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HoWBRd W P/fMlNO 
550 1 F?r(i Cj £■ pQfjci ' 

Be R ke ) ^ y. 




Qlft 
Dr. Howard Fleming 




CAMBRIDGE BIOLOGICAL SERIES. 

Qensral Editor :— Arthub E. Shipley, M.A. 

FELLOW AND TUTOR OF CHRIST'S COLLEGE, CAMBRIDGE. 



ZOOLOGY 



llonl^on: C. J. CLAY and SONS, 

CAMBRIDGE UNIVERSITY PRESS WAREHOUSE, 

AVE MARIA LANE, 

AND 

H. K. LEWIS, 
136, GOWER STREET, W.C. 




eiasgoto: 60, WELLINGTON STREET. 

l.eip>is: F. A. BROCKUAUS. 

j^rtg fiorfc: THE MAC3IILLAN COMPANY. 

SombaB ^ Colnttta: ^UCMILLAN & CO. Ltd. 



[All Eights reserved.] 



ZOOLOGY 



AN ELEMENTARY TEXT-BOOK 



BY 



A. E. SHIPLEY, M.A., 

FELLOW AND TUTOR "OT CHRIST'S COLLEGE, CAMBRIDGE, 
AND UNIVERSITY LECTURER IN ADVAS'CED MORPHOLOGY OF THE 

INVERTEBRATA; 

AND 

E. W. MacBRIDE, M.A. (Cantab.), D.Sc. (Lond.), 

SOMETIME FELLOW OF ST JOHN'S COLLEGE, CAMBRIDGE, 
PROFESSOR OF ZOOLOGY IN M<=GILL UNIVERSITY, MONTREAL. 



SECOND EDITION 



CAMBRIDGE : 
AT THE UNIVERSITY PRESS 

1904 



• ■ • 



Fir$t Edition 190L 
Second Edition 1904 



• • * • 



* k » 



V V V « • W 1 

* He 






PREFACE. 



WE have tried in the following book to write an ele- 
mentary treatise on Zoology which could readily be 
understood by a student who had no previous knowledge of 
the subject. We have endeavoured to explain the technical 
terms as they occur, and since one of the difficulties proper to 
the science of 2iOology is the enormous number and the pro- 
digious length of these terms, we have in many cases given 
derivations which may help the beginner to fix them in his 
mind. 

The attempt to construct the book on the plan that each 
section is built on what precedes it, has rendered it impossible 
to keep the treatment of the various groups at the same level, 
for much space is taken up in the earlier chapters in ex- 
plaining processes a knowledge of which is assumed in the 
later. A book such as we have aimed at is bound to be 
progressive, and the later chapters will be intelligible to the 
beginner only if he has read the eailier. Thus the part of 
this book which deals with the Yertebrata is in many respects 
more advanced than those which deal with the several Inverte- 
brate groups. 

In order to give some account of the leading types of 
animal structure within a book of moderate compass, we have 
been compelled to make but scanty reference to Histology, 
Embryology and Palaeozoology ; in fact the book in the main 
deals with the normal strncture of the adult forms of recent 

92006 



ri PBEFACEL 

^ ffiimaJn Wberei'er poatdble we luiTe endeaToored to exhibit 
this structure w the outcxHne of funetioD and hahiL We have 
tried to show that ZcxAogy deals at least as much with liTing 
as with dead ofgsxuKma. 

In tracLDg the relatkmship of the animals described to one 
aaotiiier we have at times put forward hypotheses which we fear 
will not commend themselves to all aoologists, bat we have 
thought it better to run the risk of submitting views which 
further research may compel us to abandon rather than leave 
the student with the idea that the object of zoological study 
is the mere collection of &cts. We try everyiidiere to make 
it dear that the ultimate end of the sdence is tiie disooveiy 
of the laws underiying and binding togethtf the fftcta. 

At the end of the sections dealing with each phylum a 
short table of classification is given. These tables do not 
attempt to be complete but are intended to indicate the posi- 
tion of the animals mentioned in the text in the general scheme 
of cUfisification, and since the book appears in hotik an English 
and an American edition, these examples are in most cases 
drawn from the British and North American Faunaa 

In one respect thiM book differs from many of the ele- 
mentary treatises which have appeared within the last few 
years. It has been drawn up with an eye to no examination 
and does not claim to correspond with any of the numerous 
syllabuses and schedules, issued from time to time by the 
various Boards of Examiners scattered through the United 
Kingdom and North America. 

Many of the illustrations are new and whatever merit 
they possess is due to the skill of Mr E. Wilson of Cambridge: 
and Mr F. M. Hewlett of Christ's College. We owe om 
grateful thanks to Mr S. H. Reynolds for permission to us 
many of the illustrations of his book on "The Vertebrat 



PREFACE. VU 

Skeleton/' and to Slessrs Macmillan & Co. and Messrs A. and 
C. Black and the Council of the Royal Agricultural Society for 
granting the use of certain other illustrations. We are also 
much indebted to many friends for help in special chapters. 
Mr E. S. Thompson and Mr J. Graham Kerr have read through 
the proof-sheets, a most tedious task, as the authors can 
abundantly testify ; Dr Harmer, Dr Gadow, Dr Anderson, Dr 
Hopkins, Mr G. P. Bidder, Mr J. Stanley Gardiner, Mr R. Evans^ 
Mr H. H. Brindley, and Mr C. Warburton, have most freely 
given us the help of their special knowledge. Whilst saving us 
from many mistakes they are by no means responsible for those 
that remain. We tender them all our sincere thanks. 

A. E. S. 
E. W. M. 

August y 1901. 



8. (&M. 



PREFACE TO THE SECOND EDITION. 

TN issuing a second edition of this Zoology, which has been 
-■- for some time out of print, the authors desire to thank 
many critics who have pointed out errors in the book. They 
are again indebted for much help to the gentlemen mentioned 
in the former Preface, and above all they owe thanks to 
Mr H. H. Brindley of St John's College, who has kindly 
read the whole of the proofs and whose critical power has 
been most unreservedly placed at the use of the authors. 
In the subjects which they have made their own Dr Gadow 
and Mr L. A. Borradaile of Selwyn College have given the 
writers much valued help. 

A. E. S. 
E. W. M. 

J/arcA, 1904. 



TABLE OP CONTENTS. 



CHAP. PAGE 

I. Introduction 1 

II. Phylum Protozoa 13 

III. Phylum Ooelenterata 42 

IV. Phylum Porifera 74 

y. Introduction to the Coelomata 83 

VL Phylum Annelida 89 

VIL Phylum Arthropoda 118 

VIII. Phylum MoUusca 210 

IX. Phylum EchinodermaU 249 

X. Phylum Brachiopoda 291 

XL Phylum Polyzoa 297 

XIL Phylum Chaetognatha 302 

XIII. Introduction to the Phylum Vertebrata; Sub-phyla 

I — III. Ilemichordata, Cephalochordata and Uro- 

chordata 306 

XIV. Introduction to Sub-phylum IV. Craniata. Di?ision I. 

Cyclostomata 333 

XV. Craniata. Division II. Gnathostomata. Class I. Pisces 368 

XVI. Craniata. Division II. Gnathostomata. Class II. Am- 

phibia 417 

XVII. Craniata. Division II. Gnathostomata. Class III. Rep- 

tilia 457 

XVIII. Craniata. Division II. Gnathostomata. Class IV. Aves 495 

XIX. Craniata. Division II. Gnathostomata. Class V. Mam- 
malia 520 

XX. Phylum Platyhelminthes 605 

XXI. Phylum Nemertinea 626 

XXII. Phylum Rotifera 631 

XXIII. Phylum Nematoda 639 

Index 645 

62 



LIST OF ILLUSTRATIONS. 



riO' PAGE 

1. Amoeba protetu 14 

2. D\ffliigia urcedata 18 

3. Arcella discoides 19 

4. Gramia oviformis .... .... 20 

5. Polystomella crtspa 22 

6. ffeliosphaera tnermis 24 

7. Chondrioderma difforme 26 

8. Actinophryi sol 27 

9. Actinosphaerium eiehhornii 28 

10. Vorticella microstoma 29 

11. Diagram of Vorticella 30 

12. Opalina ranarum 33 

13. Paramecium caudatum 34 

14. Eugletta viridie 36 

15. Clepndrina longa 38 

16. Hydra fusca 43 

17. Longitudinal section through the body of Hydra . 44 

18. Transverse section of Hydra fusca 46 

19. Cnidoblast from the body- wall of Hydra fusca ... 47 

20. Section tlirough body-wall of Hydra fusca .... 49 

21. Obelia helgolandica 50 

22. Part of a branch of Obelia 51 

23. Free-swimming Medusa of Obelia 52 

24. Bougainvillia fructuosa 53 

25. (1) Eye of Lizzia koellikeri; (2) Radial section through the 

edge of the umbrella of Carmarina hastata ... 55 

26. Planula of Clava squamata 56 

27. Part of a colony of Akyonium digitatum .... 60 

28. Transverse section through a polyp of Alcyonium digitatum 

below the level of the oesophagus 61 

29. Transverse section through a polyp of Alcyonium digitatum 

through the region of the oesophagus . . 61 

30. Semi-diagrammatic view of half a simple Coral ... 64 

31. Aurelia aurita 67 



Xll LIST OF ILLUSTRATIONS. 

FIG. PAGE 

32. Strobilization of Aurelia aurita ...••• ^^ 

33. Hormiphora plumosa 70 

34. View of a branch of LeucosoUnia showing osculum . 75 

35. Vertical section through an osculum of Leticosolenia . 76 

36. Section of a flagellated chamber of Spongilla lacustris . 77 

37. Section of a portion of Ghrantia extusarticulata ... 78 

38. Three transverse sections through a developing Amphioxm 

to show the origin of mesoblast 84 

39. Two stages in the early development of a common fresh-water 

mollusc, Planorbis, to show the origin of the mesoderm . 85 

40. Latero-ventral view of an Earthworm, Lumbrictu terrestrnt . 90 

41. Anterior view of the internal organs of Lumbricus ierrestrU . 93 

42. Six segments from the intestinal region of Lumbricus ierrestrU 

dissected so as to show the arrangement of the parts . . 94 

43. Diagram of the anterior end of Lumbrictu herculeus to show 

the arrangement of the nervous system .... 99 

44. Transverse section through Lumbricus terrestris in the region 

of the intestine 102 

45. View of the reproductive organs of Lumbrictu terrestris . 105 

46. Nereis pelagica 109 

47. Transverse section through Nereis cultrifera . . . 110 

48. Hirudo medicinalis Ill 

49. View of the internal organs of Hirudo medicinalis . 114 

50. Three stages in the emergence of the adult Dragon-fly from the 

larval skin 120 

51. A Centipede, Lithobius forficattu 122 

52. A male Cockchafer, Melolontha vulgaris . . . 123 

53. The Oarden Spider, Epeira diadema, sitting in the centre 

of its web 124 

54. The mouth appendages of Gammariu neglectus . 125 

55. Pedipalp of the large House-spider, Tegenaria guyonii . 126 

56. Left mouth-appendages of the Crayfish, Astaciu fluviaiilis 127 

57. Asellus aquaticus 128 

58. View of the nervous system of the Cockchafer, Melolontha 

vulgaris • . . . . 129 

59. Sections through the central and lateral eyes of a Scorpion, 

Euscorpius italicus 131 

60. View of internal organs of male Crayfish, Astacus fluviatilis^ 

seen from the side 132 

61. Views of the gills of the left side of a Prawn, Penaeus semi- 

culcatus 135 

62. View of internal organs of the male Cockchafer, Melolontha 

vulgaris 136 

63. Horizontal section through the abdomen of a Spider, Argy- 

roneta 137 



LIST OF ILLUSTRATIONS. XUl 

FIG. PAGE 

64. Longitudinal section through the operculum and gills of a 

King-crab, LimtUtis 138 

65. Longitudinal section through the lung-book of a Spider . 138 

66. Male reproductive organs of Astacut flumatilU . . . 141 

67. Female reproductive organs of Astacus flumatUis . 141 

68. Dorsal view of a female Branchipus 144 

69. Side view of male Simocephalia $ima 145 

70. Side view of female Simocephalus sima .... 145 

71. Lateral view of Cyprii Candida 147 

72. Ventral view of a male Cyclops 148 

73. Dorsal view of a female Cyclops 149 

74. View of Lepat anatifera cut open longitudinally . . 151 

75. A Schizopod, Nyctiphanes norwegica 154 

76. The Shore-crab, Cardnus maenas 156 

77. Female of Diastylis stygia, one of the Cumacea . 158 

78. Side view of Gammariu neglectus showing internal organs 159 

79. A Wood-louse, Porcellio scaher 160 

80. Peripatus capensis 161 

81. Peripatus capensisy male, dissected to show the internal 

organs 162 

82. A Centipede, Lithohitu forficatus 164 

83. Lilhobius forficatus ^ dissected to show the internal organs . 166 

84. Iidus terrestris 167 

85. Two views of a male Cockroach, Stylopyga orientcUis . 169 

86. Mouth appendages of /Stylopyga 171 

87. Female Cockroach dissected to show the viscera . . 174 

88. A Grasshopper, Pachytylus migratorius . . . . 182 

89. Larva of Bombyx mori, the Silkworm . . . . 184 

90. Cocoon of Boffnbyx tnori 185 

91. Silkworm moth, Bombyx tnori 185 

92. The Lady-bird, Coccinella septempunctata, and its larva . 187 

93. Male, Female and Neuter of the Wood- ant, Formica rufa 187 

94. Drone, Queen and Worker of the Honey-bee, Apis mellifi>ca . 188 

95. A Wasp, Polistes tepidiis, and its nest . . . . 188 

96. The Tsetse-fly, Glossina morsitans 189 

97. The Hessian-fly, Cecidomyia destructor . . . . 189 

98. The Garden Spider, Epeira diadema, sitting in the centre 

of its web 192 

99. Front view of the head of a Spider, Textrix denticukUa . 192 

100. Pedipalp of the large House-spider, Tegetiaria guyonii . 193 

101. Lateral view of the internal organs of a Spider, Epeira 

diademata 194 

102. Digrammatic view of a palpal organ 196 

103. A Phalangid or Harvestman, Oligolophus spinosus . . 197 

104. Male and female of the Cheese-mite, Tyroglyphus siro . 198 



XIV LIST OF ILLUSTRATIONS. 

PIG. PAGE 

105. Dorsal and ventral views of the Indian Scorpion, Scorpio 

swammerdami 200 

106. Dorsal view of the King-crab, Limulus polyphemus . 202 

107. Ventral view of the King- crab, LimuLuB polyphemus . 203 

108. Side view of a Snail, Helix potnatia, the animal being expanded 21 1 

109. Dorsal view of a Snail, Helix pomatia, after removal of the shell 213 

110. Helix pomatia, with the pulmonary chamber cut open . 214 

111. Longitudinal section of the head of Helix to show the radula 215 

112. Dissection of the Snail, Helix pomcUidk, to show the internal 

organs 216 

113. View of the nervous system of Helix jwmcUia . . . 218 

114. Optical section through the auditory vesicle of Pterotrachea 

friederid 219 

115. Nervous system of the Pond-suail, Litnnaea . . 219 

116. Nervous system, osphradium and gills of Haliotis . 220 

117. The Pond-mussel, Anodonla mutabilis, with foot expanded 

and the empty shell of the same 224 

118. Right side o( Anodonta mutabilis with mantle cut away and 

gills folded back 226 

119. Diagrammatic transverse sections of Anodonta . . . 227 

1 20. Right side of A nodonta mutabilis dissected to show the viscera 229 

121. Dorsal view of Anodonta mutabilis^ with the upper wall of 

the pericardium removed to show the heart . . 231 

122. Solen vagina^ the Razor-shell 233 

123. Diagrams of a series of Mollusca to show the relations of the 

foot and visceral hump to each other and to the antero- 
posterior and dorso-ventral axes 234 

124. Posterior view of a male Cuttle-fish, Sepia officinalis^ with 

the mantle-cavity opened 235 

125. A diagram showing the relation of the kidneys to the peri- 

cardium in Sepia 237 

126. View of heart and chief blood vessels of Sepia cultrata 239 

127. Diagrammatic longitudinal section of Sepia to show the 

relation to one another of some of the principal viscem . 240 

128. Lateral view of the central nervous system of Sepia officinalis 241 

129. Ventral view of Sepia qfficinalis dissected so as to show the 

nervous system 243 

130. Side view of the pearly Nautilus, Nautilus pompilius 245 

131. Oral view of a Star-fish, Echinaster sentus .... 250 

132. Dissection of the common Star-fish, Asterias rubens, so as to 

show the motor, digestive and reproductive organs . 252 

133. A Star-fish, Eckinaster sentus, in the act of devouring a 

mussel 254 

134. Diagram of a transverse section of the arm of a Star-fish . 255 

135. Pedicellariae from Asterias glacial is ..... 259 



LIST OF ILLUSTRATIONS. XV 

PIO. PAOB 

136. Dorsal view of a Brittle-star, Ophioglypha bullaia . . 262 

137. Diagram of a transverse section of the arm of a Brittl^star . 263 

138. Diagram of a longitudinal vertical section through a Brittle- 

star 264 

139. Oral view of a portion of Ophioglypha bullata . . 265 

140. Dorsal view of a Sea-urchin, Strongylocentrui drdbaehiemis, 

with the tube-feet expanded 267 

141. Dorsal view of the dried shell of the common British Sea- 

urchin, Echintu esctUentus 268 

142. A gemmiform pedicellaria from Echintu escidentus . . 269 

143. Dissection of Echinus escuientus so as to show the structure 

of "Aristotle's lantern" 270 

144. Diagram of a longitudinal vertical section of a Sea-urchin 271 

145. Transverse sections through the madreporite and the radius 

of Echinus escuientus 273 

146. Dissection of a Sea-urchin so as to show the course of the * 

alimentary canal 274 

147. The oral field or peristome of Echinus escuientus . . 275 

148. The aboral system of plates, or periproct and calyx of 

Echinus escuientus 277 

149. Dissection of a Sea-cucumber, Holothuria tubulasa, so as to 

show the arrangement of the viscera .... 280 

150. A Feather-star, Antedon acoela 283 

151. Dia*rram of a longitudinal vertical section of the common 

Feather- star, Antedon rosacea 284 

152. A stalked Feather-star, Rhizocrinus 285 

153. Ventral view of the larva of a Sea-cucumber . . . 286 

154. Shell of a fossil Brachiopod, TerebratiUa semiglobosa . 292 

155. Section through the shell of Waldheimia flavescens . . 292 

156. Dissection of Waldheimia australis so as to show the in- 

ternal organs 293 

157. Longitudinal vertical section of Argiope neapolitana . . 294 

158. Portions of two Polyzoan colonies 297 

159. Longitudinal vertical section of Plumatella fungosa . 298 

160. An avicularium of Bugula 300 

161. Ventral view of Sagitta hexaptera 303 

162. Transverse sections of Siigitta bipunctata and of Spadella 

cephaloptera 304 

163. Dolichoglossus kowaleeskiiy a species of Balanoglossus . 307 

164. Longitudinal vertical section of Glossobalanus . 309 

165. Longitudinal horizontal section of Glossobalanus . . 310 

166. Amphioxus lanceolatus seen from the left side . . 311 

167. Views of the velum and of the oral cartilages of Amphioxus 311 

168. Diagram of longitudinal section through a young embryo of 

Amphioxia 312 



XVI LIST OF ILLUSTRATIONS. 

FIG. PAOB 

169. Anterior region of a young Amphioxus seen from the left 

side 313 

170. Diagrammatic transverse section through the (>haryngeal 

region of a female Amphioxus 314 

171. Transverse section through the intestinal region of a young 

Amphioxus 315 

172. Front end of a young transparent Amphioxus . . . 816 

173. Anterior portion of the nerve-cord of Amphioxus . 316 

174. Longitudinal vertical section through the cerebral vesicle of 

Amphioxus 317 

175. Transverse section through the middle region of the nerre- 

cord of Amphioxus 318 

176. A nephridium of Amphioxus, belonging to the left side of 

the body 319 

177. Portion of a transverse section through the pharynx of Am^ 

phioxusy in order to show the relations of the excretory 
tubule 320 

178. Ventral view of an Amphioxus dissected so as to show the 

reproductive organs 321 

179. Diagrammatic transverse section of Amphioxus to show the 

relations of the excretory and genital organs . . . 322 

180. Side view of the anterior end of a larva of Ascidia . 324 

181. Dorsal view of the anterior end of a larva of Ascidia 324 

182. Diagrams showing the changes undergone by a larval Ascidian 

in its metamorphosis . . 326 

183. Ciona intestinalis 327 

184. Dissection of Ciona intestinalis so as to show the internal 

organs 328 

185. Portion of a colony of Botryllus vioUiceus .... 330 

186. Dorsal view of a fully developed asexual person of Salpa 

democraiica 331 

187. Longitudinal vertical section of ScUpa 332 

188. Views of the brain of a Dogfish, Scy Ilium ccUtUus, from 

various aspects 335 

189. Transverse section through the snout of a Dogfish, ScyUium 

canicula 338 

190. Ear of Chimaera monstrosa 339 

191. Section of an ampulla of the internal ear .... 340 

192. Transverse section through the head of an embryo Chick 

of the third day of incubation in order to show the origin 

of the retina and lens of the eye . . . . . 341 

193. Diagram to illustrate the structure of the retina . . 342 

194. Diagram of the arterial system of the Dogfish, ScyUium . 351 

195. Diagram of the venous system of the Shark, Mustelus ant- 

arcticus 354 



LIST OF ILLUSTRATIONS. XVU 

FIG. PAOB 

196. Dissection of the muscles of the eye of Scyllium canicula 355 

197. Dingrams illustrating the development of the excretory and 

reproductive organs of Craniata 357 

198. Diagram of a transverse section through a hypothetical an- 

cestral Elasmobranch in order to show the origin of the 

excretory and genital organs 359 

199. The Musk Lamprey, Petromyzon tcilderi, in the act of 

spawning 361 

200. Longitudinal vertical section through a female Lamprey, 

Petromyzon tnarinus 362 

201. Three views of the skull of Petromyzon marinus^ from 

different aspects 363 

202. Section through the skin of an Elasmobranch showing the 

origin of a scale .... .... 370 

203. Diagrammatic transver8e section of the jaw of a Shark, Odoftr 

taspU americantu 371 

204. Lateral view of the skull of a Dogfish, Scyllium canicula . 372 

205. Dorso-lateral view of the pectoral girdle and fins of a Dogfish, 

Scyllium, canicula 374 

206. Dorsal view of the pelvic girdle and fins of a male Dogfish, 

Scyllium canicula 375 

207. Dissection of Scyllium canicula^ so as to show the viscera 

viewed from the ventral aspect 378 

208. Dissection of Scyllium canicula^ so as to show the viscera 

seen from the right side 380 

209. Adult Scyllium canicula, and opened egg-case of the same 382 

210. A Ray, Raia mactdata 384 

211. Skull of a male Chimaera monstrosa 385 

212. Chimaera mtmstrosa 386 

213. Lepidoiiren paradoxa 387 

214. Dorsal and ventral views of the cranium of Ceratodus miolepis 388 

215. Lateral view of the skeleton of Ceratodus miolepii . . 389 

216. Diagram of the arterial arches of Ceratodus . . . 390 

217. Diagram of the venous system of a Dipnoan . . 391 

218. Polypterus 393 

219. The Sturgeon, Acipenser sturio 396 

220. The right half of the pectoral girdle and the right pectoral 

fin of a Cod, Oadus morrhua 398 

221. Dorsal and ventral views of the cranium of a Salmon, Salmo 

solar, from which the membrane bones have been removed 400 

222. Lateral view of the cartilaginous cranium of a Salmon, Salmo 

solar 402 

223. Mandibular and hyoid arches of a Cod, Gadus morrhua . 403 

224. Lateral view of the skull of a Salmon, Salmo solar . 404 

225. Dissection of a Roach. Leuciscus rutilus, to show the viscera 406 



XVlll 



LIST OF ILLUSTRATIONS. 



FIG. 

22fS. A Cat-fish, Amiunu cat us 

227. The Plaice, Pleuronectes platessa 

228. The Sea-horse Hippocampus 

229. Skeletons of the anterior and posterior limbs of a Newt, 

Molge cristata 

230. Skeleton of Mdge cristata^ seen from the side . 

231. Male and female specimens of Molge cristata . 

232. Dorsal, ventral and lateral views of the skull of Molge 

cristata 

233. Visceral arches of Molge cristata 

234. Ventral and lateral views of the pectoral girdle and sternum 

of Molge cristata 

235. Skeletons of (a) right fore-arm and hand of the Salamander, 

Salamandra maculosa^ and ip) the right ankle of the Newt, 

Molge cristata 

23d. Pelvic girdle of Molge cristata 

237. Dissection of a male Molge cristata 

238. Diagram of the venous system of a Urodele 

239. Diagram of the arterial arches of Molge 

240. Dorsal view of the brain of Molge cristata 

241. Excretory and reproductive organs of a female Molge cristata 

242. Excretory and reproductive organs of a male Molge cristata 

243. Larva of Molge cristata 

244. Dorsal and ventral views of the cranium of the common Frog, 

Rana temporariay from which the membrane bones have 
been removed 

245. Dorsal and ventral vieivs of the skull of Rana temporaria 

246. Lateral and posterior views of the skull of Rana temporaria 

247. Visceral arches of (a) a Tadpole, (b) an adult Frog . 

248. Pectoral girdle and sternum of (a) a very old male specimen 

of Rana temporaria, (b) an adult female Docidophryne 
gigantea 

249. Diagram of arterial arches of a Frog 

250. Dorsal view and dissections of the heart of a Frog . 

251. Dorsal view of the brain and spinal cord of a Frog . 

252. The excretory and reproductive org^s of (a) a male, and 

ip) a female Frog 

253. Tadpole of Rana esculenta 

254. Section through the scale of a Lizard 

255. Ventral view of the pectoral girdle and sternum of a Lizard, 

Loemanctus longipes 

256. Lateral view and longitudinal section of the skull of a Lizard, 

Varanus varius 

257. Diagrams of the cranial roof in a Stegocephalan, various 

types of reptile and a bird 



PAOS 

407 
409 
410 

418 
420 
423 

426 
427 

428 



429 
429 
430 
432 
433 
434 
435 
436 
437 



442 
443 
444 
445 



446 
447 

448 
449 

450 
451 

458 

461 
462 
464 



LIST OF ILLUSTRATIONS. XIX 

FTO. PAGE 

258. Lateral view of the pectoral girdle of a Lizard, Varanus . 466 

259. View of the interior of the mouth of Far anus indicus 468 

260. Diagram of the arterial arches of Chamaeleo 469 

261. Diagram of the venoas system in Antira and ReptUia 470 

262. Excretory and reproductive organs of a male Lizard 471 

263. Lateral, dorsal, ventral and posterior views of the skull of 

Sphenodon puncUUus 473 

264. The hWndL-Ytorm^ AnguU fragilisy a limbless Lizard . 475 

265. Dorsal and ventral views of the skull of the common Shake, 

Tropidonotus natrix 477 

266. Diagram of the arterial arches of a Snake .... 478 

267. The Texas Rattlesnake, Crotaliu atrox .... 480 

268. (1) Dorsal and ventral views of the carapace of a loggerhead 

Turtle, Thalassoc/ielys caretta ; (2) the plastron of a 

green Turtle, Chelone mydas 482 

269. Ventral view of the skeleton of the green Turtle, Chelone 

mydat 484 

270. Longitudinal vertical section of the skull of the green Turtle, 

Chelone mydas 485 

271. Diagram of the arterial arches of a Turtle . 486 

272. Ventral view of the skull and dorsal view of the lower jaw 

of an Alligator, Caiman latirostris 488 

273. The first four cervical vertebrae of a Crocodile, Crocodilus 

vulgaris 489 

274. Sternum and associated membrane bones of a Crocodile, 

Crocodilus palustris 490 

275. (a) Left half of the pectoral girdle and (b) the pelvis and 

sacrum of an Alligator, Caiman latirostris . . 491 

276. Diagram of the arterial arches of a Crocodile . . 492 

277. Section through the skin of a Bird showing the developing 

feather 497 

278. Skeleton of the right wing of a Gannet, Sula alba . . 498 

279. Pectoral girdle and sternum of a Peacock, Paco cristatus 499 

280. Dorsal and ventral views of the wing of the Wild- Duck, 

Anas boschas 500 

281. Lateral view of the pelvic girdle and sacrum of the Duck, 

Anas boschas 502 

282. Skeleton of the common Fowl, Oallus bankina . . 503 

283. Lateral and dorsal views of the brain of the Pigeon, Columba 

Una 506 

284. Anterior, posterior and dorsal views of the third cervical 

vertebra of an Ostrich, Struthio camelus . . . 507 

285. Diagram of the arterial arches of a Bird .... 508 

286. Diagram of the venous system of a Bird .... 509 

287. Dissection of the Pigeon, Columba lima . . . . 510 



XX LIST OF ILLUSTRATIONS. 

no. PAOE 

S88. The lungs, kidneys and reproductive organs of the Pigeon, 

Columba lima 514 

SHO. Hection through the skin of a Mammal showing the develop- 
ing hair 521 

990. Ventral view of the skull of the Dog, Canit familiarU . 523 

]|i)l. Dorsal view of the skull of the Dog, Canis familiarU . 524 

902. Dentition of the Dog, Canis familiaris .... 526 

993. Dorsal and ventral views of the brain of the Rabbit, Lepui 

eunicului 529 

994. Hternum and sternal ribs of the Dog, Canis familiaris . 530 
99A. Hkeleton of the Rabbit, Lepus cuniculia .... 532 
990. Diagrams of arterial arches of Mammals .... 534 
997. Diagmm of the venous system of a Mammal . 535 
99H. The Duckbill, OmitfMrhynchuM anatinus .... 538 
999. Diagram to show the arrangement of the female genital ducts 

in Prototheria 538 

300. Ventral view of the pectoral girdle and sternum of a Duckbill, 

Ornithi>rhynchu» paradoxiu 539 

801. Diagram to show the arrangement of the female genital ducts 

in Metatheria 540 

302. The Rock Wallaby, Petrogale xanthoptu, with young in 

tlie pouch 541 

HOH. Hkull of Lesuenr^s Kangaroo-rat, Bettongia lesueuri . . 542 

•104. The banded Ant-eater, Myrmecobius fasciattu . . . 543 
300. Diagrams to show (he arrangement of the female genital 

ducts in the Rabbit and Man as types of Entheria . 545 

306. Tamandua Ant-eater, Tamandua tetradactyla . 546 

807. The six-banded Armadillo, Datypus sexcinctus . . 547 

80H. The white-bellied Pangolin, Manis tricuipis . 548 

309. Lateral view and longitudinal section of the skull of a young 

Ca'ing Whale, Globicephaltu melas 551 

310. BkuU of the African Manatee, Manatus senegalentis . . 553 

811. Front views of the head of the American Manatee, Manatus 

americanus 553 

812. An African Jumping-shrew, Macroscelides tetradactylta . 555 

313. The Russian Desman, MyogcUe moschata 556 

814. Vortical longitudinal section of the skull of a Dog, Canis 

familiaris 569 

315. The common Skunk, Mephitis mephitica .... 562 

316. The Patagonian Sea-lion, Otaria jvbata .... 563 

317. The skull of Hyrax (Procavia) dorsalis .... 565 

318. Left view of skull of a young Indian Elephant, Elephas indi- 

cia, witli the outer sides of the jawbones removed so as 

to expose the roots of the teeth 566 

319. Bones of. the right fore-foot in living Perissodactyles 567 



LIST OF ILLUSTRATIONS. XXI 

FIO. PAGE 

320. The Indian Rhinoceros, Rhinoceros unicornis . . . 569 

321. Stomach of a Sheep cut open so as to show the different 

compartments 572 

322. Skeleton of a Cape Buffalo, Bubalus caffa .... 573 

323. The African Ghevrotain, Dorcatherium aquaticum . . 575 

324. The Musk-ox, Ovibos moschatus 577 

325. Side view of the skull of the Rabhit, Lepus cuniculus . 578 

326. Dorsal view of the skull of the Rabbit, L^us cuniculus . 579 

327. The African Flying Squirrel, Anomalurus /ulgens . . 581 

328. The Musquash, Fiber zibethicus 582 

329. Skeleton of a fruit-eating Bat, Pteropus medium . . 583 

330. Female with young of a Bat, Xantharpyia coUaris . . 585 

331. Skulls of an old and of a young specimen of the Gorilla, 

Ghrilla savagei 586 

332. The Ring-tailed Lemur, Lemur catta 587 

333. The Orang-utan, Simia satyrus, sitting in its nest . . 589 

334. A transparent Turbellarian, Mesostoma splendidum^ show- 

ing the viscera 608 

335. Planaria polychroOy with everted proboscis . . . 612 

336. Diagram of the reproductive and nervous systems of a 

Trematode, Distoma hepaticum 614 

337. Diagram of the digestive and excretory systems of a Trema- 

tode, Distoma hepaticum 616 

338. A Tape- worm. Taenia solium 619 

339. Transverse section through a mature proglottis of Ta^enia, 620 

340. Diagram of a ripe proglottis of Taenia solium . . . 621 

341. A Nemertine Worm, Linens geniculatus .... 627 

342. A young transparent Nemortino Worm, Cerebratulus fuscus 629 

343. A Rotifer Fhsctdaria ; (a) female of Floscularia comuta, 

(P) male of Floscularia campanulala .... 632 

344. Diagram to show the arrangement of the viscera in Flos- 

cularia 633 

345. Diagram of a median longitudinal section of a free-swimming 

Rotifer 635 

346. Ventral view of Hydatina senta 636 

347. Dissection of female Ascaris lumhricoides so as to show the 

arrangement of the viscera 640 

348. Dissection of male Ascaris lumbricoides so as to show the 

arrangement of the viscera 642 

349. Trichina spiralis, encysted amongst muscular fibres . 643 



CHAPTER L 
Introduction. 

The word Zoology (Gr. {<3ov, an animal; Xoyo?, an account) 
denotes the science which concerns itself with animals. 

Life* 

endeavouring to find out what they are and how they 
came into being. It is a branch of the wider science of Biology 
(Gr. /?tbs, life, Aoyo?, a discourse) \ which deals with all living things, 
plants as well as animals. Before any progress can be made with 
the study of Zoology, it is necessary to get clear ideas on two points: 
firstly, as to what is meant by life and living things ; and secondly, 
as to how an animal is to be distinguished from a plant 

The idea implied in calling a thing living, is that in some 
respects its existence is similar to our own. Our own existence is 
the only thing immediately known to us, the standard with which 
we compare everything else. Every material object has certain 
points of resemblance to our bodies, inasmuch as all are composed 
of matter obe3ring the same laws of chemical affinity, gravitation, 
and so forth ; it is necessary therefore to define the amount of re- 
semblance which constitutes life. Now everyone knows that human 
beings grow, that is, increase in size at the expense of matter 
different from themselves called food, and that further, they give 
rise at intervals to firesh human beings. These two fundamental 
characteristics — the power of growth and of multiplication — define 
life ; everything that can increase its bulk by building up foreign 
matter into itself and that reproduces its like is said to be alive. 

The idea originally underlying the word animal was a self- 
moving object, as distinguished from a plant which was regarded 
as motionless*. This distinction, however, will not stand close 

^ This tenn is too well established to admit of alteration but it implies a 
mistranslation of plot. This does not mean Mile' in the physiological sense hot 
a period of life^ a career, a life-time or oiroamstanoes of life, environment. 

' It is trae that to all general statements of Zoology, as to this, exceptions 
eoold be fonnd. The rule followed in this book is to have regard only to the 

8. A IL 1 



2 INTRODUCTION. [CHAP. 

examination. Plants as well as animals move, and although the 
motions of animals are conspicuous and such as to catch the eye, 
whilst those of plants are usually slow and imperceptible, yet there 
is no essential difference between the nature of the movements in 
the two cases. 

The fundamental difference between animals and plants is to be 

Distinction fouud iu the uature of their food. Animals can only 

animal* and ^^® ^^ complex substaucos, uot Very diflFerent in 

pianu. chemical composition fix)m their own bodies, and 

further, they can live on solid food. Plants, on the other hand, 

build themselves up out of carbon di-oxide and other gases and water 

with a few simple salts in solution, and they only take in fluids or 

gases. There are, however, a certain number of living beings which 

combine the characters of animals and plants, and the question in 

which division they should be ranked is a matter to be determined 

only after a study of the special circumstances of each case. 

It has been pointed out that our own existence is the original 
type from which the idea of life is derived. But we know ourselves 
not only as bodies in which growth and reproduction occur, but also 
as conscious, thinking beings, and we are naturally inclined to 
imagine that animals at least, which not only grow and multiply^ 
but in many other respects also resemble us, are likewise conscious. 
How &r this belief is well-founded is open to serious question, if by 
consciousness we mean anything at all resembling our own inner life — 
the only consciousness we know anything about. The movements 
of the higher animals suggest that they experience the feelings of 
fear, anger, desire, etc., and it would be foolish to deny all similarity 
between them and man in these respects, but the habit many people 
have of uncritically attributing purely human feelings to dogs, 
cats, horses, etc., is apt to lead us into serious error. Our fore- 
fathers went further than even we are inclined to do and supposed 
all natural objects, the sun, wind, trees, etc., to have spirits, that is, 
to be conscious. Since we can never learn much about the conscious- 
ness of beings with whom we cannot speak, zoologists content them- 
selves with looking at animals entirely from the outside, without 
enquiring as to whether or no they are conscious ; animals are for 
them bodies in which certain changes take place, changes such as 
growth, reproduction, movement, and others. 

vast bulk of normal cases which gave rise to the idea. The reasons for classify, 
ing abnormal oases in one category or another are not general but special, and 
have to be considered in each case. 



L] PROTOPLASM. 8 

A dose study of animals reveals the fact that though the 
chemical constitution of no two is exactly alike, yet all contain certain 

highly complex substances of very obscure chemical 
composition, known as proteids. These substances 
occur in the form of a thick, viscous fluid, in which are suspended 
iiot only numerous solid granules of most varied composition, but 
also minute drops of other fluids. Such a mixture is called by 
chemihts an emulsion, and it is the emulsion just'describe'd which is 
the seat of all those processes which we call life. This emulsion is 
termed protoplasm (Gr. irpturo^, first ; vXaa-fia, a thing moulded). 

Further, it has been found that, so long as any sign of life is 
visible, this protoplasm is in a continual state of slow combustion, 
absorbing oxygen from outside and decomposing with the liberation 
of energy, and whilst some of the products of decomposition are 
cast off, others apparently reconstitute the original substance by 
combining with some of the materials of the food. The energy 
liberated is the cause of the movements which constitute the visible 
manifestation of life. 

An animal then is only the more or less constant form of a flow 
of particles ; it may be compared to a flame, which has a constant 
form, although the particles which compose it vary from moment to 
moment ; unbumed particles coming in at one end and the oxidised 
products escaping at the other. 

The deepest insight which can be obtained into the nature of 
^ . ,, life viewed as a series of changes in the shape and 
position of bodies reveals to us this continual 
chemical change as the ultimate cause of all manifestations of life. 
It is known by the convenient name of metabolism (Gr. fi€TaPo\rj^ 
chMige, changing). The ultimate object of Zoology is therefore 
to discover the nature, cause, and conditions of the metabolism in 
the case of every animal ; but the means of attaining this object 
are still to seek, and for the most part the zoologist has to be con- 
tent with describing and comparing with one another the outer and 
visible effects of the metabolism in various cases. 

The proteids, which form the essential basis of protoplasm, 
consist of carbon, nitrogen, hydrogen, oxygen, and sulphur ; besides 
these elements phosphorus, chlorine, potaasium, sodium, magnesium, 
calcium and iron are constantly found in the bodies of animals, and 
some of them are doubtless chemically combined with the proteid. 
Phosphorus is a constituent of nucleic acid, a substance which 
in combination with proteid is characteristic of the nucleus (see 

1—2 



i 



4 DJTRODUCTION. [CHAP. 

p. 16). Proteids have a percentage composition which yariea 
somewhat, though not widely, in different cases. 

Carbon Ax)m 50 to 55 per cent 

Hydrogen „ 6 5 to 7*3 „ „ 

Nitrogen „ 15 to 17 "6 „ „ 

Oxygen „ 19 to 24 „ „ 

; Sulphur „ -3 to 2-4 „ „ 

The size of the molecules of which proteids are composed is un- 
doubtedly a large one. It is difficult if not impossible to determine 
exactly how many atoms are contained in a molecule of a particular 
proteid because it is difficult to obtain one such substance in a pure 
condition free from admixture with others. The best determinations 
which have been made show however that at least 1000 atoms must 
be contained in the molecule. But the proteids known to the 
chemist are of course taken from the dead bodies of animals and 
are themselves to be regarded as products of the decomposition of 
the molecules which existed during life. The proteid as the seat 
of life has probably a decidedly different composition from the dead 
substance. To avoid confusion, we may call the living molecules 
biogens. 

The biogen molecule is continually absorbing oxygen from the 
outside. This process is called respiration or breathing. It 
decomposes and some of the products are no longer capable of being 
built up again into other biogen molecules and are therefore got rid 
of, since otherwise they would interfere with the chemical action, 
just as accumulating ashes will eventually put out a fire. The 
process of ejecting these waste products is called excretion, the 
waste substances themselves, excreta, and the chemical changes 
which lead to their production, katabolism (Gr. KaTafiokij, deposi- 
tion). The commonest excreta are water, carbon dioxide, urea, 
and uric acid; the last two substances containing nitrogen. But 
it is not necessary that in all cases excreta should be ejected. 
They may remain within the bounds of a mass of protoplasm; 
if they are removed from the sphere of chemical action of the 
protoplasm this is sufficient. In some animals uric acid is 
stored up in this way. Many of the excreta, though injurious 
if they remain in the protoplasm, are indirectly usefrd to the 
animal after ejection. Such useful excreta are called secre- 
tions. Thus, all the hard skeletons of animals are really insoluble 
excreta. On the other hand, the gastric juice which digests the 
food in the human stomach, and the slime or mucus, which 



LJ MOVEMENT. 5 

prevents a frog from clr3ring up when taken out of water, are fluid 
excreta. A part of the body specially adapted to produce a secre- 
tion is termed a gland. 

Other products of decomposition reconstitute, as we have seen, 
the original molecule by combining with the necessary elements 
from the food; this process is known asanabolism (Gr. dva^SaXXctv, 
to put back or up) or assimilation. Inasmuch as, generally speak- 
ing, from the breaking up of one molecule more than one residue is 
produced capable of regeneration, there is an increase in the number 
of biogen molecules causing an increase in bulk of the protoplasm, 
or growth*. 

The regeneration of the biogen takes place at the expense of the 
food. Taking in food is called eating, or ingestion. Since how- 
ever, the food must penetrate to every portion of the protoplasm 
it must be dissolved — a process effected by the chemical action 
of certain products of the decomposition of the biogens, known 
as ferments. The process is called digestion. The casting out of 
an insoluble remnant of the food is called defae cation, and inasmuch 
as such remnants have never formed part of the biogen molecule, this 
process is carefully to be distinguished from excretion. The accumu- 
lation of excreta soon stops metabolism, whereas the intermission 
of defalcation need only interfere very slightly with metabolism. 

Of the numerous solid particles found in protoplasm some are 
secretions, others are solid deposits of partly assimilated food, which 
act as reserve stores, others are indigestible remains. The fluid 
drops consist largely of water — some have in solution excreta or 
secretions ; others contain the results of digestion. 

Animals, as we have seen, possess the power of executing move- 
ments ; this power is exercised in order to seek their 
food and escape their enemies. However complicated 
these movements may be, they are all found to be dependent on the 
capacity of protoplasm to alter its shape, suddenly contracting and 
then slowly expanding. By contraction is meant such an altera- 
tion of shape of the moving part as will tend to diminish its surface 
but not its bulk; that is, the contracting part tends to assume 
a spherical shape; by expansion, on the other hand, is meant 
an alteration of shape leading to increase of surface. A bird 
flies by contracting the muscles first on one side of the wing, then 
on the other; a fish swims by alternate contractions of the two 
sides of the fleshy tail. Any part of an animal fitted to execute 

1 See Verworn, General Physiology (Engl. Edition), 1899, p. 486. 



6 INTRODUCTION. [CHAP. 

movements more quickly in one direction than in another and 
80 to bring about the movement of the whole animal, is called a 
locomotor organ. Protoplasm in which the power of contraction 
u highly developed is called muscle. 

A contraction is the result of an explosive decomposition of the 
living substance; there have been a great many theories as to 
how the chemical change brings about the change of shape but, 
since all of them account for some of the facts and none of them for 
all, there is no need to mention any of them here. 

The sudden chemical change which brings about contraction, 
although dependent on the unstable character of the biogen 
molecule, must be precipitated by some change occurring either 
in the living matter itself or in the surrounding medium, just 
as an explosion of gunpowder is not brought about without a spark. 
In either case the change causing the contraction is known as a 
stimulus, and the capacity of contracting under the influence of 
stimuli is known as irritability. Thus when a moth flies into a 
flame it is acting under the stimulus of light ; when a hungry lion 
in the Zoological Gardens rises up and commences running violently 
round its cage it is obeying the stimulus of hunger. In the first 
case we have to deal with an external stimulus, in the second with 
an internal one. Of course since all internal changes are ultimately 
due to changes in the surrounding medium, — e.g. hunger to a dis- 
appearance by digestion of the food in contact with the stomach, — 
the distinction between external and internal stimuli, though con- 
venient, cannot be sharply drawn. The power of protoplasm to 
originate movement through internal changes is called automa- 
tism. In the case of external stimuli we can often observe that 
the disturbance caused at the point of application of the stimulus 
is propagated to widely difi'erent parts of the animal. Nerves 
contain protoplasm in which this power of transmission is powerfully 
developed. 

We have seen that at some period in the life of all animals 

^ ^. when food is abundant, more living matter is formed 

than is broken down ; in a word, that the animal 
increases in size, grows. But whereas volume increases proportion- 
ately to the cube of the length (or breadth), surface increases only 
proportionately to the square of the same dimension. Hence the 
amount of volume per unit of surface continually ijicreases, and thus 
the chemical action between the internal portions of the protoplasm 
and the surrounding medium, which can only go on through the 



I.] EEPRODUCTION. 7 

surface, is slowed down ; in other words, the activity of growth is 
checked and when a certain size is reached waste becomes equal to 
repair. At this stage there is a tendency for the protoplasm to 
divide into two or more pieces of smaUer size. This division into 
smaller pieces is called Reproduction, and it is a necessary result 
of growth. When an animal divides into two equal portions, the 
process is called fission, but when one portion is very much smaller 
than the other, the process is known as gemmation ; the smaUer 
portion is called the germ, and the larger the parent, since the 
latter is — somewhat illogically — regarded as identical with the origi- 
nal animal before division. A germ very rarely resembles the parent; 
usually it has to undergo a series of changes during growth by 
which it at last attains the shape of the animal which gave rise to 
it ; this series of changes in shape and size is known as Develop- 
ment. 

Reproduction in the higher animals is closely associated with 
R rod ti another process called Conjugation or Sexual 

Union. This process consists in the coalescence with 
one another of two portions of living matter. Conjugation probably 
occurs in all animals, but the interesting thing about the higher 
animals is that they give rise to special germs of two kinds, called 
ova (eggs) and spermatozoa respectively, which cannot develope 
without first conjugating, one of the first kind uniting with one of 
the second. 

The ovum is devoid of the power of movement and has a larger 
or smaller amount of undigested or at any rate unassimilated food 
stored in it; this reserve material is called yolk. The spermato- 
zoon, on the other hand, has no such reserve and is in consequence 
very much smaller than the ovum, but it possesses in nearly every 
case the power of movement by which it is enabled to seek and find 
the ovum. Reproduction, which thus requires conjugation before 
development can take place is called Sexual Reproduction. In 
most cases ova and spermatozoa are developed in different individuals. 
The individual giving rise to ova is called the female, that giving 
rise to spermatozoa the male. In this case the animals are said 
to be bisexuaL When both ova and spermatozoa are developed in 
the same individual it is spoken of as hermaphrodite. 

It is obvious to the most casual observation that there is an 

amazing variety of animals in the world. Closer 

observation reveals the fact that while no two 

animals are exactly alike, all can be nevertheless sorted into a 



8 INTRODUCTION. [CHAP. 

number of kinds called species, the individuals composing which — 
apart from the difference between males and females — resemble 
each other exceedingly closely. Where the observation has been 
made, it is always found that the members of a species conjugate 
freely with one another; and indeed this is assumed to be the 
case in every species ; that is, we group a number of specimens into 
a species under the assumption that they can conjugate with one 
another, and that young like themselves will develop as the result. 
If this can be shown to be not the case, we conclude that a mistake 
has been made and that two or more species have been confounded 
with one another. It follows that the vast majority of species rest on 
provisional hypotheses ; these hypotheses nevertheless possess a very 
high degree of probability, for by the use of them only can the great 
resemblance between the individuals grouped together in the same 
species be accounted for. When, as occasionally happens, members 
of different species are fertile inter se, the offspring is termed a 
hybrid, and hybrids may or may not be fertile. 

It has been pointed out, that whereas germs are in most cases 

H r dit exceedingly different from their parents, they never- 

and theless in process of growth come to resemble them. 

This tendency to reproduce the characters of the 

parent is called heredity. If the germ undergoes a large part of 

its development within a hard case, like a chick within the eggshell 

or in a cavity of the parent's body, it is called an embryo ; if it 

moves freely about, it is termed a larva. 

In the case of the development of an animal which has originated 
sexually, that is from the coalescence of two germs, the tendency is 
for it to assume characters intermediate between those of the two 
parents. Thus it is easy to see how sexual reproduction tends to 
annul the differences existing between members of the same species, 
by constantly producing means between them. When therefore a 
large number of individuals are found with very close resemblances, 
it is a reasonable supposition that the agent, which has caused this, 
is sexual reproduction; in other words, that they constitute a 
species. It is not however to be assumed that in every case 
conjugation results in the production of an animal exactly inter- 
mediate in character between the parents. Sometimes the child 
resembles closely the father or the mother, a result denoted by 
the term prepotency of the father or of the mother. Some- 
times in an unexplained way an exaggeration of a character found 
in one or both parents is produced. Sometimes even an apparently 



I.] CLASSIFICATION. 9 

entirely new character arises. Such deviations of the offspring from 
the average of the parents constitute variation. If the difference 
is striking the individual exhibiting it is called a sport. 

It is obvious that so vast a science as Zoology must be divided 

into various branches, since the different questions 

ofzooiogy. i*' seeks to solve require that special attention should 

be given to each side of the subject Thus, the 
nature and conditions of the metabolism and the mechanism by 
which movements are effected, etc, constitute the subject-matter of 
Physiology ; the investigation of the structure of individuals and 
of the differences in structure between the various species and the 
search for the causes of these differences is termed Morphology ; 
whilst Bionomics is the name given to the study of the means 
whereby an animal obtains its food and orders its life, in other 
words, of its habits. But it must be remembered that all such 
divisions are purely arbitrary, and indeed no great progress can be 
made in any one department if the others be ignored. Bionomics, 
when followed to its sources, passes into Physiology, and in trying 
to explain the different structures studied in Morphology constant 
recourse must be had to both Physiology and Bionomics. 

Of aU divisions of the subject, that of Physiology has been most 
neglected; it has indeed only been studied systematically in the case 
of man and of a few of the higher animals. Hence this work will be 
mainly concerned with the questions of Morphology and Bionomics. 
Of these questions, by far the greatest is the problem how the 
distinctions between the various species are to be explained. The 
question of the " Origin of Species " involves nearly all others in 
Zoology. 

The distinctions between species are of very different degrees, 

so that for convenience species closely resembling 

each other are collected into genera — genera into 
families — families into orders — orders into classes — and classes 
into phyla. These are the names in commonest use, but often the 
nature of the subject requires the introduction of further grades of 
difference, and the number of grades actually employed depends to 
a large extent on the point to which the analysis is pushed. 

The only theory of the origin of species which has so far 

commanded any considerable agreement amongst 
^^pecies. naturalists is the fekmous theory of Charles Darwin. 

According to this theory, the resemblances between 
a number of living species are due to the fact that these species 



10 



DTTBODUCnON. [CHAP. 



dtfcended fitnn a common ancestral species which possessed the 
Mnmon features as characters of its own. Therefore, the degree of 
likeness between species is the expression of a nearer or remoter 
bkvd relationship, and it logically follows that, since no pwrt of 
rK« tnimal kingdom is without resemblances to the rest, if we 

•^e far enough in time we reach a period when all the animals 
in the world constituted one species. 

To a certain extent Darwin's theory was only the expression of 
idns that had first occurred to Greek philosophers, and had in one 
nirm or other been put forward by many naturalists before him. 
His si«cial merit lies in that he pointed out various processes 

nresi^nt going on in nature which must lead to the modification 

(C ^^ecics. 

He i^called attention to the well-known fact, that although the 

iFmrinff in general resemble the parents, yet this resemblance is 
never exact, and further that the young of one brood often differ 
-inite wivepribly from one another, and that these differences are 
rtften inherited by the offspring of the individuals showing them, 
^nch differences, as has been mentioned above, are denoted by the 
rerm Variation. 

Acaiu, another fact well-known but usually ignored, was em- 
Tihii^^ by Darwin : viz., that if the state of the animal population 
(%i the globe remains fairly constant, out of all the young produced 
b\- a pftir ^^ parents during their lifetime on an average only two 
^]1 gorvive, since if more were to live the species would inevitably 
inrrtwse in numbers. Hence since each animal tends to multiply 
nt % late at which if unchecked it would soon overrun the globe, a 
^vvmpetition must result between the members of each species both 
inr food and in the escape from enemies, as a result of which the 
'fittest'' will survive. So long as the surroundings of the species 
fftmain the same, this struggle for existence will only weed out those 
individuals least perfectly adapted to their environment, so that the 
{Mecies will be kept up to a high level of adaptation to its surround- 
ings. This elimination of imperfect individuals which results in the 
arrival of the fittest is known as Natural Selection. Thus 
^ can well imagine that if white-haired individuals turned up 
mnongst hares, they would be more conspicuous and hence more 
^$;aly discovered by the animals which prey on hares. If however 
ihe circumstances of a species change, a different class of individuals 
^U survive. For instance, if for the greater part of the year the 
country inhabited by tiie liares were covered by snow, aa is the case 



l] origin op species. 11 

in the North of Canada, the whitest-haired indiyidnals would have 
the best chance, and from generation to generation would be selected 
until the colour of the hare was totally changed. The progressive 
modification of species by the agency of natural selection is called 
Evolution. If the modification tends towards simplification of 
structure it is called Degeneration, if on the contrary it tends 
towards great complexity it is spoken of as Differentiation. 

So far the theory shows how a species will become slowly modified 
as its surroundings change. But it has been postulated that distinct 
species have arisen firom the same ancestors. It is of course not 
difficult to see that if a species is distributed over a wide area the 
conditions in difi'erent portions may vary independently of one 
another, and hence the species may become modified in one place 
in one direction and in another situation in a different direction by 
the agency of natural selection. So long however as the species 
inhabits a continuous area this tendency to split up into divergent 
groups will be checked by inter-breeding between the sections of 
the species which are thus becoming modified in different directions. 
But if through geographical changes the species becomes divided 
into groups of individuals cut off from access to another, then no 
interbreeding can take place and in time two species will be formed. 
Thus when birds have been blown far out to sea and have colonised 
a distant island they have often given rise to a new species. 
The same result may be brought about by the sea overflowing a 
part of the area inhabited by the species, an event which we 
know firom geology to have often occurred. The important fact to 
be borne in mind is that at bottom the evolution of several species 
out of one is due to the formation of colonies, and that the same 
causes which have led to the differences between the American and 
the Englishman have acted again and again in the world's history so 
as to produce the marvellous variety of species inhabiting the globe, 
the only difference between human and animal colonies being that, 
in the latter case, the divergence has become so great that animal 
colonists will no longer breed with the original race. Thus, accepting 
Darwin's theory, we find it possible to give a rational explanation of 
those resemblances between animals which are expressed in a system 
of classification \ If the theory be rejected these resemblances are 
pure figments of the human mind, and the species must be regarded 

* Most of the names employed in classification were in use before Darwin's 
▼lews were accepted. The word phylum (Gr. ^DXok, tribe or stock) is howeyer 
an exception. This term expresses the central idea of the eyolution theory, 
and its proper use is to denote the whole of a group of animals characterised 



12 INTBODUCnON. [CHAP. L 

as just as independent of one another as are the chemical atoms. 
Hence since it is a choice between this explanation or none, the 
Darwinian theory is accepted by the oyerwhelming majority of 
naturalists. 

One or two interesting consequences follow from the acceptance 
of this theory. The structural features of animals are to be regarded 
as adaptations to their surroundings, since they have been built up 
by natural selection. Hence an isolated resemblance in a particular 
feature between two species need not necessarily indicate that this 
feature was present in the common ancestral species, for similar 
surroundings may have evolved a similar modification in two 
animals only remotely related. Such similarities are called Homo- 
plasy, whereas resemblances believed to indicate blood-relationships 
are grouped under the term Homology. 

Again, the inmiature forms of some animals are found to exhibit 
strong resemblances to the adults of others, and the eggs of all the 
highest animals show the strongest general resemblance to the 
simplest animals — the so-called Protozoa (Gr. Trpwros, first, iwov, 
animal). If these resemblances are to be interpreted in the same 
way as those prevailing between adults — and it is illogical to refuse 
to do so — ^then we are driven to conclude that most animals in their 
development pass through stages when they exhibit many characters 
once possessed by their ancestors, commencing at the stage of the 
Protozoa. These latter animals, since they are about as simply 
constructed as we can imagine living matter to be, may be looked 
on as slightly modified survivors of the first animals which appeared 
on the globe. 

This method of interpreting the changes which occur during 
development is what is known as the Recapitulation Theory, 
because during Ontogeny (Gr. ov, ovro?, being) or the development 
of the individual, nature recapitulates to some extent the develop- 
ment of the species in past time, Phylogeny (<;(>vXov, a stock, a race). 
There are, however, a great many other factors which have modified 
development, and the determination of these and their separation 
from the hereditary factor is a task requiring careful study and one 
which is as yet far from complete. 

by haying the same groand-plan of structure and beUeyed to be the descendants 
of a common ancestor, from whom no other living animals are descended. 
The essential feature about a phylum is its isolation, in the present state 
of our knowledge, from other phyla. Of course it is believed that at bottom 
all living beings constituted one phylum, but there are enormous differenoes 
in structure which can only be bridged by imaginative hypotheses. 



13 



CHAPTER n. 

Phylum Protozoa. 

The Protozoa are distinguished from all other animals (1) by 
the fact that they do not produce ova and spermatozoa but that the 
whole animal engages in the processes of conjugation and repro- 
duction, and (2) by the fact that the protoplasm of the body is never 
di£ferentiated into tissues nor exhibits cellular structure (see p. 27) ^ 
The higher animals are often grouped under the name Metazoa 
(Gr. furd, after; C^ov, an animal) in order to contrast them with 
the Protozoa, but whereas the Protozoa, since they have a common 
structural ground-plan, constitute a phylum in the sense defined 
in the last chapter the same is by no means true of the Metazoa. 
Hence the name Metazoa does not denote a phylum but is a mere 
conyenient collective term. 

The term Invertebrata is also a mere collective name; it 
is employed to designate all animals which do not belong to the 
phylum Vertebrata. Like the name Metazoa its convenience in 
promoting terseness of expression is its only justification. The 
Protozoa are thus Invertebrata and the Vertebrata are Metazoa. 

The phylum Protozoa includes the simplest and lowest 
members of the animal kingdom. With few exceptions the members 
of this phylum are too small to be seen by the naked eye, and yet 
many of them are of great importance in the economy of nature. 

In order to fix our ideas we may select one of the simplest 
Protozoa as a type for examination. Amoeba, some- 
times called the Proteus animalcule, from its power 
of continually changing its shape, is found in the mud at the 

^ These statements are true of the vast majority of animals classed as 
Protozoa. The exceptions are for convenience classified as Protozoa, but it 
seems to the authors that in the light of a fuller knowledge they may turn out 
to be survivors of that great series of forms which must, if the evolution theory 
be true, have intervened between the Protozoa and the Metazoa. 



14 PROTOZOA. [chap. 

bottom of ditebes, ponda and pools of atagDant water. There are 
several species TaiyiQg somewhat in size included under the 
generic name Amoeba, all of them, however, are so small as to 
necessitate the use of a microscope for their examination. When 
magnified an Amo^ki appears like a small, almost transparent lamp 
of jelly, in which we can distinguish a thin outer rind and inner 



./^ / 




'b: 






Fia. 1. Amoeba proUutx S80. From Qniber. 



sabstancfli. The firrt, called the ectoplasm, is almost abeolatelr 
transparent, the second, called the endoplaam, has usually a 
grayish tinge, due to the presence of minute solid partacles or 
grsQules, and is therefore described as granular. Often indeed, 
good sized objects of various shapes and generally of a green or 
yellow colour, can be seen in the endoplasm ; these an the 
undigested remains of the microscopic plants which the animal has 
eaten and are snrronnded by babbles of water, termed vaoaoles. 
Amoeba frequently engnlb particles of sand, though fin: what 
purpose is unknown ; possibly to render itself less palatable to 
animals which might eat it If the Amofba is healthy we shall 
see it move. The txanqiarent ectoplasm slowly sends out a 



il] amoeba. 16 

projection, and then the granular eodoplaam flows into it As of 
conrse the size of the animal does not alter, when a process is 
thrust out in front, the rest of the animal must follow it by 
shrinking away behind ; indeed it would no doubt be more correct 
to say, that it is the shrinking or contraction of the animal's body 
behind, which forces out the projection in front, for the movement 
of an Amoeba, like the movement of every other kind of animal, is 
brought about by a series of contractions. 

These projections are called by the awkward name of pseudo- 
podia (Or. ^cvSi79, false , ttoSiov, a little foot) ; the adjective pseudo- 
implies that they are not fixed organs like our own limbs, but are 
made at any part of the surface of the body. When Amoeba comes 
across anything it desires to eat, it throws out pseudopodia on each 
side of it ; these then unite beyond the object, and so the latter 
becomes engulfed, so to speak, in the body of the animal, where 
it is digested. It may thus be said, that Amoeba flows round its 
prey. Once the prey is inside, it is surrounded by a drop of water 
poured out of the surrounding protoplasm or enclosed with the food. 
There is probably some substance secreted into this water which 
acts on the prey and dissolves it. 

One of the most marked features in which Amoeba differs from 
other animals, from ourselves for instance, is, that it possesses no 
separate parts or organs, such as stomach, hearty lungs, etc., fitted 
to perform the separate vital actions, or functions as they are 
called. It breathes, that is, absorbs oxygen and gives off carbon 
dioxide all over the body ; and it likewise excretes, that is, gets rid 
of the oxidized protoplasm, at all points of the surface. I^ however, 
we are so fortunate as to come across a large Amoeba, which is 
at the same time comparatively clear of granules, and moving only 
sluggishly, we may be able to make out two definite objects in the 
endoplasm. The first of these is called the contractile vacuole 
(2, Fig. 1) ; this is a clear round space, which slowly enlarges and 
then suddenly vanishes, and then reappears in the same place and 
goes through the same series of changes. It is believed that the 
cause of this appearance is that at a certain point in the endoplasm 
a substance is produced by katabolism with a strong affinity for 
water; this substance attracts to itself from the surrounding 
protoplasm water, carrying in it the soluble waste products, in fact 
draining the protoplasm and forming a drop. This drop swells 
until it, so to speak, bursts the covering of protoplasm separating it 
from the outside water; the space it occupies then collapses^ but 



16 PROTOZOA. [chap. 

as soon as the fluid hais escaped the rent in the protoplasm joins 
up again, and as the excretory process continues the drop of fluid 
again accumulates. 

The other object which we may perceive is the nucleus. This 
Nucleus ^ ^ spherical body consisting apparently of the same 
kind of material as the endoplasm, only slightly denser 
(1, Fig. 1). If we, however, kill the animal by running in some iodine 
under the coverslip, the nucleus stands out at once in contrast to the 
rest of the protoplasm by its property of taking up more iodine and 
appearing stained a much deeper colour, and this happens in the 
case of any Amoeba, whether we have been able to see the nucleus 
whilst it was living or not. The material contained in the nucleus 
is an essential part of the body: when deprived of it metabolism 
within the protoplasm slackens and finally stops. Nearly all living 
things, animals or plants possess one or more nuclei, though in 
some rare cases the essential nuclear material is dispersed through- 
out the protoplasm. The bigger the plant or animal, the more 
nuclei it possesses. The so-called " Flowers of Tan " (Mycetozoa), 
which creep over the hides in tan-pits, are some of the few Protozoa 
which are distinctly visible to the naked eye ; they may be com- 
pared to gigantic Amoebae with branching pseudopodia, and 
they have thousands of nuclei. In the case of certain Protozoa 
it has been proved that if the animal be broken in pieces, 
those bits which contain a nucleus can repair themselves and 
continue to live, eventually growing to form an animal like the one 
of which they are fragments; but those bits which contain no 
nucleus, though they continue to live for a short time, have no 
power of feeding themselves nor of growth. On the other hand if 
the nucleus be fireed firom protoplasm it dies ; life depends on the 
mutual reactions of protoplasm and nucleus. 

The reproduction of Amoeba ordinarily takes place by the simplest 
Re reduction couceivablo proccss ; the animal divides itself into 
and Encyst- two. This proccss is Called fission : and it is found 
"**" * that the nucleus always divides into two before the 

body as a whole shows any signs of the process. When Amoebae 
are exposed to unfavourable conditions, such as the drying up of 
their surroundings, they have the power of enclosing themselves in 
a cyst. They draw in all their pseudopodia and assume a spherical 
form, and the cyst appears as a membrane on the outside which 
then thickens. Once enclosed within its cyst. Amoeba can be blown 
about like a particle of dust, and in this way we can account for 



n.] LOBOSA. 17 

the hot that we sometimes find Amoeba in infusions, that is, solu- 
tions made by allowing some animal or vegetable matter to stand in 
water exposed to the air. If we put some hay or meat into perfectly 
pure water and expose it to the air, it will putrefy ; this is due to 
the development of minute microscopic plants called Bacteria, the 
spores of which are carried by the air : at a later stage, various 
Protozoa and sometimes Amoebae will appear. At one time it was 
supposed that both Bacteria and Protozoa were spontaneously 
developed out of the dead meat, but it has been shown that if the 
water and meat be boiled, so as to kill any spores which may be in 
them, and the mouth of the vessel plugged with cotton-wool whilst 
steam is issuing, so that the air penetrating from outside through 
the interstices of the wool has all the spores it may carry strained 
off before it comes in contact with the water, neither Bacteria nor 
Protozoa will appear. The cyst which invests the body of the 
Amoeba is the first instance we have met with of what is called a 
secretion. A secretion has already been defined as dead substance 
which is of use to the animal, and which is produced by the 
decomposition of protoplasm. 

In one or two cases an Amoeba enclosed within its cyst has been 
seen to break up into a number of rounded germs which were 
eventually set free by the breaking of the cyst and each of which 
then took on the form of a minute Amoeba. This process is called 
sporulation and the germs to which it gives rise spores. It is 
unknown whether every species of Amoeba sporulates and if so 
under what conditions this occurs. 

When we were describing the endoplasm of Amoeba above, we 
called it granular, owing to its containing solid particles. When 
the highest powers of the best microscopes are used, it appears that 
both endoplasm and ectoplasm have a structure comparable to that 
exhibited by a mass of soap-bubbles. The walls of the bubbles 
consist of the actual living substance which is probably composed 
of the biogen molecules ; the cavities are filled with water which 
has in solution the products of digestion, from which the living 
framework repairs itself, and likewise the excretory products. This 
water also conveys in solution the oxygen necessary for life and 
removes the carbon dioxide. It is only by means of a structure like 
this that the complicated chemical changes which constitute life 
can be perfectly carried out by every particle of the living substance. 
The granules are temporary deposits in a solid form, either of 

S. (ft M. 2 



18 PBOTOZOA. [chap. 

matter reBulting from katabolism, or of nntritdoiu mttter not yet 
aBsimilated. 

We must DOW glance at some animals allied to Amoeba, in orda 
to gain some idea of the group Protozoa as a whole. 

Diffiugia and Arcella are both found in the mud of pools and 

ponda; they resemble Amoeba in general structure 

but differ from it in being provided with shells. In 

consequence of having these they are only ^le to put out 

pseudopodia at one spot, the mouth of the shelL The shell of 

Diffluffia is composed simply of grains of sand stuck together with a 

secretion ; it haa the shape of a pointed egg with the thick end cut 

off (1, Fig. 2). Areeih, on the other 

1 hand, makes its shell entirely out of 

its own secretion ; this is colourless 

when thin, but as the animal grows 

older the shell becomes thicker and 

acquires a characteristic brown colonr, 

and we are enabled to recognize that 

it consists of chitin. This is really a 

name for a class of substances which 

are constantly met with in the animal 

kingdom and which are probably allied 

in composition to uric acid. Out of 

chitin, for instance, all insects construct 

their hard cases. It seems probable, 

that the self-destruction of protoplasm, 

which results from the ordinary vital 

1. Shell ootopoBed of partiolu functions, may in many cases give rise 

Mim»L TSioJdL"' "" *° ^^^'^ ^^ '***' V<»^Y» in Arcella, 
the shell is at once a protection and 
the ordinary excretion. The shape of this shell is like a watch-glass 
with a flat lid testing on it, and in the middle of the lid there is a 
round hole through which the pseudopodia come out Sometimes 
gas bubbles (7, Fig. 3) can be seen in the body of the animal, which 
t«nd no doubt to baUnce the weight of the shelL Owing to the blunt 
character of the pseudopodia, Atao^, Diffiugia, Areola and similar 
forms are united into a class tenned Lobosa (Gt. Xofiot, a lobe). 

The Frotoztion Gromia possesses a thin membranous shell, 

FonmiDihn. ^^P*^ somewhat like that of Diglagia: bat the 

animal shows two important differences; first, liie 




n.] FOBAlflNIFEBA. 19 

protoplasm 4^ which the body U composed, besides fillmg the shell, 
extends in a thin layer all over its outer suriace (2, Fig. 4), and 
secondly, the pseudopodia, vhicli are given off from this layer, are 
thin and delicate threads which join and interlace with each other 
■0 as to form a network. Gromia seizes its prey by entangling it in 
these fine psendopodia ; these then flow together and form a little 
island of protoplasm surrounding the captive, which is thus digested 
quite outside the main part of the body ; the products of digestion 




Pia. B, AntUa Utoide* x 500. From Laid;. 
. Bean fMin abova, B. Saen from tha ddo, optioal wotioii. 1. Bhell. 
3. Puadopodia. 8. Edge of opening Into uieU. 4. Thread atUahmg 
•nimkl to inner fnrboe of ^all. 0. Nnoleai. 0. Food VMnole. 

7. Om TaeaoU. 



being carried along the psendopodia into the protoplasm which is 
inside the shell 

We may next consider a rather laiger ProtozSon, allied to 
Gromia and like it possessing a shell, which hower^ is composed 
not of chitis bnt of calcium carbonate. The name of this animal 
is PolyitotiuUa (Fig. 5). Like Gromia it possesses delicate inter- 
woreu psendopodia idiich spring from the whole surface, since 
there is a thin layer of protoplasm covering the ontside of the 
■hell ae well ■■ the main mass inside it. Unlike Gromia, however, 

S— 8 




. 4. Qromia (rr(fonni« x 250, bat tbe paeadopodia &» leu than one-thlid 

their raUtiTe nstnral length. From U. 8. Sohnltze. 
Shell. 2. Protoplasm •urronndine shell. 3. Paeudopodia, fnalng 
together in placea and Barroanding food partioles anoh ae diatoms. 



CHAP, il] fobaminifera. 21 

PolystomeUa has a shell which is perforated by a large nmnber of 
minute holes, through which pass cords of protoplasm, connecting 
the inner and outer parts of the animal Polystomella is therefore 
a typical example of the Foraminifera (Lat. foramen^ a hole ; 
fero, to carry), a class which includes countless varieties of microscopic 
shells, generally composed of calcium carbonate, less frequently 
of flint (silica). Gromia is included in the Foraminifera, though it 
does not possess the peculiarity indicated by the name, because its 
structure as a whole shows that it is really the same kind of animal 
There is another most instructive feature of the structure of 
Polystomella wherein it differs from Gromia. K we examine the 
shell with the low power of a microscope, we shall see that it is 
shaped like a rather flat snail shell or the shell of the Pearly 
Nautilus. If, however, we dissolve away the shell with dilute acid 
so as to expose the proper body of the animal, it will be seen that 
this is made up of separate parts, united to each other by two or 
three little bridges of protoplasm, and arranged one behind the 
other in a spiral series. This is the first example we have met with 
of the repetition of similar parts in a definite order, but upon this 
principle of the repetition of similar parts the bodies of the most 
complicated animals are built up. It is no doubt fundamentally 
the same thing as reproduction, only the various units which are 
produced, instead of separating frooi each other and leading separate 
existences, remain connected, and, as we say, are co-ordinated 
to form an individual of a more complex kind. In Polystomella 
the various parts are called chambers; a name which properly 
belongs, and was first applied to, the segments of the shell enclosing 
them. It is worthy of note that it is only the protoplasmic body of 
Polystomella which shows this composition out of definite units 
arranged in a definite order; there may be one large nucleus or 
a considerable number of smaller nuclei, but they are not arranged 
in correspondence with the chambers. 

The group Foraminifera, of which Gromia and Polystomella are 
examples, is of an enormous extent, and includes an immense variety 
of forms, the variety being brought about by differences in the 
number of chambers and the way they are arranged in series. The 
Foraminifera almost all live in the sea; some, like the two we 
have described, creep about amongst the sand and debris at the 
bottom of pools or other places where the water is quiet; many 
others float at the surfeu^e of the ocean, the protoplasm which clothes 
the outside of the shell having numerous vacuoles filled with fluid 



22 FBOTOZOA. [chap. 

probably less dense than the aesrwater, and tbos serving sa floats, 
la auch iQconceivable myriads do these floating Foiamiiiifera exists 




Fia. S. Polyitomella eriipa. Highly magnified, 
1. Shell. 2. Paeiidopodia. 



that their empty shells form thick banks of impalpable white chalky 
mnd at the bottom of the ocean, and tbe familiar white chalk of 



il] radiolabia. 23 

our English cliffs and hills is largely made up of the shells of 
Foraminifera. 

The Foraminifera show the same two methods of reproduction, 
viz. fission and sporulation which were mentioned in the case of the 
Lobosa. Indeed the progress of research has rendered it probable 
that they are found in all classes of Protozoa. The presence of a 
hard skeleton has however produced modifications in the process of 
fission. When such a form as Polystomella for instance is about to 
reproduce by fission the protoplasm emerges firom the old shell and 
divides repeatedly into a number of pieces each as large as the 
central chamber of an ordinary individual These then secrete 
round themselves shells and begin to bud off new chambers and 
gradually acquire the size and shape of adults. When on the 
contrary Polystomdla sporulates the protoplasm whilst still within 
the shell divides into a large number of small rounded pieces — these 
acquire hair-like projections called flagella which can be moved 
to and fro and by means of which they swim and then escape from 
the shell and move freely about. They coalesce with one another 
in pairs, and the resultant mass or zygote acquires a shell and 
begins to bud off new chambers. Since in spite of the fact that 
the zygote has resulted from the union of two spores it is much 
smaller than the product of fission — the adult rasulting from the 
growth of a zygote is distinguished from that resulting from fission 
by the small size of the initial chamber. Hence we can distinguish 
a microspheric form resulting from sporulation from a megalo- 
spheric form resulting firom fission. Lister has shown that in 
Polystomelia there is an alternation of fission and sporulation, 
probably more than one generation of fission intervening between 
two periods of sporulation. 

We may pass now to the consideration of some Protozoa, 
which show a good deal of resemblance in many 
points to the Foraminifera, though they have very 
marked peculiarities of their own. These are the Radiolaria; 
they have delicate threadlike interlacing pseudopodia, and their 
protoplasm is divided into two parts — an inner and an outer — 
by a membranous case having pores, through which the two parts 
communicate with each other. This case, the central capsule, 
may be compared to the Foraminiferan shell, but the interesting 
fiict is that these Badiolaria have in addition to this another skeleton 
composed not of chalky — calcareous — but of flinty — siliceous — 
substance, as are also some of the shells of the Foraminifera. This 



24 FBOTOZOA. [chap. 

iinty skeleton may conBist simply of isolated needles sticking out 
on all sides from the centre ; oftener, however, it consists of a 
beautiful basketwork as in HeliosplKura inermU (Fig. 6), and aome- 




Fi9. e. Bebotphatra inermis kZSO, From Bfltschli. 
1. Skeleton. 3. Oenttal eapsule. S. Nnclena. 

times we find several of these baskets one within the other, like the 
Chinese ivory ball. The Radiolaria, like the free-swimming Fora- 
minifera, have a bubbly outer protoplasm, and often drops of oil in 
the inner protoplasm; these structures serve to sustain them and 
they are found floating at the sur&ce of the sea amongst the Fora- 
minifera. At the bottom, in medium depths, their flinty skeletons, 
though mixed with the calcareous shells of the Foraminifera, do not 
affect the general character of the chalky mud (called the Ghbigerina 
OOte, from the name of one of the commonest Foraminifera found 
in it), but at greater depths, owing to the enormous pressure, the 
quantity of carbonic acid dissolved in the water increases very 
much — on the same principle that the preaaure inside a soda-water 
bottle keeps the gaa dissolved — and all the shells composed of 
ohalky matter are dissolved, only the flinty skeletons being left. 
The bottom mud here entirely changes its character and is called 
Radiolarian ooze. 

The next group of Protozoa to be considered is a very remark- 
able one, including the largest forms known. The 

yee io». go.pj]]g^ "FloweTB of Tan" (Mycetozoa) are brightly 
coloured patches, which may be seen on the surface of the oak-hark 
used in tan-pits. Similar patches may be seen on old tree stumps 



n.] MYCET020A. 25 

utd on thfl sattaM of beanstalks which havQ been wet for a con- 
dderable time. These patches under the microscope are seen to 
tesemble enonnous Amoebae with thin branching pseadopodia, 




Tta. 7. Yariooi stagM of Chondriademia difformt. From StTasbiiTger. 

A. FlsgellulA laftviiut ajai. B & C. Flogellulu. D young and E older 
•moebnlaa. B. Amoeholite fasing to (orm plumodiam. AU x G40. 
Q, FlMmodlomxtKL 1. NucUdb. 

which are apt to join one another to form networks, although these 
networks are muc^ coarser than in the case of the Poraminifeia. 
The fluid endoplasm is seen to have a regular flow alternately 
backwards and forwards in these pseudopodia ; the movement of 
the whole mass in any direction beiog due to the predominance of 
the forward flow over the backward, or vice versa. When stained 
the protoplasm is seen to include thousands of veiy small nuclei. 
The name Mycetczoa literally means Fungus animals (from Gr. p-vicip, 
a fungus, Z<^, animals). They are also often called Myxomycetee — 
literally Slime-Fuugi, both names having been suggested because 
their special mode of reproduction leads some naturalists to con- 
sider them to be plants. Their power of encystment is very 
marked, the slightest tendency to drought calls it into action, and 
then a mass will break up into numerous cysts, which will remain 



26 PROTOZOA. [chap, 

perfectly passive until wetted. Before reprodnction, the same 
process occurs, but the contents of the cyst divide repeatedly so 
as to form a mass of small germs — spores — which acquire walls 
of cellulose, a constant product of plant Hfe. Such spores arc 
called chlamydospores. Some of the protoplasm not used in the 
formation of spores forms long threads of cellulose, called collectively 
a capillitium, which when wetted expands and so expels the 
spores. The appearance of cellulose was the only justification for 
regarding these animals as in any way allied to plants, and it is 
known that cellulose is quite a constant product in some groups of 
animals. The contents of the spore escape as a germ propelling 
itself by a vibratile thread called a flagellum (B and C, Fig. 7), 
the germ itself being termed a flagellula. This thread is soon 
withdrawn and the germ takes on the form of a small Amoeba and 
is then called an amoebula (D, E, and F, Fig. 7). Many of these 
amoebulae coalesce to form the adult form, which is called the 
Plasmodium (0, Fig. 7), a name given to the result of the fusion 
of a number of originally separate animals. 

The Sun animalcules, or Heliozoa (6r. 17^.109, the sun), which 
inhabit, with few exceptions, fresh water, were formerly 
confounded with the Radiolaria, but they are in 
reality very different from these. They are spherical in shape and 
have a large number of stiff pointed pseudopodia sticking straight 
out all round them, like the conventional rays in pictures of the 
sun. The common and scientific names are taken from this circum- 
stance. Since these animals float about, it is not surprising to 
find much the same structure in the outer protoplasm as we found 
in Badiolaria and the floating Foraminifera. The pseudopodia are 
different in character from those of the Badiolaria, since they do 
not interlace, nor do they run together when they seize prey ; the 
captured food is simply pressed in towards the body by the 
bending of the pseudopodia, and when it is brought quite close, a 
broad irregular pseudopodium, like one of those of Amoeba, shoots 
out and engulfs it. Pseudopodia were defined in the case of 
Amoeba as irregular projections shot out at intervals from the body 
and soon withdrawn, and the question arises how far we have any 
right to call by the same name these stiff projections of the Heliozoa. 
They are, however, true pseudopodia, for if the animal be sub- 
jected to strong irritation they are all withdrawn. These unimalR 
show a most interesting example of the repetition of parts. The 
species Actinophrys sol and Actinospkaerium eichhomii are both 



n.] HELIOZOA. 27 

comparatively cominoQ inhabitants of our ditehes. The fitst is, 
howerer, exceedingly miante, not more than -^innr^b of sn inch 
in diameter and poasesses only a single nucleus, whereas the second 
is large enoogh to be jost visible to the naked eye, and h&s about 
SOO nuclei There ia here repetition of the nuclei, but no division 
of the protopUem, vheieas in Polyttomdla there ia segmentation of 




Tio. 8. Aetinophryt lol x about 800. Ttom Bionn. 
1. EetoplMm. 9. EndopUnn. B. Contnctils Tacaole. 4. Food vMniole. 
t. Naoleos. 6. Axit of a paeudopodiQm, Btiffei tban the protoplum 

«hieh oofen it. 

the protoplasm, but no corresponding multiplication of the nucleL 
If both were to occur simultaneously and to correspond so that the 
body were to consist of a number of segments of protoplasm, each 
with its nndens, the animal would be said to be multicellular, 
each unit being spoken of as a cell ; but it would no longer be a 
Frotoioon. 

So far we have been considering animals which, however much 
they may differ in details, are all essentially naked 
masses of protoplasm, and in them no very definite 
organs are set apart for Uie performance of special functions. We 
most now examine some Protozoa of a distinctly higher grade of 
Btmcture, wherein definite organs exist ; by the word organ being 
meant a part of the body definitely fitted to perform some special 
function for the general benefit of the whole. If we examine the 







<^ V <i!<«£ tv wnMiwis ^KoiHiw x a BhMt Waiifiil animal 
1^ ■■•r.r-jA. '^wu/mIu mh wODfla^ tbr ifa^« ijf « blne-bfiU 
«tr -: • .DiNfr ji » -untf ttiik-aw ta^ lai a Ma-^ioiied body; 
w>M»- a ut --ukI;. .fc « iw 7i *nat ifiiiew «fpc«t. such as, 
. 7in.-«^w<). »m. Tut ^met «f d» body oort»- 
i)T iC Vii Mil V brjai ami taned iwhrarda ; 
- Ill, ^intA. » uSai thr f>mst*M«^ dwn ia a 



n.] 



CILIATA. 



29 



Fio. 10. VoHieella 

microstoma x about 

200. 

From Stein. 



flattened projection called the disc. Between the peristome and 
disc there is a groove, and in this we can make out some short 
hair-like structures waving to and fro; there is 
a circle, or rather one twist of a spiral of these, 
as we can see when the animal turns the surface 
of the disc upwards. By the regular rhythmical 
bending of these cilia (Lat. cilium, an eyelash) 
as they are called, and possibly by the move- 
ment of a rolled membrane which projects into 
the mouth, a vortex is produced in the water, 
which draws particles of food to the Vorticella, 
The cilia and the stalk are definite permanent 
organs, the first of the kind we have met with. 
But the possession of these organs is not by any 
means the o)|ly difference between Vorticella 
and the lower Protozoa. The shape of the 
body, though it varies slightly with the state 
of expansion or contraction, is practically con- 
stant ; no pseudopodia are given out. This is the 
result partly of the possession of a firm membrane covering the 
whole of the body, called the cuticle, which is a protective secretion 
like the shell of Arcella only much thinner and so intimately con- 
nected with the protoplasm under it as to be inseparable; but the 
constancy of shape is also due to the fact that the outermost layer 
of the protoplasm itself is finely striated, constituting a specially 
contractile sheet surrounding the body and distinctly marked off 
from the inner protoplasm. This sheet is called the cortical 
layer and the striation is caused by the differentiation of the 
protoplasm into parallel strings called fibrils, embedded in different 
material acting as a cement This arrangement of protoplasm 
makes its appearance whenever the contractile power is specially 
developed. The stalk, which is entirely composed of this layer, 
might almost be regarded as a muscular fibre. The stalk is slightly 
twisted and attached in a long spiral to the inner side of an elastic 
tube of cuticle. When contraction occurs the stalk is necessarily 
thrown into the most evident spiral curves, like a corkscrew ; the 
restoration of the form after contraction is due to the elasticity of 
the tube of cuticle. 

Vorticella possesses a contractile vacuole and a nucleus just as 
Amoeba does ; in small nearly transparent specimens both are easily 
detected during life; in fact, if the specimen under observation only 



30 PBOTOZOA. [chap. 

keeps modemtely Btill, we can follow the ezpwisioQ and oontnctioB 
of the Tocnole with tiie greatest eaasi The sadeiia is -nry large 
and hss more or less the shape of a horee-shoe, thon^ the two ends 
are generally at different levels, so that in reality it forms part of a 
spiraL If we mn in some iodine it at once absorbs the stain and 
stands ont Teiy distinctly ; the VorUceUa, however, frequently shows 




1. Diio, 3. Mouth. S. Perutomial groove. i. VibrMile msmbiane in 
moDth. 6. Gortical layer. 6. Eudoplaim. 7. Food-Taouoles. Tbe 
lut ol the tood-TBouolea is neahng the podtioD ol the taai. B. Pharynx 
■liowing tonoation of food-Tacuolsa. 3. Controotile vacaole. 10. Per- 
nuneDt reeeptade into which the ooDtraotile Taonole openg. 11. Hioio- 
unoUoi. 13. Nuolena. 13. Oontrootile flbrili mnaing into mnsole 
in italk. 11. St«lk oontrMted (the axial fibre ahonld touch tbe oatiele 

in pl40««). 

its dislike to the operation by contractiag its body into the sliape of 
a ball and snapping itself off from the stalk : it is then apt to gel 
washed away from its position by the inflowing iodine and we may 
have to search over the slide to find it When a Vorticella Is 
irritated, the peristomial lip is turned in so as to lie against tiie disc, 



II.] CILIATA. 81 

and thus the groove in which the cilia lie becomes converted into a 
tube and so they are efficiently protected. 

We have seen above that VorticeUa uses its cilia in order to 
produce a miniature whirlpool in the water by means of which 
particles of edible matter — ^whether living or not — are drawn 
towards it. Since, however, it possesses a firm cuticle and in 
addition a specialized outer layer of protoplasm, the question arises 
how the food is taken into the interior of the body. If we run 
some Indian ink under the cover-glass we shall have a demonstra- 
tion of how this is managed. The black particles are caught in 
the whirlpool made by the cilia, they course round and round and 
finally accumulate in a pit which opens into the ciliated groove 
and from the bottom of tiiis they pass one by one into the internal 
protoplasm of the body. This pit which obviously passes through 
both the cuticle and the outer protoplasm — the cortical layer as it 
is called — is termed the pharynx and its opening the mouth. 
The particles of Indian ink which have passed into the body are 
surrounded by little drops of fluid which are partly swallowed with 
it and partly secreted by the protoplasm, in order to effect the 
solution of the particle, that is, its digestion. Such a drop is called 
a food-vacuole to distinguish it firom the contractile vacuole, 
which, as we have seen, has probably an excretory office to perform. 
As there is nothing nutritious in Indian ink, the VorticeUa soon 
gets tired of trying to digest it, and the particles after having 
travelled through the body in a more or less definite tract are 
thrust out into the ciliated groove. Since this takes place at only 
one spot, there must be a permanent hole in the cuticle here though 
we cannot discern it, and this opening may be called the anus. 
Therefore in contradistinction to Amoeba, where food can be taken 
in and undigested remnants cast out at any spot on the surface, 
in Vortkdla it is only at one particular spot that either action 
can take place. 

The reproduction of VorticeUa is a most interesting process. It 
takes place by longitudinal splitting, or, as it is technically called, 
fission. The disc splits into two, and the cleft soon reaches right 
down to the beginning of the stalk, so that for a time we have two 
bodies attached to the same stalk. One of these acquires a new row 
of cilia round its base ; soon after the original circle of cilia and 
the peristomial groove disappear; the animal then breaks loose 
firom the stalk and swimming by means of its new circle of cilia 
seeks a new place of rest. The other body remains on the original 



w WWTOZOA. [chap. 

.ik.^ .»«iw ' «j»4iMM.o^ it«. viffluwuf life. This simple mode of reprodnc- 
^ ^ .«^ .^o .'ik -Oi h l(/ii^ ^ine unchecked, but experiments made on 
^uiv*. >v;iv«.wi* 'u\»i^ v»^ I«e« allied to Vorticella show that it has a 
.u^;. v^^vii v««) wDf^ speaking of reproduction in the introductory 
a«^'«v». H«» -U4ttlth>^«^i that in sexual reproduction two germs had 
V tt*v vj^iUiur th^&«« they could give rise to a new indiyiduaL 
V^^^«^^ -^^^ lihii* has to happen at intervals in the case of 
' i^ .M.x<,a» lU OMCC that the reproduction by division may go on in 
« H.«uiAV liKUUMT. When this process (conjugation) is about to 
.^c >fiwo» ^\k^ individual divides repeatedly by longitudinal division 
HU4K'<iv .uiv ^' the new individuals produced breaking away from 
i^w xulk, ^ that we have a bunch of minute VorticeUae attached to 
'.iw s4^uo sUiilk. Tliis rapid division is clearly comparable to the 
(Uvsvaa icrui^ ^^sporulation" which has been already described in 
uIk* omm> s.^* the Lobosa and the Mycetozoa. The " spores," as we 
iiu^ UH'iii the small Fc^r^zW/o^^ become free and swimming away 
c^iUK'h (hem^lves to the sides of the large stalked individuals. There 
Uwii ouAU^ 9k\\ interchange of substance between the two individuals 
^hioh thuai ailhere to one another; the large nucleus of each breaks 
up tuid di^p|>ears and a small subsidiary nucleus, the micronucleus 
vU» b^. U), exceedingly difficult to detect at ordinary times, now 
vHomM iuto view. It also breaks up and many of the portions 
diMAppear, but a part of the micronucleus of the large one passes into 
th^ little one and vice versa. In the case of the allies of Vorticella, 
wh^ the two individuals which thus conjugate are of the same 
SUM), they separate afber the operation, and each goes on dividing on 
it« own account. In Vorticella, however, the small individual 
«eem8 to be exhausted by the process and is absorbed into the 
body of the other. It appears that this process of conjugation is 
only effective when it takes place between individuals of different 
parentages ; if care is taken to exclude all VorticeUae of foreign 
stock from a collection consisting of the descendants of a single one, 
either no conjugation takes place, or, if it does take place, it fails 
to produce the results which normally follow, viz., increased vigour 
of reproduction and other vital processes. When conjugation with 
individuals of different parentage is prevented the individuals which 
are produced by fission, after a certain number of generations, are 
said to be badly formed, unable to feed themselves and die, and 
this is the only instance of natural death which is met with amongst 
the Protozoa. 

like Amoeba, Vorticella encysts and it appears stiU more 



n.] CILIATA. 33 

frequently than Amoeba in infusions. The Vorticellae which are 
fannd under these circumstances are usually small and transparent 
and more favourable for observation than those occurring in 
ditches. The genus occurs both in fresh and salt water. 

Vorticella is but one example of a large cla^s of Protozoa 
termed Ciliata, which agree nith it in all the essential points of 
structure, but differ in the arrangement of cilia, the absence of a 
stalt, and more rarely in the absence of & mouth aud pharynx. 
This last feature is found only in those species which live in places 
where the surrounding fluid contains dissolved nutriment which can 
percolate in at any spot. An example of such a Oiliate is Opalina, 
found in the intestine of the Frog. This ^^ 

animal is thin and plate-like and covered '*^\^ 2 

all over with cilia of the same size ar- ^ ^'" 

ranged in regular lines; this arrangement 
of dlia, which is called the holotrichous t 

arrangement (oAot, entire ; dpif, hair), is 
always associated with the absence of a 
stalk and with a &ee-swimming life. Vor- 
ticella, on the other hand, is said to be 
peritrichous (xtpi= around). Opalina 
is farther remarkable for possessing a large 
number of nuclei (1, Fig. 12), which is 
a rare occurrence amongst the Ciliata. Fio. 12. Opalina 
When, however, division commences it From Brona. 

continues until the resulting pieces have 1. NnoUi. 3. Ectoplasm. 
only one nucleus each ; they then grow 

and do not divide again till they acquire the size they had before 
division took place and also the same number of nuclei Hence 
we might regard the multipHcation of the nuclei as the real repro- 
duction of this form, the division of the protoplasmic body being 
of lesser importance and setting in later. 

Paramecium is one of the commonest (ree-swimming Ciliata. 
It is of an elongated oval outline; seen sideways it has a thin 
scoop-like anterior end and a thick posterior part, so that it is 
nsnaUy described as slipper-shaped. It is holotrichous like Opalina, 
but like Vorticella it possesses only two nuclei, one large and easily 
visible and one, the micronucleus, small and difficult to detect. It 
haa a well-developed mouth and deep pharynx situated on one 
side and lined with specially long cilia. Paramecium is a beautiful 
form in which to study the contractile vacuole ; there are two 

8. am; 3 




34 PBOTOZOA. [chap. 

of these preeeot, one in the anterior and another iii the posterior 
portion of the aoimaL If one of these vacuoles be vatched it can 
be seen to contract and then slowly to re-appesr. Id the process 
of reappearance five or eiz isolated drops are seen which elongate 
into streaks arranged like the rays of a star. These streaks coalesce 
with one another and soon form a perfectly spherical drop. 

Paramecium poseessee pecoliar organs named trichocysts 
(&, Fig. 13) embedded in the outer layer of the protoplasm. These 




FlQ. 13. Faramieiwn 




ftboat250. After BitUohU. 



1. Moath at bottom of groove. 3, Oeaopbagng. 8. Food vacuole just 

being formed. i. Coctraetile Taoaoles. S. TriohoojBtB which hftve 

exploded: tlie nnexploded ones line the mitiole. 6. Cilia. 7. NaeleDB. 
6. Micron Qcleua. 9. Contractile fibrils. 

look like minute rods. When the Paramecium is irritated — as 
for instance if it is deluged with dilute iodine — or approaches 



IT,] FLAGELLATA. 36 

prey it wishes to seize, these are suddenly shot out, assuming the 
form of long threads, and they appear to exercise a stunning effect 
on any small animal with which they come in contact. 

The process of conjugation has been carefully worked out in the 
case of Paramecium^ so that some more details may here be given 
of what happens in that case. Two individuals about to conjugate 
become attached to one another, the larger nucleus in each breaks 
up into a number of minute pieces which are apparently used as 
food by the rest of the protoplasm — at any rate they gradually 
disappear. The smaller nucleus breaks up in eight pieces in each 
case, and of these seven are cast out, the eighth divides into two 
and of these two one passes over into the body of the other Para- 
mecium and fuses there with the remaining piece of the corresponding 
smaller nucleus. In this way each Paramecium becomes a zygote. 
The two individuals separate, the single nucleus in each divides 
twice so that four nuclei are produced, the Paramecium then divides 
transversely so that each half has two nuclei, one of which becomes 
the large and the other the small nucleus of the new individual 

Passing from the Giliata, we next come to a small group called 
the Suctoria, which are allied to the Ciliata, for 
their buds commence life as holotrichous forms. 
When they grow up they frequently become stalked like Vorticella, 
lose their cilia, and acquire instead a number of stiff rod-like 
outgrowths etiding in knobs ; these structures are termed tentacles. 
These are able in some way we do not understand to seize small 
animals and suck out their contents. Some secretion must be 
produced which eats its way through the cuticle and dissolves 
the contents of the prey. 

The next group of the Protozoa we shall consider is a very large 
Fi eUatft *^^ important one. It is called the Flagellata. 
The members of it agree with the Ciliata in having a 
fixed shape and a firm cuticle and probably (though this is difficult 
to make out in the smaller ones) a specialized cortical layer of 
protoplasm; they differ, however, in not having cilia, but in pos- 
sessing instead, one or two — ^rarely more — whip-like organs called 
flagella, which lash about in the water, and drag the animal after 
them by a spiral screw-like motion, just as a steamship is dragged 
by the screw when the engines are reversed. We may take as a 
type Euglena viridiSy one of the numerous inhabitants of ditches. 
This animal has a narrow elongated shape, pointed at one end, and 
at the other — which is its front end — it possesses the vestige of 

a— 2 



36 



PROTOZOA. 



[chap. 



ik phBr3nix, which is exceedingly narrow. It has a flagellum which 
aziaes from the phar3mgeal wall near its inner end. Slightly behind 
the pharynx there is a small contractile vacuole, and at the one side 
of this a small red spot, which may very possibly be associated with 
a smatiTeness to light About the middle of the body is a nucleus 
which can sometimes be made out as a clear spot in the living 
animal, bat which is most satisfactorily observed when the animal is 
killed with oemic acid and stained with picrocarmine. 




Fio. 14. Englena viridii, 

A X 100. B, 0, D, E, F X 200 showing the different shapes assumed by the 
aiiiuial during the euglenoid movements. 1. Pharynx. 2. Contractile 
Vfuniolo. 8. Pigment spot. 4. Nuclens. 

Two features in HJuglena, however, will strike us as very peculiar. 
Oue is, that in spite of possessing a cuticle and a cortical layer of 
l>n)t^>l>laHm, it is able to change its shape. It does not possess the 
IKiwor of throwing out pseudopodia, but it bends its body in the 
uioHt (extraordinary way, and contracts it till it is almost spherical 
*V\w {HMMiliar wriggling movements which it thus executes are so 
uiiliku anything else that they have been called euglenoid. The 
rtmMoii Inr their possibility is no doubt that the cuticle is flexible and 
tho (Mirtical layer powerfuUy contractile. The other peculiarity is 
Mtill iiinni Htriking, and it is that the protoplasm is coloured bright 
yrooii and tliat it contains particles of a substance very like starch. 
Now (liiiHii tilings indicate that Euglena feeds itself like a plant, and 
ilifit it coHHtructs its protoplasm out of carbon dioxide and mineral 
ballM iliMMolvod in water in the presence of sunlight. The only points 
lliMi'itfnrn that can be suggested in which it differs from plants are 
llial' it han a llagollum and moves, and that it does not possess a 
(uivming fif futlliilose. These supposed differences, however, will not 
Mliihft ifKitniination ; the germs of many undoubted plants, such as, 



il] flaqellata. 37 

for instance, the sea-weeds, have no cell wall and propel themselves 
by means of flagella What justification then, it may be asked, 
have we for reckoning Euglena as an animal? What do we mean by 
so classing it? It must indeed be admitted that when we come 
to deal with the simple Flagellata, the animal and plant kingdoms 
merge into one another, and the only valid line of division we can 
draw is between forms which feed on solid food, and those which 
absorb dissolved nutriment ; and amongst the latter we call those 
forms animals which we believe to have been derived from ancestors 
which fed on solid food. Now the pharynx in Euglena takes 
in solid particles from time to time and these passing into the 
protoplasm are apparently digested. We might therefore imagine 
either that Euglma is a plant which has acquired a pharynx and is 
commencing to live like an animal or else that it is an animal that 
has acquired chlorophyll and has commenced to live like a plant 
The fact that the pharynx is smaU and of little use is against the 
idea that it is an organ which has been newly acquired — as all 
organs are acquired — on account of its usefulness. It has the 
appearance of being the vestige of a once useful organ and therefore 
we conclude that Euglena is an animal which has begun to live like 
a plant. 

The reproduction of Euglena and of the Flagellata in general is 
quite similar to that of the Ciliata ; they increase by longitudinal 
division, but they also divide when in an encysted condition into 
two or four, or a larger number of germs ; these germs are not 
killed by drought When dry they are blown about, and so appear 
in infusions. In infusions Bacteria appear first, then Flagellata, 
and finally Ciliata. 

Many flagellata are devoid of a pharynx altogether, but these 
rarely have chlorophyll and subsist on the nutritive substances which 
are dissolved in the fluid in which putrefying matter is soaking. 
These are reckoned as animals on no very good grounds ; for it is 
well known that plants can lose their green matter when they can 
get the materials of protoplasm without building it up from carbon 
dioxide. How entirely arbitrary the decision is is best shown 
by the fact that many forms are claimed by both botanists and 
xoologists. For this reason it is convenient to have a name which 
denotes simply a living thing without prejudging the question as to 
^vhether it is an animal or a plant Such a name is supplied by the 
word organism, which is frequently used. 



88 PBOTOZOA. [CHAF. 

The hat group of Protozoa, the Sporosoa, agree widi the 

SiKnoioa Ciliata in posoessing a &m catide and a highly 

developed contractile cortical layer, bnt differ in 

nerer havbg any organs Bncb as cilia or flagella ; their movements, 

which are rery alnggiBh, are carried ont entirely hy contnetioiu 




Fto. 16. Cltpttdrina iimga, ttoia larrt of Tipaia, the Uaddy-long-lagB. 
Highl; magQitieJ. From Uget. 
A, B, C, D, E. Stages of the devclopineiit of C. iunaa, at first vithin and then 
pDshiDg its vay oQt of one ot the cells of the intestine of the Tipvla larva. 
F. Mature form. O. Two forms conjngating. 1. Cell of intestine 
of host. 3. Its nnelcuB. 

of the cortex, which give riae to worm-like wrigglbgs. The name 
Sporowa which was suggested by the frequently recurring sponUa- 
tion which is a marked feature in the life-histoiy is inappropriate, 
OS we have seen reason to believe that something analogous occurs 
in all Protozoa. All the Sporozoa are parasitic ; that is to say all 
live at the expense of some other animal which is termed the host 
AH as a matter of fact pass the first period of their existence 



II.] SPOROZOA. 89 

embedded in the protoplasm of some animal. Some — the Coccidea 
— remain throughout life in this position, but others when fully 
grown become at any rate partly free, adhering to their hosts only 
by one end. Only fluid nourishment is absorbed and consequently 
there is neither mouth nor anus. There is never more than one 
nucleus, although the body may be divided by partitions running 
across the protoplasm into two or even three portions, placed one 
behind the other. Reproduction takes place after encystment, and 
this encystment is in most cases preceded by conjugation, so that two 
individuals are enclosed by a common cyst. The contents of the cyst 
break up into spores, which surround themselves with flinty cases and 
hence are called chlamydospores (x^/^vs^a cloak). The proto- 
plasm inside these spores is sometimes liberated as a small amoeba- 
like creature which usually divides into two worm-like forms, which 
wander into suitable positions and become metamorphosed into 
the adult form. In very many cases, however, from two to eight 
worm-like forms are formed by the division of the contents of the 
spore before the case breaks. The name falciform embryo has 
been given to these germs. 

The best known Sporozoa are Monocystis found in the vesicula 
seminalis of the earthworm, with a long worm-like undivided body, 
Ckpsidrina blattarum found in the intestine of the Cockroach and 
C, longa in the intestine of the larval grub of the Daddy-long-legs, 
which lives in damp soil. This last form is quite free when adult and 
is distinctly divided into two portions. 

The ffaemamoeba and Haemomenas which cause Malaria in 
man and allied forms which infest other vertebrates are classed with 
the Sporozoa. 

The Coccidea is the name of one of the sub-divisions whose 
members remain entirely enclosed in the protoplasm of the infected 
animals throughout life, and in some cases cause disease. 

The larger Sporozoa are often easily detected by the intense 
opaque white colour of the protoplasm, due to the inclusion of an 
immense number of granules. This is in marked contrast with the 
translucent protoplasm in which they are embedded. 



40 PROTOZOA. [chap. 

Phylum PROTOZOA. 

The Protozoa are classified as follows : — 

Class Gymnomyxa. 

Naked forms without distinct cortical layer and capable of 
emitting pseudopodia. 

Order 1. Lobosa. 

Simple forms with blunt pseudopodia which do not form 
networks ; with or without a shell. 
Ex. Amoeba, Dijlugia, Arcella. 

Order 2. Reticularia. 

Protozoa with thread-like pseudopodia which form networks ; 
a shell is formed and protoplasm covers the outside as well as 
the inside. 

Suborder (a) Poraminifera. The shell consists of one 
or a series of chambers composed of lime or flint. 
Ex. Gromia, Folystoniella, Globigeriiia. 

Suborder (h) Radiolaria. The shell is a single sac of 
membrane. In almost every case there is an additional skeleton 
of flinty needles often joined so as to form complicated basket- 
works. 

Ex. Tkalassicola, Heliospkdera. 

Order 3. Myoetozoa. 

Protozoa with branching pseudopodia forming coarse net- 
works and devoid of skeleton. Reproduction by means of 
spores coated with cellulose. The contents of many spores 
coalesce to form one individual 

Ex. Chondrioderma. 

Order 4. Heliozoa. 

Protozoa with stiff" radiating pseudopodia. Skeleton when 
present only in the form of isolated needles. 
Ex. Actinosphaeriwn, Actinophrys, 

Class CORTICATA. 

Protozoa with a distinct cuticle and almost alwajrs a distinct 
cortical layer. 



II.] CLASSIFICATION. 41 

Order 1. CiUata. 

Forms provided with cilia. 

Ex. Vorticella, Paramoectum, Opalina. 

Order 2. Suctoria. 

Forms provided with sucking tentaclea 
Ex. Acineta. 

Order 3. Flagellata. 

Forms provided with flagella. 
Ex. Euglena, 

Order 4. Sporozoa. 

Parasitic forms, devoid of mouth, cilia, flagella or tentacles. 
The younger stages at least are cell-parasites. 

Ex. MonocystiSy Clepsidrina, Coccidium, Haemamoeba, 



42 



CHAPTER m. 
Phylum Coelenterata. 

It is difficult to say what idea the originator of the name 
Coelenterata meant to convey. Most animals have hollow 
insides (Or. icotXos, hollow ; cktc^ov, inside) ; the Coelenterata how- 
ever are distinguished from all the more highly organized groups in 
the animal kingdom by containing inside only one set of spaces, 
which all communicate with each other and with the exterior 
through the moutL 

The Coelenterate of simplest structure is undoubtedly the 
common fresh-water Poljrp {Hydra), (Pig. 16). K a 
mass of weed and other debris from a ditch or even 
the edge of a river be placed in a glass vessel along with some of 
the water in which it was grown and allowed to settle, a niunber of 
these small animals frequently termed polyps will usually be found 
collected on the side of the vessel nearest the light. Several 
distinct species are collected under the name Hydra. There are 
three species recognized in Great Britain; Hydra fusca, about a 
third of an inch long when expanded and of whitish yellow colour. 
Hydra viridis, a quarter of an inch long, of a green colour, and 
Hydra vulgaris, which is almost colourless. Similar species to the 
fiiBt two, if indeed they are not identical, are common in Lower 
Canada. Hydra fusca may be selected as a tjrpe. 

The shape of this animal is that of a minute cylinder. The base 
or foot is attached to the surface of the glass by an adhesive disc, 
whilst the other extremity carries a circle of delicate thread-like 
appendages called tentacles. In the centre of these, near their 
point of origin, we can with a lens detect a minute conical ele- 
vation, the oral cone (2, Fig. 16), at the end of which is the mouth. 
The mouth is the only opening in the body and it leads into a space 
which occupies the whole extent of the animal^ so that we might 



CHAP. III.] 



HYDRA, 



43 



with justice say that the poljrp is really simply a tube closed at 
one end and open at the other : further, the tentacles can be seen 
with the microscope to be nothing but thin hollow tubes, opening 
into the central one (Fig. 17). The central space is often termed a 
stomach, and in the case of Hydra the idea suggested by this 




Fio. 16. Hydra futca x about 12. 

A. Expanded condition. This specimen ia badding off a young Hydra, It 
contains a large food mass in its ooelenteron, probably a Daphnia or some 
other fresh-water Crustacean. B. Retracted condition. 1. Mouth. 
3. Oral cone. 8. Tentacles. 4. Bud. 5. Endoderm. 6. Foot 



term is correct. In other Coelenterata the space performs other 
functions besides those of the human stomach, and the term 
coelenteron, which does not imply any function, is preferable. 
With the microscope, however, we can make out a number of 
farther points. If the edge of the animal be carefully focussed it 
can be seen that the body-wall consists of two layers, an outer 



COELENTERATA. [CHAP. 




Fra. 17. A tongitndiiuU seotion through the body of a Hydra: tomewhAt 

diagramnutio, the det4ila of th« oelli iMing omitted. Mtgnified. 
1. Month. 9. Foot. S. Tentaole. 4. Ooet^taron ot digeative Mvltjr. 

6. Ectoderm. 6. Endodeim. 7. HesogloM or itnutnreless lamelM. 

a Battaries of UuMd mUi. 9. Twtit. 10. OtH7 with nngle onm. 



III.] HYDRA. 45 

clear one, termed the ectoderm (Gr. cktos, external ; 8€pfjLa, skin), 
and an inner one called the endoderm (6r. IvSov, inside), which 
is green in Hydra viridis and brownish in Hydra fusca ; so 
that we may speak of a skin as distinct from the lining of the 
coelenteron (Figs. 17 and 18). It is further possible to make out 
under the microscope that at any rate the outer layer is not 
homogeneous, but is composed of separate small pieces. It is 
necessary, however, to examine thin sections of specimens which 
have been hardened by being soaked in corrosive sublimate or some 
similar reagent, before one can reaUy get a good idea of the 
structure of the "skin" and of the "inner lining" of the poljrp. 
Then it is seen that both are made up of the repetition of similar 
parts, and that in each of these parts there is a single nucleus. 
Such a portion of protoplasm, marked off from the surrounding 
parts by a definite boundary^ is called by Zoologists a cell The 
wall or boundary of the cell probably consists of a tliin layer of 
some secretion, in many cases, if not in most, traversed by bars or 
sheets of protoplasm connecting the cell with its neighbours. 
Around this term " cell " many battles have been waged and its 

indiscriminate use has led to much misconception. 

It used to be said, for instance, that the Coelenterata 
were multicellular animals, as opposed to the Protozoa, which 
were unicellular. Now it has already been pointed out that the 
centre of the vital processes is the nucleus, which controls the 
processes going on in the protoplasm, and that in some of the 
Protozoa, such as Actinosphaeinum and Opalina, this essential organ 
is repeated several hundred times. But an Actinosphaerium or an 
Opalina certainly does not correspond to a so-called cell of Hydra, 
with its single nucleus ; the relation between them may rather be 
defined by saying that, whereas in Actinosphaerium the areas of 
control of the various nuclei are not visibly delimited from each 
other, in Hydra, on the other hand, this delimitation has to some 
extent taken place, leading to the appearance of cell-structure. 
But not only are cells to be detected in Hydra ; the cells are not 
all of the same kind. Those forming the endoderm are very 
big and often have great watery vacuoles near their inner ends ; 
they also contain the coloured granules to which the colour of the 
animal is due. The cells of the ectoderm or outer skiu, on the 
contrary, are much shorter than those of the endoderm and are 
more or less pear-shaped, the broader end being turned out. Be- 
tween their narrower bases we find groups of very small round cells 



46 COELENTEBATA. [CHAP. 

(2, Fig. 18). These so-called interatiti&l cells are young cells, 
which partly, do doubt, become developed into ordinary ectodeno 
cells as the older ceUs die and drop off, and in certain seasons of 
the year they increase very much in nnmber at certain spots and 
form the reproductive oi^ans (9 and 10, Fig. 17). The two kinds 
of organs, male and female, are borne by the same individual ; in 
the male organ or testis all the cells remain small and become 
converted into the small spermatozoa; in the female organ or 
ovary one cell increases very much in size at the expense of the 
rest and becomes the egg-cell or ovum (1<^ Fig. 17). There is. 




Fig. IB. TronaTerBS Beetion of Hydra fuiea. 

1. Ectoderm cells (mjv-epithelial). 2. InlAtetitial ceUB. 8. KemAtoorstt. 
4. CoetenteroD. G. Endoderm cells. 6. YoonolM. T, ?ood 

granulea. 8. Nuclei. 

however, a third change which these interstitial cells may undergo, 
which is of the utmost importance to the animaL Some of them 
move outwards and become wedged between and even embedded in 
the large ectoderm cells near the surface, e&(!h developing in its 
interior an oval bag filled with fluid. One end of this bag is 
turned into the interior of it, forming a long hollow thread. The 
whole bag is called a thread-capsnie or nematocyst (Gr. vijiia, 
a l^iread ; Kv'trnt, a bladder) (Fig. 19). If now the cell in which 
the thread-capsule is situated contracts, since the fluid in the 
o^enle is incompressible, the hollow thread must be quickly turned 



m.] HYDBA. 47 

inside out and thaa thrust out of the capenle. If the irritation 
of akin continnes the whole capanle will be pressed out by the 
animaL These thiead-capsoles are most abundantly dereloped in 
the tentacles, and a small amount of observation of the habits 
of Hydra will show how they are used. If a small Crustacean, 
or othei anim a l, approaches too near a Hydra, the latter makes 




Fia. 19. CnidobUtt vith luge Nematocjet f^om tba body-irsU of Bydra fiuea. 

Ei^; mftguiSed. From Schneider. 
A. Unexp1od«d. B. Exploded. I. CuidoblBSt. 2. Nuolens of onido- 

blMt B. Cnidoail. 4. MasoQlar aheath. S. W&ll of 

nematocyit. 6. Thread. 7. Beflezed procesBee. 

one swift lash with its tentacles and the luckless water-flea is seized 
and at the same time paralysed. If we now remove and examine 
the prey, we shall find it covered with exploded thread-capsules, 
the threads of which have entered its body, and exerted a poisonous 
action on it It is possible to induce a Hydra which is beinj; 
observed under the microscope to eject its thread-capsules : we 



48 COELENTERATA. [CHAP. 

have only to irrigate it with a little ten per cent. aolntioQ of common 
salt, and from all parts of the skin we shall see first the threads 
shot ont, and then the capsules follow. 

In the case of a fluid like salt solation, the stimulating action is 
no doubt exerted over the whole surface of the animal, but an 
examination of the tentacle when it is extended reveals an ar- 
rangement for bringing about the explosion of the thread-capsule. 
The surface of the tentacle is seen to be covered with little 
swellings, in which are collections — one might say, batteries — of 
thread-capsules (8, Fig. 17); and from the surface of the ectoderm, 
in which they are embedded, delicate hair-like rods project out into 
the water (3, Fig- 19). These rods are called cnidocils (6r. kviBtj^ 
a nettle; Lat. cilium, an eyelash) and are the simplest form of 
sense-hairs met with in the animal kingdom. If one of these be 
touched, it transmits a stimulus to the cell containing the thread- 
capsule, the cuidoblast (6r. kviBtj, a nettle ; jSXaoros, a sprout), as 
it is termed ; in response to this stimulus the cell contracts, presses 
on and explodes the capsule. 

In the first chapter it was pointed out that protoplasm, when it 
effects movements, always does so by contracting. We saw, for 
instance, that the extrusion of the pseudopodia of Amoeba could be 
accounted for by supposing that part of the outside protoplasm 
contracted and, so to speak, squeezed out part of the more fluid 
interior. In the life of Hydra the principal movements which 
occur are the shortening and lengthening of the body and the 
tentacles (B, Fig. 16). Now it has been found that in these 
movements, the shortening is effected by the contraction of the 
ectoderm in a longitudinal direction, and the lengthening by the 
contraction of the endoderm in a transverse direction, in con- 
sequence of which the animal is rendered thinner and longer. It 
has been further ascertained, by the examination of very carefully 
prepared longitudinal sections, that each ectoderm cell possesses 
at its base a tail running vertically, which is embedded in the thin 
layer of jelly sometimes called the structureless lamella or 
mesogloea (Gr. /ncVos, intermediate; yXottx, glue), which separates 
ectoderm and endoderm (Fig. 20). The endoderm cells similarly 
possess short tails, embedded in the jelly, but these run transversely. 
These tails then are instances of the tendency of protoplasm, which 
contracts regularly in one direction, to be drawn out into fibres in 
that direction, or, in other words, we have before us the first step 
in the conversion of an ordinary cell into a muscle ceU. * Cells 



III.] HTDBA. 49 

showing this inodiBcation are termed myo-epithelial (Or. /ivf-* 
moscle) : the word epithelial is used to eiguify the arrangement of 
cells ID a layer to fonn a pavement or mosaic. 

The most important function of the endoderm cells is to digest 
the prey which is captured by the tentades and thtnst into the 
coeleateron. For this purpose they secrete a fluid which has a 
great power of dissolving protoplasm. This fluid, termed digestive 
juice, ie poured forth into the coelenteron and a large portion of 
tiie prey is dissolved by it and passes by diffuaion into the endo- 
derm cells, from which part is transferred in a similar manner to 




1. Ectoderm. 2. Endodemi. 3. Meaogloes of BtrDctaieleea lamellft. 

4. Kenuito^at. S. Cnidoci!. 6. Unsele-fibres o( actoderm cells 

eat MTOH. 7. Naaleas of eatoderm cell. 8. IntarBtitial cella. 

9. Cutide. 10. Figment granule. 11. Food grsDale. 12. Maclens 
of andi>d«im cell. 13. Flaeellam. 11. Water lacuole. 

the ectoderm. Certain portions of the prey, consisting of some of 
tiie proteids, resist the aotion of this juice. These are seized by 
psendopodia emitted by the endoderm cells and bodily engulfed, 
to be subsequently slowly digested in food vacuoles. Any insoluble 
parte of the prey, such as cuticle, skeleton, etc., are ejected by the 
mooth. Some of the endoderm cells also bear flagella, whose move- 
ment doubtless aids the circulation of the fluid in the coelenteron. 

We have already seen that Hydra at certain seasons of the 
year, vit., the late autumn, produces egg-cells (ova) and male 



60 



COELENTEBATA. 



[chap. 

germs (spermatozoa). Tke l&Uer are Bhed oat into the water, and 
eventually some of them reach the egg-<»llB and unite with them. 
This process is called feTtiliiation, and the fertilized egg-cella 
cover themselves with spiny coats and drop off into the mud. 
Here they remain through the winter ; in the spring the hard coat 
cracks and out issues a minute Hydra. 

But Hydra is hy no means limited 
to this method of sexual reprodaction 
in its power of multiplying itaelC All 
tJirough the spring and summer, if it 
be well fed, it buds or reproduces itself 
by Gemmation. A small swelling 
makes its appearance on the side of 
the body; this is really a hollow pouch 
containing -a cavity in communication 
with the Goelenteron (4, Fig. 16). The 
walls of the pouch are merely continua- 
tions of the body-wall of the Hydra, 
and hence consist of the same two 
layers. The pouch rapidly lengthens, 
and after a while a circle of tentacles 
sprouts out from its free end, and a 
mouth is formed in the centre. We 
thus have a daughter Hydra still in 
close connexion with the parent, the 
coelentera of the two being in open 
communication ; later, however, this 
communication becomes closed and the 
o£bpring separates from the parent 
and leads a free existence. A third 
method of reproduction, which probably 
rarely occurs except artificially, is 
Fission. If a Hydra be divided into 
two halves, each half will grow up into 
r individual 

A large number of the Goelenterata, called the Hydromednsae, 
agree with Hydra in iJl essential points of structure ; 
the most important point of difference is that in 
them the buds do not become separated, but remain 
permanently in connection with the parent, and thus compH- 
cated colonies are built up (Fig. 21). Other differences of less 




Fid. 31. Obetia helgolandica 
X 1. From HutUub. Thia 
is the hydroid ^eoention 
natmal size u it appears 
to the Daked eje. 



Hydro- 



m.] HTDROHEDUSAE, 61 

importance are that there is a horny shell, the perisarc (Gr. «pi, 
aronod ; irdpi, flesh) (Fig. 32), secreted by the ectoderm at any rate 
on the lower portion of the body, also that the tentacles are nearly 




1. Eotodenn. 3. Endodemi. 8. Uoatb. i. Coelettteron. 

B. Coonouio. B. Peruare. 7. Hydrotbeca, prolonged at base ot 
Hydioid u ■ ahelf. 8. BlutOBtyle, a moathlesB hjrdroid bmrlng medau- 
bndi. 9. MedosB-liiid. 10. Qonotheca, part of perisaro wbioh proteete 
ths madnn-bnda. 

always solid, containing, instead of tubular outgrowths of the endo- 
deim, a solid ooid of cells (Fig. 22) with firm outer membranes and 



52 



COELENTEBATA. 



[chap. 



Medusa. 



partially fluid contents, so that the cells have the same kind of 
stiffness as a well-filled water-pillow. These cords likewise bnd ont 
from the endoderm, but, as apparently the animal does not need the 
tentacle cavity which exists in the Hydra, it has disappeared, and 
the solid axis is essentially a strengthening or skeletal structure. 
As in Hydra, there is an oral cone; and in some species of 
Hydromedusae, at any rate, there is an additional row of short 
tentacles at the tip of this. It has been stated above that the 
buds do not become detached, but there is one kind of bud differing 
much in shape from the rest which does become detached. In 

such a bud, the whole body becomes very much shorter 
and at the same time much flattened out in its lower 
portion, so that the main circle of tentacles is widely separated 
from the oral cone ; at the apex of the latter there is sometimes a 
second circle of small tentacles. The flattened part of the body 
becomes concave on the side towards the mouth so as to assume the 
form of a bell or umbrella, and, owing apparently to this circum- 
stance, the part of the coelenteron which it contains becomes so 
pressed together, that by the adhesion of its upper and lower walls, 
its cavity for the most part disappearing, it becomes converted into a 
concave layer, called the endodermal lamella. Along four lines, 
however, the cavity does not disappear (4, Fig. 23), and it also 

remains open just beneath the circle 
of larger tentacles at the edge of 
the bell, so that in this way we have 
a circular or marginal canal 
established, communicating by four 
radial canals with the part of the 
coelenteron that still persists in the 
oral cone, and opening to the exterior 
by the mouth (1, Kg. 23). The 
upper surface of the bell is styled 
the exumbrella or aboral surface 
(Lat. ab, away from ; os, oris, the 
mouth) the lower the subumbrella 
or oral surface. 

The great mass of the bell is 

composed of the jelly intervening 

between the outer ectoderm on the 

convex side and the endoderm. In this jelly solid strings sometimes 

appear which give it a firmer consistence. The modification of the 




Fio. 28. Free-swimming Medusa 
of Ohelia sp. 

1. Mouth at end of manubrium. 

2. Tentacles. 

8. Beproduotive organs. 

4. Badial canals. 

5. Auditory organ. 



m.] 



HYDBOUEDUSAE. 




Fia. Si. Bougainvitliafmctuoia, xftboat 12. From Allman. 
A. The fixed hTdcoid foim with nimieroaB bydroid polTpea and medasae in 
-nriotit ittges of development. B. Tbe free swimmiDg sexual Medusa 

which hM broken away from A. 



64 COELENTERATA. [CHAP. 

base of the animal into the shape of an umbrella causes the oral 
cone to resemble the handle, hence the name manubrium (Lat. 
a handle), by which it is usually designated in a bud of this kind 
(1, Fig. 23). Just above the circular canal in most Medusae a fold of 
the outer skin grows in towards the oral cone, so as to form a broad 
circular shelf: this structure is called the velum (Lat. an awning) 
(B, Fig. 24 ; 1, Fig. 25). The bud now breaks loose and swims by 
contractions of the bell, aided by vibrations of the velum. Anyone 
would now recognise it as a minute jelly-fish, though it really is 
quite different in many points from the larger and better known 
animals denoted by that term. Zoologists speak of it as a Medusa, 
and speak of the stock from which it was budded as a colony 
consisting of medusoid and hydroid persons, the latter term 
denoting the ordinary buds which resemble Hydra, The terms polyp 
and hydranth are also often used to denote a hydroid person. 
A medusoid is in many respects more highly developed than 
the hydroid person. The ectoderm cells composing the velum 
and those forming the lining of the under side of the bell or 
sub-umbrella are strongly drawn out into processes which are 
muscular. In the velum these are arranged so as to form two 
bands running round the edge of the bell or umbrella, one band 
being in connection with the upper and another with the lower 
layer of cells composing the fold of ectoderm of which the velum 
consists. Just, however, where the velum is attached to the bell, 
its cells— upper and lower — undergo another and more interesting 
modification (4 and 5, Fig. 25). At their bases a tangle of delicate 
threads of almost inconceivable fineness appear; these threads are 
outgrowths of the cells, but far more delicate than those which 
alr^y in Hy&ra we recognised as the forerunners of muscles; the 
threads we are now considering are, in fact, nervous in nature, 
and the tangles of them connected with the upper and lower layers, 
respectively, of the velum, constitute an upper and a lower nerve 
ring. Each thread is to be regarded as the tail of an excessively 
small ectoderm cell 

In Hydra we found the earliest appearance of sense hairs; and 
the cells of which they are processes, viz., the cnidoblasts, may be 
called sense cells. In the Medusa we meet with definite collections 
of sense cells aggregated so as to form sense organs. These are 
found close to the position of the nerve ring, either on the velum 
itself or immediately outside it at the bases of the tentacles, so 
that the stimuli which they receive are easily transmitted to the 



UI.] HTDKOHEDUSAB. 53 

nerve Hd^. Two main kindB of sense o^;ang are frequeotly foand, 
whidi may be ronghly called eybs and ears ; never, howevn, both 
kinds in one Medosa. The 'eyes' are little coloured patches of skin; 
some of the cells of which end in clear rods while others secrete a 
coloured aubatance or pigment Both pigment and rods are neces- 
sary if tiiere is to be vision, though we do not understand why. The 
eara are little pits in the base of the velum ; they may be open or 
tbeb edges may come together, so that the ectoderm lining them is 
entirely shut off from the outer skiiL In either case, some of tlie 




Fto. 25. I. A. Eya of lififa Ito^Hffari seen from (he Bide magnified B The 
ume Been from in front. C. JsoUted oella of the ums Fiom O & B 

1. Lena. Z. Fignient cells. S. Percipient cells. 
n. Bldiat section through the edge of the umbrella of Carmarina kaitata 
■howing Benie-organ and velnm. 
1. Yeltun. 3. Jell;. 8. Ciroolar mascles of velam. 4. Upper nerra 
ring. C. Lower nerye ring. G. NematocjMs. 7. Badisl Teuel 
running into cinmlar TeBsel, lioth lined bj endodem. 8. ContinnatioD 
of endodena along abonl aarfaee. 9. Sense orean or tentaoitlocTat. 

10. Anditory nerve. 

cells forming the walls of the pits secrete particles of lime, others 
close to them develope delicate sense hairs. The result is that 
vibrations in the water, if they come with a certain frequency, will 
affect the heavy particles, and their vibrations ui turn will affect 
the sense hairs. There is another kind of infonnation, however, 
which organs like ^eee give their possessor, and this is probably still 
more impOTtant to the floating Medusa, namely, information as to 
the position of the animal with regard to the vertical. In other 



56 OOELENTERATA. 

words, the Medn.^ leams from them whether it is moving upwards 
or downwarda or sideways : for when the aoimal shifts it« poaition, 
the heavy particles in the ear-sacs are shifted couformably aod affect 
different sen^ cells. 

Through these different sense organa etimuli are continnallf 
pouring in from the external world. If the etimnli only affected 
the contractile cells nearest them irregular movements would result. 
The function of the nerve ring, as of all nervous systems, is to co- 
ordinate the stimuli, that ia to collect and rearrange and rapidly 
distribute them to the whole animal so that a definite reaction of 
the whole contractile tissue results, not a series of local reactions 
interfering with one another. 

The Medusa is very voracious and rapidly increases in sixe. It 
feeds on the small organisms of all kinds, both plants and ammftla, 
which are found at the surface of the sea. After some time it e 




Fia. 96. The ciliated lana or Ptonola oF a BTdrompdngoD, Clara tqvomata. 

Magnified. From AUmaii. 
A ilE B. Swimming aboat in the sea. C. Coming to tet,t on a rock. 

D. Deveioping tenUolea, oral oone and atolon. 1. Tentacles. 3. Oral 

coDf. 8. Stolon. 

mences to give rise either to eggs or to spermatozoa, which usually 
develope in exactly the same way in which they developed in Hij^ra. 
ie., from the interstitial cells of tlie ectoderm. The ticcumulations 
of these cells, called gonads or generative organs, are borne 
either on the under side of the bell (3, Fig, 23). or on the sides of 
the manubrium, and it is a curious fact that those Medusae which 
have them in the former position usually possess ear-sacs, whereas 
when the gonad is situated on the oral cone, ear-sacs are never 
present, but eyes may be. The eggs and spermatozoa are both shed 



ni.] HYDROMEDUSAE. 57 

out into the water and coalesce there, and the fertilized egg developes 
into a little oval larva, termed a Planula (Fig. 26), without 
tentacles or mouth, and covered all over with cilia. It consists at 
first of a hollow vesicle of ectoderm cells, later becoming filled with 
a solid plug of endoderm. This little creature swims about for a 
while and then attaches itself by one end to a stone or a piece of 
sea-weed. The attached end flattens out (C and J), Fig. 26), but 
the rest of the animal lengthens and a mouth and tentacles appear 
at the firee end and the endoderm becomes hollowed out, so that 
the creature takes the form of an unmistakeable hydra-like organism. 
It then begins to bud out a branch called a stolon which creeps along 
the substratum. From this other polyps will arise, each of which 
has only to bud in order to reproduce the colonial stock fi-om which 
its parent, the Medusa, was separated. The free-swimming young 
or planulae famish good examples of what is meant by the term 
larva. This name is given to the young form of any animal when 
it is very different to the fully-grown animal and leads a free life. 
We have thus learnt that a Medusa gives rise to an egg which 
develops into a Hydroid person, which after a time in turn buds off a 
Medusa; such an alternation of generations is very characteristic 

of a large number of Coelenterata. The Medusa re- 
of Genwations. prcsouts a soxual generation, the Hydroid an asexual 

generation, and inasmuch as the Medusoid is often 
only produced as a bud of the third or fourth order (ie. is budded 
from a Hydroid person which has produced similarly from another 
Hydroid person), it will be seen that several asexual generations 
intervene between two sexual ones. One explanation of this life- 
history is that the Medusa is only a specially modified Hydroid, which 
has acquired the power of locomotion, in order to disperse the eggs 
over a large area, and thus avoid the overcrowding of a limited area 
with one species. The swimming beU and velum are contrivances 
to enable the bud which bears the eggs to move about. If, however, 
this explanation be adopted, it is a most remarkable fact that in 
many species the Medusae are very imperfectly developed and 
never become free. Such Medusae are usually more or less de- 
generate and are termed gonophores. Since the gonophore fails 
to fulfil the purpose for which we believe the Medusa to have been 
developed we must assume that conditions have now so far changed 
that the same wide scattering of the eggs is not now so necessary as 
formerly, possibly because the species in question are restricted to 
particular strips of the shore. Tubularia larynx found growing on 



^S COELENTERATA. [CHAP. 

M^w«>0d is a good example of a form with degenerate Medusae, 
H^m^^im'illia or Obelia of forms with free Medusae. 

The Hydromedusae include a large number of families, most of 
whioh are represented by small plant-like forms resembling the 
iS^uera just mentioned, but there are several groups which show 
warktHl {H'ouliarities and have been regarded by many zoologists as 
of oo<e(i\ial rank with the order although they have probably been 
d<^rive<l from ordinary Hydromedusae. Of these we may name 
vi) tlie IVachymedusae, (ii) the Narcomedusae, (iii) the Siphonophora 
M\i\ (iv) the Hydrocorallinae. In the first two groups the eggs 
doYolope from the planula stage directly into Medusae, missing out 
tJ\e hydroid stage completely. In both cases also the sense-organs 
are s)H>oially modified tentacles which are suspended like minute clubs 
round the edge of the belL In the Narcomedusae these clubs are 
tVtH^ly exposed and the wide baggy stomach occupies the whole under- 
nurface of the umbrella, whereas in the Trach3rmedusae the sensoiy 
clubs are enclosed in pits (Fig. 25) and the stomach is small and 
Huspended from the umbrella by a stalk traversed by the radial canals. 
The name Trachymedusae (Gr. Tpa^w, rough) is derived firom the 
oirtmmstance that the umbrella is 8ti£fened by numerous ribs of 
ondoderm cells and the edge has a thick rim of ectoderm. The 
members of the third group are stocks consisting both of medusoid and 
hydroid persons which are not attached to any support but which freely 
swim or float in the sea. Some of the medusoid persons known as 
noctocalyces have taken on the function of locomotor organs and 
by their rapid pulsations not only drive themselves through the sea, 
but draw after them the rest of the stock much as an engine draws 
a train of carriages. A few forms, however, like the Portuguese 
Man-of-war, Phi/salia, have no nectocalyces and float passively about. 
The popular name of this genus is derived from the shape of the huge 
air-containing float from which the persons of the colony are sus- 
pended. It has been plausibly suggested that the Siphonophora 
have been derived from planulae which attached themselves to the 
surface-film of the water instead of to a solid support The surface- 
film in consequence of its physical properties acts like an elastic 
membrane, and in artificial cultures it can often be seen that some 
planulae do attach themselves to this, and in consequence perish. 
But if by favourable variations, such as a tendency to cupping of 
the base and an inclusion of air-bubbles in the cavity, the stock 
were enabled to remain suspended, then it would be placed in a very 
favourable position for getting food, and thus it has been suggested 



III.] ACTINOZOA. 59 

the simply floating Siphonophora have been evolved from Hydro- 
medusae. If this view be taken, the three chief divisions of Siphono- 
phora represent three successive stages in the adaptation of the group 
to a pelagic life. Thus the Fhysaliidae simply float, the Physo- 
phoridae float and swim by nectocalyces, whilst the Calycophoridae 
have lost the float and trust entirely to their powerful nectocalyces. 
The Siphonophora are remarkable for the varieties of person 
which compose their colonies. As varieties of the hydroid person 
may be named the palpons or tactile persons devoid of a mouth, 
but showing their equal rank with the nutritive person by the 
possession of similar tentacles. To the category of medusoid persons 
belong not only the nectocalyces but the bracts — transparent 
sheath-like structures sometimes present, which shelter groups of 
persons. This extreme variety of persons is foreshadowed in the 
ordinary Hydromedusae. Hydractinia for instance, which grows at 
the mouth of whelk shells inhabited by hermit crabs, has palpons 
amongst its hydroid persons, but in no case is such extreme diversity 
attained as among the Siphonophora. The Hydrocorallinse hydroids 
form large colonies and are divisible into nutritive polyps or gastro- 
sooids and tentacle-like polyps, the dactylozooids. The skeleton 
is massive and they form encrusting growths. The medusoid 
persons attain varying degrees of perfection. 

The Sea-Anemones are representatives of a second division of 
the Goelenterata, which show a decidedly more com- 
plicated structure than the animals just considered. 
Unfortunately it is very difficult to obtain the ordinary sea- 
anemones in a sufficiently expanded condition to make out their 
structure, since when irritated they contract so much as to 
throw their internal structures into great confusion. Another 
animal belonging to the same group is the 'colonial' species 
Alcy(mium digitatum, sometimes called "Dead men's Angers.'' 
It is comparatively easy to paralyse the members of the colony 
or i>olyp8 by adding cocaine, or some similar reagent, to the water 
in which the colony is living (Fig. 27). If then an expanded polyp 
be cut off and examined with a lens, we shall be able to make out 
most of its structure. We notice to begin with that there is a 
single circle of eight tentacles, each of which has a double row of 
short branches, so that it looks like a miniature feather; within the 
circle of tentacles there is, however, no trace of an oral cone ; there 
is instead a flat disc, slightly sunken in the centre, where we find 
the slit-like mouth. If we look in at the lower cut end of the 



60 



COELESTERATA. 



polyp we shall see that the interual cavity or cnelenteron, instead of 
being a simple cylindrical space like that of Hijdra, is partiallj 
divided into compartiucDts by folds stretching in towards the centre, 
but not meeting. These folds are called mesenteries, and there are 
eight of them, coiresponding ia Dumber (but not iu position) with the 




Fart Qf a colony or Atcyaniwn digllatum x 8, abowing thirteen p 

IS BttkgeB at retroctioo uiid eipansioD. 
1. Mriutb, 3. MeseDterisB with repmduotiTe csIIb. 

8. Oesopbagae. 4. Festhered tentnclea. 

tentacles (Fig. 28). We shall further see that the mouth does not, 
as in Hydra, open directly into the coelenteron, but leads into a 
flattened tube which projecta into the interior of the body. This 
tube, the so-called oesophagus or gullet, ia really lined by the 
ectoderm, which is merely tucked in at the mouth. Such a tube is 
known as a stomodaeum\ The mesenteries, although they end 
fieely below, are attached to the Bides of the stomodaeum above, 
BO that in this region the coelenteron b divided into a number 
of compartments, each of which is prolonged int^ one of the hollow 
tentacles (Fig. 29). 

' "I have proposed ia deBienata tills ingrowth,. .thoEtomodaeum (arJjuafaiM', 
like wXiiaiOt, the road couaecled nith a galewsf ) and Bimilorlj to call snothei 
iDgiowth which accompaaiei the rotmation of the second orifice (the anus) of 
lh« enteron, the pioctodaeum " (v^nrAt, the anos). Bay Lankesler. 





TeotTBl 
Fra. 39> Tmurena teotion throngh a polyp of Alej/onium digitatua, through 

the legion ot the oesophagns x abont 130. From Hickeoii. 
1, Ckvlty of oeuphagiiB. 3. Sipbonogljph. 8. Eotoderm. 4. Meso- 
gloM 01 jallj. 5. Endodenn. 6. MQBClel in meunterieB. 7. Inter- 



52 



COELENTERATA. 



[chap. 



Medusa. 



partially fluid contents, so that the cells have the same kind of 
stiffness as a well-filled water-pillow. These cords likewise bud out 
from the endoderm, but, as apparently the animal does not need the 
tentacle cavity which exists in the Hydra, it has disappeared, and 
the solid axis is essentially a strengthening or skeletal structure. 
As in Hydra, there is an oral cone; and in some species of 
Hydromedusae, at any rate, there is an additional row of short 
tentacles at the tip of this. It has been stated above that the 
buds do not become detached, but there is one kind of bud differing 
much in shape from the rest which does become detached. In 

such a bud, the whole body becomes very much shorter 
and at the same time much flattened out in its lower 
portion, so that the main circle of tentacles is widely separated 
from the oral cone ; at the apex of the latter there is sometimes a 
second circle of small tentacles. The flattened part of the body 
becomes concave on the side towards the mouth so as to assume the 
form of a bell or umbrella, and, owing apparently to this circum- 
stance, the part of the coelenteron which it contains becomes so 
pressed together, that by the adhesion of its upper and lower walls, 
its cavity for the most part disappearing, it becomes converted into a 
concave layer, called the endodermal lamella. Along four lines, 
however, the cavity does not disappear (4, Kg. 23), and it also 

remains open just beneath the circle 
of larger tentacles at the edge of 
the bell, so that in this way we have 
a circular or marginal canal 
established, communicating by four 
radial canals with the part of the 
coelenteron that still persists in the 
oral cone, and opening to the exterior 
by the mouth (1, Kg. 23). The 
upper surface of the bell is styled 
the exumbrella or aboral surface 
(Lat ab, away from ; 05, oris, the 
mouth) the lower the subumbrella 
or oral surface^ 

The great mass of the bell is 

composed of the jelly intervening 

between the outer ectoderm on the 

convex side and the endoderm. In this jelly solid strings sometimes 

appear which give it a firmer consistence. The modification of the 




Fio. 28. Free-swimming Medusa 
of Ohelia sp. 

1. Moath at end of manabrinm. 

2. Tentacles. 

8. Beprodnotive organs. 

4. Badial canals. 

5. Auditory organ. 



m.] 



BYDROMEDUaAJL 




Fio, Si. Bousainelllia /mefuoia, x about 13. From Allman. 
A. The fixed Iiydroid form vith numeroua hj'droid pol;p«8 and meduBoa in 
wioiu itBgea of derelopment. B. Ibe free BvrimmiDg aeiaal Meduea 

vhloh has brokea away from A. 



54 COELENTERATA. [CHAP. 

base of the animal into the shape of an umbrella causes the oral 
cone to resemble the handle, hence the name manubrium (Lat. 
a handle), by which it is usually designated in a bud of this kind 
(1, Fig. 23). Just above the circular canal in most Medusae a fold of 
the outer skin grows in towards the oral cone, so as to form a broad 
circular shelf: this structure is called the velum (Lat an awning) 
(B, Fig. 24 ; 1, Fig. 25). The bud now breaks loose and swims by 
contractions of the bell, aided by vibrations of the velum. Anyone 
would now recognise it as a minute jelly-fish, though it really is 
quite different in many points from the larger and better known 
animals denoted by that term. Zoologists speak of it as a Medusa, 
and speak of the stock from which it was budded as a colony 
consisting of medusoid and hydroid persons, the latter term 
denoting the ordinary buds which resemble Hydra. The terms polyp 
and hydranth are also often used to denote a hydroid person. 
A medusoid is in many respects more highly developed than 
the hydroid person. The ectoderm cells composing the velum 
and those forming the lining of the under side of the bell or 
sub-umbrella are strongly drawn out into processes which are 
muscular. In the velum these are arranged so as to form two 
bands running round the edge of the bell or umbrella, one band 
being in connection with the upper and another with the lower 
layer of cells composing the fold of ectoderm of which the velum 
consists. Just, however, where the velum is attached to the bell, 
its cells —upper and lower — undergo another and more interesting 
modification (4 and 5, Fig. 25). At their bases a tangle of delicate 
threads of almost inconceivable fineness appear; these threads are 
outgrowths of the cells, but far more delicate than those which 
already in Hydra we recognised as the forerunners of muscles; the 
threads we are now considering are, in fact, nervous in nature, 
and the tangles of them connected with the upper and lower layers, 
respectively, of the velum, constitute an upper and a lower nerve 
ring. Each thread is to be regarded as the tail of an excessively 
small ectoderm celL 

In Hydra we found the earliest appearance of sense hairs; and 
the cells of which they are processes, viz., the cnidoblasts, may be 
called sense cells. In the Medusa we meet with definite collections 
of sense cells aggregated so as to form sense organs. These are 
found close to the position of the nerve ring, either on the velum 
itself or immediately outside it at the bases of the tentacles, so 
that the stimuli which they receive are easily transmitted to the 



III.] HTDBOHSDUSAE. 55 

nerre Ting. Two mam kinds of sense organs are frequently found, 
wliich may be ronghly called eyfes and ears ; never, hovever, both 
kinds in one Medosa. Tbe 'eyes' are little colouied patches of skin ; 
some of the cells of which end in clear rods while others se^ete a 
colonred snbatauce or ingment Both pigment and loda are neces- 
sary if there is to be Tision, though we do not undeistand why. The 
ears are little pits in the base of the velum; they may be open oi 
their edges may come together, so that the ectoderm lining them is 
entirely abut off from the outer skin. In either case, some of the 




Fio. 2S. I. A. B7eof£f»iatof(IfJbfri Been from the tide, magalQcd. B. The 
Eaine ieen ttom in front. C. Isolated celli of the ume. From 0. & B. 

1. Lena. 3. Pigoient cells. S. Percipient cells. 
n. BmUaI section throngh tlie edge of the umbrella of Carmarina haitata 
■hoviog HDM-OTijan and velnm. 
1, Telam. 9. Jell?. S. Ciicnlir muscles of velum. 1. Upper nerve 
ring. 0. Lower nerve ring. G. Nemntocjsts. 7. Bstlial vessel 
ttmning into cironUr vessel, both lined by endoderm. 8. Contittuation 
of andoderm along ftbortl scrfooe. fi. SeiiM organ or tentacnlocj'at. 

10. Anditory nerve. 

ceQs forming the walla of the pits secrete particles of lime, others 
close to them develope delicate sense hnirs. The result is that 
Tilmriions in the vaiet, if they come with a certain frequen<^, will 
affect the heavy particles, and their vibrations in tarn will affect 
the sense hairs. There is another kind of ioformation, however, 
which organs like these give their possessor, and this is probably still 
more important to the floating Medosa, namely, information as to 
the position of the animal with regard to the vertical. In other 



66 



COELENTERATA. 



[chap. 



words, the Medusa learns from them whether it is moving upwards 
or downwards or sideways : for when the animal shifts its position, 
the heavy particles in the ear-sacs are shifted conformably and affect 
different sense cells. 

Through these different sense organs stimuli are continually 
pouring in from the external world. If the stimuli only affected 
the contractile cells nearest them irregular movements would result. 
The function of the nerve ring, as of all nervous systems, is to co- 
ordinate the stimuli, that is to collect and rearrange and rapidly 
distribute them to the whole animal so that a definite reaction of 
the whole contractile tissue results, not a series of local reactions 
interfering with one another. 

The Medusa is very voracious and rapidly increases in size. It 
feeds on the small organisms of all kinds, both plants and animals, 
which are found at the surface of the sea. After some time it com- 




— I 



Fio. 26. The ciliated larva or Pianola of a Hydromednsan, Clava tquamata. 

Magnified. From Allman. 

A <fe B. Swimming about in the sea. G. Coming to rest on a rock. 
D. Developing tentacles, oral cone and stolon. 1. Tentacles. 2. Oral 
cone. 3. Stolon. 

mences to give rise either to eggs or to spermatozoa, which usually 
develope in exactly the same way in which they developed in Hydra, 
Le., from the interstitial cells of the ectoderm. The accumulations 
of these cells, called gonads or generative organs, are borne 
either on the under side of the bell (3, Fig. 23), or on the sides of 
the manubrium, and it is a curious fact that those Medusae which 
have them in the former position usually possess ear-sacs, whereas 
when the gonad is situated on the oral cone, ear-sacs are never 
present, but eyes may be. The eggs and spermatozoa are both shed 



m.] HYDROMEDUSAE. 67 

out into the water and coalesce there, and the fertilized egg developes 
into a little oval larva, termed a Planula (Fig. 26), without 
tentacles or month, and covered all over with cilia. It consists at 
fiist of a hollow vesicle of ectoderm cells, later becoming filled with 
a solid plug of endoderm. This little creature swims about for a 
while and then attaches itself by one end to a stone or a piece of 
sea-weed. The attached end flattens out (C and D, Fig. 26), but 
the rest of the animal lengthens and a mouth and tentacles appear 
at the firee end and the endoderm becomes hollowed out, so that 
the creature takes the form of an unmistakeable hydra-like organism. 
It then begins to bud out a branch called a stolon which creeps along 
the substratum. From this other polyps will arise, each of which 
has only to bud in order to reproduce the colonial stock from which 
its parent, the Medusa, was separated. The free-swimming young 
or planulae furnish good examples of what is meant by the term 
larva. This name is given to the young form of any animal when 
it is very different to the fully-grown animal and leads a free life. 
We have thus learnt that a Medusa gives rise to an egg which 
develops into a Hydroid person, which after a time in turn buds off a 
Medusa; such an alternation of generations is very characteristic 

of a large number of Coelenterata. The Medusa re- 
of oenmtions. pwsGiits a soxual generation, the Hydroid an asexual 

generation, and inasmuch as the Medusoid is often 
only produced as a bud of the third or fourth order (i.e. is budded 
from a Hydroid person which has produced similarly from another 
Hydroid person), it will be seen that several asexual generations 
intervene between two sexual ones. One explanation of this life- 
history is that the Medusa is only a specially modified Hydroid, which 
has acquired the power of locomotion, in order to disperse the eggs 
over a large area, and thus avoid the overcrowding of a limited area 
with one species. The swimming bell and velum are contrivances 
to enable the bud which bears the eggs to move about. If, however, 
this explanation be adopted, it is a most remarkable fact that in 
many species the Medusae are very imperfectly developed and 
never become free. Such Medusae are usually more or less de- 
generate and are termed gonophores. Since the gonophore fails 
to fulfil the purpose for which we believe the Medusa to have been 
developed we must assume that conditions have now so far changed 
that the same wide scattering of the eggs is not now so necessary as 
formerly, possibly because the species in question are restricted to 
particular strips of the shore. Tubularia larynx found growing on 



-^ COELENTERATA. [CHAP. 

«^iftv^M '^ A ^Hxl t^xample of a fonn with degenerate Medusae, 
j(^MCt-%^««''-^M or (>Mi<i of forms with free Medusae. 

^v*V tV>>i^>^^i^^^ include a large number of families, most of 
^.V\A 4^v iiv|VN»tM>uted by small plant-like forms resembling the 
^v**^^ .^U'iC luoutioned, but there are several groups which show 
-*^v:Xnn( isvuliiiritieii and have been regarded by many zoologists as 
.^ sv Aiu>'^l muk with the order although they have probably been 
<^>i,A\v^( n\KUi ikfdinary Hydromedusae. Of these we may name 
^i^ V^N> t\««ohyuicHtu8ae, (ii) the Narcomedusae, (iii) the Siphonophora 
^uvt \\\) tht) llydrocorallinae. In the first two groups the eggs 
>^\v^ts'|K» fu^m Uie planula stage directly into Medusae, missing out 
uW kvv^tv\»i\l Htage completely. In both cases also the sense-organs 
MW »|KH>i4^11y modified tentacles which are suspended like minute clubs 
vv'iAUv) tht) ixlge of the belL In the Narcomedusae these clubs are 
iVsH)|v t)\|Kkd6(i and the wide baggy stomach occupies the whole under- 
>uvUvve i»f the umbrella, whereas in the Trachymedusae the sensory 
4ub2i are enclosed in pits (Fig. 25) and the stomach is small and 
4Vu)^iuitHl from the umbrella by a stalk traversed by the radial canals. 
'i\\s imiue Trachymedusae (Gr. rpaxys, rough) is derived firom the 
car\Huatitauco that the umbrella is stifiened by numerous ribs of 
vuuUult)nu cells and the edge has a thick rim of ectoderm. The 
luuiuberd uf the third group are stocks consisting both of medusoid and 
liy droiil persons which are not attached to any support but which freely 
ttwiui or iioat in the sea. Some of the medusoid persons known as 
u t) 1 e a I y c e s have taken on the function of locomotor organs and 
b) tlioir rapid pulsations not only drive themselves through the sea, 
but draw after them the rest of the stock much as an engine draws 
a train of carriages. A few forms, however, like the Portuguese 
Man-of-war, Physalia, have no nectocalyces and float passively about. 
The popular name of this genus is derived firom the shape of the huge 
air-containing float firom which the persons of the colony are sus- 
pended. It has been plausibly suggested that the Siphonophora 
have been derived from planulae which attached themselves to the 
surface-film of the water instead of to a solid support The surface- 
film in consequence of its physical properties acts like an elastic 
membrane, and in artificial cultures it can often be seen that some 
planulae do attach themselves to this, and in consequence perish. 
But if by fikvourable variations, such as a tendency to cupping of 
the base and an inclusion of air-bubbles in the cavity, the stock 
were enabled to remain suspended, then it would be placed in a very 
favourable position for getting food, and thus it has been suggested 



III.] ACTINOZOA. 59 

the simply floating SiphoDophora have been evolved from Hydro- 
medusae. If this view be taken, the three chief divisions of Siphono- 
phora represent three successive stages in the adaptation of the group 
to a pelagic life. Thus the Physaliidae simply float, the Physo- 
phoridae float and swim by nectocalyces, whilst the Calycophoridae 
have lost the float and trust entirely to their powerful nectocalyces. 
The Siphonophora are remarkable for the varieties of person 
which compose their colonies. As varieties of the hydroid person 
may be named the palpons or tactile persons devoid of a mouth, 
but showing their equal rank with the nutritive person by the 
possession of similar tentacles. To the category of medusoid persons 
belong not only the nectocalyces but the bracts — transparent 
sheath-like structures sometimes present, which shelter groups of 
persons. This extreme variety of persons is foreshadowed in the 
ordinary Hydromedusae. Hydractinia for instance, which grows at 
the mouth of whelk shells inhabited by hermit crabs, has palpons 
amongst its hydroid persons, but in no case is such extreme diversity 
attained as among the Siphonophora. The Hydrocorallinse hydroids 
form large colonies and are divisible into nutritive polyps or gastro- 
zooids and tentacle-like polyps, the dactylozooids. The skeleton 
is massive and they form encrusting growths. The medusoid 
persons attain varying degrees of perfection. 

The Sea-Anemones are representatives of a second division of 
the Coelenterata, which show a decidedly more com- 
plicated structure than the animals just considered 
Unfortunately it is very difficult to obtain the ordinary sea- 
anemones in a sufficiently expanded condition to make out their 
structure, since when irritated they contract so much as to 
throw their internal structures into great confusion. Another 
animal belonging to the same group is the 'colonial' species 
Alcyanium digitatum, sometimes called ''Dead men's fingers." 
It is comparatively easy to paralyse the members of the colony 
or polyps by adding cocaine, or some similar reagent, to the water 
in which the colony is living (Fig. 27). If then an expanded polyp 
be cut off and examined with a lens, we shall be able to make out 
most of its structure. We notice to begin with that there is a 
single circle of eight tentacles, each of which has a double row of 
short branches, so that it looks like a miniature feather; within the 
circle of tentacles there is, however, no trace of an oral cone ; there 
is instead a flat disc, slightly sunken in the centre, where we find 
the slit-Iike mouth. If we look in at the lower cut end of the 




Ventra! 

n diffitatam below the 
level of tha oeeophagns x a^ut* 120. from Hickson. 
. CoelenteroD. 2. Meseuteiy with free edge. 8. Ectoderm. i. Meao- 
gloea or jelly. S. Endoderm, 6. Husdee in mewntery. 

DoitAl 




Ventral 
Fia. 29. Transvarse section through a polj'p of Alcj/ouiuvt digitatum, thcongh 

the region of the oesophagna x about 120. From Elckgon. 
I. Cavity of oeeophagna, 3. BiphonoglTph. 3. Ectoderm. t. Meso- 
gloea or jcily. S. Bndoderm. 6. Mnsclea in meienteriea. 7< Inter* 
'--0 catily. 



ft <»• 



*.. 



r/iKiXSTKRATA. [CHAP. 

i. 

tH^w' ts>nTuti. rt- snri A polyp shows us several other 

*"' -^^^ IV; i«« J^M we have to deal with the same 

^♦^wwtii. ^ ^^ ^^ ^^ Hydra, skin (or ectoderm) and 

*""^" ^ I .. » /f,- w.i'viucaiX Between them, however, there is 

- ''"""jKr-.^ ^«««rJ« an exceedingly fine membrane in 

" " 1 «j.,,. iT**i;.'» ii:okened, formed the substance of the 

. *) V«*«r«h '^*** .'^^^>' ^^ fairly thick in the minute sea- 

^r f»v«niii'.'^. Aud here contains cells which have 

. , , h-r^ :rjv ATtiHlerm. Some of these cells have the 

- sp.^rJo. «-^'***'J' "^^8 ^f ^°^®> termed spicules. These 

,^» «siin^it where the pol3rp merges into the general 

- 1», -r.:%»r *> that they form a kind of stiflF protecting 

♦•i **«*' ^'^^ ^^® polyp and over the surface of the 

^., ^M^^ «ii«> polyps rise. In the oi^an-pipe coral, 

♦I, ^s•%v*i.•v'» in the lower parts of the polyps are so 

^,.-^^ *^»*5 thoy form a set of parallel tubes, suggesting 

., . - - sx<wi; only the upper part of the polyp, where the 

t,- ..♦ « '^ ^^'5 oli>sely aggregated, being capable of movement. 

I , ^-s^vviA aIh»vo of the colony as distinct from the polyps, 

. ^, ,.„ .^i tho word demands some justification. When we 

."•w 't^ *t-'^ <l*o Ilydromedusae, we used the word colony in 

. ^ 10 ^lu»lo mass of the polyps which cohered together, 

.t;,t kkI AriMOU by the growth of one original polyp. Now 

.... ..:».«* aiul its allies, budding does not take place in quite 

jL,. aj luHuiuT in which it occurs in Hydra and its allies. 

, . X ■• ^*Ak^ IH'lyp growing directly out of another, the coelen- 

^. . .iio i^u'tuit Monds out a tube lined only by endoderm. This 

V ^«''>'^»# |itishing the ectoderm before it ; but, as between the 

,^:^t«ui JkwA ciuloderm there is a thick jelly interposed, the endo- 

^^.^^«w but>o oan bniuch without the ectoderm becoming indented. 

^;V<v^ ^^^^ ^^^^^^ ^^nds of these tubes reach the surfSekce, there fresh 

y^\i\i ^vo ilfvelopod, mesenteries and oesophagus making their 

^^x\uaikoo. Something like these tubes does, in fact, occur 

•.M^'Oi^^i tlydrouuHliisao: a complete colony is found to consist of 

, uumkior (if upright branches ending in polyps but connected at 

*vu lia.">nsi I'.v tuluM called stolons which creep along the sea Hoor : 

uSo uuiluilonual tiiboM of Alcyonium may be compared to these 

lU^Kiusi. tlhi great liitVonMioo being that in their case, owing to the 

tUu'iiiitti-^ i>f tltt^ J^^IIy. the ectiHlorm is stretched uniformly over a 

iuuttji uf tubed, iuNteail of each tube having its own ectodermal 

OiiViuiug ud in tlie llydnmiedusao. 



m.] ACTINOZOA. 63 

Still examining a section of the poljrp the next point we notice 
is the structure of the mesenteries. These end in a free edge 
below which is much thickened and folded, and since it stands out 
in contrast with the rest of the mesentery as if it were an inde- 
pendent structure it has been called a mesenteric filament 
(Figs. 28 and 30). The cells composing six of these filaments are 
very tall and secrete a juice which digests the prey : the remaining 
two filaments are composed of ciliated cells of moderate height 
which maintain a constant outward current of water. The surface 
of the mesenteries is covered by cells which become very much 
folded so as to produce a marked projection from the face of the 
mesentery. The cells of the folded area are all produced into 
vertical muscle-tails so that together they give rise to one of the 
powerful longitudinal muscles (Figs. 28 and 29), by which the 
sudden retraction of the polyp is brought about. The slow 
expansion is efiected by the reaction of the elastic jelly or mesogloea 
and perhaps also by the pressure exerted on the fluid contained in 
the body by a layer of circular muscles developed as outgrowths 
firom the endoderm cells of the intermesenteric chambers. 

A second difierence is found in the position of the eggs and 
sperm cells. These are developed from the endoderm on the face 
of the mesenteries, very low down in the base of the polyp and 
nearer the free edge than the longitudinal muscle. The eggs when 
ripe are cast out into the coelenteron and so out by the mouth, 
though in many species they come in contact with the male cells 
whilst in the coelenteron of the parent. 

The gullet has at one side a deep indentation or groove which 
is lined by powerful cilia (2, Fig. 29). The groove is termed a 
siphonoglyph (Gr. o-t^wv, a tube; y\v<l>ta, to hollow out) and its 
cilia keep up an inward current of water whilst the rest of the 
gullet is choked with prey, and so fresh supplies of water chaiged 
with oxygen are brought in contact with the lining of the coelenteron 
and enable it to respire. The two mesenteries with which the lower 
end is connected are called the directive mesenteries, they are 
situated opposite to the two ciliated mesenteric filaments. By the 
cooperation of the latter with the siphonoglyph complete circulation 
of the water in the coelenteron is maintained. The ectoderm of 
course gets its oxygen directly from the surrounding water. 

The ordinary sea-anemones or Zoantharia differ from Alcyo- 
nium in very many points. The tentacles, hollow as before, are 
never feather-like but always perfectly simple and round, and there 



iM COELEMTERATA. [CHAP. 

is. iMuitlly a lai^ namber of them amtoged in several coDcentric 
>,-uvW The mesenteries also are unmeronB, and extend invards 
Iv ditfwvut lengths, bo that we cui distiDgnish primary mesea- 
turie* wluch join the gullet from secondary ones which do not. 
ttuitk primaiy and secondary are usually ananged in pairs, but 
thoM is much variety and all that can be universally asserted 
is that they never exhibit exactly the arrangement shown in 
Alcyonari^ A very common arrangement is to have six pairs 
<.it' primary mesenteries and two siphonogtyphs, one at each end. 
S^uoules are never developed and in the ordinary anemones of 
our coasts there is no skeleton whatever. These commoner forms 
sometimes, though rarely, bud, but there is another large class 
of anemones which do form colonies, the bads occasionally arising 
as in the Hydromedusae from the body of the parent directly. 
These colonial anemones form the hard stony masses called coral 
(Pig. 30). If we look at a piece of coral we can see in it cups 




Fio. 80. Semi-diogranimatio view of half » limple CoTal, ptirtly after G. C- 
Boaraa. On the right tids the tuaues ue Tepreaeated aa trangpareDt to 
ahow tha ammgemeut of the theca and septa ; on ijie left side a 



I. Tentacle. 2. Month. S. Oeaophagtu. 4. MeMntei;. 5. Heaen. 
teric filamanta, free edge of mesentorj. 6. Eotoderm. 7. Endoderm 
e. Basal plate. 9. Theca. 10. Columella. 11. Beptmu. 

with partitions radiating inwatds, the whole reminding one of the 
structure of a sea-anemone: and it was a natural mistake to- 
suppose, as the earlier naturalists did, that the hard skeleton was 



III.] CORAL. 65 

formed inside the body of the poljrps, the partitions representing 
the mesenteries. Of course it is difficult to imagine how the animal 
could move if it had all that mass of stone inside it. How the 
corallum or stony skeleton is formed is a matter of dispute. It 
is certainly situated outside the ectoderm, but whether it is secreted 
by the ectoderm as a kind of sweat which hardens, or whether the 
ectoderm cells are calcified and thrown off, or the ammonium car- 
bonate, secreted by all animals, precipitates the calcium carbonate 
of the sea water and so forms the skeleton, is not finally decided. 
At any rate a calcareous cup is formed in which the polyp sits and 
the partitions of the cup indent the base of the animal, pushing 
before them folds of the body wall, which project into the coelen- 
teron between mesenteries, so that the action of the longitudinal 
muscles is not interfered with. 

Under the name Cored the skeletons of quite a number of 

^^ different kinds of Coelenterata were included besides 

Zoantharia. 

The so-called Millepore Corals belong to the first division of the 
Coelenterata, the Hydrozoa, for Millepora itself gives ofi* quite 
typical Anthomedusae and the other genera have gonophores. 

The Hydrocorallinae are really distinguished by the fact that the 
perisarc only which covers the basal stolons is thick and calcareous. 
After a while the stolons enclosed in the skeleton die, but fresh 
stolons are thrown out at higher levels, so the skeleton grows in 
thickness. The hydroid persons are of two kinds, nutritive persons, 
gastrozooids, short and with wide mouths, and tactile persons, 
dactylozooids, which surround each gastrozooid in a circle and 
which are long and mouthless. Both kinds have short rudimentary 
tentacles looking like knobs. 

The so-called Organ-pipe coral is, as has been already explained, 
an Alcyonarian in which the spicules cohere. Various fossil so- 
called corals, e.g., Syringopora, belong to the same category. The 
red Neapolitan coral of which ornaments are made is also an 
Alcyonarian, the spicules of which are of a bright pink or red 
colour and cohere to form a rod in the axis of the colony. In some 
spots off the coast of Australia the Alcyonaria with coherent spicules^ 
are so numerous that they form reefs. 

Coral-forming anemones are found all over the world, — one 
genus, Caryophyllia, being actually found at low spring tides on 
the south-west coasts of England : but it is only in the tropics that 
those species are found which keep on buddiug and growing with 

S. i&M. 5 



66 COELEKTERATA. 

Bnfficieut persistence to build up the great teefa which form the 
fsmoua coral islands of warmer seas. Of course as soon m the 
reef is built up to the surface the polyps cease to grow, and then 
the breakers soon pile up broken off pieces ia eufficieot qoancity W 
raise the reef above the tide-marks. 

A third group of the Coelenterata is constituted by the Acale^^ 
phae {Gr. dia\>j4i>i, & nettle). These animals ai^^ 
the Inrger and better known jelly-fish. They are U> 
some extent intermediate in charai-ter between the Hydrosoa and 
the Actiunzoa. Like the latter their genital cells are developed 
from endoderm, and in the larval condition there are meaenieries. 
but they do not possess a etomodaeuin. 

A common British species, Aurelia aurita, is in snmmer often 
cast by thousands on the southern shores of Great Britain. Viewed 
from the outside it very much resembles the modusoid persons of 
the Hydromedusae. Like them it possesses a awimming bell with 
a circle of tentacles at the margin. There is aiso a prumiueut oral 
cone or manubrium. This however does not bear real tentacles, hut 
the four corners of the rectangular mouth are drawn out into long 
frilled lips (3, Fig. 31), along the inner sides of which are open 
grooves leading into the gullet. Perhaps the most marked difference 
is that the reproductive organs are here, as in the anemones, 
swellings of the stomach lining: the eggs and spermatozoa are shed 
into the coelenteron and escape by the moutL The generative 
oigans have the shape of four semicircular ridges, and along the 
inner side of each of these there is a row of filaments composed of 
cells somewhat similar to the ceils on the edges of the mesenteries 
in the anemones (11, Fig, 31). Nothing like these gastral filar- 
meats, as they are called, are found in the Hydromedusae. There 
is, further, no velum in the Acalephae, and there is also no nerve 
ring. Sense oi^aus however of an exceedingly Interesting kind are 
present. 

In Aurelia, for instance, there are eight minute tentacles which 
stick out from the edge of the bell and are covered by special little 
hoodlike outgrowths of the same (9, Fig. 31). Each of these 
tentacles contains a hollow outgrowth from the circular canal lined 
like it by endoderm. The endoderm cells at the tip secrete a mass 
of calcareous particles : the skin cells at the base of the tentacle 
have produced nervous fibrillae from their bases and so the tentacle, 
as it is caused to sway in one direction or another by the weight of 
its heavy end, affects now some of the nerve fibrillae and now 



lU.] ACALEPHAB. 67 

others, uid so produces the same effect as the stones in the ear 
Mc of a medusoid, though the coustniction of the Acalephan organ 
is quite different In the TiachTinednsae and Nsrcomedusae, how- 
ever, sense tentacles dmilar to those of the Acalephae are found. 
Here the edges of the hood often join so as to form a sac enclosing 

^ihe organ, whence the name tentaculocj'st (9, Fig. 25). 

" It has been proved experimentally that the ordinary stimuli 
which cause the rhythmical pulses of the bell proceed &om these 




Fto. 31. Aurelia aurita, Somevhat reduced. 

1. HoDth. 3. Circnmoral prooesgee. 3. Tentadea on the edge of the 
□mbrdla. 4. One of tbs branching pemtdinl camilB. There are foar of 
theie, and foot timilar interradial canaU; the perradial conala coireapond 
to th« primu? ttomaoh pouches of tha Hydra-tubB, the interradial 
mltematA with these. 6. Ooe of the uubranched odradiftl ennftle. 

8. The circnlar oannl. 9. Mareinal lappeta hiding tentBoulocyata. 

11. Quti&l fllunentB, just outside theae are the genital ridges. 

tentacuhx^sts, so that they act like minute brains. How the 
co-ordination of the sUmuU proceeding from the eight centres is 
brought about we do not know, but it b probably due to the 
presence of a very thin diffuse sheet of nerve fibiillae on the under 
surface of the bell. 

6—2 



68 COELEKTEBATA. [CHAP. 

It has been mendoiied aboTG that tiiie reprodactive organs are 
swellings of the endoderm. The ceutial space or "stomach" is a 
wide sac occupying the centre part of the bell and not, as in the 
Hydromediisae, confined to a large extent to the oral cone. In 
Aurelia this space is prodaced into four lobes, and in the floor of 
each lobe is one of the reprodactiye organs. From the edges of the 
stomach a number of branching canals lead oat into the circular 
canal (4, Fig. 31), all these tnbes being, as it were, burrows in a 
continuous sheet of eudoderm cells, which stretches out to the edge 
of the disc and really represents a part of the coelenteron, the 
cavity of which has been obliterated. It thus corresponds exactly 
to the endodermal lamella of the Hydromedusae. 

When the %gs fall out of the mouth they are caught in little 
pockets and there develope into little Planulae. These, as usual. 




Fis. S2. StrobUizatioQ ot AureUa awrita. From Bftn. 
A, Hydra-tuba on stolon which ii meepiag on a Laminam. The atoloD is 
fonning new bads at 1 and 2. B. I^tet stage or Sanihistoma k 1. The 
strobil^ation has began. G. Strobilization foitber adraiicedxS. 

D. Free swimming Ephyra stage x T'5, se«D from below. E. The same 
seen in pcoGle x T6. 

become free and swim about, and finally each fixes itself and 
developes into a little polyp, called a Hydra-tuba, not unlike a 
Hydra in appearance (A, Fig. 83), but there are nevertheless 
important points of difference. l%as there is no oral cone but a 
flat oral disc in the centre of which the mouth opens into the 
coelenteron. The latter has four ridges projeotiog into it, the 



iil] ctenophoba. 69 

lower edges of which are free while the upper ones are joined to the 
gullet These ridges being produced by the folding of the endoderm 
layer they are double and contain between their two limbs a space 
filled with jelly. Into this space a prolongation of the ectoderm of 
the mouth disc grows down so as to form a '' septal funnel." The 
cells composing the septal funnel secrete longitudinal muscular 
fibrils, and thus four powerful septal muscles are formed which 
senre to shorten the Hydra-tuba. The hydroid persons of the 
Hydromedusae have also longitudinal muscles but these are dis- 
posed in a uniform sheet round the polyps in question and belong 
to the ectoderm cells forming the sides. During a large part of the 
year the Hydra-tuba multiplies by budding, just as a Hydra does, 
but at certain seasons it undergoes a very remarkable change (B and 
C, Fig. 32). The oral disc flattens out very much and its edges 
become drawn out into lobes, the tentacles at the same time drop- 
ping off. A short oral cone is developed from the centre of the disc, 
the mesenteries become perforated and finally the whole flattened-out 
top of the Hydra-tuba breaks off and swims away. This is known 
as an Ephyra larva (D and £, Fig. 32). It leads a free life and 
gradually develops into a large jelly-fish. But long before the 
primary oral disc has become free, the part of the Hydra-tuba next 
below has been growing out so as to produce a similar disc. This 
process, called Strobilization (Gr. a-TpoPiXos, a whorl), is repeated 
until the Hydra-tuba resembles a pile of saucers, in which state it is 
called aScyphistoma (Gr. <rKv<t>os, a saucer). 

We can get some idea as to how this extraordinary development 
may have arisen on the following hypothesis: — The original Aca- 
lephan wad probably an organism like an anemone with a wide top 
and narrow base. In this top the generative organs were developed, 
and when the eggs became ripe it broke off and wandered away in 
order to disperse the species. The lower part of the polyp regene- 
rated the head, exactly as a Hydra can do if the head with its ring of 
tentacles be cut off. Later this process of renewal became hurried 
on until it commenced before the separation of the head was 
complete and thus we have the Scyphistoma stage. It is a strong 
support of this theory that there exists a large coral-forming 
anemone, Fungia, in which there is a flat top and a stalk, and the 
flat top periodically falls off and is renewed. 

A third great division of the Coelenterata is constituted by the 

animals called Gten ophora (Gr. ktci?, ktcvo?, a comb). 

These are widely different from both Hydromedusae 
or Actinozoa, even including Acalephae. They never bud and with 



■ ^ COELCKTERATA. [CHAP. 

,jM;nai jisMptioa htm bo thread cells. In i^Me of these 
.ML^.>oi. »«>^*^<^ * •dhesive cells coveited with a secreticm by 
<.«Ml'A >M» -Wa«m w the prey. They ue often npped off in the 
■».*•*"*■-"* ■''* P"**' "^^^y *" ^"^^ provided with an eLutac tail 
4t.'Cv^<Miii. -1^ 'JM i^^ which pnlls in the object to which they have 
,^;kw>VM t*^ Ctooophon are free Bwinuning but their locomotion 
> NUtKiuiM iwc by the agency of muscular bands bat by eight 
<^^ .-^ 'iiiJti whigh na like meridians of longitude over the 




Fiq. SS. Bemiphora fbimoia. After Chun. Side riev. 
1. Moalh leading inla itonuch. 3. Abonl pcda with leiue orgtn. 

8. FDDoeL 4. PudeastriB canal mnninK back toiraids otal poU- 

5. One of tfaa eight band] of fuced cilia. 6. One of the eight euub 
tanninR tovarda 5. 7. A tentat^ular poach. 6. A tentacle. 

0. OelatiDoni linua. 

generally oval body from the mouth to the opposite pole (&, 
Fig. 33). The cilia in each band are arranged in short transverse 
rows, and the cilia in each row are joined at the base and free at 



III.] CTENOPHORA. 71 

the tip. So each row has the form of a comb, and thus the name 
Ctenophora, comb-bearer, is seen to be appropriate. Further the 
principal sense organ is situated in the centre of the end of the 
animal opposite to the mouth at the spot where the bands of 
thickened ectoderm which carry the combs converge. These 
thickened ridges of cells are often termed " ribs." If we compare 
the animal to a globe, the end at which the mouth is may be 
called the oral pole, the opposite end the aboral pole (2, Fig. 38). 
The sense organ at the aboral pole is a plate of thickened ecto- 
derm the cells of which have developed nerve tails. Similar nerve 
tails are developed by the bands of ectoderm which carry the combs. 
The cells at the edge of the plate carry cilia fused with one another 
which arch over the plate and cover it like a tent. Inside is a 
calcareous ball supported on four curved bars, each made of con- 
joined cilia, borne by some of the inner cells. This ball acts as a 
balancing sense-organ. If the animal inclines to one side the ball 
will bear heavily on the support on that side, and stimulate thus 
the corresponding ribs, which will thus act more vigorously than the 
rest and tend to restore the vertical position. 

Like Actinozoa, Ctenophora have a well-marked stomodaeum, and 
the true coelenteron is represented by a series of branching canals, 
the central one being termed the funnel (3, Fig. 33). The funnel 
and stomodaeum are both flattened but in planes at right angles to 
one another. The funnel gives off (1) two canals, the so-called 
excretory canals, which open at the sides of the sense-organ; 
(2) two canals, paragastric, running back towards the mouth 
parallel with the stomodaeum (4, Fig. 33); (3) two canals running 
each to a branched tentacle, which can be retracted within a pouch 
(7, Fig. 33). This branched tentacle is covered with adhesive cells, 
there is one on each side of the animal Each tentacle canal gives 
off four branches (6, Fig. 33) which lead into the meridional canals 
running under the ribs, from the cells lining which both ova and 
spermatozoa are produced, Ctenophora being hermaphrodite. The 
commonest British form is Hormiphora plumosa, which sometimes 
appears in shoals in the seas washing the Atlantic coast of Britain 
on the one hand and America on the other. The Ctenophora are 
good examples of what are called pelagic organisms, that is to say, 
organisms which pass their whole life from the egg to the adult 
condition floating at or near the surface of the sea. Such organisms 
are the only ones which are found in mid-ocean. Nearer the shore 
the waters are filled by a profusion of other animals, but these turn 



72 COELENTEBATA. [CHAP. 

out on examination to be largely composed of forms which in some 
period of their existence are adherent to or creeping on the bottom. 
Other purely pelagic groups are the Siphonophora, the Trachy- 
medusae and the Narcomedusae. 

The Gtenophora contain many forms which difier widely in 
appearance from Hormiphora — for instance the Cestum venei-isy or 
Venus's girdle, a beautiful transparent ribbon-like creature, a foot 
or 80 in length and two or three inches wide. On close examination 
the reason of this diversity of shape is found to be that the Cteno* 
phora are not really radially symmetrical, but doubly bilaterally 
symmetrical That is to say, not only right and left sides are like 
one another but also the back and belly are alike, but at the same 
time different from the sides. The difference is slight in Hormi' 
phora but very strongly marked in Cestum veneris. 



Phylum COELENTERATA. 

The classification of the Goelenterata is as follows : — 

Class I. Hydrozoa. 

Goelenterata without mesenteries or gullet lined by ectoderm: 
genital cells derived from ectoderm. 

Order 1. Hydrida. 

Only hydroid persons present, not permanently attached 
but capable of locomotion : the buds become free. 

Order 2. Hydromedusae. 

Composite fixed colonies of hydroid persons from which 
medusoid persons are budded off. 

Suborder (1) Qjrnmoblastea. Perisarc confined to the 
base of the hydroids: medusoids have eyes and bear gonads on 
the manubrium. 

Suborder (2) Calyptoblastea. Perisarc expanded to form 
cups called hydrothecae, into which heads of hydroid persons 
can be retracted : medusoids have ears and bear gonads on 
under side of umbrella. 

Order 3. Narcomedusae. 

Only medusoid persons present. The manubrium poorly 
developed, the wide stomach occupying the under side of the 
belL The sense-organs are reduced tentacles projecting at the 
edge of the bell. 



III.] CLASSIFICATION. 73 

Order 4. Trach3rmedusae. 

Forms in which like the foregoing group only medusoid 
persons are present, but there is a long manubrium traversed by 
the radial canals, and the stomach is only at the bottom of it. 

Order 5. Siphonophora. 

Free-swimming colonies consisting of hydroid and medusoid 
persons in which the base is modified into a float or some of 
the medusoids are transformed into swimming organs, or both 
arrangements are combined. 

Order 6. Hydrocorallinae. 

Composite fixed coloDies of hydroid persons : medusoid 
persons budded off" in only one or two genera. 

Perisarc thick and calcareous, surrounding chiefly the stolons 
which are given off at various levels and form a thick mass. 

Class II. ACTINOZOA. 

Solitary or colonial Coelenterata with gullet lined by ectoderm : 
coelenteron provided with radiating mesenteries : genital cells de- 
rived from endoderm. 

Order 1. Alcyonaria. 

Eight mesenteries and eight fringed tentacles : spicules 
in the jelly. 

Order 2. Zoantharia. 

Mesenteries usually in pairs, either six pairs or some 
multiple of six : tentacles conical : no spicules but often an 
external calcareous skeleton formed by the ectoderm. 

Class III. AOALEPHAE 

Coelenterata with alternation of generations : mesenteries present 
in young, but later becoming absorbed : oral part breaks loose and 
becomes developed into a free-swimming organism externally resem- 
bling a medusoid : the stalk of the original polyp reproduces the 

lost parts. 

Class IV. Otenophora. 

Very widely different from the two preceding divisions : free- 
swimming animals with a sense organ and nervous disc of skin at 
the pole opposite the mouth: swim not by muscular contractions 
but by vibrations of eight longitudinal bands of cilia radiating from 
nervous disc, which bands consist of successive transverse rows of 
cilia, the cilia of each row fused at base so as to form a comb-like 
structure : only two tentacles, a gullet lined by ectoderm : stomach 
represented by a system of branching tubes. 



74 



CHAPTER IV. 
Phylum Porifera. 

'l^v >;roup of the sponges or Porif era occupies an almost isolated 

position in the animal kingdom. Sponges agree, it is 

.u\*rv!vuii, ^^®» ^^ Coelenterata in exhibiting cellular structure 

and having their protoplasm arranged in tissues ; and 
luilhor iu the fact that all the internal cavities of the body are in 
i oiuuiuuication with one another, so that both Coelenterata and 
t'mifom might be described as systems of branched tubes. A 
\ K»iK)r iuHpection however reveals the fact that the tissues of the 
l'urif<&nk are very diflferent from those of Coelenterata and originate 
iu a different way from the larva, so that the opinion is gaining 
^ivuuil that whereas most, if not all, of the higher groups of 
uuiiaals have descended from ancestors which had we seen them 
vkv ahould have classed as Coelenterata, Porifera on the other hand 
liAVO been independently derived from Protozoa. 

lu Coelenterata the colonies can be analysed into persons 
^lutnlusoids or hydroids) and stolons, and many of the Porifera show 
a like aggregation of persons. But in many forms it is impossible 
W t»uggest how many individuals are contained in the branch 
system of a single aggregate since all distinctness of individuals is 
Uwit. Further analysis shows that the apparent persons or units, 
even when most clearly demarcated, are of very varying morpho- 
logical value. 

The salient peculiarities of sponges will be best appreciated by 
, , a short description of one of the simplest types 

Lcucoftolenia. *^ iijr »«fi- 

known, a sponge called Leucasolenta, which is 
common on most clean rocky shores. 

In this animal we can recognise a foundation consisting of a 
network of horizontal stolons, adherent to some foreign object, 
from which a number of upright tubes spring. Each upright tube 



CHAP. IV,] 



LEDCOSOLBKIA. 



75 



ends in a large opening, the OBonlnm (1, Fig. 34), which can be 
eloeed if the animal be irritated and which in LeuamUma is 
partly closed by a perforated membrane. This opening, which at 
first sight recatls the moatli of Hydra, is really need for a qnite 
different purpose. It is an efferent opening (Lat. ^ero, to 
carry ont) and from it the water which has passed through the 
animal is expelled. Water enters the internal cavity through a 
multitude of very fine pores in the walls of the tube (Fig, 85, and 
1, Fig. 37): it is the universal presence of these pores which gives 
the name Porif era to the grou' 




FlO. M. Tiew of ft bnneh of I^tueetoUnta ep., flhowiDg the sieve-like mem. 
brane which Btretchea MToag the oBoalDm, The lower part of the sponge 
■bowi spioolee only x 10. From Uinohm. 

1. Biere-like membrane. 



The wall of the tube is made up of two layers, but we must 
guard ourselves against rashly comparing these with the layers of 
the body waU of Hydra, and hence it is better to avoid the names 
ectoderm and endoderm and adopt the terms dermal and gaatral 
layers. 

The dermal layer consists of fiat cells which cover the external 
surface and extend for a short distance inside the osculnm, and of 
cells termed omoebocytes from the resemblance of their movements 
to those of an AmoAa ; the whole of the rest of the tube and the 
stolons are lined by a tissue consisting of peculiar cells called 
eboanocytes (Gr. -jpai/vi, a funnel; kvto;, a hollow vessel), or 



76 



POBIFERA. 



[chap. 



collar cells, which alone constitute the gastral layer (3, Fig. 35 
and Fig. 36). Each of these is cylindrical and provided with a 
funnel-shaped transparent rim called the collar, turned towards 
the cavity of the tube. The collars of adjacent cells are not 




Fio. 85. Yertioal section throngh an osonlnm with sieT^-like membiana, and 
a tube of Leueowlenia sp. Highly magnified. From Minchin. 

1. Sieve-like membrane. 2. Outer layer. 3. FlageUated or collar cells 
(choanocytes). The pointer shoold have been continued to indicate the 
cells lining 5. 4. Spicules. 5. Internal cavity 



normally in contact, and the outer part of the cell bodies are widely 
separate, so that here the distinctness of the elements of a cellular 
tissue is carried to an extreme. From the centre of each collar 
a long flagellum arises, and it is by the action of these flagella 



IT.] LEDCOSOLENIA. 77 

thftt water is drawn in throngh the pores. The sponge lives on 
the organisms carried in bf the current ; these appear to be carried 
within the collars b7 the miuute whirlpools produced by the 
individual flagella: they adhere to the collars and are swallowed 
by them and digested by the cells. It will be seen that the collar is 
a real Uving structure, not a caticular tube, such as the hydrotheca 
of a calyptoblaatic hydroid, and this is further illustrated by the 
&ct that it is withdrawn by the collar-cell under certain conditions. 
The water after being exhausted of its food is expelled through 
the osculum, carrying with it all excreta. 




1. NdoIbhc 3. Tkenole. 6. Opeoing into the inner space of the Bponge. 

The outer layer of the body-wall consists, in the ordinary con- 
dition of the sponge, of fattened cells. These however, especially 
in the region of the osculum, have the power of changing their 
shape so aa to become shorter and thicker ; in a word they can 
contract, although they show no trace of the fibrillae found in all 
muscle and in the muscle tails of the contractile cells of Goelen- 
terata. The contraction is slow, not quick, as in true muscle. 

It has been proved that the pores are formed by specially large 
cells, the porocytee, which extend from the outer layer and push 
aside the choanocytes, and then become hollowed out. 

Between the two layers is Found a certain amount of secretion 
which may be termed jelly, in which in many sponges a large 



78 PORIFERA. [chap. 

namber of cells is found. These form a portion of the dermal layer, 
aod are, for the most part, amoeboid. Some of these probably act 
as camerB of food and possibly of excreta from one layer to the 
other. Others at first very similar give rise to ova and sperma- 
tozoa. A third class called scleioblasts — derived in iMtcotolmia 
from the fiat cells which cover the surface, but not bo derived in all 
sponges — secrete the rods which form the skeleton and which are 
termed spicules (Fig. 34, and 6, Fig. 37). In Leucosolmia these 
ore calcareous and hare three rays, more or less in one plane — a 




Fia. 37. Seotion of a portion of Orantia txttuartiealata. Hlglilj minified. 

From Dendj. 

1. Opeoings of the inhal&nt oaoalB. 3. Inhalant oanal. 3. Opentng* of 

inhaUnt canals into flagellated chamber (proBop;)e). i. Flagellated 

chamber. G. Flagellated or eoUar-oells (ohoanoajtes). 6. Bpiculea. 

7. Eihalant opening of flagellated chamber. 

shape technically named triradiate. One limb is usually directed 
parallel with the long axis of the tube, and often bears a fourth ray 
or spine making a qoadriradiate spicule. The spicules although 
remaining unconnected aie numerous enough to form a loose mesh- 
work. 

The most important points in which the higher sponges differ 

from Leucosolenia are the folding of the outer and 
a^jElf* inner layer, the restriction of the choanocytes to 

small portions of the latter, and the differentiation of 
the body into distinct regions. 



IV.] COMPLEX SPONQEa 79 

A common sponge on the British coast, Sycon (Grantia) com- 
pressutn, will illustrate the first step in this complication. This 
animal has the form of a series of flattened thick- walled upright 
tubes. The layer lining the central cavity consists of flattened ceUs, 
but from this cavity pouches lined by choanocytes extend out into 
the substance of the wall. These flagellated chambers, as 
they are often called, communicate with the exterior by a series of 
inhalant canals which intervene between them and into which 
the pores open (Fig. 37). 

When a sponge becomes still more complicated the central 
cavity becomes broken up into a series of branching canals, which 
are termed ezhalant or efferent, and the ciliated chambers become 
small and rounded (Fig. 36), each often connected only by a single 
opening or prosopyle (Gr. Trpdo-o), forwards ; irvAiy, a gate) with the 
afferent system of canals. Numerous oscula are found in one 
sponge mass, so that no pretence of discriminating the individual 
can be made. 

A still further complication arises from the presence of sub- 
dermal spaces. These are wide cavities immediately beneath the 
surCeu^e of the sponge into which the inhalant pores open and from 
which the inhalant canals take their origin. In this way a rind or 
crust of the sponge can be separated from a deeper part containing 
the flagellated chambers. Sponges are by some of the best authori- 
ties divided into two main classes, viz. : 

Glass I. Caloabea. 

This group includes all those sponges with calcareous spicules 
and comparatively large flagellated chambers. 
It is divided into two main orders : 

Order 1. Homocoela. 

Sponges consisting of tubes lined throughout with choano- 
cytes. 

Order 2. Heterocoela. 

Sponges in which the choanocytes are restricted to special 
chambers — which may be cylindrical as in Grantia or spherical 
as in Leucandra. 

Class II. Hexaotineludae. 

Sponges in which the skeleton consists of a coherent network of 
siliceous spicules each consisting of three axes placed at right angles 



80 PORIFERA. [chap. 

to one another. The flagellated chambers are large and cylindrical 
but are separated from the central space by a system of canals. 
The central space may be deep and narrow and covered with a plate 
pierced by numerous oscula, or short, open and shallow. 

These sponges inhabit as a rule very deep water and most species 
are provided with a tuft of long needle-like spicules which root them 
in the soft mud which forms the bottom of the sea at these depths. 

Class III. Demospongiab. 

These sponges derive their name from the fact that their spicules, 
which are always siliceous, are arranged in cords so as to form a 
network traversing the substance of the sponge. The spicules com- 
posing these cords are nearly always cemented together by a homy 
elastic material called spongin. The flagellated chambers are always 
extremely small and there is never a central chamber. Besides the 
skeletal spicules, as those composing the cords are called, smaller 
ones called flesh spicules are scattered singly in the intervals of the 
network. 

There are several exceptional genera in which interesting modi- 
fications occur. 

Oscarella is totally devoid of any skeleton and has the appear- 
ance of a whitish yellow scum on the rocks to which it adheres. 
Empongia possess spongin cords but no spicules in them, and for 
this reason it can be employed for domestic purposes. 

Two fresh-water species, namely, Spongilla lacustris with a bush 
like appearance and Ephydatia fluviatilis with an encrusting form, 
are often found growing on the side of canals and on the timbers of 
river-locks or weirs in Great Britain. The two species are bright 
green when they grow in the light, but they are pale flesh-colour 
when they grow in the shade. In Canada similar species adhere to 
stones in the river St Lawrence. 



The larvae of sponges are best understood by a short description 
of the simplest form, viz. the larva of Oscarella, This 
has the form of a simple hollow sphere of ciliated cells 
like the planula of Coelenterata in its first stage. The cells at one 
pole lose their cilia, become pigmented and granular and then the 
larva fixes itself by the ciliated pole. The whole animal flattens and 
the granular cells extend over the ciliated cells which become tucked 



IV.] LARVA OF PORIFERA, 81 

into the interior and there arranged as an inner lining to a cavity. 
The flageUated chambers of the adult arise as small pocket-shaped 
outgrowths irom this cavity and the osculum is a later perforation. 
The ciliated cells are eventually restricted to these chambers where 
they form the choanocytes and all the rest of the sponge is formed 
from the granular cells. Other larvae differ from that of Oscarella 
in the early multiplication of granular cells which form a solid mass 
at one end of the larva and often — indeed generally — of such extent 
as to project into the interior. 

To compensate for greater dead-weight, so to speak, the ciliated 
layer — the locomotor organ of the larva — becomes extended to 
surround the granular material, so that we are presented with the 
remarkable phenomenon of the internal layer of the larva burst- 
ing forth and becoming the outer layer of the adult. This is the 
case in the larva of Leucosolenia. In the larvae of other calcareous 
sponges, the ciliated cells at first surround the granular cells, but 
the latter are afterwards exposed and the larva in this form has 
been called an amphiblastula. In the case of most of the Demo- 
spongiae the ciliated cells nearly, but not quite, surround the 
granular cells, and these last often contain a number of spicules 
ready formed in a central bundle which are scattered in all direc- 
tions when the sponge flattens on fixation. Comparing the develop- 
ment of a sponge with that of the planula of a Coelenterate we see 
that in the first the ciliated cells form the internal layer, in the 
second the external layer of the adult ; in the first the animal fixes 
itself by the pole at which the invagination or intucking of the cells 
destined to form the inner layer takes place — in the Coelenterata at 
the opposite pole — so that if Coelenterata and Porifera had an 
ancestor in common it could only have been an animal like the 
organism Vohxxt, consisting of a single sphere of cells — in a word it 
would have been classed were it living now as a Protozoon. 

The study of the development of sponges like Si/con shows that 
at first, after the metamorphosis, the sponge has the form of Leuco- 
solenia by a simple cylinder lined by choanocytes. The flagellated 
chambers arise as horizontal cylindrical branches on the primitive 
chamber and soon become so numerous that their walls come into 
contact and the afferent or inhalant canals are simply the crevices 
left between these chambers. As the chambers develop, flattened 
cells come inwards from the pores and displace the choanocytea 
except in the chambers. 

S. (bU. 6 



82 POBIFERA. [chap. IV. 

Porifera then may be defined as animals consisting of branch- 
systems of tubes, the principal openings of which are exhalant, 
whereas the inhalant openings are minute perforations of the walls. 
The wall consists of two layers ; some cells of the inner layer have 
the form of choanocytes, whilst the skeleton consists of siliceous or 
calcareous needles formed by cells of the outer layer which wander 
in, or of spongin. There are never any thread-cells or differentiated 
muscle or well-marked nerve-cells, nor any such organs as tentacles. 



83 



CHAPTER V. 
Introduction to the Coelomata, 

The last two groups of animals studied, although very different 
from one another in most respects, yet agree in this that the 
groundwork of their structure is a set of tubes branched in dif- 
ferent ways and with walls of varying thickness but consisting 
always of two layers with an intervening jelly. It would not be 
straining the truth to assert that in the Coelenterata and the 
Porifera we find but two tissues, an outer more or less differentiated 
skin— the ectoderm with the underlying jelly,— and an inner layer 
mainly digestive in function. 

The phyla which are next to be considered, and which may be 
^ grouped together under the name Coelomata, differ 

from the two mentioned above in the possession of 
an important organ termed the coelom (Gr. KoiXcofia, a thing 
hollowed out). This is often described as a space intervening 
between the ectoderm and endoderm, and the term coelomic 
cavity or body cavity has been used to describe it. In spite of 
the etymological difficulty we propose in the following pages to deal 
with this organ under the term coelom, and its cavity under the 
term coelomic cavity. In reality it consists of one or more pairs of 
sacs with perfectly defined walls lying at the sides of the endo- 
dermic tube. In the adult these sacs join each other above and 
below the endoderm, and the adjacent walls entirely or partly break 
down, and thus one continuous cavity results. The wall of the 
coelom and the tissues derived from it are known as the mesoderm. 
To describe the coelom as a split or space is to describe it negatively : 
with as much justice the endodermic tube might be described as a 
split. In each case the real object of consideration is the wall. 

If we leave out of account cases in which the facts of develop- 
ment have not been fuUy elucidated and confine our attention to 

6—2 



8i INTRODUCTIOK TO THE COKlXMiT^ [CHAP, 

those instances where the wliole histoiy of the coelora has been 
ezbaustively worked out, we find that this important oi^an arises 
in one of two ways, either (1) by the fonnation of poaches of 
the endodennic tube, which become nipped off (Pig. 38) ; or (3) by 
the budding of two large cells, formed themselves by budding from 
the endoderm (Fig. 39), these cells subsequently growing rapidly 
and dividing so as to form bands, the so-called germinal bands, 
which subsequently become hollowed out. These initial cells are 
termed pole-cells. 



The ectoderm U deeply shaded, tbe mesodenn U lightlj shaded, the endederm 
— ftlimentarj cansl sad Dotocboid — ia anshaded. A. sbowa the origin 
of the paired meaodennal ponchea from the areheDteroii ; the cavity — 
coelomio — of tba focmer ia atill id aommnnication with the cavity of the 
alimentary oanaJ. The notochotd is ariaing in the middle line from the 
stidodetm, and the tabular nerroui system above it ii already separated 
from tbe ecCodeim. B. shows the meBodennalpouabeaooinplel^Tahut off; 
the; each enclose a cavity, the coelom, and each ooneiBtg of an outer wall 
next the ectoderm, the 'somatopleur,' and an inner wall next the endodenn, 
the ' Bplancbnopleur.' C. ehowe the mesodermal ponches extending ventrally 
beneath the notochord, now completely eeparated from the wall of the 
alimentary canal and also round the alimentary canal. The ooelomia 
space ia laigsr, and the splancbnopleor is beginnijiB to form mosole-ceUs. 

A sharp controveiay haa raged round the question which of 
these two processes gives us the best representation of what 
occurred in the evolution of Coelomata &om simpler Coelenterata- 
like ancestors. 

If however we recall the fact that in the Actinozoa the endo- 
dermic sac has the form of a series of pouches ranged round a 
central cavity, and that the walls of these pouches become coa- 
verted into musclea and generative cells exactly as in the case of 
the coelom, and that pores exist in many cases placing the cavity of 
these pouches in communication with the outside world, we shall 



T.] lNTftODnCTIO» TO THE COELOHATA. 85 

be indaced to oonclade that the coelom woe probably evolved ftom 
l&teral pouches of the gnt and that the mesodenn is therefore 
dnived from the primittTe endoderm. Where pole-cells ocout the 
cavity of the alimentary canal ia small in proportion to the thickness 
of its wall, and the pole-cell might be looked on as a solid pouch. 




Fut. S9. T«o sUgea in the earl; deidopment of a oomiDoa fresh-water mollaBO, 
Planorbii, to show the origin of the mesoderm cells x S20. From BabL 

The wtoderm cells eie deeplj shaded, the endoderm oella ate tuiBhaded. 
k. Totmg stage in which the endoderm has not hegnn to be invagmaled ; 
it is a lateral optical section. B. Older stage, optical section seen in 
front Tiev; the endoderm oella are invaginating, and the two mesoderm 
oellfl are aeett on each side. 1. Mesoderm or pole-ceils ; in B, each has 
budded ofl another mesoderm cell. 

In most Coelomata the mesoderm forms by far the greatest 
portion of the body, and it may be roughly stated that the 
mesoderm gives rise in the fully developed animal to " the muscles, 
the bones, the connective tissue, both arteries and veins, capillaries 
and lymphatics, with their appropriate epithelium," and to the ex- 
cretory and generative oi^ns. 

In the Porifera and in the mora complex Coelenterata, where 
the thin structnreleBS lamella of Hydra has swollen into a bul^ 
mass of jelly such as we find in a Medusa, cells begin to wander 
into it &om the ectoderm, and thus a kind of tissue is formed to 
which the name mesoderm has unfortunately been applied. We 
■ee that its origin and nature are quite different, although both in 
Bpongea and Coelenterata it may give rise to the skeleton, and in 



86 INTRODUCTION TO THE COELOMATA. [CHAP. 

this respect at least it may be regarded as a forerunner of the tme 
mesoderms. 

The endoderm, after the separation from it of the mesoderm, 
forms the lining epithelium of the digestive tube and of its ap- 
pendages, which in the higher Vertebrata are the organs known as 
lungs, liver, pancreas, and urinary bladder. The basis of the 
skeleton of Vertebrata, the gelatinous rod called the notochord, 
also arises from it. 

In Coelenterata there is one opening only to the digestive sac, 
which is used both as a mouth to take in food and 

Can^L*'***^ *8 ^^ ^^^^ ^ cast out indigestible material. In the 

overwhelming majority of the Coelomata there is a 
second opening, the anus, the mouth being restricted to the 
function of taking in food. As a consequence the digestive sac 
takes the form of a tube open at both ends, and is known in the 
higher groups as the alimentary canaL Often this endodermic 
tube is much longer than the ectoderm, and so in order to be 
contained in the bounds of the ectoderm it has to be bent and 
looped on itself. 

Hound both mouth and anus the ectoderm is generally tucked 
in, so as to form as it were vestibules to the true alimentary canaL 
Of these, the ectodermic vestibule to the mouth is called the 
stomodaeum, and is found amongst the Actinozoa, where it has 
been already described. The proctodaeum is the term applied to 
the vestibule around the anus. Although not strictly parts of the 
alimentary canal stomodaeum and proctodaeum are usually included 
in descriptions of it, and indeed in some cases — Crustacea — ^they 
form by far the greatest portion of the apparent digestive tube. 

The internal anatomy of the lower animals was first studied by 
physicians and others who were primarily interested in human 
anatomy. An unfortunate consequence is that a large number of 
names are used in the description of simpler animals which are 
based on fanciful resemblances between their organs and those of 
man. As a consequence many of these names are quite misleading. 
To give some instances: the word stomach in the Lobster denotes 
part of the stomodaeum, in the Vertebrata it signifies part of the 
endodermic tube. The pharynx of an earthworm is the stomo- 
daeum, in a fish it includes both stomodaeum and the first part of 
the endodermic tube. The term liver has also been much abused. 

The names taken from the anatomy of the higher animals 
which are customarily used in the description of the alimentary 



v.] SHAPE OF THE BODY. 87 

canal are as follows: mouth- or buccal-cavity, pharynx, 
oesophagus, stomach or crop, gizzard, intestine, and 
rectum. They are applied generally to parts of it succeeding 
one another in the order above given. The significance of these 
will be explained in each case : it would perhaps be more logical to 
sweep away altogether these and a host of similar terms employed 
to designate other parts of the body, but so deeply are they 
engrained in zoological literature that such a course would render 
unintelligible most anatomical descriptions of species that we 
possess. 

Besides forming the outer layer of the skin or epidermis of 
the animal and the stomodaeum and proctodaeum, the ectoderm 
gives rise to the brain and nervous system and to the essential cells 
of the sensory organs. 

In the group Coelenterata a general circular outline of the body 
Qcner.. predomiBatea. the principal external organs being 

Shape of the arranged like the spokes of a wheel around the 
^°^^' mouth as a centre. Such an arrangement is spoken 

of as a radial symmetry, and is in all probability connected with 
the fixed life so common amongst Coelenterates, a condition of 
affairs which renders it advantageous to have organs developed so 
as, to use a familiar phrase, to be on the look-out all round. 

The firee-swimming Medusae, it is true, move always with the 
apex of the bell directed forwards, but as has been pointed out (see 
p. 55) they are to be regarded as specially modified hydroids, and 
they have not modified the radial symmetry so deeply impressed on 
them by the habits of ancestral Coelenterata. 

In the higher groups of animals there is usually one fixed part 
of the body which moves first when the animal changes its position ; 
and when such a portion is definitely set aside to move first we can 
distinguish the front end of an animal from the hind end. It is 
usual to term the former the head or anterior end and the latter 
the posterior end, and when an organ such as a man's arm lies 
nearer the anterior end (head) than another, as for instance his leg, 
we say the former is anterior to the former, and that the latter, 
i.e. the leg, is posterior to the former, i.e. the arm. In describing 
the parts of an appendage such as the arm it is usual to speak of 
the part nearest the base of attachment as proximal and the part 
further away as distaL 

Corresponding with the appearance of the head as distinct 
from the rest of the body, which is in contradistinction termed the 



88 INTRODUCTION TO THE COELOMATA. [CHAP. V. 

trunk, we find a difference arising between the surfaces of the 
body. With few exceptions animals, whether creeping, swimming, 
flying or walking, keep the same surface turned towards the eartL 
This lower surface is termed the ventral (Lat. venter, belly), 
whilst the upper surface or back is termed the dorsal (Lat. 
dorsum, back). As a rule the difference in their relationship to 
their surroundings induces a difference in the aspect of these two 
surfaces, and it is seldom difficult to determine which is the ventral 
and which the dorsal surface of the body. As a general rule the 
ventral surface of an animal is much lighter in colour than the 
dorsal In most Coelomata the nervous system is for the most part 
ventral and the chief blood-vessel dorsal, but in the Vertebrata the 
reverse is the case. In both the alimentary canal lies between the 
chief blood-vessel and the main nervous system. 

The two sides of an animal, the right and left, are however 
exposed to ]uuch the same conditions and as a rule resemble each 
other very closely. When this is the case an animal is termed 
bilaterally symmetrical, and it may then be divided in one 
plane — and in one plane only — in such a way that each half forms a 
reflected image of the other, such as we should see if we held half 
the animal up to a looking-glass. This bilateral symmetry may 
extend to all the internal organs, as it does in an earthworm or 
a crayfish, or it may be confined to the external features and 
some of the internal organs only, as in insects or in most verte- 
brates, where the coiling of the alimentary canal, etc.', interferes 
with the bilateral symmetry of the internal organs. 

In some animals, and these are for the most part such as move 
sluggishly or have become permanently attached to some sub- 
stratum and do not move at all, this bilateral symmetry has been 
lost and the two sides do not resemble one another. Such animals 
are called asymmetrical The Snail is a familiar example of 
such asymmetry. Amongst the Echinodermata (Star-fishes and 
Sea-urchins) this asymmetry is replaced by a radial symmetry. 



89 



CHAPTER VL 
Phylum Annelida. 

The name Annelida (Lat. annulus, a ring) means ringed, and 
refers to the fact that the bodies of the creatures grouped under 
this name are built up of a series of parts more or less resembling 
each other placed one behind another. This division of the 
body into more or less similar parts is called segmentation ; 
each part is called a segment (or somite), and the animal is 
said to be segmented. Like the symmetry, the segmentation 
may be merely external or may affect both the exterior and 

a greater or less number of the internal organs. 

Sometimes, however, as in the case of the longer 
half of an earthworm's body, the segmentation affects all the 
organs, and the likeness of one segment to another is so great that 
it would be impossible to say what part of the body any given 
isolated segment was taken from. More often, however, one or 
another of the organs of the body differs in shape or size in 
successive segments, and this is the case with the internal organs of 
the first twenty segments of the earthworm's body, so that if these 
segments were all separated it would not be very difficult to place 
them together in their natural order. 

K we take an earthworm and kill it by placing it in alcohol for 

a few minutes and examine it carefully, we shall see 
Earthworm. that the body is composed of some 150 rings, each 
fcSurel! ^^ which corresponds with a segment The rings are 

separated from one another by slight grooves. At 
each end of the body there is an opening, the mouth (2, Fig. 40) 
in front and the anus (3, Fig, 40) behind. Besides these, two 
slit-like pores with rather swollen lips, situated on the under sur- 
face of the fifteenth segment (5, Fig. 40), may be seen. These 
are the pores through which spermatozoa are discharged, and 
are consequently known as the male genital openings. The 



iO 



ANNELIDA. 



[chap. 



uLhur opeaings into the body are minnte and 
roquire the aid of a lens to make them out 
Thure are paired openings on each segment, 
except the first three and the last, situated 
lutero-ventrally; these are the openings of the 
tubes known as nephridia (Gr. vc^piStov, a 
little kidney), which act as kidneys; in addi- 
tion to these a median dorsal pore opening into 
the body-cavity is situated in each groove behind 
the tenth segment (11, Fig. 44). The earthworm 
is hermaphrodite, that is, it contains both 
male and female organs in its body. Through 
two slit-like openings in the ventral surface of 
the fourteenth segment the eggs are discharged: 
these are called the female generative 
openings. Two pairs of pouches called 
spermathecae, which are reservoirs for sper- 
matozoa received from another worm (v. p. 107), 
open, one pair between the ninth and tenth, 
the other between the tenth and eleventh 
segments, all on the ventral surface. 

If a worm killed in alcohol be drawn through 
the fingers a certain roughness may be felt 
along the sides and lower surface. This rough- 
ness is due to the presence of a number of small 
bristles, called chaetae (6r. x^^"^* hair), which 
project from the body (7, Fig. 40, and Fig. 43). 
Each segment bears eight of these chaetae ar- 
ranged in four pairs, one pair on each side being 
lateral and the other nearer the ventral middle 
line. Itisbymeansof the chaetae that the worm 
crawls about; since by protruding the chaetae 
and implanting them in the soil a fixed point 
is obtained from which the anterior end of the 



Fio. 40. Latero-ventral yiew of Lumhricut terrestrit, 
slightly smaller thftn life-size. From Hatschek 
and Cori. 

1. Prostomiom. 2. Mouth. 8. Anus. 4. Opening 
of oviduct. 5. Opening of vas deferens. 6. Geni- 
tal chaetae. 7. Lateral and ventral pairs of chaetae. 

zv. xuui. and xxxvn. are the 15th, 82nd, and 87th seg- 
ments. The 82nd to the 87th form the CliteUnm. 



«v^-^ 






3 
Fio. 40. 



VI.J 



LUMBBICD8. 



body can be pushed forward and to which the hinder end of the 
body can be drawn up. 

The colour and thickness of the body from the thirty-second to 
the thirty-seventh segment differ in adult worms from those of the 
ttegments which lie before and behind this band. This la due to 
the presence in this region of certain ectodermal glands whose secre- 
tion forms tlie cocoons in whicli the eggs are laid. This region of 
the body is called the Clitellnm (xxxn — xxxvn, Fig. 40), 

The surface of the body of an earthworm is glistenipg and 
somewhat slippery. This is dtio to the cuticle, which is a thin 
membrane secreted by the ectoderm cells of the stin ; if a dead 
earthworm be soaked in water for a few hours the cuticle can be 
easily stripped off the body. In the crayfish, insects, etc.. a similar 
cuticle is present, but it is much harder and forms an external 
protective skeleton; even in the earthworm, where it is soft, it acts 
as a protection to the underlying cells, and its smooth surface 
enable-s the worm to creep into narrow holes without hindrance. 
The chaetae are simply large local thickenings of the cuticle: they 
protrude from pockets called chaeta-saca, each of which is a 
portion of the ectoderm tucked in. In the bottom of each sac is a 
specially large cell which rapidly secretes a column of cuticle and 
builds up the chaeta. 

If we cut through the skin of an earthworm we do not make 
our way into the cavity of the alimentary canal but 
Ai.t^y! '"'o ^^^ coelomic cavity, in which not only the 

alimentary canal but the blood-vessels, kidneys, re- 
productive organs, apparently lie. The relation of the alimentary 
canal to the body-cavity might be roughly represented by introducing 
a piece of glass tubing loosely into an india-rubber pipe. The 
alimentary canal would be represented by the glass tube and the 
body-cavity by the space between the glass and the india-rubber. 

The coelomic cavity is a very important feature in all the higher 
animals; it may become very reduced, as in the Arthropods, but it 
is always present, although it may not at first sight be easy to 
recognise. There are, however, certain features which it always 
presents : (i) it always possesses a proper wall, never being a mere 
slit intervening between various organs, and it is always surrounded 
by mesoderm ; (ii) its walls give rise to the cells which form the 
reproductive cells; (iii) the kidneys, which are primitively tubes 
with open ends, open into it. 

There is no difficulty in recoguiaing the body-cavity of an 



92 ANNELIDA. [CUAP. 

earthworm. It is comparatively spacious and is divided by a 
iteries of partitions into a number of chambers which correspond in 
number and position with the segments of the body. These par- 
titions or septa (8, Fig. 41) are pierced by the alimentary canal, 
the nervous system and blood-vessels ; they are not complete but 
are provided with holes so that the space in one segment is not shut 
off from the spaces in the neighbouring segments. Fundamentally 
in Annelida the body-cavity consists of a series of pairs of sacs 
interposed between the skin (ectoderm) and the gut- wall (endoderm) ; 
tliere is in the embryo a pair in each segment, but the walls of 
these come into contact above and below the alimentary canal and 
then break down, so that the cavities of the right and left sacs 
open into one another and a ring-shaped space results. This space 
has distinct inner and outer walls of its own which are known col- 
lectively as the peritoneum (Gr. ircpt, around ; toVos, a stretched 
band). 

The septa are formed where the adjacent walls of two sacs, 
placed one behind the other, come in contact. If this description 
of the relations has been followed it will be seen that the coelom 
in the adult consists of a series of ring-shaped spaces, and that 
the alimentary canal is not truly in the coelom nor, it may be 
added, is tlie nervous system or the blood-system. 

Like all similar spaces in animals the body-cavity of an earth- 
worm contains a fluid, and in this fluid certain cells float which 
change their shape as an Amoeba does, and hence are called 
amoebocytes. As a rule the body-cavity is completely shut off 
from the outside world, but in the earthworm it opens to the 
exterior by means of the dorsal pores (11, Fig. 44), and at times 
the fluid which it contains escapes through these holes and pours 
over the cuticle. This fluid has a certain poisonous action on 
bacteria, and helps to keep the outside of the body clean and 
free from parasites. Somewhat similar pores leading from the 
exterior to the body-cavity are found in certain fishes. 

The first segment is divided into two parts, a lobed lip or prosto- 
mium (1, Fig. 43), overhanging the somewhat crescent-shaped mouth, 
and a peristomium containing the mouth which leads into an oral 
cavity extending through three segments (Fig. 41). There are no 
teeth in this cavity and the food is probably sucked in by the action 
of the muscular stomodaeum, called the pharynx, which succeeds it 
and reaches back to the sixth or seventh s^pnent. This is followed 
by the true endodermic tube. The first part is narrow and is 



VI.] LUMBRICTJS. 

calleil the oeaophaguB; it reaches to the twelfth segment and has ' 
Uiree pairs of lateral pouches developed on its walla. These pouches 
secrete calcareous particles, and hence are tenaed calclferous 
glands. The oesophagus dilates behind into a thin-walled sac, 
called the crop, situated in the region of segmenta thirteen to six- 
teen, and this is separated by a groove from a thick-walled sac, 
with bard, homy walls, termed the gizzard, which extends to about 
the tnentieth segment. The exact segment in which the above- 
mentionedparta of the alimentary 
canal lie varies with the amount 
of food they contain, the septa 
wiiich are pierced by them being 
Btretched forward or backward 
according to their state of fulness 
or empdneas. 

Behind the twentieth segment 
the intestine stretches without 
change to the anus. It is a thin- 
walled tube, supported by the 
septa between each segment and 
sweUing out slightly in each 
segment, so that it presents an 
outline like a string of beads. 
A deep fold, called the typhlo- 
sole (Gr. rv<j,\4i. blind; Ti^h^y, 
a gutter), runs along the upper 
surface of the intestine, project- 
ing into its cavity. Its presence 
causes the wall of the intestine to 
be pushed in, and thus the inter- 
nal absorbing portion b increased 
(7, Fig. 42). The intestine is 
covered everywhere by a number 
of cells of a yellow colour. 
These form the inner wall of the 
uoelomic sac and are actively en- 
gaged in excretion. 

The exact part that each of 
the above-mentioned parts of the alimentary canal plays in digestion 
ia not thoroughly understood. The pharynx helps to take food in 
by a tinckiug action which ia caused by the contraction of the 




Fid. 41. Anterior view of the InteniHl 
organs ot in Earthworm, Lumhricui 
IflTMtrii. filigbtl; magniSed. From 
Bataobeh and Cori. 

1. Central gaaglion orbraia. 3. Mus- 
calu pharjQi. S. OcEophngQB. 
i. Crop. S. UaBoulut gizinrd. 
6. lukitine. 7. Nepbridk (the 
rebrence lines ionul quil«icftohtbe 
nepbri<lja|, S. 8«pta. B. Dond 
blood-Tcsael. 10. Htaita. 11. 
Spermathecne. 13. Tesionlae 

■eminaleB. 

TbaBomaii UgorMicfeitotheuumbet 
of Ibe •cguionls. 



miiscleB ruDniDg from it to the body-wall, resulting in an enlarge- 
meut of tlie cavity of the pharynx eo that food may pass in by 
stmoBpheric pressure. The food posses down the oesophagus, 
being propelled by a seriea of contractions of the walls of the 
alimentut7 cana! which push it along; on its passage it is mixed 
with the secretions of the calciferous glands. The crop serves 
as a resting-place in which the food accumulates before passing into 
the gizzard. The hard, horny walls of the last-named chamber help 
to grind up the food and render it lit for the action of the juices 
which digest it. The process of digestion, or the rendering of the 
food soluble, probably takes place in the intestine, and through the 
walls of this portion of the alimentary canal the soluble products of 
digestion soak, and are taken all over the body by the blood-vessels 
and probably also to some extent by the fiuid in the coelom. 




Eatadiek and Cori, 

Septa. 2. Nephridia. 3, Ventral aarve-cord. 4. Sub-neural 

blood-vessel, 6. Nephroalonies, internal fuunel-shaped openings of 

Iiepliriilia, 6. Inleatine, 7. Tj^ihlosole. e._ Circular bloodv 
9. Yenlial or Bub-ialeatinal blood-TeBsel. 



10. Dorsal blood-vesseL 



The series of contractions which squeeze the food onwards 
towards the anus are known as peristalsis; they constitute the 
sole moveaieuts of which tlie alimentary canal is capable and are 
carried out by muscles developed from the cells of the inner wall 
of the coelom, which pass round the canal like a series of nngt 
or tight india-rubber hands. 



LUMBRICC3. 

The earthworm oats earth and manages to find sufEcient 
nouriflhment for its needs in the small amount of organic matter, 
broken-domi debris of leases, etc., which is contained in the earth. 
The actual minerals of the earth are not digested but are passed 
oat of the body in the form of those coiled and thread-like castings 
which are so commonly seen on a lawn in the early morning. 
Earthn-onna also eat fallen leaves and to this end they drag the 
leaf-atalka into their burrows, and on autumn mornings it is a 
common sight to see lawns studded with the stalks of horse- 
chestnut leaves or the needles of fir trees, the stalks having been 
dragged a little way into the burrows by the wonna. The burrows 
that they make admit both air and rain to the deeper layers of the 
soil, and Ihe earth which they swallow in their burrows is brought 
to the GorfacQ and spread about in the form of castings. Thia is 
carried on to such an extent that the whole surface of the soil soon 
becomes covered by a layer of earth brought up from below. It is 
thus clear that the earthworm is of great use as an agricultural 
agent. 

All the blood-vessels are for the moat part merely crevices 
between the coelomic wall on the one haad and the ectoderm and 
endoderm on the other. Those described are merely the larger 
channels in a continuous network of spaces. The contractile power 
which some, like the hearts, dorsal vessel, and sub-intestinal vessel, 
possess is due to the presence of a special wall of muscular cells 
derived from that part of the coelomic wall which lies next them. 

The earthworm is the first animal that we have studied posseas- 
ing a distinct and well-marked body-cavity; it is also the first in 
which we find a distinct blood-system. In the Coelenterata the 
cavity in which digestion is carried on permeates the body in all di- 
rections, and the soluble products of digestion are never far from the 
tissues or cells which may need them. But in the earthworms the 
Alimentary canal is a straight tube separated from a number of the 
other systems of organs by a space or coelomic cavity, and hence a 
Tascolar system is of great use in conveying the digested products 
to where they are most needed. Thus the blood serves to take up 
the nutriment from the intestine and distribute it to all the active 
cells in the body. The blood is also the medium by which the 
waste products resulting irom katabolism are collected and taken to 
the appropriate organs whose duty it i» to separate them from 
the blood and ca£t them out of the body of the animaL Amongst 



96 ANNELIDA, [CHAP. 

the products which do not contain nitrogen the most important is 
carbon dioxide, which is carried by the blood to the skin and got 
rid of through the ectoderm, at the same time as the oxygen 
needed for respiration is absorbed. 

The dark streak which runs along the body of the worm from 
head to tail in the middle line is caused by the dorsal blood- 
vessel (10, Fig. 42), in which the blood flows forward. A parallel 
sub-intestinal vessel in which the blood flows backwards under- 
lies the intestine, and a third but smaller vessel, the sub-neural, lies 
still more ventrally under the nerve-cord. The dorsal vessel receives 
blood from the yellow cells covering the intestine by two pairs of 
minute vessels in each segment, and anteriorly it breaks up into a 
network of small vessels which branch over the pharynx. But by 
far the larger x)art of the blood from this vessel passes into the sub- 
intestinal vessel by means of five pairs of loops, called hearts, 
situated in the seventh, eighth, ninth, tenth, and eleventh segments 
(10, Fig. 41). Each pair of these hearts encircles the oesophagus 
and contracts at regular intervals from above downwards. Their 
contractility has suggested the name heart. As they pass from 
the dorsal vessel into the sub-intestinal the effect of their con- 
tractions is to drive the blood which is passed forvrard on the dorsal 
side of the animal into the ventral system, whence it passes toward 
the tail. These contractile hearts thus take a large share in main- 
taining the circulation of the blood. The sub-intestinal vessel 
gives off a special vessel in each segment to the nephridia, and the 
blood which is purified in these organs is returned to the dorsal 
vessels by another series of vessels. The dorsal vessel and the 
sub-neural vessel are put into communication in each segment by 
two lateral vessels which lie on the outer wall of the coelom 
and which receive numerous small vessels from its substance. 

The earthworm breathes through its skin. The blood-system 
sends up into the skin innumerable minute vessels or capillaries 
which come so near the outer surface of the worm that the oxygen 
can pass in from the air into the blood. The name capillary (Lat. 
capillus, a hair) was suggested by a comparison of the exceedingly 
small calibre of these vessels with the diameter of a human hair. 

The blood is red, and the red colour is due to the same substance 
which colours our blood, haemoglobin, but there is this difference, 
that whereas in Vertebrates the haemoglobin is contained in certain 
cells which float in an almost colourless fluid, in the earthworm it 
is dissolved in the fluid itself. This substance has a strong attrac- 




VI.] LUMBBICUB. 

tion for oxfgen whicli it takes up &oin the air that comes into 
the neighbouiliood of the skiQ-capillaTies, formiDg a bright red 
compound called oxy-haemoglobin. This compound is unetable, 
and when the blood in its course round the body encounters a cell 
hungry for oxygen, the oxy-haemoglobin is decomposed : the reduced 
haemoglobin is purplish in colour. At the same time the cell gives 
up carbon dioxide to the blood. The rolatioQs of this gas in the 
blood are less understood than those of the oxygen, but like the 
latter it is in loose chemical union, though not with the haemo- 
globin. In Vertebrate animals the sodium of the blood provides 
the means of conveying the carbon dioxide to the respiratory oi^&ns. 
When the blood again approaches the skin carbon dio:[ide is 
got rid of, oxy-baemoglobin being again formed by fresh oxygen 
taken in. 

In the Vertehrata the excretion of the waste nitrogenous 
material is performed by a pair of compact organs, the kidneys. 
In the earthworm this function is carried out by the nephridia, 
which fundamentally reaemlile the tubules composing the kidney of 
Vertebrates, but are not compai'ted into a solid organ. They are 
distributed throughout the body, one pair being situated in each 
segment, except the last segment and the first three, which have 
no nephridia (7, Fig. 41, and 2, Fig. 42). Each nephridium is a 
minute tube, opening at one end on to the surface of the worm 
near the outer chaeta of the more ventral pair, and at the other end 
into the body-cavity. This inner opening or iiephrostome has 
cilia on its funnel-shaped rim, and these Hicker with an untiring 
movement The nephrostome does not lie in the same segment aa 
the rest of the tube but pierces the anterior septum, and projects 
into the cavity of the segment in front, somewhere near the sub- 
intestinal vessel Thus each segment contains a funnel-ahaped 
opening and a tube which opens externally, but they do not belong 
to the same nephridium, The tabe is. not straight but is coiled 
and lies aa a white glistening tangle close to the muscular body- 
wall. Each nephridium is to be regarded as a portion of the 
coelomic sac into which it opens internally. It is, bo to speak, a 
tail of this sac which projects backwards into the next one — not, of 
couree, pieR'iug it, but indenting, so to speak, its anterior wall. 

When we examine a nephridium through a microscope we see 
that the waCs of the tube are very richly supplied with minute 
blood vessels. The tube is really a cord of glandular cells placed 
end to end and traversed by a minute cavity. It is these cells 



i 



98 ANNELIDA. [CHAP. 

which take up the waste nitrogenous matter from the blood and 
convey it out of the body. The part of the nephridium nearest the 
external opening is swollen so as to form a bladder. The cavity 
is here intercellular instead of piercing the cells themselves, and 
surrounding it is a muscular wall by the contraction of which the 
contents are from time to time expelled. 

The blood thus takes digested food to the living cells all over 
the body and brings from them certain nitrogenous excreta to the 
nephridia, which cast them out of the body. But the nephridia 
also exert some action on the other great fluid of the body — the 
coelomic fluid — which bathes all the organs of the body. It 
has been mentioned above that the funnel-shaped ciliated openings 
of the nephridia open into the coelom, so that the fluid of this 
cavity can pass out of the body not only by the dorsal pores but by 
the tubular nephridia. This fluid has suspended it in numerous 
amoebocytes (v, p. 92), and these corpuscles act as scavengers, 
taking up into themselves any foreign bodies, such as bacteria, 
which have made their way into the coelom, and breaking them up. 

The yellow cells (7, Fig. 44), which surround the gut and 
form the inner wall of the coelom, are also actively engaged in 
extracting nitrogenous waste from the endoderm cells and the blood- 
vessels which pass near them. When the excreta have accumulated 
to a certain extent in a yellow cell it dies, and its remains fall 
out into the coelomic fluid, where they are eaten by the amoebocytes. 
These latter then wander to the nephridium and become pressed 
close against its wall, the cells of which extract the excreta from 
the amoebocytes and pass them into the cavity of the nephridial 
tube. The funnel of the nephridium is too small to admit the 
amoebocytes — it serves as a flushing apparatus, since its cilia draw 
in water from the coelom which is swept down the tube and carries 
the excreta into the terminal bladder whence they are from time to 
time expelled. 

It is probable that the yellow cells represent a primitive mode 
of excretion and that originally the whole coelomic wall undertook 
this function, the products escaping either by simple pores or by 
being taken up by amoebocytes which forced their way out through 
the skin, as in Echinodermata. The yellow cells and the nephridia 
are then to be regarded as portions of the coelom in which the 
power of storing up excreta is specially developed, and in this 
limitation of this power to a special area we have the first type of 
an excretory organ. A localized excretory organ requires some 



VI.] LCMBEICU8. 99 

means of bringing to it the prodnctg of katabolism of all portiotiB 
of the body — since poisonous excreta are produced by all living 
protopldsm— and this means is supplied in the earthworm by the 
blood-system and the nmoebocytes. 

The earthworm, although it lives in earth, has a clean, glistening 
look, and this is partly due to the fact that the coeloraic fluid 
is poured out from the doranJ pores (U, Fig. 44) aud keeps the 
skin moist and lubricated. This fluid is also antiseptic in its 
at.-tion and thus its presence prevents foreign organisms, Buch as 
bacteria, which swarm in the mould in which the worm lives, 
settling npon the skin and growing there. Numerous glandular 
calls belonging to the ectoderm also jinur forth n secretion through 
minute pores in the cuticle. 




t. ri. nt. :v. Tba Urst, geuoud, third, and tuiirth aegmentB. 

1. Tlie prostomium. 2. The wrebrni gBQKlia. 3, The oiroumornl oam- 
miasnre. 4. The 6rsl veDtral gnuglitin. 6. Tlie mouth. 6. The 
ptuuynx. 7. The donwl and ventral pair of cboetoe. 8. The tactile 
DiirTeH lo tlie pcaKtomium. !!. The anterior, middle and ptiflterior 

donftl uervea. 10. The auturiiit, middle and poaterior Tenlral neryes. 

If WB cut Open an earthworm by a median dorsal incision aud 
attentively examine the upper surface of the pharynx 
we shall find at its anterior end, tucked awiiy between 
it and the skin, two little whitish knobs lying close to 
one another. These are the cerebral or supra-pharyngcal 



The Ncrvoui 



100 ANNELIDA. [CHAP. 

ganglia (1, Fig. 41 ; 2, Fig. 43). At their outer ends the supra- 
pharyngeal ganglia pass into two cords (3, Fig. 43). If we now 
cut away the pharynx and remove the alimentary canal we can 
trace these two cords towards the ventral middle line where they 
unite and form the first sub-pharyngeal ganglion (4, Fig. 43) : 
from this a long white cord — the ventral nerve-cord — ^runs back to 
the extreme posterior end of the animal If we examine these 
structures with a lens we shall be able to see that the supra- 
pharyngeal ganglion gives off small nerves to the sensitive pro- 
stomium, and that the ventral nerve-cord swells out between each 
pair of septa, that is, in each segment, into a thicker portion which 
gives off both dorsally and ventrally and on each side three pairs of 
nerves to the surrounding parts. Each of these swellings is termed 
a ganglion (6r. yayyXiov, a knot^) (4, Fig. 43). 

The nervous system is made up of a number of cells termed 
neurons. These, as proved by a study of the development are 
ectoderm cells which have become pushed inwards from amongst 
the others. Each neuron consists of a body with a comparatively 
large nucleus difficult to stain. From the body in one direction is 
given off a tuft of root-like processes (which some suppose to be 
actual retractile pseudopodia) called receptive dendrites, by 
means of which stimuli are received into the cell In the opposite 
direction is given off a long straight process called an axon which 
may branch once or twice, the branches being called collaterals. 
The axon itself and its branches end finally in tufts of root-like 
processes which are in close contact but apparently not in continuity 
with either a muscle-fibre or the receptive dendrites of another 
neuron and are called terminal dendrites. 

Through the axon and its branches stimuli are transmitted to 
other neurons and to the muscles. 

A bundle of collateral branches of axons bending outwards to 
convey stimuli to a group of muscles is known as a motor peri- 
pheral nerve. 

The nervous system of an earthworm thus consists of two 
supra-pharyngeal ganglia situated in the third segment, a pair 
of connecting cords called commissures which form a ring 
round the pharynx, and a ventral cord which swells out into a 
ganglion in every segment behind the third. The ring round 
the mouth and the solid nature of the nervous system is common 

^ TdYY><top was ased by the old medical writers to indicate the swelling or 
'* knot" in a muscle caused by cramp. 



^n^ LUMBBICtJS. iUl 

to nearly all the Invertebrata, and in those which have a bilateral 
aymmetry and are segmented there are Bupra-pharyiigeal ganglia 
and a ventral nerve-cord bearing Begnientally repeated ganglia. 

The earthworm baa no specialized aense-orgnn^, it haa neither 
eyes to see, nor nose to smell, nor ears to hear with. Still, 
although it in apparently deaf, it is not devoid of the power of 
appreciating those stimuli which in us excite the sensation of sight 
or smell. A strong light suddenly turned on the anterior end of 
the body will cause the worm instantaneously to withdraw into its 
burrow, and worms readily recognise the presence of such favourite 
food as oniona and raw meat. Their sense of toui^h is well 
developed and they are very sensitive to vibrations ; for instance, 
a stamp of the foot on the ground wOl cause all those in a. 
certain radius to disappear into their burrows. It is further 
possible that earthworms |>oaaess other senses with which we are 
totally unaoiuaiuted. 

In each segment of the worm scattered here and there amongst 
the etrtoderm cells are a, number of aense-cells. Ea^h of these has 
a minute sense-hair which projects upwards througli a hole in the 
cuticle, and by meiuiB of this hair stimuli of various kinds are 
received by the outer world. The body of the cell is small — ^juat 
large enough to contain the nucleus — and from the base proceeds 
an axon which runs inwards and terminates inside the central 
nerve-cord in a brush of tenninal dendrites in close contact with 
tlie receptive dendrites of a neuron. In tliia way the neurons 
receive impressions from the outside world. A bundle of the axons 
of eenae-cells proceeding inwards is known as a sensory peripheral 
nerve. 

llie swelling called a ganglion is due to an aggregation of a 
number of the bodies of neurons, so that in this region the nerve- 
cord is broader than at other places, though everywhere some bodies 
can be seen in transverse section of the cord. 

The nervous system is one of the most important organs of the 
body. It governs and controls the action of every tissue and cell. 
It receives and registers impressions from the outside world and 
co-ordinates the movements and activities of every jmrt of the body. 
It further serves to put each organ and eatth part of each organ in 
communication with all the others, and thus this vast accumulation 
of tissues and cellm acts in an orderly way and towards a set 
«Dd. 

A transveree section of an earthworm, such as can be cut by a 




microtome &om a specimen embedded io paratHn wax, is most 
inatTucdve, in exhibiting the relation to one another of the various 
tissues which make up the body of the earthworm. The outermost 
boundary is cou3titut«d by the cuticle (I, Fig. -U), a hardened 
secretion poured out by the ectoderm (2, Fig. 44), The ectode rm 




. Onticle. 2. Eatodcrm or epidcimis. 3. Ciiculat mnncloa, 4. Doiaal 
nerve. S. l/ODgitudinal muaeleB. B. Smuatic cpitbelium. 

7. SplnDchnic epitbelium or ;elIow uellH. S. Eododena or epithalium 

liDing tbe mteatiDe. 9. Caelnm. 10. Nvphiidium cat iu suctioD. 

11. Dorukl pore. 12. Dorsal blood-vcsael lyiiit; along tbe typhloKile 

or groove in Ibe wall at intestine. 13. Suh-intcBtinal blood- vessel. 

U. Ventral nerve-cord. 15. Snb-neuml blood-vesHel. 16. Ventral nerve. 

The dorsal aad veattul net 



is composed of tall cylindrical celb, amongst which are isolated 
"goblet cells" — that is, cells with a round body situated beneath 
the level of the rest and with a long neck. The name is 
suggested by their shape. In the body of these cells mucus is 
secreted, which is poured forth through a hole in the cnticle 



opposite the end of the cell-neck and helps to keep the ii^urface of I 
the wonn moist. 

Beneath the ectoderm is a thin and hardly perceptible layer of ' 
jelly fonning a bed on which the ectoderm cells rest. This founda- 
tion in called the dermis, and is included inth the ectoderm in tha 
ordinary conception of the "skiQ." In contradistinction to the 
dermis the ectoderm is often spoken of as the epidermis {Gr. 
ftri. Upon). 

Beneath the dermis cornea a layer of circular musclea {3, Fig, 44), 
and beneath these again a much thicker layer of longitudinal 
muscles. The circular muscles consist of a few layers arranged to 
form rings round the section. The longitudinal muscles are arranged 
very regularly, and in the section they have the form of a series of 
feathers (5, Pig. 44), since the individual fibres appear arranged 
in oblique rows between which tongues of jelly extend, giving off 
Ut«ral branches on which the fibres rest. 

Both sete of muscles are composed of muscle-cells. These are 
long fibre-like stnictures pointed at both ends. Most of the proto- 
plasm is differentiated into fine fibrillae, which indicate (see p. 29) 
contractile power. In the centre of the cell is a patch of un- 
modified protoplasm with a nucleus. The whole cell may be 
compared to a myo-epithelial cell of fiffdni in whtcli the epithelial 
part has diminished in size and the tail increased. Nor is this a 
fanciful comparison, for the study of development teacher us that 
the cell is actually derived in tliis way from the originally »iimple 
cells of the wall of the coelomic sac or in the case of the circular 
muscles from an ectoderm cell. 

T)ie movements of the earthworm can be more easily under-fl 
stood when the arrangement of the muscles is known. Tho'l 
longitudinal muscles serve to shorten tha body, and as the coelomiG 
fluid, like water, is practically incompressible, the diameter of the 
animal must be increased, and thus the chaetae can be driven into 
the sides of tha burrow. On the other hand, the circular muscles 
dimiuigli the diameter of the coelom, and the contained duid being 
forced to move in a longitudinal direcrtion stretches the body out. 
Tlia holes in the septa equalize the pressure in the various segment* i 
by permitting the fluid to escape from one into the next. 

Mention has been made above of "jelly" as forming a support^ 
for the ectoderm and the longitudinal muscles. It forms also the ' 
main part of the substance of the septa. In the worm and higher 
animals generally jelly fulfilling this function is known as con- 



104 ANNEUDA. [chap. 

nective tissue. Its nature will be more fully dealt with in 
the section relating to Arthropoda. 

Within the longitudinal muscles there is a layer of cells called the 
somatic peritoneum (6, Fig. 44) which forms the immediate wall 
of the coelom. As the coelom in a segment of the worm has a ring- 
shaped form there is an inner as well as an outer wall of the coelom ; 
the former, since it closely invests the alimentaiy canal, is called the 
visceral or splanchnic (Lat. viscus; Gr. onrXayx^w, entrail) 
peritoneum (7, Fig. 44) — the latter the parietal or somatic 
(Lat. paries, an outer wall; 6r. awfia^ body) peritoneum. The 
parietal peritoneum is composed of flattened cells ; the visceral 
peritoneum, on the other hand, consists of large cubical cells, the 
yellow cells already described. 

Beneath the visceral peritoneum there is a thin layer of circular 
muscles, the splanchnic muscles derived from the peritoneal 
layer and forming the agency by which the peristalsis («. p. 94) of 
the gut is carried out. 

The endoderm (8, Fig. 44) consists of a single layer of long 
cylindrical cells bent in dorsally to form the typhlosole. Within 
the limbs of this fold the splanchnic peritoneum is very much 
thickened. 

The dorsal blood-vessel can be seen embedded in the yellow 
cells lying in the typhlosole (12, Fig. 44), whereas the ventral vessel 
is attached by a membrane to the ventral side of the intestine. 
This membrane is really a part of the partition which separated 
the two coelomic sacs which originally existed in the segment. 

The nerve-cord, apparently lying loosely in the coelom, is sur- 
rounded by a layer of cells similar to those forming the somatic 
peritoneum of which they once formed a part (14, Fig. 44). Hence 
the coelom has extended in a ring-shaped manner round the nerve- 
cord exactly as it has surrounded the gut. At the sides of and below 
the nerve-cord may be seen sections of vessels, the sub-neural and 
latero-neural vessels. The mass of the nerve-cord is made up of the 
sections of axons, whilst the nuclei of neurons can be seen forming 
a sheath on the outer border of the cord. The fibres are divided 
into two bundles by a septum of connective tissue. On the dorsal 
surface of the cord there are seen three apparent tubes, these are 
sections of the so-called "giant" fibres — colossal axons which are 
outgrowths of correspondingly large neurons. 

Sections of chaeta-sacs and nephridia may be seen in favourable 
sections. 



It bas been mentioned above that it is one of the cbaracteristica 
of the coelom that the cells lining it should produce 
OrB»n«." "* the reproductive cells. This does not mean that 
any cell lining the coelom can become an ovum or 
a spermatozoon, but that at certain spotti the cells fonnbgpart 
of the ooelomic wall turn into either female or male generative cells. 
Id the earthworm the paired ovaries (6, Fig. 45) are situated in 
the thirteenth segment and may be seen by cutting through the 
iutestiike about the region of the gixzard and gradually lifting it up 
from behind forwards ; when it is freed up to the twelfth segment 




I'krt of the veeiuuU seiuiuiUis U uul u 
t«ctto lUid the iaaec opening of the 
Froiu Hatiohek and Cori. 



'ay uu tlie Uii &iJe to expoae the 
ras dslerena. Slii;htly iniLgnified. 



1. Speruiatbeuoo. '2. Fannel-Hliaped iatemal opeaingB of the vas deferens, 
S, Anterior testis. 4. Vmiicalae seminutes. 5. Ovary altavhpil lo 

poateriar wall of aeptum Beporating xti and mi. S. Oviduct traversing 
aeptam separating xni and iiv. 7. Vas deferene. S. UUnda in the 
tkia. 9. Tentnil nerve. cord. 10. Suptum. 

The BoDian Qgures iadioaCe the number of the segmenta. 

the ovaries may be seen as minute white pear-shaped bodies lying 
one on each aide of the nerve-cord. They are attached by their 
broad end to the posterior wall of the septum separating segment 
twelve from segment thirteen, and they nre formed by the accii- 
malation and growth of some of the cells which cover this septum, 
that is, from cells lining this portion of the coelom. 

If one of the ovaries he removed and examined under a micro- 
scope it will be seen that many of the i^ells composing it are large, 
spherical and crowded with granule.^. The largest lie in the 
narrow end of the ovary which waves about in the coelomic duid. 



106 ANNELIDA. [CHAP. 

These cells are the full-grown eggs or ova and when ripe they drop 
off from the ovary into the coelom, but are probably at once taken 
up by the wide funnel-shaped openings of the oviducts, one of 
which is situated opposite each ovary. Like the nephridia, the two 
oviducts pierce a septum, the one between the thirteenth and the 
fourteenth segments. They are short tubes which open into the 
coelom by the above-mentioned funnel-shaped opening in the thir- 
teenth segment and to the exterior by a small pore just outside the 
inner pair of setae on the fourteenth (6, Fig. 45). They bear on 
their course a diverticulum or sac which is called the receptac- 
ulum ovorum, in which the ova collect until the earthworm 
is ready to make a cocoon to receive them. 

The male reproductive cells are formed in the testes, of which 
there are two pairs situated in a similar position to the ovaries but 
in the tenth and eleventh segments (3, Fig. 45). They are in many 
respects similar to the ovaries but are hand-shaped, the broad end 
of the hand being attached and the fingers free. Their ducts which 
convey away the spermatozoa are called the vasa deferentia(Lat. 
vas, vessel; deferens, carrying away). They have similar funnel- 
shaped openings to those of the oviducts and they traverse the 
septum behind the segment in which these openings lie, but they 
do not at once open to the exterior. The two ducts of each side 
unite in the twelfth segment, and the common duct thus formed 
runs back to open by a pore with swollen lips on the fifteenth seg- 
ment, the one behind that on which the oviducts open (7, Fig. 45). 

There is, however, one great difference between the male and 
female organs. Whereas the ovaries lie freely in the body-cavity 
and can be seen readily if the intestine be removed, each pair of 
testes and the corresponding inner funnel-shaped openings of the 
vasa deferentia are concealed by a certain sac or bag called the 
vesicula seminalis, and it is only by cutting away the wall 
of this sac that these structures come into view (4, Fig. 45). 
Each vesicula seminalis is a flat, oblong bag extending backwards 
from the front wall of the segment in which it lies and situated 
beneath the alimentary canal. The angles of the front vesicula 
seminalis are produced into two long pouches which project upwards 
at the sides of the alimentary canal, and are often called lateral 
vesiculae seminales, though they ought to be termed lateral horns of 
the anterior vesicula seminalis. A similar projection is produced 
from the hinder angles of the posterior vesicula seminalis, so that on 
opening a worm three pairs of greyish white sacs are seen at the 



VI.] LUMBRICUS. 107 

sides of the gut. The study of the way in which the vesicula 
seminalis is formed shows that the space it contains is really part 
of the coelom which has become cut off from the rest \>y the out- 
growth of folds frx>m the septa, so that, although at first sight the 
testes seem to differ from the ovaries and to be exceptions to the 
general rule that reproductive cells have their origin from the walls 
of the coelomic cavity, a closer examination shows that this apparent 
divergence is not a true one. 

Every earthworm has grown up from an egg which has been 
fertilized by a spermatozoon. As the earthworm is hermaphrodite, 
that is to say, contains both male and female organs, it might be 
thought that the spermatozoa of an individual would fertilize its 
own ova, but this is not the case. Cross fertilization or the 
fertilization of the ova of one individual by the spermatozoa of 
another is the rule in Nature, and the earthworm is no exception 
to the rule. The method by which the spermatozoa reach the ova 
is not clear in all its details, but it is something like this. The 
ceUs which are to form the spermatozoa break off from the testes 
and whilst lying in the fluid contents of the vesicula seminalis 
they divide and the products of the division or spermatozoa de- 
velope each a long vibratile tail by whose aid they swim actively 
about. Two earthworms then approach each other and the 
spermatozoa pass down the funnel-shaped opening and vasa defe- 
rentia of each and into the spermathecae of the other. The 
earthworms then separate, each carrying away the spermatozoa of 
the other. 

The spermathecae in which the earthworm stores up the 
spermatozoa received from another individual are pockets of the 
skin (1, Fig. 45). They belong, strictly speaking, to the female 
reproductive system. Seen from the interior of the animal, they 
appear as four small white spherical bodies, lying one pair near the 
hind end of segment nine, and the other pair near the hind end of 
segment ten, and each pair opens by a very short neck or duct on 
the grooves between segments nine and ten and ten and eleven, just 
inside the outer pair of chaetae. It is through these ducts that 
the spermatozoa from another worm enter. 

Earthworms lay their eggs in cocoons, which at one time were 
mistaken for the eggs themselves. These cocoons are usually 
brown and horny and vary in size in different species of earthworm ; 
some are about as large as rape seed, others almost equal in bulk to 
a small grain of wheat. They are formed from the secretions of the 



\^ ^csccsuupUL [chap. 

lihKuiiMr ^tihlens cell^ fbund m tdbo clitellmn and at first have 
;4 riag-likd shape. The aecretBona hardm when in contact with the 
i^K 'Phe tiuimal b^^ins to wriggle out of the band, which at first 
aU4i'roaudB it& body in the neighbonriiood of the thirty-eecond to the 
tlurty-::iidveuth segment As the band passes over the openings of 
th^ ovkducU in the fourteenth segment it carries awajwith it a 
cvirtaiu uumber of oYa» and as it passes the orifices of the sperma- 
UuKae between the eleventh and tenth and tenth and ninth 
!H)^m^its, some of the sp^matozoa which have been received from 
«Miother individual are squeezed out Besides ova and spermatozoa 
tho oocoou contains a certain amount of a milky and nutritive fluid 
in which these cells float ; this is probably supplied by certain other 
glauda in the skin of the earthworm. At the moment the last 
Hei^ment, that is, number one, is withdrawn, the anterior end of the 
iHHHH>u contracts and closes, and as the posterior end of the band- 
like ring passes over the head it also closes, so that the cocoon lies 
in the earth as a closed vesicle containing eggs, spermatozoa and a 
nutritive fluid. The spermatozoa fuse with the ova and firom the 
fertilized ova, by division into a number of cells and by the 
ditt'erentiation of the cells into muscle cells, epithelial cells, 
iligestive cells, nerve cells, etc., a young earthworm is built up. 
Before being hatched out of the cocoons the young embryos are 
nourished by the milky nutritive fluid in which they float. 

lu Great Britain there are several species of earthworm, which 

are grouped into two genera, viz. Allolobophora, with 
fiSVrm. fourteen species, which, with one exception, have the 

prostomium not dove-tailed into the peristomium ; 
and Lumbricus, with five species, in which the prostomium is com- 
pletely dove-tailed into the peristomium. The above account has 
been taken from the anatomy of L. Aerculeus, the largest of our 
indigenous species, but with the exception of a few minor details 
the account applies to most British earthworms. 

Order I. Oligochaeta. 

The sub-order to which earthworms belong, the Terricolae, 
are for the most part inhabitants of the land, and occur widely 
distributed over the Earth, being, as a rule, only absent firom 
sandy and desert soils. Some of them are aquatic but not 
many. On the other hand the allied sub-order the Limicolae 
ave for the most part denizens of fresh water. A few Limicolae 



VI.] 



POLYCHAETA. 



109 



possess gUls or finger-like processes well supplied with blood-vessels 
which tftke up oxygen from the surrounding water. Both sub-orders 
contain numerous genera and families; together they form the order 
Oligochaeta, which is characterised by being hermaphrodite, by 
haying the reproductive organs few in number and definite in 
position, by developing directly from the egg without the inter- 
vention of any larval stage, and lastly by the absence of certain 
structures which are very characteristic of the other great division 
of the true worms or Ghaetopoda. 



Order 11. PolychaetaN 

The Polychaeta differ from the Oligo- 
chaeta, as their name implies, by possessing a 
large number of chaetae on each segment. 
The sides of each segment are further as a 
rule drawn out into hollow flaps or lobes called 
parapodia, which bear the chaetae. Each 
parapodium may be divided into a dorsal half, 
the Notopodium, and a ventral half, the 
Neuropodium (15 and 16, Fig. 47). Both 
notopodium and neuropodium carry bunches of 
chaetae, and each has as a rule one particu- 
larly large chaeta, the aciculum, completely 
concealed in a very deep chaeta-sac, which is 
moved by muscles attached to its base and 
serves as a kind of skeleton for the parapodium. 
There is usually above the notopodium and 
beneath the neuropodium a process called a 
cirrus. The dorsal cirrus may be modified 
into a gill, and both dorsal and ventral cirri 
are absent in some cases. 

The coelom is often divided into three 
longitudinal compartments by two muscular 
partitions (5, Fig. 47) which run from the dorso- 
lateral line towi^s the median ventral line near 
the nerve-cord. The septa which divide the 
coelom in one segment from that in the next 
are in many forms incomplete or absent. 

As a rule Polychaets have a certain num- 
ber of the anterior segments modified to form a head, which usually 



Fio. 46. Nereis pel- 
agicGf L. After 
Oersted. 



1 10 ANNELIDA. [CHAP. 

wrriww teatftoles and organs for absorbing oxygen from the water, 
ckUed braaohtae or gills. They are generally of separate sexes, 
uul thtt e^gs develop into a larva nhich swims in the sea and 
KrwluiJty uhaug«s and grows np into a worm. This gronp includw 
N vury ttrvmt variety of forms, almost all of which are marine. 
Wittt tuw uAcuptious they form burrows for themselves, which most 




t'liiikilu. S. K|>IJ«tiiu». 3. CirenUr moaclra. 4. IiOii{[itadiDBl 
iiiiiuilw. A. Obliijup mnM^lM tomuDg & partition. 6. Bommtio 

Lsiii i4 •pllhrlium. 7. Gwloin. 8. SpUuehnia Ujw of peritonenm, 
<) ritut; of iiiii-iluiv. 10. Doml Uood-ve^Ml. 11. Toitnd blood- 
\iu«'l- Xi- Yfittnl iwrve-coKL 13. Nepbridiom oat in aeotioii. 

11. K^\m. 1>^' KotopodiuiD. 16. N«an>podiiim. IT. Donal 

ciirtw. ItL Vnitnl cimis. 19. CbMtae. 30. Aoionliun with 



iif ihciu (K<iMuii)»Hlly dosert in order to seek prey and to discharge 
tliu i'u]iroiliii-iivti ivUs. Some however never leave the burrows, 
ttliii^ti ill lliin cuHP oFt«D take the form of tubes composed of a 
nutiluUxU iif ihn octodorm. 



VI] 



HIBUDINEA. 



Older III. Hlmdlnea. 

BeaideB the Oligochaata and Polychaeta the order Hirudinea, 
the members of which are popularly known as leeches, is included 
amongst the Chaetopodo. They were for some time regarded as a 
distinct order of Annelida, since the great majority of species possess 
no chaetae and have other peculiari- 
ties ; but the recent discovery of 
species possessing chaetae, and the 
close resemblance between the develop- 
ment of all Hirudiaea and that of 
Oligocbaeta, renders it evident that 
t^ey are true Cbaetopoda and that tlie 
absence of chaetae is a secondary 
characteristic. 

There is little doubt that the 
LcHhH. Hirudinea are closely al- 
Extcrnai lied to the Oligochaeta ; 
indeed there are certain 
families which it is not easy to assign 
definitely to either group ; but the 
more ^ical forms are easily distin- 
guished. Externally leeches may be 
recognized by the possession of a sucker 
at each end of the body, the anterior 
one being formed by the mouth, whilst 
the posterior one is a special organ. 
By alternately attaching and releasing 
these suckers and bending the body 
the animal crawls along. 

With the exception of BranckeU 
lioa, which bears tufted gills, the bodies 
of leeches are without external pro- 
cesses. There are no parapodia, as in 
the Polychaeta, and no branchiae or 
tentaclea, and only one genus of the 
family has any chaetae. The body is 
segmented, and recently it has been 

shown that the number of segments is always thirty-three. Some 
however of the segments are fused together ; thus for example the 




Fio. 46. Hirado medieinalU, 
ftbout LCe Bize. 

1. Mouth. 3. Posterior 

Bucker. 8. SeoaOTy papillae 
on the antetior aanulus ot 
each Hegment. The remain- 
iag four annuli whioh mtJcs 
up each tine Begmeut are in- 
dicated b; the maikingi on 
the doiBol Burlooe. 



112 ANNELIDA, [CHAP. 

posterior sucker contains traces of six or seven true segments. The 
best test of the number is to count the ganglia on the ventral nerve- 
cord. But even this is not decisive, because although there are 
twenty-one free ganglia in the centre of the body a certain number, 
some say five and some six, are fused into the sub- pharyngeal 
ganglion, and a certain number, some say seven and some say six, 
coalesce to form the ganglion of the posterior sucker. Whichever 
view is taken the total number of segments is thirty-three. 

The body of the leech is ringed or divided into a number of 
annuli. These do not, however, represent the segments, but a 
number, varying in the different genera, make up a segment. In 
HirudOf the medicinal leech, there are five annuli to a true 
segment; in Clepstne, a common fresh- water leech, the number is 
three. The real segmentation is, however, to some extent indicated 
by markings on the skin. 

The animal is covered like the earthworm by a thin cuticle 
secreted by the outermost cells, and the ectoderm contains numerous 
goblet cells which are especially well-developed over the segments 
abutting on the generative orifices. Here they form a clitellum, 
and the secretion the cells pour out forms a cocoon in which the 
eggs are laid. 

The nervous system of a leech does not differ in essentials from 

that of the earthworm, but the nephridia, of which 

OrglfcM?** there are in Hirudo seventeen pairs, are peculiar. 

They are no doubt a modified form of the same 

organ as the nephridium of the earthworm, and they consist of 

coiled cellular tubes. The outer end communicates with the exterior 

through a muscular vesicle. The inner end, or so-called testis lobe, 

lies near the testis in the genital segments. The whole is traversed 

by a ramifying network of chambers opening by minute pores on 

the testis lobe. 

The other systems of organs are still more unlike what has been 
described in the case of the earthworm and deserve a short account. 
Leeches live by sucking the blood or juices of other animab, 
usually of Vertebrates. They are divided into two large groups — 
(a) the Rhynchobdellidae, which pierce the tissues of their hosts 
by means of a fine protrusible stomodaeum, the so-called proboscis, 
and {b) the Gnathobdellidae, which bite their prey by means of 
homy jaws. The medicinal leech is one that bites, and the tri- 
radiate little scar which its three teeth make in the skin was well- 
known to our forefathers in the times of bleeding and cupping. 



▼l] hirudinea. 113 

The three teeth, which are notched like a saw, are really only 
thickenings of the cuticle borne by the wall of the pharynx, which 
contains many unicellular glands whose secretion prevents blood 
from coagulating. Thus the leech when fixed on to its victim by 
the oral sucker readily obtains a full meaL 

From the pharynx a short narrow tube, the oesophagus, leads 
into an enormous dilatation, the crop. This extends to the four- 
teenth s^ment and gives o£f on each side a series of eleven pouches 
or caeca (Lat eaea^n, blind) which increase in size from before 
backward. The posterior caeca are very large and reach back to 
the level of the anus, lying one on each side of the intestine. The 
leech has the habits of a boa-constrictor. It makes a hearty meal, 
absorbing as much as three times its own weight of blood, and the 
blood it absorbs is stored up for many months in this enormous 
crop. It slowly digests the food in a small globular stomach 
situated just behind where the posterior caeca leave the crop. The 
stomach opens into a short intestine which ends in the anus, a 
minute pore situated dorsally between the posterior sucker and the 
body (Fig. 49). 

In one genus at least, Acanthobd^lla, the coelomic cavity is 
almost as well-developed as in an earthworm, and 
is divided up by septa as in that animal. In other 
leeches the cavity tends to disappear, becoming in fact filled up 
by a great growth of tissue, and thus reduced to a few narrow 
channels. In many leeches it contains a fluid closely resembling 
the true blood, so that unless very careful microscopic examination 
be made these channels may be mistaken for true blood-vessels. 
TBe capsules in which the ovaries and testes lie are also parts of 
the coelom. 

The medicinal leech, owing to a great growth of this above- 
mentioned tissue, is almost without a coelomic cavity. When the 
body is opened a narrow vessel full of a red fluid is seen running 
along the middle dorsal line above the alimentary canaL This is 
the dorsal sinus, a remnant of the true coelomic cavity; a similar 
sinus runs along the ventral surface underneath the alimentary 
canal, which is called the ventral sinus. It communicates with 
the dorsal sinus by lateral channels which run between the intestine 
and the posterior caeca of the crop. It surrounds the ventral nerve- 
cord, which thus seems to float in blood but really lies in the red 
coelomic fluid, and it gives off lateral sinuses which surround 
the inner openings of the nephridia. The true blood-vessels 
& AIL 8 






\ \ 4 ASXEUDA. [CHAP. 

vvM^^rtf * xyoMl niNMiif: iin Mdk side of the body and con* 
uvsimA ifu^tMiKt h^ vn»»T9r» fanuiche« which ran from side to 
»le Mow tlie Tentnl Buns. The 
Umml nnels (dither anpi4y capillaries 
to the nephridia. alimentary canal, 
KpiwlactiTe organs, ete., and a very 
extensive system to the skin where the 
haemoglobin of the blood takes np 
oxygen. Except in BranclteUion, which 
has special gills, the reepiiation of 
leeches is carried on by the skin. 

Leeches are, like the earthworm, 

Kcproduc- hennaphiodite, but their 

"""■ reproductive o^ans differ 

in some respects from those of that 

animal. 

In Lumbricua the testes are re- 
peated in two segments only, but in 
Hlrudo there are usually nine pairs of 
testfis. The cavities of both the testis 
and of the ovary are to be regarded as 
part of the original coelom ; in strictness 
the testes probably correspond to the 
vesiculae seminales in an earthworm, 
which are part of the coelom, and 
enclose the true testis and the sperm- 
fiiniiel. Each testicular sac produces 
spermatozoa on one side and on the 
other side is ciliated. The ciliated 
tract is the sperm-funnel and leads into 
a short transverse duct which passes 
into a lon^tudinal canal termed the 
vas deferens, there being one such 
canal on each side of the body. At its 

II. 41). View of the internal orgnna ai Hirudo medidnalu. On the left side 
thu alimentary canal ia shown, but the right hall or this organ hu been 
removed to show the eioretor; and reproductive organs. 
Head with e;e spota. 2. Mneoolar pharynx. 3. lat diverticulum o( 
the erop. i. 11th diverticulum of the crop. S. Stomach. 

(1. Itectum. 7. Auub. 6. Cerebral ganglia. 9. Ventral nerve- 
curd. 10. Nephridium. 11, Lateral blood-veasel. 12. Teitie. 
18. Vaa deferens. 14, Prostate glaud, IS. Penis, 16. Ovary. 
17. Uterui— a dilatation formed bj the conjoined oviducts. 



yi.] HIRUDINEA. 116 

imterior end each vas deferens passes into a convolated mass of 
tabes — the so-called epididymis — whose walls secrete a substance 
which binds the spermatozoa together into packets called sperma- 
tophores. It is to be remembered that the names epididymis, 
prostate, etc., are given firom fanciful resemblances to parts in the 
anatomy of man by no means homologous with the organs bearing 
the same name in the leech. From each epididymis a short duct 
passes towards the middle line, and these two ducts fuse and enter 
the base of the penis, which is protruded from the segment which 
contains the sixth distinct post-oral ganglion. 

The penis is simply the muscular end of the conjoined male 
ducts or yasa deferentia; it is the organ by which the spermatophore 
is deposited in the body of another leech. The spermatozoa in 
Ctepsine seem to penetrate the skin at any point and make their way 
to the ovaries, where they fertilize the eggs. In other species the 
spermatozoa enter in the usual way by the female genital pore. 

As in the earthworm, there is but one pair of ovaries. These 
are minute filamentous bodies each enclosed in a small coelomic sac. 
From each sac a short oviduct proceeds and uniting with its fellow 
forms a twisted tube surrounded by many glands. This finally opens 
by a median pore on the segment behind the one bearing the male 
opening. 

Thus in leeches, unlike the condition in the earthworm, the 
genital pores are single and median. The medicinal leech lays 
its eggs in a cocoon and buries them in holes in the banks of the 
ponds it inhabits. Clepsine, one of the Rhynchobdellidae which 
is very common in Britain, attaches its eggs to some stone or water- 
plant, or in some species carries them about on its ventral surface. 
It has developed a quite maternal habit of brooding over the eggs, 
and when the young are hatched it carries them about and they 
feed on some secretion from its body. 

Of the Gnathobdellidae, Hirudo medidnalis is found in Great 
Britain, but is commoner in some parts of the Con- 
tinent. It is cultivated in some districts, but the 
demand for it is decreasing with ihe disappearance of blood- 
letting. It becomes mature in three years. In the young stages 
it sucks the juices of insects. Another common but small 
Gnathobdellid leech is the brownish Nephelis, which frequents our 
ponds and pools ; it feeds on snails and planarians. A large species 
of the same genus is common in the shallows of the St Lawrence, in 
Canada. In waimer climates many leeches take to living on land, 

8—2 



116 ANNELIDA. [CHAP. 

and are a source of great annoyance to travellers whose blood 
they suck. Even water-forms do much damage unless carefully 
guarded against. Certain species make their way with drinking 
water into the throat and back of the mouth, on which they fasten, 
and so cause great suffering both to man and cattle. 



Phylum ANNELIDA. 

This phylum includes segmented animals with, as a rule, a well- 
developed coelom and metamerically repeated nephridia. The 
cuticle is always thin and flexible, and the nervous system consists 
of a pair of supra-oesophageal ganglia, a nerve collar and a ventral 
nerve-cord which has a ganglionic swelling in each segment. 

Class I. Chaetopoda. 

Annelida which possess bristles (chaetae) embedded in pits in 
the skin and serving as organs of locomotion, or which are believed 
to have once possessed such organs and to have lost them. 

Order 1. Oligochaeta. 

Chaetopoda which have the chaetae arranged singly or in 
pairs and which have neither parapodia nor tentacles : the 
generative organs are definitely localized and the sexes are 
united in the same individual : development is practically 
entirely embryonic : the group inhabits fresh water or damp 
earth. 

Ex. Lumbrictis, Alhlobopkora, 

Order 2. Polychaeta. 

Chaetopoda which have the chaetae arranged in bundles of 
some size, almost always borne on conspicuous lateral out- 
growths of the body termed parapodia : the prostomium has, 
as a rule, tactile organs, known as tentacles and palps : there 
are no localized generative organs, ova and spermatozoa being 
developed from wide stretches of the coelomic wall ; the 
sexes are separate : in the development a well-marked larval 
stage occurs : with few exceptions the group is marine. 

Ex. Nereis, 



VI.] HIRUDINEA. 117 

Order 3. Hiradinea. 

Chaetopoda in which chaetae and parapodia are absent and 
which move by means of a muscular sucker developed on the 
under surface of the posterior segments : there are no tentacles 
and the mouth acts as an anterior sucker: the coelom is 
reduced to capsules surrounding the genital cells and to a few 
narrow channels : the animals are hermaphrodite, and the 
genital pores single and median : the members of this order 
live on the juices of other animals, and there are both fresh 
water and marine species : development is entirely embryonic. 

Ex. Hirudo, Nepkelis, Clepsine. 



118 



CHAPTER VIL 
Phtluh Abthbofoda« 

One of the most striking features of the AnnelidA is the hxt 
that they are segmented, that is to say their body is divided into a 
number of similar parts placed one behind the other like coaches in 
a train, each of which to a greater or less extent resembles the part 
in firont of it The likeness of the parts to one another yaries. 
In some worms we might easily detect from which region of the 
body any given segment was taken. In the Earthworm, except 
in the region of the clitellum, there is little external difference ; 
nevertheless if we consider the internal organs we can dis- 
tinguish any of the first twenty segments from any other behind 
these and can easily arrange them in their proper order ; but no 
matter how long the worm is, all the segments behind the twentieth 
resemble one another so closely that it is impossible to assign any 
to their right place, except the last of all (v. p. 89). 

The animals included in the group of the Arthropoda are 
segmented like the Annelida, but with few exceptions 
^ the number of segments is small and does not exceed 

twenty. The segments have also become more highly differentiated 
from one another in consequence of being modified to perform 
various functions, and they are more frequently fused together than 
is the case in the Annelids. 

The Arthropoda have jointed outgrowths called limbs or ap- 
pendages. Tliese are always arranged in pairs, and at least one 
pair is modified so as to assist in holding and crushing the food* 
This character of possessing jointed limbs is what is indicated by 
the name Arthropoda (Gr. Sp^pov joint ; toxs foot). 

The Arthropoda may be dividoti into three classes : — 

L The Crustacea, which includes all the Crabs, Lobsters^ 
.^^^^^ Cray-fish, Barnacles, Wood-lice, etc, besides oonntless 
small forms s\ich as the Water-flea, Cydops, and 
many others which inhabit both salt and fi>dsh water. 




ARTHROPODA. 

II. The A3«TKNNATA, which lEclude all Arthropoda posseBaing 
one pair of feelers — antennae — and breathing by means of air tubes 
or tracheae. This group is divided into three sub-classes, viz. : 

The Prototracheata, a group containing the genus 
Peripatus. an animal not found in Europe or in North America, 
but which must be mentioned because it seems to be a survival from 
an earlier age and because its structure has given us a clue to 
mach that was obscure in the anatomy of Arthropods ; it is in 
lact in many respects intermediate between tbe Annelids and the 
air-breathing Arthropoda. 

B- The Myriapoda or Centipedes, the commonest British 
examples of nhich are the chestnut-coloured centipede LithMiis 
Jorficatut and the block "wire-worm"' Iidua terreslris. 

C. The Insecta, the largest group in the Animal Kingdom. 
It contains about 250,000 named species, and includes all those 
creatures such as Beetles, Flies, Dragon-flies, May-flies, Moths, Beee, 
Anla, Wasjis, eh;., which we are accustomed to call insects. 

III. The Aiuoui^iDA, includiug the Spiders, Harvestmen, 
Mitm and certain larger forms such ag the Scorpion, and Limulus, 
the King-crab. 

If we go into an old garden and turn over a stone or look 
between the bark and the trunk of a decaying tree or examine the 
leaves, we may find representatives of each of the four larger classes 
mentioned above. The Crustacea may be represented by a Wood- 
louse (Fig. 79), the Myriapoda by a Centipede (Figs, 51 and 84), 
the Insecta by a Beetle (Fig. 92), and the Arachnida by a Spider 
(Fig 53). If we compare these creatures one with another we shall 
see that they resemble each other in certain fundamental particulars. 

I'o begin with they are all clothed in a hard coating consisting 
largely of the homy substance called obitin which 
does not form a simple chamber or house in which 
the body of the animal lies as a snail lies in its shell, but which is 
moulded accurately over all the body and even tucked into all the 
openings ao that it forma an exact cast of the soft parts underneath. 
Tliis covering is to be regarded as an exaggeration of the cutiole found 
in Annelida, and it is called an Exoskeleton in order to dis- 
tdoguish it from the internal framework of hard parts found in the 
Vertebmta. 






:o be confused with the larva of a beetle, Elattr liaeatue, wMch 
irini-irorm " Itf the BritiBh agrianUurirt. 



k 



Vn.] ECDYSIS. ^ 121 

Fio. 50. 

A. The anterior portion of the body of a Dragon-fly, Ae$ehna cyanea, freed 
from the larval ahell. B. The tail being extricated. 0. The whole 
body extricated. J>, The perfect insect, ^e wings having acquired 

their fall dimensions, resting to dry itself preparatory to the wings being 
horizontally extended. 

Were this hard exoskeleton of the same consistency all over the 
body it would be impossible for the animal to bend its body or 
to move at all, but at certain spots, as may be well seen between 
the s^fments of a Centipede or between the members of a Beetle's 
legs, the exoskeleton has remained soft like the leather joints in a 
suit of mail-armour, and thus a certain amount of flexibility is 
given to the whole body ; for instance the Armadillo wood-louse 
and the Pill-millepede can roll themselves up into spherical balls. 

Not only is a hard exoskeleton a hindrance to unlimited 
movement but it also interferes with growth. It is 
impossible to increase in size when shut up in a hard 
unyielding case. Now growth is one of the common characters 
of all animals, and the way the obstacle presented by the exoskeleton 
of Arthropods to growth is overcome is as follows. At certain 
stated times the outer skin or ectoderm of the animal loosens itself 
from the inside of the cuticle or investment, which splits or cracks, 
usually along the middle of the back ; through the opening thus 
formed the body of the animal begins to appear, and gradually 
withdrawing each limb from its case it works its way out. The 
exoskeleton thus c&st off forms a most accurate mould of tha 
animal which has left it, and even includes those portions which are 
folded in at the mouth and anus and other openings of the body. 
A dragon-fly emerging and freeing itself from its cast skin is shown 
in Figure 50. 

The skin of the animal when it steps out of its old casing 
is quite soft, and it remains so for a varying time, a few hours in 
the case of some insects, one to three days in a Cray-fish. During 
this period the animal grows. After a longer or shorter time 
during which the body remains soft and capable of extension the 
secretions of the skin commence to harden, and very soon the 
animal is again enveloped in a hard case which rapidly assumes the 
colour and appearance appropriate to the species in question. 

This moulting of the skin in the Arthropoda is termed the 
ecdysis. It takes place at more or less regular times in each 
species, in the Cray-fish three or four times or even oftener during 



Hi 



AETHEOPODA. 



[CHiP. 



tie first year, the period of most active growth, later but once 
uiDUAlly, usually about Midsummer ; the Cockroach moults three 
times during the first j'ear, after which the moulta are Annual, but 
it does not become adult till after the seventh ecdysta, when it ia 
four years old. 

If we further examine onr wood-loDSS, 
DiviiioDi centipede. beetle, and spider, 
of B«i/. ^g gj^ notice at once that 

they are all. like the Annelids, bilaterally 
symmetrical ; and it may as well be stated 
at once that with few exceptions this ia 
true of the internal organs as well aa of 
the exoskeleton. Another feature oap- 
mon to them all b that they posaeai 
jointed limbs or appendages. These may 
occur in all the segments, as in the wood- 
louse and centipede, or the limbs may be 
reduced in number and confined to defi- 
nite regions of the body, as in the beetle 
and spider, but they always exi^ and an 
always jointed. 

In the body of the centipede we ean 
recognise but two regions (Fig. 51), ft 
head and a triiuk ; the trunk consisting 
of a number of segments, the head appa- 
rently of a ningle rounded one whose 
really composite nature is shown by the 
fact that it carries not one but several 
pairs of a])pendagea. 

The same may be said of the wood- 
loHse (Fig. 79), though here the trunk is 
divisible info two parts by the character 
of the appendages. The anterior part or 
thoras bears walking legs, the posterior 
part or abdomen plate-like appendages 
which act as respiratory organs. In 
neither of these creatures nor in the spider is there any constriction 
between the head and the trunk, thai is to say there is no neck. 
In the beetle however there is a well-marked neck separating the 
head from the rest of the body. The three following segments in 
the beetle are again separated from those which come after and 




Vm. SL A Centipede, Li- 

thobiui fiiTficalui. Dotial 
upeot K 13. 
1. Aiiteiin>«. 9. Poison 
cIbwi, 5tb ptir of append- 
sgira. i. Firit paii of 
iraUiing Ug«. 



VII.] DIVISIONS OF BODY. 123 

fonn what ia called the thorax. This part bears the three pairs 
of walking limbs and the two pairs of wings. The hindermost 
segments, often ten in number in insects, constitute the ahdomen ; 
this part of the body ia devoid of jointed limbs, though doubtless 
the ancestors of insects once possessed them on all the segments. 




Fia. 62. A Male Gookoli&ler, MOohnlha vvlgaHi, Men from kboTe mA Oigh&j 

enUcged. After Togt uid Tung. 
1. Head, atretchftd forwud. 2. Prothorax. S. Howthom. 

4. Uetatbomi. 6. Abdomen. 6. Anterior viDg (eWtron) of rigbt 

aide, tamed fomrd. 7. Posterior wing of right side, eipsnded. 

B. Uaiillu; palps. 9. Femnr of tbiid right leg. 10. Tibia of third 

right lag. 11. Tanas of third r^;ht leg. 

In the Insecta the abdomen may be constricted off from the 
thorax aa it is in wasps (Fig. 95), or there may be no constrictioo. 
If WQ now turn to the spider we shall see that the division of the 
body into regions has gone along different lines, and we can 
recognise only two principal parts, a so-called cephalo-thorax 
or prosoma to which all the appendages are attached, and a 
stalked abdomen or fused meso- and meta-soma behind which 
is devoid of obvious limbs, though certain little kuobs at its hinder 
end, from the summit of which are spun out the silken threads 
used in making the web, have been shown to be rudimentary limbs. 
The abdomen has lost all trace of external segmentation. In the 
harrestmen— Phalangids — long-legged creatures resembling spiders 
and found only during the summer months, the constriction between 
the cephalo- thorax and abdomen is absent, but the latter is distinctly 
divided into segments (Fig. 103). 

Thus in the Arthropods the body is divided into segments, and 
these s^ments are not all equal and alike, but they have become 
variously modified and some of them have fused together, as in the 



124 ABTHHOFODA. [CHAP. 

head of inBectK. and the abdomen of apiders, so that certun r^ODs 
of the body may be distinguished, and this is one of the moBt 
ohaiacteristic features of the group. 

In all Arthropoda certain of the appendages have lost tha 

function of locomotion and are bent round and 

brought into connexion with the month. These 

moatii or oral appendages assist in catching and holding the food, 

and to some extent in biting and tearing it into small pieces. 

With the exception of the Arachnida, in which the anangement 




Via. fiS. The Ouden Spidt 



is somewhat different, and of Peripatua, which has only one pair of 
jaw-limbs, the first pair of oral appendages is termed the mandible, 
the second is the first maxilla and the third the second maxilla; 
the last-named however in the Myriapoda retains the appearance of 
a walking leg, although too short to be used for walking. Un the 
odter hand in Crustacea a varying number of the appendages im- 
mediately encceeding the second maxilla are often turned forward 
and assist in the feeding. When this is the case they are termed 
maxillipedes (Figs. M and 56). 



vn.] 



AFPENDAGEa 



125 





The modification which all these appendages nndergo is similar 
in kind. The first stages of it are seen in the Arachnida. Here 
the first pair of appendages is always a pair of little claws placed 
in front of the mouth, the last joint shutting down on the next 
diyision of the limb like a knife-blade 
on the handle. After this pair come 
others, more leg-like, of which some- 
times all and in any case one pair have 
inwardly directed projections on their 
lowest joints termed gnathobases, 
so that when the limbs are brought 
together they act like a pair of nut- 
crackers (Fig. 55). Or to take another 
example, as we pass from the segment 
of the Gray-fish or of a Gammarus 
which bears the great claws forwards 
through the maxillae to the mandible 
we find the outer parts of the limb 
dwindling in size, and the basal pro- 
jection growing bigger (Figs. 54 and 
56). In the mandible only a minute 
rudiment of the other joints remains, 
and is called the palp. In the mandible 
of the insect even this has disap- 
peared. Limbs which have undergone 
these characteristic changes are called 
gnathites (6r. yva^os, a jaw). 

If we cut open the body of an 
Earthworm, a Starfish or 





Cavitiea of 

Bpdy. ^ Vertebrate, we lay open 

a chamber in which the alimentary 



Fio. 54. The month append- 
ages of Gammarus negUctus, 
From Lcackart and Nitsche, 
after G. 0. Sars. 

1. The left mandible. 

2. Its palp. 

8. Ist maxilla of left side. 

4. 2nd maxilla of left side. 

5. Maxillipede of each side to- 
gether forming an under lip. 



canal and many other organs appar- 
ently lie. This chamber is the coelom 

or primary body cavity, which has no connexion with the blood 
system, though amoebocytes float in the fluid it contains. If we 
cut open the body of an Arthropod or of a Mollusc we also open 
up a chamber which may be spacious, as in an Insect or a Snail, or 
which may be much reduced and filled up by the various organs of 
the body and by muscles, as in a Cray-fish or a Mussel. This 
cavity however, as development shows, is not similar in its nature to 
the coelom of an Earthworm or a Vertebrate, and it further difiers 



126 



ARTHROPODA. 



[chap. 



in that it contains blood and is continuous with the cavity of the 
heart and Urge blood-vessels. A special name has been given to 
this cavity and it is termed the Haemocoel (blood-cavity). The 
presence of this secondary body cavity or Haemocoel instead of a 
Coolom introduces at once a peculiarity in the physiology of the 
circulation of the Arthropoda. Instead of the oxygen-bearing and 
food-carrying blood being conveyed all over the body in minute 
capillaries which ramify in every part of every tissue, in the animals 
in question the tissues are floating in and bathed by the blood, 
which surrounds the organs on all sides and is kept in action by 
the contraction of a muscular tube — the heart — which opens freely 
into the body-cavity or Haemocoel. 

A true coelomic cavity 
(as is proved bot^ by its 
origin and its relation to 
the excretory and repro- 
ductive organs) is however 
found in Arthropods in the 
cavities of the reproductive 
organs (Fig. 67) and in 
certain vesicles connected 
wit^ the inner ends of 
some of the excretory 
organs, such as are found 
in the coxal glands of 
Arachnids. It is however 
obviously much reduced 
and takes a smaller part 
in the economy of these 
animals than it does for instance in the Annelida or Echinodermata. 
The skin of an Arthropod like that of an Earthworm includes 
Skin and Cm. R^t Only the ectoderm, but a firm support for the 
MctivtTiMUft. g|^^^ caUed the dermic The dermis in this case is 
(onued of well-developed connective tissue and this tissue also 
form^ an investment for every organ in the body, so that,as Huxley 
remarks, if all the organs were dissolved away there woold remain 
a ci>mpIote ca^l of them in connective tissue. The same statement 
i;» true of MoUusca and Vertebrata : it is therefbie important to 
obtain a clear idea what connective tissue is. 

lt» gTv>undwv>rk is a jeUv-like secretion interrening between 
eclKKlcrm and me^xietm> or meiscdeim and endoderm> and thereibre 




Fio. 55. Pedipalp of Tegenaria ffuyomi, the 
laxge hoase-spider. 

1. Coxa. 9. MaziUa, tbe gnathobnse. 

S. TroehADter. 4. Femur. 5. PuteUm. 
e. Tibi*. 7. TATsas. 8. Palpal organ. 



vil] 



CONNECTIVE TISSUE. 



127 



to some extent oomparable with the BtrnctureleBS lamella of Hydra 
ox the jelly of a Mednea. Into this substance cells are budded 
from the adjacent layers, chiefly from the meeodenn. These cells 
add to the secretion, in which fibres soon make their appearance, 
crossing each other at various angles. These fibres are to be 
looked on as more or less solid precipitates. The cells are found 
often flattened against them and connected with neighbouring cells 




Fi«. H. Left nonth Rppandagei of Attaeut Jluviatilii, alightl; mRgnified. The 

other appendage* are shown in Fig. ttO. 
I. Ifandibl*. n. Knt maxlUa. in. Beoond maiiUa (Scaphognathite). 

IT. Fint masillipad. V. Second maiilliped. VI. Third maiillip^. 

at. Bndopodile. ix. Biopodile. ep. Epipodite. &£ tt <y in in 

form a woop Ua dmilatiag vaUr over the ((illA. 

by delicate protoplasmic threads. The connective tissue round 
the ends of a muscle is modified to form tendon. Here the 
fibres aU pursue a parallel course and great tensile strength is 
the remit 

Oburration <A derelopment shows that connective tissue, 
, and blood-vessels all generally arise firom the same 



i!*J; 



ABTHBOFODA. 



[chap. 



^^JuiiiMit in tba embryo, which may be compared to the jelly of 
V'tMltttttonta. Blood is s portioD of it where the jelly ia fluid, 
ttt* fibres tue not developed ud ihe amoebocytes remain molnle. 
la. oooneotive tissue the jelly becomes more solid, fibrea ue de- 
veloped and the amoebocjrtea become stationary, being converted 
isto the ao-oalled connective-tisane corpuscles. It is interesting to 
oote that under the abnormal circumstances of a wound the blood 
of many animals can develop fibres ; this properly causes what is 
known as clotting. 




Fio. S7. JirJlui aquatiew. 
Mnls viewed from above. 
From Lauakul and NitGclie, 
after (i. 0. Sin, 

1. Anterior antenoBe. S. Fob- 
terior aQteimBe. 3^9. Tho- 
Taoic limbs. 10. The last 
pair o[ abdominal limbs. 
11. Tesles withtbeir effer- 

•^Btcni li Bbowu black. 



The muscular system of the Arthropoda is highly modified as 
ThcMuicuiir Compared with the primitive arrangement found in 
Byaum. jj^^ Annelida, Instead of a continuous sheath of 

muscle there are special bundles of muscular fibres for the purpose 



til] MDSCnLAB AND NEBV0TJ8 SYSTEMS. '^V 

of moving the Tarioiw hard parte on one another. Each joint of 

each limb, for iiutance, ie provided with a piur of muscles which 

move it on the next joint. One of these ie called the flexor, or 

bmder, and the other the extensor, or straightenei. Heie aa in 

Annelida the mueoles are derived irom ^ 

epithelial celle, bat all trace of this 

origin is lost in the adult In the 

moscle-cell in many cases the QDclens 

has divided, giving tise to several nuclei 

which are eurtounded by unmodified 

protoplasm. All the reet of the proto- 

pksm is converted into fibrillae which 

consist of alternately dark and light 

Btretches. Such muscles are said to be 

striped, and they have the power of 

contracting with much greater rapidity 

than mnecles of the type found in 

Annelida. Muscles of the latter kind 

ate called smooth mosclee, and in 

Artfaropoda exist in the wall of the 

gnt and some other places. 

The nervous system of the Arthro- 
Ncrtoo* poda is built upon the 
»>•"'»■ same plan as that of the 
Annelids. It consists in its least 
modified form of a pair of closely ap- 
proximated supra-oesopbageal ganglia 
forming a brain situated in front of the 
mouth. Id the head iu Insects, and in 
tlie anterior part of the body in those 
Arthropods which have no distinct 
head. This brain supplies nerves to 
certain sense organs, and gives off two 
atout corde, one of which passes to the 
left, and the other to the right of the oesophagus. These para- 
o€eophageal (Gr. vapa, alongside) cords unite together behind the 
oeaophague, and where they unite they form a pair of sub-oesophageal 
ganglia which send nerves to some or all of the mouth appendages 
(Figs. 57 and 58). Behind this comes a chain of ganglia, normally 
one pair for each eegment, which supply nerves to the organs and 
^ipendages of the segment in which they lie; each pair being 
n. AK. 9 




FiQ. 68. View of nwrom sjb- 
tern of the Cockohftfer, Mtlo- 
lontha wlgaTti, After Togt 
aiid fang. 

1. CerebiBl ganglioo. 

2. fiub-oesophageal ganglion. 

3. IsC thoiooio ganglion. 

4. Slid tboiacio ganglion. 

5. 3rd thoracio ganglion. 

6. Faa«d abdominal ganglia. 

7. Nerves to antennae. 

8. Optic HE rvea. 9. Origin 
of sympathetic nerves. 

10. Abdominal nerves, a pair (o 
each segment, which apht 
into an anterior and pos- 
terior branch. 



130 ARTHROPODA, [CHAP. 

connected with their successor by a double nerve-coid. These 
ganglia do not always remain distinct but show a tendency to fuse 
together. Thus in the Cray-fish the supra-oesophageal ganglion is 
shown by its development to be formed by the fusion of seyeral 
pairs of ganglia ; and the mandibles, both maxillae and the 1st and 
2nd maxillipedes are supplied from a single ganglionic mass, the 
sub-oesophageal ganglion, which is the result of the fusion of five 
primitive ganglia; again in some Flies and in Spiders all the 
ganglia behind the mouth have fused into one large nervous mass 
situated in the thorax. This fusion takes place to a less extent in 
some beetles; for instance in the Cockchafer, Melolantha wUgaris, 
the thoracic ganglia remain distinct, but the abdominal have fused 
into one mass which has been drawn up into the thorax (6, Fig. 58). 

The Crustacea are normally provided with two pairs of long 
feelers or antennae (though occasionally, as in the case of the Wood- 
louse, one pair may be entirely lost) and the Insects and Centipedes 
have a single pair, very conspicuous in some Butterflies and Beetles. 
These are organs of touch and frequently of smell ; and in some 
cases, such as the Lobster, they also act as hearing organs. They 
are supplied with nerves from the brain. No such antennae exist 
in the Spiders or Mites, and this serves at once to distinguish the 
Arachnids from the other two classes. 

The eyes of Arthropoda are peculiarly modified areas of the 
ectoderm. Over a certain area some of the ecto- 
^*' derm ceils become modified into visual sense-cells. 

In these there is a gelatinous rod developed from the outer end 
of the cell, and situated on the one side of it (Fig. 59 a) ; whilst 
from the base of the cell a nerve-fibre is developed. Usually 
several visual cells are pressed together in such a way that their 
rods cohere, and in this way a fluted spindle-shaped rod termed the 
rhabdome is built up (6, Fig. 59 A and e). This rod, like all such 
structures formed in visual cells, is cross-striped or in other words 
consists of layers of difiierent densities. Other cells of the visual area 
remain as supporting cells, being longer than the surrounding ecto- 
derm cells but otherwise unmodified. Pigmeut — universally present 
in visual organs — ^is secreted either by these cells (4, Fig. 59 a) 
or by amoebocjrtes which have wandered out from the underlying 
dermis (8, Fig. 59 b). The cuticle covering the visual area becomes 
transparent and greatly thickened, and so acts as a condensing 
apparatus or lens. In the larger eyes each group of visual cells 
secreting a rhabdome is sharply marked off from the rest and covered 
by a separate lens or thickening of the cuticle. Such a group is 



▼n.] BEH8E OBOANa 131 

termed aretinnl&; and as it is suiroanded by a sheath of pigment 
it can only be affected hy light coming from an object directly in 
front of it and priq>agated parallel to ita axis. Hence to an 
Artbn^Kxl possessing an eye vith many letinulae the outer world 
will be presented as a mostuc of light and shade, each letinula giving 
an impresnon depending on the intensity of light in the field of view 
directlf in front of it. Such a mosaic is an image, and this image 




A. TertiMl moUod through a lateral eye of a Scorpion, Eiucorpiui iUtliaa. 
B. Diagram of retiaola of a Soorpion'a oentral eye. C. D. E. Tniiii- 
*arM leotioD of B taken at diSeieal Isvels. From Lankeater and Bourne. 

1. Gntieolar lens. 2. EpiderraiB of the general bodr-gnrface. 8. Base' 
ment membrane. 4. Epidermal cells which coatnin pigment. S. Merre 
end-eella with nnolei. 6. Rhabdome. 7. Fibr^ of optio nerre. 

8. Pigment contained in eonnective tiesne cells. 



will be obviously the more detailed and definite the greater the 
number of retinuiae in a given area. In some Arthropods, 
ench as the common Fly, the eyes cover the greater part of the 
head. Eyes with numerous well-defined retinuiae are known as 
compound eyes: and they uaually present the appearance of 
numerous facets of hexagonal outline, owing to the fact that there 
is s luu corresponding to each retinula. In many cases the retinuls 

9—2 



ARTHBOFODA. 



(|<9AP. 




AUMENTABY CANAL. 



1. AnteoDiile. 3. Antenna. 3. Mandible. i. Mouth, 6. Scale 
oreqiuniB of uilcDDa (Hiopodite). 6. Anus. 7. TbIsod. 8. Opening 
at TU deleieDB. g. Chals. ID. let nalkiDK leg. 11. 2nd valking 
leg. 13. 'itd calking leg. 13. ith walking leg. 14. lei BbdominsI 
leg, modiGed- 15. 2ad abdominal leg, sligbtl; modified. 16. 3ril 
Kbdomiual leg. 17. 4th abdominal leg. IB. £th abdomimil leg. 
19. 6th abdominal leg, forming vith tetaaa the swimming paddle. 
30. OesopbasQS. 31. Stomach. 23. Meuejiteron or mid-eaC 

3S. Cenical groove. 2i. Intestine. SS. Cerebral uanglion. 

86. Pata-oesophogtal nerve-oordB. 27. Ventral nerve-cotiJ. 3H. Eye. 
29- Beort. SO. Sternal artery. 31. Sopro-intcstinol artery. 

S3. Sub-ial«>tinal artery in abdomen. '63. Sub-in tea tinal artery in 
thorax. S4. Ophthalmia artery. 35. Antennary artery. 36. Hepaiio 
artery. 37. Testis. 3S. Vas defeirnB. 39. Intental ekeleton. 

40. Qreen gtand. 41. Bladder. 42. Eiteroai opening o[ green gland. 

is depres-^ed beneath the general surface, and the adjacent ectoderm 
eells meet above it. These cells secrete clear glassy rods which 
cohere to form a crystalline cone. Thia also happens in some 
simple eyes, such as the centra! eyes of the Spider ; here the clear 
roda remain unconnected, and the whole upper layer of ectoderm 
cells is known as the vitreous layer. In the Cray-fish the compound 
eyes are carried on the ends of moveable eye-stalks ; and in Spiders 
the eyes, which in these animals are always simple, are sometimes 
elevated on a little prominence like a lighthouse, borne on the head 
and thorai. The number and position of the eyes in the Spiders 
are points of great use in identifying the various species. 

Ab a rule the alimentary canal of the Arthropods is about as 
DisenivB long as the body, so that it is straight; the Insects 
*"'^- however form an exception to this rule, since in their 

case the canal is longer than the body, and consequently has to 
be coiled or twisted in order to tuck it away in the limited space. 
]t was mentioned above that the chitinous exoskeleton of Arthro- 
pods is tncked in at both the mouth (stomodaeum or fore-gut) 
and anus (proctodaeum or hind-gut), and in many this lining 
extends BO far in as to leave but a small part of the ahnientary 
canal free from it In some species the hinder end of the stomo- 
daeum secretes deposits of chitin, as in the cockroach, or is 
hardened by calcareous deposits, as in the lobster or cray-fish, and 
thus teeth are formed which lie inside what has been termed the 
stomach or gizzard. When the moult or ecdysis of the shell takes 
place the linings of the fure- and hind'gut are also cast oET. 

The presence of chitin lining parts of the alimentary canal 
(orved to discriminate those parte of the canal which ore to be 



134 ABTHROPODA« [CHAP. 

looked on as merely parts of the skin — the stomodaeum and 
proctodaeum — &om the true endoderm. The stomodaeum is usually 
divided into a narrow portion leading from the mouth, called the 
oesophagus (20, Fig. 60), and an expanded portion containing teeth, 
called the stomach (21, Fig. 60). The proctodaeum is usually a 
straight cylindrical tube, called the intestine (24, Fig. 60). The 
small piece intervening between them is called the Mid-gut or 
Mesenteron; it alone corresponds to the human gullet, stomach, 
and intestine, and to the whole alimentary canal of a worm behind 
the pharynx. In it digestion is carried on and into it opens the 
so-called liver, i.e., one pair — rarely more — of glands consisting 
of great tufts of branching tubes lined by yellowish-brown cells. 
These secrete a fluid that assists in digestion. The food passes in 
part into these glands and some of it is there digested. 

In most animals the heart is a muscular sac which opens into a 
Circulatory systcm of tubcs with muscular walls, called arteries, 
apparatus. through which blood is driven to all parts of the 
body, finally passing into narrow tortuous passages, the so-called 
capillaries, whence it reaches the thin-walled veins through which 
it returns to the heart Thus, excepting such fluid as soaks through 
the thin walls of the capillaries, the blood is entirely confined within 
definite channels which do not open into the body cavity. But in 
the Arthropods the state of things is different; the heart (Figs. 60 
and 101), which lies in the middle line just below the skin of the 
back, opens by a series of slits called ostia into the body-cavity 
(haemocoel), and when the heart expands the blood which is in the 
body cavity enters these slits, but cannot pass out again through 
them when the heart contracts, as each slit has a valvidar arrange- 
ment which prevents this. When the heart contracts the blood is 
therefore forced forwards and leaves the heart by a vessel — ^the 
aorta — or by vessels with various names, which sooner or later 
open again into the haemocoel, and so the circuit is completa 
The part of the haemocoel in which the heart lies and into which 
the ostia open is called the pericardium, and it is separated fit>m 
the remainder ol the haemocoel by a horizontal septum called the 
pericardial septum, in which however there are perforations. The 
pericardium of Arthropoda thus contains blood, and is consequently 
widely different from the pericardium of Mollusca or Vertebrata, 
which is in both cases part of the coelom. 

In those Arthropods which have a localised respiratory system, 
and in which the blood takes part in respiration, there is a more 



til] CIEOULATION and BE3PIEATI0N. 135 

definite course for the blood than that sketched above. The 
extent to which blood-vessela with definite walls are developed 
varies in the different members of the group ; thus the Scorpion and 
King-crab have a mnch more specialized circulatory system than 




Fw. 6L 



Lett tide of & L&ttk of the Prawn. Ptnaeui, to ehow the origin of the 
gills. SliRhtl; magnified. From Claua. L, to L,. Tlie firBt to fiflh 

uiibi>l»tat7 limbg. M, to M,. The first to third maiiilipedB. la. 2a, Sa, 
7a. Podobranoha. lb, 6b. Anterior arthrob ranch b. lo. 2o. 7o. POB- 

terior artliiobranchi. Id, Gd, Td. Plenrobranchg. Of these rudiments 
of gillB onl; Dineteen develope. B. heli eide of a fully-grovn Prawn, 

Penaau lemieuleatiu, to show faUj-grown giUs. Slightlj magnified. 
8. Siopodita of eecond maxilla, which flaps to and fro and ao oaOBes a 
ODirent oTei the gills. 9. Eiopodlte of fourth ambulator; limb. 



have the Spider and Mites. The blood is very rarely red, but 

is usnally slightly tinged with a bluiith colour by a substance acting 

in tile same way as haemoglobin, hut differing from it in composition. 

In the lower and more simply organized Crustacea, such as the 

Kcspiratory wfiter-ffeas, there are no special breathing organs, but 

die blood is able to absorb its oxygen and give off its 




136 ABTHBOPODA. [CHAP. 

carbonic acid ob it courses under the thin skin. But in the larger 
and more complicated Crustacea, such as the shrimp and lobster, a 
special apparatus is present in the form of gills. These gills are 
thiu-walled extensions of the akin which project from the surfiEu» 
of the body near the base of the limb or on the side of the thorax 
into the surrounding water; inside them the blood flows to and firo 
and a cuirent of water washes tbem on 
the outside. The gills are classified 
according to their point of origin, being 
termed podobranchs(la,Fig.61) when 
they arise from the proximal joint of 
tbelimb, artbrobrancbs(lb,Fig. 81) 
when they are outgrowths of the thin 
membrane covering the articulation 
between the appendages and the body, 
and pleurobranchs (Id, Fig. 61) when 
they arise from the side of the body 
above the insertion of the appendage. 
In order to increase the surface of die 
gill, it is usually much folded or pro- 
duced into a number of small processes. 
In a lobster the gills are borne on the 
sides of the cephalo-thorax, as the 
fused head and thorax is called. There 
are twenty on each side, and they are 
4. Fused abdominal RiQglis. protected from injury by a broad flap 

6. OeBophagus. 6. Mid-gat. „ . i. , i ■ / .^ , . f 

7. Small intcBiiDB. 8. Colon, called the branchiostegite, which 
9. Bectnm. 10. Malpighian has grown down from the back and 
tabnUs, brown portion with - , lll. -.iirj 
oaeoa. 11. Malpighian tn- formed a chamber between itself and 
bales, dlitfd end. 12, Trachea thesideof the body ; in this cavity the 
with TBBioles. 13. TeBteg, ,,, ,, , , ,, ,, , . . r 
opening into coiled vaw defo- P^^ "^ concealed. At the iront end oi 
tentia. 1*. Penis. 16. Bingla this chamber lies a small paddle, which 

ispart of the second maxilla or scapho- 
gnathite (Fig. 56 m), which throws out the water from the front 
end of the gill chamber two or three times every second and thus 
keeps a current of fr«sh water passing into this space behind. This 
may be easily demonstrated by adding some coloured granules, such 
as carmine or indian-iuk, to the water in which lobsters are living. 

Many of the Arthropoda breathe air and their respiratory 
mechanism is very peculiar and unlike anything else in the animal 
kingdom. Instead of the blood being taken to a gill or lung and 



Fro, 69. View of male Cock- 
abater, ileloUmtha vatgaTii, 
from which the dorsal integu- 
ment and heart have been re- 
moved to ahow the internal 
organe. After Yogt and Yung. 

1. Cerebral ganRlioo. S. let 
thoracic ganglion. 3. 2nd 
kndSrdthoracicgangliatosed. 



BESPIBATOSr STSTEH. 



137 



til] 

there purified and then driven with its oxygen all over the body to 
every organ and tiasne, the air itaelf is introdaced into the body and 
is carried by minnte tubnles to every tissne and cell (12, Fig. 62). 
Tboa in these ^nimuli! the blood has loet one of its main functions — 
the respiratory — and remains simply a nutritive fluid. The fine 
tubules through which the air travels are termed tracheae (Gr. 
Tpaxvf, Tpaxfui, Tough, comigated), and the three groups of Myria- 
poda, ArachnidB, and Insects are sometimes coUectiveiy termed the 
Tiacheata, in contradistinction to the Groatacea, though there is 
reason to believe that the tracheae in Arachnida have arisen in a 
different way to those in the other two groups. The tracheae open 
to the exterior at certain definitely arranged pores termed stigmata, 
usnalljr found at the sides of 
the body. From these pores 
the tracheae pass inwards, 
dividing into smaller and 
smaller branches which ulti- 
mately end in the various 
tissues. Each trachea is 
reaUy a pouch of skin tucked 
into the body, and hence is 
lined by chitin continuoua 
with the exoskeleton cover- 
ing the rest of the body. 
The tracheae are kept bom 
collapsing by a thickened 
ridge of the chitinous lining 
which coib round inside the 
tube like a spiral of wire 
inside a water hose. Oxygen is thus absorbed by all the parts 
of the body directly from the air and not ftx)m the blood. This 
peculiar mode of respiration has had a profound influence on 
Insect stmctuie. 

A very primitive Arachnid, lAmulus, the King-crab, which lives 
in the sea, breathes by means of what are called gill-books ; these 
are piles of delicate leaf-like plates placed one over the other like 
the leaves of a book and attached to the posterior surface of the 
appendages of the hinder part of the body, which are flattened 
(Figs. 64 and 107). When these plates are moved up and down the 
leaves of the gill-books fly apart and the water gets in between 
them, and oxygen passes from it into the blood which circulates in 




Fio. 63. Horizontal seotion throngh the 
abdomen of a Spider, dTgyrontta. After 
MacLeod. Magnified. 
1. Opening to eitetior, tracheal stigma. 
2. Terminal traoheae. 8. Lateral 
tre^heae. 4. Long books. 



138 A&THBOPODA. [CHAP. 

the substanca of the leaves. In the Scorpiom, which &re geologioally 
the oldest and most primitive group of the Atachnids ^t live od 




Fio. M. Seetton throngb the opercnlnii) and gills of k Eiug-cnb, Lfmtdw. 
X abont 16. The normKl nmnber of gills id b Limoltu is five, the seotioa 
from whjob thla drawing is made shoved only fonr. 



land, the arrangement is the 
same, only the gill-books and 
the plate-like appendages 
which carry them are ankaller, 
and the books are packed 
away into pits on the under 
aide of the body, whilst the 
highly modified appendages 
extend horizontally below so 
as to floor in the pits, leaving 
only a slit through which air 
enters. This arrangement 
prevents the gill-book»— now 
called lung-books — from dry- 
ing up. In other Aiachiuds 
the gill or lung'book has been 
lost, and only the pit remains^ 
and this is enlarged and hor- 



Pra. 65. Londtndiua] aeation throngh the _.__ j„i.-, ♦!,„ i . * ■ 

lung bool of > Spider. MagSified. "'" *"*" '°5 "^7' forming 




From MacLaod. 
Opeuiog to the exterior or atigma. 
free edge of the pulmonary lesTes. 
SpM« in which the air oiroalateB. 
BpMS in trhioh the blood oiraiilateB. 



tracheae which may i 
all the appearance of Insect 
tracheae. The Spiders form 
an interesting link, for some 



vil] excretory system. 139 

of their Inng-books have been thus replaced by tracheae and others 
remain as in the Scorpion (Fig. 63). The tracheae of the Anten- 
nata have however developed in a different manner. In Peripatus, 
the oldest and most primitive member of the group, the stigmata 
or openings of the tracheae are scattered irregularly all over the 
surface of the body. Each leads into a short straight tube which 
ends in a bunch of diverging tracheae. In the Myriapoda there is 
usually only one pair to each segment, and the same is the case 
with the Insecta, but in them tracheae belonging to successive 
segments usually join so as to form longitudinal trunks which may 
even (as in the Flies) become swollen so as to form reservoirs of air. 
In some of the smaller Crustacea where the cuticle is thin the 
exchange of gases between the blood and the surrounding medium 
seems to take place all over the surface of the body. Another 
mode of exchange is the so-called anal respiration which is met 
with in many Phyllopods and Copepods, in Gammarus and Aselltis, 
in the larvae of Decapods and in certain Insect larvae. In these 
animals the rhythmic contraction of the muscular walls of the 
rectum alternately pumps in and expels water carrying oxygen in 
and out of the anus. In the Insect larvae the walls of the rectum 
are richly supplied with tracheae. 

The excretion of the nitrogenous products of katabolism from an 
animal's body is a function of fundamental importance. 
excrcUon."**"' W® ^avo scou in the Earthworm that this is per- 
formed in a series of little tubes called nephridia, one 
pair in each segment, and the same is roughly true of Peripatus. 
But in the other Arthropods, with few exceptions, where the primi- 
tive segmentation is much changed and modified, such structures 
do not exist in each segment, but a single pair of excretory organs 
suffices for the whole body. In the simpler Crustacea each of these 
is a tube with glandular walls (i.e. walls composed of cells which are 
filled with excreta), and each tube opens at the base of the second 
maxilla, on each side. They are termed the maxillary or shell- 
glands (Figs. 69 and 70). In the larger and more complex forms 
corresponding organs open on the second antenna (40, Fig. 60). 
Here the organ consists of a network of parallel glandular tubes 
joining each other at intervals and opening into a thin bladder 
which communicates with the outside. The whole is called the 
antennary or green-gland. In a few cases both antennary 
and maxillary glands co-exist, at least for some time of the animal's 
life. 



140 ARTHROPODA. [CHAP. 

A typical nephridium has been already defined as a tube which 
opens at its inner end into the coelont Since the excretory glands of 
Crustacea have in some cases at their inner ends thin-walled dilata- 
tions which are by most zoologists regarded as portions of the true 
coelom the Arthropoda may be said to possess modified nephridia. 

In Insects and Mjrriapods the excretory system, like the 
breathing apparatus, is peculiar; the waste nitrogenous matter is 
taken up by certain tubules called Malpighian tubules, after a 
celebrated Italian anatomist named Malpighi (Figs. 62 and 83). 
These lie in the body cavity surrounded by the blood ; they do not 
open directly into the exterior but into the front end of the procto- 
daeum, and through this their excreta leave the body. 

In the Arachnids the excretory apparatus is of two kinds which 
may coexist. In Scorpions and Spiders there are Malpighian 
tubules superficially resembling those of Insects, only shorter and 
less numerous, but they open into the endodermic tube or 
mesenteron and are therefore endodermic, not ectodermic structures. 
There are also organs which, like the green- and shell-glands of the 
Crustacea, are to be regarded as modified nephridia, which open at 
the one end into a space which is a remnant of the coelom and at 
the other to the exterior; they are termed coxal glands, since 
they lie mainly in the coxal or proximal joints of the legs. The 
contrast between the ordinary earthworm with its numerous pairs 
of nephridia and the larger and more active crayfish with its single 
pair is a very striking one for which some explanation is sought. 

Eisig experimenting on some of the marine Polychaeta discovered 
that the ectoderm performed part of the function of excretion. He 
fed the animals on a substance (indigo carmine) which was soluble 
and was consequently digested but which was got rid of by the 
excretory organs. This substance was found in the nephridia, 
and also in the ectoderm from which it was secreted into the cuticle 
and into the chaetae which, as has been shown (p. 91), are special 
developments of the cuticle. Hence we may conclude that in the 
Arthropoda with their enormous production of cuticle this function 
of the ectoderm has been so strengthened that nephridia have 
become superfluous. Peripatus, which alone retains nephridia in 
every segment, has indeed the thinnest cuticle of any Arthropod. 

With few exceptions Arthropods are bisexual. The reproductive 

organs are comparatively simple. Both the ovary 

J^IZ^'"'"'^^ (Fig. 67) and the testis (Fig. 66) are continuous with 

their ducts, which, in the Crustacea, Arachnids and 



some MjTiapods, UBuaJly open to the exterior on the under BUrface 
of the middle region of the body und at tlie posterior end of the 
body ID Insects and one dirision of the Myriapods. The apace 




Flo. 6S. Mate reproductive orgtiiH of Aitacat JluvialiluiiB.hont !}. From 
Howes. 1. Right antorior lobo of teatia. 3. Utdiaii posterior lobe of 
teHtin. S. Tas defeiena. i. Bitemal opening of vas defereuB. 

G. Kight fourth uobulataiy leg in wlucb the vbb deferens opens. 

Pin. 67. Female reproductive organs of Aitaeut Jlui'iatUitu about 2. From 
Howes. 1. Kight oTiduot. The left ovidnct ia sliowo parti; opened. 

3. Btghl lobe ot ovary. 3. Left lobe of ovary with Ibe upper half re- 
moved to show the cavity of ovary or ooelom into vhieh the ripe ova drop. 

4. Bitemul opening of ovidnot. a. Bight second ambulator; leg on 
irhioh the oviduct opens. 

inaide thei^e glands, lined by the reproductive cells, is regarded as 
part of the coelomic cavity. 

The foregoing account of the Arthropoda enables us to give the 

following definition of the group r — The Arthropods 
tht'owup" "^ *■* bilateral animals with segmented bodies. The 

segments are not all alike and frequently fuse one 
with another ; some at least hear a pair of jointed limbs, of which 
those in the region of the mouth are modified to catcii and bite the 
food. The nervous system consists of a supra-oesophage&l mass or 
brain, a nervous ring round the oesophagus and a ventral chain of 



142 CRUSTACEA. [CHAP. 

ganglia sometimes fused into a single mass. A heart is usually 
present above the alimentary canal and blood enters it through a 
series of paired valvular slits from the haemocoel or blood-cavity. 
The sexes are usually distinct. The coelom is much reduced. 

This definition is in the main true of all Arthropods, whether 
insect, spider, centipede, or crab. We must now consider however 
how the various subdivisions of this great group may be dis- 
tinguished one from another. 



Class I. Crustacea, 

The Crustacea are with few exceptions, such as the wood-louse, 
inhabitants of the water, and they breathe either through the 
general surface of the body or by means of gills. They have as a 
rule two pairs of antennae and these as well as their other jointed 
limbs are typically biramous, that is, they consist of a basal portion 
bearing two prolongations. They have at least three pairs of ap- 
pendages converted into jaws. 

The Crustacea are usually divided into two groups, the En- 
tomostraca (Gr. €VTOfio^, cut in piec^ ; oa-rpaKov, a shell) and the 
Malacostraca (6r. /xoAaKo?, supple) ; and each of these is again 
divided into four and three Orders respectively. 



Sub-class A. Entomostraca. 

This group may be regarded as a lumber-room for all Crustacea 
which are not included in the well-defined division Malacostraca, 
and the only character which can be attributed to all the members 
is that of not possessing the marks of Malacostraca. 

For the most part they are small Crustacea of simple structure. 
The number of their segments varies within wide limits ; some Ostra- 
coda having only seven pairs of limbs, whilst in Apus there are sixty- 
eight pairs. The dorsal part of their head has, in many cases, grown 
backwards and downwards like a mantle to form a large hood or shell, 
termed the carapace, which may cover a large part of the body, 
and in some cases this becomes divided into two lateral halves hinged 
together like a mussel's shell. In many descriptions of Entom- 
ostraca the words 'Hhorax'' and "abdomen" are used to describe 



Vn.] PHYLLOPODA. 143 

regions of the body. Such terms are in strictness applicable only to 
the higher Crustacea, where the trunk is sharply differentiated into 
two regions distinguished by the character of their appendages. 
Amongst the Entomostraca however the appendages of the trunk 
form a uniform series : often it is true the last segments are devoid 
of appendages, and to these the term abdomen (16, Fig. 68) is usually 
applied, but to us this seems an unjustifiable and misleading use of 
a term which has an exact significance only amongst Malacostraca. 

Entomostraca have no internal teeth in their stomacL As a 
rule the young are not like their parents but are larvae of a special 
kind called Nauplii; these after a number of ecdyses, during 
which the number of segments increases, grow up into adults. 

The Nauplius possesses an oval, unsegmented body, a median 
simple eye, three pairs of appendages and a large upper lip. The 
first pair of limbs representing the first antennae of the adult are 
simple and unjointed, the other two pairs have a basal piece and 
two branches. The inner branch of one or both pairs has a hook 
for masticatory purposes. These two pairs of appendages become 
the second antennae and mandibles of the adult ; both are at first 
placed behind the mouth. 

The Entomostraca consists of the following Orders : 



Order I. Phyllopoda. 

As the name implies the Phyllopoda (6r. <t>v\kov, a leaf; ^ov?, 
a foot) are characterized by possessing flattened leaf-like swimming 
limbs. Of these there are at least four pairs but there may be 
many more. The larger Phyllopods are not uncommon in Britain ; 
one genus, Artemia, taken at Lymington, flourishes in salt-pans in 
which the salt is so concentrated as to be fatal to other animals. 
Branchipus (Fig. 68) is devoid of the carapace and has an elongated 
heart extending throughout the body. It occurs in stagnant water, 
and has been recorded in several localities in the south of England. 
It is often found in the vicinity of Montreal, in Canada, in the 
pools of rain water which have accumulated in disused quarries. 
Apu8 is another of the larger forms which was formerly found in 
Britain but has not been met with for some years and is possibly now 
extinct in this country. It has a large carapace, and its flattened 
leaf-like appendages are regarded as primitive types of the Crustacean 
limb from which all the numerous modifications of the higher forms 



144 CBUSTACEA, [CHAP. 

may be derived. Of these eleven psirs are situated in front of the 
genital opening and are often termed "thoracic," one pair being 
attached to each of the pre-genital seg- 
ments of the trunk. Behind the genital 
opening there are fifty-two so-called 
"abdominal" pain of legs, of which 
several pairs are attached to each post- 
genital segment except the last two or 
three. 

The genera Simocephalu» and Dapk- 
nia, common in pondA and ditches, both 
in England and America, differ from 
the foregoing in having fewer s^ments 
and in possessing a bivaived catapaca 
which completely encloses the body. 
The first antennae, or, as they are 
generally called, the antennulee, are 
small and simple, but the second anten- 
nae are very large and forked and pn>> 
ject from the shell, and by their htshing 
movement carry the animal through the 
water (Figs. 69 and TO). The carapace 
is to a certain extent transparent, and 
through it the beating of the heart, 
the circulation of the blood and the 
movements of the thoracic leaf-like 
appendages may be made out. Within 
tiie substance of each valve of the 
carapace a coiled glandular tube may 
be detected ; this is the shell-gland or 
topical excretory o^;an of the Entom- 
oatrsca which opens on to the exterior 
in the region of the second maxilla. 
The male (Fig. 69) is usually smaller 
than the female (Fig. 70), and is cer- 
tainly very much rarer. The females 
lay two kinds of eggs, (i) unfertilized 
eggs, which develop in the space inter- 
vening between the dorsal side of the 
body and the shell which acts as a 
brood-pouch, and (ii) fertilized eggs, which are larger and become 




Fia.68. Dorsal view of female, 
Branchipiu Bp. foaad in a 
poDd in SuBBei x about 10, 

1, Antennae. 2. Head. 3. Ejen. 
4 — 14. The eleven "thoracio" 
limbn. IS. The caudal forks. 
16. The fifth "abdominal" 




w of male Simocephalui , 
nalM. 2. ADtennae. 3. TeetiB. 
divertiODlam. 11. H«art. 14 

17. Neok OTgao. 
Fio, 70. Side riew of female Simocephalvt tima, magnified 1 

as Fig. 69. From CDDniDgton. 1. AntenmileH. 

3. Mandibles. 1. MaiilUa. 6. Itit pair of legs, 
of legs. 7. 8rd pair of legs. 8. 4th pair of lega. 
lega. 10. Hepatic diverticolum. 11, Heart. 

18. Brood-pouoh. 14. fihell-gland. 16. Brain. 
17. Neck organ. 

A AM. 



HirIiIj magnified. 1. AliteD- 
Vaa deferens. 10. Hepatic 
:bell-glaDd. 16. Mid-gut. 



ti. 2nd pair 

0. Gth pair ol 

12, OTarj-, 

16. U id-gut. 



146 CRUSTACEA. [CHAP. 

surrounded by a special modification of the brood-pouch called the 
ephippium. The nature of the eggs produced is regulated by 
favourable or unfavourable conditions of life. At a suitable tem- 
perature and with a sufficiency of food and water, the unfertilized 
eggs are produced in large numbers at short intervals. Periods of 
drought or the cold of winter bring about the formation of eggs 
which are fertilized and enclosed in the ephippium. Sheltered by 
this case the eggs are enabled to withstand freezing or desiccation 
and with a return of suitable conditions a young Daphnia hatches 
out from each Qgg to continue the cycle of lifq. 

The Phyllopoda are divided into two Sub-orders, viz. : — 

Sub-order 1. Branchiopoda. 

Long-bodied forms devoid of a brood-pouch and not using 
the second antennae as swimming organs. 

Ex, Apus, Artemia, Branchipus. 

Sub-order 2. Cladocera. 

Short-bodied Phyllopoda with a dorsal brood-pouch and 
long second antennae. 

Ex, Simocephalus, Daphnia, 

The Branchiopoda live in fresh-water and as a rule in the stand- 
ing water of pools and ponds; they are more rarely found in 
brackish or salt water. They swim actively about by means of 
the vibrations of their flattened limbs. As a rule aquatic animals 
swim with the upper surface towards the surface of the water, but 
the Branchiopoda seem very indifferent to this rule, and are quite 
frequently seen swimming upside down. The Cladocera also fre- 
quently swim upside down, the genus Daphnia however usually in 
a vertical position with the head uppermost. 

Order II. Ostracoda. 

This Order (6r. oo-rpaicioSiy?, shell-like) contains a great number 
of species which do not differ greatly from one another. In form 
they resemble Daphnia^ but the head does not protrude from 
between the valves of the carapace, and some of the internal organs 
of the body, viz., the ovary or testis and branches of the liver, are 
prolonged into the valves of the carapace. This latter is a very 
characteristic structure, consisting, like the shell of the Mussel, of 
two valves. It opens by an elastic ligament which tends to pull 



VII.] COPEPODA. 147 

the Yalves apart, and it closes by the contraction of a muscle which 
runs across the body from one vaJve to another. The whole body 
is included in the carapace, antennae and all. 

Ostracoda have fewer ap- 
^n g pondages than any other group 

■^ I of Crustacea ; besides the anten- 

1-J^^I!?^ M ^^N. nules, antennae, mandibles and 

fix ^h// r^ \ \ *^^ P*^^ ^^ maxillae, they possess 

V^(^wa^^^^^ jkl..i ^ only two pairs of limbs, and 

^"~ ^^ — ^"^^^^^5^0/ these are stout and cylindrical 

iv^ * 6 ^^fr- ^z 1^ marked contrast to the ap- 
"'^^ pondages of the Phyllopoda. The 

1. Antennoles. 2. Antennae. m • 

3. Mandibles. 4. ist maxillae. Two pairs of excretory organs 

6 2ndmaidllae. 6. Ist pair ^ been described in some 

of legs. 7. 2nd pair of legs. . r /n, ^ , ^i_ , „ 

8. Tail. 9. Eye. species of Ustracoda, the shell- 

glands common to all Entom- 
ostraca opening at the base of the second maxillae and a pair of 
antennary glands opening at the base of the second antennae. The 
last named are seldom found except in the Malacostraca. 

Both pairs of antennae are jointed, unbranched appendages, 
which are used in swimming, another most important distinction 
frt)m Daphnia and its allies. 

The males differ from the females, which either (Cypris) lay 
their ^gs on water-plants or {Cypridina) carry them about within 
their shells. The majority of species are found in the sea but 
others occur in fresh-water. They are flesh eaters, and as they 
exist in great numbers they fulfil the important duty of scavenging 
on a small scale, and thus they prevent the accumulation of dead 
oiganic matter in the water. 

Order III. Copepoda. 

This Order (Gr. Kowny, oar ; ^rov?, foot) is also a large one and its 
free swimming members exhibit a very characteristic structure and 
appearance. The body is of an elongated pear shape, and consists 
of a large round head and a tapering trunk of comparatively few 
segments. The carapace so characteristic of the preceding order 
is entirely absent. The head bears a single median eye in front, 
the lateral compound eyes so conspicuous in most Crustacea being 

10—2 



148 



CRUSTACEA. 



[chap. 



absent. Attached to the head are 
five pairs of appendages, two pairs 
of nnbranched antennae, a pair of 
mandibles and two pairs of max- 
illae. The head is not separated 
from the trunk by any constriction. 
The latter bears four pairs of swim- 
ming feet of a typical forked pattern. 
Each of these appendages is some- 
thing like a X- ^^^ ^^^ ^^ ^^^ 
limb consists of one or two joints 
and is called the protopodite. It 
splits at its free end into two 
prolongations, the inner of which 
is known as the endopodite 
and the outer as the exopodite. 
Both are flattened, consisting of 
stout joints each of which bears 
spines, and the whole forms a 
convenient paddle. Each limb 
is also joined to its fellow of the 
opposite side by a transverse 
moveable ridge so that the right 
cannot move without the left. 
By the simultaneous action of all 
the limbs of the trunk the animal 
is enabled to execute a series of 
swift darts through the water; 
by the action of the second an- 
tennae a slow, gliding movement 
is carried out, whilst the max- 
illipedes by sweeping movements 
search the water for food. A 
forked limb is characteristic of 
the Crustacea, and is not met 
with in other groups of the 
Arthropods. It appears over and 
over again in all the Orders, 
retaining its primitive form in 
some instances, as in the ab- 
dominal appendages of a Cray- 




Fio. 72. Ventral view of male Cyclops 
sp. Magnified. 

1. Antennule. 2. Antenna. 

3. Mandible. 4. 1st maxilla. 
5. The two halves of the 2nd 
maxillae sometimes called inner 
and outer maxillipedes. 6-9. Ist- 
4th thoracic limbs. 10. Eye. 
11. Bristles near male generative 
opening. 12. Caadal fork. 

13. Mouth. 14. Copula or 

plate connecting the right and 
left limb of each pair. 



VII.] COPEPODA. 149 

fish, but m<ae oiWi by the enppreseion of one part (uBually the 
exopodite) and by the development and modification of others, the 
origioal form becomes masked and difficult to recognise. When 




Fro. TS. DorMl viev of female Cyclopt Ep. MagniGed. Partly after Hartog. 



I«t AntsDoa. 3. Sad AnteDna. 3. Eye 
6. Oridact. 7. Sp«nnathecB or pouch 

of the male. 8. Egg-attcr. ». Caudal ( 
11. Componad segment, oonaiating of the laF 
opening) and the flrst abdominal. 



4. Ovary. 5. Utenii, 
!or receiving the epennatozoa 
rk. 10. Position of anus, 
thoracic (bearing the genital 



both forks are conspicuously developed the limb is said to be 
biramonB. The fonr or five last segments of the Copepod'e body 



150 CRUSTACEA. [chap. 

bear no appendages. The last is produced into two processes, 
forming a caudal- or tail-fork. 

The sexes in the free-living species are not markedly different, 
but if we examine specimens of such a genus as Cyclops, which is 
common in our fresh-water pools, we shall find that in the breeding 
season the female carries about with her two egg-sacs (Fig. 73). 
These are attached to her body just behind the last pair of append- 
ages and project freely at the side. Each egg-sac may contain four 
or five dozen eggs which are glued together by a cement-substance. 
Such egg-sacs are very characteristic of the Copepoda and are 
found even in the parasitic members of the order. Most of the 
latter live on fish, and some have acquired the name of ''fish-lice." 
Their mouth appendages have lost their biting function and have 
become adapted for piercing the tissues of the host on which they 
live. Their segmentation is suppressed and their appendages are 
reduced and the body has grown out into all sorts of curious 
processes. The male is often much smaller than the female and 
as a rule retains the crustacean characters more than she does. 
Occasionally they are found on the skin of a fish, but more often 
they occur in the mouth and on the gills, sometimes half and 
sometimes almost wholly embedded in the flesh of their host. 

Order IV. Cirripedia. 

Some of the Copepods have become so modified by their parasitic 
habits that unless we were able to trace their development, in- 
cluding the larval forms through which they pass before becoming 
adult, we should have difliculty in assigning them to their proper 
place amongst the Crustacea. A somewhat similar modification 
occurs in the Cirripedia (Lat. cirrus, a tuft of hair ; pes, a foot) and 
is associated with a fixed or sessile habit of life. After passing 
through a variety of free-swimming larval forms the animal comes 
to rest and attaches itself by the anterior end of the body to a stone 
or rock, the bottom of a ship, or some other object submerged in 
the sea, and then becomes adult. 

Like that of the Ostracods tlie body of a Cirripede is enclosed 
in a carapace consisting of two valve-like folds which have grown 
out from the region of the head, but these are usually strengthened 
by five calcareous plates, a right and left scutum, a right and 
left tergum and a median carina, and in Balamis, the common 
acorn-barnacle of our sea-shores, a further armour of triangular 
plates developes in an additional outer fold of skin which encircles 



til] cibbipbdia. 151 

the body. In Lepas, the barnacle usaally found in clusters on the 
bottom of ships, which often seriously impedes their progress, this 
ring is absent, but the anterior end of the head bearing the first 
Antennae at its end has grown out into a long stalk which lodges 




Fia. 7-4. A visw o{ Ltpat aTutHfera, cat open iDDgitudinally to show the iliapo- 
sition of tile oTguii. From LeuckarC and Nitache, pertly after CIbdb. 

1. StaJk. 2. CariDB. 3. Targam. 4. Scutum. 5. 1st antennae. 
6. liandible with " palp " in front. 7. 1st maxilla. 8. 2nd maxilla. 
9. The fliz pairs of biramoua thoracic limbs. 10. Labrum. 11. Month. 
12. Oesophagns. 13. Liver. 14. Inleetme. 15. Anas. 16. Ovary. 
17. Ovidnot. 18. Testes. 19. Vas deferens. 20. Penis, 

21. Cement gland and duct. 22. Adductor scntorum muscle, which 

closes the carapace. 23. Mantle cavity, i.e., the epaoe iatervening 

between the carapace and the body. 

some of the internal organs of the body. The second antennae 
though present in the larvae are lost in the adult. The rest of the 
body is enclosed within the carapace. Around the mouth are a pair 
of mandibles and two pairs of maxillae, and the thorax carries six 



162 CRUSTACEA. [CHAP. 

pairs of biramous many-jointed limbs beset with nnmerons hair- 
like spines, the lashing of which kicks food particles towards the 
mouth (Fig. 74). These limbs are slender and flexible and thns 
differ firom the corresponding limbs of Gopepods. 

Like some Gopepods the Cirripedes are without a heart, and 
the existence of special respiratory organs is doubtfuL Unlike 
other Crustacea they are, as a rule, hermaphrodite, the male and 
female reproductive organs being united in one individual A few 
species are parasitic, chiefly on other Crustacea, and these have 
reached a very extreme stage of degeneration. 



Sub-class B. Malacostraoa. 

The second of the two large groups into which the Crustacea are 
divided contains most of the more familiar forms, such as Crabs^ 
Lobsters, Shrimps, Wood-lice, etc For the most part the Malac- 
ostraca are larger than the Entomostraca and the number of their 
segments is a fixed one. With the exception of the first Order, 
Leptostraca, which is really a connecting-link between true Malac- 
ostraca and the lower forms, this number is nineteen and there are 
nineteen pairs of appendages. One of the most marked characters 
in the Malacostraca is the differentiation of the trunk into two 
distinct regions, the thorax and the abdomen. It is true, as is 
mentioned above, that many authors speak of an abdomen in the 
Entomostraca, but by this they mean with a few exceptions the 
hindermost segments which are devoid of limbs. In any En- 
tomostracan if we examine the series of limbs behind the jaws we 
shall find that they constitute a continuous series without any 
sudden change in their character. In a Malacostracan, on the 
other hand, we find an abrupt change at one point in the character 
of the limbs. The hinder limbs or swimmerets (pleopods) are 
markedly different from the front limbs, for whereas in the swim- 
meret both forks of the limb, endopodite and exopodite, are equally 
developed, in the last five limbs or the thorax (peraeopods) the 
endopodite is large and the exopodite is small or absent. It is this 
difference in character which defines the abdomen. 

Although the division between the head and thorax is not always 
apparent, as a rule we may assign five segments to the head, eight 
to the thorax and six to the abdomen, which ends in an unseg- 
mented flap called the telson. The reason for this want of & 



vil] thoracostraca. 153 

definite bonndaTy between head and thorax in the Malacostraca 
is that the carapace, which as we have seen is an outgrowth of 
the head, has become fiised with the dorsal surface of the thoracic 
segments, whilst at the sides it forms freely projecting flaps, which 
since they cover the gills are known as branchiostegites (6r. cnrcya), 
to cover). 

The excretory organ of the Malacostraca opens at the base of 
the second antennae and not as in the Entomostraca on the 
second maxilla. As a rule the typical larva — the Nauplius — of the 
last-named group is not present in the life-history of the Malac- 
ostraca, which may hatch out from the egg in a practically adult 
condition or may pass through several larval stages, the first of 
which 18 the Zoaea, a larva with many appendages, possessing eyes 
and in all ways more differentiated than the Nauplius. 

Order I. Leptostraca. 

The order Leptostraca (Gr. Xcirros, slight, small) contains but 
three genera, which are interesting because they form an inter- 
mediate stage between the Malacostraca and the Entomostraca. 
Like many of the latter they are provided with a bi-valve carapace 
which, unlike that of all other Malacostraca, is not fused with the 
thoracic segments. Behind the six appendage-bearing segments of 
the abdomen there come two more segments without limbs and 
the hindermost bears two diverging filaments constituting a " caudal 
fork," such as is commonly found amongst the Entomostraca. The 
thoracic limbs are flattened and leaf-like, as in Apus, but the 
mandible bears a three-jointed feeler or palp and the eyes are 
stalked, — ^both, on the whole, Malacostracan characters. The 
excretory organ opens on the second antenna, but in the larva the 
shell-gland or maxiUary excretory organ is found and traces of it 
exist in the adult 

The order is marine, and very widely distributed throughout the 
ocean. Its members are capable of living and thriving in very foul 
water, so foul as to be fatal to most other animals. Nebalia is 
the best known genus. 

Order IL Thoracostraca. 

The Thoracostraca (Gr. ^wpaf, a breast-plate) form a large 
group and contain many very different forms. They are placed 
together because the carapace has become fused with several of the 



IM CEUSTACEA. [CHAP. 

tfaoracio aegmeata, so as to form a region known as the cephalo- 
thorax, the exoskeletou coTering which is cot jointed and is not 
bivalved as in Nebalia. The eves of the Thoncostraca are 
compound and almost always are home on moveable stalks. The 
Older is divided into four sub-orders. 



Sub-order 1. Schlzopoda. 

This sub-order includes the lowest of the Thoracostraca. The 
name is suggested by the circumstance that all the eight pairs of 
thoracic limbs are biramous ; the first and sometimes the second 
pair are reduced in size and provided with gnathobases ; they assist 
the manibles and maxillae niid hence are termed maxillipedes. It 
will occur to most observers that the thoracic feet of the Schizopod 
resemble the ordinary form of swimmeret or abdominal appendages 




FiQ, 76. Nyetiphanet wjratgica, a Schizopod. Slight!; magnified. From 
WatMi. Thu black dots indicate the phosphorescent organi. The gillB 
ore Men betneen the uephalothoracio and the abdomin&I appendagaa. 

in the more familiar Lobster or Crayfish. This is so ; the swim- 
merets of a Schiiopod are however sharply distinguished from 
the thoracic limbs by their smaller size. It appears probable 
that the first step in the evolution of an abdomea was the 
reduction in size of the appendages so as to transform the hinder 
part of the body into a powerful swimming fin, and many Schizopods 
only use the abdomen in this way, since most of the swimmerets 
are very small and appear to be practically functionless. The last 
one however is broad and assists the tail in its vigorous strokes. 
Some Schizopods have a series of phosphorescent oi^ans which 
under certain conditions emit a pale but very perceptible light 



VII J DEC APOD A. 155 

like iha,t of a glow-worm. This light seems to be controlled by 
the animal but its use is not very clear. 

There are very interesting differences amongst the genera com- 
posing the Schizopoda. The genus Euphausia for instance has 
long feathery gills attached to the basal joints of the thoracic legs 
and the eggs are not borne about by the mother but hatch out into 
Nauplii, which pass through a series of metamorphoses before 
becoming adult. In Mysis on the other hand the gills are few and 
simple and the eggs are borne under the thorax on flat plates 
termed oostegites, which project inwards from the hinder thoracic 
appendages. In these two genera we see the beginning of two 
tendencies which have led the descendants of primitive Schizopoda 
to differentiate themselves in two different directions. One group 
have taken to carrying the embryos about until they are fully de- 
veloped ; at the same time the gills are reduced and the carapace, 
which is essentially a gill-cover, tends to disappear. This group 
includes the Stomatopoda, Cumacea and Arthrostraca. 

In the other group the gills and carapace are retained, and 
though the eggs are for a time carried about attached to the 
swimmerets the young one passes through a larval stage before 
becoming adult. This group includes the Decapoda. 

Sub-order 2. Decapoda. 

The Decapoda (6r. Ukol, ten) derive their name from the circum- 
stance that the first three pairs of thoracic appendages have become 
maxillipedes, that is to say have been modified so as to assist in 
mastication, leaving five pairs of large conspicuous limbs which 
have lost all trace of an exopodite, for prehension and locomotion. 

This group includes the lobsters, cray-fish, shrimps, prawns, 
hermit-crabs and crabs, etc. In the division of the crabs, the 
Brachyura (Gr. jSpa^vs, short ; ovpa, tail), the abdomen is reduced 
in size and turned up and closely applied to the under surface of the 
thorax, except when the animal is '' in berry '' and then the masses 
of eggs force the abdomen away from the thorax. As a rule crabs 
are broader than they are long and the breadth is partly due to the 
large gill chambers on each side of the body. The gills are really 
outside the body, but are in a special chamber bounded by the 
branchiostegite or free edge of the carapace. 

The Anomura are in some respects intermediate between the 
foregoing and the following divisions. As in the crabs the 



156 CRUSTACEA. [cHAP. 

abdomen Ib folded somewhat forwaida but the taO-fin is not so much 
reduced. The last pair or last tvo pairs of the thoracic limbs are 
reduced and turned dorsalwarda. Some species — the heimit-crabs — 




shelter themselves in the empty shells of moUascs. to these cases 
the abdomen does not develope a hard covering as the animal is 
sufficiently protected by its lodging. It remains soft and acquires a 
spiral twist as it moulds itself to the interior of its borrowed shelL 



VII.] STOMATOPODA. 157 

In the lobsters, shrimps, etc., which form the division Macrura 
{Gr. fiaKpo^ long ; ovpd, tail), the tail is relatively large and is not 
folded up against the thorax. 

Some Decapods have left the sea and taken to live on land and 
this has in some cases involved a change of structure, the gills 
which breathe water being supplemented by the soft vascular 
lining to the gill-cavity covered by the branchiostegite which, like a 
lung, breathes air. 

The group is for the most part marine, though the well-known 
fresh-water cray-fishes, Ast(ictis in Europe and Cambarm in North 
America, form striking exceptions. 

Sub-order 3. Stomatopoda. 

The Stomatopoda (Gr. o-ro/xa, a mouth) are a sharply defined 
group with few genera, and may be regarded as an offshoot from 
the primitive Schizopoda peculiarly specialized. The members 
attain a considerable size, some eight inches or more in length. 
The carapace is small and only covers the anterior five thoracic 
segments ; the appendages of these segments are turned forward 
towards the mouth and take part in feeding, and so are termed 
maxillipeds. They end in a claw, the last joint shutting down 
on the penultimate one like a knife blade into its handle. They 
are thus very different from the maxillipeds of Schizopoda or 
Decapoda, which are really limbs on the way to become jaWs and 
have developed gnathobases. The maxillipeds of Stomatopoda 
are grasping, not chewing organs, and have undergone the same 
modification as the great claw of the lobster, which might just as 
reasonably be called a maxilliped. A fertile source of confusion 
in the study of the Arthropoda is the use of names like maxillipede, 
thorax, abdomen, etc., to denote different things indifferent groups. 
The last three thoracic limbs are for walking ; they are very feeble 
and retain a rudiment of the outer fork or exopodite. The 
abdomen is large and bears six pairs of flattened swimming limbs, 
each of which carries a gill in its outer branch. 

Unlike most other Crustacea, Squilla and the other members 
of the group do not carry their eggs about with them, but lay them 
in the burrows in which they live, and by sitting over them and 
moving their abdominal limbs they keep up a current of water which 
aerates the eggs. They are exclusively marine and live buried in 
the sand or hidden in crevices of the rock. They move actively 
and are difficult to catch. 



GEUSTAOU. 



6nb-ordeT 4. Cnmaces. 



The membera of the sub-order Cuinac«a (Gr, xvfta, a. wave, or 
billow) are mostly smali; they live in the sea on sandy bottoms at 
considerable depths, but come to the surface at night. They an 
especially interesting because to a certain extent they are int«i^ 
mediate in character between the Thoracoetraca and the Arthn- 
straca. Thus their paired eyes are not elaikMi aod are some- 
times fu8&l U^ether to form a single eye ; the carapace is reduced 
BO as to leave several s^meots of the thorax uncovered (Fig. 71) 
and on some of the thoracic legs there is a small exopodit«. 




Fio. 77. FemiJo DitalylU ttygia k G. After Sm. The o&npace ia repis- 
■ented aa trsaepuect to show the gilt 

I. CftnpOM. 3. First uiteniia. S. First ieg. 4. Gill borne on 

Bnt mkxilliped. 5, 6, 7 anil 8, Beeoad to fifth leg. 9. Free put at 
thorax. 10, Abdomen. II. Appendage of the last BecmeDt of tha 
abdomea. 

liave a single pair of gills borne by the first thoracic limb. In 
the female the abdomen, which is long, has lost all the limbs except 
the last pair. 



Order III. Arthrostraca. 

The membera of the Arthrostraca (Gr. afiSpov, a joint; Sm-paKov, 
a sheU) have sessile eyes, i.e., without stalks. The carapa*^. which 
in most of the above-mentioned groups covers the segmenU of 
the thorax, ia absent, and consequently seven of the latter are 
usually freely moveable on one another, the first and in rare cases 
the second thoracic segment remaining immoveably fused with the 



VII.] 



ARTHROSTRA.CA. 



159 



bead. They thua represent a further stage in the same process 
which we found going on in Cumacea and Stomatopoda. Only one 
of the thoracic appendages is modified so as to form a maxilliped; 
there are consequently seven pairs of w&Mng legs attached to the 
thorax. In the female these legs hear inwardly directed processes 
which together form a hrood pouch in which the eggs develop©. 




profile. From 

. Tbe 



i-Ti. Cepbalothorax. th-uii. Free thoraoio EegmeDta. siv-ui 

tix ftbdominsl segmeDta. I. Anterior aDtenna. 3. Poaterior sdiciidb. 
B. Haudiblei. i. let msiilk. 6. 2nd maxilla. 6. Maxilliped, 
7 — IS. ThorMio limbs. 14 — 16. Three anterior abdominal limbB for 
nrimming. IT — 19. Three posterior abdominal limbs for jumping. 

20. Heart with six pain of ostia. 31. Ovary. 23. Hepatio diverti- 
onla. 38. Pottenor divertienla of the alimentar? canal 34. Median 
dorsal diirertiaalam. 35. AlimeDtor; canal. 26. Nervoaa system. 
37. Ots in egg poach, formed from lamellae on tbe coioe of the aeoood, 
tbild and fourth thoracic limbs. 



The Arthiostraca are mostly small animals, living in either salt 
or ftesh water ; they assume very different forms, some of them 
having a rudimentary abdomen. They are divided into the sub- 
orders Amphipoda (Gr. A/i^t, on both ends; n-oSa, feet), which are 
for the most part compressed or flattened from side to side 



160 ANTENHATA. [CBAP. 

(Fig. 78) and carry their gills on their thoracic iqtpendages, and 
the Isopoda (Gr. utm, equal ; roSa, feet), which are depressed or 
flattened bom above downwards (Figs. 57 and 79), and whose gills 
are the modi&ed endopoditea of the appendages of the abdomen. 
A typical example of the last named snb-order is the Hog-water 
Louse, AseUut aquaticiu (Fig. 57), common in out ponds and 
streams, but many of the groups are parasitic and lose most of their 
characteristic Crustacean features. As in the case of the Decapods 
some genera of Isopoda have forsaken 
the sea for a life on land, amongst 
which the wood-lice, OnUcua and Por- 
cellio, exhibit certain peculiaritjea 
usually associated with Insects ; thus 
the mandible has no palp, one pair 
of antennae is usually lost, and there 
are certain tubular air passages be- 
lieved to be respiratory burrowed in 
the abdominal endopoditea which recall 
by their structure the tracheae of air- 
breathing Arthropods. This is another 
proof that any attempt to group to- 
gether all animals possessing tracheae 
leads to absurdities. 




Fio. 79. A Wood-louse, Por- 
ceiiio tcaber x Kboat 2. 
From Caviec 



Class 11. Antehnata. 
Sub-class A, Peototraoheata. 

A short account of the genus Penpatus must be given, as this 
animal is of a very primitive nature and both its adult structure 
and the mode of its development throws much light upon the origin 
and anatomy of Myriapods and Insects and indeed on tJie ArUiropods 
generally. 

The different species of Peripatta are differently coloured, but 
they mostly possess a beautiful velvety coat. In shape they 
resemble caterpillars but carry two large antennae on their heads, 
and at the base of each antenna is an eye. On the under surface 
of the head is the mouth and tucked into it on each side is a 
toothed jaw. This is an appendage which has been modified so as 
to form a true gnathite. At each side of the mouth is a third ptur 
of appendages, the oral papillae, from the tips of which a stioky 
sUme can be ejected which entangles the insects and spiders on 



VII.] ANTENNATA, 161 

which the animal lives. The other appendages, which vary in 
number in the different species from seventeen pairs to over forty, 
have the form of soft cylindrical papillae ending in two claws and 
function as walking legs (Fig. 80). The anus is posterior, and at 
the base of each leg is a slit-like pore, the opening of a nephridium. 
The genital pore is in front of the anus. 




Fio. 80. Peripatus eapentU x very slightly. From Sedgwick. 

The body cavity is a spacious haemocoel divided into three longi- 
tudinal compartments by two bands of muscles which run from its 
outer upper angle towards the middle ventral line. The lateral 
compartments are continuous with the cavities of the limbs and 
lodge the nephridia, the salivary-glands and the nervous system. 
The alimentary canal, slime-glands and generative organs lie in the 
middle compartment. 

The mouth leads into a large muscular pharynx, such as is 
found in many Chaetopods. The salivary glands open near this. 
They are interesting structures, as development has shown that in 
origin they are derived from nephridia, a state of things which 
recalls the fact that in certain Oligochaets some of the nephridia 
open into the oesophagus. The pharynx leads by a short oesophagus 
into a roomy endodermic stomach which reaches back nearly to the 
anus, a short proctodaeum only being interposed (Fig. 81). 

The structure of the heart and pericardium closely resembles 
that of the same organs in Myriapods and Insects. 

The animal breathes by bunches of tracheae or short tubes which 
pass from the exterior into the tissues and convey air. Their 
external openings or stigmata are partly in two rows above and 
between the legs and partly scattered irregularly. 

At the base of each leg is a nephridium which ends internally in 
a vesicle. Embryological research has shown that this vesicle is 
a remnant of the true coelom which is spacious in the embryo, but 
becomes displaced as development proceeds by the haemocoel. 
Enclosed in the proximal part of the leg there is a gland called the 
crural gland. 

Peripatus is bisexual, and again embryology has demonstrated 

& AIL 11 



162 



ANTENNATA. 



[chap. 



that the cavity of the sexual 
oTgauB is coelomic The male 
deposits its spermatozoa io 
packets in the body of the 
female. It is not known how 
they reach the ova but they 
are usually found in the ovaiy 
and possibly bore their way 
through the tissues, as they do 
in some leeches. The ducts 
of the reproductive organs are 
believed to be modified ne- 
phridia. 

The nervons system con- 
sists of abrain and two ventral 
cords, which however do not 
approach one another but lie 
wide apart. They are con- 
nected by nine or ten trans- 
verse commissures in each 
segment (Fig. 81). Posteriorly 
the two ventral nerve-cords 
fose abovB the proctodaeum, 
aa arrangement which recalls 
what occurs in certain primi- 
tive Mollusca. 

There are many species of 
Peripatus, which are by some 
antborities grouped into three 
ot four genera. They are 
found in widely separated 
parts of the world and afford, 
as is often the case wit-h ar- 
chaic animals, an excellent 
example of "discontinuous 
distribution." They have beeu 
found in South America ami 
the West Indies, in South 
Africa, in Australia, New 
Zealand and in some of the 
Islands of the Malay Archipelaj 




After BalToac. 



1. AoteTtDoe, abowicg aateaaiaj nerve. 
a. Oral papilla. 3, 3', S». lit, Ind 
and 10th U^ of right side. 4. Bnin 
and ejes. 6. Ciromn-oeaophageal 

cord. 6. Veotial Deive-eoTd of nght 
side, ahowing the tranHverae oommia- 
euree. 7. Pharjni. 8. Stomach. 
9. Adqs. 10. Mole generative open- 
ing. 11. Salivar; glands. 12. Slime 
^lanila aad reservoir. 13. Enlarged 

crunil glaod of the 17th leg. U*. 
1J'°. 4tli aod lOlh nephiidia of right 



;o and in Lower Siam. 



VII.] MYRIAPODA. 163 

The deyelopment of Peripatus first definitely solved the problem 
of the nature of the various spaces in the Arthropod body and has 
also thrown much light on some of the peculiarities of insect 
embryology. Some species lay eggs and some produce living young, 
and thus the genus afifords favourable opportunities for testing 
theories as to the effect of development within the body of the 
parent on the embryology of the offspring. 

That the animal is a most interesting '' missing link '' becomes 
evident if we attempt to sum up the Annelidan and the Arthropodan 
features of its anatomy. Thus Peripatus resembles Annelida in the 
nervous system, the muscular pharynx, the structure of the eyes, 
the serially repeated nephridia, the shortness of the stomodaeum 
and of the proctodaeum, the thinness of the cuticle and the hollow 
nature of the paired appendages ; but in the indications of joints 
in the appendages, the reduction in size of the coelomic spaces, 
the presence of a wide haemocoel and of tracheae, the nature of 
the antennae and of the heart and pericardium, the position of the 
genital pore and the presence of true gnathites, Peripatus approaches 
the Myriapods and Insects. 

In habits these animals are shy and inconspicuous, hiding under 
bark or stones and preferring a moist surrounding. They avoid 
the light and move with deliberation, testing the ground as they 
advance with their antennae. 



Sub-class B. Myriapoda. 

The Myriapoda (6r. fivpioq, countless) are characterised by the 
possession of a head distinct from the rest of the body, bearing 
antennae, mandibles and one pair of maxillae, followed by a large 
number of segments bearing simple leg-like appendages. 

Compared with the Crustacea or Insecta the group is a small 
one, yet it contains some thousands of species which, if we except a 
few small families, fall readily into two subdivisions (I) Chilopoda, 
and (II) Diplopoda. The subdivisions differ markedly from one 
another especially in the position of their reproductive openings 
which in the Diplopoda are on the third segment behind the head 
and in the Chilopod<i are terminal. For this reason some naturalists 
break up the sub- class and associate the Chilopoda with the 
Insecta. 



[chap. 



Order I. ChUopoda. 



The very active, lithe, chestnut-brawn, rather Serce-lookiiig 
little centipede, LithoHus forficatua, which is very common during 
the summer months under the bark of old trees, under leaves and 
other rubbish, is a good example of the CMlopoda (Gr. x^'*h ^ 
thoasand) or Centipedes. In the winter it bories itself in the soil 
The female lays her eggs from June till August and hastens to 
cover each with a thin layer of earth ; otherwise the egg is seized 
and devoured by her mate. 

If we examine a little more closely 
one of these Centipedes, we 
fcft?r™'' ^^^ see that the body is 
divided into a head followed 
by a very narrow segment and then by 
fifteen other segments of varying siie. 
The head bears a pair of long antennae, 
the first appendages, which are constantly 
waving about Close behind the point of 
origin of these antennae lie the eyea. If 
we turn the animal over and observe the 
under surface of the head we shall at 
once see a pair of lai^e vicious-looking 
clawa — the poison claws or fifth pair of 
appendages. The tip of each of these is 
pierced and, as it strikes the prey, a 
drop of poison is squeezed out which soon 
kills any insect or larva the Centipede 
wishes to eat. Although the tips of the 
poison claws are turned forwards beneath 
the head, yet these appendages really 
spring from the first segment of the trunk, 
which is enlarged and is known as the 

^ „ „ . , basilar segment. If we separate with a 

Fio. 82. A Centipede, Li- ■ - . . ji ^ 

thoMm /arficatai. Dorsal P*"' Of mounted needles these poison 

Mpectxi2. claws we shall see attached to the head 

1. Anteouae. 3. Poison the fourth pair of appendages, sometimes 

walking le(-s. resemble legs and have undergone little 

modification. They are reduced in size 

and each has a blunt functionlees gnathobase. In front of these 

the third pair of appendages or first maxillae take the form of a 




VII.] MYRIAPODA. 165 

lobed plate, being united with one another in the middle line. 
These cover in their turn the second pair of appendages or man- 
dibles which, like those of Insects and unlike those of Crustacea, 
consist simply of the blade, there being no palp or feeler. 

The fifteen segments Vrhich succeed the one carrying the poison 
claws each bear a pair of seven-jointed running legs ending in 
a pair of claws. 

Near the base of some of the legs — not of all — in the soft skin 
uniting the hard dorsal and ventral plates of chitin, 
stmcture^ is an oval opening. This leads into a chamber from 
which the tracheae pass off. These tubes divide and 
subdivide into smaller tubes, which run all over the body and traverse 
all the tissues, entering even the smallest cell. They are lined with 
a cuticle of chitin and are kept from collapsing by the presence of a 
fine spiral thickening of this cuticle which gives them a very charac- 
teristic appearance when seen through a microscope. They are full 
of air and constitute the respiratory apparatus of the Centipedes. 

The existence of such a breathing apparatus, which is confined 
to Peripatus, the M3nriapods, the Insects and certain of the 
Arachnids, is associated with certain striking features in the in- 
ternal anatomy of the body. In most animals the oxygen of the 
air is taken up by the blood and carbon dioxide is given out at 
certain fixed points called gills or lungs, and the vascular system is 
arranged so as to drive the blood through these specialized respira- 
tory organs. The blood takes the oxygen to the various tissues 
and takes from them the carbon dioxide which is removed from the 
body at the same centres. In the Tracheata however the air is 
itself conveyed by means of the tracheae to all the cells of the body 
and the gaseous exchange takes place on the spot. The blood has 
in the Tracheata lost one of its chief functions, the respiratory one, 
and exists chiefly as a nutritive fluid bathing the alimentary canal 
and taking up from it the soluble food which it conveys to the other 
tissues. It is kept in circulation by a contractile heart which lies 
along the middle dorsal line of the animal. In Lithobius this heart 
has a pair of ostia or openings in each segment, into which the 
blood from the pericardium pours, only to be sent out of the heart 
again at its anterior end into the general cavity of the body, for 
here the heart has an opening and there is no system of smaller 
vessels or capillaries. 

One of the peculiarities associated with the above-mentioned 
method of breathing is the nature of the excretory organs which rid the 



166 



AKTEMNATA. 



[OHAP. 



body of its nttrogenouB waste. 
In Peripattts ne find more or 
less tjpical nephridia and we 
meet with mod ificationa of these 
ID the coxal-glands of some 
Arachuids, bat in the Myria- 
poda and in the Insecta these 
organs are wanting and their 
place is taken by certain out- 
growths from the proctodaeum, 
called after iin Italian anatomist 
Ualptghian tubules. In 
iiitAobitis there arc two such 
tubules, blind at their free end, 
and at the other opening near 
the hind end of the alimentary 
canal (6, Fig, 83). Their walls 
contain traces of uric acid 
and urates which they have 
taken np from the blood and 
which they presumably excrete 
through the alimentary canaL 

The laBt-named organ is a 
straight tube which runs from 
one eud of the body to the 
other. A pair of salivary glands 
pour their secretion into it near 
the mouth, but no other di- 
gestive glands exist. Lithobius 
is carnivorous, living chiefly 
upon insects, their larvae and 
on earthworms. 

A large part of the spuce in 
the head is occupied by the 
bilobed brain which supplies 
the antennae and the mouth 
appendages. Tliis hrain is con- 
nected by means of para- 
ganglionated ventral cord which suppli 
rest of the body (Fig. 83). 

Myriapods are bisexual, the ovary and testis are continuous with 




Fid. S3. Lithobitu JoTfieatu; diaaecled 
to show intetasl org&nB x about 2. 
Arier Togt and Tung. 

I. AntflDDa. 3. Poison claw. 

3. Salirary gland. 1. Wallcing legi^ 
5. Ventral uerre-cord. S. HbJ- 

pigbiau tubule. 7. TMioula semin- 
alis. 8. Small accesBory glaud. 

9. Large accessory gland. 10. Un- 
paired testis. 11. AlUnentarj canaL 



commissures with & long 
i nerves to the legs and the 



VII.] MYRIAPODA* 167 

tbeir dacts which open to the exterior on the ventral surface of the 
last segment. 

Order II. Diplopoda. 

The second large subdivision of the Myriapoda is well ilhistrated 
by the black " wire-worm," lultis terrestris, very commonly found in 
Great Britain curled up under stones or burrowing in the soil, where 
it is said to do much damage by gnawing the tender roots of 
plants, for all Diplopods (Gr. 6(irAdo$, double) are vegetarians. 

The wire-worm is a black, shiny cylindrical animal with an 
enormous number of legs, in spite of which its movements are much 
slower than are those of Lithobius. The terga or dorsal shields are 
in this sub-group very much enlarged, whilst the sterna or ventral 
shields are very much reduced, and thus it comes about that the 
bases of each pair of legs, instead of being separated by the width 
of the body, are close together. Another peculiarity in this sub- 
division is that each tergum corresponds with two segments and 
that each apparent segment bears two pairs of legs and has the 
internal organs also duplicated. This double arrangement however 
only begins at the fifth segment behind the head. 




Pio. 84. lulua terrestrist Bometimes caUed the *' Wire- worm.*' From Koch 

xabont 8^. 

1. Antennae. 2. Eyes. 8. Legs. 4. Pores for the escape of the 

excretion of the stink-glands. 

The appendages on the head are : — (i) Short, usually clubbed- 
shaped antennae; (ii) mandibles; (iii) a single pair of maxillae 
fused into a lobed plate. 

Both the first two segments behind the head bear but one pair 
of legs, the third has no legs but carries the opening of the 
generative ducts. The fourth free segment has one pair of legs, the 
remainder two pairs. 

The female lulus lays its eggs, some 60 to 100, in an earthen 
receptacle she has prepared beneath the surface of the ground. 



1 68 ANTENN ATA. [CHAP. 

Sub-class C. Insecta 

The immense group of Insects far outnumbers in species any 
other group of animals and in all probability exceeds in number all 
the species of the rest of the animal world. New insects are con- 
stantly being discovered and although some quarter of a million 
have already been named and to some extent described, it is 
believed that at least as many more remain unrecorded. 

In spite of these numbers Insects are as a whole a uniform 
group and show less diversity in size and structure than many of 
the smaller groups, as for instance the Crustacea or the Mollusca. 
Probably their great number and small range of structural variation 
is not unconnected with the fact that they have found a new 
medium in which to pass some part, at any rate, of their life. The 
other group of animals which have taken to flying and lead an 
aerial life — the birds — show a somewhat similar range of species 
accompanied by a uniformity of structure which in their case is 
even more marked. 

Insects may be characterized by their body being divided into 

three distinct regions, the head, the thorax and 

feiaurei"** the abdomeu (Fig. 85). The head bears one pair 

of antennae and three pairs of gnathites. The 

thorax consists of three enlarged segments, each of which bears 

on the ventral surface a pair of legs and the two hindermost of 

wliich bear on the dorsal surface a pair of wings. The abdomen 

consists of a varying number of segments, ten being perhaps the 

usual number but fewer often occur. The abdomen bears no 

appendages except at the posterior end, where a pair of rod-like 

outgrowths — the anal cerci — are often found. 

Owing to the similarity of Insects to one another and their great 
number the study of them has become a very special branch of 
Zoology, which is termed Entomology. The necessity of ex- 
tremely detailed study is due to the same cause and a great number 
of technical terms are in use for describing the numerous structures 
which build up the body of the Insect. 

In this short book it will only be possible to indicate a few 
points about the anatomy of Insects and we will take as a type the 
common Cockroach because it is both a generalized form and not 
too small for dissection. 

The common Cockroach of the British kitchen is Stylopyga 
arierUalis, but a larger form, Periplaneta americana, is often met 




I. B, Sida view. 

1. AnteDD*. 3. Palp oC HchI maxilla. 3. Protliorai. 4. Anledor 
vingt. 6. Femnr of aecood leg. 6. Tibia. 7. Tusus. 6, Cerci 
kDftlM. 9- Stflea. 

1. Antenna. 2. Haul. S. Piothocai. 4. Anterior wing. 

5. Soft skin between terga and Bterna. 6. Sixth abdominal teteum. 
7. Bplit portion of tenth abdominal tergam. 8. Cerci analea. 9. States. 
10. Coxa of third leg. 11. Trochanter. la. Femnr. 13. Tibia. 

14. Tarans. IG. Claws. 



170 ANTENNATA, [CHAP. 

with on ships and from them makes its way to the docks ; it is also 
often found in zoological gardens, etc. PhyUodromia germanica, 
a small species, is becoming increasingly common in England. 

The whole body of the Cockroach is covered by a chitinous 
covering which varies in thickness, from the black hard head to the 
thin whitish areas which exist at the joints and which permit 
movement of the harder parts on one another. Except in the 
head the segments of the body can be detected externally, and, as 
in other members of the Arthropoda, the segmentation affects some 
only of the internal structures, such as the heart, the tracheae, the 
muscles and the nervous system, the other organs of the body not 
being influenced by it. 

The head of a Cockroach is a flattened structure placed at right 
angles to the axis of the body. It is oval in outline, its upper edge 
being considerably broader than the lower. It is loosely jointed to 
the thorax by a neck which permits considerable movement (Fig. 
85). This neck enters the head near its upper edge and below it 
the head hangs free. On the upper and outer edge of the head are 
a pair of kidney-shaped, facetted eyes of a shining black colour, on 
the inner curve of which the antennae or feelers have their origin. 
These are long whip structures, often as long or longer than the 
body ; they are made up of many joints and during life are in 
active movement, now stretched downward as if trying the ground 
on which the creature moves and now waving aloft as if testing 
the air. 

The mouth is on the lower edge of the head and is covered in 
front by a small moveable flap called the labrum or upper lip. 
At the sides it is protected by the first and the second pairs of 
appendages, and behind the fusion of the right and left third pair 
it forms a plate called the labium, which completes the boundaries 
of the mouth behind (Fig. 86). 

If the first pair of mouth-appendages or mandibles be removed 

from the head and examined through a lens, each is 

.pp.°„"iS,... seen to be a single-jointed stont jaw with a toothed 

inner edge which bites against the corresponding part 
of its fellow. It is characteristic of the mandibles of all Antennata 
to have no palp or remnant of the distal joints of the limb, such as 
is almost universally present in Crustacea. 

Behind the mandibles and like them situated on each side of 
the mouth, are the first maxillae. Each consists of a number of 
joints and each joint has a special name. Like the typical 



VII.] INSECT A. 171 

gnathite of other Arthropods we may regard them as consisting 
of a limb-like appendage with out-growths from the basal joints 
biting against corresponding processes — gnathobases — of the 
fellow appendage. There are two of these gnathobases, the hard 
pointed lacinia and an outer portion, the softer galea (B, Fig. 86). 
The lowest joints form an L-shaped hinge which, when opened out, 
protrudes the jaw. The outer portion of the first maxilla is many- 
jointed and is sensory in function, constantly touching and testing 
the ground as the animal moves about. It is called the maxil- 
lary palp. 

The second maxillae are united across the median line and thus 
constitute a fold or plate called the labium, which bounds the 





ABC 

Fio. 86. Mouth-appendages of Stylopyga. Magnified. 

A. Mandible. B. Ist maxilla. 1. Cardo. 2. Stipes. 3. Lacinia. 
4. Galea. 6. Palp. C. Bight and left 2nd maxillae fused to form 
the labium. 1. Submentnm. 2. Mentum. 3. Lignla, corresponding 
to the lacinia. 4. Paraglossa, corresponding to the galea. 5. Palp. 

mouth behind as the labrum bounds it in front (G, Fig. 86). Each 
half may be resolved into elements similar to those of the first 
maxillae, the fused basal joints of the pair of appendages form the 
mentum and sub-mentum, the galea being represented by the 
paraglossa, whilst the inner gnathobase corresponds with the 
lacinia and is termed the ligula. As in the case of the first 
maxilla the outer joints of the appendage which have a tactile 
frmction are termed the labial palp. 

The thorax is built up of three segments, the pro-, meso- and 
meta- thorax. The skeleton of each segment con- 
sists of a dorsal hard piece, the tergum, united 
with a ccmesponding ventral piece, the sternum, by a soft in- 



172 ANTENNATA. [CHAP. 

tervening pleural membrane. The tergam of the first thoracic 
segment is clearly visible, but the meso- and meta-terga are 
concealed by the wings. The two pairs of wings are formed by 
folds of the skin arising from the terga of the meso- and meta- 
thorax respectively, and in a state of rest conceal the dorsal surface 
of the animal behind the prothorax. The front pair are termed 
elytra ; they are hard and horny, one overlaps the other, and they 
probably serve more as protectors to the delicate hind wings than 
as organs of flight. The posterior wings are thin and membranous 
and are of greater area than the elytra, and they constitute the 
effective organs in the rare flight of the CockroacL At rest they 
are folded like a fan and concealed by the elytra. In Stylopyga 
orientalis the wings — as is not uncommon amongst Insects — are 
rudimentary in the female. 

The ventral plate or sternum of each thoracic segment bears a 
pair of legs by means of which the Cockroach scuttles rapidly about. 

Each leg consists of a number of joints, viz., a thick flat coxa 
applied to and articulating with the sternum ; a minute triangular 
joint, the trochanter; a stout joint called the femur; a more 
slender one termed the tibia, armed with spines ; then a piece con- 
sisting of five short joints called the tarsus, with a whitish hairy 
patch under each joint which acts as a sole ; and finally a pair of 
terminal claws (Fig. 85). These names, as is too often the case in 
Zoology, were suggested by fanciful and misleading comparisons 
with the parts of the limb of a vertebrate. 

The abdomen consists of ten segments and here the terga and 
sterna can be easily seen as they are not obscured by 
the insertion of the wings and legs. The eighth and 
ninth terga are however both tucked under the seventh and are not 
readily seen until the animal is artificially stretched. The tergum 
of the tenth or last segment stands out from the hind end of the 
animal and is cleft into two lobes. The sterna are equally distinct 
but the first is small The abdomen is broader in the female than 
in the male, and the seventh sternum is shaped like the bow of a 
boat and projects backwards, hiding the posterior sterna and sup- 
porting the lower surface of a roomy pouch or cavity in which the 
egg-case is formed. In the male the seventh sternum conceals the 
eighth and ninth. 

The stigmata leading to the tracheae are placed in the soft 
pleural membrane connecting terga and sterna. 

The abdomen is usually regarded as being without appendages^ 



VIL] INSECTA. 173 

but a pair of jointed cerci anales emerge below the edge of the 
tenth tergum in each sex, and in the male the ninth sternum bears 
a pair of anal styles. The claim of these structures to be 
reckoned as appendages of the same rank as the antennae, the 
gnathites and the legs, was at one time not generally conceded, 
but appears to be now fairly established for the cerci anales. 

In the soft tissue between the tenth tergum and the last visible 
sternum, at the hind end of the body, is placed the anus, below 
it the single genital pore is situated. The anus is supported by 
certain thickened plates in the skin, known as podical plates, 
and around the genital orifices are arranged certain rods and bars, 
symmetrical in the female but asymmetrical in the male, whose 
functions and meaning are obscure, but which are connected with 
the processes of copulation and of egg-laying. As we have seen, 
this region is in the female enlarged into a genital sac by the 
growth and modification of the seventh sternum and the tucking 
into the space so formed of the skin carrjdng the eighth and ninth 
sterna. The opening of the oviduct is on the eighth sternum and 
on the ninth is the single opening of the receptaculum seminis or 
spermatheca, which consists of two pouches of unequal size composed 
of intumed ectoderm : these are always found full of spermatozoa 
in the fertilized female. The spermatozoa apparently leave the 
spermathecae when the eggs are being laid and fertilize the ova 
whilst they are in the genital sac. Two glands, consisting of 
branching tubes, — called colleterial glands — open separately 
behind the spermatheca. They secrete a fluid which hardens to 
form the egg-capsule in which the eggs are laid. This is moulded 
in the genital sac and may often be seen half-protruding between 
the distended seventh sternum of the mature female. 

When the skin is removed from the dorsal surface of the Cock- 
roach the cavity laid open is not a coelom but a 
structure! haemocool, and it is largely filled by a loose white 

tissue, known as the fat-body, which surrounds the 
various internal organs. If the alimentary canal be disentangled 
from this it is at once evident that in Insects, unlike other Arthro- 
pods, the intestine is longer than the body and the larger portion 
of it which lies in the abdomen is coiled in order to stow it away. 
Like the digestive tube of other Arthropods a large part of its length 
consists of the stomodaeum and proctodaeum. The former consists 
of an oesophagus which quickly passes into a large crop in which 
the food is stored for a time. The lining of both these regions 



174 antennatjL [chap. 

bears haira and the muscles in their walla are striped. The crop 
is followed by a gizz&rA, which bears intenially six hard chitinotu 
teeth, and behind them are fine hairs which act as straiDeis, so 
that only finely divided food can pass on into the meeenteron or 
chylific ventricle, as the part of tho alimentary canal ie called 
which alone is lined by endoderm and is capable of absorbing 
noarishment. This tabe is produced in front into seven or eight 




Fio.'.ST. A Female Cookroaah, Stylopyga, with the doraal exoakeletoo removed 

ftnU diasected to show the viscera. Magnified about 2. 
1. Head. 2. Labmm. 3. Anlenna cat short. 4. E;e. 6. Crop. 

6. Nervous system of crop. 7. Gizzard. 8. Hepatic caeca. 9. Mid-gut 

or mesenteroQ. 10. Malpigfaian tubules. II. Colon. 13. Bectum. 

13. Salivary glands. 14. Salivary receptacle. 15. Brain. 

16. Ventral nerve-cord with ganglia. 17. Ovary. 18. Spermatheca. 

19. Oviduct. 20. Genital pouch in which the egg..coooon is found. 

21. Colleteriul glands. 22. Anal cercus. 

pouches, the so-called hepatic diverticula. The mesenteron, the 
limit of which ia marked by the insertion of the Malpighian tubules, 
is succeeded by the intestine or proctodaeum, a long coiled tube 
which enlarges posteriorly and opens by the anus. The enlarged 
portion is called the rectum (Fig, 87), the anterior coiled [Kirtioii 
the colon. 



Vn.] INSECTA. 175 

Lying along the crop on each side is a pair of branched glands 
— the salivary glands — and a bladder termed the salivary 
reservoir. All three are provided with long ducts. Those of the 
two glands on each side unite to form a single tube which then 
receives the duct of the reservoir, and the common ducts of the two 
sides open behind the mouth but in front of the second maxillae. 
The saliva converts starch into sugar. The secretion of the hepatic 
diverticula emulsifies fats and turns insoluble proteids into the 
soluble forms (peptones). This secretion seems to pass forward into 
the crop and there true digestion is effected. The digested and 
dissolved food then passes through the filter of the gizzard into the 
mesenteron, where absorption of the nutritious parts is effected, the 
undigested portions passing on to the intestine and so out of the 
body. The cells of the mesenteron undoubtedly exert some action 
on the products they absorb, though we are ignorant of its precise 
nature, but in the end they pass on the products of digestion — altered 
no doubt — to the blood which everywhere bathes the alimentary 
canal, and by it the new material is conveyed all over the body. 

The blood is kept in circulation by the heart which lies in the 
middle dorsal line close under the skin ; in fact it can be seen 
through the skin in the region of the abdomen. It is a long vessel 
made up of thirteen chambers corresponding with the three thoracic 
and ten abdominal segments, each chamber opening into the one in 
front and the whole somewhat resembling a row of funnels fitted one 
into another. At the broader hinder end of each chamber is a pair 
of ostia or holes through which the blood enters from the peri- 
cardium, and there are valves which prevent the blood being forced 
out of the ostia or forced backward when the heart contracts, so 
that its only course is to flow forward. The pericardium is separated 
from the rest of the haemocoel by the pericardial septum, in which 
however there are certain holes which permit an interchange of con- 
tents between the two cavities. When at rest the pericardial septum 
is arched upwards, but it is pulled outwards and flattened by the 
periodic contraction of certain muscles attached to its sides called 
the alary muscles, thus enlarging the pericardium and causing an 
inflow of blood into it from the rest of the haemocoel. This blood 
enters the heart when it is relaxed and the ostia are open, and hence 
by the alternate contractions of the alary muscles and the muscular 
wall of the heart the circulation is maintained. 

The anterior end of the heart, called the aorta, opens by a 
trumpet-shaped orifice into the haemocoel and the blood pours out 



176 ANTENNATA. [CHAP. 

of this and bathes all the organs of the body. It thus takes np the 
soluble food which has left the alimentary canal and conyejrs it to 
those parts of the body where it is needed, and in a similar way it 
yields up its superfluous fat to be stored up in the fat bodies and 
gives up its waste nitrogenous materials to the Malpighian tubules, 
whence they are passed out of the body. It is noticeable that just 
as water-plants are as a rule not of a compact shape, but finely 
subdivided so as to expose as large a surface as possible to the 
surrounding medium, so in the body of the Cockroach the various 
organs are long, tubular or diffuse structures offering a large 
surface to the nourishing and purifying blood. 

The heart contracts almost as frequently as the normal human 
heart, Le., about seventy-two times a minute when the Cockroachi is 
at rest, but at other times its rate of contracting varies a good deal 
The blood is colourless and contains amoeboid corpuscles. It is 
slightly alkaline. 

It is obvious from the above account that in the Cockroach the 
blood is mainly a means of conveying nutriment to the organs and 
taking certain waste matter from them, and that unlike what is 
usual in other animals, its respiratory function is at a minimum. 
Owing to the nature of the tracheae, the air with its oxygen is 
taken directly to each organ, almost to each cell, without the 
intervention of the blood. 

The tracheal system opens to the outer air by ten pairs of oval 
pores or stigmata. These lie in the soft integument between the 
terga and sterna, one pair just in frx>nt of the mesothorax, one 
pair just before the metathorax and eight pairs just in trout of 
each of the first eight abdominal segments, so that they seem to be 
intersegmental in position. These openings lead into tubes or 
tracheae which soon bifurcate and divide. The larger branches 
have a definite and symmetrical arrangement. There are dorsal 
arches running up towards the heart and ventral arches descending 
towards the nerve-cord. These arches are connected with one 
another longitudinally by trunks which run on each side of the 
pericardium and the nerve-cord. Large trunks also are given 
off to the alimentary canal. It follows that should one stigma 
become blocked the organs which its tracheae supply are still 
provided with air. The finer branches become smaller and smaller 
until they become veritable capillaries which penetrate every tissue. 

The tracheae being full of air present a glistening silvery 
appearance which is unmistakeable. They are prevented from 



vil] insecta. 177 

collapsing by the presence of a spiral thickening of the chitinous lining 
which runs round the interior of the tube just as the wire spiral 
strengthens the india-rubber tube of some kinds of garden hose. 
Respiration is elTected by the alternate arching up and flattening of 
the abdomen, resulting in an alternate increase and diminution of 
its volume, which, since the blood is incompressible, secures an 
alternate inrush and expulsion of air from the tracheae. In all 
probability only the contents of the larger tracheae are affected by 
this process ; by diffusion the oxygen from this ** tidal air '' is handed 
on to the finer tracheae. 

In Insects the tracheal mode of respiration reaches its highest 
development and it is accompanied by a correlative diminution and 
simplification of the circulatory apparatus, the oxygen of the air 
being conveyed directly to and the carbon dioxide removed directly 
ttom the cells to the outer air, whilst the blood loses its respiratory 
function. It is doubtful how far this state of things is connected 
with the peculiar disappearance of the coelom — which presumably 
exists only in the cavity of the reproductive organs — because a 
similar replacement of the coelom by a haemocoel is found in 
Crustacea and Mollusca, where the blood is respiratory and the gills 
are compact and active organs ; but probably it is correlated vdth 
the modification which the excretory system undergoes and this 
again is undoubtedly influenced by the state of the body-cavity. 

Peripatus, the most primitive Arthropod, has typical nephridia, 
each of which opens internally, not into a general coelom, but into 
a small sac which is really a special part of the coelom not com- 
municating with any other coelomic space but belonging only to 
the nephridium which opens into it. The main body-cavity is a 
haemocoel Mollusca also retain a pair of typical nephridia or in the 
case of NaiUilus two pairs. These organs open into special coelomic 
spaces which are as a rule small, the more spacious cavities of the 
body being haemocoelic. Crustacea probably retain nephridia, those 
of the fourth segment — shell glands — being persistent in the En- 
tomostraca, whilst those of the second segment — green glands — 
persist in the Malacostraca. It is not absolutely proved but it 
seems probable that the inner ends of these glands represent 
coelomic spaces. But even amongst Crustacea, in certain Amphipods 
for instance, we find the transference of the function of the nephridia 
to outgro?rth8 of the alimentary canal. In Arachnids the nephridia 
or coxal-glands show very varying degrees of development, but on the 
whole they are tending to die out and to be replaced by Malpighian 

a. AM. \^ 



178 ANTENNATA. [CHAP. 

tubules or diverticula of the intestine. Finally in Mjrriapods and 
Insects, where the true coelom is reduced to its minimum and 
where the tracheal mode of respiration attains a very high degree of 
development, all traces of nephridia have disappeared and the waste 
nitrogenous matter is excreted solely by the Malpighian tubules. 

These are very fine long caecal diverticula, — so fine as to be but 
just visible to the unaided eye,— 60 — 80 in number, arranged in six 
bundles which open into the beginning of the narrow part of the 
proctodaeum from which they are outgrowths. They float in the 
blood, winding about amongst the abdominal viscera (Fig. 87). 
They contain crystals, probably of urate of soda, which are taken up 
from the blood and which leave the body through the intestine. 

The nervous system of the Cockroach is constructed on the same 
plan as that of the other segmented Invertebrates with which we have 
had to do. There is a large supra-oesophageal ganglion or brain 
giving off commissures, which encircle the oesophagus and unite 
below in a sub-oesophageal ganglion. Together these occupy a 
considerable portion of the cavity of the head. The supra-oeso- 
phageal ganglion supplies paired nerves to the eyes and to . the 
antennae and is thus the sensory centre. The sub-oesophageal 
ganglion supplies the mandibles and both pairs of maxillae. From 
it two cords pass backward and bear three pairs of ganglia in the 
thorax and six in the abdomen. This difference between the number 
of nerve ganglia and the number of segments is carried to a much 
greater extent in some Insects where, as in Spiders, all the post- 
oesophageal ganglia tend to fuse into a common nervous mass in 
the thorax (Fig. 58). 

The only specialized sense organs are the eyes and the antennae. 
The eyes have fundamentally the same structure as those of the 
Crustacea : the antennae are the seat of the senses of smell and 
taste, and are in addition very delicate tactile organs. The 
maxillary palps are also tactile and are constantly touching and 
testing the ground on which the Cockroach is moving. 

Cockroaches are bisexual. The ovaries in the female consist of 
two sets of eight tubes, each of which has developed 
from a coelomic sac in the embryo. They unite at 
their anterior end into two cords which pass to the dorsal wall of the 
thorax and become attached to the pericardial septum, and at their 
posterior end they fuse into two short oviducts which join to form a 
small uterus (Fig. 87). Each of the sixteen tubes contains cells, 
some of which become ova, and as they approach the oviduct the ova 



I'Tn.i 



INSBOTA, 



betMine arranged in a single row. At the same time they increase 
greatly in size by the deposition of yolk in the egg, ho that an oviun 
just before it leaves the body is of considerable size. 

The eggs are apparently fertilized after leaving the uterus by 
spermatozoa which emerge from the spermatheca (in which they 
have been deposited by t)ie male) situated behind the opening of 
the utcni.s. Whilst still in the genital pouch the fertilized eggs 
are surrounded by the secretion of the coUeteria! glands which 
o]>en behind the spermatheca, and this secretion hardens into the 
egg-capsule or cocoon. 

The paired testes of the male are functional only during youth 
»nd as they diminish in size after they cease to function, they aro 
only to be found with difficulty. They he concealed by the fat body 
below the terga in the region of the fourth, fifth and sixth aMominal 
eegmciits. They look somewhat like elongated bunches of cherries, 
their translucent colour strongly contrasting with the opaque 
white of the fat body. Two vasa defereutia lead from the testes to 
B pair of large reuervoira colled vesiculae semiuaies, which together 
form the "mushroom-shaped gland." Iiit() these at an early age 
the cells destined to form spermatozoa pass Irum the testes and 
there they undei^o their further development. The mushroom- 
shaped gland opens to the exterior by means of a short muscular 
tube, the ductus ejaculatorius, which has its orifice jnat below the 
-anus. The name of the glaud is derived from its form ; it has a 
tliick stalk surrounded by a crown of branches. Fertilization and 
ovipositioii take place during the summer. 

Sixteen eggs are laid in eai^h egg-capsule, and for some seven 
or eight days, until the mother finds sumo warm aud secluded 
hiding-place to deposit her load, she carries about the capsule half- 
protruding from the genital pouch. When the embryos in the eggs 
art! fiilly formed, which takes about twelve months, it is said that 
they secrete some fluid, probably saliva, which dissolves the upper 
^rt of the capsule and so permits of their escape. In Phyllodromia 
ggrmaiiirn the mother is said to take a part in freeing her oflspring 
from their temporary imprisonment. When they first appear they 
are while with dark eyes, but the integument soon thickens and 
darkens. They have no wings, but in other respects they resemble 
their parents and thus there is no metamorpliusis such as occurs in 
the Butterflies and many other Insects. They run actively about, 
devouring any starchy food they can find, and when in time they 
{[row too large for their coat of mail it splits and a soft Cockroach 



180 ANTENNATA. [CHAP. 

extricates itself therefrom. The integument soon hardens again. 
This casting of the sldn or ecdysis takes place seven times, and after 
the seventh moult, when the insect is four years old, it is adult 

The Insects are usually subdivided into eight Orders, which 
are mainly based (i) on the structure of the gnathites ; (ii) on the 
nature of the wings ; (iii) on the amount of Metamorphosis which 
the life-history of the Insect presents. A short account of each of 
the criteria is therefore subjoined. 

The Mouth' Parts {Gnathites) of Insects. 

The mouth-parts of Insects can in almost all cases be resolved 
into a pair of mandibles which never bear palps and two pairs of 
maxillae which are usually provided with palps. In the diflferent 
orders of Insects and in different members of the orders these 
mouth-appendages show many modifications and are put to a veiy 
great variety of uses. One or other part may be suppressed and 
disappear, others may coalesce, as is the case with the right and left 
second maxillae of the Cockroach, but as a rule traces of all the 
gnathites may be found though often much altered. The mouth- 
parts of insects have been grouped as follows : (i) Biting, This kind 
of mouth is found in the Aptera, the Orthoptera, the Neuro- 
ptera and the Coleoptera (v. below), in which orders there is as a 
rule no very great difficulty in recognizing the various parts which 
have been described in the Cockroach. As an example of the great 
variety presented by the mandible, those of the male Stag-beetle, 
Lucanus cervus, may be mentioned. In this animal the mandibles 
may equal in length the whole of the rest of the body, (ii) Suckino, 
This kind of mouth is found in the Lepidoptera or Butterflies. 
Here the mandibles are rudimentary, but the gnathobases of the 
first maxUlae are much elongated and frequently coiled into what is 
sometimes termed the proboscis. Each half is grooved and so 
applied to the other as to form a tube, and in some cases the two 
halves of the tube are locked together by minute hooks. The palp 
of this maxilla is absent or rudimentary. The labium composed of 
the second maxilla is an important structure in the larva or 
caterpillar, as it forms the spinnerets through which the silk of 
the cocoons is excreted, but in the adult it is practically absent 
although its palps persist as large hairy structures. The hollow 
tube formed by the maxillae is well adapted to suck up the fluids 
on which the Lepidoptera live. The suction is performed by a 



VII.] INSECTA. 181 

X)owerful muscular sac called the suctorial-stomach, which is a 
lateral outgrowth of the oesophagus and communicates with it. 
(iii) Piercing and Sucking, The Diptera or Flies possess both 
sacking and piercing organs, which are as a rule somewhat unequally 
developed. The basal portion of the labium or second maxilla is 
much elongated and takes the form of a fleshy protuberance which 
to some extent ensheaths the other parts. In the house-fly, 
Musea domestica, the piercing organs are fused to the labium and 
act only as supporting rods for it, but in the Gnat or Mosquito 
they are iree and reach a high degree of development. These parts 
are overlaid by a somewhat enlarged labrum and between this and 
within the grooved labium the pointed stylets lie. Amongst these 
a sharp style called the hypopharynx situated behind the mouth 
may be distinguished. Two pairs of lateral stylets, identified by 
some as the modified mandibles and first maxillae, also exist and 
the maxillary palp acts as a sensory organ. The Hemiptera or Bugs 
have a very similar set of gnathites in correspondence with their 
habit of boring into animals or plants to feed on their juices. 
In this Order the mouth-parts when not in use are bent under the 
body and lie along the under surface of the thorax. The labium 
is jointed and its edges are curved so as to form an incomplete tube, 
only the base of which is partly covered by the labrum. Within 
this groove four sharply pointed styles — the mandibles and first 
maxillae — work. They are as a rule finely toothed like little saws 
and are well adapted for piercing the skin. There are no palps, 
(iv) Biting and Sucking, The Hymenoptera or Bees and Wasps 
have mandibles not unlike those of a Cockroach and use them for 
biting and moulding their food of pollen and the wax they secrete 
from their bodies. The laciniae of the maxillae form blade-like 
structures and their palps have much diminished in size. The labial 
palps however are large, and the conjoined median outgrowths from 
the labium (corresponding to the ligula of the Cockroach, 3, Fig. 
86 c) form a kind of grooved tongue along which the nectar in the 
flowers is sucked up. 

It thus appears that although the mouth-parts of Insects are 
highly modified in connexion with the kind of food they live on and 
the modes in which they obtain it, nevertheless the various mouth- 
parts have a common ground plan, and although the authorities 
differ as to details a fundamental similarity runs through these 
appendages in the different Orders. 



ANTENNATA. 



IVingt Q/' IiniiKls. 



1 



The wingB of Insects are folds of the integanient, ftfttteiied 
HO that r.lie two sides are iu coutuct oq their inneT surfaces. At 
certain places however »1its are left and throagh these tracheae 
pass taking air to the wings. The ectoderm of both upper and 




Flo. H8. Pach<ilj/tui 



r. A OtaBsUopper. 



under layers is also thickened along certain lines and the prepuce 
of these thickenings divides the wing up into a number of areas. 
These lines are nsintlly called wing-reins or win(r-nerves, but as 
they are neither veins nor nerves it is better to call them nervures. 
The presence and disposition of the nervures is of the highest 



Vn.] . INSECT A. 183 

importance in classification. Wings may be thin, membranous, and 
transparent, as in the Grasshopper (Fig. 88) or Dragon-fly (Fig. 50), 
where there is an enormous number of nervures, or in the Flies 
and Bees, where there are few nervures, or they may be thickened 
and strongly chitinised as in the front wings of Beetles. In this 
last group the anterior wings are called elytra (6r. cXvrpov, a cover) 
and they always meet together in a median longitudinal line, 
80 that when they are closed the insect appears to be wingless 
(Fig. 92) : in some few cases they have fused together so that the 
posterior or flying wings are rendered useless. In one great 
division of the Hemiptera one half of the anterior wing is horny 
and strongly chitinous, the other and posterior half membranous. 
In the Cockroach, as we have seen, the anterior wings tend to 
become homy and are of little use in flight (Fig. 85). The posterior 
wings of the same insect when at rest are folded together something 
like the leaves of a shut fan and many species in several of the 
orders fold up their hind wings when not in use. The tucking 
away of these wings under the small elytra is a complicated affair 
in some Insects such as the Earwig, where the nippers at the end 
of the body are said to aid in the process. Many Insects however, 
such as the Dragon-flies, Plant-lice, Butterflies and Moths, Flies 
and Bees and numerous others, do not fold up their wings but either 
bear them erect or lying depressed on the body. In some cases the 
wings are quite transparent, but in the Moths and Butterflies they 
are covered with a dense fur of flattened scales which can readily 
be brushed off as a fine powder oi dust (Fig. 91). It is these scales 
which give rise to the beautiful and in some cases gorgeous colouring 
of the Lepidoptera. 

Two pairs of wings are present as a rule, but the order Diptera 
has only the anterior pair, the hind wings being replaced by certain 
stalked structures called balancers or hal teres. In the Hymeno- 
ptera, the two wings of each side are clamped together by means of 
hooks on the hind wing which fit into a ridge on the hinder edge 
of the front wings (Figs. 93, 94, and 95). The two wings of each 
side thus move as one. It is not uncommon to find isolated 
species in which the wings are not developed, for instance, the 
females of many Plant-lice and some Moths ; while Fleas, which 
are sometimes placed amongst or near the Order Diptera, never 
possess wings, though their absence is compensated for by a special 
development of the powers of jumping. 



184 



ANTENNATA. 



[chap. 



Metamorphosis, 

A very large number of animals that live in the water, whether 
in the sea or fresh water, hatch out from the egg in a lanral condi- 
tion. That is to say, the being which leaves the egg is veiy 
unlike the adult in structure and habits, but by growth and a series 
of accompanying changes in time passes over into the adult which 
is capable of reproducing. These changes constitute the meta- 
morphosis. In the life-history of land animals such lanral stages 
are rare and indeed hardly exist outside the Insecta and the 
Amphibia. As we have seen, the egg of a Cockroach gives rise to a 
young Cockroach which differs but little from the adult and gradu- 
ally grows into it by a series of small changes, but which never at 
any time undergoes a long period of profound rest. But if we con- 
sider the case of a Moth or Butterfly we shall find that the egg does 
not give rise to an animal resembling a minute Butterfly but to a 
worm>like larva or caterpillar, which has no wings and in other 
respects is very unlike the Insect which produced the egg. This 
caterpillar as a rule eats voraciously and grows rapidly with little 
change in form until its fourth ecdysis, when a sadden change 
occurs, and the so-called pupal stage supervenes. In this stage 
with a few exceptions the animal now called a pupa (Lat. pupa, a 
puppet) is motionless and ceases to feed. It may be uncovered and 

protected only by the hard- 
ened integument or it may 
be enclosed in a casing or 
cocoon. In the case of 
the Silkworm Moth and 
some others, this is con- 
structed of silk. During 
the pupal stage, the ani- 
mal undergoes a profound 
change, many of its organs 
and tissues being broken 
up and new ones con- 
structed. When this process is completed the pupa casts its skin, 
makes its way out of the cocoon and emerges as an imago (Lat. 
imago, an image) or perfect insect. 

The various orders of Insecta diff^er in the degree to which meta- 
morphosis occurs. In the Aptera there is no metamorphosis and 
the development is said to be direct. In Orthoptera and Hemi- 




Fio. 89. Larva of Bomhyx mori, the Silk- 
worm. Life size. 



itera there is no quiescent pupa stnge and the chief difference 
the larva aad adult is the absence of wings. Here the 
metamorphosiB ia said to be incomplete. The 
is true of most of the Neuropt«ra. a 
Tery varied assembly of Insects, in some forma 
pf which however, e.g. the Caddis-flies, a pupal 
eatists. Amongst the Lepidoptera, Coleo- 
ptera. Hymenoptera and Diptera there is a, well- 
Biarked pupal stage, and these orders are said 
to have omplete metamorphosis. Various names 
have been given to the larvae of Insects without 
Tery precise definitions. Those of the Lepido- 
ptcra are usually called Caterpillars, They are 
often gaudily coloured and bear tufta and 
lionches of hair. Besides the three pairs of legs 
which are found on the three segments following 
the head and which correspond with the legs of 
the imago, certain of tlie alidominal segments bear fleshy stumps 
called abdominal legs. The larvae of some of the l^aw-flies (Hymeno- 
ptera) have a similar bright colouring and resemble Caterpillars, and 
like them feed exposed on leaves, etc. Tiie larvae of Beetles and 
Jnost Hymeuoptera are aa a rule hidden underground or in galls 
wax-comb. They are whitish in colour and unattractive, and 




Fig. 90. Cocoon of 
Bombt/T auiri, from 
wbicb siltc is span. 
About Ute BiKB. 




Pio. 91. Si!k-worm motli. Iinmhy.r mori. 

A. Feanle. B, Male. 

are often termed gruba, whilst the footless white larvae of the 

Diptfira, which are for the most part deposited in some organic 

hilntVLce — whether alive or not — are usually called maggots. 

In the following account of Insect Classification we can only 
indicate the chief charat'ters of each Order and mention the names 
of one or two common members of each. 



186 ANTENNATA. [cHAP. 

Order I. Aptera. 

Wingless Insects, with scales and hairs coyering the body. 
The mouth-parts are adapted for biting. They move by running or 
by springing by aid of a caudal style which is kept bent forwards 
under the abdomen and retained in this position by a yentral hook. 
When released from this hook the recoil of this style hurls the 
insect into the air. The segments of the thorax are not fused 
together and there is no metamorphosis. 

Not all wingless Insects belong to this Order. The name 
Aptera (Or. airrcpos, wingless) refers to the belief that the ancestors 
of these Insects never had wings and that thus they represent a 
lower stage of evolution than the rest of the sub-class. 

For the most part the Aptera are minute Insects living in 
retired spots under leaves or rubbish, in root-gutters, etc., but 
they are widely distributed over the world. One of the best known 
is the Silver-fish, Lepisma, which hides in disused cupboards, old 
chests of drawers, sugar barrels, etc. It runs with great rapidity. 

Order 11. Orthoptera. 

The Orthoptera (Gr. 6p66^, straight ; TrrcpoV, a wing) have mouth- 
parts adapted for biting. The anterior wings are as a rule stiff, 
and when the Insect is at rest one overlaps the other, and both 
usually cover and conceal the large membranous hinder wings with 
which the creature flies. There is an incomplete metamorphosis, 
the young being at first without wings. 

This Order is a very varied one and doubts exist as to whether 
it is a natural one. It includes the Cockroach, whose anatomy has 
already been described ; the Earwig, Forfictda ; the Mantis ; the 
leaf and stick insects, Phyllium and Phasma ; the Grasshopper 
(Fig. 88), Acridium, Pachytylus ; the Locust, Locusta ; the Cricket, 
Gryllus \ and many others. 

Order III. Neuroptera. 

The Neuroptera (6r. vtvpovy a tendon and hence a nervure) have 
biting mouth-parts. Both pairs of wings are membranous and used 
in flight, and the '' veins " of the wings form a more or less close 
network. Metamorphosis complete or incomplete. 

This Order, like the preceding, contains many families which, 
except as regards the structure of the wings, have little resemblance 
to one another. The following are a few of the more widely known 



VII.] 



INSECTA. 



187 



species: the White-ant, Termes; the May-fly, Ephemera', the 
Dragon-fly, Libettula or Aeschna (Fig. 50) ; the Ant-lion, Myrmeleo ; 
the Aphis-lion, Hemerobius ; the Caddis-fly, Phryganea, which in 
some respects approaches the Lepidoptera ; and the Thrips, an insect 
which injures com crops and certain flowers and is sometimes 
elevated to the position of a distinct Order. 

Order IV. Coleoptera. 

The Coleoptera (Gk. koXco?, a sheath) have mouth-parts adapted 
for biting. The anterior wings are hard and homy and fit together 
in the middle line with a straight suture. The hind wings are 
membranous and folded. The metamorphosis is complete, that is, 
there is an active larval stage (Fig. 92), followed by a quiescent 
stage during which extensive changes in the internal anatomy take 
place. 

Unlike the preceding, this order is clearly defined and its 
members are on the whole very like one another. It has always 
been a favourite order with Ento- 
mologists and the number of species 
named and described is far greater 
than in any other Order. It in- 
cludes all the Beetles. For the 
most part these Insects are dull in 
colour but their firm exoskeleton 
gives them a very definite outline 
and renders their preservation and 
identification comparatively easy, 
which may to some extent account for their popularity with 
collectors. 

Order V. Hymenoptera. 

The Hymenoptera (6r. vftcio-nrcpos, membrane-winged) have 
mouth-parts adapted for biting and sucking. The ligula of 
the labium is long and grooved, whilst the paraglossae are smaU. 




Fio. 92. In the centre Coccinella 

septempunctata, the Lady-bird 

X about 2^, with the larva to the 

left X about 2^, and the adult 

beetle, natural size, to the right. 





3 1 

Fio. 98. Formica rufa, the Wood-ant. 
1. Female. 2. Male. 3. Neuter. 



The mandibles are well-dereloped and the l&ciDi&e of tl 
maxillae large. The four wings are alike, membraDoua in t 
and the liiad wingH are hooked od to the anterior in siu 




way that the two wings of each side move together. They differ 
from the wings of the Neuroptera in possessing fewer veins. The 
metamorphosis is complete, 

This group comprises the Ants, Bees and Wasps. Many of 
them live in highly complex vommunities and in their social habtte 
and general intelligence they reach a level which is only surpassed 
by man himself. The group iucladea the Wood-wasp, Sirv^ ; 



assed 
; the J 



FiK. 05. Politttt (epidtu unci nest. 

Saw-fly, Tenthredo; the Gall-fly, Ci/nips; the Ichneumon; the 
Ant, Formica ; the Wasp and Hornet, Vfspa ; the Humble -bqa, 
liombus ; and the Honey-bee, Apig. 



Order VI. Hemiptera. 



j-bfl^ 



The Hemiptera (Gr. ijfu, half) have mouth-part^ arranged for 
piercing and auckiug. The basal part of the labium is elongated 



VII.] INSECTA. 189 

and tubular and the mandible and first maziUa form sharp pointed 
stales. The two pairs of vings may be alike or may differ and 
the ant6ri(n' pair are is some cases half homy and half membranous. 
The metamorphosis is incomplete, there being no quiescent stage. 

The membera of Una Order present very great divergence both 
of fonn and of size ; they are colloquially known as Bugs and Lice. 
Amongst the commoner forms are the Water-boatman, Notonecta ; 
the Water-scorpion, N^epa ; the Bed-bng, Acanthia ; the Cicada, 
remarkable for its chirping noiae ; the Frog-hoppers, including the 
Cuckoo-spit, Apkrophora ; the Plant-louse, Aphis ; the Phylloxera, 
which destroys vines ; Scale Insects and lice. 

Order VII. Diptera. 

The Diptera (Or. Sfnr^pa, two -winged) have mouth-parts 
arranged for piercing and sucking. The only difference in this 
respect from the Hemiptera consists in the 
fact that the sucking tube is partly formed 
by the labrum and that the first maxillae 
retain palps. Only one pair of wings, the 
anterior, are present; the posterior are re- 
presented by a pair of short knobs called 
baUncers or halteree (Fig. 97). The meta- 
morphoeis is complete. 

The Diptera or Plies form one of the 
largest of the Insect Orders, probably as 
large as the Coleoptera, although at present 
the nimiber of species of Beetles 
named and described is far greater 
than that of Flies. Amongst the 
commoner genera are the Gnats and 
Mosquitoes, Culex ; the Daddy- 
long-legs, Tipala; the Gall-fly, 
Ceeidomyia (Fig. 97); the Horse- 
fly, Ta&anus ; the Bot-fly, Oestrus ; 
the common House-By and Blue- 
bottle, Mutca, and many othera. 
The Flea, PaJex irritans, which is 
wingless but endowed with con- 
siderable powers of jumping, is 
sometimes placed in a Sub-order nf 
the Diptera and sometimes in a separate Order. 




». 97. Cteidomyia dettructor, 

the Heasian.fly. 
Insect. 2. Larva. 3. Pnpa, 
ir "flax seed." All magDified. 



190 ARACHNIDA. [CHAP. 

Order VIII. Lepidoptera. 

The Lepidoptera (Gk. Xcttis, a scale, '7rT€p6v, a wing) have mouth- 
parts adapted for suckiog only. The two pairs of wings are similar 
in appearance and covered with scales (flattened spines) which give 
rise to the beautiful pattern on the wings but are easily rubbed off. 
None of the wings fold up and when not in use are either held 
erect or are depressed on each side of the body. The metamor- 
phosis is complete. 

This Order is very clearly defined and the members show a 
marked resemblance one to another. It includes the Butterflies 
and Moths, and all of them exhibit a very definite and complete 
metamorphosis. The eggs give rise to worm-like larvae known as 
caterpillars, which consume much food, generally of a vegetable 
nature (Fig. 89). After a considerable time, varying from a few 
weeks to three years, the caterpillar comes to rest, and in such 
cases as the Silk-worm Moth, Bombyx mori, surrounds itself by a 
case or cocoon spun by itself, which furnishes the material silk 
(Fig. 90). Within this cocoon, or in some species without forming 
a cocoon, the caterpillar forms a pupa, and whilst in this state it 
undergoes a very thorough reorganization and gradually the mature 
Insect is built up ; after a certain time this emerges and occupies 
its comparatively short life in the propagation of its species (Fig. 91). 
The female usually deposits its eggs on or near the plants which 
serve as food for its offspring. 

Class III. Arachnida. 

The third large group of the Arthropoda is a very varied one 
and contains many animals which differ markedly in their structure 
one from another. Perhaps the most distinctive features of the 
External Arachuida (Gk. apax*^» * spider ; cTSos, shape) are 
features. ^j^ There are no true gnathites. No appendage loses 

all other functions and becomes exclusively a jaw, although the 
proximal joints of several are prolonged inwards towards the mouth 
and help to take up food ; in a word some of the limbs have 
developed gnathobases; (ii) The most anterior appendages are 
never antennae but always a pair of nippers, termed chelicerae ; 
(iii) The active catching and walking legs of the fore part of the 
body orprosoma are strongly contrasted with the plate-like modi- 
fied limbs of the middle part of the body or mesosoma when the 



VII ] . ARANEIDA. 191 

latter exist, but in many cases these have disappeared and in others 
have become so modified that they are no longer recognisable as 
limbs. Nearly all Arachnids moreover agree in having the anterior 
end of the body, the prosoma^ as it is called, marked off from the 
rest and covered by a single piece, the carapace. The rest of the 
body or abdomen is in some forms differentiated into two regions, 
the mesosoma and metasoma, but in other cases this distinction 
does not exist ; it may be segmented or it may not. The prosoma 
bears six pairs of appendages and of these the last four are usually 
walking legs. The appendages of the abdomen are connected with 
the respiratory function and are much modified, often — in the 
terrestrial forms — forming floors for the respiratory chambers. The 
breathing apparatus may be tracheal, or in a few marine forms 
branchial, or may take the form of respiratory chambers, the last 
named and the gills having a peculiar form found only amongst 
Arachnida. They consist of '' books " of thin superposed lamellae 
attached to the posterior aspect of an appendage. When modified 
for breathing air these "books" are called lung-books. When, 
as is the case in Limulus, they breathe oxygen dissolved in water 
they are called gill-books. The genital orifice is usually on the 
anterior end of the abdomen and ventral : the group is bisexual. 
Many different Orders are included in the Arachnida, the best 
known being perhaps those which include the Spiders, the Harvest- 
men, the Mites and the Scorpions. The last named are found only 
in warm climates and Mites are too small for investigation with the 
naked eye, so that we will take the Spider as an example of 
Arachnid structure. 

Order I. Araneida. 

Spiders belong to the Order Araneida (Lat. aranea, a spider), 
in which the abdomen is unsegmented and soft. The second pair 
of appendages, the pedipalpi, are leg-like and modified in the male 
in connexion with the fertilization of the female. The abdomen 
bears certain modified appendages called spinnerets, on which 

1 The name cephalothorax is often applied to this region, but the term is 
too misleading to be used. The cephalothorax of Decapod Crustacea includes 
the first thirteen segments of the body: the prosoma of Arachnida only includes 
six, and therefore corresponds roughly to the ''head" of the higher Crustacea. 
Similar criticism might be launched against the use of the word ''abdomen,'' 
bat here the error is too deep-rooted for correction since the term is used in 
describing both Cmstacea and Insecta, and in each case in a different sense. 



192 ARACHHIDA. [CHAP. 

Open the glanda, the secretion of which prodnces the Spider's web. 
If we examine such a Spider as Epeira diademata, which is common 
enough in English gardens, sitting on or near its wheel-shaped web 




Y^^F-^' 



Via. 98. The Oarden Spider, Epeira diademata, utting in th« oentre of its mb. 
Arter BUnohard. 

(Fig. 98), we notice that behi&d the proeoma 
there is a slender waist and that this ia 
followed by a large swollen abdomen with 
no outward trace of division into segments, 
or into meeo- and meta-soma. 

Tbere are six pairs of appendages, and 
Bitcrnai >'' ^^ ^^ °^*^ noticoable that 
■tnicture. \^eiQ are no antennae or feelers 
to act as sensory organs. Their function is 
to some extent taken over by the long walk- 
ing legs. The first pair of limbs are called 
chelicerae: in i^tra these are two-jointed, 
the terminal joint being pointed and folded 
1. Head. 2. Eyes, down against the basal joint except when 

3. Btual joint of che- , . . ,n.- n,.i ni- c 

Ijoerae. being used (Fig. 99). This pair of appen- 

4. Claw of ohelioene. dages contains poison glands and the poison 




Front Tiow of 
the head of a Spider, 
Tfxtrix denticulala. 
Magnified. From 

Warburton. 



VII.] ARANEIDA. 193 

escapes through an opening at the point of the second joint. By 
means of it the Spider can kill insects and seriously hurt larger 
animals. 

The second pair of i^pendages in the Arachnida are called 
pedipalpi (Fig. 100). In Epeira they resemble the walking legs, 
but in the male at the final moult the last joint becomes altered 
and forms a hollow sac — the palpal organ — which plays an im- 
portant part in fertilizing the female. 

Then follow four pairs of walking legs each with seven joints 
and terminated with two or three claws ; in some species they are 
provided with a pad of short hairs called a scopula, which helps 
the animal to run on walls and ceilings. 

The mouth is very minute, for the Spider does not swallow solid 
food but sucks the juices of its prey. It lies between the bases 
of the pedipalps, and the basal joint of each of these appendages 
has a cutting blade termed the "maxiUa" (2, Fig. 100). It is a 
common feature of the Arachnids that the basal joints of one or 
more of the pairs of appendages are produced inwards towards 
the mouth and act as jaws, but the modification never goes so 
far as to obscure the limb-like form of the appendage and so pro- 
duce a true gnathite. 

On the ventral surface of ' /a#--7 

the abdomen just behind i^llJS^ 

the waist is situated the m^r 

genital opening, protected by / ; W^Si 

a plate which is the result of \ lUjl^^p- 

the fusion of a pair of ap- \\ \^«(lJ^. e 
pendages, and on each side of N^^ftj^ 
this is the slit-like orifice of ^^v \s v\A\\a\V 

a lung-book. The lung- ^4^ . , 

books are very remarkable ^ »^^^^^^^^^^ 

structures. Each opens to the 4 ^ 

exterior by a pore through ^ 

which the air enters, and con- Fig. 100. Pedipalp of TegenaHa guyoniiy 
. ^ - , V •!. ^ the large house- spider. 

sists of a sac the cavity of 

which is largely occupied by \.Co-^„ ^^-3. ^TrS".' *'!" F^'^ 

a number of thin plates in 5. Patella. 6. Tibia. 7. Tarsus. 

the substance of which the ^- ^^^P*^^ ^^«*"- 

blood circulates, and is thus 

brought into close relationship with the air which passes in and 

out between the neighbouring plates ; the sac is floored in by a 

s. <& M. \^ 




194 A,IUCHNIDA. [chap. 

Bpeciid plate which is & modified appendage (Fige. 63 and 65). Sach 
a breathing apparatus is peculiar to the Arachnida. In some 
Spiders we find a second pair of Inng-books placed behind ihe 
others, and io other species this second pair is replaced by a pair of 
tracheae recalling the respiratory mechanism of the Myriapoda or 
the Insects (Fig. 63). They have however been independently de- 
veloped, and probably owe their origin to the sac of a lung-book 
from which the lamellae have disappeared. 




1. Month. 2. SnckinR stomach. 3. Ducts at liver. 4. Malpigfaiui 
tubaloa. S. Storoorsl pockal. 6. Anua. 7. Dorsal muscle of 

sucking etomach. 8. Coecal prolongation of etomoch. 9. CerebnU 
giuiglioii giving off nerves to eyes. 10. Bub-oesophogeai ganglionic mass. 
11. Heart with three lateral openiiiRS or ostia. 12. Lung-book. 

13. Ovary. li. Acioate and pyriform ulk glands. 15. Tubiiliform 
silk gland. 16, Ampulliform silk gland. IT. Aggregate or dendrifonn 
Bilk glands. 18. Spinnerets or mammillae. 19. Distal joint ofcheli- 
cero. 20. Poison gland. 21. Eye. 23. Pericardinin. 38. Teasel 
bringing blood from lung sao to pericardium. 31. Arteij. 

Near the binder end of the abdomen are four tubercles or 
spinnerets, and if these be pushed aside, two more, 
shorter in length, come into view. These are the 
oif^B which form the web and they have been shown to be vestiges 
of abdominal appendages. They are very mobile and are pierced 
at their ends by hundreds of minute pores through which the 
silk exudes as a fluid, hardening on exposure to the air (18, 
Fig. 101). 

The silk is secreted by a large number of glands which have 
their exit at the above-mentioned pores. Of these in K diademcUa 
there are five different sorts and each secretes a special kind of 
thread ; for the various lines in a Spider's web differ considerably 



VII.] ARANEIDA. 195 

one from another, in accordance with the use they are put to. 
The circular lines are sticky and help to catch insects for the 
Spider's food, the radial lines are stout and form a framework for 
the support of circular lines ; the threads with which the Spider 
binds up its captured prey di£fer from these, and there is still 
another kind of thread with which it constructs its cocoons, and 
each kind of line is supplied from diiferent sets of glands. 

The dissection of a Spider requires much care, since the organs 
almost fill the hody and are completely embedded in the large 
masses of the digestive and reproductive glands. The oesopha- 
gus, which leads from the mouth, opens into a strong sucking 
"stomach," which is really like the stomach of the Cray-fish, a 
stomodaeum. This is attached by muscles to the chitinous exo- 
skeleton, and when the muscles contract its cavity is enlarged and 
thus a sucking action is induced at the mouth (Fig. 101). Behind 
this is an endodermic portion of the alimentary canal which gives 
off certain caeca or blind tubes, followed by an intestine which 
traverses the abdomen and is frirther provided with a number of 
ducts which collect the products of a very capacious digestive gland 
or " liver." The intestine, which is also lined by endoderm, is fol- 
lowed by a short proctodaeum, the proximal portion of which is 
swollen up into a pouch called the stercoral pocket. It ends in an 
anus situated close behind the spinnerets. 

Spiders possess two kinds of organs which excrete waste nitro- 
genous material : (i) the Goxal glands, which are true nephridia, 
i.e., glandular tubes running between a reduced coelom and the 
exterior, and (ii) Malpighian tubules, a pair of simple pouches opening 
into the endodermal intestine and thus in their origin differing 
from those of Insects. The coxal glands are better developed in 
some species, such as the common House-spider, Tegenaria derhatnii, 
than is the case in E. diademata^ where they are very degenerate 
and where their frinctions seem to have largely passed to the 
Malpighian tubules. In fact these structures are an interesting 
example of a set of organs degenerating and of their functions 
being assumed by another set. 

The heart of the Spider is of the same general type as that of 
Myriapods ; it is a tube with paired slit-like openings — ostia — at 
the sides, through which the blood enters to be driven out again 
through certain rather ill-defined vessels to circulate in the spaces 
between the various organs. 

The nervous system is concentrated ; there is a bi-lobed ganglion 



196 ARACHNIDA. [CHAP. 

above the oesophagus which gives off nerves to the eyes and the 
chelicerae ; this is connected by two lateral cords, which pass one on 
each side of the oesophagus, with a large nervous mass situated in 
the thorax. From this, nerves pass off to supply the remaining five 
pairs of limbs and two nerves arise which pass backward and 



1.. 




Fia. 102. Diagrammatic view of Palpal Organ. 

1. Tarsus. 2. Bulb. 3. Yesicula seminalis. 4. Opening of vesicula 
semiualis. 6. Conductor. 6. Haematodocha. 7. Alveolus. 

supply the abdomen. The only conspicuous sense organs of Spiders 
are the eyes, which are ** simple " ; of these in E, diademata there 
are four large eyes arranged in a square on the top of the head and 
two small ones on each side of the square. This number, eight, is 
not uncommon in Spiders, where both the number of eyes and their 
disposition are much used in systematic classification. 

The male, as is not uncommon amongst the Araneida, is 
smaller than the female. The ovaries and testes lie in the abdomen 
and have the form of a network of tubes, a form characteristic 
of ^Vrachnida ; the spermatozoa are conveyed to the palpal organs 
of the pedipalpi of the male and by them introduced into pouches, 
the spermathecae of the female. The eggs are fertilized before they 
are laid, which latter event usually takes place in October, when they 
are enclosed in cocoons of yellowish silk. The young are hatched 
out in the following spring and at once begin spinning. By means 
of the minute threads they secrete they weave a kind of nest about 
the size of a cherry-stone which hangs suspended from some twig or 
leaf. At the least disturbance the hundreds of young Spiders in 
the nest begin to disperse ; the spherical nest breaks up as into 
dust, but when the disturbance is at an end the minute Spiders, 



VII.] PHALANGIDA. 197 

SO small as to be almost mvisible, re-assemble and agaia form their 
little spherical nursery. 

The number of species of Spiders is very great and their habits 
are very diverse and well worthy of study. 

Order 11. I^alangida. 

The Phalangids (Gr. ij/SXayY^oy, a venomous kind of spider) or 
Harvestmen are in common talk usually classed viith Spiders, but 




F:ij. 103. A PhalsDgid or Harvestman, OUgolophui tpii, 
I. Chelioeru. 



lu, adalt malex 2, 
L, second, third, and 



they difTer from the latter in having no waist, that is, the abdomen 
is not separated from the prosoma by a constriction, and they 
breathe entirely by tracheae. They have four long and very 
slender pairs of legs, which easily break, and their eyes are some- 



198 ABA.CHNIDA. [CHAP. 

times elevated &boTe the sur&ce of tihe head on a tubercle like a 
look-oat tower. The abdomen is distinctly divided into segments. 

Aa a rule these creatures are nocturnal and are nsnally met witli 
in dark comers or amongst the stalks of hay or grass. Their long 
legs enable them to steal with a gliding spring upon their prey, 
foi the most part insects or spiders, for they are carnivorous. They 
are dull in colour, grey, brown or blackish, as becomes an animal 
that loves the dnsk. About twenty-fonr species have been recorded 
in Qreat Britain. Fhalangids die down as winter sets in, but the 
eggs last through the cold weather and give rise to a new generation 
in the spring. 

Order III. Acarina. 
The Acarids (Gr. axSpU, a morsel) or Mites form an enormous 
order whose function in life is to a large extent to pl&y the 




1. Pedipolpi. 2. Chelioerae. 3. 4, 5, 9. First, aecoad, third and fODrtb 
volking legs. 7. Chitinoiu thiokeninga mpporting legs. 8. Farrow ~ 
roand bodj. 9. Reprodactive opening, flanked bj two saokeiB on each 

side. 10. Adob. 11. Sockera at liila of anna. 

scavenger, and the terrestrial forms confer the same benefits on t^e 
dwellers on the Earth that the Ostracoda and many of the smaller 
Crustacea do on the aquatic fauna. Many of them however have 
adopted parasitic habits and cause disease amongst larger animals, 



Vn.] ACARINA. 199 

while some induce the formation of galls and other deformities 
amongst plants. Most of the Mites, as their name indicates, are of 
minute size; but the female Ticks, belonging to the family Ixodidae, 
which live amongst the undergrowth of forests on the look-out for 
some vertebrate prey, can when they become attached to their 
hosts — man, cattle, or even snakes — ^by distending their bodies with 
the blood they suck, swell out to the size of hazel-nuts. 

Anatomically they are difficult to characterize. like the 
Phalangids, they have no waist, and when special breathing organs 
are present they take the form of tracheae ; they differ however 
from the Phalangids in never showing signs of segmentation. The 
chelicerae may be clawed or chelate, like a lobster's claws (Fig. 104), 
but they often take the form of piercing stylets and the pedipalpi 
may form a sheath to protect them. 

The number of species is very great ; amongst the commoner 
forms may be mentioned, Tetranychus telarius, often known as 
the Bed Spider, which spins webs under leaves in which whole 
colonies shelter. This species is believed to do great damage in 
hot-houses. Tyroglyphus siro, the Cheese-mite, which burrows in 
decaying cheese, and the genus Phytoptus, which causes the conical 
galls on lime-trees, maples, etc., are also familiar. 

Order IV. Scorpionida. 

Scorpions are not found in Great Britain, though they are common 
on the Continent of Europe around the Mediterranean basin and 
generally in warm climates. They retain a more marked segmenta- 
tion than is the case with the other Arachnids we have considered. 
The abdomen is very long, distinctly segmented and differentiated 
into two portions ; (a) the mesosoma, consisting of seven segments 
of the same diameter as the prosoma, bearing the respiratory 
appendages ; (b) the metasoma, a much narrower part, consisting 
of five segments and a curved spine like a tail at the apex of which 
is the opening of a poison gland. The mesosoma has six pairs of 
appendages. The first of these forms the genital operculum, a plate 
bearing on its posterior aspect the genital pore in both sexes ; the 
second are " pectines," curious comb-shaped structures, whose exact 
function is not yet determined, but which are morphologically 
reduced and thickened gill-books. The third, fourth, fifth and sixth 
segments bear each a pair of lung-books, and it has already been 
explained that the floors of these are formed of highly modified 



ARACHNIDA. 



[chap. 




1. Chelicera. 

walking lege. 
Bide of bod;, plei 



2. Pedip&lp. 3, 4, S, 6. 3id to 6th appendages, oi 

, Lateral ejen. 8. Median eyea, 9. Soft tissae »1 
0. 10. The poison stiog or telson. 



1 — 6 &s in A. 7. The genital operculum. B. The pectinea. 9, 10, 
11, 12. The tour right stigmata leadiog to the (our luug-bookB. 18. The 
last segment of the mesoeoma. 14. The third segment of metasoma. 
15. The teUon. la eaoh oase the metasoma, which is usually carried bent 
forward over the meio- and pro-soma, has been strughtened out. 



VII.] 8C0RPI0NIDA. 201 

plate-like appendages which in the adult have lost all trace of 
their origin from limbs. The seventh segment of the mesosoma 
shows no traces of limbs and tapers to join the first segment of 
the metasoma. At the posterior end of the fifth metasomatic 
s^ment, on the ventral surface, is situated the anus, and behind 
this is a conical pointed joint which contains the poison glands 
and which forms a very efficient and powerful sting. The whole of 
this tail is very mobile and the sting can readily be directed to any 
point In life the tail is usually borne turned forward over the 
body so that the sting threatens the head. 

Both the chelicerae, which are small and short, and the pedipalpi, 
which are long and six-jointed, end in nippers, the latter recalling 
the appearance of the claws of a lobster. The four pairs of walking 
legs end in claws. 

The mouth is very minute, for like the Spiders Scorpions only 
suck the juices of their prey. They feed for the most part on 
Insects and Spiders. The basal joints of the first two pairs of 
appendages, like those of the pedipalps in Spiders, are all produced 
towards the mouth, forming gnathobases which probably help to 
hold their food. 

Scorpions usually hide under rocks and stones during the day, 
being often very intolerant of heat, but they creep out as dusk 
comes on and run actively about. The Scorpion is viviparous, the 
young being bom in a condition resembling their parents. 

Order V. Xiphosura. 

A very peculiar aquatic Arachnid called Limulits, or popularly 
the '* King-crab," inhabits the warm seas on the Western side of the 
Pacific Ocean and along the shores of the Western Atlantic. It is 
a littoral form, that is to say, it lives not far from the shore ; it 
burrows in sand or mud at a depth of firom two to six fathoms, often 
lying with only its eyes, which are on the top of the body, exposed. 

The shape of the body is something like a half-sphere with a 
piece cut out and a long spine is attached to the truncated side. 
This spine has given the name Xiphosura (Gr. f «^os, a sword ; ovpd, 
a tail) to the Order. The half-sphere is hiaged, and the part in 
front of the hinge is the prosoma ; the rest is the abdomen or 
meso- and meta-soma. On the upper surface of the half-sphere are 
a pair of simple eyes near the middle line, and there is a pair of 
compound eyes situated further back nearer the edge. The under 
surface of the half-sphere is partially hollowed out and concealed in 



202 -ARACHlflDA. [chip. 

this hollow on each edde of the middle line of the prosomA are six 
pairs of appendsgeB. The most anterior of these are typical nipper- 
like chelicerae, the next is not specially modified to form a pedipalp, 
but it and the remaining four pairs are walking legs. All of them 




Fjq. 106. BoraAl view of the EiDg-crabj Linmlui polyphtt 



send inwards a spiny gnathobase, which helps to form the border 
of the mouth. The sixth pair of limbs end in some flattened blade- 
like structures which aaaist in digging and burrowing in the sand 
and in extracting the worms which form the principal item of the 
diet of the King-crab. The aeventii appendages, or the first on the 



VII.] XIPHOSOBA. 208 

meBoeomti, take the form of a flattened plate or operculnm which 
bears the reprodnctive pores on ita posterior siir&ce. It is bent back 
and underlies the eighth, ninth, tenth, eleventh, and twelfth pairs of 
appoidages, which are aUo plate-like and each of which bears on 
its posterior snrface a gill-book. There is a striking aimilarity 




Fta. 1D7. T«iitral view of the King-crab, Limtilui polyphemat x \. 
1. Cuapue aoTering pidBoma. 2. Meso- and meta-aoma. 3. TelsoQ. 
4. Cbelioent. 6. P«dipalp. 6, T, S. 9. 3rd to 6th appendages, 

atnbnlatoi; limtw. 10. Qenital opecoulam tarned forward to sboir the 

genital aperture. II, 12, 13, 14. IS. Appendages bearing giU-books. 

16. Anna. 17. Month. 18. ChUaria. 

between these organs and the " lung-books " of the Scorpion ; 
the latter, however, do not project, but are sunk in pite. The 
I terminates at the anus, but behind it a long sword-like 



204 ARACHNIDA. [CHAP. 

tail projects. This post-anal tail corresponds with the swollen 
stinging tail or telson of the Scorpion. It is used by the animal 
to right itself when it is upset by the motion of the waves. 

A curious plate of fibro-cartilage to which muscles are attached 
lies inside the body near the ventral surface. It is formed of modi- 
fied connective tissue in which a cheesy material termed chondrin 
has been deposited in the ground substance, and is largely built up of 
interlacing tendons of muscles so that it acts as an internal support- 
ing structure or endoskeleton. It is called the endosternite. 
Possibly it was a feature of primitive Arthropoda, as similar 
endostemites occur in many other Arachnida and in some of the 
more primitive Crustacea. 

The internal anatomy differs in many points of detail from that 
of the Spider, but in essentials there is a fairly close resemblance. 
Unlike the Scorpion, Limulus lays eggs and these are fertilized 
in the water and pass through what may be termed a larval stage. 

In many respects Limulus seems to be related to the extinct 
Eurypterina, whose fossil forms are so abundant in the Upper 
Silurian and Old Red Sandstone formations ; and like some species 
of Limulus they attained a great size, two feet or more in length 
being not uncommon. The Eurypterines were aquatic and indeed 
seem to form an intermediate stage between the Scorpion and 
Limulus^ and confirm us in the conclusion drawn from the anatomy 
of Limubis that this animal retains in many points the habits and 
structure of the marine ancestors of Arachnida. 



Phylum ARTHROPODA. 

Bilaterally sjrmmetrical Coelomata whose coelom has undergone 
great change. Segmented animals with the segments usually arranged 
in groups. Paired hollow and jointed limbs on some of the segments. 

Class I. Crustacea. 

Aquatic Arthropods usually breathing by gills, with two pairs of 
antennae. A limb-bearing thorax usually fused with the head and 
followed by a segmented abdomen which may be limbless. 

Sub-class A. Entomostraca. 

Small, simple Crustacea with varying number of segments. The 
stomach has no teeth. The larva is a Nauplius. 



VII.] CLASSIFICATION. 205 

Order 1. Phyllopoda. 

Long-bodied and usually well segmented with a shield-like 
shell protecting head and thorax and sometimes abdomen ; with 
leaf-like swimming appendages. 

Sub-order i. Branchiopoda. 

Large-bodied forms with no dorsal brood-pouch, second 
antennae not enlarged for swimming, numerous swimming 
appendages. 

Ex. Apus, Branckipus, Artemia. 

Sub-order ii. Cladocera. 

Small, short forms with bivalved shell, a dorsal brood- 
pouch and enlarged swimming second antennae. 

Ex. Daphnia, Simocephalus. 

Order 2. Ostracoda. 

Usually small forms with body unsegmented. At most 
seven pairs of appendages and a rudimentary abdomen, all shut 
up in a bivalve shell. 

Ex. CypriSy Cypridina, 

Order 3. Copepoda. 

Usually elongated and clearly segmented but often much 
modified by parasitism. Four or five pairs of biramous thoracic 
appendages. 

Ex, Cyclops, Argulus, 
Order 4. Cirripedia. 

Sessile animals whose not clearly segmented body is enclosed 
in a fold of skin strengthened by calcareous plates. Usually 
five biramous thoracic appendages. Hermaphrodite as a rule. 

Ex. LepaSf Baktmis, 

Sub-class B. Malacostraca. 

Large Crustacea as a rule with five segments in the head, eight 
in the thorax and six in the abdomen. Nauplius larva very rare. 

Order L Leptostraca. 

Bivalve shell covering the eight free thoracic segments but 
not fused with them, abdomen of eight apparent segments with 
anal forks. Thoracic limbs leaf-like. 

Ex. Nebalia. 



206 ARACHNIDA. [CHAP. 

Older 2. Thoracostraca. 

All or most thoracic s^ments fiised with head and covered 
by a cephalothorax. Eyes as a role stalked. 

Sub-order i. Schizopoda. 

Eight pairs of biramoos thoracic appendages. Eyes 
stalked. 

Ex. Mysis. 
Sub-order ii. Decapoda. 

Thoracic segments fused with head. Last five thoracic 
appendages uniramous and used for walking. Eyes stalked. 

Division a, Macrura. 

Abdomen long. 

Ex. Astaais, 

Division b, Brachjrtura. 

Abdomen short. 

Ex. Cancer^ Carcinus. 

Sub-order iii. Stomatopoda. 

Cephalothoracic shield short. Five pairs of maxillipeds. 
Abdomen large and bearing gills on its appendages. 

Ex. Squilki. 
Sub-order iv. Cumacea. 

Four or five free thoracic segments. Two pairs of 
maxillipeds. Eyes sessile. 

Ex. Cumay Diastylk, 

Order 3. Arthrostraca. 

Seven, rarely six, free thoracic segments. No cephalo- 
thoracic shield. Eyes sessile. 

Sub-order i. Amphipoda. 

Body laterally compressed. Gills on thoracic appen- 
dages. 

Ex. Gammarua, 
Sub-order ii. Isopoda. 

Body dorso-ventrally compressed. Gills on abdominal 
appendages. 

Ex. Asellus, Parcellioy Oniscu$. 



VII.] CLASSIFICATION. 207 

Class II. Antennata. 
A single pair of antennae and with tracheal respiration. 

Sub-class A. Prototracheata. 

Soft, caterpillar-like bodies with numerous pairs of appendages. 
Nephridia present. 

Ex. Peripatus, 

Sub-class B. Myriapoda. 

Terrestrial, with head well marked off from body, which consists 
of many similar segments bearing six- or seven-jointed appendages. 

Order 1. Chilopoda. 

Animal flattened dorso-ventrally, bases of legs wide apart : 
to each tergum corresponds one pair of legs : the segment 
following the head has a large pair of poison claws : genital 
opening between the last pair of legs. 

Ex. Lithobius, 

Order 2. Diplopoda. 

Animal cylindrical, bases of legs close together : to each 
tergum behind the fourth correspond two pairs of legs: no 
poison claws : genital opening on the third segment behind 
the head. 

Ex. lulus. 

Sub-class C. Insecta. 

Body divided into three regions, head, thorax and abdomen. 
Head bears the antennae and three pairs of persistent mouth parts ; 
thorax three pairs of walking appendages and usually two pairs of 
wings ; abdomen as a rule without appendages. 

Order 1. Aptera. 

Wingless insects with hairy and scaly bodies ending in 
anal filaments. No metamorphosis. 

Ex. Lepisma, 

Order 2. Orthoptera. 

Jaws biting, wings usually unalike. Metamorphosis in- 
complete. 

Ex. Forfictda^ Stylopygay Phasma^ Acridium^ Gryllus. 



208 ARACHNIDA. [CHAP. 

Order 3. Neuroptera. 

Jaws bitiDg, sometimes sucking. Wings alike, membranous, 
with many nervures. Metamorphosis varies. 

Ex. Teimes, Ephemera, Libellula, Phryganea, 

Order 4. Coleoptera. 

Jaws biting. Anterior wings hard and curving together 
with a median, straight suture. Metamorphosis complete. 

Ex. Coccinettay Melolontha. 

Order 5. Hymenoptera. 

Jaws biting and licking. Four membranous wings with 
few nervures. Metamorphosis complete. 

Ex. Formica^ Apis, Vespa, 

Order 6. Hemiptera. 

Jaws piercing and sucking. Wings alike or different. 
Metamorphosis incomplete. 

Kx. Acanthia, Cicada, Aphis. 

Order 7. Diptera. 

Jaws piercing and sucking. Hind-wings reduced, front- 
wings membranous. Complete metamorphosis. 

Ex. Culea:, Musca, 

Order 8. Lepidoptera. 

Jaws sucking. Four similar wings covered with scales. 
Metamorphosis complete. 
Ex. Bambyx. 

Class III. Arachnida. 

No antennae and no true gnathites. Frosoma of six appendage- 
bearing segments followed by a meso- and meta-soma whose appen- 
dages are when present usually much modified. 

Order 1. Araneida. 

Meso- and meta-soma soft, unsegmented. Four to six 
spinnerets, two to four lung-books. 

Ex. Epeira. 



Vn.] CLASSIFICATION. 209 

Order 2. Fhalangida. 

No waist between pro- and meso-soma which latter with 
meta-soma is segmented. Tracheate. 

Ex. Oligolophus. 

Order 3. Acarina. 

No waist. Minute and often reduced forms mostly tracheate. 
Ex. Tyroglyphus, Tetranychus. 

Order 4. Scorpionida. 

Meso-soma seven segmented in adult, meta-soma five seg- 
mented and ending in a post-anal poisonous telson. Four 
lung-books. 

Ex. Scorpio, 

Order 5. Xiphosura. 

Shield-shaped carapace covers prosoma. Meso- and meta- 
soma fused. Gill hooks. The telson forms a spine. 

Ex. lAmulus. 



S.ftM 14 



210 



CHAPTER VIIL 
Phylum Mollusca. 

MoLLirscA (Lat. mollis^ soft) is the name which is given to one 
of the largest and most important phyla of the animal kingdom. 
In it are included not only our terrestrial snails and slugs and 
many fresh-water species but also the oysters, mussels, periwinkles, 
whelks and countless other species of "shell-fish," bivalve and 

univalve, which crowd the rocks laid bare at low- 
descriptfon. Water around our coasts: and in addition to these, 

the extraordinary Octopuses, Squids and other forms 
of Cuttle-fish belong to the same great phylum. The name Mollusca 
seems to have been suggested by the fact that the members of the 
phylum do not possess any internal hard parts such as are found in 
Man and other vertebrates. This softness of internal constitution 
is shared by other classes with no relation to the Mollusca, as for 
instance the great group of the Arthropoda. The Arthropods 
however possess a horny covering which closely invests them and 
following every irregularity of their outlines, so that it seems a 
real part of themselves. This is the exoskeleton or cuticle, which 
constitutes one of the great difi'erences between them and the 
Mollusca. The latter, it is true, possess also an exoskeleton 
composed principally of calcareous matter, but this adheres only 
to a part of the surface. It is usually very thick and easily 
detached, and so it is frequently looked on as a separate thing 
&om the animal and is known as the shell. The shell is to be 
looked on as a secretion produced by a part of the skin only: 
this part of the skin, which almost always projects from the rest 
of the body as a flap, is called the mantle. The space between 
the mantle-flap and the rest of the body is known as the mantle- 
<javity. The mantle-cavity shelters the gills or organs of respira- 
tion, and into it open the kidney or kidneys and the anus, and 
usually also the genital ducts. 



tiil] oastebofoda. 



Class I. Qastebopoda. 

In order to fix our ideas we may take the common ] 

garden snail, Helix aspersa, which has also established 

o( 3^11^''*'" ^t^olf throughout considerable areas in North America, 

or, if procurable, the larger Helix pomatia, which on 

account of its size la easier to dissect, as a type of the Mollnsca. 

In Lower Canada the genus Helix is not very abundant, and the 



1. MoDth. 3. Anterior tentacles. 8. Eye tentaoles. 4. Edge of muitle. 
6. Bespintor; poie. 6. Ann*. 7. Apei of shell. 8. Foot. 

9. Bepiodaotive apertitra. 

latest species, Helix ctStolabrut, ia rather small for convenient 
dissectioD. Livmaea stagnatis, the large river-snail, is however 
common and easy to obtain, and its structure is similar in its main 
outlines to that of Helix. 

The shell is coiled into a spiral form ; the body contained in it 
consisto of a visceral hump, coiled like the shell and closely 
adhering to it, and of a poTtion which we call the head, neck, and 
foot, which can be drawn within the openiug of the shell if the 
animal is alarmed, but which under ordinary circumstances is quite 
outside it The snail is devoid of anything in the nature of 
legs, — an important character of the MoUusca as contrasted with 
the Artbropoda, — but the part of the body next the ground is 
a flat muscular surface called ^e foot. By means of wave-like 
contractions of the longitudinal muscular fibres of this organ the 
snail moves along, always preparing the ground for itself by de- 
poeitJDg a layer of slime on it. This slime is poured forth irom a 
gland which opens in Iront of the foot, just beneath tlie mouth 

11—2 



212 MOLLUSCA, [chap. 

(14, Fig. 111). The foot is one of the most important oigans of 
the Mollusca; it takes different shapes in the different groups but 
always assists locomotion. In the pond-mussel, for instance, it is 
shaped like a wedge, in order to force a path through the soft mud 
at the bottom of the ponds in which the animal lives. The 
different shapes which the foot assumes afford the chief basis for 
the classification of Mollusca. 

The head of the snail bears two pairs of feelers, or tentacles, 
which are hollow outgrowths of the body-wall (2, 3, Pig. 108) : these 
when irritated are protected by being pulled outside in, and so 
are brought into the interior of the body. The first or shorter pair 
are supposed to be the chief seat of the sense of smell : the second 
and longer pair have at their tip a small pair of black eyes. These 
eyes are merely minute sacs, the walls of which are made of light- 
perceiving cells, connected at their bases with a nerve which leads 
to the brain; in the cavity of the vesicle is a horny lens which 
nearly fills it up. The eyes of nearly all the Mollusca are con- 
structed on the same plan, but in the Cuttle-fish not only is the 
vesicle large and spacious and the lens proportionately smaller, but 
there is in addition a series of folds of skin surrounding the place 
where the eye comes to the surface, which constitute an outer 
chamber, and outside this, eyelids, so that the whole organ acquires 
a superficial similarity to the human eye. 

If we carefully pick away the shell of the animal and lay bare 
the visceral hump, brushing away any mucus which may adhere to 
the body, we shall see on the right side of the animal a round hole 
(5, Fig. 108). A bristle passed through this reaches into a large 
cavity separated from the outside by an exceedingly thin walL 
This space is nothing but the mantle-cavity, which, as explained 
above, is the space comprised between the projecting mantle flap 
and the rest of the body. The peculiarity about the snail is that 
the mantle edge has become fused to the back of the neck so as to 
shut the mantle-cavity off from the exterior, leaving only this little 
hole of communication. The mantle-cavities of the marine allies of 
the snail, such as the whelk and periwinkle, are not so completely 
shut off, inasmuch as in them the mantle flap merely lies against 
the neck but is not fased to it, and inside the mantle-cavity there 
is a gill. This gill consists of a hollow axis bearing on one or both 
sides a close set row of thin plates inside which the blood circulates 
and receives oxygen from the water by diffusion. Fresh supplies of 
water are drawn into the mantle-cavity by the action of myriads of 



VnL] GASTEROPODA. 213 

cilia which cover the gill. A gill of this nature is called a 
ctenidinm, owing to its comb-like appearance (Gr. mi'iSun-, a small 
comb). Now, since the snail breathes air, not water, it has lost the 
gill, bnt to compensate for the loss it has changed the whole mantle- 
cavity into a lung. The floor of the mantle-cavity, really the back 
of the neck, is arched and composed of muscles: when these con- 
tract the floor flattens and thus the mantle-cavity is enlarged and 
air is drawn in. 

The blood is contained in 
large vessels running in the 
thin roof of the mantle-cavi^ : 
these are clearly eeen when the 
mantle flap is clipped away 
from the neck and turned over 
to the right (9, Fig. 109, and 
Fig. 110). These vessels are 
seen to all converge to the 
heart, which consists of two 
small oval sacs placed end to 
end. That into which the vein 
eDt«Ts ia thin-walled and is 
called the auricle: the other 
thicker one is called the ven- 
tricle (Fig. 110); it is the more 
muscular of the two and drives 
the blood through two arteries 
to the body. One of these passes 
up to the visceral hump, and 
tlie other forward to the head 
and neck. In Molluscs which 
have gills the auricle always 
receives the blood from the gill : 
when there is one gill, as is the 
case with nearly all the uni- 
valves, there is only one auricle : 
but where, as in the bivalves 
and cuttle-fish, there are two or 
even four gills (as in Nautilus) 
there are likewise two or four auricles. The heart is surrounded 
by a space called the pericardium, which really corresponds to 
the body-cavity or coelom of Vertebrates, AnneUds and Echino- 




Fta. 109. Htlis pomatia. The animal 

ueD from the dorsal side after removal 
oftbe shell. From Hatechek and Cori. 
1. Auricle of the heart reoeiving pal. 
monaryTeiii. 9. An torior tentacles. 
3. Bye tentacles. 4. Edgeotmantte. 
5. Nephridinm. 6. Liver. 7. Al- 
bnmoD gland. 6. PQlmonair vein. 
9. Pool. 



214 MOLLUSCA. [chap. 

denuB, for into it the excretory organ opens and in tiie embiyo 

the genital cells are budded from its walL Othsr large spaces 

flxiBting in the head and neck have no connexion with tlie 

coelom but are really parts of the blood Bystem. Since there are 

no regular veioB, except those which ran in the mantle-roof, the 

arteries open into irregular spaces. It will be remembered that the 

space called pericardium amongst the Arthropods is really a 
blood space and that the heart opens into it by 
openings called ostia: the coelomic character of the 
pericardium of Mollusca is then another distangnishing 

feature of the group. It 

opens by a narrow ciliated 

passage, the reno-peri* 

cardial canal, into the 

kidney, which ib seen in 

the mantle-roof beside the 

pericardium {5, Fig. 109). 

The kidney looks like a 

solid yellow organ; but in 

reality it ia a vesicle into 

the cavity of which nu- 
merous folds project, cover- 
ed by the peculiar cells 

which have the power of 

extracting waste products 

from the blood, which fiows 

in spaces in the kidney 

wall The kidney com- 
municates with the exterior 

by a narrow thin-walled 

tube, the ureter, which 

runs along the right side of 

the body and opens on 

the lip of the respiratory 

opening, just above the Fm. no. Beiixpam 

opening of the anus (10, 

Fig. 112). 

The kidney in Mollusca 

varies a good deal in 

structure, but is always 

built on the Bame fuoda- 




nith tbe upper wall 
of the polmoiiary ehambar cot open aud 
folded back. From Hatschek and Cori. 
1, Venlcicle. 2. Anterior tentaclsB. 3. Eje 
teatacles. 4. Cat edge of maDtle. 

6. Respiratory pore. 6. Aoua. 7. Open- 
ing of ureter. 8. Foot 9. Amiole 
receiving pulmonar; yeln. 10. Bectuin. 
11. Nephridium. 12. Upper wall of 
palmoDar; chamber. 



Vin.] GASTEROPODA. 215 

mental plan u that of the SdoU. Where there are two gills there 
ue liluwise two kidney b. Often there is no ureter, but the kidney 
opens directly to the exterior, as in the cuttle-fish, the whelk 
{Bweinum), the limpet (Patella), in the cuttle-fish instead of 
irregular spacea there are regular veins in its walls and the folds 
coTBied with special cells are only developed over the coarse of 
these veins. 

Turning now to the digestive system of the snail we notice 
several very interesting pecnliaritiee. The mouth is situated in 
front, beneath the small pair of tentacles, and there is a cnrved 
homy bar, the jaw, in the roof of the mouth. Against the jaw 
worlra a rasp-Uke tongue, called the radula, the surface of which 
is a homy membrane covered with myriads of minute, recurved 
teeth. TTndemeath tiiis membrane there are certain small pieces oi 
cartilage to which muscles are attached which pull the cartilages 
and the membrane covering them alternately downwards and for- 
wards and upwards and btickwards, so that the tongue is worked 
against the jaw. Thus the snail is enabled to tear pieces out of 




Fio. 111. Inau view o[ right halt ot head of Helix, to show Che arraDgement 
of the radula x 2. 

1. Moath. 2. Horny jaw. 3. Baduln. 4. Cartilaginoai piece »up- 
porting ndnla. 6. Badola MO from which radula grovs. 6. Mnsole 
which retraoM the hocoal msae. 7. IntiioBic musclea which rotate the 
radola. 8. Cerebial gaoglioo. 9. Pedal and Tisceral ganglia. 

10. Oeuiphagni. 11. Anterior tentaclti. 12. Eye tentacle. 13. OriGca 
of daet ol Mlivaiy gland. 14. Mucous gland which rona along toot and 
openi jQEt imder the month, 

the leaves on which it feeds (Pig. 111). A similar organ is found 
in all Mollusca, except the Bivalves or Lamellibranchi&ta, and the 
number, shape and arrangement of the teeth are an important help 
in clasaification. The homy membrane ia continued backward into 



2lfi MOLLUSCA. [chap. 

a little blind pouch, called the raduU sac: here is its growing' 
point, where new teeth are continually being fonned u the old ones 
wear away. In the limpet {Patella), this raduia sac is extn- 




1 eipoMd. 

FharyDi 3 Oesophagaa 3 Salivar; gUnda with dact. 

4 Stomscb G Liver 6 Bectum T Anna 6 Eiiluej. 

9 laflaled commencemen oF nreter 10 Opening ot reter to eitarior. 
11 Ventt ole 12 Anr cle 13 Pu monaiy yb d II Opening 

of uephndium mto penca dium 15 o test b 16 Commoa duct 

of ovo testis IT Albumen g and 18 Fema a duot IS IfaU 

dnot 30 SpenDittheea 21 Flagellum 22 Accessory glands. 

23 Fetiu. 24 Dart sac 26 Vag nk 26 Eye tentacle 

retracted 27 Ante o tentacle retcacl«d 28 Uusde* irhicl) 

rebut the bead, phai;iii teutaela eta 



Vni.] GASTEROPODA. 217 

ordinarilj long, attaining a length two or three times greater than 
that of the body. In the cuttle-fish the radala is present and the 
jaw is developed into upper and lower beaks, like those of a parrot, 
with which the animal tears its prey to pieces. The Bivalves have 
lost all trace of both jaws and radula: they live on the microscopic 
organisms brought to them in the currents of water which they 
produce, and so they do not need to masticate their food. 

The radula sac and the muscles and cartilages belonging to the 
radula, form a swelling which is called the buccal mass. Behind 
this comes the oesophagus or gullet, which appears narrow by 
comparison, but its cavity is really as large as the space inside the 
buccal mass. The gullet soon widens out into the first stomach or 
crop, which is used for storing the food. On the outside or surface 
of this two branching whitish structures are seen, the salivary 
glands. They secrete a juice which runs forwards through two 
tubes, the salivary ducts, opening into the buccal mass. The 
saliva mingles with the food as it is being masticated. The crop is 
situated in the hinder part of the neck, and behind it the ali- 
mentary canal passes under the mantle-cavity and up into the 
visceral hump. The great mass of this hump is occupied by a 
brownish looking organ, called the liver. This, like the similarly 
named organ in the Arthropoda, is a great mass of tubes lined by 
cells of a deep brown colour: the tubes join together and event- 
ually open by two main tubes, one above and one below, into a 
dilatation of the alimentary canal. This swelling is the true 
digestive stomach. It is probable that the 'Miver" assists di- 
gestion by preparing a fluid which is poured into the stomach: 
its function is thus not the same as that of the human liver. In 
fact it must be confessed that the name liver has been recklessly 
given by the older naturalists to any brown-coloured organ found 
near the stomach of an Invertebrate. The part of the alimentary 
canal behind the true stomach is called the intestine. It takes 
a turn in the liver substance and then runs out of the visceral 
hump along the right side of the body to open by the anus, which, 
as we have seen, is placed just behind the respiratory opening. 

The central nervous system resembles that of the Annelida in 
being made up of ganglia, each of which might be compared to a 
miniature brain, connected together by means ol commissures, that 
is, bands of nerve fibres. The two largest ganglia, which are placed 
above the oesophagus one at each side and connected by a com- 
missure, are called the supra-oesophageal or cerebral ganglia. 



21» 



[CHA?. 



or sometimes the bnuQ {1, Pig. 113); bat there is no reason to think 
that they are any more important to the animal than tJie others. 
Underneath the oesophagns there is what at first sight seems to 
be a compact nervous mass, connected with the sapia-oesophageal 
ganglia by a commisaure on each side forming a nerve collar 
(Fig. 113). Closer inspection shows that this mass is perforated 
by a hole through which passes the 
great anterior artery from the ventricle, 
and that from both the lower and 
upper halves a separate nerve comes 
ofT to go to the cerebral ganglia. Thos 
the apparently simple nerve collar con- 
sists of two commissures on each side 
united in a common sheath. Between 
them a minute nerve paases down, to 
end finally in a minute membranous 
sac hned by ciliated cells and cells 
with sense hairs and containing fluid 
in which a little ball of carbonate of 
lime floats. This sac, the otocyst, is 
the only other important senae-oigan, 
besides the eyes, which the snail pos< 
sesses. It is difficult to dissect, but if 
the small bivalve Cyclas be taken, 
the shell opened and the foot cut off !■ Cercbrnl gangUou. 3. Ped«1, 
and slightly compressed, or if one of ' "" "° "•"" - 

the transparent Molluscs, such as 
Pterotrachea, which float at the sur- 
face of the sea, be examined, it is 
perfectly easy to see both otocysts 
with the microscope. It used to be 
supposed that the function of this organ was to perceive sound, but 
whilst it is probable that some vibrations of the air affect it, it is 
nearly certain that, like the otocysts of Medusae and Arthiopoda, 
its main function is to enable the Mollusc to keep its iialance by 
allowiug it to perceive whether it is leaning on one side or not. 
As the snail changes its position the little ball inside rolls about 
and affects different parts of the wall of the vesicle, and hence 
probably different fibres in the nerve which supplies it 

Not all Mollusca possess eyes, but all, except perhaps the 
Oyster, which never moves, possess otocysts. The experiment has 




6. Olfactory nerve. 
6. Optio nerve, 7- Plenro- 
Derebrol commissure. 8. Pe- 
do-cerebnil commisaure. 9. 
OeniUl nerve. 10. Nerve to 
mantle. 11. Nerve to vi»- 




Vni.] OASTKROPODA. 219 

been nude in the Gatde-fifih of cattmg them out, and it ia then 
found that the animai loses its 
power of keeping its balance in 
the water and tumbles about 

To return to the central 
nervous syatenL In the pond- 
Boail, Limnaea, the hinder part 
of the Bub-oesophageal nervous 
masses consists of no less than 
five ganglia, strung together on 
a short loop of nervous fibres, 
which is called the visceral 
loop. Of these a pair nearest 
the head are called the pleural 
ganglia, the next are called 
the visceral ganglia, and 
the one at the end the ah- 
dominal ganglion (Fig. 115). 
The &ont and lower part of t^ 
_ _ sub-oesophageal nervous mass 

G. Supporting wila, coQsists of the pedal ganglia, 
which send nerves exclusively 
to the foot. Pleural and 
visceral ganglia can be re- 
cognised in the young snail, 
but they become indistiu- 
guiahably joined in the 
adult. In other Molluscs, 
such aa the Sea-hare 
{Aplysia), or the Ear-shell 
{Haliotis)y the visceral loop 
is long and the ganglia 
widely separated. In these 
animals it can be seen that 
the pleural ganglia send 
nerves to the sides of the 

' body, and that from the 

Fid. IIS. Nsrvoai ■vatemotLtmnaeA. After ■ < ,. 

laoBze-Datbiere. Visceral gangha nerves 

1. Cerebnl Baaglion. 3. Pedal ganglioa. COme off which go tO the 

guiftlion. 6. Abdominal gangUoa. ""^ ,°' *™ P" °^ Bl"*- 

6. ViMBial gaaglian. At the base of each gill 



Fio. 114. OptJeal «eotion throngb the 
nnditoty veiiote oc ear of Pterotraekea 
fiitdrriei, a traaepaieDt pelagia Mol- 
Inso X BboDt 150. After Clana. 

1. Auditory nerve. 3. Ciliated cells. 
S. AnditOT7 celli. ' ~ 
aaditoi7 cell. 
a. OtoUth. 




220 



MOLLUSCA. 



[chap. 




there is a patch of thickened skin, called an osphradium (Gr. 
6(r<l>paivofiai, to smell), provided with numerous sense-cells, which 
enables the animal to test the water which enters its mantle-cavity. 
Of course no such organ exists in the Snail The muscles of the 
radula are supplied by nerves from a special pair of small ganglia 
placed on the buccal mass — the buccal ganglia — connected with the 
supra-oesophageal ganglia. 
We thus see that the 
nervous system of the i 

snail consists of a pair of 
supra-oesophageal ganglia 
connected by commis- 
sures with (a) a pair of 
pedal ganglia supplying 
the muscles of the fo«t 
with nerves, (b) an ex- 
tremely short visceral 
loop, the ganglia on which 
are so closely placed as 
to become practically con- 
fluent with each other, 
whence nerves go to all 
parts of the body, and 
(c) a small pair of buccal 
ganglia supplying the 
buccal mass. The ner« 
vous systems of all Mol- 
lusca are built on this 
plan : in the bivalves, 
however, where there is 
no radula, not only are 
the buccal ganglia absent, 
but the pleural and cere- 
bral are fused with one 
another, and, as the vis- 
ceral loop is long, we find 

three widely separated pairs of ganglia, — cerebro-pleural, pedal and 
visceral— the last named often termed " parieto-splanchnic," in 
different parts of the body. The Cuttle-fish have a closely massed 
nervous system, like the snail, which is protected in a kind of 
rudimentary skull, made of cartilage. 



— . 9 



Fio. 116. Nervoas gystem, osphradiam (ol- 
factory organ) and giUs of Haliotit, After 
Lacaze-Dothiers. 

1. Cerebral ganglion. 2. Pedal ganglion. 
8. Osphradial ganglion. 4. Pleural 

ganglion. 5. Abdominal ganglion. 

7. Nerves to mantle. 8. Gills. 9. Pedal 
nerves. 



VIII.] GASTEKOPODA. 221 

The only organs of the snail which remain to be mentioned are 
the reproductive organs. These are exceedingly complicated in this 
Molloac, both sexes being united in the same individual, a condition of 
affairs which is known as hermaphroditism. The essential genital 
organ is the ovotestis, a small yellowish patch of delicate tubes 
spread out on the surface of the liver, on the inner side of the 
uppermost coil of the spire (Fig. 112). This organ produces both 
eggs and spermatozoa and both travel down a single tube. Before 
the duct reaches the neck it receives the secretion of a large organ, 
called the albumen gland. This secretion consists of a fluid 
which has proteids in solution and is of high nourishiDg value. 
Beyond the albumen gland although externally simple the duct is 
divided by a septum into two passages, one for the eggs and one for 
the spermatozoa, and still further on it becomes completely divided 
into two separate tubes. The female portion opens to the exterior 
by a thick-walled muscular part, the vagina, into which a tuft of 
tubes — the mucous glands — opens. The vagina also receives the 
opening of an organ called the spermatheca, which is a round 
sac at the end of a long duct in which the spermatozoa received 
from another individual are stored up. In addition to this, a 
thick-walled sac called the dart- sac also communicates with the 
vagina. .In this sac is found a calcareous rod which is thrown out 
into the body of another individual about the time of fertilization. 
The male duct opens also into a muscular organ called the penis, 
which can be partly everted, that is, turned inside out, and so 
protruded. The function of this organ is to transfer the sperma- 
tozoa to another individual ; it has a blind pouch projecting 
inward beyond the place where the male duct enters it called 
theflagellum; intiiisthe spermatozoa are massed together into 
bundles called spermatophores. Both penis and vagina have 
a common genital opening far forward on the right side of the neck 
(Fig. 108). 

Few MoUusca have such complicated generative organs as the 
snaiL One large group of marine snails, the Opisthobranchiata^ 
resemble Hdix in being hermaphrodite, but none possess the dart- 
sac, and in many the generative opening is placed further back and 
connected with the opening of the penis by a groove called the 
seminal groove. Hence the penis is obviously derived from a 
muscular pit on the side of the head into which the spermatozoa 
trickled and was at first unconnected with the generative opening. 
In another group of maiine snails, the Prosobranchiata, there is 



222 MOLLUSCA. [chap. 

a separation of the sexes and the albumen gland is absent. The 
penis is not a sac which can be turned inside out^ but a projecting 
lobe of the body, often of great size. In the most primitiye 
Mollusca — the Solenogastres — the genital organ remains through- 
out like a thickening of the wall of the pericardium or coelom ; the 
eggs and spermatozoa drop into the pericardium and find their way 
out by the nephridia, just as is the case with Annelida. 

This is the case also in Cephalopods, where, however, there were 
originally four kidneys, and the one or two which serve as generative 
ducts are specialized for this purpose; thus the duct is in the male 
prolonged into a papilla which serves as the penis. A commoner 
case is for the generative organ to be closely connected with one 
kidney and to burst directly into it. This is found in the simpler 
Prosobranchiata, such as the Limpet {PateUa), the Ear-shell 
(HcUiotis) and their allies. In Nucula and the simplest bivalves 
there are two generative organs and they open into both kidneys; 
in the Pond-mussels {Anodonta and C7ni(?), and the more modified 
forms, they open independently close to the kidney opening. There 
is little doubt that in all Mollusca the tube convejdng away the 
generative products was originally a kidney or a part of one. 

Having got some idea of the arrangement of the organs of the 

snail we must proceed to consider certain points about 

of^dy."*^'^ the form of the body considered as a whole. If we 

except the genital opening, the head and neck of the 
snail are exactly bilaterally sjonmetrical in their outer form; on 
each side there is a taste-tentacle and an eye-tentacle and the 
mouth and the opening of the mucous gland are exactly in the 
middle line. Most of the ordinary animals we see — birds, quadrupeds, 
fishes, insects, worms, etc. — are bilaterally symmetrical with regard 
to the exterior and many with regard to the whole body. The 
peculiarity of the snail is that, while it follows the ordinary rule as 
far as the head, neck and foot are concerned, it departs from it 
with respect to the visceral hump and the included organs. The 
shell is, as we all know, spiral, but this shape is due to the shape of 
the visceral hump contained within it, by the activity of the skin of 
which the shell is produced. This spiral shape again is simply due 
to one side being longer than the other, and it is connected with 
the shortness of the right side that we find the opening of the vent 
on the right side. In all bilaterally symmetrical ani'mftlq this 
opening is situated in the middle line, but in some of the marine 
allies of the snail — the whelk, limpet and others forming the group 



VIII.] GASTEROPODA, 223 

Prosobranchiata — ^the inequality of the tivo sides of the visceral 
hump is carried to such an extent that the anus is brought right 
round so as to open nearly over the middle of the neck ; and where, 
as in the Ear-shell {Haliotis), there are two gills, the left becomes 
pushed over to the right side and the gill belonging to the right 
side becomes displaced to the left. Since the visceral ganglia are 
connected with the bases of the gills, one side of the visceral loop 
becomes pulled over the other in consequence of the displacement 
of the gills (Fig. 116). This condition of the nervous system is 
called the streptoneurous condition; it exists in all the groups 
which are ordinarily termed "sea-snails/' i.e., Prosobranchiata, and 
though most of these have only one gill, the twisting of the visceral 
loop may be regarded as a proof that they originally had two. In 
another large division of the sea-snails, the Opisthobranchiata, the 
shell is generally small or has quite disappeared, and since where 
this has taken place there seems to have been a tendency to undo 
the twisting, the anus becomes pushed back to nearly the middle 
line and the visceral loop becomes straightened out and shortened. 
There is reason to believe that this last process has gone on in the 
snail, though it has kept its shell. It appears then that the curious 
spiral form of part of the body and the inequality of the sides 
have something to do with the possession of a large shell by a 
crawling animal We do not understand very clearly how the one 
thing has brought about the other, but we can understand that 
there would be a tendency in a tall visceral hump to topple over 
to the one side or the other and thus exercise a greater strain on 
one side than on the other. Certain it is, at any rate, that the 
only existing Mollusca which possess large coiled shells and yet are 
bilaterally symmetrical, are the pearly Nautilus and another rare 
Cuttle-fish (Spirula), which do not crawl but swim. 

The class or primary division of the Mollusca to which the snail 
belongs is called the Gasteropoda, on account of the flat smooth 
foot or crawling surface which they all possess (Gr. yaor^p, the belly ; 
irov9, iroSo?, foot). The shell is typically composed of a single piece, 
never of paired pieces ; and from this circumstance is derived the 
general term ** univalve" often applied to the Gasteropoda by 
collectors ; in one small division of the class (the Isopleura) the 
shell is represented by eight pieces placed one behind the other in 
the middle line. 



LaMELLIBBANCBUTA = PELEOYPODA, 



Q_l 



The characters mentioned ivt the end of the last section shaiply 
(separate the Gaattropodn from another clasa oS 
Mmei"."™'" Molksca, the Lnmellibranchiata or Pelecypoda, to 
which the coinmoii mussel and innumerable marine 
forms, such as the oyster, clam, cockle, etc , belong. The Moliases 
belonging to this class have a shell composed of two similar pieces, 
the right and left valres, united by a horny flexible piece, the hinge 
(Fig. 117). The foot is typically formed like a wedge or axe-hettd, 




' miuolM 

pDlDt of 



Fm. 117. Shell contsiniiig Anodonla mulahilii. and behind it tba mi 
so empty left nhell. 

1. Points of inwrtion of the unterior protraclor (above) and retractc 
iMon) of the abell. I. t'oiat of uissrtioQ at tbe anieiior addaat 
3. Point of inBettioa of the poBlerior pioliaoloi of the ibell. i 
iniertion of the posterior adductor musole. 6. Lines formed by 
ftltachioents of the mantle. 6. Umbo. 7. Dorsul ajphos. i 
■ipbOD. 9. Foot protruded. 10. Liaee of growth. 



whonoe the name Pelecypoda (6r. jriKiKVi, a hatchet), and is ooed 
as a plough to force a way through tbe mud in which the creatuni 
live. There are many species of pond- or river-mneselG in Ntnlb 
America : Jntxlontn cii'/iiuea ie perhaps tbe commonest in En^and^ 
but in places Unto pictorum is abundant; A. cygnaea occtitB io 
Canada and the United States and in these countries Unio eom- 
pianatus is also common. Auy one of these forms will serve our 



viil] lamelubranchiata. 225 

purpose. The shell is about four inches long and two inches high, 
-^ -^ .. and is covered with a black horny layer, the so-called 

The SheU. , ^ ^ ^ i 

periostracum. The shell is apt in places to be 
eroded by the action of the carbonic acid in the water. Under- 
neath it is a tiiick slightly translucent layer of crystals of carbonate 
of lime, called the prismatic layer. The inner part next the 
mantle is composed of thin layers placed one above the other. 
This is the mother-of-pearl or nacreous layer, which in many 
Molluscs has an iridescent sheen, owing to its action on light. 
These three layers are also present in the shell of the snail and in 
all other Mollnscan shells, but they are very easily made out in the 
shell of the pond-mussel. To the periostracum the colour of the 
MoUuscan shell is mainly due. The periostracum and prismatic layers 
are formed by the edge of the mantle and if destroyed they cannot be 
replaced. The nacreous layer is deposited by the whole surface of 
the mantle. If by chance a grain of sand gets wedged in between 
the mantle and the shell it is apt to become covered with layers of 
mother-of-pearl, and in this way a pearl is formed. The more costly 
pearls however arise within the soft parts of the body, usually 
encysting around some parasitic larva. The shell is marked by a 
series of curved lines running parallel to one another. These lines 
mark the limits of growth attained in each year, the amount inter- 
vening between two lines being the amount of growth accomplished 
in a year. It will be seen that the common focus around which the 
curves run is not in the centre of the hinge line, but decidedly 
nearer one — the anterior — end. This common focus is called the 
umbo, and it represents the shell with which the Unto started life 
(6, Fig. 117). 

As might be expected from the shape of the shell, the mantle 
has the form of two great flaps hanging down at the sides of the 
body. The flaps have a free edge in front, below and behind, but 
pass into the general wall of the body, with which they fuse, above. 
The edges of the mantle flaps are very much thickened and closely 
adherent to the shell ; as stated above, it is by these edges alone 
that the periostracum and the prismatic layer are formed. 

The hinge is strictly speaking part of the shell ; it is secreted 

by the ectoderm of the back of the animal between the two mantle 

lobes. When the valves of the shell are pressed closely together 

the hinge is bent out of shape and by its elasticity it tends to 
throw the valves apart; hence when a mussel is dead the valves 

always gape. 

The two valves in Unio articulate with one another by means 

& AH. 15 



226 HOLLUSCA. [chap. 

of teeth. There are a p&ir of stoat teeth a little in front of the 
umbo, on the left valve, working on either aide of one tooth on the 
right valve; these are called the cardinal teeth. A long ridge 
on the right valve, woridng between two ridgea on the left viln, 
ifl called the lateral tooth. Atiodonta derives its name (6r. aV-, 
not ; iSovt, ihevrot, a tooth) from the circnmetiuice that the sheD ia 
devoid of teeth. 




Vta. lis. Bight side ot inodonto nmlabilii with tha mftntls cat ain; H)d tlte 
right gilla folded btdk k About 1. From HatBcliek uid Cori. 

L Uoath. 2. Addb. 8. Corebro-pIeiiTBl ganglion. 4. Anterior aJdaetov 
miucle. 6. Anterior protractor moscle of the shell. S. Betnetor 
miucle. T. Dorsal siphon. B. Inner labial p&lp. 9. Foot. 10. Ex- 
ternal opening of nejmridinni or organ of BojanoB. 11. Opening of 
genital dacL 12. Outer right gill-plate. 13. Inner right gill-i£ue. 
14. Tsntral siphon, IG. EpibrSDChial obamber, tha inner lamellae of 
the right and left inner gills having been slit apart. 16. Posterior pro- 
tractor mus«Ie. 

When the shell is removed from the animal the cut ends of the 
fibres of two large muscles are seen. These mnscles, which run 
transversely from the one valve to the other, are called 
the anterior and posterior adductors respectively, 
and it is by means of them that, when danger threatens, the animal 
closes the valves and shelters foot, gilla and body, within. Just 
behind the anterior adductor is a pair of small muscles running into 
the foot, and these are the anterior protractors of the shell. A 
similar pair, the posterior protractors, are found just in front of the 
posterior adductor, and by the combined action of the four the shell is 
drawn forward, the foot being (relatively) fixed in the mud (Figs. 118, 
121). The foot is thrust forth by the forcing of blood into it, through 
the contraction of the muscles which underlie the skin in various parts 
of the body. The retractors (of the shell) enable the animal to 



LAMELL1BHANCHIAT4, 

I move backwards wheu neceaaaiy. A small group of musclea ranniag 
from the mantle to be attached to the shell near the umbo puU the 
flhell downward and help to plough a furrow in the mud. The 

I ftulmal moves by forcing out the foot ajid wedging it in the mud in 
front and then drawing tlis body after it. 

At the sides of the body on each side we find the hrancbia or 
gill, or ctenidium, which as in the Gasteropoda consists of a hollow 

\ axis bearing two rows of plates. The cteuidium is. however, highly 




1. 119. A. Iiiag»mraittia sectiim thioaeh Aiuitlanta to abow the eiraulaUon 
o( tilo blood. B. Beclion througb Anodonta uoar the posterior edga of 
the tool. From Hones. 

1. Bight auricle. 2. TeDtrlctc. 3. Kebcr'a orgnit. i. 'Veaa, cava, 
S. E9«ieiit bmDcbitU trank. 6. Efferent pallial vessel. T. Efferent 
bnndiukl vecael. 8. Biaachiae. 0. Afferent branchial vessel. 

10. ESecent lenal vessel. 11. Afferecl biKncbial trunk. 12. AHeceitt 
ISD&I TuaaL 13. Keotum . 
1. Bi^t auricle. 2. Epibranchial Ehamber. 3. VoDtriole. 4. Vena 
cava. 6. Non- glandular part of the kidney. 6. Olaadular part of 
the kidne]'. 7. Intestine in foot. S. Perioardioni. 9. Shell. 
10. Ligament of shell. 

ItDodified in Unio. The axis is attaohed high up to the side of 
2ie body in front but projects freely into the mantle cavity behind, 
rhe plates hare become narrowed bo as to form long filaments, 
ind the ends of each row are bent up and are, in the case of the 
outer row, fused to the mantle lobe. The bent-up ends of the 
ir row are joined to the foot in front and to the corresponding 
a of the utenidium of the other side behind, but in the middle 
15—2 



228 MOLLUSCA. [chap. 

they are free, at least in Bome species (Fig. 119). Saccessive fila- 
ments of one row are welded together into a plate, called a lamella^ 
by the fusion of their adjacent edges, leaving only occasional holes 
for the percolation of the water, so that individual filaments appear 
like ridges on a ploughed field (Figs. 118, 120). The descending 
and bent-up ends of the same filament are tied together by cords or 
narrow plates of tissue traversiug the space between them. These 
cords and plates are called interlamellar junctions, since they 
unite two lamellae. The pieces of tissue uniting the filaments are 
called interfilamentar junctions, or collectively, subfilamentar 
tissue. Gill-plate is the name given to the whole mass composed 
of one row of V-shaped filaments : there is thus an outer and an 
inner gill-plate on each side, and each gill-plate has two lamellae 
formed from the descending and ascending limbs of the filaments, 
respectively (Fig. 119). It is this peculiar modification of the 
ctenidia which has suggested the name Lamellibranchiata for the 
class. 

Each V-shaped filament is clothed on its outside, that is, the side 
looking away from the concavity of the V, with high ectoderm cells 
carrying powerful cilia. By the action of these a strong indraught 
of water is produced, the current entering between the posterior 
borders of the mantle lobes, which normally gape slightly. On this 
current the animal depends both for respiration and for nutrition, 
since the food consists entirely of the minute Aninntla and plants 
swept in with the water. The normal position of the Mussel is to 
have the anterior end deeply embedded in the sand or mud and the 
posterior end protruding ; the animal moves only when for some 
cause the water becomes unsuitable for its purposes. 

Since the upturned ends of the inner rows of filaments of both 
ctenidia are united behind the foot, a bridge is formed dividing the 
mantie cavity into an upper or epibranchial division and a lower 
or hypobranchiaL The gaping opening between the mantle lobes 
at the posterior end is similarly divided into an upper portion, the 
dorsal siphon, and a lower, the ventral siphon. Since it is the 
outer lower surfaces of the filaments which are clothed with cilia, 
it is into the ventral siphon and hypobranchial chamber that the 
current passes. The lips of both siphons — especially the ventral 
siphon — are plentifully beset with small papillae, which are sensitive 
to light and shade. If the shadow of the hand be allowed to pass 
over them the mantie edges are instantiy drawn together and the 
siphons thus closed. In the scallop (Pecten) similar papillae are 



LAMELLIBRANCHIATA. 

Ideveloped into well-formed eyes. Part uf the water passes through 
I the small holes left between the gill-fiJamentB and bo into the 
K epibraocbial chamber and escapes by the dorsal siphon, carrying 
1 with it the matter cast out from the kidneys and the anus. As it 
I percolates throagh the gills the blood which circulates in these 
gtws receives oxygen and gets rid of its carbonic acid. 
A large part of the water, however, pursues a different i;our3e. 
I In front of the gills there are situated two organs called labial 
I palps, on each side of the anterior part of the animal (8, Fig. 118). 



s-^- 



I ], Cerebro-pleuiaJ iiftnglion 2 Cerobio pedal c(iiiiiiiisaur« 3 Oesopliagns. 
i. Anlenot proHtuitor moscla 6 Li%er 6 Stomach 7. AorM. 
8 Ext«raiil opening of orpnn of Bcijbqds or cephndium 9 Nephio- 
■ti>m« or intPraol openniR of the tMne 10 FencuTditim IL Bight 
aancle IJ lostt-rior end of vsutride passing into posterior aorta. 

13. Bectam. 14. Oluxidular part of nephridiam. 16. Adob, 

m. Op«DiD|i of epibnnchial c1]aiiib«r. IT. Teatral aiphoD. 18. Edge 
of Bliiill. 19. Cerebio-vUceral commisatire. 20. loCeitiiie. 31. Foot. 
23. ReprodnctiTe orgaDS. S3. Pedal gaoglioii ot right side. 24, Motitb. 
95. Opeomg of the raproductivs oi^an. 

These are triangular flaps, an upper and lower on each side, the 
Burfaces of which are covered with grooves clothed with abundant 
L cilia on the sides turned towards one another. The two superior 
I labial palps are connected by a narrow ridge crossing above the 
I mouth ; the two inferior labial palps by a similar ridge beneath it, 
[ The mouth thus lies at the bottom of a trough, — the lips of which 
[ are formed by the superior and inferior labial palps respectively. 



230 MOLLUSCA. [chap. 

The mouth is situated beneath the great anterior adductor muscle 
which projects beyond it like a forehead. The action of the labial 
palps is to direct a large portion of the incommg current into the 
mouth, and thus the animal obtains its food. 

The alimentary canal shows a considerable resemblance to that 
of the snail No trace, however, of radula, buccal mass, crop or 
salivary glands, is to be seen. A short oesophagus leads at once 
into the stomach, which is a wide sac receiving right and left the 
ducts of the two lobes of the liver. The intestine runs vertically 
down into the foot, makes several loops there and then turns back 
and reaches nearly to the point from which it started, Le,, the 
hinder end of the stomach. Thence it pursues a straight course 
through the pericardium and over the posterior adductor muscle, to 
end in an anal papilla which projects into the epibranchial mantle- 
cavity. For pare of its course the ventral wall is infolded towards 
the cavity so as to produce a ventral typhlosole comparable to the 
dorsal typhlosole in the worm. The straight concluding portion of 
the intestine is called the rectum. The pericardium is situated 
in the mid-dorsal line posterior to the stomacL The fact that it 
surrounds the rectum is the consequence of its origin as a pair of 
sacs in the embryo lying to the right and left of the intestine, 
which later meet above and below this organ. 

There are, as mentioned above, two kidneys or nephridia in the 
mussel These, frequently termed the organs ofBoj anus, are 
dark coloured bodies situated beneath the floor of the pericardium 

on either side of the vena cava. Each consists of a U-shaped tube 
lying horizouiaUy, with one limb placed vertically above the other 
and the bend directed backwards. The deeper limb is the active 
part; it has numerous folds projecting into it which are covered 
with dark cells. It opens into the pericardium in front by a 
curved slit lined with powerful cilia which produce an outward 
current. This of course is the reno-pericardial duct such as has 
been already described in the snail The outer and upper limb is 
wide and smooth-walled and opens into the deeper limb beneath 
the posterior adductor. In front it opens to the exterior through 
a pore with thick lips placed just above the place where the 
upturned ends of the inner row of filaments are attached to the foot 
(Pig. 120). 

The kidney, since it is a tube lined with excretory cells and 
communicating internally with the body-cavity, is a nephridium 
comparable to that of the worm, Lumbrkus, In the worm the 



LAMEi.LIBRANCmATA. 



231 



functioD of the internal opening is to convey to the exterior the 
fluid in the body-carity, which contains excretory matter thrown 
out by the cells lining the coelom. 
The anterior end of tlie perif ardiiim 
of the Mussel has a. brownish ted 
colour and is prodac«d into nu- 
merous httle pockets lined by 
jiecaliar cells, which ore excretory 
in function (Fig. 119), This por- 
tion of the pericardial wall is called 
Keber's organ, and the excreta 
thrown out by it pass down the 
reno -peri cardial canal 

The heart consists of a ventricle 
which surrounds the rectum, and 
tno flat triangular ati rides, the 
broad ba^es of which are inserted 
into the wall of the pericardium 
just over the place where the bent- 
np ends of the outer filaments of 
the ctenidium are attached to the 
mantSe. From the ventricle blood ia 
driven forwards by an anterior aorta 
dorsal to the rectum, and backwards 
by a posterior aorta ventral to the 
rectum. From these arteries it 
finds its way into a multitude of 
irregular spaces in the foot and the 
other portions of the body, and 
eventually reaches a vessel, called 
the vena cava, lying under the 

floor of the i>ericardium in the 

beiwMn leit uid tight msDile middle lino, between the right and 
lfX„"r.'?,l,."'ptaS lef »PPe' li-bs ot the two Iddneji 
ohaniber. It. Keber'a orgao. From the vena cava the blood 
13. NepbroBioDiB or iawmal streams out through many chan- 




Fia. 131. Dorsal view or Jiiodonrn 
mutabtU; witlt tha upper wnll 
<it tha pericArdinm rcmoveJ to 
■how the heart k about I. Alter 
HatMhsk and Cori. 

1. Foot. 2. Antarior addnclct 
miiaclc. 9. Bslmtor mDaclo, 
4. DepraSBorlniisalea. S, Pos- 
terior protractor ranacle. 6, 
Poatnior adduolor muscle. 7. 
SotmL aiphoQ. e. Ventrnl 

tiphon. 0. Anua. ID. Split 



opening of organ of Bojanos. 
14. Ventricle. IG. Left auriale. 
16. Anterior protnuilur mUBCle. 



in the wall of the kidney and 
reaches the axis of the ctenidium, 
whence it makes its way into the 
filaments, especially those of the outer row. From the upturned 
edgos of these htst it reaches the mantle and from this tlie auricle. 



232 MOLLUSCA. [chap. 

Some blood is sent to the lobes of the mantle, as in the snail, and 
through the thin skin absorbs oxygen ; this blood is returned direct 
to the auricle without passing through the gill ; from this fact it 
appears that the mantle lobe as well as the gill is a respiratory 
organ. 

In the nervous system the cerebral and pleural ganglia on each 
side are generally regarded as coalesced, but a distinct pleural 
ganglion has been observed in some cases on the cerebro-visceral 
commissure anterior to the pericardium. There is a long visceral 
loop, ending in two closely conjoined visceral ganglia, placed beneath 
the posterior adductor (Fig. 120). On either side of these, just 
where the axis of the ctenidium becomes free from the body, is a 
thickened patch of yellow ectoderm — the osphradium. This is a 
peculiar sense-organ, the function of which, it is believed, is to test 
the water passing over the gill as to suitability for respiration. 
There is a pair of large otocysts in the foot. 

Mussels are male and female : their productive organs are 
paired, and consist on each side of a bunch of tubes spreading 
through the foot. The ducts are continuous with the walls of the 
ovary or of the testis. They open by slit-like orifices just in front 
of the opening of the nephridia on each side of the foot The 
spermatozoa are swept out by the water passing through the dorsal 
siphon and are sucked in by the inhalant currents of female indi- 
viduals. The eggs when cast out are detained between the two 
lamellae of the outer gill-plate and there fertilized. They develope 
into peculiar larvae called Glochidia, provided with a sticky thread 
or byssus. A bivalve shell is developed but not the foot. When 
a fish passes by the mother expels the Glochidia from the gills, and 
they seize hold of the tail or fins of the fish and embed themselves 
therein. They develope there for some weeks and change gradually 
into the adult form. They show a remarkable sensitiveness to the 
presence of fish, but if they fail to attach themselves to one they fall 
to the bottom of the water and perish. 

Lamellibranchiata as a group have very uniform habits: the 
principal points in which they differ from one another are (1) the 
degree of complexity which that all-important organ the ctenidium 
has attained, and (2) the extent to which the animal is able to burrow. 

The simplest forms, such as Nucida, have ctenidia like those of 
a Gastropod, a fact which suggests the view now generally held, 
that the Lamellibranchs are descended from some primitive type of 
Gastropod. 

In others, such as the Sea-mussel {Mi/Hlas)» the ctenidia have 



-m.] LAMELLIBRANCHUTA. 233 

be BUDe eztemal appearance as those of Unio, but the filaments 
re vei7 loosely united with one another and their upturned ends 
n not fused to the mantle. The foot is small and tongue-shaped, 
the animal never burrows and rarely 
moves, being fixed by a cord of mucus 
caJlod the byssus, secreted byagland 
in the hinder part of the foot. 

In the Oyster {Ostrea) the foot has 
disappeared and the animal passes its 
life resting on one side. In the 
Scallop (Pectm) the foot has also 
atrophied, but the animal is able to 
swim through the nater by flapping 
the valves of the shell. The Cockle 
(Cardium) has a iat^e and powerfol 
foot hy which it is enabled to execute 
leaps. 

Ml/a (sometimes known as the 
Clam, tiiough this term is applied to 
many Bivalves) and its allies burrow 
deeply in the sand and have the edges 
of the mantle behind drawn out into 
two long tubes closely apposed to one 
another, termed the dorsal and ventral 
siphons. By means of these tubes 
they keep up a connexion with the 
surface, BO that the currents of water 
are not interrupted. Similar tubular 
funnels, though not so much drawn 
out, are seen in the Eazor-shell {SoUn) 
(Pig. 122). Pkolaa and some others are 
able to burrow in rock ; this is said 
in some cases to be efTected by an acid 
secretion poured out by the flat disc- 
like end of the cylindrical foot 
Teredo, the ship-worm, burrows in 
timber ; the siphons are very long and 
covered with a shelly deposit; the 
origiiutl valves of the shell are very small compared to this 
secondary shelly tube. This animal is very destructive to sub- 
merged woodon structures; a wooden pile supporting a pier in 




Fia. 123. Boltn vagina, the 
Buar-ahell, the shell ii open- 
ed ancl the poeterioi pftrt of 
Um Diuitle it torn to admit 
of thu. 

1. Shell. 9. Foot, S. Lb- 
U»l palpa. 1. OillB. 

5. Tom portdon of muitle. 

6. Brutle in ventnl siphon. 

7. Bristle in dorsal siphon. 



VancouTer was hi eighteen mouths reduced to a mere spongevnl 
of vood b; ita ravages. 

Class in. Cephalopoda. 

A third dasi of the MoDnsca, very differently constitated from 
the LamellibraDchiata, is that of the Cuttle-fish, or Cephalopoda. 
This pamdoxicftl name, 
literallj "head-footed" 
(Qr. Kt<t>aki}, head ; tovs, 
iroScJs, a foot), is suggested 
by the circumstance that 
the foot has grown for- 
ward and upwards at each 
side of the head, and that 
these two extensions have 
met and coalesced, so to 
speak, on the back of the 
neck. The edges of this 
part of the foot, which 
may be called the fore- 
foot, are drawn out in- 
to strap-iike processes, 
which are the arms by 
nhicli the animal seizes 
its prey. The e<lgos of 
the hinder port of the 
foot, OQ the other hand, 
have become bent round 
aud joined beneath the 
auimal, so as to form a 
tube, the funnel, through 
which water is ejected 
from the mantle-cavity. 
The best known Brit- 




lo. 123. DiigTBTna ol s Kties of HoUoaca 
to ahow tbe [orm of Iha foot and it* 
regiooB and the lelKtioDs of tbe Tisoet*! 
hamp to tliD UDtero. posterior and dono- 
veulral aire. After Lanki'ster. 
A PraBobrancb Goatropod. IL La- 

tDellibranoh. III. A Cepbalopod. 

A. Anterior autfoce. P. Poilerior ani- 
face. D. Dor«J aoiface. V. Ventral 
BurTace. 1. Month. 3. Antw. 

Sepi.. ""' ^l"""" 8. Mantle-oavity. t. Foot, 

lopoda are 
the Squid, LoUgo forbesi, often caught by trawlers, to whom it is 
known as the 'ink-fish'; Sepia ojficiiialis, the cutlle-fish, taken in 
the southern waters, is abundant in the Mediterranean, where it is a 
favourite article of food ; Moschitea cirrom with eight arms and a 
single row of suckers; Polypus (or Octopus) vulgaris almost c 




Plo, 124, Posterior viev of mnls or Sep a qffieinalti x 1 The mantU-M*iV 

has been opvued to expose ts contenta 
!■ Long ann half pratraded 2 Ash rt arm tlua one a beato-oot.vUzed. 

8, Lips Buirouud og liomj jaws moutli i t iterDol opcDiQg of 

fniinel. 5 E;o 6 OartiloBinooa kcob on mantle wliivh Qta mta<1 

ibt lOcket 9 7 O 11 S Soeket tor 6 Auua. 

prwBorinttBoIoof IbafunQol II Hep odu tveport IJ. Bightki 
pftpilU, 131 \ iBcerai maae. 14. I m. 



236 MOLLUSCA. [chap. 

fined to the south coast and commoner on the French shores and in 
the Mediterranean. Another squid, Ulex illecebrosa, is common in 
the Oulf of St Lawrence and on the shores of the eastern United 
States. The body of S^ia appears to be composed of a swollen 
head separated by a neck from a tapering trunk. When closely 
examined, however, the body is seen to be nothing but a long 
pointed visceral hump, like that of the snail, but it is not twisted 
and is unprotected by an external shelL The mantle, as in the 
snail, is a skirt-like fringe of skin, the space between its inner 
surface and the visceral hump forming the large mantle-cavity. In 
order to compare the animal with the snail it must be placed with 
the point of the hump projecting upwards and backwards (Fig. 123). 
The so-called head includes the true head, with two enormous eyes 
of almost human aspect, surrounded by the fore- foot. The fore-foot 
is drawn out into eight short pointed arms, thickly covered on their 
inner sides with stalked suckers (Fig. 124), and two very long arms 
bearing suckers only at their expanded ends. These latter can be 
pulled back into two large pits situated at their bases, and when so 
retracted they are completely hidden from view. 

The sucker is a cup with a homy rim which keeps the opening 
from collapsing. In its base there is a swelling, which is the 
end of a muscle running into the stalk, and by the contraction 
of this, when the cup is applied to any object, a partial vacuum 
is produced. By means of the suckers the Squid can take a firm 
hold of its prey. 

The hind-foot is the tube, known as the funnel (Figs. 124 and 
127). The posterior end of this is overlapped by the hind end of 
the mantle; in other words, it projects into the mantle-cavity. The 
mantle is veiy muscular; by the contraction of longitudinal 
muscles running towards the apex of the hump the mantle-cavity is 
widened ; by the contraction of circular muscles it is narrowed, and 
by the alternate action of these two sets of muscles, water is sucked 
in and forced out of the mantle-cavity. 

When, however, the mantle-cavity is contracted, two projecting 
pegs on the inner side of the mantle fit into sockets on the outer 
side of the funnel (6 and 8, Fig. 124). No water can then escape 
over the free edge of the mantle, and all is ejected in a narrow and 
forcible stream through the funnel The funnel itself is muscular 
and by contraction aids the process ; there is a valve-like projection 
inside it which prevents water from being driven back into the 
mantle-cavity (Fig. 127). 



VIII.] 



CEPHALOPODA. 



237 



Since water is sucked in gently and ejected forcibly the animal 
is propelled in the opposite direction, that is backwards, by the 
reaction of the stream against the surrounding water. Sepia can 
however also swim gently forward by wave-like undulations of the 
two lateral fins. These fins are flaps of skin projecting from the 
sides of the visceral hump (14, Fig. 124). 

It has been said that the visceral hump is unprotected by an 



7— . 




Fio. 125. A diagram ahowing the relation of the kidneys to the perioardinm 

in Sepia, 

1. External opening of the kidney into mantle-oavity. 2. Internal opening 
of the kidney into the pericardial eoelom. 8. Opening of the right kidney 
into the donal sao and henoe into the left kidney. 4. Left kidney, 

yentral portion. 6. Beno-perioardial canal. 6. Pericardinm (part of 
the eoelom). 7. Branchial heart. 8. Dorsal sao common to both kidneys. 
A. Arrow passing into left kidney by external opening from mantle cavity. 
D. Arrow passing into right kidney through external opening into median 
lobe. 0. Arrow passing into external opening and then into internal 
opening, and so into pericardial eoelom. The extension of the eoelom in 
which the generative cells arise is not shown in this diagram. 

external shell This is not strictly true. On the anterior surface 
of the hump there is an oval plate-like shell completely hidden in 
a sao formed by the meeting over it of upturned flaps of skin (14, 
Fig. 127). From its upper surface project an innumerable number 
of delicate calcareous plates parallel to one another, the spaces 
between them being filled with gas so as to give lightness. 

In one or two living Cephalopoda, as for instance the Pearly 
Nautilus (Nautiltis pompilius), and in very numerous extinct forms> 



238 MOLLUSCA. [CHAP. 

there was a large tabular external shell which might be straight or 
coiled, but which always had the peculiarity of having a large 
number of septa or transverse plates dividing up its cavity into 
chambers, only the last of which contained the visceral hump, the 
rest being filled with gas (Fig. 130). In Sepia it is supposed that the 
chambers have become so small and shallow that the last one simply 
appears as a plate situated on port of the surface of the hump. The 
other chambers contain gas as in Nautili^. 

Sepia possess tv?o well-formed ctenidia, each consisting of an 
axis bearing two rows of thin plates. The axis is suspended 
from the body by a membrane, and the ctenidia project forwards 
and downwards instead of backwards as in the Lamellibranchiata 
(7, Fig. 124). 

As in Lamellibranchiata, there are two kidneys which open by 
little papillae placed just in front of the bases of the ctenidia 
(12, Fig. 124). Just inside the papilla is a narrow opening, the 
lips of which are folded so as to make it appear like a rosette. 
This is the internal opening of the kidney: it leads into a lateral 
prolongation of the pericardium, which is the reno -pericardial 
canal (5, Fig. 125). The kidney has the form of a wide sac and may 
perhaps be compared to a U-shaped kidney, like that of Unio, in 
which the two limbs have become merged in one another. 

The wall of the kidney is smooth, except over the course of the 
large veins which run beneath its upper and inner walL Here the 
epithelium is folded and consists of tall cells which are actively 
engaged in extracting excreta from the blood, as is shown by the 
rows of granules with which they are filled. From the anterior ends 
of the kidney two outgrowths project which inmiediately fuse into 
one and constitute a great pouch called the dorsal sac (8, Fig. 125) 
stretching upwards and backwards just underneath the shelL This 
peculiar extension gives to the two kidneys the form of M, between 
the median V and outer limbs of which lies the pericardium. 
Posteriorly the cavities of the right and left kidneys also com- 
municate with one another. 

The pericardium is a wide sac lying between the dorsal sac of 
the kidneys and their ventral parts. At the sides it gives ofif the 
reno-pericardial canals, whilst the stomach and intestine project 
into its roof. 

The ventricle of the heart is a spindle-shaped sac Ijang trans- 
versely (1, Fig. 126). Into its two ends open the tubular thin-walled 
auricles which receive the blood from the ctenidia. From its 



viil] 



CEPHALOPODA. 



239 



anterior wall a i>oweiful artery, the anterior aorta, is given oflf 
ninning forward above the oesophagus to the head, and a smaller 
artery or posterior aorta goes backwards and upwards to supply the 
stomach and genital organ. These arteries have regularly formed 
branches from which the blood enters definite veins. This formation 
of well-defined channels for the Uood is characteristic of the Cephalo- 
poda. Of these veins the most important are : (1) the anterior 
vena cava ; this is a charmel in the mid- ventral line in front of the 




-8 



1. 



Fio. 126. View of heart and chief blood-vessels of Sepia euUrata. 

Partly after Parker and Haswell. 

Ventricle. 2. Anrldle. 8. Otenidinm. 4. Anterior aorta. 5. Pos- 
terior aorta. 6. Anterior yena cava. 7. Vein from ink-sac. 
8. Genital yein. 9. Branchial vein. 10. Branchial heart. 11. Bight 
abdominal vein. 12. Vein from the mantle. 



kidneys, which forks and sends a branchial vein over each kidney to 
the base of each gill ; (2) the abdominal veins ; these are a pair of 
large channels which come from the mantle, especially the upper 
part of it, and join the forks of the vena cava just before they enter 
the gills ; (3) the genital vein ; this is a trunk draining the genital 
organ, it runs along the ventral wall of the dorsal pouch of the 
kidney and joins the right fork of the vena cava ; (4) a vein from the 
ink-sao joins the same fork, and (5) on each side a smaller vein from 
the mantle joins the main venous system at the branchial hearts 



[OHAK 




. Ovary. 3. OeDiCal part of the ooelom. S. Perioardial part of tlw 
ooelom ; the teference line touchea the heart. 4. Mantle^avitT. 6. See- 
tion of inteatine. 6. Incomplete septum between genital aad perioaidial 
ooelom. 7. Ventral limb of kidney. 8. Glaitdalftr tiMOB of kidney. 
9. Dorsal limb of kidney. 10. Stomach. 11. Liver dnct 

13. " Panoreatia " oaeoB. 13. Liver. 14. Shell. IS. Shell uo. 
16. Dorsal homy jaw. 17. Anterior opening ol fonneL 18. Talvs 
infimnel. 19. Badula. 20. Lips. 31. Tentral homy jaw. 



viil] 



CEPHALOPODA. 



241 



(Fig. 126). Where these yeins come in contact with the kidney 
wall the special excretory tissue mentioned above is developed. 

A peculiar feature in the circulatory system is the presence of a 
pair of muscular swellings of the forks of the vena cava just before 
they enter the gills. These are the branchial hearts, the 
function of which is to drive the blood into the gills, whereas the 
auricles drain it out of them. Each branchial heart projects on the 
one side into the kidney and on the other side into the reno- 
pericardial canal, and the epithelium of the latter where it covers 
the heart is greatly thickened so as to form a cushion, the function 
of which is excretory. This, like Eeber's organ in the river-mussel, 
is a remnant of the primitive excretory function which probably all 
the cells of the coelom once possessed 




Fza. 128. Lateral view of the central nervons Bystem of Sepia offlcinalU, 

Magnified. From Cheron. 

1. Upper buccal ganglion. 2. Nerves connecting bacoal ganglion with 

cerebral ganslion. 3. Brachial ganglion. 4. Infundibular ganglion. 

5. Pleural ganglion. 7. Supra-oesophageal ganglion. 8. Out 

end of optic nerve. 9. Superior ophthalmic nerve. 10. Pallial nerves. 
11. Visceral nerve. 12. Anterior nerve to the funnel. 14. Auditory 
nerve. 15. Inferior ophthalmic nerve. 16. Nerves to the arm. 

The dotted outline represents the buccal mass and the oesophagus. 



The alimentary canal of Sepia is constructed on very much the 
same plan as that of the snaiL The mouth is situated in the centre 
of the arms and surrounded with a frilled lip (Fig. 127). There is a 
large buccal mass containing the radula and there are a pair of 
powerful jaws shaped like a parrot's beak, moveable on one another, 
of which the ventral is the larger. There are two salivary glands 
and a loug narrow oesophagus but no crop. The oesophagus widens 
behind into the- stomach, which receives, as is usual in Mollusca, the 
ducts of the liver. With the stomach is connected a side pouch 

a <ftii. 16 



242 MOLLUSCA. [chap. 

spirally coiled. The liver is enormous, occupying all the anterior 
portion of the visceral hump ; the ducts traverse the dorsal 
extension of the kidneys, and are in this position covered extern- 
ally with excretory tissue which by the older naturalists was termed 
''pancreatic caeca" (12, Fig. 127) from a mistaken comparison with 
the human pancreas. The intestine is slightly bent on itself and 
ends in an oval papilla. A peculiar sac, the ink-bag, the cells 
lining which secrete the pigment known as Indian ink or Sepia, 
opens by a long duct on this papilla (Fig. 129). When the Cuttle- 
fish is alarmed it ejects this ink and darkens the water so much 
as completely to escape from view. 

The nervous system consists of ganglia even more closely massed 
than in the case of the snail. The supra-oesophageal ganglia 
form one rounded mass ; they are produced at the sides into the 
very much larger optic ganglia which are in close relation to the 
eyes (Fig. 128). The pedal ganglion is divided into an anterior 
ganglion called the brachial, and a posterior ganglion sometimes 
called the infundibular. The brachial ganglion supplies a stout 
nerve to each arm, and each nerve swells out into a small ganglion 
just where it enters the arm (Fig. 129). The infundibular ganglion 
supplies the funnel. 

Posterior to the infundibular ganglion are the two pleural 
ganglia fused together. These give rise to a visceral loop which 
supplies the various internal organs and the gills. From the 
same ganglion two short nerves run to the mantle and terminate 
in the two large stellate ganglia, which underlie the skin and 
supply nerves to all the muscles of the mantle (Fig. 129). The 
buccal mass is supplied by two ganglia, superior and inferior, each 
representing a pair joined by a minor nerve collar running round 
the oesophagus. The inferior ganglion corresponds to the buccal 
pair in the snail, the superior is a separated part of the cerebral. 

It has been already stated that Sepia possesses complicated 
eyes. In the embryo these are like the eyes of the snail, merely 
sacs lined by visual cells and containing a transparent homy 
secretion which serves as a lens. In fact, in the embryo, the sac is 
at first a pit which gradually closes up. Immediately over the spot 
where the first pit closed a second pit is formed in which a second 
homy lens is formed just over the first one, so that the lens consists 
of two pieces, and as in the eye of the Vertebrate, there is an anterior 
and a posterior chamber in the eye separated from one another by 
the lens. 



CEPHALOPODA. 



243 



Into the aaterior chamber a circular fold projects called the 
iris, fulfilling exactly the a&me fimctioa as the iris of the human eye 
or the diaphragm in a phot<^aphic camera. Outside and around 
the eye a circolar fold acts aa au eyelid. 




B ay B tern, ventral view. 

1. Three nerves to the arma diesected out. 2. AudUoFy nerve. S, Anterior 
nerve to the tuoael. i. Nerve to vena cava. 5. Posterior nerve 

to fonoel. 6. Continaation of this nerve. 7. AcoeaHory nerve to 

mantle. B. Left nerve to mantle. 9. Stellate ganglion. 10. Com. 
mon tmnk o( the visceral loop. 11. Left branch of the viseeisl loop. 

13. Nerve to mneolea. 13. Nerve to vUcera. 14, QanRlion en 

branchial heart. IG. Nerve of ctenidinm. 16. Ink-bag. IT. Duct 
of ink.bag. 18. Lett nidamental gland. 10. Branchial heart. 

30. Position of anne. 91. Eitemal opening of kidneys. 

The cerebral, pedal and pleur^ ganglia are surrounded by very 
tough connective tisane, in which the fibres, although still visible, 



244 MOLLUSCA. [CJHAP. 

have a cheesy consistence, the tissue being called fibro-cartilage. 
In this way a kind of skull is formed, which sends a scoop-like 
extension on either side over the back of the eye, covering the 
optic ganglia. The edges of the scoop pass into ordinary connec- 
tive tissue round the rest of the eye. This tissue forms the wall 
of the eyeball. 

The otocysts are branched and embedded in the ventral wall 
of the skull. They receive nerves from the cerebral ganglia, but as 
the nerve fibrils traverse the pedal gauglia they appear to arise from 
the latter. 

The genital organ in Sepia occupies the apex of the visceral 
hump. It is, as examination of young specimens shows, a thicken- 
ing of the wall of the coelom, behind the pericardial coelom. The 
space into which the eggs and spermatozoa are shed is only shut o£f 
from the pericardium by an incomplete partition, so that in Sepia 
a state of things persists throughout life which is found only in the 
embryos of some other forms (Fig. 127). 

The genital duct is present only on one side, the left, and is 
produced into a prominent papilla which in the male is used as a 
penis (Fig. 124). The outermost part of the duct in the male is 
A wide pouch in which the spermatozoa are welded into masses and 
•enveloped in cylindrical cases called spermatophores. Just 
beyond this the duct receives the excretion of two glands termed 
prostate glands, and then narrows into a very fine tube which 
opens internally into the coelom. Since in some cuttle-fish the 
genital duct is paired and in Nautilus there are two pairs of 
kidneys, while the genital ducts of that animal appear to be portions 
of the kidney ducts split ofi*, it has been suggested that the genital 
duct of Sepia is all that is left of a missing pair of kidneys. 

The oviduct is simple, but in the female there are four glands, 
the nidamental glands, situated on the wall of the mantle cavity 
just outside the kidneys (18, Fig. 129). From the secretion of these 
glands tough egg-shells resembling india-rubber cases are made. 
The egg is about the size of a pea and when the young cuttle-fish 
emerges it is already like the adult. 

Cuttle-fish feed chiefiy on crabs, shrimps and other Arthropoda, 
using their beaks to break the hard sheU. Some are large enough 
to attack men and this circumstance has given rise to many legends. 
Gigantic species are sometimes cast dead on the shores of Nova 
Scotia, the length of body being ten feet, and of the arms over fifty 
feet. The phylum MoUusca finds its climax in the cuttle-fishes. 



Vni.] CEPHALOPODA. 245 

Modem cnttle-fish hwe a. fairly uniform structure. The two 
long tentacnl&r arms are absent in the Octopoda. In Lotigo the 
shell is horny, in Polypus it is entirely absent. In Ommatnatrephes, 
tiie common cuttle-fish of the Gulf of St Lawrence, the anterior 
chamber of the eye is open and the lena bathed with, sea- 
water. 

Navtilvs is a remarkably interesting cuttle-fish, widely difierent 
from Sepia and the others, but closely allied to most 
of the extinct fonns. The arms are short, broad, ill- 
defined lobes, the suckers being represented by tentacles with raised 
ridges round their bases. There is a large external shell coiled 



I. lAst oompleted chamber of the shell. 2. Hood part of foot. 3. Shell 

maacte. i. Mantle cut away to eipooe (5) the piD-hole fj'e. 6. Outer 

mil a! ibetl, some of which U out awa; to bIjow tlie chambeis. 7. Siphon. 
8. TentkcnliferoiiB lobes of the foot. 9. Funnel. 

forwards in the median plane over the animal's head, so to speak 
(Fig. 130). The visceral hump is enclosed in the last chamber and 
ftt>m the apex of the hump a membraneous tube called the 
siphuncle is given off, which nms through all the other chambers, 
piercing the septa. There is a fold of the coantle turned back over 
the anterior edge of the shell, the firRt foreshadowing of the shell 
sac o{ Sepia. 

There are four gills and four kidneys and four auricles in the 
heart. Thus Nautilus shows traces of segmentation. The papillae 



246 MOLLUSCA. [chap. 

of the posterior kidneys are split, one half leading directly to the 
reno-pericardial canal, the other into the sac-like kidney. The 
reno-pericardial canals thus open directly to the exterior, and the 
genital ducts are in such a relation to the anterior kidneys as to 
make it probable that they are the reno-pericardial canals belonging 
to these, which have acquired independent communication with the 
exterior. 



Phylum MOLLUSCA. 
MoUusca are classified as follows : 

Class I. Gasteropoda, 

Mollusca with a fiat foot adapted for crawling. There is a buccal 
mass and radula; distinct pleural ganglia are present and the shell 
is never composed of paired pieces. 

Sub-class I. ISOPLEUBA. 

Bilaterally symmetrical forms with a shell composed of eight 
median plates situated in a longitudinal series. Numerous pairs of 
ctenidia. 

Ex. Chiton, 

Sub-class 11. Anisopleura. 

Asymmetrical forms, with the left side of the visceral hump long 
in comparison to the right, the anus, kidneys and ctenidia being 
shifted forwards. 

Division I. Streptoneura. 

The arms and ctenidia shifted so far forward that the visceral 
loop is pulled into the shape of an eight and the gill is anterior to 
the heart. 

Order 1. Aspidobranchiata. 

Usually two kidneys, two auricles, two ctenidia. The axis 
of the ctenidia free and both rows of plates present. 

Ex. Haliotis, Patella. 
Order 2. Pectinibranchiata. 

One kidney, one auricle, one ctenidium in which the axis is 
adherent to the mantle and only provided with one row of 
plates. 

Ex. Buccinum, the Whelk. 



VUI.] CLASSIFICATION. 247 

Division 11. Euthyneura. 

The viflceral loop is untwisted and the gill is posterior to the 
heart 

Order 1. Opisthobranchiata. 

Marine forms with a ctenidium and mantle. 
Ex. Aplysia. 

Order 2. Pulmonata. 

Land and fresh-water forms breathing air, having the 
mantle cavity converted into a lung and the ctenidium aborted. 

Ex. Helix. 

Glass II. SOLENOGASTRES. 

Degenerate worm-like Mollusca devoid of shell and foot and 
with a ventral ciliated groove. There is a rudimentary radula and 
the genital organs burst into the pericardium, the nephridia serving 
as genital ducts. 

Ex. Proneomenia, Neomenia. 

Class III. SCAPHOPODA. 

Mollusca with a tubular shell and mantle and a long cylindrical 
foot ending in three processes. A buccal mass and pleural ganglia 
are present The genital organ opens into the left nephridium. 

Ex. Dentalium, 

Class IV. Lamellibranchiata (Pelecypoda). 

Mollusca with a shell composed of two valves united by a hiuge, 
and a mantle of two lobes. The foot is usually wedge-shaped and 
the plates of the ctenidia are fused to form gill-plates. No buccal 
mass. Pleural ganglia fused with the cerebral. 

Sub-class I. Protobranchiata. 

Small Lamellibranchiata with a simple ctenidium like that of 
Gasteropoda, and large labial palps. 

Ex. Nucula. 

Sub-class II. Filibranchiata. 

Lamellibranchiata in which the filaments of the ctenidium are 
loosely united with one another and their bent-up ends are not 
united to the mantle. 
Ex. Mytilus, 



248 MOLLUSCA. [chap. VIII. 

Sub-class IIL Eulamellibranchiata« 

Lamellibranchiata in which the filaments are welded into a 
" lamella " or plate and their bent-up ends are joined to the mantle. 

Ex. UniOf Anodonta, 

Class V. Cephalopoda. 

Mollusca in which the front part of the foot surrounds the head 
and is drawn out into sucker-bearing arms whilst the hind portion 
of the foot forms a muscular tube. The ganglia are massed together 
and protected by a skull : there is a buccal mass with a radula and 
two jaws. 

Sub-class I. Tbtrabranchiata. 

Cephalopoda with four ctenidia, four kidneys, four auricles, a 
large external shell, no suckers and very short arms. 

Ex. Nautilus. 

Sub-class II. Dibranchiata. 

Cephalopoda with two ctenidia, two kidneys and two auricles. 
The shell enveloped in the mantle and the arms are long and 
provided with suckers. 

Order I. Decapoda. 

Dibranchiata with two long and eight short arms. 
Ex. Ommatostrephes, Sepia. 

Order II. Octopoda. 

Dibranchiata with eight arms of equal length. 
Ex. Polyptis (Octopus). 



249 



CHAPTER IX. 

Phylum Echinodermata. 

The class of animals known as the Echinodermata comprises 
the well-known Star-fish or Five-fingers, the equally well-known 
Sea-urchins, the less familiar Sea-cucumbers and Brittle-stars, 
lastly the graceful Feather-stars. The name is derived firom two 
Greek words, ^ivos, which means hedgehog (and was also used for 
the sea-urchin), and Scp/ia, the skin. The prickles and spines with 
which many members of this Phylum are covered constitute a 
very prominent feature in their appearance. Spines, it is true, are 
sometimes absent, but in every case, whether this is so or not, the 
skin contains a skeleton consisting either of plates or of rods, and 
the spines are merely rods belonging to this skeleton projecting 
outwards and still covered by the skin which they push before them. 

Class I. ASTEBOIDEA. 

The most familiar of all the British Echinoderms is probably 
the common star-fish, Asterias rubens, which may be found at low 
water on almost any part of the coast where shell-fish, its favourite 
food, abound. Very similar species, Asterias vulgaris and Asterias 
polaris, abound on the American coast, the first-named on the New 
England coast, the second further north in the Oulf of St Lawrence. 
The species represented in Fig. 131 belongs to a different genus, 
Echinaster, but in all essential features of its anatomy it agrees 
with Asterias. Echinaster sentus is common on the N. American 
coast. The name ''star-fish" denotes the shape. The body is 
produced into five arms or lobes which are arranged like the spokes 
of a wheel round the centre of the body or disc, on the under side 
of which the mouth is situated. These arms are termed radii, and 
the re-entrant angles between them interradiL 



250 ECHINODKRMATA. [uKiP 

The Btar-fish creeps abimt with its moath downwaids: iti 

motion is effected b^ meaiia of numerutia ddictte 

semi-traDsparent tentacles. These ore sitostdd in 

6*6 grooves which run &long the aoder side of the atius and oon- 

verge tow&rds the mouth, where they merge into a 




FiQ. 131. Onl view of Echinatlrr ientiii with labe-reet eilended x&boat 1. 

Fram Agaeaii. 

surrounding that oiwning, These grooves are termed the ambo- 
lacral grooves: the tentacles situated in them are called the 
tube-feet, and the depressed space round the month in wliicb 
all the grooves unite is called the buccal membrane (Lat bueea. 
the cheek) or the peristome (Gr. Ttpi, around, aud trrdfia, moutli). 



ABTEROIDEA, 

So far as we have yet seen the Echinnderms aeem to differ from I 
Coeleitterates, which are also radiate aiiimaU, in the details of the ] 
arrangement of the organs rather than iu any fundamental features. 
The skeleton no doubt is pecnSiar in being embedded in the skin : 
bnt the spicules of the Atcyonaria occupy a similar position, although 
they rarely cohere to form the definite rods and plates like those 
characteristic of Echinodenna. When the soft parts are dissolved 
away from one of these rods or plates by caustic |>otash it is s 
consist of a delicate network of calcinm carbonate ; and it is found 
by ohsenration of the developing young that such a plate is formed - 
by a little heap of cells coming together and secreting a lime- 
stone rod between them : this rod then branches at both ends and 
the branches bifurcate again so that the twigs of the second or 
third degree approach each other and joining form a mesh, and this ' 
process of bifurcating and joining is repeated luitil the plate or 
spine is built up. The growth of the primitive rod into the mesh- 
work is rendered possible only by the growth of the cells which shed 
out the calcium carbonate. These cells remain throughout life, 
more or less modified, as a kind of living network interpenetrated by 
the skeletal one. 

When however we cut a star-fish open we see that the animal 
aptiarently cousista of two sacs plained one within the 
other. The iunennoet sac or alimentary canal opens 
in the centre of the upper surface by a minute opening, the anus, 
through wliich undigested matter is thrown out, and on the under 
surface by the month. The space or sac which apparently a urronuds 
the digestive cavity of the star-fish ia a true coelom : like the 
coelom iu a segment of an annelid it has been formed by the union 
of two sacs which in the embryo lay right and left of the digestive 
tube. Prom its walls the muscles are developed, tlie generative 
cells, and also the cells which give rise to the skeleton. Between 
the outer wall of the body-cavity and the tnie external skin which , 
corresponds to the ectoderm, there is a mass of more or less 
gelatinous substance exactly corresponding to the jelly of a Medusa 
or the connective tissue of an Arthropod, which constitutes the 
substance of the body-walL Into this material wander cells budded 
from the wall of the coelom: these cells from their power of move- 
Uieut and appearance can be recognised as amoebocytes. It is 
Irom these cells that the skeleton is formed in the way we have 
described above: some of them, however, retain their primitive 
character and wander about, probably carrying food to the various , 



IX.] DIGESTION. 253 

The alimentary canal can most easily be examined by carefiilly 
Aiimentaiy Cutting away all the upper parts of the five arms in 
canal. qj^q pioco, Cutting along both sides of each arm, then 

laisiDg the upper part of the animal and clipping through the 
intestine near the anus. By this means the animal is separated 
into an upper and lower half and all the internal organs are dis- 
played in one piece or the other. The alimentary canal is then 
seen to consist of several regions clearly distinguished from one 
another. It begins with an exceedingly short gtdlet which passes 
at the lips into the buccal membrane already mentioned : the gullet 
widens out above into an exceedingly loose baggy stomach produced 
into ten short pouches, two situated in the beginning of each arm. 
Above the stomach and communicating with it by a wide aperture 
lies a flattened pentagonal bag, called the pyloric sac, and from 
each of the five angles of this sac there is a tube given off which 
runs into each arm, where it is soon divided into two parallel sacs, 
each produced into a multitude of little, short pouches. These sacs 
are called the pyloric caeca: caecum, Latin "blind,'' being a con- 
venient zoological term for a blind poucL The pyloric caeca are 
tied to the upper side of the arm, each by two bands of transparent 
membrane called mesenteries. From the centre of the pyloric 
sac a short straight tube runs to the upper surface of the animal 
where it opens by a minute anus : this tube is called the rectum, 
a name, as we have seen, commonly given to the last portion of the 
digestive tube. The rectum has attached to it two branched 
tubes of a brown colour which open into it, called the rectal 
glands. 

The reason of the division of the digestive sac into various 
parts is of course the different uses to which they are put in the 
life of the animal ; and we may stop for a moment to enquire what 
these uses are. 

Star-fish feed chiefly on bivalve shell-fish, such as mussels, 
cockles and clams, though they will attack almost any animal. 
Their mode of seiziog their prey is very curious. If they are 
attacking a bivalve, they bend all their five arms down round it, 
thus arching up the central portion of the body. Then the stomach 
is pushed out, — this being rendered possible by the turning inside 
out of its .edges, which as we saw above, are loose and baggy — and 
wrapped around the fated mollusc. The pushing out is effected by 
the contraction of some muscle fibres in the body-wall : these tend 
to diminish the space which the coelom occupies, and as this is 



ECBINODERMATA. 

iilled with incompressible fluid, the stomach must be preeeed dqL 
After some time has elapsed the stsr-fish relaxes its hold aod iti< 
then seen that the shell of the mollusc is completely empty saA u 
clean as if it had been scrajved with a knife. It was long a p 
how the star-lish succeeded in foix-ing its vii.-tim to relax its mnsdei 
and allow the valves to open. It was supposed that the aUmoA 
secreted a paralysing poison, but it huv been conclusively | 
that this is not the case, but that the stat-hsh drags the valvei of 
its victim ajiart by main force, often actually breaking the addactn 
muBcles, The pull exercised by the suckers is not nearly strong 





Fiii. l.^iS sua b h !■ h aitrr m tu n the act f deToiiring a Mussel. 
Madrepono p ate. 
enough to open the valves at on e but the star tish has staying 
power and eventually the m issel si wly fo ed open. The cells 
lining the sto acb n lule u large u nber of goUet^cells (v. p. lOS) 
swollen by drop of lear tl ud those of the pyloric sac, on the 
other hand 7 esent a d fferent appearan e These are full of minut« 
granules and re all the appeara ce of the ells a other animals 
which conta n the act ve d gest ve | r nc pie Hence it seems 
reasonable to suj pose that the mussel s d gested by the seeretion 
of the pylonc sa and its me lage which tious downwards into 
the stomal h Kay po t n ren ain ni; und ^esttd s expelled through 
the rectum no food ever penetrates uto the pylo c caeca. 

The locomot on of the star fish s ffected n the following 
manner. The tube-feet which crowd the ambulacra! grooves 1 



nt] 



WATER-VASCULAR SYSTEM. 



255 



during life continually extended and retracted. At their ends are 
■tcc'vu- flat circular discs, and these disc:^ are pushed against 
■yitcm. jjjg etone or rock or whatever else the star-fish is 
clinging to. Then by the contractioa of their muscles t}ie centre 
of the disc is pulled upwards, and so it is made to adhere in exactly 
the same way in which a boy uiakei^ a leather "sucker" adhere to a 
■tone. When once the disc is firmly fixed the contraction of the 
tube-foot draws the animal after it. 




I. IM. mtftia ol a 

3. JoUf. S. FeribnuiabiBl i. 

4. Peritoiiwl lining of bodj-csTity. 6. A bianoliia. 

ckaeiuii. 7. Hesenteiy HupportiDg a caecnm. 8. Spine. 9. Otusiole 
in Bkio. 10. Fsdiaelluia. II. Ambnl&cnl OBsiele. IS. Adunba- 
laom] oaricle. 13. B&dud trunk of WHtet.VBBoulnr system. 

11. Badul leptam Sfparnling the tvo perihaemal spacta. IS. BadisJ 

aenv-eoii. • tbieltened band of ectoderm nitb a plexus of nerra-fibrils 
noderljiTig it. 16. Aiopulta of tube-foot. 17. Tnbe>foot. 

IS. PeribMinal space. 19. Coelooi. 

We found that in order to examine the alimentary canal it was 
advisable to divide the star-fish into an upper and a lower half. If 
WB now cut away the tube-feet and look at the roof of the ambu- 
laoml grixjve from wtiich they project, it will be seen that the 
groove is roofed in by a double series of calcareous rods, meeting 
each other at an angle like the beams of a church-roof (II, Fig. 131). 
Tbeae are called the ambulacral ossicles. They can be drawn 
together by muscle fibres running from one of a pair to its 
fellow just wider the spot where they meet. By this action the 
unbtdacral groove is narrowed ; and at the same time, inwardly 



256 ECHINODBRMATA [OHAP. 

projectiDg spines lining its edges are made to meet, so that the tabe- 
feet are entirely protected by a trelliswork of spines. These spines 
are attached to rods, called the adambulacral ossicles, firmly 
bound to the outer edges of the ambulacral ossicles (12, Fig. 134). 
Inside the animal, between the ambulacral plates, a series of pear- 
shaped transparent bladders tensely filled with fluid project into 
the coelom (Figs. 132 and 134). These are really the swollen upper 
ends of the tube-feet and are termed ampullae. They act as 
reservoirs into which the fluid contents of the lower part of the foot 
are driven when the longitudinal muscles of the tube-foot contract 
The bladder-like upper end of the foot has only circular muscles, 
and when these contract the fluid is driven back into the lower 
part of the tube-foot and it is expanded. The tube-feet, though 
from the above description it would seem as if each was capable of 
acting without the others, are really all parts of one system : they 
are connected by short transverse tubes, with a canal running along 
the whole length of the arm immediately under the ambulacral 
ossicles, called the radial water-vessel (13, Fig. 134). This 
canal and its branches can easily be seen in microscopic sections of 
the arms of young star-fish, or they can readily be demonstrated 
by cutting oflf the tip of the arm of a fully-grown specimen, 
finding the end of the radial tube on the cut surface and inject- 
ing it with coloured fluid by means of a fine pipette. The five 
radial tubes are connected with each other by a ring-shaped 
canal lying just within the peristome, which is called the water- 
vascular ring. There are nine small pouches called Tiede- 
mann's bodies projecting inwards from the ring canaL In these 
are formed the amoebocjrtes which are found floating in the fluid of 
the canal, and which arise by budding from the wall of each poucL 
From the ring canal also in one interradius, where the tenth 
Tiedemann's body if it existed would be found, a tube is given ofi* 
which leads to the upper surface of the disc, where it opens by a 
sieve-like plate, pierced by numerous minute pores, called the 
madreporite (Figs. 132 and 133). This vertical tube receives the 
awkward name of the stone-canal because its walls are stiffened 
by calcareous deposit ; its cavity is reduced to a mere slit by the 
projection into it of an outgrowth of its wall shaped in section 
like a T with coiled ends, which is also strengthened by lime. 
Although, as we have said, the cavity of the stone-canal is a mere 
slit, yet it is lined by long narrow cells carrying most powerful 
cilia. In many species of star-fi^h, although not in Asterias, stalked 



IX.] NERVOUS SYSTEM. 257 

sacB resemUing greatly enlarged ampullae are attached to the water- 
yascular ring. These appear to act as reservoirs of fluid for it: 
they are known as Folian yesicles after Poli, the naturalist who 
first described them. 

Now since all the moyements of a tube-foot can be accounted for 
by the action of the longitudinal muscles of its lower part and the 
circular muscles of the ampulla, the question arises as to what is 
the purpose of this apparatus of radial and circular tubes, stone- 
canal and madreporite ? There is one interesting little mechanism 
which supplies a valuable clue to the answer to this question. This 
is a pair of valves placed in the tube-foot at the entrance of the 
transverse canal, which unites it with the radial tube. These valves 
swing open into the tube-foot when the pressure in the radial tube 
is greater than the pressure in the tube-foot, but 'when the pressure 
in the latter is the higher they close, so that under no circumstances 
can water escape from the tube-foot into the radial canal. So it 
appears that there is an arrangement which allows fluid to pass into 
the tube-foot but which prevents its return, and this implies that 
under ordinary circumstances there must be a loss of fluid from the 
tube-foot. We must in fact suppose that when the tube-foot is 
driven out by the contraction of the ampulla, the contained fluid 
slowly transudes through its thin walls and the loss is supplied from 
the radial canaL The pressure in the radial and circular canals is 
kept up by the action of the cilia in the stone-canal, by means of 
which a slow but steady current is produced, setting in from the 
outside through the madreporite. 

The function of the whole system of tubes therefore is to keep 
the tube-feet full of fluid and thus tense and rigid, so that they can 
perform their frinctions properly. 

The nervous system of the star-fish is one of the most interesting 
Nervous fcaturcs in its anatomy. The ectoderm consists of long 

system. delicate cells bearing flagella and interspersed with 

goblet-cells similar in appearance to those lining the stomach. 
The slime which these cells manufacture covers the surface of the 
animal and no doubt protects it from the attacks of bacteria and 
microscopic algae. But the chief point of interest is that at the 
bases of the long delicate cells there is an indescribably fine tangle 
of delicate nerve-fibres which are doubtless outgrowths of some of 
the cells. Here and there a nucleus is seen amongst them which 
belongs to a neuron — that is, an ectoderm cell which has lost its 
connection with the rest and has become pushed down into the 

8. <& M. 11 



258 ECHINODERBfATA. [CHAP. 

fibrillar layer. The ectoderm all over the body is therefore under- 
lain by a nervous sheath and is very sensitive, but there are certain 
places where the nervous sheath becomes very much thickened and 
it is these areas which constitute the true sense-organs and the 
central nervous system. 

Isolated sense-cells, that is, cells having a stiff protruding hair, 
are scattered all over the surface ; but the only spot where they are 
collected in groups so as to form true sense-organs is on the tips of 
the tube-feet The tube-feet are then practically the only sense- 
organs, and since the radial water-tube ends at the tip of an arm in 
a ireely projecting tentacle, we might regard the whole radial tube 
as a huge, branched, sensitive tentacle. There is the more justifica- 
tion for doing this when it is found that the radial tube with its 
freely projecting tip is in the young star-fish quite independent of 
the outgrowth of the body called the arm, and only secondarily 
becomes applied to it. At the base of the end-tentacle there is a 
thick cushion of nervous matter in which are excavated a number 
of ectodermal pits lined by cells containing orange pigment. 
These pits are organs of vision: and it has been experimentally 
shown that a star-fish deprived of these organs is insensible to 
Ught. 

The central nervous system consists of five thick bands of 
nervous tissue situated one above each ambulacral groove under- 
neath the radial water-tube (Fig. 134). They are termed the 
radial nerve- cords and are joined by a circular band of a similar 
nature, called the nerve- ring, lying under the water-vascular ring. 
Intervening between the radial nerve-cord and the radial water-tube 
there are two canals lined by flattened cells and separated from one 
another by an imperfect septum (14, Fig. 134). They are caUed 
the radial perihaemal canals and are outgrowths firom the 
coelom. From their upper walls are derived the muscles which 
move the ambulacral ossicles on one another : from their lower walls 
a layer of ganglion cells and nerve-fibres, which may be termed the 
coelomic nervous system in order to distinguish them bom the 
main mass of ganglion cells and fibres which are derived from the 
ectoderm. This coelomic nervous system, which is very thin in 
the star-fish, seems to serve as the channel by which impulses bom 
the radial nerve-cord reach the ambulacral muscles. The five pairs 
of radial perihaemal canals are connected with one another by a 
circular canal lying above the nerve-ring called the outer perihae- 
mal ring. Inside this is another circular canal called the inner 



IX.] PEDICEILASUE. 259 

perih&em&l ring, vliich is ui espansioQ of the foot of the axial 

smns (see p. 360). 

The apper or abonl sorface of the star-fish is provided with two 
moBt interesting groupB of organs, pedicellariae and 
dermal branchiae. The former are minute pincers, 

composed of two or rarely three blades moving on a basal piece. 




Fia. 135. Pedioellariae from Aiteriat glaciali$. From Cnfoot. 

A. CroBBed form X 100. 1. Eotoderm. 2. Bam of leR "jaw." 3. Muscle 
olomog the "jaws." 4. Basal ossicle. 6. Hnscle opening the 

" jawB." 6. Fibrous bend coDcecting the basal ossicle with ooe of tlie 

rods of ths skeleton. 7. Fibres of pedimale. 



B. Straight form. 



4. Mnsole douDg the "jav 



These close when the skin of the back is irritated; their main 
purpose appears to be to keep the surface of the animal clear from 
loopbytes and other small encrusting organisms. They cover the 
thickened bases of the blunt spines with which the back is beset 
iMTgBt pedic«ll&riae are scattered in the inter-spacea between the 
qnnes and are distinguished &om the smaller by the hct that the 
blades do not cross as is the case with these. The larger kind 
are also found on the adambnlacral spines. 

The pedioellariae are probably little spines of the second order. 
In the small blunt-armed star-fish, Asterina gibbosa, there are no 
true pedioellariae, but the plates on the back bear small spines 
arranged in twos or threes, which act somewhat like pedicellariae 
when the akin is irritated. 



260 ECHINODERMATA. [CHAP. 

The dermal branchiae (5, Fig. 134) are conspicuous in a 
star-fish when alive; they are very difficult, on the 
other hand, to detect in preserved specimena They 
are in fact thin spots on the body-wall, where it consists only of 
the ectoderm and the wall of the coelom — closely apposed, the 
jelly, fibres and skeletal rods being absent These spots project 
like little finger-shaped processes and their purpose is to facilitate 
respiration. The fluid in the coelom or body cavity being separated 
from the external water by a very thin membrane, the dissolved 
oxygen is able to pass from the one fluid to the other with great 
ease. 

There is no localized excretory organ in the star-fish or indeed 
in any Echinoderm. Throughout the phylum so far as is known 
this function is performed by the amoebocytes which float in the 
coelomic fluid and have been produced by the budding of the cells 
forming the wall of the coelom. When charged with excreta the 
amoebocytes endeavour to make their way out. This in the star-fish 
they effect by accumulating at the base of the dermal branchiae 
and working their way through the thin body-wall and so escaping 
into the ocean. 

The organs of sex in the star-fish are very simple. Both kinds 
iieproductive ^^ gorm Cell are aggregated in great feather-shaped 
organs. glauds situated in pairs in the bases of the arms and 

opening in the angles between the arms or in the inter-radiL The 
ten ovaries in the female and ten testes in the male are connected 
by a circular cord of immature germ cells called the genital 
rachis running round the disc just dorsal to the coelom. This is 
embedded in the wall of a tube called the aboral sinus which like 
the other spaces in a star-fish, apart from those of the digestive 
canal, is an outgrowth of the coelom. The rachis is in turn con- 
nected with a pillar of similar cells running alongside the stone-canal 
which used to be called the heart, under a mistaken idea of its 
function, but which we shall term the genital stolon. The genital 
rachis is formed as an outgrowth from the genital stolon and the 
latter is an outgrowth from the coelomic wall, so that the genital 
cells are derived from the coelomic cells as in other Coelomata. 
The genital stolon is interposed between the general coelomic cavity 
of the animal and a special division of the same which is called 
the axial sinus, and which runs parallel to the stone-canal. 
The axial sinus is derived from the anterior portion of the 
coelom in the larva. Underneath the madreporite there is still 



IX.] OPHIUROIDEA. 261 

another diyision of the coelom completely shut oflF from the rest, 
which may be termed the madreporic vesicle. It apparently 
represents a rudimentary second water-vascular system, since in 
exceptional cases it may develope the rudiments of radial canals. 
The genital stolon projects into the axial sinus ; it has a brown 
colour which no doubt suggested the connection with the blood- 
system to the earlier anatomists, but true blood-vessels do not exist 
in Echinodermata. The ova and spermatozoa are thrown out into the 
water by pores situated on the under or oral surface at the base of 
the arms and unite with each other there. The young lead a free- 
swimming existence, and are so unlike the star-fish that no one would 
ever dream of suspecting that the two had anything to do with 
each other. As however these peculiarities are fundamentally the 
same in each of the groups of the Echinoderms they will be dealt 
with later when the characters of these other groups have been 
studied. 

The other species of star-fish, which are all grouped together in 
the Glass Asteroidea (Gr. dcmjp, a star; €1809, shape), diflfer but little 
in really fundamental points from Asterias rubens, Pedicellariae 
may, as we have seen, be absent; the arms may be short so that 
the shape almost becomes that of a pentagon and the arrangement 
of the plates and spines constituting the skeleton may vary very 
mucL In one family, the Astropectinidae, there is no anus, the 
rectum ending blindly, and the tube-feet have pointed ends. These 
star-fish do not climb but run over the surface of the sand. 

The number of arms is most often five, but not only do indivi- 
dual variations from this rule occur in species where five is the 
normal number, but species and even genera and families are 
characterised by having a larger number: the common Sun-star, 
Sohuier papposus, for instance, has from eleven to thirteen arms. 

Class II. Ophiuroidea, 

The next order of Echinoderms is termed the Ophiuroidea (Gr. 
o^tovpos, serpent- tailed ; cISo?, form) or the Brittle-stars. These like 
the star-fish have a body with five arms diverging from a central 
disc on all sides like the conventional representation of a star. The 
arms are, however, sharply marked off from the central disc, and 
they do not, as in the true star-fish, insensibly merge into it, but 
are continued along grooves on the under surface to the immediate 
neighbourhood of the mouth : further they are exceedingly long 



262 ECHINODERMATA. [CHAP. 

and flexible sod totally unlike the stiff anne of Atteriat or ita 
alliea. 

The habits of the aDimal too are veiy different from those of t^ 
star-fiab. Instead of creeping slowly along by the action of the 
tube-feet it springs along by muscular jerks of the arms, s 




pushing with four anus and seizing hold in front with one, sometimee 
pushing with three and hauling itself along with two. The name 
Brittle-star is derived from the readiness with which, if irritated, 
t^e animal will snap off an arm. 

As might naturally be expected, the most striking differences 
from the stat-fish are seen in the arms. No smbulacial groove is 



OPHIUBOIDEA. 

^parent; the arm being encased in a cuirass consisting of four eeriea 
of plates, an upper row, two lateral rows each bearing a row of spines 
on ita edge, and an under low (Fig. 137). On close inspection the 
short pointed tube-feet may be seen protruding from minute pores 
at the sides of the under row of plates. A thin section of the ann 
reveals the fact that there really is a apace correapouding to the 
ambulacral groove of the star-fish, but that by the approximatiou of 
its edges it has become closed off from the outer world so that it 
forms a canal, the so-called epineural canal (i, Fig. 137). Above 
this canal, at the spot one would term the apex of the ambulacral 




Flo. 137. Section Ihroagh 



Ditigraniinatio, mBgnifled. 



RuiiAl nsrve-card. 3. Bftdial perihaemaL cilduI. 3, Badial watar- 

viuoolar oanal. 4. Epioeural cantiL 6. Ventral plate. 0. Tabe- 
fool. 7. Pedal BanBlion. 8. Lateral plate. 9. Spine, 

10. Donal plate. 11. Coelom. 13. Lonijitiidiiisl maaale. 

IS. "Veitebni." 14. Baft tiaaue Bupporting pluiea. 



groove in a star-fiah, there ia a ridge of nervous matter covered on 
the lower side by cells exactly resembling the skin cells covering 
the nerve ridges in Asterias. This is the radial nerve, and above 
this again ia a large rounded disc of calcareous matter, the ao-csUed 
vertebra. This really corresponds to a pair of ambulacral plates 
which have become fused together. So much might be inferred 
from the fact that the radial water-tube runs in a groove on its 
under Borface, and it is clearly proved by examining young speci- 
mens. Each vertebra ia very short, and it not only has rounded 
knobs and cupa in order to enable it to sUde on its auccessor and 
predecessor, but ia connected to each of them by four great muscles. 



264) EGUXNODBBMA.TA. [CBAP. 

by the oontractioa of which the arm is moved in uiy direction 
(Fig. 137). If ihe two side musclee contract the arm is moved 
toward that side, if the two upper, upwards, and so on. These 
muscles are the seat of the chief activities of the animal, and it is 
not surprising to find that a pair of large nerves comes off between 
each two vertebrae to supply them, and that where these nerves 
are given off the nerve-cord u thickened and the nerve-cells 
increased, so that a string of ganglia is produced strongly recalling 
the ventral nerve-cord of the Earthworm, Between the vertabnM 
and tlie radial nerve-coid there is a single canal (2, ^. 137), lepn- 
senting the pair of radial perihaemal canals in a similar position in 




Fio. tSS. A diograEiimatia T«itical wetiOD of sn Ophittroid After Lndwig. 
The oircamoral Bjatema of oTgaoa are seen to the left cut aoroBa, their 
radial proloDgationB out longitadinallj to the nght 

1. Body-wall. 2. Month. 3. Coelom. 3'. Goelom of the arm. 

4. Mouth papillae, 5. Toma angalatis. 6. Oral plate. T'. lat am- 
balacral Oiainle. 7*, 7', 7*. 2nd to 4th ambulaersl oaaicle or " TBrt»>irae." 
8>, 8>, 8*. lat to 9rd Teotral plate. 9. lat oral foot. 10. Tiaiu- 

Terse muBcle of the 2Qd joint. 10*. Eiteraal inteiradiat muacle. 

10'. Internal iuterradial mnade. (The line should point to the dotted 
tiaaue.) 11. Water-vasaolar aTatem ; to the left the oironmoral ring, 

to the right the radial Teaael. 12. Folian veaiole. IS. Nerre-Tiug at^ 
radial nerve ; the Raoglia on the latter are not ahowu. 14. Genital 
lachia. 15. Badial perihaemal oanal, 

Asteroidea. From the ventral wall of the canal the coelomio 
nervous system is formed ; and it is by the greater development of 
this system where the nerves to the ambulacral muscles ue given 
off Uiat the ganglionic swellings of the nerve-cord are produced. 
The vertebrae and these muscles nearly completely fill the arm, 
leaving only a small canal above the vertebrae (11, Fig. 137): this 
is an outgrowth of the body cavity or coelom, but there is no 
branch of the alimentary canal continued into it, as was the case 
with the star-fiah. 

The digestive sac is here a simple flattened bag lined by cells 
somewhat like those lining the pyloric sac of the star-fish. There 



OPBIDROIDEA. 

is no anus, and the edges of tha stomach cannot be pushed out. 
How then, it may be asked, does the Brittle-star eat and of nbat 
does its food consist ? 

It must be confeHaed that, in spite of their qnick movements and 
highly developed nervous system, BrittJe-stars belong in general to 
the great army of mud-eaters and scavengers. Where they live — 
nsually at the bottom of sea poob and at such depths of the ocean 
as to be in still water — the mud or sand is impregnated vrith 
decaying animal and vegetable mattor, and the Brittle-stars shovel 
tiaa material into their mouths by means of the two pairs of tube- 
feet of each arm nhich he nearest the mouth and are called the 



oral tube-feet. The interradii between the arms project inwards 
over the mouth, as the mouth-angles ; those are lined along their 
edges and at their tips with broad blunt spines called teeth and 
month papillae, so tltat they form an effi<Ment strainer and prevent 
coarse particles entering the stomach (Figs. 138 and 139). The 
calcareous plat-e at the apex of each mouth-angle which bears these 
sptoea is called the torus angularis (5, Fig. 138). 

We saw that in the star-fish the whole surface is covered 
vith a sensitive akin, but that the tube-feet act aa sense-organs 



266 ECHINODERMATA. [CHAP. 

as well as being locomotor in function. In the Brittle-stars the 
sole purpose of the tube-feet is to serve as sense-organs; they are 
often covered with little warts consisting mainly of sense-cells with 
their delicate hairs sticking out all round, just like the batteries of 
cnidoblasts in Hydra, and in all cases there is a special nervous 
swelling surrounding the base of each tube-foot called the pedal 
ganglion (7, Fig. 137). As, however, these tiibe-feet have lost their 
power of attaching themselves by a sucking action to objects and 
hence are of no use for locomotion, the ampullae have disappeared ; 
and as the action of the ampullae is probably the chief cause of the 
loss of fluid in the tube-feet of the star-fish, in the Brittle-star, where 
the loss must be very smaU, the stone-canal is excessively narrow 
and the madreporite instead of being a regular sieve has two pores 
only, rarely more. It is very curious to find that the madreporite 
is on the underside of the animal; in the young Brittle-star it is 
on the edge of the disc, but in each interradius the upper surface 
grows more rapidly than the ventral and so it is forced round on to 
the underside. To the water-vascular ring are attached four large 
Polian vesicles, the interradius occupied by the stone-canal alone 
being without one. The tube-feet are the only sense-organs, in a 
sense still more real than is the case with star-fish, for in the 
Ophiuroidea the rest of the ectoderm, after having given rise to a 
cuticle, has disappeared, the solid mail of plates which the animal 
possesses appear to render it impervious to sensations of contact. 

The organs of sex are very simple ; they are situated in the disc 
in the interradii and consist, in each interradius, of several short 
pouches. These open into ten sacs, called the genital bursae ; 
one pair being placed in each interradius. These sacs are merely 
invaginations of the ectoderm which does not here disappear as over 
the rest of the body ; they are lined by ciliated cells which keep up 
a constant current of fresh water pouring into them and thus they 
fulfill the same function as the "dermal branchiae " of Star-fishes. 

Class III. ECHINOIDEA. 

The general appearance of the dried skeleton of a Sea-urchin or 
Echinoid, is familiar to most people, but many would fail to recognize 
any resemblance to a star-fish in the slightly flattened sphere 
covered with spines. If, however, we are fortunate enough to see 
one living, we at once perceive that along five meridians the sphere 
is beset with beautiful semi-transparent tube-feet, ending in suckers, 



ECniNOIDEA. 

exactly like those of the star-fish. In fact, the Sea-urchin might be 
desciibed as a star-fiah in which the upper surface had shrank to 
insignificant proportious, being repre^iented. by a small patch of 
leathery skin at the upper pole : or, if no regard the whole radial 
tube with ita tube-feet as one immense branched tentacle and the 
arm as ite support, we should say that the arm had been again 




Flo. HO. Strongijlofcnirui ilrirbachiciuii^l, Aliutal surfnoo. From AgUBsiz, 
I. Expanded tabe-fect. 2. Spines. 

merged iu the body ao that the radial tube was bent back in a 
curved course. As a matter of fact the end of the radial tube 
projecle very slightly beyond the general surface und bears at its tip 
a mass of pigment which corresponds with the eye of the star-fish, 
though no eye-structure liiis been detected in it. 'lliia is situated 
near tiie upper end of the body, just outside the small area of 
lesthery skin mentioned above. 



268 ECHINODERMATA. [CHAP. 

The skeleton of the Sea-urchin is a cuirass of plates fitting edge 
to edge, with two openings. Of these the upper (already referred to) 
is covered with leathery akin and has the small anus in the centre 
of it and is called thepeiiproct (Fig. 148): it is this area which 
corresponds to the whole upper surface of the star fish The other 
opening is in the centre of the lower surface and is likewise 
covered by flexible skin it wrrounds the mouth and is called the 
peristome (Fig 147) 




1, Adus 2 Leathery akin roand anuB penproct 3 Modreponc plate. 
4 Oenitiil plate with gen tal pora S Ooalar plate with eye. 

6. Line of junction of ambulocral nod ittterambulactal pUtes. T. Ambn- 
lacrnm. 8. Pores through whioh tube-feet piotnide. 9. Boasea 

which bear the spiaes. 

The cuiraas itself is called the corona and consists of twenty 
strips, each made up of s row of plates. Corresponding to each 
tube-foot area or radius there are two rows of so-called ambnlscral 
plates, and each intervening area or ioterradius is similarly covered 
by two rows of large plates. As in Opliiuroids, there is no ambulacral 
groove visible from the outside : it is represented by the epioeural 
canal, immediately inside which there is the radial nerve-cord. 



IX.] 



ECHINOIDEA. 



269 



It 18 necessaiy to bear this in mind when the term ambulacral 
plate is used ; the so-called ambulacral plates of an Urchin do not 
correspond to the similarly named plates in the star-fish, as they 
do not roof in the ambulacral groove, but form a floor for it. 
Inside the nerve-cord there is a single radial perihaemal canal as in 
the Brittle-star (2, Fig. 145 b). As the plates of the skeleton are 
not movable on one another nothing corresponding to the ambu- 
lacral muscles of the star-fish exist at least over most of the radius, 
and the radial perihaemal canal is separated from the general 
coelom only by a thin septum in which the radial water-tube is 
embedded For the same reason there is no recognizable coelomic 
nervous system. 

If the continuous cuirass of the Sea-urchin and the closed 

ambulacral groove remind one of an 
Ophiuroid, the resemblance ends there; 
for in the Urchin the ectoderm, consist- 
ing of long slender cells with a tangle of 
nerve-fibres at the base, is spread over 
the whole surface outside the skeleton, 
just as in a star-fisL This sensitive 
layer controls, it is found, the move- 
ments of the spines, which are among the 
most important organs of the Urchin. 
These spines, unlike the spines of the 
star-fish or Brittle-star, have hollow 
bases, which articulate with smooth 
rounded bosses on the plates (B, Fig. 
145). They are tied to these bosses by a 
sheath of muscle-fibres, so that by the 
special contraction of any side of the 
sheath they can be moved in any direc- 
tion. The skin covering the sheath has 
developed a specially thick nervous layer. 
Sea-urchins such as we have been 
describing live on stony or rocky bottoms, 
over which they slowly creep by means 
of their tube-feet The spines are 
pressed against the substratum and keep the animal from rolling 
over under the pull of the tube-feet and also help to push it 
on. The spines are usually of two distinct sizes, longer primary 
spines, and shorter secondary spines. The forest of spines has 
a kind of undergrowth of pedicellariae. These are of several 




Fzo. 142. A glandular or 
gemxniform Pedioellaria 
from E. escuUntuixlQ, 
From Chadwiok. 



270 ECHINODBRHATA. [CHAP. 

kinds and are much more highly finished organs than those of 
star-fishes ; they have a long stalk, which is portly stafiianed by 
a delicate calcareous rod, aad the jaws are three in nomber. One 




Fia. 143. AriEtotte's Lantern at E. eievlentat x 3. Pnrtlj from Chadvick. 
1. Tipper end of tooth enveloped in lantern membrane. 2. BadEns. 

S. Tranaverse muscle of radii, elevator. 4. Depressor mnscles of radina. 
S. Jaw. 6. Retractor muscles of the jaws. 7. Protractor of jaws. 

8. ADricala. 9. Ampnllae of tnbe-fect. 10. Jnter-ambitl&eral plate. 
11. Tooth. 12. Circular watec-vBHcuIar vessel. IS. EpiphTsis. 

14. Polian veaiclea. 15. Oesophagus. 16. 'Vontial"blood"-vessel. 

17. Oenital stolon. IB. StoDe-oanal. 19. Bectum. SO. Uadre- 

poriaplftte. 



IX,] ARISTOTLE'S LANTERN. 271 

kind hu §hott stumpy jaws, each with a poison bag at its base and 
a stiff stalk; these are doubtless weapons of defence and enable the 
Urchin to give any unwelcome Tisitor which may come too close a 
wann reception. Such pedioeilariae are called gemmiform (Fig. 142). 
Another kind, termed tridactyle, has long jaws and a flexible 
stalk. It was supposed that these helped the animal to climb by 
seiiiiig hold of waving fronds of sea-weed till the tnbe-feet could 
get a hold, but this is proved not to be the case. It has been 




1. Month, a. iDteatine cut ahort. 3. Siphon. 4. Itectum. S. Anas. 
6. VentTftl " blood "-vesael on intestine. T. Dorsal " blood "-vessel 

on intastiiu. S. Stone-canal. 9. Madreporio plate. 10. Qenital 
rsohiB. II. Water-TasonUr ring. 12. Nerve-ring. 13. Tube-foot 
with ampulla. 14. Radial nerve. IS. Radial water-vessel. 16. Polian 
veaicle. 17. Moscles ; those on the left pull Aristotle's Lantern out- 

wards, those on the right retract it. 18. Ocular plate. 

shown that a gentle movement in the water excites the tridactyle 
pedicellariae while a stronger movement calls the gemmiform into 
activity. Besides these there are two other kinds of pedicellariae. 
It seems most probable that these elaborate organs are for the 
purpose of protecting the sea-urchin against the attacks of certain 
animals which in their absence would either fix themselves on 
tiie skin of the Echinoid or even burrow into it. The number 
and variety of these organs are an indication of the danger that exists 
from this eouicfl. 



272 ECHINODERMATA. [CHAP. 

The Urchin is provided with five white chisel-like teeth, each of 
which slides on a pair of grooved pieces called alveoli, meeting in 
a point below. Each pair of alveoli meet in a point where they 
clasp the tooth. Above they are united by two pieces called 
epiphyses (13, Fig. 143) which meet in an arch. A pair of 
alveoli with their epiphyses are often spoken of as a jaw, and 
adjacent jaws are joined by stout, inwardly projecting rods called 
rotulae. The whole apparatus of five jaws has received the name 
of 'Aristotle's Lantern.' This can be pushed out or pulled in 
by muscles attached to arches called auriculae, rising from the 
inner side of the skeleton (8, Fig. 143). Through the auriculae the 
radial water-tube and nerve pass, and thus they correspond in 
position to the ambulacral plates of star-fish. 

The food of the Urchin consists ordinarily of seaweed which it 
gnaws with its teeth. No doubt the little worms and molluscs 
always 'found in abundance on the surface of the weed add a flavour 
to the repast The alimentary canal is exceedingly unlike those of 
the Echinoderms so far studied. The gullet ascends vertically 
between the teeth and passes into the intestinal tube which runs in 
a spiral right round the body and then turns sharply back and 
describes one turn of a spiral in the opposite direction, after which 
it bends inwards and runs straight up to the anus (Fig. 146). For 
the first part of its course a small tube, the so-called siphon, runs 
parallel to it, opening into it at both ends. 

The water-vascular ring is situated above the masticatory ap- 
paratus and is thus widely separated from the nerve-ring, which is 
situated below it : the radial tubes, in consequence, run downwards 
along the ''lantern" before bending outwards under the auriculae 
(see Fig. 144). The water-vascular ring bears small pouches which 
have been termed Polian vesicles. They seem, however, to corre- 
spond to Tiedemann's bodies in an Asteroid. The first pair of 
tube-feet in each radius are difi'erent to the rest, in that they are 
short and not capable of extension, and that their discs are ovaL 
These tube-feet protrude through the peristome and are called the 
buccal tube- feet; they function as tasting organs, and are 
thrown into violent excitement if a piece of eatable matter is put 
near them (Fig. 147). 

In describing the Asteroidea it was mentioned that the genital 
stolon or "dorsal organ '' had been mistaken by former authors for a 
heart, and that true blood-vessels were unknown amongst Echino- 
dermata. If by blood-vessel is meant a tube with well-defined 



n.] 




Flo. 115. SeotiODS through ports ol Echinut eicuUntui. 

A. A. wctioa at right ftnglw to the i)Une of the Uadreporio plats x 16. 
From Chadwiok. 1. Madiepoiic ptate. 2, Pores in the game. 
3. Uadreporio Teside. i. Ampulla of ntadreporio plate or dilatation of 
Btone-canal into which the pores open. 6. Madrepotia tabe. 6. Oeni- 
tal stolon. 7. Axial bIhub. 

B. A i«etion at right angles to an ambolacral aiea. 1. Badial nerre-cord. 
3. Badialperihaemal oanal. 3. Badial water-canal. 4. Epineuial 
oanal. 6. Ampulla. 6. Cavitj ol tube-foot. 7. Ambnlaoral plate. 
B. Boh for articnlation of spine. 9. Spine. 10. Uasolea which move 
th« spine. 11. Ectoderm. 

LAX. V& 



274 ECHINQDERHATA. [CHA?. 

walls in which there is a definite circulation of fluid thia ia strict^ 
true. 

It roust be remembered, as was pointed out in the chapter on 
Arthropoda, that blood-vessels aad connective tissne hare been 
derived from the same primitive tissue, which may be compaied to 
the jelly of Goelenterata. Now Echinodennata probably lepresent a 
stage before the evolution of either blood-vessels or proper coDoective 
tissue. Apart from the plates of the skeleton the subetance of the 
body-wall has little more consistence than the jelly of an Aurtlia, 




FiQ. 146. View of aeft-Urchin, with part of the ihell removed to show the 

coarse of the slimeotnrj oansl. From Leackart, after Coiier. 
1. Mouth BarTOQDded by five teeth (displaced), 2. Laotem of Ariatotle. 

3, Oesophagne, coiled intestine and rectum. 4. Ovaries with ovidactB. 

5. The siphon. 6. " Blood -ring." 7. Fold of peritoneum supporting 

genital rachis. 8. "Blood-vessel" aooompanjiDg intestine. 9. Ampnllae 

at base of tube-feet. 

aud readily degenerates into slime. The ground substance has 
remained so fluid that it is still traversed by arooebocytes which 
cany excreta to the exterior. In Echinoidea along two tracts, one 
situated on the same side of the oesophagus as the stone-canal and 
the other on the opposite side, the jelly intervening between the 
inner wall of the coelom aud the oesophagus has undergone the first 
stage in the change to a blood-vesBel. The fibres an scantily 



coMXEcnva tissve. 

I developed and the amoebocytes are preaent in immense numbers, 
whilst the )^und substance has become more fluid and probably 
contaiua proteidn, since it stainn with ciirmine like protoplasm. 
Tbese tracts are termed dorsal and Tentr&l " blood "-vessels. The 
dors«] vessel is on the side next the stoue-canaL These tracts 
have not the form of tubes, but are uetworka of irregular iHissages 
devoid of proper walls. They accompany the alimentary canal 
throughout most of its courHe and it seems as if the products 
[ of digestion were areumulated in them. A so-called "blood-ring" 
I ctf similar character surrounds the oesophagus just above the water- 




I Fw. 1*7. Oral field of Krhinu, fteiil-^Mue. Magnieed. Fiom Kiiktnlliol. 
AmboUcniiu with tobe-rmt and spiues. 
Ariatotlc'a lantern. 4. Buccal tubp-reet. 
mouth > the Perutome. 

Bcolar ring and into this the two " vessels " open. A similar ring 
It been described in Asteroidea and Ophiuroidea ; iii some species 
t the former class a tract of similar substance appears. to run down 
e arm just above the uerve-cord in the septum separating the two 
ihaemal canals, and the name of these canals (Ur. irtpi. around ; 
[, blood) has been suggested by this circumstance. 



276 ECHINODERMATA. [CHAP. 

Breathing, as one might expect, is carried out wherever the 
body-wall is thin enough to allow the oxygen to diffuse through, 
that is to say by the tube-feet and by the peristomal membrane. 
The tube-feet, as in the star-fish, are provided with large ampullae 
which project freely into the great spacious body cavity. Oxygen 
thus taken into the fluid filling the tube-feet can be passed into the 
body cavity through the ampullae, and there is a curious arrange- 
ment to facilitate this. Where the tube-foot passes through the 
skeleton it is split into two parallel tubes which reunite below 
(B, Fig. 145) : so that on the dried shell we see on the ambulacra! 
plate several pairs of pores, each pair corresponding to a single 
tube-foot. As the cells lining the inside of the latter are ciliated, 
the splitting of the tube is apparently to facilitate the separation of 
the upward and downward currents of water. 

The peristome has ten branched pouches, situated one pair in 
each interradius and projecting outwards. These are the gills, 
but it is unreasonable to suppose that all the breathing is done by 
them. They communicate not with the general body cavity, but 
with a part of it, called the lantern coelom, shut off from the 
rest by a septum stretched between the teeth and jaws. Embedded 
in the upper wall of this are certain rods called radii, which are 
connected with each other and the auriculae by muscles, and by 
means of these the upper wall of the lantern coelom can be raised 
or depressed and so the pressure inside altered. When these 
rods are depressed water is driven out into the gills and there 
absorbs oxygen. When they are raised the water is sucked back 
into the lantern coelom and the oxygen passes through the thin 
wall of the latter into the general coelom. 

The organs of sex are alike in external appearance in both sexes 
(Fig. 146). They have the form of five great bunches of tubes 
hanging down into the body cavity and opening by five small holes 
placed in plates called the genital plates, forming the summit of the 
interambulacral series on the corona and situated just outside the 
periproct (Figs. 141 and 148). In the young Sea-urchin there is a 
genital rachis connecting them together, and throughout life there 
is a genital stolon alongside the stone-canaL The genital stolon is 
relatively much larger in Echinoidea than in Asteroidea, and sur- 
rounds the axial sinus in the lower part of its course, so that this 
space appears like a cavity excavated in the stolon. Hence by one 
author the axial sinus and stolon were mistakenly described as a 
nephridium with gkndular walls, and the madreporic vesicle, which 



IX.] CLASSIFICATION. 277 

IB here mnch eolarged and extends parallel with the axial sinus, 
was called by the same author the "accessory kidney." 
The EchiDoidea are divided into three Orders : — 
I. The ordinary Sea-urchins, such ae we have described, con- 
stitute the Order Endoctclica, which live chiefly on rocky and stony 
ground. The other two Orders live in sand or mud and have under- 
gone singular modiflcations in order to fit them for this kind of life. 




Fia. 148. Aboral B;item of platei o( EMnoi ficuUntut x i. From Chadnicb. 



They are termed the Irregular or Exocyclic Sea-urchins, because 
whereas the anus has become shifted from the upper pole of the 
body down one side to the edge, or even to the under-surface of the 
more or less flattened body, the madreporite and genital plates still 
retain their position. In both Orders the tube-feet of the upper 
part of the ambulacra are the main breathing organs, and are 
greatly flattened and expanded at the base, while the pores 
through which they pass are arranged in two converging curves on 
each ambulacrum, the figure produced being compared to a petal 
of a flower, hence the name applied to them, viz., petaloid ambulacra. 
The special characters of these two Orders are as follows : 

II. Cltpbaotboidea or Cake-urchins. They live at or near the 
surface of the sand. They still retain their teeth, which are placed 
almost horizontally, and they use them as spades to shovel the sand 



278 ECHINODERBCATA. [CHAP. 

into the moutL All the spines covering the upper surfiftce are 
ciliated and so a constant current of water sweeps over the expanded 
tube-feet which act as gills. In addition to these tube-feet the 
whole aboral surface, radii and interradii included, is covered with a 
multitude of minute tube-feet provided with suckers. Similar tabe- 
feet are found on the oral surface, but they are confined to the 
radii. This immense multiplication of tube-feet is of course due to 
the small purchase that any one of them is able to get on such a 
yielding material as sand. In a word, the animal moves itself by a 
multitude of minute pulls instead of by a lesser number of stronger 
pulls as do the Endocyclica. There are calcareous piUars stretching 
from the upper to the lower surface of the shell or test, apparently 
to enable them to withstand rough usage, since in many cases they 
live within reach of the breakers. The best known British species 
is Echinocyamus pusillus, a little oval Sea-urchin about the size of a 
pea, whence the common name applied to it, viz. Pea-urchin. On 
the east coast of North America one species, Echinarachnius parma^ 
the Sand-dollar, is very common ; this is an extremely flattened 
Urchin of circular outline, the shape and size of which have suggested 
a comparison with the famous silver-dollar of the United States 
currency. 

III. Spatangoidea or Heart-urchins. These live buried at 
depths of a few inches to a foot beneath the surface of the mud, 
and the body is more or less oval or egg-shaped, slightly flattened 
underneath. The mouth is sometimes in the centre of the under- 
surface and sometimes nearer one end, and is usually crescentic 
and always without any trace of jaws. These Urchins have usually 
only four of the ambulacra ''petaloid"; the fifth has a few long 
tube-feet with expanded fringed discs. In the case of the familiar 
British species Echinoca/rdium cordatum it is known that the Urchin 
extends these tube-feet from its burrow right up to the surface of 
the sand and collects with them deca3dng organic matter lying on 
the surface. This is pushed within reach of the buccal tube-feet 
and so reaches the mouth. 

The Spatangoidea do not use their tube-feet to walk with, but 
move by means of spines which are provided with flattened tips, 
and so the small tube-feet present in such multitudes in the 
Cl3rpeastroidea are absent. Besides these spines they possess peculiar 
lines of very small spines covered with cilia, which cause a current 
to pass over the gill-like tube-feet. Such rows of ciliated spines are 
termed fascioles. 



IX.] HOLOTflUROIDEA. 279 



Class IV. Holothuroidea. 

The fourth group of the Echinoderms is termed the Holothu- 
roidea or Sea-cucumbers, and consists of animals of a more or less 
sausage-shaped form, with the mouth at one end and the large anus 
at the other. 

These animals have undergone the same essential modification 
as the Sea-urchins, the arms having been re-absorbed into the body 
so that the radial tubes run down the side of the body and end 
near the vent The nervous system also is situated beneath the 
surface, the ambulacral groove being represented by the epineural 
canaL The stdn, as in Echinoidea, retains its well-marked ecto- 
derm with nervous sheath. 

They are however distinguished by some most marked character- 
istics: — 1. The skeleton has almost entirely disappeared, being 
represented only by grains and prickles of various shapes completely 
buried in the skin. 2. The muscular system of the body-wall is 
most powerfully developed: there is a pair of strong longitudinal 
muscles running inside each radial tube, and transverse muscles run 
across each interradius. 3. The buccal tube-feet are highly modi- 
fied and are the means by which the animal feeds itself. 4. The 
anus is wide and the concluding portion of the intestine termed the 
cloaca is strongly muscular, and it is used as a breathing organ, 
water being sucked in at the anus and thrown out again. 5. The 
stone-canal does not reach the exterior, but terminates in a sieve 
plate hanging down into the interior of the body. 

The broathing by means of the anus is carried out by certain 
organs called gill-trees. These are two great branched tree- 
like outgrowths of the hinder part of the intestine, reaching right 
through the body cavity to near the mouth (Fig. 149). Water is 
taken in by the anus and forced up into the finest branches of these 
and no doubt diffuses through into the body cavity under the 
pressure set up by the contraction of the muscles of the anus. Hence 
it is that the animal is able to dispense with an external madreporite, 
and also to obtain the fluid necessary to keep its tube-feet tense 
from its own body cavity. From the water-vascular ring one or 
moro long-stalked Polian vesicles hang down into the body 
cavity. 

The muscular body-wall has a very curious effect on the economy 
of the animal. When it is irritated it contracts the muscles, and 




Fill. 149. View of Bolotkttrta tuJmtoia somewhat diminiBhed. The animal 
is opened along the left doreal interradioB and (he viecera are expoeed. 
After Ludwig. 

1. Tentaclel. 2. Ampnllae of tentacles. 3. Water-vosoulac ring. 

4. Polian Teaiole. G. Stone-canals. 6. fiadial water-Tees^ 

7. Badial longitadinat mosole parti? out away. 3. Beproductive 

organ. 9, AJimentary oanaL. 10. Clo&ce., 11. Bwpiiatory trees. 
13. Fuitnl "Uood-TeeseL" IB. DcumI "^\i«A-Biewi».'' 



IX.] HOLOTHUROIDEA. 281 

since the fluid in the body cavity is practically incompressible, the 
effect is to set up a tremendous pressure. As a result of this the 
wall of the intestine near the anus tears and a portion or the whole 
of the intestine is pushed out. The gill-trees are the first to go, and 
in some species the lower branches of these are covered with a 
substance which swells up in sea-water into a mass of tough white 
threads in which the enemies of the animal are entangled. A 
lobster has been seen rendered perfectly helpless as a consequence of 
rashly interfering with a Sea-cucumber. These special branches are 
termed Cuvierian organs. 

A Holothuroid is only temporarily inconvenienced by the loss of 
its internal organs. After a period of quiescence it is again 
furnished with the intestine and its appendages. Some species, which 
are able to pull in the mouth end of the body with their tentacles, 
when strongly irritated snap oflF even this, and yet are able to repair 
the loss. 

The intestine is a simple looped tube which has three limbs. 
One limb runs down towards the anus, the next turns up again 
towards the mouth and then bends back into the final limb which 
goes towards the anus. These limbs are attached by mesenteries 
to different interradii of the body, the first to that which in the 
ordinary position of the animal is mid-dorsal, the next to the left 
ventral, and the third to the right ventral (Fig. 149). 

Accompanying the alimentary canal are so-called dorsal and 
ventral "vessels" similar to those of the Echinoidea, and there is 
also a ''blood-ring" like that described in the same class. In 
Holothuroidea the ventral vessel is close to the alimentary canal 
but the dorsal vessel is borne on a little ridge projecting from the 
intestine. The alimentary canal is enswathed by minor branches of 
the network of which the dorsal and ventral vessels form merely the 
large trunks. The whole system thus assumes a very complicated 
appearance, but even here it has been shown that there is no 
circulation nor even a proper wall to the spaces. The longitudinal 
vessels indeed often do not appear to communicate with the blood- 
ring. 

The buccal tube-feet form a crown of from ten to twenty-five great 
branched tentacles, and their different shapes are used to classify 
the various families of the Sea-cucumbers. Most species feed 
on sand or mud, but one Order can be described only as anglers. 
In them the tentacles are long and delicately branched so that 
they resemble pieces of sea-weed. The animal stretches them 



282 ECHINODERBfATA. [CHAP. 

oat, and they become the resting-place of numbers of the mmate 
animals which swarm in sea-water. When one tentacle has got a 
sufficient freight it is bent round and pushed into the mouth which 
is closed on it. It is then forcibly drawn out through the closed 
lips so that all the living cargo is swept ojQf. 

The organs of sex are similar in nature to those of the Urchins, 
but are represented by only one mass of tubes which all unite in a 
common opening near the tentacle region, and it is in this region 
that the stone-canal opens in the one or two rare cases where it 
still opens to the exterior. Hence it appears that whereas in the 
irregular Sea-urchins the genital openings and madreporite have 
remained fixed while the anus has been shifted, here the anus has 
remained in its original position while the genital opening has 
been shifted towards the mouth. 

The Holothuroidea are divided into the following five Orders. 

1. Elasipoda: Sea-cucumbers whose tentacles have shield- 
shaped ends drawn out into short processes, devoid of gill-trees, 
with the tube-feet of the upper surface of the body modified 
into stiff respiratory processes. Live only at great depths in the 
ocean. 

2. Aspidochirotae : Sea-cucumbers with shield-shaped ends to 
the tentacles, — these have also large ampullae so that they can 
be individually retracted. With gill-trees and often Cuvierian 
organs. 

3. Dendrochirotae : Sea-cucumbers with long delicately-branched 
tentacles without ampullae. The whole front end of the body can 
be pulled in by means of special muscles. Gill-trees present 

4. Molpadidae : Sea-cucumbers with tentacles unbranched or 
with two or four small lateral branches, and no other tube-feet 
except a circle of papillae round the anus. Gill-trees present. 

5. Sjmaptidse : Sea-cucumbers in which the tentacles have two 
rows of short branches. No tube-feet except these, the radial 
canals having also disappeared. No gill-trees. The body-wall is 
thin and transparent and oxygen can diffuse through it. 



Class V. Crinoidea. 

The last group of the Echinoderms is termed the Crinoidea (Gr. 
Kplvov, a lily), animals long familiar to collectors of fossils under the 
name of lily-encrinites. They differ from other Echinoderms in 



IX.] CRINOIDEA. 283 

that from the centre of what corresponds to the upper or aboral 
surface of other orders, there springs a 
jointed stalk by which the animal is 
moored to the substiatnm. 

Animals of this type were much 
more common in past times than now. 
La^ masses of limestone are actu- 
ally made up of their skeletons. The 
modem order of Crinoidea mcludes a 
few species sorTiving at great depths 
in the ocean, and about the mode of 
life of these we know little. There 
are, however, besides these a number 
of species not sharply marked off from 
each other assigned to a family, the 
Comatulidae, containing two genera, 
Antedon and Actitwmetra, which lire 
at moderate depths in the ocean and 
which have been thoroughly studied. 
These however are exceptional in that 
they break off from the stalk when 
they are mature and swim about by 
muscular movements of the long anns. 
The stump of the lost stalk forms a 
knob called the centro-dorsal ossi- 
cle, which is provided with grasping 

processes called ci rri, by means of which the animal can temporarily 
attach itflel£ 

We may select for our type the common Antedon rosacea, 
which can easily be captured by the dredge at moderate depths. 
This animal has a small disc and ten extremely long arms. It 
reminds one of a star-fish, in the fact that on the oral sides of 
these arms there are open grooves converging to the mouth, and 
that the skin lining these grooves is modified to form nervous bauds 
uniting in a ring round the mouth. These ambulacral grooves are 
further lined by powerful cilia which cause currents of water carry- 
ing small animals to flow towards the mouth, and thus the animal is 
fed. The tube-feet are small and apparently of use only as gilla, 
those springing from the grooves on the disc alone retaining their 
sensory frinction. 

The skeleton is peculiar. The ventral side of the body is 




Fio. ISO. Anttdan acotla. Car. 
A;oDnfcindividD&l X 1^, Alter 
CBtpenter. 



284 BCHINODERHATA. [CH&P. 

covered by a leathery skin but on the aboral side there are first the 
centre dorsal ossicle a round knob representing the uppermost 
joint of the stem and then five rows of plates called radials 
radiating from t These five rows show ua that here as in most 
other Echinoderma we have to do with five primitive arms or 
radii these ladii however bifurcate the moment they becoms 




Fia. 151. Transverse aeation through the disc and base of an arm of 
Anttdon Toiacta. After Ludwig. 
1. Month. 2. Various sectious of alimeDlArj oaual. 3. Epithelium of 
ambulftcral grooTS. i. Nervona lajer of ftmbulaoral groove. 6. Bailial 
water-canal. 8. Circuraoral water- vascular ring. 7. Slone-eanals, 

e. Pore-canals. 9. Trabeculae trsTeraing the coelom. 10. Axial 

coelom oommuniaatiDf; with 11. 11. Coelom of arm. 12. Cini. 

IS. Genital stolon giving ofT 14. 14. Branches of genital stolon in 

oirn. 16. Badial nerve of dorsal nervous system. 16. "Chambered 
organ" in centre of dorsal nervous system, 17, Calcareous reds 

developing into trabeculae. 18. Muscles connecting radial and brachial 

plates. CD. Centro.dorsal piece. B', K^, R>. First, second and third 
radial plate. Br,', Br.' First and second brachial plate. 

free from the disc, and bo there are ten arms : the uppermost 
plate in each of the five rows having a double facet, on to which 
fits the lowest of the rows of plates supporting the arras. 

The arms really bifurcate again and again, but in each case one 
of the forks does not develops further and forms a pinnule. If in 
tiie case of any of the bifurcations both forks were to develope we 



IX.] CRINOIDEA. 

sfaotUd h&Te an increase in the number of 
arms, and indeed speciea of Antedon with 
twenty, forty and even one hundred arms 
are known. 

There is no madreporite, but the whole 
of tiie upper soft integument ia riddled 
with isolated pores which lead into the 
body cavity and are lined by ciliated cells. 
The water- vascular ring has hanging down 
from it a large number of stone- canals, 
which also open freely into the body 
cavity. Only one pore and one stone- 
can^ exist in the stalked young, but their 
position, comparatively near the mouth, 
is utterly different from that in any other 
Echinoderm. In the adult the cavity of 
the coelom is traversed in every direction 
by cellular cords called trabeculae. 

The anus is situated on the ventral 
side of the body in an interradius, the 
alimentary canal being coiled in a simple 
spire in the disc. We have spoken above 
of the ambulacral grooves being lined by 
nervous cells, like those forming the radial 
nerve in star-fish. This is indeed so, but 
the Crinoid possesses another and much 
more important nervous system. From 
the body cavity five canals are given off 
which penetrate the stalk. Iliese canals 
swell up in the substance of the ceotro- 
dorsal ossicle into chambers, and in the 
permanently stalked forms like Bhizo- 
erinns or Pentacrinm they form similar 
chambers wherever the stalk bears cirri. 
The coelomic cells which form the walls 
of these canals develope great masses of 
nervous fibrillae. In Antedon of course 
only the five uppermost chambers remain 
when the stalk disappears — they are 
termed collectively the chambered 
organ — and the nervous lining of these 




286 ECHINODERIUTA. [CHAP. 

constitutes a kind of brain (Fig. 151), This " brain "ia separated 
from the body cavity of the calyx by a shelf-like fold strengthened 
by a calcareous plate called the rosette, which represents a circle 
of five plates alternating with the columns of radials clearly seen 
in more primitive crinoids. From this brain cords go off to the 
cirri, and live great cords run upwards perforating the radial rows of 
plates and eventually bifurcating pass into the arms perforating the 
plates which form the skeleton of the latter. These cords are at 
first tubular outgrowths from the 
brain, the cells forming the walls 
of which become converted into 
nervous matter. It has been ex- 
perimentally proved that it is this 
nervous system which controls the 
muscles moving the arms, and 
that if the whole soft part of 
the disc including the ambulacral 
nervous system be removed the 
animal swims just as well as 
before. 

The organs of sex are rounded 
masses found in the pinnules and 
are really, as in Asteroidea, 
Ophinroidea and Echinoidea, 
swellings on branches of a genital 
rachis. There is also a genital 
stolon, which however has no 
connection with any of the stone- 
canals, but rises from the lachis 
to the centre of the dorsal wall of the coelom. The young are 
carried on the pinnules for some time and have a very short fr«e 
swimming life, very soon settling down and developing into little 
pentacrinoids with a jointed stalk. The name " pentacrinoid " 
is suggested by their resemblance to Penlticrhiivt. These stalked 
young present interesting features in the skeleton found in many 
living and fossil Grinoidea but absent in the adult Antedon. Thus 
the mouth is guarded by five inter-radial valves each supported 
t^ an oral plate, and the rosette is represented by five ossicles. 

Leaving aside the Grinoidea, the development of which is known 
only in one case and is there evidently much modified, the eggs of 




Fio. 153. Ventral view of a Lutk ot 
a Holothuriaii taken at Marseillea 
X about 100. From Job. Miiller. 
Moath. 3. Oesophagns. 

Stomach. 4. Rectam. 
AnuB. 6. Coelomio sac. 

Rndiment of vater-vaacalar Bj'stem. 
Ciliated band. 



IX.] DIPLEUBDLA. 

the othw four groups of EchiDoderms develope into free swiraining 
aoimala which for periods varyiog from a fortnight to six weeks lead 
a free life at the surface of the ocean. These young are called 
Dipleurula larvae and they are, as mentioned, utterly different to 
adult Echinoderms : nnlike these, but like most other animals, they 
are bilaterally symmetrical (Fig. 1S3). They swim by means of a 
powerful longitudinal ciliated ring, drawn out into a number of 
arms or processes. They possess a complete alimentary canal, 
consisting of oesotihagus, stomach and rectum, while the cnelom is 
represented by two sacs lying one at each side of the digestive 
tube. These sacs are, as a study of the early development teaches, 
portion:^ of the alimentary canal budded off from the rest. One of 
the moat interesting features in the development is the fact that 
these sacs undergo traDsverse division in the same way as do the 
germinal bands of an Annelid. On each side three segments are 
formed. The most anterior on each side often coalesce to form a 
median sac into which the originally single madreporic pore opens 
on the left side : a portion of this sac becomes the axial sinus of 
the adult. The middle sections on each side in exceptional cases 
deveIo|e each into the nidimente of a water-vascular system showing 
tliat this structure was originally paired. That on the right side, 
however, normally remains small, whilst the left one takes on the 
form of a wreath with five projections, by the union of the two ends of 
which the water- vascular ring with the rudiments of the five radial 
canals is formed. This is called the by drocoele. The most posterior 
divisions form the body cavity of the adnit ; the left one grows in a 
ring-shaped manner, encircling, as with a wider ring, the ring of the 
hydrocoele, while through the centre of both rings the oesojihagus 
of the adult grows. The oesophagus of the larva is usually cast 
off, but liometimes, as in the Ophiuroidea, it is directly converted 
into that of the adult by being shifted to the left before the rosette 
of the water-vas;:ular sy.stem becomes a ring. 

The Dipleurulae of the Asteroidea fix themselves at the conclusion 
of their larval life by the anterior end of the body, using the prae- 
oral lobe as a stalk. The tised stage is omitted in other Eleutherozoa, 
but the larva of Antedon nimcea — the only Grinoid whose develop- 
ment is known — also converts the prae-oral lobe into a stalk. But 
in the case of the Asteroidea the body becomes bent on the stalk in 
such a way that the stalk springs from dose to the month of the 
adult. The stalk is eventually absorbed and the star-tish commences 
itfl adult life, breaks loose from its attachment and moves away. 



II tiie ^H 
away. ^^| 



288 ECHINODERMATA. [CHAP. 

In the Crinoid, on the other hand, the mouth becomes rotated away 
from the stalk, and the latter seems to spring from the aboral surEace. 

The whole of this development seems to point to the conclusion 
that the radially symmetrical Echinodermata are derived bom a 
bilaterally symmetrical ancestor with traces of metameric s^men- 
tation ; that the acquisition of radial symmetry was due in the first 
instance to the assumption of a fixed mode of life, followed by the 
dwindling of the organs of the right side and the compensating 
greater growth in those of the left. The Crinoidea seemed to have 
retained the original mode of feeding by means of the current 
produced by cilia, and thus their mouth became shifted upwards 
away from the stalk into a position favourable for the capture 
of floating prey. In the Asteroidea and the other Eleutheroxoa 
food is obtained by seizing it with the tube-feet, and hence in 
these the mouth was bent downwards so that the stalk seems to 
spring from the oral surface. 

This development is not only interesting on account of the 
extraordinary metamorphosis which the young undergo, but also on 
account of the fact that whilst the adult is utterly unlike any of the 
other Coelomata, the structure of the young is reconcilable with the 
fundamental structure of Annelida and MoUusca, etc. The only 
plausible explanation of this is to be found in the hypothesis that 
the young represent in a rough sort of way the ancestor fix>m which 
the Echinoderms were derived. 

When the Vertebrata are dealt with it will be pointed out that 
the larvae of the most primitive forms bear a striking resemblance 
to those of the Echinodermata, and that in the embryos of many 
Vertebrata the coelom undergoes at first a similar division to what 
occurs in the Dipleurula, suggesting the conclusion that the hi^est 
groups in the animal kingdom are also sprung from the same 
ancestor as gave rise to the Echinoderms. 

The Phylum Echinodermata is classified as follows : 

Sub-Phylum A. Pelmatozoa. 

Echinodermata which are fixed to some foreign object during the 
whole or part of their existence by a jointed stalk springing from 
the centre of the aboral surface. 

Class I. Crinoidea. 

Pelmatozoa with five long arms which repeatedly fork. The 
genital organs are borne in the tips of the branches. 



IX.] CLASSIFICATION. 289 

Sub-Phylum B. Eleutherozoa. 

Echinodermata which are free during the whole of their adult 
existence and rarely fixed even during the larval condition. When 
the immature form is fixed the stalk springs from the oral surface 
near the mouth and is not jointed. 

Class I. ASTEROIDEA. 

Eleutherozoa with arms (free radii) containing outgrowths of the 
alimentary canal and open ambulacral grooves. The arms have 
feebly developed muscles and locomotion is effected entirely by the 
tube-feet. 

Ex. Asterias, Echinaster, 

Class 11. Ophiuroidea. 

Eleutherozoa with arms sharply marked off from the central disc. 
The arms do not contain outgrowths of the alimentary canal and 
have closed ambulacral grooves. They have highly developed 
muscles, and locomotion is entirely effected by the arms, the tube- 
feet being purely tactile. 

Ex. Ophioglypha, 

Class III. ECHINOIDEA. 

Eleutherozoa in which the arms have coalesced with the body, 
the radii being arranged like meridians on a sphere. The ambulacral 
grooves are closed. The body has a complete armour of closely 
adjusted plates and the spines are movably articulated with these 
and assist in locomotion. 

Order 1. Endocyclica. 

Echinoidea in which the anus is in the centre of the aboral 
pole and teiBth are present. 

Ex. Echinvs. 

Order 2. Clypeastroidea. 

Echinoidea in which the anus is excentric, the dorsal tube- 
feet are flattened and teeth are present. 

Ex. Cfypeaster, Echinocr/amus, Eckinarachnius, 



290 ECHINODERMATA. [CHAP. IX. 

Order 3. Spatangoidea. 

Echinoidea with an excentric anus and flattened dorsal 
tube-feet but without teeth. 

Ex. Spatangus, 

Class IV. HOLOTHUROIDEA. 

Eleutherozoa resembling Echinoidea in the anus and ambulacra! 
grooves ; but with rudimentary skeleton, highly developed muscular 
body-wall and greatly enlarged buccal tube-feet by means of which 
all the food is obtained. 

Ex. Hohthuria, 



291 



CHAPTER X. 
Phylum Brachiopoda. 

Brachiofods (Or. Ppaxunv, the arm; ttov^, voSo^, the foot) are 
tnie Goelomata and retain the coelom in a primitive and typical 
condition. Like the Mollusca, they are not segmented, and the 
only trace of a repetition of parts in the group is in a genus called 
BkynchoneUa in which the nephridia are repeated, so that we find 
two pairs. A similar repetition of the same organs occurs amongst 
the Mollusca in Nautilus. 

Brachiopods are exclusively marine. They have a shell con- 
sisting of two valves, so that at first sight they appear 

features, to resemblo the Felecypoda, but in Brachiopods the 
shells are placed ventrally and dorsally, and not on 
the two sides of the animal as in Mussels. In a few cases such as 
that of the primitive genus LingtUa the two valves of the shell are 
nearly alike in size and shape and consist largely of homy matter or 
chitin. In most cases however the shell is calcareous, and since in 
Brachiopoda, as in most bilaterally sjrmmetrical animals, the two 
sides resemble one another whilst the back and front are unlike, 
each valve of the shell is symmetrically shaped, but the dorsal valve 
differs from the ventral, the latter being usually the larger. In a 
few cases, such as that of Crania, a British form common in certain 
localities, the ventral valve is flat and attached by its whole surface 
to the substratum ; all that is then seen is the arched dorsal valve. 
Since in the overwhelming majority of Pelec3rpoda the two valves of 
the shell are similar in appearance, while each is asymmetrical in 
shape, the umbo being situated near the anterior end, it is easy to 
distinguish at a glance the shells of the Felecypoda from those of 
the Brachiopoda. 

The posterior end of the body terminates in a stalk which in 
Lingula helps to keep the animal in the holes of the sand in which 
it lives. In other forms the stalk is shorter and it is firmly glued 

19—2 



to a rock so that vheo it has 




Fio. 164. Tertbratula lemigloboi 
Braohiopod HheU x |. 
Dorsal view. B. Lateral 



BRACUIOPODA. [CHAP. 

fixed itself a Bracbiopod cannot 
change ite place of resideoce 
(Fig. 157). Each valve of 
the shell is lined hj the body- 
wall of the animal, but tlie 
body does not occupy the 
whole of the space between 
the two valyes ; it is produced 
ioto two folds or flaps called 
the mantle-flaps, which an 
foBHU g^d hollow Mid contain ex- 
tensions of the coelom. These 

a. posterior, b. anterioT end. The line secrete the larger part of 

between a and bifl called the length, it j^e Yalves of the ghelL In 

traversea tha aperture through which , . , . 

the etalk pTojects. The line between Ltngula and BOme otlieTS tfae 

°!f'f.?';>,'''t'"^'^w '^'T"! free edges of these mantfe- 

and f the thickneBS, and between g and ° n i . i i 

h is the hiDge-line. folds, lying panllel with the 

free edges of the shells, bear a 

number of cttaetae which recall those of the ChaetopodL It v 

by no means certain that the shell of Brochiopods is an external 

secretion like that of Mollusca : it seems possible that it b really 

deposited in the connective tissue under the ectoderm. 

In most of the thick-shelled forms 
the shell is traversed by processes of 
the mantle, which nourish it, so that 
in dned Brachiopods the shell seems 
perforated with a number of pores. 

If ne slightly open the valves of 
the shell of a living Bracbiopod (so as 
^ I fiavetefiu ^ avoid tearing the tissues) and look 
in we shall see between the ventral 
and dorsal mantle-folda the anterior 
body wall of the creature. This some- 
tunes runs almost horizontally across 
between the space within the valves, 
but often slopes obliquely from the 
ventral to the dorsal valve of the shell. Part of this wall is modified 
to form two long ridges, the ends of which project freely and are called 
the arms i they are coiled and are beset with tentacles (Fig. 156). 
Running close to the origin of the tentacles is a little lip or flange 
BO placed that the two form a groove or gutter. The groove is 




shell at II aldheim 
Uagnificd 
1 Pnsmatio layer formed ii 
nective tissue b Epidermal 
ia;er e Outer calcareous 

lajer ddt The expanded 
outer ends of the tubes which 
traverse the shell. 



lined with cilia and so is the timer face of each tentacle. The 
whole of this apparatus is called the lophophore. It might be 
described as a ring of tentacles, the ends of which are drawn out so 
as to form the arms. It is never quite bo simple as the above 
account would lead one to suppose, for the rin); is often produced 
into two minor lobes forming the lesser arms situated between the 
main ones, and in many genera the two main anna aro raised up 
from the level of the body-wall and each is twisted into a spiraL 
The dorsal shell may be prolonged into a series of plates and even 
into elaborate bands and loops which serve to support such 
lophophores. 




1. Month. 3. Lophophore. 3. Stomach. 4. I.irer tubes. 6. Median 
ridge on dorsal shell. C. Heart. T. loteatine. eliding; blindly. 

S. MuHole fiom dorsul valve of shell to atalk. !t. lulernitl funnel -ah aped 
opening at □ephridium. 1(1, Stallc. 11. Bodj-watl. 13. Tentacles. 
13. Coil or lip, 14. Terminal Lenloctea. 

The mouth lies on the middle line at the bottom of the gutter 
between the lip in front and the tentacles behind. 
Mruetur^ '^^^ lophophore ia thus an organ for catching food and 

passing it into the mouth. The cilia which cover the 
inner surface of the tentacles and line the gutter set up small 
whirlpools in the water so that minute animals and algae becoming 
involved in these are swept into the month- In many species the 




BBACmtWODA. 

tentacles can be protmded between the valves of the shell, uid thiu 
the area they affect is enlarged. 

The mouth leads into a simple stomach which ends in a flbott 
intestine. Both atomach and iutestiae are ciliated, 
gland called the liver consisting 
of a number of branching tubes 
opens on each aide into the 
stomach, and as is the esse in 
the Crustacea much of the di- 
gestion takes place inside these 
glands. In the genera of Brach- 
iopuiia which have a hinged 
shell the intestine ends blindly, 
but in those which have no 
hinge there is an anus which 
may open in the middle line as 
in Crania, or on the right side 
of the body as in Lingula. 

On the dorsal surface of the 
stomach is a small, muscular, 
contractile vesicle, the heart 
(Fig. 156). This givea off a 
number of vessels, amongst 
others one which passes to 
each tentacle, which there- 
fore possibly act as respiratory 
organs. 

The chief part of the nervous 
system retains its primitive re- 
lation to the ectodenn. Just 
in front and just behind the 

mouth there are thickenings of Fro. 157. A iongitudinalvertieal median 
the ectoderm forming a supra- °" '"" 




Bection tiuough Argiope ntapoliU 
1. Veuttal «heU. 3. Catial oooUiainft 



and 3ub-06sophageal ganglion 
respectively. The latter con- 
trary to the usual rule being 
much the larger (Fig. 157). 
They are connected by two 
lateral cords and give off a 
number of nerves, one of which runs to each tentacle. No sense 
organs, such as ears or eyes, are known ^ and indeed the fixed 



Sub-oesopbogekl 
4. Moatb. 
Stalk. 7. neiUB 
of btoud- vessels. 8. Meilian oreit 
on dorsal shell. 9. UembntM 

which ilie separftted from shall during 
the process of liec&lcificatian. 



to Bit still" and sweep little 
luth, lias but little need of special- 



Bnchiopod, whose "strengtli i 
particles of food towards its n 
ized sense-organs. 

The carit; of the coelom is reduced by the presence of the 
alimentary cana], the digestive gland and the heart, but it is still 
spacious. It is partially divided up by certain meaenteries which 
support the alimentary canal, and it is traversed by several pairs of 
musclea Some of these muscles run from valve to valve, and when 
they contract close the shell, others beiag situated behind the hinge, 
so that when they contract the valves slightly open. Others again 
nm from the valves to the inner surface of the stalk and their con- 
traction bends the body one way or another and may even serve to 
slightly rotate it. 

There is— except in one genus — but one pair of nephridia. 
These are short tubes which open by large trumpet-shaped openings 
(9, Fig. 156) into the coelom ; while their external openings are 
situated at the sides of the body behind the lophophore. The cells 
lining the nephridia are some of them ciliated, while others are 
crowded with coloured granules. As already mentioned, the genua 
BhyncKonella possess two pairs of nephridia. 

As a rule, in Brachiopods the sexes are separated. The cells 
destined to form ova or spermatozoa are derived from those lining 
the body cavity. At certain places, usually four in number, the 
coelomic cells multiply and build themselves up into ovaries or testes 
according to the sex. When they are ripe they fall off into the 
coelom and make their way to the exterior through the nephridia. 
The spermatozoa are cast into the water by the male, and the female 
must bring them within the valves of her shell by the action of the 
current set up by the cilia, because the eggs are almost certainly 
fertilized as soon as they leave the nephridia. The eggs develope 
in certain brood-pouches situated at the sides of the animal which 
are formed by a bulging in of the body-wall. A larva is ultimately 
formed which leaves the body of the mother and swims about in 
the sea by means of a band of ciha. As it is extremely minute, 
although it swims quickly it does not get very far, and this probably 
accounts for the fact that Brachiopods are usually found in large 
□umliers in one place. 

Brachiopods are found in all seas. Abont eleven genera have 

been dredged around the British Isles, moat of them 

■Bd ciuiifica- in comparatively shallow water. lAngvla is usually 

'""' found between tide-marks or in shallow watery it lives 



296 BRACHIOPODA. [CHAP. 3L 

in a tube in the sand, and the bristles round the mouth of the shell 
doubtless serve to keep out particles of sand which might otherwise 
injure the animal It is found along the East coast of America, in 
the Pacific and other places. One species of Brachiopod, Ter^- 
bratula teyviUeit has been dredged from a depth of close upon 3000 
£ithoms. 

Perhaps the chief interest of the group is that it includes an 
enormous number of fossil forms which had a very wide distribu- 
tion. The extinct forms far surpass both in variety and number 
the existing forms. Some species have lived on, — as fiax as we can 
judge from the shell, — unchanged from the time when the earliest 
fossil-bearing rocks were laid down. They may thus claim to be 
one of the oldest groups with which we are acquainted. 

The Brachiopoda are classified as follows : — 

Glass I. ECABDDTES. 

Shell with no hinge and no internal skeleton. The alimentary 
canal has an anus. 

Ex. Lingula, Crania. 

Class II. Testicardines. 
Shell with hinge and internal prolongations, chiefly calcareous. 
No anus. 

Ex. TerebrattUa, Argiope, Waidheimia, BhynchoneUct, 



CHAPTER XL 



Phylum Polyzoa. 



Thb group includBs a great number of species, the iodividuals 
of which are bo small as to be barely visible to the naked eye, but 
they are with hardly an exception colonial in their habits and the 
ooloniee nsually attain a fair size. These colonies take many 
shapes, some branching like a tree, others being flattened like a 
leaf, while others again are discoidal ; often they are encrusting, 
that is to say they form a layer on some sea-weed or rock, for the 
m^ority of them are marine. 

If one of these colonies be 
dried so that the organic mat- 
ter shrivels up, a hard skeleton 
remains, and this is then seen 
to consist of a number of 
chambers or " cells," each of 
which opens to the exterior 
by an orifice, and, as a rule, 
communicatos with its neigh- 
bouiB. The skeleton may be 
calcareous, or chitinons, or 
the colony may be gelatinous 
in consistency. The dried cell 
may be open or closed by a 
lid termed the operculum. 
Dniing life each of these cells lodges part of the body of a person 
of the colony, the "cell" being indeed the cuticle; part however of 
the person is not dotbed with cuticle and is normally stretohed 
out above the cell — the opening in the dried cell is in fact the 
place where the flexible part of the peraon begins. At the end of 
this flexible part is the mouth surrounded by a ring of ciliated 




Vio. 168. PortlonB of two P0I710UI 
oolonieB. Magnifl«d. 

A. Smiltia UmdiborovH. a. ATlcnUriQin. 

m. Orlfloe of sell. o. Ooeoinm or 
Donoh in which the <^ deTelopei. 

B. fiUml^ora plumoia, AftarHinokg. 



298 POLTZOA. [ORAK 

tentacles: on one side is the anua. The flexible put is tensed 
the polypide, and the cell the zooecium. If the polj|>ide be 
retracted, which occurs when it 
is irritated, the anterior end is 
inverted and forms the ten- 
tacle sheath in which the 
tentacles lie. The operculum 
when present is a movable fold 
of the body-wall thrown back 
when the polypide is pushed out, 
and covering the opening of the 
tentacle sheath when it is re- 
tracted. 

The animal within the cell 
has a U-shaped alimentary canal, 
the anna being situated not far 
from the mouth, but it is sepa- 
rated from it by a ring of tenta- 
cles in the centre of which the 
mouth lies. This crown of 
tentacles, called the lopho- 
phore, is not always circular, 
jgnt may. be drawn out into a 
horseshoe shaped structure (Fig. 
IfiS), and in the species in which 
it undergoes this modification 
there is a small projection, called 
the epistome, which overhangs 
the mouth, being situated inside 
the tentacle ring. The tentacles 
are ciliated, and the action of the 
cilia brings food to the month, 
which leads into an oesophagus 
also ciliated, and this enlarges 
into a rounded stomach uaaally 
produced into a caecum (Fig. 
1S9). From this a small intestine, 
parallel with the oesophagus, 
leads to the anus. From the 

abor&l side of the stomach a cord of mesodermic tissue, called the 
funicle, usually passes to the body-walL 




Fio. 1S9. Viev of right hilt of Pttt- 

matelia flmgaia, aligliU; diaema- 
matic. After *"'"°" and Nitioha. 
1. Lophophore. 3. Mnuth. 8. Epi- 
Bloms. i. Anna. 6. Nerve gaaglioa. 
6. OesopbagD*. 7. Stomach. 

8. IntcBtiite. 9. Edge of fold of 
body-wall. 10. WaU of taba. 

11. Muscles. 13. Fanicnlni. 

18. Bod;-walL 14. Testil. 

16. Teitia, more mature, IS. Stato- 
bl*Bt. 17. Orat;. 18. Bpermato- 
Eoa free in body-eavity. IS. Ten- 



The body cavity ia regarded aa truly coelomic, It contains a 
fluid in which cella float; it is traversed by the fuoicle and by 
numerous strands of meaodermic tissue. The funicle may be the 
remains of a median mesentery which om^e separated the coelomic 
sacs of the two sides. Sume of the cells of its walla give rise 
to reproductive cells, and the body cavity opens to the exterior 
in cfirtain individuals which possess an ovary by a short tuhulor 
duct, the so-called inter-tentacular organ. This functions as 
an oviduct and has Ijeen regarded by some authors as a modihed 
uephridium. A portion of the Ixxly ctivity is separated fnim the 
rest by a horizontal septum, and forms a space at the base of the 
tentacles- Tliis may open into the other part or may be completely 
abut off from it. 

No heart or blood-veasele are present. It is possible that some 
of the nitrogenous waste matter may be got rid of by means of the 
inter-tentacular organ, but it has ulso been suggested that these 
wa«te products are stored up in certain cells on tlie funicle. From 
time to time the tentacles, alimentary canal and nervous system 
of an individual undergo degeneration and form a brown mass, 
c-alled the brown-hody, which forms a conspicuous feature in the 
colony. After a time the bixly-wall, which has not disintegrated, 
forms a new set of organs and the brown-body may come to lie in 
the stomach of the reconstituted individual. Thence it })asses to 
the exterior through the anus. It ia thought that much of the 
yraale matter which has accumulated in the body of the animal is, 
in this way, eliminated. In certain Phylactolaemata there are ii 
jiair of small nephridia opening betweeu the mouth and the anus. 

A nerve -ganglion lies between the mouth and anus, situated in 
that' part of the body cavity which run.s round the base of the 
lophophore. 

Polyiioan colonies are usually hermaphrodite. The testes are 
as a rule formed by the multiplication of the coelomic cells which 
lie at the aide of the body-wall, while the ovary originates from the 
funicle or from the body-wall. They may be found iu the same 
individual or in different individuals of the same colony. As a rule 
the eggs develope within some part of the parent colony, hut they may 
be laid, escaping from the body cavity through the inter-tentacular 
canal, and then they pass at once into the sea-water. More usually 
the early stages of development are passed through in the t«ntac]i 
iibeath or iu a special pouch called an ooecium (Kig. i^^), or in 
certain " cells " which contain nidinientary individuals. A free 



L 



J 




300 POLTZOA. [chap. 

swimming larval form is usnally found, which after a time comee to 
rest and by budding fonns a new colony. 

Just as in the colonies of Hydrozoa we found different 
individuals set apart to perform different functions, so in Folyioi 
we find a similar specialization. Certain individuals may be 
modified to accommodate and protect the developing egg, bat 
perhaps the most remarkable modifications are the Vibracula and 
Avicularia of the CfaeUostome Polyzoa. The vibracula are long 
i hair-like processes which sweep 

through the water; the avicnlam 
are two snapping jaws provided 
with powerful muscles, like the 
claws of a lobster or the be^ 
of a parrot (Fig. 160). They are 
modifications of a "cell" and its 
operculum. The avicularia oc- 
casionally catch worms, crustacea 
and other animals whose pre- 
sence might interfere with the 
colony, and by their action they 
probably prevent the larvae of 
encrusting animals settling on the 
Polyzoan colony. 

Besides the sexual method of 
reproduction just mentioned cer- 
tain internal buds termed stato- 
blasts are formed in the group Phylactolaemsta. Masses of 
cells arise from the funicle and become enclosed between two 
watchglass shaped chitinous shells whose edges are kept together 
by a special ring of cells. As a rule, the Phylactolaemata die 
down during the winter, but the statoblasta persist and when spring 
recurs give rise to new colonies. A somewhat analogous process 
ensures the perpetuation of the species in certain fresh-water 
Ctenostomata. 

Polyzoa are widely distributed throughout the sea, many occur- 
ring in shallow water, but others have been dredged at great 
depths. The Phylactolaemata and a few genera from other sub- 
divisions are fresh-water. Fossil forms are numerous and the 
Coralline Crag, a tertiary deposit, takes its name &om the luge 
number of coral-like calcareous forms, sometimes described as 
" corallines," which are found in it. 



Fio. 160. An AvionUriDm of fiu^ula. 
MagQiGed. From Hincks. 

b. Beak. c. Chamber reprmenting 
the bodj.oaiity of the modified in- 
dividual, dm. Muscle irhich opens, 
om. muBcle which closea the man- 
dible on the bei^. md. Mandible, 
the opecoulum of the modified oeU. 
p. Stalk. 



XL] POLYZOA. 301 

There is a small class of Polyzoa with a solid body, i.e., no 
coelomic space, and with both ends of the alimentary canal included 
in the ring of tentacles, termed the Entoprocta. In this Class 
the body consists of a stalk and a *'cup." The edges of the latter are 
fringed with a short row of ciliated tentacles surrounding a disc on 
which both the mouth and the anus open. When irritated these 
tentacles are bent inwards and the contraction of a sphincter muscle 
causes the edge of the disc to be drawn over them exactly as happens 
in a Sea-anemone. Sometimes in Pedicellaria the '* cup " falls off 
and a new one is formed. Beneath the disc is situated a nerve- 
ganglion and the genital organs are continuous with a short duct 
which opens in the centre of the disc. The excretory system is 
either one or two blind, ciliated canals opening between the mouth 
and the anus. 

All other Polyzoa are grouped together as Ectoprocta, and 
these are subdivided into 

Order I. Oymnolaemata. 

With a few exceptions marine and having a circular 
lophophore. Devoid of an epistome. 

Suborder (1), Cheilostomata. An operculum covers the 
orifice of the cell Avicularia and vibracula often present. 
Skeleton with more or less calcareous deposit in it. 

Suborder (2), Cyclostomata. Cells tubular and ending in 
circular mouths. No operculum. Calcareous skeleton. 

Suborder (3), Ctenostomata. Body-wall soft The orifice 
of the cell is closed by the coming together of a fringed mem- 
brane. 

Order II. Fhylactolaemata. 

Fresh-water forms with a horseshoe shaped lophophore 
(except Fredericella), an epistome, and statoblasts. 



302 



CHAPTER Xn. 
Phylum Chaetognatha, 

The Ghaetognatha (xaCrrf, hair; yvdSo^, jaw) are small cylindrical 
animals which swim at the surface of the sea- The name is sug- 
gested by the circumstance that at the sides of the mouth are two 
rows of curved movable bristles by means of which they seize their 
prey (e, Fig. 161). 

The body has a small rounded head in front and tapers to a tail 
posteriorly; it is provided with one or two pairs of flat, lateral 
expansions termed fins ; the general shape resembles that of a 
torpedo, if we leave the head out of account. The head is sur- 
rounded by a fold of the skin forming a hood which is most 
prominent at the sides and dorsal surface. Within the hood the 
head bears a row of sickle-like hooks whose points when at rest 
converge around the mouth, but are capable of being widely di- 
varicated. The head also bears one or more rows of stout spines 
whose number and arrangement are of importance for the sjrstem of 
classification (/, Fig. 161). 

The coelom is well developed and contains a fluid in which cells 
float. In strictness there are three pairs of coelomic sacs separated 
bom one another by transverse and longitudinal partitions. In 
the head the coelomic space is practically obliterated by the great 
development from its walls of the muscles which move the hooks. 
The coelom of the trunk and tail is further divided into right and 
left halves by a vertical mesentery, which in the trunk region 
supports the alimentary canal (Fig. 162). This mesentery is pierced 
by numerous small holes. 

The skin is covered by an epithelium more than one layer thick, 
some of the cells of which are modified to form sense-organs, while 
others project from the surface of the body and are known as 
adhesive cells (Fig. 162). Beneath the epithelium is a thin layer 




ia. IGl. A venual iii>n of 
f Sagilu hexapUTii x 3j. 
I Piom O, Hertweg. 
' , MoDtb. 



, Onry. 



d. Ventral 

e. Movable 
[ho head. 

•a tbe head. 
h. Oviduc 



ANATOMY. SOa 

cif jelly called the basemeDt membrane, 
and beneath this a layer of muscleB. 
Anteriorly the muscles are broken up into 
numerous bundles which fill the cavity 
of tbe bead, but in the trunk and tail the 
mosclesform four distinct bundles, bilater- 
ally airauKed, two dorsal and two ventral. 

The nervous system consiats of a 
dorsally placed ganglion in the head 
which gives off two lateral nerves ; these 
pa»9 round tbe alimentary canaJ and end 
in » ventral ganglion situated (Figs. 161 
and 162) near tbe centre of the body. 
The cerebral ganglion gives off nerves to 
the eyes, the olfactory organ, muscles, etc., 
and both it and the ventral ganglion 
are connected with a tangle of nerve- 
fibrils lying at tbe base of the ecto- 
derm. A pair of eyes exist on the 
upper part of tbe head, and behind 
the eyes an organ to which an olfactory 
function has been assigned. This con- 
sists of a ring of modified ciliated epi- 
thelium. Gumps of isolated tactile cells 
with long hairs surrounded by supporting 
cells are scattered over the body and fins 
(Fig, 1G2). 

The alimentary canal is simple and 
straight The mouth — with one excep- 
tion — is ventral and it leads into a 
pharynx which traverses the bead ; this 
passes into an intestine lined by a single 
layer of ciliated epithelium amongst 
which are some glandular cells. The anus 
is situated at the junction of the trunk 
and tbe ttiil and — witn one possible ex- 
ception — is ventral. 

In Spaddla maritmi, the exception 
mentioned in the preceding paragraph, 
there is a glandular structure in the head 
which may be connected with the escre- 



304 CHABTOONATHA. [CBAF. 

tjon of vaste nitrogenons material, but no oHiei flzcretotr organ it 
known and no special respiratory or ciicnlatory organs exist. 

The C^iaetognatha are heriDaphtodite. The paired oTariee 
(Figs. 161 and 162) lie in the bnink F^on of the body caiitj 
supported by a lateral mesentery. When matnie th^ almost fill 
the cavity. The oviduct traverses the ovaiy. It is not known hov 
the ova make their way into it, but spermatozoa are eometiiiiet 
found inside it, so that it acta as a receptaculum seminis. The 
oviducts open to the exterior at the upper surface of the lateral fin, 
just wliere the trunk passes into the tail 




t-^V^^ 



Fio. 163. A. Transverae aection throngh a ^adeila etphaloptera in the reiciOD 
of the Tentr&l ganglion x abont 200. B. TransTerse aeotion throngh a 
Sagitta bipwtctata in tha region of the ovary x about 120. 

a. InteHtine. i. in A. Ventral ganglion. b. ia B. Ovary. e. in A. A 
ciliated aenae-organ. c. in B. Base of the left fin vhioh haa been eat off. 
d. in A. Adbesiia celU. d. in B. Lett Tentral masele. e. in A. Ecto- 
derm, t. in B. Tentral mesentery, /, Dorsal mesentery, g. Bight fin. 

The median mesentery of the trunk region is continued through 
the tail, dividing its cavity iuto two ; and in each of these lateral 
cavities the cells lining the body-wall are heaped up and form a 
testis (j. Fig. 161). The cells divide up into spermatotoa, which 
float in the coelomio fluid and are kept in motion by some of the 
ciliated cells lining the body cavity. The spermatosoa escape 
through a pail of short vasa deferentia which open on the one band 
into the coelom and on the other to the exterior on the tail 
Each has on its course a well-marked vesicula seminalis (Fig. 161). 

The ova are tranqHirent and pelagic The cells destined to form 
the reproductive organs of the adult are early set apart and 
distinguishable. The development is entirely embryonic, no larval 
form being recognizable. 



n.] CLASSIFICATION. 305 

The Chaetognatha consist of three geneia, Sagitta, Spadella 
Qd Krohnia, amongst which some twenty-three species are divided, 
lie genera differ from one another chiefly in the arrangement of 
ie armature on the head and in the disposition of the fins, which are 
lwa3rs horizontally placed and supported by fine skeletal rods. A 
mdal fin exists in addition to one (Spadella and Krohnia) or two 
Sagitta) pairs of lateral fins. 

The Chaetognatha are with hardly an exception pelagic, that 
1 they live near the surface of the sea, and as is usually the case 
ith animals which frequent the surface of the ocean they are 
lansparent. At certain times of the year they are found in 
icredible numbers swimming on the surface by the muscular con- 
"action of their bodies, their fins acting as balancers and having no 
lovement of their own. At other seasons they descend and are 
iken at depths varying from 100 to over 1000 fathoms. The cause 
r their descent is unknown. Their food consists of Infusoria, small 
orae and small Crustacea. 

The zoological position of the Chaetognatha is obscure. They 
low no relationship with any of the larger groups ; possibly their 
earest existing allies are to be found amongst certain aberrant 
Fematoda, such as CluMtosomay but at present too little is known 
> make any close comparison possible. 



& c&M 20 



806 



CHAPTER XnL 

Introduction to the Phylum Vertebrata, Sub-Phyla 
Hemichordata, Cephalochordata and Ubochordata. 

The Vertebrata comprise almost all the larger anhnah, indadiiig 

Man. The name simply means jointed (Latw Tertehra, 

of thePhylom. ^ jouit, and ospociaUy a bone of the spinal column), 

and refers to the possession of a jointed internal 
axis as the main part of the skeleton. In the lowest forms this 
axis is not developed, but in place thereof there is a smooth elastic 
rod, which has received the name of notochord, literally bad^- 
string (6r. vwtov, back ; x^P^» string). In all the members of the 
phylum this notochord is present at some stage of development, 
although in the higher forms it subsequently becomes sanonnded 
and obliterated by the jointed rod or vertebral column. Hence 
the name Chordata, which has been proposed for the group, is really 
more appropriate ; but as the term Vertebrata has been sanctioned 
by long usage it is inadvisable to depart from it 

Besides the possession of the notochord there are two other 
features by which the Vertebrata are distinguished. They all 
possess at some period of their lives slits in the wall of the front 
part of the alimentary canaL These slits in the lower forms allow 
the water which is taken in at the mouth for purposes of respiration 
to escape, and hence they are called gill -slits. Further, the 
nervous system takes on the form of a dorsal strip of sensitive 
skin — the medullary plate, which becomes wholly or partly 
enroUed to form a tube, the neural canal or spinal cord. 

There are in all about 32,000 known spedes of Vertebrata, 
including aU the more familiar animals— fish, frogs, reptOes, Urds 
and mammals ; so that the word animal to the ordinary mind 
generally calls up the idea of a vertebrate. Nevertheless the 
number of spedes is not much more than half that ci the Mollasca 
and is not a tenth that of the described species of Arthropoda. 



CHAP. Xni.] HEMICHOKDATA. 



Snb-Phylmn L HEMICHOKDATA. 

The most primitiTe membeis of the phylum are certain worm- 
like tanna, in which it hu taken speciml research to discover tnc«8 




Fro. 163. A Dolichoglm4ut ko^uUnkii n I. From Speng«l. 
I. ProboMii. 2. CoUu. 3. Tnmk. 4. Mouth. 3. Gill-sLts. 

of Vertebrate Btmetnre. They are marioe and live in mnd, passiDg 
it through their iutestinee and extracting nutriment from the 
ofganic matter it contains : thus they feed and move forwards by 
die same process. These animab are termed Hemichordata (Gr. 
Tfu, half) on acconnt of the short mdimeutar>' notochord which they 
SometimeB they ate called Enteropneosta (6r. wrtv/ui. 



308 HEMICHORDATA. [CHAP. 

breath) because, like all Vertebrata, they use the anterior portion of 
the gut for breathing. There are several genera, DolickoglostuSj 
Chlamydothorax, Glossobalanus and others ; Balanoglossus, although 
used as a generic name, may also be used as a semipopular name 
for any member of the Enteropneusta. 

The body of the animal is divided into three portions : (1) & 
conical anterior part in front of the mouth, the proboscis; (2) a 
swollen cylindrical portion immediately behind the mouth, the 
collar ; and (3) a long trunk, at the end of which is the anus. 

The proboscis contains one, and the collar and trunk each a pair 
of special sections of the coelom or body cavity. The coelomic sacs 
of the proboscis and collar communicate with the exterior by ciliated 
tubes, the proboscis and collar pores (Figs. 164 and 165). The 
cilia produce currents setting inwards : thus the collar and pro- 
boscis are kept swollen up and tense with water and form efficient 
burrowing instruments. If a Balanoglossus be removed from the 
water and laid upon damp sand it is incapable of burrowing and 
wriggles helplessly about. As soon however as it is covered by 
water the proboscis and collar are seen to dilate and become stiff, 
and the proboscis is then inserted into the sand, soon followed by 
the collar, whilst the trunk is dragged passively after them. As the 
walls of both proboscis and collar are highly muscular the water can 
be expelled through the pores and the volume of these r^ons 
of the body diminished, but the action of the cilia soon swells them 
up again. On the hinder wall of the proboscis cavity there is a 
puckered membrane richly supplied with blood-vessels, which is 
called the glomerulus and appears to act as a kidney. When the 
water in the cavity has become impregnated with excretory products 
it is expelled as explained above by a muscular contraction. 

The alimentary canal runs straight from the mouth on the 
anterior surface of the collar region to the posterior end of the 
trunk ; there is neither stomodaeum nor proctodaeum. In most 
species in the anterior part of the trunk the canal has an Q-shape in 
section, being partially constricted into two tubes, an upper or 
branchial into which the gill-slits open, and a lower or oesophageal 
along which the mud is passed which the animal has swallowed for 
food. The notochord is a hollow tube of cells surrounded by a 
tough membrane much thickened beneath (Fig. 164). This tube 
opens into the alimentary canal in the collar region and projects 
forward into the proboscis as a support for this organ, which is 
attached by a very narrow neck to the collar. The whole skin is 



NKBTOUB BTSTKM ASD OILL-SLITB. 

ensitive, since there is everywhere a layer of nerve-fibriJB under- 
fing the ectoderm cells, Uit this fibrillar layer is especially thickened 
long the mid-dorsaJ and inid-ventral hnea of the trunk, these two 
egions being connected by a ring of nervous tissue immediately 
lehind the collar, The dorsal thickening alone is continued into 
he collar region, and here it becomes rolled up so as to constitute 
I short ueural tube (Kig, 164) which becomes detached from the 




a. lU. LongitndiiiRi Tsctical nectioo through the middle Une of GU>»inhaliiniu. 

Di&icraiilmiitic. 
Probowiis. 3. Collar. 3. Traak. i. Prolianeis cavit;. 6. GlomeMituB. 
8, Periowrdinin. 7. Heart. 8. I'toboMia pore, 9. Collar cavil;. 
10. Moath. 11. Notoohord. 12. Donal blood- vessd. 13. Oeao- 
pho^eal portion of >limeiitar;r caiml. 14. Branohinl legioD of alimentary 
canal. ia, VeDtral blood- veBsel. 16. Giil-sUts shoving external and 
intornal opeainsB; Hie oatlioeB of tho eitsrnol opeoinKa are dotted. 
17. CcDtrnl nervoas system. 18. Dorsal roole ol nervoas Bjstem. 

19. Veotrol pocket of proboscis cuvit;. 

Ktoderm and aasnmes a deeper position, it may retain however n 
lonnection with the ectoderm through several strings of cells with a 
Bbrillar sheath, known aa dorsal roota (18, Fig. 164). 

There are very numerous gill-slits opening int4) the alimentaiy 
anal, in the front part of the trunk region ; they onght rather to be 
lalled pouches with a small outer and a large inner opening, The 
inner opening of each pouch is divided almost into two by a tongue 
irojecting down from its dorsal edge, the so-called tongue-bar. 
'bill tongue-bar is specially richly supplied with blood-vessels and 
may be regarded as the principal respiratory organ. The blood- 
Vessels are destitute in most cases of any proper wall : they are 
B it were mere crevices between tlie epithelial walla of the gut, 
Doelom and skin. There is however a well-detined contractile dorsal 
Bbaunel ninning forward into the kidney, the contractility being 
confined to the front end in the proboscis where there ia a closed 
c with muscular walls, which pulsate rhythmically, situated above 
■he blood-vessel. The mc is termed the pericardium, and the 



I 




Fin. ItiS. LoDRitudiiiBl hori' 
ZDDtftl aectiou through Glui- 
tabaUtnut. Diagram matic 

1. ProboBcia. 2. Collar. 3. Trunk. 
4. Proboscis cavity. 5. Olo- 
merulue. 6. Pericuidium. 
7. Heart. 8. Probosois pore. 
9. Collar cavity. 10. Peri- 
baemal cavit;. II. Collar 
pora. 12. Dorsal blood- 
TeBBel. 13. Alimentary canaL 
14. Branchial sac wiUi exter- 
nal openiDg. IS. Beproduc. 



CBPHALOCHOBDATA. [CBAP. 

dilated part of the blood-veBsel beknr 
it, the heart. This dorsal vecael om- 
nituucat«s with a ventr^ vesael in the 
tmnk region by two dewending corred 
veesels at the aides o£ the collar. Baeh 
of the coelomic cavitiea of the tnak 
seuda forwards into the collar region a 
narrow toogue lying at the side of the 
blood-vessel. These tubes from theii 
relation to the vessel are called peii- 
II haemal tubes (Gt. rtpi, around; aljia, 

blood). 

The sexual organs or gonads ue 
mere packets of cells in the gill region 
and behind it, developed from the wall 
of the trunk coelom (Fig. 16S). Each 
when ripe forms its own opening through 
the body-wall. 

One point of interest attaching 
to the Hemicbordata is that they may 
commence life aa free-swimming larvae, 
resembling the larvae of the Echino- 
dermata, and suggesting the thonght 
that perhaps two such different groups 
as the Vertebrata and Bchinodermata 
may have descended by different paths 
from the same simple free-swimming 
ancestors. 



Sub-Phylum II. CEPHALOCHORDATA. 

Leaving the Hemicbordata we next come to some small fish-like 
aninuds, the Gephalocbordata, which were formerly all included 
under the name Amphioitus, and indeed there is no very strong 
reason for breaking up this old genus. The name Amphioxw (<V^ 
at both ends ; it^, sharp) refers to the shape of the body, which 
is long, flattened and pointed ^t both ends. It is remarkable that 
we here meet for the first time with a shape very common among 
Vertebrates, but almost absolutely unknown elsewhere in the animal 
kingdom, viz. a laterally compressed form with narrow ventivl and 



NOTOCBORD. 



I dors&l regions and deep sidee. It is common to find animala with 
I broad backs and belliea and narrow sides, but only Vertebrates show 



a. ISfl. AmphioxvM lanceolntua from tha lett siile, about twice nKtOTnl sixe. 
After Luikesler. The gooadio pouohes are seen by trangpaiBooy through 
the bodf-will ; the aCriam h eipanded so that ita QooF prajeots below ^e 
iDetapleural fold ; the GD-rays of the veotral Bn aie indioated between the 
atrial pore and aliaB. The dock spot at the base of the Qfty-soooud 
mjotome represents the anua. 



the reverse condition. In con- 
sequence of this peculiar 
shape Amphioiria falls on its 
side when it ceases moving. 
It burrows in the sand, lying 
with its mouth just protrud- 
ing, and as its tips are fringed 
with ciliated rods (Fig. 167) 
a current is produced which 
brings new water to the gills 
and with it small swimming 
organisms which serve as food, 
At night Amphioxus often 
leaves its burrow and e 
about, returning instantly to 
the sand if alarmed. It can 
burrow with either the head 
or tail 

The notochord is a 
smooth cylindrical rod lying 
above the gut and running 
from end to end of the animal. 
It consists of cells, much of 
whose body is changed into 
a gelatinous substance, and 
which are surrounded by an 
exceedingly firm membrane 
termed the chordal sheath. 
In the embryo the notochord 




Via. 167. 
Telmn of Amphiotiu aeon from the 
iniide of the pharyni. Alter luiQ- 

[J. hphinoter mnBOle of Tolum. 

. leiar lentnclva Ijiug octane the ora! 
Dpenitig. 

Oral irartilageB of AmpMoxiu, After 
J. Mailer, The btual piecea lie end 
lo end in thv oiariiiii of the oral 
bood, and each baaal piece wnds up 
■D axial procoae into the correapuod- 
ing oral cirrus. 




312 



CEPHALOCHORDATA. 



[CHAP. 



first appears as a groove in the dorsal wall of the gat, so that we 
may say that the notochord of the Hemichordata retains a form 
which is passed through in development by that of Amphioxw. 
In the very young embryo also an indication is seen of the 
division of the body into the same three r^ons as we found in the 
Hemichordata. Just as in the embryo of Balanogloesus so here, the 
embryonic gut gives rise to five outgrowths firom which the coelom of 




Fio. 168. Diagrammatic longitudinal section of an embryo of Amphiaxut. 

1. Neuropore — anterior opening of the nenral canaL 2. Neural canal. 

3. Neurenterio canal. 4. Coelomio groove. 5. Somite divided off 
from coelomio groove. 6. Collar-cavity. 7. Head-cavity. 8. Ali- 
mentary canal. 

the adult is derived. These outgrowths are (1) a median anterior 
unpaired pouch, the so-called head-cavity corresponding to the 
proboscis-cavity of Balanoglossus ; (2) an anterior pair of pouches, 
the collar-cavities, corresponding to the similarly named sections 
of the coelom of Balanoglossus ; and finally (3) a pair of groove-like 
extensions of the dorso-lateral angles of the gut-cavity, called the 
coelomic grooves, developed only at the hinder end of the gut 
From the last-named the coelom of most of the body arises, and 
they correspond to the trunk-coelom of Balanoglossus (Fig. 168). 
The proboscis or prae-oral region is however very small and bent 
down ventrally ; its cavity becomes more or less obliterated in the 
adult. Dorsally the collar region is narrow from before backwards, 
but it extends obliquely downwards and backwards, causing a 
slight ridge to appear at the side of the body. This ridge grows 
out on each side into a flap, which meets its fellow beneath the 
ventral wall of the body and thus they enclose a space, the so- 
called atrial cavity (Figs. 170, 171). This still commuidcates 
with the exterior through the atrial pore. The gill-slits, 
which occur in the front part of the trunk region, open into 



COELOU. 313 

bia atrial cavity. The atrial flaps, enclosing the atrial cavity, are 
Ka obvious arrangement by which the slits are saved from being 
looked with the sand in which the creature lives. lu the Hemichor- 
data the hinder end of the collar region may extend over one or two 
gill'Slita ; the arraDgement in Amphioxiin may he regarded aa a 
farther development of this state of aflairB. As in the case of 
Hemichordata, the slits of Amplmxus are divided into two, by a 
tongue-bar reaching from the upper margin almost to the bottom 
of the slit Each slit thus becomes U-shaped (Figa. 169, 172). 




M. Atrial cavity, ei. Oral cirri. eft, Notoohord. d.f. Dorsal fin-chambers. 
(. Eye-spat. end, Eadoatyle. hfp. Oiiterowing liver i tlie index 

tine passes throa^ih one of J. Miiller's "renal papillae." met. Metaplenral 
(old. nph. Nephridla. nt. Spinal cord. otf. Olfactory pit. 

ph.b. FBriphaiyngeal ciliated band. tb. Tongae-bais. vei. Telnrn. 



The upper and anterior [lortion of the collar-cavity becomes 
separated from the rest: its inner walls thicken and develops into a 
powerful longitudinal muscle which forms the hist myotome 
(Or. /ivt, motise, muscle ; ro/iot, a division). 

The trunk coelomic cavity breaks up from the beginning into a 
iea of pouches called somites, each of which subsequently divides 
into au upper and an under part. The inner wails of the upper 
parts undergo a similar change to that experienced by the cor- 
rraponding port of the coUar-cavity forming a series of myotomes. 
The name myotome is given to each of the inelnmericnliy arranged 
bundles of muscle-fibres. Each myotome is separated from the ne^t 
liy a connective- tissue partitiott. In Ampkioxus the myotomes of 
the right side alternate with those of the left, so that the centre of 
ft myotome on one side is opposite the connective-tissue partition 
.on the other. Each is V-shaped, and they are arranged so <^g . 
Hence in a transverse section several myotomes are seen on each 




ClCrHA LOCHOKD ATA, 



ade of the lic>dy Ihaa we lave two great Benes of Inngitadinol 
muscles broken up into my trmes une on each side of the animal, 
by the altercate c ntra tion of which powerful side-strokes of the 




lavereo apction tlirongL pliarjiitjeal regioD 
feiaalo Amphioxiu, Alter Lankeatec and BoTeri, from R. Hertwig. 
. Atriid oavity. c. Doraol oo^lom, separated (Tom atrial cavity bj the doabU- 
layeccd meoibraue ICBDitD as the ligamenttivt dtiiUcul-ilum. eh. Nolo- 
ohord. d.n. Senarirf nerve. e, Eodoelyle, below whioh is tha 

eodostylai oaelnm ooalaining the veiitral aorta. /. Fin-ra; of dorsal 
fin. g. Gonadic pouoh ooatniniiig ova. H.v. Beiiatie vein lying in 

the narrow coelomio epace which Biicrouutis I, the liver or ItepaliB eoeoum. 
l.a. Iisft aorU separated Irom the right norta by the hyperpbarrne^ 
(epibranohial) gtixtve. tu, Lympb-Bpao?. tap, Metapleural fold, 

my. LoDeitudiQfll mnBoles of myotomes ; over against the dorsal coelom 
theae mOHoles are atraoged Tertically, and form the rectus abdominis of 
Schaeider. n l. Spmal cord. p. Pharyni. r. Nephridium. 

t,ffl. Transverse or Buhalrial mascles. c.ii. Motor Fpinal oerre. the 

fibres of which have the appearutoe of passing directly into the maBola- 
flbtM. N.B. The coaiiecuvo tiseue (cutis, notochordal aheath, et«.] and 
the coclomic epithelium are indlcaled by the black lines. 



fiat boJj propel the aniina! lorwurds. The elaatdcity of the notochord 
acts like a fly-wheel in storing 
■J/' the force during the latter 

part of each stroke and re- 
inforcing each striike at ita 
comiaencement. The cavity 
of the upper division of the 
somite persists throughout 
life, ajid is known as the 
myocoel (15. Fig. 179), the 
fold separating it from the 
cavity of the lower division 
being termed the iutercoe- 
lic membrane {i.m. Fig3. 
171 and 177). 

The lower portions of the 
somites fiise with one another 
and form a continuous body 
cavity round the hinder part 
of the alimentary canal i&.c. 
Fig. 171). In front, owing to 
the presence of gill-slita, there 
are formed a right and a left 
dorsal eoelomio canal, and a 
ventral coelomic tube, or eu- 
dostylar coelom, the dorsal 
and ventral portions com- 
municating with one another 
by spaces iu the gill-bara, that 
is, in the pieces of body-wall 
intervening between the gill- 
si ita. These spaces are termed 
branchial coelomic canals. 
Although at first formed from 
backward prolongations of the collar region, the atrial fliips soon 
become invaded by the lower ends of the trunk myotomes ; the 
ventral muscle however running across the under surface of the 
atrial cavity (Fig. 170), which by contracting diminishes its size and 
thus espels water, originates from the walls of the lower portions of 
tbe collar cavity. 

Tha mouth is originalty some distance behind the anterior end, 




Fio. 171. TraQ8ver6e Bcctlon tbrougli 
poBt. pharyngeal legioo of jonng AmyM- 
ozut, to show groapB of remU ct^lla id 
floor of Htriuui. After Laakester and 
WUUy. 

ad. Aorta. dI. Atrinl eavit;. K.C. Body- 
eatitf (ooelom). c.e. Central oannl of 
Qerve-ootd (n.c). ch. Notoehonl. 

d./.c. Fio-oavitv, i.m. Interooelio 

tDembrsoe. inC. Intesline. (.m. and 
r.n. Left and riflht metapleural foldj. 
r.p. One of J. Mil Iter's "renal pspiltae." 
#.f.t>. Sub-inleBtinal Teiii. 



31ti CKFEULOCHORDATA. [CEAP. 

tuiti ou the left side, so that there is a prae-onl portioD of the hoiy 
which in the emtxyo is occupied by an aoterior division of die 




^r 



t'lu. 173. AnWrior portion ot body of jrouog 
tiiiiiiipuniut Atnpht'oxiu. After J. Milller, Blight- 
ly ullerod. 

ch. Notuchord. ct. Oral rim. t. Eye-tpat. 
end. BuduBtyle. /.r. Fin-rays. gj, Gill- 
ulits ; th« skeletal roda ot the gill-baFB are 
iudicated by black linea. n(. Spinal cord, with 
piguiout granalee Dear its base. r.a. Down- 

t^ivwth from right anrta lying to the right 
ot vtl. the velum, with Telar teotocleB pro- 
jecting back into pharynx. ir,o. Ciliat«d 
epithelial tracts on inner surface of oral 
hood. 



■4 



^. 



■H 



1^ 



coelotn corresponding to the proboBcie 
cavity of the Hemichordata. Subee- 
quently however the atrial flaps extend Fw. 173. Anterior portion ot 
.,,... . . J ,1 , spinaloordot .ImfcKtJw; 

right to the antenor end, so that a new seen from abors. After 
terminal mouth is formed leading into a Schneider, 
chamber which is clothed by ectoderm Between^ '^^^^ ^^t^* 
and which is therefore to be regarded as 
the stomodaeum. The opening of the 
stomodaeum now fonns the apparent 
mouth, and the lips of this secondary 
mouth grow out into rods supported by 
gelatinous material and covered with cilia, 
the so-called oral cirri, the function 
of which has been already explained (Fig. 167). The walls of 
the stomodaeum are known collectively as the oral hood. The 
position of the primary month is stiU marked by a projecting lip, 
the velum, which is produced into ten or twelve delicate tentacles. 
These form a filter to prevent coarse material from reaching the 
alimentary canal. 



eye-Hpot; one of the 
branches of the second 
pair of cranial nerres 
sometimeH arises directly 
from the Hpinal oord as 
shown on the right ; tu- 
ther back are seen the pig- 
ment spots of the nene- 



NEKVOCa STSTKM. 

The neiToiis system is a simple tube with thick Trails and very 
oaTTOw cavity. It is almost as extended as the notochord, and lies 
above it. It does not however quite reach the front end of the 
body. Its extreme front tip is called the cerebral vesicle ; it ha^ a 
wide cavity with thin nails, ao that the total diameter is not 
iacieued. There is a pit reaching down to it from the external 




Pdj. 174. Hediui TsrticBl seotion throogh the nersbial vesicle of Amphiox'it. 
After Eupffer. 
I. Gttit; of oerebral reaiole, «. Eje-ipot. g.c. Dorenl groap of gasRlion- 
cdla. in/' lolundibaliuii. l.o. Olfaotory lobe. Ip. ITnberculum 

bIcid, pog.sibly anidimeutary olfactory organ (Fig. 174), and in tlie 
wall of the vesicle itself is a maaa of pigmented cells forming an 
eye-apot. In the young larva! Amphio-rus this part of the nerve- 
tube remains for a considoralile time as an uncovered medullary 
plate, and one is inchned to imagioe that it corresponds to the 
Benaitive nervous surface of the proboscis in the Hemichordata, 
i in the larvae of these animals there is a sensitive plate with 
two eye-spots at the apex of the prae-oral lobe, In the wall of the 
nerve-tube are to he found two kinds of nerve-cells, via., (a) ordinary 
Bmall nerve-colls, the processes of which soon pass outwards into 
the peripheral nerves, and (6) very large nerve-cells, the processes of 
'hich extend almost thronghout the entire length of the nerve-tube. 
The processes of the Utter kind of cell are called "giant fibres' 
(g./* and g./^, Fig. 175): they appear to have to do with 



J 



318 CEPHALOCHOBDATA. [CHAP. 

cooidiDating the huisciiIat movements of the khjihhL Bemdee uie 
nerve-cella, aa in all nervous systems, there are a certain number of 
anpportmg cells (a/. Fig. 175). In the embryo of Ampiioana Hie 
whole wall of the nerve-tube consists of a single layer of cells, all of 
which abut on the cavity of the tube ; loany of theee cella becone 




JTio. ITS. TroaBveTM section through the ipinal cord of Amphioxui in tbe 

middle region oF the body. After Bohde. 
a. Ciiant fibre. e.e. Central canal. g./K Oiant nerre-fibree, which taaTeiH 

tbe Bpinal cord from before beckwerdB. g.f*. Oiant nerre-fibre*. which 

traverse tbe spinal cord from behind forverde. ia.p, Moeole-platea, ue. 

terminations of the nerva-flbras on the muscles. m-r. Motor nerre-flbies. 

n.f. Longitudinal nerve-fibres cut across, t./. Supporting ecUfc 

ih, Sheutli of Di:tvB-cord. 

afterwards transformed into small round nervs-cells, and recede from 
the cavity, ansnroing a more peripheral position : but others retain 
their connection with the cavity and become drawn out into fibre- 
like supporting cells. From the nerve-cord aie given off two kinds 
of nerves, but not at the same level, ao that in a transverse section 
one kind only is seen. These are: — (I) sensory nerves, going 
directly to the skin nnd hnving a dorsal origin ; (2) motor nerves, 
going to the myotomes. The nervous tube and the alimentary 
caual at first both reach to the extreme posterior end of the body 
and here are connected by a vertical tube, the neurenteric canal 
On tbe course of tliis tube the anus is formed. As development 
proceeds the anus slowly shifts forwards and the neurenteric canal 



m 

wmes a aolM atring of cells and disappears. Thus is initiated the 
fitrmatioa of a tail, by vhich terni is denoted a portion of the body 
Rdevoted entirely to locomotion and freed from all part of the giit, 

■ being filled only with muscles. The tail of Amphloxus ncqu'iTea only 

■ ft very limited development, but it soon becomes fiurrounded by a 
1 tail-Go, at lirst merely made up of the enlarged skin cells, but soon 
I becoming a 0ap contaiuiug gelatinous material. A similar fold along 

the middle line of the back 
forms the dorsal fin, in 
which, iu the larva, there 
are a series of metameric- 
ally arranf,'ed cavities lined 
hy distinct epithelium, pro- 
bably derivatives of the 
myocoelic cavities (rf./c. 
Fig. 171). There are also 
low fin folds projecting from 
I t ■'5' -1 ^■7^.^-_t ^^& thesideaoftheatrialcavity 

L ''^ ■H''-A '^' ^^iSB^^^L "'^'^ constituting the lateral 

P'-'-:^v\!'" 'jif^^HH^^ \ '^^ metapleural fins (Fig. 
'' ^^B^B^^fc 170), and a median ventral 

fold between the atrial pore 
and anus, called the ven- 
tral fin (Fig. 166). The 
dors.il and ventral fins are 
stilleued by a number of 
l^'elatinous rods mora num- 
erous than the myotomes 
of the corresponding re- 
gions of the body. 

The alimentary canal of 
Ampkioxus is a perfectly 
straight tube consisting of 
stonodaeum or mouth 
gut, pharynx or braii- 
lehial gut, and intestine or digestive gut The pharynx has 
faking both dorsal and ventral middle lines grooves lined with cilia 
luected with each other by a circular groove just inside the velum 
Por trne mouth. The ventral groove is called the endostyle or 
rkypopharyngeal groove, the upper groove is termed the hy- 

■ perpharyngeal, and the connecting groove the peripharyngeal 
^aod (Figs. 169,170, 172). The function of these grooves is cuhouB. 




I ?ia. 176. AmphioxuM. Nephiidium of the 
left lids, with Ibe Detnhboariiitt portion of 
tha phuyugeBl vail, at Been in the iiviDg 
ooDdition. The round bodies in the aiiil 
of the tubule r^ieseut carmine gtaouliis. 
Highlj' magnilied. AFMr Boveii. 



320 CEPHALOCHOfiDATA. [CHAP. 

The endoBtyle produces ft cord of mucua which is worked fonraida 
hj its ciUa and pressed up the sides of the periph&ryugeal btnds. 
Here it is caught b; the icrushing cuireut of water produced hj the 
cilia of the oral cini and swept back along the hyperphaiyngeal 
groove to the opening of the intestine, entangling in its passage tlw 
small plants and animals carried by the water ; the latter of oonise 
escapes into the atrial cavity through the hundred or so long narrow 
gill-slits. The intestine is prolonged forward on the right side of the 
pharynx into a blind pouch, the so-called liver {I, Pig. 178), which 
probably secretes a digestive juice. 



Fio. 177. Portion at transverBe section through the phsryni ot Amphioxtu, to 
show position of excretory tnbule. After Weiss. 

ao Lett aorta. at. Atrial caTity. at.t. Atrial epithelium. c. Coelom, 
eh. Notochord. i.m. Id tereoclic membrane. l.d. Docsal wall of atrial 
CBTity nph. Nephriiiium. p.b. Gill -bar. ph.t. Bpithelinm 

of hyperpharyngeal Rroove. ph.f. Fold attached to giU-bar conUming 

branchial coelomic canal, s.fft. Sheath of notochord. (.6. Tongue-hw. 

The exeeretory organs of Amphiorus are small and have only 
recently been discovered. We have seen that in the region of the 
pharynx the coelom has become reduced to a narrow canal, 
beneath the pharjnx, and to two dorsal canals at the sides of tiie 
notochord (Figs. 170, 177). These latter canals have been described 
as having at the level of each tongue-bar a wide funnel leading into 
a short tube connecting them with the atrial cavity. The edges of 
the funnel are supported by strings crossing the coelomic canal and 
inserted in its wall. Goodrich has recently re-examined these organs 



HI.} 



EXCRETORr AKD VASCULAR SYSTEMS. 



321 



Und according to his account they do not open into tlie coelom but 
iid intera&Uy in branches beset with aoleuocytes, or cells provided 

I the choanocytea of Spongei^ with a 
Dollar inside which a flagelhiin flickers. 
ITbe same author lias desiMibed -similar 
Btmctuies in the Polycbaeta, and holds 
them to be radically distinct from the 
wide-moutlied, simple nephridia of the 
Uollusca and of some Ohaetopoda which 
e terms coelonioducts. These tubes 
re the nephridia, and taken collective- 
r constitute the kidney. It has been 
proved that carmine injected into the 
Boimal is excreted by them. Besides these 
■ number of thickened patches of the atrial 
epithelium, discovered by Johannes Miiller 
wad called by him renal papillae, are 
thought to assist in excretion (Pigs. 169 
md 171). 

The blood system is exceedingly sim- 
ile. The blood from the elementary canal 
B brought back by a 8ub-int«stinal vein, 
which like a broad river is often subdivided 
into two or three parallel channels which 
then reunite with one another ; in a word 
it is more a plexus than a tube. It 
runs to the tip of the liver on its outer 
ride, returns on its inner side, and pursues 
3 then as a single channel under 
the pharynx, where it is called the V e n t ral 
<torta. In this region it is contractile, 
dmiviog its muscles from the walla of the 
Tsntial coelomtc tube. Vertical branchial 
TeaMls called arterial arches are given 
flS; tliese ascend in the gill nepta, that 
u, the portions of the wall of the pharynx 
intervening between the gill-alits. Arriv- 
: at the dorsal line of the pharsmx 
a vessels empty into two longitudinal 
8, the dorsal aortae, which further 
: unite into one (Figs. 170 and 171). The tongue-bars abn 




Fio. ITS. Amph'oxw lUt- 
Bscted from the ventral 
side X 3. After ItAthke, 
sligbtl; Altered. 

an. AnuB. at. PositioD of 
atrial pore; thoexteDBiOQ 
oF the atriam behind this 
point is indicated by the 
dotted line puBing over 
to the light lid* of i, the 
intestine. '. £ndOBtyle. 
;;. Gonadic poaches. 1. 
Liver, m. EntraQ(>a to 
mouth with the oral cirri 
lying over it. p. Pharynx. 



tx 



3SS CEPHALOCHOBDATA. [CHAF. 

coDtain Teasels emp^ring into the dorsal aortae ; then commomcate 
with the branchial vessels tbrougti what are called synaptionlae, 
that is, cross pieces tying the tongue-bar to both sides of the gill- 
slit which it divides (Fig. 172). 

Both gill-bar and tongue-bar are strengthened with rods of 
gelatinous tissue. These are the precursors of the visceral 
arches, which form such an important part of the skeUton in the 




1. Nerre-cord. 2, Notochord. 3. Myotome. 4. HoUaur wderotome. 
S. OoQ&dio ponch. 6. Dorsal coelomic oanaL T. Nephridiam. 

8. Branchial costomio canal in gill-bar. 9. Bndoa^lu ooelom. 

10. PhaijDi. II. Hyperphuyngeal grooie. 13. Endoatjle. 

13. Dorsal aorta. 14. Atrial cavity. 15. Uyoooel. 

higher Vert«brata. Similar gelatinous tissue forms the lays of 
the dorsal and ventral fins, the sheaths of the notochord and 
nerve-cord and the dermis. It differs from ordinary connective- 
tisfiue in that although it consiBts of a ground substance with a 
deposit of fibres in it, it contains no amoebocytee or " connective- 
tisBue corpuscles." The fibres in ordinazy connective-tissue are 



lai^Iy if not entirely produced by the metabolic activity of the 
Wnoebocj'tes ; but in Amp/'i'irus it appears that tbey are produced 
by the cells of the coelom, ectoderm and endoderm which adjoin 
the connective-tiasue. Thus the sheath of the notochord is deposited 
partly by the cells of the notochord, but chiefly by a hollow out- 
growth from the myotome called the sclerotome (4, Pig. ITS), the 
fin-ray by a coolomic sac (dj'.c. Fig. 171) which disappears in the 
adult, the rods of the gill- and tougue-bars largely by the epithelium 
of the pharynx. The connective-tissue of the Cephalochordata is 
therefore in a peculiarly interesting primitive condition ; and that 
of the Hemichordata has the same structure. 

The reprodnctive organs ore very simple in construction. The 
es are separate, and ovaries an<I testes closely resemble each 
other in external appearance (Fig. 178). They take the form of 
squarish masses, called guuadic pouches, embedded iu the outer 
walla of the atrial cavity. Wlieu ripe they burst into the atrial 
cavity, the eggs escape through the mouth or atrial pore, the 
spermatozoa through the atrial pore. The fertilized egg developes 
ito a free-swimming larva of a remarkable form. There are 
Lo atrial folds covering the gills, but one set of slits is developed 
long before the other, and the mouth appears on the left side. It 
has been proved that the sexual organs are outgrowths from the 
lower ends of the myotonies, and remain throughout hfe connected 
with these hy strings of ceils (Fig. 179). 

The Hemichordata and C-ephalochordata are found all over the 
tropical and temperate regions of the world wherever a suitable 
aubstratum is found. The Hemichordata burrow in mud rich in 
decaying matter, but the Cephalochordata prefer clean sand, their 
fooiJ as we have seen consisting of swimming organisms, 

Sub-Phylum HI. TUNICATA on I'ROCHORDATA. 

By many the group of Tuuicata or Urochordata would be 
considered the lowest portion of the phylum Vcrtebrata, and if we 
had regard only to the adult structure this could not well be denied, 
for in the adult hardly a trace of the Vertebrate relationship is 
discernible. But the Tunicate commences life as a larva showing a 

redeveloped structure in several important points than .4 mpA/oj-«s 
jposseaHes at any period of its life-history, and hence we must regard 
" 9 simple organization of the adult as a degraded rather than a 
primitive condition of affairs. 

1\— 1 



DROCHOBDATA. 



[chap. 




ID. ISO. Side view of the {interior end of a larvft of Aicidia which has been 
frte-BwimiuiDg for two dajs k 8TS. Fio. IBl. Dorsal Tiew of the Mine. 
After Eowaleweky. 

. Moath. 3. The connection o( the bmin with the itomodaenm. 

3. EndoB^le. 4. Intestine. 6. Branchial oavit^. 6. let gill-slit 
7. 3nd gill-Blit. 8. Atrial opening. 9. Blood oorpnaoUs, 10. CaTit; 
of biun. 11. Dorial nerre-tnbe. 12. Kotooboid. IS. Hnscles. 
14. Filing organs. IG. Otocfst. 16. Eye, 



till.] LARVA. 325 



f 

H The typical Tuniijate larva is often called the Ascidian tadpole 
I . becauBe ita form recalls that of the well-known larva 

^P of the frog. It attains a leDf^h of about a quarter of 

Bu inch, and consists of a small round trunk and a tUu vertical 
F tail four or five times au long aa the trunk. The tail is the organ 
of locomotion, and is provided with a sheet of muscles on either side 
by the alt4?mat6 contraction of which powerful aide-strokes are 
Metuted and the animal is propelled forwani (13, Fig. 181). The 
tftil is stitfeued by a well -developed notochord— which does not 
:8xteQd into the trunk, hence the name " Urochonlata " (Ur. oipii, 
.tail ; x°P^' " string). A uniform flap of skin, a continuous 
fin, forma a liorder to the tail. The trunk contains the enlarged 
ryus which opens hy a narrow mouth in front : and laterally 
communicates with the exterior by two ciliated openings — the gill- 
riits. Its ventral wall is swollen out into a pocket which causes the 
vnder lip to protrude as a bulky chin. In this pocket we find 
developed a ciliated groove, theendostyle. having the same position 
s the organ similarly named in Amp&ioa^us. On the chin outside 
le three peculiar warts which secrete a sticky slime and which are 
used by the larva to fix itself to surrounding objects. The pharynx 
Wds l>ehind into a short iutestijte which is attached to the ectoderm 
liigh up on the side far in ailvance of the root of the tail. Hence 
'■ the priwcas of shifting forward the anus and the corresponding 
development of a purely muscular tail have been carried much 
further in the Ascidian tadpole thau in AmpAioiPus. 

The nervons syKtem in the tail is a simple neural tube ; but in 
the trunk it expands into a thin walled vesicle, the so-called sense- 
vesicle, which is the rei>reseutative of the cerebral vesicle of Ampki- 
ojvt and the forerunner of the brain of the higher Vertebrata. 
As in the larva of A mphtarun, the sense-vesicle opens to the exterior, 
but the spat where this occurs is involved in the invagination which 
forma the stomodaeum. The tube connecting the sense-vesicle and 
the stomodaeum is called the neuropore (2, Figs. I8U and 181). 
Part of the side-wall of this vesicle is modified so as to form a cup- 
shaped eye with a .-iimplu cuticular lens directed inwards. From the 
roof liangs a ball of lime suspended by a pillar of celts ; this acts aa 
an otiiiith, and the whole forms a rudimentary ear. 

Tlius both in the structure of tlie nervous system and the 
position of the anus, the Ascidian tadpole is more advanced than 
the Ampkiiixiii. 

Although, as we have seen, both mouth and anus are present, 



yet they c&niiut I 
gel&tinonM matter, 
ectodenn c«lls and 



UBOGHOSDATA. 

nfiod, tor tltey are closed by a sheet of 

This is the test which is secreted by the 

invelopes the whole body, bo that during iu 

brief free-swimniing life the 

Asciilian takes no food. 

After swimuiing for a 
nhort time tlie larv-a fi^es 

itself by ttt 
phalli"'"' chin-warla to a 

suitable sub- 
stratiun and undergoes a 
very rapid metamorphosiB. 
Tlje tail Hlirinks and is ab- 
sorbed , notticboni and nerre- 
tulje disappear : the srose- 
veaicle also disappears, only 
its hinder thickened wall 
persisting as the adult gan- 
glion (6, Pig. 182). 'ITie 
neuropore however peisista 
and developes into a maaa 
of tubes underlyiog the 
ganglion, which is called 
the sub-neural gland. It« 
opeiung acquirer a i-reseent 
form with thickened lips, 
and is called the dorsal 
tubercle. Meanwhile the 
i-liin grows enormously, so 
as to rotate the mouth up 
and away from the sab- 
stratum, and thus the long 

axis of the pharynx becomes 
Fro. 192. DiaETain of the liximi luid obuieea .■ i - . j r i. - _ 

undergone by a Wval J,ri<K«n. Prom vertical instead of horuMD- 
Lanke»tef- tal. The skin of the region 

1. HoDtb. 2. Anns. s, oill-Blita. nhere the anus becomes 
i. In A, Dotochord: in B ftnd 0. vnniBhmi! -. . i ■ ji i 

tail. 6. In A, taU. 6. Brttia. Situated 18 depressed so as 

to form a groove. This be- 
comes confluent with the outer parts of the two gill-slits, so ivs to 
form a single dorsal cavity termed the atrial-cavity, the opening of 
which is not far from the mouth. It must be noticed that this 




STRUCTURE. 

cavity does not correspond to the simi- 
larly named cavity In AmpMoj^is: in 
the case of the lost-uauied animal the 
atrial walls originate &om the dorsal 
edges of the gill-slits and meet one 
aaother beneath the animal ; whereas 
iu the ITrocliordata they arise (roiu the 
ventral edges of the slits, and are united 
with one another on the dorsal edge. 
Thegill-alitstliemselvesbecomeL^hanged 
by tlie growth of numerous |iartitiona, 
transverse to the axis of the phatynx, 
into a series of narrow slits ; and then 
by the formation of another series of 
stronger bars parallel to the long axis, 
into a veritable ciliated trellis- work. 
All this trellis-work is supported by 
homy rods like the gill-bars of Amphi- 
(uriu. The t«st thickens enonncualy 
and becomes invaded by a finger-like 
outgrowth from the hinder part of the 
body, which carries blood-vessels to it 
and buds off cells into it which nourish 
it and change its character. With these 
changes the adult form is attained. 
Few would see any resemblance to a 
Vertebrate in the motionless sac-like 
body fixed to a stone or rock and look- 
ing more like a plant than an animal 
(Fig. 183). 

Nevertheless, the Tunicate in some 
points, even when adult, 
recalls the structure of 
A mpktoirws. Thus we 
eBCOunter a ring of delicate toutatles 
a short tUstance inside the 
mouth strikingly recalling 
the velar tentacles of Am- 
phivxva. Asinthat animal 
also there is a lung hypo- 
pharyngeal groove or 



structure of 




animal at 



inUtU<uiii4n 1. Tbe live 
Id teat I some □[ the oig&nB 
can he seen, an the teat ia semi.transpAreDt. 
Man til. 3. Atri&l (iriflct. B. Anus. 
1. Ueaitalpore. 5. MusoleB. 6. Stomaoh. 
T> Iiitotitiiie. 8. Ueptodactive organB. 
0, Stalk attached to a roak. 10. Tentacular 
ring. 11. Peripharyogoal ring. 12. Bittin. 




Tio. 184, TieiT or Ciona iniatinalit lying on its right side. Both the liraiu 
and the atridl oavitieB haTe been opened lij longitudinal inoiaiona. 

1. Mouth. 2. T^ntaclel. 3. Periphorfngeal groove. 4. Fer- 

(orutiid wallB of braQchial aao. 5. Endostjle. 6. Oesophageal 

opening lending tiova the brsoohial sac to tbe gtomaob, rather diagram- 
matic 7. Stomach. 8. Intestine slioning tjphlosole; port of it remofed 
to show Enbjacent atractoies. 9. Bectnm. 10. Anna. II. Atiial 
aperlnte. 12. Inner anrface of mantle ahontng longitodinal and trans- 
Terse muscle Gbres. 13. Dorsal tuberale. 14. Subnenral gland and 
brain. 15. Cut edge of brauohial aac. IC. Heul IT. Oval?. 
16. Fore of vas deferens. The openings of the oviduct and the vaa defer«DK 
are shown enlai^ed to the right. 19. Tealioular tubes on intestine. 

80. Oviduct. 31. Beptnin ahatting off that port of the bodj-okviQ 

which containa the heart, atomacli and generative organa. ^^^_ 



endoBtyle passing in front iuto a peripharyngeal band, and 
8e(.^reting a cord of mucus which is worked forward. This muctis is 
torn into strings by the inrustiing current of water and swept back- 
wards to the opening of the oesophagus, entangling in it food 
particles jnst aa in Amphioxug. Instead of a hyperpharyngeal 
groove, there is a seriei< of tags hanging down from the dorsal wall 
of the pharynx, called languets. These in life curve round so 
■8 to form a row of hooks supporting and dire'^ting the mucous 
Btriuga. 

The oesophagus leads into a dilated stomach which bends on 
itself and leads into an intestine which after ono or two coils runs 
forward and opens into the atrial ciivity. Its ventral wall is folded 
inwards, forming a typhloaole similar to that of an Earthworm. Aa 

lal the straight terminal portion of the intestine is called the 
rectum. Near the anus open the ducts of the ovary and testes, 
for the animals are hermaphrodite. These organs are branched 
clumps of tubes, the testis being spread over the surface of the 
Btotuach, the ovary forming a mass between the stomach and 
intestine. Oviduct and vas deferens are closely applied to one 
another, the vas deferens being the more superficial The latter 
opens by a rosette of small pores, the ovary hy a broad opening, and 
IK> the water from the gill-slits, as it parses out of the atrial cavity, 
sweeps away the sexual cells. 

On the ventral side of the pharynx is a V-shaped heart, which 
is enclosed in a space called the pericardium. The heart is only 
a specially thickened part of a ventral blood-veasei, which lies im- 
mediately under the endostyle and communicates through a network 
of vessels in the gill trellis-work with the dorsal blood-vesseL 
Waves of contraction pass over the heart so as to drive the blood 
forward. After a certain interval the direction of these waves ia 
Teveraed, so tliat the blood alternately goes to the dorsal vessel from 
the heart and vice term. With the e-\ce]>tion of the heart, however, 
the blootl-veasels do not seem to have definite walls, and are really, 
an in the Enteropneusta, crevices left between various organs. 

The sluggish life of the Ascitlian lias as its only external 
nanifestatiou the sudden closing of the mouth and atrial cavity 
Vy spliincters, and the consequent ejection of water — whence the 
popular name Sea Squirt. In consequence metabolism is at a low 
level and not much waste is prodiK'ed. A good deal of tbis 
waste is probably got rid of by the throwing off of the mantle from 
time to time, but for the rest no definite excretory organ is required. 



nROCHOBDATA- 

T he nilrogenona excretion is stored up as crystals of insoluble unn 
acid in little vesicles attached to the hinder part of the intestine. 
These vesicles, together with the cavities of the genital organs and 
the pericardium, may be looked on as the remnants of the coelom, so 
that here a similar phenomenon has taken place (■<> what was met 
with in the case of ArthropodB, namely an obliteration of the coelom 
through the expansion of blood-vessels. 

The Tunicata or Urochordata abound on every rocky shore and 
exhibit a surprising diversity of form. Their priucipal divisiuni 
are ns follows. 

Class 1. CoPELATA or Lahvacea. 
Small forms which retain the larval condition thronghoul 
life. The gill-slits are undivided and the anus ventrB,l, There 
is no atria] cavity : each of the two j^ilUslits opens <lirectly 
to the exterior. The tail is usually carried bent forward at a 
ttharp angle with the body. A temiwraty teat devoid of bl( 
vessels is found : the animal when disturbed wriggles out 
and forms another. 

Class II. The AaiPA. 

Forms which have lost the 
tail with its nerves and mnsclea. 
These are divided into 

Order I. The Aacidl- 
aceae, fixed forms. 

Older II, Tlie Thali- 
aoeae, which have second- 
arily acquired the power of 
swimming by contractions of 
the whole body carried out 
by transverse bauds 
muscle. 

The Ascidiaceae constil 
the great bulk of the Urochor- 
data. Some of them, such as the 
Fm. iftS. Two gronps ot individuals form taken as a type in the 
of Bwryiiuj pioiiw^u., afier Milna general description given above, 
Edwards. Magnified. " i- n ^ 

1. Moutb opening;. 2. Conunoi 



at a 

'1^ 



oat . 
)hor- I 



olooea ot the gtaup. 



remain solitary throughont life, 
but otbetB bud and form colonua 



XIIL] 

embedded in a common teat ; these ore called Compound Ascidians. 
But the group is not a, natural one, since budding is carried out in 
diflf^Tont ways in different Emilias, and han therefore probably 
originated several times. The 
commonest method is by the 
outgrowth of a hollow finger- 
shaped process of the pharynx, 
called a stolon, arising at the 
hinder end of the endostyle, 
which becomes divided into 
pieces, each forming a bud. 
On tlie other hand in Botnj)- 
tug a difTerunt method b fol- 
lowed, since in this la.'^e the 
buda originate simply as little 
pockets of the atrial-wall of 
tlie parent. Bofri/llua is one 
of the most beautiful colonial 
forms : in it the bitds are 
arranged in circles ; the atrial 
openingE of the members of 
a circle open into a common 
pit in the centre called the 
cloaca- Pyrosoma is a free 
floating colonial form, with 
the shape of a cylinder open at 
both ends, the atrial cavities 
of the constituent persons 
opening on the inner surface, 
their mouths on the outer. 

The Thaiiaceae are extra- p,^ ^^_ j,„^ ^^^ ^, ^ iMj-growr, 
flpecimen of the aoHUiy form of Sntpa 
dtmoeratica X ahoxxt 10. Tvum Biuulu, 





I 



ordinary forms. They have 

tlie shape of cylinders with 

lkenouth.t.„e«d=„dtl,« '-pL^lti'"! ao^r™ 'o"'' 

atrial opening »t the other, 

and tlieir body is surrounded 

wholly or partly with muscular 

hoiips like tlie hoops encasing 

a tiurrel. The commonest 

form is Stilp/e, which at intervals may be seen in countless 

numbers swimming at the surface of the sea. In this anima 



preaeDliog 
pharynx, the Bo-oalled "gin." !l. En- 
dDBtfle. 4. Peripharyngeal band. 

5. Brain. 6. Ciliated pit. 8. "Nu- 
oleuB," consiBtini; □( stomaeh, iiver. in- 
letline. 9. Stolon or row of fooiiB. 
ID. Processes of muDtle. 11. Uonlh. 



332 UROCHORDATA. [CHAP. XUL 

the test is of a glassy transparency. The two original atrial 
openings or gill-slits of the larva do not become divided by 
partitions, but develope into two huge vacuities in the side walls 
of the pharjmx, reducing its dorsal wall to a mere band, the so- 
called " gill." There are two distinct forms of this animal, a sexual 
and an asexual, one giving rise to the other, so that here we have 
a case of "alternation of generations.'' In the asexual form we 
find an endostyle-process or stolon which gives rise to a chain of 



Fio. 187. Semi-diagrammatic view of left side of Salpa, From Herdman. 

1. Branchial aperture. 2. Atrial aperture. 3. Anus. 4. Branchial sac. 
5. "Gill." 6. Sub-neural gland. 7. Endostyle. 8. Heart. 

9. Oesophagus. 11. Languet. 12. Muscle bands. 13. Nerve 
ganglion. 14. Embryo in ovisac. 15. Peribranchial cavity. 

16. Peripharyngeal band. 17. Stomach. 18. Testes. 19. Test. 
20. Sub-neural gland. 

small sexual forms which one by one drop ofif. Each sexual form 
produces only one egg. This when fertilised does not give rise to 
a tailed larva, but becomes attached to the atrial wall of the 
mother by a knob of maternal tissue containing blood-vessels, called 
the placenta, which is embedded in a disc of embryonic tissue, 
through which nourishment diffuses from mother to embryo. In 
this position it grows up into an asexual form and eventually 
breaks loose and swims away. 



333 



CHAPTER XIV. 

Introduction to Sub-Phylum IV, Craniata. 

The Cyclostomata. 

All the remaming Vertebrata are distinguished by possessing a 
skuU and brain, and are grouped together as Craniata. The Craniata 
are separated by a deep gap from the lower forms : but they them- 
selves present a fedrly continuous and graded series from the lowest to 
the highest forms, and their comparative anatomy, especially when 
we take into account the fossil representatives of the sub-phylum, 
gives us a fairly good idea of the course which the evolution of 
Vertebrata has pursued ; so much so indeed, that the group might 
be compared to the fairly reliable and complete records of a country 
during the historical period, whilst the Hemichordata, Cephalo- 
chordata and Urochordata represent the few scattered and scarcely 
decipherable documents of prehistoric epochs. 

The Craniata are defined, as we have seen, by the possession of 
a skull and a brain, though these are only two of the 
many characters which distinguish them from the 
other Vertebrata. The skull is composed of either cartilage or 
bone; and even in cases where the adult skull is completely bony, in 
the embryo the bone is partly, at any rate, represented by cartilage. 
Cartilage and bone are really only two peculiar modifications of 
connective tissue whose fundamental characters it may be 
useful to recall There is in every case a gelatinous ground 
substance traversed by fibres, and applied to these fibres are cells, 
which are connected with one another by delicate threads of 
protoplasm and which secrete the greater part of the ground sub- 
stance and fibres contained therein. In cartilage, the ground 
substance becomes cheesy in consistence, the fibres being masked, 
and the cells are arranged by twos and threes in little pockets. 
In bone, on the other hand, the cells remain single while the 



334 INTRODUCTION TO CRANIATA. [CHAP. XIU. 

ground substance becomes hardened by depositions of phosphate 
and carbonate of lime. The spaces occupied by the cells are 
known as lacunae, and the delicate processes which connect the 
cells give rise to the capillary canals known as canaliculi in the 
dried bone, whilst the spaces occupied by blood-vessels traversing 
the bone are known as Haversian canals. 

In the simplest form the skull consists of two pairs of pieces of 

cartilage, one pair embracing the front end of the 

skuii!"***^* notochord and termed the parachordals. In front 

of these is the second pair, the trabeculae, united 
behind and before with each other but diverging in the middle 
so as to embrace between them the pituitary body, which is 
described with the brain. The parachordals develope ridges which 
wall in the sides of the brain and may form a roof over its hinder 
portion. 

The brain is only the enlarged and modified anterior end of the 
neural tube, and the existence of a skull is correlated with the 
presence of neural arches protecting the hinder part of the 
nervous system. These arches consist of paired pieces of cartilage 
meeting above the neural tube. They have been shown to be formed 
as solid outgrowths of the myotomes which represent the hollow 
sclerotomes of Amphiaxtts, and hence it may be that the cranium 
itself is derived from the walls of the most anterior myotomes which 
early become fused with one another and otherwise modified. 

Haemal arches, paired pieces of cartilages with their upper 
ends implanted in the sheath of the notochord and their outer ends 
directed downwards, are also always present, and like the neural 
arches are derivatives of the myotomes. In the region of the tail 
the haemal arches meet each other so as to form a V beneath the 
notochord, but in the trunk they simply project out between 
adjacent myotomes as transverse processes, the ends of which 
may become movable on the basal parts and are then known as 
ribs'. 

The brain of all Craniata is sharply divisible into three primaiy 
regions called fore-brain, mid-brain and hind- 
brain (Fig. 188). Of these the first is certainly 
the enlarged and highly developed representative of the sense 
vesicle of the Urochordata and of the cerebral vesicle of Amphioxus. 

^ This statement applies to the transverse processes of the lower Craniata : 
those of the higher Craniata are secondary outgrowths from the neural arches. 




BefUtum eatu!iu. DiBseotion of thebnin •nd of soma of the oraoial 
as. A. VentcBl oiow. B. Dorsal view. C. Lougitadmal median 
Mctiou. D. Diagmm of embrfoiiic hnia nhowinH the Ihree primuy 

, Cerebmm. 3. Pineal stalk. &. Olfactory lobe. i. Cerabial hemi' 
cphcza. G. ThalameDcephulou. 6. Pituitary body. 7. Optio lobea. 
~ "Iptia lobes. 9. Cerebellum. 10. Root of the hind-brain, 

laperiot obliqne muBcle. 12. Internal rectas muscle. 13. Sup- 
■otiu muMle. 14. Eiternal rcclun muicle. 16. Kinth or 

pfaaryngeal nerre. 16. Branch of vaguH Derre to aeoond 

1 clefi. 16a. Branch of vagus nerve to tliiid branchial 

'. Main tnittk of vuriu to fourth and fifth gill-slits, to 
blnal line and to viscpta. ii. Optio aerve. im. In A, oplie ahiaama. 
tv, V, VI, VD. VIII, IX and x. Hoots of faurih to tenth l^rBIlial nerves. In 
I), I, II, III rciireBent the first, second and third primaty veBtoIes of tbo 
-cnbfToiue brain. 



336 INTRODUCTION TO CRANIATA. [CHAP. 

In the embryo it is a simple thin-walled vesicle, the lateral walls of 
which become changed into the retina or the essential s^isoiy 
portion of the eye. This, as is the case in the Ascidian 
tadpole, has its perceptive surface turned inwards towards the 
brain cavity. The nerves by which the eyes are connected with the 
brain are really the narrowed connections of the lateral portions of 
the fore-brain with the central portion. The roof of the fore-brain 
remains thin throughout life and from it a stalk arises leading to a 
third median eye, the so-called pineal body, vestigial in all living 
forms. From the front wall of the fore-brain an outgrowth takes 
place, giving rise to a bilobed vesicle termed the cerebrum, each 
of the two lobes of which it is composed being termed a cerebral 
hemisphere. This in the higher Craniata is the seat of the more 
complex mental processes, but in the lower it appears to be intimately 
connected with the organ of smelL The cerebrum in these cases 
remains thin-roofed, but its base thickens owing to a great develop- 
ment of nervous matter. In order to distinguish it fix>m the 
cerebrum the original fore-brain is denoted by the name thal- 
amencephalon. This pituitary body is compounded of a 
downgrowth of nervous tissue from the fore-brain with a portion of 
tissue evaginated and constricted ofif from the lining of the buccal 
cavity. It represents the sub-neural gland of the Urochordata, and 
in the higher Vertebrates produces a substance which is of im- 
portance to the normal metabolism of bone and connective tissue, 
and recent research suggests that its secretion also influences the 
activity of the kidneys. 

The mid-brain acquires thick lateral pouches, the so-called optic 
lobes : the hind-brain remains thin-roofed, except in front where a 
transverse nervous band, the cerebellum, is formed. The cere- 
bellum is believed to be the portion of the brain intimately con- 
nected with the semicircular canals of the ear and to have for its 
function the control of the muscles so as to maintain the equilibrium 
of the body. The rest of the hind-brain is termed the medulla 
oblongata or spinal bulb; it controls the beating of the heart, the 
respiratory movements and other vital processes. The hinder part 
of the neural tube is known as the spinal cord, and it developes 
thick walls, so that its cavity is exceedingly smaU. 

The essential element in the nervous system of Vertebrata, as in 
all other nervous systems, is a kind of cell which has been variously 
styled nerve-cell, ganglion-cell and neuron. This last name is 
undoubtedly the best, as it avoids the old misapprehension that re- 



garded ibe d 



'E STRUCTUKK OK NERVOUS SYSTEM. 



rve-cell and neTve-fibre as two indepeDdent stmctureB. 

On page 54 it was pointed out that the Dcrve-fibre 

ia a very fine basal outgrowth of a modified ecto- 
S'tem?"'"' denii cell which is the Derve-cell. The cell, inchiding 

its outgrowth, is termed the neuron. Important 
disroveriee Iiave recently been made on the minute atructure of the 
Dervons system of Vertebrata, and we are now able to form a 
simple and connected idea of the principles on which it ia built np. 
Ongiuating as a simple strip of ectoderm which becomes rolled up 
so as to form a tube, it k at first composed of cells which extend 
through its entire thickness and which all abut on the cavity of 
the tube. Some retain this position Ijut developc branches and 
deposit a large amount of cuticnlar suhstance in their protoplasm: 
these, constituting the supporting elements of the system, are termed 
collectively neuroglia. Other cells retire from the cavity of the 
tuhe, becoming more or less rounded in form, but developing a 
number of outgrowths: these cells are the neurons. Each neuron 
is provided with a number of branching processes, sometimes 
arising from a single thick stem; these are cailed receptive den- 
drites (Gr. &€v&pov. a, tree), and they receive impulses. Impulses 
&T« transmitted through one long basal process, called the axis- 
cylinder process or a.\on, which ends in a tuft of processes 
often thickened at the tips, which are called terminal dendrites. 
Tlie name axis-cylinder is suggested by the circumstance that 
amongst Vertebrata this process is in many cases surrounded by 
a fatty sheath of a conspicuous white colour, called the myelin ; 
a proccis with or without this sheath making up what is known 
as a nerve-fibre. The tuft of dendrites in which the axon ends is 
found to be in close contiguity either with the receptive dendrites 
of another neuron, by which means the impulse is transmitted 
from one neuron to another, or else with the muscle-plate of 
a mnscle-fibre, hy which means the fibre is stimnhited. The 
muscle -plate is a disc of protoplasm with several nuclei situated 
at tlie side of the muscle-fibre. The axon may give off several 
branches termed collaterals. These like the main stem end in 
tufts of dendrites ; in this way an impulse may spread over several 
paths. The receptive dendrites of a neuron may also receive 
impulses from the terminal tufts of several axons, and in this way 
impulses are co-ordinated and c-ombincd. 

As mentioned above, the skull and brain are by no means the 
only characters which distinguish the Craniata from other Ghordata. 
8. Si HI. 22 



338 INTRODDCTION TO CRANIATA. [CHAP. 

PeihapB the next in importance is the posaeesion of three well- 
developed paits of aenee-orguis, nose, eyes and eats. 
ar^"~ Of these the noae is the most simply conatructed. 

It consists merely of a pair of pits in the skin at the 
most anterior portion of the body, the lining of vhich developes 
ridges covered with sensory cells, having an olfactory function 
(Fig. 189). The essential element in all sense-organs is the sense- 
cell, which resembles the neuron in possessing a basal procesB 
terminating in a tuft of dendrites by which the stimulus is trans- 
mitted as an impulse through a neuron, for in Craniata a sense-cell 



is never in direct communicatioD with a muscle-fibre. An olfactory 
sense-cell differs from a neuron in possessing one or more stiff 
peripheral processes projecting from the surface of the body, by 
which stimuli are received from the external world. These are 
termed sense-hairs, and they are excessively delicate in structure. 
Sense-cells are never combined by themselves into an epithelium: 
they are always intermixed with stiff supporting cells which usually 
have at the base several root-like branches. The front end of tJte 
brain comes in direct contact with the wall of the nasal sac and the 
axons of the sensory cells stretch into the brain, thus constituting 
the olfactory nerve (Fig. 188). 



XIT.] AUDITORT ORGAN. 339 

The ears are alao at first pits of the skin placed further back at 
the sides of the hind-brain. In the lower forms these pite retain a 
narrow connectioD with the exterior throughout life through a long 




Fia. 190. Eur of Chimaera mimitro$a L. x about 4. From BetziuB. Seen 
from the inner side. 

1. External apeitnre od roof of skull. Tbe wall of 2, the "duotuB endo- 

lymphaticus," ig partlj removed to show that it ie a tube. 3, Anterior, 

4, poHterior, and d, horizontal getmciroiiler canals. 6, Anterior, 

7, external, and 8, posterior ampnllae. 9. Sacenlue. 10. Auditor; 
or 8th nerve. 

tube called the dnctaa endolymphaticus (2, Pig. 100). In the 
hi^ier forms this tube is still recognizable bat no longer opens to 
iha exterior. Each pit contains lyioph and becomes constricted in 

22—2 



340 INTRODUCTION TO CBANIATA. [CHAP. 

the middle into an upper portion, the utricaluB, and a lowec 
portion, the sacculus. With the exception of the Cycloetomata 
the former gives rise to three flat outgrowths placed in planet 
at right angles to one another (Fig. 190). liiese oatgrowtha 
become converted into half-rings hy the meeting of their vails 
in the middle of each, and in this way three semicircnlar 
canals are formed, called respectively anterior, posterior and 
horizontal. The primary function of the whole organ, like that 
of the otocyats of Medunae, Crustacea and Molluscs, is to enable 
the animal to perceive its position. Where each semicircQlar 
canal arises from the utriculus it is swollen, and the swelling 



FiQ. 191. SectioD of ui ampulla of the intemaJ Eai. 
1. Senae-cell bearing a long faair. 2. SenBe-hair. S. Nerre tenm- 

nation blanching round haae of seose-cell (dendrites of a deeply plaoed 
□earon). 4. Interstitial cell. S. OelatinoQB cap in vhich the 

senso -hairs are embedded. 

is termed an ampulla. The wall of each ampulla projects 
inwards, and the projection contains cells with exceedingly long 
sense-hairs which project into the cavity of the ampulla (Fig. 191). 
The free ends of these hairs are embedded in a gelatinous cup, 
and thus the whole organ is admirably adapted to record change of 
position in any direction, since any change of position can be 
completely analysed into movements in three planes. The lower 
part of the organ or sacculus has cells adapted to be stimulated 
by vibrations in the snrrounding lymph. It often contains cal- 
careous ' ear-stones.' In higher forms it gives off a spiral tube, the 
cochlea, which contains the true aaditory sense-cells. These form 
the organ of Corti, a more complex structure than the sensory 
epithelium of the sacculus and ampullae, but resembling it in 
consisting of hair-cells which are embraced by the receptive den- 



THE ETE, 

diites of DeuroD8. The grouiied cells of the neurona of all these 
■ensoiy atnieturea form the several auditory ganglia. Both 
id ear have cartilaginous or bony coats which become finnlf 
coQoected with the skull; these are kuowu as the sense-capsules. 
The eye is the most complicated, and in the higher Craniata by 
&r the moat important, of the sense-organs. In its origin, as we 
have seen, it is the lateral portion of the fore-brain which when 
constricted off is known as 
the primary optic vesicle (Fig. 
192). Tlie outer wall of this 
becomes miwlified into a sen- 
sory epithelium called the 
retina. This consists of a 
row of visual cells, their free 
ends dire<;ted inward towards 
the brain and produced into 
the characteristic striated 
rods. Beneath these sense- 
cells lie a number of neurons, 
the dendrites of which, miiig- 
Ung with the dendrites of the 
sensory cells, give rise to a 
ci)mparntively thick bed of 
nervous tissue. Long, however, 
before the sense-cells are de- 
veli>ped, the primary vesicle of 
the eye has completely altered 
ita shape. The outer wall has 
become pushed in on the inner 
so as to completely reverse the 
•hape of the sac (Fig. 19^). 
Its cavity is reduced to a 

slit, and it takes on the form of a very deep double cup with 
its concavity directed outwards. This is the cavity of the eyeball, 
or so-called secondary optic vesicle, the clear gelatinous connec- 
.tive tissue inside which is known as the vitreous humour. The 
connective tissue surrounding the vesicle peripherally fonns a tough 
fibrous or even cartilagioous capsule called the sclerotic coat, 
lined by a thin vascular tissue, the choroid coat. The sensitive 
.and nervous outer layer of the primary vesicle is known as the 
retio&, the other layer (which becomes loaded with pigment) as 




Flo. 192. Tranaveise lectioD (hraugli a 
third diij Chick to show origin o( the 
l«ti|]a from the brain and of the lens 
from the eotoderm. Highly msgnilied. 

1. Cavity Qf brain. 2. Ontar lajer 

of retina surrODndioR the black, 
thicker layer which will form the rods 
nud coneu. S. LeiiB uriniuR ns a 

hol}ow invagination. 4. Pineal 

boilj' origiuiiting. 5. Embrjonio 



342 



INTEODUCnON TO CBAKIATA. 



[CHAP. 



die pigment epithelium of the retina. If we analyse the 
structure of the retina, we find that it has fundamentally the 
game structure as the central nervous system of which, as ita 
origin shows, it is leally a part. Thus there are a uTimber of 
branched and cuticularized supporting cells called fibres of 
Mil Her, extending throughout the whole thickness of the retina, 
and the main mass of the retina is made up of nenroug. There 
is, however, in addition a layer of characteristic visnal cells; 







Fio. 193. Diagram to illugtnte structure of & retina. The wrerftl "lajren" 

are indicated by the DumerftU III, Ac. in order from within (vitreous 

humour) ontwaraa. 
1. Gone. 2. Bod. 3. NncleoB of rod-sell. 4. Small aearon. 

5. Large neuron. 6. Pigment epithelium. 7. Fibie of MiiUer or a 

supporting cell. 

that is to say, of sense-cells, with a comparatively thick striated 
rod in place of the ordinary sense-hair. Visual rods have already 
been described in the eyes of Anthomedusae (p. 55) and of 
Arthropoda (p. 131); they occur wherever the capacity for vision 
b developed. In the retina of Graniata there are two varieties of 
visual cell, called respectively rod-cells and cone-cells. In t^ 
first, the visual rod is narrow and cylindrical, and the body of the cell 
beneath is filamentous with a rounded swelling for the nucleus; the 
basal process ends in an unbranched knob, that is to say, in a single 



THE RETINA. 

dendrite, In the cone-cell the rod is conical with a broad base, to 
hich the body of the cell coutainiug the nucleus is immediately 
applied; the btisal process ends in the normal manner in a tuft of 
dendrites. The baaal processes of both kinds of sense-cell are is 
cloae relation to the receptive dendrites of a layer of neurons with 
small cell bodies; the axis-cylinder processes of these in turn end 
close to the receptive dendrites of a layer of neurons with large 
cell bodies situated close to the outer basal surface of the retina, 
which give rise to the fibres ci^ostitiiting the optic nerve. Taking 
ft general view therefore we may say that the retina is a sensory 
epithelium cousistiug of a layer of sense-cells underlaid by 
two layers of neurons. Before its structure was thoroughly under- 
stood, however, the appearance of the retina in transverse section 
was a hewilderiug mass of fibres and nuclei, iu which for descrip- 
tive purposes different layers were distinguished. These, reckoning 
them iu the order proceeding from the inner side of the eyeball 
towards the lens, were as follows: — (a) the layer of rods and 
cones; (/>) the outer nuclear layer (ti. Pig. 193) consisting of the 
bodies of the visual cells containing their nuclei; (e) the outer 
molecular layer (v. Pig. 193) consisting of sections of the basal 
processes of the visual cells and of the receptive dendrites of the 
neurons with small cell bodies ; (of) the inner unclear layer (fv. Fig. 
193) consisting of the bodies of the above neurons; (e) the inner 
molecular layer (in. Fig, 19.')) consisting of sections of the baaal 
processes of the above neurons und of the receptive dendrites of the 
neurons with large cell boilies; (/) the layer of nerve-cells consist- 
ing of the bodies of the last-named neurons ; and finally (g) the 
layer of nerve-fibres consisting of the basal processes of the last- 
named neurons which constitute the optic nerve. 

The remainder of the eye Is to be looked on as a part of the 
skin of the side of the hea<l which has been rendered transparent 
in order to allow light to reach the retina. It consists of a lens 
■nd cornea, separated by a chamber containing the aqncoug 
Humour. The lens is an originally hollow plug of ectoderm cells, 
which breaks loose from the skin and lies in the mouth of the 
idary optic vesicle (Pig. 192). The skin outside the lens forms 
the cornea, which is transparent The cornea being joined to 
the edges of the sclerotic completes the boundary of the eyeball, 
as the fully -elaborated sense-organ may be termed, If the above 
description ha.'; been followed it will be seen that in a Craniate 
light must reach the visual cells through their basal aud not 



344 INTRODUCTION TO CRANIATA. [CHAP. 

through their^ visual ends. As this is contrary to the almost 
universal rule obtaining throughout the animal kingdom, we cannot 
believe it to be a primitive arrangement. Bather we must believe 
that when the eye was being evolved the rods of the visual cells were 
directed towards the light, and that the epithelium of which they 
form a part was exposed and not rolled up into a neuial tube ; in a 
word, that the front portion of the nervous system of Vertebrata at 
any rate was once a plate of sensitive skin. It is most su^estive 
to note that in the larva of the Hemichordata we find such a 
plate with two eye-spots at the apex of the prae-oral lobe. 

The external layer of the skin or ectoderm of Graniata is quite 
peculiar in the animal kingdom, in that it consists not of one, but of 
many layers of cells. On closer inspection, however, it is seen that 
the deepest layer, consisting of columnar cells alone, really repre- 
sents the ectoderm of the other phyla. This layer 
instead of becoming directly converted into cuticular 
substance, as, for example, in the Arthropoda, buds off flattened 
cells from its outer surface which become bodily converted into 
homy matter and scale off. The ectoderm rests on a specially firm 
bed of connective tissue called the dermis. 

A very peculiar feature in the Craniata is the character of the 
scattered sense-cells of the skin. These end in 
sense filaments embedded in the ectoderm, not pro- 
jecting beyond it. These filaments have however grown enormously, 
and with their growth the bodies of the cells with the nuclei have 
come to lie deep down in the body. Here they form segmentally 
arranged packets of cells lying at the side of the nerve-cord and 
known as the spinal ganglia. They are connected with the nerve- 
cord by their basal outgrowths or nerve-tails, which constitute the 
dorsal roots of the spinal nerves corresponding to the dorsal sensory 
nerves of Amphioxus. To each myotome a motor nerve is given off, 
as in Amphioxus y but in the Craniates the fibres of this nerve are 
bound up for a certain distance with the long peripheral hairs of the 
sense-cells constituting the spinal ganglia, so as to form a compound 
sensory-motor nerve, which is then said to have a dorsal sensory 
and a ventral motor root 

The power of transmitting and modifying impulses, characteristic 
of the nerve-cell, is merely one of the fundamental properties of all 
protoplasm, specially developed. It therefore probably resides to a 
small extent in all cells. In the ectoderm from its exposed con- 
dition this function has been largely exercised, and hence the 



CRANIAL NERVES. 

nervous system of most aniniaU coDsists of modiiied ectoderm 
cells. But the eudodenuic tube is likewise stimulated by the 
passage of food through it and it is therefore not surprising to 
learu that some of its cells deveiope nerve-tails and are 
converted into sinaJJ neurons. In this way a tangle or plesua 
of fibres with intermixed cells is formed, which is the basis of the 
nervous system of the gut, or 'sympathetic' system. In most groups 
of animals the endodermic nervous system is never developed beyond 
this point ; but in Craniata this plexus is connected with portions 
of the spinal ganglia at regular intervals which early separate from 
the rest and are called the sympathetic gangha. These ganglia 
retain their connectiou with the spinal cord by nerves imlled the 
rami communicantes, in which motor fibres going to the 
alimentary canal are included. Successive sympathetic ganglia 
are connected by a longitudinal commissure, and so there is a 
chain of sympathetic ganglia on each side of the spinal cord. 

It is usual to reckon ten liairs of nerves as appertaining to the 
brain, but these are of very unequal value. The first or olfactory 
pair are really drawn-out portions of the cerebrum. In the lower 
Craniata these parts have the shape of swellings connected by 
narrow stalks with the brain, and these stalks were confused with 
nerves (Fig. I8S). The terminal swelling comes into close contact 
with the epithelium of the nasal sac, and a large number of small 
nerves— the true olfactory nerves — connect the two. The second 
or optic nerve is formed by nerve-fibres growing along the stalk 
uniting the primary optic vesicle with the brain (Fig. 192). The 
nerve fibrils which run in this stalk go mainly but not entirely to the 
opposite side of the brain. Thus in the Boor of the thalamencepbalou, 
or primitive fore-brain, there is a crossing of fibres proceeding from 
the two eyes. This part of the floor becomes nipped off as a groove 
from the rest — and is known as the optic chiasma. The chiasma 
is connected with the combination of the stimuli received by the 
two eyes so as to produce single vision, each side of the biain 
recfliviag impulses from both eyes. The third or motor oculi, 
the fourth or patheticus, and the si:cth or abduceus nerves are 
motor nerves, supplying the eye muscles derived from tho head 
cavity, the collar cavity and tlie first myotome respectively. The 
fifth or trigeminal, and seventh or facial, are most interesting 
nerves, being sensory as well as motor, and the sense-organs they 
supply iu the lower Craniata are peculiar. These organs are scattered 
over the prae-oral part of the body or snout and the sides of the 




hend, and are known as the mucona caoaU. On the snout they 
have the shape of deep tubes swelling out at the bottom into sacs ot 
aiiipiilloe ; and on the head, of canals communicatii^ at intervals witli 
the exterior by vertical tubes. Certain of the cells lining these tnbes 
devetope blunt, freely projecting sense-hairs, recalling the oharai'ter of 
the auditoty cells, whilst others secrete the mucus with which the 
tubes are filled and whence they derive their namei. It is probable 
that the function of these organs is somewhat allied to that of the 
ear, balancing combined with hearing (or at any rate, perception of 
vibrations), for it has been proved that a &sh deprived of its eyes is 
still able to guide itself along tortuous passages so long os this 
organ remains intact, and this is explicable only on the assnuipdoa 
that the reflected pulses of the water are felt by these organs The 
branches of the fifth and seventh nerves wliich supply them are 
usually for some distance in close juxtaposition and are known as 
the ophthalmic nerves. The eighth or auditory cranial nerve 
goes to the ear, and arises in such close proximity to the seventh 
that it may be regarded as a specialized branch of it, the ear itself 
being very possibly a highly specialized mucous canal. The motor 
divisions of the fifth and seventh are distributed to the region of 
the mouth and to that of the first gill-slit respectively. They both 
fork ; the upper branch of the lifth goes to the upper jaw and the 
lower to the lower jaw, while one branch of the seventh passes in 
front and the other behind the spiracle. The ninth or glosso- 
pharyngeal nerve is similarly forked round the first true gill- 
slit (Fig. 1R8). The tenth or vagus or pneumogastric nerve, 
which is certainly a compound one, gives off a branch to each of 
the remaining slits, to which it bears a relation similar to that 
borne by the ninth nerve to the second slit. The main stem of the 
nerve passes along the alimentary canal and sends nerves to its 
muscles and to those of the heart, alt these muscles being develop- 
ments of the inner or splanchnic wall of the unsegmented coelom. 
The tentli nerve has also in the lower Craniata a sensory dii-ision. 
This separates from it soon after it leaves the brain and posse? 
backward, supplying an immensely long mucous canal, called the 
lateral line, which extends from head to tail along the mid-lateral 
portion of the body and is provided with a series of openings to the 
exterior. On account of its extensive area of distribution the tenth 
nerve has received the name of vagus (wandering). 

The alimentary canal exhibits a marked ditt'erence from 1 
condition found in the lower Chordato. The gill-slits are redi^ 



ALIMENTARY CANAL. 

tn Dumber, there being as a. rule uot more than eight: it would 
indeed be more correct to speak of them as gilli 
t«n«i"""*^ pouches. Ill this reapect Oaniata agree with the 
Hemichordata in contrast tfl the Cephalochordata and 
Hrochordata. No trace of a tongue-bar has however been found in 
any Craniate. 

The endostyle bet-omes shut nif from the pharynx and 
thus loses entirely its original fuuction ; it branches and forma 
a mass called the thyroid gland. The evil results attendant on 
its removal or diseased condition and experiments on liring animals 
show that it secretes into the system a substance which has a 
beneficial influence on metabolism, especially as regards the " tone " 
of the nerviius system and the growth of connective tissue. 

The sub-neural gland of the tlrochordata, on the other hand, 
seems to be represented by a structure called the jiituitary body. 
This, like the sub-neural gland. Is a dorsal pocket of the etomodaeum, 
but it becomes cut off from all connection with the mouth and 
intimately associated with a downgrowth of the brain, called the 
infundibulum, tn form an organ having an influence on the 
well-being of the animal (see p. 336). Since in the case of the 
Urochordata the sub-neural gland is fashioned out of the persistent 
communiuatinn of the sense vesicle with tbe exterior, one is tempted 
to regard the clo^e connection of the infundibulum and the pituitary 
body as remnants of the former connection of the brain and stomo- 
daeum in the aucestora of the Craniata. Some authors maintain 
that a rudiment of the infundibulum is to be seen even in ths 
cerebral vesicle of Amphiarus (see Pig. 174). 

Except in the lowest forms the alimentary canal Is differentiated 
into several well-marked divisions. There is to begin with a 
stomodaeum lined by an epithelium consisting of many layers 
similar to that forming the epidermis. The first division of tho 
endodermal tube is called the pharynx, and into this the gill-slita 
open, Tbe line of demarcation between ectixlerni and endoderm 
is entirely obliterated in the adult. Following on the pharynx 
is a tube of narrow diameter, termed the oesophagus or 
which leads into the stomach. The stomach, consists of the 
first of the loops into which the alimentary canal is bent 
sequence of being longer than tho body, it is a greatly dilated 
portion of the canal and in it the food is stored until a large 
amount of digestion is accomplished. As in other animals, the food 
ia moved from place to place by peristaltic contractions of the vis- 



I 



348 INTRODUCTION TO CRANIATA. [CHAP. 

ceral muscles derived firom the inner wall of the coelom. There is a 
particularly powerful girdle of these called the pyloric sphincter, 
which by remaining contracted keep the distal end of the stomach, 
the so-called pylorus, closed until the work of digestion is 
accomplished, when they relax and allow the food to pass on into 
the next division of the canal, the intestine. The walls of the 
proximal part of the stomach are produced into small pouches, the 
lining cells of which secrete a substance called pepsiji, which has 
the power of turning the proteid of the food into soluble peptone. 

Pepsin is an example of the class of substances known as 
digestive ferments or enzymes : these are complex substances of 
unknown constitution which have the power of effecting a large 
amount of chemical change without themselves undergoing a 
permanent alteration. The object of their action on food-stuffs is 
to render them soluble, and therefore fitted for absorption by the 
wall of the canal. Pepsin is active only in an acid medium, and 
free hydrochloric acid is found in the contents of the stomach in 
small quantities, produced by special cells in the waUs of the 
pouches just mentioned. 

An organ called the liver is very conspicuous (Pig. 207). 
It consists of a ventral outgrowth of the gut, arising just behind 
the stomach, which extends forwards and branches into an immense 
tree-lik^ mass of tubes welded together by connective tissue into a 
solid mass extending forwards and nearly obliterating the front part 
of the body cavity. Whether this organ really performs the same 
function as the so-called liver in Amphioxus is doubtful. It has 
been proved that the function of the Craniate liver is largely the 
elaboration of an alkaline fluid called the bile. This is partly 
excretory in nature, but has an important influence upon the 
processes of digestion and absorption in the intestine. The main 
stem of the liver tubes is called the bile-duct; there is often a 
lateral outgrowth from this which acts as a reservoir for the bile, 
called the gall-bladder. Besides this, the liver cells can form from 
the sugar brought to it from the intestine a substance called 
glycogen, allied to starch in composition, which acts as a reserve of 
carbohydrate material available for the system as needed. Among 
other influences which the liver exercises on the chemical processes 
of the body is the very important one of transforming the nitro- 
genous waste products into a suitable form (urea or uric acid) for 
excretion by the kidneys. 

Another outgrowth from the intestine arises sometimes just 



behind the opening of the bile-rtuct, sometimes from the duct 
itself. This ontgrowtli, like the liver, branches into a tree of tubes 
whi'-h are hound together by connective tissue to form a solid mass, 
though one of much smaller size than the bver. This organ is 
called the pancreas and it produces a secretion called pancreatic 
juice, by which the process of digestion is completed. This juice 
contains three ferments : these are amylo-pftin, which converts 
Starch into soluble sugar; trypsin, which, acting only in an 
aikahne medium, converts proteid into |>eptone and simpler de- 
rivatives ; and ateaps in, which splits np fat into soluble fatty 
acids and glycerine. The fatty acids unite with the alkalis present 
in the mixed contents of the intestine to form soluble soaps, and 
these are absorbed along with the glycerine, a reconstruction into 
fat taking place in the intestinal epithelium. 

The intestine is always somewhat longer than the body. Hence 
it must be to some extent looped or twisted (Fig, 207), though this 
may express itself only in a slight curvature. A fold projecting into 
it in some forma represents the typhloaole of the Urochordata and 
19 known as the spiral valve, since it shares in the twisting. In 
the intestine ihe digested food is absorbed and transferred to the 
blood-vessels and lymph-canals. The last portion of the intestine 
ia usually of larger diameter than the rest and is called, when thus 
distinguishable, the large intestine. In it the indigestible 

, material is elaborated into faeces for expulsion by the anus. 
The blood system of the Craniate is distinguished by the posses- 
sion of a large and well-developed heart, which, like 
■yBWrn.*""" '''^ heart of the Urochordata, is an enlargement and 
specia.hzation of part of the ventral vessel. The space 

■ in which it apparently lies — really, into which it protrudes, — is called 
the pericardiuin, and is only an anterior part of the coelom shut 
off from the rest by the development of a, transverse septum. The 
heart is constricted into four chambers, becoming successively more 
thirk-walled aa we proceed forwards, and named, beginning from 
behind, the sinus venosuK. the atrium, the ventricle and the 
COBUs arteriosus (Fig. 195). It is bent into an S-shape, so 
that the sinus venosus is dorsal and posterior, the atrinm dorsal and 
anterior, the ventricle ventral and posterior, and the conus arteriosus 
ventral and anterior, The conns arteriosus leads into the ventral 
aorta, which gives off the arterial arches ; the.te are branches 
which ascend between the gill-sacs and ramify on their walls. From 
the gills the blood collects into epibranchial vessels which join 



350 INTRODUCTION TO CRANIATA. [OHAP. 

to form two longitudinal vessels on the dorsal wall of the pharynx, 
the roots of the dorsal aorta. These unite behind the pharynx 
into a single dorsal aorta, giving blood to all the hinder part of the 
body. The forward extensions of the two longitudinal epibranchial 
vessels carry blood to the head and are known as the carotid 
arteries. There can be little doubt that the impulse leading to the 
evolution of the heart came from the necessity of having a strong 
force to drive the blood through the capillary channels on the walls 
of the gill-^acs. 

In the embryos of all Graniates the number of these paired 
connections between the ventral aorta and the roots of the dorsal 
aorta is six, but the two anterior pairs, viz., those traversing the 
wall of the pharynx parallel with those parts of its supporting 
skeleton known as the mandibular and hyoidean visceral arches 
respectively (see pp. 371 and 372), are found in adult forms only 
as remnants in connection with the carotid arteries. Whatever 
may have been the case in primitive forms, these first two arterial 
arches have now no part in aerating the blood, this function being 
performed by the succeeding four pairs of arches, along whose 
course only are gill-sacs developed. We shall see that the arterial 
system near the heart is in all groups of Craniata a modification of 
the six pairs of arterial arches now described. 

The fore-limb is supplied by a vessel called the subclavian 
artery, but the origin of this differs in the several classes of 
the phylum. In Amphibia, Lizards and Mammalia other than 
Getacea, it arises from the epibranchial artery near or behind the 
sixth branchial arch, while in Crocodiles, Turtles, Birds and 
Cetaceans its origin is from the ventral end of the third branchial 
arch. As in both Lizards and Cetaceans these two vessels exist 
side by side, but only one of them supplying the fore-limb, it is 
clear that the subclavian arteries are not homologous throughout 
the group. 

Each chamber of the heart is separated from the one behind by 
valves, which are flaps of membrane free to move in one direction so 
as to open and admit blood from behind, but restrained by tendinous 
chords from being driven further back than so as just to meet when 
the chamber contracts, and thus prevent any backward movement of 
the blood. In the con us there may be several transverse rows of 
pocket valves. These valves as their name implies are loose 
pockets of membrane which are pressed flat against the wall of the 
conus during the forward movement of the blood, but which when 



XIT.] CIBCULATOHT SY8TBU. 351 

the coDUB cODtnctB become filled with blood and swollen out ao as to 

meet one another and prevent the reflux of blood into the ventricle. 

The development of Uie liver has exerciaed a profound influence 




and their branobe* ii 



352 INTRODUCTION TO CRANIATA. [CHAP. 

on the afferent part of the blood system coiresponding to the 
hinder part of the sub-intestinal vein of Amphioams. The vast 
mass of tubes projecting into it has broken it up into a network of 
capillary channels called the hepatic portal system. In front of 
this, where it enters the sinus venosus, it is known as the hepatic 
vein ; behind, branches from the walls of the intestine so overshadow 
the original ventral trunk that this, embedded between the limbs 
of the spiral valve, appears as merely a small branch of the com- 
posite trunk or portal vein. 

The blood from the muscles and kidneys, in a word, from the 
dorsal and outer parts of the coelom, collects into two longitudinal 
channels called the cardinal veins. These empty into the sinus 
venosus by transverse trunks called ductus CuvierL These trans- 
verse trunks divide the veins into anterior cardinals returning 
blood from the head, and posterior cardinals returning it from 
the rest of the body. In the tail the two posterior cardinals are 
represented by the median caudal vein, which further forward splits 
into two. Just as the course of the original sub-intestinal vein has 
been obstructed by the growth of the liver, so that of the posterior 
cardinal has been choked by the growth of the kidney tubes. 
The blood from the tail and hind limbs is forced to filter amongst 
these in a series of narrow channels called the renal -portal 
system. The front part of the vein retains the name posterior 
cardinal : the hinder part is called the renal-portal vein. Since 
the kidney tubes also receive blood from the dorsal aorta they, like 
the liver, have a double supply. 

The blood of Craniata has in addition to the ordinary amoebo- 
cytes a much larger number of oval or round cells impregnated 
with haemoglobin, called red blood-corpuscles. Haemoglobin 
has been mentioned when describing LumbricuSj in which worm 
it is found diffused in the blood fluid. The great characteristic 
of haemoglobin is its power of forming a bright red, unstable 
compound with oxygen. This compound is formed in the respira- 
tory organ and carried by the circulation to all parts of the 
body. In the capillaries it is broken up and the oxygen absorbed 
by the tissues. The haemoglobin having lost its oxygen changes in 
colour, and the impure blood which leaves the tissues is in conse- 
quence bluish. From the tissues the blood takes up carbon dioxide 
which, like the oxygen, is conveyed in loose chemical combination, 
though with the sodium of the blood instead of with the haemo- 
globin. The carbon dioxide is set free in the respiratory organs. 



CIRCULATORY SYSTEM. 

On page 128 it was pointed out that both blood and connective 
tissue have been derived from a jelly-like secretion auch as is found 
in Coelenteiata. This in the embryo ooelomate animal fills up the 
interstices between ectoderm, endoderm and uoelomic sacs, these 
interstices being collectively termed the primary body-cavity or 
haemacoeL It was also pointed out there, that whereas in the 
part nf the jelly which was converted into connective tissne a large 
mimber of fibres were developed, in the portion destined to form 
Uood, on the contrary, no fibres appeared and the jelly remuned 
fluid, and in consequence the amoebocytes which had wandered into 
H from the neighbouring epithelia were able freely to move about. 
In Annelida, Arthropoda and Mollusca certain of the blood-Bpaees 
acquire muscular walla derived from the adjacent coeloniie sacs, 
Ud thereby attain contractility which may be specially localized in 
t dilatation called the heart, 'i^he spaces with muscular walls are 
Ithe arteries. In CVaniata a further differentiation has taken place : 
we find not only a definite heart and arteries leading away from it, 
litit also eipially definite veins leading into it as described above, 
ftnd arteries and veins are c-onnected with one another by narrow 
channels called capillaries with well-marked walls. Heart, 
ftrteries, veins and capillaries are all lined by a single layer of 
flattened cells called an endothelium, which has been developed 
from the flattening out and union of a certain number of amoebo- 
eytes. The capillaries possess no other wall, but arteries and veins 
intside this a wall of elastic and fibrous connective tissue in 
which is embedded a zone of circular muscle-fibres. These structures 
fie all derived from the adjacent coelomic sacs. The muscles of 
blood vessels do not contract rhythmically and spontaneonsly like 
ihosi! of the heart, but are in a state of continued contrai-tion called 
tone, This t«ne is under the control of the nervous system through 
Ihe medium of special " vasomotor " fibres, and thus the supply of 
blood to an organ can be varied according to its needs. 

In Craniatii however, outside the definite arteries, veins and 
flBpiUaries, there exists a large portion of the haemocoel in the 

1 of irregular channels and interstices, in many cases without 
definite walls, an endothelium being found only in the larger trunks. 

I system of spaces is known as the lymphatic system. It 
eontainR a clear fluid in which amoebocytes float, but no haemo- 
|[lubin -containing cells, and at one or several points the main 
trunks of the system open into the large veins. The finer branches 
ipf the system ramify amongst all the organs of the body. There 



354 INTRODUCTION TO CBANIATA. [CHIP. 

is no circulatory onrrent in tKe lymph canals except in thoea 

belonging to the viBcera, but there are v&lves arranged so that vidi 

eTei7 contracUon of neigbbounng mnwles some flnid can pu 

foTwarda in one direction but 

not backwards. 

It will be aeon Hat in 
Oraniata, unlike Aitfaropoda 
and MoUnsca, the blood, being 
everywhere confined to veaaeli 
with definite walls, does not 
directly bathe the tiaanes of 
any organ ; bnt that materials 
most first diffuse through the 
walla of the blood- veesels into 
the lymph-spaces before they 
can reach the tissue. One 
explanatioD of the separatioti 
of the lymph-eystem from the 
blood-system is that the hae- 
moglobin is not diffused in 
the fluid of the blood, bnt is 
carried in cells which have no 
power of moTement in them- 

Flo. I9S. Dugnm ol the yeaoa» 
Bjatem of MaMtelUM aiilarelicut. 
From T. J, Pm)t«r. 

1. Orbital fiaas. 3. Hroidnn 
vein. 3. Daotiis Cn*iei. 

4. Anteiioi caidinal Tciii. 5. 
JngDlar vein. 6. Codqb arteii. 
osos. 7. Ventricle. 8. Atrium. 
9. SIddb TenoooB. 10. Hep»I)0 
Tein. 11. LiTVT. 19. Hepatic 
vein. 13. Hepatic port&l Teiii. 
14. Left cardiDal TCin. 15. Bra- 
chial Tetn. 16. Sab-claTiall 
teiD. IT. Gimmi. IS. Poa- 
lerior cardiDal reiti. 19. Sper- 
matic Tein. SO. lateral Tein. 
31. Reoal-portal niiH bom 
eaodal Tein to kidne;. 33. 
Bigfat poatnioT cardinal vein. 
33. AlimeulaiT oanaL M. Vein 
coDuei^tinf: orbital sinases. 35. 
Sub-iDtettinal vein. 36. Eidnef. 
" Pelvic tvin. 34. Cloacal 




Caodaln 



39. FemonJ n 



30. 



XIV.] 



355 



selves Did these cells enter the iTrnph-eystem they would speedily 
block ita finer channels. 

The supply of smoebocytes to both blood uid lymph is provided 
for by widely distributed actively growing nodules of cells which 
bud off amoebocytes into the adjacent lymph-chaDuels. These 
packets of cells are called lymphatic glands : the largest collection 
is in the spleen, an organ having several other functions, which is 
attached to the mesentery just dorsal to the posterior end of the 
stomach. 




Fiu. 196. Dorsal view of bead of Sct/llium caiiicula x 1, The right orbit has 
been exposed so as to ahoir the masolea that move tbe eye and the aecoad 
and fourth nervea. 

1. LeoB of the eye. 3. Superior reotua moRcle ot tbe eyeball. 8. Ei- 

ternal (or posterior) rectus muscle. 4. Inferior rectus muscle, S. In- 
ternal {or anterior) rectuE muscle. 6. Inferior oblique muscle. 
7. Superior oblique muaole ; the slender nerve eiit«riiiR this muscle is tbe 
fomrUi enuiial. 8. Second cranUl or optic nervo, the nerve of sight. 

The muscles of tlie Crani&ta like those of the Cephalochordata 
■ ate developed from the inner walls of a, aeries of dorsal 

coelomic pockets, in a word, from myotomes. Unlike 
Uiat of iha Cephalochordata the trunk coelom does not become at 
first completely divided into separate sacs, the ventral portions of 
which fuse later. In the Graniata this stage is Bkip]>ed in develop- 
ment, and the coelom appears tVom the first as a pair of elongated 
sacs undivided below, but segmented above. After the complete 

23—2 



356 INTRODUCTION TO CRANIATA. [CHAP. 

sepaxation of the dorsal portions as myotomes the ventral parts of 
the two sacs unite beneath the intestine, whilst above it their walls 
become apposed, forming the vertical sheet of tissue known as the 
mesentery, in which the intestine is slung. 

It is necessary of course for the efficient action of the eyes that 
they should be movable, and this is brought about by the space 
around the eyeball becoming converted into a cavity called the 
orbit, which in the lower Craniata is continuous with the anterior 
cardinal vein, and thus contains blood (Figs. 195 and 196). To 
each eyeball six muscles are attached, two arising from the anterior 
part of the orbit and inserted one above and one below the eyeball, 
and named respectively the superior and inferior oblique ; and 
four arising close together from the posterior comer of the orbit and 
inserted on the eyeball, one above and one below, the superior and 
inferior recti, and one antero-laterally, the internal or anterior 
rectus, and one postero-laterally, the external or posterior 
rectus. 

The proboscis and collar coelomic cavities of the Hemichordata 
are represented in the Craniata by two cavities found in the embryo 
on each side, in advance of all the myotomes, termed the head- 
cavities. The most anterior, termed the pre-mandibular, is 
joined to its fellow by a narrow canal running underneath the eyes 
— the pair really constitute a bilobed cavity — from whose walls the 
inferior oblique, superior, inferior and internal recti muscles are 
developed. 

The collar-cavities are represented by the mandibular- 
cavities, a pair of long, narrow cavities running down the sides of 
the mouth and curving up over the eye on each side. From the 
wall of this portion of the cavity the superior oblique muscle is 
derived. The external rectus muscle arises from the first myotome. 
The muscles derived from the anterior head-cavity are supplied by 
a common nerve, the third cranial ; the superior oblique is supplied 
by the fourth, and the external rectus by the sixth cranial. 

Most of the muscles which compress or expand the gill-sacs are 
derivatives of the wall of the unsegmented ventral portion of the 
coelom. From the inner wall of this part of the coelom all the 
muscles of the alimentary canal, which in Craniata are longitudinal 
as well as circular, arise, as do the muscles in the walls of the 
blood-vessels. From the myotomes are derived the muscles by 
which the locomotion of the animal as a whole is carried out. In 
the lower Craniata these have the same simple arrangement as was 



XIV.] URINO-aENITAL OSQANS. 357 

found in &e cue of Amphtams, but in the higher forma where the 
roovemente are complicated by the development of limbs, these 
muscleB are divided into numerous bundles with a very complex 
airangement. All the muscles derived from the myotomes are 
composed of striated fibres. Most of those governing the move- 
ments of the alimentary canal and blood-vessels are composed of 
smooth fibres, but to this statement the muscles of the heart form 
an exception. 




Fia. 197. Dugrama illDBtrating the development of the urino-genital argaui 

of Craniato. [For full etplaDBtiou see seotioat on Elatmobrouobii and 

Amphibia.] 
A. l>avelopment of pronephroB and Begmental duct, B. Atrophy of pio- 

nephroB, development of mesonaphroB. C. OifferentiatioD of pro- and 

meBonephric dacta. D. Development of toetanephroa, male t^pe. 

E. Female type. 1. AUantoio bladder. 4. Gonad. 6. iDteatine. 

7. HeBOnephrio duct. 8. Nephrostome. 9. Metanephrio duot. 

10. Metsnephros. 12, Ovan', 13. Oviduot. 14. Fronephroa. 

16. Archinephrto daot. 16. Testia. 

The excretory and reproductive organs are closely related 

in development, and by recent research their relation 

omna''*" ** *** those of the Cephalochordata has been made 

tolerably plain. The unit in the excretory system 

is a tube opening into the body-cavity at one end and at the other 

into a longitudinal duct which opens into the proctodaeum behind. 



358 iNTBODucrrioN to crakiata. [chap. 

If it opened to the exterior directly it would be essentially identical 
with the nephridium which constitutes the excretory organ of 
Annelida and Mollusca. Of these tubes in the Graniata there are 
two kinds: the first or pronephric tubules, called collectively the 
pronephros, develope in continuity with the duct into which they 
open. This is called the archinephric duct, or sometimes the 
Wolffian duct, after Caspar Wolff, who first saw it develope in 
the embryo. The pronephric tubules are situated at the upper and 
outer angle of the unsegmented coelom, a position exactiy corre- 
sponding to that of the nephridia of Amphioxus, except that in the 
Craniata they are not developed in the shortened branchial region 
but immediately behind it From the root of the mesentery 
opposite the inner openiDgs of the pronephric tubule a swelling 
projects into the body-cavity. It is covered by a thin layer of 
peritoneum and is richly supplied with blood-vessels by branches 
from the aorta. The cells of the peritoneum appear to extract 
water and excreta from the blood and pour it into the coelom, whence 
they are excreted by the pronephric funnels. This structure is 
termed a glomerulus. 

If the homology of the two sets of organs be accepted — and in 
view of Goodrich's researches it must be regarded as doubtful — the 
somewhat startling conclusion follows that the atrial cavity of 
Amphioxus must be the homologue of the archinephric duct This 
conclusion, however, must not be expressed in the form that the 
archinephric duct is derived from the atrial cavity, but rather that 
both seem to be developments of a primitive groove, overhung by a 
longitudinal ridge which may be called the Wolffian ridge and 
which corresponds to the atrial fold in Amphioxus, This view is 
to some extent supported by recent observations on the development 
of the archinephric duct It appears probable that this arises as a 
solid ingrowth of ectoderm, a method of development which, from 
the study of other cases where it occurs, may legitimately be regarded 
as a modified form of invagination or intucking of ectoderm. The 
exceptional development of the Wolffian ridge in Amphioxm, so as 
to form a veil over the gills and wall in the atrial cavity, is perhaps 
advantageous in a burrowing animal. Van Wijhe, to whom we owe 
the first clear account of the head cavities, is a strong supporter of 
the view of the ectodermal origin of the archinephric duct just 
mentioned. It ought however in justice to be mentioned that other 
observers do not accept this view and confidentiy assert that the 
archinephric duct is derived firom the mesoderm. They regard it as 



developed out of a series of secondary connections between the 
nepUridia juat as the ureter is formed, and similar to connections 
between the nephridia found in some Polychaeta. 

The pronephric tubules are developed only in the larval form, 
and so far as is known they persist in no adult Craniate. They 
become replaced by the second kind of tubules, termed the meso- 
aephric tubules, which, like the pronephric, open into the coulom, 




Diftgiamnintic tmnsverBe Beclion of ii hypothetionl unoeBtnl Elftsmo- 
sh to hIiow origin of tddbI and genital orgluiB. On the left side & later 



1. Ncm-cord. 3. Notoohord. S. Mjutome. 

6. Ventral nolarotome. B. Nephrotome. 7, 

6. Archinephriii duel as an open RrooTe. 9. 

gland. 10. Aorta. 11. AlimenUiy cannJ. 

coelom. 13. Mesouephric tcbuie. 11. 

in. Seminal tubule. 



I. Iiiiraal aolerotome- 

Pronephric tubale. 

Radiment of genital 

12. Uii»ei;ment«d 

Archiuephrio duct. 



but unlike these Hwell out immediately beyond this, forming thin- 
walled capsules, termed the Malpighian capsules, into which a 
glomerulus, that is a thin-wallecl plug coutainiug a plexus of 
blood vessels, projet'ts. Its function is similar to that of the 
glomenilas opposite the pronephric tubules. 

The mesonepluric tuhules are developed from the necks connect- 
ing the myotome with the general coelom, that in, from the lower 
end of the myotome, a position corresponding to the ]ioiat of origin 



■Wi jsrniuocccRMP to craniata« [chap. 

t d« ^smioaL iQaw^ ii JLjimiMfftui. These necks, which may be 

«im«i«^ :% !^u.i-v sxi mjdeis% mmL j»mt ^nt the myotomes, curve roond 

>a«fc ^^tiirrr rMKUMC^ Jtcu 'SM icrr&iKfiiiAf AucL It IS the central 

•«M.tr a tiM tts.>uj.« VBKii jwetld out to fotm the Malpighian capsule. 

^ 'M» -^vifr ><ng p»oiw i ^bac die mesonephiic tsbules are the homo- 

i^i*«0^ i an 4!t»uaii sacs of Aw^kiaru*^ and this suggestion 

«^t«i»«q^ ^(4iurt 3um the &ct that the genital cigans in the lower 

rtba«<^«jk rtftcinace from the lower ends of the ncphrotomes, just 

«a«4> .OftiM euctf the general coelom, and oonseqnently the genital 

«li(:teiifr iM :U tiTst segmented. The change of function suggested 

vo^w^ ?«Muit» at first sight a little forced, but ve must remember 

U14U lu v'iui^^^ Vertebrata, as in Annelida, the whole coelomic 

^^^il 'Mkl pcubably an excretory function, and henoe if permanent 

'.u^Maii ot temporary pores for the discharge ot genital products 

s%u>» tormed, these pores would allow the escape of excreta thrown 

Mjii, by the ueighbouring portions of the coelom, the excretory 

tuucciou of which would therefore be stimulated. 

The genital rudiments soon coalesce to form continuous ridges 
Licvjcv-'tiug into the general coelom on either side of the root of the 
lucdoiitery. lu the female the sexual cells or ova, when ripe, drop 
into the coelom and are conveyed to the exterior by an oviduct 
opouiug into the body-cavity with a wide funnel The oviduct 
luriaea from a groove in the dorsal coelomic wall, the edges of which 
uioet. Posteriorly it opens into the proctodaeunu In the male, on 
the other hand, the sexual cells arrange themselves in tubes, the 
:iOiniuiferous tubules, which retain in most Craniata a perma- 
uent connection with certain of the mesonephric tubules through a 
set of tubes called the testicular network which like the organs 
they connect are derived from the nephrotomes. The archinephric 
Uuot therefore acts as vas deferens or male duct. Since through 
the opening of the proctodaeum not only the faeces are expelled but 
also the excretory products and spermatozoa from the archinephric 
duct, and ova from the oviduct, this aperture has received the 
uame of cloaca, the Roman name for 'sewer.' 



Division I. Cyclostomata. 

Tiie C-raniata are divided into two main groups, namely, the 
DyuluMtomata and the Gnathostomata. The former division, 
(litttingiiished by the absence of true visceral arches and of jaws, 
iucliulas at the present day only a few, probably degenerate, worm- 



like animalH, with short tails like Impfiuxus, and with naked 
akina. The name Cyclostomata means Unuud mouthed (6k. kvkKo^ 
a circle ; tTro/ia, mouth) and alludes to the cmumstaDces that the 
edges of the mouth are atiffeued by a ring shaped 
cartiluge the aunulaj- cartilage, bo that the mouth 
cannot be closed (2 Fig 201) There la a piston shaped tongue i 
supported by a lingual cartilage, and the whole is protruded hj 




a miucle attached to the annular cartilage in the lips (Fig. 200). 
Both the tip of the tongue and the walls of the stomodaeum are 
Iwset with horny teeth, developed from the agglutinated cells of 
the akin. The expansion of the atomodaeum causes the mouth to 
act like a sucker, and the whole animal is thus enabled to adhere 
to some foreign body, such as a stone, or to some victim, nsually 
a fish, in which case the rasp-like tongue works a hole in the fiesh 
of tlie prey. The atomodaeum is greatly elongated and is liupported 
in ite roof by several broad cartilages, the so-caIIe<l labial carti- 
lages ; in consequence the eyes and gill-slits appear to be pushed 
very far back. 



I 



^^^^^881 CTCLOSTOXATA. 


^P 


1^ IV conditiaB of ths mfM-orgimij U 


one of the roost marked 




I ^ /^j^ ^^ 


nil! 


H 


,' 


;; jK^^ 




J ■ X " 1 


^H 






1 tll^ 


1 


1 


il 
n 


1 


-2 


1 iJ\ 

! . ^-1 

1- lis" 


^^^^H Kfl 




mSUfrj-~ 


=■ s^»a 


^^H N 


» 


r^H/-/ 5 


;- ii-n 


^^^1 Q 


I iSUl 


« n .' 


^H 1 


SMflh 


ir:ii^ 




■a- f ■ 




E„ S ..9 




fS .12 S 




t^ 111 




1 ii 


^^^^^^^^^H \ nCr% Ji^or '^ 


^ l-tf 


^^^^^H '^ \ «u^ 


i I'i^sl 




£ 


^^^^^^^^ 




■^ single sac placed far back in coD8e<iuence of the elongation of the 


K Btomodaeam, as above explained. This 


single Bac is diawit «i» 



B-OBGANS AND GILL-SLITS. 

into a loDg tube passing beneath the brain, and in one Order, 
the Myxinidae or Hag-fisbes, this opens into the roof of the i 
atomodaeum. The tube-like prolongattOQ is really the pituitary 
body, which in the embryo developes close to the nasal sai-. The 
groove counectiDg the tivo organs becomes closed so as to form a 
canal, and then by the great development of the suctorial mouth 
the external openings of the two orgiius are widely removed from 
one another. 

The eye developes no proper cornea or aijueous humour, the 
lens remaining in connection vitb the skin. The ear is represented 
either by two semicircular canals and a vestibule (or sacculus) 
in the Lamprey, or by a single membranous tube in the Hag-fish. 

The gill-slits, usnally aeven in number, have the form of regular 




1. Honiy teeth. 2. Annular cartilaga. 8. Anterior Iftbial curtiU^. 

4, FoBlerior Isbiftl cHrtilaire. 5. Naaal capEnls. H. Auditory aapsnle, 
T. Don&l portion of trabeoulne. 8. Lutenil distal labial cartilage. 

fl. Lingual flartilajK, 10. Brnnohial bikskct. 11. Cartil agin hub cup 
■upporting perionrdimn. 13. 8)ie«tb at notocbonl. 13. Anteciur 

neural orchea taeed together. 

gill-sai's, recalling those of the Hemichordata, only williout the 
tongiie-bara. The external opening is circular; the connection 
with the gullet, on the other hand, is a vertioiti slit (Pig. 200). 
The whole set of sacs is sti])ported on a framework of cartilage 
consisting of longitudinal dorsal and ventral bars, connecting c 
pieces pass between the sat^s and give otf branches encircling their 
outer openings (Pig. 2iil). The whole of the branchial basket, 
as it 18 called, is a development of the dennb and baa nothing to 
do with the visceral arches of the Craniata, as will be shown 
later. 



364 CTCLOSTOKATA. [CHAP. 

The commencemeiit of the txne alimentary canal is marked, as 
in Amphioxus, by a velum. What corresponds to the hyper- 
pharyngeal groove in that animal is in many species of Cyclostomata 
completely constricted off from the remainder of the goUet and is 
known as the oesophagus, though this word is used in a different 
sense from that in which it is used in the case of the Gnathostomata. 
The lower part of the gullet which communicates with the gill-slits 
ends blindly behind and is called the respiratory tube. 

The hinder part of the alimentary canal is a nearly straight 
tube, the spiral valve having a very slight deviation from a straight 
course. There is no dilatation of any kind in its course. The 
large liver empties its secretion by the bile-duct, which opens into 
the intestine a short distance behind the branchial region. 

The skull consists of the simplest elements, viz. the trabeculae, 
with a wide hole for the infundibulum, and the parachordals, 
forming only a slender arch over the hinder part of the brain^ but 
developing a low side wall throughout their extent with which the 
simple auditory capsule is fused. The nasal capsule is represented 
by cartilage stiffening the nasal tube. The brain is remarkable 
for having a thin membranous roof except just at the front end 
of the hind brain where a narrow band of nervous matter represents 
the cerebellum. 

The only fins present consist of a fringe of skin similar to that 
found in Amphioxus surrounding the hinder end of the body in the 
vertical plane. This fringe is divided by a notch into an anterior 
(or dorsal) and a caudal fin. The dorsal fin is supported by 
cartilaginous rays situated above the neural arches which protect 
the spinal cord ; the caudal fin has, in addition to these, rays situated 
below the haemal arches. A caudal fin of this description, which 
the notochord divides into two equal lobes, is called diphycercal. 

Besides the neural arches (13, Fig. 201) and small haemal arches 
in the tail no other cartilage is developed in connection with the 
axial skeleton, the notochord with its thick fibrous sheath persisting 
unchanged throughout life. 

The pericardium is not completely separated from the remainder 
of the body-cavity, and the genital organs take the form in both 
sexes of a single median ridge projecting into the body-cavity (21, Fig. 
200). No connection of the testis tubules with the kidney tubules 
exists, nor is there any trace of an oviduct, both ova and sperma- 
tozoa being freely shed into the body-cavity and escaping by two 
abdominal pores or simple openings in the body-wall placed 



Tentrally to the openings of tfie kidneys. loasmucli as these latter 
open directly to the exterior and are quite independent of the 
opening of the intestine, which is placed more ventrally, we may 
Btate that no cloaca has yet been developed. 

Living Cyoloatomata, represented by a single class which may 
be called Maraipobranchii (Gr. ^a'po-iTros, a pouch), 
are divided into two families : (i) Petkomyzohtidae, 
(ii) Myxinidae. 

(i) In the first family, familiarly Isniura as the Lampreys, the 
pituitary body appears a-s a blind ])rocess from the nasal sac : each 
gill-sac opens directly to the exterior, and the hyperpharyngeal 
groove is separated fmm the rest of the alimentary canal aa a 
distinct tii1>e, the so-called oesophagus. 

The Lampreys {Petromi/zon) are conspifuous in the early spring, 
when they ascend small brooks to spawn. Several sjwcies inhabit 
the rivers of Great Britain, Canada and the United States, but the 
differences between them are trifling, depending mainly on the 
development of the homy teeth covering the tongue. One species, 
Prtromt/zim marintts, attaining a much larger size than the others, 
inhabits the sea. It may reach a length of three feet, whereas the 
other forms do not grow longer than from ten to twelve inches. 
Tlie e^'gs of Lampreys develope into a moat interesting larval 
form which stands in many respects nearer to the other Craniates 
tlian does the adtdt and supplie.s an intermediate stage between 
Amphiii^iie and an ordinary Craniate. This larva is called the 
Ammocoetes, and its mode of life resembles on the whole that of 
Amphioj:u». Like that animal the Ammocoetes lives ou what the 
cturentB of water, prrjduced by the cilia inside the velum, bring. 
The thyroid gland, which, as we have seen, represents the 
endostyle, remains open, and still performs its primitive funution of 
Becreting a cord of mucus, which is c-iirriod up dorsally by a ciliated 
groove, the peripharyngeal band, situated just behind the velum. 
Tile hyjierpharyngeal groove is represented by a doraa! strip of 
ciliated cells, the current produced by which sweeps the mucus 
backward into the alimentary canal just as it does in Amphioj-ug, 

The tubular suctorial stomodaeum is represented by a hood-like 
upper lip and a distinct short under lip, and when the mouth is 
contracted the velum is produced into tentacles just as in the 
Urochordata and in AmpAioaiiB. The lateral eyes are exceedingly 
rudimentary, but there is a large pineal eye, and the nasal sac 
has a median septum. 



rifti 



366 ONATHOSTOMATA. [CHAP. 

(ii) The Myxinidae are characterued by the persistent connectioii 
of the pituitary body with the stomodaeam, so that there is a tabe 
leading from the nasal sac to the moutL There are eight tentacles 
called barbels at the sides of the mouth, and there is no special 
oesophagus distinct from the rest of the gullet. The skin has a 
double series of mucous glands placed at the sides of the body, and 
so much mucus can be thrown out that a large amount of water can 
be rendered semi-solid. The intestine has no spiral valve. The 
Myxinidae are the animals known as Hag-fish. They adhere to fish 
on whose flesh they feed, but, uulike the Lampreys, they can 
actually burrow into their victims so that the stomodaeal region is 
completely buried. In connection with these habits the stomodaeal 
region is enormously elongated, and the eyes remain in a rudimmitaiy 
condition, whilst the gill openiogs are pushed very far back. 

The Myxinoids include two genera, BdeUostoma and Myaine, 
In the former, which is a genus inhabiting the southern Atlantic 
and Indian Oceans, the gill-sacs are seven in number on each side 
and open separately ; in Myxine, on the other hand, each external 
opening of the six gill-sacs is drawn out into a long tube, and the 
tubes of each side curve back and unite to open by a common 
atrial pore placed so far back that the animal can insert almost half 
its length into the body of its victim without interfering vrith its 
breathing. The portal vein is rhythmically contractile. Myxine 
is common on both the Atlantic and Pacific coasts of North 
America and on the European coast. 

Division II. Gnathostomata. 

The great division of the Gnathostomata includes all the remain- 
ing Craniata, and is characterised by the development of definite 
visceral arches, jaws and paired limbs. The visceral arches 
are jointed rods developed from the inner or splanchnic wall of the 
coelom ; they cannot therefore be considered as corresponding to 
the branchial basket of Cyclostomata. They are placed in the 
forms which retain gill-slits between these openings, and hence are 
often called gill-bars. The first pair of visceral arches lie in the 
sides of the mouth, and consist on each side of two pieces, hinged 
on one another and called the upper and underjaws respectively. 
By the motion of these on one another the mouth can be opened 
and closed. The nose is always represented by two sacs and the 
ear has three semicircular canals. 



XIV.] CLASSIFICATION. 367 

The Gnatho8tomata are divided into five classes. In the first 
three of these the temperature of the body varies with that of the 
surrounding medium. 

Class I. Pisces. 

Gnathostomata with fins supported by fin-rays and breathing 
chiefly by gills. 

Class II. Amphibu. 

Gnathostomata with pentadactyle or five-fingered limbs and 
without fin-rays. Gills and gill-slits functional in the young but 
generally entirely lost in the adult. An amnion is not formed in 
the embryo. The skin is soft and moist. 

Chiss III. Reptilu. 

Gnathostomata with pentadactyle limbs. The young are bom 
similar to the adult and in embryonic life develope an amnion. 
Skin with horny scales. 

Class IV. AvBS. 

Gnathostomata agreeing with Reptilia in most points, but 
having a constant temperature independent of that of the surround- 
ing medium : the skin is provided with feathers instead of scales 
and the fore limb is used as a wing. 

Class V. Mammalia. 

Gnathostomata agreeing in many points with Reptilia, but 
clothed with hair instead of scales. The body, like that of Aves, 
has a constant temperature independent of that of the surrounding 
medium. The young are nourished after birth by the secretion 
of certain glands of the mother termed milk glands or mammary 
glands. 



368 



CHAPTER XV. 
Sub-Phylum IV. Craniata. 

Class L PiscBSL 

The class Pisces, or true Fishes, are not, as many would imagine, 
characterized by their gills (since some Amphibia 
retain these throughout life), but by their Hns. In 
addition to the vertical flap of skin with which we have become 
acquainted in the case of the Cephalochordata and the Gyclostomata, 
we find typically two pairs of lateral flaps, an anterior pair called 
the pectoral fins, and a posterior pair known as the pelvic fins 
(Figs. 205 and 206). Both from a study of their development and 
their condition in the oldest fishes, it is believed that the paired fins 
are derived from the division of two originally continuous lateral 
flaps, of which the intermediate portions have disappeared. In the 
embryo the remains of these ridges are known as the Wolffian 
ridges, which can be with some probability identified with the 
flaps overhanging the groove that, as is held by some, becomes 
converted into the primitive kidney duct. If this be so, we have 
representatives of the lateral fins in the walls of the atrial cavity of 
Amphioxus, of which the rudimentary folds known as the meta- 
pleural folds form part If we accept this view it follows, since 
Gyclostomata possess a kidney duct, that they once possessed either 
a continuous lateral fin or the two pairs possessed by modem fishes. 

While the possession of paired fins discriminates Pisces from 
the lower Vertebrata, the forms of these members equally sharply 
mark Pisces off from the class with which they are most nearly 
allied, namely. Amphibia. In all Pisces the limb or fin is a blade- 
like organ which never exhibits the slightest resemblance to the 
typical form familiar to all in the human limb, but Amphibia 
have as the representative of the paired fins limbs in which the 



Ian of the human arm and leg can be at once recognised. The 
lade-like type of fin ia known as the ichthyopterygium (ix^vt, 

fish; TTTfpvytov, a little wing), the other type of limb aa the 

leirnpterygium (x"'pi the band). Piacea therefore are detioed 

' the poeseasioQ of iehthyopterygia. 
The m&diau and the paired fins are stretched on a skeleton with 

two-fold origio, (i) a aeries of borny or cartilaginoua rods or 
iterygiophores which auppiirt the baaal part of the fins, and 
!) a series of homy fibres or bony dermal fin-rays which support 

le distal part of the fioa. In moat Orders of fishes these are not 
xternally diacernible aa they are covered by muscles and akin, but 
1 the median fins of the Teleostiimi the former series have sunk 
ito the body, and the dermal fin-rays being covered with a thin 
'anslucent skiu without scales become more or less apparent. 

The class Pisces is divisible into four Orders, namely, the 
ilaamobranchii, the Holocepbali, the Dipnoi and the Teleo- 
tomi. These Orders can also be distinguished in the case of fossil 

sh, though the differences become less marked as we proceed back- 
pard in time, and many indications point to the conclusioo that the 
lon ancestors of the four Orders would, if we could examine 
hem, be classed as Etasmobranchii. Hence the Elasmobranchti may 
termed the basal group of the Pisces, although modem Elasmo- 
ranchii, like all modem auimals, are ^ipecialized in many respects. 

Order I. Elasmobranchii. 

The Elasmobranchii are dii^tinguisbed (i) by not possessing any 

gill-cover or operculum, as it is called, each gill-sac 

opening separately to the surface ; (ii) by the absence 

' an air-bladder opening into the alimentary canal ; and (iii) by 

le absence of large bones in the skeleton. In addition to these 

^ative characters, they are distinguished (iv) by the possession of 

peculiar scJiIe (inito characteristic of the Order. 

This scale, tlie so-called placoid scale, consists of a httle spike 

attached to a small plate at its inner end. The plate 

Er""* consists of true bone : the sjiike of a modtticatiou of 

tme bone called dentine. Dentine is distinguished 

Dm bone by possessing no HaveD^ian canals or spaces occupied 

f blood-vessels, nor even lacunae, since the cells of the connective 

gsue, out of which it is formed, remain external to the dentine. 

heir protoplusmic processes known as dentine fibres do however 



370 ELASHOBBAHCHII. [CHAP. 

penetrate it and give rise to canale called dentinal canals. The 
core of soft connective tissue is called the dentinal pulp. 

The spike therefore may be described as a little wart of dermis 
calcified on the ontside. It pushes the ectoderm befor« it, and it 
becomes encrusted with crystals of carbonate of lime fonning the 
enamel layer (Pig. 20*2). These closely set crystals are secreted by 
the inner or basal ends of the ectoderm cells. One would natnrsll; 
expect that stnicturea like scales, which are closely arranged all ovet 
the body, would also invade the stomodaenm, which is merely a put 
of the skin. This we find to be the case, but here the scales are 
very greatly enlarged in size and changed in function ; they are the 
well-known teeth which are used for the purpose of retaining and 
lacerating prey which has been seized. The spike of the tooth is 
usually flattened and blade-like, and provided with strongly serrated 




Fio. 202. S«ctioD thtoQgh the skia of ui Elasmobraneb HbowiDg (ormfttion 
of s dermal spine. Highl; magnified. 

1. Horny lajer o( eotodenn. 2. Miilpigliian layer. 3. Colunmu cells 
of ectoderm secretiDR 4. 4. Ensmel. 5. Dentine (black). S. Dentinkl 
palp. 7. Bony bsasi plate. 8. ConnectiTe lissae. 

edges. Fusions of several teeth can occur. The teeth are developed 
in a deep fold of skin, part of the stomodaeum, situated just inside 
the lower jaw, and usually speaking only the outermost row are in 
use at one time, the skin working forward the next set as each row 
wears out. 

The skull is much better developed than is the case in the 
Cyctostomes. lu the cranium the parachordals and 

^''""' trabeculae give rise to a firm continuous plate, in 

which the pituitary fossa is reduced to a minute hole ; there b a 



bigb and wcU-developed side wall and the roof extfiiids a long 
distance forward. The sense-tap sulea, nasal and auditory, are well 
developed and tirmly united with the cranium, 'i'he eyes are 1arg« 
;»nd highly developed, and the aide wall of the cranium is indented 
to make room for the spacious orbits in whi«h the eyes move. There 
W a considerable part of the head in front of the brain, which uHually 
■Iso projects in ^nt of the mouth. Thi.s is the rostrum or snont, 
And it is supported by three cartilaginous rods, one ventral and two 
dorsal, projecting from the front end 
of the cranium. These rods are the 
forerunners of the ethmoidal region 
in other forma. In most species the 
'Opening i.if the nasal sai- is connected 
with the mouth by a groove called the 
Ofo-naaal groove (Fig- 307). There 
are nsually six gill-clefts and seven 
visceral arches in Elaamobranchs. The 
■first cleft, sometimes called the spi- 
racle, is rudimentary and in some 
cases entirely absent. On the other 
hand there is one family, the Noti- 
danidae, with two extra clefts behind, 

3 that there are in all eiglit clefts and 

ine visceral arches in this family. 

The first pair of visceral arches 
Ebnn as we liave seen the jaws. The 
upper jaw is known as the palato- 
pterygo-<[uadrate bar, a compound 
Appellation derive<l from the names of 
lihe bones by which it is represented in 
bhe higher forme; tiie ti-rm is sometimes 
ihorteoed to pterygo-ijuadr&te. The lower jaw is called Meckel's 

trtilage or mandibular bar: in front a strong ligament, the 
iD^alled elhmo-jialatino ligament, attaches the upper jaw to the skull. 
The second pair of an^hes is spoken of as the hyoid. and this too is 
divided into two portions, an upper, the hyomandibnlar, which is 
firmly cimnectcd to the cranium just below the auditory cajwule, 
and a lower, the ceratohyal (Fig. SO-i), The upper jaw is con- 
nected with the cranium either directly, by articulation with the 
brariinm iu front of the auditory region, an arrangement called 
kutostylic mid prevailing among recent HIasmobrauchs only in 




Fio. 203. Diagrnm of a Beotion 
ttirougli tlie jnvt of u Shark, 
Odtmliapid amrrieonu; show- 
luK the Bucoession oC teeth. 
From Bcjnolda. 

1. Teeth in use. a. Teeth in 
*. Carli. 
Uge i>r the jaw. S. ICooruBt- 
itig calcibcatioii of caruliige. 
6. Connective tissue " " 
toderia liniug the m 



372 ELASHOBRANCHU. [CEAT. 

the Notidanidae ; or the upper jaw has lost ita articnlating proem 
with the craninm and is instead firmly connected or slung b; 
ligament into the hyomandibular, which thos suspends t^e jaw 
from the skull. This arrangement, called hyostylic, is that seen m 
the majority of fishes. A modification, termed am phistylic,occnn 
in Cestracion, where the jaw is slung by the hyomandibular but alsD 
has acquired direct articulation with the skull behind the oibii 
The remaining visceral arches have only a muscular connection with 
the skull and are termed the branchial arches, since to their mdea 




Fto. 304. Lateral view of the akull of ■ Dogfish {SeyUium eanieuia) x j. From 
Bejmolds. 

1. Nasal capsule. 3. Rostmni. 3. laterorbitol aimal for the pftssage of 
a blood-Teese). 4. Foramen far h;aidean artery. 6. Foramen for the 
exit of the ophthalmic branches of Vth aod Tilth nerves. 6. Foramen 
throoRh which the external carotid leaves the orbit. 7. Orbilo-nowl 

foramen vhioh allowH a blood-vessel to reaoh the noBe. 8. Auditory 

capsule. 9. Foramen through vhich the external carotid enters the orbit. 
10, Ethmo- palatine ligament. 11. Fatato-pteryEO-quadrate har. 

12. Meckel's cartilage. 13. Hyomandibular. 14. Cel»to-b];al. 

16. Pharj'ngo- branchial. 16. E pi -branchial. 17. CeiAto.brsnchial. 
18. Gill-rH3's ; nearly all have been out off short for the sake of clear- 
neaa. 19. E»tra. branchial. U. III. IV. V. Va. VDa. IX, X. foratnioa 

are attached the gills. The branchial arches are jointed into several 
pieces, which are placed in an oblique position and so arranged that 
when they are raised by the levatores arcnum — muscles attaching 
them to the skull— they diverge and expand the gill-sacs lying 
between them. The segments of each branchial arch are typically 
four in number, named respectively pharyngo-branchial, epi- 
branchial, cerato-branchiat, and hypo-branchial. The first- 
named are situated in the dorsal wall of the pharynx and are 
horizontal in direction ; the epi- and cerato-branchial stifien the 
sides of the pharynx — the cerato-branchial being the main portion 



XV.] VI30BRAL ARCHES. 373 

of the arch, whilst the hj'po -branchial pieces are found in the 
veDtral wall of the pharynx and converge to unite in a median 
plate, the basi-branchial. To the ceruto- brunch iaJs are attached 
a number of thin rods of cartilage which run outwards in the wall 
of the gill-eac and are called gill-rays. Lying outside the visceral 
arches are a varying number of cartilaginous rods. Those situated 
at the sidea of the f^pe are called labial cartilages, those external 
to the hinder visceral arches extra-branchials (11), Fig. 204). 
They are equivalent to gill-rays which have become detached from 
the arches. 

The lirst gilt-slit, called the spiracle, is situated between the 
jaw and the hyoid just outside the internal ear (Fig. 208). It is 
a narrow tube, and its use in the more typical forms appears to be 
to allow vibrations to come more closely in contact with the ear, and 
in some cases to admit the wat«r for breathing. The other slits are 
really flattened sacs, the walls of which are raised up into thin folds 
richly supplied with blood-vessels, which are the true gills and are 
supported hy the gill-rays. A rudimentary gill, the pseudobranch, 
is sometimes developed ou the front wall of the spiracia No gill ia 
developed on the posterior wall of the last gill-sac. 

In EUsmobrauchs wo find, as in Cyclostomata, well -developed 

dorsal (or neural) and ventral (or haemal) archas, 
column. '* with their ends deeply embedded in the thick sheath 

of the notochurd. This sheath has been converted 
into cartilage by amoebocytes wandering into the gelatinous layer 
secreted by tlie cells of the notochord, and it is divided into 
separate pieces called centra. Between the centra the sheath 
remoina membranous, and in the middle of each centrum the 
uotOL-hord becomes very much narrowed, so that instead of being 
a uniform rod it ia like a row of beads. The haemal arches meet 
beneath in the tail, but further forward they stretch out horizon- 
tally and become jointed; ttieir outer segments are the ribs, thia 
is the ftrst apjiearance of these organs. There are usually twice as 
many neural arches as there are centra, and every alternate one is 
small and does not meet its fellow, and hence is called an inter- 
calary piece: the haemal arches are as numerous as the centra. 
The cranium, visceral arches and centra are all strengthened by a 
calcareous deposit in the ground substance of the cartilage. This 
calcihed cartilage is to lie carefiiUy distinguished from trite bone, 
represented in Klasmobranchs by the bases of the scales and t«eth. 
The primitive tail-fin of Vertebrata, as we have seen, ia a fringe 



374 ra.ASUOBRANCHU. [CHiP. 

surrounding the end of the taiL Only a smaD and narrow rem- 

nant of this persists in Etasmobranchs, the whip-like end of tbs 

tail being bent up ; beneath it there is a well-marked fin, and Uiii 

together with the remains of the primitive caudal fin constitute & 

secondary tail-fin, which is now denominated heterocercal, since 

the axial skeleton does not divide it into two equal parte (Fig. 208). 

The paired fiua are attached to hoops of cartilage (tlie limb 

arches), called respectively Uie pectoral and pelvic 

giidles, the pectoral being situated just behind the 

last gill-cleft, the pelvic just in front of the anns. The pectonl 



Fro. 205. Doi«o-l>tersl view of the pecloni girdle and fins of a Dof^ita, 
Seyllium canicula, > ]. From Reynolds. The gaps betweeo tb« ladialis 
are blackened. 

1. Hollow in the midTentral part of the pectoral girdle which iapporta the 
pericardiam. 2, Dorsal (Hcapalar portion) of pectoral girdle. 3. Mett- 
pteryginm. 4. MeBO-pterygium. 6. Pro-pteJTginm. 6. Pro-pterTgial 
radial. 7. Meso>pter>')iial radial. 8. Meta-ptei7gial radial. 9. Out- 
line of the distal part of the fin which is supported b; homy fin'raya. 

girdle extends a considerable distance up the side of the animal: 
the pelvic b little more than a transverse bar. llie fins in modeni 
Elasmobrauchs are of what is called the uniseriate type, ihat is 
to say, there is a thick jointed main axis with cardlagiaons rays 
attached only to its anterior border. Fossil Elasmobrauchs show in 
one case, PUaracantkus, a bisetiate fin with rays attached to boUi 



borders; and in another, C' ladon^tttc Ae^ a atili more primitive con- 
ditiou, where the fin is merely a lateral flap supported by parallel 
bats of cartilage. By the coalescence of these at the base the axis 
was formed, and later by the disappearance of the rays on one side, 
the imiseriate fin. 

In the pectoral tin the basal portions of some of the rays 
coalesce to form two large cartilages called propterygium and 
tnesopterygium, whilst the axis ilself is called the metaptery- 
gium. In the pelvif fin of the male the axis bears distally a 
grooved rod which is termed the clasper, and ia used in trans- 
ferring spermatozoa to the female. The axis is called the basi- 
pterygium. The distal joints of the 
rays in both pectoral and pelvic fins 
are made up of numerous small carti- 
lages called radialia. 

Tlie brain of Elasmobranchs is re- 
markable for tlie great 
development of the ol- 
factory lobes, which are in close contact 
with the nasal sac and are attached 
by a narrow stalk to the cerebrum. 
This is only imperfectly divided into 
two hemispheres and has nervous 
tissue on its roof as well as it« floor. 
The cerebellum is developed into a 
great flap which projects back and 
covers the thin roof of the medulla 
oblongata (Fig. 188). It ha» also 
lateral ontgruwths called cerebellar 
lobes. 

The alimentary canal is consider- 
ably longer than the ' body and is 
consequently folded. It has, as a matter of fact, a U-shape: 
the first limb and a jiart uf the next constitute the 
c«n»i"'"'"^ stomach, which is marked off' from the intestine by 
a constriction and a powerful development of the 
circular muscles forming a sphincter or circular muscla To the 
posterior aspect of the loop is attai^hed the prominent spleen. The 
intestine, although outwardly straight, is probably derived from a 
corkscrew coil by the adhesion of successive turns : for the " spiral 
valve " which, as we said, is merely a ventral unfolding, has a very 



I 




icj, 20C. Dorsal view of tb« 
))elvic girdle and &n> ot a 
male Dogfinh. .SeijUium d 
rula. From Beynolda. 

Peine Rinlle. 2. B 

jitervgiuiiK 3. CldBper. 

i. ItiiilialiiL. 




376 ELASMOBHANCHIL [CHAP. 

strongly marked spiral coarse. The liver opens by tiie bile-duct 
into the beginning of the intestine, and close to its opening k 
situated that of the duct of the pancreas. A small gland of unknown 
function, the rectal gland, opens into the hinder end of the 
intestine. 

The pericardium is almost completely separated from the rest of 
the coelom, communicating only by two narrow holes with it. Hbt 
heart has the tjrpical structure described in the last chapter (see 
p. 349). In the conus there are at least two transverse rows of 
pocket- valves, occasionally more. The arterial arches arising from 
the ventral aorta run up between successive-gill sacs and break 
up into capillaries on the surface of the gills : from these the blood 
is collected by vessels in the form of loops completely surrounding 
the gill-sacs. From these loops four pairs of epibranchial vessels 
arise and run backwards in the dorsal wall of the pharynx con- 
verging to form the single dorsal aorta, which supplies blood to all 
the hinder part of the body. The last gill-sac has a gill only on 
its anterior border; the blood from this does not reach the dorsal 
aorta directly but is connected by a transverse vessel with the loop 
surrounding the preceding gill-sac. The dorsal aorta gives off on 
each side a subclavian artery to the pectoral fin and then four 
median arteries which run down through the mesentery and supply 
the alimentary canal. These are named the coeliac, anterior 
mesenteric, lieno-gastric and posterior mesenteric arteries 
respectively (Fig. 194). The most anterior, the coeliac, has two 
important branches, (1) one supplying the liver and the proximal 
part of the stomach with arterial blood, and (2) the other supplying 
the anterior part of the intestine and the pancreas. The anterior 
mesenteric artery supplies the greater part of the intestine and 
sends branches to the reproductive organs. The lieno-gastric 
supplies the posterior part of the stomach and the spleen and 
part of the pancreas. The posterior mesenteric supplies the rectal 
gland. After giving branches to the genital organs, kidneys and 
pelvic fins, the aorta continues its course into the tail as the caudal 
artery. From the two most anterior branchial loops a pair of vessels 
arise running forward in the dorsal wall of the pharynx and at the 
same time converging. These are the common carotid arteries, 
which supply blood to the head. Each divides into two main 
branches, an external carotid, which pierces the floor of the 
orbit and supplies the eye and the jaw, and an internal carotid, 
which pierces the floor of the skull near the middle line and supplies 



ABTERIES AMD VEINS. 377 

the brain. The paeudo-branch on the front wall of the spiracle 
Teceives its blood from the hyoidean artery which, branchiug 
from the loop BurroundiDg the first gill-sac, ruus forward in the 
Toof of the mouth parallel with the common carotid artery and 
eventually joins the internal carotid. In the venous system the 
anterior portion of the sub-intestinul vein is represented hy 
I pair of hepatic veins returning the blood from the liver, 
opening into the sinus venosus close to the middle line, whilst 
the posterior portion has dwindled to a small vein embedded 
between the folds of the spinil valve; this however is joined by 
branches from the sides of the intestinal wall to form the main trunk 
of the portal vein. Both anterior and posterior cardinal 
veins are represented by wide, somewhat irregular spaces, E&ch 
Hiterior cardinal has an expansion called the orbital sinus which 
enrrounds the eye. The two orbital sinuses communicate by an 
interorbita,! canal tunnelled m the base of the skull The blood 
from the ventral sides of the gill-sacs and pharynx is returned to 
the Ductus Cuvieri by a pair of independent triinks called the 
jugular veins. These are each connected with the anterior cardinal 
, of its side by the hyoidean vein lying in a groove on the 
byoiuaudibular cartilage {Fig. 195). The blood from the tail is 
returned by a median caudal vein lying beneath the caudal artery 
and like it enclosed between the centra and the united ventral ends 
of the haemal ari'hes. At the level of the posterior etid of the 
Ifidneys the caudal vein divides into the two renal portal veins 
lying on the outer edges of the kidneys. These veins, as has been 
alreoily explained (see p. 353), are the hinder portions of the 
poeterior cardinal veins which break up into the renal [HDrtal system 
of capillaries. These filter amongst the kidney tubules and reunite 
DQ the inner side of the kidney to form the spacious posterior 
canlinal sinuses, as the front portions of the posterior cardinals are 
Darned. These two sinuses lying ventrally to the kidneys partly 
coalesce. Each sinus curves forwards and outwards to join the 
Ductus Cuvieri and at tliis point it is met by the large subclavian 
vein returning blood from the pectoral fin. The pelvic vein receives 
the blood from the side of the cloaca by the cloacal vein and the 
blood from the pelvic fin by the femoral vein. It then opens into 
a longitudinal trunk, called the lateral vein, which runs along the 
BJde of the body beneath but parallel to the posterior cardinal vein, 
The lateral vein iu front receives the brachial vein from the ventral 
tide of the t>ectoraI fin (not to be confounded with the subclavian 




t. Left noris. 2. Moutb. 3. Fectornl Go. i 

S. Aperture of cloaca. G, Pericardia! cavity. 7. Vtotriole. 
arleriogiiE. S. Auricle. 9. SiuiiB venosus. 10. Coelomic openii 
of OTidnets. 10'. PnlcLtorni ligament. 11. Shell-gland. 12. O^ 
duct. 13. Ovar; refltcted over to the right to as to show 13, which lii 
external to the attachment of the ovary. 14. Liver. 15, Proximal ' 
limb of stomach. 16. Distal limb of stoniach. 17. Intestine. 

18. Reotam. 19. Spleen. 2Q. Pancreas. 21. Pancreatic duct. 
22. Bile-dad. 23. Dorsal fio. 04. Spinal cord. 26. Noto- 

ohord in centrum of vertebra. 36. Oaudal artery. 27, Caudal 
2S. Lateral line. 39. Myotomes. SO. Abdominal poree. a. 
patio artery. ft. Inteslinul branch of anterior mesenterio artery. 

c. Lieno-gastric artery. d. OaBtric branch of lieno-fcutric artery 

{posterior gastric artery}. t. Splenic brancli of lieno. gastric arterji 
/. Portal vein. 3. InteHtiniil vein. ft. Splenic veic 

from the dorsal side of the same organ) and then opens into the 
Ductus Cavieri. Tlie ckiacal veins farther give off median branches 
which tmite and then distril>ute blood to the viscera, so that some 
blood from the pelvic fin may also return to the heart through a 
portal system. 

The ovary is a single ridge of the dorsal coelomic wall; the 
oviducts are long and united far iu front so as to open by 
common internal opening, situated ventral to the liver (Fig. 207). 
In the middle of its length each oviduct baa an enlargement caused 
by a thickening of its walls due to the development of gland 
cells. This is called the oviducal gland, and its function is to 
secrete the pillow-shaped elastic egg-shell. In all cases a consider- 
able amount of development take^ place before the egg is laid : in 
many cases development goes so far that the egg-sbell is absorbed, 
and the embryo takes in nutriment from the wall of the oviduct 
so reaching a very large size before birth. The egg is large and 
well charged with yolk. The oviducts unit* posteriorly to open into 
the proetodaenm or cloaca behind the anus. There are two large 
tcsUs, and those are united anteriorly and connected to the front 
end of each of the kidneys, which e.ttend along the entire length of 
the abdominal coelom. The anterior region or mesouephros (for 
no pronephros is developed) is narrow and its excretory function haa 
almost disappeared. The testis is connected with the front end of 
the meaonephros by vasa efferentia uniting into a single coiled 
tube or epididymis, which structures are derived from mesonephrio 
tubules- The arcbinephric duct ha.s also lost its original function 
and become a vas deferens, which lies on the ventral surface of the 
kidney and conveys spermatozoa from the epididymis to the cloaca. 
It enluges at iu hind end into a vesicula seminalia, The 



Fio. 208, ScjfUiHTB eanienla J. View of vi 



a from the right side. 



1. Mnath. 2. Spiracle. 3. Qill-sUta. 1. Oall-bladder. 5. Oeaophagua. 
C. Pectoral &d out oft. 7- Veeioula flemiualis lying on melapbros. 

8. Tratig. 9. Anterior domal (iu. ID. Pnsterior dorejil &n. 

11. Meditui Tsntrsl fin. 12, Dorsal lobe of caadal fin. IU. Ventral 
lube of caudal &□. H. Bight lobe of liiei. 15. Proiimsl limb ot 

, Btomaoh. 16. Diatal limb of stomaoh. 17. luteHtine. IS. Reatum. 
IB. Spleen. 20. Pancrons. 31. RbcHI gland. 22. Bile-duol. 
33. Clagpere. 24. Lignment carrying the vasa tfferentio. 2S, Vaa 
deferens. a. . Coeliac nrter;. b. Hepatic arter;. c. Anterior gastric 
briery. d. Pancreatic branch of the coeliac artery. r. Anterior 

mesenterSo artery. /. LieOQ-gastric arlerj. g. Poaterior meaenterio 
artery. h. Splenic artery and vein. j. Puatetior mesenteric artery. 
k. Portal vcio. I. Intestinal vein. 

I»gterior and fiinctioDo.! part, of the Iddiiey ia the metanephros, 
sad ita tubules unite iuto about six niaiu ducts, which converge to 
form a [iiet»Dephric duct or ureter. There ia also a blind sperm 
sac into whose posterior end the veeicula seminalis opens and which 
'immediately after receives the ureter. The compound duct thus 
formed meets its fellow in the middle line and so there is a single 
uriiingenital sinus which opens into the cloaca behind the anus. 
In the female the mesouephros is more vestigial than in the male 
and itA duct (archinephric duct) is in &ont a very fine tube which 
lower down dilates and meets its fellow to form a median urinary 
sinus. This receives the ducts from the metanephros, and opens 
into the cloaca behind the oviduct. 

Actual sexual congress or copulation takes place in the Elasmo- 
brauchs; the most posterior rays of the pelvic fins called the 
claspers are enlat^ed, and used to distend the cloaca of the female 
to allow of the entrance of spermatoEoa (Fig. 208). This is cor- 
tL-latiHl with the large size and small number of the eggs and their 
long retention in the oviduct. In the mate the spermatozoa are stored 
in a swollen portion of the vas deferens, the vesicula seminalis, or 
ill special pouches termed the sperm -sacs. It is probable that the 
clasiter-, the large eggs and the division of the kidney into two parte 
are specializations peculiar to modern Elasmobrancha. 

The Elasmobranchs are the Sharks, Dog-fish, Skates and Rays 
of our seas. They are almost exclusively marine and are a group 
timch detested by tishenuen, since they are excessively voracious 
*and their flesh is of little value. 

They are divided into two sub-orders, the Selochoidei and the 
fiatoidei. The first consists of powerful swimmers with cylindrical 
bodies, well-developed tail-fins and moderate pectoral tins ; the 
Utter ore ground fish with broad backs and bellies and narrow sides. 



whip-like taiU with rudiment&ry tail-Qj), and enormous pectoral &as 
extending forward to the extreme end of the snout. 




Flu. aiW. A. SriilUum canicula. Rediicod. From Pay. 
B. Egg-ease opened lo dio« soan^ ombrso wiih yolk sac. 



CT^SSIKICATIOS. 

The Sbl&choidei are known as dog-fiahes or sharka, according to 
their size. The common Englisli dog-Sab, Srylliu 
canioulit, is about two feet long (Piga, 207, 208, and 
H)9); another kind, the Spiny dog-fish, Squalus ucunlhia», ia 
itinguiahed by haviog a spine, a greatly enlarged Bcale, in front 
vf each of the two dorsal fina, This latter genua ia very common 
the Atlantic coast of North America, where it ia known aa 
he Spiny-dog, The American Smooth dog-fiah. Galea* canis, is 
listingmshed from Scyllium by being viviparous. Amongst the 
iharks the most remarkable are Zygaena, the Hammerhead, in 
irhich the roofs and fioora of the orbits are produced outwarda, 
that the eyes are set as it were on peduncles; and Carchiroilon, 
ihe great White shark, which has lost its spiracles and poaseases 
tail-fin with crescentic under lobe. Owing to their powerful 
Iwimming capacities, sharks are as a rule not limited in distribu- 
Carchnrodon is the dreaded man-eater of the Adriatic and 
ihe warmer seas everywhere. Z;igarna occasionally carries terror 
uto the bay of Naples, and apeeic^s of both genera are found ulT the 
.merican coast. The Notidanidae are a family with many in- 
teresting traits. They poaaeaa one (Huxniirhtig) or two (fleptaiicAus) 
txtra gill-clefts, and the upper jaw directly articulatea with the skull 
Khind the orbit. Teeth of the same cbaracter as those borne by 
iring representatives of this family have been found in tlie Lias 
ibales of England. The Port Jaeksoii shark of Australia, Cmtradoii. 
H the sole aurviviug ty[)e of another family, representative a of which 
tre common in the Coal Measures. In it the snout is reduced so 
ihat the mouth ia thrust forward and the jaw is attached to the 
ikull in front of the orbit. The teeth are flat and pavement-like 
id adapted for cruabing the Molluscs on which the auitnal feeds. 

The BatoHiei or Rays are, aa we have said, ground feeders. All 
ive the true gill openings on the underside of the body: the 
liracle alone opens on the dorsal side and is enlarged. It has in 
Sftct in this group taken on the function of pumping water into 
^e pharynx, a duty which cannot be conveniently uuderbaken by 
ihe mouth when this is burrowing in the mud at the bottom. Ruia 
the common akate on both sides of the Atlantic : it has no caudal 
in but two dorsals. Turpi-dn is distinguished by a more elongated 
lody. The muscles on either side of the head are converted into 
electric organs, consisting of batteries of vertical hexagonal tubes 
llled with a clear gelutiuous fluid, each tube representing a meta- 
lorphoeed uiuscle-hbre. By means of these organs it con inflict 




Tha pecUifiU fins are joined in front of the anout. Prinlia 
saw-Gsh. It has an immensely elongated rostrum, at the aides ( 
which lai^e pointed teeth are set; the body is elongated, but i 



ahows all the essential features of the Bntoidei. The teeth in the 
month, like those of other Batoidei, are flattened. Printis is found 
both in the Mediterranean and Caribbean Seaa and elsewhere. In 
some of the extinct representatives of the family the upper jaw ia 
Attached to the cranium behind the orbit. This variation in the 
pUh.'e of attachment indicates tliat the connection between the two 
structures is secondary. 

The two most interesting fossil representatives of the groups 
are C^gdotelacis and Pleiiracuulhoif whose fins are desi'ribed above 
(p. 375). "" 

Order II. Holocephali. 
The second Order of Pisces, the Holocephali, differ from Elasmo- 
knchs chiefly in the skeleton ; in the viscera they resemble them 
ly closely. The Holocephali are distinguished by having the 




Fia. ail. SkoU of a u>le Chi 
KkuJ wpsole. 



Hubrecht. 



Cartilae>Dous appendage to the ethmoid regioD, 
n of ElaamobrikDcliii. 3. Breotile ikpp«iid>{te. 

Formara by whioh the ophthsJmio nervea lesve the orbit. 5. Forunen 
by «hich the opbthalmia branch of the Vtb nerve enters the orbit, 
ft. Auditory eapaiile. 7, Interorbitn.) septum. 8. Meckel's CRrtiloge 
KrtimlKtiDic with oil ontgronth From the poBterJor part ut tliu palftto- 
pterjRO-quadmte cartilage. 9. Teeth. 10. Lsblal cartilage, 11. 
III. V. VII. IX. X. foramina for the pasaage of cruoial nerves. 

Upper j»w completely confluent with the cranium, a condition called 

mo tostylic (Fig. 211) : the orbits are so deeply indented that the 

1 ii pressed hack from between them, and tlieir two cuvities are 

ily separated by a vertical plate of cartilage, called the inter- 

25 



HOLOCEPHALL 

orbital septum. There is no spiracle and the Uet gill-cleft is ■!» 
oloeed. A fold of skin, called the operoulam. extends back over 
the gill-slits. The gills are, however, still borne on the valla of 
sacs. 'I'he aaout or praeoral part uf the body is aiui^b rednc«d in 
sixe and supported only by a siugle rod of cartilage. 

The scales bave almost entirely disappeared and are reprcseoted 
otUy by the great spine, tbe su-caited icthyudorulite, irhiek 
stitTens tbe front edge <^ 
the dorsal tin, by the teetil 
and by the prickles on > 
peculiar tentacle situatol 
on the snout of the male. 
The teetb are confluent, 
FonuiDg ridges of dentine 
covered with enamel. OF 
these there are a pntr in 
the lower jaw, called den- 
tary plates, and two paits 
in the upper, termed vo- 
merine and palatine 
plates respectively, placed 
one behind the other. Each 
plate has certain areas, 
where the dentine is espe- 
cially thickened, called 
tritors. The arrangement 
of these tritoiB is used in 
classifying tbe fossil spe- 
cies. Tbe ]>eculiar tentacle 
on the head of the male 
arises from a pit situated 
in the middle line of the 
Buout^ and bears sharp 
tooth-like scales at its tip. 
The QotiK-hordnl sheath 
is not broken up into 
centra, but in CMmaera it 
has developed nitbin it a 
large number of calcified 

„„,„,, , rings, three to five times 

Fio. 213. Chimaera moTutrota, L. °' , , 

,, . , „ , , as numerous as the neural 

Male with pTMBsH on snout. B«dueea, , 

aicnes. 




The Holocephafi were once a niimerons group ; now they are 
represented by three closely allied genera, of which the best known 
is Chimatra, sfiuietimes called the Rabbit-fiah, common to the 
Mediterranean and bi:i the Atlantic coust of Europe and A&ica. 
C. mofistrosa ie found on the East coast of N. America. On the 
Pai'ifio coast C. colki occurs in such Euml(ers aa to be a serious 
naifiitnce to fishermen. It eata the baits off their tines. It ie 
known as the Rat-fish, in allusion to the shape of the tail. Callo- 
rkjfttckaa ocours in the temjierate waters of the Southern Hemi- 
q»hffl«5. The third genus, IlaiTtolta, is a deep-sea form. 

Order III. Dipnoi. 

The third Order of Fishes, the Dipnoi, are very interesting 
animals, inasmuch as they afford suggestions as to how land animals 
were evolved from fishes, They are distinguished by possessing true 
lungs, in the form i>f oqo or two sacs opening by a common lube 
into the ventral side of the oesophagus. The blood is supplied to 




the Kings by vessels given off from the last two paiia of efferent gill- 
arteries (Fij;. 216), and it is returned not into the general circula- 
tion, but direct to the heart, where it oj)enx into a special section 
of the atrium, the left auricle, cut off from the rest by a septum. 
Afl in the Holocephali, the upper jiiw is fused with the cranium. 
There are four or five gill-clefts and no spiracle. There is a large 
gill-cover which completely covers the clefts; it is strengthened in 
Crratftt/ug by two strong membrane- hones, tiie so-called squamosal 
or pre-operculum, and the operculum; there is also a third 
smaller bone called the inter-operculum. The oro-nasal groove 
has become closed so as to form a canal, the end of which opens 
vithin the stomodaeum as the posterior naris or choana. Owing 
to the forward growth of the jaws the mouth has become terminal. 

Tlie cranium is not narrowed between the orbits, aud the cartilage 
behind is replaced by true boue. there being two exoccipital 



k 



bonsB at etch aide of the hole called the foramen ma^aum bm 
which the spinal cord isaues. Above the foramen magnnm tin 
craaium is quite obviously coinjioGed of fused neural and veaUil 
arches ; the spiuee of the former and the riba of the latter arc quite 
distinct. As in Chimagra the teeth have coalesced to form grett 
deutary ridges in the lower jaw and in the roof of the moath, 
the fio-called vomerine and i>alatine plates, the timt named being 
anterior (Fig. 214). 

It will be observed that iu the Dipnoi we for the first time meet 
with large bones, and it is instructive to notice under what ciTcam- 
stances they appear. The exoccipital bones are quite distinct &om 



CkrtilBginoufl put ot tlie qnBilrate with nhioh tha nuadible utieoUl*, 
8. a, 4, Booting membnuie-lKioBH. 3. Nares. 6. Orbit. T. Pn- 
opercular (a(|UBmoBftl). 8. Second rib. 9. First rSb. 10. Vonnriiii 
deoUl piste. 11. Palatine dental pUte. 13. Palato-pletTguii 

13. Purasphenoid. 14. Inleropcruuiar. 



the calcifications of the cranium met with in the 1 

In the latter case there was hardening of tha 
Bmi"'"""" "' ground substance of the cartilage; here the bone 
is formed in the membrane surrounding the caitiUgt 
and eats into and destroys the cartilage. This sort of bane is Cne 
cartilage-bono. Besides such bones, however, we have m^l 
which are traceable to the fusion of the suiull bony bases of ^ 
typical scales which we found in Elaamobranchs. Theee Uttle bonj 
plates fuse together to form large structures which are then tsotd 



XV.] SKELETON. 

m em brane- bones. A lirat trace of this 
process is seen in the fusion of the teeth 
to form compound plates, as in the Holo- 
cephali. 

The Dipnoi are eel-like tish with elon- 
gated whip-like paired fins, and they are 
covered all over with thin rounded cycloid 
scales. These scales are equivalent to the 
enlarged bases of the scales of Elasmo- 
branchs without the apikes. On the upper 
side of the head these scales have joined 
to form median and lateral bones ; in 
front near the nasal sacs there are two 
smaller bones. The Dipnoi are quite 
peculiar among Craniata in having an 
unpaired series of large roofing bones on 
the top of the head. The palatine dental 
plates are supported by a bone which sur- 
ronnds and replaces the cartilaginous upper 
jaw,and is called the palato-pterygoid. 
The roof of the month is sheathed by a great 
plate of bone called the parasphenoid, 
derived from the bases of vanirfied teeth 
(Fig. 211). Beside these there is a 
large membrane bone outside the pec- 
toral girdle, which is the first trace of a 
collar bone or clavicle, and the two 
bones already mentioned in the opercular 
flap. The sheath of the notochord is 
converted into cartilage, but is not di- 
vided into centra. 

The pured fins are remarkable for 
having a long jointed axis and two rows 
of rays; they are in a word biseriate 
(Fig. 215) like those of the extinct Elas- 



Fio. 21S. Lateral view of the skeleton of Ceratodia mioltp'u. After Oiinther. 

I, 3, S. Hoofing membrane-boneB. 4. Cartilaginous posterior part of oranium. 
E. Pre-operoDlar (squamosal). 6. Opercular. 7. SuborbitaL 

6. Orbit. 9. Pectoral girdle. 10. Proximal cartilage of peotoral Gn. 
II. Pectoral &□. 13. PeWic girdle. 13. Pelvic Sn. 14. Spinal 
ooloniD. IS. Candal fin (diphyoercal). 



390 



DIPNOL 



[chap. 



mobranch Pleuracanthus. A fin of this type is called an archi- 
pterygium. The tail-fin is of the primitiye type found in Amp^ 
oxus and Cyclostomata, in which the fringe of skin supported by the 

fin-ray is equally developed 
above and below the noto- 
chord. It is in fact a diphy- 
cercal tail. 

As was to be expected, 
the blood-system has under- 
gone interesting modifica- 
tions. The conus arteriosus 
is long, and as in Elasmo- 
branchs and Holocephali hu 
several transverse rows of 
pocket valves. From its 
anterior end four arteries 
are given ofi* in a bunch on 
each side to the gill arteries 
to supply the gills : there is 
no ventral aorta. From the 
last eflferent vessel on each 
the artery going to the lungs 
arises. An oblique septum 
divides the cavity of the 
conus into two in such a 
way as to cut ofi* the open- 
ings of the last gill arteries 
from the front ones, so that 
the blood passing to the 
lungs does not mingle with 
that going on to the head. 
Thia is very like the ar- 
rangement found in Am- 
phibia. The likeness to the 
Amphibian blood-system is 
increased by the presence 
of a median *Weua cava" 
which returns the blood fix)m one kidney directly to the heart. One 
posterior cardinal vein persists, the other has atrophied except at its 
origin from the kidney (Fig. 217). 

The lungs are long, wide sacs, extending between the intestine 




17 

Fig. 216. Diagram of the arterial arches 
of CeratoduSf viewed from the venlral 
side. 

I. II. in. IV. V. VI. First to sixth arterial 
arches. 7. Gills. 8. Epibranchial. 
10. Anterior carotid. 11. Posterior 

carotid. 17. Dorsal aorta. 19. Pul- 
monary. 24. Coeliac. 



XV.] 



CLASSIFICATION. 



391 



Classification. 



and notochord, although their opening into the gullet is ventral. 
The gill-arches and gills are on the other hand very small, and 
the opening between the gill-cover and the body is narrow. 

Only three species of Lung-fish are still living, but the group 
has very many fossil representatives. The Australian lung-fish, 
Ceratodus /orsteri, has well-developed paired fins (Fig. 215). It 

inhabits rivers which at 

certain seasons become 
foetid with decaying vegetation, and 
during this time it breathes air. The 
African and South American lung- 
fishes {Protopterus annectens and 
Lepidosiren paradoxa) have whip-like 
paired fins consisting of little more 
than the axis of the limb skeleton 
(Fig. 213). They bury themselves in 
mud during the dry season, a necessary 
precaution, since they inhabit swamps 
which dry up. Lepidosiren, the South 
American form, has been shown to have 
larvae witli long feathery external gills, 
strikingly recalling the larvae of Amphi- 
bians. The young Protopterus has 
similar structures and retains traces of 
its external gills throughout life. Cera- 
iodus, on the other hand, has a develop- 
ment practically completed within the 
egg-shelL The fossils referable to this 
order are very interesting. They occur 
in a great variety of forms. Some of 
them — referred to a Sub-order called 
the Arthrodira — having not only the 
head but also the anterior part of the 
trunk clothed in great bony plates. 
The head skeleton articulated with the 
trunk skeleton by ball-socket joints. 

The occurrence of Dipnoi in great 
numbers in the rocks immediately pre- 
ceding in the geological series those in which the first remains of 
Amphibia are found is very suggestive. 




Fia. 217. Diagram to show ar- 
rangement of the principal 
veins in a Dipnoan. 

1. Sinus venosus — graduaUy 
disappearing in the higher 
forms. 2. Ductus Cuvieri 
= superior vena cava. 3. In- 
ternal jugular= anterior car- 
dinal sinus. 4. External 
jugular = sub-branchial. 5. 
Subclavian. 6. Posterior 
cardinal, front part = venae 
azygos and hemiazygos. 7. 
Inferior vena cava. 8. Benal 
portal = partly hinder portion 
of posterior cardinal. 9. Cau- 
dal. The hepatic portal 
system is omitted. 



392 TELEOSTOMI. [CHAP. 



Order IV. Teleostomi. 

The fourth Order of Fishes, the Teleostomi, is by far the largest 
and contains the overwhelming majority of living fishes. They 
differ irom the Dipnoi, in that the lung or air-bladder, as it is 
called, receives its blood from the dorsal aorta and returns it to the 
general circulation, so that the organ is as a rule not so mudi 
respiratory as hydrostatic. The lung is undivided and its opening 
has in most cases apparently become shifted up the side of the 
throat to the mid-dorsal line. Since the opening of the air-bladder 
is dorsal some authorities have held that it is quite a different 
organ from the lung, the opening of which is ventral. But it is 
very difficult to believe that we have to deal with two totally 
distinct organs in Polypterus and Lepidosteus, more especially as 
in both these fishes it is probable that the air-bladder subserves 
respiration. The air-bladder is never paired : if we suppose it to 
represent one lung we can imagine how the opening could be 
gradually shifted dorsally. The anus of Urochordata has, we know, 
undergone such a shifting. 

Another point of difference, distinguishing Teleostomi from the 
Dipnoi, is that there are no median membrane-bones on the head, 
all the bones being originally paired. Further, a set of membrane- 
bones bearing teeth appears in the sides of the mouth outside the 
primitive jaws, or, as we may express it, in the lips. These lip-bones 
functionally replace the true jaws, and as the mouth has now 
received its full armature, the name Teleostomi, or Perfect-mouthed 
Fish (tcXcios, perfect; oTo/ia, mouth) has been given to the Order. 
As in Elasmobranchs, the upper jaw is joined to the skull only by 
the upper half of the hyoid arch, which is ossified as the hyo- 
mandibular. There is a strongly developed gill-cover, armed 
with several bones, called the operculum. The septa between the 
gill-sacs are reduced to narrow bars, so that there are gill-clefts, 
not gill-pouches. The gills themselves develope into long triangular 
processes freely projecting and attached only at the base. Ofiben there 
is a rudimentary gill attached to the posterior aspect of the hyoid 
arch. This is called the pseudobranch or sometimes the oper- 
cular gilL This is not to be confounded with the pseudobranch 
of Elasmobranchii, which is attached to the first visceral arch. The 
opening of each nasal sac is divided into two by a bridge and there 
18 no oro-nasal groove. The cloaca is divided into two openings; 



CBOSSOPTEBYGn. 

an tuiterior, the anus, cammiini eating with the intestine, and a 
posterior, serving for the discharge of the genital products and 
excreta of the kidneys. 

It was formerly customary to divide 
the fisliea here grouped as Teleostomi into 
two Orders, the Teleoatei, or completely 
bony tish (oo-Tt'oi', ft bone), and the Ganoidei, 
or tiah with shining scales (yavo^, glitter, 
brightness). As, however, there is far more 
difference between dilTerent families in- 
cluded in the Ganoids than there is be- 
tween some Ganoids and some Teleostei, 
this arrangement is really unnatural. 
The Ganoids are in fact composed of 
widely different families, retaining certain 
priiuitive ehara^'tenstics once shared by 
all Teleoetomi. A far more rational divi- 
sion of the Teteostomi is that now usually 
adopted into Croasopterygii and Actino- 
pterygii. 

Sub-Order A. Croasopterygii. 

The Crosaoptorygii include only two 
living genera, Poli/plerii« and C'alamo- 
ichtkyi, which inhabit the rivers of Africa. 
In former geological periods, however, the 
Crossopterygii constituted an immense 
group. They are distinguished by retain- 
ing the biseriate paired fin with a shortened 
and broadened axis covered by scales. 
This scaly lobe is consequently fringed by 
the rays, whence the name (Kpoairoi, a 
fringe; vTfp\i$. wing or fin). The so-called 
air-bladder is paired and the two halves 
open on the underside of the pharynx. 
The spiracle still persists, having a special 
little gill-cover strengthened by several 
Bmall bones. It has recently been proved 
that the air-bladder is a respiratory organ 
and tliat the expired air escapes by the 
epirocle. 




394 TELEOSTOML [CHAP. 

The whole body is covered with lozenge-shaped scales covered 
by a thick layer of shining enamel, and the dorsal fin is subdivided 
into a large number of finlets, each supported by a large stiff ossified 
fin-ray (Fig. 218). The head is covered with numerous membrane- 
bones arranged in pairs, replacing the cartilage of the cranium. 
There are not only exoccipital bones, but also a basi-occipital 
situated beneath the foramen magnum, and the front wall of the 
auditory capsule is ossified by a pro-otic bone. The notochord 
is narrowed in part by the formation of bony centra ; these however 
are not, as in the Elasmobranchs, mere rings in the notochordal 
sheath, but are formed of the coalesced expanded bases of the bony 
neural and haemal arches. The term chorda-centra has been 
proposed for centra such as those of Elasmobranchii, formed by the 
segmentation of the sheath of the notochord; whilst centra such 
as those of the Crossopter}'gii are called arco -centra. The centra 
have concavities before and behind, and the space between two 
vertebrae is filled with an expanded section of the notochord, which 
in the middle of the vertebrae is reduced to a mere thread if not 
obliterated altogether. Vertebrae hollowed on each side are said 
to be amphicoelous (Gr. d/x</>t, both; koiAo?, hollow). 

It is obvious that an axis composed of vertebrae is a much more 
efficient organ of support than the flexible notochord with its loosely 
adherent neural and haemal arches. Hence it is not surprising to 
find that vertebrae have been independently developed in Fishes, 
Amphibia and Reptiles, and have had an independent origin even 
in difierent families of the same order — the Halecomorphi and 
Lepidosteidae, for example. Different investigators therefore in 

their endeavours to find exactly corresponding parts 
coTumn!*'^** in the vertebral columns of various animals have 

arrived at discrepant results. After years of research 
however Gadow has been enabled to give a consistent account 
of the evolution of the column in all the Graniata, and his account 
seems on the whole the most probable. 

According to Gadow, all arco-centrous vertebrae originate firom 
four pairs of cartilaginous pieces ; some of which may be entirely 
suppressed in some vertebrae, but all of which are represented in 
some part or other of the column. These are (1) basi-dorsals, 
the expanded bases of the neural arches; (2) basi-ventrals, the 
expanded bases of the haemal arches (to these when present the rib 
is always attached); (3) inter-dorsals, cartilaginous pieces situated 
between successive basi-dorsals, and derived from the segmentation 



ACTINOFTEBTail. 

of the apical ends of the haemal arches which have extended upwards 
roimd the notochord; {4) iater-veutrala, cartilagiiimis pieces 
pla>.'ed beoeath the notochord alternating with the basi-veiitraU, 
and derived from the segmentation of the apical ends of the 
neuT&l arches which have extended downwards round the sheath 
of tiie notochord. 

In an ancestral Craniate therefore corresponding to eat^h myotome 
there were ou each side of the notochord four cartilaginous pieces. 
These pieces In diSereut groups of animals have been variously 
combined with their successors and predecessors, the joint between 
two successive vertebrae being formed not between two pieces but 
by the absorption of cartilage in the middle of & piece at the ptaee 
where the maximum bending occurs. 

The Croasopterygii are probably in some respects more nearly 
related to the ancestors of Am])hibia than are modern Dipnoi. 
Thia view is strengthened by the fact that the air-bladder is 
occasionally used as a lung. The young have one large external 
gill attached to the operculum. As in the Dipnoi the mouth is 
terminal. 



Sub-Order B. Actinopterygii. 

The Actinopterygii are distinguished by having a uniseriate fin, 
the base of which is not covered by scales, avid by having the 
opening of the air-bladder dorsal. Among the moat primitive 
subdivision is that of the Cuondrumtei, or Stnrgeons. In these hsh 
there is a long shovel-shaped snout or prue-oral part of the head, 
which is used for shovelling up the mud at the bottom of rivers in 
search of prey. This old-fashioned feature is not found in any other 
family of the Actinopterygii, all of which have terminal mouths, 
The notochord of the Sturgeons has a thick sheath without any 
trace of centra, and the only cartilage bones in the cranium are small 
ossifications on ita side walls, called the orbitosphenoid and the 
alisphenoid bones, and a pro-otic in the front wall of the 
auditory capsule. The hyomandibular segment of the hyoid arch is 
ossified by two bones — a hyomandibular where it articulates with 
the skull, and a symplectic below where it joins the first visceral 
arch. A large part of the cranium is covered with a number of 
membrane-bones which pa.'-s insensibly into the great scutes or 
bony plat«a which cover the body. These latter, like the hones of 
the head, are derived from the fusion of scales. Though the mouth 



Fio. SIP. Acipent 
the Stargeoo. From Day, 



is often toothless, there is a 
maxilUry bone in the upper li 
the pterygoid hone which supports tiie 
pterygo<iuadrat« cartilage. A great para- 
i^phenoid Hupports the base of the sknil. 

The coinmun stuigeon Aripeiifur is 
found in the Pacific and North Atlantic, 
and enters the rivers of Europe Mid 
America. The ovary forms the RusgioD 
deUcacy known as caviare. The apcwn- 
hill Polynioii, found in the Missis.*in)i, 
hae a very broad snout and has lost 
nearly all its scutes, but it retains some 
at the sides of the tail and a series form- 
ing a comb-like fringe along the dorwil 
edge of the upjwr lobe of the tail, called 
fuK-ra, The extinct members of this 
subdivision were clothed all over with 
ganoid scales and the membrane-lHines 
on the head were fewer and more regular 
than in recent forms. 

Tlie subdivision of the H.u:,e(xh(08ph[ 
is represented at the preseut day by only 
one species, Amui ritliti, the Bow-fin of 
the St Lawrence and other American 
rivers. Like /'oti/ji/mis this species has 
Lomplet* ampliit'oelous centra formed by 
the ftiaion of the expanded bases of the 
neural and haemal arches: the craninm 
is also largely replaced by bone, not only 
behind where the foramen magnum is 
encircled by four hones, the supra-occi- 
pital, two ex-oceipitals and n basi- 
oeoipitai, hut at the sides and in the 
snout where a mesethmoid is formed. 
In addition there is a complete helmet of 
membrane-bones surrutinding the cranium 
above and at the sides, and the fiiH have 
above a pre-niaxiUa and a maxilla, 
and below a dentary, all well proTided 
with teeth. The whole body is 



XV.] ACTINOPTERYGII. 397 

with thin rounded scales. Amia is a voracious fish ahout two 
feet long. 

The next family, that of the Lepidosteidae, are the most bony 
of all fish. They are the Bill-fish or Gar-spikes of the Northern 
and Central American and Cuban lakes and rivers. The whole 
body is covered with scales like those of Polypterus^ coated with a 
thick layer of ganoiu. The jaws are long, and in both the upper 
and under lips there is a series of three or four bones bearing teeth, 
a rare condition. The skull is as bony as that of Amia, The 
vertebral centra have become opisthocoelous, that is to say, each 
centrum is convex in front, fitting into a concavity of the hinder 
surface of the one in front of it, so that the notochord has almost 
disappeared. The ovary, by the fusion of its free edge with the 
coelomic wall, has become converted into a sac, to which the funnel 
of the oviduct has become adherent, so that ovary and oviduct are 
continuous. The spiral valve on the intestine is rudimentary. 

The Gar-pike are said to lie in wait among the reeds on the 
banks of the lakes, in order to seize small animals visiting the swamp 
in their long pincer-like jaws. 

The remaining families of the Actinopterygii are grouped to- 
gether as Teleostei and have certain well-marked characters in 
common which distinguish them from the preceding families grouped 
together as the Ganoidei. The first of these characters is the 
structure of the tail. In all the Actinopterygii so far mentioned 
the tail is heterocercal, though in some, such as Amia, the ventral 
lobe is very predominant. In the Teleostean families the ventral 
lo))e forms the whole of the tail and is placed in a straight line with 
the rest of the body, the end of the notochord being sharply bent 
up almost at a right angle with the remainder and surrounded by 
a bony sheath. Such an apparently symmetrical tail-fin is called 
homocercal, and it is to be sharply distinguished from the really 
symmetrical diphycercal tail of the Dipnoi. 

Another important point in the anatomy of Teleostei is the 
structure of the heart. The conus arteriosus with its several rows 
of valves has become completely absorbed into the ventricle. Only 
the anterior row of valves remains, separating the enlarged ventricle 
from the ventral aorta, at the origin of which there is a non- 
contractile swelling, the bulbus arteriosus. 

The stomach is distinctly sac-like, the two limbs of the loop of 
the alimentary canal of which it is constituted showing a tendency 
to coalesce. 



398 TELEOSTOML [CHiF. 

The iDtefltaue has become longer and more coiled aitd hn 
completely lost its spiral ralve. Cloee to the entrance of the bile- 
dact there is a aeries of short, blnnt diTerticula of tlie intestiDe, 
called pyloric caeca. These were supposed to be a modified tnu 
pancreas, bnt the trae pancreas has recently been discovered in tlie 
form of a number of very delicate tubes intermixed with the pyloric 
caeca. Id do Teleostean, so br aa is known, does the pancreu 
form a compact gland. The ov&ry is a hollow orgMi continooiu 
with its duct; in this point, it is true, Lepidotteta has Teleostean 
characters, bnt in Teleostei the testis is also contjnnons with its 
duct, which shorn do relation to the kidney; bo tliat Id the male 




I, 2. So pn -clavicle. 3. ClaTicle. 4. Cortcoid. 5. Bckpnla. 

6, Accessory piece. 7. OMiSed radialia of the flu. 8. Dcsmal 

there is a complete departure from the normal Craniate arrange- 
ment. The cloaca has undergone more division than even in the 
primitive Actinopteiygii, for the conjoined kidney ducts have an 
opening behind and distinct from that of the united genital ducts. 

In the brain the roof of the cerebrum is a thio noD-nervous 
membrane, but the optic lobes and cerebellum are greatly devdqied. 



SKELETON, 399 

'Jthe interlacing of the fibres of the two optic nervea, which was 
lUliided to above as the optic chiasma, has entirely disappeared, 
D that the optic nerve from the left side proi'eed^ Htraiglit to the 
ight eye, crossing but not interlocking with those from the right 
ade to the left eye. 'flie eyes are exceedingly large, so that in 
learlyalt Teleost«i, as in the Holocephali, an inter-orbital eeptuni 
B formed, aloog the upper edge of which the olfactury stalks run. 

In the skeleton there is hardly a feature which is not ahared by 

(ome or other of the members of the more primitive families of 

Actinopteiygii, but in the coin bin a lion of features it is 

sharao ten Stic, and it is of such an uniform type throughout a 

Kge number of families that it merits a special description. 

The notochord is constricted by the formation of amphicoelous 
centra, which have developed four ridgea projecting 
outwards in two planes at right angles to one another. 
In the intervals between these ridges the ends of the Deural and 
haemal arches are inserted, all being converted into bone. The 
pectoral arch is represented by a small plate of cartilage on each 
Bide, in which are two bony centres, an upper scapula and a lower 
coracoid (Pig. 220). Outside this there is a strong curved mem- 
brane-bone, strengthening the hinder border of the opercular slit, 
^e clavicle. Thia is connected with the skull by additional 
bars, whilst from its lower end a pre-clavicle runs forwanl beneath 
bhegilla. Prom tu hinder edge a post- clavicle often projects back 
Isto the muscles. The anterior paired lins are attached to the girdle 
where the scapula ao<! caracoid join. In them the cartilaginons 
skeleton is almost all absorbed, the basal portions of the fin rays 
»)ming changed into true membrane-bone and articulating directly 
with the pectoral girdle. The pelvic fin has a similar structure, and 
tiie pelvic girrlle has quite disappeared. 

The cranium is completely covered with bones. Even where 
CATtilage persists it is covered with the Iwnea wluch 
iu other allied forms have displaced and absorbed the 
underlying cartilnge. The cranium, as already mentioned, is so 
•trongly compressed between the orbits that it is divided iuto two 
portions, an ethmoid region in front of the eyes and an occipital 
tegioii behind them, connected by a narrow sphenoid isthmus 
running between them. In the carttlagiuoua roof there are several 
membranous windows or " fontanelles," The anterior fontauelle 
'lies between the nasal sacs ; the posterior fontanelles are a pair of 
windows situated one at each side of the aupra-occipital bone. 



400 TELEOSTOUI. [CRAP. 

The supra-occipital, two exoccipitaU aod & b&si occipitil 
form the hindermost portion of the braiD case suirooiiding die 
foramen magnum (Fig. 321) The supra occipital, which is sita- 
ated in the mid-dorsal line has a great vertical ndge to which Uk 
powerful longitud nal muscles of the body are attached. The 
auditory capsule is completely replaced by bone above it is 




Fia 221 A donal and B ventral view of the craniam of a SmlmoD Salmo 
lala from which moat of the membnuie bones hare been removed After 
Fu-ker Cart laKe a dott«d 

1. SupmoucipiUI. '2. Epiotic. 3. Pterotic. 4. Sphenotic. 5. Frontal. 
6. Meitiun ethmoid. T. Parietal. 8. Lateral ethmoid. 9. Pan- 
Bphcnoid. 10. Vomer. 11. Eioceipital. 12. Opisthotio. 

IS. Aliflphenoid. 14. Oibitospbenoid. 16. Foramen foi p«8aB^ 

of an artery. 17. Prootic, 18. Articalar Hnrface for hyomandibolar. 
II. VII. IX. X. foramina for the passage of cranial nerres. 

covered by a pointed epiotic, in front and below by the pro- 
otic, behiud by the opisthotic, and on the out«r side by the 
pterotic with which the hyomandibuiar articulates. Od its inner 
side the capsule is only separated from the brain by membrane. In 
the inter-orbital septum there is an orbito-sphenoid in front, an 
alisphenoid behind and a sphenotic above partly extending into 
the auditory capsule, while below and behind in the base of tin 



I skull there is a Y-shaped hone, the hasi-spheooid, just m front of 



" the basi-o 



r the 



li-occipital (Fig. 292). The ethmoid region i 
eyes is the part in which most cartilage persists; it is ossified 
above by the median ethmoid and on the aides by the lateral 
ethmuids. Connected with the cartilaginous upper jaw there is a 
lai^e quadrate bone developed where it articulates with the lower 
jaw : an ectopterygoid bone replacing the cartilage on the outer 
and an entopterygoid on the inner side; lastly a metaptery- 
goid above and in front of the quadrate. The palatine hone is 
situated in front of the pterygoid and articulates with the lateral 
ethmoid. It ossifies around the anterior part of the on},'ina!]y 
Cartilaginous upper jnw. In the lower jaw we have a bone, 
'e articular, which moves on the quadrate. In the second 
"hyoid" visceral arch we find, as in al! hyoatylic lish, 
upper segment, the hyomandibular, articulating with the 
skull, and a lower segment, the ceratohyal, which supports the 
opercular flap and from which in Teleostei bony rays, called 
l>Tnncliiostegal rays, extend, on which the membranous part of 
the membrane is stretched, as an umbrella on its ribs, The hyo- 
landibular segment is formed, as in almost aU Actinopterygii, of 
two bones, an upper hyomandibular, Mmu stricto, uniting with 
the sloill. and a lower symplectic joined to the quadrate. The 
ceratohyal segment likewise is represented by three bones, a main 
ceratohyal, a small interhyal uniting the latter to the symplectic, 
aod a hypohyal extending iuwards from its lower end. The two 
liypohyals are joined by a median bono, the glossohyal. This 
mpporte a projection of the floor of the pharynx which is the 
wdtment of the tongne. The other visceral arches are eacli com- 
post of four bones, except the last, which is rudimentary. The 
nppermost segments extend inwards on the roof of the pharynx 
tud bear teeth ; they are called the superior pharyngeal bones. 
Owing to the forward slope of the visceral arches these bones are 
directly above the seventh or last pair of arches, which consist on 
each side of a small bone bearing teeth, the inferior pharyngeal 
What^jver chewing is done by fish is effected by these bones, 
the teeth in the front part of the mouth are chiefly used for 
fetaining prey. The presence of teeth on the gill-arches is not 
f ta explain. Teeth, as we have seen, are structures develo[)ed 
£roiu the dermis and ectoderm, while the gill-arches support tlie 
(raiU of the pharynx, which are certaiidy eudodermal. We must 
either sup|)ose that some portion of the ectoderm has migrated 



402 TELBOSTOMI. [CHtP. 

inwards through the gill-slite or that the endodenn has acqniied 
tlie power of producing teedi : on the whole the first suppodtian 
seems the more probable. 

The membrane-bones in the Teleostean skull are nnmeioas and 
are traceable to two sources, namely, the scales of the akin and the 
bases of the teeth. To the former category belong tJie loofiog bona 




1. Supra-occipital. 9. Epiotie. S. Plerotk. 4. Opiathotic. 

5. Eioccipiul. 6. Ban-oedpiuL T. PampkcBoid. B. Sphenotic. 
9. .Uupbenoid. 10. Orbilosphenoid. 11. Ectethmoid. 

13. Olfaclorc pit : ibr Tomerine WMfa kre Geen jan bdov. 14. Prootic. 
13. Bku«pbenoi<L 16. Foremen for the paua^ of ui uterv. 17. Ant. 
eriot fonUHlk. lA Po«lvrioi fonUneUe. L IL Y. TIL IX. X. 

Fonminm for the p43np* of cmiial nerrcs. 

of the skuU. the parietals at each side of the sapia-oecipital, 
and the great frontals which cover the antowH- fontandle, but 
vhioh al$o extend outwards and roof in the oibita. A pair of 
delicate nasal bones lie od the nasal sacs. On tbe »de of the bead 
there is a I'hain of bones suTTOunding the eje, called tbe orbital 
ring, and four Wne» stiffening the apfa pait of the opercular 
flap, a pre-operi-nlom in front, an opercnlnm abon and 
behind, a sub->.>peronlum below it, and an inler-operculnm 
betwwn the pr*-opeiviiloin and the suK^jfiwcahuiL 

Tit the fosioD i«f the baf«s of teeth w« mast ask-tibe the ot^iB of 
the ^rm: parasphenoid Kine. whkb sttfteits the roof of the Donth 
and exTeni:: Wk nnder tb« basi->wipiial. althoodi ia ma TdeoeCean 
d<>ei» it bear leeih. SimilariT. in fn<flt ><£ it thoe is tbe Tomer 
mmit <Si tw\i K>aee jtMned in tbe middle ttM^ wbicb stall bcir 



XT.] SKELETON. 403 

teeth in their anterior portions. The palatine and pterygoid 
are also traceable to the fusion of groups of teeth placed more 
laterally, but in this case the membrane-boDe has become a 
cartilage bone by extending into and replacing the c&rtilagiDous 
bar beneath it. 

The membrane-bones in the lips, which are essentially character- 
istic of the Teleostomi, have probably had a double origin— scales of 




Fia. 323, Mandibular and by aid arohea of a Cod, Oadu4 morrhiia x }. 

Palatine. 2. Ento -pterygoid. 3. Pterygoid. 4. Qaadrate. 

S. Sympleotio. 6. Meta- pterygoid. 7. Eyomaodibnlar. 8. Angnlar. 
9. Articalar. 10. Den tar;. 11. Inlerhyal. 12. Epibyal. 

13. Ceratobyal. 14. Hypohjal. 15, OloBHohyal. 16. BraDChioBtegal 



the outer skin united with teeth developed jnst inside the mouth. 
There are, in the upper lip, the pre-maxilla in front, the max- 
illa behind, and occasionally behind these a third bone, the jugal. 
As we have already seen, the number of these upper lip bones is 
greater in the Lepidosteidae. In the lower lip there are in &ont 
the dentary, behind and on the inner side the aplenial, and be- 
hind and below, the angular. The branchioategal rays mentioned 
above are membrane-bones, and between those of opposite sides 
there is a mediae bone called the basi-branchiostegal. 

TaUng a general view of the skeletons thus Car studied, we see 



404 TKLEOSTOMI. [0H4P. 

that the replftcement of cartUage by cartilage bone tends to take 
place first nherevor there i» a joiut. Thns in Di{iuoi we find the 
liinder part of the cranium rcpluced by excK-cipitali^, whereas in the 
C'hondroatei, where all the sktill and front part of th<> notochurdal 
sheath form one solid mass, uo siinh boneg are developed. Again, 
in Teleostei, the oidy repW-ement of the lower jaw by cartilage bnne 




1, Bopm^OMipitiil. a. Epiotic, 3. rtcrotip. *. Bphunotic. 5, Prubfal 
6. Madim ethmoid. 7. Purietal. 8. Mukl. 9. LbcIiitiiiiJ. 

10. Sub-orbiul. II. Sapr&-orbitaI, 13. Citrtil»eiDi>aH B«l«TDtio. 

18, Ossification in »olorotiij. li- Bnto-pterygoia, 15. Meta-pt«rj'goid. 
16. Pulatine. 17. Jugal. 19. Qnadtnte. 19. MuilU. iO. Prt- 
maiilla. 31- Alticulai'. 93. Angular. 33. Dsntary. W. Hyonuui- 
dibulnr. 35. Rymplectio. 26. Epihyal. 37. Ceratahnl. 

ae. Hypohyal, 39. aloaaohyal. 30. Opercular. 81. Snb-opvroular. 
83. Infra. oporauUr. 33. Pro-operonlar. 31. SapratempontL 

35. Branohioategal rajB. ao. BaBi-bronohiiwtegiJ. 

takes place at its proidmal end, where it joins the upper jaw. Then 
again we find the lateral ethmoid develoi«d where the front part 
of the palatine articulates with the skull, and the articulation 
of the hyouiandibiilar has probably had something to do with 
bringing about the oasiticatitm of the auditory capsule. 

With the exception of the Gaiioidei all the Teleostomi differ 
fundanientally from the Elasmobrsnchii and Holocephali in bavii 



avra^J 



XV.] DEVELOPMENT AND CLASSIFICATION. 405 

small eggs. These rapidly develope into larvae with diphycercal 

tails and other primitive features which ally modem 
lanra^ *"** fish with the aocieut fossil forms. The development 

into the adult is very slow and dangerous, and 
during this period an enormous number of the young perish. The 
habits, distribution and food of these young form one of the most 
important economical problems that zoologists of the present day 
are seeking to solve. 

Since in the animal kingdom small eggs and a prolonged larval 
life constitute the more primitive arrangement, and eggs charged 
with yolk and cared for by the parent a secondary modification, we 
conclude that in this matter the Teleostei have retained primitive 
habits. This throws some light on a question which must often 
arise in the minds of all students of zoology, namely, how it is that 
comparatively primitive forms, like the Elasmobranchs, and highly 
modified forms, like the Teleostei, exist side by side? We can 
see that in each case there has been some great modification of the 
primitive arrangement, in Elasmobranchs in the egg and young, in 
Teleostei in the skeleton and scales. It is improbable that any 
living animal preserves all the characters of the common ancestor 
of the group to which it belongs ; some traits are preserved in one 
case, others in another. 

There are some 10,000 species of fish included amongst the 
Teleostei. The limits of this work forbid us to 

Systematic. , . . - mi i 

mention more than a very few, and these will be 
chiefly the commoner food-fish. It is possible to separate a number 
of families, as Physostomi, distinguished by the two ancient char- 
acters of having the air-bladder opening into the oesophagus and 
having the pelvic fins abdominal in position, that is to say, placed 
far back near the vent. All the rest are termed Physoclisti ; in 
them the air-bladder is a closed sac having lost its connection with 
the oesophagus, and the pelvic fins have been in nearly every case 
shifted forwards and are either thoracic, that is, placed just behind 
the pectoral fins, or jugular, that is, placed in front of them. 

The families of the Physostomi which we shall mention are the 
Cat-fish (Siluridae), the Eels (Anguillidae), the Herrings (Clu- 
peidae), the Salmon and Trout family (Salmonidae) and the Carp 
family (Cyprinidae). 

The Cat-fish receive their name from the peculiar appendages 
called barbels which hang down from their upper and under lips. 
Other fish may have them on the under lip, but in the Siluridae 



TELEOSTOMX 



[cHir. 




^ iii 

3 "c » — ■ 



i-r. 

2 is 



I ''a 



llll 

i ll 

-^ till 



the maxilla bears no teetli and forms only a support for the great 
barbel& The Cat-tiahea are uainly a tropical group abounding 
in South America and Africa and nearly all are fresh-water. The 
forms with bony scutes are confined to South America, others 
are naked and without scales. These as a, rule wallow in the 
mud at the bottom of the streams they live in. The teeth are 
feeble. The skull is not narrowed between the orbita ; the anterior 
vertebrae are fused together and the sub-opercular bone is wanting. 
The first ray in the dorsal and in the pectoral fin ia replaced by a 
strong spine. Several however of the naked species are found 
in North America, two l>eing marine Sea-Oats. Iclalurug is thi- 
Whits Cat, an excellent food-fish ranging north to New England, 
trus is common in the Eastern States, and one species, the 
Bnll-head, A. nebulosus, b fonnd in the St Lawrence. 




The CypRiBibAK, including the Carp, Gudgeon, Barbel, are allied 
to the Silnridae with which they uj,Tee in having the " Weberian 
oi^an." This is a modification of the anterior ribs and vertebrae 
to fonn a movable chain of bones connecting the air-bladder and 
the internal ear and thus enabling the animal to receive impressions 
of the pressure in the bladder. The Cyprinidae differ from the 
Biluridae in having typical scales and in wanting the maxillary 
barlwl and in having a typical skull. Caras/iius awratiis, the Gold- 

, belongs here. 

The Anocillidae or Eels are long cylindrical creatures with 
either small deeply imbedded scales or none. The dorsal, ventral 
and caudal fins are contiguous, the pelvic fins being absent The 



408 TELEOSTOMI. [CHAP. 

skull resembles that of the Cat-fish in not being narrowed between 
the orbits, and in having only one or two bones in the giU- 
cover. Eels spawn in the sea. Anguilla, the common eel, ascends 
streams and even crosses wet grass to get to isolated ponds; 
it has small scales. Echelus {Conger)^ the conger eel, is entirely 
marine, and devoid of scales ; it attains a length of six feet The 
eggs of both species develope into a peculiar ribbon-shaped larva 
with colourless blood and slightly developed tail called Lepto- 
cephalus. 

The Clupeidae or Herrings are distinguished by the fSsu^t that 
their maxillae bear teeth and form part of the edge of the jaw, and 
further that the maxilla is really composed of two or three pieces 
placed end to end, recalling the condition in the Lepidosteidae. 
The tail-fin is forked. The genus Clupea, with compressed belly 
edged by projecting scales, includes not only the Herrings, which 
come in shoals to the English and Scotch and American coasts to 
spawn, but also the Shad, a high-backed herring, which frequents 
the coasts of Canada and New England and which spawns in the 
rivers. The eggs of Clupeidae, as a rule, are large and heavy and 
sink to the bottom, unlike those of most Teleostei, which float at 
the surface of the sea. The Anchovy, with a projecting snout, and 
a rounded belly, the Pilchard and the Sprat also belong to this 
family. ITie Sardine is the young Pilchard. 

The Salmonidae have a toothed maxilla and a jugal bone in 
the upper lip and a small soft fin devoid of rays behind the dorsal, 
called the adipose fin. In the female the oviduct has disappeared, 
the eggs escaping by two pores. The genus Salmo includes the 
well-known Salmon, which ascends rivers to deposit its spawn, and 
also the Brown River Trout of Europe, which is permanently 
confined to fresh-water. The brook-trout of North America belongs 
to a diiferent genus, Salvelinus, and the great King-Salmon of 
British Columbia, canned in such enormous quantities, is Oneo- 
rki/ncus tschawytscha. From a sportsman's point of view it is 
distinguished from the true Salmon by the circumstance that it 
does not take the fly. Coregonus, the White Fish of the great 
American lakes, much esteemed for its delicate flavour, also be- 
longs to this group. The habit of ascending the rivers to spawn 
probably points to the conclusion that the whole group was origin- 
ally fresh-water, and that the Salmon is a river fish which has 
taken to the sea, whereas the Eel is a salt-water fish which has 
taken to fresh-water. Whether all the Herring family were ever 



XV.] 



CLASSIFICATION. 



409 



firesh-water or not is doubtful, but it is interesting to note that 
the Pilchard has a small floating egg. 

The great division of the Physoclisti includes five main sub- 
divisions, the Anacanthini, the Acanthoptebj, the Pharyn- 
oooNATHi, the LoPHOBRANCHii and the Plectoqnathi. The 
last two groups are in some respects aberrant. 

I. The Anacanthini have all the fin-rays soft and flexible 
and the pelvic fins are shifted forward in front of the pectorals. 
The Oadidae are the Cod family and include the Cod, Haddock, 
Whiting and Pollack. These fishes spawn out at sea, one female 
cod producing as many as 9,000,000 eggs. The Pleuronectidae, or 
flat fish, also belong here. They are fish with compressed backs 
and bellies, and broad sides ; they habitually swim on one side, and 
the eye belonging to the side kept downwards is twisted on to the 
upper side which deforms the bones of the skuU. The anus is very far 
forward, the dorsal and ventral fins both being about equally long ; 
the air-bladder is absent The most valued member of this family 
in British water is the Sole, Solea^ distinguished by its elongated 
shape. Other food-fish belonging to this family are the Flounder, 
the Plaice, the Turbot, the Brill and the Dab. On the coasts of 
Europe and of North America is found the immense Halibut, 
HippoglcsstiSy which may attain a length of 6 feet and a weight of 
400 pounds. 




Fio. 227. PUuronectes platessa, the Plaice, found from the coast of France to 

Iceland. 



, 410 TELEOSTOML [CHlP. 

II. The AcANTHOPTERi are the spiny-nyed fiah in which the 
first rays of the dorsal and ventral fins are converted into bony 
dermal Bpines. Many fiunilies of most varied stroctnre are inc^aded 
in this subdivision : the tvo best known are the Scokbbidae, in- 
cluding the Mackerel and the Pekcidab or Perches. Tha first are 
distinguished by the Bmall dorsal fin 
supported by spines only, followed by 
a long dorsal fin the end of which ii 
broken into finleta. In the Percidae 
the scales have a toothed posterior 
border, that is, they are ctenoid and 
the first BpiDOUB dorsal fin is long. 
The American fish called Baas belong 
to two families, the river-bass being 
one of the Centbarcuidae, dieton- 
guished by high compressed body and 
nndivided dorsal, whilst the sea-basa 
belongs to the Serkanidab, a family 
closely allied to the Perchea 

III. Tha PHARYIfGOaifATHlhwt 

the lower pharyngeal bones firmly 
united with one another. This division 
includes the Labridag or Wrasses, dis- 
tinguished by their thick lips and 
protrasible prem&xillae ; the Triglwae 
or Gurnards, which walk on the spines 
of the pectoral fin ; the Gobudae aod 
many other small fish. 

IV. The LopaoBRANCHii are 
peculiar fish covered with rings of 
lai^e plates ; their gill processes are 

h„ b„„ morf ISStS, cl«b-sh.ped instead of tri.ngul.,, .nd 

tbe gillB. are attached in tuits to the side of the 

1. Branchial aperture. 2. Pee- clefts ; there are no pelvic fins and 

S™i,«,.ch: Tom;: thet.il-fioi.rudimenl.ry. The j... 

bones, the pterygoid and maxilla, are 

elongated so as to form a long muzzle at the end of which is the 

tiny mouth. St/ngnathtis is the Pipe-fish which moves slowly 

amongst the long green fronds of the green sea-weed Zostera, which 

it resembles, picking off minute Crustacea and Molluscs. It b 

common both on the British and American coasts. Hippocampu», 




Fio. 22a. A. The Sea-horse, 
Hippocampus 9p. B. Head 
of the B 



XV.] CLASSIFICATION. 411 

the Sea-horse, has the muzzle bent down at an angle with the rest 
of the body so as to present a whimsical resemblance to a horse's 
head. It anchors itself by curving its tail round weeds, and swims 
slowly but with dignity by means of the dorsal fin. This genus is 
found in the Mediterranean, on the southern parts of the American 
coast and elsewhere. The Lophobranchii show a peculiar mode of 
caring for the young ; the male has a brood pouch enclosed by two 
folds of skin on the underside of his body in which he carries the 
eggs until they are hatched. 

V. The Plectognathi resemble the Lophobranchii in being 
covered with plates instead of scales and in having lost the pelvic 
fins. The pelvic fins are represented by spines or are entirely 
absent. The pre-ma2dlla and hyomandibular are immovably fused 
to the skull, while the inferior pharyngeal bones remain distinct. 
In Ostracion, the trunk-fish, the plates form as compact a cuirass 
as the shell of an Echinus : the only flexible spots being around the 
articulations of the fins and the lower jaw. In Diodon and Tetro- 
don the teeth have coalesced to form great transverse ridges of 
enamel, and the dermal plates bear spines which are usually directed 
backwards, but which are erected when the body is rendered tense 
by swallowing air into the gullet and stomach. These extraordinary 
fish are confined to tropical waters ; they haunt small crevices of 
the rocks, in which, when the tide retires, very small quantities of 
water are left, and it appears that the giUs absorb oxygen fi*om the 
air they swallow. 

The Pisces are classified as follows : 
Order 1. Elasmobranohii. 

Pisces devoid of air-bladder or lung ; with placoid scales ; 
no bones developed except at the bases of these scales. 
Cartilaginous centra formed by the division of the notochordal 
sheath and not corresponding to the neural arches. The jaws 
slung to the skull by the second arch. No operculum : well- 
developed gill-sacs present. 

Sub-order (1). Selaohoidei. 

Elasmobranohii of cylindrical form with well-developed tail- 
fin and pectoral fins of moderate size. Spiracle small or absent. 

Ex. Carcharodon^ Scyllium, Acantkias. 
Sub-order (2). Batoidei. 

Elasmobranchii of flattened form, the tail whip-like and the 



412 TELEOSTOIO. [CHAP. 

taQ-fin rndimeiitaiy, the pectoral fin veiy large and joined to 
the skulL Spiracle veiy large and opening on the doial 
snrface, openings of the other gill-slits ventral. 

Ex. Rata, Trygon, 

Order 2. Holocephali. 

Pisces devoid of air-bladder or lung ; the skin naked, a 
series of slender bony rings in the onsegmented notochordal 
sheath, besides this no other bones. Upper jaw completely 
confluent with the skolL An operculum present, well-developed 
gill-sacs. 

Ex. Ckimaera. 

Order 3. Dipnoi. 

Pisces with a large lung, sometimes divided into two, 
opening by a ventrally situated glottis into the oesophagus: 
the atrium of the heart divided into two, the left division 
receiving blood from the lung only. The body is covered with 
thin flat scales. Membrane-bones covering the skull and roof 
of the moutL Cartilage bones in the upper jaw and in the 
hinder region of the skull, but the notochordal sheath is 
undivided. The upper jaw completely fused with the skulL 
An operculum present, the septa between the gill-sacs reduced 
so that they become gill-slits. 

Ex. Ceratodus, Lepidosireti, Protopterus, 

Order 4. Teleostomi. 

Pisces with an air-bladder which returns blood into the 
cardinal veins, the atrium of the heart being undivided. The 
sheath of the notochord sometimes remains undivided, but 
when centra are present they are formed by the fusion of the 
expanded bases of the neural and haemal arches, not by 
the segmentation of the sheath of the notochord. Membrane- 
bones in the skull and roof of the mouth, and in addition 
a series in both upper and lower lips bearing teeth, situated 
outside and replacing functionally the original jaws. Cartilage 
bones in the jaws and skull. The upper jaw slung to the skull 
by means of the hyoid arch. A well-developed operculum 
present : the septa between gill-sacs are so narrowed that they 
form gill-slits with long branchial processes. The cloaca divided 
into two openings. 



XV.] CLASSIFICATION. 413 

Sub-order 1. Crossopterygii. 

Teleostomi in which the pectoral and pelvic fins have the 
form of a lobe covered with scales fringed anteriorly and 
posteriorly with fin-rays. The air-bladder is bilobed and its 
opening is ventral. The scales are rhomboidal and covered 
with enamel 

Ex. Polyptertis. 

Sub-order 2. Actinopterygii. 

Teleostomi in which the paired fins bear rays only on their 
posterior borders and in which the base of the fin is never 
covered with scales. The opening of the air-bladder is dorsal. 

Division A. Ganoidei. 

Actinopterygii which retain the optic chiasma, several rows 
of valves in the conus arteriosus and a spiral valve in the 
intestine. A heterocercal tail 

Subdivision (1). Chondrostei. 

Actinopterygii clothed with large bony plates which 
pass uninterruptedly into the membrane-bones of the 
head, the notochordal sheath undivided ; very few cartilage 
bones. A long snout projecting in front of the mouth. 
Teeth rudimentary. 

Ex. Acipenser. 

Subdivision (2). Halecomorphi, 

Actinopterygii with thin scales ; many cartilage bones 
and the notochordal sheath surrounded by well-formed 
amphicoelous vertebrae. Mouth terminal. 

Ex. Amia, 

Subdivision (3). Lepidoateidae, 

Actinopterygii with rhomboidal scales covered with 
enamel; the skull completely ossified and the vertebrae 
opisthocoelous. Jaws very much elongated, each carrying 
a row of long teeth. 

Ex. Lepidosteus. 



414 TELE0S1X)MI. [CHiF. 

Division B. Teleostei. 

Actinopterygii in which the optic nerves cross without 
intermingling, the conus is absorbed into the ventricle, leaving 
one row of valves. No spiral valve in the intestine. A 
homocercal tail. 

Section 1. Physostomi. 
Teleostei which retain the opening of the air-bladder 
into the alimentary canal ; the pelvic fins are abdominal in 
position. 

Family (1). Siluridas, 

Physostomi with a skin naked or covered with bony 
plates ; the skull unconstricted between the orbits, maxilla 
without teeth and bearing a long barbel. Weberian chain 
present. 

Ex. Amiurus, Ictalurus. 

Family (2). Cyprinidae. 

Physostomi with scales, the skull is constricted between 
the orbits, maxilla without teeth or barbeL Weberian 
chain present. 

Ex. Cyprinus, 

Family (3). Anguillidae, 

Physostomi devoid of scales^ skull not constricted 
between orbits. Weberian chain absent. No reproductive 
ducts. A continuous dorsal-caudal-anal fin and no pelvic 
fins. 

Ex. EcheluSy Anguilla. 

Family (4). Salmonidae. 

Physostomi with thin scales, the skull constricted 
between the orbits, the maxilla forming part of the edge 
of the jaw and bearing teeth. A small soft dorsal fin 
behind the main dorsal. 

Ex. Salmo, Salvelinus, Oncorhyncus, 

Family (5). Clupeidae, 

Physostomi in most points resembling the last family ; 
the maxilla consists of several pieces and there is no soft 
dorsal fin. 

Ex. Clupea, 



XV.] CLASSIFICATION. 415 

Section 2. Physoclisti. 

The remaining sections of the Teleostei have lost the 
opening of the air-bladder into the alimentary canal, so that 
it becomes a closed vesicle. 

Sub-section 1. Anaoanthini. 

Teleostei in which the fin-rays are all soft and flexible, and 
the pelvic fins are shifted forward anterior to the pectorab. 

Family (I). Gadidas, 

Anacanthini of 83nnmetrical shape and not especially 
compressed. 

Ex. Gadtis. 

Family (2). Pleuronectidae. 

Anacanthini very much compressed laterally, which 
swim always on one side : the eye belonging to the lower 
side being rotated on to the upper side. 

Ex. Solea^ Flatessa, Hippoglossus, 

Sub-section 2. Aoanthopteri. 

Teleostei in which some at least of the fin-rays of the 
median fins are hard and unjointed. 

Family (1). Scombridae. 

Elongated Acanthopteri in which there is a short 
spinous dorsal fin followed by a long softer one, the hinder 
portion of which is broken up into finlets. 

Ex. Scomber, 

Family (2). Percidae. 

Short stout Acanthopteri with one long dorsal fin fol- 
lowed by a short one and having a toothed posterior border 
to the scales. 

Ex. Ferca, 

Family (3). Serranldae (Sea Bass). 

Closely allied to the Perctdcw^ but distinguished by 
having the dorsal fin undivided. Teeth large and numer- 
ous ; a large pseudobranch. 



416 TELEOSTOMI. [CHAP. XV. 

Family (3). Centrdrchidae (River Bass). 

Acanthopteri which have a laterally compressed body 
and undivided dorsal fin. Teeth small; a small pseudo- 
branch. 

Sub-section 3. Pharyngognathi. 

Teleostei in which the bones (rudimentary fifth gill-arches) 
bearing the pharyngeal teeth are firmly united together. 

Ex. Lahrus, 

Sub-section 4. Lophobranchii. 

Teleostei covered with bony plates, the &cial bones elon- 
gated so that the jaws are at the end of a tube-like proboscis. 
The branchial processes are arranged in tufts and thickened at 
their firee ends. The pelvic fins absent. 

Ex. Syngnathus, Hippocampus. 

Sub-section 5. Plectognathi. 

Teleostei covered with bony plates, the pre-maxilla and 
hyomandibular immovably joined to the skull. The gills 
normal. Pelvic fins absent or represented by spines. 

Ex. Ostracion, Diodon, Tetrodon, 



417 



CHAPTER XVL 
Sub-Phylum IV. Craniata. 

Glass II. Amphibia. 

The class Amphibia includes the familiar frogs and toads, the 

less-known newts and salamanders, and some very 

ofthe'cuM!* curious worm-like tropical forms which burrow in 

the earth. The name means double life (Gr. oifi<l>i, 
double ; fiio^, manner of living), and refers to the fact that all the 
typical members of the class commence their lives as fish-like larvae, 
breathing by gills, and afterwards become converted into land 
animals, breathing by lungs. This strongly marked larval type of 
development is one of the great distinctions between the Amphibia 
and the only other class of Vertebrata with which they could be 
confounded, viz., the Reptiles. In the Reptiles, as in the Birds, a 
large egg abundantly provided with nutritive material is produced, 
and the young animal practically completes its development within 
the egg-shell and is bom in a condition dififering from the adult 
chiefly in size. 

It might at first sight be thought that the fact that Amphibia 
breathe air in their later life and live on land would be sufiicient to 
mark them off from the fisL But we have already seen that one 
order of fisli — ^the Dipnoi — possesses lungs and breathes air, and on 
the other hand some Amphibia retain gills throughout life and 
rarely if ever leave the water. 

The unbridged gap between true fish and Amphibia is to be 
found not in the breathing organ but in the structure of the limb. 
Fish possess fins — median and paired — which are in both cases 
supported by homy rays, as well as an intemal skeleton ; and the 
paired fins have an intemal skeleton which has the form of a jointed 
axis bearing similar rays on one or both sides (Figs. 205 and 215). 

& &M. 27 



418 



AMPHIBIA. 



[chap. 



B 



The Amphibian limb, on the other hand, is what is known as a 
pentadactyle limb; that is to say, it is constructed on the fiamiliar 
type of the human limb, and the median fin when present has no 
fin rays (Figs. 229 and 231). 

The pentadactyle or five-fingered limb (Gr. tcktc, five ; SaKrvXoi a 
finger), also called the cheiropterygium (Gr. x^^* ^ hand; vrcpv- 
yiov, little wing, hence an appendage), consists of three segments, a 
proximal, containing one long bone ; a middle, containing two bones 
placed side by side and occasionally fused into one ; and a distal, 
containing a series of small squarish cartilages or bones arranged in 
lines so as to give rise to a series of diverging rays ; the last- 
mentioned constitute the skeleton of the fingers and toes. In the 
proximal part of this lowest segment the bones are much crowded 
together and the rays tend to coalesce : this part has received a 
special name, as has also the portion where the rays although 
separate are embedded in the same muscular mass. 

The fore-limb is called 
the arm, and its divisions 
the brachium or upper 
arm, the ante-brachium 
or fore -arm, and the 
manus or hand (b, Fig. 
229). The hind limb is 
the leg, and its divisions 
are the femur or thigh» 
the crus or shank, and the 
pes or foot (a, Fig. 229). 

The manus is divided 
into three regions, viz. : 
(a) the carpus or wrist 
where the rays tend to 
coalesce; (6) the meta- 
carpus or palm where the 
rays although separate are 
bound together by flesh 
and skin; (c) the digits 
or free ends of the rays. 
The pes is similarly divided into tarsus or ankle, metatarsus 
or sole, and digits or toes. 

The bone of the brachium is called the humerus, that of the 
femur bears the same name as the segment to which it belongs; 





Fio. 229. A. A skeleton of a right posterior, 
and B of a right anterior limb of a Newt, 
Molge cristata x 1). 

1. Femur. 2. Tibia. 3. Fibula. 

4. Tibiale. 5. Intermedium. 6. Fibu- 
lare. 7. Gentrale of tarsus. 8. Tarsale 1. 
9. Tarsalia 4 and 5 fused. I. U. III. IV. 
y. Digits. 10. Humerus. 11. Radius. 
12. Ulna. 13. Badiale. 14. Intermedium 
and ulnare fused. 15. Gentrale of carpus, 
the pointing line passes across carpale 2. 
16. Carpale 3. 17. Carpale 6. 



['thaw of the ante-brachium are called radius and ulna, those M 
the crus tibia and fibula (Fig. 229). 

The akeletonfl of the pes and manua are typically exactly the 
same. Situated proximally close to the middle segment of the limb 
is a transverse row of three small bones, the central one being called 
the intermedium in both limbs, whilst the outer and inner are 
named after the bones of the middle segment of the limb adjacent 
to them. Thus we find in the wrist a radiate and ulnare and 
in the iiuUle a tibiale and fibnlare. Beyond this row of bonea 
there is a tingle central bone which probably belongs to the middle 
ray, and beyond it a row of five small bones corresponding to the 
digits. This last row are denominated carpalia in the wrist and 
tarsalia in the ankle. The individual bones are called carpale 
(or tarsale) 1—5 in accordance with the digits opposite which they 
are situated. 

In almost every case this typical skeleton of nine bones has 
onderguoe some modification, owing either to the absenL-d of some 
bones or the fusion of others, but in the hind-limb of the lower 
Amphibia it la exactly typical. In the higher Amphibia not only 
has great reduction of the elements taken place but the radius and 
vlna in the fore-limb and the tibia and fibula in the hind-limb have 
coalesced, a groove only being left to show their primitive distinct- 
ness. 

The primitive position of the limbs with reference to the trunk 
is, from the study of development, assumed to be one in which they 
are stretched out at right angles to it, with the inner surface of the 
hand and the sole of the foot directed ventrally and iu such a 
position that a line joining the tips of the fingers is parallel to tbe 
long ana of the body. If we suppose an imaginary line or axis to 
run down ttie centre of each limb, we sball be able to distinguish a 
pre-axial from a post-axial aide. In the lower Amphibia the only 
change from this position tliat has taken place in the hind-limb is 
tliat each segment of the limb is bent at right angles on tiie one 
vhich follows it. The fore-lirab is bent similarly, but it is also 
rotated backwards so that its upper segment is almost parallel to 
the axis of tlie body, and the etbow points backwards. If this 
position were maintained the first digit would become e.\teriial; but 
the manus in most cases is at the same time twisted forwards so 
that the lower end of the radius lies intertial to that of the ulna, 
and the raditia thus crosses the ulna in its course. In the higher 
Vertebraia this iwialing can be undone and tlie hauii reverteti to 

27-2 



420 



AMPHIBIA. 



[CHAP. 



an untwisted position. This movement is known as supination, 
the reverse movement being known as pronation. 

The hind-limb in the higher Amphibia and 
other Vertebrata is likewise rotated forward so 
that the knee points forward and the first digit 
is internal, but this does not occur in the lower 
Amphibia, such as Molge. 

The pectoral girdle is not essentially dif- 
ferent in the lower Amphibia and the more 
primitive Teleostomi, but the pelvic girdle is 
firmly joined to the transverse process of one of 
the vertebrae, which is called the sacraL This 
is one of the most distinctive features of all 
pentadactyle animals ; it is a consequence of 
the adaptation of the pentadactyle limb to raise 
the body from the ground (Fig. 230). It is 
necessary for this purpose that the limb shoidd 
have a firm purchase on the axial skeleton. 
Consequently when we find some Amphibia 
which never use their limbs for crawling but 
only for swimming, we assume that this is a 
secondary degenerate condition. 

Next to the character of the limb one of 
the most distinctive features of Amphibia is 
the nature of the skin. Indeed the five great 
classes of Qnathostomata— Fishes, Amphibia, 
Keptiles, Birds, and Mammals — are each per- 
fectly characterised by the nature of their skin. 
In a typical Amphibian the skin is soft and 
moist and devoid altogether of any ossifications 
like the scales of fishes. The skin is a most 
important breathing organ, since the lung alone 
cannot meet the demand for oxygen, and if the 
skin becomes dry and consequently incapable 
of absorbing oxygen the animal dies. The 
necessary moisture is supplied from a series of 
pockets, to form which the ectoderm is pouched 
inwards — or to use a more convenient term 
* invaginated* — at various points, and the ceUs 
lining these pouches have the power of secreting 
great quantities of mucus. As the cells become 



Fio.230. Skeleton 
Triton, Molg€ cm 
(otaxl. 



lO 



SKULL AND VERTKBBAL COLCMN. 



421 



broken np into mucua, Dew e«lls take their place, being biiddeJ off 
from tLe underlying Malpighian layer just &s the horny cells are. 
These pouches are known as dermnl glands. 

The skiill and brain are very characteriptic, recalling in many 
points those of the Dipnoi. The axis of the brain appears straight, 
as in fishes; in higher Vertebrates this axis is more or less folded. 
In contrast, however, with fishes, the cerebral hemispheres of the 
fore-brain are relatively large, whereas the cerebellum, usually so 
large in fishes, is reduced to a mere band (Fig. 240). 

The skull always articulates by two pegs — the occipital con- 
dyles — with the first vertebra (Fig. 2a2). It is remarkable for its 
extremely flattened shape; the jaws are widely bent outwards so 
that the large eyes in no way compress the cranium, which is 
thus evenly cylindrical Both membrane- and cartilage -bones are 
present, but the ossification is by no means complete. The exact 
arrangement of the bones will be given when a type is studied. 

The vertebrae are either procoelous (Qr. xjio, in front; koiXot, 
hollow), or opisthocoelous (Gr. ointrflo-, behind), that is to say either 
concave in front and convex behind, or vice mrsil, and the arrange- 
ment may difi'er in allied genera, while amphicoelous vertebrae also 
occur. 

The vertebrae articulate with one another, not only by the 
centra but also by facets called aygapophyses (Gr, ^uyo'v, a yoke), 
on the aides of the neural arches. The anterior facets, pre-zygapo- 
physes, look upwards and are covered by the posterior facets or 
jiost-zygapophyses of the vertebra in front, which look downwards. 

The circulatory system closely resembles that of the Dipnoi. 
The atriiira is divided into two auricles, and the blood from the lungs 
returns direct to the left auricle by the pulmonary veins. A median 
▼ein, the inferior cava, returns the blood from the kidneys directly 
into the sinus venosus, receiving in its course the hepatic vein. 
The anterior portions of the posterior cardinals are much reduced 
in size and may be altogether absent. 

The lungs open by a common Ht«tn. the laryngeal chamber, into 
the throat. The opening is called the glottis, and it« sides are 
stiffened with cartilage. 

The kidneys and reproductive organs show essentially the same 
arrangement as in the Elasmobranchs, the kidney being divided into 
B, sexual part connected with the testis and a posterior non-sexual 
part. There is one opening for all cjecta. the cloaca. 

The ventral wall of the cloaca, however, is produced outwards 



422 URODELA. [chap. 

into a great thin-walled sac, the allantoic bladder, in which when 
the cloaca is closed the urine accumulates. This organ acquires 
immense importance in the development of the higher animals and 
is found in no fish. 

In the larva, which is to all intents and purposes a fish, there are 
present those peculiar sense organs called mucous canals, suppUed 
by the 5th and 10th nerves, but these are usually lost in the adult 

Living Amphibia are divided into three well-marked Orders, vi^ 
the Urodela, the Anura and the Afoda. The 
Urodela (Gr. ovpd^ tail; 817X09, conspicuous) have 
long cylindrical bodies and long flattened tails. The limbs are 
short and comparatively feeble, barely strong enough to lift the 
belly from the ground. Both pairs of limbs are about equal in 
size. The Anura (Gr. av-, no; ovpd, tail) have much broader and 
shorter bodies; the tail is totally lost and the hind limbs are 
powerfully developed and adapted for jumping. The Apoda (Gr. 
a-, no, voSa, feet) have lost both pairs of limbs and their cylindrical 
bodies give them a worm-like appearance; their habits heighten 
the resemblance since they burrow in moist earth. They have 
embedded in the skin small bony plates, relics of the scales which 
their Stegocephalous-like ancestors once possessed. The tail has 
in these animals almost disappeared. 

In the Carboniferous rocks the remains of a large number of 
Amphibia have been found which have been called Stegooephala 
(Gr. arcyos, a roof ; #c€<^aXi7, the head) firom the circumstance that 
the head is covered with a compact mosaic of membrane-bones 
extending from the mid-dorsal line of the cranium outwards to 
the lips. Similar small boDes or scales are found on the ventral 
surface. These features bear resemblances t6 what is found in 
Dipnoan or Crossopterygian fish from which Amphibia have probably 
descended, and the small scales of the Apoda seem to be the last 
remnants of this armature. Stegocephala include both long and 
short tailed forms, and while some of their descendants — the Laby- 
rinthodonta — became highly specialized in the structure of their 
teeth and died out in the next geological period, others, in all 
probability, gave rise to modern Amphibia. 

URODELA. 

Returning to the Urodela, which are the most primitive of 
modern Amphibia, we find that in Great Britain they are represented 
by three species, all belonging to the genus Molge {Triton) and 




poiiularly known aa efts or newts. Mutge crletata, the warty eft, 
and Molge vulgarU, the lonimou eft, are found m ponds and ditc)ies 
all oyer the country, but Slolqg pitlmata is nmuh more local. We 
may sele<.'t Molt/e cristala, the greater r warty ett or creeted newt, 
as a type of the aaatomy of T r iel Fit, i 1 



Via. 231. Motge criilala, the Warty Eft. From Qadov. 
Female. 2. Male at the breeding aeaHoD with the frills well developed. 

llie auimal in about five or six iDcbes long, half the length being 
made up of the tail, which has a continuous fringe of akin, the 
median fin. This tin in the male extends forwards to the head 
dorsally and is greatly enlarged in the breeding season, but it is at 
all times devoid of tin rays, 

The akin is clammy, owing to the secretion of the dennal glands: 

) dark coloured above and yellow sjjotted with black below. The 
oitening of the cloaca is placed behind the hind leg^: it is a 
longitudinally placed oval slit which in the male has thickened lips. 

The fore-limbs have only four lingers, the innermost corresponding 
to the human thumb being wanting, but there are five toes in the 
hind-limb. The animal when out of water crawls feebly along, but 
it awim.s actively in the wat«r by means of its vertically flattened 
tail. The head is flattened dorao- ventral! y and of somewhat oval 
outline, and the gape is of moderate e-vtent. The eyes are small 
and project but little. The nostrils are very small and situated 
at the extreme front end of the snout. 

If the newt be carefully watched when out of the water the skin 
of the underside of the head between the two sides of the lower 
jaw will be seen to throb at regular intervals, being alternately 



424 URODEUL [chap. 

paffed out and drawn in. It can be further seen that the nostrils 
are closed when the skin is drawn in and opened when it is paffed 
out. These movements constitute the mechanism of breathing in 
the newt. As in the case of the Dipnoi, the paired nasal sacs 
communicate with the interior of the mouth by an opening called 
the choanae or internal nares, and the air passes through these 
from the nostril when the cavity of the mouth is enlarged. When 
the cavity of the mouth is compressed the nostril is closed by a flap 
of skin constituting a valve, and the air is forced through ihe open 
glottis into the lung, whence it is forced out again by the elastic 
recoil when the pressure is removed. 

If the animal be laid on a board with the ventral side uppermost 
and skinned, a thin sheet of muscles, the mylo-hyoid, will be seen 
stretching between the two halves of the lower jaw. When this 
muscle is relaxed the floor of the mouth is arched upwards and the 
underside of the head consequently becomes concave. When the 
muscle contracts and straightens the cavity of the mouth enlarges 
and air is drawn in. Above the mylo-hyoid (underneath from the 
point of view of the dissection) are two longitudinal muscular 
bands, and in these are embedded the reduced remains of the 
visceral arches to which the gills of the larva were attached 
(Fig. 233). These muscles are called genio-hyoid in front of the 
arches, sterno-hyoid between them and the pectoral girdle, and 
they are continued backwards along the belly as the straight muscles 
of the abdomen, the recti abdominis. 

These sterno-hyoid muscles can draw the visceral arches 
downwards and backwards and probably assist the mylo-hyoid in 
depressing the floor of the mouth. The genio-hyoid muscle on 
the contrary pulls the arches forwards and helps to restore them 
and the floor of the mouth with them to their old position. In this 
action muscles called petro-hyoid, which run from the arches to 
the outer surface of the auditory capsule, also take part. These 
muscles are representatives of the levatores arcuum of fish, and 
they raise the arches and consequently the floor of the mouth. 

The glottis or opening into the lungs is stiffened at the sides by 
a pair of cartilages, which it seems probable are the remains of a 
hinder pair of visceral arches : and these cartilages have masdes 
attached to their sides which drag them apart and which belong to 
the same series as those which raise the arches. Hence the same 
muscular action which lifts the floor of the mouth opens the glottis 
and admits air into the lungs. 



XTLl 



425 



The remaining muscles of the body are not much altered from 
those of the fish. In the tail and the ventral part of the trunk 
there are V-shapeil myotomes, but this arrangement is distnrbed in 
the neighbourhood of the limbs. 

Turning oon to the skeleton we find that the vertebrae bear 
stout transverse processes with which are articulated 
short rib? (Fig. -^30), The ribs borne by the sacral 
vertebra are expanded in accordance with the strain put on them 
by the attachment of the ilium. Of the vertebrae those of the tail 
are the most primitive since they are composed of all the four 
arcualia ; but of these only the basi-dorsalB and the basi-ventraJB 
become ossified, and joining together form the bulk of the vertebra, 
vhile the inter-dorsals and inter-ventrals, although likewise fusing 
together, remain cartili^nous and form the inter- vertebral cartilage. 
This either remains continuous and owing to its Bexibility acts as a 
joint, or it becomes more or less separated into a cup-and-ball portion. 
Joint)! in which the cup belongs to the posterior end of the vertebrae 
are called opiathocoelous, eg., in Desmotfitatkim trlton. The basi- 
veutrals of the tail vertebrae form long downward haemal arches. 

In the trunk the basi-ventrals occur only in early larvae ; in the 
Bdult they have disappeared so that the bulk of such vertebra is 
formed only by the i>air of" basi-dorsals which alone cany the ribs, 
and these to compensate the loss of their capitular process have 
guued a new process dorsally &om the tuberculum. 

It is of importance to note that in many of the extinct 
Stegocephala, e.g., Arctwiiimtunia, the caudal vertebrae were repre- 
sented by four pairs of distinct arcualia. wiiile tlie trunk vertebrae 
consisted of three separate pairs of pieces, namely the basi-dorsal, 
the inter-dorsal and the basi-ventral ; but that in the typical 
Lahyrinlhodouta, the highest of the Stegocephala, all these constitnent 
pieces were formed into solid vertebrae ; lastly, that in some of the 
loweBt, e,g., in Branckiosaurus, each vertebra consisted of a thin 
shell of bone surrounding the chorda, and composed of the basi- 
duntals and basi-ventrala, which met each other, forming a broad- 
based section along the side of the vertebra, both partaking in the 
formation of a transverse process which carried tiie rib. The haemal 
•rches of the tail are, like the rfba, outgrowths of the basi-ventral, 
bat they do not exactly correspond to the ribs, for they are placed 

rer the middle line. 

In the skull the cranium is cylindrical, being (juite uncom- 
pressed between the eyes. The bones of the jaws and £ace are 



426 UBODELA. [chap. 

widely arched outwuds, bo that the whole skull has a flattened 
shape. The nasal aud auditory capsules form easily rect^iuUt 
buttresses projecting from the cranium. 

Id both the floor aod roof of the cartilaginous craoinm the 
proper wall is largely deficient. The deficiency of the roof is the 




Fto. 333. A doraal, B ventrtl, and C lateral ti^wb of the skull of a Newt, 
Molge ciiitala x 2}. After Parker. 

The oartilage ie dotted, the cartilSfte-boiieB are marfced with dotg and daahea, the 
membrane- bones are left white. 

1. Premaiilla. 3. Anterior narea. 3. Posterior narCK 4. NaaL 
G. Frontal. 6. Parietal. 7. Prefrontal. B. Maxilla. 9. FoKd 
Tomer and palatine. 10. Paraaphenoid. 11. Orbito«pbenoid. 

13. Pterygoid. 13. Squamosal. 14. Pro-otia region of fiiaed eioocipital 
and prO'Olic. 15. Quadrate. 16. Calcified cartilage forming the 

articular eiirface of the quadrate. 17. Eioceipital regioD of foied 

eioccipital and pro-otio. 18. Articalar. 19. Artienlat outilagb 

30. Dentary. 21. Splenial. 23. Middle narial passage, a cleft in 

the cartilage of the snont filled with connectiTe limoe. IL T. Til. IS. 

X. Foramina for the eiit of cranial nerves. 

anterior fontanelle, in the floor the greatly enlai^ed pituitary fossa. 
But these deficiencies are not seen in the uninjured aknll, because 
the hole in the roof is closed in by two pairs of membraoe-bones, the 
front&ls and the parietals, and that in the floor ia nnderiaid by 
a broad parasphenoid membrane-bone (Fig. S3S). 

Unly at its extreme front and hind ends is the wall of the (^aniam 
converted into cartil^-bone. In Jront there is oa each aide w 



XTt] VISCBBiX AIICHSS. 427 

urbito-Bpheo Old bone, in the side wall, extending ioto tlie roof and 
floor and ossifying also the hind wall of the nasal sac ; behind, two 
exocoipital bones' areplacedat the sides of the forfunen magnum, 
which Uiey nearly encircle (Fig. 232). These bones bear the two 
condyles, so characteristic of Amphibia, for articulation with the 
vertebral column. 

The iirat viac^al arch, which constitutes the cartilaginous jaws, is 
almost entirely cartilaginous. It consists of an upper part immov- 
ably attached to tJie skull, corresponding to the ptery go-quadrate 
l)ar or upper jaw of Fish, and a lower part, Meckel's cartilage, 
forming the basis of the lower jaw. It will thus be seen that 
Amphibia, like Holocephali and Dipnoi, are autostylic. The same 
is true of all the higher groups of the Craniata. The upper jaw 
consists of two regions, the suspensorium which is fused with the 
skull and to which the tower jaw is attached, and the pterygoid 
process, a spur of cartilage which runs 
forward towards the nasal capsule. 
Both suspensorium and the articular 
end of Meckel's cartilage are slightly 
calciHed. They are denominated quad- 
rate and articular in Fig. 2S2, but 
there is no true bone present in either 
case. The front of the auditory capsule 
is ossified by a large bone, the pro-otie, 
which in fully adult specimens becomes 
confluent with the exoccipital. The 
hinder visceral arches in the adult are 
present in a very degenerate condition. 
Traces of three remain (Fig. 233). 

It is usual to speak of the hinder 
visceral arches of Amphibia and higher 
Vertebrata nn the hyoid apparatus, 

simply as the hyoid. The name suggests a misleading com- 
parison with the second visceral arch of Fish; it is distiaclly to be 
Temembered that the hyoid bone of even Man contains more than 
this second arch; a good definition of the hyoid of Amphibia and 
higher animals would be "the degenerate remains of the hinder 
visceral arches," 

Turning now to the membrane-bones of the skull, we find that it 




Fin. aU3. Visfleral archea of 
Molfff crittata. The ngsified 
parts are Klightl; shaded, the 
cartilage ia white. From 
Parker. 

2. Hyoid arah, 3. First 

ViranchiBl arch. 4. Second 
branchial aroh. 8. Oopula, 
i.e. the median piece coDoeat- 
Ing sucoeBaive sccheB. 



are equiralent to the lateral o 
It oooipitalia exteroa. 



tipilaU. Tlia 



428 



[chap. 



is roofed by three p&ire, viz., the n&saU, frontala uid p&rietali. 
The nasals of course roof in the nasal saca. In the palate then ti 
one median bone, the parasphenoid, and three pairs of lateiil 
bones, viz., the vomers in front of the posterior uares, Ha 
palatines fiised with them and ruouing along the edges of the 
parasphenoid, and lastly the pterygoids underlying tJu pt^- 
goid process. Some of tbeee bones are actually built up by the 
fusion of the bases of mioute conical teeth in the larva. The 
vomers and palatines retain their teeth in the adult, whilst the 
parasphenoid loaee them. 

The upper lip has tooth- 
bearing pre-mazillary snd 
maxillary bones developed, 
the lower has a dentary on 
the outside of Meckel's car- 
tilage and a splenial on tlie 
inner. Above the maxilla then 
is a small pre-frontal bone. 
If we examine the skeleton 
of the limbe we find that 
the pectoral girdle consists of 
two plates of cartilage which 
Blightly overlap in the mid- 
ventral line. The lower half 
of each is forked, the foriu 
being called precoracoid and 
coracoid respectively, 'Hie 
centre of each half of Uie 
girdle has a hollow termed the 
glenoid cavity for the articu- 
lation of the arm. All arotmd 
the glenoid cavity the gitdle 
is converted into bone; there 
la a bone termed the scapula 
above, and a coracoid bone 
below. The nuossified part 
of the coracoid is simply 
termed the coracoid carti- 
lage. The upper part of the 
ginlle dorsal to the sci^nlar 
bone is called the supra- 




Fio. 234. A, ventral, and B, Uteial Tiew 
of the Blionlder girdle and Btemum of 
an old male Created Newt, Molge Crii- 
tata X 3. After Parker. 

1. Scapula. 2. Supra- ncapula. B. Cora- 
ooid. 4, Oleooid oavity. 6. Pre- 
ooracoid. 6. Btammn. 



LIMBS AND VISCERA. 



XVI.] 

scapula. It remains cartil^inons, but 
eoncoids are fastened behind to a small 
median cartilage called the sternum. 
The meaning of this is discussed later. 

The manus has only four fingers, 
the thumb and the corresponding small 
bone in the nrist or carpus having 
disappeared and the ulnare and inter- 
medium being fused, although they are 
distiactinthelar?a(Fig.335A). Other- 
wise the limb corresponds to the scheme 
given in the beginning of the chapter. 

The pelvic girdle on each side is 
finnly joined to the rib of the sacral 
vertebra, and the two halves meet in the 
mid-ventral line. The upper part of 
the girdle above the cavity for articula- 
tion of the thigh is a bone, the ilium; 
below this cavity, which is termed the 
acetabulum, is a so-called "ischio- 
pubic " cartilage, in the hinder part of 
which a small bone, the ischium, is 
developed. In the mid-ventral line, in 
front of the union of the two halves of 
the pelvic girdle, there is a forked piece 
of cartilage, the epipubis (Fig. 236). 
In the pes the only departure from the 
fusion of the tarsalia 4 and 5. 

If the muscles be carefully cut 
through inthemiddleline 

ViKCTK. in > . , , 

and reflected, the body 
cavity and the organs contained therein 
will be exposed. In general the dif- 
ference from the arrangement of the 
organs in a dog-fish is only in the 
relative size of the organs, in a word, 
in details. 

The alimentary canal is thrown 
into a number of loops. The oeso- 
phagus is not in any way sharply 
marked off from the stomach, and 



is often calcified. The two 




Fio. 23S. A, BigLt antebra- 
chinm and MiinuB of a larval 
Balamandar, Salamandra ma- 
caloia. Arter Gegenbaur. 

B, BigLt TarBQB aod adjoiDiii([ 
Bones of Mntgt tp. AfMr 
Oegeobaut. 

1. lUdiae. 2. Ulna. S. Bad. 
iale. 4. Intertnedium. 

B. Uluare. 6. Centrale. 
7. Carpale 2. B. Carpale S. 
9. Carpalel. 10. Carpale 
5. 11. Tibia. IS. Fibula. 
13. Tibials. 14. Id termed- 
iam. 15. Fibnlare. 

16. Centrale. 17. Tareale 1. 
18. Tarsalia 4 and 6 faeed. 
I. II. III. IV. V. Digiw. 

typical arrangement is the 




430 nSODBLA. [CHiF. 

the latter ia nearty etnight, extending only a short way roand ths 
bend of tbe first loop. There u a well marked large inteatbe or 
rectum, ventral to which hee the bladder The spleen is u 




o eipoee the inteniBl orgkDS, 

Mflo-hyoid iQiiBcle with genio-hjoid UDdemeith. 3. ConaB krUrilMlli. 

3. YeDtriole. 4. Aariole. 5. BinnB renoiilB. 6. C«rotid midi. 
7. 5f Btemic arch. S, Pulmoatur Arteiy. 9. Anterior venm cftva of left 
Bide. 10. CoraooidB polled oatwards. 11. Liver. 13. Oall-hUdder. 
13. Lung. 14. Bpleen. 16. Stomach. 16. lntestin«. IT. B«otnin. 
18. Allantoio bladder. 19. Fst-bod;. 20. TeBlea. 31. AnlerKH- 
abdominal Teui, dispUoed. 33. Kidney with dact. i ' 



1-] 



CIRCULATION. 




I red body lying at the side of the gtomach and attached to 

pie DieacDtery. The ducts of the pancreas and liver coalesce into 

coininon atem before opening into the intestine. 

The newt feeds on small worms aad aquatic insects, which it 

fceizes with its jaws. Both upper and lower jaws are armed with 

■ minnto teeth, and there are in addition two longitudinal rows of 

I teeth on the roof of the mouth borne by the conjoined vomer and 

I jialatine on each side. The function of these teeth is not so much 

to crush as to retain a hold of the prey, which is swallowed whole. 

The tongue is a circular cushion on the floor of the mouth, 

supported by the second visceral arch. Its hinder edge is partially 

free. The lungs are long, smooth-walled, tube-like elastic sacs, 

attached to the liver and other organs at their base, but their tips 

float freely in the body-cavity. 

The heart lies far forward, between the roots of the lungs, 
enclosed in the pericardium. Externally all the four 
syutm." divisions of the piscine heart are visible, viz., sinus 
venoeua, atrium, ventricle, conus. The venous syHtem 
is essentially that of the dog-fish, only the veins are indicated hy 
names borrowed from human anatomy. Thus the blood from the 
head is returned by two internal jugular veins, representing the 
anterior cardiuals of the fislu These are joined by external 
jugulars from the auperticial part of the throat and face and hy 
a 8ub-clavian vein from each arm. The common trunk formed 
by the nnioM of all three is, of course, the Ductus Cuvieri, but it is 
called the superior vena cava, and it receives on each side close 
to the middle line a posterior cardinal vein. As in fishes, this vein 
in its course breaks up into capillaries through the kidney, and 
along the outer edge of the kidney, its posterior portion, the renal 
portal, may be matie out. The two renal portals when followed 
further back are found to coaiesce in the caudal vein wliich 
returns the blood from the tail: each receives a sciatic vein from 
the ilorsal side of the leg joined by a femoral from the ventral 
surface of the limb. 

The increased importance of the hind limb has brought with it 
this increase in the vessels draining it, wliich are represented only 
by the smsJI pelvic vein in fishes. 

'Inhere are certain vessels, however, unrepresented in any fishes 
except the Dipnoi. These are: first, the pulmonary veins, 
which receive the blood from the lungs and open directly into the 
left side of the atrium, which is separated from the rest by a septum 



432 URODELA. [chap. 

and constitutes the left auricle; secondly, the inferior Tens 
cava, a large trunk situated in the median dorsal line jtut beneidi 
the aorta, which receives most of the blood that has traversed the 
kidnejs and conyeys it into the sinus venosus just between the 
openings of the two superior venae cavae. The inferior cava 
coalesces with the hepatic vein) 
returning blood from the liver: 
these thus lose their independeat 
openings into the sinus veoosiu 
which they had in the Dog-fish. 
In its hindermost portion between 
the kidneys the vena cava joins 
the posterior cardinal. 

So far the peculiarities of the 
Newt are shared by the Dipnoi : 
but there remain two veins highly 
characteristic of Amphibia. The 
muBculo-cutaneons vein re- 
ceives blood from the skin and 
pours it into the subclavian ; we 
have already seen that the skin 
is a very important breathing 
organ, and this vein returns the 
blood which has been oxygenated 
in the skin to the heart The 
anterior abdominal vein 
arises ou the ventral side of the 




Fta. 336. Diagram to show urange- 
ment of the priQcipsl Veins ol tm 
Urodele. 

1. SiDusTeDOBQi, gradually disappear- 
iDK it 



Cuvii 



temal jngular = anterior oardioal 
diiius. *■ Eitemal jtigular= 

Bub-branchial. 5. SubclaviaD. 

6. Posterior cardinal, front part. 

7. Inferior vena cava. 8. Renal 
portal — hinder part of posterit 
dinal. 9. Caudal. 10. Sciatic. 
11. Femoral. 12. Anterior ab- 
dominal. 



1 of two forks given off by 
the femoral veins ; it runs for- 
ward in the mid- ventral line, 
eventually joining branches of 
the portal vein and entering the 
liver. This vein is found also in 
the lower Reptiles and in the 

embryos of Mammalia, where it is of the utmost importance in both 

nutrition and respiration. 

When the veins are cut away it is possible to follow out the 

arteries. There 18 no ventral aorta, since on each side three arterial 

arches arise in a bunch from the &out end of the tubular conns. 

The first of these is called the carotid arch, and is derived from 



XVI.] 



CIRCULATION. 



433 



IV 

14- 

V 
VI 



-14 



the third arterial arch of the embryo, but unlike its equivalent 
in Dipnoi it does not communicate with the dorsal aorta. It gives 
off a lingual artery to the 
tongue and throat and then 
passes up round the gullet, 
to which it gives off some 
twigs and continuing as the 
common carotid supplies 
the upper part of the head 
and brain. Just after giv- 
ing off the lingual artery 
the arch swells up into a 
little knot, called the 
carotid gland. In this 
structure the channel of 
the artery is broken up into 
a network of fine passages 
and its function is believed 
to be that of holding back 
the blood from entering the 
head until, at the close of 
the contraction of the ven- 
tricle, the blood has return- 
ed from the lungs. The 
second arch, derived from 
the fourth embryonic arch, 
supplies most of the blood 
to the root of the dorsal 
aorta, and on this account 
is called the systemic 
arcL The fifth and sixth 
embryonic arches in later 
stages unite on each side 
into one trunk, which pass- 
ing round the gullet joins 
the systemic arch. From 




Fio. 239. Diagram of arterial arches of Molge, 
viewed from the ventral aspect. 

I. II. III. IV. v. VI. first to sixth arterial 
arches. 9. Carotid gland. 12. Lingual 
^ventral carotid). 13. Common carotid 
(dorsal carotid). 14. Systemic arch. 

17. Dorsal aorta. 19. Pulmonary. 

22. Subclavian (dorsal type). 23. Cuta- 
neous. 24. Coeliaco-mesenteric. 

the sixth arch is given off 

the pulmonary artery which supplies the lung. On this account it 
is called the pulmonary arch. The systemic arch on either side 
gives off a subclavian artery to the fore-limb : and fix)m its place of 
origin it will be seen that this subclavian is of the dorsal type 

8. AM. <2!^ 



434 . UBODELA. [chap. 

(p. 350). The subcl&Tians originate close to the junction of die 
two srstemic arcbes and each gives off a large branch to the otbv 
breathing organ, the skin, which is known aa the cutaneoas arterj. 
In Molge the fifth arterial arch disappears, as it does in aU Verte- 
brates above the Amphibia, bat in 
the allied genus Salamandra it is 
retained in the adult 

It is comparatiTelf easy to un- 
cover the brain and 
•jF«em^'^' spinal cord of the 
newt owing to the 
thinness of the bones which cover 
them. The cerebral hemispherea 
are long and cylindrical, and devoid 
of any other connection with one 
another than that by way of the 
thalamencephalon ; through the thin 
roof of the latter two thickenings in 
' its floor, the optic thalami, can 
be clearly seen. The mid-brain is 
a simple smooth vesicle, and the 
cerebellum is a alight inconspicuous 
transverse band (Pig. S40). 

The olfactory lobe of Amphibia 
difTers from that of Pisces in being 
separated from the cerebrum only 
by a slight constriction. From its 
anterior end a brush of nerves is 
Fi«. 240. Brain o( Triton, Molge given off which goes to the nasal 

sac. The so-called olfactory nerve 
1. Ollutoij DerreB, lepretentiag 
the olftctoty lobea of the Dog- 
Bah. 2. Olfactory lobea. 

3. Cerebral hem ilpbere. 1. Thin 
roof of thalamenoephalon. 5. 
Optic thalami. 6. Pineal bod;. 
7. Mid-brain. 8. Cerebellum. 
9. Medulla oblongata. Fiom 
Barckhardt. 




of the Dogfish which is the stalk 
connecting the olfactory lobe and 
the cerebrum is unrepresented in 
the Amphibia. 

The coarse of the cranial nerves 
is substantially the same as in the 
Dogfish ; owing, however, to the 
loss of the gills and the mucous canals in the adult, the branches 
are simplified. The 9th or glossopharyngeal, as its name implies, 
is distributed to the pharynx and tongue. The vagus supplies the 
Jaiynx and glottis, but its main stem runs on to the heart and stomach. 



XVI,] 



DBINO-OENITAL ORGANS. 



435 



The firBt spinal nerve comes out &ona beliiud the ^rat vertebra und 

is called the hypoglossal ; it runs 

directlyto the respiraU>ry muscle, the 

niylo-hyoid, crossing the vagus and 

glossopharyogeal in its course. At 

the t^ides of the dorsal aorta the tvro 

chains of sympathetic ganglia 

can be made out, connected by cross 

branches with the spinal nerves. 

To turn now to the excretory 

system, the kidney 

uotBani?" '^^^ ^^ ^^^ wlien the 

alimentary canal is 

removed. It is a long narrow strip 

on each ^ide adjacent to the aorta. 

In front it tapers to the merest 

tiiread, but behind, close to the 

cloEu:s, it thickens somewhat. Along 

itft outer edge runs the arcbine])hric 

duct, and external to the archi- 

nepbric duct is situated the long 
viduct. 
' The tubules which compose the 

kidney retain throughout life the 

ciliated openings into the body- 
cavity, and if the narrow part of 

the kidney be cut off and mounted 

in a bttle salt solution it in possible, 

at leiiat in small specimens, under 

a low power of the microscope, to 

see the funnels and to observe the 

whirlpools due to the currents pro- 

daccil by their cilia. 

The genitAl gland in both se.xes 

is represented by a pair of ridges 

suspended to tlie inner edges of 

the front parts of the kidney by 

slings of peritoneum similar to the 
L mesentery suspending thegut,and on 
B this account called mesenteries. 
Lin the female the oviduct opens 




Fio. 'J41. Urino-Rpnital organs of 
a Feraala Mnlgt criifnfo x »bout 5. 
I, O^Bry. 2. Bemnftnt of TBM 
eflereutift. 3. Remoaotof longi- 
tudinal caDnl ooDDeoting tbe T&aa 
iifferentisi. 4. Sexual portion 
of kidne}'. S. Arobinephric 
duct. 6. Ofiduet. 7. Pos- 
terior non.nexnal portion ot 
kidney. S. Opening of arohi- 
nepbric duct. 11. Internal open- 
in|{ ot oviJuct. 12, SuspenBoijr 
ligaiDEnt. 13. External upea- 
ing of oviduct. 



436 UBODBLA. [CHiP. 

by a ciliated fannel adjointog the root of the lung. The fdnnd 
leads into a long convoluted tnbe running back to open into 
the cloaca. The testis, which 
takes t^e form of two conicil 
bodies with their broad eatdi 
apposed, or aometimes a m 
of tiiree rounded lobes, com- 
municates by a nnmbei of 
-rasa efferentia with the ant- 
erior part of the kidn^, 
which is on tins accooot 
termed the sexna] portion at 
mesonephros. In the mile 
the kidney tabules belong- 
ing to tite hinder non-sexiul 
portion, or metanephros,ue 
split off from the archinephiic 
duct and unite into a short 
common trank, Mud ureter, 
which joins the archinephric 
duct juBt before the latter 
enters the cloaca 

It has been stated above 
that the genital glands ate 
a pair of ridges. In the 
larva the inner portions of 
the ridges degenerate, the 
cells becoming largely coo- 
verted into fat-bodiee. In 
the adult these &t-bodie8 
appear running parallel to the 
genital o^ans on the inner 
side. They serve as a stote 
of nourishment for the eggs 
which develope during the 
winter-sleep. The Newt, like 
other Amphibia, passes the 
winter buried in the mud at 




Fia. 212. Urino'geoit*) orguii of ft Male 
Molge eriitala x kbout 5. 

1. Testes. 2. Vun efferentia. 8. Longit- 
udinal canal eonnecting the Tasa effer- 
entia. 4. Beiaal portion of kidney 
showing nephroslomea. 6. Wolffian 
duct. 6. BudimeDtar; oviduct. 7. Non- 
aemal portion of kidney. 8. Eilemal 
opening oC the archinephrio dnot which 
has received the ureter 9 made np of 
a number of duets tram the posterior the bottom of ponds and takes 
part of the kidney. 10. Fat-body. ^^ j^ ^^ convenriou of 

Bome of the possible eggs into &t to feed the rest is simply an 



n.] 



487 



example of the aame principle as tlie sacrifice of some of the dogs in 
■a Arctic expedition to feeii the rest. 

The development of Mf)lg« is interesting. The male emita 

the spermatozoa in & bundle which the female then 

introdnces into her cloaca, and the eggs commence 

their development in the body of the mother. Soon afterwards they 

are laid and attached to water plants. After some time larvae are 

hatched out wiiich in many respects resemble fishes. They are 

provided with three long feathery appendages on each side of the 

neck, in which there is a rich blood supply and active circulation. 

hese are the external gills found only in Amphibia, Dipnoi and in 

'olffpterua. There is also a pair of curious rod-like organs in front 

F the gills attached to the sides of the head. These "balancers," 

I they are termed, are possibly a first pair of external gills peculiarly 

lodifiod. They have mucous cells at the tip, and by means of 




^em the young larva suspends itself for hours at a time to plants. 
There is a long fish-like tail, the organ of locomotion, with a (ringed 
%a. The fore-limbs are tiny buds. No trace of hind-limbs exists 
I the gill-alits are not open. 

As deveIo]>nient proceeds the fure-Hmbs make their appearance 
provided with only two toes. The gill-clefts, three in number, 
appear on each side. AJt«r a considerable time the third finger 
l^pears and the hind legs sprout out as buds; still later the fore- 
mbs get all four fingers and the hind-limbs five. The animal has 
Bow attained the appearance of the adult except in so far aa the 
plls are concerned. These are retained for a long time, and excep- 
tiially, in Switzerland in high Alpine localities, the larva may 
Kcome sexually ripe and never leave the water. More usually with 
e closing of the gill-slits and the shrivelling of the ext«rual gilU 
fche adult state is attained. 

The Urodela have for a long time been divided into two main 
poups, according to the presence during adult life of gill-slits 



438 UBODELA. [chap. 

and gills. Huxley thus divided them into Iohthtoedba and Sau- 

MAi^DROiDEA. But this has been criticized as not 

ClasBincation. ,.,, t» ^ it 

being based upon fundamental characters. Huxley s 
IcHTHTOiDEA are those which retain throughout life gill-slits or 
external gills or both. Invariably the limbs are reduced in size, 
the animals rarely if ever leaving the water. In one case the hind- 
limbs have totally disappeared. 

North America is the great head-quarters of the Ichthyoidea. 
Menopoma {Cryptobranchus) retains one gill-slit throughout life. 
This animal attains a length of 18 inches. It is fSedrly common on 
the Mississippi and its tributaries. An allied species found in 
Japan, and attaining a length of two feet, is the largest living 
Amphibian. 

Amphiuma is a snake-like animal about 18 inches long, with 
one gill-slit It is found in the same region as Menopoma. The 
limbs are exceedingly rudimentary, each having only two toes. 

Necturus, the Mud-puppy, has small but well-developed limbs. 
It retains throughout life two gill-slits and three external gills on 
each side. Necturus is abundant in the shallows of the St 
Lawrence, wriggling in and out around the roots of aquatic plants. 
A somewhat similar animal, Proteus, with more rudimentary limbs, 
is found inhabiting the limestone caverns of Camiola in Austria. 
Lastly, there is the aberrant Siren, which has a homy beak en- 
sheathing the premaxilla and dentary; it has no hind-limbs, but 
is similar to Necturus in its gills: it is found inhabiting the 
swamps of the Southern United States. 

Since the Ichthyoidea possess both gills and lungs it is tempting 
at first sight to regard them as the little modified descendants of an 
animal just making the transition from water-breathing to air- 
breathing life. There are however insuperable difficulties in the 
way of such an explanation. If we turn to other groups of the 
animal kingdom we find that the first step in fitting an animal for a 
land life is the covering up of the respiratory organ so as to protect 
it against drying up. But in hardly any fish are the respiratory 
organs so exposed as in Necturus, Proteus and Siren, 

Further, it was pointed out that the great gap between fishes 
and Amphibia is to be found in the structure of the limb. But the 
Ichthyoidea do not in any way assist in bridging the gap. On the 
contrary their limbs are obviously degenerating, a fact which seems 
to show that the aquatic life has been re-acquired. Now when the 
similarity between say Necturus and the late larva of Molge is 



CLASSIFICATION. iwt J 

borne in mind, and the further fact that these larvae may abnormally 
become aexiiallf ripe, the conclusion is irresistibly suggested that 
the Ichthyoidea are larvae in which the ailult stage has been 
suppressed. In the case of one large American newts Amblystoma 
tiffrinum, the Urva (the " AxolotI ") often breeds nnder certain 
oircumatances and was at one time regarded aa a distinct genus 
ISiredon). 

The second division of Urodela, the Salamandhoidba, are in 
general very similar to Molge, both in appearant^e, anatomy and size. 

As in Ichthyoidea, so likewise North America is very rich in 
Salamandroidea. These have been divided into families on grounds 
of difTerencea in the skeleton which have little effect on the external 
appearance. Tlie most abundant are the Amblyrtomatinae repre- 
aented by the genus Amldifstoma of which there are many species, 
nine being found in the Eastern States and Canada. The members 
of this family are distiDguishcd by having the palatine bones 
directed transversely, ho that the vomero-palatine rows of teeth 
run aiTOBS the roof of the mouth instead of along it, and by 
having amphicoeloua vertebrae.. Motga ( Diemi/eti lu^) mridescetis 
ia the common Water-Newt of Lower Canada. It is a member of 
the same genus as the English Newt which has been selected for 
detailed description, but unlike its English congener the American 
species does not develope a crest in the breeding season. These 
Newts are representatives of the Salam-^ndhinae distinguished by 
having the vomero-palatine teeth in a longitudinal row and by 
possessing opisthocoelous vertebrae. The family Dbsmuonatuinae 
are closely allied to the Amblystomatinue, but differ from the latter 
in possessing a cluster of teeth on the parasphenoid in addition 
to the transverse row of vomero-palatine teeth and in having 
opisthocflcloua vertebrae. The species of this family are common 
Water-Newts in the Eastern United States. Desmogntttlms nigra, 
the black Salamander, occurs near Montreal. The Plbtrodon- 
TINAE iucludca tlie American Cave- and Laud-Newts which rarely 
enter water but wriggle about actively on land. Tliese Newts re- 
semble the Desmognathinae in tlieir teeth, but differ in possessing 
amphicoeloua vertebrae. Although the most terrestrial in their 
habits of the New World Urodela, these animals and some of the 
Desmognathinae have undergone an extratirdtnary modification in 
their respiratory system. The lungs have disappeared and the 
septum between the auricles has l>ccome absorbed : so the animals 
depend for their oxygen entirely on their skin and the lining of the 



440 URODELA. [chap. 

pharynx, the walls of which still execute active respiratoiy movem^ts. 
This curious association of terrestrial habits with the absence of longs 
suggests the idea that the lung in such Urodela as retain it may be 
chiefly used as a hydrostatic organ like the air-bladder of fish, for 
were it of prime importance as a respiratory organ it would be diffi- 
cult to explain its disappearance in terrestrial forms. SpeUrpes 
includes the Cave-Newts, of which there are twenty species in 
America and one isolated species in Italy. In these animals the 
tongue is long and not adherent to the floor of the mouth. It can 
be suddenly protruded and is used to catch insects in the same 
way as the tongue of the Anura. This is an exceptional action 
amongst Urodela, most of which seize their prey with the jaws. 
PUthodcn erythronotus has the typical tongue. This is the common 
Land-Newt in the neighbourhood of Montreal, being found under 
old logs and in other damp situations. 



II. ANURA. 

The Anura or Batrachia are at once recognized by their broad, 
flattened, tailless bodies and their powerful hind-limbs. 
These limbs are not only efficient in jumping but also 
in swimming, and the toes are connected with one another by a thin 
web of skin in order to aid them in performing this function. The 
toes are stretched apart in the back stroke to present a large surface 
to the water, in the forward stroke they are folded together and 
offer little resistance. 

Anura are much more abundant than Urodela and are found all 
over the world, whereas the Urodela are restricted to the Northern 
hemisphere. They are in fact the dominant Amphibia of the present 
day, but they are highly specialized, and the Urodela give a much 
better idea of the relation of the Amphibia to the Fishes on the one 
hand and the Reptiles on the other, for which reason Molge was 
selected as the type. 

Besides the absence of a tail, the powerful character of the 
hind-limbs and the diff'erences in the skeleton connected therewith, 
Anura diff^er from Urodela in the skull and jaws, in the pectoral 
girdle, in the heart and lungs, and in the kidneys, genital organs 
and development. 

Two genera and four species of Anura occur in the British Isles. 
Rana temporaria, the common frog, and B. esculenta, the edible 
frog (the last named is thought by some not to be indigenous but 



1] 



ANITHA. 441 

» have been introduced), represent the family Rasidae, while the 
IryoNiDAE or toads are represented by Bvfo vulgnng, the common 
oud, and by B. ealamita, the Natterjack, whioh occurs in numbers 
It certain restricteil localities, as a rule those with a Bandy soil. 

As the Common Frog, Rana tumporaria, is easily attainable, 
the principal points in which it differs from Molge 
will be briefly described. 

The animal when at rest normally .squats on its haunches, 
Opporting itself slightly on its palms. Under these circumstances, 
he pelvic ^rdle makes a considerable angle with the vertebral 
olonin and the powerful iliac bones raise the skin of the back into 
well-marked homp, the ao-ealled sacral prominence. 

The gape is enormous, and is caused by the lower end of the 
nspensorium, or part of the skull to which the lower jaw is 

^d, slanting backwards instead of projecting directly downwards 
I in Urodela. The tongue is fixed to the floor of the mouth in 
ront, but is free behind ; it can be ra|>idly thrust out of the mouth 
\f bending the posterior end forwards and it can be ils rapidly 
etracted. It is used to whisk the insects on which the aniinal 
nds into the mouth. 

Behind tlie eye is a circular patch of thin, tightly stretched skin, 
Phis is the ear-drum or tympanic membrane, which closes ex- 
eniatly the Eustachian pouch of the gullet. It is believed that 
his pouch or tympanum is the remains of the first gill-cleft, the 
pirach) of Elasmobrauch fishes. Sound impinging on the ear-drum 
I conveyed to the wall of the ear capsule by a row of several small 
Utilages, the so-called columellar chain of the ear. In the 
trodela sound has to find its way as best it can through the skin 
Dd muscle of the iiead to the auditory organ. All Anura possess 
Bnstachian pouches and a. columella auris, but all do not have 
, well-developed ear-drum. 

The skin is most loosely attached to the muscles underneath. 
ATge spaces containing lymph are interposed between tliem. 
Siese lymph spaces form a protection against the danger of 
lying up. There arc two pairs of sacs placed, one pair just 
etween the upper ends of the pectoral girdle, and another pair 
t at the sides of the rudimentary stump of a tail, which have the 
ower of contraction and pump the surplus lymph into the veins of 
be neighbourhood. These are called the anterior and posterior 
urs of lymph-hearts. 

Turning now to the skeleton we observe many points of 



442 JLNUEJL [CXiP. 

difference between the Frog and the Newt The ribs in the Fnig 
are indiBdnguiahably fused with the transverse prooeasea; in laj 
few Annra are they distinct and they are always radimentaiy. TU 
vertebrae differ from those of the Urodela in the entire snpprenon 
of the inter-ventral element so that the centnim is constracted ont 
of basi-dorsal, inter-dorsal and bad-vential elements, the last 
named being very rudimentary. In some Annra the baai-ventnl 
piece is entirely absent, and in this case, since the centmm i) 
constructed entirely of dorsal elements, the notochord is found for 
a considerable period of development lying in a groove on its under 
surface. This is the so-called epichordal type of development. 




Fia. 2H. A, Dorsal, and B, Ventnl view of the Cmiiam ot a Cominon Fnc, 
Ratta lemporaria, from which the membrsne'bonea have most); betn 
removed X 2. After Psrket. 

1. Bpbenethmoid. 2. Palatine. 3. PleiTiioid. 4. Smapciuorian. 

5. Colamella. 6. Eiocoipital. 7. Ventral oortUsfiiiioua waltof 

cranium. 8. Pro.otio. 9. Anterior fontanelle. 10. Bi^t 

posterior fontanelle. 11. Quadratojngal. 19. Ntual capsok. 

n. V. VI. IX. X. tonimiDB for exit of cranial nerrea. 

The tail vertebrae are represented by a bony style, the nrostyla 
Besides it there are only nine vertebrae. The transverse processes, 
or " diapophyses " of the ninth or sacral vertebn, to which ii 
attached the ilium, are either cylindrical as in Rana, or ^ey in 
more or less wide and flat as is the case in Bnfo and ffyla. In 
most cases to the distal end of the diapophyais is attached a nodols, 
the rudimentary rib, which may either fiise with the diapophysis la 
in Bana or remain distinguishable throughout life as in AlyUi. 

The skull is constructed on the same plan as tliat of Molgi, 
but it is broader and flatter; this is due to the wide arcfabf 
ont of the upper jaws, leaving a very lai^ opening between them 



avi.] 



443 



sniJ the cranium. The cause of this again is to he sought in the 
large protruding prominent eyes, ao marked a feature of all Anura. 
The floor of the cartit.iginou.t cranium is complete in the Frog, the 
pituitary' fossa having shrunk to insignilicant dinieusions. The 
orbitoBphenoids have coalesced to form a box-like bone which ossifies 

only in the front part of the cranium but also in the hinder parU 
of the nasal aae, and is called the spheuethmoid. The parietal 
IB fused with the fronlal. 

Tlie suspensorinm sends fonvard a pterygoid process which 




Spbeaethmoiil. 3. FrnDtti-piu-ietaJ. 3. Pterygoid. 
6. ExoocipitBl, 7. PaiAKpheaoid. B, Prn-otii. 9. QuitdraU>jni!Bl. 
10. Maxilla. 11. NhhbL 13. Premaxilla. IS. Anterior D»res. 
14. Vomi^r. 15. Posterior naiM. 16. Palittine. 19. Colnniells. 
1». Qa*d»te. 20, Oooipital oondyle, U. Optio foramen. V. VII. 
Foramen (or exit ol trigeminal and facial nerveB. IX. X. Foramen [or 

exit of gtORSopharyngeai anJ pnoumogaatric nerveH. 

becomes attached to the skull in the uasul region. Underneath the 
posterior part of the pterygoid process there is a pterygoid bone 
which Burroiinda it and partly replaces it. The pterygoid sends out 
ft fork which underlies that part of the auspensorium which forms 
ftn articulation for the lower jaw. The front part of the pterygoid 
process where it bends in to rejoin the skull in ossitied by the 
palatine, which like the pterygoid iias ijecome a cartilage bone. 
The palatine is transverse to the axis of the nktill, as in Ambty- 
ttoma. Neither palatine nor pt«rygoid bears teeth, but the vomers 



444 ANUBA. [chap. 

bear a little group of teeth towards their hinder edge. Then 
vomerine teeth are used for crushJDg the food. 

The upper lip has a series of three bones on each aide, readung 
completely to the suapensorium, an additional qnadrsto-jugal 
being added to the two present in the NewL The presence of tJiii 
bone suggests that the ancestors of the Annra are to be sought 
amongst that highly modified group of the Stegocephala termed the 
L&byrinthodonta. In them u 
B > I , in the Annre the interventral 

element was absent but at 
any rate in the older forms 
the basi-dorsals, the basi-ven- 
trals and the inter-dorsals 
were distinct pieces. Iq all 
Anura there is a la^e mem- 
brane-bone of a characteristic 
T-shape, known as the squa- 
mosal, lying outside the sua- 
pensonuuL In the lower Up 
there is a splenial and a 
deutary, whilst in front the 
cartilaginous lower jaw is re- 
placed by a pre-deotary 
bone. In the frog only the 
premaxilla and maxilla and 
vomer bear teeth. MostAnuia 
agree with the Frogs in this, 
but the Toad, Btffo, and its 
allies are entirely toothless. 

The hinder visceral arches 
are reduced to a still more 
rudimentary condition than 
those of Molge. They are 
represented by a thin plate 
of cartilage called the basi-liugual with short blunt processes, of 
which only the last pair, which embrace the glottis, are ossified 
(Fig. 247). This pair are termed the thyio-hyals. The whole 
"hyoid" is thus the remains of the visceral arches. 

The pectoral girdle is much more strongly developed than in 
the Urodela. The coracoid and pre-coracoid processes are joined 
at their inner ends by a longitudinal bar, the epicoracoid, so as to 




Fio. 246. A, LaMnl view at the Skull, 
B, Posterior view of the Crsninin, of k 
ConiDion Frog, Rana temporaria n 2. 
Alter Parker. 

1. Sphenethmoid. 2. Fronto- parietal. 
3. Pterygoid, i. Squamosal. 6. Tjm- 
panic membrane. 6. ExoccipitaJ. 

7. Paraepbenoid. B. Pro -otic. 

9. Quadiatojugal. 10. Maxilla. 

II. Nasal. 12. PremaiiUa. 13. An- 
terior nares. 14. Pre-dentary. 
15. Dentary. 16. Splenial. IT. Basi- 
lingual plate, 19. Quadrate. 
30. Columella. 21. Occipital condyle. 
22. Anterior coma of tbe hyoid (cerato- 
hyal). 23. Foramen magnum. II. 
IX. X. Foramioa for the eiit of cranial 



XVI.] SKELETON. 445 

enclose a space called the coracoid foramen. The two epicoracoids 
are in the frog firmly united in the middle line. In many Anura 
however they merely overlap (Fig. 248, B). 

The upper portion of the pectoral girdle is ossified by a bone 
called the scapula. As in Urodela, however, the cartilage projects 
a long way beyond it, and this portion is called the supra-scapula 
and may become partially ossified. There is a distinct coracoid 
bone ossifying the coracoid process, and the pre-coracoid is under- 
lain by a membrane-bone called the clavicle. In firont of the 
pectoral girdle in the middle line lies a small rounded piece of 
cartilage called the episternum, followed by a bony piece, the 
omosternum. Behind the girdle in a similar position is a carti- 

B 





Fiu. 247. Visceral arches of Amphibia. A. Rana temporaria adult. After 
Parker. B. Tadpole of Bana, Affcer Martin St Ange. 

In A the ossified portions are slightly shaded, while the cartilaginous 

portions are left white. 

1. Basilingual plate. 2. Hyoid arch. 3. First branchial arch. 4. Second 
branchial arch. 5. Third branchial arch. 6. Fourth branchial 

arch. 7. Th7roh7al= fourth branchial arch. 

laginous bar with a flattened end, ensheathed by a bone called 
the sternum; the flattened end is called the xiphisternum. 
The omosternum has proved to be composed of a portion budded 
off by the conjoined epicoracoids. The sternum is supposed to be 
the first sign of the breast-bone of higher Vertebrates, but as their 
breast-bone originates in connection with long ribs, which meet one 
another in the mid-ventnd bone, this must be considered doubtful. 

In the arm the two points to be noticed are the complete fusion 
of the radius and ulna into one bone, and the reduction of the 
carpus, in which there are only six bones, three of the distal small 
bones having coalesced and the centrale being absent. The first 
digit or poll ex is rudimentary. 

In the pelvic girdle there is no epipubis: the ilium is a very 
long cylindrical bone : the ischium ossifies most of the ischio-pubic 



446 ANDRA. [chap. 

cartilage and is closely apj^ied to its fellow. In the leg the tilnt 
and fibula are fused into one bone, vhicb is about the same Iwgtli 
as the femur. The ankle is remarkably elongated, the tibiale aod 
the iibiale being long cylindrical bones, easily mistaken for tiu 
middle segment of the limb. The distal bones of the tarsus ban 
nearly disappeared, only two or three small nodules being present 
on tjie axial side. The longest toe is the fourth, that correspond- 
ing to the human big toe (hallux) is the shortest It is a matta 
of great interest to see on the inner side of tite foot a spur 
supported by a small bone which may be the vestige of a sixth 




Fia. 248. Shoulder- girdle and Steninni of 
An old male CommoD Frog, Bana ttmporaria. 

An adull lemsle, Docidopkryiu gigantta. After Parker, to illnitiat* the 
'a Arcifera. 



In both A and B the left Buprasoapola is remOTed. The parte tuiahBded 
are ossified; those marked vith email dota causiet of hjaline cartilage, thoie 
marked vith large dota of calcified oartilage. 

1. Caloifled cartilage of auprascapula. 2. OaaiSed poitioa of enprasoaptda. 
3. ScapQla. 4. Coracoid. 5. Epicoraooid. 6. Praooraooid. 

7. ClaTicle. 8. Oleooid oarit;. 9. Coracoid foramen. 10. Epi- 
stemnm. 11. Omostemum. IS. Slemam. 13. Xiphiatemom. 

digit. It is a common occurrence for the number five to be 
diminished, but very rare for it to be increased. It is believed that 
the pentadactyle limb is derived Irom a Jin like that of the Dipnoi 
by a shortening of the main axis and a reduction in the number of 
rays, and it would be not unnatural to expect to find in the lower 
groups of land animals traces of extra rays. 

The main difierences between the circulatory system of the Frog 
and that of the Newt we to be found in the arterial system. Not only 
aa in the Newt does the fifth arterial arch of the embryo disappear 
altogether, but the sixth becomes entirely cut off from the aorta and 



jcvl] circulation. MV 

in addition to supplying the lung it sends a large branch to the stdn, 
for which reason it is called the pulmo-cutaneoue arch. The 
conus arteriosus, na in Moiije, has two transverse rows of pocket 
TiJves, one near the heart and one near the outer end, but in the 
Frog there is in addition a longitudinal valve with a free 
ventral edge running j^omewhat obliquely from the one row of 
valTea to the other. When 
the ventnde tontructs it 
18, at hrst, full of venous 
blood from the right auricle 
At this stage the conus is 
relaxed a condition fthich 
inges the longitmiinul 
valve in such a way as to 
divert the blood almost 
exclusively into the pul 
nionary (las.^ages whose 
width and shortneis also 
&T0UTS itt flow intn 
them As these beiome 
filled the i onus Lontract" 
and this ha? the effect i>f 
jnakmg the longitudimil 
valve he against the open 
inga into the pulmonary 
anhes and ao ]irevi.titing 
any more blood entering 
them while at the same 
time the path into the 
systemic arches is widely 
opened By this time Mjote 
of the blood has returned 
from the lungs to the left 
auricle, and so mixed blood 
jmaHes tothehinderportion j jj jjj jy y y| 
of the body. When the arches. 9. Carotid gland. 
pressure in the ventricle 
rises to its highest point, 
the last blood, which is 
almost completely arterial, 
— aU from the right auricle having been driven out, — is able to 




iitb artarial 
12. Lingual 
tral carolid). 1.1. Common c?arotid 

(doraa! carolid). U. Syatemic arch. 

17. Dorsal Borta. 19. Pulino-cutaiieoas 
artery. 22. Subclaviaa (dorsal type). 

2i. Coeliaoo-" """ '" 



448 



[cBir. 



overcome the reeistance in the carotid gl&nd and go to the hud, 
which coQtaiuB the oi^ns having the greatest need tar thoronj^ 
oxygenated blood. 

The posterior cardinal veins are represented only by their faindet 
portions, the renal portab, all the blood from the kidneys being 
earned by the infenor vena cava. 

The brain of the Frog and of Annra in general is more hi^r 




B. The SI 
the ail 



IB venoBua baving been opened np, to alio* 



C. The same, disaeeted from (be front, the ventral nU together wilt one at 
the auiicuIo-veDtrioular valves having been removed. 

1, Ventricle. 2. Bight auricle. S. Left auriole. 4. Tmncna arter. 
iOBQB. S. Carotid arch. 6. Lingual artery. T. Carotid kIsoiL 

6. Carotid artery. 9. Systenuc arch. 10. Pnlmoentaneoui arch- 

il. Innominate vein. 12. Subclavian vein. IS. Vena cava inferior. 
14. Venn cava superior. 16. Opening of sLnua venoana into right auricle. 
16. Fulmonai; vein. IT. Aperture of entry of pnlmonary veia. 

18. Semi-lunar valvea. 19. Longitndinal valve. 20. Point ot 

origin of palm ocatsn ecus arch. 21. Rod passed from ventricle into Uir 
truncus arterioBua, indicating the courae taken by blood which flows into 
the carotid and aortic archea. 

developed than that of the Urodela. Thua (Fig. 251) the olfactoiy 
lobes of the cerebral hemispheies are connected together, and Uie 
optic lobes of the mid-brain are well developed. 

It was pointed out (p. 368) that the limbs of Vertebrates are in 
all probability derived frvim two lateral flaps of skin — two longi- 
tudinal fins. The muscles in these fins were originally prolongations 



I«f the myotomeB, and the nerves 
of couiee branches of the 
motor nerves going to the myo- 
tomea. Now as these longitudinal 
Baps were converted into paired 
fins, and these by a continual 
narrowing of their bases acquired 
greater distinctness from the body, 
the jmrtioiis of the myotomes 
supplying the innsuulature and 
the nerves in connection there- 
with became so to speak bunched 
togetlier nt the base of the liuib. 
In adult Craniata all trace of the 
original nietameric arrangement 
of the limb muscles ia lost ; but 
the metamerism of the nerves 
can still lie seen, and the bundles 
of these supplying tiie pectoral 
and the pelvic limbs are known 
as the brachial and the sciatic 
plexus respectively. In the 
Prog, where the limbs are of far 
greater importance to the life of 
the animal than are the fins to 
fish, the nerves forming the 
brachial and the sciatic plexus 
»re powerful trunks (Fig. 251, 
2. 3, and 7—10). 

The InngB are shorter than 
in tile Newt but much 
wider, and their inner sur- 
face ia covered with a 
network of low ridges 
which much increases their 
area. The kidney is a 
comparatively short and 
broad organ, very different 
from the long tajieriiig 
organ of the Newt. The 
testis ia connected by vasa 
s. AM. 




Fis. 9.51. Brain and Spinnl Cord ot a gene- 
mJiiied Annmn. In tbe PhADerogloafla the 
Ist Epiool nervH is euppresaed. x sbODt 3. 

a. Cerebral heminpbpre. b. OlfaoCorj' lobe, 
c. Bye. d. Tbttlamencfphalon. e. Optio 
lobeii. (. Cfrelielluin. g. Medulla 

□bloDgata. h. Fourth ventricle, i. 8pmal 
oord. I. Olfactory nerves. II. Optio 
nerve, in. Oculomotor nerve. IV. Fath- 
fltip-aB, V. Fiftli nerve. VII, Faoial 
nerve. VIII, Auditory nerve, IX, OIodbo. 
pharyngeal norve. X. V%Ka» necve. 

1—10, First to tenth spinal nervea, 2 and 
U imile to form the brachial, and 7, 8 and 
tt, to form the Kiiatia plexua. 



450 ANURA. [can. 

efferentia with certain special tubulea of the kidney. These tubnka 
do not open into the archinephric duct, but into a special dnct 
which runs along the surface of the kidney and opens into tike 
archinephric behind. Thus in a somewhat different way the 
separation of urine and spermatozoa is carried out quite u 




Fra. 252. The Frog. 

A. The QriDo-geniUl argoDB of the m&le, diaaected from the froat, ahet 
removal from the bod;. From Howes. 

B. The urino-geniUI organs ol the female, dealt with in the same manner u 
the above, except that, in order to eliow the natural relations of the month 
of the oviduct, the left lung and a poition of Iha oesoph^OB were also 
removed from the body. 

A- 1. Fat-body. 2. Fold of peritoneum supporting tbe teatis. 8. Effereot 

dnctB of testis. 4. Ducts of veeicula tiemmalia. 6. Veaioola seminaUa. 

6. Archinephiio duct. T. Cloaca. 8. Orifio* of ureter. 

9. Proctodaeom. 10. AUaatoic bladder. 11. Rectum. 12. Kidney. 

13. TeatiB. 14. Adrenal body. 
B. 1. Oeaophagaa. 2. Moutb of oviduct. S. Left Inng. 4. Corpui 

odipoaua. 5. Left ovary. G. Atchiaephrio dnet. 7. Ovidact. 

a. Allantoic bladder. 9. Cloaca. 10. Aperture of ovidnot 

11. Aperture of archinepbrio duct. 12, Prootodaeom. 13. Fold of 

peritoneum supporting the ovary. 14. Kidney. 

efficiently as m the Newt. The archinephric duct has a number 
of pouches developed on its walls which collectively form the 
vesicula seminalis in which the spermatOEoa are stored op. In 
Bombinator the vaaa efferentia apparently open directly into the 
archinephric dnct in front of the kidney. 



:ti.] 



REPRODUCTION. 



461 



Lying on the ventral surface of the kidney near its inner edge is 
Q elongated body called the adrenal body (Fig. 252 a, 14). This 
Drg&Q is found under various forms in most Vertebrates ; it has 
been recently shown to be derived from a peritoneal furrow which 
nes shut off from the general coelom and loses its cavity, 
[ortniag a solid rod of cells. Experiments made ou higher animals 
knd the obHcrvatioa of cases where it is attacked by disease, show 
tiiat the adrenal bodies, like the thyroid, produce an "internal 
secretion. " The substances [wured into the blood by both these 
organs are essential to the proper conduct of metabolism, that of 
the adrenal bodies being stimulating to the muscular tissues in 
particular. 

The eggs develope entirely outside the body, and there is a large 
thiu-walled swelling of the oviduct in which the ripe eggs accumulate 



'. Maloicl. From 




a. 'Jd3. Tadpole of liaiut r 



I. PoFBiJ Bm. '2. Tail ebciwbg lo.Totomea. 3. Hinii-limb. 

iust before being discharged. The male clasps the female round 
waist and remains in this ijoaition sometimes for weeks, uttering 
loud croaks at intervals until the eggs are discharged. When 
the eggs are discharged he emits the spermatozoa on to them. The 
eroaks are made by pumjiing the air from the lungs through the 
glottis into the pharynx and vice versa. The pharynx has usually 
two side pouches, the vocal sacs, which become inflated with air. 
It ia thus possible for the frog to croak when under water. 

The development is in many respects different from that of 
Urodela, Soon after the young are hatched they acquire, it is true, 
three external gills on each side, hut there is no trace of Hmbs and 
the gili-alits are closed, and as the mouth does not open into the 
alitneutary canal no food is taken. Later the gill-sHts appear; but 
Hap of skin, the gill-cover, grows back from the second visceral 
arch (the hyoid) and covers up the gill-slits and the external gills. 
The external gills then bood disappear. The two gill-covers 



452 ANURA. [chap. 

unite with one another beneath the animal, so only one little 
opening to the gill-chamber remains, usually on the left aide. The 
mouth has by this time opened into the alimentary canal, and it is 
provided with two horny ridges, one above and one below, besides 
rows of little homy prickles. The homy jaws crop the water-weeds 
upon which the tadpole lives. 

The larva is now the well-known tadpole, with a rounded body 
and a long flat tail, with which it swims. The limbs gradually grow, 
but for a long time the front limbs are hidden beneath tiie gill- 
cover. When they finally burst through the animal sheds its homy 
jaws and leaves the water. For a short time the tail is retained, 
but absorption soon removes all trace of it and the development is 
complete. 

The Anura are divided into two main groups according to the 

development of the tongue. In the Aglossa it is 

alio""* ^ entirely absent and the two Eustachian tubes have a 

common opening into the phaiynx. This curious 
group only includes two genera. In one species, Pipa americana, 
the Surinam toad, the eggs are emitted from the protruded oviduct 
on to the back of the female, and here the young pass through 
the tadpole stage enclosed in deep pockets of the moist skin. This 
species as its popular name implies is an inhabitant of S. America. 
In the PhanerogloBsa, on the other hand, the tongue is well- 
developed, being usually free behind, and in this case used to 
flick the prey, which consists of insects, into the capacious mouth. 
The Eustachian tubes are separate. The Phaneroglossa are divided 
into the Arcifera and the Firmistemia. In the fiurst division 
the two epicoracoids of each side overlap (Fig. 248, B); in the 
second they are firmly united in the middle line (Fig. 248, A). 
The first division includes several families, but the two largest and 
most important are those of the toads or Bufonidae and the tree 
frogs or Hylidae. 

The toads have no teeth whatever: their wrinkled skin is beset 
with wart-like poison glands in the upper parts, while numerous 
little homy spines occur superficially in the epidermis. They only 
enter the water at the breeding season and toads are in many 
respects more adapted to a land life than are frogs. Two species 
live in Great Britain; Bt{fo vulgaris, found everywhere, and £u/o 
calamita, the natterjack, a species with comparatively feeble hind- 
limbs, which crawls and (}oes not jump. The natterjack fr^uents 
sandy places and is thus local in its distribution. 



OUSSIFIOA.TION. 

One species of Bufo (li. amertcana) is found in the north of North 
America. But besides the Bufonidae another family of the Arcifera, 
the pRLOBiTiDAE, whioh have teeth, is represented by Scaphiopu^i, & 
bnrrowing species, provided with a sharp spur on the inner side of 
each foot, whence the name " spade-foot " toad. 

The HvLiDAE have teeth on the vomers and on the upper jaw, 
but their moat remariiable peculiarity consists in the possession of 
fleshy cushions underneath tlie terminal joints of the digits, the 
bones of which are bent up atid claw-like. By means of these 
cushions the Hylidae are able to adhere to smooth vertical surfaces, 
and so climb trees, in which they mostly live, only approaching the 
water for the purpose of laying their eggs. There is no species of 
this family in Great Britain and only one in Europe. In North 
America there are several species belonging to three genera; ffgta, 
Churopkiliuf, and Acrts. 

The FiRMisTBRmA have the two epicoracoids fused in tlie 
middle line and include the Frogs or Ranidae, There is only one 
epecies, Rana temporariit, which is here taken as the tyi)e of the 
Ainira, really native to Great Britain, but there exist a few 
eolonies of the common European species, Rana esculenta, mostly 
in the Eastern Counties. The frogs of this species are most powerful 
croakers, and as their name implies they are used as food. It is 
believed that they were introduced by monks from Europe, who 
before the Reformation used to pay periodical visits to England to 
supervise their property. 

In Cauada and the Northern United States there are eight 
Bpecies of frogs. A species believed to be identical with Rana 
temporarla is found, hut the two commonest are Rana virescens, of 
a green ground colour with lines of velvety black patches, and 
the great Bidl-frog, Rana catesbiana,, which attains three or four 
times the size of Rana temporttria, and is of browuish-yellow 
colour, peppered over with minute black dots. 



HI. AFODA. 

The order Apoda is, as has already been mentioned, dis- 
tioguished by the entire absence of limbs and the worm-like 
sp]iearance and habits of its members. In the skeleton the reten- 
tion of a complete roof of bones over the space between cranium 
and upper lip, known as the temporal fossa, and the existence of 



454 APODA. [chap. 

minute bony scales embedded in the dennis, are featnres retained 
from the Stegocephala. In accordance with their retiring bnrrowiDg 
habits the members of this Order have very small eyes, which in 
some cases are rendered quite functionless by being concealed under 
the skin. The internal anatomy is in many respects like that of 
the Urodela, but the pulmonary arterial arch does not in all cases 
join the aorta. These animals often live at some distance from 
water and the larval development is passed through inside the 
egg-shell, but even there the embryo developes large external gills. 
The species of this family are restricted to the tropics ; Ichthyaphi 
is found in India, Coecilia in South America, and Hypageophis in 
Africa. The extinct Stegocephala have been alluded to many times. 
Under this comprehensive head are comprised all the fossil Amphibia, 
remains of which are found in the Goal Measures and the Red 
Sandstones overlying them. It has been already pointed out that 
some of them, like Branchiosaurus, appear in the structure of the 
vertebral column to be the forerunners of the Urodela, while others, 
like the Lahj/rinthadonta, appear to lead on to the Anura. Besides 
these, limbless forms are also known, and there seems to be some 
probability that these were the ancestors of the Gymnophiona. 
Hence within this ancient group the beginnings of the division of 
the Amphibia into the three Orders by which it is now represented 
had already shown themselves. 

The class of recent Amphibia is divided as follows : 

Order 1. URODELA. 

Amphibia retaining throughout life a long tail. 

Family (1) Amphiumidae. 

Both the upper and lower jaws are furnished with teeth 
Fore and hind limbs small Eyes small and devoid of lids. 
The gill-slits are in a vanishing state, the gills disappear in 
the adult. 

Ex. Amphiumay Ctyptobratichus japanicus, G. (Menopoma) 
aUeghaniensis the Hell-bender. 

Family (2) Salamandridab. 

Both the upper and lower jaws are furnished with teeth. 
Eyes with movable lids. No gills or gill-slits in the adult 

Ex. Molge, Salamandra, Desmognathus, Plethodon, Am- 
blystoma. 



XVI.] CLASSIFICATION. 456 

FamUy (3) Protetdae. 

Both the upper and lower jaws with teeth. Eyes without 
lids. Maxillary bones absent. With permanent gills. 

Ex. Proteus, Necturus, 

Family (4) Sirenidae. 

Both jaws are toothless. The hind limbs, the maxillary 
bones and the eyelids are absent. With permanent external 
gills. 

Ex. Siren. 

Order 2. ANURA. 

Amphibia which lose when adult all trace of tail, hind-limb 
much more powerful than the fore-limb and used for leaping. 

Group I. Arcifera. 

Phaneroglossa in which the epicoracoids of opposite 
sides overlap. 

Family (1) Discoglossidae. 

Arcifera with a round disc-shaped tongue, adherent at the 
whole of its base ; vertebrae opisthocoelous. Teeth in the 
upper jaw only. 

Ex. Discoglossus, Bambinatar, 

Family (2) Pelobatidae. 

Arcifera with a protrusible tongue, dilated sacral ribs and 
with teeth in the upper jaw only. 

Ex. Pelobates, Scaphiopus. 

Family (3) Bufonidae. 

Like the previous family, but without any teeth. 
Ex. Bt{fo. 

Family (4) Hylidae. 

Arcifera with dilated sacral ribs, with teeth in the upper 
jaw and adhesive discs on the fingers and toes. 

Ex. Hyla, Charqphilus, Acris. 

Family (5) Cystignathidae. 

Arcifera with cylindrical sacral ribs. 
Ex. Psetidis, Ceratophrys. 



456 AMPHIBIA. [chap. XVL 

Group II. Firmistemia. 

PhaneroglcMssa in which the epicoiacoids are firmly 
united in the middle line. 

Family (6) Engyotomatidae. 

Finnistemia with dilated sacral ribs. 
Ex. Engystoma. 

Family (7) Ranidab. 

Firmistemia with cylindrical sacral ribs. 
Ex. Rana, 

Order 3. APODA. 

Amphibia of worm-like appearance, without limbs or tail 
and with vestigial eyes. 

Ex. Coecilia, Hypogeophis, Ichthyophis. 



467 



CHAPTER XVII. 
Sub-Phylum IV. Craniata. 

Class III. Reptiles. 

The name Reptile denotes literally anything that creeps (Lat. 

General Char- ^^P^ ^r r^to, to cTawl). Zoologically the term 

cteristica. denotes cold-blooded quadrupeds which are covered 

rith horny scales and which lay large eggs, inside the shells of 

irhich the whole development is completed. 

But it is not merely the size of the egg nor even the character 
)f the embryonic development which distinguishes Reptiles from 
\jnphibia. There are isolated cases of species of Amphibia in 
irhich the development is practically completed within the egg-shell, 
mt in all Amphibia the whole egg becomes converted into the body 
if the larva. In Reptiles on the other hand part of the egg is made 
nto a hood termed the amnion, which is wrapped around the body 
if the embryo. This structure is cast off entirely at birth and the 
round caused by its tearing is healed. With it is also cast off a 
portion of the urinary bladder — ^the allantois in the stricter and 
iriginal sense — which extends into the amnion and appears to 
ubserve respiration during embryonic life. In strictness, therefore, 
•nly a part of the egg is converted into the body of the embryo. 

This peculiar mode of development is shared by Birds and 
Mammals, for which reason these two classes are often included with 
he Reptiles in the term Amniota. 

Next to the development perhaps one of the most characteristic 
eatures of Reptiles is the nature of their skin. They are typically 
overed with scales which are widely different from the scales of 
isL The latter are essentially areas of the dermis hardened by the 
leposition of lime with sometimes the addition of a layer of crystals 
rom the basal ends of the ectoderm cells (enamel). 



438 BEPTILIA. [chap. 

The scale of the Reptile on the contrary is nothing bat an ana 
of the homy layer of the ekin where the cells are coDrerted into 
horn or Keratin and are adherent to one another. In the mass tS 
the scale the horn is rendered brown by the presence of pigment, 
bat the outermost layer is composed of clear cells and is known as 
the epitrichial layer. A corresponding layer covers the embiyot 
of Birds and Mammals, but is shed before birth. A stonghing 
or ecdysis of the scaly epidermis ia a constant feature of the 
Reptilia. It may take place bit by bit, or as is the case widi 
many Sauria the whole ' skin ' is cast in one piece. 

The dermal glands so characteristic of the Amphibia ha*e 
almost totally disappeared, being restricted to a small area, as, for 




Fia. 364. Section through the Scale of a LiBurd. 



1. Epitrichial layer. 2. Heavily comiBed oellB forming the scale. 8. Pig- 
ment cell, i. Ordinary cells of horn; Uyer. 6. Innermost Halpigbian 
layer. 6. Dermis. 

instance, the front of the thigh in a lizard. It follows that a Keptale 
is essentially a dry-skinned animal and by no means a "slimj 
beast." 

Besides the structure of the skin Reptiles are distinguished from 
Amphibia by many other points in the anatomy. Thus the skull 
has a larger number of cartilage bonee, and includes what maf 
be considered primitively part of the vertebral region dnce the 
hypoglossal nerve (see p. 435) is now a cranial nerve. The skull 
articulates with the vertebral column by one condyle. In the heart 
the conuH> arteriosus has disappeared and the ventricle is partly 
divided. The sexual part of the kidney is entirely disjoined from 
the asexual 

The lungs have to some extent acquired a spongy texture, and 
the mechanism for inhaling and exhaling air is usnally to be fmnd 



CLASSIFICATION. f^H 

in t^e ribs, not in the hyoid or remains of the hinder visceral 
KTcbes as in the Amphibia. 

Living Rflptiles are divided into five Orders, of which one 
consists only of one species, Sphenodon punctatug, found iu New 
Zealand. This animal is the type of the Order (i) Rhyncho- 
oephaJa, and is especially intere.^tiDg as not only being to some 
extent intermediate in stniL'ture between other Orders of living 
Reptiles, but as recalling very closely the structure of some of the 
oldest fossil Reptiles known to us ; indeed it retains in many 
respects a structure which we believe wa^ possessed by the common 
ancestors of the remaining four groups. These are the (ii) LacertUla 
(Lizards), the (iii) Ophidia (Snakes), the (iv) Chelonia {Turtles 
and Tortoises) and lastly the (v) Crocodilia (Alligators and 
Crocodiles). Of the five Orders only the secoud and third are repre- 
sented in Great Britain and these by very few species ; in North 
America the last four are well represented. The Lacertilia and the 
Ophidia are the most closely related and they are often grouped 
together under the same term SAURIA. 

Ab type we may select the i-ommon lizard, Lacrta vlvipara, 
which may be seen on very warm days disporting 

^_''"'' itself iu sandy and stony places in the south of 
England. On the Continent it and allied species 
»re far more abundant ; iu the South of Europe in summer the 
whole country is alive with lisards. Almost every step in the 
country causes two or three specimens to rush rapidly away into some 
retreat, either a hole under a stone or a cleft in the bark of a tree. 

The English Lizard has roughly the shape of a Newt, but there 
is a distinct neck region in front of the fore-limb, and the limbs are 
sufficiently powerful to completely raise the belly well above the 
ground and also to run at a compariitively rapid rate. Both mauns 
id pes have five digits which end in aharji claws. The body is 
covered all over with minute scales (Fig. 254), of which the prevail- 
ing colour is reddish-brown above, and orange passing into yeUow 
benoath. On the ventral surface and the top of the head the scales 
are larger and arranged in pairs. The ear-drum is situated at the 
bottom of a slight pit, which is the first appearance of the outer 
ear. It is not developed in all Reptiles. 

The anal opening is a transverse slit at the root of the tail 
behind the hind pair of legs. In front of the thigh the scales are 
perforated by a row of pores, the opt-nings of the only dermal glands 
which the lizard possesses. 



i60 REFTILIA. [chap. 

TorDing at once to the skeleton, we find that the TCTtebnl 
column consists of procoeloos yertebrae. All the 
vertebne articalate with one another by oyeriapping 
&cets called pre- and post-zygapophyses as in Amphibia. 
Although extemaUy similar to the vertebrae of Amphibia, the 
▼ertebrae of the Lizard and of Reptiles generally are fonned of 
different elements. Thus the basi-dorsal and inter-doisal have been 
suppressed, while the basi-ventral forms an interyertebral disc of 
cartilage which in the tail bears a pair of processes united with one 
another to form a Y-shaped chevron bone, recalling in its sh^ 
the haemal arches of fish. In the neck r^ion the basi-ventrals each 
bear a bony wedge which is called the sub-vertebral wedge bone. 
The centrum is formed by the united pair of enlarged inter-ventrals, 
while the basi-ventrals are partly converted into intervertebral pads, 
and partly into small parts which occasionally are ossified as the 
so-called intercentra. The rib has shifted its position so that the 
capitulum is attached to the front end of the centrum (inter- ventral) 
behind the basi-ventral to which it belongs. The tubercular attach- 
ment is represented by ligament. There are two sacral vertebrae 
which have expanded transverse processes with which the ribs are 
fused. Behind these come the vertebrae of the tail — the caudal 
vertebrae. Each of these bones is made up of two halves, an 
anterior and a posterior, which are but loosely connected with 
one another. The consequence is that when a Lizard is seized by 
the tail this organ in many species snaps in two, one of the vertebrae 
breaking into an anterior and a posterior half. 

All vertebrae in front of the sacrum, except the first two, have 
distinct ribs attached to their transverse processes. The first two 
are called respectively the atlas and axis vertebra. The first is as 
in the case of Amphibia a mere ring. It is composed of the first 
pair of neural arches united with the first pair of basi-ventrals, and 
is therefore to a certain extent homologous with one of the inter- 
vertebral discs. The second has a well-marked centrum, to the 
front of which is attached a peg-like process — the so-called odon- 
toid process — which projects through the ring of the atlas. This 
odontoid process is quite unrepresented in the Amphibia, but it is 
characteristic of all Reptiles and Birds and Mammals. It is formed 
by the first pair of inter-ventrals and is therefore the first centrum. 
In young specimens it can be seen to be separated from the centrum 
of the second vertebra by an unossified disc representing the second 
pair of basi-ventrals. 



The ribs in front of the jjectoral girdle remain quite short — this 
region is the cervical or neck region. Immediately beliind the 
pectoral girdle the riba are very long 
and curved so as to half encircle the 
body like the hoops of a barrel. 
The foremost have attached to their 
lower ends cartilaginous bars — the 
sternal ribs — which are in turn 
united with a cartila^nous sternum 
in the middle line. This structure 
has the form of a lozenge -shaped 
plate with a hole in the middle, 
ending behind in tvro forks to which 
some of the posterior sternal ribs 
are attached. The whole sternum 
has arisen from the junction of the 
sternal ribs one with another. First 
those of the same side unite to form 
a eternal band, and then these two 
bands unite in front hut remain 
separate behind. The hole in the 
middle also is a place where they do 
Dot unite. 

The flkuU is distinguished from 
the Amphibian skull by many 
features. The jaws do not arch 
outwards at the sides of the cranium, 
as in the frog, but are bent inwards **• S"*"!*! ^'^•^^ not united. 

1 1 ■ r, 1 ■ 1 1 ■! J- SloniBl nh. 

Bnderneath it. Belnud. the cartilage 

of the cranium is completely replaced by four bones — by the supra- 
occipital above the foramen magnum, the ex-occipitals at the sides 
of this opening, and the basi -occipital beneath. This last bone 
bears a single knob or condyle which articulates with the atlas 
Tertebra. To t!ie formation of this condyle the ex-occipitals in 
some degree coutribute. The basi-occipital and the single condyle 
and the supra-occipital are highly characteristic of all Reptilia — as 
is also the basisphenoid bone. This is a bone replacing the 
eartitaginous floor of the cranium just in front of the basi-occipital. 
The paraspheinoid so characteristic of Amphibia is reduced to a 
mere splint attached t« the front of the basisphenoid. 

'ITie anterior part of the cranium is so compressed between the 




Via. 255. Ventral view of the 
Shoalijer~Giritle &diI Sternuni of 
a Lizard. LofTnanttiii longipei 
« 2. After Parker. 

1. Interclsviole. , 2. Clavicle. 
S. Scapula. 4. Coracoid. 

5. Precoraeoidal process. 6. Glen- 



^^m 



462 REPTILCA. [CB&P. 

lai^e eyes that ita c&vity completely disappears and it becomes re- 
placed by a vertical sheet of membrane, the inter- orbital septan. 
It follows that in the dried skull the two orbits apparently open 
widely into one another. Almost the entire brain is poshed back 




Fio. 25G. A, Lateral v 



1. Premnxilla. 2. MaiilU. 3. Nasal. 4. Lateral ethmoid. 6. Sapia- 
orbilal. 6. Lachrymal. 7. Frontal. S. PoatlrontBl. 9. PrefrontaL 
10. BasiaphcDoid. 11. Pro-otic. 12. Epi.otic. 13. Pterygoid. 
14. Epipterygoid (colmuella oraoii). 15. Jugal. 16. TraDBvene bone. 
17. I'araspheDoiil. 18. Quadrate. 19. Parietal. 20. SqnamoeaL 
21. Supratempoial. 22. Bxoccipital. 23. Dentar?. 34. SpleDiaL 
25. Sapra-angnlar. 26. An^^laF. 27. Coronoid. 28. AiiicaUT. 
29. Tomer. 30. Basi-ocoipital. 31. Orbitoapbenoid. 

behind the eyes into the hinder part of the cranitim. Only the 
olfactory stalks run through holes in the npper part of the septum. 
The orbitosphenoid of Urodela and the spbenethmoid of the frog 



XVII.] SKELETON. 463 

are quite unrepresented, though in some of the larger lizards allied 
to Lacerta there is a minute orbitosphenoid bone in the upper part 
of the inter-orbital septum (31, Fig. 256). The inter-orbital septum 
is certainly a characteristic of the primitive Reptilia. It has how- 
ever been lost in some of the most recent and highly modified forms. 

The auditory capsule as in Teleostei is completely converted 
into bone, but it is ossified by three bones only, anepi-otic above, 
which fuses with the supraoccipital, an. opisthotic behind, which 
joins the exoccipital, and a pro-otic which remains distinct. 
There is no trace of the pterotic bone so characteristic of Teleostei. 

As in Amphibia the first visceral arch is represented by an upper 
half consisting of a suspensorium with a pterygoid process, and a 
lower half — Meckel's cartilage. In the upper half, however, the 
cartilage is completely replaced by bone. The suspensorial portion 
forms the quadrate bone, which is attached to the side of the 
auditory capsule. The pterygoid process is completely ossified by 
the pterygoid bone behind and the palatine in front. 

A curious bone characteristic of the Lacertilia excluding the 
Amphisbaenidae, and called the epipterygoid or columella 
bone, nins from the pterygoid vertically up to the parietal. This 
bone is however found only in some Lacertilia, not in Reptiles 
generally. 

The two pterygoid bones however, instead of arching outwards 
converge, under the base of the cranium, and they articulate with 
outgrowths from the basisphenoid, called basipterygoid processes. 
The palatines are united in front with the floor of the nasal capsule : 
tliey bear on their inner sides slight ridges which project somewhat 
into the cavity of the mouth. These ridges support a flap of the 
lining of the mouth, the palatal flap, which is most characteristic 
of all Reptilia but is not found in any Amphibian (Fig. 259). It is 
a first trace of the process which ends in the higher animals and 
even in some Reptiles in the division of the mouth-cavity into an 
upper air-passage and a lower food-passage. 

The end of MeckeFs cartilage which articulates with the quad- 
rate is converted into a bone, the articulare. Three pairs of the 
hinder visceral arches are preserved. These retain their rod-like 
form as in Urodela, the median connecting pieces (copulae) remain- 
ing small. 

The membrane-bones of the skull are one of its most character- 
istic features. The roofing bones are the same as in the Urodela — 
paired nasals, frontals and parietals. On the roof of the mouth 



464 KEPTILIft. [OHtf. 

there are two vomers and a parasplieiioid. The vomers, howevw, 
are rod-like, toothless and placed close together, and the pa>»- 
gpheuoid is a small rndiment. 

The bones of the side of the bead and the npper lip fonn a moet 




Stegooephalan {Mattodomaunu giganteut, ftbont ODe-flflMoth lut. liM, 
after E. Fraasj. B. (ieneralized Rbjnobooephftlaa uid CrooodUUn. 

C. (ieoeialized LRcertilUn, often losing even the ucade here iodinUd. 

D. Generalized Bird. /r. Frontal. j. Jogal. I. Lktenl temponl 
tossa, la. Lachrymal, mx. Maxilla, n. Naiial opening, na. NmoL 
0. Orbit, pa. Parietal, jitnx. Prenuiiilla. pr/. Prsfrontal. ptf. FoM- 
troDtal. pto. PoBt-orbital. qj. Quadrato-jagoi. qu. Quftdiste. i. Sopia- 



peculiar scaffolding which is widely separated from the craninm. 
The Lizard is in an intermediate condition between the Stegocephak, 
where a continuous sheet of bones extends from the craoiom to 
the upper lip, and modem Amphibia, where all those boDes have 



JtVn.] SKELETON. 465 

disappeared, leaving a large vn^utty between the craDium and upper 
Kp. 

In front in the upper lip there is a premaxilla bearing teeth 
ibllowed by a maxilla in which there are alao teeth. The maxilla 
joined to the pterygoid by an eetopterygoid or transverse 
bune. Between the maxilla and frontalon the side of the face are two 
bones known tm prefrontal and lachrymal. The line of t>OQei< in 
the npper Up is continued by the jugal. This nnJtes with a bone 
placedbehiud the eye termed the post frontal, which joins both the 
frontal and parietal. Thus the eye is surrounded by a ring of bone. 
The squamosal is a characteriHtii- V-ahaped bone. The apex 
of the V artioidates with the upper aide of the quadrat* : of the 
•two arms one is directed forwards and meets the postfrontal, thus 
forming a bony bar parallel to the cranium which is called the upper 
emporal arcade, Tlie other limb in directed backwards and 
awards and meets a cre^t on the parietal eo that a bridge is formed 
ixtending over the hinder part of the cranium, The space in the 
iried skull existing between this bridge and the iTanium is called 
he post- temporal fossa. In many reptiles, including most 
jacertilia, there is a similar sgiace between the cranium and 
the lateral bridge formed by the junction of the squamosal and 
postfrontal. This space ia roofed over in Lacerta by two membrane- 
buues calleil supratemporals, but when uncovered it is known as 
the supratemporal fossa. 

Finally, the space intervening between the quadrate and jngal on 
fcbe side of the face is known as the latero-temporal fossa. In 
f^iAenadoH, Crocoditia and a very large number of extinct Beptilea it 
B bounded below by a quadrato-jugal Iwne which joins the jugal 
» the quadrate. When the ipiadrato-jugal is present the series of 
>anee consisting of maxilla, jugal am) quadrato-jugal is known as the 
ower temporal arcade. The upper temporal arcade is formed 

Bwe have seen by the postfrontal and the squamosal. The loss of 
e quadrato-jugal in Lacertilia is doubtless connected with the 
■eater mobility of the jaws. In some lizards, notably in Geckos, 
the quadrate can move slightly on its articulation with the skull, 
can also the pterygoid on tlie basipterygoid process. When 
tlte lower jaw is pulled downwards and backwards by its depressor 
nuscle it tends to throw the lower end of the quadrate slightly 
forwards: the pterygoid slides on the basigphenoid, and pushing 
the ectopterygoid tilts the maxilla wtightly upwards. With the 
tnaxilla all the other bones of the face move, and the membranous 

K. * .M, 30 



466 SEFTILIA. [CBiP. 

iaterorbital eeptam pennits the ethmoidal re^on of the craoinm to 
be slightly bent on the hinder portion. 

The cartilaginous lower jaw is ensheaUied bf five distinct 
membrane bones. The dentary and splenial occnpy the same 
positions as in Teleostomi and Urodela. The angular clamps the 
under aide of the articulare, the cartilage bone replacing die upper 
end of the cartilaginoiui jaw. The supra-angular lies above the 
angular on the outer side of the articular. The coronoid is a 
small projection on the upper edge of tiie jaw. 

The pectoral girdle is at first sight exceedingly complicated, but 
in reality it consists of the same parts as in the Anura. Above the 
c&rity for articulation of the arm — the glenoid cavity — there is the 
cartilaginous scapula ; below the girdle forks into a coracoid and 
precoracoid united by an epicoracoid. The cartilage bones pieaent 
are the scapula, precoracoid and coracoid. The cartilage abow 
the scapular bone ia slightly calcified but is not converted into 
bone; this region as in Am^Jiibia is called the supra-scapnl^ 
Along the inner edge of the supra-scapula, scapula and precoracoid 
TMns a strong membrane-bone, the 
clavicle which reaches a median bone, 
the T-shaped interclavicle. Thi» 
bone underlies the sternum. The two 
epicoracoid cartilages join the ant«riiH 
edges of the sternum (Figs. 255 and 
258). 

The space between the coracoid 
and precoracoid is called the coracoid 
fontanelle. Since in the Urodela it 
is not closed by an epicoracoid it ma; 
be regarded as a hay or indentation m 
the lower half of the originally sim[^ 
pect^iral girdle. The condition of 
affairs in Urodela throws considerable 
tight on what occurs in certaia other 
Lacertilia, such as the American 
Iguana. There we find that a similar deep indentation has become 
developed on the inner side of both scapula and coracoid, so that 
projections are formed to which the names mesosc&pula and 
mesocoracoid have been given. These are not ossified by sepa- 
rate bones but are regions of the scapula and coracoid bones. 

The fore-limb of the lizard might be taken as the tgrpe of the 




t, Supraac&pala. 2. Sc&pulft. 
3. OleDoid cavity. 4. Co- 
racoid. £■ Clavicle, 
terclavicle. 7. J 

coidal process. 



pvn.] SKELETON. 467 

I pentadai;ty]e limb, since there are five tiogers and the i-arpns has 
r ftll the nine bones developed. 

) The pelvic gir'Ue differs markedly from that of any Amphibian, 
, in that iu ite lower portion there is a hole called the obturator 
' foramen, corresponding to the cora«oid foramen in the pectoral 
> girdle. The girdle is ossified by three bones, viz., a vertical ilium 
I articulating with the ribs of the sacral vertebrae, a pubis ossifying 
the anterior limb of the lower half of the girdle and an ischium 
osaifyiog the posterior limb. Both pubis aiid iscliium meet their 
I fellows in the middle line ; such a union is termed a symphysis. 
^The two obturator foramina are closed below and at the same time 
Keparated from one another by a loiigitudiual ligament which may 
^kave u certain amount of ossification in it. All three Iranes con- 
^Bibut« to the formation of the acetabulum, the cavity for the 
Huiiculation of the femur. 

W The presence of the obturator foramen and a distinct pubis is 
characteristic of all Reptiles, Birds, and Mammals, and at once 
distinguishes them from Amphibia. 

On the hinder edge of the pubis there is a projection which is 
called the lateral process. In some e.\tinct reptiles this process 
was extraordinarily long and ossified by a distinct bone, which has 
been called the post-ptibis. It is the post-pubis which forms the 
BO-called pubiis of Birds and Miimmals. 

The most marked feature of the hind limb is the formation of a 
shaqily- marked "ankle" joint. There is one place and one only 
where the foot bends on the shank : whereas in Urodela bending 
can (H'cur at any place in the mosaic of small bones which forms 
the tarsus. 

In the lizard all the three upper bones of the tarsus are joined 
to form a horizontal bar. The lower bones have almost entirely 
coalesced with the corresponding metatarsals, only the third and 
fourth of the series being distinguishable. Thus the lizard has what 
has been called an inter-tarsal joint — an arrangement which is 
highly characteristic of many Reptiles and of all Birds. 

All trace of the division of the muscles into myotomes has 
disappeared, but the innermost layer of the muscles 
of the flanks has become divided secondarily into a 
aeries of bands connecting eacli rib with its successor. Thesd 
bands are termed the intercostal muscles and each consists of an 
external and an internal layer of fibres. The fibres of the external 
layer slope upwards aud forwards and, in contracting, cause the nbs 



468 BEPTILIA. [CHiF. 

to rotate forwards ; the fibres of the inner layer slope npwvds and 
backwarda aad have the reverse effect. Respiration is effected by 
the pulling forwards and backwatdi 
of the ribs by these intercostal 
ausclee. In their relaxed cod- 
ditioD the rit» slant strongly back- 
wards. When they are pulled 
forward by musclee attaching thea 
to the anterior vertebrae and bf 
the external intercostals, diey rot- 
ate forwards so as to stand out 
at right angles to the vertebnl 
column and thus enlarge the cmvi^ 
of the chest, that is, the ooeloDL 
The diminution of pressure in tin 
air-tight cavity at once causes u 
inrush of air through the glfrttii, 
the elastic luogs are expanded and 
their walls closely follow the chat 
wall It will be noticed that the 
mechanism of inspiration is voj 
different from that of Amphibisai 
(p. 424). The network of low ridgei 
which is found already on the aaa 
side of the Frog's lung has iu tin 
Reptile greatly increased in com- 
plexity. The primary ridges ire 
much higher, and between them an 
lower secondary and even tertiai; 
ridges : the cavity of the lung is ai 
it were partly filled up by a spongr 
mass. In all Sanrians however the 
central cavity is easily rect^niud 
as a wide space : whilst in Croco- 
diles and Tortoises, still more so in 
Birds and Mammals, it is lepn- 
sented only by the bronchial 
tubes. 

The lungs are connected widi 
the glottis by a comparatively long 
stalk, the trachea or windpipe, which is stiffened if rings </ 




1. Posterior or internal nares, 
2. Palatal folds. S. Internal 
opening of EDstachian tabes. 



XVII.] ORAL CAvnr. 469 

csrtilage. A aimil&r structure is found amoDgst Amphibia in the 

Gyninophiona and in a few Urodela. Immediately below the glottis 

the trachea is enlarged. The enlarged portion is stiffened by a 

large, broad, ring-shaped 

cartilage, the cricoid, to 

which are articulated two 

arytenoid cartilages. The 

whole structure consisting 

of the dilatation of the 

trachea and iu cartilages 

ia called the larynx. 

The Lizard like the 
Prog lives principally on 
insects and ia provided 
with a long mobile tongue 
cleft at the tip, by means 
of which the prey are 
whisked into the mouth. 
The tongue is free in front 
and attached behind, the 
opposite arrangement to 
what is found in the Frog. 
The teeth are simple and 
conical, and are implanted 
in a groove on the inner 
side of the bones bearing 
them. As the Lizard grows 
they become actually fused 
with the bone along the 
aide of the groove. 

When a Frog's mouth 
is forced open, amongst the 
most striking features of 
the roof of the mouth are 
the two large eyeballs shin- 
ing through. When we 
open the mouth of a lizard 
nothing of the eyes can be 
seen. There is projecting 

inwards from the upper lip on each side a flap, the palatal flap. 
This does not meet it« fellow in the middle line, a cleft existing 




I. II. ni. IV. V. VI. First to siith arterial 
■rchea. 12. Tracheo-litigLiftl (veotral 

carotid). 13. Common carotid (doreal 
carotid). lH. Bigbt ajstemlo arch. 

16. Lett ajBleniio »roh. 17. Dorsal 

aorta. 19. Fulmonary. 20. In- 

nomiaate. 21. Soapular (equivalent 

of subclaTian ofveatraltype). 22. Sub- 
clavian (dorsal type). 24. Coeliao. 



470 



REPTILIA. 



[chip. 



between them. These flaps conceal the eyeballs and the ww* 
openings of the Eustachian tubes which lead up to the ear-dmin. 
Palatal flaps as already mentioned are found in iJl Beptilia. 

Turning now to the circulatory system we find that the conns 
arteriosus no longer exists as such, having been cleft into three 

trunks down to its commencemeiit 
in the yentricle. One of these 
trunks is ventral and slightly pos- 
terior to the others, and gives rise 
to the two arterial arches, which 
as pulmonary arteries supply the 
lungs and have no connection 
with the aorta. The other two 
arterial tninks form the right and 
left roots of the aorta. They cross 
each other at their origin, that 
which passes to the right of the 
oesophagus arising from the left 
of the ventricle and vice ver9d. 
The third pair of arterial arches, 
corresponding to the carotid arches 
of Amphibia, are well developed 
in the lizard. They have a 
common stem which arises from 
the right systemic arch. In some 
Lizards the longitudinal epibrsn- 
chial vessel of the embryo persists 
between the carotid and systemic 
arches on either side, so that in 
this respect a Lizard may be even 
more primitive than a Newt. In 
others, as in all other Reptiles, this 
connecting link has disappeared. 

The ventricle has projecting 
into its cavity two imperfect par- 
titions or septa. One is the con- 
tinuation of the division between 
the two auricles, the other is a ridge which arises from the ventral 
side and tends to separate the opening of the pulmonary arteries 
frrom that of the right and left aortic arches. When the ventricle 
at first begins to contract it is full of venous blood fit)m the right 




Fio. 261. Diagram to show arrange- 
ment of the principal veins in the 
Anura and Reptilia, 

1. Sinus venosus, gradually disappear- 
ing in the higher forms. 2. Duc- 
tus Cuvieri= superior vena cava. 
3. Internal jugular = anterior card- 
inal sinus. 4. External jugular 
= sub-branchial. 5. Subclavian. 
6. Posterior cardinal, front part 
= vena azygos. 7. Inferior 

vena cava. 8. Renal portal = 
hinder part of posterior cardinal. 
9. Caudal. 10. Sciatic = internal 
iliac. 11. Pelvic. 12. Anterior 
abdominal. 18. Femoral = 

external iliac. 



t : by the time arterial blood has commenced to euter it from 
le left auricle, the veutral septum meiitiooed above has been driven 
against the opposite wall, ao aa to shut off the pulmonary trunk 
from the rest of the ventricle and prevent its receiving any more 
blood. The left aortic arch, which axiaes on the right, receives 
mostly venous blood from the right auricle, the right aortic arch 
arterial blood from the left auriule, and it is from this arch, aa 
mentioned above, that the 
carotid arteries ari^. Hence 
the head receives compara- 
tively arterial blood, and 
all the rest of the body 
minted blood. The lingual 
artery of Amphibia is re- 
presented in Reptiles by a 
vessel {tracheo-lini^nal or 
"ventral carotid") which 
arises from the carotid arcli 
oear tlie middle bne and 
supplies the tongne tra' hea 
and muscles of the neck 
and shoulder. 

The vessels supplying 
the fore-limb arise together 
from the right s>stemic 
arch in the case of Lizards 
instead of na in Anura from 
both right and left arches 
tliey are subclavians of the 
dorsal type (see p. 33u), 
but in Chelonians and 
Crocodiles the eubt-lavians 
are ventral in origin, com- 
ing off from tiie carotid 
trunk on either side close 
to its division into ventral 
and dorsal carotids. In 
Lixarda this "ventral subclavian" is represented by the scapular 
artery which runs to the shoulder region. 

The veins, on the whole, flosely resemble those of Molge, 

There is however no large cutaneous vein, and the anterior part of 

I the posterior cardinal, now called the vena azygos, is found only 




Fio. 362. UrinogenitiiJorgnnBotMiiteLitard, 
1. Testis. 2. Vns deferens — archinephria 
dact. 3. EpiJidyiniB^ (msNOQephras). 

4. Kidney — motajiephrOH. 3. Uruter. 

d. Bkdtler. 7. Rectam out an<] turned 
back. 6. Cloftca laid open. t). Open* 
inK of ViiB daferena. 10. Groove Uading 
to opening of penis. 11. Pfnia. 13. Dor- 
sal fiorla. 



472 REPTILIA. [chip. 

on the right side, where it receives the numerous intercostal veins, 
returning the blood fix)m the muscles connecting the ribs. The 
renal-portal, sciatic, femoral and anterior abdominal veins have the 
same arrangement as in the Urodela. 

The brain is distinguished by the comparatively laige sixe of 
the cerebral hemispheres, which overlap the thalamencephalon above 
and at the sides. They end in fix)nt in large pear-shaped olfactoiy 
lobes. The cerebellum is a high vertical ridge and is thus much more 
prominent than in any Amphibian. The remainder of the hind 
brain, the medulla oblongata, includes a longer portion of the spinal 
cord than it does in Amphibia, for the hypoglossal nerve arises from 
its side and escapes through an aperture in the exoccipital bone. 
This nerve is reckoned the twelfth cranial, not the eleventh, for 
there is a trunk called the spinal accessory or eleventh cranial 
This arises by several roots from the side of the medulla oblongata, 
joins the vagus in a ganglion, and then leaving the skull supplies 
some of the neck muscles. In the Ophidia this nerve is not dis- 
tinguishable from the vagus. 

In the genital organs, the Lizards and Reptiles generally are 
distinguished from Amphibia by the complete separation of the 
mesonephros from the metanephros or functional part of the kidney. 
The persisting part of the mesonephros, now known as the epididy- 
mis, is only developed in the male, where it is closely connected 
with the testis. As in the Newt it receives the vasa efferentia. In 
the female the oviduct is shorter and has a wider internal funnel 
than in the Amphibia, and it is also placed further back so as to 
be rather nearer to the ovary. This is an arrangement suited to 
the large size of the eggs, which are too heavy to be drawn any 
distance by the current produced by the cilia of the oviduct. 

The egg is fertilized whilst still in the oviduct The male lizard 
has two organs called copulatory sacs or penes, situated, one on 
each side, on the hinder wall of the cloaca. These, when not in 
use, are hollow pouches opening into the cloaca. When in use they 
are turned inside out, and are then seen to have grooves leading to 
the openings of the vasa deferentia or archinephric ducts. 

Most lizards lay their eggs in crevices amongst stones and 
allow them to be hatched by the heat of the sun. In all cases a 
considerable amount of development goes on before they are laid. 
In the English species Lacerta vimpara the young burst through 
the egg-shell and use up all the yolk whilst they are still in the 
oviduct, so that in common parlance they are bom alive, that is, 
as little lizards and not as eggs. 



Order I. Rhynchocephala. 

As mentioned above, the order Hhynchoi-eph&ls ts represented 
by the single species, Sph/rtioJon punrtatm, found only in New 
Zealand. This is a very Lizard-like animal. The back is covered 
with small scales which in the middle line form a comb-Uke crest : 



A 




Fla. 3G3. Skull of Splienudo/i pmielatut v 1. 
, Lateral. B. Dorsal. C. Venlra!. D. Posterior. After toq Zittel. 
Promaiills. S. Nasftl. 8. Prefronliil. i. Prontnl. 5. Poal- 
froDtol. 6. Parietal. 7. (iiituinioBnl. 6. Quadratojugal, 9. Quad- 
rate. 10, Postorbital. II. Jugal. IS. Maxilla. 13. Vonii 
14. Palstine. 16. Pte(7(!oid. IK. Eato pterygoid or traDaTerse bor 
17. EioooipiCtil. IB. EpipterfROi^ 19' BaKiBphenoid. 30. Bnpra- 
(emporal foaua. 21. Latt^ral temporal fossa. 23, Orbit. 23. Post- 
temporal fosBn. 34. Korainen magnum. 25. Anterior nari 
26. Inlprpftrii-lal foramea, 27. Denlnrj. 28. Supra-angulf 
00. Articular. 

tiie belly is covered with large square scales. In the skeleton and 
m&le genital organs, however, Spheaodon is widely different from 
the IiiEard. The ({uadrate in the skull is quite immovable, beiug 
firmly clamped by the Bi|uamosal anil iiiLtdnttojiigal. I'he latero- 
temporal fossa is thus completely bounded below and the supra- 
temporal fossa is uncovered. Between the parietals is a gap called 



474 REPnLiA. [chap. 

the interparietal foramen : in this is sitoated the tip of the 
pineal body which has here all the characters of a simple eye. 

The animal has teeth when young, but they become worn away, 
while the edges of the maxilla and premaxilla become converted 
into cutting edges. 

The vertebrae are amphicoelous, and the basi-ventrak, repre- 
sented by the sub-vertebral wedge bones and chevrons, are placed 
beneath the interspaces between the vertebrae throughout the neck, 
trunk and tail, and not as in Lizards in the neck and tail regions 
only. 

The ribs have three divisions, there being a small intermediate 
piece intercalated between the dorsal and sternal rib. From the 
dorsal rib a hook-like outgrowth, the uncinate process, projects 
backwards, which overlaps the next rib as in Crocodiles and Birds. 

Behind the sternum there is a long series of rod-like bones, the 
so-called abdominal ribs, embedded in the muscles of the belly. 
They are placed parallel to the direction of the sternal ribs, that is, 
they slope obliquely forwards and inwards. They are regarded as 
membrane bones and supposed to correspond to the ventral bony 
scales of the Stegocephala. 

All these peculiarities of the skeleton are found in many of the 
oldest fossil reptiles. 

There is no proper copulatory organ : the cloaca is used for this 
purpose as in the Urodela. 

From a condition in many respects represented at the present 
day by Sphenodon, the ancestors of living reptiles appear to have 
diverged in two directions. 

On the one hand, the original stock gave rise to descendants 
with long flexible bodies and extensible jaws — this latter feature 
involving of course a movable quadrate. The cloacal opening 
became converted into a transverse slit and copulatory organs 
became developed behind it. This stock includes the Snakes and 
Lizards which are often included in the one comprehensive Order 
the Sauria. 

On the other hand, the descendants of the common ancestral 
form diverged in the direction of heavily armoured forms, in which 
membrane bones underl}ing the scales were developed and in which 
the jaws are very powerful, the quadrate remaining immovably 
clamped by the quadratojugal. The cloacal opening became a 
longitudinal slit and developed the single median copulatory organ 
on its front walL This stock includes the Turtles and Crocodiles. 



SAURIA. 

We now tarn to the Saubia. Moat people would imagioe that 
the task of distinguishing a lisard from a snake was an easy one. 
But if we were to collect together all the limbless species of 
Reptiles we should find not only that they differ very much from 
one another in the structure of the skull and in other points, but 
that they are more nearly related to different families of lisards 
than to one another. There is no doubt that the snake-like forms 
have been derived from four-limbed reptileB like lizards, for some of 




Ftn 964 A limblHBi Llaord ttffuti jragUn the bltod worm hIi^I I ly reduced. 

tham have rudimentary vestiges of limhs. It is evident then that 
there must be an advantage in certain situations in getting rid of 
limbs, and it is further evident that the effect of this advantage 
has been that not only in one but in many families of liiurds 
some species have lout their limbs. The kind of life to which a 
snake-like form is suited is a lurking one amongst crevices in 
atones, or thick vegetation, or in the aoil, where movement is heat 
effected by wrigghng and limbs wtudd be in the way. 

Under these circumstances, we mut^t either class together all 
limbless Sauria as snakes, and thus give up the idea that the 
tnembers of an Order must necessarily be descended from the same 
ancestral species, or else we must select one group as the true 
Bnakos (Ophidia), the members of which have many other characters 
in common besides the negative one of having no limbs. This 



476 REPnuA, [chap. 

latter coarse is that which has been adopted by Huxley, who defines 
true snakes somewhat arbitrarily as those forms which have lost all 
trace of the pectoral girdle and of the urinary bladder, although 
they may retain traces of hind-limbs. 



Order II. Lacertilia. 

The Lacertilia then include all species of Sauria which have the 
right and left halves of the mandibles connected by a sutural 
symphysis and which retain a urinary bladder and some trace of the 
pectoral girdle. In all other characters they are a very diversified 
group. Most of them possess well developed limbs, movable 
eyelids and movable quadrate bones, but a good many species 
belonging to specialised burrowing families have no limbs and 
scarcely a trace of the pectoral girdle, while the eyes are concealed 
beneath the skin and the quadrate has become more or less im- 
movable. Some, e.g., Draco volans, have the hinder ribs expanded 
so as to press out two expansions of skin and form a parachute-like 
expansion on each side, by means of which they are supported as 
they flit from tree to tree in great leaps. Most feed on insects, 
worms, &c. like the English lizards ; some are large enough to seize 
mice and birds and frogs. The limbless forms are represented in 
England by the Blind- or Slow- worm, Anguis fragilis, and in North 
America by the allied Glass-snake, Gphisaurus ventralis. These 
animals have skulls like that of Lacerta and rudiments of pectoral 
girdles. Besides the Blind-worm, the Common Lizard, Lctcerta 
vivipara, and the Sand- Lizard, Lacerta agUis, are British. 

In North America four families of Lizards are represented, one 
being that of the limbless Anguidae, while the most remarkable 
of the others is that of the Iquanidae. These animals have 
short thick tongues and overlapping scales which form a crest of 
spines on the head and back and round the throat. Phrynosoma 
douglasiy the horned ''toad," is found all through the Central 
States and even penetrates into Ontario ; it is the sole lizard 
found in Eastern Canada. 

Order III. Ophidia. 

The Ophidia, or true snakes according to definition, have the 
right and left halves of the mandible connected by an elastic band ; 
they are also devoid of a urinary bladder and of any trace of a 



XYll'] OPHIDIA. 477 

pectoral girdle. Beaides this however they have a large immber 
of other characters which severally are shared by some families 
of Lizards but which collectively are found only in the Ophidia, 
The vertebrae in additiou to the zygapophyses on the sides of 
the nenrsl arch have median bosses and pits by wluch they &t Into 
one another, called respectively zygantra and zygosphenes (Gr. 
avTpov, a cave or hollow ; o-'^ijV, a wedge). There are no stem&I ribs 




nu: 383. Dorsal (Id the left) und ventral (to the tight) Tiews o( the akull of tite 

Cammoii Snnke, TVopiifonudu tialrix. After Porker. 
1. Preioixilloe (fu»d). 2. Anterior niiree. 3. XaaaL 4. PrefroDtsL 

6. Frontal, 6. Pftrietiil. 7. Maxilla, S. TmnBrerm bone. 

n, Palatine. ID, PCerytioid, 11. Pro.otio. 13. Eiocoipitol. 

IB. Supra-nccipital. 14, Opinthotic. 15. Epi-otic. 10. Quadrate. 

17. ParBsphr>Doiil. 18. BasiBphenoid. 19. BaBJ-oeoipital. 20. Ocolp- 

ital condyle. 21. Splenial, 22. DeaCarv. 23. Angular. 24. Artionkr, 

25, Snpra-anKular. 26. Coronaid. 27. Vomer. 2B. Squaniaesl. 

IX. X. Forumina for the oiDth and Eanth oraoial nerveB, 

or sternum, but the dorsal ribs are elongated and curved ventrally, 
and a snake literally walks on the ends of them ; it is in a sense a 
vertebrate centipede. 

In the skull the chief point to be noticed is the extreme mobility 
of the jaws. The jugal aa well as the riuadrat^jugal have disap- 
peared, the pterygoids no longer articulate with the base of the 
skull, and the quadrate itself is pushed away from the cranium by 
the w^uamosal, which is a rod-like bone (Fig. -265). Some authoritiea 
hold that this bone is not the representative of the s'luamosal, but 
represents the aupra-temporal of Lawrtu. 'I'he result of this 
airangement is that when the lower jaw is pulled down, the 



*78 REPTILU. [chip. 

quftdrate is quite free to thrust the pterygoid forwaid amd push np 
the maxilla by means of the transverse bcme ; that is to say there 
is the same mechanism aa was described in the lizard, only more 
easily set in motion and 
capable of much more more- 
ment. Hie halvfls of the 
mandible, or lower jaw, are 
connected 1^ elastic fibres, 
and thus they can be widely 
separated. The restilt of this 
is, that a snake has an enot^ 
mouB gape and can swallow 
prey almost as large as itself. 
Snakes of qnite moderate 
size dispose of &ogs, birds, 
&c. The large Pythons of 
India can cmsh an ttnimal 
lai^r than a half-grown sheep 
into a shapeless mass by 
coiling themselves around it, 
and they then swallow it 
whole. 

The hyoid, including 
under that name the remains 
of all the hinder visceral 
arches, is vestigial, consisting 
of a single bone on each 
side. This permits of the 
pulling of the glottis far 
forward between the halves 
of the mandible when Uie 

Flo. 266. DiasTam of Arterial Arohes ot . i ■ i ■ n _ 

Bnake Tiewed fram the ventral «pect. ">"»»! is engaged m swaUow- 

inif its prey, this shifting of 

I. 11. ni, IV. V. VI, First to siith arterial **.,. ^ ,■'.' ^, 

archea. 12. Tracheal (Tentral carotid), position being necessary to 

13. CommoQ carotid (dornal oarotidj. IS. prevent choking. 

Bigbt ByBtemic arch. 16. Lett tyeteiaie r it i ii iL l • 

arch. 17. Dorsal aorta. 19. Pol- ^^ "le StuU tne Drain 61- 

monarj. 24. CoaUac. fends forwards between the 

eyes and there is consequently 

no interorbital septum. That this is a secondary and not a 

primary state of afTaii^ is shown by the fact that the front part of the 

brain is protected at the sides by downward extensions of the frontkl 




Ixvil,] OPHIDIA. 47» 1 

I and parietal bones, whereas iu aiiinials such as the Urodela andV 
I Mammalia, where an iiiterorbital septum has never been formed, the > 
!>ide-walls of the cranium are constituted by the orbitoephenoid ftod 
aJisphenoid bones. It is curious to tind this aljsenue of an interorbitsl 
septum in a family of limbless lizards, the Auphisgaenidae. What 
relation, if any, it has to the anake-like habits it is hard to guess. 

The two eyelid)^ have coalesced to fonn an extra guard in front I 
of the eye, but there is a traus]>areiit i>ortion in the lower one " 
through which the animal can see. The outer covering of scales is 
Khed periodically, half-a-dozen times every year or oftetier, and 
replaced by a new set formed by the activity of the ectoderm, and 
during this process, since the covering of the eye is affected, the J 
snake is blind. 

Due lung is small, and the other (the right) greatly elongated, I 
the hinder part being i^uite smooth. 

The heart resembles that of Lizards both in structure and the 1 
mode of distributing the arterial and venous blood. The differences 
l)etween the vascular systems of a Snake and a Lizard depend chiefly 
on the absence of limbs and the correlated great development of the 
vertebral column, ribs and their musi'nlature as organs of locomotion 
in the Snake. Thus the subclavian arteries are absent from the 
right systemic arch, while the vertebral and caudal arteries e 
veins are well devehiped. Another diD'erence is that the I 
pulmonary artery is very slightly developed, in connection with 
the reduced condition of the left lung. 

Snakes are divided into many families, of which two are repre- 
sented in Great Britain and three in the temperate parts of North 
America. A rough classification would divide them according to 
their habits into: ('() those which poison their prey, {!>) those which 
crush their prey, and (r) those which swallow their prey directly. 

Those which crush their prey are confined to the tropics ; those 
which swallow their prey directly are the non-venomous snakes, 
and ate representetl in both England and North America by the 
family Coluiiridak. In this family the maxilla is long and bears 
numerous teeth, as do also the pterjgoid and the lower jaw. 
The head is much broader behind than at the muzzle. There are 
about thirty species belonging to eighteen genera in North Amcr 
of which y^rfpidiimttin tnrtalis, the garter-snake freijuently met 
with in Canada, is one of the commonest ; and in Kngland the 
family is represented by two species, the smootli-snake, CortmtUa 
ianms, and the grass- or ring-snake, Tropidoiuitua luitn-x. 



480 R&PTTLu. (cur. 

I'be veiiamciiH snakes in America belong b> two families. In 
the first, the ELAPruAE, the maxilla ia a long bone and beats in 
front two large teeth which are gruoved, to allow the secretion of 
glanda in the lip to trickle down into the wounil which they make. 
The teeth behind are not grooved. The American Harlequin 
Snake, ICJapg fahius. belongs to this family. This anake receiva 
its name from its brilliant colours; it has iieventeeu crimsoD tings 




bordered with yellow. Another family is that of tite Vipkridjie. 
The maxilla is much shortened and bears one enormous fang, 
which when the mouth is closed lies against the roof of the mouth : 
when the mouth is opened the maxilla is rotated by means of the 
ecto-pter>'goid, so as to erect the tooth. The typical Rattlesnake— 
Crolalut horridat oi C airox — derives its name from an apj 



Xvn.} CBELONIA. 481 

of about H t<) 9 looaely coEnected homy rings which it bears at the 
end of its tail, the shaking of which makes a noise like a rattle. 
This is one of the most deadly snakes known : it is found all over 
the Unitod Stales in mountainous places and enters Canada. Like 
ftll Ckotalinak or Pil-vipers it lias a sensory pit between eye and 
nose- The English Adder, Vipera berm, is, like all the Old World 
Viperinae, devoid of such pits. 

Order IV. Chelonia. 

The Chelonia or Turtles are the most pecidiar order of the 
Beptilia. In some respects they are nearest to the Amphibia, but 
they are highly specialized. Their leading peculiarity is the pos- 
session of two great shields, a dorsal the carapace and a ventral 
the plastron, composed of bones firmly connected together, so that 
most of the organs of the body are enclosed in a box. The homy 
scales which cover in this box are very large and form what is 
known as tortoise-shell. The carapace is formed of a central row of 
neural plates which are espansions of the spines of the dorsal 
vertebrae with a nuchal plate in front of these and a pygal 
behind, the two last-named being of dermal origin. 

On each side there are costal plates ; this name is given to 
broad expansions of the outer surfaces of the ribs (Fig. 2(i8). The 
ribs curve inwards to join the centrum, and since this, as in all 
Reptiles, is formed by the interventral, each rib is nearly opposite 
the interspace between two centra and sometimes itnites with 
them both. The transverse process is represented by the exi>anBioQ 
of the neural plate which meets the costal plate. The almost 
horizontally directcil outer ends of the ribs are received into a 
aenes of dermal bones called marginals, which form the edge 
of the carapace. 

The plastron is formed of one unpaired and several paired bones 
(Fig. 269). The median bone, called the entoplastron. is believed 
to correspond to the intcn.'lavicle of other Reptiles. The first pair 
are called epiplastra and probably represent the clavicles of 
other forms. The posterior pairs are called hyoplastra, hypo- 
3)lastra and xiphiplastra respectively; they are firmly joined to 
'the marginals. 

In front and behind the plastron and carapace are 8e|mrated by 
joft flexihie skin ; their edges project so as to form roof and floor 
to cavities into which the bead and neck and arms in front and the 



[CHif. 




1. 266 AD 
To T a 
Buperh a Lo 

composed are 
Nuchn pate 
4. Marginal ] 
8. First verCc 


aa and B 

och y 


nt a wo 
ta Afte Ow 


n In A the outlines of tb> 
ne» I which the CBrapiee i« 

S. b. 7. TboiMia vertdm. 
ield. 


■hil tb 
penned 
2 F 

^bral ehield. 


B h b 
bj gh 

9. Costal Bb 



C. The Plastron of a Orcen Turtle, Chelone mydat x f. (Cunb. Una.) 
1. Epiplastron (clavicle). 2, EQtoploatron (interolaviole). 3, HroplMtnii 
[cleitbiDm]. 4. HjcopIoBtrai), 6. XipbiplutiOD. 



XVII.] OHELONIA. 488 

legB and tail behind can be withdrawn. A Btudy of the develop- 
ment of modem Chelonia and of the anatomy of fo.sail siieciea makes 
it plain that the anceaitora of the preHent furms were provided with 
a carapace composed entirely of dermal bonee underlying the homy 
ales, jnst as is the case with Crocodilia. This dennal carapace 
however was gradually replaced by the development of bony ex- 
LDsionB of the ribs and neural arches ; though remnants of it 
t in the nuchal, pygal and marginal plates. 

There is no trace of sternal ribs or sternum ; but the pectoral 
and pelvic girdles o<M;upy the peciJiar position of being within 
instead of outside the ribs, a conaeiiuence of the almost horizontal 
direction of these. The girdles are in fact converted into pillars or 
struts which keep the plastron and carapace apart. In front the 
scapula forms a vertical pillar which has a ventral process — the 
acromion — projecting inwards beyond the articulation with the 
coracoid. This process is unique amongst recent ReptiHa but existed 
in the Ple^iosauria. The coracoid slopes backwards and inwards. 
The ilium and pubis serve to support the carapace posteriorly. The 
pelvic girdle is similar to that of a Lizard but the pertoral girdle 
has no epicoracoid. The limbs are essentially similar to those of 
the Lizard but the toes are shorter and blunter. The neck is extra- 
ordinarily flexible ; the vertebrae composing it fit one another by 
cup and ball joiots, one is ami)hieoelous, another is biconvex. The 
dorsal vertebrae have flat faces. 

The skull is devoid of teeth and the premaxilla and maxilla are 
short. Both they and the dentary have sharp cutting edges en- 
she&tbcd in horn so as to form a beak. In all species the orbit is 
iiicin'led with a bouy ring and the ectopterygoid or transverse bone 
B wanting. The jmlatal crest on the palatine is hardly perceptible. 
The squamosal does not usually join either the jiostfrontal or 
parietal, hence the upper temporal arcade is absent and there is 
no distiui-tion between the supra-temporal and latero-temporal 
fossae. In the marine C'hehne and its allies however the post- 
frontal, si[uamosal, parietal and quadratojugal coalesce to form a 
sheet of bone from the crest of the skull to the lip. roofing over a 
cavity lying at the side of the cranium and containing muscles. 

Breathing is performed as in Amphibia, by a mylohyoid muscle 
and other muscles causing movements of the hinder visceral arches, 
of which there are three pairs. 

The heart in structure and mode of action resembles that of 
Lizards and Snakes, the left-hand gnteim^uG^cjmTgm^^oo^^ 



484 EEPnUA. [chap. 

chiefly Tenons to the viscera, while the right-hand one sapplies the 
head, trunk and limbs with blood which is much more arterialised 
than that in the other arch (p. 470). The fore-limbs are howerer 
supplied by a different vessel &om the subclavian of the Limttl 




Lower jaw or mandible. 3. Nuchal plate. 3, Ventral piocess of sopuU. 
the Acromion. 4. Scapula (much forcahortened). S. Marginal bolK. 
6. Coracoid. 7. Ilium. 6. Pubis. 9. IscliiaiD. 10. Centrum 

o( vertebra. 11. Humerus. 12. Kadiua. 13. Dlna. It. Cupo). 
IS. Femor. 16. Tibia. 17. Fibula. 



486 

We have already aoen (page 350) that in some vertebrates the 
Mtery to the fore-limb arises from the systemjc atuh on its dorsal 
Gounte to join its fellow, while in others tbe fore-lioib receives its 
blood from an artery given off from the ventral end or commencement 
of the systemic arch or else Irom the ventral end of the third arch 
near its division into dorsal and ventral carotids. As tbe vessel to 
Hie fore-hmb is always called a subclavian artery it is convenient to 
sxpress the fact that this vessel is not homologous throughout the 







W 



10 



Fill. 'iTO. IianRiludinal vertical section throttgb tbe Cranium of a, Gtec^n Turtle. 
Clithme mydat x |. 
Pitrietkl. 8. Squaniosal. 3. Quadrate. 4. Bosisphenoid. 5. Baid- 
ocoipital 6. QuadrntojuR&l. 7. Pro-ulic. S. Opialhotic. 9. Pter^j^id. 
10, Pftlatine. 11. KikI paaaed into uuriol passage. 12. Eioccipital. 
13. Epi'Otio fused to nupra-ocoipitftl. 14. Supra-occipilal. 16. Pre- 
lunxiUa. 10. Maxilla. IT. Jiigal. IH. Fostrroiital. 19. Tomer. 
20. PretfouUI. ai. PfOQtal. T 1 * 3. VII. Vni, IX, X. XI. XU. 

foTiuuiiia tor the i^iiC of uiunial oenea. 

rertebrate groups by the terms "dorsal subclavian" and "ventral 
vobclavian." In Amphibians and Lizards the subclavian is of the 
dorsal type, bat in Chelonians and, as we shall see, in (.Crocodiles 
•bo the arm is supplied by a ventral subclavian, a vessel which is 
homologous with the "scapular" artery to tbe shoulder muscles in 
■ A Liwird. The venous .system in all chief respects is like that already 
d&Dcribed m the Lizard. 

The copulatory organ is a grooved rod attached to the front 

rati of the cloaca. The groove leads to the openings of the male 

hicts, the vasa deferentia, 



486 



HEPTILIA. 



[CHiP. 



The members of the order Che1oni& have very Tarioas habits and 
modes of life. Some are vegetable feeders, others purely aiunuL 
None are found in Great Britain, but the representatives of six 
groups are fouod in temperate North America. These are 

(1) The TBSTCDnriDAE 
or Land Tortoises. 

(3) The Emtdid&e w 
Fond Turtles. 

(3) The CiNoarBESiDAE 
or Box Turtles. 

(4) The Chbltdsidie 
or Snapping Turtles. 

(5) The Trioittchidai 
or Mud Turtles. 

(6) The Chblonidax or 
Marine Turtles. 

The TBSTUDiNiDAEhaTe 
a very arched carapace snd 
short club-like limbs io 
which the toes are taghUj 
bound together by "I"" 
Only a few species, TettuSo 
pdyphemus, the burrowing 
Gopher, and Ci^ado Caro- 
lina, the Box Tortoise, ue 
known in temperate North 
America. 

The Emtdidae are re- 
presented by many qtedo. 
In this &mily the carapm 
has a wide horizontal nu^ 
gin and the toes are con- 
nected by a web. Most of 
the species are aquatic, s 
few however are almost u 
terrestrial as the Tem- 
DiNiDAB. Ckrytemys pida, 
the painted Pond Turtle, 

ranges north into the St Lawrence. 

The CmusTERHiDAE have a long and narrow carapace widi the 

margins produced downwards; it is highest behind. The fixmt pu^ 




I. II. III. IV. V. VI First to eiith arterial 
niches. 12 Tracheal (Tentral carotid). 
13. Common carotid (dorsal carotid]. 15. 
BiRbt systemic arch Ifi Left Byst^mic 

arch. 17. Dora&l aorta. 19. Fuluonar?. 
20. InnomiDate. 21. Sabclavian (veutial 
type). 24. Coeliac. 



tnd sometimes the bind part, of the plostroD move like a liinge od 
the rest and close in the head and tail, whence the name Box 
Turtle. Sole genua ChtDstemum, e.g., pt^nnsj/lvnnicnm. 

The CiiELYDitiDAE are the so-called Alligator- or Snapping- 
Turtles. The head, nei'k and tail are all large and cannot be 
completely protected between the uarapace and plastron. The 
carapace is highest in front. The jaws are hooked and powerful 
and the animals are very vicious. Chdijdra SfTjmitina, the 
~'itiapp<.>r," is one of the commonest of American turtles. It is 
ibund everywhere from Canada to the tropica. 

The Trionvchidae or Mud Turtles have no horny scales ; both 
carapace and plastron are covered with leathery skin. There is a 
soft pig-like Bnout ; only the three centre toes have claws. They 
Beek their food by burrowing in tlie bottom of ponds. 

The Chblosih.ie are distinguished liy their peculiar akuU and 
the absence of many or all of the nails. Their e.xtremities have 
become flattened and form very efficient paddles. 

Order V. Crocodilia. 

The last and higliest order of the Reptilia is the Crocodilia. 
These animals agree with the Chelouia in haviug a series of bony 
plates underlying the horny scales of the akin, also in having an 
immovable quadrate and a single median copnlatory organ. 

'llie Crocodiles are of large size and are decidedly Lii^ard-like in 
their general appearance, the chief observable external difference 
between them and the Liwertilia being in the jawa, which are 
exceeduigly long in companaon with the rest of the skull, so tb&t 
the gape is very wicle. 

The dermal plates form rings on the tail, but on the body, as in 
Clielonia, they form a dorsal and a ventral shield separated by inter- 
vening softer skin. In many Crocodiles the ventral shield is very 
rudimentary. 

Ill the (general arrangement of the bones and the temporal 
fossae the skull resembles that of Spheuodon : but there are great 
differences in the jaws and palate. The maxilla is very long and is 
armed with conical teeth which are implanted in distinct sockets 
or alveoli, the bone having grown up round their bases. 

The two palatal folds have met so as to completely divide the 
upp«r air passage from the lower food passage : both the palatines 
and the pterygoids being completely iinited in the middle line 



488 REPTILIA. [CHAP. 

(Fig. 374). The cboanae or poaterior ntireg are therefore aitiuted 
very far l>ack direcUy over the glottis, whilst the external nostril it 
at the tip of the snout 

In consequence of this position of the external nostril the 




1. Premaiilla. 2. Mmilla. 3. FalatiDe. 4. Pterygoid. S. Pofterioi 

narea, 6. TmnflverBe bone. 7. Posterior palatine vaonity, 8. Anterior 
palatiiie laoQity. 9, Basi-occipital. 10. Openio;; of median 

Euatachian canal. II. Jugal. IS. Quadra tojogal. 13. Qnsdnte. 
14. Dentary. 15. Splenial. 16. Coronoid. IT. Sapra-angolar. 
18. Angular. 19. Articular. 20. Lateral temporal fosHa. 91. Open- 
ings For the passage o( blood-vesaela supplying tbe alveoli of the teeth. 

crocodile can lie for hours hidden under the water with only the tip 
of the snout exposed, and so surprise any nnwary animal coming 
to the water to drink. 

All the cervical and trunk vertebrae and some even of the 
caudal vertebrae bear ribs. The manner in which these ribs an 
articulated to the atlas and axis vertebrae throws much light on 
tbe relation of these peculiar vertebrae to the rest. Thns we 
observe that the first pair of ribs are articulated with their heads 



to the lower part of tlie atlait, showing that this represents a basi- 
Tentral homologous with the intervertebral cartilaginous pads of the 
rest of the column. The head^ of the second pair of riba are united 
to an intervertebral cartilage separating the odontoid proceas from 
the centrum of the second vertebra. This cartilage ia therefore the 
Becond basiventral, and the odontoid process is the first interventral, 
'lomoiogous with the centra of all succeeding vertebrae. The 
tubercle of each of the second pair of ribs has also an attachment 
to the odontoid process lying obliiiuely above and behind the 
capitular attachment and hence the centrum of the axis vertebra 
has no rib attached to it. The third pair of ribs has shifted its 
capitular attachment back ou the centrum of the third vertebra. 
This backward sliifting of 
the capitular attachment 
lias taken place in all suc- 
iceeding vertebrae, and the 
liead of thp rib is attached 
directly under its tubercle. 
In the trunk, as we proceed 
backwards, the capitular 
atlachmL>ut to the centrum 
gra<iually raised till it 
reaches the transverse pro- 

and is confounded with 
the tubercular attachment, 
uid the hindennost ver- 
tebrae are single headed. 

There are abdominal 
ba,aa in SpAfTtodon; they 
Are arranged in transverse 
TOWS, each row on each side 
Oonaisting of three or four bones (Fig. 374). 

The pectoral girdle consists of simply a scapula and coracoid, 
tiie latter reaching the sternum, nhich is cartilaginous but protected 
rentrally by an interclavicle (Fig. 275). In the fore-limb the carpus 
has retained three bones in the pro.ximal row, but the distal row 
consists of ft block of cartilage representing the first nnd second 
carpalia and a bone representing the remaining tliree. There ia 
consequently an intercarpal wrist-joint corresfionding to the inter- 
tarsal joint common to Reptiles. 

The pelvic girdle is very peculiaj. The ilium is broad and 




Fio. 273. Ficflt four Cervical Vertebrae of a 
Crocodile, C, vutgnri: Partly after von 
Zittcl. 

1. Menral spioe of atlas, 'i. Lateral portion of 
atlas. 3. Odoutoid proocBB. 4. Ventral 
portion of atlas. 6. Neural spine of aiis, 

6. PoBtEjRSpophyaia of fonrth vertebra. 

7. Tubercalai portion of fourth cervical rib. 

8. First cervical rib. 9. Second cervical 
rib. 10. Conveic posterior surface of 
centram of fonrtli vertebra. 



490 REPTIUA. [chap. 

rounded above and joins the two sacral vertebrae. The tnie pidn« 
is a small round bone inserted and fused with the anterior edge of 
the ischium and ilinm but what is ordinarily called the pubis, or 
better the epipubis, is ■ 
bone directed fonntrds 
which does not meet ito 
fellow, nor does it foim 
any part of the socket 
for the femur or aceta- 
bulum (Fig. 275). TTiiB 
so-called pubis or epi- 
pubia can be compared 
only to that of TJrodeli 
and Marsupials. Ihe 
tarsus, like the carpus, 
is much reduced and 
modified. It consists d 
a proximal row of two 
bones, one of which, dte 
fibulare or caloanenm, 
forms a distinct heeL 
The distal row consisto 
of two bones, one repre- 
senting the first, second 
and third tarsalia, the 
other the fourth and 
fifth. 

The heart of the 
Crocodile ts remarkable 
for the fact that the 
septum in the ventricle 
has grown forwards bo 
as to completely divide 
it into two halves, the 
right and left Tentricle& 
The left root of the aorta 
arises from the right 
ventricle and crosses the 
right root, which arises from the left ventricle and gives off the 
two carotids. The left root therefore receives venous blood from 
the right auricle and the blood sent to the trunk is mixed. In 




Fia. 274 Sternum and asBooiated MembTane. 

bones of a Crocodile, C. palvttrii x }. 
The last pair of aUIomiDal ribs vhich are united 

with the epipnboB by a plate of cartilage Lave 

been omitted. 
I. luterclavicle. 2. Steraam, 3. Sternal 

rib. 4. Abdominal splint rib. E. Sternal 



CHOCODIUA. 

addition there is a small passage, the foramen of PaniEza, joining 
the two trunks where they cross, so that the blood leaving the right 
«rch to go to the carotid is also somewhat mixed. The right 
common or dorsal carotid is very reduced, the left-hand vessel 
supplying both aides of the head. The fore-limb receives blood by 
a subclavian of the ventral type, as iu Chekmiatts. The lung is no 
longer a simple sac, but has thick spongy walls and the central passage 
is reduced to a narrow tube. In the brain the cerebellum is large 
and oylindrical. 




., Bcapuls. S. Cur&ooid. S. lal^^rotariule. 4. tilenoid Mvi^. 

PelvJR and Haorum of an Alligator, Caiman laliroilrh x 1- 
2. Isuhiuni. !l. True pubis. 4. Epipiibis (so-called pubis], 
irnl vertatrae. 7. Union 
prezygapophjsiB. 

All these peculiarities of the iotenial organs may be termed 
foresliadowings of what is found in Birds and Mammals, and hence 
Crocodiles are styled rightly the highest of the Reptiles. 

Crocodiles are inhabitants of rivers and swamps and spend most 
of their life in the water. The best known and classical example is 
the Crocodile of the Nile, Crocodilitx nilotiont. There is but one 
Biiecies in the southern states of North America, the Alligator, 
j\ lliifiitor misiisaipiensh, which has a much shorter and broader 
snout than the Crocodile. This animid lies for hours absolutely 
motionless at the surface of the water so as to greatly resemble a 



492 BEPTir-u. 

log, and thus entrap any unwary animal which may renture near. 
The Gavial, Gavlaiia gangettcua. in India is remarkable for iu 
excessively long and narrow jaws. 

So far OS we can learn from fossils the Reptjlea seem to have 
been the dominating type 
of land animals in the agei 
whiih intervened between 
the close of the Coal e^Hich 
" \ '2 '2 II y. and the end of the Chalk 

period when the white lime- 
Btoue which uoustitutes the 
Southern cliffs of Britain 
wajn depoeiterl as a Bediment 
in the ijuiet waters which 
covereil what is now West 
eru Europe. A rough sketch 
of the history of the CI— 
lis deduced from fossils 
be given here. 

I'he Reptilia i 

have arisen from the St«g(V 

CL'phala. At least included 

in the latter group are saae 

forms like Eryopn and Oi- 

rotas in which the bune« 

flanking the notoohord. 

which have not yet united 

80 ns to form vertebrTio, Kt 

represented by basidor^a, 

basiventrals, and inlerveii- 

Fi.., aw. DingiMB of Arlerial AreliB" of tfO'ls, the interdorsal as ill 

CrcModile viewed from the ventral ispeot. a|l Reptilia being BUpprtiss- 

I. n. UI. IV. V. n. First to dxth Brterinl p^. while in the skull the 

UToheB. 13. Tmoheiil (ventmt oarotiil). , . - -. i . • 

13. Common pfttotid (dotstil oarotid) [righl basl-OCCipital region 13 0881- 

tide nearly atrophied], 15. Biwht nys- fied. In the Sandstones 

temic Broh. Ifi. Left (jetomio woh. , . , i_ (> , - 

17. Dorsal aorta. 19. Pulmonary. 80. In- 'ymg above the Coal in- 

nominale. 21. SubcUviao (ventral type), dubitable lleptiles with 

fuUyfonued vertebrae make 

their appearance. Some of these, termed the Pariasauria, still recall 

the Stegocephala in possessing a comjilete roof of dermal bones 

covering the skull. At the same time allied forms termed Then- 




theO^I 
'ossilij^^H 

eee^^H 



XVII.] CROCODILIA. 493 

morpha showed gaps in this armour of dermal bone corresponding 
to the supratemporal and laterotemporal fossae, the latter being 
exceedingly small and often not present The limbs in all these 
early Reptiles were short and stout, the fore and hind limbs being of 
nearly the same size. Some of the Theromorpha in possessing teeth 
divided into incisors, canines and molars, and in having the lower 
jaw partly supported by the squamosal, approached the characters 
of Mammalia. Another allied group were the Dicynodontia, which 
agreed with the Theromorpha in having a large supratemporal 
fossa but differed in having the teeth reduced to two tusks in front 
or completely absent. It is believed that these were the forerunners 
of the Chelonia. 

In the same period we meet a large number of forms with supra- 
temporal and laterotemporal fossae equaUy developed, these were 
the Rhynchocephala of which Sphenodon now is the sole survivor. 
From this group in the following age were developed (a) Water- 
Reptiles — the Plesiosauria — with long swan-like necks and limbs 
transformed into flippers by the shortening of the bones of the 
arm and leg, and (6) Land-Reptiles — the Dinosauria — with greatly 
developed limbs ; in some cases the whole weight being borne by 
the hind-limbs, the fore-limbs being short and used for prehensile 
purposes only. In a still later period from the less specialized 
Dinosauria were developed (1) the Crocodilia, which reverted to 
the water but retained limbs fit for progression, and (2) Pterosauria, 
possibly flying reptiles in which the "wing" was a flap of skin 
supported by the greatly elongated 5th finger. The forerunners of 
modern Sauria are found only in the Chalk period in the form of 
long-bodied aquatic Reptiles with however the characteristic loss of 
the quadrato-jugaL There remain to be mentioned the whale-like 
Ichthyosauria, which since they possess only a supratemporal fore-arm 
seem to be sprung from the Theromorpha. The limbs were converted 
into flippers more thoroughly than in the case of the Plesiosauria, 
the limb-bones being reduced to round nodules and the fingers 
increased in number by forking. There was no neck and no sternum 
but a series of ** abdominal ribs " as in Rhynchocephala. 

The class Reptilia is classified as follows : 

Order 1. Rhynchocephala. 

Reptilia devoid of special copulatory organs and with an 
immovable quadrate. 
Ex. Sphenodon. 



494 BEPTILIA. [chap. XVIL 

SAURIA. 

Order 2. Lacertilia. 

Reptilia with two copulatory sacs on the posterior 
wall of the cloaca : with a moyable quadrate, with a 
pectoral girdle, and with the rami of the lower jaw united 
by a symphysis. 

Ex. La-certa, Anguis, 

Order 3. Ophidia. 

Reptilia with two copulatory sacs on the posterior wall 
of the cloaca : with a movable quadrate but with no trace 
of a pectoral girdle ; with the rami of the lower jaw 
united by ligament. 

Ex. Crotalus, Vipera, Tropidanotus, 

Order 4. Chelonia. 

Reptilia with one median copulatory organ on the anterior 
wall of the cloaca and an immovable quadrate. No sternum 
and the ribs expanded horizontally to form a dorsal shield: 
a ventral shield of dermal bone. No teetL 

Ex. Testudo, Chelone, 

Order 5. Crocodilia. 

Reptilia with one median copulatory organ on the anterior 
wall of the cloaca and an immovable quadrate. A well-developed 
sternum, joined by the ribs. With many alveolar teeth. 

Ex. Crocodilus, AUigator, Gavialis, 



495 



CHAPTER XVIII. 
Sub-Phylum IV. Craniata. 

Class IV. AvBs. 

It is probable that if the first child one met were asked to 

describe a bird, he would say that birds were animals 

act*erirtic8. **^ which Were covered with feathers and had wings to 

fly with. Though it often happens that the marks 
by which the ordinary person distinguishes one animal from another 
are not those which seem most important to a Zoologist, yet in this 
case the Zoologist could not find more important features to serve 
as the basis of a definition of the class Aves or Birds. 

Birds then are vertebrate animals in which the fore-limb is 
modified into a wing or flying organ and in which the body is 
covered with feathers. Bats likewise have the fore-limb con- 
verted into a wing, but they are covered with hair, not feathers, 
and their wing is not constructed on the same plan as that of the 
bird. 

Birds are sometimes classed along with the Reptiles as Saurop- 
6 id a, since they have a good many features in common with them, 
and are thus contrasted with the Mammalia, or ordinary quadrupeds. 
This, however, gives a wrong view of the relationships of the three 
groups. Both Birds and Mammals are believed to be descended 
from Reptilian-like ancestors, and it is an open question whether 
the changes which Birds have undergone are not at least as im- 
portant as those which have taken place in Mammals in the process 
of their evolution from ancestors which, had they lived now, would 
have been termed Reptiles. 

Birds agree with Reptiles in that they lay large eggs from which 
the young are hatched in a form closely resembling the parent; 
they are like Reptiles also in the structure of their jaws — the lower 



496 AVES. [chap. 

jaw consisting of five bones and articulating with a quadrate 
bone — and in the structure of the hinder part of their skulls, of 
their breast-bones and of their ankle-joints. The yertebrae very 
rarely have epiphyses like those of Mammals, though these are 
found in Parrots. As in Reptiles, the number of neck vertebrM 
is variable. Like Reptiles, Birds have nuclei in the red corpuscles 
of the blood, and the sole remaining complete systemic arch goes to 
the right (Fig. 285), like the principal arch in Reptiles. On the 
other hand, they are ** warm-blooded," that is to say, the temperature 
of the body remains practically the same whether the surrounding 
air gets hot or cold ; it is in fact higher than that of any mammal : 
the ventricle of the heart is completely divided into two, and in 
addition to the wings and feathers, the structure of the leg and 
hip-bones and of the brain, distinguishes them firom any living 
Reptile. 

Strange as the statement may appear, it is true, nevertheless, 
that the feathers are really scales like those found in 
lizards, immensely developed and with the edges 
frayed out. Like scales, they are epidermal, that is, developments 
of the outer or homy layer of skin. The area which is about to form 
the feather becomes raised into a little finger-shaped knob by the 
growth of the deep layer of the skin or dermis which carries the 
blood-vessels. The little knob thus formed is in turn sunk in a 
pit called the follicle, the skin immediately surrounding it being 
depressed. Thus the lowest part of the feather is a little hollow 
tube of horny cells formed round the knob of dermis, but the 
upper part, like the scale of a lizard, is formed only on one side 
of the knob, and this part as it is pushed away by the growth of 
the deeper parts becomes frayed out so as to form the vane of 
the feather. In the latter we can distinguish a central stem or 
rhachis, and two rows of lateral branches or barbs, which are 
kept in position by a number of secondary processes or barbules. 
The barbules bear little hooks which interlock with one another. 
Down consists of small feathers growing between the bases of 
the larger ones. Li these the barbules are absent, so that the 
barbs are not held together but float freely about, forming a kind 
of fluff. When a bird is plucked it is seen that the feathers are 
confined to certain tracts (pterylae) separated by others called 
apteria devoid of feathers or covered only with down feathers. 
Thus in most birds the mid-ventral and mid-dorsal lines are 
apteria. The colour of the feathers is partly due to coloured 



XTni.] FEATHERS. 497 

snbstftncee or pigments in the epidermal cella imd partly to minute 
structural detail which causes interference of the light reflected 
from them. 

The ving is the foreleg of the Inid. One can easily recognise 
the parts corresponding to upper arm, fore-arm and hand, but the 
latter is highly modified and specialised for the important function 
of canying the long primaries or hand quills. When the wing 
is at rest the upper arm extends backwards, the fore-arm is sharply 




Epidennii. 2. Malpighiao Ibtbi of the epidenuiB. 3. Dennis. 

4. Toang luther. S. Follicle round base of feather. 6. Dermal 

papilla whioh developee blood veaaels and ia the organ of i: 
the feather. 



bent up on this, while the wrist is sharply bent down. When 
the wing is expanded these are partially, but never entirely, 
straightened out, so that a bird begins the down-stroke of the 
wing with the arm bent in a very similar way to that in which 
a swimmer's arm is bent when he strikes back with it. In the 
hand we find as a role three digits, the first, second and third. 
These have thdr fint joints, the metacarpal bones, closely united 
8. AH. 32 



498 ATES. [CHiP. 

together. Id Man Uie metacarpals of tlie variooB fingere are onitod 
by skill and fiesh which conBtitute ihe palm, bnt thety are mov- 
able on one another, whereas in the bird the metacarpals of the 
second and third digits are firmly joined at both ends. The index 
has in addition to the metacarpal, in most birds, three other smiQ 
bones called phalanges, of which the end one sometitnes catrica 
a daw : the third digit has only one bone or phalanx besides the 
metacatpal The metacarpal of the first digit or thamb is Toy 
small, but is likewise completely fused 
with the other metacarpals. Besides this 
the thumb has two joints and often • 
claw. 

Compared with the arm or fore-leg <^ 
other animals the arm of a bird strikes one 
as having very little fiesh. This is be- 
cause the muscles, especially those on the 
fore-arm, have comparatively short bellies 
but very long tendons, in correlatioa with 
the often very much lengthened bones, 
one of which, the ulna, serves as support 
of the secondaries or arm-qoills. 

The movements which constitute fly- 
ing, namely, the powerful down-stroke of 
the whole arm and the slower up-stroke, 
are carried out by the immensely developed 
Wing of a Oumet, Sula pectoral muscles, great fieshy masses which 
"*■ cover the breast-bone or sternum. This 

'a.""™- 2; sSi bon.ha..more„rle..p»r-,ih.i,edo«t- 
metaoarpal. 0. Tbird line, rounded in front and pointed behind, 
S'SS*"' 7°' KS "le ribs eodiDg in it. sides (Pig. S79). 
digit ^ 8. jE'hird digit. Jn accordance with the tendency in ill 
birds to develope the body into a long 
neck and a rounded trunk, we find evidence 
that the number of ribs encircling the bodjr 
and joining the sternum has been reduced. 
Not only do we find small free ribs connected with the hinder 
cervical vertebrae, but attached to the sternum are outgrowths 
called costal and xiphoid processes (Fig. 279) which are regarded 
as the remains of sternal ribs the dorsal halves of which ate 
vestigial or lost If we picture to ourselves the pectoral giidle 
being thrust backwards and the pelvic girdle forwards so as to crowd 




Flo. 276. Bones of the right 



Ihe distal phalangea of 
the thmnb and oeeond 
digit were wanting in the 
specimen from which this 
£gure was drawn. 



XTHI.] 



3KELBT0N. 



the viscera into n small space ve shall realize the meaning of the 
differences between the skeleton of the trunk of a Reptile and that 
of a Bird. From the middle line of the sternum projecte a great 
vertical crest Btrettrhing outviards, called the carina or keet, and it 

B from the sides of this mainly that the pectoral muscles take their 

origin. There are two main muacleE on each side. First the pect- 

oralis major on the surface, which passes into a tendon attached 

» the upper end of the humerus. The contraction of this muscle 

lings alwiit the down-stroke of the wing, the effective stroke in 
flying. Underneath the pectoralis major is situated the pectoralia 
much smaller muscle. Its tendon passes underneath 
tbe arch formed by the clavicle 
&nd the coracoid bone, the latter of 
which, as in Reptiles, connects the 
fihoulderhUde firmly with the ster- 
Having passed through this 
arch the tendon is attached to the 
back of the humerus, so that the 
contraction of the muscle pulls the 
humerus and thus the wing upwards 
uid backwards and not downwards, 
the upper end of the coracoid acting 
as a pulloy round which it passes. 

Returning to the wing, we must 
now notice how the feathers are 
uraoged. The great (juill feathers 
are attached chiefly to the upper 
and posterior edge of the hand, but 
there are also a large number which 
! implanted in the posterior aur- 
e of the ulna. These two groups 

i feathers are pushed one over the 
other when the wing is folded, just 
like the silk of a closed umbrella, but when the wing is stretched 
out they oidy overlap very slightly, and thus a coherent and 
practically air-tight surface is formed. Those feathers which are 
attached to the hand are called primaries (6, Fig. 280. G), those 
arising from the ulna, secondaries (8, Fig, 280, C); a few arising 
from the upper arm are called tertiaries; any air which might 
escajie between the bases of the long feathers is stopped by an 
uppn layer of shorter feathers, called coverts (1, 2, 3. 5 and 7, 
~^~" ' 32— »., 




1. Cftiina of tha stemuiD. 2. Cora- 
coid. 3. Soapula. 4. Clavicle. 
6. Costal process. 6. SurfuieB 
tor BrtieDldtioD with tbe etemnl 
ribfl. 7. PoBli-riot (xiphoid) 

atid oblique proceaseB. 



{CBU. 




XVni.] FLIGHT. 501 

Fio. 280. Wing of a Wild Daok, Ancu bosehas x ^. 

A. Bight wing seen from the dorsal side, with the coverts removed. B. Left 
wing disarticalated and seen from the ventral side, with the coverts 
removed. G. The dorsal side of a right wing. D. The ventral side 
of a left wing. From Wray. 

In A and B. 1. Humems. 2. Badins. 8. Ulna. 4. Radial carpal. 
5. Ulna carpal. 6. First phalanx of first digit. 7. Second metacarpal. 
8. Third metacarpal. 9. First phalanx of second digit. 10. Second 
phalanx of second digit. 11. Vestigial qaill. 12. Tertiariee. 

13. Secondaries. 14 — 17. Primaries. 

In G and D. 1, 2, 8, 6, and 7. Coverts. 4. Bastard wing. 6. Primaries. 
8. Secondaries. 9, 10. Tertiaries. 

Fig. 280, C and D). Air is prevented from escaping in front by the 
hand, which is stretched out in a vertical plane, and by two folds of 
skin, one in the angle between fore-arm and upper arm, the other 
between the upper arm and the body. The name bastard wing 
is given to a tuft of feathers borne by the thumb (4 Fig. 280, G 
and D). 

The full mechanical explanation how the down-stroke of the 
wing not only prevents a bird from falling but urges 
it onwards is not completely understood, and much 
of what is generally accepted is too complicated for an elementary 
text-book, but the broad principles involved may be simply set 
forth. A bird when it is in the air, like any other heavy body, 
is continually falling : the blow of the wing has therefore not only 
to effect a forward impulse, but also an upward one sufficient 
to compensate for the distance the bird has fallen between two 
strokes. These impulses are derived from the elastic reaction of 
the air compressed by the down-stroke of the wing. When the 
wing is expanded, it is slightly convex above and concave beneath. 
This arises from the fact that the quill feathers are attached to 
the upper edge of the webbed limb and project gently downwards 
and backwards, so that there is a space left which is bounded 
behind by the quills and in front by the bones and web of the 
limb. Now if this space had a symmetrical shape the air would 
be compressed in such a way that the resultant impulse would be 
directly upwards ; but it is not symmetrical, for its roof has a very 
steep slope in front and a very gentle one behind, and the air is 
compressed in such a way that an oblique reaction results, a 
reaction which we can resolve by the parallelogram of forces into 
an upward and an onward one. So much for the flight of a bird in 
still air. The air is, however, very rarely still, and the currents 
which exist are never quite horizontal, but generally inclined 



502 ATsa [chap. xvm. 

slightly upwards, since the lowest layer of air is checked by firiction 
against the ground, and birds which are good flyers can, by in- 
clining their wings at the proper angle, obtain quite sufficient 
support from the play of the current against the wing without 
exerting themselves to any great extent. This is called soaring, 
and can be seen beautifully in the flight of the Gkuinet. In this 
manoeuvre birds are assisted by the tail, which is really a fan- 
shaped row of strong feathers attached to the coccyx, that y&j 
snuST vestige of a true taU or portion of the ver^biid column 
extending behind the anus, which modem birds possess (Fig. 282). 
In this region the vertebrae are thin discs, several of which may be 
soldered together so as to form a bone called the pygostyle. 




Fio. 281. Lateral view of the Pelyis and Saomm of a Dock, Aruu boscJuu x {. 

1. Iliam. 2. Isohiom. 8. Pubis. 4. Pectineal process, the radiment 
of the prepabis corresponding to the pubis of the Lizard. 5. Ace- 
tabulum. 6. Ilio-ischiatic foramen. 7. Fused rertebrae. 8. Facet 
on which the projection on the femur, the trochanter, plays. 

The legs of birds can be shown to be constructed on essentially 

the same type as those of Heptiles, but modified so as to 

position. ^ enable them to support the body in an upright position. 

The arrangements to efiect this are very interesting, 
as they differ markedly from those found in the human skeleton. 

In the pelvic girdle the ilia are lengthened so as to be 
attached to a considerable number of vertebrae, six or more, and 
so a firm attachment of the limb to the main skeleton is effected. 
In Reptiles only two vertebrae are joined to the ilium, but in 
their case the weight of the body is supported on all four limbs, 
whereas in a Bird the whole vertebral column has to be balanced 
about two points of support, and hence the ilium must be quite 




Fio, SS9. Skeleton of the Common Fowl, e , GaUui bantiva x i. 
1, Fremuills. 2. Nasal. 3. Lachrymal. 4. Frontal. E. MitDdfble. 
6. Lover temporal arcade in region of qaodratoiiiga]. 7. Tympanio 

nvi^r. S. Ceiriool vertebraa. %. Ulna. 10. Humeraa. 

IL BadioB. 13. Carpo-metaoarpHB. 13. First phalanx c( aeoood 

digit. 14. Third digit. 16. Second dJgiL 16. Qinm. 

17. Cio-isohiatia foramen. 16. Pygoatyle. 19. I'emnr. 

30. Tibio-Ursaa. 21. Fibnla. 22. Patella. 23. Tarso- 

netatantu. 24. First toe. 25. Second toe. 26. Third toe. 

27. Fourth toe. 26. Spoi. 29. Pubis. SO. IschiQin. 

SI. Olaviole. S2. Coraooid, S3. Keet of steraom. 34. Xiphoid 
proeesi. Th« forked boas jast In ^nt of 7 la the QnaAnte. 



504 AVES. [chap. 

immovably strapped to the vertebral column. The resalt of this 
has been atrophy of some of the hinder ribs, and the ventnd 
halves of some of these form the xiphoid processes of the stemnm. 
The ischium is directed backwards parallel to the hinder part of 
the ilium, and often fused with it so as to surround a space 
called the ilio-ischiatic foramen. The pubis is a very slenda 
bone which is also directed backwards. It is in fact a post^ubis 
corresponding to the lateral process on the pubis of the lizard (see 
p. 467). Except in the Ostrich the two pubes never unite with 
one another ventrally to the cloaca, as they do in Beptiles and 
Mammals, the absence of a pubic symphysis facilitatiug the laying 
of the egg, which is very large relatively to the size of the animal 
The thigh is bent sharply forwards and the shank backwards, and 
the ankle is raised to a coDsiderable height above the ground by 
the great length and upward direction of the bones of the sole or 
metatarsals (Fig. 282). Thus a Bird walks on its toes and like 
Heptiles possesses an intertarsal ankle-joint. In Birds however, in 
order to give firmness to the leg, the metatarsals are closely united 
together and the small bones of the tarsus have entirely disap- 
peared, the proximal row having been incorporated with the tibia, 
while the distal bones have fused with the metatarsals. Thus in 
an adult Bird the ankle-joint is a simple hinge between two 
compact bones, the upper being a tibio-tarsus, the lower a tarso- 
metatarsus. There are usually four toes, but the first, corre- 
sponding to the human great toe, is sometimes, like the fifth, 
absent, while its metatarsal remains distinct from the other three. 
This toe, except in Steganopodes, is generally directed backwards. 
The raised sole of the foot really constitutes the visible "leg" of 
most birds, the thigh being altogether, and the shank mostly, buried 
in the feathers. In many birds the sole is plated by scales which 
are raised homy plates of skin, similar to the scales of Reptiles. 
The most characteristic features about a bird, next to the limbs 

and feathers, are certainly the head and neck. The 
Neck! *" skull is high and arched behind in order to make 

room for the comparatively large brain; in front it 
slopes gradually downwards to the pointed beak, which is encased 
in a hard homy sheath. The bones which underlie this beak are 
(above) the premaxilla and (below) the dentary bone of the lower 
jaw. No modem bird possesses teeth, and the maxilla, which 
usually carries most of the teeth in animals which have them, is 
very small and confined to the cheek behind the gape, whereas the 



XVIII.] SKELETON. 505 

premaxilla is very large. Behind the maxilla two other slender 
bones, the jugal and quadratojugal, complete the lower temporal 
arcade as in Ghelonia and Crocodilia, but the jugal never sends up 
a process behind the orbit and the post-orbital is a mere process of 
the frontal bone, so that the orbit and the temporal fossa open into 
one another. The eyes are of great size : a bird has little or no sense 
of smell, and governs its life mainly by the sense of sight : in corre- 
spondence with this the orbits or eye-sockets are so enlarged that the 
skull between them is reduced to a thin vertical plate, the inter- 
orbital septum, in which there is no brain cavity. This great 
development of the eye-sockets and the obliteration of the brain 
cavity between them is not, however, confined to Birds : it is found 
as already mentioned in many Heptiles also, and is indeed one 
of the several points in which a bird's skull may be said to be 
Reptilian. It is however characteristic of Birds, as opposed to 
Reptiles, that this interorbital septum is largely converted into 
bone. In its hinder and upper portions it is composed of orbito- 
sphenoid bones, like those found in Teleostean fishes, which support 
the exit of the optic nerve, but in its lower part it is composed of a 
vertically compressed presphenoid bone corresponding to that 
which ossifies the front part of the floor of the cranium in 
Mammalia. In front the interorbital septum is continuous with 
the intemasal or ethmoid septum : this latter is ossified by a 
mesethmoid bone, which unites, but not quite immovably, with the 
presphenoid. The hinder part of the floor of the cranium is ossified 
by the basioccipital and basisphenoid bones, and the front of the 
latter is drawn out into a long spur called the basisphenoidal rostrum. 
Underlying the basisphenoid there is a membrane-bone called the 
basitemporal, a relic of the parasphenoid of Fishes and Amphibians. 
In some Reptiles traces of the front part of this bone remain, but 
never any of the hinder portion, and this is an indication that Birds 
are descended from a type of Reptile more primitive in some respects 
than any now existing. Other points of resemblance to Reptiles 
are that the lower jaw is made up of no less than five distinct 
bones interlocking with each other ; and that instead of there being 
a direct hinging or articulation of the lower jaw to the skull, a 
bone called the quadrate is interposed, as in Reptiles, which 
articulates on the one hand with the lower jaw and on the other with 
the skull. This quadrate bone is movable, and to it in front are 
jointed the bones of the palate, the pterygoids and palatines, 
which slide on, but are not fixed to, the base of the sktdL Hence 



506 



AYES. 



[chap. 



when the lower jaw is opened, ie., palled down, these bones ue 
pushed forward, and the upper beak, to which they are fastened m 
front, is slightly tilted up, thus increasmg the width of the g^ie. 
In parrots the front part of the skull, including the bones of the 
face, has an actual joint with the hinder part of the skull. Thus it 
follows that in spite of the presence of a quadratojugal the quadrate 
is movable. It is to be remembered however that the quadrato- 
jugal is here a small flexible bone, very unlike the great bony bar 
of Ghelonia and Crocodilia. The hyoid apparatus consists of the 
second and third pairs of visceral arches. The second pair, which 




a.... 




.«- 



Fio. 283. Brain of Pigeon, Columba livia x about 2. 

1. Olfactory lobes. 2. Cerebral hemispheres. 8. Pineal