ula  (diminutive  oiflagelluni),  the 
flagellate  germ  of  one  of  the  lower 
organisms  (often  called  zoospores,  51,  54 

FlageU'um  (fiagellum,  a  whip)  :  defined! 
25  :  transition  to  pseudopod,  52,  231 

Floral  receptacle,  the  abbreviated  axis  of 
an  angiospermous  flower  378 

Flower,  a  specially  modified  cone  (<;.?'.), 


having  a  shortened  axis,  which  bears 
perianth-leaves  as  well  as  sporophylls, 
378  :  often  applied  to  the  cone  of  Gymno- 
sperms,  373 

Food-current,  Mussel  320,  322 

Food-vacuole,  a  temporary  space  in  the 
protoplasm  of  a  cell  containing  water 
and  food-particles,  n,  112 

Foot  :  of  Mussel,  320  :  of  phyllula  of  fern 

FORAMINIF'ERA  (foramen,  a  hole  \fero 
to  bear),  148  :  Figures,  149,  150,  151 

Fragmenta'tion  of  the  nucleus,  120 

Free  cell  formation,  377 

Fruit  of  Angiosperms,  384 

Func'tion  (functio,  a  performing),  mean- 
ing of  the  term,  9 


Gam'ete  (yaixew,  to  marry),  a  conjugating 
cell,  whether  of  indeterniinate  or  deter- 
minate sex  : — Heteromita,  41  :  Mucor, 
165  :  Spirogyra,  198  :  Vaucheria  173 

Gamoblum  (yd/no?,  marriage  :  0t'ps,  life), 
the  sexual  generation  in  organisms  ex- 
hibiting alternation  of  generations  (.q.v.)  : 
progressive  subordination  of,  to  agamp- 
bium  in  vascular  plants,  361,  384 

Ganglion  (yayyAiov,  a  tumour),  a  swelliffi| 
on  a  nerve-cord  in  which  nerve-cells  ^j| 
accumulated,  319,  323 

Gastric  juice  (yao-njp,  the  stomach),  pr 
perties  of,   12 

Gast'rula  (diminutive  of  yaonjp,  the 
stomach),  the  diploblastic  stage  of  the 
animal  embryo  in  which  there  is  a  diges- 
tive cavity  with  an  external  opening  : 
characters  and  Figure  of,  265  :  contrasted 
with  phyllula,,  360 

Gemma 'tion  (gemma,  a  bud).  See  Bud- 
ding. 

Genera'tion,  asexual,  See  Agamobium. 
Sexual.  See  Gamobium. 

Generations,  Alternation  of.  See  Al- 
ternation of  generations. 

Generalized,  meaning  of  term,  140 

Ge'nus  (genus,  a  race),  generic  name, 
generic  characters,  8,  139 

Germ-filter,  99 

Ger'minal  spot,  the  nucleolus  of  the 
ovum,  259 

Germina'tion  (germinatio,  a  budding), 
the  sprouting  of  a  spore,  zygote,  or 
oosperm  to  form  the  adult  plant  :  for 
germination  of  the  various  types  see 
under  their  names. 

Gill,  an  aquatic  respiratory  organ.  318,  323, 
324,  329 

Gland .(glows,  an  acorn),  an  organ  of  se- 
cretion (q.v.)  '.  gland-cells,  231,  232,  285, 

Glochid'ium,  323 

Gon'ad  (yot/os,  offspring,  seed),  the  essen- 


400 


INDEX  AND  GLOSSARY 


tial  organ  of  sexual  reproduction, 
whether  of  indeterminate  or  determinate 
sex,  i.e.  an  organ  producing  either  un- 
differentiated  gametes,  ova,  or  sperms  ; 
see  under  the  various  types,  and  espe- 
cially 172,  198,  209,  214 

Gon'aduct  (gonad.  and  ittlco,  to  lead),  a 
tube  carrying  the  ova  or  sperms  from  the 
gonad  to  the  exterior,  295,  313,  319,  323, 
3£o 

Grapping-lines,  Diphyes,  251 

Growing  point  :  Nitella,  211,  Moss,  535  : 
Fern,  350 

Growth,  13 

Guard-cells  of  stomates,  353 

Gullet,  the  simple  food-tube  of  Infusoria, 
47,  no  :  or  part  of  the  enteric  canal  of 
the  higher  animals,  280 

GYM'NOSPERMS  (yu/ou/os,  naked  :  oWpjoia 
seed)  :  Figure,  37^:  general  characters, 
373  :  structure  oflBkies  and  sporophylls, 


373,  375,  :  reductron  of  gamobium  (pro- 
thalli  and  gonads),  376,  :  pollination  and 
fertilization,  376,  377  :  formation  of  seed 
and  development  of  leafy  plant,  377 
Gyncec'ium  (yvv^,  a  female  :  ot/cos,  a 
dwelling),  the  collective  name  for  the 
female  sporophylls  in  the  flower  of 
ngiosperms,  381 


H 


m'atochrome  (al^a,  blood  :  xp^Ma> 
lour),  a  red  colouring  matter  allied  to 
chlorophyll,  26 

H^EMATOCOp'CUS  (aIMa,  blood  :  KOKKOS 
a  berry): — Figure,  24:  general  characters, 
23  :  rate  of  progression,  23  :  ciliary  move- 
ments, 25,  33  :  colouring  matters,  26  : 
motile  and  stationary  phrases.  28  :  nutri- 
tion, 28  :  source  of  energy,  30  ;  reproduc- 
tion, 35  :  dimorphism,  35  :  animal  or 
plant  ?  180 

Haemoglobin  (al/ua,  blood  :  globus,  a 
round  body,  from  the  circular  red  cor- 
puscles of  human  blood),  58  :  properties 
and  functions  of,  283 

Head-kidney :  trochosphere,  299 

Heart  :— Crayfish,  318  :  Mussel,  323  : 
Dogfish,  329 

Heat,  evolution  of,  by  oxidation  of  proto- 
plasm, 17 

Heat-rigor  (rigor,  stiffness),  heat-stiffen- 
ing, 21 

Heliotropism,  168 

Heredity  (hereditas,  heirship),  147 

Hermaph'rodite  (epju.a<J>p6StTo?,  from 
Hermes  and  Aphrodite).  See  Monoecious. 

Heterpgen'esiS  (erepos,  different  :  -yeVecri?, 
origin),  meaning  of  term,  102  :  supposed 
cases  of,  103  :  not  to  be  confounded  with 
metamorphosis  or  with  evolution,  104 

HETEROM'ITA  (eVepos,  different  :  /uuVos, 


a  thread)  : — Figure  38  :  occurrence  and 
general  characters,  36  ;  movements,  37  : 
nutrition,  37  :  asexual  reproduction,  40  : 
conjugation,  41  :  development  and  life- 
history,  42,  43  :  animal  or  plant  ?  181 

High  and  low  organisms,  106 

Higher  (triploblastic)  animals,  uniformity 
in  general  structure  of,  307 

Higher  (vascular)  plants,  uniformity  in 
general  structure  of,  363 

Histol'Ogy  (tcrrtW,  a  thing  woven  :  Aoyo?, 
a  discussion),  minute  or  microscopic 
anatomy. 

Holophyt'ic  (6Aos,  whole  :  ^UTOI',  a  plant), 
nutrition,  defined,  31 

Holozo'ic  (oAo?,  whole  :  <Jwoi',  an  animal), 
nutrition,  defined,  31 

Homegen'esis  (6/ot6s,  the  same  :  yeVco-t? 
origin),  meaning  of  the  term,  102 

Homol'OgOUS  (ojaoAoyos,  agreeing),  applied 
to  parts  which  have  had  a  common 
origin,  243 

Homomorph'ism    homomorph'ic    (6/u6? 

the  same  :  ju.op</»j,   form),  existing  under 

a  single  form,  139 
Host,  term  applied  to  the  organism  upon 

which  a  parasite  preys,  123 
HYDRA  (vSpa,  a  water-serpent)  :  Figures, 

222,  226,  228,  235  :  occurrence  and  general 
characters,  221  :    species,    223  :     move- 
ments, 223,  224  :  mode  of  feeding,  224  : 
microscopic    structure,     225 :   digestion, 
232  :  asexual,  artificial,  and  sexual   re- 
production 233,  235  :  development,  236 

Hydr'anth  (vSpa,  a  water-serpent  :  avdos 
a  flower),  the  nutritive  zooid  of  a  hydroid 
polype,  239,  243 

Hydroid  (vSpa,  a  water-serpent  :  elfio? 
form)  Polypes  (rroAvTrov?,  many-footed), 
compound  organisms,  the  zooids  of  which 
have  a  general  resemblance  to  Hydra, 
237 

Hyper 'trophy  (iiTrep,  over  :  rpo<j>rf,  nourish- 
ment), an  increase  in  size  beyond  the 
usual  limits,  118 

Hyph'a  (u(J>cuVco,  to  weave)  applied  to  the 
separate  filaments  of  a  fungus  :  they 
may  be  mycelial  (see  mycelium),  sub- 
merged, or  aerial :  Mucor,  160,  168, 
Penicillium,  185,  188 

Hyp'odermis  (  VTTO,  under  :  8e'pju.a,  skin), 
Fern,  345,  348 

Hyp'OStome  (UTTO,  under  :  erro/xa,  mouth), 

223,  239 


Immortality,  virtual,  of  lower  organisms, 

21 
Income  and  expenditure  of  protoplasm, 

18 

Individual.    See  Zooid. 
Individuation,  meaning  of  the  term.  233. 

254 


INDEX  AND  GLOSSARY 


401 


Indus  ium  (indusium,  an  under-garment), 

Inflores'cence    (floresco,    to    begin    to 

flower),     an    aggregation    of   cones    or 

flowers,  373 
Infusoria  (so  called  because  of  their  fre- 

quent occurrence  in  infusions),  107 
Ingesta  (ingcro,  to  put  into)  and  Egesta 

(e^cro,  to  expel),  balance  of,  32 
Ingestion  (ingcro,  to  put  into),  the  taking 

in  of  solid  food,  n,  58 
Insola'tiOU  (insolo,  to  place  in  the  sun), 

exposure  to  direct  sunlight,  94 
Integ'ument  (integii  mention,  a  covering) 

of      megaspore  :     Gymnosperms,     375  : 

Angiosperms,  382 

Inter-cellular  spaces,  347 
Inter-muscular  plexus  (TrAe/cw,  to  twine), 


Leaf,  structure  of: — Nitella,  208,  209 
213  :  Moss,  335  :  Fern,  344,  352  :  limited 
growth  of,  214 

Leaflet,  Nitella,  209 

Lept'othrix  (ACTTTOS,  slender ;  0pif ,  a  hair), 
filamentous  condition  of  Bacillus,  89 : 


288 
Internode    (inter,     between  :    nodus,    a 

knot),    the  portion'  of  stem  intervening 

between  two  nodes,  208 
Intersti'tial    (interstltiiiin,    a  space  be- 

tween)   cells,     Hydra,    227  :    growth, 

Spirogyra,  198 
Intestine  (intestlnus,  internal),  part    of 

the  enteric  canal  of  the  higher  animals, 

280 
Intus-suscep'tion  (intus,  into  :   snscipio, 

to  take  up),  addition  of  new  matter  to 

the  interior,  13 
Iodine,  test  for  starch,  27 
Irritability    (irritabllis,    irritable),     the 

property  of  responding   to  an  external 

stimulus,  10 


Jaws  :  Crayfish,  315  :  Dogfish,  324,  328 


K 


Karyokines'is  (/capuof,  a  kernel  or  nu- 
cleus :  KiVijo-is,  a  movement),  indirect 
nuclear  division,  67 

Katab'olism  (Kara/SoAr),  a  laying  down), 
18.  See  Metabolism,  destructive. 

Kat'astates  (*<xTacrrTJi/ai,  to  sink  down), 
1 8.  See  Mesostates,  katabolic. 

Kidney  :— Crayfish,  318  :  Dogfish,  330 


LAMINARIA     (lamina,    a    plate),     203 
C  Figure),  204 

Labial  palps,  Mussel,  322 

Larva,  the  free  living  young  of  an  animal 

in  which  development  is  accompanied  by 

a  metamorphosis,  299 
Larval  stages,  significance  of,    Polygor- 

dius,  301 


(after  Lesson,  the  French 
naturalist),  204  (Figure) 

Leuc'ocyte  (Aeu/cos,  white  :  KV'TOS,  a  hollow 
vessel,  cell),  a  colourlessblood  corpuscle  : 
— structure  of,  in  various  animals 
(Figures),  57  :  ingestion  of  solid  par- 
ticles by,  58  :  fission  of,  58  :  formation 
of  plasmodia  by,  58 

Leuwennoek,  Anthony  van,  discoverer 
of  Bacteria,  97 

Life,  origin  of.     See  Biogenesis. 

Life-history,  meanin^of  the  term,  43 

Lignin  (lignum,  wo^),  composition  and 
properties  of,  348 

Linear  aggregate,  an  aggregate  of  cells 
arranged  in  a  single  longitudinal  series, 

LinnaBUS,  C.,  introducer  of  binomial  no- 
menclature, 8,  139 
Liver,  Dogfish,  328 


M 


MACROCYSTIS  (ju,a*p6s,  long  :  KV 
bladder),  204 

Mad'reporite  (from  its  similarity  to  a 
madrepore  or  stone-coral),  311 

Mantle,  Mussel,  320 

Manub'rium  (mauubrium,  a  handle)  of 
Medusa,  241 

Matura'tion  of  ovum,  259 

Maximum  temperature  of  amoeboid 
movements.  21 

Medulla  or  medullary  substance  (me- 
dulla, marrow):  in  Infusoria,  no 

MedUS'a  (Me'Sovo-a,  name  of  one  of  the 
Gorgons),  the  free-swimming  reproduc- 
tive zooid  of  a  hydroid  polype,  239-243  : 
derivation  of  a,  from  hydranth  (Figure), 
241 

Medus'oid,  a  reproductive  zooid  having 
the  form  of  an  imperfect  Medusa, 
Dijjhyes,  251 

Meg  agam'ete  (fxeya?,  large  :  ya/xew,  to 
marry),  a  female  gamete  (q.v.)  distin- 
guished by  its  greater  size  from  the  male 
or  microgamete,  132 

Meg'anucleus  (/xe'yas,  large:  nucleus,  a 
kernel),  in,  128 

Meg'asporan'gium  (/u.eya?  large  :  '<nropd, 
seed  :  (iyyeioi/,  a  vessel),  the  female 
sporangium  in  plants  with  sexually  di- 
morphic sporangia,  usually  distinguished 
by  its  greater  size  from  the  male  ur 
micro-sporangium  : — Salvinia,  368  :  Sela- 
D  D 


402 


INDEX  AND  GLOSSARY 


ginella,  371  :  Gymnosperms,  375  :  Angio- 
sperms, 381. 

Meg  aspore  (jj.eya.s,  large  :  <riropd,  a  seed), 
the  female  spore  in  plants  with  sexually 
dimorphic  spores,  al-ways  distinguished 
by  its  large  size  from  the  male  or  micro- 
spore  : — Salvinia,  368:  Selaginella,  371  : 
Gymnosperms,  375  :  Angiosperms,  382 

Megazo'oid  (/At'ya?  large  :  r£o>oi/,  animal : 
eldos,  form),  the  larger  zooid  in  unicel- 
lular organisms  with  dimorphic  zooids, 
35>  132 

Mer  istem.  (/u.epumj|u.a,  formed  from 
/aepujou,  to  divide),  indifferent  tissue  of 
plants  from  which  permanent  tissues  are 
differentiated,  350 

Mes'entery  (ju.e'<ros,  middle  :  evrepov,  in- 
testine), a  membrane  connecting  the  en- 
teric canal  with  the  body-wall, 279  :  de- 
velopment of,  302 

Mes'oderm  (/meo-os,  middle  :  Sepfia,  skin), 
the  middle  cell-layer  of  triploblastic 
animals  :  Polygordius,  278 .  develop- 
ment of,  299:  splitting  of  to  form  somatic 
and  splanchnic  layers,  302 

Mesoglce'a  (ju.eVos,  middle  :  y\oia,  glue), 
a^transparent  layer  between  the  ecto-  and 
endo-derm  of  Coelenterates  : — in  Hydra, 
225  :  in  Bougainvillea,  244 

Mes'ophyll  (ju,e'pos,  middle :  <j>v\\ov,  a 
leai),  the  parenchyma  of  leaves,  352 

Mes'ostates  (ju,e'cro?,  middle  :  trrrfvai,  to 
stand),  intermediate  products  formed 
during  metabolism  (q.v.)  and  divisible 
into  (a)  anabolic  mesostates  or  ana- 
states,  products  formed  during  the  con- 
version of  food-materials  into  proto- 
plasm ;  and  (b)  katabolic  mesostates 
or  katastates,  products  formed  during 
the  breaking  down  of  protoplasm,  18 

Metab'olism  Oxera/SoArj,  a  change),  the 
entire  series  of  processes  connected  with 
the  manufacture  of  protoplasm,  and 
divisible  into  (a.)  constructive  meta- 
bolism or  anabolism,  the  processes  by 
which  the  substances  taken  as  food  are 
converted  into  protoplasm,  and  (b)  de- 
structive metabolism  or  katabolism, 
the  processes  by  which  the  protoplasm 
breaks  down  into  simpler  products,  ex- 
cretory or  plastic,  17 

Met'amere  (/u.eYa,  after  :  /mepos,  a  part),  a 
body-segment  in  a  transversely  seg- 
mented animal  such  as  Polygordius,  271, 
273  :  development  of,  301  :  limited  num- 
ber and  concrescence  of  in  Crayfish, '314 

Metamorphosis  (/mtTa^op^oxris),  a  trans- 
formation applied  to  the  striking  change 
of  form  undergone  by  certain  organisms 
in  the  course  of  development  after  the 
commencement  of  free  existeuce  : — Vor- 
ticella,  133  :  Polygordius,  306 

Mic  robe  (/ui/cpos,    small :  /Si'os,  life).     See 


MICROCOC'CUS  (jai/cpos,  small :  KOKKOS, 
a  berry)  (Figure),  86 

Mlcrogam'ete  (/uuxpos,  small  :  -ya^e'cu,  to 
marry),  a  male  gamete  (g.v.),  distin- 
guished by  its  smaller  size  from  the 
female  or  megagamete,  132 

Micro-millimetre,  the  one-thousandth  of 
a  millimetre,  or  i-25,oooth  of  an  inch,  84 

Micro-organism.     See  Bacteria. 

Micronucleus  (  juu*p6s,  small :  micleits,  a 
kernel),  in,  128 

Micropyle  (jul/cpoj,  small :  irvArj,  an  en- 
trance), 375 

Micro-sporan'giumGu/cpos,  small :  <rnopd, 
a  seed  :  dyyeiov,  a  vessel),  the  male 
sporangium  in  plants  with  sexually  di- 
morphic sporangia,  usually  distinguished 
by  its  smaller  size  from  the  female  or 
mega-sporangium  : — Salvinia,  368  :  Se- 
laginella, 371  :  Gymnosperms,  376  :  An- 
giosperms, 381 

Mic'rospore  (juuKpo?,  small  t  OTropa,  a 
seed),  the  male  spore  in  plants  with 
sexually  dimorphic  spores,  always  dis- 
tinguished by  its  small  size  from  the 
female  or  mega-spore  : — Salvinia,  368  : 
Selaginella,  371  :  Gymnosperms,  376  : 
Angiosperms.  381 

Microzo'oid  (jul^po?,  small :  ^Voov,  an  ani- 
mal :  clSo?,  form),  the  smaller  zooid  in 
unicellalar  organisms  with  dimorphic 
zooids,  35,  132 

Midrib  of  leaf,  Moss,  335 

Minimum  temperature  for  amoeboid  move- 
ments, 21 

Mollusca,  the,  309 

Monoecious  (/uoyo?,  single  :  OIKOS,  a 
house),  applied  to  organisms  in  which 
the  male  and  female  organs  occur  in  the 
same  individual,  199,  234 

Monopod'ial  (/xo^os,  single  :  TTOVS,  a  foot), 
applied  to  branching  in  which  the  main 
axis  continues  to  grow  in  a  straight  line 
and  sends  off  secondary  axes  to  the 
sides.  138 

MONOSTROMA  (/xdvos,  single  :  o-rpw/xa, 
anything  spread  out),  202  (Figure) 

Morphol'Ogy  (juop^,  form  :  Adyos  a  dis- 
cussion), the  department  of  biology 
which  treats  of  form  and  structure,  9 

Mor'ula  (diminutive  of  vionnn,  a  mul- 
berry) See  Polyplast. 

MOSSES  :— Figures,  333,  338  :  general 
characters,  332  :  structure  of  stem,  334  : 
leaf,  335  :  rhizoids,  335  :  terminal  bud, 
335 1  reproduction,  336 :  development 
of  sporogonium,  337 :  of  leafy  plant, 
339,  340 :  alternation  of  generations, 
340  :  nutrition,  340,  341 

Mouth  : — Kuglena,  47  :  Paramoccium,  no: 
Hydra,  223 :  Medusa,  241  :  Polygordius, 
271:  backward  shifting  of  in  Crayfish, 

MUCOR  (mncor,  mould) '.—Figure,   159: 


INDEX  AND  GLOSSARY 


403 


.-  5° 
Multicellular,  formed  of  many  cells,  61, 


occurrence  and  general  characters,   158  : 
mycelium  and    aerial    hyphse,  160-163  : 
sporangia  and  spores,  160-162-165  ;  transi- 
tion  from  uni-  to  multi-cellular    condi- 
tion,  162  :  development  of  spores,   163  ; 
conjugation,  165  :  death,  166  :  nutrition, 
167  ;  parasitism,  167  :  ferment-cells,  168 
Mucous  membrane 
[ulti 
162 
Muscle  (ww-r^/w-s-ja  little  mouse,a  muscle), 

nature  of.  130,  131 
Muscle-fibres,  Bougainvillea,  244 
Muscle-plate,    Polygordius,  development 

of,  305 

Muscle-process,  Hydra,  227,  232 
Mushroom.     See  Agaricus. 
MUSSEL  (same  root  as    muscle),    Fresh- 
water :  —  Figure,  321  :  general  characters, 
320  :  mantle,  shell,  and  foot,  320  :  food- 
current,    320  :   enteric  canal,  322  :  gills 
and  blood-system,  323  :  nephridia,  gon- 
ads,  and  nervous  system,  323 
Mycelial  hyphse,  the  hyphae  interwoven 

to  form  a  mycelium. 

Mycel'ium  (/IXVKTJS,  a  fungus),  a  more  or 
less  felt-like  mass  formed  of  interwoven 
hyphae  :—  Mucor,  160  :  Penicillium,  185 
YCET'OZOA  (MV'KTJS,  a  fungus  :  £faov, 


MY  MVKTJS,   a     ungus 

an     animal)  :  —  Figure,    53  :    occurrence 
and  general   characters,    52  :   nutrition, 

54  :    reproduction   and   life-history,    54, 

55  :  animals  or  plants?  181 
My'ophan  (/u.us,  mouse,  muscle  :  <f>aiVco.  to 

appear),  no 

Myxomyce'tes  (nvt-a,  mucus  :  nvxys,   a 
fungus).     See  Mycetozoa. 


N 


Norn  atocyst  (I^/IAO.,  a  thread  :  KVOTIS,   a 

bag),  229 

Nephrid'iopore  (i/e$po?,  a  kidney  :  Tropos, 
a  passage),  the  external  opening  of  a 
nephridium,  285 

Nephrld'ium  (i/e^pos,  a  kidney),  structure 
of,  Polygordius,  285  (Figure)  :  develop- 
ment of,  304  :  Mussel,  323 :  Dogfish, 
33° 

Neph'rostome  (ve<f>po?,  a  kidney :  0-TOju.a, 
a  mouth),  the  internal  or  ccelomic  aper- 
ture of  a  nephridium,  285 
Nerve,  afferent  and  efferent,  functions  of, 

288 

Nerve-cell,  230,  245 

Nervous  system,  central  and  peripheral  : 
— Medusa,  245  :  Polygordius,  286  :  func- 
tions of,    288 :    Starfish,  313 :    Crayfish, 
319  :  Mussel,  323  :  Dogfish,  330 
Neur'OCOele  (vevpov,    a    nerve:    KOI'ATJ,    a 
hollow),  the  central  cavity  of  the  verte- 
brate nervous  system,  330 
NITELL'A    (niteo,    to    shine) :— Figures, 


207,  212,  215,  217,  219 :  occurrence  and 
general  characters,  206 :  microscopic 
structure,  209  :  terminal  bud,  211  :  struc- 
ture and  development  of  gonads,  209, 
214:  development,  219:  alternation  of 
generations,  220 
Node  (nodus,  a  knot),  the  portion  of  a 

stem  which  gives  rise  to  leaves,  208 
Not'OChord  (VWTOS,    the   back:    x°P^>    a 

string),  328 
Nucel'lus    (diminutive    of   nucleus,    the 

name  formerly  applied),  375,  382 
Nuclear  division,  indirect :  64  (Figure) : 

65,  67  :  direct,  67 
Nuclear  membrane,  62, 
Nuclear  protoplasm.    See  Achromatin. 
Nuclear  spindle,  65,  66 
Nucle'olUS  (diminutive  of  nucleus),  8 
Nu'cleus  (nucleus,  a  kernel),  minute  struc- 
ture of,  63  ;  Amoeba,  7,  8  :  Paramcecium, 
in,    114:     Opalina,     121:     Vorticella, 
128  :   Nitella,   210,   213  :    fragmentation 
of,  1 20 
Nucleus,  secondary,  of  megaspore,  An- 

giosperms,  382 
Nutrient  solution,  artificial,  principles  of 

construction  of,  78, 

Nutrition :— Amoeba  (holozoic),  n  :  Hse- 
matococcus  (holophytic),  28 :  Hetero- 
mita  (saprophytic),  37  :  Opalina  (type  of 
internal  parasite),  123 :  Mucor  167  : 
Penicillium,  190 :  Polygordius  (type  of 
higher  animals),  273,  281 :  Moss  (type 
of  higher  plants),  340 


Ocellus  (ocellus,  a  little  eye),  structure 
and  functions  of,  Medusa,  241,  246 

CEsoph'agUS  (oicro4>avos,  the  gullet).  See 
Gullet. 

Ontog'eny  (O^TOS,  being  :  yeyeerts,  origin), 
the  development  of  the  individual : 
a  recapitulation  of  phylogeny  (y.v.), 
146 

Oogen'esis  (u>6v,  an  egg  :  yei/e'<ris,  origin), 
the  development  of  an  ovum  from  a 
primitive  sex-cell,  255,  258 

Oogon  ium  (J>oV,  egg :  yo^o?.  produc- 
tion), the  name  usually  given  to  the 
ovary  of  many  of  the  lower  plants. 

Oosperm  (u>oV,  egg :  <77repju.a,  seed),  a 
zygote  (q.v).  formed  by  the  ovum  and 
sperm:  a  unicellular  embryo,  173: 
origin  of  nucleus  of,  263 

Oosphere  (wov,  an  egg  :  cr<£cupa,  a  sphere), 
a  name  frequently  given  to  the  ovum  of 
plants. 

Oospore  (taov,  an  egg :  criropa,  a  seed),  a 
name  frequently  applied  to  the  oosperm 
of  plants. 

OPALIN'A  (from  its  opalescent  appear- 
ance) : — Figure,  122  :  occurrence  and 
general  characters,  121-123  •  structure 

fl 

i 


404 


INDEX  AND  GLOSSARY 


and  division  of  nuclei,  121  :  parasitic 
nutrition,  123  :  reproduction,  124  :  means 
of  dispersal,  124  :  development,  125 

Opt'inium  (pptimus,  best)  temperature  for 
amoeboid  movements,  21  :  for  sapro- 
phytic  monads,  40 

Organ  (opyavov,  an  instrument),  a  portion 
of  the  body  set  apart  for  the  perform- 
ance of  a  particular  function,  291 

Or'ganism,  any  living  thing,  whether 
animal  or  plant,  5 

Ossicle  (diminutive  of  <>s,  a  bone),  311 

Ov'ary  (ovum,  an  egg),  the  female  gonad 
or  ovum-producing  organ  ;  see  under  the 
various  types  and  especially  Vaucheria, 
172  :  atrophy  of,  in  Angiosperms,  382. 
The  name  is  also  incorrectly  applied  to 
the  venter  of  the  pistil  of  Angiosperms, 
381 

Ovi'duct  (ovum,  an  egg  '•  ditco,  to  lead), 
a  tube  conveying  the  ova  from  the  ovary 
to  the  exterior,  295 

Ovum  (ovum,  an  egg),  the  female  or 
megagamete  in  its  highest  stage  of  dif- 
ferentiation :  general  structure  of,  68, 
69  :  minute  structure  and  maturation  of, 
258,  259 :  see  also  under  the  various 
types  and  especially  Vaucheria,  172  : 
formation  of,  in  Angiosperms,  382 

Ov'ule  (diminutive  of  cnntni),  the  name 
usually  applied  to  the  megasporangium 
of  Phanerogams. 

Oxidation  of  protoplasm,  15 

OXYTRICH'A  (6£vs,  sharp  :  0p<:£,  a  hair), 
1 20  (Figure) 


Pancreas  (rra-yicpeVr,  sweetbread),  328 

Pandorina,  266  (Figure),  267 

Param'ylum  (napd,  beside :  aju.vA.oi/,  fine 
meal,  starch),  46 

PARAMCE'CIUM :— Figures,  108,  115: 
structure,  107:  mode  of  feeding,  112: 
asexual  reproduction,  114  :  conjugation, 
114 

Par'asite,  parasitism  (Trapao-tro?,  one 
who  lives  at  another's  table)  :  --Opalina, 
123  :  Bacteria,  92  :  Mucor,  167 

Paren'chyma  (  napeyxv^a,  anything 
poured  in  beside,  a  word  originally  used 
to  describe  the  substance  of  the  lungs, 
liver,  and  other  soft  internal  organs), 
applied  to  the  cells  of  plants  the  length 
of  which  does  not  greatly  exceed  their 
breadth  and  which  have  soft  non-lignified 
walls,  6c  :  ground-parenchyma,  345,  347 

Pari'etal  (paries,  a  wall),  applied  to  the 
layer  of  ccelomic  epithelium  lining  the 
body-wall,  277,  278 

Parthenogenesis   (Trapflt'cos,   a   virgin : 

•yeVeeris,  origin),  development  from  an 
unfertilized  ovum  or  other  female 
gamete,  200 


Parthenogenet'ic  ova,  characteristics  ot, 

262 
Pasteur,  Louis,  researches  on  yeast,  78- 

80 

Pasteur's  solution,  composition  of,  76 
Pedal  (pes,  the   foot)   ganglion,    Mussel, 

PENICILL'IUM  (pcnicillnin,  a  painter's 
brush,  from  the  form  of  the  fully-deve- 
loped aerial  hyphae)  : — Figure,  186  :  oc- 
currence and  general  characters,  184: 
mode  of  growth,  185  :  microscopic 
structure,  185  :  formation  and  germina- 
tion of  spores,  189  :  sexual  reproduction, 
190:  nutrition,  190:  vitality  of  spores, 
191 

Pepsin  (TreVrco,  to  digest),  the  proteolytic 
or  pepsonizing  ferment  of  the  gastric 
juice,  12,  80 

Peptones,  12 

Perianth  (irepi,  around  :  ai'flos,  a  flower), 
the  proximal  infertile  leaves  of  a  flower, 
380 

Perisperm  (rrepi,  around  :  <r;repju,a,  seed), 
nutrient  tissue  developed  in  the  nucleus 
of  the  seed,  380  (description  of  figure) 

Peristom'e  (rrepi,  around  :  <rr6juia.,  the 
mouth),  Vorticella,  128 

PeriStom'ium  (nepi,  around  :  O-TO/XIOI/,  a 
little  mouth),  the  mouth-bearing  segment 
of  worms,  273,  207 

Peritone'um  (Trepiroi/aioi/),  the  membrane 
covering  the  viscera,  325 

Pet'alS  (TreTaXov,  a  leaf),  the  inner  or  dis- 
tal perianth  leaves  in  the  flower  of 
Angiosperms,  380 

Phar'ynx  ($apvy£,  the  throat)  : — Poly- 
gordius,  280  :  Dogfish,  328 

Phloem  (0Aoi6s,  bark  or  bast),  the  outer 
portion  of  a  vascular  bundle,  349 

Phyla  (</>CAoi',  a  tribe)  of  the  animal  king- 
dom, 307  :  of  the  vegetable  kingdom, 
364 

Phyll'ula  (diminutive  of  $vAAov,  a  leaf), 
the  stage  in  the  embryo  of  vascular 
plants  at  which  the  first  leaf  and  root 
have  appeared,  360  :  contrasted  with 
gastrula,  360 

Phylog  eny    (<t>v\oi>,    a    race : 

origin),  the    development   of   the    race, 

Physiol'ogy  (<(>vcns,  the  nature  or  proper 
of  a  thing  :  Ao-j/ps,  a  discussion),  the  < 
partment    of   biology    which    treats 
function,  9  ct  seq. 

Pigment-spot,  Euglena, 

PileUS  (pllcus,  a  cap),  AgaHCtlS,  191 
Pinna  (pinna,  a  feather),  of  leaf,  352 
Pistil  (pistillutx,  a  pestle,  from  pinso,  to 

pound.)    See  Gyncecium. 
Plan'ula  (diminutive  of  7rAai/os,  a  wander- 
ing  about),    the    mouthless  diploblastic 
larva  of  a  hydroid,  248 
Plant,  definition  of,  176 


INDEX  AND  GLOSSARY 


4°5 


Plants,  classification  of,  364 

Plas'ma  (TrAacr/xa,  anything   moulded),  of 

blood,  56 
Plasmo'dium  (TrAao-yaa,  any  thing  moulded), 

52-55  :  comparison  of  with  zygote,  54 
Plastic  (TrAeun-iAcos,  formed  by  moulding) 

products,  products  of  katabolism  which 

remain  an  integral  part  of  the  organism, 

Pod'omere  (TTOV'S,  a  foot :  /ixepos,  a  part),  a 
limb-segment,  314 

Polar  cells,  formation  of,  262 

Pollen  grain  (pollen,  fine  flour),  a  name 
given  to  the  microscope  of  Phanero- 
gams. 

Pollen-sac,  a  name  given  to  the  microspo- 
rangium  of  Phanerogams. 

Pollen-tube,  376,  383 

Pollina'tion,  376,  383 

POLYGORD'IUS  (TroAvs,  many  :  TopSios, 
King  of  Phrygia,  inventor  of  the  Gordian 
knot)  : — Figures,  272,  274,  285,  287,  294, 
296,  298,  300,  303  :  occurrence  and  gene- 
ral characters,  271,  274  :  metameric  seg- 
mentation, 271-273  :  mode  of  feeding, 
273  :  enteric  canal,  273,  277  ;  cell-layers, 
276-278  ;  ccclome,  273,  277  :  distribution 
of  food,  281  ;  blood-system,  282  :  nephri- 
dia,  284  :  nervous  system,  286  :  differen- 
tiation of  definite  organs  and  tissues, 
291  :  reproduction,  293  :  development  and 
metamorphosis,  299-306 

Polymorphism  (TroAvs,  many  :  M°P<W, 
form),  existing  under  many  forms,  251 

Pol'yplast  (rroAi)?,  many:  TrAaerros,  formed, 
modelled),  the  multicellular  stage  of  the 
embyro  before  the  differentiation  of  cell- 
layers  or  organs  : — Hydroids,  248  :  Moss, 
337  :  Fern,  359 

PORPITA  (TTOPTTYJ,  a  brooch),  253  :  (Figure), 
253 

Primor'dial  utricle,  196,  210 

ProctodaB'um  (Trpco/cros,  the  anus  :  6Scuo?, 
belonging  to  a  way),  an  ectodermal 
pouch  which  unites  with  the  enteron  and 
forms  the  posterior  end  of  the  enteric 
canal,  its  external  aperture  being  the 
permanent  anus,  298 

Pro-embryo,  chara,  219  (Figure) 

Pro-nucleus,  female,    262 :    male,    263  : 

conjugation  of  male  and  female,  263 
Pl'OStom'ium  (Trpo,  before  :  (TTOjuiov,  a  little 
mouth),  the  first  or  pre-oral  segment  in 
worms,  Sic.,  271,  296 

PROT'AMCEBA  (n-pwros,  first :  a/uoi/Sos, 
changing),  9  (Figure). 

Prothal'lus  (n-po,  before  :  0aAA6?,  a  twig), 
the  gamobium  of  vascular  plants  :— Fern 
355  :  dimorphism  of  in  Equisetum,  367 
reduction  of  in  Salvinia,  369 ;  Selagi 
nella,  371,  and  Gymnosperms,  376,  378 
retarded  development  of  in  Angiosperms 

ProthallUS,  secondary,  Selaginella,  371 


Prot'eidS  (n-pwro?,  first),  composition  of,  5 
ProtiSt'a  (7rpum<rros,  the  first  of  all),   the 
lowest   organisms   intermediate   between 
the  lowest  undoubted  animals  and  plants, 
182 
ProtOCOC'CUS  (7rpo)TO?,  first  :  KOKKOS,  berry). 

See  Haematococcus. 

PROTOMYX'A  OU.PCOTOS,  first  :/nv£a,  mucus): 

Figure,  50 :  occurrence  and  general  cha- 

acters,    49 :    life-history,    51 ;    animal   or 

plant?     181 

Protonem'a(7rpa»TOS,  first :  i>rj/u,a,  a  thread), 

Moss,  336,  339 

Prot'oplasm  (Trpwros,  first :  TrAacr/ixa,  any- 
thing moulded),  composition  of,  5  :  pro- 
perties of,  5-7  :  micro-chemical  tests  for, 
7,  8  :  minute  structure  of,  62,  63  :  con- 
tinuity of  in  Fern.  350 :  in  Polygordius, 
292 :  intra-  and  extra-capsular,  Kadio- 
laria,  152 

Protozoa, the,  308 

Proximal  (proximus.  nearest),  the  end 
nearest  the  point  of  attachment  or  or- 
ganic base,  e.g.  in  the  stalk  of  Vorticella, 
126 

Pseud'opod1  (vl/evSrjs,    false  :   TTOVS,    foot), 
described,  4:  comparison  of  with  cilium, 
34,    52  :  in  columnar  epithelium,  59  :  in 
endoderm  cells  of  Hydra,  231 
Pteris.     See  Ferns. 
Punctum  vegetationis.     See   Growing 

point. 

Putrefac'tidn(/«£«?/«6-/Y7,  to  make  rotten) 
nature  of,  82  :  a  process  of  fermentation, 
91  :  conditions  of  temperature,  moisture, 
&c.,  93,  94 

Putres'cent  (putresco,  to  grow  rotten) 
Solution,  characters  of,  37,  82 

Putres'cible  infusion,  sterilization  of,  99- 

IO2 

Pyren'oid  (n-upryi/,  the  stone  of  stone-fruit  : 
elSos,  form),  a  small  mass  of  pr^teid 
material  invested  by  starch,  27 


Radial  symmetry,  starfish,  309 

RADIOLAR'IA  (radius,  a  spoke  or  ray):— 
Figures,  152,  153  :  occurrence  and  general 
characters,  152  :  central  capsule,  152  : 
intra-  and  extra-capsular  protoplasm, 
152 :  silicious  skeleton,  152 :  symbiotic 
relations  with  Zooxanthella,  154 

Rect'UUl  (intestinum  rectum,  the  straight 
gut),  the  posterior  or  anal  division  of  the 
enteric  canal,  281 

Redi,  Francisco  (Italian  savant},  experi- 
ments on  biogenesis,  97 

Reducing  division,  257,  262 
Reflex  action,  289 
Reproduction,  necessity  for,  19 
Reproductive  organ.    See  Gonad. 


406 


INDEX  AND  GLOSSARY 


Reservoir  of  contractile  vacuole,  Euglena, 
47 

Respiration  : — Amoeba,  17  :  Polygordius, 

284 
Respiratory  caeca,  Starfish,  312 

Rhiz'oid(pi£a,  root  telSos,  form):— Nitella, 
206,  214  :  Moss,  335  :  prothallus  of  Fern, 

Root,  Fern,  344,  353 

Root-cap,  354 

Root-hairs,  353,  357 

ROSS,  Alexander,  on  abiogenetic  origin  of 
mice,  insects,  &c.,  96 

Rotation  of  protoplasm,  210 

Rudiment,  rudimentary  (mdimentum,  a 
beginning),  the  early  stage  of  a  part  or 
organ  :  often  used  for  a  structure  which 
has  undergone  partial  atrophy,  but  in 
such  cases  the  word  vestige  (q.  v.)  is 
more  suitable. 


s 


SACCHAROMY'CES  (<ra.Kxapov,  sugar: 
/ixu'/crjs,  fungus): — Figure,  72  :  occurrence, 
71  :  structure,  71  :  budding,  73  :  in- 
ternal fission,  74 :  nutrition,  75 :  alco- 
holic fermentation  caused  by,  75,  79,  80  : 
experiments  on  nutrition  of,  78-80  :  ani- 
mal or  plant?  182 

SALVIN'IA  :— Figure,  369  :  general  cha- 
racters, 368  :  mega-  and  micro-sporangia 
and  spores,  368  :  male  and  female  pro- 
thalli  and  gonads,  369,  370  :  development 
and  alternation  of  generations,  370 

Saprophyt'ic  (o-a.7rp6s,  putrid  :  fyvrov,  3. 
plant)  nutrition,  defined,  39 

Schulze's  solution,  test  for  cellulose,  28 : 
for  lignin,  348.  349 

Scleren'chyma  (ovcAijpo?,  hard :  e-yxv/u-a, 
infusion)  : — Moss,  334 :  Fern,  345,  348, 

Secre'tion  (secrctus,  separate),  nature  of, 
231  :  formation  of  cell- wall  a  process  of, 
*4 

Seed,  formation  of,  377,  384  :  germination 
of,  377 

Seg'ment  (segmentwm,  a  piece  cut  off),  in 
plants  a  node  together  with  the  next 
proximal  internode,  208  :  in  animals  the 
name  is  variously  applied.  See  Meta- 
mere,  Podomere. 

Segment'al  cell:  Nitella,  211:  Moss, 
335  :  Fern,  350 

Segmentation,  metameric.  See  Meta- 
mere, 

SELAGINELL'A  (o-eAa-yeou,  to  shine):— 
Figure,  372  ;  general  characters,  371  : 
cone,  sporangia,  and  spores,  371  :  pro- 
thalli  and  gonads,  371  :  development  and 
alternation  of  generations,  372,  373 

Self-fertilization,   applied  to   the   sexual 


process  when  the  gametes  spring  from 
the  same  individual,  199 
Sep'alS  (separ,    separate),    the    outer    or 
proximal  perianth-leaves  in  the  flower  of 
Angiosperms,  378 

Sep'tum  (septttm,  a  barrier) :— In  plants 
187  :  in  Polygordius,  280  :  development 
o  f,  302 

Set'a  (seta,  a  bristle),  290 
Sex-cells,  primitive,  255 :   origin  of  in 

Hydroids,  247  :  in  Polygordius,  293 
Sexual   differentiation,    illustrated    by 

Vaucheria,  172  :  by  Spirogyra,  199 
Sexual  generation.    See  Gamobium. 
Sexual  reproduction,  nature  of,  42 
Shell,  Mussel,  320 
Shoot,  in  plants,  an  axis  of  the  second  or 

any  higher  order  with  its  leaves,  209 
Sieve-tubes  and  plates,  350 
Sinus  (sinus,  a  hollow),  a  spacious  cavity 

3i8 

Skeleton.     See  Endo-  and  Exo-skeleton. 
Slime-fungi,     See.Mycetozoa. 
Solid  aggregate,  203 

Somatic  (o-w/u.a,  the  body),  applied  to  the 

layer  of  mesoderm  which  is  in  contact 

with  the  ectoderm  and  with  it  forms  the 

body-wall,  278 

Sor'us  (eriopo?,  a  heap),  an  aggregation  of 

sporangia,  354,  368 

Species  (species,  a  kind),  meaning  of  term 
illustrated,    8,    137  ;   definition   of,    139 : 
origin  of,  141,  144 
Specific  characters,  specific  name,  8, 

139 

Specialized,  meaning  of,  140 
Sperm  (o-irep/ota,  seed),  the  male  or  micro- 
gamete  in  its  highest  stage  of  differentia- 
tion :  structure  and  development  of,  255  : 
see  also  under  the  various  types,  and 
especially  Vaucheria,  172,  173 
Spermatozo  id,  spermatozo'on  (o-7rep/u.a, 
seed  :  £a>ov,  animal,  from  the  actively 
moving  sperms  of  animals  having  been 
supposed  to  be  parasites),  synonyms  of 
sperm. 

Spermary  (orn-ep/ota,  seed),  the  male  gonad 
or  sperm-producing  organ  :  see  under  the 
various  types,  and  especially  Vaucheria, 
172 

Sperm'iduct  (<T7re'pju.a,  seed  :  duco,  to  lead), 
a  tube   conveying   the    sperm   from  the 
'    spermary  to  the  exterior,  295 
SpermatOgen'esiS  (o-Trep/u-a,  seed  ;  yeVecrts, 
origin),  the  development  of  a  sperm  from 
a  primitive  sex-cell,  255,  256  (Figure). 
Spinal  cord,  Dogfish,  330 
Spiral  vessel.    See  Vessel. 
SPIRILL'UM  O^Vrt,  a  coil)  86,  88  (Figure) 
SPIROGYRA  (spira,  a  coil :  gyms,  a  revo- 
tion) : — Figure,     195  :     occurrence    and 
general     characters,     194  :     microscopic 
structure,  194  :  growth,  197  :  conjugation, 
198  :  development,  200  :  nutrition,  200 


INDEX  AND  GLOSSARY 


407 


Splanch'niC  (<rn \dyxvov,  intestine  or  vis- 
cus),  applied  to  the  layer  of  mesoderm 
which  is  in  contact  with  the  endoderm 
and  with  it  forms  the  enteric  canal,  278 

Spontaneous  generation.  See  Abio- 
genesis. 

Sporan'gium  (a-iropd,  seed  :  dyyelov,  a 
vessel),  a  spore-case: — Mucor,  160  :  Vau- 
cheria,  171:  Fern,  354.  See  also  Mega- 
and  Micro-sporangium. 

Spore  (o-Tropd,  a  seed),  an  asexual  repro- 
ductive cell :  see  under  the  various  types 
and  especially  Heteromita,  42  :  Saccha- 
romyces,  74  :  Bacteria,  89  :  vitality  of 
in  Bacteria,  99,  101 :  Pemcillium,  189 : 
Moss,  339  :  Fern,  355.  See  also  Mega- 
and  Micro-spore. 

Sporogon'ium  a-nopd  seed  :  yovos,  pro- 
duction), the  agamobium  of  a  moss,  337 

Spor'ophyll  (a-nopd,  seed  :  <j>v\\ov,  leaf), 
a  sporangium-bearing  leaf: — Equisetum, 
366 :  Selaginella,  371 :  Gymnosperms, 
373i  375  '  Angiosperms,  381 

Stamen  (stamen,  a  thread),  a  male  sporo- 
phyll,  373,  381  ^ 

Starch,  composition  and  properties  of,  27 

STARFISH ;— Figure,  310:  general  cha- 
racters, 309-311 :  radial  symmetry,  309: 
tube-feet  and  ambulacral  system,  311, 
313  :  exoskeleton,  312 

Stem,  structure  of: — Moss,  334  ;  Fern.  345 

Sterig'ma  (o"r»jpry^.a,  a  support) :  Penicil- 
lium, 188  ;  Agaricus,  193 

Sterilization  of  putrescible  infusions,  99- 

IO2 

Stigma  (o-Tiy/ma,  a  spot),  the  receptive  ex- 
tremity of  the  style,  381 

Stimulus,  various  kinds  of,  289 

Stock.     See  Colony. 

Stom'ate  (crro^ia,  mouth),  353 

Stomodse'um  (a-ro/xa,  mouth  :  ofiaios,  be- 
longing to  a  way),  an  ectodermal  pouch 
which  unites  with  the  enteron  and  forms 
the  anterior  end  of  the  enteric  canal,  its 
aperture  being  the  permanent  mouth, 
298 

Stone-canal,  Starfish,  313 

Style  (stylus,  a  column),  the  distal  solid 
portion  of  the  female  sporophyll  or  of  the 
entire  gyncecium  in  Angiosperms,  381 

QT  VT  nxrvr'iTT'T  A  /       ~  \  '     .,       i. 

01  iJ-iUJN  iu±l  lA(<7TvAos,  a  column  :  ow£, 

a  claw),  Figure,  117:  occurrence  and 
general  characters,  116  :  polymorphism 
of  cilia,  118-119 

Sub-apical  cell.     See  Segmental  cell. 

Superficial  aggregate.  202 

Supporting  lamella.     See  Mesoglcea. 

Suspensor:  Selaginella,  373:  Gymno- 
sperms, 377  :  Angiosperms.  383 

Sweet  Wort,  composition  of,  75 

Swimming-bell,  Diphyes,  251 

Symbio'siS  (cru/x/3t'ojcrt?,  a  living  with),  an 
intimate  and  mutually  advantageous 
association  between  two  organisms,  154 


Syner'gidffi  (ovvepyos,  a  fellow  worker), 
382 

Sys'tole  (cruo-ToATj,  a  drawing  together, 
contraction),  the  phase  of  contraction  of 
a  heart,  contractile  vacuole,  &c.}  in 


Teeth,  Dogfish,  328 

Temperature,  effects  of  on  protoplasmic 

movements,  20,  21 
Tentacles  :—Hydra,  223  :    Bougainvillea, 

239 ;  Polygordius,  271  :  development  of, 

302 
Terminal  bud  :— Nitella,  208,  210:  Moss 

TestiS  (the  Latin  word),  generally  used  for 
the  spermary  in  animals. 

Thermal  death-point.  See  Ultra-maxi- 
mum temperature. 

Tissues,  differentiation  of: — Polygordius, 
291  :  Fern,  353 

Tracheides  (rpa^vs,  rough  :  eiSos,  form). 
See  Vessels  of  Plants,  349 

Transpiration,  the  giving  off  of  water 
from  the  leaves  of  plants,  341 

Trich'ocyst  (0pif,  a  hair  :  KVO-TIS,  a  bag), 
JI3 

TliploblaSt'iC  (rpiTrAoos,  triple  :  /SAao-ros, 
a  bud),  three-layered  :  applied  to  ani- 
mals in  which  the  body  consists  of  ecto- 
derm, mesoderm,  and  endoderm,  244, 
278 

TrOCh'OSphere  Orpoxos,a  wheel, in  reference 
to  the  circlet  of  cilia  :  o^cupa,  a  sphere), 
the  free-swimming  larva  of  Polygordius, 
&c. : — characters  of,  296  (Figure)  ;  origin 
of  from  gastrula,  297,  298  ;  metamorpho- 
sis of,  299 

Tube-feet,  Starfish,  311,  313 

U 

Ultra-maximum  temperature,  for  amoe- 
boid movements,  21  ;  for  monads,  40  ;  for 
Bacteria,  93 

ULVA  (jilva,  an  aquatic  plant),  203 

Umbell'ate  (inubella,  a  sun-shade,  um- 
brella) applied  to  branching  in  which 
the  primary  axis  is  of  limited  growth  and 
sends  off  a  number  of  secondary  axes 
from  its  distal  end,  138 

Unicell'ular,  formed  of  a  single  cell,  61  ; 
connection  of  uni-  with  multi-cellular 
organisms,  264-270 

Ureter  (ovprjr^p,  the  Greek  name),  the 
duct  of  the  kidney,  330 


Vac'uole  (vacuus,  empty),  contractile,  13 

in  :  non-contractile,  71 
Variability,  147 
Variation,  individual,  140,  147 


408 


INDEX  AND  GLOSSARY 


Variety,  an  incipient  species,  147 

Vasc'ular  (vasciiltun,  a  small  vessel) 
bundles,  345,  348 

Vascular  plants,  365 

VAUCHERIA  (after  J.  P.  E.  Vaucher,  a 
Swiss  botanist) : — Figure,  170  :  occur- 
rence and  general  characters,  169  :  minute 
structure,  169  :  asexual  reproduction,  171: 
sexual  reproduction,  172  :  nutrition,  175 

Veins  of  Dogfish,  329  :  of  leaves,  352 

Vel'um  (velum,  a  veil)  of  medusa,  241 

Vent,  the  aperture  of  the  cloaca,  324 

Venter  (venter,  the  belly),  of  ovary  of 
Moss,  336,  and  Fern,  358  :  of  the  female 
sporophyll  or  of  the  entire  gynoecium  of 
Angiosperms  (so-called  ovary)  381 

Ventral  nerve-cord :— Polygordius,  286 : 
development  of,  301  :  Crayfish,  319 

Ventricle.     See  Heart. 

Vermes,  the,  308 

Ver'tebral^T^ra,  a  joint)  centra  and 
column,  Dogfish,  328 

Vertebrata,  the,  309 

Vessels  :— of  plants,  spiral  and  scalariform, 
348,  349  :  of  animals,  see  Blood-vessels. 

Vestige,  vestigial  (vestigium,  a  trace), 
applied  to  any  structure  which  has  be- 
come atrophied  or  undergone  reduction 
beyond  the  limits  of  usefulness,  118 

Vib'riO  (vibro,  to  vibrate),  86,  88,  (Figure) 

•ViSC'eral  (viscns,  an  internal  organ),   ap- 
plied to  the  layer  of  coelomic  epithelium, 
or  of  peritoneum,  covering  the   intestine 
and  other  internal  organs,  277 
Visceral  ganglion,  Mussel,  323 

Vitelline  (vitellus,  yolk)  membrane,  the 

cell-membrane  of  the  ovum,  259 
Volvox    (volvo,   to  roll),    267,    268,    269. 

(Figure*) 

VORTIOELLA  (diminutive  of  vortex, 
eddy) : — Figure,    127  :     occurrence    .. 
general  characters,  126  :  structure,  126  : 
asexual  reproduction,  131  :  conjugation, 
132  :  means  of  dispersal,  132-136  :  ency-<- 
tadon,     spore-formation,    development, 
and  metamorphosis,  133 


W 

Waste-products,  33 
Water  of  organisation,  5,  29 
Whorl  of  leaves,  208 
Wood.     See  Xylem. 
Work  and  Waste,  14 


Xylem  (£uAoi/,  wood),  the  inner  portion  o. 
vascular  bundle,  349 


Yeast,  7r 

Yeast-plant.    See  Saccharomyces. 
YellOW-cells  of  Radiolaria,  154 
Yolk-granules  or  spheres,  68,  235, 


Zoogloe'a  (£<aov,  an  animal :  -y/Aoi'a,  glue), 
85 

Zooid  (£a>oc,  an  animal*  etSos,  form),  a 
single  individual  of  a  compound  organism, 
137,  237 

Zootham'nium  (^(aov,  an  animal  :  0t.,xi;o?, 
a  bush): — Figures,  134,  138:  OCL  -rence 
and  general  characters,  135  -.dimorphism 
ofzooids,  135:  means  of  dispersal,  136: 
characters  arid  mutual  relations  of  species, 

Zooxanthell'a  (fo>oi>  an  animal  :  £ai'06?, 
yellow),  154 

Zyg'ospore(c;iryoi>,  a  yoke  :  <ntopa,  a  seed), 
applied  to  a  resting  zygote  formed  by  the 
conjugation  of  similar  garnet  ,  166 

ZygOte  (£vywTos,  yoked),  the  p.oducts  of 
conjugation  of  two  gametes  : — Hetero- 
mita,  41  :  Vorticella,  133  :  Mucor,  165  : 
Vaucheria,  174  :  Spirogyra,  198-200. 


THE   END 


RICHARD    CLAY   AND   SONS,    LIMITED,    LONDON    AND    BUNUAY. 


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