SS ‘ornell University Libra QL 47. 148 1916 wwii ; CORNELL UNIVERSITY LIBRARY BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND GIVEN IN 1891 BY HENRY WILLIAMS SAGE RETURN TO ALBERT R. MANN LIBRARY Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003404161 OUTLINES OF ZOOLOGY OUTLINES OF ZLOOLOGY BY ry Ke ARTHUR THOMSON, M.A. REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN 3 JOINT-AUTHOR OF ‘‘ THE EVOLUTION OF SEX”; ‘THE STUDY OF ANIMAL LIFE,” ‘THE SCIENCE OF LIFE,” ‘‘THE PROGRESS OF SCIENCE,” ‘“‘ HEREDITY,” ** DARWINISM AND HUMAN LIFE,” ETC. AUTHOR OF FIFTH EDITION, REVISED, WITH q20 ILLUSTRATIONS NEW YORK D. APPLETON AND COMPANY 1916 107 PREFACE TO THE SIXTH EDITION —>— THis book is intended to serve as a Manual which students of Zoology may use in the lecture room, museum, and laboratory, and as an accompaniment to several well- known works, cited in the Appendix, most of which follow other modes of treatment. To numerous authorities I acknowledge an: obvious indebtedness, a detailed recognition of which would be out of place in a book of this kind. I must, however, acknowledge that in the preparation of a previous edition I had throughout the able assistance of Miss Marion Newbigin, D.Sc., and I have also been aided by sugges- tions from various kindly critics, especially Professor W. C. M‘Intosh, Professor P. J. White, the late Dr. Ramsay Traquair, Dr. John Beard, the late Mr. J. G. Goodchild, Dr. Arthur Masterman, Dr. John Rennie, Dr. W. D. Henderson, Mr. E. S. Russell, Mr. W. P. Pycraft, Mr. C. Tate Regan, and Professor H. J. Fleure. For most of the figures I am indebted to my artist friends, Mr. William Smith, Miss Florence Newbigin, vi PREFACE TO THE SIXTH EDITION. Miss E. M. Shinnie, and the late Mr. George Davidson. In almost every. case the illustrations have been derived from original memoirs and works of reference, or drawn from specimens. ° J. A. T. THE UNIVERSITY, ABERDEEN, March 1914, CONTENTS. ° —+— GENERAL CHAPTER I ‘ PAGE GENERAL SURVEY OF THE ANIMAL KINGDOM 5 « FE CHAPTER II PHYSIOLOGY c , ‘ i . é * 2 CHAPTER III MORPHOLOGY. . 7 . : . » 34 CHAPTER IV EMBRYOLOGY . : A . . ' » 52 CHAPTER V PALLONTOLOGY . . . . . » 97 CHAPTER VI DocTRINE OF DESCENT . ‘ a i " . 84 vill CONTENTS. INVERTEBRATES CHAPTER VII PROTOZOA . CHAPTER VIII SPONGES . F “ : ‘ . : CHAPTER IX COLENTERA . ‘ P : * . . CHAPTER X UNSEGMENTED ‘‘ WorRMS” i P F % CHAPTER XI ANNELIDS ' . . CHAPTER XII ECHINODERMS . ‘ CHAPTER XIII ARTHROPODA . . CHAPTER XIV - ONYCHOPHORA OR PROYOTRACHEATA, Myrioropa, INSECTA ; ‘ ‘i ‘ % CHAPTER XV ARACHNOIDEA AND PALAOSTRACA . . . ' CHAPTER XVI MOoLuuscs . ‘ ‘ ‘ ‘ ql PAGE 88 124 - 137 179 209 « 280 AND 318 363 + 380 HEMICHORDA TSC HORS CEPHALOCHORDA STRUCTURE AND -CYCLOSTOMATA FISHES . AMPHIBIA REPTILES. BIRDS ‘ MAMMALS DEVELOPMENT OF VERTEBRATES CONTENTS. VERTEBRATES CHAPTER XVII CHAPTER XVIII CHAPTER XIX CHAPTER XX CHAPTER XXI CHAPTER XXII CHAPTER XXIII CHAPTER XXIV CHAPTER XXV CHAPTER XXVI ix PAGE 434 443 459 473. 516 529 578 610 647 692 x CONTENTS. GENERAL CHAPTER XXVII DIsTRIBUTION CHAPTER XXVIII THEORY OF EVOLUTION. APPENDIX ON BOOKS INDEX LIST OF ILLUSTRATIONS © ON ONAWNHNG . Fertilisation in Ascaris megalocephala—after Boveri . . Modes of Segmentation : : : ‘ ; . Life history of a coral, Monoxenia darwinii—from Haeckel . . Embryos—(1) of bird; (2) of man—after His. The latter —>— . Duckmole (Ornithorhynchus). z /’henacodus, a primitive extinct Mammal—after Cope Extinct moa and modern kiwi—-after Carus Sterne . Crocodiles . é Salamander, an Amphibian . Queensland dipnoan (Ceratodus) . Alancelet, Amphioxus—after Haeckel . Ascidian or sea-squirt—after Haeckel . Cephalopod (paper nautilus, female) . : : : . Fresh-water crayfish (Astacus), a Crustacean—atter Huxley . « @, Caterpillar; 4, pupa; c, butterfly . . : . Spider . . . Crinoid or feather-star . Earthworm ‘i ‘ ; . Bladderworm stage of a Cestode—after Leuckart 3 . Sea-anemones on back of hermit-crab—after Andres . ‘ . Fossil Foraminifera (Nummulites) in limestone—after Zittel . . Diagrammatic expression of classification in a genealogical tree. B indicates possible position of Balanoglossus. D of Dipnoi, S of Sphenodon or Hatteria x % . Diagram of Vertebrates . Diagram of Invertebrates 7 . Diagram of cell structure—after Wilson . Structure of the cell—after Carnoy . Fertilised ovum of Ascarzs—after Boveri . Diagram of cell division—after Boveri te . ‘Karyokinesis—after Flemming é . . . Diagrammatic expression of alternation of generations . Diagram of ovum, showing diffuse yolk granules. . Forms of spermatozoa (not drawn to scale) . : . Diagram of maturation and fertilisation. (From “Evolution of Sex”) . F : about twenty-seven days old F : a) > a a WOON ANU WW N LIST OF ILLUSTRATIONS. - Gradual transitions between Paludina neumayré (a), the oldest form, and Paludina hernesd (7)—from Neumayr . . Life history of Amaba . Actinophrys sol (Sun- animalcule)—after Grenacher . Polystonella, megalospheric form, with large central chamber (AZ) and one nucleus (4/)—after Lister Polystomella, microspheric form, with large central chamber (c.c.), numerous nuclei (), bridges of- protoplasm between chambers (8)—after Lister a ‘ . Paramecium—after Biitschli . . Conjugation of Paramecium aurelia — four stages—after Maupas . . Diagrammatic expression of process of conjugation i in Para- mectum aurelia—after Maupas . . . Vorticella—after Biitschli , . : ‘ . Volvox globator—after Cohn . i 3 . . Life history of MWonocystzs—after Biitschli a . . Life history of Gregarina—after Biitschli . . . End-to-end union of Gregarines—after Frenzel - . Life history of Coccidium—after Schaudinn. . . . . Diagram of Protomyxa aurantiaca—after Haeckel . . . Formation of shell in a simple Foraminifer—after Dreyer . A Foraminifer (Polystomel/a) showing shell and pseudopodia —after Schultze . . A pelagic Foraminifer—Hastigerina. (Globigerina) Murrayi after Brady . Optical section of a Radiolarian (Actinomma)—after Haeckel . Glossina palpalis, tse-tse fly ‘i . Trypanosoma gambiense . A colonial flagellate Infusorian—Proterospongia haechelii— after Saville Kent . . Simple sponge (Ascetta primoriialis)—afier F Haeckel ss . A sponge colony : . Sponge spicules . Section of a sponge—after FE. Schulze F . Diagram showing types of canal system—after Korschelt and Heider. The flagellate regions are dark throughout, the mesogloea is dotted, the arrows show the direction of the currents. All the figures represent cross-sections through the wall . : . Development of Sycandra raphanus—after F, E. Schulze . Diagrammatic representation of mag susan of Oscarella /obularis—after Heider . . A, Young ae i a Whitman. “RB, Female Orthonectid (Khopalura giardii)—after Julin . . Salinella—after Frenzel . Diagram of Ccelenterate structure, endoderm ‘darker through: out . Colony of Hydractinia on back of a Buccinum shell tenanted by a hermit-crab . ‘ . . : . 93- - Diagram of a typical Hydrozoon polyp—after Allman . Hydra hanging from water-weed—after Greene é . Minute structure of Aydra—after T. J. Parker and sebes . Development of ydra—after Brauer . Bougainvillea—after Allman . Structure of a Medusoid—after Allman . Surface view of Aurelia—from Romanes . Vertical section of Aure/a—after Claus ; . Diagram.of life history of Aure/ia—after Haeckel . . Lucernaria—after Korotneff . Diagram of Lucernaria—after Allman . External appearance of Tealia crassicornis. . . Vertical section of a sea-anemone—after Andres . Section through -sea-anemone (across arrow in Figure 79)— . Alcyonarian Spicules . Diagram of a gymnoblastic Hydrotd—after Allman . . Graptolites ; ; . Hydroids—after Hincks : . Campanularian Hydroid—after Allman . . Diagram: of a Ctenophore—after Chun . Hydroctena. A medusoid with hints of Ctenophore structure LIST OF ILLUSTRATIONS. after Andres . Z, Diagrammatic section of Zoantharian ; re of Alcyonarian —after Chun . The formation of a coral- shell (Astroides)—after Pfurtscheller . Structure of Antipatharians . Diagrams of Types of Alcyonarla—after Hickson Coralium rubrum, a corner of a Ss Lacaze- Duthiers Commensalism of sea-anemones and hermit-crab_ . . 934. Portions of excretory system of flat-worms a . 94. 95. Diagram of Turbellarian—after Lang i E - Structure of liver fluke—after Sommer. From ventral surface. The branched gut’(g.) and the lateral nerve (/.7.) are shown to the left, the branches of the excretory vessel (e.v.) to the right - . Reproductive organs of liver ‘fluke—after Sommer : . . Life history of liver fluke—after Thomas. : . . Diagram of life cycle of liver fluke - Diagram of reproductive organs in Cestode joint—Constructed from Leuckart . . . Life history of Zenda solium—after Leuckart . ‘ . Diagram: of life history of Zaxza solium . Diagrams of bladder-worms . , . Diagrammatic longitudinal section of a Nuedtoas (Amphi- porus lactifloreus), dorsal view—after M‘Intosh . Transverse section of the Nemertean Drepanophorus latus —after Birger . . Transverse section of a simple Nemertean (Car inella)—after Biirger . . : . . . . Xilf PAGE 141 143 145 148: 150 ISI 153 155 150 158. 158 160- 161 162. 163. 165 166. 167 168. 169: 170 17I 172 173 174 175 177 178 181 184 185 187 189: 192 193 194 196: 197 198 19% LIST OF ILLUSTRATIONS. . Cross section through Ascarés . . . Illustrating the structure of a Nematode , . Trichine in muscle, about to be encapsuled—after Leuckart . Trichine in muscle, encapsuled. There may be 12,000 in a gramme of pig’s muscle—after Leuckart . Earthworms. . . . . Anterior region of earthworm—after Hering A . Transverse section of earthworm. . . . Reproductive organs of earthworm—after Hering . . Stages in the development of earthworm—after Wilson F . Arenicola marina . Anterior part of nervous system in Arenicola—after Vogt and Yung ‘i ‘ . Dissection of lob-worm from “dorsal surface ‘ . Cross-section of Arenzcola—after Cosmovici . Development of Polygordzus—after Fraipont . . . Parapodium of ‘‘Heteronereis” of MVerezs pelagica—after Ehlers . . . Free-living Polycheete (Nereis cultrifera) 5 . Transverse section of leech—after Bourne F . Alimentary system of leech—after Moquin- -Tandon . 7 ‘ . Dissection of leech—after Bourne . : 5 ‘ . A nephridium of leech—after Bourne . : . Development of Sagitta—after O. Hertwig. Illustrating formation of a body cavity by pockets from the archen- teron ; also the early separation of reproductive cells. . Actinotrocha or larva of Phoronds—after Masterman . Phoronis . Diagram of an Ectoproctous "Polyzoon (Plumatella) . Interior of Brachiopod shell, showing calcareous support for the ‘‘arms ”’—afler Davidson . . Pluteus larva of Ophiuroid, with rudiment of adult—after Johannes Miiller . Star-fish 5 . . . Alimentary system of star-fish—after Miiller ‘and Troschel . . Diagrammatic cross-section of star-fish arm—after Ludwig . . Ventral surface of disc of an oes (Ophiothrix pica —after Gegenbaur ji . . Apical disc of sea-urchin : . . . Dissection of sea-urchin 3 F . . Spicules of Holothurians—after Semon . . Dissection of Holothurian (Holothuria tubulosa) from the ventral surface . . Diagrammatic vertical section through disc and base of one of the arms of Antedon rosacea—after Milnes Marshall . . Stages in development of Echinoderms—after Selenka . Appendages of Norway lobster : . Section of compound eye of A/ysés vulgaris—after Grenacher . Longitudinal section of lobster, showing some of the organs . Male reproductive organs of crayfish—after Huxley . Mouth appendages of cockroach—after Dufour . Transverse section of insect—after Packard . . Head and mouth parts of bee—after Cheshire . Nervous system of bee—after Cheshire 2 . Food canal of bee—in part after Cheshire . Hive-bees and the cells in which they develop . Mouth-parts of mosquito—from Nuttall and Shipley . Young may-fly or ephemerid—after Eaton . . Diagrams of insect embryo—after Korschelt and Heider . Life histories of insects . Life history of the silk-moth ‘(Bombyx mori) . Development of blow-fly ne erythrocaphala) —ateer LIST OF ILLUSTRATIONS. . Female reproductive organs of crayfish—after Siskow : - Section through the egg of, Astacus aftér the completion of segmentation—after, Reichenbach . Longitudinal section of later embryo of Astacus—after Reichenbach . Section through cephalothorax ofa crab—after Pearson . Dorsal aspect of swimming crab (Portunusy . Dorsal aspect of shore crab (Carcinus) . Ventral aspect of female shore crab . - Dorsal surface of. Apus cancriformis — from “Bronn’s _ Thierreic! . . Daphnia Cypris . Cypris, side view, after removal of o1 one valve—after “Zenker, . Cyclops type . . Acorn-shell (Balanus tintinnabulum)—after Darwin . Development of sail china (Not drawn to scale). . ‘ . 2 . Nebatia—after Sars... . . . Anaspides—after Calman. 5 ‘ : . An Amphipod (Caprella linearis) . ; . ‘ . Hermit-crab withdrawn from its shell. The anterior ap- «, pendages are broken off ‘ A : . . Mysis flexuosa, from side . Nervous system of shore-crab ( Carcinus manas)—afier Bethe . Zozea.of common shore-crab (Carcinus menas)—after Faxon . External form of Peripatus—after Balfour . . Dissection of Peripatus—after Balfour . Embryos of Peripatus capensis, showing closure of blasto- pore and curvature of i ala Korschelt and Heider . A millipede . . a . . . . Acentipede . . 3 . . Female cockroach (P. ortentalis) ane : Male cockroach (P. orzentalzs) 1 Ventral aspect of male cockroach with the wings: extended. An imaginary median line has been inserted . Leg of cockroach, Thomson Lowne 5 XV. PAGE 293 294 295 296 297 297 298 299 300 xvi FIG. 188, 189. 190. 191. 192. 193. . 194. 195. 196. 197. 198. 199. 200. 201. 202. 203. 204. 205. 206. 207. 208, 209. 210, 2m. 212. 213. 214. 215. 216, 217 218, 219. 220, 221. 222, 223. 224, 225. 226. 227). 228, 229. LIST OF ILLUSTRATIONS. Mosquito—from Nuttall and Shipley : Anurida maritima (after Imms), one of the primitive wing- less Collembola s . . . Acerentomon, a very primitive. insect . Scorpion. f ‘ : . . Garden spider Dissection of Mygale from the ventral surface—after Cuvier Section of lung-book—after Macleod A Follicle-mite (greatly enlarged) Itch-mite (Sarcoptes scabiez) (greatly “enlarged) Tick (Zxodes réduvius, female), dorsal surface (after Wheler), showing the oval shield. . Tick (Zxodes riduvius, nie ventral surface—after Wheler . i ; . 7 Limulus or King-crab i 5 : Young Lzmulus—after Walcott Trilobite (Cosocephadétes)—after Barrande . Vertical cross-section of a Trilobite (Calymene)—after Walcott . Sea-spider (Pycnogonum littorale), from the dorsal surface . Male of Nymphon—after Sars é P 3 . Ideal mollusc—after Ray Lankester . . . Stages in molluscan development . 5 . . Roman snail (Helix pomatza) i Vertical section of the shell of a species of Helix . Dissection of snail Reproductive organs of Helix pomatia—after Meisenheimer Snail (Helix pomatia), laying its eggs—after Meisenheimer Diagram of larva of Paludina—after Erlanger The fresh-water mussel ( Uzo) f . Structure of Axodonta—after Rankin . z Development of Anodonta—after Goette Side view of Sepza—after Jatta ‘ External appearance of a cuttlefish (Lotigo) . Diagram of the structure of Sef7a.—Mainly after Pelseneer | Diagram of circulatory and excretory systems in a i as like Sepia—after Pelseneer Male of Avgonauta (after Jatta), showing ‘« hectocotylus ” arm ; compare Fig. 9 of female . Bunch of Sepia eggs attached to plant—after Lieto 3 Common buckie (Buccinum undatum) ‘ Bivalve (Panopea norvegica), showing siphons Nudibranch (Dendronotus arborescens), showing dorsal out- growths forming adaptive gills . Ventral surface of Patella vulgata—after Forbes and Hanley Chiton—after Prétre : Dorsal view of nervous system of Chiton—after Pelseneer . Anatomy of Chiton . A Pteropod (Cymbulia peronii), showing the wing- ‘like ex- pansions (pteropodial lobes) of the mid-foot . s LIST OF ILLUSTRATIONS. FIG. 230. Stages in molluscan development . . 231. Proneomenta. Nervous system—from Hubrecht | . 232. Section of shell of Nautilus—after Lendenfeld . . 233. The Pearly Nautilus (Mautdlus pompelius)—after Owen 234. Male of Balanoglossus (Dolichoglossus) howalevskit—after Bateson. 235. Transverse section through. gill- -slit region of Piychodera minuta—after Spengel .: 236. Direct development of Dolichoglossus—after Bateson 237. Tornaria larva, from the side—after Spengel _—_—. 238. Piece of a colony of Cephalodiscus, showing the tubes in- habited by the animals—after Ridewood i 239. An individual Cephalodiscus—after Ridewood ? 240. Dissection of Ascidian—after Herdman . P 241. Diagram of Ascidian—after Herdman 242. Young embryo of Ascidian ( Clavelina)—after Van Beneden and Julin 7 243. Embryo of Clavelina—modified after Seliger ‘ . 244. “Nurse” of Doliolum miilleri—after Uljanin : 245. Sexual individual of Dololum miilleri—after a é 246. Diagram of Salpa africana . . 247. Anatomy of Appendicularia—after Herdman E 248. Lateral view of Amphioxus—after Ray Lankester . , 249. Transverse section through phenees region of Amphioxus —after Ray Lankester . 250. Development of atrial chamber ‘in Amphioxus—alter Lankester and Willey . 251 and 252. The nephridia of ‘Amphioxus—after Boveri 2 253. Early stages in the development of Amphioxus—after _ Hatschek 254. Sections through embryos of Amphioxus, to illustrate de- velopment of body cavity . 254A. Portions of excretory systems of Phyllodoce and Api oxus—after Goodrich 255.°Transverse section through an Elasmobranch "embryo (diagrammatic)—after Ziegler . 4 ase and 257. Ideal fore and hind limb—after Gegenbaur : 258. Partial section of brain of young Vertebrate. c 259. Vertical section of the ne eye in an embryo of Spheno- don—after Dendy 260. Diagram of parts of the brain in Vertebrates — after Gaskell . a . 5 : 261. Diagrammatic section of spinal cord 262. Diagram of spinal cord of man, thoracic region— after John- ston ; 263. Diagram showing the ear and related parts in a young cat . 264. Diagram of the eye . 265. Development of the eye—after Balfour and Hertwig 266. Origin of lungs, oe and Meret in the chick—after Goette . . . : ‘ xviii LIST OF ILLUSTRATIONS. FIG, 267. Section through a young newt . : ' 268. Blood corpuscles of Vertebrates . 269. Diagram of circulation—after Leunis 270. Development of excretory system of Vertebrate—in part after Boveri . . i . 271. Urogenital system . . . 272. Mammalian ovum—after Hertwig 273. Median longitudinal section of anterior end of Myxine— after Retzius and Parker r . : 274. Respiratory system of hag, from ventral surface. 275. Bdellostoma stouti (Californian hag), aiitiriaiie in sheath of mucus—after Bashford Dean 276. The lamprey (Petromyzon marinus) 277. Longitudinal vertical section of anterior "end of larval lamprey—after Balfour . 278. Restored skeleton of Palaospondylus gunni—after Traquair 279. Pterichthys milleré. Lateral view—restored by Traquair" 280. Under surface of skull and arches of skate—after W. K. Parker . 7 : 281. Side view of skate’s skull—after W. K. Parker F ‘ 282, Skeleton of skate—from a preparation . . 283. Dissection of nerves of skate ~ 284. Side view of chief cranial nerves of Blasmobranchs—sightly modified from Cossar Ewart. ‘ : 285. Spiral valve of skate—after T. J. Parker . . Upper part of the dorsal aorta in the skate—after Monro . Heart and adjacent vessels of skate—in part after Monro . . Urogenital organs of male skate . : . Urogenital organs of female skate—in part after Monro ‘ . Elasmobranch development—after Balfour . Embryo dogfish in egg-case (‘* Mermaid’s purse °) which has been cut open to show contents . The haddock . . External characters of a Teleostean—a carp (Cxprinus carpio)—after Leunis . Caudal vertebra of haddock . . Disarticulated skull of cod—from Edinburgh Museum of Science and Art . Pectoral girdle and fin of cod—from Edinburgh Museum of Science and Art Diagram of a Teleostean gill i in section . ‘ . Diagram of Teleostean circulation—after Nuhn_ . . The early development of the salmon ‘i ‘. ri . Development of eel—after Smit . j . : . Young skate—from Beard . . Lateral view of dogfish (Scyd/éum catulus) . ‘: . Outline of Acanthodes sulcatus—after Traquair . . Larva of Polypterus (after Budgett), 14 inch in length . Sturgeon (Aczpenser sturio) ’. . . P . The goldfish (Cyprinus auratus) . . i z FIG. 307- 308. 309. 310. 311. 312. 313. 314. 315. 316. 317. 318. 319. 320, 321. 322, 323. 324. 325. 326. 327. 328, 329. 330. 331. 332. 333+ 334+ 335+ 37 338. 339: 340. 341. 342. 343- LIST OF ILLUSTRATIONS, Lepidosiren (after Graham Kerr), showing (22.7) pectoral fin and the tufted pelvic fin (Pv.f,) of the mature male Skeleton of Ceratodus fin—from Gegenbaur . Head region of Protopferus—from W. N, Parker . Larva of Protopterus—after Budgett . . Larva of Lepédosiren—after Graham Kerr . : The edible frog (Rana esculenta) j i Vertebral column and pelvic girdle of bull- -frog A Skull of frog—upper and lower surface—after W. K. Parker Skeleton of frog. The half of the pectoral girdle, and fore- and hind-limb of the right side are not shown . . Pectoral girdle of Rana esculenta—after Ecker , Side view of frog’s pelvis—after Ecker . F Brain of frog—after Wiedersheim . . . . Nervous system of frog—after Ecker . . . Arterial system of frog . : : . ’ Venous system of frog Urogenital system of male edible frog—after Ecker . Urogenital system of female frog—after Ecker . Division of frog’s ovum—after Ecker Section of frog embryo—after Ziegler’s model and Marshall Dissection of tadpole—after Milnes Marshall and Bles ‘ Life history of a frog—after Brehm . i ‘ Cecilian (/chthyophis) with eggs—after Sarasin ‘ External appearance of tortoise Skull of turtle és F Carapace of tortoise Pectoral girdle of a Chelonian Internal view of bones of the plastron of the Greek tortoise Scales on ventral surface of plastron of Greek tortoise Infernal view of skeleton of tortoise . ‘ Dissection of chelonian heart—after Huxley . . Heart and associated vessels of tortoise—after Nuhn . Hyoid apparatus of a chelonian % f Lateral view of brain of Hatteria punctata—after Osawa Hatleria or Sphenodon—after Hayek - . Side view of skull of Lacerta—after W. K. Parker . 3 Heart and associated vessels of a lizard—after Nuhn P Lung of Chameleo vulgaris, showing air-sacs—after Wieders- heim. . . . : 344. Anterior view of python’ s vertebra r ‘ ‘ 345. Posterior view of python’s vertebra . ‘ ‘ 346. Snake’s head—after Nuhn .. - . 347. Skull of grass-snake—from W. K. Parker . : 348. Lower surface of skull of a young crocodile . . : 349. Cervical vertebra of crocodile . . . . 350. Crocodile’s skull from dorsal surface . . 351. Pectoral girdle of-crocodile . : . 352. Half of the pelvic girdle of a young crocodile . : 353. Origin of amnion and allantois—after Balfour ° PAGE XX FIG. LIST OF ILLUSTRATIONS. 3534. Vertical section through backbone and ribs of a Chelonian 381. and a Mammal—in part after Jaekel . Position of organs in a bird—after Selenka . : . Fore-limb and hind-limb compared . . . . Diagrammatic section of young bird—after Gadow . . . A falcon : . Young bearded griffin (Gypiictus barbatus)—after Nitzsch . . Young feather and filoplume—after Nitzsch 7 . Types of feathers . . a 2 e . Parts of a feather—after Nitzsch . Entire skeleton of condor, showing the relative positions of the chief bones . . Disarticulation of bird’s skull—after Gadow. Membrane bones shaded : Under surface of gull’s skull : ‘ . Wing of dove i : “ 2 : . Side view of pelvis of cassowary é : . . - Bones of hind-limb of eagle . 7 . : . . Brain of pigeon . . . Diagrammatic section of cloaca of male bird—after 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—from Marey . F ‘ ‘ . Wings coming down—from Marey . . Wings completely depressed—from Marey . : . Stages in development of chick—after Marshall . Diagrammatic section of egg—after Allen Thomson . . Diagrammatic section of embryo—after Kennel Hesperornts—after Marsh . is ‘ 381A. Fore and hind limbs of rabbit 382. 393: 394. . Side view of rabbit’s skull . . . . Dorsal view of rabbit’s skull , . . . Under surface of rabbit’s skull i ‘i i » Skull of capybara . - . - Dorsal view of rabbit’s brain . Under surface of rabbit's brain—after Krause . Diagram of ceecum in rabbit « . Duodenum of rabbit—from Krause, in part after Claude Diagram of skull bones (partly after Flower and Weber), the membrane bones shaded. Bernard ‘ z a , . Circulatory system of the rabbit Vertical section through rabbit’s head—from a section, with help from Parker’s Zootomy and Krause F . Urogenital organs of male rabbit. : . . Urogenital organs of female rabbit . ‘ . . LIST OF ILLUSTRATIONS. FIG. 395. Segmentation of rabbit’s ovum—after Van Beneden : 396. Development of hedgehog. Three early sists alist Hubrecht 397. Embryo of Perameles with its foetal membranes—after Hill” 398. Two stages in segmented ovum of hedgehog—after Hubrecht 399. Development of foetal membranes—after Hertwig . - 400, Diagram of foetal membranes—after Turner FA 401, View of embryo, with its foetal membranes—after Kennel . 402. Pectoral girdle of Echidna . . : : : 403. Pelvis of Echidna . q : 4 . : 404. Lower jaw of kangaroo 5 é : 405. Foot of young kangaroo. 406. Side view of sheep’s skull, with roots of back teeth exposed 407. Stomach of sheep—from Leunis a‘ ‘ ‘ ‘ 408. Side view of lower part of pony’s fore-leg . z 5 409. Side view of ankle and foot of horse . . 410. Side view of horse’s skull, roots of teeth exposed . . 411. Feet of horse and its predecessors—from Neumayr - 412. Left fore-limb of Balenoptera . 413. Fore-limb of whale (Afegaptera longimana)—alter Struthers 414. Pelvis and hind-limb of Greenland whale alee eas Struthers 7 415. Vertebra, rib, and sternum of Balenoptera—from specimen in Anatomical Museum, Edinburgh . . . 416. Skull of tiger, lateral view . * : i 2 417. Lower surface of dog's skull 3 : . 418. Skull of Orang-Utan . F . . . 419. Skull of gorilla ; . ‘ A , j 420. Skeleton of male gorilla . . , . . BIRDS. Placentals. MAMMALS. Marsupials. SIMPLEST ANIMALS. Flying Birds. Running Birds. Monotremes. uw : ie) : a Snakes. Lizards. REPTILES. Crocodiles. Tortoises. a is Dipnoi. AMPHIBIANS. = Bony Fishes. Newt. Frog. 2 FISHES. a) “Ganoids.”” CYCLOSTOMES. > Elasmobranchs, Lamprey. Hag-fish. < : LANCELETS. TUNICATES. = | BALANOGLOSSUS. e Cuttle-fishes. a © [Insects. Arachnids. ANNELIDS. Gasteropods. 4 > Myriopods. MOLLUSCS. N Peripatus. : ° Bivalves. > ARTHROPODS. c oa Crustaceans. “WORMS.” Feather-stars. i Brittle-stars. HH Star-fish. Z, Ctenophores. Jelly-fish. Sea-anemones. Corals. Loe! CCQELENTERA. Medusoids and Hydroids. SPONGES. Infusorians. Rhizopods. Sporozoa, VOZ -OLOU OUTLINES OF ZOOLOGY CHAPTER I GENERAL SURVEY OF THE ANIMAL KINGDOM In beginning the study of Zoology, it is natural and useful to try to get a bird’s-eye view of the “ Animal Kingdom.” Without this, one is apt to miss the plan in studying the details. But the survey can be of little service unless the student has the actual animals in his mind’s eye. VERTEBRATES, OR BACKBONED ANIMALS Mammals.—We begin our survey with the animals which are anatomically most like man—the monkeys. But neither we nor the monkeys are separated by any structural gulf from the other four-limbed, hair-bearing animals, to which Lamarck gave the name of Mammals. For although there are many different types of Mammals—such as monkeys and men; horses, cattle, and other hoofed quad- rupeds ; cats, dogs, and bears ; rats, mice, and other rodents ; hedgehogs, shrews, and moles, and so on—the common possession of certain characters unites them all in one class, readily distinguishable from Birds and Reptiles. These distinctive characters include the milk-giving of the mother mammals, the growth of hair on the skin, the general presence of convolutions on the front part of the brain, the occurrence of a muscular partition or diaphragm between the chest and the abdomen, and so on, as we shall I 2 GENERAL SURVEY OF THE ANIMAL KINGDOM. afterwards notice in detail. Most mammals are suited for life on land, but diverse types, such as seals, whales, and sea- cows, have taken to the water. In another direction the bats are markedly adapted for aerial life. Among the mammalian characteristics of great import- ance are those which relate to the bearing of young, and even a brief consideration of these shows that some mammals are distinguished from others by differences deeper than those which separate whales from carnivores, or rodents from bats. These deep differences may be stated briefly as follows:—(a) Before birth most young mammals are very closely united (by a complex structure Fic. 1.—Duckmole ( Ornzthorhynchus). called the placenta) to the mothers who bear them. (4) But this close connection between mother and unborn young is of rare occurrence, or only hinted at, in the pouched animals or Marsupials, which bring forth their young in a peculiarly helpless condition, as it were prematurely, and in most cases place them in an external pouch, within which they are sheltered and nourished. (c) In the Australian duckmole and its two relatives, the placental connection is quite absent, for these animals lay eggs as birds and most reptiles do. These differences and others relating to structure warrant the division of Mammals into three sub- classes :— (a) Eutheria, Monodelphia, or Placentals—those in which there is a close (placental) union between the unborn embryo and its mother, e.g. Ungulates, Carnivores, Monkeys. BIRDS. 3 (4) Metatheria, Didelphia, or Marsupials—the prematurely bearing, usually pouch-possessing kangaroos, opossums, etc. {c) Prototheria, Ornithodelphia, or Monotremes—the egg-laying duckmole (Ornzthorhynchus), Echidna, and Proechidna. aS Fic. 2.—Phenacodus, a primitive extinct Mammal.—After Cope. Birds.— There can be no hesitation as to the class which ranks next to Mammals. For Birds are in most respects as highly developed as Mammals, though in a different direc- tion. They are character- ised by their feathers and wings, and many other adaptations for flight, by their high temperature, by the frequent spongi- ness and hollowness of their bones, by the tend- ency to fusion in many parts of the skeleton, by the absence of teeth in modern forms, by the fixedness of the lungs and their association with ‘numerous air sacs, and so on. Fic. 3.—Extinct moa and modern er eed kiwi.—After Carus Sterne. But here again different grades must be distinguished—(1) There is the vast majority—the flying birds, with a breast-bone keel or carina, to 4 GENERAL SURVEY OF THE ANIMAL KINGDOM. which the muscles used in flight are in part attached (Carinatz) ; (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, Avcheopteryx, 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. Fic. 4,.—Crocodiles. Reptiles do not form a compact class, but rather an assemblage of classes. In other words, the types of Reptile differ much more widely from one another than do the types of Bird or Mammal. Nowadays there are five dis- tinct types:—the crocodilians, the unique New Zealand, “lizard” (Sphenodon), the lizards proper, the snakes, and the tortoises. But the number of types is greatly increased when we take account of the entirely extinct saurians, who had their golden age in the inconceivably distant past. The Reptiles which we know nowadays are scaly-skinned animals; they resemble Birds and Mammals in having during embryonic life two important “foetal membranes” (the amnion and the allantois), and in never having gills ; they differ from them in being “cold-blooded,” and in many other ways. AMPHIBIANS. 5 Amphibians.—The Amphibians, such as frogs and newts, were once regarded—e.g. by Cuvier—as naked Reptiles, but a more accurate classification has linked them rather to the Fishes. Thus Huxley grouped Birds and Reptiles together as Sauropsida ; Amphibians and Fishes together as Ichthyopsida—for reasons which will be afterwards stated. Amphibians mark the transition from aquatic life, habitual Fic. 5.—Salamander, an Amphibian. 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 very muscular posterior region of the body usually forms Fic. 6.—Queensland dipnoan (Ceratodus). the locomotor organ, and we say that a fish swims by bending and straightening its tail. The limbs have the form of paired fins—that is, they are limbs without digits. There are also unpaired median fins supported by fin rays. All have permanent gills borne by bony or gristly arches. 6 GENERAL SURVEY OF THE ANIMAL KINGDOM. There is an exoskeleton of scales, and the skin also bears numerous glandular cells and sensory structures. In many ways Fishes are allied to Amphibians, especially if we include among Fishes three peculiar forms, known as Dipnoi, which show the beginning of a three-chambered heart, and have a lung as well as gills. Ordinary Fishes have a two-chambered heart, containing only impure blood, which is driven to the gills, whence, purified, it passes directly to the body. Apart from the divergent Dipnoi, there are two great orders of Fishes—the cartilaginous Elasmobranchs, such as shark and skate ; and the Teleosteans or bony fishes, such as cod, herring, salmon, eel, and sole. There are several smaller orders of great importance, some of which, é.g. the sturgeons, are often called ‘* Ganoids.”” Primitive Vertebrates.—Under this title we include— (1) the Roundmouths or Cyclostomata; (2) the lancelets or Cephalochorda; (3) the Tunicates, some of which are See Ses SSSR WW Ay uice, LE Fic. 7.—A lancelet, Amphioxus.—After Haeckel. called sea-squirts; and (4), with much hesitation, several strange forms, especially Balanoglossus, which exhibit structures suggestive of affihity with Vertebrates. The Cyclostomata, represented by the lamprey (Pe/vo- myzon) and the hag (AZyxine), and some other forms, probably including an interesting fossil known as FPalgo- spondylus, are sometimes ranked with fishes under the title Marsipobranchii. But they have no definitely developed jaws, no paired fins, no scales, and are in other ways more primitive. The lancelets or Cephalochorda are even simpler in their general structure (see Fig. 7). Thus there is an absence of limbs, skull, jaws, well-defined brain, heart, and some other structures. The vertebral column is represented by an unsegmented (or unvertebrated) rod, called the noto- chord, which in higher animals (except Cyclostomes and some fishes) is a transitory embryonic organ afterwards replaced by the backbone. PRIMITIVE VERTEBRATES. y) The Tunicata or Urochorda are remarkable forms, the majority of which degenerate after larval life (Fig. 8). In the larvee of all, and in a few adults which are neither peculiarly specialised nor degenerate, we recognise some of the fundamental characters of Vertebrates. Thus there is a dorsal supporting axis (or notochord) in the tail region, a dorsal nervous system, gill - clefts opening from the pharynx to the exterior, a simple ventral heart, and so on. Of Balanoglossus and its allies (Hemichorda or Enteropneusta) it is still difficult to speak with confidence. The possession of gill-clefts, the dorsal position of an important part of the nervous system, the occurrence of a short supporting structure on the anterior dorsal surface of the pharynx, and other features, have led many to place them at the base of the Vertebrate series. Characteristics of Vertebrata.—At this stage, having reached the base of the . Vertebrate series, we may seek to define a Fic. 8.—Ascidian or Vertebrate animal, and to contrast it with sea - squirt, — After Invertebrate forms. Haeckel. The distinction is a very old one, for even Aristotle distinguished mammals, birds, reptiles, amphibians, and fishes as ‘‘ blood-holding,” from cuttle-fish, shell-bearing animals, crustaceans, insects, etc., which he regarded as ‘‘ bloodless.” He was, indeed, mistaken about the bloodlessness, but the distinctiveness of the higher animals first mentioned has been recognised by all subsequent naturalists, though it was first precisely expressed in 1797 by Lamarck. Yet it is no longer possible to draw a boundary line between Verte- brates and Invertebrates with that firmness of hand which characterised the early or, indeed, the pre-Darwinian classifications. We now know—(1) that Fishes and Cyclostomata do not form the base of the Vertebrate series, for the lancelets and the Tunicates must also be in- cluded in the Vertebrate alliance ; (2) that Balanoglossus, Cephalodiscus, and some other forms, have several Vertebrate-like characteristics ; (3) that some of the Invertebrates, especially the Chzetopod worms, show some hints of affinities with Vertebrates. The limits of the Vertebrate alliance have been widened, and though the recognition of their characteristics has become more definite, not less so, the apartness of the sub-kingdom has disappeared. 8 GENERAL SURVEY OF THE ANIMAL KINGDOM. It does not matter much whether we retain the familiar title Verte brata, or adopt that of Chordata, provided that we recognise—(1) that it is among Fishes first that separate vertebral bodies appear in the supporting dorsal axis of the body ; (2) that, as a characterdstzc, the backbone is less important than the notochord, which precedes it in the history alike of the race and of the individual. Nor need we .object to the popular title backboned, if we recognise that the adjective “bony ” is first applicable among Fishes, and not to all of these. The essential characters of Vertebrates may be summed up in the following table, where they are contrasted, somewhat negatively, with what is true of Invertebrates :— ‘BACKBONELESS,” INVERTEBRATE or Non-CHORDATE. ‘“BaCKBONED,” VERTEBRATE oR CHORDATE. If there is a nerve-cord, it is ventral, No internal dorsal axis. No gill-slits. The eye is usually derived directly from the skin. The heart, if present, is dorsal. The central nervous system—brain and spinal cord—is dorsal and 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, they are associated with gills, and are useful in respiration; in higher forms they are transitory and functionless, except when modified into other structures. The essential parts of the eye are formed by an outgrowth from the brain. The heart is ventral. INVERTEBRATES, OR BACKBONELESS ANIMALS Molluscs.—If we take the concentration of the nervous system as a useful criterion, the highest backboneless animals are the Molluscs. This series of forms includes Bivalves, such as cockle and mussel, oyster and clam; Gasteropods, such as snail and slug, periwinkle and whelk ; Cephalopods, such as octopus and pearly nautilus. Unlike Vertebrates, and such Invertebrates as Insects and Crustaceans, Molluscs are without segments and without appendages. A muscular protrusion of the ventral surface, known as the “foot,” serves in the majority as ap organ of locomotion. In most cases a single or double fold of skin, called the “ mantle,” makes a protective shell. The nervous system has three chief pairs of nerve centres or ganglia. In many cases there are very characteristic free-swimming larval stages. ARTHROPODS, 9 Fic. g.—Cephalopod (paper nautilus, female). Arthropods. — This large series includes Crustaceans, Myriopods, Insects, Spiders, and other forms, which have segmented bilaterally symmetrical bodies and jointed Fic. 10.—Fresh-water crayfish Fic. 11.—a, Caterpillar ; (Astacus), a Crustacean,— 6, pupa; ¢, butterfly. After Huxley. 10 GENERAL SURVEY OF THE ANIMAL KINGDOM. appendages. The skin produces an external, not-living cuticle, the organic part of which is a substance called chitin, associated in Crustaceans with carbonate of lime. The nervous system con- sists of a dorsal brain, connected, by a nerve-ring around the gullet, with a ventral chain of ganglia. Echinoderms. This is a well- defined series, including star-fishes, brittle-stars, sea-urchins, sea-cucum- bers, and feather - stars. The symmetry of the adult is usually radial, though that of the larva is Fic. 12.—Spider. bilateral. A peculiar system, known as the water-vascular system, is characteristic, and is turned to various uses, as in locomotion and respiration. There is a marked tend- ency to deposition of lime in the tissues. The develop- ment is strangely circuitous or “indirect.” Segmented ‘‘worms.” —It is hopeless at present to arrange with any definiteness those heterogeneous forms to which the title ‘worm ” is given. For this title is little more than a name for a_ shape, assumed by animals of varied nature who be- gan to move head foremost and to acquire sides. There is no class of “worms,” but an assemblage—a mob Fic. 13.—Crinoid or feather-star. —not yet reduced to order. It seems useful, however, to separate those which are ringed or segmented from those which are unsegmented. The former are often called Annelids, and include two chief classes :— UNSEGMENTED ‘‘WORMS.” 1s (1) Cheetopoda or Bristle-footed worms, ¢.g. earthworm. and lob-worm ; and (2) Hirudinea or Leeches. ee Tf, Wa Ge? Unsegmented ‘“worms.”—These differ from the higher “worms” in the absence of true segments and appendages, and resemble them in their bilateral symmetry. There is a motley lot :—the free-living Turbellarians or Planarians ; the parasitic Trematodes or Flukes ; the parasitic Cestodes. or Tape-worms; the Nemer- teans or Ribbon-worms; the frequently parasitic Nematodes or Thread-worms; and several smaller classes. As to some other groups, such as the sea-mats (Polyzoa or Bryozoa), the lamp-shells (Brachiopoda), the worm-like Sipunculids, and the wheel- animalcules or Rotifers, we must confess that they are still a, Early stage with head inverted. incert@ seats. &, Later stage with head everted. But the general fact is not without interest, that in the midst of the well-defined classes of Invertebrates there lies, as it were, a pool from which many streams of life have flowed; for among the heterogeneous “worms” we may find in diverse types. affinities with Arthropods, Molluscs, Echinoderms, and. even Vertebrates. Contrast of Coelomate and Ccelenterate.—At this stage we may notice that in all the above forms the typical symmetry is. bilateral (in Echinoderms, the superficial radial symmetry belongs only to the adults); that in most types a body cavity or coelom 1s. developed ; that the embryo consists of three germinal layers (external: Fic. 15.—Bladderworm stage ofa Cestode.—After Leuckart. 12 GENERAL SURVEY OF THE ANIMAL KINGDOM. ‘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 (Ccelentera and Sponges) the conditions are different, -as may be expressed in the following table :— Sponces AND CG&LENTERA. HIGHER ANIMALS (C@&LOMATA). There is no body cavity. There is but | There is a body cavity or cwlom be- one cavity, that of the food canal. tween the food canal and the body- wall. But this is often incipient, or degenerate. Except in ctenophores, there is no | There is a distinct middle layer of cells definite middle layer of cells (meso- (mesoderm) between the external derm), but rather a middle jelly ectoderm and the internal endo- (mesogloea), and the embryo is derm. The embryo is triploblastic. diploblastic. The radial symmetry of the gastrula | The adults are usually bilateral, in some embyro is usually retained in the cases asymmetrical, in echinoderms adult, and the ‘longitudinal (oral- superficially radial. aboral) axis of the adult corresponds to the long axis of the gastrula. 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 a few are marine. The body may be a tubular polyp, or a more or less bell-like “ medusoid,” and PORIFERA. 13 in some cases the two forms are included in one life cycle. Budding is very common, and many of the sedentary forms. —‘‘corals””—have shells of lime. Porifera.—Sponges, or Porifera, are the simplest many- celled animals. In the simplest forms, the body is a tubular, two-layered sac, with numerous inhalant pores by which water passes in, with a central cavity lined by cells bearing lashes or flagella, and with an exhalant aperture. But budding, folding, and other complications arise, and there is almost always a skeleton, calcareous, siliceous, or “horny.” Apart from one family (Spongillidze), all sponges. are marine. Contrast of Metazoa and Protozoa.—aAll the animals hitherto- mentioned have Jdodzes built up of many cells, but there are other. animals, each of which consists of a single cell. These simplest animals- are called Protozoa. Every animal hitherto mentioned, from mammal or bird to sponge, develops, when reproduction takes its usual course, from « fertilised. egg-cell. This egg-cell or ovum divides and redivides, and the daughter cells cohere and are differentiated to form a ‘‘ body.” But the Protozoa form no ‘‘ body”; they remain (with few exceptions) single cells, and when they divide, the daughter cells almost invariably go apart as independent organisms. Here, then, is the greatest gulf which we have hitherto noticed— that between multicellular animals (Metazoa) and unicellular animals. (Protozoa). But the gulf was bridged, and traces of the bridge remain. For—(a) there are a few Protozoa which form loose colonies of cells, and (4) there are a few multicellular animals of great simplicity. Protozoa.— The Pro- tozoa remain single cells, with few exceptions. Thus they form no “body”; and necessarily, therefore, they have no organs in the ordinary sense. They illustrate the deginnings of sexual reproduction, and ‘N they are Met subject to Fic, 17.—Fossil Foraminifera natural death in the same (Nummulites) in limestone. — degree as Metazoa are. After Zittel. The series includes— tA cell may be defined as a unit corpuscle or unit area of living matter, typically controlled by a single nuclcus. 14 GENERAL SURVEY OF THE ANIMAL KINGDOM. (a) Rhizopods, with outflowing threads or processes of living mattey, é.g. the chalk-forming Foraminifera (Fig. 17). (6) Infusorians, with actively moving lashes of living matter. (c) Sporozoa, parasitic forms, usually without either lashes or out- flowing processes. Note on Classification. We always group together in our mind those impressions which are 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 blood-relationships of animals, to group together those whose affinities are shown by their being like one another in architecture or structure. It must, therefore, be based on the results of comparative anatomy— technically speaking, on ‘‘ homologies,” z.e. resemblances in funda- mental structure and in mode of development. Whales must not be ranked with fishes, nor bats with birds. To a classification based on structural resemblances, two corrobora- tions are of value, from embryology and from paleontology. On the -one hand, the development of the forms in question must be studied : thus no one dreamed that a Tunicate was a Vertebrate until its life- history was worked out. On the other hand, the past history must be inquired into : thus the affinity between Birds and Reptiles is confirmed by a knowledge of the extinct forms. In classification it is convenient to recognise certain grades or degrees of resemblance, which are spoken of as species, genera, families, orders, ‘classes, and so on. To give an illustration, all the tigers are said to form the species Felis tégrés, of the genus Fes, in the family Felide, 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 eé/is ; broader still are the resemblances between all members of the cat family Felidz ; still wider those between cats, dogs, bears, and seals, which form the order Carnivora; and Jastly, there are the general resemblances of structure which bind Mammals together in contrast to Birds or Reptiles, though all are included in the series or phylum Vertebrata. It must be understood that the real things are the individual animals, and that a species includes all those individuals who resemble one another so closely that we feel we need a specific name applicable to them all. And as resemblances which seem important to one naturalist may seem trivial to others, there are often wide differences of opinion as to the number of species which a genus contains. But while no rigid definition can be given of a species, certain common-sense considerations should be borne in mind :— CLASSIFICATION. 15 1. No naturalist now believes, as Linnzeus did, in the fixity of species ; swe believe, on the contrary, that one form has given rise to another. At the same time, the common characteristics on the strength of which we deem it warrantable to give a name to a group of individuals, must BIRDS SOF Snakes Ligand, coylians a Ss S cero? Rants Fishes i. Amphibian = é 78 ancelet ’ cycloto™ B wna d (< pba opods ‘ / motu Z A Syerons Bos, wy onltgee SS Unsectg S25. forms] -- fs} a D F BORO SE _ | echinodermy we = Ter astaceans iS @eflenfdra Mesozoa~y, A Spon ges, (nfusonians —, ; w_Gregarines do. Prrotoza Plants. per ee Fic. 18.—Diagrammatic expression of classification in a genealogical tree. B indicates possible position of Balano- glossus, D of Dipnoi, S of Sphenodon or Hatteria. not be markedly fluctuating. The specific characters should exhibit a certain degree of constancy from one generation to another. 3 2. Sometimes a minute character, such as the shape of a tooth or the marking of a scale, is so constantly characteristic of a group of indi- viduals that it may be safely used as the index of more important 16 GENERAL SURVEY OF THE ANIMAL KINGDOM. characters. On the other hand, che distinction between one species and another, should always be greater than any difference between the members of a family (using the word family here to mean the progeny of a pair). For no one would divide mankind into species according to the colour of eyes or hair, as this might lead to the absurd conclusion that two brothers belonged to different species. Thus it is often doubly unsatisfactory when a species is established on the strength of a single specimen—(a) because the constancy of the specific character is undeter- mined ; (4) because the variations within the limits of the family have not been observed. Indeed, it has happened that one species has been made out of a male, and another out of its mate. 3. Although cases are known where members of different species have paired and brought forth fertile hybrids, this is not usual. Zhe members of a spectes are fertile inter se, but not usually with members of other species. In fact, the distinctness of species has largely depended on a restriction of the range of fertility. TABULAR SURVEY.—(for Future Reference) METAZOA CHORDATA Eutheria. Bd Mamma.ta. Metatheria. Marsupials. aa Prototheria. Monotremes. Oviparous.) & & Carinate. Keeled flying birds. ‘ Aves Qdontolcz. Extinct toothed birds. = Ratite. Keel-less running birds. Extinct reptile-like birds. Crocodilia. Crocodiles and alligators. | Ophidia. Snakes. Lacertilia. Lizards. Rhynchocephalia. Sphenodon. Chelonia. Tortoises and turtles. Extinct Classes. Anura. Tail-less frogs and toads. Urodela. Tailed newts. Gymnophiona, e.g. Cecilia. Labyrinthodonts and other extinct Amphibians. c {eles Mud-fishes. Sauropsida. REPTILia. Gnathostomata (z.e. jawed). Craniota (with skulls). AMPHIBIA, Ichthyopsida. Teleostomi. Bony fishes, etc. Elasmobranchii. Cartilaginous fishes. Hag-fish (A/yxine), and Lamprey (Petromyzon). CErHALOCHORDA. Amfphioxus. } Pisces. CycLOsTOMATA. { Urocuorpa. Tunicates. Hemicuorpa. Balanoglossus, Cephalodiscus TABULAR SURVEY OF CHIEF CLASSES. 17 METAZOA NON-CHORDATA Gasteropoda. Snails. Lamellibranchiata, Bivalves. ‘Two smaller classes :—Scaphopoda and Solenogastres Cephalopoda. Cuttle-fishes. Mo ttusca. Insecta. ae Myriopoda. , Centipedes and millipedes. Prototracheata. Pertpatus. Crustacea. Palzostraca :—Trilobites, Eurypterids, and King-crabs. Some smaller classes. Arachnoidea. Spiders, scorpions, mites ARTHROPODA. Crinoidea. Feather-stars. (Cystoids and Blastoids, éxtinct.) Ophiuroidea. _ Brittle-stars. Asteroidea, Star-fishes. Echinoidea. Sea-urchins. Holothuroidea. Sea-cucumbers. EcHINODERMA. Annelids or Discophora. Leeches. Annulata. { Chetopoda. Bristle worms. } Some smaller classes. ( Brachiopoda. Lamp-shells. | Potyzc, e.g. Sea-mat (Flustra). Sipunculoidea, eg. Sipunculus. Worn: Nematoda. Thread-worms. Acanthocephala. Nemertea. Ribbon-worms. Rotifera. Wheel-animalcules. Cestoda. Tape-worms. {Brematoaa. Flukes. Platyhelminthes. \ (Turbellaria. Planarians. Ctenophora, ¢.g. Beroé. ‘ Actinozoa or Anthozoa. Sea-anemones. Alcyonarians and re- CGLENTERA. lated corals. Scyphomedusz or Acraspeda. Je Hydrozoa. Zoophytes and medisoids. PoRIFERA. Sponges. Calcareous and non-calcareous. PROTOZOA Inrusorta. Ruizopopa. SPporozoa. Simplest forms of animal life. VERTEBRATES. 18 “(yemure py snosoATuae3) WD PUA 21 “(prtq Burky PIPey) Suypwig cor (ajndey) pavzry °6 -(ueiqrydwy) Borg “g -‘(aayiweiq a]qnoq) ysypny *£ ‘(awojsoa]a7) “dieg ‘9 “(qouesqomsyyy) ysysoq “S “(aui0ysoj9AD) Aeidwey -y “jajsouery “€ ‘soyeIqoyaA JO wWeIsVIq—‘6l ‘DIg “uvIplosy “% sayeotuny-sjodpey, *= INVERTEBRATES. *Csn]oy podoiaysey) jreug “Zr ‘apids uapresy ‘gt sdseyy “St ssugudiaag “br ‘(uvaovjsniy) umurg ‘fr “(plyauuy) UMOMYWeR "ZI yoaary rr *(uIIapoulyo) eqs - apg sor "OYNY AAry 6 “(a7 e189 [20 suoWsUY-Bag * esnpay “4 “plosnpayy g = -eipAzT “S ‘eZuods adung “b -(eozoj01g) wntosurereg *€ Taper % ee te I “Sa]BIGs}IOAU] Jo WeIseIq—'oz ‘Olt CHAPTER it 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 f2o master activities in the animal body, those of muscular and those of nervous parts. To these the other internal activities—digestion, respiration, excretion, and the like—are subsidiary. LIVING AND NOT LIVING. 21 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 either parts of itself.or special germ-cells 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 grows at the expense of material different from itself, while the crystal—one of the few not- living things which can be said to grow—increases only at the expense of material chemically the same as itself. (4) 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. The physical basis of life invariably includes com- plex compounds known as gvoteids, built up chiefly of Carbon, Hydrogen, Oxygen, and Nitrogen, and these are continually being broken down and made anew. (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 is a self-stoking, and, within limits, a self-repairing engine, and 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 creature is a more or less perfect :ntegrate, it has a unzfied behaviour, it gives effectzve response to external stimuli. (e) 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 activities of growth and reproduction. 22 THE FUNCTIONS OF ANIMALS. Division of labour.—All the ordinary functions of life are exhibited by the simple unicellular animals or Protozoa. Thus the Amceba 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 areas or cells. A cell is a unified area of living matter almost always with a definite centre or nucleus. It is impossible for these cells to remain the same, for as they increase in number they become diversely related to the outer world, to food, to one another, and soon. Division of labour, consequent on diversity of conditions, is thus established in the organism. In some cells one kind of activity predominates, in others a second, in others athird. And this division of labour is associated with that complication of structure which we call differentiation. Thus in the fresh-water Aydra, 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, Phénomenes de la vie communs aux animaux et aux végétaus, The beech- tree feeds and grows, digests and breathes, as really as does PLANTS AND ANIMALS. 23 the squirrel on its branches. In regard to none of the main functions (except excretion) 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 movement. 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, and many other plant structures exhibit marked sensitiveness. (2) Resemblance in structure.—The 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 organisms have a cellular structure. This general conclusion is part of the Cell Theory or Cell Doctrine 1838 : (c) ne in 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. 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 which animals are able to utilise. Thus most plants derive the carbon they require from the carbon dioxide of the air, while only a few (green) animals have this power; all the 24 THE FUNCTIONS OF ANIMALS. 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 carbon dioxide, and in the manufacture of starch and proteids, the kinetic energy of sunlight is transformed by the living matter into the pctential chemical energy of complex foodstuffs. 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 foodstuffs 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, 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. 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. The unit in nervous reaction in any highly organised animal is the vefex. It requires three structures, a receptor (end-organ), a conductor, and an effector (muscle). The conductor consists of two or more nerve cells or neurones which span the distance between receptor and effector by means of their long processes. Stimulation of the receptor causes a nervous impulse to be transmitted along the conductor to set the effector in action, The whole nervous system is essentially a connected series of such reflex-arcs, all intricately joined up with one another. There are two chief kinds of stimuli which are transmitted to the central nervous system—stimuli from without the 23 CHARACTERISTICS OF PLANTS AND ANIMALS. ‘sossaooid , o1joquue,, 1 “sur -primq-dn ‘aarjonajsu0d jo aoueiapuod ‘aid gazguzaz e ‘wusejdojoid 10 ra}yeU -| Surat] Jtey1 YA peazeloosse sasueyo [eHA eq} ur moys pue ‘aatssed Apue -urwopaid are ‘y10M [eUIA}xa IO UOT}OUL ur AS19ua ayy Ajaaneredusoo . pued -x9 “(apIxorp uogied Jo) sasonpar A]TE013 -Sliajoeieys are Avy} { saourysqns xed ‘W109 asaq3 Jo ABrous [eotuayo yeryUejod ayz oyur yySuns jo ABiousa DVOUTy ayy y19au09 Aayy { saoueysqns xazdwoo Jo Suray ojur jetayeur pooy afduns Ayjeormays ‘epnio dn pring Aayy, ‘sassaooid ,, oToqeIey,, 10 “suryeeaq -umop ‘aandnisip jo 20uerapuodaid sauppas @ ‘uise[dojoid 10 Jaqyeur Surat] | Hay} YM payeroosse sasueyd [eA ay ur Moys pue ‘aanoe ATUeUTWOp -aid ore ‘siasipixo AT[vorsiiajorreys are Aayy {yJom yeutoyxa pue uo} -owloz0]_ ul ASisue INAUTY OJUT ABi0UD Jenuajod siqz yraauoo Aay3 { spewrue Jayjo Aq 10 sjurjd Aq dn paxiom Apeaye jeuajyeu poos asin Aayy, *InOqv[ JO UOISTAIp ssay yon ‘aSvioav uz uo “IqIyxa s[[eo eYyT, *OTISLIO}OVAVYO ST S{]29 OT} Suowe sinoqry jo uoIsIAIp paxieyl “s]]R9 peyeu Yseay ye ourty & 10g ‘saey szuvjd ajduris surog “yoreys 0} pale Ajyecturayd = Terwayeur wv ‘asoynijeao Aq Ul pay[@M sv sjjao yueuoduIOD ayy, *asO]NT[99 JO advI3 Aue MOYsS I9AaT ysowye pue ‘sourjzsqns [[99 947 Woy yuarayip A[qeryjsuourep [erzayeur Jo wey savy Ajarer ‘s[jum [29 oqUuysp Ar0A ou aaey UaIjO s{jao JusuodwoD oY, “suvIPIOse Jo sj1inbs seas aatssed ayy jo ajoiyno 4O dTUN} BY} Jo JsowW suUIOF pue ‘suvliosnjuy awos ut AN330 0} SUI33S sso[n[sD. ‘TAqdoro]y9 ou savy soqsvied auios pue 1sun 7 , *saouvysqns xajduioo dn Zurpiing ur pue ‘(uashxo jo uonesaqy qjM) eprxorp uoqies Suronpar ur ‘IYySI] -uns jo AZ1au9 ay} sasiiqn Joyeur Suraty aq} yoryay jo pre Aq quawsid use aq} ‘qAydorojyo ssassod Aysofvur aqy, = yAyd -o10,y9 Aue Ajarex Aaa aavy Aat[y, sq -O10[YD YW eoUspr 310 snosojeue Ajasojo juswSid uaais aavy ‘s7pi7e waphyy ‘g8uods royeM-Yseqy 941 *voz0j01g BWOS *s"9 “May! “uOrTAINU T1943 ul jeuondaoxe jred ur aie ‘sayiseied owos pue ‘ung sjue[d snoroatuiea ‘uresy *sjonpoid aise snouasoijyiu jo pir jad you op Aaqy ‘Ylos 943 jo sajeijia ayy Alpeioedsa ‘spunoduiod snouasoiu ajdwis woy uaSoiyu aysinber aq} urejyqo Aayy, “syonpoid aysem snouasoi}IU Jo pli 393 0] UMOUY aie Way} Jo JOT “susTULS -10 19q30 Aq apew ‘sprajoid ueyy said -wis jou ‘spunodwios snouasoij1u wor uasontu eysmber aq} ureyqo Aayy, “sjuryd aA Pees 07 s1qe A[quqoid are B0z0j}0Ig awios ‘ules “Addns -uoqivd jo saoimos 10410 puy ‘sazisered awos pue ‘1Zun gq ‘syueyd snosoatures *X9JeM UT P3apos -SIp JO Iv ay Ul aprIxorp voqies Woy uoqivd aysinbax 9y3 ulejqo Asyy, “syewue 13430 Aq 10 sjued Aq apeur Soja Gey faesns ‘yore3s wor uogivo aqisinbar oyz urejqo Aayy, “(oradygozoy) op syueid se 9pIxXoIp uoqievs asin 0} aTqe aq 0} Waas ({°979) BOz0}01g Uuse1s = swWOS *pooy afqnjos qrosqu Ay yz, : “pooy PHOS sseT JO ex0ur uo peasy AoYT, *qiosqe Ajduns sous -vicd pue voz0j01g awog - "SNOLLATIXY ANOS “SINVId dO SOILSIMALIVAVHD “STIVAINY JO SOILSINALIVUVHD s “SNOILdSOXY ANOS 26 THE FUNCTIONS OF ANIMALS. body, which make the organism aware of changes in its environment; and stimuli from within the body, which make it aware of the dispositions of its organs, eg. the stimuli transmitted by the afferent nerves of the muscles, tendons, etc. The chief functions of the nervous system are, then, to make the animal aware of its environment and to co- ordinate and integrate all its bodily functions and activities. As we ascend in the scale, we find that in addition the brain possesses, to an increasing extent, the power of correlating present and past experiences, and of originating or inhibiting action in accordance with this correlation. In whatever part there is activity, there is necessarily waste of complex substances and some degree of exhaustion; and it is interesting to notice, aS a triumph of histological technique, that Hodge, Gustav Mann, and others have succeeded in demonstrating in nerve cells the structural results (cellular collapse, etc.) of fatigue, and that in such diverse types as bee, frog, bird, and dog. Muscular activity—The movements of a unicellular animal are due to the contractility of the living matter, or of special parts of the cell, such as lashes or cilia. In sponges specially contractile cells begin to appear; in most higher animals such cells are aggregated to form the muscles. A piece of typical muscle consists of numerous fine transparent tubes or fibres, each invested by a sheath or sarcolemma, while the whole muscle is surrounded by connective tissue. It usually runs from one part of the skeleton to another, and is fastened to the skeleton by tendons or sinews. It is 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, CO,, 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 CHIEF FUNCTIONS OF THE ANIMAL BODY. 27 changes of “electric potential” associated with each con- traction. Beside muscular movement we must rank ciliary, amoeboid, and epithelial movement. Under the last head- ing are included active non-amceboid contractions and expansions of covering cells. Digestion.—The energy expended in work or in growth is balanced by the energy of the food-stuffs :—proteids, carbohydrates, fats, water, and salts, in varying pro- portions. In some of the lower animals, such as sponges, the food particles are engulfed by certain cells with which they come in contact, and digested within these cells (éxtracellular digestion). In most cases, however, the food is digested within the food canal, by ferments made by the secretory cells of the gut or of associated glands. The peculiarity of these ferments is that a small quantity can act upon a large mass of material without itself undergoing any apparent change. However digestion be effected, it means dissolving the food and making it diffusible. In a higher vertebrate there are many steps. (a) The first ferment to affect the food, masticated by the teeth and moistened by the saliva, is the Atyalim of the salivary juice, which changes starch into sugar. The juice is formed or secreted by various salivary glands around the mouth. (6) 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, ¢.g. trypsin, and affects all the different kinds of organic food. It continues the work of the stomach, changing proteids into peptones and peptones into much simpler compounds such as amino-acids; 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 28 THE FUNCTIONS OF ANIMALS. said to have slight power of converting starch into sugar; by its alkalinity it helps the action of the trypsin of the pancreas (which, unlike pepsin, acts in an alkaline fluid) ; 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 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 of water also goes on; so that by the time the mass has reached the rectum, it is semi-solid, and is known as feeces. These contain the indigestible and un- digested 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 mus red blood corpuscles. Absorption begins in the stomach by direct osmosis into the capillaries or fine branches of blood vessels in its walls, and_a similar absorption, especially of water, takes place along the whole of the digestive tract. But lining the intestines there are delicate projections called villi; they contain capillaries belonging to the portal system (blood vessels going to the liver), and small vessels known as lacteals connected with lymph spaces in the wall of the intestine. The lacteals lead into a longitudinal lymph vessel or thoracic duct, which opens into the junction of the left jugular and left subclavian veins at the root‘of the neck. The contents of the duct in a fasting animal are clear; after a meal they become milky ; the change is due to the matters discharged into it by the lacteals. It is probable that nearly all the fat of a meal is absorbed from the intestines by the lacteals, but it is not certain in what measure, if at all, this is true of the other dissolved foodstuffs ; the greater part certainly passes into the capillaries of the portal system, which are contained in the villi. The digested proteid, chiefly in the form of amino-acids, passes into the blood of the portal vein, either directly or through the intermediary of leucocytes, which flock to the intestine when proteid food is being digested. Function of the liver—We now know the fate of the fats, and of the proteids of the food, and the manner in CHIEF FUNCTIONS OF THE ANIMAL BODY. 29 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 regulation 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 harmful substances which are formed in the course of digestion are changed by the liver into harmless com- pounds. The excess of sugar, we have already noted, is stored in the liver. It is synthesised 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 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. 30 THE FUNCTIONS OF ANIMALS. The oxygen of this air combines with a substance called hemoglobin, contained in the red corpuscles of the blood, and is thus carried to all parts of the body. From the blood it passes to the tissues usually through the medium of the lymph. It is used in the tissues for oxidation. The carbon dioxide formed as a waste product is ab- sorbed by the serum of the blood, or enters in part into loose chemical combination with its salts, 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 carbon dioxide. But there is much waste resulting from tissue changes, which is not gaseous. It is cast into the blood stream by the tissues, and has to be got rid of in some way. This is effected by the kidneys, which are really filters introduced into the blood stream. But they are the most marvellous filters imaginable, and give us a good example of the intricacy of life processes. For the kidneys not only take out of the blood all the waste products that result from the metabolism of proteids and contain nitrogen, they also maintain the composition of the blood at its normal, rejecting any stuffs that vary from that normal, either qualitatively or quantitatively, doing this work according to laws quite different from the simple 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 protect, 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 (4) of the removal of MODERN CONCEPTION OF PROTOPLASM. 31 He waste products which are formed as the ashes of life. There are indeed some organs which we have not mentioned, such as the spleen, which seems to be an area for the multiplication of red blood corpuscles (fishes, newts, embryo-mammals) or for the destruction of worn- out corpuscles (mammals), 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 whén 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 suppose that the function of such a ‘‘ liver” is the same as in Mammals, for comparatively little investigation into the physiology of the lower types of animal 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 (hzemo- globin) of the blood, or of some pigment which is not red, or occurring without the presence of any blood at all, always means that oxygen is absorbed almost like a kind of food by the tissues, and that the carbon dioxide which results from the oxidation of part of the material of the tissues is removed. MopERN CONCEPTION OF PROTOPLASM The activities of animals are ultimately due to physical and chemical changes associated with the living matter or protoplasm. This is a mere truism. We do not know the nature of this living matter; perhaps our most certain knowledge of it is, that in our brains its activity is associated with consciousness. 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 32 THE FUNCTIONS OF ANIMALS. 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. What is known in regard to the structure of protoplasm does not help the physiologist very much. The microscopists discover an in- tricate structure which pervades cach unit of living matter, but no physiologist dreams of explaining the life of a cell in terms of its microscopically visible structure. 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 content 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. 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 is highly probable that there is no one substance which should be called protoplasm, but that vital phenomena depend upon the inter- actions of several complex substances. As Verworn says, ‘‘ The life- process consists in the metabolism of proteids.” Generalising from his studies on colour sensation, Professor Hering was led to regard all life as an alternation of two kinds of activity, both induced hy stimulus, the one tending to storage, construction, assimilation of snaterial, the other tending to explosion, disruption, disassimilation. MODERN CONCEPTION OF PROTOPLASM. 33 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 répair, of discharge and restitution, of activity and recuper- ative rest. But there is no certainty as to the precise nature of this twofold process. ; CHAPTER 11] THE ELEMENTS OF STRUCTURE (MorpHotocy) 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 paleontologist is also a student of morphology. FORM AND SYMMETRY. 35 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 marphological, though he has also to inform us, if he can, about the physiology of develop- ment. 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. As regards symmetry, animals may be distinguished as—(a) radially symmetrical; (4) bilaterally symmetrical ; (c) asymmetrical. In a radially symmetrical animal, such as a jelly-fish, the body can be halved by a number of vertical planes—it is symmetrical around a median vertical axis. That is, it is the same all round, and has no right or left side. In a bilaterally symmetrical body, such as a worm’s, there is but one plane through which the body can be halved. In an asymmetrical animal, such as a snail, accurate halving is im- possible. , Radial symmetry is illustrated by simple Sponges, most Ccelentera, and by many adu/t 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 Ccelentera. 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. The formation of a ‘‘brain” is correlated with the habit of moving head foremost. Among many-celled animals, some worm type prob- ably deserves the credit of beginning the profitable habit of moving head foremost. Had some one not taken this step, we should never have known our right hand from our left. 36 THE ELEMENTS OF STRUCTURE. II. Organs.—We give this name to any well-defined part of an animal, such as: heart or brain. The word sug- gests a piece of mechanism; but the animal is more than a complex engine, and many organs have several different activities to which their visible structure gives little clue. 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 Aydra, 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 Aydra will reproduce the whole animal, but this is not true of the frog. The struc- tural result of this physiological division of labour is difer- entiation. The animal, or part of it, becomes more complex, more heterogeneous. If we contrast a bird and a sponge, another great fact in regard to the evolution of organs is illustrated. The bird is more of a unity than a sponge; its parts are more closely knit together and more adequately subordinated to the life of the whole. This kind of progress is called zntegration. Differentiation involves the acquisition of new parts and powers, these are consolidated and harmonised as the animal becomes more integrated. Correlation of organs.—It is of the very nature of an organism that its parts should be mutually dependent. The organs are all partners in the business of life, and if one member changes, others also are affected. This is especially ORGANS. 37 true of certain organs which have developed and evolved together, and are knit by close physiological bonds. Thus the circulatory and respiratory systems, the muscular and the skeletal systems, the brain and the sense organs, are very closely 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, omologous. Thus the nineteen pairs of appendages on a crayfish are all homologous; the three pairs of “jaws” in an insect are homologous with the insect’s legs ; and it is also true that the fore-leg of a frog, the wing of a bird, the flipper of a whale, the arm of a man, are all homologous. The wing of a bird and the arm of man exhibit the same chief bones, blood vessels, muscles, and nerves, and they begin to develop in the same way ; they are homologous but not analogous. The wing of a bird and the wing of an insect, which resemble one another in being organs of flight, are not the least alike in structure ; they are analogous but not homologous. Yet two organs may be doth 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 burrow- ing amphibian, a burrowing lizard, and a burrowing snake resemble one another in being limbless, but this ‘“ conver- gence,” or “homoplasty,” of form does not indicate any relationship between them. Change of function.—Division of labour involves restric- tion of functions in the several parts of an animal, and no higher Metazoa could have arisen if all the cells had 38 THE ELEMENTS OF STRUCTURE. remained with the many-sided qualities of Amcebe. Yet we must avoid thinking about organs as if they were necessarily active in one way only. For many organs, e.g. the liver, have several very distinct functions. In addition to the main function of an organ, there are often secondary functions ; thus the wings of an insect may be respiratory as well as locomotor, and part of the food canal of Tunicates and Amphioxus is almost wholly subservient to respiration. Moreover, in organs which are not very highly specialised, it seems as if the component elements retained a consider- able degree of individuality, so that in course of time what was a secondary function may become the primary one. Thus Dohrn, who especially emphasised this idea of function change, says: ‘Every function is the resultant of several components, of which one is the chief or primary function, while the others are subsidiary or secondary. The diminution of the chief function and the accession of a secondary function changes the total function ; the secondary function becomes gradually the chief one; the result is the modification of the organ.” The contraction of a muscle is always accompanied by electric changes, and in the electric organs of fishes we see the electric changes in the modified muscular tissue composing the organs becoming more important than the contractility. The structure known as the allantois is an unimportant bladder in the frog, in Birds and Reptiles it forms a foetal membrane (chiefly respiratory) around the embryo, and in most Mammals it forms part of the placenta which effects vital connection between off- spring and mother. ; Substitution of organs.—The idea of several changes of function in the evolution of an organ, suggests another of not less importance which has been emphasised by Kleinen- berg. An illustration will explain it. In the early stages of all vertebrate embryos, the supporting axial skeleton is the notochord,—a rod developed along the dorsal wall of the gut. From Fishes onwards, this embryonic axis is gradually replaced in development by the vertebral column or backbone; the notochord does not become the back- bone, but is replaced by it. It is a temporary structure, around which the vertebral column is constructed, as a tall chimney may be built around an internal scaffolding of ORGANS, 39 wood. Yet it remains as the sole axial skeleton in Amphioxus, persists in great part in hag and lamprey, but becomes less and less persistent in Fishes and higher Vertebrates, as its substitute, the backbone, develops more perfectly. Now, what is the relation between the notochord and its substitute the backbone, seeing that the former does not become the latter? Kleinenberg’s suggestion is that the notochord supplies the stimulus, the necessary condi- tion, for the formation of the backbone. Of course we require to know more about the way in which an old- fashioned structure may stimulate the growth of its future substitute, but the general idea of one organ leading on to another is suggestive. It is consistent with our general conception of development—that each stage supplies the necessary stimulus for the next step; it also helps us to understand more clearly how new structures, too incipient to be of use, may persist, and why old structures should linger though they have only a transitory importance. Rudimentary organs.—In many animals there are struc- tures which attain no complete development, which are rudimentary in comparison with those of related forms, and seem retrogressive when compared with their promise in embryonic life. But it is necessary to distinguish various kinds of rudimentary structures. (a) As a pathological variation, probably due to some germinal. defect, or to the insufficient nutrition of the embryo, the heart of a mammal is sometimes incompletely formed. Other organs may be similarly spoilt in the making. They illustrate arrested development. (6) Some animals lose, in the course of their life, many 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, which 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. (d) But among “rudimentary organs ” we also include structures somewhat different, e.g. the gill- 40 THE ELEMENTS OF STRUCTURE clefts which persist in embryonic reptiles, birds, and mammals, though most of them serve no obvious purpose, or the embryonic teeth of whalebone whales. These are “vestigial structures,” traces of ancestral history, and in- telligible on no other theory. The gill-clefts are used for respiration in all vertebrates below reptiles; the ancestors of whalebone whales doubtless had functional teeth. Classification of organs.—We may arrange the various parts of the body physiologically, according to their share in the life. Thus some parts have most to do with the ex¢ermal/ relations of the animals ; such as locomotor, prehensile, food-receiving, protective, aggressive, and copulatory organs. Of zz¢ernal parts, the skeletal structures are passive ; the nervous, muscular, and glandular parts are active. The repro- ductive organs are distinct from all the rest. They are conveniently called ‘‘ gonads,” which is a better term than reproductive glands. For by a gland we mean an organ which secretes, whose cells produce and liberate some definite chemical substance, such as a digestive ferment ; whereas the gonads are organs where there is periodic multi- plication of certain cells, kept apart from the specialisation character- istic of most of the ‘‘body cells” or ‘‘somatic” cells, It is true, however, that an accessory glandular function is often associated with the gonads. Another classification of organs is embryological, z.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—(1) the outer skin or epidermis ; (2) the nervous system ; (3) much at least of the sense organs: the inner layer of the embryo (the endoderm or hypoblast) forms at least an important part (the ‘‘ mid-gut ”) of the food canal, and the basis of outgrowths (lungs, liver, pancreas, etc.) which may arise therefrom, and also the notochord of Vertebrates: the middle layer of the embryo (the mesoderm or mesoblast) forms skeleton, connective swathings, muscle, lining of body-cavity, etc. 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 générale, in which he carried the analysis further, showing that the organs were composed of tissues, contractile, nervous, glandular, etc. In 1838-39, Schwann and Schleiden formulated the “cell theory,” in which was stated the result of yet deeper analysis—that all organisms have a cel/wlay structure and origin. The simplest animals (Protozoa) are typically single cells or unit masses of living matter; as such all animals begin; but all, TISSUES. 41 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. 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) Epdthelial 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 formed 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 4 sweated-off cuticle, susceptible of many modifications (especially of protective value). (6) Connective tissue.—This term includes too many different kinds of things to mean much. It represents a sort of histological lumber- room. The embryologists help us a little, for they have shown that almost all forms of connective tissue are derived from the mesoderm or middle layer of the embryo. As this mesoderm usually arises in the form of outgrowths from the gut, or from (‘‘ mesenchyme”) cells liberated at an early stage from either (?) of the two other layers of the embryo (ectoderm or endoderm), we inay say that connective tissue is primarily derived from epithelium. The general function of ‘‘ connective tissue” is to enswathe, to bind, and to support, but the forms assumed are very various. : The cells may be without any intercellular ‘‘ mortar” or matrix. They may be laden with fat or with pigment. In other cases the cells of the connective tissue lic in a matrix, which they secrete, or into which they in part die away. Sometimes the matrix becomes secondarily invaded by cells. The connective cells are very often irregular in outline, and give off, in most cases, fine processes, which traverse the matrix as a network. ‘They may secrete long fibres, as in the various kinds of fibrous tissue. The fibrous tissue 42 THE ELEMENTS OF STRUCTURE. ot 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 concentric layers, usually around a cavity or a blood vessel. (There are no blood vessels in cartilage.) The matrix becomes very rich in lime salts (especially phosphate); and the cartilage foundation, if there was one, is quite destroyed by the new formation. Here we may also note two important fluid tissues, the floating corpuscles or cells of the blood, and those of the body cavity or ‘* perivisceral” fluid, which is often abundant and important in back- boneless animals. (c) Muscular tzssue.—The single-celled Ameba 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 Aydra 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 Coelentera. In the majority, the muscle cells arise on the walls of the body cavity, and their origin may often at least be described as epithelial. But in other cases the muscles arise from those wandering ‘‘ mesen- chyme” 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. A striped muscle fibre is a cell the greater part of which is modified into a set of parallel longitudinal fibrils, with alternating ‘‘clear and dark ” transverse stripes. A residue of unmodified cell substance, with a nucleus or with many, is often to be observed on the side of the fibre, and a slight sheath or sarcolemma forms the ‘‘cell wall.” Many muscle fibres closely combined, and wrapped in a sheath of connective tissue, form a muscle, which, as every one knows, can contract with extreme rapidity when stimulated by a nervous impulse. (d@) Nervous tissue.—Beginning again with the Ameba, we recognise that it is diffusely sensitive, and that a stimulus can pass from one part of the cell to another. ' TISSUES 43 In some Ccelentera a few of the external cells seem to combine contractile and nervous functions. Therefore they are sometimes called ** neuro-muscular.” But in Hydra there are superficial sensory cells, whose basal pro- longations are connected either directly with contractile cells, or with deeper ganglion-cells, some of which give off motor processes to the contractile cells. In sea-anemones and some other Ccelentera there is a more sharply defined division of labour. Superficial sensory cells are connected with subjacent nerve- or ganglion-cells, from which fibres pass to the contractile elements. In higher animals the sensory cells are mostly integrated into sense organs, the ganglionic cells into ganglia, while the delicate fibres. which form the connections between sensory cells and 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. They are of very varied shape, but consist always of a cell-body which gives off one or more processes. One of these processes is long, branches very sparingly, and is known as the axis-cylinder. There are usually present other processes which ramify like the branches of a tree and are called dendrites. The cell-body contains a nucleus, distinct granules, and a network of fine fibrils. The nervous system is built up of such ‘‘neurones.” In the ganglia they are supported and held apart by much-branched neuroglia cells. In all but a few of the simplest Metazoa, the nerve fibres (axis- cylinders) are surrounded by a sheath called the neurilemma, said to be formed by adjacent connective tissue. Several nerve fibres may com- bine to form a nerve, but each still remains ensheathed in its neuri- lemma while fibrous sheaths bind the nerve fibres together. In Verte- brate animals each nerve fibre usually has in addition a medullary sheath. But even in the higher Vertebrates, ‘‘non-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. A nerve fibre contains 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. The nerve fibres arise as prolongations of the ganglion cells, which extend themselves in the embryo like Amoebze sending out pseudopodia. 44 THE ELEMENTS OF STRUCTURE. IV. Cells.—In discussing tissues, it was necessary to refer to the component cells. Let us now consider the chief characteristics of these elements. A cell is a unit mass or area of living matter usually with a nucleus. Most of the simplest animals and plants (Protozoa and Protophyta) are single cells; eggs and male elements are single cells; in multicellular organisms the cells are combined into tissues and organs. Most cells are too small to be distinguished except through lenses; many Protozoa, ¢.g. large Ameebee, are just visible to our unaided eyes; the chalk -forming Foramin- ifera are single cells, whose shells are often as ‘large as pin-heads, and some of the extinct kinds were as big as half-crowns (see Fig. 17); the bast cells of plants may extend for i several inches; the largest Fic. 21.—Diagram of cell structure. animal cells are eggs dis- atte WAG Rs tended with yolk. Pi. Plastids in cytoplasm. The typical and primi- cc. Centrosome. ate Beclealis tive form of cell is a ‘W, Nucleus sphere—a shape naturally ct. General cytoplasm. assumed by a complex V. Vacuole. f Gr. Granules. coherent substance situ- ated in a medium different from itself. Most egg-cells and many Protozoa retain this primitive form, but the internal and external conditions of life (such as nutrition and pressure) often evolve other shapes,—oval, rectangular, flattened, thread-like, stellate, and so on. As to the structure of a cell, we may distinguish (see Fig. 21)— (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 (metaplasm) ; CELLS. 45 (4) A specialised nucleus, with a complex, structure, and important functions ; : (c) One or more specialised bodies called central corpuscles or centrosomes, which seem to be centres of activity during cell division ; : (d) A cell wall, which occurs in very varied form, or may be entirely absent. (a) As to the cell substance, it 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 distinguish the genuinely living protoplasm, which may be a mixture of proteids, from other materials of simpler chemical composition, such as carbohydrates, fats, pigments, etc. Some of these may be nutritive materials in process of elaboration into more complex substances; others are disruptive products of the metabolism. (6) As to the nucleus, one at least is present in almost every cell. It used to be said that some very simple animals, which Haeckel called Monera, had no nuclei, but in many cases the nuclei have now been demonstrated. In other cases, e.g. some Infusorians, the nuclear material seems to be diffused in the cell substance. The red blood cells -of Mammals seem to be distinctly nucleated in their early stages, though there is no nucleus in those which are full grown. The nucleus is a very important part of the cell, but it is not yet possible to define precisely what its importance is. In fertilisation an essential process is the union of the nucleus of the spermatozoon or male cell with the nucleus of the ovum or female cell (Fig. 23). In cell division the nucleus certainly plays an essential part. Cells bereft of their nuclei die, or live for a while a crippled life. Accord- ing to some,. the nucleus is important in connection with the nutrition of the cell; according to others, it is of special importance in connection with the respiration of the cell. It is certain that there are complex actions and reactions’ 46 THE ELEMENTS OF STRUCTURE. between the living matter of the nucleus and that of the cytoplasm.’ Cytoplasm and nucleoplasm form a “cell firm,” potent in their co-operation. In many cells it has been shown that fragments or extensions of the nucleus pass into the cytoplasm, forming what is called a “ chromidial appar- atus,” which seems to be of much functional importance. The nucleus often lies within a little nest in the midst of the cell substance, but it may shift its position from one part of the cell to another. It has a definite margin, but this. may be lost, e.g. before cell division begins. Inter- nally, it is anything but homogeneous (see Fig. 22); at any rate, homogeneous nuclei are rare. Twisted strands or Fic. 22, —Structure tubes of “linin” bear a more stainable of the cell.—After material called “chromatin,” and when Carnoy. the cell is preparing to divide the W, Nucleus with chro- strands assume the form of a definite matin coil; note pro- toplasmic reticulum. number of separable rods or loops or granules, the “chromosomes.” Sur- rounding the linin and chromatin is the nuclear sap. Sometimes a linin thread shows a row of minute chromatin bodies {microsomata), like jewel-stones embedded on a belt. Weismann maintains that the chromosomes or idants of the germ-cells are the vehicles of the heritable qualities. He has made a hypothetical scheme, according to which the chromosomes or zdazés are built up of zds, and the ids of determinants, and the determinants of dzophors. Many nuclei also contain little round bodies or nucleoli, or sometimes a single nucleolus. 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 collected. (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 separa- tion extremely fine “archoplasmic” threads pass from the centrosomes to the chromosomes. The centrosomes are therefore regarded as “division organs,” or as “dynamic centres.” They also occur, in most cases singly, in resting CELLS. 47 cells, and it seems likely that they are present in most animal cells, at least in those which retain the power of division. (d) As to the cell wall, it seemed of much moment to the earlier histologists, who often spoke of cells as little bags or boxes. It is, however, the least important part of the cell. In plant cells there is usually a very distinct wall, consisting of cellulose. This 1G. 23.—Fertilised ovum of is a product, not a part, of Ascarts.—After Boveri. chr., Chromatin elements, two the protoplasm, though some from ovum nucleus and two protoplasm may be intimately ee Bielend ) Gee Fy . ‘* . rom whic associated with it as long as its “archoplasmic” threads growth continues. In animal dadinte, wartly Sods elivom: a me: cells there is rarely a very distinct wall chemically distinguishable from the living matter itself. But the margin is often different from the in- terior, and a slight wall may be formed by a superficial 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 sub- stance. Again, animal cells may secrete a superficial “cuticle,” e.g. the chitin formed by the ectoderm cells in Insects, Crustaceans, and Fic. 24.—Diagram of cell other Arthropods. division.—After Boveri. In animals, as well as in plants, chr. Chromosomes forming adjacent cells are often linked an cquatorial' plate; ¢s by intercellular bridges of living matter, which may be paths for the passage of materials or of disturbances from cell to cell. In many cases, ¢.g. of gelatinous tissue, a matrix arises out- side of and between the cells, as an exoplasmic product. In regard to cell division, the most important facts are the 48 THE ELEMENTS OF STRUCTURE. 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 karyokinesis or mitosis, and these are much the same everywhere, though different kinds of cells have their specific peculiarities. Occasionally, however, both in Protozoa and Metazoa, the nucleus divides by simple constriction (direct or amitotic division). This is a quicker process than the other, and occurs especially when there is rapid growth or frequent replacement of cells. Another departure from the ordinary scheme is seen when the nucleus shows a multiple division, while the cell remains undivided. This occurs normally in some marrow cells. The eventful changes of karyokinesis are as follows :— (a) The resdeng stage of the nucleus shows a network or complete coil of filaments (chromatin elements) (Fig. 22). (4) 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. 25, 1). (c) Astroid stage.—The chromatin elements bend into looped pieces (or chromosomes), which are disposed in a star, lying flat at the equator of the cell, the free ends of the U-shaped loops being directed outwards. Meanwhile a centrosome has appeared and divided into two separating halves, between which a spindle of fine achromatin threads is formed. This seems to form (at least part of) what is called the nuclear spindle. The centrosomes separate until one lies at each pole of the cell, surrounded by radiating ‘archoplasmic” threads which become attached to the chromosomes (Fig. 25, 2). J (d) Diviston and separation of the loops.—Each of the loops which make up the star divides /ongztudinally 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. 25, 2-4). (e) Déastrocd.—The single star thus forms two daughter stars, which separate farther and farther from one another towards the opposite poles of the cell, remaining connected, how- ever, by delicate threads (Fig. 25, 3-5). (7) 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: 25, 6). i CELLS. 49 (g) Each daughter nucleus then passes into the normal resting phase. The spindle disappears, and the centrosomes may also vanish. The essential fact is the exact partition of the nuclear material between the two daughter cells : Flemming gives the following summary of karyokinesis :— MOTHER NUCLEUS DauGHTER NUCLEUS (progressive changes). (regressive changes). a Resting stage. Resting stage. z & Coil. Coil. w ¢ Astroid. Diastroid. é » d Division of Astroid and its loops ——> (Prophases) (Metakinesis) (Anaphases). Fic. 25.—Karyokinesis.—After Flemming. 1. Coil stage of nucleus ; ¢.c., central corpuscle. 2. Division of chromatin elements into U-shaped loops, and longitudinal splitting of these (astroid stage). ‘ , 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. Besides the ordinary indirect division just described, the net result of which is that each of the two daughter cells gets an equal number of chromosomes, a precise half of each of the chromosomes in the original cell, there is another kind of cell division (meiotic or reducing division) which occurs only in the maturation of the ovum and 4 . 50 THE ELEMENTS OF STRUCTURE. oe spermatozoon, and has for its net result the reduction of the number of chromosomes to a half of the normal number. We are far from being able to give even an approximate account of the “mechanism” of cell division. The whole process is vital, and cannot, at present at least, be re- described in terms of matter and motion. On the other hand, Leuckart, Spencer, and Alexander James have given a general rationale of cell division. Why do not cells grow much larger? why do they almost always divide at a definite limit of growth? ‘The answer is as follows :—Suppose a young cell has doubled its original volume, that means that there is twice as much living matter to be kept alive. But the living matter is fed, aerated, purified through its surface, which, in growing spherical cells, for instance, only increases as the square of the radius, while the mass increases as the cube. The surface growth always lags behind the increase of mass. Therefore, when the cell has, let us say, quadrupled its original volume, but by no means quadrupled its surface, difficulties set in, waste begins to gain on repair, anabolism loses some of its ascendancy over katabolism. At the limit of growth the cell divides, halving its mass and gaining new surface. It is true that the surface may be increased by out- flowing processes, just as that of leaves by many lobes; and division may occur before the limit of growth is reached, but, as a general rationale, applicable to organs and bodies as well as to cells, the suggestion above outlined is very helpful. The ratio of the amount of nuclear material in the cell to the amount of cytoplasmic. material seems also to have a determining influence upon cell division (R. Hertwig). Protoplasm. —Morphological as well as physiological analysis passes from the organism as a whole to its organs, thence to the tissues, thence to the cells, and finally to the protoplasm itself. But although we may define protoplasm as genuinely living matter—as “the physical basis of life” —we cannot definitely say how much or what part of an Ameeba, or an ovum, or any other cell, is really protoplasm. We are able to make negative statements,.eg, the yolk of an egg is not protoplasm, but we cannot make positive ‘statements, or say, This is protoplasm, and nought else. PROTOPLASM. 51 Thus what is spoken of as the structure of protoplasm is really the structure of the cytoplasm. It is often.specifically different in different cases. 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, ¢.g. Frommann, describe a network or reticulum, with less stable material in the meshes; others, ¢.g. Flemming, describe a manifold coil of fibrils; and others, ¢.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 Biitschli’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°-60° C., an acid from the oil splits up the potassium carbonate, liberates. carbon dioxide, and forins an extremely fine emulsion. Drops of this show a structure not unlike that of cytoplasm, exhibit movements and streamings not unlike those of Amcebze, and are, in short, mimic cells. Just as a working model may help us to understand the circulation, so these oil-emulsion drops | may help us to understand the living cell, by bringing the strictly vital phenomena into greater prominence. Tt cannot be said, however, that subsequent research has corroborated the conclusion that cytoplasm has, in general, a vacuolar structure. There is increasing evidence of specific architectural organisation in different kinds of cells, and of the significance of infinitely small bodies—the plastosomes—which are included in the general cyto- plasmic matrix, and appear to be the vehicles of particular properties or formative potencies. What is certain is that the cell-substance is not homogene- ous like white-of-egg, but very heterogeneous and intricate. 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 recogni- tion of what seems to us so evident, that almost every animal arises from an egg-cell or ovum which has been fertilised by a male cell or spermatozoon. The exceptions to this fact are those organisms 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. 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 ; (4) the union or amphimixis of two different kinds of germ cells, ova and spermatozoa, which come to nothing unless they unite. Furthermore, these dimorphic reproductive cells are produced by two different kinds of individuals (females and males), or from different organs of one individual, or at different times within the same organ (hermaphroditism). It is conceivable that organisms might have gone on REPRODUCTION. 53 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 amphimixis or fertilisation of dimorphic germ cells, usually produced by different organs in one individual or by distinct male and female individuals. Liberation of special germ cells,x—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 Aydra, however, the buds are set adrift ; the same is true of not a few worms. This liberation of buds takes us nearer the sexual process of liberating special germ cells. But unless the organism is in very favourable nutritive conditions, in which over- growth is natural, the liberation of buds is an expensive way of continuing the life of a species. Not only so, but we can hardly think of budding even as a possibility, in very complex organisms, like snails or 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 “ 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 54 REPRODUCTION AND LIFE HISTORY. which may befall the “body” which bears them. And, finally, in the mixture of two units of living matter which have had different histories, an opportunity for new permuta- tions and combinations, in other words, for variation, 18 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. SumMMARY OF MopEs 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). 5 * (3) The formation of one bud at a time (very common). (4) The ordinary division into two daughter cells at the limit of growth. (5) Repeated divisions within limited time and within limited space’ (a cyst). This results in what is called spore-formation (e.9. in Sporozoa). B. Ln 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.) (4) The formation of definite buds which may or may not be set free. (c) Various forms of fission and fragmentation. (Sexual) The liberation of special reproductive or germ cells, which have not taken part in the formation of the body, and which retain the essential qualities of the original germ cell from which the ' parent arose. These special germ cells—the ova and sperma- tozoa—are normally united in fertilisation, but some animals have (parthenogenetic) ova which develop without being fertilised. Evolution of sex.—A further problem is to account for the two facts—(a) that most animals are either males or temales, the former liberating actively motile male elements or spermatozoa, the latter ‘forming and usually liberating more passive egg cells or ova; and (4) that these two EVOLUTION OF SEX, 55 different kinds of reproductive-cells usually come to nothing unless they combine. “te Bs The problem is partly solved by a clear statement of the facts. Let us begin with those interesting organisms which are on the border line between Protozoa and Metazoa, the colonial Infusorians, of which Volvox is a type. The adults are balls of cells, and the component units are con- nected by protoplasmic bridges. From’such a ball of cells reproductive units are sometimes set adrift, and these divide to form other individuals without more ado. In other con- ditions, however, when nutrition is checked, a less direct mode of reproduction occurs. Some of the cells become large, well-fed elements, or ova; others, less successful, divide into many minute units or spermatozoa. The large cells are fertilised by the small. - Hete we see the formation of dimorphic reproductive cells in different parts of the same organism. But we may also find Volvex 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 (Aafabolic) processes and con- structive (azabolic) processes in the protoplasmic metabolism varies from type. to type. It may be that the contrast between the sexes is another expression of this fundamental alternative of variation. Stages in the history of fertilisation. —While it is not difficult to see the advantage of fertilisation as a process which helps to sustain the standard or average of a species and as a source of new variations, we can at present do little more than indicate various forms in which the process occurs, (a) Formation of Plasmodia, the flowing together of numerous feeble cells, as seen in the life-history of those very simple Protozoa called Proteomyxa, ¢.¢. Protomyxa, and Mycetozoa, ¢.g. flowers of tan (4 thaliwm septicum), . (8) Multiple conjugation, in which more than two cells unite and fuse together temporarily, as in some Sporozoa and in the sun- animalcule.(Actinospherium). a wa . 56 REPRODUCTION AND LIFE HISTORY. (¢) Ordinary conjugation, in which two similar cells unite, with fusion of their nuclei, observed in Sporozoa, Heliozoa, Flagel- lates, and Rhizopods. In ciliated Infusorians, the conjugation may be merely a temporary union, during which nuclear elements are interchanged, (d) Dimorphie 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. 42 and Fig. 47). (e) Fertilisation, in which a spermatozoon liberated from a Metazoon unites intimately with an ovum, usually liberated from another individual of the same species. Divergent modes of sexual reproduction.—(2) 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 (autogamy). Certain facts, such as the occurrence of hermaphrodite organs as a transitory stage in the development of the embryos of many unisexual animals (e.g. frog and bird), suggest that hermaphroditism is a primitive condition, and that the unisexual condition of permanent maleness or femaleness is a secondary differentiation. Other facts, such as the hermaphroditism of many parasites, where cross- fertilisation would be difficult, suggest that the bisexual condition may have arisen as a secondary adaptation. It seems likely that there is both primitive and secondary hermaphroditism. (4) Parthenogenesis, as we know it, is a degenerate form ofsexuals reproduction, in which ova produced by female organism develop without being fertilised by male elements. It is well illustrated by Rotifers, in which fertilisation is the MODES OF SEXUAL REPRODUCTION. 57 exception (in some genera males have never been found), by many small Crustaceans whose males are absent for a season; by Aphides, from among which males may be absent for the summer (or in artificial conditions for several years) without affecting the rapid succession of female generations ; by the production of drones in the bee-hive from eggs which are never fertilised. (c) Alternation of generations. A fixed asexual hydroid or zoophyte often buds off and liberates sexual medusoids or swimming-bells, whose fertilised ova develop in- ee, ee to embryos which become I . fixed and grow into hydroids 4 tor or (Fig. 71, p. 150). This is the simplest illustration of alternation of generations, Grow which may be defined as .p © the alternate occurrence in a ar" one life-cycle of two (or more) different forms differently ¥ic. 26.—Diagrammatic expression produced (Fig. 26): of alternation of generations. The liver-fluke (Distomum 1. Hydromeduse, ov. Fertilised ovum gives rise to an hepaticum) of the sheep asexual form 4, which, by bud- produces eggs which, when fae Gane Be one Cee fertilised, grow into embryos. medusz, A is represented by Within the latter, certain eo ee cells (which might be called 7 pertlioet ovum gives rise to spores) grow into numerous paae e see Nad i, from Dp special spore-like cells pro- ee Ee = ae a eae the sexual orm. ithin these e fluke (5). same process is repeated, and finally the larvee thus produced grow (in certain con- ditions) into sexual flukes (Fig. 98, p. 189). In this case, reproduction by special cells, like undifferentiated precocious ova, alternates with reproduction by ordinary fertilised egg- cells. So, too, the vegetative sexless “fern-plant” gives rise to special spore cells, which develop into an inconspicuous bisexual “ prothallus,” from the fertilised egg-cell of which a “fern-plant” springs. Various kinds of alternation are seen in the life-cycle of the fresh-water sponge, in the stages of the jelly-fish Auzelza, 58 REPRODUCTION AND LIFE HISTORY. 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 essential qualities of the fer- _tilised ovum from which the parent animal ‘was devel- oped. The ovum has the usual characters of a cell; its substance is traversed by a fine protoplasmic net- work ; its nucleus or germinal vesicle con- Fic. 27.—Diagram of ovum, showing diffuse yolk granules. tains the usual chro- g.v., Germinal vesicle or nucleus ; chv., chromatin matin elements 3 it elements. has often a store of reserve material or yolk, and a distinct sheath representing a cell wall (Fig. 27). . ; 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. EMBRYOLOGY. 59 formation is restricted to distinct regions, and usually to definite organs—the ovaries. The young ovum is often amceboid, and that of Hydra retains this character for some time (Fig. 70, p. 148). 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 accumulated in the centre as in the eggs of Insects and Crustaceans; or it may be very copious, dwarf- ing the formative protoplasm, as in the eggs of Birds, Reptiles, and most Fishes (Fig. 31). Round the egg there are often sheaths or envelopes of various kinds—(a) made by the ovum itself, and then very delicate (e.g. the vitelline membrane); (4) formed by ad- jacent cells (e.g. the follicular envelope) ; or (¢) 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 we find a minute aperture (micropyle) or several of them through which’ the sperma- tozoon can enter. The hard calcareous shells round the eggs of Birds and Tortoises, or the mermaid’s purse en- closing the egg of a skate, are of course formed after fertilisation. Egg-shells must be distinguished from egg capsules or cocoons, ¢.g. of the earthworm, in which several eggs are wrapped up together. Male cell or spermatozoon.—This is a much smaller and usually a much more active cell than the ovum. In its minute size, locomotor energy, and persistent vitality, it resembles a flagellate Monad, while the ovum is comparable to an Amoeba or to one of the more encysted Protozoa. A spermatozoon has usually three 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 of the centrosome. The spermatozoa of Thread- 60 REPRODUCTION AND LIFE HISTORY. worms and most Crustaceans are sluggish, and inclined to be ameeboid (Fig. 28 (6, 7)). Both ova and spermatozoa are true cells, and they are complementary, but the spermatozoon has a longer history behind it (Fig. 29). The homologue of the ovum is the mother sperm cell or spermatogonium. This segments as the ovum does, but the cells into which it divides have little coherence. They go apart, and become spermatozoa. There is often a resemblance between the different ways in which a mother sperm cell divides and the various kinds of segmentation in a fertilised ovum.. In most cases the 75 Fic. 28.—Forms of spermatozoa (not drawn to scale). z1and 2. Immature and mature spermatozoa of snail; 3. of bird; 4. of man (4., head; 7., middle portion ; ¢., tail); 5. of sala- mander, with vibratile fringe (4); 6. of Ascaris, slightly ameeboid with cap (c); 7. of crayfish. spermatogonium divides into spermatocytes, which usually divide again into spermatids 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 bodies, which come to nothing, though they may linger for MATURATION OF OVUM. 61 atime in the precincts of the ovum, and may even divide. The second division follows the first without the intey- vention of the “resting stage” which usually succeeds a nuclear division. In most cases the division which forms the first polar body is a reducing or meiotic division, the number of chromosomes being reduced to half the number characteristic of the cells of the body. The extrusion of polar globules and the associated reduction is almost universal in the history of ova, but in most parthenogenetic ova only one polar body is formed, and there is no reduc- tion in the number of chromosomes. In some other cases B Fic. 29.—Diagram of maturation and fertilisation. (From Zvolution of Sex.) A. .Primitive sex cell, supposed to be ameeboid. B. Ovum; C. formation of first polar body (1. 4.2.); D. formation of second polar body (2. 4.4.). B’, Mother sperm cell; C’. the same divided (sperm-morula). 2’. Ball of immature spermatozoa: sA., liberated spermatozoa. __ £. Process of fertilisation; #. approach of male and female nuclei within the ovum. the parthenogenetic ovum passes through the meiotic phase and forms two polar bodies. The second of these, however, is not liberated, but remains within the ovum and re-uniting with the reduced nucleus restores the normal number of chromosomes. Reducing or Meiotic Division.—In each kind of animal there is a definite number of chromosomes, say , in each of the body-cells. In the ripe germ-cells, however, there is half the normal number, 2, so that when spermatozoon and ovum unite in fertilisation the normal number is restored. In the history of the germ-cells, therefore, in one way or another, at one stage or another, the number of chromo- 62 REPRODUCTION AND LIFE HISTORY. somes undergoes reduction to half the normal. In eps cases this reduction comes about through a “ heterotypic ” meiotic division. We give a condensed account of what happens in a large number of cases. The germ-cells grow relatively large ; the nuclear material takes the form of a definite number of chromatin loops; at a certain stage it is seen that the number is half what it was in more immature stages of the germ-cells, and half what it is in the somatic cells of the species under consideration. If the normal number be z, it is reduced to 4. There has been 2207 of chromosomes. The chromatin-loops contract away from the nuclear membrane (synapsis) ; the chromatin granules divide so that each loop appears doubly-beaded ; the ends of each loop are separated, and there are now n bodies with chromatin, each equivalent to a chromosome. The ends of the loops move apart, and, with or without a second synapsis, they change in shape, unite end to end, and form 2 twin- bodies or gemini, sometimes rod-like, sometimes like two brackets, sometimes like four dots. The nuclear membrane disappears, the = meiotic gemini are set free, they become arranged on the division-spindle at right angles to the equatorial plane (not flat as in ordinary karyokinesis), with their axes parallel to the axis of the spindle. They halve as if transversely, separating into two parts which go to the two poles of the spindle. Thus each daughter-cell has 7 chromosomes. In the case of the ovum the meiotic division usually occurs in the formation of the first polar body, so that it and the reduced nucleus of the ovum have each 7 chromosomes. There is no further reduction in the formation of the second polar body, which involves an ordinary equation-division. The first polar body often divides into two. Thus the result is one viable cell (the mature ovum) and three non-viable cells (the polar Lodies), each with 2 = chromosomes. In the spermatogenesis or production of spermatozoa the meiotic division is usually the second-last. A ‘“‘mother-sperm cell” or spermatogonium divides into spermatocytes with z chromosomes, each of these divides into 2 spermatocytes with = chromosomes, and these again divide into spermatocytes which differentiate into spermatozoa. The result is that from each of the penultimate generation of spermato- cytes there arise four spermatozoa, each with = chromosomes. Thus there is a close parallelism in the maturation process in the two sexes, That the fertilisation of the ovum restores the number to the normal is obvious. Part of the significance of the long circuitous process of meiotic division is that it affords opportunity for fresh permutations and combinations of hereditary qualities, for it seems probable that the chromosomes are the bearers of these.. FERTILISATION. 63 It is important to understand that in ordinary mitosis or cell-division, each daughter-cell gets an absolutely similar half of each chromosome of the mother-cell, whereas in meiotic division the daughter-cells get dissimilar halves. ’ A very important fact, discovered by Farmer, Moore, and Walker, is that the meiotic phase occurs among the cells of malignant growths (cancer). ‘*Through the action of one or several different causes at present unknown, certain cells of the soma, passing out of co-ordination, go through the meiotic phase and produce a number of generations of cells that live upon the parent organism in a parasitic manner.” Fertilisation.—In the seventeenth and eighteenth cen- turies, some naturalists, nicknamed “ ovists,” believed that the ovum was all-important, only needing the sperm’s awakening touch to begin unfolding the miniature model which it contained. Others, nicknamed “animalculists,” were equally confident that the sperm was essential, though it required to be fed by the ovum. Even after it was recognised that both kinds of reproductive elements were essential, many thought that their, actual contact was un- necessary, that fertilisation might be effected by an aura seminalis. Though spermatozoa were distinctly seen by Hamm and Leeuwenhoek in 1679, their actual union with ova was not observed till 1843, when Martin Barry detected it in the rabbit. i 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 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. fans (2) The union of spermatozoon and ovum is very intimate ; the nucleu$ of the spermatozoon and the reduced nucleus of the ovum approach one another, combining to form a unified nucleus. ; (3) The ovum centrosome disappears before fertilisation, and it is a centrosome introduced by the spermatozoon that 64 REPRODUCTION AND LIFE HISTORY. divides into the two which play an important réle in the cleavage or segmentation of the fertilised ovum. (4) When the combined or segmentation nucleus begins the process of development by dividing, each of the two daughter nuclei which result consists partly of material derived from the sperm nucleus, partly of. material derived from the ovum nucleus. In other words, the union is Fic. 30.—Fertilisation in Ascarzs megalocephala, —After Boveri. 1. Spermatozoon (s.) entering ovum, which contains reduced nucleus (1), having given off two polar bodies (4.4. 1 and 2). 2. Sperm nucleus (the upper), and ovum nucleus (JV), each with two chromatin elements or idants, and with centrosomes (c.s.). 3. Centrosomes (c.s.) with ‘‘archoplasmic” threads radiating outwards in part to the chromosomes of the two approximated nuclei. 4. Segmentation spindle before first cleavage. orderly as well as intimate, and the subsequent division is so exact, that the qualities marvellously inherent in the sperm nucleus (those of the male parent), and in the ovum nucleus (those of the mother animal), are diffused through- out the body of the offspring, and persist in its reproductive cells, (5) Some eggs, eg. of sea-urchins, can be artificially induced to develop without fertilisation (by being immersed for a couple of hours in a mixture of sea water and solution SEGMENTATION. 65 of Magnesium chloride, and by other means). It seems, therefore, justifiable and useful to distinguish in ordinary fertilisation, (a) the mingling of the hereditary qualities of the two parents, and (4) an exciting or liberating stimulus which induces the ovum to divide. It should be noted that the chromosomes of the spermatozoon do not fuse with the chromosomes of the ovum when fertilisation occurs. There is some evidence for the view that they remain distinct from one another until maturation again takes place, and one theory of the reduction in the number of chromosomes which takes place at maturation, is that it involves the fusion in pairs of the paternal chromosomes with the maternal. In some insects there is an accessory chromosome present in one half of the spermatozoa. It has been interpreted as an element whose presence or absence determines whether the.offspring is to be male or female. 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. A. COMPLETE Divis1on—Holoblastic Segmentation (1) Eggs with little and diffuse yolk material divide completely into approximately equal cells, [or, Ova which are alecithal (z.e. without yolk) undergo approxi- mately equal holoblastic segmentation]. This is illustrated in most Sponges, most Ccelentera (Figs. 31 (1) and 32), 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, 5 66 REPRODUCTION AND LIFE HISTORY. Fic. 31.—Modes of Segmentation. x. Ovum, with little yolk, segments totally and equally into a blastosphere, ¢.g. Hydra, sponge, sea-urchin. 2. Ovum, with a considerable amount of yolk (y.) at lower pole, segments totally but unequally, e.g. frog ; (y.s.) larger yolk- laden cells. 3. Ovum, with much yolk (y.) at lower pole, segments partially and! discoidally, forming blastoderm (42), e.g. bird, most fishes. 4, Ovum, with central yolk, (y.) segments partially and peripherally, e.g. most Arthropods BLASTULA AND GASTRULA. 67 [or, Ova with a considerable amount of deutoplasm lying towards. one ‘ag (telolecithal), undergo unequal holoblastic segmenta- tion]. This is illustrated in some Sponges, some Ccelentera (e.g. Ctenophora), some ‘‘ Worms,” many Molluscs, the lamp- rey, Ganoid Fishes, Dipnoi, Amphibians (Fig. 31 (2)). B, ParTiaL DivistoN—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 merobiastic and discoidal segmentation]. This is illustrated in all Cuttle-fishes, all Elasmobranch and Teleostean Fishes, all Reptiles and Birds (Fig. 31 (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. 31 (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 establishment of the two primary germinal layers, the outer ectoderm and the inner endoderm, or the epiblast and the hypoblast. One hemisphere of the hollow ball of cells may be appar- ently dimpled into the other, as we might dimple an india- rubber ball which had a hole init. Thus out of a hollow ball of cells, a two-layered sac is formed—a gastrula formed by invagination or emébolé (Fig. 32). The mouth of the gastrula is called the blastopore, its cavity the archenteron. 68 REPRODUCTION AND LIFE HISTORY. But where the ball of cells is practically a solid morula, the apparent in-dimpling cannot occur in the fashion de- scribed above. Yet in these cases the two-layered gastrula Fic. 32.—Life history of a coral, Monoxenia darwiniz, —From Haeckel. A, B, Ovum. C, Division into two. D, four-cell stage. E, Blas- tula. F, Free-swimming blastula with cilia. , Section of blastula. H, Beginuing of invagination. I, Section of com- pleted gastrula, showing ectoderm, endoderm, and archenteron , Free-swimming ciliated gastrula. ORIGIN OF ORGANS. 69 is still formed. The smaller, less yolk-laden cells, towards the animal pole, gradually grow round the Jarger yolk-con- taining cells, and a gastrula is formed by overgrowth or epibole. In various ways the ectoderm and the endoderm are established, either by some form of gastrulation, or by some other process, such as that called delamination (see p. 163). 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 Ccelentera it is not a distinct layer except in Cteno- phora, being usually represented by a gelatinous material (mesoglea) 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 (eg. in the earth- worm’s development); or from numerous “ mesenchyme” immigrant cells, which are separated from the walls of the blastula or gastrula (e.g. in the development of Echino- derms); or as celom pouches—outgrowths from the en- dodermic lining of the gastrula cavity (eg. in Sagé?tta, Balanoglossus, Amphioxus); or by combinations of these and other modes of origin. The mesoderm lies or comes to lie between ectoderm and endoderm, and it lines the body cavity, one layer of mesoderm (parietal or somatic) clinging to the ectodermic external wall, 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 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 Ccelentera are possessed by ectoderm and endoderm, the only two layers distinctly represented in these classes. In a backboned animal the embryological origin of the organs is as follows :— (a) From the ectoderm or epiblast arise the epidermis and epidermic outgrowths, the nervous system, the “7O REPRODUCTION AND LIFE HISTORY. most essential parts of the sense organs, infoldings at either end of the gut (fore-gut or stomodeum and hind-gut or proctodzeum). (b) From the endoderm or hypoblast arise the mid-gut (mesenteron) and the foundations of its out- growths (e.g. the lungs, liver, allantois, etc., of higher Vertebrates), also the axial rod or noto- chord. (c) From the mesoderm or mesoblast arise all other struc- tures, ¢.g. dermis, muscles, connective tissue, bony skeleton, the lining of the body cavity, and the vascular system. ‘This layer aids in the formation of organs originated by the other two. With it the reproductive organs are associated. Con- nective tissues, vascular system, and unstriped muscles are formed by mesenchyme cells which are budded off from the true mesoderm. 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 the fertilised egg-cell divide, how does the yolk affect segmentation, what are the conditions of the infolding Fic. 33.-—Embryos—(1) of bird ; (2) of man.—After His. The latter about twenty-seven days old. y.s., Yolk-sac ; A2., placenta. which forms the endoderm, and of the outfolding which makes the coelom pouches ; in short, what are the immediate conditions of each step in the familiar process by which, out of apparent simplicity, cbvious complexity arises? Generalisations.—(1) Zhe ovum theory or cell theory.— All many-celled animals, produced by sexual reproduction, GENERALISATIONS. zr begin at the beginning again. “The Metazoa begin where the Protozoa leave off”—as single cells. Fertilisation does not make the egg cell double; there is only a more com- plex and more vital nucleus than before. All development takes place by the division of this fertilised egg-cell and its descendant cells. (2) Zhe gastrea 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 gastrea. The gastrula is, on this view, the individual animal’s recapitula- tion of the ancestral gastreea. Rival suggestions have been made: perhaps the original Metazoa were balls of cells like Volvox (Fig. 43), with a central cavity in which repro- ductive cells lay; perhaps they were like the planula larvie of some Ccelentera—two-layered, externally ciliated, oval forms without a mouth. (3) The idea of recapitulation.—It is a matter of experi- ence that we recapitulate in some measure the history of our ancestors. Embryologists have made this fact 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 | (1) The first stage of development cells (Protozoa). is a single cell (fertilised (2) The next simplest are balls of ovum). cells (¢.g. Volvox). (2) The next is a ball of cells (3) The next simplest are two- (blastula or morula). layered sacs of cells (e.g. | (3) The next is a two-layered sac Hydra). of cells (gastrula). 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 72 REPRODUCTION AND LIFE HISTORY. is narrowed. The rabbit begins like a Protozoon as a single cell; after a while it may be compared to the young stage of a 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 un- mistakably a young rabbit. Herbert Spencer expressed the same idea, by saying that the progress of development is from homogeneous to heterogeneous, through steps in which the individual history is parallel to that of the race. But Haeckel has illustrated the idea more vividly, and summed it up more tersely, than any other naturalist. His ‘fundamental biogenetic law” reads: “Ontogeny, or the development of the individual, is a shortened recapitulation of phylogeny, or the evolution of the race.” It is hardly necessary to say that the young mammal is never like a worm, or a fish, or a reptile. It is at most like the embryonic stages of these, and it may also be noticed that, as our knowledge is becoming more intimate, the individual peculiarities of different embryos are be- coming more evident. But this need not lead us to deny the general resemblance. Moreover, the individual life 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 “larvee” which are very unlike their parents, often exhibit characters which are secondary adaptations to modes of life of which their ancestors had probably no experience. In short, the individual’s recapitulation of racial history is general, but not precise. It is seen rather in the stages in the development of organs (organogenesis) than in the development of the organism as a whole. (4) Organic continuity between generations.—Heredity.— Everyone knows that like tends to beget like, that offspring resemble their parents and their ancestors. Not only are the general characteristics reproduced, but minute features, idiosyncrasies, and pathological conditions, inborn in the parents, may recur in the offspring. HEREDITY 73 At an early stage in the development of the embryo the future reproductive cells of the organism are often dis- tinguishable from those which are forming the body. These, the somatic cells, develop in manifold variety, and, as division of labour is established, they lose their likeness to the fertilised ovum of which they are the descendants. The future reproductive cells, on the other hand, are not implicated in the formation of the “body,” but, remaining virtually unchanged, continue the protoplasmic tradition. unaltered, and are thus able to start an offspring which will resemble the parent, because it is made of the same protoplasmic material, and develops under similar con- ditions. An early isolation of reproductive cells, directly con- tinuous and therefore presumably identical with the original ovum, has been observed in the development of some “worm types” —(Sagitéa, Thread-worms, Leeches, Polyzoa), and of some Arthropods (e.g. Aoima among Crustaceans, Chironomus among Insects, Phalangidee among Spiders), in Micrometrus aggregatus among Teleostean fishes, and with less distinctness in some other animals. A cell which will give rise to the germ-cells can be recognised in the gastrula stage of Cyclops, and in the very first segmentation stages of the thread-worm Ascaris. In many cases, however, the reproductive cells are not recognisable until a relatively late stage in development, after differentiation has made considerable progress. Weismann gets over this difficulty by supposing that the continuity is sustained by a specific nuclear substance— the germ-plasm—which remains unaltered in spite of the differentiation in the body. It is perhaps enough to say that, as all the cells are descendants of the fertilised ovum, the reproductive cells are those which retain intact the qualities of that fertilised ovum, and that this is the reason why they are able to develop into 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 74 REPRODUCTION AND LIFE HISTORY’. doing, or as dints from external forces. The “body” is thus changed, but there is much doubt whether the repro- ductive cells within the “body” are affected specifically by such changes. Weismann denies the transmissibility of any characters except those inherent in the fertilised egg- cell, and therefore denies that the influences of function and environment are, or have been, of direct importance in the evolution of many-celled animals. Such influences affect the dody, and produce what are technically called ““ modifications,” but these modifications do not affect the reproductive cells—at least not in a specific representative way. Therefore modifications are not likely to be trans- mitted, and there seems no good evidence to show that they are. Many of the most authoritative biologists are at present of this opinion. On the other hand, many still maintain that profound changes due to function or environ- ment may saturate through the organism, and affect the seproductive cells in such a way that the changes or modifications in question are in some measure transmitted to the next generation. The question remains under dis- ‘cussion, but the probabilities are strongly against the transmissibility of acquired characters. It is important to try to distinguish different modes of hereditary resemblance. The characters of the two parents may be d/ended in the offspring, or those of one parent may find predominant expression (exclusive inheritance), or the characters of one parent may be expressed in one part of the offspring and those of the other parent in another (particulate inheritance). Another important inquiry is into the share that the various ancestors have ox an average in forming any indi- vidual inheritance. The inheritance of an animal repro- duced in the ordinary way is always dual, partly maternal and partly paternal, but ¢zrvovgh the parents there come contributions from grandparents, etc. Galton’s Law of Ancestral Inheritance states that ‘‘The two parents con- tribute between them, on the average, one half of the total heritage; the four grandparents, one quarter; the eight ‘great-grandparents, one eighth, and so on.” Another generalisation of great interest is Mendel’s Law, which seems to apply to certain cases, ¢.g. peas, stocks, HEREDITY 75 mice, and rabbits. In its simplest expression the law may be stated as follows :—If A be a well-established, pure-bred variety with a certain character, e.g. of stature or colour, and # be another well-established variety in which the corresponding character is different, and if A and & are crossed, the hybrid offspring (4) will usually resemble one of the parents in the particular distinguishing character. The character which finds expression is called the dominant ; the character which remains latent in the hybrids is called the recessive. Now, if the hybrids are bred together, their descendants will be of two kinds, some like the dominant grandparent, some like the recessive grandparent. When those like the recessive grandparent are in-bred, they yield only recessives. When those like the dominant grand- parent are in-bred, some yield pure dominants only—that is, forms which if in-bred yield only dominants ; but others yield apparent dominants like the original hybrid—that is, with the power of throwing off when in-bred more pure dominants, more pure recessives, and more apparent dominants like the original hybrid. The results tend to be always in the proportion 14+2A (B)+1B, as regards the two contrasted characters of A and B. Two diagrams (after T. H. Morgan and R. C. Punnett) may make the matter clearer. A B A (B) io" s Re 1A 2A (B) 1B \ 76 REPRODUCTION AND LIFE HISTORY. : D x R D I vi a % oe “te 6 D(R) R = # a Sy y, e £ i & * vil D D(R) R R Heredity may be defined as the relation of genetic continuity between successive generations, and inheritance as all that the organism is or has to start with in virtue of this hereditary relation. Development is the expression or realisation of the heritable qualities. which have their physical basis in the germ cells, and it presupposes an ap- propriate environment of nutrition and “liberating stimuli,” —‘ nurture” in the widest sense. What the organism becomes is the resultant of two components, inherited “nature” and external “nurture.” CHAPTER V PAST HISTORY OF ANIMALS (PALZONTOLOGY) 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. In the early days of the Evolution theory the modern science of Embryology was still in its infancy, and could furnish few arguments, and it was the opponents of the new theory rather than its sup- porters who appealed to Paleontology. They asserted that the palzontological facts refused to lend the support which the theory demanded. To their attacks the evolutionists usually replied by pointing out that the geological record was very incomplete. The numerous investigations which have since been carried on on all sides now show con- clusively that it was imperfection rather of knowledge than of the record which produced the negative results. We must, however, still acknowledge that, except in a few cases, there is but little certainty as to the precise pedi- gree of living animals, and seek for reasons to explain this. “Imperfection of the geological record.”—If we re- member the rule of modern Geology, that the past is to be interpreted by the aid of the present, there can be no difficulty in realising that the chances against the preserva- tion of any given animal are very great. Many are destroyed by other living creatures, or obliterated by chemical agencies. Except in rare instances, only hard parts, such as bones, 78 PAST HISTORY OF ANIMALS. 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. 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 Amcebee ; while the Foramin- ifera and the Radiolaria, having hard structures of lime or silica, have been well preserved. The flinty Sponges are well represented by their spicules and skeletons, Of the Coelentera, except an extinct order known as Graptolites, only the various forms of coral had any parts readily capable of preservation, and remains of these are very abundant in the rocks of many ancient seas. But, strange as it may seem, some beautiful vestiges of jelly-fish have been discovered. Of the great series of ‘‘ worms,” only the tube-makers have left actual remains; the others are known only by their tracks, while of any that may have lived on the land there is no evidence. The Echinoderms, because of their hard parts, are well represented in all their orders, except the Holothurians, where the calcareous structures characteristic of the class are at a minimum. “ PALAZEONTOLOGICAL SERIES.” 79 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 tham any other animals, since nearly all of them are possessed of a shell specially suitable for preservation. Among the Vertebrates some of the lowest are without scales, teeth, or bony skeleton ; such forms have therefore left almost no traces. Fishes, which are usually furnished with a firm outer covering, or with a bony internal skeleton, or with both, are well represented. The primitive Amphibians were furnished with an exoskeleton of bony plates, and are fairly numerous as fossils. ‘The bones and teeth of the others have been fossilised, though more rarely. Of some the only record is their footprints. The traces of Reptilia depend upon the habits of the various orders,. those living in water being oftenest preserved, but the strange flying Reptiles have also left many skeletons behind them. Of the Birds, the wingless ones are best represented, and then those- that lived near seas, estuaries, or lakes. The history of Mammals is very imperfect, for most of them were terrestrial. But the discoveries of Marsh, Cope, and others show how much may be found by careful search. The aquatic Mammals are- fairly well preserved. “Paleontological 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 fossilised 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. 34), and not less strikingly among those extinct Cuttle-fishes which are known as. Ammonites, and have perfectly preserved shells. Similarly, though less perfectly, the modern crocodiles are linked by many intermediate forms to their extinct ancestors, for it is- impossible not to call them by that name. In short, as knowledge increases, the evidence from Paleontology 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 80 PAST HISTORY OF ANIMALS 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 almost the whole mass of the oldest rocks, known as Archean or Pre-Cambrian, has 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. Careful searching in Pre-Cambrian beds has revealed the presence of several Molluscs, a Eurypterid, and a fragment of Trilobite. There are also “annelid tracks” indicative of life. Fic. 34.—Gradual transitions between Paludina neumayrzt (a), the oldest form, and Paludina hernest (j).—From Neumayr. 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 than real: the species lives on in its modified descendants, “different species” though they be. But, on the other hand, there are not a few fossil animals which have become wholly extinct, having apparently left no direct descendants. Such are the Graptolites, the ancient Trilobites, their allies the Eurypterids, two classes EXTINCTION OF TYPES. 81 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 them- selves 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 animats tn 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 miust fully realise that they are not isolated facts, that the present lay hidden in the past and has gradually grown out of it. Of the relative length of the periods represented here we know almost nothing, and we are also ignorant of the earliest ages in which life began. But the general result is clear. We find that in the Cambrian rocks, before Fishes appeared, the great Invertebrate classes were represented, though as yet but feebly. As we pass upwards they increase in number and in differentiation, Again, Fishes precede Amphibians, Amphibians are historically older than: Reptiles, and many types of Reptiles are much older than Birds. In short, in the course of the ages life has been slowly creeping upwards. [TaBLes. 82 PAST HISTORY OF ANIMALS. Quaternary or Post-Tertiary. Pliocene. ¢ coe santecee aces Bases ans sees Beale oesienteess I >| eee] Cree RS : a 3 3 g Ea g () 8 Miocene. 4 a 2 = z & <3 2 ek i : raf < G ee! fey wee 9 cece ee . oe € ween % ed . = er es 5 < Modern Eocene. Types Placentals. Cretaceous, cious rien Modern rogened and steans. | Types. orms: A RS gala armenia es eieanens ee Corie) Career (ee ee eee Ger civtd piseinjwieies oa o% as ; 8 : J) Marsupials « 8 Jurassic. ee and Mono- § g pleryx." tremes (?) GY wscseuising Gace pamanead es Paces: Pees eeerer i onbatradanad oostenonennen Laie mahae neon Triassic. Few primi- tive types. Permian. “i Laby- Carboniferous. tint donts, Devonian or Old ae ; Red Sandstone. Dipnoi. MS viecuors i pcennedle uxpaauaens Paseeneh ntenee yeaa o18s~enpa ies Redeeen ee a Beat g Ahoy Eye : 8 8 Ganoids 8 ‘Naxians and s Silurian. Elasino- $ branch Q i) Seren Serine i ier rr i a ei cr eae 8 . & Ordovician. x es Pee ee) e! Ce es oe naee Representa- tives of all . the chief Cambrian. lasses. GE Inverte- brates. Pre-Cambrian or Archzan. APPEARANCE IN TIME. 83 Coelentera. Echinoderma. Arthropoda. Cephalopoda. —~ ———, os EET T Quaternary or Post- Tertiary. Pliocene. Sepia and recent forms. Miocene. Tertiary or Cainozotc. Eocene. , Cretaceous. & sy : a A . ere eee eee eee PSP E Eee (ree pee eee 2 A fs vee ac oe le AX : al 41-30-37 oF" os Sg] 8 SHS Sere] SiS] SVS) | TELE SS Jurassic. Bs Fel el ed Ae NRG Bt I eal | eS ee | gs Of 413] S75)... 8] 8 Ele 3 eee ee eet eaten ees Pee) Cee bed nee (ei pas Fea] saedee elbagig feed camila; g, coment interesting analogy Poni to the processes of maturation and fertilisation in the higher animals. © : ; A number of individuals become joined up in a common gelatinous cyst. Each loses its pseudopodia and forms a membranous cyst. These cysts become associated in pairs. The nucleus of each ‘cyst divides mitotically.and a polar body is extruded from each, after which the nucleus returns to the resting condition. The cysts now fuse in pairs, with complete arid intimate union of their nuclei and cell-bodies. The zygote so formed rests for a short period, then divides up into two daughter cysts from which emerge two new individuals of Actinophrys, 92 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. In the allied genus Actinospherium, with very numerous nuclei, there is a strange and complicated formation and fusion of cysts within a single individual. Third Type of Protozoa—POLysTOMELLA Polystomella (see Fig. 50) is a type of Foraminifera with a calcareous perforate shell or test. Description.— Polystomella crispa is common on the shore, especially among Zostera. It looks like a miniature of an Ammonite shell, and Foraminifera were indeed classified by the older naturalists with the Ammonites. The test forms a close spiral with beautifully chiselled surface; only the last whorl is visible from the outside. The test is made up of a series of chambers which com- municate with one another and with the exterior by fine pores. Granular protoplasm fills up the chambers and forms also a thin layer on the outside. Long slender pseudopodia issue from the openings in the test and are given off also by the external protoplasmic layer. They frequently branch and anastomose with one another, and their granular protoplasm exhibits marked streaming movements. The pseudopodia serve to catch and en- tangle the diatoms and Infusoria on which the Foraminifer feeds. Like many other Foraminifera, Polystomella shows a remarkable dimorphism. It occurs in two forms, outwardly indistinguishable, but differing in internal struc- ture. In the megalospheric form the central chamber is large (a megalosphere), and there is a single large nucleus, placed about the middle of the series of chambers; in the microspheric form the central chamber is small (a microsphere), being about one-tenth of the diameter of the megalosphere, and there are numerous small nuclei. The megalospheric individuals are about thirty times as numerous as the microspheric indi- viduals, Life history.—The microspheric form has its nuclei replaced by chromidia (chromatin bodies detached from the nuclei into the protoplasm). These chromidia form the centres of amceboid nucleated spores which leave the POLYSTOMELLA. 93 shell or are liberated by the protoplasm creeping out and forming a halo of anastomosing threads round the deserted test. The spores secrete a shell and grow into the typical megalospheric forms. When the megalospheric form is about to reproduce, its nucleus disintegrates and is replaced by numerous scattered nuclei formed around chromidia. protoplasm segregates into little masses, each centred in a nucleus. Each of these nuclei divides by mitosis into two, then into four, and the division of the nucleus is followed by the division of the protoplasmic mass, The- so that hosts of e provided with flagella, swim out Fic. 37.—Polystomella, megalo- spheric form, with large central chamber (JZ) and one nucleus (W).—After Lister. tiny cells are Fic. 38.—Polystomella, central chamber (c.c.), microspheric form, with small umerous nuclei (V7), bridges of protoplasm between chambers (B).—After Lister. 94 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. into the water, leaving behind them the empty test, and there conjugate in pairs, not with one another but with similar “gametes” from another megalospheric individual. The “zygote” so formed becomes the initial chamber of a microspheric individual. In a more direct way—by fission — the megalospheric individual may give rise to another like itself. There is therefore in this complex life history of Polystomella an alternation between a sexual and an asexual generation. Fourth Type of Protozoa—PaRAM@CIUM Paramecium, a type of ciliated Infusorians, especially of those which are uniformly covered with short cilia (Holotricha). Fic. 39.—Paramecium.—After Biitschli. ad. Adult form, showing cilia, ‘‘ mouth,” contractile vacuoles, ete. div. Transverse division. con. Conjugation. Description.—Specimens of Paramecium may be readily and abundantly obtained by leaving fragments of hay to soak for some 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 PARAMGCIUM. 95 abundant in most stagnant pools, and are just visible when a test-tube containing them is held between the eye and the light. Their food consists of small vegetable particles. : ; The form is a long oval, with the blunter end in front; Fic. 40.—Conjugation of Paramecium aurelia—four stages. —After Maupas. 1. Shows macronucleus (/V) and two micronuclei (z) in each ot the two conjugates. z. Shows breaking up of macronucleus, and multiplication of micronuclei to eight. 2 3+ Shows the fertilisation in progress; the macronucleus is vanishing. : 4. Shows a single (fertilised) micronucleus in each conjugate. 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 Fic. 41.—Diagrammatic expression of process of ‘conjugation in Paramecium aurelia. —After Maupas. A. The two micronuclei enlarge. B. Each divides into two. C. Eight micronuclei are formed. 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 macronucleus and two micronuclei. to the mouth, from which undigested residues are got rid of. The surface is covered with cilia, in regular longitudinal rows; these serve both for locomotion and for driving food particles towards the mouth. Among the cilia there are small cavities in the cortex, in which lie fine protrusible. 96 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. threads (‘“trichocysts”). These, though parts of a cell, suggest the thread cells of Ccelentera, 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 asin the Ameba. 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 division into two (Fig. 39, @v.). One half includes the “mouth,” the other has to make one. As well as this simple fission, a process of transient conjugation 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. 40). This process is necessary 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. aure/éa is summarised diagrammatically in Fig. 41, 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 macronuclei of the two daughter-cells into which the Infusorian proceeds to divide (H); the other two form the micronuclei, but before another division occurs each has again divided. Thus each daughter-cell contains a macronucleus and two micronuclei. Fifth Type of Protozoa—VorTicELLA Vorticella, or the bell-animalcule, is a type of those ciliated Infusorians in which the cilia are restricted to a region round the mouth (Peritricha). VORTICELLA. 97 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 QE Let KA Tt a WAC RR lia: Ny ue oO Fic. 42.— Vorticella.—After Biitschli. 2. Structure. JV., Macronucleus; ., micronucleus; ¢.v., con- tractile vacuole ; 7z., mouth; /v., food vacuole; v., vestibule. 2. Encysted individual. 3. Division. 4. Separation of a free-swimming unit—the result of a division. 5. Formation of eight minute units (#g-). 6. Conjugation of microzooid (#zg.) with one of normal size - 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 Carchestum the stalk is branched, and each branch terminates in a bell. Up the stalk there runs, in a slightly wavy curve, a contractile filament, which, in shortening, gives the non-contractile sheath a spiral form. -This contractile filament, under a high power, may exhibit a fine striation, (A similar striated structure is seen in 7 98 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. some Amcebee, Gregarines, spermatozoa, etc., and of a much coarser type in striped muscle fibres. It seems to be some structural adaptation to contractility.) The bell has a thickened margin, and within this lies a disc-like lid; in a depression on the left side, between the margin and the disc, there is an opening, the mouth, which leads by a distinct passage into the cell. On the side of this passage there is a weak spot, the potential anus, by which useless débris is passed out. The cilia are arranged so as to waft food particles into the mouth and down the passage. There is a large and horseshoe-shaped macronucleus, and a small micronucleus. Food vacuoles and contractile vacuoles are present as usual. Sometimes a Vorticella bell jerks itself off its stalk and swims about; in other conditions it may form a temporary cyst; normally, the cilia are very active, and the move- ments of the stalk frequent and rapid. Multiplication may take place by longitudinal fission—a bell divides into similar halves; one of these acquires a basal circlet of cilia and goes free, ultimately becoming fixed. Or the division may be unequal, and 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 spermatozoon) fuses intimately with a larger passive cell, which may be compared to an ovum. Sixth Type of Protozoa—VoLvox Volvox is a type of flagellate Infusorians, especially of those with flagella of equal size. Volvox is found, not very commonly, in fresh-water pools, 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 amceboid in VM. globator, more spherical in V. aureus; each contains a nucleus and a contractile vacuole. VOLVOX. 99 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, z.e. it feeds as a plant does and builds up starch granules. In its method of reproduction Vo/vox is of much biological interest and importance. As Klein, one of its best describers, says, it is am Fic. 43.—Volvox globator.—After Cohn. a, Balls of sperms ; 4, immature ova ; c, ripe ova. epitome of the evolution of sex. Some of the colonies are asexual. In these a limited number of cells possess the power of dividing up to- form little clusters of cells; these clusters escape from the envelope of the parent colony, and form new free-swimming colonies. In other colonies there are special reproductive cells, which may be called ova, and spermatozoa. : In V. globator the two kinds of reproductive cells are usually formed in the same colony, the formation of spermatozoa generally preceding that of the ova, Technically the colony may then be described as a protandrous hermaphrodite. too PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. In V. aureus the colony is oftenest unisexual or dicecious, z.e. either male or female. But it may be moncecious or hermaphrodite, and is then generally protogynous, z.e. producing eggs first. Whether in a hermaphrodite or in a unisexual colony, the sex cells appear among the ordinary vegetative units ; the ova are distinguishable by their larger size, the ‘‘sperm mother cells” divide rapidly and form numerous (32-100 or more) slender spermatozoa, each with two cilia. In V. globator their bundles may break up within the parent colony ; or, as always occurs in V. aureus, they may escape intact, and swim about in the water. In any case, an ovum is fertilised by a spermato- zoon, and, after a period of encystation and rest, segments to form a new colony. Occasionally, however, this organism, so remarkable a condensation of reproductive 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 reproductzve cells, and occurring in asexual, hermaphrodite, and unisexual phases. Klein records no less than twenty-four different forms of V. aureus from the purely vegetative and asexual to the parthenogenetic, for there may be almost entirely male colonies, almost entirely female colonies, and other interesting transitional stages. Klein has also succeeded to some extent in showing that the occurrence of the various reproductive types depends on outside influences, Seventh Type of Protozoa—Mownocystis Monocystis, a type of Sporozoa in which the cell is zot divided into two parts by a partition. Description.—Two species (AZ agilis and MZ. magna) infest the male reproductive organs of the earthworm almost 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 projection which resembles the cap of Gregarina, otherwise unrepresented in AZonocystis. As in Gregarina, and many _ other parasitic forms, a contractile vacuole is absent. Life history.—The young form of JZ. agitis is parasitic within one of the sperm mother cells of the earthworm. It grows, and becomes free from the cell as a trophozoite. In the free stage, two individuals may unite in the curious MONOCYSTIS. 101 end-to-end manner observed also in Gregarina. In repro- duction two individuals (gametocytes) become associated inside a common cyst. The nucleus of each divides up repeatedly, and the daughter nuclei migrate to the surface of the cell, where each becomes surrounded by a little mass of protoplasm. Each of the gametocytes has thus given rise to a number of gametes, while there remains over a mass of residual protoplasm which has not been used up during this process. The wall between the two gametocytes now breaks down and the gametes conjugate in pairs, forming zygotes. It is probable that of each pair of conjugating gametes one is derived from each gametocyte. Each zygote Fic. 44.—Life history of Monocystis.—After Biitschli. 1, Young Gregarine lying within a sperm mother cell of earthworm. 2. Association of two Gregarines within a cyst, ready to form gametes. 3. Numerous spore-cases (pc. pseudonavicelle) within a cyst. 4. A spore-case with eight spores (s.) and a residual core (74). secretes a membrane and becomes a spore-case. The nucleus divides up, and eight elongated spores are formed round a residual core. The spore-case now takes its typical shape and is known as a pseudonavicella. The spores are considerably larger than those of Grvegarina. Eventually, in the alimentary canal of another earthworm the cyst bursts, the spore-cases are extruded, the spores emerge from their firm chitinoid cases. The young spore (sporozoite) is like a bent spindle (falciform), and seems next door to being flagellate. It bores into a mother sperm cell, and from this it afterwards passes as an adult into the cavity of the seminal vesicles. Intracellular parasitism and copious food naturally act as checks to activity, and the adult is sluggish. The allies of Monocystis occur chiefly in “Worms,” Tunicates, and Arthropods; none are known in Vertebrates. 102. PHYLUM PROTOZOA—THE SIMPLEST ANIMALS Eighth Type of Protozoa—GREGARINA Gregarina, a type of 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 foodstuffs, such Fic. 45.—Life history of Gregarina.—After Biitschli. x. Young forms (a, 4, c) emerging from intestinal cells (é.c.);3 .7., nucleus of intestinal cell. 2. Two forms conjugating (G. dlattarum). 3. Spore formation within a cyst. 4. Adult with deciduous head-cap (c.c.), and a cuticular partition dividing the cell into an anterior part (A) and a posterior part (B); ., the nucleus. 5. A spore within its spore-case (sf.c.). as peptones and carbohydrates, from their hosts, and store up glycogen within themselves. In many the size is about one-tenth of an inch. There is a firm cuticle of ‘“ proto- elastin,” which grows inwards so as to divide the cell into a larger nucleated posterior region and a smaller anterior region, and also, in the young stage, forms a small anterior cap. The cell substance is divided into a firmer cortical layer and a more fluid central substance. The protoplasm often presents a delicate fibrillar appearance, suggesting that of striated muscle. The nucleus is very distinct, but there are no vacuoles. We may associate the absence of GREGARINA—COCCIDIUM SCHUBERGI 103 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, re- mains for a time still attached to it by the cap (Fig. 45, a, 4,c); later this is cast off, and the individual 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 together. ‘The details of spore-formation are similar to those in Monocystis. All the 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 = - Bnd firm case. Eventually the cyst bursts, the ect Sieh spore-cases are liberated, and from within of Gregarines, each of these eight spores emerge to be- —After Fren- come cellular parasites. The adult of G. el. (Porospora) gigantea is sometimes three- quarters of an inch in length—enormous for a Protozoon. Ninth Type of Protozoa-—-CocciD1UM SCHUBERGI Coccidia are intracellular parasitic Sporozoa, attacking mainly the epithelial cells of the gut or associated organs. They are found chiefly in insects, myriopods, molluscs, and vertebrates. Coccidium schubergt infests the intestinal epithelium of the centipede Lithobius forficatus. The adult is a minute 104 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. oval or spherical cell with a nucleus. It lives a quiescent life within the host cell, growing and absorbing nourishment until the resources of the cell are exhausted. Life history.—The coccidium enters the host cell as a minute sickle-shaped body, pointed at the anterior end, and Fic. 47.—Life history of Coccidium.—After Schaudinn. 1. Sporozoite; 2. Sporozoite entering a cell and becoming a trophozoite ; 3-4. Schizont, forming merozoites; 5. Merozoites entering another cell; 6°. Merozoite forming macrogamete; 6%. Merozoite forming microgametes ; 7. Free microgamete ; 8-9. Fertilisation of macrogamete by microgamete; 10. Zygote within odcyst; tx. Formation of spores within odcyst ; 12. Spores forming sporozoites. more blunt posteriorly. This is the spovozoite stage of the life history; it is liberated from a cyst (odcyst) when the latter is swallowed by the centipede in its food. When freed in the gut the sporozoite progresses by forward gliding movements, alternating these by flexions, bending itself like COCCIDIUM. 105 a bow and straightening out again. When about to enter an epithelial cell it presses the anterior end through the cell wall and wriggles its way in. Once within the cell in which development is to proceed, its movements gradually cease, but it may pass through several cells before coming to rest. Within the host cell the coccidium—now in the ¢rophozotte stage—becomes oval in form, and in about twenty-four hours has reached full size and has exhausted the host cell contents. This is the completion of the trophozoite period, and the parasite now enters the schizon¢t stage, where its nucleus divides into a number of daughter nuclei. These arrange themselves around the periphery of the cell, whilst the protoplasm breaks up to form along with them bodies of a shape similar to the sporozoites. There are important structural differences, however, apart from the difference in origin. The parasites, now known as merozoites, rupture the host cell, move in the gut cavity after the manner of the sporozoites, enter fresh epithelial cells, and repeat the fore- going cycle until ultimately the greater part of the gut epithelium is destroyed. In about five days, however, owing perhaps to the failing capacity of the host to nourish, the limit of asexual reproductivity is reached, and the parasite now enters upon a spore-forming stage. Certain merozoites grow more slowly than the others, and instead ot becoming schizonts give rise to elements of two types, viz. microgametes, slender cells bearing a flagellum at each end, which are male, and macrogametes, larger bean-shaped cells, which are female. The latter after maturation free them- selves from the host cell, and in the cavity of the gut are fertilised by a male element. After fertilisation, a trans- parent membrane forms around the zygote (fertilised cell). This membrane in the first instance serves to exclude all microgametes after the first, and later, becoming very tough and resistant, forms a pretecting envelope or odcys¢. After the odcyst is formed the parasite may pass from the host to the exterior or remain for some time longer within it. The nucleus cf the zygote within the odcyst now divides into four, around which the protoplasm aggregates itself to form the speres. There are thus four spores within a cyst. Each spore divides, forming two sporozoites, which on the arrival of the odcyst in the gut of a fresh host are liberated, _ 106 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. and attacking the lining epithelium recommence the life history. GENERAL CLASSIFICATION OF PROTOZOA Since the Protozoa are unicellular organisms (except the few which form loose colonies), their classification should be harmonious with that of the cells in a higher animal. This is so. Thus (a) the Rhizopods, in which the living matter flows out in changeful threads or “‘ pseudopodia,” as CLASSIFICATION OF PROTOZOA {CorTICATA.) (GYMNOMYXA.) (CORTICATA.) Predominantly F Predominant] ciliated and ey encysted nat active. passive. INFUSORIANS. RHIZOPODS. SPOROZOA. ACINETARIA. RADIOLARIA. FORAMINIFERA. CILIATA. SPOROZOA LABYRINTHULIDEA, RHYNCHOFLAGELLATA HELI0z0A. OR DINOFLAGELLATA. LOBOSA. GREGARINES. FLAGELLATA. PROTEOMYXA and MyYcETOzOA, PRIMITIVE FoRMS. in the common Ameba, are comparable with the white blood corpuscles or leucocytes, many young ova, and other “ameboid” cells of higher animals; (4) the Infusorians, which have a definite rind and bear motile lashes (cilia or flagella), e.g. the common Paramecium, may be likened to the cells of cé//ated epithelium, or to the active sperma- tozoa of higher animals; (c¢) the parasitic Sporozoa, which SYSTEMATIC SURVEY. 107 have a rind and no motile processes or outflowings, may be compared to degenerate muscle cells, or to mature ova, or to “‘excysted” 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; (4) 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 amceboid ovum accumulating yolk becomes encysted, the ciliated cells of the windpipe may, to our discomfort, sink into amceboid forms. The same is true of the Protozoa ; thus in various conditions the ciliated or flagellate unit may become encysted or amceboid, while in some of the simplest forms, such as Protomyxa, there isa ‘‘ cell-cycle ” in which all the phases occur in one life history. SYSTEMATIC SURVEY A. Primitive forms.—Under this heading may be included two classes : (1) the Proteomyxa, primitive, insufficiently known forms often without a nucleus, though nuclear material may be present in the form of scattered granules (chromidia), and (2) the Mycetozoa, organisms with somewhat complex fructifications, often classed as plants allied Fic, 48.—Diagram of Protomyxa aurantiaca.—After Haeckel. x. Encysted; 2. Dividing into spores; 3. Escape of spores, at first flagellate, then amceboid ; 4. Plasmodium, formed from fusion of small amoebz. to Fungi. As examples of the Proteomyxa, we have the interesting Protomyxa in four phases: (a) encysted and breaking up into spores, which (4) are briefly flagellate, (c) sink into amceboid forms, and (d) flow together into a composite ‘‘ plasmodium ” ; Vampyrella, parasitic on fresh-water Algz ; and many others. The Mycetozoa are well illustrated by Fudigo or 4thalium septicum, “¢ flowers of tan,” found in summer as a large plasmodium on the bark of the tan-yard. The coated spores are formed in little capsules which rise from the surface of the plasmodium. The spores may be first flagellate, then amceboid, or amoeboid from the first ; the characteristic plasmodium is formed by the fusion of the amcebee, 108 PHLYUM PROTOZOA—THE SIMPLEST ANIMALS. B. Predominantly amceboid Protozoa.—Rhizopoda.—The simplest Rhizovods generally resemble Amada, and are ranked in the class (3) Lobosa, They may reproduce simply by division, as does Ameba itself, or may liberate several buds at once (Avcel/a), or form spores which conjugate (Pelomyxa). Warious forms, such as A7cella, are furnished with a shell. (4) The Labyrinthulidea are represented by forms like Labyrznthula on Algee, 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. Fic. 49.—Formation of shell in a simple Foraminifer. —After Dreyer. In A the shell has one chamber ; B, C, and D show the formation of asecond. Note outflowing psuedopodia and the enclosure of the shell by a thin Jayer of protoplasm; note also the nucleus in the central protoplasm. As (5) Heliozoa are classified the sun-animalcules (Actenospherium, Actinophrys sol), and others, in which there are stiff processes radiating froma spherical body. Reproduction may be by division or by spore formation ; skeletal structures may be represented by spicules. The (6) Foraminifera or Reticularia include an interesting series of shelled forms in which the peripheral protoplasm forms branching interlacing threads. A few simple forms occur in fresh water ; the great majority occur on the floor of the sea at varying depths; some SYSTEMATIC SURVEY. 109 families are abundantly represented on the surface. The shell is usually calcareous, more rarely arenaceous or chitinous. There is sometimes dimorphism. Multiplication occurs by fission, or by the formation of swarm-spores (amoeboid or flagellate). Foraminifera are common as fossils from Silurian rocks onwards, and at the present day are very important in the formation of calcareous ooze ; in this respect Globigerina, with a chambered shell, is especially important. Species of Gromia are found in both fresh and salt water; Halphysema, a form utilising sponge-spicules to cover itself, was once mistaken for a minute sponge. Most kinds of chalk consist mainly of the shells of Foraminifera Fic. 50.—A Foraminifer (Polystomella) showing shell and pseudopodia.—After Schultze. accumulated on the floor of ancient seas; Mummudlztes (Fig. 17) and related fossil forms were as large as shillings or half-crowns. More complex are the (7) Radiolaria, which are divided by a chitinoid membrane into an inner central capsule (with one or more nuclei), and an outer portion, gelatinous and vacuolated, giving off radiating thread- like pseudopodia, which very rarely interlace. There is usually a skeleton in the form of a siliceous lattice-work or regularly disposed spicules outside the central capsule, but in some cases the shell is formed of a horn-like substance called acanthin, which is probably a complex silicate. Radiolarians multiply by fission, which sometimes includes a halving of the skeleton, and by spores, which in some cases are dimorphic. Most Radiolarians include unicellular Algz (yellow 110 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. cells), with which they live in intimate mutual partnership (symbiosis). Most Radiolarians float on the surface of the sea; others live below the surface at varying depths; and some are abyssal. They are abundant as fossils, and of much importance in the formation of the ooze of great depths. Examples.—Thalassicola, Eucyrttdium, and the colonial Collozoum and Spherozoum, Fic. 51.—A pelagic Foraminifer— Hastigerina (Globigerina) murrayt.—After Brady. Note central shell, projecting calcareous spines with a protoplasmic axis ; also fine curved pseudopodia and vacuolated protoplasm. C. Predominantly active forms (ciliate and flagellate), generally called Infusorians.—Protozoa, with a definite rind and with 1-3 undulating flagella, are included as (8) Flagellata, a very large group, among which are such familiar forms as the common Euglena of ponds ; the Monads; Volvox, a colonial form ; Codosdga, a colony in which the individual cells are furnished with a collar (Choano- flagellata), The Hzemoflagellata are important blood parasites, generally called Trypanosomes (see p. 121), SYSTEMATIC SURVEY, I1t Modified flagellate torms are included in the groups Dinoflagellata and Cystoflagellata, in both of which there are two flagella, differ- ently placed in the two cases. In the first are included Pertdindum and Ceratium ; in the latter, the large phosphorescent Moctdluca. They form an important part of the plankton of lakes and sea. As (9) Ciliata are included a very large number of forms, more or less closely resembling Paramecium or Vorticella, and very abundant in infusions ; some, such as Ofaina, in the intestine of the frog, are parasitic, As specially modified Ciliata are included (10) Acinetaria, highly specialised forms, ciliated when young, but usually furnished when adult Fic. 52.—Optical section of a Radiolarian (Acténomma). —After Haeckel. a, Nucleus; 4, wall of central capsule; c, siliceous shell within nucleus ; cl, middle shell within central capsule ; ¢?, outer shell in extra-capsular substance. Four radial spicules hold the three spherical shells together. with suctorial tentacles, They are fixed in adult life, and feed on other Protozoa. As examples may be given Acéneta ; Dendrosoma, forming. branched colonies; and Oshryodendron, without suctorial tentacles. Some, like Spherophrya, are minute and parasitic. D. Predominantly encysted Protozoa,—Sporozoa.—Forms. like Gregarina and Monocystds are included in a group of the (11) Sporozoa, the Gregarinida in the strict sense. They are parasites in the gut or body cavity of many Invertebrates, especially Arthropods. Cocctdium is a type of the Coccidiidea, which are intracellular parasites. occurring in Arthropods, Molluscs, and Vertebrates. A very im- portant group, with a life cycle essentially similar to that of the Coccidiidea, are the Hzemosporidia, which are parasitic in the red blood corpuscles of Vertebrates. The malaria parasites belong to this 112, PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. group. In many of the Hzemosporidia 1 part of the life cycle takes place in an intermediate host, usually a mosquito or a tick. Other groups of the Sporozoa are the Myxosporidia, with peculiar nematocyst-like organs (Invertebrates and cold-blooded Vertebrates), and the Sarcosporidia, which are found inside the striped muscles of warm-blooded Vertebrates. GENERAL NOTES ON THE FUNCTIONS OF PROTOZOA Movement.—The simplest form of movement is that termed amceboid, as illustrated by an Amada. 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 Vortzce//a 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, or, more often, are held stretched out in front, and by a curious rotatory movement draw the cell along. They are then more aptly termed ¢vacte//a. It seems probable that cilia and flagella consist of an elastic core surrounded by a sheath, which may be uniformly contractile, or may have one contractile band, or two opposite contractile bands, and so on. Considered generally, the movements are of two kinds: either (1) re- flex, z.e. responses to external stimulus, as when the Protozoon moves towards a nutritive substance ; or (2) automatic, z.e. such movements as appear to originate from within, without our being able to point to the immediate stimulus, e.g. the rhythmical pulsations of contractile vacuoles. Actively moving Protozoa usually show the following motor reaction to stimulus:—they move backward, turn over on one side structurally defined, and then move forward again. Sensitiveness.—The Amceba 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. FUNCTIONS OF PROTOZOA, ~~ °° 113 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 amceboid cells) of a Metazoon crowd towards an in- truding parasite or some irritant particle. Nutrition.—The Amada expends energy as it lives and moves ; it regains energy by eating and digesting food particles. Most of the free Protozoa live in-this manner upon solid food particles; a few, such as Volvox, in virtue of their chlorophyll, are holophytic, ze. they feed like plants ; the parasitic forms usually absorb soluble and diffusible substances from their hosts., Respiration.—Oxygen is simply taken up by the general protoplasm from the surrounding medium, into which the waste carbonic acid is again passed. The bubbles which enter with the food particles assist in respiration. In parasitic forms the method of respiration must be the same as that of the tissue cells of the host. Excretion.—Of the details of this process little is certainly known, but the contractile vacuoles are, without doubt, primitive excretory appliances. In the more specialised forms they appear to drain the cell substance by means of fine radiating canals, and then to burst to the exterior. Uric acid and urates are said to be demonstrable as waste products. BS get 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 conclusior that the cells are symbiotic Algze, so-called Zoochlorelle. According to some, the “chlorophyll corpuscles” seen in the primitive Avcherina, in some flagellate forms, as Huglena, and in many Ciliata, as Stentor, Stylo- nichia, one species of Paramacium, Volvox and the. allied forms, are 114 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. also symbiotic Algee, 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 Vortzcella 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 datomin 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 hemoglobin of the host. Psychical life.—Protozoa often behave in a way which suggests control, but it should be noted that cut-off fragments sometimes behave just as effectively as the intact units. Verworn has decided, after much labour, that the Protozoa do not exhibit what even the most generous could call intelligence; but this is no reason why he or any other evolutionist should doubt that they have in’ them the indefinable rudiments of mind. Jennings has shown that the behaviour of some Infusorians corresponds to what may be called the method of trial and error; they “try” one kind of response after another until, in some cases, they give the effective answer. GENERAL NOTES ON THE STRUCTURE OF PROTOZOA The Protozoa are sometimes called “structureless,” but they are only so relatively. For though they have not stomachs, hearts, and kidneys, as Ehrenberg supposed, they are not like drops of white of egg. The cell substance consists of a living 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 bodies which are essential to the life and multiplication of the unit. In the Protozoa there are several conditions under which the nucleus may exist :— (1) In some adult forms, and in many spores or young forms, no definite nucleus has yet been discovered. It is, however, unnecessary to preserve the term ‘‘ Monera” for such simple forms, as it is probable that nuclear material does exist in the form at granules. NOTES ON THE STRUCTURE OF PROTOZOA. 115 (2) In the majority of cases, notably in the Sporozoa, the nucleus is single, often large, and placed centrally. From a consideration of the cells of Metazoa we may call this the typical case. _(3) In many of the Ciliata, e.g. Paramecium, there are two dimorphic nuclei. There is a large oblong nucleus, and beside it a smaller spherical one. he (4) In some Ciliata the macronucleus exists in the form of powder scattered through the protoplasm, ¢.g. in Ofalinopsds. The granules may collect to form a compact nucleus when fission is about to take place. (5) In Ofalina, from the intestine of the frog, and a few other forms, there are very numerous nuclei, arranged in a symmetrical manner in the cell substance. In some cases these isolated nuclei have been observed to unite to form one large nucleus just before binary fission takes. place. Of these various cases the diffuse condition is apparently very primitive. , The nucleus, when stained and examined under high powers, is observed to be complex in structure. It consists of a nuclear network, or a coil of chromatin threads. Karyokinesis has been observed in some cases. While we cannot at present define the physiological import of the nucleus, we must recognise its importance. Thus Bruno Hofer has shown that when an Amada is cut in two, the part with the nucleus lives and grows normally, while the part without any nucleus sooner or later dies ; and Balbiani has observed that in the case of Infusorians cut into pieces, those parts which have nuclei survive, while if no nucleus is present in the fragment, the wound may remain unhealed, and death ensues. The outer part of the cell substance (‘ectoplasm”) is often clearer and less granular than the inner part (‘‘ endo- plasm”). In corticate Protozoa there is a more definite rind or thickened margin of cell substance. Outside this there may be a “cuticle” distinct from the living matter, sometimes consisting of chitin, or gelatin, or rarely of cellulose. The cuticle may form a cyst, which is either a protection during drought, or a sheath within which the unit proceeds to divide into numerous spores. Moreover, the cuticle may become the basis of a shell formed from foreign particles, or made by the animal itself of lime, flint, or organic material. In the cell substance there may be bubbles of water taken in with food particles (food vacuoles), contractile vacuoles, fibres which seem to be specially contractile (in Gregarines), spicules of flint or threads of horn-like material, which may build up a connected framework, and the pigments raealdy mentioned. 116 PHYLUM PROTOZOA——-THE SIMPLEST ANIMALS. . 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. 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 (é.g. rcella, 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, z.e. in narrowly limited time and space, the result is the formation of a considerable number of small units or spores. In the great majority of cases, each result of division is seen to include part of the parent nucleus. A many-celled animal multiplies in most cases by liberating reproductive cells—ova and spermatozoa — different from the somatic cells which make up the “ body.” A Protozoon 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 (4) the fertilisation of ova by spermatozoa. As Protozoa have no “body”—though the beginnings of one are seen in the colonial forms—they cannot be said to exhibit sexual reproduction in the first sense (a), yet many of them (especially the Sporozoa) give origin by division to special reproductive cells. And although many Protozoa can live on, dividing and multiplying, for prolonged periods without the occurrence of anything like fertilisation, processes corresponding to fertilisation are of general occurrence. For in many of the Protozoa there occurs at intervals a process of “conjugation” in which two individuals unite REPRODUCTION OF PROTOZOA. 117 either permanently or'temporarily. This is an incipiently sexual process; it is the azalogue of the fertilisation of an ovum by a spermatozoon. In many cases, moreover, there is a difference between the two conjugates, analogous to the difference between ovum and spermatozoon. (1) It is one of the recurrent phases in the life history of some of the simplest Protozoa (Proteomyxa and Mycetozoa) (see p. 107), that a number of amceboid units flow together into a composite mass, which has been called a ‘‘ plasmodium.” ; (2) It is known that more than two individual Sporozoa and other forms occasionally unite. To this the term ‘‘ multiple conjugation ” has been applied. (3) Commonest, however, is’ the union of two apparently similar individuals, either permanently, so that the two fuse into one, or temporarily, so that an exchange of material is effected. Permanent conjugation has been observed in several Rhizopods, Infusorians, and Sporozoa. ‘Temporary conjugation is well known in not a few ciliated Infusorians, and it is possible that a curious end-to-end union of certain Sporozoa is of the same nature, or it may be of the nature of a ‘‘plasmodium ” formation. The formation of small spores (gametes) which conjugate is not uncommon. (4) There are some cases where one of the conjugating individuals is larger and less active than the other. Thus in Vorézcella, a small free-swimming form unites and fuses completely with a stalked indivi- dual of normal size. This ‘‘dimorphic conjugation” is evidently analogous to the fertilisation of a passive: ovum by an active sper- matozoon. In Volwox this is even more obvious, for the small: and active cells, both in shape and method of formation, recall the sper- matozoa of higher fornis. : Significance of Conjugation.—The precise interpretation of conjugation is uncertain. 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 characters 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 (Onychodromus grandis) were engaged in conjugation, a single individual had,. by ordinary asexual division, given rise to a family of from forty thousand to fifty thousand individuals. Moreover, the intense internal changes preparatory to conjugation, and the general inertia during subsequent reconstruction, not only involve loss of time, but expose the Infusorians to great risk. Conjugation seems to involve danger and death rather than to conduce to multiplication and birth. 118 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. 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. j We thus see that without normal conjugation the whole family becomes senile, degenerates both morphologicaliy 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. It must be noted, however, that some subsequent investigators have watched over two hundred asexual generations of ciliated Infusorians without seeing the slightest trace of senile degeneration. Calkins has cultivated Paramecium for over six hundred generations without conjugation by giving beef extract when degeneration threatened to set in. The same result was obtained by stimulating with alcohol, strychnine, etc. Ecology. — Many Protozoa raise organic débris once more into the circle of life, and many form part of the food of higher animals. Thus those pelagic Foraminifera and Radiolarians, which sink dying to the great oceanic depths, form along with more substantial débris the fundamental REPRODUCTION OF PROTOZOA. TI9 food supply in that plantless world. Fundamental, since it is plain that the deep-sea animals cannot all be living on one another. Almost every kind of nutritive relation occurs among the Protozoa. Predatory life is well illustrated by most In- fusorians, and thoroughgoing parasitism by the Sporozoa ; Opatina in the rectum of the frog may serve as a type of those which feed on decaying débris, and Volvox of those which are holophytic. Radiolarians, with their partner Algee, exhibit the mutual benefits of symbiosis, the plants utilising the carbon dioxide of their transparent bearers, the animals being aérated 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 fatally injurious to higher animals. Though Protozoa may be seriously infected by Bacteria, by Acinefa parasites, by some fungi, like Chytridiwm, etc., fatal infection is rare, because of the power of intracellular digestion which most Protozoa possess. “The parasite,” Metchnikoff says, “makes its onslaught by secreting toxic or solvent substances, and defends itself by paralysing the digestive and expulsive activity of its host; while the latter exercises a deleterious influence on the aggressor by digest- ing it and turning it out of the body, and defends itself by the secretions with which it surrounds itself.” With this struggle should be compared that between phagocytes and Bacteria in most multicellular animals. History.—Of animals so small and delicate as Protozoa, we do not expect to find distinct relics in the much-battered ancient rocks. But there are hints of Foraminifer shells even in the Cambrian ; 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 part of chalk deposits. There seem at least to be sufficient relics to warrant Neumayr’s generalisation in regard to Foraminifera, that the earliest had shells of irregularly agglutinated particles (Astrorhizidze), that these were succeeded by forms with regularly agglutinated shells, exhibiting types of architecture which were subsequently expressed in lime. 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. ~ 120 PHYLUM. PROTOZOA—THE SIMPLEST ANIMALS. Protozoa and Disease.—The discoveries of recent years have shown that the study of Protozoa is an inquiry of great practical importance. Numerous Protozoa— representing the main divisions of the group—are known at some stage of their life history to be parasitic in the human body or in domestic animals. Some of them are associated with serious and fatal diseases. Thus, Amaba (Entameba) histolytica causes an inflammation of the intestinal mucous membrane and liver abscesses. Several flagellates of the genus Zrypanosoma are serious Fic. 53.—Glossina palpalis, tse-tse fly. parasites of the blood affecting man, horses, cattle, camels, and other domestic animals in both the old and new worlds. TZ7ypanosoma gambiense (Fig. 54) is the parasite causing the fatal “sleeping sickness,” a human disease disseminated by tse-tse fly, Glossina palpalis, in Africa (Fig. 53). In the fully formed Trypanosome, the flagellum is expanded into an undulating membrane which extends down the edge of the cell. In this membrane there are eight fine contractile threads or myonemes, which are connected at the lower end with a modified nucleus called the blepharoplast. The so- PROTOZOA AND DISEASE. 121 called Leishman-Donovan body, the parasite of dum-dum fever or splenomegaly, a disease occurring in India and Africa, has recently been shown to be a stage in the life history of a flagellate protozoon. Of great import- ‘ance, also, is the family of Spirochztes, one of which, Treponema (Spirochete) pallidum, is the organism which causes syphilis. Another highly important genus is Fivo- plasma (Babesia), a sporozoon. These are blood parasites, Fic. 54. 2. Trypanosoma gambiense, showing nucleus, blepharoplast, and flagellum. 2and 3. Individuals undergoing longitudinal fission. 4. Leucocyte engulfing a trypanosome. causing Texas fever in cattle and analogous diseases in horse, sheep, dog, and possibly man also. The parasite of Texas fever is transmitted through two generations of ticks. Lastly may be mentioned the parasites of ‘malaria, JLaverania and Plasmodium, whose compli- ‘cated life histories in mosquito and man are now well ‘known. j , General zoological interest.—The Protozoa illustrate, in 122 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. free and single life, forms and functions like those of the cells which compose the many-celled animals. T 'ypically, 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; (4) the beginnings of fertilisation, from ‘‘ the flowing together of exhausted cells” and multiple conjugation, to the specialised sexual union of some Infusorians, Heliozoa, Sporozoa, etc..—where two individuals become closely united; along with this, the beginnings of maturation, as shown in the formation of polar nuclei in some Heliozoa, Sporozoa, Flagellata, and Lobosa ; (¢) the beginnings of sex, in the difference of size and of constitution sometimes observed between two con- jugating units (e.g. in Coccidium); (d) the beginnings of many-celled animals, in the associated groups or colonies which occur in several of the Protozoan classes. These colonies show a gradation in complexity. Raphidiophrys and other Heliozoa form loose colonies, which arise by the want of separation of the products of fission. Among the Radiolarians there are several colonial forms; in these the individuals are united by their extra-capsular protoplasm, but are all equivalent. In /voterospongia the cells show GENERAL ZOOLOGICAL INTEREST. 123 considerabie 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 repro- ductive units. Fic. 55.—A colonial flagellate Infusorian— Proterospongia haeckeliz.—After Saville Kent. There are about qo flagellate individuals. a, nucleus ; 4, contractile vacuole; c¢, amoeboid unit in gelatinous matrix ; 3 @, division ‘ of an amoeboid unit; e, flagellate units with collars contracted ; J, hyaline outer membranes ; g, unit forming spores. Lastly, in their antithesis of passivity and activity, con- structive and destructive preponderance, anabolism and katabolism, the Protozoa illustrate the phases of the cell- cycle, and so furnish a key to the variation of higher animals. CHAPTER VIII PHYLUM PORIFERA—SPONGES Class I. CALCAREA. Class 11. HEXACTINELLIDA. Class III. DEMOSPONGIA. SPONGES seem to have been the first animals to attain marked success in the formation of a “body.” For though their details are often complex, their essential structure is simpler than the average of any other class of Metazoa, and some of the simplest forms do not rise high above the level of the gastrula embryo. A “body” has been gained, but it shows relatively little division of labour or unified life ; it is a community of cells imperfectly integrated. The cells of the body show an arrangement in two distinct layers, which is one of the most essential characters of the Metazoa. There are no definite organs, and the tissues are, as it were, in the making. Sponges are passive, vegetative animals, and do not seem to have led on to anything higher; but they are successful in the struggle for existence, and are strong in numbers alike of species and of individuals. GENERAL CHARACTERS Sponges are diploblastic (two-layered) Metaszoa, the middle stratum of cells, the mesoglea, not attaining to the definiteness of a proper mesoderm. There is no celom or body cavity. The longitudinal axts of the body corresponds to that of the embryo; tn other words, the general symmetry of the gastrula ts retained. In these three characters the Sponges \ STRUCTURE OF SPONGES. 125 agree with the Celentera, and differ from higher (trt- ploblastic and celomate) Metazoa. The body varies greatly in shape, even within the same species. It ts traversed by canals, through ° which currents of water bear food in- wards and waste outwards. Numerous minute pores on the surface open into afferent canals, leading into a cavity or cavities lined by flagellate cells, many or all of which have a goblet shape with a delicate collar through which the flag- ellum rises (‘‘choanocytes”). To the activity of the flagella the all-important water currents are due. The internal cavity may be a simple tube, or it may have radially outgrowing chambers, or tt may be represented by branched spaces, Jrom which afferent canals lead to the exterior. When there is a distinct central Fic. 56.—Simple cavity there ts usually but one large sponge (Ascetta exhalant aperture (osculum), but in other hig age 1 oe cases there are many exhalant apertures. ae A delicate outer layer covers the body, Nee aie aie and is perhaps continued into the affer- uaeaite pores in the ent canals. Beneath the covering layer ~*~ there is in all but the simplest forms a mass of cells (the mesoglea) which may be very varied in its composition. Thus there are scleroblasts making the skeleton of lime, flint, or Spongin ; amebotd cells or phago- cytes, important in digestion and excretion , reproductive cells, and other elements. This median mass of cells ts traversed by the afferent canals and by the diverticula of the central cavity or the branches of poecre the original central cavity, lined Fic. 57.—A sponge colony. Sy flagellate cells, It is difficult to call this cavity or system of cavities the gut or enteron, or to call the layer which lines it the endoderm, or the outer covering layer the ectoderm. In ERO 126 PHYLUM PORIFERA—SPONGES. fact, the sponges are very different Jrom other Metazoa, and represent a cul de sac in evolution. Budding is very common, and in a few cases buds are set adrift. Both hermaph- rodite and unisexual forms occur. The sexually-produced embryo ts almost always developed within the mes- oglea, and leaves the sponge as a ciliated larva. With the exception of one family, Fic. 58.—Sponge spicules. 1, Monaxon ; 2, triod; 3, triaxon; 4, tetraxon 3 5, anchor 2 6, polyaxon ; 7, a kind of amphidisc, all are marine. Description of a simple sponge. — A sponge, such as Ascetfa, is very simple 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. In the calcareous sponges, the pores are minute perforations in single cells (porocytes). The walls consist of—(1) a flat covering layer; (2) a mesogloea containing triradiate calcareous spicules, phagocytes, and reproduc- tive elements; and (3) a layer lining the central cavity, and composed of collared flagellate cells, like some of the monad Infusorians (cf. Fig. 55): 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 (Zuspongia). Let us consider the origin of complications. Fic. 59.— Section of a sponge.—After F. E. Schulze. Showing inhalant canals, flagellate chambers, a gastrula forming in the mesogloea, etc. (a) Sponges—long regarded as plants—are plant-like in being sedentary and passive. They seem also to feed STRUCTURE OF SPONGES. 127 easily and well. Like plants, they form buds, the outcome of surplus nourishment, a rose-bush, often ac- quire some apparent independence, and: the sponge looks like many vases, not like one. Moreover, as they grow these buds may fuse, like the branches of a tree tied closely to- gether. Thus the struc- ture becomes more in- tricate. (6) In the simple sponge the cavity of the vase is completely lined by the collared flagellate cells (Ascon type). But the inner layer may grow out into radial chambers to which the choan- ocytes are restricted (Sycon type), and the walls of these may also be folded into side aisles (Leucon type). The out- growing of the inner layer into the mesogloea may be continued even further, and the cells may become pavement- like except in the minute flagellate cham- bers, where alone the characteristic choano- cytes are retained (see Fig. 60). It may be that the characteristic folding or ‘These buds, like the suckers of Fic. 60.— 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. Simple Ascon type (Zc., outer layer; Az., inner layer; /Zg., mesoglcea). Se type, with flagellate radial chambers ree the main radial chambers. Still more complex type, with small flagel- late chambers (/.ch.). A. B. C. Leucon ‘type, with flagellate side, aisles on D. outgrowth of the inner layer is necessitated by the fact that the component cells are better 128 PHYLUM PORIFERA—SPONGES. nourished and multiply more rapidly than ‘those of the outer’ layer. (c) By infoldings of the outer layer and a subjacent sheath of mesoglcea—subdermal spaces may be formed; an outer cortex may be distinctly differentiated from the internal region in which the flagellate chambers occur; the pores may collect into sieve-like areas, which open into dome-like cavities; these and many other complications are common. (d) The covering layer usually consists of flat epithelium, but flask-shaped cells have also been observed (Bidder). It may be folded inwards, as we have noticed, and, accord- ing to some, it also lines the inhalant or afferent canals in whole or in part. In a few cases, e.g. Oscarella lobularis, it is ciliated, and its cells may also exhibit contractility, as around the osculum of Ascetfa clathrus, though the con- tractile elements usually belong to the mesoglcea. The inner layer consists typically of collared flagellate cells, but in the more complex sponges these are replaced, except in the flagellate chambers, by flat epithelial cells, with or without flagella. The mesoglea contains very varied elements, and illus- trates the beginnings of different kinds of tissue. Thus there are migrant amceboid cells (phagocytes); irregular connective tissue cells; spindle-shaped connective tissue cells, united into fibrous strands; contractile cells, e.g. those forming a sphincter around the oscula of some forms, such as Pachymatisma; skeleton-making cells; pigment- containing cells; supposed nerve cells, projecting on the surface, and believed to be connected internally with multipolar (ganglion?) cells; and lastly, the reproductive cells, (e) The skeleton consists of calcareous or siliceous spicules, or of spongin fibres, or of combinations of the two last. A calcareous spicule is formed of calcite, with a slight sheath and core of organic matter; a siliceous spicule is formed of colloid silica or opal; the spongin is chemically - somewhat like silk. Uniradiate, biradiate, triradiate, quadri- radiate, sexradiate, and multiradiate spicules occur, and they are effective in keeping the meshes open and in giving the body architectural stability. In every pole scaffolding ORDINARY FUNCTIONS. 129 we see, as it were, huge hexactinellid spicules, spliced to- gether with ropé. It is convenient to distinguish the large macroscleres from the small microscleres. Each spicule begins to. be formed by one or more “scleroblasts,” and may be speculatively regarded as an organised intra- cellular excretion. ‘ During its growth,” Professor Sollas says, “the spicule slowly passes from the interior to the exterior of the sponge, and is finally (in at least some sponges—Geodia, Stelletta) cast out as an effete product.” The fibres of spongin are formed as the secretions of mesogloea cells, known as spongioblasts. Ordinary functions.—Excepting the fresh-water Spong- illide, all sponges are marine, occurring from between tide marks to great depths. After embryonic life is past, they live moored to rocks, shells, seaweeds, and the like. Their motor activity is almost completely restricted to the lashing movements of the flagella, the migrations of the phagocytes, and the contraction of muscular mesogloeal cells, especially around the exhalant apertures. In the closure of the inhalant pores, sponges show sensitiveness to injurious influences, but how far this is localised in specialised cells is uncertain. The most important fact in the life of a sponge is that which Robert Grant first observed—that currents of water pass gently in by the inhalant pores, and more forcibly out by the exhalant aperture or apertures. This may be demonstrated by adding powdered carmine to the water. ‘The instreaming currents of water bear dissolved air and supplies of food, such as Infusorians, Diatoms, and particles of organic débris. 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 flagellate cells; (4) in the adjacent phagocytes of the mesogloea ; (c) in the phagocytes surrounding the sub- dermal spaces, if these exist. It is uncertain whether the epithelium of the subdermal spaces or the flagellate lining of the deeper cavities is the more important area of absorp- tion, but it is certain that the phagocytes play an important part in engulfing and transporting particles, in digesting those which are useful, and in getting rid of the useless. 9 130° PHYLUM PORIFERA—SPON GES. 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 zoonerythrin (familiar in lobsters) is common. Some pigments, such as floridine, may help 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 Spongillidze 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 suffers from the cold and the scarcity of food, and dies away. But throughout the moribund parent gemniules 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 genera- tions. 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 the 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 amceboid 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 DEVELOPMENT. 131 the latter case, however, either the production of ova or the production of spermatozoa usually preponderates, probably in dependence upon nutritive conditions. Development.—lIt 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, usu- ally lies near one of the canals, and is fertilised by a spermato- zoon 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. Fic. 61.—Development of Sycandra raphanus.—After F. E. Schulze. x. Ovum. 2. Section of 16-cell stage. 3. Blastula with 8 granular cells (gxc.) at lower pole. 4. Free-swimming amphiblastula, with -upper hemisphere’ of flagellate cells (fc.), and lower hemisphere of granu- lar cells (gv.c.). 5. Gastrula stage settled down. c., outer layer; Zw, inner layer; 42., closing blastopore ; a@7.Z., mooring, amceboid processes. , 132 PHYLUM PORIFERA—SPONGES. (a) In the small calcareous sponge Sycandra raphanus (Fig. 61), as described by F. E. Schulze, the segmentation results in a hollow ball of cells—the J/astula. 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 invaginated, forming what is called a “‘ pseado-gastrula.” This leaves the parent, and the cells forming the lower hemisphere of the embryo become rounded and non-flagellate. The embryo swims for a time actively, but the flagellate cells of the upper hemisphere are invaginated into or overgrown by the large granular cells, and thus what is generally called the gastruda stage results. This soon settles down, on rock or seaweed, with the blastopore or gastrula mouth down- wards, and is moored by amceboid 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 entrance of water enables the inner cells to récover their flagella, and an exhalant aperture is ruptured at the upper pole. Fic. 62.—Diagrammatic re- presentation of development of Oscarella lobularis. — After Heider. Bl, Free-swimming blastula with flagella; G., gastrula settled down. Next figure shows folding of inner layer (Fx.); £c., outer layer. Lowest figure shows radial cham- bers (2.C.); Mesoglea (AZg.); inhalant pore (/.); exhalant osculum (0.). hemispherical gastrula, which settles mouth downwards. The young sponge is now in an Ascon stage, from which, by the outgrowth of the inner layer into radial chambers, it passes into the permanent Sycon form, grows into a cylinder, and becomes differentiated in detail (Fig. 61). (6) In Oscarella (Halisarca) lobularis (Fig. 62), a sponge without any skeleton, the ovum segments equally into a blastula, which is flagellate all over. This free-swimming stage may’ be in- vaginated from either pole to form a Pores, an osculum, and the mesogloea are formed as before, and the inner layer becomes folded into flagellate chambers. The main features of sponge embryology are thus summarised by Minchin :— “I, The larva is composed of three classes of cell-elements: (1) Columnar flagellated cells, forming the outer covering or localised at CLASSIFICATION. 133 the anterior pole ; (2) rounded, more or less amoeboid elements, rarely flagellated, forming the inner mass or aggregated at the posterior pole ; and (3) the archzeocytes, usually scattered in the inner mass, and often represented by undifferentiated blastomeres... . “TI, The larva fixes and undergoes a metamorphosis whereby the flagellated cells become placed in the interior, while the cells of the inner mass come to surround them completely. “JIT. (1) The flagellated cells of the larva become the choanocytes of the adult (gastral layer), acquiring a collar; . . . (2) the inner mass _gives rise to the dermal layer in its entirety; . . . (3) the archzeocytes become the wandering cells of the adult, from which the reproductive cells arise.” It is interesting to note that the primitive germ-cells are early set apart. Classification. __ Class I.—Catcarea. With skeleton of calcareous spicules :— Grade I.—Homoccela. — Continuous’ internal layer of collared flagellate cells, e.g. Ascetta, Leucosolenia. Grade II.—Heteroccela.—Collared flagellate cells restricted to radial tubes or chambers, ¢.g. Sycou (Grantia), Class II.—HEXACTINELLIDA, or Triaxonia, with sexradiate siliceous spicules (triaxons). The members live chiefly in deep water, e.g. Wenus Flower-Basket (Zzp/ectella) and the Glass-Rope Sponge (Ayalonema). Class III. —Demospongiz. Skeleton of siliceous spicules, but never triaxons, or of spongin fibres, or of spongin fibres and siliceous spicules, or absent. Grade I.—Tetraxonida, typically with tetraxon spicules, e.g. Pachymatisma, Tetzlla. Grade II.—Monaxonida, with monaxon spicules, sometimes with spongin in addition, ¢.g. Mermaid’s Gloves (Cha/ina oculata), Crumb-of-Bread Sponge (Halzchondria or Amorphina pantcea), Fresh-Water Sponge (Spongzl/a). Grade III.—Ceratosa, ‘‘ horny” sponges with or without spicules, e.g. the Bath-Sponge (Zuspongia). Grade IV.—Myxospongida, without any skeleton, eg. Halisarca and Oscarella. A very remarkable form called /er/za seems to have both a siliceous and a calcareous skeleton. History.—Sponges, as one would expect, date back almost to the beginning of the geological record. Thus the siliceous Protospongia occurs in Cambrian rocks, and in the next series—the Silurian—the main groups are already represented. From that time till now they have continued to abound and vary. The division between calcareous and siliceous sponges goes deep down to the very roots of the phylum, and the siliceous branch must have divided very early into Triaxonia and Tetraxonia. Ccology.— Sponges are living thickets in which many small animals play hide-and-seek. Many of the associa- 134 PHYLUM PORIFERA—SPONGES. tions are harmless, but some burrowing worms do the sponges much damage. The spicules and a frequently strong taste or odour doubtless save sponges from being more molested than they are; the numerous phagocytes wage successful war with intruding micro-organisms. Some sponges, such as C/iona on oyster-shells, are borers, and others smother forms of life as passive as themselves. Several crabs, such as Dromia, are masked by growths of sponge on their shells, and the free transport is doubtless advantageous to the sponge till the crab casts its shell. A compact orange-coloured sponge (Suderites domuncula) of peculiar odour often grows round a whelk-shell tenanted by a hermit-crab, and gradually dissolves the shell-substance. Within several sponges minute Algz live, like the “yellow cells” of Radiolarians, in mutual partnership or symbiosis. One of the cuttlefishes, Ross¢a glaucopis, puts its eggs care- fully into pockets in the substance of a siliceous sponge. Finally, sponges deserve mention as factors in human civilisation. General zoological position. — Sponges form the first successful class of Metazoa. They illustrate the beginnings of a “body,” and the beginnings of tissues. Along with the Ccelentera, they differ markedly from the triploblastic, Ccelomate Metazoa, which do not retain the radial symmetry of the gastrula. In their germinal layers and in their internal cavity they differ so much from Ccelentera and all other Metazoa, that they must be regarded as on a by-road of evolution. This has been emphasised by Professor Ray Lankester in the term ‘‘ Parazoa” ; he speaks of them as a sterile stock. Their origin is wrapped in obscurity; it may be that they are the non-progressive descendants of primitive gastrula-like ancestors with a sluggish constitution. The presence of choanocytes suggests a relationship with certain of the flagellate Protozoa (Choanoflagellata), and Protero- spongia (Fig. 55) may possibly be regarded as a connecting link. InceRT& SEpDis. MEsozoA The title Mesozoa was applied by Van Beneden to some simple orginisms which appear to occupy a very humble position in the .MESOZOA. 135 Metazoan series. He regarded them as intermediate between Protozoa and Metazoa; but others have remarked on their resemblance to Platyhelminthes, and especially to the sporocysts of certain Flukes. They may perhaps be regarded as precociously reproductive sporocysts. It will be enough here merely to notice four types :— 1. Dicyemidz (type Dzcyema) occur as parasites in Cephalopods ; Fic. 63.—A. Young Décyema.— Fic. 64.—Salinella. — After Whitman. 3B. Female After Frenzel. Orthonectid (Ahopalura giar- Pee en eee dzz).—-After Julin. = stron e ‘ pape terior. ¢., Ectoderm ; e7., inner endoderm cell with nucleus (z.); and embryo (e77.). Note the segmentation and the fibrillation supposed to be muscular. z. Transverse section. 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. 63, A). 2. Orthonectide (type RAopalura) occur as parasites in Turbellarians, Brittle-stars, and Nemerteans; the body is slightly ringed, and con- sists of a ciliated outer layer, a subjacent sheath of contractile fibres, and an internal mass of cells, among which ova and spermatozoa appear. ‘The sexes are separate and dimorphic (see Fig. 63, B). * \ 136 MESOZOA. 3. Professor F. E. Schulze discovered a small marine organism — Trichoplax adherens—in the form of a thin, ¢hvee-layered, externally ciliated plate ; and Monticelli records a similar form under the title Treptoplax adherens. But Trichoplax is now said to be the planula of the Hydromedusan Eleuthera, 4. Professor J. Frenzel discovered. in brine solutions a minute Turbellarian-like organism—Salénella salve—whose body consists of one layer of cells (Fig. 64). There is an anterior mouth, a ciliated food canal, and a posterior anus. The ventral surface is finely ciliated, the other cells bear short bristles. The animal reproduces by trans- verse fission, but conjugation and encystation also occur. It must be confessed that some corroborative evidence in regard to this peculiarly simple animal is much to be desired. CHAPTER IX PHYLUM CQ@Q:LENTERA Class 1. HYDROZOA. Class 3. ANTHOZOA or Hydroids and ACTINOZOA. Medusoids. Sea-anemones, Class 2. SCYPHOMEDUS or Madrepore-corals, ACRASPEDA. Alcyonarians, etc. Jelly-fishes. Class 4. CTENOPHORA. Tue Ccelentera—including zoophytes, swimming-bells, jelly- fish, sea-anemones, Alcyoriarians, corals, and the like—form a very large series of Accelomate Metazoa, 7.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 (¢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 co-operation and division of labour. With the exception of three or four fresh-water forms like Hydra, all are marine. GENERAL CHARACTERS The Calentera are almost always radially symmetrical animals in which the primary long axis of the gastrula becomes the long axis of the adult. There is no body cavity, or ceelom, distinct from the digestive cavity (enteron) and its outgrowths. In the lower members of the phylum, the 138 PHYLUM CELENTERA. primary opening of this cavity becomes the mouth of the adult, but in the more specialised types there ts an (ectodermic) oral invagination, which forms a gullet-tube or stomodeum. Between the ectoderm and endoderm of the body wall there 7s a supporting layer, or mesoglea, often of jelly-like con- sistency, In Ctenophora, however, a more definite mesoderm zs established at an early stage in development. In the simplest cases the mesoglea is a secretion quite devoid of cells, but secondary cells may migrate into tt from the endoderm. Stinging cells of varying complexity are almost always present, but in most of the Ctenophora their place ts taken by adhesive cells, The Celentera exhibit two types of structure—polypotd and medusoid—which recur in modified forms throughout the group, and may be both present in the course of one life history, when they illustrate the phenomenon of alternation of generations, or metagenesis. The more primitive type is the sessile tubular polyp, which, at its simplest, may be com- pared toa gastrula fixed by one end, and furnished with a crown of tentacles round the central aperture of the other pole. The other derived form, which has. become specialised in various directions, 1s the active medusoid or jelly-fish type. In several divisions the formation of a calcareous “ skeleton” by the polypoid type results in the production of “corals.” Multiplication by budding ts common, and often results in the JSormation of colonies, some of which show considerable adivt- sion of labour. The preservation of the primary axts, the absence of true mesoderm and of a ceelom, are often said to distinguish Calentera and Sponges from the other Metazoa (Celomata), but the results of recent researches on the nature 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-anemones which cluster in the nooks of the rocks, and the active jelly- fish which swim on the waves, are but different expressions GENERAL SURVEY. 139 of the antithesis between sedentary polypoid and active medusoid types which is characteristic of the phylum. The delicate iridescent globes, which represent the class Ctenophora, illustrate the climax of activity, and have no hint of a sedentary phase. : In our preliminary survey of the series, we may begin with the little fresh-water Hydra (Fig. 68), which is often Ne Fic. 65.—Wiagram of Ccelenterate structure, endoderm darker throughout. 1. To left, shows longitudinal section of Hydra; to right, of sea-anemone, g., gut; g., incipient gullet. z. 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; gZ, gullet. Note anatomical correspondence of the polypoid and medu- soid forms. to be found attached to the stems and leaves of water plants. The structure here is extremely simple, but the simplicity is probably due to degeneration. In favourable conditions the polyp may give off daughter buds, which remain for a time attached to the parent, and then separate as independent polyps. The bud itself, before leaving the parent, may also bud, so that three generations are present. If we picture this process of gemmation, but with 140 PHYLUM C@LENTERA., imperfect separation of the units, continued indefinitely, we can understand the formation of hydroid colonies, such as the zoophytes. In such cases the colony is usually sup- ported by an organic sheath (Jerisarc) of varying complexity. But the members of such a colony do not usually remain similar and equivalent. In Mydractinia, for example, which often grows on a Gastropod shell tenanted by a hermit- crab, the colony consists of polyps of varied structure and function. Some of the polyps are nutritive “persons,” like Hydra in appearance ; some are reproductive “ persons,” with rudimentary tentacles, with or without a mouth; others Fic. 66.—Colony ot Hydractenza on back of a Buccinum shell tenanted by a hermit-crab. are long, slender, mobile, sensitive, ften abundantly fur- nished with stinging cells; while he little protecting spines at the base of the colony may perhaps be abortive “persons.” All these polyps are united by connecting canals at the base. Thus Aydractinia exhibits polymorphism among the members of the colony, and a tendency towards more or less division of labour is common in the Ccelentera. In most hydroid colonies the division of labour only amounts to dimorphism; there are reproductive “ persons,” different from the ordinary polyps. These are in many cases sessile and mouthless, or they may after a time GENERAL SURVEY. 141 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. _ Again, just as the predominance of passivity is exhibited in Aydractinia 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 medus- oids, e.g. Geryonia, where the sedentary hydroid stage is omitted, and the embryo becomes at once medusoid. Finally, the medusoids _ themselves may become colonial, and we have active float- ing colonies, like those of the Portuguese man- of-war, which show, on a different plane, as much polymorphism as Aydrac- asi Fic. 67.—Diagram of a typical The same general con- Hydrozoon polyp.—After Allman. clusions apply tothe jelly- 5c, Ectoderm; £W., endoderm; C., the fish and sea-anemones. cavity of the gut (coelenteron); G., a re- The jellyfish present a prosuctive tnd: 7g tenadies #4. hype strong resemblance to the medusoids, but are distinguished from them by their usually greater size, as well as by greater complexity and several anatomical differences. It is in accordance with this increased complexity that the alternation of active and passive forms, though as real, is less obvious. But even here we find one type (Zéedagia) always locomotor, another (Auzelia) whose early life is sedentary, and others (Lu- cernarians) which in their adult life are predominantly passive, and attach themselves by a stalk. 142 PHYLUM C@LENTERA. 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 polyps bear to the swimming-bells (Fig. 65). They are, however, much more complicated in structure than the hydroids. Solitary forms are much commoner than in the hydroids, but the colonial type is nevertheless very frequent. The colonies may be supported by an organic framework only, but very commonly: there is a tendency to accumulate lime in the tissues, which results in the formation of “corals.” It should be noted, however, that various quite distinct polypoid types may form “corals.” Thus, while the most important reef-building corals are included in the Anthozoa, the Millepore-corals are hydroids. Finally, as the corals are predominantly passive, so there is a climax of activity in the Ctenophores, which move by cilia united into combs, and often shine with that ‘ phos- phorescence” which is an expression of the intensity of life in many active animals. As to diet, many of the larger forms, ¢.g. sea-anemones and jelly-fish, are able to engulf booty of considerable size ; the active Ctenophores are carnivorous, attaching them- selves by adhesive cells to one another, or to other small animals; but most Ccelentera feed on small organisms, in seizing and killing which the tentacles and stinging cells are actively used. Stinging cells or cnidoblasts are so characteristic of Ccelentera that they deserve particular notice. They occur, in all Coelentera except the Ctenophores, and even there they have been detected in Zuchlora rubra. They also occur in some Turbellarian worms, and in the papillz of A®olid nudibranchs amongst molluscs; but it has been shown that these animals obtain their nematocysts from the Ccelentera on which they feed. Each cnidoblast contains a capsule or nemato- cyst, which encloses a coiled lasso lying in an irritant gelatinous substance. The nematocyst fills most of the cell, 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 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 TYPES OF CELENTERA—HYDRA. 143 threads is mechanical and chemical. They fix, e.g. by the stilets, into the victim, and the secretion poisons the wound, paralysing or killing small animals, and sometimes acting as a solvent. Many seem to be prehensile threads rather than weapons, .TyPES OF C@LENTERA first Type—Hynra, a simple representative of the Class Hyprozoa General life. —The genus 7ydra—cosmopolitan, like many other small fresh-water animals—is represented by several species, e.g. the green Hydra viridis, the brownish H. oligactis or fusca, and the orange ZH. vulgarts or grisea, widely distributed in fresh water. They are among the simplest of Ccelentera, for the body is but a two-layered tube, with a crown of (6-10) hollow tentacles around the mouth, and with no organs except those concerned in reproduction. The body is usually fixed by its base to some aquatic plant, often to the l lower surface of a duckweed. It @ff may measure 4~4 inch in length, but it is as thin as a needle, and contracts ue Prien Saree into a minute knob. — After Greene, The animal sways its. body and tentacles in the water, and it can also : loosen its base, lift itself up by its tentacles, stand on its head, or creep by looping movements. According to some observers, its movements are helped by fine pointed pseudopodia protruded from the ectoderm cells of the tentacles and base, and by threads ejected from large cylindrical stinging cells. Usually, however, the Hydra prefers a quiet life. It feeds on small animals, which are paralysed or killed by stinging cells on the 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 the ffydra may sometimes be seen struggling fiercely with a small Annelid worm (Zwudzfex). Tpfusorians (Euplotes, ov., Ovary; Z., testes. 144 PHYLUM C@LENTERA. etc.) are often seen wandering to and fro on the surface of the Hydra, but these wonted visitors do not provoke the stinging cells to action. So simple is Hydra, that a cut-off fragment may grow into an entire animal. Thus the Hydra may be multiplied by being cut in pieces. The two conditions of a fragment regenerating a whole are—(1) that the fragment be not too small, and (2) that it bea fair sample of the various kinds of cells in the body. Thus neither a little corner off the base nor the tip of a tentacle will grow into a new Aydra. If the animal be turned inside out (a delicate operation), the status guo is soon restored. The Abbé Trembley, who first made this experiment, thought that the out-turned endoderm assumed the characters of the ectoderm, and that the inturned ectoderm assumed the characters of endoderm. But this is not the case. Either the animal rapidly rights itself by turning outside in, or,. if this be prevented, the inturned ectoderm disappears internally, and by growing over the out-turned endoderm, from the lips downwards, restores the normal state. In favourable nutritive conditions, the Hydra forms buds, and on these a second generation of buds may be developed. A check to nutrition or some other influence causes the buds to be set ‘adrift. Sometimes a Aydra divides across the middle, and each half grows into a complete polyp in afew days. Besides these asexual modes of multiplication, the usual sexual reproduction occurs. ; General structure.—The tubular body consists of two layers of cells, ze. the animal is diploblastic. The cavity is the gut, and it is continued into the hollow tentacles. These, when fully extended, may be much longer than the body. The mouth is slightly raised on a disc or hypostome. Of the two layers of cells, the outer or ectoderm is trans- parent, the inner or endoderm usually contains abundant pigment. On the tentacles especially, even with low power, one can see numerous clumps of clear stinging cells. The male organs appear as ectodermic protuberances a short distance below the bases of the tentacles; the ovary, with a single ovum, is a larger bulging farther down. Both male and female organs may occur on the same animal, either at one time or at different times, but often they occur on TYPES OF C@LENTERA—AHVDRA. 145 different individuals. Abundant food favours the develop- ment of female forms ; when food is scarce males are more abundant. The buds have the same structure as the parent body ; in origin they appear to be mainly due to multiplica- tion of interstitial cells. Minute structure.—The outer layer or ectoderm includes the following different kinds of cells :— that b Fic. 69.—Minute structure of Hydra.—After T. J. Parker and Jickeli. A. Ect., ectoderm ; #g., mesogleeal plate 5 s¢.c. stinging cell; Zxd., endo- derm with flagella and amceboid processes. B. 2.c., nerve cell, and st.c., stinging cell. C. Stinging cell with ejected thread; ., nucleus. D. Mesoglceal plate (#g.) with contractile roots resting on it. E. m.c., muscular cell with contractile roots, ¢c.7. (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 cztzcle, z.e. a pellicle no longer living, produced by the underlying cells. (Ia) Many of these large cells have contractile basal processes, or roots, running parallel to the long axis of the body, and lying on a 10 146 PHYLUM CELENTERA. middle lamina which separates ectoderm from endoderm (Fig. 69, 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 presence of special nerve cells shows that even in Aydra there is a differentiation of the two functions of contractility and irritability. (2) Stinging cells or cnidoblasts occur abundantly on the upper parts of the body, especially on the tentacles. Each contains a protrusible 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 stinging 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. With the help of the barbs they penetrate through even a chitinous membrane, and the secreted fluid has a solvent action. The victim is held fast and drawn closer. Besides the ordinary stinging cells, there are others of small size which coil into a spiral after explosion. (3) There is to the inner aspect of the covering cells a network of ganglion cells and nerve processes. More superficially there are minute sensory cells, some of them connected by fine fibres with the ganglion cells. (4) Small interstitial or indifferent units fill up chinks in the ecto- derm, and seem to grow into reproductive, stinging, and other cells. (5) Granular glandular cells on the basal disc or ‘‘ foot” probably secrete a glutinous substance. They are also said to put out pseudo- podia, and so move the animal slowly. The endoderm is less varied. Its cells are pigmented, often vacuolated, and most of them are either flagellate or amceboid. The pigment bodies in A. wirédzs are like the chlorophyll corpuscles of plants ; it seems almost certain that they are unicellular Algae. When a green Hydra liberates an egg while kept in the dark, that egg gives rise to a white Hydra, which is supposed to imply that the partner Algze do not migrate into the egg when there is no light. In the other species of Hydra, the pigment is quite different from chlorophyll. The active lashing of the flagella causes currents which waft food in and waste out. If some small animal, stung by the tentacles, is thus wafted in, it may be directly engulfed by the amceboid processes of some of the cells, and it has been noticed that the same cell may be at one time flagellate and at another time amceboid (cf. the cell-cycle, p- 107). After this direct absorption the food is digested within the cells, and while some of the dark granules seen in those cells may be decomposed pigment bodies, others seem to be particles of indigestible débris. Thus Aydra 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 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- TYPES OF C@&LENTERA—AVDRA. 147 verse or circular direction. A few cells near the mouth and base are described as glandular, and the presence of a few stinging cells has been recorded, though some suggest that the last are discharged ecto- dermic nematocysts which have been swallowed. The middle lamina, representing the mesogloea, is a thin homogene- ous plate, bearing on its outer and inner surfaces the muscular roots of ectodermic and endodermic cells (Fig. 69, D). It is historically interesting to notice the important step which was made when, in 1849, Huxley definitely compared the outer and inner layers of the Coelentera with the epiblast and hypoblast which embry- ologists were beginning to demonstrate in the development of higher animals, Not long afterwards, Allman applied to the two layers of hydroids the terms ectoderm and endoderm. Tie 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 :— OutTER LAYER. MrippLe Layer. INNER Laver. In Hydra the ectoderm forms— , Covering cells, stinging cells, nerve cells, muscle cells, etc. None in Hydra, apart from the middle lamella. In Hydra the endoderm forms— Digestive cells lining the food canal, and also muscle cells, etc. The embryonic epiblast of higher animals grows into epidermis, nervous system, and essential parts 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. of sense organs. The reproductive organs.—(a) From nests of repeatedly dividing interstitial cells, several (I-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 Aydra, 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. 70, 1). (4) Usually there is but one female organ or ovary, but in A. fusca as many as eight have sometimes been observed. The ovary arises, like the testes, from a nest of interstitial cells, in the centre of which, distinct from the start, the single ovum lies. In rare cases in A. viridis, H. fusca, and H. grisea there are two ova; in . diecéa there may be several. Development.—The ovum of Hydra is the successful central cell in the ovary. It is at first amceboid, and becomes more and more rich at the expense of its neighbours. Their remains (perhaps nuclei) 148 PHYLUM C@LENTERA. accumulate within the ovum as “yolk spherules” or ‘‘ pseudo-cells.” Some yolk-granules, formed within the ovum, may coalesce in ‘‘ pseudo- cells” of another type. With increase of size the ovum changes its 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, and two polar bodies are extruded at the distal pole. There are twelve chromosomes to begin with, and by the reduction division in forming the first polar body, the number is reduced to six. Thereafter the ectoderm of the parent Aydra yields to the increasing strain put upon it, and Fic. 70.—Development of Aydra.—After Brauer. 1. Sf., spermatozoa. 3 2. Amceboid ovum; g.v., germinal vesicle or nucleus; y.s., yolk spherules. 3. Ovum with lobed envelope (sz.) around it. 4. Ovum protruding ; ., the nucleus ; ecf., the ruptured ectoderm ; end., the endoderm. 5. Section of blastosphere—Zct., ectoderm;. Znd., endoderm— being formed. 6. Section of larva. Zct., ectoderm; Exd., endoderm; g.c., gut cavity: sk., ruptured envelopes. ruptures, allowing the ovum to protrude. By abroad base it still remains, however, attached to the parent, and in this state it is fertilised, the spermatozoon entering by the distal pole (Fig. 70, 4). The segmentation which follows is total and equal, and results in the formation of a blastosphere (Fig. 70, 5). By inwandering, or by division 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 filled up, and the two layers become differentiated from one another, The outer or ectodermic layer forms—(a) an external “ chitinoid ” shell of several layers; (4) an internal membrane, homogeneous, thin, TYPES OF C@LENTERA—A MEDUSOID. 149 and elastic ; and (c) the future ectoderm of the adult. In Aydra fusca the egg is separated from the parent before the shell is formed, and is fastened by its gelatinous sheath to aquatic plants; in A. werddés and 1. grisea the egg falls off after the outer shell has been formed. In all species the separation from the parent appears to be followed by a period of quiescence lastirfg from one to two months. It is probable that this resting-stage is carried by wind and birds from one water basin to another. Within the shell differentiation at length recommences, but it pro- ceeds slowly. Interstitial cells arise in the ectoderm; a middle lamella is formed ; a gastric cavity begins to appear in the midst of the endoderm. Thereafter the shell bursts, and development proceeds more rapidly. The embryo elongates, acquires a mouth by rupture at the distal (sometimes called vegetative) pole. The inner sheath is also lost, and the young “ydra 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 (Mécrohydra ryder?) is known to liberate free-swimming medusoids. A fresh-water Medusoid Limnocodium was found in the Victoria Regia tanks in the Botanic Gardens, Regent’s. Park, London. Its native habitat is unknown. Another species, L. kawaiz, has been found in the Jantszekiang in China, 1000 miles from its mouth, A related form, Limnocndda, occurs in Lakes Tangan- yika and Victoria Nyanza, and in the river Niger. A strange simple polype—Lolypodium—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 CELENTERA.—A Medusoid. Class HypRozoa Hydra is too simple to be thoroughly typical of the Hydrozoa. The class includes the hydroid colonies or zoophytes, which may be compared to Aydre with many buds, and also free medusoid forms, which may be (a) liberated members of a hydroid colony, or (4) 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 (¢.g. Zudularia). In some of the hydroid colonies, notably the Millepores and Aydractinia, the polyps are very dissimilar to one another, and have become specialised for the performance of different functions. 150 PHYLUM C@LENTERA. 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. The margin of Fic. 71.—Bougainvillea.—After Allman. A. Asmall piece of a hydroid colony. p-, Perisarc ; #., medusoid bud; 4., hydranth or polyp head. B. A medusoid ; #a., manubrium; ~.c., radial canal; s., sense- organ. the bell also bears tentacles, usually hollow, and abundantly furnished with stinging cells (Fig. 65, 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 canal, and extends also into the TYPES OF C@&LENTERA—A MEDUSOID. 151 tentacles. The mesogloea 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. A plate of endoderm lies in the mesogloea between the radial canals. Digestion is intracellular, and probabiy 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 gang- lionic cells, which apparently control the muscular contrac- tions. Sete rr IG. 72.— structure of a a miter bet ot ey eee M alse —After ee base of the tentacles (Ocellatz), Ae T ernen ee. coded or in the form of “auditory” ference canal; G., gonad; &.C., . ix ahe radial canal; 4WV., endoderm; vesicles developed as pits in the | ZC., ectoderm; 4/G’, mesoglea. velum (Vesiculatz). The reproductive organs develop either in the manu- brium or on the radial canals. The products always (?) ripen in the ectoderm, and often seem to arise there; but Weismann and others have shown that the reproductive cells of a medusoid derived from a hydroid, or of the reduced and fixed 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 152 PHYLUM C@LENTERA. a fixed asexual hydroid colony, the planula settles down, loses its cilia, buds out tentacles, and develops into a new hydroid. ; In many hydroid colonies, as has been already noticed, the sexual members are not set free, but remain as buds attached to the parent. These fixed “gonophores” show many stages of degeneration ; some, notably in the floating colonies of Siphonophora, differ little structurally from true medusoids, while others, as in Wydractinia, are simply small closed sacs enclosing the genital products (Fig. 87). Third Type of CELENTERA.—The common Jelly-fish —Aurelia aurita. Class SCVPHOMEDUSZ This Medusa is almost cosmopolitan, and in the summer months occurs abundantly around the British coasts. It swims by pulsating its disc, and also drifts along at rest without any pulsations. They often occur in great shoals, and hundreds may be seen stranded on a small area of flat sandy beach. The glassy disc usually measures about four inches in diameter, but may be twice as large. The jelly- fish feeds on small animals, such as copepod crustaceans, which are entangled and stung to death by the long lips. External appearance.—The animal consists of a gela- tinous disc, slightly convex on its upper (ex-umbrellar) surface, and bearing on the centre of the other (sub- umbrellar) surface a four-cornered mouth, with four long much-frilled lips. The circumference of the disc is fringed by numerous short hollow tentacles, by little lappets, and by a continuation of the sub-umbrella forming a delicate flap or velarium. Conspicuously bright are the four re- productive organs, which lie towards the under surface. Nor is it difficult to see the numerous canals which radiate from the central stomach across the disc, the eight marginal sense organs, and the muscle strands on the lower surface (Fig. 73). The three layers.—The ectoderm which covers the external surface bears stinging cells, sensory and nerve cells, and muscle cells. The ectoderm seems also to be invagin- ated to form the gullet or stomodeum. The endoderm lines the digestive cavity, is continued out into its radiating TYPES OF C@LENTERA—AURELIA AURITA. 153 canals, and is ciliated throughout. The mesogloea 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 10 per cent. of the total weight. Yet some jelly-fish (species of Rhopilema) are used as food in Japan! Nervous system.—The nervous system consists—(a) of a special area of nervous epithelium, associated with each of the eight sense organs, and (4) of numerous much-elongated bipolar ganglion cells lying beneath the epithelium on the under surface of the disc. This condition should be con- trasted with the double nerve-ring in Craspedote medusoids, but too much must not be made of the contrast, for a nerve-ring is described in Cubo- medusz, one of the orders of Acraspedote 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 pig- Fy, 73.—Surface view of Aureloa,— mented spot, a club-shaped From Romanes. projection with numerous Showing four genital pockets in centre, 6c ; m4 much branched radial canals, eight peri- calcareous otoliths -_ pheral niches for sense organs, and peri- its cells, and a couple of _ pheral tentacles. apparently sensitive pits or grooves. The sense organs arise as modifications of tentacles, and are often called “‘tentaculocysts” or “rho- palia.” 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. 154 PHYLUM CQ@LENTERA. 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 gullet or ‘‘manubrium” connects the mouth with the digestive cavity in the centre of the disc. From this central chamber sixteen gastro-vascular canals of approximately equal calibre radiate to the circumference, where they open into a circular canal, with which the hollow tentacles are connected. Eight of the radial canals are straight, but the other eight are branched, and thus in an adult Aurelia the total number of canals is large. These canals are really due to a partial obliteration of the gastric cavity by a fusion of its ex-umbrellar and sub-umbrellar walls along definite lines. They are all lined by ciliated endoderm. Where the gullet passes into the central digestive cavity, there are four strong pillars of thickened sub-umbrellar material. Beside these pillars, there are four patches where the sub-umbrellar surface remains thin. These are the gastro-genital membranes, lined internally by germinal epithelium (Fig. 74, &.). To the inside of these genital organs, within the digestive cavity, are four groups of mobile gastric filaments (g.f,, Fig. 74), which are very characteristic of jelly-fish. In appear- ance these are very similar to the small tentacles of the margin, and, like them, are hollow. ‘They are covered with endoderm—with ciliated, glandular, muscular, and stinging cells. The body is mapped out into regions by the following convention : The first tentacles to appear in the larva are four in number, and correspond to the four angles of the mouth; the radii on which they appear are called ‘‘perradial,” marked by the four lips. Half-way between these, four ‘‘interradials” are then developed, marked by the gonads and gastric filaments. Then eight ‘‘adradials” may follow, between perradii and interradii, marked by the eight unbranched radial canals. Reproductive system.—The sexes are separate. The reproductive organs—ovaries or testes—consist of plaited ridges of germinal epithelium, situated on the four patches already mentioned, within sacs which are derived from and TYPES OF C@ELENTERA—AURELIA AURITA. 155 communicate with the floor of the gastric cavity. They are of a reddish violet colour, and at first of a horseshoe shape, with the closed part of the curve directed outwards. Afterwards the ridges become circular, and surround the walls of the sacs in which they lie. But the sub-umbrellar surface is modified beneath each genital sac in such a way that the sac comes to lie in a sub-genital cavity com- municating with the exterior (g.4., Fig. 74). The con- tractions of the umbrella produce a rhythmic movement of the water which enters the sub-genital cavities, and this constant renewal of the water suggests some respiratory significance for the sacs. The genital sacs containing the plaited ridges of germinal epithelium communicate with the gastric cavity only, while the sub-genital cavities containing water and enveloping the geni- tal sacs communicate with the exterior only. The ova and = sper- Fic. 74.—Vertical section of Aurelia,— matozoa pass from the After Claus. frills of germinal epi- mM, Mau st., stomach ; Tbs radial canal; ‘ ‘ +) Teproductive organs; g./., gastric thelium into the sacs, filaments; g.Z., sub-genital cavity; 2, and thence into the gas- "aging entices cs, sense organs tric cavity. They find exit by the mouth, but young embryos may be found swimming in the gastro-vascular canals, and also within the shelter of the long lips. Variations.—The jelly-fish often exhibits variations, i.e. inborn changes of germinal origin which result in the organism being different from the norm or average of its species. It is normally tetrapartite, but sexpartite, penta- partite, and, more rarely, tripartite forms occur; and the detailed variations are manifold. Life history 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 156 PHYLUM C@ELENTERA. processes the embryo elongates, the outer cells become ciliated, and the mouth closes. Thus the embryo becomes a free-swimming oval planula, After a short period of free life, this planula settles down on a stone or seaweed, attaching itself by the pole where the mouth formerly opened, Ata 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 ectoderm is formed, which hangs freely in the general cavity. During this process there are formed first two and then four diverticula of the Fic. 75.—Diagram of life history of Aure/ia.—After Haeckel. 1. Free-swimming embryo ; 2-6, various stages of Hydra-tuhba ; 7, 8, Strobila stage; 9, liberation of Ephyre; ro, 11, growth of Ephyra into Meduse. 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 teeniolee. 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 teeniolze (contrast the ezdodermic muscles of Anthozoa). In contrasting this development with that of the hydroid polyp, Goette specially TYPES OF CELENTERA—AURELIA AURITA. 157 emphasises the fact that the radial symmetry is first indicated by the gut pockets, and the tentacles are late in development. Goette describes a quite similar process of development_in certain sea- anemones, and claims to have found there rudiments of septal pockets and ectodermal muscles, thus confirming his view of the intimate relation between the Anthozoa and Scyphomedusz. The larva now forms a ‘‘ Hydra-tuba” or ‘‘Scyphistoma”; it is about an eighth of an inch in height. By lateral budding, or by the formation of creeping stolons, it may givé rise to larve 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. (4) Sometimes, in unfavourable conditions, 4 furrow appears round the upper region of the Scyphistoma, the upper portion is converted into an Ephyra, and floats away, while the lower portion re-forms its oral region by 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 Ephyre, 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 Ephyre 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 Ephyree are liberated. Each of these grows into a sexual jelly-fish, producing ova or spermatozoa. Relatives of Aurelia.—The Meduse, or true jelly-fish, include forms which agree with the Anthozoa in relative complexity of structure as compared with Hydrozoa, and in the possession of an ectodermal gullet, but differ in possessing ectodermal septal muscles and in some histological features. If Goette’s discovery of rudimentary ectodermal muscles in the larve of certain sea-anemones be confirmed, however, it would greatly increase the probability of a close relationship between the two sets. Among the Scyphomedusz 158 PHYLUM CE@LENTERA. closely allied to Auveléa some, e.g. Pelagéa, have a direct development without the intervention of Scyphistoma or Strobila stages, but this may occur exceptionally in Azrelia, Cyanea is often very large, Fic. 76.—Lucernarta.—After Korotneff. “‘it may measure 74 ft. across the bell, with tentacles 120 ft. long.” Chrysaora is hermaphrodite, and has diffuse sperm sacs even upon the arms. In the Rhizostome, eg. Cass¢opeca and Pilema, the Fic. 77.—Diagram of Lucernaria.— After Allman, C., Cavity of gut (ccelenteron); #, gastric fila- ments; /7., hypostome; G., gonad; 7., tentacle; ¢ ¢., circumference canal. mouth is obliterated, and replaced by numerous small pores on the four double arms. Lzcernaria and its allies are interesting sessile forms which have been compared to sexual Scyphistomas, that is, are regarded as persistently larval forms, TYPES OF C@LENTERA—A SEA-ANEMONE. 159 Contrast between Medusoids (Hydromeduse) and Medusa (Scyphomedusa) Mepusorps. (CRASPEDOTA.) Mepusm&. (ACRASPEDA.) The majority are small ‘‘swimming- ” A flap or velum (craspedon) projects in- wards from the margin of the bell. No teeniole, nor gastric filaments. A double nerve-ring around the margin. Naked sense organs either optic or audi- tory. hey 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- medusa, all arise as the liberated reproductive persons of hydroid colonies. 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 teniole, 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 polyp stage (or Scyphistoma) in the course of thei life history. Probably more nearly related to Anthozoa than to Hydrozoa. Fourth Type of CELENTERA.—A Sea-Anemone, such as Tealta crassicornis. Class ANTHOZOA Most sea-anemones live fixed to the rocks about low- water mark. All these fixed forms have a distinct basal disc, and may, like Zealia crassicornis, be half buried in sand and gravel; others, without a basal disc, are loosely inserted in the sand, e.g. Edwardsia and Certanthus. All are able to shift their positions by short stages. Some reef-anemones (Cvadactis) can crawl about on their tentacles. They feed on small animals — molluscs, crustaceans, worms—which are caught and stung by the tentacles. Many» depend on minute organisms; others may be seen trying to engulf molluscs decidedly too large for them. A few anemones, without pigment or with little, have symbiotic Algz in their endoderm cells; the bright pigments of many others seem to help in respiration. Besides the sexual reproduction (in which the young are 160 PHYLUM C@LENTERA. 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 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, Fic, 78.—External appearance of Zealia crassicornis. 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 anemones through the walls of the body (Fig. 79). General structure.—- The Anthozoon polyp differs markedly from the Hydroid polyp—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 mesenteries extend from the body wall towards this gullet. Some of the partitions are “complete,” ze. they reach the gullet; others are “in- TYPES OF C@LENTERA—A.SEA-ANEMONE. 161 complete,” ze. do not extend so far inwards. The complete mesenteries are attached to the oral disc above, to the side of the gullet, and to the base, and all the mesenteries are ingrowths of the body wall. The cavity of the anemone is thus divided into a number (some multiple of six) of. radial chambers. These are in communication at the base, so that food particles from the gullet may pass into any of the chambers between the partitions. Moreover, each partition is perforated, not far from the mouth, by a pore, besides which there is often another nearer the body wall. The tentacles are continuous with the cavities between the mes- enteries, and thus all the parts of the body are in communication. The mouth is usually a longi- tudinal slit, and its two corners are often richly ciliated. The gullet is marked with longitudinal grooves, two of which, the ‘“siphonoglyphes,” correspond to the corners of the mouth, and are z., Tentacles; o., mouth; @s., cesophagus; especially broad and c.,¢c’., apertures through a mesentery; 4.,a., = acontia; g., genital organs on mesentery; deep. Along these two ro, meen Ene filaments ; 7.2., longitudinal grooves, and by these two muscles; s., primary septum or mesentery ; . s’., secondary septum; s”., tertiary septum 3 corners, food particles 2,’ basal disc. : usually pass in; but in some, one side is an incurrent, the other an excurrent channel. Occasionally only one corner of the mouth and side of the gullet is thus modified. The gullet often extends far down into the cavity of the anemone. It admits of a certain-amount of extrusion. The mesenteries bear—(a) mesenteric filaments; (4) retractor muscles; (c) II Fic. 79.—Vertical section, of a sea- anemone.—After Andres, 162 PHYLUM C@LENTERA. ridges of reproductive 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 uncentralised, and consists of superficial sen- sory cells connected with a plexus of sub- epithelial ganglion cells. The layers of the body.— The ectoderm which clothes the ‘ exterior is continued down the inside of the gullet. The endo- derm lines the whole of the internal cavity, including mes- enteries and tentacles. The mesogloea is a supporting plate between these two layers, and forms a basis for their cells. The ectoderm consists of ciliated, sensory, stinging, and glandular cells, and also of sub- epithelial muscle and ganglion Fic. 80. — Section anemone (across arrow in Figure 79).—After Andres. A, B, Directive septa; mf, mesenteric through _ sea- filaments; g., genital organs; 7.., longitudinal muscles; s., primary sep- tum ; s’:, secondary septum ; s”., tertiary septum. The arrow enters between two primary septa (an intra-septal cavity), and passes out between two tertiary cells based on the mesogloea, but mainly restricted to the circum- oral region. The endoderm consists mainly of flagellate cells, with muscle septa. fibres at their roots. These form the chief muscle bands of the wall, the mesenteries, 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 appeat in pairs. Two pairs of mesenteries, however, differ from all the rest—those, namely, which are attached to.the two corners of the mouth and to the corresponding grooves of the gullet. These two pairs of mesenteries are called ‘‘ directive,” and they divide the animal into bilaterally sym- metrica ‘halves. Anatomically, a pair of directive mesenteries differs from the other paired mesenteries, because the retractor muscles, which TYPES OF C@LENTERA—A SEA-ANEMONE. 163 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 teeniolee 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 Ccelentera 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 KD oH 5 A Fic. 81.—Z, Diagrammatic section of Zoantharian ; 4, of Alcyonarian.—After Chun. The line S-S in Z is through the siphonoglyphes (a), the line 7-T passes through two inter-septal spaces. The retractor muscles are represented by dark thickenings on the mesen- teries—all on one (the ventral) side in the Alcyonarian. The line S-S in A represents the axis of symmetry. ° 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 Goette, the development is in essentials the same as that of the Hydra-tuba. The larva of Cerianthids is for a time pelagic, and used to be recognised as a distinct genus, Avachnactis. 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 Alcyonium and the related forms. This contrast is not very satis- factory, but it rests on such distinctions as the following :— 164 PHYLUM C@LENTERA. ANTHOZOA OR ACTINOZOA ZOANTHARIA, HEXACORALLA, ¢.g. SEA-ANEMONE. ALcYoNARIA, OCTOCORALLA, 6.2% Drap-MEn’s-FINGERS. Many are simple, many colonial. The polyps of a colony may give rise to others directly by fission or budding. Tentacles usually simple, usually some multiple of six, often dissimilar. Mesenteries usually some multiple of six, complete and incomplete. Retractor muscles never as in Alcyo- naria. Two gullet grooves or siphonoglyphes, or only one. No dimorphism. Calcareous skeleton, if present, is derived from the basal ectoderm. Examples. Sea-anemones—eg. Tealia and Actinia. Madrepore corals, many of them reef- building. Antipatharians. An aberrant Anti- patharian, Dendrobrachia fallax, has e7ght feathered tentacles, All colonial, except a small family in- cluding Monoxenia and Haimea. The polyps of a colony give rise to others not directly, but through stolons or solenia. Tentacles eight, feathered, uniform. Mesenteries eight, complete. Retractor muscles always on one (ven- tral) side of each mesentery (see Fig. 81). One (ventral) gullet groove (siphono- glyplfe or sulcus), or none. Frequent 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 rubrum (Red coral), with an axis of fused spicules. Pennatula (Sea-pen), a free phosphor- escent colony, witha ‘‘horny” axis, possibly endodermic. ZOANTHARIA The Zoantharia include many orders, ¢.g. the primi- tive Cerianthidea (Cerianthus, etc.) and Edwardsiidea (Zdwardsia), the Actiniidea (including the typical sea- anemones and the Madreporaria), and the divergent Anti- pathidea. Making of a typical coral.—Although the term “ coral” is applied to many different Ccelenterate types with substantial calcareous skeletons, e.g. to Millepores which are Hydrozoa, and to “blue corals” and “red corals” which are Alcyonarians, the corals par excellence are the Madreporarians. They form the coral rock and “coral islands” found in many parts of the globe, but rarely north or south of a belt extending 30° on each side of the equator, and rarely below the 4o-fathom line. ZOANTHARIA. 165 In a general way a Madrepore polyp is like a sea-anemone in structure, and the “coral” it forms is its external shell rather than its skeleton. It is altogether a product of the ectoderm. From one polyp others usually arise by budding or by division, e.g. Astr@a and Madrepora and Lophohelia (North Sea), but there are solitary forms such as Pungia and Caryophyllia (British). The first part of the “shell” to be formed is the dasal plate between the ectoderm of the base and the substratum. DIN" {! Ne ' ‘ , We » Fic. 82.—The formation of a coral shell (Astroides).— After Pfurtscheller. st., Stomodzeum ; 7zs., mesentery ; s., calcareous septum ; &., basal plate. On this plate a number of radially arranged vertical ridges (septa or cnemes) are then formed, and as they grow in height they push the ectoderm of the base up before them (see Fig. 82). An external wall or ¢heca is then formed, partly by the fusion of the outer margins of the septa and partly by a circular upgrowth from the basal plate. This theca pushes the body wall before it, as the septa pushed the base. Sometimes a second external wall or efztheca is formed outside of and concentric with the theca. By the coalescence of septa in the central line a colume//a or median 166 PHYLUM C@LENTERA. pillar may be formed. ‘The outer wall of the theca may bear vertical ridges or cost, and these may be connected with neighbouring coste of other polyps by horizontal shelves or dissepiments. Both septa and costz correspond to intermesenteric spaces. (See Shipley’s Zoology of the Invertebrata, pp. 68-71.) ANTIPATHARIANS Usually arborescent, sometimes whip-like colonies, of wide distribu- tion in most seas, often called ‘‘black corals.” A spinose hollow horny axis is covered with coenenchyma and regularly arranged polyps, ae wees Fic. 83.—Structure of Antipatharians. 1. A group of polyps—J/., mouth ; ¢., tentacles. 2. Axis without polyps and ccenenchyma, covered with spines S; 3. Vertical section of a polyp—A., axis; ¢., tentacle; g., gullet ; m., mesentery ; 0., ovary ; #2.., mesenteric filaments. 4. Cross section of a polyp—ZC., ectoderm; /., mesoglcea ; EN., endoderm ; G., gullet; 17S., mesenteries. without any trace of spicules. A polyp is usually oval in section, with its long diameter in the line of the axis, and its gullet elongated at right angles to this. There are usually six simple non-retractile tentacles, ten mesenteries, and two ill-defined siphonoglyphes. The mesenteries are without muscle-banners. The two longest, running at right angles to the elongated stomodeum, bear gonads. The develop- ment is unknown. ALCYONARIA. 167 Examples :— Antipathes (arborescent). Cirripathes (whip-like). Leiopathes (with twelve mesenteries). Dendrobrachia (with eight pinnate retractile tentacles). : ALCYONARIA In the Alcyonarian polyp there are al- ways eight fzmmate tentacles and eight mesenteries attached to the stomodzum or gullet. There is one longitudinal at Fic. 84.—Diagrams of Types of Alcyonaria.—After Hickson. Types of Alcyonaria :—I. Of Stolonifera ; II. of Aleyonacea; III. of Axifera; IV. of Stelechotokea. ciliated groove (siphonoglyphe or sz/cus) in the stomodzeum 168 PHYLUM C@LENTERA. (ventrally). The mesenteries bear retractor muscles, all situated on the sulcar aspect (see Fig. 81), and each mesentery bears a mesenterial filament. The two dorsal (asulcar) mesenteries are long, ciliated, and non-glandular ; they are respiratory in function and cause an upward current, that in the sulcus being downward. Many Ale cyonarians are dimorphic, having in addition to the typical polyps (autozooids) dwarf siphonozooids, with suppressed \ Fic. 85.—Corallium rubrum, a corner of a colony.— After Lacaze-Duthiers. A,, Anthocodia or retractile portion of a polyp; 7.2., com- pletely retracted polyp, with the verruca or calyx portion left protruding ; C., coenenchyma; 7., pinnate tentacles. tentacles, strongly developed sulcus, no mesenteric fila- ments, and often ill-developed mesenteries. Their function is to drive currents of water through the canal systems of the colony, and they are sometimes reproductive as well. With the exception of one small family of solitary forms (Haimeide), the Alcyonarians form colonies which are in various ways supported by spicules, or by spicules and an axis. The spicules, which take the most diverse forms, seem to be begun at least by ectodermic cells (a pair to ALCYONARIA, ; 169 each spicule), but they usually pass into the mesoglcea. The nematocysts are usually small. A number of Alcyon- arjians are viviparous ; the embryo is usually a planula. Colonies are formed’ in different ways. (1) A parent polyp gives off hollow stolons or so/enza, which bud off new polyps, and the whole forms a spreading network or flat plate, 2g. Clavularia, a type of Stolonifera (Fig. 84, I.). (2) The polyps may be crowded together so as to form bundles raised on a stalk, or lobose fleshy growths with the polyps projecting on the surface of a dense mesogloeal mass honeycombed by solenia, 4g. Xenia and Alcyonium, types of Alcyonacea (Fig. 84, II.). (3) Or the colony may raise itself in the water by forming 2 Fic. 86,—Alcyonarian spicules. common upright coenenchyma, in which the polyps are embedded, and the medullary part of which may form a substantial axis of cemented spicules, ¢.g. Corallium, a type of Pseudaxonia. (4) Or the vertical extension of the colony may be effected by a horny secretion from the polyps, which comes to form an axis, really outside of the polyps though encrusted by them. This axis may be purely horny or in part calcareous, e.g. Gorgonda and Acanella, types of Axifera (Fig. 84, III.). (5) Fifthly, the vertical extension may be due to a great elongation of a single primary polyp which gives off solenia bearing numerous secondary polyps, e.g. Pennatula, a type of Stelechotokea (cf. Fig. 84, IV.). An altogether aberrant type is represented by the blue coral (Heliopora) and its extinct relatives (Hedoltes, etc... 170 PHYLUM C@ELENTERA. GENERAL SURVEY OF CQ@ELENTERA Before we proceed to the systematic survey, we may contrast the essential structural features of the four classes of Ccelentera. I. In the Elydrozoa or Hydromeduse there is no inturned ectodermic gullet or stomodzum; there are no partitions or mesenteries; there are no special digestive organs; in the body wall the ectodermic muscles are mostly longitudinal and the endodermic muscles circular ; the sex cells are usually produced in the ectoderm; there is very frequently a combination of polypoid and medusoid phases in the life history ; the circumference of the medusoid bears a muscular velum of ectoderm and mesoglcea ; there is no calcareous ‘secretion (except in Millepores). : II. In the Scyphomeduse there is an inturned ectodermic gullet or stomodzeum ; there are hints of mesenteries ; there are special digestive filaments ; the sex cells are endodermic; there is no velum; there is often a non-sexual sedentary stage ; there is no calcareous secretion. III. In the Anthozoa there is an inturned ectodermic gullet or stom- odzeum ; there are distinct mesenteries or partitions from body wall to gullet wall; there are often digestive fila- ments; in the body wall the ecto- dermic muscles are circular (except in Cerianthus), and the endodermic muscles longitudinal; the sex cells are endodermic ; there is no medusoid hase. IV. The Ctenophora are very di- vergent and apart from the other classes, ¢.g. in rarely having any stinging cells, and in having a well- defined mesoblast, SYSTEMATIC SURVEY lass I. HyvpRozoa Fic. 87.—Diagram of a gymno- c : oa blastic Hydroid.—After All- Solitary polyps like Hydra, hydroid man. a., Stem; 4,, root 5 ¢., gut cavity; @., Peete (dark); @., Rarer ., horny perisarc; g, ra-like Unaraon* Chydranths pom the same, contracted; 4., hypostome bearing mouth; &%., sac-like repro- ductive bud (sporosac); ., a modified hydranth (blastostyle) bearing sporosacs; 2, medusoid “* person.” i colonies or zoophytes with medusoid reproductive buds, medusoids without sedentary stages, colonies of modified medusoids. 1. Order Hydromedusze. — Simple or colonial forms in which the sexu- ally reproductive persons are either liberated as free-swimming medusoids or are sessile gonophores. SYSTEMATIC SURVE Y—HYDROZOA. 172 (2) Hydrophora,—Two types are included here. The first includes the Tubularians, Aydvactinza, and other forms in which the polyps are not enclosed in the protective perisarc which often surrounds the colony (gymnoblastic), and in which the free medusoid forms, when present, have their genital organs placed in the wall of the manubrium (Anthomedusz), and are furnished with ‘ ocelli placed at the base of the tentacles, ffydra and its allies may be included here. An unattached marine hydroid—Ayfolytus peregrinus—has been described, and as it bore gonophores it was obviously mature, which is doubtful as regards two other unattached forms, Protohydra leuckartiz and Halermita cumulans, which may turn out to be larval. The hydroid stages of Pelago- hydra and Margelopsés are free-swimming. Examples :— Syncoryne sarstz, the free medusoid of which is called Sarsza tubulosa. Bougatnvillea ramosa \iberates the medusoid Margelis ramosa. Cordylophora lacustris and Tubularia larynx have sessile gonophores or sporosacs. The second type includes Campanularians and Sertularians along one line; Halecids and Plumularians along another line. The protective perisarc surrounding the colony is continued into little cups (hydrothecz) enclosing the polyps (calyptoblastic). These < hydrothecze are stalked in Campanularians, sessile in Sertularians and Plumularians. The free medusoids have their gonads placed in the course of the radial canals (Lepto- medusze), and are either ‘‘ocellate” or “* vesiculate.” Examples :— Plumularia, with hydrothece on one side of the branches, and Sertudlaria, AI. with hydrothecze on both sides of the dL. branches. The Campanularian Odelia geniculata ic. 88.-—Graptolites. liberates the medusoid Odea gent- I. Monograptus. culata. II.’ Diplograptus. (2) EHydrocorallinze. —Colonial torms which suggest the Hydractiniz in their polymorphism and division of labour, but are distinguished by their power of taking up lime, and so forming ‘*corals.” The colonies are complex and divergent, the reproductive persons are either sessile gonophores or simple medusoids. A/¢//egora, Stylaster. (c) Trachymedusze.—These exist as a rule only in the medusoid form, 172: PHYLUM C@LENTERA. Fic. 89.—Hydroids.—After Hincks. I, Tubularia, II A. Piece of Sertularia. II B. A fragment enlarged, showing sessile hydrothece (/7.) on both. sides of the twigs. IIT A. Plumularia. III B. A fragment enlarged, showing hydrothece (H.) on one side of each twig, an epillary penothees (G.) and minute nemato- phores. IVA. Campanularian. B. A fragment enlarged, showing stalked hydrothece (H.), a gonotheca (G.); C., the coenenchyma; P., the perisarc ; S., a stalk. SYSTEMATIC SURVEY—SCYPHOMEDUSA. 173 and are divided into two groups, Trachomeduse and Narcomeduse, according to the position of the gonads. The fresh-water medusz Limnocodium and Limno-- cntda may possibly belong by? to this group. \\y f far Geryonia, Carmarina, - Pies Ey] Cunina, Aeginopsis. 2. Order Siphonophora. —Free-swimming colonies of modified medusoid per- sons (medusomes), with much division of labour. Physalia _ (Portuguese man-of-war), Dephyes, Vel- ella, Porpita. Incerte ‘sedis. Grapto- lites.—Extinct unattached colonies with a rod-like axis found in Upper Cambrian, Ordovician, and Silurian systems, The colony is usually linear, and consists of cup-shaped hydrothecze borne on one, two, or four sides of the solid axis (wz~gula). Each opens into a common median canal. At the proximal free end there is a- minute triangular or dagger-shaped body —the szcu/a—which re- presents the embryonic skeleton. Some repro- ductive bodies or gon- angia have been found. -The animals were prob- Fic. 90.—Campanularian Hydroid.— After Allman. ably free-swimming in #H., Hydrotheca or polyp-cup; AY; hy- muddy seas, and of a dranth, or polyp-head; G., gonotheca, Hyd. d 0 enclosing a reproductive polyp producing ydromedusan nature, medusoid buds; J/., a liberated medu- soid ; S7., basal stolon. Class II. ScypHoMEDUS (= Acraspeda) Jelly-fish with gastric filaments, sub-genital pits, and no velum— (1) Lucernarize.—Sedentary forms. Lucernaria, Haliclystus, and Depastrum. (2) Discomedusee.—Active forms, often with complicated life history. Aurelia, Pelagia, Cyanea, Rhizostoma. 174 PHYLUM C@LENTERA. (3) Cubomedusze.—Forms with broad pseudo-velum, and other peculiar features. Charybdea. (4) Peromeduse.—Forms with four inter-radial tentaculocysts only. Pericolpa. Class III. AntHozoa (= Actinozoa) Polypoid forms with well-developed gullet and septa, and circumoral tentacles. (1) Zoantharia or Hexacoralla. (a) Actiniaria. Sea-anemones. Actinia, Anemonia, Tealia, Cerianthus. (4) Madreporaria. Stone or reef corals. Astrea, Madrepora, Fungia, Meandrina, (c) Antipatharia. ‘‘ Horny” black corals. 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 J/yzostoma, 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 dicecious forms with small males, ¢.g. JZ. pulvinar. 2. Hermaphrodite forms and true males, which remain males, ¢.g. M. glabrum. 3. Hermaphrodite forms and males, which, retaining their positions on the hermaphrodites, afterwards become female, e.g. JZ. alatum. 4. Hermaphrodite forms, in which the males have lost their dorsal position, and have either become extinct or converted into protandric hermaphrodites, e.g. AZ. cérriferum. Class Hirupingea or DiscopHoRA. Leeches This class includes forms in which the body is oval and flattened, usually devoid of sete or gills, and marked ex- ternally by vings which are much more numerous than the true segments. The body cavity ts much reduced and broken up (except in Acanthobdella), and may communicate 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 236 PHYLUM ANNELIDA. by the mouth. Almost all are hermaphrodite,—the male organs are numerous and segmentally arranged, and special genital ducts are present. The genital openings are median. The development is direct. Most live in fresh water or on land, but a few are marine. Type, the Medicinal Leech (Airudo 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 blood-letting 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. Watched 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 development, the young leeches emerge and make for the water. External features.—The leech is usually from 2 to 6 inches in length, amd appears cylindrical or strap-like, according to its state of contraction. The slimy body shows over one hundred skin-rings ; 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. The hind sucker is supposed to consist of 7 fused segments, making the total number 33. MEDICINAL LEECH. 237 These segments may be recognised externally by conspicuous. pigment spots (‘‘segmental papillee”), 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 there is a pair of ‘‘eyes” on the Ist, 2nd, 3rd, 5th, and 8th rings; these are homologous with ‘*segmental papille,” and therefore in this region eight rings corre- spond to five segments. The penis is protruded on the middle véntral line between rings 30: and 31; the aperture of the female duct lies five rings farther 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 9-11 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 carbon dioxide is readily effected. Opening between the epidermal elements, but really situated much deeper, are numerous long-necked, flask-shaped glandular cells, secret-: ing the mucus so abundant on the skin. Underneath the epidermis there is much connective tissue, besides 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. 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 238 PHYLUM ANNELIDA. supra-cesophageal) ganglia are connected with the most anterior (or sub-cesophageal) pair on the ventral chain, by a narrow nerve-ring surrounding the beginning of the gut. The sub-cesophageal ganglia represent about five pairs of ganglia fused together. From the dorsal ganglia nerves proceed to the “eyes” and anterior sense spots ; from the ventral centres the general body is innervated. Special Fic. 122.—Transverve section of leech.—After Bourne. c., Cuticle; ¢., epidermis; c.7., dermis and outer muscles (circular and oblique); 2.., longitudinal muscles (the peculiar connective tissue is hardly indicated); ».#., radial muscles; Zv., lateral blood vessel; d@.s., dorsal sinus; vs., ventral sinus enclosing nerve-cord (x.); g., median part of crop, with lateral pockets (.)3 z., testis ; £, nephridial funnels; v.@., vas deferens. nerves from the dorsal ganglia 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 fifth skin-ring. The eyes are arranged round the edge of the mouth, and look like little black spots. Microscopic , 2. ECHINOIDEA. Sea-urchins. I SuB-PHYLUM 5, 3. ASTEROIDEA. Starfishes. J ELEUTHEROZOA. >» 4. OPHIUROIDEA. Brittle-stars. >, 5. CRINOIDEA. Feather-stars, », 6 EDRIOASTEROIDEA. Extinct. SuB-PHYLUM 3» 7 BLASTOIDEA. Extinct. PELMATOZOA. >, 8 CYSTIDEA. Extinct. In contrast to the “worms,” the Echinoderms form a well- defined series. They may be described as sluggish marine animals, generally with superficially radial symmetry, with a tendency to form limy skeletons. The radial symmetry led the older zoologists to place the Echinoderma near Ccelen- tera, but there seems to be no real affinity. Moreover, the larval Echinoderm is bilateral in its symmetry. It seems likely that the Echinoderms represent an offshoot of some “worm” stock. As in Ccelentera, the nervous system shows a marked absence of -centralisation, which may be connected with the absence-of a definite head region, and this again with the sedentary or sluggish habit. GENERAL CHARACTERS The Echinoderms are celomate marine animals in which the bilateral symmetry of the larva is replaced in the adult by more or less marked radial symmetry. In addition to the dominant radial symmetry, the adults show to a varying extent a tendency towards the bilateral type, but this is never the same as that of the larva, nor is it equivalent in the different forms. Lime is always deposited in the mesodermic GENERAL NOTES ON STRUCTURE. 253 tissues (mesenchyme), and in consequence there is frequently a very complete skeleton. From the primitive gut of the larva, pouches grow out to form the usually spacious celom and the characteristic water vascular system (hydrocel), which may have locomotor or respiratory functions.or both. The branches of this system, together with the nerves, exhibit in most cases a typical five-rayed arrangement. In addition to the water vascular system, there ts an ill-defined lacunar system of blood vessels, In the hemal vessels, water vessels, and celom, there are abundant migratory amebocytes. Well-defined excretory organs are absent. Gonads arise on the lining of the body cavity, and are radt- ately disposed except in Holo- thurians. The sexes are almost always separate. There ts usually a striking circuitous- ness or indirectness in develop- ment. The larve are almost always free-swimming, and exhibit a metamorphosis. The diet ts vegetarian (most sea- urchins), or carnivorous (star- Jishes), or consists of the organic particles found in sand and mud, the. Holothurians in par- ticular practising this worm- like mode of nutrition. . Most Echinoderms have toa ana oh a Aral remarkable extent the power After Johannes Miiller. of casting off and regenerating . portions of their body. This power is probably one of their means of defence, but they often mutilate themselves as a consequence of unfavourable conditions of life. This self- mutilation, or autotomy, seems to be reflex, and not voluntary. GENERAL NoTES ON STRUCTURE The Echinoderma, in spite of the numerous fossil representatives, form an exceedingly well-defined group, showing no close relation to any other, and exhibiting certain striking peculiarities. The skeleton is generally well developed; in Holothurians it consists of isolated spicules, but elsewhere of a series of plates which may be firmly united 254 PHYLUM ECHINODERMA. together, as in most sea-urchins, or may be capable of movement upon one another. Apart from the skeleton proper, lime may appear in almost any of the organs of the body. With this deep-seated tendency to form skeletal substance may perhaps be associated the sluggish habit of the majority, and the absence of definite excretory organs, Except in Holothurians, where the calcareous plates are diffusely scattered, the parts of the skeleton show much regularity of arrangement. The primitive skeleton is believed to have consisted of two series of plates, constituting respectively the oral and apical systems. These, especially the latter, were of much importance in the formation of the skeleton of the extinct Blastoids and Cystoids, but in modern Echinoderms they are absent or unimportant, and are functionally replaced by accessory plates, such as those which form the ‘‘test” of sea-urchins, The oral system consists of five plates surrounding the mouth, and in living forms it is fully developed only among Crinoids. The apical system in the Pelmatozoa typically forms a cup or calyx enclosing the viscera, and consists of a central plate to which a stalk may be attached, and three sets of plates arranged around this, five infra-basals, five basals, and five radials. In the larva of Aztedon this apical system is fully represented, except that the infra-basals are reduced to three, but in other Crinoids and in the adult Azzedon there tends to be reduction. Among other Echinoderms the apical system is best represented among sea-urchins, where there are often five basals (the genitals) around the anus. The ‘‘oculars” seem to correspond to the ‘‘terminals” at the tips of starfish arms. In Ophiuroids the apical system is sometimes re- presented both by basal and radial plates, but often only by radials ; in starfishes it is typically absent in the adult, though more or less clearly shown in the larva. The other most striking characteristic of Echinoderms is the peculiar water vascular system. This arises in development from the ccelom, and consists typically of the following parts:—An external opening or madreporite opens into a canal with calcified walls, called the stone canal; this opens into a ring canal around the mouth, which has often connected with it little vesicles and glandular bodies; the ring canal opens into five radial canals which run in the radii of the body, and give off branches to the protrusible tube-feet which project on the surface of the body, and may be furnished with suckers; the radial canals are also often connected with internal reservoirs or ampull. The tube-feet are very characteristic, and have different functions in the different classes. In Asteroids, in most Holothurians, and in part in Echinoids, they are primarily locomotor ; in Ophiuroids, in Crinoids, and in part in Echinoids, they are respiratory, tactile, or used for food- catching. But there is great variety of structure and functions; thus in many Holothurians the tube-feet are represented only by a ring of tentacles around the mouth, Class ASTEROIDEA. Starfishes Star-like or pentagonal Echinoderms more or less flattened at right angles. to the main axis of the body ; usually with ASTEROIDEA. 255 well-defined simple arms containing the gonads and prolonga- “tions of the gut, and with a ventral ambulacral groove supported by paired ossicles and bearing the tube-feet.; with regularly disposed calcareous, often spinous, plates on the skin ; with an external madreporite (occasionally multiple), always on the uppey surface of the disc in living forms; with a mouth at the centre of the lower surface, and usually with an anus at the opposite pole. Description of a Starfish. The description applies especially to the common five- rayed starfish (Asterias or Asteracanthion rubens). It is often seen in shore pools exposed at low water, but its haunts are on the floor of the sea at greater depths. There it moves about sluggishly by means of its tube-feet. Each of the five arms bears a deep ventral groove in which the tube-feet are lodged. The mouth is in the middle of the ventral surface, the food canal ends about the centre of the dorsal disc. With this flat, five-rayed form, the 11-13 rayed sun-star (So/as/er), the pincushion- like Porania, and the flat pentagonal Padmipes, should be contrasted. Between two of the arms lies the perforated madreporic plate, thus defining the d/vium, while the three other arms constitute the, ¢vivdum. | The body is covered by a ciliated ectoderm, beneath which lies a mesodermic layer. In association with the latter there is developed on the ventral surface of each arm a double series of sloping plates. These meet dorsally, like rafters, in the middle line of the arm, forming an elongated shed. The rafter-like plates are called ambulacral ossicles ; the groove which they bound lodges the nerve-cord, the water vessel, and the tube-feet of each arm. In association with the outer mesodermic layer of the integument, numerous smaller plates are developed, e.g. the adambulacrals, which articulate with the outer lower ends of ambulacrals. The dorsal surface bears a network of little ossicles, and many of these bear spines. Peculiarly modi- fied spines, known as pedicellariz, look like snapping scissor-blades mounted on a single soft handle. They 256 PHYLUM ECHINODERMA. have been seen gripping Alge and the like, and probably keep the surface of the star-fish clean. . A starfish is not very muscular, but it often bends its arms upwards by means of a muscular layer in the body wall. . Other muscles affect the size of the ventral grooves, and muscular elements also occur on the protrusible part of the stomach, and in connection with the water vascular system. GEE we Rey is iN ea Fic. 132.—Starfish. I. Ventral surface; 4.4, tube-feet extended; a.g., the ambulacral groove with the tube-feet retracted; 2., the mouth. II. Dorsal surface, showing the position of the madrepore (J7.); the two adjacent arms form the bivium. Underneath the ciliated ectoderm lies a network of nerve fibrils, with some ganglion cells. But besides these diffuse clements there is a pentagon around the mouth, and a nerve along each arm. The system is not separable from the skin. Ganglion cells are developed also on certain parts of the wall of the ccelom. A red eye spot, sensitive to light, lies on the terminal ossicle at the tip of each arm, and is usually upturned. It is a modified tentacle, bearing numerous little cups, lined ASTEROIDEA. 257 by sensitive and pigmented cells, containing clear fluid, and covered by cuticle. The skin is diffusely sensitive. The term#nal tube-foot of each ray seems to be olfactory. The starfish may be found with part of its stomach extruded over young oysters and other bivalves. This protrusible portion of the stomach is glandular and saccu- FIG. 133.—Alimentary system of starfish. —After Miiller and Troschel. The dorsal surface has been removed ; the digestive czeca and the stomach are shown. lated, and bulges slightly towards the arms; it is followed by an upper portion, giving off five branches, each of which divides into two large digestive ceeca,—a pair in each arm (Fig. 133). These glands are comparable to a pancreas; their secretion contains three ferments, which convert proteids into peptones, starch into sugar, and break up fats into fatty acids and glycerine. From the short tubular 17 258 PHYLUM ECHINODERMA. intestine between the stomach and the almost central dorsal anus two little outgrowths are given off, perhaps homologous with the “respiratory trees” of Holothurians (Fig. 139, 7.2). Some parts of the food canal are ciliated. The ccelom is distinct, though not much of it is left unoccupied either in the disc or in the arms. It is lined by ciliated epithelium, and contains a fluid with amceboid cells. A few of these have a pigment which probably aids in respiration; others are phagocytes, which get rid of injurious particles through the “skin-gills”; others con- tinue the work of digestion. When a starfish is crawling up the side of a rock, scores of tube-feet are protruded from the ventral groove of each arm; these become long and tense, and their sucker-like terminal discs are pressed against the hard surface. There they are fixed, and towards them the starfish is gently lifted. The protrusion is effected by the internal: injection of fluid into the tube-feet; the fixing is due to the pro- duction of a vacuum between the ends of the tube-feet and the rock. As to the course of the fluid, it is convenient to begin with the madreporic plate, which lies between the bases of two of the arms (the bivium). This plate is a complex calcareous sieve, with numerous perforating canals and external pores. It may be compared to the rose of a watering-can, but the holes are much more numerous, and lead into small canals, which converge into a main ciliated canal, the stone canal. This, as usual, opens into a ring canal around the mouth. The ring canal gives off nine glandular bodies (Tiedemann’s bodies), and, five radial tubes, one for each of the arms. Considerations of symmetry suggest that there should be ten glandular bodies, but in the inter-radius containing the stone canal there is only one. In many starfishes there are five or ten little reservoirs (Polian vesicles) opening into the circumoral ring, but in Asterzas rubens these are hardly dis- tinguishable from the first ampullee of the radial vessels. These run along the arms, and lie in the ambulacral groove beneath the shelter of the rafter-like ossicles. From them branches are given off to the bases of the tube-feet, but from each of these bases a canal ascends between each pair of ambulacral ossicles, and expands into an ampulla or reservoir on the dorsal or more internal side (see Fig. 134). The fluid in the system may pass from the radial vessels into the tube-feet, and from the tube-feet it can flow back, not into the radial vessel, but into the ampullz. There are muscles on the walls of the tube-feet, ampulla, and vessels. At the end of each arm there is a long unpaired tube-foot, which seems to act asa tactile tentacle, and has also olfactory significance. ASTEROIDEA. 259 With regard to the vascular system there is considerable uncertainty There is probably no definite vascular system at all. The organ de- scribed asa heart is really the ‘‘ genital stolon.” There is a ‘‘ pseud- heemal sinus” surrounding the stone canal, leading into a circum- cesophageal ring, which gives off a vessel along each ray. From the dorsal surface and sides of a starfish in a pool, numerous transparent processes may be seen hanging out into the water. They are the simplest possible respiratory structures, contractile outgrowths of the skin with cavities L, Td. a bv. I, Fic. 134.—Diagrammatic cross-section of starfish arm.— After Ludwig. #., Radial nerve; J.v., radial blood vessel according to Ludwig, septum in pseud-hezmal vessel according to others; w.v., radial water vessel; az., ampulla; 7, tube-foot; g.c., a pyloric caecum cut across ; s.f., a calcareous spine; g., askin- gill; Zac., spaces in the wall; go., ova in ovary ; @.0., ambu- Tacral ossicle. continuous with the ccelom, and are called ‘“skin-gills.” It is likely that pigmented cells of the body cavity fluid act like rudimentary red blood corpuscles; the water vascular system may help in aeration; and the whole body is, of course, continually washed with water. The “skin-gills” are said to have an excretory function ; for phagocytes, bearing waste, seem to traverse their walls. It may also be that excretion is somehow concerned in 260 PHYLUM ECHINODERMA. forming the carbonate of lime skeleton, but facts are wanting. , The sexes are separate, and they are like one another, both externally and internally. The gonads develop periodi- cally, and lie in pairs in each arm. Each is branched like an elongated bunch of grapes, and is surrounded by a “blood sinus.” Each has a separate duct, which opens on a porous plate, between the bases of the arms on the dorsal surface. In Asterina gibbosa, however, the eggs are extruded ventrally. In the same species there is an in- teresting sexual variability: many are first males and then females (protandric), others are simply hermaphrodites, others seem exclusively of one sex. The eggs of starfishes are fertilised in the water, and the free-swimming larva is known as a Bipinnaria or as a Brachiolaria. Other Starfishes Parental care is incipient among Asteroids. A species of Asterias has been seen sheltering its young within its arms: there is a definite brood-pouch in the form of a sort of tent on the dorsal surface of Preraster. Many Asteroids break very readily, or throw off their arms when these are seized. The lost parts are slowly regenerated, and strange forms are often found in process of regrowth. Thus the “comet form” of starfish occurs when a separated arm proceeds to grow the other four. There are many deep-sea forms, such as the ophiuroid- like Brisinga, the widely-distributed Aymenaster, and the blue Porcellenaster ceruleus; but the majority occur in water of no great depth. Asteroidea first occur in Silurian strata. Classification.— Order I. Phanerozonia. With strongly developed marginal plates, the upper and lower marginals in contact ; with skin- gills restricted to the dorsal (abactinal) surface; with broad ambulacral plates; with prominent adambulacrals in the peri- stome, with pedicellariz sessile (if present), with two rows of tube-feet. ; e.g. Astropecten, Luidia, Porania, Asterina, Palmipes. Order II. Cryptozonia, - With indistinct or rudimentary marginal plates in the adults, often with intermediate plates between the OPHIUROIDEA, 261 upper and lower marginals, with skin-gills not restricted to the dorsal (abactinal). surface, with narrow ambulacral plates, with ambulacrals or adambulacrals prominent in the peristome, with pedicellariz sessile or stalked (if present), often with apparently four rows of tube-feet. e.g. Asterias, Solaster, Henricia, Brisinga. Class OpHiuRoIDEA. Brittle-stars, e.g. Ophiopholis aculeata ~ Echinoderms with a stellate flattened body, nearly related to starfishes, but usually differing from them in having the arms (sometimes branched) sharply marked off from the Fic. 135.—Ventral surface of disc of an Ophiuroid (Ophiothrex fragilis).—After Gegenbaur. &, Openings.of genital pockets or burse; 7., mouth; v., ventral plates of arms; s/., spines of arms ; ¢/, tube-feet—at the right side these are represented as retracted; 0., the openings through-which they are protruded ; #., plates around mouth bearing the so-called teeth; one of these plates is perforated, and functions as the madreporite. : central disc, no ambulacral groove on the ventral surface of the arms, the digestive organs and gonads restricted to the disc, and the madreporite ventral. There is no anus. There are deep respiratory clefts on the dise at the insertion of the 262 PHYLUM ECHINODERMA. arms. They agree with starfishes in being free, in having radially disposed gonads, in having the tube-feet restricted to the under surface, and in other features. The body of a brittle-star differs from that of a star- fish in the abruptness with which the arms spring from the central disc (cf. Brisinga). These arms are muscular, and useful in wriggling and clambering; they do not con- tain outgrowths of the gut, nor reproductive organs. Moreover, there is no ambulacral groove, and the tube-feet which project on the sides are usually very small. They are often of locomotor service, adhering even to vertical surfaces, but in many cases they seem to be only sensory. Each segment of the arm includes a central “vertebral ossicle,” with four plates forming a tube round about it. There is a complete oral skeleton. .The madreporic plate is situated on the ventral surface, usually on one of the plates around the mouth. The food canal ends blindly. Some brittle-stars have small luminescent glands, e.g. Amphiura squamata. The reproductive organs lie in pairs between the arms, and open into pockets or bursze formed from inturnings of the skin, which communicate with the exterior by slits opening’ at the bases of the arms. Water currents pass in and out of these pockets, which probably have both respiratory and excretory functions. The free-swimming larva is a luteus, very like that of Echinoids (see Fig. 131). Ophiuroids are first found in Silurian strata. The Ophiuroids are usually classified according to the characters of their ossicles and covering plates. Some common genera are Ophiothrix, Ophiocoma, Ophiopholis, Ophiura. In the deep-water Astrophyton and Gorgonocephalus the arms are repeatedly branched. In =a Ce Fic. 139.—Dissection of Holothurian (Holothuria tubulosa) from the ventral surface. #., Tentacles surrounding the mouth; 44, scattered tube-feet of ventral surface ; ¢., calcareous ring surrounding the food canal ; a., ampulle of tentacles (modified tube-feet); ~., circular vessel surrounding the gullet, giving off the branched stone canal (s#.), the single Polian vesicle (o.), and the five radial canals (7c.), which run forwards, pass through the calcareous ring, and then curve outwards to run on the surface of the longitudinal muscles (¢.7.) along the radial areas. Of the five longitudinal muscles, one only is marked. gl., The gut cut through at the beginning of the first loop; 2., 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 ; 7.4., the right respiratory tree—the left is cut short mle to its origin; ov., the ovary. The blood vessels are not shown. 272 PHYLUM ECHINODERMA. some others the body cavity serves as a brood-pouch. This illustrates how the same result may be reached in a great variety of ways. The calcareous plates of Holothurians are found as far back as Carboniferous strata. As “trepang” or “béche-de-mer,” the Holothurians of the Pacific form an important article of commerce, being regarded as a delicacy by the Chinese. Classification.— Order 1. Actinopoda. The radial water vessels are associated with external tentacles, tube-feet, and ambulacral papilla, but the tube-feet and papilla may be absent. There are several families, e.g. the deep-sea Elasipoda, markedly bilateral, almost always flattened ventrally, often with an external pore for the stone canal, e.g. Hipzdia and Kolga; the Aspidochirote, e.g. Holothuria and Stzchopus, and Dendrochirote, e.g. Cucumaria, Thyone, Psolus, with tube-feet as well as tentacles; the Molpa- diidze with tentacles only, eg. Molpadia; the Pelagothuriidze containing the free-swimming Pelagothuria. Order 2. Paractinopoda or Apoda. The only external outgrowths ot the water-vascular system are the pinnate tentacles around the mouth. One family, Synaptide, e.g. Syzapta and Chzridota. There are no tube-feet or respiratory trees or Cuvierian organs. The calcareous bodies are usually beautiful anchors and plates. Many are hermaphrodite. Class Crinoipea. Feather-stars Usually stalked forms, with five jointed, often branched arms (“brachia”), growing out from a central cup or “theca,” and bearing pinnules; the arms arise from a ‘orresponding number of thecal plates or “radials,” below which there is a circlet of alternating “‘basals,” often with “infra-basals” alternating again with them; below the “Bbasals” or “infra-basals” there is usually a jointed stem anchored to the substratum by “ cirri.” The feather-stars or sea-lilies differ from other Echino- derms in being fixed permanently or temporarily by a jointed stalk. The modern Comatulids, eg. 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 CRINOIDEA. 273 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.s 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— 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 “Il vil i Hl “ « i om i Pye Fic. 140.—Diagrammatic vertical section through disc and base of one of the arms of Axmtedon rosacea.—After Milnes Marshall. The section is inter-radial on the left, radial on the right. 74., Cili- ated openings in body wall; %., sub-epithelial ambulacral nerve ; Z., water-vascular canal; %, tentac'e; ~, mouth; s., intestine ; &, central plexus, with ‘‘chambered organ” at its base; f, ceelom ; #1.-R3,, radial plates ; B., brachial plates ; ~., muscle ; a,, axial nerve-cord; @., central capsule; C.D., centro-dorsal plate; Z., cirri; ¢., nerve branches from central capsule to cirri. the class, eg. Comatula, are infested by minute parasitic “worms” (Myzostomata) allied to Chzetopods, which form galls on the arms. A lost arm can be replaced, and even the visceral mass may be regenerated completely within a 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 (I) 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 lowest part of the cup is supported by a pentagonal 18 274 PHYLUM ECHINODERMA **centro-dorsal” ossicle, bearing the cirri; this conceals the coalesced ‘*basals” of the larva; above this are three tiers of ‘‘ radials,” whence spring the ‘‘ brachials” of the arms. The oral disc, turned upwards,. is supported by plates. Here 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 many ferns, and as each altcrnate 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, Hyocrinus, and in the stalked stage of Antedon, the stalk bears lateral cirri. The nervous system consists (a) of a circumoral ring with ambulacral nerves, and (4) of axial coelomic nerves up the ossicles on the opposite side of each arm and connected with » peculiar ‘‘ chambered organ” in the interior of the centro-dorsal plate. 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 on a papilla. The last part of the gut is expanded to form an anal tube, which during life is in con- stant movement, and has apparently a respiratory function. From the cup, where the body cavity is in great part filled with connective 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 circum-cesophageal 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 Aztedon 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 ampullz, 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. The reproductive organs extend as tubular strands from the disc along the arms, but are rarely functional except in the pzznules, from each of which the elements burst out by one duct in females, by one or two fine canals in males. The oval ciliated larva of Amtedon, the only one known, is less peculiar than that of other Echinoderms, 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. DEVELOPMENT OF ECHINODERMS. 275 The recent forms include the stalked Pentacrinus, Rhizocrinus, etc., and the free Comatulids, which pass through a stalked Pentacrinus stage, e.g. Antedon. Class EDRIOASTEROIDEA. Wholly extinct These extinct Pelmatozoa had a sac-like theca of an indefinite number of irregular plates, with a mouth in the centre of the upper surface, with at most a short stalk. Ordovician, Silurian, and Devonian. ‘* They are alone among Pelmatozoa in presenting a type of ambulacrum from-which the holothurian, stellerid, and echinoid types may readily be derived” (F. A. Bather). Class BLastorpEa. 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. Class CystipgEa. Wholly extinct The Cystidea 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. DEVELOPMENT OF ECHINODERMS The ovum undergoes total segmentation, and a hollow ball of cells or blastosphere results. A typical gastrula is formed by invagination. The mesoblast has a twofold origin: (a) from ‘“ mesen- chyme” cells, which immigrate from the invaginated endo- derm into the segmentation cavity ; (4) from the outgrowing of one or more ccelom pouches (vaso-peritoneal vesicles) from the gastrula cavity or archenteron. From these vesicles the body cavity and the rudiments of the water- vascular system arise. The larva is, first of all, a slightly modified, diffusely ciliated gastrula. In Holothuroids, Echinoids, Asteroids, and Ophiuroids, it becomes quaintly modified by the outgrowth of external processes, and the formation of 276 PHYLUM ECHINODERMA. special ciliated bands. These are at first simply pre-oral and pre-anal rings, but they become drawn out along variously disposed and shaped processes. The larva of Crinoids (of Anfedon) 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 on one side. The arms or pro- cesses peculiar to the larva are absorbed or in part thrown off. Only in a few forms which have brood-chambers or Fic. 141.—Stages in development of Echinoderms.—After Selenka. 1. Section of blastula of Synapta digitata (Holothuroid), with a hint of gastrulation. 2. Section of gastrula of 7o.xopneustes brevispinosus (sea- urchin) ; ec., ectoderm ; ez., endoderm ; 7., segmentation cavity with mesenchyme cells init. 3. Section of larva of Asterina gibbosa (star- fish) ; BZ., blastopore ; g., archenteron; v.Z., vaso-peritoneal vesicle ; yr. and Z., right and left sides. are viviparous is the development direct, and without free- swimming larvee. The celebrated comparative anatomist and physiologist, Johannes Miiller, was the first to show that the various types of Echinoderm larvee 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, a persist only in restricted regions or ciliated bands” (Korschelt and eider). Crinoids.—The simplest Echinoderm larva is that of Azedon, a somewhat modified oval, with five transverse rings of cilia (the most 277 RELATIONSHIPS OF ECHINODERMA. “y} Iolajsod & pue SBulL pazeriod asreasuesy aay YIM ‘padvys-jerzeq st uopazupy JO ealey ony “9n3gn] J @—vare'y *DLLDIOLYIDAG & 10 wiemuuigig €—ealey ‘Snagn] J C—BAIe'T “DIADINIAN YF UP —PeAleT ‘a19q) uado pur ‘saynu -uid ay} 03 pezojsar are sue3i0 aaijonpoidar ay? jo syed yeuonsuny ayy, “sure ayy jo saseq ay} 38 Ajyerpestejur uado pue ‘Apoq aq} ural] suv310 aaonpoidar = ayy, ‘Aypetperzeq ur uado pur ‘sure aq} ur al] suvsi0 satjonpoidal aq], ‘saqeyd peorde uo Ajyeipeizayut uedo pue ‘uorsa1 Teolde ayy Japun at] suvsio aatjonpoiderayy, ‘quswWasuele pafel-aay & y1qiyxs jou op pue ‘sajoejuaz jo ywaIM aya jo aseq ayy seau uedo sayy ! Aytazo Apog ay3 UI saqn} peyouriq are suesio aaljonpoider ayy *BUITIN}29-Pooj ul ySTsSU pur ‘sapejuay Aloyendsar are jaaj-aqny ayy, ‘saiod snorauinu Aq sia}ua r3yeA yorya our ‘Az1avo Apoq 34} YUM syeuvd perlaaas ‘yews pue ‘je1aze_ ‘pazutod are jaay-aqn} ayy, *sazeid [eIo ayy Jo BuO ‘soSIp UI pua yeoy-oqny auL “SOSIP UT pue Jaaj-aqn} ayy, “OSIp Teorde ay} ur sazeyd pequed : “yynour ay} puno.ins yoryM sapoezUE} jo 9]OIID OY} UOJ 0} payrpow sAemye Ie eau0S *sOsIp [EUIUIIA} JNO wjided a1aur ueyjo pue ‘pazorsjser uayo are J99}-9qn} ay} { passoiddns aq] UO Tayjoue suo Jesu are snue pue yo, St o1ay} { peruse pue yeajUaA sryynour oy, uaqm ‘snug ay} { [erjUe. pur [e1jU9A St ynow sy, -INs [21]USA dy} Jo a[pprul ayy Ul St yjnow sayy Aq sayeotunwiurod = uraq | uo ATyensn ‘fex3uaa st] ‘[eIpeiajur pu [esiop st | aay 347 Jo suo uO st Sut | Ajyensn st ynq ‘A4jtavo Apog oy3 jut -sAS Ie[noseA-Jayem ayy, | eyejdowodaipeureyy, | ae[d stsodaspem ayy, ~ | -uado owodaspem oyy | uedo Aeur ayeyd stiodezpeu ayy, “Sule 94} UT AIT 0RJ} aAIISOSIp 9y3 JO suots | -ajod aytsoddo 9y3 reat 10 ‘sovjins pauinjdn ‘snue ou | -UaIxy —*yesiop “yuasaid | ye Ayyensn st snue a3 { 90%; *J9qIJO 9q} AwaU JO Ww snuE 943 $ ajod auo ‘1vau Jo je st ‘sajou} -ua} Aq pepunoiins ‘ynour sy J, “sue310 asuas [eIdeds ON ‘Asosuas Ayureu st WpIqM ‘wa3zsds [eloenquie ensn 943 pu ‘urazsAs snoa | -Jau peoe[nqurezue Arosuas pue Jojoure st ary, “sueSio suas jeroads ou ale aiaqy fwoysfs SnoAIou = FedOV[Nqure yensn aq} st aq], “suze 943 jo diy ay} ye saka are oI0q} pue ‘uiayshs snoarou yeioejnqg -we j[ensn 94} ST oto L, S249,, are aay, ‘saqouriq jeiper YIM ‘Surt-aarou ye1ouUNoIID B St slay, ,, SO’S-Tea ,, 218 219q} SaWIeMIOg ‘seyruEIq [eIPel WIT Suri-aarou [RIOWNIII & ST a19G], *syye3s yo do} ay3 uo suze 11943 AVMs S1dYy}0 By} ‘Ayues wns sprnyewoo self I0q30 pue wopazupy “surIe qejnosnur 9y} Surys -311m Aq aaour Aayy, *799}-9qn7 9qy jo suvout fq oaow Ao y, ‘soutds aq} Aq apa] ® papre Yeaqj-sqn3 942 jo sueaw Aq aaour Ay, *ya2j-9qn} aq jo suvaw Aq Apyred ‘sZurqztia aenosnu Aq Apjied saow Aaqy *spremjno peaids satnuutd TeI972[ WIT ste paqouviq | gory wow ‘duo xafdutoo e *Ayidniqe ayeiper ‘sacoi1s = [eIDE[NquIe Aue ynoqyim ‘sure paid aay ory “quaseid are wr1e][adtpad pue “oya ‘saporaqny ‘sazeid Aury Aueur siveq urys aq], “QA00IS [BIOL[NQUIE [BIIV2A daap & aaey sure ay, *juasaid are RWEPIpeg ‘saurds aiqe -AOUL Ivaq pue ‘Yays prs Ajrensn v urioy saqejd Aur] *‘pappaquis are saqejd Awty qaqa ur ‘Uys Ie(MIsnut YSnor e& YIM ‘IIT sreaq 4[eIS paquiol Arerod | woy ‘ostp yeuoSejuad | -ajeyjays Jo ‘jeuoSeyuad | :pauayzeg 10 ‘padeys-jreay -wa} Jo jusuewisd y | seg B st Apo ayy ‘pauaiey st Apoq eq yz ‘re[nqojs st Apoq oy J, ‘VACIONIND *VACIOUNIHAO “vadIOuaLsy “VACIONIHOD -UIOM pue pazesuoya SI Apog ayy, | "VAdCIOUNHLO TOF SWUACGONIHOY AO SASSVID LNVLXY AZAIA AHL NAAMLAL SLSVYLNOO AWOS 278 PHYLUM ECHINODERMA. anterior is less distinct), and a posterior terminal tuft. Eventually the posterior end is elongated to form, in the pentacrinoid stage, an attach- ing stalk, which is afterwards absorbed. As all the extinct Crinoids are permanently stalked, there is here an instance of Recapitulation. Holothuroids.—The larva of Holothuroids (an Aurécularza) is much quainter. Its diffuse cilia are succeeded by a wavy longitudinal band, which in the fuga stage breaks into transverse rings, usually five in number. ‘The pre-oral region becomes large. Asterotds,—Nearest the Auriculariéa is the larva of starfishes, which has the same enlarged pre-oral region. There are ¢wo ciliated bands, of which the ad-oral is smaller, the ad-ana] much larger. They are extended peripherally by the development of soft bilateral arms, and such a larva is known as a Szpznnaréa. But another larval form in Asteroids is the Avrachzolarta stage, in which three warty arms are formed at the anterior dorsal end, independently of the ciliated bands. Ophiuroids and Echinotds.—In the Pluteus larve (Fig. 131) 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 amceboid 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 Axtedon is regarded by many naturalists as a point of much importance. RELATIONSHIPS OF ECHINODERMA The Echinoderms form an exceedingly well-defined phylum, but the Holothurians especially show how many of the significant char- acters may be lost. In that class we see how the power of forming a calcareous skeleton, the characteristic tube-feet, and the greater part of RELATIONSHIPS OF ECHINODERMA. 279 the peculiar water-vascular system, may all disappear ; it is conceivable that further modification of the same kind might eliminate all the dis- tinctively 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 often suggested. It is conceivable that Holothurians of the worm-like Syxag¢a type are nearest the primitive stock of Echinoderma. But there are strong arguments in favour of the view that the free forms, the Eleutherozoa, have been derived from attached Pelmatozoic ancestors. The extinct Edrioasteroidea are in some ways intermediate between the Cystidea and the Eleutherozoa. CHAPTER X11 PHYLUM ARTHROPODA Chief Classes—CRUSTACEA, PROTOTRACHEATA, MyRIOPODA, InsEcTA, ARACHNOIDEA, PALAOSTRACA More than half the known species of animals are included in the Arthropod phylum, 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 suc- cessive 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 tracheze somewhat similar to those of Myriopods and Insects, has nephridia like those of some Annelids ; and the biramose appendages of a simple Crustacean like Afus may be compared with the parapodia of an Annelid. It is difficult to discern the relationships of the various classes included in the Arthropod phylum. Crustaceans, most of which are aquatic and breathe by gills, are often opposed to the Prototracheata, Myriopods, Insects, and Arachnoids, most of which are terrestrial or aerial, and breathe by trachez, or possible modifications of these. Three divergent groups—the King-crabs (Zimu/us), and the extinct Eurypterids and Trilobites—may be conveniently referred to a separate class—Palzostraca. General Characteristics of Arthropods (to which primitive, parasitic, and degenerate forms present exceptions) The body ts bilaterally symmetrical, and consists of numer- ous segments variously grouped, Several or all of the segments CRAYFISH. 281 bear paired jointed appendages variously modified. The cuticle is chitinous. Ciltated epithelium is almost always absent. The dorsal brain 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 celom is always small in the adult; the apparent body cavity is of secondary origin, and has ina great part a blood-carrying or vascular Junction. The sexes are almost always separate, the reproduc- tive organs and ducts are usually paired. There is often some metamorphosis in the course of development. In habit the Arthropods are predominantly active. Class CRUSTACEA General Characteristics of Crustaceans (to which primitive, parasitic, and degenerate forms offer exceptions) With few exceptions, e.g. land-crabs, wood-lice, and sand- hoppers, Crustaceans live in water. They breathe by gills or cutaneously, The head carries two pairs of antenne in addition to other appendages, e.g. at least three pairs of jaws; the thorax, sometimes distinct from, and sometimes Jused to the head, bears various kinds of limbs ; the abdomen zs usually segmented, and often has appendages. The typical appendage consists of two branches and a basal portion, to which gills may be attached. To the chitin of the cuticle, carbonate of lime ts added. A Type of CRUSTACEA. The fresh-water Crayfish (Astacus fluviatilis) (Most of the following description will apply also to the Lobster (Homarus), to the Rock Lobster (Paiimurus), and to the Norway Lobster (Wephrops norvegicus), 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 not found in 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 282 PHYLUM ARTHROPODA. creep forwards on their “ walking legs.” Their life is toler- ably 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 mineteen segments bears a pair of appendages. Among other external characters may be noticed the stalked movable 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. (1) The external shell or cuticle, composed of various strata of chitin, coloured with pig- ments, hardened with lime salts ; ‘The Bopy WALL } (2) The ectoderm, epidermis, or hypodermis, consists of— 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. It is a layer not in itself living or cellular, made by the underlying living skin. As it cannot expand, it has to be moulted periodically as long as the animal continues to grow. The old husk becomes thinner, a new one is formed beneath it, a split occurs across the back just behind the shield, the animal with- ‘draws 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 gastric mill, and the tendinous CRAYFISH. 283 inward prolongations of the cuticle to which some of the muscles are attached, are all got rid ofand renewed. The moults occur in the warm months, eight times in the first year, five times in the second, 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. It is the necessary tax 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 setze 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 pvofopodite, and two jointed branches rising from this—an internal exdopodite and an external exopodite ; but in many the outer branch disappears. The protopodite has usually two joints—a basal or proximal coxo- podite, 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 identification 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 antenniform structure suggest that the eye-sta/k may be of the nature of an appendage. With many of the thoracic appendages, gills, plate-like epipodites, and sete 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. 284 PHYLUM. ARTHROPODA. THE APPENDAGES OF THE CRAYFISH Head (s) Thorax (8). Abdomen (6). No. ™ NAME. FuncTION, STRUCTURE. Antennules (pre- oral ?). Antenne oral ?). Mandibles. (pre- ust Maxillee. and Maxille. Tactile, olfactory, with ear-sac at base. Tactile, opening of kidney at base. Masticatory. i Produces respira- tory current. 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 proto- podite). 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. ist Maxillipedes (foot-jaws). Walking Legs (chelate). 2nd Maxillipedes. 3rd Maxillipedes. Forceps (chelate). Masticatory. Fighting, seizing. Walking. Genital opening in female. Genital opening in male, Thin protopodite, small en- dopodite, large exopodite. Two- jointed protopodite, five - jointed endopodite, long exopodite. Two - jointed protopodite, | large five-jointed eidopo- 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 chela. ” Modified swim- merets in male; in female, rudi- mentary, Modified swim- merets in male, Swimmerets. ” . Great paddles. normal in female. Serve in the male as canals for the seminal fluid. oars, and carry the eggs in the female. Important in swim- ming. ee slightly like [Protopodite and endopodite form acanal; no exopodite. All the three parts, Fic. 142.— Appendages of Norway lobster. £x., Exopodite : Ex., endopodite ; protopodite dark throughout ; #/., epipodite. 1. Antennule—Z., position of ear; 2. antenna—X.., opening of kidney; 3. mand- ible—P., palp; 4. first maxilla; 5. second maxilla—B., baler ; 6. first maxilli- pede ; 7. second maxillipede ; 8. third maxillipede—the basal joint of the proto- poditeis called coxopodite, the next basipodite ; the five joints of the endopodite are called—ischiopodite (z.) ; meropodite (7z.) ; carpopodite (¢.) ; propodite (z.) ; dactylorodite (d.) ; 9. forceps—(7) coxopodite ; (6) basipodite, the joints of the endopodite are numbered ; 10-13. walking legs; 14. modified male appendage; 15-18. small swimmerets ; 19. large paddles. 286 PHYLUM ARTHROPODA. {t is likely that some of the crowded mouth-parts, e.g. the first maxillz, are almost functionless. The hard toothed knob which forms the greater part of the mandible is obviously weil 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 (éfzs¢oma) in front of the mouth, and the slight upper and lower lips ; and the lateral flaps of the body wall which project the gills. Each posterior segment consists of a dorsal arch (¢ergzmz), side flaps (pleura), a ventral bar (sternum), while the little piece between the A/euron and the socket of the limb is dignified by the name of efzmeron. The hindmost piece (e/son), 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 appendages (e.g. the mandibles), and of the ventral region of the thorax, is folded inwards, forming chitinous ‘‘ tendons” or insertions for muscles, and, above all, constituting the complex, apparently, but not really, internal, ‘“‘endophragmal” skeleton of the thorax, protecting the ventral nerve- cord and venous blood sinus. 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—(1) 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-cesophageal 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-cesophageal ganglia are three-lobed, and give off nerves to eyes, antennules, antennz, and food canal, besides the commissures to the sub-cesophageal centres. They act as a true brain. The sub-cesophageal 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 CRA VFISH. 287 ventral chain is double, and at one place, between the fourth and fifth ganglia, an artery (sternal) passes between the two: 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-cesophageal ganglia, nerves are given off to the food canal, forming a complex visceral or stomato-gastric system, Simi- larly, from the last ganglia of the ventral chain, nerves go to the hind-gut. If the brain be regard- ed as the fusion of two pairs of ganglia, as the development sug- gests, and the sub-cesophageal as composed of six fused pairs, ther these, along with the eleven other pairs of the ventral chain, give a total of nineteen nerve-centres, —a pair for each pair of append- ages, ‘ Sensory system.—A skin ‘ clothed with chitin is not Fic. 143.—Section of compound eye likely to be in itself very of Adyses vulgards.—After Gren- sensitive, but some of the cher. ae sete are, and some Ob- “Jdllings in the course ‘of the® optic servers describe a perl- nerve; 7.5. the nerve fibrils passing up to the retinule; +/4., the rhabdoms; pheral plexus of nerve-cells ve., elements of retinule; 4., band of beneath the epidermis. ements ca crete ee sacle The sete are not mere outgrowths of the cuticle, but are continuous with the living epidermis beneath ; and though some are only fringes, both experiment and histological examination show that others are ¢actdle. On the under surface of the outer fork of the antennules there are special innervated sete, which have a smelling function. Other specialised setee have sunk into a sac at the base of the antennules, and are spoken of as auditory. The sac 288 PHYLUM ARTHROPODA, opens by a bristle-guarded slit on the inner upper corner of the expanded basal joint, and contains a gelatinous fluid and small “ otoliths,” which appear to be foreign particles. This “ear” seems to be an equilibrating organ, connected with directing the animal’s movements. In some other Crustaceans the auditory hairs are lodged in an open de- pression; this has become an open sac in the crayfish, a closed bag in the crab. Small sete on the upper lip of the mouth have been said to have a tasting function. The stalked eyes, which used to be regarded as append- ages, 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 epidermal cells. Then follows a focussing 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 crystalline cones are surrounded by the retinula cells. Each retinula consists of five elongated cells arranged about a central axis. Distally, this axis is formed by the crystalline cone, proximally by a little rod or rhabdom. The rhibdom 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 deeply pigmented. Thus each element consists of corneal facet, crystalline cone, and retinula, and the retinula consists of internal rhabdom and external retinula cells. Between the individual optic elements lie some pigment cells. The retinule image is erect, not inverted as in the eyes of Vertebrates. Alimentary system.—The food canal consists of three distinct parts—a fore-gut or stomodaum developed by an intucking from the anterior end of the embryo, a hind-gut or proctodzeum 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 CRAYFISH. 289 end of the body, so that the antennules and antennz lie far in front of it. The fore-gut, which is lined by a chitinous cuticle, includes a short “gullet,” on the walls of which there are small glands, hypothetically called ‘‘salivary,” and a capacious gizzard, which is distinctly divided into two regions. In the anterior (cardiac) region there is 4 complex mill; in the posterior (pyloric) region there is a sieve of numerous hairs. The mill Fic. 144.—Longitudinal section of lobster, showing some of the organs. #., Heart; AO., ophthalmic artery; a@a., antennary artery; ah. hepatic artery ; S7Z., sternal artery; SA., superior abdominal artery; J7G., mid-gut ; DG., digestive gland; AG., hind-gut ; £x., extensor muscles of the tail; #2, flexor muscles of the tail: IA., inferior abdominal artery; G., gizzard ; C., cerebral ganglia P., pericardium ; 7., testes. is very complex ; there are supporting ‘‘ossicles” on the walls with external muscles attached 1o 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, setee, 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 19 290 PHYLUM ARTHROPODA. 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 czeca, 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 amceboid cells. These interspaces seem to represent enlarged blood sinuses (a heemoccele), 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 (wa 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 antennz, and a pair to the digestive gland (hepatic). Posteriorly there issues a single vessel, which at once divides into a 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 CRAYFISH. 291 several smaller apertures, which admit of entrance but not of exit. The blood contains amceboid cells, and the fluid or plasma includes a respiratory pigment, hemocyanin (bluish when oxidised, colourless when deoxidised), and a lipochrome pigment, called zoonerythrin. 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 epepodzie. The membranes between the basal joints of the appendages and the body, from the second maxillipede to the fourth large limb inclusive, bear a second set, the avthrobranchs, which have no epipodites. In 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 setze. 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. 292 PHYLUM ARTHROPODA. 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 phagocytosis, that important process in which wandering ameeboid cells resist infection and help to repair injuries (cf. possible function of thymus in Fishes). In not a few inverte- brates there are scattered groups of excretory cells or nephrocytes, and it seems that the endothelial cells of the lymphatic vessels and renal capillaries in tadpoles have a similar function. Reproductive organs.— The male crayfish is distin- guished from the female by his slightly slimmer build, of crayfish.—After Huxley. and by the peculiar modi- 4, Testes; vd., vas deferens; va’., open- fication of the first two pairs ing of vas deferens on last walking leg. 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 in front of the heart, and of a median lobe extending backwards. 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, Fic. 145.—Male reproductive organs CRAYFISH. 293 and becomes thicker in its passage through the genital ducts. 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. Fic. 146.—Female reproductive organs of crayfish, — After Suckow. ov., Ovaries ; ov’., fused posterior part ; od., oviduct ; vz., female aperture on the second walking leg. Before this, however, sexual union has occurred. The male seizes the female with his great claws, throws her on 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 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 294 PHYLUM ARTHROPODA. 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 division is not followed by division of the 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 un- segmented yolk enveloped by a peripheral sphere of rapidly dividing Fic. 147.—Section through the egg of Aséacus after the com- pletion of segmentation.—After Reichenbach. st., Stalk of the egg ; c/., chorion envelope ; /., peripheral blastoderm within which are the yolk pyramids (dark). cells. In the central yolk, free nuclei are frequently found ; these are the so-called yolk nuclei. Such a type of segmentation is called peri- pheral or centrolecithal, and is very characteristic of Arthropod eggs. Overa particular region of the segmented egg, known as the ‘‘ ventral plate,” the cells begin to thicken ; at this region an invagination occurs, 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 CRAYFISH. 295 into themselves in 4 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 endo- dermic mid-gut. The ear-sac 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. Fic. 148.—Longitudinal section of later embryo of Astacus.—After Reichenbach. £c., Ectoderm ; #z., mesoderm cells; ¢.g., cerebral ganglia; s¢., stomodeum; A., anus; 7., telson; g., ventral ganglia; s.s., sternal sinus ; Ad., proctodeum; %., heart ; #zg., mid-gut ; yolk pyramids dark. ‘ As usual, the nervous system arises from an ectodermic thickening. The eyé arises partly from the optic ganglia of the ‘‘ brain,” partly 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 like the 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 completed within the egg-case, and that it is continuous without metamorphosis. The shortened life history of the crayfish is interesting 296 PHYLUM ARTHROPODA. 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. THE CRAB It is instructive to contrast the crab-type with that of the crayfish or lobster. The cephalothorax is broadened by a hollow extension of the gill-covering (branchiostegite) region. The abdomen is greatly re- duced, with a soft sternal region, and is bent permanently upwards and forwards in a groove in the thoracic sterna. In the male there are only two pairs of abdominal limbs, which have a reproductive function ; in the female there are four pairs, which carry the eggs. Fic. 149.—Section through cephalothorax of a crab, — After Pearson. #7., Heart; Te., extension of the tergum ; raeea sternum ; PL., pleuron; T., tendons; rst W.L., insertion of first walking leg; Bv., gill in gill- chamber; g., gut; d.a., descending artery; A., afferent branchial ; £., efferent branchial. The eye-stalks lie in sockets of the carapace; the bases of the reflexed antennules are also in sockets; the antennze are short and straight. The third maxillipedes are broad and flat and form a kind of oper- culum over the five preceding pairs of appendages. The great claws are relatively very large, the other thoracic legs are non-chelate, and in the swimming crabs, ¢.g. Portunus (see Fig. 150), the fifth pair of thoracic legs have their last joint adapted as a paddle. As to the soft parts, there is a noteworthy change in the nervous system. From the cerebral ganglia a pair of cesophageal commissures extend to a large ganglionated mass sheltered by the endosternal skeleton. It is composed of numerous pairs of ganglia fused together, and gives off nerves to maxillze, maxillipedes, and thoracic limbs. It is perforated by the sternal artery. The cesophageal commissures are united by a transverse commissure just behind the gullet, and in front of this cross junction there are two small ganglia giving off nerves to the mandibles. On the lower surface of the anterior part of the gizzard there are two small gastric ganglia innervated from the cerebrals. 7.N \ h 4) 4 M7 | Y bs AN Fic. 150.—Dorsal aspect of swimming crab (Portunzs). &., Paddle; Ada., abdomen; A1.,antennules ; 42., antennz ; Z., eyes ; /., forceps Fic. 151.—Dorsal aspect of shore crab (Carcznus). Abd., Abdomen; A}, antennules; A%., antenne; Z., eyes; ., forceps. 298 PHYLUM ARTHROPODA. When the-branchial chamber is opened the large pyramidal gills are seen, also the long sword-shaped epipodite (flabellum) of the first maxillipede which seems to help the “baler,” the smaller and mobile epipodites borne by the second and third maxillipedes, and the broad Fic. 152.— Ventral aspect of female shore crab. Aéd., Abdomen ; #x#., third maxillipede. scaphognathite of the second maxilla which bales the water forwards and outwards, It must be clearly understood that the branchial chamber is entirely outside of the body, being formed by the lateral extension of a hollow reduplicature from the tergal region. The large gizzard, the enormous greyish-yellow hepatopancreas, the transparent pericardium, and other organs are readily seen. SystEmaTIC SURVEY OF THE Ciass CRUSTACEA - (1) Entomostraca, lower forms. They are usually small and simple. The number of segments and ap- pendages is very diverse. The larva is generally hatched as a simple unsegmented Mauplius. There is no gastric mill. The excretory organ is associated with the second maxillz. (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 Naupiius. There is often a gastric mill. The excretory organ is usually associated with the antennz, but maxillary glands may be present in the larvee, and may even per- sist in adults. ENTOMOSTRACA. First Sub-Class. ENTOMOSTRACA 299 Order 1. Phyllopoda,—In these at least four pairs of leaf-like swimming feet bear respiratory plates. and is protected by a shield-like or bivalve shell. are without palps, and the maxille are rudimentary. (a) Branchiopoda. or more) foliaceous append- ages with respiratory plates. The shell is rarely absent, usually shield-like or bi- valved. The heart is a long dorsal vessel with numerous openings. The eggsare able to survive prolonged desicca- tion in the mud. Branchipus, a beautifully coloured _ fresh - water form, with hardly any shell. Artemia. Brine - shrimps. Periodically partheno- genetic. By gradually changing the salinity of the water, Schmanke- witsch was able, in the course of several gen-. erations, to modify 4. salina into A. mil- hauseniz, and vice versa. Artemia fertilts is one of the four animals known to occur in the dense waters of Salt Lake. Apus, an archaic fresh- water form with a large dorsal shield. The body is generally well segmented, The mandibles The body has numerous segments and (10-20. Fic. 153.—Dorsal surface of Apus cancriformts. —From Bronn’s. Thierretch. In the anterior region are the two com- pound eyes, and behind them the simple unpaired eye. The whip-like outgrowths of the first thoracic ap- pendage project laterally. Afus is over an inch in length, a giant among Entomostraca. It has an almost world-wide distribution. numerous and mostly leaf-like. The appendages are very They may be regarded as. representing a primitive type of Crustacean limb. Professor Ray Lankester enumerates them as follows :— I. Antenna, Pre-oral. 2. Second antenna. Oral. 4. Maxilla. 3. Mandible. 5. Maxillipede. (This is sometimes absent, and apparently always in certain species, ) 300 PHYLUM ARTHROPODA. 6. First thoracic foot (leg-like). 7-16. Other ten thoracic feet (swimmers). The 16th in the female carries an egg-sac or brood- 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. Thoracic (Pregenital). Fic. 154.—Daphnia, £., Eye; A.®, second antenna; A.1, first antenna; de., digestive cxca; SLs; shell gland; go., gonad; 4., heart in. pericardium ; o., ovum; L.p., brood-pouch; sf., spine; /, furca; s., setae; Aé., rudimentary abdomen ; z., caudal fork; g., gut; 745, thoracic limbs. 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. There is periodic parthenogenesis. ENTOMOSTRACA. 30r (6) 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 antennie 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 maxillz. 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, Sida, Polyphemus, Leptodora, 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. In Daghnia the appendages are:—antennules, antennz, mandibles, first maxille, second maxille (disappearing in the larva), and five thoracic limbs. The abdomen is turned down- wards and forwards, and shows three segments and a telson. Order 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 ~qules, antennz, mandibles, first maxillze, é two pairs of thoracic limbs. Parthenogenesis- is often prolonged. Examples.—Cygris (fresh water), Cyprédina (marine). Fic. 155.—Cypris. M., Marks of adductor muscle; £., eye seen through the shell (S/.); A.J, first antennz ; A.2, second antenne ; F., thoraciclegs. 302 PHYLUM ARTHROPODA. Fic. 156.—Cypris, side view, after removal of one valve.— After Zenker. : z., Eye; A.J, first antennz ; 4.2, second antenne; AZ.V., mandibles ; mx.1, first maxilla ; 72.2, second maxilla; /.7, £2, thoracic legs 5 Aé., rudimentary abdomen, Fic. 157.—Cyclops type. JA,, first antenna; // A., second antenna; OV., ovary; R.S., receptaculum seminis; OS., ovisac; /., caudal fork. ENTOMOSTRACA. 303 Order 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. Most Copepods move very actively in the water, jerking themselves rapidly by means of their thoracic legs, or swim more gently by means of their second antennz. 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. Its appendages are:—antennules, antennz, mandibles, first maxille, second maxille, four pairs of flattened biramous thoracic legs united across the middle with those of the opposite side, another rudimentary pair, and probably the genital valve. Cetochzlus, Calanus, free and abundant in the sea. In Chondracanthus, as in many other cases, the parasitic females carry the pigmy males attached to their body. Caligus, a very common genus of “‘fish-lice.’”’” In the carp-lice (Arvgu/us) the mouth is a sucker with sharp stilets and the second maxillze form adhesive discs. Lernea, Penella, etc. The adult females are parasitic, and almost worm-like. The males and the young are free. Order 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 antennz 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 particles into the depressed mouth. The abdomen is rudimentary. There is no heart. The sexes are usually combined, but dimorphic unisexual forms alsos occur. The hermaphrodite individuals occasionally carry pigmy or ‘‘complemental” 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 antennee and: the front lobe of the head. The second antennz are lost in larval life. The mouth region bears a pair of small mandibles and two pairs of small maxillz,—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 304 PHYLUM ARTHROPODA. anteriorly, two ¢evga at the free posterior end. The nervous system consists of a brain, an oesophageal 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 maxillze, 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. Fic. 158.—Acorn-shell (Bdlanus tintinnabulum). —After Darwin. T., tergum ; CR., thoracic legs; F., outer shell in section ; D., aper- ture of oviduct; #., mantle cavity; ¥., depressor muscle of tergum; AX., antenne; OV., ovary; G., depressor of scutum; #., oviduct ; AM., adductor muscle of scuta; S., scutum. The life history. Nauplius larve escape from the egg-cases, and, after moulting several times, become like little Cyprids. 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. 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 ecdysis is much restricted, except in one genus. Neither the valves, nor the uniting membranes, nor the envelope of the stalk, are ‘marks. It may be ‘described, ENTOMOSTRACA. 305 moulted, though disintegrated portions may be removed in flakes and renewed by fresh formations. In the allied genus Scalpellum, some are like Lepas, hermaph- rodites, without complementary males (Sc. dalanozdes); others are hermaphrodite, with comple- mentary males (Sc. v2llosum) ; and others are unisexual, but the males are minute and para- sitic (Sc. regezm). Balanus, the acorn-shell, en- crusts the rocks in great numbers between high and low water 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 Leas, by a fold of skin, which forms a rampart of six or more cal- careous plates, and a fourfold lid, consisting of two scu¢a and two ¢erga. 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 acorn-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 ac- quires a firmer shield, a longer spined tail, and stronger legs. Then it passes into a Cyprds stage, with two side eyes, six pairs of swimming legs, a bi- valve shell, and other organs. Fic. 159.—Development of Saccudina. As it exerts itself much but does ae Delage. (Not drawn to not feed, it is notunnatural that °° €.) it sHould sink down as if in 4., Sereswimnting Tapplius, with tees : . . pairs of appendages ; B., pupa stage; C., ek d re ees gout protruding from the abdomen of a ? crab. by the secretion of the cement gland. Some of the structures, ¢.g, the bivalve-shell, are lost; new 20 306 PHYLUM ARTHROPODA. 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 and habits are gradually assumed, At frequent periods of continued 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 objects, and even to whales and other animals. On the ventral surface of the abdomen of crabs, Sacczlina, one of the most degenerate of all parasites, is often found. Its 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 into numerous ‘‘ roots,” which ramify within the body of the crab, and by them the Saccz/7a obtains nutrition and gets rid of its waste products; it is practically an exdopfaraszte. The larvee leave the brood-chamber as Nauplii; they moult rapidly and become Cyprid larvae. These fix themselves by their antennz to young crabs, at the uncalcified membrane round the base of large bristles. The thorax and abdomen are cast off; the structures within the head region contract; eyes, tendons, pigment, the remaining yolk and the carapace, are lost; a little sac remains, which passes into the interior of the crab. It reaches the abdomen, and, as it approaches maturity, the integuments of the crab are dissolved beneath it, and the sac-like body protrudes. It appears to live for three years, during which time the growth of its host is arrested, and no moult occurs, Forms allied to Sacculina are grouped together as Rhizocephala. One of them—Sesarmaxenos—occurs on a fresh-water crab, Sesarma, in the Andamans; all the rest are marine. Second Sub-Class. ManacosTRaca Series I. Leptostraca, Division Phyllocarida. Marine Crustaceans of great systematic interest, retaining in many ways the simplicity of ancestral forms, and linking Malacostraca and Entomostraca. The most important genus is Vebal/za. 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 free 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. There are both antennary and maxillary excretory organs. Veda/éa and its MALACOSTRACA. 307 congeners are probably related to certain ancient fossil forms from Paleozoic strata, e.g. Hymenocaris from the Cambrian. Fic, 160.—Nebalia.—Alter Sars. SH., Shell; 4.2, first antenne; A.2, second antenne; 7H., 8 thoracic limbs; 4é.4, 44.6, fourth and sixth abdominal limbs. Fic. 161.—Anaspedes.—After Calman. A.s, A.2, antenne; £x., rudimentary exopodite ; G., respiratory lamina PR.7, PR.S, seventh and eighth thoracic limbs or pereiopods; PL.s, 2, 6, first, second, and sixth abdominal limbs or pleopods. 308 PHYLUM ARTHROPODA. Series IJ. Eumalacostraca. Division 1. Syncarida, the order Anaspidacea, primitive fresh-water forms, without a carapace; with the eight thoracic segments all distinct (Azzasfzdes), or with the first one fixed to the head (Koonunga) ; with stalked eyes in Anaspides, sessile eyes in Koonunga; with lamellar branchize on the thoracic legs, whose slender exopodites are also respiratory. Division 2. Peracarida, with a carapace that leaves at least four of the thoracic segments free, with the first thoracic segment always fused to the head, with usually sessile eyes, with a brood-pouch on the thoracic appendages of the female, with an elongated heart, with direct development. Numerous orders including :—the pelagic Mysi- dacea (formerly united with Euphausiacea as Schizopods), e.g. dZysis; the pelagic and deep-water Cumacea, e.g. Cuma and Diastylzs ; the Isopods, with dorso-ventral flattening of the body, a posterior heart, and respiratory organs on the abdominal limbs, ¢.g. the terrestrial wood-lice (Por- cellio, Oniscus, etc.), which show minute trachea-like respiratory tubes in the abdom- inal limbs, and corresponding forms on the shore (e.g. Ligza, Jdotea) ; the Amphipods, with lateral flattening of the body, an anterior heart, and respiratory organs usually on the thoracic limbs, e.g. Gam- marus locusta in the shore pools, G. pulex in fresh water, and sandhoppers like Talitrus and Orchestia; the ‘‘no body” crabs, Caprella; Phronima, living inside the glassy case of the free-swimming Tunicate Pyrosoma. : : Division 3. Hoplocarida, with a carapace Fic. 162.—An Amphipod that leaves et feast four of the thoracie (Caprella linearis), segments free, with stalked eyes, with the The two anterior thoracic seg- eggs carried in a chamber formed by the Be dan we cee maxillipedes, with an elongated heart, duced and without append- 2nd with a complicated metamorphosis. ages; the fourth and fifth Order :—Stomatopods, e.g. Sguzl/a, with thoracic segments bear only the second maxillipedes forming very large respiratory plates. raptorial organs. Division 4. Eucarida, with a cephalo-thoracic shield uniting the head and thorax segments; with stalked eyes; with a saccular heart ; with eggs attached to the abdominal endopodites; with spherical spermatozoa showing peculiar radiating pseudopodia; usually with a complex metamorphosis. Order 1. Euphausiacea :—shrimp-like surface and deep-water forms, with biramous thoracic limbs as in Mysids, e.g. Huphausza. Order 2. Decapoda:—with the three anterior thoracic limbs turned forward as maxillipedes, with the other thoracic limbs almost always uniramous, ; MALACOSTRACA., 309 Sub-order Macrura.—Abdomen long. Homarus (lobster) ; Meph- vops (Norway lobster, sea crayfish); As¢acus (fresh-water crayfish) ; Palinurus (rock lobster), whose larva was long known as the glass- crab (Phyllosoma) ; Peneus, a shrimp which passes through Nauplius, . Zoza, and Mysis stages ; Lucéfer and Sergestes are also hatched at a stage antecedent to the Zoza 3; Crangon vulgaris (the British shrimp) ; Palemon, Pandalus, Hippolyte (prawns); Galathea (with the abdomen Fic. 163.—Hermit-crab withdrawn from its shell. The anterior appendages are broken off. hd., Head; ¢., thorax ; aéd., abdomen. bent forwards) ; Pagurus, Eupagurus (hermit-crabs) ; Bzrgus latro (the terrestrial robber or palm-crab), in which the upper part of the gill- cavity is shut off to form a “‘lung,”.the walls having numerous vascular plaits. ; Sub-order 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 310 PHYLUM ARTHROPODA. into. an oval mass. Cancer (edible crab); Carcénus manas (shore- crab); Portunus (swimming-crab); Dromia (often covered by a sponge).; Pinnotheres (living inside bivalves); Ze/phusa (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, e.g. the Branchiopod Zs¢herda, living from Devonian ages till now, are remarkably persistent and successful. How the class arose we do not know ; itis probable that types like Anaspzdes and Nebalza give us trustworthy hints as to the ancestors of the higher Crustaceans ; it is likely that the Phyllopods, e.g. Afus, bear a similar relation to the whole series ; the Copepods also retain some primitive ELK: Niet Fic. 164.—Mysis flexuosa, from side. &., Brood-pouch borne on posterior thoracic limbs ; 0., otocyst in tail. Note eight pairs of similar biramose thoracic feet. a last two thoracic segments are not covered by the shield, characteristics; but it is difficult 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 imagine how a develop- ment of cuticular chitin would tend to produce a flexibly jointed limb out of an unjointed parapodium ; how the mouth might be 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 includes animals so diverse as crabs, lobsters, shrimps, ‘ beach-fleas,” ‘“ wood-lice,” barnacles, acorn-shells, and “‘ water-fleas,” it is difficult to state general GENERAL NOTES ON CRUSTACEANS. 3Ir characteristics, other than those facts of structure which we have already summarised. Admitting the parasitism of many Crustaceans, 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 seg- mented. 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 Arthropeds ; as special characteristics we notice the two pairs of antennz, the presence of Carbdon- ate of lime in the cuticle, and the nature of the respiratory organs —these, with few excep- i i Fic, 165.—Nervous system of shore-crab ee sea usnrtes (Carcinus menas).—After Bethe. 2 br., The supra-cesophageal mass; g., gullet While these characters surrounded. by gv, the gullet ring; mt. , the remain constant through- sub-cesophageal mass representing a fusion of S the thoracic ganglia of the crayfish, and out the group, there is giving off merc to the lint behind i it : 1 j 7 a short strand representing the abdominal aly almost infinite variety ganglia of the crayfish. a1., antennules ; in detail. In regard a@?., antenna ; é., eye. 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 specialisation. In the primitive Phyllopods the body con- sists 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 312 PHYLUM ARTHROPODA. segments, which exhibit marked division of labour; a comparison of Mebalia, Mysts, Euphausia, Penaeus, WVephrops, 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 append- ages, like the parapodia of Annelids, are primitively organs of locomotion, usually adapted as swimming organs. In Phyllopods the great majority of the appendages remain permanently at this level. Ita ice_ that in the Naupli i d in free-swimming copepods, ig antenna themselves aie—suumming Organs, Just as, however, in the Annelid head the locomo tion of the parapodia becomes subordinated to the sensory one, so also in Crustacea the anterior appendages of the head become specialised as sense organs. Again, the append- ages in connection with the mouth become modified in connection with alimentation, and the further processes of specialisation which differentiate the regions of the body are reflected in the appendages of these regions. It is this specialisation of certain appendages to function as mastica- tory 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- sented 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. 165). Sense organs are usually well developed, and are not confined to the head region; thus many Mysids have “auditory” organs in the tail (Fig. 164). Dhemelimentanmeneanal runs straight throughout the body; it consists of fore-gut, id-gut, and hind-gut. e-gut and hind are GER OaET POSLA Pagina ae ectoderm, and are always large, especially in ostraca. In the higher Malacostraca the fore-gut is furnished with a gastric mill: The mid-gut or archentéfon is always short, barns con nected with it diverticula which form the so-called hepato- pancreas. In the Entomostraca there is usually only a single pair of outgrowths; in Mysids, Cumacea, and larval GENERAL NOTES ON CRUSTACEANS. 313 ‘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 is a pair of blind tubes functioning as excretory organs, and presenting an interesting similarity to the Malpighian tubes of insects, which, however, are in con- nection with the hind-gut. The body cavity is never lage being mainly filled up with muscles and organs, and, as in In the blood, hemocyanin is the commonest pigment, ut is not universal. respiration 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 excsesery—ergans of the Entomos- traca 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 possibly modified nephridia. There are many peculiarities connected with reproduc- tion—thus parthenogenesis for prolonged periods is common among “water-fleas” ; _herma hroditism is frequent, occur- often complicated_by the simultaneous existence o y often very diverse. The spermatozoa are often exceptional i in being very slightly motile. Some appendages are often modified for copulation or for carrying the eggs. Development.—The ova of most Crustacea show con- siderable similarity to those of Astacus, and the segmenta- tion is typically of the kind already described. But while this is the most typical case for Crustacean, and, indeed, for Arthropod development, it is possible, within the limits of the class Crustacea, to trace out a complete 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 gastrulation is usually much disguised, there are many modes, from 314 PHYLUM ARTHROPODA. an invagination of the simplest embolic type (Zucifer), and through the condition described for Asfacus, to the forma- tion of endoderm by the ingrowth of a solid plug of cells. Compared with Astacus, 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 (WWVebalia, Mysis) a metamorphosis takes place within the egg-cases, and in the few forms in which develop- ment seems to be direct, slight traces of meta- morphosis are found. Almost all the lower Crustaceans and some higher forms, e.g. Euphausia and Peneus, 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 Astacus), The Nauplius is char- acterised by a typically Fic. 166.—Zozea ot common shore-crab (Carcinus menas).—After Faxon. rounded. Dad, and. by a pakeacy the presence of three e appendages are numbered ; ¢., gills; : z., alimentary canal, pairs of appendages, F which are the only obvious indications of segmentation. The first pair of appendages; are unbranched, and bear larval sense organs, the next two are biramose swimming 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 antenne and the mandibles of the adult. The head region of the f GENERAL NOTES ON CRUSTACEANS. 315 Nauplius becomes the head region of the adult; the posterior region also persists; the new growth of segments. and appendages takes place (with numerous moultings) in the region between these. The second important form of larva is the Zozea, which has all the appendages on to the last maxillipedes inclusive, a segmented abdomen, and two lateral compound eyes, in addition to the unpaired one of the Nauplius stage. Most Decapoda are hatched in the Zozea stage. (a) The crayfish (Astacus) is hatched almost as a miniature adult. The development is therefore very direct in this case, (4) The lobster (Homarus) is hatched in a AZysds stage, in which the thoracic limbs are two-branched and used for swimming. After some moults it acquires adult characters. | (c) Crabs are hatched in the Zoga form, and pass with moults through a Megalopa stage, with the abdomen in a line with the cephalo- thorax. The abdomen is stibsequently tucked in under the thorax. : (d) Penaeus (a kind of shrimp) is hatched as a Mazplius, becomes a Zoea, then a Myszs, then an adult. Its relative Luczfer starts as a Meta-Nauplius with rudiments of three more appendages than the Nauplius. Another related form, Sevgestes, is hatched as a Protozoea, with a cephalothoracic shield and an unseg- mentedabdomen. Thus there are two grades between Nauplius. and Zoza. Three facts must be borne in mind in thinking over the life histories. of crayfish, lobster, crab, and Pexeus: (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 larve , exhibit characters which are related to their own life rather than to that of the adult ; (3) it is a gezera/ 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 Zoea stage is antecedent to the A/yszs, etc.,—yet it does not follow that ancestral Crustaceans were like Nauplii. On the contrary, the Vauzplcus must be regarded as a larval reversion to a type much simpler than the ancestral Crustacean. C&cology.—Most Crustaceans are carnivorous and pred- atory; others feed on dead creatures and organic débris in the water ; a.§4inOrifgdepend upon plants. Many of the smaller forms play a very important part in the economy of nature—in the circulation of matter—for while they feed on 316 PHYLUM ARTHROPODA. animalcules and débris, they are themselves the food of larger animals such as fishes. ; Parasitism occurs in ovetZe0 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. eater, weend_zamifving—absorptive roots through the ody of the host. Sometimes the parasitism 1s temporary ( Areulus); sometimes only the females are parasitic (e.g. in Lernea). 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 (Fig. 16). The hermit-crab is concealed and protector vy ane Sea anemons—thelaiter is camied 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 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 seaweeds 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. Many will “voluntarily” part with a leg to save themselves from their enemies. 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 colouring”; in many cases, they are brilliantly phosphorescent. Yet more abundant are the pelagic Crustaceans (especially Entomostraca and Mysids) ; they are often transparent except the eyes, often G@COLOGY. 317 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 repre- ‘sented in fresh water, in pools, streams, and lakes. A few Crustaceans, such as wood-lice and land-crabs, are terrestrial, and some blind forms occur in caves. CHAPTER XIV PHYLUM ARTHROPODA—(continued) Classes (continued)—ONYCHOPHORA or PROTOTRACHEATA ; Myriopopa; and INSECTA ‘THESE three classes form a series of which winged insects are the climax. The type Levipatus is archaic, and links the series to the Annelids: the Myriopods lead on to the primitive wingless insects. All breathe by trachese—tubes which carry air to the organs of the body—and all have antenne ; hence they are often united under the title ‘Tracheata Antennata. First Class of Tracheata Antennata.—ONYCHOPHORA or PROTOTRACHEATA GENERAL CHARACTERS The body ts 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, which development shows to be true appendages, numerous pairs of short, imperfectly jointed legs, each with two claws, and a pair of anal papille, which are rudt- mentary appendages. The legs contain peculiar (crural) glands. Respiration is effected by numerous unbranched trachee with openings irregularly scattered. The heart ts an elongated dorsal vessel with valvular ostia, There ts a series of nephridia in the legs. The halves of the ventral nerve-cord are widely separate. All are viviparous. ONYCHOPHORA OR PROTOTRACHEATA. 319 In its possession of trachee and nephridia this type is an interesting connecting link; in many ways it seems to be an old-fashioned survivor of an archaic stock. There are about half a dozen genera very widely distributed. The Onychophora are very beautiful animals. Prof. Sedgwick says: “The exquisite sensitiveness and continu- ally changing form of the antehnz, 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, with a great dislike to light. They seek out damp places under leaves and among rotting wood. They feed on insects, which they catch by the ejection of slime from the oral papille. The slime is also squirted out when they are irritated. 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. They have been seen to climb up vertical glass plates. When at rest or irritated they coil up in a circle. ; Fic. 167.—Ex- Like some other archaic types, ¢.g. Dipnoi, the ternal form of Onychophora have a very wide range of distribution, Peripatus,— which may be briefly indicated :—Ferdpatus (tropi- After Balfour. cal America and tropical Africa) ; Hoperipatus (Indo- N 5 Malay) ; Perzpatoddes and Ooperdpatis (Australasia) ; “gapleleee Opesthopatus (Chili and South Africa) ; Paragpertpatus (New Britain); Perzpatopszs (Central Africa). 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. There is a clear dorso-median line. 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 antennz ; then follow the sickle-like jaws in the mouth cavity; a little farther back are two oral papillee from which slime is exuded. Then there are the 14-42 stump-like legs, each with two terminal chitinous claws. 320 PHYLUM ARTHROPODA. Skin.—The chitinous cuticle, ordinarily thick in Arthropods, is delicate. It is subject to moulting. -The epidermis is a single layer of cells. Beneath it there is a dermis. i 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 vesicles. Striped, rapidly contracting muscles are characteristic of Arthropods, but in Perdpatus the muscles are unstriped, excepting those which work the jaws and are perhaps the most active. The true ccelom 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 ceso- phageal ring with the two- widely separate latero-ventral nerve-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. Sense organs are represented by two simple eyes on the top of the head. These are most like the eyes of some marine Annelids. Alimentary canal.—Round about the mouth papillae seem to have fused to form a ‘‘ mouth cavity,” which includes the mandibles, « 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 cesophagus 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 proctodeum, 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 ‘*hzmo- coele.” Respiratory system.—Very long and fine unbranched trachez are widely distributed in the body ; a number open together to the exterior in flask-like depressions. These openings or stigmata are irregularly, distributed. Excretory system.—aA pair of nephridia lie in each segment. Each consists of an internal mesodermic terminal funnel, a looped canal, and a wide vesicle which opens near the base of each leg, the two last parts being invaginations of the ectoderm. Nephridia are not known in any other Tracheate. The salivary glands and the genital ducts seem to be modified nephridia. It may be noted, too, that the same is perhaps. true of the ‘‘coxal glands” of ZLzwewlus and of the antennary glands of Crustaceans, ONYCHOPHORA OR PROTOTRACHEAT. 321 Coxal or crural glands lie in the legs and open to the exterior. They can be in part evaginated, and they probably help in respiration. In the male of P. capgenszs the last pair are very long (Fig. 168, a.g.). The large mucous glands, which pour forth slime from the oral papillz, are regarded as modified crural glands. eproductive system.—(a) Female (of P. edwardszz).—From the two ovaries, which are surrounded by one connective tissue sheath, and arise, as usual, from the coelomic 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 pair of receptacula seminis (for storing the spermatozoa slg. | >: Use sid. 4 08 vw.| co ewe wae ph. Z s.0.17] cl at, : A A_A oh Dr tat taal Suited, p 5.0.4 i 5.0.5 oe “06.60 eT ag as Fur Fa Fic. 168.—Dissection of Perdpatus.—After Balfour. az,, Antenne; o7.4,, oral papille; c.g., cerebral ganglia; sé.d., duct of slime gland (s/.g.); 5.0.8, eighth segmental organ or nephridium 3 v.c., ventral nerve connected by transverse com- missures (co.) with its fellow; s.o.77, seventeenth nephridium ; £.0., genital aperture ; A., anus; 4.d.c., posterior commissure ; f.17, seventeenth appendage ; a.g., last crural gland—that of the opposite side is marked v.g.; 7.1, 7.2, first and second legs; 0¢.co., esophageal nerve commissure ; ve., cesophagus ; ~A., pharynx—the remainder of the gut is removed. 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 « ‘‘ placenta,” so suggestive is it of mammalian gestation. The older embryos lose this ‘‘ placenta,” but each lies constricted off from its neighbours. When born the young resemble the parents except in size and colour. In P. capemszs the period of gestation is thirteen months. (4) Male (of P. edwardsiz7).—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 ? 21 1 322 PHYLUM ARTHROPODA. of the anus. In the ejaculatory duct the spermatozoa, which are thread-like, are made into spermatophores which are attached to the female. It is uncertain how the spermatozoa get into the female. Fertilisation is ovarian. : While it is characteristic of Arthropods, in which chitin is so pre- dominant, that ciliated epithelium is absent, it seems that in Perzpatus, which is much less chitinous than the others, ciliated cells occur in some parts of the reproductive ducts, Development.—There is some variety of development in different species. Thus there is much yolk in the ovum of P. xove zealandie, extremely little in that of P. capenszs. In P. 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 proto- plastic 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 Prof. Sedgwick this singular fact suggested the theory that the Metazoa may have begun as multinucleate Infusorian-like ani- mals, The gut appears from a fusion of vacuoles within the multinucleated mass, and a gastrula stage is thus established. A very interesting feature is that the blastopore or mouth of the gastrula is first elon- gated, then dumb-bell shaped, then closed except at the two ends which é form the mouth and the anus. In the ova of P. nove zealandie, Fic. 169.—Embryos of Perépatws which have much yolk, a superficial capensis, showing closure of multiplication of nuclei forms a sort blastopore and curvature of of blastoderm, which spreads over embryo.—After Korschelt and almost the entire ovum. The seg- Heider. mentation in this case has been called a., Anus} 82., blastopore; #., mouth; centrolecithal (the type characteristic DeSoy Faracive segments; w., zone of of Arthropods), but it is again proliferation. 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 » process of crystallising out 27 széw from a mass of yolk, among which is » proto- plasmic reticulum containing nuclei.” Zoological positi »n.—The synthetic characters of Pertpatus and ils allies may be thus summarised :— MYRIOPODA. 323 ANNELID CHARACTERISTICS. Segmentally arranged nephridia as in Chaetopods. The muscular ensheathing of the body. The cilia in the genital ducts. Less important are the stump-like hollow legs and the simple eyes. ARTHROPOD AND TRACHEATE CHARACTERISTICS. The presence of tracheze, The nature of the (a tube with paired ostia communicat- ing with a pericardium) and the lacunar circulation. The modification of appendages uth organs. The form of the salivary glands. The smallness of the genuine ceelom; the cavity of the body is heemoccelic. The Onychophora differ from other Tracheata Antennata in the simplicity and diffuseness of the trachez, in having only one pair of jaws, in the absence of external segmentation, in the nature of the body wall, and so forth. The ladder-like character of the ventral nervous system (cf. primitive Molluscs, Phyllopod Crustaceans, and Nemerteans) is probably primi- tive. 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 antennz, jaws, and oral papille of Per¢patus precisely correspond to the antennz, mandibles, and first maxille of Insects. Our general conclusion is that Perdpatus is an archaic type, a sur- vivor of forms which were ancestral to Tracheata and closely related to Annelids. Second Class of Tracheata Antennata.—MyRriopopa. Centipedes and Millipedes The centipedes and millipedes, which are grouped together in the class Myriopoda, are usually elongated, somewhat vermiform animals, with a distinct head and a very uniform segmented trunk. The head bears eyes (groups of eye-spotsAngt)¥compound eyes like those of insects, except in Scutigera), jomted antenne, and two or three pairs of jaws. The segments of the trunk bear six- or seven-jointed legs with terminal claws, very similar through- out. The nervous system, the_tr t the ex- cretory tubiles, etc., are like those of Insects, Tt cannot 324 PHYLUM ARTHROPODA. be said that the centipedes (Ciidggaga) and the millipedes (Diplopoda) are very closely related to one another, and -therearetwo other distinct orders, Symphyla and Pauropoda. The resemblances are in part resemblances of convergence, not of genuine affinity. Simple wingless insects, known as Collembola and Thysanura, are closely approached by such Fic. 170.—A millipede. Fic. 171.—A centipede Myriopods as Scolopendredla ; and it is likely that Myriopods and Insects are divergent branches from a common stock. Centipedes and millipedes are characteristically texrestr#l. Most are very shy animals, lurking in dark places and avoiding the light, but it is interesting to note that at least two Myriopods—Geophilus submarinus and Linotenia maritima—occur on British coasts. INSECTA, 325 MYRIOPODA CENTIPEDES. MILLIPEDEs. CHILOPODA, DipLopopa (or CHILOGNATHA). Carnivorous. Vegetarian. Harmless. popes: ody usually flat. One pair of appendages to each segment. The stigmata do not correspond in number to the segments; they often occur on alternate segments. Many-jointed antennz. Toothed cutting mandibles. | Two pairs of maxille, usually with palps. Body cylindrical. By the imperfect separation of the segments, all but the first three behind the head seem to have two pairs of appendages each, and also two paired ganglia, and two pairs of stigmata (tracheal openings). Seven-jointed antenne. . Broad masticating mandibles. A pair of maxille fused in a broad plate, usually four-lobed. No poison claws. | he first pair of legs modified: as poison claws. — ee A single genital aperture on the | second last segment. Examples. —Scolopendra, Lithobius, Geophilas. teriorly. Examples.— /zlus. Polyxenus. Glomeris. More Genital apertures open ap- i, In the order Symphyla (Scolofendrella) there are not more than twelve segments, and there is only one pair of trachez, which open on the head. Scolopendrella is in several ways like the primitive insects known as Thysanura. In the order Pauropoda (Fauropus), there are ten segments, and the antennz are branched. 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. 326 PHYLUM ARTHROPODA. GENERAL CHARACTERS Like other Arthropods, Insects have segmented Ladies, jointed __legs, chitinous armature, and a vertwat-chain of ganglia linked to a dorsal brain. Compared with Peripatus and Myrtopods, adult insects show concentration of the body segments, decs&éease in the number and ucrease in the quality of the appendages, and wings tn the great majority. SoS Insects are teryestrial and aerial, and xanely aguatie— animals ; usually winged as adults, breathing by means of Le aa RS Lia ZL Fic. 172.—-Female cockroach Fic. 173.—Male cockroach (P. orientalis). (P. ortentalds), trachea, and often with a metamorphosis tm the gourse gf their » life history. <> arpeed 43 FEMS Th, — ; Atiddebdgmen. The head bears a pair of gre-oral antenge, and th : endages ; 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; theabdomen has no appenda dimentary modifications of these be re- presented by stings, ovipositors, ett. aaa First Type of Insects, Periplaneta (or Blatta).— The Cockroacu Habits.—The cockroaches in Britain are immigrants from the East (P. orientalis), or from America (P. americana). COCKROACH. EXTERNAL CHARACTERS REGIon. APPENDAGES, OTHER STRUCTURES. The head is ver- tically elongated and separated from the thorax by a neck. The insect’s head seems to consist of seven fused segments— ocular,antennary, intercalary, man- dibular, maxillu- lar, maxillary, and labial. The thorax con- sists of three seg- ments— % prothorax, 4) mesothorax, Q metathorax. Each segment is bounded by a dorsal tergum and ventral ster- num.) The abdomen consists of 10 (or x1) distinct segments, with terga and sterna as in the thorax. The first sternum is rudimentary in both sexes, and in the female the eighth and ninth segments are con- cealed by the large seventh. 1. The antenna (probably homologous with appendages), long, slender, many-jointed, tactile. 2. A pair of stout toothed mandibles work- ing sideways. 3. The first maxille, each consisting— (a) of a basal piece or protopodite with two joints : a basal cardo, a distal stipes ; (2) 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 maxilla, 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 ears— (4) a double endopodite (ligula) consisting of an inner lacinia and an outer paraglossa ; (c) an exopodite or labial palp, consisting of three joints. (a) First pair of legs. (4) Second pair of legs. (c) 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 ina pair of claws (Fig. 175). 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. The large black compound | eyes. The “ upper lip” or labrum, in front of the mouth. The white oval patches near the bases of the antennz, pos- sibly sensory. In some primitive insects a minute pair of appendages, known as maxillulz, occurs be- | tween the mandibles and the first maxille. (4) Apair of wing-covers (modi- fied wings), rudimentary in female of P. orientalis. (c)A_ pair of membranous wings, sometimes used in flight, folded when not in use, absent in female of | P. orientalis. Between the segments of the thorax are two pairs of respira- tory 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 sperma- theca—the female’s receptacle for spermatozoa —lies on the ninth sternum of the abdomen. 328 PHYLUM ARTHROPODA, 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 same order as locusts and grasshoppers. The hatched young are like miniatures of the adults, except that wings are absent. If there are wings, they appear at the last moult, when the cockroach becomes sexually mature. Fic. 174.—Ventral aspect of male cockroach with the wings extended. An imaginary median line has been inserted. A., antenne; £., eye; P.7., prothorax; W1, first pair of wings ; W*, second pair of wings; C., cercus; St., style; Co., coxa; Tr., trochanter; #., femur; 772., tibia; 7a., tarsus. Skin.—There is an external chitinous cuticle and a subjacent cellular Jayer—the epidermis or hypodermis— from which the cuticle is formed. The newly hatched cockroaches are white, the adults are dark brown. Moulting, which involves a casting of the cuticle, of the internal lining of the trachez, etc., occurs some seven times before the cockroach attains in its fifth year to maturity. The muscles which move the appendages, and produce COCKROACH. 329 the abdominal movements essential to respiration, are markedly cross striped. They are in many cases attached to special tendons, which arise as cuticular invagina- tions, and are lost and re- placed at each moult. Nervous system.—A pair of supra-cesophageal or cere- bral ganglia lie united in the head. As a brain they receive impressions by antennary and optic nerves. By means of a paired commissure surround- ing the gullet, they are con- nected with a double ventral Fic. 175.—Leg of cockroach. ¢., Broad expanded coxa; ¢~, troch- anter; f, femur; ¢z., tibia; ¢a., six- jointed tarsus with terminal claws and adhesive cushions. chain of ten ganglia. Of these, the first or sub-cesopha- geal pair are large, and give off nerves to the mouth-parts, Fic. 176.—Moutn appendages of cock- roach.—After Dufour, I, Aln., mandibles ; II. first maxilla ; C., cardo ; Sz, stipes; Z., lacinia; G., galea; mx p., maxillary palp; III. second maxille or la- bium; S7., submentum; #., mentum; Z., lacinize ; Bo paraglossa 3 2, poy labial palp. etc. ; from each of the ganglia of the thorax and the abdomen nerves are given off 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 cesophageal commissures visceral nerves are given off to the gullet, crop, and gizzard. Besides the large compound eyes, there are other sensory structures —some of the setee on the skin, the maxilla (to some extent organs of oo the antennse (tactile and olfactory), the anal cerci (tactile and possibly the oval white patches on the head. 330 PHYLUM ARTHROPODA. Alimentary system.—(1) The fore-gut (stomodzum) 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 ; (0) the narrow gullet or cesophagus ; (c) the swollen crop; (@) 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 Fic. 177.—Transverse section of insect.—After Packard. ‘., Heart; g., gut; #., nerve-cord; sz., stigma; ¢r., trachea; w., wing; J, femur of leg. is short and narrow, and with its anterior end seven or eight club-shaped digestive (pancreatic) outgrowths are connected. (3) The hind-gut (proctodzum) is lined by a chitinous cuticle. It is convoluted and divided into narrow ileum, wider colon, and dilated rectum with six internal ridges. Respiratory system.—The tracheal tubes, which have ten pairs of lateral apertures or stigmata, ramify throughout the body, and have a spirally thickened chitinous lining. 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 COCKROACH. 338 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 been already mentioned. REPRODUCTIVE SYSTEM OF THE MALE. OF THE FEMALE. 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 ‘*mushroom - shaped gland”) open into the top of the ejacu- latory duct. This duct opens between the gth and ioth sterna. Beside the aperture, there are copulatory structures (gonapophyses). With the ejaculatory duct a gland is associated. Large branched tubular glands secrete a volatile alkaline sub- stance, with a strong mousy odour, probably offensive to enemies, 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. On the gth abdominal sternite a pair of arborescent glands pour out their cementing secretion by two apertures. The sper- matheca is a paired sac opening between the 8th and the gth abdominal sternum. Sixteen ova, one from each ovarian tube, are usually enclosed within each egg-capsule. The latter is formed from the partly calcareous secretion of the arborescent. 332 PHYLUM ARTHROPODA. 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. 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 mellifica) 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 4s, 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 larvee, and the older foraging bees, which gather food for the whole colony. In consideying the relation between the life of the Hive- Bee and that of many allied forms (Boméus, etc.), it is important to notice that the habit of laying up stores of food material for the winter enables the colony, and not BRITISH HIVE-BEE. 333 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 antennz, - which are composed of a long # basal and numerous smaller joints. They are marvellously sensitive, serving to communi- cate impressions, and also con- taining 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 maxille the maxillary palps are aborted, and the appendage con- sists of an undivided lamina at each side, borne on a basal piece consisting as usual of’ stipes and cardo. The second pair of maxillee form as usual the labium or so-called lower lip, and are much modified. The united basal joints form the mentum and sub-mentum. From the mentum at either side springs Fic. 178.—Head and mouth parts the long labial palp, which re- of bee.—After Cheshire. presents the outer fork of the a., Antenna; ., mandible; g., labrum or typical appendage. The endo- epipharynx; #7x.., rudiment of maxil- podite at each side is divided iy Pale gig apne tenes ies into two parts, but the inner two The parabltees ie couvecied berwees thie (lacinize) are united, much elon- basal portions of the labial palps and gated, and form the tongue or _ the ligula. ligula of the bee. The outer halves form the paraglossee, 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 334 PHYLUM ARTHROPODA. 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 antennz ; this is present in all three forms, and varies with the size of the antenne. 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. 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 employed 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 segment, 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. Ramify- ing through the abdomen are found the two slender coiled tubes which constitute 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. Nervous system,—In the adult this exhibits considerable BRITISH HIVE-BEE. 335 fusion of parts. The supra-cesophageal 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 two last pairs of legs. In the worker there are five Fic. 179.—Nervous system of bee.—After Cheshire. A, of larva, B, ofadult. a, Antenna; wx., maxilla; 7., mandible ; w., origin of wing ; 1-5, abdominal ganglia. pairs of abdominal ganglia, but in the queen and drone only four. The sense organs are the simple and compound eyes, and the antenne, which are furnished with numerous sensitive structures. Alimentary system.—The csophagus is a narrow tube which runs down the thoracic region. In the abdominal region it expands into the crop or honey-sac. The crop opens by a complicated orifice, with a remarkable stopper 336 PHYLUM ARTHROPODA. arrangement, into the digestive region or chyle stomach, which is separated by a pylorus from the coiled small intestine. The inner wall of the small intestine bears numerous rows of chitinous teeth set in longitudinal ridges, and is perforated by the apertures of the excretory tubules. At the junction of the small with the large intestine there are six brownish plates, perhaps functioning as valves. Inconnection with the anterior region of the gut there is a very complicated series of glands. First we have, in the workers only, on either side of the head, a long coiled gland which is intracellular in type. Itis largest in the so-called ‘‘ nurses” which feed the young, and diminishes in size later. According to Mr. Cheshire, this gland secretes a nitrogenous fluid which is fur- nished to all the larva in their early stages, but is supplied to the future queen during the whole of the feeding period, and also during the period of egg- laying ; this secretion was form- erly termed ‘‘royal jelly.” In addition to this pair of glands, there are in the worker three other gland systems. Of these, the second and third pairs have a common central outlet on the mentum, and secrete the saliva, which is plentifully mixed with the nectar during suction. The fourth pair is small, and the ducts open just within the mand- Fic. 180.—Food canal of bee,—In ible. The last three pairs of part after Cheshire. glands are found also in drone and queen. mx., Maxilla; @., antenna; ¢., eye; sg, salivary glands; o¢., cesophagus; 4.5., ‘ re honey-sac; s., stopper; c.s., chylifie | The method of feeding in stomach ; 7.2, Malpighian tubules; s.2., the bee di : small intestine; 42, large intestine; : e bee differs considerably st., sting. in the three types. In the ; worker, the honey sucked up from flowers is mixed with saliva, passes down the gullet into the crop, thence by the opening of the “stomach BRITISH HIVE-BEE. 337 mouth” it may reach the true stomach and so be digested, or may be carried in the crop to the hive, and there emptied into the cells by regurgitation. The pollen, which is frequently mixed with the honey, is separated from the latter by means of the stomach mouth, and is digested. Before impregnation, the queen, like the worker, feeds on pollen and honey; after it, she is always fed by the Fic. 181.—Hive-bees and the cells in which they develop. D., Drone cells; W., worker cells; Q., queen cell, open and closed ; ., drone; w., worker; g., queen. attendant workers. The drones, like the young workers, avail themselves of the general food-supply of the colony, and do not themselves collect honey. Other systems.—The respiratory system is represented by the ramifying tracheal tubes. They open to the ex- terior by the lateral spiracles, which can be completely closed. In connection with the trachee there are large air-sacs. The circulatory system is in essentials the same as that 2? 338 PHYLUM ARTHROPODA. of. the cockroach. The blood contains a few nucleated amceboid corpuscles. The excretory system consists of numerous fine Mal- pighian tubules which open into the small intestine. Reproductive system.—In the drone the reproductive organs consist of a pair of testes, each furnished with a narrow vas deferens, expanding at its distal end into a seminal vesicle. The seminal vesicles open into the ejacu- latory duct, and at their junction a large paired mucus gland opens. When maturity is reached, the testes diminish in size, while the spermatozoa accumulate in the terminal expanded part of the ejaculatory duct, and there become aggregated into a compact spermatophore. With the ter- minal portion of the male duct copulatory organs are associated. Mating takes place only once in the life of the queen, and is followed by the death of the drone. In the queen the large ovaries occupy considerable space in the abdominal region. As usual, each consists of numerous (100-150) ovarian tubes, containing ova in various stages of maturity. The ovarian tubes open into the right and left oviducts, which again unite to form the common oviduct. With the anterior portion of the common duct the globular spermatheca is associated. In connection with it there is a gland corresponding to the mucus gland of the male. The oviduct terminates in a copulatory pouch. Previous to laying, the eggs are fertilised by sperms set free from the spermatheca. In the case of drone eggs, this liberation of sper- matozoa does not take place, and the eggs in consequence are partheno- genetic. Queens which have never mated, or which have exhausted their stock of male elements, habitually lay drone eggs, but those which are laying abundant fertilised eggs at times also lay unfertilised eggs. This withholding of spermatozoa is said to be ‘‘ voluntary,” and related to the needs of the colony, but the physiological reason is unknown, The workers possess female organs similar in type to those of the queen, but of an extremely rudimentary nature. The eggs are laid singly in the cells of the comb, at the rate of about two per minute, for weeks together. They are of the usual insect type. According to the size of the cell in which it is deposited, and the food with which it is furnished, the fertilised ovum develops into a worker or into a queen. The development takes place within the cell, and includes a complete metamorphosis. GENERAL NOTES ON INSECTS. 339 CLASSIFICATION OF INSECTS I. Primitive wingless insects, Apterygota or Aptera, including Thysanura, e.g. Machilis, Campodea, Lepisma; Collembola, Springtails, e.g. Podura, Smynthurus. II. Winged insects, Pterygota (in some degenerate forms the wings have been lost). A. With mouth-parts usually adapted throughout life for biting (Menognathous), with no metamorphosis (Ametabolic) or with incomplete metamorphosis (Hemimetabolic). e.g. Orthoptera (cockroach, locust, cricket, etc.) ; Corrodentia (Termites, bird-lice); Odonata (Dragon-flies) ; Ephemerida (May-flies); and Dermaptera (Earwigs). B. With mouth-parts adapted in the main as suctorial organs (Menorhynchous), usually with no metamorphosis (Ametabolic). e.g. Rhynchota or Hemiptera, ¢.¢. Phylloxera, aphides, coccus insects; Cicadas; bugs; water-scor- pions, lice. C. With complete metamorphosis (Holometabolic), with mouth-parts always adapted for biting (Menognathous), or adapted at first for biting and afterwards for sucking (Metagnathous). e.g. Coleoptera (beetles); Diptera (two-winged flies) ; Lepidoptera (butterflies and moths); Hymen- optera (ants, bees, and wasps). GENERAL Notes ON INSECTS The main characteristics of insects have already been described in the two types chosen, but we here revise them in general terms. Form.—The body of an adult insect may be divided into three distinct regions :— 1. The head, probably consisting of seven fused segments. 2. The median thorax, divided into pro-, meso-, and meta-thoracic segments, each with » pair of legs, the last two often with wings. 3. The abdomen, usually with ten to eleven segments, withnever —inorethan.izansformed traces of appendages. Within these limits there is great variety of form, e.g. the long dragon-fly with its large outspread wings, the compact cockchafer, the thin-waisted wasps and long-bodied butterflies, the house-fly and cricket, the large moths and beetles, and the almost invisible insect parasites, 340 PHYLUM ARTHROPODA. Appendages.—Insects feel their way, test food, and apparently communicate impressions to one another, by means of the antenne. Then follow the mandibles, first A at SSOaS 4s Be $65 1 i q Fic, 182.—Mouth-parts of mosquito.—After Nuttall and Shipley. 4., labium ; 7.Z., maxillary palps; cé., clypeus; cs., head scales. a., Antennz ; Zre., labrum and epipharynx ; 7., mandibles ; 4., hypopharynx 3 #x., first maxillz ; maxillz, and second maxille, on the head; the three pairs of legs on the thorax; and sometimes vestiges of legs on the abdomen. GENERAL NOTES ON INSECTS. 341 It was a step of some importance in morphology when Savigny showed that the three pairs of anpendages about the mouth are homologous with the other appendages, z.¢. are masticatory legs. (1) Farthest forward lie two mandzb/es, the biting and cutting jaws. These are single-jointed, and thus differ from the organs of the same name in the crayfish, which bear a three-jointed palp in addition to the hard basal part. In those insects which suck and do not bite, e.g. adult butterflies, the mandibles are reduced. (2) Next in order is the first pazr of maxdllg. Each maxilla consists of a basal piece (protopodite), an inner fork (endopodite), and an outer fork (exopodite). The entomologists divide the protopodite into a lower joint, the cavdo, and an upper, the s¢zZes ; the endopodite into an internal Jacézza and an external galea; while the exopodite is called the maxzllary palp. (3) The last pair of oral appendages or second maxille are partially fused, and form what is called the /adcum. The lower and upper joints of their fused protopodites are called sadmentum and mentum ; the endopodites on each side are double, as in the first maxille, and consist of internal /acénza and external Zaraglossa ; the exopodites are called the labzal palps. The three pairs of thoracic legs consist of many joints, are usually clawed and hairy at their tips, and differ greatly according to their uses, as may be seen by comparing, for instance, the hairy feet by aid of which the fly runs up the smooth window-pane, the muscular limbs of grasshoppers, the lank length of those which characterise ‘‘ daddy-long- legs,” the bees’ legs with their pollen baskets, the oars of water-beetles. Wings.—These arise as flattened hollow sacs, which grow out from the two posterior segments of the thorax. They are moved by muscles, and traversed by “ veins” or ‘‘ nervures,” which include air-tubes, nerves, and vessel-like continuations of the body cavity. Most insects have two pairs, but many sluggish females and parasites, like lice and fleas, have lost them. On the other hand, there is no reason to believe that the very simplest wingless insects, known as Collembola and Thysanura, ever had wings. There are many interesting differences in regard to wings in the various orders of Insects. Thus in beetles the front pair form wing- covers or elytra; in the little bee parasites—Strepsiptera—they are twisted rudiments ; in flies the posterior pair are small knobbed stalks (halteres or balancers); in bees the wings on each side are hooked together. When the insect is at rest, the wings are usually folded neatly on the back ; but dragon-flies and others keep them expanded ; butter- flies raise them like a single sail on the back; moths keep them flat. Many wings bear small scales or hairs, and are often brightly coloured. It is well known that the colours also vary with sex, climate, and surroundings. Most interesting are those cases in which the colours of an insect harmonise exactly with those of its habitat, or make it a mimetic copy of some more successfully protected neighbour. 342 PHYLUM ARTHROPODA. As to the origin of wings, it may be mentioned that in many cases they are of some use in respiration as well as in locomotion, and the theory seems plausible that wings were originally respiratory outgrowths, which by and by became useful for aerial locomotion. New organs seem often to have arisen by the predominance of some new function in organs which had some prior significance. Moreover, we can fancy that an increase in respiratory efficiency brought about by the out- growths in question would quicken the whole life, and would tend to raise insects into the air, just as terrestrial insects can be made to frisk and jump when placed in a vessel with relatively more oxygen than there is in the atmosphere. Finally, we must note that the aquatic larve of some insects, e.g. may-flies, have a series of respiratory outgrowths from the sides of the abdomen, the so-called ‘‘ tracheal gills,” which in origin and appearance are like young wings (Fig. 183). Insects excel in locomotion. “They walk, run, and jump with the quadrupeds ; they fly with the birds ; they glide with the serpents, and they swim with the fish.” They beat the elastic air with their wings, and though there cannot be so much complexity of movement as in birds where the individual feathers move, the insect wing is no rigid plate, and its up-and-down / x motions are complex. They can Fic. 183.-Young may-fly soar rapidly, but their lightness ercphemsnds ice Eaton. often makes horizontal steering sno peatngahonta them difficult. The wind often helps as well as hinders them; thus the insects which fly in and out of the windows of express trains are probably in part sucked along. Marey calculates the approximate number of wing strokes per second at 330 for the fly, 240 for the humble-bee, 190 for the hive-bee, r10 for the wasp, 28 for the dragon-fly, 9 for a butterfly. For short distances a bee can outfly a pigeon. Skin.—As in other Arthropods, the epidermis (or hypo- dermis) of Insects forms a firm cuticle of chitin, which in the exigencies of growth has sometimes to be moulted. This cuticle is often finely marked, so that the animal seems iridescent ; and there are many different kinds of scales, oe GENERAL NOTES ON INSECTS. 343 hairs, and spines. Chitin is not favourable to the develop- ment of skin glands. Most insects have “salivary glands” opening in or near the mouth. Bees have wax-making glands opening on the abdomen; aphides have glandular tubes ; not a few have poison bags; and many larvz besides silkworms have organs from which are exuded the threads of which a cocoon is made. Muscular system.—In very active animals like Insects, we of course find a highly developed set of rapidly contract- ing striped muscles. These work the wings, the legs, and the jaws. The resulting movements have this further significance, that they help in the respiratory interchange of gases, and in the circulation of the blood. Nervous system.—As in other Arthropods, the nervous system consists—(a) of a dorsal brain or supra-cesophageal ganglionic mass, and (4) of a double ventral nerve-cord with a number of paired ganglia, of which the most anterior (the sub-cesophageal) are linked to the brain by a ring com- missure around the gullet; and (c) of nerves given off from the various ganglia to the sense organs, the alimentary canal, and the other organs. In many of the higher insects the ganglia of the ventral nerve-card are in some degree con- centrated, and in the adults are usually more centralised than in the larvee. Sensory structures.—Animals so much alive as Insects, and in surroundings so stimulating as many of them enjoy, have naturally highly-developed sense organs. Two compound eyes are present on the head of all adults except the primitive Collembola, the degenerate lice, the likewise parasitic fleas, and blind insects which live in caves or other dark places. Each eye contains a large number of similar elements, in each of which we can distinguish—(1) a cuticular or corneal facet; (2) a glassy lens-like portion ; (3) a retinal portion in association with which are fibres from the optic nerve; and there are also pigmented cells between the elements. In addition to the compound eyes, simple eyes or ocelli are present in the adults of many insects, e.g. ants, bees, and wasps; they occur without the accompaniment of com- pound eyes in Collembola, lice, and fleas, and they are usually the only eyes possessed by larve. They have only 344 PHLVYUM ARTHROPODA. one lens (monomeniscous), whereas the compound forms have many lenses (polymeniscous). In the simple eye each retinal unit is a single cell, of which the distal part is unpig- mented. In the compound eye the recinal unit consists of six cells around an axis. The stricture of ocelli varies greatly, and their use is very uncertain. Auditory (or chordotonal) organs have been found in all orders of Insects (except as yet the Thysanoptera), and occur both in the larve and in the adults. Their essential structure is as follows :—A nerve ends in a centre or ganglion near the skin; some of the cells of this ganglion " grow out into long sensitive rods enclosed in a tiny sheath ; the rods are directly or indirectly connected with the epidermis above them. ‘‘ They are found in groups of 2-200 in various parts of the body, antennz, palps, legs, wings, in the halteres of Diptera, and upon the dorsal aspect oftheabdomen.” Quite different from these, and occurring in flies alone, on the hind end of the larva, or at the base of the adult’s feelers, are little bags with fluid in which clear globules float. In addition to the ‘‘eyes” and ‘‘ears,” there are innervated hairs (tactile, tasting, olfactory) on the antenne and mouth-parts of many insects. Not a few insects seem to possess a diffuse or dermatoptic sense, by which, for instance, they can, when blinded, find their way out of a dark box. Many Insects produce sounds. We hear the whirr of rapidly moving wings in flies; the buzz of leaf-like structures near the openings of the air-tubes in many Hymenoptera ; the scraping of legs against wing ribs in grasshoppers; the chirping of male crickets, which rub one wing against its neighbour; the piping of male Cicadas, which have a complex musical instrument; the voice of the death’s-head moth, which expels air forcibly from its mouth. The death-watch taps with its head on wooden objects, as if knocking at the door behind which his mate may be hidden. In some cases the sounds are simply auto- matic reflexes of activity; in many cases they serve as alluring love calls ; and they may also serve as expressions of fear and anger, or as warning alarms. In the case of hive-bees there is definite evidence of a sense of direc- tion. They return straight to the hive from a distance of over a mile, even when they have been blinded and robbed of antenne, even when they have been carried afield in a box. Alimentary system.—The diet of Insects is very varied. Some, such as locusts, are vegetarian, and destroy our crops; others are carnivorous (we need not specify the homceopathist’s leech), and suck the blood of living victims, or devour the dead; the bees flit in search of nectar from flower to flower, while the ant-lion lurks in his pit of sand for any unwary stumbler; the termites gnaw decaying wood; some ants keep aphides as cows (‘“vacce formi- carum,” Linnzeus called them), whose sweet juices they GENERAL NOTES ON INSECTS. 345 lick; and a great number of larve devour the flesh and vegetables in which they are hatched. Many modifications of mouth organs, and of the ali- mentary canal, are associated with the way in which the insect feeds. The alimentary canal consists of fore-gut, mid-gut, and hind-gut, but in many cases it seems very doubtful if the mid-gut has its typically endodermic character. The fore-gut conducts food, and includes mouth cavity, pharynx, and cesophagus, the latter being often swollen into a storing crop, or continued into a muscular gizzard with grinding plates of chitin. The mid-gut is digestive and absorptive, often bearing a number of glandular outgrowths or ceca, and varies in length (in beetles at least) in inverse proportion to the nutritive and digestible quality of the food. The hind-gut is said to be partly absorptive, but is chiefly a conducting intestine, often coiled and terminally expanded into a rectum with which glands are frequently associated. In association with the alimentary canal are various glands :— (a) The salivary glands, which open in or near the mouth. They are usually paired on each side, and provided with a reservoir. They arise as invaginations of the ectoderm near the mouth. Their secretion is mainly diastatic in function, z.¢. it changes starchy material into sugar by means of a ferment. Along with these may be ranked the ‘spinning glands” of caterpillars, etc., which also open at the mouth. They secrete material which hardens into the threads used for the cocoon. (6) From the beginning of the mid-gut blind outgrowths sometimes arise (in some Orthoptera, etc.), which are apparently digestive. They are sometimes called pyloric caca. In other cases (some beetles) there may be more numerous and smaller glandular outgrowths resembling villi in appearance. Respiratory system.—The body of an insect is traversed by a system of air-tubes (trachez), which open laterally by special apertures (stigmata), and by means of numerous branches conduct the air to all the recesses of the tissues. In animals which breathe by gills or lungs the blood is -carried to the air; in insects the air permeates the whole body. But how does the air pass in and out? In part, no doubt, there is a slow diffusion ; in part the movements of the wings and legs will help; but there are also special 346 PHYLUM ARTHROPODA. expiratory muscles. We see their action when we watch a drone-fly panting on a flower. Inspiration is passive, as in birds, and depends on the elasticity of the skin and of the tracheal walls ; expiration is active, and depends upon these muscles. They are chiefly situated in the abdomen, but in some beetles (at least) they are also present in the metathorax. The tracheze seem to arise as tubular ingrowths of skin, and, primitively, each segment probably contained a distinct pair ; but their number has been reduced, and they are often in part connected into a system. With the doubtful excep- tion of one of the primitive Collembola, and the certain exception of caterpillars, no insects have any tracheal openings in the head region. There are rarely more than two pairs in the thorax; there are often six to eight pairs in the abdomen ; the maximum total is ten pairs. Each trachea is kept tense throughout the greater part of its course by internal chitinous thickenings, which apparently have a spirai course. The branches of the trachez penetrate into all the organs of the body, carrying oxygen to every part. The very efficient respiration of insects must be kept in mind in an appreciation of the general activity of their life. As the conditions of larval life are often different from those of the adult insects, the mode of respiration may also differ in details. In insects without marked metamorphosis, and even in some beetles in which the metamorphosis is complete, the young insect and the adult both breathe by tracheze with open stigmata. Both are said to be ‘“holopneustic.” When the larvee live in water, the tracheal system is closed, other- wise the creatures would drown. This closed condition is termed ‘“apneustic.” These larvee (of dragon-flies, may-flies, and some others) breathe by ‘‘ tracheal gills” (see Fig. 183)—little wing-like outgrowths from the sides of the abdomen, rich in trachese—or by tracheal folds within the rectum, in and out of which water flows. In either case, an interchange of gases between the tracheze and the water takes place. In adult aerial life the trachez of the body acquire stigmata, and the insect becomes ‘‘holopneustic.” In most insects with complete metamorphosis, the larva (e.g. cater- pillar or grub) has closed stigmata on the last two segments of the thorax (those which will bear wings), but there is a pair of open stigmata on the prothorax. In the adult the reverse is the case. There are some other modifications—for instance, what obtains in the parasitic larvee of some flies, e.g. gadflies. In these the stigmata are open only at the end of the body. In all cases, however, the stigmata of the adult are already present as rudiments in the larva, though they may not open till adolescence is over. CIRCULATORY SYSTEM. 34) Circulatory system.—As the respiratory system is very efficient, air passing into the inmost recesses of the body, it is natural that the blood-vascular system should not be highly developed. Within a dorsal part of the body cavity, known as the pericardium, the heart lies, swayed by special muscles. It is a long tube, usually confined to the abdomen, and with eight chambers, with paired valvular openings on its sides, through which blood enters from the pericardium. The blood is driven forwards, the posterior end of the heart being closed, and there is usually an anterior aorta or main blood vessel. But, for the most part, the blood circulates in spaces within what is commonly called the body cavity. Such a circulation is often described as lacunar. The blood may be colourless, yellow, red, or even greenish, and, in some cases, hemoglobin, the characteristic blood pigment of Vertebrates, has been detected. The cells of the blood are amoeboid. Body cavity.—It is necessary to distinguish the primitive ccelom from the apparent body cavity of the adult. In discussing the develop- ment of Peripatus, Sedgwick notes the following characteristics of a true coelom :—It isa cavity which—(1) does not communicate with the vascular system ; (2)does communicate by nephridial pores with the exterior ; (3) has the reproductive elements developed on its lining; (4) develops either as one or more diverticula from the primitive enteron (or gut), or as a space or spaces in the unsegmented or segmented mesoderm. Now, in Arthropods the apparent body cavity of the adult is not a true ccelom: it consists of a set of secondarily derived vascular spaces ; it has been called a pseudoccel or a heemoccel. The true ccelom of Arthropods is very much restricted in the adult. The apparent body cavity in which the organs lie, and in which the blood circulates, is well developed in Insects. It includes, zter alia, a peculiar fatty tissue, which seems to be a store of reserve material, which is especially large in young insects before metamorphosis, and is also interesting as one of the seats of ‘‘ phosphorescence.” Excretory system.—Although no structures certainly homologous with nephridia have yet been demonstrated in Insects, the excretory system is well developed. From the hind-gut (proctodzeum), and therefore of ectodermic origin, arise fine tubes, or in some cases solid threads, which extend into the apparent body cavity. Their number varies from two (in some Lepidoptera, for instance) to one hundred and fifty (in the bee). They twine about the organs in the abdominal cavity, and their excretory significance is inferred from the fact that they contain uric acid. g 348 PHYLUM ARTHROPODA. Reproductive system.—Among Insects the sexes are always separate and often different in appearance. The males are more active, smaller, and more brightly coloured than the females. Darwin referred the greater decorative- ness of the males to the sexual selection exercised by the females. The handsomer variations succeeded in courtship better than their rivals. Wallace referred the greater plain- ness of females to the elimination of the disadvantageously conspicuous in the course of natural selection. There may be truth in both views, but both require to be supplemented by the consideration, in part accepted by Wallace, that the “secondary sexual characters” of both sexes are the natural and necessary expressions of their respectively dominant constitutions. The organs consist of :— MALE. FEMALE. The paired testes, usually formed of many small tubes. Two ducts (vasa deferentia) con- ducting spermatozoa (perhaps in part comparab’e to neph- ridia). An unpaired terminal and ejacula- tory duct, paired and with two apertures in Ephemeridsonly ; sometimes formed by aunion of the vasa deferentia, sometimes by an external invagination meeting the vasa deferentia. From the vasa deferentia or from the ejaculatory duct, opens a paired or unpaired seminal vesicle for spermatozoa, Various accessory glands, whose secretion sometimes unites the spermatozoa into packets or spermatophores. Sometimes a copulatory penis. Often external hard pieces. The paired ovaries, usually formed ofmanysmall tubes(ovarioles). Two ducts (oviducts) conducting the ova (perhaps in part com- parable to nephridia). An unpaired terminal region or vagina, paired and with two apertures in Ephemerids; usually formed from an ex- ternal invagination meeting the united ends ofthe oviducts. Near or from the vagina, opens a receptaculum seminis for storing spermatozoa received froma male during copulation. Various accessory glands, ¢.g. those which secrete the material sur- rounding the eggs. Sometimesa special bursacopula- trix in the vagina. Often external hard pieces, e.g., ovipositor. SOME PECULIARITIES IN REPRODUCTION. 349 Some peculiarities in reproduction.—Many Insects, such as aphides, silk-moth, and queen-bee, are exceedingly prolific. The queen termite lays thousands of eggs, ‘‘at the rate of about sixty per minute” ! The store of spermatozoa received by the female, and kept within the receptaculum seminis, often lasts for a long time,—for two or three years in some queen-bees, Parthenogenesis, or the development of ova which are unfertilised, occurs normally, for a variable number of generations, in two Lepidop- tera and one beetle, in some coccus insects and aphides, and in certain saw-flies and gall-wasps. It occurs casually in the silk-moth and several other Lepidoptera, seasonally in aphides, in larval life in some flies (Afiastor, Chironomus), and partially or ‘‘ voluntarily ” when the queen- bee lays eggs which become drones. A few insects hatch their young within the body, or are “‘ viviparous.” This is the case with parthenogenetic summer aphides, a few flies, the little bee parasites Strepsiptera, a few beetles and cockroaches, Development of the ovum.—The tubes which compose the ovaries and lead into the oviducts begin as thin fila- ments, the ends of which are usually connected on each side. These thin filaments consist of indifferent germinal cells, all of them potential ova, and of mesodermic epithelial cells, which form the ovarian tubes, etc., and are connected anteriorly to the pericardial wall. But in most cases only a minority of these cells be- come ova, the others become nutritive cells which are absorbed by the ova, and follicle cells which line the walls of the ovarian tubes and help to furnish the egg- shells. There may be, indeed, ovarian tubes without nutritive cells (e.g. in Orthoptera), and then each tube is simply a bead-like row of ova, which become larger and larger as they recede from the thin terminal filaments and ap- proach the oviducts. In other cases the bead-like row consists of ova alternating with clumps of nutritive cells (e.g. in Hymenoptera and Lepidoptera). In other cases the nutritive cells mostly remain in the terminal region, but their products pass down to the receding ova. As there are numerous ovarian tubes in each ovary, and as the same process of oogenesis is going on in each, numerous eggs are ready for liberation at the same time, and are simultaneously discharged into the oviduct of each side. The eggs are large and contain much yolk. In relatively 350 PHYLUM ARTHROPODA, few cases yolk is almost absent, as, for example, in the summer eggs of the Aphides, which are hatched within the body, and in some forms where the young are endoparasitic. The ovum is surrounded by a vitelline membrane, and also by a firm chitinous shell, secreted by the follicular cells, which is often sculptured in a characteristic manner. ‘This shell is pierced by one or more minute holes (micropyles). Through a micropyle the spermatozoon finds entrance, Fic. 184. Diagrams of Insect embryo.—After Korschelt and Heider. A transverse section before the union of the amnion folds, and a longitudinal median section after the union of the folds. a., Anterior end of blastoderm; Z., posterior end of blastoderm; af., in the left-hand figure, the beginning of the amnion fold ; am., amnion; @.c., amniotic cavity; s., serosa; ¢c., ectoderm; 2., lower germinal layer; y., yolk. The amniotic cavity marks me ae ventral region of the embryo, so that the yolk mass lorsal, sometimes (as in the cockroach) after moving round and round the shell in varying orbits. The ripe egg usually consists of a central yolk-containing mass, sur- rounded by a thin sheath of protoplasm. As is usual in Arthropods, the segmentation is peripheral or centrolecithal. The central nucleus divides up into several nuclei, which, being united by protoplasmic cords, form fora time a central syncytium. Later, these nuclei emigrate into the peripheral protoplasm, which segments around them ; thus.a peripheral layer of similar epithelial cells is formed. Some of the nuclei DEVELOPMENT OF THE OVUM. 351 may be left behind in the central yolk to form the yolk nuclei, or, what is probably the more primitive condition, these are formed by subse- quent immigration from the blastoderm. The next process is the appearance of differentiation among the similar cells of the blastoderm. Over a special area—the ventral plate—(cf. Astacus) the cells increase in number and become cylindrical in shape ; over the rest of the egg the cells flatten out and become much thinner. In the middle of the ventral plate a slight groove is formed by rapid multiplication of the cylindrical cells. This represents the disguised gastrulation, the open roof of the groove being the much-elongated blastopore. The surrounding cylindrical cells unite over this open roof, the groove usually flattens out, and thus we have formed a two-layered germinal streak which spreads forwards and backwards over the egg, and early exhibits externally transverse division into segments. The upper layer is the ectoderm ; the lower includes the rudiments of both mesoderm and endoderm. Meanwhile another very important event has taken place. We saw that while the cells of the ventral plate increased in depth, the remain- ing cells flattened out laterally; at the point where the two kinds of cells unite, on either side of the ventral plate, a double fold arises. The two folds unite over the surface of the ventral plate, forming a mem- branous arch over it. The internal fold is called ‘‘ amniotic,” the outer ‘‘serous,” from their resemblance to the similar envelopes in the embryos of higher vertebrates. The folds take no direct part in the development of the embryo. We must now return to the germinal streak. The gastrula groove may persist as a tube after closure of the blastopore, but it is usually compressed by the ectoderm, or never exists as a distinct cavity. The greater part of the lower stratum of the germinal streak consists of mesoderm. This becomes divided into successive segments at each side, each containing a primitive coelomic cavity, perhaps continuous with the gastrula cavity. The endoderm arises as paired clusters of cells, found only at the anterior and posterior ends of the primitive streak. These clusters increase rapidly and form long endodermal streaks, which curve downwards so as to enclose the yolk. The streaks meet and fuse, first ventrally and later dorsally, thus constituting the mid-gut. The yolk nuclei previously mentioned have meanwhile increased rapidly, forming yolk cells which absorb the yolk. These cells are included in the endodermic mid-gut, and there break up. As the endoderm grows round the yolk, it is accompanied by a layer (splanchnic) of the mesoblast. Fore- and hind- gut are formed by invaginations which fuse with the mid-gut. In the later stages of development the primitive coelomic pouches lose their cross partitions, become filled with mesenchyme cells, and practically obliterated. The body cavity of the adult is formed by the appearance of lacunze amid the cells of the mesenchyme. The trachez arise as segmentally repeated invaginations of the ecto- derm. The openings of the invaginations form the stigmata. From the hind-gut arise the Malpighian tubules, which are therefore ecto- dermic. The development of the other organs is similar to that of the Crustacea. 352 PHYLUM ARTHROPODA. In summarising the development of Insecta, one must specially note the peripheral segmentation, the formation of the two-layered germinal streak, the presence of an over- arching blastodermic fold, the segmentation of the meso- derm, and the formation of the mid-gut by the union of endodermic bands. : Metamorphosis of Insects.—(1) In the lowest Insects, namely, in the old-fashioned wingless Thysanura and Collembola, the hatched young are miniatures of the adults. By gradual growth, and after several moultings, they attain adult size. Similarly, the newly hatched earwigs, young of cock- roaches and locusts, of lice, aphides, termites, and bugs, are very like the parents, except that they are sexually immature, and that there are no wings, which indeed are absent from some of the adults. These insects are called ametabolic, te. they have no marked change or metamorphosis. (2) In cicadas there are slight but most instructive differences between larve and adults. The adults live among herbage, the young on the ground, and the diversity of habit has associated differences of structure, as in the burrowing fore-legs of the larva. Moreover, the larva acquires the characters of an adult after a quiescent period of pupation. The differences between larva and adult are more striking in may-flies, dragon-flies, and the related Plecoptera (eg. ferla), for in these the larva are aquatic, with closed respiratory apertures, and with tracheal gills or folds, while the adults are winged and aerial, and breathe by open tracheze. These insects are called Lemimetabolic, z.e. they have a partial or incomplete metamorphosis. (3) Very different is the life history of all other sets of Insects—ant-lions, caddis-flies, flies, fleas, butterflies and moths, beetles, ants, and bees. From the egg there is hatched a larva (maggot, grub, or caterpillar), which lives a life very different from the adult, and is altogether unlike it in form. The larva feeds voraciously, grows, rests, and moults. Having accumulated a rich store of reserve material in its “fatty body,” it finally becomes for some Fic. 185.—Life histories of Insects. L., P., and A., larva, pupa, and adult respectively of water-beetle (Dytiscus marginalis); 1, p., @., larva,‘pupa, and adult of blue-bottle fly (Musca vomitoria); 1.1, 2.1, 4.1, larva, pupa, and adult of Cossus ligniperda, 23 384 PHYLUM ARTHROPODA. time quiescent, as a pupa, nymph, or chrysalis, often within the shelter of a cocoon. During this period there are great transformations ; wings bud out, appendages of the adult pattern are formed, reconstruction of other organs is effected. Finally, out of the pupal husk emerges a miniature winged insect of the adult or imago type. These insects are called holometabolic, i.e. they exhibit a complete metamorphosis. Two kinds of larvee occur among insects. (a) In many ametabolic and hemimetabolic forms the larva is somewhat like one of the lowly Thysanuran insects (Campodea), and is A 6 Fic. 186.—Life history of the silk-moth (Bombyx mor). A, caterpillar ; B, pupa; C, imago; the cocoon is cut open to show the pupa lying within. In the caterpillar note the three pairs of true legs in the anterior region, and the four pairs of pro-legs in the posterior region. therefore called campodeiform. It has the regions of the body well defined, three pairs of locomotor thoracic limbs, and mouth-parts adapted for suction. (4) The other type is worm-like or eruciform, ¢.g. the caterpillars of Lepidoptera (Fig. 186, A), with distinct head and limbs; the more modified grubs of bees, etc., with distinct head, but without limbs ; and the degenerate maggots of flies (Fig. 187, A), etc., not only limbless, but with an ill-defined head. A typical cater- pillar has a cylindrical body often “hairy,” a distinct hard head, simple eyes, small antennze, mouth-parts suited for biting, three pairs of jointed clawed thoracic limbs (corre- METAMORPHOSIS OF INSECTS. 355 sponding to those of the butterfly), and four ot five pairs of unjointed clasping abdominal “ pro-legs.” Other abdominal appendages are known on the larve of other insects, and even in the embryos of some whose larve are campodei- form. These facts make it likely that the primitive form had many legs. The larvee of Insects vary enormously in habit and in structure, and exhibit numerous adaptations to conditions of life very different from ‘hose of the parent. Thus caterpillars, which are usually plump and Fic. 187.—Development of blow-fly (Calliphora erythrocephala), —After Thompson Lowne. The lower figure (A) shows the adult larva (maggot). Note, as compared with the caterpillar, the absence of appendages, except those about the mouth; %., the large hooks connected with the maxille ; J/., pro-legs. The upper figure (B) shows the pronymph removed from the pupa- case. In the abdominal region the imaginal discs are shown ; 2., rudiments of legs; w., of wings. tense, so that a peck from a bird’s bill may cause them to bleed to death, even if no immediate destruction befall them, are protectively adapted in many different ways. Their colours are often changed in harmony with those of their surroundings; some palatable forms are saved by their superficial resemblance to those which are nauseous ; a few strike “ terrifying attitudes” ; while others are like pieces of plants. Internal metamorphosis.—In Insects with no marked metamorphosis, or with merely an incomplete one, the organs of the larve develop gradually into those of the adult. But in Insects with complete metamorphosis there 356 PHYLUM ARTHROPODA. is a marvellous internal reconstruction during the later larval, and especially during the quiescent pupal stage. The more specialised larval organs are disrupted, their débris being used in building new structures. In some cases, such as flies, phagocytes play a very important part in this metamorphosis; in other cases there is no true phagocytosis. Parts of larval organs which have not been highly specialised form the foundations of new adult structures. Of special importance are certain ingrowths of the. larval skin (the epi- or hypo- dermis) which form what are called “imaginal discs,” ze. embryonic or germinal areas, from which arise the wings, legs, etc., of the adult insect. The reconstruction is very thorough ; most of the musculature, much of the tracheal system, part of the mid-gut, etc., are gradually replaced by the corre- sponding organs of the adult. There is first a disruptive process of histolysis, and then a reconstructive process of histogenesis. Yet in most cases the disruption and replacement of organs is very gradual. CEcology.—The average insect is active, but between orders (¢.g. ants, bees, and wasps versus aphides, coccus insects, and bugs), between nearly related families, between the sexes (e.g. male and female cochineal insect), between caterpillar and pupa, we read the constantly recurrent antithesis between activity and passivity. The average length of life is short. Queen-bees of five years, queen-ants aged thirteen, are rare exceptions. In many cases death follows as the rapid nemesis of repro- duction. But though the adult life is often very short, the total life may be of considerable length, as in some Ephemerids, which in their adult life of winged love-making may be literally the flies of a day, while their aquatic larval stages may have lived for two years or more. The relation between the annual appearance of certain insects and that of the plants which they visit, the habits of hibernation in the adult or larval state, the occasional “dimorphism” between winter and summer broods of butterflies, should be noticed. The prolific multiplication of many insects may lead to local and periodic increase in their numbers, but great increase is limited by the food-supply and the weather, by GCOLOGY, 357 the warfare between insects of different kinds, by the numerous insects parasitic on others, by the appetite of higher animals,—fishes, frogs, ant-eaters, insectivores, and, above all, birds. There is a great variety of protective adaptation. The young of caddis-flies are partially masked by their external cases of pebbles and fragments of stem; many caterpillars and adult insects harmonise with the colour of their environ- ment; leaf-insects, ‘walking sticks,” moss-insects, scale- insects, have a precise resemblance to external objects which must often save them; a humming-bird moth may resemble a humming-bird; many palatable insects and larvee have a mimetic resemblance to others which are nauseous or otherwise little likely to be meddled with. Many insects may be saved by their hard chitinous armour, by their disgusting odour or taste, by their deterrent discharges of repulsive formic acid, etc., by simulation of death, by active resistance with effective weapons. Many flowers depend for cross-fertilisation upon insects, which carry the pollen from one to another. Many insects depend for food on the nectar and pollen of flowers. Thus many flowers and insects are mutually dependent. But many insects injure plants, and many plants exhibit structures which tend to save them from attack. On the other hand, there may be “partnerships” between insects and ‘plants—as in the “myrmecophilous” (ant - loving) plants, which shelter a bodyguard of ants, by whom they are saved from unwelcome visitors. And again, the formation of galls by some insects which lay their’ eggs in plants, and the insect-catching proclivities of some carnivorous plants, should be remembered. Most insects are terrestrial and aerial; the majority live in warm and temperate countries, but they are represented almost everywhere, even above the snow-line, in arctic regions, in caves. Even on the sea the Challenger explorers found the pelagic Halobates, a genus of bugs. The distribution of insects is mainly limited by food- supplies and climate, for their powers of flight are often great, and their opportunities of passive dispersal by the wind, floating logs, etc., are by no means slight. Many insects are more or less parasitic, either externally 358 PHYLUM ARTHROPODA. as adults, eg. fleas, lice, bird-lice, plant-lice, etc., or in- ternally as larve, eg. the maggots of bot-flies in sheep, and a great number of borers within plants. We need only mention Hessian-fly, phylloxera, Colorado beetle, weevils, locusts, to suggest many more which are of much economic importance as injurious insects. On the other hand, our indebtedness to hive-bee and silk-moth, to cochineal and lac insects, to those which destroy injurious insects, and to those which carry pollen from flower to flower, is obvious. Finally, we must at least mention that in ants, — Ly ra iy Vi Tos id ) a Fic. 188.—Mosquito,—After Nuttall and Shipley. wasps, and termites we find illustration of various grades of social life, and marvellous exhibitions of instinctive skill as well as some intelligence. INSECTS AND DISEASE As carriers of disease-germs insects play a very im- portant part. The réle of flies as mechanical distributors of anthrax, plague, and other bacterial diseases has been clearly proved. Besides carrying bacilli upon their bodies and leaving them on wounds or food, they also swallow germs, and subsequently deposit them in their excreta in similar situations. Undoubtedly, however, the most serious cases are those of the blood-sucking Diptera which act as PEDIGREE, 359 hosts as well as carriers of disease-producing parasites. The gnats or mosquitoes (Culicide) are perhaps the most important in this respect. Human malaria is conveyed by at least twelve different species of mosquito, of which those belonging to the genus Axopheles have the widest dis- tribution. Anopheles maculipennis occurs all over Europe, in many parts of Africa, North America, and India, and in all these countries it carries malaria (see Fig. 182). Proteosoma, the malaria parasite of birds, is carried by a Culex, a related genus. The unknown parasite of yellow fever is transmitted by the bite of another mosquito, Stegomyia fasciata, It occurs in all parts of the world between the parallels 40° N. and S. “It is a most vicious biter both by day and night, and breeds in small artificial collections of water, such as barrels, puddles, cisterns, and even in such small receptacles as sardine tins” (Theobald). Culex fatigans' and C. pipiens act as carriers of Filaria bancrofti or F. sanguinis hominis nocturna, the parasite of the human disease filariasis. The African Tsetse flies, Glossina palpalis (Fig. 53) and G. morsitans, convey the parasites (Trypanosomes) of sleeping sickness and Nagana respectively. The latter disease, which is com- municable to horses, cattle, goats, sheep, and other domesticated animals, is probably also conveyed by other species of Tsetse flies. In general, one may say that wild animals, which appear to be un- affected by the parasites which they contain, are the source of the fatal infection of new-comers. PEDIGREE Insects must have appeared relatively early, for remains of a cockroach-like form have been found even in Silurian strata. The higher forms with complete Fic. 189.—- Anurida metamorphosis appear much later (e.g. i date ga oe beetles in the Carboniferous ages); but it primitive wingless seems that the Palaeozoic insects were Collembola. 360 ORDERS OF INSECTS. (‘stsoydiourejaur ajaqdutoo & aavy sj9as -uI snov09 afem ayy) ‘aor, ‘suotdioos-rayem ‘sdnq fsepeoro £ sjoasut sno009 ‘sapryde ‘v4axoyhyg 8a ‘wsaggemapy 10 nzoysudy yy “S]]IS Teayoer] savy pure ‘19}eM Ul dal] SOUITJIWIOS VAIL] ayy ‘semarou Aueur yim ssurm Asseys yo sired omy, ‘udouay] ‘SOI PesuIM-aoel pue suoT-juy “v4azdo1nayy *re]idiayeo & ay]—earey ‘auou samy -ouIOS JO ‘SSUIM SnouvIquIsW moreu jo sited OM], ‘usoUDW, ‘sory-uoidi09g = ‘wzvguoung ‘orsnoude are pur ‘saseo yeteds uryjim szayem ut oat Ayyensn ‘ayzy-zreypidioyeo yeymouuos aie xarep ayy, «*Apeos Ajarer ‘Airey stApog ayy, “Suey OHI] PaplOoF toq £ sButm asoy ay] UeY] Josie] ATfensn ssutm pulpy “Usouey] ‘Sol-SIPpeg *v-4azdoy72.4. 7, “pare 4qqeqoad axe viaqdisdans soysered aaq ayy ay, “399F WITM Ayyeraues ‘asioarp Ada WAIVE = ‘aS UL}OU May papjoy ssurM puly ‘s1aA09-SUIM OJ PIYlpoul SSuUIM BOT . ‘USeJoyW Aparer *-udouaT ‘satjoag «= *v.zazfoazop *jossem ssapjooj—ealvyT ‘oniseredoyoq ‘sada punodwos ‘sSuIM ON “‘Sutosaid jo ramod yyIM osye yng ‘usejaT ‘seal ‘weaggeuvygp 10 vsaggouoyg?s “pray JOULISIp B NoYTM “oSseutr ssapjooy w A[Tensn—vare'] _SeaIeY,, 10 ,slaoue|eq,, 1oL1aysod pur ‘sum pappojun yuared -SUBI]IOMaUBOMI, “SUIIG Jo Jamod YIM sauITjawOs yng “-USeIaTAT ‘yeu ‘oSprum ‘Ag-pes ‘Ap-asnozy ‘sary pasurm-omy, "v4az¢77 ‘reypidiayeo e—eare'] ‘sBurm Ayeos “wrosiun jo sured om], “USeIOT, + ‘syjowl pue soigreyng ‘vazgopzdaT ‘sdsem owios ut ydeoxa ‘sqnis ssoy -JOoy are warey ‘sS8urm uaredsues moj YIM ATyensq ‘sayeys asay} UsaMyaq ostworduIOD Jo OS B JO “UdeIaTAL 10 “USOUSTL ‘o]0 ‘saty-aes ‘sary-y[es ‘sdsem ‘saaq ‘sjuy ‘v-razouaru dy oN *8 IPI 6 PPIO ‘OI EPIC) ‘II PIO ‘EI IIPIO, ‘€1 OPO ‘VI IPI SI IBplO ‘QI PIO *(stsoydiourejour ou *2‘z) VIOdVLANY ‘| ‘(snyeredde Suryons hq pooejdar smef Surytq ‘3‘7) VHLVNOVLA Bie) (swef Suriq yt skemye *9*z) VHLVNOONTI saute ‘(stsoydiowejaur ajapduioo YIM *9*z) VIO@VLI *O -(sux10} ayerauesap amos Surjdaoxa) sjasuy pesutm “VLODANALd | ‘(Buey] pue Janevig 103j8) SLOASN] 40 SHaaduO 361 ORDERS OF INSECTS. “xhGof ‘sueysuyy ‘outsedaT ‘vapoduin Ba ‘vanunshy ‘“snanyudus ‘oanpog 8a ‘siyeyBursdg ‘vzoguapjo7 "I PIO ‘e PIO “syoasu] ssopBuIAA sal ‘VLODAYALIV = ‘sdaoI0} JoMa\sog — “aSTMssox0 pue Aljeurpnyrsuoy yjoq papfoy anq ‘asiel sum pury “[eus sdurm ronajuy “Surq soy paydepe syred-yjnow, = -quiawy i ‘sSimieq ‘vcazdoucaay “sudo ynout Suriq savy pue ‘sq[ES [eoyoes} Aq ayyearq ‘197eM UTOAT[ are] “jUasqe Jo [Tews ssurm pury ‘oSre] Sumo, “pasn [axed pur ‘ayerauaSapyeymauiosynpeyo sjred-yjnow ‘qejowtwazy ‘salp-Aeyl ‘vpecamay dy “SP[OF JO ST[IS yeayoeny Aq ayyeoiq pure “rayem ut oat] earl ou, “SSUIM papfosun o31e] jo sured omy, *Suryiq 10; paydepe syed-yyjnoyy = -qujatutua zy ‘salp-uoseiq ‘vynu0pcC “WNPE ayy ur ysistad usyo yorya ‘syrs peayoen 4q oypearq pure ‘royem ur Sar] wary oy LL ‘auou 10 ssurm 31x] jo sured omy, “Suriq 10; paydepe syed-ymoy ‘quiowrwazy “wyAagd 3'a ‘n4azgoIa/q ‘Uasqe souuTjawos are sired YJog -xBIOY] ay} Jo Jsar 9U3 Wor JOUNSIP pue podojaaap A[Suorjs xeIoyjo1g *sI9A09-SUIM OV! peyrpour Jo “puryaq asoy} uey} r9WAY pue ra}I0Ys Aqensn ssuim JonaqUy “Burjtq soy poydepe sjred-ymoy = ‘quiawy cc FeO] Supe ,, .YOHs Sur “TBM ,, “Jayorso-aJour “Jaxr9 Ysnsoy yoeoIyIoD “sa ‘vsazgoyz10 *sado punodutoo ou aaey ao1-priq ayy, “Sut -JUBM Uayo sBury, “Suryiq 107 paydepe syred-qynoyy ‘quay ‘soyUlta} ‘SoT]-piig, Fa “wezuaposs07 “wrayshs snoarau payexjuaouos) "eyeutarys jo sired moj 10 as3y3 AtUQ ‘yuasqe io Areyuawmpns uayo ‘momen AIaA sSurjy ‘sueS30 ynow Tevtoyong = ‘qujauty ‘sGity 7 8-9 ‘vasaggouvsdy 7 ‘ayerauasap syoadsaz yeraaas ur are pue ‘sata punod “WOd OU aavy SUUIOF ONISeIed BYJ, ‘auOU IO SBUIM Jo sired OM L ‘Surosaid yySys 10y pue Suryons Joy paydepe are sjzed-yjnow ayy, ‘I JaPIO) ‘t PIO *€ rapIO “b IZPIO ‘$ 19pPIO ‘9 PIO) *L JapiO *(smel Suntq yuajstszed YIM +3:2) VHLVNOONT[ ‘(stsoydiourejout aja, wroout yy *9"2) - VIOGVLAWINAL] pue ‘(sisoydrowejau OU YIM *2"2) VIO@VLANY “V | *(sueBi0 [etIojons Juaysisiad YIM *2°2) VHONAHUYONT 362 PHYLUM ARTHROPODA. mostly generalised types, prophetic of rather than referable to the modern orders. As to the pedigree of insects, the wingless Collembola and Thysanura are doubtless primitive. In /votopteron, for instance, there are appendages on the first four seg- ments of the abdomen, and the genital apertures are paired. Similarly, Aceventomon is a little blind creature, without antenne, without cerci, without stigmata, with suctorial mouth-parts, with eleven abdominal segments, with a peculiar anal segment, with an unpaired genital aperture Fic. 190.—Acerentomon, a very primitive insect. H, Head; TH.1, TH.?, TH.%, terga of thoracic segments ; 7, 2, 7, the thoracic legs; 4.1, 4.2, 4.3, A.4, abdominal appendages; P.A., eighth abdominal tergum ; G., genital aperture ; AP.P., post-anal appendix, on the eleventh urosternite. For Acerentomon, Acerentulus, and Zosentomon (with stigmata) the special order Protura has been proposed. These and similar primitive forms lead us back to some of the less specialised Myriopods (e.g. Scolopendrella), back further to the level represented by Leripatus, which helps to link the Tracheate to the Annelid series. But though the primitive wingless insects, the simple types of Myriopods, and /erpatus, represent ascending steps in evolution, what the actual path has been we do not know. CHAPTER XV PHYLUM ARTHROPODA—(continued) Classes ARACHNOIDEA (Spiders, Scorpions, Mites, etc.) and PaL#ostTraca (King-crabs, Eurypterids, Trilobites) Tue class Arachnoidea is far from being a coherent unity. Its subdivisions are numerous and diverse, and a statement of general characters is consequently difficult. The anterior segments, about seven in number, are usually Jused into a cephalothorax, with six pairs of appendages. The most anterior of these 5 sible may be turned in front- of the mouth, but there are al antenne as in [nsects. The first two pairs of appendages (cheliceree and_pedipalps) generally have to do with seizing and holding the food ; the pthers are walking legs. But although six pairs occur in most, there may be more or less. but-not. always, without..appendages ; it may be segmented or unsegmented ; tt is generally distinct from, but may be fused to the cephalothorax. A plate-like internal skeleton, called the endosternite, ts often present. The elaborate compound eyes of Insects are notrepuescuted, the-eyes-being almost always spixation may be by tubular trachea, or..by lung: books (chambered trachee?), or by both, or cutaneous, and many would include the branchiate Paleostraca along with Arachnoidea. In the tracheate forms there are never more than four pairs of stigmata. Within all or some of the legs lie coxtaluglands,. 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, In most cases the newly hatched young are essentially like the adults—that 1s to say, there is no metamorphosis, 364 PHYLUM ARTHROPODA. Order 1. SCORPIONIDE 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 smail 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 backwards. 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 surrounded by fire or otherwise fatally threatened, but it has been answered that they do not sting themselves, that they could not if they would, and that, even if they could, the poison would have Fic. 191.—Scorpion. no effect! ch. Chelicere; Af. pedipalps; 0, The body is divided into— genital operculum; /., pectines; Ss. stigma of a lung-book on the (1) a cephalothorax or “ pro- seinen ey See AP BS soma’ of six gepments, 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 cuticle of chitin, and also an interesting internal piece of skeleton (the endosternite), partly chitinoid, but also SCORPIONS. 365 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 chelicerze 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-jointed, 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 sub-cesophageal 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 simpler than the median pair, and the type is in some ways inter- mediate between the simple eye and the compound eye. There is, as in ocelli, a single crystalline-lens-like portion, below which there are a few groups of retinal cells. Each group has five cells, and the outer part of each cell is pigmented. There is no crystalline cone. Scorpions seize small animals with their pedipalps, hold them close to the small mouth by their chelicerze, 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; 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 so-called Malpighian tubes of Bzthzs europeus are really the ducts of the liver. The cavity of the body 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 amceboid corpuscles and the respiratory pigment hemocyanin. An eight-chambered heart, within «a pericardium, lies along the back of the mesosoma. It gives off lateral arteries from the posterior end of each of its chambers, is continued backwards in a posterior aorta, and forwards in an anterior aorta. The latter supplies the head and divides into two branches, encircling the gullet and 366 PHYLUM ARTHROPODA. 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 lamellz or partitions. 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 miniatures of the adults, and adhere for some time’ after birth to the body of the mother. , In Euscorpio etalicus 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 Opisthophthalmus 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. In many ways, eg. in their appendages, endosternite, and coxal glands, the scorpions link the Arachnoids to the King-crabs, and thus to the Trilobites. Order 2. PSEUDOSCORPIONID&. ‘‘ Book-Scorpions,” eg. Chelifer, Chernes 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. There is a cephalothorax with six . pairs of appendages ; the chelicerze are minute and chelate, with | openings of spinning glands, the pedipalps like those of scorpions. The abdomen is broad, with ten to eleven segments. They breathe | by tubular trachez. Order 3. PEDIPALPI. ‘‘ Whip-Scorpions,” e.g. Zhelyphonus, Phrynus Small animals, found in warm countries. There is a cephalothorax with six pairs of appendages ; the abdomen is depressed, well-defined SPIDERS. 367 from the thorax, and has eleven to twelve segments. The chelicerz are simply clawed, but are poisonous ; the pedipalps are simply clawed or else truly chelate. The first pair of limbs are like antenne. Respiration is by two pairs of abdominal lung-sacs. In Thelyphonus there is a long terminal whip. Order 4. PHALANGID (or OPILIONINA). ‘‘ Harvest-men,” e.g. Phalangium The small, spider-like ‘‘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 constricted off from the unsegmented cephalothorax ; the chelicerze are chelate ; the pedipalps are like legs. Respiration is by tubular trachee. The harvest-men do not trouble us, but feed on small insects. Order 5. SOLPUGID or SOLIFUGA, ¢g. Galeodes or Solpuga Active, pugnacious, venomous, nocturnal animals, found in the wariner parts of the earth, The head and abdomen are distinct from the thorax. The thorax has three segments, the abdomen nine or ten. The chelicetz are large and chelate, the pedipalps like long‘legs. The respiration is by means of tubular tracheze. The presence of distinct segments on the thorax is remarkable. Several other small orders of Arachnids must be recognised, e.g. Palpigradi for a very interesting minute form, Kezenza, with the last two joints of the cephalothorax free, and with an abdomen of eleven segments ending in a long-jointed whip. Order 6. ARANEID. Spiders Spiders are found almost everywhere upon the earth, and a few are at home in fresh water, eg. Avgyronefa, and on the seashore, eg. Désis, Desidiopsis. Most of them live on the juices of insects, and many form webs in which their victims are snared. They may be divided, accord- ing to habit, into the wanderers who spin little, and the sedentary forms who spin much. - The body of a spider is very distinctly divided into two parts: the cephalothorax and the abdomen, connected 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 chelicerze or falces, whose terminal joint or fang 368 PHYLUM ARTHROPODA. bends down on the basal joint in ‘‘sub-chelate” fashion, and is per- forated 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. ae Four pairs of terminally clawed 7-jointed walking legs. The most anterior pair are much used as feelers. The spinnerets at the end of the abdomen are modified abdominal legs. Besides these the embryo has four pairs of abdominal appendages which abort. Fic. 192,—Garden spider. I., Female garden spider; II., end view of head of the same showing the simple eyes, the poison fangs (ch.), and the pedipalps (A.); III., pe end of body showing two pairs of spinnerets (s#.), with anus above. 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. 193), a condition comparable to that in crabs. There are two or three rows of simple eyes on the cephalothorax, whose focal distance is very short, spiders trusting most to their exquisite sense of touch, by which they discriminate SPIDERS. the various vibrations on a web line. 369 The senses of smell, hearing, and taste are also present, but little is known in regard .to the organs. Body cavity, endosternite, and coxal glands generally resemble 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 plates, and worked by muscles. From the small mid-gut arise five pairs of long ceca, a pair running forwards and a pair passing into the bases of each - pair of legs, and then back again. These ceca sometimes anastomose. Farther back the mid-gut gives off numerous digestive outgrowths, which fill a large part of the abdomen. Their secretion digests pro- teids. Terminally there is a large cloaca, and where the intestine joins this, four much- branchedexcretory Malpighian tubes are given off, which are said to be endodermal in origin. 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 Fic. 193.—Dissection. of AZygale from the ventral surface. —After Cuvier. 1, Chelicerze; 2, pedipalps cut short ; 3-6, walking legs; g.1, large thoracic ganglion; g.2, ganglion at base of abdomen; c.#., chambered trachez or lung-books—at the left side the anterior is cut open to show the lamellee (2.); 2., muscle of abdomen; stl and sz.2, stigmata of lung-books ; ov., ovary ; Sf., spinnerets. 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 AZygale (Fig. 193). two lung-books, and tubular trachez in addition. 24 In the vast majority (Dipneumones) there are The 370 PHYLUM ARTHROPODA. stigmata of the lung-books lie on the anterior ventral surface of the abdomen; the trachez open posteriorly near the spinnerets, or just behind the opening of the lung-books, or at both places. The spinnerets (4-6) lie just in front of the anus. They are movable and perforated by numerous (often many hundred) tubes or “spinning spools,” each of which is connected with a compressible gland secreting silk. There are various kinds of glands; both the amount and the nature of the secretion are under control. The spinnercts are transformed abdominal appendages (a new organ from an old—as is so often the case); and the glands are ectodermic invaginations. Many spiders have at the base of their spinnerets. a transverse surface or cribrel- um perforated by spinning tubes, and from this they comb out a peculiar curled silk with the help of a row of stiff bristles or calamistrum on each posterior leg. The males are usually Fic. 194 —Section of lung-book. smaller and often more “Hitter Map IRou brightly coloured than their d., Dorsal; v., ventral; 2., lamella; ., mates. From the paired posterior; @., anterior; d.c., dorsal 7 , chamber; x., posterior wall; s¢., testes, in the anterior part stemes fg one of the interlamellar of the abdomen, two vasa deferentia pass to a com- mon 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 recep- tacula 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 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. There is no metamorphosis ‘but SPIDERS. 371 spiders at birth are often very different in details from their later stages. Spinning.—Muscular compression of the 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, as has been mentioned, a special comb (calamistrum). One of the most important ways in which the secreted threads are used is in forming a web. The common garden spider (Zfezra) 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 ‘* yay,” which it fixes firmly; another and another is formed, all inter- secting in one centre. Thirdly, 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. Fourthly, 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 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 tiptoe 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. : The distribution of spiders, e.g. on islands, does not appear to be much affected by the absence of wings. Many young forms are aeronauts, and many are carried about by the wind apart from ballooning. Courtship.—The males are often much smaller than the females. The disproportion is sometimes stich as would be observed if a man 6 ft. high and 150 Ib. in weight were to marry a giantess 76-90 ft. high, 200,000 Ib. in weight. The smallness of the males may be due to the fact that they are males; others say 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. Wallace spoke of the brilliancy of males-as due to their greater vitality, and xeferred the relative plainness of the females to their greater need for protection. Darwin referred the greater decorative- ness of males to the fact that those which varied in this direction found 372 PHYLUM ARTHROPODA. 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. Inthe Zvolution 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 associated with the more fundamental qualities of maleness and femaleness. Classification of Spiders 1 Tetrapneumones or Mygalomorpha, with four lung-books and no trachez ; the fangs of the cheliceree move vertically, parallel to each other, e.g.— Mygale, a large lurking spider which has been known to kill small birds, but usually eats insects; Atypus, Cleniza, and others make neat trap-door nests. z. Dipneumones or Arachnomorpha, with two lung-books and trachece as well; the fangs of the chclicereze move somewhat horizontally toward each other. The web-spinners, eg. Hfezra; wolf-spiders, e.g. Lycosa, Tarantula, the latter with poisonous qualities which have been much exaggerated ; jumping spiders or Attide, e.g. Altus salticus. The common house spider is Tegenarza domestica; the commonest garden spider is petra diademata. Agyroneta aguatica fills an aquatic silken nest with bubbles of air 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 débris. The abdomen is fused with the cephalothorax, but there is sometimes a clear boundary line; both are unsegmented except in Of¢/oacarus, which has a segmented abdomen. 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 tracheze 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 tracheze. Cheese- mite (Zyroglyphus). Itch - mite (Fig. 196) (Savcoptes scabtec), causing “itch” in man; S. MITES AND TICKS. 373 canis, causing ‘‘mange” in dogs. Follicle-mite (Demodex Solliculorum), common in the hair follicles of man and domestic animals (Fig. 195). Gall-mites (Phytoptids), forming dimples and pouches on plants. (4 With tracheze. Harvest - mites (7rombidium), whose minute hexapod larvee are troublesome parasites in summer on ’ Fic. 195.—Follicle-mite Fic. 196.—Itch-mite (Sarcopies scabiet) (greatly enlarged). (greatly enlarged). insects, many mammals, and man. The so-called ‘red spider” (Zétvanychus teleartus) spins webs, and lives ' socially under leaves. Water-mites, e.g. Hydrachna on water-beetles, and Azar on gills of fresh-water mussels. Beetle-mites (Gamasus), often found on carrion beetles. There is a common red mite on the shore-rocks, known as Molgus (Bdella) littoralis. Ticks (Ixodidee, etc.) are the largest Acarina. They show a movable “‘capitulum ” bearing serrated cutting chelicerze and strong four-jointed pedipalps. They are responsible for spreading the germs of some diseases affecting man and beast, eg. human ‘‘tick-fever” on the 374 PHYLUM ARTHROPODA. Congo, spread by Ornithedoros moubata ; a spirocheet disease in, Diiry, borne by Argas reflexus and A. persicus ; Texas fever or “‘red wy ‘em in cattle, carried by Boophzlus annulatus. The common sheep, ic in Bi io \t | Fic, 197.—Tick (Jxodes riduvius, Fic. 198.--Tick (/avdes " LUTUS, female), dorsal surface, showing the female), ventral surface -\_Aftey oval shield (.S/.).—After Wheler. Wheler. an H., Hypopharynx ; P., palp; Z./., L.ZV. R., Rostrum; P., palp; Go cent first and fourth leg. mee Pe , , aperture ; ST. stigma ; ae . < Britain is /xodes ricinus. It may be noted that mites have been fourg inside human tumours, and there are many facts suggesting that some 5¢ the small Acarines may share in spreading disease germs. Eve, _Demodex may play its part. \ Aberrant Orders or Classes ( i Order LINGUATULIDA or PENTASTOMIDA, ¢.g. Pentastomum teniotdes This strange animal is parasitic in the nasal and frontal cavities, etc., ‘ of the dog and wolf. It is worm-like in form, externally ringed, : without any oral appendages, but with two pairs of movable hooks near ' the mouth. The muscles are striated. The alimentary canal is very ; simple, without Malpighian tubes. A narrow circumcesophageal nerve- ! ring, without a brain, is connected with a single ventral ganglion. There are no sense organs nor trachese, nor is there any heart. The sexes are separate ; the males smaller than the female. 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 or \ THE KING-CRAB. 375 lung. There they éncyst, moult, and undergo metamorphosis. The final larval 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. There is very little reason for ranking them along with Arachnoids. Order TARDIGRADA. Water-bears or Sloth-animalcules, eg. Macrobiotus Microscopic animals, sometimes found about the damp moss of swamps or even in the roof-gutters of houses. Some occur in fresh water, others in the sea. The unsegmented body is somewhat worm- like, with four pairs of unjointed clawed limbs like little stumps, with mouth-parts resembling those of some mites, and adapted for piercing and sucking. The muscles are unstriped. 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 Tardigrada, even as adults, have great powers of successfully resisting desiccation, but sometimes only the eggs do so, developing rapidly when favourable conditions return. There is very little reason for ranking them along with Arachnoids. Perhaps, as the seta-like ‘‘ claws” and the cirri of some types suggest, they are nearer to Annelids. ; ; Class PALHOSTRACA 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. The recently discovered antennz 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 1. XIPHOSURA There is one living genus, the King-crab or Horseshoe- crab (Limulus). 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 376 PHYLUM ARTHROPODA. body consists of a vaulted cephalothorax shaped like a horseshoe, and an almost hexagonal abdomen ending in a long spine. Burrowing in the sand, Zému/us 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. Fic. 199.—Lémudlus or King-crab. ch., Chelicerz ; of., operculum ; @., anus. 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 repre- sentative of an old race, 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 meso-and meta-soma; 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— (t) A little pair of three- jointed chelicerze in front of and bent towards the mouth. (2) A pair of pedipalps lateral to the mouth. (3-6) Four pairs of walking legs, the bases of which surround the mouth, and help in mastication. Be- hind these, still on the cephalothorax, there is a pair of small appendages called chilaria, Then follows on the abdomen a double ‘‘ operculum” with the genital apertures on its posterior surface, Under the operculum lie five pairs of flat plates bearing remark- able respiratory organs (‘‘gill-books”). These appendages show hints of the exopodite and endopodite structure character- istic of Crustaceans. Each ‘‘ gill-book” looks like a much-plaited gill, or like a book with EURVPTE RINA—TRILOBITA. 377 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 often compared to the leaves of the insunk lung- books of scorpions. Spawning occurs in the spring and summer months. The ova and spermatozoa are deposited in hollows near high-water mark. Some of the early stages of development present considerable resem- blance to corresponding stages in the ‘scorpion. In the larve, 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 Hemi- aspidee, e.g. Hemiaspis, Bélinurus. Order 2. EURYPTERINA (=Mero- stomata or Gigantostraca), eg. Lurypterus Large extinct forms found from Cambrian to Carboniferous strata. The body is divided into head, thorax, and abdomen. The head is small and unsegmented. The thorax is composed of six distinct segments, the abdomen of six with a terminal telson. On the head are borne six pairs of appendages .of varying shape, two lateral compound eyes, and two . median ocelli. On the ventral surface F1G. 200.—Young L2mulus.— of the thorax there are five pairs of After Walcott. gills covered by flat plates, of which the most anterior pair are very large, and form the so-called operculum (cf. Ldveulus), The surface of the body was covered with scales. Some of the Eurypterids reached a length of 6 ft. The oldest Merostomes are referred by Walcott to a sub-order Limulava somewhat divergent from other Eurypterids. This order is sometimes placed near the Crustacea, but the general opinion is that they are linked through Zzmzlus to Arachnoids. Order 3. TRiLopiTA. 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 three parts—the unsegmented head shield, often prolonged back- wards 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. 378 PHYLUM ARTHROPODA. Traces of limbs are only rarely preserved. In the head region there Fic. 201.—Trilobite (Conoceph- alttes).— After Barrande. h.s., Head shield ; 4/., pleura of thoracic region; Ay., pygidium. are four pairs, apparently simple. Antennz have been recently found in this region. The thorax and abdomen are furnished with biram- ose appendages, with long-jointed endopodite, short exopodite, and a gill (or epipodite ?) of varying shape. In the abdominal region the gills were perhaps rudimentary. Trilobites are often found rolled up in a way that reminds one of some wood-lice. So abundant are they in some rocks that even their development has been studied with some success. The limbs seem to be more like those of Crustaceans than those of Arachnoids, and the occurrence of antenne, observed by Linnzus (1759), and recently corroborated, accentuates the reseniblance. The affinities with Zzmzlus, according to the views of other authorities, justify the association of Trilobites and Arachnoids. A compromise may be perhaps effected by regard- ing the Trilobites as an offshoot from a stock ancestral to both Arachnoids and Crustaceans. Fic. 202,—Vertical cross-section of a Trilobite (Calymene). —After Walcott. z., Intestine; s., shield ; Z., endopodite; ¢., exopodite; 4., epipodial parts. Incerte Sedis Class PYCNOGONIDA, PANTOPODA, or PODOSOMATA Marine Arthropods, sometimes called sea-spiders. They may be ranked between Crustaceans and Arachnoids.. Many climb about PANTOPODA OR PYCNOGONIDA. 379 seaweeds and hydroids near the shore, but some live at great depths. The body consists of an anterior proboscis, cephalothoracic region with three fused and three free segments, and an unsegmented rudi- mentary abdomen. Four some- what primitive eyes on an anterior hillock, are nearer to the eyes of Arachnoids than to those of any other class. There are typically seven pairs of appendages. 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 walking legs. Into them, and Fic. 203.—Sea-spider (Pycnogonum littorale), from the dorsal surface. into the chelicerze when these are present, out-growths of the mid-gut extend. The sexes are separate» The larvee are at first unsegmented, with three pairs of appendages, Fic. 204.—Male of Nymphon.—After Sars. PR., Proboscis; CH., chelophores; P., pedipalps; Z., eggs carried on ovigerous legs; A., rudimentary abdomen. Examples.—Lycnogonum, Nymphon, A mmothea. In Pentanymphon and Decolopoda there is.an extra pair of long walking legs. CHAPTER XVI PHYLUM MOLLUSCA Classes :—1. GASTEROPODA, ¢.g. Snails, 2. SOLENOGASTRES—A small class of doubtful worm-like forms, e.g. Meomcenia, 3. SCAPHO- PoDA—A small class, e.g. Dentalium. 4. LAMELLIBRANCHIATA —Bivalves. 5. CEPHALOPODA—Cuttle-fiskes. THE series of Molluscs is in many ways contrasted with that of Arthropods; thus the body of the Mollusc is un- segmented, 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 Arthro- pods are active. In the frequent presence of a trochosphere larva, in the nerve-ring around the gullet, and in some other features, Molluscs resemble Annelids, but it is probable that they took their origin from a still lower level. GENERAL CHARACTERS Molluscs are unsegmented and without appendages. The symmetry ts fundamentally bilateral, but this is lost in most Gasteropods. The “foot”—a muscular protrusion of the ventral surface—ts 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 pallium (Fig. 205, ¢.), which often secretes a single or bilobed shell covering the viscera, and roofs in a space—the mantle cavity—within which lie the gills. But both mantle and shell may be absent. There are three chief pairs of ganglia—cere- brals, pedals, and pleurals—with connecting circum-esophageal commissures, and there ts also a visceral nervous system con- GENERAL CHARACTERS. 381 sisting typically of (a) a loop connecting the two pleurals and provided with two visceral ganglia, and (b) a stomato-gastrie ope f zl J? gab, Fic, 205.—Ideal mollusc.—After Ray Lankester. m., Mouth ; g.c., cerebral ganglia; c., edges of mantle skirt; z.g., duct of right lobe of digestive gland ; s., pericardial cavity ; /, edges of shell-sac; w., ventricle of heart; %., nephridium 3 az., anus ; #., posterior part of the foot ; 2., opening of nephridium ; &., genital aperture; g.ad., abdominal ganglion on visceral loop ; g.v., visceral ganglion ; 2.2., left lobe of digestive gland ; B., foot ; g.fe., pedal ganglion; g.A/. pleural ganglion. loop connecting the cerebrals below the gullet and provided with two buccal ganglia (Fig. 205). Lxcept in Lamelli- branchs, in which the head region is degenerate, there is in the Fic. 206.—Stages in molluscan development. D, Larva of Heteropod (after Gegenbaur); sk., shell covering visceral hump; v., velum; 7, foot. E, Larva of Atlanta (after Gegenbaur); v., velum; s%., shell; J, foot ; of., operculum. mouth a chitinous ritbon or radula, usually bearing numerous small teeth, and moved by special muscles, the whole structure - being known as the odontophore. There is much unstriped muscle, but the more rapidly contracting muscles have cross- 382 PHVLUM MOLLUSCA., striped fibres, or fibres with unstriped fibrils twisted in a spiral. A portion of the true body cavity or celom usually persists as the pericardium at least (Fig. 205, S.), and communicates with the exterior through the nephridium or nephridia. The rest of the cavity of the body is hemoceltc. The vascular system is almost always well developed, but part of the circulation ts in most cases lacunar, the heart typically consists of a ventricle and two auricles. Respiratory organs are most typically represented by gills or ctentdia, consisting of an axts attached to the body and bearing lamella, but the gills may have simpler forms, or may be absent, and in the terrestrial snails the mantle cavity 1s adapted for aerial respiration. 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 Trochosphere, which resembles the same stage in some Annelids, and the more characteristic Veliger (Fig. 206) ; but the development ts often direct. First Type of Motiusca. The Snail (/e/ix), one of the terrestrial (pulmonate) Gasteropods Habits.—The common garden snail (47. aspersa), or the larger edible snail (A fomatia), which is rare in England SONS AI AH 0 Fic. 207.—Roman snail (//elix pomatia). Note shell covering visceral hump; Z.a¢., pulmonary aperture (including anus and opening of ureter); 7, the foot; g.ap., genital aperture ; #., mouth ; ¢., eye on long horn; s.4., one of short horns. but abundant on the Continent, serves as a convenient type of this large genus of land-snails. They are thoroughly THE SNAIL. 383 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 mcuths 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. Fic. 208.—Vertical section of the shell of a species of Helix, : ™., Mouth of shell; A., apex; C., columella. 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 384 PHYLUM MOLLUSCA. 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 forwards to the right. But still farther 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 (conchin). The outermost layer is coloured, without 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 made, the inner walls of the coils form a central pillar (columella), as’ on a staircase, to which the animal is bound by a strong (columellar) muscle. Many Gasteropods bear on the foot a lid or operculum, of conchin or of lime, which closes the mouth of the shell. In Ae/¢x there is 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 the operculum to the foot of the whelk, and is loosened off in the mildness of spring. Sinistral shells, with left-handed spiral, occasionally occur as variations. The shell, held with its summit towards the observer, has its aperture to the left. The internal organs are inverted, and at the start there is a reversal of the cleavage planes of the egg. 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 contrasted—(a) with the thick collar of the mantle; () 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 MUSCULAR AND NERVOUS SYSTEMS. 385 (the collar) is fused to the body-wall. The result is to form a respiratory cavity, which is as much outside the body as is the gill-chamber of the crayfish. It is important to realise that the great rupture-like hump of viscera on the dorsal surface 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 the shell. Muscular system.—Among the important muscles are— (a) those of the foot; (4) those which retract the animal into its shell, and are in part attached to the columella ; (c) those which work the radula in the mouth; (d) the retractors of the horns; and (e) the retractor of the penis. The muscle fibres usually appear unstriated. There is much connective tissue, some of the cells of which contain glycogen, pigment, and lime. Nervous system.—This is concentrated in a ring around the gullet. Careful examination shows that this ring con- sists dorsally of a pair of cerebral ganglia, connected ventrally with a pair of pedals and a pair of pleuro-viscerals, which, . according to some authorities, have a median abdominal ganglion lying between them. The cerebrals give off nerves to the head, eg. to the mouth, tentacles, and otocysts, and also two nerves which run to small buccal ganglia, lying beneath the junction of gullet and buccal mass. The pedals give off nerves to the foot; the pleuro-viscerals to the mantle and posterior organs. Sense organs.—An eye, innervated from the brain, is situated on one side of the tip of each of the two long horns _It is a cup invaginated from the epidermis, lined posteriorly by a single layer of pigmented and 25 386 PHYLUM MOLLUSCA. non-pigmented retinal cells, filled with u clear vitreous body perhaps equivalent to a lens, closed in front by a transparent ‘‘cornea,” and strengthened all round by a firm “sclerotic.” How much a snail sees we do not know, but it detects quick movements. Though the eye is by no means very simple, the snail soon makes another if the original be lost, and this process of regeneration has been known to occur twenty times in succession. The otocysts appear as two small white spots on the pedal ganglia. Each is a sac of connective tissue, lined by epithelium which is said to be ciliated in one region, containing a fluid and a variable number of oval otoliths of lime, and innervated by a delicate nerve from the cere- bral ganglia. Though no osphradium or smelling-patch, comparable to that which occurs at the base of the gills in most Molluscs, has been discovered in Helix, the snail is repelled or attracted by odours; it shrinks from tur- pentine, it smells strawberries from afar. This sense of smell seems to be located in the horns, for a dishorned snail has none. The tips of both pairs of horns bear sensory cells connected with ganglionic tissue and nerve-fibres within. Other sensory cells, probably of use in tasting, lie on the lips; and there are many others, which may be called tactile, on the sides of the foot, and on various parts of the body. In short, the snail is diffusely sensitive. Alimentary system.—lIn cutting a piece of leaf, the snail uses two instruments—the crescentic jaw-plate on the roof of the mouth, and the toothed ribbon or radula on the floor. This radula is like a flexible file——a short and broad strip of membrane, bearing several longitudinal rows of minute chitinoid teeth. It rests on a cartilaginous pad on the floor of the mouth cavity, and is moved (backwards and forwards, and up and down) in a curve by protractor and retractor muscles. The whole apparatus, including teeth, mem- brane, and pad, is called the odontophore. The radula wears away anteriorly, but is added to posteriorly within a radula sac which projects from the floor of the buccal cavity. Its action on leaves may be compared very roughly to that of a file, but its movements within the mouth also produce a kind of suction which draws food particles inwards. In this suction the muscular lips and the cilia in the mouth cavity assist. The ducts of two large salivary glands open on the dorsal surface of the buccal cavity, and there are numerous distinct glandular cells close to the entrance of the two ducts. The salivary glands are large lobed structures, and extend far backward on the crop. They consist of hundreds VASCULAR SYSTEM. 387 of glandular cells or unicellular glands, which secrete a clear fluid. This travels up the ducts, and is forced, in part at least, by muscular compression, into the buccal cavity. While some say that this fluid converts starch into sugar (after the usual fashion of saliva), other authorities deny that it has any effect upon the food. Similar glands are found in all Gasteropods, while they are entirely absent in Lamellibranchs. In some boring Gasteropods the secretion contains 2-4 per cent. of free sulphuric acid. The gullet extends backward from the buccal cavity, and expands into a storing-crop; this is followed by a small stomach surrounded by the digestive gland; thence the intestine extends, and, after coiling in the visceral hump, passes forward to end on the right side anteriorly beside the respiratory aperture. The digestive tract is muscular, and in part ciliated internally. A large part of the visceral spiral is occupied by the so- called “‘liver.” This gland has two lobes, each of which opens by a duct into the stomach. The left lobe is again imperfectly divided into three. Besides producing juices which digest all kinds of food, the gland makes glycogen, stores phosphate of lime, and contains a greenish pigment. It is thus more than a “liver,” more even than a ‘‘hepato- pancreas,” it is a complex digestive gland, producing several digestive ferments.. The phosphate of lime may possibly be used to form the autumnal epiphragm. Vascular system.—The blood contains some colourless amoeboid cells, and a respiratory pigment called hzemo- cyanin, which gives the oxidised blood a blue tint, and is very common among Molluscs. The heart, with a ventricle and a single auricle, lies in a pericardial chamber on the dorsal surface, to the left side, behind the mantle cavity. The average number of pulsa- tions in Gasteropods is about one hundred per minute, but in the hibernating snail the beating is scarcely perceptible. From the ventricle: pure blood flows by cephalic and visceral arteries to the head, foot, and body, passes into fine ramifications of these arteries, and thence into spaces among the tissues. From these the blood is collected in larger venous spaces, and eventually in a pulmonary sinus around the mantle cavity, on the roof of which there is a 388 PHYLUM MOLLUSCA. network of vessels. There the blood is purified. Most of it returns directly to the auricle by a large pulmonary vein, but some passes first through the kidney. Respiratory system.—Most Gasteropods, ¢.g. the dog- whelk (Purpura), the buckie (Buccinum), the periwinkle (Littorina), breathe by gills covered by the mantle. The snail being entirely terrestrial, has a pulmonary or lung cavity, formed by the mantle fold. On the roof of this cavity the blood vessels are spread out. Air passes into and out of the pulmonary chamber by the respiratory aperture. When the animal is retracted within its shell, the freshening of the air in the pulmonary chamber takes place by slow diffusion, but when the snail extends itself at full length, the chamber is rapidly filled with air, and it is even more rapidly emptied when the body is withdrawn into the shell. Excretory system.—There is a single triangular greyish kidney behind the pulmonary chamber, between the heart and the rectum. It is a sac with plaited walls, and excretes nitrogenous waste products, which pass out by a long ureter running along the right side of the pulmonary chamber, and opening close beside the anus. There are two sources of blood supply to the kidney —(a) from the pulmonary chamber, and (4) from the heart by a renal artery. As in most other Molluscs, the kidney communicates by a small aperture with that part of the ccelom which forms the pericardial sac. Thus, as in earthworm, lobworm, etc., the coelom has a nephridial connection with the exterior. Reproductive system.—The snail is hermaphrodite, and its reproductive organs exhibit much division of labour. (2) The essential reproductive organ (the ovofestis) is a whitish body near the apex of the visceral spire. It consists of numerous cylindrical follicles, in each of which both ova and spermatozoa are formed, but not at the same time. (6) A much-convoluted hermaphrodite duct of a white colour conducts the sex cells from the ovotestis, and leads to the base of a large yellowish albumen gland. (c) This tongue-shaped albumen gland varies in size with the age and sexual state of the snail. It forms gelatinous proteid material, which envelops and probably nourishes the ova. (2) The ova and spermatozoa pass from the hermaphrodite REPRODUCTIVE SYSTEM. 389 duct towards the head along a common duct, but not at the same time. Moreover, their paths are different, for the portion of the duct down which the ova travel is much plaited, while the path which the spermatozoa follow is a Fic. 209.—Dissection of snail. T., Short horn; 77., long horn with eye; WV., cerebral ganglia; S.G., salivary glands on the crop; #., foot; M., columellar muscle; .C., visceral coil; O.7., ovotestis; V., ventricle of heart; R. rectum; U., ureter; B.V., blood vessels returning to the auricle from the mantle; A., pulmonary aperture ; J7A., edge of the mantle. less prominent groove, incompletely separated from the other. Both paths are glandular, and the glands on the male side are often called prostatic. (e) At the base of this common duct, a distinct vas deferens diverges to the left and leads into a muscular fezs, 390 PHYLUM MOLLUSCA. which can be protruded at the single genital aperture and retracted by a special muscle. Before the vas deferens enters the penis, a long process or flagel/um is given off. It is like the lash of a whip, and is as long as the common duct. Its secretion is used in forming a sperm-packet or spermatophore of a large number of spermatozoa, which are OT D.S Fic. 210.—Reproductive organs of Helix pomatia.— After Meisenheimer, O.T., Ovotestis ; H.D., hermaphrodite duct; 4.G., albumen gland; F.D., female side of common duct; J7.D., male side of common duct; O., oviduct; &.S., receptaculum seminis; JZ.G., mucus glands; D.3S., dart-sac; V.D., vas deferens; FL., flagellum; P., penis; AZ., retractor muscle of penis ; A/., genital aperture. compacted together at the time of sexual union partly in the flagellum, partly in the penis. Thé spermatophore is transferred by the penis into the genital aperture of another snail. (f) Continued from the oviducal side of the common duct, there is a separate ciliated ovéduct. This has a short course, and ends in the common genital aperture. Before REPRODUCTIVE SYSTEM. 391 it reaches this, however, the oviduct is associated with two structures. The first of these is a long process, as long as the common duct beside which it runs, in appearance suggesting the flagellum, but expanding at its free end into a globular sac—the veceptaculum seminis or spermatheca. In Helix aspersa a long slender diverticulum is given off from the duct of the receptaculum. This is also occasionally seen in Helix pomatia. A spermatophore from another Fic. 211.—Snail (He/zx pomatza) laying its eggs. — After Meisenheimer. snail passes into the receptaculum, and is there dissolved after some days, liberating hundreds of spermatozoa. By these spermatozoa the ova of the snail are fertilised. It seems likely that the place of fertilisation is in a small diverticulum at the upper end of the oviducal side of the common duct, whither the spermatozoa are said to find : their way. The second structure associated with the female duct is a conspicuous mucus gland, formed of two sets of finger-like processes. The secretion is very abundant during copulation, and as it contains not a little lime, it is possible that it may form the calcareous shells of the eggs. 392 PHYLUM MOLLUSCA. It seems to serve as a lubricant which facilitates the expulsion of a calcareous dart and the copulation. (g) Finally, between the entrance of oviduct and penis into the terminal aperture there lies a firm cylindrical structure, larger than the penis and with muscular walls. It is the Cupid’s Dart Sac, and contains a pointed calcareous arrow (spiculum amoris), which is jerked out previous to copulation. The dart is sometimes found adhering to the foot of a snail, and after copulation the sack is empty, soon, however, to be refilled. When two snails pair, the genital apertures are dilated, the protruded penis of one is inserted into the aperture of the other, and the spermatophore of each snail is trans- ferred to the recepta- culum of the other. The large eggs are laid in the earth in June and July. Each is sur- Fic. 212.—Diagram of larva of Pala. rounded by gelatinous dina.—After Erlanger. material acquired in the Ec., Ectoderm; £x., endoderm 5 v, velum, oviduct and by an elastic with cilia; g., gut-cavity; S.c., segmenta- tion cavity; c.Z., coelom pocket from gut; but calcareous shell. bi.g., blastopore groove closed, except at Segmentation is total 6l., which becomes the anus. The origin » of ‘the mesoderm from a gut-pocket has as but slightly unequal. AS yebooly Been ilespabed 1 in Paludina among the snail is a terrestrial Gasleropod, there is no trochosphere larva, nor more than a slight hint of the char- acteristic Molluscan velum. A miniature adult is hatched in about three weeks. The study of development may be more profitably followed in the pond-snail Zimncaus, where gastrula, trochosphere, and veliger can be readily seen. Second Type of Mottusca. The Fresh-water Mussel (Anodonta cygnea), one of the Lamellibranchiata Habit.—The fresh-water mussel lives in rivers and ponds. It lies with its head end buried in the mud, or moves slowly along by means of its ploughshare-like foot. Its food FRESH-WATER MUSSEL. 393 consists of minute plants and animals, which are wafted in at the posterior end by the currents produced by the cili- ated gills. What is noted here in regard to Anodonta will also apply, for the most part, to Unio and other fresh-water mussels. External appearance.—The bivalve is 4 to 6 in. long; its valves are equal and united in a dorsal hinge by an elastic ligament, an uncalcified part of the shell; on the ventral surface when the valves gape the foot protrudes ; the anterior end is rounded, the posterior end is more pointed, and it is there that the water currents flow in (ventrally) and out (dorsally). In bivalves the ligament is generally posterior to the dorsal knob or wmso—the oldest part of the shell—and the umbo generally points towards the anterior end. The greenish brown soft (“horny”) layer of the shell is often worn away near the umbo on each side, and then -displays the median layer of lime. This is called prismatic, since the lime salts are deposited in prisms, transversely varicose or striated, like those which form the enamel of our teeth. Internally there is a pearly layer. Lines of growth on the shell mark the position of the margin in former years, the newest part being obviously at the edge. The shell is a cuticular structure, ze. it is made by the epidermis of the mantle. It consists, as in the snail, of calcium carbonate plus conchiolin or conchin. Thus the composition of a Pinna shell is:—Lime salts, 89°2 ; organic matrix, 1°3; water, 9°5. Internal appearance.—When the right half of the shell is folded back, the anterior and posterior closing muscles having been carefully cut close to the gently raised valve, the mantle folds are seen lining the shell, and forming posteriorly the ventral inhalant and dorsal exhalant lips. The ventral lips have papillary processes. Internal to the mantle there are two gill-plates on each side; projecting from between these is the foot, muscular ventrally, softer dorsally ; the median dorsal pericardium is just beneath the ligament; the ventricle shines through its walls, and the dark-coloured kidneys are*seen through its floor. Below the anterior adductor muscle is the large mouth, bordered beneath by two lip processes (labial palps) on each side. pl. Vv Fic. 213.—The fresh-water mussel (U/20). The uppermost figure represents the bivalve in motion in the mud with protruded foot (/.) ; note inhalant and exhalant apertures, The middle figure shows the inside of the shell(left valve). The lower figure shows the outside (right valve). «#., The umbo; Z., the ligament; ¢.4., lateral téeth; @.a., anterior adductor mark ; a.7,, mark of protractor of the foot; 4.2. pallial line ; ~-@., posterior adductor mark ; 4,.~., mark of posterior retractor of the foot ;4g., a line of growth; 4., anterior (the blunter end); P., posterior; V., ventral. ar FRESH-WATER MUSSEL, 395 These resemble the gills in appearance, and are probably modified portions of the gills. The anus is above the posterior closing muscle. The whole space between the two mantle flaps is called the mantle cavity, and it is divided by a slight partition at the bases of the gills into a large ventral infra-branchial chamber and a small dorsal supra- branchial chamber which ends at the exhalant orifice. On the surface of the valves of the shell a few small pearls may be seen; they are formed by the enclosure of some minute grains of sand in the prismatic layer. There are two teeth in front of the umbo in Uzio, but not in Anodonta. The following muscles are inserted on the shell, and leave impressions :— (a) The anterior adductor. (4) The posterior adduetor. (c) The anterior retractor of the foot continues with (a). (d) The protractor of the foot a little below (a). (e) The posterior retractor of the foot continues with (4). As the shell grows, the insertion of the muscles and the attachment ot the mantle change, and the traces of this shifting are visible. Skin.—There is much ciliated epithelium about Azodonta, especially on the internal surface of the mantle, on the gills, and on the labial palps; and little pieces cut from an animal incompletely dead (e.g. from the oyster swallowed half-alive) have by means of their cilia a slight power of motion. The skin of the foot is not ciliated but glandular ; on the mantle edge sensitive and glandular cells are abund- ant, but usually in inverse ratio to one another. Muscular system.—The shell is closed and kept closed by the action of the two adductor muscles. When these are relaxed under nervous control, the elasticity of the hinge ligament opens the valves. The foot is a muscular protru- sion of the ventral surface, under the control of three muscles—a retractor and a protractor anteriorly, and a posterior retractor. Its upper portion 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 pre- vented 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, 306 PHYLUM MOLLUSCA. and leaves a furrow in its track. The muscle fibres, as in the snail, are mainly of the slowly contracting non-striped sort, but those of the adductor and of the heart show oblique cross-striping. In that part of the adductor muscle of Pecten (and some other bivalves) that effects the rapid closing of the valves, and hence the swimming, the muscle- fibres are transversely cross-striped, and the same is true of those found in the margin of the mobile mantle. There is here therefore a good instance of the connection between striation and rapidity of contraction and relaxation. Nervous system.—There are three pairs of nerve- centres :— : (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 (4) and (c), by long paired connect- ives, and giving off some nerves to mantle, palps, etc. (2) 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 cerebral ganglion. (c) Visceral 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” (osphradium) at the bases of the gills. Sense organs.—Unlike not a few bivalves, which have hundreds of “eyes” on the mantle margin, Azodonta 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, ete. 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, which ALIMENTARY SYSTEM. 397 waft food into the mouth. 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 a paired digestive gland. Part of the food digested by Fic. 214.—Structure of Azodonta.—After Rankin. a.a., Anterior adductor; ¢..g., cerebro-pleural ganglia; sz, stomach; v., ventricle, with an auricle opening into it; 4, kidney, above which is the posterior retractor of the foot; vy, rectum ending above posterior adductor; wg., visceral ganglia with connectives (in black) from cerebro-pleurals ; ¢., gut coiling in foot ; Z.g., pedal ganglia in foot, where also are seen branches of the anterior aorta and the reproductive organs ; 2.g., labial palps behind mouth. At the posterior end the ex- halant (upper) and inhalant (lower) apertures are seen. ~ these juices in the stomach is compacted in autumn into a “ crystalline style ””—a mass of reserve foodstuffs, and similar but less solid material is found in the intestine. On this supply the mussel tides over the winter. The intestine, which has in part a folded wall like that of the earthworm, coils about in the foot, ascends to the pericardium, passes 398 PHYLUM MOLLUSCA, 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 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- 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 larve (a reproductive function). The water may pass ¢hrough 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 respiration. In many cases, e.g. Lutraria elliptica, the posterior end of the mantle gives origin to a contractile respiratory siphon, a REPRODUCTIVE ORGANS. 309 double tube, the upper half of which is expiratory and the lower half inspiratory. A cross-section shows a cuticular investment of conchin, a layer of epidermis, a narrow zone of circular muscle-fibres, a thick zone of longitudinal muscle- fibres, a narrow zone of circular muscle-fibres, an internal epithelium, and the two canals. The white circular muscle- fibres are unstriped; the longitudinal muscle-fibres, which are greyish yellow, show a lozenge-shaped marking as in the more opaque fibres of the adductor muscles. 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 ; (4) 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 ; (Z) 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; (ce) this mode of origin, and the much leéss 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 of united and reflected gill filaments. On the gills there are often parasitic mites (Uzonccola or Atax ypsilophorus). 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 400 PHYLUM MOLLUSCA. 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 larva 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 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 spermatozoa 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 larve feed for some time on mucus secreted by the gill. Development and life history.—The development of Asodonta differs in certain details from that of most bivalves, perhaps in adapta- 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 spermatozoon enters. Segmentation is total but unequal. A number of small clear yolkless cells are rapidly divided off from a large yolk-containing portion, which is slower in dividing. Eventually a hollow ball of cells or blastosphere results (Fig. 215). 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. 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. 215). The shell-sac forms an embryonic shell, and many of the mesoderm 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 sed 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 DEVELOPMENT AND LIFE HISTORY. 401 sensory 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” Fic. 215.—Development of Azodonta.—After Goette. x. Section of blastosphere. s.d., Shell gland; ¢.d., ciliated disc; e., beginning of ectodermic invagination, Note mesoderm cells in the cavity. 2. Later stage. 2., Mesoderm. 3. Embryonic shell has appeared. 4. Glochidium larva; note byssus threads, and teeth on shell valves. and the embryonic ‘‘ byssus glands” vanish, but a true 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. : 26 402 PHYLUM MOLLUSCA. Third Type of Mortusca. 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, Seféa 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 conditions and internal emotions; and a plentiful discharge of ink Fic. 216.—Side view of Sesza.—After Jatta. often covers, its retreat from an enemy. Its food includes fish, other molluscs, and crabs. In spring the female attaches her encapsuled eggs to seaweeds 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 Seféa measures about to in. in length and 4 to 5§ in breadth; the body, fringed 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; CUTTLEFISA. 403 within the mantle cavity lie the anus and the openings of the nephridia and genital duct. : The true orientation of the different regions in Sedza is not obvious. If the “arms” surrounding the mouth be divided portions of the anterior 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 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 postertor 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 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 -contracts when these relax. It is probable that these chromatophore 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. Many of the muscles show double oblique striping. The animal 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. 404 PHYLUM MOLLUSCA. 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 bya strange double hook-and-eye arrange- ment; 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. The suckers on the arms are muscular cups, borne’ on little stalks (unstalked in Octopus, etc.) well innervated, and able to grip with a tenacity which in giantcuttlefishis dangerous even to men. The inner edge of the cup margin is supported 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 attach- ment from slipping. It seems likely that the arms represent a_ pro- podium, and the siphon a mesopodium, and a Fic. 217.—External appearance of Valve within the siphon a cuttlefish (Zo/igo), 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 over the eyes, enclosing the “‘ears”; (4) 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. NERVOUS SYSTEM. 405 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 ysed 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 Sepa 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, foilowed 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. 218), 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 off nerves to the mouth and lips, and is connected also with an ‘‘ infra- pharyngeal” ganglion. The cerebral ganglia are also connected by 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. 406 PHYLUM MOLLUSCA. 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. is (2) Ten nerves to the ‘‘arms” are given off by the pedal ganglion, and this is one of the reasons which have led most morph- ologists 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 posterdor 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 silvery 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 ‘‘ olfactory 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, Apart from sight altogether, an octopus can find a dead fish at a distance of over a yard in a few minutes, and even slight movements in the water are detected. In many Decapods there are luminous organs, usually on the ventral surface in diverse positions, and often buried. They may serve as recognition-marks or as search-lights. They may be glandular or ALIMENTARY SYSTEM. 407 non-glandular, and those of the second type are often somewhat eye-like, with pigment layer, reflector, lens, and diaphragm, or with some of these structures. . Alimentary system.—The cuttlefish eats food which 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 p-Z narrow gullet passes through the Ch ganglionic mass, and leads into st, the globular stomach, lying near 4 the dorsal end of the body. The stomach is followed by a cecum or pyloric sac, and the intestine curves headwards again, to end far forward in the mantle cavity. There do not seem to be any glands on the walls of the food g canal; the stomach has a hard = § cuticle ; the digestion which takes place there must therefore be due to the digestive juices of the glandular annexes. Of these the Fic. 218.—Diagram of the structure of Sepza.— Mainly most important is usually called the liver; it is bilobed, and lies in front of the stomach, attached to the cesophagus. Its two ducts conduct the digestive juice to the region where the stomach, “pyloric sac,” and intestine meet; and these ducts are fringed by numerous vascular and glandular appendages, which are called “ pancreatic,” after Pelseneer. a., Eight short arms around mouth ; Z.a., one of the two long arms; b., beak of the mouth; c.g., cere- bral ganglia, with commissures to the others; £., eye; 2, gullet; ag., digestive gland (the “salivary glands” are not repre- sented); sz., stomach; @., anus} she, shell-sac with sepiostaire } &, kidney; &., reproductive organ; 4r.., branchial heart ; fy a zill; 2.6., ink-bag; 712.¢., mantle cavity; “, funnel. and arise from the wall of the unpaired portion of the nephridia. 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 408 PHYLUM MOLLUSCA. Cephalopods, but that of Octopus has a poisonous, paralys- ing 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 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 papillee. ; Vascular system.—The blood of Sega is bluish, owing to the presence of hemocyanin in the serum; the blood cells are colourless and ameeboid. 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 enigmatical 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. 409 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 two papillz 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 appendices into the nephridial sacs. Fic, 219.—Diagram of circulatory and excretory systems in a Decapod-like Sefza.—After Pelseneer. x, 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” ; 11, glandular appendix of branchial heart; 12, renal appendices of branchial vein; 13, external aperture of kidney; 14, vena cava; 15, anterior aorta; 16, bifurcation of vena cava; 17, reno-pericardial aperture, E 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 the body cavity. It surrounds the heart and other organs, and is often called the viscero-pericardial cavity. Through the kidneys or nephridial sacs it is in communication with the exterior. Associated with the branchial hearts there are numerous diminutive cells which contain ammoniacal salts, phosphates, pigment, etc. ; these waste products are probably passed into the blood and got rid off by the kidneys, just as, in 2 Vertebrate, the urea formed in the liver passes by the blood to the kidneys. In Invertebrates there is often this co-operation between ‘‘closed. kidneys” and ‘‘ open kidneys.” 410 PHYLUM MOLLUSCA. Reproductive system.—The sexes are separate, but there is not much external difference 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 modification on his fifth left arm. The essential 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- 7 Phore is like an automatically explosive bomb; within the transparent shell there lies a bag of spermatozoa, and a complex spring-like arrangement. Even on the scalpel or slide these strange but efficient bombs will explode. The liberated spermatozoa are of the usual type. 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 accessory 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 en es pass from the genital duct of the male to the fifth left arm, which becomes’ covered with them and quaintly modified. This modification of one of the arms is usual among cuttlefish; indeed, Fic. 220.— Male of Argo- in some, e.g. Argonazita and Trem- nauta (after Jatta), show- octopus, the modified arm, with its ing ‘‘hectocotylus” arm; load of spermatozoa, is discharged compare Fig. 9 of female. bodily into the mantle cavity of the GENERAL NOTES ON MOLLUSCS. 4it female. There its discoverers described it’ as a parasitic worm, “* Hfectocotylus.” 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. Fic, 221.—Bunch of Sefza eggs attached to plant.—After Jatta. 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 Mo.Luuscs From the description of these three types a general idea of the structure of Mollusca may be obtained, but it should be noted—(r) that all the three types are specialised ; (2) that two small classes, the Solenogastres and the Scaphopoda, are unrepresented in the descriptions; (3) that in the three classes to which the types belong there is much diversity of structure, 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 412 PHYLUM MOLLUSCA. 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. Best developed in Gasteropods and Cephalopods, the head region may elsewhere be represented, as in Dentalium, 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 Fic. 222.—Common buckie (Buccénum undatum), e., Eye; s., respiratory siphon ; ¢., operculum ; 7, foot. condition, in C/zton (Fig. 228). This condition is retained in many Gasteropods, and in the simplest Lamellibranchs, like Solenomya. In most Lamellibranchs, however, in adaptation to a more or less passive life in the sand, the foot became wedge-shaped, and the characteristic byssus gland, which secretes attaching threads, is developed. In the Cephalopods the foot became greatly modified, and in those related to Sefza 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 emp‘oyed 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 Phy//irhoé among Gasteropods, in GENERAL NOTES ON MOLLUSCS. 413 the sedentary oyster among Lamellibranchs ; in the pelagic Pteropods part of it forms lateral wing-like lobes used in swimming, while in Janthina, which has a similar habit, its chief use is to secrete a “float” to which the egg-capsules are attached. In various Lamellibranchs, and in Dextalium, itis 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 Fic. 223.—Bivalve (Panopea norvegica), showing siphons. e., Exhalant aperture ; z., inhalant aperture. 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 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 forms the pulmonary chamber, which opens to the exterior 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. 414 PHYLUM MOLLUSCA. Typically the Mollusca are bilaterally symmetrical animals, and this symmetry is marked in the Solenogastres, the Lamellibranchiata, and occurs to a less extent in the Cephalopoda (cf. the unpaired genital organs). In most Gasteropoda it is completely lost. This seems to be in some way associated with the dorsal displacement of the viscera in Gasteropods to form the (usually coiled) visceral hump. In Cephalopods there is a somewhat similar dis- Fic. 224.—Nudibranch (Dendronotus arborescens), showing dorsal outgrowths forming adaptive gills. 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 internal organs of Mollusca must be inferred from the description of the types, but the nature of the respiratory organs may be briefly noted. Typically, these consist of two feathery gills, or ctenidia, with an axis attached to the body and bearing a double row of lamellae. These are sheltered beneath the mantle, and bear at their bases two osphradia or smelling patches. Gills of this typical form occur in Cuttles (4Vawéi/us has four), in the simplest Gasteropods (but many other Gasteropods have a simple unpaired gill), and in the lowest Lamellibranchs (Solenomya, Nucula, etc.). The respiratory organs in other Mollusca show much diversity when compared with this primitive type. Thus the gills may be totally suppressed and the mantle may directly take on a respiratory function. This occurs in many marine Gasteropods, for example, in GENERAL NOTES ON MOLLUSCS. 415 the common limpet (fate//a) (Fig. 225), 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 respira- tion, the gills being perhaps of most importance in connec- tion 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. 224). The osphradia are absent in Cephalopods, except in Nautilus, and one at least is usually suppressed in Gas- teropods. Shell.—On the dorsal surface of almost every mollusc em- bryo there is a little shell-sac in which an embryonic shell is begun; the adult shell, how- ever, is always started and increased by the mantle. Like other cuticular products, it has an organic basis (conchiolin or - deed , conchin), along | with which Pie, ant carbonate of lime is associated. 3,4 Hatley. There is a thin outer “horny 7 Note simple eyes at base of tentacles, layer, a, thick median “pris- ~ mouth, median foot, and vascular matic” see oi a Seale. mantle replacing the an internal mother-of-pear layer, which may be divided into two strata by a clear intermediate layer, well seen in the fresh-water mussel, Margaritana margaritifera. My. 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 shell-making is expressible in a chemical reaction of this simplicity, but it 416 PHYLUM MOLLUSCA. 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. Pearls are formed in sacs of the external epithelium of the mantle, sometimes around a centre of a periostracum-like substance, sometimes around the larva of a Trematode or Cestode. They are to be dis- tinguished from concretions formed around an intruded irritant particle. The latter do not show the characteristic lamination of pearls. Some pearl-like structures are fixed to the shell; true pearls are free. While some investigators insist on the parasitic origin of pearls, others are equally emphatic in declaring that they may arise independently. But all are agreed that they are pathological products. Larve.—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. 206). 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 /e/ix, partly because it is terrestrial; and not in Sega, partly because the eggs are rich in yolk. CLASSIFICATION OF MOLLUSCA Leaving aside the difficult Solenogastres, which may not be Molluscs at all, we may rank as lowest the Isopleura, bilaterally symmetrical Gasteropods with many primitive characters. Some of these forms, like Chiton, are probably not far removed from the primitive Mollusca. From primitive forms, related perhaps to Chzfon, Mollusca have diverged in two directions. In Gasteropoda, Scaphopoda, and Cephalopods, the head region becomes well developed, and the radula present in the primitive Isopleura is retained, except in rare cases, such as one of the species of Hudéma, a semi-parasite. These three classes are therefore often placed together as Glossophora or Odontophora, in contrast to the Lamellibranchiata (Lipocephala or Acephala), GASTEROPODA. 417. where the radula has disappeared, and the head region remains un- developed. As already seen, however, the lowest Lamellibranchs have a flattened creeping foot and simple feathery gills, in these respects resembling Gasteropods. There is also much reason to believe that the’ Scaphopota arose from a stem common to them and the lowest Gastero- pods, 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. Class I. GASTEROPODA Molluscs with a usually well-developed head region with tentacles and odontophore. The foot is usually a flat median sole on which the animal creeps; it ts often divided into pro-, meso-, and meta-podium. Most are unsymmetrical, but there ts a primitive bilateral symmetry in. Isopleura and a secondary superficial bilateral symmetry in some pelagic forms such as Fleteropods. The manile or covering of the visceral sac usually Jorms a well-marked fold or flap where the visceral sac joins the head and foot, and thus encloses a mantle cavity. In most cases the shell ts a single piece; in Chitons there are eight pieces; in many cases the shell ts rudimentary or absent. There is usually a trochosphere and veliger larva, except in terrestrial forms. Sub-class I. GasTEROPODA ISOPLEURA The [sopleura are marine Gasteropods more or less elongated in form, with bilateral symmetry. The symmetry ts not only seen in the form of the body, but in the numerous ctenidia, the paired nephridia, auricles, and genital ducts. The shell con- sists of eight pieces. The mouth ts anterior ; the anal and” nephridial apertures are posterior. The mantle, 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 (pedal and visceral), with ganglionic cells but at most very slightly developed ganglia, which run the whole length of the body. Of these paired cords the pedals are connected by numerous cross-commissures, and the viscerals or pallials are united posteriorly by a commissure above the rectum. The bilateral symmetry ts shown internally, 27 418 PHYLUM MOLLUSCA. eg. Fic. 226.—Chiton,— After Prétre, Fic. 227.—Dorsal view of nervous system of Acanthochiton.—After Pelseneer. x, Upper buccal commissure ; 2, upper buccal ganglion ; 3, stomatogastric commis- sure; 4, labial commissure; 5, sub-radular ganglia; 6, anterior pedal commissure } 7, pedal nerve with pallio- pedal connections; 8, supra-rectal pallial com- missure ; 9, pallial nerve; 10, anastomosis of branches ofpedalnerves; r1,stomato- gastric ganglia; 12, ceso- phageal nerves; 13,cerebral ; cominissure, in the paired nephridia, auricles, and genital ducts. The class, is of ancient origin, dating from the Silurian, There is one order—Polyplacophora, ©. Chiton. - The Isopleura or Polyplacophora are represented on British coasts by several species of Chzton, sluggish, usually vege- tarian, animals, occurring 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. 226), perforated, in many cases at least, “by numerous sensory organs, which are 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 gangli- onic 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 numer- ous and 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 pos- terior, and consists of a ventricle and two to eight auricles. There are two symmetrical nephridia opening posteriorly, and consisting of much-branched tubes, The sexes are separate; a single repro- ductive organ extends dorsally between gut and aorta 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 re- tained for some time by the female be- tween the mantle and the gills. The .segmentation is holoblastic, and a gastrula is formed by invagination. GASTEROPODA. 419 / -_ UTR Fic. 228.—Anatomy of Chzton. A, ventral surface (after Cuvier), B, dorsal view of alimentary canal (after Lankester). C, genital and excretory organs from dorsal surface (after Lang and Haller, diagrammatic). 7., mouth ;a@., anus ; 6v., numerous simple gills ; 7, foot ; 4., buccal mass ; 4., liver 3 z., intestine ; ao., aorta; v., ventricle of heart ; ya and Za., right and left auricles ; ov., ovary ; od., oviduct ; od'., opening of oviduct ; ., part of nephridium, represented in black throughout ; ~o., external opening of nephridium; Z., outline of pericardium. Sub-class II. GasTEROPODA ANISOPLEURA, 6.2. Snail, Whelk, Limpet In these more or less asymmetrical Gasteropods, the head region, which is well developed, remains symmetrical, and so does the foot, which ts typically a flat creeping organ. But the visceral mass_or hump, with its mantle fold, is more or less twisted forwards and to the right. Thus the pallial, anal, nephridial, and genttal 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 ts usually lost. Similarly, one of the nephridia, probably that which is morphologically the left, tends to disappear, and in most cases only one persists— topographically on the left side. The main torsion must be distinguished from the spiral twisting which the visceral hump often exhibits, and from the frequently associated spiral coiling of the untvalve shell, Moreover, a superficial secondary bilateral symmetry tends to be acquired by free-swimming Jorms, e.g. Heteropods. “There are never more than two gills 420 PHYLUM MOLLUSCA. of the ctenidium type. The shell is usually in one piece ; but it is sometimes rudimentary or absent. The foot usually contains a mucus gland, and tends to be divided into three regions—the pro-, meso-, and meta-podium. There ts a singl: reproductive organ and genital duct. Branch A. STREPTONEURA In the torsion of the body one limb of the visceral loop crosses the other in a figure 8. Order 1, ZYGOBRANCHIATA The atrophy of the primitively left-side gills and nephridia is not carried out, or only partially, e.g. Halzotds (ear-shell) ; Azsszzella (key- hole limpet) ; Pated/a (limpet). Order 2, AZYGOBRANCHIATA The originally left gill and the originally left nephridium have been lost. Heart with single auricle, one gill, one nephridium ; operculum present. : Fa Periwinkle (Lzttorina), buckie (Buccznum, Fig. 222), dog-whelk (Purpura), Ianthina, and the majority of the marine Gasteropods with coiled shells, together with some fresh-water forms. The pelagic Heteropods are also included here :—Ad¢/anéa, shell well developed ; Cardnaria, -with small shell; P/ezo- trachea, with no shell. Branch B. EUTHYNEURA The visceral loop does not share in the torsion of the visceral hump. Order 3. OPISTHOBRANCHIATA The visceral loop is euthy- neural, 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 rudi- mentary; there is Fic. 229.—A_ Ptleropod (Cymbulia a well-developed perontt), showing the wing-like expan- mantle fold and sions (pteropodial lobes) of the mid-foot. a single gill, e.g. MODE OF LIFE. 421 Bulla, Aplysta, 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 bilateral symmetry, and- swim by two large lateral lobes of the foot. They often swim actively in shoals, and occur in all seas. They afford tood 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 related to Bulla and its allies. Examples.—Hyalea, Cymbulia. (4) Gymnosomata, without mantle fold or shell in the adult. Closely allied to Aflys¢a and its allies. Actively carnivorous, ¢.g. Clio, Pueumoderma, B. Nudibranchiata. Shell, mantle fold, and true gill are absent ; various forms of ‘‘ adaptive gills” may be present, or there may be no special respiratory organs, ¢.g. sea-slugs, Dords, Lolis, Dendronotus (Fig. 224). Order 4. PULMONATA The visceral loop is short and untwisted, gills are absent, and the mantle cavity functions as a lung ; all are hermaphrodite, e.g. the snail (Helix); the grey slug (Zzmax) ; the black slug (dréon) ; fresh-water snails, such as Lemnea, Planorbis, and Ancylus. 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 Cephalopods, 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 open-sea, the gliding of fresh-water snails (Zzmuca) 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 (/a¢e//a), 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. 422 PHYLUM MOLLUSCA. Of this myriad, about gooo 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 Fic. 230, —Stages in mol- luscan development. A., Blastula of limpet (after Patten). £, Gastrula_ of Paludina vivipara (after Tonniges); v., beginning of velum; a@vc., archenteron ; m., mesoderm cells. C, later stage of the same; w, velum; 7#., mouth inva- gination; a7c., _archen- teron; a., anus; f, begin- ning of foot; sh.g., shell gland. —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 unnotched shell mouth, feed on plants, ¢g. the seaweed-eating peri- winkles (Litforina). Most land snails and slugs are vegetarian. Many Gasteropods, both marine and terrestrial, are voracious and indis- criminate in their meals; others are as markedly specialists or epicures. Some marine forms partial to Echino- derms have a salivary secretion of dilute sulphuric acid, which changes the carbonate of lime in the starfish into the more brittle and readily pulverised sulphate. About ten genera are parasitic on or in Echino- derms, e.g. Stylifer, Turtonia, Thyca, and the extremely degenerate Ex/o- concha, within the Holothurian Syz- afta. Some species of Lu/ima also live a semi-parasitic life on certain Echinoderms. Life history.—The eggs of Gasteropods are usually small, without much yolk, but surrounded by a jelly, the surface of which often hardens. is an egg-shell of lime. In the snail and some others there LIFE HISTORY—@GCOLOGY. 423 Sexual union occurs between hermaphrodites as well as between separate sexes, and fertilisation is effected inside the genital duct. Development sometimes proceeds within the parent, but in most cases the fertilised eggs are laid in gelatinous clumps, or within special capsules. The free- swimming Janthina 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 vases or : 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 maturity,—those that get the start eating the others as they develop. The development is usually simple and iynical 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. 230). 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 increasé. Most of the Paleozoic 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 Paleozoic Agee, and the terrestrial'air-breathers — are comparatively modern. 3 CEcology.— As voracious animals, with irresistible raspers, Gasteropods commit many atrocities in the struggle for existence, 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. mer) 424 PHYLUM MOLLUSCA. Class II. SOLENOGASTRES The members of this class are worm-like animals, in which the mantle envelops the whole body and bears numerous spicules, but no shell. It is somewhat doubtful if they are Molluscs at all. There are two families—Neomeniidze and Cheetodermidee. Of Neomeniide, six genera are known, e.g. Meomenia and Pro- neomenta, They have a longitudinal pedal groove, an intestine without distinct digestive gland, two neph- cbc wea ridia with a common aperture, and ae ip hermaphrodite reproductive organs. VY, The Cheetodermidz, represented by EF one genus Chetoderma, are cylin- ry drical in form, without a pedal groove, rae ot OO with a radula bearing one tooth, with 1 a distinct digestive gland, and with TY, two nephridia opening separately into rT a posterior cavity, which also contains rt: two gills. The sexes are separate. —1-T mA Class III. ScaPHOPODA a a Very different in many respects from 7 26. Gasteropoda are the Scaph opoda, of ma which Deztalzum (Elephant’s tooth- Esl > shell) is the commonest genus. They | g are apparently related to the Zeugo- me branchiate Gasteropods, and also to fo the simplest Bivalves. They burrow wa we in the sand at considerable depth off wz the coasts of many countries. The a w p es mantle has originally two folds, which fuse ventrally, and the shell becomes cylindrical, like an elephant’s tusk. Fic. 231.—Proncomenia, Ner- It is open at both ends. The larger vous system.—From Hubrecht. OPEnns (directed downwards in the es Coccbeal arigtist sie gout fueueli sand) is anterior, the concave side of a'p.g., anterior pedal; ",A.g., pos- the shell is dorsal. The mouth opens terior pedal; A.z.g., posterior, vis- at the end of a short buccal tube, cerals ; sZ., sublingual connectives; at the base of which is a circle of ees ,,cerebro-pedal connective; #e. ciliated tentacles. The foot is long ongitudinal pedal nerves ; /a., long- ~. . 2 ftadinal lateral nerves: with three small terminal lobes. It is used in slow creeping, and is pro- truded 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 animals. There isa much reduced heart, and colourless blood circulates in the body cavity. There are two nephridial apertures, one LAMELLIBRANCHIATA. 425 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.—Dentalium, Entalium. About forty widely distributed species are known. Dentalium entale occurs off British coasts. The genus occurs as a fossil from Devonian strata onward. Class IV. LAMELLIBRANCHIATA or BIVALVES (Synxonyms—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 move or less ploughshare-like. The head (or prostomium) region remains undeveloped, and without tentacles ; radula, horny jaws, and salivary glands are absent, but there ts a pair of labial palps on each side of the mouth. The mantle skirt ts divided into two flaps, which secrete the two valves of the shell, now lateral instead of dorsal in position. The values 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 (hence the title Lamellibranch). There are usually three pairs of ganglia: (a) cerebropleurals 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 coelomic 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 fresh water ; the general habit ts sedentary or sluggish. 426 PHYLUM MOLLUYSCA.: Classification.—That of Pelseneer is based on the structure of the gills. Order 1. PROTORRANCHIA.—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. Mucula, Solenontya. Order 2. FILIBRANCHIA.—The gill filaments are greatly elongated and reflected, so that they consist of an ascending and a descending limb, e.g. Arca (Noah’s-ark shell), AZy¢zdus (edible mussel), AZodzo/a (horse- mussel). Order 3. PsEUpO-LLAMELLIBRANCHIA.—The successive gill filaments are loosely connected together to form gill-plates/ ¢.g. Pecten (scallop), Ostrea (oyster). Order 4. EULAMELLIBRANCHIA.—The separate filaments are no longer discernible ; the gills form double flattened plates. The great majority of Bivalves are included here, eg. Anodonta, Venus, Pholas (a boring form), JZya. GENERAL NoTES ON LAMELLIBRANCHS Structure.—The organs which show most variety in. bivalves are the foot, the gills, the adductor muscles, and the mantle skirt. The foot shows much diversity 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 (J/y2z/us). The gills show a 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 Avxodonta. These processes are 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. 223). These siphons sometimes attain a considerable length ; they occur especially in forms such as AZya, which live buried in sand or mud, or which burrow in wood or stone, e.g. Pholas._ The diversities in the adductor muscles afford one basis for classification. We may associate with the sluggish habits and sedentary 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 anterior eyes, biting or rasping organs) with the conditions of life. In some Lamellibranchs, ¢.g. Mytilidee, small eyes occur at the base of the most anterior filament of the inner gill-plate; 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 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 (AZytz/us 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 GENERAL NOTES ON LAMELLIBRANCHS. 427, slowly up from estuary to river, from river to lake; Dvrezssensia poly- morpha 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 Cyc/as ; (4) on the other hand, it is more probable that the fresh-water mussels (U/7zo, Anodonta, etc.) are relics of a fauna which inhabited former inland seas, of which some lakes are the freshened residues. Between the active Zzma 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 (Cardizum) uses its bent foot to take small jumps on the sand ; the razor-fish (Soe) not only bores in the sand, but may swim backwards by squirting out water from within the mantle cavity ; many (e.g. Leredo, 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 Algze, Infusorians and other Protozoa, minute Crustaceans and organic particles, which the cilia of the gills and palps sweep towards the mouth. The bivalves are them- selves eaten by worms, starfishes, gasteropods, fishes, birds, and even mammals. Several commensal bivalves (Montacutidz) are known,—Montacuta on heart-urchins, Z7zova/va in the gullet of Synaptids, Sczoderetza on a sea-urchin, and Jousseaumdella on a Sipunculid. Life history.—The eggs are sometimes laid in the water, either freely or in attached capsules, or 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 Unionidz the embryos are retained within the cavities of the outer gills; in Cyclas and Pzszdium there are special brood-chambers at the base of the gills. In Cyc/as the embryos are nourished by the maternal epithelial cells. Seginentation is always unequal ; a gastrula may be formed by invagina- tion or by overgrowth, the two cases being connected 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 Dredssensia polymorpha, in which the habit is recently acquired, do not possess free-swimming larvze ; this must be regarded as an adapta- tion. 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 remarkable are large twisted shells, called Azppurdtes (Rudistze), whose remains are often very abundant in deposits of the chalk period. 428 PHYLUM MOLLUSCA. Class V. CepHALopopa. Cuttlefish Examples.— Sepia, Octopus (Polypus), Loligo, Nautilus The Cephalopods, are bilaterally symmetrical and free- swimming. The head is surrounded by numerous “ arms” bearing tentacles or suckers. These arms seem to be equivalent to processes of the margin of the foot. Another portion of the foot forms a partial or complete tube—the “siphon” or “ funnel” —through which water ts forcibly expelled from the mantle cavity.. The muscular mantle flap which shelters well-developed plumose gills is posterior in position; the visceral hump shows no trace of spiral coiling, but is elongated in a direction anatomically dorsal and posterior, though it may point forwards when the animal propels itself through the water. Except in the pearly Nautilus, the shell of modern forms has been enclosed by the mantle, and ts, in most cases, only hinted at. There is a very distinct head region, jurnished with eyes and other sensitive structures, and the mouth has strong beak-like jaws, as also a well-developed radula, The nervous system shows. considerable specialisa- tion; the chief gangla are concentrated in the head, and sheltered by cartilage. The true body cavity, pericardium of other Molluscs, ts usually well developed, and frequently surrounds the chief organs. Except in the Nautilus, it com- municates with the exterior by the nephridia. The nephridia are disposed on the walls of the afferent branchials. The vascular system is well developed, and, except in the Nautilus, there are accessory branchial hearts. The sexes are separate. The gonad ts in a celomic sac and not directly continuous with the gonoduct. The ovum undergoes incom- plete segmentation. Development is direct. ln habit, Cephalopods are predominantly active and predatory ; in diet, carnivorous. 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 is called “pearly” on account of the appearance of the CEPHALOPODA. 429 innermost layer of the shell. This is exposed after the soft organic stratum and the median porcellanous 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. All the compart- ments are in communication by a median tube of skin—the siphuncle—which is in part cal- careous. , 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 Z Sree Satie may the dorsal integument so as to “Fe Ee ee eld. fill up the space previously occupied by the animal.” There can be no confusion between the beautiful shell of the cuttlefish called the paper Nautilus (Avgonauta argo) 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 seems to be formed by two of the arms. It is instructive to compare the Nautilus shell with that of some Gasteropods, for there also chambers are occasion- ally, formed. But these arise from secondary.alterations of an originally continuous spiral. The Gasteropod shell is 430 PHYLUM MOLLUSCA. usually unsymmetrical, and the foot (ventral) is turned towards the internal curve of the coil, while in Nautilus the dorsal part of the animal is towards the internal surface of the chamber. Fic. 233.—The Pearly Nautilus (Mazte/us pompilius). —After Owen. The shell is represented in section, but the animal is not dissected. Part of the mantle has been removed. c¢., Last or body chamber, separated by a septum (se.) from the compartment behind ; s., the siphuncle traversing all the compartments ; 7z., the portion of the mantle which is reflected over the shell; 4., the hood ; ¢., the eye with its opening to the exterior; 7, the lobes which bear the sheathed tentacles (#.); sz., the incomplete sipaons mit, the shell muscle ; ., the position of the nidamental gland. There are only about half a dozen living species of NVautilus, 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 | CEPHALOPODA. 431 suspecting that these belong to the Dibranchiate section of Cephalopods. The following table states the chief points of distinction between Mautilus and the other series of Cephalopods :— CEPHALOPODA TETRABRANCHIATA (Wautdlus). D1BRANCHIATA (Sepia, Octopus, etc.) Allextinct 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 patches at the bases of the gills. phridia; two genital ducts (the left rudimentary). 4 The ccelom sac -(pericardium) opens directly to the exterior by two aper- tures. The heart has two pairs of auricles, and there are no branchial hearts. No ink-bag. No salivary glands. “‘osphradia”” or smelling | “Two pairs of gills; two pairs of ne- ;, Eledone moschata; in others an un- 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 was internal even in the extinct Belemnites, and in modern forms it occurs in various degrees of degeneration (cf. Spzrula, 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 alens, 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 Oigopsida; two vasa deferentia in . paired genital duct. The ccelom opens into the nephridia by two pores, and thus to the ex- terior. The heart has two:auricles, and there are branchial hearts. Usually with an ink-bag. Salivary glands. CLASSIFICATION OF CEPHALOPODA. Order I. Tetrabranchiata (see Table). Family I. Nautilide. Nautilus alone alive; but a great series of fossil forms, Orthoceras— Trochoceras. Family II. Ammonitide. 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. Bactrztes, Ceratites, Baculites, Turrilites, Heteroceras, and the whole series of genera formerly classed as Ammonttes. 432 PHYLUM MOLLUSCA. 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.” Sfzvula; extinct Bel- emnites ; Sepia, With organic internal ‘ shell.” (a) Eyes with closed cornea, Myopsida, e.g. Loligo. (4) Eyes with open cornea, Oigopsida, ¢.g. Ommeastrephes. 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 Argonauta, eg. Octopus (Polypus), Eledone (Moschites), Argo- naula, Ctrroteuthts (with cirri on the arms and no radula). The classification given above is that usually adopted, but it is not certain that the Ammonites should be included in the Tetrabranchiata. The Nautiloids began in the Cambrian and died out at the end of the Paleozoic period, except the Orthoceras and MNautclus-like types. The genus Vauti/us appeared in the Cretaceous. The Ammonite series lasted from the Silurian to the early Tertiary. The Cephalopods are the most specialised of the Molluscs, and present much diversity of type. They swim freely in the sea, or creep sluggishly 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 formid- able adversaries. Many live at considerable depths, and their chief foes are the toothed whales, some of which, like the sperm whale (Pfyseter), and the bottlenose (Ayperoo- don), subsist almost entirely on cuttles. Some deep-sea forms have highly evolved luminous organs. Shells of Cephalopods.—A chambered external shell, serving as a house, is present in Mazedéz/us alone among living Cephalopods. 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. In the majority of shells of the Ammonitid series, the septa between the chambers are convex towards the aperture (the opposite in the Nautilus); the siphuncle is marginal or ventral; the septal necks of the siphuncle project forwards (not backwards as in the Nautilus); there is CEPHALOPODA. 433 an initial chamber or protoconch at the apex of the spiral (per- haps represented by a cicatrix in the Nautilus); the suture lines marking the chambers tend to be lobed. There is often a single or paired ** Aptychus,” perhaps of the nature of an operculum. Most of the modern forms seem to be more active than their ancestors, and their shells have degenerated. But the line of degeneration is 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 farther, and, where a shell is present, it is 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, and forming the ‘‘ guard” or rostrum. The conical chambered shell, with a siphuncle, is known as the phragmacone. It is produced anteriorly into a gladius or pro- ostracum. In the extinct Spirudérostra the shell was spiral and mostly internal; it has a guard. In Spzrz/a the shell can be caught sight of in the young animal, but it becomes surrounded by the secondary mantle folds that form the mantle-sac.. It is a spiral chambered shell, with a ventral siphon. Its relation to the dorsal and ventral surface of the animal is the opposite of that of the Mazdédlus. The shell is inside the animal; in Maztd/us the animal is inside the shell. It seems that Spzru/a is a swift swimmer at great depths ; though the empty shells are often cast ashore, the creature itself is rarely seen. In Sefza, the narrowed tip of the ‘‘bone” probably represents the remains of the phragmacone; the bulk of the ‘‘ bone” probably corresponds to the pro-ostracum in the Belemnites. Besides lime there is chitin in the ‘‘Sepia-bone.” In Lo/zgo there is no deposit of lime, an organic chitinous pen only being left. In Octopus there is no trace of shell at all, and no mantle-pocket, save a trace, in the very young animal, 23 CHAPTER XVII PHYLUM CHORDATA SUB-PHYLUM HEMICH ORDA UNnbeER the title Hemichorda are included a number of interesting types which seem to have affinities with Verte- brates. These affinities are clearest in certain worm- like animals with distinct gill-clefts, eg. Balanoglossus and tychodera, which form the class Enteropneusta. Perhaps allied to these are two peculiar types,—Rhabdo- pleura and Cephalodiscus, which may be united in the class Pterobranchia. Very doubtfully in this alliance is Phoronis. GENERAL CHARACTERS OF ENTEROPNEUSTA The worm-like body has three regions—a pre-oral “ pro- boscis,” a “collar” around and behind the mouth, and a trunk, the anterior part of which bears gill-slits. A dorsal and in part tubular nerve-cord arises from the ectoderm along the middle line, and ts connected, by a ring round the pharynx, with a ventral cord. In the skin, which is covered with ciliated ectoderm, there 1s also a nerve plexus. From the anterior region of the gut a diverticulum grows forward for @ short distance, becomes a firm support for the proboscis, and: 7s often called the “notochord.” The gill-slits open dorsally, are very numerous, and increase in number during life. The mesoblast is formed by the outgrowth of five caelom pouches Jrom the -archenteron. An unpaired anterior pouch forms the pre-oral or proboscis cavity of the adult; there are two collar cavities and two trunk cavities. There are about 30 species in 9 genera, ¢.g. Balanoglossus, BALANOGLOSSUS. 435 Dotichoglossus, Piychodera, Schizocardium, and Glandiceps. They are very widely, though locally, distributed, and most occur in the littoral area. DESCRIPTION OF BALANOGLOSSUS Form and habitat.—The species which form this genus are worm-like marine animals, burrowing in sand and mud in almost all seas. They vary in length from about 1 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 externally by slight differences in colour. The body con- sists of a prominent turgid and muscular “proboscis,” a firm “collar,” 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 Fic. 234.—Male of Balanoglossus (Do- glands. With theaddi- “choglossus) kowalevskit.—After Bate- tion of grains of sand, S ; the mucus sometimes Nos. t"Gperculum behind the collar; then the forms a tube round the _ region with gill-slits; zs., testes ; a., anus. 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 and internal radial and longitudinal muscles. The fibres are unstriped. There is great regenerative capacity. Nervous system.—The dorsal nerve-cord is most de- veloped in the collar, but is continued along the whole length. It arises as a longitudinal groove of ectoderm and often remains tubular, like a typical spinal cord. The dorsal nerve-cord is connected by a band round the collar with a ventral nerve. There is also a nervous plexus Mo., Mouth; o/., 436 SUB-PHYLUM HEMICHORDA. beneath the epidermis. There are no special sense organs in the adult. In the larvae of some species there are two eye-spots. Alimentary system.—The permanently open mouth is on the ventral surface between the proboscis and the collar. Sand seems to pass into it during the wriggling movements of the animal, which are greatly aided by the turgidity of the proboscis and collar. The pharynx is divided into a dorsal and ventral region, of which the former is respiratory (Fig. 235, g.1), and connected with the exterior by many gill-slits, while the latter is nutritive (Fig. 235, g.), and conveys the food particles onwards. 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 diverti- culum from the dorsal wall of the gut in the collar region. Beneath the notochord there is a chitinous ‘proboscis skeleton.” The septa between the gill-slits are supported by chitinous “forked primary” bars; and each slit, at first circular, is split into a V-shape by the growth downwards of a double rod of chitin called a “tongue bar”; the whole is suggestive of Amphioxus. The body cavity.—The body cavity consists of five distinct parts, all of which are lined by mesoderm, and arise as pouches from the archenteron. (a) There is first the unpaired cavity of the proboscis, which communicates with the exterior by a dorsal pore at the' base of the pro- boscis next the collar. (d) In the collar region there are two small paired coelomic cavities, 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. BALANOGLOSSUS. 437 Respiratory and vascular systems.—The respirator} system consists of many pairs of ciliated gill-slits. They open dorsally by minute pores behind the collar. In development they begin as a pair, increase in number from in front backwards, and they go on increasing long after gs an Uv.Y. un, Fic. 23 speereide digs 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; @.z., dorsal nerve; d.z., dorsal vessel; v.7., ventral nerve; v.v., * ventral vessel; £., hutritive part of gut; g-1, respiratory part of gut; ¢., lateral coelomic spaces; ¢.77., longitudinal muscles; 2., reproductive organs. As the gill-slits are oblique, the whole of one could not be seen in a single cross-section. 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 is a main dorsal blood vessel, which, at its anterior end, forms a heart lying adove the notochord, and below a closed contractile dilatation, sometimes called the “ peri- cardium.” Beside the latter there is a paired ‘“ proboscis gland,” formed from the ccelomic epithelium. There is a ventral vessel beneath the gut; and numerous smaller vessels. The almost colourless blood flows forwards dorsally, backwards ventrally. This system should be contrasted with that of Amphioxus. Excretory and reproductive systems.—No nephridia are 438 SUB-PHYLUM HEMICHORDA. known, but from the region of the collar two ciliated funnels open to the exterior, and the enigmatical proboscis gland is possibly excretory. The sexes are separate. A number of simple paired genital organs lie dorsally in a series on each side of the body cavity in and behind the region with gill-slits (Fig. 235, &.). They open by minute dorsal pores. Development.—The eggs are fertilised outside of the body. Segmentation is complete and approximately equal ; Fic. 236.—Direct development of Dolichoglossus.—After Bateson. The mesoderm is represented by the broken dark line. In the upper row, from the left— Section of blastula ; beginning of gastrulation, Zzd., endoderm ; section of gastrula, 4/., blastopore; Ac., Archenteron; S.c., segmentation cavity ; closure of blastopore, outgrowth of five ccelom pouches (/¥/.). In the lower row, from the left— Longitudinal section, showing the five parts of the body cavity (bc.1, bc.2, bc.3) or ccelom. Cross-section, C.V.S., central nervous system ; Vch., notochord ; bc.2, body cavity in collar region. Section at a later stage, D.d.v., dorsal blood vessel. a blastosphere results; this is invaginated in the normal fashion, and becomes a gastrula. The development may be direct without a larval stage, as in Dolichoglossus howalevskii, or indirect with a Zornaria larva, as in Balanoglossus biminiensis, In the direct development the blastopore of the gastrula narrows and closes; the external surface of the gastrula becomes ciliated; the DEVELOPMENT. 439 endoderm lies as an independent closed sac within the ectoderm, Meanwhile the embryo has become or is becoming free from the thir egg envelope, and begins to move about at the bottom in shallow water. It elongates and becomes more worm-like ; there is an anterior tuft and'a posterior ring of cilia; the primitive gut forms five coelomic pouches ; a mouth and an anus are perforated; there seem to be no fore-gut nor hind-gut invaginations. Two gill-slits appear; the regions of the body are defined at a very early stage. In the indirect development, there is a Tornaria larva, at first bell- shaped. A ventral mouth opens into the curved gut, which is furnished with a posterior terminal anus. A ‘‘dorsal pore” leads into a thin- walled sac which becomes the proboscis cavity of the adult. There are external bands of cilia, something like those of an Echinoderm larva, and also an apical sensory plate (like that of many Annelid trochospheres), with. two eye - spots. The Tornaria is a,pelagic form. During its period of free pelagic life it gradually loses its distinctive bands of cilia, be- comes diffusely ciliated, acquires a pro- boscis and two gill-slits, and thus ap- proaches the form already described. It is elongated in the post-oral region, and becomes a creeping form. The Tornaria must be regarded as the more primitive larval form; the temporary absence of mouth and anus in the other type is probably an adaptation acquired after the pelagic habit was lost. Johannes Miiller, ranked the Tornaria larva, whose adult form was not then known, beside the larvz of Echinoderms, and the resemblance has been recently emphasised by Willey. The ciliated bands of the Tornaria resemble those of Echinoderm larvee, but this is only a superficial characteristic. The an- terior pouch, which forms the cavity a Fic. 237.— Tornaria larva, from’ the side. — After Spengel. M., mouth; g.,- gut; «., anus; h., heart; g., pore entering proboscis cavity ; ¢.., anal ring of cilia: s.¢.7., secondary anal ring. he dark wavy line in- dicates the margin of the lobes of the larval body with their bands of cilia. Note also the apical spot with cilia and sense organ, of the proboscis and communicates with the exterior, has also been com- _ pared with the beginning of the water-vascular system in Echinoderms, and it is true that in both several independent ccelom pouches grow out from the primitive gut. The anterior body cavity in Balano- glossus communicates with the exterior by a pore, which becomes the proboscis-pore of the adult, and this has been compared with the water-pore, or outlet of the water-vascular system of Echinoderms, which similarly opens from an anterior enteroccel to the exterior. On the other hand, the presence of an apical plate—a structure almost invariably absent in Echinoderms—suggests an affinity with an Annelid trochosphere. ‘ 440 SUB-PHYLUM HEMICHORDA. Affinities with Vertebrates (especially emphasised by Bateson). (1) ‘* Motochord.”—A dorsal outgrowth from the anterior region of the gut grows forward for a short distance into the pro- boscis, and becomes a solid supporting rod (Fig. 236, Vcz.). It may be compared with the notochord of Vertebrates, which also arises dorsally from the gut. But it lies delow the main dorsal blood vessel, is of very limited extent, and may be merely an analogue of the notochord. (2) Gell-séits.’—Numerous gill-slits (Fig. 234) open from the anterior region of the gut to the exterior, and are separated from one another by skeletal bars, which in some ways vesemble the framework of the respiratory pharynx in Amphioxus, There are, however, many differences in detail, —thus the slits open dorsally, not laterally; the skeletal bars are differently disposed; the blood supply is different. (3) ‘* Dorsal nerve-cord.”—A dorsal median insinking (Fig. 235, d.n.) of ectoderm, especially strong in the region of the collar, may be compared with the medullary canal of Verte- brates. But it must be noticed that there is also a ventral nerve-cord (Fig. 235, v.7.). (4) ‘* Zhe celom.”—The development of five enteroccelic pouches is very suggestive of affinities with Amphioxus, Affinities with Annelids (after Spengel). The larva (Tornaria) (Fig. 237) may be regarded as a modified Trochosphere, but this points at most to a far-off common stock. Moreover, the nephridia, usually present in the Trochosphere, are unrepresented in the Tornaria. The heart lies, as in some Annelids, dorsal to the gut, not ventral as in Vertebrates; the dorsal vessel carries blood forwards, the ventral backwards, as is usual in Annelids. But the double nervous system is essentially different from that of Annelids; and the gill-slits are unrepresented there, though Salensky has described cesophageal pockets opening to the exterior in four Annelid types—Polygordius, Saccoctrrus, Spio fuliginosus, and Polydora cornuta. In the last there are five pairs in the larva, and two persist. If there be a relationship between Enteropneusta and Annelids, it must be a very distant one, perhaps restricted to origin from some common stock. : Class PTEROBRANCHIA. (1) Cephalodiscus Cephalodiscus dodecaiopaus was dredged by the Challenger in the Magellan Straits. Others are known from Japan, the Malay Archi- pelago, South Africa, and the Antarctic. It was at first described by M‘Intosh as a divergent Polyzoon, but the researches of Harmer point to relationship with Balanoglossus. PTEROBRANCHIA. The minute individuals are associated together within a gelatinous investment ; the colony may attain a size of 9 in. by 6 in. The gut is curved, the anus being beside the mouth, beneath which are 4-6 pairs of arms with ciliated tent- acles. These two characters, formerly supposed to indicate Polyzoan affinities, may perhaps be adaptations to the sedent- ary life. With Balanoglossus this type has been compared, on account of the possession Fic. 239.—An individual Cephalo- discus.—After Ridewood. b., Buds: sé#., stolon; go., to the left, bulging of the body caused by the gonad ; ga., to the right, bulging of the body caused by the stomach ; J.s., pos- terior lobe of buccal shield; ~.2., a red line on the buccal shield; J.s., dark edge of the buccal shield ; 22. tentacular plumes. Fic, 238.—Piece of a colony of Cephalodiscus, showing the tubes inhabited by the animals. — After Ridewood. of the following characters :—(a) The body is divided into three regions, which correspond to the proboscis, collar, and trunk of Balanoglossus; this is especially obvious in the young bud; (4) each of the three regions contains a coelomic cavity, the most anterior being single, while the other two are divided by a median par- tition; (c) the anterior pre-oral cavity’ opens to the exterior by two pores (cf. proboscis pore of Balanoglossus); (d) the collar region is also furnished with two collar-pores; (2) in the collar region the dorsal nervous system is also placed, and is continued to some extent into the proboscis ; (f) beneath the nervous system lies a diverticulum from the gut, which extends towards the pro- boscis region; this has been compared to the ‘‘ notochord” of Balanoglossus ; (g) the anterior’ region of the gut is perforated by 442 SUB-PHVLUM HEMICHORDA. , a pair of lateral gill-slits. The gonads lie between anus and pharynx Buds are given off from a lateral stalk. (2) Rhabdopleura This genus is found at considerable depths in the North Sea and Atlantic. Like Cephalodiscus, the individuals are minute and stalked, and occur in a colony; in this case, however, they remain attached to one another by a common stolon, instead of being united only by an investment. The proboscis or buccal shield makes a thin annulated tube within which the polyp moves up and down. In the head region there are two hollow lateral arms bearing numerous ciliated tentacles, which have a skeletal support. The gut, as in Cephalodéscus, has a U-shaped curvature and an anterior diverticulum (‘‘ notochord”). There are five coelomic cavities, and two collar-pores. There are no gill-slits, CHAPTER XVIII PHYLUM CHORDATA SUB-PHYLUM UROCHORDA or TUNICATA (Ascipians, SEA-SQUIRTS, ETC.) Tue Tunicates are remarkable animals, which seem to stumble on the border line between Invertebrates and Vertebrates. They were classified with Polyzoa and Brachiopoda as Molluscoidea, until, in 1866, Kowalevsky described the development of a simple Ascidian, and correlated it, step by step, with that of Amphioxus. He showed that the /arval Ascidian has a dorsal nerve-cord, a notochord in the tail region, gill-slits opening from the pharynx to the exterior, and an eye developing from the brain. It is true that in most cases the promise of youth is unfulfilled ; the active larva settles down to a sedentary life, loses tail and notochord, nerve-cord and eye, and becomes strangely deformed. Nevertheless we must now class Tunicates along with the Chordates. Of their possible relations to simpler forms nothing definite is known. GENERAL CHARACTERS The Tunicates ave marine Chordata, but the chordate characteristics—dorsal tubular nervous system, notochord, gill-slits, and brain eye—are in most cases discernible only in the free-swimming larval stages. They usually degenerate in the course of their development, and the adults, which are in most cases sedentary, tend to diverge very widely from the Vertebrate type. Thus the nervous system 1s generally re- duced to a single ganglion placed above the pharynx. The 444 SUB-PHYLUM UROCHORDA OR TUNICATA. body is invested by a thickened cuticular test, which contains cellulose. The relatively large pharynx 1s perforated by twe (tn Larvacea), or (in the majority) by numerous ciliated gill- slits, and is surrounded to a greater or less extent by a peribranchial chamber, which communicates with the exterior by a special dorsal (atrial) opening. The ventral heart ts simple and tubular, and there is a pertodic reversal in the direction of the blood current. Nephridia are absent, and the renal organs have no ducts. All are hermaphrodite. There 7s usually a metamorphosis in development. Colonies are Jrequently formed. Type of Tunicata—a simple Ascidian (Ascdta mentula) An adult Ascédia is an irregular oval of 3 to 4 in. in length ; one end is attached to stones or weed; the other, more tapering, bears the 8-lobed mouth; close beside this, on the morphological dorsal surface, lies the 6-lobed ex- halant or atrial aperture. During life, water is constantly being drawn in by the mouth and passed out by the atrial opening. If irritated, the animal may drive a jet of water with considerable force from both apertures, whence the name “sea-squirt.” Test.—The whole body is clothed in a thick test, some- times called a tunic, though this name is more frequently applied to the underlying body wall. From this body wall the test can be readily removed, the two being unattached except at one spot, where blood vessels pass into the test, and also to a less degree at the two openings. To begin with, this test is a true cuticle, produced by secretory prolongations of the ectoderm cells; but soon after its formation mesenchyme cells migrate into it, and give rise to patches of connective tissue cells. These cells apparently retain throughout life some phagocytic importance. In Ascidia outgrowths of the body wall with prolongations of blood channels enter the test, ramifying in all directions. In some Ascidians this is carried further, so that the test becomes an important accessory organ of respiration. The test consists in great part of a carbohydrate identical with the cellulose of plants. This “cellulose” or “tunicin” is common throughout the group, but the relative amount ASCIDIA. 445 produced varies markedly in the different forms. In some forms the “test ”-cells make calcareous spicules. . In. ap. Fic. 240.—Dissection ot Ascidian.—After Herdman. Jn. ap., Inhalant aperture; 7., test, cut away below to show mus- cular layer, pharynx, etc.; Zx., endostyle or ventral groove of pharynx. Note removal of pharynx to show, on the other— the left—side, stomach (S7.), intestine (with fold seen at inci- sion), and reproductive organs (G.); H., opening of pharynx into cesophagus; G.D., genital duct; 4., anus; C/., cloacal chamber; £x. ap., exhalant aperture; Gz., lies above the ganglion, which is seen between the two apertures ; beneath it is the sub-neural gland and its duct. : Body wall and muscular system.—The body wall, mantle, or tunic, disclosed by peeling off the test, is a structure of considerable complexity. Its outer surface is covered by a 446 SUB-PHYLUM UROCHORDA OR TUNICATA, single layer of ectoderm cells, which secrete the test. Beneath these there lies a gelatinous matrix containing numerous connective tissue cells, blood-carrying spaces, muscle cells forming slender fibres, and so on. A true coelom has been described in some embryos, but it is afterwards almost suppressed, being represented at most by the pericardium and small lacunar spaces. The apparent body cavity of the Ascidian—the space between gut and body wall—is, as we shall see, lined throughout by ectoderm. The muscular system is not well developed. The muscle cells are much elongated and unstriped ; they are aggregated into fibres of varying thickness, which form an irregular net- work on the right side of the body, while they are virtually absent on the left. Special sets of fibres form sphincters round the apertures. Alimentary and respiratory systems.—The mouth opens into a short stomodzum, separated from the branchial sac itself by a sphincter muscle, whose posterior border is furnished with numerous simple elongated tentacles. Behind this lies a ciliated peripharyngeal groove. In the living animal the tentacles form a sort of sieve over the opening of the branchial sac. This sac is morphologically the pharynx, and extends almost to the posterior end of the body. It is separated from the mantle by a space whose dimensions vary greatly in the different regions of the body. This space is the peribranchial chamber, which is formed from the ectoderm, and communicates with the exterior by the atrial opening, and with the branchial sac by innumer- able slits). The remainder of the alimentary canal lies on the left side of the body, between pharynx and mantle, and consists of a short cesophagus leading from the pharynx to the fusiform stomach, and of an intestine which describes an S-shaped curve, and then crosses the atrial chamber, to end in an anus iying beneath the exhalant opening. The absorbing surface of the intestine is increased by a marked infolding, corresponding to the typhlosole of the earthworm. A mass of tubules connected by a duct with the cavity of the stomach is possibly a digestive gland. The structure of the pharynx is exceedingly complex, for it has a double function—respiratory and nutritive. More- NERVOUS SYSTEM AND SENSE ORGANS. 447 over, the breathing organs of sedentary animals tend to be elaborate. The water which enters by the branchial aper- ture is not only used in. respiration, but brings with it the minute food particles. Similarly, the outgoing current carries with it the water used in respiration, the undigested residue of the food, and the spermatozoa and ova. The water of respiration passes from the pharynx through its numerous gill openings to the peribranchial chamber, and so to the exterior. On its way it purifies the blood in the vessels running in the complex framework of the pharynx wall. The water-current is produced and maintained by the action of the ciliated cells lining the gill-slits, and its force necessitates special arrangements to prevent the food particles being swept out before they have entered the digestive region of the gut. In this connection there is a longitudinal glandular groove or endostyle along the ventral surface of the pharynx, and a ciliated fold on its dorsal— ‘the regions being defined by the nerve ganglion. According to Willey, the minute alge and the like of the food are entangled in the abundant mucus secreted by the ventral groove or endostyle, and are swept forward in a cord of slime, until at the anterior end of the endostyle they reach the ciliated peripharyngeal groove, whose two halves sur- round the pharynx, and unite’ to form the dorsal lamina or fold. The food particles passing round the peripharyngeal groove are swept backwards by the cilia of the dorsal lamina until they reach the cesophageal opening. In many Ascidians the dorsal lamina is replaced by a series of pro- cesses—the dorsal languets, which may be sensory, as well as food-wafting structures. Nervous system and sense organs.—In the adult both of these show marked degeneration. In the larva there is a slightly developed brain continued into a dorsal nerve-cord, and having connected with it a median eye and an otocyst. The two latter are completely absent in the adult, and the nervous system consists merely of a ganglionic mass lying between the two apertures, giving off a few nerves forwards and backwards. A structure of doubtful utility, but of considerable morphological interest, is the small sub-neural gland which lies beneath the ganglion, and communicates by a ciliated duct with the pharynx. The opening’ 448 SUB-PHYLUM UROCHORDA OR TUNICATA. is usually complex, and forms the so-called dorsal tubercle, which is very distinct on the wall of the pharynx. It lies at the point where the two halves of the ciliated groove, or peripharyngeal band, already described, converge dorsally to form the dorsal lamina. In Ascidia the sub-neural organ is ventral to the brain, and partly RU STULANRUNLAN A LAN a: Fic. 241.—Diagram of Ascidian.—After Herdman. The arrows indicate the two openings; the dark border the test. Ph., Pharynx, with gill-slits; G., reproductive organs; H., heart, with blood vessels; G.D., genital ducts; &., rectum, ending in cloacal chamber. Surrounding the pharynx the peribranchial cavity is shown. glandular in character, and so it is in many; in some cases, however, it is dorsal in position, and its glandular portion is reduced to nil, It is probable that the sub-neural gland and its duct correspond to the olfactory pit of Amphzoxus, and perhaps to the hypophysis of Vertebrates. VASCULAR AND REPRODUCTIVE SYSTEMS 449 It is probable that the pigment spots between the lobes of the apertures, the tentacles in the branchial siphon, and the dorsal lamina, or its representatives, the languets, have some sensory function. ' Vascular system.—The simple tubular heart lies in a pericardial space at the ventral side of the lower end of the pharynx. In development, two diverticula grow out from the pharynx; these meet and fuse, forming the pericardium. The heart arises as an invagination from its dorsal wall, and is thus endodermal in origin, and probably not homologous with the heart of the other Vertebrates. A periodical reversal of the direction of the waves of contraction is discernible in the heart ; for a certain number of beats the blood is driven upwards, and then the direction is reversed. This same reversal also occurs in Phoronis. According to Herdman, the ventro-dorsal contractions occasion the following circulation :—The blood, which is spread out on the walls of the pharynx in vessels lying between the slits, collects into one large (branchio-cardiac) vessel, which, after receiving a vessel from the test, enters the ventral end of the heart. From the dorsal end it is poured into a great (cardio-visceral) trunk, which sends one branch to the test, and then breaks up among the viscera. From the visceral lacunze the blood is again collected (in a branchio-visceral) to be distributed to the branchial sac. At the reversal of the contractions this circulation is also reversed. The reversal occurs every couple of minutes or so. The blood is very colourless, but usually contains a few pigmented corpuscles. Excretory system.—In the loop of the intestine there lies a mass of clear yesicles containing uric acid and other waste products. This, therefore, seems to be a renal organ, lout there is no duct. Bacteria are usually found in the vesicles, and their activity may make diffusion easier. It is interesting to find such a plant-like method of storing up, instead of eliminating, waste products in these very passive animals. It has been suggested that the sub-neural gland may have some-renal function. Reproductive system.—Tunicates are hermaphrodite. The reproductive organs (Fig. 240, G.) are very simple, and lie in the loop of the intestine. The ovary is the larger, and contains a cavity into which the ova are set free, and from which they pass outwards along an oviduct which opens into the cloacal chamber. The testis surrounds the ovary, and is mature at a different time (dichogamy) ; its 29 450 SUB-PHYLUM UROCHORDA OR TUNICATA. duct runs by the side of the oviduct. In some forms, where the gonads are near the cloaca, there are no ducts. The ova are surrounded by follicular cells, and probably fertilised in the cloaca. Development.—The fertilised ovum divides completely and almost equally. The spherical blastosphere becomes slightly flattened, and ultimately forms a two-layered gastrula, Along the dorsal median line of the gastrula the ectoderm cells form the medullary groove, the sides of which arch together and form a canal—the medullary canal. This opens anteriorly to the exterior by Fic. 242.—Young embryo of Ascidian (C/aveléna).—After Van Beneden and Julin, NP., Neuropore; NC., neural canal; WCH., notochord; Z., ectoderm ; J7., mesoderm; A., archenteron. the neuropore, and posteriorly communicates with the archenteron by the neurenteric canal. : With regard to the origin of mesoblast and notochord, there is more difficulty. Both originate from the endoderm in the region of the blastopore, and for a time grow forward together. The notochord lies in its usual position on the roof of the gut, from a specialisation of which it arises; but its forward extension is limilted,—it never extends into the anterior region, and in the posterior region—the future tail—it increases at the expense of the primitive gut, whose lumen it obliterates. The mesoderm, on the other hand, extends right forward, and becomes divided into two regions—a posterior, ultimately forming the muscula- ture of the tail, and an anterior, giving rise to the blood, connective tissues. body muscles, excretory and genital organs. According to Van GENERAL NOTES ON TUNICATA. 451 Beneden and Julin, the mesoderm primarily originates in the form of two pockets, which grow out from the gut, as in. Amphioxus, and whose cavity is the true ccelom. According to the majority of investi- gators, it originates as solid blocks of cells, and the body cavity is only represented by spaces produced by the subsequent separation of these cells. The further processes of development result in the formation of a tadpole-like larva, with dorsal nervous system, notochord in the tail region, and well-developed sense organs. Two ectodermal in- vaginations form the original double peribranchial chamber, and small diverticula from the pharynx meet these and form the first gill- slits,” : For some kours the larva enjoys a free-swimming life, using its tail as an organ of locomotion. Then it fixes itself by papillee on its head, & /P Fic, 243.—Embryo of Clavelina.— Modified after Seeliger. J#-, Fixing papilla; ef£, ectodermic fold; c.g., ciliated groove; en., endostyle; s.o., cerebral vesicle with sense organs; g's., gill-slits ; May nerve-cord beginning to degenerate; ch., noto- chord; g., gut curving upwards towards atrial opening. The atrial invagination is marked bya dotted line ; the mouth and atrial opening are indicated by arrows. and begins almost immediately to degenerate. The tail shrinks and disappears, being consumed by phagocytes. The nerve-cord is lost, and with it the larval sense organs, while simultaneously a change of axis results in the adult relation of parts. The peribranchial chamber becomes greatly enlarged, and its two openings fuse together to form the single atrial aperture of the adult. The gill-slits increase greatly in number, the increase being due both to the formation of new slits and to the division of those first formed, and the whole animal under- goes 2 metamorphosis which is one of the mast signal instances of degeneration. GENERAL NOTES ON TUNICATA The description of Ascidia given above is, in its general outlines, applicable to all the simple Ascidians, which are 452 SUB-PHYLUM UROCHORDA OR TUNICATA. abundantly represented on British coasts. As contrasted with this type, we have in other members of the class most remarkable diversity in structure, habit, and life history. The simple Ascidians are usually sedentary, growing fixed to stones, shells, or weed, and are widely distributed, occur- ring on or near the coasts of all seas. With the exception of the so-called social Ascidians (e.g. Clavelina), they do not reproduce by budding, but are often gregarious, great masses being found together. To the compound Ascidians (e.g. Botryllus) those simple forms are linked by Clavelina, where each individual is surrounded by its own test, but is united to its fellows by a common blood system. In the compound Ascidians, on the other hand, many individuals are enveloped in a common test, and all like C/avelina possess the power of reproducing asexually by budding. There is, however, no doubt that the so-called compound Ascidians are an artificial group, whose members diverge widely in structure, though all dis- play the two characters mentioned. Some of the compound Ascidians are not fixed, but form floating colonies. These forms lead up to the beautiful Pyrosoma or phosphorescent, fire-flame, where the whole colony with its numerous individuals swims as one creature. All these belong to the Ascidian series, and display interesting diversity in their methods of development. The simplest case is that already described for Ascidia, where the tailed larva gives rise to a sexual adult without any power of budding. This occurs in almost all simple Ascidians, but even here there are indications of possible complication. ‘Thus, on the one hand, in some, eg. Aol gula, there is a tendency towards abbreviation—the larval stage being suppressed, while, on the other, the adult acquires the power of reproducing asexually, eg. C/avelina. Both processes are carried further in the compound Ascidians. In these the eggs have usually a considerable amount of yolk, and development takes place either in the atrial cavity of the mother, or in special brood-pouches. In consequence, the development, especially in the early stages, shows considerable modification, although the larval stage is quite distinct. Again, the tailed larva develops into an adult which has no sexua! organs, but forms a colony by GENERAL NOTES ON TUNICATA. 453 budding. The individuals of the colony then give rise to eggs and so to larvae. The development thus includes a distinct alternation of generations. Budding takes place in many different ways in the com- pound Ascidians. In one set (the Diplosomide) the tailed larva is precociously reproductive, giving rise to buds before undergoing metamorphosis. This forms an_ interesting transition to the condition seen in /yrosoma, where the fertilised egg gives rise to a rudimentary larva (cyathozooid), from which a young colony of four individuals arises by budding. These individuals again bud, until a Jarge colony is formed, the members of which become sexual. The ova are few in number, a statement which is generally true for the pelagic Tunicates, as contrasted with sedentary forms. ' While the Ascidians in the narrow sense include all the more typical Tunicates, there are two other sets, few in number both as regards genera and species, but of great theoretic importance. The one set includes the free-swimming genera Sa/pa and Dotiolum, together with the aberrant deep-water genus Octacnemus ; the other, a few active free-swimming forms, which exhibit throughout life many of the characteristics of the larval Ascidian. Of these, Appendicularia is the most familiar type. Both Sapa and Doléolum are pelagic in habit, and differ markedly in structure from the Ascidians. The body is fusiform (Sa/ga) or barrel- shaped (Do/olum), and wholly or partially encircled by definite muscle bands, which replace the scattered fibres of the Ascidians, The mouth is at one end of the body, and the atrial aperture at the other; the animals swim by forcing the water out of the peribranchial chamber posteriorly. Many of the most marked signs of specialisation in the Ascidians are here absent. Thus the test may be, as in Dolzolem, very thin and devoid of cells, and the branchial sac is relatively simple in structure ; the cilia on its walls are never so important in producing the respiratory current as in the Ascidians, and the gill-slits may be few in number, or, as in Sa/pa, may be represented by two large holes in the walls of the pharynx. Further, the hermaphroditism is modified by the occurrence of very marked protogyny, and the ova are never numerous —in Sa/pa each sexual individual usually produces only one. On the other hand, the development exhibits marked alternation of generations, both solitary and colonial forms being included in one life history. In Doliobin the fertilised egg gives rise to a tailed larva, which develops into an asexual ‘‘nurse,” possessing the power of budding (cf. SUB-PHYLUM UROCHORDA OR TUNICATA. 454 Compound Ascidians). The ventral stolon of the nurse gives rise to a P QO : M Do I i { _FTE cl c =| 10). : Nf 4 -E =F a En F Hoe D Fic. 244.—‘‘ Nurse” of Doliolum miiller?.—After Uljanin. I, Inhalant, E., exhalant aperture; C., ciliated band round peas (P.); En., endastyle ; O., ‘‘ otocyst” ; N., nerve-gang- ion; H., heart ; C&., cesophageal opening; D., stomach; A., anus; Cl., cloaca; DO., dorsal organ; M., muscle bands. number of primitive buds, which migrate over the body until they reach a dorsal outgrowth, apparently well supplied with blood. Here they N M P B — OE En H G Fic. 245.—Sexual individual of Doliolum miller. — After Uljanin. G., gonads ; B., gill-slits ; other letters as before. reference line points to the stomach. The unlettered fix themselves and divide up to form three series of buds—two lateral and one median. All these buds develop into individuals belonging to GENERAL NOTES ON TUNICATA. 455 the sexual generation, but only a few become truly sexual. The two .. lateral series develop into nutritive forms, which supply the nurse with food. The nurse itself loses its alimentary and respiratory organs, and becomes a mere organ of locomotion. The median buds develop into ‘* foster mothers,” which ultimately go free, bearing with them other buds destined to develop into the solitary sexual forms. In these, first ova and then spermatozoa are produced, which start the life cycle afresh. It is thus obvious that there is considerable division of labour in the sexual form, accompanied by polymorphism ; the whole process presents some curious analogies to the conditions seen in the Ccelentera, In Sala the single egg is fertilised within the body of the mother, and becomes attached to the wall of the peribranchial chamber. Here the developing egg is nourished by means of a “placenta,” and the development is in consequence much abbreviated, the tailed larva not being represented. This embryo gives rise to a solitary ‘‘nurse” form, ana aon SAU UY \ Fic. 246.—Diagram of Salpa africana. o.a., Oral aperture ; @.¢., dorsal tubercle; Ze., tentacle; g., ganglion; ., muscle bands; ady., atrium; &.v., blood-vessel; az., anus; a@.a., exhalant aperture; v.7., visceral nucleus; 4., heart; s¢., stolon; 2.2, dorsal lamina; Z., endostyle; s.2.g., sub-neural gland; 2%., pharynx ; p.p.., peri-pharyngeal band. which by budding produces a chain of embryos. This chain is set free, jts members become sexual, and, either while still united or after separation, give rise to the eggs which develop into the nurse form, The remaining order of Tunicates includes minute simplified forms like Appendicularia, also pelagic in habitat, but without any power of budding, and never forming colonies. These forms have a distinct tail, which is bent at an angle to the body, and is the main-organ of locomo- tion. The mouth is at the anterior end; the anus, which is distinct from the atrial openings, is at the root of the tail. These atrial openings lie slightly behind the anus, and are merely small ectodermic invagina- tions communicating with the two gill-slits of the pharynx. They correspond to the similar invaginations in the Ascidian larva. The test may form a large investing ‘‘ house,” but it does not contain cells, and is periodically cast and renewed. The important points as regards internal structure are the presence of the notochord throughout life, and the structure of the nervous system. The latter consists of a lobed 456 SUB-PHYLUM UROCHORDA OR TUNICATA. ganglionic mass above the mouth, and a dorsal nerve-cord extending backward from this into the tail, where it is furnished with other ganglia. In connection with the cerebral ganglion there is a pigment spot, an otocyst (auditory ?), and a tubular process communicating with the pharynx, and corresponding to the sub-neural gland and the ciliated duct of other Tunicates. We have already noted the simple structure of the pharynx, which has but two gill-slits communicating directly with the exterior. The same simplicity of structure is observable in the heart, which is without any associated vessels. The hermaphrodite 7? oe 78 - ae ot iz ov — br ors SO st 2x if ieee . Nat - : end app les ee A younstpur 413,\ “syuawsas aantuuid = aary “don -tyUaIWIBaS ‘aha uleig, ‘auUON, *£I0} ~ISUBI} sosvo jsou UT “QUON, ‘ako ulelg: “yreay [esa A, *yrvay ayUyap ONT -ASopoumoy [Npiqnop jo yavay jeqjuaa aiduusg por Suyjszoddns ayy dAoqe pur [esiop St yreay afduus ayy “yreay [erusA -Aroyerdsar jou aie pue ‘sits payeloosse ou savy Aayy suviqryduy aaoqe suof ul ‘SIJa]o -[[18 7YySto uey} a10w JON *snorauInN “SITS Arepuosas snoxawinu Aq sad{y qsou ut paorjd -o1 ‘ared Arewd y “SNOJOUIN NT “syP[O-ID sguoqyaeq ayy Aq paoeydar yred jsow ay} 10; St proys -oJou ay} ‘seysy atmos pue eyemojsopod_ ut ydaoxq ‘dn 0} dy wojy pioysojou VW ‘sad 4 ysow ur Aroyrsueyy ‘[1e} at} ur por Sursoddns y ‘por Sunioddns Joua}UB WYSYs V “SIxe [BSIOC *proo yeutds pue urerg, “proo yeurds pue ureiq waeAjaq UONeNUarIay “Jip jo yury yysys A194 “uoll -Sue3 apsurs 2 01 saqei -ouadap waysks snoaiau yesiop ay} ysow uy “Sp1OS-9AIOU [BI] -udsA puB [vsIo”d, *ulaysAs sno -ALOU [SIO(] © *VIVINVUO, “SQXOIHANY “VIVOINOL “SASSOTIONV IVY, SOLLSINALOVUVHO ALVUAALYAA AO AONVUVAGIV TVOAGVYD GENERAL CLASSIFICATION. 475 General Classification of Phylum Chordata { aj Carinatze i ass J} Odontolcz (extinct). Birps, {ei (running). Class Mammats, Saururz (extinct). : ‘ ‘ 3. Eutheria, Placentalia, Monodel- A Crocodilia (crocodiles, phia: the higher placental ° etc.). mammals. Ophidia (snakes). : Seed : Lacertilia (lizards, | 2, Metatheria, Marsupialia, Didel- Cl etc.). phia: Kangaroos, etc. ; young Rep oS Rhynchocephalia— born precociously, usually nur- | wp SEMtiLes. Sphenodon. tured in pouches. 3 2 Chelonia (tortoises, 2; etc.). s 4 a Extinct Reptiles —|S-1. Prototheria, | Monotremata, Or- 3 é . (maity orders). nithodelphia: oviparous, Ornz- an thorhynchus and Echidna. aziz ~- —- < 7 Sauropsida. Mammalia. z\oO . < in Amniota, embryos with amnion and allantois. OT§ = YE] Class ; 5 {2 | Fisues.—e.g. Dipnoi(double-breath- Class = Oo ing nud-fishes). —|S-AMPHIBIANS.— A, Teleostomi (modern bony Anura (tailless frogs, etc.). By fishes and ‘‘ Ganoids”). Urodela (tailed newts, etc:). > Elasmobranchii (skate, Gymnophiona (worm-like Ca- a shark, etc.). cilia, etc.). Extinct Stegocephali (Lady: rinthodon, etc.). Ichthyopsida (fishes and amphibians). Class Hypostomata (extinct). Class Cyc.osromata (Round Mouths), without true jaws. Myxine, hag-fish. Petromyzon, lamprey. F Salpa type. Sus-Puytum | Ascidian type (sea- Urocuorpba or squirts). « TUNICATA. Afppendicularia (lay- val type persistent). Sus-PHyLuM CEPHALOCHORDA.—A miphi- oxus or Lancelet. Surviving offshoots of ancestral Vertebrates. Puytum HemicHorpa or ENTEROpNEuSTA (offshoots of incipient Vertebrates ?). Balanoglossus, etc.; probably Cephalodiscus ; possibly Rhabdopleura, Ancestry of Vertebrates It is not at present possible to trace the path along which Vertebrates have evolved, though our faith in the doctrine of evolution—as a modal 476 STRUCTURE OF VERTEBRATA. theory of origins—leads us to believe that Vertebrates arose from forms which were not Vertebrates. But even when we recognise that Amphzoxus is a Vertebrate very simple in its generad features, and that the Tunicata, especially in their youth, are Vertebrates, we must admit that these are specialised not very primitive types. The Enteropneusta carry us a little farther back. For, while many of their alleged Vertebrate characteristics are debatable, one cannot gainsay, for instance, the possession of pharyngeal gill-slits. But the affinities of the Enteropneusta with Invertebrate types are quite obscure. We have, in fact, to acknowledge that the pedigree of Vertebrates remains unknown, though alleged affinities have been discovered among Annelids, Nemerteans, Arachnids, Crustaceans, Palzostraca, etc. There is almost no great class of Invertebrate Metazoa whose characters have not been ingeniously interpreted so as to reveal affinities with Vertebrates. It will be enough to select one illustration. Annelid affinities.—Dohrn, Semper, Beard, and others maintain that Annelids have affinities with Vertebrates. (t) Both Annelids and Vertebrates are segmented animals. (2) The segmental nephridia of Annelids correspond to the primi- tive kidney-tubes of a Vertebrate embryo. (3) The ventral nerve-cord of Annelids may be compared (in altered position) to the dorsal nerve-cord of Vertebrates. Both cords are bilateral, and it is possible that the tubular character of the spinal cord and brain is the necessary result of its mode of development, and without much morphological importance. (4) Segmentally arranged ganglia about the appendages of some Cheetopod worms may correspond to the branchial and lateral sense organs of Ichthyopsida, and the ganglia asso- ciated with some of the nerves from the brain. (5) The formation of the oral part of the pituitary body is suggestive of the way in which the mouth of Annelids is sometimes formed. Perhaps the pituitary body represents an old lost mouth and its ancient innervation, To minor points, such as the red blood and well-developed body cavity of many Annelids little importance can be attached. STRUCTURE AND DEVELOPMENT OF VERTEBRATES Having separately discussed the Hemichorda, Urochorda, and Cephalochorda, we propose in this chapter to discuss the general structure of Craniata and the development of some of the important organs. Skin.—This forms a continuous covering over the surface of the body, serves as a protection to the underlying tissues, in some instances retains its primitive respiratory sig- THE SKIN. 477 nificance, and is frequently concerned in the excretion of waste and the regulation of the body temperature. As one or other of its many functions predominates, there are cor- responding structural modifications. One function which we find oftenest emphasised, at the expense of the others, is that of protection, and yet the extinct G/yptodon, the sluggish Chelonia, the decadent ‘“‘ Ganoids,” seem to indicate that this, in itself, or in its correlated variations, is not con- ducive to the continuance of the species. The skin includes— é (a) The epidermis, usually in several layers, the outer, ‘‘ horny” stratum corneum, the inner ac- tively growing stratum Malpighii, or mucosum ; both derived from the ectoderm or epiblast of the embryo. (6) The dermis, cutis, corium, or under-skin, derived from the meso- derm or mesoblast of the embryo. From the epidermis are de- rived feathers, hairs, and some kinds of scales. The dermis, as is natural when we consider its origin from the mesoblast (mesenchyme) or vascular layer, : assists in nourishing these Fic. 255.—Section through Elasmo- epidermic structures. In the branch embryo.—Ziegler. case of feathers and the scales C.,nervecord; WV., notochord; AU., aorta}. of Reptiles, the dermic papilla gut; V7 subintestina! vei MC es is of primary importance, but fin; C., celom: U., segmental duct; in the case of hairs it arises %.-M., myotome; MP., muscle plate; ~ late and is always small. SK., skeletogenous cells around noto- Fiowi the dermis are derived “Hott ) 2E= Setoderm : the bony shields of armadillos, and a few related mammals, the bony scutes of crocodiles and some other reptiles, and the scales of most bony fishes. This again is readily explained by the fact that the mesoblast is also the skeletal layer of the embryo. The ordinary teeth of Vertebrates, as well as the superficial or skin-teeth of gristly fishes, are largely formed-from the dermis, but are usually covered by a thin coating of ectodermic enamel. The mesoderm is divided in the embryo into (1) a series of dorsal segments or somites, with a transient cavity (the myoccel), and (2) an unsegmented ventral portion or ‘‘lateral plate.” The dorsal part 478 STRUCTURE OF VERTEBRATA. gives rise to the myotomes forming all the segmented muscles, to out- growths into the limbs, to the cutis or dermis, to a sheath round the notochord, etc. The ventral part gives rise to the splanchnic or visceral muscles (usually unstriped), to the coelomic epithelium, etc. Skeletal system.—Apart from the exoskeleton of skin- teeth, scutes, shields, etc., the skeleton consists of the following parts :— The skull and its associated “ arches.” (2) Axial The backbone and associated ribs. Skeleton. (The notochord is transitory except in the simplest Vertebrates.) (4) 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 trabeculee lying in front. The parachordals grow round and above the notochord, producing the basilar plate, while the trabe- cule 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 nine. The most anterior is the mandibular arch which bounds the mouth, the second the Ayord; these two are of great importance in the development of the skull. The others, in Fishes and at least young Amphibians, bound open gill- SKULL. 479 slits and support the pharynx; above Amphibians, they are less completely developed. In the Elasmobranch fishes, the mandibular and hyoid arches do not form any direct part of the cartilaginous brain-case, but in the Tele- osteans 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 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. 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 with- out 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 centres of ossification appear, and we thus have ‘‘ cartilage” bones 480 STRUCTURE OF VERTEBRATA. SUMMARY OF THE DEVELOPMENT OF THE SKULL Onicin. REsuLTs. ELEMENTS. I. Parachordals and _ trabeculze, aided in some, cases by the end of the notochord. Their precise relations, ¢.g.to the notochord, are unknown. Occipital region, with four bones—basi-occi- ital, two ex-occipitals, and a supra-occipital lin 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. Sensecapsules. (a) Nasal. (4) Auditory. From cartilage surrounding the ectodermic pits which form the foundation of nose and ear. , (a) Unite with ethmoidal region. (4) May give origin to five bones—pro-, sphen-, pter-, epi-, and opisth- otics, or to the single periotic of Mammals, III. Arches, (2) Mandibular. (4) Hyoid arch. These arches, like those which follow them, are supports of the pharynx, lying between primit- ive or persistent gill-slits. (2) 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, (4) Upper part or hyo-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, (az) From the roof of the skull. (4) On the floor of the skull, z.e. from the roof of the mouth. (c) On the sides of the skull, (d) On upper jaw (e) On lower jaw. Originally of the nature of external bony plates, tooth structures, and the like, (2) Parietals, frontals, nasals, ete. (4) Vomer, parasphenoid, ete. (c) Lachrymal, squamosal, orbitals, ete. (d) Premaxilla, maxilla, jugal, and quadrato- jugal (in part). (e) Dentary, splenial, angular, supra-angular, coronoid. Some recognise also a gonial, often fusing with the articular. ‘VERTEBRAL COLUMN. 481 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. The segmentation of the head, in contradistinction to the unseg- mented skull, is expressed, although indistinctly, by the muscle seg- ments and by the nerves supplying these, perhaps also by the lateral sense organs, the ganglia, and the arches. There are three pro-otic head-segments (pre-mandibular, mandibular, and hyoid), which correspond to the orbital region, their walls forming the six eye-muscles. Behind the auditory capsule there are ten or eleven head-segments. 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 meso- blastic origin. It develops as the substitute of the noto- chord, and not from it, but from a skeletogenous sheath surrounding it. According to Kleinenberg, the notochord supplies the necessary growth stimulus for the rise of its substitute, the backbone. 31 482 STRUCTURE OF VERTEBRATA. A vertebra generally consists of sevzral 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 the articular processes. The ribs which support the body wall usually articulate with the transverse processes, or with the transverse pro- cesses and centra. Amphibians are the first to show a breast-bone or sternum. It arises from two cartilaginous rods in a tendinous region on the ventral wall of the thorax. The sternum of some Reptiles, and of all Birds and Mammals, arises from a cartilaginous tract uniting the ventral ends of a number of ribs. Limbs and girdles.—The pectoral girdle consists of a dorsal scapula, a ventral coracoid, and a forward growing membrane-bone, the clavicle or collar-bone. According to Broom, frogs and some primitive Reptiles show a coracoid and a pre-coracoid ; lizards and birds only a pre-coracoid ; the Monotremes a coracoid and « pre-coracoid; other mammals a coracoid only. 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 com- posed 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, z.e. without digits. Distinct from the other bones are a few little sesamoids of occasional occurrence, ¢.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; (0) the peri- NERVOUS SYSTEM—BRAIN. 483 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 Fics. 256 and 257,—Ideal fore and hind limb.—After Gegenbaur. H., Humerus; R., radius; U., ulna; ~., radiale; w’., ulnare; ¢., inter- medium; ¢., centrale; 1-5, carpalia bearing the corresponding digits with metacarpals (zc.) and phalanges (£%.). SJ, Femur ; 72., tibia ; 72., fibula ; z., intermedium ; 4., tibiale (astragalus); JS» fibulare (os calcis); ¢., centrale; 1-5, tarsalia bearing the corre- sponding digits with metatarsals (#¢.) and phalanges (f/.). 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 of the neural tube still persists as a little ciliated canal in the centre of the cord, and as the internal cavity of the brain. Brain.—At an early stage, even before the closing-in 484 STRUCTURE OF VERTEBRATA. 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 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 asa 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. Fic. 258.—Partial section of a Vertebrate brain (diagrammatic). OLF., Olfactory lobe ; CH., cerebral hemispheres ; C., wall of cerebrum cut to show ventricle, behind this the figure is that of a median sec- tion; PA., parietal organ arising from thalamencephalon; //., pineal organ; /VF., infundibulum descending from thalamen- cephalon; #., hypophysis; OL., optic lobes; C4., cerebellum ; CPL., choroid plexus on roof of fourth ventricle ; AZO., floor of the medulla oblongata ; CC., central canal of spinal cord. We must now follow the metamorphosis of the primary brain vesicles. The first vesicle gives rise anteriorly to the cerebral hemi- spheres, while the remainder forms the region of the optic thalami 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 PITUITARY BODY—PINEAL BODY. 485 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 which resembles 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 outgrowths, which form the optic nerves and some of the most essential parts of the eyes. The original cavity persists as the third ventricle of the brain ; the thin roof gives off the dorsal pineal outgrowth or epi- physis, and, uniting with the pia mater, or vascular brain membrane, forms a choroid plexus; the lateral walls become much thickened (optic thalami); the thin floor gives off a slight ventral evagination, or infundibulum, which bears the enigmatical pituitary body or hypo- physis. The infundibulum also bears in most Teleosts a peculiar posterior saccus vasculosus, which seems to be a sense organ. It is not developed except in Fishes. The pituitary body.—This is derived partly from a downgrowth from the thalamencephalon and partly from an upgrowth from the roof of the mouth. The two parts unile to form a complex little organ, whose morphological nature is very puzzling. It produces an internal secretion of importance, and a pathological state of the organ is associated in man with certain diseases, e.g. acromegaly. The pineal body.—The dorsal upgrowth from’ the roof of the thalamencephalon is represented, though to a varying extent, in all Vertebrates. It consists of two parts, a pineal organ or epiphysis proper, and a parietal organ, which arises as a rule from the epiphysis but may have an independent origin in front of it. It is probable that they were originally right and left members of a pair. The parietal organ may become atrophied, but in some cases, especially in Reptiles, it 1s terminally differentiated into a little body known as the pineal body. This was entirely an enigma until De Graaf discovered its eye- like structure in Anxguzs, and Baldwin Spencer securely confirmed this in the New Zealand “‘ lizard” (Sphenodox), where the pineal body shows distinct traces of a retina. In /etromyzon both the epiphysis 486 STRUCTURE OF VERTEBRATA. and the parietal organ show an eye-like structure, most marked in the case of the epiphysis. Fic. 259.—Vertical section of the pineal eye in an embryo of Spheno- don.—After Dendy. E., Epidermis; D., dermis; Z., lens; /.W., inner wall of the eye; O.W. outer wall of the eye; PA.N., parietal nerve; PA.S., parietal stalk; C., cartilage. of the parietal organ) receives a nerve from a ‘‘ parietal centre” near the base, but independent of the epiphysis; this nerve is transitory in Amguts, more or less persistent in /gwana. Above Reptiles the pineal stalk is relatively short, and its terminal portion is glandular. Among mammals the epiphysis is absent in the dugong and some Cetaceans; the pineal body is absent in Dasypus and the dolphin. The significance of the pineal body is uncertain. According to some, 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) degenera- tion. It may be, however, that the optic function is not primitive, but the result of a secondary transforma- tion, In Elasmobranchs the pineal process (epiphysis) is very long, and, perforating the skull, terminates below the skin in 4 closed vesicle. In the young frog it also comes to the surface above the skull, but degenerates in adoles- ‘cence. In Sphenodon the stalk passes through the skull by the ‘parietal foramen,” so that the ‘‘eye” itself, developed from the parietal organ, lies close beneath the skin, the scales of which in this region are specialised and transparent. In Jgwana, Anguis, Lacerta, etc., the epiphysis loses connection with the ‘‘eye” portion; and it is also to be noticed that in Amguzs and Jguana the pineal body (on the end Fic. 260.—Diagram of the parts of the brain in Vertebrates. — After Gaskell. c.4., Cerebral hemispheres; c.f2., choroid pisses; o.th., optic thal- ami; 0.4, optic lobes; ¢é., cere- bellum; ¢.f2, choroid plexus; 4M.0., medulla oblongata; S.C., spinal cord. THE BRAIN 487 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. 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. Summary (1) Cerebral hemispheres, prosencephalon, or fore-brain. Note commissures, olfactory lobes and nerves, and first and second First Embryonic | ventricles. Vesicle. (2) Optic thalami, thalamencephalon, or tween- brain. Note—(a) optic, (4) pineal, (c) pituitary outgrowths, and the third ven- tricle. (3) Optic lobes, mesencephalon, or mid-brain. Note crura cerebri, and the aqueduct of Sylvius. Median Embryonic | | (4) Cerebellum,-metencephalon, or hind-brain. Vesicle. Note pons Varolii. (5) Medulla oblongata, myelencephalon, or after-brain. Note rudimentary roof, fourth ventricle, and origin of most of the cranial nerves. Third Embryonic Vesicle. 483 STRUCTURE OF VERTEBRATA, Enswathing the brain and spinal cord, and following its irregularities, is a delicate membrane—the pia mater—rich in blood vessels, which supply the nervous system, Outside this, in higher Vertebrates, there is another membrane—the arachnoid—which does not follow the minor irregularities 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 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. Roor. Cavity. Spinalcord.| | Anterior grey} White and] Posteriorcom-| Central canal. and white com- | grey substance. | missure. missure. : Myelen- ‘ Epithelium of | Posterior part of cephalon. Medulla oblongata. choroid plexus. | fourth ventricle. Meten- Commissural Pedunculi of | Cerebellum. Anterior part of cephalon. | part. crura cerebri, fourth ventricle. Mesen- Crura cerebri. Cortex of] Anterior com-|, Aqueduct of Syl- cephalon. optic lobes. missure, velum | vius and _ lateral of Sylvius. extensions. Thalamen-| Infundibulum, Inner part of | Epiphysis ani) Third ventricle. cephalon. | hypophysis, | optic lobes and | epithelium — of chiasma, optic thalami. | choroid plexus. Corpus callo- sum. Anterior com- missure. Prosen- Corpus stria- Lateral ven- cephalon. Kiana : tricles. amina ter- . é finalis, Cerebral hemispheres. Olfactory lobes. SPINAL CORD—CRANIAL NERVES. 489 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. 254, 7e.c.), and though it is only temporary, its frequent occurrence is of much interest. The wall of the medullary canal becomes very much thickened, 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. * 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 :— [TaBLE, 490 STRUCTURE OF VERTEBRATA. Name. OriGIN. DistTRIBUTION. Noves. x. Olfactory. s.* - Front of fore- | Olfactory organ. Quite Jer se. rain. z. Optic. s Opticthalami.| Eye. Quite fer se. 3. Oculomotor or ciliary. 92.* 4. Pathetic or trochlear. 7, . Trigeminal. s. and m. 6. Abducens. mz. 7. Facial. s. and 7. 8. Auditory. S. g. Glossopharyn- geal. s. and 7. Vagus or Pneu- mogastric. s. and a. Io. Floor of mid brain. From pos- terior part of optic lobes. Medulla ob- longata. All the muscles of a eye but two, Superiar oblique muscle of the eye. me Pptinatnne te ee Maxillary to the upper jaw, etc. s. (3) Mandibular to lower jaw, lips, etc. m. and s. External rectus of eye. (x) Hyoidean and spiracular. 1 (2) Palatine. (3) Buccal, facial, and auditory. Ear. First gill arch. Posterior gills and arches, lungs, heart, gut, and body generally. 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 ophthalmicus profundus, often in- cluded with 5, is pro- bably the dorsal com- ponent of 3. Perhaps belongs to 7,asaventral branch. Ganglia at the roots of 7 and 8. Apparently a com- plex, including the elements of four or five nerves. In a Vertebrates there are two others, the spinal accessory (11).and the hypo- lossal (12: S The fourth or pathetic nerve is peculiar among motor nerves in that it appears 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 bas lost most of its sensory branches, and become chiefly motor. * The letter s. is a contraction for sensory or afferent, z.¢. transmitting impulse froma sensitive area to the centre 5 and WM. is a contraction for motor or e! erent, Le. transmitting impulses from the centre to the body. There is much uncertainty in regard to the morphological value of the various cranial nerves, but the following conclusions may be stated :— (1) Like the spinal nerves, the cranial nerves are primarily seg- mental, and there are probably about seven of them,—three pro-otic and four metotic. The olfactory and optic nerves are quite by themselves and not segmental. (2) Like the spinal nerves, the cranial nerves have primarily two roots,—a dorsal and a ventral, but the ventral roots do not join the SPINAL NERVES. 491 dorsals, which have a more superficial course and include numerous motor fibres (correlated with the great development of visceral musculature in the head). (3) The pre-mandibular primitive segment (I.) was probably supplied by the oculomotor (ventral) and the ophthalmicus profundus (dorsal). The mandibular primitive segment (II.) was probably supplied by the pathetic (ventral) and the trigeminal (dorsal). The hyoid primitive segment (III.) was probably supplied by the abducens (ventral) and the facial (dorsal). The auditory, glosso- pharyngeal, and vagus nerves have no ventral roots. Spinal nerves.—Each spinal nerve has two roots—a dorsal, posterior, or sensory, and a ventral, anterior, or motor. These arise separately and independently, but Fic. 261.—Diagrammatic section of spinal cord. @4-, Posterior fissure; g.c., posterior column of white matter; @.f.s., dorsal, posterior, sensory or afferent root ; g-, ganglion ; ¥.a.12., ventral, anterior, motor or efferent root; c.#., compound spinal nerve with branches; s.g., sympathetic ganglion; @.c., anterior column—the anterior fissure is exaggerated; g.c., ganglion cells ; g.22., grey matter ; z.7., white matter. 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. According to most authorities, the sympathetic ganglia are offshoots from the same rudiment as that from which the dorsal ganglia arise, 492 STRUCTURE OF VERTEBRATA. They are usually connected in a chain, which is linked anteriorly to cranial nerves. They are also connected by fine fibres with the ventral roots, They give off nerves to blood vessels and viscera, Fic, 262,—Diagram of spinal cord of man, thoracic region.—After Johnston, S.S., Somatic sensory; V..S., visceral sensory ; S.J7., somatic motor ; V.M.and U.M., visceral motor; d@.r., dorsal root; U.&., ventral root. Sense organs.—The ectoderm or epiblast 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 likeli- hood primitively connected with gill-clefts. In Sauropsida and Mammals these branchial sense organs are no longer distinct as such. The nose.—lIt is possible that the sensory pits of skin which form the nasal sacs were originally two branchial sense organs. They are lined by epithelium in great part sensory, and innervated by the olfactory nerves. In Fishes the nasal sacs remain blind posteriorly, but there is a peculiar condition in Dipnoi, where the grooves from SENSE ORGANS. 493 Pd anterior nares to mouth are arched over and open posteriorly into the front of the mouth. In Amphibians, and in all the higher Vertebrates, the nasal chambers open posteriorly into 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 (Servanus, 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 farther in, and the ori- ginally wide opening to the exterior becomes a long narrow tube. In Elasmobranchs, which exhibit many primitive features, this condition is usually 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. In Anura the ductus endolymphaticus gives rise to a long sac dorsal to the spinal cord giving off outgrowths in which the “ calcareous bodies” lie. 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 im- portant 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. 494 STRUCTURE OF VERTEBRATA. 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« 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 Fic. 263.—Diagram showing the ear and related parts in a young cat. P., Pinna; Sg., squamosal: £.A.M., external auditory meatus; 7., tympanum; 47., malleus; /., incus; S7., stapes abutting on foramen ovale; Z., bulla of tympanic bone; Se., a septum in the bulla; £.7., eustachian tube leading from the tympanic cavity to the back of the mouth; B.O., basi-occipital: C., cochlea; S., sacculus; U., utriculus ; D.£., ductus endolymphaticus; .V., auditory nerve; S.C., semi- circular canal; PZ., periotic bone. change in the direction or velocity of movement. Tow far the ears of Invertebrates (¢.g. Crustacea and Mollusca) 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 perception 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 farther into the recesses of the skull, and a special path for the sound is present. In Elasmobranchs, SENSE ORGANS. 495 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 wail 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 interpretation has been stated on p. 480; the following is Hertwig’s :— Malleus = Articular + angular elements of Meckel’s cartilage. Incus = Palato-quad- rate of lower Verte- brates. Stapes of Mammals has a double origin, being formed from the upper part of hyoid arch + an ossi- fication from the wall of the ear cap- sule = (wholly?) col- umella of Birds, Reptiles, and Am- phibians. Fic. 264.—Diagram of the eye. . C., Cornea; @.4., aqueous humour; ¢.é,, ciliary The eye.—There is body; 4, lens; 7, iris; Sc., sclerotic; Ch., 2 3 choroid; #., retina; v.4., vitreous humour; no eye in Amphioxus, y.sp., yellow spot; #., optic nerve. 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, ¢.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 subterranean amphibians like Cac/za, very small in a few snakes and lizards, and its nerves are abortive in the mole. 496 STRUCTURE OF VERTEBRATA. 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, innetvated 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; Fic. 265.—Development of the eye.—After Balfour and Hertwig. x. Section through first embryonic vesicle, showing outgrowth of optic vesicles (of.v.) to meet the skin; 7.., thalamencephalon ; G., the gut. : 2-4. Sections illustrating the formation of the lens (2) from the skin, and the modification of the optic vesicle into an optic cup; &., retina; v.4., vitreous humour. 5. External aspect of embryonic eye; Z., lens. together these are often called the choroid. The vascular part may be compared to the pia mater covering the brain, and like it is derived from mesoblast. Outside of the choroid is a protective 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 clear ball derived directly SENSE ORGANS. 497 from the skin. 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 con- junctiva—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 is alterable by the action of accommodating ciliary 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 ina knot-like structure in the lens. Itacts as an “‘accommodator.” The retina isa 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 farthest 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 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 32 498 STRUCTURE OF VERTEBRATA. one next the cavity of the cup becomes the retina, while the outer forms the pigmented epithelium and the muscles of the iris. Meanwhile, surrounding 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 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, eg. 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 stomodzum. Finally, there is usually a slight posterior invagination of ectoderm, forming the anus. ‘This is the hind-gut or proctodzum, but it is practically absent in Vertebrates. Associated with the mouth cavity or stomodzum are—(a) teeth (ectodermic rudiments of enamel combined with a mesodermic papilla which forms dentine or ivory); (4) 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 Gegenbaur suggested 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 all higher animals (above Fishes), 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 ALIMENTARY SYSTEM. 499 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 frequent 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 stomodzum, of which the so-called nose of AZyxine 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 (Zopado- rhynchus), in which the larval stomodzeum 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 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 ¢hyroid 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 gut-gland. As to its morphological nature, its mode of origin suggests com- parison with the hypobranchial groove in Amphioxus and the endostyle of Ascidians. 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 500 STRUCTURE OF VERTEBRATA. 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, through some specific secretion. The ¢hymus 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 rudi- ments. In Mammals it often seems to degenerate after youth. In the rabbit it has its maximum weight in the fourth month, and thereafter begins to be rapidly reduced. As it has from its first origin a distinct lymphoid nature, and apparently forms leucocytes, it has been inter- preted (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 tonsilsy which are believed by some (Stohr, Killian, Gulland) to have a similar phagocytic function. The pharynx leads into the gullet or cesophagus, which is a conducting tube, and this into the digestive stomach, which is followed by the diges- tive, absorptive, conducting intestine, ending in the rectum and anus. From the cesophagus the air- or swim- bladder of most Fishes, and the lungs of higher Verte- brates, grow out. The air- bladder usually lies dorsally and is almost always single; the lungs lie ventrally and are double, though connected with the gullet by a single tube. The beginning of the intes- tine gives origin to the liver, saa which regulates the composition ss er ee - "t. of the blood and secretes bile, chick. —After Goette. and to the pancreas, which a itera coe is shaded ; the endo- secretes oe juices. The erm dark. ancrea: i ég., One of the lungs; S¢., stomach ; P s has often a multiple Z., liver ; 4., pancreas, rudiment. ALIMENTARY SYSTEM. 501 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 vascular foetal membrane con- cerned with the respiration or nutrition of the i or both, Fic. 267. Beri through a young newt. c.2,, Connective tissue ; Z., epidermis; D., dermis; S.C. spinal cord; J7., muscle; JV., notochord Sh, mesodermic sheath of notochord; K., kidney; 2, lung; S 5 spleen 3 3 ST., stomach; Pe., peritoneum 3 L., liver; a@., duct of the pancreas (P); G.B., gall-bladder ; Bex dorsal aorta. . Cilia are very common on the lining of the intestine in Invertebrates, but.they are much rarer in Vertebrates. Yet as they occur in Amphioxus, lampreys, many fishes, Proto- pterus, some Amphibians, and in embryonic Mammals, it 502 STRUCTURE OF VERTEBRATA. seems not unlikely that the alimentary tract was originally a ciliated tube. At the posterior end an ectodermic invagination or proctodzeum 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 iri 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. In some Fishes and Amphibians the anus is formed directly from the blastopore. om 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 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, czeca, and coils, which would be justified by the increase of absorptive and digestive surface. There are regular longitudinal folds in AZyxzne, cross-folds traversing these would form crypts, which may be exaggerated into the pyloric caeca of Teleosteans and Ganoids, while other modifications would give rise to: ‘*spiral valves” and the like. In the same way it may be suggested that the numerous important outgrowths of the mid-gut, such as lungs, iver, 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 Mecdurus, 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 (Stdhr). 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 outgrowths 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. [TaBLE. ALIMENTARY SYSTEM—BODY CAVITY. 503 ALIMENTARY SySTEM.—SUMMARY REGION OF THE GuT. OuTGROWTHs. ASSOCIATED STRUCTURES. Mouth cavity, Oral part of the} Teeth. or Stomodzum, hypophysis. Salivary glands. i or Fore-gut, Tongue. : originating as an ectodermic invagination. Pharynx, gullet or ceso-| Thyroid\and the| With the several out- phagus, stomach, small in-}| Thymus f gill-clefts. | growths the surrounding me- testine, large intestine, and| Air bladder; lungs. | soderm becomes associated, rectum ;=the mesenteron or| Liver. often to a great extent. mid-gut, originating from} Pancreas. Note also the origin of the cavity of the gastrula, Allantois. the notochord as an axial the archenteron or primitive] The pancreas is | differentiation of cells along gut; lined by endoderm. usually the result of | the mid-dorsal line of the two ventral © out- | embryonic gut. growths and a dorsal one. In Cyclostomes Anal region, and Elasmobranchsit | In some Fishes, all Amphi- or Proctodzum, seems to have but bians, all Sauropsida, and or Hind-gut, one rudiment; in the | the Prototherian Mammals, originating as an ectodermic | Sturgeon four. the terminal part of the invagination. gut is a cloaca or common chamber, into which the rectum, the urinary, and the genital ducts open. Body cavity.—In Amphioxus the ccelom arises as pouches from the archenteron (enzerocelic). In the other Vertebrates, owing to modified processes of development, probably first arising from the presence of much yolk, solid cell masses grow out in place of hollow sacs, but the cavities which appear later, apparently by splitting of the cell mass (schizocelic), are in reality the retarded cavities of true coelom-pouches. A dorsal segmented portion (protoverte- bree) becomes separated off from a ventral unsegmented portion (Fig. 255). It is this ventral portion which forms the body cavity of the adult. In the adult it is divided into an anterior pericardial and a posterior peritoneal portion. The body cavity may form part of one or all of the following systems : —(1) excretory, voiding waste by abdominal pores or by nephrostomes ; (2) reproductive, receiving the liberated genital elements; and (3) lymphatic, receiving transudations from visceral and abdominal organs. 504 STRUCTURE OF VERTEBRATA. It is probably never quite closed, but may communicate with the exterior by abdominal pores (or through nephrostomes) opening into the renal system. Both occur together in some Elasmobranchs, but they are usually mutually exclusive. In the higher Teleostei, in some Saurians, and in Mammals, there are neither abdominal pores nor nephrostomes, but only openings (stomata) into the lymphatic system. Vascular system.--From Cyclostomata onwards the blood fluid contains red cor- puscles, z¢. cells coloured with heemoglobin—a pigment which readily forms a loose union with oxygen, and bears it from the exterior (through gills or lungs) to the tissues. These pigmented cells are usually oval and nucleated. In all Mammals except Camelidz they are circular. Moreover, the full-grown red corpuscles of Mammals have no visible nuclei. The blood fluid also contains uncoloured Fic. 268.—Blood corpuscles. x. Amphibian, seen on the flat, oval, bi-convex disc (nucleated); 2, am- phibian, in profile; 3, mammalian (non-nucleated), circular, bi-concave disc; 4, mammalian, in profile; 5, camel’s (non- nucleated), oval; 6, mud-fish (Lepidosiren) in_ section, like Amphibian ; 7, Lepidosiren, seen on the flat ; 8, an amoeboid leucocyte with lobed nucleus and large gran- ules; 9, a leucocyte with non-lobed nucleus and minute granules; 10, a leucocyte dividing into two; 11, a flat amoeboid corpuscle or blood platelet inequality of pressure which makes the blood flow. nucleated amceboid cells, the white corpuscles or leuco- cytes, of much physiological importance. Some of them, specialised as phagocytes, form “a body-guard,”. at- tacking and destroying micro- organisms within the body. The heart receives blood from veins, and drives it forth through arteries. Its contrac- tions in great part cause the It lies in a special part of the body cavity known as the peri- cardium, and develops from a single (sub-pharyngeal) vessel in Cyclostomata, Fishes, and Amphibians, from a pair in Reptiles, Birds, and Mammals. The receiving region of the heart is formed by an auricle or by two auricles; thence the blood passes into the VASCULAR SYSTEM 505 muscular ventricle or ventricles, and is driven outwards. Except in adult Birds and Mammals, the veins from the body enter the auricle (or the right auricle if there are two) by a porch known as the sinus venosus. In Fishes (except Teleosteans) and in Amphibians the blood passes from the ventricle into a valved conus arteriosus, which seems to be a continuation of the ventricle. In Teleosteans there is a superficially similar structure, but without valves and non- contractile, and apparently developed from the aorta, not from the ventricle; it is called the bulbus arteriosus, and may occur along with the conus arteriosus in other Fishes. In Vertebrates higher than Amphibians there is no distinct conus. In Cyclostomata, and in all Fishes except Dipnoi, the heart has one auricle and one ventricle, and contains only impure blood, which it receives from the body and drives to the gills, whence purified it flows to the body. In Dipnoi the heart is incipiently three-chambered. In Amphibians the heart has two auricles and a ventricle. The right auricle always receives venous or impure blood from the body, the left always receives arterial or pure blood from the lungs. The single ventricle of the amphibian heart drives the blood to the body and to the lungs. ; In all Reptiles, except Crocodilia, the heart has two auricles and an incompletely divided ventricle. The partition in the ventricle secures that much of the venous blood is sent to the lungs; indeed, the heart, though possessing only three chambers, works almost as if it had four. In Crocodilia there are two auricles and two ventricles. But the dorsal aorta, which supplies the posterior parts of the body, is formed from the union of two aortic arches, one from each ventricle. Therefore it contains mixed blood. In Birds and Mammals the heart has two auricles and two ventricles, and ome aortic arch supplies the body with wholly pure blood. his aortic arch always arises from the left ventricle, but in Birds it curves over the right bronchus, z.e. is a right aortic arch, and in Mammals over the left, ze. is a left aortic arch. Impure blood from the body enters the right auricle, passes into the right ventricle, is driven to the lungs, returns purified to the left auricle, enters the left ventricle, and is driven to the body. ; The arterial system of a fish consists of a ventral aorta continued forwards from the heart, of a number of afferent vessels diffusing the impure blood on the gills, and of efferent vessels collecting the purified blood into a dorsal aorta. In the embryo of higher Vertebrates the same arrangement persists, though there are no gills beyond Amphibians. From a ventral arterial stem arches arise, which are connected so as to form the roots of the 506 STRUCTURE OF VERTEBRATA. dorsal aorta. This aorta gives off vessels to the body, while in embry- onic life it sends important vitelline arteries to the yolk, and (in Reptiles, Birds, and Mammals) equally important allantoic arteries to the allantois. Returning to the arterial system of a fish, we must consider the arches more carefully, and compare them with those of Sauropsida and Mammals, where they are no longer connected with functional gill-clefts, and also with those of Amphibians, where the complications due to lungs, etc., begin (see the following Table). SUMMARY AS TO AORTIC ARCHES SAUROPSIDA AND FISHES. AMPHIBIANS. MAMMALS. (a) Mandibular aortic} Aborts, or is not} At most merely em- arch usually aborts; | developed. bryonic. there isa persistent trace in Elasmo- branchs (spiracular artery). (6) Hyoid aortic arch | Aborts. At most merely em- aborts, or is rudi- bryonic. mentary. (c) Ist branchial. Carotid. Carotid. (d@) 2nd branchial. Systemic arches, | Systemic. Onlythe right | unite to form} persists in Birds; only dorsal aorta, the left in Mammals. (e) 3rd branchial. Rudimentary or | Disappears. disappears in most forms. (f) 4th branchial (gives | Pulmonary. The pulmonary. off artery to “lung” of Dipnoi). The important features in the development of the venous system are as follows :— (a) In the embryo the vitelline veins bring back blood from the yolk-sac, at first directly to the heart, and later to the liver. Into these veins, blood returned from the intestine is poured in increasing quantity by other veins. In the adult these persist to form the hepatic portal system, by means of which blood from the stomach and intestine is carried to the liver, and not directly to the heart. VASCULAR SYSTEM. 507 (6) At an early stage in development the blood is brought back from the anterior region by the superior cardinal veins, from the posterior region by the inferior cardinals. The two cardinals on each side unite to form the short transverse ductus Cuvieri, the two ducts entering the sinus venosus of the heart. In Fishes the superior car- dinals persist, the inferior cardinals bring back blood from the kidneys, and also to some extent, by means of their union with the caudal vein, from the pos- terior region of the body. In some cases this union with the caudal is only in- direct, through the medium of the kidney (Elasmo- branchs); in this way the renal portal system is con- stituted. In higher Verte- brates, before development is completed, the superior cardinals are replaced by the superior venze cave (into which the superior cardinals open as external jugulars). The inferior car- dinals at first return blood from the Wolffian bodies and the posterior region ; later they atrophy, and are replaced by an unpaired inferior vena cava which eo agli peg Fic. 269.—Diagram of circulation. from the liver (hepatics), —After Leunis, and from the hind-limbs ~.2., Right auricle receiving superior vena except when there is a @y% {0 aoe ences a, ae renal portal system. The monary artery to lungs (Z.); .v., right azygos vein of Mammals pulmonary vein; 4da., left auricle; is a persistent remnant 4., left ventricle; @o., aortic arch; fi i : d.ao., dorsal aorta giving off arteries to of the inferior cardinals. liver (dé.), to gut (g.), to body (B.); (c) In Amphibia 4 vein known —4o.v., portal veins; 4.v., hepatic vein. as the epigastric (anterior abdominal) carries blood from the hind-limbs into the hepatic portal system. This vein also receives blood from the allantoic bladder, a fact which is of great theoretical importance. In all higher Vertebrates in embryonic life, the blood from the allantois passes through the liver, and to a greater or less extent into its capillaries, on its way to the heart. In aoa dao. 508 STRUCTURE OF VERTEBRATA. Reptiles the allantoic veins persist throughout life as the epigastric vein or veins. In Birds and Mammals, on the other hand, they atrophy completely at the close of foetal life. In Birds, however, a vein is developed which connects the veins coming from the posterior region with the allantoic veins; this persists when the remainder of the allantoic veins atrophy, and thus in Birds as in Amphibia there is a con- nection between the components of the inferior vena cava and the portal system. In Mammals no such connection occurs. According to many authorities, the vascular system is de- veloped in the mesoblast from the hollowing out of strands of cells, the outer cells forming the walls of the vessels, the inner forming the constituents of the blood. The heart, with the exception of its endothelial lining, is a tubular de- velopment of the splanchnic mesoderm. : Associated with the vascular system is the spleen, which ‘appears to be an area for the multiplication or destruction -of blood corpuscles. The lymphatic system, developed in mesoblastic spaces, is a special part of the vascular system. It consists of fine ‘tubes which end blindly in the tissues and drain off fluids, of larger vessels which the tubes combine to form, and which open into veins. The lymph vessels contain amce- boid cells, and have associated lymphatic glands in which these lymphocytes are produced. Respiratory system.—In Balanoglossus, Tunicates, and Amphioxus, the walls of the pharynx bear slits, between ‘which the blood is exposed in superficial blood vessels to the purifying and oxygenating influence of the water. In Cyclostomata, Fishes, all young and some adult Am- ‘phibians, there are not only clefts on the walls of the pharynx, but gills associated with these. On the large ‘surface of the feathery or plaited gills, the blood is exposed _and purified. In Reptiles, Birds, and Mammals, traces of gill-clefts occur in the embryos, but without lamellz or respiratory function. In the embryo the blood is purified, as will be explained afterwards, by aid of the foetal sac known as the allantois; and after birth the animals breathe by lungs. All adult Amphibians also have lungs, to which the lung or ‘swim-bladder of Dipnoi is physiologically equivalent. " The gill-clefts arise as outgrowths of the endodermic gut which meet the ectoderm and open. The ventral paired EXCRETORY SYSTEM. 50 lungs arise from an outgrowth of the gut, as does also. the swim-bladder of many Fishes, though it usually lies on the dorsal surface, has rarely more than a hydrostatic function, and usually has a blood supply different from that of the lungs. In Dipnoi and some “Ganoids” it is supplied by a pulmonary artery arising from the sixth aortic arch. There is probably a homology between lung and swim-bladder. Excretory system.—The development of this is always compli- cated. In the embryos of Vertebrates at an early stage there are always. traces of a pronephros, or so-called head-kidney. This is perhaps seen: in its most primitive condition in Amphioxus, where, as already de- scribed, there is a series of tubules, segmentally arranged, opening on the one side into the body cavity by several flame-cells, and on the other into the atrial chamber, ze. the exterior. On the surface of each tubule # vessel connecting the sub-intestinal vein with the dorsal aorta forms a vascular plexus—the so-called glomus. Such a con- dition of parts is never in its entirety found in the Craniata. There the tubules open not directly to the exterior, but into a longitudinal pronephric or segmental duct, and they are usually few. in number ;. but in their segmental arrangement, as shown by the blood supply,. and in the presence of glomera, they agree entirely with those of 4m- phioxus. In connection with the glomera, it may be noted that while the blood supply usually comes directly from the dorsal aorta, it has been. shown by Paul Mayer and Riickert that in the embryos of Selachians connecting vessels occur between the dorsal aorta and the sub-intestinal vein, which form rudimentary networks on the tubules of the pronephros. This shows a very striking correspondence with the conditions seen in. Amphioxus. The pronephros develops from the parietal mesoblast at the junction. of the muscle segments and the unsegmented body cavity (see Fig. 270) in the anterior region, and varies greatly in its degree of development. In AGyxine and Bédellostoma it persists in adult life, though apparently, at least in part, in a degenerate condition, and is said to be the functional excretory organ of the little (degenerate ?) fish /zerasfer and some other Bony Fishes. In most Bony Fishes, and in Amphibia, it is merely a larval organ, but is then large and important. In Elasmobranchs and Amniota, except Crocodiles and Turtles, it is from the first rudimentary and functionless. The origin of the segmental or pronephric duct is still undetermined. It usually arises from the mesoblast, in some cases growing backwards. directly from the rudiment of the pronephros, while in others the sur- rounding mesoblast takes an important part in its formation ; in Elasmo- branchs, in Mammals, and in the chick, a connection with the epiblast has been described by various observers. Riickert is of opinion that it originally arose by the fusion of the outer ends of the pronephric tubules, and that the occasional connection with the ectoderm indicates. the position of former excretory pores (cf. Amphioxus). 510 STRUCTURE OF VERTEBRATA. At a late period in those types in which the pronephros is a functional larval organ, but much earlier in the higher Vertebrates, another series of tubules is differentiated from the mesoblast, and, acquiring a con- nection’ with the segmental duct, constitutes the mesonephros, or mid- kidney. The tubules arise usually, though not invariably, nearer the posterior end of the body than the pronephros, and are formed from the portion of the mesoblast which connects the muscle segment and the lateral plate (see Fig. 270). Below the Amniota the mesonephros forms the permanent excretory organ. In higher forms another series of nephridial tubules arises still farther back in the body, and forms the metanephros, or perma- nent kidney. The mesonephric and metanephric tubules resemble each other closely, but the relation of the former to the pronephros is still a debated point. When fully developed, a mesonephric tubule consists of—(1) an internal ciliated funnel (nephrostome), which opens into the body cavity, but is only rarely represented; (2) a Fic. 270.—Development of excre- tory system of Vertebrate.—In part after Boveri. In I. the primitive segments are not separated off from the lateral plate, and the pronephros (/.) is seen arising from the lower part of the primitive segment. In II. the pronephros is com- pletely separated off from the primi- tive segment and lateral plate. In III. the origin of the mesonephric tubules is seen. They arise from the upper part of the lateral plate, which is now completely separated from the primitive segment, and curving round the pronephric duct come tg open into it. w.c., nerve cord; zch., notochord; Amn., pronephros; g., gut; /.s., primitive segment; es., mesonephric tubule; gn.d., pronephric duct; 4.c., body cavity; @o., aorta; szv., sub-intestinal vein, with vessel to the aorta. SUPRARENAL BODIES. 511 small cavity (Malpighian capsule) derived from the ccelom, and con- taining a mass of capillaries which project into the cavity of the tubule ; and (3) a coiled tube in part excretory, in part a conducting canal for the waste filtered from the blood. The metanephric tubules have a quite similar structure, but the nephrostome is never present. : In all Vertebrates the primitive nephridia open into a pair of longitudinal ducts, developed like the nephridia as special portions of the ccelom. These ducts open into the end of the gut. According to their connections with the nephridia these longitudinal ducts are called pronephric, mesonephric, or metanephric ducts, and they are also called segmental ducts. In Elasmobranch fishes a Miillerian duct is separated off from in front backwards from the longitudinal duct and forms the oviduct of the female, a rudiment in the male. After the separation of the Miillerian duct, the longitudinal duct (now called mesonephric or Wolffian) forms in the male the vas deferens and also receives the tubes from the permanent kidney (mesonephros). Tn the female the Wolffian duct has this last function. In general it may be said that the original longitudinal duct becomes the vas deferens in the male Vertebrate, and that another duct—the Miillerian—whose development is obscure except in Elasmobranchs, forms the oviduct. The meta- nephric duct, developed in part from the hinder end of the mesonephric duct, is the ureter of the permanent kidney in Amniota. , Suprarenal bodies.—These are found in most Vertebrates near the reproductive organs and kidneys. They seem to increase in importance as we ascend the series. Typically, each shows a dis- tinction into a cortical and a medullary zone. It is usually asserted that-these two areas have a different origin, the medullary region being derived from the sympathetic nervous system, the cortex from the coelomic epithelium. There is much evidence (morphological and physiological) that the suprarenals of Elasmobranchs correspond to the medullary part in Mammals, while the interrenals of Elasmobranchs and the suprarenals of Teleosts and Ganoids correspond to the cortical portion in Mammals. With regard to function, there is some uncertainty. The suprarenal bodies are relatively very large in embryonic life, but fail to maintain their primitively rapid rate of growth. A substance, adrenalin, can be extracted from them which has a remarkable action upon the parts innervated by the sympathetic system, producing on injection the same effects as stimulation of the sympathetic would have, é.g. constriction of the arterioles, and consequent heightening of the blood pressure, STRUCTURE OF VERTEBRATA. 512 A SMUAPIPIde “ez fsuasiajap sea “pra fenuaiays vsea “aa fsiisoy iz $Apoq ueyjom “oy { Areao ag fyonp URLOTINI “py S$ 3ONp uegjonwy “py ! sorydauosaur Joraysod ‘sxe ,rajain ,, $ vowO[D ‘72 $ eipraydeu Jo saqny [ejuawsas “4r {syuauSas yuasaidar saul] pai10p ay3—sarpoq uviysidieyy “py Ssawoysorydeu ‘9-47 fJonp [eueuZas do yeuipnysuoT “ps (anoyyeg seye ¢-z) -ajeur ynpe Ul jusweasuviry “PF ‘ayeway y[npe ul quaWeSULIy “E ‘ofIqua qouesqouse|y Uy wayshs $10jaIOxa Jo ayeys sag *z ‘CIeysieH{ Foye) wiayshs Asoyaiox9 jo ainjonys ajduns Ajeopy ‘x *wiaysds [eyUasoiQ—'i Le ‘OTT “U'SUL “U'Sut REPRODUCTIVE SYSTEM. 513 Reproductive system.—The ovaries and testes are developed from a ridge formed by a part of the epithelium lining the abdominal cavity, this ridge constituting the so-called germinal epithelium. In the male the proliferating germinal epithelium is divided by embryonic connective tissue into numerous follicles. The cells of the follicles form seminal mother- cells, which, by their ultimate divisions, give rise to sper- matozoa. From the mesonephros, tubules grow out to the embryonic testes; these form the collecting tubes of the organs and open into the Wolffian duct, the vas deferens of the adult. : In the female the ovary is similarly divided up into follicles. In this case, however, differentiation sets in among the originally equivalent cells of the follicle. One cell in each follicle is more successful than its neighbours, which are sacrificed to form an envelope of follicular cells around the single large ovum cell. The ova are usually shed into the body cavity, and pass thence to the exterior by the Miillerian ducts or oviducts. “In many cases, between the follicular cells and the ovum there is 4 membrane, the zona radiata, which is traversed by fine pores, and, in consequence, has a striated appearance ; other egg membranes, more or less transitory in nature, also occur. In the lower Vertebrates the layer of follicle cells is single, but in Mammals (except in Monotremes) it is multiple, and a quantity of clear fluid accumulates between the cells and the ovum. The whole forms a ‘‘ Graafian follicle,” which bursts when the ovum is liberated. Before fertilisation takes place, the ovum undergoes a process of maturation, during which extrusion of polar bodies typically occurs ; the technical difficulties in the way of the definite observation of this fact are, however, often very great. The ovaare fertilised outside the body in Cyclostomata, Ganoids, Teleosteans, Dipnoi, and tailless Amphibians ; internally in the other Vertebrates. Hermaphroditism occurs as a normal state in Tunicata, most of which are first functionally female and then male (protogynous) ; in AZyxine (g.v.), which is first male and then female (protandrous); in some species of the Teleostean genera Chrysophrys and Serranus, of which the latter is regularly self-fertilising ; and in a solitary Batrachian. It occurs casually in some Selachians, in the sturgeon, in about a score ot Teleosteans, ¢.g. cod, in various Amphibians, and more rarely in Amniota. There are also embryological facts which suggest that the embryos of higher Vertebrates pass through a state of hermaphroditism before the unisexual condition is reached. On these grounds it has often been suggested that the original Vertebrate animals were hermaphrodite. . 33 514 STRUCTURE OF VERTEBRATA. The quantity of yolk present in the egg varies very greatly in Vertebrates, and its presence or absence exercises a profound influence upon the processes of development. Following Hertwig, we may notice that the presence of yolk has both a physiological and a morphological effect. Physiologically, the presence of a store of nutriment enables the developmental process to be carried on uninterruptedly, and the period of independent life to be postponed until more or less complexity of organisation has been attained. Morphologically, the yolk acts as a check to the activity of the protoplasm, and by substituting an embryonic mode of nutrition for that for which the adult organism is fitted, tends to prevent a speedy establishment of the adult form. When much yolk is present, it usually forms a hernia-like yolk-sac, hanging down from the embryonic gut. Asa further consequence, we may notice the tendency to the production of embryonic organs useful only during embryonic life. We must consider the formation of an organic connection between mother and unborn young as a further step in the same direction as the acqui- sition of yolk. This is hinted at in some Fishes and Reptiles, but cul- minates in the placental Mammals, It may be looked at in two differ- ent ways. On the one hand, the diversion of the nourishment from the ovary, during the period of gestation, tends to starve the remain- ing ovarian ova, and this check to Fic. 272.—Mammalian ovum.— owt is further prolonged during ‘After Hertwig. actation (Ryder); on the other : hand, the chance of survival is ov., Ovum; /,, follicular capsule; /z., much increased, and the maternal follicle cells ; fc., follicle cells form- ° 26 Daan f ‘ ing discus proligerus; £2, cavity Sacrifice finds its justification in occupied by liquor folliculi. the increased specialisation of the offspring. In accordance with the effect of the presence of yolk as noted above, we find that segmentation is total (holoblastic) in the ova of the lam- prey, the sturgeon, Ceratodus, Amphibians, and all Mammals except the Monotremes. In the ova of Elasmobranchs, Teleosteans, Reptiles, Birds, and Monotremes, the activity of the protoplasm is not sufficient to overcome the inertia of the yolk, and segmentation is partial (meroblastic). . Similarly we find that a gastrula is formed, in part at least, by dis- tinct invagination in the development of the lamprey, the sturgeon, and Amphibians (though the occurrence of invagination has been denied for the frog); it is more modified in Teleosteans and Elasmobranchs, whose ova have more yolk; it is much disguised in Sauropsida and Mammals. Most Vertebrates lay eggs in which the young are hatched outside of the body, and to all these forms the term ovi- REPRODUCTIVE SYSTEM. 515 parous is applied. In some sharks, a few Teleosteans, some tailed Amphibians, a few lizards and snakes, the young are hatched before they leave the body of the mother. To these cases the awkward term ovo-viviparous is applied, but there is no real distinction between this mode of birth and that called oviparous, and both may occur in one animal (e.g. in the grass-snake) in different conditions. In the placental Mammals there is a close organic connection between the unborn young and the mother, and the parturition in this case is usually called wvigarous. But all the three terms are bad. CHAPTER XXI PHYLUM CHORDATA SUB-PHYLUM CRANIATA CLass CYCLOSTOMATA (Synonym, MARSIPOBRANCHII) Tue hag (A@yxine), the lamprey (Petromyzon), and a few others like them, differ in so many ways from Fishes, that they must be ranked in a distinct class. They represent an archaic type, whose interest has been enhanced by the discovery of Paleospondylus in the Old Red Sandstone. GENERAL CHARACTERS Unlike all higher Vertebrates (Gnathostomata), the Cyclostomata have round suctorial mouths, without distinctly developed jaws. They are also without paired fins and without scales. Their respiratory system consists of paired gill-pouches, to which the term Marsipobranch refers. The body is vermiform, the unpaired fins have no true fin-rays. In the extant forms the skeleton is wholly cartilaginous, and the notochord persists unconstricted. The nasal organ ts unpaired, there is no sympathetic nervous system, no conus arteriosus, no distinct pancreas, no spleen, no genital ducts, and the segmental duct persists as such. Their geographical distribution ts wide. First Type. M€dyxine—The Hag The glutinous hag (AZyxine glutinosa) is not uncommon off the coasts of Britain and Scandinavia, the Atlantic coast of America, etc. It lives in the mud at depths of MYXINE, 517 40 to 300 fathoms. It often lies buried with only the nostril protruding from the mud, but it can swim gracefully and rapidly in eel-like fashion in search of prey. : It eats the bait off the fisherman’s long: lines, and it also enters and devours the cod, etc., which have been caught on the hooks. According to some, the hag also bores its way into free- swimming fishes, but the evidence is not satisfactory. Ac- cording to Mr. J. T. Cunningham, the young animals are hermaphrodite, containing immature ova and ripe sper- matozoa, while older forms produce ova only. If the same form is first functionally a male and afterwards functionally a female, the term “protandrous hermaphroditism” is justified, and Nansen corroborated Cunningham’s dis- covery, which is, however, disputed by Bashford Dean. A somewhat similar “ protandrous” hermaphroditism is known elsewhere, eg. in the Nemertean Stichostemma eilhardit, in the aberrant AZyzostoma, and in the crustacean Cymothoide. Hag are said to spawnin lateautumn. Of the development and early history nothing is known. Porm, skin, and muscular system.—The body is eel- like; measuring 15 to 24 in. in the adult. The colour is pinkish, the red blood shining through an unpigmented skin. There is a slight median fin around the tail; beside the mouth and nostril there are four pairs of sensitive barbules. There are no paired fins. The cloacal opening is near the posterior end of the body. The skin is scaleless, and rich in goblet cells, which secrete mucus. There is also a double row of glandular pits, partly embedded in muscle, and arranged segmentally on each side of the ventral surface along its entire length. Each opens by a distinct pore, and so much mucus is rapidly secreted that the ancients said the hag “could turn water into glue.” This makes the hag difficult to grip, and its function is doubtless in part protective. The mucus chiefly consists of strange spiral threads which uncoil when ejected from the sacs. The zigzag muscle segments or myomeres are traceable. The rasping teeth are worked by a powerful muscular structure, sometimes called a “tongue.” A section of this shows a strong muscular cylinder surrounding a cartilage. 518 CYCLOSTOMATA. The skeleton.—The skeleton is wholly cartilaginous. The notochord persists unsegmented within a firm sheath, the skull is a simple unroofed trough, jaws are not distinctly developed, there is only a hint of the complicated basket-work which supports the gill-pouches of the lamprey ; but the tongue, the barbules, etc., are supported by cartila- ginous rods. The tail is protocercal. Nervous system.—The brain has the usual parts, but the cerebrum and cerebellum are little more than rudiment- ary: It is much compressed, with practical obliteration of the ventricles. The fore-brain seems to agree with that of “ Ganoids” and Teleosteans in having a non-nervous roof. Fic. 273.—Median longitudinal section of anterior region of Myxine.—After Retzius and Parker. &., Barbule; 4, nasal aperture ; V7., nasal tube with rings of cartilage ; NC., nasal capsule; BR., brain; SC., spinal cord; ., notochord ; G., gut; 7., cartilage of “tongue”; M7U., muscle; WTT., posterior part of nasalsac; 7.,atooth plate; A77., median tooth on roof of mouth (/Z.). The spinal cord is somewhat flattened, and is sheltered simply by fibrous tissue. Throughout at least a portion of the cord there are two dorsal roots for each ventral root. The union of dorsal and ventral roots is only partial, and there is no sympathetic system. There is no lateral line system. The eye is without lens, cornea, iris, or muscles, and is hidden beneath the skin; the optic nerves do not cross until they enter the brain; the ear has only one semi- circular canal, The single nasal sac (with paired folds of olfactory epithelium in Adedlostoma, an American relative) opens dorsally at the apex of the head, and. communicates posteriorly with the pharynx by a naso-palatine duct. It MYVXINE, 519 may be, as in the lamprey, a combination of olfactory and pituitary involutions. The absence of pigment and sensory structures in the skin, and the simple state of the eye and ear, may be partly associated with the hag’s mode of life. It seems probable that the simplicity is primitive rather than degenerate. Alimentary system. — The mouth is suctorial. There is a median tooth above, and two rows of teeth are borne on each side of the muscular “tongue.” These teeth are entirely “horny,” but sharp. Into the mouth, just in front of a fringed velum which separates it from the pharynx, the nasal, or, as some would say, the naso-pituitary, sac opens. Thus water passes from the nostril into the pharynx. It may be, as Beard suggests, that this passage is‘a per- sistent “old mouth,” the palzeo- stoma of Kupffer. From the gullet open six respiratory pouches, each of which has an efferent tube, and the six efferent tubes of each side unite in a common exhalant ori- fice. The gut is straight and Fig. 274,—Respiratory sys- uniform, with wavy longitudinal tem of hag, from ventral ridges internally, with a two-lobed _ surface. liver and a gall-bladder, but with- 4 Barbules; m., mouth opening. out the usual pancreas. The ¢2, first eill-pouch cat open anus lies within an integumentary {fshowjpterna) lamella se #.% cloacal chamber. canal of first gill-pouch ; ts Respiratory system.— Water {tosinon calen geen may enter by the nasal sac or by the mouth. It passes into the pharynx, down the gullet, into the six pairs of respiratory pouches and their efferent tubes, and leaves the body by the single aperture at each side. The respiratory pouches have much-plaited internal walls, on which the blood vessels are spread out. On the left side, behind the sixth pouch, a 520 CYCLOSTOMATA. tube (the cesophago-cutaneous duct) opens from the cesophagus to the exhalant aperture. Perhaps some water enters by it in inspiration. Vascular system.—The blood contains the usual ame- boid leucocytes and red blood corpuscles, elliptical in form (circular in the lamprey). It is collected from the body in anterior and posterior cardinals, passes through a sinus venosus into the auricle of the heart, thence to the ventricle, thence along a ventral aorta, which gives off vessels to the respiratory pouches. From these the purified blood passes dorsalwards in efferent branchial vessels, which unite posteriorly, to form the dorsal aorta, while from the most anterior a branch goes to the head. The portal vein has a contractile sinus which drives blood through the liver. The pericardium is in free communication with the general body cavity. es Excretory system.—The segmental pronephric ducts persist, and give off short lateral tubules, metamerically arranged, ending in globular malpighian capsules. The pronephros is functional in the young form, and at least part of it persists throughout life, e.g. in a lymphoid structure beside the pericardium. The ducts end by separate pores on a papilla within the integument- ary cloaca, Reproductive system.—//yxine is a protandrous herma- phrodite, spermatozoa being formed at an early period, and ova afterwards. The reproductive organ is simple, unpaired, and moored by a median dorsal fold of peri- toneum. Owing to the large size of the ova, the ovary is very conspicuous in full-grown forms. When the ova are freed from the ovary, they pass into the body cavity. Each has an oval horny membrane, with a circlet of knobbed processes at each end. By these they become entangled together. There are no genital ducts, but just above the anus there is a large genital pore opening from the body cavity into the integumentary cloaca. The development is still unknown. Besides Myxcne glutinosa, two other species are known—one from Japan, another from the Magellan Straits. The southern JZ, australis lives in shallow water close by the shore, but the others live in deep water. The genus Bdellostoma, from the Pacific coasts America, off the Cape of Good Hope, etc., is nearly allied. PETROMYZON. The best-known species, Bdellostoma dombeyz, resembles the hag in many ways. It lives at the bottom of the sea, at depths of a hundred fathoms or more, and is often found inside caught halibut, etc. The gill- pouches have separate openings, and are extraordinarily. variable in number, from six to fourteen on either side—a variability per- haps pointing to ancestral reduc- tion from a larger number (cf. Amphioxus). Large eggs are laid on a shelly or rocky bottom, become connected by polar hooks in chains or clusters, are fertilised after deposition, and exhibit merdblastic discoidal segmentation and direct devel- opment. Ayers’ experiments show that the removal of one or both ears in this form does not materially affect equilibration. SECOND TYPE Letromyzon—The Lamprey There are three British species—the sea lamprey (Petromyzon marinus), over 3 ft. in length; the river lampern (Pf. ftuviatilis), nearly 2 ft. long; and the small lampern or “stone- grig” (P. planeri). They eat worms, small crustace- ans, insect larve, dead animals, etc. ; but they also attach themselves to living fishes, and scrape holes in their skin. As their names suggest, they also fix their mouths to stones, and some draw these together into nests. 521 ss a. Teh 0, Fic. 275.—Bdellostoma stout? (Cali- fornian hag), enveloped in sheath of mucus.—After Bashford Dean. 4., Barbules ¢.,eyes m. mucus; eg., eggs. 522 CYCLOSTOMATA. The spawning takes place in spring, usually far up rivers. Before laying the eggs, the lamprey seems to fast (cf. i, yf Fic. 276.—The lamprey (Petromyzon marinus). I, The entire animal ; note the seven gill-slits of which the first is marked g.s., the nostril #., and the unpaired median fins. II. Ventral pee of the head ; 2.2. pest teeth; 22, lower teeth; ., the piston in the mouth. Uppers surface of the head ; z., the nostril with the pineal Bi be ‘ind it ; ¢., the eye. salmon, Protopterus, frog), and its muscles undergo a granular degeneration (cf. Protopterus, tadpole, etc.). Soon PETROMYZON. 523 after spawning the adults of both sexes die. For reproduc- tion is often the beginning of death as well as of life, though in higher animals the nemesis may be slow. The young are in many ways unlike the parents, and after 2-4 years. pass through a striking metamorphosis. To the larve before metamorphosis the old name Ammocetes is applied. Form, skin, and muscles.—The body is eel-like, with two unpaired dorsal fins, and another round the tail. The skin is scaleless, slimy, and pigmented. Its structure, like that of AZyxine, is complex. Sensory structures occur on the head and along the sides, and form a lateral line system. Fic. 277,.—Longitudinal vertical section of anterior end -of larval lamprey.—After Balfour. m., Mouth; th., thyroid ; ¢.., one of the gill-pouches ; v.a0., ven- tral aorta; 4., heart; V., notochord; S.C., spinal cord; £., auditory. vesicle ; cb., cerebellum; 4.4, pineal body; c.z., cerebral hemispheres ; off, olfactory invo, ution The muscle segments or myomeres are well marked. The suctorial mouth and the rasping “tongue” are very muscular. The skeleton.—The skeleton is wholly cartilaginous. The notochord persists unsegmented, but its firm sheath forms rudimentary neural arches. The skull is imperfectly roofed. There are no distinct jaws, but a cartilaginous ring supports the lips of the mouth. There is a complex basket- work around the gill-pouches, but it is zof likely that its elements correspond to visceral arches. Endoskeletal cartilaginous rods, not comparable to the dermal fin-rays of fishes, support the dorsal and caudal fins, and other skeletal parts occur about the “tongue.” The caudal end of the notochord is quite straight. $24 CYCLOSTOMATA. Nervous system.—The brain has the usual parts, but is small and simple; the roof of the fore-brain is composed of non-nervous epithelium; there is a distinct pineal body, with hints of an eye; the oral part of the hypo- physis is developed from in front of the mouth, and becomes closely connected with the involution of epiblast which forms the nostril. A unique peculiarity in the brain is that the middle part of the roof of the ztev is simply epithelial. The spinal cord is flattened; the dorsal and ventral roots of the spinal nerves alternate and do not unite ; there is no sympathetic system. Though the larva sometimes receives the name of “ nine- eyes ”—which expresses a popular estimate of the branchial apertures—it is blind, for the eyes are rudimentary and hidden. In the adult they rise to the surface, and are fairly well developed. The optic nerves do not cross until they enter the brain. The ear has only two semicircular canals instead of the usual three. The single nasal sac does not open posteriorly into the mouth as it does in Myxine ; though prolonged backwards it ends blindly. Its external opening is at first ventral, but is shunted dorsally and posteriorly. Alimentary system.—The oral funnel, at the base of which the mouth lies, has numerous horny teeth. It is applied to the lamprey’s victim, and adheres like a vacuum sucker ; the toothed “tongue” works like a piston; both flesh and blood are thus obtained. From the floor of the pharynx an endostylar groove is constricted off to form the thyroid. From the gullet of the young larva seven gill-pouches open directly to the exterior; in the adult this larval gullet becomes wholly a respiratory tube. It is closed pos- teriorly, and opens anteriorly into the gullet of the adult, which is a new structure. At the junction of the respira- tory tube with the gullet of the adult lie two flaps or vela. The rest of the gut is straight and simple, with a single- lobed liver, but with only a hint of a pancreas. The gall- bladder and bile-duct disappear-in the adult, and the whole intestine is partially atrophied. There is a slight spiral fold in the intestine. REPRODUCTIVE SYSTEM 525 Respiratory, vascular, and excretory systems.—Seven gill-pouches with plaited walls open directly to the exterior” on each side, and communicate indirectly with the gullet. Water enters the pouches partly v/a the mouth, partly by the external apertures (spiracula), and the movements of the branchial basket and of the tongue-piston aid greatly in the process. In the larva there is an eighth most’ anterior pouch which does not open to the surface. It corresponds to the spiracle of Elasmobranchs. With each of the seven open pouches in the larva four thymus rudiments are associated. The vascular system is essentially the same as in the hag. The red blood cells are biconcave, circular, nucleated discs. ; ; The segmental or pronephric ducts persist as ureters, and are connected with lateral mesonephric tubules forming a kidney more complicated than that of the hag. The pronephros, which is functional in the larva, entirely dis- appears. The ureters unite terminally in a urogenital sinus (not present in the hag), into which there open two genital pores from the body cavity. The sinus opens, like the anus, into an integumentary cloacal chamber. Reproductive system.—The sexes are separate, but ova sometimes occur in the testes. The reproductive organ is elongated, unpaired, and moored by a median dorsal mesentery. There are no genital ducts. The ova and spermatozoa are liberatéd into the body cavity, and pass by two genital pores (true abdominal pores) into the uro- genital sinus, and thence to the exterior. In the male there is an ejaculatory structure, or so-called “penis.” There are many more males than females. Development of P. planeri.—The ripe ovum has a considerable quantity of yolk, but segmentation is total though slightly unequal. A blastosphere is succeeded by a gastrula. The blastopore persists as the anus of the animal, and there is no neurenteric canal. The formation of the central nervous system is peculiar, for the sides. of the epiblastic infolding remain in contact instead of forming an open. medullary canal. In the head region, where the gut is not surrounded by yolk-cells, the mesoblast is formed from hollow folds in ‘‘ enteroccelic” fashion ; but in the trunk region the cushions of hypoblastic yolk-cells change gradually into mesoblast, and acquirea coelom cavity in ‘‘ schizoccelic’” 526 CYCLOSTOMATA. fashion. Thus the two main ways in which a body cavity arises— (a) from ccelom pouches of the archenteron, (4) from a splitting of solid mesoblast rudiments—are here combined. : Metamorphosis of Lampreys.—The larve live wallowing in the sand or mud of streams, and feed on minute animals. Those of 2. planeri are so unlike the adults that they were once referred to a dis- tinct genus Avmocetes, and though a Strasburg fisherman, Baldner, is said to have discovered their true nature about two hundred years ago, the fact was overlooked until August Miiller traced the metamorphosis in 1856. Inthe small lampern the change to the adult state is some- times postponed until the autumn of the fourth or fifth year, when it completes itself rapidly. Less is known about the metamorphosis of the other species. In the Ammocetes, or larva before metamorphosis, the head is small, the dorsal fin is continuous, the upper lip is semicircular, the lower lip is small and separate, the mouth is toothless and not suctorial, the brain is long and narrow, the eyes are half made and hidden beneath the skin; the future gullet, as distinguished from the respiratory tube, is not yet developed. - Contrast between Hag and Lamprey Hac (Myxine). Lamprey (Petvomyzon). Exclusively marine. The fin is confined to the tail. Numerous large glands in the com- plex, slimy skin. Mouth with barbules, no lips, few teeth. Skull without any roof. . Skeletal system less developed than in the lamprey. Only a hint of a branchial basket. Cerebrum and cerebellum rudiment- ary. Eyes hidden and rudimentary. Ear with one semicircular canal. Nasal sac opens posteriorly into the mouth cavity. Six pairs of gill-pouches, opening directly into the gullet, less directly to the exterior. Longitudinal ridges in the intestine. No urogenital sinus; one genital pore. Ova large and oval, with attaching threads; meroblastic in Bdellostoma. Development unknown in Myxine; direct in Bdellostoma. In rivers and seas. Two unpaired dorsal fins. Sensory structures in the complex, slimy, pigmented skin. No barbules (except in the larva), but lips, and many teeth. . Skull very imperfectly roofed. Hints of vertebral arches. Cartilaginous basket- work around gill-pouches. All the usual parts of the brain are distinct. Eyes hidden and retarded in the larva, exposed and complete in adult. Ear with two semicircular canals. Nasal sac ends blindly. Seven pairs of gill-pouches, opening directly to the exterior, less directly into the adult gullet. A slight spiral fold in the intestine. A urogenital sinus, and two genital pores. Ova small and blastic. Development with metamorphosis. spherical; holo- PALAZZOSPONDYLUS. 527 Lampreys are distributed in the rivers and seas of north and south temperate regions. They are often used as food. Besides Petromyzon there are several related genera, e.g. Mordacia and Geotria, from the coasts of Chili and Australia, and /chthyomyzon, from the west coast of N. America. Certain structures called ‘‘conodonts,” from very ancient (Silurian) strata, have been interpreted as teeth of lampreys or hags. Palzospondylus gunni.— Under this title Dr. Traquair has de- scribed a very remarkable fossil form from the Old Red Sandstone of Caithness. He'speaks of it as a “strange relic of early verte- brate life.” It is a dainty little creature, somewhat tadpole-like at first sight, usually under an inch in length. The following characters point strongly to its affinities with Cyclostomata :— (1) ‘The skull is apparently formed of calcified cartilage, and devoid of discrete ossifications.” An anterior part is comparable:to the trabecular and palatal region of a lamprey’s skull; a posterior part is comparable to the parachordal region and auditory cap- sules. (2) **There is a median opening or ring, surrounded with cirri, and presum- ably nasal, in the front of the head” (Fig. 278, ‘n.)s (3) ‘*There are neither jaws nor limbs.” (4) “The rays which support the caudal fin expansion, apparently spring- ing from the neural and hzemal arches, are dichotomised (at least the neural ones), as are the corresponding rods in the lamprey.” Just behind the head lie two small oblong plates (Fig. 278, x.), closely apposed to the commencement of the vertebral column, one on each side. The notochordal sheath is calcified in the form of ring-shaped or hollow verte- Fic.278.—Restored skeleton of Palaospondylus gunni. —After Traquair. d.c., Cirti of dorsal margin 3 Zc., long lateral cirri; v.c., cirri of ventral margin ; 2., nasal ring ; “.p., anterior ‘trabeculo- -palatine part of cranium; 4., anterior depression or fenestra ; 3 C-, pos- terior depression or fenestra ; 3 @., lobe divided off from anterior part; #.@., posterior or para- chordal part of cranium; ., post-occipital plates, 528 HYPOSTOMATA. bral centra with neural arches. . Towards the tail the arches are produced into slender neural spines, opposite which are shorter heemal ones. : Class HyPOSTOMATA or OSTRACODERMI Extinct forms without jaws, without a segmented axial skeleton in the trunk, without any trace of girdles, with complex dermal armature, Fic. 279.—Pterichthys miller?. Lateral view. —Restored by Traquair. with a head shield; Silurian’and Devonian, e.g. Pteraspzs and Cepha- laspis, both without paired limbs; and Pterichthys and Bothriolepi's, with strange armoured paddles (probably not limbs in the ordinary sense) fixed to the antero-lateral angles of the body-shield. Their systematic position is very doubtful. They are the oldest known Vertebrates, : CHAPTER XXII Ciass PISCES—FISHES Sub-Class I. ELASMOBRANCHII :-— . Order Plagiostomi (skates and sharks). Order Holocéphali (Chimera and Callorhynchus). Several extinct orders, e.g. Acanthodei. Sub-Class II. TELEOSTOMI :— Order Crossopterygii (Polypterus), Order Chondrostei, ¢.g. sturgeon. Order Holostei, ¢.g. bony pike. Order Teleostei, the great majority of living fishes, Sub-Class III. DipNor :— Ceratodus, Protopterus, and Lepidosiren, and many extinct forms.. Fisues form the first markedly successful-class of Verte- brates. For though the Tunicates are numerous, most of them are degenerate; the level attained by the lancelets. is represented by, at most, two or three closely related. genera; and the Cyclostomes are also few in number. In the possession of a vertebrate axis and central nervous. system, in the general integration of their structure, and in. their great fecundity, Fishes have an easy pre-eminence. over their Invertebrate inferiors. With their typically wedge-like bodies, supple muscular tails, fin-like limbs, and. the like—they are well adapted to the medium in which they live. Their success may be read in the immense number of individuals, species, and genera, not only now, but in the: past; in the geological record which shows how the cartilaginous Elasmobranchs have persisted strongly from. Silurian ages, or how the mysterious decadence of the. “Ganoids” has been followed by a yet richer predomin- ance of the modern Bony Fishes; and, furthermore, im 34 530 PISCES—FISHES. the plasticity with which many types appear to have assumed particular specialisations, such as the lungs of Dipnoi, which point forward to the epoch-making transition from water to dry land. GENERAL CHARACTERS Fishes ave aquatic Vertebrates, breathing by gills,—vascular outgrowths of the pharynx, bordering gill-clefts and supported by gill-arches. In Dipnot a single or double outgrowth from the gut—the air- or swim- bladder—functions as a lung, air being inspired at the surface of the water. In most Teleo- stomes the same structure ts present, but though occasionally of some use in respiration, ts typically hydrostatic. Two pairs of non-digitate limbs, i.e. in the form of fins, ave usually present, and there are also unpaired median fins, supported by dermal fin-rays (dermotrichia). There are two chief types of paired fin, but no hint of the pentadactyl type of higher Vertebrates. In Dipnoi, and in some extinct forms, the fin has a median segmented axis, which (e.g. Ceratodus) bears on each side a series of radial pieces. In other fishes the radials diverge outwards on one side from several basal pieces, and there ts no median axis. The skin usually bears numerous scales, mainly or wholly due to the dermis, but covered by a layer of epidermis, which may produce enamel. They vary greatly in form and texture, are suppressed in electric fishes, and rudimentary in eels and some other forms. Numerous glandular cells occur in the skin, but these are not compacted into multicellular glands, except in Dipnot and a few poisonous fishes. The skin also bears sensory structures, usually aggregated on the head, and arranged in one or more “lateral lines” along the trunk. There are no muscular elements in the dermis. The muscle segments or myotomes persist as such in adult life. In many the gut ends in a cloaca, in others a distinct anus lies in front of the genital and urinary aperture, or apertures. The nostrils are paired, and do not communicate with the mouth by posterior nares, they are exclusively olfactory organs. There is no tympanic cavity or tympanum, or ear- ossicles. The heart is two-chambered, and contains only venous THE SKATE. 531 blood, except in the Dipnot, where it shows hints of becoming three-chambered, and receives pure blood from the lungs as well as impure blood from the body. Apart from the Dipnot, the heart has a single auricle receiving impure blood from the body, and a ventricle which drives this through a ventral aorta to the gills, whence the purified blood flows to the head and by a dorsal aorta to the body. In addition to the two essential chambers of the heart, there is a sinus venosus, which serves as a porch to the ‘auricle, and there is often a muscular conus arteriosus in front of the ventricle, or a Sulbus arteriosus at the base of the ventral aorta. Except in Dipnot, there is no vein which resembles what is known in all higher Vertebrates as the inferior vena cava, i.e. a single vessel receiving hepatic veins from the liver, renal veins Srom the kidneys, and others from the posterior region. Its place is taken by paired posterior cardinals. The kidney ts usually a persistent mesonephros. There ts no distinct indication of an outgrowth from the find end of the gut comparable to that which forms the bladder of Amphibians or the allantois of higher Vertebrates. Most fishes lay eggs which are fertilised in the water. Compared with Cyclostomes, the true fishes show a distinct advance. Thus we may note—the jaws formed from the first visceral arch, the limbs, the dermal exoskeleton of scales, the frequent occurrence of bone, the true teeth, the paired nostrils, the three semicircular canals, the renal-portal system, the spleen, and the genital ducts. First type of Fisues. The Skate (#aj2)—one of the Elasmobranchii The smooth skate (2. datis), the thornback (2. clavata), and the ray (2. maculata), and other species, are common off British coasts. They are very voracious fishes, and live on the bottom at considerable depths. External characters.—The body is flattened from above downwards or dorso-ventrally, unlike that of the bony flat- fishes, such as plaice and flounder, which are flattened from side to side. The skate rests on its ventral surface, the flounder on its side. The triangular snout, the broad pectoral fins, the long tail with small unpaired fins, are 532 PISCES—FISHES. obvious features. On the dorsal surface the skin is pig- mented and studded with placoid scales; on the top of the skull there are two unroofed areas or fontanelles ; numerous jointed radials support the pectoral fins. Behind the lidless eyes are the spiracles—the first of the obvious: gill-slits, opening dorsally, containing a rudimentary gill, and communicating posteriorly with the mouth cavity. On the ventral surface are seen the sensory mucus canals, the transverse mouth, and the nostrils incompletely separated from it, as if in double harelip, the five pairs of gill aper- tures, the cloacal aperture and two abdominal pores beside it. Pectoral and pelvic girdles support the fore- and hind- fins. In the male the hind-fins are in part modified into complex copulatory “ claspers.” The skin.—On the dorsal pigmented surface, embedded in the dermis, there are many “skin-teeth,” or ‘dermal denticles,” or “placoid scales.” Each is based in bone, cored with dentine or ivory, tipped with enamel. The enamel is mainly, if not wholly, due to the ectoderm (epidermis), the rest to the mesoderm (dermis) ; the whole arises as a skin papilla. The enamel is practically in- organic, the cells having been replaced by lime-salts ; dentine has 34 per cent. of organic matter (apart from water); the bone is more obvious cellular tissue. On the ventral unpigmented or less pigmented surface there are numerous mucus canals or jelly tubes, sensory in function. Some are also present on the dorsal aspect, especially about the head. Most of the slime exudes from glandular goblet cells in the epidermis. Muscular system.~—In the posterior part of the body and in the tail, the segmental arrangement of the muscles may be recognised. The large muscles which work the jaws are noteworthy. Professor Cossar Ewart has described a small electric organ in the tail region of Raja datis and R. clavata, apparently too small to be of any use, probably incipient rather than vestigial. Electric organs are best developed in two Teleostean fishes—a S. American eel (Gymnotus) and an African Siluroid (Ala/apterurus), and in the Elasmobranch Zorpedo, In Gymnotus they lie ventrally along the tail, in Malapterurus they extend as a sheath around the body, and in Torpedo they lie on each side of the head, between the gills and the anterior part of the pectoral fin. In other cases where they are THE SKELETON. 533 slightly developed (certain Elasmobranchs and Teleosteans), they lie in the tail. Separated from one another by connective tissue partitions are numerous ‘‘electric plates,” which consist of strangely modified muscle substance and numerous nerve-endings. The electric discharge is very distinct in the three forms noted above, and is controlled in some measure at least by the animal. The skeleton.—The skeleton is for the most part cartil- -aginous, but here and there ossification has begun, as a crust over many parts, more deeply in the vertebre, teeth, and scales. The vertebral column consists of an anterior plate not divided into vertebrze, and of a posterior series of distinct vertebrae. Each of these has a biconcave or amphiccelous centrum. From each side of the centrum a transverse process projects outwards, and bears a minute hint of a rib. From the dorsal surface of each centrum two neural processes arise. Between each two vertebrae there is at each side a broad interneural plate, which not only fills what would be a gap between the neural processes and the slightly developed neural spine, but also links the vertebrz together. In the caudal vertebree, what seem to be the transverse processes are directed downwards to fofm a heemal arch enclosing the caudal artery and vein. In the lozenge- shaped spaces between the vertebrz lie gelatinous remains of the notochord. In Selachians and Dipnoi amoeboid cartilage cells from the arcualia (paired nodules of cartilage in the mesenchyme or embryonic connec- tive tissue outside the sheath of the notochord, which form neural and hheemal arches) migrate into the sheath of the notochord and convert it into a cylinder of cartilage (segmented into centra in Selachians), This is called a chordacentrous vertebral column. In Teleostomes and higher Vertebrates, the expanded bases of the arcualia fuse to form cartilaginous (eventually bony) centra, outside the sheath of the noto- chord. This is called an arczcentrous vertebral column. The skull is a cartilaginous case, with a spacious cavity for the brain, a large posterior aperture or foramen magnum through which the spinal cord passes, two condyles working on the end of the vertebral plate, a large ear capsule on each side posteriorly, a similar nasal capsule on each side anteriorly, a long rostrum in front, two fontanelles on the roof. Compared with the skull of a cod or of a higher Vertebrate, that of a skate is simple; it is not ossified, nor divided into distinct regions, nor has it anything corre- sponding to the investing membrane bones, which in higher animals are added to the original foundations of the skull, nor do the visceral arches in the skate take part in forming the skull, which arises, as usual, from parachordals, trab- ecule, sense capsules, etc. 534 PISCES——FISHES. The visceral arches are primitively supports for the N \\ an Sz, i) wy Za 2D ln. A. br J. V4 Fic. 280.—Under surface of skull and arches of skate.—After W. K. Parker. 41, First labial cartilage; 2., rostrum; ¢., trabecular region ; #.c., nasal capsule; @.0., antorbital cartilage ; 2tg-s palato-pterygo-quadrate ; M.c., Meckel’s car- tilage; .%., hyo-mandibular; 4.47.1-5, hypobran- chials ; ¢.47.5, fifth ceratobranchial ; ¢.4,., cerato-hyal ; 4.2-4, labial cartilages. wall of the anterior part of the food canal, but the first two of them are much modified in connection with the jaws. THE SKELETON. 535 The upper jaw of the skate is a strong transverse bar, formed from the union of two _palato-pterygo-quadrate cartilages. The lower jaw is a similar bar formed from the union of two Meckel’s cartilages. From the ear capsule to the articulation of upper and lower jaw there extends on each side a club-shaped cartilage, which connects the jaws with the skull, known as the hyo- mandibular or suspensorium. It is the upper half of the second arch. Attached to it is a slender four-jointed rod— the lower half of the hyoid arch. Fic, 281. —Side view of skate’s skull. —After W. K. Parker. éx., First labial cartilage; 2.c., nasal capsule; @.o., antorbital; p.pt.g., palato-pterygo-quadrate; .c., Meckel’s cartilage; h.m., hyo-mandibular; ¢.4., epi-hyal; ¢.4., cerato-hyal; 4./., hypo-hyal ; %.67.1-5, ypobranchials ; ¢.d., ceratobranchial ; e.6r., epibranchial ; 4.47.1., first prebranchial ; 2.4., inter-hyal ; m.pt., meta-pterygoid ; 2, 5, 7, foramina of exit of the corre- sponding nerves, Then follow five -branchial arches, each primarily four- jointed, forming the framework of the gill-bearing region. Of less importance are the labial cartilages about each nasal capsule, an antorbital cartilage uniting the nasal capsule with the end of the pectoral fin, and a spiracular or meta-pterygoid cartilage supporting the rudimentary gill in the spiracle. The pectoral girdle forms an almost complete hoop of 536 PISCES——FISHES. eeeaagg Ge” \<3 Lp Fic. 282,—Skeleton of skate. —From a preparation. In the skull notice the anterior rostrum, the nasal capsules (t.0.) with the antorbital cartilages projecting laterally; the palato- pterygo-quadrate cartilage (f.g.) or upper jaw; Meckel’s THE SKELETON. 537 cartilage attached dorsally to the crest of the vertebral plate. The ventral region is distinguished as the coracoid, and is separated from the dorsal or scapular region by three facets, to which the three basal pieces of the pectoral fin are fixed. A separated portion of the girdle forms the supra-scapula, which connects the scapula with the crest of the vertebral plate. Of the three basal pieces of the fin, the anterior or propterygium and the posterior or metapterygium are jarge, the median or mesopterygium is small. All bear jointed radials, which are parts of the endoskeleton; a few radials articulate directly with the shoulder-girdle (see Fig. 282). The true fin-rays, comparable to the dermal rays in the fins of Bony Fishes, are represented by “horny” (ox, more strictly, elastoidin) fibres. These are intercellular products of mesoderm (mesenchyme) cells. -The pelvic girdle is simpler than the pectoral, and is not fixed to the vertebral column. Its dorsal region is pro- longed into an iliac process, while anteriorly a prepubic process projects from the ventral (pubic) bar. The girdle bears two articulating facets, to the posterior of which the strong basal piece or metapterygium of the hind-limb is attached. From this, and from the anterior facet of the girdle, the jointed radials proceed. The claspers of the males are closely connected with the posterior part of the hind-limb, and ‘have a complex cartilaginous skeleton and an associated gland. The brain.—The brain (see p. 483) has the following parts :— i. The fused cerebral hemispheres or prosencephalon, with a nervous roof, and without ventricles. : cartilage (JZ.) forming the lower jaw; and the hyo-mandibular (4.m.) which suspends the jaws to the skull. A little farther back are seen the five branchial arches and the anterior hyoid arch ; 4.47., the fifth hypobranchial ; v.f/., the vertebral plate. At the right is seen the skeleton of the paired ‘fins, at the left the surface of the skin with the sensory tubes (s.4.); sc., the scapular region of the shoulder-girdle, with the scapular fontanelle ; c., the coracoid region; 2.f2., the ‘anterior basal cartilage or pro-pterygium ; 7./., the meso-pterygium ; 77.f4., the meta-pterygium—all three bear jointed radials, while a few, as shown here, articulate directly with the shoulder-girdle ; pu., pubic bar of pelvic girdle ; s¢., stomach; s.v., spiral valve of intestine. 538 PISCES—FfISHES. c t Py BRANCHIALS Fic, 283.—Dissection of nerves of skate. CH., Cerebral hemispheres; O.TH., optic thalami; OL., optic lobes ; M., medulla; 4V., posterior part of cerebellum, covering CRANIAL NERVES. 539 2. The thalamencephalon or region of the optic thalami, with a thread-like pineal body above, infundibulum and pituitary body below, thinly roofed third ventricle within. 3. The mesencephalon or mid-brain, with the optic lobes above, the crura cerebri below, the iter passing between. 4. The cerebellum, with an anterior and a posterior lobe, both marked by ridges and grooves. 5. The medulla oblongata, with thin vascular roof, with dorso- lateral extensions called “‘ restiform bodies.” The region beneath the thalamencephalon bears—(a) two ovoid inferior lobes ; (6) the infundibulum, which carries the pituitary body; and (c)a thin-walled three-lobed saccus vasculosus, situated between the pituitary body and the inferior lobes. Cranial nerves.!—Owing to the flat form of the skate and its frequently large size, the dissection of the cranial nerves is perhaps easier than in any other Vertebrate. Expecting practical verification, we shall describe their distribution in some detail, following in regard to certain points the investigations of Professor Cossar Ewart. I. The o/factory, rising from the olfactory lobes of the cerebral hemispheres, extend to the nostrils, and there expand in olfactory bulbs, which give off small nerves to the nostrils. II. The oftic, leaving the region of the optic thalami, cross in an optic chiasma, and extend to the retina of the eye. III. The oculomotor or ciliary, arising from the crura cerebri, near the mid-ventral line, supply four of the six muscles of the eye. There is a ciliary ganglion in connection with III and also with the ganglion of the ophthalmicus profundus. 1] have to acknowledge indebtedness to Dr. Beard for his kindness in helping me to state the distribution of these nerves. fourth ventricle; OB., olfactory bulb ; OC., olfactory capsule ; SO.,.superior oblique muscle ; E., eye; SR., superior rectus; ER., external rectus ; SO.VII., superficial ophthalmic branch of VII. ; SO.V., superficial ophthalmic branch of V. ; OP., oph- thalmicus profundus; A.C., auditory capsule; B.Pl., brachial plexus ; R.F., recurrent facial ; C.T., chorda tympani; F.P., facial proper ; Hy., hyoidean ; Hyomn., hyomandibular ; E.M., external mandibular ; M.M., mandibular muscle ; Sp., spiracle ; P.sp., prespiracular ; Pl., palatine ; O.B., outer buccal ; Mn., mandibular; Mx., maxillary; 1.B., inner buccal; L., lateral branch of X.; Py., pyloric branch; C., cardiac branch. 7540 PISCES— FISHES, IV. The pathetic or trochlear are small nerves emerging dorsally from between the mid- and hind- brain, and supplying the superior oblique muscles of the eye. VY. The trigeminal, or nerve of the “mouth-cleft,” arising from the medulla oblongata (as do all that follow), has a (Gasserian) ganglion on its root, and three main branches—the sensory maxillary, which unites with the inner buccal of VII.; the motor mandibular, which inner- vates the muscles of the jaws; and the sensory superficial ophthalmic (or orbitonasal), which runs over the eye to the snout, closely united (inside the same sheath) with a similar branch of VII. ; Parallel to these superficial ophthalmics, internal to and above the inner buccal of VII., there is a ganglionated ophthalmicus profundus, which sends branches to the eyeball, snout, etc. VI. The abducens, a slender nerve, arising near the mid-ventral line, adjacent to V. and VIII., and hidden beneath the former, supplies the external rectus muscle of the eye. WII. The facial, the nerve of the spiracular cleft, supplies all the five groups of ampullz on the head, and has numerous branches. 1. The ophthalmicus superficialis runs over and past the eye, in intimate association with the similar branch of V., and supplies ampulla on the snout. 2. The inner buccal runs under the eye, through the nasal capsule, to inner buccal ampulle. The - outer buccal runs under the eye, external to the olfactory capsule, to outer buccal ampullee. 3. The large hyomandibular runs directly outwards behind the spiracle to hyoid ampulle. It gives off minor hyoidean nerves. 4. The external mandibular runs behind and outside of the mandibular muscle to mandibular ampulle, and is a branch of the hyo-mandibular. 5. The palatine descends in front of the spiracle to the roof of the mouth. Close beside it there is a prespiracular. CRANIAL NERVES. 54D 6. The ‘‘ facial proper,”’ apparently os from 3, supplies the muscles of the hyoid arch 7. The ‘‘chorda tympani,” apparently arising from 3,. runs under the spiracle to the inner side of the- jaw. With the loss of the sensory ampullee, the seventh: nerve of higher Vertebrates becomes restricted to: the last three branches (5, 6, and 7). A recurrent branch of the facial also runs externah to the auditory capsule to IX., and is equivalent. to Jacobson’s anastomosis in higher forms. VIII. The auditory, arising just behind VIL, is the nerve: of the ear. IX. The glossopharyngeal, the most typical of all, is the nerve of the first functional gill-cleft. Its root passes through the floor of the auditory capsule, and bears a ganglion above the cleft. Its. branches, as named by Beard, are :— I. Peal to the muscles of the first branchial! arch ; 2. Pree-branchial, arches over the cleft and runs along. its front wall; 3. Intestinal or visceral, to the pharynx ; 4. Supra-branchial or dorsal, to a few sense organs om the mid-dorsal line of the head. X. The vagus, apparently made up of several cranial nerves, has numerous roots, and divides into six main ganglionated portions, which supply the four posterior clefts and arches, the posterior jelly-tubes, and the heart and stomach. It thus. consists of :—~ 1. Ganglionated roots with nerves to the clefts and’ arches (2 to 5 inclusive), with post-branchial, pree-branchial, and pharyngeal branches as in IX. 2. A ganglionated root, arising in front of all the others, from which arises the lateral branch innervating all the posterior sensory tubes. 3. From the fourth branchial branch arises the gang- lionated intestinal which innervates the heart and the stomach. The spinal cord lies in the ee neural archwad! above the vertebral column, is divided by deep dorsal any ventral fissures, and gives off numerous spinal nerves,. formed as usual from the union of dorsal (sensory) and 542 PISCES—-FISHES. ventral (motor) roots. The first sixteen or eighteen nerves form the brachial plexus, which supplies the pectoral fin. The sympathetic system consists of a longitudinal gang- lionated cord along each side of the vertebral column. of ch ch Ino Fic. 284.—Side view of chief cranial nerves of Elasmobranchs, —-Slightly modified from Cossar Ewart. olf., Over olfactory nerve; ch., over cerebral hemispheres ; cd., over cerebellum ; 7z.0., over medulla oblongata; 7., mouth; mx., maxillary branch of 5; 2.5, mandibular branch of 5 3 2.7, mandibular branch of seventh nerve ; @.1~5, groups of ampulla ; 0.8.5, superficial ophthalmic of § ; 0.f., ophthalmicus profundus ; 0.8.7, superficial ophthalmic of 7; JV., nostril; 3, oculomotor ; e.g., Ciliary ganglion; 5, trigeminal; 7.4., inner buccal; 0.d., + outer buccal ; 74., buccal of 7; 2., palata: of 7; sf., spiracle ; ch., chorda tympani; 7.47., hyomandibular of 7; 8, auditory; £., ear; 9, glossopharyngeal ; 10, roots of vagus ; ¢.10, lateral nerve of vagus ; 7.10, intestinal nerve of vagus ; 1’-s’, gill-clefts. Sense organs.— (a) The eyes (see p. 495). The iris has a fringed upper margin. (6) The ears (see p. 493). The vestibule is connected with the sur- face by a delicate canal—the aqueductus vestibuli—a remnant of the original invagination. A small part of the wall of the auditory capsule is covered only by the skin, forming a kind of tympanum. Within the vestibule are calcareous otolithic par- ticles surrounded by a jelly. (c) The nasal sacs are cup-like cavities with plaited walls. (d@) The sensory tubes are best seen on the ventral surface, where they lie just under the skin. At their internal ends lie ampullee, containing sensory cells. At their outer ends there are pores. It is probable that they are organs partly of touch, and partly of ‘‘ chemical sense.” Alimentary system.—The mouth is a transverse aperture ; the teeth borne by the jaws are numerous, and those worn away in front are replaced by fresh ones from behind ; naso- ALIMENTARY SYSTEM. 543 buccal grooves connect the nostrils with the corners of the mouth ; the spiracles, which open dorsally behind the eyes, communicate with the buccal cavity; from the gullet five gill-clefts open ventrally on each side. The stomach, lying to the left, is bent upon itself; the large brownish liver is trilobed, and has an associated gall-bJadder, from which the © bile-duct extends to-the duodenum—the part of the gut immediately succeeding the stomach ; the whitish pancreas lies at the end of the duodenal loop, and its duct opens opposite the bile-duct. The intestine is exceedingly short, but it contains an internal spiral fold—which greatly increases the absorptive surface. ; The development of this spiral intestine is of general interest. The well-nourished gut grows quickly, but its increase in calibre is hindered by the peritoneal mesodermic sheath, and the growth is expressed in an internal invagination or fold. But as the growth continues in length as well as in calibre, and as the gut is fixed at both ends, twisting or coiling or both must result. In Mammals, for instance, the result is a coiled in- testine. But in Elasmobranch fishes the coiling or twisting takes place w2thzz the peritoneal sheath, not along with it. In the case of the skate and some other Elasmobranchs, close twisting occurs, and the so-called spiral valve is mainly due to the fusion of the walls of adjacent twists. A small “rectal gland” of unknown ces hh significance arises as a vascular diverti- —After T. J. culum from the end of the gut. The end Parker. of the gullet and the anterior portion of the stomach and the rectum are supported by folds of peri- toneum,—the membrane which lines the body cavity; the rest of the gut lies freely. Rectum, ureters, and genital ducts all communicate with the. exterior through the common terminal chamber or cloaca. An abdominal pore opens on each side of the cloacal aperture, and puts the body cavity in direct communication with the exterior. Excepting mouth cavity and cloaca, the gut is lined by endoderm. Respiratory system.—The first apparent gill-clefts—the spiracles—open dorsally behind the eyes. Each contains a rudimentary gill on the anterior wall, supported by a 544 PISCES—FISHES. spiracular cartilage. Through the spiracles water may enter or leave the mouth. There are other five pairs of gill-clefts, separated by com- plete partitions (Elasmobranch), and with ventral apertures. The first is bounded anteriorly by the hyoid arch, posteriorly by the first branchial arch. The hyoid bears branchial filaments on its posterior surface ; the first four branchials bear gill filaments on both surfaces; the fifth branchial bears none. Each set of branchial filaments is called a half- gill; and as the first four branchial arches bear a half-gill on Fic. 286.—Upper part of the dorsal aorta in the skate. —After Monro. @.a., Dorsal aorta; ¢., coeliac artery; #., superior mesenteric; s.cl., subclavian ; ¢.d., efferent branchial vessels, three formed from the union of nine; w., vertebral ; c., carotid. each side, and the hyoid arch a half on its posterior surface, there are four and a half gills in all. There is no operculum or gill cover. Circulatory system.—The impure blood from the body enters the heart by a bow-shaped sinus venosus, opening into a large thin-walled auricle. Thence through a bivalved aperture the blood passes into the smaller muscular ventricle, and from this it is driven through a contractile conus arteriosus, with three longitudinal rows of five valves, into the ventral aorta. CIRCULATORY SYSTEM. 545 The ventral aorta gives off a pair of posterior innominate arteries, which take blood to the three posterior gills, and a pair of anterior innominate arteries, which supply the anterior gill and the hyoid half- gill on each side. The purified blood passes from each half-gill by an efferent branchiah artery. To begin with, there are nine of these on each side, but by union they are reduced first to four and then to three efferent trunks, which combine to form the dorsal aorta. From the efferent branchial of the hyoid arch a carotid arises, which: divides into internal and external branches supplying the brain: and head. The two internal carotids unite, and pass through a small hole: ‘i ae Fic. 287.—-Heart and adjacent vessels of skate.—In part after Monro. v., Ventricle; ¢.@., conus arteriosus ; #.t., posterior innominate ; Ua, ventral aorta ; @.7., anterior innominate ; 7’4., thyroid; M., ‘mouth ; a, auricles S.%., sinus venosus 3 S.C, precaval sinus or sinus of Cuvier ; "he Sey hepatic. sinus ; j., jugular; dn, brachials ; cdi, cardinal ; efg., epigastric. on the ventral surface of the skull. Just after the first and second mair efferent branches have united, a vertebral is given off, which passes. through a hole in the vertebral plate to the spinal cord and brain. The dorsal aorta gives off—(1) a subclavian to each pectoral fin ; (2) a. coeliac to the stomach, duodenum, and liver; (3) a superior mesen- teric to the intestine, pancreas, and spleen; (4) spermatic arteries to the reproductive organs; (§) an inferior mesenteric to the rectum ; (6) renal arteries to the kidneys; (7) arteries to the pelvic fins. It ends in the caudal artery. At each end of the bow-shaped sinus venosus there is a precaval sinus. This receives venous blood as follows :—(a) from the head by 35 546 PISCES——FISHES., a jugular vein ; (4) from the liver by a hepatic sinus, which runs from one precaval sinus to the other like the string of the bow; (c) froma large posterior cardinal sinus (between the reproductive organs) by a cardinal vein on each side; (@) from the hind-fin by an epigastric, with which brachials from the fore-limb unite anteriorly. The great cardinal sinus receives blood from the hind-limbs, the kidneys, and other posterior parts. Blood asses zo the liver (a) from the cceliac artery, and (4) by portal veins from the intestine (the hepatic portal system) ; blood /eaves the liver by hepatic veins which enter the hepatic sinus. Blood fasses znto the kidneys (a) from the renal arteries, and (4) by renal portal veins from the caudal, pelvic, and lumbar regions (the renal portal system); blood eaves the kidneys by posterior cardinal veins, which enter the cardinal sinus. Into the precaval sinus there also opens the lymphatic trunk. The heart lies in a_ pericardial cavity, which is connected with the abdominal cavity by two fine canals, and is an anterior part of the ccelom. The blood contains, as usual, red blood corpuscles and leucocytes. The dark red spleen lies in the curve of the stomach. The red thyroid gland lies just in front of the anterior end of the ventral aorta. The thymus is represented by a whitish body dorsal to each of the first four gill-clefts. | Each begins as a patch of endoderm, and this is invaded by migratory mesenchyme cells which multiply as lymphocytes. Fic. 288.—Urogenital organs of male skate. E : is Teter why, wolaiagiiiaY wt, xcretory and reproductive vas. deferens + °K idney § aoe systems.—The dark red kid- ee tec ainey CA, eloace, neys lie far back on each side of the vertebral column. They are developed from the hind part of the mesonephros. Several tubes from each kidney combine to form a ureter. The two ureters of the male open into the urogenital sinus, whence the waste products pass out by the cloaca; in the female they open into little bladders,—the dilated ends of the Wolffian ducts,—and thence by a common aperture into the cloaca. _ EXCRETORY AND REPRODUCTIVE SYSTEMS. 547 The segmental duct of each side divides into Wolffian and Miillerian ducts. The Wolffian duct becomes in the male the vas deferens, in the female it is an unimportant Wolffian duct; the Miillerian duct becomes in the female the oviduct, in the male it is a mere rudiment. The muscles and other organs of Elasmobranchs retain considerable quantities of nitrogenous waste products. There can be no doubt that the body cavity helps in excretion, and gets rid of waste through the two abdominal pores. In some Elasmobranchs these are replaced by openings (neph- rostomes) into the kidney. Occasionally there are both nephrostomes and abdomi- nal pores. pon ae The male organs or testes lie on each side of the cardinal sinus, moored by a fold of, peritoneum. Sperma- tozoa pass from the testis by vasa effer- entia into a tube sur- rounded anteriorly by epididymis. The tube of the epididymis is continued into the F!G. 289.—Urogenital organs. of female vas deferens, which bee nee A nee page a . : @g., Aperture of united oviducts; W.D, olffan is dilated posteriorly “uct; ov, ovary; O.D.G., oviducal gland ; info @ seminal vee ai. Sivan deter ied icle and an adjacent kidney (arrow from base of oviduct into cloaca). sperm - sac. Finally, ; the two vasa deferentia open into the urogenital sinus, whence the spermatozoa pass into the cloaca. Thence, in copulation, they pass into the complex “claspers”.of the male, which are said to be inserted into. the cloaca of the female.. The female organs or ovaries lie on each side of the car- $99 7» 2 2.2. 548 PISCES—FISHES. dinal sinus, moored by a fold of peritoneum. In young skates they are like the young testes, but in the adults they are covered with large Graafian follicles, each containing an ovum. The ripe ova burst into the body cavity, and enter the single aperture of the oviducts, which are united an- teriorly just behind the heart. About the middle of each oviduct there is a large oviducal gland, which secretes the horny “purse”; the elastic lower portions open into the cloaca. _ Development.— The ripe ovum which bursts from the ovary is a large sphere, mostly of yolk, with the for- mative protoplasm concen- trated at one pole. The formation of polar bodies (maturation) takes place at an early stage. Fertilisation occurs in the upper part of the oviduct. Some observers have de- scribed the occurrence of polyspermy. As the ovum descends farther, it Fic.290.—Elasmobranch develop- ment.—After Balfour. Uppermost figure shows blastoderm at an early stage. £f., Epiblast; sg.c., segmentation cavity; ., yolk-nuclei. Middle figure shows the invagination which forms the gut. .x., Blastopore ; &-, archenteron. Mesoderm dark. Lowest figure, a longitudinal section at a later stage. Zf., epiblast; ~.c., neural canal; #e.c., neurenteric canal ; is surrounded first by albuminous material, and then by the four- cornered ‘‘mermaid’s purse” se- creted by the walls of the oviducal gland. This purse is composed of &-y gut; #., notochord. Mesoderm keratin—a common skeletal sub- ark. stance which occurs for instance in hair and nails. Its corners are produced into long elastic tendrils, which may twine round’ seaweed, and thus moor the egg. Rocked by the waves, the embryo develops, and the young skate leaves the purse at one end. Development is very slow, and takes perhaps the greater part of a year. The egg-case of some sharks, e.g. the Port Jackson shark (Cestracton philippz), has elastic spiral fringes, and is found securely wedged among the rocks ; that of a neighbour species (C. ga/eatzs) has reduced spirals. ending in a couple of tendrils, which may be go in. in length, and serve very effectively to entangle the egg among seaweed. The segmentation is meroblastic, being confined to the disc of formative protoplasm. From the edge of the DEVELOPMENT, 549 blastoderm, or segmented area, some nuclei (so-called “merocytes”) are formed in the outer part of the subjacent yolk (Fig. 290, ~.). It seems most probable that these are hypoblast elements which assist in the preparation of the yolk for absorption, and eventually degenerate in the empty external yolk-sac. At the close of segmen- tation the blastoderm is a lens-shaped disc with two: strata of cells. It is thicker at one end—where the em- bryo begins to be formed. ‘Towards the other end, be- tween the blastoderm and the yolk, lies a segmentation cavity (Fig. 290, 5g.c.). At, the embryonic end the outer layer or epiblast undergoes a slight invagina- tion (Fig. 290, x.), beginning to form the roof of the future gut (g.); in other words, establishing the hypo- blast. This inflected arc of the blastoderm corresponds to the blastopore or mouth een F ire of the _gast tula, which is Gua et hich much disguised by the pres- has been cut open to show con- ence of a large quantity of tents. yolk. As the invagination eg. ‘‘External” gills; dA, dorsal fin proceeds, the segmentation {14s 2, yelksacs at, Salk of yolk cavity is obliterated. The case By mene Ot niet it is ed 2 floor of the gut is formed by ee ee ee infolding of the lateral walls. Along the mid-dorsal line of the epiblast a medullary groove appears—the beginning of the central nervous system. Its sides afterwards arch towards one another, and meet to form a medullary canal (Fig. 290, #.c.). A posterior communication between this dorsal nervous tube above and the ventral alimentary tube persists for some time as the neurenteric canal (Fig. 290, 7e.c.). 550 PISCES—FISHES. The mesoblast arises as two lateral plates, one on each side of the medullary groove. The plates seem to arise as a pair of solid outgrowths from the wall of the gut. They are afterwards divided into segments. Between the meso- blast plates, along the mid-dorsal line of the gut, the notochord is established (Fig. 290, 7.). Besides the internal establishment and differentiation of layers, there are two important processes,—(a) the growth of the blastoderm around the yolk, (4) the folding off of the embryo from the yolk. The result of the two processes is that the yolk is enclosed in a yolk-sac, with which the embryo is finally connected only by a thin stalk—the umbilical cord. The history of the yolk is briefly as follows:—It is accumulated by the ovum from neighbouring cells, and from the vascular fluid ; it is partly prepared for absorption by the merocytes or yolk-nuclei ; it is at first absorbed by the blood vessels of the yolk-sac ; at a later stage, absorption by blood vessels becomes less and less important, and the yolk passes inside the embryo and into the gut, where it is digested. Then the yolk-sac, empty of all but merocytes, degenerates, shrivels, and disappears. Second type of Fisoes. The Haddock (Gadus eglefinus) —A type of Teleosteans with closed swim-bladder (Physoclysti). Form and external features.—The elongated wedge-like form is well adapted for rapid swimming. The lower jaw bears a short barbule,—long in the cod (G. morrhua), absent in the adult whiting (G. merlangus). The nostrils, situated near the end of the snout, have double apertures. The eyes are lidless, but covered with transparent skin. Over the gill chamber and the four gills lies the operculum, supported by several bones. Distinct from one another, but closely adjacent, are the anal, genital, and urinary apertures,—named in order from before backwards. Along the sides of the body runs the dark lateral line containing sensory cells. There are three dorsal and two anal fins, and an apparently symmetrical tail fin. Skin.—The small scales are developed in the dermis, and consist of flexible structureless bone (vitrodentin). THE HADDOCK. 55? Their free margin is even, a characteristic to which the term cycloid is applied, in contrast to ctenoid, which n.@., Nasal apertures (double on each side); @. 7.1, df.2, 2,/.3, dorsal unpaired fins ; c.f, the caudal fin of the homocercal tail. &., Barbule; 9f., operculum covering the four gills; 47.7., con- tinuation of the gill-cover forming the branchiostegal mem- brane ; Av. /, pelvic fin(=hind-limb)—note its jugular position in front of J.f£, the pectoral fin (=fore-limb). a., Anus; g, genital aperture; #., urinary aperture; @./.1, @,/.2, unpaired anal fins. : describes those scales which have a notched or comb-like Fic. 293.—External characters of a Teleostéan— a carp (Cyprinus carpio).—After Leunis. R., Dorsal unpaired fin; S., homocercal caudal fin; A., anal fin; ~ B., B., pectoral and pelvic paired fins. Note also the lateral line and barbule. free margin. Over the scales extends a delicate partially pigmented epidermis. Appendages.—The pectoral fins are attached to the 552 PISCES— FISHES. shoulder-girdle just behind the branchial aperture. The pelvic or ventral fins, attached to what is at most a rudiment of the pelvic girdle, lie below and slightly in front of the pectorals—far from the normal position of hind-limbs. Muscular system.—The main muscles of the body are disposed in segments,—myotomes or myomeres, separated by partitions of connective tissue. The effective swimming -organ is the posterior body and the tail, as contrasted with the pectoral fins in the skate. Skeleton.—The vertebral column consists of biconcave ‘or amphiccelous bony vertebrae, and is divided into two regions only, caudal and pre-caudal. The spaces between the vertebre are filled by the remains of the notochord. Each cen- trum in the trunk region bears superior neural processes, uniting in a neural arch crowned by a neural spine, and transverse processes projecting from each side. Artic- ulated to the distal ends of the transverse processes are the downward curving ribs, and also more delicate intermuscular bones which curve upwards. In the caudal verte- bre (Fig. 294), the centra (¢.) bear not only superior neural processes (7.@.), but also inferior heemal processes (4.a.); they are of course without ribs. Fic.294.—Cau- At the end of the vertebral column lies dal vertebra q fan-shaped hypural bone which helps to ofhaddock. support the tail, and is developed from an hee pee enlarged hemal arch. The fin-rays are et jointed flexible rods, which in the dorsal and . anal fins are attached to the ends of inter- spinous bones alternating with the neural and hzemal spines, and connected with them by fibrous tissue. The skull includes the following bones, which may be ‘grouped in the following regions (the membrane bones in italics) :— (2) Around the foramen magnum: basi-occipital, two ex-occipitals, and a supra-occipital. (4) Along the roof: sepra-occipital, paréetals, frontals, mesethmoid, nasals. Beneath the parzeta/s lie the alisphenoids. SKELETON. 553 {c) Along the floor: basi-occipital, Jarasphenotd, vomers. (@) Around the ear on each side: sphenotic, pterotic, and épiotic (above), prootic and opisthotic (beneath). 4e) In front of and around the orbit: parethmoid, Jachrymal, orbitgls. Thus the haddock’s skull shows in two respects an ad- Fic. 295.—Disarticulated skull of cod. S.O., Supra-occipital ; Pa., parietal; 77, frontal; 47.Z., meseth- moid; WV., nasal; P.#., parethmoid; Oz, otics; #.O., ex-occi- pital; B.O., basi-occipital; Pa.S., parasphenoid; V., vomer; L., lachrymal; ord., orbitals; A.4Z., hyomandibular; S., symplectic; Q., quadrate; Pz., pterygoid ; w.pz., metaptery- goid; ms.f¢., mesopterygoid; PZ, palatine; AZx., maxilla; Pmy., premaxilla; Av., articular; Am., angular; D., dentary; u.h., urohyal ; 2.4., hypohyal; ¢.4., ceratohyal ; ¢f.4., epihyal ; i.h., interhyal; Of., opercular; S.of., sub-opercular; z.op., inter-opercular ; 2.0%., pree-opercular. vance upon that of the skate: first, in the ossification of the primitive cartilage ; and second, in the addition of membrane bones. Of the latter, the parietals and frontals cover over the spaces which in the skate form the fontanelles. 554 PISCES—FISHES. The first or mandibular arch is believed by many to form Meckel’s ° cartilage beneath, and the palato-pterygo-quadrate cartilage above. Meckel’s cartilage becomes the foundation of the lower jaw, and bears a large tooth-bearing membrane bone—the dentary, a small corner bone—the angular, while the articular element is a cartilage bone. Of the bones associated with the upper part, the palatine lies in front, the quadrate articulates with the lower jaw; while between palatine and quadrate lie the pterygoid, the mesopterygoid, and the meta- pterygoid. The second or hyoid arch is believed by many to form the hyo- Fic. 296.—Pectoral girdle and fin of cod. fr., Fin-rays ; 8.0., brachial ossicles; cor., coracoid; se., scapula; ed., clavicle; .cZ., post-clavicle ; s.cZ., supra-clavicle ; 4.2., post- temporal. mandibular and the symplectic above, and various hyoid bones beneath. The hyomandibular, and its inferior segment the symplectic, connect the quadrate with the side of the skull. Of the six hyal bones, the largest and most important is the ceratohyal, which bears seven long branchiostegal rays. It is important to note that the bones formed in connection with these arches do not yet form an integral part of the skull. The toothed premaxilla forms the upper part of the gape, while the maxilla which articulates dorsally with the vomer, and nearly reaches the quadrate posteriorly, does not enter into the gape. Both are mem- brane bones. NERVOUS SYSTEM. 555- In the opercular fold are four membrane bones. _ There are four pairs of complete branchial arches, which are divided into various parts. Of these the most interesting are the two superior pharyngeal bones, which lie in the roof of the pharynx and bear teeth, and are formed by the coalescence of the dorsal elements of the arches. Their teeth bite against those of the inferior pharyngeal bones,, which lie on the floor of the pharynx, and represent the fifth branchial arches. The limbs and girdles.x~The dermal rays of the pectoral fin are attached to four small brachial ossicles ; these articulate with a dorsal scapula and a more ventral coracoid ; both of these are attached to the: inner face of a large clavicle or cleithrum, which almost meets its fellow in the mid-ventral. line of the- throat. From the clavicle a slender: post-clavicle extends backwards and downwards; while a stout supra- clavicle extends from the dorsal end of the clavicle upwards to articulate with a forked post-temporal, which articulates with the back of the skull. It must not be assumed that the elements of this girdle are directly comparable to those of a higher Vertebrate, although the nomenclature is the same. The pelvic girdle seems to be absent, as in almost all Teleostomes, but its place is taken by a thin plate of bone, apparently due to a fusion of some basal elements of the pelvic fins. Nervous System.—The relatively undifferentiated fore- brain with defective cortical region, the thalamencephalon with its inferior lobes and infundibulum, the large optic lobes, the tongue-shaped cerebellum which conceals most of the medulla oblongata, have their usual general relations. Each of the olfactory nerves is at first double; their bulb- like terminations lie far from the brain behind the nasal sacs. The large optic nerves cross one another without Jusion at a slight distance from their origin ; otherwise the nerves generally resemble those of the skate. The eyes are large but lidless; the small nasal sacs with plaited walls have double anterior apertures ; the vestibule of the ear contains a large solid otolith, and another very small one in a posterior chamber. The dark lateral line, covered over by modified scales, lodges sensory cells, and is innervated by a branch of the vagus. Alimentary system.—Teeth are borne by the premaxille, the vomer, and the superior pharyngeal bones above, by the dentaries and the inferior pharyngeal bones beneath. There are no salivary glands, no spiracles, nor posterior nares. A small non-muscular tongue is supported by a ventral part of the hyoid arch. Five gill-clefts open from the pharynx; their inner margins are fringed by horny gill- 556 PISCES—FISHES. rakers attached to the branchial arches and serving as ‘strainers; they prevent the food from being swept out with the respiratory current. The gullet leads into a curved stomach ; at the junction of stomach and duodenum numerous tubular pyloric czeca are given off; into the duo- denum opens the bile-duct from the gall-bladder and liver; the coiled intestine passes gradually into the rectum, which has an aperture apart from those of the genital and urinary ducts. There is no spiral valve, and there are no abdominal pores. A pancreas is absent ; perhaps the py- loric ceeca take its place. (In some Teleosteans the pancreas, apparently absent, is combined with the liver.) The peritoneum is darkly pigmented. Respiratory system.—Water that passes in by the mouth may pass out by the gill-clefts; the branchial chamber is also washed by water which passes both in and out under the operculum. The gill-filaments borne on the four anterior branchial arches are long triangular processes, whose free ends form a double row. As there are no partitions between the five gill-clefts, the filaments pro- ject freely into the cavity covered by the operculum. On the internal surface of the operculum lies a red patch, the pseudobranch or rudi- Section ofa mentary hyoidean gill. Inspiration Ba aay . and taking food into the mouth G.F,, Gill-filament; A., artery are associated with the retraction of Gengua nog) eR imgaues the hyoid apparatus ; expiration and swallowing are associated with the protraction of the hyoid arch. The usual retractor of the lower jaw is absent in Teleosts, and the lowering of the lower jaw comes about automatically in the retraction of the hyoid arch and the raising of the operculum,—in short in the inspiratory phase. A large and quaint parasitic copepod— Lernea branchialis—is often found with its head deeply CIRCULATORY SYSTEM. 557 buried in the tissues of the gills and head. Many related forms are common on fishes. The swim-bladder lies along the dorsal wall of the abdomen; the duct which originally connected it with the gut has been closed. The dorsal wall of. the bladder is so thin that the kidneys and vertebrae are seen through it; the ventral wall is thick, and bears anteriorly a large vascular network or rele mirabile, which receives blood from the mesenteric artery and returns blood to the portal vein. Circulatory system.—The heart lies within a pericardial chamber, separated by a partition from the abdominal cavity. The blood from the body and liver enters the heart by the sinus venosus, passes into the thin-walled auricle, and thence to the muscular ventricle. From the ventricle it is driven up the ventral aorta, the base of which forms a .white non-contractile bulbus arteriosus. The ventral aorta gives off, on each side, four afferent branchial vessels to the gills. Thence the blood is collected by four efferent trunks, which unite on each side In an epibranchial artery. The two epibranchials are united pos- teriorly to form the dorsal aorta, while anteriorly they give off the carotids, which are united by a transverse vessel closing the “ cephalic circle.” Blood enters the sinus venosus veins, and by hepatics from the liver. Fic. 298.—Diagram of Teleostean circulation.— After Nuhn. ‘ A., auricle; V., ventricle; .a.,. bulbus arteriosus ; v.@., ventral aorta; a.dr., afferent branch- ials; ¢.d7., efferent branchials; e.¢., cephalic circle; ¢., caro- tids; 4.c.v., anterior cardinal’ veins; P.C.V., posterior car- dinal veins; d.c., ductus Cuvieri; d@.a., dorsal aorta;. ¢.v., caudal vein; ¢.@., caudal artery; K., kidney. by two vertical precaval Each precaval vein is. 558 PISCES—FISHES, formed from an anterior cardinal from the head and a posterior cardinal from the body. The posterior cardinals extend along the kidneys, and are continuous with the caudal vein, but the middle part of the left cardinal is obliterated. The circulation of the blood seems to be helped, in some fishes at least, by the respiratory movements and by the muscular contractions in swimming. Excretory system.—The kidneys are very long bodies, extending above the swim-bladder under the vertebral ‘column. The largest parts lie just in front of and just behind the swim-bladder. From the posterior part an unpaired ureter extends to the urinary aperture, before reaching which it gives off a small bilobed bladder. ‘The pronephros degenerates; the functional kidney is a mesonephros. Reproductive system.—The testes are long lobed organs, conspicuous in mature males at the breeding season; there is no epididymis. The ovaries of the female are more compact sacs, more posterior in position. Two vasa deferentia combine in a single canal. The likewise single oviduct is continuous with the cavity of the -ovaries. The genital aperture in either sex is in front of, but very close to, that of the ureter. The oviducts of most Teleosts seem to be backward extensions of the ‘ovarian sacs, but they may be disguised Miillerian ducts. In salmonids the eggs are shed into the coelom, and escape by a pair of pores opening together behind the anus. Development.—The ova of the haddock, like those of ‘other Teleosteans, contain a considerable quantity of yolk, .are fertilised after they have been laid, and undergo meroblastic segmentation. The eggs float, ze. are pelagic ; while those of the herring sink, ze. are dimersal. At one pole of a transparent sphere of yolk lies a disc of formative protoplasm of a light terra-cotta colour. The ovum is surrounded by a firm vilelline membrane. After fertilisation the formative disc divides first into two, then into four, then into many cells, which form the blastoderm. From the edge of the blastoderm certain yolk-nuclei or periblast-nuclei are formed, which afterwards have some importance. At the end of segmentation the blastoderm lies in the form of a doubly ‘convex lens in a shallow concavity of the yolk. The blastoderm extends for some distance laterally over the yolk ; ‘the central part raises itself, and thus forms a closed segmentation Fic. 299.—The early development of the salmon. x, The fertilised egg ; 2, the egg just before hatching ; 3, the newly hatched salmon ; 4 and 5, the larval salmon nourished from yolk- sac (y.s.) which is diminishing while the fish is increasing in size; 6, the salmon with yolk absorbed (about six weeks old). The small figures to the right indicate the actual sizes. 560 PISCES—FISHES, cavity ; one radius of the blastoderm becomes thicker than the rest, and forms the first hint of the embryo ; an inward growth from the edge of the blastoderm forms an invaginated layer—the dorsal hypoblast or roof of the gut; the periblast forms the floor of the gut, and afterwards aids. the mesoblast, which appears between epiblast and hypoblast; the medullary canal is formed as usual in the dorsal epiblast. It is likely that the edge of the blastoderm represents the blastopore or mouth of the gastrula, much disguised by the presence of yolk. The newly hatched larva is still mouthless, and lives for awhile om the residue of yolk, which, by its buoyancy, causes the young fish to be suspended in the water back downwards. GENERAL NOTES ON THE FUNCTIONS, HABITS, AND Lire Histories oF FISHES Movement.—A fish may well compare with a bird in its mastery of the medium in which it lives. Thus a salmon travels at the rate of about eight yards in a second, or over sixteen miles an hour. The motion depends mainly on the powerful muscles which produce the lateral strokes of the tail and posterior part of the body. It may be roughly compared to the motion of a boat propelled by an oar from the stern, So energetic are the strokes that a fish is often able to leap from the water to a considerable height. In some cases undulating movements of the unpaired fins, and even the rapid backward outrush of water from under the gill-cover, seem io help in movement. The paired fins are chiefly used in ascending and descending, in steering and balancing. The large pectoral fins of the flying-fish (Dactylopterus and Exocetus) are used rather as parachutes than as wings during the long skimming leaps. Shape in relation to habit.—The characteristic form of the body, as seen in herring or trout, is an elongated laterally compressed spindle, thinning off behind. like a wedge. In most cases the trunk passes quite gradually into head and tail. This torpedo-like form is. well adapted for rapid progression. Flat-fishes, whether flattened from above downwards, like the skate, or from side to side like the plaice and sole, usually live more or less on the bottom; eel-like forms often wallow in the mud, or creep in and out of crevices ;. globe-fishes, like Dzodon and Zetrodon, often float passively, Colour.—The colours of fishes are often very bright. They depend partly on the presence of pigment cells in the skin, partly on the physical structure of the scales. The common silvery colout is. due to small crystals of guanin in the skin. In many cases the colours of the male are brighter than those of his mate, as in the gemmeous dragonet (Caddonymus lyra) and the stickleback (Gasterosteus), and this is especially true at the breeding season. The colours of many fishes change with their surroundings. In the plaice and some others. the change is rapid. Surrounding colour affects the eye, the influence passes from eye to brain, and from the brain down the sympathetic nervous system, thence by peripheral nerves to the skin, where the GENERAL NOTES ON FISHES. 561 distribution of the pigment granules in the cells is altered. In shallow and clear water this power of colour-change may be protective, but an appreciation of the protective value of colouring demands careful attention to the habits and habitat of the fishes, to the nature of the ig in which they live, and to the enemies which are likely to attack them. Food.—The food of Fishes is very diverse — from Protozoa to Cetaceans. Sharks and many others are voraciously carnivorous ; many engulf worms, crustaceans, insects, molluscs, or other fishes; others browse on seaweeds, or swallow mud for the sake of the living and dead organisms which it contains, Their appetite is often enormous, and cases are known (4g. Chéasmodon niger) where a fish has swallowed another larger than its own normal size. Many fishes follow their food by sight ; many by a diffuse sensitiveness, to which it is difficult to give a name; a few, it would seem, by a localised sense of smell. It is important to realise that fishes depend very largely on small crustaceans, and these again on unicellular plants and animals, Just as we may say that all flesh is grass, so we may say that all fish is Diatom. Senses, etc.—Fishes do not seem to have much sense of taste or of smell, but diffuse sensitiveness to touch, chemical stimuli, etc., is well developed, especially on the head and along the lateral line. Though there is no drum, and the ear is deeply buried, a few seem to hear. Some experiments suggest that the semicircular canals of the fish’s ear are indispensable in the direction or equilibration of movement. The sense of sight is, on the whole, well developed, and many have “¢ darkness eyes.” As to the intellectual powers of their small brains we know little, but many show quickness in perceiving friends or foes, a few give evidence of memory, and many of their instincts are complex. At the breeding season there is sometimes an elaborate expression of excitement, well seen in the stickleback. Reproduction. — Hermaphroditism is constant in some bony fishes, e.g. Chrysophrys auratus (dichogamous), and three species of Serranus (autogamous); almost constant in Pagellus mormyrus ; very frequent in Box salga and Charax puntazzo; and exceptional in over a score of fishes, such as sturgeon, cod, herring, pike, and carp. The simplicity of the genital organs and their ducts may in part explain why casual hermaphroditism is more frequent in Fishes than in higher Vertebrates. In many cases the males are smaller, brighter, and less numerous than the females. Courtship is illustrated by the sticklebacks (Gasterosteus, etc.), the paradise- fish (Macropodus), and others; and many male fishes fight with their rivals. ‘ Most fishes lay eggs which are fertilised and develop outside of the body. They may be extruded on gravelly ground, or sown broadcast in the water. Sturgeon, salmon, and some others ascend rivers for spawning purposes, while the eels descend to the sea, In the case of trout, Barfurth has observed that the absence of suitable spawning ground may cause the fish to retain its ova. This results in ovarian disease, and in an inferior brood next season. Except in Elasmo- branchs, the ova are relatively small, and large numbers are usually 36 562 PISCES—FISHES. laid at once. In Elasmobranchs the egg is large, and in the oviparous genera it is enclosed in a ‘‘ mermaid’s purse.” Most sharks and a few Teleosteans, ¢.g. Sebastes marinus, Zoarces viviparus, are viviparous, the eggs being hatched in the lower part of the oviduct in sharks, in the ovary or oviduct in Teleosteans. In two viviparous sharks (A/ustelus levis and Carcharias glaucus) there is a union between the yolk-sac and the wall of the oviduct, to be com- pared with a similar occurrence in two lizards, and with the yolk-sac placenta of some Mammals. As to fertilisation, the usual process is that the male deposits spermatozoa or ‘‘ milt” upon the laid eggs or ‘‘ spawn,” but fertilisation is of course internal when the eggs are enveloped in a firm sheath, or when they are hatched within the mother. Most fishes have a great number of offspring, and parental care is proportionately little. Moreover, the conditions of their life are not suited for the development of that virtue. When it is exhibited, it is usually by the males,—e.g. by the sea-horse (Azpfocampus) and the pipe-fish (Syzgnathws), which hatch the eggs in external pouches, and ‘*the male of some species of Arcus, who carries the ova about with him in his capacious pharynx.” The female of Aspredo carries the eggs on the under surface of the body until they are hatched, much in the same way as the Surinam toad bears her progeny on her back ; while in Solenostoma a pouch for the eggs is formed by the ventral fins and skin. At least a dozen kinds of fishes make nests, of which the most familiar illustration is that of the male stickleback, who twines grass stems and water-weeds together, glueing them by mucus threads exuded as semi-pathological products from the kidneys, which are compressed by the enlarged male organs. Fishes have a less definite limit of growth than most other Vertebrates, and it is rare for a fish to exhibit any of the senile changes associated with old age in other Vertebrates. But surroundings and nutrition affect their size and colour very markedly. Some, such as the flounder, seem almost equally at home in fresh or salt water, but many are sensitive to changes of medium. Many can endure prolonged fasting, and some may survive being frozen stiff. Lowered temperature may induce torpor, as seen in the winter sleep of the pike, while in the dry season of hot countries the mud-fishes, the Siluroids, and others, encyst themselves in the mud, and remain for a long time in a state of ‘‘ latent life,’ Life histories.—The life histories of fishes form the subject of an endless chapter, of which we can only give a few illustrations. We know how the lusty salmon return from the sea to the possibly safer rivers, and after a period of fasting deposit their eggs and milt on the gravelly bed of the stream. A similar migration is true of the sturgeon. In great contrast to these cases is the life history of the eel, the mystery of which has been at least partially removed. From the inland ponds and river-stretches the female eels migrate on autumn nights seawards, meet their mates lower down the rivers, and descend to very deep water in the sea (250 fathoms or more). There the eggs are laid, and there in all probability the parents die. Thence the GENERAL NOTES ON FISHES. 563 transparent larvee (Leptocephalz) rise to the surface and are for a year or so pelagic. From the open sea the young eels or elvers migrate up the streams in a marvellous procession or eel-fare, the females ap- parently going farther inland than the males, Inter-relations.—Commensalism is illustrated by some small fishes which shelter inside large sea-anemones, and by Fverasfer, which goes in and out of sea-cucumbers and medusz. On the outside or about the gills of Fishes, parasitic Crustaceans (fish-lice) are often found ; various Flukes are also common external parasites, and many Cestodes in bladder-worm or tape-worm stage infest the viscera. The immature stages of Bothriocephalus latus occur in pike and burbot ; a remarkable TY PT PUN UL LTO TT ANNA} AIN DION Fic. 300.—Development of eel.—After Smit. Change from Leptocephalus shape (I.) to “‘ Elver” shape (V.). hydroid (Polypodium) is parasitic on the eggs of a sturgeon; the young of the fresh-water mussel are temporarily parasitic on the stickleback ; and the young of the Bitterling (Rhodeus amarus) live for a time within the gills of fresh-water mussels. Distribution in space.—There are about 2300 species of fresh- water fishes, three or four Dipnoi, about thirty ‘‘ Ganoids,” and the rest Teleosteans, over a half being included in the two families of carps (Cyprinidee) and cat-fishes (Siluridz), Among marine fishes, about 3500 species frequent the coasts, rarely descending below 300 fathoms. A much smaller number, including many sharks, live and usually breed in the open sea. About 100 genera have been recorded from great depths. In regard to the last, Dr. Giinther has shown that in forms living at 564 PISCES— FISHES. depths from 80 to 200 fathoms, the eyes tend to be larger than usual, as if to make the most of the scanty light ; beyond the 200-fathom line small-eyed forms occur with highly developed organs of touch, and large-eyed forms which have no such organs, but perhaps follow the gleams of ‘‘ phosphorescent” organs; finally, in the greatest depths some forms occur with rudimentary eyes. Many of these abyssal fishes are phosphorescent; the colouring is usually simple, mostly blackish or silvery ; the skin exudes much mucus; the skeleton tends to be light and brittle; the forms are often very quaint; the diet is necessarily carnivorous. GENERAL NOTES ON THE STRUCTURE OF FISHES Fins.—Along the dorsal and ventral median line of some fishes, e.g. flounder, there is a continuous fin—a fold of skin with dermal fin-rays (dermotrichia) and deeper skeletal supports (somactids), In the embryos of many fishes the same continuous fringe is seen, while the adults have only isolated median fins. There is no doubt that these isolated median fins—of which there may be two dorsals, a caudal, and an anal or ventral—arise, or have arisen, from a modifica- tion of a once continuous fin. Now, the paired fins, which correspond to limbs, often resemble unpaired fins in their general structure, and in their mode of origin. It is possible that the paired fins may have arisen by a localisation of two once continuous lateral folds. According to another theory, the origin of paired fins is to be found in the visceral arches. The paired fins are supported by dermic fin-rays (dermmotrichia) and by endoskeletal pieces (somactéds or radials), some of which are articulated to the girdles and are then called dasa/za. Two main types of fish fin are distinguishable—(a) that best illustrated among living fishes by Ceratodus, in which a median jointed axis bears on each side a series of radial rays—a form often called an archipterygium ; and (4) the commoner type, in which the radials arise on one side of the basal pieces (an ichthyopterygium). In the bony fishes the support of the fin beyond the base seems mainly due to dermal rays. Tail.—In Dipnoi and a few Teleosteans, e.g. the eels, the vertebral column runs straight to the tip of the tail, dividing it into two equal parts. This perfectly symmetrical condition is called diphycercal or protocercal. ; In Elasmobranchs, Holocephali, cartilaginous and many extinct ** Ganoids,” the vertebral column is bent dorsally at the end of the tail, and the ventral part of the caudal fin is smaller than, and at some little distance from, the upper part. This asymmetrical condition is called heterocercal. In most Teleostei, and in extant bony ‘‘Ganoids,” the end of the vertebral column is also bent upwards, but the apex atrophies, and, by the disproportionate development of rays on the ventral side, an apparent symmetry is produced. The vertebral column usually ends in a urostyle,—the undivided ossified sheath of the notochord. Most of the fin really lies to the ventral side of this. The condition is termed homocercal. GENERAL NOTES ON STRUCTURE OF FISHES. 565 The effect of a stroke with the heterocercal tail is to force the anterior region downwards, and thus the heterocercal tail in fish is associated with a ventral mouth and the habit of ground-feeding. The movement of the homocercal tail, on the other hand, drives the body straight forwards, and is associated with a terminal mouth. _ Scales.—(1) In Elasmobranchs the scales (placoid) have the form of skin-teeth (dermal denticles), tipped with enamel, cored with dentine, and based with bone sunk in the dermis, They arise from skin papillee, the (ectodermic) epidermis forming the enamel, the (mesodermic) dermis forming the rest. In other fishes the scales are almost wholly dermic, in marked contrast to those of Reptiies. (2) ‘*Ganoid” scales, as in Lepzdosteus, are plates of bone with an enamel-like covering called ganoin. . (3) In most Teleosts the scales are relatively soft dermic plates of thin bone. In the sturgeon and many Teleosts the scales are substantial bony plates. The typical ‘‘soft” Teleost scales are called cycloid or ctenoid, as their free margins projecting from sacs in the dermis are entire or notched. The concentric rings on the scales indicate periods of growth, like the rings on a tree stem, and it is possible in some cases to tell the age of a fish from its scales, as also from the otoliths in the ear when these have a layered structure. _ The scales “of Elasmobranchs are homologous with teeth, and a number may fuse into'a plate just as teeth often do. ~ Swim-bladder.—The swim-bladder of fishes is one of the numerous outgrowths of: the gut. It is absent in Elasmobranchs and some Teleosteans, such as most flat-fish, and it forms the lung of Dipnoi. Unlike a lung, it opens dorsally into the gut, except in Dipnoi and the Ganoid olypterus, where the aperture is ventral. The original duct communicating with the gut may remain open, as in Physostomatous Teleosteans, or it may be closed, as in Physoclystous Teleosteans. The bladder is usually single, but it is double in Protopterus, Lepidosiren, and Polypterus. In regard to the use of the swim-bladder, there is still considerable uncertainty. Where it is abundantly supplied with impure or partially purified blood, as in Dipnoi, Polygterus, and Ama, and where the gas within is periodically emptied arid renewed, it is doubtless respiratory. But what of other cases, where its supply of blood is arterial, and what especially where it is entirely closed? In such cases it is usual to speak of its function as hydrostatic. In greater detail the function of the air-bladder is—(1) to render the fish, bulk for bulk, of the same weight as the medium in which it lives ; moreover (2), the volume of the contained gas varies with increased secretion and absorption, and seems to adjust itself to different external pressures as the fish descends or ascends, There is sometimes a well- developed gas-gland with a rich blood-supply on the inner wall of the bladder. (3) In many fishes the bladder may help indirectly in respiration by storing the superabundance of oxygen introduced into the blood by the gills. (4) There is in several Teleosteans a remarkable connection between the swim-bladder and the ear, sometimes by an anterior process of the bladder, as in the herring and perch-like fishes, sometimes by 4 chain of bones, as in Siluride. This has suggested 566 PISCES—FISHES. the view that the connection serves to make the fish aware of the varying tensions of gas in the bladder, due to the varying hydrostatic pressure. CLASSIFICATION OF FISHES Sub-Class I. ELASMOBRANCHII Cartilaginous Fishes, e.g. Sharks and Skates Voracious carnivorous fishes, with cartilaginous skeleton, placoid scales, usually heterocercal tails, “claspers” on Fic. 301.—Young skate.—From Beard. The yolk-sac has been cut off, the yolk-stalk is left. 1., Mouth; o/.0., nostril, e.g. ‘external gills”; a., cloaca; c¢., claspers. the pelvic fins of the males. Except in Holo- cephali there 1s no cover over the (5-7) gill-aper- tures; anterior to these there is often a spiracle —the first gill-cleft— with a rudimentary gill. The gill-clefts are separated by complete septa, and the gill- filaments are attached throughout their length to the septa. The mouth extends transversely on the under side of the head. The nostrils are also ventral. There is no air-bladder. A spiral fold extends along the internal wall of the large intestine. Into the ter- minal chamber (or clo- aca) of the gut the genital and urinary ducts also open. The ventricle of the heart has a con- tractile conus arteriosus. Fertilisation is internal. The ova are few and large, z.e. with much yolk. ELASMOBRANCHII, 567 Large egg-purses are common, but some Elasmobranchs are viviparous. ‘The embryos have gill-filaments projecting out of the gill-clefts, so-called external gills. They are really elongated internal gills. Elasmobranchs retain more embryonic features, e.g. the naso-buccal groove and auditory opening, than other fishes. Order 1. PLAGIOSTOMI or SELACHII With transverse ventral mouth, pre-oral rostrum, uniserial paired fins, claspers, heterocercal tail, usually five pairs of open gill-clefts. Subdivisions.—(1) The older Selachoidei, with approximately cylindrical bodies and lateral gill-openings, as in shark and dogfish ; (2) the more modified Batoidei, with flattened bodies, ventral gill- openings, and pectoral fins joined to the head, as in skates or rays. Mustelus, Carcharias, Squalus, Torpedo, Acanthias, and others, are Fic. 302.—Lateral view of dogfish (Scy/zum catulus). Note ventral mouth with naso-buccal groove, heterocercal tail, and unpaired fins. gs., Gill-slits; Zc., pectoral fins; 4v., pelvic fins. viviparous ; Raja, Scylléum, Cestracton, and others, are oviparous. In most species of AM/ustelus there is a placenta-like connection between the yolk-sac of the embryo and the uterus of the mother. In several viviparous genera long filaments are developed from the inner surface of the uterus which secrete a nutritive fluid. In some cases the nutriment seems to be afforded by degeneration of the uterine wall. In Acanthias vulgaris there is no nutritive material, and the young are unattached. ‘This is intermediate between oviparous and specialised placental conditions. Zygena has a peculiar hammer-like head expan- sion; Se/ache reaches a length of 40 ft. ; Przstés has the snout prolonged” in a tooth-bearing saw ; Zorgedo has a powerful electric organ. The Greenland Shark (ZLemargus borealis) is unique in having small eggs, without egg-cases, perhaps fertilised in the water. In the eel-like deep-water Japanese Shark (Ch/amydoselachus) the mouth is anterior, the nostrils lateral, the vertebral column is imperfectly segmented, _ there is a slight opercular fold, and there are six pairs of gill-openings and arches. In the large viviparous Notidanide, e.g. Hexanchus (six , 568 PISCES—FISHES. gills) and Heptanchus (seven gills), the mouth is almost inferior, the vertebral column is imperfectly segmented with persistent notochord. History.—The Elasmobranchs appear in the Upper Silurian, are very abundant from the Carboniferous onwards, but are now greatly outnumbered by the Bony Fishes. An increasing calcification of the axial skeleton is traceable through the ages, and in some of the ancient forms the exoskeleton was greatly developed, often including long spines or ichthyodorulites firmly fixed on the dorsal fins or on the neck. Order 2, HOLOCEPHALI The Holocephali are represented by the sea-cat or Chimera from northern seas, and Callorhynchus from the south. There is a fold or operculum covering the (4) gill-clefts and leaving only one external opening on each side; there is no spiracle; the vertebral column is unsegmented ; the upper jaw is fused to the cartilaginous skull, and thus the hyoid does not help in its suspension (azéostylic) ; the skin is naked except in the young, which have some dorsal placoid spines. There is a urogenital aperture separate from the anus. In general the Holocephali most nearly resemble Plagiostomi, but they have many affinities with Dipnoi, ¢.g. in the autostylic skull. : Teeth (of Ptychodus, Rhynchodus, etc.), which have been referred to Chimeroids, occur in Devonian rocks, and some at least of the detached spines of Carboniferous age may have belonged to fishes of this order. Undoubted Mesozoic Chimeroids are Sgualoraja, Myriacanthus, Chimeropsis, Ischyodus, etc., while others, including the recent genus Chimera, are found in strata of Tertiary age. The other recent genus, Callorhynchus, is also represented by a Cretaceous species, C. hector?. EXTINCT ORDERS Order 3. PLEUROPTERYGII Devonian, Carboniferous, and Permian. Forms with unconstricted notochord, heterocercal tail, terminal mouth, paired fins with unseg- mented parallel radials. C/adoselache. Order 4. ICHTHYOTOMI Lower Carboniferous to Permian. Forms with unconstricted noto- chord, diphycercal tail, and pectoral fins with a segmented axis of basals bearing biserial radials. Plewracanthus, Order 5. ACANTHODEI Another interesting extinct group, whose position was for long a matter of dispute, but which is now usually placed near Elasmobranchii, is that of the Acanthodei. These flourished principally in Devonian times, but lived on through the Carboniferous to the Lower Permian. TELEOSTOMI. 509 They are usually rather small fishes, with minute rhomboidal shagreen- like scales, and a strong spine in front of each fin, except the caudal. In some genera (Parexus, Climatius) there are two rows of small intermediate spines between the proper pectorals and the pelvics. d FIG. 303.—Outline of Acanthodes sulcatus.—After Traquair. é., Pectoral fins; v., pelvics.; a., anal; d., dorsal. Sub-Class II. TELEoOsToMI Fishes with more or less ossified skeletons, especially as regards skull, jaws, operculum, and -pectoral girdle. The skull is hyostylic, the jaws being supported by the hyoman- dibular. The pelvic girdles are usually rudimentary or absent. The mouth is usually terminal; the scales are in the majority soft and cycloid. There is always a gill- cover; the inter-branchial septa are much reduced; the gill-filaments project freely from the gill-arches. There is usually a swim-bladder. There are no claspers, no naso- buccal grooves; there is no cloaca. The fore-brain has a non-nervous roof. The ova are small and numerous, usually meroblastic, sometimes holoblastic. Fertilisation is usually external. Order 1. CROSSOPTERYGII Ancient forms with pectoral fins obtusely lobate and uniserial or acutely lobate and biserial; with scales and dermal skull bones often covered with enamel-like ganoin ; with a pair of jugular plates between the rami of the lower jaw. All are extinct except Polypterus and Calamoichthys from African rivers. Examples, Osteolepis (Lower Devonian), Holoptychius (Devonian), Megalichthys (Carboniferous), In Polypterus, the body is covered with rhombic ganoid scales; there are numerous dorsal fins; the tail is diphycercal; the pectoral fin has three basal pieces as in Elasmobranchs, then two rows of radials, and then the dermal fin-rays or dermotrichia; the air-bladder is double and is used in respiration, its duct opens ventrally into the pharynx ; the young form has an external gill on the operculum; the 570 PISCES—FISHES. oral part of the hypophysis retains its opening into the mouth, The genus Calamoichthys has very similar characters, but no pelvic fins. These two forms may almost be called living fossils. AT gs ee Se PAGS ad nn mama Fic. 304.—Larva of Polypterus (after Budgett), 14 inch in length, e.g., Large external gill of the hyoid arch; Pc., pectoral fins; Pv., pelvic fins. ‘The larva is drawn in a very characteristic attitude. The following three orders are often grouped as Actino- pterygii, with the following characters. The paired fins are never lobate, they have short basal pieces, and are mainly supported by dermal fin-rays. Order 2, CHONDROSTEI—Wwith cartilaginous internal skeleton Living examples :—Sturgeon (Acipenser), Polyodon, Sca- phirhynchus. Fic. 305.—Sturgeon (Aczpenser sturio). Note the elongated snout, the barbules bounding the ventral mouth, the operculum covering the gills, the rows of bony scutes, the markedly heterocercal tail. Extinct examples :— Chetrolepis, Pal@oniscus, Chondrosteus. In the sturgeon (Acépenser) the skin bears five rows of large bony scutes; the tail is heterocercal; the notochord is unsegmented. A snout, with pendent barbules, extends in front of the ventral mouth, which is rounded and toothless. Sturgeons feed on other fishes, TELEOSTE, 578 which they swallow whole. They are the largest fresh-water fishes, for A. sturdo may attain a length of 18 ft. and a weight of 600 lb., while the 4. uso of Southern Russia may measure 25 {t. and weigh nearly 3000 lb, !' Most of the species are found both in the sea and in rivers or lakes. The roes or ovaries form caviare; the gelatinous internal layer of the swim-bladder is used as isinglass. The genus Scaphirhynchus is represented in Asia and the United States ; Polyodon or Spatularia spatula is the paddle-fish or spoon-bill of the Mississippi. Order 3. Ho.ostE1 !—with bony skeleton Living examples :—Lepidosteus and Amia. Extinct examples :—Lepidotus, Pycnodus, Aspidorhynchus. The N. American bony pike—Zefidosteus—is covered with rows of “ganoid” scales; the whole skeleton is well ossified, and the vertebral bodies are opisthoccelous; the swim-bladder is like a lung in structure, and to some degree in function. The bow-fin, Ama calva, frequenting still waters in the United States, has a similar lung-like swim-bladder. Its scales are similar to those of a Teleost. Order 4. TELEostTe1. The “ Bony Fishes” This order includes most of the fishes now alive. Though comparatively modern fishes, they are older than was formerly supposed, as several Jurassic genera (Zhrissops, Leptolepis, etc.), which used to be classed as “ Ganoids,” 4 must be considered as actual Clupeoids, or herring-like Teleostei. It is, however, not until the Upper Cretaceous and Tertiary epochs that they assume among fishes that overwhelming preponderance in numbers which they possess at the present day. The physostomous type of Teleostean is the most ancient, and probably stands in a continuous genetic line with the Holostei. The skeleton is well ossified, with numerous investing bones on the skull, others in the operculum, and on the shoulder-girdle. There is always a supra-occipital in the 1 The term ‘ Ganoids,” which we abandon, is often used to include Crossopterygii, Chondrostei, and Holostei. Though they agree in having « conus arteriosus with many valves, as opposed to the Teleostean bulbus, an optic chiasma, as opposed to the decussate condition in Teleosts, and an intestinal spiral valve which is absent in Teleosts, they do not seem to form a natural division. 572 PISCES—FISHES. skull. The tail is sometimes quite symmetrical or diphycercal, but in most cases it is heterocercal at first, and acquires a secondary symmetry termed homocercal ; for while the end of the notochord in the young forms is bent upwards as usual, the subsequent development of rays produces an apparent symmetry. The scales are in most cases relatively soft. The roof of the fore-brain is without nervous matter. The optic nerves are remarkable, because they cross one another without interlacing (decussate). The partitions between the gill-clefts disappear ; so, instead of the pouches seen in Elasmobranchs, there is, on each side, one branchial chamber, covered over by an opercular fold. Into this chamber the comb-like gills, borne by the branchial arches, project freely. There is usually a Wtedaaaes Ny Be) a Fic. 306.—The goldfish (Cyprinus auratus). rudimentary gill or pseudobranch associated with the hyoid. There is no spiracle. In most, a swim-bladder is developed from the dorsal side of the gullet. The duct of the swim- bladder may remain open (Physostomous), as in herring, salmon, and carp; or it may be closed (Physoclystous), as in perch and cod. There is no spiral valve in the intestine, and the food canal ends in front of, and separate from, the genital and urinary apertures or aperture. The base of the ventral aorta is swollen into a non-contractile bulbus arteriosus, but there is no conus, unless very exceptionally, as in Butirinus. A remarkable peculiarity is that the gonads are usually continuous with their ducts. The ova are numerous, usually small and fertilised in the water. The segmentation is meroblastic, and there is usually a distinct larval stage. DIPNOL 573 The Teleosts include the great majority of living fishes, which are classified in thirteen sub-orders and numerous families, ¢.g. Clupeidee (herrings); Salmonide (salmon, trout); Cyprinidze (carps) ; Murzenidze (eels); Esocidee (pike); Gasterosteide (stickle- backs) ; Syngnathidze (pipe-fish and sea-horses) ; Gadidze (cod-fishes) ; Percidee (perch) ; Scombridze (mackerels) ; Pleuronectidse (flat-fishes) 5. Cottidee (bull-heads) ; Triglidee (gurnards) ; Lophiidz (anglers) ; Tetrodontidz (globe-fishes). Sub-Class III. Drpnor. ‘ Mud-Fishes” Fishes with a lung—the modified swim-bladder—as well as gills ; the paired fins are of the archipterygium type, with a long segmented axis, sometimes bearing a series of lateral pieces on each. side, with overlapping cycloid scales, with’ multicellular skin-glands, with a diphycercal tail. The notochord persists, and its sheath is unsegmented ; the skull is autostylic and is largely a persistent chondrocranium with the addition of some membrane bones; there are large compound grinding teeth. The external nares are on the ventral surface of the snout, or even within the upper lip, and the arching over of the nasal grooves leads to the formation of separate internal nares. The heart is incipiently three-chambered, containing mixed blood, with a spiral conus arteriosus with numerous valves; there is a vein resembling the inferior vena cava of higher vertebrates. There is a spiral valve in the intestine. The eggs are large and exhibit total unequal segmentation, as in Amphibians. The Dipnoi, whose name means double breathers, are now represented by three genera—Ceratodus, from two. rivers of Queensland; /votopierus, from certain African rivers, e.g. the Gambia; and Lefidosiven, from the Amazons. The wide distribution is noteworthy. They are very ancient forms, for Ceratodus existed in Triassic and Jurassic times (though no _ post-Jurassic remains are known). There were also undoubted Dipnoi far back in Paleozoic times, such as Dypterus and Phaneropleuron of the Devonian, Cienodus and Uronemus of the Carboniferous. The living Dipnoi are probably the survivors of an archaic. group; in their teeth and autostylic skull they resemble Holocephali; in their fins and air-bladder. they 574 PISCES—FISHES. recall Crossopterygii; in their cartilaginous skeleton and persistent notochord they are primitive ; in their lung, heart, in- ferior vena cava, multicellular skin- glands, and eggs they approach Amphibians. The Dipnoi are physiologically transitional between Fishes and Amphibians, having, for instance, acquired lungs while retaining gills, but it does not follow that they are morphologically transitional. They are intermediate, but that is not to say that they are ¢Ze connect- ing links. Ceratodus.— The genus Cera- todus is abundantly represented by fossils in the Mesozoic beds of Europe, America, Asia, and Aus- tralia, but the living animal is now limited to the basins of the Burnett and Mary rivers of Queensland (see Fig. 6). Like that other old- fashioned animal the duckmole, Ceratodus frequents the still, deep places of the river’s bed, the so-called ‘water-holes.” At the bottom of these it lies sluggishly, occasionally rising to the surface to gulp in air. Its diet was for- merly supposed to be exclusively vegetarian, but Semon holds that it crops the luxuriant vegetation of the river-banks only for the sake of the associated animal life— larvee and eggs of insects, worms, molluscs, amphibians, and _ fishes. Though Ceratodus is quite unable to live out of water, its air-breath- ing powers enable it to exist in water which is laden with sand or rotten vegetable matter. ie: \) " Fic. 307. —Lepidosiren (after Graham Kerr), showing (7¢,/.) pectoral fin and the tufted pelvic fin (Pv.f.) of the mature male. PROTOPTERUS. 575 Ceratodus sometimes attains a length of 6 ft. The body is elongated and compressed, and bears a continuous vertical fin. The. paired fins are trowel-like. There are five gill-clefts, four internal gills, and a hyoid half-gill. There are no external gills. The swim-bladder or lung issingle. It is supplied with blood from the fourth branchial arches, as is the swim-bladder of Polypterus and Ama. It arises ventrally, but lies dorsally, and is divided into compart- ments. The auricle of the heart has a dorsal fibrous ridge hinting at a divi- sion, A similar incomplete septum occurs in the ventricle, and the sinus venosus is divided into a left pulmonary and a right systemic portion. The conus arteriosus is peculiarly twisted, and contains a short longitudinal spiral valve and numerous large ‘‘ pocket” (or ‘* Ganoid ”) valves. Protopterus. — This mud-fish lives in the Gambia, Quilimane, and some other African rivers. It is mainly but not exclusively carnivorous, and attains a length of 2 to 3 ft. or more. It has extraordinary vitality, surviving severe wounds, long fasting, and desiccation. It appears to be most active at night, and to prefer shallow water, swimming rapidly with powerful tail-strokes, or “walking” slowly along the bottom with its filamentous fins moving alternately on each side, somewhat like the legs of a newt. At short Fic. 308.—Skeleton of Cera- intervals it comes to the surface “sen fin.—From Gegen- to take mouthfuls of air, which 0 asics» eadiale: passes out again through the 7", ‘fasal pies, opercular aperture. As the dry season approaches, Protogerus burrows into the earth to a depth of about 18 in., coils itself up, and secretes abundant mucus from its skin glands. This secretion forms a cocoon or capsule, with adherent earth externally, with moist slime internally, and with a lid, on which there is always a small aperture. Thus encapsuled, the 576 PISCES—FISHES, animal may remain dormant for many months, ¢.g, from August to December. The air seems to pass directly from the mouth of the burrow, through the aperture of the capsule-lid (which is produced Fic, 309.—Head region of Protopterus.—From W. N. Parker. ” sat, Sensory tubes ; Z.2., lateral line ;.¢.d7., external gills ; fc.2., pectoral fin ; of., operculum. inwards in a short pipe) to the nostrils, and thence to the lungs. The nourishment appears to be derived from a store of fat deposited in the lymphoid tissue around the reproductive organs and kidneys, and among the lateral muscles of the tail (cf. fatty bodies in Fic. 310.—Larva of Protopterus.—After Budgett. e.g. external gills; Pe., pectoral fin; Pv., pelvic fin. caterpillars, amphibians, etc.). Moreover, some of the muscles are replaced by fat, and others undergo a pathological granular degenera- tion (cf. lamprey). To a certain extent, therefore, the dormant animal lives on its own tail. It is probable that leucocytes aid in the absorption and transportation of the degenerated muscles (cf. tadpoles): LEPIDOSIREN. 577 These capsules, with the surrounding earth, have often been transported from Africa to Northern Europe, without injury to the dormant fish within, The fish makes .a nest which is guarded by the male. The larvze have four pairs of external gills, and a crescentic sucker like that of an Amphibian tadpole. Fic. 311.—Larva of Lepidostren.—After Graham Kerr. Lepidosiren.—This mud-fish from the Amazons has an eel-shaped body, with a continuous vertical fin. The limbs are reduced to the axis only. There is a well-developed septum in the auricle, an all but complete septum in the ventricle, and a complete septum in the conus. The lung is double. The eggs are laid in burrows, and the male remains curled up beside them. The young are hatched with external gills. CHAPTER XXIII Ciass AMPHIBIA Order I. STEGOCEPHALI (extinct). », II. GYMNOPHIONA or APODA (a small order). », III, URODELA or CAUDATA, ¢.g. Newts and Salamanders. », IV. ANURA or EcauDATA, ¢.g. Frogs and Toads. AMPHIBIANS made the transition from aquatic to terrestrial life. But almost all have lagged near the water. Certain acquisitions, such as lungs and a three-chambered heart, incipient in the Dipnoi, are here firmly established. As regards bodily size, the Amphibian race has dwindled since the days of its prime, but it seems to have been progressive, for some of its members show affinities with Reptiles. GENERAL CHARACTERS Amphibia are Vertebrates in which the visceral arches of the larva almost always bear gills, which may be retained throughout life, though the adults normally possess functional lungs. Whence it follows that the nostrils, through which the air enters, must open into the mouth. When limbs ave present, they have distinct digits. . The unpaired fins, fre- guently present both in larve and adults, are without fin-rays. In existing forms there ts rarely any exoskeleton, but some extinct forms had an armour of bony plates. The skull has two occipital condyles. The heart is three-chambered, with two auricles and a ventricle,—and a conus arteriosus. The gut ends in a cloaca, into which the ducts from kidneys and reproductive organs also open. A bladder, growing out from ~ the hind region of the gut, ts probably homologous with the allantots of the embryos of higher Vertebrates. The ova are small, numerous, usually pigmented, and with yolk towards one pole. They are almost always laid in water; the seg- ‘AMPHIBIANS. 579 mentation is holoblastic, but unequal, metamorphosis in development, Huxley was the first to recognise the affinities between Fishes and._ Amphibians, and to unite the two classes under the title Ichthyopsida. Of the characters common to the two classes, the following are important : Gill-slits are functional in respiration, but in Amphibians they may disappear after larval life, the Eustachian tube excepted ; gills are always present, but they may be restricted to the larval stages in Amphibians ; in fishes and larval Amphibians a single ventral aorta leaves the heart; there is no amnion, and at most a homologue of the allantois (in Amphibians); there are only. ten pairs of cranial nerves ; there are lateral sensory structures, such as the ‘‘ branchial sense organs” and those of the ‘‘ lateral line,” but these may be dim- inished in the adults; unpaired fins are almost always represented, but may not persist in the adult life; there is a functional pronephros in early stages. ; From the higher Vertebrates or Amniota the Ichthyopsida are clearly distinguished by the presence of gills (in youth at least) and by the absence of amnion and functional allantois. For though the bladder of Amphibians may be homologous with an allantoic outgrowth, it does not function as such, ze. it does not aid in the respiration or the nutrition of the embryo. ‘ _ Itis more difficult to distinguish between Fishes and Amphibians, more _ especially if we include the Dipnoi in the former class. The most obvious differences are the absence of fin-rays and the development of fingers and toes. In the following table the two classes are contrasted :— There is usually a FISHES. AMPHIBIANS. Gills persist throughout life. The swim-bladder functions as a lung in Dipnoi and less markedly in some ‘‘Ganoids,” but in most cases its respiratory significance is slight. The heart is two-chambered (incipiently three-chambered in Dipnoi). There is no inferior vena cava, except in Dipnoi. The limbs are fins. The unpaired fins are supported by fin- rays (dermotrichia). The skull has, in most cases, one occipital condyle. There is usually an exoskeleton of scales or scutes. There are no true posterior nares. There is no certain homologue of the allantois. Gills may disappear as the adult form is attained. Lungs are always developed in the adults.- It is doubtful whether they are directly comparable with the swim-bladder. The heart has three chambers. There is an inferior vena cava, and paired posterior cardinals are seen only in the larva. The limbs have digits. There are no fin-rays. There are two occipital condyles. A columella runs from the tympanum to a fenestra ovalis in the ear capsule. There is no exoskeleton, except in a few cases, and in extinct forms. There are posterior nares opening into the cavity of the mouth. The cloacal bladder seems to be the homologue of the allantois ~ 580 AMPHIBIA. THE FROG AS A TYPE OF AMPHIBIANS The common British frog (Rana temporaria) and the frequently imported continental species (A. esculenta) agree in essential features. Though aquatic in youth, they often live in dry places, hiding in great drought, reappearing when the rain returns. Every one knows how they sit with humped back, how they leap, how they swim. They feed on living insects and slugs. Fic. 312,—The edible frog (Rana esculenta). These are caught by the large viscid tongue, which, being fixed in front of the mouth and free behind, can be jerked out to some distance, and with even greater rapidity re- tracted. When a frog is breathing, the nostrils are alternately opened and closed, the under side of the throat is rhythmically expanded and compressed, the mouth re- mains shut meanwhile. The males trumpet in the early spring to their feebly responsive mates. In our British species the pairing takes place soon after; the young are familiarly known as tadpoles, and a notable metainorphosis takes place. In winter the frogs hiber- THE FROG 581 nate—buried in the mud of the ditches and ponds, mouth shut, nose shut, eyes shut—and breathe through their skin. Form and external features.—The absence of neck and tail, the short fore-limbs almost without thumbs, the longer hind-limbs with five webbed nailless toes and with a long ankle region, the apparent hump-back where the hip-girdle is linked to the vertebral column. There is a very rudi- mentary thumb, and there is a horny knob at the base of the hallux or “great toe.” At pairing time the skin of the first finger is modified in the males into a rough cushion, darkly coloured in 2. temporaria. The wide mouth, the valvular nostrils, the protruding eyes, the upper eyelid thick, pigmented, and slightly mov- able, the lower rudimentary and immovable, the third eyelid or nictitating membrane semi-transparent and moving very freely, the circular drum of the ear, the slightly dorsal cloacal aperture. Skin.—The smooth, moist skin is loosely attached at intervals to the muscles by bands of connective tissue, which form the boundaries of over a score of lymph-sacs. These contain fluid partly absorbed through the skin, and open into the veins by two pairs of lymph-hearts. The skin consists of a two-layered (ectodermic) epidermis, and an internal (mesodermic) dermis. The transparent outer layer of the epidermis is shed periodically, and swallowed by the frog. The dermis differs markedly from that of a fish, for- there is no exoskeleton, though this was present in the extinct Labyrinthodonts; there are multicellular glands, whose secretion keeps the skin moist and is in part poisonous; and there is a stratum of unstriped muscle fibres. Pigment cells occur in the dermis, and’ some extend between the cells of the epidermis. The colour changes a little according to the state of these cells, the protoplasm expanding and contracting partly through the direct influence of light and moisture on the skin, partly by a more complex reflex action in which the eyes, the brain, and the sympathetic nervous system are all implicated. In the larval salamander the pigment cell seems to contract and expand as a whole, but this is not usually the case. There are cutaneous blood vessels, by means of which the frog can, to a certain extent, breathe by its skin. The 582 AMPHIBIA, tadpole has sensory cells in distinct lateral lines, but of this regularity the adult retains little trace, though it has many nerve-endings and “touch-spots” in various parts of its skin. The axial skeleton.—The vertebral column consists of nine vertebra, and an unsegmented urostyle or coccyx. The first vertebra bears two facets for the two condyles of the skull, and an odontoid process which lies between the condyles. It has no transverse processes, and its arch is incompletely ossified. Each of the next six has an anteriorly concave or proccelous centrum, a neural arch sur- rounding the spinal cord, a transverse process from each side of the base of the arch, an anterior and a posterior pair of articular processes, and a short . neural spine. The eighth vertebra has a biconcave or amphiccelous centrum. The ninth is convex in front, with two convex tubercles behind, and _ bears large: transverse processes with which the hip-girdle articulates. The uro- style, formed by the fusion of several vertebre, has anteriorly a dorsal arch enclosing a prolongation of the spinal cord; but both arch and nerve-cord soon disappear posteriorly. FIG. 313. — Vertebral The notochord, around which the column and pelvic vertebral column has developed, is girdle of bull-frog. finally represented only by the ves- 4p., Transverse processes tiges in the centra of the verte- of sacral vertebra; /2, ilium; U., urostyle ; Fe., bree. ea 5 4seh., ischiae ~The skull consists — (a) of the persistent parts of the original car- tilaginous brain-box or chondrocranium, developed, as in the skate, from parachordals and trabecule, plus nasal and auditory capsules ; (4) of ossifications of parts of the chondrocranium, cartilage bones; (c) of membrane or investing bones; and (d) of associated visceral arches Two ex-occipitals bounding the foramen magnum and forming the condyles, two pro-otics or ossifications of the original auditory capsule, THE AXIAL SKELETON. 583. and an unpaired sphenethmoid forming the front of the brain-case, are cartilage bones. or jugals are also cartilage bones. Probably the slendet rods known as quadrato-jugals: Two parieto-frontals and two nasals above, a paired vomer and an unpaired dagger-shaped parasphenoid beneath, and two. lateral hammer-shaped squamosals (para- quadrates) are membrane bones. There is no basisphenoid ossifica- tion. To these are added the small premaxillze in the very front of the skull, and the long maxillze on each side. The quadrato-jugal connects the maxille with a minute nodule which represents the quadrate bone. On the roof of the mouth, ex- tending from the quadrate forwards to near the vomers, are the triradiate pterygoids, while at right angles to the anterior end of the parasphenoid’ and behind the vomers are the palatines. Each half of the lower jaw, based on Meckel’s cartilage, con- sists of three pieces,—the largest an articular angulo-splenial, out- side this a thin dentary, and anteriorly uniting with its fellow a minute mentomeckelian. A delicate rod—the columella auris— extends from the tympanum to the fenestra ovalis in the inter- nal capsule of the ear. According to Parker, it represents the upper part of the hyoid arch, the lower portion of which forms the car- tilaginous ‘or partially ossified hyoid plate, which lies in the floor of the mouth and is produced into two anterior and two posterior cornua. The teeth are borne by the pre- maxillze, maxillee, and vomers. There is no parietal foramen, but in the Labyrinthodonts it is always distinct. Fic. 314.—Skull of frog—upper and lower surface.—After W. K. Parker. Upper surface— Puix., premaxilla ; 1V., nasal; J7., max- illa; Sy., squamosal; Q.j., quadrato- jugal ; ¢.0., ex-occipitals ; PA, parieto- frontals ; S#2.Z., sphenethmoid ; P.O., pro-otic, Lower surface— Pmzx., premaxilla; JZ, maxilla; Q.7., quadrato-jugal; @., quadite Pt, pterygoid; Ps., parasphenoid; P.O., pro-otic; SJ.Z., sphenethmoid; Pd, palatine ; ., voiner ; ¢., columella. The cartilage which bears the quadrate at its lower end, and runs between pterygoid and squamosal, connecting the articulation of the lower jaw with the side of the skull at the auditory capsule, is called the suspensorium, In Elasmobranchs the hyomandibular is the sus- 584 AMPHIBIA. pensorium ; in Teleosteans the name is applied to the hyomandibular and symplectic ; in Sauropsida the quadrate occasionally gets the same confusing title. When the lower jaw is connected with the skull wholly by elements of the hyoid arch, as in most Elasmobranchs and Ganoids, and all Teleosteans, the term hyostylic is used. When the connection is due to a quadrate element only, as in Amphibia and Sauropsida, it is called autostylic. When there is both a hyoid and a quadrate element, as in Lepzdosteus among Ganoids, or a hyoid and a palato-quadrate, as in Cestracton among Elasmobranchs and perhaps also in Holocephali, the term amphistylic is used. Finally, it may be noted here that in Mammals the lower jaw articulates with the squamosal. The first or mandibular arch gives origin inferiorly to Meckel’s cartilage, which forms the basis and persistent core of the lower jaw, and superiorly to the palato-pterygo-quadrate cartilage which is represented in the adult by the minute quadrate bone, by the suspensorial cartilage, and by other cartilages which are invested by the pterygoid and palatine bones. The second or hyoid arch gives origin inferiorly to the hyoid plate ; superiorly, according to Parker, to the columella. Of the four posterior branchial arches, there are in the adult some persistent remnants, ¢.g. in the larynx. The limbs and girdles.—The shoulder-girdle consists of a dorsal portion—the scapula and the partially cartilagi- nous supra-scapula, and of: a ventral portion—the coracoid and the pre-coracoid. With the latter, according to most authorities, a thin clavicle is associated. The glenoid cavity, with which the humerus articulates, is formed by the junction of scapula and coracoid. Between the median ends of the coracoids lie two fused cartilaginous epicoracoids, behind which is a bony part of the sternum, prolonged posteriorly into a notched cartila- ginous xiphisternum. Anteriorly lies a bony portion called the omosternum, which is prolonged forwards into an epi- sternum cartilage. This sternum does not arise like that of higher Vertebrates, from a fusion of the ventral ends of ribs. Indeed, there are no ribs in the frog, unless they be minute rudiments at the ends of the transverse processes. The true frogs (Ranidz) have what is called a firmdsternal pectoral arch, in which precoracoid and coracoid nearly abut on the middle line, and are only narrowly separated by the epicoracoids. In toads, tree-frogs, etc., the arch is arcéferal, the precoracoid and coracoid being widely separated medianly, and connected by a large arched epicoracoid, over- lapping its fellow. The skeleton of the fore-limb consists of an upper arm Fic. 315.—Skeleton of frog. The half of the pectoral girdle, and fore- and hind- limb of the right side are not shown. gmx., premaxilla; mx., maxilla; ., nasal; s4%., sphenethmoid ; PS, Pparieto-frontal; P.O., pecs 24., pterygoid; g.7., a quadrato-jugal ; sg., squamosal; Q., quadrate; ¢., columella auris ; 4., atlas ; ¢.g., transverse process ; S.7., sacral vertebra 3 U., urostyle ; S.sc., supra scapula; H., humerus; 2.U., radio ulna; C%., carpals ; Mc., metacarpals ; /2,, ilium ; Zs., ischium ; f., femur; 7./., tibio-fibula; Ca., calcaneum; As., astra- galus; C., calcar; .4¢., metatarsals. 586. AMPHIBIA. or humerus, a fore-arm in which the inner radius and the outer ulna are fused, a wrist or carpus including two Fic. 316.—Pectoral girdle of Rana esculenta. —After Ecker. The cartilaginous parts are dotted. Z%., Episternum ; 07., omo- sternum; £%.c., epicoracoids ; sz., sternum $ Bey xiphisternum 3 cl., clavicle with underlying precoracoid cartilage 3 3 €0., Cora coid ; 3 Se., scapula; S.sc., supra-scapula; GZ, glenoid cavity for humerus. proximal and three distal elements, and a central piece wedged in between them, five metacarpal bones, of which the first—corresponding to the absent thumb—is very Fic. 317.—Side view of frog’s pelvis. —After Ecker. i2., Uium; Zs., ischium ; Pd., pubis; 4c., acetabulum. small, and four fingers, of which the two innermost have two joints or phalanges, while the two others have three. The pelvic girdle is shaped like a V, or like a pair of tongs. The ends are cartilaginous and articulate with the THE LIMBS AND GIRDLES. 587 expanded transverse processes of the ninth or sacral vertebra. Each limb of the V is an ilium; the united posterior part consists of a fused pair of ischia, and a ventral cartilaginous pubic portion. Ilium, ischium, and pubis unite in bounding the deep socket or acetabulum with which the femur articulates. : The skeleton of the hind-limb consists of a thigh bone or femur, a lower leg formed from the united tibia and fibula, an ankle region or tarsus including two long proximal elements—the astragalus or tibiale and the calcaneum or Fic. 318.—Brain of frog.—After Wiedersheim. I. DorsAL AspEcT.—o.2., Olfactory ‘lobes; ¢.4., cerebral hemi- spheres; P., pineal body, rising from region of optic thalami ; op.i., optic lobes ; cd., rudimentary cerebellum; JZ.0., medulla oblongata. IL. Ventrat Asrect.—The numbers indicate the origins of the nerves. ch., Optic chiasma; Z.c., tuber cinereum (infundib- ulum); Z., hypophysis. Ill. Horizontal SECTION.—Zz. 4y I and 2, lateral ventricles of cerebrum ; 7.72. foramen of Monro; V., 3 and 4, third and fourth ventricles ; Aq., cavities of optic lobes and aqueduct of Sylvius from third to fourth ventricle. fibulare—and three imperfectly ossified distal elements, five métatarsal bones, and’ five toes.- The first toe or hallux has two phalanges, the second also two, the third three, the fourth four, the fifth three, and, finally, outside the hallux there is a “calcar,” which looks like an extra toe, and con- sists of three pieces. The astragalus is in line with the first toe. The long bones of the skeleton show readily separable calcified terminal caps. 588 AMPHIBIA. Muscular system.—The muscles are enswathed in con- nective tissue. They consist of bundles of striated fibres, and at their ends or at one of them they are usually con- tinued into’ tendons, which are more or less directly attached to parts of the skele- ton. For an account of the musculature of Vertebrate types, the student is re- ferred to the guides to practical work cited in the Appendix. Nervous system.— The brain, covered with a darkly pig- mented pia mater, has the usual five parts. The elongated cerebral hemi- spheres have “olfactory lobes” in front of them, and are con- nected by an- terior and ByMbne eof A Fic. 319.—Nervous system of frog.—After posterior com- Ecker. ee missures, and 1-10, The cranial nerves ; oc., eyes; cvd., in front of 7 optic chiasma; /o., optic’ tract’; syit., sympa- by a hint of a thetic system; #zsf., spinal cord; sé., spinal oF corpus cal- nerves. losum ” (?). The thalamencephalon gives origin dorsally to a pineal outgrowth. The pineal body lies outside the skull in the tadpole, but is partially atrophied in the adult, so that little more than the stalk is left. On the ventral side will be seen the chiasma or interlaced crossing of the optic nerves, and a tongue-shaped mass (the tuber SENSE ORGANS. 589 cinereum or infundibulum), to which the pituitary body _ is attached. : The optic lobes, a pair of oval bodies, between and below which is the iter. The cerebellum, a very narrow transverse band. The medulla oblongata, on the roof of which the pia mater forms a very vascular “ choroid plexus.” The cavities of the brain and the canal of the spinal cord are in the adult lined by ciliated epithelium. _ The cranial nerves are, as usual, on each side the following :— (1) Olfactory, from the olfactory lobe to the nose ; (2) Optic, crossing and interlacing with its fellow ; (3) Oculomotor, to four muscles of the eye ; (4) Pathetic, to the superior oblique eye muscle ; (5) Trigeminal, with ophthalmic, maxillary, and mandibular branches ; (6) Abducens, to the external rectus eye muscle ; (7) Facial, arising along with the auditory, with a ganglion uniting with the Gasserian ganglion of,the trigeminal, with a palatine branch to the roof of the mouth, and a hyoid branch to the lower jaw ; (8) Auditory, to the ear ; (9) Glossopharyngeal, to the tongue and some of its muscles; with a ganglion which unites with that of the tenth ; (10) Vagus, with branches to lungs, heart, stomach, etc. The spinal cord gives origin to ten pairs.of spinal nerves, and is swollen at the origin of those which go to the limbs. Around the union of the anterior and posterior roots lie sacs with crystals of carbonate of lime. The sympathetic system consists of about ten pairs of ganglia—(a) united by branches to the spinal nerves; (4) united to one another by longitudinal trunks which accompany the dorsal aorta and the systemic arches, and end anteriorly in the Gasserian ganglion; (c) giving off branches to the heart, the aorta, and the viscera in the pelvic region. Sense organs.—The eyes project on the top of the head and on the roof the mouth. There is a third eyelid. The transparent cornea in front, the firm sclerotic surround- ing the eyeball, and the sheath of the optic nerve, are as usual continuous. The next layer includes the vascular and pigmented choroid and the brilliant iris. Internally is the sensitive retina, while vitreous humour fills the cavity behind the lens. The internal ears have the usual parts, and lie within the auditory capsules, which are in great part bounded by the 590 _ AMPHIBIA. pro-otics. Connecting the fenestra ovalis of the ear with the tympanic membrane, which is flush with the skin, there is a delicate bony rod—the columella. This lies in the Eustachian tube, which opens into the mouth at the corner of the gape. The nostrils open into small nasal cavities, with folded walls of sensitive membrane; the posterior nares open into the front of the mouth. There are taste papilla on the tongue, and touch-spots on the skin. Alimentary system.—The frog feeds in great part on insects, which it catches dexterously with its tongue. This is fixed in front and loose behind. There are teeth on the premaxille, maxilla, and vomers. Into the cavity of the mouth the nasal sacs open anteriorly, and the Eustachian tubes posteriorly. The males of Rana esculenta have a pair of resonating sacs which open into the mouth cavity at the angle of the jaw, and are dilated during croaking. The tongue bears numerous taste papille. Behind the tongue on the floor of the mouth is the glottis, the opening of the short larynx which leads to the lungs. The larynx is sup- ported by two arytenoid cartilages, and also by a ring; with the arytenoids the vocal cords are closely associated. The lungs lie so near the mouth that laryngeal, tracheal, and bronchial regions are hardly distinguishable. On the floor of the mouth is the hyoid cartilage, which serves for the insertion of muscles to tongue, etc. Of the (4) gill-clefts which are borne on the walls of the pharynx in the tadpole, there are no distinct traces in the adult. The lungs develop as outgrowths from the gullet. The gullet leads into a tubular stomach, which is not sharply separated from it. There is a pyloric constriction dividing the stomach from the duodenum, or first part of the small intestine. After several coils the small intestine opens into the wider large intestine or rectum, which enters the cloaca. The liver has a right and a left lobe, the latter again sub- divided. ‘The gall-bladder lies between the right and left lobes; bile flows into it from the liver by a number of hepatic ducts, which are continued onwards to the duodenum ina common bile-duct. The pancreas lies in the mesentery VASCULAR SYSTEM. 591 “between stomach and duodenum, and its secretion enters the distal portion of the bile-duct. The bladder is a ventral -outgrowth of the cloaca, has no connection with the ureters, and seems to bé homologous with the allantois of Reptiles, Birds, and Mammals. , Vascular system.—The heart, enclosed in a pericardium, is three-chambered, consisting of a muscular conical ven- tricle, which drives the blood to the body and the lungs, of a thin-walled right auricle receiving impure blood from the body, and of a thin-walled left auricle receiving purified blood fromthe lungs. From each of the auricles blood enters the ventricle. The two superior vene cave which bring back blood from the anterior regions of the body, and the inferior: vena cava which brings back blood from the posterior parts, unite on the. dorsal surface of the heart in a thin-walled sinus venosus, which serves as a porch to the right auricle. From the ventricle the blood is driven up a truncus arteriosus, which is at first single (the py/angium) and then multiple (the syzangium). Thus we may distinguish five regions in the heart,—the ventricle, the right auricle, the left auricle, the sinus venosus, and the truncus arteriosus. The sinus venosus is the hindmost, the truncus arteriosus the most anterior part. The opening of the pylangium into the ventricle is guarded by two semilunar valves ; the cavity of the pylangium is incompletely divided by a longitudinal valve; there are also valves separating pylangium from synangium, and in the cavity of the latter. The complex mechanism is interesting because it determines the course of the blood: leaving the ventricle. The truncus arteriosus corresponds, in part at least, to the conus arteriosus of many fishes. As the heart continues to live after the frog is really dead, its contrac- tions can be readily observed. The sinus venosus contracts first, then the two auricles simultaneously, and finally the ventricle. Although the ventricle receives both impure and pure blood, the structural ar- - rangements are such that most of the impure blood jis driven to the lungs, the purest blood to the head, and somewhat mixed blood to the body. The blood contains in its fluid plasma—(a) the oval “red” corpuscles, with a definite rind, a distinct nucleus, and the pigment hemoglobin; (4) white corpuscles or leucocytes, like small amcebe in form and movements ; (c) very minute bodies, usually colourless and variable in shape. When the blood clots, the plasma becomes a colourless serum, traversed by coagulated fibrin filaments, 592 AMPHIBIA, the red corpuscles often arrange themselves in rows, and the white corpuscles are entangled in the coagulated shreds. When the web of a living frog is examined under the micro- scope, it will be seen that the flow of blood is most rapid in Fic. 320,—Arterial system of frog. Z, Lingual; c., carotid; s., systemic; cz., cutaneous; Z., pulmon- ary; v., occipito-vertebral; 47, brachial; c.m#., cceliaco- mesenteric ; ~., renal ; 7/., common iliacs ; 4., hemorrhoidal. the arteries, more sluggish in the veins, most sluggish in the capillaries or fine branches which connect the arteries and the veins, The red corpuscles are swept along most rapidly, and are often deformed by pressure; the leucocytes tend to ARTERIAL SYSTEM. 593 ‘cling to the walls of the capillaries, and may indeed pass through them (diapedesis). The arterial system.—Each branch of the truncus arteri- osus is triple, and divides into three arches :— Fic. 321.—Venous system of frog. - m., 1., Mandibular and lingual ; ¢.7., external jugular 3 2.7., internal jugular ; scf., subscapular ; z7., innominate ; sc/., subclavian; ér., brachial; m.c., musculo-cutaneous; %.v., hepatic vein; 4.p., hepatic portal; @.a., anterior abdominal; ~4., renal- portal ; 4.v., pelvic; sc., sciatic ; 4, femoral ; z.v.¢., inferior vena Cava; ¢., cardiac vein. I. The carotid arch, the most anterior, corresponding to the first efferent branchial of the tadpole, gives off— A lingual artery to the tongue ; A carotid artery, which bears near the origin of the lingual a spongy swelling (the ‘‘carotid gland”), and gives off an 38 594 AMPHIBIA. external carotid to the mouth and the orbit, and an internal carotid to the brain. II. The systemic arch, the median one of the three, corresponding to the second efferent branchial in the tadpole, gives off— The laryngeal artery to the larynx ; The cesophageal to the cesophagus ; The occipito-vertebral to the head and vertebral column ; The subclavian or brachial to the fore-limb. From the left aortic arch, just as it unites with its fellow of the other side to form the dorsal aorta, or from the’ beginning of the dorsal aorta, there is given off the cceliaco- mesenteric to the stomach, intestine, liver, and spleen. Farther back the dorsal aorta gives off— The renal arteries to the kidneys, and the genital arteries to the ' reproductive organs; | The inferior mesenteric to the large intestine. Then it divides into two iliacs, each of which supplies the bladder (hypogastric), the ventral body wall (epigastric), and the leg (sciatic). III. The pulmocutaneous arch, the most posterior, corresponding to the fourth efferent branchial in the tadpole, gives off— the cutaneous artery to the skin, and the pulmonary artery to the lungs. The venous system.—I. Each superior vena cava is formed from the union of three veins, and each of these three is formed from two smaller vessels. External ace from the mouth and tongue. jugular. Mandibular from the lower jaw. Internal jugular from the inside of the skull. Superior | Innominate. 4; Subscapular from the back of the arm and vena cava. the shoulder. Brachial from the arm. Subclavian. + Musculo-cutaneous from the skin and sides \ , of the body. ” II. The inferior vena cava begins between the kidneys, and ends in the sinus venosus.. Its components are as follows :— Inferior Genital veins from the reproductive organs, vena cava. Efferent renal veins from the kidneys. Efferent hepatic veins from the liver. LYMPHATIC SYSTEM. 595 The renal portal system, by which venous blood from the posterior region filters through the kidneys on its way back to the heart, is as follows on each side :— A posterior branch of the femoral vein from the hind-limb forms the renal portal vein, which receives the sciatic from the back of the leg, and the dorso-lumbar veins from the dorsal wall of the body, and oviducal veins in the female. Renal portal system. The anterior branch of the femoral vein is called the pelvic, and unites with its fellow of the opposite side, and gives origin to a median vein which runs to the liver—the anterior abdominal, By means of an anastomosing branch, the anterior branch of the femoral is also connected to the sciatic. The hepatic portal system, by which venous blood from the posterior region and from the gut passes through the liver on its way back to the heart, is as follows :— Anterior abdominal vein, from the union of the se two pelvics, receiving tributaries from the Hepatic portal bladder, ventral body wall, and _ truncus system. arteriosus. Hepatic portal vein, from the union of veins from the stomach, intestine, and spleen. III. The pulmonary veins, which bring back purified blood from the lungs, unite just before they enter the left auricle. There are numerous valves in the veins of the frog. Lymphatic system,.—The lymph is a colourless fluid, like blood without red corpuscles. It is found m the spaces between the loose skin and the subjacent muscles, in the pleuro-peritoneal cavity in which heart, lungs, and other organs lie, in ‘a sub-vertebral sinus extending afong the backbone, and in special lymphatic vessels which pass fatty materials absorbed from the intestine’ into the venous system, There are two pairs of contractile ‘‘lymph’ hearts” at two regions where the lymphatic system communicates with the veins. A pair lie near the posterior end of the urostyle; the other two lie between the transverse processes of the: third and fourth vertebrae, Their pulsations can be seen on the back of the living frog. Mechanism of the heart.—The right half of the ventricle, being nearer the right auricle, contains more impure blood, and it is from the right side of the ventricle that the truncus arteriosus arises, The middle of the ventricular cavity contains mixed blood. The left corner contains pure blood received from the pulmonary veins. 596 AMPHIBIA, The various valves and the conditions of pressure are such that the venous blood passes by the pulmonary artery to the lungs, the next quantum of blood enters the systemic arches, and the nearly pure arterial blood from the left side of the ventricle passes into the carotids. To understand the mechanism, it is necessary to consult some book with a complete anatomical decenipten, especially Gaupp’s edition of Ecker and Wiedersheim’s Anatomie des Frosches (1899). Spleen, thyroid, and thymus.—The spleen is a small red organ lying in the mesentery near the beginning of the large intestine. The thyroid is represented by two little bodies near the roots of the aortic arches. The thymus, perhaps originally associated with the gill-clefts, lies on each side just behind the angle of the lower jaw. Respiratory system.—The larval frog breathes at first through its skin, then by gills. The adult frog breathes chiefly by its lungs, but some cutaneous respiration is still retained, for even without its lungs a frog may live for some time, and it does not use them when hibernating. The lungs arise as outgrowths of the cesophageal region of the gut, and are connected with the back of the mouth by a short laryngo-tracheal tube, whose slit-like aperture is the glottis. Each lung is a transparent oval sac, with muscle fibres in its walls. The cavity is lessened by the spongy nature of the internal walls, which form numerous little chambers bearing the fine branches of blood vessels. In respiration the mouth is kept shut, and air passes in and out through the nostrils. A frog will die of asphyxia if its mouth be artificially kept open for a considerable time. When the floor of the mouth is lowered, and the buccal cavity thus increased, air passes in. When the nostrils and the opening of the gullet are shut, and the floor of the mouth at the same time raised, air is forced through the glottis into the lungs. When the pressure on the lungs is relaxed, and when the muscles of the sides of the body contract, the air passes out. e Excretory system.—The paired kidneys are elongated organs situated dorsally and posteriorly beside the urostyle. The waste products which they filter out of the blood pass backward by two ureters which open separately on the dorsal wall of the cloaca, and are not directly connected with the bladder. The ureter or Wolffian duct is seen as a white line along the outer side of each kidney; in the male it functions also as the duct of the testis. On the ventral surface of each kidney is a longitudinal yellowish REPRODUCTIVE SYSTEM. >» 597 streak, the adrenal gland, and little spots mark ciliated apertures or nephrostomes, which remain as communica- tions between the abdominal cavity and the renal veins, though they are originally connected with: the urinary tubules. There are also, as in higher Vertebrates, open- ings from the abdominal cavity into the lymphatic system. female frog.—After Ecker. f4b., Fatty bodies; v.c., vena cava; ovd., Opening of oviduct ; ov., ovary ; T., testis; K., kidney; w.d., Wol- /.4., fatty body; X., kidney; U4, . ffian duct ; c/., cloaca; B., bladder. uterus; Uyv., opening of ureters into x cloaca (cZ.), in front of the openings of the oviducts. Reproductive system.—The males are distinguishable from the females by the swollen cushions on the first fingers. At the breeding season in spring, they trumpet to their mates. The male clasps the female with his fore-limbs, and retains his hold for several days, fertilising the ova as they pass out into the water. The paired testes are oval yellowish bodies lying in front of the kidneys; the spermatozoa pass by vasa efferentia 598. AMPHIBIA, through the anterior part of the kidney into the Wolffian duct, which functions both as a ureter and as a vas deferens. In the male of &. esculenta the vas deferens is.dilated for some distance after leaving the kidney; in &. temporaria it bears on the outer side near the cloaca a dilated glandular mass or ‘‘seminal vesicle.” In the males, rudiments of the Miillerian ducts are sometimes seen. In the male toad a small rudimentary ovary, known as Bidder’s organ, occurs at the anterior end of the testis. The paired ovaries when mature are large plaited organs, bearing numerous follicles or sacs containing the pigmented ova. The spawn laid by a single frog may consist of several thousand eggs. The ripe ova are liberated into the body cavity, and moved anteriorly towards the heart, near which the oviducts open. The movement of the ova is mainly due to the action of peritoneal ciliated cells, which converge towards the mouths of the oviducts, but partly to muscular contraction, including the beating of the heart. The oviducts are long convoluted tubes, anteriorly thin-walled and straight, then glandular and coiled, terminally thin- walled and dilated. In the median part the ova are surrounded with jelly; the terminal uterine parts open on the dorsal wall of the cloaca. In the females the Wolffian ducts act solely as ureters. Attached to the anterior end of the reproductive organs are yellow, lobed, ‘fatty bodies,” largest in the males. It has been suggested that they contain stores of reserve material, which is absorbed at certain seasons. They seem to be fatty degenerations of the anterior part of the genital ridges. The head kidney or pronephros persists for some time in the embryo, but event- ually degenerates. It does not seem to have anything to do with the fatty bodies. Development of the frog.—The ripe ovum exhibits “polar differentiation”; its upper portion is deeply pig- mented, the lower has no pigment and contains much yolk. This yolk-containing hemisphere is’ the heavier, and conse- quently is always the lower half of the egg, however this may be turned about. Round the ovum there is a delicate vitelline membrane, and this is again surrounded by a gela- tinous investment which swells up in water. The formation of polar bodies takes place before the liberation of the eggs. * DEVELOPMENT OF THE FROG. 599 The spheres of jelly preserve the eggs and embryos from friction, prevent their being eaten by most birds, appear to be distasteful to Gammarids, and often enclose in their interspaces groups of green Algz, which help in aeration. The spheres may also be of use in relation to the absorption and radiation of heat. Fertilisation occurs immediately after the eggs are laid. The spermatozoa, which exhibit the usual features of male elements, work their way through the gelatinous envelopes, and one fertilises each ovum. The first cleavage is vertical, and divides the ovum into a right and a left half. If one of these two cells be punc- Fic. 324.—Division of frog’s ovum.—After Ecker. The numbers indicate the number of cells or blastomeres. tured, and the ovum be kept still, the other half will, according to Roux, form a one-sided half-embryo. At a certain stage Roux’s half-embryo regenerated the missing half, usually by re-vitalising the remains of the cell which was punctured. If the ovum be shaken about after punctur- ing, a readjustment of material is effected, and a half-sized embryo is formed (Morgan). The second cleavage is also vertical, and at right angles to the first, dividing an anterior from a posterior half. The third cleavage is equatorial, at right angles to the first two, dividing the dorsal region from the ventral. The segmentation is total but unequal, and results in the formation of a ball of cells, those of the upper hemisphere being smaller and more numerous than the yolk-laden cells 600 AMPHIBIA, 2 below. Within there is a small segmentation cavity. Since the presence of yolk acts as a check on the activity of the protoplasm, we can understand why the smaller cells continue to divide much more rapidly than the large yolk-containing cells, and so how the smaller epiblastic cells gradually spread over the egg, covering in the larger ones. At one point, where upper and lower cells meet, a groove is formed. This groove represents the dorsal lip of the blastopore. It becomes crescentic and moves as a whole down over the large yolk-cells. Invagination of the small cells of the upper hemisphere goes on rapidly all round this crescentic groove, and the archenteron is thus formed. The horns of the cres- cent meetata point near the lower pole of the egg to form the ventral lip of the blastopore. The _ blastopore now becomes re- duced, by the in- growing of its mar- gins, to a small Fic, 325.—Longitudinal vertical section of . a frog embryo, shortly before closure of blasto- = C1 cular area which pore.—After Ziegler’s model and Marshall. appears white, the FB., fore-brain; EC., ectoderm; N., notochord; SC. j canal of spinal cord ; NVE., neurenteric canal; B. colour being due blastopore; J7., mesoderm cells; ¥., Yolk-laden toa plug of yolk- cells; 4ZV., mesonteron; /., beginning of pituitary cells which almost invagination. obliterates its opening. The whole egg now rotates backwards through a little more than a right angle, so that the blastopore is carried up into the position previously occupied by the first trace of its dorsal lip. The blastopore now marks the posterior end of the embryo. The archenteron has by this time greatly enlarged, and has pushed the segmentation cavity almost out of existence. The embryo elongates slightly, but the mass of yolk-laden cells which lie on the floor of the gut prevents the body acquiring at once the fish-like shape. Along the mid-dorsal line the usual neural plate forms the medullary canal. At the posterior end this communicates with the archenteron for a time by the neurenteric canal. DEVELOPMENT OF THE FROG, 601 Internally, a differentiation of hypoblast forms the notochord along the mid-dorsal line of the archenteron, At each side of this lie masses of mesoblast which have been split off from the hypo- blast. Each of these divides into the primitive segments (proto- vertebre) above, and the un- segmented lateral plates below. The lateral plates split into two layers, the splanchnic or inner investing the gut, the somatic or outer layer being applied to the epiblast ; the space between the two layers is the body cavity. The body now becomes dis- tinctly divided into regions, the eyes bud out from the brain, a rudiment of the gills appears, and the larva, still within its gelatinous case, exhibits peculiar lashing movements of the tail. ~ Eventually, about a fortnight after the eggs are laid, the larva escapes from the surrounding jelly and swims in the water. At this stage and for some time the ectoderm is ciliated. There is a cloacal opening, but the mouth is not yet more thana dimple. A glandular crescent, often mis- named a sucker, lies on the under surface of the head, and secretes a sticky slime, by means of which the tadpole attaches itself to foreign objects. The protruding gills soon become branched. -There are three of them -on each side, the first the largest. They are covered with Fic. 326. — Dissection of tadpole. — After Milnes Marshall and Bles. DL., Lower lip; #., ventricle of heart; DZ., cesophagus; 1VA., head kidney; A.; aorta; K., kidney; AU., ureter; DO., cloaca; ZLH., hind-limb; XV., opening of ureter into cloaca >. GR., genifal ridge; GF., fatty body; L/., fore-limb; OG., gills ; a, epidermis ; 4, dermis. ectoderm, and are borne on the outside of the first 602 AMPHIBIA. three branchial arches. The mouth, which has pre- viously been merely a blind pit, opens into the gut, the gut itself lengthens rapidly, and becomes coiled like a watch-spring; the larve feed eagerly on vegetable matter and increase in size. The glandular crescent forms . two small discs, which gradually disappear as the power of locomotion increases. About the time when the mouth is opened, four gill clefts open from the pharynx to the exterior. A second period, the true tadpole stage, now begins. A skin-fold or operculum covers the external gills, which then atrophy, and are replaced by “internal” gills developed on the ventral halves of four branchial arches. These gills, though called internal, are covered with ectoderm Wke their predecessors, and are com- parable not to ordinary fish-gills, but to the external gills of Polypterus, Protopterus, and Lepidosiren. The mouth acquires horny jaws, and the fleshy lips bear horny papilla. By the continued growth of the opercular fold the gillchambers are closed, with the exception of a single exhalant aperture on the left side. Through this opening, the water which is taken in by the mouth in respiration passes outwards, having washed the gills’ on its way. In the third period the rudiments of the limbs appear. The fore-limbs are concealed within the gill-chambers, and so are not obvious until later; but the hind-legs may be watched in the progress of development from small papillz to the complete limb. The lungs are developed as outgrowths from the ceso- phagus, even before hatching, but grow very slowly. After the appearance of the hind-legs, the larvee come to the surface of the water to breathe, showing that the lungs are now to some extent functional. At this stage the tadpoles, now about two months old, are at the level of Dipnoi. The changes in the relations of the blood vessels, which accompany the successive changes in the methods of respiration, and render these possible, are somewhat com- plicated. When respiration is by the gills only, the circulation DEVELOPMENT OF THE FROG. 603 is essentially that of a fish. From the two-chambered heart the blood is driven by afferent branchials to the . gills ; from these it collects in efferent vessels which unite on each side to form two aorte. The aorta’*send arteries to the head, and passing backwards unite to form the single dorsal aorta which supplies the body. For a time there are two dorsal aorte. When the first set of gills is replaced by the second set, new gill- capillaries are developed, but the circulation remains the same. As in Cevatodus, a pulmonary artery arises from the fourth efferent branchial. At the time when the hind-legs begin to be developed, a direct com- munication is established between afferent and efferent branchial vessels, so that blood can pass from the heart to the dorsal aorta without going through the gills. As the pulmonary circulation becomes increasingly important, the single auricle of the heart becomes. divided into two by a septum, and the pulmonary veins are established. At the time of the metamorphosis an increasing quantity of blood avoids the gills in the manner indicated above, and these, being thrown out of con- nection with the rest of the body, soon atrophy, while the lungs become the important respiratory organs. The fate of the various branchial arteries is shown in the table on the following page. The tadpole has by this time grown large and strong, feeding in great part on water-weeds. Now it seems to fast, but the tail,.which begins to break up internally, furnishes, with the help of phagocytes, some nourishment to other parts of the body. The habit becomes less active, the structural adaptations to the aquatic life disappear. “The horny jaws are thrown off; the large frilled lips shrink up; the mouth loses its rounded suctorial form and becomes much wider; the tongue, previously small, increases considerably in size; the eyes, which as yet have been beneath the skin, become exposed ; the fore-limbs appear, the left one being pushed through the spout-like opening of the branchial chamber, and the right one forcing its way through the opercular fold, in which it leaves a ragged hole” (Marshall). 604 AMPHIBIA, ! SKELETAL CLEFTs. Aortic ARCHES AorTic ARCHES ARCHES. : IN THE Empryo. IN THE ADULT. 2 ; Mandibular. ay Late in develop- | Only a trace per- ment vessels sists. appear which re- present a modifi- cation of those of a branchial arch. Eustachian tube. Hyoid. The arch is repre- | Disappears en- sented in a less tirely. modified form. First cleft. Carotid arch. F.rst branchial. First branchial arch. Second cleft. Second branchial. Second i Systemic arch. Third cleft. Third branchial. Third a Atrophies. Fourth cleft. Fourth branchial. Fourth a Pulmo-cutaneous. While these changes are in progress, and as the supply of food afforded by the tail begins to be exhausted, the tadpole recovers its appetite, but is now exclusively carnivorous, feeding on any available animal matter, or even on its fellows. The change is not, however, so great as it seems, for even at a very early stage animal food is eagerly devoured. With the change of diet, the abdomen shrinks, stomach and liver enlarge, the intestine becomes relatively narrower and shorter. The- tail shortens moré and more, and as it does so the disinclination for a purely aquatic life seems to increase. Eventually it is completely absorbed, the hind- Iimbs lengthen, and the conversion into a frog is completed. In the reduction of the tail the epidermis thickens and is partly cast, partly dissolved ; the muscles break up, and their substance undergoes intracellular digestion or is dissolved in the body juices; the notochord is repeatedly bent on itself and is also disrupted; the same is true of nervous system and blood vessels. It is a pathological process which has become normal. Some credit the phagocytes with playing a very important part in the reduction of the tail; but others restrict their function to engulfing solid particles, such as pigment granules, and say that most of the material degenerates until it becomes almost liquid, when it passes directly into the vascular fluid. In many respects the development of the tadpole is very interesting, especially because it is a modified recapitulation CLASSIFICATION OF AMPHIBIA. 605 of that transition from aquatic to aerial respiration which must have marked one of the most momentous epochs in the evolution of Vertebrates. Fic. 327.—Life history of a frog.—After Brehm. 1-3, Developing ova; 4, newly hatched forms hanging to water- weeds; 5, 6, stages with external gills; 7-10, tadpoles during emergence of limbs ; 11, tadpoles with both pairs of limbs appa- rent ; 12, metamorphosis to frog. CLASSIFICATION OF AMPHIBIA Order ANURA or ECAUDATA The adults have no tail or external gills or open gill-clefts, There are always four limbs. z Sub-order Phaneroglossa.—Tongue present; the Eustachian tubes open separately into the pharynx. : Series A. Arcifera (see p. 584), ¢.g. the toothless toads (Bz/fo) ; the tree-frogs (Hy/a), with adhésive glandular discs on the ends of the digits; the obstetric frog (Alyzes) ; Bombinator, Pelobates, and others. Series B, Firmisternia (see -p. 584), the frogs proper (Ranidz), eg. the gtass-frog (R. temporaria), the edible frog (R. esculenta), the N. American bull-fiog (R. catesbiana), sometimes 8 in. in length, and with a sonorous croak. 606 AMPHIBIA, Sub-order Aglossa.—Tongueless; the Eustachian tubes have a common median aperture into the pharynx. The Surinam toad (Pipa americana), and the allied African genus Xenopus, Order URODELA or CAUDATA The tail persists in adult life ; the larval gills and gill-slits may also persist ; the limbs are weak when compared with those of Anura, and the hind pair may be absent. Family 1. Amphiumide.—The N. American Amphiuma, with two pairs of rudimentary legs, with a slit persisting in adult life as a remnant ofthe gilled state; Cryptobranchus maximus, the largest living Amphibian, found in Japan and Thibet, attains a length of over 3 ft. : } Family 2. Salamandridse.—Sal/amandra maculosa and S. atra, both European, both viviparous; the usually oviparous newts —Tritton or Molge—of which Triton alpestris becomes sexually mature while still larval (sedogenesis). Desmognathus Jusca, the common /umgless water salamander of the United States, lays its eggs in a wreath which the female twines round its body. The N. American Ammdblystoma, with its sometimes persistent larval form the Axolotl, formerly thought to be a different species. Family 3. Proteide.—With persistent gills. Several species of Proteus inhabit the caves of Carinthia and Dalmatia. There are two pairs of limbs. The eyes are degenerate-and the skin white, as we should expect in cave-animals. Two species of Necturus (or Menobranchus) occur in N. American rivers and lakes. Family 4. Sirenidee.—Two extant genera, Szven and Pseudobranchus, both N. American, both with persistent gills, and only anterior limbs. Papillae in the lower dermic layer in Szrez, hidden by looser superficial dermis and epidermis, look like vestiges of ancestral scales, Order GYMNOPHIONA or APODA Worm-like or snake-like forms, subterranean in habit; without 1imbs or girdles; with extremely short tail; with dermic calcified scales concealed in transverse rows in the skin ;.in at least some forms (Aypogeophis) external gills are present in the very young stages, but disappear before hatching ; there may be no larval stage ; if there is, the respiration is pulmonary, There are many other striking peculiarities : —the eyes are small, covered up, and functionless; there is no tympanum or tympanic cavity; there is « peculiar protrusible tentacle in a pit behind the nostril; there are only two pairs of aortic arches (systemic and pulmonary). The notochord is largely persistent ; the vertebre are amphiccelous ; the frontals are distinct LIFE OF AMPHIBIANS. 607 from the parietals; the palatines are fused with the maxille. The eggs are large and meroblastic. They are altogether peculiar archaic Amphibians. Examples :—Cacd/¢éa (S. America) ; Zchthyophis (Ceylon, India, Malay); Aypogeophis (E. Africa); Siphonops, without scales (America). Order STEGOCEPHALI Extinct forms, occurring from Carboniferous to Triassic strata, The earliest known digitate animals, Dermal armour is present, the teeth are frequently folded in a complex manner (Labyrinthodonts). MJastodonsaurus, Dendrerpeton, Archegosaurus, Branchiosaurus. : /LIFE. OF AMPHIBIANS Most Amphibians live in or near fresh-water ponds, swamps, and marshes. They are fatally sensitive to salt. Even those adults which have lost all trace of gills are usually fond of water. The tree- toads, such as Ay/a, are usually arboreal in habit, while the Gymnophiona and some toads are subterranean. The black salamander (Sa/a- mandra atra) of the Alps lives where pools of water are scarce, and instead of bringing forth gilled young, as its relative the spotted salamander (S. mzaczlosa) does, bears them as_ lung- breathers, and only a pair at a time. The unborn young have os . gills which are pressed against Fic. 328.—Cecilian (Zchthyophis) the vascular wall of the uterus. with eggs. —After Sarasin. It is said that the respiration (and : nutrition) of the young is helped by crowds of red blood corpuscles which are discharged from the walls of the uterus; the débris of unsuccessful eggs and embryos seems also to be used for food. Species of Hylodes, such as A. martinicensis of the West Indian Islands, live in regions where there are few pools. In such cases the development is completed within the egg-case, and a lung-breathing tailed larva is hatched in about fourteen days. a In some Mexican and N. American lakes there is an interesting amphibian known as Amélystoma or Siredon. It has two forms—one losing its gills (Ambystoma), the other retaining them (Axolotl). Both these forms reproduce, and both may occur in the same lake. Formerly they were referred to different genera. But the fact that some Axolotls kept in the Jardin des Plantes in Paris lost their gills when their surroundings were allowed to become less moist than usual, led 608 AMPHIBIA. naturalists to recognise that the two forms were but different phases of one species. It has been shown repeatedly that a gilled Axolotl may be transformed into a form without gills ; and this metamorphosis seems to occur constantly in one of the Rocky Mountain lakes. Abundant food and moisture favour the persistence of the Axolotl stage. Amphibians are very defenceless, but their colours often conceal them. Not a few have considerable power of colour-change. The secretion of the'skin is often nauseous, and therefore protective. Ina few cases, such as Ceratophrys dorsata, there is a bony shield on the back made of a number of small pieces arising as ossifications of the inner stratum of the dermis and of the subcutaneous connective tissue. It is interesting to notice the occurrence of numerous hair-like filaments on the sides and thighs of the males of a Kamerun frog (Astylosternus robustus). Many Amphibians liye alone, but they usually congregate at the breeding seasons, when the amorous males often croak noisily.. Alike in their love and their hunger, they are most active in the twilight. Their food usually consists of worms, insects, slugs, and other small animals, but some of the larval forms are for a time vegetarian in diet. They are able to survive. prolonged fasting, and many hibernate in the mud. Though the familiar tales of ‘‘ toads within stones” are for the most part inaccurate, there is no doubt that both frogs and toads can survive prolonged imprisonment. Besides having great vital tenacity, Amphibians have considerable power of repairing injuries to the tail or limbs. Although the life of Amphibians seems to have on an average a low potential, even the most sluggish wake up in connection with re- production. The males often differ from their mates in size and colour. Some of their parental habits seem like strange experiments. Thus in the Surinam toad (Pifa americana) the large eggs are fertilised internally and placed by the everted cloaca of the female upon the back, the male apparently helping in the process. The skin becomes much changed—doubtless in response to the strange irritation —and each fertilised ovum sinks into a little pocket, which is closed by a gelatinous lid. In these pockets the embryos develop, perhaps ab- sorbing some nutritive material from the skin. They are hatched as miniature adults. In Mototrema the female has a dorsal pouch of skin opening posteriorly, and within this tadpoles are hatched. In Rhzvo- derma darwinzi the male carries the ova in his capacious croaking-sacs. In the case of the obstetric toad (Alytes obstetricans), not uncommon in some parts of the Continent, the male carries the strings of ova on his back and about his hind-legs, buries himself in damp earth until the development of the embryos is approaching completion, then plunges into a pool, where he is freed from his living burden. In the Anura the ova are fertilised by the male as they leave the oviduct ; in most Urodela fertilisation is internal, sometimes by approxi- mation of cloace, sometimes by means of complex spermatophores which the male deposits in the water close to the female. The eggs of the frog are laid in masses, each being surrounded by a globe of jelly ; those of the toad are laid in long strings ; those of newts are fixed singly to water-plants; those of some tree-toads, such as Hylodes, are \aid on or under leaves in moist places. LIFE OF AMPHIBIANS. 609 There are about goo living species of Amphibia, most of them tail- less. All are averse to salt water, hence their absence from almost all oceanic islands. The anura are well-nigh cosmopolitan; the Urodela are almost limited to the temperate parts of the northern hemisphere. History.—lIt is likely that Amphibians were derived from a Piscine stock related to. the Dipnoi and perhaps also to the Crossopterygians. The Stegocephali were the first pentadactyl animals (Lower Carboni- ferous). Of living forms, the Gymnophiona are more old-fashioned than the others. The modern types gradually appear in Tertiary times. Some of the extinct forms were gigantic. Huxley emphasised the following affinities between Amphibians and Mammals :—The Amphibia, like Mammals, have two condyles on the skull; the pectoral girdle of Mammals is as much amphibian as it is sauropsidian ; the mammalian carpus is directly reducible to that of Amphibians. In Amphibians only does the articular element of the mandibular arch remain cartilaginous; the quadrate ossification is small, and the squamosal extends down over it to the osseous elements of the mandible, thus affording easy transition to the mammalian con- dition of these parts. But Mammals are, on the whole, more nearly related to Reptiles. There are some remarkable affinities between the Stegocephali and some of the extinct Reptiles, such as the Anomodonts, which in their turn have affinities with Mammals. 39 CHAPTER XXIV Ciass REPTILIA CHELONIA. RHYNCHOCEPHALIA. LACERTILIA. OPHIDIA. CrocopiniaA. Many ExTINcT ORDERS THE diverse animals—Tortoises, Lizards, Snakes, Croco- dilians, etc.—which are classed together as Reptiles, are the modern representatives of those Vertebrates which first became independent of the water, and began to possess the dry land. While almost all Amphibians spend at least their youth in the water, breathing by gills, this is not necessary for Reptiles, in which embryonic respiration is secured by a vascular foetal membrane known as the allantois. As in still higher Vertebrates, gill-slits are present in the embryos ; but they are not functional, and are without gills. Reptiles seem to form among Vertebrates a great central assemblage, like “ worms” among Invertebrates, more like a number of classes than a single class, exhibiting close affinities with Birds and Mammals, and more distant affinities with Amphibians. Reptiles, Birds, and Mammals are distinguished, as Amniota, from Amphibians and Fishes, which are called Anamnia, the terms referring to the presence or absence of a protective foetal membrane—the amnion—with which another, the allantois, is always associated. Among other common characters the following may be noted :—the generally terrestrial habit, the absence of gills, the absence of a conus arteriosus, the breaking ‘up of the ventral aorta, the presence of twelve cranial nerves, the importance of the hyo-mandibular gill-cleft. ICHTHYVOPSIDA, SAUROPSIDA, AND MAMMALIA. 611 “uoIyeJUSUE '|-Bas onsejqojoy ym ‘y[OA ou JO ATVI, WIM ‘jeuls ore Bao ay} ‘sauiazj0U0WW ur Jdaoxy *snoredtara aie [le ‘souezjouoyy }daoxq ‘usmIOpge WoIy xeIOY sajeiedas wSeiqdeip renosnur ev frends sXemyje ysoure pue ‘payeaponu-uou are saposnd -109 ‘poojq par ay} § (Ja] 243 03) Gore oI0e aUuo SI a1aq} § paraquieyo-inoj st Jivay oy, *BO¥O/D ONI} & alayj SI SaUIDIJOUOW UI AUG *SdAJoU [BIUPID sA[OM] OIE SID], *episdoineg jo 324} YIM snosojouloy St wmNtie3s aq} { Are}UeUIIPNA St plooeI0D 343 ‘samaljouoyl Ul ydaoxY ,"SayeIqa11aA Jamo] JO ae[ngipuewody pue ‘ayerpenb ‘repnoijze ayy 07 puodsai109 sdeqied qorgm—sadejs pus ‘snout ‘sno[feui—sapoisso AlojIpne = SIyst19]0eTeYyo 99143 aie at0q} fyesourenbs 943 yA Suge, -noijie ‘auog aus ¥ SI J[Mpe oy} jo a[qrpueul ay? fyeqdiooorseq 9q3 Jo djaq aq} qarm sew -sulos ‘sfeyidiov0xe 94} wolf pauioy ATyensn ‘sajApuod om] ere a10q3 fooled xafdur0o 9Y} WIOJ 03 VsNJ SauOg J1V0 ayI { wAqaz19A [eo -IA199 WaAas a1¥ 91943 ‘suoT|dadxe 921T]} IO OM7 YIM $ sauo0g 94} JO ISOUI Jo Spus aqj Jz Os[e pus ‘(elualig pure saularjouo yy ur 3deoxa) e17099 [eq -37194 343 JO spua a3 ye soshydida ore aTEY, miaysAs aul] [eIEIz[ JO VdeI] JOUNSIP ON *sopeWay aq? 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Juasqe Jo [Tews St proueydsiseq ay} f pro -usydsered o31y] & uayjo St a10q3 f ystszed Avur x0q-UTeIq SNOUISETIII 947 30 YonyT *sesAyd -1da do suOT}edyISsO ayeredas ou arv a1aq3 ‘930 *erjuad [eIqe}19A ay} JO Spua ayy Uo { payisso Ajaye[duroour waqjo st wuINOo [eIqazIEA By, sayty Ajzea Burinp 3se9} ye ‘sueZio asuas [eie}e[ Jo wajshs © SI a10qT, *U0}2]94SOX2 OU ST araq? suviqiydury uzepour [je ysomye ur apy ‘oruiiap jied year ur ore saysy jo sopeos ayy, “qseay 3@ at] Ajzea Surmp ‘s][id are 10 T, ‘sueiqiqdury jo rappeyq yedeoyo aq Aq poyuasaidar s1.1aqqe] ay} Se ey OS ur jdaoxa ‘stoywey[e Jou uoTUWe OU SI aJ9y 7, ‘SIVINAOV 1g 29 ‘STVIdNSUVJ ‘SHNAALONOJW “VIIVNW VL “sdulg GNV SAMLdayY *vaisdounvs “SNVIGIHANY ONV SHHSI *VOISdOAH LHS] VIIVWNVW GNV ‘VCISdOUNVS ‘VCISAOAHLHOI NAAM SLSVULNOO AHL AO ANOS 612 REPTILIA. Some of the main contrasts between living Reptiles and Birds are summarised in the following table :— REPTILES. Birps. The exoskeleton consists of horny epidermal scales, sometimes augmented by bony dermal scutes. , The centra of the vertebrae are rarely like those of birds. When there is a sacrum, its vertebrze (usually two in number) have large ex- panded ribs with the ends of which the ilia articulate. The cartilaginous sternum may be- come bony, but is not replaced by membrane bones, unless perhaps in Pterodactyls. When there is an interclavicle or epi- sternum, it remains distinct from the clavicle and sternum. The hand has more than three digits, and at least the three radial digits are clawed. In living reptiles the ilia are prolonged farther behind than in front of the aceta- bulum ; the pubes slope downward and forward; there are usually pubic and ischiac symphyses. There are often five toes; the tarsals and the metatarsals remain distinct. At least two aortic arches persist ; only the Crocodilia have a structurally four-chambered heart; more or less mixed blood always goes to the pos- terior body. The body has approximately the tem- perature of the surrounding medium. The optic lobes lie on the upper surface of the brain. There is an outer covering of feathers, and though there may be a few scales, there are never scutes. ‘The centra of the cervical vertebra | have usually a saddle-shaped terminal curvature. The two sacral vertebre have no expanded ribs, they fuse with others to form a long composite ‘‘synsacrum.” The cartilaginous sternum is replaced by membrane bone from several centres. When there is an interclavicle, it is confluent with the clavicles. The hand has not more than three digits, and at most two digits are clawed. The fore-limbs are modified as wings; some carpals fuse with the fused metacarpals. The ilia are greatly prolonged in front of the acetabulum, the inner wall of which is membranous. The pubes slope backwards, parallel with the ischia; only in Struthio is there a pubic symphysis, only in Rhea is there an ischiac one. There are not more than four toes; the proximal tarsals unite with the tibia, forming a tibio-tarsus; the first metatarsal if present is free, but the three others are fused to one another and to the distal tarsals, forming a tarso-metatarsus. There is but one aortic arch, to the right; the heart is four-chambered ; the blood sent to the body is purely arterial. The body temperature is high and almost constant. The optic lobes lie on the sides of the brain. The lungs have associated air-sacs. The sutures between the bones of the skull are usually obliterated at an early stage. The right ovary atrophies. CHELONIA. 613 Order CuEtonia. Tortoises and Turtles GENERAL CHARACTERS.—TZhe broad trunk is encased in bones which form a dorsal and a ventral shield, within the Fic. 330.—Skull of turtle. §.0O., supra-occipital; PAR., parietal; #R., frontal; P.F., pre- frontal; PO.F., post-frontal; SQ., squamosal; PALX., pre- maxilla; M2X., maxilla; /., jugal; Q./., quadrato-jugal ; Q., quadrate; D., dentary; AW., angular; AR., articular; S., surangular. shelter of which the head and nech, tail and limbs, can be more or less retracted. The dorsal carapace ts usually formed 614 REPTILIA. from—(a) the flattened neural spines (plus dermal scutes) ; (b) expanded and more or less coalesced ribs (plus costal dermal scutes); (c) a series of dermal marginal scutes around the outer edge. In the Athece the dorsal vertebra and ribs are not fused to the dermal plates which form the carapace. The ventral shield or plastron ts formed of nine or so dermal bones. There ts no sternum. Overlapping, but not corresponding to the bony plates, there are (except in Trionychia and Athece) epidermic horny plates of “tortoise shell,” which, though very hard, are not without sensitiveness, numerous nerves ending upon them. The quadrate ts immovably untted with the skull. There is only a lower temporal arcade. The jaws are covered by a horny sheath, and are without teeth, though hints of these have been seen in some em- bryos. There is a single anterior nasal opening. The scapular arch is internal to the ribs. The limbs are pentadactyl, but often in the form of paddles. The average life of Chelonians is sluggish. Perhaps this is in part due to the way in which the ribs are lost in the carapace, for this Fic, 331.—Carapace of yyust tend to make respiration less tortoise. “active. The lungs are divided into The dark contours ae these of a mumber of compartments. Citas siesta of theme: . Tae slamcal aperture is usually which have been removed. longitudinal, never transverse, the copulatory organ is unpaired. All are oviparous. The eggs have firm, usually calcareous, shells. Some Peculiarities in the Skeleton of Chelonia The (10) dorsal vertebrae are without transverse or articular processes, and along with the ribs are for the most part immovably fused in the carapace. The tail and neck are the only flexible regions. There are two sacral vertebrae. The greater part of the dorsal shield is due to a coalescence of eight ribs with eight costal plates derived from the dermis, SKELETON OF CHELONIA. 615 Similarly, the median pieces are the result of fusion between median dermal bones and the neural spines of the vertebre. The plastron usually consists of nine dermal bones, and the three anterior pieces perhaps represent clavicles and interclavicle (or episternum). The eight cervical vertebrse have at most little rudiments of ribs, are remarkably varied as regards their articular faces, and give the neck many possibilities of motion, There are no lumbar vertebree. The bones of the skull are immovably united; there is only a lower temporal arcade, formed by jugal and quadrato-jugal; there are no ossified alisphenoids, but downward prolongations of the large parietals Fic. 332.—Pectoral girdle of a Chelonian. G., Glenoid cavity; SC., scapula; P.C., procoracoid fused to the scapula; C., coracoid; Z.C., epicoracoid cartil- age; L., ligament. : ‘ take their place ; neither presphenoid nor orbitosphenoids are ossified ; there are no distinct nasal bones in modern Chelonians, their place being taken by the prefrontals ; the premaxille are very small; there are no teeth. ; : There is no sternum. The pectoral girdle on each side consists of a ventral coracoid and a dorsal scapula attached to the carapace. The ‘ scapula bears an anterior process of large size, usually regarded as a ‘* precoracoid” or procoracoid. ; The pelvic girdle consists of dorsal ilia attached to the carapace, posterior ischia, and anterior pubes, with pre-pubic processes and an epi-pubic cartilage. There is a pubic and an ischiac symphysis. The girdles originally lie in front of, or behind the.ribs, but are over- arched by the carapace in the course of its development. 616 REPTILIA. Some Peculiarities in the Organs of Chelonia In Chelonians and in all higher animals except serpents, there are twelve cranial*nerves, for, in addition to the usual ten, a spinal accessory to cervical muscles, and a hypoglossal to the tongue, are ranked as the eleventh and twelfth. The gullet of the turtle shows in great development what is hinted at in others, long horny papillae pointing downwards ; it is probable that these help to tear up the food (seaweed in the case of the turtle).