Outlines of Zooloc^v. T- Arthur Thomson, 8th edition. 28 + 972 pj). '528 ligs. Oxford Uni- versity Press. 1929. This admirable survey of the animal kinodom from Amoeha to man is remarkable for the wealth of information systematically arranged in a volume of handy size. It contains essentiallv ^ - tlie information that the l)ej^innin2: zoologist w should have, and the skillful use of three points £ ^ of type gives a sense of perspective that is im- ■p portant, especiall\- in a work of this kind. In the ^ 03 ])resent edition the author has had the help of ^ his son, Dr. D. L. Thomson, in addni.e^ more ^'^' ])hysiolocrical material, and of J\Ir. R. M. Neill on H<; the structure and development of the mud-fishes. Hie first six chapters deal in a general wav w"th Q>w physiology, morphology, palaeontology, the doc- "^^ trine of descent, etc. Then follows an account P o . ' (DO of each of the principal phyla, including general *^^ characters, followed by descriptions of typical (D forms of special interest, then classification, struc- ^ ture, life-history, ecology, and other topics, suchj as parasitism and relation to disease. The illus- trations are clear and significant, and for the most part are original. The final chapters deal with geographical distribution and the factors in or- ganic evolution ; then follow test questions, an ex- cellznt list of books of reference, and an index. — R. P. BiGELOW. ^'^ ■\ Reviewed in the "Collecting Net' August 22, 1931, by Dr. R, P. Bigelow, and presented to the Liorary of the Marine Biological Laboratory OUTLINES OF ZOOLOGY / / - OUTLINES OF ZOOLOGY BY J. ARTHUR THOMSON, M.A, LL.D. REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN ; AUTHOR OF " THE STUDY OF ANIMAL LIFE," " THE SCIENCE OF LIFE, «'THE BIOLOGY OF THE SEASONS," "HEREDITY," "DARWINISM AND HUMAN LIFE," " THE SYSTEM OF ANIMATE NATURE," " THE NEW NATURAL HISTORY " EIGHTH EDITION, REVISED, WITH 528 ILLUSTRATIONS HUMPHREY MILFORD OXFORD UNIVERSITY PRESS London Edinburgh Glasgow Leipzig New York Toronto Melbourne Cape Town Bombay Calcutta Madras Shanghai 1929 First Edition i8q2 Second Edition i8q§ Third Edition . . iSgg Fourth Edition Decetnber IQ07 Fi/th Edition November igro Sixth Edition April igi4 „ ,, Second Impression . . December jqi6 „ „ Third „ . . January iQig ,, ,, Fourth ,, . . October iqiq „ ,, Fi/th „ . . September IQ20 Seventh Edition March ig2i Eighth Edition October igzg PRINTED IN GREAT BRITAIN BY MORRISON AND GIBE LTD., LONDON AND EDINBURGH \ PREFACE TO THE EIGHTH EDITION In this new edition more account has been taken of the physiological aspect, and in this connection I have been helped by my son, Dr. David Landsborough Thomson. More space has also been devoted to the structure and development of certain types, such as the Mud-fishes, which were too briefly discussed in previous editions ; and here I have been indebted to the assistance generously given by Mr. R. M. Neill, M.C., M.A., Lecturer on Zoology in this University. I have also to thank Prof. W. Rae Sherriffs, D.Sc, for many useful suggestions. Numerous figures have been added, mostly drawn from specimens. J. A. T. The University, Aberdeen, August 1929. PREFACE TO THE SEVENTH EDITION This book is intended to serve as a Manual which students of Zoology may use in the lecture room, museum, and laboratory, and as an accompaniment to several well- known works, cited in the Appendix, most of which follow other modes of treatment. To numerous authorities I acknowledge an obvious indebtedness, a detailed recognition of which would be out of place in a book of this kind. I must, however, acknowledge that in the preparation of a previous edition I had throughout the able assistance of Miss Marion Newbigin, D.Sc, and I have also been aided by sugges- tions from various kindly critics, especially Professor W. C. M'Intosh, Professor P. J. White, the late Dr. Ramsay Traquair, Dr. John Beard, the late Mr. J. G. Goodchild, Dr. Arthur Mastermait, Dr. John Rennie, Dr. W. D. Henderson, Mr. E. S. Russell, Mr. W. P. Pycraft, Mr. C. Tate Regan, and Professor H. J. Fleure. For most of the figures, I am indebted to my artist friends, Mr. William Smith, Miss Florence Newbigin, vu Vlll PREFACE TO THE SEVENTH EDITION Miss E. M. Shinnie, Miss Alice M. Davidson, and the late Mr. George Davidson. In almost every case the illustra- tions have been derived from original memoirs and works of reference, or drawn from specimens. J. A. T. The University, Aberdeen, October 1920. CONTENTS GENERAL CHAPTER I PAGE General Survey of the Animal Kingdom . . i CHAPTER H Physiology ....... 20 CHAPTER HI Morphology ....... 4° CHAPTER IV Embryology ....... 60 CHAPTER V^ Palaeontology ....... 98 CHAPTER VI Doctrine of Descent . . . . .105 iz 37951 X CONTENTS INVERTEBRATES CHAPTER VII PAGE Protozoa. ....... 109 CHAPTER VIII Sponges . . . . . . . -153 CHAPTER IX CCELENTERA . . . . . . . 167 CHAPTER X Unsegmented Worms . . . . . .210 CHAPTER XI Annelids ....... 244 CHAPTER XII ECHINODERMS ....... 289 CHAPTER XIII Arthropoda ....... 319 CHAPTER XIV Onychophora or Prototracheata, Myriopoda, and Insecta ....... 361 CHAPTER XV Arachnoidea and Pal^ostraca . . . .412 CHAPTER XVI Molluscs ....... 433 CONTENTS XI VERTEBRATES CHAPTER XVII Hemichorda • • PAGE . 489 CHAPTER XVIII Urochorda • • f • 499 CHAPTER XIX Cephalochorda • * • t • . 515 CHAPTER XX Structure and Development of Vertebrates . .529 Cyclostomata CHAPTER XXI . 574 Fishes CHAPTER XXII • • . 587 Amphibia CHAPTER XXIII 649 Reptiles CHAPTER XXIV . 683 Birds CHAPTER XXV 725 Mammals CHAPTER XXVI . 776 Xll CONTENTS GENERAL CHAPTER XXVII Distribution . . . . ' . . . 895 PAGE CHAPTER XXVIII Theory of Evolution. ..... 908 TEST QUESTIONS FOR STUDENTS . . .915 APPENDIX ON BOOKS . . . . .931 INDEX 937 LIST OF ILLUSTRATIONS -♦ after Huxley FiG. 1. Duckmole {Ornithorhynchus) ., . . • 2. Phenacodus, a primitive extinct Mammal — after Cope 3. Extinct moa and modern kiwi — after Carus Sterne 4. Crocodiles ..... 5. Salamander, an Amphibian 6. Queensland Dipnoan (Ceratodus) 7. Lancelet, Amphioxus — after Haeckel 8. Ascidian or sea-squirt — after Haeckel . 9. Cephalopod (paper nautilus, female) 10. Fresh-water crayfish {Astacus), a Crustacean 11. a. Caterpillar ; b, pupa ; c, butterfly 12. Spider ..... 13. Crinoid or feather-star 14. Earthworm .... 15. Bladderworm stage of a Cestode — after Leuckart 16. Sea-anemones on back of hermit-crab — after Andres . 17. Fossil Foraminifera (Nummulites) in limestone — after Zittel 18. Diagrammatic expression of classification in a genealogical tree ...... 19. Diagram of Vertebrates .... 20. Diagram of Invertebrates. 21. Diagrams of reflex actions — modified from Bayliss 22. Three ciliated cells. .... 23. A smooth or unstriped muscle-cell, slowly contracting 24. A piece of striped muscle fibre with its nerve-endings 25. Diagram of cell structure — after Wilson 26. Structure of the cell — after Carnoy . " 27. Fertilised ovum of Ascaris — after Boveri 28. Diagram of cell division — after Boveri . 29. Karyokinesis — after Flemming . 30. Diagrammatic expression of alternation of generations 31. Diagram of ovum., showing diffuse yolk granules xiii PA.GE 2 3 3 4 5 5 6 7 9 9 9 10 10 II II 12 13 15 18 19 27 48 49 50 52 54 55 55 57 65 67 XIV LIST OF ILLUSTRATIONS FIG. 32. Diagram of a typical spermatozoon 33. Forms of spermatozoa (not drawn to scale) 34. Diagram of maturation and fertilisation. (From " Evolution of Sex ") . 35. Oogenesis and spermatogenesis — after Boveri . 36. Fertilisation of egg-cell — after Fol 37. Diagram showing relative size of an egg-cell and a sperm-cell 38. Fertilisation in Ascaris megalocephala — after Boveri . 39. Modes of Segmentation ..... 40. Life-history of a coral, Monoxenia darwinii — from Haeckel 41. Embryos — (i) of bird ; (2) of man — after His. The latter about twenty-seven days old 42. Larvae of common eel ..... 43. Mendelian inheritance illustrated in wood snail {Helix nemoralis) ...... 44. Gradual transitions between Paludina neumayri (a), the oldest form, and Paludina hcernesi (/) — from Neumayr 45. Structure of Amoeba proteus — after Lucy A. Carter 46. Life-history of ^wcB&fl ..... 47. Actinophrys sol (Sun-animalcule) — after Grenacher 48. Polystomella, megalospheric form, with large central chamber (M.) and one nucleus (N.) — after Lister . 49. Polystomella, microspheric form, with small central chamber (c.c.)> numerous nuclei (A^.), bridges of protoplasm between chambers {B.) — after Lister 50. Paramcecium in longitudinal optical section, and dividing — after BUtschli ..... 51. Conjugation of Paramcecium aurelia — four stages — after Maupas ...... 52. Diagrammatic expression of process of conjugation in Para moecium aurelia — after Maupas 53. Vorticella — after Biitschli .... 54. Volvox globator — after Klein and Janet 55. Life-history of Monocystis — after Biitschli 56. Life-history of Gregarina — after Biitschli 57. End-to-end union of Gregarines — after Frenzel 58. Life-history of Coccidium — after Schaudinn 59. Diagram of Protomyxa aurantiaca — after Haeckel 60. Formation of shell in a simple Foraminifer — after Dreyer 61. A pelagic Foraminifer — Hastigerina {Globigerina) murrayi — after Brady ....... 62. The trumpet-shaped ciliated Infusorian, Stentor — after Stein LIST OF ILLUSTRATIONS XV FIG. 63. 64. 65. 66. 67. 68. 69. 70. 71- 72. 73- 74- 75. 76. 77- 78. 79- 80. 8i. 82. 83- 84. 85. 86. 87. 88. 89. 90. 91. 92. 93 Optical section of a Radiolarian (Actinomma) — after Haeckel A Monad Infusorian — after Saville Kent Diagram of the structure of Noctiluca . Spore -formation in Noctiluca — after Roule Glossina palpalis, tsetse fly . . . Life-cycle of Plasmodium vivax . Trypanosoma gambiense .... Colonial Infusorian — Ophrydium sessile — after Saville Kent A colonial flagellate Infusorian — Proterosponia haeckelii — after Saville Kent .... Simple sponge (Ascetta primordialis) — after Haeckel A sponge colony ..... Sponge spicules ..... Section of a sponge — after F. E. Schulze Diagram showing types of canal system — after Korschelt and Heider. The flagellate regions are dark throughout, the mesogloea is dotted, the arrows show the direction of the currents. All the figures represent cross-sections through the wall ...... Diagram of sponge structure .... Development of Sycandra raphanus — after F. E. Schulze Diagrammatic representation of development of Oscarella lobularis — after Heider Diagram of early fixed stage of sponge . A, Young Dicyema — after Whitman. B, Female Orthonectid {Rhopalura giardii) — after Julin Salinella — after Frenzel ..... Diagram of Coelenterate structure, endoderm darker through out ...... Colony of Hydradinia on back of a Buccinum shell tenanted by a hermit-crab ..... Diagram of a typical Hydrozoon polyp — after Allman Diagram of stinging-cells or cnidoblasts, the one to the right undischarged ...... Hydra hanging from water-weed — after Greene Minute structure of Hydra — after T. J . Parkier and Jickeli Development of Hydra — after Brauer . Bougainvillea — after Allman .... Structure of a Swimming-bell or Medusoid, Obelia geniculata budded off from a Companularian Hydroid Structure of a Medusoid — after Allman , Surface view of A urelia — from Romanes h PAGE 135 138 141 143 145 146 148 154 154 156 157 161 162 163 165 165 169 170 171 173 174 177 179 181 182 183 185 XVI LIST OF ILLUSTRATIONS FIG. FAG 94. Vertical section of i4wr^/ia — after Claus . . . j86 95. Diagram of life -history of ^wr^/io — after Haeckel . . 188 96. Lucernaria — after Korotneff . . . . .189 97. Diagram of Lucernaria — after Allman . . .189 98. External appearance of Tealia crassicornis . . . 191 99. Vertical section of a sea-anemone — after Andres . .192 100. Section through sea-anemone (across arrow in Figure 99) — after Andres. ...... 193 loi. Z, Diagrammatic section of Zoantharian ; ^, of Alcyonarian — after Chun . . . . . . 194 102. The formation of a coral shell (Astroides) — after Pfurtscheller 196 103. Structure of Antipatharians ..... 197 104. Diagrams of Types of Alcyonaria — after Hickson . .198 105. Corallium rubrum, a corner of a colony — after Lacaze- Duthiers . . . . . • • 199 106. Alcyonarian spicules ...... 200 107. Diagram of a gymnoblastic Hydroid — after Allman . . 202 108. Graptolites ....... 202 109. Hydroids — after Hincks ..... 203 no. Campanularian Hydroid — after Allman . . . 204 111. Diagram of a Ctenophore — after Chun . . . 206 112. Hydroctena. Amedusoid with hints of Ctenophore structure 207 113. Commensalism of sea-anemones and hermit-crab . . 208 114. Portions of excretory system of flat-worms . . . 211 115. Diagram of Turbellarian — after Lang . . . .213 116. Structure of liver fluke — after Sommer. From ventral sur- face. The branched gut (g.) and the lateral nerve {l.n.) are shown to the left, the branches of the excretory vessel (e.v.) to the right ..... 117. Reproductive organs of liver fluke — after Sommer . 118. Life-history of liver fluke — after Thomas 119. Diagram of life-cycle of liver fluke .... 120. Male and female Bilharzia — Schistosomum hcsmatobium — after Looss ....... 121. Front end of the head of TcBnia solium .... 122. Diagram of reproductive organs in Cestode joint — con- structed from Leuckart ..... 123. Life-history of T^nm so/mw — after Leuckart 124. Diagram of life-history of Tania solium 125. Diagrams of bladder-worms ..... 126. Diagrammatic longitudinal section of a Nemertine [Amphi- porus lactifloreus), dorsal view — after M'Intosh. . 231 216 217 219 221 222 225 226 227 228 230 LIST OF ILLUSTRATIONS XVll riG. PAGE 127. Transverse section of the Nemertine Drepanophorus latus — after Biirger . . . • • .232 128. Transverse section of a simple Nemertine {Carinella) — after Biirger ....... 233 129. Cross-section through Ascaris ..... 237 130. Diagram of the structure of a male Nematode . . . 238 131. Trichinae in muscle, about to be encapsuled — after Leuckart 241 132. Trichinae in muscle, encapsuled. There may be 12,000 in a gramme of pig's muscle. After Leuckart . . 241 133. Earthworms ....... 246 134. Anterior region of earthworm — after Hering . . . 247 135. Transverse section of earthworm .... 251 136. Reproductive organs of earthworm — after Hering . . 253 137. Stages in the development of earthworm — after Wilson . 255 138. Arenicola marina . . . . . • '2 57 139. Anterior part of nervous system in Arenicola — after Vogt and Yung ....... 259 140. Dissection of lob-worm from dorsal surface . . . 260 141. Cross-section of Arenicola — after Cosmovici . . . 261 142. Diagram showing structure of a Trochosphere of Eupomatus — after Shearer ...... 263 143. Development of Polygordius — after Fraipont . . • 265 144. Parapodium of " Heteronereis " of Nereis pelagica — after Ehlers ....... 267 145. Free-living Polychaete {Nereis cuUrifera) . . . 268 146. Sex dimorphism in Bonellia viridis . . . • .269 147. Transverse section of leech — after Bourne . . • 274 148. Alimentary system of leech — after Moquin-Tandon . . 275 149. Dissection of leech — after Bourne .... 276 150. A nephridium of leech — after Boiume .... 277 151. Development of Sagitta — after O. Hertwig. Illustrating formation of a body cavity by pockets from the archen- teron ; also the early separation of reproductive cells . 281 152. Diagram of a Rotifer {Hydatina) — after Plate . . 283 153. Actinotrocha or larva of Phoronis — after Masterman . 285 154. Phoronis . . . . .... . . 285 155. Diagram of an Ectoproctous Polyzoon {Plumatella) . . 286 156. Interior of Brachiopod shell, showing calcareous support for the " arms " — after Davidson .... 287 157. Pluteus larva of Ophiuroid, with rudiment of adult — after Johannes Miiller ...... 290 158. Starfish ........ 293 XVlll LIST OF ILLUSTRATIONS FIG , PAGE 159. Dissection of A sterias rubens. From a specimen . . 294 160. Diagrammatic cross-section of starfish arm — after Ludwig . 296 161. Regeneration of a starfish from a separated arm — so-called " comet form "....,. 297 162. Ventral surface of disc of an Ophiuroid {Ophiothrix fragilis) — after Gegenbaur . . . . . .299 163. Apical disc of sea-urchin . . . . . 301 164. Dissection of sea-urchin ...... 302 165. Spicules of Holothurians — after Semon . . . 306 166. A small sea-cucumber ...... 307 167. Dissection of Holothurian {Holothuria tuhulosa) from the ventral surface ...... 309 168. Diagrammatic vertical section through disc and base of one of the arms of Antedon rosacea — after Milnes Marshall . 311 169. General appearance of a stalked Crinoid (P^n/acnmfs) . 313 170. Stages in development of Echinoderms — after Selenka . 316 171. Appendages of Norway lobster ..... 324 172. Central nervous system of the crayfish . . . 325 173. Section of compound eye of Mysis vulgaris — after Grenacher 326 174. An ommatidium of a compound eye . . . .327 175. Longitudinal section of lobster, showing some of the organs . 329 176. Male reproductive organs of crayfish — after Huxley . 332 177. Female reproductive organs of crayfish — after Suckow . 333 178. Section through the egg of Astacus after the completion of segmentation — after Reichenbach . . -334 179. Longitudinal section of later embryo of Astacus — after Reichenbach ...... 335 180. Section through cephalothorax of a crab — after Pearson . 336 181. Dorsal aspect of swimming crab (Portunus) . . . 337 182. Dorsal aspect of shore crab (Carcinus). . . . 337 183. Ventral aspect of female shore crab .... 338 184. Dorsal surface of Apus cancriformis — from Bronn's " Thierreich "...... 339 185. Daphnia . . . . . . . .341 186. Cypris ........ 342 187. Cyan's, side view, after removal of one valve — after Zenker . 342 188. Cyclops type ....... 343 189. Two barnacles hanging from a ship / . . . 344 190. Acorn-shell [Balanus tintinnabulum) — after Darwin . . 346 191. Development of Sacculina — after Delage. (Not drawn to scale) ....... 347 192. Sacculina as a parasite on a female crab — after Delage . 348 LIST OF ILLUSTRATIONS XIX FIG. 193. Nehalia — after Sars ...... 194. Anaspides — after Caiman ..... 195. An Amphipod [Caprella linearis) .... 196. Dorsal view of a wood-louse {Oniscus) — after Webb and Sillem ....... 197. Hermit-crab withdrawn from its shell. The anterior ap- pendages are broken off . 198. Mysis flexuosa, from side ..... 199. Nervous system of shore-crab {Carcinus mcenas) — after Bethe 200. Partial peripheral segmentation of the ovum — characteristic of Arthropods ...... 201. Zoaea of common shore-crab {Carcinus mcenas) — after Faxon 202. External form of Peripatus — after Balfour 203. Dissection of Peripatus — after Balfour 204. Embryos of Peripatus capensis, showing closure of blasto pore and curvature of embryo — after Korschelt and Heider 205. A millipede 206. A centipede 207. Mouth-parts and poison claws of a centipede 208. Mouth-parts of a millipede 209. Female cockroach (P. orientalis) 209A. Male cockroach (P. orientalis) 210. Mouth-parts of cockroach 211. Ventral aspect of male cockroach with the wings extended An imaginary median line has been inserted 212. Leg of cockroach ..... 213. Transverse section of insect — after Packard . 214. Head and mouth-parts of bee — after Cheshire 215. Nervous system of bee — after Cheshire 216. Food canal of bee — in part after Cheshire 217. Hive-bees and the cells in which they develop 218. Mouth-parts of mosquito — after Nuttall and Shipley 219. Young may- fly or ephemerid — after Eaton 220. Life-histories of insects . 221. Life-history of the silk-moth {Bombyx fnori) 222. A typical caterpillar 223. Development of blow-fly {Calliphora erythrocephala) — after Thompson Lowne .... 224. Fly about to emerge from pupa-case — after Hayek 225. Anufida maritima (after Imms), one of the primitive wing less Collembola ...•., PAGE 349 349 350 351 352 353 355 357 358 362 363 364 366 366 367 368 370 370 371 373 374 375 378 380 381 383 386 387 398 399 400 401 402 404 XX LIST OF ILLUSTRATIONS FIG. PAGK 226. Silver fish {Lepisma saccharina), one of the Thysanura . 405 227. Acerentomon, a very primitive insect .... 408 228. Mosquito — after Nuttall and Shipley . . . .411 229. Scorpion, ventral surface . . . . .413 230. Scorpion, dorsal surface . . . • • -415 231. Sex dimorphism in a spider (iVg/?/ii7a mgm) — after Vinson . 418 232. Dissection of My gale from the ventral surface — after Cuvier . 419 233. Section of lung-book — after Macleod .... 420 234. Follicle-mite (greatly enlarged) .... 423 235. Itch-mite {Sarcoptes scahiei) (greatly enlarged) . . 423 236. Tick [Ixodes riduvius, female), dorsal surface (after Wheler), showing the oval shield ..... 424 237. Tick {Ixodes riduvius, female), ventral surface — after Wheler ....... 424 238. Mouth-parts of a tick ...... 425 239. Limulus or King-crab, ventral aspect .... 427 240. Limulus or King-crab, dorsal aspect .... 428 241. Yowng Limulus — after Walcott. .... 429 242. Trilobite (Conocephalites) — after Barrande . . . 430 243. Vertical cross-section of a Trilobite (Calymene) — after Walcott . . . . • • -430 244. Under-surface of a Trilobite — after Beecher . . -431 245. Sea-spider {Pycnogonum littorale), from the dorsal surface . 432 246. Male of Nymphon — after Sars ..... 432 247. Ideal mollusc — after Ray Lankester .... 434 248. Stages in molluscan development .... 435 249. Roman snail (Helix pomatia) ..... 436 250. Vertical section of the shell of a species of Helix . . 437 251. Dissection of snail ...... 442 252. Reproductive organs of Wg/j;t /jowa^za — after Meisenheimer . 443 253. Snail (//e/jA;^owai!Ja) laying its eggs — after Meisenheimer . 444 254. Diagram of larva of Paludina — after Erlanger . . 445 255. The fresh-water mussel (C7mo) ..... 447 256. Structure of Anodonta — after Rankin .... 45i 257. Glochidium larva of the fresh-water mussel . . . 454 258. Side view of Se^m — after Jatta .... 456 259. External appearance of a squid {Loligo) . . . 458 260. Diagram of the structure of Sepia — mainly after Pelseneer . 459 261. Diagram of circulatory and excretory systems in a Decapod like Sepia — after Pelseneer .... 463 262. Male of Argonauta (after Jatta), showing " hectocotylus " arm ; compare Fig. 9, showing the female . . 464 LIST OF ILLUSTRATIONS XXI FIG. 263. Bunch of Sepia eggs attached to plant — after Jatta 264. Common buckie {Buccinum undatum) . 265. Bivalve {Panopcea norvegica), showing siphons 266. Nudibranch {Dendronotus arbor escens), showing dorsal out growths forming adaptive gills ... 267. Ventral surface of Patella vulgata — after Forbes and Hanley 268. Chiton — after Pretre ..... 269. Dorsal view of nervous system of Acanthochiton — after Pelseneer ....•• 270. Anatomy of Chiton ..... 271. A Pteropod [Cymhulia peronii), showing the wing-like ex pansions (pteropodial lobes) of the mid-foot 272. Stages in moUuscan development 273. Proneomenia. Nervous system — from Hubrecht 274. Section of shell of Nautilus — after Lendenfeld 275. The Pearly Nautilus {Nautilus pompilius) — after Owen 276. Section of the shell of Spirula .... 277. Spirula, a small Decapod cuttlefish 278. Male of Balanoglossus {Dolichoglossus) kowalevskii — after Bateson ...... 279. Dissection of Balanoglossus .... 280. Transverse section through gill-slit region of Ptychodera minuta — after Spengel .... 281. Direct development of Dolichoglossus — after Bateson 282. Tomaria larva, from the side — after Spengel . 283. Piece of a colony of Cephalo discus, showing the tubes in habited by the animals — after Ridewood 284. An individual Cephalodiscus — after Ridewood 285. Dissection of Ascidian — after Herdman 286. Diagram of Ascidian — after Herdman 287. Young embryo of Ascidian (Clavelina) — after Van Beneden and Julin ...... 288. Embryo of Clavelina — modified after SeeUger 289. Part of a colony of Botryllus, showing two individuals em bedded in a gelatinous matrix and with a common exhalant aperture . . .^ . 290. Asexual reproduction in Salpa .... 291. " Nurse " of Doliolum mUlleri — after Uljanin. 292. Sexual individual of Doliolum millleri — after Uljanin 293. Diagram of Salpa africana .... 294. Structure of Appendicular ia — after Herdman 295. Lateral view of Amphioxus — after Ray Lankester . PAGE 465 466 467 468 469 471 471 472 475 476 478 483 484 486 487 491 492 493 494 496 497 497 5or 504 506 507 508 509 510 510 511 512 516 XXll LIST OF ILLUSTRATIONS FIG . PAGE 296. Transverse section through pharyngeal region of Amphioxus — after Ray Lankester . . . . .518 297. Development of atrial chamber in Amphioxus — after Lankester and Willey . . . . .519 298. Sections through embryos of Amphioxus, to illustrate de- velopment of body cavity — after Hatschek . . 522 299. The nephvidia. of Amphioxus — after Boveri . . . 524 300. Small portions of excretory organs of Amphioxus (A) and the Polychaete Phyllodoce (B) — after Goodrich . '525 301. Early stages in the development of Amphioxus — after Hatschek . . . . . . .526 302. Larval Amphioxus, from the right side — after Willey . 527 303. Diagrammatic section through a Vertebrate animal . -536 304. Ideal fore- and hind-limb — after Gegenbaur . . . 538 305. Partial section of a Vertebrate brain (diagrammatic) . . 540 306. Vertical section of the pineal eye in an embryo of Spheno- don — after Dendy . . . . . -541 307. Diagram illustrating the development of the nervous system in an Elasmobranch fish ..... 544 308. Diagrammatic section of spinal cord .... 546 309. Diagram of spinal cord of man, thoracic region — after Johnston ....... 546 310. Diagram showing development of ear in a Vertebrate (cartilaginous fish) ...... 548 311. Diagram showing the ear and related parts in a young cat . 549 312. Diagram of the eye ...... 550 313. Development of the eye — after Balfour and Hertwig . 551 314. Diagram of embryonic pharynx in a Mammal — after Goette . 555 315. Origin of lungs, liver, and pancreas in the chick — after Goette ....... 556 316. Section through a young newt ..... 557 317. Section through Elasmobranch embryo — after Ziegler . 559 318. Blood corpuscles . . . . . . .560 319. Diagram showing the valves {VA.) common in veins . .561 320. Diagram of circulation — after Leunis .... 563 321. Development of excretory system of Vertebrate — in part after Boveri ...... 566 322. Urinogenital system of Chordata Vertebrata . . . 567 323. Mammalian ovum — after Hertwig .... 572 324. Median longitudinal section of anterior region of Myxine — after Retzius and Parker ..... 576 325. Respiratory system of hag, from ventral surface . . 577 LIST OF ILLUSTRATIONS XXlll FIG. PAGE 326. Bdellostoma stouti (Californian hag), enveloped in sheath of mucus — after Bashford Dean .... 579 327. The lamprey {Petromyzon marinus) .... 580 328. Longitudinal vertical section of anterior end of larval lamprey ....... 583 329. Restored skeleton of Palceospondylus gunni — after Traquair 585 330. Pterichthys milleri. Lateral view — restored by Traquair . 586 331. Diagram of the placoid scale of an Elasmobranch fish . 590 332. Diagram of the " soft " scales of a Bony Fish . . -591 333. Under surface of skull and arches of skate — after W. K. Parker ....... 592 334. Side view of skate's skull — after W. K. Parker . . 593 333. Skeleton of skate — from a preparation . . -594 336. Dissection of nerves of skate ..... 596 337. Side view of chief cranial nerves of Elasmobranchs — slightly modified from Cossar Ewart . . . 600 338. Spiral valve of skate — after T. J. Parker . . . 601 339. Heart and adjacent vessels of skate — in part after Monro . 602 340. Upper part of the dorsal aorta in the skate — after Monro . 603 341. Urinogenital organs of male skate .... 604 342. Urinogenital organs of female skate — in part after Monro . 605 343. Elasmobranch development — after Balfour . . . 606 344. Embryo dogfish in egg-case ("mermaid's purse") which has been cut open to show contents . . . 608 345. External features of Acanthias vulgaris . . . 610 346. Acanthias vulgaris. Longitudinal section showing organs . 611 347. Side view of brain of Acanthias vulgaris — after Purser . 612 348. Dorsal view of brain ol Acanthias vulgaris — after Purser . 613 349. Arterial system of Acanthias — after O'Donoghue . . 614 350. The haddock ....... 616 351. External characters of a Teleostean — a carp {Cyprinus carpio) — after Leunis . . . . .617 352. Lines of growth on salmon's scale — from J. Arthur Hutton . 6x8 353. Diagram of the zigzag myotomes in a fish . . . 6x8 354. Caudal vertebra of haddock ..... 6x9 355. Disarticulated skull of cod ..... 620 356. Pectoral girdle and fin of cod . . . . .621 357. Section of a Teleostean gill ..... 622 358. Diagram of Teleostean circulation — after Nuhn . . 624 359. The early development of the salmon . . . 625 360. Development of eel — after Schmidt .... 629 361. Young skate — from Beard ..... 633 Xxiv LIST OF ILLUSTRATIONS PAGE FIG. 362. Lateral view of dogfish {Scyllium catulus) . . . 634 363. Polypterus bichir . ,....• 637 364. Larva of Polypterus (after Budgett), li inch in length . 637 365. Sturgeon {A cipenser sturio) ..... 638 366. Bony pike {Lepidosteus osseus) ..... 639 367. The Queensland lung-fish {Ceratodus forsteri) . . . 642 368. Protopterus, the African mud-fish .... 643 369. Larva of Protopterus — after Budgett .... 643 370. Lepidosiren (after Grahana Kerr), showing (Pc.F.) pectoral fin and the tufted pelvic fin {Pv.F.) of the mature male . . • • • • 371. 'Laxvdi oi Lepidosiren — after Graham Kerr 372. Skeleton of Ceratodus fin — from Gegenbaur . 373. The edible frog (Rana esculenta) 374. Vertebral column and pelvic gurdle of bull-frog 375. Skull of frog — upper and lower surface — after W. K. Parker 376. Pectoral girdle of Rana esculenta — after Ecker 377. Skeleton of frog. The half of the pectoral girdle, and fore and hind-limb of the right side, are not shown 378. Side view of frog's pelvis — after Ecker 379. Brain of frog — after Wiedersheim 380. Nervous system of frog — after Ecker . 381. Arterial system of frog . 382. Venous system of frog . 383. Urinogenital system of male edible frog — after Ecker . 384. Urinogenital system of female frog— after Ecker 385. Division of frog's ovum — after Ecker . 386. Section of frog embryo— after Ziegler's model and Marshall 387. Dissection of tadpole— after Milnes Marshall and Bles 388. Life-history of a frog— after Brehm . 389. Male and female of the Crested Newt {Triton or Molge cr (status) . . . • • 390. The Axolotl {Amhly stoma tigrinum) 391. Proteus anguineus .... 392. Limbless subterranean Amphibian {Ichthyophis) 393. Caecilian {Ichthyophis) with eggs— after Sarasin 394. Foetal membranes in Amniota — after Roule . 395. Sphenodon punctatus. The New Zealand "Lizard" 396. Lateral view of brain of Sphenodon punctatus — after Osawa 397. External appearance of tortoise 398. Skull of turtle ...••• 399. Carapace of tortoise ..... 645 645 647 651 653 654 655 656 657 658 659 663 664 668 668 671 672 674 676 678 678 679 679 681 684 685 685 687 687 688 LIST OF ILLUSTRATIONS XXV FIG. PAGE 400. Pectoral girdle of a Chelonian ..... 689 401. Internal view of the plastron of the Greek tortoise . . 690 402. Scales on ventral surface of plastron of Greek tortoise . 690 403. Internal view of skeleton of tortoise .... 691 404. Dissection of Chelonian heart — after Huxley . . .691 405. Heart and associated vessels of tortoise — after Nuhn . 692 406. Hyoid apparatus of a Chelonian .... 692 407. Roof of the skull of a lizard (Varanid) . . . 695 408. Side view of skull of Lacerta — after W. K. Parker . . 696 409. Pectoral girdle of a lizard ..... 698 410. Heart and associated vessels of a lizard — after Nuhn . . 699 411. Lung of ChamcsleG vulgaris, showing air-sacs — after Wieders- heim ........ 700 .}i2. Slow-woxia {Angiiis fragilis), alimhlesslizard . . . 702 413. Anterior view of Python's vertebra .... 704 414. Posterior view of Python's vertebra .... 704 415. Snake's head — after Nuhn ..... 705 416. Side view of skull of non-poisonous Pythonid snake . . 706 417. Side view of skull of a poisonous snake .... 707 418. Skull of grass-snake — from W. K. Parker . . . 708 419. Lower siurface of skull of a young crocodile . . . 710 420. First vertebra of crocodile . . . . .711 421. First five cervical vertebrae of crocodile . . .712 422. Cervical vertebra of crocodile . . . . .713 423. Crocodile's skull : dorsal surface .... 714 424. Pectoral girdle of crocodile . . . . .715 425. Half of the pelvic girdle of a young crocodile . . 715 426. Origin of amnion and allantois — after Balfour . .718 427. Vertical section through backbone and ribs of a Chelonian and a Mammal — in part after Jaekel . . .721 428. Position of organs in a bird — after Selenka . . . 727 429. Anterior view of a dorsal vertebra of an ostrich . . 728 430. Fore-limb and hind-limb compared . . . 729 431. D iagrammatic section of young bird — after Gadow . . 730 432. A falcon ........ 731 433. Young heaxded grifhn (Gypaetus barbattis) — after Nitzsch . 732 434 . Young feather and filoplume — after Nitzsch . . . 733 435. Diagram showing a stage in the development of a feather . 733 436. Types of feathers ...... 735 437. Parts of a feather — after Nitzsch .... 736 438. Entire skeleton of condor, showing the relative positions of the chief bones . . . . . . 738 XXVI LIST OF ILLUSTRATIONS FIG. 439. Disarticulation of bird's skull — after Gadow. Membrane bones shaded 440. Vertebral or upper part of the rib of a bird 441. Under surface of gull's skull 442. Pectoral girdle and breastbone of an eagle 443. Wing of dove .... 444. Side view of pelvis of cassowary 445. Bones of hind-limb of eagle 446. Pelvic girdle and hind-limb of a fowl . 447. Brain of pigeon .... 448. Diagrammatic section of cloaca of male bird — after Gadow 449. Heart and arterial system of pigeon 450. Heart and venous system of pigeon 451. Diagram of air-sacs of a pigeon. 452. Female urinogenital organs of pigeon 453. Male urinogenital organs of pigeon 454. Pectoral girdle and sternum of swan 455. Position of wings in pigeon at maximum elevation — from Marey ..... 456. Wings coming down — from Marey 457. Wings completely depressed — from Marey 458. Stages in development of chick — after Marshall 459. Diagrammatic section of egg — after Allen Thomson 460. Diagrammatic section of embryo — after Kennel 461. Restoration of Archaeopteryx — by permission of Mr. W. P Pycraft ..... 462. Flightless Apteryx or Kiwi of New Zealand . 463. Pectoral girdle of an ostrich 464. Hesperornis — after Marsh 465. Pectoral girdle and adjacent parts in a gibbon 466. Diagram of skull bones (partly after Flower and Weber) the membrane bones shaded 467. Occipital region of cat's skull . 468. Fore-limb and shoulder-girdle and hind-limb of rabbit 469. Atlas vertebra of a Mammal, side view 470. Front view of Mammalian atlas 471. Side view of the axis of a Mammal 472. Side view of rabbit's skull 473. Upper surface of rabbit's skull . 474. Under surface of rabbit's skull . 475. Skull of capybara 476. Scapula of a rabbit PAGE 739 740 741 742 743 744 744 745 746 748 750 751 753 755 755 757 758 759 759 766 767 768 769 771 772 773 779 781 782 785 786 787 788 789 790 791 792 794 LIST OF ILLUSTRATIONS XXVU FIG. 477- 478. 479- 480. 481. 482. 483- 484. 485. 486. 487. 488. 489. 490. 491. 492. 493- 494. 495- 496. 497- 498. 499. 500. 501. 502. 503. 504 505 506 507 508 509 510 511 512 after Claude a section, with Pelvic girdle and terminal vertebrae of a gibbon Dorsal view of rabbit's brain ..... Under surface of rabbit's brain — after Krause Median vertical section through Mammal's brain — after Edinger .... Diagram of the alimentary tract of a rabbit Diagram of caecum in rabbit Blood corpuscles . Duodenum of rabbit — from Krause, in part Bernard Circulatory system of the rabbit Structure of mammalian heart . Vertical section through rabbit's head — from help from Parker's Zootomy and Krause Urinogenital organs of male rabbit Urinogenital organs of female rabbit . Dentition of a dog Segmentation of rabbit's ovum — after Van Beneden . Development of hedgehog. Three early stages — after Hubrecht ....... Embryo of Perameles with its foetal membranes — after Hill . Two stages in segmented ovum of hedgehog — after Hubrecht Development of foetal membranes — after Hertwig Diagram of foetal membranes — after Turner . View of embryo, with its foetal membranes — after Kennel . Diagram of foetal membranes in a rabbit — in part after Bonnet Pectoral girdle of Echidna Pelvis of Echidna. Lower jaw of kangaroo . , Foot of young kangaroo . Side view of sheep's skull , Stomach of sheep — from Leunis . Side view of lower part of pony's fore-leg . Side view of ankle and foot of horse . Side view of horse's skull . Feet of horse and its predecessors — from Neumayr . Vertical almost median section through elephant's skull . Left fore -limb of Balcenoptera ..... . Fore-limb of whale {Megaptera longimana) — after Struthers . . Pelvis and hind-limb of Greenland whale (Balcena) — after Struthers .....•• PAGE 796 798 798 799 801 802 803 803 804 805 808 810 810 817 818 819 820 821 822 823 826 828 834 836 838 842 849 850 853 853 855 855 858 861 861 863 XXVlll LIST OF ILLUSTRATIONS FIG. 513. Vertebra, rib, and sternum of Balcenoptera — from specimen in Anatomical Museum, Edinburgh 514. Lower jaw of a rodent .... 515. Skull of tiger, lateral view 516. Lower surface of dog's skull 517. Side view of the brain of a dog . 518. Fore -arm and hand of the mole 519. Outline of a bat's wing .... 520. Skeleton of one of the large bats (Megachiroptera) 521. Skull of orang-utan .... 522. Skull of gorilla ..... 523. Fore-limb of a small monkey . 524. Skeleton of male gorilla .... 525. Hind-limb of a gibbon .... 526. Restoration of head of Pithecanthropus erectus — after MacGregor ..... 527. Restoration of head of Homo neanderthalensis — after MacGregor ..... 528. Zoo-geographical regions of the earth's surface PAGE 865 866 868 869 870 875 877 878 886 887 888 889 891 892 893 906 C/5 < Pi pq W > C/3 Pi w > ^2; O BIRDS. Flying Birds. Running Birds. Placentals. MAMMALS. Marsupials. Monotremes. Snakes. Lizards. REPTILES. Crocodiles. Tortoises. FISHES. Dipnoi. Bony Fishes. "Ganoids." Elasmobranchs. AMPHIBIANS. Newt. Frog. CYCLOSTOxMES. Lamprey. Hag-fish. LANCELETS. TUNICATES. Insects. Arachnids. Myriopods. Peripatus. ARTHROPODS. Crustaceans. BALANOGLOSSUS. ANNELIDS. "WORMS." UNSEGMENTED WORMS. Cuttle-fishes. Gasteropods. MOLLUSCS. Bivalves. Feather-stars. Brittle-stars. Star- fish. ECHINODERMS. Sea-urchins. Sea-cucumbers. Ctenophores. Jelly-fish. Sea-anemones. CCELENTERA. Medusoids and Hydroids. Corals. SPONGES. Infusorians. Rhizopods. Sporozoa. SIMPLEST ANIMALS. OUTLINES OF ZOOLOGY CHAPTER I GENERAL SURVEY OF THE ANIMAL KINGDOM In beginning the study of Zoology, it is natural and useful to try to get a bird's-eye view of the " Animal Kingdom." Without this, one is apt to miss the plan in studying the details. But the survey can be of little service unless the student has the actual animals in his mind's eye. Vertebrates, or Backboned Animals Mammals. — We begin our survey with the animals which are anatomically most like man — the monkeys. But neither we nor the monkeys are separated by any structural gulf from the other four-limbed, hair-bearing animals, to which Lamarck gave the name of Mammals. For although there are many different types of Mammals — such as monkeys and men ; horses, cattle, and other hoofed quad- rupeds ; cats, dogs, and bears ; rats, mice, and other rodents ; hedgehogs, shrews ^ and moles, and so on — the common possession of certain characters unites them all in one class, readily distinguishable from^Birds and Reptiles. These distinctive characters include the milk-giving of the mother mammals, the growth of hair on the skin, the general presence of convolutions on the front part of the brain, the occurrence of a muscular partition or diaphragm between the chest and the abdomen, and so on, as we shall 2 GENERAL SURVEY OF THE ANIMAL KINGDOM afterwards notice in detail. Most mammals are suited for life on land, but diverse types, such as seals, whales, and sea-cows, have taken to the water. In another direction the bats are markedly adapted for aerial life. Among the mammalian characteristics of great import- ance are those which relate to the bearing of young, and even a brief consideration of these shows that some mammals are distinguished from others by differences deeper than those which separate whales from carnivores, or rodents from bats. These deep differences may be stated briefly as follows : — {a) Before birth most young mammals are very closely united (by a complex structure Fig. I. — Duckmole (Ornithorhynchus). called the placenta) to the mothers who bear them, {b) But this close connection between mother and unborn young is of rare occurrence, or only hinted at, in the pouched animals or Marsupials, which bring forth their young in a peculiarly helpless condition, as it were prematurely, and in most cases place them in an external pouch, within which they are sheltered and nourished, (c) In the Australian duckmole and its two relatives, the placental connection is quite absent, for these animals lay eggs as birds and most reptiles do. These differences and others relating to structure warrant the division of Mammals into three sub- classes : — (a) Eutheria, Monodelphia, or Placentals — those in which there is a close (placental) union between the unborn embryo and its mother, e.g. Ungulates, Carnivores, Monkeys. BIRDS (6) Metatheria, Didelphia, or Marsupials — the prematurely bearing, usually pouch-possessing kangaroos, opossums, etc. (c) Prototheria, Ornithodelphia, or Monotremes— the egg-laying duckmole (Ornithorhynchus), Echidna, and Proechidna. Fig. 2. — Phenacodus, a primitive extinct Mammal. — After Cope. Birds. — There can be no hesitation as to the class which ranks next to Mammals. For Birds are in most respects as highly developed as Mammals, though in a different direc- tion. They are character- ised by their feathers and wings, and many other adaptations for flight, by their high temperature, by the frequent spongi- ness and hollowness of their bones, by the tend- ency to fusion in many parts of the skeleton, by the absence of teeth in modern forms, by the fixedness of the lungs and their association with numerous air sacs, and so on. Fig. 3. — Extinct moa and modern kiwi. — After Carus Sterne. But here again different grades must be distinguished — (i) There is the vast majority — the flying birds, with a breast -bone keel or carina, to 4 GENERAL SURVEY OF THE ANIMAL KINGDOM which the muscles used in flight are in part attached (Carinatae) ; (2) there is the small minority of running birds (ostriches, emu, cassowary, kiwi, and extinct moa), with wings incapable of flight, and with no keel (Ratita?) ; and (3) there is an extinct type, Archcpopteryx, with markedly reptihan affinities. Reptiles. — There are no close relationships between Birds and Mammals, but the old-fashioned Monotremes have some markedly reptilian features, and so have some aberrant living birds, such as the Hoatzin and the Tinamou. Moreover, when we consider the extinct Mammals and Birds, we perceive other resemblances linking the two highest classes to the Reptiles. Fig. 4. — Crocodiles. Reptiles do not form a compact class, but rather an assemblage of classes. In other words, the types of Reptile differ much more widely from one another than do the types of Bird or Mammal. Nowadays there are five dis- tinct types :— the crocodilians, the unique New Zealand " lizard " (Sphenodon), the lizards proper, the snakes, and the tortoises. But the number of types is greatly increased when we take account of the entirely extinct saurians, who had their golden age in the inconceivably distant past. The Reptiles which we know nowadays are scaly-skinned animals ; they resemble Birds and Mammals in having during embryonic life two important " foetal membranes " (the amnion and the allantois), and in never having gills ; they differ from them in being " cold-blooded," and in many other ways. AMPHIBIANS Amphibians. — The Amphibians, such as frogs and newts, were once regarded — e.g. by Cuvier — as naked Reptiles, but a more accurate classification has linked them rather to the Fishes. Thus Huxley grouped Birds and Reptiles together as Sauropsida ; Amphibians and Fishes together as Ichthyopsida — for reasons which will be after- wards stated. Amphibians mark the transition from Fig. 5. — Salamander, an Amphibian. aquatic life, habitual among Fishes, to terrestrial life, habitual among Reptiles ; for while almost all Amphibians have gills — in their youth at least — all the adults have lungs, and some retain the gills as well. In having limbs which are fingered and toed, and thus very different from fins, they resemble Reptiles. But the two fcetal membranes characteristic of the embryonic life of higher Vertebrates are not present in Amphibian embryos, and the general absence of an exoskeleton in modern forms is noteworthy. Fishes. — The members of this class are as markedly adapted to life in the water as birds to life in the air. The very muscular posterior region of the body usually forms Fig. 6. — Queensland Dipnoan {Ceratodus). the locomotor organ, and we say that a fish swims by bending and straightening its tail. The limbs have the form of paired fins — that is, they are limbs without digits. There are also unpaired median fins supported by fin rays. All have permanent gills borne by bony or gristly arches. 6 GENERAL SURVEY OF THE ANIMAL KINGDOM There is an exoskeleton of scales, and the skin also bears numerous glandular cells and sensory structures. In many ways Fishes are allied to Amphibians, especially if we include among Fishes three peculiar forms, known as Dipnoi, which show the beginning of a three-chambered heart, and have a lung as well as gills. Ordinary Fishes have a two-chambered heart, containing only impure blood, which is driven to the gills, whence, purified, it passes directly to the body. Apart from the divergent Dipnoi, there are two great orders of Fishes — the cartilaginous Elasmobranchs, such as shark and skate ; and the Teleosteans or bony fishes, such as cod, herring, salmon, eel, and sole. There are several smaller orders of great importance, some of which, e.g. the sturgeons, are often called " Ganoids." Primitive Vertebrates. — Under this title we include — (i) the Roundmouths or Cyclostomata ; (2) the lancelets or Cephalochorda ; (3) the Tunicates, some of which are Fig. 7. — Lancelet, Amphioxus. — After Haeckel. called sea-squirts ; and (4), with much hesitation, several strange forms, especially Balanoglossus, which exhibit structures suggestive of affinity with Vertebrates. The Cyclostomata, represented by the lamprey (Petro- myzon) and the hag {Myxine), and some other forms, probably including an interesting fossil known as Palceo- spondylus, are sometimes ranked with fishes under the title Marsipobranchii. But they have no definitely developed jaws, no paired fins, no scales, and are in other ways more primitive. The lancelets or Cephalochorda are even simpler in their general structure (see Fig. 7). Thus there is an absence of Umbs, skull, jaws, well-defined brain, heart, and some other structures. The vertebral column is represented by an unsegmented (or unvertebrated) rod, called the noto- chord, which in higher animals (except Cyclostomes and some fishes) is a transitory embryonic organ afterwards replaced by the backbone. CHARACTERISTICS OF VERTEBRATES The Tunicata or Urochorda are remarkable forms, the majority of which degenerate after larval life (Fig. 8). In the larvae of all, and in a few adults which are neither peculiarly specialised nor degenerate, we recognise some of the fundamental characters of Vertebrates. Thus there is a dorsal supporting axis (or notochord) in the tail region, a dorsal nervous system, gill - clefts opening from the pharynx to the exterior, a simple ventral heart, and so on. Of Balanoglossus and its allies (Hemichorda or Enteropneusta) it is still difficult to speak with confidence. The possession of gill - clefts, the dorsal position of an important part of the nervous system, the occurrence of a short supporting structure on the anterior dorsal surface of the pharynx, and other features, have led many to place them at the base of the Vertebrate series. Fig. 8. — Ascidian or sea - squirt. — After Haeckel. Characteristics of Vertebrata. — At this stage, having reached the base of the Vertebrate series, we may seek to define a Vertebrate animal, and to contrast it with Invertebrate forms. The distinction is a very old one, for even Aristotle distinguished mammals, birds, reptiles, amphibians, and fishes as " blood-holding," from cuttle-fish, shell-bearing animals, crustaceans, insects, etc., which he regarded as " bloodless." He was, indeed, mistaken about the bloodlessness, but the distinctiveness of the higher animals first mentioned has been recognised by all subsequent naturalists, though it was first precisely expressed in 1797 by Lamarck. Yet it is no longer possible to draw a boundary line between Verte- brates and Invertebrates with that firmness of hand which characterised the early or, indeed, the pre-Darwinian classifications. We now know — (i) that Fishes and Cyclostomata do not form the base of the Vertebrate series, for the lancelets and the Tunicates must also be in- cluded in the Vertebrate alliance ; (2) that Balanoglossus, Cephalodiscus, and some other forms, have several Vertebrate-like characteristics ; (3) that some of the Invertebrates, especially the ChaBtopod worms, show some hints of affinities with Vertebrates. The limits of the Vertebrate alliance have been widened, and though the recognition of their characteristics has become more definite, not less so, the apartness of the sub-kingdom has disappeared- 8 GENERAL SURVEY OF THE ANIMAL KINGDOM It does not matter much whether we retain the familiar title Verte- brata, or adopt that of Chordata, provided that we recognise — (i) that it is among Fishes first that separate vertebral bodies appear in the supporting dorsal axis of the body ; (2) that, as a characteristic, the backbone is less important than the notochord, which precedes it in the history alike of the race and of the individual. Nor need we object to the popular title backboned, if we recognise that the adjective " bony " is first applicable among Fishes, and not to all of these. The essential characters of Vertebrates may be summed up in the following table, where they are contrasted, somewhat negatively, with what is true of Invertebrates : — " Backboneless," Invertebrate OR Non-Chordate. " Backboned," Vertebrate OR Chordate. If there is a nerve-cord, it is ventral. No internal dorsal axis. No gill-slits. The eye is usually derived directly from the skin. The heart, if present, is dorsal. The central nervous system — brain and spinal cord — is dorsal and iubttlar. There is a dorsal supporting axis or notochord, which is in most cases replaced by a backbone. Gill-slits or visceral clefts open from the sides of the pharynx to the exterior. In fishes, and at least young amphi- bians, they are associated with gills, and are useful in respiration ; in higher forms they are transitory and functionless, except when modified into other structures. The essential parts of the c^-e are formed by an outgrowth from the brain. The heart is ventral. Invertebrates, or Backboneless Animals Molluscs. — If we take the concentration of the nervous system as a useful criterion, the highest backboneless animals are the Molluscs. This series of forms includes Bivalves, such as cockle and mussel, oyster and clam ; Gasteropods, such as snail and slug, periwinkle and whelk ; Cephalopods, such as octopus and pearly nautilus. Unlike Vertebrates, and such Invertebrates as Insects and Crustaceans, Molluscs are without segments and without appendages. A muscular protrusion of the ventral surface, known as the " foot," serves in the majority as an organ of locomotion. In most cases a single or double fold of skin, called the " mantle," makes a protective shell. The nervous system has three chief pairs of nerve centres or ganglia. In many cases there are very characteristic free-swimming larval stages. ARTHROPODS Fig. 9. — Cephalopod (paper nautilus, female). Arthropods. — This large series includes Crustaceans, Myriopods, Insects, Spiders, and other forms, which have segmented bilaterally symmetrical bodies and jointed Fir,. 10. — Fresh- water crayfish {Astacus), a Crustacean. — After Huxlev. Fig. II. — a, Caterpillar ; b, pupa ; c, butterfly. 10 GENERAL SURVEY OF THE ANIMAL KINGDOM Fig. 12. — Spider. appendages. The skin produces an external, not-living cuticle, the organic part of which is a substance called chitin, associated in Crustaceans with carbonate of lime. The nervous system con- sists of a dorsal brain, connected, by a nerve - ring around the gullet, with a ventral chain of ganglia. Echinoderms. — This is a well- defined series, including star-fishes, brittle-stars, sea - urchins, sea- cucumbers, and feather-stars. The symmetry of the adult is usually radial, though that of the larva is bilateral. A peculiar system, known as the water-vascular system, is characteristic, and is turned to various uses, as in locomotion and respiration. There is a marked tend- ency to deposition of lime in the tissues. The de velopment is strangely circuitous or " indirect." Segmented "worms." — It is hopeless at present to arrange with any definiteness those heterogeneous forms to which the title " worm " is given. For this title is little more than a name for a shape ^ assumed by animals of varied nature who began to move head foremost and to acquire sides. There is no class of " worms," but an as- semblage — a mob — not yet reduced to order. It seems useful, however, to separate those which are ringed or segmented from those which are unsegmented. The former are often called Annelids, and include twQ chief classes ; — ^ Fig. 13. — Crinoid or feather-star. WORMS I I (i) Chaetopoda or Bristle-footed worms, e.g. earthworm and lob- worm ; and (2) Hirudinea or Leeches. Fig. 14. — Earthworm. Unsegmented * ' worms. ' ' — These differ from the higher " worms " in the absence of true segments and appendages, and resemble them in their bilateral symmetry. There is a motley lot : — the free-living Turbellarians or Planarians ; the parasitic Trematodes or Flukes ; the parasitic Cestodes or Tape-worms ; the Nemer- teans or Ribbon-worms ; the frequently parasitic Nematodes or Thread-worms ; and several smaller classes. As to some other groups, such as the sea-mats (Polyzoa or Bryozoa), the lamp-shells (Brachiopoda), the worm-like Sipunculids, and the wheel- animalcules or Rotifers, we must confess that they are still incertce sedis. But the general fact is not without interest, that in the midst of the well-defined classes of Invertebrates there lies, as it were, a pool from which many streams of life have flowed ; for among the heterogeneous " worms " we may find in diverse types affinities with Arthropods, Molluscs, Echinoderms, and even Vertebrates. Contrast of Ccelomate and Coelenterate. — At this stage we may notice that in all the above forms the typical symmetry is bilateral (in Echinoderms, the superficial radial symmetry belongs only to the adults) ; that in most types a body cavity or ccelom is developed ; that the embryo consists of three germinal layers (external Fig. 15. — Bladderworm stage of a Cestode. — After Leuckart. a, Early stage with head inverted. b, Later stage with head everted. 12 GENERAL SURVEY OF THE ANIMAL KINGDOM ectoderm or epiblast, internal endoderni or hypoblast lining the gut, and a median mesoderm or mesoblast lining the body cavity). In the next two classes (Ccelentera and Sponges) the conditions are different, as may be expressed in the following table : — Sponges and Ccelentera. Higher Animals (Ccelomata). There is no body cavity. There is but one cavity, that of the food canal. Except in ctenophores, there is no definite middle layer of cells (meso- derm), but rather a middle jelly (mesoglcea), and the embryo is diploblastic. The radial symmetry of the gastrula embryo is usually retained in the adult, and the longitudinal (oral- aboral) axis of the adult corresponds to the long axis of the gastrula. There is a body cavity or ccelom be- tween the food canal and the body- wall. But this is often incipient, or degenerate. There is a distinct middle layer of cells (mesoderm) between the external ectoderm and the internal endo- derni. The embryo is triploblastic. The adults are usually bilateral, in some cases asymmetrical, in echinoderms superficially radial. Ccelentera. — This series includes jelly-fishes, sea- anemones, corals, zoophytes, and the like, most of which are Fig. 1 6. — Sea-anemones on back of hermit-crab. — After Andres. equipped with stinging cells, by means of which they paralyse their prey. All but a few are marine. The body may be a tubular polyp, or a more or less bell-like " medu- PROTOZOA 13 soid," and in some cases the two forms are included in one life cycle. Budding is very common, and many of the sedentary forms — " corals " — have shells of lime. Porifera. — Sponges, or Porifera, are the simplest many- celled animals. In the simplest forms, the body is a tubular, two-layered sac, with numerous inhalant pores by which water passes in, with a central cavity lined by cells bearing lashes or flagella, and with an exhalant aperture. But budding, folding, and other complications arise, and there is almost always a skeleton, calcareous, siliceous, or " horny." Apart from one family (Spongillidae), all sponges are marine. Contrast of Metazoa and Protozoa.— All the animals hitherto mentioned have bodies built up of many cells,i but there are other animals, each of which consists of a single cell. These simplest animals are called Protozoa. Every animal hitherto mentioned, from mammal or bird to sponge, develops, when reproduction takes its usual course, from a fertilised egg-cell. This egg-cell or ovum divides and redivides, and the daughter cells cohere and are differentiated to form a " body." But the Protozoa form no "body"; they remain (with few exceptions) single cells, and when they divide, the daughter cells almost invariably go apart as independent organisms. Here, then, is the greatest gulf which we have hitherto noticed — that between multicellular animals (Metazoa) and unicellular animals (Protozoa). But the gulf was bridged, and traces of the bridge remain. For — (a) there are a few Protozoa which form loose colonies of cells, and (6) there are a few multicellular animals of great simplicity. Protozoa. — The Pro- tozoa remain single cells, with few exceptions. Thus they form no " body " ; and necessarily, therefore, they have no organs in the ordinary sense. They illustrate the beginnings of sexual reproduction, and they are not subject to natural death in the same degree as Metazoa are. The series includes — 1 A cell may be defined as a unit corpuscle or unit area of living matter, typically controlled by a single nucleus. Fig. 17. — Fossil Foraminifera (Nummulites) in limestone. — After Zittel. 14 GENERAL SURVEY OF THE ANIMAL KINGDOM (a) Rhizopods, with outflowing threads or processes of hving matter, e.g. the chalk-forming Foraminifera (Fig. 17). (b) Infusorians, with actively moving lashes of living matter. (c) Sporozoa, parasitic forms, usually without either lashes or out- flowing processes. Note on Classification We always group together in our mind those impressions which are hke one another. In this hes the beginning of all classification, whether that of the child, the savage, or the zoologist. For there are many possible classifications, varying according to their purpose, according to the points of similarity which have been selected as important. Thus we may classify animals according to their habitats or their diet, without taking any thought of their structure. But a strictly zoological classification is one which seeks to show the blood-relationships of animals, to group together those whose affinities are shown by their being like one another in architecture or structure. It must, therefore, be based on the results of comparative anatomy — technically speaking, on " homologies," i.e. resemblances in funda- mental structure and in mode of development. Whales must not be ranked with fishes, nor bats with birds. To a classification based on structural resemblances, two corrobora- tions are of value, from embryology and from paleontology. On the one hand, the development of the forms in question must be studied : thus no one dreamed that a Tunicate was a Vertebrate until its life- history was worked out. On the other hand, the past history must be inquired into : thus the affinity between Birds and Reptiles is confirmed by a knowledge of the extinct forms. In classification it is convenient to recognise certain grades or degrees of resemblance, which are spoken of as species, genera, famihes, orders, classes, and so on. To give an illustration, all the tigers are said to form the species Felis tigris, of the genus Felis, in the family FeHdae, in the order Carnivora, within the class Mammalia. The resemblances of all tigers are exceedingly close ; well marked, but not so close, are the resera blances between tigers, lions, jaguars, pumas, cats, etc., which form the genus Felis ; broader still are the resemblances between all members of the cat family Felidae ; still wider those between cats, dogs, bears, and seals, which form the order Carnivora ; and lastly, there are the general resemblances of structure which bind Mammals together in contrast to Birds or Reptiles, though all are included in the series or phylum Vertebrata. It must be understood that the real things are the individual animals, and that a species includes all those individuals who resemble one another so closely that we feel we need a specific name applicable to them all. And as resemblances which seem important to one naturalist may seem trivial to others, there are often wide differences of opinion as to the number of species which a genus contains. But while no rigid definition can be given of a species, certain common-sense considerations should be borne in mind : — GENEALOGICAL TREE 15 I. No naturalist now believes, as Linnaeus did, in the fixity of species ; we believe, on the contrary, that one form has given rise to another. At the same time, the common characteristics on the strength of which we deem it warrantable to give a name to a group of individuals, must Gregoj-ine^ Fig. 18. — Diagrammatic expression of classification in a genealogical tree. B indicates possible position of Balano- glossus, D of Dipnoi, S of Sphenodpn or Hatteria, P of Peripatus. not be markedly fluctuating. The specific characters should exhibit a certain degree of constancy from one generation to another. 2. Sometimes a minute character, such as the shape of a tooth or the marking of a scale, is so constantly characteristic of a group of indi- viduals that it may be safely used as the index of more important i6 GENERAL SURVEY OF THE ANIMAL KINGDOM characters. On the other hand, the distinction between one species and another should always be greater than any difference between the members of a family (using the word family here to mean the progeny of a pair). For no one would divide mankind into species according to the colour of eyes or hair, as this might lead to the absurd conclusion that two brothers belonged to different species. Thus it is often doubly unsatisfactory when a species is established on the strength of a single specimen — (a) because the constancy of the specific character is unde- termined ; {b) because the variations within the limits of the family have not been observed. Indeed, it has happened that one species has been made out of a male, and another out of its mate. 3. Although cases are known where members of different species have paired and brought forth fertile hybrids, this is not usual. The members of a species are fertile inter se, but not usually with members of other species. In fact, the distinctness of species has largely depended on a restriction of the range of fertility. Tabular Survey. — {For Future Reference) METAZOA CHORDATA Mammalia. AVES. Reptilia. Amphibia. Pisces. Cyclostomata. I'Eutheria. -| Metatheria. Marsupials. \ Prototheria. Monotremes. Oviparous. {Carinatae. Keeled flying birds. Odontolcae. Extinct toothed birds. Ratitae. Keel-less running birds. Extinct reptile-like birds. I'Crocodilia. Crocodiles and alligators. Ophidia. Snakes. Lacertilia. Lizards. "j Rhynchocephalia. Sphenodon. I Chelonia. Tortoises and turtles. ^Extinct Classes. Anura. Tail-less frogs and toads. Urodela. Tailed newts. Gymnophiona, e.g. CcBcilia. I Labyrinthodonts and other extinct \ Amphibians. /"Dipnoi. Mud-fishes. -[ Teleostomi. Bony fishes, etc. V. Elasniobranchii. Cartilaginous fishes., / Hag-fish [Myxine), and Lamprey \ {Petromyzon). \ Hi Cephalochorda. Amphioxus. Urochorda. Tunicates. Hemichorda. Balanoglossus, Cephalodiscus. 'J) o C/2 Oh c ni ■*-» Ti O o 2'^ ) .2 « r- O-r Invertebrates ^1 METAZOA NON-CHORDATA MOLLUSCA. i Cephalopoda. Cuttle-fishes. Gasteropoda. Snails. Lamellibranchiata. Bivalves. Two smaller classes : — Scaphopoda and Solenogastres. Arthropod.'^. ECHINODERMA. /Arachnoidea. Spiders, scorpions, mites. I Insecta. I Myriopoda. Centipedes and millipedes. I- Prototracheata. Peripatus. Crustacea. Palaeostraca : — Trilobites, Eurypterids, and King-crabs. ,^Some smaller classes. ^Crinoidea. Feather-stars. (Cystoids and Blastoids, extinct. ) Ophiuroidea. Brittle-stars. -' Asteroidea. Star-fishes. I Echinoidea. Sea-urchins. V Holothuroidea. Sea-cucumbers. " Worms." Chaetopoda. Bristle worms. ^ . ,. i Discophora. Leeches. I Annelids or Some smaller classes. / ^nnuiata. I" Brachiopoda. Lamp-shells. I Polyzoa, e.g. Sea-mat (Flusira). \ Sipunculoidea, e.g. Siptinciilus. Nematoda. Thread-worms. Acanthocephala. Nemertea. Ribbon-worms. Rotifera. Wheel-animalcules. t Cestoda. Tape-worms. ^ I Trematoda. Flukes. '• > VTurbellaria. Planarians. J CcELENTERA. / Ctenophora, e.g. Beroe. Actinozoa or Anthozoa. Sea-anemones. Alcyonarians and re- lated corals. Scyphoniedusae or Acraspeda. Jelly-fishes. Hydrozoa. Zoophytes and medusoids. PORIFERA. Sponges. Calcareous and non-calcareous. PROTOZOA Infusoria. Rhizopoda. Sporozoa. Simplest forms of animal life. i8 GENERAL SURVEY OF THE ANIMAL KINGDOM INVERTEBRATE ANIMALS 19 • M o q^ O ^ — ^ a* ^ 03 OS l-l > oi u o 6 in ■a a, o <« ^ So "Si i . 6 "^ o • o 6 c 0! ™ « y CO « 2 • o [? ^ a m . • ^^ ^ . — .S rt "a; IH (b r; «C/3 q re S3 .t£o ■2 3 '.S y.„ J2'3 "y en tn " y ^2 3 They build up crude, chemically simple food material into living or complex substances ; they convert the kinetic energy of sunlight into the potential chemical energy of these com- plex substances ; they are characteris- tically reducers (of carbon dioxide), ex- pend comparatively little energy in motion or external work, are predomin- antly passive, and show in the vital changes associated with their living matter or protoplasm, a relative pre- ponderance of constructive, up-build- ing, or " anabolic " processes. a) < S Z < b tn H (/■■ 5 u H y <; ft! <: 3 U ."2 1 tn in _y Ih 2 6 -a y y M-i >. y They obtain the requisite carbon from starch, sugar, fat, etc., made by plants or by other animals. They obtain the requisite nitrogen from nitrogenous compounds, not sim- pler than proteins, made by other or- ganisms. Most of them are known to get rid of nitrogenous waste products. They have very rarely any chloro- phyll. The component cells often have no very definite cell walls, rarely have them of material demonstrably different from the cell substance, and almost never show any trace of cellulose. bo 3 6 re i-i 3 .0 re "^ .y M-i *■+-• .in *i-i .2 " in 2 '> re •^ _3 -O y ■73 •'- i-> "y re S y .3 They utilise food material already worked up by plants or by other animals ; they convert this potential energy into kinetic energy in locomo- tion and external work ; they are characteristically oxidisers, are pre- dominantly active, and show in the vital changes associated with their living matter or protoplasm, a relative preponderance of disruptive, down- breaking, or " katabolic " processes. t/i Z H Cu U y fc) S !/3 Some Protozoa and para- sites simply absorb. Some green Protozoa (etc. ?) seem to be able to utilise carbon dioxide as plants do {holopkytic). Again, some Protozoa are probably able to feed like plants. A few, e.g. some Protozoa, have chlorophyll. Others, e.g. the fresh-water sponge, Hydra viridis, and Convo- luta, have symbiotic .\lgae with chlorophyll. Cellulose seems to occur in some Infusorians, and forms most of the tunic or cuticle of the passive sea- squirts or ascidians. ■ 26 THE FUNCTIONS OF ANIMALS the aid of catalysts, which may be compared to lubricants. Thus pure hydrogen does not combine with oxygen except at very high temperatures, when it does so with explosive violence ; but the two gases can be got to com- bine at a low temperature by the introduction of finely- divided platinum as a catalyst. The essential features of a catalyst are that it is not used up in the course of the reaction which it promotes, that accordingly a small quantity of catalyst is sufficient for a great mass of reactant, and that only reactions in which energy is set free can be promoted — a catalyst cannot supply energy. Within living cells oxidations are aided by catalysts of different kinds. In the first place, the enormous surfaces of the microscopic and ultra-microscopic constituents of the protoplasm are of importance, since in their neigh- bourhood are local peculiarities of concentration and electrical forces. In the second place, there are certain fairly simple organic substances, such as glutathione, which are easily oxidised and reduced again, and in their transmutations assist in the oxidation of more resistant compounds. In the third place, there are catalysts of a special type, peculiar to living organisms, known as enzymes or ferments. The chemical nature of these enzymes is obscure ; they are very sensitive to changes in the medium in which they act (acidity, etc.), and are easily destroyed ; and they are markedly specific. Each enzyme — a large number being known — catalyses only one reaction or a small group of reactions. Their activity is not restricted to oxidations ; as we shall see, they play a most important part in the processes of digestion. Chief functions of the animal body. — There are two master activities in animals, those of muscular and of nervous structures ; the other vital processes, always ex- cepting growth and reproduction, are subservient to these. Let us now consider these master and subsidiary functions, as they occur in some higher organism, such as man. Nervous activities. — Life has been described as action and reaction between the organism and its environment, and it is evident that an animal must in some way become aware of surrounding influences. An external influence stimulates a sensory cell or its n nu Wr\-^ / HI su ^ h.c ,^" N.C SPG ii±f^v:^MI A ^\ i^F 21. — Diagrams of reflex actions. — Modified from Bayliss's Principles of General Physiology. I. In a sea-anemone a stimulus from a sensory nerve-cell or neurone (S.C.) at the surface (SU.) may pass directly bv a sensory fibre (S.F.) to a muscle (MU.). II. In some cases, e.g. in the earthworm, the stimulus passes into the ventral nerve-cord (N.C.) and directly affects a motor nerve-cell or neurone (M.C.). Thence an impulse travels by a motor fibre (M.F.) to the muscle (MU.). III. More usually in the earthworm and similar types there are associative neurones (A.) interpolated in the nerve-cord (N.C.) between the branches of the sensory nerve fibre (S.F.) and the dendrites of the motor neurones (M.C.). IV. In Vertebrates from a nerve-ending (N.E.) on the surface (SU.) the stimulus passes by a sensory fibre to the sensory neurones in the spinal ganglia (on the dorsal roots of the spinal nerves) and thence into the spinal cord (SP.C). Associative or internuncial neurones (A.) connect the branches of the sensory nerve-fibre with the den- drites of the motor neurone (M.C.). The impulse travels along the axis cylinder or motor fibre to the muscle (MU.). 28 THE FUNCTIONS OF ANIMALS ending, and a message travels by a sensory fibre to the nerve-cord. The inner end of the sensory fibre is con- nected with the branches or dendrites of an associative (communicating or internuncial) cell. Thence the message is passed, still within the cord, to the dendrites of a motor nerve-cell. Thence an efferent impulse travels down the axis-cylinder or motor nerve-fibre of the motor neurone to an ending on a muscle fibre, which is thus commanded to contract. The whole nervous system is essentially a con- nected series of such reflex-arcs, all intricately joined up with one another. The passage of an impulse along a nerve-fibre may be compared to the passage of a flame along a train of gun- powder. In each case the strength of the initial stimulus does not affect the strength of the transmitted stimulus — the " all-or-none law " is obeyed. In each case the passage of an impulse prevents a second impulse from following close behind it, although the " refractory period " of a nerve-fibre, during which recovery or repair takes place, is extremely short. Oxidation is involved in both cases, though the oxygen consumption of nerves is small. But whereas the train of gunpowder serves to transmit a difference of temperature from one place to another, the nerve-fibre transmits a difference of electric potential from point to point. There are two chief kinds of stimuli which are trans- mitted to the central nervous system — stimuli from without the body, which make the organism aware of changes in its environment ; and stimuli from within the body, which make it aware of the dispositions of its organs, e.g. the stimuli transmitted by the afferent nerves of the muscles, tendons, etc. The chief functions of the nervous system are, then, to make the animal aware of its environment and to co- ordinate and integrate all its bodily functions and activities. As we ascend in the scale, we find that in addition the brain possesses, to an increasing extent, the power of correlating present and past experiences and of originating or inhibiting action in accordance with this correlation. In whatever part there is activity, there is necessarily waste of complex substances and some degree of exhaustion ; and it is CONTRACTION OF MUSCLE Zg interesting to notice, as a triumph of histological technique, that Hodge, Gustav Mann, and others have succeeded in demonstrating in nerve cells the structural results (cellular collapse, etc.) of fatigue, and that in such diverse types as bee, frog, bird, and dog. Muscular activity. — The movements of a unicellular animal are due to the contractility of the living matter, or of special parts of the cell, such as lashes or cilia. In sponges specially contractile cells begin to appear ; in most higher animals such cells are aggregated to form the muscles. There are two distinct types of muscle, with some inter- mediate forms. The most highly developed is the cross- striated or skeletal muscle, which typically consists of numerous fine transparent tubes or fibres, each invested by a sheath or sarcolemma, while the whole muscle is surrounded by connective tissue. It usually runs from one part of the skeleton to another, and is fastened to the skeleton by tendons or sinews. It is controlled by motor nerves, which may bring about a sharp " twitch " con- traction, a powerful maintained contraction (tetanus), or a steady *' postural " state of tension (tonus). The fibres of smooth or involuntary muscle are of an attenuated spindle shape ; their contractions are more sluggish, and they frequently encircle hollow organs without being bound to the hard parts of the body. They are not under voluntary control, though they have their motor nerves and take part in reflexes ; but they have some inde- pendence of the nervous system, and can maintain a tonic tautness or even carry on a rhythmic series of contractions automatically. Muscle displays to some extent the phenomena of the all-or-none law and the refractory period. When a muscle contracts, usually under a stimulus propagated along a motor nerve, it and each of its fibres becomes shorter and broader. The contraction of the fibres is itself a physical rather than a chemical pheno- menon, like a change in the state of a spiral spring. In the actual contraction there is no using up of oxygen or output of CO2, but lactic acid is set free from glycogen within the muscle, and this acts by its acidity on the fibres, causing them to contract. Relaxation takes place when the lactic acid is neutralised, chiefly by bases set free from combina- 30 THE FUNCTIONS OF ANIMALS tion with the muscle proteins. When oxygen is available, about one-fifth of the neutraUsed lactic acid is burnt to CO2 and water, and so much energy is obtained by this reaction that the rest of the lactic acid can be resynthesised into glycogen. The process of contraction is therefore much more economical in the presence of oxygen, and fatigue is deferred. Besides the chemical change and the change of shape, there are also changes of " electric poten- tial " associated with each contraction. Beside muscular movement we must rank ciliary, amoeboid, and epithelial movement. Under the last heading are included active non- amceboid contractions and expansions of covering cells. Digestion: — The energy expended in work or in growth is balanced by the energy of the food-stuffs : — proteins, carbo- hydrates, fats, water, and salts, in varying proportions. In some of the lower animals, such as sponges, the food particles are engulfed by certain cells with which they come in contact, and digested within these cells {intracellular digestion). In most cases, however, the food is digested within the food canal, by ferments made by the secretory cells of the gut or of associated glands. The peculiarity of these ferments is that a small quantity can act upon a large mass of material without itself undergoing any apparent change. However digestion be effected, it means dissolving the food and making it diffusible. In a higher vertebrate there are many steps. {a) The first ferment to affect the food, masticated by the teeth and moistened by the saliva, is the p^alin of the salivary juice, which changes starch into sugar. The juice is formed or secreted by various salivary glands around the mouth. {b) The food is swallowed, and passes down the gullet to the stomach, where it is mixed with the gastric juice secreted by glands situated in the walls. These walls are also muscular, and their contractions churn the food and mix it with the juice. In the juice there is some free hydrochloric acid and a ferment called pepsin : these act together in turning proteins into peptones. The juice has also a slight solvent effect on fat, and the acid on the carbohydrates. (c) The semi -digested food, as it passes from the stomach into the small intestine, is called chyme, and on this other juices act. Of these the most important is the secretion of the pancreas, which contains various ferments, e.g. trypsin, and affects all the different kinds of organic food. It continues the work of the stomach, changing proteins into peptones and peptones into much simpler compounds such as amino-acids ; it continues the work of the salivary juice, changing DIGESTION 31 starch into sugar ; it also emulsifies the fat, dividing the globules into extremely small drops, which it tends to saponify or spUt into fatty acids and gl3'cerine. {d) Into the beginning of the small intestine the bile from the liver also flows, but it is not of great digestive importance, being partly of the nature of a waste product. It has a very important action in lowering surface tension so that the fatty constituents of the chyme can form a finely divided emulsion, readily attacked by the digestive ferment from the pancreas, and it also aids in the absorption of the digested fat by the cells lining the intestine. In some animals it is said to have shght power of converting starch into sugar ; by its alkalinity it helps the action of the trypsin of the pancreas (which, unlike pepsin, acts in an alkaline fluid) ; and it is said to have various other qualities. (e) In addition to the liver and the pancreas, there are on the walls of the small intestine a great number of small glands, which secrete a juice which seconds the pancreatic juice ; this contains the ferment erepsin, which completes the sphtting of peptones into amino-acids, and ferments which spht the more complex sugars, such as cane sugar. The digested material is in part absorbed into the blood, and the mass of food, still being digested, is passed along the small intestine by means of the miiscular contraction of the walls known as peristaltic action. It reaches the large intestine, and its reaction is now distinctly acid by reason of the acid fermentation of the contents. The walls of the large intestine contain glands similar to those of the small intestine, and the digestive processes are completed, while absorption of water also goes on ; so that by the time the mass has reached the rectum, it is semi-sohd, and is known as faeces. These contain the indigestible and undigested remnants of the food, especially cellulose ; residues of the secretions of the digestive glands ; and enormous numbers of bacteria (mostly dead) from the large intestine, with the products of their activity. The digestive processes of Invertebrates are, in a general way, similar : for instance, an alternation of acid and alkaline reactions in digestion is common, though not so well marked as in Vertebrates. There is, however, much adaptation to the diet, both in the structure of the alimentary canal and in the ferments secreted. Un- necessary ferments are dispensed with ; for example, many carnivorous insects have lost the power to digest starch. New ferments are evolved -to digest substances of which other groups can make no use ; thus the clothes- moth larva can digest the very resistant keratin of hair. Cellulose and some other insoluble carbohydrates, which constitute the hard parts of plants, can be digested by some protozoa, and by snails and perhaps a few other families ; but in many groups, for instance in some wood-boring 3^ THE FUNCTIONS OF ANIMALS insects, these substances are split up not by secreted ferments but by the action of symbiotic microbes within the food-canal, or in close relation to it. Absorption. — But the food must not only be rendered soluble and diffusible, it must be carried to the different parts of the body, and there incorporated into the hungry cells. It is carried by the blood stream, and in part also by what are called lymph vessels, which contain a clear fluid resembling blood minus red blood corpuscles. Absorption begins in the stomach by direct osmosis into the capillaries or fine branches of blood vessels in its walls, and a similar absorption, especially of water, takes place along the whole of the digestive tract. But lining the intestine there are delicate projections called villi ; they contain capillaries belonging to the portal system (blood vessels going to the liver), and small vessels known as lacteals connected with lymph spaces in the wall of the intestine. The lacteals lead into a longitudinal lymph vessel or thoracic duct, which opens into the junction of the left jugular and left subclavian veins at the root of the neck. The contents of the duct in a fasting animal are clear ; after a meal they become milky ; the change is due to the matters discharged into it by the lacteals. It is probable that nearly all the fat of a meal is absorbed from the intestines by the lacteals, but it is not certain in what measure, if at all, this is true of the other dissolved food-stuffs; the greater part certainly passes into the capillaries of the portal system, which are contained in the villi. The digested protein, chiefly in the form of amino-acids, passes into the blood of the portal vein, either directly or through the intermediary of leucocytes, which flock to the intestine when protein food is being digested. Functions of the liver. — The absorbed products of the digestion of proteins and carbohydrates are carried from the intestine to the liver by the portal vein, which splits up into fine channels (sinusoids) in close connection with the liver cells. In digestion the more complex carbo- hydrates, such as starch, glycogen, and cane sugar, are split into molecules of simple sugars such as glucose. A large part of the glucose absorbed after a meal is stored in the liver in the form of glycogen ; the muscles of the body also contain glycogen, from which lactic acid is pro- duced, and as their glycogen stores are depleted they draw upon the glucose of the blood. One of the functions of the liver is to maintain the concentration of glucose in the blood at a constant level, by mobilising its glycogen stores as required. These equilibria are partly controlled by a substance (insulin) formed in the pancreas. DIGESTION 33 The end-products of the digestion of proteins are the amino-acids, which contain nitrogen in the form of amino or NHo groups. In the Hver these are spHt off to form ammonia, which combines with carbon dioxide to form ammonium carbonate. By removal of water, probably in the liver cells, this compound is converted into urea, which is excreted from the body by the kidneys. The nitrogen-free residues of the amino-acids have either a carbohydrate or a fatty character, and yield energy to the body by being oxidised. Although the fats absorbed from the intestine do not pass through the portal system directly, the liver plays a part in their metabolism. Some of the links of the long carbon-atom chains of the fatty acids are weakened, so that their subsequent oxidation is facilitated ; and fatty acids are also combined with phosphoric acid to form lecithin, which is more easily transported about the body than the true fats. There is no special organ for the regulation of the amount of fat ; the drops pass through the walls of the capillaries and are stored in connective tissue cells. The liver has many other functions, for example, in the preparation of the bile, which contains both valuable adjuvants to digestion and useless waste products ; it is, in short, the most important chemical clearing-house of the body. Many Invertebrates, such as Molluscs and Crustacea, possess a large digestive gland called the " liver " or hepatopancreas. It combines the function of the verte- brate pancreas, in secreting digestive enzymes, with that of the liver in storing absorbed food and probably in carrying out various chemical reactions and setting free waste products ; and very often there takes place intracellular digestion and assimilation of particles of food brought up from the alimentary canal proper — a function unknown in Vertebrates. The relative importance of these activities varies from group to group. Respiration. — There is another most important material to be noticed, namely, the oxygen which is absorbed from the air by the lungs. We may picture a lung as an elastic sponge-work of air chambers, with innumerable blood 3 34 THE FUNCTIONS OF ANIMALS capillaries in the walls, enclosed in an air-tight box, the chest, the size of which constantly and rhythmically varies. When we take in a breath, the size of the chest is increased, the air pressure within is lowered, and the air from without rushes down the windpipe until the pressure is equalised. The oxygen of this air combines with the coloured iron- containing protein called haemoglobin, contained in the red corpuscles of the blood, and is thus carried to all parts of the body. From the blood it passes to the tissues usually through the medium of the lymph. It is used in the tissues for oxidation. The carbon dioxide formed as a waste product is temporarily combined with bases set free from the blood proteins, especially haemoglobin, and so in time reaches the lungs. But as the partial pressure of the carbonic acid in the air is lower than it is in the serum, the gas escapes from the latter into the air chambers of the lungs. When the size of the chest is decreased, the pressure is increased, and the gas escapes by the mouth or nose until the pressure is equaUsed. Many very different types of respiratory organ are met with in the animal kingdom. Fundamentally, a respiratory organ is a region of the body-surface, usually either an in-folding or an out-folding, at which the external air or water is brought into close relation with either the body-fluids or with the cells themselves, so that diffusion of gases takes place readily. But different animals have solved the problem in different ways, and within each group many adaptations to the conditions of various environments may be demonstrated. There may be deep differences in the physiology of respiration between different phyla. In birds and mammals, which maintain a constant body temperature, there is no direct relation between the external temperature and the amount of oxygen consumed ; but in most cold- blooded animals metabolism increases with rising tempera- ture until the heat becomes harmful. It is usually found that the consumption of oxygen by the body is, within wide limits, independent of the concentration of oxygen in the external medium ; but in some Invertebrates, in which increase of size has not been balanced by the de- velopment of efficient respiratory and circulatory systems, EXCRETION 35 there is a regular relation between metabolism and the partial pressure of oxygen. It has been explained that muscle cells derive energy from the splitting of glycogen, without oxidation, though they effect an economy by using oxygen when they can. Many bacteria derive all their energy from such reactions ; and it has been suggested that some Invertebrates, such as intestinal worms, which normally live in a medium extremely poor in oxygen, do the same. But the most recent researches (of Slater and others) make it unlikely that any of the Metazoa have so fundamentally adapted their physiology that they can thrive all their lives without oxidations ; though undoubtedly many Invertebrates have an amazing power of surviving the absence of oxygen for long periods. Excretion. — We have seen that the blood carries the digested food to the various parts of the body, and that it is also the carrier of oxygen and of the waste carbon dioxide. But there is much waste resulting from tissue changes, which is not gaseous. It is cast into the blood stream by the tissues, and has to be got rid of in some way. This is effected by the kidneys, which are really filters introduced into the blood stream. But they are the most marvellous filters imaginable, and give us a good example of the intricacy of life processes. For the kidneys not only take out of the blood all the waste products that result from the metabolism of proteins and contain nitrogen, they also maintain the composition of the blood at its normal, rejecting any stuffs that vary from that normal, either qualitatively or quantitatively, doing this work according to laws quite different from the simple laws of diffusion or solubility : thus sugar and urea are about equally soluble, and yet the sugar is kept in the body, while the urea is cast out. Even substances as insoluble as resins are removed from the blood by the living cells of the kidneys. A considerable quantity of water, plus traces of salts, fats, etc., leaves the body by the skin, but its chief use is to protect, and to regulate the temperature by variations in the size of its blood vessels. Some special substances are 36 THE FUNCTIONS OF ANIMALS excreted into the alimentary canal in the bile or by the cells lining the large intestine. This completes our sketch — (a) of the process by which the food becomes available for the organism as fuel for the maintenance of its life energies, and (b) of the removal of the waste products which are formed as the ashes of life. Some organs have not been mentioned, such as the spleen, an accessory reservoir for blood, also an area for the multiplication of red blood corpuscles (fishes, newts, embryo-mammals) or for the destruction of worn- out corpuscles (mammals) ; and the various ** endocrine " glands, which make and pour into the blood specific substances called hormones, whose function it is to regulate the activity of cells in other parts of the body. Thus the thyroid glands form thyroxin, a general stimulant of metabolism. But what we have said is perhaps enough to convey a general idea of the processes of life in a higher animal. In conclusion, it is perhaps useful to remark that when in the course of further studies the student meets with organs which are called by the same name as those found in man or in Mammals, as, for example, the " liver " (^f the Molluscs, he must be careful not to suppose that the function of such a " liver " is the same as in Mammals, for comparatively little investigation into the physiology of the lower types of animal hfe has as yet been made. At the same time, he must clearly recognise that the great internal activities are in a general way the same in all animals ; thus respiration, whether accomplished by skin, or gills, or air-tubes, or lungs, by help of the red pigment (haemo- globin) of the blood, or of some pigment which is not red, or occurring without the presence of any blood at all, always means that oxygen is absorbed almost like a kind of food by the tissues, and that the carbon dioxide which results from the oxidation of part of the material of the tissues is removed. Modern Conception of Protoplasm The activities of animals are ultimately due to physical and chemical changes associated with the living matter or protoplasm. This is a mere truism. We do not know the nature of this living matter ; perhaps our most certain knowledge of it is, that in our brains its activity is associated with consciousness. PROTOPLASM 37 When more is known in regard to the chemistry and physics of Uving matter, it may be possible to bring vital phenomena more into line with the changes which are observed in inorganic things. At present, however, it is idle to deny that vital phenomena are things apart. Not even the simplest of them can be explained in terms of chemistry and physics. Even the passage of digested food from the gut to the blood vessels is more than ordinary physical osmosis ; it is modified by the fact that the cells are living. There are some processes going on in the body of which a complete account can be given in chemical and physical terms, but we cannot, at present at least, give in chemical and physical terms an adequate account of any distinctively vital action, nor of growth and development, nor of behaviour. But though we cannot analyse living matter, nor thoroughly explain the changes by which the material of the body breaks down or is built up, we can trace, by chemical analysis, how food passes through various trans- formations till it becomes a usable part of the living body, and we can also catch some of the waste products formed when muscles or other parts are active. Living protoplasm is in a colloidal state, i.e. ultra- microscopic solid particles and immiscible droplets are in suspension and free movement in a fluid. There is a complex mixture of proteins, carbohydrates, fats, and some inorganic constituents, and 70-90 per cent, of water. In this mixture there is a complex play of forces, such as those of surface-tension and electrical charge, and a great variety of chemical changes, summed up in the term " metabolism." Diflterent kinds of chemical changes go on in close prox- imity to one another, yet with some degree of separateness, like eddies in a stream. Perhaps the localisation of par- ticular processes within the cell depends on the deposition of more stable, less labile constituents, forming a sort of framework — the furniture of the living laboratory. When the substance of a cell is fixed and stained, it often shows an intricate reticular, fibrillar, or alveolar structure, but this seems to be mainly a post-mortem effect. However this may be, the cytoplasm of living cells is 38 THE FUNCTIONS OF ANIMALS certainly far from homogeneous. The complex colloidal system is delicately poised between two states, the one truly fluid, though viscous, the other truly solid, though gelatinous. The fluctuations between these ' gel ' and ' sol ' states are largely governed by local variations in concentra- tion of the electrically charged ions of acids, bases, and inorganic salts. Within the cell certain areas, the nucleus, for example, seem to be enclosed by invisibly thin semi- solid membranes, and a similar membrane, more easily studied, forms the external boundary of the cell. This " plasma membrane " exhibits the phenomenon of semi-permeability — that is to say, it permits the passage of some substances, but retains others. Molecules of water and of gases pass through readily, and so also do many organic substances of the class of the " fat-solvents," such as alcohol, ether, chloroform, benzene. All these soak into the substance of the plasma membrane and have a narcotic action, depressing the activity of the cell ; for, as Lillie has said, the plasma membrane is more than a mere partition : it is a sensitive intermediary between the cell and the external world. On the other hand, water- soluble organic substances, such as sugars and amino- acids and urea, and all inorganic salts, are held back by the healthy plasma membrane. But the plasma membrane is a most delicate structure, whose semi-permeable pro- perties are easily impaired ; for example, a pure solution of sodium chloride is harmful to most cells, but the addi- tion of a little calcium chloride '' balances " the solution and renders it harmless ; sea water is a perfectly balanced solution. The question of the permeability of the plasma membrane enters into almost every physiological problem, and will be referred to again. Generalising from his studies on colour sensation, Professor Hering was led to regard all life as an alterna- tion of two kinds of activity, both induced by stimulus, the one tending to storage, construction, assimilation of material, the other tending to explosion, disruption, dis- assimilation. Generalising from his studies on nervous activities, Professor Gaskell was led to regard all life as an alterna- tion of two processes, one of them a running down or dis- ANABOLISM AND KATABOLISM 39 ruption (katabolism), the other a winding up or construc- tion (anaboUsm). All physiologists are agreed that in life there is a twofold process of waste and repair, of discharge and restitution, of activity and recuperative rest. But there is no certainty as to the precise nature of this twofold process. CHAPTER III THE ELEMENTS OF STRUCTURE (Morphology) Animals may be studied alive or dead, in regard to their activities or in regard to their parts. We may ask how they Hve, or what they are made of ; we may investigate their functions or their structure. The study of hfe, activity, function, is physiology ; the study of parts, architecture, structure, is morphology. The first task of the morphologist is to describe structure (descriptive anatomy) ; the second is to compare the parts of one animal with those of another (comparative anatomy) ; the third is to try to state the " principles of morphology," or the laws of vital architecture. But just as the physiologist investigates life or activity at different levels, passing from his study, of the animal as a unity with certain habits, to consider it as an engine of organs, a web of tissues, a city of cells, and a whirlpool of living matter ; so the morphologist has to investigate the form of the whole animal, then in succession its organs, their component tissues, their component cells, and finally, the structure of protoplasm itself. The tasks of morphology and of physiology are parallel. Morphology thus includes not only the description of ex- ternal form, not only the anatomy of organs, but also that minute anatomy of tissues and cells and protoplasm which we call histology. Moreover, there is no real diff"erence between studying fossil animals which died and were buried countless years ago, and dissecting a modern frog. The anatomical palaeontologist is also a student of morphology. Finally, as the greater part of embryology consists in study- 40 FORM AND SYMMETRY 4I ing the anatomy and histology of an organism at various stages of its development, the work of the embryologist is also in the main morphological, though he has also to inform us, if he can, about the physiology of development. Morphology has been defined by Geddes as " the study of all the statical aspects of organisms," in contrast to physiology, which is concerned with their vital dynamics. In this chapter we shall follow the historical development of morphology, and work from the outside inwards. I. Form and symmetry. — The form of an animal is due to the interaction of two variables — the protoplasmic material which composes the organism, and the environ- ment which plays upon it. In some measure, an animal takes definite form as a mineral does : in both the shape is determined by the nature of the stuff and by the surround- ing influences. But the form of an animal is also aflPected by function, i.e. by action and reaction between the organism and its surroundings. As regards symmetry, animals may be distinguished as — {a) radially symmetrical ; [b) bilaterally symmetrical ; (c) asymmetrical. In a radially symmetrical animal, such as a jelly-fish, the body can be halved by a number of vertical planes — it is symmetrical around a median vertical axis. That is, it is the same all round, and has no right or left side. In a bilaterally symmetrical body, such as a worm's, there is but one plane through which the body can be halved. In an asymmetrical animal, such as a snail, accurate halving is im- possible. Radial symmetry is illustrated by simple Sponges, most Coelentera, and b)' many adult Echinoderms. As it is the rule in the two lowest classes of Metazoa, and as it is characteristic of the very common embryonic stage known as the gastrula (an oval or thimble -shaped sac consisting of two layers of cells), it is probably more primitive than the bilateral symmetry characteristic of most animals above Coelentera. Radial symmetry seems best suited for sedentary life, or for aimless floating and drifting. Bilateral symm^etry probably arose as it became advantageous for animals to move energetically^ and in definite direc- tions, to pursue their prey, avoid their enemies, and seek their mates. The formation of a " brain " is correlated with the habit of moving head foremost. Among many-celled animals, some worm type prob- ably deserves the credit of beginning the profitable habit of moving head foremost. Had some one not taken this step, we should never have known our right hand from our left. Axial gradient. — A physiological analogue to symmetry is to be found in the " axial gradients " studied by Child 42 THE ELEMENTS OF STRUCTURE and others. In a bilaterally symmetrical animal, such as a flat-worm, the head is the region of greatest physiological activity, e.g. most intense metabolism and greatest sus- ceptibility to external influences. Behind the head the activity decreases towards the relatively inert middle part of the body ; the tail is a secondary centre of activity, less intense than the head. It is possible to demonstrate these gradients, and the physiological dominance of one region over another, in many ways and in all types of animals, as well as in organs and sometimes in single cells. These studies provide a new point of view for the better under- standing of many evolutionary processes, especially for the nervous system, and of many problems of behaviour, regeneration, etc. 11. Organs. — We give this name to any well-defined part of an animal, such as heart or brain. The word sug- gests a piece of mechanism ; but the animal is more than a complex engine, and many organs have several different activities to which their visible structure gives little clue. Dijferentiation and integration of organs. — When we review the animal series, or study the development of an individual, we see that organs appear gradually. The gastrula cavity — the future stomach — is the first acquisition, though some would make out that it was primitively a brood-chamber. To begin with, it is a simple sac, but it soon becomes complicated by digestive and other out- growths. The progress of the individual, and of the race, is from apparent simplicity to obvious complexity. We also notice that before definite nervous organs appear there is diffuse irritability, before definite muscular organs appear there is diffuse contractility, and so on. In other words, functions come before organs. The attainment of organs implies specialisation of parts, or concentration of functions in particular areas of the body. If we contrast a frog with Hydra, one of the great facts in regard to the evolution of organs is illustrated. Among the living units which make up a frog, there is much more division of labour than there is among those of Hydra. An excised representative sample of Hydra will reproduce the whole animal, but this is not true of the frog. The structural result of this physiological division of labour is HOMOLOGOUS ORGANS 43 differentiation. The animal, or part of it, becomes more complex, more heterogeneous. If we contrast a bird and a sponge, another great fact in regard to the evolution of organs is illustrated. The bird is more of a unity than a sponge ; its parts are more closely knit together and more adequately subordinated to the life of the whole. This kind of progress is called integration. Differentiation involves the acquisition of new parts and powers, these are consolidated and harmonised as the animal becomes more integrated. Correlation of organs. — It is of the very nature of an organism that its parts should be mutually dependent. The organs are all partners in the business of life, and if one member changes, others also are affected. This is especially true of certain organs which have developed and evolved together, and are knit by close physiological bonds. Thus the circulatory and respiratory systems, the muscular and the skeletal systems, the brain and the sense organs, are very closely linked, and they are said to be correlated. A variation, for better or worse, in one system often brings about a correlated variation in another, though we cannot always trace the physiological connection. Homologous organs. — Organs which arise from the same primitive layer of the embryo (see Chapter IV.) have some- thing in common. But when a number of organs arise in the same way, from the same embryonic material, and are at first fashioned on the same plan, they have still more in common. Nor will this fundamental sameness be affected though the final shape and use of the various organs be very different. We call organs which are thus structurally and developmentally similar, homologous. Thus the nineteen pairs of appendages on a crayfish are all homologous ; the three pairs of " jaws " in an insect are homologous with the insect's legs ; and it is also true that the fore-leg of a frog, the wing of a bird, the flipper of a whale, the arm of a man, are all homologous. The wing of a bird and the arm of man exhibit the same chief bones, blood vessels, muscles, and nerves, and they begin to develop in the same way ; they are homologous hut not analogous. The wing of a bird and the wing of an insect, which resemble one another in being organs of flight, are not the least alike in structure ; 44 THE ELEMENTS OF STRUCTURE they are analogous but not homologous. Yet two organs may be both homologous and analogous^ e.g. the wing of a bird and the wing of a bat, for both are fore-Umbs, and both are organs of flight. Sometimes two organs or two organisms — deeply different in structure — have a marked superficial resemblance, simply because both have arisen in relation to similar conditions of life. Thus a burrow- ing amphibian, a burrowing lizard, and a burrowing snake resemble one another in being limbless, but this ** conver- gence," or " homoplasty," of form does not indicate any relationship between them. Change of function. — Division of labour involves restric- tion of functions in the several parts of an animal, and no higher Metazoa could have arisen if all the cells had remained with the many-sided qualities of Amoebae. Yet we must avoid thinking about organs as if they were necessarily active in one way only. For many organs, e.g. the liver, have several very distinct functions. In addition to the main function of an organ, there are often secondary functions ; thus the wings of an insect may be respiratory as well as locomotor, and part of the food canal of Tunicates and Amphioxus is almost wholly subservient to respiration. Moreover, in organs which are not very highly specialised, it seems as if the component elements retained a consider- able degree of individuality, so that in course of time what was a secondary function may become the primary one. Thus Dohrn, who especially emphasised this idea of function change, says : " Every function is the resultant of several components, of which one is the chief or primary function, while the others are subsidiary or secondary. The diminution of the chief function and the accession of a secondary function changes the total function ; thesecondary function becomes gradually the chief one ; the result is the modification of the organ." The contraction of a muscle is always accompanied by electric changes, and in the electric organs of fishes we see the electric changes in the modified muscular tissue composing the organs becoming more important than the contractility. The structure known as the allantois is an unimportant bladder in the frog, in Birds and Reptiles it forms a foetal membrane (chiefly respiratory) around the embryo, and in most Mammals it forms part of SUBSTITUTION OF ORGANS 45 the placenta which effects vital connection between off- spring and mother. Substitution of organs. — The idea of several changes of function in the evolution of an organ, suggests another of not less importance which has been emphasised by Kleinen- berg. An illustration will explain it. In the early stages of all vertebrate embryos, the supporting axial skeleton is the notochord — a rod developed along the dorsal wall of the gut. From Fishes onwards, this embryonic axis is gradually replaced in development by the vertebral column or backbone ; the notochord does not become the back- bone, but is replaced by it. It is a temporary structure, around which the vertebral column is constructed, as a tall chimney may be built around an internal scaffolding of wood. Yet it remains as the sole axial skeleton in Amphioxus, persists in great part in hag and lamprey, but becomes less and less persistent in Fishes and higher Vertebrates, as its substitute, the backbone, develops more perfectly. Now, what is the relation between the notochord and its substitute the backbone, seeing that the former does not become the latter ? Kleinenberg's suggestion is that the notochord supplies the stimulus, the necessary condi- tion, for the formation of the backbone. Of course we require to know more about the way in which an old- fashioned structure may stimulate the growth of its future substitute, but the general idea of one organ leading on to another is suggestive. It is consistent with our general conception of development — that each stage supplies the necessary stimulus for the next step ; it also helps us to understand more clearly how new structures, too incipient to be of use, may persist, and why old structures should linger though they have only a transitory importance. Rudimentary organs. — In many animals there are struc- tures which attain no complete development, which are rudimentary in comparison with those of related forms, and seem retrogressive when compared with their promise in embryonic life. But it is necessary to distinguish various kinds of rudimentary structures, (a) As a pathological variation, probably due to some germinal defect, or to the insufficient nutrition of the embryo, the heart of a mammal is sometimes incompletely formed, Other organs may be 46 THE ELEMENTS OF STRUCTURE similarly spoilt in the making. They illustrate arrested development, (b) Some animals lose, in the course of their life, many of the prominent characteristics of their larval Hfe ; thus parasitic crustaceans at first free-living, and sessile sea-squirts at first free-swimming, always undergo degenera- tion, which can be seen in each lifetime, {c) But the little kiwi of New Zealand, with mere apologies for wings, and many cave fishes and cave crustaceans with slight hints of eyes, illustrate degeneration, which has taken such a hold of the animals that the young stages also are degener- ate. The retrogression cannot be seen in each lifetime, evident as it is when we compare these degenerate forms with probable ancestors, (d) But among '* rudimentary organs " we also include structures somewhat different, e.g. the gill- clefts which persist in embryonic reptiles, birds, and mammals, though most of them serve no obvious purpose, or the embryonic teeth of whalebone whales. These are " vestigial structures,^' traces of ancestral history, and in- telligible on no other theory. The gill-clefts are used for respiration in all vertebrates below reptiles ; the ancestors of whalebone whales doubtless had functional teeth. Classification of organs. — We may arrange the various parts of the body physiologically, according to their share in the life. Thus some parts liave most to do with the external relations of the animals ; such as locomotor, prehensile, food-receiving, protective, aggressive, and copulatory organs. Of internal parts, the skeletal structures are passive ; the nervous, muscular, and glandular parts are active. The repro- ductive organs are distinct from all the rest. They are conveniently called " gonads," which is a better term than reproductive glands. For by a gland we mean an organ which secretes, whose cells produce and liberate some definite chemical substance, such as a digestive ferment ; whereas the gonads are organs where there is periodic multi- plication of certain cells, kept apart from the specialisation character- istic of most of the " body cells " or " somatic " cells. It is true, however, that an accessory glandular function is often associated with the gonads. Another classification of organs is embryological, i.e. according to the embryonic layer from which the various parts arise. Thus the outer layer of the embryo (the ectoderm or epiblast) forms in the adult — (i) the outer skin or epidermis ; (2) the nervous system ; (3) much at least of the sense organs : the inner layer of the embryo (the endoderm or hypoblast) forms at least an important part (the " mid-gut") of the food canal, and the basis of outgrowths (lungs, liver, pancreas, etc.) which may arise therefrom, and also the notochord of Vertebrates : the middle layer of the embryo (the mesoderm or mesoblast) forms skeleton, connective swathings, muscle, lining of body-cavity, etc. TISSUES 47 III. Tissues. — Zoological anatomists, of whom Cuvier may be taken as a type, analyse animals into their com- ponent organs, and discover the homologies between one animal and another. But as early as 1801, Bichat had published his Anatomie generate, in which he carried the analysis further, showing that the organs were composed of tissues, contractile, nervous, glandular, etc. In 1838-39, Schwann and Schleiden formulated the " cell theory," in which was stated the result of yet deeper analysis — that all organisms have a cellular structure and origin. The simplest animals (Protozoa) are typically single cells or unit masses of living matter ; as such all animals begin ; but all, except the simplest, consist of hundreds of these cells united into more or less homogeneous companies (tissues), which may be compacted, as we have seen, into organs. If we think of the organism as a great city of cells, the tissues represent streets (Hke some of those in Leipzig), in each of which some one kind of function or industry predominates. The student should read the introductory chapters in one of the numerous works on histology, so as to gain a general idea of the character of the different tissues. There are four great kinds — epithelial, connective, muscular, and nervous. (a) Epithelial tissue is illustrated by the external layer of the skin (epidermis), the internal (endothehal) lining of the food canal and its outgrowths, the lining of the body cavity, etc. ; by the early arrange- ments of cells in all embryos ; and by the simplest Metazoa, such as Hydra, whose tubular body is formed by two layers of epithehum. Embryologically and historically, epithelium is the most primitive kind of tissue. It may be single layered or stratified ; its cells may be columnar, scale-like, or otherwise. The cells may be close together, or separated by intercellular spaces, and they are' often connected by bridges of living matter. Nor are the functions of epithelium less diverse than its forms, for it may be ciliated (effecting locomotion, food-wafting, etc.), or sensitive (and as such forming sense organs), or glandular (liberating certain products or even the whole contents of its cells), or pigmented (and thus associated with respiration, excretion, and protection), or covered externally with "'a sweated-off cuticle, susceptible of many modifications (especially of protective value). (b) Connective tissue. — This term includes too many different kinds of things to mean much. It represents a sort of histological lumber- room. The embryologists help us a little, for they have shown that almost all forms of connective tissue are derived from the mesoderm or middle layer of the embryo, As this mesoderm usually arises in the form of 48 THE ELEMENTS OF STRUCTURE outgrowths from the gut, or from (mesenchyme") cells liberated at an early stage from either (?) of the two other layers of the embryo (ectoderm or endoderm), we may say that connective tissue is primarily derived from epithelium. The general function of " connective tissue " is to enswathe, to bind, and to support, but the forms assumed are very various. The cells may be without any intercellular "mortar" or matrix. They may be laden with fat or with pigment. In other cases the cells of the connective tissue lie in a matrix, which they secrete, or into which they in part die away. Sometimes the matrix becomes secondarily invaded by cells. The connective cells are very often irregular in out- line, and give off, in most cases, fine processes, which traverse the matrix as a network. They may secrete long fibres, as in the various kinds of fibrous tissue. The fibrous tissue of tendons and the different kinds of gristle or cartilage illustrate connective tissue with much matrix. Cartilage is sometimes hardened by the deposition of lime salts in its substance, and then has a slight resemblance to another kind of " connective tissue" — bone. But bone, which is restricted to Verte- brate animals, is quite different from the cartilage which it often succeeds and replaces. It is made by strands or layers of special bone-forming cells C, Cilia; B., basal corpuscle at tlic root of (osteoblasts), which mav rest •each cilium; -F-i, fi"^ intracellular hbnls cartilage foundation, or corresponding to the cilia ; A ., nucleus « .« ^ f, with chromosomes darkly stained; (Y., may be quite mdepondent. general cytoplasm of the cell. These osteoblasts form the bone matrix, and some of them are involved in it, and become the permanent bone cells. These have numerous radiating branches, and are arranged in concentric layers, usually around a cavity or a blood vessel. (There are no blood vessels in cartilage.) The matrix becomes very rich in lime salts (especially phosphate) ; and the cartilage foundation, if there was one, is quite destroyed by the new formation. Here we may also note two important fluid tissues, the floating corpuscles or cells of Fig. 22. — Three ciliated cells. MUSCULAR TISSUE 49 the blood, and those of the body cavity or " perivisceral " fluid which is often abundant and important in backboneless animals. (c) Muscular tissue. — The single-celled Amoeba moves by flowing out on one side and drawing in its substance on another. It is diffusely contractile, and it has also sensitive, digestive, and other functions. In Hydra and some other Coelentera the bases of some of the epithelial cells which form the outer and inner layers are prolonged into contractile roots. Here, then, we have cells of which a special part discharges a contractile or muscular func- tion, while the other parts retain other powers. In other Coelentera the muscular cells are still directly connected with the epithelium, but become more and more exclusively con- tractile. In all other animals the muscular tissue is derived from the mesoderm, which, as we have already mentioned, is not dis- tinctly present in Coelentera. In the majority, the muscle-cells arise on the walls of the body cavity, and their origin may often at least be described as epithelial. But in other cases the muscles arise from those wandering " mesenchyme " cells to which we have already referred. Smooth or unstriped muscle fibres are elongated contractile cells, externally homo- geneous in appearance. They are especially abundant in sluggish animals, e.g. Molluscs, and occur in the walls of the gut, bladder, and blood vessels of Vertebrates. They are less perfectly differentiated than striped muscle fibres, and usually contract more slowly. A striped muscle fibre is a cell the greater part of which is modified into a set of parallel longitudinal fibrils, with alternating " clear and dark " transverse stripes. A residue of unmodified cell substance, with a nucleus or with many, is often to be observed on the side of the fibre, and a slight sheath or sarcolemma forms the " cell wall." Many muscle fibres closely combined, and wrapped in a sheath of connective tissue, form a muscle, which, as every one knows, can contract with extreme rapidity when stimulated by a nervous impulse. (d) Nervous tissue. — Beginning again with the Amoeba, we recognise that it is diffusely sensitive, and that a stimulus can pass from one part of the cell to another. In some Coelentera a few of the external cells seem to combine contractile and nervous functions. Therefore they are sometimes called " neuro-muscular." Fig. 23. — A unstriped smooth or muscle-cell. slowly contracting. A'^., Nucleus ; FL., longitudinal intracellular fibrillation. 50 THE ELEMENTS OF STRUCTURE But in Hydra there are superficial sensory cells, whose basal pro- l(jngations are connected either directly with contractile cells, or with deeper ganglion-cells, some of which give off motor processes to the contractile cells. In sea-anemones and some other Coelentera there is a more sharply defined division of labour. Super- ficial sensory cells are connected with subjacent nerve- or ganglion- cells, from which fibres pass to the contractile elements. In higher animals the sensory cells are mostly integrated into sense organs, the ganglionic cells into ganglia, while the delicate fibres which form the connections between sensory cells and gang- lionic cells, and between the latter and muscles, are compacted to form well-developed nerves. So far as we know, nervous tissue always arises from the outer or ectodermic layer of the embryo, as we would expect from the fact that this is the layer which, in the course of history, has been most directly subjected to external stimulus. I-et us consider first the gang- lionic cells which receive stimuli and shunt them, which regulate the whole life of the organism, and are the physical conditions of " spon- taneous " activity and intelligence. They are of very varied shape, but consist always of a cell-body which gives off one or more processes. One of these processes is long, branches very sparingly, and is known as the axis-cylinder. There are usually present other processes which ramify like the branches of a tree and are called dendrites. The cell-body contains a nucleus, distinct granules, and a network of fine fibrils. The nervous system is built up of such "neurones." In the ganglia they are supported and held apart by much-branched neuroglia cells. In all but a few of the simplest Metazoa, the nerve fibres (axis- cylinders) are surrounded by a sheath called the neurilemma, said to be formed by adjacent connective tissue. Several nerve fibres may com- bine to form a nerve, but each still remains ensheathed in its neuri- 5TR Fig. 24. — A piece of striped muscle fibre with its nerve-endings. STR., Striations of tlie muscle fibre ; N., nuclei of the muscle fibre ; M.N., a motor nerve giving off motor nerve fibres (M.F.), which lead to branching motor-endings (M.A.). S.A. is a sensory nerve-ending, from which impulses are carried by sensory fibres (S.F.) to a sensory nerve. CELLS 51 lemma, while fibrous sheaths bind the nerve fibres together. In Verte- brate animals each nerve fibre usually has in addition a medullary sheath. But even in the higher Vertebrates, " non-meduUated " or simply contoured nerve fibres are found in the sympathetic and olfactory nerves, and this simpler type alone occurs in hag, lamprey, and lancelet, as well as in all the Invertebrates with distinct nerves. A nerve fibre contains numerous fibrils Uke those seen within a ganglion cell. These are regarded by some as the essential elements in conducting stimuli, while others maintain that the essential part is the less compact, sometimes well-nigh fluid stuff between the fibrils, or that the fibrils are but the walls of tubes within which the essentially nervous stuff lies. The nerve fibres arise as prolongations of the ganglion cells, which extend themselves in the embryo like Amoebae sending out pseudopodia. IV. Cells. — In discussing tissues, it was necessary to refer to the component cells. Let us now consider the chief characteristics of these elements. A cell is a unit mass or area of living matter usually with a nucleus. Most of the simplest animals and plants (Protozoa and Protophyta) are single cells ; eggs and male elements are single cells ; in multicellular organisms the cells are combined into tissues and organs. Most cells are too small to be distinguished except through lenses ; many Protozoa, e.g. large Amoebae, are just visible to our unaided eyes ; the chalk-forming Fora- minifera are single cells, whose shells are often as large as pin-heads, and some of the extinct kinds were as big as half-crowns (see Fig. 17) ; the bast cells of plants may extend for several inches ; the largest animal cells are eggs distended with yolk. The typical and primitive form of cell is a sphere — a shape naturally assumed by a complex coherent substance situated in a medium different from itself. Most egg-cells and many Protozoa retain this primitive form, but the internal and external conditions of life (such as nutrition and pressure) often evolve other shapes — oval, rectangular, flattened, thread-like, stellate, and so on. As to the structure of a cell, we may distinguish (see Fig- 25)— (a) The general cell substance or cytoplasm, which con- sists partly of genuinely living stufi^ or protoplasm, and partly of complex materials not really living (metaplasm) ; 52 THE ELEMENTS OF STRUCTURE (b) A Specialised nucleus, with a complex structure, and important functions ; (c) One or more specialised bodies called central corpuscles or centrosomes, which seem to be centres of activity during cell division ; {d) A cell wall, which occurs in very varied form, or may be entirely absent. (a) As to the cell substance, it appears in living cells to be Fig. 25. — Diagram of cell structure. — After Wilson. PL., Plastids in cytoplasm ; CC, centrosomes in centrosphere ; n., nucleolus ; N., nucleus ; CHR., chromosomes ; CT., general cytoplasm ; v., vacuole ; GR., granules. clear, colourless, structureless, and more or less fluid. There are great variations in viscosity from cell to cell, from time to time, and even from place to place within a single cell at one instant. In cells " fixed " and prepared for microscopic study the cytoplasm has an artificial reticular or fibrillar structure. The cytoplasm often contains numerous inclusions. Granules, watery droplets or vacuoles, and oily globules are present in varying numbers ; they are regarded as non- NUCLEUS 53 living aggregates of material — stores of nutritive material, or products of the cell's activity, either useful or useless. The scattered thread-like or rod-like mitochondria and the knot-like Golgi apparatus are more often supposed to form part of the living protoplasm. The first-named at least may be seen in living cells, but both are destroyed by the usual methods of fixation, and require special demonstra- tion. {b) As to the nucleus, one at least is present in almost every cell. It used to be said that some very simple animals, which Haeckel called Monera, had no nuclei, but in many cases the nuclei have now been demonstrated. In other cases, e.g. some Infusorians, the nuclear material seems to be diffused in the cell substance. The red blood cells of Mammals seem to be distinctly nucleated in their early stages, though there is no nucleus in those which are full grown. The nucleus is a very important part of the cell, but it is not yet possible to define precisely what its importance is. In fertilisation an essential process is the union of the nucleus of the spermatozoon or male cell with the nucleus of the ovum or female cell (Fig. 27). In cell division the nucleus certainly plays an essential part. Cells bereft of their nuclei die, or live for a while a crippled life. Accord- ing to some, the nucleus is important in connection with the nutrition of the cell ; according to others, it is of special importance in connection with the respiration of the cell. It is certain that there are complex actions and reactions between the living matter of the nucleus and that of the cytoplasm. Cytoplasm and nucleoplasm form a " cell firm," potent in their co-operation. In many cells it has been show^n that fragments or extensions of the nucleus pass into the cytoplasm, forming what is called a " chromidial apparatus," which seems to be of much functional im- portance. The nucleus often lies within a little nest in the midst of the cell substance, but it may shift its position from one part of the cell to another. It has a definite margin, but this may be lost, e.g. before cell division begins. Inter- nally, the living resting nucleus appears to be fluid and quite homogeneous ; but if injured mechanically or 54 THE ELEMENTS OF STRUCTURE chemically (in fixation) it coagulates readily in a pattern which is anything but homogeneous (see Fig. 26). Twisted strands or tubes of " linin " bear a more stainable material called " chromatin," and when the cell is preparing to divide the strands assume the form of a definite number of separable rods or loops or granules, the " chromosomes." The number of chromosomes is in general constant for each species of animals and plants. Surrounding the linin and chromatin is the nuclear sap. Sometimes a linin thread shows a row of minute chromatin bodies (microsotnata), like jewel-stones embedded on a belt. Weismann suggested that the chromosomes or idants of the germ-cells are the vehicles of the heritable qualities or of some of them ; and this view is generally accepted. They carry the hereditary " factors " or " genes," apparently in a linear arrangement. Many nuclei also contain little round bodies or nucleoli, or sometimes a single nucleolus. The term is applied somewhat vaguely to little aggrega- tions of chromatin, and more properly to vacuole-like bodies, in which some believe that the waste products of the nucleus are collected. {c) As to the centrosomes, it may be note protoplasmic notcd that whcu an animal cell divides, SJJedb^'fixhit'S these bodies play an important part. The chromatin elements of the nucleus are divided, and separate to form the two daughter nuclei. In this separation extremely fine " archoplasmic " threads appear to pass from the centrosomes to the chromosomes. The centrosomes are therefore regarded as " division organs," or as " dynamic centres." They also occur, in most cases singly, in resting cells, and it seems likely that they are present in most animal cells, at least in those which retain the power of division. {d) As to the cell wall, it seemed of much moment to the earlier histologists, who often spoke of cells as little bags or boxes. It is, however, the least important part of the cell. In plant cells there is usually a very distinct wall, consisting of cellulose. This is a product, not a Fig. 26. — Structure of the cell. — After Carnoy. N ., Nucleus with chromatin coil CELL WALL 55 Fig. 27. — Fertilised ovum of Ascaris. — After Boveri. chr., Chromosomes, two from ovum nucleus and two from sperm nucleus ; cs., centro- some from which " archo- plasmic " threads i^adiate, partly to the chromosomes. part, of the protoplasm, though some protoplasm may be intimately associated with it as long as its growth con- tinues. In animal cells there is rarely a very distinct wall chemically distinguishable from the living matter itself. But the margin is often different from the interior, and a slight wall may be formed by a superficial physical alteration of the cell substance, com- parable to the formation of a skin on cooling porridge. In other cases, especially in cells which are not very active, such as ova and encysted Protozoa, a more definite sheath is formed around the cell substance. Again, animal cells may secrete a superficial " cuticle," e.g. the chitin formed by the ectoderm cells in Insects, Crustaceans, and other Arthropods. The " plasma membrane," which fornis the outer boundary of the protoplasm, is invisibly thin ; its exist- ence and properties are deduced indirectly from experi- ments on its impermeability to various substances. The " micro-dissection " experiments of Chambers have made it more real to us in the last few years, and have emphasised its great im- portance in the life of the cell. In animals, as well as in plants, adjacent cells are often linked by intercellular bridges of living matter, which may be paths for the passage of materials or of dis- turbances from cell to cell. In many cases,- e.g. of gelatinous tissue, a matrix arises outside of and between the cells, as an exo- plasmic product. In regard to cell division, the most important facts are the following : — There is a striking similarity in most cases, and the nucleus plays an essential part in the process. The chr cs Fig. 28. — Diagram of cell division. — After Boveri. chr., Chromosomes forming an equatorial plate ; cs., centro- some. 56 THE ELEMENTS OF STRUCTURE dividing nucleus usually passes through a series of complex changes known as karyokinesis or mitosis, and these are much the same everywhere, though different kinds of cells have their specific peculiarities. Occasionally, however, both in Protozoa and Metazoa, the nucleus divides by simple constriction (direct or amitotic division). This is a quicker process than the other, and occurs especially when there is rapid growth or frequent replacement of cells. Another departure from the ordinary scheme is seen when the nucleus shows a multiple division, while the cell remains undivided. This occurs normally in some marrow cells. The eventful changes of karyokinesis are as follows : — (a) The resting stage of the nucleus shows a network or complete coil of filaments (chromatin elements) (Fig. 29). {[■>) First stage. — As division begins, the membrane separating the nucleus from the cell substance disappears, and the chromatin elements are seen as a tangled or broken coil (Fig. 20, I). (c) Astroid stage.— The chromatin elements bend into looped pieces (or chromosomes), which are disposed in a star, lying flat at the equator of the cell, the free ends of the U-shaped loops being directed outwards. Meanwhile a centrosome has appeared and divided into two separating halves, between which a spindle of fine achromatin threads is formed. This seems to form (at least part of) what is called the nuclear spindle. The centrosomes separate until one lies at each pole of the cell, surrounded by radiating " archoplasmic " threads which become attached to the chromosomes (Fig. 29, 2). {d) Division and separation of the loops. — Each of the loops which make up the star divides longitudinally into two, and each half separates from its neighbour. They lie at first near the equator of the cell, but they are apparently drawn, or driven, to the opposite poles (Fig. 29, 2-4). (e) Diastroid. — The single star thus forms two daughter stars, which separate farther and farther from one another towards the opposite poles of the cell, remaining connected, how- ever, by delicate threads (Fig. 29, 3-5). (/) Each daughter star is reconstituted into a coil or network for each daughter cell, for the cell substance has been con- stricted meanwhile at right angles to the transverse axis of the spindle. The halves separate in the case of Protozoa, but in most other cases, e.g. growing embryos, they remain •-. adjacent, with a slight wall between them (Fig. 29, 6). (g) Each daughter nucleus then passes into the normal resting phase. The spindle disappears, and the centrosomes may also vanish. KARYOKINESIS 57 The essential fact is the exact partition of the nuclear material between the two daughter cells. It may be added that these various complexities of structure can be seen in living cells as well as in fixed and stained material. Flemming gives the following summary of karyokinesis : — Mother Nuclei's Daughter Nucleus (progressive changes). (regressive changes). a Resting stage. Resting stage. g ^ b Coil. Coil. / c Astroid. Diastroid. e i d Division of Astroid and its loops ^ (Prophases) (Aletakinesis) (Anaphases). c c C'C Fig. 29. — Karyokinesis. — After Flemming. 1. Coil stage of nucleus ; ex., central corpuscle. 2. Division of chromatin elements into U-shaped loops, and longitudinal splitting of these chromosomes (astroid stage). 3. 4. Recession of chromosomes from the equator of the cell (diastroid). 5. Nuclear spindle, with chromatin elements at each pole, and achromatin threads between. 6. Division of the cell completed. Besides the ordinary indirect divisipn just described, the net result of which is that each of the two daughter cells gets the normal number of chromosomes, a precise half of each of the chromosomes in the original cell, there is another kind of cell division (meiotic or reducing division) which occurs only in the maturation of the ovum and spermatozoon, and has for its net result the reduction of the number of chromosomes to a half of the normal number. 58 THE ELEMENTS OF STRUCTURE We are far from being able to give even an approximate account of the " mechanism " of cell division. The whole process is vital, and cannot, at present at least, be re- described in terms of matter and motion. On the other hand, Leuckart, Spencer, and Alexander James have given a general rationale of cell division. Why do not cells grow much larger } why do they almost always divide at a definite limit of growth ? The answer is as follows : — Suppose a young cell has doubled its original volume, that means that there is twice as much living matter to be kept alive. But the living matter is fed, aerated, purified through its surface, which, in growing spherical cells, for instance, only increases as the square of the radius, while the mass increases as the cube. The surface growth always lags behind the increase of mass. Therefore, when the cell has, let us say, quadrupled its original volume, but by no means quadrupled its surface, difficulties set in, waste begins to gain on repair, anabolism loses some of its ascendancy over katabolism. At the limit of growth the cell divides, halving its mass and gaining new surface. It is true that the surface may be increased by out- flowing processes, just as that of leaves by many lobes ; and division may occur before the limit of growth is reached ; but, as a general rationale, applicable to organs and bodies as well as to cells, the suggestion above outUned is very helpful. It is supported by an experiment due to Hart- mann, who kept an Amoeba alive and healthy for over four months, without any division, by amputating small portions of the cytoplasm each day so that the size remained constant. Amoebae which were not operated on divided every second day. The ratio of the amount of nuclear material in the cell to the amount of cytoplasmic material seems also to have a determining influence upon cell division (R. Hertwig). Protoplasm. — Morphological as well as physiological analysis passes from the organism as a whole to its organs, thence to the tissues, thence to the cells, and finally to the protoplasm itself. But although we may define protoplasm as genuinely living matter — as " the physical basis of life " — we cannot definitely say how much or what part of an Amoeba, or an ovum, or any other cell, is really protoplasm. PROTOPLASM 5 9 We are able to make negative statements, e.g. the yolk of an egg is not protoplasm, but we cannot make positive statements, or say. This is protoplasm, and nought else. Thus what is spoken of as the structure of protoplasm is really the structure of the cytoplasm. Sections of fixed and stained cells often show a considerable com- plexity of structure, and various appearances have been often described. Thus some, e.g. Frommann, describe a network or reticulum, with less stable material in the meshes ; others, e.g. Flemming, describe a manifold coil of fibrils ; and others, e.g. Biitschli, describe a foam- like or vacuolar structure. Hardy has imitated these structures by treating perfectly homogeneous colloidal solutions, of egg-white, for example, with various fixatives. Professor BUtschli's belief that the cytoplasm has a vacuolar structure is corroborated by his interesting experiments on microscopic foams. Finely powdered potassium carbonate is mixed with olive oil which has been previously heated to a temperature of 5o°-6o'' C, an acid from the oil splits up the potassium carbonate, liberates carbon dioxide, and forms an extremely fine emulsion. Drops of this show a structure not unlike that of cytoplasm, exhibit movements and streamings not unlike those of Amoebse, and are, in short, mimic cells. Just as a working model may help us to understand the circulation, so these oil-emulsion drops may help us to understand the living cell, by bringing the strictly vital phenomena into greater prominence. More recent work, especially with the ultra-microscope, points to the conclusion that the reticular, fibrillar, and other complexities are, in the main, post-mortem effects. There are definite formed bodies, such as mitochondria and various plastids, in many cells, and there is often a deposition of less labile material by the ever-changing protoplasm, but the important fact is that protoplasm is a heterogeneous mixture in a colloid state. CHAPTER IV THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS I. Reproduction In the higher animals the beginnings of individual hfe are hidden, within the womb in Mammals, within the egg-shell in Birds. It is natural, therefore, that early preoccupation with those higher forms should have hindered the recogni- tion of what seems to us so evident, that almost every animal arises from an egg-cell or ovum which has been fertilised by a male cell or spermatozoon. The exceptions to this fact are those organisms w^hich multiply by buds or detached overgrowths, and those which arise from an egg- cell which requires no fertilisation. Thus Hydra may form a separable bud, much as a rose-bush sends out a sucker ; thus drone-bees " have a mother, but no father," for they arise from parthenogenetic eggs which are not fertilised. Sexual reproduction. — There is apt to be a lack of clearness in regard to sexual reproduction, because the pro- cess which we describe by that phrase is a complex result of evolution. It involves two distinct facts— {a) the liberation of special germ cells from which new individuals arise ; {b) the union or amphimixis of two different kinds of germ cells, ova and spermatozoa, which come to nothing unless they unite. Furthermore, these dimorphic reproductive cells are produced by two different kinds of individuals (females and males), or from different organs of one individual, or at different times within the same organ (hermaphroditism). It is conceivable that organisms might have gone on multiplying asexually, by detaching overgrown portions of 60 GERM CELLS 6 1 themselves which had sufficient vitahty to develop into complete forms. But a more economical method is the liberation of special germ cells, in which the qualities of the organism are inherent. This is the primary characteristic of sexual, as opposed to asexual, multiplication. It is also conceivable that organisms might have re- mained approximately like one another in constitution, and at all times very nearly the same, and that they might have liberated similar germ cells capable of immediate develop- ment. Such a race would have illustrated the one char- acteristic of sexual reproduction, the liberation of special germ cells ; but it would have been without that other characteristic of sexual reproduction — the amphimixis or fertilisation of dimorphic germ cells, usually produced by different organs in one individual or by distinct male and female individuals. Liberation of special germ cells. — One must think of this as an economical improvement on the method of start- ing a new life by asexual overgrowth or by the liberation of buds. Asexual reproduction, as Spencer and Haeckel point out, is a mode of growth in which the bud, or whatever it is, becomes distinct or discontinuous from the parent. The buds of a sponge, of a coral, of a sea-mat, or of many Tunicates, remain attached to the parent. If there be a keen struggle for subsistence, this may be disadvantageous ; but in som.e cases, doubtless, the colonial life which results is a source of strength. In the case of Hydra, however, the buds are set adrift ; the same is true of not a few worms. This liberation of buds takes us nearer the sexual process of liberating special germ cells. But unless the organism is in very favourable nutritive conditions, in which over- growth is natural, the liberation of buds is an expensive way of continuing the life of a species. Not only so, but we can hardly think of budding even as a possibility in very complex organisms, like snails or bitds, in which there is much division of labour. Moreover, the peculiarity of true germ cells is that they do not share in building up the " body," and that they retain an organisation continuous in quality with the original germ cell from which the parent arose ; they are thus not very liable to be tainted by the mishaps which may befall the " body " which bears them. 62 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS And, finally, in the mixture of two units of living matter which have had different histories, an opportunity for new permutations and combinations, in other words, for variation, is supplied. Thus it is not surprising to find that the asexual method of liberating buds has been re- placed in most animals by the more economical and ad- vantageous process of sexual reproduction. Summary of Modes of Reproduction A. In Single-celled Animals (Protozoa) (i) The almost mechanical rupture of an amoeboid cell, which has become too large for physiological equilibrium. (2) The discharge of numerous superficial buds at once {e.g. Arcella and Pelomyxa). (3) The formation of one bud at a time (very common). (4) The ordinary division into two daughter cells at the limit of growth. (5) Repeated divisions within limited time and within limited space (a cyst). This results in what is called spore-formation {e.g. in Sporozoa). B. In Many-celled Animals (Metazoa) {Asexual) {a) The separation of a clump of body cells, e.g. from the surface of some Sponges. (A crude form of budding.) {b) The formation of definite buds which may or may not be set free, (c) Various forms of fission and fragmentation. {Sexual) The liberation of special reproductive or germ cells, which have not taken part in the formation of the body, and which retain the essential qualities of the original germ cell from which the parent arose. These special germ cells — the ova and sperma- tozoa — are normally united in fertilisation, but some animals have (parthenogenetic) ova which develop without being fertilised. Evolution of sex. — A further problem is to account for the two facts — {a) that most animals are either males or females, the former liberating actively motile male elements or spermatozoa, the latter forming and usually liberating EVOLUTION OF SEX 63 more passive egg-cells or ova ; and {b) that these two different kinds of reproductive cells usually come to nothing unless they combine. The problem is partly solved by a clear statement of the facts. Let us begin with those interesting organisms which are on the border line between Protozoa and Metazoa, the colonial Infusorians, of which Volvox is a type. The adults are balls of cells, and the component units are con- nected by protoplasmic bridges. From such a ball of cells reproductive units are sometimes set adrift, and these divide to form other individuals without more ado. In other con- ditions, however, when nutrition is checked, a less direct mode of reproduction occurs. Some of the cells become large, well-fed elements, or ova ; others, less successful, divide into many minute units or spermatozoa. The large cells are fertilised by the small. Here we see the formation of dimorphic reproductive cells in different parts of the same organism. But we may also find Volvox balls in which only ova are being made, and others with only spermatozoa. The former seem to be more vegetative and nutritive than the latter ; we call them female and male organisms respectively ; we are at the foundation of the differences between the two sexes. All through the animal series, from active Infusorians and passive Gregarines to feverish Birds and more sluggish Reptiles, we read antitheses between activity and passivity, between lavish expenditure of energy and a habit of storing. The ratio between disruptive {katabolic) processes and con- structive {anabolic) processes in the protoplasmic meta- bolism varies from type to type. It may be that the contrast between the sexes is another expression of this fundamental alternative of variation. Stages in the history of fertilisation. — While it is not difficult to see the advantage of fertilisation as a process which helps to sustain the standard or average of a species and as a source of new variations, we can at present do little more than indicate various forms in which the process occurs. (a) Formation of Plasmodia, the flowing together of numerous feeble cells, as seen in the life-history of those very simple Protozoa called Proteomyxa, e.g. Protomyxa, and Mycetozoa, e.g. flowers of tan {/Ethalium septicum). {b) Multiple conjugation, in which more than two cells unite and fuse 64 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS together temporarily, as in some Sporozoa and in the sun- animalcule (A ctinosplun-iiim). (c) Ordinary conjugation, in which two similar cells unite, with fusion of their nuclei, observed in Sporozoa, Heliozoa, Flagel- lates, and Rhizopods. In ciliated Infusorians, the conjugation may be merely a temporary union, during which nuclear elements are interchanged. (d) Dimorphic conjugation, in which two cells different from one another fuse into one, a process well illustrated in Vorticella and related Infusorians, where a small, active, free-swimming (we may say, male) cell unites with a fixed individual of normal size, which may fairly be called female (see Fig. 53). (e) Fertilisation, in which a spermatozoon liberated from a Metazoon unites intimately with an ovum, usually liberated from another individual of the same species. Divergent modes of sexual reproduction. — (a) Herm- aphroditism is the combination of male and female sexual functions in varying degrees within one organism. It may be demonstrable in early life only, and disappear as male- ness or femaleness predominates in the adult. It may occur as a casualty or as a reversion ; or it may be normal in the adult, e.g. in some Sponges and Coelentera, in many " worms," such as earthworm and leech, in barnacles and acorn-shells, in one species of oyster, in the snail, and in many other Bivalves and Gastropods, in Tunicates and in the hag-fish. In most cases, though these animals are bisexual, they produce ova at one period and spermatozoa at another (dichogamy). It rarely occurs (e.g. in some parasitic worms) that the ova of a hermaphrodite are fertilised by the sperms of the same animal (autogamy). Certain facts, such as the occurrence of hermaphrodite organs as a transitory stage in the development of the embryos of many unisexual animals (e.g. frog and bird), suggest that hermaphroditism is a primitive condition, and that the unisexual condition of permanent maleness or femaleness is a secondary differentiation. Other facts, such as the hermaphroditism of many parasites, where cross- fertilisation would be difficult, suggest that the bisexual condition may have arisen as a secondary adaptation. It seems likely that there is both primitive and secondary hermaphroditism. (b) Parthenogenesis, as we know it, is a degenerate form of sexual reproduction, in which ova produced by a female ALTERNATION OF GENERATIONS 6s organism develop without being fertilised by male elements. It is well illustrated by Rotifers, in which fertilisation is the exception (in some genera males have never been found) ; by many small Crustaceans whose males are absent for a season ; by Aphides, from among which males may be absent for the summer (or in artificial conditions for several years) without affecting the rapid succession of female generations ; by the production of drones in the bee-hive from eggs which are never fertilised. {c) Alternation of generations. — A fixed asexual hydroid I. ^-^ ^R rr;^ /^r r r^ — ^ Fig. 30. — Diagrammatic expression of alternation of generations. 1. HydromedusaB. ov., Fertilised ovum (ov.) gives rise to an asexual form A, which, by budding, produces sexual form or forms S ; in the case of Hydromedusae, A is represented by hydroid (H), and S by medusoid (M). 2. Liver Fluke. ov., Fertilised ovum (ov.) gives rise to asexual stages (A), which, from special spore-like cells (R), produce eventually the sexual fluke (S). or zoophyte often buds off and liberates sexual medusoids or swimming-bells, whose fertilised ova develop into embryos which become fixed and grow into hydroids (Figs. 90 and 107). This is the simplest illustration of alternation of generations, which may be defined as the alternate occurrence in one life-cycle of two (or more) different forms differently produced (Fig. 30). The liver-fluke (Distomum hepaticum) of the sheep produces eggs which, when fertilised, grow into embryos. Within the latter, certain cells (which might be called 5 66 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS Spores) grow into numerous other larvae of a different form. Within these the same process is repeated, and finally the larvae thus produced grow (in certain con- ditions) into sexual flukes (Fig. ii8). In this case, reproduction by special cells, like undifferentiated pre- cocious ova, alternates with reproduction by ordinary fertilised egg-cells. So, too, the vegetative sexless " fern- plant " gives rise to special spore cells, which develop into an inconspicuous bisexual " prothallus," from the fertilised egg-cell of which a " fern-plant " springs. Various kinds of alternation are seen in the life-cycle of the fresh-water sponge, in the stages of the jelly-fish Amelia, in the history of some " worms" and Tunicates. They illustrate a rhythm between asexual and sexual multi- plication, between parthenogenetic and normal sexual reproduction, between vegetative and active life, between a relatively " anabolic " and a relatively " katabolic " preponderance. II. Embryology Egg-cell or ovum.^ — Apart from cases of asexual repro- duction and parthenogenesis, every multicellular animal begins life as an egg-cell with which a male cell or sperma- tozoon has entered into intimate union. The most important characteristic of the reproductive cells, whether male or female, is that they retain the essential qualities of the fertilised ovum from which the parent animal was developed. The ovum has the usual characters of a cell ; its cyto- plasm is a complex colloidal mixture of substances ; its nucleus or germinal vesicle contains the usual chromatin elements ; it has often a store of reserve material or yolk, and a distinct envelope representing a cell wall (Figs. 31 and 37). In Sponges the ova are well-nourished cells in the middle stratum of the body ; in Coelentera they seem to arise in connection with either outer or inner layer (ectoderm or- endoderm) ; in all other animals they arise in connection with the middle layer or mesoderm, usually on an area of the epithelium lining the body cavity. In lower animals THE EGG-CELL 67 they often arise somewhat diffusely ; in higher animals their formation is restricted to distinct regions, and usually to definite organs — the ovaries. The young ovum is often amoeboid, and that of Hydra retains this character for some time (Fig. 89, 2). The ovum grows at the expense of adjacent cells, or by absorbing material which is contributed by special yolk glands or supplied by the vascular fluid of the body. The yolk or nutritive capital may be small in amount, and distributed uniformly in the cell, as in the ova of Mammals, earthworm, starfish, and sponge ; or it may be Fig. 31.— Diagram of ovum, showing diffuse yolk granules. g.v., Germinal vesicle or nucleus ; chr., chromatin elements or chromosomes. more abundant, sinking towards one pole as in the egg of the frog, or accumulated in the centre as in the eggs of Insects and Crustaceans ; or it may be very copious, dwarf- ing the formative protoplasm, as in the eggs of Birds, Reptiles, and most Fishes (Fig. 39). Round the egg there are often sheaths or envelopes of various kinds — (a) made by the ovum itself, and then very delicate (e.g. the vitelline membrane) ; [b) formed by ad- jacent cells {e.g. the follicular envelope) ; or (c) formed by special glands or glandular cells in the walls of the oviducts {e.g. the " shells " of many eggs). The envelope is often 68 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS firm, as in the Fig. 32. — Diagram of a typical sper- matozoon. //., The so-called " head " ; A., acro- some, which first attaches itself to the egg-cell in fertilisa- tion; CHK., chromo- somes, the vehicles of many hereditary factors, if not of all ; Cy., the protoplasm of the head ; M.P., " middle piece " of the spermatozoon, including the ccntro- some (C.) ; T., the locomotor tail, with an axial filament {A.F.) running down the middle. chitinoiis coat around the eggs of many Insects, and in these cases we find a minute aperture (micropyle), or several of them, through which the sperma- tozoon can enter. The hard calcareous shells round the eggs of Birds and Tor- toises, or the mermaid's purse enclosing the egg of a skate, are of course formed after fertilisation. Egg-shells must be dis- tinguished from egg capsules or cocoons, e.g. of the earthworm, in which several eggs are wrapped up together. Male cell or spermatozoon. — This is a much smaller and usually a much more active cell than the ovum. In its minute size, locomotor energy, and persistent vitality, it resembles a flagellate Monad, while the ovum is comparable to an Amoeba or to one of the more encysted Protozoa. A spermatozoon has usually three dis- tinct parts : the essential " head," con- sisting mainly of nucleus, and the mobile " tail," which is often fibrillated, and a small middle portion between head and tail, which is usually the bearer of the centrosome. The spermatozoa of Thread- worms and most Crustaceans are sluggish, and inclined to be amoeboid (Fig. 33 (6, 7))- Both ova and spermatozoa are true cells, and they are complementary, but the spermatozoon has a longer history behind it (Fig. 34). The homologue of the ovum is the mother sperm cell or spermatogonium. This segments as the ovum does, but the cells into which it divides have little coherence. They go apart, and become spermatozoa. There is often a resemblance between the difl^erent ways in which a mother sperm SPERMATOZOA 69 cell divides and the various kinds of segmentation in a fertilised ovum. In most cases the spermatogonium Yic. 33.— Forms of spermatozoa (not drawn to scale). I and 2. Immature and mature spermatozoa of snail ; 3- of bird ; 4. of man (h., head ; m., middle portion ; /., tail) ; 5- 01 sala- mander, with vibratile fringe (/.) ; 6. of Ascaris, slightly amoeboid with cap (c.) ; 7. of crayfish. divides into spermatocytes, which usually divide again mto spermatids or young spermatozoa. YiG, 34. — Diagram of maturation and fertilisation. (From Evolution of Sex.) A. Primitive sex cell, supposed to be amoeboid. B. Unripe ovum ; C. formation of first polar body (i. p.b.) ; D. forma- tion of second polar bodv (2. p.b.). B'. Mother sperm cell ; C the same divided (sperm-morula). D'. Ball of immature spermatozoa ; sp., liberated spermatozoa. E. Process of fertilisation ; F. approach of male and female nuclei within the ovum. Maturation of ovum. — When the egg-cell attains its definite size or limit of growth, it bursts from the ovary or 70 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS from its place of formation, and in favourable conditions meets either within or outside the body with a spermatozoon from another animal. Before the union between ovum and spermatozoon is effected, generally indeed before it has begun, the nucleus or germinal vesicle of the ovum moves to the periphery and divides twice. This division results in the formation and extrusion of two minute cells or polar bodies, which come to nothing, though they may linger for a time in the precincts of the ovum, and may even divide. The second division follows the first without the inter- vention of the " resting stage " which usually succeeds a nuclear division. In most cases the division which forms the first polar body is a reducing or meiotic division, the number of chromosomes being reduced to half the number characteristic of the cells of the body. The extrusion of polar globules and the associated reduction is almost universal in the history of ova, but in some parthenogenetic ova only one polar body is formed, and there is no reduc- tion in the number of chromosomes. In some other cases the parthenogenetic ovum passes through the meiotic phase and forms two polar bodies. The second of these, however, is not liberated, but remains within the ovum and re-uniting with the reduced nucleus restores the normal number of chromosomes. Reducing or Meiotic Division. — In each kind of animal there is a definite number of chromosomes, say n, in each of the body-cells. In the ripe germ-cells, however, there is half the normal number, ", so that when spermato- zoon and ovum unite in fertilisation the normal number is restored. In the history of the germ-cells, therefore, in one way or another, at one stage or another, the number of chromo- somes undergoes reduction to half the normal. In many cases this reduction comes about through a " heterotypic " or meiotic division. We give a condensed account of what happens in a large number of cases. The immature germ-cells, whether oocytes or spermatocytes, show n chromosomes, half of which are of paternal, and half of maternal, origin. At a certain stage in the ripening or inatnration there is a conjugation of the chromosomes in pairs, and the two forming a pair seem to be of maternal and paternal origin. MEIOTIC DIVISION 71 In the reducing, meiotic, or maturation division each daughter-cell gets one or the otlier member of each pair of homologous chromosomes. In the case of the ovum the meiotic division usually occurs in the formation of the first polar body, so that it and the reduced nucleus of the ovum have each " chromosomes. There is no further reduction in 2 the formation of the second polar body, which involves an ordinary equation-division. The first polar body often divides into two. Thus Y^mT- -Vt/. la • • I \l 'V V V,, • • • IPi \f\ Fig. 35. — Oogenesis and Spermatogenesis. — After Boveri. I. and I. A. Primordial germ-cells. II. -IV. and II.A-IV.A. Multiplication of germ-cells (oogonia and sperma- togonia). V. An immature full-grown egg-cell. VI. It gives off the first polar body (P.B.i) by a meiotic division, and the first polar body may divide again (i, 2). VII. The reduced oocyte gives off a second polar body (3) by an equation division, and thus becomes the ripe egg (4). V.A. A spermatogonium which divides by a meiotic division to form two spermatocytes (VI. a). Each of these divides again by an equation division, forming four sperma- tids, which differentiate into spermatozoa (1A-4A). the result is one viable cell (the mature ovum) and three non-viable cells (the polar bodies), each with - chromosomes. In the spermatogenesis or production of spermatozoa the meiotic division is usually the second-last. A " mother-sperm cell " or spermatogonium divides into spermatocytes with n chromosomes, each of these divides into 2 spermatocytes with ^ chromosomes, and these 72 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS again divide into spermatocytes which differentiate into spermatozoa- The result is that from each of the penultimate generation of spermato- cytes there arise four spermatozoa, each with - chromosomes. Thus there is a close parallelism in the maturation process in the two sexes. That the fertilisation of the ovum restores the number to the normal n is obvious. Part of the significance of meiotir division is that it affords oppor- tunity for fresh permutations and combinations of hereditary qualities, for chromosomes are the bearers of at least some of these. It is important to understand that in ordinary mitosis or cell-division, each daughter-cell gets an absolutely similar half of each chromosome of the mother-cell, whereas in meiotic division each daughter-cell gets half of the total number. If we compare the nucleus and its chromosomes to such a common- place thing as a box of matches we may make the difference between the two kinds of division obvious. We might halve the matches by putting half of them into another box (meiotic division) ; or we might take a knife and split each match longitudinally and put one of the sets of halves into another box (ordinary equation division). Fertilisation. — In the seventeenth and eighteenth cen- turies, some naturaUsts, nicknamed " ovists," beHeved that the ovum was all-important, only needing the sperm's awakening touch to begin unfolding the miniature model which it contained. Others, nicknamed " animalculists," were equally confident that the sperm was essential, though it required to be fed by the ovum. Even after it was recognised that both kinds of reproductive elements were essential, many thought that their actual contact was un- necessary, that fertilisation might be effected by an aura seminalis. Though spermatozoa were distinctly seen by Hamm and Leeuwenhoek in 1679, their actual union with ova was not observed till 1843, when Martin Barry detected it in the rabbit. Of the many facts which we now know about fertilisa- tion, the following are the most important : — (i) Apart from the occurrence of parthenogenesis in a few of the lower animals, an ovum begins to divide only after a spermatozoon has united with it. After one sper- matozoon has entered the ovum, the latter ceases to be receptive, and other spermatozoa are excluded. If, as rarely happens, several spermatozoa effect an entrance into the ovum, the result is usually some abnormality. It is said, however, that the entrance of numerous spermatozoa (polyspermy) is frequent in insects and Elasmobranch fishes. FERTILISATION 73 (2) The union of spermatozoon and ovum is very in- timate ; the nucleus of the spermatozoon and the reduced nucleus of the ovum approach one another, combining to form a unified nucleus. (3) The ovum centrosome disappears before fertilisation, but a centrosome is introduced or evoked by the spermato- zoon. It divides into the two which play an important role in the segmentation of the fertilised ovum. (4) When the combined or segmentation nucleus begins the process of development by dividing, each of the two daughter nuclei which result consists partly of material 11 m Fig. 36.— -Fertilisation of egg-cell. — After Fol. I. Shows a minute hillock of protoplasm rising from the ovum towards the approaching spermatozoon. II. Shows how the head of the successful spermatozoon has entered the ovum. III. The tail is nipped off when the head has entered. A pellicle — the fertilisation membrane — is seen around the o\'um. derived from the sperm nucleus, partly of material derived from the ovum nucleus. In other words, the union is orderly as well as intimate, and the subsequent division is so exact, that the qualities marvellously inherent in the sperm nucleus (those of the male parent), and in the ovum nucleus (those of the mother animal), are diffused through- out the body of the offspring, and persfst in its reproductive cells. (5) The spermatozoon may be able to enter the egg only through pre-existing apertures or micropyles, or it may be restricted to a particular region of the egg where the cyto- plasm is not too heavily charged with yolk ; in the simplest cases, as, for example, in annelids, echinoderms, and 74 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS mammals, it may enter at any point. Entrance is effected partly by the boring motion of the spermatozoon, but partly, in some cases at least, by an active engulfing action on the part of the egg ; the whole process is usually com- plete within one minute. In the most studied case, the sea-urchin egg, the first visible change in the egg follows almost immediately : the delicate vitelline membrane becomes detached from the surface of the egg to form the Fig. 37- — Diagram showing relative size of an egg-cell and a sperm -cell (S.). A'., Nucleus ; C, colloidal cytoplasm ; CHR., portions of chromosomes ; G.S., the nucleolus, which used to be called the germinal spot. fertilisation membrane ; the space between this and the egg itself is filled with a clear fluid. There is a striking increase in the permeability of the surface of the ovum during these changes, so that dissolved substances in the cytoplasm may escape altogether and be lost, and the egg is most sensitive to abnormalities in the external medium. Moreover, there is a great increase in the metabolism of the ovum after fertilisation : increased consumption of oxygen, increased evolution of COg, and increased heat production. FERTILISATION 75 (6) Some eggs, e.g. of sea-urchins, can be artificially induced to develop without fertilisation (by being im- mersed for a couple of hours in a mixture of sea-water and solution of magnesium chloride, and by many other means). It seems, therefore, justifiable and useful to distinguish in ordinary fertilisation, (a) the mingling of the hereditary qualities of the two parents, and {b) an Fig. 38. — Fertilisation in A^caris megalocephala. — After Boveri. 1. Spermatozoon (sp.) entering ovum, which contains reduced nucleus (N.), having given off two polar bodies {p.b. i and 2). 2. Sperm nucleus (the upper), and ovum nucleus (A^.). each with two chromosomes, and with centrosomes {c.s.). 3. Centrosomes (c.s.) with " archoplasmic " threads radiating outwards in part to the chromosomes of the two approximated nuclei. 4. Segmentation spindle before first cleavage. exciting or liberating stimulus which induces the ovum to divide. In one interesting case (the thread-worm Rhabditis) the spermatozoon has the latter function, but not the former ; it enters the ovum and stimulates it to divide, but degenerates without fusing with the egg-nucleus. But in more normal cases, it is found that if the fertilised egg is cut in two before the nuclei have united, the half containing the spermatozoon nucleus may divide and 76 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS develop {merogony), but the half containing only the ovum nucleus degenerates. It should be noted that the chromosomes of the spermatozoon do not fuse with the chrom.osomes of the ovum when fertiUsation occurs. They are associated together and divide together in all the cell- • divisions, whether of body-making or of the germ-cell lineage. In some of the divisions of the germ-cell lineage there seems to be an interesting interchange or " crossing over " of pieces of the members of a pair of chromosomes. A special chromosome of the germ-cells seems often to have to do with sex, whether as determiner of maleness or femaleness, or as an index of these two physiological conditions. Segmentation. — The different modes of division ex- hibited by fertilised egg-cells depend in great measure on the quantity and disposition of the passive and nutritive yolk material, which is often called deutoplasm, in con- trast to the active and formative protoplasm. The pole of the ovum at which the formative protoplasm lies, and at which the spermatozoon enters, is often called the animal pole ; the other, towards which the heavier yolk tends to sink, is called the vegetative pole. In the floating ova of some fish, however, the yolk is uppermost, and the embryonic area lowest. In contrasting the chief modes of segmentation, it should be recognised that they are all connected by gradations. A. Complete Division — Holoblastic Segmentation (i) Eggs with little and diffuse yolk material di\ide completely into approximately equal cells, [or, Ova which are alecithal {i.e. without yolk) undergo approxi- mately equal holoblastic segmentation]. This is illustrated in most Sponges, most Coelentera (Fig. 39 (i)), some " Worms," most Echinoderms, some Molluscs, all Timicates, Amphioxiis, and most Mammals. (2) Eggs with considerable yolk material accumulated towards one pole, divide completely, but into unequal cells, [or, Ova with a considerable amount of deutoplasm lying towards one pole (telolecithal), undergo unequal holoblastic segmenta- tion]. This is illustrated in some Sponges, some Coelentera {e.g. Ctenophora), some " Worms," many Molluscs, the lamp- rey. Ganoid Fishes, Dipnoi, Amphibians (Fig. 39 (2)). CLEAVAGE OF THE OVUM 77 V • ♦ • «• * •* • * * • -V Fig. 39. — Modes of Segmentation. 1. Ovum, with little yolk, segments totally and equally into a blastosphere, e.g. Hydra, sponge, sea-urchin. 2. Ovum, with a considerable amount of yolk (y.) at lower pole, segments totally but unequally, e.g. 'frog ; {y.s.) larger yolk- laden cells. 3. Ovum, with much yolk (y.) at lower pole, segments partially and discoidally, forming blastoderm [bl.], e.g. bird, most fishes. Ovum, with central yolk (y.), segments partially and peripher- ally, e.^. most Arthropods. 78 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS B. Partial Division — Meroblastic Segmentation (3) Eggs with a large quantity of yolk on which the formative protoplasm lies as a small disc at one pole, divide partially, and in discoidal fashion, [or. Ova which are telolecithal. and have a large quantity of deutoplasm. undergo meroblastic and discoidal segmentation]. This is illustrated in all Cuttle-fishes, all Elasmobranch and Teleostean Fishes, all Reptiles and Birds (Fig. 39 (3)), and also in the Monotremes or lowest Mammals. (4) Eggs with a considerable quantity of yolk accumulated in a central core and surrounded by the formative protoplasm, divide partially, and superficially or peripherally, [or, Ova which are centrolecithal undergo meroblastic and peri- pheral segmentation.] This is illustrated by most Arthropods (Fig. 39 (4)), and by them alone. Cleavage pattern. — After fertilisation, and before the division of the egg into the two first " blastomeres," there may be a visible rearrangement of the materials in the cytoplasm. The subsequent cleavages very often follow so regular a pattern that it may be possible to point to a particular region of the cytoplasm of the ovum and predict the part which it is to play in the formation of the embryo, if development follows its normal course. But we must be chary of supposing that any such region is specialised from its surroundings except by position, for it is often possible to obtain complete embryos from fragments of eggs, or from isolated blastomeres from the two-cell or four-cell stage. Eggs which have this power of readjust- ing their organisation are called " regulative " in contrast to the " mosaic " eggs in which there is more evidence of the presence of a fixed structural pattern. Even here, however, it is too much to speak of " organ-forming substances " in the egg ; for it is usually found that the visible pattern of the cytoplasm may be completely changed by whirling the eggs in a centrifuge, without marked abnormalities in the subsequent development. The plane of the first cleavage is typically a meridian running through the two poles of the egg, and its exact situation is determined by the path of the spermatozoon nucleus and centrosome in the cytoplasm. For subse- quent cleavages, the simplest type is seen in the sea-urchin EXPERIMENTAL EMBRYOLOGY 79 and the frog, where the first three cleavage-planes are at right angles to each other, the first two being meridians and the third equatorial. In other cases the blastomeres may be unequal in size, so that a spiral type of cleavage results, and many intermediate forms are known. In insects and crustaceans there may be many nuclear divisions without division of the cytoplasm, though ultimately the nuclei separate and each becomes the centre of a cell. The two first blastomeres frequently correspond to the right and left halves of the future embryo, but exceptions to this are very common, and there may be much variation in this respect even within a single species. Conse- quently the problem of the origin of the bilateral sym- metry of the embryo is not the same as the problem of the determination of the first cleavage plane. The eggs of squids and of many insects have a bilateral shape and structure before fertilisation, and the axes of symmetry of the egg are the same as those of the embryo : cases have been described in which insect eggs are laid in lines, all pointing in the same direction, and the development can be followed through the transparent shell until there is an Indian file of unhatched larvae. Such eggs are enveloped in stiff membranes, and it is therefore likely that their symmetry is not inherent in their own structure but is imposed upon them by the maternal cells which produce the shell. In most other eggs, which are radially symmetrical before fertilisation, it seems that bilaterality is determined by the point at which the spermatozoon enters ; this holds, for instance, for the sea-urchin and frog. Experimental embryology. — Experiments on the power of isolated blastomeres to produce complete dwarf embryos have been made in almost all classes of animals. The sea-urchin's egg is an example of the most " regulative '* type : a blastomere representing only^ 73V of the egg has been known to develop normally for some time, and it seems possible that it is smallness of size rather than any regional differentiation that prevents the perfect develop- ment of these and smaller blastomeres. In most Coelenter- ates the blastomeres are " totipotent/' i.e. capable of producing a whole embryo, up to the four-cell stage, but 8o THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS in the Ctenophores certain structures are missing from the larvae which develop from isolated blastomeres. The eggs of Nematodes and Ascidians are of the mosaic type, and isolated blastomeres will not develop properly ; one member of the latter group, Styela, may be regarded as an extreme case ; the egg contains a yellow substance which is necessary for the formation of muscle-fibres, and if this substance is redistributed by centrifuging the development is upset. The case of the Amphibian egg, which has been much studied, is interesting. Diametrically opposite the point where the spermatozoon enters there forms a '* grey crescent " which eventually gives rise to the dorsal lip of the blastopore (see Fig. 39). This grey crescent is usually divided by the first cleavage plane, and in this case the blastomeres, if isolated, can give rise to whole embryos ; but if the cleavage-plane falls elsewhere, the blastomere which contains none of the material of the grey crescent will not develop. It is possible also to prevent or delay the development of one blastomere. at the two-cell stage, by injuring it with a hot needle ; the uninjured blastomere then behaves as if the sister-cell in contact with it were segmenting normally, and develops *' mosaically " into a half-embryo. Again, it may be possible to effect a partial separation of the two first blastomeres, for example by gentle shaking in the case of the egg of Amphioxus (Wilson) ; this may give rise to " Siamese- twin " or two-headed " Janus " embryos. The normal distribution of the nuclei in segmentation, e.g. in the frog or sea-urchin, may be completely upset by gentle pressure, yet normal embryos are produced ; this proves that during cleavage the nuclei of the blastomeres are identical, as far as their efl^ect on differentiation is concerned. Variations in the chemical composition of the medium may greatly affect the development of eggs. For example, the fish Fundulus in a solution containing magnesium chloride develops a single median " cyclopic " eye instead of the usual pair of eyes. If calcium, is absent from the medium, the blastomeres of t\\t sea-urchin's egg will not stick together, but become separated. In solutions GASTRULA 8 1 containing lithium sea-urchin eggs fail to gastrulate properly ; no invagination takes place, and abnormal hour- glass-shaped larvae are produced. Blastosphere and morula. — The result of the division is usually a ball of cells. But when the yolk is very abundant a disc of cells — a discoidal blastoderm — is formed at one pole of the mass of nutritive material, which it gradually surrounds. As the cells divide and redivide, they often leave a large central cavity — the segmentation cavity — and a hollow ball of cells — a blastosphere or blastula — results. But if the so-called " segmentation cavity " be very small or absent, a solid ball of cells or morula, like the fruit of bramble or mulberry, results* Gastrula. — The next great step in development is the establishment of the two primary germinal layers, the outer ectoderm and the inner endoderm, or the epiblast and the hypoblast. One hemisphere of the hollow ball of cells may be appar- ently dimpled into the other, as we might dimple an india- rubber ball which had a hole in it. Thus out of a hollow ball of cells, a two-layered sac is formed — a gastrula formed by invagination or embole (Fig. 40). The mouth of the gastrula is called the blastopore, its cavity the archenteron. But where the ball of cells is practically a solid morula, the apparent in-dimpling cannot occur in the fashion de- scribed above. Yet in these cases the two-layered gastrula is still formed. The smaller, less yolk-laden cells, towards the animal pole, gradually grow round the larger yolk-con- taining cells, and a gastrula is formed by overgrowth or epibole. In various ways the ectoderm and the endoderm are established, either by some form of gastrulation, or by some other process, such as that called de lamination. Mesoderm. — We are not yet able to make general state- ments of much value in regard to the origin of the middle germinal layer — the mesoderm or mesoblast. In Sponges and Coelentera it is not a distinct layer except in Cteno- phora, being usually represented by a gelatinous material {mesogloea), which appears between ectoderm and endoderm, and into which cells wander from these two layers. In the 6 82 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS Other Metazoa, the middle layer may arise from a few Fk;. 40. — Life-history of a coral. Monoxenia darwinii. — From Haeckcl. A, B, Ovum. C, Division into two. D, Four-cell stage. E, Blas- tula. F, Free-swimming blastula with cilia. G, Section of blastula. H, Beginning of invagination. I, Section of com- pleted gastrula, showing ectoderm, endoderm, and archenteron. K, Free-swimming ciliated gastrula. primary mesoblasts or cells which appear at an early stage ORIGIN OF ORGANS 83 between the ectoderm and endoderm {e.s;. in the earth- worm's development) ; or from numerous " mesenchyme " immis^rant cells, which are separated from the walls of the blastula or gastrula {e.g. in the development of Echino- derms) ; or as ccelom pouches — outgrowths from the en- dodermic lining of the gastrula cavity {e.g. in Sagitia, Balanoglossus, Amphioxus) ; or by combinations of these and other modes of origin. The mesoderm lies or comes to lie between ectoderm and endoderm, and it lines the body cavity, one layer of mesoderm (parietal or somatic) clinging to the ectodermic external wall, theother(visceral or splanch- nic) cleaving to the endodermic gut and its outgrowths. Origin of organs. — From the outer ectoderm and inner endoderm, those organs arise which are consonant with the position of these two layers, thus nervous system from the ectoderm, digestive gut from the endoderm. The middle layer, w^hich begins to be developed in " Worms," assumes some of the functions, e.g. contractility, which in Sponges and Coelentera are possessed by ectoderm and endoderm, the only two layers distinctly represented in these classes. In a backboned animal the embryological origin of the organs is as follows :- - {a) From the ectoderm or epiblast arise the epidermis and epidermic outgrowths, the nervous system, the most essential parts of the sense organs, infoldings at either end of the gut (fore-gut or stomodasum and a trace of hind-gut or proctodasum). {b) From the endoderm or hypoblast arise the mid-gut (mesenteron) and the foundations of its out- growths {e.g. the lungs, liver, allantois, etc., of higher Vertebrates), also the axial rod or noto- chord. {c) From the mesoderm or mesoblast arise all other struc- tures, e.g. dermis, muscles, connective tissue, bony skeleton, the lining of the body cavity, and the vascular system. This layer aids in the formation of organs originated by the other two. With it the reproductive organs are associated. Con- nective tissues, vascular system, and unstriped muscles are formed by mesenchyme cells which are budded off from the true mesoderm. 84 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS Our knowledge of the origin of organs has been greatly- added to by the researches of Spemann, Mangold, and others on the development of Amphibians. Before gastrulation it is possible to exchange for a fragment of ordinary epidermis a fragment of that region of the epi- dermis which would normally be folded in to form the nerve-cord ; these fragments are found to be still " plastic," and the " presumptive " nerve-cord becomes ordinary skin, and the presumptive skin normal nerve- cord ; such transplants can even be made from one species to another. After gastrulation, however, although no visible differentiation has taken place, the plasticity has been lost and the exchange can no longer be made success- fully ; the fate of these regions of epidermis has been settled and sealed, presumably by some chemical alteration of the tissues. The " axial structures " on the back of the embryo, i.e. the nerve-cord, notochord, and somites, arise in the meridian of the dorsal lip of the blastopore as it travels backwards towards the vegetative pole. If, before this time, the upper part of the blastula is " scalped " off and replaced at right angles to its original position, the axial structures maintain their relation to the meridian of the dorsal lip, although the cells from which they are formed have been rotated. The position of the axial structures is determined by the dorsal lip of the blastopore, which, as we have seen, arises from the grey crescent of the egg. Still more remarkable, it is found that if the dorsal lip from one embryo is grafted into the flank of another, it will there induce the formation of an extra and imwanted set of axial organs, which would otherwise never have arisen ; these do not arise from the cells grafted-in, but from the cells of the host embryo under the influence of the implant. It is not even necessary that the implant should come from an embryo of the same species or genus as the host. The dorsal lip of the blastopore, which provokes the formation of a set of axial organs in normal or abnormal situations, is called the " organiser." The evidence suggests that from it chemical substances diffuse out into the surrounding tissues. The organiser cannot make its TRANSPLANTING EXPERIMENTS 85 influence felt across a cut where there is no contact ; and it is possible to graft indifferent tissue into contact with an oFganiser and infect it with organising properties. Similar transplanting operations have given much in- formation about the differentiation of various organs. For instance, the rudiment of the eye may be transplanted into such unlikely situations as the wall of the abdomen, and will there differentiate by itself into a typical optic cup. On the other hand, the development of the lens of the eye is determined by the presence of an optic cup behind the ectoderm from which the lens arises. Thus if the rudiment of the cup is transplanted, no lens arises in the " proper " place on the head whence the primordium has been removed, but a lens does form from the ectoderm covering the optic cup in its new situation in the abdomen. There may be curious differences in these respects between closely related species. Thus in one frog, Rana esculenta, ordinary epithelium will not form a lens ; but the optic cup of this species can provoke lens-formation from the ordinary epithelium of another species, such as R. fusca, in which lens-formation is dependent on the optic cup as described above. At a later stage in differentiation the function of the organs begins to play a part in differentiation. Thus the length of the intestine in tadpoles may be influenced by the diet. Again, in early stages the veins and arteries are alike, and it is possible to transplant a section of vein into the course of an artery, where eventually it becomes thick- walled and elastic under the influence of the higher blood- pressure ; whereas veins, where the blood-pressure is lower, are ordinarily thin-walled and flaccid. Generalisations. — (i) The ovum theory or cell theory. — All many-celled animals, produced by sexual reproduction, begin at the beginning again. " The Metazoa begin where the Protozoa leave off " — as single cells. Fertilisation does not make the egg-cell double ; there is only a more com- plex and more vital nucleus than before. All development takes place by the division of this fertilised egg-cell and its descendant cells. (2) The gastrcea theory. — As a two-layered gastrula stage occurs, though sometimes disguised by the presence of 86 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS much yolk, in the development of the majority of animals, Haeckel concluded that it represents the individual's re- capitulation of an ancestral stage. He suggested that the simplest many-celled animal was like a gastrula, and this hypothetical ancestor of all Metazoa he called a gastrcea. The gastrula is, on this view, the individual animal's recapitulation of the ancestral gastraea. Rival suggestions have been made : perhaps the original Metazoa were balls of cells like Volvox (Fig. 54), with a central cavity in which reproductive cells lay ; perhaps they were like the platmla larvae of some Coelentera — two-layered, externally ciliated, oval forms without a mouth. Fig. 41. — Embryos — (i) of bird ; (2) of man. — After His. The latter about twenty-seven days old. y.s., Yolk-sac ; pi., placenta. (3) The idea of recapitulation. — It is a matter of experi- ence that we recapitulate in some measure the history of our ancestors. Embryologists have made this fact very vivid, by showing that the individual animal develops along a path the stations of which correspond to some extent with the steps of ancestral history. (i) The simplest animals are single cells (Protozoa). (2) The next simplest are balls of cells {e.g. Volvox). (3) The next simplest are two- layered sacs of cells {e.g. (i) The first stage of development is a single cell (fertilised ovum). (2) The next is a ball of cells (blastula or morula). (3) The next is a two-layered sac of cells (gastrula). Hydra). Von Baer, one of the pioneer embryologists, acknow- ledged that, with several very young embryos of higher RECAPITULATION 87 Vertebrates before him, he could not tell one from the other. Progress in development, he said, was from a general to a special type. In its earliest stage every organism has a great number of characters in common with other organisms in their earliest stages ; at each successive stage the series of embryos which it resembles is narrowed. The rabbit begins like a Protozoon as a single cell ; after a while it may be compared to the young stage of a simple vertebrate ; then to the young stage of a higher vertebrate ; afterwards, to the young stage of almost any mammal ; afterwards, to the young stage of almost any rodent ; eventually it becomes un- mistakably a young rabbit. Herbert Spencer expressed the same idea, by saying that the progress of development is from homogeneous to heterogeneous, through steps in which the individual history is parallel to that of the race. But Haeckel has illustrated the idea more vividly, and summed it up more tersely, than any other naturalist. His " fundamental biogenetic law " reads : " Ontogeny, or the development of the individual, is a shortened recapitulation of phylogeny, or the evolution of the race." It is hardly necessary to say that the young mammal is never like a worm, or a fish, or a reptile. It is at most like the embryonic stages of these, and it may also be noticed that, as our knowledge is becoming more intimate, the individual peculiarities of different embryos are be- coming more evident. But this need not lead us to deny the general resemblance. Moreover, the individual life-historv is much shortened compared with that of the race. Not merely does the one take place in days, while the other has progressed through ages, but stages are often skipped, and short cuts are dis- covered. And again, many young animals, especially those " larvae " which are very unlike their parents, often exhibit characters which are secondary adaptations to modes of life of which their ancestors had probably no experience. In short, the individual's recapitulation of racial history is general, but not precise. It is seen rather in the stages in the development of organs (organogenesis) than in the development of the organism as a whole. 88 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS (4) Organic continuity between generations. — Heredity. — Everyone knows that like tends to beget like, that offspring resemble their parents and their ancestors. Not only are the general characteristics reproduced, but minute features, idiosyncrasies, and pathological conditions, inborn in the parents, may recur in the offspring. At an early stage in the development of the embryo the j^B*^ .^^^iv^^mi^^iXS:;^^^^) *..... ..lI^L ^ ^ uW^'m>H ^- ^ ^:^^.I^;^-;^..^^^^^^ Fig. 42. — Larvae of Common EeL (Not drawn to scale.) I. Smallest l-nown larva, 7 mm. in length. II. and III. Laterally compressed transparent Leptocephalus stages. I\'. Change, when about two years old, from knife-blade-like to cylindrical shape. \'. Young elver, about three inches long, about two and a half years old. future reproductive cells of the organism are often dis- tinguishable from those which are forming the body. These, the somatic cells, develop in manifold variety, and, as division of labour is established, they lose their likeness to the fertilised ovum of which they are the descendants. The future reproductive cells, on the other hand, are not implicated in the formation of the " body," but, remaining virtually unchanged, continue the protoplasmic tradition unaltered, and are thus able to start an offspring which SEGREGATION OF GERM-CELLS 89 will resemble the parent, because it is made of the same protoplasmic material, and develops under similar con- ditions. An early isolation of reproductive cells, directly con- tinuous and therefore presumably identical with the original ovum, has been observed in the development of some *' worm types" — {Sagitta, Thread -worms, Leeches, Polyzoa), and of some Arthropods {e.g. Moina among Crustaceans, Chironomus among Insects, Phalangidae among Arachnids), Micrometnis aggregatus among Teleo- stean fishes, and with less distinctness in some other animals. A cell which will give rise to the germ-cells can be recognised in the gastrula stage of Cyclops, and in the very first segmentation stages of the thread-worm Ascaris. In many cases, however, the reproductive cells are not recognisable until a relatively late stage in development, after differentiation has made considerable progress. Weismann got over this difficulty by supposing that the continuity is sustained by a specific nuclear substance — the germ-plasm — which remains unaltered in spite of the diflferentiation in the body. It is perhaps enough to say that, as all the cells are descendants of the fertilised ovum, the reproductive cells are those which retain intact the qualities of that fertilised ovum, and that this is the reason why they are able to develop into off'spring like the parent. Finally, it may be noticed in connection with heredity, that there is great doubt to what extent the " body " can definitely influence its own reproductive cells. Animals acquire individual bodily peculiarities in the course of their life, as the result of what they do or refrain from doing, or as dints from external forces. The " body " is thus changed, but there is much doubt whether the repro- ductive cells within the " body " are affected specifically hy such changes. Weismann denied the transmissibility of any characters except those inherent in the fertilised egg- cell, and therefore denied that the influences of function and environment are, or have been, of direct importance in the evolution of many-celled animals. Such influences affect the body, and produce what are technically called " modifications,'' but these modifications do not affect the 90 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS reproductive cells — at least not in a specific representative way. Therefore modifications are not likely to be trans- mitted, and there seems no good evidence to show that they are. Many of the most authoritative biologists are at present of this opinion. On the other hand, many still maintain that profound changes due to function or environ- ment may saturate through the organism, and affect the reproductive cells in such a way that the changes or modifications in question are in some measure transmitted to the next generation. The question remains under dis- cussion, but the probabilities are strongly against the transmissibility of acquired characters. It is important to try to distinguish different modes of hereditary resemblance. The characters of the two parents may be blended in the offspring, or those of one parent may find predominant expression {exclusive inheritance), or the characters of one parent may be expressed in one part of the offspring and those of the other parent in another {particulate inheritance). Another important inquiry is into the share that the various ancestors have on an average in forming any indi- vidual inheritance. The inheritance of an animal repro- duced in the ordinary way is always dual, partly maternal and partly paternal, but through the parents there come contributions from grandparents, etc. Galton's Law of Ancestral Inheritance states that " The two parents con- tribute between them, on the average, one half of the total heritage ; the four grandparents, one quarter ; the eight great-grandparents, one eighth, and so on." Mendelian inheritance. — Of the greatest practical and theoretic importance in the study of heredity are the laws discovered by Mendel in 1865, but almost ignored until 1900. In their original form these laws are empirical formulations of the average results of breeding experi- ments ; but since 1900 the hypothetical basis suggested by Mendel to explain these laws has become much more concrete and definite. If black Andalusian and white Andalusian fowls be bred together, the offspring (the first filial or " Fj " generation) bear a finely divided pattern of black-and- white markings which gives a blue effect. But if two of MENDELIAN INHERITANCE QI these blue Andalusian fowls be mated, their offspring (the " F2 " generation) are not all blue ; some are black and others are white. On the average, in this generation, there will be equal numbers of pure black and pure white, and twice as many of the mixed or blue form. Far more commonly, however, it is found that there is no blending of the contrasted parental characters, but that one prevails over the other. Thus when a black guinea-pig is mated with a white one, the offspring in the Fi ccf) Fn /^ Aa A A A a ^^ . > r ^— AA AA Aa aA aa AA Aa aA 0.0. aa Fig. 43. — Mendelian inheritance illustrated in wood snail [Helix nemoralis). P., The parents, bandless [A), dominant, and banded {a), recessive. F.I., First filial generation, all bandless (.-ta). F.ii., Second filial generation, 25 per cent, pure bandless {A A), yielding bandless offspring in the next generation (F.iii.). 25 per cent, pure banded {aa), yielding banded offspring in the next generation (F.ni.). 50 per cent, bandless {Aa), with the banded character recessive as in F.i. These, if inbred, yield in the F.iii. generation the same ratio : lAA + zAa + iaa. F^ generation are not intermediate in colour, but perfectly black ; blackness is said to be dominant over whiteness or albinism, which is a recessive character. In the Fg generation the majority are again pure black and the minority pure white, the ratio between blacks and whites being three to one. Now if these white guinea-pigs of the Fg generation be inbred, their offspring will always be pure white ; in the same way either the black or white Anda- lusian fowls may be inbred without any other shade of 92 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS colour appearing. But if the black guinea-pigs of the Fg generation are inbred, a fraction of their offspring will be white. Further examination will show that the F2 genera- tion of guinea-pigs, with their three-to-one ratio of black to white, is more comparable than appears at first sight to the F2 generation of the Andalusian fowls, with their one-two-one ratio of black, blue, and white. For if the blue Andalusian fowls happened to be more like the black ones than they are, so like that they could not be distin- guished, then it would not be possible, except by chance, to inbreed the pure blacks of the Fg generation and obtain only pure black offspring ; one would be more likely to have chosen one or two of the indistinguishable mixed forms, and a fraction of the offspring would be white. Thus it appears that of the black guinea-pigs of the Fg generation one-third are " pure " black, and if by chance these can be inbred, their offspring will all be black; but two-thirds of the Fg black guinea-pigs are in reality mixed forms, and if two of them can be bred together, their offspring will be blacks and whites again in the ratio three-to-one. But the pure blacks and the mixed blacks are indistinguishable in appearance, and only by lengthy breeding experiments can the one be told from the other. The matter may be made clearer by a diagram, in which " D " represents the dominant character and " R " the recessive, which is completely masked in the presence of the dominant. P generation D x R Fi generation /D(R) \ V \ F2 generation D D(R) D(R) R / ■ ^ \ D D D(R) D(R) R R To explain these results it is suggested that the mixed forms of the F^ generation receive a " factor " for blackness from the one parent, a factor for whiteness from the other, though the presence of the latter is masked. But the recessive factor remains present, and the germ-cells of the MENDELISM 93 Fj generation are of two different kinds in each sex ; thus the Fi females have two kinds of ova, equal numbers of each : one kind containing the factor for blackness only, the other that for whiteness only. Similarly the sper- matozoa of the males are of two kinds, containing either factor, and again equal numbers of each. In the inbreed- ing of the Fj generation these germ-cells may be supposed to come together at random, and the result will be the formation of equal numbers of four kinds of fertilised egg-cells : *' black " ovum and " black " spermatozoon, '' black " and " white," " white " and " black," *' white" and " white." The second and third of these are identical, and the animals developing from them will be identical in appearance with those developing from the first kind of fertilised ovum ; only by continued breeding will the presence of the recessive " white " factor become manifest. In short, if the constitution of the F^ generation is expressed as BW, the germ-cells are either J5 or Win the female, B or W in the male, and the following combinations result : BB, BW, WB, WW, all the forms in which the dominant character is present being apparently identical. When Mendel's work was rediscovered it was seen that precision could be given to the theory by the assumption, since then abundantly justified, that the hereditary " factors " were carried in some way by the chromosornes. For we know that in the cells of the F^ generation the chromo- somes are half of paternal, half of maternal origin ; and that in the development of the spermatozoa a reducing division takes place, in which the chromosomes are sorted out into two lots, and the paternal chromosome which bears the factor for colour is separated from the corre- sponding one of maternal origin, so that there will be two kinds of spermatozoa, differing in this particular ; and that a similar reducing division in the maturation of the ovum will have a similar effect, producing equal numbers of two kinds of ova, if it is a matter of chance whether it is the paternal or the maternal chromosome that is thrown out in the first polar body. The factor for a given character carried by a chromosome is now often called a '* gene " and regarded as a specific material substance, but speculations as to its nature or mode of 94 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS action are as yet hardly profitable. In the case discussed we know that the black pigment of the guinea-pig's coat (melanin) is formed by the action of an enzyme (tyrosinase) on a colourless precursor, and that in the albinos the enzyme is lacking ; we may perhaps regard the " gene " for blackness as a substance essential for the formation of the enzyme, and the character for whiteness as merely the absence of this substance ; then any animal which has received the enzyme-forming gene, from either parent or from both, will be black. It is demonstrable that sex is inherited on Mendelian principles, and the idea has important applications. The inheritance of sex is to be compared with the result of crossing a mixed or DR form with a pure recessive or RR form, a " back-cross." Then the germ-cells of one parent are of two kinds, of the other parent all alike, and the offspring will again be of two kinds, DR or RR, in equal numbers. It has been demonstrated for a large number of species that the cells of the female contain two recognisable " X " chromosomes, while those of the male contain only one X-chromosome and, corresponding to the other, a small Y-chromosome : so that the males may be regarded as mixed forms in this respect, and every mating as a '* back-cross " between XX and XY. In other cases the Y-chromosome may be altogether absent ; and in the Lepidoptera and probably in birds it is the females that are mixed, so that they have two kinds of eggs, male- producing and female-producing, while other animals have two kinds of spermatozoa. Mendel's second great discovery was that when parents differing in two characters were mated, the offspring inherited these characters independently. Thus, in guinea-pigs, " rough-coat " is dominant over " smooth- coat " : so that when a black smooth-coated guinea-pig is crossed with a white rough-coated guinea-pig, all the members of the F^ generation are black and rough-coated. In the F., generation the rough-coats outnumber the smooth- coats in the ratio three to one, just as the blacks, pure and apparent, outnumber the whites ; but the two characters are independent of each other. Thus of every four black F.J guinea-pigs, three are rough-coated ; and of DIHYBRIDISM 95 every four smooth-coated, only one is white, on the average. The Fo generation cannot be truly represented by a group of four animals any longer, but only by a group of sixteen : of these, nine show both dominant characters, blackness and rough-coat ; three are black and smooth-coated, three are white and rough-coated, and only one shows both recessive characters, whiteness and smooth-coat. On the chromo- some theory it is supposed that the two contrasting pairs of characters are borne by different chromosomes, and further that in the reducing divisions it is a matter of chance whether it is the paternal or the maternal coat-deter- mining chromosome that accompanies the paternal colour- determining chromosome when it separates from its partner. Thus it follows that in the Fj generation there are four kinds of ova and four kinds of spermatozoa, and consequently sixteen possible combinations, though many of these are identical in effect. This may be represented by a diagram, in which B represents black, W white, R rough-coat, and S smooth-coat ; small letters are used for the recessive characters where they are present, but concealed by the presence of the dominant. p BBSS X WWRR Fi B wR? F2 BBRR BBRs; BwRR BwRs BBRs BBSS BwRs BwSS BwRR BwRs WWRR WWRs • BwRs BwSS WWRs WWSS When the chromosome theory was applied to Mendelism, Boveri predicted that exceptions to Mendel's second law would be found, for in cases where the genes for two different sets of characters happened to lie on the same chromosome they w^ould not be separated in the reducing divisions, and the two characters would not be inherited independently, but together. This was confirmed by Bateson and Punnett, who, like Mendel, studied the inheritance of characters in the pea, and subsequently by many other workers on other subjects. The vinegar-fly, Drosophila melanogaster and related species, has been much used for such studies by Morgan and his colleagues, for it has the advantages of being prolific and easily reared, of being remarkably variable in very many characters, and 96 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS of having a simple chromosome complex (only four pairs). For instance, in Drosophila, the characters for black body- colour, as opposed to the grey wild type, and for vestigial or badly developed wings, both recessives, are linked together and are thus known to lie on the same chromosome; one never occurs without the other. Further analysis shows that all the variable characters of Drosophila can be arranged in four linkage groups, corresponding to the four chromosome pairs. There is a special interest in the cases, long known in man though not understood, where characters lie upon the " X " or sex-chromosome, and are said to be sex-linked. Thus if a man suffers from haemophilia, a congenital tendency to excessive bleeding, his spermatozoa will be of two kinds : half carry the X-chromosome and with it the gene for haemophilia (which is a recessive character), and may give rise to females who themselves do not show the condition but pass it on to half of their male offspring, and half the spermatozoa carry the Y-chromosome, on which there are few or no genes ; these may give rise to males, who will be free from the haemophilia altogether, so that it cannot appear in their offspring. The studies on Drosophila have demonstrated that exceptions to the law of linkage may occur. Before the reducing division in the maturation of the ova the chromo- some-pairs are tightly interlaced, and an exchange of material or " crossing-over " may take place between them. In this way a gene may pass from the chromosome on which it originally lay to the partner chromosome, and two linked characters may be separated. As the genes do not pass over singly but in groups, the frequency of separa- tion of linked characters is a measure of the distance between their genes on the chromosome, and in this way Morgan and his colleagues have been able to prepare chromosome maps for Drosophila^ showing the position on the chromosomes of the genes of over three hundred mutant characters and the relative distance between them. The chromosome maps of related species are closely similar. Conclusion. — Heredity may be defined as the relation of genetic continuity between successive generations, and HEREDITY 97 inheritance as all that the organism is or has to start with in virtue of this hereditary relation. Development is the expression or realisation of the heritable qualities which have their physical basis in the germ-cells, and it pre- supposes an appropriate environment of nutrition and " hberating stimuli " — " nurture " in the widest sense. What the organism becomes is the resultant of two com- ponents, inherited " nature " and external " nurture." CHAPTER V PAST HISTORY OF ANIMALS (Palaeontology) In the two preceding chapters we have noticed two of the great records of the history of animal Hfe — that preserved in observable structures, and the modified recapitulation discernible in individual development ; in this we turn to the third — the geological record. In the early days of the Evolution theory the modern science of Embryology was still in its infancy, and could furnish few arguments, and it was the opponents of the new theory rather than its sup- porters who appealed to Palaeontology. They asserted that the palaeontological facts refused to lend the support which the theory demanded. To their attacks the evolutionists usually replied by pointing out that the geological record was very incomplete. The numerous investigations which have since been carried on on all sides now show con- clusively that it was imperfection rather of knowledge than of the record which produced the negative results. We must, however, still acknowledge that, except in a few cases, there is but little certainty as to the precise pedi- gree of living animals, and seek for reasons to explain this. "Imperfection of the geological record." — If we re- member the rule of modern Geology, that the past is to be interpreted by the aid of the present, there can be no difficulty in realising that the chances against the preserva- tion of any given animal are very great. Many are destroyed by other living creatures, or obliterated by chemical agencies. Except in rare instances, only hard parts, such as bones, teeth, and shells, are likely to be preserved, and this at once greatly limits the evidential value of fossils. The primitive 98 FOSSILS 99 forms of life would almost certainly be without hard parts, and have left no trace behind them. A number of ex- tremely interesting forms, such as many worms and the Ascidians, are, for the same reason, almost unrepresented in the rocks. Finally, we cannot suppose that such an external structure as a shell can always be an exact index of the animal within. After fossilisation has taken place, the rock with its con- tents may be entirely destroyed by subsequent denudation, or so altered by metamorphic changes that all trace of organic life disappears. Of those fossils which have been preserved only a small percentage are available, for vast areas of fossiliferous rocks are covered over by later deposits, or now lie below the sea or in areas which have not yet been explored. With all these causes operating against the likelihood of preservation, and of finding those forms that may have been preserved, it is little wonder if the geological record is incomplete ; but such as it is, it is in general agreement with what the other evidence, theoretical and actual, leads us to expect as to the relative age of the great types of animal life. Further, those specially favourable cases which have been completely worked out have yielded results which strongly support the general theory. Probabilities of ** fossils." — But it will be useful to note the probabilities of a good representation of extinct forms in the various classes of animals. Thus among the Protozoa the Infusoria have no very hard parts, and have therefore almost no chance of preservation, and the same may be said of forms like Amoeba ; while the Foramin- ifera and the Radiolaria, having hard structures of lime or silica, have been well preserved. The flinty Sponges are well represented by their spicules and skeletons. Of the Coelentera, except an extinct order known as Graptolites, only the various forms of coral had any parts readily capable of preservation, and remains of these are very abundant in the rocks of many ancient seas. But, strange as it may seem, some beautiful vestiges of jelly-fish have been discovered. Of the great series of " worms," only the tube-makers have left actual remains ; the others are known only by their tracks, while of any that may have lived on the land there is no evidence. The Echinoderms, because of their hard parts, are well represented in all their orders, except the Holothurians, where the calcareous structures characteristic of the class are at a minimum. The Crustacea, being mostly aquatic, and in virtue of their hard shells, are fossilised in great numbers. lOO PAST HISTORY OF ANIMALS The Arachnida and the Insects, owing to their air-breathing habit, are chiefly represented by chance individuals that have been drowned, or enclosed within tree-stumps and amber. The Molluscs and Brachiopods are perhaps better preserved than any other animals, since nearly all of them are possessed of a shell specially suitable for preservation. Among the Vertebrates some of the lowest are without scales, teeth, or bony skeleton ; such forms have therefore left almost no traces. Fishes, which are usually furnished with a firm outer covering, or with a bony internal skeleton, or with both, are well represented. The primitive Amphibians were furnished with an exoskeleton of bony plates, and are fairly numerous as fossils. The bones and teeth of the others have been fossilised, though more rarely. Of some the only record is their footprints. The traces of Reptilia depend upon the habits of the various orders, those living in water being oftenest preserved, but the strange flying Reptiles have also left many skeletons behind them. Of the Birds, the wingless ones are best represented, and then those that lived near seas, estuaries, or lakes. The history of Mammals is very imperfect, for most of them were terrestrial. But the discoveries of Marsh, Cope, and others show how much may be found by careful search. The aquatic Mammals are fairly well preserved. " Palaeontological series." — In spite of the imperfec- tion of the " geological record," in spite of the conditions unfavourable to the preservation of many kinds of animals, it is sometimes possible to trace a whole series of extinct forms through progressive changes. Thus a series of fossilised fresh -water snails (Planorbts) has been worked out ; the extremes are very different, but the intermediate forms link them indissolubly by a marvellously gradual series of transitions. The same fact is well illustrated by another series of fresh- water snails {Paludina, Fig. 44), and not less strikingly among those extinct Cuttle-fishes which are known as Ammonites, and have perfectly preserved shells. Similarly, though less perfectly, the modern crocodiles are linked by many intermediate forms to their extinct ancestors, for it is impossible not to call them by that name. In short, as knowledge increases, the evidence from Palaeontology becomes more and more complete. In a general way it is true that the simpler animals pre- cede the more complex in history as they do in structural rank, but the fact that all the great Invertebrate groups are represented in the oldest distinctly stratified and fossili- ferous rocks — the Cambrian system — shows that this corre- EXTINCTION OF TYPES lOI spondence is only roughly true. To account for this, we must remember that almost the whole mass of the oldest rocks, known as Archaean or Pre-Cambrian, has been so profoundly altered, that, as a rule, only masses of marble and carbonaceous material are left to indicate that forms of life existed when these rocks were laid down. Careful searching in Pre-Cambrian beds has revealed the presence of several Molluscs, a Eurypterid, and a fragment of Trilobite. There are also ** annehd tracks " indicative of life. Extinction of types. — Some animals, such as some of the lamp-shells or Brachiopods, have persisted from almost Fig. 44. — Gradual transitions between Paludma neumayri (a), the oldest form, and Paludina hcernesi (;'). — From Neumayr. the oldest ages till now, and most fossilised animals have modern representatives which we believe to be their actual descendants. That a species should disappear need not surprise us, if we believe in the " transformation " of one species into another. The disappearance is more apparent than real : the species lives on in its modified descendants, " different species " though they be. But, on the other hand, there are not a few fossil animals which have become wholly extinct, having apparently left no direct descendants. Such are the Graptolites, the ancient Trilobites, their allies the Eurypterids, two classes of Echinoderms (Cystoids and Blastoids), many giant Reptiles, and some Mammals, 102 PAST HISTORY OF ANIMALS It is almost certain that there has been no sudden extinction of any animal type. There is no evidence of universal cataclysm, though local floods, earthquakes, and volcanic eruptions occurred in the past, as they do still, with disastrous results to fauna and flora. In many cases the waning away of an order, or even of a class of animals, may be associated with the appearance of some formidable new competitors ; thus cuttle-fish would tend to exter- minate Trilobites, just as man is rapidly and often inex- cusably annihilating many kinds of beasts and birds. Apart from the struggle with competitors, it is con- ceivable that some stereotyped animals were unable to accommodate themselves to changes in their surroundings, and also that some fell victims to their own constitutions, becoming too large, too sluggish, too calcareous — in short, too extreme. Appearance of animals in time. — Such tables as those given here are apt to be misleading, in that they convey the impression that the great types of structure have appeared suddenly. It must be noted that any apparent abruptness is merely due to incompleteness of knowledge or inaccuracy of expression. The table is a mere list of a few important historical events, but one must fully realise that they are not isolated facts, that the present lay hidden in the past and has gradually grown out of it. Of the relative length of the periods represented here we know almost nothing, and we are also ignorant of the earliest ages in which life began. But the general result is clear. We find that in the Cambrian rocks, before Fishes appeared, the great Invertebrate classes were represented, though as yet but feebly. As we pass upwards they increase in number and in differentiation. Again, Fishes precede Amphibians, Amphibians are historically older than Reptiles, and many types of Reptiles are much older than Birds. In short, in the course of the ages life has been slowly creeping upwards. [Tables GEOLOGICAL SUCCESSION 103 Quaternary or Post-Tertiary. Pliocene. > Ganoids and Elasmo- branchs. •1 Ordovician. Cambrian. Representa- tives of aD the chie) classes oi I n v e r t e- brates. Pre-Cambrian or Archaean. 104 PAST HISTORY OF ANIMALS Coelentera. Echinoderma. Arthropoda. Cephalopoda. ' S . ^ > , ■ > ' — ^ Quaternary or Post- Tertiary. - - — - Pliocene. in B u a d o u c n « •S 2 Miocene. H 'O • • ; Eocene. *o a; ■4-t < (A 'o 1 u5 o o u Cretaceous. O . - <0 ^^ -^ *^ O •^ p Jurassic. ^5 "S . . . rt u _o a. o < 'c S to Triassic. Permian. Carboniferous. in 1-1 C "c '/i ... u u o Devonian or -1 Old Red Sandstone. a. o ^ ' Silurian. •*4 c/5 u 'c ■4-* 'u r-' K ft, Ordovician. Cambrian. Pre-Cambrian or Archaean. CHAPTER VI THE DOCTRINE OF DESCENT When we ask, as we are bound to ask, how the hving plants and animals that we know have come to be what they are — very numerous, very diverse, very beautiful, marvellous in their adaptations, harmonious in their parts and qualities, and approximately stable from generation to generation — we may possibly receive three answers. According to one, the plants and animals that we know have always been as they are ; but this is at once contradicted by the record in the rocks, which contain the remains of successive sets of plants and animals very different from those which now live upon the earth. According to another, each successive fauna and flora was destroyed by mundane cataclysms, to be replaced in due season by new creations, by new forms of life which arose after a fashion of which the human mind can form no conception. Of such cataclysms there is no evidence, and if it be enough to postulate one creation, we need not assume a dozen. The third answer is, that the present is the child of the past in all things : that the plants and animals now existing arose by a natural evolution from simpler pre-existing forms of life, these from still simpler, and so on back to a simplicity of life such as that now represented by the very lowest organisms. This third theory is really an old one ; it is merely man's application of his idea of human history to the world around him. It was maintained with much concreteness and power by Buffon (1749), by Erasmus Darwin (1794), and by Lamarck (1801). Yet in spite of the labours of these thoughtful naturalists and of many others, the general idea of the natural descent of organisms from simpler ancestors was not received with favour until Darwin, in his Origin 105 I06 THE DOCTRINE OF DESCENT of Species (1859), made it current intellectual coin. By his work, and by that of Spencer, Wallace, Haeckel, Huxley, and many others, the doctrine of descent, the general fact of evolution, has been established, and is now all but universally recognised. The chief arguments which Darwin and others have elaborated in support of the doctrine of descent, according to which organisms have been naturally evolved from simpler forms of life, may be ranked under three heads — {a) structural, (b) physiological, {c) historical. Evidences of evolution. — {a) Structural. — Some say that there are over a million living animals of different species. In any case, there are many myriads. These species are linked together by varieties which make strict severance often impossible ; they can be rationally arranged in genera, orders, families, and classes, between which there are not a few remarkable connecting links ; there is a gradual increase of complexity from the Protozoa upwards along various hues of organisation ; it is possible to rank them all on a hypothetical genealogical tree (Fig. 18). A little practical experience makes one feel that the facts of classification favour the idea of common descent. Throughout vast series of animals we find in different guise essentially the same parts twisted into most diverse forms for different uses, but yet referable to the same funda- mental type. It is difficult to understand this " adherence to type," this " homology " of organs, except on the theory of natural relationship. There are many rudimentary organs in animals, especially in the higher animals, which remain very slightly developed, and which often disappear without having served any apparent purpose. Such are the " gill-slits " or " visceral clefts " in Reptiles, Birds, and Mammals, the teeth of young whalebone whales, the pineal body (a rudimentary eye) in Vertebrates. Only on the theory that they are vestiges of structures which were of use in ancestors are these rudi- ments intelligible. They are relics of past history, com- parable, as Darwin said, to the unpronounced letters in many words. (b) Physiological. — Observation shows that animals are to some extent plastic. In natural conditions they usually EVIDENCES OF EVOLUTION IO7 exhibit some measure of changefulness from generation to generation. This is especially the case if one section of a species be in any way isolated from the rest, or if the animals be subjected in the course of their wanderings to novel conditions of life. The evidence from domesticated animals is very convinc- ing. By careful interbreeding of varieties which pleased his fancy or suited his purpose, man has produced numerous breeds of horses, cattle, sheep, and dogs, which are often distinguished from one another by structural differences more profound than those which separate two natural species. In great measure, however, domestic breeds are fertile with one another, while different species rarely are. The numerous and very diverse breeds of domestic pigeons, which are all derived from the rock-dove {Columba livia), vividly illustrate the plasticity or variability of organisms. It sometimes happens that offspring resemble not so much the parent as some other form believed or known to be ancestral. Thus a pigeon like the known ancestor Columba livia may be hatched in the dovecot, and a few instances are known of similar reversions to a presumed ancestor. {c) Historical. — Among the extinct animals disentombed from the rocks, many form series by which those now existing can be linked back to simpler ancestors. Thus the ancient history of horses, crocodiles, and cuttle-fish is known with a degree of completeness which makes it almost certain that the simpler extinct forms were in reality the ancestors of those which now live. Moreover, many con- necting links have been discovered in the rocks, and the higher animals appear gradually in successive periods of the earth's history The facts of geographical distribution, and the history of the diffusion of animals from centres where the presumed ancestral forms are or were most at home, favour the doctrine of descent. The individual life-history of an animal — often strangely circuitous or indirect — is interpretable as a modified re- capitulation of the probable history of the race. Such, in merest outline, is the nature of the evidence which leads us to conclude that the various forms of life I08 THE DOCTRINE OF DESCENT have descended or have been evolved from simpler an- cestors, and these from still simpler, and so on, back to the mist of life's beginnings. None of the evidence is logically demonstrative ; we accept the evolution idea because it is a plausible interpretation which is applicable to many orders of facts, and is contradicted by none. In accepting the evolutionist interpretation naturalists are unanimous ; but in regard to the manner in which the transformation of species or the general ascent of life has been brought about, there is much difference of opinion. The fact of evolution is admitted ; debate goes on with regard to the factors (see Chapter XXVIII). CHAPTER VII PHYLUM PROTOZOA— THE SIMPLEST ANLMALS Chief Divisions Rhizqpods : Classes — Lobosa, Heliozoa, Foraminifera, Radio- LARIA, etc. Infusorians : Classes — Flagellata, Ciliata, Acinetaria, etc. Sporozoa : Several Classes. The Protozoa are the simplest animals, and they are of pecuhar interest on this account. They throw light upon the beginnings of organic structure and vital activity, and they give us hints as to the nature of the first forms of life, of which we can know nothing directly. Almost all the Protozoa are single cells, iinit masses of living matter ; and in virtue of their simpUcity, they are in some measure exempt from natural death, which is " the price paid for a body." In their variety they exhibit, as it were, a natural analysis of the higher animals, w^hich are built up of many diverse cells. General Characters The Protozoa^ the simplest and -most primitive animals, are usually very small single cells. Most of them feed on small plants or on other Protozoa, or on debris, and not a few are parasitic. Most of them live in water, hut many can endure dryness for some time. In one series (Rhizopods) the living matter is without any rind, and flows out in more or less changeful threads and lobes, by the movements of which the animals engulf their food and glide along. The others 109 no PHYLUM PROTOZOA — THE SIMPLEST ANIMALS have a definite rind, which in a large number (Infusorians) bears motile cilia or flagella, but in the others (Sporozoa) is usually without locomotor structures. But these three phases — amoeboid, ciliate or flagellate, and encysted— may occur in the life-history of one form ; and the three main lines of evolution — Rhizopods, Infusorians, and Sporozoa^are marked by the predominant occurrence of the amoeboid, ciliate or flagellate, or encysted phase of celt life. Many have a skeletal framework— of lime, flint, or other material— while within the cell there is a special kernel or nucleus, or there may be several. There are also other less constant structures. A Protozoon multiplies by dividing into two daughter units, or into a large number ; and two individuals often unite, temporarily or permanently, in conjugation, which is analogous to the union of ovum and spermatozoon in higher animals. A few types, instead of remaining single cells, form by division or budding loose colonies, taking a step, as it were, tozvards the Metazoa, but never forming differentiated tissues. First Type of Protozoa — Amceba Amoeba, a type of Rhizopods, especially of those in which the outflowing processes of living matter (pseudopodia) are blunt and finger-like (Lobosa). Description. — Amoeba proteus and some other species are found in the mud of ponds ; A. terricola occurs in damp earth. Some are just large enough to be seen with the unaided eye. The diameter is often about one-hundredth of an inch. Each is a unified corpuscle of living matter, and glides over the surface of stone and plant by protruding and retracting the pseudopodia. As they move the shape constantly changes, whence the old (1755) name of " Pro- teus animalcule." Round the margin, which may show an apparent radial striation, the cell substance is firmer and clearer than it is in the interior, where it is more fluid, but contains very abundant granules, some of which are of a protein, and others of a fatty nature. In the centre of the cell lies the usually single nucleus. The food consists of minute Algae, such as diatoms, or of vegetable debris. There is reason also to suspect cannibalism. The food is surrounded by the finger-like processes, and engulfed STRUCTURE OF AMCEBA III along with drops of water, which form food vacuoles in the cell substance. Into these vacuoles digestive ferments flow ; D Fig. 45. — Amceba proteus. — After Lucy A. Carter. EC, Clear ectoplasm, sometimes showing fine radial striation. EN., More granular endoplasm. The granules are partly nutritive substances in reserve, partly waste products and undigested debris. F.V., Food vacuoles, droplets of water surrounding food particles. A solid particle is shown in process of digestive disintegration. C.V., A contractile vacuole with excretory function. D., A diatom that has been engulfed as food. PS., A pseudopodium. N., The nucleus showing chromatin bodies (represented as dark granules). the contents of the vacuoles are first acid and then alkaline, which recalls the change of reaction in the alimentary canal in mammals ; but it is doubtful if there is true 112 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS digestion during the acid phase in Amoebae. The fer- ments secreted attack proteins, carbohydrates including cellulose, but possibly not fats. After the digestible parts of the food have been absorbed, the undigested residue is got rid of at any point of the protoplasm. One or more contractile vacuoles are visible in the cell substance. They have an excretory function, and serve to get rid of the finer waste products, and also of the w^ater which must be continually drawn into the cell, whose contents have a higher osmotic pressure than the media in which the Amoebae usually live. Life-history. — In favourable nutritive conditions the Amoeba grows. At the limit of growth it reproduces by Fig. 46. — Life-history of ^mQ?&a. I. Amoeba with pseudopodia ; n., nucleus ; c.v., contractile vacuole. 2. Division in two. 3. Encystation. 4. Escape of Amoeba from its cyst. dividing into two. In disadvantageous conditions, such as drought, it may become globular, and, secreting a cell wall or cyst, lie dormant for a time. The cyst wall is said to be chitinoid. With the return of favourable conditions the Amoeba revives, and, bursting from the cyst with renewed energy, recommences the cell-cycle. The conjugation of two Amoebae has been observed, and spore-formation occasionally occurs. Second Type of Protozoa — Actinophrys The Sun-animalcule, Actinophrys sol, is a type of the Heliozoa. Description. — Like most other Heliozoa, Actinophrys lives in fresh water, floating about or rolling over the SUN-ANIMALCULE 113 bottom. It is spherical and minute, measuring at most 0-05 mm. in diameter. Long stiff pseudopodia radiate out from the body. A clear axial filament runs up each pseudopodium, and the small organisms on which Actino- phrys feeds are paralysed when they come in contact with the pseudopodia. The body consists of ectoplasm and endoplasm. The ectoplasm is a thick external layer closely packed with large vacuoles, which are non-contractile and contain a clear fluid. But food vacuoles are formed as in other Fig. 47. — Actinophrys sol (Sun-animalcule). — After Grenacher. »., Nucleus ; f.v., food vacuole ; v., contractile vacuole ; ps., pseudopodium. Protozoa, and there is also a single contractile vacuole. The endoplasm forms the central mass. It is not vacuo- lated, and contains the large, centrally placed nucleus. Life-history. — An Actinophrys may withdraw its pseudopodia and divide into two,^ with or without the formation of a cyst. A number of individuals may unite for a time by the ectoplasm alone, and separate without any nuclear fusion having taken place (plastogamy). But Schaudinn has described a true sexual process which offers an interesting analogy to the processes of maturation and fertilisation in the higher animals. A number of individuals become joined up in a common 8 114 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS gelatinous cyst. Each loses its pseudopodia and forms a membranous cyst. These cysts become associated in pairs. The nucleus of each cyst divides mitotically and a polar body is extruded from each, after which the nucleus returns to the resting condition. The cysts now fuse in pairs, with complete and intimate union of their nuclei and cell-bodies. The zygote so formed rests for a short period, then divides up into two daughter cysts from which emerge two new individuals of Actinophrys. In the allied genus Actinosphceriiim, with very numerous nuclei, there is a strange and complicated formation and fusion of cysts within a single individual. Third Type of Protozoa — POLYSTOMELLA Polystomella (see Fig. 48) is a type of Foraminifera with a calcareous perforate shell or test. Description. — Polysto- mella crispa is common on the shore, especially among Zostera. It looks like a miniature of an Ammonite shell, and Foraminifera were indeed classified by the older naturalists with the Ammonites. The test forms a close spiral with beautifully chiselled surface ; only the last whorl is visible from the outside. The test is made up of a series of chambers which com- municate with one another and with the exterior by fine pores. Granular protoplasm fills up the chambers and forms also a thin layer on the outside. Long slender pseudopodia issue from the openings in the test and are given off also by the external protoplasmic layer. They frequently branch and anastomose with one another, and their granular protoplasm exhibits marked streaming movements. The pseudopodia serve to catch and entangle the diatoms and Infusoria on which the Foraminifer feeds. Fig. 48. — Polystomella, megalo- spheric form, with large central chamber (M.) and one nucleus (iV.).— After. Lister. POLYSTOMELLA 115 Like many other Foraminifera, Polystomella shows a remarkable dimorphism. It occurs in two forms, outwardly indistinguishable, but differing in internal struc- ture. In the megalospheric form the central chamber is large (a megalosphere), and there is a single large nucleus, placed about the middle of the series of chambers ; in the microspheric form the central chamber is small (a microsphere), being about one-tenth of the diameter of the megalosphere, and there are numerous small nuclei. The megalospheric individuals are about thirty times as numerous as the microspheric individuals. Fig. 49. — Polystomella, microspheric form, with small central chamber {c.c), numerous nuclei {N.), bridges of protoplasm between chambers {B.). — After Lister. Life-history. — The microspheric form has its nuclei replaced by chromidia (chromatin bodies detached from the nuclei into the protoplasm). These chromidia form the centres of amoeboid nucleated spores which leave the shell or are liberated by the protoplasm creeping out and forming a halo of anastomosing threads round the deserted test. The spores secrete a shell and grow into the typical megalospheric forms. When the megalospheric form is about to reproduce, its nucleus disintegrates and is replaced by numerous scattered nuclei formed around chromidia. The proto- Il6 PHYLUM PROTOZOA^THE SIMPLEST ANIMALS plasm segregates into little masses, each centred in a nucleus. Each of these nuclei divides by mitosis into two, then into four, and the division of the nucleus is followed by the division of the protoplasmic mass, so that hosts of tiny cells are formed. These become provided with flagella, swim out into the water, leaving behind them the empty test, and there conjugate in pairs, not with one another but with similar " gametes " from another megalo- spheric individual. The " zygote " so formed becomes the initial chamber of a microspheric individual. In a more direct way — by fission — the megalospheric individual may give rise to another like itself. There is therefore in this complex life-history of Polystomella an alternation between a sexual and an asexual generation. Fourth Type of Protozoa — Paramcecium Paramaecium, a type of ciliated Infusorians, especially of those which are uniformly covered with short cilia (Holotricha). Description. — Specimens of Paramoecium may be readily and abundantly obtained by leaving fragments of hay to soak for some days in a glass of water. A few in- dividuals have been lying dormant about the plant ; they revive and multiply with extraordinary rapidity. They are also abundant in most stagnant pools, and are just visible when a test-tube containing them is held between the eye and the light. Their food consists of small vegetable particles. The form is a long oval, with the blunter end in front ; the outer portion of the cell substance is differentiated into a dense rind or cortex, with a delicate external cuticle, perforated by cilia. There is a definite opening, the so- called mouth, which serves for the ingestion of food particles ; and there is also a particular anal spot posterior to the mouth, from which undigested residues are got rid of. The surface is covered with cilia, in regular longitudinal rows ; these serve both for locomotion and for driving food particles towards the mouth. Paramoecium rotates like a rifle bullet as it swims ; its track is not straight, but an open spiral. If it strikes a solid object or enters an PARAMCECIUM 117 unfavourable medium it " reverses " for a short distance, turns on its side, and goes forward at an angle to the original path. Among the ciUa there are small cavities in the cortex, in which lie fine protrusible threads (" tricho- cysts "). These, though parts of a cell, suggest the thread cells of Coelentera, but are probably of the nature of mooring threads effecting attachment to solid objects. AAA Fig. 50. -Paramcecium in longitudinal optical section, and dividing. — After Butschli. C.V., Contractile vacuole ; MY., longitudinal " myophan " striations ; MA., macronucleus ; MI., one of two micronuclei ; F.V., food vacuole ; CA., a canal in the cytoplasm entering the contractile vacuole which is bursthig through the cortex ; TR., trichocysts at the roots of the cilia (CI.) ; MO., " mouth " leading into gullet. In the right-hand figure D.L. is the transverse dividing line ; the dumb- bell-like elongations of the macronucleus (MA.) and micronuclei (MI.) ; P. A., a "potential anus or weak spot," where debris may be got rid of. The cortical layer is contractile, an^i is distinctly fibrillated. In the substance of the cell lie two nuclei, the smaller " micronucleus " lying by the side of the larger " macro- nucleus." Food vacuoles occur as in the Amoeba, and the digestive process appears to be similar ; but Paramcecium is remarkable for the strength of the acid which it secretes into the vacuoles. There are two contractile vacuoles, from which fine canals radiate into the surrounding proto- ii8 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS plasm ; these discharge into the vacuole, which then bursts to the exterior. Life-history.— Growth is followed by obliquely transverse division into two (Fig. 50, D.L.). One half includes the "mouth," the other has to make one. As well as this simple fission, a process of transient Pjq t^i. — Conjugation of Paramcecium aurelia- stages. — After Maupas. -four Shows macronucleus (A^.) and two micronuclei (m.) in each of the two conjugates. Shows breaking up of macronucleus, and multiplication of micronuclei to eight. Shows the fertilisation in progress ; the macronucleus is vanishing. Shows a single (fertilised) niicronucleus in each conjugate. conjugation also occurs. Two individuals approach one another closely, the two nuclei of each break up, an exchange of pieces of the micro- nucleus takes place ; the two then separate, each to reconstruct its two nuclei (Fig. 51). This process is necessary for the continued health of the species. Fig. 52. — Diagrammatic expression of process of conjugation in Paramcecium aurelia. — After Maupas. The two micronuclei enlarge. Each divides into two. Eight micronuclei are formed. Seven disappear ; one (darkened) divides into two. An interchange and fusion occurs, and the con- jugates separate. The fertilised micronucleus divides into two. Each conjugate begins to divide, the micronucleus of each half dividing into two, one of which becomes the macronucleus, while the others form the two normal micronuclei. The top line repre- sents four individuals, each with a macronucleus and two micronuclei. JLJL ' ' X 1. 1 fl IJ T 4 ' A. B. C. D. E. — ivWv/W/\\ F. / The details of the conjugating process have been worked out with great care by Maupas and others. They differ slightly in different species ; what occurs in P. aurelia is summarised diagrammatically in Fig. 52. The micronuclear elements are represented by two minute bodies. As conjugation begins, these separate themselves from the macronucleus. The macronucleus degenerates, and each micronucleus increases in VORTICELLA II9 size (A). Each divides into two (B) ; another division raises their number to eight (C) ; seven of these seem to be absorbed and disappear, the remaining eighth divides again into what may be called the male and female elements (D) ; for mutual fertilisation now occurs (E). After this exchange has been accomplished, the Infusorians separate, and nuclear reconstruction begins. The fertilised micronucleus divides into two (F), and each half divides again (G), so that there are four in each cell. Two of these form the macronuclei of the two daughter-cells into which the Infusorian proceeds to divide (H) ; the other two form the micronuclei, but before another division occurs each has again divided. Thus each daughter-cell contains a rhacronucleus and two micronuclei. In a "pure line," all descended from one, there is no conjugation. But there is a periodic, usually iflohthlry,: occurrence, as Woodruff and Erdmann have shown, of a reriiarkable p/rocess called endomixis. The nuclei break down as if thei;e.;was _goiDjg to be con- jugation, and then there is re-organisation. ^V >!J- -'/^^ Fifth Type of Protozoa — Vorticella Vorticella, or the bell-animalcule, is a type of those ciliated Infusorians in which the cilia are restricted to a region round the mouth (Peritricha). ,>'| Description. — Groups of Vorticella,^ or of the compound form Carchesium, grow on the stenis of fresh-water plants, and are sometimes readily visible to the unaided eye as white fringes. In Vorticella each individual suggests an inverted bell with a long flexible handle. The base of the stalk is moored to the water-weed, the bell, swings in the water, now jerking out to the full length of its tether, and again cowering down with the stalk contracted into a close and delicate spiral. In Carchesium the stalk is branched, and each branch terminates in a bell. Up the stalk there runs, in a slightly wavy curve, a contractile filament, which, in shortening, gives the non-contractile sheath a spiral form. This contractile filament, under a high power, may exhibit a fine striation. (A similar striated structure is seen in some Amoebae, Gregarines, spermatozoa, etc., and of a much coarser type in striped muscle fibres. It seems to be some structural adaptation to contractility.) The bell has a thickened margin, and within this lies a disc-like lid ; in a depression on the left side, between the margin and the disc, there is an opening, the mouth, which leads by a distinct passage into the cell. On the side of this passage there is a weak spot, the potential anus, by which useless 120 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS debris is passed out. The cilia are arranged so as to waft food particles into the mouth and down the passage. There is a large and horseshoe-shaped macronucleus, and a small micronucleus. Food vacuoles and contractile vacuoles are present as usual. Sometimes a Vorticella bell jerks itself off its stalk and swims about ; in other conditions it may form a temporary -nig- 4 5- Fig. 53. — Vorticella. — After Biitschli. Structure. A'^., Macronucleus ; «., micronucleus ; c.v., con- tractile vacuole ; m., mouth ; f.v., food vacuole ; v., vestibule. Encysted individual. 3. Division. Separation of a free-swimming unit — -the result of a division. Formation of eight minute units (tng.). 6. Conjugation of microzooid (mg.) with one of normal size. cyst ", normally, the cilia are very active, and the move- ments of the stalk frequent and rapid. Multiplication may take place by longitudinal fission — a bell divides into similar halves ; one of these acquires a basal circlet of cilia and goes free, ultimately becoming fixed. Or the division may be unequal, and a microzooid, or as many as eight, may be set free. These swim away by means of the posterior girdle of cilia, and each may conjugate with an VOLVOX 121 individual of normal size. In this case a small active cell (like a spermatozoon) fuses intimately with a larger passive cell, which may be compared to an ovum. Sixth Type of Protozoa — Volvox Volvox is a type of flagellate Infusorians, especially of those with flagella of equal size. Volvox is found, not very commonly, in fresh-water pools, Fig. 54. — Volvox globator. — After Klein and Janet. I. and v. Biflagellate individuals. II. Ripe ovum. III. A ball of sperms. IV. A daughter colony developing. and is usually classed by botanists^ as a green Alga. It consists of numerous biflagellate individuals, connected by fine protoplasmic bridges, and embedded in a gelatinous matrix, from which their flagella project, the whole forming a hollow, spherical, actively motile colony. In V. globator the average number of individuals is about 10,000 ; in V. aureus or minor, 500-1000. The individual cells are stellate or amoeboid in V. globator^ more spherical in V. 122 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS aureus ; each contains a nucleus and a contractile vacuole. At the anterior hyaline end, where the flagella are inserted, there is a pigment spot ; the rest of the cell is green, owing to the presence of chlorophyll corpuscles. In consequence of the presence of these, Volvox is holophytic, i.e. it feeds as a plant does and builds up starch granules. In its method of reproduction Volvox is of much biological interest and importance. As Klein, one of its best describers, says, it is an epitome of the evolution of sex. Some of the colonies are asexual. In these a limited number of cells possess the power of dividing up to form little clusters of cells ; these clusters escape from the envelope of the parent colony, and form new free-swimming colonies. In other colonies there are special reproductive cells, which may be called ova and spermatozoa. In V. globator the two kinds of reproductive cells are usually formed in the same colony, the formation of spermatozoa generally preceding that of the ova. Technically the colony may then be described as a protandrous hermaphrodite. In V. aureus the colony is oftenest unisexual or dioecious, i.e. either male or female. But it may be monoecious or hermaphrodite, and is then generally protogynous, i.e. producing eggs first. Whether in a hermaphrodite or in a unisexual colony, the sex cells appear among the ordinary vegetative units ; the ova are distinguishable by their larger size, the " sperm mother cells " divide rapidly and form numerous (32-100 or more) slender spermatozoa, each with two cilia. In V. globator their bundles may break up within the parent colony ; or, as always occurs in V. aureus, they may escape intact, and swim about in the water. In any case, an ovum is fertilised by a spermato- zoon, and, after a period of encystation and rest, segments to form a new colony. Occasionally, however, this organism, so remarkable a condensation of reproductive possibiUties, may produce ova which develop parthenogenetically. Here, then, we have an organism, on the border line between plant and animal life, just across the line which separates the unicellular from the multicellular, illustrating the beginning of that important distinc- tion between somatic or body cells and reproductive cells, and occurring in asexual, hermaphrodite, and unisexual phases. Klein records no less than twenty-four different forms of V. aureus from the purely vegetative and asexual to the parthenogenetic, for there may be almost entirely male colonies, almost entirely female colonies, and other interesting transitional stages. Klein has also succeeded to some extent in showing that the occurrence of the various reproductive types depends on outside influences. Seventh Type of Protozoa — Monocystis Monocystis, a type of Sporozoa in which the cell is not divided into two parts by a partition. MONOCYSTIS 123 Description. — Two species (M. agilis and M. magna) infest the male reproductive organs of the earthworm. The full-grown adults are visible to the naked eye — flattened worm-like cells ; the shape alters during the sluggish move- ments. Peripherally there is a porous cuticle, a clear cortical zone, and a network of myoneme fibrils. In a more fluid medullary substance, the large nucleus floats. There are numerous granules of protein, carbohydrate, and other sub- stances. In one species there is an anterior projection like the cap of Gregarina, otherwise unrepresented in Monocystis. As in Gregarina, and many other parasitic forms, a con- tractile vacuole is absent. Life-history. — The young form of M. agilis is parasitic sp.c Fig. 55. — Life-history of Monocystis. — After BUtschli. 1. Young Gregarine lying within a sperm mother cell of earthworm. 2. Association of two Gregarines within a cyst, ready to form gametes. 3. Numerous spore-cases (sp.c, pseudonavicellae) within a cyst. 4. A spore-case with eight spores (sp.) and a residual core {rb.). * within one of the sperm mother cells of the earthworm. It grows, and becomes free from the cell as a trophozoite. In the free stage, two individuals may unite in a curious end-to-end manner observed also in Gregarina. Quite diflFerent is the association of two individuals (gametocytes) inside a common cyst. After a process of " reduction " the nucleus of each divides repeatedly, and the daughter nuclei migrate to the surface of the cell, where each is surrounded by a little mass of protoplasm. Each of the gametocytes thus gives rise to a number of gametes ; there remains a mass of residual protoplasm. The wall between the two gametocytes now breaks down and the gametes conjugate in pairs, forming zygotes . In each pair of conjugating gametes one is probably derived from each gametocyte. Each zygote secretes a membrane and becomes a spore-case. The 124 PHYLUM PROTOZOA^ — THE SIMPLEST ANIMALS nucleus divides up, and eight elongated spores are formed round a residual core. The spore-case now takes its typical shape and is known as a pseudonavicella. The spores are considerably larger than those of Gregarina. Eventually, in the alimentary canal of another earthworm the cyst bursts, the spore-cases are extruded, the spores emerge from their firm chitinoid cases. The young spore (sporozoite) is like a bent spindle (falciform), and seems next door to being flagellate. It bores into a mother sperm cell, and from this Fig. 56. — Life-history of Gregarina. — After Butschli. 1. Young forms {a, b, c) emerging from intestinal cells (i.e.) ; i.n., nucleus of intestinal cell. 2. Two forms conjugating (0. blattarum). 3. Spore formation within a cyst. 4. .\dult with deciduous head-cap (c.c), and a cuticular partition dividing the cell into an anterior part (A ) and a posterior part (B) ; n., the nucleus. 5. A spore-case (sp.c). it afterwards passes as an adult into the cavity of the seminal vesicles. Intracellular parasitism and copious food naturally act as checks to activity, and the adult is sluggish. The allies of Monocystis occur chiefly in " Worms," Tunicates, and x\rthropods ; none are known in Vertebrates. Along with Monocystis we take Gregarina, a type of Sporozoa in which the cell is divided into two regions by a partition. Various species occur in the intestine of the lobster, cockroach, and other Arthropods. When young they are intracellular parasites, but later they become free in PLASMODIUM 125 the gut. They feed by absorbing diffusible food-stuffs, such as peptones and carbohydrates, from their hosts, and store up glycogen within themselves. In many the size is about one-tenth of an inch. There is a firm cuticle of " proto- elastin," which grows inwards so as to divide the cell into a larger nucleated posterior region and a smaller anterior region, and also, in the young stage, forms a small anterior cap. The cell substance is divided into a firmer cortical layer and a more fluid central substance. The protoplasm often presents a delicate fibrillar appearance, suggesting that of striated muscle. The nucleus is very dis- tinct, but there are no vacuoles. We may associate the absence of locomotor pro- cesses, " mouth," and contractile vacuoles, as well as the thickness of the cuticle and the general passivity, with the parasitic habit of the Gregarines. The young Gregarine is parasitic in one of the hning cells of the gut ; it grows, and, leaving the cell, remains for a time still attached to it by the cap (Fig. 56, «, b, c) ; later this is cast off, and the individual be- comes free in the gut, w^hile still increasing in size. Two or more individuals attach themselves together end to end, but the meaning of this is obscure. Encystation occurs, involving a single unit or two to- gether. The details of spore-formation are similar to those in Monocystis. Eventually the cyst bursts, the spore-cases are liber- ated, and from within each of these eight spores emerge to become cellular parasites. G. {Porospora) gigantea is sometimes three-quarters of an inch in length — enormous for a Protozoon. to Fig. 57. — End- to-end union of Gregarines. — After Frenzel. The adult of Eighth Type of Protozoa — Plasmodium vivax Plasmodium, one of the Haemosporidia, is parasitic in the red blood cells of man and other Vertebrates. P. vivax benign tertian " malaria in man. The Hfe- causes 126 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS history falls into two parts : (i) asexual or schizogony stage, passed in man ; (2) sexual or sporogony stage, passed in a mosquito belonging to the genus Anopheles. Life-history. — (i) The Plasmodium enters its verte- brate host as a minute and very slender spindle-shaped creature in the saliva which mosquitoes inject when they bite. It is then in its sporozoite stage. It burrows into a red blood cell and gradually comes to rest within it. This is the trophozoite stage. The Plasmodium becomes a rounded body with a single large nucleus. A large vacuole frequently develops, pushing the nucleus to one side and giving a characteristic ring-like appearance (" signet-ring form "). At the conclusion of the trophozoite period the parasite enters on the schizont stage. Multiple fission of the nucleus takes place, and the cytoplasm, save for a small residual amount containing pigment granules, aggregates round the daughter nuclei to form 15-20 merozoites. The red blood cell then bursts, and the merozoites are set free in the blood. The merozoites attack fresh red blood cells, and the cycle, which takes forty-eight hours, is repeated. After about five cycles have been passed, the amount of toxin liberated into the blood from the shattering of the red blood cells sets up fever in the human host, and attacks recur every forty- eight hours, corresponding with the escape of the mero- zoites. Eventually the second stage in the life-history becomes inaugurated. Certain merozoites grow more slowly than the others, and do not produce schizonts, but, entering a red blood cell, become rounded oflF without developing a vacuole, and after reaching their limit of growth, become free in the blood as male and female gametocytes, the latter somewhat larger in size than the former,"but with a smaller nucleus. (2) If at this stage some of the gametocyte-containing blood is sucked in by an anopheline mosquito, the gametocytes alone survive digestion, and sporogony begins. The male gametocytes give rise to 4-8 slender active microgametes, which swim rapidly about until contact occurs with a macrogamete produced after maturation from a female gametocyte. The tiny microgamete enters the macrogamete, their nuclei fuse, and a zygote (fertilised cell) is formed. The COCCIDIUM 127 zygote becomes surrounded with a thin pelUcle, becomes pointed at both ends, and works its way into the m.osquito's gut wall, where it burrows through the lining and comes to rest, developing a globular envelope or oocyst. Great growth now takes place, both of oocyst and contents. Ultimately there are formed within the oocyst an enormous number of elongated sporozoites. The oocyst then bursts, and the sporozoites are set free in the mosquito's blood spaces. Very many of them accumulate in the salivary glands, and are passed out with the salivary juice to start the life-cycle anew. Ninth Type of Protozoa — Coccidium schubergi Reference may here be made to the common Coccidia, intracellular Sporozoa, attacking mainly the epithelial cells of the gut or associated organs. They are found chiefly in insects, myriopods, molluscs, and vertebrates. Thus Coccidium schubergi infests the intestinal epithelium of the centipede Lithobius forficatus. The adult is a minute oval or spherical cell with a nucleus. It lives a quiescent life within the host cell, growing and absorbing nourish- ment until the resources of the cell are exhausted. Life-history. — The coccidium enters the host cell as a minute sickle-shaped body, pointed at the anterior end, and more blunt posteriorly. This is the sporozoite stage of the life-history ; it is liberated from a cyst (oocyst) when the latter is swallowed by the centipede in its food. When freed in the gut the sporozoite progresses by forward gliding movements, alternating these by flexions, bending itself like a bow and straightening out again. When about to enter an epithelial cell it presses the anterior end through the cell wall and wriggles its way in. Once within the cell in which development is to proceed, its movements gradually cease, but it may pass through several cells before coming to rest. Within the host cell the coccidium — now in the trophozoite stage — becomes oval in form, and in about twenty-four hours has reached full size and has exhausted the host cell contents. This is the completion of the trophozoite period, and the parasite now enters the schizont stage, where its nucleus divides into a number of daughter nuclei. These 128 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS arrange themselves around the periphery of the cell, whilst the protoplasm breaks up to form along with them bodies of a shape similar to the sporozoites. There are important structural differences, however, apart from the difference in origin. The parasites, now known as merozoiteSy rupture Fig. 58. — Life-history of Coccidium. — After Schaudinn. I. Sporozoite ; 2. Sporozoite entering a cell and becoming a trophozoite; 3-4. Schizont, forming merozoites ; 5. Merozoites entering another cell ; 6a. Merozoite forming macrogamete ; 6b. Merozoite forming microgametes; 7. Free microgamete ; 8-9. Fertilisation of macrogamete by microgamete ; 10. Zygote within oocyst ; 11. Formation of spores within oocyst ; 12. Spores forming sporozoites. the host cell, move in the gut cavity after the manner of the sporozoites, enter fresh epithehal cells, and repeat the fore- going cycle until ultimately the greater part of the gut epithelium is destroyed. In about five days, however, owing perhaps to the failing capacity of the host to nourish, the limit of asexual reproductivity is reached, and the Classes of protozoa 129 parasite now enters upon a spore-forming stage. Certain merozoites grow more slowly than the others, and instead of becoming schizonts give rise to elements of two types, viz. microgametes, slender cells bearing a flagellum at each end, which are male, and macrogametes, larger bean-shaped cells, which are female. The latter after maturation free themselves from the host cell, and in the cavity of the gut are fertilised by a male element. After fertilisation, a trans- parent membrane forms around the zygote (fertilised cell). This membrane in the first instance serves to exclude all microgametes after the first, and later, becoming very tough and resistant, forms a protecting envelope or oocyst. After the oocyst is formed the parasite may pass from the host to the exterior or remain for some time longer within it. The nucleus of the zygote within the oocyst now divides into four, around which the protoplasm aggregates itself to form the spores. There are thus four spores within a cyst. Each spore divides, forming two sporo.zottes , which on the arrival of the oocyst in the gut of a fresh host are liberated, and attacking the lining epithelium recommence the life- history. General Classification of Protozoa Since the Protozoa are unicellular organisms (except the few which form loose colonies), their classification should be harmonious with that of the cells in a higher animal. This is so. Thus (a) the Rhizopods, in which the living matter flows out in changeful threads or " pseudopodia," as in the common Amoeba^ are comparable with the white blood corpuscles or leucocytes, many young ova, and other " amoeboid " cells of higher animals ; (b) the Infusorians, which have a definite rind and bear motile lashes (cilia or flagella), e.g. the common Paramoecium^ may be likened to the cells of ciliated epithelium, or to the active sperma- tozoa of higher animals ; {c) the parasitic Sporozoa, which have a rind and no motile processes or outflowings, may be compared to degenerate muscle cells, or to mature ova, or to " encysted " passive cells in higher animals. This comparison has been worked out by Professor Geddes, who also points out that the classification represents the three physiological 130 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS ■possibilities: (a) the amoeboid units, neither very active nor very passive, form a median compromise ; {b) the ciHated Infusorians, which are usually smaller, show the result of a relative predominance of ex- penditure ; (c) the encysted Gregarines represent an extreme of sluggish passivity. But, as Geddes and others have shown, the cells of a higher animal often pass from one phase to another — the young amoeboid ovum accumulating yolk becomes encysted, the ciliated cells of the windpipe may, to our discomfort, sink into amoeboid forms. The same is true of CORTICATA.) Predominantly ciliated and active. Infusorians. Classification of Protozoa (GVMNOMYXA.) Predominantly amoeboid. Rhizopods. (CORTICATA.) Predominantly encysted and passive. Sporozoa. ACINETARIA. RADIOLARIA. CILTATA. Rhynchoflagellata. DiNOFLAGELLATA. FLAGELLATA. FORAMINIFERA. Labyrinthulidea. Heliozoa. LOBOSA. SPOROZOA OR GREGARINES. Proteomyxa and Mycetozoa. Primitive Forms. the Protozoa ; thus in various conditions the ciliated or flagellate unit may become encysted or amoeboid, while in some of the simplest forms, such as Proiomyxa, there is a " cell-cycle " in which all the phases occur in one life-history. Systematic Survey A. Primitive forms.— Under this heading may be included two classes : I ) the Proteomyxa, primitive, insufficiently known forms often without a nucleus, though nuclear material may be present in the form CLASSES OF PROTOZOA I3I of scattered granules (chromidia), and (2) the Mycetozoa, organisms with somewhat complex fructifications, often classed as plants allied to Fungi. As examples of the Proteomyxa, we have the interesting Protomyxa in four phases : (a) encysted and breaking up into spores, which (6) are briefly flagellate, (c) sink into amoeboid forms, and {d) flow together into a composite " Plasmodium " ; Vampyrella, parasitic on fresh -water Algae ; and many others. The Mycetozoa are well illustrated by Fuligo or yEtkalium septicum, " flowers of tan," found in summer as a large plasmodium on the bark of the tan-yard. The coated spores are formed in little capsules which rise from the surface of the plasmodium. The spores may be first flagellate, then amoeboid, or amoeboid from the first ; the characteristic Plasmodium is formed by the fusion of the amoebae. B. Predominantly amoeboid Protozoa. — Rhizopoda. — The simplest Rhizopods generally resemble Amoeba, and are ranked in the class (3) Lobosa. They may reproduce simply by division, as does Amceba itself, or mav liberate several buds at once (Arcella), or form Fig. 59. — Diagram of Protomyxa aurantiaca. — After Haeckel. I. Encysted ; 2. Dividing into spores ; 3. Escape of spores, at first flagellate, then amoeboid ; 4. Plasmodium, formed from fusion of small amoebae. spores which conjugate {Pelomyxa). Various forms, siich as Arcella, are furnished with a shell. (4) The I.abyrinthulidea are represented by forms like Labyrinthula on Algae, and Chlamydomyxa on bog-moss, which consist of a mass of protoplasm spread out into a network, and of numerous spindle-shaped units, which travel continually up and down the threads of the living net. As (5) Heliozoa are classified the sun-animalcules [Actinosphcsrium, Actinophrys sol), and others, in which there are stiff processes radiating from a spherical body. Reproduction may be by division or by spore formation ; skeletal structures may be represented by spicules. The (6) Foraminifera or Reticularia include an interesting series of shelled forms in which the peripheral protoplasm forms branching interlacing threads. A few simple forms occur in fresh water ; the great majority occur on the floor of the sea at varying depths ; some families are abundantly represented on the surface. The shell is usually calcareous, more rarely arenaceous or chitinous. There is sometimes dimorphism. Multiplication occurs by fission, or by the formation of swarm-spores (amoeboid or flagellate). Foraminifera are common as fossils from Silurian rocks onwards, and at the present day are very important in the formation of calcareous ooze ; in this respect Globigerina, with a chambered shell, is especially important. Species 132 PHYLUM PROTOZOA— THE SIMPLEST ANIMALS of Gromia are found in both fresh and salt water ; Haliphysema, a form utilising sponge-spicules to cover itself, was once mistaken for a minute sponge. Most kinds of chalk consist mainly of the shells of Foraminifera accumulated on the floor of ancient seas ; Nummulites (Fig. 17) and related fossil forms were as large as shillings or half-crowns. More complex are the (7) Radiolaria, which are divided by a chitinoid membrane into an inner central capsule (with one or more nuclei), and Fig. 60. — Formation of shell in a simple Foraminifer. — After Dreyer. In A and B the shell has one chamber ; C and D show the formation of a second. Note outflowhig pseudopodia and the enclosure of the shell by a thm layer of protoplasm ; note also the nucleus in the central protoplasm. an outer portion, gelatinous and vacuolated, giving off radiating thread- like pseudopodia, which very rarely interlace. There is usually a skeleton in the form of a siliceous lattice-work or regularly disposed spicules outside the central capsule, but in some cases the shell is formed of a horn-like substance called acanthin, which is probably a complex silicate. Radiolarians multiply by fission, which sometimes includes a halving of the skeleton, and by spores, which in some cases are dimorphic. Most Radiolarians include unicellular Algae (yellow cells), with which they live in intimate mutual partnership (symbiosis). Most Radiolarians float on the surface of the sea ; others live below CLASSES OF PROTOZOA 133 the surface at varying depths ; and some are abyssal. They are abundant as fossils,' and of much importance in the formation of the ooze of great depths. • , ^ „ Examples.— Thalassicola, EucyrUdtum, and the colonial Collozoum and Sphcerozoum. ,1 , v C. Predominantly active forms (ciliate and flagellate), Fig. 61.— a pelagic Foraminifer — Hastigerina {Globigerina) murrayi. — After Brady. Note central shell, projecting calcareous spines with a protoplasmic axis ; also fine curved pseudopodia and vacuolated protoplasm. generally called Infusorians.— Protozoa, with a definite rind and with 1-3 undulating flagella, aie included as (8) Flagellata, a very large group, among which are such familiar forms as the common Euglena of ponds ; the Monads ; Volvox, a colonial form ; Codosiga, a colony in which the individual cells are furnished with a collar (Choano- flagellata). The Ha^moflagellata are important blood parasites, gener- ally called Trvpanosomes (see p. i47)- Modified flagellate forms are included in the groups Dmoflagellata 134 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS and Cystoflagellata, in both of which there are two flagella, differ- ently placed in the two cases. In the first are included Peridinium and Fig. 62. — The trumpet -shaped cihated Infusorian, Stentor. — After Stein. A^., Beaded iiiacronucleus ; «., divided niicronucleus ; C.V., contractile vacuole ; AD., large adoral cilia, spirally arranged ; C7., small cilia all over ; P., a particle being wafted into the mouth, which is near the centre of the whirlpool. Numerous food vacuoles are seen in the general cytoplasm. Cerattum ; in the latter, the large phosphorescent Noctiluca. form an important part of the plankton of lakes and sea. They CLASSES OF PROTOZOA 1 35 As (9) Ciliata are included a very large number of forms, more or less closely resembling Paramcecium or Vorticella, and very abundant in infusions ; some, such as Opalina, in the intestine of the frog, are more or less parasitic. As specially modified Ciliata are included (10) Acinetaria, highly specialised forms, ciliated when young, but usually fmrnished when adult with suctorial tentacles. They are fixed in adult life, and feed on other Protozoa. As examples may be given Acineta ; Dendrosoma, forming branched colonies ; and Ophryodendron, without suctorial tentacles. Some, like Sphcerophrya, are minute and parasitic. D. Predominantly encysted Protozoa. — Sporozoa. — Forms like Gregarina and Monocystis are included in a group of the (11) Fig. 63. — Optical section of a Radiolarian (Actinomnia). —After Haeckel. a Nucleus ; b, wall of central capsule ; c, siliceous shell within 'nucleus; c', middle shell within central capsule; c2, outer shell in extra-capsuleir substance. Four radial spicules hold the three spherical shells together. Sporozoa, the Gregarinida in the strict sense. They are parasites in the gut or body cavity of many Invertebrates, especially Arthropods. Coccidiiun is a type of the Coccidiidea, which are intracellular parasites occurring in Arthropods, Molluscs, and Vertebrates. A very im- portant group, with a life-cycle essentially similar to that of the Coccidiidea, are the Haemosporidia, which are parasitic in the red blood corpuscles of Vertebrates. The malaria parasites belong to this group. In many of the Haemosporidia a part of the hfe-cycle takes place in an intermediate host, usually a mosquito or a tick. Other groups of the Sporozoa are the Myxosporidia, with peculiar nematocyst-hke organs (Invertebrates and cold-blooded Vertebrates), and the Sarcosporidia, which are found inside the striped muscles of warm-blooded Vertebrates. 136 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS General Notes on the Functions of Protozoa Movement. — The simplest form of movement is that termed amoeboid, as illustrated by an Amoeba. In ordinary conditions it is continually changing its shape, putting forth blunt lobes and drawing others in. Surface tension phenomena occur on the outermost zone of the cytoplasm, and also beneath that — often along the dorsal surface in the direction of motion, then over the front end, then along the ventral surface next the substratum, then again at the posterior end dorsally. Thus there is a complex " caterpillar- wheel "-like stream.ing movement of the granules. No final explanation of the whole process in physico-chemical terms has yet been given. A more defined contraction, like that of a muscle cell, is illustrated in the contractile filam.ent of the stalk of Vorticella and similar Infusorians ; and not less definite are the move- ments of cilia and flagella, by means of which most In- fusorians travel swiftly through the water. Cilia in movement are bent and straightened alternately ; while flagella, which are usually single mobile threads, exhibit lashing movements to and fro, or, more often, are held stretched out in front, and by a curious rotatory movement draw the cell along. They are then more aptly termed tractella. It seems probable that cilia and flagella consist of an elastic core surrounded by a sheath, which may be uniformly contractile, or may have one contractile band, or two opposite contractile bands, and so on. Considered generally, the movements are of two kinds : either (i) re- flex, i.e. responses to external stimulus, as when the Protozoon moves towards a nutritive substance ; or (2) automatic, i.e. such movements as appear to originate from within, without our being able to point to the immediate stimulus, e.g. the rhythmical pulsations of contractile vacuoles. Actively moving Protozoa usually show the following motor reaction to stimulus : — they move backward, turn over on one side structurally defined, and then move forward again. Sensitiveness. — The Amoeba is sensitive to external in- fluences. It shrinks from strong light and obnoxious materials ; it moves towards nutritive substances. This sensitiveness is, so far as we know, diffuse — a property of PHYSIOLOGY OF PROTOZOA 1 37 he whole of the cell substance ; but the pigment spots of some forms are specialised regions. Many Protozoa well illustrate a strange sensitiveness to the physical and chemical stimuli of objects or substances with which they are not in contact. Thus the simple amoeboid Vampyrella will, from a con- siderable distance, creep directly towards the nutritive substance of an Alga, and the Plasmodium of a Myxomycete will move towards a decoction of dead leaves, and away from a solution of salt. The same sensitiveness, technically termed chemotaxis, is seen when micro- organisms move towards nutritive media or away from others, when the spermatozoon (of plant or animal) seeks the ovum, or when the phago- cytes (wandering amoeboid cells) of a Metazoon crowd towards an in- truding parasite or some irritant particle. Nutrition. — The Amoeba expends energy as it lives and moves ; it regains energy by eating and digesting food particles. Most of the free Protozoa live in this manner upon solid food particles ; a few, such as Volvox, in virtue of their chlorophyll, are holophytic, i.e. they feed like plants ; the parasitic forms usually absorb soluble and diffusible substances from their hosts. Respiration. — Oxygen is simply taken up by the general protoplasm from the surrounding medium, into which the waste carbonic acid is again passed. The bubbles which enter with the food particles assist in respiration. In parasitic forms the method of respiration must be the same as that of the tissue cells of the host. Excretion. — Of the details of this process little is certainly known, but the contractile vacuoles are, without doubt, primitive excretory appliances. In the more specialised forms they appear to drain the cell substance by means of fine radiating canals, and then to burst to the exterior. Uric acid and urates are said to be demonstrable as waste products. Colour. — Pigments are not infrequently present in the Protozoa. We have already noticed the presence of chlorophyll in some forms ; with Radiolarians the so-called " yellow cells " are found almost constantly associated. Each of these cells consists of protoplasm, surrounded by a cell wall, and containing a nucleus. The protoplasm is impregnated with chlorophyll, the green colour of which is obscured by a yellow pigment. Starch is also present. The cells multiply by fission, and continue to live after isolation from the protoplasm of the Radiolarian. All these facts point to the conclusion that the cells are symbiotic Algae, so-called Zoochlorellce. According to some, the " chlorophyll corpuscles " seen in the primitive Archerina, in some 138 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS flagellate forms, as Euglena, and in many Ciliata, as Stentor, Stylo nichia, one species of Paramcecium, Volvox and the allied forms, are also symbiotic Alga), which have lost the power of independent exist- ence. The evidence for this is, however, insufficient, and this explana- COL ■ Fig. 64. — A Monad Infusorian. — After Saville Kent. N., Nucleus; C.V., contractile vacuole; F.V., food vacuole ; F.P., an external food particle ; the arrows indicate how the particles are swept ; COL., trans- parent collar in the middle of which the flagcUum (FL.) works. tion will not apply in cases like that of VorticeUa viridis, where the green colouring matter is uniformly distributed through the protoplasm. In many cases there is, besides the chlorophyll, a brown pigment, identical with the diatomin of Diatoms. In many of the Flagellata there are one or more bright pigment spots at the anterior end of the STRUCTURE OF PROTOZOA 1 39 cell ; these may be specially sensitive areas. . In some of the simpler Gregarines the medullary protoplasm is coloured with pigment which is apparently a derivative of the hsemoglobin of the host. Psychical life. — Protozoa often behave in a way which suggests control, but it should be noted that cut-off frag'ments sometimes behave just as effectively as the intact units. Verworn has decided, after much labour, that the Protozoa do not exhibit what even the most generous could call intelligence ; but this is no reason why he or any other evolutionist should doubt that they have in them the indefinable rudiments of mind. Jennings has shown that the behaviour of some Infusorians corresponds to what may be called the method of trial and error ; they " try " one kind of response after another until, in some cases, they give the effective answer. General Notes on the Structure of Protozoa The Protozoa are sometimes called " structureless," but they are only so relatively. For though they have not stomachs, hearts, and kidneys, as Ehrenberg supposed, they are not like drops of white of egg. The cell substance consists of a living colloidal mixture, often with vacuoles. In many cases there are numerous granules, some of which are food fragments in process of digestion, or waste products in process of excretion. The cell substance includes one or more nuclei, special- ised bodies which are essential to the life and multiplication of the unit. In the Protozoa there are several conditions under which the nucleus may exist : — (i) In some adult forms, and in many spores or young forms, no definite nucleus has yet been discovered. It is, however, unnecessary to preserve the term " Monera " for such simple forms, as it is probable that nuclear material does exist in the form of granules. (2) In the majority of cases, notably in, the Sporozoa, the nucleus is single, often large, and placed centrally. From a consideration of the cells of Metazoa we may call this the typical case. (3) In many of the Ciliata, e.g. Pararncecium, there are two dimorphic nuclei. There is a large oblong nucleus, and beside it a smaller spherical one. {4) In some Ciliata the macronucleus exists in the form of powder scattered through the protoplasm, e.g. in Opalinopsis. The granules may collect to form a compact nucleus when fission is about to take place. 140 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS (5) In Opalina, from the intestine of the frog, and a few other forms, there are very numerous nuclei, arranged in a symmetrical manner in the cell substance. In some cases these isolated nuclei have been observed to unite to form one large nucleus just before binary fission takes place. Of these various cases the diffuse condition is apparently very primitive. The nucleus, when stained and examined under high powers, is observed to be complex in structure. It consists of a nuclear network, or a coil of chromatin threads. Karyokinesis has been observed in some cases. While we cannot at present define the physiological import of the nucleus, we must recognise its importance. Thus Bruno Hofer has shown that when an Amceha is cut in two, the part with the nucleus lives and grows normally, while the part without any nucleus sooner or later dies ; and Balbiani has observed that in the case of Infusorians cut into pieces, those parts which have nuclei survive, while if no nucleus is present in the fragment, the wound may remain unhealed, and death ensues. The outer part of the cell substance (*' ectoplasm ") is often clearer and less granular than the inner part ('' endo- plasm "). In corticate Protozoa there is a more definite rind or thickened margin of cell substance. Outside this there may be a " cuticle " distinct from the living matter , sometimes consisting of chitin, or gelatin, or rarely of cellulose. The cuticle may form a cyst, which is either a protection during drought, or a sheath within which the unit proceeds to divide into numerous spores. Moreover, the cuticle may become the basis of a shell formed from foreign particles, or made by the animal itself of lime, flint, or organic material. In the cell substance there may be bubbles of water taken in with food particles (food vacuoles), contractile vacuoles, fibres which seem to be specially contractile (in Gregarines), spicules of flint or threads of horn-like material, which may build up a connected framework, and the pigments already mentioned. Some marine and fresh-water Protozoa secrete bubbles of oxygen which buoy them up in the water. Reproduction of Protozoa Growth and reproduction are on a different plane from the other functions. Growth occurs when income exceeds expenditure, and when constructive or anabolic processes REPRODUCTION IN PROTOZOA 141 are in the ascendant. Reproduction occurs at the hmit of growth, or sometimes when nutrition is checked. As it is by cell division that all embryos are formed from the egg, and all growth is effected, the beginnings of this process are of much interest. (a) Some very simple Protozoa seem to reproduce by what looks like the rupture of outlying parts of the cell substance. (&) The production of a small bud from a parent cell is not uncommon, and some Rhizo- pods (e.g. Arcella, Pelomyxa) give off many buds at once, (f) Com- moner, however, is the definite and orderly process by which a unit Fig. 65. — Diagram of the structure of Noctiluca. FL., Large locomotor flagelluin ; fl., small food-inwafting flagellum ; M., mouth ; iV., nucleus ; P., central protoplasm around nucleus, with granules and food- particles ; R., a ridge in the "gullet" — a con- tinuation of the mouth opening ; V.C., much vacuo- lated general cytoplasm ; CU., cuticular pelUcle. divides into two — ordinary cell division, {d) Finally, if many divisions occur in rapid succession or contemporaneously, and usually within a cyst enclosing the parent cell, i.e. in narrowly limited time and space, the result is the formation of a considerable number of small units or spores. In the great majority of cases, each result of division is seen to include part of the parent nucleus. A many-celled animal multiplies in most cases by liberating reproductive cells — ova and spermatozoa- different from the somatic cells which make up the " body." A Protozoon multiphes by dividing wholly into daughter 142 PHYLUM PROTOZOA — tHE SIMPLEST ANIMALS cells. This difference between Metazoa and Protozoa in their modes of multiplication is a consequence of the difference between multicellular and unicellular life. Each part of a divided Protozoon is able to live on, and will itself divide after a time, whereas the liberated spermatozoa and ova of a higher animal die unless they unite. By sexual reproduction we mean — (a) the liberation of special reproductive cells from a " body," and (b) the fertilisation of ova by spermatozoa. As Protozoa have no " body " — though the beginnings of one are seen in the colonial forms — they cannot be said to exhibit sexual reproduction in the first sense (a), yet many of them^ (especially the Sporozoa) give origin by division to special reproductive cells. And although many Protozoa can live on, dividing and nujltiplying, for prolonged periods without the occurrence of anything like fertilisation, processes corresponding to fertilisation are of general occurrence. For in many of the Protozoa there occurs at intervals a process of " conjugation *' in which two individuals unite either permanently or temporarily. This is an incipiently sexual process ; it is the analogue of the fertilisation of an ovum by a spermatozoon. In many cases, moreover, there is a difference between the two conjugates, analogous to the difference between ovum and spermatozoon. (i) It is one of the recurrent phases in the Hfe-history of some of the simplest Protozoa (Proteomyxa and Mycetozoa) (see Fig. 59), that a number of amoeboid units flow together into a composite mass, which has been called a " Plasmodium.'" (2) It is known that more than two individual Sporozoa and other forms occasionally unite. To this the term " multiple conjugation " has been applied. (3) Commonest, however, is the union of two apparently similar individuals, either permanently, so that the two fuse into one, or temporarily, so that an exchange of material is effected. Permanent conjugation has been observed in several Rhizopods, Infusorians, and Sporozoa. Temporary conjugation is well known in not a few ciliated Infusorians, and it is possible that a curious end-to-end union of certain vSporozoa is of the same nature, or it may be of the nature of a " Plasmodium " formation. The formation of small spores (gametes) which conjugate is not uncommon. (4) There are some cases where one of the conjugating individuals is larger and less active than the other. Thus in Vorticella, a small free-swimming form unites and fuses completely with a stalked indivi- dual of normal size. This " dimorphic conjugation " is evidently CONJUGATION 143 analogous to the fertilisation of a passive ovum by an active sper- matozoon. In Volvox this is even more obvious, for the small and active cells, both in shape and method of formation, recall the sper- matozoa of higher forms. Significance of Conjugation. — Conjugation is an episode in repro- duction, not a mode of multiplication. It promotes variability in a stock and it helps to ward off senescence. All the descendants of a single individual form what is called " a pure line," and conjugation does not occur among the members of a pure line. Asexual multiplica- tion may continue in ideal conditions for ten years and through thou- FiG. 66. — Spore-formation in Noctiluca. — After Roule- From minute nucleated hillocks on the surface of the cell peculiar flagellate spores (SP.) are given off. The long flagellum becomes the short oral flagellum of the adult. The peak- like process develops into the large locomotor flagellum of the adult (see Fig. 65). sands of generations. In ideal conditions of experimental isolation there is no falling off in the vigour of the stock. But in such cases, as also in certain species of Paramoecium in natural conditions, there is a periodic, approximately monthly occurrence of a process called by Woodruff and Erdmann " endomixis." There is a nuclear disintegra- tion, followed by reintegration. It is similar to the preliminary stages leading on to conjugation, but no conjugation occurs. Endomixis is a process that assists rejuvenescence. Ecology. — Many Protozoa raise organic debris once more into the circle of life, and many form part of the food 144 PHYLUM PROTOZOA — THE SfMPLEST ANIMALS of higher animals. Thus those pelagic Foraminifera and Radiolarians, which sink dying to the great oceanic depths, form along with more substantial debris the fundamental food supply in that plantless world. Fundamental, since it is plain that the deep-sea animals cannot all be living on one another. Almost every kind of nutritive relation occurs among the Protozoa. Predatory life is well illustrated by most In- fusorians, and thoroughgoing parasitism by the Sporozoa ; Opalina in the rectum of the frog may serve as a type ot those which feed on decaying debris, and Volvox of those which are holophytic. Radiolarians, with their partner Algae, exhibit the mutual benefits of symbiosis, the plants utilising the carbon dioxide of their transparent bearers, the animals being aerated by the oxygen which the plants give off in sunlight, and moreover nourished by the carbohydrates which they build up. Some of the parasitic forms, especi- ally among the Sporozoa, are fatally injurious to higher animals. Though Protozoa may be seriously infected by Bacteria, by Acineta parasites, by some fungi, like Chytridium, etc., fatal infection is rare, because of the power of intracellular digestion which most Protozoa possess. " The parasite," Metchnikoff says, " makes its onslaught by secreting toxic or solvent substances, and defends itself by paralysing the digestive and expulsive activity of its host ; while the latter exercises a deleterious influence on the aggressor by digest- ing it and turning it out of the body, and defends itself by the secretions with which it surrounds itself." With this struggle should be compared that between phagocytes and Bacteria in most multicellular animals. History. — Of animals so small and delicate as Protozoa, we do not expect to find distinct relics in the much-battered ancient rocks. But there are hints of Foraminifer shells even in the Cambrian ; riiore than hints in the Silurian and Devonian ; and an abundant representation in rocks of the Carboniferous and several subsequent epochs. The shells of calcareous Foraminifera form an important part of chalk deposits. There seem at least to be sufficient relics to warrant Neumayr's generalisation in regard to Foraminifera, that the earliest had shells of irregularly agglutinated particles (Astrorhizida;), that these were succeeded by forms with regularly agglutinated shells, exhibiting types of architecture which were subsequently expressed in lime. PROTOZOA AND DISEASE H5 Remains of siliceous Radiolarian shells are known from Silurian and from Devonian strata onwards. From the later Tertiary deposits of Barbados earth, Ehrenberg described no fewer than two hundred and seventy-eight species. Protozoa and Disease. — The discoveries of recent years have shown that the study of Protozoa is an inquiry of great practical importance. All three main divisions of the Protozoa contain important disease-producing parasitic forms, especially the flagellated Infusorians called Haemoflagellata (Trypanosomes) and the Sporozoon Fig. 67. — Glossina palpalis, tsetse fly. group of Haemosporidia, to w^hich Plasmodium vivax (see Fig. 68) belongs. I . Various species of Amoebae are parasitic in the human food canal, e.g. Entamoeba colt, E. histolytica, lodamoeba butschlii, Endolimax nana, and Entamoeba gingivalis in ill-kept teeth, but the only pathogenic form is Entamoeba histolytica. This Amoeba eats into -the wall of the lower intestine, causing ulceration and, in severe cases, amoebic dysentery as well as abscesses on the liver and elsewhere. A clear cyst may be formed within which the nuclei divide usually into four. The cyst is passed out of the intestine, and should it find its way into the food canal of another human being, the cyst breaks and sets free the contained daughter Amoebae. 10 INFECTIVE MOSQUIToA 6iTes nm. D SCHIZOC-OMY IN MAPS oo riO60UITO B TAhES I- >nGfinE:rocYTes. SPOROGOhY no5Quiro. nOSQUlTObinFECTlVE:. 5toaO20lTES ENTER jQ a. PiQ, 58. — Life-cycle of Plasmodium vivax. — Bas?d]on Sehaudinn. I. Sporozoites; 2-7. Schizogony in human red blood cells ; 7a,8. Formation of gametocytes in red blood cells ; 9. Gametocytes set free in stomach of mosquito ; 10. Production of gametes; 11. Union of i and 9 gametes; 12. Zygote; 13. Zygote burrows through stomach lining of mosquito ; 14. Encysts; 13, 16. Growth and development of sporo- zoites; 17. Rupture of oocyst ; sporozoites set free. SLEEPING SICKNESS 147 2. The Trypanosomidae are flagellate Protozoa, chiefly parasitic in the blood of higher Vertebrates and the ali- mentary canal of Invertebrates. A typical fully formed trypanosome is seen in Fig. 69 (i), but there may be great variation of shape at different stages of the life-history, even rounded non-flagellate stages occurring {Leishmania) . The curved-blade-like cell has a single flagellum rising from a base termed the blepharoplast, and for part of its length joined to the rest of the cell by a thin undulating membrane. Near the blepharoplast is a small nucleus- like body. There is also a prominent central nucleus. Vacuoles and granules may be present in the cytoplasm. A delicate " periplast " covers the outside of the cell. Reproduction is by longitudinal division, beginning at the basal end of the flagellum. Trypanosomes have been found in the blood of many mammals, including mice, voles, rabbits, cavies, squirrels, various bats, moles, shrews, ant-eaters, badgers, marmosets, monkeys, armadillos, as well as the better-known hoofed animals. They are also found in birds, reptiles, amphibians, and a great many fishes. They are spread from one host to another by means of an intermediate host, usually a blood-sucking insect or leech, within which a phase of the life-history is passed. A few occur in plants ! T. gamhiense multiplying in the blood causes African sleeping sickness. It is transmitted by a tsetse fly. T. brucei, also carried by a tsetse fly, and a most virulent trypanosome, causes Nagana in, chiefly, domestic stock. Its " natural " hosts are certain of the bigger African game animals which seem to be unaffected by it. T. equiperdum causes dourine in horses. T. evansi, a trypanosome affecting horses, camels, mules, domestic cattle, and dogs in tropical countries, causes the disease known as " Surra," especially deadly amongst horses. _,In 1907 T. cruzi was discovered by Chagas in South America. It chiefly affects children and adolescents, causing Chagas' disease. It is transmitted by a bug. The commonest trypanosome is that found in the blood of rats — T. lewisi — and transmitted from rat to rat by fleas. When infected blood is sucked in by the flea, the trypanosomes pass from the cavity of the flea's stomach and burrow into the lining cells of 148 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 3\ UTU Fig. 69. I. Trypanosoma gambiense, showing nucleus (n.) ; blepharoplast (Bl.) ; so-called kineto-nucleus {Bk.) ; undulating membrane (u.m.) ; free end of flagellum (F/.). IL Individual undergoing longitudinal fission. III. and IV. Individual dividing into spores. MALARIA ORGANISM 1 49 the Stomach wall. Here an individual trypanosome may multiply eight or ten fold, till the cell is a mere envelope containing a moving skein of trypanosomes. These then break out into the stomach, and may invade other cells. Some migrate to the rectum and hind gut, whence they are expelled with the excrement. Rats are infected by lick- ing the excrement of the flea while cleaning their fur. Leishmania donovani is a rounded non-flagellate stage of a flagellate parasite, and occurs in the lining cells of blood vessels, causing splenomegaly or Kala azar in tropical and sub-tropical countries. Many attempts have been made to discover the intermediate host of this parasite. Many possible carriers, especially fleas and bugs, have been in- vestigated, but so far without success. L, tropica is the cause of the skin lesions known as oriental sore, a widely distributed disease in warmer countries. Ulcerating wounds develop chiefly in exposed parts of the body, such as hands, feet, and head. The parasites are carried by sand flies. 3. Plasmodium vivax, the organism of benign tertian malaria, has already been dealt with (pp. 125 and 146). As in most Haemosporidia, the schizogony phase of the life- history is intracellular within a red blood cell, while the spo'rogony phase is passed in a carrier insect. The two species P. malaria and P. falciparum cause quartan malaria and malignant tertian malaria respectively. The life-history in each case is very similar to P. vivax. Plas- modium prcecox (Proteosoma) , the parasite of bird malaria, is transmitted by the mosquito, Culex fafigans, and an allied parasite of the pigeon, Hcemoproteus columbce, by a biting fly, Lynchia maura. In Piroplasma {Babesia) bovis the very characteristic first part of the life-history is within the red blood cells of cattle and other animals, the second part — not yet fully understood — within a tick. This minute pear-shaped parasite is the cause of Texas or Red-Water Fever, a formidable cattle disease in certain parts of America and in Australia. Among the other parasitic Sporozoa are various coccidia (Eimeria), which are found in the intestine and related parts of horses, pigs, sheep, and other mammals, of 150 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS grouse, pheasants, and other common birds, as well as a large number of Invertebrates. They are intracellular parasites. Gregarines are common gut parasites of most animals, at first intracellular, later becoming free in the gut cavity. The Spirochaetes form a group of often ex- tremely minute spiral organisms which some regard as Bacteria, others as Protozoa. Some of them are among the most formidable parasites of man. Treponema {Spiro- chceta) pallidum is the cause of syphilis, one of the heaviest taxes on civilisation. Many Spirochastes are Fig. 70. — Colonial Infusorian — Ophrydium sessile. — After vSaville Kent. (Enlarged 100 times.) The individuals are embedded in a jelly-like matrix (/.). M., Mouth ; B., band round oral disc ; C, circumoral cilia. blood parasites, e.^. S. duttoni^ conveyed by a tick, is the cause of African relapsing or tick fever. Another species of Spirochaete, S. recurrenlis, is transmitted in more northern countries in the haemocoele of the body-louse, and causes what is termed " European " relapsing fever. General zoological interest. — The Protozoa illustrate, in free and single life, forms and functions like those of the cells which compose the many-celled animals. Typically, they show great structural or morphological simplicity, but great physiological complexity. Within its single cell the Protozoon discharges all the usual functions, while in a higher animal distinct sets of cells have been specialised for BEGINNINGS ILLUSTRATED BY PROTOZOA 151 various activities, and each cell has usually one function dominant over the others. The Metazoan cells, in acquir- ing an increased power of doing one thing, have lost the Protozoan power of doing many things. The Protozoa remain at the level represented by the reproductive cells of higher forms, and are comparable to reproductive cells which have not formed bodies. In the sexual colonies of Volvox, however, we see the beginning of that difference between reproductive cells and body cells which has become so characteristic of Metazoa The Fio. 71. — A colonial flagellate Infusorian — Proterospongia haeckelii. — After Saville Kent. There are about 40 flagellate individuals, a, Nucleus ; b, contractile vacuole ; c, amoeboid unit in gelatinous matrix ; d, division of an amoeboid unit ; e, flagellate units with collars contracted ; /, hyaline outer membranes ; g, spore-formation. Protozoa are self-recuperative, and in normal conditions they are not so liable to " natural death " as are many-celled animals. Weismann and others maintain that they are physically immortal. They illustrate — (a) the beginnings of reproduction, from m.ere breakage to definite division, either into two, as in fission, or'in limited time and space into many units, as in the formation of spores within a cyst ; (b) the beginnings of fertilisation, from " the flowing together of exhausted cells " and multiple conjugation, to the specialised sexual union of some Infusorians, Heliozoa, Sporozoa, etc. — where two individuals become closely united ; along with this, the 152 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS beginnings of maturation, as shown in the formation of polar nuclei in some Heliozoa, Sporozoa, Flagellata, and Lobosa ; {c) the beginnings of sex, in the difference of size and of constitution sometimes observed between two con- jugating units (e.g. in Coccidium) ; {d) the beginnings of many-celled animals, in the associated groups or colonies which occur in several of the Protozoan classes. These colonies show a gradation in complexity. Raphidiophrys and other Heliozoa form loose colonies, which arise by the close coherence of the products of fission. Among the Radiolarians there are several colonial forms ; in these the individuals are united by their extra-capsular protoplasm, but are all equivalent. In Proterospongia the cells show considerable morphological distinctiveness ; some are flagellate, some amceboid, some encysted and spore- forming. Again, in Volvox, as we noticed above, the cells of the colonies show a distinction into nutritive and repro- ductive units. Lastly, in their antithesis of passivity and activity, con- structive and destructive preponderance, anabolism and katabolism, the Protozoa illustrate the phases of the cell- cycle, and so furnish a key to the variation of higher animals. CHAPTER VIII PHYLUM PORIFERA— SPONGES Class I. Calcarea. Class II. Hexactinellida. Class III. Demospongi^. Sponges seem to have been the first animals to attain marked success in the formation of a " body." For though their details are often complex, their essential structure is simpler than the average of any other class of Metazoa, and some of the simplest forms do not rise high above the level of the gastrula embryo. i\ " body " has been gained, but it shows relatively little division of labour or unified life ; it is a community of cells imperfectly integrated. The cells of the body show an arrangement in two distinct layers (diploblastic). There are no definite organs, and the tissues are, as it were, in the making. Sponges are passive, vegetative animals, and do not seem to have led on to anything higher ; but they are successful in the struggle for existence, and are strong in numbers alike of species and of individuals. General Characters Sponges are diploblastic {two-layered) Metazoa, the middle stratum of cells, the mesogloea, not attaining to the definiteness of a proper mesoderm. There is no coelom or body cavity. The longitudinal axis of the body corresponds to that of the embryo ; in other words, the general symmetry of the gastrula is retained. In these three characters the Sponges agree with the Coelentera, and differ from higher (triplo- blastic and coelomate) Metazoa- '53 154 PHYLUM PORIFERA — SPONGES The body varies greatly in shape, even within the same species. It is traversed by canals, through which currents of water bear food inwards and waste outwards. Numerous minute pores on the surface open into afferent canals, leading into a cavity or cavities lined by flagellate cells, many or all of which have a goblet shape with a delicate collar through which the flagellum rises (" choano- cytes "), To the activity of the fiagella the all-important water currents are due. The internal cavity may be a simple tube, or it may have radially outgrowing chambers, or it may be represented by branched spaces, from which efferent canals lead to the ex- terior. When there is a distinct central cavity there is usually but one large exhalant aperture {psculum), but in other cases there are many exhalant apertures. A delicate outer layer covers the apical oscuium, the inhai- bodv , and IS perhaps inturucd iuto the ant pores in the walls. rr , 7 t^ 7 .1 afferent canals. Beneath the covering layer there is in all but the simplest forms a mass of cells (the mesogloea) which may be very varied in its composition . Thus there are scleroblasts making the skeleton of lime, flint, or spongin ; amoeboid cells or phago- cytes, important in digestion and excretion ; reproductive cells, and other elements. This median mass of cells is traversed by the afferent canals and by the diverticula of the central cavity, or the branches of the original central cavity, lined by flagellate cells. It is difficult to call this cavity or system of cavities the gut or enteron, or to call the layer which lines it the endoderm, or the outer covering layer the ectoderm. In fact, the sponges are very different from other Metazoa, Fig. 72. — Simple sponge {Ascetta primordialis). — After Haeckel. Note the vase-like form, the Fig. 73. — A sponge colony. STRUCTURE OF SPONGES 155 and represent a cul de sac in evolution. There are no nerve- cells — a fatal defect. Budding is very common ^ and in a few cases buds are set adrift. Both herma- phrodite and unisexual forms occur. The sexually-pro- duced embryo is almost always developed within the mesogloea, and leaves the sponge as a ciliated larva. Except the family of Spon- gillids, all are marine. Description of a simple sponge. — A very simple sponge, such as Ascetta, is a hollow vase, moored at one end to rock or seaweed, with a large exhalant aper Fig. 74.— Sponge spicules. Monaxon ; 2, triod ; 3, triaxon ; 4, tet- raxon ; 5, anchor ; 6, polyaxon ; 7, a kind of amphidisc. ture at the opposite pole, and with numerous minute inhalant pores penetrating the walls. In the calca- reous sponges, the pores are minute perforations in single cells (poro- cytes). The walls consist of — (i) a flat covering layer ; (2) a mesogloea con- taining triradiate calcareous spicules, phagocytes, and reproductive ele- ments ; and (3) a layer hning the central cavity, and composed of collared flagellate cells, like some of the monad Infusorians (cf. Fig. 71). More complicated forms. — But a description of a simple sponge like Ascetta conveys little idea of the structure of a complex form such as "■flSS.e'°'"chamber';!"""a the bath-sponge (FAcspongiu). Let us gastruia forming in the cousidcr the Origin of compUcatious mesogloea, etc. / \ r~i i j j {a) Sponges — long regarded as plants — are plant-like in being sedentary and passive. They seem also to feed easily and well. Like plants, Fig. 75. — Section of sponge. — After F. Schulze. is6 PHYLUM PORIFERA — SPONGES CS»^. they form buds, the outcome of surplus nourishment. These buds, Hke the suckers of a rose-bush, often acquire some apparent independence, and the sponge looks like many vases, not like one. Moreover, as they grow these buds may fuse, like the branches of a tree tied closely together. Thus the structure becomes more intricate. {b) In the simple sponge the cavity of the vase is completely lined by the collared flagellate cells {Ascon type). But the inner layer may grow out into radial chambers to which the choano- cytes are restricted (Sycon type), and the walls of these may also be folded into side aisles {Leucon type). The outgrowing of the inner layer into the mesogloea may be con- tinued even further, and the cells may become pavement-like except in the minute flagellate chambers, where alone the char- acteristic choanocytes are retained (see Fig. 76). It may be that the characteristic folding or outgrowth of the inner layer is necessitated by the fact that the com- FiG. 76. — Diagram showing types of canal system. — After Korschelt and Heider. The flagellate regions are dark through- out, the mesogloea is dotted, the arrows show the direction of the currents. All the figures represent cross-sections through the wall. A. Simple Ascon type {Ec, outer layer ; En., inner layer ; Mg., mesogloea). B. Sycon type, with flagellate radial chambers {r.c). C. Leucon type, with flagellate side aisles on the main radial chambers. D. Still more complex type, with small flagellate chambers (f.ch.). STRUCTURE OF SPONGES 157 ponent cells are better nourished and multiply more rapidly than those of the outer layer. (c) By infoldings of the outer layer and a subjacent sheath of mesoglcea — subdermal spaces may be formed ; an outer cortex may be distinctly differentiated from the internal region in which the flagellate chambers occur ; the pores may collect into sieve-like areas, which open into dome-like cavities ; these and many other complications are common. {d) The covering layer usually consists of flat epithelium, but flask-shaped cells have also been observed (Bidder). It may be folded inwards, as we have noticed, and, accord- ing to some, it also lines the inhalant or afferent canals in R.C Fig. 77. — Diagram of sponge structure. R.C., A flagellate chamber into which water passes by an inhalant pore (LP.). O., An osculum, into which the exhalant canals open ; the arrows indicate the directions of the currents. SD.SP., A subdermal cavity or porch, into which the inhalant pores may open. whole or in part. In a few cases, e.g. Oscarella lobularis, it is ciliated, and its cells may also exhibit contractility, as around the osculum of Ascetta clathrus, though the con- tractile elements usually belong to the mesoglcea. The inner layer consists typically of collared flagellate cells, but in the more complex sponges these are replaced, except in the flagellate chambers, by flat epithelial cells, with or without flagella. The mesogloea contains very varied elements, and illus- trates the beginnings of different kinds of tissue. Thus there are migrant amoeboid cells (phagocytes) ; irregular connective tissue cells ; spindle-shaped connective tissue cells, united into fibrous strands ; contractile cells, e.g. those forming a sphincter around the oscula of some forms, 158 PHYLUM PORIFERA — SPONGES such as Pachymatisina ; skeleton-making cells ; pigment- containing cells ; and lastly, the reproductive cells. {e) The skeleton consists of calcareous or siliceous spicules, or of spongin fibres, or of combinations of the two last. A calcareous spicule is formed of calcite, with a slight sheath and core of organic matter ; a siliceous spicule is formed of colloid silica or opal ; the spongin is chemi- cally somewhat like silk. Uniradiate, biradiate, triradiate, quadriradiate, sexradiate, and multiradiate spicules occur, and they are eflfective in keeping the meshes open and in giving the body architectural stability. In every pole scaffolding we see, as it were, huge hexactinellid spicules, spliced together with rope. It is convenient to distinguish the large macroscleres from the small microscleres. Each spicule begins to be formed by one or more " scleroblasts," and may be speculatively regarded as an organised intra- cellular excretion. " During its growth," Professor Sollas says, ** the spicule slowly passes from the interior to the exterior of the sponge, and is finally (in at least some sponges — Geodiiiy Stelletta) cast out as an effete product." The fibres of spongin are formed as the secretions of mesogloea cells, known as spongioblasts. Ordinary functions. — Excepting the fresh-water Spong- illidas, all sponges are marine, occurring from between tide marks to great depths. After embryonic life is past, they live moored to rocks, shells, seaweeds, and the like. Their motor activity is almost completely restricted to the lashing movements of the flagella, the migrations of the phagocytes, and the contraction of muscular mesogloeal cells, especially around the exhalant apertures. In the closure of the inhalant pores, sponges show sensitiveness to injurious influences, but how far this is localised in specialised cells is uncertain. The most important fact in the life of a sponge is that which Robert Grant first observed- — that currents of water pass gently in by the inhalant pores, and more forcibly out by the exhalant aperture or apertures. This may be demonstrated by adding powdered carmine to the water. The instreaming currents of water bear dissolved air and supplies ^i food, such as Infusorians, Diatoms, and particles of organic debris. The outflowing current carries away NUTRITION OF MARINE ANIMALS 1 59 waste. When a sponge is fed with readily recognisable substances, such as carmine or milk, and afterwards sectioned, the grains or globules may be found — (a) in the collared flagellate cells ; {b) in the adjacent phagocytes of the mesogloea ; (c) in the phagocytes surrounding the sub- dermal spaces, if these exist. It is uncertain whether the epithelium of the subdermal spaces or the flagellate lining of the deeper cavities is the more important area of absorp- tion, but it is certain that the phagocytes play an important part in engulfing and transporting particles, in digesting those which are useful, and in getting rid of the useless. In extracts of several sponges, Krukenberg and others have found digestive ferments, probably formed within the phagocytes, but digestion is wholly intracellular. Many sponges contain much pigment ; thus the lipo- chrome pigment zoonerythrin (familiar in lobsters) is common. Some pigments, such as floridine, may help in respiration. The green pigment of the fresh-water sponge is due to the presence of green symbiotic algae {Chlorella), which in their holophytic activity probably supply food- stuflFs to the host. Nutrition of marine animals. — Much discussion has centred round the thesis maintained by Putter, that marine animals find a valuable source of energy in organic com- pounds present in solution in the water. He maintains, for instance, that although a sponge may pass five times its own weight of water through its canals in an hour, yet the particulate, solid food contained in this amount of water is insufficient for the sponge's needs, so that there must be absorption of dissolved food-material. Similar arguments are advanced for higher animals, including fishes, with the corollary assumption that such forms are able to absorb organic substances through their gills or elsewhere. Nearly every fink in Putter's chain of argument has been violently assailed, though it has also found many supporters. It is admitted that sea-water may contain considerable quantities of dissolved organic compounds, but it is uncertain whether any of these are valuable as food-stuffs. The view that there is an insufficiency of solid food is disputed, and unfortunately there has 'been much confusion in the controversy, some l60 PHYLUM PORIFERA — SPONGES workers regarding only whole organisms, and others including colloidal particles, as solid food. There is good evidence that even vertebrates can absorb and utilise dissolved food-materials, if present in relatively high concentration, but it is not clear that such absorption takes place except in the alimentary canal ; in molluscs, it appears to cease if the mouth is stopped up. Owing to the difficulty of excluding bacteria, the experimental evidence is not truly decisive ; and it is very doubtful whether dissolved substances play an important part in nutrition. On a rather different level are the experiments of Peters, who got various Infusoria to thrive on nutrient solutions, apparently free from soUd particles or bacteria. Nitrogen and carbon were present in the solution only in the re- latively simple compound ammonium glycero-phosphate ; rnost higher animals undoubtedly require more complex nitrogen- and carbon-containing compounds (amino- acids, at least). Further work on these lines would be very valuable. Reproduction. — If a sponge be cut into pieces, these may regenerate the whole — a fact which illustrates the relatively undifferentiated state of the sponge body. It is possible that fission may sometimes occur naturally. Ordinary budding is a mode of continuous growth, but when small buds are set adrift, e.g. in Donatia and Tethya, there is a form of asexual reproduction. In the fresh-water Spongilhdae there is a pecuUar mode of reproduc- tion by statoblasts or gemmules. A number of mesogloeal cells occur in a clump, some forming an internal mass, others a complex protective capsule, with capstan-like spicules, known as amphidiscs. According to W. Marshall, the life-histor\^ is as follows : In autumn the sponge suffers from the cold and the scarcity of food, and dies away. But throughout the moribund parent gemmules are formed. These survive the winter, and in April or May they float away from the dead parent, and develop into new sponges. Some become short-lived males, others more stable females. The ova produced by the latter, and fertilised by spermatozoa from the former, develop into a summer generation of sponges, which, in turn, die away in autumn, and give rise to gemmules. The Ufe-history thus illustrates what is called alternation of genera- tions. Interpreted from a utihtarian point of view, the formation of gemmules is a Hfe-saving expedient. As Professor SoUas says, " the gemmules serve primarily a protective purpose, ensuring the persistence of the race, while as a secondary function they serve for dispersal." DEVEI.OPMENT OF SPONGES l6l All sponges produce sex cells, which seem to arise from amceboid mesoglcea cells retaining an embryonic character. In the case of the ovum, the amoeboid cell increases in size, and passes into a resting stage ; in the case of the male elements, the amoeboid cell divides into a spherical cluster of numerous minute spermatozoa. The similar origin of the ova and spermatozoa is of interest. Most sponges are unisexual, but many are hermaphrodite. In the latter case, however, either the produc- tion of ova or the production of spermatozoa usually preponder- ates, probably in dependence upon nutritive conditions. Development. — It is not sur- prising to find that there is great variety of development in the lowest class of Metazoa ; it seems almost as if numerous experi- ments had been made, none attended with progressive success. The minute ovum, without any protective membrane, usually lies near one of the canals, and is fertilised by a spermatozoon borne to it by the water. It exhibits a certain power of migration, as in some Hydroids. Previous to fertilisation, the usual extrusion of polar bodies has been observed in a few cases, Fig. 78. — Development of Sycandra raphanus. — After F. E. Schulze. 1. Ovum. 2. Section of i6-cell stage. 3. Blastula with 8 granular cells {gr.c.) at lower pole. 4. Free-swimming amphiblastula, with upper hemisphere of flagellate cells (f.c.), and lower hemisphere of granular cells {gr.c). 5. Gastrula stage settled down. Ec, Outer layer ; En., inner layer ; bl., closing blastopore ; am.p., mooring, amoeboid processes. II ■gf.C. am.p 1 62 PHYLUM PORIFERA — SPONGES and is doubtless general. Segmentation is total and usually equal, and results in a spherical or oval embryo more or less flagellate. This leaves the parent sponge, swims about for a time, then settles down, and undergoes a larval meta- morphosis often diflicult to understand. It is peculiarly difficult to bring the history of the germinal layers in sponges into line with that in other Metazoa. {a) In the small calcareous sponge Sycandra raphanus (Fig. 78), of the Mediterranean, whose development is very similar to that of the common British Sycon ciliatum, the segmentation results in a hollow ball of cells — the blastula. A few cells at the lower pole remain large, and are filled with nutritive granules ; the other cells divide rapidly and become small, clear, columnar, and flagellate. The large granular cells be- come invaginated, at first, \mtil the embryo is free of the parent, when the rounded form is restored in the amphi- blastula (Fig. 78, 4). This swims for a time actively, but the flagellate cells of the upper hemisphere are invaginated into or overgrown by the large granular cells, and thus what is generally called the gastrula stage results. This soon settles down, on rock or seaweed, with the blastopore or gastrula mouth down- FiG. 79.— Diagrammatic re- wards, and is moored by amoeboid pro- presentation of develop- messes from the granular cells, which ment of Oscarella lobularis. likewise obliterate the blastopore. The After Heider. granular cells lose their granules, for the Bl., Free-swimming blastula with larva is not yet feeding ; the flagellate flagella ; G., gastrula settled ^ells begin to acquire the characteristic NexTTgure shows folding of inner collar ; a mesoglcea with spicules begins layer (En.) ; Ec, outer layer. to be formed between the inner and outer Lowest figure shows radial cham- layer, probably by migrants from the bers {R.C.) ; Mesoglcea (Mg.) ; i„4.*p_ Pores ooen through the walls inhalant pore (P.) ; exhalant ^atier. rores open xnrougn ine waus, osculum (0.). water is drawn m by the action of the flagella, and an exhalant aperture is ruptured at the upper pole. The young sponge i^ now in an A scon stage, from which, by the outgrowth of the inner flagellate layer into radial chambers, it passes into the permanent Sycon form, heightens, and becomes differentiated in detail. (b) In Oscarella (Halisarca) lobularis (Fig. 79), a sponge without any skeleton belonging to the Demospongise, the ovum seements equally into a blastula, which is flagellate all over. During this free-swimming DEVELOPMENT OF SPONGES 163 stage the cells at one pole lose their flagella and become granular, and an amphiblastula results. This invaginates to form a hemispherical gastrula, which settles mouth downwards. Pores, an osculum, and the mesogloea are formed as before, and the inner layer becomes folded into flagellate chambers. The main features of sponge embryology are thus summarised by Minchin : — " I. The larva is composed of three classes of cell-elements : (i) Columnar flagellated cells, forming the outer covering or localised at the anterior pole ; (2) rounded, more or less amoeboid elements, rarely flagellated, forming the inner mass or aggregated at the posterior pole ; and (3) the archaeocytes, usually scattered in the inner mass, and often represented by undifferentiated blastomeres. . . . " II. The larva fixes and undergoes a metamorphosis whereby the Fig. 80. — Diagram of early iixed stage of sponge. D., Dermal layer of cells ; G., gastric layer of cells ;• BL., cavity of blastula disappearing ; A., archen- teron ; P., attaching processes of the outer layer cells. This early stage is somewhat like a thimble, fastened mouth (M.) downwards to the substratum. flagellated cells become placed in the interior, while the cells of the inner mass come to surround them completely. '■ III. (i) The flagellated cells of the larva become the choanocytes of the adult (gastral layer), acquiring a collar ; . . . (2) the inner mass gives rise to the dermal layer in its entirety : . . . (3) the archaeocytes become the wandering cells of the adult, from which the reproductive cells arise." It is interesting to note that the primitive germ-cells are early set apart. Classification. — Class I. — Calcarea. With skeleton of calcareous spicules : — Grade I. — Homoccela. — Continuous internal layer of collared flagellate cells, e.g. Ascetta, Leucosolenia. Grade II. — Heterocoela. — Collared flagellate cells restricted to radial tubes or chambers, e.g. Sycon (Grantia). 164 PHYLUM PORIFERA — SPONGES Class II. — Hexactinellida, or Triaxonida, with sexradiate siliceous spicules (triaxons). The members live chiefly in deep water, e.g. Venus Flower-Basket (Euplectella) and the Glass-Rope Sponge (Hyalonema). Class III. — Demospongi^. Skeleton of siliceous spicules, but never triaxons, or of spongin fibres, or of spongin fibres and siliceous spicules, or absent. Grade I. — Tetraxonida, typically with tetraxon spicules, e.g. Pachymatisma, Tetilla. Grade II. — Monaxonida, with monaxon spicules, sometimes with spongin in addition, e.g. Mermaid's Gloves {Chalina oculata), Crumb-of-Bread Sponge (Halichondria or Amorphina panicea), Fresh-Water Sponge (Spongilla). Grade III. — Ceratosa, " horny " sponges with or without spicules, e.g. the Bath-Sponge {Euspongia). Grade IV. — Myxospongida, without any skeleton, e.g. Halisarca and Oscarella. A very remarkable form called Merita seems to have both a siliceous and a calcareous skeleton. History. — Sponges, as one would expect, date back almost to the beginning of the geological record. Thus the siliceous Protospongia occurs in Cambrian rocks, and in the next series — the Silurian — the main groups are already represented. From that time till now they have continued to abound and vary. The division between calcareous and siliceous sponges goes deep down to the very roots of the phylum, and the siliceous branch must have divided very early into Triaxonida and Tetraxonida. Ecology. — Sponges are living thickets in which many small animals play hide-and-seek. Many of the associa- tions are harmless, but some burrowing worms do the sponges much damage. The spicules and a frequently strong taste or odour doubtless save sponges from being more molested than they are ; the numerous phagocytes wage successful war with intruding micro-organisms. Some sponges, such as Cliona on oyster-shells, are borers, and others smother forms of life as passive as themselves. Several crabs, such as Dromia^ are masked by growths of sponge on their shells, and the free transport is doubtless advantageous to the sponge till the crab casts its shell. A compact orange-coloured sponge {Suberites domuncula) of peculiar odour often grows round a whelk-shell tenanted by a hermit-crab, and gradually dissolves the shell-sub- stance. Within several sponges minute Algae live, like the " yellow cells " of Radiolarians, in mutual partnership or symbiosis. One of the cuttlefishes, Rossia glaucopis, puts its eggs carefully into pockets in the substance of a siliceous POSITION OF SPONGES 165 sponge. Finally, sponges deserve mention as factors in human civilisation. General zoological position. — Sponges form the first successful class of Metazoa. They illustrate the beginnings of a *' body," and the beginnings of tissues. Along with B Fig. 81. — A. Young Dicyema.— After Whitman. B. Female Orthonectid (Rhopalura giar- dii). — After Julin. e., Ectoderm ; en., inner endoderm cell with nucleus [n.) ; and embryo [em.). Note the segmentation and the fibrillation supposed to be muscular. Fig. 82. — Salinella. — After Frenzel. 1. Longitudinal section — a., anterior ; p., pos- terior. 2. Transverse section. the Coelentera, they differ markedly from the triploblastic, coelomate Metazoa, which do not retain the radial sym- metry of the gastrula. In their germinal layers and in their internal cavity they differ so much from Coelentera and all other Metazoa, that they must be regarded as on a by-road of evolution. This has been emphasised by 1 66 PHYLUM PORIFERA — SPONGES Sir E. Ray Lankester in the term '' Parazoa." He also speaks of them as a sterile stock. Their origin is wrapped in obscurity ; it may be that they are the non-progressive descendants of primitive gastrula-like ancestors with a sluggish constitution. The presence of choanocytes suggests a relationship with certain of the flagellate Protozoa (Choanoflagellata), and Protero- spongia (Fig. 71) may possibly be regarded as a connecting link. Incertte Sedis. Mesozoa The title Mesozoa was applied by Van Beneden to some simple organisms which appear to occupy a very humble position in the Metazoan series. He regarded them as intermediate between Protozoa and Metazoa ; but others have remarked on their resemblance to Platyhelminthes, and especially to the sporocysts of certain Flukes. They may perhaps be regarded as precociously reproductive sporocysts. It will be enough here merely to notice four types : — 1. Dicyemidse (type Dicyema) occur as parasites in Cephalopods ; the body consists of a ciUated outer layer, enclosing a single multi- nucleate inner cell, within which egg-like germs develop, apparently without fertilisation, into dimorphic embryos (see Fig. 81, A). 2. Orthonectidas (type Rhopalura) occur as parasites in Turbellarians, Brittle-stars, and Nemerteans ; the body is slightly ringed, and con- sists of a ciliated outer layer, a subjacent sheath of contractile fibres, and an internal mass of cells, among which ova and spermatozoa appear. The sexes are separate and dimorphic (see Fig. 81, B). 3. Professor F. E. vSchulze discovered a small marine organism — Trichoplax adhcsrens — in the form of a thin, three-laiyered, externally ciliated plate ; and Monticelli records a similar form under the title Treptoplax adhcsrens. But Trichoplax is now said to be the planula of the Hydromedusan Eleutheria. 4. Professor J. Frenzel discovered in brine solutions a minute Turbellarian-like organism — Salinella salve — whose body consists of one layer of cells (Fig. 82). There is an anterior mouth, a ciliated food canal, and a posterior anus. The ventral surface is finely ciliated, the other cells bear short bristles. The animal reproduces by trans- verse fission, but conjugation and encystation also occur. It must be confessed that some corroborative evidence in regard to this peculiarly simple animal is much to be desired. CHAPTER IX PHYLUM CCELENTERA Class I. Hydrozoa. Hydroids and Medusoids. Class 2. ScYPHOMEDUSvE or ACRASPEDA. Jelly-fishes. Class 3. Anthozoa or ACTINOZOA. Sea-anemones, Madrepore-corals, Alcyonarians, etc. Class 4. Ctenophora. The Coelentera — including zoophytes, swimming-bells, jelly-fish, sea-anemones, Alcyonarians, corals, and the like — form a very large series of Acoelomate Metazoa, i.e. multicellular animals without a body cavity. Their simplest forms are not much above the level of the simplest sponges, but the series has been more progressive. Thus many illustrate the beginnings of definite organs. In their variety they seem almost to exhaust the possibilities of radial symmetry, and some types {e.g. Ctenophora) may be regarded as pioneers of the yet more progressive bilateral " worms." Many are very vegetative, deserving the old name of zoophytes (which should rather be read back- wards — Phytozoa), and in their budded colonies afltord interesting illustrations of co-operation and division of labour. With the exception of three or four fresh-water forms like Hydra, all are marine. General Characters The Coelentera are almost always radially symmetrical animals in which the primary long axis of the gastriila becomes the long axis of the adult. There is no body cavity, or coelom, distinct from the digestive cavity {enteron) and its outgrowths. In the lower members of the phylum, the 167 1 68 PHYLUM CCELENTERA primary opening of this cavity becomes the mouth of the adult ^ but in the more specialised types there is an (ectodermic) oral invagination, which forms a gullet-tube or stomodteum. Between the ectoderm and endoderm of the body wall there is a supporting layer, or mesogloea, often of jelly-like con- sistency. In Ctenophora, however, a more definite mesoderm is established at an early stage in development. In the simplest cases the mesogloea is a secretion quite devoid of cells, but secondary cells may migrate into it from the endoderm. Stinging cells of varying complexity are almost always present, but in most of the Ctenophora their place is taken by adhesive cells. The Coelentera exhibit two types of structure — polypoid and medusoid — which recur in modified forms throughout the group, and may be both present in the course of one life- history, when they illustrate the phenomenon of alternation of generations , or metagenesis . The more primitive type is the sessile tubular polyp, which, at its simplest, may be com- pared to a gastrula fixed by one end, and furnished with a crown of tentacles round the central aperture of the other pole. The other derived form, which has become specialised in various directions, is the active medusoid or jelly-fish type. In several divisions the formation of a calcareous " skeleton " by the polypoid type results in the production of " corals.'* Multiplication by budding is common, and often results in the formation of colonies, some of which show considerable divi- sion of labour. The preservation of the primary axis, the absence of true mesoderm and of a ccelom, are often said to distinguish Coelentera and Sponges from the other Metazoa {Coelomata), but the results of recent researches on the nature of the meso- derm seem to rob this distinction of part of its precision. General Survey The CcElentera or " Stinging animals " include a large number of familiar and beautiful forms. The graceful zoophytes which fringe shells and stones, and the tiny transparent bells which float in the pools ; the sea-anemones which cluster in the nooks of the rocks, and the active jelly- fish which swim on the waves, are but different expressions CCELENTERATE STRUCTURE 169 of the antithesis between sedentary polypoid and active medusoid types which is characteristic of the phylum. The dehcate iridescent globes, which represent the class Ctenophora, illustrate the climax of activity, but two or three give indications of sedentariness. In our preliminary survey of the series, we may begin with the little fresh- water Hydra (Fig. 87), which is often Fig. 83. — Diagram of Coelenterate structure, endoderm darker throughout 1. To left, shows longitudinal section of Hydra ; to right, of sea-anemone, g.. Gut ; gl., incipient gullet. 2. To left, shows cross-section of Hydra ; to right, of sea- anemone, in the region of the gullet ; mesenteries not shown. 3. To left, shows vertical section of Craspedote Medusoid (with velum) ; to right, of Acraspedote Medusa (with- out velum), g.. Gut ; g/., gullet. Note anatomical correspondence of the polypoid and medu- soid forms. to be found attached to the stems- and leaves of water- plants. The structure here is extremely simple, but the simplicity is probably due to degeneration. In favourable conditions the polyp may give off daughter buds, which remain for a time attached to the parent, and then separate as independent polyps. The bud itself, before leaving the parent, may also bud, so that three generations are present. If we picture this process of gemmation, but with 170 PHYLUM CCELENTERA imperfect separation of the units, continued indefinitely, we can understand the formation of hydroid colonies, such as the zoophytes. In such cases the colony is usually sup- ported by an organic sheath (perisarc) of varying complexity. But the members of such a colony do not usually remain similar and equivalent. In Hydr actinia, for example, which often grows on a Gastropod shell tenanted by a hermit- crab, the colony consists of polyps of varied structure and function. Some of the polyps are nutritive " persons," like Hydra in appearance ; some are reproductive " per- FiG. 84. — Colony of Hydractinia on back of a Buccinum shell tenanted by a hermit-crab. sons," with rudimentary tentacles, with or without a mouth ; others are long, slender, mobile, sensitive, often abundantly furnished with stinging cells ; while the little protecting spines at the base of the colony may perhaps be abortive " persons." All these polyps are united by con- necting canals at the base. Thus Hydractinia exhibits polymorphism among the members of the colony, and a tendency towards more or less division of labour is common in the Coelentera. In most hydroid colonies the division of labour only amounts to dimorphism ; there are reproductive " persons," different from the ordinary polyps. These are in many COLONIES OF CCELENTERA 171 cases sessile and mouthless, or they may after a time become detached and float away as deHcate, pulsating swimming-bells. These swimming-bells are male and female, they give rise to male and female elements, and so to embryos, which, after a time, settle down and form new zoophyte colonies. This is an instance of alternation of generations. Again, just as the predominance of passivity is exhibited in Hydractinia and some zoophytes, where the active swimming - bell stage is left out of the life-history, so the pre- dominance of activity is exhibited in the per- manent medusoids, e.g. Geryonia^ where the sedentary hydroid stage is omitted, and the em- bryo becomes at once medusoid. Finally, the medusoids themselves may become- colonial, and we have active floating colonies, like those of the Portuguese man-of-war, which show, on a diflferent plane, as much polymorphism as Hydractinia. The same general con- clusions apply to the jelly-fish and sea-anemones. The jelly-fish present a strong resemblance to the medusoids, but are distinguished from them by their usually greater size, as well as by greater complexity and several anatomical differences. It is in accordance with this increased complexity that the alter- nation of active and passive forms, though as real, is less obvious. But even here we find one type {Pelagia) always locomotor, another (Aurelia) whose early life is sedentary, and others (Lucernarians) which in their adult life are Fig. 85. — Diagram of a typical Hydrozoon polyp. — After Allman. EC, Ectoderm ; EN., endoderm ; C, the cavity of the gut (coelenteron) ; G., a re- productive bud ; T., a tentacle ; H., hypo- stome or oral cone ; M., mouth. 1-72 PHYLUM CCELENTERA predominantly passive, and attach themselves by a stalk. The sea-anemones and their numerous allies may be regarded as bearing to the jelly-fish a relation somewhat similar to that which the hydroid polyps bear to the swimming-bells (Fig. 83). They are, however, much more complicated in structure than the hydroids. Solitary forms are much commoner than in the hydroids, but the colonial type is nevertheless very frequent. The colonies may be supported by an organic framework only, but very commonly there is a tendency to accumulate lime in the tissues, which results in the formation of " corals." It should be noted, however, that various quite distinct polypoid types may form " corals." Thus, while the most important reef-building corals are included in the Anthozoa, the Millepore-corals are hydroids. Finally, as the corals are predominantly passive, so there is a climax of activity in the Ctenophores, which move by cilia united into combs, and often shine with that " phos- phorescence " which is an expression of the intensity of life in many active animals. As to diet, many of the larger forms, e.g. sea-anemones and jelly-fish, are able to engulf booty of considerable size ; the active Ctenophores are carnivorous, attaching them- selves by adhesive cells to one another, or to other small animals ; most Ccelentera feed on minute organisms, in seizing and killing which the tentacles and stinging cells are actively used. Stinging cells or cnidoblasts are so characteristic of Coelentera that they deserve particular notice. They occur in all Coelentera except the Ctenophores, and even there they have been detected in Euchlora rubra. They also occur in some Turbellarian worms, and in the papillae of ^olid nudibranchs amongst molluscs ; but it has been shown that these animals obtain their nematocysts from the Coelentera on which they feed. Each cnidoblast contains a capsule or nemato- cyst, which encloses a coiled lasso lying in an irritant gelatinous substance. The nematocyst fills most of the cell. At the distal end of the cell there may be a trigger-like cnidocil or a fringe of bristles. At the proximal end there may be fixing processes. In some Anthozoa the coiled lasso is simply ruptured out, but in most cases it is evagin- ated. The basal part of the lasso is often stronger than the rest, and may bear barbs or stilets ; spirally arranged roughnesses and bristles are also frequent on the thread itself. The explosion of the cnidoblast is believed to be due to aji entrance of water, which causes STINGING CELLS 173 the gelatinous substance to swell up. According to others, the cnidoblast contracts as a whole. The action of the threads is mechanical and chemical. They fix, e.g. by the stilets, into the victim, and the secretion poisons the wound, paralysing or killing small animals, and sometimes acting as a solvent. Many seem to be pre- hensile threads rather than weapons. The nervous system of the Coelenterates is of a primitive type : a network of nerve-fibres runs diffusely and almost uniformly through the body ; thus, as Romanes showed, it is possible to cut a jelly-fish into a fantastic pattern, a long ribbon for instance, without preventing the conduc- FiG. 86. — Diagram of stinging-cells or cnidoblasts, the one to the right undischarged. I, Nucleus ; 2, cytoplasm ; 3, lasso or nematocyst, with barb-like processes at its base ; 4, the fluid-containing cavity of the cell in which the undischarged nematocyst lies coiled up ; 5, the trigger or cnidocil. tion of nervous impulses from one end to the other ; whereas in Vertebrates an injury to the spinal cord at once cuts off the lower part of the body from all nervous com- munication with the brain. The Coelenterates have no central nervous system, but only a nerve-net ; but in the Anthozoa and Scyphomedusae there may be regions over which the nerve-net does not extend, there may be differ- entiated organs of special sensitiveness, and there are usually certain nerve-tracts in which conduction of the impulse takes place rapidly and in a determined direction — hinting at the definite localised nerves of higher phyla. 174 PHYLUM CCELENTERA Many points in the behaviour of the Ccelenterates may be deduced from the structure of the nervous system and the absence of any co-ordinating centre ; they are " reflex repubhcs," in which any excited portion may acquire temporary dominance over the rest. Nerve-networks of a similar nature are found in the walls of the viscera of Vertebrates, e.g. Auerbach's plexus. Types of Ccelentera First Type — Hydra, a simple representative of the Class Hydrozoa General life. — The genus Hydra — cosmopolitan, like many other small fresh-water animals — is represented by several species, e.g. the green Hydra viridis, the brownish H. oligactis or fusca, and the orange H. vulgaris or grisea, widely distributed in fresh water. They are among the simplest of Ccelentera, for the body is but a two-layered tube, with a crown of (6-10) hollow tentacles around the mouth, and with no organs except those concerned in re- production. The body is usually fixed by its base to some aquatic plant, often to the lower surface of a duckweed. It may measure J- J inch in length, but it is as thin as a needle, and contracts into a minute knob. The animal sways its body and tentacles in the water, and it can also loosen its base, lift itself up by its tentacles, stand on its head, or creep by looping movements. According to some observers, its movements are helped by fine pointed pseudopodia protruded from the ectoderm cells of the tentacles and base, and by threads ejected from large cylindrical stinging cells. Usually, however, the Hydra prefers a quiet life. It feeds on small animals, which are paralysed or killed by stinging cells on the Fig. 87. — Hydra hang- ing from water-weed. — After Greene. ov., Ovary ; t., testes. STRUCTURE OF HYDRA I75 tentacles, and are swept into the tubular cavity of the body by the action of flagella on the internal cells. Sometimes animals as large as water-fleas {e.g. Daphnid) are caught, and the Hydra may sometimes be seen struggling fiercely with a small Annelid worm (Tubifex). Infusorians (EtiploteSy etc.) are often seen wandering to and fro on the surface of the Hydra, but these wonted visitors do not provoke the stinging cells to action. So simple is Hydra, that a cut-oflF fragment may grow into an entire animal. Thus the Hydra may be multiplied by being cut in pieces. The two conditions of a fragment regenerating a whole are — (i) that the fragment be not too small, and (2) that it be a fair sample of the various kinds of cells in the body. Thus neither a little corner off the base nor the tip of a tentacle will grow into a new Hydra. If the animal be turned inside out (a delicate operation), the status quo is soon restored. The Abbe Trembley, who first made this experiment, thought that the out-turned endoderm assumed the characters of the ectoderm, and that the inturned ectoderm assumed the characters of endoderm. But this is not the case. Either the animal rapidly rights itself by turning outside in, or, if this be prevented, the inturned ectoderm disappears internally, and by growing over the out-turned endoderm, from the lips downwards, restores the normal state. In favourable nutritive conditions, the Hydra forms buds, and on these a second generation of buds may be developed. A check to nutrition or some other influence causes the buds to be set adrift. Sometimes a Hydra divides across the middle, and each half grows into a complete polyp in a few days. Besides these asexual modes of multiplication, the usual sexual reproduction occurs. General structure. — The tubular body consists of two layers of cells, i.e. the animal is diploblastic. The cavity is the gut, and it is continued into the hollow tentacles. These, when fully extended, may be much longer than the body. The mouth is slightly raised on a disc or hypostome. Of the two layers of cells, the outer or ectoderm is trans- parent, the inner or endoderm usually contains abundant pigment. On the tentacles especially, even with low power, one can see numerous clumps of clear stinging cells. The 176 PHYLUM CCELENTERA male organs appear as ectodermic protuberances a short distance below the bases of the tentacles ; the ovary, with a single ovum, is a larger bulging farther down. Both male and female organs may occur on the same animal, either at one time or at different times, but often they occur on different individuals. Abundant food favours the develop- ment of female forms ; when food is scarce males are more abundant. The buds have the same structure as the parent body ; in origin they appear to be mainly due to multiplica- tion of interstitial cells. Minute structure. — The outer layer or ectoderm includes the following different kinds of cells : — (i) Large covering or epithelial cells, within or between some of which lie the stinging cells. The epithelial cells are somewhat conical, broader externally than internally, and in the interspaces lie interstitial cells. By certain methods, a thin shred can be peeled off the external surface of the ectoderm cells. This is a cuticle, i.e. a pellicle no longer living, produced by the underlying cells. (la) Many of these large cells have contractile basal processes, or roots, running parallel to the long axis of the body, and lying on a middle lamina which separates ectoderm from endoderm (Fig. 88, E). The cells themselves are contractile, but there are these special con- tractile roots. Like the muscle cells of higher animals, they contract under certain stimuli, and are often called " neuro-muscular." But the presence of special nerve cells shows that even in Hydra there is a differentiation of the two functions of contractility and irritability. (2) Stinging cells or cnidoblasts occur abundantly on the upper parts of the body, especially on the tentacles. Under stimulus, whether directly from the outside or from a nerve cell, the cnidoblast explodes and the nematocyst is thrown out. With the help of the barbs they penetrate through even a chitinous membrane, and the secreted fluid has a solvent action. The victim is held fast and drawn closer. Besides the ordinary stinging cells, there are others of small size which coil into a spiral after explosion. (3) There is to the inner aspect of the covering cells a network of ganglion cells and nerve processes. More superficially there are minute sensory cells, some of them connected by fine fibres with the ganglion cells. (4) Small interstitial or indifferent units fill up chinks in the ecto- derm, and seem to grow into reproductive, stinging, and other cells. (5) Granular glandular cells on the basal disc or " foot " probably secrete a glutinous substance. They are also said to put out pseudo- podia, and so move the animal slowly. The endoderm is less varied. Its cells are pigmented, often vacuolated, and most of them are either flagellate or amoeboid. The pigment bodies in H. viridis are like the chlorophyll corpuscles of plants ; it seems almost certain that they are unicellular Alga;. When a green Hydra liberates an egg while kept in the dark, that egg gives STRUCTURE OF HYDRA 177 rise to a white Hydra, which is supposed to imply that the partner Algse do not migrate into the egg when there is no hght. In the other species of Hydra, the pigment is quite different from chlorophyll. The active lashing of the flagella causes currents which waft food in and waste out. If some small animal, stung by the tentacles, is thus wafted in, it may be directly engulfed by the amoeboid processes of some of the cells, and it has been noticed that the same cell may be at — ^■-..^■ End /TI JtC -«^^^^-;; B Fig. 88. — Minute structure of Hydra.— Ki\.QX T. J. Parker and Jickeli. A. Ect., Ectoderm ; mg., mesogloeal plate ; %i.c., stinging cell ; End., endo- derm with flagella and amoeboid processes. B. M.C., Nerve cell, and st.c, stinging cell. C. Stinging cell with ejected thread ; n., nucleus. D. Mesogloeal plate (mg.) with contractile roots resting on it. E. m.c. Muscular cell with contractile roots, c.r. one time flagellate and at another time amoeboid (cf. the cell-cycle, Fig. 59). After this direct absorption the food is digested within the cells, and while some of the dark granules seen in those cells may be decomposed pigment bodies, others seem to be particles of indigestible debris. Thus Hydra illustrates what is called intracellular digestion, such as occurs in Sponges, some other Coelentera, and some simple " worms." But experiments show that some of the protein of the food may be digested in the gut cavity, and subsequently absorbed. Thus it seems that both intracellular and extracellular digestion occur. 12 178 PHYLUM COELENTERA There is no fundamental physiological difference between the two processes ; a food-vacuole is as certainly a " dead space " as a cavity lined by cells is. In some flagellate Protozoa digestion takes place in a single vacuole of very large size ; these may be regarded as physiologically transitional types. Some of the endoderm cells have muscular roots like those of the ectoderm. They lie on the inner side of the middle lamina, in a trans- verse or circular direction. A few cells near the mouth and base are described as glandular, and the presence of a few stinging cells has been recorded, though some suggest that the last are discharged ecto- dermic nematocysts which have been swallowed. The middle lamina, representing the mesogloea, is a thin homo- geneous plate, bearing on its outer and inner surfaces the muscular roots of ectodermic and endodermic cells (Fig. 88, D). It is historically interesting to notice the important step which was made when, in 1849, Huxley definitely compared the outer and inner layers of the Coelentera with the epiblast and hypoblast which embry- ologists were beginning to demonstrate in the development of higher animals. Not long afterwards, Allman applied to the two layers of hydroids the terms ectoderm and endoderm ; and these are now used embryologically. The division of labour among the cells of Hydra is not very strict, but already the essential characteristics of ectoderm and endoderm are evident. We use ectoderm and epiblast, endoderm and hypoblast, as synonymous. Outer Layer. Middle Layer. Inner Layer. In Hydra the ectoderm forms — Covering cells, stinging cells, nerve cells, muscle cells, etc. None in Hydra, apart from the middle lamella. In Hydra the endoderm forms — Digestive cells lining the food canal, and also muscle cells, etc. The embryonic epiblast of higher animals grows into epidermis, nervous system, and essential parts of sense organs. The mesoblast of higher animals becomes muscu- lar, connective, and skele- tal tissue. The embryonic hypo- blast of higher animals always lines the digestive part of the food canal. The reproductive organs. — [a) From nests of repeatedly dividing interstitial cells, several (1-20) simple male organs or testes are formed. Each consists merely of a clump of male elements or spermatozoa, bounded by the distended ectoderm. Through this the spermatozoa are extruded at intervals, and one may fertilise the ovum of the Hydra. In other words, self-fertilisation, which is very rare among animals, may occur. The spermatozoon is a motile cell, with a minute cylin- drical " head " consisting of nucleus, a more minute middle-piece, and a long thread-like vibratile tail (Fig. 89, i). (ft) Usually there is but one female organ or ovary, but in H. fusca as many as eight have sometimes been observed. The ovary arises, like DEVELOPMENT OF HYDRA 179 the testes, from a nest of interstitial cells, in the centre of which, distinct from the start, the single ovum lies. In rare cases in H. viridis, H. fusca, and H. grisea there are two ova ; in H. dicecia there may be several. Development. — The ovum of Hydra is the successful central cell in the ovary. It is at first amoeboid, and becomes more and more rich at the expense of its neighbours. Their remains (perhaps nuclei) accumulate within the ovum as " yolk spherules " or " pseudo-cells." Some yolk-granules, formed within the ovum, may coalesce in " pseudo- cells " of another type. With increase of size the ovum changes its \. ' '.«•' '•;'.••• '-'W' -5 o >rf3 » » • • » • • t , • • • . j . x» ••• •-/ ' -x •/ / . . • 1^>k "■+ end Fig. 89. — Development of Hydra. — After Brauer. 1. sp.. Spermatozoa. 2. Amoeboid ovum ; g.v., germinal vesicle or nucleus ; y.s., yolk spherules. 3. Ovum with lobed envelope (sh.) around it. 4. Ovum protruding ; w.. the nucleus; ec/., the ruptured ectoderm; end., the endoderm. 5. Section of blastula or blastosphere — Ect., ectoderm ; End., endoderm — being formed. 6. Section of young Hydra. Ect., Ectoderm ; End., endoderm ; g.c, gut cavity ; sh., ruptured envelopes. form from amoeboid to cake-like, and from that to spherical. Around the spherical ovum a gelatinous sheath is formed. When the limit of growth is reached, the nucleus or germinal vesicle divides twice, and two polar bodies are extruded at the distal pole. There are twelve chromosomes to begin with, and by the reduction division in forming the first polar body, the number is reduced to six. Thereafter the ectoderm of the parent Hydra yields to the increasing strain put upon it, and ruptures, allowing the ovum to protrude. By a broad base it still remains, however, attached to the parent, and in this state it is fertilised, the spermatozoon entering by the distal pole (Fig. 89, 4). l8o PHYLUM CCELENTERA The segmentation which follows is total and equal, and results in the formation of a blastula (Fig. 89, 5). By inwandering, or by division of the cells of the blastula, an internal endoderm is formed, and this formation takes place on all sides. In a word, it is multipolar. The segmentation cavity of the blastula is thus filled up, and the two layers become differentiated from one another. The outer or ectodermic layer forms — (a) an external " chitinoid " shell of several layers ; (b) an internal membrane, homogeneous, thin, and elastic ; and (c) the future ectoderm of the adult. In Hydra fusca the egg is separated from the parent before the shell is formed, and is fastened by its gelatinous sheath to aquatic plants ; in H. viridis and H. grisea the egg falls off after the outer shell has been formed. In all species the separation from the parent appears to be followed by a period of quiescence lasting from one to two months. It is probable that this resting-stage is carried by wind and birds from one water basin to another. Within the shell differentiation at length recommences, but it pro- ceeds slowly. Interstitial cells arise in the ectoderm ; a middle lamella is formed ; a gastric cavity begins to appear in the midst of the endoderm. Thereafter the shell bursts, and development proceeds more rapidly. The embryo elongates, acquires a mouth by rupture at the distal (sometimes called vegetative) pole. The inner sheath is also lost, and the young Hydra fixes itself and begins to live as its parent or parents did. Forms like Hydra. — Even simpler than Hydra is Protohydra, without tentacles, occurring both in the sea and in fresh water. An American fresh-water form {Microhydra ryderi) is known to liberate free- swimming medusoids {Limnocodium) which have been found in Europe, e.g. in the Victoria Regia tanks in the Botanic Gardens, Regent's Park, London. Another species, L. kawaii, has been found in the J angtszekiang in China, 1000 miles from its mouth. A related form, Limnocnida, occurs in Lakes Tanganyika and Victoria Nyanza, and in the river Niger. A strange simple polyp — Polypodium — has been found as a parasite on the eggs of sturgeons. Further details in regard to all these forms are much wanted. Second Type of Ccelentera. — A Medusoid. Class Hydrozoa Hydra is too simple to be thoroughly typical of the Hydrozoa. The class includes the hydroid colonies or zoophytes, which may be compared to Hydrce with many buds, and also free medusoid forms, which may be (a) liberated members of a hydroid colony, or {b) independent organisms. Besides these there are complex colonies of medusoid forms (Siphonophora). The hydroid type, except in minor details, usually resembles Hydra. In some cases the tentacles are solid, HYDROIDS AND MEDUSOIDS l8l instead of hollow as in Hydra, and they may be arranged in two circles — an outer and an inner {e.g. Tubularia). In some of the hydroid colonies, notably the Millepores and Hydr actinia, the polyps are very dissimilar to one another, and have become specialised for the performance of different functions. The medusoid type is like an inflated hydroid adapted Fig. 90. — Bougainvillea. — Alter AUman. A. A small piece of a hydroid colony, p., Perisarc ; m., medusoid bud ; h., hydranth or polyp head. B. A medusoid. ma., Manubrium; r.c, radial canal; s., sense organ. for swimming. It is bell-shaped, and down the middle of the bell hangs a prolongation — the manubrium — which terminates in the mouth. Around the margin of the bell there is a little shelf, the velum or craspedon, which projects inwards, and is furnished with muscle cells. The margin of the bell also bears tentacles, usually hollow, and abundantly furnished with stinging cells (Fig. 83, 3). l82 PHYLUM CCELENTERA On the convex surface of the hell the ectoderm forms simply an epithelial layer ; on the concave surface it is differentiated into muscle cells on the velum, the manu- brium, and the tentacles, nerve cells at the base of the velum, and stinging cells on the tentacles. The endoderm Fig. 91. — Structiire of a Swimming-bell or Medusoid, Obelia geniculata, budded off from a Campanularian Hydroid. M., Mouth on the short manubrium ; R.C., one of the four radial canals from the central stomach to the circumference canal (C.C.) round the margin ; G., gonad on radial canal (Leptomedusoid) ; T., numerous marginal tentacles, which have small internally projecting vesicles (.S.) at their base. These are not to be confused with 8 minute spherical balancing organs or statocj'sts (ST.), situated adradially on the margin. is ciliated ; it lines the food canal, and extends also into the tentacles. The mesoglcea forms a thickened jelly, present more especially on the convex (ex-umbrellar) surface. The mouth opens into the canal of the manubrium, which leads to the central cavity of the dome. With this a varying number of unbranched radial canals communicate ; these open into a marginal circular vessel, which communicates STRUCTURE OF MEDUSOID 183 with the cavities of the tentacles. A plate of endoderm lies in the mesoglcea between the radial canals. Digestion is intracellular, and probably goes on throughout the whole of this " gastro-vascular " system. The movements of the bell are caused by the contrac- tions of the ectodermic muscle cells. The nervous system consists of a double ring of nerve fibres around the margin of the bell. With these are associ- ated ganglionic cells, which apparently control the muscular contractions. Sense organs may be present, in the form of " eyes," at the base of the tentacles (Ocellatae), or in the form of " auditory " (probably balancing) vesicles developed as pits in the velum (Vesiculatae). The reproductive organs develop either in the manu- brium or on the radial canals. The products always (?) ripen in the ectoderm, and often seem to arise there ; but Weismann and others have shown that the reproductive cells of a medusoid derived from a hydroid, or of the reduced and fixed repro- ductive persons in many hydroids, have considerable powers of migration, and may originate (sometimes in the endoderm) in the hydroid colony at some distance from the place where they are matured within the medusoid bud. The sexes are usually separate. The commonest kind of free-swimming larva is the planula, which is oval, ciliated, and diploblastic, devoid of an open- ing, and usually without a central cavity. In the case of those medusoids which arise as liberated sexual members of a fixed asexual hydroid colony, the planula settles down, loses its cilia, buds out tentacles, and develops into a new hydroid. In many hydroid colonies, as has been already noticed, Fig. 92. — Structure of a Medu- soid. — After Allman. ST., Stomach ; M., manubrium ; V., velum; T., tentacle; C.C., circum- ference canal ; G., gonad ; R.C., radial canal ; EN., endoderm ; EC, ectoderm ; MG., mesoglcea. 184 PHYLUM CCELENTEKA the sexual members are not set free, but remain as buds attached to the parent. These fixed " gonophores " show many stages of degeneration ; some, notably in the floating colonies of Siphonophora, differ little structurally from true medusoids, while others, as in Hydr actinia, are simply small closed sacs enclosing the genital products (Fig. 84). Third Type of Ccelentera. — The common Jelly-fish — Aurelia aurita. Class ScYPHOMEDUSi^ This Medusa is almost cosmopolitan, and in the summer months occurs abundantly around the British coasts. It swims by pulsating its disc, and also drifts along at rest without any pulsations. They often occur in great shoals, and hundreds may be seen stranded on a small area of flat sandy beach. The glassy disc usually measures about four inches in diameter, but may be twice as large. The jelly- fish feeds on small animals, such as copepod crustaceans, which are entangled and stung to death by the long lips. External appearance. — The animal consists of a gela- tinous disc, slightly convex on its upper (ex-umbrellar) surface, and bearing on the centre of the other (sub- umbrellar) surface a four-cornered mouth, with four long much-frilled lips. The circumference of the disc is fringed by numerous short hollow tentacles, by little lappets, and by a continuation of the sub-umbrella forming a delicate flap or velarium. Conspicuously bright are the four re- productive organs, which lie towards the under surface. Nor is it difficult to see the numerous canals which radiate from the central stomach across the disc, the eight marginal sense organs, and the muscle strands on the lower surface (Fig- 93)- The three layers. — ^The ectoderm which covers the external surface bears stinging cells, sensory and nerve cells, and muscle cells. The ectoderm seems also to be invagin- ated to form the gullet or stomodaeum. The endoderm lines the digestive cavity, is continued out into its radiating canals, and is ciliated throughout. The mesoglcea is a gelatinous coagulation containing wandering amoeboid cells froin the endoderm. The whole animal is very watery ; indeed, the solid parts amount to not more than 10 per STRUCTURE OF JELLY-FISH i8s cent, of the total weight. Yet some jelly-fish (species of Rhopilema) are used as food in Japan ! Nervous system. — The nervous system consists — {a) of a special area of nervous epithelium, associated with each of the eight sense organs, and {h) of numerous much-elongated bipolar ganglion cells lying beneath the epithelium on the under surface of the disc. This condition should be con- trasted with the double nerve-ring in Craspedote medu- soids, but too much must not be made of the contrast, for a nerve-ring is described in Cubo-medusae, one of the orders of Acraspedote jelly-fish. In Aurelia the sense organs are less diff^erentiated than in many other jelly-fish. Each of the eight organs, protected in a marginal niche, consists of a pig- mented spot, a club-shaped projection with numerous calcareous " otoliths " in its cells, and a couple of apparently sensitive pits or grooves. The sense organs arise as modifications of tentacles, and are often called " tentaculocysts " or " rhopalia." The.r ^^ ^-ZllI^rRotrl''"'*'' cavities are in tree com- munication with branches of the radial canals. Muscular system. — Between the plexus of nerve cells and the sub-umbrellar mesogloea there are cross-striped muscle fibres, each of which has a large portion of non-contractile cell sub- stance attached to it. They lie in ring-like bundles, and by their contractions the medusa moves. Unstriped muscle fibres are found about the tentacles and lips. Alimentary system. — The four corners of the mouth are extended as four much-frilled lips, each with a ciliated groove and stinging cells, and with an axis of mesogloea. They exhibit considerable mobility. Their crumpled and mobile bases surround and almost conceal the mouth. A Showing four genita pockets in centre, much -branched radial canals, eight peri- pheral niches for sense organs, and peri- pheral tentacles. 1 86 PHYLUM CCELENTERA short gullet or *' manubrium " connects the mouth with the digestive cavity in the centre of the disc. From this central chamber sixteen gastro-vascular canals of approximately equal calibre radiate to the circumference, where they open into a circular canal, with which the hollow tentacles are connected. Eight of the radial canals are straight, but the other eight are branched, and thus in an adult Aurelia the total number of canals is large. These canals are really due to a partial obliteration of the gastric cavity by a fusion of its ex-umbrellar and sub-umbrellar walls along definite lines. They are all lined by ciliated endoderm. Where the gullet passes into the central digestive cavity, there are four strong pillars of thickened sub- umbrellar material. Be- side these pillars, there are four patches where the sub-umbrellar sur- face remains thin. These are the gastro - genital membranes, lined in- ternally by germinal epithelium (Fig. 94, R.). To the inside of these genital organs, within the digestive cavity, are four groups of mobile gastric filaments {g.f., Fig. 94), which are very characteristic of jelly-fish. In appear- ance these are very similar to the small tentacles of the margin, and, like them, are hollow. They are covered with endoderm — with ciliated, glandular, muscular, and stinging cells. The body is mapped out into regions by the following convention : The first tentacles to appear in the larva are four in number, and correspond to the four angles of the mouth ; the radii on which they appear are called " perradial," marked by the four lips. Half-way between these, four " interradials " are then developed, marked by the gonads and gastric filaments. Then eight " adradials " may follow, between perradii and interradii, marked by the eight unbranched radial canals. Reproductive system. — The sexes are separate. The reproductive organs — ovaries or testes — consist of plaited Fig. 94. -Vertical section of Aurelia. — After Claus. m.. Mouth ; st., stomach ; r.c, radial canal ; R., reproductive organs ; g.f., gastric fila- ments ; g.p., sub-genital cavity ; /., marginal tentacle ; s., sense organ ; the shaded part is mesogloea. STRUCTURE OF MEDUSA 1 87 ridges of germinal epithelium, situated on the four patches already mentioned, within sacs which are derived from and communicate with the floor of the gastric cavity. They are of a reddish violet colour, and at first of a horseshoe shape, with the closed part of the curve directed outwards. Afterwards the ridges become circular, and surround the walls of the sacs in which they lie. But the sub-umbrellar surface is modified beneath each genital sac in such a way that the sac comes to lie in a sub-genital cavity com- municating with the exterior {g.p.^ Fig. 94). The con- tractions of the umbrella produce a rhythmic movement of the water which enters the sub-genital cavities, and this constant renewal of the water suggests some respiratory significance for the sacs. The genital sacs containing the plaited ridges of germinal epithelium communicate with the gastric cavity only, while the sub-genital cavities con- taining water and enveloping the genital sacs communicate with the exterior only. The ova and spermatozoa pass from the frills of germinal epithelium into the sacs, and thence into the gastric cavity. They find exit by the mouth, but young embryos may be found swimming in the gastro-vascular canals, and also within the shelter of the long lips. Variations. — The jelly-fish often exhibits variations, i.e. inborn changes of germinal origin which result in the organism being different from the norm or average of its species. It is normally tetrapartite, but sexpartite, penta- partite, and, more rarely, tripartite forms occur ; and the detailed variations are manifold. Life-history. — The fertilised ovum divides completely, but not quite equally, to form a blastula, with a very narrow slit -like cavity. From the larger-celled hemisphere, single cells migrate into the cavity, and fill it up with solid endoderm. The archenteron or primitive gut cavity arises as a central cleft in this cell mass, and opens to the exterior temporarily by the primitive mouth. During these processes the embryo elongates, the outer cells become ciliated, and the mouth closes. Thus the embryo becomes a free-swimming oval planula. After a short period of free lif% this planula settles down on a stone or seaweed, attaching itself by the pole where the mouth formerly opened. At a very early stage the mesogloea appears between the two layers. At the free pole an ectodermic invagination next occurs, an opening breaks through at its lower end, and thus a gullet lined with ectoderm is formed, which hangs freely in the general cavity. During 1 88 PHYLUM CCELENTERA this process there are formed first two and then four diverticula of the general cavity, which are arranged round the gullet above, and open freely into the digestive cavity below. In the gullet region these are separated by broad septa, which are continued into the lower region of the body as four interradial ridges or taeniolae. The tentacles bud out from the region of the mouth, the first four corresponding in position to the four pouches. Interradially above the four septa, four narrow funnel-shaped invaginations arise ; these are produced by the ingrowth of ectoderm, which then forms the muscle fibres which run down the Fig. 95. — Diagram of life-history of Aurelia. — After Haeckel. I, Free-swimming embryo ; 2-6, various stages of Hydra-tuba ; 7. 8, Strobila stage ; 9, liberation of Ephytje ; 10, 1 1 growth of Ephyrae into Medusae. taeniolee (contrast the endodermic muscles of Anthozoa). In contrasting this development with that of the hydroid polyp, Goette specially emphasises the fact that the radial symmetry is first indicated by the gut pockets, and the tentacles are late in development. Goette describes a quite similar process of development in certain sea- anemones, and claims to have found there rudiments of septal pockets and ectodermal muscles, thus confirming his view of the intimate relation between the Anthozoa and Scyphomedusae. The larva now forms a " Hydra-tuba " or " Scyphistoma " , it is about an eighth of an inch in height. By lateral budding, or by the formation of creeping stolons, it may give rise to larvas like itself. DEVELOPMENT OF AURELIA 189 The gradual widening of the central cavity renders the gullet tube less obvious, and results in an increasing resemblance to the medusa type. In late autumn, however, a more fundamental change occurs in the history of the Hydra-tuba, (a) Occasionally, by a " telescoping," the Scyphistoma becomes detached ^M° Fig. 96. —Lucernaria. — After Korotneff. and converted into a free-swim- ming Eph>T:a, which in turn be- comes a jelly-fish, {b) Sometimes, in unfavourable conditions, a furrow appears roimd the upper region of the Scyphistoma, the upper portion is converted into an Ephyra, and floats away, while the lower portion re-forms its oral region by regeneration, and pro- duces another Ephyra. (c) In ordinary conditions the Scyphi- stoma elongates, and displays a succession of annular constric- tions. This stage, often compared to a pUe of discs or saucers, is called a Strobila. Each disc is separated off in its turn as a free-swimming Ephyra, which becomes a jelly-fish. The still undivided basal portion may rest for a time, and then undergo further constric- tion. This is probably an abbreviation of the primitive mode of de- velopment. In the conversion of the Scyphistoma into the Eph\T:se, the diverticula coalesce into a general cavity, the entrances to the septal invaginations probably persist as the sub -genital pits, the gastric filaments sprout out from the remains of the septa, and so mark the place where the ecto- dermal gullet passed into the endodermal cavity. The first Ephyra differs from those which come after it in bearing the original tentacles of the Hydra-tuba. From its margin eight bifid lobes grow out, each embracing the base of a perradial or interradial tentacle. The bases of these eight tentacles become the sense organs or rhopalia. The other eight adradial tentacles atrophy. On the Ephyrae which follow there are at first no tentacles, only the eight bifid marginal lobes which bear the sense organs in their niches. Fig. 97. -Diagram of Lucernaria. — After AUman. C, Cavity of gut (ccelenteron) ; F., gastric fila- ments ; H., hypostome ; G., gonad ; T., tentacle ; C.C., circumference canal. 1 90 PHYLUM CCELENTERA This development illustrates alternation of generations. From the fertilised ovum a fixed asexual Scyphistoma results. This grows into a Strobila, from which transverse buds or Ephyrae are liberated. Each of these grows into a sexual jelly-fish, producing ova or spermatozoa. Relatives of Aurelia. — The Medusa?, or true jelly-fish, include forms which agree with the Anthozoa in relative complexity of structure as compared with Hydrozoa, and in the possession of an ectodermal gullet, but differ in possessing ectodermal septal muscles and in some histological features. If Goette's discovery of rudimentary ectodermal muscles in the larvae of certain sea-anemones be confirmed, however, it would greatly increase the probability of a close relationship between the two sets. Among the Scyphomedusae closely allied to Aurelia some, e.g. Pelagia, have a direct development without the intervention of Scyphistoma or Strobila stages, but this may occur exceptionally in Aurelia. Cyanea is often very large, " it may measure j}, ft. across the bell, with tentacles 120 ft. long." Chrysaora is hermaphrodite, and has diffuse sperm sacs even upon the arms. In the Rhizostomae, e.g. Cassiopeia and Pilema, the mouth is obliterated, and replaced by numerous small pores on the four double arms. Lucernaria and its allies are interesting sessile forms which have been compared to sexual Scyphistomas, that is, are regarded as persistently larval forms (Figs. 96 and 97). Contrast between Medusoids {Hydromedusce) and Medusce {Scyphomediisce) Medusoids. (Craspedota.) The majority are small " swimming- bells." A flap or velum (craspedoii) projects inwards from the margin of the bell. No taeniolcB, nor gastric filaments. A double nerve-ring around the margin . Naked sense organs either optic or "auditory." They are usually derived from the skin, but the " auditory " sacs may be modified tentacles. Reproductive organs on the radial canals or by the side of the manubrium. The reproductive cells are usually derived from the ectoderm. With the exception of the Trachy- medusae, all arise as the liberated reproductive personae of liydroid colonies. {N.B. — "Auditory" organs are prob- ably for balancing or equilibration.) Medusa. (Acraspeda.) Many are large " jelly-fish." No velum. (The velarium of Aurelia is a mere fringe, very inconspicuous in tlie adult, and not inturned.) In the Scyphistoma there are four taeniolffi, from part of which the gastric filaments of the adult grow. Eight separate nervous centres beside the sense organs, and a sub-umbrellar nervous plexus. Sense organs are modified tentacles, and probably have almost always a triple function. They are usually protected by a hood. Reproductiv'e organs in special pockets on the floor of the gastric cavity. The reproductive cells arise in the endoderm. Have no connection with hydroids, but may have a small sedentary polyp- stage (or Scyphistoma) in the course of their life-history. Probably more nearly related to Anthozoa than to Hydrozoa. SEA- ANEMONES IQI Fourth Type of Ccelentera. — A Sea-Anemone, such as Tealia eras sic or tits. Class Anthozoa Most sea-anemones live fixed to the rocks about low- water mark. All these fixed forms have a distinct basal disc, and may, like Tealia crassicornis, be half buried in sand and gravel ; others, without a basal disc, are loosely inserted in the sand, e.g. Edwardsia and Cerianthiis. All are able to shift their positions by short stages. Some \iV^ /A Fig. 98. — External appearance of Tealia crassicornis. reef-anemones {Cradactis) can crawl about on their tentacles. They feed on small animals — molluscs, crustaceans, worms — which are caught and stung by the tentacles. Many depend on minute organisms ; others may be seen trying to engulf molluscs decidedly too large for them. A few anemones, without pigment or with little, have symbiotic Algae in their endoderm cells ; the bright pigments of many others seem to help in respiration. Besides the sexual reproduction (in which the young are sometimes developed within the parent), some sea-anemones also multiply asexually by detaching portions from near the base, and fission occurs in a few forms. 192 PHVLtJM CCELENTERA External appearance of a fixed Anemone. ^ — The cylindrical body is fixed by a broad base ; it bears whorls of hollow tentacles around the oral disc ; the mouth is usually a longitudinal slit. The tentacles are contracted when the animal is irritated, and the whole body can be much reduced in size. Just below the margin of the oral disc there is a powerful sphincter muscle ; this contracts, and pulls together the body wall over the mouth and retracted tentacles. Water may pass out gently or other- wise by a pore at the tip of each tentacle, and long white threads, richly covered with stinging cells, can be ejected in many anemones through the walls of the body (Fig. 99)- General structure.- — The Anthozoon polyp differs markedly from the Hydroid polyp — not only because an in- vagination from the oral disc inwards has formed a gullet tube, which hangs down into the Fig. 99. — Vertical section of a sea- anemone. — After Andres. /., Tentacles ; o., mouth ; oes., oesophagus ; c, c'., apertures through a mesentery ; a., a'., acontia ; g., genital organs on mesentery ; m.f., mesenteric filaments ; jn.l., longitudinal muscles ; s., primary septum or mesentery ; tCbasarS'"^^'™' ^"•' t^^t^^-^y ^^Pt""^ ; general cavity, but also because a number of partitions or mesenteries extend from the body w^all towards this gullet. Some of the partitions are " com- plete," i.e. they reach the gullet ; others are " incomplete," i.e. do not extend so far inwards. The complete mesenteries are attached to the oral disc above, to the side of the gullet, and to the base, and all the mesenteries are ingrowths of the body wall. The cavity of the anemone STRUCTURE OF SEA-ANEMONE 193 mf ml is thus divided into a number (some multiple of six) of radial chambers. These are in communication at the base, so that food particles from the gullet may pass into any of the chambers between the partitions. Moreover, each partition is perforated, not far from the mouth, by a pore, besides which there is often another nearer the body wall. The tentacles are continuous with the cavities between the mesenteries, and thus all the parts of the body are in communication. The mouth is usually a longi- a tudinal slit, and its two corners are often richly ciliated. The gullet is marked with longitudinal grooves, two of which, the *' siphonoglyphs," correspond to the corners of the mouth, and are especially broad and deep. Along these two grooves, and by these two corners, food particles usually pass in ; but in some, one side is an incurrent, the other an excurrent channel. Occasionally only one corner of the mouth and side of the gullet is thus modified. The gullet often extends far down into the cavity of the anemone. It admits of a certain amount of extrusion. The mesen- teries bear — {a) mesenteric filaments ; [h] retractor muscles ; {c) ridges of reproductive cells, almost always either ova or spermatozoa, rarely both ; and {d) in some cases oflfensive threads or acontia. The mesenteric filaments seem to be closely applied to the food, and perhaps secrete digest- ive ferments. Intracellular digestion also occurs. Sea- anemones have no sense organs ; the sapphire beads, which are so well seen at the bases of the outermost tentacles of the common Actinia mesembryanthemum, are batteries of 13 Fig. 100. — Section through sea- anemone (across arrow in Figure 99). — After Andres. A, B, Directive septa; m.f., mesenteric filaments ; g., genital organs ; m.L, longitudinal muscles ; s., primary sep- tum; s'., secondary septum; s"., tertiary septum. The arrow enters between two primary septa (an intraseptal cavity), and passes out between two tertiary septa. 194 PHYLUM CCELENTERA Stinging cells. The nervous system Is iincentralised, and consists of superficial sensory cells connected with a plexus of sub-epithelial ganglion cells. The layers of the body. — The ectoderm which clothes the exterior is continued down the inside of the gullet. The endoderm lines the whole of the internal cavity, including mesenteries and tentacles. The mesogloea is a supporting plate between these two layers, and forms a basis for their cells. The ectoderm consists of ciliated, sensory, stinging, and glandular cells, and also of sub-epithelial muscle and ganglion cells based on the mesogloea. but mainly restricted to the circumoral region. The endoderm consists mainly of flagellate cells, with muscle fibres at their roots. These form the chief muscle bands of the wall, the s s Z A Fig. ioi. — Z, Diagrammatic section of Zoantharian ; A, of Alcyonarian. — After Chun. The line S-S in Z is through the siphonoglyphs (a), the line T-T passes through two inter-septal spaces. The retractor muscles are represented by dark thickenings on the mesen- teries — all on one (the ventral) side in the Alcyonarian. The line S-S in A represents the axis of symmetry. mesenteries, and the gullet. Nor are glandular and even nerve cells wanting in the endoderm. The mesenteries. — In sea-anemones and nearly related Anthozoa, twelve primary mesenteries are first formed. These are grouped in pairs, and the cavity between the members of a pair is called intra- septal, in contrast to the inter-septal cavities between adjacent pairs. In these inter-septal chambers other mesenteries afterwards appear in pairs. Two pairs of mesenteries, however, differ from all the rest — those, namely, which are attached to the two corners of the mouth and to the corresponding grooves of the gullet. These two pairs of mesenteries are called "directive," and they divide the animal into bilaterally sym- metrical halves. Anatomically, a pair of directive mesenteries differs from the other paired mesenteries, because the retractor muscles, which extend in a vertical ridge along them, are turned away from one another, and run on the inter-septal surfaces, whereas in the other mesenteries SEA-ANEMONES 195 the retractor muscles run on the intra-septal surface — those of a pair facing one another. The arrangement of these muscles is of great im- portance in classifying Anthozoa. It is possible that the mesenteries are homologous with the tasniolae of jelly-fish, and the mesenteric with the gastric filaments. But some embryologists maintain that the mesenteric filaments are derived from the ectoderm of the gullet. From the above description it will be noticed that the fundamental radial symmetry of the Coelentera has here become profoundly modified. Development. — From the fertilised ovum a blastula may result which by invagination becomes a gastrula. In some cases the ovum segments into a solid morula ; this becomes a free planula, in which a cylindrical depression at one pole forms the mouth and gullet. Or the two layers may be estabhshed by a process known as delamination, in which a single layer of cells is divided into an inner endodermic and an outer ectodermic layer. The planula settles down by the aboral pole, and develops like a Hydra-tuba. The larva of Cerianthids is for a time pelagic, and used to be recognised as a distinct genus, Arachnactis. Related forms. — The sea-anemones are classified in the sub-class Anthozoa or Actinozoa, and along with many corals are distinguished as Zoantharia or Hexacoralla from the Alcyonaria or Octocoralla, like A Icyonium. Anthozoa or Actinozoa Zoantharia, Hexacoralla, e.g. Sea-Anemone. Many are simple, many colonial. The polyps of a colony may give rise to others directly by fission or budding. Tentacles usually simple, usually some multiple of six, often dissimilar. Mesenteries usually some multiple of six, complete and incomplete. Retractor muscles never as in Alcyo- naria. Two gullet grooves or siphonoglyphs, or only one. No dimorphism. Calcareous skeleton, if present, is derived from the basal ectoderm. Examples. Sea - anemones — e.g. Tealia and Actinia. Madrepore corals, many of them reef- building. Antipatharians. An aberrant Anti- patharian, Dendrobrachia fallax, has eight feathered tentacles. Alcyonaria, Octocoralla, e.g. Dead-Men's-Fingers. All colonial, except a small family including Monoxenia and Haimea. The polyps of a colony give rise to others not directly, but through stolons or solenia. Tentacles eight, feathered, uniform. Mesenteries eight, complete. Retractor muscles always on one (ven- tral) side of each mesentery (see Fig. loi). One (ventral) gullet groove (siphono- glyph or sulcus), or none. Frequent dimorphism among members of a colony. There are usually calcareous spicules (of ectodermic origin) in the mesogloea. Examples. Alcyonium (Dead-men's-fingers), with diffuse spicules of lime. Tiibipora (Organ - pipe coral), with spicules fused into tubes and trans- verse platforms. Coraliium rubrum (Red coral), with an axis of fused spicules. Pennatula (Sea-pen), a free phosphor- escent colony, with a " horny " axis, possibly endodermic. 196 PHYLUM CCELENTERA ZOANTHARIA The Zoantharia include many orders, e.g. the primi- tive Cerianthidea {Cerianthus, etc.) and Edwardsiidea (Edwardsta), the Actiniidea (including the typical sea- anemones and the Madreporaria), and the divergent Anti- pathidea. Making of a typical coral. — Although the term Fig. 102. — The formation of a coral shell {Astroides). — After Pfurtscheller. St., StomodiBum ; ms., mesentery ; s., calcareous septum ; B., basal plate. " coral " is applied to many different Coelenterate types with substantial calcareous skeletons, e.g. to Millepores which are Hydrozoa, and to " blue corals " and " red corals " which are Alcyonarians, the corals par excellence are the Aladreporarians. They form the coral rock and " coral islands " found in many parts of the globe, but rarely north or south of a belt extending 30° on each side of the equator, and rarely below the 40-fathom line. In a general way a Madrepore polyp is like a sea-anemone in structure, and the " coral " it forms is its external shell ZOANTHARIA 197 rather than its skeleton. It is altogether a product of the ectoderm. From one polyp others usually arise by budding or by division, e.g. Astrcea and Madrepora and Lophohelia (North Sea), but there are solitary forms such as Fungia and Caryophyllia (British). The first part of the " shell " to be formed is the basal plate between the ectoderm of the base and the substratum. FiG. 103. — Structure of Antipatharians. 1. A group of polyps — M., mouth ; t., tentacles. 2. Axis without polyps and ccenenchyma, covered with spines [Sp.). 3. Vertical section of a polyp — ^., axis; ^, tentacle; g., gullet; m., mesentery ; 0., ovary ; m.f., mesenteric filaments. 4. Cross-section of a polyp — EC, ectoderm; M., mesogloea ; EN., endoderm ; G., gullet ; MS., mesenteries. On this plate a number of radially arranged vertical ridges (septa or cnemes) are then formed, and as they grow in height they push the ectoderm of the base up before them (see Fig. 102). An external wall or theca is then formed, partly by the fusion of the outer margins of the septa and partly by a circular upgrowth from the basal plate. This theca pushes the body wall before it, as the septa pushed the base. Sometimes a second external wall or epitheca is formed outside of and concentric with the theca. By the 198 PHYLUM CGELENTERA coalescence of septa in the central line a columella or median pillar may be formed. The outer wall of the theca may bear vertical ridges or costae, and these may be connected with neighbouring costae of other polyps by horizontal shelves or dissepiments. Both septa and costae correspond to intermesenteric spaces. Antipatharians. — Usually arborescent, some- times whip-like colonies, of wide distribution, often called " black corals." A spinose hollow MMWis tmtft I. M. \J Fig. 104. — Diagrams of Types of Alcyonaria. — After Hickson. Types of Alcyonaria : — I. Of Stolonifera ; II. of Alcyonacea ; III. of Axifera; IV. of Stelechotokea, horny axis is covered with coenenchyma and regularly arranged polyps, without any trace of spicules. A polyp is usually oval in'section, with its long diameter in the line of the axis, and its gullet ALCYONARIANS 199 elongated at right angles to this. There are usually six simple non- retractile tentacles, 6-10 mesenteries, and two ill-defined siphonoglyphs. The mesenteries are without muscle-banners. The two longest, running at right angles to the elongated stoniodacum, bear gonads. The development is unknown. Examples : — Antipatkes (arborescent). Cirripaihcs (whip-like). Lciopaihes (with twelve mesenteries). Dendruhrachia (with eight pinnate re- tractile tentacles). Alcyonaria In the Alcyonarian polyp there are eight tentacles, almost always pinnate, and eight mesenteries attached to -rp Fig. 105. — Corallium rubrum, a corner of a colony. — After Lacaze-Duthiers. A., Anthocodia or retractile portion^ of a polyp; r.p., com- pletely retracted polyp, with the verruca or calyx portion left protruding ; C., ccenenchyma ; T., pinnate tentacles. the stomodaeum. In Bourne's Acrossota the tentacles have no pinnules. There is one longitudinal ciliated groove (siphonoglyph or sulcus) in the stomoda?um (ven- trally). The mesenteries bear retractor muscles, all situated on the sulcar aspect (see Fig. loi), and each mesen- tery bears a mesenteric filament. The two dorsal (asulcar) 200 PHYLUM CCELENTERA mesenteries are long, ciliated, and non-glandular ; they are respiratory in function and cause an upward current, that in the sulcus being downward. Many Alcyonarians are dimorphic, having in addition to the typical polyps {autozooids) dwarf siphonozooids , with suppressed tentacles, strongly developed sulcus, no mesenteric filaments, and often ill-developed mesenteries. Their function is to drive currents of water through the canal systems of the colony, and they are sometimes reproductive as well. With the exception of one small family of solitary forms Fig. io6.— Alcyonarian spicules. (Haimeidae), the Alcyonarians form colonies which are in various ways supported by spicules, or by spicules and an axis. The spicules, which take the most diverse forms, seem to be begun at least by ectodermic cells (a pair to each spicule), iDut they usually pass into the mesogloea. The nematocysts are usually small. A number of Alcyon- arians are viviparous ; the embryo is usually a planula. Colonies are formed in different ways, (i) A parent polyp gives off hollow stolons or solenia, which bud off new polyps, and the whole forms a spreading network or fiat plate, e.g. Clavularia, a type of Stolonifera (Fig. 104, I.). (2) The polyps may be crowded together so as to form bundles raised on a stalk, or lobose fleshy growths with the polyps projecting on the surface of a dense mesogloeal mass honeycombed by solenia, e.g. Xenia and Alcyonmm, types of Alcyonacea (Fig. 104, II.). CLASSES OF CCELENTERA 201 (3) Or the colony may raise itself in the water by forming a common upright coenenchyma, in which the polyps are embedded, and the medullary part of which may form a substantial axis of cemented spicules, e.g. Corallium, a type of Pseudaxonia. (4) Or tiie vertical extension of the colony may be effected by a horny secretion from the polyps, which comes to form an axis, really outside of the polyps though encrusted by them. This axis may be purely horny or in part calcareous, e.g. Gorgonia and Acanella, types of Axifera (Fig. 104, III.). (5) Fifthly, the vertical extension may be due to a great elongation of a single primary polyp which gives off solenia bearing numerous secondary polyps, e.g. Pennatida, a type of Stelechotokea (cf. Fig. 104, IV.). An altogether aberrant type is represented by the blue coral {Heliopora) and its extinct relatives {Heliolites, etc.). General Survey of Ccelentera Before we proceed to the systematic survey, we may contrast the essential structural features of the four classes of Ccelentera. I. In the Hydrozoa or Hydromedusae there is no inturned ectodermic gullet or stomodaBum ; there are no partitions or mesenteries ; there are no special digestive organs ; in the body wall the ectodermic muscles are mostly longitudinal and the endodermic muscles circular ; the sex cells are usually produced in the ectoderm ; there is very frequentlv a combination of polypoid and medusoid phases in the life- history ; the circumference of the medusoid bears a muscular velum of ectoderm and mesogloea ; there is no calcareous secretion (except in Millepores). II. In the Scyphomedusae there is an inturned ectodermic gullet or StomodaBum ; there are hints of mesenteries ; there are special digestive filaments ; the sex cells are endodermic ; there is no velum ; there is often a non-sexual sedentary stage ; there is no calcareous secretion. III. In the Anthozoa there is an intiurned ectodermic gullet or stomodffium ; there are distinct mesenteries or partitions from body wall to gullet wall ; there are often digestive filaments ; in the body wall the ectodermic muscles are circular (except in Cerianthus), and the endodermic muscles longitudinal ; the sex cells are endodermic ; there is no medusoid phase. IV. The Ctenophora are very divergent and apart from the other classes, e.g. in rarely having any stinging cells, and in having a well- defined mesoderm. SYSTEMATIC SURVEY Class I. Hydrozoa Solitary polyps hke Hydra, hydroid colonies or zoophytes with medusoid reproductive buds, medusoids without sedentary stages, colonies of modified medusoids. 202 PHYLUM CCELENTERA I. Order Hydromedusae.— Simple or colonial forms in which the sexually reproductive persons are either liberated as free-swimming medusoids or are sessile gonophores. (a) Hydrophora.— Two types are included here. The first mcludes the Tubularians, Hydractinia, and other forms in which the polyps are not enclosed in the protective perisarc which often surrounds the colony (gymnoblastic), and in which the free medusoid forms, when present, have Fig. 107. — Diagram of a gymno- blastic Hydroid. — After All- man. a. Stem ; b, root ; c, gut cavity ; d, endodcrni (dark) ; e, ectoderm ; /, horny perisarc ; g, hydra - like " person " (hydranth) ; g', the same, contracted ; h, hypostome bearing mouth ; k, sac-like repro- (hictivebud (sporosac) ; /.medusoid " person " ; m, a modified hydranth (blastostyle) bearing sporosacs. I. i Fig. 108. — Graptolites. I. Monograptus. II. Diplograptus. their genital organs placed in the wall of the manubrium (Antho- medusa;), and are furnished with ocelli placed at the base of the tentacles. Hydra and its allies may be included here. An unattached marine hydroid — Hypolytus peregrinus — has been described, and as it bore gonophores it was obviously mature, which is doubtful as regards two other unattached forms, Protohydra leuckartii HYDROID ZOOPHYTES 203 Fig. 109. — Hydroids. — After Hincks. Tubularia. II. A. Piece of Sertularia. II. B. A fragment enlarged, showing sessile hydrothecae (H.) on both sides of the twigs. III. A. Plumularia. III. B. A fragment enlarged, showing hydrothecae IH.) on one side of each twig, an axillary gonotheca (G.) and minute nemato- phores. IV. A. Campanularian. IV. B. A fragment enlarged, showing stalked hydrothecaB (H.), a gonotheca (G.) ; C, the ccenenchyma ; P., the perisarc ; S., a stalk. 204 PHYLUM CCELENTERA and Halermita cumulans, which may turn out to be larval. The hydroid stages of Pelagohydra and Margelopsis are free -swimming. Examples : — Syncoryne sarsii, the free medusoid of which is called Sarsm tubulosa. Bougainvillea ramosa liberates the medusoid Margelis ramosa. Cordylophora lacustris and T uhularia larynx have at- tached gonophores or sporosacs. The second type includes Campanularians and Ser- tularians along one line ; Halecids and Plumularians along another line. The protective perisarc sur- rounding the colony is con- tinued into little cups (hydrothecae) enclosing the polyps (calyptoblastic). These hydrothecae are stalked in Campanularians, sessile in Sertularians and Plumularians. The free medusoids have their gonads placed in the course of the radial canals (Lepto- medusae), and are either " ocellate " or " vesicu- late." Examples : — Plumularia, with hy- drotheca; on one side of the branches, and S ertul aria, with hydrothecae on both sides of the branches. The Campanularian Obelia geniculata liberates the medu- soid Obelia genicu- lata. {b) Hydrocorallinae. — Colonial forms which suggest the Hydractiniae in their polymorphism and division of labour, but are distinguished by their power of taking up lime, and so forming " corals." The colonies are com- plex and divergent, the reproductive persons are either sessile gonophores or simple medusoids. Millepora, Stylaster. (c) Trachymedusae. — These exist as a rule only in the medusoid form, and are divided into two groups. Trachomtdusa! and Narcomedusae, Fig. no. — Campanularian Hydroid. — After Allman. H., Hydrotheca or polyp-cup ; HY., hydranth or polyp-head ; G., gonotheca, enclosing a repro- ductive polyp producing medusoid buds ; M., a liberated medusoid ; ST., basal stolon. SURVEY OF CCELENTERA 205 according to the position of the gonads. Examples : Geryonia, Car- marina, Cunina, Aeginopsis. (Tlie fresh-water medusoid Limnocodiutn or Craspedacusta is budded oft" from the North American Microhydra ryderi). 2. Order Siphonophora. — Free-swimming colonies of modified medusoid persons (medusomes), with much division of labour. Physalia (Portuguese man-of-war), Diphyes, Velella, Porpita. Incertce sedis. Graptolites. — Extinct unattached colonies with a rod-like axis found in Upper Cambrian, Ordovician, and Silurian systems. The colony is usually linear, and consists of cup-shaped hydrothecae borne on one, two, or four sides of the solid axis {virgula). Each opens into a common median canal. At the proximal free end there is a minute triangular or dagger-shaped body — the sicula — which represents the embryonic skeleton. Some reproductive bodies or gonangia have been found. The animals were probably free- swimming in muddy seas, and of a Hydromedusan nature. Class II. ScYPHOMEDUS^ ( = Acraspcda) Jelly-fish with gastric filaments, sub-genital pits, and no velum — (i) Lucernariae. — Sedentary forms. Lucernaria, Haliclystus, and Depastrum. (2) Discomedusae. — Active forms, often with complicated life- history. Aurelia, Pelagia, Cyanea, Rhizostoma. (3) Cubomedusae. — Forms with broad pseudo-velum, and other peculiar features. Charybdea. (4) Peromedusse. — Forms with four inter -radial tentaculocysts only. Pericolpa. Class III. Anthozoa (=Actinozoa) Polypoid forms with well-developed gullet and septa, and circumoral tentacles, (i) Zoantharia or Hexacoralla. {a) Actiniaria. Sea-anemones. Actinia, Anemonia, Tealia, Cerianthus. {b) Madreporaria. Stone or reef corals. Asircsa, Madrepora, Fimgia, McBandrina. (f) Antipatharia. " Horny " black corals. Antipathes. (a) Alcyonaria or Octocoralla. Alcyonium (Dead-men's-fingers), Tubipora (Organ-pipe coral). Cor allium (Red coral)," Sea- fans, Pennatula (Sea- pen), Monoxenia (non-colonial). Class IV. Ctenophora Delicate free-swimming organisms, generally globular in form, moving by means of eight meridional rows of ciliated plates, or comb- like combinations of cilia. The stinging cells are almost always replaced by " adhesive cells." The mouth is at one pole, and leads 2o6 PHYLUM CCELENTERA into an ectodermic gullet. The gastric cavity is usually much branched. The mesodermic layer is well developed, and includes muscular and connective cells. At the aboral pole there is a sensory organ, including an " otolith," which seems of use in steering. Here, also, there are two excretory apertures. Except in Beroe and its near relatives, there are two retractile tentacles. All are hermaphrodite. The development is direct. They are pelagic, very active in habit, carnivorous in diet, and often phosphorescent. According to some, they lead on to Polyclad worms, especially through Ctenoplana and Coeloplana, two Fig. III. — Diagram of a Ctenophore. — After Chun. M., Mouth ; 5., sensory organ ; T., tentacle cut short ; SH., pouch of tentacle ; C, ciliated combs ; F., funnel or central canal ; SV., paragastric canal running parallel with stomodaeum ; G., other canals of the gut ; V., one of the meridional canals, bear- ing gonads, running under the bands of ciliated combs. curious flattened forms which crawl like Planarians. Mortensen's remarkable sessile Tjalfiella corroborates this affinit3\ Examples : — (a) With tentacles, Cydippe and the ribbon-shaped Venus' Girdle (Cestus veneris), (b) Without tentacles, Beroe. History of Coelentera. — Of corals, as we would expect, the rocks preserve a faithful record, and we know, for instance, that in the older (Palaeozoic) strata they were represented by many types. We often talk of the imperfection of the geological record, and rightly, for much of the library has been burned, many of the volumes are torn, whole chapters are wanting, and many pages are blurred. But this imperfect record sometimes surprises us, as in the quite distinct remains of ancient jelly-fish, which animals, as we know them now, are appar- ently little more than animated sea-water. We should also grasp the PEDIGREE OF CCELENTERA 207 conception, with which Lyell first impressed the world, of the uniformity of natural processes throughout the long history of the earth. Thus in connection with Coelentera we learn that there were great coral reefs in the incalculably distant past, just as there are coral reefs still. So in the Cambrian rocks, which are next to the oldest, there are on sandy slabs markings exactly like those which are now left for a few hours when a large jelly-fish stranded on the flat beach slowly melts away. On the other hand, some forms of life which lived long ago seem to have been very different from any that now remain, as is well shown by the abundant Graptolite fossils, which, though probably Coelentera, do not fit well into any of the modern classes. As to the pedigree of the Coelentera, the facts of individual life- history, and the scientific imagination of naturalists, help us to construct a genealogical tree — a hypothetical statement of the case. Thus it Fig. 112. — Hydroctena. A medusoid with suggestions of Ctenophore structure, but a true medusoid none the less. ab.o., Aboral sensory organ ; T., retractile tentacle ; v., velum ; M., month ; ST., stomach. seems very Ukely that the ancestral many-celled animals — ancestral to Sponges, Coelentera, and all the rest — were small two-layered tubular or oval forms. The many-celled animals must have begun as clusters of cells ; the question is, what sort of clusters — spheres of one layer of cells, or mouthless ovals, or little discs of cells, or two-layered thimble- like sacs ? Possibly there were many forms, but Haeckel and other naturalists were led to fix their attention especially on the two-layered sac or gastrula, because this form keeps continually cropping up as an embryonic stage in the life-history of animals, whether sponge or coral, earthworm or starfish, mollusc or even vertebrate, and also because this is virtually the form which is exhibited by the simx^lest sponges (Ascons), the simplest Coelentera {Hydra), and even by the simplest " worms " (Turbellarians). If we begin in our survey with such a gastrula-like ancestor, the probabilities are certainly in favour of the supposition that it was a free- 2o8 PHYLUM CCELENTERA swimming organism. A gradual perfecting of the locomotor character- istics might yield the two medusoid types of which we have already spoken. But we know that the common jelly-fish Aurelia has a prolonged larval stage which is sedentary, vegetative, and prone to bud. If we suppose with W. K. Brooks that many forms, less constitutionally active than others, relapsed into this sedentary state, with postponed sexuahty, and with a preponderant tendency to bud, we can understand how polyps arose, and these of two types, one nearer the jelly-fish and «! — J?> i-V. '-^t»- f :> » n -DK ' ■ Fig. 113. — Commensalism of sea-anemones and hermit-crab. Lucernarians and leading on to sea-anemones and corals, the other nearer the swimming-bell type and leading on to a terminus in Hydra. It is certainly suggestive that we have jelly-fish wholly free {Pelagia), jelly-fish with a sedentary larval life {Aurelia), jelly-fish predominantly passive (Lucernaria), and related polyps (Sea-anemones, etc.), which only occasionally rise into free activity ; while in the other series we have medusoid types always free (Trachymedusaa), others which are liberated from (Campauularian and Tubularian) sedentary hydroida, other (Sertularian and Plumularian) zoophytes whose buds though often medusoid-like are not set free, and finally Hydra, which, though it ECOLOGY OF CCELENTERA 209 may creep on its side, or walk on its head, is predominantly a sedentary animal, without any youthful free-swimming stage. Ecology. — The Coelentera are almost all marine. In fresh water we find the common Hydra, the minute Micro- hydra without tentacles, the strange Poly podium, which in early life is parasitic on sturgeons' eggs, the compound Cordylophora, occurring in canals and in brackish water, and the fresh-water Medusoids {Limnocodium and Limnocnida). Most of the active swimmers are pelagic, but there are also a few active forms in deep water. Many polyps anchor upon the shells of other animals, which they sometimes mask, and there are m.ost interesting constant partnerships between hermit-crabs and sea-anemones, e.g. between Eupagurus prideauxii and Adamsia palhata. The hermit-crab is masked by the sea-anemone, and may be protected by its stinging powers ; the sea-anemone is carried about by the hermit-crab, and may get crumbs from its abundantly supplied table. This illustrates a mutually beneficial external partnership or commensalism. In some other animals it may degenerate into parasitism (see Fig. Another kind of partnership is illustrated by many sea- anemones and Alcyonarians. Minute unicellular Algae (Zoochlorellae) live within the cells of the animals in close physiological partnership with them (symbiosis). A spatial partnership in which one animal finds habitual shelter within or near another is not infrequent; e.g. small horse-mackerels (Carangida;) swimming in shelter of large jelly-fish; a small fish [Amphiprion bicinctus) inside a giant sea-anemone (Crambactis arahica) which has a diameter of a foot ; another fish (Fierasfer) thai goes in and out of the hind-gut of Holothurians ; another that lives among the very long hair-like spines of the Red Sea rock-urchin {Diadema saxatile) ; and another {Apogonichthys strombi) that spends part of its time in the mantle cavity of the large sea-snail {Strombus gigas) of the Bahamas. The quaint little hydroid Lar sabellarum^Yives at the mouth of the tubes of the worm Sabella ; another hydroid {StylacHs minoi) grows all over the skin of a rock-perch [Minotis) from the Indian Ocean ; Stylactis vermicola was found on the back of the worm Aphrodite at the great depth of 2900 fathoms. M CHAPTER X UNSEGMENTED WORMS Phylum Platyhelminthes : Chief Classes — Turbellaria, Trematoda, Cestoda. Phylum Nemertea. Phylum Nemathelminthes : Chief Classes — -Nematoda, Nematomorpha, Acanthocephala. The title " worms " is hardly justifiable except as a con- venient name for a shape. The animals to which the name is applied form a heterogeneous mob, including about a dozen classes whose relationships are imperfectly known. It is likely that certain " worms " were the first animals definitely to abandon the more primitive radial symmetry, to begin moving with one part of the body always in front, to acquire head and sides. And if one end of the body constantly experienced the first impressions of external objects, it seems plausible that sensitive and nervous cells would be most developed in that much-stimulated, over- educated head region. But a brain arises from the insinking of ectodermic cells, and its beginning in the cerebral ganglion of the simplest " worms " is thus in part explained. Worm types begin the series of triploblastic coelomate animals, i.e. of those which have a well-defined mesoderm, and a coelom or body cavity lined with mesoderm and distinct from the gut. It must be noted, however, that the appearance of a well-developed ccelom and mesoderm is very gradual ; thus there is practically no coelom in the Platyhelminthes, and the mesoderm is sometimes not more definite than in Ctenophora. 2 lO FLAT-WORMS 211 Phylum Platyhelminthes The Platyhelminthes or flat-worms include three chief classes — Turhellarians^ Trematodes, and Cestodes — which form a related series. The body is flattened from above downwards ; the mesoderm forms a compact mass of cells or parenchyma without a definite coelom ; there is the be- FiG. 114. A. A minute portion of the branched excretory system of a Platyhehninth, showing longitudinal duct (I.), with cilia (C.), its branches (IL and IIL), and the terminal flame-cells (IV.). B. One of the characteristic hollow flame-cells, leading into the excretory tubule (i), showing the long cilia (2), the excretory globules (3), the nucleus (4), and pseudopodia-like processes (5) passing among adjacent cells. ginning of a head-brain ; the excretory system consists of a pair of lateral canals, giving off many branches, whose twigs end in peculiar " flame-cells " ; almost all are hermaphrodite. There is no doubt that the three classes, Turbellarians or Planarians, Trematodes or Flukes, and Cestodes or Tape- worms, are related to one another. A fourth class of Temnocephalids must also be admitted. It is interesting 212 UNSEGMENTED WORMS to notice that the Turbellarians and TemnocephaUds are free-Uving, except in the case of a few marine Turbellarians which have taken to parasitism ; that the Trematodes are all parasitic, either external hangers-on (ectoparasites) or internal boarders (endoparasites) ; and that the Cestodes are altogether endoparasitic. It is probable that the flukes and tape-worms arose from Turbellarian-like ancestors which adopted parasitic habits. Attention must be directed to the flame-cells which are characteristic of Platyhel- minthes. Each terminal twig of a branch of an excretory canal leads into a large hollow cell, from the base of which a bunch of cilia — with rapid movements suggesting a flickering flame — projects into the cavity towards the lumen of the twig. Class TuRBELLARiA. Planarians, etc. Turbellarians are tinsegmented " worms'' usually leaf- like, living in fresh, brackish, or salt water, or in moist earth. Almost all are carnivorous, a few are parasitic. They represent the beginning of definite bilateral symmetry. The ectoderm is ciliated, often glandular, often with peculiar rod-like bodies (rhabdites) which may be discharged on irrita- tion. A pair of ganglia in the anterior region give off lateral nerve-cords, and there are usually simple sense organs. The food canal has a protrusible muscular pharynx, is often branched, and is always blind ; digestion takes place partly or wholly within the lining cells. There are no special respiratory or circulatory organs ; the body cavity is not represented, unless it be by intercellular lacunce in the parenchyma ; the excretory system usually consists of two longitudinal canals, whose branches end internally in flame- cells. The Turbellarians are almost always hermaphrodite ; and the reproductive organs usually show some division of labour, e.g. in the development of a yolk gland, which may have arisen as an over-nourished {hypertrophied) part of the ovary. The eggs are usually enclosed in shells or cocoons, and the development may include a metamorphosis. Some forms multiply by fission. There seem to be affinities between Turbellaria and Coelentera, especially the CtenoPhora. CHARACTERS OF TURBELLARIA 213 The Turbellarian worms form an exceedingly interesting group ; they are often beautiful, and the ciliated ectoderm and well-developed muscles enable them to move with singular grace. Although the bilateral symmetry and the distinction of anterior and posterior ends is quite marked, the " mouth " or single opening of the food canal is often near the middle of the ventral surface. The anterior region is usually furnished with tactile processes. The shape of the body in the aquatic Fig. 115. — Diagram of Turbellarian. — After Lang. C, Cerebral ganglia ; E., eye ; T., tentacle ; PH., pharynx ; Mo., mouth ; M., male aperture ; P., female aperture ; the ovaries and testes are branched organs on both sides, represented b^ dots. forms is flattened and leaf-like, as in the delicate Leptoplana, the " living film " found on the shore-rocks. Fresh-water forms are usually small and often minute, but those living in the sea may attain a length of six inches, though most are small. Land Planarians are elongated and more worm-like in shape ; they may measure a foot or more in length, and are most abundant in tropi-zal countries. Some, like Planaria, have so much regenerative capacity that half a dozen or more may be produced by cutting one into pieces. 214 UNSEGMENTED WORMS Classification. — Order i. Rhabdocoelida — small fresh-water and marine forms. The food canal is very slightly branched, or quite straight, or blocked. Rhabdocoela. With straight intestine, e.g. Microstoma, a fresh- water genus. It is first male and then female (protandrous hermaphrodite) ; it forms temporarily united asexual chains, sometimes of sixteen individuals, suggesting the origin of a segmented type. Grafilla and Anoplodium are parasitic on Gastropods. Among the Vorticidae allied to Grafilla we may notice Provortex tellincB in Tellina and a related form in the cockle. Alloiocoela. With irregular caeca on the gut, e.g. Allostoma. All marine except one from Swiss lakes {Plagiostoma lemani) and Bothrioplana. Accela. Without intestine, e.g. Convoluta, which contains green symbiotic alg«. Marine. Order 2. Tricladida. Elongated flat " Planarians " with three main branches from the gut, e.g. Planaria and Dendroccelum (fresh-water), the former sometimes dividing transversely ; Polycelis nigra, a common fresh-water form ; Gunda (Procerodes) segmentata (marine), showing hints of internal segmentation ; Geodesmus and Bipalium (in damp earth) ; Bipalium kewense is an import often found in Britain. Order 3. Polycladida. Large leaf-like marine " Planarians," with numerous intestinal branches diverging from a central stomach, e.g. Leptoplana (not uncommon on the seashore), Thysanozoon. Class Temnocephaloidea The Temnocephalids are flattened forms, e.g. Temnocephala, found clinging to fresh-water animals, especially Crustaceans ; there is a large ventral sucker ; the epidermis is a nucleated syncytium (i.e. without distinct demarcation into cells) which secretes a thick cuticle, contains rhabdites, and rarely bears cilia. The class seems to be intermediate between Rhab- docoelid Turbellaria and Trematodes. Symbiotic algse. — Of all the numerous Invertebrates which harbour symbiotic algae within their bodies the best studied is the Acoelan Convoluta, thanks especially to Keeble and Gamble. In C. roscoffensis the algae are green {Zoochlorellce) ; like other green plants, they utilise the energy of sunlight to build up complex organic com- pounds from carbon dioxide, with evolution of oxygen. Both the oxygen and the elaborated compounds (food- stuffs) are valuable to the host ; for the greater part of its life C roscoffensis does not feed for itself, but lives on the SYMBIOSIS 215 products of its symbionts. In C. paradoxa the algae are yellow {Zooxanthellce), but their function is the same ; this species does seek food on its own account, but it cannot live without the help of the algae. Zoochlorellae occur in many Rhizopods and Ciliates, in fresh-water Sponges, in Chlorohydra, and in some Rotifers ; Zooxanthellae occur in many Rhizopods and most Radiolarians, and in very many Ccelenterates, especially Anthozoa. In many cases the host can probably survive without the symbionts, but they undoubtedly help it, especially in times of starva- tion. Sometimes, in unfavourable circumstances, the host will kill the goose that lays the golden eggs by digesting the algae. In return for their services to the host, the algae make use of the carbon dioxide and nitrogenous waste products of the host's metabolism. Class Trematoda. Flukes, etc. The Trematodes are leaf-like^ or sometimes cylindrical external or internal parasites. With their parasitic life may be associated the absence of cilia on the surface of the adults, the thick " cuticle'' the presence of attaching suckers {occasion- ally with hooks), and the rarity of sense organs. After embryonic life the ectoderm degenerates, ceases to be distinctly cellular, and sinks inwards. It is likely that they have arisen from free Turbellarian-like ancestors, and they resemble the Turbellarians in being unsegmented, in having anterior ganglia, from which nerves pass backward and forward, in the rudimentary nature of the body cavity, in the ramifying system of fine excretory canals, in the hermaphrodite and usually complex reproductive system. The excretory and nervous systems are, however, more complex than those 0^ Turbellaria. The alimentary canal is usually forked, often much branched, and always ends blindly. In many cases the animals are self -impregnating, but cross- fertilisation also occurs. The development of the external parasites is usually direct, of the internal parasites usually indirect, involving alternation of generations. They occur on or in all sorts of Vertebrates, but those which have an indirect development, and require two hosts to complete their life-cycle, often pass part of their life in some Invertebrate. 2l6 UNSEGMENTED WORMS Type The Liver Fluke {Distomum hepaticutn) The adult fluke lives as a parasite in the liver and bile ducts of the sheep, causing liver-rot." flukes, it occasional sometimes e.v FiG.i 1 6. —Structure of liver fluke. — After Sommer. From ventral surface branched gut (g.) and the lateral nerve [l.n.) are shown to the left, the branches of the excretory vessel {e.v.) to the right. m.. Mouth ; ph., pharynx ; g., lateral head ganglion ; v.s., ventral sucker ; c.s., position of cirrus sac. An arrow indicates the ex- cretory aperture. Unlike most has many hosts — it occurs in cattle, horses. deer, camel, antelopes, goat, pig, beaver, squirrel, kangaroo, and rarely in man. The animal is flat, oval, and leaf-like, almost an inch in length by half an inch across the broadest part, reddish brown to greyish yellow in colour. As the word Distomum suggests, there are two suckers — an an- terior, perforated by the mouth ; a second, im- perforate, a little farther back on the mid-ventral line. There is a muscular pharynx and a blind alimentary canal which sends branches through- out the body. The food is the blood sucked from the liver of the host. From a ganglionated The collar round the pharynx, nerves go forward and backward ; of those which run backward, the two lateral are most im- portant. Although the larva has eye spots to STRUCTURE OF LIVER FLUKE 217 Start with, there are no sense organs in the adult. The body cavity is not represented unless it be by minute P- c.s — ^ -<^' t^ ^^^-^ Fig. 117. — Reproductive organs of liver fluke.— After Sommer. ov. Ovary (dark). ut. Uterus. c.s. Cirrus sac. /. Female aperture. s.v. Seminal vesicle. y.gl. Diffuse yolk glands. sh.g. Shell gland. v.d. Vas deferens. T. Testes (anterior). p. Penis. m. Mouth. g. Anterior lobes of gut. intercellular spaces in the body parenchyma. Into these there open the internal ciliated ends of much-branched excretory tubes (see Figs. 114 and ii6), which unite 21 8 UNSEGMENTED WORMS posteriorly in a terminal vesicle opening to the exterior. The reproductive system is hermaphrodite and complex. From much-branched testes, spermatozoa pass by a pair of ducts (vasa deferentia) into a seminal vesicle lying in front of the ventral sucker. Thence they are expelled by an ejaculatory duct, which passes through a muscular protrusible penis. The retracted penis and the seminal vesicle lie in a space or " cirrus sac " between the ventral sucker and the external male genital aperture. The ovary is also branched, but less so than the testes. The ova pass from its tubes into an ovarian duct. Nutritive cells are gathered from very diffuse yolk glands, collected in a reservoir, and pass by a duct into the end of the afore- said ovarian duct. At the junction of the yolk duct and the ovarian duct there is a shell gland, which secretes the " horny " shells of the eggs, and from near the junction a fine canal (the Laurer-Stieda canal) seems to pass direct to the exterior, opening on the dorsal surface. The meaning of this is still somewhat uncertain. In some flukes it is said to be a copulatory duct ; in others it is regarded as a safety valve for overflowing products. From the junction of the ovarian duct and the duct from the yolk reservoir, the eggs (now furnished with yolk cells, accompanied by spermatozoa, and encased in shells) pass into a wide convoluted median tube, the oviduct or uterus, which opens to the exterior at the base of the penis. Self-fertilisation is probably normal, but in some related forms cross-fertilisation has been observed. Life-history. — The fertilised and segmented eggs pass in large numbers from the bile duct of the sheep to the intestine, and thence to the exterior. A single fluke may produce about 50,000 embryos, which illustrates the prolific reproduction often associated with the luxurious conditions of parasitism, and almost essential to the con- tinuance of species whose life-cycles are full of risks. Outside of the host, but still within the egg-case, the embryo develops for a few weeks, and eventually escapes at one end of the shell. Those which are not deposited in or beside pools of water soon die. The free embryo, known as a miracidium, is conical in form, covered with cilia, provided with two eye-spots, and actively locomotor. By means of its cilia it swims actively in the water for some hours, but its sole chance of life depends on its meeting a small amphibious water-snail (Limnceiis truncatulus or minutus), into which it bores. In an epidemic among horses and cattle in the Hawaiian Islands, the host was L. oahuensis ; in the same locality the host may be DEVELOPMENT OF DISTOMUM 219 Fig. 118. — Life-history of liver fluke. — After Thomas. I, Developing embryo in egg-case ; 2, free-swimming ciliated larva ; 3, sporocyst ; 3a, shell of LimncBus truncahdus ; 4, division of sporo- cyst ; 5, sporocyst with redis forming within it ; 6, redia with mor rediae forming within it ; 7, tailed cercaria ; 8, young fluke. 220 UNSEGMENTED WORMS L. peregra ; in Victoria Bulimus tenuistriatus . This diversity of host, also remarkable in the adult, is very unusual. Within the snail, e.g. in the pulmonary chamber, the miracidium settles down, loses its cilia, increases in size, and becomes a sporocyst. The sporocyst is a hollow sac, with a slightly muscular wall and with the beginnings of an excretory system. Sometimes this sporocyst divides transversely (Fig. ii8 (4)). Within the sporocyst a few cells behave like partheno- genetic ova. Each segments into a ball of cells or morula, which is invaginated into a gastrula, and grows into another form of larva — the redia. These rediae burst out of the sporocyst, and migrate into the liver or some other organ. Each sporocyst usually forms at a time 5-8 rediae ; each of these forms 8-12 more rediae ; and each of these forms 14-20 cercariae. In the winter a sporocyst may give rise to cercariae directly. A redia is a cylindrical organism with a short alimentary canal, excretory canals with " flame cells," and a pair of blunt locomotor processes posteriorly. A cercaria has a bifurcated gut, two suckers, a locomotor tail, and the beginnings of gonads (Fig. 118 (6)). The cercariae emerge from the rediae, wriggle out of the snail, pass into the water, and after swimming for a short time, moor themselves to stems of damp grass. There they lose their tails and become encysted. If the encysted cercaria on the grass stem be eaten by a sheep, the cyst is dissolved in the stomach, and the young fluke makes its way up the bile duct and its tributaries. In about six weeks it grows into the adult sexual fluke. It will be noted that the sporocyst is the modified embryo, but that it has the power of giving rise asexually to redia?. These develop, however, from special cells of the sporocyst, which we may compare to spores or to precociously developed parthenogenetic ova. Though the reproduction is asexual, it is not comparable to budding or division. The same power is possessed by the rediae, and there are thus several (at least two) asexual generations between the embryo and the adult. The disease of liver -rot in sheep is common and disastrous. It has been known to destroy a million sheep in one year in Britain alone. Classification. — Order i. Heterocotylea, with a posterior ad- hesive organ, often with a pair of accessory suckers beside the mouth. LIFE-HISTORY OF LIVER FLUKE 221 RtDlAE 5POROCY5T Fig. 119. — Diagram of life-cycle of liver fluke. Upper quadrant, adult in sheep ; />/;., pharynx ; s., sucker; g., gut. Right quadrant, free-swimming larva with eye-spots {e.). Lower quadrant, sporocyst and rediae in water-snail ; R., redia within sporocyst or within redia ; g., gut in redia ; C, cercariae in redia. Left quadrant, free cercaria ; g., gut ; s., sucker ; t., tail. 222 UNSEGMENTED WORMS Most are ectoparasitic. The development is direct and associated with one host (monogenetic). e.g. Polystonium integerrimum, with many posterior suckers, often in the bladder of the frog. It attaches itself in its youth to the gills of tadpoles, passes thence through the food canal to the bladder. Gyrodactylus, on the gills and fins of fresh-water fishes. It is Fig. 1 20. — Male and female Bilharzia — Schistosomum hcemato- bium. — After Looss. The male is about three- fifths of an inch long ; the female (F.), carried in the ventral groove or gynaeco- phoric canal {G.C.), is four- fifths. S., Anterior suctorial mouth ; P.S., the adhesive sucker. On the surface of the male's body there are numerous minute papillae. viviparous, but the embryo, before it is extruded, itself contains an embryo, and this in turn another. Diplozoon paradoxum consists of two individuals united. The single larva (Diporpa) is at first free-swimming, but becomes a parasite on the gills of a minnow, and there two individuals unite very closely and per manently. Tristomum, with three suckers, on some marine fishes. ORDERS OF TREMATODA 223 Order 2. Aspidocotylea, with a large sucker occupying most of the ventral surface. Development is direct, and there is one host. e.g. Aspidogasier in Molluscs. Order 3. Malacotylea, with never more than two suckers. The development is indirect and requires two hosts, the adult usually frequenting the gut of a vertebrate. e.g. Disiomum, with numerous species. Schistosomum (Bilharzia) hcematohium, a parasite of man, widely distributed in Africa, e.g. in Egypt. It occurs in the portal vein, the blood vessels of the bladder, etc., 'causing inflammation, ha;maturia, stone, etc. The embryos are passed out in the urine. The intermediate host is a fresh-water snail {e.g. Biilinus). There is no redia. The bifid microscopic Cercaria usually enters man by the skin. The pain is due to the sharp corners of the egg-shells, which have terminal spines. Another species, S. mansoni, is intestinal, and the eggs, which have a lateral spine, pass out with the faeces. The young stages occur in Planorbis, etc. The cercariae die in 36 hours in water kept quite still, or may be killed by a little sulphate of soda. Monostomum, with one sucker ; adult in ducks, young in fresh-water snail, Planorbis. The relationships of the Trematodes are on one hand with the free-living Turbellarians, on the other hand with the parasitic Cestodes. Class Cestoda. Tape-worms The Cestodes are internal parasites., whose life-history includes a bladder-worm (proscolex) and a tape-worm (strobila) stage, the former in a Vertebrate or Invertebrate host, the latter {with one exception) in a Vertebrate. In a few cases the body is unsegmented, e.g. Archigetes and Caryophyllaeus, with one set of gonads ; in a few others, e.g. Ligula, there is a serial repetition of gonads without distinct segmentation of the body ; in most cases, e.g. Taenia and Bothriocephalus, the body of the tape-worm forms a chain of numerous joints or proglottides, each with a set of gonads. Thus the class in- cludes transitions from unsegmented to segmented forms, but the latter are imperfectly integrated? The general form of the body is tape-like and bilaterally symmetrical, with anterior hooks, grooves, or suckers ensuring attachment to the gut of the host. The body wall consists of a cuticle and a well-innervated epidermis, within which there is parenchymatous connective tissue, often with cortical deposits of lime, and at least two sets {longitudinal and transverse) of unstriped muscles. The nervous system consists of two or more longitudinal nerve- 224 UNSEGMENTED WORMS Strands and anterior commissures ; there are no special sense organs. There is no alimentary system ; the parasite floating in the digested food of its host absorbs soluble material by its general surface. There is no vascular nor respiratory system, and a body cavity is represented merely by irregular spaces in the solid parenchymatous tissue. In some of these spaces there are '' flame- cells,'' which lie at the ends of the fine branches of lofigitudinal excretory tubes, which are united in a ring in the head, are connected transversely at each joint, and open terminally by one or more pores. All tape-worms are hermaphrodite, and most, if not all, are probably self -fertilising . The male reproductive organs in- clude diffuse testes, a vas deferens, and a protrusible terminal cirrus. The female organs include a pair of ovaries, yolk glands, a shell gland, a vagina by which spermatozoa enter, a receptacle for storing spermatozoa, and a uterus in which the ova develop. The embryo develops within another host into a proscolex or bladder-worm stage, which forms a " head " or scolex. When the host of the bladder-worm is eaten by the final host, the scolex develops into an adult sexual tape-worm. With the conditions of endoparasitic life may be associated the occurrence of fixing orgafis, the absence of sense organs, the low though somewhat complex character of the nervous system, the entire absence of a food canal, and the prolific reproduction. Life-history of Taenia solium. — This is one of the most frequent of the tape- worms infesting man. In its adult state it is often many feet in length, and is attached by its " head " to the wall of the intestine. The head bears four suckers and a crown of hooks, and buds off a long chain of joints, which develop complex reproductive organs as they get shunted farther and farther from the head. The last of the joints or proglottides is liberated (singly or along with others), and passes down the intestine of its host to the exterior. It has some power of muscular contraction and of movement, and it is distended with little embryos within firm egg-shells. When the proglottis ruptures, these are set free. In certain circumstances, the embryos, within their firmly resistant egg-shells, may be swallowed by the omni- vorous pig. Within its alimentary canal the egg-shells are dissolved, and embryos (hexacanths) bearing six anterior TAPE-WORMS 225 hooks are liberated. They bore their way from the in- testine into the muscles or other structures, and there encyst. They lose their hooks, increase in size, and become passive, vegetative, asexual " bladder-worms." A bud from the wall of the bladder or proscolex grows into the cavity of the same, and forms the future " head " or Fig. 121. Front end of the head of Tcenia solium, showing four adhesive suckers and a circle of fixing hooks. The natural size is that of an ordinary pin's head. scolex. This is afterwards everted, ajid then the bladder- worm consists of a small head attached by a short neck to a relatively large bladder. When man unwittingly eats " measly " pork — that is, pork infested with bladder-worms — an opportunity for further development is afforded. The bladder is lost, and is of no importance, but the '* head " or scolex fixes itself to the wall of the intestine. There it is copiously and 15 226 UNSEGMENTED WORMS richly nourished, and buds off asexually a chain of joints. As these joints are pushed by younger interpolated buds 7'.S Fig. 122.- -Diagram of reproductive organs in Cestode joint. Constructed from Leuckart. ov., Ovary, with short oviduct ; «/., " uterus " ; /., diffuse testes ; sh.g., shell glared; y.g., yolk gland; v.d., vas deferens; v., vagina ; r.s., receptaculum seminis ; I.e., longitudinal excretory ducts ; I.e., transverse bridges connecting these. The dotted lines above and below represent the anterior and posterior borders of the proglottis. Note that the so-called uterus is blind ; it opens to the exterior in a few tape-worms, and is perhaps the homologue of the Laurer-Stieda canal of Trematodes. farther and farther from the head, they become sexually mature. The ova are fertilised, apparently by spermatozoa LIFE-CYCLE OF TAPE-WORM 227 from the same joint ; the joint becomes distended with developmg embryos. These ripe joints are hberated, the Fig. 123— Life-history of Tcznia solium.— Aitev Leuckart '' S'^,h^°'^'^d embryo in egg-case ; 2, proscolex or bladder-worm atef 'hrad T^T'^^^'^^^J 3, bladder-worm wit?"g™ embryos are set free by rupture, and the vicious circle may recommence. Happily, however, the chances are 228 UNSEGMENTED WORMS many millions to one against the embryo becoming an adult. The above history is true, mutatis mutandis, for many other tape- worms. The embryo grows into a proscolex or bladder, which buds off a scolex or head, which, in another host, buds off the chain of proglottides. HEAP Fig. 124. — Diagram of Ufe-history of TcBnia solium. First chapter : Tape-worm in man ; H., head ; PR., proglottides. Second chapter : Free proglottis and egg-cases ; ut., uterus ; g.a., genital aperture ; embryo within the egg-case. Third chapter : Within the intermediate host, the pig ; H., hexacanth embryo ; p.sc, proscolex or bladder-worm ; m., muscle of pig ; sc, scolex or head, everted in final host. As it is virtually the same animal throughout, the life-history does not include an " alternation of generations." It is doubtful, however, what term should be applied to those cases in which the bladder-worm (Ccenurus and Echinococcus) forms not one head only but many, each of which is capable of becoming an adult tape-worm. The only known eTtception to the fact that sexual tape-worms are parasites of Vertebrates is Archigetes sieboldii, a simple cestode which is sexual within the small fresh-water oligochaete Tubifex rivulorum. TAPE-WORMS 229 Representative Life-Histories Adult, Sexual, or Tape-worm Stage. 1. Tcenia solium, in man, with four suckers and many hooks. The joints are elongated ; the ripe uterus shows coarse branching. 2. TcBnia saginata, in man, with four suckers, but no hooks. The joints are markedly elongated ; the ripe uterus has many slender branches. 3. Bothriocephalus latus (Dibothrio- cephalus), in man, with two lateral groove-like suckers, but no hooks, with less distinct separation of the pro- glottides than in Tcenia. The joints, which are short and wide, show less distinct separation. The ripe uterus is somewhat stellate. The total length of the chain may be as much as 11 yards. Common in Finland and Switzer- land. 4. Echinococcifer echinococcus, in dog, wolf, jackal. Very small, with three joints behind the head, which bears four suckers and two rows of barbed booklets. 5. Tcenia ccenurus, in dog. 6. Tcenia serrata, in dog. 7. Dipylidium caniniim (T. cucumer- ina), in cat and dog ; head with hooks and four suckers ; joints ovoid, with genital aperture at both margins. 8. Moniezia, the broad tape-worm of sheep and cattle. Non-Sexual, Proscolex, or Bladder- worm Stage. 1. Cysticercus cellulosce, in muscles of the pig. 2. Bladder-worm in cattle. 3. The ciliated, free-swimming em- bryo becomes a parasite in the muscles of pike, trout, burbot, etc., but without a distinct bladder-like stage. It is worm-like in appearance, and called a plerocercoid larva. 4. Echinococcus veterinonim, in sheep, cattle, pigs, etc., and sometimes in man, producing brood capsules, which give rise to many " heads." 5. Cosmirus cerebralis, causing sturdie or staggers in sheep, with numerous " heads." Also in cattle, goat, horse, etc. 6. Cysticercus pisiformis, in rabbit. 7. In lice and fleas. 8. Life-history unknown. Zoologically the cestodes are interesting, on account of their life- histories, the degeneration associated with their parasitism, the pre- valence of self-impregnation, and the complexity of the reproductive organs. Practically they are of importance as parasites of man and domestic animals. Classification. — The class Cestoda includes a number of families : — Cestodariidae. No joints, one set of gonads. e.g. Archigetes, Caryophyllceus, Amphilina, Gyrocotyle, Bothriocephalidae. Two weak fiat suckers ; genital openings usually on the flat surfaces. e.g. Bothriocephalus ; Ligula, with no suckers or joints but with serial gonads. 230 UNSEGMENTED WORMS Tetrarhynchidse. With four protrusible proboscides armed with hooks, parasites of fishes. Found also in Sepia. e.g. Tetrarhynchtis. The finest pearls in the Ceylon pearl oyster are formed round a larval Tetrarhynchus. Tetraphylhdae. With four very mobile suckers. e.g. Echeneibothrium, Phyllo- bothrium. Taeniidae. With four suckers, often with apical hooks, with margi- -j^ nal genital apertures. e.g. Tcsnia. General Note on Platyhelminthes The four classes, Turbellaria, Trematoda, Cestoda, and Temno- cephaloidea, constitute the Platy- helminthes or Flat -worms — an in- teresting group, because its members illustrate so well the progressive degeneration associated with increas- ing parasitism, and also because of the relatively great simplicity. The four classes are nearly related, for forms like Temnocephala connect Turbellaria and Trematoda, and the " monozoic " Cestodes like Archi- getes, Amphilina, CaryophyllcBus, and Gyrocotyle connect Trematoda and Cestoda. It is probable that both Cestodes and Trematodes arose from a Turbellarian stock. Among the most striking of the Platyhelminth characters are the nature of the excretory and repro- ductive organs and the condition of the mesoderm. The excretory system, with its longitudinal trunks, its ramifying canals, and " fiame-cells," is characteristic. The reproductive Fig. 125. — Diagrams of bladder-worms. I. The ordinary Cysticercus type, with one head (H.). II. The Coenurus type, with many heads. III. The Echinococcus type, with , , j- ■ • many heads, and with brood organs are complex, show division capsules producing many of labour, and are furnished with ^^^^s- ducts of their own, unconnected with the excretory system — a condition not common in worms. The presence of shells around the eggs is another point of interest. It becomes of great importance to the parasitic flukes and tape-worms, but occurs also in the free-living Turbellaria. The formation of yolk cells from a specialised part of the ovary (yolk gland) is also noteworthy. There is no true body cavity, RIBBON-WORMS 231 —po the space between gut and body wall being filled with a packing tissue ; the absence of an anus is also important, the two characters taken together being held to indicate affinity with the Ctenophora. Class Nemertinea. Phylum Nemertea. The ribbon-worms or Nemertincs are interesting in many ways, e.g. in being the simplest animals ^^^^^^^—pp to have an open gut, a closed blood- system, and, occasionally, haemoglobin ; in having some very peculiar structures, notably a protrusible proboscis and ciliated head slits ; in being in many cases extraordinarily extensile and liable to break into pieces. The Nemertines are worm-like animals, unsegmented and generally elongate in form ■ they are almost all marine, and most, if not all, are carnivorous. The ectoderm is ciliated. There is a remarkable retractile proboscis, uncon- nected with the alimentary canal, and forming a tactile organ or a weapon. The nervous system consists of a brain, a com- missure round the proboscis, and two lateral nerve-cords ; in connection with the brain there is a pair of ciliated pits. The gut terminates in a posterior anus, and is furnished with lateral pockets. ' There is no body cavity in the adult, but the closed vascular system is probably of coelomic origin. The excretory system is apparently of the Platyhelminth type. The sexes are usually separate and the si -^ Fig. 126. — Diagrammatic longitudinal section of a Nemertine (Amphiporus ladifloreiis), dorsal view. — After M'Intosh. p.p., Proboscis pore ; b., brain giving off the latera^ nerve-cords (n.) ; po., oesophageal pocket ; p., pro- boscis Iving within its sheath ; St., stilet of proboscis ; m., retractor muscles of proboscis ; g., gut shown in outline at the sides of the proboscis ; e., the three main longitudinal blood vessels, which unite both anteriorly and posteriorly. 232 UNSEGMENTED WORMS organs simple. The development is in some cases direct, while in others there is a peculiar pelagic larva. General Account of Nemertines In appearance most Nemertines are ribbon- or thread-like, and the cross-section is generally a flattened cylinder. They show a greater diversity of size than any other " worms " — from a Linens, 12 or more d.n P Fig. 127. — Transverse section of the Nemertine Drepanophorus latus. — After Biirger. d.n., Dorsal or proboscis nerve ; P.s., proboscis sheath ; P.c, proboscis cavity ; P.s'., sac of proboscis cavity ; d.v.m., dorso-ventral muscles ; cm., circular muscles ; l.m., longitudinal muscles ; l.n., lateral nerve with branches ; P., parenchyma ; g., gut ; l.v., lateral blood vessel, beside which lies an excretory vessel ; E.p., excretory pore ; d.v'., dorsal blood vessel ; Ep., epidermis. feet in length (25 metres has been recorded for an extended Lineus longissitnus), to the pelagic Pelagonemertes, which is under an inch. The colours are often bright, and tend to resemble those of the sur- roundings. The ectoderm is covered with numerous short cilia, and many of its cells are also glandular, secreting the mucus, which often forms a tube around the animal, or is exuded in movement. Beneath the epidermis there is a parenchyma, consisting in part of connective tissue, and often in part gelatinous. The body is remarkably con- tractile, and in some cases the spasms result in breakage. The muscles are circular and longitudinal, and often also diagonal. The fibres are striped. In the adult there is no distinct coelom, the space between RIBBON-WORMS 233 the gut and the body wall being filled up with gelatinous connective tissue. In the larva?, however, a body cavity may be seen, either as an archiccfile, i.e. the persistent segmentation cavity {IJneus obscurus), or as a schizocoele, i.e. a space formed by the cleavage of the mesoderm into two layers (Pilidium-larvse). In the adult only the blood spaces and the cavity of the proboscis sheath are coelomic. The nervous system" consists of a brain generally four-lobed — the two lobes of each side being closely united and connected with those on the other side by a commissure above and by another below the proboscis cavity. From the lower lobes two longitudinal nerve-stems run along the sides, and are sometimes united posteriorly above the anus (Fig. 126, n.). In some forms there is in addition a dorso-median nerve, and sometimes a ventro-median nerve. On each side of the head there is a ciliated pit communicating with the exterior through an open slit or groove, and communi- cating internally either with the brain itself or with ad- jacent nervous tissue. In those cases in which the de- velopment has been studied, these so-called lateral organs arise from ectodermic insink- ings and oesophageal out- growths. In the most primi- tive genus, Carinella, they are absent, except in one species. It has been sug- gested that they conduce to the respiration of the brain, which is rich in haemoglobin, and they have even been compared with gill-slits. In some forms the groove through which they open to the exterior is rhythmically gested that they are sensory are very sensitive; ficial nerve plexus. eyes and eye spots are general ; and in some there are otocyst-sacs. Apart from the cephalic $hts, the head also bears sensory pits and grooves and terminal sensory spots. In some there is a pair of lateral sense organs in the (anterior) excretory region. The mouth is ventral, and leads into a plaited glandular fore-gut or cBsophagus, which is followed by a straight, ciliated mid-gut (stomach and intestine), usually with regularly arranged lateral casca. Between the caeca run transverse muscle partitions. The anus is in most cases terminal. In a cavity along the dorsal median line there hes the remarkable proboscis. It is protruded and retracted through an opening above, or, in a few cases, within the mouth. It arises in the body wall and is surrounded by a cavity (rhynchocoelom) bounded by Fig. 128. — Transverse section of a simple Nemertine {Carinella). — After Biirger. d.n., Dorsal nerve ; p.c, proboscis cavity ; g., gut ; cm., circular muscles ; l.m., longitudinal muscles ; d.v.m., dorso-ventral or diagonal muscles ; l.v., lateral blood vessel. contractile. It has also been sug- Apart from these organs, Nemertines and in many this is associated with a super- Tactile papillae and patches are often present ; 234 UNSEGMENTED WORMS a muscular proboscis sheath. The proboscis is a muscular, richly innervated tube hned with glandular epithelium, sometimes protruded with such force that it separates from the body. It has been compared in its retracted state to a glove-finger drawn in by two threads attached to its tip, the threads being retractor muscles which are fastened posteriorly to the wall of the proboscis sheath. But in front of the attachment of the retractor muscles there is a non-eversible glandular region which secretes an irritant fluid. In many cases there are stilets at the tip of the eversible portion, and if these be absent, there are adhesive papillae. There is a hint of a similar structure in some Rhabdoctjel Turbellarians, and the organ may be interpreted as origin- ally tactile, secondarily aggressive. It is protruded by the muscular contraction of the walls of the proboscis sheath, which forms a closed cavity surrounding the proboscis, and containing a fluid with corpuscles (Fig. 126). In the majority there are three longitudinal blood vessels or spaces, a median and two laterals, which unite anteriorly and posteriorly, and also communicate by numerous transverse branches. The vessels or spaces are remnants of a ccelom. The blood is a coloiirless fluid, sometimes at least with nucleated elliptical corpuscles in which haemo- globin may be present. The excretory system usually consists of two coiled ciliated canals opening in the anterior region by a varying number of ducts. They are said to divide up internally into numerous fine branches ending in flame- cells, or in blind ampuUee embedded in the walls of the blood vessels. The sexes are usually separate, and the reproductive organs are always simple. A few species (of Geonemertes and Prosadenophorus) are hermaphrodite, and some species of Tetrastemma are protandrous. The organs consist of simple sacs, arranged in a series on each side between the intestinal cseca, and communicating with the exterior by fine pores. The ova are often laid in gelatinous tubes, and are probably fertilised shortly before or at the time of expulsion. In three or four forms {Prosorhochmus, a fresh- water Tetrastemma, a species of Linens) known to be viviparous, the fertilisation must, of course, be internal. Segmentation is total and almost always equal ; a complete or partial gastrula is formed, and development may be direct or indirect. In Cerebratulus, etc., the larva is adapted for pelagic life, and is known as the Pilidium. " In external shape it resembles a helmet with spike and ear lobes, the spike being a strong and long flagellum or a tuft of long ciha, the ear lobes lateral ciliated appendages " (Hubrecht). Out of this, somewhat abruptly, the adult form arises. Relationships. — The Nemertines are probably nearly related to Turbellaria, but show some very distinct marks of advance. Of these, the most noticeable are the presence of an anus, of a closed vascular system, of a coelom at least in the larva. The presence of flame-cells in connection with the excretory system confirms the idea of Platy- helminth affinities ; but it is to be noticed that the reproductive system is strikingly different. Professor Hubrecht has suggested that Nemertines exhibit affinities with Vertebrates, comparing proboscis sheath with notochord, and so forth. RIBBON-WORMS 235 Classification. Order Protonemertini. Brain and lateral nerves outside the muscular layers ; mouth behind brain ; no stilets. Carinella, Hubrechtia. Order Mesonemertini. Lateral nerves in the muscular layer ; mouth behind brain ; no stilets. Carinoma, Cephalothrix. Order Metanemertini. Mouth in front of brain, usually opening along with proboscis ; usually with stilets ; lateral nerves internal to the muscular layers ; usually with an intestinal caecum. e.g. Amphiporus, Drepanophorus, Tetrastemma. An isolated form, Malacobdella, parasitic in bivalves, has a posterior sucker, a coiled intestine, and other peculiarities. Order Heteronemertini. Mouth behind brain ; no stilets ; three layers of muscle, the outermost and innermost longitudinal ; lateral nerves outside circular muscular layer. e.g. Lineus, Cerebratulus.^ Habits. — Most Nemertines are marine, creeping about in the mud, under stones, among seaweed, and the hke ; many, e.g. Cerebratulus, are able to swim ; Pelagonemertes and Planktonemertes are leaf-Hke hyaline forms of pelagic habit ; two or three species of Prostoma live in fresh water ; seven species of Geonemertes are terrestrial ; Malacobdella and a few others live in the mantle-cavity of marine bivalves, and some others are found as commensals in Ascidians ; Cephalothrix galathece destroys the eggs of its host — the crustacean Galathea. Most seem to be carnivorous, eating annelids, molluscs, and even small crustaceans. Many break readily into pieces when irri- tated, and some are able to regenerate what they lose in this way. The fresh-water Prostoma lumbricoides forms a pro- tective cyst of mucous threads in unfavourable conditions, and Tetrastemma dorsale often does the same along stems of the hydroid Tubularia. 236 unsegmented worms Phylum Nematohelminthes Class Nematoda, e.g. Ascaridae. Class Nematomorpha, Gordiidae. Class Acanthocephala, e.g. Echinorhynchus Class Nematoda. Thread-worms, Hair-worms, etc. The Nematodes are unsegmented, more or less thread-like " worms,"" some free-living and others parasitic. The body is covered by a cuticle, often thick, usually striate, often subject to moulting ; the muscular system consists of elongated muscle- cells arranged longitudinally, and usually leaving two free " lateral lilies." From a nerve-ring around the gullet, six or so nerves go forwards and also backwards. The gut is usually well developed, with mouth and anus, and is divided into three regions. Vascular and respiratory systems are unrepresented ; the cavity of the body is not coelomic ; the remarkable excretory system consists of two lateral canals opening anteriorly by a single pore. The sexes are usually separate and the reproductive organs simple ; there is distinct sexual dimorphism . The males have usually copulatory spicules, and sometimes a membranous bursa. The vulva may be any- where on the ventral surface, often well forward. The life-history is often intricate. There are many remarkable features such as the sluggish amoeboid spermatozoa, the almost complete absence of cilia and flagella, and the absence of migratory phagocytes. Type, Ascaris megalocephala, the Round- worm of the horse This round-worm occurs in the small intestine of the horse, while other species similarly infest man, ox, pig, etc. The body is cylindrical in cross-section and tapering at each end. The colour is dead-white, the absence of pigment being very characteristic of Nematodes. Some of the small thread-worms, e.g. Trichostrongylus pergracilis in the caeca of the grouse, are quite transparent- and almost invisible when alive. At the anterior end is the mouth, furnished with three lips bearing sense papillae ; the anus is posterior and ventral. The male is smaller than the female, and has a recurved tail furnished with two horny spines and numerous sense papillae. It is usually about seven inches long, while the female may be as much as seventeen. ROUND-WORMS 237 {a) Most externally there is a thick chitinoid cuticle, perhaps of protective value. With its presence may be associated the scarcity of cutaneous glands, and the absence of cilia, (b) Beneath this is the sub-cuticula or epidermis, thickened along four longitudinal lines — median, dorsal, ventral, and lateral — and consisting of a protoplasmic matrix without distinct cell-limits. Except at the tail-end the nuclei are confined to the longitudinal lines, and are most Tl.C ovo Fig. 129. — Cross-section through Ascaris. — From a speoimen. DN., Dorsal nerve; n.c, non-contractile portion of muscle cells; C, cuticle: £., epidermis; LL., lateral line; EV., excretory vessel ; A/.', contractile portion of muscle cells ; VN., ventral nerve ; OV., ovary ; LT., uterus ; G., gut. numerous laterally. The epidermis makes and remakes the cuticle, which is periodically moulted, (c) Beneath the epidermis is a layer of remarkable muscle cells, lying in groups defined by the lines mentioned above. Many of the Nematodes are very agile. Around the pharynx there is a nerve-ring from which six nerves run forwards and six backwards. One runs along the median dorsal line — a unique position in an 238 UNSEGMENTED WORMS Invertebrate. Here and there on the ring and on the nerves there are ganghonic cells, but there is but little aggregation of these into ganglia. Sense organs are represented by the papillae already mentioned. As the food consists of juices from a living host, it is not surprising to find that the alimentary canal has but a narrow cavity. It consists of three parts — a fore-gut or oesophagus, lined by the inturned cuticle, a mid-gut or mesenteron of endodermic origin, and a usually short hind-gut or rectum lined by the cuticle. When the external cuticle is shed, so is that of the fore- gut and hind-gut (cf. Crayfish). There is a distinct space between gut and body wall, but it is lined externally by the muscle cells, internally by the endoderm of the gut, which has no mesoblastic coat ; the space is therefore not strictly coelomic. It contains a clear fluid, which probably discharges some of the functions of blood. There are no free amoeboid phagocytes. Embedded in each lateral line there is a longitudinal canal. These unite anteriorly, and open in a ventral excretory pore near the head. They seem to be associated internally with fixed phagocytic cells. In the species dis- cussed there are four giant branched cells situated anteriorly, which are especially con- nected with taking up waste particles. The relation of this excretory system to that of other Invertebrates is unknown. The sexes are separate. In the male the testis is unpaired — a coiled tube Fig. 130. — Diagram of the structure of a male Nematode. M., Mouth; CE., oesopha- gus; GA., nerve ring; B., bulb at lower end of fore-gut ; G., mesen- . _p - . . teron ; SP., spme with gradually difterentiatmg mto vas de- ejSuiatory 'duett Vs. "i fcrcus, scmiual vcsiclc, and ejaculatory seminal vesicle;' T.', duct. The genital apcrturc is close to testis ' £, i , ion*^itucli- O 1 nai excretory tube, cut the auus. The spcrmatozoa have not short; EP., excretory ^^^ typical form, and are sluggish. Their movement within the female ducts appears to be due to flagella-like villous processes from the walls. In the female the ovary is a paired tube, pore. NEMATODES 239 which passes gradually into an oviduct and a uterus at each side, and a short unpaired vagina. The genital aperture is ventral and anterior. The ova meet the spermatozoa at the junction of uterus and oviduct. Segmentation is total, and resuhs in the formation first of a blastula and then of a gastrula. The germ-cells are distinguishable very early from the body- cells. Blastopore and archenteron are obliterated, the mid-gut arising as a secondary splitting between two rows of endoderm cells. The eggs pass out of the gut of the host and probably hatch in water, and are thus re-intro- duced. No intermediate host is known. There is evidence that the larvae of Ascaris in some hosts exhibit an extensive migration within their host before settling down to mature in the intestine. The same may be true in man. Though parasitism is exceedingly common among Nematodes many are free-Hving for at least a part of the life-cycle, and feed on putre- fying organic matter. Although the number of individuals which may infest one host shows how successful the parasitism is, yet Nematodes exhibit few of the ordinary adaptations to a parasitic Ufe, and there is no sharp structural line of demarcation between free and parasitic forms. Some, like Ascaris, secrete an irritating toxin. Among histo- logical pecuharities, the practically complete absence of cilia — paralleled elsewhere only among the Arthropods — the nature of Hhe muscle-cells, the condition of the subcuticular layer, are to be noticed. Among the grosser structural pecuharities, the nature of the excretory system, of the cavity of the body, and of the nervous system, are worthy of special note. Sense organs are never well developed, but in the free-living forms simple eyes may occur. The alimentary canal is usually completely developed, but may, as in Sphcsrularia, be degenerate. Of the relationships nothing is known. Life-Histories 1. The embryo grows directly into the adult, and both live in fresh or salt water, damp earth, and rotting plants— Enoplida, e.g. Enoplus. 2. The larvce are free in the earth, the sexual adults are parasitic in plants, or in Vertebrate animals, e.g. Tylenchus scandens, a common parasite on cereals ; Strongyhis and Dochmius in man. 3. The sexual adults are free, the larvae are parasitic in insects, e.g. Merniis. The fertilised females of Sphcerulana bombi pass from the earth into the body cavity of humble-bee and 240 UNSEGMENTED WORMS wasp, whence their larvae bore into the intestine and eventually emerge. 4. The larvae are parasitic in one animal, the sexual adults in another which feeds on the first. Thus Ollulanus passes from mouse to cat, Cucullanus from Cyclops to perch. There are other life-histories, and many degrees of parasitism. The most remarkable form is Angiostomum (or Ascaris or Leptodera) nigrovcnosum. In damp earth males and females occur, the progeny of which pass into the lungs of frogs and toads. There they mature into hermaphrodite animals (the only example among Nematodes), which produce first spermatozoa and then ova. They are self-impregnating, and the young pass out into the earth as males or females. Here there is alternation of generations : and a somewhat similar story might be told of Rhahdonema strongyloides from the intestine of man, and Leptodera appendiculata from the snail. There are several quaint reproductive abnormalities, thus — the female SphcBvularia bonibi, which gets into the body cavity of the humble-bee, has a prolapsed uterus, larger than itself : the male of Trichodes crassicauda passes into the uterus of the female. Parasitic Nematodes Trichinella (Trichina) spiralis is a formidable parasite in man, pig, and rat, but it has been introduced experimentally into hedgehog, fox, dog, cat, rabbit, ox, and horse. The sexual forms live in the intestine, the female about 3 mm. in length, the male less than half as long. After impregnation the female brings forth numerous embryos viviparously, sixty to eighty at a time, and altogether about 1500. These are produced in the wall of the intestine, or in the adjacent lymphatic spaces. Most of them find their way into lymph and blood vessels, and are swept by the blood stream to the muscles ; occasion- ally some seem to migrate actively, boring their way especially through connective tissue. The migration causes inflammation and fever. In the muscle fibres they grow, coil themselves spirally, and become encysted within a sheath, at first membranous and afterwards cal- careous (Figs. 131 and 132). The cyst is partly due to the muscle, and partly to the parasite. The infected muscle fibre degenerates. In these cysts, which may be sometimes counted in millions, the young Trichinae remain passive, unless the flesh of their host be eaten by another — pig eating rat, man eating pig. In the ahmentary canal of the new host the capsule is dissolved, the embryos are set free, and become in two or three days reproductive. The male seems to die after copulation. Among the numerous other parasitic Nematodes the following may be noted : — The giant pahsade worm {Eustrongylus gigas) occurs in the renal region of domestic animals, etc. ; the female may be 3 ft. long. The armed palisade worm {Strongylus armatus) occurs in the intestine and intestinal arteries of horse, causing aneurysms, colic, etc. The THREAD- WORMS 241 young forms are swallowed from stagnant water, bore from gut into arteries, become adult, return to gut, copulate and multiply. Various other species of Strongylus occur in sheep, cattle, etc. Of the genus Ascaris alone, over 200 species have been found in all types of Verte- brates : — A. megalocephala in horses, A. lumbricoides in man, A. mystax in cats and dogs. Syngamus trachealis occurs in the trachea of birds, gapes," e.g. in poultry and pheasants. It pierces the wall causmg Fig. 131 . — Trichinae in miiscle, about to be encapsuled. — After Leuckart. Fig. 132. — Trichinae in muscle, encapsuled. There may be 12,000 in a gramme of pig's muscle. — After Teuckart. of the trachea, and " actually clenches the teeth with which its mouth is provided in the tracheal rings." A remarkable large form, Ichthyo- nema grayi, is found inside sea-urchins. Various species of Tylenchus, especially T. devastatrix and T. scandens (or T. tritici), destroy cereal and other crops. Various species of Heierodera (especially H. schachtii and H. radicicola) infest the roots of many cultivated plants, e.g. turnip, radish, cabbage. There is evidently a great variety of habit and habitat among Nematodes, and yet the general structure is very uniform. They seem to represent a homogeneous class, very much by themselves, and not nearly related to other types, even to the other Nematohelminthes. 16 242 UNSEGMENTED WORMS Some of the most Important Forms Parasitic in Man Name. Position. i History. Kesvlt on Host. A scaris lumbri- Usually in small Repeated experi- Conuiionest in coides, maw-worm intestine. ment has shown that children ; rarely (common). infection results if dangerous, unless [A . mystax, com- the eggs (with their very numerous, or mon in dogs and outer envelope en- maturing in other cats, has also been tire) are swallowed parts of the body, found in man. J along with vegetable e.g. respiratory food or otherwise. tract, bile duct, .\fter hatching, the vermiform appen- larva? may be dis- dix. Like others, tributed in the blood it may puncture stream. the wall of the gut and liberate patho- From food or genic bacteria. Oxyuris vermi- Frr.ni stomach Rarely more than cular is (common). to rectum, mostly water. discomfort. in colon. Trichocephalus Colon ; more dispar or trichi- rarely appendix iirus, the whip- and small intestine. worm (common). A nchylostomum Small intestine. The larva^ live Ulceration, hae- diiodenaie (widely freely in the earth. morrhage,and dan- distributed). Infection usually gerous anaemia. Necator, another through the skin. Serious sapping of closely related vitality in warm " Hookworm." Filaria bancrofti Mature female countries. Larvae in a mos- Elephantiasis and (Australia, China, (80-100 mm.) in quito, haematuria. | India, Egypt, and lymphatic glands, Brazil). embryos in blood. .Males rare (30-45 mm.). Dracuncuhis (Fil- The female is 1-6 Larvae in a Cy- Subcutaneous aria) medinensis ft. long, encysts clops, abscesses. | (Guinea-worm), in beneath skin, es- .Arabia, Egypt, pecially of back or Abyssinia, etc. legs. Male rarely seen. Trichinella spir- Becomes sexually From " trichi- Inflanunatory pro- lii':, widely dis- mature in the in- nosed " pig's mus- cesses, often fatal, tributed. testine ; embryos. cle to man. are brought about produced rapidly by the migration and viviparously. of the young worms find their way to [ from intestine to muscles, and be- muscles. 1 come encysted. 1 1 CLASSES OF NEMATOHELMINTHES 243 Class Nematomorpha The Gordiidae (e.g. Gordius aquaiicus — the horse-hair worm) are so different from true Nematodes that they must be ranked in a separate class. There are no lateral lines. Three nerve-strands lie close together in the mid-ventral line. In the adult Gordius the mouth is shut and the food canal is partly degenerate. The adult Gordiidae usually live freely in fresh water ; larval forms occur in aquatic molluscs, young insects, etc. ; later stages usually occur in carnivorous insects, whence they emerge to become adult in the water. One form, Nectonema agile, is marine. Class Acanthocephala For a few genera, of which the best known is' Echinorhynchus, whose larvae live in Arthropods, and the adults in Vertebrates, a special class, Acanthocephala, has been established. They may be placed beside Nematodes, but the relationship does not seem to be very close. Mouth and gut are absent. The anterior end bears a protrusible hooked proboscis used in boring in the intestinal wall of the host. In the minute swellings at the ends of the two much-branched excretory organs of E. gigas, there are ciliated cells — the only case known among Nematohelminthes. Echinorhynchus proteiis of pike, minnow, trout, etc., larva in the Amphipod Gammarus pulex. „ angustafus of perch, larva in the Isopod Asellus aquaticus. „ moniliformis of rat, etc., larva in larval beetles {Blaps). „ gigas of pig, larva in grubs of cockchafer, etc. Desiccation Many of the smaller Nematohelminthes are able to survive prolonged drying up or desiccation. The body may become quite brittle, and yet replacement in water brings about revivification — even after years. This state of latent life is of great theoretical interest, for the living matter loses most of its water-content and passes out of the colloid state. CHAPTER XI PHYLUM ANNELIDA Chief Classes — Ch^topoda, Hirudinea or Discophora The Annelids or Annulata are segmented worms, in most of which the segmentation of the body is visible externally. The head usually consists of a pre-oral " prostomium " and a post-oral peristomium. The body wall has several layers of muscles , and many, e.g. Chcetopods, have setce embedded in the skin. In most, there is a well-developed caelom, communicating with the exterior by paired nephridia. The nervous system consists typically of two dorsal cerebral ganglia, a commissural ring round the gullet, and a ventral ganglionated chain. The gonads arise on the coelomic epi- thelium. Not infrequently the nephridia function also as genital ducts. The development may be direct or indirect, and if indirect it usually includes a larval Trochosphere stage. In habit, form, and structure the AnneUds exhibit much diversity. The Chaetopods, represented by the famihar earthworms and by the marine worms, are most typical. With these may be included the aberrant Echiuridae, e.g. Echiurus and Bonellia. A few primitive forms (Archi- Annelida), and the Myzostomata (parasitic on Crinoids), may also be appended. The leeches (Discophora) are divergent. Further, some zoologists include Chaetognatha in this series as Annelids with three segments, and also the Rotifers (Rotatoria), whose adult form somewhat resembles the Trochosphere larvae of many Annelids. Finally there are associated in an uncertain way Sipunculids, Gephyreans, Phoronids, Polyzoa, and Brachiopods. 244 BRISTLE-FOOTED WORMS Class Ch^topoda. Annelids with Bristles 245 Segmented animals with setce developed in little skin-sacs, either on a uniform body wall or on special locomotor pro' trusions known as parapodia. The segments, indicated externally by rings, are often marked internally by parti- tions running across the body cavity, which is usually well developed. The nervous system generally consists of a double ventral chain of ganglia, connected with a pair of dorsal cerebral ganglia by a ring round the beginning of the gut. Two excretory tubes or nephridia are typically present in each segment, and they or their modifications may also function as reproductive ducts. The reproductive elements are formed on the lining membrane of the body cavity. The development is either direct or with a metamorphosis. Type of Oligoch^ta. The Earthworm (Lumbricus) Habits. — Earthworms eat their way through the ground, and form definite burrows, which they often make more comfortable by a Hning of leaves. The earth swallowed by the burrowers is reduced to powder in the gut, and, robbed of some of its decaying vegetable matter, is discharged on the surface as the familiar " worm-castings." By the burrowing the earth is loosened, and ways are opened for plant-roots and rain-drops ; the internal bruising reduces mineral matter to more useful form ; while, in covering the surface with earth brought up from beneath, the earth- worms have been ploughers before the plough. Darwin calculated that there were on an average over 53,000 earthworms in an acre of garden ground, that 10 tons of soil per acre pass annually through their bodies, and that they cover the surface with earth at the rate of 3 in. in fifteen years. He was therefore led to the conclusion that earthworms have been the great soil-makers, or, more precisely, that the formation of vegetable mould was mainly to be placed to their credit. Though without eyes, earthworms are sensitive to light and persistently avoid it, remaining underground during the day, unless rain floods their burrows, and reserving 246 PHYLUM ANNELIDA their active life for the night. Then, prompted by " love " and hunger, they roam about on the surface, leaving on the moist roadway the trails which we see in the morning. smmm^;v \f 'i o -t-> l-l CO O More cautiously, however, they often remain with their tails fixed in their holes, while with the rest of their body they move slowly round and round. The nocturnal peregrina- tions, the labour of eating and burrowing, the transport of STRUCTURE OF EARTHWORM 247 leaves to their holes, the collection of little stones to pro- tect the entrance to the burrows, include most of the activities of earthworms, except as regards pairing and egg- laying, of w^hich something will afterwards be said. When an earthworm is halved with the spade, it does not neces- sarily die, for the head portion may grow a new tail, while a decapitated worm may even grow a new head and brain. Phagocytes help as usual in the regeneration. The earth- worm is much persecuted by numerous enemies, e.g. centi- pedes, moles, and birds. The male reproductive organs are always infested by uni- cellular parasites — Gregar- ines of the genus Monocystis ; and minute thread - worms (Pelodera pellio) usually occur in the nephridia and body cavity, and often in the ventral blood vessels. Form and external characters. — The earthworm is often about 6 in. long, with a pointed head end, and a cylindrical body rather flat- tened posteriorly. The successive rings seen on the surface mark true segments. The mouth is overarched by a small lobe called the pro- stomium, and the food canal ter- minates at the blunt posterior end. The skin is covered by a thin trans- parent cuticle, traversed by two sets of fine lines, which break up the light and produce a slight iridescence. On a region extend- ing from the 31st to the 38th ring, the skin of mature worms is swollen and glandular, forming the clitellum or saddle, which helps the worms, as they unite in pairs, and also forms the slimy stuff which hardens into cocoons. The middle line of the back is marked by a special redness of the skin. On the sides and ventral surface we feel and see four rows of tiny bristles or setaj, which project from little sacs, are worked by muscles and assist in locomotion. These bristles are fixed like pins into the ground, at times so firmlv that even a bird finds it difficult to pull the worm from its hole. " As each of the four longitudinal rows is double, there are obviously eight bristles to each ring. On the skin of the ventral surface there are not a few special apertures, which should be looked for on a full-grown worm ; but careful examination of several specimens Fig. 134. — Anterior region of earthworm. — After Hering. Note the eight setaj (s.) on each segment. R.S., Spots between 9-10, lo-ii. indicate openings of receptacula seminis ; Ovd., openings of oviducts on segment 14 ; y.^., openings of vasa deferentia on segment 15. 248 PHYLUM ANNELIDA is usually necessary. Almost always plain on the 15th ring are the two swollen lips of the male ducts, less distinct on the 14th are the apertures of the oviducts through which the eggs pass, while on each side, between segments 9 and 10, 10 and 11, are the openings of two receptacula seminis or spermotheca; into which male elements from another earthworm pass, and from which they again pass out to fertilise the eggs of the earthworm when these are laid. Each segment con- tains a pair of excretory tubes, which have minute ventral-lateral apertures, while on the middle line of the back, between the rings, there are minute pores, through which fluid from the body cavity may exude on to the skin. Skin and bristles. — The thin cuticle is produced by the cells which lie beneath, and is perforated by the apertures previously mentioned. The epidermis clothing the worm is a single layer of cells, of which most are simply supporting or covering elements, while many are slightly modified, as glandular or mucous cells, and as nervous cells. As the latter are connected with afferent fibres which enter the nerve-cord, the skin is diffusely sensitive. In a few species the skin is slightly phosphor- escent. The chitinous bristles, which are longest on the genital segments, are much curved, and lie in small sacs of the skin, in which they can be replaced after breakage. Muscular system and body cavity. — The earthworm moves by the contraction of muscle cells, which are arranged in circular hoops and longitudinal bands under- neath the skin. The special muscles about the mouth and pharynx have considerable powers of grasping, while less obvious muscular elements occur in the wall of the gut, in the partitions which run internally between the segments, and on the outermost portions of the excretory tubes. Unlike the leech, the earthworm has a very distinct body cavity, through the middle of which the gut extends, and across which run the partitions or septa incompletely separating successive segments. In this cavity there is some fluid with cellular elements, of which the most numerous are yellow cells detached from the walls of the gut. Possible communications with the exterior are by the dorsal pores, and also by the excretory tubes, which open internally into the cavities of the segments. Nervous system. — ^Along the middle ventral line lies a chain of nerve-centres or ganglia, really double from first NERVOUS SYSTEM OF EARTHWORM 249 to last, but compactly united into what to unaided eyes seems a single cord. As the segments are very short, the limits of the successive pairs of ganglia are not very evident, especially in the anterior region, but they are plain enough on a small portion of the cord examined with the micro- scope, when it may also be seen that each of the pairs of ganglia gives off nerves to the walls of the body. Anteriorly, just behind the mouth, the halves of the cord diverge and ascend, forming a ring round the pharynx. They unite above in two dorsal or cerebral ganglia, which are situated in the peristomium or first ring, and not, as in Polychaetes, in the prostomium. These form the earthworm's " brain," and give off nerves to the adjacent pre-oral lobe or pro- stomium, on which are numerous sensitive cells. These, coming in contact with many things, doubtless receive impressions, which are transmitted by the associated nerves to the " brain." As Mr. Darwin observed that earthworms seized hold of leaves in the most expeditious fashion, taking the sharp twin leaves of the Scotch fir by their united base, we may credit the earthworms with some power of profiting by experience ; moreover, as they deal deftly with leaves of which they have no previous experience, we may even grant them a modicum of intelligence. From the nerve- collar uniting a dorsal ganglia with the first pair on the ventral cord, nerves are given off to the pharynx and gut, forming what is called a " visceral system." The earth- worm has no special sense organs, but there are abundant sensitive cells, especially on the head end. By them the animal is made aware of the differences between light and darkness, and of the approaching tread of human feet, not to speak of the hostile advances of a hungry blackbird. The sense of smell is also developed. The afferent or sensory nerve fibres from the nervous cells of the skin enter the nerve-cord and bifurcate into longitudinal branches, which end freely in the nearest ganglia. In this the earth- worm's nervous system suggests that of Vertebrates. The nerve cells, instead of being confined to special centres or ganglia, as they are in Arthropods, also occur diffusely along with the nerve fibres throughout the course of the cord. Along the dorsal surface of the nerve-cord there run three peculiar tubular " giant fibres,"' with firm walls and clear contents. They are probably comparable to the medullated nerve fibres of Vertebrates. 250 PHYLUM ANNELIDA Alimentary system. — Earthworms eat the soil for the sake of the plant debris which it may contain, and also because one of the modes of burrowing involves swallowing the earth. In eating they are greatly helped by the muscular nature of the pharynx ; from it the soil passes down the gullet or oesophagus, first into a swollen crop, then into a strong-walled grinding gizzard, and finally through a long digestive and absorptive stomach-intestine. There are three pairs of oesophageal glands. Canals from the posterior two pairs open into the anterior pair, and thus into the gullet. Their contents are Hmy, and perhaps counteract the acidity of the decaying vegetable matter. It may be that they are in part excretory ; or it may be that they serve to fix some of the carbon dioxide formed by the animal. The long intestine has its internal surface increased by a dorsal fold, which projects inwards along the whole length. In this " typhlosole," and over the outer surface of the gut, there are crowded yellow cells. There is no warrant for calling the yellow cells hepatic or digestive. Structurally they are pigmented cells of the peritoneal epithelium, which here, as in most other ani