^ CX LIBRIS UNIVERSITATIS ALBERT/ENSIS A MANUAL OF ZOOLOGY MANUAL OF ZOOLOGY BY T. JEFFREY PARKER, D.Sc., F.R.S. PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO, DUNEDIN, N. Z. AND WILLIAM A. HASWELL, M.A., D.Sc., F.R.S. PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF SYDNEY, N.S.W. RE VISED AND ADAPTED FOR THE USE OF AMERICAN SCHOOLS AND COLLEGES THE MACMILLAN COMPANY LONDON ; MACMILLAN & CO., Ltd. 1917 All rights reserved Copyright, igcxs, By the MACMILLAN COMPANY. Set up and electrotyped January, igoo. Reprinted September, igcxi; March, igo3 ; January, igos ; January, igo6; February, igo7; March, igog ; December, igio; September, igia; August, igi6; September, igiy. UNIVERSITV LiBRARt UNIVERSITV of alberta PREFACE In planning the present work the aim of the authors has been to provide a manual embodying a course of study adapted to the requirements of the student chiefly in higher classes of schools, and to some extent in junior classes of universities. To make this, within the necessarily narrow limits of space imposed, anything more than a bare synopsis, it has been necessary to restrict the extent of the ground covered. This has been done (i) by leaving out altogether certain classes of existing animals; (2) by omitting all de- scriptions of extinct groups ; (3) by dealing only very briefly with embryology. Opinions must differ as to the best selec- tion of groups for an elementary manual of this kind. But broadly, there can, it has appeared to us, be little doubt that what should be omitted, or only briefly dealt with, are the groups of rare occurrence and uncertain relationships, the greater part of the space being devoted to the more familiar representatives of the large phyla. A course of laboratory and museum instruction, supple- mented by work in the field and on the seashore, is abso- lutely necessary in order that any sound knowledge of PREFACE zoology may be attained. The present manual does not provide such instruction, but is intended to be used in association with it, and the examples selected for de- scription are such as may under most circumstances be readily obtained. The general plan is similar to that followed in the Text- Book of Zoology by the same authors, but the restricted space has necessitated considerable modifications. We have not adopted the method, followed in various recent manuals, of beginning with one of the larger Invertebrata or with a vertebrate, and working from that upwards and downwards. The reasons given for such a mode of treat- ment we understand to be that if we begin with the simplest animals, the Protozoa, we discourage and embarrass the beginner by introducing him at once into a world entirely new to him, requiring him at the same time to learn the use of an entirely unfamiliar instrument the microscope. But in our opinion, the difficulty is much less than is alleged by the advocates of the alternative method, and the advantage of presenting the facts at the outset in a natural and logical order by far outweigh any such disadvantages. We are con- vinced that any general acquaintance which the student may possess beforehand with a rabbit or a crayfish will be of little real value to him when he begins to take up seriously the study of its structure. Moreover an elementary knowledge of the use of the microscope is absolutely essential to any adequate study of Zoology as an intellectual discipline, and this difficulty, such as it is, may as well be met first as last. PREFACE vii Owing to the lamented death of Professor T. Jeffrey Parker, at a time when but little progress had been made with this work, his actual share in it has been but slight : but as it was planned between us, and the earlier parts had the advantage of his revision, and more especially as it owes a great deal to his work in the Text- Book it has been thought right to let it appear under our joint names as origi- nally intended. I have to express very great indebtedness to Professor W. Newton Parker for the pains he has taken in revising the proof-sheets and for many valuable suggestions which he has made during the progress of the work. William A. Haswell. PREFACE TO THE AMERICAN EDITION This American edition of Parker and Haswell’s useful and concise “ Manual of Zoology ” has been adapted for the use of American schools. Common American forms closely similar to the European or Australasian ones described in the English edition, have been mentioned, so that the student can use the book in examining the allied typical forms from his own country. In the majority of cases the European species differ only in trivial characteristics, so that one general description will answer for both. In a very few cases the American editor has revised and corrected views or statements not believed to be correct. For example, few, if any, American zoologists would regard Limulus as an undoubted Arachnidan. A few additional American animals have been referred to and figured, while a few cuts not reproduced in the English edition have been copied from Parker and Haswell’s “Text-book of Zoology,” and also from Sedgwick’s “Text- book of Zoology.” ix CONTENTS PAGE Preface to the English Edition v Preface to the American Edition . . . . . ix List of Illustrations xvii Introduction i Definition of zoology i Binomial nomenclature 2 Individual variations 3 Hybrids 3 Definition of morphology 4 Definition of histology 4 Definition of embryology 4 Classification 5 Phyla . . . . . . . . . . . 7 Organic evolution 9 Genealogical tree 9 Palaeontology 10 Fossils . 10 Succession of life in time 1 1 Phylogeny 1 1 Distribution of animals, in space and in time . . .11 The plankton 12 The necton 12 The benthos .12 Definition of a fauna 12 Zoo-geographical regions 12 Definition of physiology 12 Definition of ethology or binomics 13 CONTENTS SECTION I Phylum Protozoa Class I. The Rhizopoda Example of the Class — Aniceba proteus . Class 2. The Mastigophora .... Example of the Class — Euglena viridis . Class 3. The Infusoria Example of the Class — Parmttcecium caudatutn Class 4. The Sporozoa Example of the Class — Monocystis agilis . SECTION II The Metazoa Oosperm or egg Male cell or sperm Fertilisation Segmentation of the oosperm .... Germinal layers . Tissues, epithelium Glands, ducts Connective tissues Fibrous tissue Fat Cartilage Bone Muscular tissue . Nerve tissue, ganglia, nerves .... Organs ........ Exoskeleton Endoskeleton Organs of digestion .* Organs of respiration The blood Blood vascular system Heart Organs of excretion PAGE 14 14 14 34 34 45 45 55 55 59 59 60 60 60 62 63 63 65 65 65 66 66 66 66 66 67 67 68 70 70 70 71 71 CONTENTS The brain Reproduction . The phyla of the animal kingdom Tabular view of the phyla . . . . . SECTION III Phylum Porifera . Example of the Phylum and Class — Sycon gelatinosum . SECTION IV Phylum Ccelenterata Class I. The Hydrozoa Example of the Class — Obelia Class 2. The Scjrphozoa ....... Example of the Class — Aurelia aurita . . . . Class 3. The Actinozoa Example of the Class — Tealia crassicornis Class 4. The Ctenophora Example of the Class — Horntiphora plumosa . SECTION V Phylum Platyhelminthes Class I. The Trematoda Example of the Class — Disiomum hepaticum . Class 2. The Turbellaria Example of the Class — Planaria torva .... Class 3. The Cestoda Example of the Class — Tcenia solium .... Class 4. The Nemertinea Example of the Class — Tetrastemma .... SECTION VI Phylum Nemathelminthes . . ... Class. The Nematoda Example of the Class — Ascaris lumbricoides . Xlll PAGE 71 72 74 75 76 76 90 91 91 108 108 114 1 14 125 125 129 129 129 137 138 138 138 145 145 149 149 149 XIV CONTENTS SECTION VII PAGE Phylum Echinodermata 157 Class I. The Asteroidea 157 Example of the Class — Asterias vulgaris . . -157 Class 2. The Ophiuroidea 169 Example of the Class — Ophioglypha lacertosa . .169 Class 3. The Echinoidea 170 Example of the Class — Strongylocentrotus . . .170 Class 4. The Holothuroidea .173 Example of the Class — Holothuria edulis . . .173 Class 5. The Crinoidea 174 Example of the Class — Antedon 174 SECTION VIII Rotifera, Polyzoa, and Bracfiopoda 178 Class I. Rotifera 178 Example of the Class — Brachionus rubens . . . 178 Class 2. Polyzoa 181 Example of the Class — Bugula avicularia . . . 181 Class 3. Brachiopoda 184 Example of the Class — Magellania Jlavescens . . .184 SECTION IX Phylum Annulata 188 Class I. The Chaetopoda 188 Example of the Class — Nereis dumerilii .... 189 Class 2. The Hirudinea 203 Example of the Class — Hirudo medicinalis . . . 203 SECTION X Phylum Arthropoda 212 Class I. The Crustacea 213 Example of the Class — Astacus Jluviatilis . . .213 Class 2. The Onychophora 236 Example of the Class — Peripatus 236 CONTENTS XV PAGE Class 3. The Myriapoda 239 Example of the Class — Scolopendra morsitans . . . 239 Class 4. The Insecta . . . . . . . 241 Example of the Qass — Periplaneta americana . . 241 Class 5. The Arachnida 254 Example of the Class — Scorpio ..... 254 SECTION XI Phylum Mollusca 264 Class I. The Pelecypoda 265 Example of the Class — ^Anodonta cygnea . . . 265 Class 2. The Amphineura 281 Example of the Class — Chiton spinosus .... 281 Class 3, The Gastropoda . 284 Example of the Class — ^ Helix nemoralis .... 284 Class 4. The Cephalopoda 296 Example of the Class — Nautilus pompilius . . . 298 SECTION XII Phylum Chordata 310 Subphylum i. The Adelochorda 31 1 Example of the Subphylum — Balanoglossus . . . 311 Subphylum 2. The Urochorda 314 Example of the Subphylum — Ascidia . . . . 314 Subphylum 3. The Vertebrata 322 Examples of the Subphylum — Amphioxus, Petromyzon, Pisces, etc 322 Division A. The Acrania 323 Example of the Division — Amphioxus lanceolatus . . 323 Division B. The Craniata 328 Examples of the Division, the dogfish, lizard, and rabbit . 328 Class I. Cyclostomi ........ 360 Example of the Class — Petromyzon marinus . . . 360 Class 2. Pisces 365 Examples of the Class - — Sharks, perch, and lung fishes . 365 XVI CONTENTS PAGE Subclass I. Elasmobranchii 366 Example of the Subclass — Scyllium and Mustelus . . 366 Subclass 2. Holocephali (omitted) 394 Subclass 3. Teleostomi 394 Example of the Subclass — Saltno fario .... 395 Subclass 4. Dipnoi 405 Example of the Subclass — Ceratodus fosteri . . . 405 Class 3. Amphibia 407 Example of the Class — Rana tejnporaria . . . 408 Class 4. Reptilia . . . . . . . . 433 Examples of the Class — Lizards, turtles, crocodile . . 434 Class 5. Aves . . 456 Example of the Class — Columba livia .... 457 Class 6. Mammalia . .491 Example of the Class — Lepus cuniculus .... 491 LIST OF ILLUSTRATIONS FIG. FACE 1. Amoeba proteus . 15 2. Amoeba polypodia 18 3. Quadrula, Hyalosphenia, Arcella, Difflugia .... 20 4. Forms of Foraminifera 22 5. Shells of Foraminifera 25 6. Actinophrys sol 26 7. Actinosphserium eichhornii 27 8. Forms of Heliozoa 29 9. Liteocircus annularis 31 10. Actinomma asteracanthion .32 11. Collozoum inerme 33 12. Euglena viridis » • 35 13. Forms of Mastigophora -37 14. Forms of Choanoflagellata 39 15. Forms of Dinoflagellata 40 16. Noctiluca miliaris . . .41 17. Volvox globalor 43 18. Paramoecium caudatum 47 19. Forms of Ciliata 49 20. Forms of Tentaculifera *51 21. Forms of Ciliata 53 22. Vorticella 54 23. Monocystis agilis 56 24. Gregarina 57 25. Ovum of a sea-urchin 60 26. Diagram of maturation and fertilization or ovum . . . 6l 27. Segmentation of the oosperm ...... 62 28. Forms of epithelium .... . 64 29. Diagram illustrating the structure of glands .... 65 xvii LIST OF ILLUSTRATIONS xviii FIG. PAGE 30. Bones of arm with biceps muscle . . 67 31. Viscera of male frog 69 32. Hydra . . . . . 73 33. Sycon ciliatum .... 76 34. Sycon gelatinosum 77 35. Sycon gelatinosum, magnified .. 78 36. Sycon gelatinosum, transverse section * 80 37. Ascetta primordialis 0 1 84 38. Section of Spongilla 85 39. Skeleton of sponges 87 40. Sponge spicules .... 88 41. Obelia colony .... 93 42. Nematocysts of Hydra . 96 43. Dissection of Medusa . , 97 44. Development of Laomedea and Eudendrium 99 45. Structure of Hydra 100 46. Petasus and Glossocodon 102 47. Bougainvillea ramosa 103 48. Physalia 106 49. Physalia arethusa .... 106 50. Halistemma tergestinum 107 51. Aurelia aurita, partly dissected 109 52. Aurelia aurita, development . . , III 53. Tessera princeps .... 113 54. Tealia crassicornis 115 55. Sea-anemone, in sections 116 56. Common sea-anemone . 118 57. Corallium rubrum .... 118 58. Alcyonium palmatum II9 59. Tubipora musica .... 120 60. Pennatula sulcata .... 121 61, Flabellum curvatum 122 62. Astrsea pallida .... 123 63. Dendrophyllia nigrescens and Madrepora aspera 124 64. Cancrisocia on back of a cra,b 125 65. Hormiphora plumosa 126 66. Hormiphora plumosa, section of a tentacle 127 67. Idyia roseola ..... 128 LIST OF ILLUSTRATIONS dissected FIG. 68. Distomum hepaticum, natural size 69. Distomum hepaticum, anatomy . 70. Distomum hepaticum, development 71. Trematodes: Amphistomum and Homalogas 72. Structure of a triclad turbellarian 73. Planaria polychroa 74. Taenia solium . . 75. Taenia solium, head magnified 76. Taenia solium, proglottis 77. Development of tape-worm 78. Cyst of Taenia echinococcus with daughter-cyst and scolices 79. Diagram of organs of a Nemertine 80. Tetrastemma, structure 81. Ascaris lumbricoides .... 82. Ascaris lumbricoides, dissection of female 83. Diagram of nervous system of Nematoda 84. Ascaris lumbricoides, posterior end of male 85. Trichina spiralis . 86. Starfish, showing tube feet 87. Starfish, vertical section through an arm 88. Starfish, diagrammatic sections 89. Asterias rubens, digestive system 90. Ambulacral systems of a starfish 91. Anthenea, dorsal surface 92. Anthenea, ventral surface 93. Ophioglypha lacertosa . 94. Strongylocentrotus 95. Corona of sea-urchin . 96. Apical systems of plates of sea-urchin 97. Cucumaria planci 98. Antedon 99. Metacrinus interruptus 100. Brachionus rubens 101. Bugula avicularia 102. Plumatella . 103. Pedicellina . 104. Magellania flavescens . 105. Magellania lenticularis, sagittal section xix PAGE 130 131 135 136 137 138 139 140 141 142 144 146 147 150 151 152 153 155 158 I6I 164 165 166 167 168 169 171 172 172 174 175 177 179 182 183 184 185 186 XX LIST OF ILLUSTRATIONS FIG. PAGE 106. Nereis dumerilii, natural size 189 107. Nereis dumerilii, parapodium 190 108. Nereis dumerilii, setae 191 109. Nereis dumerilii, anatomy . . . . , . .192 no. Nereis dumerilii, transverse section ..... 194 111. Section through the eye of Nereis 196 1 12. Brain and connecting nerves of Nereis .... 197 1 13. Serpulas in their tubes 198 1 14. Trochosphere of Eupomatus 199 115. Lumbricus agricola 200 1 1 6. Lumbricus, setae 201 1 1 7. Hirudo medicinalis . 204 1 18. Head of Hirudo medicinalis, showing the three jaws . . 205 1 19. Head of Hirudo quinquestriata ...... 206 120. Nephridium of the inedicinal leech ..... 208 1 21. Transverse section of Hirudo 209 122. Diagram of blood-channels of leech . . . . .210 123. Astacus fluviatilis 214 124. Appendages of Astacus 217 125. Astacus fluviatilis, dissection from right side . . .221 126. Respiratory organs of Astacus fluviatilis . . . . 223 127. Thorax of crayfish, transverse section 225 128. Diagram of the circulation in the crayfish .... 226 129. Nervous system of Astacus fluviatilis 227 130. Reproductive organs of Astacus fluviatilis .... 229 1 31. Cancer pagurus 230 132. Pagurus bernhardus 231 133. Apus glaciahs 232 134. Development of Apus 233 135. Cyclops and Calocalanus 234 136. Lepas anatifera 235 137. Peripatus capensis 237 138. Peripatus capensis, head, etc 237 139. Internal organs of Peripatus 238 140. Scolopendra 240 14 1. Periplaneta americana 242 142. Mouth-parts of the cockroach 243 143. Pieris rapse, larva, and pupa 244 LIST OF ILLUSTRATIONS xxi FIG. PAGE 144. Carpet beetle, larva, and pupa • 245 145. Culex and larva . 246 146. Internal organs of cockroach . 247 147. Periplaneta, its tracheal system . . 248 148. Periplaneta, nervous system . 249 149. Honey bee, queen, worker, and drone • 253 150. Red ant, male, worker, and female . 253 1 5 1. Euscorpio . 255 152. Scorpion, ventral side .... • 256 153. Scorpion, internal organs . 258 1 54. Epeira diadema . 259 155. Cattle tick . 260 156. Itch mite . 260 157. Limulus, ventral view .... . 261 158. Anodonta cygnea, entire animal . . 265 159. Anodonta cygnea, right valve, and animal . 268 160. Anodonta, section of shell and mantle . 269 1 61. Anodonta cygnea, animal . . 270 162. Anodonta cygnea, dissection from left side • 273 163. Anodonta- cygnea, sections of gills . 274 164. Anodonta, diagram of circulatory system . 276 165. Anodonta, embryo and glochidium . 278 166. Mytilus edulis . 279 167. Teredo navalis 168. Chiton spinosus . 281 169. Chiton, ventral view .... . 282 170. Chiton, nephridial and genital systems . 283 171. Helix nemoralis . 285 172. Triton nodiferus, shell . 286 173. Triton nodiferus, median section of shell . 287 174. Solarium perspectivum, under side . 288 175. Terebra oculata, shell .... . 289 176. Cyprgea moneta, animal expanded, in its shell . 290 177. Doris tuberculata .... . 290 178. Shell-bearing Pteropoda . 291 179. Patella vulgata, animal, ventral view . . 292 180. Limax, lung-cavity, etc. • 293 181. Triton nodiferus . ... . . 294 xxii LIST OF ILLUSTRATIONS FIG. PAGE 182. Sepia cultrata 297 183. Nautilus pompilius 298 184. Nautilus pompilius, section of shell 299 185. Spirula peronii 301 186. Sepia cultrata, shell 301 187. Loligo vulgaris 302 188. Argonauta argo 303 189. Chromatophore of Sepia 304 190. Sepia cultrata, dissected 305 191. Nautilus pompilius, anatomy 306 192. Sepia officinalis, jaws . 307 193. Sepia officinalis, enteric canal 307 194. Nautilus pompilius, oral surface of male and female . . 309 195. Balanoglossus . -311 196. Balanoglossus, diagrammatic sagittal section of anterior end . 312 197. Ascidia 314 198. Ascidia 315 199. Ascidia, diagram of longitudinal section . . . • 317 200. Ascidia mammillata, larva . 320 201. Diagram of metamorphosis of larva into fixed Ascidian . 321 202. Botryllus violaceus . . 322 203. Amphioxus lanceolatus, ventral and side view . . . 323 204. Amphioxus, diagram of anatomy 325 205. Amphioxus lanceolatus, sections 327 206. Dogfish, fins, etc. 329 207. Lacerta viridis 330 208. Lepus cuniculus, lateral view of skeleton with outline of body . 33 1 209. Scyllium, vertebrae 334 210. Lizard, vertebrae of 335 21 1. Lepus cuniculus ......... 336 212. Scyllium canicula 338 213. Lacerta agilis, three views of skull 340 214. Fore and hind limbs of vertebrate, diagram . , . 341 215. Tooth, longitudinal section, semi-diagrammatic . . . 343 216. Scyllium canicula, dissection 345 217. Lacerta agilis, viscera in their natural relations . . . 348 218. Circulation of a fish, diagram 351 219. Scyllium canicula, brain, dorsal view 353 LIST OF ILLUSTRATIONS xxiii FIG. PAGE 220. Eye of man, diagrammatic horizontal section . 357 221. Petromyzon marinus 222. Myxine glutinosa, head . . . 362 223. Petromyzon marinus 224. Scyllium canicula, side view of skull . • 371 225. Scyllium, pectoral arch .... • 373 226. Scyllium canicula, dissection • 376 227. Diagram of the vascular system of a fish . 378 228. Scyllium catulus . 382 229. Dogfish, egg-case . 385 230. Scyllium, embryo, with gills, etc. . 386 231. Lamna cornubica • 387 232. European sting-ray (Urolophus cruciatus) . . 388 233. Skeleton of Urolophus testaceus . . 390 234. Heptanchus, side view of skull . 391 235. Salmo fario, fins, etc • 395 236. Salmo fario, caudal end of vertebral column • 397 237. Pleuronectes cynoglossus .... • 398 238. Ctenoid and ganoid scales .... . 399 239. Polypterus birchir ..... . 399 240. Skull of sturgeon 241. Salmo fario, entire skull, left side] . 401 242. Premaxillse of Sargus 243. Hippocampus (sea-horse) .... . 404 244. Ceratodus fosteri • 405 245. Ceratodus fosteri, anterior portion of skeleton . 406 246. Rana temporaria 247. Rana temporaria, skeleton .... . 412 248. Rana temporaria, skull, different views . 413 249. Rana esculenta, shoulder girdle . . 415 250. Rana esculenta, pelvic girdle from right side . 416 251. Rana temporaria, dissection from left side . . 417 252. Rana temporaria, heart with cavities laid open . 420 253. Rana temporaria, arterial system, etc. . . 421 254. Rana temporaria, venous system, etc. . . 424 255. Rana esculenta, brain from above and below . 426 256. Rana esculenta, urinogenital organs of male . . 428 257. Rana esculenta, urinogenital organs of female . 429 xxiv LIST OF ILLUSTRATIONS FIG. PAGE 258. Rana temporaria, stages in life-history .... 430 259. Salamandra maculosa 432 260. Siren lacertina ......... 433 261. Pygopus lepidopus . .' 437 262. Hatteria punctata 438 263. Grecian tortoise, Testudo graeca 439 264. Skeleton of crocodile 441 265. Cistudo lutaria 442 266. Chelone midas ■ . . . . 443 267. Skull of rattlesnake ........ 445 268. Pectoral arch and sternum of Lacerta agilis . . . 446 269. Heart of monitor, Varanus 448 270. Brain of alligator, from above 449 271. Pineal eye of Hatteria punctata, section . . . *451 272. Poison apparatus of rattlesnake 454 273. Columba livia, diagram with most of feathers removed . 458 274. Columba livia, feather 460 275. Pterylosis of Columba livia 462 276. Columba livia, bones of the trunk 463 277. Columba livia, cervical vertebra 464 278. Columba livia, sacrum of nestling 465 279. Columba livia, skull of young 466 280. Columba livia, hyoid apparatus 467 281. Columba livia, bones of left wing 468 282. Columba livia, bones of left manus of nestling . . . 469 283. Columba livia, left innominate of nestling .... 470 284. Columba livia, bones of left hind-limb . . . .471 285. Columba livia, part of left foot of embryo .... 472 286. Columba livia, muscles of left wing 473 287. Columba livia, dissection from right side .... 475 288. Heart of pigeon, dorsal aspect 478 289. Columba livia, brain, different views 479 290. Eye of pigeon 480 291. Columba livia, right membranous labyrinth of ear . . 481 292. Columba livia, male urinogenital organs .... 482 293. Columba livia, female urinogenital organs .... 482 294. Feather of cassowary 484 295. Wing of nestling of Opisthocomus and of adult Apteryx . 486 LIST OF ILLUSTRATIONS XXY FIG. PAGE 296. Callus banLiva, domestic fowl, egg at time of hatching . 489 297. Lepus cuniculus, side view of skeleton with outline of body 492 298. Lepus cuniculus, atlas and axis 494 299. Lepus cuniculus, skull, side, and ventral view . ' . . 497 300. Lepus cuniculus, shoulder girdle 501 301. Lepus cuniculus, distal end of fore-leg and carpus . . 502 302. Lepus cuniculus, innominate bones and sacrum . . . 503 303. Lepus cuniculus, bones of hind foot ..... 504 304. Lepus cuniculus, lateral dissection of head, neck, and thorax 506 305. Lepus cuniculus, stomach, intestine, and liver, etc. . . 508 306. Lepus cuniculus, heart, from right side .... 509 307. Lepus cuniculus, the vascular system 51 1 308. Lepus cuniculus, larynx, ventral and dorsal views . *513 309. Lepus cuniculus, brain, dorsal and ventral view . . .514 310. Lepus cuniculus, two dissections of brain . . . .516 31 1. Lepus cuniculus, longitudinal vertical section of brain . 518 312. Lepus cuniculus, urogenital organs 519 313. Lepus cuniculus, anterior end of vagina, with right uterus, etc. 521 314. Duck-bill, Ornithorhynchus anatinus ... . . 526 315. Spiny ant-eater. Echidna aculeata ..... 527 316. Virginian Opossum, Didelphys virginiana . . . .528 317. Dasyure, Dasyurus viverrinus 529 318. Rock wallaby, Petrogale xanthopus 530 319. Koala, Phascolarctos cinereus . . . . . • 53^ 320. Unau, or two-toed sloth 532 321. Tatu armadillo, Dasypus sexcinctus 533 322. Scaly ant-eater, Manis pentadactyla 534 323. Aard-vark, Orycteropus capensis 534 324. Killer, Orca gladiator 535 325. Section of upper jaw, with baleen plates, of Balsenoptera . 536 326. Harbor seal, Phoca vitulina 540 327. Bat, Synotus barbastellus 542 MANUAL OF ZOOLOGY INTRODUCTION Zoology, the branch of Natural History which deals with animals, is one of the two subdivisions of the great science of Biology, which takes cognizance of all organisms, or things having life, as distinguished from such lifeless natural objects as rocks and minerals. The second of the two subdivisions of Biology is Botany, which deals with plants. The subject-matter of Zoology, then, is furnished by the animals which inhabit the land-surface, the air, and the salt and fresh waters of the globe ; the aim of the science is to find out all that can be known of these animals, their structure, their habits, their mutual relationships, their origin. The first step in the study of Zoology is the recognition of the obvious fact that the innumerable individual animals known to us may be grouped into what are called species, the members of which resemble one another so closely that to know one is to know all. The following example may serve to give the reader a fairly accurate notion of what zoologists understand by species, and of the method of naming species which has been in use since the time of the great Swedish naturalist Linnaeus. B 1 2 MANUAL OF ZOOLOGY The domestic cat, the European wild cat, the ocelot, the leopard, the tiger, and the lion are animals which agree with one another in the general features of their organisation — in the number and form of their bones and teeth, in the possession of retractile claws, and in the position and characters of their internal organs. No one can fail to see that these animals, in spite of differences of size, colour, markings, etc., are all, in the broad sense of the word, “cats.” This is expressed in the language of systematic Zoology by saying that they are so many species of a single genus. According to the system of binomial nomenclature intro- duced by Linnseus, each kind of animal receives two names — one the generic name, common to all species of the genus ; the other the specific name, peculiar to the species in question. Both generic and specific names are Latin in form, and are commonly Latin or Greek in origin, although frequently modern names of persons or places, with Latinised terminations, are employed. In giving the name of an ani- mal, the generic name is always placed first, and is written with a capital letter, the specific name following it, and being written, as a rule, with a small letter. For instance, to take the examples already referred to, the domestic cat is called Felis domestica, the European wild cat F. catus, the leopard F. pardus, the tiger F. tigris, the lion F. leo. Thus the systematic name of an animal is something more than a mere appellation, since it indicates the affinity of the species with other members of the same genus : to name an animal is, in fact, to classify it. It is a matter of common observation that no two indi- viduals of a species are ever exactly alike : two tabby cats, for instance, however they may resemble one another in the general characters of their colour and markings, invariably INTRODUCTION 3 present differences in detail by which they can be readily distinguished. Individual variations of this kind are of universal occurrence. Moreover, it often happens that the members of a species are divisible into groups distin- guishable by fairly constant characters : among domestic cats, for instance, we find white, black, tabby, gray, and tor- toiseshell cats, besides the large long-haired Persian breed, and the tailless Manx cat. All these are distinguished as varieties of the single species, Fetis domestica. It is often difficult to decide whether two kinds of ani- mals should be considered as distinct species or as varieties of a single species, and no universal rule can be given for determining this point. Among the higher animals mutual fertility is a fair practical test, the varieties of a species usually breeding freely with one another and producing fer- tile offspring, while -distinct species either do not breed together or. produce infertile hybrids or mules. Compare, for instance, the fertile mongrels produced by the union of the various breeds of domestic dog with the infertile mule produced by the union of the horse and ass. But this rule is not without exception, and in the case of wild animals is, more often than not, impossible of application : failing it, the only criterion of a “good species” is usually the pres- ence of constant differences from allied species. Suppose, for instance, that a naturalist receives for description a number of skins of wild cats, and finds, after an accurate examination, that in some specimens the tail is two-thirds the length of the body and the skin of a uniform reddish tint with a few markings on the head, while in the rest the tail is nearly half as long as the body and the skin tawny with black stripes. If there are no intermediate grada- tions between these two sets of individuals, they will be placed without hesitation in distinct species ; if, on the 4 MANUAL OF ZOOLOGY Other hand, there is a complete series of gradations between them, they will be considered to form a single variable species. As, therefore, animals have to be distinguished from one another largely by structural characters, it is evident that the foundations of a scientific Zoology must be laid in Morphology, the branch of science which deals with form and structure. Morphology may be said to begin with an accurate examination of the external characters ; the divi- sions of the body, the number and position of the limbs, the characters of the skin, the positions and relations of the mouthy eyes, ears, and other important structures. Next the internal structure has to be studied, the precise form, posi- tion, etc., of the various organs, such as brain, heart, and stomach being made out : this branch of morphology is distinguished as Anatomy. And, lastly, the various parts must be examined by the aid of the microscope, and their minute structure, or Histology, accurately determined. It is only when we have a fairly comprehensive knowledge of these three aspects of a given animal — its external charac- ters, its rough anatomy, and its histology — that we can with some degree of safety assign it to its proper position among its fellows. An accurate knowledge of the structure of an animal in its adult condition is not, however, all-sufficient. Nothing has been made more abundantly clear by the researches of the last half-century than that the results of anatomy and histology must be checked, and if necessary corrected, by Embryology — i.e. by the study of the changes undergone by animals in their development from the egg to the adult condition. A striking instance is afforded by the common barnacles which grow in great numbers on ships’ bottoms, piers, etc. The older zoologists, such as Linnaeus, grouped INTRODUCTION 5 these creatures, along with snails, mussels, and the like, in the group Mollusca, and even the great anatomical skill of Cuviejr failed to show their true position, which was made out only when Vaughan Thompson, about fifty years ago, proved, from a study of the newly hatched young, that their proper place is among the Crustacea, in company with crabs, shrimps, and water-fleas. Given a sound knowledge of the anatomy, histology, and embryology of animals, their Classification may be attempted — that is, we may proceed to arrange them in groups and sub-groups, each capable of accurate definition. The general method of classification employed by zoolo- gists is that introduced by Linnaeus, and may be illustrated by reference to the group of cats which we have already used in the explanation of the terms genus, species, and variety. We have seen that the various kinds of true cat — domes- tic cat, lion, tiger, etc. — together constitute the genus Felis. Now there is one member of the cat- tribe, the cheetah, or hunting leopard, which differs from all its allies in having imperfectly retractile claws and certain peculiari- ties in its teeth. It is therefore placed in a distinct genus, Cynalurus^ to mark the fact that the differences separating it from any species of Felis are of a more fundamental char- acter than those separating the species of Felis from one another. The nearest allies of the cats are the hyaenas, but the presence of additional teeth and non-retractile claws — to mention only two points — makes the interval between hyaenas and the two genera of cats far greater than that between Felis and Cynaelurus. The varying degree of differ- ence is expressed in classification by placing the hyaenas in a separate family, the Hyanida, while Felis and Cynaelurus 6 MANUAL OF ZOOLOGY are placed together in the family Felidcs. Similarly the civets and mongooses form the family Viverridce ; the dogs, wolves, jackals, foxes, etc., the family Canidce; bears, the family Ursidce; and so on. All the foregoing animals have sharp teeth adapted to a flesh diet, and their toes are armed with claws. They there- fore differ fundamentally from such animals as sheep, deer, pigs, and horses, which have flat teeth adapted for grinding vegetable food, and hoofed feet. The differences here are obviously far greater than those between any two of the families mentioned above, and are emphasised by placing the flesh-eater in the order Carnivora^ the hoofed animals in the order Ungulata. In the same way gnawing animals, such as rats, mice, and beavers, form the order Rodentia ; pouched animals, such as kangaroos and opossums, the order Marsupialia; and so on. Carnivora, Ungulata, Rodentia, Marsupialia, etc., although differing from one another in many important respects, agree in the possession of a hairy skin and in the fact that they all suckle their young. They thus differ from birds, which have a covering of feathers, and hatch their young from eggs. The differences here are considerably more important than those between the orders of quadrupeds referred to, and are expressed by placing the latter in the class while birds constitute the class Aves. In the same way the scaly, cold-blooded lizards, snakes, tortoises, etc., form the class Reptilia; the slimy-skinned, scaleless frogs, toads, and sala- manders the class Amphibia , and the finned, water-breathing fishes the class Pisces. Mammals, birds, reptiles, amphibians, and fishes all agree with one another in the possession of red blood and an inter- nal skeleton — an important part of which is the backbone or vertebral column — and in never having more than two INTRODUCTION 7 pairs of limbs. They thus differ in some of the most funda- mental features of their organisation from such animals as crabs, insects, scorpions, and centipedes, which have colour- less blood, a jointed external skeleton, and numerous limbs. These differences — far greater than those between classes — are expressed by placing the backboned animals in the phylum or sub-kingdom Chordata, the many-legged armoured forms in the phylum Arthropoda. Similarly, soft- bodied animals with shells, such as oysters and snails, form the phylum Mollusca, polypes and jellyfishes the phylum C(zlenterata. And, finally, the various phyla recognised by zoologists together constitute the kingdom Animalia. Thus the animal kingdom is divided into phyla, the phyla into classes, the classes into orders, the orders into families, the families into genera, and the genera into species, while the species themselves are assemblages of individual animals agreeing with one another in certain constant characters. It will be seen that the individual is the only term in the series which has a real existence : all the others are mere groups formed, more or less arbitrarily, by man. To return to the animal originally selected as an example, it will be seen that the zoological position of the domestic cat is expressed as follows : — Kingdom — Animalia. Phylum — Chordata. Class — Mammalia. Order — Carnivora. F amily — Felidce. Genus — Felis, Species — F. domestic a. The object of systematic zoologists has always been to 8 MANUAL OF ZOOLOGY find a natural as opposed to an artificial classification ol animals. Good instances of artificial classification are the grouping of bats with birds on the ground that both possess wings, and of whales with fishes on the ground that they both possess fins and live in the water. An equally good example of a natural classification is the grouping of both bats and whales under the head of Mammalia because of their agree- ment, in all essential points of anatomy, histology, and embryology, with the hairy quadrupeds which form the bulk of that class. With the older zoologists the difficulty was to find some general principle to guide them in their arrangement of animals — some true criterion of classification. It was believed by all but a few advanced thinkers that the in- dividuals of each species of animal were descended from a common ancestor, but that the original progenitor of each species was totally unconnected with that of every other, having, as Buffon puts it, ‘‘participated in the grace of a dis- tinct act of creation.” To take an instance : all wolves were allowed to be descended from a pair of ancestral wolves, and all jackals from a pair of ancestral jackals, but the original pair in each case was supposed to have come into being by a supernatural process of which no explanation could or ought to be offered. Nevertheless it was obvious that a jackal was far more like a wolf than either of them was like a tiger, and that in a natural system of classification this fact should be expressed by placing the wolf and jackal in one family, the tiger in another. All through the animal kingdom the same thing occurs : no matter what group we take, we find the species com- posing it resemble one another in varying degrees, or, as it is sometimes expressed, have varying degrees of relationship to one another. On the view that each species was sepa- INTRODUCTION 9 rately created, the word relationship was used in a purely metaphorical sense, as there could, of course, be no real relationship between two groups of animals having a totally independent origin. But it was assumed that creation had taken place according to a certain scheme in the Divine Mind, and that the various species had their place in this scheme like the bits of glass in a mosaic. The problem of classification was thus to discover the place of each species in the pattern of the unknown design. The point of view underwent a complete change when, after the publication of Darwin’s Origin of Species in 1859 the Doctrine of Descent or of Organic Evolution came to be generally accepted by biologists. A species is now looked upon, not as an independent creation, but as having been derived by a natural process of descent from some pre- existing species, just as the various breeds of Domestic Fowl are descended from the little jungle- fowl of India. On this view the resemblances between species referred to above are actually matters of relationship, and species are truly allied to one another in varying degrees, since they are descended from a common ancestor. Thus a natural classification becomes a genealogical tree, and the problem of classifica- tion is the tracing of its branches. This, however, is a matter of extreme difficulty. Repre- senting by a tree the whole of the animals which have ever lived on the earth, those existing at the present day would be figured by the topmost twigs, the trunk and main branches representing extinct forms. Thus the task of arranging animals according to their relationships would be an almost hopeless one but from two circumstances : one, that remains of many extinct forms have been preserved : the other, that the series of changes undergone by an ani- mal in its development from the egg often forms an epitome MANUAL OF ZOOLOGY lO of the changes by which, in the course of ages, it has been evolved from an ancestral type. Evidence furnished by the last-named circumstance is, of course, furnished by embry- ology : the study of extinct animals constitutes a special branch of morphology to which the name Palaeontology is applied. The solid crust of the earth is composed of various kinds of rocks divisible into two groups: (i) Igneous rocks ^ such as granite and basalt, the structure of which is due to the action of the internal heat of the globe, and which originate below the surface and are not arranged in layers or strata ; (2) Aqueous or sedimentary rocks, which arise by the disin- tegration, at the surface of the earth, of pre-existing rocks, the fragments or debris being carried off by streams and rivers and deposited at the bottom of lakes or seas. Being formed in this way by the deposition of successive layers or strata, the sedimentary rocks have a stratified structure, the lowest being in every case older than the more superficial layers. The researches of geologists have shown that there is a general order of succession of stratified rocks ; that they may be divided into three great groups, each representing an era of time of immense but unknown duration, and that each group may be subdivided into more or fewer systems of rocks, each representing a lesser period of time. Imbedded in these rocks are found the remains of various extinct animals in the form of what are called fossils. In the more recent rocks the resemblance of these to the hard parts of existing animals is perfectly clear; we find shells hardly differing from those we pick up on the beach, bones easily recognisable as those of mammals, birds, or fishes, and so on. But in the older rocks the fossils are in many cases so different in character from the animals existing at the present day as to be referable to no existing order. We INTRODUCTION n find birds with teeth, great aquatic reptiles as large as whaleSj fishes, molluscs, Crustacea, etc., all of an entirely different type from any now existing. We thus find that the former were in many cases utterly unlike the present animal inhabi- tants of the globe, and we arrive at the notion of a succession of life in tune, and are even able, in exceptionally favourable circumstances, to trace back existing forms to their extinct ancestors. By combining the results of comparative morphology, embryology, and palaeontology we get a department of Zoology called Phylogeny, the object of which is to trace the pedigrees of the various groups. There are, however, very few cases in which this can be done with any approach to exactness; most “ phylogenies ” are purely hypothetical, and merely represent the views at which a particular zoolo- gist has arrived after a more or less exhaustive study of the group under discussion. Animals may also be studied from the point of view of Distribution. One aspect of this study is inseparable from Palaeontology, since it is obviously necessary to mention in connection with a fossil the particular system or systems of rocks in which it occurs : thus we distinguish geological distribution or distribution in time. The distribution of recent forms may be studied under two aspects, their horizontal or geographical distribution, and their vertical or bathymetrical distribution. To men- tion the latter first, we find that some species exist only on plains, others — hence called alpine forms — on the higher mountains ; that some marine shells, fishes, etc., always keep near the shore {littoral species), others live at great depths {abyssal species), while others (^pelagic species) swim on the surface of the ocean. Among aquatic animals, moreover, whether marine or fresh-water, three principal modes of life 12 MANUAL OF ZOOLOGY are to be distinguished. There are animals such as jelly- fishes, which float on or near the surface of the water, and are carried about passively by currents ; such forms are included under the term Plankton. Most fishes, whales, and cuttle-fishes, on the other hand, are strong swimmers, and are able to traverse the water at will in any direction ; they together constitute the Nekton. Finally, such animals as crabs, oysters, sponges, zoophytes, etc., remain permanently fixed to or creep over the surface of the bottom, and are grouped together as the Benthos. Under the head of geographical distribution we have such facts as the absence of all land-mammals, except bats in New Zealand and the Polynesian Islands, the presence of pouched Mammals, such as kangaroos and opossums, only in some parts of America and in Australia and the adjacent islands, the entire absence of finches in Australasia, and so on. We find, in fact, that the fauna — i.e. the total animal inhabitants — of a country is to a large extent independent of climate, and that the faunae of adjacent countries often differ widely. In fact, it is convenient in studying the geo- graphical distribution of animals largely to ignore the ordi- nary division into continents, and to divide the land-surface of the globe into what are called zoo-geographical regions. There are still two departments of zoological science to be mentioned. As it is impossible to have a right under- standing of a machine without knowing something of the purpose it is intended to serve, so the morphological study of an animal is imperfect without some knowledge of its Physiology, i.e. of the functions performed by its various parts, and the way in which they work together for the welfare of the whole. Not only may we study the action of a given animal’s organs, but also the actions of the animal as a whole, its INTRODUCTION 13 habits, its relations to other animals, whether as friends, as enemies, or as prey, to the vegetable kingdom, and to its physical surroundings, such as temperature, humidity, etc. In a word, the whole question of the relation of the organism to its environment gives us a final and most important branch of Natural History which has been called Ethology or Bionomics. SECTION I.— PHYLUM PROTOZOA 1. THE RHIZOPODA The simplest members of the animal kingdom are for the most part, too small to be visible without the aid of a micro- scope, or at least so small as to appear to the unassisted eye as extremely minute specks, not distinguishable, unless in unusually active movement, from small particles of non-living matter. Representatives of this class of simple minute ani- mals are to be found living under a variety of different con- ditions ; they are abundant in fresh water, running or stagnant, and they are equally numerous in the sea, while they are also to be found living in the fluids of cavities in the bodies of higher animals. An example which will serve to illustrate some of the main features of the class is the Proteus animalcule or Amoeba. Amoeba (Fig. i) is some- times to be found by searching with the aid of the micro- scope in water from stagnant pools. To the unpractised beginner it is a difficult task to discriminate between the microscopic particles of non-living matter which form the main part of the sediment at the bottom of such a pool — debris of animals, vegetable or mineral nature — and the object of which he is in search. Numerous minute bodies will doubtless be seen which their active movements among the motionless particles show to be endowed with life. But 14 SECT, I PHYLUM PROTOZOA 15 Amoeba is not one of these. It is to be recognised as a glassy-looking, irregularly shaped particle with a definite out- line. From a particle of some crystalline mineral substance, to which such a description would equally well apply, Amoeba would soon be distinguishable owing to the cir- cumstance that it is constantly changing its shape. This change is effected by the pushing out of projections or processes, called pseudopods or pseudopodia which undergo various alterations of size and shape, and may be- come withdrawn, other similar processes being developed in their place. At the same time careful watching shows that the Amoeba is also, with extreme slowness, changing its position. This it effects by a kind of streaming motion. A projection forms itself on one side, and the entire substance of the Amoeba gradually streams into it ; a fresh projec- tion appears towards the same side, the streaming move- ment is repeated, and, by a constant succession of such movements, an extremely gradual locomotion, which it often takes very close watching to detect, is brought about. In these movements, it is to be noticed, the Amoeba is influenced i6 MANUAL OF ZOOLOGY SECT. to some extent by contact with other minute objects ; when the processes come in contact with small grains of sand or other similar particles, their movements are modified in such a way that the Amoeba, in its slow progress onwards, passes on one side of them, so that it might be said to feel its way among the solid particles in a drop of sediment. Judging from the nature of the movements, we are obliged to infer that the substance of which this remarkable object is composed must be soft and semi-fluid, yet not miscible with the water, and, therefore, preserving a sharp contour. These and other characteristics to be mentioned subsequently enable us to conclude that we have to do with the substance of complex chemical composition termed protoplasm, which constitutes the vital material of all living organisms whether animals or plants. In Amoeba the protoplasm is clearly dis- tinguishable into two parts, an outer homogeneous, glassy- looking layer completely enclosing a more granular internal mass. Examination of the Amoeba with a fairly high power of the microscope reveals the presence in its interior of two objects which with a low power we should be likely to overlook. One of these is a small rounded body of a homogeneous appear- ance, which preserves its form during all the changes which the Amoeba as a whole undergoes. This is termed the nucleus (Fig. i, nu) ; it is enclosed in an extremely delicate membrane, and consists of a protoplasmic material differing from that which forms the main bulk of the Amoeba in con- taining a substance which refracts the light more strongly and which has a stronger affinity for certain colouring matters. The other minute object to be distinguished in the interior appears as a clear rounded space (c. vac) in the protoplasm. When this is watched it will be observed to increase gradually in size till it reaches a maximum of, let us i PHYLUM PROTOZOA say^ a fifth of the total diameter of the Amoeba, when by a sudden contraction of its walls, it suddenly disappears, to reappear presently and gradually grow again to its maxi- mum size. This pulsating clear space is the contractile vacuole. By watching the Amoeba carefully for some time we may be enabled to observe that the movements of the proto- plasm of the body not only effect locomotion, but are con- nected also with the reception of certain foreign particles of organic nature — /., spicules. (From Biitschli’s Protozoa, after Schulze and Greeff.) I 30 MANUAL OF ZOOLOGY sect. But in some genera the process of fission under some circum- stances remains incomplete, the two protoplasmic bodies to which the fission gives rise remaining connected together by a bridge or isthmus of protoplasm, instead of becoming separated off in the shape of two independent animals, as in Amoeba. Further, these two bodies may each in turn divide in the same incomplete way, so that four Heliozoans are developed, all remaining connected together; and by further repetitions of the same process a structure may be formed consisting of a large number of units all connected together by living substance. A structure of this kind, formed as a result of repeated incomplete division (or, in other cases, budding) from an original simple animal, is termed a colony, and the elements or units of which it is composed are termed zooids. How such a colony of unicellular Protozoa is to be distinguished from a multicellular animal or Metazoan (p. 19) will be explained at a later stage. It will at once be apparent that the compound Foraminifera are of the nature of colonies of unicellular zooids, each occupying one of the chambers of the shell, formed as the result of a process of repeated budding. In addition to the process of .multiplication by fission multiplication also takes place in some Heliozoa by a pro- cess known as the formation of spores. In spore-formation (a form of which has already been referred to as occurring in the Foraminifera) the protoplasm breaks up into numerous small parts, each of which eventually develops into the form of the parent. Usually the Protozoan passes into a qui- escent condition before this takes place ; the pseudopodia become withdrawn, and the whole becomes enclosed in a firm envelope or sporocyst ; this process is known as encysfa- tion. The spores in some of the Heliozoa, when set free, are provided each with two flagella (Fig. 8, j, b) which PHYLUM PROTOZOA 31 subsequently become lost, pseudopodia appearing in their place. The Radiolaria are marine Rhizopoda which have exceed- ingly delicate, thread-like pseudopodia (P'ig. 9, psd') and a skeleton usually composed of silica. This skeleton may be composed of loosely woven needle-like bodies or spicules ; more usually it is in the form of a globular, conical, star- shaped, or disc-shaped shell, perforated by numerous open- ings, and often supported by spines which radiate out from Fig. 9. — Liteocircus annularis, cent, caps, central capsule; ext. caps, pr, extra- capsular protoplasm; int. caps, pr, intra-capsular protoplasm; nu, nucleus; psd, pseudopods; skel, skeleton; z, cells of Zooxanthella. (After Biitschli, from Parker’s Biology.') the centre ; sometimes (Fig. 10) there are several such shells one within the other. In some Radiolaria the skele- ton is composed not of silica, but of a chitinoid substance called acanthin. Embedded in the protoplasm is a perfor- ated membranous sac, the central capsule (Figs. 9 and 10, cent, caps), in the protoplasm within which is a single nucleus or a number of nuclei, and a number of oil-drops. There is no contractile vacuole, but in many Radiolaria the protoplasm outside the central capsule contains numerous non-contractile vacuoles,- the presence of which gives it a frothy appearance. 32 MANUAL OF ZOOLOGY SECT. Radiolaria which give rise to colonies are exceptional, but a few cases occur. In these (Fig. ii) the central capsule divides again and again giving rise to a number of central capsules which remain embedded in a firm gelatinous sub- stance — the vacuolated protoplasm outside the central cap- sules. Such a mass, which may attain considerable size, floats about freely in the sea. f PHYLUM PROTOZOA 33 In addition to reproduction by simple binary fission, spore-formation also occurs in some of the Radiolaria. The protoplasm contained in the central capsule breaks up into small masses, each of which becomes a flagellula provided with a flagellum (Fig. 44, F). In most of the Radiolaria there occur in the 'extra-capsular protoplasm minute yellow cells (Fig. 9, z), which multiply Fig. II. ~ Collozoum inerme. A — C, three forms of the entire colony, nat. size; D, a small colony showing the numerous capsules (c. caps') and extra-capsular protoplasm with vacuoles {vac) ; E, spores containing crystals (c) ; mega- and microspore. (From Biitschli’s Pvotozoa, after Hertwig and Brandt.) independently by binary fission. It has been proved that these are microscopic unicellular plants (^Zooxanthella) of the class Algse, which live in the substance of the protoplasm of the living Radiolarian. Such an intimate association between two living organisms is known as symbiosis. There can be no doubt that this association is beneficial both to the Radiolarian and to the Alga. It is characteristic of the plant cell that under the action of light and in the presence o 34 MANUAL OF ZOOLOGY SECT, of the specially vegetable green colouring matter, chlorophyll, it is able to utilise for its nutrition the carbon dioxide or “carbonic acid gas” present in the air. The carbon is seized and made use of by the plant cell for the building up of such compounds as starch and sugar, while the oxygen is set free. The animal cell, on the other hand, is continually using up oxygen and giving off carbon dioxide in the process of respiration, while it is unable, in the absence of chloro- phyll, to manufacture such substances as starch and sugar. Thus in this close association or symbiosis between the Zoox- anthella and the Radiolarian, the latter benefits the former by supplying it with carbonic acid and other substances by which it i^ nourished, while the Alga contributes to the respiration of the Radiolarian by the oxygen which it gives off, and to its nutrition by the sugar and other substances which it forms. 2. THE MASTIGOPHORA We have seen that the spores by which multiplication is effected in some of the Rhizopoda (Heliozoa, Radiolaria) are characterised by the presence of slender whip-like appendages — the flagella. In a great number of Protozoa such a flagellate condition of the cell is not merely a tempo- rary larval one, as in the cases already dealt with, but is the ordinary and permanent condition of the adult animal. These permanently flagellate Protozoa constitute the class Mastigophora — a very numerous group, mostly of very small size. A good example of this class, very abundant in fresh-water pools, in which it may be present in such enormous numbers as to impart to the water a distinct green colour, is Euglena viridis (Fig. 12). Another species or variety of Euglena viridis, is so abundant at times as to colour the water blood-red (Stokes). I PHYLUM PROTOZOA 35 The body of Euglena {E, H) is spindle-shaped, and has at the blunt anterior end a depression, the gullet {F. ces), from the inner surface of which springs a single long flagellum (/). The organism is propelled through the water by the forwards ; it can also perform slow, worm -like movements of contraction and expansion {A — Z>), but anything like the 36 MANUAL OF ZOOLOGY SECT. I free pseudopodial movements which characterise the Rhizo- poda, is precluded by the presence of a very thin skin or cuticle which invests the body. There is a nucleus {nu') near the centre of the body, and at the anterior end a con- tractile vacuole c. vac), leading into a large non-con- tractile space or reservoir (r) which discharges into the gullet. The greater part of the body is coloured green by the characteristic vegetable pigment, chlorophyll, and contains grains of paramylum p), a carbohydrate allied to starch. In contact with the reservoir is a bright red speck, the stigma {pg), formed of a pigment allied to chlorophyll and called hoemato chrome. It seems probable that the stigma is a light-perceiving organ or rudimentary eye. Euglena is nourished like a typical green plant ; it de- composes the carbon dioxide of the air dissolved in the water, assimilating the carbon and setting free the oxygen. Nitrogen and other elements it absorbs in the form of min- eral salts in solution in the water. But it has also been shown that the movements of the flagellum create a whirl- pool by which minute fragments are propelled down the gullet and into the soft internal protoplasm. There seems to be no doubt that in this way minute organisms are taken in as food. Euglena thus combines the characteristically animal {holozoic) with the characteristically vegetable {holo- phytic) mode of nutrition. Sometimes the active movements cease; the animal comes to rest and surrounds itself with a cyst or cell-wall of cellulose (the characteristic material of the cell- wall of plants), from which, after a quiescent period, it emerges to resume active life. It is during the resting condition that reproduction takes place by the division of the body in a median plane parallel to the long axis ((?). Under certain circumstances 11<0inobryon 12.Syncry |)to 13. An^hoJ)hysa 14.Rhi|)idodendron Fig. 13. — Various forms of Mastigophora. In a, flagellate {a) and amoeboid {l>) phases are shown; in 5, flagellate {a) and heliozoan {!>) phases; in 8 are shown two stages in the ingestion of a food particle {/)\ chr, chromatophores; c. vac, contractile vacuole; f, food particle ; g, gullet; nu, nucleus; I, lorica; protoplasm ; peristome; z/. z, vacuole of ingestion. (Mostly from Biit- schli’s Protozoa, after various authors.) 37 MANUAL OF ZOOLOGY SECT. 3S multiple fission takes place, and flagellulce, i.e., young pro- vided with flagella, are produced, which, sometimes after passing through an amoeboid stage, develop into the adult form. In the other Mastigophora the body may have a shape similar to that of Euglena, or may be longer and narrower, or, on the other hand, may be short and thick, ovoid or globular. Anterior and posterior ends are nearly always distinguishable, the former being that which is directed forwards in progression. Usually there are distinct dorsal and ventral surfaces, the former being that which is habitually directed upwards. In most cases the body is equal-sided or bilaterally symmetrical, i.e., is capable of being divided into two equal lateral portions along the median vertical plane ; but sometimes it is unsymmetrical, one side differ- ing more or less from the other. In most the body is, as in Euglena, naked ; but some have a chitinoid shell or lorica, while others have a firm cell-wall of cellulose which may present an elaborate pattern of strips, dots, etc., and may be produced into long processes. Most of the Mastigophora are, like Euglena, free-swimming, but some are permanently attached by means of a slender stalk (Fig. 13, jo, ij, 14', Fig. 14, 7, j). The number and arrangement of the flagella vary greatly. The number may be one, as in Euglena, or two, three, or four. In forms with two flagella these are both attached at or near the anterior end, and often take on different functions, one of them, directed forwards, being alone used in locomotion, while the other is trailed behind when the animal is swim- ming freely, or is used to anchor it to various solid bodies. In one large group of Mastigophora, the Choanoflagellata (Fig. 14), there is, surrounding the base of the flagellum, a remarkable vase-like prolongation of the protoplasm, ex- PHYLUM PROTOZOA 39 ceedingly delicate and transparent, called the collar. This is contractile, and, though its precise functions are not yet certainly known, there is evidence to show that its move- ments cause a flow of water, with minute particles in sus- pension, up the outside of the collar and down the inside, 40 MANUAL OF ZOOLOGY SECT the solid particles being then ingested in the soft protoplasm between the base of the flagellum and that of the collar. Both collar and flagellum may be withdrawn, and the animal pass into an Amceba-like or amoeboid form. In another group — the Dinoflagellata (Fig. 15) — there are two fla- gella, one springing from a longitudinal groove extending along the anterior half of the body, and the other lying in a 1 PHYLUM PROTOZOA 41 transverse groove which completely encircles the body ; the former alone acts as an organ of locomotion, the latter lies habitually in the groove and performs undulating move- ments. Noctiluca (Cystoflagellata) (Fig. 16), which is the largest member of the class, being about half a millimetre in diameter, has two flagella, one of which is modified in a remarkable manner. The body of Noctiluca is globular, with a cleft along one side so that it resembles a miniature peach. From this sprmgs a very large and stout flagellum I Fig. 16. — Noctiluca miliaris. a, the adult animal; d, c, flagellulse; tentacle; f, flagellum; mouth; nucleus. (From Lang, after Butschli.) or tentide, which is marked with a number of transverse lines or striations ; and a second flagellum, of comparatively small size, lies in the gullet. Though all the Mastigophora are characterised by the possession of flagella, there are a few, such as Mastigamoeba (Fig. 13, 4), which also possess pseudopodia, and may be capable of amoeboid movements. Nutrition is effected in a variety of different ways. Some forms live in decaying organic infusions, not taking in solid food-particles, but absorbing nourishing matter in a dis- 42 MANUAL OF ZOOLOGY SECT, solved form from the substances in the infusion. Others, living in internal cavities of the bodies of higher animals, receive, in a similar way, nourishment from the juices of the animal they infest. Organisms, whether plants or animals, which receive their nourishment in the former of these two ways, are said to be saprophytic as regards their nutrition, while such as obtain it from other living organisms are said to be parasitic. But a large proportion of the Mastigophora are neither saprophytes nor parasites, and are nourished in one of two other ways, or in both of them. Many take in minute solid particles of organic matter, usually in the form of minute living organisms. In many such cases, there is, as in Euglena, an aperture, the mouth, opening into a short passage, the gullet, by which the food is received into the protoplasm in the interior of the body; but this is not always present, and in such cases (Fig. 13, 8) the food-parti- cles are taken in by a process not unlike that which we have seen to occur in Amoeba. But, on the other hand, many of the Mastigophora are not distinguishable from plants by their mode of nutrition ; and on that ground, taken in connection with their structure, which is in nearly all respects that of a typical unicellular plant, have almost equal claims to be ranked in either the vegetable or the animal kingdom. They have a cell-wall of cellulose like a plant cell, they contain chlorophyll or a red colouring matter, hcemato chrome, of similar composition, and they have no mouth. They must, therefore, be nourished precisely after the manner of a green plant, and, if they are assigned to the animal kingdom instead of to the vegetable, it can only be because the possession of flagella seems to ally them with forms that are of undoubted animal character. Colonies are of frequent occurrence among the Mastigo- phora. Sometimes there is a branching slender stalk I PHYLUM PROTOZOA 43 bearing a single zooid or a group of zooids at the end of each of the branches (Fig. 14, J), the whole colony being fixed by the base of the main stalk, and the flagellum serving for the capture of food-particles and not for locomotion. Fig. 17. — Volvox globator. A, entire colony, enclosing several daughter-colonies; B, the same during sexual maturity; C, four zooids in optical section; Di— D®, asexual formation of daughter-colony; E, zooid which has become converted into a mass of microgametes; F, microgamete ; G, megagamete surrounded by microgametes; H, zygote; a, early stages in the formation of daughter-colonies; Jl, flagellum; ov, ovy, megagametes; pg, pigment spot; spy, zooids containing microgametes. (From Parker’s Biology, after Cohn and Kirchner.) Sometimes (Fig. 17) the colony is of a more massive char- acter, the zooids being embedded in a clump of gelatinous material, with the end bearing the flagellum projecting on the exterior ; usually such colonies are free-swimming. 44 MANUAL OF ZOOLOGY SECT. Multiplication is effected most commonly by the simple process of binary fission (Fig. ii, which may take place either in the active or in an encysted condition. In some cases the fission is multiple, the protoplasm dividing not merely into two, but into a greater number of parts, each destined to develop into the adult form. We also meet in the Mastigophora with what may be regarded as the simplest mode of sexual reproduction. In some forms two individuals come together and become completely fused, the process being known as conjugation^ and the body formed by the union of the cells being known as a zygote. The protoplasm of the latter divides by mul- tiple fission into very minute spores. These, when first liberated by the rupture of the zogote, are mere granules, but soon the flagella are developed. In some cases the con- jugating cells or gametes are of two sizes, union always taking place between a large cell or megagamete and a small cell or microgamete. In Volvox, which is a free-swimming spheri- cal colony (Fig. 17, E, F, G) this difference between the two sets of conjugating cells reaches its extreme, pro- ducing a condition of things closely resembling what we find in the sexual reproduction of higher forms. Certain of the zooids enlarge and form megagametes, others divide repeatedly and give rise to groups of microgametes, each of the latter having the form of a rod-like body with two flagella. The microgametes escaping, swim about freely and conjugate with the motionless megagametes to form a zygote, which, after a time divides to give rise to a new colony. Mastigophora occur under the most various conditions, to some of which reference has been already made. Many kinds live in fresh water ; others are abundant in the sea, Noctiluca and others among the marine forms are phos* 1 Conjugation has also been observed to occur in many Rhizopods. t PHYLUM PROTOZOA 45 phorescent, and are usually the agents by means of which the diffuse phosphorescence of the sea is produced. Others, again, are saprophytes, while others are parasites of higher animals. 3. THE INFUSORIA Often to be found in great numbers, in stagnant pools, organic infusions, etc., is Paramoecmm, the “ slipper-shaped animalcule,” a Protozoan of comparatively large size, about \ mm. in length, which moves about very actively like Euglena, but with a more regular and more rapid move- ment, and by means of organs of locomotion differing in character from the flagellum of the latter. The body of Paramoecium, (Fig. i8. A, B) is covered with what appear under the microscope like small delicate hairs arranged in longitudinal rows. These are the cilia; they are in inces- sant to-and-fro vibration, and it is by their means that the Paramoecium moves about and obtains its food. In shape the body is somewhat cylindrical, rounded at the anterior and bluntly pointed at the posterior end. On one side, the ventral, is a large oblique depression, the buccal groove {buc. gr), leading into a short gullet {gul), which, as in Euglena, ends in the soft internal protoplasm. The proto- plasm is differentiated into a firmer superficial layer, the cortex {corf), and a semi-fluid central mass, the medulla {med), and is covered superficially by a thin cuticle. The cilia are prolongations of the cortex, and perforate the cuticle. In the cortex are found two nuclei. One of these, the meganucleus {nu), is a comparatively large ovid body; the other, the micronucleus {pa. mi), is a small rounded body closely applied to the meganucleus. Two contractile 46 MANUAL OF ZOOLOGY SECT. 1 vacuoles {c. vac) are present. Each is connected with a series of radiating spindle-shaped cavities in the protoplasm which serve as feeders to it; after the contraction of the vacuole these cavities are seen gradually to fill, apparently receiving water from the surrounding protoplasm ; they then contract, discharging the water into the vacuole, the latter rapidly enlarging while they disappear from view ; finally the vacuole contracts and discharges i|s contents externally. The cortex contains minute radially-arranged sacs called trichocysfs {trek). When the animal is irritated, more or fewer of these suddenly discharge a long delicate thread (C), which, in the condition of rest, is very probably coiled up within the sac. Food, in the form of small living organisms, is taken in by means of the current caused by the cilia of the buccal groove. The food-particles, enclosed in a globule of water, or “ food-vacuole ” (/. vac), circulate through the proto- plasm, where the soluble parts are gradually digested and assimilated. Effete matters are egested at a definite anal spot posterior to the mouth, where the cortex and cuticle are less resistant than elsewhere. The whole feeding pro- cess can readily be observed in this and other Infusoria by placing in the water some insoluble colouring matter, such as carmine or indigo, in a fine state of division, the minute particles of the colouring matter, which are taken into the mouth in the way described, being readily observed as they become received into food-vacuoles and circulate in the central protoplasm. Multiplication takes place by transverse fission (D), the division of the body being preceded by that of both nuclei. It has been proved, however, that multiplication by binary fission cannot go on indefinitely, but that after it has been repeated a certain number of times, it is interrupted by 47 48 MANUAL OF ZOOLOGY SECT. 1 conjugation. In this very remarkable and characteristic process two Paramoecia become applied by their ventral faces, but do not fuse \ their meganuclei break up and dis- appear, and an interchange of the substance of the micro- nuclei of the two conjugating individuals takes place, with the result that each develops a new meganucleus, and a new micronucleus, partly formed of the substance of its own micronucleus, partly that of the other Paramoecium. The possession of cilia is the distinctive feature of the class Infusoria among the Protozoa. But in one section of the class — the order Tentaculifera — cilia are only present in the young, their place in the adult being taken by append- ages known as tentacles. The form of the body in the Infusoria (Fig. 19) is very varied; it may be globular, ovoid (7), kidney-shaped (2), trumpet-shaped (j), vase- shaped (p), produced into a long, flexible, neck-like pro- cess (5), or into large paired lappets (d), flattened from above downwards, or elongated and divided into a series of segments. Most are free-swimming, but many are fixed, usually by means of a slender stalk (p). The arrangement of the cilia also varies greatly. Some, like Paramoecium, have small cilia of uniform character distributed over the entire surface. Others have different kinds of cilia on different parts of the surface, while in others the cilia are entirely confined to certain regions. An instance of the latter arrangement is the common stalked form Vo7-ticella, with its allies such as Epistylis (p), in which the cilia are confined to the free extremity. These cilia produce rapid currents, and the Epistylis, says Stokes, select from them anything they may want, and let the rest sweep by. In another group, again, the body, which is of flattened shape, bears on its dorsal surface a small number of very fine motionless cilia, while on its ventral the cilia are Fig. 19. — Various forms of Ciliata. qa shows part of a colony, b a single zooid, and tf a couple of nematocysts; anus; c. vac, contractile vacuole; f. vac, food vacuole; gullet; meganucleus; ;;zz. micronucleus; mt/i, mouth; nu, nucleus; nematocyst; tentacle; ti. mb, undulating membrane; vac, non-contractile vacuole; vst, vestibule. (From BUtschli’s Protozoa, after vari- ous authors.) 49 5° MANUAL OF ZOOLOGY SECT, very strong, and are modified into the shape of hooks, bris- tles, or plates with fringed ends. The hooks and plates do not vibrate rhythmically like ordinary cilia, but are moved as a whole at the will of the animal, such Infusoria being able, in addition to swimming freely through the water, to clamber by the aid of these specially modified cilia over the surface of weeds, etc. Tentacles may be present in addition to cilia (14), and a number of other exceptional modifications {lo-ij) occur which cannot be specially re- ferred to here. In addition to cilia, many genera possess delicate sheets of ox undulating membranes {u. mb) in connec- tion with the peristome. These contract so as to pro- duce a wave-like movement which aids in the ingestion of food. The tentacles, which, in the Tentaculifera (Fig. 20), take the place of cilia in the adult, are elongated cylindrical structures, capable of protrusion and retraction, and having the distal end expanded into a sucker. The tentacle is practically tubular, the core consisting of a semi-fluid proto- plasm, while the outer part is tolerably firm. Infusoria and other organisms are caught by the tentacles, the cuticle of the prey is pierced or dissolved where the sucker touches it, and the semi-fluid protoplasm can then be seen flowing down the tentacle into the body of the captor. A single tentacle alone may be present (j), or the tentacle may be branched {4), the extremity of each of the branches being suctorial. In some forms (j) the tentacles are devoid of sucker-like extremities, and can be moved about actively to catch the prey. The meganucleus is often ovoid, as in Paramoecium. In other cases it may be long and band-like (Fig. 19, j, mg. nu), horseshoe-shaped (p), very long and constricted PHYLUM PROTOZOA Fig. 20. — Various forms of Tentaculifera. la and b, two species of Podophrya; c, a tentacle much enlarged; 2a, Acineta jolyi ; 2b, A. tuberosa; in 6 the ani- mal has captured several small Ciliata; 8a, a specimen multiplying by budding; 5(5, a free ciliated bud ; pa, the entire colony: qb, a portion of the stem; qc, a liberated bud; a, organism captured as food; b. c, brood cavity: bd, bud; c. vac, contractile vacuole; mg. nu, meganucleus; mi. nu, micronucleus" t, tentacle. (After Butschli and Seville Kent.) 52 MANUAL OF ZOOLOGY SECT, at intervals so as to look like a string of beads, or branched. In nearly all cases one or more micronuclei are present. In Vorticella and others there is a single contractile vacuole which opens, as in Euglena, through the intermediation of a reservoir into the gullet. In other Infusoria there may be one, two, or many contractile vacuoles. In some instances the protoplasm is hollowed out by numerous non-contractile vacuoles. Trichocysts mainly occur in the forms with a uni- form coating of cilia ; more complicated bodies of similar character termed neniatocysts (p, c) occur in some cases. A mouth is absent in many parasitic forms, and nourish- ment is obtained by the absorption of the digested food of the animal in which the infusorian is parasitic. In the Tentacu- lifera, in which a mouth is also absent, nourishment is drawn in by means of the tentacles in the manner already described. In the rest there is a mouth and gullet, usually situated, as in Paramoecium, at the end of a buccal groove, or peristome. In Vorticella and its allies (Fig. 19, 9, and Fig. 22) the body is in the shape of a wine-glass, the stem of which is represented by a slender stalk {si), while the rim is the equivalent of the peristome {per) ; in the area which the peristome encloses is an elevated disc of protoplasm, be- tween which and the peristome on one side is the opening of the mouth {mth) : the only cilia present run in a spiral band round the peristome, round the edge of the disc, and down into the gullet {gull). An anal spot is present in Vorticella and many other forms ; in a few there is, instead, a distinct anal aperture (Fig. 19, 2 a). A chitinoid skeleton (Fig. 21) occurs in a few forms; usually it is bell-shaped, sometimes it is perforated by a number of apertures (/) so that it resembles in appearance the skeleton of some of the Radiolaria. A chitinoid lid or operculum {2, J, op) may be fixed to the edge of the peri- I PHYLUM PROTOZOA 53 stome, and when the animal is retracted into its case, accu> rately closes the mouth of the latter. Colonies occur in many of the Infusoria. Some allies of Vorticella (Fig. 19, p) develop highly complex colonies, the slender stalk branching again and again, and each terminal branch ending dn a zooid. A remarkable colonial form is 54 MANUAL OF ZOOLOGY SECT. Dendrosoma (Fig. 20, p), one of the Tentaculifera : it has a creeping stem from which branches spring upwards, each terminating in a zooid with suctorial tentacles ; and Fig. 22. — Vorticella. A, B, living specimens in different positions; C, optical section; D^, D^, diagrams illustrating coiling of stalk; E^, E-, two stages in bina^ fission; E^, free zooid; F^, F^, division into mega- and microzooids; G’, G^, conjugation; multiple fission of encysted form; H®, develop- ment of spores; ax. f, axial fibre; cort, cortex; cuticle; c. vac, contractile vacuole ; disc ; gullet; m, microzooid; mouth; meganucleus; per, peristome, (From Parker’s Biology, partly after Saville Kent.) I PHYLUM PROTOZOA 55 the single nucleus extends as a narrow branching cord throughout the axis of the entire colony. Transverse fission is the universal method of reproduc- tion ; and budding also occurs. Spore-formation has been observed in a few forms. Conjugation, in the form of a temporary union of two individuals, with interchange of the substance of the micro- nuclei, occurs in many of the ciliate Infusoria. In some forms the conjugating individuals become completely fused. The effect of the process of conjugation seems to be in- creased activity in multiplication by fission. In mode of life the Infusoria are as varied as the Mastigo- phora. Some are holozoic, some saprophytic, some parasitic. Of the parasitic forms some give rise to definite diseases in the bodies of their hosts. The skin affection known as eczema, for example, seems to be caused by the presence ^ of parasitic Vorticellae. 4. THE SPOROZOA In the interior of certain organs, termed the seminal vesicles, of the earthworm will often be found a parasitic Protozoan — Monocystis agilis (Fig. 23) — which exempli- fies another of the classes of the phylum, the class Sporozoa. It is flattened, elongated, pointed at both ends, and performs slow movements of expansion and contraction {A, E), reminding us of those of Euglena. There are neither pseudopodia, nor flagella, nor cilia. There is a firm cuticle, and the protoplasm is divided into a denser superficial portion, the cortex, and a central semi-fluid mass, the medulla. There is a large clear nucleus, but no trace of contractile vacuole, or of mouth or gullet. Reproduction takes place by a peculiar and characteristic process of spore- formation. Either a single individual, or two individuals 56 MANUAL OF ZOOLOGY SECT. closely applied together, but not actually fused, become encysted. Multiple fission then takes place, the proto- plasm becoming divided (C) into an immense number of spindled-shaped spores, each surrounded by a strong chitinoid coat, and thus differing markedly from the naked spores of Rhizopoda and Mastigophora. The protoplasm of each spore then undergoes fission, becoming divided I PHYLUM PROTOZOA 55 into a number of somewhat sickle-shaped bodies, which are arranged within the spore-coat somewhat like a bundle of sausages. By rupture of the spore-coat these falciform young, as they are termed, are liberated, and at once begin active movements, the thin end of the body moving to and fro like a clumsy flagellum. They enter the clumps of devel- oping sperms of the earthworm, and afterwards escape into the cavity of the seminal vesicle and grow into the adult form. 58 MANUAL OF ZOOLOGY SECT. I All the Sporozoa are parasitic, and all are characterised by the absence of pseudopodia, flagella, and cilia; and of mouth and gullet, and by the formation of spores enclosed in chitinoid coats. Gregarina (Fig. 24) differ from Mono- cystis in having the medullary part of the protoplasm divided into two sections, known as the protomerite (pr), and deutomerite {deu), by a sort of partition, with, in the young condition, a third division, the epimerite (ep) in front; and in the more complex form of the cysts, which have delicate canals or sporoducts (spd) through which the spores escape. Some of the Sporozoa ( Coccidium and others) are parasites, not like Monocystis and Gregarina, in the cavities of organs, but in the interior of cells, such as the cells lining the intes- tine of higher animals. The various forms of the disease known as malaria in Man have been proved to be due to the presence of a Sporozoan {Hczmamoeba laverani ) which in- vades and destroys, at a certain stage in its life-history, the red corpuscles of the blood. Another form (Apwsoma bigeminum) causes the Texas fever in cattle, the infection being carried by ticks. These parasites cause high fever, anaemia, bloody urine, and the number of red-blood corpus- cles is diminished in one week to one-sixth of the normal amount. Babesia bovis in the blood of the ox causes the dis- ease known as hsemoglobinurea, and another form produces a similar disease in sheep. A parasite of the tzetse fly, which is a flagellate hsematozoan, is the cause of the tzetse disease in southern Africa. These organisms live in the marrow and lymphatics, and flush at intervals into the general blood stream. The disease is communicated by the tzetse fly from the wild game, the herds of which are the fester spots which maintain the disease. The silkworm disease called pebrine is due to one of the Myxosporidia, Glugea bombycis, which inhabits all the tissues of the caterpillar of Bombyx mori. SECTION II. — THE METAZOA While the Protozoa are predominantly unicellular, and of extremely simple structure, the rest of the animal kingdom, grouped together under the comprehensive title of Metazoa, are all multicellular in the adult condition, and have, except in some of the lowest groups, a more or less elaborate struc- ture owing to the presence of complicated systems of organs for carrying on the various functions of animal life. Such an animal as a lobster or a frog, for example, may readily be ascertained to be made up of a complicated system of parts, — skeleton, muscles, digestive organs, blood vessels, and so on, — and it requires only the most superficial micro- scopic examination of the substance of these various parts to render it evident that each is built up of an immense multitude of cells. A lobster or a frog, however, or any other Metazoan, consists, in the earliest stage of its exist- ence, of a single cell, the oosperm, formed by the union of a male cell or sperm with a female cell or ovum. The ovum (Fig. 25) is usually spherical in shape, with one or more enclosing membranes, with cell-protoplasm enclosing a large nucleus {germinal vesicle, as it is often termed in this case), in which are contained one or more small, rounded bodies {germinal spot or spots'). The ovum may contain, in addition to the protoplasm, a quantity of non- protoplasmic material or yolk. 59 6o MANUAL OF ZOOLOGY SECT, Before the changes begin which lead to the formation of the multicellular Metazoan, another cell, the male cell or sperm, has to unite with the ovum or female cell. Before this takes place, the ovum throws off portions of its substance (Fig. 26, /(?/) in the form of two little rounded bodies — the polar bodies^ This preliminary process is known as the maturation of the ovum. The male cell or sperm is a relatively small cell, usually motile, which penetrates into Fig. 25. — Ovum of a Sea-Urchin, showing the radially striated cell-membrane, the protoplasm, containing yolk-granules, the large nucleus (germinal vescicle), with Us network of chromatin and a large nucleolus (germinal spot). (From Bal- four’s Embryology, after Hertwig.) the ovum, and coalesces with it — the coalescence being what is termed fertilisation or impregnation — and the immediate result being that, instead of separate ovum and sperm, we have a compound body, the oosperm, formed by their union, but not differing at first in any marked degree from the simple ovum, and containing a single nucleus representing both the nucleus of the sperm and that of the ovum. On impregnation follows the process of segmentation of the oosperm. The nucleus first divides into two ; then the n THE METAZOA 6l Fig. 26. — Diagram illustrating the maturation and fertilisation of the ovum. A, formatw,.i o' first polar globule; B, beginning of fertilisation, sperms _ approaching the micropyt , c / aperture in the enclosing membrane of the ovum through which the sperm enters; C, forma tion of the male pronucleus; D, approximation of the male and female pronuclei; E, forma- tion of segmentation-nucleus; 5 ce7it, female centrosome; cf cent, male centrosome (tla centrosomes are cell-structures not further referred to in this work); egg-membrane. 77iicrop, micropyle; pol, polar bodies; 9 pro7i, female pronucleus; ^ pro7i, male pronu cleus; seg, nucl, segmentation nucleus. 02 MANUAL OF ZOOLOGY SECl substance of the protoplasm becomes cleft into two parts (Fig. 27), each half containing one of the nuclei, so that two complete cells result. This process, it will be observed, is essentially the same as the binary fission of Amoeba and other Protozoa : in the Metazoan, however, the two cells do not become separated from one another as the two parts of the divided Amoeba do, but remain in contact and undergo further changes. Each of them divides (Fig. 27) Fig. 27. — Various stages in the segmentation of the oosperm. (From Gegenbaur’s Comparative Anatomy.) in the same manner into two — four cells being thus formed; the four divide to form eight, the eight to form sixteen, and so on ; until, by this process of division and sub- division, the oosperm becomes segmented into a large number of comparatively small cells. In this mass of cells an arrangement into layers, the germinal layers, becomes by and by discernible ; and from these layers of cells are developed eventually all the parts of the body of the Metazoan. II THE METAZOA 63 This mode of development is, however, not entirely with- out parallel among the Protozoa. In the colonial Volvox (p. 43, Fig. 17) it will be remembered that male cells or microgametes (sperms) and female cells or megagametes (ova) are developed, and that by the coalescence of a microgamete with a raegagamete a compound cell, the zygote (oosperm), is formed, which undergoes division to give rise to an adult Volvox. As the various parts become gradually moulded from the cells of the germinal layers, the form and arrangement of the cells of the different parts become modified in different ways, so that the cellular structure comes to differ widely; and, as a result, we find in the fully formed animal a variety of different kinds of material, — tissues, as they are termed, — such as muscle, bone, gristle, nerve, all derived from the cells of the germinal layers. Of such tissues the following are the most important. An epithelium is a thin stratum of cells covering some surface, external or internal ; it may be one cell thick, or several cells thick. The cells of which an epithelium is composed vary greatly in form in different cases (Fig. 28) : they may be beset at their free surfaces with cilia {a), like the cilia of the Infusoria, or with flagella, like those of the Mastigophora (/), or may be amoeboid {Ji), sending out pseudopodia like a Rhizopod. The epithelium which covers the- outer surface is known as the epidermis or deric epithelium ; that which lines the interior of the digestive organs is the enteric epithelium. Glands (Fig. 29) are formed by modification of epithe- lial cells. In many cases a single cell of the epithelium forms a gland, which is then termed a unicellular gland {A, B) . The secretion (or substance which it is the func- tion of the gland to form or collect) gathers in such a case in the interior of the cell, and reaches the surface of the 64 MANUAL OF ZOOLOGY SECT, Fig, 28. — Various forms of epithelium, a, ciliated epithelium; b, columnar: d, sur- face view of the same; c, tessellated; the same from the surface; y, flagellate epithelium with collars; g, flagellate epithelium without collars; h, epithelium of intestine with pseudopodia; z', stratified epithelium; k, deric epithelium of a marine planarian with pigment cells, rod cells, and sub-epithelial glands. (From Lang’s Comparative A7tatomy.) n THE METAZOA 65 epithelium through a narrow prolongation of the cell, which serves as the duct of the gland. In other cases the gland is niulticellular {D, G), formed of a number of cells of the epithelium, lining a depression or infolding, simple or com- plex in form, of the latter. In the central cavity of such a Fig. 29. — Diagram to illustrate the structure of glands. A, unicellular glands in an epithelium; B, unicellular glands lying below epithelium and communicating with the surface by narrow processes (ducts) ; C, group of gland cells; Z>, group of gland cells lining a depression; JE and F, simple multicellular gland; G, branched multicellular gland. (From Lang.) gland the secretion collects to reach the surface through a passage, the duct. The general name of connective tissues is applied to a number of tissues which play a passive part in the economy of the animal, connecting and supporting or protecting the various organs. Sometimes’ connective tissue is gelatinous in character, sometimes fibrous. Fat or adipose tissue is F 66 MANUAL OF ZOOLOGY SECT. usually developed by modification of fibrous connective tissue, the cells becoming distended with oily matter. Cartilage is a firm but elastic material, readily cut with a knife, which forms an important constituent of the skeleton in higher animals. Bone differs from cartilage in being much denser and harder, owing to its being strongly impregnated with limey matter (carbonate and phosphate of lime). Muscular tissue is the material by means of which nearly all the movements of the Metazoa are effected. It consists of bundles of microscopic fibres, which in the living condi- tion have the special property of contractility, contracting, i.e. becoming shorter and thicker, when stimulated. Bundles or bands of these form the organs known as muscles. Nerve tissue, which is the sensitive, conducting, and stimulating tissue of the body, consists of nerve-cells and nerve-fibres ; groups of the former constitute nerve- ganglia ; bundles of the latter form nerves. Associated with the multicellular character of the Metazoa is the possession of a variety of different parts or organs adapted to carrying out different functions in the life of the animal. Such a formation of organs is faintly fore- shadowed in the unicellular body of the Protozoa ; the contractile vacuoles, the nucleus, the pseudopodia, flagella, and cilia, the gullet, etc., are all to be looked upon as organs subserving certain functions. But in the Metazoa, with the exception of some of the lower groups, the development of organs for the carrying on of the functions of animal life — organs of locomotion, organs for protection and support, organs of digestion, respiration, and reproduction — is carried much further. Some of the chief functions which are caixied on in the body of an animal have already been briefly referred to in II THE METAZOA 67 the account of the Protozoa. The special study of these constitutes, as already pointed out in the Introduction, the science of Physiology, which forms accordingly an important part of the study of Zoology, and a part to which frequent reference will be made in dealing with the structure of the various groups of animals. The various internal parts of an animal are supported and protected by the skin and the skeleton. The skin or in- tegument consists of a layer of cells — the epidermis — with, Fig. 30. — Bones of the human arm and fore-arm with the biceps muscle, showing the shortening and thickening of the muscle during contraction and the conse- quent change in the relative position of the bones — viz., flexion of the fore-arm on the upper arm. (From Huxley’s Physiology superficial to it, in many animals, a non- cellular layer known as the cuticle, and below it usually a fibrous layer, the dermis. The skeleton is, as already explained in the section on the Protozoa, a system of hard parts, external or internal, serving for the protection and support of the softer substance of the body. When these hard parts are external they form an exoskeleton, when internal an endoskeleton. An exoskeleton is formed by the thickening and hardening of portions of one or other of the layers of the integument, — cuticle, epidermis, or dermis. An endoskeleton usually 68 MANUAL OF ZOOLOGY SECT. consists either of cartilage, or of bone, or of both. The parts of the skeleton in the higher animals, whether external or internal, usually consist of a number of distinct pieces which are movably articulated together, and these have the additional important function of serving for the attachment of muscles, constituting a jointed framework on which the muscles act in bringing about the various movements of the body and its appendages (Fig. 30). The nutrition of the Metazoa is in some cases, as in some of the Protozoa, effected by food being absorbed in a dissolved form through the general surface. In the great majority, however, the food, liquid or solid, is received through an opening — the mouth — into a cavity in the interior of the body — the digestive or enteric cavity. In most cases this has the form of a longer or shorter tube or canal, beginning at the mouth and ending at a second exter- nal opening — the anus. This digestive or enteric canal consists usually of a number of different parts, through which the food passes in succession, each part having its special function to perform in connection with nutrition. In most cases there are organs in the neighbourhood of the mouth serving for the seizure of food ; these may be simply tentacles, or soft, finger-like appendages, or they may have the form of jaws, by means of which the food is not only seized but torn to pieces, or ground into small fragments, in the process of mastication. In general we can distinguish in the enteric canal a buccal cavity, a pharynx, an oesophagus or gullet, a stomach, and an intestine. It is in the stomach and anterior part of the intestine that the food becomes acted upon by certain digestive secretions, the effect of which is to render the various ingredients soluble, and thus fitted to be absorbed through the wall of the enteric canal, so as to reach the various parts of the body and supply them with nourish- II THE METAZOA 69 ment. These digestive secretions are partly produced by the cells of the epithelium of the canal, which are modified to form unicellular or multicellular glands (p. 65), partly by certain large special digestive glands, salivary glands, liver, and pancreas. The nutrient parts of the food are by this means so acted upon that they are ready to be absorbed, and in most animals pass into the blood, to be distributed Fig. 31. — General view of the viscera of a male frog, from the right side, a, stomach; b, urinary bladder; c, small intestine; cl, cloacal aperture; d, large intestine; liver; y, bile duct; gall bladder; spleen; f, lung; /r, larynx; I, fat body; m, testis; n, ureter: o, kidney; /, pancreas: s, cerebral hemi- sphere; sp, spinal cord; t, tongue: u, auricle; ur, urostyle; v, ventricle; v.s, vesicula seminalis; w, optic lobe; x, cerebellum; y, Eustachian recess: z, nasal sac. (From Marshall.) throughout the body. The insoluble and indigestible ingre- dients of the food pass on through the posterior part of the intestine, and reach the exterior through the anal aperture as the fmces. A supply of oxygen is necessary for the carrying on of the chemical changes in the tissues on which vital activity is dependent. At the same time, as a result of these changes, 70 MANUAL OF ZOOLOGY SECT. carbonic anhydride (carbonic acid gas) is constantly being produced. The taking in of oxygen and giving off of car- bonic anhydride is the process of respiration. The task of facilitating the entry of oxygen and the passage outwards of carbonic anhydride is in most of the Metazoa performed by a set of organs known as organs of respiration ; but in many respiration takes place through the general surface, and special organs for carrying on this function are absent. When organs of respiration are present, they are either processes or gills {branchicE) adapted for the respiration of air dissolved in water ; or lungs or other cavities which are adapted for the direct respiration of air. Through the thin membrane lining the gill, or lungs, the oxygen passes and enters the blood in vessels immediately underneath the membrane, to be conveyed, like the food, throughout the system and supplied to the several parts. At the same time the carbonic anhydride, brought to the gill or lung by the same means, passes outwards into the surrounding water or air, and is thus got rid of. The blood consists of a fluid plasma, in which float numerous cells — the blood corpuscles. Sometimes the blood is colourless j usually it is bright red, owing to the presence of a red colouring matter, termed hcemoglobin, which is sometimes confined to certain of the corpuscles, sometimes diffused throughout the plasma. Haemoglobin has a strong affinity for oxygen, and is thus of importance in connection with respiration. In order to carry on its functions as a conveyer of nutriment and of oxygen throughout the body, the blood flows in a system of vessels — the blood vascular system — which ramify throughout all the organs. Through this system of vessels it is driven in a more or less regular course, either by pulsating contractions of the muscular u THE METAZOA 71 walls of the blood-vessels themselves, or by the agency of a special organ, the heart. The heart is essentially a sac with muscular walls. Its cavity is in communication with the main blood-vessels, and its walls contract regularly and drive the blood through the system of vessels, the direction of flow being regulated by a system of valves. The nitrogenous waste-matters which are produced as a result of the chemical changes that accompany vital action in the various organs, are separated out and got rid of by a system of organs known as the organs of excretion, or renal organs — this process of elimination being known as the process of renal excretion. It is by means of the nervous system that the animal receives impressions from the exterior and from the internal organs, and that the various internal parts are brought into vital communication with one another. The nervous system extends as a complicated system of nerves or bundles of nerve-fibres throughout all parts' of the body. Large aggregations of nerve-cells and nerve-fibres forming the centres of the system are known as nerve-ganglia. When one of these, or a group of them, situated towards the anterior end, preponderates in size over the others, it is termed the brain. Forming an important part of the nervous system are the organs of the special senses, — sight, hearing, smell, and taste, each of which is an organ adapted for the reception of impressions of a special kind from the exterior, — the impressions of light, of sound waves, of the particles and substances that produce the sensations of smell and taste. The less specialised sense of touch and of heat and cold is diffused generally over the integument, in which there are frequently special cells, or groups of cells, with nerve-fibres terminating in them, that are concerned with such sensations. 72 MANUAL OF ZOOLOGY SECT. The organs of sexual reproduction are the gonads, in which male and female cells or sperms and ova are produced, with the gonoducts or canals by which these cells reach the exterior. The gonads in which male cells or sperms are produced are called testes, and their ducts are the sperm-ducts. The gonads in which female cells or ova are formed are called ovaries, and their ducts oviducts. Sometimes testes and ovaries occur in distinct r/iale and female individuals, when the animal is said to be unisexual, or to have the sexes distinct. In other cases both ovaries and testes occur in the same individual, when the animal is said to be hermaphrodite, or to have the sexes united. In some instances the same gonad produces both sperms and ova — assuming the character of a hermaphrodite gonad or ovo-testis. In many animals the ova are fertilised by the sperms after they have passed out from the body, and the develop- ment takes place externally — a condition which is known as oviparity. But in others the ova are fertilised while still in the ovary or oviduct of the parent, and the development may take place in the oviduct, usually in a special dilated part of the latter — the uterus — so that the young only escape to the exterior after they have attained a compara- tively advanced stage of their development — when the animal is said to be viviparous. Besides the sexual process of reproduction by means of ova and sperms, there are in many classes of animals various asexual modes of multiplication. One of these — the process of simple fission — has been already noticed in connection with the reproduction of Protozoa. The forma- tion of spores is an asexual mode of multiplication which occurs only in the Protozoa, and has been described in the account of that group. Multiplication by budding takes II THE METAZOA 73 place in a number of different classes of animals. In this form of reproduction a process or bud (Fig. 32, bd^ is given off from some part of the parent animal ; this bud sooner or later assumes the form of the complete animal, and may- become detached from the parent either before or after its Fig. 32. — Fresh-water polype (hydra), two specimens, the one expanded the other contracted, showing multiplication by budding, bd'^ 6d^ bd^, buds in various stages of growth. (From Parker’s Biology.') development has been completed, or may remain in perma- nent vital connection with it. When the buds, after becoming fully developed, remain in vital continuity with the parent, a sort of compound animal, consisting of a greater or smaller number of con- nected units, is the result. Such a compound organism is 74 MANUAL OF ZOOLOGY SECT. termed a colony, and the component units are termed zooids. In some cases such a colony is produced by a process which is more correctly termed incomplete fission than budding. The various systems of organs, — digestive, circulatory, nervous, excretory, etc., — present under one form or an- other in all the higher groups of animals, are variously arranged and occupy various relative positions in different cases, producing a number of widely different plans of animal structure. According as their structure conforms to one or another of these great plans, animals are referred to one or another of the corresponding great divisions or phyla of the animal kingdom. That animals do present widely differing plans of structure is a matter of common knowledge. We have only to compare the true fish, such as cod, haddock, etc., in a fishdealer’s shop with the lobsters and the oysters, to recognise the general nature of such a distinction. The first named are characterised by the possession of a backbone and skull, with a brain and spinal cord, and of two pairs of limbs (the paired fins) : they belong to the great vertebrate or backboned group — the division Vertebrata of the phylum Chordata. The lobsters, on the other hand, in which these special verte- brate structures are absent, possess a jointed body enclosed in a hard jointed case, and a number of pairs of limbs also enclosed in hard jointed cases, and adapted to different purposes in different parts of the body — some being feelers, others jaws, others legs : their general type of structure is that which characterises the phylum Arthropoda. The oysters, again, with their hard calcareous shell secreted by a pair of special folds of the skin constituting what is termed the mantle, and with a special arrangement of the nervous system and other organs which need not be described here, are referable to the phylum Mollusca. Other familiar II THE METAZOA 75 animals are readily to be recognised as, belonging to one or other of these great phyla. A prawn, a crab, a bluebottle fly, a spider, are all on the same general plan as the lobster : they are jointed animals with jointed limbs, and they have the internal organs occupying similar positions with relation to one another. They are all members of the phylum Arthropoda. Again, a mussel, a snail, a squid, are all to be set side by side with the oyster as conforming to the same general type of structure ; they are all members of the phylum Mollusca. Finally a dog, a lizard, a fowl, are obviously nearer the fish : they all have skull and backbone, brain and spinal cord, and two pairs of limbs ; they are all members of the great group Chordata. Altogether twelve phyla are to be recognised, viz. : — I. II. III. IV. V. VI. Protozoa VII. Molluscoida Porifera- VIII. Echinodermata Cmlenterata IX. Annulata Platyhelmin thes X. Arthropoda Nemathehninthes XL Mollusca 2 rochelminthes XII. Chordata SECTION IIL — PHYLUM PORIFERA The Porifera, or sponges, belong to the lowest group of the Metazoa. They live fixed to the surface of rocks, or to submerged timber or seaweeds, so as to be incapable of locomotion ; and have, in most cases, a general form which suggests the vegetable rather than the animal kingdom. But, in essentials, as will presently become evident, the sponges are distinctly animal in character, and the resemblances to plants are entirely superficial. The majority of sponges are compli- cated and difficult to understand, owing to their elaborate mode of branching and the fusion of the branches, and to the exceed- ingly intricate character of the skeletal parts, aside from their cellular structure. Some, however, are free from these com- plications; and it is in one of these that (After Hyatt.) the main characteristics of sponges are best studied. Such a simple form is Sycon, a small sponge living attached to rocks on the seashore towards or below low- water mark. Sycon gelatinosum ^ has the form of a tuft, one to three inches long, of branching 1 This is an Australasian species, but the following account will apply in all essential respects to Sycon ciliatum (Fig. 33) and S. clarkii of the coast of New England. 76 SECT. Ill PHYLUM PORIFERA 77 cylinders (Fig. 34), all connected together at the base, where it is attached to the surface of seaweeds, rocks, or other solid bodies submerged in the sea. It is flexible, though of tolerably firm consistency. On the outer surface are to be detected, under the microscope, groups of minute pores — the inhalant pores . At the free end of each of the cylindrical branches is a small but distinct opening, sur- rounded by what appears like a delicate fringe. When the branches are bisected longitudinally (Fig. 35), it is found that the terminal openings (o) lead into narrow passages, Fig. 34. — Sycon gelatinosum . Entire sponge, consisting of a group of branching cylinders (natural size). wide enough to admit a stout pin, running through the axis of the cylinders ; and the passages in the interior of the various branches join where the branches join — the pas- sages thus forming a communicating system. On the wall of the passages are numerous fine apertures which require a strong lens for their detection. The larger apertures at the ends of the branches are the oscula of the sponge, the pas- sages the paragastric cavities. If the living Sycon is placed in sea-water with which has been mixed some carmine pow- der, it will be noticed that the minute particles of the carmine seem to be attracted towards the surface of the 78 ^lANUAL OF ZOOLOGY SECT. sponge, and will often be seen to pass into its substance through the minute pores already mentioned as occurring in groups between the elevations on the outer surface. This would appear to be due to the passage of a current of water Fig. 35. — Sycon gelatinosum. _ A portion slightly magnified; one cylinder (that to the right) bisected longitudinally to show the central paragastric cavity opening on the exterior by the osculum, and the position of the incurrent and radial canals; the former indicated by the black bands, the latter dotted, ip marks the position of three of the groups of inhalant pores at the outer ends of the incurrent canals; o, osculum. into the interior of the sponge through these minute open- ings dotted over the surface ; and the movement of the floating particles shows that a current is at the same time flowing out of each of the oscula. A constant circulation of in PHYLUM PORIFERA 79 water would thus appear to be carried on — currents moved by some invisible agency flowing through the walls of the sponge to the central paragastric cavities, and passing out again by the oscula. If a portion of the Sycon is firmly squeezed, there will be pressed out from it first sea-water, then, when greater pressure is exerted, a quantity of gelatinous-looking matter, which, on being examined microscopically, proves to be partly composed of a protoplasmic material consisting of innumerable, usually more or less broken, cells with their nuclei, and partly of a non-protoplasmic, jelly-like substance. When this is all removed there remains behind a toughish, felt-like material, which maintains more or less completely the original shape of the sponge. This is the skeleton or supporting framework. A drop of acid causes it to dissolve with effervescence, showing that it consists of carbonate, of lime. When some of it is teased out and examined under the microscope, it proves to consist of innumerable, slender, mostly three-rayed microscopic bodies (Fig. 36, sp') of a clear, glassy appearance. These are the calcareous spicules which form the skeleton of the Sycon. Covering the outer surface of the sponge is a single layer of flattened, scale-like cells — the ectoderm (Fig. 36, ec^ — through which project regularly arranged groups of needle- like and spear-like spicules {sp''). The paragastric cavities are lined by a layer of cells {en), which are like those of the ectoderm in general shape ; this is the endoderm of the paragastric cavity. Running radially through the thick wall of the cylinders are a large number of regularly arranged straight passages. Of these there are two sets, those of the one set — the incurrent canals {IC) — nar- rower, and lined by ectoderm similar to the ectoderm of the surface ; those of the other set — the radial 01 Fig. 36. — Sycon gelatinosum. Transverse section through the wall of a cylinder (parallel with the course of the canals), showing one incurrent canal (,/C), and one radial (I?) throughout their length; sp, triradiate spicules; j/', oxeotc spicules of dermal cortex (dc) ; s/>" , tetraradiate spicules of gastral cortex (g'cj; ectoderm; endoderm ; /;«, pore membrane; prosopyles; a/, apopyle ; d/, diaphragm; ejcc, excurrent passage; PG, paragastric cavity; em, early embryo; em' , late embryo. The arrows indicate the course of the water through the sponge. 80 SECT. Ill PHYLUM PORIFERA 8i flagellate canals (Jl) — rather wider, octagonal in cross- section, and lined by endoderm continuous with the lining of the paragastric cavity. The incurrent canals end blindly at their inner extremities, not reaching the paragastric cavity ; externally each becomes somewhat dilated, and the dilations of neighbouring canals often communicate. These dilated parts are closed externally by a thin membrane — the pore membrane, perforated by three or four openings — the inhalant pores already referred to. The flagellate canals are blind at their outer ends, which lie at a little distance below the surface ; internally, each communicates with the paragastric cavity by a short, wide passage, the excurrent canal (exc). Incurrent and flagellate canals run side by side, separated by a thin layer of sponge substance, except at certain points, where there exist small apertures of com- munication — ■ the prosopyles ( // ) — uniting the cavities of adjacent incurrent flagellate canals. The ectoderm lining of the incurrent canals is of the same character as the ectoderm of the outer surface. The endoderm (i?) of the flagellate canals, on the other hand, is totally different from that which lines the paragastric cavity. It consists of cells of columnar shape, ranged closely together so as to form a continuous layer. Each of these flagellate endoderm cells, or collared cells, as they are termed, is not unlike one of the choanoflagellate Protozoa (p. 38) ; it has its nucleus, one or more vacuoles, and, at the inner end, a single, long, whip-like flagellum, surrounded at its base by a delicate, transparent, collar-like upgrowth, similar to that which has already been described as occurring in the Choanoflagellata. If a portion of a living specimen of the sponge is teased out in sea-water, and the broken fragments examined under a tolerably high power of the microscope, groups of these collared cells will be detected here and S2 MANUAL OF ZOOLOGY SECl. there, an.d in many places the movements of the flagella will be readily observed. It is to these movements that the formation of the currents of water passing along the canals is due. The short passage or excurrent canal, which leads inwards from the flagellate canal to the paragastric cavity, differs from the former in being lined by flattened cells similar to those of the paragastric cavity ; it is partly separated from the flagellate canal by a thin diaphragm (Fig. 36, di), perforated by a large circular central aperture — the apopyle {ap) — which is capable of being contracted or dilated ; its opposite aperture of communication with the paragastric cavity, which is very wide, is termed the gastric ostium of the excurrent canal. The effect of the movement of the flagella of the cells in the flagellate canals is to produce currents of water running from without inwards along the canals to the paragastric cavity. This causes water to be drawn inwards through the prosopyles from the incurrent canals, and, indirectly, from the exterior through the perforated membranes at the outer ends of the latter. Between the ectoderm of the outer surface and of the incurrent canals, and the endoderm of the inner surface and of the flagellate canals, are a number of spaces filled by an intermediate layer — the mesoderm or mesoglcea — in which the spicules of the skeleton are embedded. The spicules (Fig. 36, spP), each of which is developed in a single cell of the middle layer, are regularly arranged, and connected together in such a way as to protect and support the soft parts of the sponge. Most are, as already noticed, of triradiate form. Large numbers, however, are of simple spear-like or club-like shape {sp'). The sexual reproductive cells — the ova (Fig. 36, ov') and sperms — are developed Ill PHYLUM PORIFERA 83 immediately below the flagellate endoderm cells of the flagellate canals, and in the same situation are to be found developing embryos {em, em'). The simplest sponges are vase-shaped or cylindrical in form, either branched or unbranched, and, if branched, with or without anastomosis or coalescence between neigh- bouring branches. But the general form of the less sim- ple sponges differs widely from that of such a branching cylinder as is presented by Sycon (Fig. 34). From the point to which the embryonic sponge becomes attached, it may spread out horizontally, following the ir- regularities of the surface on which it grows, and forming a more or less closely adherent encrustation like that of an encrusting lichen. In other cases the sponge grows at first more actively in the vertical than in the horizontal direction, and the result may be a long, narrow structure, cylindrical or compressed, and more or less branched. Sometimes vertical and horizontal growth is almost equal, so that event- ually there is formed a thick, solid mass of a rounded or polyhedral shape, with an even, or lobed, or ridged surface. Very often, after active vertical growth has resulted in the formation of a comparatively narrow basal part or stalk, the sponge expands distally, growing out into lobes or branches of varying forms, and frequently anastomosing. Sometimes after the formation of the stalk with root-like processes for attachment, the sponge grows upwards in such a way as to form a cup or tube with a terminal opening. Sometimes the sponge grows from a narrow base of attach- ment into a thin flat plate or lamella ; this may become divided up into a number of parts or lobes, which may exhibit a divergent arrangement like the ribs of an open fan. 84 MANUAL OF ZOOLOGY SECT. Sycon belongs to a type of sponges intermediate between the very simplest forms on the one hand, and the more complex on the other. The simplest and most primitive of known sponges is one named Ascetta primordialis (Fig. 37). It is vase-shaped, contracted at the base to form a sort of stalk, by the expanded extremity of which it is attached; at the oppo- site or free end is the circular osculum. So far there is a considerable resemblance to Sycon gelatinosum ; but the structure of the wall in Ascetta is extremely simple. Regu- larly arranged over the sur- face are a number of small rounded apertures, the in- halant or incurrent pores ; but, since the wall of the sponge is very thin, these apertures lead directly into the central or paragastric cavity, the long passages or canals through which the communication is effected in Sycon being absent. The wall consists of the same three layers as in Sycon ; but the middle one, though it contains a small number of spicules, is very thin ; the endoderm, which lines the Fig. 37 Ascetta primordialis. A portion of the wall of the vase-like sponge removed to show the para- gastric cavity. (After Haeckel.) Ill PHYLUM PORIFERA 85 paragastric cavity, consists throughout of flagellate collared cells similar to those of the flagellate canals of Sycon. The majority of sponges, however, are more complicated in structure than Sycon. One of the causes of their complexity being that the canals, instead of being simple and straight, become branched, forming a system, often highly complicated, of ramifying channels. In these more complex sponges the flagellate collared cells are confined to Ex Fig. 38. — Vertical section of a fresh-water sponge (Spongilla) , showing the arrange- ment of the canal-system. C, ciliated chambers; DP, dermal pores: Ex, excurrent canals; GO, openings of the excurrent canals; PG, paragastric cavity; SD, subdermal cavities; O, osculum. (Modified from Leuckart and Nitsche’s diagrams.) certain rounded dilatations of the canals — the flagellate chambers. Moreover, in the more complex forms the development of branches from the originally simple sponge, and the coalescence of neighbouring branches with one another, greatly obscure the essential nature of the sponge as a colony of zooids similar to the branches of Sycon ; and this effect is increased by the development of a variety of infoldings of the ectoderm which appear in the higher forms. 86 MANUAL OF ZOOLOGY SECT. The elements of the skeleton differ in character in the two sub-classes into which the sponges are divided. In the Calcarea, of which Sycon is an example, they consist of calcareous spicules, usually triradiate in form. In the Non-Calcarea the skeleton either consists of spongin fibres alone (Fig. 39, A), or of siliceous spicules alone, or of a combination of spongin fibres with siliceous spicules {B) : in some (Myxospongise) skeletal parts are altogether absent. Spongin is a substance allied to silk in compo- sition ; the fibres are exceedingly fine threads, which branch and anastomose, or are woven and felted together in such a way as to form a firm, elastic supporting structure.- The siliceous spicules (Fig. 40) are much more varied in shape than the spicules of the Calcarea, and in a single kind of sponge there may be a number of widely differing forms of spicules, each form having its special place in the skeleton of the various parts of the sponge-body. In most Non- Calcarea siliceous spicules and spongin fibres combine to form the supporting framework, the relative development of these two elements varying greatly in different cases. But in certain groups of the Non-Calcarea, including the common washing sponges, spicules are completely absent, and the entire skeleton consists of spongin. In some Non-Calcarea which are devoid of spicules, the place of these is taken by foreign bodies — shells of Radiolaria, grains of sand, or spicules from other sponges (Fig. 39, C). In others, again, such as the Venus’s flower-basket {Euplectellci), the glass- rope sponge (Hyalonema), and others, the skeleton consists throughout of siliceous spicules bound together by a siliceous cement. Reproduction in the Sponges is effected either sexually or asexually. The process by which, in all but the simplest forms of sponges, a colony of zooids is formed from the Ill PHYLUM PORIFERA 87 originally simple cylinder or vase, may be looked upon as an asexual mode of reproduction by budding. Asexual Fig. 39. — Microscopic structure of the skeleton in various sponges. A, Euspongla, network of spongin fibres; B, Pachychalina, spongin strengthened by siliceous spicules; C, Spongelia, spongin strengthened by various foreign siliceous bodies, fragments of spicules of other sponges, etc. (After Vosmaer.) 88 MANUAL OF ZOOLOGY SECT. multiplication also assumes the form in some cases of a process of production of internal buds in the shape of groups of cells called ge7nmules, which eventually become detached and develop into new individuals. In the fresh- water sponges {Spongillidce) multiplication takes place very actively by means of such gemmules, each of which is a spherical group of cells enclosed in an envelope composed of peculiarly shaped siliceous spicules, termed amphidiscs (Fig. 40, right side). All sponges multiply by a sexual process — by means of male cells, or sperms, and female cells, or ova. Ova and sperms are developed in the same sponge, but rarely at the same time. The cell destined to form sperms divides into a number of small cells, giving rise to a rounded mass of sperms. The latter, when mature, have oval or pear-shaped heads and a long taper- ing appendage or tail. Each cell destined to form an ovum enlarges, and eventually assumes a spherical form. After a sperm has penetrated into its interior and effected impreg- nation, it usually becomes enclosed in a brood-capsule formed for it by certain neighbouring cells, and in this situation, still enclosed in the parent sponge, it undergoes Ill PHYLUM PORIFERA 89 the earlier stages of its development. Eventually it becomes free as a ciliated larva, which pursues a free existence for a time, swimming about by the agency of the cilia, till after a time it becomes fixed and develops into the adult form. Fresh-water sponges {Spongillida) live in rivers, lakes, etc. Marine sponges occur in all seas, and at all depths, from the shore between tide marks to the deepest abysses of the ocean. Sponges do not appear to be edible by fishes, or even the higher crustaceans or molluscs. Countless lower animal forms, however, burrow in their substance, if not for food, at least for shelter, and the interior of a sponge is frequently found to be teeming with small crustaceans, annelids, mol- luscs, and other invertebrates. None of the sponges are true parasites. The little boring sponge, Cliona, burrows in the shells of oysters and other bivalves, and even into solid limestone, but for protection and not for food. But the sponge frequently lives in that close association with another animal or plant to which the term messmateism, or commensalism is applied — associations which benefit one or both. Thus some species of sponge are never found grow- ing except on the backs or legs of certain crabs. In these cases the sponge protects the crab and conceals it from its enemies, while the sponge benefits by being carried from place to place, and thus obtaining freer oxygenation. Cer- tain cirripede crustaceans (members of the order to which the barnacles and acorn-shells belong) are invariably found embedded in certain species of sponge. Frequently a sponge and a zoophyte grow in intimate association, so that they seem almost to form one structure. Thus the glass-rope sponge {Hyalonema) is always found associated with a zoophyte {Pafyfhoa) , and there are many other in- stances. Sponges often also grow in very close association with certain low forms of plants (A/gcs). SECTION IV. — PHYLUM CGELENTERATA In the previous section we saw that the simplest type of sponge has the general character of a cylinder, closed at one end and open at the other, and having the walls perforated by minute pores, and composed of three layers, — ectoderm, mesogloea, and endoderm, the last consisting of collared flagellate cells. In such an organism as this, imagine the pores to disap- pear, the internal cavity thus coming to communicate with the exterior by a single terminal aperture ; the mesogloea to be replaced by a very thin, structureless layer containing no cells ; the endoderm cells to lose their collars ; and a circlet of arm-like processes, or tentacles, formed of the same layers as the body-wall, to be developed round the terminal aperture. The result would be a polype, and would serve as a type of the general structure of the group of animals with which we are now concerned. The most familiar examples of Coelenterata are the horny, seaweed-like hydroids, or, as they are sometimes called, “zoophytes,” to be picked up on every sea-beach, jelly- fishes, sea anemones, and corals. The phylum is divided into four classes as follows : — Class I. — Hydrozoa, including the fresh- water polypes, zoophytes, many jellyfishes, — mostly of small size, — and a few stony corals. 90 SECT. IV PHYLUM CGELENTERATA 91 Class 2. — Scyphozoa, including most of the large jelly- fishes. Class j. — Actinozoa, including the sea-anemones, and the vast majority of stony corals. Class 4. — Ctenophora, including certain peculiar jelly- fishes known as “comb-jellies.” 1. THE HYDROZOA Obelia, which is a good example of the class, is a common zoophyte occurring in the form of a delicate, whitish, or light brov/n, almost fur-like growth on the wooden piles of piers and wharfs. Obelia coni77iissuralis occurs on the coast of New England almost at low- water mark, being exposed only at the lowest tides. With it, north of Cape Cod, may be found Obelia gelatmosa, a rather stouter species, but similar in general appearance. Obelia geniculata is abundant on Laminaria or the “ devil’s apron,” giving the fronds when submerged a downy appearance. The following account refers to a common European species : It consists of branched filaments about the thickness of fine sewing cotton; of these, some are closely adherent to the timber, and serve for attachment, while others are given off at right angles, and present at intervals short lateral branches, each terminating in a bud-like enlargement. The structure is best seen under a low power of the microscope. The organism (Fig. 41) is a colony, consisting of a common stem or axis, on which are borne numerous zooids. The large majority of the zooids have the form of little conical structures (/*, i-P, 4), each enclosed in a glassy, cup-like investment or hydrotheca {htli), and produced dis- tally into about two dozen arms or tentacles {t) : these zooids are the polypes or hydranths. Less numerous, and 92 MANUAL OF ZOOLOGY SECT. IV found chiefly towards the proximal region of the colony, are long cylindrical bodies or blastostyles {bis), each enclosed' in a transparent case, the gonangium or gonotheca (gih), and bearing numerous small lateral offshoots, varying greatly in form according to their stage of development, and known as medusa-buds {ni.bd). By studying the development of these structures, and by a comparison with other forms, it is known that both blastostyles and medusa-buds are zooids, so that the colony is trbnorpJiic, having zooids of three kinds. To make out the structure in greater detail, living speci- mens should be observed under a high power. A polype is then seen to consist of a somewhat cylindrical, hollow body, of a yellowish colour, joined to the common stem by its proximal end and produced at its distal end into a conical elevation, the 7nanubnu7ti or hypostome {mnb), around the base of which are arranged the twenty-four tentacles in a circle. Both body and manubrium are hol- low, containing a spacious cavity, the enteron {ent), which communicates with the outer world by a mouth {mth), an aperture placed at the summit of the manubrium. The mouth is capable of great dilatation and contraction, and accordingly the manubrium appears now conical, now trumpet-shaped. Under favourable circumstances small organisms may be seen to be caught by the tentacles and carried towards the mouth to be swallowed. The hydro- theca {hth) has the form of a vase or wine-glass, and is perfectly transparent and colourless. When irritated — by a touch, or by the addition of alcohol or other poison — the polype undergoes a very marked contraction : it suddenly withdraws itself more or less completely into the theca, and the tentacles become greatly shortened and curved over the manubrium {P. 2) . Fig. 41. — Obelia sp. A, portion of a colony, with certain parts shown in longi- tudinal section; B, medusa; C, tha same, with reversed umbrella; D, the same, oral aspect; Bd. 1, 3, buds; bis, blastostyle ; cce, coenosarc ; ect, ectoderm; end, endoderm \ ent, enteric cavity: gth, gonotheca (gonangium) ; hth, hydro- theca; I, lithocyst ; m.bd, medusa-bud; mnb, manubrium; msgl, mesogloea; mth, mouth; p, perisarc; P, i, 2, 3, 4, polypes; rad.c, radial canal; t, tentacle ; vl, velum. 93 94 MANUAL OF ZOOLOGY SECT. The various branches of the common stem show a very obvious distinction into two layers : a transparent, tough, outer membrane, of a yellowish colour and horny con- sistency, the perisarc (p), and an inner, delicate, granular layer, the ccenosarc {coe), continuous by a sort of neck or constriction with the body of each hydranth. The coenosarc is hollow, its tubular cavity being continuous with the -icavities of the polypes, and containing a fluid in which a flickering movement may be observed, due to the presence of vibrating cilia. In the blastostyle both mouth and tenta- cles are absent, the zooid ending distally in a flattened disc ; the hydrotheca of the polype is represented by the gono- theca {gth), which is a cylindrical capsule enclosing the whole structure, but ultimately becoming ruptured at its distal end to allow of the escape of the medusa-buds. These latter are, in the young condition, mere hollow off- shoots of the blastostyle : when fully developed they have the appearance of saucers attached by the middle of the convex surface to the blastostyle, produced at the edge into sixteen very short tentacles, and having a blunt process, the manubrium, projecting from the centre of the concave sur- face. They are ultimately set free through the aperture in the gonotheca as little medusae or jellyfish (B-D), which will be described hereafter. The microscopical structure of Obelia reminds us, in its general features, of that of such a simple sponge as Ascetta, but with many characteristic differences. The body is composed of two layers of cells, the ectoderm and the endoderm, the latter ciliated ; between them is a very delicate transparent membrane, the mesoglcea or supporting lamella, which, unlike the intermediate layer of sponges, contains no cells and is practically structureless. The perisarc or transparent outer layer of the stem shows ’■V PHYLUM CCELENTERATA 95 AO cell-structure, but only a delicate lamination. It is, in fact, not a cellular membrane or epithelium, like the ecto- derm and endoderm, but a cuticle, formed, layer by layer, as a secretion from the ectoderm cells (see p. 67). It is of chitinoid or horn-like consistency, and, like the lorica of many Protozoa, serves as a protective external skeleton. Embedded in the ectoderm are numerous clear, ovoid bodies, the stinging- capsules or nematocysts (Fig. 42), serving as weapons of offence. Each consists (A) of a tough, ovoid capsule, full of a gelatinous material, and invaginated at one end in the form of a hollow process continued into a long, coiled, hollow thread. The whole apparatus is developed in an interstitial cell called a cnidoblast {cnb), which, as it ap- proaches maturity, migrates towards the surface, and becomes embedded in one of the large ectoderm cells. At one point of its surface the cnidoblast is produced into a delicate pro- toplasmic process, the cnidocil or trigger-hair {cnc) : when this is touched — for instance by some small organism brought into contact with the waving tentacles — the cnidoblast un- dergoes a sudden contraction, and the pressure upon the stinging-capsule causes an instantaneous eversion of the thread (B), at the base of which are minute barbs. The threads or the gelatinous substance are poisonous and exert a numbing effect on the animals upon which the Obelia preys. The structure of the Medusae — formed as we have seen by the development of medusa-buds liberated from a ruptured gonangium — yet rem.ains to be considered. The convex surface of the bell or umbrella (Fig. 41, B-D) by which the zooid was originally attached to the blastost'yle, is distinguished as the ex-umbrella, the concave inner surface as the sub-umbrella. From the centre of the sub-umbrella proceeds the manubrium {mnb), at the free end of which is 96 MANUAL OF ZOOLOGY SECT. the four-sided mouth {intK). Very commonly as the medusa swims the umbrella becomes turned inside out, the sub- Fig. 42. — Nematocysts of hydra. A, undischarged; B, discharged; C, nerve- supply; cnb, cnidoblast; cnc, cnidocil; nu^ nucleus; ntc, nematocyst; 7iv.c, nerve-cell. (From Parker’s Biology, after Schneider.) umbrella then forming the convex surface, and the manu- brium springing from its apex (Fig. 41, C}. tv PHYLUM CCELENTERATA 9'; The mouth (Figs. 41, C, D, and 43, 7nth) leads into an enteric cavity which occupies the whole interior of the manubrium, and from its dilated base sends off four delicate tubes, the i-adial canals {I'ad. c'), which pass at equal distances from each other through the substance of the umbrella to its margin, where they all open into a circular canal {cir. c), running parallel with and close to the margin. By means of this system of canals the food, taken Fig. 43 — Dissection of a medusa with rather more than one-quarter of the umbrella and manubrium cut away (diagrammatic). The ectoderm is dotted, the endo- derm striated, and the mesogloea black, cir. c, circular canal; end. lam, endoderm lamella; gon, gonad; I, lithocyst; mnb, manubrium; mth, mouth; rad. c, radial canal; vl, velum. in at the mouth and digested in the manubrium, is dis- tributed to the entire medusa. The edge of the umbrella is produced into a very narrow fold or shelf, the velum (Fig. 43, vt), and gives off the tentacles (/), which are sixteen in number in the newly-born medusa (Fig. 4r, D'), but which are very numerous in the adult. At the bases of eight of the tentacles — two in each quadrant — are minute globular sacs (/), each containing a end lam I H 98 MANUAL OF ZOOLOGY SECT. calcareous particle or lithite. These are the marginal sense- organs or lithocysts : they were formerly considered to be organs of hearing, and are hence frequently called otocysts : in all probability their function is to guide the medusa by enabling it to judge of the direction in which it is swim- ming. The marginal organs, in this case, may therefore be looked upon as organs of the sense of direction. In the description of the fixed Obelia-colony no mention was made of cells set apart for reproduction, like the ova and sperms of a sponge. As a matter of fact, such sexual cells are only found, in their fully developed condition at least, in the medusae. Hanging at equal distances from the sub- umbrella, in immediate relation with the radial canal, are four ovoid bodies (Fig. 43, each containing a mass of cells which are developed either into ova or into sperms. As each medusa bears organs of one sex only (testes or ovaries as the case may be), the individual medusae are dioecious. When the gonads are ripe, the sperms of the male medusae are shed into the water and carried by currents to the females, impregnating the ova, which thus become oosperms or unicellular embryos. The oosperm undergoes complete segmentation (Fig. 44, A-F), and is converted into an ovoidal ciliated body called a planula ( G, H) . The planula swims freely for a time (AT), and then settles down on a piece of timber, seaweed, etc., fixes itself by one end (.^), and becomes converted into a hydrula or simple polype (Z, M), having a disc of attachment at its proximal end, and at its distal end a manubrium and circlet of tentacles. Soon the hydrula sends out lateral buds, and, by a frequent repetition of this process, becomes converted into the com- plex Obelia-colony with which we started. This remarkable life- history furnishes the first example we> have yet met with of alternation of generations, or metagenesis. IV PHYLUM CCELENTERATA 99 The Obelia-colony is sexless, having no gonads, and develop- ing only by the asexual process of budding j but certain of its buds — the medusae — develop gonads, and from their impregnated eggs new Obelia-colonies arise. We thus have an alternation of an asexual generation — the Obelia-colony — with a sexual generation, the medusa. Fig. 44. — Stages in the development of two zoophytes (A-H, Laomedea, I-M, Eudendrium) allied to Obelia; A-F, stages in segmentation; G, the planula enclosed in the maternal tissues; H, the free-swimming planula; I-M, fixation of the planula and development of the hydrula. (From Parker’s Biology, after Allman.) The majority of the Hydrozoa resemble Obelia in form- ing fixed colonies ; but there are a few exceptional cases, in which the animal remains simple. One of these is Hydra, the Fresh-water Polype. In Hydra the entire organism (Fig. 45) consists of a simple cylindrical body with a conical hypos- Fig. 45.- — Hydra. A vertical section of entire animal; B, portion of transverse section, highly magnified ; C, two large ectoderm cells : D, endoderm cell of H. viridis ; E, large nematocyst; F, small nematocyst; G, sperm; a, ingested diatom; bdd, bd?‘,\>\xA's,\ chr, chromatophores; cnidoblast; cwc, cnidocil; ect, ectoderm; end, endoderm; ent. cav, enteric cavity; ent. ca7'\ its prolonga- tion into the tentacles; Jl, flagellum; hyp, hypostome or manubrium: int c, in- terstitial cells; m.pr, muscle processes; mth, mouth; mesogloea ; net, large, and small nematocysts; nucleus; t>», ovum; tizy/, ovary; psd, pseudopods; spermary ; z'air, vacuole. lOO SECT. IV PHYLUM CCELENTERATA loi tome and a circlet of from six to eight tentacles surrounding the mouth. It is ordinarily attached, by virtue of a sticky secretion from the proximal end, to weeds, etc., but is capable of detaching itself and moving from place to place after the manner of a looping caterpillar. The tentacles are hollow, and communicate freely with the enteron. There is no perisarc. Buds are produced which develop into Hydras ; but these are always detached sooner or later, so that a permanent colony is never formed. There are no special reproductive zooids, but simple ovaries (czry) and testes are developed, the former nearer the proximal, the latter nearer the distal end of the body. In nearly all the remaining Hydrozoa that do not form colonies the form assumed is not that of the polype^ but that of the medusa (Fig. 46), a polype stage never being developed, and the animal resembling in all essential respects the medusae of Obelia ; the chief difference of importance being the presence of sense-organs in the form of hollow, club-shaped appendages, the tentaculo cysts, con- taining calcareous bodies of lithites. These simple free- swimming medusiform Hydrozoa {Trachylmce) develop ova and sperms which give rise to free-swimming ciliated larvae ; but the latter, instead of becoming fixed and developing into plant-like colonies, remain free, and develop directly into medusae like those from which they originated. The fixed zoophyte stage is thus absent in the life-history, and an alternation of generations is not recognisable. In the colonial Hydrozoa, which constitute the great majority of the class, the colony in most instances resem- bles that of Obelia in being a fixed structure consisting of a slender branching stem, covered over by perisarc, and bear- ing zooids and blastostyles. In many the perisarc is produced to form hydro thecae and gonothecae for the 102 MANUAL OF ZOOLOGY SECT. protection of the polypes and blastostyles respectively ; but in others (Fig. 47) these protecting structures are absent. The polypes resemble those of Obelia in all es- sential respects, but differ in the number and arrangement of the tentacles and other minor points. In many medusae are developed from blastostyles as in Obelia, and when fully formed become free. The shape of the medusa TV PHYLUM CCELENTERATA 103 differs in different forms, more particularly as regards the umbrella. There is always a manubrium, with gastric t Fig. 47. — Bougainvillea ramosa. A, entire colony, natural size; B, portion of the same magnified ; C, immature medusa ; cir. c, circular canal ; cu, cuticle or perisarc ; ent. cav, enteric cavity ; hyd, polype or hydranth ; hyp, hypostome or manubrium ; med, medusa; mtib, manubrium ; rad. c, radial canal ; t, tentacle ; V, velum. (From Parker’s Biology, after Allman.) This is closely allied to the New England B. supercilians. 104 MANUAL OF ZOOLOGY sect. cavity, and a marginal and four radial canals, and a velum is universally present. But lithocysts are not present in all, their place being taken by specks of red or black pigment — the ocelli or rudimentary eyes — at the bases of the ten- tacles. The number and arrangement of the tentacles is subject to considerable variation. The gonads are some- times, as in Obelia, developed in the radial canals, some- times in the manubrium. In size the medusae range from about I up to 400 millimetres (16 inches) in diameter. In many of the zoophytes, however, the medusae never become detached from the colony, developing the ova and sperms without becoming free. In such cases the charac- teristic medusa structure is more or less imperfectly de- veloped, and in many forms is not at all recognisable, the buds corresponding to those which in Obelia give rise to medusae merely developing into rounded outgrowths termed sporosacs, in the interior of which the ova and sperms are formed. The reproductive buds are not in all cases formed, as in Obelia, on distinct, peculiarly modified, mouthless zooids. In many instances, whether they are destined to give rise to medusae or sporosacs, the buds spring directly from the coenosarc, or from the ordinary zooids. A small group of Hydrozoa — the Hydrocorallina — in- cluding the Millepores {^Millepord) and Stylaster, form colonies, the supporting material of which, instead of being chitinoid, is of calcareous and stony character, like the substance of a coral. The colonies of Hydrozoa are not in all instances at- tached, like those of Obelia and the other hydroid zoo- phytes. In one large order, the Siphonophora, the colonies of zooids float or swim freely in the sea. In some Siphono- phora there are no organs for active locomotion, and the IV PHYLUM CCELENTERATA 105 colony drifts about, completely at the mercy of wind and tide, buoyed up by a bladder-like float or pneumatophore containing air. Such a passively floating form is the Portuguese Man-of-war {Fhysalia) (Figs. 48, 49) which has an elongated float, pointed at the ends, and produced above, along its upper edge, into a crest or sail {cr.). At one end is a minute aperture communicating with the exterior. From the under side of the float hang polypes (/), feelers, groups of medusa-buds looking like bunches of grapes of a deep blue colour, and long retractile tentacles, sometimes several feet in length, and containing batteries of stinging- capsules powerful enough to sting the hand as severely as a nettle. The male reproductive buds remain attached and take the form of sporosacs, while the female buds apparently become detached as free medusse. Physalia arethusa is common in the West Indies, and, borne northward by the Gulf Stream, is occasionally met with on the coast of southern New England, and off Nova Scotia. In such a Siphonophoran as Halistemma (Fig. 50), on the other hand, there is a long, slender, flexible stem or coenosarc, at the upper end of which is a comparatively small float. Next to this come a number of closely set, transparent structures (yici), having the general characters of unsymmetrical medusae without manubria, each being a deep, bell-like body, with a velum and radiating canals. During life these stvimming-bells or nectocalyces contract rhythmically, — i.e., at regular intervals, — thus serving to propel the entire organism through the water. Below the last nectocalyx the character of the structures borne by the stem changes completely : they are of several kinds, and are arranged in groups which follow one another at regular intervals. Some of these are unmistakable polypes (/) diflering, io6 MANUAL OF ZOOLOGY Fig. 48. — Physalia. The living animal floating on the surface of the sea. cr, crest ; p, polype ; pneumatophore, float, or air-sac. (After Huxley.) Fig. 49. — Physalia arethusa, natural size. (After Agassiz.) PHYLUM CCELENTERATA 107 . 50.— Halistemma tergestinum. A, the entire colony; B, a single group of zooids; cce, ccenosarc; dz, dactylozooid; hph, hydrophyllium or bract; net, nectocalyx or swimming-bell; ntc. battery of nematocysts; p. polype; pn, pneumatophore or float ; sporocysts; ^.tentacle. (After Claus.) io8 MANUAL OF ZOOLOGY SECT. however, from those we have hitherto met with, in having no circlet of tentacles round the mouth, but a single, long, branched tentacle (/) arising from its proximal end, and bearing numerous groups or “ batteries ” of stinging- capsules {ntc). Others are dactylozooids or feelers {^dz') — mouth- less polypes, each with an unbranched tentacle springing from its base. Near the bases of the polypes and dactylo- zooids spring groups of sporosacs (B, s, s'), some male, others female ; and finally delicate, leaf-like transparent bodies — the bracts or hydrophyllia {hph) — partly cover the sporosacs. Halistemma occurs in the Atlantic and Mediter- ranean. A closely related form {AgaMopsis cara) occurs off the coast of New England. 2. THE SCYPHOZOA Aurelia, which may be taken as an example of the Scyphozoa, is the most common of our larger jellyfishes, and is often found cast up on the sea-shore, where it is readily recognisable by its gelatinous saucer-shaped umbrella, from eight to twelve, and sometimes fifteen inches in diam- eter, having near the centre four red or purple horseshoe- shaped bodies — the gonads — lying embedded in the jelly. The general arrangement of the parts of the body (Fig. 51) is very similar to what we are already familiar with in the hydrozoan jellyfishes (Figs. 41 and 43). Most con- spicuous is the concavo-convex umbrella, the convex sur- face of which, or ex-umbrella, is uppermost in the ordinary swimming position. The outline is approximately circular, but is broken by eight notches, in each of which lies a pair of delicate processes, the marginal lappets {mg. Ip) with a peculiar sense-organ ; between the pairs of lappets the edge Fig. 51. — Aurelia aurita. Ventral view — two of the oral arms are removed; a.r.c, radial canal; gon, gonads; i.r.c, radial canal; mg. Ip, marginal lappet; mth, mouth; or. a, oral arm; p.r.c, radial canal; s.g.p, sub-genital pit; t, tentacles. IV PHYLUM CCELENTERATA 109 of the umbrella is fringed by numerous close-set marginal tentacles (/). In the centre of the lower or sub-umbrella surface is a four-sided aperture, the mouth {mth), borne at the end of an extremely short and inconspicuous manubrium : sur- rounding it are four long delicate processes, the oral arms no MANUAL OF ZOOLOGY SECT, {or. a), situated one at each angle of the mouth and uniting round it. At a short distance from each of the straight sides of the mouth is a nearly circular aperture leading into a shallow- pouch, the sub-genital pit {s. g. p), which lies immediately beneath one of the conspicuously coloured gonads (gon). The mouth leads by a short tube or gullet, contained in the manubrium, into a spacious stomach, which is produced into four wide inter-radial gastric pouches, which extend about halfway from the centre to the circumference. In the outer or peripheral wall of each gastric pouch are three small apertures, leading into as many radial canals {a.r£, i.r.c, p.r.c), which pass to the edge of the umbrella and then unite in a very narrow circular canal. Each gonad {goni) is a horseshoe-shaped frill-like structure situated on the floor of the gastric pouch. When mature, its products — ova or sperms — are discharged into the stomach, and pass out by the mouth. The sexes are lodged in distinct individuals. Lying parallel with the inner or concave border of each gonad is a row of delicate filaments supplied with stinging- capsules. These are the gastric filaments : their function is to kill or paralyse the prey taken alive into the stomach (compare Fig. 53,^./). The development and life-histor)' of Aurelia present several striking and characteristic features. The impreg- nated egg-cell or oosperm becomes converted into a closed two-layered sac or planula (Fig. 52, A), similar to that of a Hydrozoon. The planula swims about by means of the cilia with which its ectodermal cells are provided, and, after a brief free existence, settles down, loses' its cilia, and be- comes attached by one pole. At the opposite pole a mouth is formed. On two opposite sides of the mouth hollow Fig. 52. — Aurelia aurita, development. A, planula; B, C, formation of the gullet or stomodseum; D, transverse section of young scyphula; E, scyphula; F, longitudinal section of same; G, division of scyplmla into ephyrulae; H, ephy- rula from the side; I. the same from beneath. In A-D and F the ectoderm is unshaded, the endoderm striated, and the mesogloea dotted, a, lobes of umbrella; manubrium; mouth; jy, septal funnel ; j/, stomodseum; t, tentacle; tn, taenioles, or gastric ridges. (From Korschelt and Heider’s Embryology) II2 MANUAL OF ZOOLOGY SECT. processes grow out, forming the first two tentacles; soon two others appear at right angles to these. Subsequently other tentacles appear. At the same time the attached or proximal end is narrowed into a stalk-like organ of attach- ment (E). The outcome of all these changes is the metamorphosis of the planula into a polype (F), not unlike a Hydra. The Scyphozoon-polype is called a Scyphula. The Scyphula some- times multiplies by budding. After a time it undergoes a process of transverse fission (G), becoming divided by a series of constrictions which deepen until the polype assumes the appearance of a pile of saucers, each with its edge produced into eight bifid lobes. Soon the process of constriction is completed, the saucer-like bodies separate from one another, and each, — except the first topmost one, which falls off and dies, — turning upside down, begins to swim about as a small jellyfish called an Ephyrula (H, I), which grows rapidly and eventually develops into the adult Aurelia. The rest of the Scyphozoa resemble Aurelia in the gen- eral features of their structure, but there is a good deal of variation in certain points (Fig. 52). Thus the umbrella, instead of being a saucer-shaped disc, as in Aurelia, is often conical or cup-shaped or cubical. In some, tentaculocysts are not developed, and in others the oral arms are absent. Lucernaria differs somewhat widely from the rest in being attached by means of a short stalk developed from the centre of the ex-umbrella. In the RhizostomecB the mouth is obliterated by the uniop of the bases of the oral arms, the food being taken in through a large number of minute orifices scattered over the surface of the arms, and leading into a system of fine canals, which join together to form larger canals, eventually opening into the gastric cavity. Many of the Scyphozoa pass through an alternation of generations IV PHYLUM CCELENTERATA 113 similar to that which has been described in the case of Aurelia, with a fixed scyphistoma stage ; but in others the ciliated larvae developed from the ova give rise directly to tentacle: stomach; tn, taeniole or gastric ridge. Antarctic Ocean. (After Haeckel.) jellyfishes like the parent, without the intercalation of any fixed stage. The Scyphozoa are all marine, and the majority are pelagic, i.e., swim freely in the surface waters of the ocean. I MANUAL OF ZOOLOGY SECT. II4 A few inhabit the deep sea, and have been dredged from as great a depth as 3 000 fathoms. Nearly all are free- swimming in the adult state ; some, however, live on coral- reefs or mud-banks, and are found resting, in an inverted position, on the ex-umbrella ; and a few, such as Lucernaria, are able to attach themselves at will by a peduncle. Many are semi-transparent and glassy, but often with brilliantly coloured gonads, tentacles, or radial canals. In many cases the umbrella, oral arms, etc., are highly coloured, and some species are phosphorescent. They are all carnivorous, and, although mostly living on smaller organisms, are able, in the case of the larger species, to capture and digest crustaceans and fishes of considerable size. 3. THE ACTINOZOA The simplest and most familiar of the Actinozoa are the Sea-anemones, which are to be found attached to rocks, seaweeds, shells, etc., on the sea-shore. When expanded a sea-anemone has the form of a cylindrical column attached to a rock or other support by a broad base. The distal or free surface of the column, termed the disc or peristome, bears in the middle an elongated, slit-like aperture — the mouth. Springing from the disc and encircling the mouth are numerous cylindrical tentacles, disposed in circlets, their total number being some multiple of five. Obviously the sea-anemone is a polype, formed on the same general lines as a polype of the Hydrozoa. But certain important differences from the Hydrozoan polype become manifest when we examine the internal .structure (Fig. 54). The mouth does not lead at once into a spacious undivided enteric cavity, but into a short tube (gul), having the form of a flattened cylinder, which hangs downward IV PHYLUM CGELENTERATA into the interior of the body, and terminates in a free edge. This tube is called the gullet or stomodseum. Its inner surface is marked with two longitudinal grooves {sgph), known as the gullet-grooves or siphonoglyphes. The gullet does not simply hang freely in the interior cavity, but is connected with the body-wall by a number of radiating Fig. 54. — Tealia crassicornis. Dissected specimen; gon, gonads; gitl, gullet; 1. m, longitudinal muscle; Ip, lappet; mes. /, primary, mes. 2, secondary, 7nes. 3, tertiary mesenteries; mes. F, mesenteric filaments; 7uth, mouth; ost. I, ost. 2, ostia or aperture in mesenteries; p. wz, parietal muscle; sgph, siphonoglyphe ; m, sphincter muscle; /. m, transverse muscle. partitions, the complete ox primary 7nesenteries {niesl i') \ between these are incomplete secondary rnesenteries {mes. 2), which extend only part of the way from the body-wall to the gullet, and tertiary mesenteries {mes. j), which are hardly more than ridges on the inner surface of the body- Ii6 MANUAL OF ZOOLOGY SECT. wall. Thus the entire enteric cavity of a sea-anemone is divisible into three regions : (i) the gullet or storaodaeum, communicating with the exterior by the mouth, and opening below into (2) a single main digestive cavity, the stomach, which gives off (3) a number of radially arranged cavities, the inter-mesenteric chambers. The free edges of the mesenteries below the gullet are produced into curious IV PHYLUM CCELENTERATA 117 twisted cords, the mesente7Hc filaments {fnes. F), answering to the gastric filaments of Scyphozoa. Stinging- capsules occur in the ectoderm, and are also very abundant in the mesenteric filaments. They resemble in general character the nematocysts of Hydrozoa, but are of a more elongated form, and the thread is usually provided at the base with very numerous slender barbs. In virtue of possessing both stinging-capsules and gland- cells, the mesenteric filaments perform a double function. The animal is very voracious, and is able to capture and swallow small fishes, molluscs, sea-urchins, etc. The prey is partly paralysed before ingestion by the nematocysts of the tentacles, but the process is completed, after swallowing, by those of the mesenteric filaments. Then, as the captured animal lies in the stomach, the edges of the filaments come into close contact with one another and practically surround it, pouring out at the same time a digestive juice secreted by their gland-cells. Sea-anemones are dioecious, the sexes being lodged in distinct individuals. The gonads — ovaries or testes — are developed in the substance of the mesenteries (Fig. 54, gon), a short distance from the edge, and, when mature, often form very noticeable structures. The development of sea-anemones resembles, in its main features, that of Scyphozoa, but there is no alternation of generations. Our common sea-anemone, Metridium marginatum (Fig. 56), lives under stones near low- water mark. Two main divisions or sub-classes of the Actinozoa are recognised, — the Zoantharia and the Alcyonaria, the former including the sea-anemones, the Madrepores, and other stony corals, and the horny black corals ; the latter the “dead men’s fingers,” red coral, organ-pipe coral, “sea-fans,” and “sea-pens.” The principal distinguishing ii8 MANUAL OF ZOOLOGY SECT, features of the two sub-classes are, that in the Zoantharia the tentacles and mesenteries are usually very numerous, and are arranged, as a rule, in multiples of five or six, and Fig. 56. — The common Sea-anemone. Fig. 57. — Corallium rubrum, portion of (After Emerton.) colony. Enlarged twice. (After Lacaze- Duthiers.) that the tentacles are simple in form ; while in the Alcyo- naria (Fig. 58) the tentacles and mesenteries are always eight in number, and the tentacles are pinnate, i.e., each of them consists of a main stem with two rows of lateral branchlets. Only the sea-anemones (with a few exceptions) and a few Madrepore corals remain simple, the rest all giving rise to more or less extensive colonies, of a variety of differ- ent forms, by continuous budding. The structure of the zooids is similar to that of the sea-anemone in all essential respects. In many of the Alcyonaria two forms of zooids are to be distinguished in each colony {^dhtiorphism of the zooids), ordinary zooids, and siphonozooids, which are smaller, and are devoid of tentacles and of gonads. IV PHYLUM CCELENTERATA 19 None of the sea-anemones have a true skeleton ; in some, however, there is a thick cuticle, and several kinds enclose themselves in a more or less complete tube, which may be largely formed of discharged nematocysts. In some Alcyo- Fig. 58. — Alcyonium palmatum. A, entire colony natural size; B, spicules. Mediterranean Sea. (After Cuvier.) naria, such as the “ dead men^s fingers ” {^Alcyonium, Fig. 58), the skeleton consists of minute, scattered, irregular deposits of carbonate of lime called spicules. Alcyonium carneum occurs below tide-mark off the New England coast. In Tubipora (the “organ-pipe coral”) (Fig. 59) there is a con- 120 MANUAL OF ZOOLOGY SECT. tinuous calcareous tube for each polype. In the red coral of commerce (Fig. 57), which inhabits the Mediterranean Sea, there is an extremely hard calcareous branched rod which extends as an axis through the coenosarc. In the black corals {^Antipathes and allies) there is a horn-like axis ; and in Gorgonia there is a similar skeleton, some- times partly calcareous, with the addition of numerous spicules. In the sea-pens (Fig. 60) the colony is supported by an' un- branched horny axis. Pennatula aculeata lives in deep water in the North Atlantic. In the Madrepore corals we have a skeleton of an entirely different type, consisting, in fact, of a more or less cup-like calca- reous structure secreted from the ectoderm of the base and column of the polype. When formed by a solitary polype such a ‘‘cup- Fig. 59. — Tubipora musica. Skei- coral” is known as a corallite ; eton of entire colony. Natural size, //.platform. indianOcean. in the maiority of species a large (After Cuvier.) y ± o number — sometimes many thou- sands — of corallites combine to form a corallum, the skele- ton of an entire coral-colony. The structure of a corallite is conveniently illustrated by that of the solitary genus Flabellum (Fig. 61, A, B). It has the form of a short conical cup, much compressed, so as to be oval in section. Its wall or theca is formed of dense stony calcium carbonate, the proximal end pro- duced into a short stalk or peduncle. From the inner IV PHYLUM CCELENTERATA I2I surface of the theca a number of radiating partitions, the septa^ proceed inwards or towards the axis of the cup, some of them meeting in the middle to form an irregular central mass or columella^ which in some kinds of corals forms an independent, pillar-like structure arising from the middle of the base. In the living condition the polype fills the whole interior of the corallite, and projects beyond its edge to a greater or less degree according to its state of expan- sion. The septa alternate with the mesenteries, each being in- vested by an in-turned portion of the body- wall ; so that, though having at first sight the appear- ance of being internal structures, they are really external, lying alto- gether outside the enteric cavity, and are in contact throughout with the ectoderm. The almost infinite variety in form of the compound corals is due, in the main, to the various methods of budding. According to the mode of budding, massive corals are produced in which the corallites are in close contact with one another, as in Astraa (Fig. 62) ; or tree-like forms, such as Den- drophyllia (Fig. 63, A), in which a common calcareous stem, the coe.nenchyma, is formed by calcification of the coenosarc,^ and gives origin to the individual corallites. 1 See p. 94. Fig. 60. — Pennatula sulcata. Entire colony. Natural size. I, lateral branch. (After Koelliker.) 122 MANUAL OF ZOOLOGY SECT, Fig. 6i. — A, B, two views of Flabejlum curvatum. Natural size. C, semi-dia- grammatic view of a simple coral; D, portion of a corallite; E, F, diagram of a simple coral in longitudinal and transverse section; ectoderm dotted, endoderm striated, skeleton black, b.pl, basal plate; col, columella; e, th, epitheca; gul, gullet; mes., mes. i, mes. 2, mesenteries; mes. f, mesenteric filaments; sep, septa; t, tentacle; th, theca. (A and B after Moseley; C and D after Gilbert Bourne.) IV PHYLUM CCELENTERATA 123 It is by this last-named method, the coenosarc attaining great dimensions, and the individual corallites being small and very numerous, that the most complex of all corals, the Madrepores (Fig. 63, B), are produced. The Actinozoa are remarkable for the variety and brill- iancy of their colour during life. Every one must have noticed the vivid and varied tints of sea- anemones ; but in Fig. 62. — Astrsea pallida, the living colony. Natural size. Fiji Islands. (After Dana.) life the corals also exhibit a marvellously varied and gor- geous colouring ; and the same holds good of many of the Alcyonaria. Many Actinozoa, like many sponges (p. 89), furnish examples of commensalism, a term used for a mutually beneficial association of two organisms of a less intimate nature than occurs in symbiosis. An interesting example is furnished by the sea-anemone Adamsia palliata. This species is always found on a univalve shell — such as that of a whelk — inhabited by a hermit-crab. The sea-anemone is carried from place to place by the hermit-crab, and in 124 MANUAL OF ZOOLOGY SECT. this way secures a more varied and abundant food-supply than would fall to its lot if it remained in one place. On the other hand, the hermit-crab is protected from the attack of predaceous fishes by retreating into its shell and leaving exposed the sea-anemone, which, owing to its toughness. Fig. 63. — A, Dendrophyllia nigrescens; B, Madrepora aspera. Natural size, corallites ; cj, ccenosarc; polypes. Pacific Ocean. (After Dana.) and to the pain caused by its poisonous stinging-capsules, is usually avoided as an article of food. A similar case is that of Cancrisocia, of the China seas, which lives on the back of a crab (^Dorippe facchino. Fig. 64) . The crab carries, for its protection when young, a small shell over its back, which it holds in this position by IV PHYLUM CCELENTERATa 125 means of its two reversed pairs of hind legs. The sea- anemone appears to have fixed itself when young to the Fig. 64. — Cancrisocia living as a commensal on the back of a crab. (After Verrill.) shell, and afterwards, by its growth, spread over the back of the crab, taking the place of the shell. 4. THE CTENOPHORA The Ctenophora or comb-jellies are a group of free- swimming, gelatinous, transparent animals which occur, some- times in enormous numbers, in the surface waters of the sea. The animal (Fig. 65) has the appearance of a mass of clear jelly, usually of a globular shape ; and no pulsating move- ments, such as those by means of which a Medusa propels itself, are to be observed. Running over the surface, nearly from pole to pole of the globular body, there will be observed a series of eight bands of flashing points of light. These are found, when examined more closely, to consist of rows of long cilia, which run at right angles to the long axis of the 126 MANUAL OF ZOOLOGY SECT, band. The cilia of each row are cemented together at their bases, free from one another distally, so that each row is comb-shaped, the basal cemented parts of the cilia forming the back of the comb, the free portions the teeth. It is by the paddling action of the numerous swimming Fig. 65. — Hormiphora {Cydippe) plumosa. A, from the side; B, from the aboral pole, mthi mouth; s. pp swimming plates; t and b, tentacles. Natural size. Mediterranean Sea. (After Chun.) combs of these eight bands that the ctenophore is propelled through the water. Laterally there is situated a pair of long slender tentacles, each provided with numerous little tag-like processes, and having its base lodged in a sheath into the interior of which the whole tentacle can be retracted. At one pole, the oral, is an opening, the mouth : and at the opposite pole is a pair of minute pores, the excretory pores, which are the IV PHYLUM COELENTERATA 127 openings of a pair of canals given off from the enteric cavity. Between the two excretory pores is a remarkable structure, which is the nerve-centre as well as an organ of special sense. The mouth leads into a flattened tube, the gullet, and this again leads into a cavity, the infundibulum, which Fig. 66. — Hormiphora plumosa. A, transverse section of one of the branches of a tentacle; B, two adhesive cells {ad. c,') and a sensory cell (i'. c) highly magni- fied. cuticle; nucleus. (After Hertwig and Chun.) probably corresponds to the stomach of the sea-anemone. From this cavity certain canals are given off. Stinging-capsules are not developed, their place being taken by a number of peculiar cells called adhesive cells, with which the branches of the tentacles are covered. An adhesive cell (Fig. 66, B) has a convex surface, produced into small papillae, which readily adheres to any surface with which it comes in contact, and is with difficulty separated. In the interior of the cell is a spirally coiled filament, the delicate inner end of which can be traced to the muscular axis of the tentacular branch. These spiral threads act as springs, and tend to prevent the adhesive cells from being 128 MANUAL OF ZOOLOGY SECT. IV torn away by the struggles of the captured prey. An allied form is Pleurobrachia, very abundant off our shores. In some of the Ctenophora the body is produced into a pair of lateral lobes. In Bero'e, instead of being globular, it is more nearly cylindrical, with an extremely wide mouth and gullet, and without tentacles. In the “Venus’s girdle” (Ces^us), it is compressed and almost ribbon-like. All are free-swimming ; colonies are never formed ; and there is never any kind of skeleton. The Ctenophora are usually per- fectly transparent, and quite colour- less, save for delicate tints of red, brown, or yellow on the tentacles or on ridges on the inner surface of the gullet. Cestus has, however, a deli- cate violet hue, and, when irritated, shows a beautiful blue or bluish-green fluorescence ; while Beroe is coloured is of a brilliant pink. The most primitive form to be found on our coast is Idyia (Fig. 67), which is a simple oval sphere, the interior of which forms an immense digestive cavity, in which entire large animals may be engulfed. Fig. 67. — Tdyla roseola. Seen the broad side, ha)*" natural size. a. anal opening; d, lateral tu'be; c, circular tube; d, e,f,g, h, rows of paddles. ( After Agassiz.) rose-pink, and Idyia SECTION V. — PHYLUM PLATYHEL- MINTHES The Platyhelminthes or Flat- worms are a group of animals which, though of a low type of organisation, yet show in many cases a great advance on the Coelenterata, in the possession of systems of organs of a more or less elaborate character for the carrying on of the various functions. Many are internal parasites of higher animals ; others are parasites on the outer surface (external parasites) j others again are non-parasitic. 1. THE TREMATODA A good and easily procurable example of the flat-worms is the Liver-fluke of the sheep (Distomum hepaticum), which lives as a parasite in the liver, in the interior of the larger bile-ducts of the infested animal. It is a soft-bodied worm, of flattened, leaf-like shape (Fig. 68), with a trian- gular process, the head lobe, projecting from the broader end. When the liver-fluke is compared with a zooid of Obelia, or with a Medusa or a sea-anemone, a striking difference in the general disposition or symmetry of the parts is at once recognisable. In the latter, as in the Coelenterata in general, the prevailing arrangement is a radial one, the parts being disposed in a radial manner round the main axis of the body, which is an imaginary line running through the middle of K 129 130 MANUAL OF ZOOLOGY SECT. the mouth and enteric cavity. In the fluke, on the other hand, the parts are disposed to the right and left of an imaginary median vertical plane, along which the entire animal is capable of being divided into two completely symmetrical, right and left, halves. The type of symmetry here exemplified is termed bilateral; it has already been met with in some of the Protozoa, and is characteristic of nearly all animals higher than the Coelenterata. The broader end of the body is determined as anterior, ow- ing to the mouth and the central part of the nervous system being situated at that extremity. One of the broad flat surfaces is the dorsal, the other the ventral. The mouth {ind), situated at the anterior extremity of the head- lobe, is surrounded by a muscular oral sucker, and some distance back, on the ventral surface, just behind the head- lobe, is a second much larger posterior sucker (sckr). Between the two suckers is a median aperture, the genital opening (repr), through which a curved muscular process, the cirrus or penis, may be protruded. In the middle of the posterior end of the body is a minute opening, the excretory pore (excr). The surface is covered with innumerable minute spinules, but vibratile cilia are absent. The mouth (Fig. 68, mo) leads to a small, bulb-like body, the pharynx (Fig. 69, pH), with thick muscular walls and a small cavity. From this a short passage^ the oesophagus, leads to the intestine. The latter (int) is frequently a very Natural size, excr, excretory pore; mo, mouth; ro/r, repro- ductive aperture; sckr, pos- terior sucker. V PHYLUM PLATYHELMINTHES 131 conspicuous structure, owing to its being filled with the dark biliary matter on which the fluke feeds. It divides almost immediately into two main limbs, right and left, and from each of these are given olf, both internally and ex- Fig. 69. —Distomum hepaticum. Internal organisation. General view of the anterior portion of the body, showing the various systems of organs as seen from the ventral aspect. ejaculatory duct; y, female reproductive aperture; int, anterior portion of the intestine (the rest is not shown) ; od, commencement of oviduct; ov, ovary; /, penis; ph, pharynx; ?h, shell-gland; te, testes; ut, uterus; W,, left vas deferens; vd^, right vas deferens; vit, lobes of vitelline glands; vs, vesicula seminalis. (After Sommer.) ternally, a number of blind branches or coeca, those on the inner side being short and simple, while those on the outer side are longer and branched. The two limbs of the intes- tine, with their branches, thus form a complicated branching 132 MANUAL OF ZOOLOGY SECT. system, the ramifications of which extend throughout the whole of the body. There is no anus, or aperture of com- munication between the intestine and the exterior, the only external opening of the alimentary system being through the mouth. A branching system of vessels — the water-vessels or vessels of the excretory system — ramify throughout the body. A longitudinal main trunk opens outwards by means of the excretory pore. In front it gives off four large trunks, each of which branches repeatedly, the branches giving off smaller vessels, and these' again still smaller twigs, until we reach a system of extremely fine microscopic vessels, or capillaries. Each of these ends internally in a slight enlargement situated in the interior of a large cell, a flame-cell, with a bunch of vibratile cilia, or a single thick cilium, in the interior. The fluke has a nervous system, the arrangement of which partakes of the bilateral symmetry of the body. The central part of this system consists of a ring of nerve matter, which surrounds the oesophagus, and presents two lateral thickenings or ganglia containing nerve-cells, and a single ganglion situated in the middle line below. From this are given off a number of nerves, of which the chief are a pair of lateral cords running back to the posterior end and giving off numerous branches. There are no organs of special sense. The reproductive organs are constructed on the hermaph- rodite plan, /.) are provided with long tails, with anterior and posterior suckers, a mouth and pharynx, and a bifid intestine. These escape through an aperture in the wall of the redia, and, moving actively by means of their tails, force their way out from the body of the snail. They then, losing the tail, become encysted, attached to blades of grass or herbage. The transference of the larval fluke to its final host, the sheep, is effected if the latter swallow the grass on which the cercaria has become encysted. The young fluke then escapes from the cyst, and forces its way up the bile- ducts to the liver, in which it rapidly grows, and, developing reproductive organs, attains the adult condition. The liver-fluke is an example of the class of flat-worms known as Trematoda. These are all parasitic. Some are internal parasites, and in the adult condition inhabit, for the V PHYLUM PLATYHELMINTHES 35 most part, the enteric canal, the liver, or the lungs of some animal of the Vertebrate or back-boned class (fishes, amphib- ians, reptiles, birds, or mammals), swallowing the digested ^as{ Fig. 70. — A-D, development of Distomum hepaticum. A, ciliated larva; £, sporocyst, containing redias in various stages of development; C, redia, containing a daughter redia, and cercariae; Z>, fully developed cer- caria. i-o/i, birth opening; ent, enteron of redia; eye, eye-spots; g-ast, gastrula stage of redia; germ, early stages in the formation of cercariae; int, intestine of cercaria; mor, morula stage in the development of cercariae; oes, oesophagus; or.su, oral sucker; pap, head-lobe of ciliated embryo; ph, pharynx; proc, pro- cesses of redia; vent.su, ventral (posterior) sucker. (After Thomas.) food or various secretions of their host. Others are external parasites, living on some part of the outer surface of their host, and feeding on mucus or other secretions of the in- MANUAL OF ZOOLOGY SECT. 136 tegument. The leaf-like form exemplified in the liver-fluke prevails in most (Fig. 71), but a more elongated form some- times occurs. The anterior end is distinguished from the posterior by its shape, by the arrangement of the suckers, and, in many of those Trematodes that are external para- sites, by the presence of eyes. Suckers are universal in their occurrence. They are always ventrally placed, their chief function being to fix the parasite to the surface of its Fig. 71. — Trematodes. A, Amphistomum ; B, Hovialogaster. gp, genital aperture: vi, mouth; s, posterior sucker; te, testes; vit, vitelline glands. (After M. Braun.) host in such a way as to facilitate the taking in by the mouth of animal juices and epithelial debris. Their number and arrangement vary considerably. There are nearly always present an anterior set (or, as in the liver-fluke, a single anterior sucker surrounding the mouth), and a posterior set or a single large posterior sucker. There PHYLUM PLATYHELMINTHES 137 are no cilia on the surface, and a well-developed enteric canal is always present. A remarkable series of metamorphoses, such as that which has been de- scribed in the liver-fluke, is characteristic of the internally parasitic forms ; in the ectoparasitic or externally parasitic Tre- matodes development is direct, the young animal when it escapes from the egg differing little from the adult except in size. 2. THE TURBELLARIA The Turbellaria are a class of flat-worms which, though for the most part non-parasi tic, resemble the Trematodes very closely, the chief difference being the presence of a coating of vibrating cilia, and the absence, in the majority, of suckers. The leaf form is the prevailing one (Fig. 72), but in many the body is elongated and ribbon- like, or subcylindrical. In Fig. 72. — General plan of the structure of a Triclad Turbellarian. cn, brain; e, eye; g, ovary ; t\, median limb of the intestine ; Zj, left limb; z'b, right limb ; longitudinal nerve-cord; m, mouth; od, oviduct; ph, pharynx; zf, testes; tentacles; z/zf, vas deferens; zz, uterus; cf, ejaculatory duct; $ , vagina; cT 9 > common genital aperture. (After Von Graff.) some the anterior end is retractile, and may be everted as MANUAL OF ZOOLOGY SECT. ^38 a proboscis. The mouth is never at the extreme anterior end, but is always ventrally placed, sometimes behind the middle. A few multiply by budding, and these may give rise to chains of individuals, which subsequently become separated. In the lowest Tur- bellaria the intestine is repre- sented merely by a nucleated mass of protoplasm ; in others it is a simple sac ; in the major- ity it is branched. The general structure of the other internal organs very closely resembles that of the corresponding parts in the Trematodes. Y\G.^i. — Planariapolycltroa{a), Turbellaria occur in the sea, lugubris [b) , torva (c), about ... i i i thrice the natural size. (After m iresh Water, and also m damp Schmidt, from Claus.) . ... , , localities on land. Ihe great majority are non-parasitic, their food consisting of minute aquatic animals and plants of various kinds. An example is Planaria tob'va of our fresh-water pools and streams (Fig. 73, C-). 3. THE CESTODA The class Cestoda or tape-worms are all internal parasites, and in the adult condition live in the enteric canal of verte- brates. The tape-worms are much more completely adapted to a life of parasitism than the Trematodes : they have no digestive system, and are nourished by the imbibition, through the general surface, of liquid nutriment derived from the digested food of the vertebrate host. The shape of a typical tape- worm is widely different from that of a trematode. A tape-worm (Fig. 74) is flattened like a trematode, but is extremely elongated, the length being PHYLUM PLATYHELMINTHES Fig. 74. — Taenia solium.' Human tape-worm. Entire specimen reduced, cap, head. (After Leuckart.) 140 MANUAL OF ZOOLOGY many times, often hundreds of times, the greatest breadth, so that the* animal assumes the form of a long, narrow ribbon or tape. This ribbon is not continuous, but is made up of a string of segments or proglottides. Towards one end the body becomes narrower, terminating in a rounded knob — the head or scolex. On the head (Fig. 75) is a circlet of hooks borne on a rounded prominence, the rostelluni, which is capable of being protruded and retracted to a certain extent ; at the sides are four suckers. By means of these hooks and suckers the head is attached to the wall of the intestine of the host, the elongated body lying free in its interior. The part of the body just behind the head {neck) is not divided into segments. The most anterior segments are much shorter than those further back, and not so distinctly separated off from one another. The surface is devoid of cilia, as in the Trematodes. A digestive cavity is, as already stated, absent ; but there is a distinct nervous system, and a system Fig. 75.— Head of Taenia of water-vessels with flame-cells. In the solium, magnified. . . . , , , , (After Leuckart.) postcnor region of the body each pro- glottis (Fig. 76) is found to contain a complete set of hermaphrodite reproductive organs similar in general plan to those of the liver-fluke. The ova, when fertilised, are enclosed in a chitinoid shell, and received into a uterus. In the most posterior segments the uterus is a large branched tube distended with enormous quantities of these eggs, and the other parts of the reproductive appa- ratus have become absorbed. These ripe ” proglottides, as they are termed, drop off, one by one, from the pos- V PHYLUM PLATYHELMINTHES terior end, and reach the exterior with the faeces of the host. At the same time new proglottides are constantly being formed by the appearance of new ring-like grooves behind the neck region. This dropping off of ripe proglot- tides from the posterior end, and the formation of new ones behind the neck, results in a gradual shifting backwards of the proglottides. As each proglottis passes backwards from its point of origin, it gradually develops the various parts of the reproductive apparatus in its interior, until, when Fig. 76. — A proglottis of Taenia solium with mature reproductive apparatus. can. excret, longitudinal excretory canals with transverse connecting vessels; gl. vit, vitelline glands: nerv. I, longitudinal nerves; ov,ov, ovaries; par. ge?i, genital pore; schld, shell-glands; titer, uterus; vag, vagina; vas. def, vas deferens. The numerous small round bodies are the lobes of the testes. (After Leuckart.) it has reached the posterior region, it possesses a com- plete set of reproductive organs, and, as it reaches the extreme posterior end, it has become ripe, i.e., has its uterus distended with eggs. In the interior of each of the eggs in the ripe proglottides is an embryo consisting of a rounded mass of cells bearing six chitinoid hooks — the six-hooked or hexacanth embryo (Fig. 77, A). After the egg has been discharged from the free proglottis, it has to reach the enteric canal of a second par. •va ■ne.rv.1 142 MANUAL OF ZOOLOGY SECT. kind of animal — a second or intermediate host — in order that the embryo may be enabled to enter the next phase of its life-history. In the case of some tape-worms, this second or intermediate host is, like the first or permanent host, a vertebrate animal : in the case of others it is some inver- A C D Fig. 77. — Development of Tape-WOrm. A, hexacanth embryo; B, Proscolex of Tcenia saghiata; C-E, stages in the formation of the scolex of the same; C, the invagination before the hooks and suckers have become developed; D, after th^’ appearance of the hooks and suckers; E, partly evaginated; F, fully evaginated scolex of T. solium, with caudal vesicle; G, scolex of T. serrata with the remains of the vesicle; H, young tape-worm of T. serrata. (After Leuckart.) tebrate animal such as an earth-worm, a centipede, or an insect. This transference of the hexacanth embryo to the second host is a passive migration, not an active one, as in V PHYLUM PLATYHELMINTHES 143 the case of the ciliated embryo of the Trematodes, the egg being received into the enteric canal of the second host with the water or food. The digestive fluids of this second host dissolve the egg-shell and set free the contained embryo, which bores its way by means of its hooks to some part of the body in which it is destined to pass through the next phase of its life-history, and there becomes encysted (B). The phase which follows presents two main varieties. In cases in which the second host is an invertebrate animal, the hooked embryo develops into a form to which the name of Cysticercoid is given ; when, on the other hand, the inter- mediate host is a vertebrate, the form assumed is nearly always that termed Cysticercus or bladder-worm. In both cases a tape-worm head is developed, with the rostellum, hooks, and suckers of the adult. In the Cysticercus (C-H) this is formed from the wall of a relatively large cyst or bladder into which the hooked embryo develops. In a very small number both of Cysticercoids and of Cysticerci more than one tape-worm head is formed. Thus Tcenia coenurus of the dog has a bladder-worm stage occurring in the sheep and rabbit, which gives rise to several tape-worm heads. But the most striking instance of mul- tiple production of tape-worm heads in a bladder-worm is Tania echinococcus, well known as the cause of the disease termed hydatids, common in man and in various domestic animals. In this case the hooked embryo develops into a large mother-cyst, from the interior of which daughter-cysts are budded off. Eventually from the walls of these daughter- cysts (Fig. 78) are formed numerous tape-worm heads. The transference to the first or final host is effected by the second or intermediate host, or the part of it containing the Cyticercus or Cysticercoid, being taken into the enteric canal of the final host. Sometimes, if the intermediate host is 144 MANUAL OF ZOOLOGY SECT. a small animal, such as a water-flea, this may take place “accidentally”; in other cases the intermediate host actually forms the food of the final host. Thus, to give two instances, a Cysticercoid having as an intermediate host an earth-worm is taken with the latter into the enteric canal of a sea-gull — its final host; a Cysticercus which occurs in the liver of rats and mice is received into the enteric canal of the cat. In this way the Cysticercus or Cysticercoid is set free in the enteric canal of the final host; the tape-worm head becomes attached by means of its hooks and suckers to the wall of the intestine, and the long segmented body of the tape-worm is developed behind. Fig. 78.— Cyst of Tasnia echinococcus with the developing daughter-cyst and scolices. (After Leuckart.) The commonest human Cestode parasites in the United States and Canada are Tcenia solium and T. saginata (T mediocaneUata), the latter being the more common pest. The Cysticercus stage of the former occurs chiefly in the flesh of the pig ; that of the latter in the flesh of the ox ; and the relative prevalence of these two tape-worms in different countries varies with the habits of the people with regard to flesh-eating : where more swine’s flesh is eaten in an imperfectly cooked state Tania solium is the more prevalent, where more beef, 71 saginata. V PHYLUM PLATYHELMINTHES 145 Bothriocephalus latus, a very large tape-worm without hooks, is a common human parasite in eastern countries. Its Cysticercus occurs in the pike and certain other fresh- water fishes. It has not become endemic, or naturalized, in the United States. 4. THE HEMERTIHEA The Hemerteans are non-parasitic, unsegmented worms, most of which are marine, only a few forms living on land or in fresh water. They are commonly looked upon as nearly related to the Turbellaria, and were formerly in- cluded in that class ; but they are in some respects higher in organisation than the Turbellaria, and they exhibit cer- tain special features distinguishing them from the rest of the lower worms. The body (Figs. 79 and 80) is narrow and elongated, cylindrical or depressed, unsegmented, and devoid of ap- pendages. In length it varies, in different species, from a few millimetres to as much as ten metres. The entire sur- face is covered with vibratile cilia. The mouth (m) is at or near the anterior extremity on the ventral aspect. Close to it above there is an opening through which can be protruded a very long muscular organ, the proboscis {pr), the possession of which is one of the most characteristic features of this class of worms. The proboscis is hollow : when it is extended to its utmost, a part still remains which is not capable of being everted, and at the junction between the eversible and non-eversible parts, i.e., at the extremity of the organ when it is fully protruded, there is in many of the Nemerteans a pointed or serrated stylet {¥\g. 80, st), which probably permits of the proboscis being used as a weapon : when a stylet is absent, L 146 MANUAL OF ZOOLOGY SECT. the surface of the extremity is sometimes abundantly provided with stinging-capsules ; sometimes it is beset with glandular adhesive papillce. The proboscis is capable of Fig. 79. — Diagram of the organs of a Nemertine, from below, a, anus; hr, brain; div, coeca; long, nc, longitudinal nerve-cords; m, mouth; n, nephridia; ov.^ ovaries; pr, probosis. (After Hubrecht.) being retracted within the interior of an investing sheath, the proboscis sheath. PHYLUM PLATYHELMINTHES proi op 147 brob'. brob ' aUrs pes ,op neph ■ doraves Fig. 8o. — Tetrastemma. General view of the internal organs, an, anus; ac. st, accessory stylet ; cer. g-, brain ; cil.gr, ciliated groove ; dors, ves, dorsal vessel ; lai. ne, lateral nerve; lat. ves, lateral vessel; neph, nephridium ; op. 7ieph, nephridial aperture \ prob'-, eversible part of proboscis ; prob'^', non-eversible part of proboscis; prob. ap, aperture for the protrusion of the proboscis; retr. miis, retractor muscle of the proboscis ; st, stylet. (From Hatschek’s Lekrbuch.) 148 MANUAL OF ZOOLOGY SECT. V The alimentary canal (Fig. 79) is a simple tube distin- guishable into oesophagus with longitudinally folded walls, and intestine with lateral coeca {div). It ends in an anal opening {a) situated near the posterior extremity of the body. The Nemerteans possess a system of blood-vessels with well-defined walls formed of an epithelium and a layer of muscle. There are three principal longitudinal trunks — a median dorsal and two lateral. The blood follows no regular course through the vessels, but is moved about by the muscular contractions of the body. The excretory vessels of the Platyhelminthes are repre- sented in the Nemertine worms by a pair of greatly coiled and branched tubes (Fig. 80, neph), opening on the exterior ; the fine terminal branches of the system are provided with ciliary flames, and cilia occur also in the course of the vessels themselves. The nervous system is in some respects more highly developed than in the Turbellaria. The brain (Fig. 80, cer. g) is composed of two large ganglia with lobed surfaces, connected together by two commissures, dorsal and ventral, between which pass the proboscis and its sheath. From the brain pass backwards a pair of thick nerves which run throughout the length of the body. Eyes are present in the majority of Nemerteans, and in the most highly organised species occur in considerable numbers. Most species are dioecious. The ovaries (Fig. 79, ov') and testes are situated in the intervals between the intestinal coeca. The ovary or testis is a sac, the cells lining which give rise to ova or spermatozoa ; when these are mature each sac opens by means of a narrow duct leading to the dorsal surface, where it opens by a pore. SECTION VL — PHYLUM NEMATHEL- MINTHES The Nemathelminthes or round-worms are so named because the body instead of being compressed from above downwards, as in the flat-worms, is rounded, i.e., cylindrical. The majority of the members of the phylum belong to the class of the Nematoda or round-worms in a more restricted sense. A good example of these is the common round- worm of man (Ascaris lumbricoides), which is a common parasite in the human intestine ; or the nearly allied Ascaris suilla of the pig. When fresh the animal is of a light yellowish-brown colour ; it is marked with four longitudinal streaks, two of which, very narrow and pure white in the living worm, are respectively dorsal and ventral in position, and are called the dorsal (Fig. 8i, d. /) and ventral {v. 1) lines: the other two are lateral in position, thicker than the former and brown in colour, and are distinguished as the lateral lines. The mouth is anterior and terminal in position, and is bounded by three lobes, or lips, one median dorsal {d. Ip), the other two ventro -lateral {v. Ip). A very minute aperture on the ventral side, about two millimetres from the anterior end, is the excretory pore {ex. p). At ^bout the same distance from the pointed and down-turned posterior end is a transverse aperture with thickened lips, the anus {an), which in the male serves also as a reproductive aperture, and gives exit to a pair of needle-like chitinoid 149 150 MANUAI. OF ZOOLOGY SECT. bodies, the penial seta {pn. s). In the female the repro- ductive aperture or gonopore is separated from the anus, and is situated on the ventral surface about one-third of the length of the body from the anterior end (Fig. 82, gnp'). The outer surface of the body is furnished by a delicate, transparent, elastic membrane, of a chitinoid nature, the VI PHYLUM NEMATHELMINTHES cuticle. It is wrinkled trans- versely so as to give the animal a segmented appearance. Be- neath the cuticle is a proto- plasmic layer containing scat- tered nuclei and longitudinal fibres, and representing a syn- cytial ectoderm, i.e., an ecto- derm in which the cell-bodies are not differentiated, and its cellular nature is recognisable only by the nuclei. Beneath the ectoderm is a single layer of muscular fibres of peculiar structure, arranged longitudinally, and bounding the body-cavity. The muscular layer is not continuous, but is divided into four longitudinal bands or quadrants, two dorso-lateral and two ventro-lateral, owing to the fact that at the dorsal, ventral, and lateral lines the ectoderm undergoes a great thickening and projects in- wards, between the muscles, in the form of four longitudi- nal ridges. It is this arrange- ment that gives rise to the lines seen externally. The mouth leads into the anterior division of the enteric 151 Fig. 82. — Ascaris lumbricoides. Semi-diagrammatic dissection of the female. anus; cuticle; der. e/tkm, deric epithelium; ex. excretory pore; ex. z/, excretory vessel ; gnp. gonopore; int, intestine partly cut away; lat. I, lateral line; m, muscular layer; mth, mouth; nv. r, nerve-ring; ovy, ovary, that of the right side in situ, the left spread out; ph, pharynx, partly cut away; nt. uterus. 152 MANUAL OF ZOOLOGY SECT. canal, the pharynx or Fig. 83. — Diagram of nervous system of Nematoda. c, com- missures; din, dorsal nerve; hsn, posterior lateral nerve; on, _ upper and un, under portion of nerve-ring; sg, lateral swellings; ventral nerve. (From Lang, after Biitschli.) stomodseum (Fig. 82, ph'), with very muscular walls. Posteriorly the pharynx opens into the intestine (inf), a thin walled tube, flattened from above downwards. Posteriorly the intestine narrows considerably to form the short rectum, which opens externally by the anus {an) . The food, consisting of the semi- fluid contents of the intestine of the host, is sucked in by movements of the pharynx, and is then absorbed into the system through the walls of the intestine. The food being already digested by the host, there is no need of digestive gland-cells such as occur in animals which pre- pare their own food for absorption. Between the enteric canal and the body- wall is a distinct space, the coelome or body-cavity, containing a clear fluid. The excretory system presents a certain resemblance to that of Pla- todes. It consists of two longitu- dinal canals {ex. v), one in each lateral line. Anteriorly they pass to the ventral surface, unite with one another, and open by the minute excretory pore {ex. p) al- ready noticed. The nervous system consists of a ring {nv. r) surrounding the pharynx VI PHYLUM NEMATHELMINTHES 153 and giving off six nerves forwards and six backwards (Fig. 83). Of the latter, two are of a considerable size and run in the dorsal and ven- tral lines respectively {din, vln). The reproductive organs are formed on a peculiar and very characteristic pat- tern. The testis (Fig. 84, ts') is a long coiled thread, occupying a considerable portion of the body-cavity. At its posterior end it is continuous with the vas deferens. The vas deferens, in its turn, becomes con- tinuous with a wide canal, the vesicula seminalis {vs. sent), which opens by a short, narrow, muscular tube, the ductus ejaculato- rius, into the rectum. Be- hind the rectum, and open- ing into its dorsal wall, are paired muscular sacs {s') containing the penial setce {p. ns) already noticed. The anterior end of the testis consists of a solid mass of sexual cells ; pass- ing backwards there is found a cord or rachis occupying the axis of the tube and Fig. 84. — Ascaris lumbricoides , posterior extremity of male, dissected, an, anus; cu, cuticle; der. epthm, epidermis; m, muscular layer; pn. s, penial seta; s, sacs containing penial seta; (s, testis; vs. sem, vesicula seminalis. 154 MANUAL OF ZOOLOGY SECT. VI having the sperm-cells attached to it ; still further back the sperms become gradually differentiated, and are finally set free in the vas deferens. The organs of the female (Fig, 82) resemble those of the male, but are double instead of single. There are two coiled, thread-like ovaries {pvy), each passing insensibly into a uterus {ut'). In the ovary, as in the testis, the eggs are developed in connection with an axial cord or rachis. The two uteri unite in a short muscular vagina (vag) which opens, as already seen, on the ventral surface of the body {gnp) at about one-third of the entire length from the head. The Nematodes in general vary greatly in size, from about I mm. or less to two metres (six feet) in the case of the Guinea-worm, the length always being great in proportion to the diameter, and the body being always bluntly pointed at the anterior end, and either pointed or forked posteriorly. The mouth is frequently armed with spines by means of which the worms draw blood from the intestinal blood- vessels of their host. Many free-living forms have a sharp stylet for piercing the tissues of the plants on which they feed, and a suctorial apparatus for absorbing their juices. The nervous system has in most the same general struc- ture as in Ascaris, and the same holds good of the repro- ductive apparatus. A few are hermaphrodite, but, instead of a double set of reproductive organs as in Platyhelminthes, they have organs similar to those of the female Ascaris, the gonads, producing first sperms and afterwards ova. One of the most terrible parasites of man is a nematode called Trichina spiralis (Fig. 85), a minute worm, the male (C) a little over i mm. (yU inch) in length, the female (B) about 3 mm. (i inch). In the adult or sexual condition it lives in the intestine of man, the pig, and other mammals. Internal impregnation takes place, the eggs develop in the Fig. 8s. — Trichina spiralis. A, encysted form in muscle of host; B, female; C, male, bh, connective tissue envelope; cy, cyst; de, ejaculatory duct; e, em- bryos; y, fat globules; A, testis; w. y, muscle fibre; pharynx; ovary; 1VO, gonopore; zh, cell masses in intestine. (From Lang’s Comparative Anatomy, after Claus.) 155 156 MANUAL OF ZOOLOGY sect, vi uterus of the female, and the minute young (B, second abdominal segment; ar3. ^-12, inner arthrobranchiae; arb-^. y-l2, outer arthrobranchiae: ep. 5, scaphognathite; plb. 11-13, pleurobranchiae; pdb. 7-13, podobranchs; pi. /, first pleopod; b-13, thoracic appendages. (From Lang’s Comparative Anatomy, after Huxley.) lei canals in the stem — an external, the afferent branchial vein, and an internal, the efferent branchial vein. According to their point of origin, the gills (Fig. 126) are 224 MANUAL OF ZOOLOGY SECT. divisible into three sets, — ^x?X, podobranchicz or foot-gills, springing from the epipodites of the thoracic appendages, from which they are only partially separable ; secondly, arthrobranchicz or joint-gills, springing from the articular membranes connecting the thoracic appendages with the trunk ; and thirdly, pleurobranchicz or wall-gills, springing from the lateral walls of the thorax, above the attachment of the appendages. At the base of each antenna is an organ of a greenish colour, the antennary or green gland, by which the function of renal excretion is performed. The gland is cushion- shaped ; it discharges into a thin-walled, sac or urinary bladder which opens by a duct on the proximal segment of the antenna. The glands already referred to as occurring in ' the gills are also supposed to have an excretory function. The circulatory organs are in a high state of development. The heart (Figs. 125, 127, >^) is situated in the dorsal region of the thorax, and is a roughly polygonal muscular organ pierced by three pairs of apertures or ostia ( Fig. ig6, prob) has muscular walls; its cavity opens on the exterior usually by a single minute aperture — the proboscis pore (Fig. ig6, prob. po') — rarely by two. The collar (Fig. 195, co') is also muscular, and contains one cavity or two (right and left) separated from one another by dorsal and ventral mesenteries, and completely cut off from the proboscis. The collar cavity com- municates with the exterior by a pair of collar pores — ciliated tubes leading into the first gill-slit or first gill-pouch. Fig. 195. Balanoglossus. Entire animal. branchial region; £■£•, basi-occipital ; basi-pterygoid processes; bas.sph, basi-sphenoid; col, epi-pterygoid ; cor, coronary; dent, dentary; eth, ethmoid; ex oc, ex-occipital; ext. 7tar, external nares ; /' mag, foramen magnum; fr, frontal; int. nar, internal nares; ju, jugal; Icr, lacyf- mal; max, maxilla; nas, nasal; oc. cond, occipital condyle; olf, olfactory- capsule; op.ot, opistbotic; opt n, optic nerve; pal, palatine; par, parietal; par. parasphenoid; par./, parietal foramen; p. mx, pre-maxillae; pr.fr, pre- frontal; ptg, pterygoid, pt. orb, post orbital; yu, quadrate; s. ang, stipra- angular; j. orb, supra-orbitals; sq, squamosal; supra. supra-temporal i; supra, t.'^. supra-temporal 2; trans, transverse;, supra-occipital; vom, vomer. (After W. K. Parker.) 340 SECT. XII PHYLUM CHORDATA 341 rabbit the skeleton of the limbs is constructed on a general plan, common to the limbs of all Craniata but the fishes, and known as the pentadactyle, in allusion to tho five digits in which the limb typically terminates. In the pectoral limb the upper arm has a single long bone known as the hume^'us ; at its proximal end this is movably articulated with the pectoral arch. The forearm contains two long Fig. 214. — Diagrams of the fore (A) and hind (B) limbs with the limb-girdles, actb, acetabulum; gl, glenoid cavity; p. cor, procoracoid; I-V, digits. Cartilage bones — cn. I, cn.2, centralia; COR, coracoid; dst. 5-1, distalia; FE, femur; FI, fibula; fl, fibulare; HU, humerus; IL, ilium; int, intermedium; IS, ischium; mtcp. 1-5, metacarpals; mt. ts. 1-5, metatarsals; ph, phalanges; PU, pubis; RA, radius; ra, radiale; TI, tibia; ti, tibiale; UL, ulna; ul, ulnare, membrane bone ; CL, clavicle. bones — radius and ulna — articulating proximally with the distal end of the humerus. The skeleton of the hand con- sists of three principal parts, — the carpus, the metacarpus, 342 MANUAL OF ZOOLOGY SECT. and the phalanges. The carpus or wrist consists of a num- ber of small irregularly shaped bones arranged in two trans- verse rows, proximal and distal, with a central bone between the rows. The metacarpus consists of five narrow bones forming the support of the basal parts of the five digits, and articulating proximally with the distal row of carpals. The rest of the skeleton of the digit is formed of a row of small bones, the phalanges, the last of which — ungual phalanx — is modified in shape to support the horny claw. The skeleton of the hind-limb corresponds closely with that of the fore-limb. The pelvic arch consists on each side of three bones which become firmly united together, one of these, the ilium, is dorsal in position, the other two, puhis and ischium, are ventral, the pubis being anterior to the ischium. The ilia articulate firmly with the sacral region of the spinal column ; the pubes unite ventrally in an articu- lation known as the pubic symphysis, and in the lizard the ischia are similarly connected. Laterally where the three bones unite is a cup-like cavity — the acetabulum — which forms the socket for the head of the thigh-bone. The thigh has a single long bone, the femur. The leg has two bones, the tibia and fibula, the former, which is internal, being the larger of the two, and the latter in the rabbit not being distinct from the former towards the distal end. In the foot are. a number of tarsal bones correspond- ing to the carpals of the hand, a series of metatarsals corre- sponding to the metacarpals and a series of phalanges. When the skin of the trunk of the dogfish is removed there will be found immediately beneath it a thick layer of muscle. This is distinctly divided into segments or myomeres similar to those of Amphioxus, and this, with the division of the vertebral column into segments or vertebrae (which, however, do not exactly correspond in arrangement XII PHYLUM CHORDATA 343 with the myomeres), indicates that the body, like that of Nercds or an Arthropod, is metamerically segmented. In the lizard and rabbit the metamerism of the muscular sys- tem, though distinguishable at an early stage, becomes lost in the adult, and the muscles take on a much more compli- cated arrangement. On the jaws are a series of teeth, the function of which is to seize the food, and in the rabbit cut it into fragments, and crush it into yet smaller particles, in order to prepare it for the process of digestion. In the dogfish the teeth are numerous and of uniform character throughout, small with sharp points directed backwards. At their bases they are fixed to the surface of the cartilage of the jaw by means of dense fibrous tissue. In the lizard the teeth are also of uniform character {homodont dentitioii). They are of a simple conical shape, and fixed to the bone of the jaws. In the rabbit the teeth are distinctly visible into sets, dif- fering from one another in shape and function {heterodont dentitioii). Their bases are lodged in sockets or alveoli in the substance of the jaws. The structure of the tooth is the same in all three cases. The main mass of the tooth consists of dentine, a densely calcified material permeated by delicate parallel tubules. The free surface is covered with a layer of still harder mate- rial, the enamel, and the basal portion is covered with a layer of cement, which is similar in micro scopic structure to bone. PH 215. — Longitudinal section of a tooth, semi- diagrammatic. PH, pulpcavity; /’//’.open- ing of same; ZB, den- tine; ZC, cement; ZS, enamel. (FromWieder- sheim’s Vertebrata.') 344 MANUAL OF ZOOLOGY SECT. XII The anterior part of the cavity into which the mouth leads is the buccal cavity, the posterior part is the pharynx. On the floor of the buccal cavity is, in the lizard and in the rabbit, a mobile muscular prominence, the tongue, repre- sented in the dogfish by a much less prominent and little mobile process. From this a wide tube leads backwards to open into a spacious chamber, the stomach. From the stomach the intestine, a more or less coiled tube, leads eventually to the anal aperture. In the dogfish and in the lizard the anus opens into a chamber, the cloaca, which also receives the ducts of the urinary and reproductive organs. In the rabbit a cloaca is absent, and the anus is separate from the urino- genital opening. The mucous membrane of the enteric canal contains numerous glands, the secretions of which play an important part in digestion j the most important of these secretions is the gastric juice secreted by the glands of the stomach. In addition, special large digestive glands are present producing secretions, also having the function of acting on the various components of the food in such a way as to facilitate the passage of the useful ingredients from the cavity of the alimentary canal to the blood-vessels. In the rabbit these special large digestive glands are the salivary glands, the liver, and the pancreas ; in the dogfish and lizard the salivary glands are absent, though in the latter there are numerous small glands, the buccal glands, in the wall of the buccal cavity. The secretion of the salivary glands, the saliva, enters the cavity of the mouth through the ducts of the glands. It contains a ferment, ptyalin, which has the property of converting starch into sugar. The liver is in all three a relatively large organ, fixed by folds of peritoneum to the dorsal wall of the abdominal cavity and divided by fissures into a number of lobes. Its 345 346 MANUAL OF ZOOLOGY SECl, duct, the biU duct, conveys its secretion, the bile, into the most anterior part of the intestine known as the duo- denum. The duct gives off a diverticulum which expands into a rounded sac, the gall-bladder ; this acts as a recep- tacle for the bile when it is not required. The bile has an important action on the fatty matters of the food, converting them into an emulsion and decomposing a small proportion into glycerine and fatty acid. In addition to secreting the bile the liver has another function to perform : it acts as a storehouse for surplus carbohydrates absorbed from the food. The carbohydrates — compounds of the nature of starch and sugar — are converted in the liver into a sub- stance known as glycogen or animal starch, which becomes stored up in the cells to be given out again to the blood as it is required for nutrition during the intervals of fasting ; this function of the liver is known as the glycogenic function. The pancreas, which is a much smaller gland than the liver, produces a secretion, the pancreatic juice, which has the effect of converting starch into sugar, proteids into soluble modifications known as peptones, and of assisting in the emulsification of fats. The duct of the pancreas also opens into the duodenum. The nutrient matters of the food, rendered soluble by the action of the various digestive fluids, pass into the blood contained in the blood-vessels in the wall of the enteric canal, and are thus conveyed through- out the body to be distributed. The fatty matters, however, pass into a system of minute vessels — the lacteals — which ramify in the wall of the intestine. The lacteals are not blood-vessels, but belong to the lymphatic vascular system to be referred to presently. The lacteals combine together and in the rabbit open into a large trunk — the thoracic duct — by means of which the absorbed emulsion, or chyle as it is termed, is conveyed to one of the great veins. XII PHYLUM CHORDATA 347 The body-cavity in which the enteric canal and other organs are contained is lined with a membrane, the peri- toneum. This is reflected over the surface of the contained structures, and folds of it serve to suspend the various organs and connect them together. The best developed of these folds is the mese7itery (defective in the dogfish), by means of which the intestine is attached to the dorsal wall of the body-cavity. The organs of respiration of the dogfish are gills adapted for receiving oxygen from the air dissolved in sea-water ; those of the lizard and the rabbit are lungs adapted for breathing air directly. The movements of respiration have been already referred to. In the dogfish these movements have the effect of causing water to be taken in by the mouth, and to pass out from the pharynx to the exterior through the gill-slits. In passing out, the water flows over the gills, which are sets of vascular elevations on the walls of a series of five pairs of chambers — the branchial sacs opening internally into the pharynx, and externally communicating with the surrounding water through the branchial slits. In this way the needed oxygen is constantly being taken up, and the carbon dioxide given off. The walls of the branchial sacs are supported by the hyoid and branchial arches. Inspiration and expiration of air in the lizard and rabbit take place through the nostrils. The nasal chambers into which the nostrils lead communicate internally with the mouth-cavity or the pharynx through a pair of apertures known as the internal or posterior nares. On the floor of the pharynx behind the root of the tongue is a slit-like aperture, the glottis, opening behind into a chamber known as the larynx, the wall of which is supported by cartilages. From the larynx the air passes backwards along a tube, the trachea, the wall of which is supported by numerous rings 348 MANUAL OF ZOOLOGY SECT. of cartilage. The trachea bifurcates when it enters the body- cavity, each of the two branches, or bronchi as they Fig. 217. — Lacerta agilis. General view of the viscera in their natural relations. Bl, urinary bladder; Ci, post-caval vein ; ED, rectum; GB, gall-bladder; H, heart; Lg,Lg', the lungs; AT, stomach; MD, small intestine; Oe, oesophagus; Pn, pancreas; Tr, trachea. (After Wiedersheim.) are termed, passing to the corresponding lung. In the lizard the lung is in essence a thin-walled sac with elas- tic walls. In the wall of the sac immediately out- side, the delicate internal epithelium is a rich net- work of blood-vessels, into the blood contained in which oxygen from the air in the cavity of the lung readily passes, while the carbonic acid is at the same time given off. In the rabbit the lung is of much more complicated structure, but the essential relations are the same. In the lizard the lungs lie in the anterior part of the general body-cavity. In the rabbit the anterior part of the body-cavity, containing the lungs and the heart, is separated off from the posterior part, containing the greater por- tion of the enteric canal and other organs, by a muscular partition concave posteri- XII PHYLUM CHORDATA 349 orly, — the diaphragm, — the anterior portion of the cavity being known as the cavity of the thorax, and the posterior as that of the abdomen. , The air in the lungs, as it is constantly losing oxygen and gaining carbon dioxide, requires to be frequently renewed ; and the respiratory movements which have already been referred to are the movements indicative of this renewal j in the movement of respiration air is drawn into the lungs, which become fully distended j in that of expiration, the greater part of the air is driven out again, and the lung collapses. In the rabbit inspiration and expiration are effected by the movements of the ribs and of the diaphragm, by which the dimensions of the cavity of the thorax are increased or diminished. The blood-vascular system is highly developed in all the three examples. The blood is of a red colour, owing to the presence of red corpuscles containing a red colouring matter termed hcBmoglobin. The blood-vessels are of three kinds, — arteries, veins, and capillaries. The arteries have firm and elastic walls, which do not collapse when the vessel is empty; they contain arterial blood, i.e., blood which contains abundance of oxygen. The veins have thin, non- elastic walls which col- lapse when the vessel is empty and contain valves ; the con- tained venous blood is darker in colour than the arterial, and has been deprived of oxygen in the tissues. Both arteries and veins ramify extensively, the ultimate branches being of very small size. Connecting together the ultimate branches of the arteries and the ultimate branches of the veins is a system of microscopic vessels — the capillaries. The heart is ventral and anterior in position. In the dog- fish it will be found to lie in a space, the pericardial cavity, between the two rows of gills, and separated behind from the 350 MANUAL OF ZOOLOGY SECT. general body-cavity (abdomen) in which the majority of the internal organs are contained, by a transverse fibrous parti- tion. It consists of four chambers, — the sinus venosus, auri- cle, ventricle, and conus arteriosus. The venous blood enters the sinus venosus from the great veins and passes through the other three chambers in succession in the order given. All the chambers contract rhythmically, and by their con- tractions the blood is propelled from chamber to chamber, and finally driven out from the heart, its passage in the opposite direction being prevented by the presence of valves. These are placed in the openings leading from chamber to chamber, and are so arranged that while they permit the ready passage of the blood in the direction above given, they close up the opening when pressure is exerted in the opposite direction ; thus, for example, when the auricle contracts, the valve guarding the opening leading back into the sinus venosus closes that opening, while the valve in the opening leading into the ventricle opens freely, and the blood passes readily in that direction. The ventricle is by far the most muscular of the four chambers, since it is mainly by its contractions that the blood is forced through the system of vessels. The blood which is forced out from the heart by the contractions of the ventricle passes into a series of vessels which carry it all to the gills. Here it enters a system of capillaries in the gills, and these being separated from the surrounding water only by a thin mem- brane, oxygen readily enters the blood, and the carbon dioxide collected in the various tissues and organs of the body is given off. The blood then enters a set of larger vessels, which combine to form a large trunk, the dorsal aorta. Branches from this distribute blood to all parts of the body, where it enters the systems of capillaries, and whence it is carried back again to the heart by the veins. XII PHYLUM CHORDATA 351 In the lizard the heart and the circulation are somewhat more complicated than in the dogfish. There is a sinus venosus as before. The auricle is completely divided into two chambers, right and left, by a partition. Into the right auricle the sinus venosus drives the venous blood from the great veins ; into the left open the pulmonary veins, bring- ing the oxygenated blood from the lungs. Both the auricles open into the ventricle, the cavity of which is partly divided by a septum. From the ventricle are given off the main arteries {syste?nic arteries) which branch throughout all parts Fig. 218. — Diagram illustrating the course of the circulation in a fish. Vessels containing aerated blood, red; those containing non-aerated blood, blue; lym- phatics, black. B, capillaries of the body generally; E, of the enteric canal; G, of the gills; K, of the kidneys; L, of the liver; T, of the tail. a. br. a, afferent branchial arteries; au, auricle; c. a, conus arteriosus; d. ao, dorsal aorta; e. br. a, afferent branchial arteries; h. p. 7/, hepatic portal vein; h. v, hepatic vein; Ic, lacteals; ly, lymphatics; pr. cv. v, pre-caval veins; r. p. ?/, renal portal veins; s. v, sinus venosus; v, ventricle; v. ao, ventral aorta. The arrows show the direction of the current. of the body, and the pulmonary arteries, which pass direct to the lungs. By various arrangements of the parts which need not be described at present, the venous blood from the right auricle is mainly guided into the pulmonary arte- ries, and passes to the lungs to obtain oxygen and part with its carbon dioxide ; while the arterial blood is mainly guided 352 MANUAL OF ZOOLOGY SECT. to the systemic arteries. A certain degree of mixing, how- ever, of the venous and arterial currents takes place as they pass through the ventricle. In the rabbit this mixing of the arterial and venous cur- rents is entirely prevented, owing to the ventricle being completely divided into two chambers — right and left. The right auricle opens into the right ventricle, and fills it with venous blood from the great veins. From the right auricle the blood is driven through a pulmonary artery to the lungs. From the lungs the oxygenated blood is returned by means of the pulmonary veins to the left auricle ; from the left auricle it enters the left ventricle, and from the latter is driven out through the system of systemic arteries to all parts of the body. There are thus two distinct cur- rents of blood constantly passing simultaneously through the heart, but entirely cut off from one another, viz., a venous current on the right side and an arterial on the left. The blood of the rabbit has a much higher temperature than that of the dogfish or lizard. In all the three examples the veins which carry the venous blood towards the heart from the stomach, intestine, and pancreas unite together to form a large vein, the hepatic portal, which ramifies in the substance of the liver, and forms the main source of’ the blood supply of that organ. In the dogfish and lizard, but not in the rabbit, veins con- vey blood from the posterior region to the kidneys, forming what is termed a renal portal system. The nervous system is highly developed. The central nervous system consists of the brain and spinal cord. The brain is, as already stated, contained in the cavity of the cranium ; the spinal cord, continuous with the posterior end of the brain, extends through the neural canal roofed over by the series of neural arches of the vertebrae. XII PHYLUM CHORDATA 353 The spinal cord is similar in essential respects in all three examples. It is a cylindrical cord of nerve matter, having running along the middle of its dorsal surface a fissure, the Fig, 219. — Dorsal view of the brain of Scyllium canicula. The posterior division of the brain is the medulla oblongata (NH) , on the dorsal surface of which is shown one of the central ventricles {F. rho). The large cerebellum {HH) nearly covers the optic lobes {MH), The diencephalon {ZH) shows in the middle one of the central ventricles, and the place of attachment of the pineal body {GJ>). The prosencephalon ( VH) gives off the olfactory lobes {Tro, L. ol). The following nerves are shown ; optic (//), trochlear (IV), trigeminal ( facial {VH), auditory {VIII), glossopharyngeal (iJc), and vagus {X). (From Wiedcrsheim.) dorsal longitudinal fissure, and along the middle of its ven- tral surface, a second fissure, the ventral longitudinal fissure. 2A 354 MANUAL OF ZOOLOGY SECT. Through its substance from end to end runs a narrow canal, the central canal. In the brain of the dogfish the most anterior portion is a thick mass of nerve matter indistinctly divided into two lateral portions by a shallow depression. This is the pro- sencephalon of the fore-brain. A pair of lobes given off from this in front are the olfactory lobes. The prosencepha- lon with a narrow region, diencephalon or thalamencephalon, behind it, constitute the fore-brain. Behind the fore-brain a pair of oval lobes, the optic lobes, constitute the dorsal portion of the mid-brain, which comprises, in addition, a thick mass of longitudinal nerve-fibres, lying below, and connecting the hind-brain with the fore-brain. An elon- gated median mass, indistinctly divided into lobes, is the cerebellum, the anterior portion of the hind-brain. The posterior division of the hind-brain, — medulla oblongata, — broad in front, tapers posteriorly where it passes into the spinal cord. The central canal of the spinal cord expands in the me- dulla oblongata into a wide shallow cavity, roofed over only by a thin membrane ; this is known as the fourth ventricle. From this runs forwards a narrow passage, the iter or aque- duct of Sylvius, expanding in front in the thalamencephalon into a laterally compressed cavity, the third ventricle. From this are given off a pair of lateral ventricles, passing into the prosencephalon, each giving off a prolongation into the corresponding olfactory tube. The roof of the third ventricle is very thin ; it is pro- duced into a slender process — the epiphysis or pineal body. Its side walls are formed of two masses, the optic thalami; its floor is produced into a hollow prolongation, the infun- dibulum, to the end of which a vascular body, the hypophysis or pituitary body is applied. XII PHYLUM CHORDATA 355 In the brain of the lizard the same parts are recognisa- ble as in the dogfish, the chief differences being that the prosencephalon is deeply divided by a median longitudinal fissure into two lobes, the cerebral he7nispheres, and that the cerebellum is very small. In the rabbit also we rec- ognise the same parts. But the whole brain is larger in pro- portion to the bulk of the body ; the cerebral hemispheres are much more highly developed, and the cerebellum is not only of large relative size, but is of complicated structure. The peripheral nervous system consists of the spinal and cerebral nerves given off from the spinal cord and the brain respectively, with their ramifications through all parts of the body. A pair of spinal nerves emerge from the neural canal between each adjoining pair of vertebrae. Each spinal nerve arises from the .spinal cord by two roots — a dorsal and a ventral ; the former is dilated into a ganglion. Experiments prove that the dorsal root contains the sensory fibres of the nerves, i.e., those fibres which are concerned in carrying impulses from the various parts to the nerve centres to be translated in consciousness into sensations. When, for example, the skin of some part of the body is touched, the impulse by means of which we become con- scious of the contact passes from the surface through branches of the spinal nerves, and enters the spinal cord through the dorsal root, in order to be transmitted to the brain. The ventral root, on the other hand, contains the motor fibres ; the fibres through which impulses which lead to the contraction of muscles pass outwards from the central nervous system. More or less extensive intercommunications take place between the spinal nerves that are situated opposite the origin of the limbs ; these spinal nerve plexuses give off the nerves to the limbs. 356 MANUAL OF ZOOLOGY SECT, The cerebral or cranial nerves correspond pretty closely in their general arrangement in the three examples. The olfactory nerve-fibres, which originate from the olfactory lobes, the optic nerves, which are derived from the thalamen- cephalon, and the auditory nerves which originate from the medulla oblongata, are the nerves of the special senses of smell, sight, and hearing respectively, the first ending in the epithelium of the nasal cavities, the second in the retina of the eye, and the third in the epithelium of the interior of the inner ear. Other cranial nerves supply the muscles that move the eyeball, the skin of the head, the muscles of the jaws, the tongue, pharynx, heart, stomach, etc. The structure of the eye is in all essential respects the same in all the three examples ; such diiferences as there are will be referred to later. The eye of a bullock or a sheep, being larger, may with advantage be substituted. The eyeball is globular, and is encased in a rough opaque capsule, the sclerotic. It lies in the cavity of the orbit, and is capable of being turned about in various directions by a number of muscles inserted into it. On the side of the eyeball directed towards the light, the opaque sclerotic is replaced by a transparent membrane, the cornea, which forms a window through which the rays of light enter the eye. Within the sclerotic is a more delicate pigmented layer, the choroid. Towards the cornea the choroid passes into a circular pigmented diaphram, the iris, the opening of which is known as the pupil. Through the pupil, the size of which is capable of being increased or diminished, the light is admitted into the interior of the eye. The sen- sitive part of the eye, the part on which the image produced by the rays of light proceeding from an object must fall in order to produce the sensation of sight, is a soft gray layer lining that part of the cavity of the eye which lies within the XII PHYLUM CHORDATA 357 iris. The rays of light are brought to a focus on the retina mainly by means of the crystalline lens, a firm, glassy body situated within the iris. The cornea also assists in this, as does a gelatinous substance, the vitreous humour, which fills the part of the cavity of the eyeball internal to the lens. The ear in the dogfish is imbedded in the cartilage of the posterior part of the skull (auditory region). It con- sists of a somev/hat complicated structure termed the Fig. 220. — Diagrammatic horizontal section of the eye of man. c, cornea; ch, choroid (dotted); C. P, ciliary processes; e. c, epithelium of cornea; e. cj, conjunctiva; f. c, yellow spot; /, iris; L, lens; OJP, optic nerve; OS, ora serrata; o-jc, optic axis; p. c. R, anterior non-visual portion of retina; P. E, pigmented epithelium (black); R, retina; sp. I, suspensory ligament; Scl, sclerotic; F. vitreous body. (From Foster and Shore’s membranous labyrinth, with soft walls and an internal epithelium in which the fibres of the auditory nerve termi- nate. Contained in the interior of the labyrinth is a fluid, the endolymph, in which there are suspended particles of 358 MANUAL OF ZOOLOGY SECT. carbonate of lime, the otoliths. In the lizard and rabbit there are superadded to this, the essential part of the ear, certain accessory parts. The most important of these is the tympanu7n or drum of the ear. This is a cavity to the outside of the auditory region of the skull (the region in which the membranous labyrinth is enclosed). The tym- panum communicates with the pharynx through a passage known as the Eustachian passage. Externally the cavity of the tympanum is closed by a tense, drum-like membrane, the tympanic membrane. The tympanic membrane is set in vibration by the waves of sound, and the vibrations are transmitted across the tympanic cavity by a slender rod of bone (in the lizard) or a chain of minute bones (in the rabbit). The inner end of the rod or chain of bones is inserted into a membrane covering over a small aperture in the outer wall of the auditory region of the skull, which forms the inner wall of the tympanic cavity, and by this means the vibrations are communicated to the endolymph of the membranous labyrinth and affect the terminations of the auditory nerve-fibres. In the lizard the tympanic membrane is nearly on a level with the skin of the head, and its position is conspicuously indicated by a brown patch situated behind the eye. In the rabbit the tympanic membrane is more deeply sunk, and a wide passage, the passage of the outer ear, leads to it from the exterior. The ear of the rabbit also differs from that of the lizard in the presence of the prominent auricle or pinna of the ear to which reference has been already made. The kidneys, or organs of renal excretion, though they differ in form in the three examples are not widely different in essential structure. Their function is the secretion of urine, which consists of water containing various nitrogenous waste matters in solution. Essentially the kidney is a mass XII PHYLUM CHORDATA 359 of tubules by whose agency the process of. secretion is car- ried on, the whole being richly supplied with blood-vessels. Eventually the tubules open into a duct, the ureter. In the lizard and the rabbit there is present a median thin-walled sac, the urinary bladder, in which the urine is stored, to be discharged at intervals. In the rabbit the ureters open into the bladder, and the latter opens on the exterior by a median canal, the urethra. In the lizard the ureters and the bladder have independent openings into the cloaca, and the bladder is filled only by regurgitation from the latter chamber. The sexes are distinct in all three. There are two testes, each with its duct or vas deferens. In the female there are two ovaries, which are solid bodies in which the ova lie im- bedded. In the dogfish, when mature, the ova are of large size, containing a great quantity of food-yolk. The ova of the rabbit are extremely small, while those of the lizard are of a size intermediate between those of the other two. Each ovum is enclosed in a follicle — the Graafian follicle — with a wall composed of small cells. When the ovum approaches maturity the follicle projects on the surface of the ovary, and eventually the wall becomes ruptured and the ovum escapes into the body-cavity. The oviducts, of which there are two, are not connected with the ovaries, each opening anteriorly into the body- cavity by a wide opening. In the dogfish and the lizard the oviducts remain practically distinct from one another throughout; in the rabbit the posterior parts are united to form a median chamber, the body of the uterus, and a median passage, the vagina, leading to the exterior. The ova in all three, when discharged from the ovaries, enter the wide openings of the oviducts and are impregnated during their passage backwards. In both the dogfish and the lizard each fertilised ovum becomes enclosed while in the oviduct MANUAL OF ZOOLOGY SECT. 360 in a tough shell, and is discharged when development has only begun. In the rabbit the fertilised ovum is received into the uterus and there undergoes its development, the young rabbit when born differing little, save in size, from the adult. The nourishment of the foetus or uterine young of the rabbit is effected by means of a special vascular structure known as the placenta, by means of which nutrient material passes from the blood of the mother to that of the foetus ; and after birth the young rabbit receives its nourishment for a time exclusively from the secretion of a set of glands of the mother — the manvnary or milk glands. CLASS I. CYCLOSTOMI The lowest of existing Craniate Vertebrates are certain fish-like animals known as “lampreys” and “hag-fishes,” or “ slime-fishes,” which are looked upon as constituting the class of Craniata, to which the name of Cyclostomi is ap- plied. Of them it is here possible only to make the briefest mention. The lampreys {^Petromyzon and other genera) and the hag-fishes or slime-fishes (Myxine and Bdellostoma) are somewhat eel-like in general shape, that is to say, they have a long and narrow body without marked external dis- tinction into regions, and with a soft and slimy integument. Of the fins of such a fish as the dogfish the median or un- paired series alone are represented, paired fins corresponding to the limbs of the higher Craniata being entirely absent. There is a dorsal fin divided into two in the lampreys, undi- vided in the hag-fishes, which is continued as a tail fin round the posterior or caudal extremity of the body. On the lower or ventral surface of the anterior or head-end is a deep hollow — the buccal funnel, much more conspicuous in the lampreys than in the hags, at the bottom of which the small XII PHYLUM CHORDATA 361 opening of the mouth is situated. There are no jaws, but on the inner surface of the buccal funnel and on the tongue — a fleshy (?) process below the opening of the mouth. In Myxine the funnel is edged with slender, flexible processes or tentacles. At the sides of the head are the eyes, well developed and conspicuous in the lamprey, imperfect and buried beneath the skin in Myxine, and on the upper surface is a single median aperture, the nostril. Further back at na.ap Fig. 221. — Petromyzon marinus. Ventral (A), lateral (B), and dorsal (C) views of the head. r/. /, first gill-cleft; buccal funnel ; eye, eye', mouth; na. ap, nasal aperture; /, papilla : pn, pineal area; 1. 1, t. 2, t. 3, teeth of buccal funnel; t.4, teeth of tongue. (After W. K. Parker.) bncht the sides of the head are, in the lamprey, a series of seven pairs of slits, the gill-slits, leading to the gill-pouches ; in Bdellostoma there are six pairs of small gill-slits, in Myxine only a single aperture on each side. The skeleton is very unlike that of the true fishes, and 362 MANUAL OF ZOOLOGY SECT. XII is in some respects extremely primitive. The spinal col- umn is represented merely by a thick persistent notochord, enclosed in a sheath, with, in the lampreys, small carti- •mth Fig. 222. — Head of Myxine glutinosa (A) and of Bdellostoma forsteri (B), from beneath, br. ap, branchial aperture; hr. cl. i, first branchial cleft; mth, mouth; na. ap, nasal aperture ; oes. ct. d, oesophageo-cutaneous duct. The smaller open- ings in A are those of the mucous glands. (After W. K. Parker.) laginous processes representing neural and haemal arches. The skull is cartilaginous, and is peculiarly modified. Be- hind it in the lamprey is a remarkable basket-like apparatus, 363 364 MANUAL OF ZOOLOGY SECT, composed of cartilaginous processes. This branchial basket, as it is termed, supports the gill-sacs. The gill-sacs, of which there are either six or seven pairs, are the organs of respiration, representing the gills of the true fishes. In the lamprey each of these communicates with the exterior by the corresponding gill-slit, and inter- nally opens into a common passage, the respiratory tube which leads in front into the buccal cavity. In Bdellostoma each gill-pouch has its own internal opening through a narrow tube into the pharynx, as well as its external open- ing through a small gill-slit. In Myxine, on the other hand, though each pouch has a separate internal commu- nication with the pharynx, the tubes leading outwards from the gill-pouches of each side all join to form a common tube, which opens on the exterior by the single gill-slit. The other systems of organs are not so remarkable. The alimentary canal, the heart, and the brain are not widely different from those of the true fishes. A peculiar feature is that there is only a single nasal sac (opening by the single nasal aperture already referred to) instead of the pair developed in all other Craniates ; in Myxine its cavity com- municates by a passage with the cavity of the mouth. In the lamprey, in addition to paired eyes having the typical vertebrate structure, there is connected with a lobe in the roof of the fore-brain a median or pmeal eye of simpler structure and imperfectly understood function. Lampreys live mainly in rivers and estuaries. Their food consists chiefly of small aquatic animals, such as worms, small crustaceans, etc. ; but they also sometimes attach themselves to the bodies of fishes, by means of the sucker- like buccal funnel, and rasp off portions of the flesh with the horny teeth of the tongue. Myxine actually makes its way into the interior of the bodies of large fishes, such as the XII PHYLUM CHORDATA 365 cod, consuming the flesh in its passage, and thus becomes for a time an internal parasite — the only example among the Vertebrata of such a condition. In the free state Myxine usually lies buried in the sand, with only the anterior end, with the nasal aperture, projecting on the surface. By means of the passage leading from the nasal sac to the mouth, water passes in and out through the nasal aperture, and the process of respiration is carried on while the ani- mal remains almost completely hidden. The geographical distribution of the Cyclostomi is some- what remarkable. Petrofnyzon is found on the coasts and in the rivers of Europe, North America, Japan, and West Africa. Of the allied genera one, Ichthyomyzon, occurs on the western coast of North America ; another, Mordacia, in Tasmania and Chili; a third, Geotria, in the rivers of Chili, Australia, and New Zealand. Myxine occurs in the North Atlantic and on the Pacific Coast of South America, includ- ing the Straits of Magellan ; Bdellostoma on the coasts of South Africa, New Zealand, and Chili. CLASS II. PISCES The class Pisces or Fishes includes the Elasmobranchii or cartilaginous fishes (sharks, dogfishes, and rays), the Teleo- stomi or bony fishes (such as perch, pike, mackerel, cod, sole, salmon, sturgeon, and bony pike), and the Dipnoi or lung-fishes. In these the organs both of respiration and of locomotion are adapted for an aquatic mode of life. The chief and, in the majority, the only organs of respiration are the gills, which are in the form of series of vascular processes attached to the branchial arches and persisting throughout life. The organs of locomotion are the paired pectoral and the pelvic fins, and the unpaired dorsal. 366 MANUAL OF ZOOLOGY SECT. ventral, and caudal ; these are all supported by fin-rays of dermal (p. 372) origin. A hard external covering of scales developed in the dermis is usually present. In the en- doskeleton the notochord is usually replaced more or less completely by cartilaginous or bony vertebrae ; there is a well-developed skull and a system of well-formed visceral arches, of which the first forms the upper and lower jaws, the latter movably articulating with the skull, and both nearly always bearing teeth. An air-bladder is frequently present, and in certain exceptional cases acquires the function of a lung or chamber for breathing air. Sub-class I. Elasmobranchii A dogfish may be selected as a convenient example of the sub-class and of the class Pisces. Dogfishes occur at slight depths off the coasts in all quarters of the globe. The commonest European forms are the rough hound {Scyllium Canicula), the lesser spotted dogfish {S. caiulus), the piked dogfish {Acanlhias vulgaris), and the smooth hound {Mustclus vulgaris). Allied species of the southern hemi- sphere are Scyllium, Acanlhias, and Mustclus anarcticus. On the coast of Northeastern America the common dogfish is Mustclus canis. For the description which follows, any of these species will be found to serve very well. A slight general account of the dogfish has already been given in the introduction to the Craniata ; this has now to be extended and supplemented. The general shape (Fig. 206) may be described as fusiform ; at the anterior or head-end it is broad and depressed ; posteriorly it tapers gradually and is compressed from side to side. The head terminates anteriorly in a short blunt snout. The tail is narrow and bent upwards towards the extremity. The entire surface is XII PHYLUM CHORDATA 367 covered closely with very minute hard placoid scales or dermal teeth somewhat larger on the upper surface than on the lower. These are pointed, with the points directed somewhat backwards, so that the surface appears rougher when the hand is passed over it forwards than when it is passed in the opposite direction. When examined closely, each scale is found to be a minute spine situated on a broader base. The spine consists of dentine covered with a layer of enamel ; the base is composed of bone, and the whole scale has thus the same essential structure as a tooth. Along each side of the head and body runs a faint depressed longitudinal line or slight narrow groove, — the lateral line. As in fishes in general, two sets of fins are to be recognised, — the unpaired or median fins, and the paired or lateral. These are all flap-like outgrowths, running vertically and longitudinally in the case of the median fins, nearly horizontally in the case of the lateral ; they are flexible, but stiffish, particularly towards the base, owing to the presence of a supporting framework of cartilage. Of the median fins, two — the dorsal — are situated, as the name indicates, on the dorsal surface : they are of triangular shape ; the anterior, which is the larger, is situated at about the middle of the length of the body, the other a little further back. The caudal fringes the tail ; it consists of a narrower dorsal portion and a broader ventral, continuous with one another round the extremity of the tail, the latter divided by a notch into a larger, anterior, and a smaller, posterior lobe. The tail is heterocercal, i.e., the posterior extremity of the spinal column is bent upwards and lies in the dorsal portion of the caudal fin. The ventral or so-called anal fin is situated on the ventral surface, opposite the interval between the anterior and posterior dorsals ; it resembles the latter in size and shape. 368 MANUAL OF ZOOLOGY SECT. Of the lateral fins there are two pairs, the pectoral and the pelvic. The pectoral are situated at the sides of the body, just behind the head. The pelvic, which are the smaller, are placed on the ventral surface, close together, in front of the middle of the body. In the males the bases of the pelvic fins are united together in the middle line, and each has connected with it a clasper or copulatory organ. The latter is a stiif rod, on the inner and dorsal aspect of which is a groove leading forwards into a pouch-like depression in the base of the fin. The mouth — a transverse, somewhat crescentic opening — is situated on the ventral surface of the head, near its anterior end. In front and behind it is bounded by the upper and lower jaws, each bearing several rows of teeth with sharp points directed backwards. The nostrils are sit- uated one in front of each angle of the mouth, with which each is connected by a wide groove, the nasobuccal groove. A small rounded aperture, the spiracle, — placed just behind the eye, — leads into the large mouth-cavity or pharynx. Five pairs of slits running vertically on each side of the neck, the branchial slits, also lead internally into the mouth- cavity. A large median opening on the ventral surface at the root of the tail, between the pelvic fins, is the opening leading into the cloaca, or chamber forming the common outlet for the intestine and the renal and reproductive organs. A pair of small depressions, the abdominal pores, situated behind the cloacal opening, lead into narrow passages open- ing into the abdominal cavity. The skeleton is composed entirely of cartilage, with, in certain places, depositions of calcareous salts. As in Verte- brates in general, we distinguish two sets of elements in the skeleton, — the axial set and the appendicular, the former comprising the skull and spinal column, the latter the limbs and their arches. XII PHYLUM CHORDATA 369 The spinal column is distinguishable into two regions, — the region of the trunk and the region of the tail. In the trunk region each vertebra (Fig. 209, A) consists of a centrum {c), neural arch (na), and transverse processes (tr.pr). In the caudal region there are no transverse processes, but inferior or hcemal arches (D, h. a) take their place. The centra of all the vertebrae are deeply biconcave or amphi- ccelous, having deep conical concavities on their anterior and posterior surfaces. Through the series of centra runs the notochord, greatly constricted in the centrum itself, dilated in the large spaces formed by the apposition of the amphicoelous centra of adjoining vertebrae. The concave anterior and posterior surfaces of the centra are covered by a dense calcified layer, and eight radiating lamellae of calcified material run longitudinally through the substance of the centrum itself. Each neural arch consists of a pair of rod-like neural processes, which form the sides, and two pairs of compressed neural plates (one placed opposite the centrum, the other or intercalary cartilage, opposite the interval between adjoining centra), which form the roof of the arch, together with usually two nodules — the repre- sentatives of neural spines — which form the keystones. The transverse processes are very short : connected with each of them is a cartilaginous rudimentary rib about half an inch in length. The cranium (Fig. 224) is a cartilaginous case, the wall of which is continuous throughout, and not composed, like the skulls of higher vertebrates, of a number of distinct ele- ments (bones) fitting in together. At the anterior end is a rostrum, consisting of three cartilaginous rods converging as they extend forwards and meeting at their anterior ends. At the sides of the base of this are the olfactory capsules {olf cp~), — thin rounded cartilaginous sacs opening widely 370 MANUAL OF ZOOLOGY SECT. below, — the cavities of the two capsules being separated from one another by a thin septum. The part of the roof of the cranial cavity behind and between the olfactory capsules is formed, not of cartilage, but of a tough fibrous membrane, and the space thus filled in is termed the anterioi' fontanelle ; in contact with the lower surface of the membrane is the pineal body, to be afterwards mentioned in the account of the brain. Each side wall of this part of the skull presents a deep concavity, the orbit over which is a ridge-like prominence, the supra-orbital crest, terminating anteriorly and posteriorly in obscure processes termed respectively the pre-orbital and post-orbital processes. Below the orbit is a longitudinal infra-orbital ridge. Behind the orbit is the auditory region of the skull {and. cp), a mass of cartilage in which the parts of the mem- branous labyrinth of the internal ear are embedded. On the Upper surface of this posterior portion of the skull are two small apertures situated in a mesial depression. These are the openings of the aqueductiLs vestibuli {endolymphatic ducts), leading into the vestibule of the membranous laby- rinth. Behind this again is the occipital region, forming the posterior boundary of the cranial cavity, and having in the middle a large rounded aperture, the foramen magnimi, through which the spinal cord contained in the neural canal and protected by the neural arches of the vertebrae becomes continuous with the brain, lodged in the cranial cavity. On either side of this is an articular surface, the occipital condyle, for articulation with the spinal column. A number of smaller apertures or foramina, chiefly for the passage of nerves, perforate the wall of the skull. In close Connection with the cranium are a number of cartilages composing the visceral arches (Fig. 224). These are incomplete hoops of cartilage, mostly segmented, which brr 372 MANUAL OF ZOOLOGY SECT. lie in the sides and floor of the mouth-cavity or pharynx. The first of these forms the upper and lower jaws. The upper jaw, or palato-quadrate (?/ consists of two parts, — a tarsal region directed downwards from the heel-joint, and five long, slender digits united by thin folds of skin or webs. Thus the limbs are placed in such a way that the elbow and knee face one another, and the first digit — that of the hand probably rep- resenting the second or index-finger, that of the foot, the 410 MANUAL OF ZOOLOGY SECT. hallux or great toe — is turned inwards or towards the median plane of the body. The skin is soft and slimy, owing to the secretion of mucous glands ; there is no trace of an exoskeleton. The vertebral column (Fig. 247) is remarkable for its extreme shortness ; it consists of only nine vertebrae (V. i- V. 9), the last followed by a slender, bony rod, the urostyle (UST). The second to the seventh vertebrae are similar in character. The centrum (B, cn') is somewhat depressed, and has a concave anterior and a convex posterior face — a form known as procodous. Each half of the neural arch consists of two parts, — a pillar-like pedicle {pd) springing from the centrum and extending vertically upwards, and a flat, nearly horizontal lamina (^Im), forming, with its fellow, the roof of the neural canal. The zygapophyses {a. zyg) or yoking processes are far better developed than in any fish. Laterally the neural arch gives off on each side a large outstanding transverse process {tr. pr) ; its crown is produced into a very small and inconspicuous neural spine. The first or cervical vertebra (V. i) has a very small centrum and no transverse processes. There are no anterior zygapophyses, but at the junction of centrum and arch there occurs on each side a large oval concave facet for articulation with one of the condyles of the skull (see p. 411). The eighth vertebra has a biconcave centrum ; that of the ninth or sacral vertebra (V. 9) is con- vex in front and presents posteriorly a double convexity articulating with a double concavity on the anterior end of the urostyle. The skull (Figs. 247, 248) consists of a. nsLvrow brain-case, produced behind into great outstanding auditory capsules, and in front into large olfactory capsules. The whole of the bones of the upper jaw are immovably fixed to the cranium XII PHYLUM CHORDATA 411 so that the only free parts are the lower jaw and a small plate, the hyoid apparatus, partly bony and partly cartilaginous, which supports the tongue, and is the sole representative of the entire visceral or gill-bearing skeleton of the fishes. A cartilaginous cranium comparable with that of the dog- fish, but very thin and delicate, forms the foundation of the skull of the frog ; but superadded to this are a number of cartilage bones — or bones which replace portions of the cartilage, and 7nembrane bones — or bones which are formed in membrane, independently of the cartilage. There are five cartilage bones, the paired exoccipitals and pro-oiks, and the median sphenethmoid. The exoccipitals (EX. OC) lie in the posterior or occipital region of the skull and bound the \Bxgeo-^e.nmgor foramen magnufn {for. mag') at the posterior end of the skull through which the spinal cord, contained in the neural canal enclosed by the neural arches of the vertebrae, becomes continuous with the brain, contained in the cavity of the cranium. Below the foramen magnum are a pair of oval projections, the occipital condyles {oc. cn), for articu- lation with the first vertebra of the spinal column. The second pair of cartilage bones, the pro-otics (PR. OT), are ossifications in the roof of the auditory capsule, situated just in front of the exoccipitals, with which they become firmly united. The sphenethmoid (SP. ETH) is a peculiar ossification of somewhat complex form, which lies partly in the wall of the anterior portion of the cranial cavity, partly in the wall of the posterior portions of the nasal cavities or olfactory sacs. To this cartilaginous cranium with its cartilage bones certain membrane bones are added. Covering the roof of the brain-case are two long narrow bones called the fronto-parietals {FR. FA), because they are formed by the V Fig. 247. temporaria. A, the skeleton from the dorsal aspect; the left half of the shoulder girdle and the left fore and hind limbs are removed, as also are the membrane bones on the left side of the skull. Cartilaginous parts dotted. Names of cartilage bones in thick, those bones in italic capitals, a. c. hy, anterior cornu of hyoid: actb, acetabulum; AST, astragalus; b. hy, basi-hyal; C, calcar: CAL, calcaneum; EX. OC, ex-occipital; FE, femur; fon. fon , fontanelles; FR. PA, fronto-parietal; HU, humerus; IL, ilium; MX, maxilla; ol. cp, olfactory capsule; ct. pr, otic process; p. c. hy, posterior cornu of hyoid: PMX, premaxilla; PR. OT, pro-otic; RA. UL, radio-ulna; SP. ETH, sphenethmoid; SQ, squamosal; S. SCP, supra-scapula; j?.; j, suspensorium ; TI. FI, tibio-fibula; /r, transverse process; UST, iirostyle; V. i, cervical vertebra; V. 9, sacral vertebra; VO, vomer; I-V digits. B, the fourth vertebra, anterior face. a. zyg, anterior zygapophysis; centrum; lamina; n. sp, neural spine; pd, pedicle; tr. pr, transverse process. (After Howes, slightly altered.) SECT. XII PHYLUM CHORDATA 413 union of a pair of frontals in front with a pair of parietals behind. Over the olfactory capsules are a pair of triangular nasals (NA), and applied to their ventral surfaces small paired vomers (VO). On the ventral surface of the skull is a large T-shaped parasphenoid {PA. SPH), its stem underlying the base of the cranium, while its two arms ex- tend outwards beneath the auditory capsules. 414 MANUAL OF ZOOLOGY SECT. The palato- quadrate cartilage is unossified ; but in relation to its anterior portion is a palatine {^PAL) membrane bone and to its posterior portion a pterygoid {^PTG) . The former is a slender rod-like bone directed transversely on the lower surface of the skull. The latter is a larger three-rayed bone, having an anterior, an inner, and a posterior arm. The posterior portion of the palato-quadrate cartilage, the quad- rate or suspensorium (sus), connects the lower jaw with the posterior region of the skull. Internally it is covered over by the inner and posterior arm of the pterygoid ; externally a hammer-shaped membrane bone, the squamosal (SQ) is applied to it. The upper jaw is formed by three mem- brane bones, the small premaxilla (PMX) in front, then the long narrow maxilla {MX), and -finally the short quad- rato-jugal {QU JU) which is connected posteriorly with the quadrate. The mandible contains on each side a per- sistent Meckel’s cartilage as a sort of core, ossified at its anterior end, outside which are two membrane bones. The hyoid is a squarish plate of cartilage {l>. hy) with two pairs of processes {a.e. hy,p. c.hy), the posterior of which is ossified. > The scapula is ossified and is connected by its dorsal edge with a supra-scapula (Fig. 247, S. SCP) formed partly of bone, partly of calcified cartilage, and developed from the dorsal region of the embryonic shoulder-girdle. The coracoid (Fig. 249, Co) is also ossified, while the procora- coid is represented by a bar of cartilage, having a membrane bone, the clavicle {Cl), closely applied to it. The supra- scapula overlaps the anterior vertebrae j the coracoid and procoracoid are connected ventrally by a cartilage, the epi- coracoid {Co'), which is in close contact with its fellow of the opposite side in the middle ventral line, so that the entire shoulder-girdle, like that of the dogfish, forms a single inverted arch. XII PHYLUM CHORDATA 415 Passing forwards from the anterior ends of the united epicoracoids is a rod of bone, the episternum {£p), tipped by a rounded plate of cartilage, the omosternum ; and passing backwards from their posterior ends is a similar but larger bony rod, the sternum {St), also tipped by a cartilaginous plate, to which the name xiphisternum {Kn) is applied. Fig. 249. — Rana esculenta. The shoulder girdle from the ventral aspect. Co, coracoid; Co', epicoracoid; Cl, clavicle; G, glenoid cavity; Ep, episternum; Fe, fenestra between procoracoid and coracoid; KC, cartilage separating scapula and clavicle; Kn, xiphisternum; m, junction of epicoracoids; S, scapula; St, sternum. (From Wiedersheim’s Comparative Anatomy.') The four limbs deviate from the typical structure (p. 341) chiefly in the fusion of the radius and ulna into a single radio-ulna (Fig. 247, RA. UL) and in the presence of only four complete digits with a vestigial one on the radial side. In all probability the latter represents the pollex, and the 41 6 MANUAL OF ZOOLOGY sect. complete digits are the second to the fifth of the typical hand. Six carpals only are present. The pelvic arch (Figs. 247 and 250) is very peculiarly modified ; it re- sembles in form a bird’s ‘‘ merry- thought,” consisting of two long curved bars articulating in front with the transverse processes of the sacral vertebra and uniting posteriorly in an irregular vertical disc of mingled bone and cartilage which bears on each side a deep, hemispherical acetabuluifi (Fig. 250, G) for the ar- ticulation of the thigh-bone. The curved rods are the ilia {II, P) ; they expand posteriorly and unite with one another in the median plane to form the dorsal portion of the disc and about one-half of the acetabulum. The posterior portions of the disc and the acetabulum are furnished by the ischia {Is), fused with one another in the sagittal plane, and their ventral portions by the similarly united pubes {Kn). The ilium and ischium are formed of true bone, the pubis of calcified cartilage ; the union of the elements in the median plane is called the symphysis. In the hind-limb the tibia and fibula are fused to form a single tibio-fibula (Fig. 247, TI. FI), and the two bones in the proximal row of the tarsus, namely, the tibiale or astraga- lus (AST) and the fibulare or calcaneum (CAL), are greatly elongated and provide the leg with an additional segment. There are three tarsals in the distal row, one of which ap- pears to represent a central, another the first distal carpal, and the third the fused second and third. There are five Fig. 250. — Rana esculenta. Pelvic girdle from the right side. G, acetabulum; //, /*, ilium; /j, ischium; /iTw, pu- bis. (From Wiedersheim’s Comparative A natomy.) XII PHYLUM CHORDATA 417 well-developed digits, and on the tibial side of the first is a spur-like structure or calcar {€), formed of three bones, a metatarsal and two phalanges : such a rudimentary digit is called a pre-hallux. The mouth leads into a wide buccal cavity having in its roof the posterior nares (Fig. 251,/. a pair of projec- tions due to the downward bulging of the large eyes, and the crb.h eus.t Fig. 251. — Rana temporaria. Dissection from the left side; the viscera somewhat displaced, an, anus; b. d, bile duct; b. hy, body of hyoid; bl, urinary bladder; bV , its opening into cloaca; c.art, conus arteriosus; cblm, cerebellum; cl, cloaca; cn. 3, centrum of third vertebra; cp. ad, corpus adiposum; crb. h, cere- bral hemisphere; d. ly. s, dorsal lymph sinus; if?/, duodenum; ep. cor, opicora- coid; C7CS. t, Eustachian tube; FR.PA, fronto-parietal; gl, glottis; gul, gullet; IL, ilium; is, ischium; kd, kidney; l.au, left auricle; 1. Ing, left lung; Ir, liver; m. mck, mento-meckelian; «. a. i, neural arch of first vertebra; olf.l, olfactory lobe; opt. I, optic lobe; o. ST, omo- and epi-sternum; pcd, peri- cardium; PMX, premaxilla; pn, pancreas; p. na, posterior naris; pu, pubis; ret, rectum ; r. Ing, right lung ; s. int, small intestine ; sp. cd, spinal cord ; SPH. ETH, sphenethmoid; spl, spleen; st, stomach; j. v, sinus venosus; tng, tongue; ts, testis; ur, ureter; ur' , its aperture into the cloaca; UST, urostylp; V, ventricle; v.ly.s, ventral lymph sinus; vo. t, vomerine teeth; vs. sent, vesicula seminalis. Eustachian tubes {eus: t, vide infra). On its floor is the large tongue {tng)^ attached in front and free behind, where it ends in a double point ; by means of its muscles it can be 4i8 MANUAL OF ZOOLOGY SECT. suddenly projected, point foremost, from the mouth, and is used in the capture of insects. Immediately behind the tongue is the glottis {gl). Teeth are arranged in a single series round the edge of the upper jaw, attached to the pre- maxillse and maxillae ; there is also a small patch of teeth {vo. t) on each vomer just internal to the posterior nostril. The teeth are small conical bodies, their bases ankylosed to the bones ; their only use is to prevent the polished or slimy bodies of the prey — insects and worms — from slipping out of the mouth. The buccal cavity narrows towards the pharynx, which leads by a short gullet {gul) into a stomach {si) consisting of a wide cardiac and a short, narrow pyloric division. The duodenum {du), or first portion of the small intestine, passes forwards parallel with the stomach; the rest of the small intestine is twisted into a coil. The large intestine or rectum {ret) is very wide and short, and passes without change of diameter into the cloaca {el) . The liver {Ir) is two-lobed ; between the right and left lobes lies a large gall-bladder. The pancreas {pn) is an irregular gland surrounding the bile duct, into which it pours its secretion; the spleen {spl) is a small, red globular body attached near the anterior end of the rectum. The lungs (/. Ing, r. Ing) are elastic sacs lying in the anterior part of the coelom above the heart and liver ; their size and appearance vary greatly according to their state of distention. Each contains a spacious cavity, and has its walls raised into a complete network of ridges abundantly supplied with blood-vessels. The two lungs open anteriorly into a small laryngo-tracheal chamber which communicates with the mouth by the narrow slit-like glottis. The walls of the laryngo-tracheal chamber are supported by a cartilagi- nous framework, and its mucous membrane is raised into a XII PHYLUM CHORDATA 419 pair of horizontal folds, the vocal chords, by the vibration of which the croak of the frog is produced. In breathing, the frog keeps its mouth closed, and, by depressing the floor of the mouth draws air into the buccal cavity through the nostrils. The floor of the mouth is then raised, the nostrils are closed, and the air is forced through the glottis into the lungs. The skin is also an important respiratory organ. "Wst pericardium (Fig. 2^1, pcd) is not a separate cham- ber, as in fishes, but the heart, enclosed in a pericardial membrane, lies in the general coelomic cavity between the gullet above and the epicoracoids below. The heart con- sists of a sinus venosus (Figs. 251 and 252, s,v), right and left auricles {r. au, 1. au), a ventricle {v,vt), and a conus arteriosus {c. art) . As in Dipnoi, the sinus venosus opens into the right auricle, the pulmonary veins into the left ; a striking advance is seen in the greatly increased size of the left auricle and its separation by a complete partition, the septum auricularum (Fig. 252, spt. aur), from the right. The two auricles open by a common auriculo-ventricular aperture, guarded by a pair of valves {au. v. v), into the sin- gle ventricle. The conus springs from the right side of the base of the ventricle ; it is separated from the latter by three small semilunar valves, and is traversed obliquely along its whole length by a large flap-like longitudinal valve {Iv) which springs from its dorsal wall and is free ventrally. The conus passes without change of diameter into a bulbus aortcz, the two being separated by a semilunar valve and by the free end' of the longitudinal valve. The bulbus gives off two branches, right and left, each of them divided by two longi- tudinal partitions into three vessels, — an inner or anterior, the carotid trunk {car. tr), a middle, the systemic trunk or aortic arch, and an outer or posterior, the pulmo- cutaneous 420 MANUAL OF ZOOLOGY SECT. trunk {pul. cu. tr). The carotid and systemic trunks com- municate separately with the bulbus, the two pulmo-cuta- neous trunks communicate with the anterior end of the conus by a single aperture placed just below the free end of the longitudinal valve. v.v raw s.aw.aft aur Fig. 252. — Rana temporaria. The heart from the ventral aspect with the cavities laid open, a, a', bristle in left carotid trunk; att. v. v, auriculo-ventricular valves; b,b‘ , bristle in left systemic trunk; c,c\ bristle in left pulmo-cutaneous trunk; car. carotid artery; car. carotid plexus; c. ar^, conus arteriosus; car. tr, carotid trunk; 1. au, left auricle; Ig. a, lingual artery; 1. v, longitudinal valve; pul. cu. tr, pulmo-cutaneous trunk; p7il. v, aperture of pulmonary veins; r. au, right auricle; j. a«. ap, sinu-auricular aperture; spt. atir, septum auricu- larum; z/. valves; ventricle. After being bound together in the way described for a short distance, the carotid, systemic, and pulmo-cutaneous trunks separate from one another. The carotid trunk divides XII PHYLUM CHORDATA 421 Fig. *53. — Rana temporaria. The arterial system, with the heart, lungs, kidneys, and left testis, from the ventral aspect, car, carotid artery: car. gl, carotid gland; c. art, conus arteriosus; car. tr, carotid trunk; ccel. mes, cosliaco- mesenteric artery; cu, cutaneous artery; d. ao, dorsal aorta; du, duodenal artery; gs, gastric artery; hp, hepatic artery; il, iliac artery; /nt, intestinal arteries; kd, kidney; /. au, left auricle; Ig, lingual artery; ptd, pulmonary artery: pul. cu. tr, pulmo-cutaneous trunk; r. du, right auricle; renal arteries; spl, splenic artery; jj/f. tr, systemic trunk; spin, spermatic artery; ts, testis; v, ventricle. 422 MANUAL OF ZOOLOGY SECT. into carotid (Figs. 252, car. a and 253, car') and lingual (^) arteries for the supply of the head, the former having at its base a small swelling, the carotid gland {car.gl), consisting of a plexus of blood-vessels. The systemic trunks curve round the gullet and unite with one another above it to form the dorsal aorta {d. ao), from which, or from one of the systemic trunks themselves, the arteries to all parts of the body, except the head, the lungs, and the skin, are given off. The pulmo-cutaneous trunk divides into two, a pulmonary artery {pul) to the lung, and a cutaneous arte7j {cu) to the skin. The blood from the head and fore-limbs is returned by veins which unite to form a pair of large trunks, the pre- cavals, which open separately into the sinus venosus. One portion of the blood from the hind-limb is carried forward by a vein which unites with its fellow of the oppo- site side to form the abdominal vein (Fig. 254, abd), which passes forwards, in the ventral body-wall, to the level of the sternum, where it turns inwards and divides into two branches, both breaking up into capillaries in the liver. Just as it enters the liver it is joined by the hepatic portal vein {hp.pt), bringing the blood from the stomach, intes- tine, spleen, and pancreas. The rest of the blood from the hind-limb is carried by the renal portal vein to the corresponding kidney. The blood is collected from the kidneys by the renal veins {tm), which unite to form the large unpaired post-caval vein {pt. cv). This passes forward through a notch in the liver, receives the hepatic veins {hp) from that organ, and finally opens into the sinus venosus. Thus the blood from the hind-limbs has to pass through one of the two portal systems on its way back to the heart ; part of it goes by the renal portal veins to the kidneys, and thence by the renal XII PHYLUM CHORDATA 423 veins to the post-caval, part by the abdominal vein to the liver, and thence by the hepatic veins to the post-caval. Lastly, the blood which has been purified in the lungs is returned by the pubnonary veins {pul) directly to the left auricle. It will be perceived that the blood poured into the right auricle is mostly impure or venous, that poured into the left fully aerated or arterial. When the auricles contract, which they do simultaneously, each passes its blood into the corresponding part of the ventricle, which then instantly contracts, before the venous and arterial bloods have time to mix. Since the conus arteriosus springs from the right side of the ventricle, it will at first receive only venous blood, which, on the contraction of the conus, might pass either into the bulbus aortge or into the aperture of the pulmo-cutaneous trunks. But the carotid and systemic trunks are connected with a much more extensive capillary system than the pulmo-cutaneous, and the pressure in them is proportionally great, so that it is easier for the blood to enter the pulmo-cutaneous trunks than to force aside the valves between the conus and the bulbus. A fraction of a second is, however, enough to get up the pressure in the pulmonary and cutaneous arteries, and in the meantime the pressure in the arteries of the head, trunk, etc., is constantly diminishing, owing to the continual flow of blood towards the capillaries. Very soon, therefore, the blood forces the valves aside and makes its way into the bulbus aortae. Here again the course taken is that of least resistance ; owing to the presence of the carotid gland the passage of blood into the carotid trunks is less free than into the wide, elastic, systemic trunks. These will, therefore, receive the next portion of blood, which, the venous blood having been mostly driven to the lungs, will be a mixture of venous and 424 MANUAL OF ZOOLOGY SECT. Fig. 254. — Rana temporaria. The venous system with the heart, lungs, liver, kidneys, and right testis, from the dorsal aspect. abd, abdominal vein; hr, brachial vein; frf, cardiac vein; dorso-lumbar vein; duodenal vein; yW, external jugular vein; fentoral vein; gs, gastric vein; hepatic vein; hepatic portal vein; intestinal veins; yV/, internal jugular vein; kidney; 1. au, left auricle; Ing, lung; /wr, liver; 7ns. cu, musculo- cutaneous vein; /r. pre-caval vein; pt. post-cav^l vein; /«/, pulmonary vein; /w, pelvic vein ; r. a?/, right auricle; r«, renal veins; r«. renal portal vein; sc, sciatic vein ; spl, splenic vein; spTn, spermatic vein; v, sinus venosus; ts, testis; ves, vesical veins. XII PHYLUM CHORDATA 425 arterial. Finally, as the pressure rises in the systemic trunks, the last portion of blood from the ventricle, which, coming from the left side, is arferial, will pass into the carotids and so supply the head. The lymphatic system is very well developed, and is remarkable for the dilatation of many of its vessels into immense lymph sinuses. The lymph is pumped into the veins by two pairs of lymph-hearts, one situated beneath the supra-scapulse, the other beside the posterior end of the urostyle. The brain (Fig. 255) has a very small cerebellum (HI/), large optic lobes (MI/), a well-developed diencephalon, and large hemispheres and olfactory lobes, the latter fused in the median plane. The first spinal nerve performs the function of the hypoglossal, one of the cranial nerves of higher Vertebrates supplying the muscles of the tongue : it passes out between the first and second vertebrse. The spinal cord is short, and ends in a delicate filament, the filum terminale. In correspondence with the number of vertebrae there are only ten pairs of spinal nerves, of which the second and third unite to form a brachial plexus giving off the nerves to the fore-limb, while the seventh to the tenth join to form a lumbo-sacral plexus giving off the nerves to the hind-limb. The olfactory sacs have each two openings: the anterior naris or external nostril and the posterior naris (Fig. 251, p. na), or internal nostril, which opens into the mouth immediately external to the vomer. The eye and the auditory organ have the usual structure, but in connection with the latter there is an important accessory organ of hearing not hitherto met with. Bounded externally by the tympanic membrane, and internally by the outer wall of the auditory capsule, is a considerable space, Fig. 255. — Rana esculenta. The brain. A, from above; B, from below, ch. opt, optic chiasma; HH, cerebellum; Hyp, pituitary body; Inf, infundibulum; L. ol, olfactory lobe; Med, spinal cord; MH, mid-brain; NH, medulla ob- longata; Th. opt, optic thalamus; Tr. opt, optic tract; VH, cerebral hemi- sphere; Z//, diencephalon ; / — Af, cerebral nerves; Af//. (7), hypoglossal (first spinal) nerve. (From Wiedersheim’s Cotnparative Anatotny.) 426 MANUAL OF ZOOLOGY sect. the tympanic cavity (cav), which communicates with the mouth by the short Eustachian tube already noticed (Fig. 251, eus. /), so that a probe thrust through the tympanic membrane from outside passes directly into the mouth. In the roof of the tympanic cavity lies a slender rod of bone and cartilage, the columella, its head, or extra-columella, attached to the inner surface of the tympanic membrane, its XII PHYLUM CHORDATA 427 handle united to the stapes, a nodule of cartilage which is fixed in the membrane of the fenest^'a ovalis (p. 358). Sonorous vibrations striking the tympanic membrane are communicated by the columella and stapes to the fenestra ovalis, thence to the perilymph, and thence to the mem- branous labyrinth. The connection of the Eustachian tube with the mouth obviates undue compression of the air in the tympanic cavity. The kidneys (Figs. 251 and 253, Kd, and Figs. 256 and 257, N) are flat, somewhat oval bodies of a dark red colour, lying in the posterior region of the coelom. On the ventral face of each is an elongated, yellow adrenal, and irregularly scattered nephrostomes occur on the same surface. They do not, however, communicate with the urinary tubules, but with the renal veins, and serve to propel the lymph from the coelom to the venous system. The ureters ( Ur) pass backwards from the outer borders of the kidney, and open into the dorsal wall of the cloaca {Cl). Opening into the cloaca on its ventral side is a bilobed, thin-wailed, and very delicate sac, the urinary bladder (Fig. 251, bl)^ into which the urine passes by gravitation from the cloaca when the anus is closed. The testes (Fig. 256, Ho) are white ovoid bodies lying immediately ventral to the anterior ends of the kidneys, to which they are attached by folds of peritoneum. From the inner edge of each pass a number of delicate vasa efferentia, which enter the kidney and become connected with the urinary tubules. The spermatic fluid is thus passed into the urinary tubules and carried off by the ureter, which is therefore a urinogenital duct in the male frog. A vesicula semmalis opens by numerous small ducts into the outer side of the ureter. Attached to the testis are lobed bodies of a bright yellow colour, the fat-bodies {FK). 428 MANUAL OF ZOOLOGY SECT. Cv Ao The ovaries (Fig. 257, Ov) are large folded sacs, on the surface of which the black and white ova project. A fat- body is attached to each. The oviducts {^Od^ are greatly convoluted tubes, the narrow anterior ends of which open into the coelom by small apertures ( Oi) placed close to the bases of the lungs. Their posterior ends are wide and thin- walled ( Uf) , and open into the cloaca (P) . The ova break loose from the surface of the ovary and enter the coe- lomic apertures of the ovi- ducts, the walls of which are glandular, and secrete an albuminous fluid having the property of swelling up in water. The eggs receive a coating of this substance as they pass down the ovi- ducts, and are finally stored up in the thin-walled pos- terior portions of those Fig. 256. — Rana esculenta. Urinogenital .... organs of the male. Ao, dorsal aorta; tubeS, which in the breed- er, cloaca ; Cv, post-caval vein ; FK, fat . bodies: HO, testes: N, kidneys; .S', mg season become im- apertures of ureters into cloaca; Ur, , ureters. (From Wiedersheim’s Com- mensely dilated and Serve iarative A natotnyO . „i i • -i as uteri. The eggs are laid in water in large masses ; each has one black and one white hemisphere, the former always directed upwards, and is sur- rounded by a sphere of jelly. During oviposition the male sheds his spermatic fluid over the eggs, and the sperms making their way through the jelly impregnate them. XII PHYLUM CHORDATA 429 When the embryo escapes from the egg by the rupture of the egg-membrane it swims about as a little fish-like creature or tadpole, with two pairs of branched external gills and a Fig. 257. — Rana esculenta. Urinogenital organs of the female. N, kidneys: Od, oviduct; Ot, its ccelomic aperture; Ov, left ovary (the right is removed); P, cloacal aperture of oviduct; S. S', cloacal apertures of ureters; Ui, uterine dilatation of oviduct. (From Wiedersheim’s Comparative Anatomy,') long tail. There is no mouth, and eyes have not yet become developed. On the lower surface of the head is a pair of 430 MANUAL OF ZOOLOGY SECT, suckers by which the tadpole is able to attach itself to water-weeds. Soon a pair of external gills appears, the mouth and gill-slits are formed, and the eyes appear. The mouth is small, bounded by lips beset with horny papillae. Fig. 258. — Rana temporaria. Stages in the life-history, from the newly-hatched Tadpoles (/) to the young Frog (5). 2a is a magnified view of 2. (From Mivart.) and provided with a pair of horny jaws. The enteric canal grows to a great length and is coiled like a watch-spring, and the tadpole browses upon the water-weeds which form its XII PHYLUM CHORDATA 431 staple food, though it also eats decaying leaves and the flesh of dead animals. Soon the external gills show signs of shrivelling, and at the same time mternal gills, like those of fishes, are devel- oped on the branchial arches. A fold of skin, the operculum, appears on each side, in front of the gills, growing from the region of the hyoid arch, and extends backwards until the gill-slits and external gills are covered and there is only a single small external branchial aperture on each side. On the right side the operculum soon unites with the body-wall so as to close the branchial aperture, but on the left side the opening remains for a considerable time as the sole exit of the water. At this time the tadpole is to all intents and purposes a fish. The lungs now appear, and the larva is for a time truly amphibious, rising periodically to the surface to breathe air ; the single branchial aperture, however, soon closes, and henceforth respiration is purely aerial. In the meantime the limbs are developed. The hind- limbs appear as little rounded buds, one on each side of the root of the tail. The fore-limbs arise beneath the operculum, and are therefore hidden at first ; soon, however, they emerge by forcing their way through the operculum. As the limbs increase in size, the tail undergoes a progressive shrinking. The mouth widens, the intestine undergoes a relative diminution in length, and vegetable is exchanged for animal diet. The little tailed frog can now leave the water and hop about upon land ; its tail is soon completely absorbed, and the metamorphosis is complete. The frogs and toads are all closely allied as regards all the main features of their structure — the chief differences between the many genera and species being in such super- ficial characteristics as coloration and proportions. In 432 MANUAL OF ZOOLOGY SECT. some teeth are altogether absent ; in others the webs between the toes are not developed; in some tree-frogs {Hyla and allied genera) the toes terminate in sucking discs. A less superficial point of divergence from the structure of the common American frogs is to be observed in some members of the group, such as the tree-frogs and toads (Hy/a, Bufo, and others), in which the two halves of the shoulder-girdle, instead of being firmly united in the mid- Fig. 259. — Salamandra maculosa. (After Cuvier.) die line, overlap one another. In one small group the tongue is absent. In some, again, there is no fish-like, gill- bearing larva or tadpole — the young animal emerging from the egg with the limbs formed, with no gills and no tail. All the frogs and toads are grouped together to form an order of Amphibia — the Anura or tailless Amphibia. The newts and salamanders (Fig. 259), with a number of other less widely known forms, differ from the frogs and toads in the possession in the adult of a well-developed tail. These constitute the order Urodela or tailed Amphibians. Of these tailed Amphibians, some, such as the newts and XII PHYLUM CHORDATA 433 salamanders, lose both gills and gill- slits completely in the adult : while in others (such as Proteus, Necturus, and Siren) , either gills are retained through- out life, or, as in the American Meno- poma, or hell-bender, gill-slits remain as a permanent record of their pres- ence in the larva. In some of the tailed Amphibians the limbs are well- developed j in others they are very small. Widely different in many respects from both the Anura and Urodela are a group of Amphibia — the Gymno- pJiiona — which are quite snake-like in appearance, owing to the elongated and narrow form of the body and the entire absence of limbs. The group is represented by the blind snake (C«- cilia) of Central and South America. CLASS IV. EEPTILIA ' The class Eeptilia comprises the lizards and snakes, the tuataras, the turtles and tortoises, and the alligators and crocodiles. On a superficial com- parison of these with the Amphibia, it might be inferred that there is a close alliance between the two groups j but this impression becomes weakened when a closer examination is made of the structure and development, and it 2 F Fig. 260. — Siren lacer* tina. (From Mivart.) 434 MANUAL OF ZOOLOGY SECT. at length becomes evident that in the Reptilia we have to do with a class of Vertebrates which stand on a dis- tinctly higher plane than the Amphibia. One significant feature of the Reptilia which marks them off sharply from the Amphibia is that the lungs are the sole organs of respiration, gills never being developed at any stage. Another is the development in the embryo of two struc- tures known as the amnion and the allantois, not devel- oped in lower groups of Vertebrates, but present in the embryos of all the higher. The amnion is a thin membrane which covers over the body of the embryo, the space between it and the latter being tensely filled with a watery fluid. The amnion thus forms a sort of water-cushion, protecting the delicate and fragile embryo from the effects of any shocks which may be sustained by the eggs. The allantois, repre- sented in the frog by the urinary bladder, is a membranous structure developed as a hollow outgrowth of the enteric canal at its posterior end. It becomes highly vascular, and acts as an embryonic respiratory organ. There are four well-marked orders of living reptiles : — 1. The Squamata, comprising the Lacertilia or lizards (including the iguanas, monitors, skinks, geckos, chamseleons, and others), and the Ophidia or snakes (including the vipers and rattlesnakes, pythons, boas, sea-snakes, etc.). 2. The Rhynchocephalia, including only the New Zealand Tuatara {Hatteriai). 3. The Chelonia, including the land tortoises, soft tor- toises, river and marsh tortoises, and the turtles. 4. The Crocodilia, including the crocodiles, gavials, the alligators and caimans. The most striking external difference between a typical lizard (Fig. 207) and the frog are in the covering of scales in the case of the former, the comparative smallness of its XII PHYLUM CHORDATA 435 head, and the presence of a distinct neck, the great length of the caudal region, the shortness of the limbs, and the approximate equality in length of the anterior and posterior pairs. The anterior limbs are situated just behind the neck, springing from the trunk towards the ventral surface. The fore-limb, like that of the frog, is divided into three parts, the upper-arm or brachium, the fore-arm or anti- brachium, and the hand or manus ; there are five digits provided with horny claws, the first digit or pollex being the smallest. The hind-limbs arise from the posterior end of the trunk towards the ventral aspect; each, like that of the frog, consists of three divisions — thigh or fe^nur, shank or crus, and foot or pes. The pes, like the manus, termi- nates in five clawed digits, of which the first or hallux is the smallest. The head is somewhat pyramidal, slightly de- pressed ; the openings of the external nares are situated above the anterior extremity. The mouth is a wide slit- like aperture running round the anterior border of the head. At the sides are the eyes, each provided with upper and lower opaque movable eyelids, and with a transparent third eyelid or nictitating membrane, which, when withdrawn, lies in the anterior angle of the orbit. Behind the eye is a circular brown patch of skin, — the tympanic ^nembrane, — corresponding closely to that of the frog, but somewhat sunk below the general level of the skin. The trunk is elongated, strongly convex dorsally, flatter at the sides and ventrally. At the root of the tail on the ventral surface is a slit-like transverse aperture — the anus or cloacal aperture. The tail is cylindrical, thick in front, gradually tapering to a narrow posterior extremity ; it is nearly twice as long as the head and trunk together. There is an exoskeleton of horny epidermal scales covering all parts, differing in size in different positions. 436 MANUAL OF ZOOLOGY SECT. In some groups of lizards the tail is comparatively short and thick; and in others it is depressed and expanded into a leaf-like form. In the chamseleons the long and tapering tail is used as a prehensile organ, the coiling of which round branches of the trees in which the animal lives, aids in maintaining the balance of the body in climbing from branch to branch. In the limbs there is likewise a considerable amount of variation in the different groups of the Lacertilia. Moder- ately long pentadactyle limbs, like those of Lacerta, are the rule. In the chamseleons both fore- and hind-limbs become prehensile by a special modification in the arrangement and mode of articulation of the digits. In these remarkable arboreal reptiles the three innermost digits of the manus are joined together throughout their length by a web of skin, and the two outer digits are similarly united ; the two sets of digits are so articulated that they can be brought against one another with a grasping movement much analogous to the grasping movements of a parrot’s foot or the hand of man. A similar arrangement prevails in the pes, the only difference being that the two innermost and three outermost digits are united. In some groups of Lacertilia, on the other hand, such as the blind-worms {Anguis), limbs are entirely absent, or are represented only by mere vestiges ; and numerous intermediate gradations exist between these and forms, such as Lacerta, with well-developed limbs. The limbless lizards, such as the glass-snake, and Pygopus (Fig. 261), bear a very close resemblance to the snakes, not only in the absence of the limbs, but also in the general form of the body and the mode of locomotion. The body of a snake is elongated, narrow, and cylindrical, usually tapering towards the posterior end, sometimes with, more usually without, a constriction behind the head. In XII PHYLUM CHORDATA 437 the absence of limbs, the beginning of the short caudal region is only indicated by the position of the cloacal open- ing. The fore-limbs are never represented even by vestiges ; in some pythons there are inconspicuous vestiges of hind- limbs, in the form of small clavv-like processes. The mouth of the snake is capable of being very widely opened by the free articulation of the lower jaw, and it is this mainly which distinguishes it from the snake-like lizards. But other, less Fig. 261. — Pygopus lepidopus. (After Brehm.) conspicuous, points of distinction are the absence of movable eyelids in the snake, and also the absence of a tympanum. Hatteria, the New Zealand Tuatara (Fig. 262), the only living representative of the Rhynchocephalia, is a lizard-like reptile with a well-developed laterally-compressed tail, and pentadactyle extremities, very similar to those of a typical lizard. The upper surface is covered with small granular scales, and a crest of compressed spine-like scales runs along the middle of the dorsal surface. The lower surface is covered with transverse rows of large squarish plates. 438 MANUAL OF ZOOLOGY SECT. In the Chelonia (Fig. 263) the body is short and broad, enclosed in a hard “shell,” consisting of a dorsal part or carapace and a ventral part or plastron. These are firmly united, apertures being left between them for the head and neck, the tail and the limbs. The neck is long and mobile ; the tail short. The limbs are fully developed, though short. In some (land and fresh-water tortoises) they are provided each with five free digits terminating in curved horny claws ; in the turtles the digits are closely united together, and the Fig. 262. — Hatteria punctata. (After Brehm.) limb assumes the character of a “ flipper ” or swimming paddle. The cloacal aperture is longitudinal. The Crocodilia, the largest of living reptiles, have the trunk elongated and somewhat depressed, so that its breadth is much greater than its height. The snout is prolonged, the neck short, the tail longer than the body and compressed laterally. The limbs are relatively short and powerful, with five digits in the manus and four in the pes, those of the latter being partly or completely united by webs of skin. XII PHYLUM CHORDATA 439 The eyes are very small; the nostrils placed close to the end of the snout and capable of being closed by a sphincter muscle. The cloacal aperture is a longitudinal slit. Characteristic of the Squamata is the development in the epidermis of horny plates, the scales, which cover the entire surface, overlapping one another in an imbricating manner. Sometimes they are similar in character over all parts of the surface; usually there are specially developed scales — the Fig. 263. — Grecian tortoise (Testudo graeca). (After Brehm.) head shields — covering the upper surface of the head. In the majority of snakes the ventral surface is covered with a row of large transversely elongated scales, the ventral shields. In some lizards (chamseleons and geckos) the scales are reduced and modified into the form of minute tubercles or granules. In some lizards special developments of the scales occur in the form of large tubercles or spines. In the snake-like Amphisbsenians there are no true scales. 440 MANUAL OF ZOOLOGY SECT. with the exception of the head shields, but the surface is marked out into annular bands of squarish areas. In addition to the modification of the scales, the integu- ment of the chamaeleons is remarkable for the changes of colour which it undergoes, these changes being due to the presence in the dermis of pigment cells which contract or expand under the influence of the nervous system, in a way that reminds one of the integument of the Cephalopoda. In the Chelonia, scales, when developed, are confined to the head and neck, the limbs and the tail, but in all of them, with the exception of the soft tortoises, both dorsal and ven- tral surfaces are covered by a system of large horny plates. A series of horny head-shields usually cover the dorsal sur- face of the head. Beneath the horny plates of the dorsal and ventral surfaces are the bony carapace and plastron, partly composed of dermal bones, but so intimately united with elements derived from, the endoskeleton that the entire structure is best described in connection with the latter (P- 443)- In the Crocodilia, the dorsal surface is covered with longi- tudinal rows of sculptured horny plates, beneath which are bony dermal scutes of corresponding form. The ventral surface of the body is covered with scales like those of a lizard. The horny plates of the dorsal surface of the tail are elevated into a longitudinal crest. A periodical ecdysis or casting and renewal of the outer layers of the horny epidermis takes place in all the Reptilia. Sometimes this occurs in a fragmentary manner; but in snakes and many lizards the whole comes away as a con- tinuous slough. The vertebra are always fully ossified. Only in the geckos and Hatteria are the centra amphicoelous, with XII PHYLUM CHORDATA 441 remnants of the notochord in the inter-central spaces. In most of the others the centra are procoelous, a ball-like con- vexity on the posterior surface of each centrum projecting into a cup-like concavity on the anterior face of the next. The various regions of the spinal column are well marked in most of the lizards, in the Chelonia, and in the Croco- dilia (Fig. 264) . In the snakes and many of the snake-like lizards only two regions are distinguishable — pre - caudal and caudal. In the others there is a sacral region com- prising two vertebrae, which have strong transverse pro- cesses for articulation with the ilia. The first and sec- ond vertebrae are always modi- fied to form an atlas and axis. Ribs are developed in connection with all the ver- tebrae of the pre-sacral or pre- caudal region ; in the caudal region they are usually replaced by inferior arches. In the fly- ing lizards {Draco) a num- ber of the ribs are greatly produced, and support a pair Fig. 264. — Skeleton of crocodile. C, caudal region ; D, thoracic region of spinal column; F, fibula; Fe, femur; H, humerus; 7, ischium ; L, lumbar region; R, radius; Ri, ribs; S, sacrum; Xc, scapula; Sta, abdominal ribs; T, tibia; U, ulna. (From ZitteU 442 MANUAL OF ZOOLOGY SECT. of wide flaps of skin at the sides of the body, acting as wings or rather as parachutes. In Hatteria and Crocodilia (Fig. 264) each rib has connected with it posteriorly a flat- tened curved cartilage, the uncinate. Fig. 265. — Cistudo lutaria. Skeleton seen from below ; the plastron has been removed and is represented on one side. C. costal plate; Co, coracoid; e, ento- plastron; Ep, epiplastron; F, fibula; Fe, femur; H, humerus; II, ilium; Is, ischium; M, marginal plates; Nu, nucha! plate; Pb, pubis; Pro, pro-cora- coid; Py, pygal plates: R, radius; Sc, scapula; T, tibia; U, ulna. (From Zittel.) In the Chelonia (Fig. 265) the total number of vertebrae is always smaller than in the members of the other orders. The cervical and the caudal are the only regions in which the vertebrae are movable upon one another. The vertebrae XII PHYLUM CHORDATA 443 of the trunk, usually ten in number, are immovably united together. Each of the neural spines, from the second to the ninth inclusively, is expanded into a flat plate, and the row of neural plates (Fig. 266, V), thus formed constitutes the median portion of the carapace. The ribs are likewise immovable ; a short distance from its origin each passes into a large bony costal plate (C), and the series of costal plates uniting by their edges form a large part of the cara- pace on either side of the row of neural plates. The cara- pace is made up of the neural and costal plates supplemented by a row of marginal plates (Figs. 265 and 266, M~) running along the edge, and nuchal (Nu) and pygal (jy) plates Fig. 266. — Chelone midas. Transverse section of skeleton. C, costal plate: C, centrum; M, marginal plate; P, lateral element of plastron; P, rib; F, expanded neural plate. (After Huxley.) situated respectively in front of and behind the row of neural plates. The bony elements of the plastron of the Chelonia are an anterior and median plate and six pairs of plates — the six pairs probably being of similar nature to the abdominal ribs of the Crocodilia. The sternum in the Lacertilia is a plate of cartilage with a bifid posterior continuation. In the Ophidia and Chelonia it is absent. In the Crocodilia it is a broad plate with a posterior continuation or hyposternum^ extending backwards as far as the pelvis. 444 MANUAL OF ZOOLOGY SECT. A series of ossifications — the abdominal ribs, with a mesial abdominal sternum — lie in the wall of the abdomen in the Crocodilia (Fig. 264, Stai), and similar ossifications occur also in the monitors and in Platteria. The elements of the plastron of the Chelonia are probably of a similar character. In the skull ossification is much more complete than in the Amphibia, the primary chondrocranium persisting to a considerable extent only in some lizards and in Hatteria, and the nu'mber of bones is much greater. The parasphenoid is reduced, and its place is taken by large basi-occipital, basi-sphenoid, and pre-sphenoid bones. The lower jaw articulates with the skull through the interme- diation of a quadrate bone, which is movable in the lizards and snakes, fixed in Hatteria, the Chelonia, and Crocodilia. A remarkable feature of the skull of the snakes (Fig. 267), is the free articulations of the bones of the jaws, permitting of the mouth being opened very wide so as to allow the passage of the relatively large animals which the snake swallows whole ; this wide opening of the mouth is further aided by the two halves of the mandible not being firmly fixed together anteriorly, but merely connected together by means of elastic tissue, so that they are capable of being widely separated from one another. In accordance with their purely aerial mode of respira- tion, the visceral arches are much more reduced in the Reptilia than in the Amphibia in general. The only well- developed post mandibular arch is the hyoid, and even this may undergo considerable reduction (Ophidia) . The branchial aiches, except in so far as they may contribute to the formation of the tracheal rings, are not represented in the adult, with the exception of most Chelonia, in which the first branchial arch persists. XII PHYLUM CHORDATA 445 In the lizard (Fig. 268) and crocodiles there is a cross- shaped membrane-bone, the interclavicle or episternum (epist), in relation to the pectoral arch and sternum. In the limb- less lizards the pectoral arch may be absent or may be well developed ; it is completely wanting in all snakes. In the pelvic arch the ischium is separated from the pubis by a wide space not developed in the Amphibia. Slight vestiges of hind- limbs occur in some pythons alone among the Ophidia. Fig. 267. — A, lateral view of skull of rattlesnake (Crotalus). B. O, basi-occipital; B. S, basi-sphenoid ; B. O, exoccipital ; F. O, fossa ovalis; La, conjoined lac- rymal and pre-frontal; L. f, articulation between lacrymal and frontal ; Mn, mandible ; Mx, maxilla; Na, nasal; PI, palatine; Pmx, pre-maxilla; P. Sph, pre-sphenoid; Pt, pterygoid; Qu, quadrate; Sg, squamosal ; II. V, foramina of exit of the second and fifth cranial nerves. B, transverse section at point lettered B in Fig. A; T, trabeculae. (After Huxley.) In the lizards teeth are present in the pre-maxillae, the maxillae, the mandible, and usually the palatines. These are in most lizards small and simple, and uniform in shape and atrangement. A Mexican Lizard, Heloderma^ which also occurs in Utah and Arizona, differs from all the rest in 446 MANUAL OF ZOOLOGY SECT. having teeth which are perforated for the ducts of poison- glands. In the snakes (Fig. 267) teeth are rarely devel- oped on the pre-maxillae, but are present on the maxillae, palatines, pterygoids, and sometimes the transverse bones, as well as the dentary of the mandible. They may be of the same nature throughout, solid, elongated, sharp-pointed teeth, which are usually strongly recurved, so that they have the character of sharp hooks, their function being rather to Fig. 268. — Pectoral arch and sternum of a lizard (Lacerta agilis). cl, clavicle; cor, coracoid; e/. cor, epicoracoid; episternum ; glenoid cavity for head of humerus; /r. cor, pro-coracoid; r.i — r. 4, first to fourth sternal ribs; sc, scapula; sternum; supra, sc, snpra-scapnXa. (After Hoffman.) hold the prey and prevent it slipping from the mouth while being swallowed than to masticate it. Non-venomous snakes possess teeth only of this character. In the venomous snakes, more or fewer of the maxillary teeth assume the character of poison-fangs. These are usually much larger than the ordinary teeth, and either grooved or perforated by a canal XII PHYLUM CHORDATA 447 for the passage of the duct of the poison-gland. In the vipers there is a single large curved poison-fang with small reserve-fangs at its base, these being the only teeth borne by the maxilla, which is very short ; in the venomous colu- brine snakes the poison-fangs are either the most anterior or the most posterior of a considerable range of maxillary teeth. In the vipers the large poison-fang is capable, owing to the maxilla in which it is fixed being movable on a hinge-joint, of being rotated through a considerable angle, and moved from a nearly horizontal position, in which it lies along the roof of the mouth, embedded in folds of the mucous mem- brane, to a nearly vertical one when the snake opens his mouth to strike its prey. In Hatteria there are pointed, triangular, laterally- compressed teeth, arranged in two parallel rows, one along the maxilla, the other along the palatine. The teeth of the lower jaw, which are of similar character, bite in between these two upper rows, all the rows becoming worn down in the adult in such a way as to form continuous ridges. Each pre-maxilla bears a prominent, chisel-shaped incisor, represented in the young animal by two pointed teeth. In the young Hatteria a tooth has been found on each vomer — a condition exceptional among reptiles. In the Chelonia, teeth are entirely absent, the jaws being invested in a horny layer in such a way as to form a structure like a bird’s beak. The Crocodilia have numerous teeth which are confined to the pre-maxillae, the maxillae, and the dentary. They are large, conical, hollow teeth, devoid of roots, each lodged in its socket or alveolus, and each becoming replaced, when worn out, by a successor developed on its inner side. In the enteric canal of the Reptiles the principal special features to be noticed are the muscular, gizzard-like stomach 448 MANUAL OF ZOOLOGY SECT. of the Crocodilia, the presence of a rudimentary csecum at the junction of small and large intestines in most Lacertilia and in the Ophidia, and the presence of numerous large cornified papillae in the oesophagus of the Turtles. Fig. 269. — Heart of monitor (Varanus) dissected to show the cavity of the ventricle and the vessels leading out from it. A. .^4', auricles; Ao, dorsal aorta; Ap. Ap' , pulmonary arteries; A sc, subclavian artery; Ca. Ca' , caro- tids; RA. RA, roots of dorsal aorta; Trca, innominate trunk; V, ventricle; t, right aortic arch; *, left aortic arch. (From Wiedersheim.) its aperture of communica guarded by valves. There The Reptiles have all an elongated trachea, the wall of which is supported by numerous cartilaginous rings. The anterior part of this is dilated to form the larynx, the wall of which is supported by certain spe- cial cartilages — the cricoid and the arytenoids. The trachea bifurcates posteri- orly to form two bronchi, right and left, one passing to each lung. The lungs of the Lacer- tilia and Ophidia are sim- ple and sac-like, like those of the frog. In the Croco- dilia and Chelonia they are of a more complex char- acter, being divided inter- nally by septa into a number of chambers. In the heart (Fig. 269) the sinus venosus is always distinct, and is divided into two parts by a septum; i with the right auricle is s always two quite distinct XII PHYLUM CHORDATA 449 auricles, as in the Amphibia, the right receiving the venous blood from the body, the left the oxygenated blood brought from the lungs by the pulmonary veins. But a vital point of difference between the heart of the reptile and that of the am- phibian is that in the former the ventricle is al- ways more or less com- pletely divided into right and left portions. In all the Lacertilia, Ophidia, and Chelonia the ventricle is incompletely divided by a septum which does not entirely cut off the two portions of the cavity from one another. But in the Crocodilia the cavity is completely divided, so that we may speak of distinct right and left ventricles. The brain of Reptiles is somewhat more highly organised than that of the Amphibia. The cerebral hemispheres are well de- veloped in all. The mid- brain consists usually of two closely approximated oval optic lobes. Fig. 270. — Brain of alligator, from above. B. oL, olfactory bulb ; G. p, epiphysis ; cerebellum ; Med, spinal cord : MH, optic lobes ; NH, medulla oblongata ; VH, cerebral hemispheres ; / — XI, cra- nial nerves ; 1,2, first and second spinal nerves. (After Wiedersheim.) The cerebellum 45° MANUAL OF ZOOLOGY SECT. is always of small size, except in the Crocodilia (Fig. 270), in which it is comparatively highly developed, and consists of a median and two lateral lobes. The eyes are relatively large, with a cartilaginous sclerotic in which a ring of bony plates is developed in some cases. Most reptiles have both upper and lower eyelids and nicti- tating membrane. The greater number of the geckos and all the snakes constitute exceptions, movable eyelids being absent in both these groups. In the chamseleons there is a single circular eyelid with a central aperture. The middle ear is absent in the snakes, though a colu- mella auris is present, embedded in muscular and fibrous tissue. Developed in close relation to the epiphysis there is in many lizards (^Lacerta, Varanus, Anguis, Grammatophora, and others) and in Hatteria, a remarkably eye-like organ — the pineal eye (Fig. 271), which is situated in a foramen of the cranial roof immediately under the integument, and covered over by a specially modified, transparent scale. Like the epiphysis itself, the pineal eye is developed as a hollow outgrowth of the roof of the diencephalon ; the distal end of this becomes constricted off as a hollow sphere, while the remainder becomes converted into a nerve. The nerve degenerates before the animal reaches maturity, so that the organ would appear — though evidently, from its structure, an organ of sight — to have now entirely or nearly lost its function. Though fertilisation is always internal, most Reptilia are oviparous, laying eggs clothed in a tough, parchment-like or calcified shell. These are usually deposited in holes and left to hatch by the heat of the sun. In the crocodiles they are deposited in a rough nest and guarded by the mother. In all cases development has only progressed to a very early XII PHYLUM CHORDATA 451 Stage when the deposition of the eggs takes place, and it is only after a more or less prolonged period of incubation that the young, fully formed in almost every respect, emerge from the shell and shift for themselves. Many lizards. Fig. 271. — Section of the pineal eye of Hatteria punctata. blood-vessels; h, cavity of the eye filled with fluid; k, capsule of connective tissue; I, lens; molecular layer of the retina; r, retina; st, stalk of the pineal eye; _r, cells in the stalk. (From Wiedersheim, after Baldwin Spencer.) nowever, and a few snakes are viviparous, the ova being developed in the interior of the oviduct, and the young reaching the exterior in the completely formed condition. 452 MANUAL OF ZOOLOGY SECT. The lizards are for the most part terrestrial animals, usually extremely active in their movements and endowed with keen senses. The majority readily ascend trees, and many kinds are habitually arboreal ; but the chamaeleons are the only members of the group which have special modi- fications of their structure in adaptation with an arboreal mode of life. The skinks and the amphisbsenians are swift and skilful burrowers. The geckos are enabled by the aid of the sucker-like discs on the ends of their toes to run readily over vertical or overhanging smooth surfaces. A few lizards, on the other hand, live habitually in fresh water. The flying lizards (Draco) are arboreal, and make use of their wings — or, to speak more accurately, aeroplane or para- chute (thin folds of skin supported by the greatly produced ribs) — to enable them to take short flights from branch to branch. Chalmydosaiirus and certain other genera are exceptional in frequently running on the hind-feet, with the fore-feet entirely elevated from the ground. A tolerably high temperature is essential for the maintenance of the vital activities of lizards, low temperatures bringing on an inert condition, which usually passes, during the coldest part of the year, into a state of suspended animation or hibernation. The food of lizards is entirely of an animal nature. The smaller kinds prey on insects of all kinds, and on worms. Chamaeleons, also, feed on insects, which they capture by darting out the extensile tongue covered with a viscid secretion. Other lizards supplement their insect diet, when opportunity offers, with small reptiles of various kinds, frogs and newts, small birds and their eggs, and small mam- mals, such as mice and the like. The larger kinds, such as the monitors and iguanas prey exclusively on other ver- tebrates ; some, on occasion, are carrion-feeders. Most lizards lay eggs enclosed in a tough calcified shell. These XII PHYLUM CHORDATA 453 they simply bury in the earth, leaving them to be hatched by the heat of the sun. Some, however, are viviparous ; in all cases the young are left to shift for themselves as soon as they are born. Snakes are also usually extremely active and alert in their movements ; and most are very intolerant of cold, under- going a hibernation of greater or less duration during the winter season. Many live habitually on the surface of the ground — some kinds by preference in sandy places or among rocks, others among long herbage. Some (tree- snakes) live habitually among the branches of trees. Others (fresh- water snakes) inhabit fresh water ; others (sea-snakes) live in the sea. The mode of locomotion of snakes on the ground is extremely characteristic, the reptile moving along by a series of horizontal undulations brought about by contractions of the muscles inserted into the ribs, any inequalities on the surface of the ground serving as fulcra against which the free posterior edges of the ventral shields (which are firmly connected with the ends of the ribs) are enabled to act. The burrowing blind- snakes and other families of small snakes feed on insects and worms. All the rest prey on vertebrates of various kinds — fishes, frogs, lizards, snakes, birds and their eggs, and mammals. The pythons and boas kill their prey by constriction, winding their body closely round it and draw- ing the coils tight till the victim is crushed or asphyxiated. Some other non-venomous snakes kill with bites of their numerous sharp teeth. The venomous snakes sometimes, when the prey is a small and weak animal such as a frog, swallow it alive : usually they first kill it with the venom of their poison-fangs. When a venomous snake strikes, the poison is pressed out from the poison-gland by the contraction of the masseter 454 MANUAL OF ZOOLOGY SECT. (Fig. 272, Me), one of the muscles which raise the lower jaw: it is thus forced along the duct ^Gc) to the aperture iza), and injected into the wound made by the fang. The effect is to produce acute pain with increasing lethargy and weakness, and in the case of the venom of some kinds of snakes, paralysis. According to the relative amount of the poison injected and the degree of its virulence (which differs not only in different kinds of snakes, but in the same snake under different conditions) the symptoms may result in death, or the bitten animal may recover. The poison is a Fig. 272. — Poison apparatus of rattlesnake. A, eye; Gc, poison-duct entering the poison-fang at | ; Km, muscles of mastication partly cut through at * ; Me, masseter or constrictor muscle; Me', continuation of the constrictor muscle to the lower jaw ; JV, nasal opening ; S, fibrous poison-sac ; z, tongue ; za, opening of the poison-duct ; z/", pouch of mucous membrane enclosing the poison-fangs. (FromWiedersheim. ) clear, slightly straw-coloured or greenish liquid ; it preserves its venomous properties for an indefinite period, even if completely desiccated. The poisonous principles are cer- tain proteids not to be distinguished chemically from other proteids which have no such poisonous properties. Immu- nity against the effects of the poison, and relief of the symp- toms after a bite has been inflicted, have been found to be conferred by injections of the serum of animals which have XII PHYLUxM CHORDATA 455 been treated with injections of increasing doses of the poison. The majority of snakes are viviparous. Some, however, lay eggs, which, nearly always, like those of the oviparous lizards, are left to be hatched by the heat of the sun, some of the Pythons being exceptional in incubating them among the folds of the body. Hatteria lives in burrows in company with mutton-birds (Pufifinus), and feeds on insects and small birds. It lays eggs enclosed in a tough parchment-like shell. The eggs are laid in November, and the embryos pass the winter in a state of hibernation unknown to any other vertebrate embryo, not emerging from the egg until nearly thirteen months have elapsed (Dendy). Of the Chelonia some (land- tortoises) are terrestrial; others (fresh- water tortoises) inhabit streams and ponds, while the sea-turtles and luths, or leather-backed turtles, inhabit the sea. Even among reptiles they are remarkable for their tenacity of life, and will live for a long time after severe mutilations, even after the removal of the brain ; but they readily succumb to the effects of cold. Like most other reptiles, the land and fresh-water tortoises living in colder regions hibernate in the winter ; in warmer latitudes they sometimes pass through a similar period of quiescence in the dry season. The food of the green turtles is exclusively vegetable ; some of the land tortoises are also exclusively vegetable feeders; other Chelonia either live on plant food, together with worms, insects, and the like, or are com- pletely carnivorous. All are oviparous, the number of eggs laid being usually very, great (as many as 240 in the sea- turtles) ; these they lay in a burrow carefully prepared in the earth, or, in the case of the sea-turtles, in the -sand of the sea-shore, in a round hole about fifteen or twenty inches 456 MANUAL OF ZOOLOGY SECT. deep and ten inches in diameter, and having covered them over, leave them to hatch. The crocodiles and alligators, the largest of living reptiles, are in the main aquatic in their habits, inhabiting rivers, and, in the case of some species, estuaries. Endowed with great muscular power, these reptiles are able, by the move- ments of the powerful tail and the webbed hind-feet, to dart through the water with lightning-like rapidity. By lying in wait motionless, sometimes completely submerged with the exception of the extremity of the snout bearing the nostrils, they are often able by the suddenness and swiftness of their onset to seize the most watchful and timid animals. In the majority of cases the greater part, and in some the whole, of their food consists of fishes ; but all the larger and more powerful kinds prey also on birds and mammals of all kinds, which they seize unawares when they come down to drink or attempt to cross the stream. On land their movements are comparatively slow and awkward, and they are correspondingly more timid and helpless. The Crocodilia are all oviparous, and the eggs, as large in some species as those of a goose, are brought forth in great numbers (sometimes loo or more), and either buried in the sand or deposited in rough nests. CLASS V. AVES In many respects birds are the most highly specialised of Craniata. As a class they are adapted for aerial life, and almost every part of their organisation is modified in ac- cordance with the unusual environment. The non-conduct- ing covering of feathers ; the modification of the fore-limbs as wings, of the sternum and shoulder-girdle to serve as xn PHYLUM CHORDATA 457 origins of the wing-muscles, and of the pelvic girdle and hind-limbs to enable them to support the entire weight of the body on land ; the perfection of the respiratory system, producing a higher temperature than in any other animals, — all ‘these peculiarities are of the nature of adaptations to flight. The common or domestic pigeon is known under many varieties which differ from one another in size, proportions, coloration, details in the arrangements of the feathers, and in many points of internal anatomy. The following description refers especially to the common dovecot pigeon. In the entire bird (Fig. 273) the plump trunk appears to be continued insensibly into the small, mobile head, with its rounded brain-case and prominent beak formed of the upper and lower jaws covered by horny sheaths. The head, neck, and trunk are invested in a close covering of feathers, all directed backwards and overlapping one another. Posteriorly the trunk gives origin to a number of outstanding feathers which constitute what is ordinarily called the tail. From the anterior region of the trunk spring the wings, also covered with feathers, and, in the position of rest, folded against the sides of the body. The legs spring from the hinder end of the trunk, but, owing to the thick covering of feathers, only the feet are to be seen in the living bird, each covered with scales and terminating in four digits {dg 1' — dg 4'), three directed forwards and one backwards. In order to make a fair comparison of the outer form with that of other craniate types, it is necessary to remove the feathers. When this is done the bird is seen to have a long, cylindrical, and very mobile neck, sharply separated both fron) head and trunk. The true tail is a short, conical projection of the trunk, known as the uropygium, and giving 458 MANUAL OF ZOOLOGY SECT. origin to the group of large feathers (ref) to which the word “ tail ” is usually applied. On the dorsal surface of the uropygium is a papilla bearing on its summit the opening of Fig. 273. — Columba livia. The entire animal from the left side with most of the feathers removed, ad. dg. rmx, ad-digital remex; al. sp, ala spuria; an, anus; a/, auditory aperture; cubital remiges ; r&wc\\xa\s,', b ky, .1 „ , • •, 1 basi-hyal; c. br, cerato - branchial : Ot the tympanic cavity, and ^ hy^ hyoid comu; ep. br, epi- presenting Telow a cojidyle branchial, for articulation with the mandible ; the mandible of the young bird consists of a cartilage bone, the articular (Fig. 279, ar'), and four membrane bones, which all coalesce in the 468 MANUAL OF ZOOLOGY sect. adult. The hyoid apparahis (Fig. 280), is of characteristic form, having an arrow-shaped body {b. hy) with a short pair of anterior cornua {0. hy) derived from the hyoid arch, and a long pair of posterior cornua (. mtcp, carpo-metacarpus: hu, hu- , . i j i v ^ merus; ph.i, phalanx of first digit; ph.2' , attached by ligament a sabre-shaped scapula ipcp) which extends XII PHYLUM CHORDATA 469 backwards over the ribs, and includes, with the coracoid, an acute angle, the coraco-scapular angle. The glenoid cavity {gl. cv) is formed in equal proportion by the two bones ; in- ternal to it the scapula is produced into an acromion process. In front of the coracoids is a slender V-shaped bone, the furcula {fur') or “ merrythought,” the apex of which nearly reaches the sternum, while each of its extremities is attached by ligament to the acromion and acro-coracoid processes of the corresponding side in such a way that a large aper- ture, the foramen triosseiC7n {f irs) is left between the three bones of the shoulder- girdle. The furcula is a mem- brane bone' and represents fused Clavicles and inter-' clavicle. Equally characteristic is the skeleton of the fore-limb. The humerus (Fig. 281, hu) is a large, strong bone, with a greatly expanded head and a prominent ridge for the in- sertion of the pectoral muscle. The radius {ra) is slender and nearly straight, the tilna stouter and gently curved. There are two large free car- Fig. 282. — Columba livia. Left manus of a nestling. The cartilaginous parts are dotted, cp. /, radiale; cp. 2, ulnare; mcp. l, 2,3, meta- carpals ; ph. i, phalanx of first digit : ph. 2,ph.2', phalanges of second digit; ph. 3, phalanx of third digit; ra, radius; 7il, ulna. (From Parker’s Zootomy.) pals, a radiale {rd) and an uhiare ftl'), and articulating with these is a bone called the carpo-metacarpus {cp. mtcp), consisting of two rods, that on the preaxial side strong and nearly straight, that on the postaxial side slender and curved, fused with one another at both their proximal and distal 470 MANUAL OF ZOOLOGY SECT. ends ; the proximal end is produced pre-axially, into an outstanding step-like process. The study of its development shows that this bone is formed by the union of the distal carpals with three meta- carpals (Fig. 282), the second and third of which are the two rod-like portions of the bone, the first, the step-like projection. Articulating with the first metacarpal is a single pointed phalanx 7) ; the second metacarpal bears two phalanges, the proximal one ( j>A. 2') produced postaxially into a flange, the distal one (j>A. 2") pointed ; the third metacarpal bears a single pointed phalanx {ph.j). Fig. 283. — Columba livia. Left innominate of a nestling. The cartilage is dotted. ac, acetabulum; a. tr, anti-trochanter; z7, pre-acetabular; and z/, post-acetab- ular portion of ilium; is, ischium; i. s. f, ischiatic foramen; ob. f, obturator notch; pu, pubis. (From Parker’s Zootomy.) The pelvic girdle (Fig. 283). The ilium {il') is an immense bone, attached by fibrous union with the whole of the syn-sacrum and becoming ankylosed with it in the adult. As usual it furnishes the dorsal portion of the acetab- ulum. The ventral portion of the acetabulum is furnished in about equal proportions by the pubis and ischium (Fig. 283) : it is not completely closed by bone, but is perforated by an aperture covered by membrane in the recent state. Both pubis and ischium are directed sharply PHYLUM CHORDATA 471 fe-- li.ls 7rUh.i~^ backwards from their dorsal or acetabular ends, and lie nearly parallel. Neither is- chium nor pubis unites ven- trally with its fellow to form a symphysis. In the hind-limb the femur (Fig. 284,/^) is a compara- tively short bone. Its proxi- mal extremity bears a promi- nent trocha7iter {tr) and a rounded head (^hd) . Its dis- tant end is produced into pulley-like condyles. Articu- lating with the femur is a very long bone, the tibio- tarsus {ti. ts') ; its distal end is pulley-like, not concave like the corresponding ex- tremity of the tibia of other Amniota. The study of its development shows that the pulley-like distal end of the bone (Fig. 285, //^) con- sists of the proximal tarsals, — astragalus and calcaneum, — which at an early period unite with the tibia and give rise to the compound shank- bone of the adult. The fibula (Fig. 284,7?) is very small, much shorter than the tibia, and tapers to a point at its distal end. -ts.mlts y Ph.i: I 5 tarso-metatarsus; tr. trochanter. 472 MANUAL OF ZOOLOGY SECT. Following the tibio-tarsus is an elongated bone, the tarso- metatarsus {ts, mtis), presenting at its proximal end a con- cave surface for the tibio-tarsus, and at its distal end three distinct pulleys for the articulation of the three forwardly directed toes. In the young bird the proximal end of this bone is a separate cartilage (Fig. 285, repre- senting the distal tarsals, and followed by three distinct metatarsals, belonging respectively to the second, third, and fourth digits. To the inner or preaxial side of the tarso-metatarsus, near its distal end, is attached by fibrous tissue a small irregular bone, the first metatarsal {nitts. 1) . The back- wardly directed hallux has two pha- langes, the second or inner toe three, the third or middle toe four, and the fourth or outer toe five. In all four digits the distal or ungual phalanx is pointed and curved, and serves for the support of the horny claw. A further peculiarity is the fact that the larger proportion of the bones contain no marrow, but are filled during life with air, and are therefore said to be pneumatic. The cavities of the various bones open externally in the dried skeleton by apertures called pneumatic foramina (Fig. pn.for'), by which, in the entire bird, they communicate with the air-sacs {vide infrai) . As might naturally be expected, the muscles of the fore- limb are greatly modified. The powerful downstroke of the Fig. 285. — Columba livia. Part of left foot of an un- hatched embryo (magni- fied). The cartilage is dotted. mtl. 2, second; mil. 3, third; and mtl. foiirth metatarsal; tl, tibia; il. /, proximal tarsal car- tilage; tl. 2, distal tarsal cartilage. (From Parker’s Zootomy.') XII PHYLUM CHORDATA 473 wing, by which the bird rises into and propels itself through the air, is performed by the pectoralis (Fig. 286, pet), an immense muscle having about one-fifth the total weight of the body ; it arises from the whole of the keel of the sternum {car. st) , from the posterior part of the body of Fig. 286. — Columba livia. The principal muscles of the left wing; the greater part of the pectoralis {pc£) is removed, car. st, carina sterni; ct, furcula; cor, coracoid; cor. br. br, coraco-brachialis brevis; cor. br. Ig, coraco-brachialis longiis; cp. st, corpus sterni; ext. cp. rd, extensor carpi radialis; ext. cp. ul, extensor carpi ulnaris; fi. cp. td, flexor carpi ulnaris; gl. c, glenoid cavity hu, head of humerus ; its distal end; /cif, pectoralis; its cut edge; its insertion; prn. br, pronator brevis; prn. Ig, pronator longus; pr. ptgm, pre- patagium; pt. ptgm, post-patagium ; sb. civ, sub-clavius; sb. civ', its tendon of insertion passing through the foramen triosseum, and dotted as it goes to the humerus; tns. acc, tensor accessorius; tns. br, tensor brevis; tns. Ig, tensor longus; tns. m.p, tensor membranse posterioris alae. that bone {cp. st), and from the clavicle {ct), filling nearly the whole of the wedge-shaped space between the body and the keel of the sternum, and forming what is commonly called the “breast” of the bird. Its fibres converge to their insertion {pcf) into the, ventral aspect of the humerus 474 MANUAL OF ZOOLOGY SECT. XU {hu, hu'), which it depresses. The elevation of the wing is performed, not, as might be expected, by a dorsally placed muscle, but by the sub-clavius {sb. civ), arising from the anterior part of the body of the sternum, dorsal to the pectoralis, and sending its tendon {sb. civ') through the foramen triosseum to be inserted into the dorsal aspect of the humerus. In virtue of this arrangement, the end of the foramen acting like a pulley, the direction of action of the muscle is changed, the backward puli of the tendon raising the humerus. Digestive Organs. — The niouth (Fig. 287) is bounded above and below by the horny beaks, and there is no trace of teeth. The tongue {tn^ is large and pointed at the tip. The pharynx leads into a wide and distensible {gul), which soon dilates into an immense reservoir or crop {crp) situated at the base of the neck, between the skin and the muscles and immediately in front of the sternum. In this cavity the food, consisting of grain, undergoes a process of maceration before being passed into the stomach. From the crop the gullet is continued backwards into the stomach, which consists of two parts, the prove7itriculus {pfvn) and the gizzard {giz). The proventriculus appears externally like a slight dilatation of the gullet, but its mucous mem- brane is very thick and contains numerous gastric glands so large as to be visible to the naked eye. The gizzard has the shape of a biconvex lens ; its walls are very thick and its lumen small. The thickening is due mainly to the im- mense development of the muscles which radiate from two tendons, one on each of the convex surfaces. The epi- thelial lining of the gizzard is very thick and horny, and of a yellow or green colour : its cavity always contains small stones, which are swallowed by the bird to aid the gizzard in grinding up the food. Fig. 287. — Columba livia. Dissection from the right side. The body-wall, with the vertebral column, sternum, brain, etc., are in sagittal section: portions of the gullet and crop are cut away and the cloaca is opened ; nearly the whole of the ilium is removed, and the duodenum is displaced outwards, a. ao, aortic arch; bd.i, bd.2, bile-ducts; b. fabr, bursa Fabricii; cbl, cerebellum; cce, right coecum; cpdin^ coprodaeum; cr, cere; crb. h, left cerebral hemisphere; crp, crop; cr. v. 1, first cervical vertebrae; di. coe, diacoele; dnt, dentary; duo, duodenum; eus. ap, aperture of Eustachian tubes; giz, gizzard (dotted behind the liver); gl, glottis; gul, gullet; ilm, ilium; i. orb. sp, inter-orbital septum; right kidney ; right lung; /r, liver (right lobe) ; ««, bristle pa.ssed from nostril into mouth ; obi. sep, oblique septum ; o. gl, oil gland ; pcd, pericardium ; pmx, pre-maxilla; pn, pancreas; pn. b, pineal body; pnd. 1-3, pancreatic ducts; pr. cv, right pre-caval; prdm, proctodaeum; p7'VJi, proventriculus (dotted behind liver) ; pt. cz>, post-caval; pty. b, pituitary body; pyg. si, pygo- style; r. au, right auricle; r. br, right bronchus; ret, rectum; r. vfit, right ventricle; sp. cd, spinal cord; spl, spleen (dotted behind liver); j. rhb, sinus rhomboidalis ; s. scr, syn-sacrum; si, carina sterni; syr, syrinx; th. v. i, first, and th. v.g, fifth thoracic vertebrae; tng, tongue; tr, trachea; is, right testis; ur, aperture of left ureter; urdm, urodacum; v. df, aperture of left vas deferens. 475 476 MANUAL OF ZOOLOGY SECT. The duodenum {duo') leaves the gizzard quite close to the entrance of the proventriculus and forms a distinct loop enclosing the pancreas. The rest of the small intestine is called the ilium {Urn) : it passes without change of diameter into the rectitm or large intestine {ret), the junction between the two being marked only by a pair of small blind pouches or coeca {ece) . The cloaca is a large chamber divided into three compartments. There are small buccal glands opening into the mouth, but none that can be called salivary. The liver {Ir) is large, and is divisible into right and left lobes, each opening by its own duct {b. d. I, b. d. 2), into the duodenum : there is no gall bladder. The pancreas {pn) is a compact reddish gland lying in the loop of the duodenum, into which it discharges its secretion by three ducts {pn. d. 1-3). A thick- walled glandular pouch, the bursa Fabricii {b. fabr), lies against the dorsal wall of the cloaca in young birds, and opens into the cloaca : it atrophies in the adult. The spleen {spl) is an ovoid red body, of unusually small proportional size, attached by peritoneum to the right side of the proventriculus. There are paired thyroids at the base of the neck ; and, in young pigeons, there is an elongated thymus on each side of the neck. The adrenals (Fig. 292, adr) are irregular yellow bodies placed at the anterior ends of the kidneys. glottis (Fig. 287,^/) is situated just behind the root of the tongue and leads into the larynx, which is supported by cartilages, but does not, as in other vertebrates, function as the organ of voice. From the larynx an elongated tube, the trachea or windpipe, the wall of which is supported by numerous bony rings, runs back along the ventral aspect of the neck to enter the body-cavity, where it divides into the right {r. br) and left bronchi, one passing to each of the lungs. XII PHYLUM CHORDATA 477 At the junction of the trachea with the bronchi is found the characteristic vocal organ, the syrinx {syr), occurring in no other class. The lungs (Fig. 287, Ing) are very small in comparison with the size of the bird, and are but slightly distensible, being solid, spongy organs, not mere bags with sacculated walls, as in Amphibia and many reptiles. Their dorsal sur- faces fit closely into the spaces between the ribs and have no peritoneal covering ; their ventral faces are covered by a strong sheet of fibrous tissue, the pubnonary aponeurosis or pleura, a special development of the peritoneum. Into this membrane are inserted small, fan-like costo-pulnionary mus- cles, which arise from the junction of the vertebral and sternal ribs. Each main- bronchus gives off secondary bronchi, and these branch again, sending off tubes which end blindly near the surface of the lung and give off blind dilations commonly known as alveoli. In addition to these, each main bronchus also gives off branches which end in a series of thin-walled air-sacs, which lie in the body-cavity, and are in communi- cation with the pneumatic cavities of the bones. The hea7't (Fig. 288) is of great proportional size, and like that of the crocodile consists of four chambers, i.e., the right and left auricles, and right and left ventricles. There is no sinus venosus, that chamber being, as it were, absorbed into the right auricle (Fig. 288, A, r. au). The right ventricle (Fig. 288, B) partly encircles the left, the former having a crescentic, the latter a circular cavity in transverse sections. The left auriculo-ventricular valve has the usual membranous structure consisting of two flaps connected with the wall of the ventricle by tendons, but the corresponding valve of the right side (R. V) is a large muscular fold, very characteristic of the class. 478 MANUAL OF ZOOLOGY SECT.' The right auricle receives the right and left pre-cavals {r. prc, 1. prc) and the post-caval {ptc), the left, four large pulmonary veins {p. v). The left ventricle (Fig. 288, /. vn), as in the crocodile, gives origin to the right aortic arch \jr. ao), but the right ventricle (r. V7i) gives off only one trunk, the pulmonary artery, which soon divides into two Fig. 288. — A, heart of the pigeon, dorsal aspect, a. ao, arch of aorta; br. a, brachial artery; z/, brachial vein; c. r, common carotid; yw, jugular; l.an, left auricle; 1. p. a, left pulmonary artery; 1. vn, left ventricle; pc. v, left pre- caval; post-caval ; p. z;, pulmonary veins; r. r. right auricle; r.p.a, right pulmonary artery; r. prc, right pre-caval; r. V7i, right ventricle. B, heart of a bird with the right ventricle opened. L. V, septum ventriculorum ; R. V, right ventricle; V, right auriculo-ventricular valve. (A, from Parker’s Zootomy; B, from Headley’s Birds.') B {r.p. a., L p. a). The left aortic arch is absent in the adult, and it is the right alone which is continued into the dorsal aorta. The result of this is that the systemic arteries re- ceive pure arterial blood from the left side of the heart, and the only mingling of aerated and non-aerated blood is in the JO < Tir PHYLUM CHORDATA 479 capillaries. This is perhaps the most important physiologi- cal advance made by birds over reptiles. The aortic arch curves over the right bronchus to reach the dorsal body-wall, and then passes directly backwards as the dorsal aorta. Fig. 289. — Columba livia. The brain; A, from above; B, from below; C, from the left side, cb, cerebellum; c. h, cerebral hemispheres; /, flocculus; inf, in- fundibulum; m. o, medulla oblongata; o. I, optic lobes; 0. t, optic tracts; pn, pineal body; II-XIII, cerebral nerves; j/. 7, first spinal nerve. (From Parker’s Zooiomyi) The brain completely fills the cranial cavity, and is re- markable for its short, broad, rounded form. The cerebellum 480 MANUAL OF ZOOLOGY SECT. (Fig. 289, c. b') is of great size, and has a large median portion and two small lateral lobes or fiocculi (/) ; the surface of the middle lobe is marked by grooves passing inwards in a radiating manner and carrying with them the grey matter, the extent of which is thus greatly increased. The hemi- spheres {c. h) extend backwards to meet the cerebellum, and the optic lobes {p. /) are thereby pressed outwards so as to take up a lateral instead of the usual dorsal position. Fig. 290. — The eye. A, in sagittal section; B, the entire organ, external aspect; cornea; ch, choroid; cl. pr, ciliary processes; ir, iris; I, lens; opt. nv, optic nerve; pet, pecten; rt, retina; scl, sclerotic; scl. pi, sclerotic plates. (After Vogt and Yung.) The eye (Fig. 290) is not even approximately globular, but has the form of a biconvex lens. Sclerotic plates are present, and there is a large pecten in the form of a plaited and strongly pigmented membrane projecting into XII PHYLUM CHORDATA 481 the cavity of the eye from the entrance of the optic nerve. The auditory organ (Fig. 291) is chiefly distinguished from that of reptiles by the great development of the cochlea. The tympanic cavity and columella have the same arrange- ment as in the frog ; the narrow Eustachian tubes open by a common aperture (Fig. 287, eus. ap) in the roof of the pharynx. The kidneys (Fig. 287, kd, Figs. 292 and 293, k') have a very characteristic form. Each is a flattened organ divided into three main lobes and fitted closely into the hollows of the pelvis. The ureters (ur) are narrow tubes passing directly backwards to open into the middle compart- ment, or the cloaca. The testes (Figs. 287 and 292, ts') are ovoid bodies, varying greatly in size according to the season, attached by the perito- neum to the ventral surfaces of the anterior ends of the kidneys. From the inner border of each goes ofl* a convoluted vas def- erens (vd), which passes backwards, parallel with the ureter, to open into the cloaca on the extremity of a small papilla. The posterior end of the spermiduct is slightly enlarged to form a vesicula seminalis (v.s). The female organs (Fig. 293) are remarkable for the more or less complete atrophy of the right ovary and oviduct. 21 Fig. 291. — Columba livia. The right membranous labyrinth, outer aspect; ampulla of posterior canal; /^B, posterior canal; HA, ampulla of hori- zontal canal; HB, horizontal canal; /ag-, cochlea or lagena; fftr, membrane of Reissner; />A, basilar part of cochlea; S, sacculus ; SA, ampulla of anterior canal; SB, anterior (canal. From Wiedersheim, af- ter Hasse.) 4S2 MANUAL OF ZOOLOGY SECT. The left ovary {ov') is a large organ in the adult bird, its surface studded with follicles or ovisacs, varying in size from about 15 mm. in diameter downwards, and each containing a single ovum. The left oviduct (/. od') is long and convoluted ; its anterior end is enlarged to form a wide, membranous coelomic funnel (/. into which the k adr pie fr'cU Fig. 292. — Columba livia Male urino-genital organs, adr, ad- renal; cl. 2, urodseum; cl. 3, proctodseum ; k, kidney; ts, testis, that of the right side dis- placed; ur, ureter; ur' , aper- ture of ureter ; vd, vas deferens ; vd' , its cloacal aperture; v. s, vesicula seminalis. (From Par- ker’s Zootomy.) Fig. 293. —Columba livia. Female urino- genital organs, cl. 2, urodaeum ; cl. 3, proctodmum; k, kidney; 1. od, left ovi- duct; 1. od' , its cloacal aperture; 1. od" , its coelomic funnel; 1. od'" , its coelomic aperture; ov, ovary; r. od, right oviduct; r. od' , its cloacal aperture; ur, ureter; ur' , its cloacal aperture. (From Parker’s Zootomy. ) ripe ova pass on their liberation from the ovisacs ; the rest of the tube has thick, muscular walls, lined with glandular epithelium, and opens into the urodaeum. Internal impregnation takes place. As the ova or XII PHYLUM CHORDATA 483 “ yolks ” pass down the oviduct, they are invested with the secretions of its various glands ; first with layers of albumen or “ white/’ next with a parchment-like shell-membrane, and lastly with a white calcareous shell. They are laid, two at a time, in a rough nest, and are incubated or sat upon by the parents for fourteen days, the temperature being in this way kept at about 40° C. (104° F.). At the end of incubation the young bird is sufficiently developed to break the shell and begin free life. It is covered with fine down, and is fed by the parents with a secretion from the crop, the so-called “ pigeon’s milk.” Of recent birds two main divisions are recognised — the Ratitae and the Carinatae — the former comprising only the emus (^Dromceus) , cassowaries {Casuarius) , and kiwis {Apteryx) South American ostriches {Rhea) ,ax\