n Pentlcmd's Students Manuals. OUTLINES OF ZOOLOGY. NUNQUAM ALIUD NATURA, AL1UD SAPIENTIA DIC1T. OUTLINES OF ZOOLOGY BY J. ARTHUR THOMSON, M.A. REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN THIRD EDITION , REVISED AND ENLARGED, WITH 332 ILLUSTRATIONS. EDINBURGH AND LONDON: YOUNG J. PE NT LAND. 1899. Edinburgh: printed for young j. pentland, ii teviot place, and 38 WEST SMITHF1ELD, LONDON, E.C., BY MORRISON AND GIBB LIMITED [All rights Reserved .] PREFACE TO THE THIRD EDITION. The favourable reception granted to the former editions of this book has already led to a demand for a third. I have again endeavoured to take advantage of the sug- gestions of kindly critics, especially Professor W. C. M'Intosh, Dr. Ramsay Traquair, Dr. John Beard, and Dr. Arthur Masterman. I must also acknowledge grate- fully that in the revision I have had throughout the able assistance of Miss Marion Newbigin, D.Sc., who has also written the chapter on Comparative Physiology. The 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. Many new figures have been added, and 1 wish to express my thanks to my artist friends, Mr. William Smith and Miss Florence Newbigin, for the carefulness with VI PREFACE TO THE THIRD EDITION. which they have done their work. I am also indebted to Dr. Traquair for allowing me to figure some of the specimens in the Edinburgh Museum of Science and Art. In regard to the illustrations, I may further say that in almost every case they have either been derived from original memoirs and works of reference, or drawn from specimens. Of course, no one who has worked with such excellent practical books as that by Marshall and Hurst or Parker’s Zootomy, can help being assisted by them in preparing analogous diagrams ; but I have refrained from incurring any but an absolutely necessary debt. J. A. T. School of Medicine, Edinburgh,. April 1S99. CONTENTS. — ♦ — GENERAL. CHAPTER I. PAGE General Survey of the Animal Kingdom . . i Physiology CHAPTER II. 18 ^ Morphology CHAPTER III. 32 Embryology CHAPTER IV. 49 Paleontology . CHAPTER V. > 73 Doctrine of Descent CHAPTER VI. . Xo CONTENTS. viii INVERTEBRATES. CHAPTER VII. PAGE Protozoa ....... S4 CHAPTER VIII. Sponges. . . . . . . .113 CHAPTER IX. CCELENTERA . . . . . . 1 25 CHAPTER X. Unsegmented “Worms” . . . . • JS5 CHAPTER XT. Segmented Worms or Annelids . . . .181 CHAPTER XII. Echinoderms ....... 223 CHAPTER XIII. • Crustaceans ....... 247 CHAPTER XIV. Peripatus, Myriopods, and Insects . . .281 CHAPTER XV. Arachnoidea and Pai./eostraca .... 322 CHAPTER XVI. Molluscs 337 CONTENTS. ix VERTEBRATES. CHAPTER XVII. PAGE Hemichorda 387 CHAPTER XVIII. Urochorda ....... 396 CHAPTER XIX. Cephalochorda . . . . . .411 CHAPTER XX. Structure and Development of Vertebrates . . 425 CHAPTER XXI. Cyclostomata ....... 469 CHAPTER XXII. Fishes ........ 480 CHAPTER XXIII. Amphibia ....... 530 CHAPTER XXIV. Reptiles ....... 560 CHAPTER XXV. Birds 593 CHAPTER XXVI. Mammals ....... 632 X CONTENTS. GENERAL. CHAPTER XXVII. PAGE Comparative Physiology ..... 737 CHAPTER XXVIII. Distribution ....... 757 CHAPTER XXIX. Etiology ....... 769 APPENDIX ON BOOKS . . . . -777 INDEX . 783 LIST OF ILLUSTRATIONS. FIG. PAGE 1. Duckmole ( Omithorhynchus ) ...... 2 2. Phenacodus, a primitive extinct Mammal (Cope) . • • 3 3. Extinct moa and modern kiwi (Carus Sterne) ... 3 4. Crocodiles .......... 4 5. Salamander, an Amphibian .... .5 6. Queensland dipnoan ( Cera/odits ) ..... 5 7. Amphioxus (Haeckel) ....... 6 8. Ascidian or sea-squirt (Haeckel) . ..... 7 9. Cephalopod (paper nautilus, female) ..... 9 10. Fresh-water crayfish ( Astacus ), a Crustacean (Huxley) . 9 11. a , Caterpillar; b, pupa; c, butterfly ..... 9 12. Spider .......... 10 13. Crinoid or feather-star ....... 10 1 4. Earthworm . . . . . . . .11 15. Bladderworm stage of a Cestode (Leuckart) . . .11 16. Sea-anemones on back of hermit-crab (Andres) ... 12 17. Fossil Foraminifera, Nummulites ..... 13 18. Diagrammatic expression of classification in a genealogical tree 15 19. Structure of the cell (Camoy) ...... 44 20. Fertilised ovum of Ascaris (Boveri) ..... 45 21. Diagram of cell division (Boveri) . . . . . 45 22. Karyokinesis (Flemming) ....... 47 23. Diagrammatic expression of alternation of generations 54 24. Diagram of ovum, showing diffuse yolk granules . 56 25. Forms of spermatozoa (not drawn to scale) .... 57 26. Diagram of maturation and fertilisation. (From “Evolution of Sex”) 58 27. Spermatogenesis and polar bodies (Hertwig and Weismann) 60 28. Fertilisation in Ascaris megalocephala (Boveri) ... 61 29. Modes of segmentation ....... 63 30. Life history of a coral, Monoxenia darwinii (Haeckel) . 65 31. Embryos — (1) of bird; (2) of man (His) .... 68 32. Gradual transitions between Paludina neumayri {a) and Paludina hcernesi (/) (Neumayr) ..... 76 LIST OF ILLUSTRATIONS. xii FIG. PAGE 33. Life history of Arnceba ....... 86 34. End-to-end union of Gregarines (Frenzel) .... 87 35. Life history of Gregarina (Btitschli) ..... 88 36. Life history of Monocystis (Btitschli) ..... 89 37. Paramcecium (Biitschli) ....... 90 38. Conjugation of Paramcecium aurelia — four stages (Man pas) 91 39. Diagrammatic expression of process of conjugation in Paramcecium aurelia (Maupas) ..... 91 40. Vorticella (Biitschli) ........ 93 41. Volvox globator (Cohn) ....... 95 42. Diagram of Protomyxa aurantiaca (Haeckel) ... 97 43. Formation of shell in a simple Foraminifer (Dreyer) . . 98 44. A Foraminifer ( Polystomella ) showing shell and pseudopodia (Schultze) ......... 99 45. A pelagic Foraminifer — Hastigerina ( Globigerina ) murrayi (Brady) ......... 100 46. Optical section of a Radiolarian {Actinomma) (Haeckel) . 101 47. A colonial flagellate Infusorian — Proterospongia haeckelii (Saville Kent) . . . . . . . 101 48. Simple sponge {Ascetta primordialis) (Haeckel) . . .114 48A. A sponge colony (Haeckel) . . . . . .114 49. Section of a sponge (F. E. Schulze) . . . . 1 1 5 50. Diagram showing types of canal system (Korschelt and Heider) . . . . . . . . .116 51. Development of Sycandra raphanus (F. E. Schulze) . .120 52. Diagrammatic representation of development of Oscarella lobularis (Heider) . . . . . . .121 53. A. Young Dicyema (Whitman). B. Female Orthonectid (Rhopalura giardii) (Julin) . . . . . .123 54. Salinella (Frenzel) . . . . . . . .123 55. Diagram of Coelenterate structure, endoderm darker through- out . . . . . . . . . .127 56. Hydra hanging from water-weed (Greene) . . . .130 57. Minute structure of Hydra (T. J. Parker and Jickeli) . . 133 58. Development of Hydra (Brauer) ... . 135 59. Surface view of Aurelia (Romanes) . . . . .140 60. Vertical section of Aurelia (Claus) . . . . .141 61. Diagram of life history of Aurelia (Haeckel) . . . 142 62. Lucernana (Korotneff) ....... 144 63. External appearance of Tealia crassicomis .... 145 64. Vertical section of a sea-anemone (Andres) . . . . 146 65. Cross-section through sea-anemone (Andres). . . . 147 66. Z, Diagrammatic section of Zoantharian ; A , of Alcyonarian (Chun) ......... 149 67. Diagram of a gymnoblastic Hydromedusa (Allman) . .150 68. Diagrammatic figure of a simple Titrbellarian . . . 157 69. Diagrammatic figure of part of the structure of a simple Turbellarian . . . . . . . 157 70. Structure of liver-fluke (Sommer) ..... 159 71. Reproductive organs of liver-fluke (Sommer) . . . 160 LIST OF ILLUSTRATIONS. XIII FIG. PACK 72. Life history of liver fluke (Thomas) . . . . .162 73. Diagram of reproductive organs in Cestode joint (Leuckart) 166 74. Life history of Tania solium (Leuckart) .... 167 75. Diagrammatic longitudinal section of a Nemertean, Amphi- porus lactifloreus, dorsal view (MTntosh) . . . 169 76. Transverse section of a Nemertean, Drepanophorus latus (Biirger) . . . . . . . . I71 77. Transverse section of a simple Nemertean, Carinslla (Biirger) 172 78. The structure of a Nematode, Oxyuris (Galeb) . . .176 79. Trichinae in muscle, about to be encapsuled (Leuckart) . 179 80. Do., encapsuled (Leuckart) . . . 179 81. Anterior region of earthworm ( Bering) . . . .184 82. Transverse section of earthworm (Claparede) . . .187 83. Reproductive organs of earthworm (Hering) . . . 1S9 84. Stages in the development of earthworm (Wilson) . . 191 85. Arenicola marina . . . . . . . .195 86. Anterior part of nervous system in Arenicola (Vogt and Yung) 196 87. Dissection of lob-worm from dorsal surface . . . . 197 88. Cross-section of Arenicola (Cosmovici) .... 198 89. Development of Polygordius (Fraipont) .... 201 89A.Parapodium of “ Heteronereis” of Nereis pelagica (Ehlers) 203 90. Free-living Polvchaete, Nereis cultrifera .... 205 91. Transverse section of leech (Bourne) ..... 210 92. Alimentary system of leech (Moquin Tandon) . . .211 93. Dissection of leech (Bourne) . . . . . .212 94. Nephridium of leech (Bourne) ...... 213 95. Development of Sagitta (O. Hertwig) .... 217 95. Actinotrocha or larva of Phoronis (Masterman) . . . 220 97. Interior of Brachiopod shell, showing calcareous support for the “arms” (Davidson) ...... 221 98. Pluteus larva with rudiment of adult (Johannes Muller) . 224 99. Alimentary system of starfish (Muller and Troschel) . . 227 xoo. Diagrammatic cross-section of starfish arm (Ludwig) . . 229 101. Ventral surface of disc of an Ophiuroid, Ophiothrix fragilis (Gegenbaur) . . . . . . . .231 102. Diagram of sea-urchin, Echinus (Huxley) .... 233 103. Dissection of Holothurian, Holothuria luiulosa, from the ventral surface ........ 238 104. Diagrammatic vertical section through disc and base of one of the arms of Anledon rosacea (Milnes Marshall) . . 240 105. Stages in development of Echinoderms (Selenka) . . 242 106. Appendages of Norway lobster ...... 253 io6a. Section of compound eye of Mysis vulgaris (Grenacher) . 254 io6b. A single eye element or ommatidium of the lobster (G. II. Parker) ......... 255 107. Longitudinal section of lobster, showing some of the organs 256 108. Male reproductive organs of crayfish ( Huxley) . . . 259 109. Female reproductive organs of crayfish (Suckow) . . 260 1 10. Section through the egg of Astacus after the completion of segmentation (Reichenbach) . . . . . .261 XIV LIST OF ILLUSTRATIONS. KIG. hi. Longitudinal section of later embryo of A si am s (Reichenbach) 1 12. Embryo of crayfish, flattened out, with removal of yolk (greatly magnified) (Reichenbach) .... 1 13. Dorsal surface of Apus cancriformis (Bronn’s “ Thierreich ”) 1 14. Acorn-shell [Balamis tintinnabuhtm) (Darwin) . 1 1 5. Development of Sacculina (Delage) . 1 1 6. An Amphipod [Caprella linearis) . ... . 1 17. Sthizopod (AJysisJlexuosa), from side . 118. Nervous system of shore-crab (Carcimts manas) (Bethe) 1 19. Zoasa of common shore-crab ( Carcimts manas ) (Faxon) 120. External form of Peripatus (Balfour) ..... 1 21. Dissection of Peripatus capensis (Balfour) . 122. Embryos of Peripaius capensis , showing closure of blastopore and curvature of embryo (Korschelt and Heider) . 123. A millipede ......... 124. A centipede ......... 125. Female cockroach [Periplaneta orientalis) . 126. Male cockroach ( P . orientalis ) ...... 127. Leg of cockroach ........ 128. Mouth appendages of cockroach (Dufour) . . . . 129. Transverse section of insect (Packard) . . . . 130. Id ead and mouth-parts of bee (Cheshire) . . . . 131. Nervous system of bee (Cheshire) . . . . . 132. Food canal of bee (Cheshire) ...... 133. One of the Thysanura, Campodea staphylinus (Lubbock) 134. Young may-fly or ephemerid (Eaton) ..... 135. Diagrammatic cross-section of an Invertebrate, with the coelom (be.) shaded (Ziegler) . . . . . . 136. Diagrammatic cross-section of an Invertebrate, with a coelom and a haemoccel (Ziegler) ...... 137. Diagrams of Insect embryo (Korschelt and Heider) 138. Life history of the silk-moth, Bombyx mori 139. Development of blow -fly, Calliphora eiythrocephata (Thompson Lowne) ....... 140. Scorpion .......... 141. Dissection of Mygale from the ventral surface (Cuvier) 142. Section of Lung- book (Macleod) . . . . . 143. Livmlus or king-crab ....... 144. Young Limulus (Walcott) ....... 145. Trilobite, Conocephalites (Barrande) ..... 146. Vertical cross-section of a Trilobite, Calymene (Walcott) 147. Sea-spider, Pycnogonum littoral e, from the dorsal surface 148. Ideal mollusc (Ray Lankester) ...... 149. Stages in Molluscan development ..... 150. Dissection of Helix pomatia (Leuckart) . . . . 1 5 1. Diagram of larva of Pahidina (Erlanger) . . . . 152. Nervous system of Molluscs ...... 153. Structure of Anodonta (Rankin) . . . . . . 154. Development of Anodonta (Gcette) . . . . . 155. External appearance of a cuttlefish . . . . . PAGE 262 263 26; 268 269 272 273 274 2 77 282 284 285 2 87 287 289 289 291 291 292 296 297 298 5co 302 308 309 312 315 316 323 327 32S 333 334 335 336 336 338 338 345 346 349 35i 354 356 LIST OF ILLUSTRATIONS. XV FIG. 156. Diagram of the structure of Sepia (Pelseneer) 157. Diagram of circulatory and excretory systems in a Decapod, like Sepia (Pelseneer) ....... 158. Common Buckie, Buccinum undatum . . . . 159. Bivalve, Panopaa norvegica, showing siphons 160. Nudibranch, Dendronotus arborescens, showing dorsal out- growths forming adaptive gills ..... 1 6 1 . Ventral surface of Patella vulgata (Forbes and Hanley) 162. Chiton (Pretre) ......... 163. Dorsal view of nervous system of Chiton (Pelseneer) . 164. Proneonienia. Nervous system (Hubrecht) . . . . 165. Anatomy of Chiton ....... 166. Stages in Molluscan development ..... 167. Section of shell of Nautilus (Lendenfeld) . . . . 168. The Pearly Nautilus, Nautilus pompilius (Owen) 169. Male of Balanoglossus kowalevskii (Bateson) 170. Transverse section through gill-slit region of Ptychodera minuta (Sprengel) ....... 1 7 1. Development of Balanoglossus (Bateson) . . . . 172. Tornaria larva, from the side (Sprengel) .... 173. Dissection of Ascidian (Herdman) . . . . . 174. Do. Do. 175. Young embryo of Ascidian, Clavelina (Van Beneden and Julin) .......... 176. Embryo of Clavelina (Seeliger) . . . . . . I76a.“ Nurse” of Doliolum iniil/eri (Uljanin) . . . . 1 76B. Sexual individual of Doliolum viiilleri (Uljanin) . 176c. Anatomy of Appendicularia (Herdman) .... 177. Lateral view of Amphioxus (Ray Lankester) 178. Transverse section through pharyngeal region of Amphioxus (Ray Lankester) ........ 179. Development of atrial chamber in Amphioxus i8oandi8i. The nephridia of Amphioxus (Boveri) . 182. Early stages in the development of Amphioxus (Ilalschek) . 183. Sections through embryos of Amphioxus. to illustrate de- velopment of body cavity ...... 184. Transverse section through an Elasmobranch embryo (dia- grammatic) (Ziegler) ....... 185 and 186. Ideal fore- and hind-limb (Gegenbaur) . 187. Longitudinal section of brain of young dog-fish (diagram- matic) (Gaskell) ........ 188. Origin of pineal body (Beard) ...... 189. Diagram of the parts of the brain in Vertebrates (Gaskell) . 190. Diagrammatic section of spinal cord ..... 19 1 . Diagram of the eye ........ 192. Development of the eye (Balfour and l lertwig) . 193. Origin of lungs, liver, and pancreas in the chick (Gcette) 194. Transverse section through a Teleostean embryo (diagram- matic) (Ziegler) ........ 195. Diagram of circulation (Leunis) ...... PAGE 360 362 364 365 366 366 370 370 370 371 376 382 390 391 392 398 401 403 404 407 407 408 412 413 416 418 421 422 430 437 439 440 441 445 449 450 454 456 459 XVI LIST OF ILLUSTRATIONS. FIG. 196. Development of excretory system of Vertebrata (Boveri) 197. Urogenital system ........ 198. Mammalian ovum (Hertwig) ...... 199. Median longitudinal section of anterior end of Myxine (Retzius) ......... 200. Respiratory system of hag, from ventral surface . 201. Longitudinal vertical section of anterior end of larval lam- prey (Balfour) ........ 202. Restored skeleton of Palaospondylus (Traquair) . 203. Under surface of skull and arches of skate (W. K. Parker) . 204. Side view of skate’s skull (W. K. Parker) .... 205. Skeleton of skate ........ 206. Dissection of nerves of skate ...... 207. Side view of chief cranial nerves of Elasmobranchs (Cossar Ewart) ......... 208. Spiral valve of skate (T. J. Parker) ..... 209. Upper part of the dorsal aorta in the skate (Monro) . 210. Heart and adjacent vessels of skate (Monro) 21 1. Urogenital organs of male skate . 212. Urogenital organs of female skate (Monro) .... 213. Elasmobranch development (Balfour) . . . . . 214. External characters of a Teleostean — a carp (Leunis) . 215. Caudal vertebra of haddock ...... 216. Disarticulated skull of cod . ...... 217. Pectoral girdle and fin of cod ...... 218. Diagram of Teleostean circulation (Nuhn) . . . . 219. Young skate (Beard) ........ 220. Lateral view of dog-fish, Scyllium catulus .... 221. Outline of Acanthodcs subcatus (Traquair) . . . . 222. Sturgeon ( Acipenser sturio) from side ..... 223. Pterichthys milleri , lateral view (Traquair) 224. Skeleton of Ceratodus fin (Gegenbaur) .... 225. Head region of Protopterus (W. N. Parker) 226. Vertebral column and pelvic girdle of bull-frog . 227. Skull of frog — upper and lower surface (W. K. Parker) 228. Pectoral girdle of Rana esculenta (Ecker) .... 229. Side view of frog’s pelvis (Ecker) ..... 230. Brain of frog (Wiedersheim) ...... 231. Nervous system of frog (Ecker) ...... 232. Arterial system of frog (Ecker) ...... 233. Venous system of frog (Ecker) ...... 234. Urogenital system of male frog (Ecker) .... 235. Urogenital system of female frog (Ecker) .... 236. Division of frog’s ovum (Ecker) ...... 237. Gastrula stage of newt (Hertwig) ..... 238. Dissection of tadpole (Milnes Marshall and Bles) 239. Life history of a frog (Brehm) ...... 240. Csecilian ( Ichlhyophis ) with eggs (Sarasin) .... 241. External appearance of tortoise ...... 242. Carapace of tortoise ........ PAGE 462 464 467 471 472 475 479 484 4S5 486 490 492 493 494 495 496 497 498 500 501 502 503 505 516 517 517 518 519 523 524 533 534 537 537 535 539 543 544 548 545 550 55' 552 555 558 563 564 LIST OF ILLUSTRATIONS. XVII FIG. 243- 244. 245- 246. 247. 245. 249. 250. 251. 252. 253- 254. 255- 256. 257. 258. 259- 260. 261. 262. 263. 264. 265. 266. 267. 268. 268a, 269. 269A 270. 271. 272. 273- 274. 275- 276. 277. 278. 279. 280. 281. 282. 253. 284. 285. 286. 287. 288. Internal view of skeleton of turtle Dissection of Chelonian heart ( Huxley) Heart and associated vessels of tortoise (Nuhn) . Lateral view of brain of Hatteria punctata (Osawa) Hatteria or Sphenodon (Hayek) .... Side view of skull of Lacerta (W. K. Parker) Heart and associated vessels of a lizard (Nuhn) . Lung of Chamcdeo vulgaris, showing air sacs (Wiedersheim) Snake’s head (Nuhn) .... Skull of grass snake (W. K. Parker) . Lower surface of skull of a young crocodile Crocodile’s skull from dorsal surface . Half of the pelvic girdle of a young crocodile Origin of amnion and allantois (Balfour) Comparison of pelvic girdles Position of organs in a bird (Selenka) . Diagrammatic section of young bird (Gadow) Entire skeleton of condor, showing the relative positions of the chief bones .... Disarticulation of bird’s skull (Gadow) Under surface of gull’s skull Wing of dove Side view of pelvis of cassowary Bones of hind-limb of eagle Brain of pigeon . Diagrammatic section of cloaca of male bird (Gadow) Heart and arterial system of pigeon . Heart and venous system of pigeon Female urogenital organs of pigeon . .Male urogenital organs of pigeon Pectoral girdle and sternum of swan . Position of wings in pigeon at maximum elevation (Marey) Wings coming down (Marey) .... Wings completely depressed (Marey) . Diagrammatic section of egg (Allen Thomson) . Stages in development of chick (Marshall) . Diagrammatic section of embryo within egg (Kennel) Side view of rabbit’s skull ..... Dorsal view of rabbit’s skull .... Under surface of rabbit’s skull .... Rabbit’s fore-leg ...... Rabbit’s hind-leg ...... Dorsal view of rabbit’s brain, with most of cerebellu away (Krause) . . .... Under surface of rabbit's brain (Krause) Diagram of caecum in rabbit .... Duodenum of rabbit (Krause) .... Circulatory system of the rabbit (Parker and Krause) Vertical section through rabbit’s head Urogenital organs of male rabbit b m cut PAGE 565 565 566 567 568 57i 573 575 578 580 582 584 585 588 59i 595 596 600 601 602 603 604 60 s 606 607 610 61 1 6i3 613 615 616 617 617 623 624 626 641 642 643 644 645 648 648 650 651 652 655 657 XV111 LIST OF ILLUSTRATIONS. FIG. 289. Urogenital organs of female rabbit ..... 290. Segmentation of rabbit’s ovum (Van Beneden) . 291. Development of hedgehog. Three early stages (Hubrecht). 292. Embryo of Perameles with its foetal membranes (Hill) . 293. Two stages in segmented ovum of hedgehog (Hubrecht) 294. Development of foetal membranes (Hertwig) 295. Diagram of foetal membranes (Turner) .... 296. View of embryo, with its foetal membranes (Kennel) . 297. Pectoral girdle of Echidna ....... 298. Pelvis of Echidna ........ 299. Urogenital organs of male duckmole (Owen) 300. Urogenital organs of female duckmole (Owen) . 301. Lower jaw of kangaroo ....... 302. Foot of young kangaroo ....... 303. Foot of ox ......... 304. Fore-leg of pig ......... 305. Side view of sheep’s skull, with roots of back teeth exposed 306. Stomach of sheep (Leunis) ....... 307. Side view of calfs fore-leg ....... 308. Side view of lower part of pony’s fore-leg .... 309. Side view of ankle and foot of horse ..... 310. Side view of horse’s skull, roots of teeth exposed 31 1. Feet of horse and its progenitors (Neumayr) 312. External appearance of common porpoise . . . . 313. Left fore-limb of Balanoptera ...... 314. Fore-limb of whale, Megaptera longimana (Struthers) . 315. Pelvis and hind-limb of Greenland whale, Baleen a (Struthers) 316. Vertebra, rib and sternum of Balcenoptera . . . . 317. Skull of tiger, lateral view . ...... 318. Lower surface of dog’s skull ...... 319. The common seal ........ 320. Skeleton of fox-bat, Pteropus ...... 321. Skull of Orang-Utan ........ 322. Skull of gorilla ......... 323. Skeleton of male gorilla ....... PAGE 657 666 667 668 669 670 671 675 681 683 684 684 686 690 697 697 698 699 700 702 702 703 704 708 709 710 711 713 715 716 721 725 731 732 733 OUTLINES OF ZOOLOGY. INVERTEBRATES. VERTEBRATES. BIRDS. Flying Birds. Running Birds. Placentals. MAMMALS. Marsupials. Monotremes. Snakes. Lizards. REPTILES. Crocodiles. Tortoises. Dipnoi. FISHES. Bony Fishes- Ganoids. Elasmobranchs. AMPHIBIANS. Newt. Frog. CYCLOSTOMATA. Lamprey. Hag-fish. LANCELET. TUNICATES. BALANOGLOSSUS. Insects. Arachnids. Myriopods. Peripatus. ARTHROPODS. ANNELIDS. Cuttlefish. Gasteropods. MOLLUSCS. Bivalves. Crustaceans. “ WORMS.” Feather-stars. Brittle-stars. Star-fish. ECHINODERMS. UNSEGMENTED WORMS. Sea-urchins. Sea-cucumbers. Ctenophores. Jelly-fish. Sea-anemones. CCELENTERA. Metlusoids and Hydroids. SPONGES. Infusorians. Rhizopods. Gregarines. SIMPLEST ANIMALS. Corals. METAZOA. I Proto- OUTLINES OF ZOOLOGY. + CHAPTER I. GENERAL SURVEY OF THE ANIMAL KINGDOM. In beginning the study of Zoology, it seems useful to take a general survey of the “Animal Kingdom.” Without some such bird’s-eye view — necessarily superficial — one is apt to lose sight of the plan in studying the details. But the survey can be of little service unless the student has the actual animals before him, or in his mind’s eye. Vertebrates, or Backboned Animals. Mammals.— We naturally begin a 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 i 2 GENERAL SURVEY OR THE ANIMAL KINGDOM. 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 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, while the bats are as markedly suited 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 Fig. i. — Duckmole ( Ornithorhynchus ). stated briefly as follows : — (a) Before birth most young mammals are very closely united (by a complex structure 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 : — BIRDS. 3 (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. (h) Metatheria, Didelphia, or Marsupials — the prematurely bearing, usually pouch-possessing kangaroos, opossums, etc. Fig. 2. — Phenacodus, a primitive extinct Mammal. — After Cope. (r) Prototheria, Ornithodelphia, or Monotremes — the egg-laying duckmole ( Omithorhynchus ), Echidna , and Proechuina. 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 parts of the by the absence in modern forms, the fixedness of the Fig. 3.— -Extinct moa and modern kiwi. — After Carus Sterne. in many skeleton, of teeth by the lungs and their associa- tion with numerous air sacs, and so on. 4 GENERAL SNR FEY OF THE ANIMAL KLNGDOM. 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 which the muscles used in flight are in part attached (Carinatse) ; (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 (Ratitee) ; and (3) there is an extinct type, Arcluzopleryx , with markedly reptilian 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 ” ( Hatteria ), 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 ; AMPHIBIANS. 5 they differ from them in being “cold-blooded,” and in many other ways. 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 Pishes. Thus Huxley grouped Birds and Reptiles together as Sauropsida ; Amphibians and Fishes together as Fig. 5. — Salamander, an Amphibian. Ichthyopsida — for reasons which will be afterwards stated. Amphibians mark the transition from 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 foetal 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 Fig. 6. — Queensland dipnoan ( Ceratodus ). tail usually forms the locomotor organ, and the limbs are fins. There are also unpaired median fins supported by fin rays. All have permanent gills borne by bony or gristly 6 GENERAL SURVEY OF THE ANIMAL KINGDOM. arches. 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 hints of a three-chambered heart, and have a lung as well as gills. Other 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 three great orders of Fishes — the cartilaginous Elasmobranchs, such as shark and skate ; the Ganoids, such as sturgeon and bony pike ; and the Teleosteans or bony fishes, such as cod, herring, salmon, eel, and sole. Primitive Vertebrates. — Under this title we include — (i) the class of Roundmouths or Cyclostomata; (2) the class represented by the lancelets ; (3) the class of Tunicates, Fig. 7. — Ampliioxus.- — After Haeckel. some of which are called sea-squirts ; and (4), with much hesitation, several strange forms, especially Balanoglossus, which exhibit structures suggestive of affinity with Verte- brates. The Cyclostomata, represented by the lamprey ( Pet?-o - myzon) and the hag ( Myxine ), and some other forms, probably including an interesting fossil known as Palceo- spo?idyIus, 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 lancelet ( Amphioxus ), for which the class Cephalo- chorda has been erected, is even simpler in its general structure. Thus there is an absence of limbs, skull, jaws, well-defined brain, heart, and some other structures. The vertebral column is represented by an unsegmented (or un- vertebrated) rod, called the notochord, which in higher PRIMITI VE FEE TEBRA TES. 7 animals (except Cyclostomata and some fishes) is a tran- sitory embryonic organ afterwards replaced by a backbone. The Tunica ta or Urochorda form a class of remarkable forms, the majority of which degenerate after larval life. In the larvte of all, and in the 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, for which the class Hemichorda or Enteropneusta has been established, it is still difficult to speak with confidence. The possession of gill- clefts, the dorsal position of an im- portant 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. 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. Fig. 8. — Ascidian or The distinction is a very old one, for sea ' squirt. — After even Aristotle distinguished mammals, birds, Haeckel, 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 — (1) that Fishes and Cyclostomata do not form the base of the Vertebrate series, for the lancelet 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 Chtetopod worms, show some hints of affinities with Vertebrates. The limits of the $ GENERAL SURVEY OF THE ANIMAL KINGDOM. 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. 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 even 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. The greater part of the nervous system is on the ventral surface. No corresponding structure is known. No corresponding structures are known with any certainty. The eye is usually derived directly from the skin. The heart, if present, is dorsal. “ Backboned,” Vertebrate or Chordate. The central nervous system — brain and spinal cord — is dorsal and tubular. 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, theyare associated withgdls, and are useful in respiration ; in higher forms they are transitory and functionless, except when modified into other structures. The essential parts of the eye are formed by an outgrowth from the brain. The heart is ventral. Invertebrates, or Backboneless Animals. Molluscs. — This series of forms includes Bivalves, such as cockle and mussel, oyster and clam ; Gasteropods, such as snail and slug, periwinkle and buckie ; Cephalopods, such as octopus and pearly nautilus. They may be placed highest among Invertebrates, since many of them exhibit a concentration of the nervous system greater than occurs elsewhere. 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. ARTHROPODS. 9 Fig. g. — Cephalopod (paper nautilus, female). The nervous system has three chief pairs of nerve centres ■or ganglia. In many cases the larval stages are very characteristic. Arthropods. — -This large series includes Crustaceans Fig. io. — Fresh-water crayfish ( Astacus ), a Crustacean. — •After Huxley. Fig. ii. — a, Caterpillar; b, pupa ; c, butterfly. 10 GENERAL SURVEY OF THE ANIMAL KINGDOM. Myriopods, Insects, Spiders, and other forms, which have segmented bilaterally symmetrical bodies and jointed appendages. The skin produces an external cuticle, the organic part of which consists of a substance called chitin, associated in Crustaceans with carbonate of lime. The nervous system consists of a dorsal brain, connected, by a nerve-ring around the gullet, with a ventral chain of ganglia. Fig. i2.— Spider. 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 respira- tion. There is a marked tendency to deposition of lime in the tissues. The de- velopment is strangely circuitous or “in- direct.” 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 assemblage — a mob — not yet reduced to order. It seems useful, however, to separate those which are ringed or segmented, from those which Fig. 13. — Crinoid or feather-star. UNSE GMENTED ‘ ‘ WORMS. are unsegmented. The former are often called Annelids, and include — Fig. 14. — Earthworm. Chaetopoda or Bristle-footed worms, eg. earthworm and lob-worm ; and Hirudinea or Leeches ; and some smaller classes. Unsegmented. “ worms.” — These differ from the higher “ worms ” in the absence of true segments and appendages, and resemble them in their bilateral symmetry. The series includes Turbellarians or Planarians ; the parasitic Tre- matodes or Flukes ; the parasitic Cestodes or Tape- worms ; the Nemerteans or Ribbon-worms; the frequently parasitic Nema- todes or Thread-worms ; and several smaller classes. As to certain other forms, such as the sea-mats (Polyzoa or Bryozoa), the lamp-shells (Brachiopoda), and the worm- like Sipunculids, it seems best, at this stage, to confess that they are 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 hdve flowed ; for among the heterogeneous “ worms ” we detect affinities with Arthropods, Molluscs, Echinoderms, and even Vertebrates. At this stage we may notice that in all the above forms the typical symmetry is bilateral (see p. 33) (in Echinoderms, the radial symmetry 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. belongs only to the adults) ; that in most types a body-cavity or coelom is developed ; that the embryo consists of three germinal layers (external ectoderm or epiblast, internal endoderm or hypoblast lining the gut, and a median mesoderm or mesoblast lining the body-cavity). In the next two classes (Coelentera and Sponges) the conditions are different, as may be expressed in the following table, though it is open to question whether the contrast is quite so great as it seems : — - Sponges and Ccelentera. Higher Animals (Ccelomata). There is no body cavity. There is but one cavity, that of the food canal. There is no definite middle layer of cells (mesoderm), but rather a middle jelly (mesogloea). The radial symmetry of the gastrula embryo is 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 coelom be- tween the food canal and the walls of the body. But this is often in- cipient, or degenerate. There is a distinct middle layer of cells (mesoderm) between the external ectoderm and the internal endo- derm. The longitudinal axis of the adult does not correspond to the long axis of the gastrula embryo. Coelentera. — This series includes jelly-fishes, sea-anemones, corals, zoophytes, and the like, most of which are equipped Fig. 16. — Sea-anemones on back of hermit-crab. — After Andres. with stinging cells, by means of which they paralyse their prey. All but four or five are marine. The body may PORIFERA. 13 be a tubular polype, or a more or less bell-like “ medusoid,” 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 rnany- celled animals. In the simplest forms, the body is a tubular, two-layered sac, with numerous inhalant pores perforating the walls, 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,” or both siliceous and horny at once. Water passes in by the small inhalant pores, and out by the exhalant aperture. With few exceptions they are marine. All the animals hitherto mentioned have bodies built up of many cells or unit masses of living matter ; but there are other animals, each of which consist of a single cell. These simplest animals are called Protozoa. Every' animaj 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 are arranged in various ways to form a “body.” But the Protozoa form no “ body ” ; they remain single cells, and when they divide, the daughter cells almost invariably go apart as independent organisms. Plere, 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 (b) there are 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, nor sexual reproduction in the ordi- nary sense of the phrase. The series includes — (a) Infusorians, with actively moving lashes of living matter. (b) Rhizopods, with outflowing threads or processes of living matter, e.g. the chalk-forming Foraminifera (Fig. 17). (c) Sporozoa, parasitic forms, without either lashes or outflowing processes. Fig. 17. — Fossil Foraminifera, Nummulites. 14 GENERAL SURVEY OF THE ANIMAL KINGDOM. Note on Classification. We naturally group together in the mind those impressions which are like one another. In this lies 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 natural relationships of animals, to group together those which resemble one another in their real nature or structure. It must, therefore, be based on the results of comparative anatomy — technically speaking, on “ homologies,” or real resemblances of structure. Whales must not be ranked with fishes, nor bats with birds. To a classification based on structural resemblances, two corrobora- tions are necessary, from embryology and from palaeontology. 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, families, orders, classes, and so on. To give an illustration, all the tigers are said to form the species Re l is tigris, of the genus Eelis, in the family Felidae, in the order Carnivora, within the class Mammalia. The resemblances of all tigers are exceedingly close ; well marked, but not so close, are the resem- blances between tigers, lions, jaguars, pumas, cats, etc., which form the genus Felts ; 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 re- semblances 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 is a subjective conception within which we include 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. In a handful of small shells the “ splitters ” may recognise 20 species, while the “ slumpers” see only 3. Thus Haeckel says of calcareous sponges that, as the naturalist likes to look at the problem, there are 3 species, or 21, or 289, or 591 ! But while no rigid definition can be given of a species, seeing that the conception is one of practical convenience and purely relative, there are certain common-sense considerations to be borne in mind : — 1. 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 characteristic on the strength of which CLASSIFICA TION. r5 we deem it warrantable to give a name to a group of individuals, must not be markedly fluctuating. The specific character 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 mdi- Fig. 18. — Diagrammatic expression of classification in a genealogical tree. B indicates possible position of Balano- glossus, D of Dipnoi, S of Sphenodon or Hatteria. viduals, that it may be safely used as the index of more important 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 would lead to the absurd conclusion that two 16 GENERAL SURVEY OF THE ANIMAL KINGDOM. brothers belonged to different species. Thus it is often doubly unsatis- factory when a species is established on the strength of a single specimen — (a) because the constancy of the specific character is undetermined ; ( 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. But the characters of a single specimen are sometimes so distinctive that the zoologist is safe in making it the type of a new species, or even of a new genus. 3. Although cases are known where members of different species have paired and brought forth fertile hybrids, this is not usual. The snembers 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. To sum up, a species is but a relative conception, convenient when we wish to include under one title all the members of a group of individuals who resemble one another in certain characters. There is no absolute constancy in these specific characters, and one species often melts into another, with which it is connected by intermediate varieties. At the same time, the characters, on account of which the naturalist gives a specific name to a group of individuals, should be greater than those which distinguish the members of any one family, should show a relative constancy from generation to generation, and should be associated with reproductive peculiarities which tend to restrict the range of mutual'fertility to the members of the proposed species. Tabular Survey of Chief Classes. — ( For Future Reference'). metazoa chordata. Mammalia. Aves. Reptilia. < Eutheria. -j Metatheria. Marsupials. (Prototheria. Monotremes. Oviparous. ( Carinatee. Keeled flying birds. < Ratitae. Keel-less running birds. (Extinct reptile-like birds. rCrocodilia. Crocodiles and alligators. Ophidia. Snakes. I Lacertilia. Lizards, i Rhynchocephalia. Sphenodon. I Chelonia. Tortoises and turtles. ''Extinct Classes. /'Anura. Tail-less frogs and toads. Urodela. Tailed newts. Amphibia. -! Gymnophiona, e.g. Ccecilia. Labyrinthodonts and other extinct v Amphibians. /Dipnoi. Mud-fishes. Teleostei. Bony fishes. I Ganoidei, e.g. Sturgeon, t Elasmobranchii. Cartilaginous fishes. 1 (Hag-fish ( Myxine ), and Lamprey ( Petromyzon ). Cephalochorda. Amphioxus. Urochorda. Tunicates. Hemichokda. Balanoglossus, Cep/ialodiscus. 6.2 rt "rt § 6 Pisces. Cyclostomata. 3 a C/3 rC TABULAR SURVEY OF CHIEF CLASSES. 17 Mollusca. Arthropoda. Echinoderma. “ Worms.” CcELENTERA. Porifera. METAZOA NON-CHORDATA. I Cephalopoda. Cuttle-fishes. Gasteropoda. Snails. ''Lamellibranchiata. Bivalves. Arachnoidea. Spiders, scorpions, mites. Insecta. \ Myriopoda. Centipedes and millipedes. Prototracheata. Peripatus . ^Crustacea. (Crinoidea. Feather-stars. (Cystoids and Blastoids, extinct.) Ophiuroidea. Brittle-stars. Asteroidea. Star-fishes. Echinoidea. Sea-urchins. Holothuroidea. Sea-cucumbers. I (Chaetopoda. Bristle worms. \ Annelids or (Discophora. Leeches. J Annulates. f Brachiopoda. Lamp-shells. \ Polyzoa, eg. Sea-mat ( Flustra ). v Sipunculoidea, eg. Sipunculus. Nematoda. Thread-worms. Nemertea. Ribbon-worms. {Cestoda. Tape-worms. ^ Trematoda. Flukes. ]- Platyhelminthes. Turbellaria. Planarians. J rCtenophora, e.g. Beroe. Scyphozoa. Jelly-fishes and sea-anemones. '-Hydrozoa. Zoophytes and medusoids. Sponges. PROTOZOA. Infusoria. Rhizopoda. Sporozoa. Simplest forms of animal life. 2 CHAPTER II. THE FUNCTIONS OF ANIMALS. (Physiology.) Most animals live an active life, in great part ruled by the two motives of love and hunger in their widest sense ; they are busy finding food, avoiding enemies, wooing mates, making homes, and tending the young. These and other forms of activity depend upon internal changes within the body. Thus the movements of all but the very simplest animals are due to the activity of contractile parts known as muscles, which are controlled by nervous centres and by impulse-conducting fibres, and the energy involved in these movements, and in most other vital activities, is supplied by the oxidation or combustion of the complex carbon- compounds which form a substantial part of the various organs. The work done means expenditure of energy, and is followed by exhaustion (muscular, nervous, etc.), so that the necessity for fresh supplies of energy is obvious. This recuperation is obtained through food, but before this can restore the exhausted parts to their normal state, or keep them from becoming, in any marked degree, exhausted, it must be rendered soluble, diffused throughout the body, and so chemically altered that it is readily incorporated into the animal’s substance. In other words, it has to be digested. A fresh supply of oxygen and a removal of waste are also obviously essential to continued activity. We may say, then, that there are two master activities in the animal body, those of muscular and those of nervous parts. To these the other internal activities are subsidiary LIVING AND NOT LIVING. 19 conditions, turning food into blood and thus repairing the waste of matter and energy, keeping up the supply of oxygen and the warmth of the body, sifting out and removing waste products, and so on. Besides the more or less constantly recurrent activities or functions, there are the processes of growth and repro- duction. When income exceeds expenditure in a young animal, growth goes on, and the inherited qualities of the organism are more and more perfectly developed. At the limit of growth, when the animal has reached “maturity,” it normally reproduces, that is to say, liberates parts of itself which give rise to new individuals. Living and not living. — Although no one is wise enough to tell completely what is meant by the simple word alive, it is safe to say that active life involves the following facts : — (a) The living organism g rows at the expense of material different from itself, while the crystal — one of the few dead things which can be said to grow — increases only at the expense of material chemically the same as itself. (b) The living organism is subject to ceaseless chemical change (metabolism), and yet it has the power of retaining its integrity, of remaining more or less the same for prolonged periods. (c) The living organism resembles an engine, in being a material system adapted to transform matter and energy from one form to another; but it must be granted that it is a self-stoking, and, within limits, a self-repairing engine, and that it is able to do what no engine can effect, namely, reproduce. From a physical standpoint it differs from an inanimate system in this, that the transfer of energy into it is attended with effects conducive to further transfer and retardative of dissipation, while the very opposite is true of an inanimate system. (d) A living organism exhibits five everyday activities — contractility (the power of movement), irritability (the power of feeling in the wide sense), nutrition or utilisation of food, respiration, and excretion, besides the periodic activ- ities of growth and reproduction. Division of labour.- — All the ordinary functions of life are exhibited by the simple unicellular animals or Protozoa. 20 THE FUNCTIONS OF ANIMALS. Thus the Amoeba moves by contracting its living substance, draws back sensitively from hurtful influences, engulfs and digests food, gets rid of waste, and absorbs oxygen. But all these activities occur in the Amceba within the compass of a unit mass of living matter, — a single cell, physiologically complete in itself. In all other animals, from Sponges onwards, there is a “ body ” consisting of hundreds of unit masses or cells. It is impossible for these to remain the same, for some are internal and others external, nor would it be well for the organism that all its units should retain the primitive and many-sided qualities of Amoebce. Division of labour, con- sequent on diversity of conditions, is thus established in the organism. In some cells one kind of activity predominates, in others a second, in others a third. And this division of labour is associated with that complication of structure which we call differentiation. Thus in the fresh-water Hydra, which is one of the simplest many-celled animals, the units are arranged in two layers, and form a tubular body. Those of the outer layer are protective, nervous, and muscular ; those of the inner layer absorb and digest the food, and are also muscular. In worms and higher organisms, there is a middle layer in addition to the other two, and this middle layer becomes, for instance, predominantly muscular. Moreover, the units or cells are not only arranged in strands or tissues, each with a predominant function, but become compacted into well-defined parts or organs. None the less should we remember that each cell remains a living unit, and that, in addition to its principal activity, it usually retains others of a subsidiary character. Plants and animals. — Before we give a sketch of the chief functions in a higher animal, let us briefly consider the resemblances and differences between plants and animals. (a) Resemblance in function. — The life of plants is essentially like that of animals, as has been recognised since Claude Bernard wrote his famous book, Phcnomcnes de la vie communs aux animaux et aux vegctaux. 1 he beech- tree feeds and groivs , digests and breathes, as really as does PLANTS AND ANIMALS. 21 the squirrel on its branches. In regard to none of the main functions is there any essential difference. Many simple plants swim about actively ; young shoots and roots also move ; and there are many cases in which even the full- grown parts of plants exhibit movements. Moreover, the tendrils of climbers, the leaves of the sensitive plant, the tentacles of the sundew, the stamens of the rock-rose, the stigma of the musk, are but a few instances of the numerous plant structures which exhibit marked sensitiveness. (i b ) Resemblance in structure. — 1'he simplest plants (Pro- tophyta), like the simplest animals (Protozoa), are single cells ; the higher plants (Metaphyta) and higher animals (Metazoa) are built up of cells and various modifications of cells. In short, all organisims have a cellular structure. This general conclusion is part of the Cell Theory or Cell Doctrine. (c) Resemblatice iti development. — When we trace the beech-tree back to the beginning of its life, we find that it arises from a unit element or egg cell, which is fertilised by intimate union with a male element derived from the pollen- grain. When we trace the squirrel back to the beginning of its life, we find that it also arises from a unit element or egg cell, which is fertilised by intimate union with a male cell or spermatozoon. Thus all the many-celled plants and animals begin as fertilised egg cells, except in cases of virgin birth (parthenogenesis) or of asexual reproduction. From the egg cell, which divides and redivides after fertilisa- tion, the body of the plant or animal is built up by con- tinued division, arrangement, and modification of cells. Thus plants and animals resemble one another in their essential functions, in their cellular structure, and in their manner of development. Contrasts. — But while there is no absolute distinction between plants and animals, they represent divergent branches of a V-shaped tree of life. It is easy to distinguish extremes like bird and daisy, less easy to contrast sponge and mushroom, well-nigh impossible to decide whether some very simple forms, which Haeckel called “protists,” have a bias towards plants or towards animals. We cannot do more than state average distinctions. The food which most plants absorb is cruder or chemically simpler than that 22 THE FUNCTIONS OF ANIMALS. which animals are able to utilise. Thus most plants derive the carbon they require from the carbonic acid gas of the air, while only a few (green) animals have this power ; all the others depend for their carbon supplies on the sugar, starch, and fat already made by other animals, or by plants. As regards nitrogen, most plants take this from nitrates and the like, absorbed along with water by the roots ; whereas animals obtain their nitrogenous supplies from the complex proteids formed within other organisms. Most plants, therefore, feed at a lower chemical level than do animals, and it is characteristic of them that, in the reduction of carbonic acid, and in the manufacture of starch and proteids, the kinetic energy of sunlight is trans- formed by the living matter into the potential chemical energy of complex food stuffs. Animals, on the other hand, get their food ready made ; they take the pounds which plants have, as it were, accumulated in pence, and they spend them. For it is characteristic of animals that they convert the potential chemical energy of food stuffs into the kinetic energy of locomotion and other activities. In short, the great distinction— an average one at best — is that most animals are more active than most plants. The time-honoured “distinctions between plants and animals” may be condensed in the opposite table. Chief functions of the animal body. — We have seen that there are two master activities in animals, those of muscular and of nervous structures, and that the other vital processes, always excepting growth and reproduction, are subservient to these. Let us now consider the various functions, as they occur in some higher organism, such as man, reserving comparative treatment for a subsequent chapter. Nervous activities. — Life has been described as consisting of action and reaction between the organism and its en- vironment, and it is evident that an animal must in some way become aware of surrounding influences. In a higher animal there are always parts which are specially excitable. These are the sensory end-organs : the retina of the eye for light, certain parts of the ear for sound, papillae on the tongue for taste, part of the lining of the nasal chamber for smell, tactile corpuscles of the skin for pressure and temperature. All these end-organs are associated with nerves which CO MPA R A TIVE CHA RAC 'EE RIS 7 ICS. 23 ._r-D X O i rt 0 O. rt X 3 M o - o > -j « a 3 O O “* . C * .>» a-g-sl (4 O !« 5 w.- V M a « rt T ns 0 § cut; ao 2 v v 2^ 0 E 2 3 cx >0 £ 0 0 « C S-5 cS u § g.2 rt u • - 0 DC O rt b/j.o . bOc .S 3 g .3 2 v bo 3 g*4= o 5 g*c o 9*^5 « «ir .42 bfl " a 1^- 3 •- .3 J2 ^ > *3 0.-3: u_ o.9 o_ s |1 ?Sc O S.E ■S. 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"• w ■55; M V |3 = aoa 1-Tl t- 3 c o O 3 u rt o 0 « £ _ -3 2 3 2. rt <5*n « 6 2*5 s - , V.’~ oJ -■ o 24 THE FUNCTIONS OF ANIMALS. are stimulated by the excitation of the end-organ, and conduct the stimulus inwards to what are called centres or ganglia. In vertebrate animals the brain and spinal cord contain a series of such centres, some of which serve for the perception of the changes produced in the end-organs by the stimulus, while others preside over the activities of the muscles. 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 the judgment formed. Thus nervous activities involve — (a) end-organs or sense organs ; ( b ) centres or ganglia ; and (c) the conducting nerves, some of which are afferent (or sensory) passing from end-organs to ganglia, while others are efferent (or motor) passing from centres to muscles. And in whatever part there is activity, there is necessarily waste of complex substances and some degree of exhaustion. It is 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 cilia (see p. 102). In sponges there are often specially contractile cells ; in most higher animals such cells are aggregated to form the muscles, on whose activity all movement depends. In many of the lower animals, e.g. sea-anemones and sea-squirts, the contractile strands consist of long spindle- v shaped cells which appear almost homogeneous ; these are called smooth muscle fibres. They occur in certain parts of the body in higher vertebrates, e.g. on the wall of the urinary bladder. A more specialised kind of muscle, pre- vailing in active animals, consists of fibres which show alternate light and dark cross bands ; these are called striped muscle fibres. The two kinds, unstriped and striped, may be seen to pass into one another in the same animal, and in a general way one may think of the former as slowly contracting, the latter as rapidly contracting. CHIEF FUNCTIONS OF THE ANIMAL BODY. 25 A piece of living muscle consists of 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 stimulated by motor nerves, and is richly supplied with blood. When a muscle contracts, usually under a stimulus propagated along a motor nerve, there is of course a change of shape — it becomes shorter and broader. The source of the energy expended in work done is the “ chemical explosion ” which occurs in the fibres, for the oxygen stored up (intramolecularly) in the muscle enters into rapid union with carbon compounds. Heat, C02, and water are produced as the result of this combustion, and lactic acid is also formed as a by-product. Besides the chemical change and the change of shape, there are also changes of “ electric potential ” associated with each con- traction. Digestion. — The energy expended in doing work or in growth is balanced by the potential energy of the food stuffs taken into the body. These consist of proteids, carbohydrates, fats, water, and salts, in varying proportions according to the diet of the animal. Oxygen may also be regarded as forming part of the food. In some of the lower animals, such as sponges, the food particles are directly engulfed by some of the cells with which they come in contact. Within these cells they are dissolved : this is known as intracellular digestion. In most cases, however, the food is rendered soluble and diffusible within the food canal, by the action of certain ferments made by the cells which line the gut or form the associated glands. The great peculiarity of these ferment- ing substances is that a small quantity can act upon a large mass of material without itself undergoing any apparent change. But however digestion be effected, it means making the food soluble and diffusible. In a higher vertebrate there are many steps in the process. (a) The first ferment to affect the food, masticated by the teeth and moistened by the saliva, is the ptyalin of the salivary juice, which 26 THE FUNCTIONS OF ANIMALS. 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 proteids 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 proteids into peptones ; it continues the work of the salivary juice, changing starch into sugar ; it also emulsifies the fat, dividing the globules into extremely small drops, which it tends to saponify or split into fatty acids and glycerine. (d) Into the beginning of the small intestine the bile from the liver also flows, but it is not of great digestive importance, being rather of the nature of a waste product. It seems to have a slight solvent, emulsifying, and saponifying action on the fats ; in some animals it is said to have slight power of converting starch into sugar ; by its alka- linity it helps the action of the trypsin of the pancreas (which, unlike pepsin, acts in an alkaline fluid) ; it affects cell membranes, so that they allow the passage of small drops of fat and oil ; 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 probably seconds the pancreatic juice. 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 muscular 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 also goes on ; so that by the time the mass has reached the rectum, it is semi- solid, and is known as fseces. These contain the indigestible and undigested remnants of the food and the useless products of the chemical digestive processes. 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, CHIEF FUNCTIONS OF THE ANIMAL BOD Y. 27 especially of water, takes place along the whole of the digestive tract. But lining the intestines there are special hair-like 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 foodstuffs ; the greater part certainly passes into the capillaries of the portal system, which are contained in the villi. The peptone or digested proteid, as it passes through the cells of the villi, is changed into other proteids nearly related to those of the blood, for no peptone is found in the portal vein. Function of the liver. — We now know the fate of the fats, and of the proteids of the food, and the manner in which they pass into the blood ; but we must follow the starchy material, or carbohydrates, a little further. The starch, we know, is converted into sugar, and this, with the sugar of the food, passes into the capillaries of the villi, and is carried to the liver. During digestion there is an increase of sugar in the blood vessel going to the liver from the intestine, that is, in the portal vein, but no increase in the hepatic veins, the vessels leaving the liver. The increase must therefore be retained in that organ, and we recognise as one of the functions of the liver, the regula- tion of the amount of sugar in the blood. 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. All the products of digestion, except the fat, pass through the liver, which receives everything before it is allowed to pass into the general circulation. Thus many poisons, such as metals, are arrested by the liver, and various harm- ful substances which are formed in the course of digestion are changed by the liver into harmless compounds. The excess of sugar, we have already noted, is stored in the liver. It is converted there into a substance called glycogen, which can be readily retransformed into sugar according to the needs of the system. Glycogen is stored in the muscles also, and forms an important part of the fuel for the 28 THE FUNCTIONS OF ANIMALS. supply of muscular energy and of the warmth of the body. Thus, if an animal be subjected to a low temperature, the glycogen of the liver disappears just as it does during the performance of muscular work. - Another of the many functions of the liver is that in it nitrogenous waste products begin to be prepared for their final elimination by the kidneys. Respiration. — There is another most important foodstuff 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 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 a substance called haemoglobin, contained in the red corpuscles of the blood, and is thus carried to all parts of the body. The proto- plasm of the tissues having a stronger affinity for oxygen than the haemoglobin has, removes as much as it requires. The carbonic acid gas formed as a waste product is absorbed by the serum of the blood, 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 equalised. 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 carbonic acid gas. 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 in- tricacy of life processes. For the kidneys not only take out of the blood all the waste products that result from MODERN CONCEPTION OF PROTOPLASM. 29 the metabolism of proteids, and contain nitrogen, but they 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 ones 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, and traces of salts, fats, etc., leave the body by the skin, but its chief use is to pro- tect and to regulate the temperature by variations in the size of its blood vessels. 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. There are indeed some organs which we have not men- tioned, such as the spleen, which seems to be an area for the multiplication of blood corpuscles, and the thyroid gland, which seems to have to do with keeping the blood at a certain standard of efficiency ; 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” of the Molluscs, he must be careful not to sup- pose 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 life 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 car- bonic acid gas 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 3° THE FUNCTIONS OF ANIMALS. protoplasm. This is a mere truism. We do not know the nature of this living matter ; perhaps our most certain know- ledge of it is, that in our brains its activity is associated with consciousness. When more is known in regard to the chemistry and physics of living 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. 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 transformations 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. In this way we learn that waste products are invariably formed when work is done, and that living animals have a marvellous power of rapid repair, of ceaselessly changing, and yet remaining more or less the same. Theory begins when we attempt to make the general idea of waste and repair more precise. In the study of “ protoplasm,” both morphologist and physiologist have reached their strict limits. Further analysis becomes physical and chemical, and ends in the confession that protoplasm is a marvellous form of matter in motion, or a subtle kind of motion of which we can form only a very vague conception. What is known in regard to the structure of protoplasm does not help the physiologist very much. As we shall afterwards see, the microscopists discover an intricate network which pervades each unit of living matter, but no physiologist dreams of explaining the life of a cell in terms of its microscopically visible structure. Yet, as Burdon Sanderson says, “we still hold to the fundamental principle that living matter acts by virtue of its structure, provided the term structure be used in a sense which carries it beyond the limits of anatomical in- vestigation, i.e. beyond the knowledge which can be attained either by the scalpel or the microscope.” But, in the end, this means that living MODERN CONCEPTION OF PROTOPLASM. 3i matter acts in virtue of its peculiar organisation, of which we can form only a hypothetical conception, and can give no scientific explanation. One general idea, however, the study of structure has suggested, which the conclusions of physiologists corroborate. This idea is — that a cell consists of a relatively stable living framework, and of a changeful content enclosed by it. Now, many physiologists regard the framework as the genuine living protoplasm, and the contents as the material upon which it acts. “ The framework is the acting part, which lives, and is stable ; the content is the acted-on part, which has never lived, and is labile, that is, — in a state of metabolism or chemical transformation.” This view naturally leads those who adopt it to regard protoplasm as a sort of ferment acting on less complex material which is brought to it, which forms the really changeful part of each cell. It may be recalled that the strange characteristic of a ferment is that it can act on other substances without being itself affected by the changes which it produces, and that it can go on doing so continuously with a power which has no direct relation to its amount. In these respects, therefore, living matter resembles a ferment. Somewhat different, however, is another idea, — that the protoplasm is itself the seat of constant change ; that it is constantly being unmade and remade. On the one hand, more or less crude food passes into life by an ascending series of assimilative or constructive chemical changes, with each of which the material becomes molecularly more complex and more unstable. On the other hand, the protoplasm, as it becomes active or a source of energy, breaks down in a descending series of disruptive or destructive chemical changes ending in waste products. The former view, which considers protoplasm as a sort of ferment, restricts the metabolism to the material on which the protoplasm acts. The second view regards protoplasm as the climax or central term of the constructive and disruptive metabolism. It may be, however, that there is no such substance as protoplasm, and that vital phenomena depend upon the interactions of several com- plex substances. Generalising from his studies on colour sensation, Professor Ilering was led to regard all life as an alternation 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 alternation of two processes, one of them a running down or disruption (katabolism), the other a winding up or construction (anabolism). All physiologists are agreed that in life there is a twofold process of waste and repair, of discharge and restitution, of activity and recuper- ative rest. But there is no certainty as to the precise nature of his 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 live, or what they are made of ; we may investigate their functions or their structure. The study of life, 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 difference 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. FORM AND SYMMETR Y. 33 Finally, as the greater part of embryology consists in study- 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 fact, an animal takes definite form just as a mineral does : in both the shape is determined by the nature of the stuff and by the surrounding influences. Activity, or function, also affects form ; but function is merely action and reaction between the animal and its surroundings. Such statements, however, are platitudes ; we are far from being able to explain the conditions of growth which lead to this shape or that. As regards symmetry, animals may be distinguished in an elementary way, as — (a) Radially symmetrical ; or (i b ) Bilaterally symmetrical ; or (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 our own, there is but one plane through which the body can be halved. In an asymmetrical animal, such as a snail, accurate halving is impossible. Radial symmetry is illustrated by simple Sponges, most Coelentera, and by 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 symmetry 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. Among many-celled animals, some worm type probably deserves the 3 34 THE ELEMENTS OF STRUCTURE. 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. II. Organs. — We give this name to any well-defined part of an animal, such as heart or brain. The word suggests a piece of mechanism ; but the animal is more than a com- plex engine, and many organs have several different activities to which their visible structure gives little clue. Differentiation 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 we cannot do this with the frog. The structural result of this physiological division of labour is 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 subordinate 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 ORGANS 35 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 the respiratory systems, the muscular and the skeletal systems, the brain and the sense organs, are very closely united, 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. On the other hand, 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; this is expressed by saying that they are only analogous. 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-limbs, 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 burrowing amphibian, a burrowing lizard, and a burrowing snake resemble one another in being limbless, but this “convergence,” 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 36 THE ELEMENTS OF STRUCTURE. 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, and we know how wondrously diverse are the activities in our brains. 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 considerable degree of individuality, so that in course of time what was a secondary function may become the primary one. Thus Dohrn, who has 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 ; the secondary function becomes gradually the chief one ; the result is the modification of the organ.” Thus it may be noticed that the structure known as the allantois is an unimportant bladder in the frog, that in Birds and Reptiles it forms a foetal membrane (chiefly respiratory) around the embryo, and that in most Mammals it forms part of the placenta which effects vital connection between offspring 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 ORGANS. 37 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. 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 similarly spoilt in the making. They illustrate arrested development. (l>) Some animals lose, in the course of their life, some of the prominent characteristics of their larval life ; thus parasitic crustaceans at first free-living, and sessile sea-squirts at first free-swimming, always undergo degenera- tion. The retrogression can be seen in each lifetime. 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, (c) But among “rudimentary organs” we also include structures somewhat different, e.g. the gill- clefts which persist in embryonic reptiles, birds, and mammals, though they serve no obvious purpose, or the embryonic teeth of whalebone whales. These are “ vestigial structures ,” traces of ancestral history, and intelligible on no 3§ THE ELEMENTS OF STRUCTURE. other theory. The gill-clefts are used for respiration in all vertebrates below reptiles ; the ancestors of whalebone whales doubtless had functional teeth. In regard to these persistent vestigial structures, it must also be recognised that we are not warranted in calling them useless. Though they themselves are not functional, they may sometimes be, as Ivleinenberg suggests, necessary for the growth of other structures which are useful. Classification of organs. — We may arrange the various parts of the body physiologically, according to their share in the life. Thus some parts have 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 often called “gonads,” and should never be called glands. For by a gland we mean an organ which secretes, an organ whose cells produce and liberate some definite chemical substance, such as a digestive ferment ; whereas the gonads are organs in which certain cells, kept apart from the specialisation characteristic of most of the “body cells” or “somatic” cells, are multiplied. 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, etc. It is important to adopt some order of description, and in the descrip- tions of animals given in this book, we shall follow, almost consistently, this order of treatment : — Mode of life, form, external appendages, skin, skeleton, muscle, nervous system, sense organs, food canal, body cavity, vascular system, respiratory system, excretory system, reproductive system, development. 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 gendrale,” 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 TISSUES. 39 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 (like some of those in Leipzig), in each of which some one kind of function or industry predominates. Since Leydig gave a strong foundation to comparative histology in his remarkable “ Lehrbuch der Histologie des Menschen und der Thiere” (Frankfurt, 1857), the study has been prosecuted with great energy, and has been constantly stimulated by improvements in microscopic apparatus and technique. The student should read the introductory chapters in one of the numerous works on histology, so as to gain a general idea of the characters 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 (endothelial) 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 lined by two layers of epithelium. 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-ofif cuticle, susceptible of many modifications (especially of protective value). {/>) Connective tissue. — This term is somewhat like the title “ worms.” It 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 outgrowths from the gut, or from (“mesenchyme”) cells liberated at an early stage from either (?) of the two other layers of the embryo 4o THE ELEMENTS OF STRUCTURE. (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. (а) The cells may be close together, without any intercellular “mortar” or matrix. They may contain large vacuoles, and thus produce the appearance of a network, or they may be laden with fat or with pigment. (б) In other cases the cells of the connective tissue lie in a matrix, which they exude, or into which they in part die away. Such cells are very often irregular in outline, and give off, in most cases, fine processes, which traverse the matrix as a network. 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 Vertebrate 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 (osteoblasts), which may rest on a cartilage foundation, or may be quite independent. 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 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 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 Amccba 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 Ccelentera the bases of some of the epithelial cells which form the outer and inner layers are prolonged into con- tractile roots. Here, then, we have cells of which a special part discharges a contractile or muscular function, while the other parts retain other powers. In other Ccelentera the muscular cells are still directly connected with the epithelium, but become more and more exclusively contractile. In all other animals the muscular tissue is derived from the mesoderm, which, as we have already mentioned, is not distinctly present in Ccelentera. 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 homogeneous 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. T/SSUES. 4i 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 ever)' one knows, can contract with extreme rapidity when stimulated by a nervous impulse. (d) Nervous tissue. — Beginning again with the Ama’ba, we recognise that it is diffusely sensitive, and that a stimulus can pass from one part of the cell to another. In some Ccelentera some of the external cells seem to combine contractile and nervous functions. Therefore they are sometimes called “ neuro-muscular.” But in Hydra there are special nervous cells, whose basal prolonga- tions are connected with the contractile roots already described. This is a neuro-muscular apparatus of the simplest kind. The nerve cells probably receive impressions from without, and transmit them as stimuli to the contractile elements. In sea-anemones and some other Coelentera there is an interesting complication, withal very simple. There are superficial sensor)' cells, connected with subjacent nerve- or ganglion-cells, from which fibres pass to the contractile elements. In higher animals the sensory cells are integrated into sense organs, the ganglionic cells into ganglia, while the delicate fibres which form the connections between sensor)' cells and ganglionic cells, and between the latter and muscles, are represented by 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. Let us consider first the ganglionic cells which receive stimuli and shunt them, which regulate the whole life of the organism, and are the physical conditions of “spontaneous” activity and intelligence. The simplest are prolonged at one pole into an outgrowth which branches into an afferent and efferent nerve fibre. Most, however, give off outgrowths from two poles or on all sides. Internally they consist in great part of a network or coil of fine fibrils, amid which lies the usual cell kernel or nucleus. Ganglionic cells, aggregated to form ganglia, generally lie embedded in a fibrous cellular substance called neuroglia, usually regarded as an ensheathing and supporting material. In all but a few of the simplest Metazoa, the nerve fibres are sur- rounded by a sheath called the neurilemma, said to be formed by adjacent connective tissue. Several nerve fibres may combine to form a nerve, but each still remains ensheathed in its neurilemma. In Vertebrate animals each nerve fibre usually consists of an internal “axis cylinder,” the important part, and an external unessential medullary sheath. But even in the higher Vertebrates, “ non-medullated ” 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. Furthermore, 42 THE ELEMENTS OF STRUCTURE. nerves are usually surrounded by an enveloping nucleated layer called Schwann’s sheath, or else by neuroglia. A nerve fibre consists of numerous fibrils like 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. According to some authorities, the nerve fibres arise as extensive pro- longations of the ganglion cells ; according to others, the neuroglia or other ensheathing elements contribute to the extension of the nerve fibres, or rather special neuroblast cells make both sheath and fibre. 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 of living matter. 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 pm-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 — (a) The general cell substance or cytoplasm, which con- sists partly of genuinely living stuff or protoplasm, and partly of complex materials not really living ; (b) A specialised kernel or nucleus, with a complex structure, and important, but hardly, as yet, definable functions ; (c) One or more specialised bodies called central CELLS. 43 corpuscles or centrosomata, 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 often appears at first sight almost homogeneous, but higher magnification shows con- siderable structural complexity. It is certainly not like white of egg, but shows a reticular, fibrillar, or vacuolar structure. It is usually slightly fluid, but it may be firm and compact in passive cells. It is usually translucent, but there are often obscuring granules of different kinds. In thinking of the cell substance or cytoplasm, we may distinguish the genuinely living protoplasm, of whose nature we know almost nothing, from associated substances, such as proteids, carbohydrates, fats, pigments, etc., whose chemical composition can be ascertained. But it may be that what we call protoplasm is a mixture of proteids and other complex substances. (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, but there is no trace of a 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. 20). 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, and it is generally believed that there are complex actions and reactions between the living matter of the nucleus and that of the cytoplasm. Perhaps we may venture to say that cytoplasm and nucleoplasm form a “ cell firm,” potent only in their mutual dependence. The nucleus often lies within a little nest in the midst of 44 THE ELEMENTS OF STRUCTURE. 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. Internally, it is anything but homogeneous ; at any rate, homogeneous nuclei are rare. Usually there is a network of fine, strongly stainable (chromatin) strands, with less stainable (achro- matin) substance in the meshes. But in other cells, or at another time in the same cell, the nucleus is seen to contain a coiled (chromatin) thread, or a number of chro- matin loops (Fig. 19). Weismann and others believe that these chromatin elements or chromosomes are made up of hypothetical bodies whose properties are supposed to determine the nature of an organism and its life. Many nuclei also contain one or more little round bodies or nucleoli, apparently of less importance. The term is applied somewhat vaguely to little aggregations of chromatin, and more properly to vacuole-like bodies, in which some believe that the waste products of the nucleus are col- lected. Within the nucleolus an “ endo-nucleolus ” has been discovered. Though the nuclei of different cells differ in details, there is a fundamental sameness, both of structure and activity, throughout the world of cells. (c) As to the centrosomes, it may be noted that when an animal cell divides, 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 pass from the centrosomes to the chromosomes. These 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. Fig. 19. — Structure of the cell. — After Carnoy. N , Nucleus with chro- matic coil ; note pro- toplasmic reticulum. CELLS. 45 chr- Fig. 20. — Fertilised ovum of Ascaris. — After Boveri. chr., Chromatin elements, two from ovum nucleus and two from sperm nucleus; cs., centrosoma from which 1 ‘ archoplasmic ” threads radiate, partly to the chromo- somes. In plant cells there is usually a very distinct wall, consisting of cellulose. This is a product, not a part, of the pro- toplasm, though some protoplasm may be intimately associated with it as long as its growth continues. In animal cells there is rarely a very dis- tinct wall chemically distinguish- able from the living matter itself. But the margin is often different from the interior, and a slight wall may be formed by a super- ficial compacting of the threads of the cell network, or by a physical alteration of the cell substance, comparable to the formation of a skin on cooling porridge. In other cases, especi- ally 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. In animals, as well as in plants, adjacent cells are often linked by intercellular bridges of living matter. 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 dividing nucleus usually passes through a series of complex changes known as karyo- kinesis or mitosis, and these are much the same everywhere, though different kinds of cells have their specific peculi- arities. Occasionally, however, both in Protozoa and Metazoa, the nucleus divides by simple constriction (direct or amitotic division). Fig. 21. — Diagram of cell division. — After Boveri. chr., Chromosomes forming an equatorial plate ; cs. , centrosoma. 46 THE ELEMENTS OF STRUCTURE. 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. 19). ( b ) 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. 22, 1). ( c ) Astroid stage. — The chromatin elements bend into looped pieces, which are disposed in a star, 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. 22, 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. 22, 2-4). ( e ) Diastroid. — -The single star thus forms two daughter stars, which separate further and further from one another towards the opposite poles of the cell, remaining con- nected, however, by delicate threads (Fig. 22, 3-5). if) 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. 22, 6). Each daughter nucleus then passes into the normal resting phase. The spindle disappears, and the centrosomes may also vanish. Flemming gives the following summary of karyokinesis : — Mother Nucleus Daughter Nucleus (progressive changes). (regressive changes). a Resting stage. Resting stage, g f I b Coil. Coil. f Y c Astroid. Diastroid. e d Division of Astroid and its loops f- (Prophases) (Metakinesis) (Anaphyses). We are far from being able to give even an approximate account of the ‘ £ mechanism ” of cell division. Rapidly progressive research has disclosed many mysteries, but it does not explain them. The nucleus is resolved into a chromatin framework and an achromatin matrix, but we know the nature of neither. The longitudinal division of each CELLS. 47 loop shows how thorough is the partition of the chromatin substance. The “ central corpuscles,” recently discovered, act like centres of force, and the indescribably fine threads, which pass from around these to the chromatin loops, have been credited with motive powers. Similarly the threads of the nuclear spindle are believed by some to draw or drive the chromosomes. But we do not know. The whole process is vital, and therefore inexplicable in terms of matter and motion, so long, at least, as we do not know the secret of protoplasm. On the other hand, Leuckart and Spencer have given a general rationale of cell division. Why do not cells grow much larger ? why do they almost always divide at a definite Fig. 22.— Karyokinesis.— After Flemming. 1. Coil stage of nucleus ; c.c, central corpuscle. 2. Division of chromatin elements into U-shaped loops, and longi- tudinal splitting of these (astroid stage). 3. 4. Recession of chromatin elements 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. limit of growth ? The answer is as follows : — Suppose a young cell has doubled its original mass, 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 48 THE ELEMENTS OF STRUCTURE. cube. The surface growth always lags behind the increase of mass. Therefore, when the cell has, let us say, quadrupled its original mass, 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 of Leuckart and Spencer is very helpful. 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. 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. In regard to this structure, we know that it is very complex, but we are not sure of much more. For different experts see different appear- ances, even in the same cells. 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. It seems likely that the structure is different at different times, or in different cells. Professor Btitschli’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 50°-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 like that of cytoplasm, exhibit movements and streamings not unlike those of Amoebae, and are, in short, mimic cells. Just as a working model may help us to understand the circulation, so these oil emulsions may help us to understand the living cell, by bringing the strictly vital pheno- mena into greater prominence. CHAPTER IV. THE REPRODUCTION AND LIFE HISTORY OF ANIMALS. I. Reproduction. In the higher animals the beginnings of individual life 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 recog- nition 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 which 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. Apart from these and similar cases, the “ ovum theory,” which Agassiz called “ the greatest discovery in the natural sciences in modern times,” is true — that each organism begins from the division of a fertilised egg cell. Sexual reproduction. — There is apt to be a lack of clear- ness in regard to sexual reproduction, because the process 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 ; (/>) the occurrence of two different kinds of germ cells, ova and spermatozoa, which come to nothing unless they unite (fertilisation). Furthermore, these dimorphic reproductive cells are produced by two different kinds of individuals 4 5° REPRODUCTION AND LIFE HISTORY. (females arid 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 themselves which had sufficient vitality 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 remained 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 charac- teristic of sexual reproduction, the liberation of special germ cells ; but it would have been without that other character- istic of sexual reproduction — -the existence of dimorphic germ cells, of different kinds of sexual organs, or of 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 some 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 overgrowth is natural, the liberation of buds is evidently 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 birds, 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 SUMMARY OF MODES OF REPRODUCTION. 51 “ 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. And, finally, in the mixture of two units of living matter which have had different histories, the possibility of per- mutations and combinations, in other words, of variation , is evidently supplied. Thus it is not surprising to find that the asexual method of liberating buds has been replaced in most animals by the more economical and advantageous process of sexual reproduction. Summary of Modes of Reproduction. A. In Single-celled Animals (Protozoa). ( 1 ) 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, “free-cell formation,” “endogenous multiplication” (e.g. in Gregarines). 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 liberated ; and other forms of asexual multiplication. (Sexual.) (a) The liberation of cells from a simple Metazoon, in which there is so little division of labour that the distinction between body cells and reproductive cells is not marked. (Hypothetical. ) {b) The liberation of special reproductive or germ cells, which have not taken part in the formation of the body, and which retain, more or less unaltered, the inherent qualities of the original germ cell from which the parent arose. These special repro- ductive cells — the ova and spermatozoa — are normally united in fertilisation, but some animals have (parthenogenetic) ova which develop without being fertilised. 52 REPRODUCTION AND LIFE HISTORY. 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 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. Begin with those interesting organisms which are on the border line between Protozoa and Metazoa, the colonial Infusorians of which Volvox is a type (see p. 94.) 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 metabolism varies from type to type. We believe that the contrast between the sexes is another expression of this fundamental alternative of variation. This theory may be confirmed in many ways, e.g. by contrasting the characteristic products of female life, — passive ova, with the characteristic products of male life, — active spermatozoa ; or by comparing the complex condi- tions (such as abundant food, favourable temperature) which MODES OF SEXUAL REPRODUCTION. 53 favour the production of female offspring, with the opposite conditions which favour the production of males ; or by- contrasting the secondary sexual characters of the two sexes. 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. (а) 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. Proiomyxa, and Mycetozoa, e.g. flowers of tan ( JEthalium septicum). (б) Multiple conjugation , in which more than two cells unite and fuse together, as in some Gregarines and in the sun-animalcule (A clinosphariutn ) . (e) Ordinary conjugation , in which two similar cells fuse together, observed in Gregarines 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. 40, p. 93) (e) Fertilisation , in which a spermatozoon liberated from a Metazoon unites intimately with an ovum liberated from another individual normally 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 Ccelentera, 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. Certain facts, such as the occurrence of hermaphrodite organs as a transitory stage in the development of the embryos of many 54 REPRODUCTION AND LIFE HISTORY. unisexual animals (e.g. frog and bird), suggest that hermaphroditism is a primitive condition, and that the unisexual condition of permanent maleness or female- ness 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 female organisms 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 con- ditions for several years) without affecting the rapid succession of female genera- tions ; by the production of drones in the bee-hive, from eggs which are never fertilised. (c) Alternation of generations. — A fixed asexual hydroid or zoophyte (carnpanularian or tubularian) often buds off and liberates sexual medusoids or swimming-bells, whose fertilised ova develop into embryos which become fixed and grow into hydroids (Fig. 67, p. 150). 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. 23). The liver-fluke ( Distomum hepaticum) of the sheep produces Fig. 23. — -Diagrammatic expression of alternation of generations. 1. Hydromedusae. ov. Fertilised ovum gives rise to an asexual form A, which, by bud- ding, produces sexual form or fortns .S' ; in the case of Hydro- medusae, A is represented by hydroid (//), and ^ by medu- soid (d/). 2. Liver Fluke. ov. Fertilised ovum gives rise to asexual stages (A), which, from special spore-like cells (/?), pro- duce eventually the sexual fluke (S). EMBRYOLOGY. 55 eggs which, when fertilised, grow into embryos. Within the latter, certain cells (which can hardly be called eggs) 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 conditions) into sexual flukes (Fig. 72, p. 162). In this case, reproduction by special cells, like undifferentiated precocious ova, alternates with reproduc- tion 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 Aurelia , in the history of some “worms” and Tunicates. They illustrate a rhythm between asexual and sexual multiplica- tion, between parthenogenetic and normal sexual reproduc- tion, between vegetative and animal 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 essen- tial qualities of the fertilised ovum from which the parent animal was developed. The ovum has the usual characters of a cell ; its sub- stance is traversed by a fine protoplasmic network ; its nucleus or germinal vesicle contains the usual chromatin elements ; it has often a store reserve of material or yolk, and a distinct sheath representing a cell wall (Fig. 24) In Sponges the ova are well-nourished cells in the middle stratum of the body ; in Ccelentera 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 they often arise somewhat diffusely ; in higher animals their 56 REPRODUCTION AND LIFE HISTORY. 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. 58, p. 135). The ovum grows at the expense of adjacent cells, or by absorb- ing 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 more abundant, sinking towards one pole as in the egg of the frog, or accum- ulated in the centre as in the eggs of Insects and Crust- v. aceans ; or it may be very copious, dwarfing the form- ative protoplasm, as in the eggs of Birds, Reptiles, and most Fishes (Fig. 29). Round the egg there are often sheaths or envel- opes of various kinds — (a) made by the ovum itself, and then very deli- -.5. the vitelline membrane) ; (l>) formed by adja- cent cells (eg. 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 firm, as in the chitinous coat around the eggs of many Insects, and in these cases there is often, a little aperture (micropyle) through which alone the spermatozoon can enter. The hard calcareous shells round the eggs of Birds and Tortoises, or the mermaid’s purse enclosing the egg of a skate, are ot course formed after fertilisation. Egg-shells must be distinguished from egg capsules or cocoons, e.g. of the Fig. 24. — Diagram of ovum, showing diffuse yolk granules. Germinal vesicle or nucleus ; chr. , chromatin elements. g.r. cate (e.i. MALE CELL OR SPERMATOZOON. 57 earthworm, in which several eggs are wrapped up to- gether. 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 distinct parts : the essential “head,” consisting mainly of nucleus, and the mobile “tail,” which is often fibrillated, and a small middle portion between head and tail, which is said to be the bearer Fig. 25. — Forms of spermatozoa (not drawn to scale). 1 and 2. Immature and mature spermatozoa of snail ; 3. of bird ; 4. of man (//., head ; middle portion ; tail) ; 5. of sala- mander, with vibratile fringe {/.) ; 6. of Ascaris, slightly amceboid with cap ( c .) ; 7. of crayfish. of the centrosome. The spermatozoa of Thread-worms and Crustaceans are sluggish, and inclined to be amoeboid (Fig. 25 (6, 7)). Both ova and spermatozoa are true cells, and they are com- plementary, but the spermatozoon has a longer history behind it (Fig. 27). The homologue of the ovum is the mother sperm cell or spermatogonium. This segments much as the ovum does, but the cells into which it divides have little coherence. They go apart, and become spermatozoa. There is a striking resemblance between the different ways in which a mother sperm cell divides and the various kinds of segmentation in a fertilised ovum. In most cases the REPRODUCTION AND LIFE HISTORY. 58 spermatogonium divides into spermatocytes, which usually divide again into spermatides or young spermatozoa. Maturation of ovum. — When the egg cell attains its definite size or limit of growth, it bursts from the ovary or 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 Fig. 26. — Diagram of maturation and fertilisation. (From “ Evolution of Sex.”) A. Primitive sex cell, supposed to be amoeboid. B. Ovum ; C. formation of first polar body (1. p.b .) ; D. formation of second polar body (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. bodies, the first containing half, the second a quarter of the nuclear material which composed the germinal vesicle. The second division follows the first without the inter- vention of the “ resting stage ” which usually succeeds a nuclear division. Moreover, there is this important differ- ence between the formation of polar bodies and ordinary cell division, that the number of nuclear rods or chromosomes suffers reduction, whereas in ordinary karyokinesis the daughter nuclei have as many nuclear rods as the original cell. The extruded polar bodies come to nothing, though they may linger for a time in the precincts of the ovum, and may even divide. The extrusion of polar globules from FERTILISA TION. 59 mature ova seems to be almost universal ; but observations are lacking in regard to Birds and Reptiles. Moreover, Weismann and Ischikawa have shown that in all partheno- genetic ova which they have examined, only one polar body is formed. It is said, however, that in the parthenogenetic eggs which become drones (Blochmann), and in those of a moth called Liparis (Platner), two polar bodies are formed. But in neither of these two exceptional cases is the partheno- genesis habitual; thus many of the eggs which the queen- bee lays are fertilised, and give rise to queens and workers. One of the most important results of recent investigations as to polar bodies is due to O. Hertwig and others. It may be briefly stated with particular reference to the ova of Ascaris megalocephala — the thread- worm of the horse. In one variety of this worm (var. bivalens ) the germinal vesicle of the ovum contains four nuclear rods, chromosomes, or idants. By doubling, these increase to eight (Fig. 27, B) ; the first polar body goes off with four (Fig. 27, C), and the second with two (Fig. 27, D) ; leaving two. Two “ reducing divisions ” have thus occurred. Similarly, the homologue of the ovum, the sperm mother cell, contains four chromo- somes in its nucleus (Fig. 27, A'). By doubling, these increase to eight (Fig. 27, IF), and by division the cell forms four spermatozoa, each with two. When fertilisation takes place (Fig. 28), the nucleus of the sper- matozoon, with two chromosomes, unites with the reduced nucleus of the ovum, also with two chromosomes ; and the number is thus raised to four, the normal number in the body-cells of this variety of Ascaris megalocephala. There is thus a striking parallelism in the history of the two nuclei which unite in fertilisation : both have been subjected to reducing divisions. Other cases are not so clear as that of the thread- worm, but a process of “reducing division” seems to be of general occurrence in the maturation of sex cells. If reducing division did not occur, each fertilisation would involve a doubling of the number of chromosomes. Weismann interprets the whole process as an arrange- ment by which the combinations and permutations of nuclear rods and their vital qualities are increased so as to give rise to new variations. There are, indeed, other interpretations, and the facts are difficult to understand on any theory. Thus Minot, Balfour, Van Beneden, and others have suggested that the polar bodies are extrusions of male substance from the ovum. Butschli, Giard, and others interpret the premature division of the ovum as the survival of an ancient habit, and regard the polar bodies as rudimentary or abortive ova. It may be possible to combine various interpretations : (1) the ovum divides, like any other cell — like the Protozoon ancestors — at its limit of growth ; (2) the extrusion does in some way differentiate the ovum, and renders fertilisation possible or more profitable ; (3) the peculiar reduction involved in the process makes the origin of new variations more certain. Fertilisation. — In the seventeenth and eighteenth cen- turies, some naturalists, nicknamed “ovists,” believed that 6o REPRODUCTION AND LIFE HISTORY. the ovum was all-important, only needing the sperm’s awakening touch to begin unfolding the miniature model Fig. 27. — Spermatogenesis and polar bodies. — After Hertwig and Weismann. A'. Primitive germ cell of Ascaris megaloccphala , var. bivale ns (4 chromosomes). B'. Sperm mother cell (8 chromosomes). C'. Two spermatocytes formed, each with 4 chromosomes (first reducing division). D'. Four spermatozoa formed, each with 2 chromosomes (second reducing division). A. Primitive germ cell (4 chromosomes). B. Fully developed ovum (8 chromosomes). C. Formation of first polar body (pb.i) (first reducing division). D. Formation of second polar body (pb. 2) (second reducing division). First polar body may divide into two. which it contained. Others, nicknamed “ animalculists,” were equally confident that the sperm was essential, though FERTILISATION. 61 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. Fig. 28.— Fertilisation in Ascaris megalocephala. —After Boveri. 1. Spermatozoon {sfi.) entering ovum, which contains reduced nucleus (NJ, having given off two polar bodies ( fi.b . i and z). 2. Sperm nucleus (the upper), and ovum nucleus ( N ), each with two chromatin elements or idants, and with centrosomes (c.r.). 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. Of the many facts which we now know about fertilisation, the following are the most important : — (1) 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 62 REPRODUCTION AND LIFE HISTORY. 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. (2) The union of spermatozoon and ovum is very intimate ; the nucleus of the spermatozoon and the reduced nucleus of the ovum approach one another, combining to form a single nucleus. (3) When this combined or segmentation nucleus begins the process of development by dividing, each of the two daughter nuclei which result consists partly of material 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 so 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 persist in its reproductive cells. As to the interpretation of these facts, Weismann maintains the importance of the quantitative addition which the sperm nucleus makes to the diminished nucleus of the ovum. At the same time, he finds an important source of transmissible variations in the mingling of the two nuclear substances (amphimixis). Others believe that the mingling diminishes the risk of unfavourable idiosyncrasies being transmitted from parents to offspring. Others emphasise the idea that the sperm supplies a vital stimulus to the ovum. Segmentation.- — The different modes of division exhibited 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 contrast 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, how- ever, 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. SEGMENTATION. Fig. 29. — Modes of segmentation. Ovum, with little yolk, segments totally and equally into a Ovum^wnh^onsiderabl^Tolk 00 at lower pole, segments totally but unequally, e.g. frog ; (ys.) larger yolk-laden cells. Ovum, with much yolk (y.) at lower pole, segments partially and discoidally, forming blastoderm (W.), e.g. bird. Ovum, with central yolk ( y .), segments partially and peripherally, e.g. crayfish. 64 REPRODUCTION AND LIFE HISTORY. A. Complete Division — Holoblastic Segmentation. ( 1 ) Eggs with little and diffuse yolk material divide 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 (Figs. 29 (1) and 30), some “worms,’' most Echinoderms, some Molluscs, all Tunicates, Amphioxus , 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, Ceratodns, Amphibians (Fig. 29 (2)). 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. 29 (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 super- ficial segmentation]. This is illustrated by most Arthropods (Fig. 29 (4)), and by them alone. Blastosphere and morula. — The result of the division is usually a ball of cells. But when the yolk is very abundant (3), 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 GASTRULA. 65 establishment of the two primary germinal layers, the outer ectoderm and the inner endoderm, or the epiblast and the hypoblast. Fig. 30. — Life history of a coral, Monoxenia Darwinii. — From Haeckel. 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. 5 66 REPRODUCTION AND LIFE HISTORY. 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. 30). 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 described 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-containing 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 delamination. (See p. 148.) 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 less distinct than in higher forms, and is usually represented by a gelatinous material ( mesogloea ) which appears between ectoderm and endoderm, and into which cells wander from these two layers. In the other Metazoa, the middle layer may arise from a few primary mesoblasts or cells which appear at an early stage between the ectoderm and endoderm ( e.g . in the earthworm’s development) ; or from numerous “ mesenchyme ” immi- grant cells, which are separated from the walls of the blastula or gastrula (e.g. in the development of Echinoderms) ; or as coelom pouches — outgrowths from the endodermic lining of the gastrula cavity (e.g. in Sagilta, 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 bodycavity, one layer of mesoderm (parietal or somatic) clinging to the ectodermic external wall, the other (visceral or splanchnic) 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 GENER.4LISA T10NS. 67 ectoderm, digestive gut from the endoderm. The middle layer, which begins to be developed in “ worms,” assumes some of the functions, e.g. contractility, which in Sponges and Coelentera are possessed by ectoderm and endo- derm, 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 stomodaeum and hind-gut or proctodasum). (b) From the endoderm or hypoblast arise the mid-gut (mesenteron) and the foundations of its outgrowths (e.g. the lungs, liver, allantois, etc., of higher Verte- brates), also the axial rod or notochord. According to some authorities, the blood and the vascular system of Vertebrates are in the main endodermic in origin. (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 perhaps the vascular system. This layer aids in the forma- tion of organs originated by the other two. With it the reproductive organs are associated. Physiological embryology. — Of the physiological conditions of develop- ment we know relatively little. To investigate them is one of the tasks of the future. Why does an egg cell form polar bodies, how is the sperm attracted to the ovum, why does the fertilised egg cell divide, how does the yolk affect segmentation, what are the conditions of the infolding which forms the endoderm, and of the outfolding which makes the coelom pouches, and what do the numerous larval stages mean ? Generalisations. — (1) 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 descendent cells. 68 REPRODUCTION AND LIFE HISTORY. (2) The gastraa theory. — As a two-layered gastrula stage occurs, though sometimes disguised by the presence of much yolk, in the development of the majority of animals, Haeckel concluded that it represents the individual’s recapitulation of an ancestral stage. He suggested that the simplest stable, many-celled animal was like a gastrula, and this hypo- thetical ancestor of all Metazoa he called a gastrcea. The gastrula is, on this view, the individual animal’s recapitula- tion of the ancestral gastraea. Rival suggestions have been made : perhaps the original Metazoa were balls of cells like Volvox (Fig. 41), with a central cavity in which repro- ductive cells lay ; perhaps they were like the planula larvae Fig. 31. — Embryos- — (1) of bird ; (2) of man. — After His. The latter about twenty-seven days old. y.s., Yolk-sac; pi, placenta. of some Coelentera — two-layered, externally ciliated, oval forms without a mouth. (3) The fact of recapitulatio?i. — It is a matter of experi- ence that we recapitulate in some measure the history of our ancestors. Embryologists have made this fact most 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. ( 1 ) 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. Hydra). (1) 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). GENERALISA TIONS. 69 Von Baer, one of the pioneer embryologists, acknow- ledged that, with several very young embryos of higher 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 nar- rowed. The rabbit begins like a Protozoon as a single cell ; after a while it may be compared to the young stage of a very simple vertebrate ; afterwards, to the young stage of a reptile ; afterwards, to the young stage of almost any mammal ; afterwards, to the young stage of almost any rodent ; eventually it becomes unmistakably a young rabbit. Herbert Spencer expressed the same idea, by saying that the progress of development was from homogeneous to heterogeneous, through steps in which the individual history was 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 becoming more evident. Thus Professor Sedgwick has recently said that a blind man could distinguish the early stages of Elasmobranch and Bird embryos. But this need not lead us to deny the general resemblance. Moreover, the individual life history 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 7o REPRODUCTION AND LIFE HISTORY. short, the individual’s recapitulation of racial history is general, but not precise. But we do not understand how the recapitulation is sustained. Has the protoplasm of the embryo some unconscious memory of the past ? Have the protoplasmic molecules, as Haeckel puts it, learned long since some rhythmic dance which they cannot forget ? And, to what extent must there be similarity of external conditions if the recapitulation, “ the perigenesis of the plastidules,” is to be sustained ? (4) Orgafiic co?iti?iuity between generations. — Heredity. —Everyone knows that like tends to beget like, that off- spring resemble their parents, and sometimes their ancestors (atavism). Not only are the general characteristics trans- mitted, but minute features, idiosyncrasies, pathological conditions, innate or congenital in the parents, may be transmitted to the offspring. Many attempts have been made to explain this, but the first suggestion with any scientific pretensions was that the reproductive cells, which may become offspring, consist of samples accumulated from the different parts of the body. This was a very old idea, but Herbert Spencer and Charles Darwin gave it new life. According to Darwin’s “ provisional hypothesis of pangenesis,” the reproductive cells accumulate gemmules liberated from all parts of the body. In development these gemmules help to give rise to parts like those from which they originated. This hypo- thesis has been repeatedly modified, but except in the general sense that the body may influence its reproductive cells, “ pangenesis ” is discredited by most biologists. The idea which is now accepted with general favour is, that the reproductive cells which give rise to the offspring are more or less directly continuous with those which gave rise to the parent. This idea, suggested by Owen, Haeckel, Rauber, Galton, Jager, Brooks, Nussbaum, and especially emphasised by Weismann, is fundamentally important. At an early stage in the development of the embryo the future reproductive cells of the organism are distinguishable 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 HEREDITY. 7» of the “ body,” but, remaining virtually unchanged, continue the protoplasmic tradition unaltered, and are thus able to start an offspring which will resemble the parent, because it is made of the same protoplasmic material, and develops under similar conditions. A fertilised egg cell with certain characters {a, b, c), de- velops into an organism in which these characters are vari- ously expressed ; but if, at an early stage, certain cells are set apart, retaining the characters a, b, c, in all their entirety, then each of these cells will be on the same footing as the original fertilised egg cell, able to give rise to an organism, almost necessarily to a similar organism. 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 ” — ( Sagi/ta , Thread-worms, Leeches, Polyzoa), and of some Arthropods ( e.g . Moina among Crustaceans, Chironomus among Insects, Phalangidae among Spiders), in Micrometrus aggregates among Teleostean fishes, and with less distinctness in some other animals. In many cases, however, the reproductive cells are not recognisable until a relatively late stage in development, after differentiation has made considerable progress. Weis- mann gets over this difficulty by supposing that the con- tinuity is sustained by a specific nuclear substance — the germ-plasm — which remains unaltered in spite of the differentiation in the body. But 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 offspring 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 reproductive cells within the “ body ” are affected by such changes. Weis- mann denies the transmissibility of any characters except 72 REPRODUCTION AND LIFE HISTORY. those inherent or congenital in the fertilised egg cell, and therefore denies that the influences of function and environ- ment are, or have been, of direct importance in the evolution of many-celled animals. Such influences affect the body , but do not reach its reproductive cells, and are therefore non-transmissible. Many of the most authoritative biolo- gists are at present of this opinion. On the other hand, many still maintain that profound changes due to function or environment 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 trans- mitted to the next generation. The question remains under discussion. 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 life,— that preserved in observable structures, and the modified recapitulation discernible in individual development ; in this we turn to the third — the geological record. From Morphology many conclusions as to the course of evolution have been drawn ; the study of form must indeed, by itself, in time have led to the doctrine of evolution, — that the present is the child of the past. 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 supporters who appealed to Palaeontology. They asserted that the palaeontological facts refused to lend the support which the theory demanded. To their attacks the evolutionists then chiefly sought to reply by pointing out that the geological record was very incomplete. The numerous investigations which have since been carried on on all sides now show conclusively 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 relatively few cases, little is known of the ancestors 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- 74 PAST H [STORY OF ANIMALS. 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 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. Some shells, such as Nautilus and some of the Brachiopods, occur as fossils from remote Palaeozoic ages onward, but it is impossible to believe that the animal within has never varied during this period, though we can- not now learn either the nature or the amount of the variation. 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 Amoebae ; while the Foramini- fera and the Radiolaria, having hard structures of lime or silica, have been well preserved. The Sponges are well represented by their spicules and skeletons. Of the Coelentera, except an extinct order known as PALAEONTOLOGICAL SERIES.” 75 C< Graptolites, only the various forms of coral had any parts leadily 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. 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 coveiing, 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 imperfection of the “ geological record,” in spite of the conditions un- favourable 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 fossil- ised fresh- water snails ( Planorbis ) has been worked out ; the extremes are very different, but the intermediate forms link them indissolubly by a marvellously gradual series of transi- tions. The same fact is well illustrated by another series of fresh-water snails ( Paludina , Fig. 32), and not less strikingly among those extinct Cuttle-fishes which are known as Ammonites, and have perfectly preserved shells. Similarly, 76 PAST HISTORY OF ANIMALS. 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, and the modern horse to its entirely different progenitors. In short, as know- ledge increases, the evidence from Pakeontology 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 fossiliferous rocks — the Cambrian system — shows that this correspond- ence is only roughly true. To account for this, we must remember that the whole mass of the oldest rocks, known as Archaean or Pre-Cambrian, have been so pro- foundly 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. What these early forms of life were it seems impos- sible for us to find out, although recent discoveries, for instance, of “annelid tracks” in rocks of possible Pre- Cambrian age in N.-W. Scotland, suggest that patient investigation may yet do much towards the solving of the problem. Extinction of types. — Some animals, such as some of the lamp-shells or Brachiopods, have persisted from almost 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 Fig. 32. — Gradual transitions between Paludina Neumayri (a) and Paludina Hoernesi (j). — From Neumayr. EXTINCTION OF TYPES. 77 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 ancient Trilobites (perhaps remotely connected with our king-crab), their allies the Eurypterids, two classes of Echinoderms (Cystoids and Blastoids), many giant Reptiles, and some Mammals. 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 exterminate Trilobites, just as man is rapidly and often inexcusably annihilating many kinds of beasts and birds. Apart from the struggle with competitors, it is conceivable 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 Am- phibians, Amphibians are historically older than Reptiles, and many types of Reptiles are much older than Birds. In short, in the couise of the ages life has been slowly creeping upwards. [Tables. Primary or Palaozoic. I Secondary or j ' Tertiary or 78 PAST HISTORY OF ANIMALS. Quaternary or Post-Tertiary. Pliocene. | Miocene. 8 j Eocene. Cretaceous. urassic. Triassic. Permian. Carboniferous. Devonian or Old Red Sandstone. Silurian. Ordovician. Cambrian. Representa- tives of all the chief classes of Inverte- brates. Teleo- steans. Dipnoi. Ganoids and Elasmo- branchs. Modern Types. Laby- rintho- donts. in Modern Types. Placentals. Toothed and Primitive Forms. Archaeo- pteryx. Marsupials and Mono- t r ernes (?) Few primi- tive types. Pre-Cambrian or Archaean. APPEARANCE IN TIME , 79 Coelentera. Echinoderma. Arthro- poda. Cephalopoda. Quaternary or Post- Tertian'. Pliocene. Miocene. .« S Eocene. Cretaceous. Jurassic. <0 Triassic. Permian. w "►J b Carboniferous. O Devonian or Old Red Sandstone. co Silurian. Ordovician. U Cambrian. u Pre-Cambrian or Archaean. Belemnites. Setia and recent forms~ CHAPTER VI. THE DOCTRINE OF DESCENT. When we ask, as we are bound to ask, how the living 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 evolu- tion 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 EVIDENCES OF EVOLUTION. 81 of Species” (1859), made it current intellectual coin. By his work and by that of Spencer, Wallace, Haeckel, 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) struc- tural, ( 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 (Fig. 32); 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 lines 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 fundamental type. It is difficult to understand this “ad- herence 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. (1?) Physiological. — Observation shows that animals are to some extent plastic. In natural conditions they vary in 6 82 THE DOCTRINE OF DESCENT. the course of several generations, or even in a lifetime. This is especially the case if one section of a species be in any way isolated from the rest, or if the animals be sub- jected in the course of their wanderings to novel conditions of life. Even apart from markedly changed circumstances, moreover, animals exhibit variations from generation to generation. The evidence from domesticated animals is very con- vincing. 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 livid), vividly illustrate the plasticity or variability of organisms. It sometimes happens that the offspring of an animal resemble not so much the parent as some other form be- lieved or known to be ancestral. Thus a blue pigeon like the ancestral Columba livia may be hatched in the dovecot, a foal may appear with zebra-like stripes, and in times of famine children may be born who are in some ways ape-like. Such atavisms or reversions are not readily intelligible except on the theory of descent. (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, that many connecting links have been discovered in the rocks, and that the higher animals appear gradually in successive periods of the earth’s history, are strong corroborations of the theory. It is less easy to state in a few words how the facts of geographical distribution, or 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. EF/DEJVCES OF EVOLUTION. 83 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. The embryo mammal is at one stage somewhat like an embry- onic fish, at another like an embryonic reptile ; even in details the recapitulation, if such we may term it, is some- times faithful. Such, in merest outline, is the nature of the evidence which leads us to conclude that the various forms of life have descended or have been evolved from simpler ancestors, and these from still simpler, and so on, back to the mist of life’s beginnings. None of the evidence is logically demon- strative ; 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 modification 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 XXIX.). CHAPTER VII. PROTOZOA— THE SIMPLEST ANIMALS. Chief Classes — (i) Rhizopods; (2) Sporozoa; (3) Infusoria. The Protozoa are the simplest animals, and they are of peculiar 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, unit masses of living matter ; and in virtue of their simplicity, 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, which 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 , but many can endure dryness for some time. In one set ( Rhizopods ) the living matter is without any rind, and flows out hi more or less changeful threads and lobes , by the movements of which the animals engulf their food and glide along. The others have a definite rind, which in a large number ( Infusorians ) bears motile cilia or flagella, but in the others ( Sporozoa ) is without any obvious locomotor structures. Put these three phases may occur in the life of one form ; in fact, each of the AMCEBA. §5 three great classes is marked by the predominant, and not by the exclusive occurrence of the Rhizopod-like, or the Infusorian- like, or the Sporozoon-like phase of cell life. Many have a skeletal framezvork of lime, flint, or other material, while zv it hin 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 tzvo 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 spermotozoon in higher animals. A fezv types, instead of remaining single cells, form by division or budding loose colonies, taking a step, as it were , tozvards the Metazoa. First Type of Protozoa — Amceba. Amceba, a type of Rhizopods, especially of those in which the outflowing processes of living matter ( pseudopodia ) are blunt and finger-like (Lobosa). Description. — Amceba 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 like a little sac of jelly, and glides over the surface of stone and plant by protruding and retracting the pseudopodia. As they move the shape con- stantly changes, whence the old (1755) name of “Proteus 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 proteid, and others of a fatty nature. According to Professor Ray Lankester, the formation of pseudopodia is due to the outflowing of the central fluid substance at places where the outer pellicle has been temporarily ruptured. In the centre of the cell lies the usually single nucleus, but Amoeba princeps has numerous nuclei. The food consists of minute Algae, such as diatoms, or of vegetable debris. It is surrounded by the finger-like pro- cesses, and engulfed along with drops of water, which form food vacuoles in the cell-substance. After the digestible 86 PROTOZOA THE SIMPLEST ANIMALS. 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. Life history.— In favourable nutritive conditions the Amoeba grows. At the limit of growth it reproduces- by 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 i. Amoeba with pseudopodia ; nucleus; c.v., contractile vacuole. 2. Division in two. 3. Encystation. 4. Escape of Amoeba from its cyst. energy recommences the cell-cycle. The conjugation of two Amoebae has been observed, and spore formation oc- casionally occurs. Second Type — Gregarina. Gregarina , a type of those Sporozoa in which the cell is divided into two regions by a partition. Description. — 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 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 “ protoelastin,” which grows inwards so as to divide the GREGARINA. 87 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 distinct, but there are no vacuoles. We may associate the absence of locomotor processes, “ mouth,” and contractile vacuoles, as well as the thickness of the cuticle and_the general passivity, with the parasitic habit of the Gregarines. It is not clearly understood how these and other intestinal parasites have become habituated to resist the action of digestive juices. Life history. — The young Gregarine is parasitic in one of the lining cells of the gut ; it grows, and, leaving the cell, remains for a time still attached to it by the cap (Fig. 35, a ., yg.) ; later this is cast off, and the indi- vidual becomes free in the gut, while 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, and from the division of the encysted W-Pp Hi. P m cell or cells, spores are formed. All the •JSteA- Vv -7. Fig. 34. — End- to-end union of Gregarines. — After Fren- zel. protoplasm is not always used up in forming the spores, but a residue may remain, which forms a network of threads supporting the spores. The cyst is sometimes (as in G. blattarum) complex, with “ ducts ” serving for the exit of the spores, each of which is surrounded by a firm case. Eventually the cyst bursts, the spore-cases are liberated, and from within each of these the single spore emerges to become a cellular parasite. The spore of G. gigantea is at first non- nucleated ; it gives off two processes, one of which becomes detached, vibratile, and nucleated, while the other seems to come to nothing (Fig. 35, sp2). The adult of this species is sometimes three-quarters of an inch in length — enormous for a Protozoon. 88 PROTOZOA THE SIMPLEST ANIMALS. Third Type — Monocystis. Monocystis , a type of those Sporozoa in which the cell is not divided into two parts by a partition. Description. — Two species (M. agilis and M. magna ) infest the male reproductive organs of the earthworm almost Fig. 35. — Life history of Gregarina. — After Btitschli. a.yg. Young forms emerging from intestinal cells. ad. Adult with deciduous head-cap and a cuticular partition divid- ing the cell into two. con. Two forms conjugating (G. blattaruni). sfi.f. Spore formation. sp\. Ripe spore of G. blattarum , within spore-case. sp2. Spore of G. gigantea , after escaping from the spore-case, show- ing long vibratile part which breaks of and develops into the adult. constantly. The full-grown adults are visible to the naked eye. They are usually flattened worm-like cells, but the shape alters considerably during the sluggish movements. There is a definite contractile rind, which is sometimes fibrillated, and a more fluid medullary substance, in which the large nucleus floats. In one species there is an anterior MONOCYS'J'IS. 89 projection which resembles the cap of Gregarina , otherwise unrepresented in Monocystis. As in Gregarina , and many other parasitic forms, a contractile vacuole is absent. Life history. — The young form is parasitic within one of the reproductive cells of the earthworm. It grows, and becomes free from the cell. In the free stage, two indi- viduals may unite in the curious end - to - end manner observed also in Gregarina. Encystation occurs, involving either a single individual or two together. Within the rounded cyst, orderly nuclear division results in the forma- tion of spore-forming masses. These form elliptical spore- cases, or “ pseudonavicellse,” enclosed in a firm sheath, and each spore-case seems to contain several, usually eight, Fig. 36. — Life history of Monocystis. — After Biitschli. 1. Gregarine lying within a sperm mother cell of earthworm. 2. Conjugation of two Gregarines within a cyst. 3. Numerous spore-cases (sp.c., pseudonavicellae) within a cyst. 4. A spore-case with eight spores (s/>.) and a residual core (rt>.) spores, lying around a residual core. The spores are con- siderably larger than those of Gregarina. Eventually the cyst bursts, the spore-cases are extruded, the spores emerge from their firm chitinoid cases. The young spore is like a bent spindle (falciform), and seems next door to being flagellate. It bores into a mother sperm cell, and from this it afterwards passes as an adult into the cavity of the seminal vesicles. In some allied Sporozoa the young form is first flagellate, and then amoeboid, before it becomes the sluggish adult. Intracellular parasitism and copious food naturally act as checks to activity. The species of Monocystis occur chiefly in “ Worms” and Tunicates; none are known in Arthropods, Molluscs, or Vertebrates. go PROTOZOA THE SIMPLEST ANIMALS. Fourth Type — Paramgecium. Paramaicium, a type of Infusorians, especially of those which are uniformly covered with short cilia (Holotricha). Description. — Specimens of Paramaecium may be readily and abundantly obtained by leaving fragments of hay to soak for a few days in a glass of water. A few individuals 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 Fig. 37. — Paramcecium. — After Blitschli. ad. Adult form, showing cilia, “ mouth,” contractile vacuoles, etc. div. Transverse division. con. Conjugation. 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. PARA MCE CIUM. 9i The surface is covered with cilia, in regular longitudinal rows ; these serve both for locomotion and for driving food particles towards the mouth. Among the cilia there are small cavities in the cortex, in which lie fine protrusible threads (“ trichocysts ”). These, though parts of a cell, Fig. 38. — Conjugation of Paramcecium aurelia — four stages. — After Maupas. 1. Shows macronucleus (A) and two micronuclei («) in each of the two conjugates. 2. Shows breaking up of macronucleus, and multiplication of micronuclei to eight. 3. Shows the fertilisation in progress ; the macronucleus is vanishing. 4. Shows a single (fertilised) micronucleus in each conjugate. suggest the thread cells of Coelentera, and are probably of the nature of weapons. The cortical layer is contractile, and is distinctly fibrillated. In the substance of the cell lie two nuclei, the smaller “micronucleus” lying by the side of the larger “macronucleus.” Food vacuoles occur as in the Fig. 39. — Diagrammatic expression of process of conjugation in Pciramcecium aurelia . — After Maupas. A. The two micronuclei enlarge. B. Each divides into two. C. Eight micronuclei result. D. Seven disappear ; one (darkened) divides into two. E. An interchange and fusion occurs, and the con- jugates separate. F. The fertilised micronucleus divides into two. G. 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 macrunucleus and two micronuclei. Amoeba. There are two contractile vacuoles, from which fine canals radiate into the surrounding protoplasm ; these discharge into the vacuole, which then bursts to the exterior. Life history. — Growth is followed by obliquely transverse J-J- _L._t 92 PROTOZOA THE SIMPLEST ANIMALS. division into two (Fig. 37, div.). One-half includes the “mouth,” the other has to make one. As well as this simple fission, a process of transient conjunction also occurs. Two individuals approach one another closely, the two nuclei of each break up, an exchange of pieces of the micronucleus takes place ; the two then separate, each to reconstruct its two nuclei (Fig. 38). This process is neces- sary for the continued health of the species. 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. 39. 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 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 ntacronuclei 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 macronucleus and two micronuclei. Fifth Type — V orticella. 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 stems 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. VORTICELLA. 93 This contractile filament, under a high power, may exhibit a fine striation. (A similar striated structure is seen in some Amcebse, Gregarines, spermatozoa, etc., and above all 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 1. Structure. N Macronucleus : n., micronucleus ; c.v., con- tractile vacuole ; //*., mouth \fv food vacuole ; za, vestibule. 2. Encysted individual. 3. Division. 4. Separation of a free-swimming unit — the result of a division. 5. Formation of eight minute units (jng. ). 6. Conjugation of microzooid {trig.') with one of normal size. 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 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 micro- nucleus. Food vacuoles and contractile vacuoles are present as usual. 94 PROTOZOA THE SIMPLEST ANIMALS. Sometimes a Vorticella bell jerks itself off its stalk and swims about ; in other conditions it may form a temporary cyst ; normally, the cilia are very active, and the movements 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 one, or as many as eight, microzooids may be set free. These swim away by means of the posterior girdle of cilia, and each may conjugate with an individual of normal size. In this case a small active cell (like a sper- matozoon) fuses intimately with a larger passive cell, which may be compared to an ovum. The details of the process of fertilisation are analogous to those described in Para- mtzcium. It is said that in some cases an encysted Vorticella breaks up into a number of minute spores, but this is doubtful. Sixth Type- — 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, 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 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. 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 VOL VOX. 95 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 Fig.- 41. — Volvox globator. — After Cohn. a., Balls of sperms ; b., immature ova ; c., ripe ova. 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 96 PROTOZOA THE SIMPLEST ANIMALS. condensation of reproductive possibilities, 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 transi- tional stages. Klein has also succeeded to some extent in showing that the occurrence of the various reproductive types depends on outside influences. 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 Amcefra, 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 Paramcecium , 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 possibilities — ( a ) the Amoeboid units, neither very active nor very passive, form a median compromise; {/>) the ciliated Infusorians, which are usually smaller, show the result of a relative predominance of expendi- ture ; (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 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 Protomyxa, there is a “cell-cycle” in which all the phases occur in one life history. It is also important to notice Professor Ray Lankester’s division of SYSTEM A TIC SURVEY. 97 the Protozoa into naked and corticate forms (Gymnomyxa and Corticata). The Gymnomyxa include the primitive forms and the Rhizopods ; the Corticata include the two extremes — Gregarines and Infusorians. (Corticata.) Predominantly ciliated and active. Infusorians. Classification of Protozoa. (Gymnomyxa. ) (Corticata. ) Predominantly amoeboid. Rhizopods. Predominantly encysted and passive. Sporozoa. ACINETARIA. RADIOLARIA. CILIATA. Rhynchoflagellata ] Dinoflagellata. FLAGELLATA. FORAMINIFERA. Labyrinthulidea. Heliozoa. LOBOSA. SPOROZOA OR GREGARINIDA. Proteomyxa and Mycetozoa. Primitive Forms. Systematic Survey. A. Primitive forms. — Under this heading may be included (i) the Proteomyxa, primitive, insufficiently known forms often without a Fig. 42. — Diagram of Prolomyxa 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 amoebic. nucleus, and (2) the Mycetozoa, organisms with somewhat complex fructifications, often classed as plants allied to Fungi. As examples of 7 98 PROTOZOA THE SIMPLEST ANIMALS. the Pioteomyxa, we have the interesting Protomyxa in four phases : (a) encysted and breaking up into spores, which (/>) are briefly flagellate, (c) sink into amoeboid forms,, and (d) flow together into a composite plasmodium ” ; V nnpyrella , parasitic on fresh- water Alga; ; and many others. J 1( The Mycetozoa are well illustrated by Ettligo or AEthalium 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 use from the surface of the plasmodium. The spores may be first Fig. 43. — Formation of shell in a simple Foraminifer. — After Dreyer. In A the shell has one chamber ; B, C, and D show the formation of a second. Note outflowing pseudopodia and the enclosure of the shell by a thin layer of protoplasm ; note also the nucleus in the central protoplasm. flagellate, then amoeboid, or amoeboid from the first ; the characteristic plasmodium is formed by the fusion of the amoebae. B. Predominantly Amoeboid Protozoa-Rliizopoda. — The simplest Rhizopods generally resemble Amceba, and are classified as (3) Lobosa. They may reproduce simply by division, as does A mceba itself, or may liberate several buds at once (Arcella), or more rarely from spores ( Peloinyxa ). Various forms, such as Arcella , are furnished with a shell. In Magospluera ( Catallacta ), described by Haeckel, the life history is complex. It appears as — (a) an encysted form ; ( b ) a free- swimming colony of ciliated cells (like the embryos of some sponges) ; SYSTEMATIC SURVEY. 99 ( c ) as ciliated units produced by the breaking up of (/;) ; and ( d ) as amoeboid forms resulting from the modification of the active units. (4) The Labyrinthulidea are represented by forms like Labyrinthula on Alga.', 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 ( A ctinosphizrium , 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. Fig. 44. — A Foraminifer ( Polystomella ) showing shell and pseudopodia. — After Schultze. The (6) Foraminifera or Reticularia include an interesting series of shelled forms in which the peripheral protoplasm forms a mass of interlacing threads. Most are marine, the shell is usually calcareous, more rarely arenaceous or chitinous, and encloses the central mass only. 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 Globigerma, with a chambered shell, is especially important ; others are Gromia, found in both fresh and salt water ; Haliphysema, a form utilising sponge-spicules to cover itself, once mistaken for a minute sponge, or for a very simple many - celled animal. Most kinds of chalk consist mainly of the shells of Foraminifera xoo PROTOZOA THE SIMPLEST ANIMALS. 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 mem- brane into an inner central capsule (with one or more nuclei), and an outer portion, giving off radiating thread-like pseudopodia. There is usually a skeleton in the form of a siliceous shell outside the central capsule, but in some cases the shell is formed of a horn-like substance called acanthin. Most Radiolarians include unicellular Algte (yellow Fig. 45. — 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. cells), with which they live in intimate mutual partnership (symbiosis). They are abundant as fossils, and of much importance in the formation of the ooze of great depths. Examples. — Thalassicola, Ettcyrtidium, and the colonial Collozoum and Spheerozoum. C. Predominantly encysted Protozoa-Sporozoa. — Formslike Gregarina and Monocystis are included as (8) Sporozoa or Gregarinida. SYSTEMATIC SURVEY. IOI Fig. 46. — Optical section of a Radiolarian ( Actinomma ). — After Haeckel. a., Nucleus; b., wall of central capsule; c., siliceous shell within nucleus ; c1., middle shell within central capsule ; c2., outer shell in extra-capsular substance. Four radial spicules hold the three spherical shells together. The others mostly resemble these types, but some, like Cocci dium, are permanent cell-parasites. Gregarines are parasitic in many different kinds of animals, including vertebrates. The Myxosporidia peculiarly Fig. 47. — 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. , unit forming spores. 102 PROTOZOA THE SIMPLEST ANIMALS. abundant in Fishes ; the Coccidia found in most animals ; the Sarco- sporidia inside muscle fibres, especially of Mammals ; the Hsemosporidia inside red blood corpuscles, are all classed as Sporozoa. It is probable that the organisms which cause pebrine and malaria are classifiable here. D. Predominantly active forms (ciliate and flag-ellate), generally called Infusorians. — Protozoa, with a definite rind and with 1-3 undulating flagella, are included as (9) 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. Modified flagellate forms are included in the groups (10) Dino- flagellata and (11) Rhynchoflagellata, in both of which there are two flagella, differently placed in the two cases. In the first are included Peridinium and Ceratium ; in the latter, the large phosphorescent Noctiluca. As (12) Ciliata are included a very large number of forms, more or less closely resembling Paramecium, and very abundant in infusions ; some, such as Opalina , in the intestine of the frog, are parasitic. The cilia often vary in size and distribution, and constitute a basis of classification. As specially modified Ciliata are included (13) Acinetaria, highly specialised forms, ciliated when young, but usually furnished 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. 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. With this is usually associated a streaming movement of the granules. A more defined contraction, like that of a muscle cell, is illustrated in the contractile filament of the stalk of Vorticella and similar Infusorians ; and not less definite are the movements of cilia and flagella, by means of which most Infusorians 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. Considered generally, the movements are of two kinds: either (1) reflex, i.e. responses to external stimulus, as when the Protozoon moves towards a nutritive substance ; or (2) automatic, i.e. such movements as FUNCTIONS OF PROTOZOA. 103 appear io originate from within, without our being able to point to the immediate stimulus, e.g. the rhythmical pulsations of contractile vacuoles. While all vital activity or life must remain inexplicable in lower terms until we know the chemical nature of protoplasm, it is useful to compare the movements of Amoebae with the movements of drops of fine emulsion, as Professor Btitschli has done in great detail. For in this way the strictly vital may be distinguished from what depends on known physical conditions. Dr. Verworn has speculatively suggested that the substance of the amoeboid cell is drawn out towards oxygen in the medium, that the chemically satisfied particles make way for their unsatisfied neighbour particles, that external stimulus provokes a molecular disruption, and that the exhausted particles have then to retreat to the nucleus, which he regards as a trophic centre. Sensitiveness. — The Amoeba is sensitive to external influ- ences. 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 the 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 Volvo: c, 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. — Like all living creatures, the Amoeba re- spires, that is, its complex substance is continually under- going a process of oxidation, carbon dioxide being produced as a waste product. Without oxygen none of the activities can be efficiently performed, and if it is long withheld death 104 PROTOZOA THE SIMPLEST ANIMALS. ensues. In all Protozoa 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 Zoochlorellx. According to some, the “chlorophyll corpuscles” seen in the primitive Archerina , in some flagellate forms, as Euglena, and in many Ciliata, as Stentor , Stylo- nichia , one species of Parainoecium , Volvox and the allied forms, are also symbiotic Algae, which have lost the power of independent exist- ence. The evidence for this is, however, insufficient, and this explana- tion will not apply in cases like that of Vorticella 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 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 htemoglobin of the host. Psychical life.- — Protozoa often behave in a way which suggest conscious control and intelligence, but as cut-off fragments also act with apparent reasonableness, and as the nucleus cannot be regarded as a brain, there seems no reason to credit them with more than that diffuse conscious- ness which is possibly co-extensive with life. Verworn has decided, after much labour, that the Protozoa do not exhibit what even the most generous could call intelligence ; but this NOTES ON THE STRUCTURE OF PROTOZOA. 105 is no reason why he or any other evolutionist should doubt that they have in them the indefinable rudiments of thought. 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 network or foam, in the meshes or vacuoles of which there is looser material. Included with the latter are 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 areas which are essential to the life and multiplication of the unit. In the Protozoa there are several conditions under which the nucleus may exist : — (1) In some adult forms, and in many spores or young forms, no nucleus has yet been discovered. It is, however, unnecessary to pre- serve the term “ Monera ” for such simple forms, as it is probable that nuclear material does exist in some form even in these cases. (2) In some of the Ciliata the nucleus is diffuse, that is, it exists in the form of a powder scattered through the medullary protoplasm, and is only discernible after death by means of careful staining. In Opalin- opsis the fine powder sometimes coalesces into a single nucleus. (3) 1° the majority of cases, notably in the Gregarines, the nucleus is single, often large, and placed centrally. From a consideration of the cells of Metazoa we may call this the typical case. (4) In many of the Ciliata, e.g. Paramcecium, the nucleus is double. There is a large oblong nucleus, and beside it a smaller spherical one. (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. In the division of many Protozoa, as in the cells of higher animals, it plays an important part. During division it passes from the resting to the active condition. The nuclear threads, or “chromatin filaments,” loosen themselves from their coiled state, io6 PROTOZOA THE SIMPLEST ANIMALS. and arrange themselves in a star at the equator of the cell, whence they divide into two groups, which retreat from one another, and become the daughter nuclei of two daughter cells. In short, karyokinesis has been observed here as elsewhere (see p. 46). 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 Amceba 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. There seems no reason why one may not combine the view of Weismann, that the nucleus bears the essential hereditary substances, with the view that it is a trophic, or, at any rate, a vital centre in the cell. In naked Protozoa the outer part of the cell-substance (“ectoplasm”) is often clearer and less granular than the inner part (“endoplasm”), but this difference is a physical one of little importance. 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. More- over, the cuticle may become the basis of a shell formed from foreign particles, or made by the animal itself of lime, flint, or “ horny ” 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. 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 are in the ascendant. Reproduction occurs at the limit of growth, or sometimes in disadvantageous conditions, when disruptive or katabolic processes gain some relative pre- dominance. REPRODUCTION OF PROTOZOA. 107 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. (6) 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, (c) Com- moner, however, is the definite and orderly process by which a unit 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 — ovaand spermatozoa — different from the somatic cells which make up the “ body.” A Protozoon multiplies by dividing wholly into daughter 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. It is obvious that unicellular Protozoa can show nothing corresponding to sexual reproduction in the first sense. Moreover, Protozoa can live on, dividing and multiplying, for prolonged periods without the occurrence of anything like fertilisation. So it is often stated as a characteristic of Protozoa, that “ they have no sexual reproduction.” But if this mean that the unicellular Protozoa have no special reproductive cells, then it is a truism. If, however, the statement mean that the Protozoa are without anything corresponding to fertilisa- tion, then it is not true. For in many of the Protozoa there occurs at intervals a process of “ conjugation ’’ in which two individuals unite either permanently or tem- porarily. This is an incipiently sexual process ; it is the analogue of the fertilisation of an ovum by a spermatozoon. (1) It is one of the recurrent phases in the life history of some of the simplest Protozoa (Proteomyxa and Mycetozoa) (see p. 97), that a io8 PROTOZOA THE SIMPLEST ANIMALS. 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 Gregarines and other forms occasionally unite. To this the term “ multiple conjugation ” has been applied. (3) Commonest, however, is the union of two apparently similar in- dividuals, either permanently, so that the two fuse into one, or tem- porarily, so that an exchange of material is effected. Permanent conjugation has been observed in several Rhizopods, Infusorians, and Gregarines. Temporary conjugation is well known in not a few ciliated Infusorians, and it is possible that a curious end-to-end union of certain Gregarines is of the same nature, or it may be of the nature of a “ plasmodium ” formation. (4) There are some cases where one of the conjugating indivi- duals is larger and less active than the other. Thus in Vorticella , a small free-swimming form unites and fuses completely with a stalked individual of normal size. To call this “dimorphic conjugation” is hardly necessary, since it is evidently equivalent to the fertilisation of a passive ovum by an active spermatozoon, one of the well-known characteristics of reproduction in the Metazoa. In Volvox this is even more obvious, for the small and active cells, both in shape and method of formation, recall the spermatozoa of higher forms. The conjugation of ciliated Infusorians, such as Para- mcecium, has been studied with great care by Gruber, Maupas, R. Hertwig, and others, and though their results are not quite harmonious, the main facts are secure. In many ciliated Infusorians there are two nuclear bodies — one large, the other small. The smaller micronucleus lies by the side of the larger macronucleus. The micronucleus divides into parts, while the macronucleus degenerates. Two individual Infusorians (A and B) lie side by side in close contact, a portion of the micronucleus of A passes into B, and fuses with a portion of the micronucleus of B, similarly a portion of the micronucleus of B passes into A, and fuses with a portion of the micronucleus of A. In short, mutual fertilisation occurs, the conjugating individuals separate, a new micronucleus and a new macronucleus are established in each. The precise interpretation of the process is to some extent a matter of mere opinion. We may regard it as a mutual rejuvenescence, each unit supplying some substances or qualities which the other lacks ; or we may regard it rather as a process by which the average character of the species is sustained, peculiarities or pathological variations of one individual being counteracted by other characteis in the neighbour (apparently no near relation) with which it conjugates ; or we may see in it a source of variation as the result of new combinations among the essential hereditary substances. The researches of M. Maupas have thrown much light on the facts, and some of his results deserve summary. It has been often alleged that the subsequent dividing is accelerated by conjugation ; but Maupas finds that this is by no means the case. The reverse in fact is true. While a pair of Infusorians ( Onycliodronius grandis) were engaged in conjugation, a single individual had, by ordinary asexual division, given rise to a family of from forty thousand BIONOMICS. 109 to fifty thousand individuals. Moreover, the intense internal changes preparatory to fertilisation, and the general inertia during subsequent reconstruction, not only involve loss of time, but expose the Infusoiians to great risk. Conjugation seems to involve danger and death rather than to conduce to multiplication and birth. The riddle was, in part at least, solved by a long series of careful observations. In November 1885, M. Maupas isolated an Infusorian ( Stylonichia pustulata), and observed its generations till March 1886. By that time there had been two hundred and fifteen generations pro- duced by ordinary division, and since these lowly organisms do not conjugate with near relatives, there had been no conjugation. What was the result? At the date referred to, the family was observed to have exhausted itself. The members were being born old and debilitated. The asexual division came to a standstill, and the powers of nutrition were lost. Meanwhile, before the generations had exhausted themselves, several of the individuals had been restored to their natural conditions, where they conjugated with unrelated forms of the same species. One of these was again isolated, and watched for five months. In this case, up till the one hundred and thirtieth generation, it was found that on removal to fresh conditions the organisms were capable of conjugating with unrelated forms. Later this power was lost, and at the one hundred and eightieth generation the individuals of the same family were observed making vain attempts to conjugate with each other. We thus see that without normal conjugation the whole family becomes senile, degenerates both morphologically and physiologically. Morphologically, the individuals decrease in size, until they measure only a quarter of their original proportions, the micronucleus atrophies completely or partially, the chromatin of the macronucleus gradually disappears, other internal structures also degenerate. Physiologically, the powers of nutrition, division, and conjugation come to a standstill, and this senile decay of the isolated individuals or family inevitably ends in death. The general conclusion is evident. Sexual union in those Infusorians, dangerous, perhaps, for the individual life, and a loss of time so far as immediate multiplication is concerned, is absolutely necessary for the species. The life runs in strictly limited cycles of asexual division. Conjugation with allied forms must occur, else the whole life ebbs. Without it, the Protozoa, which some have called “immortal,” die a natural death. Conjugation is the necessary condition of their eternal youth. Bionomics.— Many Protozoa raise organic debris once more into the circle of life, and many form part of the food of higher animals. Thus those pelagic Foraminifera and Radiolarians, which dying sink 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. I 10 PROTOZOA THE SIMPLEST ANIMALS. Almost every kind of nutritive relation occurs among the Protozoa. Predatory life is well illustrated by most In- fusorians, and thoroughgoing parasitism by the Gregarines ; Opalina in the rectum of the frog may serve as a type of those which feed on decaying debris, and Volvox of those which are holophytic. Radiolarians, with their partner Algse, 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 probably nourished by the carbohydrates which they build up. Some of the parasitic forms, especially among the Sporozoa, are of serious importance to higher animals. Though Protozoa may be seriously infected by Bacteria, by Acmeta 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. Few Protozoa come into direct touch with human life, but man has several Protozoon parasites, e.g. Amoeba co/i, associated with inflammation of the intestinal mucous membrane ; Coccidium oviforme , a Sporozoon affecting the liver ; and various Infusorians. 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 ; more 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 pait of chalk deposits. The famous Eozoon canadense of Cambrian rocks is regarded by most as a purely mineral formation. 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 (Astrorhizidm), that these were succeeded by forms with regularly agglutinated shells, exhibiting types of architecture which were subsequently expressed in lime. GENERAL ZOOLOGICAL INTEREST. 1 1 1 Relics of siliceous Radiolarian shells are also known from Silurian strata onwards, with, perhaps, the exception of the Devonian. Best known are those which form the later Tertiary deposits of Barbados earth, from which Ehrenberg described no fewer than two hundred and seventy-eight species. 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 various activities, and each cell has usually one function dominant over the others. The Metazoan cells, in acquiring 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 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 mere 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 ; (h) the beginnings of fertilisation, from “the flowing together of exhausted cells” and multiple conjugation, to the specialised sexual union of some Infusorians, where two individuals become closely united ; ( c ) the beginnings of sex, in the difference of size and of constitution sometimes observed between two con- jugating units ; ( 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 want of separation of the products of fission. Among the Radiolarians there are 1 1 2 PROTOZOA THE SIMPLEST ANIMALS. 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 amoeboid, some encysted and spore -forming. Again, in Volvox, as we noticed above, the cells of the colonies show a distinction into nutritive and reproductive 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. PORIFERA— SPONGES. A. B. Calcarea (Calcispongise). r Hexactinellida. Non-Calcarea. -[ l Demospongke. ( Monaxonida. ( Tetractinellida. 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 general 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. A “ body ” has been gained, but it shows relatively little division of labour or unified life ; it is a community of cells imperfectly integrated. 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 ( tzvo-layered ) Metazoa , the middle stratum of cells, the mesoglcea , not at taming to the definiteness of a proper mesoderm. There is no coelom or body cavity. The longitudmal axis of the body corresponds to that of the embryo ; in other zvords, the general symmetry of the gastrula is retained. In these three characters the Sponges agree with the Cedent era, a?id differ from higher ( triploblastic and ccdomate ) Metazoa. The body varies greatly in shape , even within the same 8 PORIFERA SPONGES. 114 species. It is traversed by canals , through which currents of water bear food i?iwards and waste outwards. Numerous minute pores on the surface open into afferent canals, leading into a cavity or cavities lined by endoderm cells, many or all of which are flagellate. To the activity of the flagella the all-important water currents are due. The endodermic or gastric 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 exterior. Where there is a dis- tinct central cavity there is usually but one large exhalant aperture ( osculum ), but in other cases there are many exhalant apertures. The ectoderm is the least developed layer ; it covers the body, and is perhaps continued into the afferent canals. The endoderm lines most of the internal cavities, and is typically flagellate. The intervening mesoglcea contains a skeleton of lime, flint, or spongin ; amoeboid cells or phagocytes, important in digestion and excretion ; reproductive cells , and other elements. Budding is very common, and in a few cases buds are set adrift. Both hermaphrodite and unisexual forms occur. The sexually - produced embryo is almost always developed within the mesoglcea , and leaves the sponge as a ciliated lari'a. With the exception of one family, 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 aperture at the opposite pole, and with numerous minute inhalant pores penetrating the walls. These walls con- Fig. 48. — Simple sponge ( Ascetta pri m o r d i a lis). — After Haeckel. Note the vase-like form, the apical osculum, the inhalant pores in the walls. Fig. 48A. — A sponge colony. MORE COMPLICATED FORMS. 115 sist of — (1) a flat ectoderm ; (2) a mesoglcea containing triradiate calcareous spicules, phagocytes, and reproductive elements ; and (3) an endoderm lining the central cavity, and composed of collared flagellate cells, almost exactly like some of the monad Infusorians. This simple sponge is not much above the gastrula level ; it agrees generally with a simple Coelenterate, such as Hydra, but differs from it in the absence of tentacles and stinging cells, and in the greater development of the mesogloea. More complicated forms. — But a description of a simple sponge like Ascetta conveys little idea of the structure of a complex form such as the bath - sponge (Euspongia). Let us consider the origin of complications. (a) Sponges — long regarded as plants — are plant -like in being sedentary and passive. They seem also to feed easily and well. Like plants, they form buds, the outcome of surplus nourishment. These buds, like the suckers of a rose- bush, often acquire some apparent independence, and the sponge looks like many vases, not like one. More- over, as they grow these buds may fuse, like the branches of a tree tied closely together. Thus the structure becomes more intricate. (fr) In the simple sponge the gastric cavity of the vase is com- pletely lined by the collared endoderm cells ( A scon type). But the endoderm may grow out into radial chambers, and the walls of these may also be folded into side aisles ( Sycon type). The outgrowing of the endoderm into the mesogloea may be continued even further, and the cells may become pavement-like, except in the minute flagellate chambers, where the characteristic collared type is retained ( Leucon type). (See Fig. 50.) [Speculatively, it may be suggested that the characteristic folding or outgrowth of the endoderm is necessitated by the Fig. 49. — Section of a sponge. — After F. E. Schulze. Showing inhalant canals, flagellate chambers, a gastrula forming in the mesogloea, etc. PORIFEKA SPONGES. x 16 fact that the endoderm cells are better nourished and multiply more rapidly than those of the ectoderm, which thus fails to keep pace A Ec Eti Fig. 50. — 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 re- present cross-sections through the wall. A. Simple Ascon type ( Ec ., ectoderm; En., endoderm ; Mg., mesogloea). B. Sycon type, with flagellate radial cham- bers ( r.c .). C. Leucon type, with flagellate side aisles on the main radial chambers. D. Still more complex type, with small flagellate chambers {/. c/t.). with the inner layer.] ( c ) By infoldings of the skin — ectoderm and a subjacent sheath of meso- gloea— 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 com- plications are common. (d) The ectoderm is usually described as a covering layer of flat epithelium, but flask- shaped cells have also been observed (Bidder). It may be folded inwards, as we have noticed, and, according to some, it also lines the incurrent or afferent canals in whole or in part. In a few cases, e.g. Oscarella lobiilaris , 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 mesogloea. The endoderm consists typically of collared flagellate cells, but in the more complex sponges these are replaced, except ORDINARY FUNCTIONS. 1 17 in the flagellate chambers, by flat epithelial cells, with or without flagella. The mesoglcea 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 embedded in a little jelly ; spindle- shaped connective tissue cells, united into fibrous strands ; contractile cells, e.g. those forming a sphincter around the oscula of some forms, such as fachymatisma ; skeleton- making cells ; pigment • containing cells ; supposed nerve cells, projecting on the surface, and believed to be con- nected internally with multipolar (ganglion ?) cells ; and lastly, the reproductive cells, which are connected by transi- tional forms with the ordinary phagocytes. ( 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 chemically somewhat like silk. Uniradiate, biradiate, triradiate, quadri- radiate, sexradiate, and multiradiate spicules occur, and in a general way it may be said that they are arranged so that they give most architectural stability. Each is formed witbin a single cell, and may be speculatively regarded as an organised 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 — Geodia, Stel/etta ) cast out as an effete product.” The fibres of spongin are formed as the secretions of mesoglcea cells, known as spongioblasts. Ordinary functions. — Excepting the fresh-water Spong- illidre, all Sponges are marine, occurring from between tide marks to great dejrths. 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. 1 1 8 PORIFEKA SPONGES. 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 of food, such as Infusorians, Diatoms, and particles of organic debris. The outflowing current carries away 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 endoderm cells ; ( b ) in the adjacent phagocytes of the mesoglcea ; (c) in the phagocytes surrounding the sub- dermal spaces, if these exist. It is uncertain whether the epithelium of the subdermal spaces or the collared endoderm is the more important area of absorption, 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 an extract of several sponges, Krukenberg found a (tryptic) digestive ferment, probably formed within the phagocytes, but digestion is wholly intracellular. Many sponges contain much pigment ; thus the lipo- chrome pigment (see Chap. XXVIII.) zoonerythrin is common, and, like some others, such as floridine, is regarded as helping in respiration. The green pigment of the fresh- water sponge is closely analogous, if not identical, with chlorophyll, and probably renders some measure of holophytic nutrition possible. 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. The frequent budding is merely a kind of continuous growth, but when buds are set adrift, as sometimes happens, we have discontinuous growth or asexual reproduction. In the fresh-water Spongillidre there is a peculiar mode of reproduc- tion by statoblasts or gemmules. A number of mesoglceal 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 history is as follows : — In autumn the sponge DE VEL OPM ENT. 119 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 life history thus illustrates what is called alternation of generations (see p. 54). Interpreted from a utilitarian point of view, the formation of gemmules is a life-saving expedient. As Professor Sollas says, “ the gemmules serve primarily a protective purpose, ensuring tfye persistence of the race, while as a secondary function they serve for dispersal. ” All Sponges produce sex cells, which seem to arise from amoeboid 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 pioduction of ova or the production of spermatozoa usually preponderates, probably in dependence on nutritive conditions. Development. - -It is not surprising to find that there is great variety of development in the lowest class of Metazoa ; it seems almost as if numerous experiments 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, and is doubtless general. Seg- mentation 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 metamorphosis often difficult 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. 51), as described by F. E. Schulze, the segmentation results in a hollow ball of 120 PORIFERA — SPONGES. Fig. 51. — Development of Sycandra raphanus. — After F. E. Schulze. 1. Ovum. 2. Section of 16 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 gran- ular cells. 5. Gastrula stage settled down. Ec., outer layer (ectoderm ?) ; En., inner layer (endoderm ?) ; bl., closing blastopore ; am.p., mooring amoe- boid processes. 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 become temporarily invaginated, forming what is called a ‘ ‘ pseudo-gaslrula. ” This leaves the parent and the granular cells right themselves, forming the posterior hemisphere of the embryo, now called an amphi-blastula. It swims for a time actively, but the flagellate cells of the anterior 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 downwards, and is moored by amoeboid processes from the granular cells, which likewise obliterate the blastopore. The granular cells lose their granules, for the larva is not yet feeding ; the now internal flagella disappear in the absence of the stimulating water ; a mesogloea with spicules begins to be formed between the inner and outer layer, probably by migrants from the latter. But this disadvantageous state of affairs cannot last. Pores open through the walls, the entiance of water enables the inner cells to recover their flagella, and an exhalant aperture is ruptured at the upper pole. The young sponge is now in an Ascon stage, from which, by the outgrowth c. (?) of the inner layer into radial chambers, it passes into tbe permanent Sycon form, grows into a cylinder, and becomes differentiated in detail (Fig- 51 2 3 4 5)- (b) In Oscarella (Hahsarca) lobul- aris (Fig. 52), a sponge without any CLASS1FICA TION. 121 skeleton, the ovum segments equally into a blastula, which is flagellate all over. This free-swimming stage may be invaginated from either pole 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. (e) Another type, seen for instance in a horny sponge, Spongelia , results in a flagellate larva, whose cavity is filled up with what may be called gelatinous connective tissue, from which mesoglcea and endoderm are subsequently dif- ferentiated. Such a larva is called a parenchymula. As these are not all the types of development which occur among sponges, the general fact is impressive, that in this lowest class of Metazoa there has been considerable plasticity in develop- ment. Classification. — A. Porifera Calcarea , with skeleton of calcareous spicules : — Order I. — Homocoela. — Endoderm wholly composed of collared flagellate cells, e.g. Ascetta , Leuco- solenia. Order II. — Heterocoela. — Endo- derm consists of collared flagellate cells in radial tubes or chambers, and of flat epithelium elsewhere, e.g. Grantia, Sycon. B. Porifera non- Calcarea, skeleton of silica or of spongin, or of both. Fig. 52. — Diagrammatic re- presentation of development of Oscarella lobularis. — After Heider. (1) Hexactinellida, with sexradiate siliceous spicules, canal system usually simple, with Sycon chambers. The members live chiefly in deep water, e.g. Venus Flower-Basket (Euplec- tella) and the Glass-Rope Sponge ( Hyalonema ). (2) Monaxonida, with siliceous spicules (which are not quadri- or sex-radiate), or with “horny” skeleton, or with both. Bl., Free-swimming blastula with flagella; G., gastrula settled down. Next figure shows folding of endoderm ( En .); Ec., ectoderm. Lowest figure shows radial cham- bers (R.C.) ; Mesogloea (Mg. ) ; inhalant pore (P.) ; exhalant osculum (O.). Order I. — Monaxona, with spicules only, e.g. Mermaid’s Gloves (Chalina oculata ), Crumb-of-Bread Sponge ( Hal ich ondria or Amorphina panicea), Fresh-Water Sponge (Spongilla). Order II. — Ceratosa, “ horny ” sponges with or without spicules, e.g. the Bath-Sponge (Euspongia). 122 PORIFERA SPONGES. (3) Tetractinellida, mostly with quadriradiate spicules, or with triaenes, in which a main shaft bears at one end three branches diverging at equal angles, e.g. Geodia, Pachyrna- tisma, Plakina. There are also a few sponges (Myxospongiae) without any skeleton, perhaps survivals of primitive types ( Oscarella, Halisarca ) or degraded forms ( Chondrosia ). 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. Bionomics. — Sponges are living thickets in which many small animals play hide-and-seek. Many of the associations are practically constant and 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 Clione on oyster-shells, are borers, and others smother forms of life as passive as themselves. Several crabs, such as Dromici, 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 eats into the shell-substance. Within several sponges minute Algae live, like the “ yellow cells ” of Radiolarians, in mutual partner- ship or symbiosis. Finally, sponges deserve mention as factors in human civilisation. General zoological interest and position. — Sponges have this great interest, that they form the first successful class of Metazoa. They illustrate the beginnings of a “ body,” and the beginnings of tissues. Along with the Coelentera, from which it is the almost unanimous opinion that they must be held distinct, they differ markedly from the triploblastic, Ccelomate Metazoa, which do not retain the radial symmetry of the gastrula. Their origin is wrapped in obscurity, though there is much to be said for the view that they are the non-pro- gressive descendants of primitive gastrula-like ancestors of INCERT.-E SERES — MESOZOA. 123 sluggish constitution. It does not seem likely that they have led on to anything higher, they rather represent a by-road in Metazoan evolution. Incertze Sedes. Mesozoa. The title Mesozoa was applied by Van Beneden to some very simple organisms which appear to occupy a very humble position in the Fig. 53. — A. young Dicyema. — After Whitman. B. Female Orthonectid (Rhopaiura Giardii). — After Julin. c., Ectoderm; en., inner endoderm cell with nucleus («.); and embryo («»/.). Note the segmentation and the fibrillation supposed to be muscular. FlG. 54. — Salinella. — After Frenzel. 1. Longitudinal section — a., anterior; p., pos- terior. 2. Transverse section. 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 : — 124 MESOZOA. 1. Dicyemidte (type Dicyema ) occur as parasites in Cephalopods ; the body consists of a ciliated outer layer, enclosing a single multi- nucleate inner cell, within which egg-like germs develop, apparently without fertilisation, into dimorphic embryos (see Fig. 53, A). 2. Orthonectkke (type RhopaLura ) occur as parasites in Turbellarians, Brittle-stars, and Nemerteans ; the body is slightly ringed, and consists 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. 53, B). 3. Professor F. E. Schulze has discovered a small marine organism - — Triclioplax adhcerens — in the form of a thin, three- layered, externally ciliated plate ; and Monticelli records a similar form under the title Treptoplax adhcerens. 4. Professor J. Frenzel has discovered in brine solutions a minute Turbellarian-like organism — Salinella salve — whose body consists of one layer of cells (Fig. 54). There is an anterior mouth, a ciliated food canal, and a posterior anus. The ventral surface is finely ciliated, the rest of the cells bears short bristles. The animal reproduces by transverse fission, but conjugation and encystation also occur. CHAPTER IX. CGELENTERA. Class i. Hydrozoa Class 2. Scyphozoa (Hydromedusae. {Siphonophora. \ Acraspeda. (Actinozoa. ( Hydro medusae. or Class i. Hydrozoa •< Siphonophora. ( Scy phomedusae. Class 2. Actinozoa. Class 3. Ctenophora. Class 3. Ctenophora. The Ccelentera — including zoophytes, jelly-fish, sea-ane- mones, 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 backwards — Phytozoa), and in their budded colonies afford interesting illustrations of organic co-operation and division of labour. General Characters. The Ccelentera are simple, almost wholly marine , fo?'ins in which the primary long axis of the gastrula becomes the long axis of the adult, which is almost ahvays radially symmetrical about this axis. There is no body cavity or cceloih, distinct from the primitive digestive cavity ( enteron ) and its outgrowths. In the lower members of the series, the primary openvig of this cavity becomes the mouth of the adult, but in the more specialised types there is an ( ectodermic ) oral invagination , 126 CCELENTERA. which forms a gullet - tube. Between the ectoderm and endoderm of the body wall there is a supportijig layer , or mesoglcea, of jelly-like consistency. In the simplest cases this is quite devoid of cells, but seco?idarily , they may migrate into it from the endoderm. Stinging cells of varying com- plexity are almost invariably present, but in almost all the Cte?iophora their place is taken by adhesive cells. The Ccelentera exhibit two divergent types of structure , which recur constantly, in modified forms, throughout the group, and may even be both present in the course of one life history, when they illustrate the phenomenon of alternation of generations or metagenesis. Of the two, the more primitive type is the sessile tubular hydroid, which may be compared to a gastrula fixed by one end, and furnished with a crow?i of tentacles placed round the central aperture of the other pole. The other de?'ived form , which has become specialised in various directions, is the active medusoid or jellyfish type. In several divisions the formation of a calcareous “ skeleton ” by the hydroid type may result in the production of “ corals .” Multiplication by budding is common , and often results in the formation of colonies, some of which shotv considerable divi- sion of labour. The preservation of the primary axis, the absence of true mesoderm a?id of a coelom , are often said to distinguish Ccelentera and Sponges from the other Metazoa (Coelomata), but the results of ?-ecent researches on the Jia/ure of the mesoderm 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-ane- mones which cluster in the nooks of the rocks, and the active jelly-fish which swim on the waves, are but different expressions of the antithesis so characteristic of the series, and illustrate, the former in the class Hydrozoa, the latter in the Scyphozoa, the two physiological tendencies of the Ccelentera. The delicate iridescent globes, which represent the third class, the Ctenophora, illustrate the climax of GENERAL SURVEY. 127 activity, for among them there is no sessile hydroid type. In our survey of the series, however, we must pass over these familiar types, and begin with the little fresh-water Hydra (Fig. 56), which is often 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 Fig. 55. — Diagram of Ccelenterate 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. 3. To left, shows vertical section of Craspedote Medusoid (with velum); to right, of Acraspedote Medusa, with- out velum, g. , gut ; gl., gullet. Note anatomical correspondence of the polypoid and medu- soid forms. degeneration. In favourable conditions the polype may give off daughter buds, which remain for a time attached to the parent, and then separate as independent Hydrce. 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 imperfect separation of the units, continued indefinitely, we can understand the formation of hydroid colonies, such as the zoophytes. 1 28 CCELENTERA. In such cases the colony is usually supported by an organic sheath of varying complexity. The members of such a colony would, however, with an exception which we will consider later, be all similar and equivalent, and this is by no means true of all hydroid colonies. In Hydractinia , for example, which is common on shells at the shore, the colony consists of polypes of varied structure and function. It may be that these differences are caused by differences in nutrition, the fact at any rate is that some of the polypes are nutritive “per- sons,” like Hydra in appearance ; some are mouthless (?) reproductive “persons,” which produce sperms and eggs, and so eventually start a new colony ; others, with a mouth, are long, slendefl, sensitive, and abundantly furnished with stinging cells ; while the little protecting spines at the base of the colony may perhaps be abortive “persons.” All these polypes are united by connecting canals at the base, and all are fed at the expense of the nutritive “ per- sons.” Hydractinia thus exhibits division of labour among the members of the colony, and a tendency towards this is common in the Ccelentera. If we now return to the simpler zoophyte colony, we find that this tendency can be recognised even here. Like Hydractinia , the colony at intervals exhibits repro- ductive “persons,” different from the ordinary polypes. These, as in Hydractinia , may be sessile and mouth- less, or they may after a time become detached and float away as delicate, 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 (see P- 54)- ... j 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 predominance of activity is exhibited in the permanent medusoids, e.g. Geryonia, where the colonial hydroid stage is omitted, and the embryo becomes at once medusoid. Finally, the medusoids themselves may become colonial, and we have active floating colonies, like those of the GENERAL SURVEY. 129 Portuguese man of- war, which show, on a different plane, as much division of labour as Hydractinia. The same general conclusions 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 distinct anatomical differences. It is in accordance with this increased complexity that the alternation 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 (Lu- cernarians) which in their adult life are predominantly passive, and attach themselves by a stalk. The sea-anemones and their numerous allies may be regarded as bearing a relation to the jelly-fish, somewhat similar to that which the hydroid polypes bear to the swimming-bells (Fig. 55). They are, however, much more complicated in structure than the hydroids. Solitary forms are much commoner than in the Hydrozoa, 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 Scyphozoa, the Millepores among the Hydrozoa also form very considerable calcareous colonies. 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 intensity of life in many active animals. The origin of the Ctenophores is still obscure ; Goette believes that they have arisen from a scyphula, the hypothetical ancestor of the Scyphozoa. As to diet, the active Ctenophores are carnivorous, attaching themselves by adhesive cells to one another, or to other small animals ; many of the larger forms, e.g. sea- anemones and jelly-fish, are able to engulf booty of considerable size ; the majority, however, feed on small organisms, in seizing and killing which the tentacles and stinging cells are actively used. 9 CCELENTE RA. 130 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 Euclilora rubra. They also occur in some Turbellarian worms, and in the papillae of /Eolid nudibranchs among molluscs. Each cnidoblast con- tains a capsule or nematocyst, which encloses a coiled lasso lying in an irritant gelatinous substance. The nematocyst fills most of the cell, but there is a nucleus, etc. , besides. At the distal end there may be a trigger-like cnidocil or a fringe of bristles, etc. 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 evaginated. The basal part of the lasso is often stronger than the rest, and may bear stilets ; spirally arranged roughnesses and bristles are also frequent on the thread itself. The explosion of the cnidoblast is believed by most authorities to be due to an entrance of water, which causes the gelatinous substance to swell up. According to others, the cnidoblast contracts as a whole. The action of the threads is both mechanical and chemical. They are fixed, e.g. by help of the stilets, into the victim, and the irritant substance poisons the wound, causing paralysis or death in small animals. Types of Ccelentera. First Type. — Hydra, illustrative of the Class Hydrozoa. General life. several species, The genus Hydra is represented by r the green Hydra viridis and the brownish Hydra fusca, both widely distributed in fresh water. They are among the simplest of Coelentera, 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 reproduc- tion. The body is usually fixed by its base to some aquatic plant, often to the underside of a duckweed. It may measure in. 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 by its tentacles, stand on its head, or creep by looping movements. Ac- cording to some observers, its movements may be helped by fine pointed pseudopodia protruded from the ectoderm Fig. 56. — Hydra hang- ing from water-weed. - — After Greene. ov Ovary; testes. TYPES OF CCELENTERA HYDRA. 131 cells of the tentacles, etc. Usually, however, the Hydra prefers a quiet life. It feeds on small organisms, which are paralysed or killed by stinging cells on the 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 . Daphnia ) are caught, and in part digested. Infusorians ( Euplotes , etc.) are often seen wandering to and fro on the surface of the Hydra , but these wonted visitors do not seem to provoke the sting- ing cells to action. So simple is Hydra, that a cut-off fragment, containing samples of the various kinds of cells in the body, and not too minute, may grow into an entire animal. Thus the Hydra may be multiplied by being cut in pieces. 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 inner layer or endoderm assumed the characters of the outer layer or 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. Besides this asexual mode 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 longer than the body. The mouth is slightly raised on a disc or hypostonre. Of the two layers of cells, the outer or ectoderm is transparent, 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 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 further down. Both male and 132 CCELENTERA. female organs may occur on the same animal, either at one time or at different times, but often they occur on different individuals. The buds have the same structure as the parent body, but in origin they appear to be wholly ecto- dermic. Minute structure. — The outer layer or ectoderm includes the following different kinds of cells : — (1) 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. 57, E). The cells themselves are contractile, but there is special contractility in the roots. Like the muscle cells of higher animals, they contract under certain stimuli, and are often called “ neuro-muscular.” But the discovery of special nerve cells (Jickeli) shows that even in Hydra there is a differentiation of the two functions of contracility and irritability. (2) Small stinging cells or cnidoblasts occur abundantly on the upper parts of the body, especially on the tentacles. Each contains a pro- trusible nematocyst. This consists of a sac, the neck of which is doubled in as a pouch, usually bearing internal barbs, and prolonged into a long, hollow, spirally coiled filament or lasso. This lasso is bathed in a fluid, presumably poisonous. On its free surface the sting- ing cell usually bears a delicate trigger hair or cnidocil. Under stimulus, whether directly from the outside or from a nerve cell, the cnidoblast explodes and the nematocyst is thrown out. Besides the ordinary stinging cells, there are others of small size which do not seem to explode. (3) Scattered about there are minute nerve cells, with fine connec- tions, especially with the muscular and the stinging cells (Fig. 57, B). (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 p6eudopodia, and so move the animal slowly. The inner layer or endoderm is less varied in structure, as is to be expected from the fact that it is not, like the ectoderm, exposed to the varying action of the environment. Its cells are pigmented, often vacuolated, and most of them are either flagellate or amoeboid. The pigment bodies in H. viridis seem comparable to the chlorophyll cor- puscles of plants ; in H. fusca they are brownish and without chloro- phyll. 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 TYPES OF CCELENTERA HYDRA. r33 of some of the cells, and it has been noticed that the same cell may be at one time flagellate and at another time amoeboid (cf. the cell-cycle, p. 96). After this direct absorption the food is digested within the cells, and while some of the dark granules seen in these 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 Ccelentera, and some simple “ worms.” But experiments show that some of the food may be Fig. 57. — Minute structure of Hydra. — After T. J. Parker and Jickeli. A. Ect., ectoderm ; mg., mesoglceal plate ; st.c., stinging cell ; End., endo- derm with flagella and amoeboid processes. B. nc., nerve cell, and st.c., stinging cell. C. Stinging cell with ejected thread ; nucleus. D. Mesogloeal plate (mg.) with contractile roots resting on it. E. m.c., muscular cell with contractile roots, c.r. digested in the gut cavity, and subsequently absorbed. Thus it seems that both intracellular and extracellular digestion occur. 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. 134 C (ELENTERA. The middle lamina, representing the mesogloea, is a thin homogeneous plate, on each side of which lie the muscular roots of ectodermic and endodermic cells (Fig. 57, 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 Ccelentera with the epiblast and hypoblast which embryo- logists 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. 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 may summarise these as follows, comparing them with the characteristics of epiblast and hypoblast in higher animals : — Outer Layer. j Median Layer. Inner Layer. In Hydra the ectoderm forms — Covering cells, stinging cells, nerve cells, muscle cells, etc. None in Hydj'a. 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, seveial (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. 58, 1). (/>) Usually there is but one female organ or ovary, but in //. fusca as many as eight have sometimes been observed. The ovary arises like the testes from a nest of interstitial cells, one of which becomes the ovum. In rare cases there are two ova. The ovum is at first amceboid and transparent, but, like many other ova, it feeds on its neighbours, loses its amoeboid form, and becomes rich in nutritive material and in pigment. The same process of exploitation is well seen in the oogenesis of Tubularia larynx, a common marine polype. It illus- trates the struggle for existence among germ cells. 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- TYPES OF CCELENTERA HYDRA. 135 cells.” With increase of size the ovum changes its form from amceboid 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 in the usual way, and two polar bodies are extruded at the distal pole. There- after 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. 58, 4). The segmentation which follows is total and equal, and results in the Fig. 58. — 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 (s/i) around it. 4. Ovum protruding ; n, the nucleus ; ect., the ruptured ectoderm ; end., the endoderm. 5. Section of blastosphere — Ect., ectoderm; Etui., endoderm — being formed. 6. Section of larva. Ect., ectoderm ; End., endoderm ; g.c., gut cavity; s/i., ruptured envelopes. formation of a blastosphere (Fig. 58, 5). By inwandering, or by divi- sion of the cells of the blastosphere, an internal endoderm is formed, and this formation takes place on all sides. In a word, it is multipolar. The segmentation cavity of the blastosphere is thus fdled up, and the two layers become differentiated from one another. The outer or ectodermic layer forms — ( a ) an external “chitinoid” shell of several layers ; (A) an internal membrane, homogeneous, thin, and elastic ; and (c) the future ectoderm of the adult. In Hydra ftt.ua 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. grt'sea the egg falls off after the outer shell has been formed. In CCELENTERA. 136 all species the separation from the parent appears to be followed by a period of quiescence lasting from one to two months. 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. This should be compared with the hydri- forrn organism believed to be connected with the fresh-water Medusoid Limnocodium found in the Victoria Regia tanks in the Botanic Gardens, Regent’s Park, London, and also in African lakes. A strange simple polype — 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 Hydro- zoa. Sub-Class Hydromedusae or Craspedota. Hydra is too simple to be thoroughly typical of the Hydrozoa. The class includes the hydroid colonies or zoo- phytes, 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, instead of hollow as in Hydra , and they may be arranged in two circles, — an outer and an inner ( Tubularia ). In some of the hydroid colonies, notably the Millepores and Hydrac - titiia , the polypes 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 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. 1 he margin of TYPES OF CCELENTERA A MEDUSOID. 137 the bell also bears tentacles, usually hollow, and abundantly furnished with stinging cells (Fig. 55, 3). On the convex surface of the bell 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 is ciliated ; it lines the food space, 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 with the cavities of the tentacles. Digestion is intracellular, and probably goes on throughout the whole of this “ gastro- vascular ” system. The movements of the bell are caused by the contractions 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 associated 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” vesicles developed as pits in the velum (Vesi- culate). 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 YVeismann and others have shown that the reproductive cells of a medusoid derived from a hy droid, or of the reduced and fixed reproductive persons of 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 opening, and usually without a central cavity. In the case of those medusoids which arise as liberated sexual members of CCELENTERA. 138 a fixed asexual hydroid colony, the planula settles down, loses its cilia, buds out tentacles, and develops into a new hydroid. In many Hydrozoa, as has been already noticed, the sexual persons are not set free, but remain as buds attached to the parent hydroid. 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 Hydractinia , are simply small closed sacs enclosing the genital products (Fig. 67). Third Type of Ccelentera. — The common Jelly-fish — Aurelia aurita. Class Scyphozoa. Sub -Class Scyphomedusse or Acraspeda. 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 muscular flap or velarium. Conspicuously bright are the four reproductive organs, which lie towards the under sur- face. 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. 59). Tlie three layers. — The ectoderm which covers the external surface bears stinging cells, sensory and nerve cells, and muscle cells. According to some, the ectoderm lines part of the mouth-tube or manubrium. The endoderm lines the TYPES OF CCELENTERA — AURELIA AURITA. 139 digestive cavity, is continued out into its radiating canals, and is ciliated throughout. The mesoglcea is a gelatinous coagulation containing wandering amoeboid cells from the endoderm. The whole animal is very watery ; indeed, the solid parts amount to not more than ten per cent, of the total weight. Nervous system. — The nervous system consists — (a) of a special area of nervous epithelium, associated with each of the eight sense organs, and (b) 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 that in Craspedote medusoids, but too much must not be made of the contrast, for a nerve-ring is described in Cubomedusse, one of the orders of Acraspe- dote jelly-fish. In Aurelia the sense organs are less differentiated than in many other jelly-fish. Each of the eight organs, protected in a marginal niche, consists of a pigmented 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 modi- fications of tentacles, and are often called “ tentaculocysts ” or “rhopalia.” Their cavities are in free communication 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 substance 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 mesoglcea. They exhibit considerable mobility. Their crumpled and mobile bases surround and almost conceal the mouth. A short tube, the “ manubrium ” or gullet, connects the mouth with the central digestive cavity, which occupies 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 140 CCF.LENTERA. 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 manubrium or tube from the mouth passes into the central digestive cavity, there are four strong pillars of thickened sub-umbrellar material. Outside each of these pillars, and still near the base of the manubrium, there are four patches where the sub - umbrellar surface remains thin. These are the gastro-genital membranes, lined inter- nally by germinal epi- thelium (Fig. 60, _/?.). To the inside of these genital organs, within the digestive cavity, are' four groups of mobile gastric filaments {g.f., Fig. 60), which are very character- istic of jelly - fish. In appearance these are very similar to the small tent- acles of the margin, and, like them, are hollow. They are covered with endo- derm — 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.” Halfway between these, four “ inter- radials” are then developed. Then eight “adradials” may follow, between perradii and interradii. Reproductive system. The sexes are separate. The reproductive organs — ovaries or testes — consist of plaited ridges of germinal epithelium, situated on the four patches already mentioned, within sacs which are derived from and Fig. 59. — Surface view of Aurelia. — From Romanes. Showing four genital pockets in centre, much branched radial canals, eight peri- pheral niches for sense organs, and peri- pheral tentacles. TYPES OF CCELENTERA — AURELIA AURITA. 141 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 extend all round 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 communicating with the exterior (g.fl., Fig. 60). The con- tractions of the umbrella produce a rhythmic movement of the water which enters the sub-genital sacs, and this constant renewal of the water suggests some respiratory significance for the sacs. It must be understood that the genital sacs containing the plaited ridges of germinal epith- elium communicate with the gastric cavity only, while „the sub - genital cavities containing water and enveloping the genital sacs communicate with the exterior only. The ova and sperma- tozoa 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. Life history of Aurelia. — The fertilised ovum divides completely, but not quite equally, to form a blastosphere, with a very narrow slit-like cavity. From the larger-celled hemisphere, single cells migrate into the cavity, and fill this up with a solid mass of endoderm. The archenteron 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 plan ala. After a short period of free life, 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 mesoglcea 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 Fig. 60. — Vertical section of Aurelia. — After Claus. /«., Mouth ; st. , stomach ; r.c., radial canal ; K., reproductive organs ; g.f., gastric filaments; g.p., genital pocket; marginal tentacle ; s., sense organ ; the shaded part is mesoglcea. 142 C (ELENTERA. ectoderm 1 is formed, which hangs freely in the general cavity. During 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 treniolre. 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. 6i. — Diagram of life history of Aurelia. — After Haeckel. i, Free-swimining embryo: 2-6, various stages of Hydra-tuba: 7, 8, Strobila stage ; 9, liberation of Ephyrm ; io, n, growth of Ephyrte into Medusae. trcnioke (contrast the endodermic muscles of Anthozoa). In contrasting this development with that of the hydroid polype, 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 1 The statement as to the ectodermic gullet is due to Goette (1SS7) : its existence is denied by Claus, who is followed by Chun. TYPES OF CCELENTERA AURELIA AURITA. 143 and ectodermal muscles, thus confirming his view of the intimate relation between the Actinozoa and ScyphomedusEe. 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 forma- tion of creeping stolons, it may give rise to larvae like itself. 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, as has been observed by Haeckel, the Scyphistoma becomes detached and converted into a free- swimming Ephyra, which in turn becomes a jelly-fish. (6) Sometimes, in unfavourable conditions, a furrow appears round the upper region of the Scyphistoma, the upper portion is converted into an Ephyra, and floats away, while the lower portion reforms its oral region by regenera- tion, and produces another Ephyra. (c) In ordinary conditions the Scyphistoma elongates, and displays a succession of annular constric- tions. This stage, often compared to a pile 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 con- striction. This is probably an abbreviation of the primitive mode of development. In the conversion of the Scyphistoma into the Ephyrse, 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 Ephyne which follow there are at first no tentacles, only the eight bifid marginal lobes which bear the sense organs in their niches. 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 Ephyra; are liberated. Each of these grows into a sexual jelly-fish, producing ova or spermatozoa. The first two cases mentioned {a and b) show how readily this alternation might pass into a “direct ” development. Relatives of Aurelia. — The Meduste, or true jelly-fish, include forms which agree with theAnthozoa, in relative complexity of structure as compared with Hydrozoa, and in the possession of an ectodermal gullet (see footnote on p. 142), but differ in possessing ectodermal septal muscles and in some histological features. If Goette’s discovery of rudimentary ectodermal muscles in the larva; of certain sea-anemones be confirmed, however, it would greatly increase the probability of a close relationship between the two sets. Among the Scyphomedusre closely allied to Aurelia , some, e.g. Pelagia , have a direct development without the intervention of Scyphistoma or Strobila stages, but this may occur 144 CCELENTERA. exceptionally in Aurelia. Cyanea is often very large, “it may measure 7i ft. across the bell, with tentacles 120 ft. long.” Chrysaora is her- maphrodite, and has diffuse sperm sacs even upon the arms. In the Rhizostomce, 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 re- garded as persistently larval forms. We may note here that Chun, while agreeing provisionally to the separation of the Acraspeda Fig. 62. — Lucernaria. — After from the Hydrozoa, strongly Korotneff. opposes their association with the Anthozoa, basing his opposition especially on the existence of ' Scyphistomas of great simplicity {e.g. Spongicola). Contrast between Hydrozoon Medusoids and Scyphozoon Medusa ?. Hydrozoon. (Craspedota.) Scyphozoon. (Acraspeda.) The majority are small “swimming bells.” A flap or velum (craspedon) projects in- wards from the margin of the bell. No taeniolae, nor gastric filaments. A double nerve-ring around the margin. Naked sense organs either optic or audi- tory. 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 persons of Hydroid colonies. True Hydrozoa. Many are large “jelly-fish.” No velum. (The velarium of Aurelia is a mere fringe, very inconspicuous in the adult, and not inturned.) In the Scyphistoma there are four tamiolac, from part of which the gastric filaments of the adult grow. Eight separate nervous centres be- side 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. Reproductive 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 polype stage (or Scyphistoma) in the course of their life history. Probably more nearly related to Anthozoa than to Hydrozoa. TYPES OF CCELENTERA — A SEA-ANEMONE. 145 Fourth Type of Ccelentera. — A Sea-Anemone, such as Tea/ia crassicornis. Class Scyphozoa. Sub -Class Anthozoa or Actinozoa. Most sea-anemones live fixed to the rocks about low- water mark. All these fixed forms have a distinct basal disc, and may, like Tenlia crassicornis , be half buried in sand and gravel ; others, without a basal disc, are loosely inserted in the sand, e.g. Edwardsia and Cerianthus , but all are able to shift their positions by short stages. They feed on small animals, — molluscs, crustaceans, Fig. 63. — External appearance of Tealia crassicornis. worms, which are caught and stung by the tentacles ; but many depend largely on minute organisms, while others may be seen trying to engulf molluscs decidedly too large for them. A few anemones, without pigment or with little, have symbiotic Algte 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. External appearance of a fixed Anemone. — The cylindrical body is fixed by a broad base ; it bears whorls 10 146 CCELENTER A. 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 otherwise by a pore at the tip of each tentacle, and long white threads, richly covered with stinging cells, can be ejected in many anem- ones through the walls of the body (Fig. 64). General structure. — The Anthozoon polype differs markedly from the Ilydroid polype — not only because an invagination from the oral disc inwards has formed a gullet tube, which hangs down into the general cavity, but also because a number of partitions or mesen- teries extend from the body wall towards this gullet. Some of the partitions are “ com- plete,” i.e. they reach Fig. 64. — -Vertical section of a sea- anemone. — After Andres. t., Tentacles ; o., mouth ; tvs., oesophagus ; c., c'., apertures through a mesentery ; a., a'., acontia ; g., genital organs on mesentery ; m.f., mesen- teric filaments; in.l., longitudinal muscles; s., primary septum or mesentery ; s'., second- ary septum ; s"., tertiary septum disc. 7'., hasal 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 in- growths of the body wall. The cavity of the anemone is thus divided into a number (some multiple of six) of radial chambers. These are in communication at the base, TYPES OF CCELENTERA A SEA- ANEMONE. 147 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 longitudinal slit, and its two corners are often richly ciliated. The gullet is marked with longitudinal grooves, two of which, the “siphonoglyphes,” corres- ^ pond 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. Occa- sionally 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 mesenteries bear — (a) mesenteric fila- mf m l ments ; (b) retractor Fig. 65. — Section through sea- anemone (across arrow in Figure 64). — After Andres. A, B, directive septa ; in./., mesenteric filaments; g., genital organs; nr./., longitudinal muscles ; s., primary septum ; r'., secondary septum ; .s'". . ter- tiary septum. The arrow enters between two primary septa (an intra-septal cavity), and passes out between two tertiary septa. muscles ; (c) ridges of repro- ductive cells, almost always either ova or spermatozoa, rarely both ; and ( d ) in some cases offensive threads or acontia. The mesenteric filaments seem to be closely applied to the food, and perhaps secrete digestive juice, 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 stinging cells. The nervous system is un- centralised, and consists of superficial sensory cells con- nected with a plexus of sub-epithelial ganglion cells. 148 CCELENTERA. 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 meso- gloea 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 mesen- teries, and the gullet. Nor are glandular and even sensory 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 cornels 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 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 teenioke of jelly-fish, and the mesenteric with the gastric filaments. From the above description it will be noticed that the funda- mental radial symmetry of the Coelentera has here become profoundly modified. Development. — Comparatively little is known in regard to the early stages of development in sea-anemones. From the fertilised ovum a blastosphere 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 established by a process known as delamination, in which a single layer of cells is divided into an inner endodermic and an outer ectodermic layer. According to Gcette, the development is in essentials the same as that of the Hydra-tuba. 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 Alcyonimn and the related forms. This contrast is not very satis- factory, but it rests on such distinctions as the following : — TYPES OF CCELENTERA A SEA-ANEMONE . 149 Anthozoa or Actinozoa. ZOANTHARTA, HeXACORALLA, e.g. Sea-Anemone. Alcyonaria, Octocoralla, e.g. Dead-Men’s-Fingers. Many are simple, many colonial. Tentacles usually simple, usually some multiple of six, often dissimilar. Mesenteries usually some multiple of six, complete and incomplete. Retractor muscles never as in Alcyonaria. Two gullet grooves or siphonoglyphes, or only one. Dimorphism only in some Antipatharia, and in one Madrepore coral. Calcareous skeleton, if present, is derived from the basal ectoderm. Types. Actiniaria. Sea-anemones. Madreporaria. Reef-building corals. Antipatharia. Black corals. All colonial, except a small family in- cluding Monoxenia and Haimea. Tentacles eight, feathered, uniform. Mesenteries eight, complete. Retractor muscles always on one (ven- tral) side of each mesentery ( sec Fig. 66). One (ventral) gullet groove or siphono- glyphe, or none. Occasional dimorphism among members of a colony. There are usually calcareous spicules (of ectodermic origin) in the mesoglcea. Examples. Alcyonium (Dead-men’s-fingers), with diffuse spicules of lime. Tubipora (Organ - pipe coral), with spicules fused into tubes and trans- verse platforms. Corallium rubrutn (Red coral), with an axis of fused spicules. /sis, with an axis of alternately limy and “ horny ” joints. Pennatula (Sea-pen), a free phosphor- escent colony, with a “ horny” axis, possibly endodermic. Heliopora , blue coral. z A Fig. 66. — Z, Diagrammatic section of Zoantharian ; A, of Alcyonarian. — After Chun. i he line 6*- 6* in Z is through the siphonoglyphes (a), the line 1 ~F passes through two interseptal 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. CCELENTERA. !5° SYSTEMATIC SURVEY. A. Class Hydrozoa. There are two types, polypoid and medusoid, which may be com- bined in one life history. The mouth leads directly into the gastric cavity. The mesoglcea is simple, and without migrant cells. The reproductive cells seem to be usually ectodermic. t. Order Hydromedusse, — Simple or colonial forms in which the sex- ually reproductive persons are either, liberated as free-swimming medusoids or are sessile gonophores. (a) Hydrophora.— Two types are included here. The first includes the Tubularians, Hydractinia, and other forms in which the polypes are not enclosed in the protective sheath which often surrounds the colony (gymnoblastic), and in which the free medusoid forms, when present, have their genital organs placed in the wall of the manubrium (Anthomeduste), and are furnished with ocelli placed at the base of the tentacles. Hydra and its allies may be included here. Examples : — Syncoryne sarsii, the free medu- soid of which is called Sarsia iubulosa. Bougainvillea ramosa liberates the medusoid Margelis ramosa. Cordylophora lacustris and Tubularia larynx have sessile gonophores. The second type includes Caiupanu- larians, Sertularians, I’lumularians, and others, in which the protective sheath surrounding the colony is continued into little cups enclosing the polypes (calyptoblastic). The free medusoids have their gonads placed in the course of the radial canals (Leptomedusae), and are either “ocellate” or “vesiculate.” Examples : — Plumularia and Sertularia have sessile gonophores. Campanularia geniculata liberates the medusoid Obeli a geniatlata. {b) Hydrocoral linK. — Colonial forms which suggest the Hydractiniae in their polymorphism and division of labour, but are distinguished by Fig. 67. — Diagram of a gymno- blastic Hydromedusa. — After Allman. a., Stem ; 6., root ; c., gut cavity ; d., endoderm (dark); c., ectoderm; /.’ . horny perisarc ; g. , hydra-like “ person ” (hydranth) ; g'., the same, contracted ; h., hypostome bearing mouth ; k., sac-like repro- ductive bud (sporosac) ; in ., a modified hydranth (blastostyle) bearing sporosacs ; /., medusoid “ person.” .S' YSTEMA TIC SUR VE V — SC YPHOZOA. '51 their power of taking up lime, and so forming “ corals.” The colonies are complex and divergent, the reproductive persons are probably sessile gonophores, but a simple male medusoid has been described. Mille- pora, Stylaster. (c) Trachymedusse. — These exist only in the medusoid form, and are divided into two groups, Trachomedusm and Narcomedusa;, according to the position of the gonads. Geryonia, Carmarina, Cunina, Aeginopsis. 2. Order Siphonophora. — Free - swimming colonies of modified medusoid persons (medusomes), with much division of labour. Thysa/ia (Portuguese man-of-war), Diphyes, Velella , Porpita. B. Class Scyphozoa. There are two types — polypoid and medusoid — very rarely occurring in one life history. The gastric cavity has partitions with gastric or mesenteric filaments, and there is an ectodermic gullet. The nresogloea generally contains migrant cells. The reproductive cells are endo- derrnic. I. Sub-class Scyphomedusse or Acraspeda — Jelly-fish with gastric filaments, sub-genital pits, and no velum — (1) Lucernariae. — Sessile forms. Lucernaria. (2) Discomedusse. — Active forms, often with complicated life history. Aurelia, Pelagia, Cyanea, Rhizostoma. (3) Cubomedusae. — Forms with broad pseudo-velum, and other peculiar features. Charybdea. (4) Peromedusae. — Forms with four tentaculocysls only. Pericalpa. II. Sub-class Anthozoa, or Actinozoa — Polypoid forms with well-developed gullet and septa, and circumoral tentacles. (1) Zoantharia or I lexacoralla. (a) Actiniaria. Sea-anemones. Actinia, Ammonia, Tealia, Cerianthus. ( b ) Madreporaria. Stone or reef corals. Astraia, Madrepora, Fungia, Mieandrina. (c) Antipatharia. “Horny” black corals, with an axial skeleton, and occasional dimorphism between nutritive and reproductive “persons.” Antipathes. (2) Alcyonaria or Octocoralla. Alcyonium (Dead-men’s-fingers), Tubipora (Organ- pipe coral), Corallinm (Red coral), Gorgonia, Pen- natula (Sea-pen), Monoxenia (non-colonial). C. Class Ctenophoka. Delicate free - swimming organisms, generally globular in form, moving by means of eight meridional rows of ciliated plates, or comb- 152 CCELENTERA. like combinations of cilia. The stinging cells are almost always replaced by “adhesive cells.” The mouth is at one pole, and leads into an ectodermic gullet. The gastric cavity is usually much branched. The mesenchyme is very 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 Lang, they have affinities with Planarian “worms,” but this is very uncertain. Examples : — ( a ) With tentacles, Cydippe and the ribbon-shaped Venus’ Girdle (Cesium 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 (Palteozoic) 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 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 Ccelentera 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 Ccelentera, 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 seems very likely that the ancestral many-celled animals — hncestral 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 simplest sponges SYSTEM A TIC SUN VE V — CTENOPHORA. 153 (Ascones), the simplest Ccelenlera {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- 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 GENERAL SCHEME OF CCELENTERA. Predominantly Passive. Predominantly Active. C. Ctenophora, e.g. Beroe, Venus’ Girdle. (Active climax.) B. ScYPHO- i ZOA. t II. Anthozoa or Actinozoa. (Zoantharia) Sea-anemones and related corals. (Alcyonaria) Dead - men’s- fingers and related corals. The embryos are free-swimmers, and a few adults also are locomotor. j (Scyphomedusae or (Acraspeda. c. Adult Lucernarians usually attached. b. Sedentary larval stage. a. No fixed stage. c. Free embryos. b. Aurelia type of jelly-fish. a. Pelagia type of jelly-fish. ANCESTRAL GASTRA3A. \ Hy zo / t. DRO-- A 'i. No fixed stage. 2. No fixed stage. 3. Many Hydroid colonies. (Campanularians and Tubularians.) 4. Many Hydroid colonies, whose reproductive per- sons are not liberated. 5. .Coralline Millepores. . 6. Hydra without any speciallj 1. Trachymedusae (always locomotor). 2. Siphonophora (locomotor colonies of modified medusoids). 3. Liberated reproductive “persons” of these colonies. 4. No free stage, except as embryos. 5. A male medusoid is known, locomotor stage. 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 sexuality, and with a preponderant tendency to bud, we can understand how polypes arose, and these of two types, one nearer the jelly-fish and Lucernarians and leading on to sea-anemones and corals, the other nearer the swimming bell type and leading on to a terminus in Hydra. 154 CCELENTERA. 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 polypes (Sea-anemones, etc.), which only occasionally rise into free activity ; while in the other series we have medusoid types always free (Trachymedusse), others which are liberated from (Campanularian and Tubularian) sedentary hydroids, other (Sertularian and Plumularian) zoophytes whose buds though often medusoid-like are not set free, and finally Hydra, which, though it may creep on its side, or walk on its head, is predominantly a sedentary animal, without any youthful free-swimming stage. It must be noticed that the most frequent larval form is the planula, so that, if we regard the gastrula as the ancestral type, the life history is not here a recapitu- lation of the race history. Bionomics. — The Coelentera are almost all marine. In fresh water we find the common Hydra , the minute Micro- hydra without tentacles, the strange Polypodium, which in early life is parasitic on sturgeons’ eggs, the compound Cordylophora, occurring in canals and in brackish water, and the fresh-water Medusoid (Limnocodium) found in a tank in the Regent’s Park Botanic Gardens, and another similar form recently discovered in Africa. Most of the active swimmers are pelagic, but there are also a few active forms in deep water. Many polypes anchor upon the shells of other animals, which they sometimes mask, and there are most interesting constant partnerships between hermit-crabs and sea-anemones, e.g. between Pagurus prideauxii and Adamsia palliata. 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 partnership or commensalism, which, however, in some other animals may degenerate into parasitism. (See Fig. 1 6). CHAPTER X. UNSEGMENTED “ WORMS.” ' A. Turbellaria, Trematoda, and Cestoda (the Platyhelminth or Flat-worm Series). Chief B. Nemertea or Nemertini. Classes C. Nematoda, Gordiacea, and Acantho- cephai.a (the Nematohelminth or Round- . worm Series). 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. But the zoological interest of the diverse types of “ worms ” is great. For amid the diversity we discern affinities with Coelentera, Echinoderms, Arthropods, Molluscs, and Vertebrates. Moreover, it is likely, as has been already noted, 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 insink- ing of ectodermic cells, and its beginning in the cerebral ganglion of the simplest “worms” is thus in part explained. Again, it may be noted that worm types begin the series of triploblastic calcinate animals, i.e. of those which have a UNSEGMENTED “WORMS.” 156 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 coelom and mesoderm is very gradual ; thus there is practically no coelom in the Platyhelminthes, and the mesoderm is only represented by “ mesenchymatous ” cells, comparable to those of Ctenophora. Class Turbellaria. Planarians, etc. Turbellarians are unsegmented “ worms,” living in fresh , brackish , or salt water , or in moist earth. Almost all are carnivorous, a few are parasitic. They represent the begintiing of defin ite bilateral symmetry. The ectoderm is ciliated , and contains peculiar rod-like bodies ( rhabdites ), and occasionally stinging cells. 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. There are no special respiratory or circulatory organs; the body cavity is represented at most by small spaces ; the excretory system usually consists of two longitud- inal cafials, whose branches end internally in ciliated (flame ) cells. Excepting two genera, the Turbellarians are her- maphrodite ; 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 ( hyperti-ophied ) part of the ovary. The eggs are usually enclosed in shells or cocoons , and the development may include a metamorphosis. The Turbellarian worms form an exceedingly interesting group ; they are often beautiful, and the ciliated ectoderm enables them to move with singular grace. Although the lateral 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 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. Land Planarians are elongated and more worm-like in shape ; they may measure a foot or more in length, and are most abundant in tropical countries. There seems little doubt that the next two classes (Trematoda and TURBELLAR1A. >57 Cestoda) have arisen from Turbellarian-like ancestors, which adopted parasitic habits, as a few marine Turbellaria have done. Classification. — A. Rhabdocoelida. — Small fresh-water and marine forms. The food canal is very slightly branched, or quite straight, or absent. (1) Accela. Without intestine, e.g. Convoluta, which contains green cells, regarded by some as symbiotic Algm. (2) Rhabdoccela. With straight intestine, e.g. Vortex ; Microstoma , l.c Ms eo. Fig. 68. — Diagrammatic figure of a simple Turbellarian. m., Mouth pharynx ; g. , digestive part of gut ; l.e., longitudinal excre- tory vessels; e.p., excretory pore; Ect., ciliated ectoderm; Ms., meso- derm ; End., endoderm. a unisexual fresh-water genus, with stinging cells, which forms tempor- arily united asexual chains, sometimes of sixteen individuals, suggesting the origin of a segmented type; Graffilla and Anoplodium , parasitic (cf. next class). (3) Alloioccela. With lobed or irregular gut. All marine except one from Swiss lakes (Plagiostoma Leinani). B. Tricladida. Elongated flat “ Planarians ” ; the mouth and tubular pharynx lie behind the middle of the body ; intestine with three Fig. 69. — Diagrammatic figure of part of the structure of a simple Turbellarian. Ect., Ciliated ectoderm ; e.g., cerebral ganglion; lateral nerve; 7’., testes ; ov., ovary. UNSEGMENTED “IVOR MS.” iS8 main branches, themselves branched ; two ovaries, numerous yolk glands and testes, a common genital aperture, e.g. Planaria and Dendroccdum (in fresh water), the former sometimes divides trans- versely ; Gunda segmentata (marine), showing hints of internal segmentation ; Geodesmus and Bipalium (in damp earth) ; Bipalium kewense is an import often found in Britain. C. Polycladida. Large leaf - like marine “ Planarians,” with numerous intestinal branches diverging from a central stomach ; with numerous ovaries and testes, without yolk glands, mostly with two genital apertures. e.g. Cycloport/s (showing beginning of anus), Leptoplana (not uncommon on the seashore), Thysanozoon. Class Trematoda. Flukes, etc. The Trematodes are leaf-like , or roundish external or internal parasites. With their parasitic life may be associated the absence of cilia on the surface of the adults , the well-formed and apparently cellular “ cuticle f the presence of attaching suckers (occasionally with hooks), and the rarity of sense organs. It is likely that they have arisen from free Turbellarian-like attcestors, and they resemble the Turbe}- l aria ns in being unsegmented , in having a/iterior fierve centres, from which nerves pass backward a?id fonvard, 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 of Turbellaria. The alimentary canal is usually forked, often much branched, and always ends blindly. In many cases the animals are self impregnating, but crossfertilisation 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. Type, The Liver Fluke ( Distomum hepatic// m or Fasciola hepatica). The adult fluke lives as a parasite in the bile ducts of the sheep. It sometimes occurs in cattle, horses, and other domestic animals, rarely in man. In the sheep it TRE.UA tod a. 1 59 causes the serious disease called liver rot. The animal is flat, oval, and leaf-like, measures almost an inch in length by half an inch across the broadest part, varies from reddish brown to greyish yellow in colour. As the word Distomum suggests, there are two suckers, — an anterior, perforated by the mouth ; a second, imperforate, a little further back on the mid- ventral line. There is a muscular pharynx and a blind alimentary canal which sends branches through- out the body. The nervous system consists of a ganglionated collar round the pharynx, from which nerves go forward and backward ; of those which run backward, the two lateral are most important. Although the larva has eye spots to start with, there are no sense organs in the adult. The body cavity is repre- sented by a few small spaces. Into these there open the internal ciliated ends of much-branched excretory tubes, which unite posteriorly in a terminal vesicle opening to the exterior. Fig. 70.— Structure of liver fluke. — After Sommer. From ventral surface. 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. hi., Mouth; ///., pharynx ; g., lateral head ganglion ; v.s., ventral sucker ; c.s., position of cirrus sac ; an arrow indicates the ex- cretory aperture. 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. i6o UNSEGMENTED ‘ ‘ WORMS. ” Tl'encc they are expelled by an ejaculatory duct, which passes through a muscular protrusible penis. The retracted penis and the seminal Fig. 71. — Reproductive organs of liver fluke. — After Sommer. f. Female aperture. s.v. Seminal vesicle. y.gl. Diffuse yolk glands. sh.g. Shell gland. v.d. Vas deferens. T. Testes (anterior). ov. Ovary (dark). vt. Uterus. c.s. Cirrus sac. p. Penis. m. Mouth. g. Anterior lobes of gut. 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 TREMA TOD A. 1 6 1 duct. Nutritive cells are gathered from very diffuse yolk glands, collected in a reservoir, and pass by a duct into the end of the aforesaid 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 he a copulatory duct ; in others it is regarded as a safety valve for over- flowing 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 half a million 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 must die. The free embryo 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 (Limriceus truncatulns or minutus), into which it bores its way. In an epidemic among horses and cattle in the Hawaiian Islands, the host was L. cahuetisis. Within the snail, eg. in the pulmonary chamber, the embryo becomes passive, loses its cilia, increases in size, and becomes a sporocyst. Sometimes this sporocyst divides transversely (Fig. 72 (4)). Within the sporocyst certain 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 redice burst out of the sporocyst, and migrate into the liver or some other organ, killing the snail if they are very numerous. Indeed, the death of the snail is probably necessary for the escape of 162 UNSEGMENTED “ WORMS i. Developing embryo in egg - case ; 2. free - swimming ciliated embryo ; 3. sporocyst ; 3a. Shell of Limtueus truncatulus ; 4. division of sporo- cyst ; 5. sporocyst with redia; forming within it ; 6. redia with more redia: forming within it ; 7. tailed cercaria ; S. young fluke. THEM A TOD A. 163 the final larvae. Each redia is a cylindrical organism with a short alimentary canal (Fig. 72 (6)). Like the sporocysts, the rediae give rise internally to more embryos, of which some are simply rediae over again, while the last set are quite different, — long-tailed cercarice, with two suckers and a forked food canal. These emerge from the rediae, wriggle out of the snail, pass into the water, and 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, it grows, in about six weeks, 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 redite. These develop, how- ever, 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 ; and in the winter 1879-80 the mortality attributed to fluke disease was estimated at three millions. It is especially common after wet seasons, and in damp districts. Classification. — Trematodes may be conveniently arranged, though not exactly classified, according to their mode of development. A. Trematodes with direct development — Monogenetic. e.g. Polystomum integerrimum. This form with many suckers is often found 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, where it develops slowly for years. Gyrodactylus, found on the gills and fins of fiesh-water fishes. It is viviparous, but the embryo, before it is extruded, itself contains an embryo, and this in turn another, so that three generations of embryos are re- presented simultaneously. Diplozoon paradoxical consists of two individuals united. The single embryo ( Diporpa ) is at first free-swimming, but becomes a parasite on the gills of a minnow, and there two individuals unite very closely and permanently. Tristomuni, with three suckers, is not uncommon on the skin of some marine fishes. B. Trematodes with indirect development — Digenetic. e.g. Distomum, with numerous species. Bilharzia heematobius, a dangerous parasite of man, widely distributed in Africa, e.g. in Egypt. It infests the urinary and abdominal blood vessels, causing inflammation, 164 UNSEGMENTED ‘ ‘ WORMS. hsematuria, etc. The sexes are separate, and the male (about half an inch in length) carries the more thread-like female (about an inch in length) inserted in a groove or gynsecophoric canal. Man is probably infected from bad water, but the history of the parasite is still uncertain. The embryos are passed out in the urine. Monostomum , with one sucker, adults in ducks, young in Planorbis. The relationships of the class are on the one hand with the free-living Turbellarians, on the other hand with the parasitic Cestodes. A few interesting simple forms like Temnocephala found in Crustacea and Vertebrates, seem not to be truly parasitic, for they live on minute organisms and only receive shelter from the host. In the ciliation of the ectoderm and in some other respects these forms closely resemble Turbellaria. Class Cestoda. Tape-worms. The Cestodes are internal parasites , whose life history incltides a bladde7'-worm(proscolex) and a tape-worm ( st?-ol>ila ) stage, the former in a Vertebj-ate or Invertebrate host, the latter ( with o?ie exception ) in a Vertebrate. Iti a few cases the body is unsegmented, e.g. Archigetes and Caryophyllteus, 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-ivorm 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 hooks, grooves, or suckers ensuring attachment to the gut of the host. The body-wall consists of a cuticle and a well-innervated epide?-mis, within which there is parenchymatous co/mective tissue, often with cortical deposits of lime, and at least two sets ( longitudinal and transverse) of unstriped muscles. The iiervous syste77i co7isists of two or i7ior e longitudinal twve- st7-a7ids and a7ite7-ior ganglio7iated co7/i/nissures ; there are no special se7ise orga7is. There is 710 almentary system: the parasite float i7ig in the digested food of its host abso7-bs soluble 77iaterial by its ge7ie7-al surface. The7-e is no vascular nor respiratory system, a7id a body cavity is represe/ited i/ierely by irregular spaces in the solid parenchymatous tissue. In so/ne of these spaces there are “flame cells,” which lie at the ends of CESTODA. 165 the fine branches ofi longitudinal 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 withiti another host into a pros co lex 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 organs, 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 further and further 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 is distended with little embryos within firm egg-shells. When the proglottis ruptures, these egg-cases are set free. In certain circumstances, the embryos, within their firmly resistent egg-shells, may be swallowed by the omnivorous pig. Within the alimentary canal of this animal the egg- shells are dissolved, and embryos bearing six anterior hooks are liberated. They bore their way from the intestine into the muscles or other structures, and there encyst. They lose their hooks, increase in size, and become passive, vege- tative, 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 scolex. This is afterwards everted, and then the bladder-worm consists of a small head attached by a short neck to a relatively large bladder. But UNSEGMENTED “WORMS.” 1 66 this remains quiescent, and without power of further develop- ment, unless the pig be eaten by some other Vertebrate. When man unwittingly eats “ measly ” pork, that is pork infested with bladder-worms, an opportunity for further Fig. 73. — Diagram of reproductive organs in Cestode joint. — Constructed from Leuckart. 07/., Ovary, with short oviduct; ut., “uterus;” t., diffuse testes ; sh.g., shell gland ; y.g., yolk gland ; v.d., vas deferens ; v., vagina ; r.s., receptaculum seminis ; l.c ., longitudinal excretory ducts ; t.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. 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 richly nourished, and buds off asexually a chain of joints. CESTODA. 167 As these joints are pushed by younger interpolated buds further and further from the head, they become sexually Fig. 74. — Life history of Tania solium . — After Leuckart. 1. Six-hooked embryo in egg-case ; 2. proscolex or bladder- worm stage, with invaginated head ; 3. bladder-worm with evaginated head ; 4. enlarged head of adult, showing suckers and hooks ; 5. general view of the tape-worm, from small head and thin neck to the ripe joints ; 6. a ripe joint or proglottis with branched uterus (cf. Fig. 73) ; all other organs are now lost. mature, developing complex hermaphrodite reproductive organs. The ova are fertilised, apparently by spermatozoa UNSEGMENTED ‘ ‘ WORMS.” 1 68 from the same joints ; the proglottis becomes distended with developing embryos. These ripe joints are liberated, the embryos are set free by rupture, and the vicious circle may recommence. Happily, however, the chances are thirty- five 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 oft a scolex or head, which, in another host, buds off the chain of proglottides. 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 exception 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 worm Tubifex rivulorum. Life Histories. Adult, Sexual, or Tape-worm Stage. 1. Teenia solium , in man, with four suckers and many hooks. 2. Teenia saginata or mediocanellata , in man, with four suckers, but no hooks. 3. Bothriocephalus latus , in man, with two lateral suckers, but no hooks, with less distinct separation of the proglottides than in Teenia. It may be n yards in length. 4. Teenia echinococcus , in dog. 5. Teenia ccenurus , in dog. 6. Teenia serrata, in dog. 7. Teenia cucumerina , in cat. 8. Teenia elliptica , in dog. Non-Sexual, Proscolex, or Bladder- worm Stage. 1. Cysticercus cellulosee , in muscles of the pig. 2. Bladder-worm in cattle. 3. The ciliated, free-swimming embryo becomes a parasite in the pike or burbot, but without a distinct bladder-like stage. 4. Echinococcus veterinorum , in do- mestic animals, and sometimes in man, producing brood capsules, which give rise to many “ heads.” 5. Ccenurus cerebralis , causing sturdie or staggers in sheep, producing numerous “ heads.” 6. Cystice?'cics pisi/ormis , in rabbit. 7. Cysticercus fasciolaris, in mouse. 8. Cysticercus , in dog-louse or perhaps in flea. 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. The medical student should consult Leuckart’s great work, “ The Parasites of Man,” part of which has been translated by W. E. Hoyle (Edin., 18S6). The three classes, Turbellaria, Trematoda, and Cestoda, taken to- gether, constitute the Platyhelminthes or Flat-worms — an interesting group, because its members illustrate so well the progressive degener- NEMERTEA. 169 Class Nemertea. Nemertines. 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, -pp ation associated with increasing parasitism, and also because of the relatively great simplicity. The three classes are undoubtedly nearly related, for forms like Temnocepkala connect Turbellaria and 1 rematoda, and the “ monozoic ” Cestodes like Arcliigetes, Ainphihna, and Caryo- phy llaits, connect Trematoda and Cestoda. Among the most striking of the Platyhel- rninth characters are the nature of the excretory and reproductive organs and the condition of the mesoblast. The excretory system, with its longitudinal trunks, its ramifying canals, and “flame cells,” is eminently characteristic, though it occurs in more or less modified condi- tion in higher forms. The reproductive organs are complex, show division of labour, and are furnished with ducts of their own, unconnected with the excretory system — a condition of affairs not common elsewhere. 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. There is no true body cavity, the space between gut and body-wall being filled with a packing tissue ; the absence of an anus is also important in this connection, the two characters taken together being held to indicate affinity with the Ctenophoia. -po -s Fig. 75. — Diagrammatic longitudinal section of a Nemertean (Amphiporus lactijloreus), dorsal view. — After M'Intosh. lew, — P pp-, I^fepscis pore; b., brain giving off the lateral nerve^brds («.); jw., oesophageal pocket; p. , pro- boscis lying within its sheath ; st., stilet of proboscis ; in., retractor muscles of proboscis ; g. , gut shown in outline at’.lhe sides of the proboscis; c., the three main _ longitudinal blood vessels which unite both anteriorly and posteriorly. 170 UNSEGMENTED “WORMS. a commissure round the proboscis , and tivo 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 organs simple. The development is in some cases direct, zvhile in others there is a peculiar pelagic larva. General Account of Nemertea. In appearance Nemertines are ribbon- or thread-like, and the cross-section is generally a flattened cylinder. They vary in size, from a Linens, 1 2 or more feet in length, to the pelagic Pelagonemertes, which is under an inch. The colours are often bright, and tend to resemble those of the surroundings. 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 dermis, consisting in part of connective tissue, and often in part gelatinous. The body is remarkably contractile, 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 the gut and the body-wall being filled up with connective tissue. In the larvae, however, a body cavity may be seen, either as an archicoele, i.e. the persistent segmentation cavity ( Linens obscurus), or as a schizocoele, i.e. a space formed by the cleavage of the mesoderm into two layers {Pilidium- larvae). 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. From the dorsal lobes a commissural ring rises and sur- rounds the proboscis sheath ; from the lower lobes two longitudinal nerve-stems run along the sides, and are some- times united posteriorly above the anus (Fig. 76, /.«.). It is interesting to find that in Drepanophonis the lateral nerve-stems are approximated ventrally, and in Langia, dorsally ; for these two approximations tend towards positions characteristic of the nervous GENERAL ACCOUNT OF NEMERTEA. 171 systems of Annelids and Arthropods on the one hand, and of Y erte- brates on the other. On each side of the head there is a ciliated pit communi- cating with the exterior through an open slit or groove, and communicating internally either with the brain itself or with adjacent nervous tissue. In those cases in which the development has been studied, these so-called lateral organs — After Burger. t i.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; c.tn.f circular muscles; 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. arise from epiblastic insinkings and oesophageal outgrowths. In the most primitive genus, Carinella, they are absent, except in one species. It has been suggested that they conduce to the respiration of the brain, which is rich in hsemoglobin, and they have even been compared with gill- slits. In some forms the groove through which they open to the exterior is rhythmically contractile. It has also been suggested that they are sensory. Apart from these organs, Nemertines are very sensitive, and in many this is associated 172 UNSE G MEN TED ‘ ‘ WORMS.” with a superficial nerve plexus. Tactile papillae and patches are often present ; eyes and eyespots are general ; and in some there are otocyst-sacs. Apart from the cephalic slits, the head also bears sensory grooves and terminal sensory spots. The mouth is ventral, and leads into a plaited glandular fore-gut or oesophagus, which is followed by a straight, ciliated mid-gut or intestine, with regularly arranged lateral caeca. Between the caeca run transverse muscle partitions. The anus is in most cases terminal. In a cavity along the dorsal median line there lies the remark- able proboscis. It is protruded and retracted through an opening above, or, in a few cases, within the mouth. It arises as an invagination from in front, and is a muscular, very richly innervated tube, some- times protruded with such force that it sepa- rates from the body, and then often retains its vitality for a long time, as if it were itself a worm. 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. 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 stinging cells or adhesive papillae. There is a hint of a similar structure in some Turbellarians, and the organ may be interpreted as one which was originally tactile, but which has become 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 con- taining a fluid with corpuscles (Fig. 75). In the majority there are three longitudinal blood vessels Fig. 77. — Transverse section of a simple Nemertean ( Carinella ). — After Burger. d. n. , Dorsal nerve ; ft.c., proboscis cavity ; g., gut; c.m., circular muscles; /.?«., longi- tudinal muscles ; d.v.tn., dorso-ventral or diagonal muscles ; l.v., lateral blood vessel. GENERAL ACCOUNT OF NEMERTEA. 173 or spaces, a median and two laterals, which unite anteriorly and posteriorly, and also communicate by numerous trans- verse branches. The vessels or spaces are remnants of a coelom. The blood is a colourless fluid, sometimes at least with nucleated elliptical corpuscles in which haemo- globin may be present. The excretory system is not fully known, but 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 ampullae 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 Prosadenophortis ) 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 creca, 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 excretion. 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 in- direct. (1) In Cerebral ulus, 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 cilia, the ear lobes lateral ciliated appendages ” (Hubrecht). Out of this, somewhat abruptly, the adult form arises. (2) In Linens there is a sedentary' larva, which has been interpreted as a reduced Pilidium, and is known as the “ larva of Desor.” (3) In Hoplonemertea the development is direct without metamorphosis. 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 recent discovery of flame cells in connection with the excretory system confirms the idea of Platyhelminth affinities ; but it is to be noticed that apart from the points mentioned above, the reproductive system is strikingly different. Professor Hubrecht has suggested that Nemer- tines exhibit affinities with Vertebrates, comparing proboscis sheath with notochord, and so forth. 174 UNSE GHENT ED ‘ ‘ WORMS. ” Classification. — t. Palseonemertea : No deep head fissure; no stilet ; mouth behind brain. e.g. Carinella, Cephalothrix, Carinoma, Folia. 2. Schizonemertea : A deep head fissure with a ciliated duct to the brain ; lateral nerves between the longitudinal and inner circular muscles ; mouth behind brain. e.g. Linens , Cerebratulus, Langia. 3. Hoplonemertea : No deep head fissures ; lateral nerves inside the muscles ; stilet present ; mouth generally in front of brain. e.g. Amphiporus, Nemertes, Drepandphorus, Malacobdella. The last has no head fissures nor spines on the pro- boscis, but bears a posterior sucker. Habits. — Most Nemertines are marine, creeping about in the mud, under stones, among seaweed, and the like ; many, e.g. Cerebratulus , are able to swim ; Pelagonemertes is pelagic ; four species of Tetra- stemma 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 irritated, and the Schizonemertea are able to regenerate what they lose in this way. Series Nematohelminthes. Class Nematoda, e.g. Ascaridte. Class Gordiacea, e.g. Gordiidae, Nectonemidte. Class Acanthocephala, eg. Echinorhynchus. Class Nematoda. Thread-worms, Hair-worms, etc. The Nematodes are unsegmented , more or less thread-like “ worms,” some of which are free-living and others parasitic. The body is covered by a cuticle , often thick ; cilia are totally absent , and the muscular system is very peculiar. From a nerve-ring around the gullet , six nerves go forwards and six backwards. The alimentary canal is usually well developed , has mouth and anus, and is divided into three regions. Vascular and respiratory systems are unrepresented ; there is a distinct body cavity ivhich is not ccelomic, and the remarkable excretory system consists of two lateral canals opening to the exterior by a single pore. The sexes are usually separate and the organs simple ; there is distinct sexual dimorphism. The life history is often intricate. NEMA TOD A. 175 Type, Ascaris ( e.g. 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, the 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. 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. (a) Most externally there is a thick chitinoid cuticle, perhaps of service in enabling the animals to resist the action of the digestive juices. With its presence may be associated the scarcity of cutaneous glands, and the entire absence of cilia. ( b ) Beneath this is the sub-cuticula or hypodermis, thickened along four longitudinal lines — median dorsal, ventral, and lateral, and consisting of a protoplasmic mass without distinct cell-limits, (c) Beneath the hypodermis 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 of the latter runs along the median dorsal line — a unique position in an Invertebrate. Here and there on the ring and on the nerves there are ganglionic cells, but any aggregation of these into ganglia is rare. 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, 176 UNSEGMENTED “ WORMS" which has no mesoblastic coat ; the space is therefore not strictly ccelomic. It contains a clear fluid, which probably discharges some of the functions of the absent blood. There are no amoeboid phagocytes. Fig. 78. — Illustrating the structure of a Ne- matode (Oxyuris). — After Galeb. Imbedded 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 in- ternally with phagocytic cells. In the species discussed there are four giant cells situated anteriorly, which are especially con- nected with taking up foreign 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 gradually differentiating into vas deferens, seminal vesicles, and ejacu- latory duct. The genital aperture is close to the anus. The spermatozoa have not the typical form, and are sluggish. In the female the ovary is a paired tube, which passes gradually into an oviduct, a uterus, and a short vagina at each side. The genital aperture is ventral and anterior. The ova meet the spermatozoa at the junction of uterus and oviduct. Segmentation is total, and results in the formation first of a blastula and then of a gastrula. The eggs pass out of the gut of the host and probably hatch in water, and are thus re-intro- duced. No intermediate host has yet been found. Moutl' * c-’ {*■ cuticu'fr The Nematoda form an important group, bulb containing teeth ; i., interesting both on account of their parasitism intestine ; T., testis \v.d., and on account of their peculiarly isolated vas deferens ; sp., penial zoological position. Though parasitism is spine at anus. exceedingly common, many are free living for at least a part of the life-cycle, and feed on putrefying organic matter. Again, although the number of indi- viduals which may infest one host shows how successful the parasitism is, yet Nematodes exhibit few of the ordinary adaptations to a parasitic life, and there is no sharp structural line of demarcation between free LIFE-HISTORIES. 177 and parasitic forms. Among histological peculiarities, the absence of cilia — paralleled elsewhere only among the Arthropods— the nature of the muscle cells, the condition of the sub-cuticular layer, are to be noticed. Among the grosser structural peculiarities, the nature of the excretory system, of the body cavity, 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 Spharnlaria , be degen- erate. 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 — Enoplidae, e.g. Enoplus. 2. The larvae are free in the earth, the sexual adults are parasitic in plants, or in Vertebrate animals, e.g. Tylenchus scan dens, a common parasite on cereals ; Strongylus and Dochmius in man. 3. The sexual adults are free, the larvae are parasitic in insects, e.g. Mermis. The fertilised females of Sphcerularia bomb i pass from the earth into the body cavity of humble-bee and wasp, whence their larva; 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 Asian's or Leptodera) nigrovenosum. 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 Rhabdonema slrongyloides from the intestine of man, and Leptodera appendiculata from the snail. There are several quaint reproductive abnormalities, thus — the female Sphcerularia bombi, which gets into the bod)- cavity of the humble- bee, has a prolapsed uterus, larger than itself ; the male of Trichodes crassicauda passes into the uterus of the female. Table. 12 178 UNSEGMENTED “WORMS.” Some of the most Important Forms Parasitic in Man. Name. Position. History. Result on Host. A scaris hunbri- coidcs (maw-worm) (common). A. my stax. com- mon in dogs and cats, has also been found in man. Usually in small intestine. Repeated experi- ment has shown that infection results if the eggs (with their outer envelope en- tire) are swallowed along with vegetable food or otherwise. Von Linstow has suggested, on theo- retical grounds, that two in y r i 0 p 0 d s, Julies guttulatus and Polydesmus complanatus , may be i n te r m e d i a t e hosts. Commonest in children ; rarely j dangerous, unless j very numerous, or through wandering into other parts of the body, such as respiratory tract, bile duct, vermi- form appendix. Oxyuris vermi- cular is (common). From stomach to rectum, mostly in caecum. From food or water. Rarely more than discomfort. T richocephalus dispar or whip- worm (common). Caecum and colon. Dochmius(A nchy- lostoma) duoden- alis (Europe, Egypt, Brazil). R hab do n e m a strongyloides. Small intestine. Associated with Dochmius. The larvae seem to live freely in the earth. Ulceration, hae- morrhage, and dan- gerous anaemia. It was common in the workers at theMont Cenis Tunnel. Filaria sanguinis hominis (Australia, China, India, Egypt, and Brazil). Mature female in lymphatic glands, embryos in blood. Larvae in a Mos- quito. Elephantiasis and haematuria. Dracunculus ( Fil- aria) medinensis (Guinea-worm), in Arabia, Egypt, Abyssinia, etc. The female is 1-6 ft. long, encysts beneath skin, es- pecially of back or legs. The male is practically un- known. Larvae in a Cy- clops. Subcutaneous abscesses. Trichina spiralis. Becomes sexually mature in the intes- tine ; embryos, pro- duced rapidly and viviparously, find their way to muscles, and be- come encysted. From “trichi- nosed ” pig’s muscle to man. Inflammatory pro- cesses, often fatal, are brought about by the migration of the young worms from intestine to muscles. Trichina. — The formidable Trichina deserves fuller notice. It is best known as a parasite in man, pig, and rat, but occurs also in hedgehog, fox, marten, dog, cat, rabbit, ox, and horse. The sexual forms live in the intestine, the female about 3 mm. in length, the male about half LIFE-HISTORIES. 179 as long. After impregnation the female brings forth numerous embryos viviparously, sixty to eighty at a time, and altogether about 1500. Most of these find their way through the wall of the intestine into blood vessels, and are swept along in the blood stream to the muscles ; occasionally some seem to migrate actively, boring their way, especially through connective tissue, to the muscle fibres. There they grow, coil themselves spirally, and become encysted within a sheath, at first membranous and afterwards calcareous (Figs. 79 and 80). The cyst is partly due to the muscle, and partly to the parasite. In these cysts, which may be sometimes counted in millions, the young T rich in a; remain passive, unless the flesh of their host be eaten by another, — pig eating rat, man eating pig. In the alimentary canal of the new host the capsule is dissolved, the embryos are set free, and become rapidly reproductive. Among the numerous other parasitic Nematodes the following may be noted : — The giant palisade worm ( Eustrongylus gigas ) occurs in the renal region of domestic animals, etc. ; the female may be 3 ft. long. The Fig. 79. — Trichinae in muscle, Fig. 80. — Trichinae in muscle, about to be encapsuled. — encapsuled. — After Leuckart. After Leuckart. armed palisade worm ( Strongylus armatus ) occurs in the intestine and intestinal arteries of horse, causing aneurisms, colic, etc. The 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 Vertebrates ; — A. megalocephala in horses, A. lumbricoid.es in man, A. mystax in cats and dogs. Syngamus trachealis occurs in the trachea of birds, causing “gapes.” Various species of Tylenchus , especially T. devas- i8o UNSEGMENTED ‘ ‘ WORMS. ” tatrix and T. scandens (or T. tritici), destroy cereal and other crops. Various species of Heterodera (especially H. schachtii and H. radici- cola) infest the roots of many cultivated plants, e.g. turnip, radish, cabbage. Class Gordiacea. The Gordiidte [e.g. Gordius aquations — the horse-hair worm ) and the Nectonemidse are so different from true Nematodes that they must be ranked in a separate class. In the adult Gordius the mouth is shut and the food canal is partly degenerate. The adults live freely in fresh water ; there are two larval forms, the first in aquatic molluscs, young insects, etc., the second in adult insects, fish, frog, etc. Class Acanthocephala. For a few genera, of which the best known is EchinorhyncJius, whose larva; live in Arthropods, and the adults in Vertebrates, a special class, Acanthocephala, has been established. We may provisionally place these forms, of which there are several hundreds, beside Nema- todes, but the relationship does not seem to be very close. Mouth and gut are absent. The anterior end bears a protrusible hooked proboscis. Echinorhynchus proteus of pike, larva in the Amphipod Gammarus pulex. ,, angustatus of perch, larva in the Isopod Asellus aqua- tions. ,, gigas °f P'g> larva in young cockchafers. CHAPTER XI. SEGMENTED WORMS OR ANNELIDA. Chief Classes : — Ch.-etopoda, Discophora. The Annelids or Annulata include segmented “worms,” in most of which the segmentation of the body is visible externally. There is usually a well-developed body cavity, which communicates with the exterior by paired nephridia. The nervous system consists typically of dorsal cerebral ganglia, a commissural ring round the gullet, and a ventral ganglionated chain. Not infrequently the nephridia func- tion also as genital ducts. The development may be direct or indirect, and then includes a larval Trochosphere stage. In habit, form, and structure, the Annelids exhibit much diversity. The Chaetopods, represented on the one hand by the familiar earthworm, and on the other by the marine worms, best exhibit the structure upon which the Annelid type is founded. With these, however, may be included the aberrant Echiuridse, e.g. Echiurus and Bo7iellia. A few primitive forms (Archi-Annelida), and the Myzostomata (parasitic on Crinoids), may also be appended to the Chaetopod class. The leeches (Discophora) are probably Annelids which have diverged in consequence of a peculiar half-parasitic habit. Finally, some zoologists include Sagitta (Chaetognatha) in this series as an Annelid with three segments, and also the Rotifers (Rotatoria), whose adult form somewhat resembles the Trochosphere larvae of many Annelids. Class Ch/ETOPODA. Worms with Bristles. Segmented, animals with setae developed in little skin-sacs, 182 SEGMENTED WORMS OR ANNELIDA. 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 or cerebral centres, by means of 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, and the development is either direct or with a metamorphosis. The two prominent divisions of this class may be con- trasted as follows : — Oligoch^eta, e.g. Earthworm. Polych.-i;ta, e.g. Nereis. With no parapodia, and with few setae. Other external appendages are also wanting, except gills in a few forms. Hermaphrodite, with complex repro- ductive organs. Development direct. Living in fresh water or in the soil. With parapodia and with numerous setae. With antennae, gills, and cirri. Sexes usually separate, and repro- ductive organs simple. A metamorphosis in development. Marine. Type of OuGOCHi'ETA. The Earthworm ( Lumbricus ). Habits. — Earthworms eat their way through the ground, and form definite burrows, which they often make more comfortable by a lining 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 earthworms 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 to tons of soil per acre pass annually through their bodies, and that they cover the surface with earth at the rate of 3 ins. in fifteen EARTHWORM. 183 years. He was therefore led to the conclusion that earth- worms 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 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. 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 leaves to their holes, the collection of little stones to protect the entrance to the burrows, include most of the activities of earthworms, except as regards pairing and egg-laying, of which something will afterwards be said. When an earthworm is halved with the spade, it does not necessarily die, for the head portion may grow a new tail, while a decapitated worm may even grow a new head and brain. Leucocytes 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 unicellular parasites — Gregarines of the genus Monocystis ; and little 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 ins. long, with a pointed head end, and a cylindrical body rather flattened posteriorly. The successive rings seen on the surface mark true segments. The mouth is overarched by a small lobe called the prostomium, and the food canal terminates at the blunt posterior end. The skin is covered by a thin transparent cuticle, traversed by two sets of fine lines, which break up the light and produce a slight iridescence. On a region extending 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 perhaps forms the slimy stuff which haidens 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 .setae, which project from little sacs, are worked by muscles, and assist in locomotion. These bristles are fixed like pins into the ground, 184 SEGMENTED WORMS OR ANNELIDA. at times so firmly that even a bi Fig. 81. — Anterior region of earth- worm.— After Hering. N ote the eight setse (s.) on each segment. R . S. , Spots between 9-10, to-ix, indicate openings of receptacula seminis ; Ovd., openings of oviducts on segment 14 ; vd. , openings of vasa deferentia on segment 15. adjacent rings, there are minute body cavity may exude. 1 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 speci- mens 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 spermatheoe 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 every two ores, through which fluid from the 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 phosphorescent. The 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 hoops underneath the skin, and in longitudinal bands more internally. The special muscles above the mouth and pharynx have considerable powers of grasping, EARTHWORM. 185 ■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 ex- cretory 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 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, es- pecially in the anterior region, but they are plain enough on a small portion of the cord examined with the microscope, 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 Polychsetes, 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 the 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- 1 86 SEGMENTED WORMS OR ANNELIDA. 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. Two facts in regard to minute structure deserve attention. 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 fibres, with firm walls and clear contents. These “giant fibres,” which have been dignified by the name of neurochord, are probably comparable to the medullated nerve fibres of Vertebrates. 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. I n 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. On the gullet are three pairs of oesophageal or calciferous glands— the products of which are limy and able to affect the food chemically, probably counteracting the acidity of the decay- ing vegetable matter. 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, the yellow cells are crowded. 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 animals, lines the body cavity and covers the gut. As to their function, we only know that they'absorb particles from the intestine, and go free into the body cavity, whence, as they break up, their debris may pass out by the excretory tubes. When a worm has been made to eat powdered carmine, the passage of these useless particles from gut to yellow cells, from yellow cells to body cavity, and thence out by the excretory tubes, has been traced. Various ferments have been detected in the gut, a diastatic ferment turning the EARTHWORM. 187 starchy food into sugars, and others— peptic and tryptic— not less im- portant. The wall of the stomach-intestine from without inwards, as may be traced in sections, is made up of pigmented peritoneum, muscles, capillaries, and an internal ciliated epithelium. In the other parts of the gut the innermost lining is not ciliated, but covered with a cuticle. Vascular system. — The fluid of the blood is coloured red with haemoglobin, and contains small corpuscles. Along the median dorsal line of the gut a prominent blood vessel dv S.n.v Fig. 82.— Transverse section of earthworm. — After Claparede. c., Cuticle; e., epidermis; c.m ., circular muscles; Lin., longitudinal muscles; s., a seta; cos., coelom \ y.c.% yellow cells; T., typhlo- sole ; v.v. , supra-neural blood vessel ; s.n.v., sub-neural vessel; d.v. , dorsal vessel. extends, another (supra-neural) runs along the upper surface of the nerve-cord, another (infra-neural) along the under surface, while two small latero-neurals pass along each side of this same cord. All these longitudinal vessels, of which the first three are most important, are parallel with one another ; the first three meet in an anterior network on the pharynx ; the dorsal and the supra-neural are linked together 1 88 SEGMENTED WORMS OR ANNELIDA. in the region of the gullet by five or six pairs of contractile vessels or “hearts.” The precise path of the blood is not known, but the distribution of vessels to skin, nephridia, and alimentary canal is readily seen. Respiration is effected by the distribution of blood on the general surface of the skin. Excretory system. — There is a pair of nephridia in each segment except the first four. Each opens internally into the segment in front of that on which its other end opens to the exterior. They remove little particles from the body cavity, and get finer waste products from the associated blood vessels. Nephridia occur in many animals, in most young Vertebrates as well as among Invertebrates, but they are never seen more clearly than in the earthworm. When a nephridium is carefully removed, along with a part of the septum through which it passes, and examined under the microscope, the following three parts are seen : — (a) An internal ciliated funnel ; ( b ) a trebly coiled ciliated tube, at first transparent, then glandular and granular ; and (e) a muscular duct opening to the exterior. Minute par- ticles swept into the ciliated funnel pass down the ciliated coils of the tube, and out by the muscular part which opens just outside of the ventral bristles. The coiled tube con- sists in part at least of a series of intracellular cavities, that is to say, it runs through the middle of the cells which compose it ; the external muscular portion arises from an invagination of skin. Reproductive system. — Like all Oligochsetes, the earth- worm is hermaphrodite and the organs complex. The complexity is produced by the specialisation of certain of the nephridia to form genital ducts and accessory organs, and by the presence of chambers (seminal vesicles) con- nected with the testes, formed by the shutting off of portions of the body cavity. The organs in the earthworm are difficult to dissect, and differ considerably in old and young specimens. (a) The Male Organs consist of two pairs of testes, three pairs of seminal vesicles, and paired vasa deferentia. (i) The testes, flattened lobed bodies, about y1^ in. in size, arise from proliferations of the peritoneal lining of the body cavity, and are invested by a delicate membrane derived EARTHWORM. 189 therefrom ; they lie near the nerve-cord, attached to the posterior surfaces of the septa between segments 9-10 and io-ii. They are minute, translucent, and difficult to see. In immature worms they lie exposed in the body cavity ; in mature worms they are concealed by the great development of — (2) The seminal vesicles, which are much-lobed struc- tures, exceedingly prominent in dissection. Small and laterally placed in young worms, in the adult the anterior two pairs fuse in the middle line and cover the anterior Fig. 83. — Reproductive organs ot earthworm. — After Hering. N., Nerve cord; T., anterior testes; S., sacs of setae; R.S., receptacula seminis ; s., seminal funnels; v.o vas deferens; ovd, oviduct; ov ., ovary; s.v ., seminal vesicles cut open; VI 1 1. -XV, segments. pair of testes and its ducts, while the posterior pair similarly conceals the second pair of testes with its ducts. Into the seminal vesicles mother-sperm-cells from the testes pass, and there divide up to form spermatozoa. Development shows that the seminal vesicles arise as outgrowths of the septa of segments 9-12, and that their lumen is a portion of the body cavity. This is of importance, for in Polychaetes the genital products mature in the general body cavity, just as the spermatozoa in the earthworm mature in the seminal vesicles. (3) From the seminal vesicles the spermatozoa are carried 190 SEGMENTED WORMS OR ANNELIDA. to the exterior by means of the vasa deferentia. The in- ternal openings of these are large and funnel-shaped, and are concealed by the seminal vesicles. Each of the four funnels opens into a duct, and the two ducts unite at each side to form the two elongated vasa deferentia, which pass backwards to open externally on the 15 th segment. (р) The Female Organs consist of two ovaries and two oviducts, each of which has a side receptacle for the eggs. (1) The two ovaries are small bodies situated near the nerve-cord on the septum between segments 12-13. Each is pear-shaped, the stalk of the pear being a string of ripe ova. They are more likely to be seen than the testes. (2) The two oviducts open internally on the anterior face of the septum between 13-14, and externally on the ventral surface of segment 14. Into the wide ciliated in- ternal mouths, which lie opposite the ovaries, the ripe eggs pass. (3) The egg-sac or receptaculum ovorum, near the internal mouth of each oviduct, is a posterior diverticulum of the septum between segments 13-14. Within it a few mature ova are stored. (с) Two pairs of receptacula seminis or spermathecae receive spermatozoa from another earthworm, and liberate them to fertilise the eggs of this one. They are white globular sacs, opening in the grooves between segments 9-10 and io-ii, and probably, like the genital ducts, arise from modified nephridia. According to some, these sper- mathecse not only receive and store spermatozoa, but make them into packets or spermatophores. Others say that the glands of the clitellum make these packets. At any rate, minute thread-like packets of spermatozoa are formed, and a pair of them may often be seen adhering to the skin of the earthworm about the saddle region. When two worms unite sexually, they, lie apposed in opposite directions, the head of the one towards the tail of the other. What happens is that the spermatozoa of the one pass into the receptacula of the other. When the eggs of an earthworm are liberated, they are surrounded by a sheath of gelatinous stuff, said by some to be secreted by the saddle. As this is peeled off towards the head, a spermatophore is also enclosed. EARTHWORM. 191 Development. — Many cocoons are made about the same time, and each contains numerous ova, and also packets of sperms, so that fertilisation takes place outside the body. These cocoons are buried in the earth a few inches below the surface. They measure about a quarter of an inch in length. The favourite time for egg- laying is during the spring and summer, though it may be con- tinued throughout the whole year. The earthworm of the dungheap ( L . ftttidus) makes this a habit, induced probably by the warmth of its environment. Of the many ova in the cocoon of L. /errestris, only one comes to maturity, while in L. fcetidus a few, and in L. communis two may do so. But in the last species the two embryos are often twins formed from one ovum, separation taking place at the gastrula stage. The whole process of growth, until leaving the egg, lasts from two to three weeks, the time vary- ing, however, with the tempera- ture. The ovum is surrounded by a vitelline membrane, and is laden with yolk granules. Segmentation Fig. 84. — Stages in the develop- ment of earthworm. — After Wilson. 1. Two-celled stage ; /.r., polar bodies. 2. Blastula ; M., a primary mesoblast. 3. Gastrula stage ; Ec., ectoderm or epiblast ; En., endoderm or hypo- blast, in process of being covered by the small ectoderm cells. Note the widely open blastopore ; M '., mesoblast cells. 4. Longitudinal section in late gastrula stage, showing germ-bands; ec., ectoderm; en., endoderm; M., mouth; si., stomodteum ; >«., primary mesoblasts ; Nb., neuro- blasts ; nc., nerve-cord; N., ne- phridioblasts ; jus., mesoderm bands ; npc., incipient nephridia. 192 SEGMENTED WORMS OR ANNELIDA. is slightly unequal (Fig. 84 (1)), and exhibits considerable variation even within the limits of a species. In about twenty-four hours a nearly spherical, one-layered blasto- sphere or blastula is formed. It consists of only about thirteen cells. During the next twenty-four hours the cells increase in number rapidly, but the blastula remains one-layered. Two cells lying together do not take part in this division ; they are rather larger than the rest, and their inner ends project into the cavity, and are soon cut off asdaughtei cells. Gradually the large cells still undergoing division begin to sink in, and at last are quite included in the cavity (Fig. 84 (2)). Thus there arise two parallel rows of cells within the blastula, and these define the longitudinal axis of the embryo. This is the beginning of the mesoblast which forms all the muscles of the trunk, and which thus takes origin from two primary mesoblasts. After five to six pairs of secondary mesoblasts have been formed, the blastula begins to flatten, and to elongate, becoming an oval disc. The cells of the lower surface become clearer, and the hypoblast is thus defined. The cells of the upper surface are smaller, and become very much flattened ; they compose the epiblast. The mesoblasts lie side by side near one end, forming two rows extending forwards and down- wards, but divergent, because of the flattening of the blastula. The hypoblast now becomes concave, and thus the blastopore arises, occupying the whole of the lower surface (Fig. 84 (3)). The sides close in and the blastopore becomes a slit, which iurther closes from behind forwards, leaving only a small opening, — the future mouth. During these processes the cells at the anterior tip of the blastopore, which will give rise to the prseoral lobe, undergo no change, but the mesoblast has been active. As gastrulation proceeds, the mesoblast rows grow forwards and upwards, until they come near each other above the anterior tip of the blastopore, while their middle portions are carried downwards until they lie on the ventral surface. Over them the epiblast is thickened in two bands. Two longitudinal rows of epiblast cells near the anterior end, and ending behind in large cells, sink in just as the primary mesoblasts did. The thickening now extends ventrally until the two bands meet, and, passing into the blastopore, forms the stomodteum. Even before this the embryo has begun to swallow the albumen in which it floats. There are now two lateral bands of cells called the germ bands, composed of three layers (Fig. 84(4)): outside is the thickened epi- blast, next the rows of cells which sank in, and innermost the meso- blast rows. The mesoblast rows have met in the middle line by dividing and widening out into a pair of flattened plates, but they still end behind in the two primary mesoblasts. Coelomic cavities develop in the plates, and the anterior ends meet above the mouth. The epiblastic rows w'hich sank in (there were eight of them, four on each side of the median line, and each ending in a large mother cell) go on growing. The mother cells are apparently carried backwards as the embryo lengthens, leaving a trail of daughter cells behind them. The cells so formed also divide, the embryo rapidly lengthening and finally becoming vermiform. Of the eight rows the innermost on each side (neuroblasts) give rise to the nervous system, the next two rows on CH.ETOPODA. 193 either side (nephridioblasts) form parts of the nephridia (Fig. 84 (4)), while of the fourth row nothing definite is known. Each row, ending behind in a single cell, widens out and deepens as it is traced forwards. The neural and mesoblastic rows can be traced round the mouth, and help to form the prostomium ; the others fade away at the sides of the stomcxkeum. Let us sum up this complex history : — Fertilised ovum. Blasiosphere or blastula. Two-layered gastrula with primitive^ mesoblasts. K pi blast or ectoderm f ( a ) The original outer layer becomes the epidermis. (/>)The secondary inner strat- um consists of neuroblasts which form the nervous system, of nephridioblasts which form parts of the nephridia, and of lateral . cells of unknown function. Mesoblast mesoderm | formed from the division of | the primitive “ mesoblasts.” Muscle. Blood vessels. Inner parts of nephridia. Reproductive organs. Hypoblast fLiningof endoderm. lmid‘Sut- General development of the organs. — The origin of the more im- portant organs may be briefly noticed. In the nervous system , while the ventral cord arises from the neuro- blasts, the two cerebral ganglia originate, according to Kleinenberg, independently from a median unpaired apical plate of ectoderm, while, according to Wilson, they arise along with the ventral cord, and have their foundations in the thickened anterior end of each of the two neural rows. The history of the excretory system is complex, (a) At the anterior end of young embryos a group of ectoderm cells, dorsal in position, forms a larval excretory organ, which wholly disappears in later stages. (6) Next appear two ciliated canals in the anterior region, closed inter- nally, but opening externally on the head. These are known as “provisional nephridia” or “head kidneys.” They degenerate as the permanent excretory organs develop, (c) The numerous permanent nephridia are for the most part ectodermic, arising from the rows of nephridial cells already described. Two parts of each nephridium, however, have a mesoblastic origin, viz. the innermost part or the ciliated funnel, and the peritoneal investment, which ensheaths the whole organ. The gastrula cavity forms the archenteron — the future mid-gut, — and elongates with the growth of the embryo. To the completion of the entire alimentary canal, however, two other processes are necessary, an intucking of ectoderm from in front — the stomodaum or “ fore-gut " — which pushes the archenteron backwards and forms the future pharynx, and a similar intucking of ectoderm from behind — the 13 194 SEGMENTED WORMS OR ANNELIDA. proctodceum or “ hind-gut” — which meets and fuses with the archenteron, and forms the anus and a small portion of the posterior gut. The mesoderm begins with the two primary mesob/asts already described. These multiply and form mesoderm bands, which, insinuat- ing themselves between ectoderm and endoderm, proceed to surround the gut. At the same time, some of the mesoderm cells become migratory, wander on to the head, and also surround the gut, before the final trunk musculature is completed. The migrator y mesoblasts of the trunk appear to form a special larval musculature precociously developed, in order to enable the embryo to manage the enormous mass of albumen (absorbed from the capsule) with which its body is dis- tended. The mesoderm bands grow in strength, and form a complete ring encircling the archenteron. They then become two-layered, and the two layers separate, the inner (splanchnic) cleaving to the gut, the outer (somatic) clinging to the body-wall. The space between them is the body cavity or coelom. But as the separation of somatic and splanchnic layers takes place, partitions are also formed transversely, to become the septa which partition off the body cavity into a series of segments. The cavity of the pre-oral region or prostomium differs some- what from that of the others, being from the first unpaired, instead of including two lateral cavities, one on each side of the gut. Type of Polychteta. The Lob-Worm ( Arenicola marina). Habits. — On the flat sandy beach uncovered at low tide, the “ castings ” of the lob-worm or lug- worm are very numerous. There the fishermen seek the worms for bait, and have to dig quickly, for the burrowers retreat one to two feet into the sand. The burrows are U-shaped tubes, lined by a yellowish green secretion from the animal’s epidermis, and the surrounding sand is often discoloured by some change which the secretion effects on the iron oxides and other constituents. The tubes are at first vertical, afterwards oblique or horizontal, and then turn vertically upwards again. The lob-worm burrows like the earthworm, not only forcing the anterior part of its body onwards, but eating the sand for the sake of the organic particles and small organ- isms which it contains. The sandy castings, which pass from the end of the food canal, and are got rid of at the mouth of the tube, fall into spiral coils. It has been calculated that in a year the average volume of sand per acre thus brought up in castings is about 1900 tons, representing a layer of 13 in. spread out over the ARENICOLA. *95 surface. This work, comparable to that of earthworms, tends to cleanse the sand and to reduce it to a finer powder. When getting rid of the casting, the worm lies with its tail upwards and its head' downwards, or with its body bent like a bow ; when the tide comes in, the mouth may protrude at the other end of the U-shaped tube. The worms that live between tide-marks seem to differ in many respects (as to colour, gills, habits, and sexual maturity) from those which occur in the Laminarian zone, which is only un- covered at low spring-tides. Ehlers states that at certain seasons the adults swim about freely, but this requires corroboration. The young stages are for a time pelagic. External appearance. — The lob-worm varies in length Fig. 85. — A reni cola marina . Entire animal viewed slightly from left side. Note anterior mouth ; setae on anterior region ; setae and gills on median region ; thinner tail region often longer than shown. from 8 in. to a foot, and at its thickest part is about half an inch in diameter. There are three regions in the body: — (a) The anterior seven segments, of which all but the first have bristles ; ( b ) the middle region of thirteen segments, with both gills and bristles ; ( . sp., apical spot; ci/., ciliated ring; neph primitive nephridia ; 10. formation of posterior segments; 11. form of adult Polygcn-dius. 202 SEGMENTED WORMS OR ANNELIDA. portionately large, but later, by an independent process of growth, becomes reduced. The larva abandons its pelagic life, and becomes adult. Comparing the development of Polychteta with this, we find that the Trochosphere is often modified, and that segmentation tends constantly to appear at an earlier stage. As a further step in the same direction, we may note that in some Polychteta the Trochosphere stage is no longer recognisable as such. A general Contrast of the Modes of Developme7it in different Annelids. A. “ Larval ” Types, as in marine Chsetopods, Polygordius, etc. Development indirect. A free-swimming Trochosphere stage, with trunk almost or wholly suppressed, with head region greatly developed, with adapta- tions to free marine life. B. “ Foetal ” Types, as in Earthworm, Leech, etc. Development direct, within egg capsule ; Trochosphere stage almost or wholly suppressed. Lumbricus type Clepsine type with little nutri- with much nutri- tive material in tive material in ovum, with gas- ovum, with gas- trula formed by trula therefore invagination (em- formed by over- bolic). growth (epibolic). General Survey of Ch^etopoda. I. Oligochasta. — The general characters may be gathered from the description of the earthwoim, but it is to be noticed that the earth- worms are specialised forms, and that the fresh-water Oligochsetes are of much simpler structure. The most essential distinction from the Polychteta is to be found in the complex reproductive organs. The absence of gills, though general, is not universal, for a few fresh-water forms, such as Dero and Branchiura, possess gills of simple structure, while the West African Alma has more complex branched retractile gills. Among other characters may be noticed the tendency to variation in the structure of the excretory system. In all, with the exception of AEolosoma, certain of the nephridia are modified to serve as genital ducts, while in the Megascolicidte the nephridia tend to be reduced to a mass of minute tubules ramifying over the inner surface of the body-wall. In general the Oligochtetes, however, show more uniformity of structure than their marine allies. They may be divided into two main groups — (i) the Microdrili, and (2) the Megadrili. The first group includes the small aquatic GENERAL SURVEY OF CHAETOPODA. 203 forms ; of these most familiar are Tubifex rwulorum , often found in the mud of brooks, and the species of Nats, remarkable for their power of asexual budding. The leech-like Braiichiobdella, which is parasitic on the gills of the fresh-water crayfish, is a somewhat aberrant member of the group. The Megadrili include the larger Oligochwtes, mostly living in earth, and commonly designated as “earthworms.” The largest form is a Tasmanian species (Megascoltdes gippslandicus ), measuring about 6 ft. in length, and said to make a gurgling noise as it letreats underground. II. Polychseta. — -As contrasted with the more or less subterranean earth- and mud- wonns, the marine ' Polychreta have a richer development of external structures and a more complex life history. The ex- ternal appearance is greatly modified by the relative degree of development of the parapodia, which are lateral outgrowths typically functioning as walking “legs,” or as swimming organs. A para- podium, when fully developed, is divi- sible into a ventral neuropodium and a dorsal notopodium. Each of these is bilobed, bears a tactile process or cirrus, and is fringed with firm bristles or seta;. Within the substance of each lobe is embedded a stout needle-shaped “ aciculum,” which functions as an internal skeleton, both by giving support and by serving as an attach- ment for muscles. With the notopodium, further, true gills containing prolongations of the body cavity are often associated. Such typical parapodia occur especially in the active free-living forms like Nereis and its allies, but in the order in general the parapodia show much variation, and may be almost suppressed, as in Arenicola. Parapodia are absent from the “ prostomium,” and are rarely fully developed on the first true segment or peristomium. In both cases, however, tactile cirri and tentacles are often present. The prostomium varies greatly in development and structure, and is of great systematic importance ; it is frequently furnished with eyes and other sense organs, but these may Fig. 89A.— Parapodium of “ Heteronereis " of Nereis pelagica. — After Ehlers. the leaf-like outgrowths; e1., notopodial 3, 4 neuropodial rt1., a‘.. supporting bristles of notopodium s., set®. acicula or and neuropodium ; 204 SEGMENTED WORMS OR ANNELIDA. also occur in other regions of the body. Apart from the parapodia, the shape and appearance of the body are most affected by the condition of the septa. In the active free-living forms (Errantia) these are usually present throughout the body, and give a characteristic worm-like appearance. In burrowing and tubicolous forms (Sedentaria) the septa tend to be suppressed. Their absence facilitates burrowing, by per- mitting free movement of the coelomic fluid, and is often associated with a division of the body into regions, and a loss of the typical uniform shape (cf. Arenicola). With regard to internal organs, the gut is frequently branched and of large calibre. In some cases (Capitellidae) it possesses an accessor}' communicating tube (Nebendarm), which is of interest, because it has been compared to the notochord of Vertebrates. The nephridia function as genital ducts ; they are often reduced in number, and may, as in the common Lattice cbnchilega, be united by longitudinal ducts, which have been compared to the segmental ducts uniting the excretory tubes of young Vertebrates. Though the sexes are usually separate, there are a few hermaphrodite forms, and the aberrant Sternaspis, where the reproductive system recalls that of Oligochsetes, is an exception to the rule that the organs are simple. There is a metamorphosis in development, and some interesting peculiarities occur in regard to reproduction. Thus several species of the common genus Nereis , when sexually mature, have the body divided into two regions, — a posterior region containing the ova or sperms, and an anterior unmodified asexual region. The posterior region is distinguished by the structure of its parapodia, which become converted into broad, flattened swimming organs, and there is sexual dimorphism. Worms of this peculiar type were long described as a distinct genus under the name of “ Heteronereis,” and even yet the subject is imperfectly understood, for there is from unknown causes much variation as regards the extent of the modification. A complete change of habit at the spawning season is probably common here as elsewhere in marine Invertebrates. In the Syllidte a phenomenon occurs similar to the formation of a “ Heteronereis,” but a process of fission may result in the division of the modified form into an anterior asexual zooid and a posterior sexual one. In this way a regular alternation of sexual and asexual generations may arise. The Polychpeta were formerly classified as active and sedentary forms, but few are permanently active, and the classification is now abandoned. It is, however, necessary to realise that while certain forms dwell habitually within tubes, others are at least at times active and free-living. The latter have usually well-developed parapodia and sense organs, the anterior part of the gut may be furnished with strong jaws, the body is more or less uniform, and the worms are carnivorous. These forms are all included in the sub-order Nereidiformia, which embraces such familiar animals as the common sea-mouse ( Aphrodite ), with its mass of iridescent bristles covering the dorsal surface, the species of Nereis and Nephthys, so common under stones on the shore, and others equally remarkable for beauty of colour. The bright colours may be due to the iridescent cuticle or to pigments. The sedentary forms lead a sluggish life within various kinds of GENERAL SURVEY OF CH.-ETOPODA. 205 tubes, — limy, sandy, papery, or gelatinous. They are not nearly related, but possess in common certain adaptive characters, such as the aggregation of gills, cirri, tentacles, and sense organs to the anterior exposed part of the body ; the reduction of the parapodia, often used solely for clambering in the tube; the absence of “jaws,” and the habit of feeding on minute Algte or other substances suspended in water. Among these are included Serpula , which forms twisted limy tubes outside shells and other marine objects ; the aberrant Sabellaria , which often builds reefs of porous rock formed of the aggregated sandy tubes ; the common Terebella or Lattice conchilega , with its tubes Fig. 90. — Free-living Polychaete ( Nereis cult rif era). Note, as compared with Arenicola , the absence of gills, and the well-developed parapodia which are absent from the peris- toinium (fie-), or first true segment. The prostomium bears eyes (e.), and the small tentacles (t.) ; the large palps, c. ; the four paired cirri borne by the peristomium ; a., the anus with two long cirri. of glued sand particles; and the strange Chte/optents, found in deep water, within its yellow parchment-like tube. III. Echiuridae. — In holes in the rocks on the warmer coasts of Europe there lives a curious “ worm” — Bonellia viridis, of a beautiful green colour, with a globular body, and a long, grooved, anteriorly forked, pre-oral protrusion. Such, at least, is the female ; but the male is microscopic in size, lives in or on his mate, and is exceedingly degenerate. His gut is without mouth and anus, the surface is covered with cilia, and the body cavity almost obliterated. Related to Bonellia , but of less anomalous shape, are a few other forms, like Thalassema and Echinrus. In all, the body in the adult shows mere traces of segmentation ; 206 SEGMENTED WORMS OR ANNELIDA. parapodia, cirri, and gills are absent, but except in the degenerate males a few setae are always present. The most characteristic structure is the elongated solid proboscis, which has the mouth at its base. The nervous system consists of a gullet-ring and a ventral cord, but the latter is unsegmented, and there is no brain. The gut is coiled, and bears a curious adjacent tube known as the “collateral intestine,” and a pair of excretory “anal vesicles,” opening from gut to body cavity, and formed in development from nephridia. The anus is terminal, there is a closed vascular system, and one to three pairs of nephridia. The sexes are separate, the reproductive elements ai'e formed on the walls of the body cavity, and are shed into it. There is a metamorphosis in development, but the nature of the larva differs markedly in the different genera. In Echiurus and Thalassema it bears a striking resemblance to a Trochosphere. Thus there is a well-developed pre-oral lobe with an apical sense organ, and pre-oral and post-oral bands of cilia. “ Head-kidneys” or provisional nephridia occur, * and the post-oral region shows distinct segmentation, the segments being marked externally by rings of cilia. As development proceeds, all trace of segmentation is lost. In Bonellia the larva shows no trace of segmentation, and is Turbellarian-like ; owing to a premature arrest of development, the male remains at this level throughout life. Appendix (i) to Cluetopoda. Primitive Forms. Archi-Chattopoda or Archi- Annelida. There are a few small, simple, marine worms, with some Annelid or Chsetopod characters, which are sometimes supposed to be ancestral forms. Thus Dinophilus is a minute Planarian-like animal found among weeds. In the young at least the body is distinctly segmented, but there are no bristles, gills, or tentacles. The nervous system consists of a brain and a ventral ganglionated cord, but it remains embedded in the epidermis. More distinctly Annelid are the marine worms Polygordius , Protodrilus, and Histriodrihis. The small body is segmented and uniform ; there are no setae, parapodia, cirri, or gills, but the head bears a few tentacles ; the pre- oral region is small, and the segment around the mouth is large ; the very simple nervous system is retained in the epidermis. Polygordius (Fig. 89 ( 1 1 )) is a thin worm, an inch or more in length, living at slight depths in sand or fine gravel, often along with the lancelet. It has a few external cilia about the mouth in a pair of head-pits, and sometimes on the body ; it moves like a worm, but has no bristles. It feeds like an earthworm, or sometimes more discriminatingly on unicellular organisms. The -females are usually larger than the males, and in some species break up at sexual maturity. The development includes a metamorphosis, and the larvae seem to throw some light on the nature of the ancestral Annelids. They are ciliated, free-swimming, light-loving, surface animals, feeding on minute pelagic organisms. PARASITIC AND DEGENERATE CH.ETOPODS. 207 seeking the depths as age advances. According to some, the larva represents a primitive unsegmented ancestral Annelid, with medusoid affinities ; according to others, the larval characteristics are adaptive to the mode of life, and without historic importance. Protodrilus is even smaller than Polygordius , with more cilia, mobile tentacles, and two fixing lobes on the posterior extremity ; the move- ments are Turbellarian-like, the reproductive organs hermaphrodite, the development direct. Hislriodrilus is parasitic on the eggs of the lobster, and its affinities are doubtful. Appendix (2) to Cluetopoda. Parasitic and Degenerate Chtetopods. Myzostomata. The remarkable forms [Myzosloma) included in this small class, live parasitically on feather-stars, on which they form galls. They are regarded as divergent offshoots from primitive Annelids, the larvffl form showing some distinctly Chtetopod characters. The minute disc-like body is unsegmented, and bears five pairs of parapodia, each with a grappling hook, with which four pairs of suckers usually alternate. There are also abundant cirri. The skin is thick, the body muscular, the nervous system is concentrated in a ganglionic mass, which encircles- the gullet, and gives off abundant branches. There is a protrusible proboscis and a branched gut ; the mouth and anus are ventral. The ova arise in the reduced body cavity, and pass by three meandering oviducts to the anal aperture. The testes are paired, branched, and ventral, with associated ducts, which open anteriorly on the side of the body. The series are united, but there is marked protandry. The very young forms, originally described as “dwarf males,” contain sperma- tozoa, and are often carried on the back of the mother ; as they grow older they become hermaphrodite, and later the power of forming spermatozoa is lost and the animals become female. It must be allowed, however, that all would not agree with the above summary. Thus Beard says: “The various kinds of parasitism presented by the numerous species of Myzoslorna, have led in some cases to the preservation of the males, in others to their extinction, in yet others to their conversion into hermaphrodites.” He distinguishes — 1. Purely dioecious forms with small males, e.g. M. pulvinar. 2. Hermaphrodite forms and true males, which remain males, e.g . M. glabrnm. 3. Hermaphrodite forms and males, which, retaining their positions on the hermaphrodites, afterwards become female, e.g. M. alatuni. 4. Hermaphrodite forms, in which the males have lost their dorsal position, and have either become extinct or converted into protandric hermaphrodites, e.g. M. cirriferum. 208 SEGMENTED WORMS OR ANNELIDA. Class Hirudinea or Discophora. Leeches. This class includes forms in which the body is oval and flattened, usually devoid of setce or gills, and marked externally by rings which are much more numerous than the true segments. The body cavity is much reduced ', and may com- municate indirectly with the well-developed vascular system. The nephridia are numerous and segmentally arranged. There are usually two suckers, one at each end of the body , the anterior being formed by the mouth. Almost all are her- maphrodite,— the male organs are numerous and segmentally arranged, and special genital ducts are present. The develop- ment is direct. Most live in fresh water or on land, but a feiv are marine. Type, the Medicinal Leech ( Hirudo medicinalis). Habits. — This is the commonest and most familiar of leeches, once so constantly used in the practice of medicine that leech became synonymous with physician. It lives in ponds and sluggish streams, and though not common in Britain, is abundant on the Continent, where leech farms, formerly of importance, are still to be seen. Leeches feed on the blood of fishes, frogs, and the like, and are still caught in the old fashion on the bare legs of the callous collector. As animals are naturally averse to bloodletting and hard to catch, leeches make the most of their opportunities. They gorge themselves with blood, and digest it slowly for many months, it may be indeed for a year. AVatched in a glass jar, the leech is seen to move by alternately fixing and loosening its oral and posterior suckers, and, on some slight provocation, it will swim about actively and gracefully. At times it casts off from its skin thin transparent shreds of cuticle, — a process which, in natural conditions, usually occurs after a heavy meal, when the animal, as if in indigestion, spasmodically contracts its body, or rubs itself on the stems of water-plants. Numerous eggs are laid together in cocoons in the damp earth near the edge of the pool. Thence, after a direct develop- ment, the young leeches emerge and make for the water. External features. — The leech usually measures from 2 to 6 ins. in length, and appears cylindrical or strap-like, according to its state of contraction. The slimy body shows over one hundred skin-rings ; HIR UDINE A OR D1SC0PH0RA. 209 its dorsal surface is beautifully marked with longitudinal pigmented bands, while the ventral surface is mottled irregularly ; the suctorial mouth is readily distinguished from the unperforated hind-sucker, above which, on the dorsal surface, the alimentary canal may be seen to end. According to Whitman’s precise investigations, there are 102 skin- rings and 26 somites or true segments. These segments may be recog- nised externally by conspicuous pigment spots (“ segmental papilla;”), which in the middle region of the body occur on every fifth ring. In type, therefore, five rings correspond to a segment, but at either end of the body the number of rings is abbreviated. In the head region a pair of “eyes” occurs on the 1st, 2nd, 3rd, 5th, and- 8th rings ; these are homologous with “ segmental papilke,” and therefore in this region eight rings correspond to five segments. The penis is protruded on the middle ventral line between rings 30 and 31 ; the aperture of the female duct lies five rings further back. Also on the ventral surface there are seventeen pairs of small lateral apertures, through which a whitish fluid may be squeezed — the openings of the excretory organs. The skin of segments 9-1 1 is especially glandular, and forms the so-called clitellum or saddle, the secretion of which forms the cocoon for the eggs. Skin. — Most externally lies the cuticle — a product of the epidermis — periodically shed, as we have already noticed. In this shedding some of the genuine epidermis cells are also thrown off. These are somewhat hammer-like units, with the heads turned outwards, while the spaces between the thick handles contain pigment and the fine branches of blood vessels. As the latter come very near the surface, a respiratory absorption of oxygen and outward passage of carbonic acid is readily effected. Opening between the epidermal elements, but really situated much deeper, are numerous long-necked, flask-shaped glandular cells, the contents of which form the mucus so abundant on the skin. Underneath the epidermis there is much connective tissue, and yellow and green, brown and black pigment. Muscular system and body cavity. The muscular system consists of spindle-shaped cells arranged externally in circular bands like the hoops of a barrel, internally in longitudinal strands like staves. Besides these there are numerous muscle bundles running diagonally through the body, or from dorsal to ventral surface, and there are other muscles associated with the lips, jaws, and pharynx. The body cavity, though distinct in the embryo, is almost obliterated in the adult leech, where the predominant con- nective tissue has filled up nearly every chink. 14 210 SEGMENTED WORMS OR ANNELIDA. Nervous system and sense organs. — The nervous system mainly consists of a pair of dorsal ganglia lying above the pharynx, and of a double nerve-cord, with twenty-three ganglia, lying along the middle ventral line. The dorsal (or supra-cesophageal) ganglia are connected with the most anterior (or sub-oesophageal) pair on the ventral chain, by a ds Fig. 91. — Transverse section of leech. — After Bourne. c., Cuticle ; c epidermis ; c.m.y dermis and outer muscles (circular and oblique) ; longitudinal muscles (the peculiar connective tissue is hardly indicated) ; r.m ., radial muscles; l.v ., lateral blood vessel; ci.s dorsal sinus; v.s., ventral sinus enclosing nerve-cord (n) ; g., median part of crop, with lateral pockets (/.) : t., testis;/, nephridial funnels ; v.d vas deferens. narrow nerve-ring surro.unding the beginning of the gut. From the dorsal centres nerves proceed to the “ eyes ” and anterior sense spots, from the ventral centres the general body is innervated, and from the beginning of the ventral chain special nerves supply the alimentary canal, forming what is called a visceral system. The sense organs of the leech are ten so-called “ eyes,” besides numerous sense spots usually occurring on every HIR UDINE A OR DISCOPHORA. 21 i fifth skin-ring. The eyes are arranged round the edge of the mouth, and look like little black spots. Microscopic examination shows them to be definite cups, surrounded by connective tissue with black pigment, and containing clear strongly refracting cells, each in connection with a fibre of the optic nerve. It has been shown (Whitman) that the eyes of leeches are serially homologous with the segmental sense organs. At the one extreme there are purely tactile organs, at the other extreme there are purely visual organs, and between these there are compound sense organs, in part 'tactile and in part visual, — a series which is full of suggestiveness in regard to the evolution of sense organs (cf. the series of sensitive sette in the crayfish). The visual organs of the leech are not able to form images of external objects, but the animals are exquisitely sensitive to alterations of light. Alimentary system. — When the leech has firmly fastened itself to its prey by the hind sucker, it brings its muscular mouth into action, pressing the lips tightly on the skin, and pro- truding three chitinous tooth-plates which lie within. Each of these tooth-plates is worked by muscles, and is like a semicircular saw, for the edge bears from 60 to ioo small teeth. Rapidly these saws cut a triangular wound, whence the flowing blood is sucked into the muscular pharynx. The process may be observed and felt by allowing a hungry leech to fasten on the arm. As the blood passes down the «•> Mouth; aA, sixth crop- , , pocket; cr^.t last crop- pharynx, it is influenced by the pocket ; v., rectum : -r., secretion of glandular cells which P°sterior sucker- lie among the muscles of the seventh, eighth, and ninth segments, and exude a ferment which prevents the usual clotting. The blood greedily sucked in gradually fills the next region of the gut — the crop — which bears on each side eleven storing pockets. These become wider and Fig. 92. — Alimentary system of leech. — After Moquin-Tandon. 212 SEGMENTED WORMS OR ANNELIDA. more capacious towards the hind end, the largest terminal pair forming two great sacs on each side of the comparatively narrow posterior part of the gut. As all the pockets point more or less backwards, it is evident that a leech to be emptied of the blood which it has sucked must be pressed from behind forwards. The pockets filled, the leech drops off its victim, seeks to retire into more private life, and digests at leisure. The digestion does not take place in the pockets, but in a small area just above the beginning of the terminal part or rectum. This rectum, running be- tween the two last pockets, is separable from the true stomach just mentioned by a closing or sphincter muscle. It ends in a dorsal anus above the hind sucker. Vascular system.— Two main lateral vessels run longitudinally, one on each side of the body. They are connected with one another by looping ves- n.c l.b.v -T.t, n v sels, give off numer- Fig. 93. — Dissection of leech. — After Bourne. ' c.g. , Cerebral ganglia; penis; s.z'., is opposite the seminal vesicle; ov., ovary; ut., uterus; vd., vas deferens ; l.l'.v., lateral blood vessel: T. 4, fourth testis; n.v., nephridial vesicle ; N. 17, last ne- phridium ; G.19, nineteenth pair of ganglia ; >i.c., nerve-cord. HIRUDINEA OR DISCOPHORA. 213 ous branches which riddle the spongy body, and have a definite muscular coat. On the dorsal surface and ventrally around the nerve-cord are two lacunar spaces, which aie really portions of the true body cavity, and not parts of the vascular system. W ith those and similar spaces, however, the blood’ vessels are connected by means of a secondarily developed series of canals, roughly corresponding to the lymphatic vessels of Vertebrates. The blood is red, and contains colourless floating cells of diverse form. Fig. 94. — A nephridium of leech.- — After Bourne. F., Internal terminal funnel ; C., glandular coil covered with blood vessels; V., external terminal vesicle. Excretory system. — There are seventeen pairs of excretory tubules or nephridia, from the second to the seventeenth segment inclusive. These open laterally on the ventral surface, voiding the waste products extracted from the blood vessels which cover their walls. From the seventh to the seventeenth, each nephridium ends internally in a ciliated “ cauliflower ” lobe, corresponding to the funnel of Oligochseta, and enclosed in a blood space, apparently part of the reduced coelom. In the first nine of these funnel- 214 SEGMENTED WORMS OR ANNELIDA. bearing nephridia the terminal lobe lies close upon and dorsal to a testis, but there is no morphological meaning in this approximation. Each consists of two parts, a twisted horseshoe-shaped glandular region, where the actual ex- cretory function is discharged, and a spherical, internally ciliated bladder opening to the exterior. Within the latter there is a whitish fluid with numerous waste crystals. The nephridia secrete a clear fluid which helps to keep the skin moist, and thus makes respiratory diffusion easier. Reproductive system. — The leech, like many other Invertebrates, is hermaphrodite, containing both male and female reproductive organs. The essential male organs or testes are diffuse, being represented by nine pairs, lying on each side of the nerve-cord in the middle region of the body. Each is a firm globular body, within which mother sperm cells divide into balls of sperms. The spermatozoa pass from each testis by a short canal leading into a wavy longitudinal vas deferens. This duct, followed towards the head, forms a coil (so-called seminal vesicle) as it approaches the ejaculatory organ or penis. From the coil on each side the sperms pass into a swollen sac at the base of the penis, where, by the viscid secretion of special (“prostate”) glands, they are glued together into packets or spermatophores. These pass up the narrow canal of the muscular penis, and leave the body on the middle ventral line between rings 30 and 31, when they are transferred in copulation to the female duct of another leech. The female organs are more compact. The two small tubular and coiled ovaries are enclosed in spherical vesicles, the walls of which are continued as two oviducts, which unite together in a convoluted common duct. This is surrounded by a mass of glandular cells, which exude a glairy fluid into the duct. Finally, the duct leads into a relatively large muscular sac — the “ uterus ” — which opens through a sphincter muscle on the middle ventral line between rings 35 and 36. The favourite breeding time is in spring. Two leeches in- seminate one another, uniting in reverse positions, so that the penis of each enters the uterus of the other. Spermatophores are passed from one to the other, and the contained sperms GENERAL NOTES ON LEECHES. 215 may remain for a long time within the uterus, or, liberated from their packets, may work their way up the female duct meeting the eggs at some point, or reaching them even in the ovaries. The development is direct, and in many respects recalls that of the earthworm. General Notes on Leeches. The leeches constitute a relatively small class, whose structure has been insufficiently worked out. The presence of suckers, the parasitic habit, the reduction of the body cavity, have led many naturalists to associate them with Flat-worms, but all recent work goes to emphasise their affinity with Annelids, especially Oligochaetes. In leeches seta: are absent, except in Acanthobdella, which has paired segmentally arranged bristles in the anterior region ; but it is to be noted that they are absent in some Oligochaetes. As in Oligochaetes, gills are usually absent, but occur in Branchellion. The condition of the body cavity affords one of the most striking contrasts to Oligochaetes ; but in Acanthobdella the adult has a typical Annelid coelom divided into regions by septa. In others, in spite of the large amount of connective tissue in the adult, there are distinct traces of segmental septa. In Hirudo the reduction is carried so far that the ccelorn is represented merely by canals without trace of septa. In all cases, however, development shows that the reduction is secondary', and that in the embryo there is a true Annelid body cavity unconnected with the vascular system. The condition of the alimentary' canal affords a basis for classification, for in one set the anterior region is protrusible, and in the other it is not, but is furnished with jaws or tooth-plates. The jaws are interesting, because they are absent from Oligochaetes, except in a few forms, like Branchiobdella ; the jawed leeches are more specialised than those without these structures. With regard to the nephridia, in Clepsine , which has a fairly well- developed body cavity, there is a direct communication between ccelorn and nephridia by means of a ciliated funnel of typical Annelid form. Where the coelom is much reduced, as in Hirudo, the funnel is represented by the blind ciliated “cauliflower lobe.” In the reproductive system, apart from the numerous male organs, the leeches differ from the Oligochaetes in the apparent continuity of the organs and ducts ; in the case of the ovaries at least, however, the connection is secondary. In the processes of fertilisation and egg-laying, in the formation of a cocoon, and in the development, the two groups show marked resemblance. Most leeches are worm-like aquatic animals, with blood-sucking propensities ; but some live in moist soil, and others keep to the open surface, while the parasitic “ vampire ” habit, familiarly illustrated by the apothecary’s ancient panacea, is in many cases replaced by carnivorous habits and predatory life. The medicinal leech ( Hirudo ) is typical of the majority, for it lives in ponds and marshes, and sucks the blood of snails, fishes, frogs, or of larger available victims. The 2l6 SEGMENTED WORMS OR ANNELIDA. giant leech ( Macrobdella valdiviana), said to measure 2h ft. in length, though this is very doubtful, is subterranean and carnivorous ; while the wiry land-leeches ( Hcemadipsa , etc.), of Ceylon and other parts of the East, move in rapid somersaults along the ground, fasten on to the legs of man or beast, and gorge themselves with blood. The hungry horse- leeches are species of Htvmopis , greedily suctorial, though the teeth, which occur in two rows, are too small and irregular to be useful in medicinal blood-letting ; but the name is also applied to species of the common genus Aulostoma, which are carnivorous in habit. Other common leeches are species of Nephelis, predacious forms with indis- criminating appetites, and the little Clepsine, also common in our ponds, notable for its habit of carrying its young about on its belly. Numerous marine forms prey upon fishes and other animals, e.g. the “skate-sucker” (Pontobdella muricata), with a leathery skin rough with knobs. This form lays velvety eggs in empty mollusc shells, and mounts guard over them for more than a hundred days. The remarkable Branchellioit on the Torpedo, has numerous leaf-like respirator}' plates on the sides of its body. Perhaps the strangest habitat is that of Lophobdclla , which lives on the lips and jaws of the crocodile. Classification. — 1. RhynchobdellidEe, in which the fore part of the pharynx can be protruded as a proboscis. There is an anterior as well as a posterior sucker. The blood plasma is colourless. The ova are large and rich in yolk ; the embryos are hatched at an advanced stage, and soon leave the cocoon, which contains no albuminous fluid. e.g. Clepsine , Pontobdella , Branchellion. 2. Gnathobdellidce, in which there is no proboscis, but the pharynx usually bears three tooth-plates. The mouth is suctorial. The blood plasma is red. The ova are small and without much yolk ; the embryos are hatched at an early stage, and swim about in the nutritive albuminous fluid of the cocoon. e.g. Hirudo , Hanuopis , H/cniadipsa, Aulostoma, Nephelis. A ppendix ( I ) to A undid Series. Class Ch^etognatha. Arrow-Worms. There are two little marine “worms,” Sagitta and Spadel/a, which are so different from all others, that they have been placed in a class by themselves. It is possible to regard them as Annelids with three segments. The translucent body, which may be nearly 3 in. long, but is usually much less, has three distinct regions, — a head bearing a ventral mouth with spines and bristles (whence the name Chmtognatha), a median region with lateral fins, and a trowel-like tail. The nervous system consists of a supra-cesophageal ganglion in the head, a sub- cesophageal about the middle of the body, long commissures between them, and numerous nerves from both ; it retains its primitive con- nection with the epidermis. There are two eyes and various patches ROTATORIA. 217 of sensitive cells. The food canal is complete and simple, and lies in a spacious ciliated body cavity. Corresponding to the external divisions, the cavities of the head, body, and tail are distinct, being separated from one another by septa ; a longitudinal mesentery supports the gut and divides the cavities into lateral halves. There is no vascular system, nor aie there any certain nephridia. It is possible that the latter may be represented by the genital ducts. The animals are hermaphrodite, and the simple reproducive organs lie near one another posteriorly. The two ovaries project into the body cavity, and their ducts open laterally where body and tail meet. The two testes project into the cavity of the tail ; and their ducts have internal ciliated funnels, and open on the tail. Two reproductive^' cells are set apart at a very early stage, and each divides into the rudi- ment of an ovary and of a testis. The eggs undergo complete segmenta- tion ; a gastrula is formed by the invagination of the blastula ; the m Fig. 95.— Development of Sagitta. — After O. Hertwig. Illustrating formation of a body cavity by pockets from the archenteron ; also the early separation of reproductive cells. Ec., Ectoderm; En„ endoderm ; nc., archenteron; E., repro- ductive cells; hi. , blastopore; a./. . coelom pouches; m., mouth ; I. section of gastrula ; 2 and 3. origin of coelom pouches. body cavity arises, in enteroccelic fashion, as two pockets fiom the archenteron. The young forms are like the adults. Appendix (2) to Annelid Series. Class Rotatoria. Rotifers. Rotifers are beautiful minute animals, abundant in fresh water, also found in damp moss, and in the sea. They owe their name and the old-fashioned title of wheel animalcules to the fact that the rapid move- ments of cilia on their anterior end produce the appearance of a rotating wheel. The food seems to consist of small organisms and particles caught in the whirlpool made by the lashing cilia. The little animals are tenacious of life, and can survive prolonged drought. If they are left dry for long, however, thev die, though the ova may survive and subsequently develop. The body is usually microscopic, and is sometimes (e.g. in Melicerta 2 1 8 SEGMENTED WORMS OR ANNELIDA. and Floscularia ) sheltered within an external tube. There is no internal segmentation, but there are sometimes external rings, and the attaching outgrowth or “foot” is sometimes segmented. The anterior end bears, on a retractile ridge, the ciliated ring or “ trochal apparatus.” The nervous system is a single dorsal ganglion with a few nerves. An unpaired eye and some tufts of sensory hairs are usually present. The food canal extends along the body in a well-developed coelom, and the fore-gut contains a mill, in which two complex hammers beat upon an anvil. The canal ends posteriorly on the dorsal surface between the body and the foot, and as the terminal portion also receives the excretory canals and the oviduct, it is called a cloaca. There is no vascular system, but a nephridial tube of a primitive type lies on each side of the body, and opens posteriorly into the cloaca. The sexes are separate ; the reproductive organs are simple. Except in the marine parasite Seison, in Rhinops vitrea, and two or three other forms, the males are dwarfed and degenerate, destitute even of a true food canal, and often “little more than perambulating bags of spermatozoa.” In many cases at least, sexual union (effected by a penis) seems to be ineffective, and there is no doubt that many, if not most, Rotifers are parthenogenetic. No males have as yet been found in Philodina, Rotifer , Callidina, or Adineta. The females lay three different kinds of eggs, according to their conditions and constitution — either small ova, which become males, or thin-shelled “summer ova,” or thick-shelled “resting or winter ova,” the two last developing into females. The so-called winter eggs may occur at any season, and seem usually to have been fertilised. Many species, however, are viviparous. We include the Rotifers beside the Annelids proper, be- cause it seems possible to regard them as derived from ancestors somewhat like Annelid larvae. Rotifers living in fixed tubes or envelopes, — Melicerta , Floscularia, Steplianoceros. Free Rotifers, — Notommata, Hydatina, Brachionus. Parasitic on the marine crustacean Nebalia, — Seison. Pedalion occupies a unique position ; it has hints of appendages and a peculiar jumping motion. At this stage it may be mentioned that there are several sets of small worm-like animals of which we know very little. It is quite possible that some of them may become of great interest to the systematic zoologist, but we do not yet understand what places in the system they should occupy. Moreover, as they are small, un- familiar, and unknown to myself, I shall simply refer the curious to what more complete works say about the Gasterotricha, Echinoderidte, Demoscolecidm, and Chmtosomidm. Appendix (3) to Annelid Series. A. Class Sipunculida:, e.g. Sipunculus, and B. Class Priapuuda:, e.g. Priapulus. These two classes were formerly united with the Echiuridm as Gephvrea, but it is improbable that the three are nearly related. The S1PUNCULID.E. 219 Echiuridse are apparently modified Chaetopods, while the position of the Sipunculidae and Priapulidte is quite uncertain. Both include marine worms, living in the sand or mud upon which they feed, having unsegmented bodies with a capacious body cavity, and an anterior protrusible proboscis or introvert, which is moved by special retractor muscles, and bears the mouth at its tip. In most other respects the two classes differ markedly from one another. In the Sipunculids, the large introvert terminates in a hollow tentacular fringe, within the cavity of which closed blood vessels run. The gut is much coiled, and the anus is dorsal and anterior. A nervous system with a distinct brain, a gullet-ring, and a ventral cord is present, but the ventral cord is unsegmented. Large nephridia or brown tubes, usually two in number, occur in the anterior region, and function also as genital ducts. The sexes are separate, and the reproductive cells develop on the lining of the body cavity. In the development, which includes a metamorphosis, several peculiarities are observable, tending to show that the animals are not primitive. The larva of Sipunculus is sometimes compared to a trochosphere, but differs from a typical trochosphere, notably in the total absence of segmentation, of “head- kidneys,” of a pre-oral band of cilia, as well as in the position of mouth and anus, and the slight development of the pre-oral lobe. The class includes eleven genera, which are widely distributed ; many of the species are large and conspicuous. It should be noticed that while typically without trace of setae [Gephyrea Achaeta], some genera, e.g. Phascolosoma , have distinct hooks on the introvert. The Priapulidae include two genera — Priapulus and Halicryptus, both almost entirely confined to the northern hemisphere. They have no tentacles, no vascular system, no brown tubes, and no brain. The gut is straight, or has a single loop ; the anus is posterior. A gullet-ring and ventral nerve-cord are present as in Sipunculus , but retain their primitive connection with the epidermis. There are complex genital ducts opening by a pore on each side of the anus, which in the young are connected with an excretoiy system of the Platyhelminth type, while in the adult they are overgrown and concealed by the repro- ductive cells. The development is unknown. In Priapulus there is a peculiar respiratory (?) appendage at the posterior end of the body. Appendix (4) to Annelid Series. Under the old term Molluscoidea are sometimes included the three classes — Phoronoidea, Polyzoa or Bryozoa, and Brachiopoda. The Molluscoidea are characterised by the presence of a true ccelom, formed in development by the folding off of pouches from the archenteron, and by the shortening of the dorsal region of the body, which results in the close approximation of mouth and anus. The mouth is typically furnished with ciliated tentacles, and is often overhung by an epistome ; both tentacles and epistome, when present, contain spaces which are part of the body cavity. Except in the Ectoprocta among Polyzoa, two or four nephridia are present, and serve also as genital ducts. There is always a metamorphosis in development, and the larvae are peculiar. 220 SEGMENTED WORMS OR ANNELIDA. The development is in most cases insufficiently known, and it is probable that further knowledge of it will remove these sets of animals from their apparently anomalous position. CNR Class Phoronoidea. This class has been erected for the single genus Phoronis, which has been associated both with the Gephyrea and with Polyzoa. With the removal of Cephalodiscus and Rhabdopleura from the last - named group to the Plemichorda, Phoronis has been left in a somewhat isolated position. Recently it has been proposed by Mr. Masterman to re- associate it with these forms and with Balanoglossns, on account of certain Chordate affinities said to be exhibited by the larva. The point will be further discussed in the chapter on Balano- glossus. The genus Phoronis includes a few species of small marine worms, social in habit, and found enclosed in fixed leathery tubes often encrusted with foreign parti- cles. Each individual is furnished with a horseshoe-shaped crown of tentacles, which are hollow and supported by an internal skeleton. The nervous system lies in the ectoderm — a very primitive character, and consists of a ring round the mouth, and of a cord down the left side of the body. An interesting point is the presence of a closed vascular system with nucleated red cells. The body cavity is well developed, and is divided into chambers. The sexes are united ; and the larva, known as Actinotroclia , under- goes a remarkable metamorphosis in the course of its conversion into the adult. Fig. 96.- — Actinotroclia or larva of Phoronis. — After Masterman. The mouth is overhung by the prominent pre-oral hood ; the anus is at the other end of the body. Behind the mouth is a ring of ciliated tentacles. SP., the nerve ganglion in the hood ; NG., the nerve gan- glion of the region called collar region by Masterman ; CNR., nerve-ring at base of tentacles. Class Polyzoa. As usually defined, the class includes two sub-classes, the Ectoprocta and the Entoprocta, but it seems almost certain that these are distinct classes. The Ectoprocta include fresh-water and marine forms, in which the anus is outside the basis of the tentacles. The nervous system is represented by a ganglion placed between the mouth and anus. There is no vascular system. Nephridia are absent. All are colonial and bud very freely ; the marine forms show considerable division of labour among the members of the colon}-. BRACHIOPODA. 221 (11., Mouth ; g. , gut cut through, and with coils omitted ; a ., anus; >na., rnadreporite; st., stone canal ; e.c., circular canal ; P. , one of the Polian vesicles ; r.v., radial vessel; am. . ampulla of tube- foot; t/., tube-foot ending in sucker; «., radial nerve given off from nerve-ring; al., alveolus, one of the parts of Aristotle’s lantern, at the left the alveolus is removed to show one of the strong teeth ( i .) ; f, falces, to which the retractor muscles (mu.) of the lantern are attached ; s/>., spines on surface of test ; pc., pedicellaria : fleshy lobes or lips ; oc., one of ocular plates. The nervous system consists of a ring around the mouth, of radial branches running up each ambulacral area, and of the superficial network. Tube-feet, sphseridia, pedicellariae, and spines are all under nervous control, and each radial 234 ECHINODERMA. nerve ends in the “eye specks” of the apical “ocular plates.” It is probable that all the tube-feet are sensory, and this is certainly the main function of ten which lie near the mouth. The alimentary canal passes through Aristotle’s lantern, and the intestinal portion lies in two and a half coils around the inside of the shell, to which it is moored by mesenteries. It contains fine gravel, sand, and some organic debris. It ends near the centre of the apical disc, whence the pedi- cellarite have been seen removing the faeces. Accompanying the first coil of the gut is a canal or “siphon,” which opens into the gut at both ends. Accord- ing to Cuenot, a current of water traverses this tube, which thus, by reason of its thin walls, carries oxygen to the corpuscles of the body fluid. The spacious body cavity is lined by ciliated epithelium, and contains a “perivisceral” fluid, whose corpuscles have a respiratory pigment (echino- chrome). When the fluid of a perfectly fresh sea-urchin is emptied out, the contained corpuscles unite in plasmodia, forming composite amoeboid clots (cf. Proteomyxa , etc.). The madreporic plate communicates with a membranous stone canal, which runs downwards into a circular vessel near the upper end of the lantern. This gives off five inter- radial transparent vesicles and five radial vessels, which run down the sides of the lantern and up each ambulacral area. Each radial vessel gives off numerous lateral branches, which communicate with the internal ampullae and thence with the external tube-feet. When the tube-feet are made tense with fluid, they extend beyond the limit of the spines, and are attached to the surface of the rock over which the sea-urchin slowly drags itself. The sucker at the tip of each tube-foot bears small calcareous plates regularly arranged ; indeed, there is hardly any part of an Echinoderm in which lime may not be deposited. Before bending upwards from the base of the lantern, each radial vessel gives off a branch to two large tentacle-like tube-feet without attaching discs. The five pairs lie near the mouth, and are sensitive. The blood vascular system is not leadily traced, and there is un- certainty as to many points. It seems to consist of a circular vessel around the gullet, connected both with five radial vessels and with two vessels lying respectively on the dorsal and ventral surfaces of the intestine, and forming a network over it. The fluid cannot be dis- HOLOTHUROIDEA. 235 tinguished from that of the body cavity ; it contains corpuscles, some of which are pigmented. On the area round about the mouth there are ten hollow outgrowths, which resemble the skin-gills of star-fishes. As already mentioned, the pigmented cells of the body cavity fluid seem able to absorb oxygen. The water vascular system plays here a very important part in respiration. Waste products seem simply to accumulate in the tissues, but Hartog maintains that the water vascular system helps in excretion. The sexes are separate, and like one another. Five branched yellow-brown ovaries or rose-white testes lie interradially under the apex of the shell, and open by separate ducts on the five genital plates. In spring the apical disc may be seen covered with orange ova or milky- white spermatozoa. The eggs are fertilised externally by sperms wafted from adjacent sea-urchins, and the free-swimming larva is called a Pluteus. Classification.— 1. Pakeoechinoidea. Extinct forms, apparently with a plastic test of overlapping and variable plates. They appear in Lower Silurian rocks. 2. Desmosticha. Regular and symmetrical sea-urchins like Echinus. In Cidaris, there are no external gills. A species of Diadema has been described as covered with compound eyes. In Cyanosoma urens the spines contain a poison apparatus. Echinothuridse have flexible tests. 3. Clypeastroidea. Shield-shaped, and often flat. The food canal ends outside the apical disc on the posterior inter-radius. e.g. Clypeaster. 4. Petalosticha. Pleart-shaped. The mouth is ex-centric, the food canal ends away from the apical disc. There are no masticat- ing organs. On the dorsal surface the ambulacral areas dilate from the apex outwards, and contract again towards the margin in the form of “petals.” The anterior area is often different from the other four. e.g. Spa/angus. Some, e.g. Hemiaster, carry their young among their spines. Class Holothuroidea. Sea-Cucumbers. rhe Holothurians do not at first sight suggest the other Echinoderms, for they are like plump worms, and the 236 ECHINODERMA. calcareous skeleton is not prominent. But closer examina- tion shows the characteristic pentamerous symmetry, and the occurrence of calcareous plates in the skin. These seem to be absent in the unique pelagic Pelagothuria. Holothurians occur in most seas, from slight to very great depths. Their food consists of small animals, and of organic particles from the sand. Some of them catch these in their waving tentacles, which are then plunged into the pharynx. The muscles of a captured Holothurian often over-contract and eject the viscera at the ends or through a side rupture ; in this way the animal may sometimes escape, and the viscera can be regrown. In Synapta the rupture of the body takes place very rapidly, and is probably defensive, the anterior portion reforming a complete individual. In some forms of Cucumaria planci the body divides by stricture, torsion, or stretching into two or three equivalent parts, each of which may regenerate the whole. In this case the autotomy seems to be reproductive. The worm-like body is often regular in form, with five equidistant longitudinal bands, along which tube-feet emerge. But three of these “ambulacral areas” may be approxi- mated on a flattened ventral sole, leaving two on the convex dorsal surface, and there are other modifications of form. In many cases the tube-feet are irregularly scattered over the surface. The walls of the body are tough and muscular, and a skeleton is represented by scales, plates, wheels, and anchors of lime scattered in the skin, and by plates around the gullet and on a few other regions. The nervous system consists of a circumoral ring in which the five radial nerves running in the ambulacral areas unite, and from which nerves to the tentacles arise. Sense organs are represented by the tentacles, which sometimes have “ ear-sacs ” at their bases, and by tactile processes on the dorsal surface of some of the creeping forms. From the terminal or ventral mouth, surrounded by five, ten, or more tentacles, the food canal coils to the opposite pole. There it expands in a cloacal chamber sometimes contractile, and from this are given off in many forms a pair of much branched “respiratory trees,” which extend forward in the body cavity. These “ trees ” are supplied HOLO THUROIDEA. 237 with fresh water by means of the rhythmic contractions of the cloaca, and are respiratory, hydrostatic, and excretory. The body fluid sometimes contains a red pigment like haemoglobin. Arising from the base of the left respira- tory tree, in some Holothurians there are remarkable “ Cuvierian organs,” consisting of numerous tubes, in most cases glandular. The Holothurian can eject these tubes through the cloaca, the wall of which is apparently ruptured in the process. The tubes are very viscid, and seem to grow longer in the water ; they will adhere to almost everything but the Holothurian itself. Those Holothurians in which the organs are well developed are often called “ cotton-spinners,” on account of the dense mass of viscid substance which they eject. A little fish, Fiercisfer , introduces itself — tail first- — into the cloaca of several Holothurians, and lives there as an innocent commensal. The water vascular system shows many peculiarities. In what, by analog}" with the other classes, may be described as the primitive condition, there is a ring canal round the mouth communicating with the exterior by a stone canal and a madreporite, with one or more Polian vesicles hanging in the body cavity, and with five radial canals. The radial canals, as in star-fishes and sea-urchins, are connected with internal ampul Ice and external tube-feet. The anterior tube-feet are greatly enlarged and modified to form the tentacles which encircle the mouth. It is, however, only rarely that the water vascular system exhibits this primitive condition. In most cases the stone canal loses its original connection with the exterior and opens merely into the body cavity ; often it is represented by numerous small canals, hanging freely in the body cavity (Fig. 103, si.). Certain of the tube-feet are always modified to form tentacles, and they may, as in Synapta , be the only representatives of the tube-feet. In regard to the function and degree of development of these, there is indeed much variation. The blood vascular system is well developed, but here as elsewhere the vessels are lacunar and without endothelial linings, and tend to form ramifying networks over the surface of the organs. The arrange- ment of the vessels is in essence the same as in sea-urchins. The sexes are usually separate. The reproductive organs do not exhibit radial symmetry, and are branched tubes which open within or just outside the circle of tentacles. I hey and other internal organs of Holothurians are often very brightly coloured. The larva is, in most cases, what is known as an Auricularia. Sometimes, how- ever, the larval stage is skipped, as in Cucamaria crocea Fig. 103. — Dissection of Holothurian ( Holothuria tubulosa ) from the ventral surface. Tentacles surrounding the mouth ; t.f.> scattered tube-feet of ventral surface; c., calcareous ring surrounding the food canal ; a., ampullae of tentacles (modified tube-feet) ; r., circular vessel surrounding the gullet, giving off the branched stone canal (j/.), the single Polian vesicle (o.), and the five radial canals ( r.c .) which run forwards, pass through the calcareous ring, and then curve outwards to run on the surface of the longitudinal muscles ( l.m .) throughout the body. Of the five longitudinal muscles, one only is marked. g*., The gut cut through at the beginning of the first loop; the mesentery which attaches the gut to the body-wall, showing the course of the gut ; g2.} the other end of the gut ; c/., the cloaca bound down by muscles ; an ., the anus ; r. t. , the right respiratory tree, the left is cut short close to its origin ; ov.t the ovary. The blood vessels are not shown. CRINOIDEA. 239 and Psolus epfuppiger, where the eggs and young are attached to the back of the mother. In Cucumaria Iceyigata there is an invaginated brood-pouch ; in Synapta vivipara and others the body cavity serves as a brood-pouch. The calcareous plates of Holothurians are found as far back as Carboniferous strata. As “trepang” or “ beche-de-mer,” the Holothurians of the Pacific form an important article of commerce, being regarded as a delicacy by the Chinese. Classification. — 1. Elasipoda : primitive deep-sea forms, bilaterally symmetrical, with tube-feet on the ventral surface only, and with papillae on the back. The stone canal often opens externally by a pore. There are no respiratory trees or Cuvierian organs. e.g. Kolga, Elpidia. 1. Pedata : with well-developed tube-feet and papillae, usually with respiratory trees and Cuvierian organs. e.g. Holothuria, Cucumaria , Psolus. Pelagothuria , a very remarkable form, is pelagic and free-swimming, and wholly without lime. 3. Apoda : without radial canals, tube-feet, or respiratory trees. e.g. Synapta, a remarkable animal, especially apt to break in pieces ; tentacles pinnate ; hermaphrodite ; with beautiful calcareous anchors and plates in the skin. Semper has described a stiange animal, Rhopalodina lageniformis , from the Congo coast. It is like a globular flask, with mouth and anus close together at the narrow end, with ten ambulacral areas. Class Crinoidea. Feather-Stars. The feather-stars or sea-lilies differ from other Echino- derms in being fixed permanently or temporarily by a jointed stalk. The modern Comatulids, e.g. the rosy feather-star (Comatula or Antedon rosacea) leave their stalk at a certain stage in life ; but the other Crinoids, e.g. Pentacrinus , are permanently stalked, like almost all the extinct stone-lilies or encrinites, once so abundant. Most of them live in deep water, and many in the great abysses. An anchorage is found on rocks and stones, or in the soft mud, and great numbers grow together — a bed of sea-lilies. The free Comatulids swim gracefully by bending and straightening their arms, and they have grappling “ cirri ” on the aboral side, where the relinquished stalk was attached. By these cirri they moor themselves temporarily. Small organisms — 240 ECHINODERMA. Diatoms, Protozoa, minute Crustaceans — are wafted down ciliated grooves on the arms to the central mouth, which is of course on the upturned surface. Some members of the class, eg. Comatula, are infested by minute parasitic “ worms ” (Myzostomata) allied to Chaetopods, which form galls on the arms. A lost arm can be replaced, and even the visceral mass may be regenerated completely within a Fig. 104. — Diagrammatic vertical section through disc and base of one of the arms of Antedon rosacea. — After Milnes Marshall. The section is interradial on the left, radial on the right, t. , Cili- ated openings in body- wall ; h., subepithelial ambulacral nerve ; water vascular canal ; k ., tentacle ; r., mouth ; j., intestine ; g., central plexus, with “chambered organ” at its base ; f.y coelom ; RP-R3., radial plates ; Br.y brachial plates ; «., muscle ; a., axial nerve-cord; d., central capsule; C.D. , centro-dorsal plate ; p cirri ; e., nerve branches from central capsule to cirri. few weeks after it has been lost. It has been suggested that the occasional expulsion of the visceral sac frees the Crinoid from parasites (Dendy). The animal consists of (1) a cup or calyx, (2) an oral disc forming the lid of this cup, (3) the radiating “arms,” and (4) the stalk supporting the whole. The oral disc, turned upwards, is supported by plates. Heie the anus also is situated. The arms usually branch in dichotomous fashion, and thus ten, twenty, or more may arise from the original five. But the growing point continues to fork dichotomously, like the leaf of mam- ferns, and as each alternate fork remains short, a double series of lateral “ pinnules ” results. The arms are supported by calcareous plates. The stalk usually consists of numerous joints, especially in extinct forms, in some of which it measured over fifty feet in length. Except in Holopus, and in the stalked stage of Antedon , the stalk bears lateral cirri. BLASTOIDEA. 241 Apart from the superficial epithelium, there are no sensory structures. The ciliated food canal descends from the mouth into the cup, and curves up again to the anus, which is usually ex-centric in position. The last part of the gut is expanded to form an anal tube, which during life is in constant movement, and has apparently a respiratory function. From the cup, where the body cavity is in great part filled with con- nective tissue and organs, four coelomic canals extend into each of the arms. They communicate at the apices of the arms and pinnules, and currents pass up one and down the other. The blood vascular system consists of a circumoral ring, which is connected with a radial vessel under each ambulacral nerve, and with a circumcesophageal plexus. The water vascular system consists as usual of a circumoral ring and radial vessels, but in several respects it shows remarkable modification. The madreporite of other forms is represented by fine pores which open from the surface of the calyx directly into the body cavity, and which may be very numerous; there are said to be 1500 in Antedon rosacea. By these pores water enters the body cavity, and from it enters the numerous stone canals which hang from the ring freely in the body cavity, and open into it near the pore canals. There are no Polian vesicles or ampullae, the tube-feet are small, are arranged in groups of three, and are connected by delicate canals with the radial vessels. Certain of them form tentacles around the mouth, and these are supplied by canals coming off directly from the ring canal. The sexes are separate, and a process suggestive of sexual union has been observed in Antedon. The reproductive organs extend as tubular strands from the disc along the arms, but are rarely functional except in the pinnules, from each of which the elements burst out by one duct in females, by one or two fine canals in males. There are about 400 living species in twelve genera, but about 1500 species in 200 genera are known from the rocks. The class is obviously decadent. It is represented in the Cambrian, and attained its maximum development in Silurian, Devonian, and Carboniferous times. The oval ciliated larva of Antedon, the only one known, is less quaint than that of other Echinoderms. The classification is a matter of considerable difficulty, but the old division into Palaeocrinoidea and Neocrinoidea must apparently be abandoned. The recent forms include the stalked Pentacrinus , Rhizo- crinus, etc., and the free Comatulids, which pass through a stalked Pentacrinus stage, e.g. Antedon. Holopus is a remarkable deep sea form, with direct ancestors in the Upper Silurian. Marsupi/es is an extinct Crinoid which had no stalk. Class Blastoidea. Wholly extinct. The Blastoids are first found in the Upper Silurian, later than Cystoids and Crinoids ; they had their golden age in the Carboniferous and Devonian times, but then disappeared. Their body was ovate, with five ambulacral areas, with each groove of which jointed pinnules were associated. 16 242 ECHINODERMA. Class Cystoidea. Wholly extinct. The Cystoids are first found in the Lower Silurian rocks, had their golden age in Upper Silurian times, and died out in the Carboniferous period. Their body was ovate or globular, sessile or shortly stalked, covered with polygonal plates often irregularly arranged. Some (according to Bell, the more primitive) types were “never fixed, and had not fixed ancestors.” They seem usually to have borne two to five feeble, unbranched arms. Development of Echinoderms. The ovum undergoes total segmentation, and a hollow ball of cells or blastosphere results. Apart from two alleged cases of delamination, the gastrula is always formed by the invagination of this blastosphere. Ectoderm and endoderm, or epiblast and hypoblast, are thus established. Fig. 105. — Stages in development of Echinoderms. — After Selenka. i. Section of blastula of Synafita digitata (Holothuroid), with a hint of gastrulation. 2. Section of Gastrula of Toxopneustes brevispinosus (sea- urchin) ; ec., ectoderm; cn., endoderm; m. , segmentation cavity with mesenchyme cells in it. 3. Section of larva of Asterina gibbosa (star- fish); Bl.y blastopore;^-., archenteron ; v.p. , vaso-peritoneal vesicle ; r. and right and left sides. The mesoblast has a twofold origin : (a) from “ mesen- chyme ” cells, which immigrate from the invaginated hypo- blast into the segmentation cavity ; ( b ) by the outgrowing of one or more coelom pouches from the gastrula cavity or archenteron. It is thus that the body cavity and the rudiments of the water vascular system arise. According to Hertwig’s fundamental thesis, this double origin is a primitive condition, and the mesenchyme here, as always, is non-epithelial, and gives rise to the connective tissues and to the vascular system. On the other hand, it has been asserted that in Echinoderms the mesenchyme is DEVELOPMENT OF E CHIN ODE RMS. 243 not purely a “packing tissue,” but may acquire a distinctly epithelial character. Many of the early mesenchyme cells are calciferous, combining to form the larval skeleton. The larva is, first of all, a slightly modified, diffusely ciliated gastrula. In Holothuroids, Echinoids, Asteroids, and Ophiuroids, it becomes quaintly modified by the out- growth of external processes, and the formation of special ciliated bands. The larva of Crinoids (i.e. of Antedon only) is not so divergent. In all cases the bilateral symmetry is preserved. The larva does not grow directly into the adult. On the contrary, the adult arises, for the most part, from new growth within the larva. The structures peculiar to the larva are absorbed or in part thrown off. Only in a very few cases is the development direct. The details of the development are so difficult that we can here only give a few notes. There is a close connection between the origin of the body cavity and that of the water vascular system. Both are the results of an outgrowth or of outgrowths from the gastrula cavity or archen- teron, into the surrounding space between endoderm and ectoderm. As they have a common origin, the outgrowth or outgrowths which give rise to enterocoel and hydroccel may be termed vaso-peritoneal. The celebrated comparative anatomist and physiologist, Johannes Muller, was the first to show that the various types of Echinoderm larvae might be derived from one fundamental form. “This fundamental type is an elongated, oval, or pear-shaped larva, which is somewhat flattened on its ventral side. It has arisen from a gastrula whose blastopore has become the anus, while the archenteron is bent towards the ventral surface, where it communicates by the larval mouth with the exterior. Besides these two apertures, the larva has a third, namely, the dorsal pore of the water vascular system. The cilia, with which the larva was at first uniformly covered, partly disappear, and persist only in restricted regions or ciliated bands” (Ivorschelt and Heider). Crinoids. — The simplest Echinoderm larva is that of Antedon, a somewhat modified oval, with five transverse rings of cilia (the most anterior is less distinct), and a posterior terminal tuft. Holothuroids.- — The larva of Holothuroids (an Auricularia ) is much quainter. Its diffuse cilia are succeeded by a wavy longitudinal band, which in the pupa stage breaks into transverse rings, usually five in number. The pre-oral region becomes large. Asteroids. — Nearest the Auricularia is the larva of starfishes, which has the same enlarged pre-oral region. There are two ciliated bands, of which the ad-oral is smaller, the ad-anal much larger. They are extended peripherally by the development of soft arms, and such a larva is known as a Bipinnaria. But this may be succeeded by a Brachiolaria 244 E CH1N0DE RMA . stage, in which three warty arms are formed at the anterior dorsal end, independently of the ciliated bands. Ophiuroids and Echinoids. — In the Pluteus larvae (Fig. 98) char- acteristic of these classes the pre-oral region remains small, while the post-anal region becomes large. There is one undulating ciliated band, the course of which is much modified by the growth of six long arms, with temporary calcareous supports. This quaint form is often compared to a six-legged easel. The development of these larval forms into the adult is very intricate. The adult is a new formation within the larva, retaining the water vascular system and mid-gut, but absorbing or rejecting the provisional larval structures. As certain parts are broken down, others are built up, chiefly through the agency of the wandering amoeboid cells of the mesenchyme. The first steps in the upbuilding of the adult, and especially of its skeleton, are to some extent parallel in the five classes. One of the most important changes is that from bilateral to radial symmetry. In connection with this, it has been conjectured that the primitive ancestor was bilaterally symmetrical, and that the radiate symmetry was acquired by early sessile or sedentary Echinoderms, such as the Cystoids. As we have already seen, the adults in the different classes tend to acquire an independent and secondary bilateral symmetry. It is very difficult to compare the Echinoderm larva;, even in their simplest form, with those of other animals. The nearest type is perhaps the Tornaria of Balanoglossus , but it again is very unique. One naturally tries to compare the Echinoderm larva with the Trochosphere of Annelids, but the differences are very marked. One of the most marked of these is the absence of the apical sense organ, so charac- teristic of the Trochosphere. The fact that this is represented in the larva of Antedon is regarded by many naturalists as a point of much importance. • Relationships of Echinoderma. The Echinoderms form an exceedingly interesting class. Well- defined as they are, the Holothurians especially show how many of the significant characters may be lost. In that group we see how the power of forming a calcareous skeleton, the characteristic tube-feet, and the greater part of the peculiar water vascular system, may all disappear ; it is conceivable that further modification of the same kind might eliminate all the distinctively Echinoderm characters, and produce an organism whose systematic position would be very' difficult to determine. This is important, because, as we have already seen, there are many “worm-like” types of whose affinities we know nothing. That some of these are related to Echinoderms has been tentatively suggested by many observers, and there is every reason to believe that the progress of research will remove the Echinoderms from their present isolated position. Concerning the exact relationships of the different classes of Echino- derma, there is still considerable doubt. The following account is based upon the views set forth by Professor Jeffrey Bell ; but the student will do well to realise that in this, as in most problems of phylogeny, there is little certainty : — RELATIONSHIPS OF E CHINO DERMA 245 246 ECHINODERMA. The Holothurians have no ab-oral system of plates, and the radial symmetry does not affect the' reproductive organs. These two negative characters, combined with some positive ones, may indicate that the Holothurians are primitive, and, as is certainly suggested by their external appearance, have affinities with the supposed “worm-like” ancestors of Echinoderms. Again, some members of the heterogeneous class of Cysloids are extremely primitive, but differ from the Holothurians in the possession of an ab-oral "system of plates, alternately radial and inter-radial. From this primitive Cystoidean stock, two branches diverge. The one leads to the sessile Cystoids, Blastoids, and Crinoids (Pelmatozoa) ; the other to the free Echinoidea, Asteroidea, and Ophiuroidea. Of these, the existing Asteroidea and Ophiuroidea are late divergences from a common stock. / CHAPTER XIII. CRUSTACEA (First Class of the Arthropod Series). More than half the known species of animals are included in the Arthropod series, for of insects alone there are said to be more species than of all other animals taken together. The Arthropods are in some ways like Annelids, — in the bilateral symmetry; in the division of the body into successive segments, some or all of which bear appendages ; in the plan of the nervous system ; and so on. Furthermore, Peripatus, which has air-tubes or tracheae somewhat similar to those of Myriopods and Insects, has nephridia like those of some Annelids ; and the biramose appendages of a simple Crustacean like Apus may be compared with the parapodia of an Annelid. But we cannot, as yet, do more than recognise certain possibilities of pedigree. It is also difficult to discern the relationships of the various classes included in the Arthropod series. Crusta- ceans, most of which are aquatic and breathe by gills, are often opposed to the others (Prototracheata, Myriopoda, Insecta, and Arachnoidea), most of which are terrestrial or aerial, and breathe by tracheae, or possible modifications of these. But besides the classes named there are three divergent types the King-crab (Litnulus), and the extinct Eurypterids and Trilobites. These have been much bandied about from Crustaceans to Arachnoids, and it seems convenient to keep them in a separate class as Palreostraca. General Characteristics of Arthropods (to which primitive, parasitic, and degenerate forms present exceptions). The body is bilaterally symmetrical, and consists of numerous segments variously grouped. Several or all of the segments 248 CRUSTACEA. bear paired jointed appendages not uniform in structure. The cuticle is chitinous. Ciliated epithelium is almost always absent. The dorsal braiti is connected by a ring round the gullet with a double chain of ventral ganglia. Above the food canal lies the heart. The true or primitive coelom is always small in the adult ; the apparent body cavity is of secondary origin , and has in a great part a blood carrying or vascular function. The sexes are almost always separate , the repro- ductive organs and ducts are usually paired. There is often some metamorphosis in the course of development. In habit the A7-thropods are predominantly active. Class Crustacea. General Characteristics of Crustaceans (to which primitive, parasitic, and degenerate forms offer exceptions). With the exceptioii of the la7id-crabs, wood-lice , and sand- hoppers, the Crustacea7is live i7i water and breathe by gills or through the shift. The head carries two pairs of antemice in additio7i to other appe7idages ; the thorax or 7iiedia7i part of the body , so7netimes distuict fro77i , and sometimes fused to the head, also bears limbs ; the posterior region or abdo7tie7i is usually seg77iented, and ofte7i furnished with appendages. The typical appendage consists of two branches a?id a basal portion , to which gills 77iay be attached. To the chitin of the cuticle, carboiiate of line is added. A Type of Crustacea. The fresh-water Crayfish (Astacus fluviatilis). (Most of the following description will apply also to the Lobsters Hoi/iarus and Palmurus, and to the Norway Lobster (Nephrops Norvegictts), often called a crayfish). Mode of life.— The fresh-water crayfish lives in streams, and burrows in the banks. It is not found in Scotland, but occurs here and there in England, and Ireland, and is common on the Continent. It is absent from districts where the water contains little lime. The food is very varied — from roots to water-rats ; cannibalism also occurs. The animals swim backwards by powerful tail strokes, or creep forwards on their “walking legs.” There life is tolerably CRA YFISH. 249 secure, but the frequent moultings during adolescence are expensive and hazardous. When hatched the young- are like miniature adults ; for a time they cling beneath the tail of the mother. External appearance. — The head and thorax are covered by a continuous (cephalothoracic) shield ; the abdomen shows obviously distinct segments movable upon one another. As indicated by the appendages, there are three groups of segments or metameres — five in the head, eight in the thorax, six in the abdomen, as well as an unpaired piece or telson on which the food canal ends. Each of the nineteen segments bears a pair of appendages. Among other external characters may be noticed the stalked mov- able eyes, the two pairs of feelers, the mouth with six pairs of appendages crowded round it, and the gills under the side flaps of the thorax. The Body-Wall consists of — ((1) The external shell or cuticle, composed of various strata of chitin, coloured with pig- ments, hardened with lime salts ; . (2) The ectoderm, epidermis, or hypodermis, which makes and remakes the cuticle ; (3) An internal connective tissue layer or dermis, with pigment, blood vessels, and nerves. \ Internal to this lie the muscles. Between the rings and at the joints the cuticle contains no lime, and is therefore pliable. As a sacrificed product of epidermic cells, it is dead and cannot expand. Hence, as long as the animal continues to grow, periodic moulting is necessary. The old husk becomes thinner, a new one is formed beneath it, a split occurs across the back just behind the shield, the animal withdraws its cephalothorax and then its abdomen, and an empty but complete shell is left behind. The moulting is preceded by an accumulation of glycogen in the tissues, and this is probably utilised in the rapid growth which intervenes between the casting of the old and the hardening of the new shell. How thorough the ecdysis or cuticle-casting is, may be appreciated from the fact that the covering of the eyes, the hairs of the ears, the lining of the fore-gut and hind-gut, the gastiic mill, and the tendinous inward prolongations of the cuticle to which some of the muscles are attached, are all got rid of and renewed. The moults occur in the warm months, eight times in the first year, five times in the second, 250 CRUSTACEA. thrice in the third, after which the male moults twice, the female once a year, till the uncertain limit of growth is reached. It is not clearly known in what form the animals procure the carbonate of lime which is deposited in the chitinous cuticle, but Irvine’s experiments have shown that a carbonate of lime shell could be formed by crabs even when the slight quantity of carbonate of lime in sea water was replaced by the chloride. Moulting is an expensive and exhausting process, and great mortality is associated with the process itself or with the defenceless state which follows. The process is a disadvantage attendant on the advantage of armature. Inequalities in the legs are usually due to losses sustained in combat, but these are gradually repaired by new growth. The surface of the body bears hairs or bristles of various kinds. These have their roots in the epidermis, and are made anew at each moult. There are simple glands beneath the gill-flaps, and on the abdomen of the female there are cement glands, the viscid secretion of which serves to attach the eggs. Appendages. — The limbs of a Crustacean usually exhibit considerable diversity ; in different regions of the body they are adapted for different work ; yet all have the same typical structure, and begin to develop in the same way. In other words, they are serially homologous organs , illus- trating division of labour. Typically each consists of a basal piece or protopodite, and two jointed branches rising from this — an internal e?idopodite and an external exopodite ; but in many the outer branch disappears. The protopodite has usually two joints — a basal or proximal coxopodite, and a distal basipodite ; the five joints which the endopodite frequently exhibits are named from below upwards — ischio-, mero-, carpo-, pro-, dactylo - podites — details of some use in the comparison and identi- fication of species. The stalked eyes are not included in the above list, since their develop- ment is not like that of the other appendages ; but cases where an excised eye has been replaced by an an tenniform structure, suggest that the eye-stalk may be of the nature of an appendage. Though the two pairs of antennae lie far in front of the mouth, it is possible that they were originally post-oral. With many of the thoracic appendages, gills, plate-like epipodites, and setae are associated. It is interesting to connect the structure of the appendages with their functions. Thus it may be seen that the great paddles are fully spread when the crayfish drives itself backwards with a stroke of its tail, while in straightening again the paddles are drawn inwards, and the outer joint of the exopodite bends in such a way that the friction is reduced. It is likely that some of the crowded mouth parts, e.g. the first maxilire, are almost functionless. The hard toothed knob which forms Abdomen Thorax Head CRA YFISH. 251 THE APPENDAGES OF THE CRAYFISH. No. Name. Antennules (pre- oral ?). Antennre oral ?). Mandibles. (pre- 1st Maxillae. 2nd Maxillae. 1. st Maxillipedes (foot-jaws). 2nd Maxillipedes. 3rd Maxillipedes. Forceps (chelate). Walking Legs (chelate). 13 x4 I 16 17 18 *9 Modified swim- nierets in male ; in female, rudi- mentary. Modified swim- merets in male, normal in female. Swimmerets. Great paddles. Function. Tactile, olfactory, with ear - sac at base. Tactile, opening of kidney at base. Masticatory. Produces respira- tory current. Masticatory. Fighting, seizing. Walking. Genital opening in female. Genital opening in male. ( Serve in the male < as canals for the ( seminal fluid. Structure. Two branches, but probably not homologous with endo- podite and exopodite. Small exopodite. Four joints, of which three form the palp (endopodite and upper joint of protopo- dite). Thin single-jointed protopo- dite, small endopodite, no exopodite. Thin protopodite, filamen- tous endopodite ; the “baler” is formed from the epipodite, probably along with the exopodite. Thin protopodite, small en- dopodite, large exopodite. Two -jointed protopodite, five - jointed endopodite, long exopodite. Two -jointed protopodite, large five-jointed endopo- dite with strong teeth on its ischiopodite, slender exopodite. No exopodite. In the claw the last joint bites against a prolongation of the second last. Without chelae. Protopodite and endopodite form a canal ; no exopodite. All the three parts. f Move slightly like J oars, and carry | the eggs in the | V female. Important in swim- ming. 252 CRUSTACEA. the greater part of the mandible is obviously well adapted to its crush- ing work. In connection with the skeleton, the student should also notice the beak ( rostrum ) projecting between the eyes ; the triangular area (epi stoma) in front of the mouth, and the slight upper and lower lips ; and the lateral flaps of the body-wall which protect the gills. Each posterior segment consists of a dorsal arch (ter gum), side flaps (pleura), a ventral bar (sternum), while the little piece between the pletiron and the socket of the limb is dignified by the name of epimeron. The hindmost piece ( telson ), on which the food canal ends ventrally, is regarded by some as a distinct segment. The most difficult fact to understand clearly, is that the cuticle of certain mouth parts (e.g. the mandibles), and of the ventral region of the thorax, is folded inwards, forming chitinous “tendons” or insertions for muscles, protecting the ventral nerve-cord and venous blood sinus, and, above all, constituting the complex, apparently, but not really, internal, “ endophragmal ” skeleton of the thorax. Muscular system. — The muscles are white bundles of fibres, which on minute examination show clearly that trans- verse striping which is always well marked in rapidly con- tracting elements. The muscles are inserted on the inner surface of the cuticle, or on its internal foldings ( apodemata ). The most important sets are — (i) the dorsal extensors or straighteners of the tail ; (2) the twisted ventral muscles, most of which are flexors or benders of the tail, which have harder work, and are much larger than their opponents ; (3) those moving the appendages ; (4) the bands which work the gastric mill. Nervous system. — The supra-oesophageal nerve-centres or ganglia, forming the brain, have been shunted far forward by the growth of the pre-oral region. We thus understand how the nerve-ring round the gullet, connecting the brain with the ventral chain of twelve paired ganglia, is so wide. The dorsal or supra-oesophageal ganglia are three-lobed, and give off nerves to eyes, antennules, antennae, and food canal, besides the commissures to the sub-oesophageal centres. They act as a true brain. The sub- oesophageal ganglia, the first and largest of the ventral dozen, innervate the six pairs of appendages about the mouth. There are other five ganglia in the thorax, and six more in the abdomen. Though the ganglia of each pair are in contact, the ventral chain is double, and at one place, between the fourth and fifth ganglia, an artery (sternal) passes between the two Fig. 106. — Appendages of Norway lobster. Ex., Exopodite; En., endopodite ; protopodite dark throughout E/., ep‘podite. I. Antennule — E., position of ear ; 2. antenna, A ., opening of ktdney , 3> mand ible— palp; 4. first maxilla: 5. second maxilla- A., baler ; ft. first maxilli- pede ; 7. second maxillipede ; 8. third maxillipede— the basal joint of the proto- podite is called coxopodite, the next basipodite ; the five joints of the endopodite are called— ischiopodite (f.) ; meropoditc (;«.) ; carpopodite (c.) , propodite(A) , dactylopodite (d.) : 9- forceps-<7) coxopodite ; (6) basipodite th»J»*o'he endopodite are numbered ; 10-13. walking legs; 14. modified male appendage , 15-18. small swimmerets ; 19. large paddles. 254 CRUSTACEA. halves of the cord. From each pair of ganglia nerves are given off to appendages and muscles, and apart from the brain, these minor centres are able to control the individual movements of the limbs. In the thoracic region the cord is well protected by the cuticular archway already referred to. From the brain, and from the commissure between it and the sub- oesophageal ganglia, nerves are given off to the food canal, forming a complex visceral or stomato-gastric system. Similarly, from the last ganglia of the ventral chain, nerves go to the hind-gut. If the brain be regarded as the fusion of two pairs of ganglia, as the development suggests, and the sub-oesophageal as com- posed of six fused pairs, then these, along with the eleven other pairs of the ventral chain, give a total of nineteen nerve- centres, — a pair for each pair of appendages. Sensory system. — A skin clothed with chitin is not likely to be in itself very sensitive, but some of the setae are, and some ob- servers describe a peri- pheral plexus of nerves beneath the epidermis. The setae are not mere out- growths of the cuticle, but are continuous with the living epidermis beneath ; and though some are only fringes, both experiment and histological examina- tion show that others are tactile. On the under surface of the outer fork of the antennules there are special innervated setae, which have a smelling function. Other likewise specialised hairs have sunk into a sac at the base of the antennules, and are spoken of as auditory. The sac opens by a bristle-guarded slit on the inner upper corner of the expanded basal joint, and contains a gelatinous Fig. io6a. — Section of compound eye of My sis vulgaris . — After Gren- adier. ?//., Musde of eye-stalk ; 1-4 ganglionic swellings in the course of the optic nerve ; n., the nerve fibrils passing up to the retinulae ; rh ., the rhabdoms ; re,, elements of retinulae; band of pigment; c crystalline cones ; co., the corneal facets with the subjacent nuclei. CKA YFJSH. 255 d.r d.r— l! fluid and small “otoliths,” which seem to be foreign particles. This “ ear ” seems to be an equilibrating organ, connected with directing the animal’s movements. In some other Crustaceans the audi- tory hairs are lodged in an open depression ; this has become an open sac in the crayfish, a closed bag in the crab. Small hairs on the upper lip of the mouth have been said to have a tasting func- tion. The stalked eyes, which used to be regarded as appendages, arise in development from what are called “ procephalic lobes ” on the head. They are compound eyes, that is, they consist of a multitude of elements, each of which is structurally complete in itself. On the outside there is a cuticular cornea, divided into square facets, one for each of the optic elements ; beneath this lie, as in other parts of the body, the nucleated epi- dermal cells. Then follows a focus- sing layer, consisting of many crystalline cones. Each crystalline cone is composed of four crystalline cells, which taper internally, and externally secrete a firm crystalline body. The bases of the crystal- line cones are surrounded by the retinula cells. Each retinula con- sists of five elongated cells arranged about a central axis. Distally, this axis is formed by the crystal- line cone, proximally by a little rod or rhabdome. The rhabdome consists of four little red rods closely apposed together, and connected by a nerve-fibre with the optic ganglion, which lies at the end of the optic nerve. The proximal ends of the retinal cells are p.r — -N Fig. io6b. — A single eye ele- ment or ommatidium of the lobster. — After G. H. Parker. c., Cornea; c.h., corneal hypo- dermis ; cp., cap of crystalline cone; co., crystalline cone and body ; d.r., distal retinula elements; p.r., proximal re- tinula elements; A’., rhab- dom ; N., nerve-fibre. 256 CRUSTACEA. deeply pigmented. Thus each element consists of corneal facet, crystalline cone, and retinula, and the retinula consists of internal rhabdome, and external retinula cells. Between the individual optic elements lie some pigment cells. The eyes are able to form images of external objects, and these images are erect, not inverted as in the eyes of Vertebrates. Alimentary system. — The food canal consists of three distinct parts, a fore-gut or stomodgeum developed by an intucking from the anterior end of the embryo, a hind-gut Fig. 107. — Longitudinal section of lobster, showing some of the organs. H., Heart ; AO., ophthalmic artery; aa. , antennary artery; ah., hepatic artery ; ST., sternal artery; SA., superior abdominal artery; MG., mid-gut; DG., digestive gland ; HG., hind-gut; Ex., extensor muscles of the tail ; FI., flexor muscles of the tail ; I A., inferior abdominal artery ; G., gizzard ; C., cerebral ganglia; P., pericardium ; T., testes. or proctodaeum similarly invaginated from the posterior end, and a mid-gut or mesenteron, which represents the original cavity of the gastrula. The mouth has been shunted backwards from the anterior end of the body, so that the antennules and antennae lie far in front of it. The fore-gut, which is lined by a chitinous cuticle, includes a short “guilet,” on the walls of which there are small glands, hypothetically called “ salivary,” and a capacious gizzard, which is distinctly divided into two regions. HG CKA YFISH. 257 In the anterior (cardiac) region there is a complex mill ; in the posterior (pyloric) region there is a sieve of numerous hairs. The mill is very complex; there are supporting “ossicles” on the walls with external muscles attached to them, and internally projecting teeth which clash together and grind the food. Three of the teeth are conspicuous ; a median dorsal tooth is brought into contact with two large laterals. On each side of the anterior part of the gizzard there are two limy discs or gastroliths, which are broken up before moulting, and though quite inadequate to supply sufficient carbonate of lime for the new skeleton, seem to have some relation to this process. The occurrence of chitinous cuticle, hairs, teeth, and gastroliths in the gizzard, is intelligible when the origin of the fore-gut is remembered, and so is the dismantled state of this region when moulting occurs. The mid-gut is very short, but outgrowths from it form the large and complex digestive gland. The mid-gut, here as always, is the digestive and absorptive region, but both processes are carried on to a large extent in the digestive gland, which communicates with the mid-gut by two wide ducts. It is roughly three-lobed at both sides, and consists of an aggregated mass of caeca, closely compacted together. The gland is more than a “liver,” more even than a “ hepatopancreas.” It absorbs peptones and sugar; like the Vertebrate liver, it makes glycogen ; its digestive juices are comparable to those of the pancreas and the stomach of higher animals. The hind-gut is long and straight. It is lined by a chitinous cuticle, as its origin suggests. There are a few minute glands on its walls. Body cavity. — The space between the gut and the body- wall is for the most part filled up by the muscles and the organs, but there are interspaces left which contain a fluid with amoeboid cells. These interspaces seem to represent enlarged blood sinuses (a htemocoele), rather than a true body cavity or coelom. One of the spaces forms the blood-con- taining pericardium, or chamber in which the heart lies. Vascular system. — Within this non-muscular pericardium, and moored to it by thin muscular strands, lies the six-sided heart, which receives pure blood from the gills {via the pericardium) and drives it to the body. The arterial system is well developed. Anteriorly, the heart gives off a median (ophthalmic) artery to the eyes and antennules, a pair of (antennary) arteries to the antennm, and a pair to the digestive gland (hepatic). Posteriorly there issues a single vessel, which at once divides into a 17 CRUSTACEA. 25S superior abdominal, running along the dorsal surface, and a sternal which goes vertically through the body. This sternal passes between the connectives joining the fourth and fifth ventral ganglia, and then divides into an anterior and posterior abdominal branch. All these arteries are con- tinued into capillaries. From the tissues the venous blood is gathered up in channels, which are not sufficiently defined to be called veins. It is collected in a ventral venous sinus, and passes into the gills. Thence purified by exposure on the water-washed surfaces, it returns by six vessels on each side to the peri- cardium. From this it enters the heart by six large and several smaller apertures, which admit of entrance but not of exit. The blood contains amoeboid cells, and the fluid 01- plasma includes a respiratory pigment, hsemocyanin (bluish when oxidised, colourless when deoxidised), and a lipochrome pigment, called tetronerythrin. Both of these are common in other Crustaceans. Respiratory system. — Twenty gills — vascular outgrowths of the body-wall — lie on each side of the thorax, sheltered by the flaps of the shield. A current of water from behind forwards is kept up by the activity of the baling portion, or scaphognathite, of the second maxilla. Venous blood enters the gills from the ventral sinus, and purified blood leaves them by the six channels leading to the pericardium. Observed superficially, the gills look somewhat like feathers with plump barbs, but their structure is much more complex. The most important fact is that they present a large surface to the purifying water, while both the stem and the filaments which spring from it contain an outer canal continuous with the venous sinus, and an inner canal communicating with the channels which lead back to the pericardium and heart. Three sets of gills are distinguishable. To the basal joints of the six appendages from the second maxillipede to the fourth large limb inclusive, the podobranchs are attached. They come off with the appendages when these are pulled carefully away, and each of them bears, in addition to the feathery portion, a simple lamina or epipodite. The membranes between the basal joints of the appendages and the body, from the second maxillipede to the fourth large limb inclusive, hear a second set, the arthrobranclis , which have no epipodites. In CRA YFTSII. 259 connection with the second maxillipede there is a single arthrobranch ; in connection with each of the five following appendages there are two ; so that there are eleven arthrobranchs altogether. There remain three pleurobranchs, one on the epimeron of the fifth large limb, and two others quite rudimentary on the two preceding segments. The bases of the podobranchs bear long setae. In Nephrops, the podobranchs are represented by a small rudiment on the second maxillipede, and by five well-developed gills on the next five appendages ; there are eleven arthrobranchs, the most anterior being small ; and there are four large pleurobranchs. Excretory system. — A kidney or “ green gland ” lies behind the base of each antenna, and its opening is marked by a conspicuous knob on the basal joint of that appendage. Each kidney consists of a dorsal sac communicating with the exterior, and of a ventral coiled tube which forms the proper renal organ. The latter is supplied with blood from the antennary and abdominal arteries, and forms as waste products uric acid and greenish guanin. Each kidney may^ be regarded as homologous with a nephridium. The crayfish has also, near the gills, small branchial glands which excrete carcinuric acid from the blood, and also help in phago- cytosis, that important process in which wandering amoeboid cells resist infection and help to repair injuries (cf. possible function of thymus in Fishes). Reproductive organs. The male crayfish is distin- guished from the female by his slightly slimmer build, and by the peculiar modi- fication of the first two pairs of abdominal appendages. In both sexes the gonads are three-lobed, and communicate with the exterior by paired ducts. The testes consist of two anterior lobes lying beneath and K ig. 108. — Male reproduc ti ve orga n s of crayfish. — After Huxley. t., Testes; vd. , vas deferens ; vd'., open- ing of vas deferens on last walking leg. 260 CRUSTACEA. in front of the heart, and of a median lobe extending back- wards. Each lobe consists of many tubules, within which the spermatozoa develop. From the junction of each of the anterior lobes with the median lobe, a genital duct or vas deferens is given off. This has a long coiled course, is in part glandular, and ends in a short muscular portion opening on the last thoracic limb. The spermatozoa are at first disc-like cells, they give off on all sides long pointed processes like those of a Heliozoon, and remain very sluggish. The seminal fluid is milky in appearance, and becomes thicker in its passage through the genital ducts. Fig. iog. — Female reproductive organs of crayfish. — After Suckow. 07’., O varies ; ov’ ., fused posterior part ; od., oviduct ; vu., female aperture on the second walking leg. It is possible that the genital ducts represent modified nephridia, and that the cavities of the gonads are coelomic. The ovaries are like the testes, but more compact. The eggs are liberated into the cavity of the organ, and pass out by short thick oviducts opening on the second pair of walking legs. As they are laid they seem to be coated with the secretion of the cement glands of the abdomen, and the mother keeps her tail bent till the eggs are glued to the small swimmerets. Before this, however, sexual union has occurred. The male seizes the female with his great claws, throws her on CRA YFISH. 261 her back, and deposits the seminal fluid on the ventral surface of the abdomen. The fluid flows down the canal formed by his first abdominal appendages, and these seem to be kept clear by the movements of the next pair, which are also modified. On the abdomen of the female the I'nj. no.— Section through the egg of Aslacus after the com- pletion of segmentation.— After Reichenbach. s/., Stalk of the egg ; ch., chorion envelope ; bl., peripheral blastoderm within which are the yolk pyramids (dark). agglutinated spermatozoa doubtless remain until the eggs are laid, when fertilisation in the strict sense is achieved. The Development has been very fully worked out, and is of interest in being direct, without the metamorphosis so common among the Arthropoda. The spherical ovum is surrounded by a cuticular vitelline membrane, and contains a considerable quantity of yolk. After ferti- lisation the segmentation nucleus divides in the usual way into two, four, eight, and so on, but this nuclear div ision is not followed by division of the 262 CRUSTACEA. plasma. Eventually the nuclei, each surrounded by a small amount of protoplasm, approach the surface of the egg and arrange themselves regularly round it. The peripheral protoplasm then segments round these nuclei, and thus we have a central core of unsegmented yolk enveloped by a peripheral ring of rapidly dividing cells. In the central yolk, free nuclei are frequently found ; these are the so-called yolk nuclei. Such a type of segmentation is called peripheral or centro- lecithal, and is very characteristic of Arthropod eggs. Over a particular region of the segmented egg, known as the 1 ‘ ventral plate,” the cells begin to thicken ; at this region an invagination occurs, P*IG. iii. — Longitudinal section of later embryo of A stacus. — After Reichenbach. Ec. , Ectoderm; ///., mesoderm cells; c.g., cerebral ganglia; st. , stomodaeum ; A., anus; 7'., telsQn; g., ventral ganglia; s.s., sternal sinus ; pd. , proctodaeum ; h., heart ; mg. , mid-gut ; yolk pyramids dark. which represents the gastrula. At the anterior lip of the blastopore the mesoderm appears, being many-celled from the first. Soon the blasto- pore closes ; the cavity of the gastrula thus becomes a closed sac — the future mid-gut. The cells of this archenteron take up the core of yolk into themselves in a way which early suggests their future digestive function. On the surface of the egg there have already appeared ectodermic thickenings, — the so-called eye-folds, — rudiments of the appendages, and of the thoracic and abdominal regions. In the later stages invaginations of the ectoderm form the fore- and hind-gut, which grow inward from opposite ends to meet the endoder- Fig. xi2. — Embryo of crayfish, flattened out, with removal of yolk (greatly magnified). — After Reichenbach. Note rudiments of eyes and appendages, and in the middle line the nervous system. 264 CRUSTACEA. mic mid-gut. The ear-sac and green gland, and the greater part of the gills, have also an ectodermic origin. From the mid-gut the digestive gland is budded out. The heart, the blood vessels, blood, and muscles are due to the mesoderm. As usual, the nervous system arises from an ectodermic thickening. The eye arises partly from the optic ganglia of the “ brain,” parti v from the “eye-folds,” and partly from the epidermis. When the young crayfishes are hatched from the egg-shells, they still cling to these, and thereby to the swimmerets of the mother. In most respects they are miniature adults, but the cephalothorax is convex and relatively large, the rostrum is bent down between the eyes, the tips of the claws are incurved and serve for firm attachment, and there are other slight differences. The noteworthy fact is that the development is com- pleted within the egg-case, and that it is continuous without metamor- phosis. The shortened life history of the crayfish is interesting in relation to its fresh-water habitat, where the risks of being swept away by currents are obviously great ; but it must also be remem- bered that the tendency to abbreviate development is a general one. There is some maternal care in the crayfish, for the young are said sometimes to return to the mother after a short exploration on their own account. Systematic Survey of the Class Crustacea. ( 1 ) Entomostiaca, lower forms. They are usually small and simple. The Humber of segments and ap- pendages is very variable. The larva is generally hatched as a simple unsegmented Nauplius. There is no gastric mill. (2) Malacostraca, higher forms. They are usually larger and more complex. The head consists of 5, the thorax of 8, the abdomen of 6 (7 in Leptostraca) segments. The larva is usually higher than a Nauplius. There is a gastric mill. {A pus, Branclii- pus, and Artemia (brine-shrimps), Daplmia , Moina , Polypkemus. 2. Ostracoda, Cypris , Cypridina. 3. Copepoda, Cyclops, Argulus, many parasites. 4. Cirripedia, acorn - shells and barnacles, e.g. Balanus and Lepas. Leptostraca, Arthrostraca, e.g. Nebalia. rAmphipods (sand- hoppers, etc.), j Isopods (wood- lice, etc.). t Curna. Si/uilla. Thoracostraca, - Mysis. Shrimp, lobster, l crayfish, crab. First Sub-Class. Entomostraca. Order 1. Phyllopoda. — In these at least four pairs of swimming feet bear respiratory plates. The body is generally well segmented, ENTOMOSTRA CA. 265 and is protected by a shield-like or bivalve shell. The mandibles are without palps, and the maxillae are rudimentary. (a) Branchiopoda. The body has numerous segments and (10-20 or more) appendages with respiratory plates. The shell is rarely absent, usually shield-like or bivalved. The heart is a long dorsal vessel with numerous openings. The eggs are able to survive prolonged desiccation in the mud. Branchipus , a beautifully coloured fresh-water form, with hardly any shell. Artemia. Brine-shrimps. Periodically parthenogenetic. By gradually changing the salinity of the water, Schmankewitsch was able, in the course of several generations, to modify A. salina into A. milhausenii, and vice versd. A rtemia fertilis is one of the four animals known to occur in the dense waters of Salt Lake. Apus , a fresh-water form with a large dorsal- shield. Periodically parthenogenetic. One species heimaphrodite. Of these, Apus is certainly the most interesting. It is over an inch in length, and therefore a giant among Entomostraca. It has an almost world-wide distribu- * IG' 1 13-; -Horsal surface of Apus tion. “ It possesses peculi- cancriformis.-Yrom Bronn s arities of organisation which lerieici. mark it out as an archaic In the anterior region are the two com- 111 . *. j* pound eyes, and behind them the Oim, probably standing simple unpaired eye. The whip-like nearer to the extinct an- outgrowths of the first thoracic an- cestors of the Crustacea pendage project laterally, than almost any other living member of the group.” The appendages are very numerous and mostly leaf-like. They may be regarded as representing a primitive type of Crustacean limb. Professor Ray Lankester enumerates them as follows : — i I. Antenna. Pre-oral. ' 2. Second antenna. (This is sometimes absent, and ( apparently always in certain species. ) j 3. Mandible. Oral. 4. Maxilla. I 5. Maxillipede. 266 CRUSTACEA. ( 6. First thoracic foot (leg-like). Thoracic | 7-16. Other ten thoracic feet (swimmers). (Pregenital). 1 The 16th in the female carries an egg-sac or brood- v. chamber. There are eleven thoracic rings on the body. Abdominal / 17-68. Fifty-two abdominal feet, to which there corre- ( Post-genital). spond only seventeen rings on the body. The large dorsal shield is not attached to the segments behind the one bearing the maxillipedes. Many of the thin limbs doubtless function as gills. The genital apertures are on the sixteenth appendages. The anus is on the last segment of the body. There is a pair of ventral ganglia to each pair of limbs ; the ventral nerve-cords are widely apart ; and the cephalic ganglion is remarkably isolated. (b) Cladocera. Small laterally compressed “ water-fleas,” with few and somewhat indistinct segments. The shell is usually bivalved, and the head often projects freely from it. The second antenna are large, two-branched, swimming appendages, and there are 4-6 pairs of other swimming organs. The heart is a little sac with one pair of openings. An excretory organ (the shell or maxillary gland) opens in the region of the second maxilla. It is the Entomostracan equivalent of the antennary green gland of Malacostraca. The males are usually smaller and much rarer than the females. The latter have a brood-chamber between the shell and the back. Within this many broods are hatched throughout the summer. Periodic parthenogenesis (of the “summer ova”) is very common. “Winter eggs,” which require fertilisation, are set adrift in a part of the shell modified to form a protective cradle or ephippium. Daphnia , Moina , S/da, Polyphemus , Lep/odora , and many other “ water-fleas,” are extraordinarily abundant in fresh water, and form part of the food of many fishes. A few occur in brackish and salt water. Older 2. Ostracoda. — Small Crustaceans, usually laterally compressed, with an indistinctly segmented or unsegmented body, rudimentary abdomen, and bivalve shell. There are only seven pairs of appendages. Examples. — Cypris (fresh water), Cypridina (marine). Older 3. Copepoda.— Elongated Crustaceans, usually with distinct seg- ments. There is no dorsal shell. There are five pairs of biramose thoracic appendages, but the last may be rudimentary or absent. The abdomen is without limbs, and of its five segments the first two are sometimes united. The females carry the eggs in external ovisacs. Many are ecto-parasitic, especially on fishes (“ fish-lice ”), and are often very degenerate. The free living Copepods form an important part of the food-supply of fishes. Cyclops, free and exceedingly prolific in fresh water. Cetocliilus , free and abundant in the sea. Sapphirina , a broad flat marine form, about a quarter of an inch long, occasionally parasitic. The male is remarkable for its brilliant “phosphorescent” ENTOAIOSTRACA. 267 colour. In Chondracanthus, as in many other cases, the para- sitic females earn- the pigmy males attached to their body. Caligus, a very common genus of “ fish-lice.” Lertuza, Penella , etc. The adult females are parasitic, and almost worm-like. The males and the young are free. That the males are often free and not degenerate, while their mates are parasitic and retrogressive, may be understood by con- sidering— (1) the greater vigour and activity associated with maleness ; (2) the fact that parasitism affords safety and abundance of nutrition to the females during the reproductive period. Older 4. Cirripedia. — Barnacles and acorn-shells, and some allied degenerate parasites. Marine Crustaceans, which in adult life are fixed head down- wards. The body is indistinctly segmented, and is enveloped in a fold of skin, usually with calcareous plates. The anterior antennae are involved in the attachment ; the posterior pair are rudimentary. The oral appendages are small, and in part atrophied. In most there are six (or less frequently four) pairs of two-branched thoracic feet, which sweep food par- ticles into the depressed mouth. The abdomen is rudimentary. There is no heart. The sexes are usually combined, but dimorphic unisexual forms also occur. The hermaphrodite individuals occasionally carry pigmy or “ com piemen tal ” males. The spermatozoa are mobile, which is unusual among Crustacea. Lepas , the ship-barnacle, is as an adult attached to floating logs and ship-bottoms. The anterior end by which the animal fixes itself is drawn out into a long flexible stalk, containing a cement gland, the ovaries, etc., and involving in its formation the first pair of antenna; and the front lobe of the head. The second antennae are lost in larval life. The mouth region bears a pair of small mandibles and two pairs of small maxillae, — the last pair united into a lower lip. The thorax has six pairs of two-branched appendages, and from the end of the rudi- mentary abdomen a long penis projects. At the base of this lies the anus. Around the body there is a fold of skin, and from this arise five calcareous plates, an unpaired dorsal carina , two scuta right and left anteriorly, two terga at the free posterior end. The nervous system consists of a brain, an cesophageal ring, and a ventral chain of five or more ganglia. There is a fused pair of rudimentary eyes. No special circulatory or respiratory organs are known. Two excretory (?) tubes lead from (coelomic) cavities to the base of the second maxilla;, and are probably comparable with shell-glands and with nephridia. There is a complete food canal and a large digestive gland. Beside the latter lie the branched testes, whose vasa deferentia unite in an ejaculatory duct in the penis. From the much -branched ovaries in the stalk, the ovi- ducts pass to the first thoracic legs, where they open into a cement- making sac, opening to the exterior. The eggs are found in flat cakes between the external fold of skin and the body. The life history is most interesting. Nauplius larva; escape from the 268 CRUSTACEA. egg-eases, and, after moulting several times, become like little Cyprid water-fleas. The first pair of appendages become suctorial, and, after a period of free-swimming, the young barnacle settles down on some floating object, mooring itself by means of the antennary suckers, and becoming firmly glued by the secretion of the cement glands. During the settling and the associated metamorphosis, the young barnacle fasts, living on a store of fat previously accumulated. Many important changes occur, the valved shell is developed, and the adult form is gradually assumed. While the early naturalists, such as Gerard (1597), regarded the barnacle as somehow connected with the barnacle-goose, and zoologists, before J. Vaughan Thompson’s researches (1829), were satisfied with calling Cirripedes divergent Molluscs, we now know Fig. 114. — Acorn-shell (Balanus tintinnabulum). — After Darwin. /., Tergum ; s., scutum ; d., opening of oviduct, the aperture is not distinct ; f. , mantle cavity ; x. , depressor muscle of tergum ; g. , depressor muscle of scutum; h., oviduct; r., outer shell in section; a., adductor muscle of scuta; cr., thoracic legs; 1, first plate of outer shell ; />., position of viscera. clearly that they are somewhat degenerate Ciustaceans. We do not know, however, by what constitutional vice, by what fatigue after the exertions of adolescence, they are forced to settle down to sedentary life. The food consists of small animals, which are swept to the mouth by the waving of the curled legs. Growth is somewhat rapid, but the usual skin casting is much restricted, except in one genus. Neither the valves, nor the uniting membranes, nor the envelope of the stalk, are moulted, though disintegrated portions may be removed in flakes and renewed by fresh formations. In the allied genus Scalpellum, some are like Lepas, hermaphrodites, without complementary males {Sc. bala- noides) ; others are hermaphrodite, with complementary males (Nr. villomtn) ; and others are unisexual, but the males are minute and parasitic {Sc. regittm). ENTOMOSTRACA. 269 Balanus. the acorn -shell, encrusts the rocks in meat numbers between high and low water marks. It may be described, in Huxley’s graphic words, as a crustacean fixed by its head, and kicking the food into its mouth with its legs. The body is surrounded, as in Lepas, by a fold of skin, which forms a rampart of six or more calcar- eous plates, and a fourfold lid, consisting of two scuta and two terga. When covered by the tide, the animal protrudes and retracts between the valves of the shell six pairs of curl-like thoracic legs. The structure of the acom-shell is in the main like that of the barnacle, but there is no stalk. The life history also is similar. A Nauplius is hatched. It has the usual three pairs of legs, an unpaired eye, and a delicate dorsal shield. It moults several times, grows larger, and acquires a firmer shield, a longer spined tail, and stronger legs. Then it passes into a Cypris stage, with two side eyes, six pairs of swimming legs, a bivalve shell, and other organs. As it exerts itself much but does not feed, it is not unnatural that it should sink down as if in fatigue. It fixes itself by its head and antennae, and is glued by the secretion of the cement gland. Some of the structures, e.g. the bivalve shell, are lost ; new structures appear, e.g. the characteristic Cirriped legs and the shell. Throughout this period, which Darwin called the “pupa stage,” there is external quiescence, and the young creature continues to fast. The skin of the pupa moults off ; the adult structures Fig. i 15. — Development of Sacculina. — After Delage. (Not drawn to scale. ) A, Free-swimming Nauplius, with three pairs of appendages ; B, pupa stage ; C, adult protruding from the abdomen of a crab. and habits are gradually assumed. At frequent periods of continued 270 CRUSTACEA. growth the lining of the shell and the cuticle of the legs are shed. In spring these glassy cast coats are exceedingly common in the sea. Acorn-shells feed on small marine animals. They fix themselves not to rocks only, but also to shells, floating obiects, and even to whales and other animals. On the ventral surface of the abdomen of crabs, Sacculina, the most degenerate of all parasites, is often found. Its complete history has been beautifully worked out by Professor Delage. It is in shape an ovoid sac, and is attached about the middle of a segment. On the lower surface of the sac there is a cloacal aperture, opening into a large brood - chamber, usually distended with eggs contained in chitinous tubes. The brood-chamber surrounds the central “visceral mass,” consisting of a nerve ganglion, a cement gland which secretes the egg- cases, and the hermaphrodite reproductive organs ; of digestive or vascular systems there is no trace. The parasite is attached by a peduncle, dividing up, within the body of the crab, into numerous “ roots,” which have been compared to the placenta of a mammalian feetus. The ‘ ‘ roots ” ramify within the body of the crab, and by them the Sacculina obtains nutrition and gets rid of its waste products ; it is therefore practically, even at this stage, an endoparasite. The larvae leave the brood-chamber as Nauplii ; they moult rapidly and become Cyprid larvae. These fix themselves by their antennae to young crabs, at the uncalcified membrane surrounding the base of the large bristles of the back or appendages. The thorax and abdomen are cast off entirely ; the structures within the head region contract ; eyes, tendons, pigment, the remaining yolk and the carapace, are all lost ; and a little sac remains, which passes into the interior of the crab. Eventually it reaches the abdomen, and, as it approaches maturity, the integuments of the crab are dissolved beneath it, and the sac-like body protrudes ; essentially, however, Sacculina is always endoparasitic. It appears to live for three years, during which time the growth of its host is arrested, and no moult occurs. Second Sub-Class. Malacostraca. Legion 1 . Leptostraca. Marine Crustaceans of great systematic interest, because they retain in many ways the simplicity of ancestral forms, and link Malacostraca to Phyllopods. The most important genus is Nebalia. A bivalve shell covers the whole of the lank body, except the last four abdominal segments ; the head is free from the thorax ; the eight segments of the thorax are fiee from one another, and the plate-like appendages resemble those of Phyllopods ; the abdomen has seven segments and a telson with two forks ; the elongated heart extends into the abdomen, and has seven 'pairs of lateral apertures or ostia. Nebalia and its congeners are probably related to certain ancient fossil forms from Palaeozoic strata — Hytnenocaris, Ceratiocaris, etc. Legion 2. Arthrostraca. (Edriophthalmata, sessile-eyed.) There is no shell-fold or shield, except in the order Anisopoda. The first thoracic segment (rarely with the addition of the second) is fused MAI.ACOSTRACA. 271 10 the head, the corresponding appendages serve as maxillipedes, the other thoracic segments (seven or six) are free. The eyes are sessile. The heart is elongated. Order 1. Anisopoda.— The fusion of the first two thoracic segments to the head, the presence of a cephalothoracic shield, and other divergent features, distinguish Tanais, Apseudes, etc., from the Isopoda. Order 2. Isopoda.- — The body is flattened from above downwards. The first thoracic segment is fused to the head, while the other six or seven are free, and there is no cephalothoracic shield. The abdomen is usually short, and its appendages, usually over- lapped by the first pair, are plate-like, and function in part as respiratory organs. The “ wood-lice " {Outsells, Porcellio ) are familiar animals w hich lurk in damp places under stones and bark, and devour vegetable refuse. Some related forms {e.g. Arma- dillo), which roll themselves up, are called “pill-bugs.” In the terrestrial forms there is obviously a departure from the ordinarily aquatic habit of Crustaceans, and the exopodites of some of the abdominal appendages have tubular air-passages. Asellus is a very common form, living in both fresh and salt water. Idotea is not uncommon among the shore rocks. The “gribble” ( Limnoria lignorum ) is a destructive marine Isopod which eats into wood. Among the marine Cymothoidm which are often parasitic on fishes, some, e.g. Cymothoe, are remarkable in their sexual condition, for they are hermaphrodites, in which the male organs mature and become functional when the oviducts are still closed, while at a later period in life the male organs are lost, and the animals become functionally female. The Bopyridre infest the gill-chambers of other Crustaceans, e.g. prawns. The pigmy males are usually carried about by their mates. Among the parasitic Cryptoniscidre we again find herma- phrodites with associated pigmy males. In not a few cases they seriously affect the reproductive organs of their male hosts. Order 3. Amphipoda. — -The body is laterally compressed. In most it is only the first thoracic segment which is fused to the head, in the “ no - body - crabs ” ( Caprellidce), and “ whale - lice ” {Cyamidce), two segments are involved. The thoracic limbs bear respiratory appendages. Of the six pairs of legs which the abdomen usually bears, the anterior three are usually more strongly developed as swimmers, while the posterior three — directed backwards — are used in jumping. Gammams pulex is very common in fresh water. Other species occur on the seashore. There also the “ Beach- fleas ” ( Talitrus and Orchestia) are exceedingly abundant. 272 CRUSTACEA. On solid ground they move on their sides in a strange fashion, but they swim very swiftly. Hyper i a, Phronima, and many marine Amphipods, have a habit of living as commensals with other animals. Caprella, a common marine gymnast on Hydroids, etc., has the trunk of the body reduced to the quaintest possible minimum. (Fig. 1x6.) Legion 3. Thoracostraca. (Podophthalmata, with stalked eyes.) Several or all of the thoracic segments are fused to the head, and there is a cephalothoracic shield overlapping the gills. The two eyes are stalked, except in Cumacea. Order 1. Cumacea. — The cephalothoracic shield is small, and four or five thoracic segments are left uncovered and free. The eyes are sessile, and adjacent or fused. There are two pairs of maxillipedes. The females have no abdominal appendages except on the last segment. The genera are marine, e.g. Cuina or Diastylis. Order 2. Stomatopoda. — The shield is still small, and does not cover the three posterior thoracic segments. The body is somewhat flattened, the abdomen is very strong. Five anterior thoracic appendages are directed towards the mouth, and serve to catch food, and to clamber. The five anterior abdominal legs carry feathery gills, the sixth pair forming swimming - paddles. The elongated heart extends into the abdomen, which also contains the reproductive organs. The genera are marine, e.g. Sqnilla. Fig. 116.— An Amphipod (< Caprella linearis). The two anterior thoracic segments are fused to the head ; the abdomen is greatly reduced and with- out appendages ; the fourth and fifth thoracic segments bear only respiratory plates. Order 3. Schizopoda. — A delicate shield covers the whole of the thorax, but there is still some freedom as to one or more of the posterior thoracic segments. The eight thoracic appendages are uni- formly biramose, but the first two may serve as maxillipedes. The abdominal appendages of the male are strongly developed ; those of the female are weak, except the last, which in both sexes form paddles. They are marine forms, e.g. Mysis (without gills on the thoracic legs), Lophogaster , and Eupkausia (with gills on the thoracic legs). The last-named starts in life as a Nauplius. As an adult it has luminous organs on the eye-stalks, thoracic legs, and abdominal segments, MALACOSTRACA. 273 Order 4. Decapoda. — The shield is large and firm, and is fixed to the dorsal surface of all the thoracic segments. Of the thoracic1 appendages, the first three pairs are maxillipedes, the five other pairs are jointed walking legs (whence the term Decapod). Sub-order 1. Macrura. — Abdomen long. Homarus (lobster) ; Nephrops (Norway lobster, sea crayfish) ; Astacus (fresh- water crayfish) ; Palimtrus (rock lobster), whose larva was long known as the glass-crab ( Phyllosoma ) ; Penceits , a shrimp which passes through Nauplius, Zorea, and Mysis stages ; Lucifer and Sergestes are also hatched at a stage antecedent to the Zotea ; Crangon vulgaris (the British shrimp) ; Palcemon , Panda/us, Hippolyte (prawns); Galathea (with the abdomen bent inwards) ; Pagtirus, Eupagurus (hermit crabs) ; Birgits lalro (the terrestrial robber or palm crab), in which the upper Fig. 117. — Schizopod [Mysis Jlexnosa), from side. />., Brood-pouch borne on posterior thoracic limbs ; 0., otocyst in tail. Note eight pairs of similar biramose thoracic feel. The last two thoracic segments are not covered by the shield. part of the gill-cavity is shut off to form a “ lung,” the walls having numerous vascular plaits. Sub-order 2. Brachyura. — Abdomen short, and bent under the thorax. It is narrow in the male, and does not usually bear more than two pairs of appendages ; it is broader in the female, and bears four paired appendages. The ventral ganglia have fused into an oval mass. Cancer (edible crab) ; Carcinus mcenas (shore crab) ; Poriitnus (swimming crab) ; Dromia (often covered by a sponge) ; Pinnotheres (living inside bivalves) ; Telphusa (a fresh-water crab) ; Gecarcinus (land-crabs, only visiting the sea at the breeding season). History. — Fossil Crustaceans are found in Cambrian strata, but the highest forms (Decapoda) were not firmly established till the Tertiary period. Some of the genera, c.g. the Branchiopod Estheria, living from Devonian ages till now, are remarkably persistent and successful. How the class arose we do not know ; it is probable that types like Nebalia 1 8 274 CRUSTACEA. give us trustworthy hints as to the ancestors of the higher Crustaceans ; it is likely that the Phyllopods, e.g. A pus, bear a similar relation to the whole series ; the Copepods also retain some primitive characteristics ; but it is difficult, apart from mere guessing, to say anything definite as to the more remote ancestry. We naturally think of a segmented worm-type as a plausible starting- point for Crustaceans, and it is not difficult to understand how a Fig. 118. Nervous system of shore crab fleas,” it is difficult to ( Carcmus meenas).— After Bethe. state general character- and the sedentary life of barnacles and acorn-shells, we must still allow that great activity characterises the class. With this may be connected the brilliant colouring, the power of colour change, and the phosphorescence of many forms. Except in the case of a few primitive and degenerate forms, the Crustacea are all segmented. In this, in the presence of hollow jointed appendages, in the reduction of the coelom, and in their firm chitinous cuticle, the Crustacea resemble other Arthropods ; as special characteristics we development of cuticular chitin would tend to produce a flexibly jointed limb out of an unjointed parapodium ; how the mouth might be 11WW LUC tUO U 111 UUgllL UC shunted a little backwards, and two appendages and ganglia a little forwards ; and how division of labour would result in the differentiation of distinct regions. General Notes on Crustaceans. Of a class that in- cludes animals so diverse as crabs, lobsters, shrimps, “ beach - fleas,” “wood-lice,” barnacles, acorn-shells, and “ water- a short strand representing the abdominal rnnrised 1 ! C , l. ^ ^ C - 1. n 1 nnfanmilnp ' * ganglia of the crayfish. a1., antennules ; a2., antennae ; e eye. Admitting the parasit- ism of many Crustaceans, GENERAL NOTES ON CRUSTACEANS. 275 notice the two pairs of antennae, the presence of carbonate of lime in the cuticle, and the nature of the respiratory organs — these, with few exceptions, being adapted for breath- ing in water. While these characters remain constant throughout the group, there is an almost infinite variation in detail. In regard to the segmentation of the body, we notice that, apart from the general tendency to reduction which is so marked in many parasitic forms, the higher forms as compared with the lower show marked specialisa tion. In the primitive Phyllopods the body consists of a large but varying number of segments, remarkably uniform in structure. The higher Crustacea, on the other hand, are characterised by their relatively few but constant segments, which exhibit marked division of labour ; a comparison of Neba/ia, Schizopods and Decapods, a series which illustrates the development of the thorax, will make this plain. The same gradual process of specialisation is observable in the appendages. Typically consisting of a basal piece and two branches, the appendages, like the parapodia of Annelids, are primitively organs of locomotion ; in the Crustacea especially, swimming organs. In Phyllopods the great majority of the appendages remain permanently at this level. It is worth notice that in the Nauplius and in Ostracods and the free-swimming Copepods, the antenna; themselves are swimming organs. Just as, however, in the Annelid head the locomotor function of the parapodia becomes subordinated to the sensory one, so also in Crustacea the anterior appendages of tire head become specialised as sense organs. Again, the appendages in connection with the mouth become modified in connection with alimentation, and the further processes of specialisa- tion which differentiate the regions of the body are reflected in the appendages of these regions. A comparison of Nebalia, Schizopods and Decapods, will again make this plain. It is this specialisation of certain appendages to function as masticatory organs which especially characterises Arthropods as compared with Annelids. In the nervous system there is always a certain amount of fusion of ganglia — these never being so numerous as the segments — but the fusion is more marked in the more specialised forms. In the Crabs the ventral chain is repre- 276 CRUSTACEA. seated by a lobed ganglionic mass in the thorax, connected with a mere rudiment, which corresponds to the abdominal portion of the cord in the crayfish (Fig. 118). Sense organs are usually well developed, and are not confined to the head region ; thus many Schizopods have “ auditory ” organs in the tail (Fig. 1 1 7). The alimentary canal runs straight throughout the body ; it consists of fore-gut, mid-gut, and hind-gut. The fore-gut and hind-gut are anterior and posterior invaginations of ectoderm, and are always large, especially in Malacostraca ; in the Malacostraca the fore-gut is furnished with a gastric mill. The mid-gut or archenteron is always short, but has connected with it diverticula which form the so-called hepato-pancreas. In the Entomostraca there is usually only a single pair of out- growths; in Schizopods, Cumacea, and larval Decapods there are three pairs ; a process of rapid growth and branching converts these into the compact digestive gland of the adult Decapods. In connection with the posterior end of the mid-gut in Amphipods and some others, there are a pair of blind tubes functioning as excretory organs, and presenting an interesting similarity to the Malpighian tubes of insects, which, however, are in connection with the hind-gut. The body cavity is never large, being mainly filled up with muscles and organs, and, as in Arthropods in general, the true coelom is virtually absent. In the blood, hgemocyanin is the commonest pigment, but is not universal. Respira- tion is carried on in many different ways. In the simple forms it may be merely by the general surface, but in the majority of cases, certain portions of the limbs, or outgrowths of the limbs, constitute definite respiratory organs, often specialised to form gills. In the excretory system the numerous nephridia of Annelids are absent. The typical excretory organs of the Entomostraca are the “shell-glands” — paired coiled tubes opening on the second maxilla ; of the Malacostraca, the antennary glands exemplified by the green glands of the crayfish. The genital ducts are pro- bably modified nephridia, and the fact that they open on different segments in the two sexes, is regarded as evidence of the former existence of a series of nephridia like those of Annelids. The process of excretion in the Crustacea is not well understood ; it is possible that shell-making GENERA I. NOTES ON CRUSTACEANS. is an organised method of getting rid of some waste products. There are many peculiarities connected with reproduc- tion— thus parthenogenesis for prolonged periods is common among “water-fleas”; hermaphroditism is frequent, occur- ring, for. example, in barnacles, acorn-shells, etc., and it is often complicated by the simultaneous existence of “ pigmy ” complemental males. When separate the two sexes are often very diverse. The spermatozoa are usually exceptional in being very slightly motile. In both sexes some appendages are often modified for copulation or for carrying the eggs. Development. — - The ova of most Crustacea show considerable simil- arity to those of Astacus, and the segmentation is typically of the kind already described. But while this is the most typical case for Crust- acean, and, indeed, for Arthropod development, rl9; ^0£ea of common ^shore crab 1 ... ,1 ( Car cm us mamas). — After taxon. The It IS possible, Within the appendages are numbered; c., gills; limits of the class Crust- alimentary canal, acea, to trace out a com- plete series, in which the first term is a segmentation of the complete and equal type, like that of a worm, and the last the purely peripheral. In the same way, though gastrula- tion is usually much disguised, there are many modes, from an invagination of the simplest embolic type ( Lucifer ), and through the condition described for Astacus , to the forma- tion of endoderm by the ingrowth of a solid plug of cells (Arthrostraca, etc.). 27S CRUSTACEA. Compared with As/acus , however, the most important point we have to notice is the frequent occurrence of a very striking metamorphosis in the life history. In other words, the larva hatched from the egg is rarely like the parent, and only acquires the adult characters after a series of profound changes. In some cases ( Nebalia , My sis) a metamorphosis takes place within the egg-cases, and in the few forms in which development seems to be direct, slight traces of meta- morphosis are found. Almost all the lower Crustaceans and the higher forms Euphausia and Penceus are hatched in a Nauplius stage. In the remaining cases the Nauplius stage is indicated within the egg by the moulting of a larval cuticle (as in A stanis). The Nauplius is characterised by a typically rounded body, and by the presence of three pairs of append- ages, which are the only obvious indications of segmenta- tion. The first pair of appendages are unbranched and bear larval sense organs, the next two are biramose swim- ming organs. There is an unpaired median eye, but no heart, and frequently no hind-gut. The three pairs of appendages become the first and second pairs of antennse and the mandibles of the adult. The head region of the Nauplius becomes the head region of the adult ; the posterior region also persists ; the new growth of segments and append- ages takes place (with numerous moultings) in the region between these. The second important form of larva is the Zotea, which has all the appendages on to the last maxillipedes inclusive, an unsegmented abdomen, and two lateral compound eyes, in addition to the unpaired one of the Nauplius stage. Most Decapoda are hatched in the Zotea stage. (а) The crayfish ( As/acus ) is hatched almost as a miniature adult. The development is therefore very direct in this case. (б) The lobster ( Hornams ) is hatched in a Alysis stage, in which the thoracic limbs are two-branched and used for swimming. After some moults it acquires adult characters. (7) Crabs are hatched in the Zocea form, and pass with moults through a Megalopa stage, in which they resemble certain Hermit crabs. The abdomen is subsequently tucked in under the thorax. (A) Penceus (a kind of shrimp) is hatched as a Nauplius, becomes a Zotva, then a My sis , then an adult. Its relative Lucifer starts as a Me/a-Nauplius with rudiments of three more appendages than the Nauplius. Another related form, Sergcs/es, is hatched GENERAL NOTES ON CRUSTACEANS. 279 as a Protozoa- a. with a cephalothoracic shield and an unseg- mented abdomen. Thus there are two grades between Nauplius and Zotea. Three facts must be borne in mind in thinking over the life histories of crayfish, lobster, crab, and Tenons : ( 1 ) There is a general tendency to abbreviate development, and this is of more importance when meta- morphosis is expensive and full of risks ; (2) there is no doubt that larvae exhibit characters which are related to their own life rather than to that of the adult ; (3) it is a general truth, that in its individual development the organism recapitulates to some extent the evolution of the race, that ontogeny tends to recapitulate phylogeny. But while there can be no doubt that the metamorphosis of these Crustaceans is to some extent interpretable as a recapitulation of the racial history, — for there were unsegmented animals before segmented forms arose, and the Zocea stage is antecedent to the Mysis , etc., — yet it does not follow that ancestral Crustaceans were like Nauplii. On the contrary, the Nauplius must be regarded as a larval reversion to a type much simpler than the ancestral Crustacean. Moreover, the idea of recapitulation offers a philosophical rather than a material explanation of the facts, and holds good only in a very general way. Bionomics. — Most Crustaceans are carnivorous and pre- datory ; others feed on dead creatures and organic debris in the water ; a minority depend upon plants. Parasitism occurs in over 700 species, in various degrees, and, of course, with varied results. Most of the parasites keep to the outside of the host ( e.g . Fish-lice), and suck nourishment by their mouths ; the Rhizocephala {e.g. Sacculina) send ramifying absorptive roots through the body of the host. Sometimes the parasitism is temporary ( Argulus ) ) sometimes only the females are parasitic {e.g. in Lerncea). The parasites tend to lose appendages, segmen- tation, sense organs, etc., but the reproductive organs become more fertile. The hosts, e.g. crabs, infested by Rhizocephala, are sometimes materially affected, and even rendered incapable of reproducing. Some Crustaceans live not as parasites, but as commensals with other animals, doing them no harm, though sharing their food. Thus there is a constant partnership between some hermit crabs and sea-anemones. The hermit crab is concealed and protected by the sea-anemone ; the latter is carried about by the Crustacean, and gets fragments of food. Masking is also common, especially among crabs. Some will cut the tunic off a sea-squirt and throw it over their own shoulders. Many attain a mask more passively, for they are CRUSTACEA. 2S0 covered with hydroids and sponges, which settle on the shell. There is no doubt, however, that some actively mask themselves, for besides those known to use the Tunicate cloak, others have been seen planting sea-weeds on their backs. The protective advantage of masking both in offence and defence is very obvious. The intelligence of crabs and some of the higher Crus- taceans is well developed. Maternal care is frequent. Fighting is very common. The loss of limbs is readily repaired. Deep-sea Crustaceans are very abundant, and often remarkable “for their colossal size, their bizarre forms, and brilliant red colourings ” ; some are blind, others are brilliantly phosphorescent. Yet more abundant are the pelagic Crustaceans (especially Entomostraca and Schizo- pods) ; they are often transparent except the eyes, often brightly coloured or phosphorescent. Many Crustaceans live on the shore, and play a notable part in the struggle for existence which is so keen in that densely-crowded region. The lower Crustaceans are abundantly represented in fresh water, in pools, streams, and lakes. A few, such as wood-lice and land-crabs, are terrestrial, and some blind forms occur in caves. CHAPTER XIV. PERIPATUS, MYRIOPODS, AND INSECTS. Arthropoda. Sub-division Tracheata Antf.nnata. Classes Prototracheata. — Peripatus. Myriopoda. — Centipedes and Millipedes. Insecta. — Insects. These three classes form a series of which winged insects are the climax. The type Peripatus is archaic, and links the series to the Annelids ; the Myriopods lead on to the primitive wingless insects. All breathe by tracheae — tubes which carry air to the organs of the body — and all have antennae ; hence the title Tracheata Antennata. First Class of Tracheata Antennata. — Prototracheata. Generai. Characters. The body is worm-like in form , soft skinned, and without external segmentation. The appendages are — a pair of prominent pre-oral antenna, a pair of jaws in the mouth, a pair of slime-secreting oral papilla, ivhich development shows to be true appendages, numerous pairs of short, imperfectly-jointed legs, each with two claws, and a pair of anal papilla, ivhich are rudi- mentary appendages. The legs contain peculiar ( crural ) glands. Respiration is effected by numerous trachea with openings somewhat scattered on the surface of the body. The heart is simply an elongated dorsal vessel with valvular openings. There is a series of excretory tubes or tiephridia. The halves of the ventral nerve-cord are widely separate. 1 82 PE RIP A TUS, MY RIO PODS, AND INSECTS. The species of Peripatus, which some refer to four genera, are numerous and widely distributed. In its possession of tracheae and nephridia it is an interesting connecting link ; in many ways it seems to be an oldfashiotied survivor of an archaic stock. The species of Peripatus are very beautiful animals. Mr. Sedgwick says : “ The exquisite sensitiveness and continu- ally changing form of the antennae, the well-rounded plump body, the eyes set like small diamonds on the side of the head, the delicate feet, and, above all, the rich colouring and velvety texture of the skin, all combine to give these animals an aspect of quite exceptional beauty.” They are shy and nocturnal, hiding under stones and among rotting wood, feeding on insects and the like, which they catch by the ejection of slime from the oral papillae. To their shy habits their persistence is possibly in part due. They are able to move quickly, somewhat after the fashion of Millipedes, especially like Scolopendrella. Young forms roll up when touched, and have been seen to climb up vertical glass plates. About a score of species are known, from S. Africa, Australia, New Zealand, West Indies, S. o' America, etc., widely distributed like some other Peripatus — archaic Tpes (cf. Dipnoi). After Balfour As the different species have similar habits, and live in very similar conditions, the differences Note antennas and between them probably illustrate purely constitu- simple feet. tional variations. A more Detailed Account of Peripatus. Form. — The body suggests an Annelid or a caterpillar, but, apart from the appendages, there is no external segmentation. Over the soft skin are numerous minute warts with small bristles. The mouth is ventral and anterior ; the anus terminal and posterior. Appendages. — The first are the large, ringed antenna; ; then follow the sickle-like jaws in the mouth cavity ; a little further back are two oral papilla; from which slime is exuded. Then there are the 14-42 stump-like legs, each with two terminal chitinous claws. In the young P. capensis the leg is said to be five-jointed, but in the adults there is PRO TO TP. 4 CHE A TA. 283 no trace of this. In respect to its legs, therefore, Peripatus is lmrdlv an Arthropod. Skin. — The chitinous cuticle, ordinarily thick in Arthropods, is delicate. The epidermis is a single layer of cells. Muscular system. — Externally there is a layer of circular muscles ; within this lies a double layer of diagonal fibres ; internally there are strong longitudinal bundles. Finally, in connection with this internal layer, there are fibres which divide the apparent body cavity into a median and two lateral compartments. The median includes heart, gut, slime glands, reproductive organs ; the laterals include the nerve- cords and salivary glands ; the legs contain nephridia and coxal or crural glands. Striped, rapidly-contracting muscles are characteristic of Arthropods, but in Peripatus the muscles are unstriped, excepting those which work the jaws and are perhaps the most active. The true coelom is represented in the embryo by the cavities of the mesoderm segments, which give origin to the muscular system. Nervous system. — The dorsal brain is connected by an oeso- phageal ring with the two widely separate latero-ventral nen'e-cords. These are connected transversely by numerous commissures, are slightly swollen opposite each pair of legs, to which they give off nerves, and are united posteriorly over the anus. There are only hints of ganglia, but there is a continuous layer of ganglionic cells. The brain is very homogeneous, simpler than that of most Insects. From the brain nerves pass to the antennae, etc., and two viscerals or sympathetics, soon uniting, innervate the anterior part of the gut. Sense organs are represented by two simple eyes on the top of the head. These are most like the eyes of some marine Annelids. Behind each there lies a special optic lobe connected with the brain, but the eye itself arises as a dimple in the skin. Alimentary canal. — Round about the mouth papillae seem to have fused to form a “mouth cavity,” which includes the mandibles, a median pad or tongue, and the opening of the mouth proper. The mouth leads into a muscular pharynx, into which opens the common duct of two large salivary glands, which extend far back along the body. Mouth, pharynx, and short oesophagus are lined by a chitinous cuticle, like that of the exterior. The long endodermic digestive region or mid- gut extends from the second leg nearly to the end of the body. Its walls are plaited. Finally, there is a short rectum or proctodseum, lined by a chitinous cuticle. Circulatory system. — The dorsal blood vessel forms a long con- tractile heart. It lies within a pericardial space, and receives blood by segmentally arranged apertures with valves. The circulation is mostly in ill-defined spaces in the apparent body cavity or “ hremocoele.” Respiratory system. — Very long and fine unbranched tracheae arc widely distributed in the body ; a number open together to the exterior in flask-like depressions. These openings or stigmata are diffuse and irregular in Peripatus edwardsii, but in P. capetisis there is a dorsal and ventral row on each side. In P. nova zealandia the trachea; are said to be branched. Excretory system. — A pair of nephridia lie in each segment. Each consists of an internal mesodermic terminal funnel, a looped canal, 2S4 PERTPA TVS, A/ YR 10 POPS, AND INSECTS. and a wide vesicle which opens near the base of each leg, the two last parts being invaginations of the ectoderm. This is the only certain case of their occurrence in a T racheate. The salivary glands and the genital ducts are probably modified nephridia. It may be noted, too, that the same is perhaps true of the “coxal glands” of Limulus and of the antennary glands of Crustaceans. Crural Glands lie in the legs and open to the exterior. Their meaning is uncertain, their occurrence is variable. Thus in P. edwardsii they occur in the males only, in P. capensis they are present in both sexes. In the male of P. capensis the last pair are very long (Fig. 12 1, a.g.). The large mucus glands, which pour forth slime from the oral papillre, are regarded as modified crural glands. Fig. X2i. — Dissection of Peripatus capensis. — After Balfour. at., Antennae; or.p., oral papillm ; c.g., cerebral ganglia; si. if. , duct of slime gland ( sl.g .) ; s.o.8, eighth segmental organ or nephridium ; v. c. , ventral nerve connected by transverse com- missures ( co .) with its fellow ; s.0.17, seventeenth nephridium ; g.o., genital aperture ; A., anus ; p.d.c. , posterior commissure ; 1. 17, seventeenth appendage; a.g., last crural gland, that of the opposite side is marked v.g. ; F. /, F.2, first and second legs; oe.co., oesophageal nerve commissure; PC. . oesophagus; fih., pharynx, the remainder of the gut is removed. Reproductive system. — {a) Female (of P. edwardsii). — From the two ovaries, which are surrounded by one connective tissue sheath, and arise, as usual, from the ccelomic epithelium, the ova pass by two long ducts leading to a common terminal vagina opening between the second last legs. These ducts are for the most part uteri, but on what may be called the oviduct portions adjoining the ovaries, there are two pairs of pouches — [a] a pair of receptacula seminis (for storing the spermatozoa received during copulation), and a pair of receptacula ovorum for storing fertilised eggs. The eggs are hatched in the uteri, and all stages are there to be found in regular order. The young embryos seem to be connected to the wall of the uterus by what has been called a “placenta,” so suggestive is ii of mammalian gestation. The older embryos lose this “placenta,” but PRO TO TRA CHE A TA. 285 each lies constricted off from its neighbours. When born the young resemble the parents except in size and colour. In P. novte scalandhc the ova pass from the ovary into the uterus in December, and the young are born in July — a long period of gestation. (b) Male (of P. edwardsti). — The male elements are produced in small testes, pass thence into two seminal vesicles, and onwards by two vasa deferentia into a long single ejaculatory duct, which opens in front of the anus. In the ejaculatory duct the spermatozoa are made into a long packet or spermatophore, which is attached to the female during copulation. While it is characteristic of Arthropods, in which the develop- ment of chitin is so predominant, that ciliated epithelium is absent, it seems that in Penpalus , which is much less chitinous than the others, ciliated cells occur in some parts of the reproductive ducts, and perhaps also at the internal funnels of the nephridia. This is- indeed what one would expect. Development. — There is great variety of development in different species. Thus there is much yolk in the ovum of P. naive zealanduc, extremely little in that of P. capensis. In the former species the yolk has a manifold origin ; it is said to arise in the protoplasm of the ovum itself from the breaking up of the germinal vesicle, from surrounding follicle cells, and from yolk present within the ovary. In P. capensis and /’. balfouri spermatozoa reach the ovary, and there probably the ova are fertilised, but in P. nova zcalandfte the spermatozoa are con- fined to the receptaculum seminis, near which fertilisation seems to occur. In the maturation of the ova of P. capensis and P. balfouri two polar bodies are extruded as usual, but none have been observed in the case of P. nova zealandia. In /’. capensis the “ segmentation ’’ is remarkable, for true cleavage of cells does not occur. The fully “ segmented ” ovum does not exhibit the usual cell limits. It is a protoplasmic mass — or syncytium— with many nuclei. Even when the body is formed, the continuity of cells persists, nor does the adult lack traces of it. To Mr. Sedgwick this singular fact suggested the theory that the Metazoa may have begun as multinucleate Infusorian-like animals. Fig. 122.- — Embryos of Peripatus capensis , showing closure of blastopore and curvature of embryo. — After Korschelt and Heider. a., Anus ; 61., blastophore ; m., mouth : p.s., primitive segments ; so., zone of proliferation. 286 PE RIP A TUS, MYRIOPODS, AND INSECTS. The gul appears as a large vacuole within the multinucleated mass, and a gastrula stage is thus established. In the ova of P. nova zealandia, which have much yolk, a superficial multiplication of nuclei forms a sort of blastoderm, which spreads over almost the entire ovum. The segmentation in this case has been called centrolecithal (the type characteristic of Arthropods), but it is again true that for a long time the cells do not exist as well-defined units. It has been said, indeed, that “ the embryo is formed by a process of crystal- lising out in situ from a mass of yolk, among which is a protoplasmic reticulum containing nuclei.” These examples may serve to show that the development of Peripatus is veiy varied. Zoological position. — Professor Lang summarises the synthetic characters of Peripatus as follows : — Annelid Characteristics. Tracheate Characteristics. Segmentally arranged nephridia as The presence of tracheae. in Chtetopods. The nature of the heart and the Segmentally arranged crural lacunar circulation. glands, like similar glands in The modification of appendages some Chietopods. as mouth organs. The muscular ensheathing of the The form of the salivary glands. body. The smallness of the genuine body Less important are the stump- cavity or coelom, like legs and the simple eyes. The ladder-like character of the ventral nervous system (cf. primitive Molluscs, Phyllopod Crustaceans, and Nemerteans)is probably primitive. That salivary glands and genital ducts are homologous with nephridia, is a fact of much morphological interest. It is possible that the slime glands are modifications of crural glands, and that the latter are homologous with the parapodial glands of some Annelids. It is not certain that the antenna:, jaws, and oral papilla: of Peripatus precisely correspond to the antenna:, mandibles, and first maxi Ike of Insects. Our general conclusion is that Peripatus is an archaic type, a survivor of forms which were ancestral to Tracheata and closely related to Annelids. Second Class of Tracheata Antennata. — Myriopoda. Centipedes and Millipedes. These animals retain a worm-like shape ; the numerous rings of the body and the appendages they bear are very uniform ; there is little division of labour. Simple wingless insects, known as Collembola and Thysanura, are closely approached by such Myriopods as Scolopcndrella ; and it is likely that Myriopods and Insects are divergent branches from a common stock. MYRIOPODA. 287 Both centipedes and millipedes live on land, but two or three of the latter occur on the seashore. Most are very shy animals, lurking in dark places and avoiding the light. The head bears a pair of antennte, and two other pairs of appendages — mandibles and maxillae. The limbs are six- or seven-jointed, clawed, and uniform. They have many more legs than insects, but they make less of them. The nervous system, heart, excretory tubules, etc., are like those of Insects. Fig. 123.— A millipede. Fig. 124.— A centipede. The development in many ways suggests and leads up to that of Insects. The two main sub-classes, which are very divergent, are contrasted in the following table : — In reference to habitat, it is interesting to note that at least two myriopods — Geophihis submarinus and Linotccnia maritima, occur on British coasts. As distinct from the two chief sub-classes, it is perhaps necessary to recognise other two — Pauropoda, c.g. Pauroput, and Symphyla, e.g. Scolopendrella. 288 PERIPATUS, MYR/OPODS, AND INSECTS. MV RIO POD A. Centipedes. Chilopoda. Millipedes. JJiplopoda (or Chilognatha). Carnivorous. Poisonous. Body usually flat. Vegetarian. Harmless. Body cylindrical. A pair of appendages to each segment. By the imperfect separation of the segments, all but the most anterior seem to have two pairs of appendages each, and also two paired ganglia, and two pairs of stigmata (tracheal openings). Many -jointed an tenure. Toothed cutting mandibles. Two pairs of maxillae, usually with palps. Seven-jointed antennas. Broad masticating mandibles. A pair of maxillae fused in a broad plate, usually four-lobed. The first pair of legs modified as poison claws. No poison claws. A single genital aperture on the last segment. Genital apertures open an- teriorly. Exam pies. — Seolopendra. Lithobius. Examples.— -Julits. Geophilus. Third Class of Tracheata Antennata. — Insecta. Insects occupy a position among the backboneless animals like that of birds among the Vertebrates. The typical members of both classes have wings and the power of true flight, richly aerated bodies, and highly developed respiratory, nervous, and sensory organs. Both are very active and brightly coloured. They show parallel differ- ences between the sexes, and great wealth of species within a narrow range. General Characters. Like other Arthropods , Insects have segmented bodies , jointed legs, chifinous armature , and a ventral chain of ganglia linked to a dorsal brain. Compared with Peripatus and Myriapods, COCKROACH. 289 adult insects show concentration of the body segments , decrease in the number and increase in the quality of the appendages , and wings in the great majority. Insects are terrestrial and aerial, and rarely aquatic animals ; usually winged as adults, breathing by means of trachea , and often with a metamorphosis in the course of their life history. The body is divided into three distinct regions, — head, thorax, and abdomen. The head bears a pair of pi'e-oral antenna, and three pairs of mouth appendages ; the thorax bears a pair of legs on each of its three segments, and, typically, a pair of wings on each of the posterior two ; the abdomen has no ! appendages, unless rudimentary modifications of these be re- presented by stings, ovipositors, etc. First Type of Insects, Feriplaneta (or B/atta). — The Cockroach. Habits. — The cockroaches in Britain are immigrants from the East (P. orientals), or from America (P. americana). They are omnivorous in their diet, active in their habits, hiding during the day and feeding at night. They are ancient insects, for related forms occurred in Silurian ages ; they. are average types, neither very simple nor very highly specialised. Their position is among the Orthoptera, in the Fig. 125. — Female cockroach ( P. orientalis ). . Fig. 126. — Male cockroach (P. orientalis). 290 PERIPATUS, MYRIOPODS, AND INSECTS. External Characters. Region. The head is ver- tically elongated and separated from the thorax by a neck. The thorax con- sists of three seg- ments— (a) prothorax, (b) mesothorax, ‘(c) metathorax. (Each segment is bounded by a dorsal tergum, and ventral ster- num.) The abdomen consists of io (or ii) distinct segments, with j terga and sterna j as in the thorax. The first sternum j is rudimentary in both sexes, and in the female the eighth and ninth segments are con- cealed by the large seventh. Appendages. Other Structures. 1. The antennae (probably homologous with appendages), long, slender, many -jointed, tactile. 2. A pair of stout toothed mandibles work- ing sideways. 3. The first maxillae, each consisting — (a) of a basal piece or protopodite with two joints : a basal cardo, a distal stipes ; (b) of a double endopodite borne by the basal piece, and consisting of an inner lacinia and a softer outer galea ; (c) of an exopodite or maxillary palp also borne by the basal piece, and consist- ing of five joints. 4. The second pair of maxillae, fused to- gether as the “ labium,” consisting — (a) of a fused basal piece or protopodite with two joints : a basal sub-mentum, a smaller distal mentum ; on each side this protopodite bears — (b) a double endopodite (ligula) consisting of an inner lacinia, and an outer paraglossa ; (c) an exopodite or labial palp, consisting of three joints. The large black compound eyes. The “ upper lip ” or labrum, in front of the mouth. The white oval patches near the bases of the antenna;, pos- sibly sensory. (a) First pair of legs. (b) Second pair of legs. (t) Third pair of legs. Each leg consists of many joints — a basal expanded “coxa” with a small “trochanter ’’ at its distal end, a “femur,” a “tibia,” a six-jointed tarsus or foot ending in a pair of claws (Fig. 127). Two cigar-shaped tactile anal cerci, at- tached under the edges of the last tergum, are possibly relics of the last abdominal appendages. The ninth sternum of the male bears a pair of styles, possibly relics of appendages. Both sexes have complex hard structures (gonapophyses) beside the genital apertures. They are possibly relics of appendages. (b) A pair of wing-covers (modi- fied wings) rudimentary in female of P. oricntalis. (c) A pair of membranous wings, sometimes used in flight, folded when not in use, absent in female of P. orient alis. Between the segments of the thorax are two pairs of respiratory apertures or stigmata. A pair of stigmata occur be- tween the edges of the terga and sterna in the first eight abdo- minal segments. The anus is terminal, beneath the tenth tergum of the abdo- men ; a pair of “ podical plates” lie beside it. The genital aperture is on the eighth segment in the female, behind the ninth sternum in the male. The opening of the spermag theca — the female's receptacle for spermatozoa — lies on the ninth sternum of the abdomen. COCKROACH. 291 same order as locusts and grasshoppers. The young are hatched as miniature adults, except that wings are absent ; in other words, there is no metamorphosis in develop- ment. Skin. — There is an exter- nal chitinous cuticle and a subjacent cellular layer — the epidermis or hypodermis — from which the cuticle is formed. The newly hatched Fig. 127.— Leg of cockroach, cockroaches are white, the c., Broad expanded coxa; tr., troch- orlnlt-t: nrp rlnrL hrnwn anter femur ; ti., tibia; to.., five- uUUllS are uaiK Drown. jointed tarsus with terminal claws and Moulting, which involves a adhesive cushions, casting of the cuticle, of the internal lining of the tracheae, etc., occurs some seven times before the cockroach attains in its fifth year to maturity. The muscles which move the appendages, and produce the abdo- minal movements essen- tial to respiration, are -mx-r> markedly cross striped. Nervous system. — A pair of supra-cesopha- geal or cerebral ganglia lie united in the head. As a brain they receive impressions by anten- nary and optic nerves. By means of a paired commissure surround- ing the gullet, they are connected with a double ventral chain of ten ganglia. Of these, the first or sub-oesopha- geal pair are large, and give off nerves to the mouth parts, etc. ; from each of the ganglia of the thorax and the abdomen nerves are given off Fig. 128. — Mouth appendages of cock- roach.— After Dufour. I. Mn., mandibles ; II. first tnaxilke ; C., cardo ; Si., stipes ; L., lacinia ; G., galea; mx.fi., maxillary palp; III. second maxilla; or la- bium; S.ttt., submentum ; in., mentum ; L., lacinia; ; fig . paraglossa ; l.fi., labial palp. 292 PERIPATUS , M YRIOPODS, AND INSECTS. to adjacent parts. There are three pairs of ganglia in the thorax, and six in the abdomen, of which the last is the largest. From the oesophageal commissures two visceral nerves are given off, which form in a somewhat complex manner the innervation of gullet, crop, and gizzard. Besides the large compound eyes, there are other sensory structures — some of the hairs on the skin, the maxillae (to some extent organs of taste), the antennae (tactile and olfactory), the anal cerci (tactile), and possibly the oval white patches on the head. Alimentary system. — (i) The fore-gut (stomodaeum) is lined by a chitinous cuticle continuous with that of the outer surface of the body. It includes — (a) the buccal or mouth cavity, in which there is a tongue - like ridge, and into which there opens the duct of the salivary glands ; (/>) the narrow gullet or oesophagus ; (c) the swollen crop ; ( d ) the gizzard, with muscular walls, six hard cuticular teeth, and some bristly pads. There is a pair of diffuse salivary glands on each side of the crop, and between each pair of glands a salivary receptacle. The ducts of the two salivary glands on each side unite ; the two ducts thus formed combine in a median duct, and this unites with another median duct formed from the union of the ducts of the receptacles. The common duct opens into the mouth. (2) The mid-gut (mesenteron) is lined by endoderm. It is short and narrow, and with its anterior end seven or eight club-shaped digestive (pancreatic) outgrowths are connected. (3) The hind-gut (proctodaeum) is lined by a chitinous cuticle. It is convoluted and divided into narrow ileum, wider colon, and dilated rectum with six internal ridges. Kespiratory system. — The tracheal tubes, which have Fig. 129. — Transverse section of insect. — After Packard. /«., Heart ; g., gut ; «., nerve-cord ; si., stigma ; tr., trachea; w., wing;/., femur of leg. COCKROACH. 293 ten pairs of lateral apertures or stigmata, ramify throughout the body, and have a chitinous lining throughout. Circulatory system. — The chambered heart lies along the mid-dorsal line of abdomen and thorax. It receives blood by lateral valvular apertures from the surrounding pericardial space, and drives it forwards by a slender aorta. The blood circulates, however, within ill-defined spaces in the body. Excretory system. — There are sixty or so fine (Mal- pighian) tubules, which rise in six bundles from the begin- ning of the ileum, and twine through the “ fatty body ” and in the abdominal cavity. The absence of nephridia in insects has already been noted. Reproductive System. Of the Male. The testes are paired organs, sur- rounded by the fatty body below the 5th and 6th ab- dominal terga. They atrophy in the adult. From the testes, two narrow ducts or vasa deferentia lead to two seminal vesicles. These seminal vesicles (the “mush- room-shaped gland ”)open into the top of the ejaculatory duct. This duct opens between the 9th and 10th sterna. Beside the aperture, there are copulatory structures (gonapophyses). With the ejaculatory duct a gland is associated. Of the Female. The ovaries are paired organs, in the posterior abdominal region , each consisting of eight ovarian tubes. These are bead - like strings of ova at various stages of ripeness. From the ovarian tubes of each side eight eggs pass at a time into a short wide oviduct. The two oviducts unite and open in a median aperture on the 8th abdominal sternum. Be- side the aperture are hard structures (gonapophyses) which help in the egg-laying. Here also a pair of “ col- leterial” glands pour out their cementing secretion by two apertures. The spermatheca is a paired sac with a single aperture on the 9th abdominal sternum. Sixteen ova, one from each ovarian tube, are usually enclosed within each egg-capsule. The latter is formed 294 PERIPATUS, MYR10P0DS, AND INSECTS. from the secretion of the colleterial glands. Each egg is enclosed in an oval shell, in which there are several little holes (micropyles), through one of which a spermatozoon enters. Spermatozoa, from the store within the spermatheca, are included in the egg-capsule. The development is similar to that of other insects, and it has already been mentioned that there is no metamorphosis. At an early stage in development some cells associated with the mesoderm are set apart as reproductive cells, and originally these have a segmental arrangement as in Annelids ; at a later stage other meso- .derm cells join these, some forming ova, others epithelial cells around the latter. The distinction between truly reproductive cells and associated epithelial cells, which is said to be late of appearing in some of the higher insects, is established at a very early stage in the cockroach. Second Type of Insects. — The British Hive-Bee (Apis mellificd). This is a much more highly specialised type than the cockroach. It belongs to the order Hymenoptera. Habits. — -The Hive-Bee (Apis mellifica) is a native of this country, and is the species most commonly found domesticated. It is the only British representative of the genus Apis , and exhibits, in its most fully-developed form, the social life which is foreshadowed among the Humble- Bees. As a consequence of this social life, there is much division of labour, which expresses itself alike in habit and in structure. The males (drones) take no part in the work of the colony, and are wholly reproductive ; the females include the queen-bees and the workers. In the workers, which perform all the work of the hive, the reproductive organs are normally abortive and functionless. In the queens, of which there is but one adult to each hive, the enormous development of the reproductive organs seems to act as a check upon the brain and other organs, which are less developed than in the workers. The workers are further divisible into nurses, which are young and do not leave the hive, being occupied with the care of the larvie, and the older foraging bees which gather food for the whole colony. In considering the relation between the life of the Hive- Bee and that of many allied forms (Bombus, etc.), it is BRITISH HIVE- BEE. 295 important to notice that the habit of laying up stores of food material for the winter enables the colony, and not merely an individual, to survive, and must thus have greatly assisted in the evolution of sociality. External features. — The body shows the usual division into head, thorax, and abdomen, and varies considerably in the three different types, being smallest in the workers. It is entirely covered with hairs, some of which are sensitive, while others are used in pollen gathering, etc. The head bears antennas, which are composed of a long basal and numerous smaller joints. They are marvellously sensitive, serving to communicate impressions, and also containing organs of special sense. A pair of compound eyes, largest in the drones, and three median ocelli, are also present in the head region. Of the other appendages of the head, the mandibles are in the workers very powerful, and used for many purposes connected with comb-building. In the first maxillae the maxillary palps are aborted, and the appendage consists of an undivided lamina at each side, borne on a basal piece consisting as usual of stipes and cardo. The second pair of maxillae form as usual the labium or so-called lower lip, and aie much modified. The united basal joints form the mentum and sub-mentum. From the mentum at either side springs the long labial palp, which represents the outer fork of the typical appendage. The endopodite at each side is divided into two parts, but the inner two (lacinke) are united, much elongated, and form the tongue or ligula of the bee. The outer halves form the paraglossae, which are closely apposed to the base of the ligula. It is the great elongation of the ligula and labial palps which especially fits the bee for nectar-gathering. The three structures can be closely apposed to one another, and then form an air-tight tube, up which, by the action of the stomach, nectar is sucked. In many of our British bees the ligula is much shorter, and more or less trowel-like in shape, and is then used largely, as in wasps, in the operation of plastering the nest. In such cases the bee can only suck those flowers in which the nectar is superficial. The hive-bees and humble-bees, on the other hand, are specially modified to enable them to extract nectar from tubular flowers. When not in use the elongated mouth-parts are folded back upon themselves, not coiled as in butterflies and moths, where there is even greater elongation. In the queen and in the drone the mouth-parts are shorter, and are not used in honey gathering. The thoracic appendages consist as usual of three pairs of legs, which have the usual parts. On the first leg, at the junction of the tibia and the first tarsal joint, there is a complicated mechanism which is em- ployed in cleaning the antenna ; this is present in all three forms, and varies with the size of the antenna. In the workers the third leg is remarkably modified for pollen gathering purposes. The first tarsal joint bears regular rows of stiff straight hairs on which the pollen grains are collected ; they are borne to the hive in the pollen basket, placed at the back of the tibia, and furnished with numerous hairs. In queen and drone, these special arrangements of hairs are absent. 296 PERIPATUS, MYRIOPODS, AND INSECTS. The second and third thoracic segments bear each a pair of wings. These are largest in the drones and relatively smallest in the queen, who flies but seldom. At the base of each wing there is a respiratory spiracle. In the adult queen and worker, the abdomen is divided into six segments ; in the drone, into seven. There are no abdominal appen- dages. On the ventral surface in the worker, but not in the queen or drone, there are four pairs of wax pockets or glands, which secrete the wax, which, after mastication with saliva, is em- ployed in building the combs. The abdomen also bears in queen and worker five pairs of spiracles, but in the drone, on account of the additional seg- ment, there are six pairs. The total number of spiracles is thus fourteen for queen and worker, and sixteen for the drone. The posterior region of the abdomen bears the complicated sting. In. the worker this consists of a hard incomplete sheath, which envelops two barbed darts. The poison flows down a channel lying between the darts and the sheath. Ramifying through the abdomen are found the two slender coiled tubes which con- stitute the poison gland. At the posterior end of the body these unite and open into a large poison sac. When a bee uses its sting, the chitinous sheath first pierces the skin, and then the wound is deepened by the barbed and pointed darts, while at the same time poison is steadily pumped down the channel mentioned above, and pours out by minute openings at the bases of the darts. The poison contains formic acid, and is fatal to the bee if directly introduced into its blood. Associated with the sting there are a pair of delicate tactile palps. In the queen the sting is curved and more powerful, but it is apparently only used in combat with a rival. In the worker the sting, and with it a portion of the gut, is usually lost after use, and, in consequence, death ensues ; the queen, on the other hand, can withdraw her sting from the wound with considerable ease. The sting is really an ovipositor adapted to a new function. Naturally, therefore, there is no trace of it in the drones. Fig. 130. — Head and mouth-parts of bee. — After Cheshire. a., Antenna; in., mandible ; g., labrumor epipharynx ; mx.fr., rudiment of maxil- lary palp ; nix., lamina of maxilla ; Ifr., labial palp; ligula ; b., bouton at end. The paraglossas lie concealed between the basal portions of the labial palps and the ligula. BRITISH HIVE-BEE. 297 Nervous system. — In the adult this exhibits considerable fusion of parts. The supra-oesophageal ganglia are very large, and send large lateral extensions to the compound eyes. This “ brain ” is best developed in the active workers. The sub-cesophageal mass is formed by the fusion of three pairs of ganglia. In the thorax there are two pairs of ganglia, of which the second supplies the wings and the Fig. 131.— Nervous system of bee. — After Cheshire. A, of larva. 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The anterior segments, about seven in number , are fused into a cephalothorax , which bears six fan's of appendages. The most anterior of these appendages may be turned in front of the mouth , but there are no pre-oral antennce as in Insects. The first two pairs of appendages ( chelicerce and pedipalps ) generally have to do with seizing and holding the food ; the others are walking legs. But although six pairs occur in most , there may be more or less. The abdomen is generally, but not always, without appendages ; it may be segmented or unsegmented ; it is generally distinct from , but may be fused to, the cephalothorax. A plate-like internal skeleton, called the endosternite, is often present. The elaborate compound eyes of Insects are not i-epresented , the eyes being almost always simple. Respiration may be by tubular trachea, or by lung- books ( chambered trachea ?), or by both, and many would include the branchiate Palaostraca along with Arachnoidea. In the tracheate forms there are never more than four pairs of stigmata. Within all or some of the legs lie coxal glands, perhaps comparable to nephridia. An elongated dorsal heart usually lies in the abdomen. The position of the genital aperture or apertures is usually on one of the anterior abdominal segments. All have separate sexes. The relation of Arachnoids to other Arthropods, and especially to Insects, is uncertain. Many believe that the Arachnoids are descended SCORPION I D/E. 32 3 from a branchiate form , and have acquired their trachea independently ; apart from the trachea , few structural resemblances to Insects are apparent. Order 1. Scorpionid^e. Scorpions are elongated Arachnoids, restricted to warm countries, lurking under stones or in holes during the day, but active at night. The Scorpio afer of the East Indies attains a length of 6 inches, but most are much smaller. They feed on insects, spiders, and other small animals. The “ tail,” with the venomous sting at its tip, is usually curved over the anterior part of the body, and can reach forward to kill the prey caught by the anterior appendages, or can be suddenly straightened to strike back- wards. When man is stung, the poison seems to act chiefly on the red blood corpuscles, and, though never or very rarely fatal, may cause much pain. It has been said that scorpions commit suicide when sur- rounded by fire Or Otherwise r/i., Chelicerm; //., pedipalps; o., r . J . . . , genital operculum; pectines ; fatally threatened, but it has •>., stigma of a lung-book on the been answered that they do anafpiece!6" ’ st' stmg 01 post' not sting themselves, that they could not if they would, and that, even if they could, the poison would have no effect ! The body is divided into — (1) a cephalothorax or “pro- soma” of six segments, whose terga fuse into a carapace, and (2) an abdomen, which includes a broad seven- segmented “ mesosoma,” and a narrow five-segmented “ metasoma.” At the end of the latter there is a post-anal curved spine or “ telson,” containing a paired, compressible poison gland opening at the sharp tip. There is a strong Fig. 140. — Scorpion. 324 ARACHNOIDEA AND PALAEOSTRACA. cuticle of chitin, and also an interesting internal piece of skeleton (the endosternite), partly chitinoid, but also resembling fibro-cartilage, which lies in the cephalothorax above the nerve-cord, and serves for the insertion of muscles. The appendages are — 1. Small, three-jointed, chelate chelicerre or falces just above the mouth, used in holding prey. 2. Large, six-jointed, chelate pedipalps. These seize the prey ; their basal joints help in mastication, and in some cases they produce rasping sounds. 3-6. Four pairs of seven -join ted, non-chelate walking legs. The basal joints of the first two pairs help in connection with the mouth. Apparently equivalent to a first pair of abdominal appendages is a small notched plate or operculum which covers or bears the genital aperture or apertures. Apparently of the nature of appendages are the comb-like, probably tactile, pectines on the second abdominal segment. Six other pairs of abdominal appendages are present in the embryo, but they abort. The nervous system consists of a dorsal brain, a ring round the gullet, and a ventral nerve-cord. The eyes are innervated from the brain, the first six appendages from the collar and the suboesophageal ganglion. Behind the latter there are seven ventral ganglia in the eleventh to seventeenth segments inclusive. There are in scorpions two to six pairs of eyes placed on the carapace. The lateral eyes are simple ocelli, but the median pair are remarkable among Arachnoid eyes, in that, although there is only a single lens, there are numerous retinulte. Scorpions seize small animals with their pedipalps, hold them close to the small mouth by their chelicerte, sting them if need be, and suck their blood and juices. The pharynx serves as a suction pump ; a narrow gullet leads to a slight enlargement, into which a pair of salivary glands open ; from the narrow mid-gut several large digestive outgrowths arise, and also one or two pairs of Malpighian tubes ; the hind-gut ends in a ventral anus beneath the base of the sting. The narrowness of the gut may be associated with the fluid nature of the food. The position of the Malpighian tubes shows that here, as in certain Crustacea, they are endodermal structures as contrasted with the (Y/odermal tubules of Insects. The body cavity is for the most part filled up with organs, muscles, and connective tissue. A pair of coxal glands, perhaps excretory and nephridial, but apparently closed in the adult, lie near the base of the third pair of walking legs. It is stated that in the embryo they open into the body cavity by internal funnels. The blood contains amoeboid corpuscles and the respiratory pigment hsemocyanin. An eight-chambered heart, within a pericardium, lies along the back of the mesosoma. It gives off lateial arteries from the posterior end of each of its chambers, is continued backwards in a SCORPTONID.E. 325 posterior aorta, and forwards in an anterior aorta. The latter supplies the head and divides into two branches, encircling the gullet and reuniting in a ventral artery above the nerve-cord. From capillaries the blood is gathered into a ventral venous sinus, is purified in the lung- books, and thence returns by veins to the pericardium, finding its way by valved lateral openings (ostia) into the anterior end of each heart- chamber. On the ninth to twelfth segments lie slit-like stigmata, the openings of four pairs of lung-books. Each lung-book is like a little purse with numerous (over a hundred) compartments. Air fills the much-divided cavity, and blood circulates in the lamellae or partitions. These lung- books or pulmonary sacs are believed by some to be chambered or plaited trachea:, while Professor Ray Lankester regards them as in- vaginated modifications of gill-books such as Limulus possesses. The testes consist of two pairs of longitudinal tubes, united by cross bridges; the vas deferens, with a terminal copulatory modification, opens under the operculum on the first abdominal segment. The ovary consists of three longitudinal tubes, united by cross ducts, and two oviducts open on the under surface of the operculum. Fertilisation is internal ; the ova begin their development in the ovary, and complete it in the oviduct. The segmentation is discoidal, the ova are hatched within the mother. The young, thus born “vivi- parously,” are like miniature adults, and adhere for some time aftei birth to the body of the mother. In Euscorpio italicus there is abundant yolk in the ovum ; in Scorpio there is little ; but the embryo of the latter seems to eat the terminal part of the ovarian tube in which it develops. In the embryo of Opilhophthalmus there are peculiar horn-like outgrowths, possibly absorptive in function. The race of scorpions is of very ancient origin, for one has been found in Silurian strata, and others nearly resem- bling those now alive are found in the Carboniferous. Order 2. Pseudoscorpioniivk. “ Book -Scorpions, ” Chelifer , C hemes. Minute animals, most abundant in warm climates, under bark, in books, under the wing-covers of insects, etc. They are like miniature scorpions, but without the long tail and sting. Their food probably consists of the juices of insects ; the chelicerce are minute suckers, the pedipalps like those of scorpions. The abdomen is broad, with ten to eleven segments. They breathe by tubular trachere, and have spinning glands. Order 3. Pedipai.pi. “ Whip-Scorpions," e.g. Thelyphonus, Phrynus. Small animals, found in warm countries. The abdomen is depressed^ well-defined from the thorax, and has eleven to twelve segments. The cheliccne arc simply clawed, but are poisonous ; the pedipalps arc simple 1 ARACHN01DEA AND PAL/EOSTRACA. 326 clawed or else truly chelate. The first pair of limbs are like antennre. Respiration is by two pairs of lung-sacs. In Thelyphomis there is a long terminal whip. Order 4. Phalangid.e (or Opilionina). “ Harvest-men,” e.g. Phalangium. The small spider-like 1 ‘ harvest-men ” are noted for their extremely long legs, by which they stalk slowly along, avoiding the glare of day. The broad six-segmented abdomen is not distinct from the thorax ; the chelicerse are chelate ; the pedipalps are like legs. Respiration is by tubular tracheae. The harvest -men are sometimes called daddy longlegs, but we reserve that name for the crane-fly ( Tipula oleracea). Nor are they to be confused with the troublesome “harvest-bugs” ( Trombidium holosericeum), which are minute red mites. The harvest-men do not trouble us, but feed on small insects. Order 5. Solpugide or Solifug/e, e.g. Galeodes or Solpuga. Active, pugnacious, venomous, nocturnal little animals, found in the warmer parts of the earth. The head and abdomen are distinct from the thorax. The thorax has three segments, the abdomen nine or ten. The chelicerse are chelate, the pedipalps like long legs. The respiration is by means of tubular trachea:. The segmentation of the thorax is remarkable. Order 6. Araneiive. Spiders. Spiders are found almost everywhere upon the earth, and a few are at home in fresh water. Most of them live on the juices of insects, and many form webs in which their victims are snared. They may be divided, according to habit, into the wanderers who spin little, and the sedentary forms who spin much. The body consists of an unseg- mented cephalothorax and -a soft unsegmented abdomen, separated by a narrow waist. The chitinous cuticle varies in hardness, hairiness, and colouring ; it has, as usual, to be moulted as the spider grows. Thus the young garden- spider moults eight times in its first year. There are six pairs of appendages — - 1. The two-jointed chelicerse or falces, whose terminal joint bends down on the other in “ sub'-chelate ” fashion, and is perforated by the duct of a poison gland. 2. The leg-like, usually six-jointed, non-chelate pedipalps, whose basal joint helps in mastication, while the terminal joint in the male expands as a reservoir for the spermatozoa and serves as a copulatory organ. 3-6. Four pairs of terminally clawed walking legs. The most ARANETD/F.. 327 anterior pair are much used as feelers. In the embryo there are four pairs of abdominal appendages which abort. The nervous system is of the usual Arthropod type, but shows much centralisation. Thus the ventral ganglia are fused into one large centre in the cephalothorax (see Fig. 141), a condition comparable to that in crabs. There are two or three rows of simple eyes on the cephalothorax, whose focal distance is fery short, spiders trusting most to their exquisite sense of touch, by which they discriminate the various vibrations on a web line. The senses of smell, hearing, and taste are also pre- sent, but little is known in re- gard to the organs. Body cavity, endosternite, and coxal glands generally re- semble those of scorpions. The spider usually sucks the • blood and juices of its prey, and behind the gullet lies a powerfully suctorial region, strengthened by chitinous Fl£‘ My&l plates, and worked by muscl From the small mid-gut five pairs of long casca, a running forwards and a passing into the bases of each pair of legs, and then back again. These caeca sometimes anastomose. Further back the mid-gut gives off numerous digestive outgrowths, which fill a large part of the abdomen. Their secretion digests proteids. Terminally there is a large cloaca, and where the intestine joins this, four much-branched excretory Malpighian tubes are given off, and are again said to be endodermal in origin. from the ventral surface. — After Cuvier. Chelicerae ; 2, pedipalps cut short ; 3-6, walking legs ; g1. large thoracic ganglion ; g 2. ganglion at base of abdomen; c.t., chambered trachea; or lung-books — at the left side the anterior is cut open to show the lamellae (/.) ; m.t muscle of abdomen ; sfl. and st2., stigmata of lung-books ; ov. , ovary ; sp. , spinnerets. 32S ARACHNOTDEA AND PALMOSTRACA. A three-chambered heart, containing colourless blood, lies within a pericardium near the dorsal surface of the abdomen. It gives off an anterior and a posterior aorta and lateral vessels; and the circulation corresponds in general to that of the scorpion. In a few forms (Tetrapneumones) respiration is effected by four “ lung-books,” e.g. in the large bird-catching Mygale (Fig. 141). In the vast majority (Dipneumones) there are two lung-books, and tubular tracheae in addition. The stigmata of the lung-books lie on the anterior ventral sur- face of the abdomen ; the tracheae open posteriorly near the spinnerets, or just behind tire openingof the lung-books, or at both places. The spinnerets (4-6) lie posteriorly a little in front of the anus. They are movable organs, perforated by numer- ous (often many hundred) fine tubes or “ spinning spools.” The tubes are connected with numerous compressible glands secreting liquid silk. There are various kinds of glands, and both the amount and the nature of the secre- tion are under the spinner’s control. The spinnerets arise from modifications of abdominal appendages, and the glands are ectodermic invaginations. The males are usually smaller and often more brightly coloured than their mates. From the paired testes, in the anterior part of the abdomen, two vasa deferentia pass to a common aperture beside the openings of the lung-books. From the paired ovary two oviducts likewise arise and open into a uterus, whose external aperture is surrounded in the mature female by a complex genital armature or epigynium. Here also in most females are the openings of two receptacula seminis, in which the sperms received from a male are stored, and from which they pass by a pair of internal ducts to the oviducts, there to fertilise the s.i. Fig. 142. — Section of Lung-book. — After Macleocl. d., Dorsal ; v., ventral ; lamellae ; posterior; a., anterior; d.c., dorsal chamber ; x., posterior wall ; si., stigma; ch ., one of the interlamellar chambers. COURTSHIP. 329 ova. The sperms of the male, after emission, may be stored up in the last joint of the palps. The ova are usually surrounded by silken cocoons, which are carried about by the mother or carefully hidden in nooks or nests. Spinning1. — Compression of the spinning-glands causes a flow of liquid silk through the fine spools of the spinnerets. The extremely thin filaments from each spinneret unite into a thread, and the thread of one spinneret is often combined with that from the others. In this way a compound thread of exquisite fineness, though rivalled by a quartz-fibre, is produced ; but two or four separate threads are often exuded at the same time. Before beginning to “spin,” the spider often presses the spinnerets against the surface to which the thread is to adhere, and draws the filaments out by slowly moving away. Often, however, the filaments ooze out quite apart from any attachment. The legs are also much used in extending and guiding the thread, and some spiders have on the hind legs a special comb of stiff hairs. One of the most important ways in which the secreted threads are used is in forming a web. The common garden spider ( Epeira ) makes a web which is a beautiful work of unconscious art, and very effective as a snare for insects. The spider first forms “foundation lines” around the selected area ; it then swings across the area with the first “ray,” which it fixes firmly; another and another is formed, all inter- secting in one centre. Secondly, it starts from the centre, and moves from ray to ray in a long wide spiral gradually outwards, leaving a strong spiral thread as it goes. Thirdly, the spider moves in a closer spiral from the circumference inwards, biting away the former spiral, replacing it by another, which is viscid and adhesive. It is to this that the web chiefly owes its power of catching insects which light there. There is usually a special thread running to the adjacent hole or nest, and the entire fabric is marvellously sensitive, for the spider feels rather than sees when a victim is caught. The spun threads are used in many other ways. They line the nest, and form cocoons for the eggs. They often trail behind the spiders as they creep ; they greatly assist locomotion, and are used in marvellous feats of climbing. Small and young spiders often stand on tip-toe on the top of a fence, secrete a parachute of threads, and allow them- selves to be borne by the wind. The fallen threads are known as gossamer. Courtship. — The males are usually much smaller than the females. It is calculated that the disproportion is sometimes such as would be observed if a man 6 ft. high and 1 50 lb. in weight were to marry a giantess of 76-90 ft. high, 200,000 lb. in weight. It may be that the smallness of the males is mainly due to the fact that they are males ; others explain it by saying that the smaller the males are, the less likely they are to be caught by their frequently ferocious mates. The males are often more brilliantly coloured than the females, perhaps, again, because they are males, though what the physiological connection between the male constitution and bright colours is in this case we cannot tell till the nature of the pigments is known. Wallace 330 A RA CHNOIDEA AND PAL/EOSTRACA. has spoken of the frequent brilliancy of males as due to their greater vitality, and refers the relative plainness common in females to their greater need for protection. Darwin referred the greater decorativeness of males to the fact that those which varied in this direction found favour in the eyes of their mates, were consequently more successful in reproduction, and thus tended to entail brilliancy on their male successors. The careful researches of Prof, and Mrs. Peckham greatly strengthen the position of those who believe in the efficacy of sexual selection. In the “Evolution of Sex,” it has been suggested that sexual selection may help to establish the brilliancy of males, and that natural selection may help to keep the females plain, but that the decorative and other differences between the sexes are primarily asso- ciated with the more fundamental qualities of maleness and femaleness. Classification of Spiders. 1. Tetrapneumones, with four lung-books and no trachea;, e.g. — Mygale, a large lurking spider which has been known to kill small birds, but usually eats insects ; Atypus, Cteniza, and others make neat trap-door nests. 2. Dipneumones, with two lung-books and trachea; as well, such as — The web-spinners, e.g. Epeira ; wolf-spiders, e.g. Lycosa, Tarantula , the latter with poisonous qualities which have been much exaggerated ; jumping spiders or Attidre, e.g. A Hits salt tens. The common house spider is Tegenaria domestica ; the commonest garden spider is Epeira diade- inata. Argyroneta aquatica fills an aquatic silken nest with bubbles caught at the surface. Order 7. Acarina. Mites and Ticks. Mites are minute Arachnoids inclined to parasitism. They occur in the earth, or in water, salt and fresh, or on animals and plants. They feed on the organisms they infest or upon organic debris. The abdomen is fused with the cephalothorax ; both are unsegmented. According to the mode of life, the mouth-parts are adapted for biting or for piercing and sucking. Respiration may be simply through the skin ; in the majority there are trachea; with two stigmata. A heart seems usually absent, but it is present in Gamasus. Many of the young have only three pairs of legs when hatched, but soon gain another pair. When some mites are starved or desiccated, and to some extent die, certain cells in the body unite within a cyst, and are able in favourable conditions to regrow the animal. Examples. — (a) Without tracheae. Cheese -mite ( Tyroglyphus ). Itch-mite (Sarcoples scabiei ), causing “itch” in man; A cants, causing “mange” in dogs. Follicle-mite (De/nodex folli- culorum), common in the hair follicles of man and domestic animals. Gall-mites ( Phytoplus ). on plants. PAL/F.OSTRA CA. 33' (/>) With trachece. Ilarvest-mites ( Trombidium ), whose minute hexapod lame are troublesome parasites in summer on insects, many mammals, and man. The so-called “ red- spider ” ( Tetrarhynchus telearius) spins webs, and lives socially under leaves. Water-mites, e.g. Hydrachna on water-beetles, and Atax on gills of fresh-water mussels. Beetle-mites ( Gamasus ), often found on carrion beetles. Ticks (Ixodes), on dogs, cattle, etc. Aberrant Orders or Classes. Order 8. Linguatulida. Pentastomum tanioides. This strange animal is parasitic in the nasal and frontal cavities of the dog and wolf. It is worm-like in form, externally ringed, without an)- oral appendages, but with two pairs of movable hooks near the mouth. There are no sense organs nor tracheae. The sexes are separate ; the males smaller than the females. Embryos within egg-cases pass from the nostrils of the dog. If they happen to be swallowed by a rabbit or a hare, or it may be some other mammal, the embryos hatch in the gut and penetrate to liver 01- lung. There they encyst, moult, and undergo metamorphosis. The final laival form has two pairs of short legs, and has been compared to a larval mite. Liberated from its encystment, it moves about within its host, but will not become adult or sexual unless its host be eaten by dog or wolf. There are a few other species occurring in Reptiles, Apes, and even man, but their history is not adequately known, and the systematic position is very uncertain. Order 9. Tardigarda. Water-Bears or Sloth-animalcules, e.g. Macrobiotus. Microscopic animals, sometimes found about the damp moss of swamps or even in the roof-gutters of houses. The body is somewhat worm-like, with four pairs of clawed limbs like little stumps, with mouth-parts resembling those of some mites, and adapted for piercing and sucking. There is no abdomen. There is a food canal, a brain, and a ventral chain of four ganglia, sometimes even a pair of simple eyes, but no respiratory or vascular organs. The sexes are separate ; the males rarer and smaller. The terrestrial Tardigarda, even as adults, have great powers of suc- cessfully resisting desiccation, but sometimes only the eggs do so, developing rapidly when favourable conditions return. Class Pal^eostraca. The three following orders, Xiphosura, Eurypterina, and Trilobita, may be united under this title. They live or lived in water, and have or had gills in association with the limbs. ARACHNOTDEA AND PALMOSTRACA. The recently discovered antennae of Trilobites, together with the markedly biramose character of some of their limbs, suggest an affinity with Crustacea, but, on the other hand, the affinities of the Xiphosura seem to be distinctly Arachnoid. Order i. Xiphosura. There is one living genus, the King-crab or Horseshoe- crab (Li 'mu his). The King-crab lives at slight depths off the muddy or sandy shores of the sheltered bays and estuaries of North America, from Maine to Florida, in the West Indies, and also on the Molucca Islands, etc., in the far East. The body consists of a vaulted cephalothorax shaped like a horseshoe, and an almost hexagonal abdomen ending in a long spine. Burrowing in the sand, Limulus arches its body at the joint between cephalothorax and abdomen, and pushes forward with legs and spine. It may also walk about under water, and even rise a little from the bottom. It is a hardy animal, able to survive exposure on the shore, or even some freshening of the water. Its food consists chiefly of worms. The King-crab is interesting in its structure and habits, and also because it is the only living representative of an old race. Since Prof. Lankester published in 1881 a famous paper entitled “Limulus an Arachnoid,” it has been generally, though not unanimously, recognised that the King-crab’s relationships among modern animals are with Arachnoidea, not with Crustacea. The hard, horseshoe - shaped, chitinous cephalothoracic shield is vaulted, but the internal cavity is much smaller than one would at first sight suppose ; the well-defined abdomen shows some hint of being divisible into rneso- and metasoma ; the long, sharp spine is (like the scorpion’s sting) a post-anal telson. On the concave under-surface of the cephalothorax there are six (or seven) pairs of limbs, as in spiders and scorpions — (i) A little pair of three-jointed chelicerae in front of and bent towards the mouth. (They are chelate in the female, simply clawed in the male. ) (2-6) Five pairs of six-jointed walking legs, the bases of which surround the mouth, and help in mastication. The last of these ends in twro flat plates, v'hich help in digging. The others are usually chelate, except the first in the male. (7) Then follow's on the abdomen a double “operculum” over- lapping the rest. The genital apertures lie on its posterior surface. Some refer this operculum to the cephalothorax. THE KING-CRAB. 333 (S-12) Under the operculum lie five pairs of fiat plates bearing remarkable respirator}' organs (“ gill - books ”). These appendages show hints of the exopodite and endopodite structure characteristic of Crustaceans. At any rate in the young they serve also as swimming organs. As in the scorpion, there is an internal skeletal structure, or endo- sternite, lying between the gullet and the nerve-ring, serving for the attachment of muscles. It should be noted, however, that an analogous structure occurs in Apus and some other Crustaceans. The nervous system. — The supra-oesophageal brain gives off nerves to the eyes. United to the brain are two ganglionated and transversely connected commissures forming a long oval oesophageal ring, giving off nerves to the limbs, and continued into a ganglionated abdominal cord. Ensheathing ring, ventral cords, and some of the nerves, are numerous blood vessels. There are two “compound” eyes lying towards the sides of the cephalothoracic shield, and in front of these two more median simple eyes. The compound eyes are cov- ered by a layer of chitin continuous with that of the shield, and the various eye elements are so remark- ably distinct from one another that the eye might be called a group of simple eyes. The food, canal. — Worms and the like, seized by some of the pincers, are partly masticated by the bases of the five posterior cephalothoracic legs. The mouth leads into a suctorial pharynx, with chitinous folds ; thence the fore-gut Fig. 143. — I.imulus or King-crab, bends upwards and forwards into a ciielicera: ; op, operculum ; a., crop. Separated from this by a valve anus, is the mid-gut, which extends along the cephalothorax and abdomen, and in the former bears two pairs of large yellow hepato-pancreatic outgrowths. The hind-gut is short, and ends in front of the base of the spine. Two large reddish glands in the cephalothorax open in young forms at the bases of the fifth appendages. They also open internally, and may be compared with the coxal glands of spider and scorpion, with the shell gland of Entomostraca, and with nephridia (?). The vascular system.—' The heart lies within a pericardium, and is partially divided into eight chambers, with eight pairs of valved 334 ARACHNOIDEA AND PAL/E0S7 RAC A. oslia. Haemocyanin is present as usual as the respiratory pigment of the blood, and there are oval corpuscles. From an anterior aorta, like that of the scorpion, two vessels are given off which bend backward, unite with lateral arteries from each chamber of the heart, and form a collateral vessel on each side of the heart. These unite in a posterior dorsal artery. From the anterior aorta two other branches unite in a ring around the nerve-collar, which gives off vessels to the limbs, and is continued backwards around the nerve-cord. From capillaries the blood is gathered into a ventral venous sinus, whence it passes to the respiratory organs, and thence to the pericardium and heart. The respiratory organs or gill-books are borne by the last five appendages. Each looks like a much-plaited gill, or like a book with over a hundred hollow leaves. The leaf-like folds are externally washed by the water, and within them the blood flows. The leaves of the gill-books are compared to the leaves of the lung-books of scorpions. If this homology is correct, the gill-books are evaginations, the lung-books in- vaginations, of the skin. The reproductive system. — The males are smaller than the females. The testes are very diffuse, the two vasa deferentia open on the internal surface of the operculum, and the spermatozoa, which are vibratile, are shed into the water. The ovaiies form two much-branched but con- nected sacs ; the oviducts are separ- ate, and enlarge before they open beneath the operculum. Spawning occurs in the spring and summer months. The ova and sper- matozoa are deposited in hollows near high-water mark. Some of the early stages of development, still imper- ectly known, present considerable resemblance to corresponding stages in the scorpion. In the larvae, both cephalothorax and abdomen show signs of segmentation, but this disappears. The spine is represented only by a very short plate, and the larva presents a striking superficial resemblance to a Trilobite. It seems likely that Limulus is linked to the extinct Eurypterids by some fossil forms known as Hemiaspidse, e.g. Hemiaspis, Belinurus. Fig, 144. — Young Limulus. After Walcott. Order 2. Eurypterina ( = Merostomata), e.g. Eurypterus, Gigantic extinct forms found from Ordovician to Carboniferous strata. The body is divided into head, thorax, and abdomen. The head is small and unsegmentcd. The thorax is composed of six distinct seg- ments, the abdomen of six with a terminal telson, which was sometimes a pointed spine, sometimes paddle-shaped. There is, however, some doubt as to the exact nomenclature of the regions. On the head are T KILOBIT A. 335 borne six pairs of appendages of varying shape, two lateral compound eves and two median ocelli. On the ventral surface of the thorax there are five pairs of gills covered by flat plates of which the most anterior pair are very large, and form the so-called operculum (cf. Limit lus). The surface of the body was covered with scales, borne of the Eurypterids reached a length of 6 ft. This order is sometimes placed near the Crustacea, but the geneia opinion seems to be that which links them through Limit bus to Arachnoids. Order 3. TrilOBITA. Trilobites, e.g. Calymene, Phacops, Asaphus. Extinct forms chiefly found in Cambrian and Ordovician strata, but extending up to the Carboniferous. The body as found is divisible into Fig. 145.— Trilobite (Couocephalites).— After Barrande. h.s., Head shield ; //., pleura of thoracic region ; py„ pygidium. three parts — the unsegmented head shield, often prolonged backwards at the angles ; the flexible thorax of a varying number of segments ; the unsegmented abdomen or pygidium. A median longitudinal ridge, or rachis, divides the body into three longitudinal portions. Traces of limbs are only rarely preserved. In the head region there are four pairs, apparently simple. Antennce have been recently found in this region. The thorax and abdomen are furnished with biramose appendages with long-jointed cndopodite, short exopodite, and a gill (or epipodite?) of varying shape. In the abdominal region the gills were perhaps rudimentary. Trilobites arc often found rolled up in a way that reminds one ol some wood-lice. So abundant are they in some rocks, that even t.heii development has been studied with some success. 336 ARACHN01DEA AND PALAEOSTRA CA. The limbs seem to be more Fig 146. — Vertical cross-section of a Trilobite ( Calymene ). — After Walcott. i., Intestine ; X., shield ; L endopodite ; c., exo- podite ; b., epipodial parts. like those of Crustaceans than those of Arachnoids, and the t occurrence of antennx, observed by Linnxus (I7S9)>and recently cor- roborated, accentuates the resemblance. The affinities with Limulus, according to the views of other authorities, justify the association of Trilobites and Arach- noids. A compromise may be perhaps effected by regarding the Trilo- bites as an offshoot from a stock ancestral to both Arachnoids and Crustaceans. Incertce Sedis. PANTOPODA OR PYCNOGONID7E. These are marine Arthropods, sometimes called sea-spiders. Their affinities are uncertain, but perhaps they may be ranked between Crustaceans and Arachnoids. Many climb about sea- weeds and hydroids near the shore, but some live at great depths. The body con- sists of an anterior proboscis, a cephalothoracic region with three fused and three free seg- ments, and an unsegmented rudimentary abdomen. There are typically seven pairs of ap- pendages. Of these the first are short and chelate, but may be absent in the adult. The next two are small and slender, and are often absent in the adult female ; the second pair may also be absent in the male, but the third in the males of all genera carries the eggs. The last four pairs of appendages are always present, and form the exceedingly long walking legs. Into them, and into the chelicerce when these are present, outgrowths of the mid-gut extend. The sexes are separate. The larvae are at first unsegmented, with three pairs of appendages. Examples. — Pycnogomun , Nymphon, Ammothea. Fig. 147. — Sea-spider ( Pycnogonum lit/ora/e), from the dorsal surface. The first two pairs of appendages are ab- sent. In the anterior region are four simple eyes. CHAPTER XVI. MOLLUSCA. Classes I. Amphineura — A small class of bilaterally symmetrical forms, e.g. Chiton. 2. Gasteropoda, e.g. Snails. 3. Scapho- poda — A small class, e.g. Dentalium. 4. Lamellibranchiata — Bivalves. 5. Cephalopoda — Cuttle fishes. The series of Molluscs is in many ways contrasted with that of Arthropods ; thus the body of the Mollusc is unsegmented, and there are no appendages. The general habit of life is also very different, for, although there are active Molluscs and sluggish Arthropods, it is true as an average statement that Molluscs are sluggish and Arthropods are active. The pedigree is unknown, but there does not seem to be any possible ancestry for Molluscs less remote than the stock from which Turbellarians and other unsegmented “worms” have sprung. General Characters. Molluscs are unsegmented and without appendages. The symmetry is fundamentally bilateral , but this is lost in most Gasteropods. The “foot" — a muscular protrusion of the ventral surface — is very characteristic ; it usually serves for locomotion, but is much modified according to habit. Typically , a projecting dorsal fold of the body-wall forms a mantle, or pallhim (Fig. 148, c.), which often secretes a single or bilobed shell covering the viscera, and roofs in a space — the mantle cavity — within ivhich lie the gills. But both mantle and shell may be absent. There are three chief pairs of ganglia — cerebrals, pedals, and pleurals — with connecting commissures, and often with accessory ganglia, especially two viscerals on a loop connecting the pleurals (Figs. 148, 152). Except in Lamelli- 22 33S MOLLUSC A. branchs , in which the head region is degenerate , there is in the mouth a chitinous ribbon or radula , usually bearing numerous Fig. 148. — Ideal mollusc. — After Ray Lankester. vi., Mouth ; g. c. , cerebral ganglia; c., edges of mantle skirt ; c.g., duct of right lobe of digestive gland ; s., pericardial cavity ; /., edges of shell-sac ; v., ventricle of heart ; u., nephridium ; an. anus ; posterior part of the foot ; opening of nephridium ; k., genital aperture; g.ab., abdominal ganglion on visceral loop ; g.v., visceral ganglion ; z.l., left lobe of digestive gland ; foot ; g.pe ., pedal ganglion ; g.pl., pleural ganglion. small teeth , and moved by special muscles , the whole structure being known as the odontophore. A portion of the true body cavity or coelom usually persists as the pericardium at least Fig. 149. — Stages in Molluscan development. D, Larva of Heteropod (after Gegenbaur); s/i. , shell covering visceral hump ; v., velum ; /., foot. p Larva of Atlanta (after Gegenbaur); v., velum; sit., shell; f.t foot; op., operculum. (Fig. 148, s.), and communicates with the exterior through the nephridium or nephridia. The vascular system is almost always well developed , but part of the circulation is in most cases through ill-defined spaces or lacuna; the heart typically THE SNAIL. 339 consists of a ventricle and two auricles. Respiratory organs are most typically represented by a pair of vascular processes of the body-wall (ctenidia or gills), but one or both of these may be absent. At the base of the gills there is generally an olfactory organ or osphradium. The sexes are separate or united. There are two common larval stages, — the Trocho- sphere, which resembles the same stage in some Annelids, and the more characteristic Ve/iger (Fig. 149); but the develop- ment is often direct. The Mollusca form a very large group, exhibiting much diversity of habit. First Type of Mollusca. The Snail (Helix), one of the terrestrial (pulmonate) Gasteropods. Habits. — The common garden snail (H. aspersa), or the larger edible snail (H. pomatia), which is rare in England but abundant on the Continent, serves as a convenient type of this large genus of land-snails. They are thoroughly terrestrial animals, breathing air directly through a pulmon- ary chamber, and drowning (slowly) when immersed in water. Their food consists of leaves and other parts of plants, but they sometimes indulge in strange vagaries of appetite. They are hermaphrodite, but there is always cross-fertilisation. The breeding time is spring, and the eggs are laid in the ground. In winter snails bury them- selves, usually in companies, cement the mouths of their shells with hardened mucus and a little lime, and fall into a state of “latent life,” in which the heart beats feebly. They have been known to remain dormant for years. General appearance. — A snail actively creeping shows a well developed head, with two pairs of retractile horns or tentacles, of which the longer and posterior bear eyes. The foot, by the muscular contraction of which the animal creeps, is very large ; it leaves behind it a trail of mucus. The viscera protrude, as if ruptured, in a dorsal hump, which is spirally coiled and protected by the spiral shell. On slight provocation the animal retracts itself within its shell, a process which drives air from the mantle cavity, and thus helps indirectly in respiration. Around the mouth of the shell is a very thick mantle margin or collar, by which the continued growth of the shell is secured. On the right 340 MOLLUSC A. side of the expanded animal, close to the anterior edge of the shell, there is a large aperture through which air passes into and out of the mantle cavity. Within the same aperture is the terminal opening of the ureter. The food canal ends slightly below and to the right of the pulmonary aperture. All the three openings are close together. The anterior termination of ureter and food canal is one of the results of the twisting of the visceral mass fonvards to the right. But still further forward, at the end of a slight groove which runs along the right side of the neck, indeed quite close to the mouth, is the genital aperture. Lastly, an opening just beneath the mouth leads into the large mucus gland of the foot. Shell. — -The right-handed spiral shell is a cuticular product made and periodically enlarged by the collar. Chemically it consists of carbonate of lime and an organic basis (conchiolin). The outermost layer is coloured, with- out lime, and easily rubbed off; the median layer is thickest, and looks like porcelain ; the innermost layer is pearly. The twisted cavity of the shell is continuous, and the viscera extend to the uppermost and oldest part. As the shell is gradually made, the inner walls of the coils form a central pillar (columella), as on a staircase, and to this the animal is bound by a strong (columellar) muscle. Many Gasteropods bear a horn-like shell-lid (operculum) on the foot, but Helix has none ; the “ epiphragm ” with which the shell is sealed in winter, consists of hardened mucus, plus phosphate and a smaller quantity of carbonate of lime. It is formed very quickly from the collar region when cold weather sets in, has no organic connection with the animal, such as binds its operculum to the foot of the whelk, and is loosened off in the mildness of spring. Appearance after the shell is removed. — If the shell is removed carefully, so that nothing is broken except the columellar muscle, many structures can be seen without any dissection. The skin of the head and foot should be con- trasted— (a) with the thick collar of the mantle ; (h) with the mantle itself, which forms the loose roof of the pulmonary chamber ; ( c ) with the exceedingly delicate, much stretched, and always protected skin of the visceral hump. The mantle is a downgrowth of the skin of this dorsal region. It is peculiar in the snail, in that its margin (the collar) is fused to the body-wall. The result is to form a respiratory MUSCULAR AND NERVOUS SYSTEMS. 34i cavity, which is as much outside the body as is the gill- chamber of the crayfish. It is important to realise that the snail has an “ enlargement of the liver ” and a great rupture- like hump of viscera on the dorsal surface, that this has been coiled spirally, and that there is the yet deeper torsion forward to the right. A great part of the hump consists of the greenish brown digestive gland, in which the bluish intestine coils ; behind the mantle chamber, on the right, lies the triangular and greyish kidney ; the whitish reproductive organ lies in the second last and third last coil of the spiral. Skin. — This varies greatly in thickness. It consists of a single-layered epidermis and a more complex dermis, including connective tissue and muscle fibres. There are numerous cells from which mucus, pigment, and lime are secreted ; those forming pigment and lime are especially abundant on the collar, where they contribute to the growth of shell. Muscular system. — Among the important muscles are — (a) those of the foot ; (/>) those which retract the animal into its shell, and are in part attached to the columella ; (i*s ; in the middle that of Helix. In the last two the position of the gullet is shown. c.p., Cerebro-pleural ganglia; pedals; v., viscerals ; c., cere- brals ; //., pleurals ; b., buccals ; s., stellate ganglion. contains some coils of gut and the reproductive organs ; its lower region is very muscular. The protrusion or extension of this locomotor organ is mainly due to an inflow of blood, which is prevented from returning by the contraction of a sphincter muscle round the veins. In moving, the animal literally ploughs its way along the bottom of the pond or river pool, and leaves a furrow in its track. The muscle fibres, as in the snail, are of the slowly contracting non- striped sort. Nervous system. — There are three pairs of nerve- centres ; — - 35° MOLLUSC A. (a) Cerebro-pleural ganglia , lying above the mouth on each side on the tendon of the anterior retractor of the foot, connected to one another by a commissure, connected to the two other pairs of ganglia (b) and (c), by long paired connect- ives, and giving off some nerves to mantle, palps, etc. (b) Pedal ganglia, lying close together about the middle of the foot, united by connectives to (a), giving off nerves to the foot, and having beside them two small ear-sacs, each with a calcareous otolith, and with a nerve said to be derived from the connective between (a) and (b). (r) Ji'sceral ganglia (also called parieto-splanchnic or osphradial), lying below the posterior adductor, connected to ( a ) by two long connectives, and giving off nerves to mantle, muscles, etc., and to a patch of “ smelling cells ” at the bases of the gills. Sense organs. — Unlike not a few bivalves, which have hundreds of “ eyes ” on the mantle margin, Anodonta has no trace of any. The ear-sac, originally derived from a skin- pit, is sunk deeply within the foot, and is of doubtful use. The “smelling patch” or “ osphradium” at the base of the gills, has perhaps water-testing qualities. There are also “tactile” cells about the mantle, labial palps, etc. Alimentary system. — The mouth lies between the anterior adductor and the foot, and beside it lie the ciliated, vascular, and sensitive labial palps, two on each side. It opens immediately into the gullet, for the pharynx of other Molluscs, with all its associated structures, is absent in Lamellibranchs. The short wide gullet leads into a large stomach surrounded by the paired digestive gland, equivalent to that of the snail. Part of the food digested by these juices in the stomach is compacted in autumn into a “crystalline style” — a mass of reserve food stuffs, and similar but less solid material is found in the intestine. On this supply the mussel tides over the winter. Some author- ities, however, maintain that the style is a glandular secretion, protecting the lining of the gut from injury. Similar structures are found in several Gasteropods. The VASCULAR SYSTEM. 35* intestine, which has in part a folded wall like that of the earthworm coils about in the foot, ascends to the peri- cardium, passes through the ventricle of the heart, and ends above the posterior adductor at the exhalant orifice. Vascular system. — The heart lies in the middle line on Fig. 153. — Structure of Anodonta. — After Rankin. a.n., Anterior adductor; c.p.g., cerebro-pleural ganglia; st. , stomach; v., ventricle, with an auricle opening into it; X\, kidney, above which is the posterior retractor of the foot ; r., rectum ending above posterior adductor; v.g., visceral ganglia with connectives (in black) from cerebro-pleurals gut coiling in foot ; p.g. , pedal ganglia in foot, where also are seen branches of the anterior aorta and the reproductive organs ; /./*., labial palps behind mouth. At the posterior end the ex- halant (upper) and inhalant (lower) apertures are seen. the dorsal surface, within a portion of the body-cavity called the pericardium, and consists of a muscular ventricle which has grown round the gut and drives blood to the body, and of two transparent auricles — one on each side of the ventricle — which receive blood returning from the gills and mantle. In bivalves the heart-beats average about twenty per minute, much less than in Gasteropods. The colour- 352 MOLLUSC A. less blood passes from the ventricle by an anterior and a posterior artery ; flows into ill-defined channels ; is collected in a “vena cava” beneath the floor of the pericardium; passes thence through the kidneys, where it loses nitrogenous waste, to the gills, where it loses carbonic acid and gains oxygen ; and returns finally by the auricles to the ventricle. The blood from the mantle, however, returns directly to the auricles without passing through kidneys or gills, but probably freed from its waste none the less. The so-called “ organ of Keber ” consists of “ pericardial glands” on the epithelium of the pericardial cavity. They seem to be connected with excretion. Many of the cells lining the blood channels secrete glycogen, the principal product of the Vertebrate liver. Respiratory system. — Lying between the mantle flaps and the foot there are on each side two large gill-plates, whence the title Lamellibranch. They are richly ciliated ; their internal structure is like complex trellis work ; their cavities communicate with the supra-branchial chamber. As in many other molluscs, the gills or ctenidia are not merely surfaces on which blood is purified by the washing water-currents (a respiratory function), but some of their many cilia waft food-particles to the mouth (a nutritive function), and in the females the outer gill-plate shelters and nourishes the young larvae (a reproductive function). The water may pass through the gills to the supra-branchial chamber and thence out again, or over the gills to the mouth, and thence into the supra-branchial chamber. It is likely that the mantle has no small share in the respira- tion. The precise structure and attachment of the gill-plates is complex, but it is important to understand the following facts : — ( a ) A cross section of the two gill-plates on one side has the form of a W, one half of which is the outer, the other the inner gill-plate ; ( b ) each of these gill-plates consists of a united series of gill filaments, which descend from the centre of the W and then bend up again ; [c) adjacent fila- ments are bound together by fusions and bridges both horizontal and vertical, so that each gill-plate becomes like a complex piece of basket work ; (d) both gill-plates begin by the downward growth of filaments from a longitudinal “ ctenidial axis,” the position of which on cross- section is at the median apex of the W ; ( e ) this mode of origin, and the much less complex gills of other bivalves, lead one to believe that there is on each side one gill, consisting of two gill-plates formed from a series DEVELOPMENT AND LIFE HISTORY. 353 of united and reflected gill filaments. On the gills there are often parasitic mites ( Atax ). Excretory system. — The paired kidney, which used to be called the “organ of Bojanus,” lies beneath the floor of the pericardium. Each half is a nephridium bent upon itself, with the loop posterior, the two ends anterior. The lower part of this bent tube is the true kidney ; it is dark in colour, spongy in texture, and excretes guanin and other nitrogenous waste from the blood which passes through it. It has an internal opening into the pericardium, which thus communicates indirectly with the exterior. The upper part of the bent tube, lying next the floor of the pericardium, is merely a ureter. It conveys waste products from the glandular part to the exterior, and opens anteriorly just under the place where the inner gill-plate is attached to the visceral mass. As already mentioned, the “pericardial glands ” probably aid in excretion, and possibly the same may be said of the mantle. The reproductive organs. — These lie in the upper part of the foot, adjacent to the digestive gland. Ovaries and testes occur in different animals, and the two sexes are distinguishable, though not very distinctly, by the greater whiteness of the testes and by slight differences in the shells. The females are easily known when the lame begin to accumulate in crowds in the outer gill-plates. The repro- ductive organs are branched and large ; there are no accessory structures ; the genital aperture lies on each side under that of the ureter. The autumn and winter months seem to be the usual periods for reproduction. The ova pass from the ovaries in the foot, and appear to be moved to the exhalant region, whence, however, they do not escape, but are crowded backward till they pass into the cavity of the outer gill- plate. At some stage they are fertilised by sperma- tozoa drawn in by the water currents, though it is difficult to believe that this is entirely a matter of chance. Development takes place within the external gill-plate, and the larvte feed for some time on mucus secreted by the gill. Development and life history.— The development of Anodonta differs in certain details from that of most bivalves, perhaps in adapta- 354 MOLLUSC A. tion to fresh-water conditions. Moreover, a temporary parasitism of the larva has complicated the later stages. The egg cell is surrounded by a vitelline membrane, and attached to the wall of the ovary by a minute stalk, the insertion of which is marked on the liberated ovum by an aperture or micropyle, through which the spermaiozoon enters. Segmentation is total but unequal. A number of small clear yolkless cells are rapidly divided off from a large yolk-containing portion, which I Fig. 154.- — Development of Aiiodonta. — After Gcette. 1. Section of blastosphere. s.d., Shell gland : c.if., ciliated disc ; e., beginning of ectodermic invagination. Note mesoderm cells in the cavity. z. Later stage, in., Mesoderm. 3. Embryonic shell has appeared. 4. Glochidium larva ; note byssus threads, and teeth on shell valves. is slower in dividing. Eventually a hollow ball o. cells or blasto- sphere results (Fig. 1 54)- On the posterior dorsal region a number of large opaque cells form an internally convex plate, — the beginning of the future shell-sac. A pair of large cells are intruded into the central cavity, and begin the mesoderm. t On the under surface posteriorly there is a slight protrusion of ciliated cells forming a ciliated disc. In front of this, at an unusually late stage, an invagination establishes the archenteron, and the embryo becomes a gastrula (see Fig. 1 54)- The shell-sac forms an embryonic shell, and many of the mesodeim SEPIA. 355 cells combine in an adductor muscle. The mouth of the gastrula closes, and a definite mouth is subsequently formed by an ectodermic invagina- tion. Gradually a larva peculiar to fresh-water mussels, and known as a Glochidium, is built up. The Glochidium has two triangular, delicate, and porous shell valves, each with a spiny incurved tooth on its free edge. The valves clap together by the action of the adductor muscle. The mantle lobes are very small, and their margins bear on each side three or four patches of sensor}’ cells. The foot is not yet developed, but from the position which it will afterwards occupy there hang long attaching threads of “ byssus,” which moor the larva. If it manage to anchor itself on the tail, fins, or gills of a fish, the Glochidium shuts its valves and fixes itself more securely, and is soon surrounded by a pathological growth of its host’s skin. In this parasitic stage a remarkable metamorphosis occurs. The sensory or tactile patches not unnaturally disappear ; the byssus and the embryonic byssus glands vanish, but a new byssus gland (which remains quite rudimentary in Anodonta) appears ; the single adductor atrophies, and is replaced by two ; the foot and the gills make their appearance ; the embryonic mantle lobes increase greatly, or are replaced by fresh growths ; and the permanent shell begins to be made. After this metamorphosis, when the larva has virtually become a miniature adult, no longer so liable to be swept away, it drops from its temporary host to the bottom of the pond or river pool. Third Type of Mollusca. The Common Cuttlefish {Sepia officinalis ), one of the Dibranchiate Cephalopods. Habits. — This common cuttlefish is widely distributed, especially in warmer seas like the Mediterranean. Unlike Octopus , which usually lurks passively, Sepia is an active swimmer ; it moves head foremost by working the fins which fringe the body, or it jerks itself energetically back- wards by the outgush of water through the funnel. It likes- the light, and is sometimes attracted by lanterns. The beautiful colours change according to external condi- tions and internal emotions ; and a plentiful discharge of ink often covers its retreat from an enemy. Its food includes fish, other molluscs, and crabs. In spring the female attaches her encapsuled eggs to sea-weeds and other objects, and often comes fatally near the shore in so doing. The cuttles are caught for food and bait. The “cuttle bone ” and the pigment of the ink-bag are sometimes utilised by man. External appearance. — A large Sepia measures about io in. in length and 4 to 5 in breadth; the body, fringed 356 MOLLUSC A. by a fin, is shaped like a shield, the broad end of which bears a narrowed head, with eight short and two long sucker-bearing arms. Besides the diffuse pigment cells, there are bands across the “back.” The large eyes, the parrot-beak-like jaws protruding from the mouth, the spout- like funnel on the neck, and the mantle cavity, are con- spicuous. Beside the eyes are the small olfactory pits ; anatomical strictness the dorsal surface tilted backwards. (As above noticed, the animal may also swim with foot and mouth in front.) The side of lighter colour, marked by the mantle cavity and the siphon or funnel, is posterior and slightly ventral ; the banded and more convex side, on which the cerebral ganglia lie in the head region, and on which the shell lies concealed in the visceral region, is anterior and slightly dorsal. Skin. — There are numerous actively changeful pigment within the mantle cavity lie the anus and the openings of the nephridia and genital duct. Fig. 155. — External Appearance of a cuttlefish. The true orientation of the different regions in Sepia is not obvious. If the “arms” surrounding the mouth be divided portions of the an- terior part of the “foot,” the ventral surface is that on which the animal rests when we make it stand on its head. We can fancy how the “ foot” of a snail might grow forward and surround the mouth, so as to bring that into the middle of the sole. Then the visceral mass has been elongated in an oblique dorso-posterior direction, so that the tip of the shield, directed forward when the cuttle jerks itself away from us, represents in SKELETAL SYSTEM. 357 cells or chromatophores lying in the connective tissue beneath the epidermis. Each cell is expanded by the contraction of muscular cells which radiate from it, and con- tracts when these relax, it is probable that these chromato- phore cells have some protoplasmic spontaneity of their own, but the controlling muscular elements are also affected by nervous impulses from the central ganglia. As the cells dilate or contract, the pigment is diffused or concentrated, and the colours change. The animal’s beauty is further enhanced by numerous “iridocysts” or modified connective tissue cells, with fine markings which cause iridescence. Muscular system. — The cuttlefish is very muscular, notably about the arms, the mantle flap, and the jaws. With great quickness it seizes its prey by throwing out its two long arms, which are often entirely retracted within pouches. With great force it jerks itself backwards by contracting the mantle cavity, and making the water gush out through the pedal funnel. This mode of locomotion is very quaint. At one time the mantle cavity is wide, and you can thrust your fingers into its gape ; when about to contract, this gape is closed by a strange double hook-and- eye arrangement ; contraction occurs, and the water, no longer free to leave as it entered, gushes out by the funnel, the base of which is within the mantle cavity. Another muscular development is interesting, that of the suckers on the arms. They are muscular cups, borne on little stalks (unstalked in Octopus , etc.), well innervated, and able to grip with a tenacity which in giant cuttlefish is dangerous even to men. The inner edge of the cup margin is sup- ported by a chitinoid ring bearing small teeth. Each cup acts as a sucker, in a fashion which has many analogues, for a retractor muscle increases the size of the cavity after the margin has been applied to some object. The external pressure is then greater than that within the cup, and the little teeth keep the attachment from slipping. It seems likely that the arms represent a propodium, and the siphon a mesopodium, and a valve within the siphon has been compared to a metapodium. Skeletal system. — An internal skeleton is represented by supporting cartilaginous plates in various parts of the body, especially — ( a ) in the head, round about the brain, arching 35S MOLL USC A. over the eyes, enclosing the “ ears ” ; (/>) at the bases of the arms ; (c) as a crescent on the neck ; (d) at the hook-and- eye arrangement of the mantle flap ; ( e ) along the fringing fins. Ramified “ stellate ” cells lie in the structureless transparent matrix of the cartilage. On the shore one often finds the “ cuttle bone ” or sepio- staire, which is sometimes given to cage birds to peck at for lime, or used for polishing and other purposes. It lies on the dorsal side of the animal, covered over by the mantle sac. In outline it is somewhat ellipsoidal, thinned at the edges like a flint axe-head, and with curved markings which indicate lines of growth. In the very young Sepia it con- sists wholly of the organic basis conchiolin, but to this lime is added from the walls of the sac. Between the plates of lime there is gas, and though the structure may give the cuttle some stability, it is probably of more use as a float. Internal appearance. — When the mantle flap is cut open and reflected, the two plume-like gills are seen, and the lower end of the siphon. The dark outline of the ink-bag, followed along towards the head, leads our eyes to the end of the food canal. Near this are the external apertures of the two kidneys and of the genital duct. On each side of the base of the funnel lies a very large and unmistakable “stellate” ganglion. Removing the skin as carefully as possible over the whole visceral region between the gills, and taking precautions not to burst the ink-sac, we see the median heart, the saccular kidneys, contractile structures or branchial hearts at the base of each gill, and the essential reproductive organs near the apex of the visceral mass. Disturbing the arrangement of these organs, we can follow the food canal, with its stomach, digestive gland, etc. Nervous system. — Three pairs of ganglia surround the gullet, — cerebral on the dorsal and anterior side, pedal and pleuro-visceral on the ventral and posterior side (Fig. 152), but lying so close together that their boundaries are defined with difficulty. All are well protected by the investing cartilages. The cerebral ganglia are three-lobed, and are connected anteriorly by two commissures with a “ supra-pharyngeal ” ganglion, which gives of! nerves to the mouth and lips, and is connected also with an “ infra- pharyngeal ” ganglion. The cerebral ganglia are also connected by ALIMENTARY SYSTEM. 359 short double commissures with the pedals and pleuro-viscerals on the ventral side of the gullet. The pedal ganglia at each side are in part divided into two, — one half forming the brachial ganglion which sends nerves to the arms, the other the infundibular which supplies the funnel. The following chief nerves arise from the central system : — (1) The very thick optic nerves are given off from the commissures between cerebrals and pleuro-viscerals, and lead to a large optic ganglion at the base of each eye. (2) Ten nerves to the “ arms ” are given off by the pedal ganglion, and this is one of the reasons which have led most morpho- logists to regard these arms as portions of the “foot.” (3) Two large nerves from the more ventral portion of the pleuro- visceral ganglia form a visceral loop, and give off many branches to the gills and other organs. From the pleural portion arise two mantle nerves, each of which ends in a large stellate ganglion. Sense organs.- — The eyes are large and efficient. They present a striking resemblance to those of Vertebrates, and, as they are not “ brain eyes,” they illustrate how superficially similar structures may be developed in different ways and in divergent groups. In cuttlefishes the eyes lie on the sides of the head, protected in part by the cartilage surrounding the brain, and in part by cartilages on their own walls. The eye is a sensitive cup arising in great part from the skin. Its internal lining is a complex retina, on the posterior surface of which the nerves from the optic ganglion are distributed. It seems likely that the Cephalopod retina corresponds only to the rods and cones (the sensory part) of the Vertebrate retina. In the cavity of the cup there is a clear vitreous humour. The mouth of the cup is closed by a lens, supported by a “ciliary body.” The lens seems to be formed in two parts — an outer and an inner plano-convex lens. The pupil in front of it is fringed by a con- tractile iris. The outer wall of the optic cup is ensheathed by a strong supporting layer — the sclerotic, which is in part strengthened by cartilage, covered by a silver}' membrane, and continued into the iris. In front of the eye there is a transparent cornea, and the skin also forms protecting lids. Round about the optic ganglion there is a strange “ white body,” which seems to be a fatty cushion on which the eye rests. The two ear-sacs, containing a spherical otolith and a fluid, sometimes with calcareous particles, are enclosed in part of the head cartilage, close to the pedal ganglia. The nerves seem to come from the pedals, but it is said that their fibres can be traced up to the cerebrals. A ciliated “ ol factor)' sac” lies behind each eye, and is innervated from a special ganglion near the optic. There are no osphradia of the usual type. Finally, there are tactile or otherwise sensitive cells on various parts of the body, especially about the arms. Alimentary system. — The cuttlefish eats food which 360 MOLLUSC A. requires tearing and chewing, and this is effected by the chitinous jaws worked by strong muscles, and by the toothed radula moving on a muscular cushion. The mouth lies in the midst of the arms, bordered by a circular lip, and opens into a large pharynx or buccal cavity (cf. the snail). The narrow gullet passes through the ganglionic mass, and leads into the globular stomach, lying near the dorsal end of the body. The stomach is followed by a caecum or pyloric sac, and the intestine curves head- wards again, to end far forward in the mantle cavity. There do not seem to be any glands on the walls of the food canal ; the stomach has a hard cuticle ; the digestion which takes place there must therefore be due to the digestive juices of the glandular appendages. Of these the most important is usually called the liver ; it is bilobed, and lies in front of the stomach, attached to the oesophagus. Its two ducts conduct the digestive juice to the region where the stomach, “py- loric sac,” and intestine meet ; and these ducts are fringed by numerous vascular and glandular appendages, which are called “ pancreatic,” and arise from a differentiated part of the digestive gland. Far forward, in front of the large digestive gland, lie two small white glands on each side of the gullet, with ducts which open into the mouth (cf. the “ salivary glands ” of the snail). A diastatic ferment has been proved in the salivary secretion of Cephalopods, but that of Octopus has a poisonous, paralysing effect on the crabs, etc., which are bitten, and also a peptonising action. At the other end of the food canal, the ink-sac, full of black pigment, probably of the nature of waste products, opens a., Eight short arms around mouth ; /.a., one of the two long arms; b. , beak of the mouth ; eg., cere- bral ganglia, with commissures to the others ; E., eye ; g., gullet; d.g., digestive gland (the “salivary glands" are not re- presented) ; st ., stomach ; a., anus ; sh. , shell-sac with sepio- staire ; k., kidney ; R., Repro- ductive organ ; br.li., branchial heart; g., a gill ; i.b., ink-bag; iit.c., mantle cavity ; funnel. EXCRETORY SYSTEM. 361 into the rectum close to the anus. This ink-sac is a much enlarged anal gland ; for, while most of the bag is made of connective tissue and some muscle fibres, a distinct gland is constricted off at the closed end, and the neck is also glandular. Beside the anus are two pointed papillae. Vascular system. — The blood of Sepia is bluish, owing to the presence of haemocyanin in the serum ; the blood cells are colourless and amoeboid. The median but some- what oblique ventricle of the heart drives the blood forward and backward to all parts of the body. It reaches the tissues by capillaries, and apparently also by lacunar spaces. The venous blood of the head region is collected in an annular sinus round the basis of the arms, and passes towards the heart by a large vena cava, which divides into two branchial veins, covered by spongy outgrowths of the nephridia. Joined by other vessels from the apical region of the viscera, each branchial vein enters a “ branchial heart” at the base of each gill. The branchial heart is contractile, and drives the venous blood through the gills, whence, purified, it returns by two contractile auricles into the ventricle. There are valves preventing back-flow from the ventricle to the auricles, or from the arteries to the ventricle. Beside each branchial heart lies an enig- matical glandular structure known as a “ pericardial gland,” possibly an excretory or incipiently excretory organ. The course of the blood differs from that in the mussel and snail in this, that none returns to the heart except from the respiratory organs. In the nephridial outgrowths around the branchial veins the interesting parasite Dicyema is found. Respiratory system. — The blood is purified by being exposed on the two feather-like gills which are attached within the water-washed mantle cavity. The water pene- trates them very thoroughly ; the course of the blood is intricate. At the base of the gills there is some glandular tissue, which those impatient with enigmas have credited with blood-making powers. Excretory system. — The excretory' system is difficult to dissect and to explain. On each side of the anus there is a little papilla, through which uric acid and other waste products ooze out into the mantle cavity, and so into the water. A bristle inserted into either of these 362 MOLLUSC A. two papillte leads into a large sac — the nephridial sac. But the two sacs are united by two bridges, and they give off an unpaired dorsal elongation, which extends as far back as the reproductive organs. The dorsal wall of each nephridial sac becomes intimately associated with the branchial veins, and follows their outlines faithfully. It is likely that waste material passes from the blood through the spongy appendages into the nephridial sacs. Into the terminal portion of each nephridial sac, a little below its aperture at the urinary papilla, there opens by a ciliated funnel another sac, which is virtually yy the body cavity. It \ LI surrounds the heart and ' “ ^25 ii other organs, and is AZ often called the viscero- pericardial cavity. Through the kidneys or nephridial sacs it is in communication with the exterior. R e p roductive system. — The sexes are separate, but there is not much external differ- ence between them, though the males are usually smaller, less rounded dorsally, and have slightly longer arms. When mature, the male is easily known by a strange modifica- tion on his fifth left arm. The essential Fig. 157. — Diagram of circulatory and excre- tory systems in a Decapod-like Sepia. — After Pelseneer. r, Gill ; 2, renal sac ; 3, afferent branchial vessel ; 4, branchial heart ; 5, abdominal vein ; 6, heart ; 7, viscero-pericardial sac (body cavity) ; 8, genital organ; 9, posterior aorta; 10, “auricle”; n, glandular appendage of branchial heart; 12, renal appendages of branchial vein ; 13, external aper- ture of kidney ; 14, vena cava; 15, anterior aorta ; 16, bifurcation of vena cava ; 17, reno-pericardial aperture. reproductive organs are unpaired, and lie in the body cavity towards the apex of the visceral mass. The testis — an oval yellowish organ — lies freely in a peritoneal sac, near the apex of the visceral mass. From this sac the spermatozoa pass along a closely-twisted duct — the vas deferens. This expands into a twofold “ seminal vesicle,” and gives off two blind outgrowths, of which one is called the “ prostate.” The physiological interest of these parts is that within them the spermatozoa begin to be arranged in packets. In this form they are found within the next region, the spermatophore sac, which opens to the exterior to the left of the anus. Each spermato- GENERAL NOTES ON MOLLUSCS. 363 phore is like an automatically explosive bomb ; within the transparent shell there lies a bag of spermatozoa, and a complex spring-like arrange- ment. Even on the scalpel or slide these strange but efficient bombs will explode. The liberated spermatozoa are of the usual sort. The ovary— a large, rounded white organ — lies freely in a peritoneal sac near the apex of the visceral mass. From this sac the eggs pass along a short direct oviduct, which opens into the mantle cavity to the left of the anus. Associated with the oviduct, and pouring viscid secretion into it, are two large “ nidamental glands,” of foliated struc- ture. Close beside these are accessor}' glands, of a reddish or yellowish colour, with a median and two lateral lobes ; while at the very end of the oviduct are two more glands. All seem to contribute to the external equipment of the egg. The spermatophores pass from the genital duct of the male to the fifth left arm, which becomes covered with them and quaintly modified. This is usual among cuttlefish ; indeed, in some, e.g. Argonauta and Tremoctopus , the modified arm, with its load of spermatozoa, is dis- charged bodily into the mantle cavity of the female. There its discoverers described it as a parasitic worm, “ Hectocoty/us The lost arm is afterwards regenerated. In Sepia, however, the modified arm is not discharged, but is simply thrust into the mantle cavity of the female. The spermatophores probably enter the oviduct, and burst there. The eggs, when laid, are enclosed within separate black capsules containing gelatinous stuff, but the stalks of the capsules are united, so that a bunch of “ sea-grapes ” results. General Notes on Molluscs. From the description of these three types a general idea of the structure of Mollusca may be obtained, but it should be noted — (1) that all the three types are specialised, the mussel in the direction of degeneration ; (2) that two small classes, the Amphineura and the Scaphopoda, are unre- presented in the descriptions ; (3) that in the three classes to which the types belong there is much diversity of struc- ture, this being especially true of the large and heterogeneous class of Gasteropods. In surveying the structure of the whole group, it is con- venient to begin with the most striking of the external characters — the absence or presence of a well-developed head region. In the Lamellibranchs or Pelecypoda the head is absent, and along with it the tentacles, the radula, and the pharynx with all its associated structures. Elsewhere a head region, usually furnished with tentacles and eyes, and con- taining within it a pharynx and radula, is always present. 364 MOLL USC A. Best developed in Gasteropods and Cephalopods, the head region may elsewhere be represented, as in Denta/ium , merely by a buccal tube fringed with tentacles. Apart from Lamellibranchs, the radula is characteristic and, with few exceptions, universal. Almost as important is the condition of the characteristic Molluscan foot. Primitively this had the form of a ventral creeping sole, as shown, for example, in its simplest condition, in Chiton (Fig. 165). This condition is retained in many Gasteropods, and in the simplest Lamellibranchs, like Sole- notnya. In most Lamellibranchs, however, in adaptation to a more or less passive life in the sand, the foot became wedge- FiG. 158. — Common Buckie ( Buccinum undatum), c., Eye ; s., respiratory siphon ; o., operculum ;/!, foot. shaped, and the characteristic byssus gland, which secretes attaching threads, is developed. In the active cuttles the foot became greatly modified, and in those related to Sepia a portion of it is specialised as the funnel — the main organ of active locomotion. That the condition of the foot cannot in itself be employed as a basis of classification, is, however, obvious, when its differences within the limits of a class are considered. Thus it is obsolete in the pelagic Phyllirhoe among Gasteropods, in the sedentary oyster among Lamelli- branchs ; in the pelagic Pteropods part of it forms lateral wing-like lobes used in swimming, while in Ianthina , which has a similar habit, its chief use is to secrete a “float” to which the egg-capsules are attached. In various Lamelli- GENERAL NOTES ON MOLLUSCS. 365 branchs, and in Dentalium , it is modified as a conical boring organ. The mantle is another important Molluscan structure, and as it secretes the shell, the shape of the latter is of course determined by it. Primitively the mantle is repre- sented by a uniform downgrowth of skin from the dorsal surface, surrounding the ventral foot, and secreting a dorsal cap-shaped shell. Such a simple condition occurs in the limpet. In the Lamellibranchs, with the lateral flattening of the body, the mantle becomes divided into right and left halves, and the shell becomes two-valved. In most Lamelli- branchs the mantle is prolonged into two tubes or siphons, through which the water of respiration enters and leaves the . e Fig. 159. — Bivalve [Pcinopcca norvegica ), showing siphons. c., Exhalant aperture ; inhalant aperture. mantle cavity. A similar but unpaired siphon is found in many Gasteropods. In Scaphopoda the mantle folds fuse ventrally to form a continuous tube. In most Gasteropods the mantle skirt is retained, and secretes a spiral shell, as well as enclosing a space in which the gills lie ; in some, both mantle and shell are absent. In the snail and its allies (Pulmonata), the mantle fuses with the body-wall and forms the pulmonary chamber, which opens to the anterior by a small aperture. In Cephalopoda the mantle skirt is well developed and muscular, and, besides sheltering the gills, is of much importance in locomotion. Typically the Mollusca are bilaterally symmetrical animals, and this symmetry is marked in the Amphineura, the Lamellibranchiata, and occurs to a less extent in the Cephalopoda (cf. the unpaired genital organs). In the 366 MOLLUSC A. Gasteropoda it is completely lost. This seems to be in some way associated with the dorsal displacement of the Fig. 160. — Nudibranch ( Dendronotus arborescens), showing dorsal outgrowths forming adaptive gills. viscera in Gasteropods to form the (usually coiled) visceral hump. In Cephalopods there is a somewhat similar dis- placement in a postero-dorsal direction, in Lamellibranchs in a ventral direction, but in neither case is it so marked as in Gasteropods. The characters of the inter- nal organs of Mollusca must be gathered from the description of the types, but the nature of the respiratory organs may be briefly noted. Typically, these consist of two feathery gills, sheltered beneath the mantle, and bearing at their bases two Fig. i6i.— Ventral surface of osphradia or smelling patches, and Hanley. Gills of this typical form occur Note simple eyes at base of tentacles, in Cuttles ( Nautilus has four), in mouth, median foot, and vascular the simplest Gasteropods (but margin of mantle replacing the f ^ j 1 absent gills. many other Gasteropods have a simple unpaired gill), and in the lowest Lamellibranchs ( So/enomya , Nucula , etc.). The respir- atory organs in other Mollusca show much variation when compared with this primitive type. Thus the gills may be SHELL. 367 totally suppressed and the mantle may directly take on a respiratory function. This occurs in many marine Gastero- pods, for example, in the common limpet ( Patella ) (Fig. 1 61), as well as in terrestrial forms like the snail, where the mantle cavity forms the pulmonary chamber. Even in Lamellibranchs, where the gills are present in much modified form, it is probable that the mantle has much importance in respiration, the gills being perhaps of most importance in connection with nutrition, and as brood-chambers. In those Gasteropods in which the gills are suppressed, there are often special respiratory organs (“ adaptive gills ”), such as the circle of plumes around the anus in Doris and its allies (Fig. 160). The osphradia are absent in Cephalopods, except in Nautilus, and one at least is usually suppressed in Gasteropods. Shell. — Mollusc shells are very beautiful, alike in form and colour. They grow larger by month and year, and mark their progress by rings of growth and changing tints. That they afford their bearers efficient protection, is shown by the appreciation which some hermit crabs exhibit for stolen whelk or buckie shells. More precise observation shows us that the shell consists in great part of carbonate of lime ; that it has a thin outer “ horny ” layer, 'a thick median “ prismatic ” stratum of lime, and an internal mother- of-pearl layer. On the dorsal surface of almost every mollusc embryo there is a little shell-sac in which an embryonic shell is begun ; the adult shell, however, begins on a separate area of the skin, and it is always lined and increased by the mantle. If the increase of the shell be carefully watched in young Molluscs, or if chemical analysis be made, it becomes plain that the shell is no mere deposi- tion of carbonate of lime. Like other cuticular products, it has an organic basis (conchiolin or conchin), along with which the lime is associated. Mr. Irvine’s experiments at Granton Marine Station suggest that the lime salt originally absorbed is not the carbonate (of which there is a scant supply in sea water), but the sulphate (which is abundant), and that the internal transformation from sulphate to carbonate is perhaps associated with the diffuse decomposition of nitrogenous waste products. Thus carbonate of ammonia, which seems to occur abundantly in the mantle of perfectly fresh mussels, would, with calcium sulphate, yield carbonate of lime and ammonium sulphate. One cannot suppose that 368 MOLLUSC A. shell-making is expressible in a chemical reaction of this simplicity, but it is certain that Molluscs do not simply absorb carbonate of lime from the sea water, and sweat it out from their skins. It is reasonable to inquire how far shell-making may express a primitive mode of excretion to which a secondary significance has come to be attached, and in what way carbonate of lime shells are associated with preponderant sluggish- ness of habit. The thickness of the shell seems often to bear some relation to the external and internal activities of the mollusc, for it is thin in the active scallop ( Pecten ) and Lima , thick in the passive oyster and Tridacna, slight or absent in the pelagic Pteropods (“sea-butter- flies”), and in the more or less active cuttlefish, but heavy in most of the slowly creeping littoral forms. But that this is only one condition of shell development is evident in many ways, — for instance, when we compare land-snails with slugs ; for the latter, though not more active than the former, are practically shell-less. In most cases, as Lang points out, the loss of the shell is justified by increased power of locomotion, by increased adaptation to peculiar habits of life, and so forth. Larvae. — In their life history most Molluscs pass through two larval stages. The first of these is a pear-shaped or barrel-shaped form, with a curved gut, and with a ring of cilia in front of the mouth. It is a “ trochosphere,” such as that occurring in the development of many “worms.” Soon, however, the trochosphere grows into a yet more efficiently locomotor form — the veliger. Its head bears a ciliated area or “velum,” often produced into retractile lobes ; its body already shows the beginning of “ foot” and mantle ; on the dorsal surface lies the little embryonic shell gland (Fig. 149). But although trochosphere and veliger occur in the development of most forms, they do not in any of the three types which we have particularly described,— not in Anodonta , partly because it is a fresh- water animal, with a peculiarly adhesive larva of its own ; not in Helix , partly because it is terrestrial ; and not in Sepia, partly because the eggs are rich in yolk. Classification of Mollusca. The classification of the Mollusca is a matter of considerable difficulty. Lowest of all should undoubtedly be placed the Amphineura, bilaterally symmetrical Mollusca with many primitive characters. Some of these forms, like Chiton , are probably not far removed from the primitive Mollusca ; but others, e.g. Proneomenia , are probably degenerate. From primitive forms, related perhaps to Chiton , Mollusca have diverged in two directions. In Gasteropoda, Scaphopoda, and Cephalopoda, the radula present in the primitive Amphineura is retained, and the head region AMPHINEURA. 369 becomes well developed ; these classes are therefore often placed together as Glossophora or Odontophora, in contrast to the Lamelli- branchiata (Lipocephala or Acephala), where the radula has disappeared, and the head region remains undeveloped. As already seen, however, the lowest LameUibranchs have a flattened creeping foot and simple feather)' gills, in these respects resembling Gasteropods. There is also much reason to believe that the Scaphopoda arose from a stem common to them and the lowest Gasteropods, which are central unspecialised forms. The Cephalopoda are the most highly specialised of all the Mollusca, and in their existing forms at least not nearly related to the other classes. The Mollusca may therefore be classified in outline as follows ; but the relation of LameUibranchs and Dentalium to Gasteropods should be kept in mind. Class Amphineura. Order 1. Polyplacophora, e.g. Chiton. Order 2. Aplacophora or Solenogastres, e.g. Neomenia. < c o tA O O Class Gasteropoda. Order 1. Prosobranchia, including the pelagic Heteropoda. Order 2. Pulmonata. Order 3. Opisthobranchiata, including the pelagic Pteropoda. Class Scaphopoda. Class Cephalopoda. Order x. Tetrabranchiata. Order 2. Dibranchiata. Lipocephala. — Class Lamellibranchiata. The characters of the trochosphere larva which occurs in many Molluscs, and many of the features of the simple Amphineura, suggest that Molluscs arose from some worm type, but beyond this all is hypothesis. Class I. Amphineura. Syn. — Gasteropoda Isopleura, e.g. Chiton. General Characters. — The Amphineura are marine Molluscs , ?nore or less elongated in form, with bilateral symmetry. They are often ranked alotig with Gasteropods. The mouth is anterior ; the a?ial and nephridial apertures are posterior. The ?na?itle, which bears cuticular spicules , covers at least a great part of the body. The nervous system consists of a cerebral commissure and two paired longitudinal cords , 24 370 MOLLUSC A. with ganglionic cells but at most very slightly developed ganglia, which run the whole length of the body. Of these Fig. 163. — Dorsal view of nervous system of Chiton. — After Pel- seneer. c., Cerebral commissure; g., gut (above all the com- missures except cerebral and supra-rectal) ; pa., pallial or visceral loop, with supra-rectal com- missure {s.r.c.) ; p., pedal nerves united by numer- ous transverse branches ; s.g-. , stomato-gastric com- missure ; s.r., subradular commissure ; , labial commissure ; v. , visceral commissure. Fig. 164. — Proneometiia, Ner- vous system. — From Ffub- recht. c.g., Cerebral ganglia ; slg., sub- lingual ; a.p.g. , anterior pedal ; p.p.g., posterior pedal ; p.v.g., posterior viscerals ; si., sublingual connectives ; cpc., cerebro-pedal connective ; pe., longitudinal pedal nerves; la., longitudinal lateral nerves. AMPHINEURA. Un- paired cords the pedals are connected l>y numerous cross- commissures, and the viscera/s or pallials are united posteriorly by a commissure above the rectum. The bilateral symmetry is shown internally, e.g. in the paired nephridia , auricles , and genital ducts. The class is of ancient origiti, dating from the Silurian. There are two orders — Polyplacophora, e.g. Chiton, and Aplacophora, e.g. Neomenia. ist Order of Amphixeura. Polyplacophora (Chitonidae). The members of this order, represented on British coasts by several species of Chiton , are sluggish, usually vegetarian, animals, occurring Fig. 165 — Anatomy of Chiton. A, ventral surface (after Cuvier). B, dorsal view ot alimentary canal (after Lankester). C, genital and excretory organs from dorsal surface (after Lang and Haller, diagrammatic), m., mouth ; a., anus ; hr., numerous simple gills foot ; 6., buccal mass ; , liver ; i., intestine ; ao., aorta ; v. , ventricle of heart ; r.a. and /.a., right and left auricles ; ov., ovary ; od., oviduct ; od' opening of oviduct ; «., part of nephridium, represented in black throughout; no., external opening of nephridium; outline of pericardium. from the shore to great depths. The foot is generally as long as the body ; the mantle covers the back and bears eight shell-plates (Fig. 162), perforated, in many cases at least, by numerous sensory organs, which may be in part optic ; numerous gills lie in a regular row along a groove on each side between the mantle and the foot. In most cases the eight shell-plates are jointed on one another, and the animal can roll itself up. The uncovered parts of the mantle bear spicules. Ganglia, in the strict sense, are scarcely developed, but there is a supra-cesophageal ganglionic commissure from which the visceral and pedal cords extend backwards along the whole length of the body. There are no special sense organs on the head, which is but slightly differentiated ; but the pallial sense organs are usually numerous and 372 MOLLUSC A. varied. A twisted gut runs through the body, surrounded by a diffuse digestive gland. There is a radula in the mouth. The heart is median and posterior, and consists of a ventiicle and two to eight auricles. There are two symmetrical nephridia opening posteriorly, and consisting of much-branched tubes. The sexes are separate ; a single reproductive organ extends dorsally between gut and intestine almost the whole length of the body ; the genital ducts are paired and open posteriorly in front of the excretory apertures. The ova, with chitinous spiny shells, are usually retained for some time by the female between the mantle and the gills. The segmentation is holoblastic, and a gastrula is formed by invagination. 2nd Order of Amphineura. Aplacophora, e.g. Neomenia, Proneomenici, and Chatodei'ma. The members of this order are worm-like animals, in which the mantle envelops the whole body and bears numerous spicules, but no shell. There are two families — Neomeniidte and Chretodermidse. Of Neomeniida:, six genera are known. They have a longitudinal pedal groove, an intestine without distinct digestive gland, two nephridia with a common aperture, and hermaphrodite reproductive organs. The Chretodermkke, represented by one genus Cluztoderma, are cylindrical in form, without a pedal groove, with a radula bearing one tooth, with a distinct digestive gland, and with two nephridia opening separately into a posterior cavity, which also contains two gills. The sexes are separate. Class II. Gasteropoda, e.g. Snail, Whelk, Limpet. General Characters. — Gasteropods are more or less asymmetrical Molluscs. The head region , which is well developed , remains symmetrical , and so does the foot , which is typically a flat creeping organ. But the visceral mass or hump , with its mantle fold , is more or less hoisted forwards and to the right. Thus the pallial, anal, nephridial, and genital apertures usually lie on the right side , more or less anteriorly. A further asymmetry is shown by the twisting of the morphologically right gill to the left side, while the original left gill is usually lost. Similarly, one of the nephridia , pro- bably that which is morphologically the left, tends to disappear, and in most cases only one persists — topographically on the left side. The mam torsion must be distinguished from the spiral hoisting ivhich the visceral himip ofte?i exhibits, and fro?n the frequently associated spiral coiling of the univalve shell. Moreover , a superficial secondary bilateral symmetry tends to be acquired by free-swimming forms, e.g. Heteropods. The GASTEROPODA. 373 foot usually contains a mucus gland, and tends to be divided into three regions — the pro-, tneso-, and meta-podium. There is a single reproductive organ and genital duct. Order i. Prosobranchiata. A shell is almost always present, and the foot frequently bears an operculum. The pleuro-visceral commissure is twisted into the form of the figure 8 (streptoneural). There is usually only one gill lying in front of the heart. When one auricle is present it lies in front of the ventricle. Sexes separate. A. Diotocardia. Primitive forms. The heart has usually two auricles, and there are two nephridia ; Zeugobranchs, with two gills, e.g. Ilaliotis ; Azygobranchs, a single gill, Turbo, Trochus, etc. ; Docoglossa, single gill, and single auricle, left nephridium degenerate, no operculum, e.g. limpet ( Patella ), without gill (Fig. 1 6 1 ) ; Acmcea, with single gill. B. Monotocardia. Heart with single auricle, one gill, one nephridium ; operculum present. Periwinkle ( Littorina), buckie {Buccinum, Fig. 158), Dog -whelk ( Purpura ), Pant kina, and the majority of the marine Gasteropods with coiled shells, together with some fresh-water forms. The pelagic Ileteropods are also included here : — Atlanta, shell well developed ; Carinaria, with small shell ; Pterotrachea, with no shell. Order 2. PULMONATA. The visceral loop is short and untwisted (euthyneural), gills are absent, and the mantle cavity functions as a lung ; all are hermaphrodite, e.g. the snail [Helix) ; the grey slug ( Limax ) ; the black slug (.-Irion) ; fresh-water snails, such as Limnceus, Planorbis, and Ancylus. Order 3. Opisthobranchiata. The visceral loop is euthyneural, as in snails ; the single auricle lies behind the ventricle ; the shell and mantle are often absent. A. Tectibranchiata. A shell is present, but may be rudimentary ; there is a well-developed mantle fold and a single gill, e.g. Bulla, Aplysia, Dolabella, Umbrella. The Tectibranchiata also include the Pteropoda, the winged snails or sea-butter- flies, which have become much modified for pelagic life. They have a secondarily acquired apparent symmetry, and swim by two large lateral lobes of the foot (“ parapodia”). They often swim actively in shoals, and occur in all seas. They afford food for whales, etc., and the shells of some are abundant in the ooze. They include — (a) Thecosomata, with mantle fold and shell, diet of minute animal or vegetable organisms, closely re- lated to Bulla and its allies. Examples. — Hyalea , Cymbulia. 374 MOLLUSC A. (/>) Gymnosomata, without mantle fold or shell in the adult. Closely allied to Aplysia and its allies. Actively carnivorous, e.g. Clio , Pneumoderma. B. Nudibranchiata. Shell, mantle fold, and true gill aie absent ; various forms of “adaptive gills” may be present, or there may be no special respiratory organs, e.g. sea-slugs, Doris, Eolis, Dendronotus (Fig. 160). It will be obvious from this table that the classification of the Gasteropods cannot be greatly simplified. The essential points may perhaps be summarised as follows : — In the order Prosobranchia are included, first, primitive forms with more or less simple conical shells and with traces of the primitive bilateral symmetry ; and, second, the greater number of these marine Gasteropods which have well-developed conical shells closed by an operculum, as well as the modified pelagic Heteropods. The Pulmonata are readily recognised, and the Opistho- branchiata include (in general terms) marine Gasteropods usually without conspicuous shells, and often much modified in external appearance, and also the aberrant pelagic Pteropods or sea-butterflies. From a form somewhat like a Chiton, but with a simple conical shell, we may consider that the Gasteropods proper have been developed. They are all more or less asymmetrical, but we must notice — (i) that this want of symmetry does not affect the head or the foot, but only the dorsal viscera, which are more or less twisted round to the right side towards the head ; (2) the torsion must be distinguished from the frequent spiral twisting of the visceral hump and of the shell ; (3) the torsion occurs in variable degree, and some forms, especially free swimmers, have a superficial symmetry. The current explanation of the asymmetry, which has been recently elaborated by Lang, is as follows If we begin with a form something like a Chiton, but with a simple shell, we must suppose the head and foot to become increasingly specialised, and at the same time to acquire an increasing freedom of movement ; during the process the viscera will tend to become more and more limited to a special region of the body, and a “visceral hump” will thus be formed. The shell becomes limited to this region, but the contractility of head and foot, which enables these to be drawn into the shell, must be correlated with the increasing size and complexity of this structure. As, however, shell and visceral hump become larger, they become too heavy to be carried in the primitive position on the back of the animal, and incline to one side. There is, therefore, a one- sided pressure, which results in an increased growth relatively of the opposite side, and so in a deep-seated twisting, which brings the originally posterior anus to an anterior position near the mouth, and produces a tendency to the suppression of one of the originally paired gills, nephridia, etc. According to Lang, during the torsion an increased growth of the upper surface of the visceral mass is necessary in order to avoid rupture, and thus the superficial spiral coiling is produced ; this is reflected in the coiling of the shell. In one series of the Gasteropods the visceral nerve loop, running from the cerebral and pleural to the visceral ganglia, is “caught in the twist,” and twines like a figure 8 MODE OF LIFE. 375 (Streptoneural condition) ; in the others, the same visceral loop is short and untwisted (Euthyneural condition). In both groups we find forms with coiled shells, but among the Euthyneura there is a tendency to lose the shell, the visceral hump becoming at the same time incon- spicuous, while a superficial appearance of symmetry is produced. The deep-seated torsion of the organs is, however, still retained. It is not very uncommon to find, either as a constant occurrence or as an occasional variation, spirally coiled shells with a reversed or left- handed spiral. In some of these cases the superficial coiling of the visceral hump, as well as the deep-seated torsion, is also left-handed ; but in others we find that the internal structure retains the normal arrangement. Mode of life. — From the number of diverse types which the class includes, it is evident that few general statements can be made about the life of Gasteropods. We are safe in saying, however, that though the majority are sluggish when compared with Crustaceans, they are active when compared with Lamellibranchs. The locomotion effected by the contractions of the muscular foot is usually a leisurely creeping, but there are many gradations between the activity of Heteropods in the open sea, — the gliding of fresh-water snails ( Limnoms ) foot upwards across the surface of the pool, the explorations of the periwinkles on the sand of the shore, and the extreme passivity of limpets ( Patella ), which move only for short distances at a time from their resting-places on the rocks. The number of terrestrial snails and slugs, breathing the air directly by means of a pulmonary chamber, is estimated at over 6000 living species, while the aquatic Gasteropods are reckoned at about 10,000, most of which are marine. Of this myriad, about 9000 are streptoneural, the relatively small minority are euthyneural Opisthobranchs and Nudi- branchs, with light shells or none. The Heteropods and some Opisthobranchs live in the open sea ; the great majority of aquatic Gasteropods frequent the shore and the sea bottom at relatively slight depths ; the deep sea forms are comparatively few. Gasteropods rarely feed at such a low level as bivalves do — indeed, some of them are fond of eating bivalves. Most Prosobranchs (streptoneural), with a respiratory siphon and a shell notch in which this lies, are carnivorous, e.g. the buckies ( Buccinum ) and “dog-whelks” {Purpura) ; on the other hand, those without this siphon, and with an un- 376 MOLLUSC A. notched shell mouth, feed on plants, e.g. the seaweed eating periwinkles ( Littorina ). The vegetarian habits of most land snails and slugs are known to all. Many Gasteropods, both marine and terrestrial, are very voracious and indiscriminate in their meals ; others are as markedly specialists or epicures. Some marine forms, partial to Echino- derms, have got over the difficulty of eating such hard food, by secreting dilute sulphuric acid, which changes the carbonate of lime in the starfish into the more brittle and readily pul- verised sulphate. A few Gasteropods are parasitic, e.g. Eulima and Stylifer on Echinoderms, and the extremely degenerate Entoconcha mirabilis , — within the Holothurian Synapta. Life history.— The eggs of Gastero- pods are usually small, without much yolk, but surrounded by a jelly, the surface of which often hardens. In the snail and some others there is an egg-shell of lime. Sexual union occurs between her- maphrodites as well as between separ- ate sexes, and fertilisation is effected inside the genital duct. Development sometimes proceeds within the parent, 1 OLU ijju.ru. ^iii ici . , « - .*1* i Tonniges) ; v., beginning of blit in ITlOSt CciSGS the I0rtlllS6Q CggS are laid in gelatinous clumps, or within special capsules. The free-swimming Ianthina carries the eggs in capsules attached to a large raft-like float towed by the foot. On the shore one often finds numerous egg-capsules of the “ buckie ” ( Buccinum undatum) united in a ball about the size of an orange. Under the ledges of rock are many little yellowish cups, the egg-capsules of the dog-whelk ( Purpura lapillus). In the buckie and whelk, and in some other forms, there is a struggle for existence — an infant cannibalism — in the cradle, for out of the numerous embryos in each capsule only a few reach Fig. 166. — Stages in mol- luscan development. A , Blastula of limpet (after Patten). B , Gastrula of Paiudina vivipara (after velum ; arc., archenteron ; m, mesoderm cells. C, later stage of the same ; v ., velum; mouth inva- gination; arc., archen- teron ; a., anus; f., begin- ning of foot; sh.g., shell gland. SCAPHOPODA. 377 maturity, — those that get the start eating the others as they develop. The development is usually simple and typical. In other words, segmentation is total though often unequal ; gastrula- tion is embolic or epibolic according to the amount of yolk present ; the gastrula becomes a trochosphere, and later a veliger. (Fig. 166.) Past history. — As the earth has grown older the Gasteropods have increased in numbers. A few have been disinterred from the Cambrian rocks ; thence onwards they increase. Most of the Palaeozoic genera are now quite extinct, but many modern families trace their genealogy to the Cretaceous period. Those with respiratory siphons were hardly, if at all, represented in Palaeozoic ages, and the terrestrial air-breathers are comparatively modern. Bionomics. — Asvoracious animals, with irresistible raspers, Gasteropods commit many atrocities in the struggle for exist- ence, and decimate many plants. Professor Stahl shows, however, that there are more than a dozen different ways in which plants are saved from snails, — by crystals, acids, ferments, etc.; in short, by constitutional characteristics sufficiently important to determine survival in the course of natural selection or elimination. As food and bait, many Gasteropods are very useful ; their shells have supplied tools and utensils and objects of delight ; the juices of Purpura and Murex furnished the Tyrian purple, more charming than all aniline. Class III. Scaphopoda. Very different in many respects from Gasteropoda are the Scaphopoda, of which Dentalium (Elephant’s tooth-shell) is the commonest genus. They are apparently related to the Zeugobranchiate Gasteropods, and also to the simplest Bivalves. They burrow in the sand at considerable depth off the coasts of many countries. The mantle has originally two folds, which fuse ventrally, and the shell becomes cylindrical, like an elephant’s tusk. It is open at both ends. The larger opening (directed downwards in the sand) is anterior, the concave side of the shell is dorsal. The mouth opens at the end of a short buccal tube, at the base of which is a circle of ciliated tentacles. The foot is long, with three small terminal lobes. It is used in slow creeping, and is protiuded at the anterior opening. There are cerebral and pleural ganglia near one another in the head, pedal ganglia in the foot, and a long untwisted visceral loop with olfactory ganglia near the posterior anus. Sense organs are represented by otocysts beside the pedal ganglia. There is an odontophore with a simple radula. The food consists of minute 378 MOLL USC A. animals. There is no heart, but colourless blood circulates in the body cavity. There are two nephridial apertures, one on each side of the anus ; and two nephridia. The sexes are separate ; the reproductive organ is simple and dorsal in position ; the elements pass out by the right nephridium. The gastrula is succeeded by a free-swimming stage, in which there is a hint of a velum and a rudimentary shell gland. Examples. — Dentaliuni, Entalium. About forty widely-distributed species are known. Dentaliuni entale occurs off British coasts. The genus occurs as a fossil from Carboniferous (or perhaps earlier) strata onward. Class IV. Lamellibranchiata or Bivalves. (. Synonyms — Acephala, Conchifera, Pelecypoda, Lipocephala, etc.) Examples. — Cockles, Mussels, Clams, and Oysters. Lamellibranchs are bilaterally symmetrical Molluscs , in which the body is compressed from side to side and the foot more or less ploughshare-like. The head (or prostomium) region remains undeveloped. , and without tentacles ; radula , horny jaws , and salivary glands are absent , but there is a pair of labial palps on each side of the mouth. The mantle skirt is divided into two flaps , which secrete the two valves of the shell , now lateral instead of dorsal in position. The valves are united by a dorsal elastic ligament , and closed by two transverse adductor muscles or by one. Internal bilateral symmetry is marked by the paired nature and disposition of the nephridia , auricles , gills , digestive gland, and reproductive organs. The gills ( ctenidia ) consist of numerous gill filaments , which typically grow together into large plates (lienee the title Lamellibranch). There are usually thi-ee pairs of ganglia : (a) cerebrofileurals in the head ; (b) pedals in the foot; (c) viscerals at the posterior end of the body. The heart consists of a ventricle and two auricles , and is surrounded by a pericardium which is ccelomic in origin , and communicates with the exterior by means of the two nephridia. Repro- ductive organs are always simple, and the sexes are usually separate. The typical development includes trochosphere and veliger stages. Most Lamellibranchs feed on microscopic organisms and particles ; the distribution is very wide, both in salt and fi-esh water ; the general habit is sedentary or sluggish. GENERAL NOTES ON LAMELLIBRANCHS. 379 Classification. — The best classification of Lamellibranchs seems to be that of Pelseneer, which is based on the structure of the gills. Order I. Protobranchia. — There are two simple posterior gills, quite similar to those of Zeugobranchs ; the foot has a flattened creeping surface ; the pleural and cerebral ganglia are distinct, e.g. Nucula, Solenomya. Order 2. Filibranchia. — The gill filaments are greatly elongated and reflected, so that they consist of an ascending and a descending limb, e.g. Area (Noah’s-ark shell), Mytilus (edible mussel), Modiola (horse- mussel). Order 3. Pseudo-lamellibranchia. — The successive gill filaments are loosely connected together to form gill-plates, e.g. Pecten (scallop), Ostrea (oyster). Order 4. Eulameli.ibranchia. — The separate filaments are no longer discernible ; the gills form double flattened plates. The great majority of Bivalves are included here, e.g. Anodonta, Venus, Pholas (a boring form), Mya. General Notes on Lamellibranchs. Structure. — The organs which most frequently vary in other bivalves, as compared with Anodonta , are the foot, the gills, the adductor muscles, and the mantle skirt. The foot varies much in size and shape ; the pedal gland of Gasteropods is often represented by a “byssus” gland, which secretes attaching threads, well seen in the edible mussel (Mytilus). The gills show an interesting series of gradations, from a slight interlocking of separate gill filaments to the formation, by complicated processes of “concrescence,” of plate-like structures such as those of Anodonta. These processes are, however, much more closely related to the method of nutrition than of respiration, which, indeed, is probably largely performed by the mantle skirt. The mantle skirt is often united to a greater or less extent inferiorly, and is often prolonged and specialised posteriorly to form exhalant and inhalant “siphons” (Fig. 159). These siphons sometimes attain a considerable length ; they occur especially in forms such as Mya, which live buried in sand or mud, or which burrow in wood or stone, e.g. Pholas. The variations of the adductor muscles afford one basis for classification. We may associate with the sluggish habits and sedentaiy life of bivalves — (1) the undeveloped state of the head region ; (2) the largeness of the plate-like gills, which waft food-particles to the mouth ; and (3) the thick limy shells. We may reasonably associate these and other facts of structure (e.g. the rarity of head-eyes, biting or rasping organs) with the conditions of life. In other words, these characteristics may be regarded as adaptations resulting from the action of natural selection on germinal variations. In thinking about the sluggishness of most bivalves, we must not forget, however, that the larval trochospheres and veligers are very active, perhaps almost too active, young creatures. In some Lamellibranchs, e.g. Mytilidae, small eyes occur on the head ; in some. other cases they are present in the larva, but not in the adult. Habit. — Most bivalves, as every one knows, live in the sea, and MOLLUSC A. 380 their range extends from the sand of the shore to great depths. They occur in all parts of the world, though only a few forms, like the edible mussel ( Mytilus edulis), can be called cosmopolitan. Some, such as oysters, can be accustomed to brackish water. The fresh-water forms may have found that habitat in two ways — (a) a few may have crept slowly up from estuary to river, from river to lake ; Dreissenia poly- inorpha has been carried on the bottom of ships from the Black Sea to the rivers and canals of Northern Europe ; and it is likely that aquatic birds have assisted in distributing little bivalves like Cyclas ; (b) on the other hand, it is more probable that the fresh-water mussels {Unto, Anodonta, etc.) are relics of a fauna which inhabited former inland seas, of which some lakes are the freshened residues. Between the active Lima and Pecten , which swim by moving their shell valves and mantle flaps, and the entirely quiescent oyster, which has virtually no foot, there are many degrees of passivity, but most incline towards the oyster’s habit. Of course, there is much internal activity, especially of ciliated cells, even in the most obviously sluggish. The cockle ( Cardium ) uses its bent foot to take small jumps on the sand ; the razor-fish (Solett) not only bores in the sand, but may swim backwards by squirting out water from within the mantle cavity ; many (e.g. Teredo, Pholas, Lithodomus, Xylophaga ) bore holes in stone or wood ; in the great majority the foot is used for slow creeping motion. The food consists of Diatoms and other Alga;, Infusorians and other Protozoa, minute Crustaceans and organic pai tides, which the cilia of the gills sweep from the posterior end of the shell to the mouth. The bivalves are themselves eaten by worms, starfishes, gasteropods, fishes, birds, and even mammals. Life history. — The eggs are sometimes laid in the water, either freely or in attached capsules, or, more frequently, they are fertilised by spermatozoa drawn in with the inhaled water, and are subsequently sheltered within the body during part of the development. In the Unionidee the embryos are retained within the cavities of the outer gills ; in Cyclas and Pisidium there are special brood-chambers at the base of the gills. In Cyclas the embryos are nourished by the maternal epithelial cells. Segmentation is always unequal ; a gastrula may be formed by invagination or by overgrowth, the two cases being con- nected by a series of gradations. A trochosphere stage is more or less clearly indicated, being most obvious in cases where the eggs are laid in the water. The free-swimming trochosphere becomes a veliger, and this is modified into the adult. The fresh-water forms, with the exception of Dreissenia polymorpha, in which the habit is recently acquired, do not possess free-swimming larvae ; this must be regarded as an adaptation. Past history of bivalves. — Even in Cambrian rocks, which we may call the second oldest, a few bivalves have been discovered ; in the Upper Silurian they become abundant, and never fall off in numbers. Those with one closing muscle to the shell seem to have appeared after those which have two such muscles. Those which, from the shell markings, seem to have had an extension of the mantle into a pro- trusible tube or siphon, were also of later origin. The present fresh- water forms were late of appearing. Of all the fossil forms the most CEPHALOPODA. 38' remarkable are large twisted shells, called Hipptintes (Rudistoe), whose remains are often very abundant in deposits of the chalk period. Class V. Cephalopoda. Cuttlefish. Examples. — Sepia, Octopus, Loligo, Nautilus. The Cephalopods are bilaterally symmetrical free-swimming Molluscs. The head is surrounded by numerous “ arms ” bearing tentacles or suckers. Part of the foot forms a partial or complete tube — the “ siphon ” or “ funnel ” — through which water is forcibly expelled fro7n the mantle cavity, driving the animal backwards. The muscular mantle flap which shelters the gills is posterior in positio?i ; the visceral hump shows no trace of spiral coiling, but is elongated in a direction anatom- ically dorsal and posterior, though it may point forwards 5 when the animal propels itself through the water. Except in the pearly Nautilus, the shell of modern forms has been e?tclosed by the mantle, and is, in most cases, only hinted at. There is a very distinct head region, furnished with eyes and other sensitive structures, and the mouth has strong beak-like iaws, as also a well-developed radula. The ?iervous system slurws considerable specialisation, and the chief ganglia are concentrated iti the head. The true body cavity, pericardium of other Molluscs, is usually well developed, and frequently surrounds the chief organs. Except in the Nautilus, it com- municates with the exterior by the nephridia. The vascular system is well developed, and , except in the Nautilus, there are accessory branchial hearts. The sexes are separate. Development is di?-ect. In habit, Cephalopods are predomhiantly active and predatory ; in diet, carnivorous. The Cephalopods are divided into two markedly distinct orders, of which the one includes Sepia and all other living cuttles except Nautilus, which is the sole living type of the second order. As Sepia has been already described, we may briefly review some of the more striking characters of the pearly Nautilus ( Nautilus pompilius). The shells of the pearly Nautilus are common on the shores of warm seas, but the animals are much less familiar. The Nautilus creeps or swims gently along the bottom at no great depth, and its appearance on the surface, “ floating like a tortoiseshell cat,” is probably the result of storms. It 3§2 MOLLUSC A. is called “ pearly ’’ on account of the appearance of the innermost layer of the shell. This is exposed after the soft organic stratum and the median layer which bears bands of colour have been worn away, or dissolved in a dolphin’s stomach, or artificially treated with acid. The beautiful shell is a spiral in one plane, divided into a set of chambers, in the last of which the animal lives, while the others contain gas. The young creature inhabits a tiny shell curved like a horn ; it grows too big for this, and proceeds to enlarge its dwelling, meanwhile drawing itself forward from the older part, and forming a door of lime behind it. This process is repeated again and again ; as an addition is made in front, the animal draws itself forward a little, and shuts off a part of the chamber in which it has been living. Thus the compartments are not suc- cessive chambers, but fractions of successive chambers, aban- doned and partitioned off as more space was gained in front. All the compartments are in communication by a median tube of skin— the siphuncle — which is in part calcareous. It has been suggested that “ each septum shutting off an air-containing chamber is formed during a period of quiescence, probably after the reproductive act, when the visceral mass of the Nautilus may be slightly shrunk, and gas is secreted from the dorsal integument so as to fill up the space previously occupied by the animal.” The only other living Cephalopod which has a shell at all like that of the Nautilus is Spiru/a. In it the shell is again chambered and spirally coiled in one plane. But it is without a siphuncle, and lies enveloped by folds of the mantle. There can be no confusion between the beautiful shell of the cuttlefish called the paper Nautilus {Argonaut a argo) Fig. 167. — Section of shell of nautilus. — After Lenden- feld. NAUTILUS. and that of our type. For it is only the female Argonaut which bears a shell ; it is not chambered, and is a shelter for the eggs— a cradle, not a house. It is usually stated to be u Fig. 168. — The Pearly Nautilus {Nautilus pompi Hus). — After Owen. The shell is represented in section, but the animal is not dissected. c., Last or body-chamber, separated by a septum (se.) from the compartment behind; s., the siphuncle traversing all the compartments ; /«., the portion of the mantle which is reflected over the shell; h., the hood; e., the eye with its opening to the exterior; 4, the lobes which bear the sheathed tentacles ft.) ; si., the incomplete siphon ; mu., the shell muscle ; the position of the nidamental gland. formed by two of the arms, but it seems doubtful whether it is not in reality due to the activity of the mantle. It is instructive also to compare the Nautilus shell with that of some Gasteropods, for there also chambers may be formed. But these arise from secondary alterations of an originally continuous spiral, and the resemblance is never 3S4 MOLLUSC A. very striking. The fresh-water snail Planorbis has an unchambered shell spirally coiled in one plane ; but in this and in similar Gasteropods the foot is turned towards the internal curve of the coil, while that of Nautilus is directed externally. There are only about half a dozen living species of Nautilus, but there are many hundred fossils of this and allied genera. This list is usually swelled by the addition of the extinct Ammonites, but there are some reasons for believing that these belong to the Dibranchiate section of Cephalopods. The following table states the chief points of distinction between Nautilus and the other series of Cephalopods Cephalopoda. Tetrabranchiata ( Nautilus ). Dibranchiata {Sepia, Octopus, etc.). All extinct except one genus — Nautilus ; the extinct forms are usually ranked as Nautiloid and Ammonoid. Shell external, chambered, straight or bent or spirally coiled. That in which Nautilus lives has been described, with its siphuncle, gas - containing compartments, etc. The part of the foot surrounding the mouth bears a large number of lobes, which carry tentacles in little sheaths, but no suckers. The two mid-lobes of the foot form a siphon, but they are not fused into a tube. The eye is without a lens, and is bathed internally by sea water, which enters by a small pinhole aperture. There are two “osphradia” or smelling patches at the bases of the gills. Two pairs of gills ; two pairs of ne- phridia ; two genital ducts (the left rudimentary). The coelom sac opens directly to the exterior t>y two apertures. The heart has two pairs of aurirles, and there are no branchial hearts. No ink-bag. No salivary glands. Numerous living genera, ranked as Decapods or Octopods ; along with the former the extinct Belemnites are included. No living Dibranchiate lives in a shell. The shell is internal even in the extinct Belemnites, and in modern forms it occurs in various degrees of degeneration (cf. Spiru/a, Sepia, Loligo ), or is quite absent (Octopoda). The part of the foot surrounding the mouth is divided into ten or eight arms, which carry suckers, stalked in Decapods, sessile in Octopods. The two mid-lobes of the foot fuse to form a completely closed tubular siphon or funnel. The covering of the eye may be per- forated, but the mouth of the retinal cup is closed by a lens. There are no osphradia, though there may be “olfactory pits” behind the eyes. One pair of gills ; one pair of nephridial sacs ; two oviducts in Octopoda and Ommastrephes ; two vasa deferentia in Eledone moscluita; in others an unpaired genital duct. The coelom opens into the nephridia by two pores, and thus to the exterior. The heart has two auricles, and there are branchial hearts. An ink-bag and salivary glands. CLASSIFICATION OF CErilALOPODA. 385 Classification of Cethai.ofoda. Order I. Tetrabranchiata (see Table). Family I. Nautilicke. Nautilus alone alive ; but a great series of fossil forms, Ortkoceras — Troclioceras. Family II. Ammonitidse. All extinct, but with shells well preserved, so that long series can be studied. They furnish striking evidence of progressive evolution in definite directions, e.g. Baclrites, Ceralites , Baculites, Turrilites, Heteroceras, and the whole series of genera formerly classed as Ammonites. Order II. Dibranchiata (see Table). Sub- Order Decapoda. Eight shorter and two longer arms. Suckers stalked and strengthened by a strong ring. Large eyes with a horizontal lid. Body elongated, with lateral fins. Mantle margin with a cartilaginous “hook- and-eye ” arrangement. Some sort of internal “ shell,” enclosed by upgrowths of the mantle. With calcareous internal “shell.” Spirilla ; extinct Bel- emniles ; Sepia. With organic internal “shell.” (a) Eyes with closed cornea, e.g. Loligo. {/>) Eyes with open cornea, e.g. Ommastrephes. Sub-Order Octopoda. Eight arms only. Suckers sessile without horny ring. Small eyes with sphincter-like lid. Body short and rounded. No “hook-and-eye” arrangement. No “shell,” except in the female A rgonaula. e.g. Octopus , Eledone , Argonaut a. The classification given above is that usually adopted, but it is not certain that the Ammonites should be included in the Tetrabranchiata. The Cephalopods are the most specialised of the Molluscs, and present much variation of type. Nautilus appeared very early and has persisted, apparently unchanged, until the present, while the Ammonites and Belemnites, once so abundant, have entirely disappeared. Among recent forms we have Squid, Calamary, Octopus, Argonaut, and many others. All swim freely in the sea, or lurk and creep passively among the rocks. They are voracious eaters, and devour very diverse kinds of animals, their parrot-like jaws and powerful odontophore, as well as the numerous suckers, rendering them formidable adversaries. Many live at considerable depths, and their chief foes are the toothed whales, some of which, like the sperm whale ( Physeter ), and the bottle-nose ( Hvperoodon ), subsist almost entirely on cuttles. 25 MOLLUSC A. 386 A chambered external shell, serving as a house, is present in Nautilus alone among living Cephalopods. In Spirula there is a spiral chambered shell, but it is very small, enclosed by the mantle, and quite useless for protection. Most of the extinct forms had large and efficient shells of very diverse shape, some straight like Orthoceras, or coiled, with chambers separated by complex septa, as in the Ammonites. Most of the modern forms seem to be more active than their ancestors, and their shells have degenerated. While the fact of the degeneration is perfectly obvious, the line along which it has taken place is difficult and still debated. In Nautilus , although the animal lives within the shell, the mantle fold is for some distance reflected over it ; in the other series of Cephalopods this process has gone further, and, where a shell is present, it is entirely enclosed within the mantle fold, and is much reduced in size. In the extinct Belemnites the internal shell was straight and chambered, but almost concealed by secondary deposits of lime, secreted by the walls of the shell-sac. In Sepia , according to one view, the central laminated region of the “bone” represents the remains of the chambered shell ; the remainder corresponds to the secondary deposits of lime in the Belemnites. In Loligo there is no deposit of lime, an organic chitinous pen only being left. In Octopus there is no trace of shell at all. According to some, the shell-sac, in which the shell or pen of Cephalopods is formed, is to be regarded as equivalent to the embryonic shell-sac plus a mantle pocket. CHAPTER XVII. Class HEMICHORDA or ENTEROPNEUSTA. Type Balanoglossus. A species of Balanoglossus was described by Delle Chiaje at the end of the eighteenth century, but it is only within the last few years that the researches of Spengel, Bateson, and others have led to an appreciation of the importance of the type, and to a recognition of its peculiar features. The class (Enteropneusta) which was erected for the reception of Balanoglossus has at present included in it a few other forms, whose more or less distinct affinities with Vertebrates are suggested by the alternative title Hemi- chorda. Taken along with Tunicates and Amphioxus , they illustrate gradual approximations towards Vertebrate characters. General Characters. The body is divisible into three regions — a pre-oral “ proboscis ,” a “ collar ” around and behind the mouth , and a trunk , the anterior part of which bears gill-slits. A dorsal nerve-cord arises from the epiblast alo?ig the middle line , and is connected , by a ring routid the pharynx, with a ventral cord. In the skin , which is covered with ciliated ectoderm , there is also a nerve plexus. From the a7iterior region of the gut a diverticulum grows forward for a short distance , becomes a solid support for the proboscis , and is often called the '‘'' notochord.'" The gill-slits open dorsally, are very numerous , and increase in number during life ; in some details of development they recall those of Amphioxus. The mesoblast 388 HEMICHORDA OR ENTEROPNEUSTA. is formed by the outgrowth of pouches from the arche7iteron ; i.e. the body cavity is etiteroccelic. An unpaired a7iterior pouch fo7'77is the p7-e-oral or proboscis cavity of the adult, and is co7npared to the anterior impaired body cavity of Amphioxus. Spengel, in his recent monograph, recognises nineteen species and four genera — Balanoglossus , Ptychodera , Schizo- cardium, and Gla7idiceps. They are very widely, though locally, distributed, but are perhaps absent on the Pacific coasts of America. Description of Balanoglossus. Form and habitat. — The species which form this genus are worm-like marine animals, found in sand and mud in the English Channel, the Mediter- ranean, Chesapeake Bay, etc. They vary in length from about i in. to over 6 in., and are brightly coloured and have a peculiar odour, like that of iodoform. The sexes are distinct, and are marked exter- nally by slight differ- ences in colour. The body consists of a pro- minent pre-oral region or “ proboscis,” a firm “ collar ” behind the mouth ; behind this, a region with gill-slits ; and finally, a long, soft, slightly coiled portion. Skin and muscles. — The epidermis is ciliated, and exudes abundant mucus from unicellular glands. In B. robmii the mucus sets firmly, and, with the addition of grains of sand, forms a tube round the body. Some species are phosphorescent. The muscular system is best developed about the proboscis and collar, which are used in leisurely locomotion through the soft sand. There are external circular Fig. 169. — Male of Balanoglossus koiva- levskii. — After Bateson. Note anterior proboscis. Mo., Mouth; op., slight operculum behind the collar ; then the region with gill-slits ; is., testes ; a., anus. THE BODY CAVITY. 3*>9 and internal radial and longitudinal muscles. The fibres are unstriped. Nervous system. — The dorsal nerve-cord is most developed in the collar, but is continued along the whole length. It arises as a solid cord of epiblast, which is continued both forwards and backwards as a hollow tube. The cavity is said to be comparable to that of the spinal cord in Vertebrates. But the dorsal nerve- cord is connected by a band round the collar with a ventral nerve. There is also a nervous plexus beneath the epidermis. There are no special sense organs, nor should we expect them in an animal which spends most of its life immersed in muddy sand. In the larvae of some species there are two eye spots. Alimentary system. — The mouth is permanently open, and is on the ventral surface between the proboscis and the collar. Sand seems to pass into it during the wriggling movements of the animal. The pharynx is constricted into a dorsal and ventral region, of which the former is respiratory (Fig. 170, g1.), and connected with the exterior by many gill-slits, while the latter is nutritive (Fig. 170, g.), and conveys the food-particles onwards. According to Willey, there is no evidence that this groove is comparable to a structure of similar appearance seen in Tunicates and the lancelet, as used to be asserted. Behind the region with gill-slits, the gut has a dorsal and a ventral ciliated groove, and bears, throughout the anterior part of its course, numerous glandular sacculations, which can be detected through the skin. The anus is terminal. The animal eats its way through the sand, and derives its food from the nutritive particles and small organisms therein contained. Skeletal system. — The skeletal system is represented by the “ notochord,” which lies in the proboscis, and arises, like the notochord of indubitable Vertebrates, as a hypo- blastic structure from the dorsal wall of the gut. Each gill- slit is furnished with a “chitinous” skeleton, which gives the slit a U-shape, on account of the growth downwards of a “tongue bar”; the whole is suggestive of Amphioxus. Beneath the branchial skeleton there lies a “chitinous” rod, which divides into two in the collar. The body cavity. — The body cavity is somewhat complex, 39° HEMICHORDA OR ENTEROPNEUSTA. — /. m consisting of five distinct parts, all of which are lined by mesoderm, and arise as pouches from the primitive gut or archenteron. (a) There is first the unpaired cavity of the proboscis, which communicates with the exterior by a dorsal pore (or sometimes by two) at the base of the proboscis next the collar. It is possible that a glandular structure, which lies in front of the heart in the proboscis, may have excretory significance, but it seems to be quite enclosed. (^)In the collar region g-.s d.v there are two small \ paired coelomic cavi- ■ A ties, from which two funnels open to the exterior. Both these cavities and that of the proboscis tend to be obliterated by growth of connective tissue, (c) Two other cavities extend along the posterior region of the body, to some extent separated by the dorsal and ventral mesentery which moors the intestine. In these there is a body cavity fluid with cells. Respiratory and vascular systems. — The respiratory system consists of many pairs of ciliated gill-slits. They open dorsally by small pores behind the collar. In develop- ment they begin as a pair, increase in number from in front backwards, and they go on increasing long after the adult structure has been attained. Water passes in by the mouth and out by the gill-slits, where it washes branches of the dorsal blood vessel. There are no gill lamellae associated with the slits. The vascular system includes a main dorsal blood vessel, v.i). D.n. Fig. 170. — Transverse section through gill- slit region of Ptychodera minuta. — After Spengel. The section, somewhat diagrammatic, shows a gill- slit (g-.s.) to left, and a septum between two slits to the right; d.n., dorsal nerve; d.v., dorsal vessel; v.n., ventral nerve ; v.v., ventral vessel ; g., nutritive part of gut ; g1., respiratory part of gut; c., lateral coelomic spaces; l.m., longitud- inal muscles; R., reproductive organs. As the gill-slits are oblique, the whole of one could not be seen in a single cross-section. DEVELOPMENT. 391 which, at its anterior end, lies above the notochord ; an anterior dilatation, which is sometimes called the “ heart ” ; a ventral vessel beneath the gut : and numerous smaller vessels. The blood flows forwards dorsally, backwards ventrally. This system should be contrasted with that of Amphioxus. Excretory and reproductive systems. — The excretory system is slightly developed. No nephridia are known, Fig. 171. — Development of Balanoglossus. — After Bateson. The mesoderm is represented by the broken dark line. In the upper row, from the left — Section of blastula ; beginning of gastrulation, End., endoderm ; section of gastrula, bl., blastopore; Ac., Archenteron ; S.c., segmentation cavity ; closure of blastopore, outgrowth of five coelom pouches (M.). In the lower row, from the left — Longitudinal section, showing the five parts of the body cavity () the atriopore in myotome thirty-six, giving exit to the water which enters by the mouth ; {c) the anus, ventral and slightly to the left, behind the atriopore, but some distance from the posterior end of the body. Along the back there is a median fin, which is continued around the tail, and along the ventral surface as far as the atriopore. In front of this region the ventral surface is flattened, and fringed on either side by a slight fin-like “ metapleural ” fold. These folds are continuations downwards of the walls of the atrial or branchial chamber, which extends from behind the mouth to the atriopore, and into which the gill- slits of the pharynx open in the adult. Skin. — -The epidermis consists of a single layer of cylin- drical cells. Some of them project slightly from the surface, and are connected at the base with nerve fibres. These are sensory cells, and may be compared to the cells of the SKIN. 413 Son The' buccal cirri. The epidemus Ires upon a thin layer of clear cutis. Ph. Fir I7g —Transverse section through pharyngeal region of Amphioxus.— After Ray Lankester. , . Soinai Cord ; nch., notochord, beneath which lie the two dorsal ^ aone • >«-, myotome; a.c.f, atrio-codomic funnel, opening h?to sub-chordal coelom; C., caecum; G., a genital sac with ova • nip- metapleural fold ; air., atrial cavity , I h., pharyn. , whh dorsal and ventral grooves, and bars between giU-sllts. Note in the primary bars and in the ventral groove the sma cadomic spaces. The ectoderm is dark throughout. lteneath this there is a layer of fine tubes, which unite in a longitudinal canal running along each metapleural fold. These metapleural canals are said by some to arise in development by a splitting of an ongmal > solid mass (schizocoelic) ; but it seems more probable that they ate morphologically portions of the true coelom— venlro-lateral extensions of the “collar-coelom” (enterocoelic). 414 CEPHALO CHORDA. Skeleton. — This is slightly developed, for there is not only no bone, but the material is not even definitely car- tilaginous. (a) The notochord runs from tip to tip. It consists of vacuolated cells, and the supporting power is probably due to their turgidity, as in many vegetable structures. Its anterior extension beyond the end of the nerve-cord is particularly characteristic. (b) The pharynx is supported by chitinoid bars, which border the numerous gill-slits. There is also a series of paired plates underlying the mid-ventral groove. (c) The margin of the pre-oral hood contains a supporting ring, segmented into about two dozen pieces, each of which sends a process into the adjacent cirrus. (, d ) The sheath which envelops the notochord and is continued round the nerve-cord, the septa of connective tissue (myocommas) which divide the muscle segments, and the numerous “ fin rays ” which support the dorsal and ventral fins, may also be noticed here. Muscular system. — The sixty -two muscle segments, myo- tomes, or myomeres, are dovetailed into one another like a succession of V-shaped plates, and are particularly strong dorsally. These produce the side-to-side wriggling move- ments by which the animal swims. On the ventral surface, between the mouth and the atriopore, there is a transverse set of fibres, which help to drive out the water from the atrial cavity. Other muscles occur in the region of the mouth, and elsewhere. Nearly all the fibres are striated. Nervous system. — The dorsal nerve-cord is shorter than the notochord, and has no definite brain. In the anterior region, however, there is some differentiation in minute structure, and the central canal widens out to form the so- called cerebral vesicle, which in the larva communicates with the anterior by a pore (the neuropore). From the nerve-cord there arise two sets of nerves, dorsal and ventral. Of these the two anterior pairs of dorsal nerves are called cranial, and do not correspond to the myotonies. Behind these a pair of dorsal nerves arise at each myotome, but, as is the case with most of the other segmentally arranged parts of the lancelet, the members of a pair are not directly opposite to one another. The ventral nerves are absent in ALIMENTARY AND RESPIRATORY SYSTEMS. 415 the region of the two first pairs of dorsals, and behind this they divide up into many minute fibres just as they leave the nerve-cord. The two sets of nerves are compared respectively to the single-rooted sensory dorsal nerves, and to the many-rooted motor ventral nerves of higher Ver- tebrates. But the dorsal nerves of Amphioxus supply the transverse muscles as well as the skin, so that they are pro- bably partly motor. Furthermore, there is no connection between the two sets, and the dorsal nerves have no ganglia, except in so far as these are represented by aggre- gations of nerve nuclei. Nor are there any sympathetic ganglia. The nervous system of the lancelet is thus very divergent from what is typical for Vertebrates : — ( 1) A brain is almost undeveloped ; (2) the ventral roots far outnumber the dorsal roots ; (3) the two sets of roots do not unite ; (4) the dorsal nerves are partly motor ; (5) there are no spinal ganglia ; (6) there are no sympathetic ganglia. The anterior region of the nerve-cord exhibits some histological dis- tinctiveness ; and with it the following structures ate associated : — (a) Slightly to the left side there is a ciliated pit, often called olfactory. The development of this is interesting. The cavity of the medullary tube opens at first to the exterior by the neuropore. Later, an invagination of the ectoderm takes place at this point, and carries the neuropoie in with it. This invagination forms the olfactory pit ; it at first opens into the neural tube by the persistent neuropore ; later this closes, and the pit becomes a blind sac. This invagination may perhaps correspond with the ciliated duct of the sub-neural gland of Tunicates, and so with part of the hypophysis of other Vertebrates. ( b ) At the end of the nerve-cord there is a pigment spot, sometimes called an eye spot. There are no true eyes, but numerous regularly arranged pigment spots on each side of the spinal cord appear to be optic. (r) On the roof of the mouth there opens a small sac, the pre-oral pit, which may have a tasting or smelling function. It seems to arise from the left of two pouches which grow out anteriorly from the gut of the embryo. The right of these pouches forms the head cavity of the adult, so that ontogenetically the pre-oral pit is the aborted head cavity of the left side. This is, however, only one of many explanations of the organ. It is likely that the most important sensory structures of the adult are the sensitive cells of the epidermis. The feeble development of sense organs may be associated with the almost sedentary habit. Alimentary and respiratory systems. — The true mouth lies within the projecting pre-oral hood. It is surrounded by a membrane called the velum, and is fringed by twelve velar tentacles, which must not be confused with the external 416 CEPHAL OCHORDA. U cirri. In the larva the hood is absent, and the mouth is flush with the surface. The mouth opens into the pharynx, which, like it, is richly ciliated. The pharynx, like that ofTunicates, and indeed of Fishes also, is modified for respiration (Fig. 178, Ph.). Its walls are perforated by numerous gill-slits on each side, and between these lie supporting bars alternately split and unsplit at their lower ends. Along the mid-dorsal and mid- ventral lines there are grooves, respectively called hyper- and hypo-branchial. The latter is comparable to the endostyle of Ascidians, by which name it is often called. As in Ascidians, two ciliated bands — the peri- pharyngeal bands — encircle the anterior part of the pharynx. The water current which enters Fig. 179. — Development of atrial cham- ber in Amphioxus. — After Lankester and Willey. In I. the metapleural folds are seen sending a slight projection inwards. In II. the two projections have united and enclose a small space (AT.), which is the rudiment of the atrial chamber. In III. this space is enlarg- ing at the expense of the body cavity, which it pushes up before it. A comparison of this figure with the cross-section of the adult (Fig. 178) will show the relation of coelom and atrial chamber. FR., coelomic space within dorsal fin ; AL., gut ; S., coelomic space of metapleural fold ; MP., metapleural fold; SA T., projection which forms floor of atrial chamber ; AO., aorta ; B.C. , body cavity ; S.I. V., sub-intes- tinal vein ; A., nerve-cord; SH., sheath of notochord; MY., myotome; C., remains of myocoel ; A T., atrial chamber. The dotted line indicates the mesodermic wall of the body cavity. BOD Y CA VI TV. 4i7 the mouth, is, as in Tunicates, connected both with respira- tion and nutrition. The food particles, entangled in mucus, are said to pass backwards along the hyperpharyngeal groove ; the water passes down the pharynx, through its numerous gill- slits to the atrial chamber, and so to the exterior by the single atriopore. In the larva the gill-slits are few in number, and open directly to the exterior ; in the adult they are con- cealed by the atrial chamber, and have greatly increased in number; there may be more than 100 pairs. The water currents are kept up by the cilia, probably assisted by the transverse muscles. The first sign of the development of the atrial chamber is the appear- ance of two lateral folds on the body-wall, which form the metapleural folds of the adult. On their inner apposed, but not united, surfaces, two ridges appear. These grow towards one another and unite, leaving only the atriopore open. Thus the floor of the atrial chamber (Fig. 179 ,11.) is produced. The chamber, as first formed, is a tube with a very small lumen, but, secondarily, it becomes enlarged, and extending upwards and inwards, constricts the coelom, until it comes almost to surround the gut. The atrium eventually becomes a cavity, crescent- shaped in cross section, surrounding the pharynx and extending back- wards as a blind pouch on the right side of the intestine. At the same time, the metapleural folds increase in size until they assume the adult appearance (Fig. 179, III.). During these processes the originally few gill-slits have been increasing in number, both by the addition of new slits and by the division of those first formed. The division is effected by the downward growth of a secondary bar or tongue-bar in the middle of each slit. The primary bars differ from these tongue-bars in being split at their lower ends, in enclosing a coelomic space, and in some other respects. The pharynx opens into the intestinal region of the gut, which is straight and simple. Near its commencement a pouch-like “liver” or caecum (Fig. 178, C.) arises, and extends forwards on the right side of the pharynx. The anus is some distance from the end of the body (cf. Fishes) ; in the larva it is close to the caudal fin. Body cavity. — This can only be understood when its development is studied (see Fig. 183). It is a fine example of what is called the enteroccelic mode of origin. From the archenteion of the embryo a hollow ridge grows out on each side, and becomes almost at once segmented into a series of small sacs. These lie one behind the other, and lose all connection with the gut. Each ultimately divides into two — a dorsal muscular portion, and a ventral thin-walled portion. The dorsal portions form the body musculature, and retain their segmentation. Their cavity, the myoccel, persists to some extent in the adult, forming 27 4iS CEPHA L 0 CHORDA . Figs. 180 and 181. — The Nephridia ol Amphioxus . — After Boveri. Both figures are lateral views of the upper region of the pharynx, the body-wall being removed. In the upper figure the atrial chamber is laid completely open by the removal of its outer EXCRE TOR F A K9 TEA/. 419 the system of lymph spaces and canals which lie below the cutis. In the ventral poitions the septa disappear, and the enclosed spaces, bounded by somatopleure and splanchnopleure, unite to form the “ splanchnocoel ” which surrounds the gut. In the adult this space is reduced anteriorly to small spaces and coelomic canals, by the develop- ment of the atrial chamber (see Figs. 178 and 179). This pushes the somatopleure up before it as it develops, and is thus enlarged at the expense of the true coelom. The coelomic spaces and canals contain coagulable fluid, and represent the lymphatic system of higher forms. Besides the main trunk origin of the ccelom, there is an anterior portion, which is separated off from the very front of the gut, and is divided into two cavities, of which the right becomes large and thin-walled, while the left becomes a small thin-walled sac, which has an opening to the exterior. This may correspond to the head coelom of Balanoglossus, and to the bilobed head cavity which lies beneath tfie eyes of fishes, and forms most of the eye muscles. Thirdly, there is a pair of pouches, which form the first pair of muscle segments, and are continued out into the atrial folds. These may correspond to the collar coelom of Balanoglossus (MacBride). Circulatory system. — The blood is colourless, with a few amoeboid cells. There is no definite heart, but the branchial artery is rhythmically contractile. This branchial artery' lies in the portion of the body cavity which is enclosed by the endostyle, and is the anterior continuation of a large hepatic vein from the caecum. From the branchial artery a series of smaller vessels arise, which pass up the primary gill-bars, and also supply the tongue-bars. These unite on the dorsal surface of the pharynx to form the right and left dorsal aortae, which join at the hinder end of the pharynx to form a single vessel running backward over the intestine, and breaking up into capillaries on its wall. From the right dorsal aorta there arises a complex < f vessels supplying the anterior region. From the capillaries of the intestine the blood is collected in a sub-intestinal vein, which again breaks up in the caecum. The cycle is completed by the capillaries which form the hepatic vein. Excretory system. — A number of structures have been credited with excretory functions. wall, which is cut through along its line of insertion. The result is to show that the chamber is prolonged dorsally into a series of bays (/n), which lie on the surface of the tongue-bars ( t.b .). Into these bays each of the nephridia («.) open- by a pore ( 0 .), while they also open internally by many funnels (_/), fringed by very large cilia ( c .). The bays are separated by ridges (o'.), formed by a downgrowth of the walls of the ccelom over the primary bars (p.b.). my., a myotome ; sy. , one of the synapticula connecting the pharyngeal bars. The lower figure is a more superficial view, to show the blood vessels which form an anastomosing plexus ( c .) over the walls of the nephridia («//;.). d. , Dorsal aorta; co\ , ccelomic space within primary bar ; b.v., blood vessel of secondary bar ; m., cut edge of the wall of the atrial chamber ; other letters as before. 420 CEPHAL0CH0RDA. ( a ) On the floor of the atrial chamber clusters of cells occur which form the so-called renal papilke. These have been experimentally shown to possess the power of taking up foreign substances introduced into the body. ( b ) Professor E. Ray Lankester discovered a pair of short pigmented funnels in the region of the twenty-seventh myotome, which open into the atrial cavity, and perhaps communicate with the dorsal coelomic space. They hardly seem to be nephridia, and their relations are doubtful. ( c ) More recently Boveri has described an elaborate system of about ninety pairs of nephridia lying in the dorso-lateral wall of the pharynx. They are short tubules, with a single opening into the atrial cavity, and also opening into the body cavity by a variable number of funnels, most numerous in the nephridia lying in the middle of the pharynx. The vessels of the primary gill-bars and of the tongue-bars form an anastomosing vascular plexus, called a glomerulus, over the tubules. In number the tubules correspond to the primary gill-clefts, and are therefore in origin segmental structures. They are regarded by their discoverer as equivalent to the pronephric tubules of Vertebrates. Their development is unknown. - ( d) Hatschek discovered in the anterior region of the larva a nephridial tube which is absent in the full-grown adult. Its significance is very' doubtful, but it perhaps represents the connection between the left of the pair of collar pouches and the gut. Reproductive system. — The sexes are separate and similar. The organs are very simple, and without ducts. They form twenty-six pairs of horseshoe-shaped sacs, lying along the inner wall of the atrial cavity in segments ten to thirty-five on each side (Fig. 177, G.). Each lies in a “genital chamber” formed in development by constriction from the cavity of the lower part of the primitive segment. In the mature female the ovaries are large and con- spicuous ; the ova burst into the atrial cavity, whence they pass out by the atriopore. The testes are like the ovaries; the spermatozoa burst into the atrial cavity, and pass out by the atriopore. The eggs are fertilised in the surrounding water. Development.— The fertilised ovum is about in. in diameter. The segmentation is complete and almost equal (Fig. 182). The first cleavage is vertical, and divides the ovum into two equal parts ; the second is also vertical, along a meridional plane at right angles to the first, and the result is four equal cells. The third cleavage is equatorial, and gives rise to four larger cells (or macromeres) below or towards the vegetative pole, and to four smaller cells (or DE VELOPMENT. 421 micromeres) above or towards the animal pole. The blasto- sphere, which is the final result of segmentation, invaginates to form a gastrula. Along the mid-dorsal line of the gastrula the ectoderm cells sink in slightly so as to form a groove. This is the medullary groove, which here follows an unusual course of development. Instead of immediately closing to form a canal, the groove sinks inwards, and the lateral ectoderm grows over it before closing takes place. Later, the groove Fig. 182. — Early stages in the development of Amphioxus. — After Hatschek. 1. Oram with germinal vesicle; 2. four-cell stage; 3. external appearance of blastula ; 4. blastula in section ; 5. beginning of gastrula stage ; 6. section of completed gastrula. forms the medullary tube, which opens into the gut by the neurenteric canal, and to the exterior by the anterior neuro- pore (Fig. 183). The cavity of the gastrula — the archenteron — becomes the gut of the adult, and gives rise to the coelomic pouches (see p. 417). The notochord arises along the mid-dorsal line of the archenteron ; its forward extension is secondary. During the early part of larval life the ectoderm cells, including those forming the medullary canal, are ciliated. 422 CEPHA L OCHORDA. At this stage the larva is much more active than the adult. The later larvae are more sedentary, lying much on the right side, and they are strongly asymmetrical. The mouth Fig. 183. — Sections through embryos of Amphioxus, to illustrate development of body cavity. On the upper line, three longitudinal sections ; on the lower line, three transverse sections, ec., Ectoderm; cn. , endoderm ; a., archenteron ; p.s., primitive segments (protovertebra;); n.c., nerve-cord ; /. , posterior end ; np., neuropore ; ne.c., neuren- teric canal ; m.p., medullary or neural plate ; c/t., notochord ; ep ., splanchnocuel, above it is the myocoel. is placed at the left side ; the gill-slits of one side appear considerably before those of the other; the primitive seg- ments of one side are not opposite those of the other, and so on. By the process known as the “ symmetrisation ” of the larva, the apparent symmetry of the adult is produced. RELATIONS OF AMPHIOXUS AND TUN [CATES. 423 The adult position of the anus and of the olfactory pit, both to the left side, and the position of the unpaired liver diverticulum, show how partial this process is. Experimental embryology. — As an illustration of experimental em- bryology, and of the developmental potentiality of the early segmentation cells, reference may be made to the experiments of Prof. E. B. Wilson. By shaking the water in which the two-celled stages floated, Prof. Wilson separated the two cells, and the result was two quite separate and independent twins of half the normal size. Each of the isolated cells segments like a normal ovum , and gives origin, through blastula and gastrula stages, to a half-sized metameric larva. If the shaking has separated the two first segmentation cells incom- pletely, double embryos — like Siamese twins — result, and also form short-lived (twenty-four hours) segmented larva;. Similar experiments with the four-celled stages succeeded, though development never continued long after the first appearance of meta- merism. Complete isolation of the four cells resulted in four dwarf blastula, gastrula, and even larva. Separation into two pairs of cells resulted in two half-sized embryos. Incomplete separation resulted in one of three types — [a) double embryos, (6) triple embryos — one twdce the size of the other two — and (c) quadruple embryos, each a quarter size. Isolated blastomeres of the eight-celled stage never formed gastrula. Flat plates, curved plates, even one-eighth size blastula were formed, but none seemed capable of full development. Thus a unit from the four-cell stage may form an embryo, but a unit from the eight-cell stage does not. For various reasons it seems likely that this is due to qualitative limitations, not merely to the fact that the units of the eight-cell stage are smaller. For although the separated cells of the eight-cell stage have considerable vitality, and swim about actively, the difference between macromeres and micromeres has by this time been established ; in fact, the cells have begun to be specialised, and have no longer the primitive indifference, the absence of differentia- tion, which explains the developmental potentiality of the separated units of the two-celled or four-celled stages. Somewhat similar experiments have been made by other investigators on the developing ova of Ascidians, sea-urchins, etc. Specialisation of segmentation cells appears to occur at different times in different animals, but it is illogical to infer the absence of specialisation from the fact that any of the first four blastomeres, let us say, can produce an entire embryo. For specialised cells may retain a power of regeneration. Relations of Amphioxus and Tunicates. The above account of Amphioxus will in its details recall to the student the description of Tunicates. It is indeed remarkable that the resemblance should be so much stronger in minor anatomical points than in broad outline, 424 CEPHAL OCHORDA . but this is in part explained by the very marked degenera- tion displayed by the adult Ascidians. The following important resemblances should be noticed : — In both cases the walls of the pharynx are perforated by numerous slits, which open, not directly to the exterior, but into an atrial or peribranchial chamber, formed from the ectoderm, and with a single external aperture. In both, the pharynx has a distinct ventral glandular endostyle, and a dorsal fold (Tunicates) or groove ( Amphioxus ), connected anteriorly to the endostyle by means of a ciliated band ; the process of food-taking seems also to be similar. Again, the olfactory pit of Amphioxus is apparently homologous with the sub-neural gland of the Ascidians, and although there is little in common between the nervous system of the adult Ascidia and of Amphioxus, yet the nerve-cord pf the larval Ascidian, in its origin, structure, and relations, shows a close resemblance to that of Amphioxus. Similarly, the larval notochord, although never attaining the development which it does in Amphioxus , is an essentially similar structure. On the other hand, the Ascidians differ from the lancelets in many ways, e.g. the sessile habit, the presence of the test, of a heart, and of genital ducts ; the absence of seg- mentation, of nephridia, and any trace of coelom in the adult ; the U-shaped alimentary canal, the power of budding, so common in sedentary animals, and the hermaphroditism. The detailed study of development yields similar series of facts — marked resemblances coupled with marked differences ; among the latter, the absence in Ascidians of the segmented coelomic pouches of lancelets is especially noteworthy. In spite of these differences, most morpho- logists are agreed that the resemblances are due to true homology, and that lancelet and Tunicates are descended from a common ancestor, which was at least nearly related to the forms from which the true Vertebrates sprang. It is noteworthy, however, that the inclusion of Amphioxus and Tunicates among the Chordata does not bridge over the gap between Vertebrates and Invertebrates, for the rela- tion of Tunicates to the latter remains obscure. It is possible that Balanoglossus and the related forms may help to bridge the gulf, but as yet there is much un- certainty. CHAPTER XX. STRUCTURE AND DEVELOPMENT OF VERTEBRATA. The distinction between higher and lower animals, between the backboned and the backboneless, was to some extent recognised by Aristotle over two thousand years ago, and was probably always more or less evident to any who cared to look with precision at the forms of animal life. Yet it was not till about a century ago that the line of separa- tion was drawn with firmness. This Lamarck did in 1797. But the doctrine of descent— the idea of organic evolu- tion— with which Darwin impressed the thoughtful in 1859, suggested inquiry into the apparently abrupt apartness of the group of Vertebrates. The inquiry bore fruit in 1866, when the Russian naturalist, Kowralevsky, wwked out the development of the Vertebrate characteristics of Amphioxus, correlated this with the development of Ascidians, and discovered the pharyngeal gill-slits of Balanoglossus. Thus the apparent apartness of the Vertebrata was annulled. General Characters. Vertebrates are ccelomaie Metazoa , with a segmental arrange- ment of parts. The central nervous system lies in the dorsal median line , and is tubular in its origin. A skeletal rod or noto- chord, formed as an outgrowth along the dorsal median line oj the primitive gut , is always present in the embryo at least , but tends to be replaced by a mesodermic axial segmented skeleton — the backbone. Pharyngeal gill-slits , which may or may not per- sist in adult life , are always developed , but above Amphibians they are restricted to embryonic life , are not directly functional, and have no associated gill-lamellce. The heart is ventral. The eye begins its development as an outgrowth from the brain. GRADUAL APPEARANCE OF VERTEBRATE CHARACTERISTICS. 426 STRUCTURE OF VERTEBRA TA. Z < ai U D X < h < u z D H "O G to .S TO ^ £ c G « G D s £. 0) -G-Q bn , b\ d *.s to cl G ^ » w — to .3 4-. D — s CD « CJ 0 X w W TJ G TO a. I o £ 3 ,G 1) qj •5 £ co T3 CD *2 Js j s* CJ u o -G o G CL .9 ‘S •G rG (D cd -9 ,G *2 O ^ CL. 3 2 - G o G co S O *%< % -*-• to ►S V > o ^ *-< o > “! -G ' b/3 TO JG CO T3 •- % C •r Lo TO u ' ID CO CLJ C? O *3 .2 D -sr4 o ^ TO CO G O Vh D G co G .G TO D .g c cG D T3 O £ TO • D -C fc/3 TO O •-< c 4-4 G G O d jz: -2 2 ’Hh^ .3 o VO G3 V 2? 1 S3 D G O TO CJ D r- £ 5 c D D TO , CO D CO CO 0 G and ven :-cords. .2 £ . TO c3 2 co TO S J g , G ^ D c -TO 'O ,5 a. c s primitive 0 G tn O p d S 0 ^ G z < < to r* co TO s- 1 D O G Q - .SP-fl ^0 0 ^ ^L <> CL OJ g 'z 1/1 73 g; »2p ^G 0 ^ H TOJ 0 None. Three gment S3 u TO D to • r-< > CO A D * £ X TO CO »G D TO >-< eye. G D 13 S S2 p 13 CO ll 1j C .3 *TO £ tn 0 5? 0 rro d ^ D . P « Q 0 > s S3 1/3 0 j2 D O -G ** I1 GENERAL CLASSIFICA TION. 427 General Classification. I I v I Carinatae (flying). Birds. ■< Ratitae (running). (Saururae (extinct). /N Reptiles. Crocod il ia (crocod i 1 es, etc.). Ophidia (snakes). Lacertilia (lizards, etc.). Rhynchocephalia — •S ' phenodon. Chelonia (tortoises, etc.). Extinct Reptiles — (many classes). Sauropsida. Mammals. 3. Eutheria, Placentalia, Monodel- phia: the higher placental mammals. 2. Metatheria, Marsupialia, Didel- phia : Kangaroos, etc. ; young bom precociously, usually nur- tured in pouches. ^-1. Prototheria, Monotremata, Or- nithodelphia : oviparous, Orni- thorhynchus and Echidna. Mammalia. Amniota, embryos with amnion and allantois. Fishes.— ^..j-.Dipnoifdouble-breath- ing mud-fishes). — Teleostei (modern bony fishes). Ganoidei (sturgeon, etc.). Elasmobranchii (skate, shark, etc.). Amphibians. — Anura (tailless frogs, etc.). Urodela (tailed newts, etc.). Gymnophiona (worm-like C) Appendicular \ Fore limbs, and pectoral girdle. Skeleton. '( Hind limbs, and pelvic girdle. Skull. — The notochord grows forward anteriorly as far as that region of the brain known as the optic thalami. Around notochord and brain the mesenchyme forms a continuous sheath, which is the foundation of the skull. As in the case of the notochordal sheath of the trunk region, so also here cartilage is formed in the primitive membranous cranium. The first cartilages to appear are the two parachordals, which lie on the lower surface of the head at the sides of the notochord, and the two trabeculae lying in front. The parachordals grow round and above the notochord, producing the basilar plate, while the trabeculae unite in front to form the ethmoid plate. The continuance of the process of cartilage formation, together with the addition of cartilaginous nasal capsules in front and auditory capsules behind, completes the formation of the primitive cartilaginous brain-box or chondrocranium of the lower Vertebrates. Also connected with the head region, and of great import- ance, are the visceral or gill arches which loop around the pharynx on either side, and separate the primitive gill-clefts. At the time when cartilage begins to be formed in the membranous cranium, the arches also become chondrified, and at the same time divided into segments. Of these arches there are never more than eight. The most anterior is the mandibular arch which bounds the mouth, the second the hyoid ; these two are of great importance in the development of the skull. The others, in Fishes and at least young Amphibians, bound open gill- slits and support the pharynx ; above Amphibians, they are less completely developed. 432 STRUCTURE OF VERTEBRATA. In the Elasmobranch fishes, the mandibular and hyoid arches do not form any direct part of the cartilaginous brain-case, but in the Teleo- steans and thence onwards, the cartilages or bones arising in connection with the mandibular and upper part of the hyoid arches contribute directly to the formation of the skull. The hyoid proper, or lower part of the hyoid arch, forms the skeleton supporting the tongue. Cartilages arising in the lower part of the third visceral arch assist in the formation of the hyoid bones of the higher Vertebrates, and parts of the two other arches appear to help in forming the laryngeal skeleton of Mammals. The mandibular arch in Elasmobranchs and frogs divides into a lower portion — Meckel’s cartilage — -which forms the lower jaw or its basis, while from the upper portion a bud grows forward, the palato-pterygo- quadrate cartilage, which forms the upper jaw in shark and skate, and has a closer union with the skull in the frog. In higher Vertebrates the lower portion of the mandibular always forms the basis of the lower jaw, a quadrate element is segmented off from the upper part, but the palato-pterygoid part seems to arise more independently. The hyoid arch also divides into a lower portion, the hyoid proper, and an upper portion, the hyo- mandibular, which may connect the jaws with the skull, or from Amphibians onwards may be more remarkably displaced and modified as a columella or stapes connected with the ear. We adhere to the old interpretation, according to which the mandibular and hyoid form two arches ; but Dohrn believes that they are equivalent to four — palato-pterygo-quadrate, Meckel’s cartilage, hyo-mandibular, and hyoid, being in his opinion independent arches. Returning now to the brain-box itself, we must notice another complication, — the development of “ membrane ” bones. If we examine the skull of the skate, we find that the brain lies within a cartilaginous capsule ; but this is not entirely closed, spaces (the fontanelles) being left in the roof, which during life are covered only by the tough skin with its numerous “ dermal denticles.” In the sturgeon, again, the small skin-teeth are replaced by stout bony plates covering over the cartilaginous capsule. From such super- ficial bony plates it is supposed that the “ membrane ” bones, or ossifications in membrane, which form so import- ant an element in the skull of the higher Vertebrate, have originated. In some bony fishes, notably the salmon, we find the brain enclosed in a double capsule. Inside there is a cartilaginous brain-case in which what are called centres of ossification have appeared, and upon this a layer of membrane bones is placed, which can be readily removed without injury to the cartilage beneath. In general, however, we must recognise that, with the appearance of membrane bones, two changes tend to occur, — first, the cartilaginous cranium tends to be reduced and to exhibit considerable openings ; second, in the remaining cartilage THEOR V OF THE SKULL. 433 centres of ossification appear, and we thus have “cartilage” bones formed. Further, in spite of the developmental differences, the mem- brane and cartilage bones become closely united to one another, or even fused, and there is thus formed “a firm, closed, bony receptacle of mixed origin,” as exemplified by the skull of any of the higher Vertebrates. We may thus say that in the evolution of the skull there is first a cartilaginous capsule, that this becomes invested to a greater or less extent by dermal ossifications, and that finally the dermal bones lose their superficial position, and, fusing with the ossified remainder of the cartilaginous cranium, form a complete bony capsule. In Cyclostomes and Elasmobranchs the brain-box is wholly cartilaginous ; above Elasmobranchs the cartilage is more or less thoroughly replaced or covered by bones. In the individual develop- ment there is a parallel progress. Although one is safe in saying that skeletal structures in Vertebrates are mostly mesodermic in origin, it should be noted — (i) that the notochord is endodermic, and (2) that in the head certain ectodermic proliferations may give rise to skeletal rudiments of a connective tissue nature which sub- sequently become differentiated into cartilage (Goronowitsch, Platt). But there is still doubt as to this last point. Theory of the skull. — Near the beginning of this century, Oken and Goethe independently propounded what is known as the vertebral theory of the skull. Regarding the skull as an anterior portion of the vertebral column, composed of three or four vertebra;, they compared the bones of the different regions to the parts of a vertebra. Thus in the hindmost region of the skull, the basi-occipital, the two ex-occipitals, and the supra-occipital were held to correspond to the centrum, the neural arches, and the neural spine of a vertebral body. This undoubtedly suggestive theory, modified in various details, per- sisted for a long period, but ultimately gave way before the advances in comparative anatomy and embryology. Huxley gave it its deathblow, and Gegenbaur replaced it by what may be called the segmental theory of the skull. To realise this theory we must go back in development to the period before the mesoblast has ensheathed the notochord. At this time the segmentation of the body is expressed, not in the skeleton (notochord), but in the primitive segments. The segments, though less obvious than in the trunk, are represented in the head region. Formerly nine were enumerated, but it appears that in Elasmobranchs they are more numerous. Subsequently brain and spinal cord become alike enveloped in the mesoblastic sheath, which gives rise to the skeleton of both head and trunk. 28 434 STRUCTURE OF VERTEBRA TA. Summary of the Development of the Skull. Elements. Origin. Results. I. Parachordals and trabeculae, aided in some cases by the end of the notochord. Their precise relations, e.g. to the notochord, are unknown. Occipital region, with four bones— basi-occi- pital, two ex-occipitals, and a supra-occipital (in part). The basi-occipital is distinct only in Reptiles, Birds, and Mammals. Sphenoidal and ethmoidal region, with basi- sphenoid and pre-sphenoid (present only in Reptiles, Birds, and Mammals), paired ali- sphenoids and orbitosphenoids, the inter-orbital septum, the lateral or ectoethmoids, the inter- nasal septum. II. Sense capsules. (a) Nasal. (b) Auditory. From cartilage surrounding the ectodermic pits which form the foundation of nose and ear. (a) Unite with ethmoida^ region. (b) May give origin to five bones — pro-, sphen-, pter-, epi-, and opisth-otics, or to the single periotic- of Mammals. III. Arches. (a) Mandibular. (b) Hyoid arch. These arches, like those which follow them, are supports of the pharynx, lying between primit- ive or persistent gill-slits. (a) Upper part = palato-pterygo-quadrate cartilage of Elasmobranchs, palatine, pterygoid, and quadrate bones in the higher Vertebrates, but in Mammals the quadrate is believed by many to become the incus of the middle ear. Lower part=Meckel’s cartilage— the basis of the lower jaw in all animals ; the part next the quadrate becomes the articular bone, which in Mammals is believed by many to become the malleus of the middle ear. (b) Upperpartorhyo-mandibular=the "sus- pensorium ” cartilage of Elasmobranchs, the hyo-mandibular and symplectic of Teleosteans, the columella auris of Amphibians, Reptiles, and Birds, the stapes of the Mammal’s ear. Lower part = the hyoid proper (cartilage or bone). IV. Investing membrane bones. (a) From the roof of the skull. (b) On the floor of the skull, i.e. from the roof of the mouth. (c) About the sides of the skull. (d) About the upper jaw. (e) About the lower jaw. Originally of the ' nature of external bony plates, tooth structures, and the like. (a) Parietals, frontals, nasals, etc. (b) Vomer, parasphenoid, etc. (f) Lachrymal, squamosal, orbitals, etc. (d) Premaxilla, maxilla, jugal, and quadrato- jugal (in part). (e) Dentary, splenial, angular, supra-angular, coronoid. VERTEBRAL COLUMN. 435 The great development of the muscle segments of the trunk region induces a secondary segmentation of the mesoblastic skeleton (vertebral column), while the slight development of the muscles of the head region exercises no such influence upon its skeleton ; this is therefore always quite devoid of segmentation. The segmentation of the head, in con- tradistinction to the skull, is expressed, although indistinctly, by the muscle segments and by the nerves supplying these, perhaps also by the lateral sense organs, the ganglia, and the arches. While it is quite certain that it is the head that is segmented and not the skull, the details of the segmentation are still much debated. Vertebral column. — A dorsal skeletal axis is character- istic of Vertebrata, and its usefulness is evident. It gives coherent strength to the body ; it is usually associated very closely with a skull, with limb girdles, and with ribs ; it affords stable insertion to muscles ; its dorsal parts usually form a protective arch around the spinal cord. To understand this skeletal axis, we must distinguish clearly between the notochord and the backbone. The notochord is the first skeletal structure to appear in the embryo. It arises as an axial differentiation of endo- derm along the dorsal wall of the embryonic gut or archenteron beneath the nerve-cord. The backbone, which in most Vertebrates replaces the notochord, has a mesoblastic origin ; it develops as the substitute of the notochord, but not from it. In Ba/anog/ossus, what is sometimes dignified with the name of notochord, is restricted to the most anterior part of the body ; in the Tunicata the notochord is confined to the tail, in Amphioxus it runs from tip to tip of the body, in Cyclostomata and Dipnoi it persists as an unsegmented gristly rod, in other Vertebrates it is more or less com- pletely replaced by its better substitute — the backbone. In Cyclostomata the notochord forms and is ensheathed by a cuticula chorche (or membrana limitans interna ) ; outside this there is a meso- blastic 01 skeletogenous sheath ; and outside this again lies a cuticula sceleli (or membrana limitans externa). It is likely that this represents a piimitive condition. What happens in most Vertebrates is that the skeletogenous or mesoblastic sheath forms the backbone, and more or less completely obliterates the notochord. The formation of cartilage takes place at regular intervals in the notochordal sheath, and the vertebral bodies thus formed alternate regularly with the primitive muscle segments. This arrangement is necessary for the proper attachment of the muscles to the future vertebrae, and makes it prob- able, as we noticed above, that the segmentation of the backbone is secondary, and was only acquired, as a mechanical necessity, when the notochordal sheath became chondrified, and so rigid. Thus we reach the conclusion that the primitive segmentation of the Vertebrates, alike in head and trunk, finds its expression in the arrangements of the 436 STRUCTURE OF VERTEBRATA. primitive segments and the nerves supplying these, and not in the skeleton, which is a later development. In the higher Vertebrates, soon after the formation of the bodies of the vertebrae, the rudiments of the neural arches appear in the mem- brane surrounding the spinal cord. Finally, centres of ossification may occur, and so produce the segmented backbone. In Amphioxus, in Myxine , and in young lampreys (known as Ammo- cates), the notochord persists, unsegmented and with a simple sheath. In the adult lamprey there are rudimentary arches of cartilage forming a trough in which the spinal cord lies. In the cartilaginous Ganoid fishes, in the Chimcera type, and in the Dipnoi, arches appear both above and below, but there are as yet no vertebral bodies. These begin in the Elasmobranchs, in which the notochord is constricted by its encroaching sheath. In the bony Ganoids the vertebrae ate ossified, as they are in all the higher Vertebrates. Moreover, the notochord is more and more completely obliterated as the backbone grows. In the oldest known vertebral column in Britain, that of Cosmopholis mitchelli, the vertebrae are annular, as in some other ancient fishes. The calcification in the notochordal sheath has simply formed a tube around the notochord, — a state which illustrates an interesting persist- ence of an embryonic phase. It will be remembered (see p. 36) that, according to Kleinenberg, the notochord supplies the necessary growth- stimulus for the rise of its substitute, the backbone. A vertebra generally consists of several more or less independent parts : the substantial centrum, the neural arches which form a tube for the spinal cord, and are crowned by a neural spine ; the transverse processes which project laterally, and are, perhaps, homologous with the inferior hsemal processes in the posterior region of Fishes and some Amphibians. The ribs which support the body-wall and usually arti- culate with the transverse processes, or with the transverse processes and centra, perhaps bear the same relation to the vertebrae that the visceral arches do to the skull. Amphibians are the first to show a breast-bone or sternum. It arises from two cartilaginous rods in a tendin- ous region on the ventral wall of the thorax, and seems to be different from that of higher animals. For the sternum which is present in some Reptiles, and in all Birds and Mammals, arises from a cartilaginous tract uniting the ventral ends of a number of ribs. Limbs and girdles. — No secure conclusion has yet been reached as to the origin of the paired limbs. According to LIMBS AArD GIRDLES. 437 Gegenbaur, the pectoral and pelvic girdles are homologous with branchial arches, while the primitive limbs are made up of modified fin rays originally like those of the unpaired fins. According to Dohrn, the limbs are residues of a longitudinal series of segmentally arranged outgrowths, Figs. 185 and 186. — Ideal fore- and hind-limb. — After Gegenbaur. H., Humerus ; R., radius ; U., ulna; r'., radiale ; ulnare ; i., intermedium ; c., centrale ; 1-5, carpalia bearing the correspond- ing digits with metacarpals {me.) and phalanges (//<.). G Femur; ti4, tibia;./?., fibula;?., intermedium; t., tibiale (astra- galus)',/., fibulare(); c. , centrale; 1-5, tarsalia bearing the corresponding digits with metatarsals (mt.) and phalanges (J>h.). perhaps comparable to the parapodia of an Annelid. According to Wiedersheim, the girdle portion is primarily due to the centripetal growth of the fin skeleton, which arose from a localisation of the supports of continuous lateral folds. The pectoral or shoulder girdle consists of a dorsal scapular portion or shoulder - blade, a ventral coracoid 43§ STRUCTURE OF VERTEBRATA. portion, with the articulation for the limb between them, and of a forward growing clavicle or collar-bone. The pelvic or hip girdle consists of a dorsal iliac portion, a ventral posterior ischiac portion, with the articulation for the limb between them, and of a ventral, usually anterior, pubic portion. The fore limb — from Amphibians onwards — consists of a humerus articulating with the girdle, a lower arm composed of radius and ulna lying side by side, a wrist or carpus of several elements, a “ hand ” with metacarpal bones in the “ palm,” and with fingers composed of several phalanges. The hind limb — from Amphibians onwards — consists of a femur articulating with the girdle, a lower leg composed of a tibia and fibula lying side by side, an “ankle” region or tarsus of several elements, a foot with metatarsal bones in the “ sole,” and with toes composed of several phalanges. In Fishes the limbs are fins, i.e. without digits. Distinct from the other bones are a few little sesamoids of occasional occurrence, e.g. the knee-pan or patella. They develop in connection with the tendons of muscles. Nervous system. — This includes — (a) the central nervous system, consisting of brain and spinal cord ; ( b ) the peri- pheral system, consisting of spinal and cranial nerves ; and (c) the sympathetic nervous system. The central nervous system first appears as a superficial groove along the mid-dorsal line of the embryo. The sides of this ectodermic groove meet, and, uniting, convert the medullary groove into the medullary canal. The greater part of this canal forms the spinal cord ; the anterior portion of it is specialised as the brain. There is at first a posterior connection between the neural canal and the primitive 'gut of the embryo ; when this is lost the cavity still persists as a little ciliated canal in the centre of the cord, and as the in- ternal cavity of the brain. In Cyclostomes and Bony Fishes the central nervous system arises as a solid cord of cells, the cavities not appearing until a later stage ; this condition does not seem to be primitive. Brain. — At an early stage, even before the closing-in process is completed, certain portions of the anterior region of the medullary canal grow more rapidly than others, and form the three primary brain vesicles. By further processes RRA1N. 439 of growth and constriction, these three form the five regions of the adult brain. When first formed the brain vesicles lie in a straight line, but as a consequence, probably, of their rapid and unequal growth, this condition is soon lost, and a marked cranial flexure is produced. In the lower forms, e.g. Cyclostomata, the flexure is slight, and is corrected later, but in the higher types it is very distinct, and causes the marked over- lapping of parts so obvious in the adult. We must now follow the metamorphosis of the primary brain vesicles. The first vesicle gives rise anteriorly to the cerebral hemi- Fig. 187. — Longitudinal section of brain of young dog-fish (diagrammatic). — After Gaskell. C./i., Cerebral hemispheres; o.th., optic thaiami ; 3 F., third ventricle; In., infundibulum : pt.b., pituitary body; o.l. optic lobes; cb., cerebellum; M.O., medulla oblongata; 4 F., fourth ventricle ; S.C., spinal cord. spheres, while the remainder forms the region of the optic thaiami or thalamencephalon. The cerebral hemispheres (prosencephalon or fore-brain) are exceedingly important. They predominate more and more as we ascend in the scale of Vertebrates, and become more and more the seat of intelligence. Except in a few cases, the prosencephalon is divided into two parts — the cerebral hemispheres — which contain cavities known as the lateral ventricles. The two hemispheres are united by bridges or commissures, which have considerable classifica- tory importance. With the anterior region of the hemi- spheres olfactory lobes are associated. In Cyclostomata, Ganoids, and Teleosteans, the fore-brain has no nervous roof, but is covered by an epithelial pallium homologous with 440 STRUCTURE OF VERTEBRA TA. what is called the choroid plexus of the third ventricle in higher Verte- brates. This choroid plexus is a thin epithelium, with blood vessels in it. But in Elasmobranchs, Dipnoi, and Amphibians the basal parts of the fore-brain have grown upwards to form a nervous roof, and this persists in higher Vertebrates. The optic thalami (thalamencephalon or tween-brain) form the second region of the adult brain. Hence arise the optic out- growths, which form the optic nerves and some of the most essential parts of the eyes. The original cavity per- sists as the third ventricle of the brain ; the thin roof gives off the dorsal pineal outgrowth or epiphysis, and, uniting with the pia mater, or vascular brain membrane, forms a choroid plexus ; the lateral walls become much thick- ened (optic thalami) ; the thin floor gives off a slight ventral evagination, or infundibulum, which bears the en- igmatical pituitary body or hypophysis. The pituitary body. — This is derived in part from the brain and in part from the mouth, and is extremely difficult to under- stand. It is apparently equivalent in part to the sub-neural gland of Tunicates, but this does not carry us much further. Dohrn con- nected it with two abortive gill-slits, but the evidence seems insufficient. Beard has inter- preted it as a residue of the original mouth which Vertebrates are supposed to have pos- sessed before the persistent one with which we are familiar was evolved, and of the in- nervation of that hypothetical structure ; but again confirmation seems wanting. Of its physiological nature we know almost nothing, beyond that a pathological state of this organ is associated in man with certain diseases, e.g. acromegaly. The pineal body. — The dorsal upgrowth (or epiphysis) from the roof of the thalamencephalon is represented, though to a varying extent, in all Vertebrates. It is terminally differ- entiated into a little body known as the pineal body. This was entirely an enigma until De Graaf discovered its eye-like structure in Angitis, and Baldwin Spencer securely confirmed this in the New Zealand “lizard” {Sphenodon), where the pineal body shows distinct traces of a retina. Fig. 188. — Origin of pineal body. — After Beard. Lowest figure — a section through the first embry- onic vesicle, while the medullary groove (g.) is still open ; o., optic out- growths. Middle figure shows beginning of pin- eal outgrowth (/.). Top- most figure shows a later stage. THE PINEAL BODY. 44' In Elasmobranchs the pineal process is very long, and, perforating the skull, terminates below the skin in a closed vesicle. In the young frog it also comes to the surface above the skull, but degenerates in adolescence. In Sphenodon the stalk passes through the skull by the “parietal foramen,” so that the “eye” itself lies close beneath the skin, the scales of which in this region are specialised and transparent. In Iguana, Anguis , Lacerta, etc., the epiphysis loses connection with the “eye” portion; and it is also to be noticed that in Anguis and Iguana the pineal body receives a nerve from a ‘ ‘ parietal centre ” near the base of, but independent of, the epiphysis ; this nerve is transitory in Anguis, more or less persistent in Iguana. Above Reptiles the pineal stalk is always relatively short, and its terminal portion forms a glandu- lar structure. In fact, the develop- ment of the pineal body is much more complicated than at first ap- peared ; thus, according to Locy’s researches on Acantkias embryos, it represents the fusion of an extra pair of eyes. The full significance of the pineal body is thus uncertain. According to one view, its primitive function is that of an unpaired, median, upward - looking eye — a function retained only in the Reptiles mentioned above, the organ having elsewhere undergone (independent) degeneration. It has also been interpreted as an “organ for the perception of warmth.” It may be, however, that the optic function is not primitive, but the result of a secondary modification. Thus one of the first interpreta- tions (Dohrn’s) connected the pineal and the pituitary outgrowths with a supposed passage of the original nerve-cord. Fig. 189. — Diagram of the parts of the brain in Vertebrates. — After Gaskell. c.h., Cerebral hemispheres; c.pl., choroid plexus; o.t/i., optic thal- aini ; o.l., optic lobes; cb. , cere- bellum; c.pl. , choroid plexus; M.O., medulla oblongata ; S.C., spinal cord. lypothelical mouth through the The second primary vesicle of the brain forms the third region, that of the optic lobes (mesencephalon or mid- brain), in the adult brain. The floor and lateral walls form the thickened crura cerebri ; the roof becomes the two optic lobes, which are hollow in almost all Vertebrates. In Mammals a transverse furrow divides each optic lobe into two (corpora quadrigemina). The cavity of the vesicle becomes much contracted, and forms the narrow iter or aqueduct of Sylvius, a canal connecting the third ventricle with the fourth. 442 STRUCTURE OF VERTEBRA TA. The third primary vesicle gives rise to the metencephalon, or hind-brain, or region of the cerebellum, and to the myelencephalon, or after-brain, or region of the medulla oblongata. In the metencephalon the roof develops greatly, and gives rise to the cerebellum, which often has lateral lobes, and overlaps the next region. In the higher forms the floor forms a strong band of transverse fibres — the pons Varolii. From the region of the medulla oblongata most of the cranial nerves are given off. Here the roof, partly over- lapped by the cerebellum, degenerates, becoming thin and epithelial, the cavity — called the fourth ventricle — is con- tinuous with the canal of the spinal cord. Sun in i ary. (1) Cerebral hemispheres, prosencephalon, or fore-brain. Note commissures, olfactory lobes and nerves, and first and second ventricles. (2) Optic thalami, thalamencephalon, or tween- brain. Note — (a) optic, ( b ) pineal, (c) pituitary outgrowths, and the third ven- tricle. (3) Optic lobes, mesencephalon, or mid-brain. Note crura cerebri, and the aqueduct of Sylvius. (4) Cerebellum, metencephalon, or hind-brain. Note pons V arolii. (5) Medulla oblongata, myelencephalon, or after - brain. Note rudimentary roof, fourth ventricle, and origin of most of the cranial nerves. Enswathing the brain, and following its irregularities, is a delicate membrane — the pia mater — rich in blood vessels , which supply the ner- vous system. Outside this, in higher Vertebrates, there is another membrane — the arachnoid — which does not follow the minor irregular- ities of the brain so carefully as does the pia mater. Thirdly, a firm membrane — the dura mater — lines the brain-case, and is continued down the spinal canal. In lower Vertebrates the dura mater is double throughout ; in higher Vertebrates it is double only in the region of the spinal cord, where the outer part lines the bony tunnel, while the inner ensheaths the cord itself. In Fishes the brain-case is much larger than the brain, and a large lymph space lies between the dura and the pia mater. An understanding of the relations of the different regions will be First Embryonic Vesicle. Median Embryonic Vesicle. Thiid Embryonic Vesicle. SPINAL CORD. 443 facilitated by a study of the following table, which Dr. Gadow gives in his great work on Birds in Bronn’s Thierreich : — Region. Floor. Sides. Rook. Cavity. Spinal cord. Anterior grey and white com- missure. White and grey substance. Posterior com- missure. Central canal. Myelen- cephalon. Medulla oblongata. Epithelium of choroid plexus. Posterior part of fourth ventricle. Meten- cephalon. Commissural part. Pedunculi of crura cerebri. Cerebellum. Anterior part of fourth ventricle. Mesen- cephalon. Crura cerebri. Cortex of optic lobes. Anterior com- missure, velum of Sylvius. Aqueduct of Syl- vius and lateral extensions. Thalamen- cephalon. Infundibulum, hypophysis, cliiasma. Inner part of optic lobes and optic thalami. Epiphysis and epithelium of choroid plexus. Corpus callo- sum. Anterior com- missure. Third ventricle. Prosen- cephalon. Corpus stria- tum. Lamina ter- minalis. Olfactory lobes. Cerebral hemispheres. Lateral ven- tricles. Spinal cord. — After the formation of the brain vesicles, the remainder of the medullary canal forms the spinal cord. The canal is for a time continuous posteriorly with the food canal beneath, so that a => -shaped tube results. The connection between them is called the neurenteric canal (Fig. 183, ne.c.), and though it is only temporary, its frequent occurrence is of much interest. The wall of the medullary canal becomes very much thick- ened, the roof and floor grow less rapidly, and thus the cord is marked by ventral and dorsal longitudinal furrows. At the same time, the canal itself is constricted, and persists in the fully formed structure only as a minute canal lined by ciliated epithelium, and continuous with the cavity of the brain. 444 STRUCTURE OF VERTEBRATA. In the cord it is usually easy to distinguish an external region of white matter, composed of medullated nerve-fibres, and an internal region of grey matter, containing ganglionic cells, and non-medullated fibres. The arrangement of the grey matter, together with the longitudinal fissures, give the cord a distinct bilateral symmetry, which is sometimes obvious at a very early stage. The brain substance is also composed of grey and white matter, but there, at any rate in higher forms, the arrangement is very complicated. Cranial nerves.— The origin and distribution of the cranial nerves may be summarised as follows : — Name. I Origin. 1. Olfactory. s .* 2. Optic. s. Front of fore- brain. Optic thalami. 3. Oculomotor or ciliary. 7;/.* 4. Pathetic or trochlear, m. 5. Trigeminal. s. and ///. 6. Abducens. m. 7. Facial, chiefly m. partly s. 8. Auditory. s. 9. Glossopharyn- geal. s. and 1/1. 10. Vagus or Pneu- mogastric. s. and m. Floor of mid- brain. From pos- terior part of optic lobes. Medulla ob-^ longata. ?> >5 Distribution. Olfactory organ. Eye. All the muscles of the eye but two. Superior oblique muscle of the eye. (1) Ophthalmic to snout. jr. (2) Maxillary to the upper jaw, etc. s. (3) Mandibular to lower jaw, lips, etc. >n. and x. External rectus of eye. (1) Hyoidean. (2) Palatine. (3) Buccal to space between jaws and snout. Ear. First gill arch and cleft. Posterior gills and arches, lungs, heart, gut, and body generally. Notes. They cross before they enter the brain, and generally unite at their intersection. A ciliary ganglion at roots. Perhaps belongs to 5, as a ventral root. Gasserian ganglion at roots. The nature of the ophthalmicus pro- fundus, often includ- ed with 5, sometimes with 3, is doubtful. Perhaps belongs to 7, asa ventral branch. Ganglion at the roots of 7 and S. Apparently a com- . plex, including the elements of four or five nerves. The fourth or pathetic nerve is peculiar among motor nerves in that it appeals to arise from the extreme dorsal summit of the brain, between the mid- and hind-brain, from the region known as the “valve of Vieussens.” In Fishes the seventh nerve is mainly a nerve of special sense ; in higher Vertebrates it has lost most of its sensory branches, and become chiefly motor. * The letter s. is a contraction for sensory or afferent, i.e. transmitting impulses from a sensitive area to the centre; and in. is a contraction for motor or efferent, i.e. transmitting impulses from the centre to the body. SPINAL NERVES. 445 There is much uncertainty in regard to the morphological value of the various cranial nerves, but the following conclusions may be stated : — ( 1 ) The nerves arise either as outgrowths of the central system or as specialisations of peripheral cells. Each spinal nerve has two roots — a dorsal and a ventral, but in most cases at least a cranial nerve has primitively a single dorsal root developing from a neural ridge of the dorsal surface of the brain. In many cases this root divides into “dorsal,” “ventral,” and other branches. As may be well studied in 9, these typically innervate a gill-arch and slit, and the branches may be therefore called (as Beard proposes) supra-branchial (dorsal), post- branchial, prae-branchial, etc. In the course of growth the nerve often shifts from the position whence its root originated. (2) Some of the cranial nerves mark distinct segments of the head, while others are secondary derivatives. It is likely that 1, 3, 5, 7, 8, 9, and several parts of 10 mark segments. It is possible that the oculo- P,c Pi Fig. 190. — Diagrammatic section of spinal cord. p.f., Posterior fissure; p.c., posterior column of white matter; d.ft.s., dorsal, posterior, sensory or afferent root; g., ganglion; v.a.m., ventral, anterior, motor or efferent root ; c.n., compound spinal nerve with branches; s.g., sympathetic ganglion; a.c., anterior column; the anterior fissure is exaggerated; g.c., ganglion cells ; g.m., grey matter ; w.m., white matter. motor is a ventral tool associated with the ophthalmicus profundus, that the trochlear is a ventral root of the trigeminal, that the abducens is a ventral root of the facial. (3) It is possible that each truly segmental nerve supplied a primitive gill-slit, as 7 supplies the spiracle, 9 the first branchial, 10 the second, third, fourth, and fifth branchials. (4) It is possible that each segmental nerve was associated with a branchial sense organ (Beard and Froriep). These organs arise above the gills, and grow thence into various parts of the head, and along the trunk as the “lateral line.” It is possible that a branchial sense organ lay over each primitive gill-cleft, and had an associated ganglion. The ganglia known as ciliary, gasserian, etc., may be the ganglia of branchial sense organs, and it seems that parts of them arise in development independently of the brain. It may be that nose and ear were originallv branchial sense organs. Spinal nerves. — Each spinal nerve has two roots — a 446 STRUCTURE OF VERTEBRATA. dorsal, posterior, or sensory, and a ventral, anterior, or motor. These arise separately and independently, but combine in the vicinity of the cord to form a single nerve. The dorsal root exhibits at an early period a large ganglionic swelling — -the spinal ganglion ; the ventral root is apparently non-ganglionated. Moreover, the dorsal root has typically a single origin (as in the cranial nerves), while that of the ventral root is often multiple. The dorsal roots are outgrowths of a continuous ridge or crest along the median dorsal line of the cord. As the cord grows the nerve roots of each side become separated. They shift sidewards and downwards to the sides of the cord. The ventral roots are later in arising ; they spring as outgrowths from the latero-ventral angle of the cord. Beard maintains that the spinal ganglia do not arise from the spinal cord, but have an independent origin from the deeper layers of the epiblast. According to most authorities, the sympathetic ganglia are offshoots from the same rudiment as that from which the dorsal ganglia arise, and it is possible that they are the more or less vagrant ganglia of the ventral roots, with which they are connected by small fibres. On this view (Gaskell’s) both roots may be said to be ganglionated. But the ganglion of the dorsal root is stationary in position, and the nerve-fibres which pass through it come both from the visceral (splanchnic) and from the peripheral somatic parts, separating from one another within the cord. On the other hand, the supposed ganglion (sympathetic) of the ventral root is more or less vagrant, and off the main line of the root, from which it receives small fibres passing to splanchnic or visceral structures. Sense organs. — The central nervous system has doubtless arisen in the course of history from the insinking of external nerve cells ; it does arise in development as an involution of ectoderm or epiblast. The same layer gives origin to the essential parts of the sense organs. The Vertebrate eye is formed in great part as an outgrowth from the brain, but as the brain is itself an involution of epiblast, the eye may be also referred to external nerve-cells. Branchial sense organs. — In many Fishes and Amphib- ians there are lateral sense organs which form the “lateral lines,” while others lie in the head, and were in all likelihood primitively connected with gill-clefts. In Sauropsida and Mammals these branchial sense organs are no longer distinct as such. The nose. — It is possible that the sensory pits of skin which form the nasal sacs are two branchial sense organs. SENSE ORGANS. 447 They are lined by epithelium in great part sensory, and are connected posteriorly with the olfactory nerves. In all Fishes, except Dipnoi, the nasal sacs remain blind ; in Amphibians, and in all the higher Vertebrates, they open posteriorly into the cavity of the mouth, and serve for the entrance of air. The peculiar nostril of hag-fish and lamprey is referred to in the chapter on Cyclostomata. The ear in Invertebrates develops as a simple invagina- tion of the ectoderm, forming a little sac, which may become entirely detached from the epidermis, or may retain its primitive connection ; so in Vertebrates, at an early stage, an insinking forms the auditory pit. In some Fishes (. Serranus , salmon) and Amphibians a common ectodermic thickening seems to form the rudiment from which the ear, the lateral line, and a pre-auditory sensory patch are derived. The auditory sac sinks further in, and the originally wide opening to the exterior becomes a long narrow tube. In Elasmobranchs, which exhibit many primitive features, this condition is retained in the adult ; in other Vertebrates the tube loses its connection with the exterior, and becomes a blind prolongation of the inner ear — the aqueductus vestibuli, or ductus endolymphaticus. The auditory vesicle, at first merely a simple sac, soon becomes very complicated. It divides into two chambers, the larger utriculus and the smaller sacculus. From the utriculus three semicircular canals are given off, except in the lamprey and hag, which have two and one respectively. From the sacculus an outgrowth called the cochlea or lagena originates ; it is little more than a small hollow knob in Fishes and Amphibians, but becomes large and important in Sauropsida and Mammals. As this differentiation of the parts of the internal ear takes place, the lining epithelium also becomes differentiated into flattened covering cells and sensory auditory cells. The auditory cells are arranged in patches to which branches of the auditory nerve are distributed. With these sensory patches calcareous concretions (otoliths) are associated, except in the cochlea of Mammals. The fact that lime salts are often deposited in the skin, and that the ear-sac arises as an insinking of epiblast, may perhaps shed some light on the origin of otoliths. The parts which we have so far considered constitute together the membranous labyrinth of the ear. Round about them the mesoblast (mesenchyme) forms a two-layered envelope. Its inner layer disin- 44§ STRUCTURE OF VERTEBRATA. tegrates to produce a fluid, the perilymph, which bathes the whole outer surface of the membranous labyrinth. Its outer layer forms a firm case, the cartilaginous or bony labyrinth, surrounding the internal ear. The membranous labyrinth itself contains another fluid, the endolymph. With regard to the function of the parts of the ear, the semicircular canals are believed by many to be concerned with the appreciation of a change in the direction or velocity of movement. How far the ears of lower Invertebrates (e.g. Crustacea and Molluscs) are adapted for any function except this, is still doubtful, and we can hardly see that any other would be of much use to purely aquatic animals. It seems likely at any rate that the primitive function of the ear was the percep- tion of vibrations, and that from this both the sense of hearing and the sense of equilibration have been differentiated. It is in accordance with the facts mentioned above that we rarely find in Fishes any special path by which impressions of sound may travel from the external world to the ear. In Amphibians and higher Vertebrates, however, the ear has sunk further into the recesses of the skull, and a special path for the sound is present. In Elasmobranchs, the spiracle, or first gill-cleft, is situated in the vicinity of the ear ; in higher forms, according to many authors, this first gill-cleft is metamor- phosed into the conducting apparatus of the ear. In development, a depression beneath the closed gill-cleft unites with an outgrowth from the pharynx, and thus forms the tympanic cavity, which communicates with the back of the mouth by the Eustachian tube. The tympanic cavity is closed externally by the drum or tympanum, which may be flush with the surface, as in the frog, or may lie at the end of a narrow' passage, which in many Mammals is furnished externally with a projec- tion or pinna. In Amphibia and Sauropsida the tympanic cavity is traversed by a bony rod — the columella, which extends from the drum to the fenestra ovalis, a little aperture in the w>all of the bony labyrinth. In Mammals this is replaced by a chain of three ossicles, an outermost malleus, a median incus, an internal stapes. The homologies of these ossicles are still uncertain. One interpreta- tion has been stated on p. 434 ; the following is Hertwig’s : — Malleus = Articular + angular elements of Meckel’s cartilage. Incus = Palato-quadrate of lower Vertebrates. Stapes of Mammals has a double origin, being formed from the upper part of hyoid arch + an ossification from the w'all of the ear capsule = (w'holly ?) columella of Birds, Reptiles, and Amphibians. The eye. — There is no eye in Amphioxus , it is rarely more than larval in Tunicates, it is rudimentary in Myxine and in the young lamprey. In higher forms the eye is always present, though occasionally degenerate, e.g. in fishes from caves or from the deep sea. It is hidden under the skin in Proteus, an amphibian cave-dweller, and in the subter- ranean amphibians like Ca’dlia , very small in a few SENSE ORO'ANS. 449 snakes and lizards, and its nerves are abortive in the mole. The adult eye is more or less globular, and its walls con- sist of several distinct layers. The innermost layer bound- ing the posterior part of the globe is the sensitive retina, innervated by fine branches from the optic nerve. It may be compared to the nervous matter of the brain, from, which, indeed, it arises. Outside of the retina is a pigmented epithelium, and outside of this a vascular membrane ; together these are often called the cho- crystalline lens, a c., Cornea; ci.h aqueous humour ; c.b ciliary It is fringed in front by a pigmented and muscular ring — the iris, which is for the most part a continuation of the choroid. The space enclosed by the iris in front of the lens is called the pupil. Protecting and closing the front of the eye is the firm cornea continuous with the sclerotic, and covered externally by the conjunctiva — a delicate epithelium continuous with the epidermis, between the cornea and the iris is a lymph space containing aqueous humour, while the inner chamber behind the lens contains a clear jelly — the vitreous humour. The lens is moored by “ ciliary processes ” of the choroid, and its shape roid. The vascular part may be com- pared to the pia mater covering the brain, and like it is derived from meso- blast. Outside of the choroidisaprotective layer or sclerotic, comparable to, and continuous with, the dura mater covering the brain, and also mesoblastic in origin. Occupying the front of the globe is the crystalline lens, a Sc. ~Ch. R. n. Fig. 19 i. — Diagram of the eye. 45o STRUCTURE OF VERTEBRA TA. is alterable by the action of accommodating muscles arranged in a circle at the junction of iris and sclerotic. In many Reptiles, and in Birds, a vascular fold, called the pecten, projects from the back of the eye into the vitreous humour. A similar fold in Fishes ( processus falciformis ) ends in a knot-like structure in the lens. The retina is a Fig. 192. — Development of the eye. — After Balfour and Hertwig. 1. Section through first embryonic vesicle, showing outgrowth of optic vesicles ( op.v .) to meet the skin ; f.b., thalamen-cephalon ; G., the gut. 2-4. Sections illustrating the formation of the lens (/.) from the skin, and the modification of the optic vesicle into an optic cup ; R ., retina ; v.h., vitreous humour. 5. External aspect of embryonic eye ; , lens. very complex structure, with several layers of cells, partly supporting and partly nervous ; the layer next the vitreous humour consists of nerve-fibres, while that furthest from the rays of light and next the pigment epithelium consists of sensitive rods and cones. The region where the optic nerve enters, and whence the fibres spread, is called the blind spot, and near this there lies the most sensitive region — the yellow spot, with its fovea centralis, where ALIMENTARY SYSTEM. 45 1 all the layers of the retina have thinned off except the cones. Among the extrinsic structures must be noted the six muscles which move the eyeball, the upper and lower eyelids, which are often very slightly developed, and the third eyelid or nictitating membrane. Above Fishes there is a lachrymal gland associated with the upper lid, and a Harderian gland associated with the nictitating membrane. In Mammals there are also Meibomian glands. The secretions of all these glands keep the surface of the eye moist. While the medullary groove is still open, the eyes arise from the first vesicle of the brain as hollow outgrowths or primary optic vesicles. Each grows till it reaches the skin, which forms a thickened involution in front of it. This afterwards becomes the compact lens. Meantime it sinks inwards, and the optic vesicle becomes invaginated to form a double-walled optic cup. The two walls fuse, and the one next the cavity of the cup becomes the retina, while the outer forms the pigmented epithelium. Meanwhile sur- rounding mesoblast has insinuated itself past the lens into the cavity of the optic cup, there forming the vitreous humour, while externally the mesoblast also forms the vascular choroid, the firm often cartilaginous sclerotic, the inner layer of the cornea, etc. Along the thinned stalk of the optic cup the optic nerve is developed. Its protective sheath is continuous with the sclerotic of the eye and the dura mater of the brain. As the nerves enter the optic thalami, they always cross one another in a chiasma, and their fibres usually interlace as they cross. Alimentary system. — The alimentary tract exhibits much division of labour, for not only are there parts suited for the passage, digestion, and absorption of the food, but there are numerous outgrowths, e.g. lungs and allantois, which have nothing to do with the main function of the food canal. By far the greater part of the food canal is lined by endoderm or hypoblast, and is derived from the original cavity of the gastrula— the primitive gut or archenteron. This is the mid-gut or mesenteron. But the mouth cavity is lined by ectoderm, invaginated from in front to meet the mid-gut. This region is the fore-gut or stomodjeum. Finally there is usually a slight posterior invagination of ectoderm, 452 STRUCTURE OF VERTEBRATA. forming the anus. This is the hind-gut or procto- dseum. Associated with the mouth cavity or stomodmum are— (a) teeth (ectodermic rudiments of enamel combined with a mesodermic papilla which forms dentine or ivoiy) ; (/») from Amphibians onwards special salivary glands; (c) a tongue, — a glandular and sensitive outgrowth from the floor. The tongue develops as a fold of mucous membrane in front of the hyoid, and afterwards becomes increased by growth of connective tissue, etc. In larval Amphibians muscle strands find their way into it, and it seems likely, as Gegenbaur has recently in- dicated, that their original function was to compress the glands. As they gained strength they became able for a new function, that of moving the tongue. In Myxine , Dipnoi, and higher animals, the nasal sac opens posteriorly into the mouth ; in some Reptiles and Birds, and in all Mammals, the cavity of the mouth is divided by a palate into an upper nasal and lower buccal portion. The origin of the oral aperture is uncertain. In Tunicates it is formed by an ectodermic insinking which meets the archenteron ; in Amphioxus it seems to arise as a pore in an ectodermic disc ; in other cases it is a simple ectodermic invagination ; or it may owe its origin to the coalescence of an anterior pair of gill-clefts innervated by the fifth nerve. If the last interpretation be true, its origin illustrates that change of function which has been a fiequent occurrence in evolution. But if the mouth arose from a pair of gill-clefts, and in some cases it actually has a paired origin, then there must have been an older mouth to start with. Thus Beard in his brilliant morphological studies dis- tinguishes between “the old mouth and the new.” The new mouth is supposed to have resulted, as Dohrn suggested, from a pair of gill- clefts ; the old mouth was an antecedent stomodseum, of which the so-called nose of Myxine and the oral hypophysis of higher forms may be vestiges. This theory harmonises with the observations of Kleinen- berg on the development of the mouth in some Annelids (Lopado- rhynchus), in which the larval stomodseum is replaced by a paired ectodermic invagination. The mouth cavity leads into the pharynx, on whose walls there are the gill-clefts. Of these the maximum number is eight, except in Amphioxus. If we exclude the hypo- thetical clefts, such as those possibly represented by the mouth, the first pair form the spiracles — -well seen in skates. In the position of the spiracles the Eustachian tubes of higher Vertebrates develop. In front of the spiracle there is sometimes a spiracular cartilage, which Dohrn dignifies as a distinct arch. The other gill-clefts are associated with gills in Fishes and Amphibians, while in Sauropsida and Mammals, in which there are no gills, four “ visceral ” clefts persist as practically functionless vestigial structures. In ALIMENTARY SYSTEM. 453 some cases their openings are very evanescent. The clefts are bordered by the branchial arches, and supplied by blood vessels and nerves. With the anterior part of the alimentary canal two strange structures are associated — the thyroid and the thymus. The thyroid gland arises as a diverticulum from the ventral wall of the pharynx. It may be single (as in some Mammals), or bilobed (as in Birds), or double (as in some Mammals and Amphibians), or diffuse (as in Bony Fishes). Only in the larval lamprey does it retain its original connection with the pharynx, and is then a true gland. As to its morphological nature, its mode of origin suggests com- parison with the hypobranchial groove in Amphioxus and the endostyle of Ascidians. According to Dohrn, it is a residue of the visceral cleft between the hyo-mandibular and the hyoid. It sometimes has accessory parts derived from a visceral cleft (fourth in Mammals). Almost the only light which has been cast on the physiological nature of the thyroid is from the pathological side. Goitre and Derbyshire neck are associated with an enlargement and diseased state of this organ, and myxcedema with its degeneration or absence. As injection of extract of sheep’s thyroid, or even eating this organ, alleviates myx- cedema, it is concluded that the thyroid must have some specific effect on the large quantity' of blood which flows through it. It is probably safe to say' that the thyroid aids in keeping the blood at a certain standard of health, perhaps through some specific ferment. The thymus arises as a dorsal endodermic thickening where the outgrowths which form the gill-clefts meet the ectoderm. It may be associated with a variable number of clefts — seven in the shark Heptanchus, five in the skate, four in Teleosteans, three in the lizard, one in the chick, and one (the third) in Mammals. In the young lamprey there are said to be no fewer than twenty-eight thymus rudiments. In Mammals it often seems to degenerate after youth. As it has from its first origin a distinct lymphoid nature, and apparently forms leucocytes, it has been interpreted (Beard) as a structure adapted for the phagocytic protection of the gills from bacteria, parasites, and the effects of injury. If this be so, we can understand its diminishing importance in Sauropsida and Mammalia, where its place may be to some extent taken by the palatal and pharyngeal tonsils, which are believed by some (Stohr, Killian, Gulland) to have a similar phagocytic function. The pharynx leads into the gullet or oesophagus, which is a conducting tube, and this into the digestive stomach, which is followed by the digestive, absorptive, conducting intestine, ending in the rectum and anus. From the oesophagus the air- or swim-bladder of most Fishes, and the lungs of higher Vertebrates, grow out. The air-bladder usually lies dorsally and is almost always single ; 454 STRUCTURE OF VERTEBRATA. the lungs lie ventrally and are double, though connected with the gullet by a single tube. The beginning of the intestine gives origin to the liver, which regulates the composition of the blood and secretes bile, and to the pancreas, which secretes digestive juices. The pancreas has often a multiple rudiment. From the hindmost region of the gut, the allantois grows out in all animals from Amphibians onwards. In Amphibians it is represented by a cloacal bladder ; in the higher Vertebrates it is a vas- cular foetal membrane concerned with the respiration or nutrition of the embryo, or both. Cilia are very common on the lining of the intestine in Inverte- brates, but they are much rarer in Vertebrates. Yet as they occur in Amphioxus , lampreys, many fishes, Protopterus, some Amphibians, and in embryonic Mammals, it seems not unlikely that the alimentary tract was originally a ciliated tube. It is often said that, in some cases at least, as in lamprey, frog, and newt, the blastopore or opening of the primitive gastrula cavity persists as the anus of the adult ; but it seems doubt- ful whether the anus is not always a new formation. In many cases, at least, an ectodermic invagination or proctodeum meets the closed archenteron, and at the junction the two epithelial layers give way, so that an open tube is formed. The formation of the anus does not take place close to the posterior end of the primitive gut, but at a point some short distance in front of this. In consequence the so-called post-anal gut is formed. This is continuous with the neurenteric canal, and so communicates with the neural canal. The post-anal gut attains in Elasmobranchs a relatively considerable length. It has been very frequently found in Vertebrates, and is probably of universal occurrence. After a longer or shorter period it becomes completely atrophied, and with it the communication between neural and alimentary canals is completely destroyed. Speculative. — The primitive gut was probably a smooth straight tube, but the rapid multiplication of well-nourished cells would tend to its increase in diameter and in length. But on increase in both Fig. 193. — Origin of lungs, liver, and pancreas in the chick. — After Goette. The mesoderm is shaded ; the endo- derm dark. lg., One of the lungs ; St., stomach ; 4, liver; p. , pancreas. ALIMENTARY SYSTEM — SUMMAR Y. 455 directions the slower growth of the general body would impose limita- tions, and in this we may find the immediate growth-condition deter- mining the origin of folds, crypts, creca, and coils, which would be justified by the increase of absorptive and digestive surface. There are regular longitudinal folds in Myxine, cross-folds traversing these would form crypts, which may be exaggerated into the pyloric cceca of Teleosteans and Ganoids, while other modifications would give rise to “ spiral valves ” and the like. In the same way it maybe suggested that the numerous important outgrowths of the mid-gut, such as lungs, liver, pancreas, and allantois, so thoroughly justified by their usefulness, may at first have been due to necessary conditions of growth — to the high nutrition, rapid growth, and rapid multiplication of the endoderm. It may be noted that in the development of the Amphibian Nec turns, there are hints of more numerous endodermic diverticula (Platt). It is also said that the hypochorda — a transitory structure — arising below and subsequent to the notochord, is in part due to a series of dorsal out- growths from the gut (Stohr). Even the notochord, which arises as a median dorsal fold, may be speculatively compared to a typhlosole — folded outwards instead of inwards. The future elaboration of the organs which arise as outgiowths of the gut, would, however, depend on many factors, such as their correlation with other parts of the body, and would at each step be affected as usual by natural selection. Alimentary System. — Summary. Region of the Gut. Outgrowths. Associated Structures. Mouth cavity, or Stomodaeum, or Fore-gut, originating as an ectodermic invagination, or from a co- alescence of two gill-clefts. Oral part of the hypophysis. Teeth. Salivary glands. Tongue. Pharynx, gullet or oeso- phagus, stomach, small in- testine, large intestine, and rectum ; = the mesenteron or mid-gut, originating from the cavity of the gastrula, the archenteron or primitive gut ; lined by endoderm. Thyroid) and the Thymus) gill-clefts. Air bladder ; lungs. Liver. Pancreas. Allantois. The pancreas is usually the result of two ventral out- growths and a dorsal one. In Cyclostomes and Elasmobranchs it seems to have but one rudiment'; in the sturgeon four. With the several out- growths-the surrounding me- soderm becomes associated, often to a great extent. Note also the origin of the notochord as an axial differentiation of cells along the mid-dorsal line of the embryonic gut. Anal region, or Proctodaeum, or Hind-gut. Where the mouth of the gastrula persists, it forms the terminal aperture of the gut, in other cases there is an ectodermic invagination or proctodaeum. In some Fishes, all Amphi- bians, all Sauropsida, and the Prototherian Mammals, the terminal part of the gut is a cloaca or common chamber, into which the rectum, the urinary, and the genital ducts open. 456 STRUCTURE OF VERTEBRA TA. Body cavity. — In Amphioxus a paired pouch grows out from the archenteron. 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