..-.;. •..:..•:;. 11111 s&i V OF UNIVERSITY; f A TEXT-BOOK OF ZOOLOGY VOL. II MACMILLAN AND CO., LIMITED LONDON . BOMBAY . CALCUTTA MELBOURNE THE MACMILLAN COMPANY NEW YORK . BOSTON . CHICAGO ATLANTA . SAN FRANCISCO THE MACMILLAN CO. OF CANADA, LTD. TORONTO A TEXT-BOOK OF ZOOLOGY BY T. JEFFERY PARKER, D.Sc., F.R.S. PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO, DUNEDIN, N.Z. AND WILLIAM A. HASWELL, M.A., D.Sc., F.R.S, PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF SYDNEY. N.S.W. IN TWO VOLUMES VOL. II WITH ILLUSTRATIONS MACMILLAN AND CO., LIMITED ST. MARTIN'S STREET, LONDON 1910 KJCHARD CLAY AND SONS, LIMITED, BREAD STREET HILL, E.G., AND BUNGAY. SUFFOLK. First Edition, 1898. Second Edition, 1910. ($L P33 : G Zoology. Vol. II. ERRATA. PAGE. 9, description of Fig. 710, for "Fig. 667 " read " Fig. 709." 17, line 8 from bottom, for " end" read " (end)'1. 19, line I, for " peribranchial " read " periphary ngeal. " 183, Fig. 840, for " Pristurua " read " Pristinrus. " 229, description of Fig. 885, for " dorsa " read " dorsal." 322, line 20 from top, for " innominiata" read " innominala." 432, lines 3 and 20 from top, for " Apertyx '' read " Apteryx." 500, line I, for " Arycteropus " read " Orycteropus." 606, line 9 from top, for " Squolodontidoe " read " Squalodontidce. 620, line 3 from bottom, for " import " read " important." . ueneral Kemarks Class II. Pisces ............ 144 Sub-class 1. Elasmobranchii . . . . . . . .144 1. Example of the Sub-class — tfcifl.liutu cxitindd or HemixcyUium modestum ........... 144 2. Distinctive Characters and Classitication ..... 166 3. General Organisation ......... 170 Q/ P33 CONTENTS SECTION XIII PAGE PHYLUM CHORD ATA . 1 Sub-phylum and Class I. Hemichorda (Adelochorda) ... 2 Sub-phylum and Class II. TJrochorda ....... 14 1. Example of the Class — Ascidia ........ 14 2. Distinctive Characters and Classification . -, . . . .21 Systematic position of the Example ....... 24 3. General Organisation ......... .24 Sub-phylum III. Euchorda ......... 43 Section I. ACUANIA ........... 44 Section II. CKANIATA . • ........ 65 Class I. Cyclostomata .......... 123 1. Example of the Class — Petromywn ....... 124 2. Distinctive Characters and Classification ..... 137 3. Comparison of the Myxinoids with the Lamprey .... 138 4. General Remarks ......... . 142 Class II. Pisces ............ 144 Sub-class I. Elasmobranchii ' ........ 144 1. Example of the Sub-class— -Scylliurii cnmc-ula or Ifemiacylliitm . ......... . 144 2. Distinctive Characters and Classification ..... 166 3. General Organisation ......... 170 vi CONTENTS PHYLUM CHOBDATA — continued. Class II. Pisces — continued. PAGE Sub-class II. Holocephali 188 Sub-class III. Teleostomi 196 1. Example of the Sub-class — Salmofario ..... 198 2. Distinctive Characters and Classification 217 Systematic Position of the Example 224 • 3. General Organisation 224 Sub-class V. The Dipnoi 246 1. Example of the Class — Cemtodus (Neoceratodus) fosteri . . 246 2. Distinctive Characters and Classification . 257 Appendix to Pisces— The Ostracodermi 261 Class III. Amphibia 264 1. Example of the Class — Rana temporaries or Rana esculenta . 264 2. Distinctive Characters and Classification 292 Systematic Position of the Example 294 3. General Organisation 294 Class IV. Reptilia 313 1. Example of the Class — Lacerta 314 2. Distinctive Characters and Classification 334 Systematic Position of the Example 337 3. General Organisation of Recent Reptilia 338 4. Extinct Groups of Reptiles 371 Class V. Aves . 378 1. Example of the Class — Columba livia 378 2. Distinctive Characters and Classification 40S Systematic Position of the Example 4J 7 3. General Organisation 417 Sub-class I. Archeeoniithes 418 II. Neornithes . . 420 CONTENTS vii PHYLUM CHORDATA — continued. PAGE Class VI. Mammalia 446 1. Example of the Class — Lepus cuniculus 446 2. Distinctive Characters and Classification of Recent Mammalia . 476 Sub-class I. Prototheria 477 II. Theria 478 Systematic Position of the Example 488 3. General Organisation 488. The Mutual Relationships of the Chordata 610 On the Mutual Relations of the Phyla of Animals . . . . , . 616 SECTION XIV DISTRIBUTION 619 1. Geographical Distribution 619 2. Bathymetrical Distribution 634 3. Geological Distribution 638 SECTION XV THE PHILOSOPHY OP ZOOLOGY 643 SECTION XVI THE HISTORY OF ZOOLOGY 668 APPENDIX— Zoological Literature 691 INDEX . 697 LIST OF ILLUSTRATIONS 705. Balanoglossus 3 706. ,, anterior end 4 707. Ptychodera bahamensis 5 708. Balanoglossus, development 7 709. Tornaria 8 710. „ 9 711. Cephalodiscus, gelatinous investment 10 712. ,, zooid . . . 11 713. ,, sagittal section 12 714. Rhabdopleura 13 715. Ascidia 15 716. ,, anatomy 16 717. ,, mesh of branchial sac 17 718. ,, diagrammatic longitudinal section 18 719. ,, transverse section 19 720. ,, dorsal tubercle, ganglion, and associated parts ... 20 721. ,, sagittal and transverse sections 21 722. Appendicularia 24 723. ,, diagram 25 724. Botryllus violaceus 25 725. Composite Ascidian, diagram of zooid 26 726. Doliolum 27 727. Salpa democratica, ventral view 28 728. ,, lateral view 28 729. Pyrosoma 29 730. ,, part of section 29 731. Salpa, lateral view of ganglion 31 732. Ascidian, mature egg 32 733. Development of Clavellina, early stages 33 734. „ „ later „ 35 735. Larva of Ascidia rnammillata 37 736. Metamorphosis of Ascidian, diagrammatic 38 737. Doliolum, tailed larva 39 738. ,, asexual stage, lateral view 40 739. „ „ ,, dorsal ,, 40 740. Salpa, late stage in development 41 LIST OF ILLUSTRATIONS 741. Amphioxus lanceolatus 45 742. ,, ,, transverse sections of pharyngeal and in- testinal regions 46 743. ,, ,, anatomy, diagrammatic .... 49 744. ,, ,, transverse section of pharyngeal region, diagrammatic 50 745. ,, ,, diagram of vascular system . ... . 51 746. ,, ,, nephridium 53 747. ,, ,, brain and cerebral nerves .... 54 748. ,, ,, anterior portion of neuron ... 55 749. ,, ,, segmentation of the oosperm ... 56 750. ,, ,, formation of gastrula 57 751. ,, ,, development of notochord, neuron, and mesoderm .58 752. ,, ,, advanced embryo 59 753. ,, ,, young larva 60 754. ,, ,, more advanced larva 62 755. ,, ,, development of atrium .... 63 756. ,, ,, ,, ,, transverse sections 64 757. Ideal Craniate 68 758. Section of skin of Fish 69 759. Muscular system of Dogfish 70 760. Ideal Craniate, anatomy 71 761. Vertebral column of embryo, transverse section .... 73 762. Diagram illustrating segmentation of vertebral column ... 74 763. Elements of embryonic cranium 75 764. Diagrams of cartilaginous skull . . .77 765. Diagrams of bony skull 80 766. Development of pelvic fins, diagram 82 767 and 768. Diagrams of limbs and limb-girdles 83 769. Structure and development of tooth 86 770. Structure of liver, diagrammatic 87 771. Diagram of gills 89 772. Diagram of vascular system of Fish 91 773. Diagram of circulation in a Fish 93 774. Diagram of vascular system of embryo of air-breathing Craniate . 95 775. Diagram of heart of Amphibian and Crocodile . . .96 776. Blood-corpuscles of Frog and Man 97 777. Transverse section of spinal cord . . . . , . . .99 778. Diagrams of Craniate brain 101 779. Diagram of cerebral and anterior spinal nerves .... 104 780. Organs of touch 107 781. Sensory canals of head and organs of the lateral line . . . 108 782. Taste-buds 109 783. Olfactory cells 109 784. Section of eye 110 785. Diagram of retina Ill 786. Development of eye 112 LIST OF ILLUSTRATIONS xi FIG. PAGK 787. A. Muscles and nerves of eye 113 787. B. Pineal eye of Sphenodon 114 788. Organ of hearing 115 789. Section of Ampulla 116 790. Urinary tubule 117 791. Diagrams of urinogenital organs 119 792. Development of mesoderm in Frog 121 793. Petroinyzon marinus, external views of head 124 794. ,, ,, skull, with branchial basket . . . .126 795. „ „ „ 127 796. ,, ,, dissection of female 129 797. ,, ,, brain 131 798. ,, ,, ,, with olfactory and pituitary sacs . 132 799. ,, ,, development of olfactory and pituitary sacs 133 800. ,, ,, auditory organ 134 801. ,, ,, transverse section of abdomen . . . 134 802. ,, ,, urinogenital sinus and related parts . . 135 803. ,, development 135 804. ,, sections of embryos 136 805. ,, nuviatilis, head of larva 137 806. Head of Myxine and of Bdellostoma 139 807. Myxine glutinosa, dissection 140 808. ,, auditory organ 141 809. Bdellostoma, kidney 141 810. Palseospondylus gunni . 143 811. Hemiscyllium modestum 145 812. Scyllium canicula, vertebrae 147 813. Hemiscyllium, skull 148 814. ,, visceral arches 150 815. ,, pectoral arch and fin 151 81(>. ,, pelvic arch and fin 152 817. ,, lateral dissection 153 818. ,, branchial sac 154 819. Scyllium, heart and branchial arteries ...... 155 820. Hemiscyllium, blood-vessels 157 821. Scyllium canicula, brain 158 822. Hemiscyllium, brain 159 823. Scyllium catulus, cranial nerves and branchial plexus . . . 160 823 bis. ,, canicula, cerebral nerves 162 824. ,, ,, urinogenital organs . . ... 164 825. Hemiscyllium, right kidney and urinary sinus . . . . 166 826. Dog-fish, egg-case 166 827. Cladoselache fyleri 167 828. Pleuracanthus ducheni 168 829. Acanthodes wardi 169 830. Chlamydoselachus anguineus 169 831. Lamna cornubica 170 832. Urolophus cruciatus 171 xii LIST OF ILLUSTRATIONS FIG. PAGE 833. Centrophorus calceus, dermal denticles 172 834. Scymnus, spinal column . 172 835. Urolophus testaceus, skeleton 174 836. Heptanchus, skull 175 837. Torpedo-Ray, showing electric organ 177 838. Cestracion galeatus, egg-case 181 839. Pristiurus, section of blastoderm 182 840. ,, formation of mesoderm 183 841. Elasmobranch embryo, sections 184 842. Scyllium canicula, embryo 185 843. Ray, embryo 185 844. Elasmobranch embryo with yolk-sac 186 845. Scyllium canicula, head of embryo 187 846. „ „ „ „ later stage 187 847. Chimsera and Callorhynchus . 189 848. ,, vertebral column 191 849. „ skull 192 850. Callorhynchus antarcticus, skull 193 851. „ „ brain 194 852. ,, ,, male urinogenital organs . . . 195 853. ,, ,, embryo in egg-shell .... 197 854. Salmo fario 198 855. „ „ head 199 856. „ ,, scale 200 857. „ ,, vertebrae 201 858. ,, „ caudal end of vertebral column 202 859. ,, skull 203 860. ,, fario, skull disarticulated 205 861. ,, salar, ,, of young individual . 208 862. ,, fario, fin-ray 208 863. ,, ,, shoulder-girdle and pectoral fin 209 864. ,, ,, pelvic fin 210 865. ,, ,, side dissection 211 866. ,, „ brain 213 867. ,, ,, eye 214 868. ,, ,, auditory organ 215 869. ,, ,, urinary organs 215 870. ,, ,, development 216 871. ,, ,, section of blastoderm 217 872. Polypterus bichir .218 873. Acipenser ruthenus . 219 874. Lepidosteus platystomus 219 875. Amiacalva 219 876. Rita buchanani 220 877. Gadus morrhua . 22 1 878. Sebastes percoides 879. Labrichthys psittacula 222 880. Ostracion . . 223 LIST OF ILLUSTRATIONS xiii FIG. PAGE -881. Hippocampus . 223 882. Pleuronectes cynoglossus . 227 883. Stomias boa 228 884. Ctenoid and ganoid scales 228 885. Polypterus, part of vertebral column 229 886. Sturgeon, skull 230 887. Polypterus, skull . 231 388. , , pectoral tin 232 889. ,, pelvic fin ' 232 890. Gymuotus electricus 233 891. Sargus, teeth 234 892. Anabas scandens . 235 893. Lepidosteus, digestive organs . . 236 894. Pseudophycis bachus, relation of air-bladder to auditory organ . 237 895. Lepidosteus, brain 238 ,, male organs 239 ,, and Amia, female organs 240 ,, segmentation 241 Polypterus, head of larva 242 9( )(>. Glyptolepis and Macropoma 243 901. Palseoniscus and Platysomus 244 902. Lepidotus and Caturus . . . 245 Ceratodus forsteri . 247 ,, ,, anterior portion of skeleton .... 248 ,, ,, skull, dorsal . . 249 ,, ,, ,, ventral 249 ,, ,, pelvic arch and fin 250 ,, ,, lung 251 ,, ,, heart and main blood-vessels .... 252 ,, ,, brain 253 ,, ,, reproductive organs, female .... 255 ,, ,, development 256 913. Protopterus annectens 258 914. ,, ,, skull, shoulder-girdle, and fore- limb . . 259 915. Coccosteus decipiens 260 916. Pteraspis rostrata 261 917. Lanarkia spinosa 262 918. Drepanaspis gemundenensis . 262 919. Cephalaspis 263 920. Pterichthys testudinarius . 264 921. Rana temporaria 265 ,, skeleton 267 skull 269 ,, ,, y, of tadpole . 271 ,, esculenta, shoulder-girdle 272 ,, ,, ,, ,, transverse section, diagrammatic 272 ,, pelvic-girdle 273 ,, muscles . . 275 xiv LIST OF ILLUSTRATIONS FK;. PAGE 929. Rana temporaria, dissection from left side 276 930. ,, esculenta, digestive organs 277 931. ,, temporaria, heart 278 932. „ ,, arteries 279 933. „ „ veins 281 934. ,, ,, course of blood and lymph, diagrammatic . 283 935. ,, esculenta, brain 285 936. ,, accessory auditory apparatus .... . . 286 937. ,, esculenta, urinogenital organs, male ..... 287 938. ,, ,, ,, ,, female 288 939. ,, development 289 940. ,, temporaria, stages in life-history 291 941. Necturus maculatus 295 942. Siren lacertina 295 943. Amphiuma tridactyla 295 944. Salamandra maculosa .... 296 945. Coecilia pachynema 297 946. Urodela, structure of vertebral column 299 947. Proteus anguinus, chondrocranium 300 948. Salamandra atra, skull 301 949. Ichthycpbis glutinosa, skull .... ... 301 950. Protriton, skull 302 951. Salamandra and Amblystoma, shoulder-girdle and sternum . . 303 952. ,, pelvic girdle 304 953. ,, heart and chief arteries, larva and adult . . . 306 954. ,, maculosa, venous system ...... 307 955. Urodela, diagrams of male and female organs 309 956. Nototrema marsupiatum 310 957. Pipa americana 310 958. Ichthyophis glutinosa 311 959. Amblystoma tigrinum (axolotl) 311 960. Lacerta viridis 314 961. Lizard, vertebra 310 962. Lacerta agilis, skull ... 318 963. ,, ,, pectoral arch and sternum 321 964. ,, ,, carpus 322 965. ,, vivipara, pelvis 322 966. ,, agilis, tarsus 323 967. ,, ,, general view of viscera 324 968. , , viridis, dissection from ventral aspect .... 325 969. Lizard, lateral dissection 327 970. Lacerta viridis, brain 329 971. ,, vivipara, brain of an embryo 330 972. ,, Jacobson's organ 331 973. ,, sclerotic ossicles 331 974. ,, viridis, membranous labyrinth 332 975. ,, ,, urinogenital organs, male ..... 3.">3 976. „ female . 333 LIST OF ILLUSTRATIONS xv PIG. 977. Chameleon vulgaris 338 978. Pygopus lepidopus 339 979. Sphenodon punctatum 340 980. Testudo greeca 341 981. Sphenodon, vertebra 343 982. Python, vertebra 343 983. Crocodile, skeleton 345 984. Sphenodon, skeleton 345 985. Crocodile, anterior vertebrae 346 986. Cistudo lutaria, skeleton 346 987. Chelone midas, transverse section of skeleton 347 988. Tropidonotus natrix, skull . 348 989. Crotalus, skull 349 990. Sphenodon, skull 350 991. Emys europsea, skull 351 992. Chelone mydas 351 993. Crocodilus porosus, skull 352 994. Crocodile, skull 353 995. Emys europsea, tarsus 354 996. Alligator, carpus .354 997. ,, pelvis . 355 998. Crocodile, tarsus 355 999. Monitor, Emys, and Alligator, tongues 357 1000. Chameleon, lungs 358 1001. Lacerta muralis, heart 358 1002. Turtle, diagram of heart 359 1003. Crocodile, heart 359 1004. Alligator, brain 360 1005. Sphenodon punctatum, pineal eye 361 1006. Alligator, early development 363 1007. Lacerta . 364 1008. Draco volans 366 1009. Rattlesnake, poison apparatus 367 1010. Belodon, skull 370 1011. Galesaurus planiceps, skull 371 1012. Plesiosaurus macrocephalus 372 1013. ,, pectoral arch 372 1014. ,, pel vie arch 373 1015. Ichthyosaurus communis . . . 373 1016. Iguanodon bernissartensis 374 1017. ,, mantelli, teeth 375 1018. Pterodactylus spectabilis 376 1019. Scaphognathus, skull 376 1020. Ramphorhynchus 377 1021. Edestosaurus 377 1022. Columba livia, external form 379 1023. ,, „ feathers 381 1024. Structure of feather . 382 xvi LIST OF ILLUSTRATIONS . FIG. FACE 1025. Development of feather 383 1026. Columba livia, pterylosis 384 1027. ,, ,, bones of trunk 386 1028. ,, ,, cervical vertebra 386 1029. ,, ,, sacrum of nestling 387 1030. ,, ,, skull of young specimen 388 1031. Diagram of Bird's skull .389 1032. Columba livia, hyoid apparatus 390 1033. ,, ,, columella auris 390 1034. ,, ,, bones of left wing 391 1035. ,, ,, manus of nestling 392 1036. ,, ,, innominate of nestling 392 1037. ,, ,, bones of hind-limb 393 1038. „ ,, foot of embryo 393 1039. ,, ,, muscles of wing 395 1040. ,, ,, dissection from right side 396 1041. ,, ,, lungs and trachea 398 1042. Diagram of air-sacs of a Bird 399 1043. Columba livia, heart 400 1044. ,, ,, vascular system 402 1045. „ ,, brain 404 1046. ,, ,, dissections of brain 405 1047. „ ,, eye 406 1048. ,, ,, auditory organ 406 1049. ,, ,, urinogenital organs, male 407 1050. „ „ „ „ female ..... 407 1051. Apteryx australis 411 1052. ,, ,, skeleton 412 1053. Hesperornis regalis ,, 413 1054. Ichthyornis victor ,, . . 414 1055. Eudyptes antipodum 415 1056. Archaeopteryx lithographica 418 1057. ,, „ skull 419 1058. ,, ,, manus 419 1059. Opisthcomus and Apteryx, wings 421 1060. Gypaetus and Ardea, pterylosis 422 1061. Casuarius, feather 423 1062. Gallus, Turd us, Vultur, Procellaria, and Casuarius, sterna . . 425 1063. Eudyptes pachyrhynchus, skeleton . . . . • . . . 426 1064. Apteryx mantelli, skull of young specimen, side view . . . 427 1065. ,, ,, ,, ,, ,, dorsal view . . 428 1066. Anas boschas, skull 429 1067- Ara ararauna, ,, 429 1068. Apteryx mantelli, shoulder-girdle 430 1069. Dinornis robustus, skeleton . 431 1070. Sterna wilsoni, fore-limb of embryo 432 1071. Apteryx australis, left innominate 432 1072. Gallus bankiva, innominate of embryo 433 LIST OF ILLUSTRATIONS xvii FIO. PAGE 1073. Apteryx oweni, hind-limb of embryo ... . 433 1074. Gallus bankiva, egg at time of laying ...... 436 1075. ,, ,, blastoderm 437 1076. ,, ,, two embryos 439 1077. ,, ,, egg with embryo and embryonic appendages . 440 1078. ,, ,, diagrams of development of embryonic mem- branes 441 1070. Diagram illustrating the relationships of the chief groups of Birds 445 1080. Lepus cuniculus, skeleton with outline of body .... 446 1081. ,, ,, vertebra 448 1082. ,, ,, skull 451 1083. ,, ,, ,, vertical section 454 1084. ,, ,, carpus with distal end of fore-arm . . . 456 1085. ,, ,, sacrum and innominates 457 1080. ,, ,, skeleton of pes 457 1087. ,, . ,, nasal region, vertical section .... 459 1088. ,, ,, lateral dissection of head, neck, and thorax . 460 1089. ,, ,, digestive organs . . . . . . . 461 1090. ,, ,, heart 462 1091. ,, ,, vascular system 465 1092. ,, ,, larynx 466 1093. ,, ,, transverse section of thorax 467 1094. ,, ,, brain . . . 468 1095. ,, ,, dissections of brain 469 1096. ,, ,, brain, vertical section 470 1097. ,, ,, urinogenital organs 473 1098. ,, ,, female organs (part) 474 1099. ,, ,, diagrammatic section of advanced embryo . 475 1100. Section of human skin 488 1101. Longitudinal section of hair 489 1102. Development of hair 490 1103. Echidna hystrix, with pouch and mammary glands . . . 491 1104. Diagrams of development of nipple 492 1105. Ornithorhynchus anatinus 493 1106. Echidna aculeata 493 1107. Didelphys virginiana 494 1108. Dasyrus viverrinus 494 1109. Petrogale xanthopus 495 1110. Notoryctes typhlops . . . 495 1111. Phascolomys wombat 496 1112. Phascolarctos cinereus 497 1113. Choloepus didactylus 498 1114. Dasypus sexcinctus . . . 499 1] 15. Manis gigantea 499 1116. Orycteropus capensis 500 1117. Orca gladiator 500 1117, bis. Hippopotamus amphibius . . . . . 503 VOL. II b LIST OF ILLUSTRATIONS 1118. Equus burchelli ........... 503 1119. Tapirus terrestris ........... 504 1120. Rhinoceros indicus .......... 504 1121. Phoca vitulina ........... 506 1122. Galeopithecus ............ 507 1123. Synotus harbastellus .......... 508 1124. Gorilla ............ .509 1125. Diagram of Mammalian skull .... ... 512 1126. Sagittal sections of Mammalian skulls, diagrammatic . . . 514 1127. Ornithorhynchus, skeleton ......... 517 1128. Echidna aculeata, skull ....... . 519 1129. Ornithorhynchus, scapula ......... 520 1130. Kangaroo, atlas ........... 521 1131. Halmaturus ualabatus, skeleton ........ 522 1132. Dasyurus, skull ........ ... 523 1133. Petrogale penicillata, skull ..... .... 523 1134. Phascolomys, skull ......... . 524 1135. Phalanger, bones of leg and foot ....... 525 1136. Macropus bennetii, bones of foot ....... 525 1137. Dasypus sexcinctus, skull ......... 526 1138. Myrmecophaga, skull, lateral ........ 526 1139. ,, ,, ventral ........ 527 1140. Bradypus tridactylus, skull ..... . . . . 527 1141. Dasypus sexcinctus, shoulder-girdle ....... 528 1142. Bradypus tridactylus, skeleton ........ 529 1143. ,, ,, shoulder-girdle ...... 530 1144. ,, ,, manus ..... . . . 530 1145. ,, ,, pes ......... 5: JO 1146. Dasypus sexcinctus, pelvis ......... ;,:;! 1147. ,, „ pes ......... 531 1148. Phocsena communis, skeleton ........ 532 1149. Balaenoptera musculus, sternum ........ 5:52 1150. Globiocephalus, skull . ......... 533 1151. Halicore australis, skeleton ......... 534 1152. Manatus senegalensis, skull ......... 535 1153. Cervus elaphus, axis .......... 536 1154. Equus caballus, posterior part of skull ...... 537 1155. Ovis aries, skull ........... 539 1156. Hyrax, skull ............ 540 1157. Elephas africanus, skull ......... 540 1158. Cervus elaphus, scapula ...... ' . . 541 1159. Tapirus indicus, manus .......... 542 1160. Equus caballus ,, ..... ..... 542 1161. Sus scrofa ,, .......... 542 1162. Cervus elaphus ,, ........ 542 1163. Equus caballus, tarsus .......... 543 1164. Cervus elaphus ,,.... . . 543 1165. Sus Scrofa , 543 LIST OF ILLUSTRATIONS lir.ii. Felis tigris, skull ........... 545 1167. ,, ,, section of auditory bulk ....... 545 1168. Canis familiaris, skull .......... 54K 1 169. Ursus ferox, section of auditory bulla ...... 546 1170. ,, americanus, carpus ........ -. 547 1171. Felis leo, digit ........... 547 1172. Phoca vitulina, skeleton ......... 548 1173. Centetes ecaudatus, skull ......... 549 1174. Pteropus jubatus, skeleton ......... 551 1175. ,, fuscus, skull .......... 552 1176. Homo sapiens, skull . . . . ...... 553 1177. Anthropopithecus troglodytes, skull ....... 555 1178. Siniia satyrus, skeleton ......... 556 1179. Oynocephalus anubis, carpus ........ 556 1180. Homo, Gorilla, and Simla, foot ........ 557 1181. Various forms of teeth, sections ........ 558 1182. Development of Mammalian teeth . • ..... 559 1183. ,, „ „ „ ....... 559 1184. Canis familiaris, milk and permanent dentitions .... 560 1185. Lagenorhynchus, teeth . . • . . • ...... 561 1186. Perameles, teeth ........... 562 1187. Phascolarctos cinereus, front view of skull ..... 562 1188. Macropus major, teeth ........... 563 1 ISM. Sareophilus ursinus, front view of skull ...... 563 1190. Didelphys marsupialis, teeth ........ 563 1191. Orycteropus, section of lower jaw and teeth ..... 564 ll'.»2. Sus si-n.fa. teeth ........... 565 1193. K(|ims eaballus, skull and teeth ........ 566 11M4. Klephas africanus, molar teeth ........ 567 HIT). l>al;enoptera rostrata, lower jaw of foetus, with teeth . . . 567 llMt't. .. section of upper jaw, with baleen .... 568 1197. Lo\\er carnassial teeth of Carnivora ....... 569 I IMS. Different forms of stomach in Mammalia ...... 571 1 IMM. Stomach of Ruminant .......... 572 12< H). .. ,, Porpoise ........ .573 1*201. Liver of Mammal, diagrammatic ....... 574 1202. Oniis familiaris, brain ...... ... 577 1203. Echidna aculeata, sagittal section of brain ..... 578 1204. Petrogale penicillata . . . ....... 578 I'JO.V ( )rnithorhynchus anatinus, brain ....... 579 1206. Echidna aculeata, brain ......... 579 T207. Macropus major ,, . ...... 580 12(18. Cogiagreyi ,, ......... 580 1209. Homo sapiens, sagittal section of nasal and buccal cavities . . 581 1210. „ „ ear ....... .... 581 1211. Female organs of Marsupials ........ 583 1212. Tteri of Eutheria ..... ..... c85 1213. Homo, sagittal section of ovary ........ 586 xx LIST OF ILLUSTRATIONS 1214. Development of Graafian follicle ....... 586 1215. Segmentation of Mammalian oosperm ...... 588 1216. Lepus cuniculus, embryonic area ....... 589 1217. ,, ,, embryos ......... 590 1218. Formation of foetal membranes of Mammal ..... 591 1219. Lepus cuniculus, embryo with membranes ..... 592 1220. Erinaceus, formation of amnion and trophoblast .... 593 1221. Formation of amnion in Mammalia ....... 594 1222. Macropus, mammary foetus ......... 596 1223. Hypsiprymnus rufescens, embryo and foetal membrane . . 596 1224. Phascolarctos cinereus ,, ,, ,, ,, . . 596 1225. Perameles obesula , , , , placenta .... 597 1226. Theria and Monotremata, blastula ....... 597 1227. Phascolotherium bucklandi, mandible ...... 601 1228. Plagiaulax becklesi, mandible ....... .601 1229. Diprotodon australis, skeleton ........ 602 1230. Nototherium mitchelli, skull ........ 603 1231. Thylcoles carnifex ... ........ 604 1232. Glyptodon clavipes, skeleton ...... . 604 1233. Mylodon robustus . .......... 605 1234. Squalodon, teeth . . . ........ 605 1235. Dinotherium giganteuin, skull ........ 607 1236. Tillotherium fodiens, skull ......... 609 1237. Diagram illustrating the mutual relationship of the Chordata . 616 1238. „ „ „ „ „ „ „ Phyla of animals . 618 1239. Map showing depths of sea between the British Isles and the Continent .......... 623 1240. Map showing depths of sea between New Zealand and Australia . 624 1241. Diagram illustrating the relations of the Zoo-geographical Regions 634 ZOOLOGY SECTION XIII PHYLUM CHORDATA. IN the arrangement which it has been found convenient to follow in the present work, the Vertebrate animals (Fishes, Amphibians, Reptiles, Birds, and Mammals), together with the Cephalochorda or Lancelets, the Urochorda or Ascidians, and the Hemichorda or Balanoglossus and its allies, are all grouped together in a single phylum — the Chordata. The main groups comprised in this assemblage, however, differ so widely from one another in certain essential points, and the common features uniting them together are so few, that it has been thought advisable to depart from the plan of arrangement followed in connection with the rest of the phyla, and to make a primary division in this case not into classes, but into sub-phyla. In accordance with this scheme the phylum Chordata is regarded as made up of three sub-phyla — the Hemichorda, the Urochorda, and the Euchorda, the last-mentioned comprising the two sections Acrania and Craniata or Vertebrata, each of which receives separate treatment. The name Chordata is derived from one of the few but striking common features by which the members of this extensive phylum are united together — the possession, either in the young condition or throughout life, of a structure termed the chorda dorsalis or notochord. This is a cord of specially modified vacuolated cells extending along the middle line on the dorsal side of the enteric cavity and on the ventral side of the central nervous system. In the lower Chordates (the Hemichorda, Urochorda, and Cephalochorda) the notochord is developed directly and unmis- takably from the endoderm, and in the first-named group it remains permanently in continuity with that layer. But in the Craniata its origin is by no means so definite, and it may originate from cells which are not obviously of endodermal derivation. It VOL, II B 1 ^l c ZOOLOGY SECT. "vtisty be ""enclosed ^m^a^ftnn sheath and thus be converted into a stiff, but elastic, supporting structure. In the Craniata (with a few exceptions among lower forms), it becomes in the adult replaced more or less completely by a segmented bony or cartilaginous axis — the spinal or vertebral column. Another nearly universal common feature of the Chordata is the perforation of the wall of the pharynx, either in-"the embryonic or larval condition only, or throughoutHife, by a system of clefts — the branchial clefts ; and a third characteristic is the almost universal presence at all stages, or only in the larva, of a cavity or system of cavities, the neurocoele, in the interior of the central nervous system. The Chordata ar^goelomata (Vol. I., p. 340), and the mode of development of tHe^ccelome in the lower sub-phyla is essentially the same as in the Echinodermata (Vol. I., p. 389), the Chsetognatha (p. 318), and the Phoronida (p. 358) : it is derived, that is to say, by direct outgrowth from the archenteron. In the Craniata this enteroccelic origin of the cavity is no longer definitely traceable, though what appear to be indications of it may be detected in some cases. The Urochopdar are not segmented l : in the Hemichorda there ig^ajivision of the coelome into three parts, each occupying a definite region of the body, so that the view is sometimes maintained that these animals are tri-segmented : in the Cephalochorda and Craniata there are numerous segments, the nature of which* will be referred to later. SUB-PHYLTTM AND CLASS I.-HEMICHORDA (ADELOCHORDA). A number of worm-like, simply organised animals possessing a structure which is commonly regarded as of the nature of a rudimentary notochord, comprising Balanoglossiis and certain allied genera, are so widely removed from the other members of the Chordata that, if we accept them as Chordates, it is advisable to consider them as constituting an independent sub-phylum, and to this the name of Hemichorda or Adelochorda has been applied. Resembling Balanoglossus in the condition of the supposed noto- chord, in the division of the body into three regions, sometimes looked upon as representing three segments, and in certain other features, are two genera of small marine animals — Cephalodiscus and Rhabdopleura. These are probably more nearly related to one another than they are to Balanoglossus, from which they are separated by well-marked differences, and the Hemichorda may, therefore, best be regarded as divisible into two classes — one, the Enteropneusta, comprising only Balanoglossus 2 and its imme- 1 Though faint indications of serial repetition of parts are traceable in certain cases. 2 The name Balanoglossus is here used as a general designation rather than as a strictly generic term. XIII PHYLUM CHORD AT A, diate allies ; the other, the Plero- branchia, including Cephalodiscus and Rhabdopleura. External Characters and Coelome of Enteropneusta.— Balanoglossus (Fig. 705) is a soft- bodied, cylindrical, worm-like animal, the surface of which is uniformly ciliated. It is divisible into three regions ; in front there is a large club-shaped hollow organ — the pro- boscis (pr.) ; immediately behind the proboscis and encircling its base is a prominent fold — the collar (co.) ; the third region or trunk is long and nearly cylindrical, but somewhat depressed. Balanoglossus lives in the sea, bur- rowing in sand or mud by means of its proboscis : one species has been found swarming on the surface of the sea. Numerous glands in the in- tegument secrete a viscid matter to which grains of sand adhere in such a way as to form a fragile temporary tube. The proboscis (Fig. 706, prob.) has muscular walls ; its cavity (pro- boscis-coelome) opens on the exterior usually by a single minute aperture —the proboscis-pore (prb.po.) — rarely by two. In some species the pro- boscis-pore does not communicate with the proboscis coelome, but ter- minates blindly, and may send off a narrow tubular diverticulum which opens into the neuroccele. The nar- row posterior part or " neck " of the proboscis is strengthened by a layer of cartilage-like or chondroid tissue, which supports the blood-vessels. The collar is also muscular, and contains one cavity, or two (right and left) separated from one another by dorsal and ventral mesenteries, and com- pletely cut off from the proboscis- cavity. The collar-cavity and also that of the proboscis are crossed by numerous strands of connective-tissue gen. FIG. 705. — Balanoglossus. En- tire animal, br. branchial region ; co. collar ; gen. genital ridges ; hep. prominences formed by hepatic cseca; pr. proboscis. (After Spengel.) B 2 ZOOLOGY SECT. of a spongy character. The collar-cavity communicates with the exterior by a pair of collar-pores — ciliated tubes leading into the first gill-slit or first gill -pouch. On the dorsal surface of the anterior part of the trunk is a double row of small slits — the gill-slits (Fig. 705, br.) — each row situated in a longitudinal furrow ; these slits increase in number throughout life. The most anterior are in some species overlapped by a posterior prolongation of the collar called the operculum. div FIG. 706.— BalanoglOBSUS. Diagrammatic sagittal action of anterior end. card s. cardiac sac ; div. diverticulum (supposed notochord) ; dors. n. dorsal nerve-strand ; dors. sin. dorsal sinus ; dors. v. dorsal vessel ; mo. mouth; f\ ob. proboscis ; prob. po. proboscis-pore ; prob. skel. proboscis-skeleton ; vent. n. ventral ner'se strand ; vent. v. ventral vessel. (After Spengel.) A pair of longitudinal genital ridges (gen.) — not recognisable in some species — which extend throughout a considerable part of the length of the body both behind and in the region of the gill-slits (branchial region), are formed by the internally situated gonads : these ridges are so prominent in some of the genera as to form a pair of wide wing-like lateral folds. Behind the branchial region are two rows of prominences (hep.) formed by the hepatic caeca. The trunk is irregularly ringed, this annulation, which is entirely xiii PHYLUM CHORDATA 5 superficial and does not correspond to an internal segmentation, being most strongly marked behind. The coelome of the trunk is divided into two lateral closed cavities by a vertical partition (dorsal and ventral mesenteries). Digestive Organs. — The mouth (Fig. 706, mo.) is situated ventrally at the base of the proboscis, within the collar. Into the dorsal half of the anterior portion of the alimentary canal open the internal gill-openings. Each of these is in the form of a long narrow U, the two limbs separated by a narrow process — the tongue —which contains a prolongation of the body-cavity. In most of the Enteropneusta the internal gill-openings lead into gill-pouches which in turn communicate with the exterior by the gill-slits. —v e*' FIG. 707.— Ptychodera bahamensis. Transverse section of the branchial region. 6. branchial part of alimentary canal ; b. , c3, coelome of trunk ; d. m. dorsal mesentery ; d. n. dorsal nerve ; d. v. dorsal vessel ; e. epidemis with nerve layer (black) at its base ; g. genital wing ; g. p. branchial aperture encroached upon by tongue (t) ; 1. lateral septum ; m. longi- tudinal muscles ; o. digestive part of oasophagus ; r. reproductive organ ; t. tongue ; v. ventral mesentery and ventral vessel ; v. n. ventral nerve. (From Harmer, Cambridge Natural History, after Spengel.) But in the genus Ptychodera (Fig. 707) there are no gill-pouches, the U-shaped internal gill-openings leading directly to the exterior. The gill-pouches are supported by a chitinoid skeleton consisting of a number of separate parts. Each of these consists of a- dorsal basal portion and three long narrow lamella, a median and two lateral ; the median, which is bifurcated at the end, lies in the se'ptum or interval between two adjoining gill-sacs ; the two lateral lie in the neighbouring tongues. In most species a number of transverse rods — the synapticulce— connect together the tongues and the adjoining septa, and are supported by slender processes of the skeleton. The posterior part of the alimentary canal is a nearly straight tube, giving off in its middle part, paired hepatic cceca, which bulge 6 ZOOLOGY SECT. outwards in the series of external prominences already mentioned. Posteriorly it terminates in an anal aperture situated at the posterior extremity of the body. In the posterior part of its extent in some Enteropneusta the intestine presents a ventral median ridge-like outgrowth of its epithelium — the pygochord. Throughout its length the intestine lies between the dorsal and ventral divisions of the vertical partition, which act as mesen- teries. As the animal forces its way through the sand, a quantity of the latter enters the digestive canal through the permanently open mouth, and is eventually passed out again by the anus in the shape of castings, which may be thrown out on the surface of the sand in a form resembling that taken by the castings of earthworms. A series of pores (gastro-cutaneous pores), variously arranged in the different genera, connect the intestine with the surface. Notochord or oesophageal diverticulum. — The dorsal wall of the part of the digestive canal immediately following upon the mouth gives off a diverticulum (div.) that runs forwards some distance into the basal part of the proboscis after giving off a short ventral branch. The diverticulum contains a narrow lumen, and its wall is composed of a single layer of long and very narrow cells each of which contains a vacuole. This layer of cells forming the wall of the diverticulum is continuous with the epithelium of the digestive canal itself, the cells being somewhat modified by the presence of the vacuoles. The diverticulum, owing partly to its structure, partly to its relations, is usually regarded as representing the notochord of the typical Chordata. In close relation with this on its ventral surface is the chitinoid proboscis-skeleton (prob. skel.) which consists of a median part of an hour-glass shape, and with a tooth-shaped process, bifurcating behind into two flattened bars which lie in the anterior region of the oesophagus and support the opening into the lumen of the diverticulum. There is a blood-vascular system with dorsal and ventral longitudinal trunks. The dorsal vessel (dors, v.) lies above the notochord, and ends in front in a sinus, the dorsal sinus or heart (dors, sin.), situated in the anterior part of the collar and the neck of the proboscis, in close contact with the notochord. From the posterior part of the sinus is given off a vessel which bifurcates to supply the proboscis. In communication with the sinus in front are a number of vessels of a bilateral plexus in the glomcrulus, a glandular organ, probably excretory, situated at the anterior end of the alimentary diverticulum. From the posterior end of each half of the glomerulus there passes backwards an efferent vessel which breaks up into a plexus ; the two plexuses unite ventrally to form a median ventral plexus XIII PHYLUM CHORDATA continuous behind with the ventral vessel. The dorsal sinus, having no definite walls, is not contractile; but a closed sac, the cardiac sac (card, s.), situated on the dorsal side of the sinus, has a muscular ventral wall, by the contractions of which the blood may be propelled. The nervous system consists of dorsal and ventral strands (dors, n., vent, n.) which extend throughout the length of the body. These are merely thickenings of a layer of nerve-fibres which extends over the entire body in the deeper part of the epidermis. Here and there are giant nerve-cells. The part of the dorsal strand which lies in the collar (collar-cord) is detached from the epidermis ; it contains a larger number of the giant nerve-cells than the rest ; in some species it contains a canal, the neuroccele, opening in front and behind ; in others a closed canal ; in most a number of separate cavities. At the posterior extremity of the collar the dorsal and ventral strands are connected by a ring-like thickening, and there is a thickening also round the neck of the proboscis. There are no organs of special sense ; but some cells of the epidermis on certain parts of the proboscis and on the anterior edge of the collar seem to be of the character of sensory cells. Reproductive Organs. — The sexes are separate, and often differ in colour ; the ovaries and testes are simple or branched saccular organs arranged in a double row along the branchial region of the trunk and further back ; they open on the exterior by a series of pores. The course of the development (Fig. 708-710) differs in different species. In some it is compar- atively direct ; in others there is a metamorphosis. Im- pregnation is ex- ternal. Segmenta- tion is complete and fairly regular, re- sulting in the for- mation of a bias- tula, which is at first rounded, then flattened. On one side of the flattened blastula an invagination takes place. The embryo at this stage is covered with short cilia, with a ring of stronger cilia. The aperture of invagination closes and the ectoderm FIG. 708.— Development of Balanoglossus. A, stage of the formation of the first grove (gr.). S, stage in which the second groove has appeared, and the first gill-slit has become developed ; co. collar ; FIG. 710.— Tornaria. Lateral view. Lettering as in Fig. 667 ; in addition, int. intestine ; mo. mouth. (After Spengel.) 10 ZOOLOGY SECT. not in organic continuity, so that, though enclosed in a common investment, they do not form a colony in the sense in which the word is used of the Polyzoa or the Hydroid Zoophytes. They have the feature in common with such a colony that they multiply by the formation of buds ; but these become detached before they are mature. With the collar-region are connected a series of usually eight to sixteen arms, each beset with numerous very fine pinnately -arranged tentacles and containing a prolongation of the collar-cavity. The pro- boscis (Fig. 713, ps.) is a shield -shaped lobe overhang- ing the mouth ; its cavity communicates with the ex- terior by two proboscis-pores (p. p.). The cavity of the collar communicates with the exterior by a pair of ciliated passages opening by the collar- pores. Behind the collar re- gion on each side is a small area in which the body-wall and that of the pharynx are coalescent ; this area is per- forated by an opening — the gill-slit. Cilia occur only on the arms, proboscis and lateral lips. A nerve- strand contain- ing nerve-fibres and ganglion- cells is situated on the dorsal side of the collar, and is pro- longed on to the dorsal surface of the proboscis and the dorsal surface of the arms. On the ventral side of this nerve-strand is a very slender cylindrical cellular cord (nch.) continuous behind with the epithelium of the pharynx : this is supposed to represent the diverticulum of Balanoglossus, and thus to be homologous with the notochord of the Chordata. A blood-vascular system with heart and cardiac sac like those of the Enteropneusta is present. The nervous system lies deeper than the epidermis ; it comprises a dorsal ganglion (collar-cord) situated in the collar. In some species of Cephalodiscus the sexes are united, in most they are separate. The posterior end of the body is FIG. 711.— Cephalodiscus. Gelatinous investment. (After Mclntosh.) XIII PHYLUM CHORDATA 11 drawn out into a sort of stalk on which the buds are developed (Fig. 712). A pair of ovaries (ov.) lie in the trunk-cavity, and there is a pair of oviducts (ovd.) (originally supposed to be eyes) lined by elongated, pigmented epithelium. The development, Fifi. 712.— Cephalodiscus. Entire zooid. (After Mclntosh.) which is direct, without free-swimming larval stage, takes place in passages in the investment. According to one account the segmentation is complete, but unequal and a gastrula is formed by invagination : according to another, the segmentation is in- complete, and a gastrula is formed by delamination. 12 ZOOLOGY SECT. Rhabdopleura (Fig. 714) occurs in colonies of zooids organically connected together, and enclosed in, though not in organic con- tinuity with, a system of branching membranous tubes connected with a creeping stolon. The collar-region bears a pair of hollow arms each carrying a double row of slender tentacles — the whole supported by a system of firm internal (cartilaginous ?) rods. There are collar-pores and proboscis-pores. The " notochord " and the nervous systems resemble those of Cephalodiscus. A single testis has been found, opening on the exterior by a pore situated near the anus. The female reproductive apparatus is unknown. FIG. 713. — CephalodiSCUS. Diagram of longitudinal section, a. anus ; be1, ccelome of pro- boscis ; be'2, ccelome of collar ; ftc3. ccelome of trunk ; int. intestine ; m. mouth ; nrh. supposed notochord ; n. s. nerve-strand ; op. operculum ; ces. oesophagus ; ov. ovary ; ovd. oviduct ; ph. pharnyx ; p. p. proboscis-pore ; ps. proboscis ; st. stomach ; stk. stalk. (After Harmer.) Cephalodiscus, of which there are twelve species, has been found at various widely separated localities in the Southern Hemisphere (Straits of Magellan, Borneo, Celebes, the Antarctic) : species occur off the coast of Japan and Korea. Some live in shallow water : none have been found at a greater depth than 245 fathoms. Rhabdopleura has been found at moderate depths in Norway, Shetland, the North Atlantic, France, the Azores, Tristan d'Acunha, Celebes, and South Australia. It seems doubtful if more than one species occurs. XIII PHYLUM CHORDATA 13 Affinities. — The inclusion of the Hemichorda in the phylum Chordata is an arrangement the propriety of which is not uni- versally admitted, and is carried out here partly to obviate the inconvenience of erecting the class into a separate phylum. On the whole, however, there seems to be sufficient evidence for the view that, if not the existing representatives of ancestral Chor- dates, they are at least a greatly modified branch, taking its origin -an -rcl -si irib FIG. 714. Rhabdopleura. A, Entire zooid. «, mouth ; 6, anus ; c, stalk of zooid ; d, pro- boscis ; e, intestine ;/, anterior region of trunk ; g, one of the tentacles. (After Ray Lankester.) B, Diagram cf the organisation : median longitudinal section, seen from the left. a. arm ; an. anal prominence ; col. collar ; col. ne. collar-nerve ; c. s. cardiac sac ; int. intestine ; m. mouth; ntc. " notochord " ; oe. oesophagus; pr. proboscis ; pr. c. proboscis-coelome ; ret. rectum ; at. stomach ; te. tentacles ; tr. c. trunk-coelome ; v. n. ventral nerve. (After Schepotieff.) from the base of the chordate tree. The presence of the pre- sumed rudimentary representative of a notochord and of the gill- slits seems to point in this direction. It should, however, be stated that by some of those zoologists by whom the members of this group have been most closely studied, their chordate affinities are altogether denied. If the Hemichorda are primitive Chordates, the fact is of special interest that they show remarkable 14 ZOOLOGY SECT. resemblances in some points to a phylum — that of the Echino- dermata — which it has been the custom to place very low down in the invertebrate series. The tornaria larva of Balano- glossus exhibits a striking likeness to an echinopsedium (Vol. I., p. 432), and, though this likeness between the larvae does not estab- lish near connection, it suggests, at least, that an alliance exists. Between actinotrocha, the larva of Phoronis (Vol. I., p. 358) and tornaria there are some striking points of resemblance ; and a pair of gastric diverticula in the former have sometimes been compared with the single notochord or oesophageal diverticulum of the Hemichorda. SUB-PHYLUM AND CLASS II,— UROCHORDA. The Class Urochorda or Tunicata comprises the Ascidians or Sea-Squirts, which are familiar objects on every rocky sea-margin, together with a number of allied forms, the Salpse and others, all marine and for the most part pelagic. The Urochorda are specially interesting because of the remarkable series of changes which they undergo in the course of their life-history. Some present us with as marked an alternation of generations as exists among so many lower forms; and in most there is a retrogressive meta- morphosis almost, if not quite, as striking as that which has been described among the parasitic Copepoda or the Cirripedia. In by far the greater number of cases it would be quite impossible by the study of the adult animal alone to guess at its relationship with the Chordata ; its affinities with that phylum are only de- tected when the life-history is followed out, the notochord and other higher structures becoming lost in the later stages of the metamorphosis. Multiplication by budding, so common in the lower groups of Invertebrata, but exceptional or absent in the higher, is of very general occurrence in the Urochorda. 1. EXAMPLE OF THE CLASS — THE ASCIDIAN OR SEA-SQUIRT. (Ascidia.) Sea-squirts are familiar objects on rocky sea-shores, where they occur, often in large associations, adhering firmly to the surface of the rock. When touched the Ascidian ejects with considerable force two fine jets of sea- water, which are found to proceed from two apertures on its upper end. The shape of the Ascidian, however, can only be profitably studied in the case of specimens that are completely immersed in the sea-water, specimens not so immersed always undergoing contraction. In an uncontracted specimen (Fig. 715), the general shape is that of a short cylinder with a broad base by which it is fixed to the rock. The free end presents a large rounded aperture, and some little distance from it XIII PHYLUM CHORDATA 15 on one side is a second of similar character. The former aperture is termed the oral, the latter the atrial. A strong current of water will be noticed, by watching the movements of floating particles, to be flowing steadily in at the former and out of the latter. When the animal is removed from the water both apertures become narrowed, so as to be almost completely closed, by the con- traction of sphincters of muscular fibres which surround them. At the same time the walls of the body contract, streams of water are forced through the apertures, and the bulk is con- siderably reduced. Body-wall and Atrial Cavity. — The outer layer of the body-wall is composed of a tough translucent substance forming a thick test or tunic (Fig. 716, test.). This proves when analysed to consist largely of a substance called tunicine, which is apparently identical with the cellulose, already referred to (Vol. I., p. 14) as a charac- teristic component of the tissues of plants, and of rare occurrence in the animal kingdom. The test of an Ascidian is frequently referred to as a cuticle, and it is a cuticle in the sense that it lies outside the ectoderm and is derived from that layer in the first instance. The cells, from the right side. however, by the action of which its substance is added to in later stages, seem to be chiefly de- rived, not from the ectoderm, but from the underlying mesoderm, from which they migrate through the ectoderm to the outer surface. These formative cells of the test are to be found scattered through its substance. Running through it are also a number of branching tubes lined with cells, each terminal branch ending in a little bulb-like dilatation. The interior of each tube is divided into two channels by a longitudinal septum which, however, does not completely divide the terminal bulb. Through these tubes (which are of the nature of looped blood-vessels) blood circulates, passing along one channel, through the terminal bulb, and back through the other channel. When the test is divided (Fig. 716) the soft wall of the body or mantle (mant.), as it is termed, comes into view ; and the body is found to be freely suspended within the test, attached firmly to the latter only round the oral and atrial apertures. The mantle (body- wall) consists of the ectoderm with underlying layers of connective- tissue enclosing muscular fibres. It follows the general shape of the test, and at the two apertures is produced into short and wide tubular prolongations, which are known respectively as the oral and atrial siphons (Fig. 718, or. siph., atr. siph.). These are continuous at 16 ZOOLOGY SECT. their margins with the margins of the apertures of the test, and round the openings are the strong sphincter muscles by which closure is effected. In the rest of the mantle the muscular fibres are arranged in an irregular network, crossing one another in all directions, but for the most part either longitudinal or transverse. Within the body-wall is a cavity, the atrial or peribranchial cavity (atr. cav.), communicating with the exterior through the atrial or.ap alrap tent gonod. mant FIG. 716.— Dissection of Ascidia from the right side. The greater part of the test and mantle has been removed from that side so as to bring into view the relation of these layers and of the internal cavities and the course of the alimentary canal, etc. an. anus ; atr. ap. atrial aperture ; end. endostyle ; gon. gonad ; gonod. gonoduct ; hyp. neural gland ; hyp. d. duct of neural gland ; mant. mantle ; ne. gn. nerve-ganglion ; oes. ap. aperture of oasophagus ; or. ap. oral aperture ; ph. pharynx ; stom. stomach ; tent, tentacles ; test, test. (After Herdman ) aperture : this is not a coelome, being formed to a great extent by involution from the outer surface. Pharynx. — The oral aperture leads by a short and wide oral passage (stomodceum) into a chamber of large dimensions, the pharynx or branchial chamber (Fig. 716, ph.). This is a highly characteristic organ of the Urochorda. Its walls, which are thin and delicate, are pierced by a number of slit-like apertures, the xiii PHYLUM CHORDATA 17 stigmata (Fig. 718, stigm.) arranged in transverse rows. Through these the cavity of the pharynx communicates with the atrial or peribranchial cavity /which completely surrounds it except along one side. The edges of the stigmata are beset with numerous strong cilia, the action of which is to drive currents of water from the pharynx into the atrial cavity. It is to the movements of these cilia lining the stigmata that are due the currents of water already mentioned as flowing into the oral and out of the atrial apertures, the ciliary action drawing a current in through the oral aperture, driving it through the stigmata into the atrial cavity, whence it reaches the exterior through the atrial aperture. The stigmata (Fig. 717) are all vertical in position; those of the same row are placed close together, separated only by narrow vertical bars; neighbouring rows are separated by somewhat thicker horizontal bars ; in all of these bars run blood-vessels. Extending across the atrial cavity from the body- wall to the wall of the pharynx are a number of bands of vascular meso- dermal tissue, the con- nectives. It has been already men- tioned that the atrial cavity does not completely sur- round the pharynx on one side. This is owing to the fact that On the side in FIG. TIT.— Ascidia, a single mesh of the branchial question, which is ventral S^bl™- S£&dU *±5 in position, the wall of the pharynx is united with the mantle along the middle line (Fig. 719). Along the line of adhesion the inner surface of the pharynx presents a thickening in the form of a pair of longi- tudinal folds separated by a groove : to this structure, consisting of the two ventral longitudinal folds with the groove between them, the term endostyle end is applied. The cells covering the endostyle are large cells of two kinds — ciliated cells and gland- cells — the former beset at their free ends with cilia, the action ot which is to drive floating particles that come within their influence outwards towards the oral aperture, the latter secreting and dis- charging a viscid and mucous matter. Anteriorly the endostyle is continuous with a ciliated ridge which runs circularly round the anterior end of the pharynx. In front of this circular ridge, and 1 A distinction is sometimes made between the lateral parts of this space (peribranchial cavities, right and left) and the median unpaired (dorsal) part, (atrial cavity, or cloaca), in which the two peribranchial cavities coalesce, and which leads to the exterior through the atrial aperture. VOL. II C 18 ZOOLOGY SECT. running parallel with it, separated from it only by a narrow groove, is another ridge of similar character : these are termed the peripharyngcal ridges; the groove between them is the peri- pharyngeal groove. Dorsally, i.e. opposite the endostyle, the posterior peripharyngeal ridge passes into a median, much more prominent, longitudinal ridge, the dorsal lamina (dors, lam.), which tent leal slont co.rd.visc FIG. 718.— Ascidia, diagram of longitudinal section from the left side, the test and mantle removed, an. anus ; atr. cav. atrial cavity; atr.siph. atrial siphon; br.car. branchio-cardiac vessel; card, vise, cardio-visceral vessel; dors.v. dorsal vessel; gonod. gonoduct ; lit. heart; hyp. neural gland ; tnant. mantle ; ne. (in. nerve-ganglion ; ces. oesophagus ; or. tsiph. oral siphon ; ov. ovary ; rect. rectum ; stiy. stigmata ; stoin. stomach ; tent, tentacles ; text, test ; tr. v. transverse vessel ; vent. r. ventral vessel ; vise. br. viscero-branchial vessel. (From Herdman, after Perrier.) runs along the middle of the dorsal surface of the pharynx to the opening of the oesophagus. In the living animal the lamina is capable of being bent to one side in such a way as to form a deep groove. The mucus secreted by the gland-cells of the endostyle forms viscid threads which entangle food-particles (microscopic organisms of various kinds); the cilia of its ciliated cells drive XIII PHYLUM CHORDATA 19 perl.br these forwards to the peribranchial groove, around which they pass to the dorsal lamina, and the cilia on the cells of the latter drive them backwards to the opening of the oasophagus. Some little distance in front of the anterior peripharyngeal ridge, at the inner or posterior end of the oral siphon, is a circlet of delicate ttntacles (Fig. 716, tent.). Enteric Canal. — The oesophagus (Figs. 716 and 718, ces.) leads from the pharynx (near the posterior end of the dorsal lamina) to the stomach (stom.), which, together with the intestine, lies embedded in the mantle on the left- gn, va.a.tj* hand side. The stomach is a large fusiform sac with tolerably thick walls. The intes- tine is bent round into a double loop and runs forwards to terminate in an anal aperture (an.) situated in the atrial cavity. Along its inner wall runs a thickening — the typhlosole. There is no liver ; but the walls of the stomach are glandular, and a system of deli- cate tubules which ramify over the wall of the intestine and are connected with a duct opening into the stomach, is supposed to be of the nature of a digestive gland. The Ascidian has a well-developed blood-system. The heart (Fig. 718, ht.) is a simple muscular sac, situated near the stomach in the pericardium — a cavity entirely cut off from the surrounding spaces in which the blood is contained. Its mode of pulsation is very remarkable. The contractions are of a peristaltic character, and follow one another from one end of the heart to the other for a certain time ; then follows a short pause, and, when the con- tractions begin again, they have the opposite direction. Thus the direction of the current of blood through the heart is reversed at regular intervals. There are no true vessels, the blood circulating through a system of channels or sinuses devoid of epithelial c 2 PIG. 719. — Ascidia, transverse section, bl. v. blood-vessels ; dors. lam. dorsal lamina ; epi. epidermis ; end. endostyle ; gn. ganglion ; hyp, neural gland ; mus. muscular layer of wall of body ; peribr. peribranchial cavity ; ph. pharynx ; test, test ; vas. tr. vascular ti-abeculse. (After Julin.) 20 ZOOLOGY SECT. , lining, and of spaces or lacunae, forming a hacmocoele : in the description that follows, therefore, the word vessel is not used in its strict sense. At each end of the heart is given off a large " vessel." That given off ventrally, the branchio-cardiae vessel (br. car.) runs along the middle of the ventral side of the pharynx below (externally to) the endostyle, and gives off a number of branches which extend along the bars between the rows of stigmata, and give off smaller branches passing between the stigmata of each row. The vessel given off from the dorsal end of the heart — the cardio- msceral (card, vise.) — breaks up into branches which ramify over the surface of the alimentary canal and other organs. This system of visceral vessels or lacunae opens into a large sinus, the viscera- branchial vessel, which runs along the middle of the dorsal wall of the pharynx externally to the dorsal lamina, and communicates with the dorsal ends of the series of transverse branchial vessels In addition to these principal vessels there are numerous lacunae extending everywhere throughout the body, and a number of branches, given off both from the branchio-cardiac and card io- visceral vessels, ramify, as already stated, in the substance of the test. The direc- tion of the circulation through the main vessels differs according to the direction of the heart's contractions. When the heart contracts in a dorso- ventral direction, the blood flows through the branchio-cardiac trunk to the ventral wall of the pharynx, and through the transverse vessels, after undergoing oxygenation in the finer branches between the stigmata, reaches the viscero-branchial vessel, by which it is carried to the system of visceral lacunae, and from these back to the heart by the cardio-visceral vessel. When the contractions take the op- posite direction, the course of this main current of the blood \is reversed. The nervous system is of an ex- tremely simple character. There is a single nerve -ganglion (Figs. 716 and 718, ne. gn., 720, gn., and 721, n.g.) which lies between the oral and atrial apertures, embedded in the mantle. This is elongated in the dorso- ventral direction, and gives off at each end nerves which pass to the various parts of the body. Lying on the ventral side of the nerve-ganglion is a body — the FIG. 720.— Ascidia. Dorsal tubercle, nerve-ganglion, and associated parts as seen from below, dct. duct of neural gland ; dors. lam. dorsal lamina ; gld. neural gland ; gn. ganglion ; hyp. dorsal tubercle ; nv., nv. nerves ; periph. peri- pharyngeal band. (After Julin.) XIII PHYLUM CHORDATA 21 neural gland (Figs. 716, 718,%?.; Fig. 720, gld., and Fig. 721, n.gl.) — which has sometimes been correlated with the hypophysis of the Craniata. A duct (Fig. 720, dct. and Fig. 721, gl.d) runs forward from it and opens into the cavity of the pharynx ; the Branchial sao d.1 B FK;. 721. — Antcro-dorsal part of Ascidia, sliowing tho relations of the layers of tlic body and of the nervous system, A, in sagittal section ; JJ, in transverse section, d. bl. s. dorsal blood- sinus ; d. 1. dorsal lamina ; d. n. dorsal nerve ; d. t. dorsal tubercle ; ect. ectoderm ; en. eiidoderm ; c.p. br. epithelium of peribranchial cavity ; . on. nerve-ganglion ; or. ap. oral aperture ; ov ovary ; peri. bd. penpharyngeal band ; pU. pharynx ; stiy. stigma ; stom. stomach ; test, testis. (After Herd- man.) a peri pharyngeal band ; but there is no dorsal lamina. Doliolum moves through the water by the contractions of the muscular bands, which have the effect of driving the water backwards out of the branchial sac. Salpa (Figs. 727-728) is nearly allied to Doliolum in its external features and internal structure. It has a fusiform body, usually somewhat compressed laterally, and with the oral and atrial cavities nearly terminal ; but the muscular bands do not form complete hoops. The pharyngeal and atrial cavities take up the greater part of the space in the interior of the body, where they form an almost continuous cavity, being separated from one another only by an obliquely running vascular band, which repre- sents the dorsal lamina of the fixed Ascidians and is frequently termed the branchia. Octacnemus, sometimes regarded as allied to Salpa, appears to be 28 ZOOLOGY SECT. fixed, is colonial in one species, and has the oral and atrial apertures •towards one end of the body, which is somewhat discoid, with its margin produced into eight tapering processes. It has no eye mua.bds e-nd or. a/a FIG. 727.— Salpa democratica, asexual form, ventral view. atr. ap. atrial aperture ; branch. dorsal lamina ; end. endostyle ; ht. heart ; mus. bds. muscular bands ; ne. gn. nerve-ganglion ; or. ap. oral aperture ; proc. processes at the posterior end ; sens. ory. sensory organ (ciliated funnel and languet); tstol. stolon. (After Vogt and Jung.) (cf. p. 30). In all probability Octacnemus is more nearly related to the social Ascidians (p. 25) than to Salpa. * Pyrosoma (Fig. 729) is a colonial Tunicate, the colony assuming the form of a cylinder, the internal cavity of which, closed at one c.c p.p end. FIG. 728.— Salpa. Lateral view of a section— which is sagittal (longitudinal, vertical and median) in the oral two-thirds, and oblique in the atrial third, at. atrial cavity ; br. branchia ; c. c. ciliated crests on the edge of the branchia ; <•. f. ciliated funnel ; d. I. dorsal lip; end. endostyle; ey. eye; gl digestive gland; tjn ganglion; ht. heart; int. intestine; I »;/. languet; mo. mouth; o;s. (esophagus; «;. ap. cesophageal aperture; ph. pharynx; pp. peripharyngeal band ; st. stolon ; st. (behind) stomach ; v. I. ventral lip. (After Dclage and Herouard.) end, open at the other, serves as the common cloaca for all the zooids. The oral apertures (Fig. 730, or. ap.) of the zooids are situated on the outer surface of the cylinder on the extremities of XIII PHYLUM CHORDATA 29 FIG. 729.— Colony of Pyrosoma. A, side view ; B, end- view. (After Herdman.) a series of papillae. The colonies of Pyrosoma, which may be from two or three inches to four feet in length, are pelagic, and are bril- liantly phosphorescent. The enteric canal in Appendicularia (Fig. 723) consists, in addi- tion to the pharynx, of a narrow oesophagus, a bilobed stomach, and a straight intestine (int.) which opens directly by an anal aperture (an.) situated on the ventral side. The alimentary canal of the simple Ascidians has already been de- scribed, and there are few differences of con- sequence in the various families, except that in some cases there is a well-developed digestive gland or "liver"; in the composite forms jfg#\ j>roc 1 the arrangement of the parts as the same in all essential respects as in the simple. In the Salpse and in Doliolum and Octacnemus the aliment- ary canal forms a rela- tively small dark mass — the so-called nucleus — towards the posterior end of the body; it consists of oesophagus, stomach, and intestine, the anal aperture being situated in the peribranchial or atrial part of the internal cavity. The heart in all has the simple structure al- ready described in the simple Ascidian. In Ap- pendicularia its wall IG. 730.— Part of a section through a Pyrosoma colony, atr. up. atrial aperture ; or. ap. oral aper- ture ; proc. processes of test on outer surface of colony ; ph. pharynx ; stol. stolon on which are de- veloped buds giving rise to new zooids ; tent, tentacles. (After Herdman.) 30 ZOOLOGY SECT. consists of only two cells. In one of the genera of Larvacea (Kowalevskia) it is absent. The nervous system in Appendicularia consists of a cerel/ral ganglion (Fig. 723, w. gn') on the dorsal side of the rnouth, of a dorsal nerve which passes from this to a caudal ganglion (ne. gn") at the root of the tail, and of a caudal nerve (ne.') which extends from this to the extremity of the tail, presenting at intervals slight enlargements from which nerves are given off. An otocyst or statocyst (oto.) and a pigment-spot are placed in close relation to the cerebral ganglion, and close to it also is a ciliated funnel ; but there is no neural gland opening into the branchial sac. In the simple Ascidians, as we have seen, there is a single flattened ganglion, representing the cerebral ganglion of Appen- dicularia, situated between the oral and atrial apertures ; and the same holds good of the composite forms. Many of the simple Ascidians have pigment-spots, probably of a sensory character, around the oral and atrial apertures. The dorsal tubercle is always present, but varies in shape in accordance with variations in the form of the ciliated funnel, which always opens on it in conjunction with the duct of the neural gland, of which it forms the terminal part. The opening may be divided into several smaller apertures by fusion of its lips : rarely the duct gives off branches with independent openings. The tubercle with the ciliated funnel is supplied with nerve- fibres from the ganglion, arid is probably a sensory organ of some kind. The neural gland is usually below the ganglion, but may be situated above it or on one side. Sometimes it coalesces with it. In Salpa and Doliolum there is also a single ganglion (Figs. 726, 727, 728, and 731) situated dorsally, giving off nerves to the various parts of the body. Salpa has a single tentacle, the so-called languet (Fig. 728, Ing.}, absent in Doliolum. In Salpa there is a median horse-shoe-shaped eye (Figs. 728, 731), with sometimes smaller accessory eyes. In Doliolum an eye is not developed, but there is a pair of otocysts or statocysts. A neural gland and duct with ciliated funnel are present in Doliolum ; but in Salpa, though there are a pair of glands which occupy a position similar to that occupied by the neural gland (Fig. 731, n. gl.), their correspondence with the latter is not established, and their ducts have no connection with the ciliated funnel. In the simple Ascidian we have seen that the renal organ consists of a number of large clear vesicles situated in the loop of the intestine and devoid of duct. In some forms the terminal portion of the sperm-duct has glandular walls in which concretions of uric acid have been found. The neural gland is by some zoologists looked upon as having an excretory function, but there is no positive evidence in favour of this view, and no definite xiii PHYLUM CHORDATA 31 conclusion has yet been reached as to the function which it performs. Reproductive System. — The Urochorda are hermaphrodite. Ovary and testis are in all cases simple organs placed in close relation with one another. In Appendicularia (Fig. 723) they are situated in the aboral region of the body. In the simple Ascidians, they may be either single or double, and their ducts, sometimes very short, sometimes more elongated, open close together into the at rial cavity. In Pyrosoma there are no gonoducts, the ovary — which contains only a single ovum — and the testis being lodged in a diverticulnm of the peribranchial cavity. In Salpa also the ovary contains usually only a single ovum : ovary and testis lie in close relation to the alimentary canal in the " nucleus," and their short ducts open into the peribranchial cavity. In Doliolum the elongated testis and oval ovary have a similar position to that cf ph JT ^S € 3 -c.c I [',. 78].— Salpa. Diagrammatic lateral view of the ganglion and neighbouring parts, at. wall of atrial cavity ; br. branchia ; br. ap. aperture of branchia ; c. c. ciliated crests of branchia ; c. j. ciliated funnel ; ey. eye ; n. t/L gland (paired) that may represent neural gland ; ph. wall of pharynx. (After Delage and Herouard.) which they occupy in Salpa, but the ovary consists of a number of ova. Development and Metamorphosis. — In the Ascidiacea Im- pregnation usually takes place after the ova have passed out from the atrial cavity. But in a few simple, and most if not all com- pound forms, impregnation takes place in the atrium or in a special outgrowth of the latter serving as a brood-sac, and the ovum remains there until the tailed larval stage is attained. In certain composite forms there is a coalescence of the investing layers of the ovum with the wall of the atrium, forming a structure analogous to the placenta of the Mammals and designated by that term. Self-impregnation is usually rendered impossible by ova and sperms 32 ZOOLOGY SECT. becoming mature at different times; but sometimes both become ripe simultaneously, and self-impregnation is then possible. A somewhat complicated series of membranes (Fig. 732) invests the ovum. The immature ovarian ovum is enclosed in a layer of flat cells — the primitive follicle- cells — derived from indifferent cells of the ovary. On the sur- face of this is developed a structureless basal membrane. The follicle cells increase by division and soon form a sphere of cubical cells. Certain of the cells migrate into the interior of the sphere so as to form a layer on the surface of the ovum. Others penetrate into the latter so as to lie in the superficial strata of the yolk. The layer of cells on the surface of the ovum are termed the test-cells (e) : they afterwards develop on the outer surface a thin and internal to them is formed which the test-cells in a de- FIG. 732.— Ascidian (Ciona). Mature egg from the oviduct, c. follicle-cells ; d, chorion ; e, test-cells ; /, ovum ; x, gela- tinous layer. (From Korschelt and Heider, after Kupffer.) structureless layer, the chorion, a gelatinous layer (x) through generated condition become scattered. Meantime, external to the follicle-cells, between them and the basal membrane, has appeared a layer of flattened epithelial cells ; this, with the basal membrane, is lost before the egg is discharged. In all the simple Ascidians, with the exception of the few in which development takes place internally, the protoplasm of the follicle-cells (Fig. 732, c) is greatly vacuolated so as to appear frothy, and the cells become greatly enlarged, projecting like papillse on the surface and buoying up the developing ovum. Segmentation is complete and approximately equal, but in the eight-cell stage four of the cells are smaller and four larger : the smaller, situated on the future dorsal side, are the beginning of the endoderm ; the four larger form the greater part, if not the whole, of the ectoderm. In the following stages the ectoderm cells multiply more rapidly than the endoderm, so that they soon become the smaller. In the sixteen-celled stage the embryo (Fig. 733, A) has the form of a flattened blastula (placula) with ectoderm on one side and endoderm on the other, and with a small segmenta- tation- cavity. The transition to the gastrula stage is in most Ascidians effected by a process intermediate in character between embolic and epibolic invagination ; in some the imagina- tion is of a distinctly epibolic character. In the former case the ectoderm cells continue to increase more rapidly than the PHYLUM CHORDATA 33 endoderm, the whole embryo becomes curved, with the con- cavity on the endodermal side, and the ectoderm extends over the endoderm, the two layers coming to lie in close contact and the segmentation-cavity thus becoming obliterated. The concavity deepens until the embryo assumes the form of a saucer-shaped gastrula with an archenteron and a blastopore which is at first a very wide aperture extending along the whole of the future B e.cf •\. 733.— Early stages in the development of Clavellina. A, flattened blastula; B, early gastrula ; C, approximately median optical section of more advanced gastrula in which the blastopore has become greatly reduced and in which the first rudiment of the notochord is discernible ; D, similar view of a later larva in which the medullary canal has begun to be closed in posteriorly. U. p. blastopore : ect. ectoderm ; end. endoderm ; med. can. medullary canal ; nerv. cells destined to give rise to the nerve-cord ; neur. neuropore ; noto. notochord ; seg. cav. segmentation cavity. (A and B from Korschelt and Heider, after Seeliger ; C and D after Van Beneden and Julin.) lorsal side. The blastopore gradually becomes constricted (Fig. 733, B) — the closure taking place from before backwards, and the opening eventually being reduced to a small pore at the posterior end of the dorsal surface. The embryo elongates in the direction of the future long axis. The dorsal surface becomes recognisable by being flatter, while the ventral remains convex. The ectoderm cells bordering the blasto- pore are distinguished from the rest by 'their more cubical VOL. II D 34 ZOOLOGY SECT. shape ; these cells, which form the earliest rudiment of the nervous system, become arranged in the form of a plate — the medullary plate — on the dorsal surface. On the surface of this plate appears a groove — the medullary groove — bounded by right arid left medul- lary folds, which pass into one another behind the blastopore. The medullary folds grow upwards and inwards over the medul- lary groove, and unite together (D), the union beginning behind and progressing forwards in such a way as to form a canal, the neuro- ccele, in the hinder portion of which is the opening of the blastopore. In this process of closing-in of the medullary groove the fold which passes round behind the blastopore takes an important part, grow- ing forwards over the posterior portion of the canal. The blasto- pore, thus enclosed in the medullary canal, persists for a time as a small opening — the neurenteric canal — by which the neuroccele and enteric cavity are placed in communication. At the anterior end of the medullary canal, owing to its incomplete closure in this region, there remains for a time an opening — the neuropore (Fig. 734, neur.) — leading to the exterior. A notochord (Fig. 733, C, D, 734 and 735, noto.) is formed from certain of the cells of the wall of the archenteron along the middle line of the dorsal side. These are arranged to form an elongated cord of cells which becomes completely constricted off from the endoderm of the wall of the archenteron, and comes to lie between the latter and the medullary groove. Laterally certain cells of the endoderm, in close relation to those forming the rudi- ment of the notochord, divide to give rise to a pair of longitudinal strands of cells — the rudiments of the mesoderm (Fig. 734, mes.\ .During this process of mesoderm-formation, there are no diverticula developed from the archenteron. The embryo (Fig. 734, B) now becomes pear-shaped, the narrow part being the rudiment of the future tail. As this narrow portion elongates, the part of the enteric cavity which it contains soon disappears, coming to be represented only by a strand of endoderm cells, which gives rise in the middle to the extension backwards of the notochord, laterally to the mesoderm of the tail, and ventrally to a cord of endoderm cells continuous with the wall of the enteric cavity in front. The caudal region increases in length rapidly, and the anterior or trunk region, at first round, becomes oval. At its anterior end there appear three processes of the ectoderm, the rudiments of the adhesive papillae (Fig. 735, adh.), organs by which the larva subse- quently becomes fixed. The ectoderm cells at an early stage secrete the rudiments of the cellulose test ; in the caudal region this forms longitudinal dorsal and ventral flaps having the function of unpaired fins. The medullary canal becomes enlarged at its anterior end. A vesicular outgrowth from this enlarged anterior portion forms the XIII PHYLUM CHORDATA 35 sense-vesicle (sens. res.). The posterior Barrow part forms the caudal portion of the central nervous system (zpincd cord). Masses of pigment in relation to the sense-vesicle early form the rudiment Tries no to FIG. 734.— Later stages in the development of Clavellina. A, approximately median optical section of a larva in which the medullary canal (neuroc<«le) has become enclosed throughout, communicating with the exterior only by the neuropore at the anterior end and with the archenteron by the neurenteric canal ; B, larva with a distinct rudiment of the tail and well- formed mesoderm-layer and notochord. Letters as in preceding figure ; in addition, meg. mesoderm. (After Van Beneden and Julin.) of the two larval sense-organs, otocyst (or statocyst) and eye. The part behind this presents a thickened wall with a narrow lumen. This is known as the ganglion of the trunk. The rudiment of the D 2 36 ZOOLOGY SECT. neural gland early appears on the ventral wall of a ciliated diverticulum (cil. gr.) of the anterior end of the archenteron (future pharynx), which subsequently unites with an outgrowth from the medullary canal. The embryonic alimentary canal consists of two regions, a wide region situated altogether in front of the notochord, and a nar- rower portion situated behind in .the region of the notochord. The wider anterior part gives rise to the pharynx, the posterior part to the oesophagus, stomach, and intestine. The mouth-opening is formed shortly before the escape of the embryo from the egg ; an ectodermal invagination is formed at the anterior end, and an endodermal diverticulum from the archenteron grows out to meet it ; the two coalesce, and the oral passage is thus formed. The rudiments of the heart and pericardial cavity first appear as a hollow outgrowth from the archenteron : this subsequently becomes constricted off and involuted to form a double- walled sac, the inner layer of which forms the wall of the heart, while the outer gives rise to the wall of the pericardium. The first beginnings of the atrial cavity appear as a pair of invaginations of the ectoderm which grow inwards and form a pair of pouches, each opening on the exterior by an aperture. There is a difference of opinion as to some points in the history of these atrial pouches, and it remains uncertain to what extent the ectoderm and endoderm respectively share in the formation of the atrial cavity. Eventually spaces, into the forma- tion of which the two ectodermal diverticula at least largely enter, grow round the pharynx and give rise to the atrial cavity ; and perforations, the stigmata, primarily two in number, place the cavity of the pharynx in communication with the surrounding space. The two openings of the atrial pouches subsequently coalesce to form one — the permanent atrial aperture. It will be useful now, at the cost of a little repetition, to sum- marise the various characteristics of the larval Ascidian at the stage when it escapes from the egg and becomes free-swimming (Fig. 735). In general shape it bears some resemblance to a minute tadpole, consisting of an oval trunk and a long laterally- compressed tail. The tail is fringed with a caudal fin, which is merely a delicate outgrowth of the thin test covering the whole of the surface ; running through the delicate fringe are a series of striae, presenting somewhat the appearance of the fin-rays of a Fish's fin. In the axis of the tail is the notochord (noto.}, which at this stage consists of a cylindrical cord of gelatinous substance enclosed in a layer of cells. Parallel with this runs, on the dorsal side, the narrow caudal portion of the nerve-cord, and at the sides are bands of muscular-fibres. In the trunk the nerve-cord is dilated to form the ganglion of the trunk, and, further forwards, expands into the sense-vesicle (sens, ves.) with the otocyst (or stato- xin PHYLUM CHORDATA 37 cyst (oto.) and eye (eye). A prolongation of it unites, as already stated, with the ciliated diverticulum from the anterior part of the pharynx. From the walls of this at a later stage are developed, on the dorsal side, the ganglion, and, on the ventral, the neural gland ; the pharyngeal opening (cil. gr.) becomes the ciliated funnel. The enteric canal is distinguishable into pharynx, oesophagus, stomach, and intestine. The pharynx opens on the exterior by the mouth : in its ventral floor the endostyle (end) has become developed ; its walls are pierced by stigmata, the number of which varies. The atrial cavity has grown round the pharynx, and opens on the exterior by a single aperture only (atr). The heart and pericardial cavity have been formed. In this tailed free-swimming stage the larva remains only a few hours ; it soon becomes fixed by the adhesive papillae and begins ciclh FIG. 735.— Free-swimming larva of Ascidia mammillata, lateral view. adh. adhesive papillae ; all. alimentary canal ; atr. atrial aperture ; cil. gr. ciliated diverticulum, becoming ciliated funnel ; etui, endostyle ; eye, eye ; med. nerve-cord (ganglion of trunk) ; noto. notochord ; oto. otocyst ; sens. ties, sense-vesicle ; stig. earliest stigmata. (From Korschelt and Heider, after Kowalevsky.) to undergo the retrogressive metamorphosis by which it attains the adult condition. The chief changes involved in the retrogressive metamorphosis (Fig. 736) are the increase in the number of pharyngeal stigmata, the diminution, and eventually the complete disappearance, of the tail with the contained notochord and caudal part of the nerve- cord, the disappearance of the eye and the otocyst, the dwindling of the central part of the nervous system to a single ganglion, and the formation of the reproductive organs. Thus, from an active free-swimming Iarv7a, with complex organs of special sense, and provided with a notochord and well-developed nervous system, there is a retrogression to the fixed inert adult, in which all the parts indicative of affinities with the Vertebrata have become aborted. The significance of these facts will be considered when we come to discuss the general relationships of the Chordata. In some simple Ascidians, and in the composite forms in which development takes place within the body of the parent, the meta- morphosis may be considerably abbreviated, but there is always, 38 ZOOLOGY SECT. so far as known, a tailed larva, except in the genus Amviella of the Molgulidee — a family of the simple forms, in which the tailed reel FIG. 736. — Diagram of the metamorphosis of the free tailed larva into the fixed Aacidian. A, stage of free-swimming larva ; B, larva recently fixed ; C, older fixed stage, adk. adhesive papillfe ; atr. atrial cavity; cil. -l. cxelome ; d. ao. dorsal aorta ; int. intestine ; mj/om ; myomeve ; nek. notochord ; ii<'H. neiuvm ; s. int. v. sub-intestinal vein. (A, from Hertwig, after Lankester and Boveri ; B, partly after Rolph.) formed of a single. layer of columnar epithelial cells, some of which are provided with sensory hairs. The epithelium of the buccal cirri presents at intervals regular groups of sensory cells, some of them bearing stiff sensory hairs, others cilia. Beneath the epi- dermis is the dermis, formed mainly of soft connective-tissue. The muscular layer (my, myom.) is remarkable for exhibiting metameric segmentation. It consists of a large number — about sixty — of muscle-segments or myomeres, separated from one another by partitions of connective tissue, the mi/womnias, and ha\ appearance, in a surface view, of a series of very open V s their apices directed forwards (Figs. 74] aud 743). Each m xm PHYLUM CHORDATA 47 is composed of numerous flat, striated * muscle-plates, arranged longi- tudinally, so that each is attached to two successive myocommas. In virtue of this arrangement the body can be bent from side to side with great rapidity. The -myomeres of the right and left sides of the body are not opposite to one another, but have an alternate arrangement. A special set of transverse muscles (Fig. 742, A) extends across the ventral surface of the anterior two- thirds of the body, lying in the floor of the atrial cavity presently to be described. One striking and characteristic feature of the muscular layer of the body-wall is the immense thickness of its dorsal portion. In the higher Worms and many other Invertebrates the muscles form a layer of approximately equal thickness surrounding the body- cavity, which contains, amongst other organs, the central nervous system. In Vertebrates, on the other hand, the dorsal body-wall is greatly thickened, and in it are contained both the nervous system and the notochord. Skeleton. — The chief of the skeletal or supporting structures of the Lancelet is the notoc.hord (Figs. 742 and 743, c, nch,), a cylin- drical rod, pointed at both ends, and extending from the anterior to the posterior end of the body in the median plane. It lies immediately above • the enteric tract and between the right and le/t myomeres. It is composed of a peculiar form of cellular tissue known as notochordal tissue, formed of large vacuolated cells extending from side toj*te of the notochord, and having the nuclei confined to its d<^p and ventral regions. Around these cells is a notochordal sheath of connective-tissue, which is produced dorsally into a canal for the nervous system. The noto- chord, like the parenchyma of plants, owes its resistant character to the vacuoles of its component cells being tensely filled with fluid, a condition of turgescence being thus produced. The oral hood is supported by a ring (Fig. 743, sk.) of carti- laginous consistency, made up of separate rod like pieces arranged end to end, and corresponding in number with the cirri. Each piece sends an offshoot into the cirrus to which it is related, furnishing it with a skeletal axis. The pharynx is "supported by delicate oblique rods of a firm, gelatinous material, the gill-rods (br. r.). These will be most conveniently discussed in connection with the pharynx itself. The dorsal fin is supported by a single series, and the ventral fin by a double series of fin-rays (dors. f. r., vent. f. r.), short rods of con- nective-tissue, each contained in a cavity or lymph-space. Digestive and Respiratory Organs. — The mouth (mth.), as already mentioned, lies at the bottom of the vestibule or cavity of the oral hood (or. hd.). It is a small circular aperture surrounded by a membrane, the velnm (?;/.), which acts as a sphincter, arid has its free er1^ produced i'lto a number of velar tentacles (vl. t.). 48 ZOOLOGY SHCT. The mouth leads into the largest section of the enteric canal, the pharynx (ph.), a high, compressed chamber extending through the anterior half of the body. Its walls are perforated"1 by more than a hundred pairs of narrow oblique clefts, the gill- slits or branchial apertures (br. cl.), which place the cavity of the pharynx in communication with the atrium (see below). From the pos- terior end of the pharynx goes off the tubular intestine (int.) which extends backwards almost in a straight line to the anus. On the ventral wall of the pharynx is a longitudinal groove, the endostyle (Fig. 742. A, «.), lined by ciliated epithelium containing groups of gland-cells. Like the homologous organ in Ascidia (p. 17), the glands secrete a cord of mucus in which food parti- cles are entangled and carried by the action of the cilia to the intestine. A somewhat similar structure, the epipharyngcal groove, extends along the dorsal aspect of the pharynx : its sides are formed by ciliated cells, which, at the anterior end of the groove, curve downwards, as the peripharyngeal bands, and join the anterior end of the endostyle. From the ventral region of the anterior end of the intestine is given off a blind pouch, the liver (Ir.) or hepatic caecum, which extends forwards to the right of the pharynx : it is lined with glandular epithelium and secretes a digestive fluid. The gill-slits (br. cl.) are long narrow clefts, nearly vertical in the expanded condition, but very oblique in preserved and con- tracted specimens — hence the fact that a large number of clefts always appear in a single transverse section (Fig. 742, A, Jed.). The clefts are more numerous than the myomeres in the adult, but correspond in number with them in the larva : hence they are fundamentally metameric, but undergo an increase in number as growth proceeds. The branchial lamellae (Fig. 743, br. sep., Fig. 742, A, kb.), or por- tions of the pharyngeal wall separating the clefts from one another, are covered by an epithelium which is for the most part endo- dermal in origin, and is composed of greatly elongated and ciliated cells. On the outer face of each lamella, however, the cells are shorter and not ciliated, and are, as a matter of fact, portions of the epithelial lining of the atrium, and of ectodermal origin. Each lamella is supported towards its outer edge by one of the branchial rods (Fig. 743, br. r.) already referred to. Those are narrow bars united with one another dorsally by loops, but ending below in free extremities which are alternately simple and forked. The forked bars are the primary (br. r. 1), those with simple ends the secondary (br. r. %) branchial rods, and the lamellae in which they are contained are similarly to be distinguished as primary lamellae (br. sep. 1) and secondary or tongue-lamella (br. sep. 2). In the young condition the two clefts between any two primary lamellae are represented by a single aperture : as development proceeds a down- PBYLUM growth takes phi the i ^c aperture, fon in ..nqglossus (p. 3». tongue which extends downwards, dividing the original cleft into two, itself becoming a ndary lamella. A further complication is product M! by tl i nation of tra or supported by rods connecting the primary septa with one .r.her at tolerably • vals. The Atrium.— The gill-clefts lend into a wide chamber occupy - 'ing most of the sp •'ween the body- wall the pharynx, and !od the ai ;gs. 74-2, B, and 7- It is crescentic in tion, surrounding the :iral and lateral ns of tl but not ils dorsal por- tion. It ends blindly in front; o] - m- Ly, behind the level of the pharynx, by the and is continued backwards by a blind, pouch-like extension Wp) lying to e right of tl tine (Fig. 742, I>, atr.\ The whole (tavity is od by an a trial epi- •thelium of .--<.>''-Tmal As in Ascidia, V cilia lining tl: • gill- .. II 50 /< )( )LOGY SECT. setting in at the mouth, entering the pharynx, passing thence by the gill-slits into the atrium, and out at the atriopore. The current, as in Tunicata and Balanoglossus, is both a respiratory 1 FIG. 744.VAmphioxus lanceolatus. Diagrammatic transverse section of the pharyn- geal Region, passing on the right through a primary, on the left through a secondary branchial lamella, ao. dorsal aorta ; c, derm; ec, enrlost3rlar portion of coelome; /'. fascia or investing layer of myomere ; //&, compartment containing fin-raj7 ; ;/. gonad ; -2.— Amphioxus lanceolatus. Embryo. A, from the side; B, in horizontal section, ak, ectoderm ; en, neurenteric canal ; dh, archenteron ; ik, endoderm ; mk, meso- dermal folds ; n, neural tube ; ud, archenteron ; us, first coelomic pouch ; ush, coelomic cavity ; V, anterior; H, posterior end. (From Korschelt and Heider, after Hatschek.) end of the archenteron : its final snout is a subsequent process; extension to the end of the The significance of these early stages in the development of Amphioxus has been variously regarded by different embryologists, and it is impossible to give here more than a statement of the ascertained facts, leaving the question of their interpretation, which can only be profitably discussed on the broadest basis, as a matter for more advanced study. Since, however, the gastrulation and mesoderm-formation in Amphioxus are, almost universally, made the starting- point in the process of interpreting the phenomenfc of early development in all Vertebrates, it may be desirable at the present stage to state that though in the foregoing account the cells which are invaginated are referred to throughout simply as endoderm, this view of their nature is not the only one that may reasonably be held. If the inner layer of the gastrula be composed of endoderm and of endoderm only, certain consequences necessarily follow : the notochord 60 ZOOLOGY SECT. must be purely endoclermal in origin, and so must the whole of the mesoderm. This is the terminology which has been followed in the preceding pages. But it may be held that the process of invagination is not so simple, and that the inner layer of the resulting gastrula is made up of two distinct parts, a dorsal part, which is ectodermal, and a ventral part, which consists of endoderm. On this view the notochord and the mesoderm, derived from the dorsal part of the invaginated layer, would both be of ectodermal origin, and only the enteric epithelium would be endodermal. These points will be referred to again at a later stage. A further point about which there may be room for differences of opinion is the detailed development of the coelome. According to one view of the facts the coelome of Amphioxus may at one stage be compared to that of Balanoglossus (p. 8) ; — with an unpaired anterior part, destined to form the head-cCBlome and representing the proboscis-cavity of Balanoglossus ; a middle pair of pouches which form the first pair of somites, and a pair of canal-like backward extensions, which may be compared to the collar- cavities ; and a posterior pair correspond- ing with the trunk -coelome of the Enteropneust — the last becoming divided up to form the ccelomic sacs. New coelomic pouches are formed in regular order from before backwards, the embryo at the same time elongating and becoming laterally compressed and pointed fore and aft. At the anterior end the mouth (Fig. 753, m) appears on the left side of the body as a small aperture, which soon increases greatly in size. On the ventral surface another small aperture, the first gill-slit (ks), makes its appearance, and soon shifts over to the right side f it forms a direct communication between the pharynx and the exterior, like the stigmata of Appendicularia (p. 24) : there is at present no trace of the atrium. The anterior end of the archenteron has meanwhile grown out into a pair of pouches, which become shut off as closed sacs : of these the right gives rise to the ccelome of the head (A), the left to a depression called the pre-oral pit (w), which opens on the exterior, and from which the groove of Hatschek and the wheel-organ are afterwards formed. Hatschek' s nephridium (Fig. 7 54, a?) is a narrow ciliated tube which opens into the anterior part of the pharynx, and runs forwards to terminate blindly in the roof of the oral hood. It appears to be developed from the narrow neck that connects the left coelomic pouch of the first pair with the archenteron. and disappears completely in the adult except in Amphioxides, in which it is said to contain solenocytes. On the floor of the archenteron in the neighbourhood of the mouth a depression appears giving rise to a structure known as the club-shaped gland (&) which may be a modified gill-cleft. Posteriorly the neurenteric canal closes and the anus appears. We left the mesoderm in the form of separate paired ccelomic sacs, arranged metamerically in the dorsal region of the embryo. The sacs increase in size, and extend both upwards and downwards, each presenting a somatic layer (Fig. 751, D, mkl) in contact with the external ectoderm, and a splanchnic layer (wi&2) in contact xin PHYLUM CHORDATA 61 with the nervous system and notochord dorsally, and with the enteric canal ventrally. At about the level of the ventral surface of the notochord, a horizontal, partition is formed in each coelomic sac (Fig. 751, D), separating it into a dorsal and ventral portion. The dorsal section is distinguished as the protovertebra (ns), and its cavity as the myocode or muscle-cavity: the ventral section is called the lateral plate, and its cavity forms a segment of the ccelome. The ventral plates now unite with one another in pairs below the enteric canal, their cavities becoming continuous : at the same time the cavities of successive ventral plates are placed in communication with one another by the absorption of their mr ch ch I sv SV FIG. 753.— Amphioxus lanceolatus. A, young larva ; B, anterior end more highly magnified, r., provisional tail-fin ; ch. notochord ; en, neurenteric canal ; d, enteric canal ; h. coelome of head ; £, club-shaped gland ; k', its external aperture ; kg, first gill-slit ; m. mouth ; mr, neuron; np, neuropore ; sv, sub-intestinal vein; w, pre-oral pit. (From Korschelt and Heider, after Hatschek.) adjacent (anterior and posterior) walls. In this way the cavities of the entire series of ventral plates, right and left, unite to form the single unsegmented coelome of the adult, their walls giving rise to the ccelomic epithelium. At the same time the cells of the splanchnic layer of the protovertebrse become converted into muscular fibres, which nearly fill the myoccele, and give rise to the myomeres : the myocommas arise from the adjacent anterior and posterior walls of the proto vertebrae. An outpushing of the splanchnic layer, at about the level of the ventral surface of the notochord, grows upwards between the myomere externally and the notochord and nerve-tube internally : from the cells lining this pouch the ZOOLOGY SECT. it ill 3j| O tt><3 connective-tissue sheath of the noto- chord and nervous system arises, and perhaps also the fin-rays. From the parietal layer of the protovertebrse is formed the derm or connective-tissue layer of the skin. The larva increases in size, and be- comes very long and narrow, with a pointed anterior end and a provisional caudal fin posteriorly (Fig. 754, c). As growth proceeds, new segments are added behind those already formed, the notochord grows forwards to the anterior end of the snout, and the eye- spot (au.) and olfactory pit appear, the latter as an ectodermal pit which com- municates with the neurocosle by the still open neuropore (np.). The mouth (m.) attains a relatively immense size, stnH-emaining on the left side. /^Additional gill-slits arise behind the one already mentioned : they all make their appearance near the middle ventral line, and gradually shift over to the right side : at first they corre- spond with the myomeres, so that the segmentation of the pharynx is part of the general metamerism of the body. Altogether fourteen clefts are produced in a single longitudinal series. Above, i.e. dorsal to them, a second longitudinal series makes its appearance, containing eight clefts, so that at this stage there are two parallel rows of gill-slits on the right side of the body, and none on the left. But as growth goes on, the first or ventral series gradually travels over to the left side, producing a symmetrical arrangement, and at the same time the first slit and the last five of the first or definitively left series close up and disappear, so that the numbers are equalised on the two sidess At first each gill-slit is simple, but before long a fold grows down from its dorsal edge, and, extending ventrallyy divides the single aperture into two : this fold is the secondary or tongue- xin PHYLUM CHORDATA 63 lamella, the original bars of tissue between the undivided slits becoming the primary lamellae. While the development of the gill-slits is proceeding, the atrium is in course of formation. Paired longitudinal ridges, the meta- pleiwal folds (Fig. 755, If. rf., Fig. 756, sf.), appear on the ventral side of the body, behind the gill -slits, and gradually extend for- wards, dorsal to the latter, their arrangement being very unsym- metrical in correspondence with that of the clefts themselves. On the inner face of each fold, i.e. the face which looks towards its fellow of the opposite side, a longitudinal sub-atrial ridyc (Fig. 756, A, si) appears, and the two sub-atrial ridges meeting and coalescing, a canal (B, p) is formed immediately below the ventral body-wall. This canal is the commencement of the atrium : it is at first quite narrow, bat gradually extends upwards on each side (C, p) until it attains its full dimensions. It is open, at first, both ap FIG. 755.— Amphipxus lanceolatus. Ventral aspect of three larvae showing the develop- ment of the atrium, ap. atriopore ; k, gill-slits ; //; left metapleufal fold ; m. mouth ; rf. right metapleural fold ; w. pre-oral pit. (From Korschelt and Heider, after Lankester and Willey.) in front and behind : the posterior opening remains as the atrio- pore : the anterior opening becomes gradually shifted forwards as the fusion of the sub-atrial ridges proceeds (Fig. 755, B and C), and is finally completely closed. In this way the gill-slits come to open, not directly on the exterior, but into a cavity formed by the union of paired ridges of the body-wall, and therefore lined by ectoderm. The mouth gradually passes to the ventral surface, and under- goes a relative diminution in size : a fold of integument develops round it and forms the oral hood, which is probably to be looked upon as a stomodseum. The endostyle appears on the right of the pharynx (Fig. 754, fl\ and is at first rod-shaped, then V-shaped : ultimately the limbs of the V unite in the middle ventral line. The gill-slits increase in number and become more and more vertically elongated. The provisional caudal fin disappears. The gonads arise from the outer and ventral regions of the 64 ZOOLOGY SKCT. protovertebire in the form of pouches, which gradually assume their permanent form. The development of the nephridia has not been fully worked out ; but what is known regarding it supports the conclusion .that these organs, while representing the nephridia of the Annulata, do not correspond to the excretorj7 organs of the Craniata (see Section II.). Distribution. — The Branchiostomida3 are very widely dis- tributed in tropical and warm-temperate seas. Ampliioxidcs has only been obtained with the tow-net and is, seemingly, of permanently pelagic habit. It differs from Amphioxus in the absence of an atrial cavity, the branchial slits openng FIG. 756.— Amphioxus lanceolatus. Diagrammatic transverse sections of three larva; to show the development of the atrium, ao. aorta ; c, dermis ; d, intestine ; /. fascia (layer of connective-tissue on inner surface of myomere) ; /&, cavity for dorsal fin -ray ; m. myomere; n, neuron;^, atrium; */, metapleural folds; *£, sub-intestinal vein; sk, sheath of notochord and neuron ; si. sub-atrial ridge ; sp, coelome. (From Korschelt and Heider, after Lankester and Willey.) directly on the exterior. No sexually-mature specimens have yefc been found. Distinctive Characters. — The Acrania may be defined as Euchorda in which the notochord extends to the anterior end of the snout, in advance of the central nervous system. There is no skull, and no trace of limbs. The ectoderm consists of a single " layer of cells which may be ciliated. The pharynx is of immense size, perforated by very numerous gill-slits, and surrounded by an atrium. The liver is a hollow pouch of the intestine. There is no heart, and the blood is colourless. The nephridia remain dis- tinct and open into the atrium. The brain is very imperfectly differentiated ; there are only two pairs of cerebral nerves ; and the xiii PHYLUM CHORDATA 65 dorsal and ventral spinal nerves do not unite. There are no paired eyes, but there is a median pigment-spot in the wall of the brain, and many others in the spinal cord ; an auditory organ is absent. 'The gonads are metamerically arranged and have no ducts. There is a typical invaginate gastrula, and the mesoderm arises in the form of metameric coelomic pouches. The ccelome is an enterocoele. Affinities. — Amphioxus has had a somewhat chequered zoologi- cal history. Its first discoverer placed it among-the Gastropoda, considering it to be a Slug. When its vertebrate character was made out, it was for a long time placed definitely among Fishes, as the type of a distinct order of that class ; but it became obvious, from a full consideration of the case, that an animal with neither skull, brain, heart, auditory organs, nor paired eyes, with colourless blood, with no kidneys in the ordinary sense of the word, and with its pharynx surrounded by an atrium, was more widely separated from the lowest Fish than the lowest Fish from a Bird or Mammal. There was still, however, no suspicion of any connection between Amphioxus and the Urochorda until the development of both was- worked out, and it was shown that in many fundamental points, notably in the formation of the nervous system and the notochord, there was the closest resemblance between the two. The likeness was further emphasised by the presence in both forms of an endostyle, an epipharyngeal groove and peripharyngeal bands, and of an atrium, and by the obvious homology of the stigmata or gill-slits of Tunicates with those of Amphioxus. The Urochorda being obviously a degenerate group, it was suggested that the peculiarities of the adult Amphioxus might also be due to a retrogressive metamorphosis. Of this, however, there is not sufficient evidence, and all recent investiga- tions have tended to bring the Acrania nearer to the Craniate Vertebrata, and to remove them further from the lower Chordata. SECTION II.— CRANIATA (VERTEBRATA). The group of the Craniata (Vertebrata) includes all those animals known as Fishes, Amphibians, Reptiles, Birds, and Mammals, or, in other words, Vertebrata having a skull, a highly complex brain, a heart of three or four chambers, and red blood-corpuscles. In spite of the obvious and striking diversity of organisation obtaining among Craniata — between, for instance, a Lamprey, a Pigeon, and a Dog — there is a fundamental unity of plan running through the whole group, both as to the general arrangement of the various systems of organs and the structure of the organs them- selves — far greater than in any of the principal invertebrate groups. 66 ZOOLOGY SECT. The range of variation in the whole of the six classes included in the division is, in fact, considerably less than in many single classes of Invertebrata — for instance, Hydrozoa or Crustacea. Hence, while the plan hitherto adopted of treating the group class by class will be followed, it will be found convenient to begin by devoting a considerable space to a preliminary account of the Craniata as a whole, since in this way much needless repetition will be avoided. The Craniata include the following classes and sub-classes :— CLASS I. — CrcLosTOMi, Including the Lampreys and Hags. CLASS II. — PISCES, True Fishes, which are again divisible into Sub-class 1 . — Elasm obranchii, Including the Sharks and Rays. Sub-class 2. — Holocepkali, Including the Cat-fish (Chimcera) and the Elephant-fish (Callorhynchus). Sub-class 3. — Teleostomi, Including the bony Fishes, such as Perch, Cod, Trout, &c., and the Sturgeons and their allies. Sub-class 4. — Dipnoi^ Mud-Fishes. CLASS III. — AMPHIBIA, Including Frogs, Toads, Newts, and Salamanders. CLASS IV. — REPTILIA, Including Lizards, Snakes, Crocodiles, Turtles, and Tortoises. • CLASS V. — AVES, Birds. 1 The animals included in Classes I. and II. are all " Fishes " in the broad sense of the word. xin PHYLUM CHORDATA H7 CLASS VI. — MAMMALIA, Including Hairy Quadrupeds, Seals, Whales, Bats, Monkeys, and Man. External Characters. — The body of Craniata (Fig. 757) is bilaterally symmetrical, elongated in an antero-posterior direction, and usually more or less cylindrical. It is divisible into three regions : the head, which contains the brain, the chief sensory organs, and the mouth and pharynx ; the trunk, to which the coelorne is confined, and which contains the principal digestive and circulatory as well as the excretory and reproductive organs ; and the tail, or region situated posteriorly to the coelome and anus, and containing none of the more essential organs. Between the head and trunk there is frequently a narrow region or neck, into which the ccelome does not extend. In aquatic Vertebrates the tail is of great size, not marked off externally from the trunk, and is the chief organ of locomotion : in terrestial forms it usually becomes greatly reduced in diameter, and has the appearance of a mere unpaired posterior appendage. The mouth (mth.} is a transverse aperture placed at or near the anterior end of the head. Near it, sometimes dorsal, sometimes ventral in position, are the paired nostrils or anterior nares (na.) — or in Cyclostomi the single nostril — leading to the organs of smell. Farther back, on the sides of the head, are the large paired eyes («.), and on the dorsal surface there is sometimes more or less indication of a vestigial median or pineal sense-organ (jpn. e.), which may take the form of an eye. Posterior to the paired eyes are the auditory organs (2. — Diagram illustrating the segmentation of the vertebral column, c. n. t. jierichordal tube; h. r. haemal ridge; h. 1. hfemal tube; i. r. f. intervertebral foramen; -,?. t. neural tube ; nch. notochord. The dotted lines indicate the segmentation into vertebrae. bone may be developed : in the Amphibia it is an independent structure ; in the higher classes it is formed by the fusion of some of the anterior ribs in the middle ventral line. In this way the anterior or thoracic region of the coelome is enclosed in an articulated bony framework formed of the vertebral column above, the ribs at the sides, and the sternum below. The ribs under these circum- stances become segmented each into two parts, a dorsal vertebral rib, articulating with a vertebra, and a ventral sternal rib with the sternum. In the tail there is frequently a haemal arch (Fig. 760, D, h. a.) springing from the ventral aspect of the centrum and en- closing the haemal canal. Thus the line of centra in the fully formed vertebral column occupies the precise position of the notochord ; the neural arches encircle the spinal portion of the cerebro-spinal cavity ; the transverse processes, ribs, and sternum encircle the coelome ; and the haemal arches similarly surround the haemal canal or vestigial coelome of the tail. As we ascend XIII PHYLUM CHORDATA 75 the series of Craniata we find every gradation from the persistent notochord of the Cyclostomata, through the imperfectly differen- tiated vertebrae of Sharks and Rays, to the complete bony vertebral column of the higher forms. The vertebra? are equal in number to the myorneres, but are arranged alternately with them, the fibrous partition between two myorneres abutting against the middle of a vertebrae, so that each muscle-segment acts upon two adjacent vertebra?. Thus, the myomeres being metameric or segmental structures, the vertebrae are intersegmental. In connection with the anterior end of the noto- chord, where no vertebrae are formed, there are developed certain elements of the bkull or cephalic skeleton, a struc- ture which is eminently characteristic of the whole craniate division, and to the possession of which it owes its name. The skull makes its first appearance in the embryo in the form of paired cartilaginous plates, the paraehordals (Fig. 763, pe), lying one on each side of the anterior end of the notochord (nch) and thus continuing forward the line of vertebral centra. In front of the para- chordals are developed a pair of curved cartilaginous rods, the trabceiilce (tr), which un- derlie the anterior part of the brain, as the parachordals underlie its posterior part : their hinder ends diverge so as to embrace the pituitary body (pty) already referred to. Cartilaginous investments are also formed around the organs of the three higher senses : a pair of olfactory capsules round the organs of smell, one of optic capsules round the organs of sight, and one of auditory capsules (au. c.) round the organs of hearing. The optic capsule, which may be either fibrous or cartilaginous, remains free from the remaining elements of the skull in accordance with the mobility of the eye ; it constitutes, in fact, the sclerotic or outer coat of that organ. The olfactory capsules are usually formed in relation to the trabeculas, and are continuous with those structures from an early stage. The auditory capsules 1 rich Fin. 703. — The elements of the cranium in an embryo Salmon, from above, au. c. auditory capsule ; nch. notochord ; pc. parachordal ; pty. position of pituitary body ; tr. trabecula. (From a model by Ziegler.) 76 ZOOLOGY SECT. , in some cases arise as outgrowths of the parachordals, in others as independent cartilages, each of which, however, soon unites with the parachordal of its own side. As development goes on, the trabeculae and parachordals become fused into a single basal plate (Fig. 764, B, b. or.) underlying the brain: the skull-floor thus formed gives off vertical up-growths on each side which finally close in above to a greater or less extent, and so give rise to a more or less complete cranium or brain-case enclosing the brain and the organs of smell and hearing, and furnishing open cavities or orbits for the eyes. In the continuous solid cranial box thus formed certain definite regions are to be distinguished : a posterior or occipital region, formed from the parachordals, united or articulated with the anterior end of the vertebral column, and presenting a large aperture, the foramen magnum (Fig. 764, B, for. mag.), through which the spinal cord becomes continuous with the brain ; an auditory region formed by the two outstanding auditory capsules (A, au. cp.) ; and a trabecidar region, including all the rest. The latter is again divisible into an interorbital region, between the orbits or eye-sockets ; an olfactory region, constituted by the olfactory capsules (olf. cp.), and by a median vertical plate, the mesethmoid (B, m. eth.), which separates them from one another; and a, p re- nasal region or rostrum (r) extending forwards from the meseth- moid and forming a more or less well-marked anterior prolongation of the cranium. The cavity for the brain (B) extends from the foramen magnum behind to the olfactory region in front ; its floor, formed from the basal plate of the embryo, is called the basis cranii (b. cr.) : its roof is always incomplete, there being one or more apertures or fontanclles (fon.) closed only by membrane and due to the imperfect union above of the side-walls. In the walls of the brain-case are apertures or foramina for the passage outwards of the cerebral nerves (vide infra). The most important of these are the olfactory foramina (nv. 1) for the nerves of smell, situated at the anterior end of the cerebral cavity, one on each side of the mesethmoid ; the optic foramina (nv. 2) for the nerves of sight, in the interorbital region ; the trigeminal foramina (nv. 5) for the fifth nerves, just in front of the auditory capsule ; the auditory foramina (nv. 8) for the nerves of hearing, in the inner wall of the auditory capsules ; and the vagus foramina (Nv. 10) for the tenth nerves, immediately posterior to the auditory capsules. In addition to the elements of the brain-case — parachordals, trabeculae, and auditory capsules — there enter into the composition of the skull another set of elements called visceral bars. These are cartilaginous rods formed in the walls of the pharynx between the gill-slits, and thus encircling the pharynx like a series of paired half-hoops (Fig. 760, B, us. b.). The corresponding right and left xnr PHYLUM CHORDATA 77 bars become united with one another below by an unpaired cartilage (Fig. 764, A, b. br.), forming a visceral arch, and the unpaired ventral pieces may unite successive arches with one another in the middle ventral line, thus giving rise to a more or less basket-like visceral skeleton. It will be noticed that the visceral skeleton has a seg- mental arrangement, being formed of parts arranged in an antero- posterior series, whereas in the cranium there is no clear indication of segmentation. There is, however, no exact correspondence between the segments of the visceral skeleton and the metameres. The visceral arches vary in number from four to nine : the fore- B b.br.s Fin. Y64.— A, diagram of cartilaginous skull from the left side ; B, cranium in sagittal section. a-u. cp. auditory capsule ; b. br. 1 — 5, basi-branchials ; b. cr. basis cranii ; b. hy. basi-hyal ; c. br. cerato-branchial ; c. hy. cerato-hyal; ep. br. epi branchial ; ep. hy. epi-hyal ;fon. fontanelle; for. mat/, foramen magnum ; A. br. hypo-branchial ; A. hy. hypo-hyal ; Aw. m. hyomandibular ; lb. 1 — U, labial cartilages ; 'nu:k."c. Meckel's cartilage ; m. eth. mesethmoid ; nv. 1—10, foramina for cerebral nerves ; olf. cp. olfactory capsule ; pat. qu. palato-quadrate ; ph. br. pharyngo- branchial ; r. rostrum ; s, t. pituitary fossa or sella turcica. most of them is distinguished as the mandibular arch, and lies just behind the mouth ; the second is called the hyoid arch, and the rest branchial arches, from the fact that they support the gills in water-breathing forms. In all Craniata except the Cyclostomes the mandibular arch becomes modified into structures called jaws for the support of the mouth. Each mandibular bar divides into a dorsal and a ventral portion called respectively the palato-quadrate cartilage (Fig. 764, A, pal. qu.) and Meckel's cartilage (mck. c.): the palato-quadrates grow forwards along the upper or anterior margin of the mouth, and unite with one another in the middle line, forming an upper j<«>- : Meckel's cartilages similarly extend along the lower or 78 ZOOLOGY SK< T. posterior margin of the month and unite in the middle line, forming the lower jaw. The quadrate (gu.\ or posterior end of the palato-quadrate, furnishes an articulation for the lower jaw, and often acquires a connection with the cranium, thus serving to suspend the jaws from the latter. Thus each jaw arises from the union of paired bars, the final result being two unpaired transverse structures, one lying in the anterior, the other in the posterior margin of the transversely elongated mouth, and moving in a vertical plane. The fundamental difference between the jaws of a Vertebrate and the structures called by the same name in an Arthropod or a Polycha^tous Worm will be obvious at once. The hyoid bar usually becomes divided into two parts, a dorsal, the hyomandibular or pharyngo-hyal (hy:m.\ and a ventral, the hyoid cornu, which is again divisible from above downwards into segments called respectively epi-hyal (epjiy.), eerato-hyal (c.hy.\ and hypo-hyal (Ji.hy.). The median ventral element of the arch, or basi-hyal (b.hy.\ serves for the support of the tongue. In some Fishes the hyoman- dibular articulates above with the auditory region of the cranium, while the jaws are connected with its ventral end. We may thus distinguish two kinds of suspensorium or jaw-suspending appara- tus— a mandibular suspensoriwn, furnished by the quadrate, and a hyoidean suspensorium formed by the hyomandibular : in the former case the skull is said to be autostylic, i.e. having the jaw connected by means of its own arch, in the latter it is called hyostylic : in a few instances an amphistylic arrangement is produced by the presence both of a direct articulation between the palato-quadrate and the auditory region of the skull, and an indirect connection through the hyomandibular. The branchial a,rches become divided transversely into dorso- ventral segments called respectively pharyngo-branchial (ph. br.) epi-branchial (ep.br.), cendo-branchial (c.br.), arid hype-branchial h.br.), and the visceral skeleton thus acquires the character of an articulated framework which allows of the dilatation of the pharynx during swallowing and of its more or less complete closure at other times. In connection with, and always superficial to the rostrum, olfactory capsules, and jaws, are frequently found labial cartilages (Jb. 1 — 4)> which sometimes attain considerable dimensions. In certain Fishes, such as Elasmobranchs, the cartilages of the skull become more or less encrusted by a superficial granular deposit of lime-salts, giving rise, as in the vertebral column of these Fishes, to calcified cartilage ; but in all the higher forms true ossification takes place, the cartilaginous skull becoming compli- cated, and to a greater or less extent replaced, by distinct bones. Of these there are two kinds, replacing or " cartilage " — and investing or " membrane " — bones. Replacing bones may begin by the de- position of patches of bony matter in the cartilage itself (endo- chondral ossification). As development proceeds, these may be xiii PHYLUM CHORDATA 79 replaced by ossification starting .within the pertchcndrium , or layer of connective-tissue surrounding the cartilage, and gradually invading the latter. More usually the bone is formed from the outset by the deposition of layers invading the cartilage from the perichondrium (or periosteum) inwards (perichondral or periosteal ossification). But in either case the bones in question are usually said to be preformed in cartilage, i.e., they replace originally cartilaginous parts. In the case of investing bones centres of ossification also appear, in constant positions, in the fibrous tissue outside the cartilage : they may remain quite independent of the original cartilaginous skull and its replacing bones, so as to be readily removable by boiling or maceration ; or they may eventually become, as it were, grafted on to the cartilage, in which case all distinction between investing and replacing bones is lost in the adult. The investing bones are to be looked upon as portions of the exoskeleton which have retreated from the surface and acquired intimate relations with the endoskeleton. The replacing bones have a very definite relation to the regions of the cartilaginous cranium. In the occipital region four bones are formed, surrounding the foramen magnum : a median ventral la si-occipital (Fig. 765, A and B, B. oc.), paired lateral ex-occipitals (EX. oc.), and a median dorsal supra-occipital (s. oc.). In each auditory capsule three ossifications commonly appear : a pro-otic (A, PR. OT.) in front, an opisthotic (OP. OT.) behind, and an cpi-otic (EP, OT.) over the arch of the posterior semicircular canal of the ear (vide infra}. In front of the basi-occipital a bone called the basi- sphenoid (A and C, B. SPH.) is formed in the floor of the skull : it appears in the position of the posterior ends of the trabeculse, and bears on its upper or cranial surface a depression, the sella turcica (s.t), for the reception of the pituitary body. Con- nected oiveach side with the basi-sphenoid are paired bones, the all-sphenoids (AL. SPH.), which help to furnish the side walls of the interorbital^region. The basi-sphenoid is continued forwards by another median bone, the prc-sphcnoid (A and D, P. SPH.), with which paired ossifications, the orbito-sphenoids (ORB. SPH), are connected, and complete the side walls of the interorbital region. The basi-occipital, basi-sphenoid, and pre-sphenoid together form the basis cranii of the bony skull. A vertical plate of bone, the mesethmoid M. ETH.), appears in the posterior portion of the car- tilage of the same name, and the outer walls of the olfactory capsules may be ossified by paired ecto-ethmoids (E, EC. ETH.). So far, it will be seen, the cranial cavity has its hinder region alone roofed over by bone, viz. by the supra- occipital : for the rest of it the replacing bones furnish floor and side- walls only. This deficiency is made good by two pairs of investing bones, the parietals (PA\ formed immediately in front of the supra-occipital and usually articulating below with the ali-sphenoids, and the frontals (FR), placed in front of the parietals, and often connected 80 ZOOLOGY below with the orbito-sphenoids. A pair of nasals (NA) are developed above the olfactory capsules and immediately in advance of the frontals ; and below the base of the skull two important investing bones make their appearance, the vomer (TO)— which may be double — in front, and the para-sphenoid (PA. SPH) behind. The result of the peculiar arrangement of replacing and invest- ing bones just described is that the brain- case, in becoming ossified, acquires a kind of secondary segmentation, being clearly divisible in the higher groups, and especially in the Mammalia, into three quasi-segments. These are the occipital segment (B) FIG. 765. — A, diagram of bony skull in sagittal section ; B, transverse section of occipital region ; C, of parietal region ; D, of frontal region ; E, of ethmoidal region. Cartilaginous parts are dotted ; replacing bones are marked in thick type, investing bones in italics, mck. c. Meckel's cartilage ; Nc. 1 — 10, foramina for cerebral nerves ; r. rostrum ; s. t. sella turcica or pituitary fossa. Replacing bones— AL. SPH. alisphenoid ; ART. articular ; B. BR. basi-branchial ; B. HY. basi-hyal ; B. OC. basi-occipital ; B. SPH. basi-sphenoid ; C. BR. cerato-bran- chial ; C. HY. cerato-hyal ; EC. ETH. ecto-ethmoid ; EP. BR. epi-branchial ; EP. HY. epi-hyal ; EX. OC. ex-occipital ; H. BR. hypo-branchial ; H. HY. hypo-hyal ; HY.M. hyornandibular ; M. ETH. mesethmoid ; OP.OT. opisthotic ; OR. SPH. orbito-sphe- nuid ; PAL. palatine ; PH. BR. pharyugo-branchial ; PR.OT. pro-otic ; PR. SPI pre-sphenoid ; PTG. pterygoid ; QXT. quadrate ; S. OC. supra-occipital. Investing bones —J)NT. dentary ; FR. frontal; MX. maxilla; NA. nasal; PA. parietal; PA.SPH. parasphe- noid ; PMX. premaxilla ; SQ. squamosal ; VO. vomer. formed by the basi-occipital below, the ex-occipitals at the sides, and the supra- occipital above1; the parietal segment (C), formed by the basi-sphenoid below, the alisphenoids laterally, and the parietals above ; and the frontal segment (D) constituted by the pre-sphenoid below, the orbito-sphenoids on either side, and the frontals above. It must be observed that this segmentation of the cranium is quite independent of the primary segmentation of the head, which is determined by the presence of myomeres and by the relations of the cerebral nerves. The cranial bones have constant relations to the cerebral nerves. The olfactory nerves (A, Nv. 1} pass out one on either side of the 1 With the occipital segment in many Fishes are amalgamated one or several of the most anterior vertebrse. xm PHYLUM CHORDATA 81 mesethmoid, the optic nerves (Nv. 2) through or immediately behind the orbito-sphenoids, the fifth nerves (Nv. 5) through or immediately behind the alisphenoids, and the tenth nerves (Nv. 10) through or immediately in front of the ex-occipitals. It will be seen that a clear distinction can be drawn between the primary cranium or chondrocranium, formed by the fusion of the parachordals, auditory capsules, and trabeculae, and consisting of an undivided mass of cartilage more or less replaced by bones, and the secondary cranium or osteocranium, modified by the super- addition of investing bones. A similiar distinction may be drawn between the primary and secondary jaws. The primary tipper jaiu, or palato-quadrate, be- comes ossified by three chief replacing bones on each side, the palatine (A, PAL.) in front, then \\iorpterygoid (PTG.), and the quad- rate (QU.) behind, the latter furnishing the articulation for the lower jaw or mindible. In the higher classes the primary upper jaw does not appear as a distinct cartilaginous structure, and the palatine and pterygoid are developed as investing bones. The secondary upper jaw is constituted by two pairs of investing bones, the pre-maxilla (P. MX) and the maxilla (MX), which in bony skulls furnish the actual anterior boundary of the mouth, the primary jaw becoming altogether shut out of the gape. The proximal end of the primary lower jaw ossifies to form a replacing bone, the articular (ART.), by which the mandible is hinged : the rest of it remains as a slender, unossified Meckcl's cartilage (Mck. C), which may dis- appear entirely in the adult. The secondary lower jaw is formed by a variable number of investing bones, the most important of which is the dentary (DNT). In Mammalia the dentary forms the entire mandible, and articulates, not with the quadrate, but with a large investing bone formed external to the latter, and known as the squamosal (SQ). In the hyoid arch a replacing bone, the hyj-mandibular (HY. M), appears in the cartilage of the same name, and ossifications are also formed in the various segments of the hyoid cornua (EP. HY, c. HY, H. HY, B. HY) and of the branchial arches (PH. BR, EP. BR, c. BR, H. BR, B. BR). In the air-breathing forms both hyoid and branchial arches undergo more or less complete atrophy, the whole gill-bearing apparatus becoming reduced mainly to a small hyM lone serving for the support of the tongue. The skeleton of the median fins is formed of a single row of cartilaginous rays orpterygioplwres (Fig. 760, C and D,/.r), lying in the median plane, and more numerous than the vertebrae. They may ossify, and may be supplemented by dermal fin-rays, of varying composition, developed in the derm along the free edge of the fin. The latter are clearly exoskeletal structures. Both pectoral and pelvic fins are supported by pterygio- phores or radialia (Fig. 766, Had.) the basal or proximal 82 ZOOLOGY SECT. ends of which are articulated with stout cartilages (Has), often replaced by bones, the basalia, which serve to strengthen the fin at its point of union with the trunk. In all classes above Fishes the paired fins are, as we have seen, replaced by five-toed or pentadactyle limbs. These are supported by bones, probably to be looked upon as greatly modified pterygiophores, FIG. 700. — Diagram of three stages in the development of the pelvic fins. In A the anterior pterygiophores on the right side (Had), have united to form a basal cartilage (Bas.) ; in B the basalia (Bas.) are fully formed and are uniting at * to form the pelvic girdle ; in C the pelvic girdle (G) is fully constituted, and at t has segmented from the basale on the right side. CL. cloacal aperture. (From Wiedersheim's Comparative Anatomy.) and obviously homologous in the fore- and hind-limbs. In the proxi- mal division of each limb there is a single rod-like bone, the humcrus (Fig. 767, HU), or upper-arm-bone, in the fore-limb, the femur (Fig. 768, FE,) or thigh-bone, in the hind-limb. In the middle division there are two elongated bones, an anterior, the radius (RA), and a posterior, the ulna (UL), in the fore-limb ; an anterior, the tibia (TI), and a posterior, the fibula (FI), in the hind-limb. Next follow the bones of the hand and foot, which are again divisible into three sets : carpals or wrist-bones, nietacarpals XIII PHYLUM CHORDATA 83 (mtcp) or hand-bones, the phalanges (ph) or finger-bones, in the fore-limb ; tar sals or ankle-bones, metatarsals (mtts) or foot- bones, and phalanges (ph) or toe-bones, in the hind-limb. The carpals and tarsals consist typically of three rows of small nodules of bone or cartilage, the proximal row containing three, the middle two, and the distal five elements. The three proximal carpals are called respectively radiate (ra), intermedium (int), and ulnare (ul), those of the middle row the first and second centralia (en. 1, en. 2), those of the third row the five distalia (dst. 1-5), the separate elements being distinguished by numbers, counting from SCP HU a.t.1 n Tnlcp.i -41 •Sf( -I JF FK.S. 707 and 70S. — Diagrams of the fore- and hind-limbs with the limb-girdles, actb. acetabulum ; fil. glcnoid cavity ; p. cor. procoracoid ; / — V, digits. Replacing bones — en. 1 , en. 2, contralia ; COR. coracoid ; dst. 1—5, distalia ; FE. femur ; PI. fibula ; fi. fibulare ; HU. humerus ; IL. ilium; int. intermedium; IS. ischium ; mtcp. 1—5, metucarpals ; mtts.l 5, metatarsals ; ph. phalanges; PU. pubis ; RA. radius ; ra. radiale ; SCF. scapula; TI. tibia ; ti. tibiale ; UL. ulna ; ul ulnare. Investing bone — CL. clavicle. the anterior or radial edge of the limb. In the tarsus the bones of the first row are known respectively as tibiale (ti), intermedium (int), and fibulare (fi), those of the second row as centralia (en. 1, en. 2), and those of the third as distalia (dst. 1-5). The meta- carpals (mtcp. 1-5) and metatarsals (mtts. 1-5) are five rod-like bones, one articulating with each distale : they are followed by the phalanges (ph), of which each digit may have from one to five. The first digit of the fore-limb (Fig. 767, i) is distinguished as the pollex or thumb, that of the hind-limb (Fig. 768, i) as the hallux or great toe; the fifth digit of each limb (v) is the minimus. In connection with the paired appendages are formed supporting 84 ZOOLOGY SECT. structures called the limb-girdles ; they occur in the portions of the trunk adjacent to the appendages and serve for the articulation of the latter. In the embryonic condition they are continuous with the basalia and are probably to be looked upon as ingrowths of the primitive fin-skeleton (Fig. 766). The shoulder- girdle or pectoral arch has primarily the form of paired bars, which may unite in the middle ventral line so as to form an inverted arch. Each bar — i.e. each half of the arch — furnishes a concave or convex glenoid surface (Fig. 767, gl.) for the articulation of the pectoral fin or fore-limb, and is thereby divided into two portions — a dorsal or scapular region, above the glenoid surface, and a ventral or coracoid region below it. The coracoid region is again divisible, in all classes above Fishes, into two portions : an anterior, ihepro-cora- coid (p. cor}, and a posterior, the coracoid proper. Each of these regions commonly ossifies — a replacing bone, the scapula (SCP), appearing in the scapular region, another, the coracoid (COR); in the coracoid region, while in relation with the pro- coracoid is formed a bone, the clavicle (CL), largely or entirely developed independently of pre-existing cartilage. The constitution of the hip-girdle, or pelvic arch, is very similar. It consists originally of paired bars, which may unite in the middle ventral line, and are divided by the acetabuhtm (Fig. 768, actb.), the articular surface for the pelvic fin or hind-limb, into a dorsal or iliac region, and a ventral or pubo-ischial region, the latter being again divisible, in all classes above Fishes, into an anterior portion, or pubis, and a posterior portion, or ischium. Each region is replaced in the higher forms by a bone, the pelvic girdle thus consisting of a dorsal ilium (IL) serially homologous with the scapula, an antero-ventral pubis (PU) with the pro-coracoid and clavicle, and a postero- ventral ischium (IS) with the coracoid. The long bones of the limbs are divisible each into a shaft, and proximal and distal extremities. When ossification takes place the shaft is converted into a tubular bone, the cartilaginous axis of which is absorbed and replaced by a vascular fatty tissue called marroiv. The extremities become simply calcified in the lower forms, but in the higher a distinct centre of ossification may appear in each, forming the epiphysis, which finally becomes ankylosed to the shaft. Digestive Organs. — The enteric canal is divisible into buccal cavity (Fig. 760, A, luc. c.), pharynx (ph.), gullet, stomach (st.), and intestine (int.), the latter sometimes communicating with the exterior by a cloaca (el.), which receives the urinary and genital ducts. The buccal cavity is developed from the stomodaBum of the embryo: the proctodaeum gives rise to a very small area in the neighbourhood of the anus, or, when a cloaca is present, to the external portion of the latter ; all the rest of the canal is formed XIH PHYLUM CHORDATA 85 from the mesenteron, and is therefore lined by an epithelium of endodermal origin. The pharynx communicates with the exterior, in Fishes and in the embryos of the higher forms, by the gill-slits (i. br. a, 1-7); it communicates with the stomach by the gullet. The stomach (st.) is usually bent upon itself in the form of a U; the intestine (int.) is generally more or less convoluted; hence the stomach and intestine are together con- siderably longer than the enclosing abdominal cavity. In the embryo the intestine is sometimes continued backwards into the haemal canal by an extension called the post-anal gut (p. a. g.\ which may perhaps indicate that the anus has shifted forwards in the course of evolution. The epithelium of the buecal cavity is usually many-layered, like that of the skin, of which it is developmentally an in-turned portion; the pharynx and gullet have also a laminated epithelium, but the rest of the canal is lined by a single layer of cells under- laid by a layer of connective-tissue, the sub-mimosa ; epithelium and sub-mucosa together constitute the mucous membrane. The mucous membrane of the stomach and sometimes of the intestine usually contains close-set tubular glands ; those of the stomach — the gastric glands, secrete gastric juice, which acts upon the proteid portions of the food only ; the secretion of the intestinal glands digests proteids, starch, and fats. Outside the mucous membrane are layers of umtriped muscle, usually an internal circular and an external longitudinal layer. Externally the intra-ccelomic portion of the canal is invested by peritoneum formed of a layer of connective-tissue next the gut and a single-layered ccelomic epithelium facing the body-cavity. In connection with the enteric canal certain very characteristic structures are developed. In the mucous membrane of the mouth calcifications in most cases appear, and form the teeth, which usually occur in a row along the ridge of each jaw, but may be developed on the roof of the mouth, on the tongue, and even in the pharynx. A tooth is usually formed of three tissues — dentine, enamel, and cement. The main bulk of the tooth is made up of dentine (Fig. 769, A, ZB), which occurs under three forms. Hard dentine consists of a matrix of animal matter strongly impregnated with lime-salts and permeated by delicate, more or less parallel, tubules con- taining organic fibrils. Vaso-dentine is permeated with blood- vessels, and consequently appears red and moist in the fresh condition. Osteo-dentine approaches bone in its structure and mode of development. The free surface of the tooth is usually capped by a layer of enamel (ZS), a dense substance, either structureless or presenting a delicate fibrillation, containing not more than 3 to 5 per cent, of animal matter, and being, therefore, the hardest tissue in the body. The cement (ZG) coats that portion of the tooth which is embedded in the tissues of the jaw, VOL. II G 86 ZOOLOGY SECT. and sometimes forms a thin layer over the enamel ; it has prac- tically the structure of bone. At the inner end of the tooth there is frequently an aperture (PH'} leading into a cavity (PH) filled in the fresh condition by the tooth-pulp, a sort of connective- tissue plug abundantly supplied with nerves and blood-vessels. In the development of a tooth (Fig. 769, B) the deep layer of the buccal epithelium becomes invaginated and grows inwards into the sub-mucosa in the form of a narrow cord, the enamel-organ (SK). The distal end of this enlarges into a flask-like form, and the bottom of the flask becomes invaginated (Ma) by the growth fe— ME ZS FIG. 769.— A, longitudinal section of a tooth, semi-diagrammatic. PH, pulp-cavity ; PIT, opening of same ; Ztf, dentine ; ZC, cement ; ZS, enamel. B, longitudinal section of developing tooth. By, submucosa ; DS, dentine ; Ma, invaginated layer of enamel-organ ; ME, epithelium of mouth ; 0, odontoblasts ; SK, stalk of enamel-organ ; ZK, tooth-papilla. (From Wieders- heim's Vertebrate.) of a conical process of the sub-mucosa, the dental papilla (ZK). Mesoderm cells accumulate on the free surface of the papilla and form a distinct layer of cells called odontollasts (0). From these the dentine is formed in successive layers, which gradually accumulate between the layer of odontoblasts and the inner or invaginated layer of the enamel-organ. The lower, or proximal, part of the papilla remains uncalcified and forms the tooth-pulp. The enamel is formed by the deposition of successive layers of calcific matter from the inner or invaginated layer of the enamel- organ, the cement by the ossification of the tissue immediately surrounding the papilla. Thus the tooth is partly of ectodermal, partly of mesodermal, origin. XIII PHYLUM CHORDATA 87 In some Fishes the scales or elements of the dermal exo- skeleton pass insensibly into the teeth over the ridges of the jaws, and agree with them in structure, so that there can be no doubt as to the homology of the two. Teeth are, in fact, to be looked upon as portions of the exoskeleton which have migrated from the skin into the buccal cavity, and even into the pharynx, and have there increased in size and assumed special functions. The tongue is a muscular elevation of the floor of the mouth, supported by the basi-hyal, and usually more or less protrusible. The roof of the buccal cavity in the embryo sends off a pouch, the pituitary diverticulum (Fig. 760, A, pty. s.), which grows upwards, and, losing its connection with the mouth, becomes attached to the ventral surface of the brain as the pituitary body (pty. ft.). It may correspond with the neural gland of Urochorda. In terrestrial Craniata buccal glands are present, opening by ducts into the mouth : the most important of these are the racemose salivary glands which se- crete a digestive fluid — saliva, cap* able of converting starch into sugar. There are also two large and highly characteristic digestive glands in the abdominal cavity, both developed as outpushings of the intestine, but differing greatly from one another in their fully developed state, both in outward appearance and in histological structure : these are the liver and the pancreas. The liver (Fig. 7 60, A, Ir.) is a dark-red organ of relatively immense size : it not only secretes a digestive juice, the bile, which has the function of emulsifying fats, but also forms an amyloid substance called glycogen or animal starch, which, after being stored up in the liver-cells, is restored to the blood in the form of sugar. The liver is formed of a mass of polyhedral cells (Fig. 770, /.) with minute intercellular spaces which receive the bile secreted from the cells and from which it passes to the ducts (b). The pancreas (Fig. 760, A, pn.) is a racemose gland, and secretes pancreatic juice, which acts upon proteids, starch, and fats. The ducts of both glands usually open into the anterior end of the intestine : that of G 2 FIG. 770. — Diagram of structure of liver. J>, a small branch of hepatic duct ; b', its ultimate termination in the intercellular spaces ; c, blood-capillaries ; ?, liver-cells. (From Huxley's Physiology.) 88 ZOOLOGY SECT. the liver (b. d.) generally gives off a blind offshoot ending in a capacious dilatation, the gall-bladder (g. b.) in which the bile is stored. We thus have one or more hepatic ducts conveying the bile from the liver and meeting with a cystic duct from the gall- bladder, while from the junction a common bile-duct leads into the intestine. Another important and characteristic organ in the abdomen of Craniata is the spleen (spl.), a gland -like organ of variable size and shape, attached to the stomach by a fold of peritoneum, but having no duct. It is formed of a pulpy substance containing numerous red blood-corpuscles, many of them in process of dis- integration : dispersed through the pulp are masses of leucocytes which multiply and pass into the veins. Two other ductless glands are formed in connection with the enteric canal. The thyroid (thd.) is developed as an outpushing of the floor of the pharynx which becomes shut off, and forms, in the adult, a gland-like organ of considerable size. Its final posi- tion varies considerably in the different classes. It has been com- pared with the endostyle of Tunicata and of Amphioxus, which, as will be remembered, is an open groove on the ventral side of the pharynx. This view is supported by the condition of the parts in the larval Lamprey (see Cydostomfita). The thymus is developed from the epithelium of the dorsal ends of the gill-clefts : in the adult it may take the form of a number of separate gland-like bodies lying above the gills, or may be situated in the neck or even in the thorax. The thymus and thyroid, by virtue of internal secretions which they produce, and which mingle with the blood, control and modify the physiological condition of various organs and tissues with which they have no immediate anatomical connection. The whole intra-abdominal portion of the enteric canal as well as the liver, pancreas, spleen, and indeed, all the abdominal viscera, are supported by folds of peritoneum, called by the general name of mesentery (Fig. 760, C, mes.) and having the usual relation to the parietal and visceral layers of the peritoneum. Two kinds of respiratory organs are found in Craniata: water-breathing organs or gills, and air-breathing organs or lungs. Gills arise as a series of paired pouches of the pharynx which extend outwards, or towards the surface of the body, and finally open on the exterior by the gill-slits already noticed. Each gill-pouch thus communicates with the pharynx by an internal (Fig. 760, B, i. br. a), with the outside water by an external bran- chial aperture (e. br. a), and is separated from its predecessor and from its successor in the series by stout fibrous partitions, the interbranchial septa (Fig. 771, i. br. s). The mucous membrane XIII PHYLUM CHORDATA 89 forming the anterior and posterior walls of the pouches is raised up into a number of horizontal ridges, the branchial filaments (br. /"), which are abundantly supplied with blood. A current of water entering at the mouth passes into the pharynx, thence by the internal gill-slits into the gill-pouches, and finally makes its way out by the external gill-slits, bathing the branchial filaments as it goes. The exchange of carbonic acid for oxygen takes place in the blood-vessels of the branchial filaments, which are, therefore, the actual organs of respiration. It will be noticed that the re- spiratory epithelium is endodermal, being derived from that of the pharynx, which, as we have seen, is a portion of the mesenteron. As already mentioned, the walls of the pharynx are supported by the visceral arches, which surround it like a series of incom- plete hoops, each half-arch or visceral bar being embedded in the inner or pharyngeal side of an interbranchial septum. Thus the visceral arches (v. b.) alternate with the gill-pouches, each being related to the posterior set of filaments of one pouch and the an- terior set of the next. In the higher Fishes, such as the Trout or Cod, the interbranchial septa be- come reduced to narrow bars enclosing the visceral arches (right side of Fig. 771), with the result that a double set of free branchial filaments springs from each visceral bar and constitutes what is called a single gill. Thus an entire gill or Uolobranch (hi. br.) is the morphologi- cal equivalent of two half-gills— hemibranchs (km. br.), or sets of branchial filaments belonging to the adjacent sides of two con- secutive gill-pouches. On the other hand, a gill-pouch is equivalent to the posterior hemibranch of one gill and the anterior hemibranch of its immediate successor. In some Amphibia water-breathing organs of a different kind are found. These are the external gills : they are developed as branched outgrowths of the body-wall in immediate rela- tion with the gill-slits, and differ from the internal gills just described in having an ectodermal epithelium. They are probably, however, of the same essential character as the endo- dermal gills. Lungs (Fig. 760, A, Ig) are found in all Craniata from the Dipnoi i.br. FIG. 771.— Diagrammatic horizontal section of the pharyngeal region of a Craniate : on the left are shown three gill-pouches (.) with fixed branchial filaments (br. /.) and separated by inter-branchial septa (i. br. s.); on the right one hemibranch (hut. br.) and two holobranchs (hi. br.) with free fila- ments, covered by an operculum (op). Ectoderm dotted, endoderm striated, mesoderm evenly shaded, visceral bars (v. b.) black. 90 ZOOLOGY SEPT. upwards. They are developed as a hollow outpushing from th ventral wall of the embryonic fore-gut or anterior part of th enteric canal; this passes backwards and upwards, usually dividing into right and left divisions, and finally coming to lie in the dorsal region of the ccelome. The inner surface of the single or double lung thus formed is raised into a more or less complex network of ridges so as to increase the surface of blood exposed to the action of the air ; and, in the higher forms, the ridges, increasing in number and complexity, and uniting with one another across the lumen of the lung, convert it into a sponge-like structure. The respiratory epithelium is, of course, endodermal. Since the lungs are blind sacs, some contrivance is necessary for renewing the air contained in them : this is done either by a process analogous to swallowing, or by the contraction and relaxation of the muscles of the trunk. In some Fishes there occurs, in the position occupied in air- breathers by the lungs, a structure called the air-bladder, which contains gas, and serves as an organ of flotation. Like the lungs, it is developed as an outgrowth of the fore-gut, but, except in four instances, from its dorsal instead of its ventral side. In many cases the air-bladder loses its connection with the pharynx and becomes a closed sac. The blood-vascular system attains a far higher degree of complexity than in any of the groups previously studied: its essential features will be best understood by a general description of the circulatory organs of Fishes. The heart (Figs. 760 and 772) is a muscular organ contained in the pericardia! cavity and composed of three chambers, the sinus vcnosus (s. v.), the auricle (au.), and the ventricle (v.), which form a single longitudinal series, the hindmost, the sinus venosus, opening into the auricle, and the auricle into the ventricle. They do not, however, lie in a straight line, but in a zigzag fashion, so that the sinus and auricle are dorsal in position, the ventricle ventral. Usually a fourth chamber, the conus arteriosus (c. art.), is added in front of the ventricle. The various chambers are separated from one another by valvular apertures (Fig. 773) which allow of the flow of blood in one direction only, viz. from behind forwards — that is, from sinus to auricle, auricle to ventricle, and ventricle to conus. The heart is made of striped muscle of a special kind — the only involuntary muscle in the I)ody having this histological character — which is particularly thick and strong in the ventricle. It is lined internally by epithelium and covered externally by the visceral layer of the pericardium. Springing from the ventricle, or from the conus when that chamber is present, and passing directly forwards in the middle line below the gills, is a large, thick-walled, elastic blood-vessel, the XIII PHYLUM CHORDATA 91 VOL. II G 2* 92 ZOOLOGY SECT- ventral aorta (Figs. 760, B, and 772, v. ao.). At its origin, which may be dilated to form a bulbus aorta?, are valves so disposed as to allow of the flow of blood in one direction only, viz. from the ventricle into the aorta. It gives off on each side a series of half-hoop-like vessels, the afferent branchial arteries (a. br. a.), one to each gill. These vessels ramify extensively, and their ultimate branches open into a network of microscopic tubes or capillaries (Fig. 773, G.), having walls formed of a single layer of epithelial cells, which permeate the connective- tissue -layer of the branchial filaments, and have therefore nothing between them and the surrounding water but the epithelium of the filaments. The blood, driven by the contractions of the heart into the ventral aorta, is pumped into these respiratory capillaries, and there exchanges its superfluous carbonic acid for oxygen. It then passes from the capillaries into another set of vessels which join with one another, like the tributaries of a river, into larger and larger trunks, finally uniting in each gill, into an efferent branchial artery (e. br. a.). The efferent arteries of both sides pass upwards and discharge into a median longitudinal vessel, the dorsal aorta (d. ao.\ situated immediately beneath the notochord or vertebral column. From this trunk, or from the efferent branchial arteries, numerous vessels, the systemic arteries, are given off to all parts of the body, the most important being the carotid arteries (Fig. 772, c. a.} to the head, the subclavian (scl. a.) to the pectoral fins, the cceliac (cl. a.) and mesenteric (ms. a.) to the stomach, intestine, liver, spleen, and pancreas, the renal (r. a.) to the kidneys, the spermatic (sp. a.) or ovarian to the gonads, and the iliac (il. a.) to the pelvic fins. After giving off the last the aorta is continued as the caudal artery (cd. a.) to the end of the tail. With the exception of the capillaries, all the vessels described in the preceding paragraph, including the dorsal and ventral aortse, are arteries. They are firm, elastic tubes, do not collapse when empty, usually contain but little blood in the dead animal, and serve to carry the blood from the heart to the body generally. The systemic arteries branch and branch again into smaller and smaller trunks, and finally pour their blood into a capillary network (Fig. 773, B, K, and T) with which all the tissues of the body, except epithelium and cartilage, are permeated. In these systemic capillaries the blood parts with its oxygen and nutrient constituents to the tissues, and receives from them the various products of destructive metabolism— carbonic acid, water, and nitrogenous waste. The systemic, like the respiratory, capillaries are micro- scopic, and their walls are formed of a single layer of epithelial cells. We saw that the respiratory capillaries are in connection with two sets of vessels, afferent and efferent. The same applies to the systemic capillaries, with the important difference that their PHYLUM CHORDATA 93 efferent vessels are not arteries, but thin-walled, non-elastic, collapsible tubes called veins. They receive the impure blood from the capillaries, and unite into larger and larger trunks, finally opening into one or other of the great veins, presently to be described, by which the blood is returned to the heart. As a general rule the vein of any part of the body runs parallel to its artery, from which it is at once distinguished by its wider calibre, by its dark colour— due to the contained bluish-purple blood seen through its thin walls — by being gorged with blood after death, by the complete collapse of its walls when empty, and by its usually containing valves. In some cases the veins become dilated into spacious cavities called sinuses ; but sinuses without proper walls 0,6 FIG. 773. — Diagram illustrating the course of the circulation in a Fish. Vessels containing aerated blood red, those containing non-aerated blood blue, lymphatics black. B. capillaries of the body generally ; E. of the enteric canal ; G. of the gills ; K. of the kidneys ; L. of the liver ; T. of the tail. a. br. a. afferent branchial arteries ; au. auricle ; r. a. conns arteriosus , d. no. dorsa aorta ; e. br. a. efferent branchial arteries ; h. p. v. hepatic portal vein ; h. v. hepatic vein ; If. lacteals ; ly. lymphatics ; pr. cv. v. precaval veins ; r. p. v. renal portal veins ; s. c. sinus venosus ; r. ventricle ; c. ao. ventral aorta. The arrows show the direction of the current. (From Parker's Elementary Bioloyy.) such as occur in many Invertebrates, are never found in the Craniata. The veins from the head join to form large, paired jugular veins (Fig. 772, y. v.) which pass backwards, one on each side of the head, and are joined by the cardinal veins (crd. v.) coming from the trunk, each jugular uniting with the corresponding cardinal to form a large precaval vein (pr. cv. v.) which passes directly downwards and enters the sinus venosus. The blood from the tail returns by a caudal vein (cd. v.), lying immediately below the caudal artery in the haemal canal of the caudal vertebra (Fig. 760, D). On reaching the coelome the caudal vein forks horizontally, and the two branches either become directly continuous with the cardinals or pass one to each kidney under the name of the renal portal veins (Fig. 772, r. p. v.). In the kidneys they break up into capillaries (Fig. 773, K), their blood mingling with that brought by the renal arteries and being finally discharged into the cardinals by the renal veins (Fig. 772, r. v.). Thus the blood from 94 ZOOLOGY SECT. the tail may either return directly to the heart in the normal manner or may go by way of the capillaries of the kidneys. In the latter case there is said to be a renal portal system, the essential characteristic of which is that the kidney has a double blood-supply, one of pure blood from the renal artery, and one of impure blood from the renal portal v«ein ; in other words, it has two afferent vessels, an artery and a vein, and the latter is further distinguished by the fact that it both begins and ends in capillaries instead of beginning in capillaries and ending in a vein of higher order. The blood from the gonads is returned to the cardinals by veins called spermatic (sp. v.) in the male, ovarian in the female. That from the paired fins takes, in what appears to be the most typical case, a somewhat curious course. On each side of the body there is a lateral vein (lat. v.), running in the body- wall and following the course of the embryonic ridge between the pectoral and pelvic fins. It receives, anteriorly, a subclavian rein (scl. r.) from the pectoral fin, and posteriorly an iliac vein (il. v.) from the pelvic fin, and in front pours its blood into the precaval. The veins from the stomach, intestine, spleen, and pancreas join to form a large hepatic portal vein (h. p. v.), which passes to the liver and inhere breaks up into capillaries, its blood mingling with that brought to the liver by the hepatic artery (h. a.), a branch of the cceliac. Thus the liver has a double blood-supply, receiving- oxygenated blood by the hepatic artery, and non-oxygenated but food-laden blood by the hepatic portal vein (Fig. 773, L). In this way we have a hepatic portal system resembling the renal portal system both in the double blood-supply, and in the fact that the afferent vein terminates, as it originates, in capillaries. After circulating through the liver the blood is poured, by hepatic vtins (h. v.), into the sinus venosus. The hepatic, unlike the renal portal system, is of universal occurrence in the Craniata. In the embryo there is a sub-intestinal vein, corresponding with that of Amphioxus, and lying beneath the intestine and the post- anal gut. Its posterior portion becomes the caudal vein of the adult, its anterior portion one of the factors of the hepatic portal vein. To sum up : — The circulatory organs of the branchiate Craniata consist of (a) a muscular organ of propulsion, the heart, provided with valves and driving the blood into (b) a set of thick-walled, elastic, afferent vessels, the arteries, from which it passes into (c) a network of microscopic vessels or capillaries which permeate the tissues, supplying them with oxygen and nutrient matters and receiving from them carbonic acid and other waste products : from the capillary network the blood is carried off by (d) the veins, thin- walled, non-elastic tuoes by which it is returned to the heart. Thus the general scheme of the circulation is simple : the arteries XIII PHYLUM CHORDATA spring from the heart, or from arteries of a higher order, and end in capillaries ; the veins begin in capillaries and end in vessels of a higher order or in the heart. Actually, however, the system is complicated (a) by the in- terposition of the gills in the course of the outgoing current, as a result of which we have arteries serving as both afferent and efferent vessels of the re- spiratory capillaries, the effer- ent arteries taking their origin in those capillaries after the manner of veins ; and (b) by the interposition of two im- portant blood-purifying organs, the liver and the kidney, in the course of the returning current, as a result of which we have veins acting as both afferent and efferent vessels of the hepatic and renal capil- laries, the afferent vessels of both organs ending in capil- laries after the fashion of arteries. In the embryos of the higher, or air-breathing, Craniata, the circulatory organs agree in essentials with the above de- scription, the most important difference being that, as no gills are present, the branches of the ventral aorta do not break up into capillaries, but pass directly into the dorsal aorta, forming the aortic arches (Fig. 774, Ab.). With the ap- pearance of the lungs, however, a very fundamental change occurs in the blood-system. The last aortic arch of each side gives off a pulmonary artery (Fig. 775, Ap.) to the corresponding lung, and the blood, after circulating through the capillaries of that organ, is returned by a pulmonary vein (lv.), not into an ordinary systemic vein of higher order, but into the heart Fi<;. 774. — Diagram of the vascular system in the embryo of an air-breathing Craniate. A, dorsal aorta and auricle ; Al>, aortic arches ; Acd, caudal artery ; All. allantoic arteries ; Am, vitelline arteries ; B, ventral aorta ; c, c1. carotid arteries ; D, precaval veins ; Ic, E, iliac arteries ; EC, cardinal veins ; KL, gill- clefts ; S. A. S, S1, roots of dorsal aorta ; Sb, subclavian arteries; £&i, subclavian veins; V. ventricle ; VC, jugular vein ; Vm, vitelline veins. (From Wiedersheim's Vertebmta.) 96 ZOOLOGY SECT. directly : there it enters the left side of the auricle, in which a vertical partition is developed, separating a left auricle (/I1), which receives the aerated blood from the lungs, from a right auricle (A), into which is poured the impure blood of the sinus venosus. Lastly, in Crocodiles, Birds, and Mammals (B) the ventricle also becomes divided into right and left chambers, and we get a four-chambered heart, having right and left auricles and right and left ventricles : at the same time the conus arteriosus and sinus venosus cease to exist as distinct chambers. The left auricle receives aerated blood from the lungs and passes it into the left ventricle, whence it is propelled through the system : the right auricle receives impure blood from the system, and passes it into the right ventricle to be pumped into the lungs for aeration. Thus the four-chambered heart of the higher Vertebrata is quite a different thing from that V FIG. 775.— Diagram of the heart A, in an Amphibian ; B, in a Crocodile. A, right auricle ; A', left auricle ; Aji, pulmonary artery ; Ir, pulmonary vein ; RA, aortic arches ; V. ventricle ; V, left ventricle ; v, v, and Ve, Ve, pre- and postcavals. (From Wiedersheim's Vertebrata.) of a Fish : in the latter the four chambers — sinus venosus, auricle, ventricle, and conus arteriosus — form a single longitudinal series, whereas in a Mammal, for instance, the four chambers constitute practically a double heart, there being no direct communication between the auricle and ventricle of the right side, or respiratory heart, and those of the left side, or systemic heart. The modifica- tions undergone by the arteries and veins in the higher Vertebrata will be best considered under the various classes. It will be noticed that there is a sort of rough correspondence between the blood-vessels of Craniata and those of the higher Worms. The sub-intestinal vein, heart, and ventral aorta together form a ventral vessel, the dorsal aorta a dorsal vessel, and the aortic arches transverse or commissural vessels. The heart might thus be looked upon as a portion of an original ventral vessel, which has acquired strongly muscular walls, and performs the whole function of propelling the blood. But in making such a comparison it xiii PHYLUM CHORD ATA 97 has to be borne in mind that the direction of the current of the blood in the Craniata is exactly the opposite of that in the Annulata. The Hood of Craniata is always red, and is specially distin- guished by the fact that the haemoglobin to which it owes its colour is not dissolved in the plasma as in most red-blooded Inver- tebrates, but is confined to certain cells called red Hood-corpuscles (Fig. 776), which occur floating in the plasma in addition to, and FIG. 776. — Surface and edge views of re$ blood-corpuscles of Frog (A) and Man (B). nu. nucleus. (From Parker's Bloloijy.) in far greater numbers than, the leucocytes. They usually have the form of flat oval discs (A.), the centre bulged out by a large nucleus (nu.), but in Mammals (B.) they are bi-concave, non-nucle- ated, and usually circular. The red corpuscles do not perform amo3boid movements. The colour of the blood varies with the amount of oxygen taken up by the hemoglobin. When thoroughly aerated it is of a bright scarlet colour, but assumes a bluish-purple hue after giving up its oxygen to the tissues. Owing to the fact that oxygenated blood is usually found in arteries, it is often spoken of as arterial blood, while the non-oxygenated, purple blood, being usually found in veins, is called venous. But it must not be forgotten that an artery, e.y., the ventral aorta or the pulmonary artery, may contain venous blood, and a vein, e.g., the pulmonary vein, arterial blood. The distinction between the two classes of vessels does not depend upon their contents, but upon their relations to the heart and the capillaries. In addition to the blood-vessels the circulatory system of Craniata contains lymph-vessels or lymphatics (Fig. 773, ly). In most of the tissues there is a network of lymph-capillaries, inter- woven with, but quite independent of, the blood-capillaries. From this network lymphatic vessels pass off, and finally discharge their contents into one or other of the veins. Many of the lower Craniata possess spacious lymph- sinuses surrounding the blood-vessels ; and there are communications between the lym- phatics and the coelome by means of minute apertures or stomata. The lymphatics contain a fluid called lymph, which is to all intents and purposes blood minus its red corpuscles. The lymph-plasma consists of the drainage from the tissues : it makes its way into the lymph capillaries, and thence into the lymphatics, which are all efferent vessels, conveying the fluid from the capillaries to the 98 ZOOLOGY SECT. veins. Leucocytes are added to the plasma in bodies called lymphatic glands, which occur in the course of the vessels. Valves may be present to prevent any flow of lymph towards the capillaries, and in some cases the flow of the fluid is assisted by lymph-hearts, muscular dilatations in the course of certain of the vessels. The lymphatics of the intestine have an important function in the absorption of fats, and are known as lacteals (lc.) The nervous system attains a complexity, both anatomical and histological, unknown in the rest of the animal kingdom. It arises, as in other Chordata, from a dorsal medullary groove the edges of which unite and enclose a tube. From the ectoderm lining the tube the whole central nervous system, or neuron, is formed ; its lumen forms the neuroccele or characteristic axial cavity of the neuron. So far the agreement with the lower. Chordata is com- plete, but a fundamental advance is seen in the fact that at an early period — before the closure of the medullary groove — the anterior end of the neuron undergoes a marked dilatation and forms the rudiment of the brain, the rest becoming the spinal cord. Moreover, as growth goes on, a space appears in the meso- derm immediately surrounding the nervous system, and forms the neural or cerebro-spinal cavity already referred to (Fig. 760, c.s. c.), so that the neuron, instead of being solidly imbedded in mesoderm, lies in a well-marked and often spacious tube enclosed by the neural arches of the vertebra?, and in front by the cranium (Fig. 760, B-D). The spinal cord (Fig. 777) is a thick- walled cylinder, con- tinuous in front with the brain. It is traversed from end to end by a narrow central canal (3\ lined by ciliated epithelium derived from the superficial layer of in-turned ectoderm cells. The dorsal surface of the cord is marked by a deep, narrow, longi- tudinal cleft, the dorsal fissure (2), the ventral surface is similarly scored by a groove, the ventral fissure (1) ; owing to the presence of these fissures a transverse section presents two almost semi- circular halves with their straight edges applied to one another and joined in the middle b}^ a narrow bridge (4, 5) in which the central canal lies. The cord is made up of two kinds of tissue. Surrounding the central canal and having a somewhat butterfly-shaped transverse section, is the grey matter (a, e) consisting of delicate, inter-twined, non-medullated nerve-fibres, amongst which are numerous nerve- cells. The superficial portion is composed of medullated nerve-fibres running longitudinally, and is called the white matter (6, 7, 8). In both grey and white matter the nervous elements are supported by a non-nervous tissue called neuroglia, formed of branched cells. From the cord the spinal nerves are given off. They arise in pairs from the sides of the cord, and agree in number with the myomeres. Each nerve arises from the cord by two roots, a XITI PHYLUM CHORDATA dorsal and a ventral. The dorsal root (Fig. 779, d. r.) is dis- tinguished by the presence of a ganglion (gn. d.r.) containing nerve-cells, and its fibres are usually wholly afferent, conveying impulses from the various parts and organs of the body to the central nervous system ; the ventral root (v. r.) is not ganglionated, and its fibres are efferent, conveying impulses from the neuron outwards. Each root arises from one of the horns of the grey matter, and the two mingle to form the trunk (sp. 1-3) of the nerve, which emerges from the spinal canal usually between the FIG. 777.— Transverse section of spinal cord. 1. ventral fissure ; 2, dorsal fissure ; 3, central canal ; 4, 5, bridges connecting grey matter of right and left) sides ; 6, 7, 8, white matter ; 9, dorsal root of spinal nerve ; 10, ventral root, a, />, dorsal horn of grey matter ; c, Clarke's column ; e, ventral horn. (From Huxley's Physiology.) arches of adjacent vertebrae. Soon after its emergence it divides into two chief divisions, dorsal (d.) and ventral (sp. 1, &c.). The spinal nerves supply the muscles and skin of the trunk and limbs, and are therefore spoken of as somatic nerves. Frequently groups of nerves unite with one another to form more or less complex networks called plexuses. Closely associated with the spinal are the sympathetic nerves (Fig. 779, sym). They take the form of paired longitudinal cords with ganglia (sym. gn.) at intervals, lying one on each side of the aorta in the dorsal wall of the coelome. They contain both 100 ZOOLOGY SECT. afferent and efferent fibres, the afferent derived from the dorsal, the efferent from the ventral roots of the spinal nerves, and both traceable, through those roots, into the grey matter of the cord. The sympathetic nerves supply the enteric canal and its glands, the heart, blood-vessels, &c., and are therefore denominated splanchnic nerves. As already mentioned, the anterior end of the nervous system undergoes, at a very early period, a marked dilatation, and is distinguished as the brain (Fig. 778). Constrictions appear in the dilated part and divide it into three bulb-like swellings or vesi- cles, the fore-brain (A,/, b.), mid-brain (m. b.) and hind-brain (h. b.). Soon a hollow outpushing grows forwards from the first vesicle (B, prsen), and the third gives off a similar hollow outgrowth (cblm.) from its dorsal surface. The brain now consists of five divisions : (the prosencephalon (prs. en.) and the diencephalon (dien.), derived from the fore-brain :^Cthe mid-brain or mesencephalon (m. b.) which remains unaltered :^ th^ epencephalon or cerebellum (cblm.), and the metencephalon or medulla oblongata (med. obi.), derived from the hind-brain.1 Additional constrictions appear in the medulla oblongata giving it a segmented appearance, but they disappear as development proceeds, and, whatever may be their significance, have, nothing to do with the main divisions of the adult organ. The original cavity of the brain becomes corre- spondingly divided into a series of chambers or veritricles, all * communicating with one another and called respectively the fore- ventricle or prosoccele, third ventricle or djacasle, mid-ventricle or mesoccele, cereoeUar ventricle or epicode, and fourth ventricle or iriefaccele. fn some Fishes the brain consists throughout life of these five divisions only, but in most cases the prosencephalon grows out into paired lobes, the right and left cerebral hcniispheres or parencephala (I-L, c.h.), each containing a~cavity, the lateral venixicle or paraccele (pa. cw.) which communicates with the diacoele (di. cos.) by a narrow passage, the foramen of Monro (f.m.). Moreover, each hemisphere gives off a forward prolongation, the olfactory bulb or rhinencephalon (plf. L), containing an olfactory ventricle or rhinoccele (rh. cos.) : when there is an undivided prosen- cephalon, the olfactory bulbs (C, D, olf. I.) spring from it. In the embryo of some forms there is a median unpaired olfactory bulb, like that of Amphioxus. The part of the cerebral hemisphere with which the olfactory bulb is immediately related is the olfactory lobe. The brain undergoes further complications by the unequal thickening of its walls. In the medulla oblongata the floor becomes greatly thickened (D, H, K), while the roof remains thin, con- 1 The prosencephalon is sometimes called the J^sncephalon, the epencephalon the KQ^tencephcdon, and the metencephalon the inyelencephalon. XIII PHYLTM CHORDATA j*iH$jfj{ o^iJFlP c^; 3 K j *._•r. 1— ~>, branchial branches ; X. c, cardiac branch ; X. . pineal body; i>n. >:. pineal eye ; sp. c. spinal cord ; sp. 1—3, ventral branches of spinal nerves ; tym. sympathetic nerve ; sym. gn. sympathetic ganglion ; r. r. ventral root. It is a very small and purely motor nerve, supplying only the superior oblique muscle of the eye. The fifth or trigeminal nerve (Fig, 779, V.) is of great size and wide distribution. It arises from the side of the medulla, fre- quently by two roots, a dorsal and a ventral, thus resembling in its origin a spinal nerve. Near its origin it enters a ganglion, the trigeminal or Gasserian ganglion (g. gn.\ which may be partly divided into two parts, an antero-dorsal and a postero-ventral. The trunk of the nerve early divides into two principal branches, the ophthalmic and the mandibular (V. md.) : the latter sends off a maxillary nerve (V. mx.), and we thus get the three divisions to which the name trigeminal is due. The ophthalmic nerve frequently divides into two branches, a superficial (V. o. s.) and a deep (V. o.p~), xin PHYLUM CHORD ATA 105 the former present in Fishes only : the latter in some Fishes a semi-independent nerve given off separately from the dorsal part of the trigerninal ganglion. The ophthalmic is purely sensory, and supplies the skin in the neighbourhood of the mouth and certain parts in the orbit. The maxillary nerve (V. mx.) is also sensory : it supplies the parts in relation with the upper jaw, including the teeth. The mandibular nerve (V. md.) is partly sensory, partly motor : it supplies the muscles of the jaws, the skin and teeth of the lower jaw, and sends off a gustatory nerve or nerve of taste to the epithelium of the tongue in the higher forms. The ophthalmic nerve is connected by a branch with the ciliary ganglion. The sixth or abducent (Figs. 779 and 787, VI.) is a small motor nerve, arising from the ventral region of the medulla, and sup- plying the external rectus muscle. of the eye. We thus have the remarkable fact that out of ten, or at the most twelve, cerebral nerves, three are devoted to the supply of the six small muscles • by which the eye-ball is moved, and of those by which the accommodation of the eye for varying distances is effected. The seventh or facial (Fig. 779, VII.) is, like the fifth, a mixed nerve in the lower Craniata, i.e., contains both sensory and motor fibres. It arises from the side of the medulla, a short distance behind the fifth, and is dilated near its origin into & facial ganglion. It has two chief branches, a palatine (VII. p.\ which passes in front of the mandibulo-hyoid gill-cleft, and supplies the mucous membrane of the palate, and a hyomandibular (VII. h.), which passes behind the same cleft and sends branches to the lower jaw and to the hyoid arch. In most aquatic Vertebrata an ophthalmic branch is given off from the trunk of the nerve, and usually accompanies the superficial ophthalmic division of the fifth. In the higher Vertebrata the seventh becomes a purely motor nerve, supplying the muscles of the face. The eighth or auditory nerve (VIII.) arises immediately behind the seventh, with which it is intimately connected at its origin. It is a purely sensory nerve, supplying the organ of hearing, i.e., the epithelium of the membranous labyrinth presently to be described. The ninth or glossopharyngcal (IX.) is a mixed nerve : it arises from the lateral region of the medulla, behind the organ of hearing, and is connected at its origin with the vagus ganglion (see below). Its trunk passes downwards and forks over the second gill-cleft, sending an anterior branch to the hyoid arch which bounds the cleft in front, and a posterior branch to the first branchial arch which bounds it posteriorly. Thus the entire nerve supplies the second gill-pouch, including both branchial filaments and muscles : its anterior branch goes to the posterior hemibranch of the hyoid arch, its posterior branch to the anterior hemibranch of the first branchial arch. In the air-breathing 106 ZOOLOGY SECT. Vertebrata, in which gills are absent, the glossopharyngeal sends a gustatory nerve to the tongue and supplies the pharnyx. In Fishes a nerve known as the lateral (X. /.) takes its origin above the glossopharyngeal, sometimes in front of the latter, sometimes behind it. It usually joins the trunk of the following or tenth nerve, but becomes separate again and runs back- wards, supplying the cutaneous sense-organs of the lateral line (see below). The tenth nerve (X.), called the vagus or pneumogastric, is dis- tinguished by its wide distribution. It arises by numerous roots from the side of the medulla, the roots uniting into a stout trunk with a vagus ganglion at its origin. From the trunk are given off, in the first place, branchial nerves (X. br. 1-5), corre- sponding in number and position to the gill-slits from the third to the last inclusive. Each branchial nerve behaves in exactly the same way as the glossopharyngeal ; it forks over the gill-pouch to which it belongs, sending one branch to the anterior, another to the posterior wall of the pouch. Thus each gill-pouch has its own nerve while each gill receives its supply from two sources; for instance, the gill of the second branchial arch has its anterior hemibranch innervated from the first, its posterior hemibranch from the second branchial branch of the vagus. The vagus also gives off a cardiac nerve (X. c) to the heart, a gastric nerve (X. g) to the stomach. In the air-breathing Craniata there are, of course, no branchial nerves; but the vagus still retains control of the respiratory organs by giving origin to pulmonary nerves to the lungs and laryngcal nerves to the larynx. The eleventh or accessory nerve (XL), which is recognisable in some Fishes as a part of the vagus, is a distinct nerve in higher forms, and consists of cerebral and spinal portions, so that it occupies an intermediate position between the purely cerebral and the purely spinal nerves. It acts in higher Craniates mainly as the motor nerve for certain muscles of the shoulder. The twelfth or hypoglossal (XII.) arises from the ventral aspect of the medulla oblongata, after the manner of the ventral root of a spinal nerve. It is purely motor, and supplies the muscles of the tongue and certain neck-muscles. In the Amphibia its place is taken by the first spinal nerve, and there is no doubt that it is to be looked upon as a spinal nerve which has become included in the cranial region : even in some Fishes it passes out through the skull. The sympathetic nerve (sym.) is continued into the head and becomes connected with some of the cerebral nerves. Sensory Organs. — The whole surface of the body forms an organ of touch, but special tactile organs are more or less widely distributed. End-buds consist of ovoidal groups of sensory cells supplied by a special nerve : touch-cells (Fig. 780, A) are nerve-cells XIII PHYLUM CHORDATA 107 occurring in the dermis at the termination of a sensory nerve : touch- corpuscles (B) are formed of an ovoidal mass of connective-tissue containing a ramified nerve, the terminal branches of which end in touch-cells : Pacinian corpuscles (C) consist of a terminal nerve- branch surrounded by a complex laminated sheath. Touch- corpuscles and Pacinian bodies are found only in the higher forms. In Fishes, characteristic sense-organs are present, known as the ncuromast-organs or organs of the lateral line. Extending along the sides of the trunk and tail is a longitudinal streak, due to the presence either of an open groove or of a tube sunk in the -A Fn;. 7 SO.- A, tactile spot from skin of Frog, a, touch-cells ; b, epidermis ; N, nerve. B, tactile corpuscle from dermal papilla of human hand, a, connective-tissue investment ; 6, touch- cells ; n, n', n", n'", nerve. C, Pacinian corpuscle from back of Duck. A, A', neuraxis ; ./A, central knob and surrounding cells; L,Q, investing layers; NS, medullary sheath of nerve. (From Wiedersheim's Vertebrata.) epidermis, and continued on to the head in the form of branching grooves or canals (Fig. 781, A). These organs and also certain others in the form of pits or of unbranched canals, are lined with epithelium (B), some of the cells of which are arranged in groups, the ncuromasts, and have the form characteristic of sensory cells (I) produced at their free ends into hair-like processes (c) : they are innervated by the lateral branch of the vagus, and in the head, by the seventh and sometimes also the ninth nerve. At their first appearance in the embryo the organs of the lateral line are distinct, segmentally-arranged patches of sensory epithelium in intimate connection with the ganglia of the third, fifth, seventh, ninth, and tenth nerves. Cutaneous sense-organs of the lateral- 108 ZOOLOGY SECT. line system, having at first a metameric arrangement, also occur in the aquatic Amphibia. The function of the neuromast-organs has been shown to be to enable the animal to detect vibrations in the water of too low a frequency to form a sound capable of perception by the ear. R f FIG. 781.— A, Sensory canals of the left side of the head of a Bony Fish. (From the Cambridge Natural History, after Cole.). B, organ of the lateral line (neuromast)in a tailed Amphibian (semi-diagrammatic) a, epidermic cells, through which are seen 6, sensory cells ; c, sensory hairs ; N, nerve ; R, hyaline tube. (From Wiedersheim's Vertebrata.) The sense of taste has for its special organs taste-buds (Fig. 782), similar in general character to the end-buds in the skin, and composed of groups of narrow rod-shaped cells. In Fishes these are widely distributed in the mouth and branchial cavities, also on the outer surface of the head, and in some Fishes XIII PHYLUM CHORDATA 109 over almost the whole surface of the body. In higher Craniates, with the exception of Birds, they are chiefly confined to the epi- Fio. 782. — A, vertical section of one of the papilla; of the tongue of a Mammal, rf, sub- imicosa; c. epithelium ; n. nerve-fibres; t. taste-buds. B, two taste-buds, c. covering cells shown in lower bud; 0. — A, part of a urinary tubule with blood-vessels, ai, artery ; yt, Malpighian capsule con- taining glomerulua ; »•. veinlet returning blood from capillary network (to the right) to vein ri ; ra, afferent vessel of glomerulus ; re, efferent vessel. B, longitudinal, and C, transverse sections of urinary tubules, a, secreting part of tubules ; 6, conducting part of tubules ; r. capillaries ; n. nuclei. (From Foster and Shore's Physiology.) pores, but more usually the pronephric duct in the female assumes the functions of an oviduct and the mesonephric duct in the male those of a spermiduct (cf. p. 120). The pronephros is almost always functionless in the adult, and usually disappears altogether. The mesonephros is generally the functional kidney in the lower Craniata, in which, as a rule, no metanephros is developed, and the mesonephric duct, in addition to carrying the seminal fluid of the male, acts as a ureter. In the higher forms the mesonephros atrophies, and the metanephros is the functional kidney, the metanephric duct becoming the ureter. The kidney — meso- or meta-nephros — of the adult is a massive gland of a deep red colour made up of convoluted urinary tubules (Fig. 790), separated from one another by connective-tissue con- 118 ZOOLOGY SECT. taining an abundant supply of blood-vessels. The tubules "e lined by a single layer of glandular epithelial cells (B, C) and each ends blindly in a globular dilatation, the Malpighian capsule (A, yl.), lined with squamous epithelium. In many of the lower Craniata, a branch goes off from the tubule, near the Malpighian capsule, and, passing to the ventral surface of the kidney, ends in a ciliated funnel-like body (Fig. 791, nst.), resembling the nephrostome of a worm, and, like it, opening into the coelome. At their opposite ends the tubules join with one another, and finally discharge into the ureter. The renal arteries branch extensively in the kidne}^ and give off to each Malpighian capsule a minute afferent artery (Fig. 790, A, va.) : this pushes the wall of the capsule before it, and breaks up into a bunch of looped capillaries, called the . glomerulus, sus- pended in the interior of the capsule. The blood is carried off from the glomerulus by an efferent vessel (ve.), which joins the general capillary system of the kidneys, forming a network over the urinary tubules : finally, the blood is returned from this network to the renal vein. The watery constituents of the urine are separated from the blood in traversing the glomerulus, and flowing down the tubule, take up and dissolve the remaining constituents — urea, uric acid, &c. — which are secreted by the cells of the tubules. The development of the kidney reveals a resemblance to the coelomoducts of Annulata which would hardly be suspected from its adult structure. The pronephros (Fig. 791, A, p. nph.) originates as two or three coiled tubes formed from mesoderm in the body- wall at the anterior end of the coelome ; they are arranged meta- merically, and each opens into the coelome by a ciliated funnel (nst.). Obviously such tubes are coelomoducts : their chief pecu- liarity is that their outer ends do not open directly on the exterior, but into a longitudinal tube, the pronephric or segmental duct (sg. d.)t which passes backwards and discharges into the cloaca. It seems probable that this arrangement is to be explained by supposing that the coelomoducts originally opened externally into a longitudinal groove, which, by the apposition of its edges, was converted into a tube. All three tubules of the pronephros open, by their ciliated funnels, into the narrow anterior end of the coelome, into which projects a branch of the aorta ending in a single large glomerulus. The pronephros soon degenerates, its tubules losing their connection with the pronephric duct (B), but in the meantime fresh tubules appear in the segments posterior to the pro- nephros, and together constitute the mesonephros or Wolffian body (B, ms. nph.), from wrhich the permanent kidney is formed in most of the lower Craniata. The mesonephric tubules open at one end into the pronephric duct (sg. d.), at the other, by ciliated XIII PHYLUM CHORDATA 119 f .lels (nst.\ into the coelome ; a short distance from the funnel each gives off a blind pouch, which dilates at the end and forms a cl mesonephros ; mt. n. d. metanephric duct; mt. nph. metanephros ; nst. nephrostome ; ovary ; p. n. d. and sg. d. pronephric duct ; p. nph. pronephros ; t. testis ; v. e. \ ov. vasa eflerentia. Malpighian capsule (m. c.),and a branch from the aorta entering it gives rise to a glomerulus. 120 ZOOLOGY SECT. In some forms the pronephric duct now becomes divided by a longitudinal partition into two tubes : one retains its connection with the mesonephrosand is known as the mesonephric or Wolffian duct(C,ms.n.d.): the other has no connection with the tubules, but opens into the coelome in the region of the vanishing pronephros, and is called the Miillerian duct (p. n. d.). In some Craniate the Miillerian appears quite independently of the Wolffian duct : the latter is then simply the pronephric duct after the union with it of the mesonephric tubules. In the higher Vertebrata, from Reptiles to Mammals, a diverti- culum (D, E, mt. n. d.) is given ofF from the posterior end of the Wolffian duct, which grows forwards and becomes connected with the hindmost tubules. In this way is formed a metanephros (mt. nph.), which forms the permanent kidney, and a metanephric duct (mt. n. d.), which gives rise to the ureter. The Wolffian body ceases to discharge a renal function, and becomes a purely vestigial organ. In many Fishes there is a dilatation of the ureter, the urinary bladder, which serves as a receptacle for the urine. In the higher Craniata the ventral wall of the cloaca sends off a pouch, the allantoic Uaddcr (al. U.\ which serves the same purpose, although morphologically an entirely different structure. The gonads (gon.) are developed as ridges growing from the dorsal wall of the coelome and covered by coelomic epithelium, from the cells of which, as in so many of the lower animals, the ova and sperms are derived. The testis consists of crypts or tubules, lined with epithelium, and usually discharging their pro- ducts through delicate vasa efferentia (D, v. e.) into the Wolffian duct, but in some groups into the coelome. The sperms are always motile. The ovary is formed of a basis of connective- tissue or stroma, covered by epithelium, certain of the cells of which become enlarged to form ova. In the majority of cases the ova are discharged from the surface of the ovary into the coelome and thus into the open ends of the Miillerian ducts (E,^>. n. d.}, which thus function simply as oviducts, having no connection in the adult with the urinary system. In some groups the ova, like the sperms, are shed into the coelome and escape by the genital pores, and in many bony Fishes, the ovary is a hollow organ, as in Arthropoda, discharging its ova into an internal cavity, whence they are carried off by a duct continuous with the gonad. A few Craniata are normally hermaphrodite, but the vast majority are dioecious, hermaphroditism occurring, however, occasionally as an abnormality. In close topographical relation with the urinogenital organs are found certain " ductless glands," the adrenals or inter- and supra- renal bodies. They are developed partly from ridges of the dorsal XIII PHYLUM CHORDATA 121 wall of the coelome — i.e., from mesoderm, partly from the sympa- thetic ganglia. There may be numerous adrenals segmen tally arranged, or a single pair. Like other ductless glands the adrenals produce an internal secretion, which mingles with the blood and produces physiological effects on other parts. Development. — The ovaof Craniata are usually telolecithal, but the amount of food-yolk varies within wide limits. When it is small in quantity segmentation is complete but usually unequal, when abundant, incomplete and discoidal. In the latter case the embryo proper is formed, as in Cephalopods, from a comparatively small portion of the oosperm, the rest giving rise to a large yolk-sac. There is never a typical invaginate gastrula, as in Amphioxus, but in some of the lower Craniata a gastrula stage is formed by a sp.c me. rig enl -> y* msd Fin. 792. — Transvcrve section of earlier (A) and later (B) embryos of Frog. caul, coelome ; cfl'. pro- longation of coelome into protovertebra ; ent. mesenteron ; med. (jr. medullary groove ; m*d. mesoderm ; nch. notochord ; pr.v. protovertebra ; s(t. d. segmental duct ; som. somatic layer of mesoderm ; up. c. spinal cord ; sj>t. splanchnic layer of mesoderm ; yk. yolk-cells. (After Marshall.) combination of inpushing and overgrowth: details will be given in the sections on the various groups. In the higher forms a gastrula cannot be recognised with absolute certainty. The mode of development of the mesoderm and of the coelome differs strikingly from the process we are familiar with in Amphi- oxus. At an early stage the mesoderm is found in the form of paired longitudinal bands (Fig. 792, A, msd.) lying one on each side of the middle line, where they are separated from one another by the medullary groove (md. yr.) and the notochord (nch.), and com- pletely filling the space between the ectoderm and the endoderm. In all probability the mesoderm is derived from both of the primi1 tive germ-layers. Each mesoderm-band becomes differentiated into a dorsal portion, the vertebral plate, bounding the nervous system and notochord, and a ventral portion, the lateral plate, 122 ZOOLOGY SECT. bounding the mesenteron. The vertebral plate undergoes meta- meric segmentation, becoming divided into a row of squarish masses, the protovertebrce or mesodcrmal segments (B, pr. v.) : the lateral plate splits into two layers, a somatic (som.}, adherent to the ectoderm, a splanchnic (&pl.)t to the endoderm. The space between the two is the ccelome (cceL), which is thus a schizocwle, or cavity hollowed out of the rnesoderm, and is, except in the head-region in the Lampreys (p. 136), at no stage in communication with the mesentercn, like the ccelomic pouches of Amphioxus. The dorsal portion of the ccelome assumes the character of a series of paired diverticula of the main ventral part, each situated in the interior of a protovertebra ; but such an arrangement is temporary, and these protovertebral cavities early disappear. From the dorsal portions of the protovertebrae the myomeres are formed, from their ventral portions the vertebra. The development of the principal organs has been described, in general terms, in the preceding account of the organs themselves : it will be convenient to defer further consideration of this subject until we come to deal with the development of the various types of Craniata, and with the embryological characteristics of the classes and sub-classes. Metamerism. — A tendency, more or less strongly marked, to a serial repetition of parts is to be observed in a number of different systems of organs. Instances of this have already been pointed out in the skeleton, and the muscular, nervous and excretory systems. This phenomenon seems to lead to the conclusion that the structure of the Craniata can be understood only when they are regarded as metamerically-segmented animals. The phase of metamerism presented by the Craniata is, however, widely different from that which prevails in the segmented Invertebrates. In the latter the segmentation is usually quite distinctly pro- nounced externally, and it may involve a metameric division ex- tending to the ccelome as well as to the various systems of internal organs. In the Craniata, on the other hand, segmentation is never visible on the exterior, and in the adult condition the ccelome never shares in the division. Even in the case of the organs which present metameric characters, the metamerism often appears indefinite and uncertain : thus, as already pointed out, the segmentation of the spinal column, which in the adult is the most pronounced of all, does not coincide with the segmentation of the muscular and nervous systems. Yet when we take the phenomena of embryonic development into account, it becomes sufficiently clear that in the Craniata we have to do with animals possessing a metameric segmentation of the same general type as that possessed by Amphioxus, and that the apparent anomalies are due to processes of secondary modification. It is in the trunk region that the metamerism is most strongly xni PHYLUM CHORDATA 123 pronounced, and that more particularly in the lower groups. In the head there is great specialisation in co-ordination with the presence in this region of the brain, the chief organs of special sense, and the mouth and jaws ; so that, though there are indications of metamerism of various parts, it is only by the study of development that it is possible to interpret the structure of the head in terms of a metameric segmentation which becomes much modified and disguised in the adult animal. When the develop- ment is followed out, it becomes evident that, as in the Arthropoda, the head in Craniata is formed as a result of a process of fusion between a number of metameres, the individual- ity of which is quite evident in early stages, more particularly among lower forms, being most pronounced in the region behind the auditory capsules. Distinctive Characters. — The Craniata may be defined as Euchorda in which the notochord is not continued to the end of the snout, but stops short beneath the fore-brain, some distance from its anterior end. A skull is always present, and there are usually paired limbs. The ectoderm is many-layered and is never ciliated in the adult, and only rarely in the larva. The pharynx is of moderate dimensions, and is perforated by not more than seven pairs of gill-slits (except in some Cyclostomes). The gill-pouches do not open into an atrium. The liver is large, massive, and not obviously tubular. There is a muscular chambered heart, and the blood contains red corpuscles. The renal tubules unite to form large paired kidneys and open into ducts which discharge into or near the posterior end of the intestine. The brain is com- plex, and there are at least ten pairs of cerebral nerves : the spinal nerves are, except in Cyclostomes, formed by the union of dorsal and ventral roots. Paired eyes of great complexity, derived in part from the brain, are present ; and there is a pair of auditory organs. There is typically a single pair of gonads, and the re- productive products are usually discharged by ducts derived from the renal system. There is never a typical invaginate gastrula, and the mesoderm arises in the form of paired longitudinal bands which subsequently become segmented. The coelome is nearly always developed as a schizocoele. CLASS I-CYCLOSTOMATA. The Cyclostomata, or Lampreys and Hags, are eel-like animals, distinguished from all other Craniata by the possession of a suctorial mouth devoid of functional jaws, by the single olfactory organ, and by the absence of lateral appendages, or paired fins. 124 ZOOLOGY SECT. 1. EXAMPLE OF THE CLASS. — THE LAMPREY (Petromyzon), Three species of Lamprey are common in the Northern Hemi- sphere : the Sea-lamprey (P. marinus), which attains a length of a metre; the Lampern, or common fresh-water Lamprey (P. fluvia- tilis), which may reach a length of about 90 cm. ; and the Sand- pride, or lesser fresh-water Lamprey (P. planeri), not exceeding 45 cm. in length. In the Southern Hemisphere the Lampreys belong to two genera : Mordacia, found on the coasts of Chili and Tasmania, and Geotria, in the rivers of Chili, Australia, and New Zealand. Both genera differ from Petromyzon in minor details only. External Characters. — The head and trunk (Fig. 793) are nearly cylindrical, the tail-region compressed or flattened from Fro. 793.— Petromyzon marinus. Yentralr(A), lateral (B), and dorsal (C) views of the head. lii. <'/. 1, first gill-cleft ; huc.f. buccal funnel ; eye, eye; mtk. mouth; iin. up. nasal aperture; p. papillse ; pn. pineal area ; tl. £'. <:f. teeth of buccal funnel ; <-». teeth of tongue. (After W. K. Parker.) side to side. At the anterior end, and directed downwards, is a large basin-like depression, the buceal funnel (buc. /.), surrounded with papillae (p) and beset internally with yellow, horny teeth (tl — t'3). At the bottom of the funnel projects a prominence, the so-called "tongue" (£4), also bearing horny teeth, and having immediately above it the narrow mouth (mth.). On the dorsal surface of the head is the single median nostril (na. ap.), and immediately behind it a transparent area of skin (pn.) indicates the position of the pineal organ. The paired eyes have no eyelids, but are covered by a trans- parent area of skin. The gill-slits (br. d. 1) are seven pairs of small apertures on the sides of the head, the first a little behind the xni PHYLUM CHORDATA 125 eyes. On the ventral surface, marking the junction between trunk and tail, is the very small anus (Fig. 802, «..), lying in a slight depres- sion, and having immediately behind it a small papilla pierced at its extremity by the nrinogcnital aperture (#.). There is no trace of paired appendages, and the only organs of locomotion are the unpaired fins. Two dorsal fins of approximately equal dimen- sions, separated by a notch, and a caudal fin are present, the second dorsal being continuous with the caudal. Lampreys prey upon Fishes, attaching themselves to the bodies of the latter by the sucker-like mouth, and rasping off their flesh with the armed tongue. They are often found holding on to stones by the buccal funnel, and under these circumstances? perform regular respiratory movements, the branchial region ex- panding and contracting like the thorax of a Mammal. The reason of this is that when the animal is adhering by the mouth the respiratory current cannot take its usual course — entering at the mouth and leaving by the gill-slits — but is pumped by muscular action both into and out of the branchial apertures. The skin is soft and slimy, mottled greenish-brown in P. marinus, bluish above and silvery on the sides in the fresh- water species. The epiderm contains unicellular glands, the secretion of which gives its slimy character to the skin. The segmented sense-organs take the form of a lateral line which is superficial, not enclosed in a canal, and of minute pits on the head. There is no trace of exoskeleton. Skeleton. — The axial skeleton of the trunk is very simple. There is a persistent notochord (Fig. "794, nch.) with a tough sheath composed of an inner fibrous and an outer elastic layer. Attached to the sides of the notochord are little vertical rods of cartilage (n. a.) arranged segmentally, bounding the spinal canal on each side, and corresponding to rudimentary neural and inter- neural arches. For the rest of its extent the spinal canal is enclosed only by tough, pigmented connective-tissue. Slender rods of cartilage support the median fins. The cranium also exhibits a very primitive type of structure. Its floor is formed by a basal plate (Fig. 795, b. pl.)t made by the union of the parachordals and trabeculse, and surrounding pos- teriorly the fore-end of the notochord. Immediately in front of the termination of the notochord is a large aperture, the basi- cranial fontanelle (b. cr.f.), due to the non-union of the posterior ends of the trabeculse; through it passes the pituitary pouch, pre- sently to be referred to (Fig. 798), on its way from the olfactory sac to the ventral surface of the notochord. Lateral walls extend upwards from each side of the basal plate, but the roof of the cranium is formed by membrane except at one point, where a narrow transverse bar (cr.- r.) extends across between the side- walls and furnishes a rudimentary roof. United with the posterior end 126 ZOOLOGY SECT. of the basal plate are the auditory capsules (au. c.), and the side- walls are pierced with apertures for the cerebral nerves (Nv. 2, Nv. 5, Nv. 8). So far the skull is thoroughly typical, though in an extremely simple or embryonic condition ; the remaining parts of it differ a good deal from the ordinary structure as described in the preceding section, and are in many cases very difficult of interpretation. The olfactory capsule (olf. c.) is an unpaired concavo-convex plate which supports the posterior wall of the olfactory sac and is pierced by paired apertures for the olfactory nerves. It is unique in being united to the cranium by fibrous tissue only. Extending outwards and downwards from each side of the basal plate is an inverted arch of cartilage, called the sub-ocular arch (Figs. 794 and 795, sb. oc. a.) from the fact that it affords a support olf:c ,, br.b.t br.b.s krb.:> I.C* * \ A /• 2 „ .. « / / / / sb.oc.a br.cU f c + L.C.S FIG. 794.— Fetromyzon marinus. Skull, with branchial basket and anterior part of verte- bral column. The cartilaginous parts are dotted, a. d. c. anterior dorsal cartilage ; a. lat. c. anterior lateral cartilage ; an. • . annular cartilage ; au. c. auditory capsule ; br. b. 1 — 7, verti- cal bars of branchial basket ; 1:: cl. 1 — 7, external branchial clefts; en. c. cornual cartilage; cr. r. cranial roof ; 1. c. 1 — 4, longitudinal bars of branchial basket ; Ig. c. lingual cartilage ; m. v. c. median ventral cartilage ; n.a, neural arch ; na. ap. nasal aperture ; nek. notochord ; Ni: 2, foramc Tor optic nerve ; olf. t. olfactory capsule ; pc. c. pericardial cartilage ; p. d. c. posterior dov.,,d c^ulage ; p. lat.c. posterior lateral cartilage ; sb. oc. a. subocular arch ; st. p. styloid process ; i>ty. c. styliform cartilage ; t. teeth. (After W. K. Parker.) to the eye. From its posterior end a slender styloid process ($t. p.) passes directly downwards and is connected at its lower end with a small cornual cartilage (en. c.}. In all probability the sub-ocular arch answers to the palato-quadrate or primary upper jaw, the styloid and cornual cartilages to the main part of the hyoid arch. In close relation with the angle of the sub-ocular arch is an upwardly directed plate, the posterior lateral cartilage (p. lat. c.). Connected with the anterior end of the basal plate is the large bilobed posterior dorsal cartilage (p. d. c.) ; it appears to be formed from the united anterior ends of the trabecul*. Below and pro- jecting in front of it is the anterior dorsal cartilage (a. d. c.\ which is probably homologous with the upper labial cartilage of some Fishes and Amphibians (see below). Also belonging to the series of labial cartilages are the paired anterior lateral cartilages (a. I. c.) XIII PHYLUM CHORD AT A 127 and the great ring-shaped annular cartilage (an. c.) which supports the edge of the buccal funnel. GL.d.C B — nc7t Cft.C FIG. 795.— Petromyzon marinus. Dorsal (A), ventral (B), and sectional (C) views of skull. The cartilaginous parts are dotted, a. •c > e« « ^ a? s -2 & s a 2 .. urn S S S3 S'.gS ISSN'S V^^ ,f\| llop^l^l!3 - >> « -^'ojj -j-^; 3 Illiall^ll § J*|-S3|S'S S^ fifl'Sl'i'^'0 a|«.s !!1!0^!> i^Pl^sIl lf*^i«-;| 61i-: 7, facial ; .AY. 8, auditory ; .AY. 10, vagus ; Nr. 13, hypoglossal ; olf. cp. olfactory capsule ; olf. I. olfactory lobe, with which the olfactory bulb is amalgamated ; olf. m. m. olfactory mucous membrane ; opt. I. optic lobe ; pn. parapineal organ ; pn. c. pineal eye ; pt>/. b. pituitary body ; pty. p. pituitary pouch ; sp. median septum of olfactory sac ; sp. 1, dorsal root of first spinal nerve. (Combined from figures by Ahlborn and Kaenische.) small left ganglion habenulse. The pineal eye is not an organ capable, like the paired eyes, of forming definite images of objects, but probably is capable of distinguishing differences in the intensity of the light. The optic nerves differ from those of most of the higher classes in the fact that a chiasma, or intercrossing of fibres between the nerves of the right and left sides, is not conspicuously developed. The spinal cord (Figs. 796 and 801, my.) is flattened and band-like. The dorsal roots of the spinal nerves alternate with the ventral roots, and do not unite with them to form a trunk : the dorsal roots are opposite the myocommas, the ventral opposite the myomeres. A sympathetic is represented. The hypoglossal is the first spinal nerve. Sensory Organs. — The external nostril (Fig. 796, na" Fig 798, XIII PHYLUM CHORDATA 133 na. ap.) leads by a short passage into a rounded olfactory sac (Fig. 796 na, Fig. 798) placed just in front of the brain and having its posterior wall raised into ridges covered by the olfactory mucous membrane (Fig. 798, olf. m. m.). From the bottom of the sac is given off a large pituitary pouch (Fig. 796, na, Fig. 798, pty. p.) which extends downwards and backwards, be- tween the brain and the skull-floor, passes through the basicranial fontanelle, and ends blindly below the anterior end of the notochord. The relations between the olfactory sac, the pituitary pouch, and the pituitary body are very remarkable. In the embryo, before n-ch Fir;. 709. — Petromyzon. Diagrams of four stages in the development of the olfactory and pituitary sacs. ///•. brain; ent. rncsenteroii ; inf. infundibulum; /. Ip. lower lip ; nch. ~noto- chord ; olf. s. olfactory sac ; pn. pineal body ; pty. s. pituitary sac ; stdm. stomodseum ; 11. l/i. upper lip. (Altered from Dohrn.) the stomodseum (Fig. 799, A, stdm.) communicates with the mesen- teron, two unpaired ectodermal invaginations appear in front of the mouth. The foremost of these is the rudiment of the olfac- tory sac (olf. s.). The other, which is situated between the olfactory sac and the mouth, is the pituitary sac (pty< s.\ which in this case opens just outside the stomoda3um instead of within it as in other Craniata : its inner or blind end extends to the ventral surface of the fore-brain and terminates just below the infundibulum (inf.). As development goes on, the olfactory arid pituitary invaginations become sunk in a common pit (B), which, by the growth of the 134 ZOOLOGY SECT. nd.s a.s.c jy.s.c sac FIG. 800.— Auditory sac of Fetromyzon. «. s. c. anterior semicircular canal ; aud.n. auditory nerve ; end. s. endolymphatic sac ; p.s.c. posterior canal ; sac. sacculus ; utr. utviculus. (After Retzius.) immense upper lip (up. /.), is gradually shifted to the top of the head (C, D), the process being accompanied by elongation of the pituitary sac, into which the olfactory sac opens posteriorly. Where the pituitary sac comes in contact with the infundibulum it gives off numerous small follicles which become separated off and give rise to the pituitary body (Fig. 798,2%. &.). Thus the entire nasal passage of the Lamprey, including its blind pouch, is a per- sistent pituitary sac into which the single olfactory organ opens. More- over, owing to the extraordinary displacement undergone during development, the pituitary sac perforates the skull-floor from above instead of from below, as in all other Craniata. The auditory organ (Fig. 800) is remarkable for having only two semicircular canals, corresponding to the anterior (a.s.c.) and posterior (p.s.c.) of the typical organ. Organs of taste are present on the wall of the pharynx between the gill-sacs. Urinogenital Organs. — The kid- neys (Figs 801 and 802, &) are long strap-shaped bodies developed from the mesonephros of the embryo. The tubules have no nephrostomes. Each is attached along one edge to the dorsal wall of the body-cavity by a sheet of peritoneum ; along the other or free edge runs the ureter (ur.\ which is the undivided pronephric duct. The ureters open posteriorly into a small urinogenital sinus (Fig. 802, u.g.s.), placed just behind the rectum, and opening, by a urino- genital papilla ^.g.p.), into a pit in which the anus (a) also lies. The side-walls of the sinus are pierced by a pair of small apertures, the genital pores (y\ which place its cavity in communication with the coelome. The gonad (Fig. 796, ov, Fig. 801, ts) is a large unpaired organ occupying the greater part of the abdominal cavity and suspended by a sheet of peritoneum. The sexes are separate, but ova have been found int FIG. 801.— Fetromyzon marinus. Transverse section of abdomen, cd. cardinal veins ; d. ao. dorsal aorta ; /. ?•. fin -rays (neural spines) ; /. t. fibrous tissue of spinal canal ; int. intestine, the line pointing to the spiral valve ; k, kidneys ; ly. sub- vertebral lymph-sinus ; m. body- muscles ; my, spinal cord ; nc. noto- chord ; n. ca. spinal canal ; ts. testis ; ur, ureter. (From Parker's Zootomy.) XIII PHYLUM CHORD ATA 135 in the testis of the male. The reproductive products are shed into the coslome and make their way by the genital pores into Fir;. 802.— Petromyzon marinus. The urinogenital sinus with posterior end of intestine and part of left kidney, a. anus ; int. intestine ; L: left kidney ; r. rectum ; u.ij.p. nrhio genital papilla; u.l. cl. blastocuile or segmentation-cavity ; k. keel ; mg. m. megameres ; mi. m. micromeres. (After Shipley and Kupffer.) large cells or megameres (mg.m.\ containing much yolk. In the blastula stage (D) the segmentation-cavity or blastoccele (bid.) is situated nearer to the upper than to the lower pole. A trans- verse semilunar groove appears which is bounded by a prominent \ ZOOLOGY SECT. rim towards the future dorsal and anterior side : this is the blastopore (blp.). The megameres become gradually enclosed by the micromeres as a result of a process which is partly invagina- tion, partly epiboly. During this process the segmentation-cavity becomes displaced by the archenfceron. The dorsal and ventral walls of the latter differ widely from one another, the ventral wall being composed of a thick mass of yolk-cells (megameres) while the roof is comparatively thin and consists of two or three layers of rounded cells. The lumen is a narrow, dorso-ventrally compressed cleft. When the process of gastrulation is completed, the blastopore takes up a position at the postero-dorsal end. The development of the central nervous system differs widely from the corresponding process in Amphioxus, and is only approached among the Craniata by the Teleostomi or Bony Fishes. The dorsal surface becomes nc ent end Fia. 804. — Fetromyzon. Sections of embryos. A, transverse section of the trunk-region. B, transverse «ection of the head-region. c&. ccelomic sacs ; ect. ectoderm, end. endoderm ; ent. enteric cavity ; so. mesoderm-strand ; m.c, mi: medullary keel and medullary cord ; nc. notochord. (From O. Hertwig ; A, after Goette, B, after Kupffer.) flattened along a narrow longitudinal area and along this a groove appears, which stops short just in front of the blastopore. The area along which the groove runs soon becomes raised up above the general surface so as to form a narrow longitudinal elevation. Sections of this stage show that the ectoderm has developed a thickening along the course of the longitudinal groove, and this comes to grow downwards towards the archenteron as a solid longitudinal medullary keel (Fig. 803, C, k ; Fig. 804, A, mk). This is the rudiment of the central nervous system. Subsequently the keel becomes separated off from the surface ectoderm, and lies below it as a solid cord. It is only at a considerably later period that a lumen appears in this cord, and gives rise to the ventricles of the brain and the central canal of the spinal cord. During the formation of the medullary keel the rudiment of the notochord is developed from the underlying endoderm very much as in Amphioxus (p. 59). On each side of the medullary cord and notochord is a group of XIII PHYLUM CHORDATA 137 l.l/ cells arranged as a longitudinal strand — the mesoderm plates. In the head-region (Fig. 804, B) a number of diverticula from the archenteron — ccelomic diverticula — are given off into the'se strands : in the trunk region (A) these are absent. The inner portion of the mesoderm on each side becomes divided up into a series of meso- dermal somites or protovertebrse, the lateral part remaining un- divided and forming the lateral plate. In this restriction of somite-formation to the part of the mesoderm immediately adjacent to the middle line, the Lamprey differs from Amphioxus and re- sembles all the rest of the Craniata. The blastopore does not close up, but is converted into the anus, so that there is no proctoda?um. The dorsal lip of the blastopore, very promin- A ent from the first, becomes produced to give rise to the rudiment of the tail region. The mouth is developed later than the anus by the formation of a stomodseal invagination. The young is hatched as a peculiar larval form called Ammocwtes (Fig. 805), which differs from the adult in several re- spects. The median fin is continuous. There is a semicircular, hood- shaped upper lip (u. /.) instead of the suctorial buccal funnel of the adult, and teeth are absent. A ciliated peripharyngeal groove encircles the pharynx in front and is continued backwards on the ventral side as a median groove opening behind into the thyroid gland, which is thus proved to be a special develop- ment of a structure corresponding to the endostyle of Amphioxus (p. 48) and the Tunicata. -The eyes are rudimentary and hidden beneath the skin ; the brain is of far greater proportional size than in the adult ; and, as already mentioned, the gill-pouches open into the pharynx in the normal manner. Fio. SOo.— Fetromyzon flu via tills. Head of larva. A, from beneath; B, from the side. br. 1, first branchial aperture ; eye, eye ; 1. I. lower lip ; na. ap. nostril ; u. L upper lip. (After W. K. Parker.) 2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. The Cyclostomata are Craniata in which the mouth lies at the bottom of a sucker-like buccal funnel, or in a depression edged with tentacles, and there are no jaws. Horny teeth are borne on the 138 ZOOLOGY SECT. interior of the buccal funnel and on the large " tongue." Paired fins are absent. There is no exoskeleton ; the skin is glandular. The vertebral column consists of a persistent notochord with a fibrous neural tube, in which rudimentary neural arches may be developed, The skull is largely or wholly roofed by membrane, and there is an extensive development of labial cartilages. The segments of the post-auditory region of the head are more distinct than in the rest of the Craniata. The enteric canal is straight, and there is no cloaca. The respiratory organs are six to fourteen pairs of gill- pouches. There is no conus arteriosus and no renal portal system. There are distinct cerebral hemispheres, which may be either hollow or solid ; the cerebellum is very small. Each optic nerve passes directly to the eye of its own side. The olfactory organ is single and median, but is supplied by paired olfactory nerves ; it opens into a large persistent pituitary sac which perforates the basis cranii from above. The auditory organ has one or two semicircular canals. The kidney is a mesonephros, the ureter a pronephric duct. The gonad is unpaired, and there are no gonoducts, the genital products making their exit by genital pores. The Class is divided into two Orders. ORDER 1. — PETROMYZONTES. Cyclostomata in which there is a well-developed dorsal fin and a complete branchial basket ; the pituitary sac terminates posteriorly in a blind pouch ; the gills open into a respiratory tube below the gullet. This order includes the Lampreys, which belong to the genera Petromyzon, Mordacia, Geotria, and Ichthyomyzon. ORDER 2. — MYXINOIDEI. Cyclostomata in which the dorsal fin is absent or feebly developed; the branchial basket is reduced to a vestige; the pituitary sac opens posteriorly into the mouth ; the gills open into the pharynx in the normal manner. This order includes the Hags or Slime-eels, belonging to the genera Myxine and Bdellostoma. 3. — COMPARISON OF THE MYXINOIDS WITH THE LAMPREY. The organisation of the Lampreys is so uniform that all that will be necessary in the present section is to indicate the principal points in which the Hags differ from them. Myxine is about the size of a fresh- water Lamprey — i.e. some forty-five cm. long : Bdellostoma is fully a metre in length. Both are remarkable for the immense quantities of slime they are capable of exuding from the general surface and from the seg- mentally arranged mucus-glands of the skin. It is said that two XIII PHYLUM CHORDATA 139 mlh nci.ein mlh specimens of Myxine thrown into a bucket of water are capable of gelatinising the whole with their secretion. The slime-glands of Myxine contain peculiar " thread-cells " containing a much- coiled thread which unwinds either before or after the discharge of the cell from the gland. Myxine approaches most nearly to the condition of an internal parasite of any Vertebrate ; it is said to attach itself to living Fishes and gradually to bore its way into the coelome, devouring the flesh as it goes. There is no true buccal funnel : the space oil which the mouth opens is edged with tentacles (Fig. 806) supported by cartilages ; there is a single median tooth above the oral aperture, and two rows of smaller teeth on the tongue. The papillae beneath the cone- like horny teeth bear a still closer superficial re- semblance to rudiments (or vestiges) of true cal- cified teeth than is the case in the Lamprey; but it appears that no odon- toblasts and no calcified substance of any kind are formed in connection with them. The nostril (na. ap.) is a large un- paired aperture situated in the dorsal margin of the buccal space, and is continued into a pas- sage, the pituitary sac, which opens into the pharynx. Myxine commonly lives nearly buried in mud, and the respiratory current passes through this passage to the gills. The only fin is a narrow caudal surrounding the end of the tail. The respiratory organs present striking differences in the two genera. In Bdellostoma there are in different species six to fourteen very small external branchial apertures (br. cl. 1) on each side, each of which communicates by a short tube with one of br.cl.i~ \ br.a-jo oes.ct.d, FIG. 806.— Head of Myxine glutinosa (A) and of Bdellostoma forsteri (B), from beneath, br. ap. branchial aperture ; br. cl. 1, first branchial cleft ; mth. mouth ; na. ap. nasal aperture ; ces. ct. d. oaso- phageo-cutaneous duct. The smaller openings in A are those of the mucus-glands. (After W. K. Parker.) 140 ZOOLOGY the gill-pouches, which is again connected with the pharnyx by another tube. Behind and close to the last gill-slit, on the left side, is an aperture leading into a tube, the aesophageo-cutaneous dud (ens. ct. d.\ which opens directly into the pharynx. In Myxine (Fig. 807) the tubes leading outwards from the gill- pouches all unite together before opening on the exterior, so that there is only a single external branchial aperture (br. ap.) on each side ; into the left common tube (c. br. t.) the cesophageo- cutaneous duct (ces. ct. d.) opens. In both genera the internal branchial apertures communicate directly with the pharynx ; there is no respiratory tube. The neural canal is over-arched merely by fibrous tissue (Fig. 807, n.t.) ; there is no trace of neural arches in the trunk, but in the posterior part of the caudal region both neural canal and notochord are enclosed in a continuous car- tilaginous plate. Similarly the roof of the skull is entirely membranous. The nasal passage (na. t.) is strengthened by rings of cartilage, and the buccal tentacles are supported by rods of the same tissue. Behind the styloid cartilage or hyoid bar (st.p.) is a rod connected below with the subocular arch ; it probably re- presents the first branchial bar. The "tongue" is supported by an immense cartilage (m. v. c.), corresponding to the lingual cartilage of the Lamprey. The branchial basket is quite rudi- mentary, being represented only by certain small irregular car- tilages, such as one in the walls XIII PHYLUM CHORDATA 141 of the cesophageo-cutaneous duct, and, in Myxine (Fig. 807, br. &.), one on the right side supporting the common external gill -tube. The myotomes of one side al- ternate with those of the other. The intestine is very wide. The liver consists of two separate portions, the ducts of which open separately into the gall-bladder. A pancreas-like gland is present in both Myxine and Bdellostoma. The brain differs considerably from that of the Lamprey, espe- cially in the larger hemispheres, absence of lateral ventricles, . 80S.— Auditory organ of Myxine. amp., amp.1 ampulla? ; cmL s. endolym- phatic sac ; s. c. semicircular canal ; utr. sac. utriculo-saeculus. (After Retzius.) and smaller mid-brain. O efferent artery. C°"lpai'ati< The dorsal and ventral roots of the spinal nerves unite instead of remaining separate. The eyes are vestigial and sunk beneath the skin, and the auditory organ (Fig 808) has only a single semi- circular canal, which, having an ampulla at each end, probably represents both anterior and posterior canals. Bdellostoma has a persistent prone- phros in the form of a paired irregularly ovoidal body situated just above the heart and consisting of a large number of tubules richly branched peripherally : the nephrostomes open into the pericardium. The tubules do not communicate with the pronephric duct in Myxine, but end blindly : in Bdellostoma they open into an incomplete longitudinal duct, which does not communicate with the per- manent kidney-duct. The functional kidney is the mesonephros, and is specially interesting from the fact that in Myxinoids it retains in the adult its primitive seg- mental arrangement. The ureter (prone- phric duct, Fig. 809, a) sends off in each segment a coiled tubule (b) with a single Malpighian capsule (c), into which a branch from the aorta (d) enters and forms a glomerulus. Myxine is hermaphrodite, the anterior part of the gonad being ovary, the pos- terior testis : in some the ovary is mature and the testis rudimentary, in others the 142 ZOOLOGY SECT. opposite condition holds good, so that, while hermaphrodite, each individual is either predominantly female or predominantly male. The eggs of both genera are of great proportional size, and those of Myxine are enclosed when laid in a horny shell bearing numerous hooked processes at each pole : by means of these the eggs are entangled together, and probably also attached to seaweed. In Bdellostoma stouti, the only Myxinoid of which the develop- ment is known, the eggs are elongated and cylindrical, and contain a large quantity of food-yolk. The segmentation is meroblastic, being confined to a germinal disc situated at one end of the elongated egg. The blastoderm thus formed extends gradually over the surface of the yolk, which it only completely encloses at a late stage, when the gill-clefts are all formed. Bdellostoma differs from Petromyzon and resembles the majority of the Craniata in the mode of development of the central nervous system, which is formed, not from a solid ectodermal keel, but from an open medullary groove the lips of which bend inwards and unite to form a medullary canal. 4. — GENERAL REMARKS. The Lampreys ai±d Hags are undoubtedly the lowest of craniate Vertebrata, but are in many respects so highly specialised that it is a matter of great difficulty to determine their affinities with the remaining classes. The structure of the vertebral column and of the Cranium are undoubtedly primitive in the extreme ; but in the development of what may be called the accessory portions of the skull, such as labial cartilages, they show a singularly high degree of specialisation. The branchial basket is quite sui generis, the theory that its vertical bars are true branchial arches, displaced outwards during development, being quite unproved. The absence of functional jaws is very remarkable, seeing that in the remaining Craniata these structures always bound the mouth at a period when the skull is in the stage of development in which it remains permanently in Cyclo- stomes : it is quite possible that their functionless condition may be due to degeneration accompanying the evolution of a suctorial mouth. The brain, in spite of its small size, is in some respects — notably in the presence of cerebral hemispheres— of a more advanced type than that of some of the true Fishes. The circumstance that the pituitary pouch perforates the skull-floor from above and becomes early associated with the olfactory sac, is unique among the Vertebrata. The kidney of Bdellostoma is of the most primitive type, and the presence of a large pronephros is a significant archaic character. The total absence of limbs may be a result of degeneration. XIII PHYLUM CHORDATA 143 The geographical distribution of the class is interesting from the fact that each order contains some genera which are mainly northern, others which are exclusively southern. Petromyzon is found on the coasts and in the rivers of Europe, North America, Japan, and West Africa; it is therefore mainly Holarctic. Ichthyomyzon is found on the western coasts of North America, Mordacia in Tasmania and Chili, Geotria in the rivers of Chili, Australia, and New Zealand. Myxine occurs in the North Atlantic and on the Pacific Coast of South America ; Paramyxine in the Pacific; Bdellostoma on the coasts of South Africa, New Zealand, and Chili. No undoubted fossil remains of Cyclo- stomes are known, but there is some reason to believe that a little fossil, Palwospondylus gunni (Fig. 810), dis- covered in the Devonian rocks of Scot- land, may be referable to this class. It is about an inch long, and shows two re- gions, the cranium and the vertebral column ; there is no trace of exoskeleton or teeth. The vertebral column is com- posed of calcified centra with neural arches ; haemal arches are present in the caudal region ; the structure of this part of the skeleton is thus of a distinctly higher type than in recent Cyclostomes, and this perhaps lends support to the view that the latter are degenerate. There is a caudal fin supported by slender, sometimes forked, rays. The cranium consists of an anterior, probably trabecular, region (t.p.), and of a posterior region (p.a.), which seems to answer to the parachordals and auditory capsules. Just in advance of the anterior region is a ring-shaped opening surrounded by cirri (c.) : this may be either the nasal aperture or the mouth. There are vestiges of upper and lower jaws, and about four branchial arches. The posterior region of the skull gives off paired plates (x.) which may perhaps represent pectoral fins. FIG. 810.— Palaeospondylus gunni (magnified), c. cirri ; p. a. parachordal and auditory region ; t.p. trabecular re- gion ; x, backward processes of skull. (After Traquair.) 144 ZOOLOGY SECT. CLASS II.— PISCES. The Pisces, including the cartilaginous Fishes, the bony Fishes, and the Dipnoi, are Craniata which have the organs both of respiration and of locomotion adapted for an aquatic mode of life. The chief, and in the majority the only, organs of respiration are the gills, which are in the form of series of vascular processes attached to the branchial arches and persisting throughout life. . The organs of locomotion are the paired pectoral and pelvic fins, and the unpaired dorsal, ventral, and caudal ; these are all supported by fin-rays of dermal origin. A dermal exoskeleton is usually present. In the endoskeleton the notochord is usually more or less completely replaced by vertebrae ; there is a well-developed skull and a system of well-formed visceral arches, of which the first forms upper and lower jaws, the latter movably articulating with the skull, and both nearly always bearing teeth. There is frequently an air-bladder, which in certain exceptional cases acquires the function of a lung or chamber for breathing air. The hypophysis is not in any way connected with the nasal chambers and lies within the cranial cavity. There is a pair of nasal chambers which only exceptionally communicate internally with the mouth- cavity. The auditory labyrinth contains the three typical semi- circular canals. The kidney is a persistent mesonephros. The first two sub-classes are nearly related to one another and are frequently regarded as sections of a single sub- class — the Chondrichthycs or Cartilaginous Fishes. Sub-Class I. — Elasmobranchii. The sub-class Elasmobranchii comprises the Sharks, Dog-fishes, and Rays. The skeleton of these fishes, like that of the Cyclo- stomes, is composed essentially of cartilage, and, though there may be calcification of the substance of the cartilage, true bony tissue, such as is found in all higher groups, is not present. The dermal fin-rays, supported on the cartilaginous skeleton of the fin, are of horn-like constitution. There is never (in recent forms) an operculum or gill-cover. A cloaca is present, the external opening of which serves as a common outlet for the rectum and the renal and reproductive ducts. Among some of the fossil repre- sentatives of this group are to be found the most primitive of all known Fishes. 1. — EXAMPLE OF THE SUB-CLASS : THE DOG-FJSH (Scyllium - canicula or Hemiscy Ilium modestum). General external features. — The general shape of the body (Fig. 811) may be roughly described as fusiforum ; at the anterior, or head-end it is broader and depressed ; posteriorly it tapers xiii PHYLUM CHORDATA 145 gradually and is compressed from side to side. The head termi- nates anteriorly in a short, blunt snout. The tail is narrow and bent upwards towards the extremity. The colour is grey with brown markings, or dark-brown above, lighter underneath. The entire surface is covered closely with very minute hard placoid scales or dermal teeth, rather larger on the upper surface than on the lower. These are pointed, with the points directed some- what backwards, so that the surface appears rougher when the hand is passed over it forwards than when it is passed in the opposite direction. When examined closely each scale is found to be a minute spine situated on a broader base. The spine consists of dentine covered with a layer of enamel ; the base is composed of bone, and the whole scale has thus the same essential structure as a tooth. Along each side of the head and body runs a faint depressed longitudinal line or slight narrow groove — the lateral FIG. 811.— Dog- Fisli (Hemiscyllium modestum). Lateral view. (After Waite.) line, marking the position of the lateral line canal, which contains integumentary sense-organs. As in Fishes in general, two sets of fins are to be recognised — the unpaired or median fins, and the paired or lateral. These are all flap-like outgrowths, running vertically and longitudinally in the case of the median fins, nearly horizontally in the case of the lateral : they are flexible, but stiffish, particularly towards the base, owing to the presence of a supporting framework of cartilage. Of the median fins two — the dorsal — are situated, as the name indicates, on the dorsal surface : they are of triangular shape ; the anterior, which is the larger, is situated at about the middle of the length of the body, the other a little further back. The caudal fin fringes the tail : it consists of a narrower dorsal portion and a broader ventral, continuous with one another round the extremity of the tail, the latter divided by a notch into a larger, anterior, and a smaller, posterior lobe. The tail is heterocercal, i.e., the posterior extremity of the spinal column is bent upwards and" lies in the dorsal portion of the caudal fin. The ventral or so-called anal fin is situated on the ventral surface, in Scyllium opposite the interval between the anterior and posterior dorsals, in Hemiscyllium behind the latter ; it resembles the latter in size and shape. VOL. II K 146 ZOOLOGY SECT. Of the lateral fins there are two pairs, the pectoral and the pelvic. The pectoral are situated at the sides of the body, just behind the head. The pelvic, which are the smaller, are placed on the ventral surface, close together, about the middle of the body. In the males the bases of the pelvic fins are united together in the middle line, and each has connected with it a clasper or copulatory organ. The latter is a stiff rod, on the inner and dorsal aspect of which is a groove leading forwards into a pouch-like depression in the base of the fin. The month — a transverse, somewhat crescentic opening — is situated on the ventral surface of the head, near its anterior end. In front and behind it is bounded by the upper and lower jaws, each bearing several rows of teeth with sharp points directed back- wards. The nostrils are situated one in front of each angle of the mouth, with which each is connected by a wide groove — the naso- buccal groove. In Hemiscyllium the outer edge of the groove is prolonged into a narrow subcylindrical appendage — the barbel. A small rounded aperture, the spiracle — placed just behind the eye — leads into the large pharynx. Five pairs of slits running vertically on each side of the neck — the branchial slits — also lead internally into the pharynx. A large median opening on the ventral surface at the root of the tail, between the pelvic fins, is the opening leading into the cloaca, or chamber forming the common outlet for the intestine and the renal arid reproductive organs. A pair of small depressions, the abdominal pores, situated behind the cloacal opening, lead into narrow passages opening into the abdominal cavity. The skeleton is composed entirely of cartilage, with, in certain places, depositions of calcareous salts. As in Vertebrates in general, we distinguish two sets of elements in the skeleton — the axial set and the appendicular, the former comprising the skull and spinal column, the latter the limbs and their arches. The spinal column is distinguishable into two regions — the region of the trunk and the region of the tail. In the trunk-region each vertebra (Fig. 812, A and B) consists of a centrum (c.), neural arch (%.«.), and tran verse processes (tr.pr). In the caudal region there are no transverse processes, but inferior or haemal arches (G,D, h.a.) take their place. The centra of all the vertebrae are deeply biconcave or amphicoslous, having deep conical concavities on their anterior 'and posterior surfaces. Through the series of centra runs the notochord (ntc.), greatly constricted in the centrum itself, and dilated in the large spaces formed by the apposition of the amphicoelous centra of adjoining vertebrae, where it forms a pulpy mass. The concave anterior and posterior surfaces of the centra are covered by a dense calcified layer, and in Hemiscyllium eight radiating lamellae of calcified tissue run longitudinally through the substance of the centrum itself. The centra, unlike XIII PHYLUM CHORDATA 147 those of the higher forms, are developed as chondrifications of the sheath of the notochord into which cells of the skeletogenous layer have migrated (p. 73). On the dorsal side of the row of centra the spinal column is represented by the series of neural arches which support the walls of the spinal canal. Owing to the presence of a series of intercalary cartilages the neural arches appear to be twice as numerous as the central Each neural arch consists on each side of a process, the neural process, given off from the centrum, and of a small cartilage, the neural plate (basi-dorsal), which becomes completely fused with the neural process in the n.c n.sp n.a h. Fia. 812.— Portions of the vertebral column of Scy Ilium canicula. A and B, from the trunk ; C and D, from the middle of the tail ; A and C, two vertebra in longitudinal section ; B and I), single vertebrae viewed from one end. li, calcified portion of centrum ; c1. centrum ; ./>>/•. foramen for dorsal, and ]',»•'. for ventral root of spinal nerve ; IL.OK haemal .arch (basi- venfral) ; h.c. haemal canal; li.xj). haemal spine; i.n.p. intercalary piece (interdorsal, or interneural plate); ii.a. neural arcnx; /i.e. neural canal; ,i.t>. neural plate (basi-dorsal) ; /'.*/<. neural spine; ntc. intervertebral substance (remains of notochord); r. proximal portion of rib ; tr.pr. transverse process (basal stump).* (From Parker's Practiced Zooloyy.) adult. Between successive neural plates, the width of each of which is only about half the length of the centrum, are interposed a series of plates of very similar shape, the interdorsal or inter- neural plates. Small median cartilages, the neural spines, fit in between both neural and interneural plates of opposite sides and form keystones completing the arches. The transverse processes are very short : connected with each of them is a rudimentary cartilaginous rib (r.) about half-an-inch in length. The cranium (Fig. 813) is a cartilaginous case, the wall of which is continuous throughout, and not composed, like the skulls of higher Vertebrates, of a number of distinct bony elements fitting in together. At the anterior end is a rostrum , consisting in Scyllium K 2 148 ZOOLOGY SECT. of three cartilaginous rods converging as they extend forwards and meeting at their anterior ends. At the sides of the base of this are the olfactory capsules (off.) — thin rounded cartilaginous sacs opening widely below, the cavities of the two capsules being separated from one another by a thin septum. The part of the roof of the cranial cavity behind and between the olfactory capsules is formed, not of cartilage, but of a tough fibrous membrane, and the space thus filled in is termed the anterior fonfanelle : in contact with the lower surface of the membrane is the pineal body, to be afterwards mentioned in the account of the brain. Each side-wall of this part of the skull presents a deep concavity — the orbit — over which netcr intcrc /1\ ffWff i7t.br. 3 e.p.br.5 FIG. 813. — Hemiscyllium, lateral view of skull with visceral arches and anterior part of spinal column ; the branchial rays are not represented. The skull and hyoid arch are somewhat drawn downwards, so that the hyoid and first branchial arch are not exactly in their natural relations, frr.1 — br. -5 branchial arches ; ccr. hy. ceratohyal ; tp. l>r. epibranchials ; ;/l. aperture for glossopharyngeal nerve ; />. hy. basihyal ; hy. run. hyomandibular ; interc. intercalary (interdorsal) plates; mck. Meckel's c; rtilage ; neur.- neural processes; olf. olfactory capsule ; oc. foramen for oculomotor ; opt. optic foramen ; pal. q. .palatoquadrate ; path, foramen for 4th nerve ; ph. i>r.1 first pharyngobranchial ; ph. br.5 fifth pharyngobranchial ; up. neural spines ; tr. transverse" processes and ribs ; tri. foramen for trigeminal nerve. is a ridge-like prominence, the supraorbital crest, terminating anteriorly and posteriorly in obscure processes termed respectively the precrbital and postorbital processes. Below the orbit is a longitudinal infra-orbital ridge. Behind the orbit is the auditory region of the skull — a mass of cartilage in which the parts of the membranous labyrinth of the internal ear are embedded. On the upper surface of this posterior portion of the skull are two small apertures situated in a mgsial depression. These are the openings of the aqueductHs vestibuli (endolymphatic ducts), leading into the vestibule of the membranous labyrinth. Behind this again is the occipital region, forming the posterior boundary of the cranial cavity, and having in the middle xin PHYLUM CHORDATA 149 a large rounded aperture — the foramen magnum — through which the spinal cord, contained in the neural canal and protected by the neural arches of the vertebrae, becomes continuous with the brain, lodged in the cranial cavity. Below this, on either side is an articular surface — the oceipttcd condyle — for articulation with the spinal column, and between the two condyles is a concavity, like that of the vertebral centra, containing notochordal tissue. A number of smaller apertures, or foramina, chiefly for the passage of nerves, perforate the wall of the skull. Behind and to the outer side of the anterior fontanelle is an aperture through which the ophthalmic branches of the fifth and seventh nerves leave the skull. Piercing the inner wall of the orbit are foramina through which the optic or second pair of cerebral nerves (opt.'), the oculomotor (oc.). or third, the pathetic, or fourth (path.), the tri- geminal, or fifth, the abducent, or sixth, and the facial, or seventh, gain an exit from the interior of the cranial cavity. Just behind the auditory region is the foramen for the glossopharyngeal, and in the posterior wall of the skull, near the^foramen magnum, is the foramen for the vagus. In close connection with the cranium are a number of cartilages composing the visceral arches (Figs. 813 and 814). These are in- complete hoops of cartilage, mostly segmented, which lie in the sides and floor of the mouth-cavity or pharynx. The first of these forms the upper and lower jaws. The upper jaw, or palatoquadrate (pal. q.\ consists of two stout rods of cartilage firmly bound to- gether in the middle line and bearing the upper series of teeth. The lower jaw, or Meckel's cartilage (mck.), likewise consists of two stout tooth-bearing cartilaginous rods firmly united together in the middle line, the union being termed the symphysis. At their outer ends the upper and lower jaws articulate with one another by a movable joint. In front the upper jaw is connected by a ligament with the base of the skull. Immediately behind the lower jaw is the hyoid arch. This consists of two cartilages on each side, and a mesial one below. The uppermost cartilage is the kyomandibutar (hv.mn.}: this articulates by its proximal end with a distinct articular facet on the auditory region of the skull ; distally it is connected by ligamentous fibres with the outer ends of the palatoquadrate and Meckel's cartilage. The lower lateral cartilage is the cerato- hyal (cer. Jiy.}. Both the hyomandibular and ceratohyal bear a number of slender cartilaginous rods — the branchial rays of the hyoid arch. The mesial element, or basihyal (b. hy.), lies in the floor of the pharynx. Behind the hyoid arch follow the branchial arches, which are five in number. Each branchial arch, with exceptions to be presently noted, consists of four cartilages. The uppermost of these — pharyngobranchial (ph. br.l-ph. br?) — lie in the dorsal wall of the pharynx, not far from the spinal column ; 150 ZOOLOGY the pharyngobranchials of the last two arches are fused together- The next in order — the epibranchicds (ep. br.) — with the exception of those of the last arch, bear a number of slender cartilaginous rods — the branchial rays— which support the walls of the gill-sacs ; and the next — the ceratobranchials (ccr. br.) — are, with the same exception, similarly provided. The hypobranchicds (hyp. br.), which succeed these, are absent in the case of the first and fifth arches. In the middle line on the floor of the pharyngeal cavity is a mesial cartilage — the basibranchial (Fig. 814, b. br.) — which is connected mck. hyp.br. f ph.br. J FIG. 814. — Hemiscy Ilium, ventral view of the visceral arches. Letters as in preceding figure. In addition — b. br. basibranchial plate ; cer. br. ceratobrancliials ; hyp. br. hypobranchials. with the ventral ends of the third, fourth, and fifth arches. A series of slender curved rods — the cxtrabranchials — lie superficial to the branchial arches, along the borders of the corresponding external branchial clefts. Two pairs of delicate labial cartilages are present at the sides of the mouth, arid a couple at the margins of the openings of the olfactory capsules. The skeleton of all the fins — paired and unpaired — presents a XIII PHYLUM CHORDATA 151 considerable degree of uniformity. The main part of the expanse of the fin is supported by a series of flattened segmented rods, the pterygiophores or cartilaginous fin-rays, which lie in close apposition : in the case of the dorsal fins these may be partly calcified. At the outer ends of these are one or more rows of polygonal plates of cartilage. On each side of the rays and polygonal cartilages are a number of slender " horny " rays or ccrato-trichia of dermal origin.1 In the smaller median fins there maybe an elongated rod of cartilage constituting the skeleton, or cartilage may be entirely absent. In the pectoral fin (Fig. 815) the fin-rays are supported on three basal cartilages articu- lating with the pectoral arch. The latter (pect.) is a strong hoop of cartilage incom- plete dorsally, situated immedi- ately behind the last of the branchial arches. It consists of a dorsal, or scapu- lar, and a ventral, or coracoid por- tion, the coracoid portions of oppo- site sides being completely con- tinuous across the middle line, while the scapular are separated by a wide gap in which the spinal column lies. Between the two portions are the three articular surfaces for the three basal cartilages. The coracoid portions are produced forwards in the middle line into a flattened process supporting the floor of the pericardial cavity in which the heart is lodged. The three basal cartilages of the fin are named, respectively, the anterior, propterygium (pro.), the middle, mesopterygium (meso.\ and the posterior, metapterygium (meta.). Of these the first is the smallest and the last the largest : the first 1 Though, 011 account of their appearance and horn-like consistency, these structures are commonly referred to as horny, they do not consist of true horn (which is always epidermal in origin), but of a substance called dastin, characteristic of elastic connective -tissue fibres, FIG. 815. — H e mi scy Ilium, pectoral arch and fin. d. r. dermal horny rays ; meso. mesopterygium ; meta. metapterygium ; pect. pectoral arch ; pro. propterygium. 152 ZOOLOGY SECT. XIII FIG. 816.— Hemiscyllium, pelvic arch and pelvic fin. pelvic arch. mtta. metapterygium ; pelr. bears only one large ray; the other two bear twelve or more rays, differently arranged in the two genera. The pelvic fin (Fig. 816) has only a single basal cartilage (meta) articulating with the 2^lvic arch, with which also one or two of the fin-rays articulate directly. The pelv pelvic arch (pelv.) is a nearly straight bar of cartilage which runs transversely across the ventral surface of the body, just in front of the cloacal opening. Enteric Canal (Fig. 817).— The mouth leads into a very wide cavity, the pharynx, into which open at the sides the in- ternal apertures of the branchial clefts and of the spiracle. From this runs backwards a short wide tube — the esophagus (ces.) — which passes behind into the stomach. The stomach is a U-shaped organ, with a long left limb continuous with the oesophagus, and a short right passing into the intestine. At the pylorus (pyl.) — the point where the stomach passes into the intestine — is a slight con- striction, followed by a thickening. The intestine consists of two parts — small intestine or duodenum, and large intestine. The former is very short, only an inch or two in length. The latter is longer and very wide; it is divisible into two portions — the colon' (ccL) in front and the rectum (rcct.) behind. The former is very wide and is characterised by the presence in its interior of a spiral valve, a fold of the mucous membrane which runs spirally round its interior, and both retards the too rapid passage of the food and affords a more extensive surface for absorption. The rectum differs from the colon in being narrower and in the absence of the spiral valve ; it opens behind into the cloaca. There is a large liver (liv.) consisting of two elongated lobes. A rounded sac — the gall-Uadder (g. U.)— lies embedded in the left lobe at its anterior end. The duct of the liver— the lile-duct (b. dct.) —runs from the liver to the intestine. Proximally it is connected with the gall-bladder, and by branch-ducts with the right and left lobes of the liver. It opens into the commencement of the colon. The pancreas (pancr) is a light-coloured compressed gland con- sisting of two main lobes with a broad connecting isthmus, lying in the angle between the right-hand limb of the stomach and the small intestine. Its duct enters the wall of the small intestine * - K -.- "8 « II a -- • >- s 154 ZOOLOGY SECT and runs in it for about half an inch, opening eventually at the point where the small intestine passes into the colon. Connected with the rectum on its dorsal aspect is an oval gland —the rectal gland (red. gl.) — about three-quarters of an inch in length. The spleen (spl.) is a dark-red or purple body attached to the con- vexity of the U-shaped stomach and sending a narrow lobe along the right-hand limb. The organs of respiration in the Dog-fish are the f/ills, situated in the five gill-pouches. Each gill-pouch (Fig. 818) is an antero- posteriorly compressed cavity opening internally into the pharynx and externally by the corresponding gill-slit. The walls of the pouches are supported by the branchial and hyoid arches with their rays, the first pouch being situated between the hyoid and first branchial arches, the last between the fourth and fifth branchial arches. On the anterior and posterior walls of the pouches are the gills, each hemibranch consisting of a series of close-set parallel folds or plaits of highly vascular mucous membrane. Separating adjoining gill-pouches, and supporting the gills, are a series of broad interbranchM septa, each containing the corresponding branchial arch with its connected branchial rays. The most anterior hemibranch is borne on the posterior surface of the hyoid arch. The last gill-pouch differs from the rest in having gill-plaits on its anterior wall only. On the anterior wall of the spiracle is a vestigial gill — the pseudcbranch or spiracular gill — in the form of a few slight ridges. Blood-system. — The heart is situated in the pericardial cavity, on the ventral aspect of the body, in front of the pectoral arch, and between the two series of branchial pouches. Its dorsal wall is supported by the basibranchial cartilage. Placing it in communi- cation with the abdominal cavity is a canal — the pericardio-peri- toneal canal. The heart (Fig. 817) consists of four chambers — sinus venosus (sin. ven.\ auricle (aur.), ventricle (vent.), and CO/IMS arteriosus (eon.\ through wrhich the blood passes in the order given. The sinus venosus is a thin-walled, transverse, tubular chamber, into the ends of which the great veins open. It communicates with the auricle by an aperture, the sinu-auricular aperture. The auricle is a large, triangular, thin-walled chamber, situated in front of the sinus veno- sus and dorsal to the ventricle. Its apex is directed forwards, and its lateral angles project at the sides of the ventricle : it commu- nicates with the ventricle by a slit-like aperture guarded by a two- lipped valve. The ventricle is a thick-walled, globular chamber, Fir,. 818.— Hemiscyllium. Branchial sac exposed from the outside. xin PHYLUM CHORDATA 155 forming the most conspicuous part of the heart when looked at from the ventral surface. From it the conns arteriosus runs forwards as a median stout tube to the anterior end of the peri- cardial cavity, where it gives off the ventral aorta. It contains two transverse rows of valves, anterior and posterior, the former consisting of three, the latter of three or four. The ventral aorta (Fig. 819) gives origin to a series of paired afferent branchial arteries (af. lr.), one for each branchial pouch. In Scyllium the two most posterior arise close together near the beginning of the ventral aorta, the third pair a little further forwards. The ventral aorta then runs forwards a little distance and bifurcates to form FIG. 810.— The heart and branchial arteries of Scyllium, from the side. af. ^/-.i— 5, afferent branchial arteries : av, auricle ; c. a. conns arteriosns ; rj.1— 5, branchial clefts ; cor. coronary artery ; d. ao. dorsal aorta ; r,i — 9, efferent branchial arteries; cp. in: ! — 4, epibraiichial arteries ; inn. mandibnlar artery ; sp. spiracle ; s. d. snb- clavian artery ; .<*. r. sinus venosus ; r. ventricle ; r. ao. ventral aorta ; v. c. ventral carotid artery. (From Parker's Practical Zoology.) the two innominate arteries, right and left, each of which in turn bifurcates to form the first and second afferent vessels (af. lr1., af. br.2) of its side. In Hemiscyllium (Fig. 820) the arrangement is somewhat different. From the gills the blood passes by means of the efferent branchial arteries. These efferent vessels (Fig. 819 (ef. br.)) form a series of loops, one running around the margin of each of the first four internal branchial clefts : a single vessel runs along the anterior border of the fifth branchial cleft and opens into the fourth loop. The four main efferent branchial vessels (epibranchials, ep. br.) run inwards and backwards from the loops under cover of the mucous membrane of the roof of the pharynx to unite in a large median trunk — the dorsal aorta (d. ao.). A dorsal carotid artery (d. c.) is given off from the first efferent branchial. A branch (hyoidean) 156 ZOOLOGY SECT. given off from the same efferent vessel supplies the pseudobranch, and the blood from the latter is taken up by ventral carotid (v. c.). Both carotids run forwards to supply the head. The dorsal aorta (Fig. 819, d. ao.) runs backwards throughout the length of the body-cavity, giving off numerous branches, and is continued as the caudal artery, which runs in the canal enclosed by the inferior arches of the caudal vertebrae. The first pair of branches are the subclavians (s. cL), for the supply of the pectoral fins ; these are given off between the third and fourth pairs of efferent arteries. The next large branch is the unpaired cceliac (Fig. 817, cceL) : this runs in the mesentery and divides into branches for the supply of the stomach and liver, the first part of the intestine, and the pancreas. iKThe anterior mesenteric artery, also median, supplies the rest of the intestine and gives off branches to the reproductive organs. The lienogastric supplies part of the stomach, the spleen, and part of the pancreas. The posterior mesenteric is a small vessel mainly supplying the rectal gland. Small renal arteries carry a small quantity of arterial blood to the kidneys, and a pair of iliac arteries, likewise of small size, supply the pelvic fins. In addition to these a number of small arteries, the parietal, supplying the wall of the body, are given off throughout the length of the aorta. The veins are very thin-walled, and the larger trunks are re- markable for their dilated character, from which they have obtained the name of sinuses, though they are true vessels and not sinuses in the sense in which the word is used in dealing with the Invertebrates (cf. p. 93). The venous blood is brought back from the head by a pair of jugular or anterior cardinal sinuses (Fig. 820, jug. v.\ and from the trunk by a pair of posterior cardinal sinuses. At the level of the sinus venosus the anterior and posterior cardinals of each side unite to form a short, nearly transverse sinus, the precaval sinus or ductus Cuvieri (Fig.820,dct.c.), which is continued in to the lateral extremity of thesinus venosus. Into the ductus Cuvieri, about its middle, opens an inferior jugular sinus (inf. jug. v.) which brings back the blood from the floor of the mouth and about the branchial region of the ventral surface. The two posterior cardinal sinuses extend back- wards throughout the length of the body-cavity ; in front they are enormously dilated, behind they lie between the kidneys. Ante- riorly each receives the corresponding siibdavian vein, bringing the blood from the pectoral fin and adjacent parts of the body- wall. The lateral vein (/.«'.), instead of joining with the sub- clavian (p. 94), opens separately into the precaval. The genital sinus discharges into the posterior cardinal sinus. There are two portal systems of veins, the renal portal and the hepatic portal (hep. port, r.), by which the kidneys and liver, respectively, are supplied with venous blood. The caudal vein, XIII PHYLUM CHORDATA 157 inf.ju-g.v which brings back the blood from the tail, running, along with the caudal artery through the inferior arches of the vertebra, divides on entering the ab- dominal cavity into right and left renal portal veins, which end in a number of afferent renal veins supplying the kid- neys. The hepatic portal vein (h. port, v.) is formed by the con- fluence of veins de- rived from the intestine, stomach, pancreas, and spleen, and runs forwards to enter the liver a little to the right of the middle line.1" In Hemiscyllium a large branch connects the genital sinus with the intestinal tribu- taries of the hepatic portal system. The blood from the liver enters the sinus ven- osus by two hepatic sinuses placed close together. Nervous System. —The fore-brain con- sists of a rounded, smooth prosence- phalon (Fig. 821, V.H.) divided into two lateral parts by a very shallow median longitudinal groove. From its antero- lateral region each half gives off a thick cord, which dilates into a large mass of nerve-matter, the olfactory bulb (L. ol.\ closely applied to the posterior surface of the corresponding olfactory \porb.v FKJ. 8-20.— Hemiscy Ilium. Diagrammatic representation of the ventral aorta and afferent branchial arteries, and of the chief veins, ali. alimentary canal ; br. v.i-br. v.& afferent branchial arteries ; caud. v. caudal vein ; dct. c. ductus Cuvieri ; Id. heart ; hep. port. r. hepatic portal vein ; hep. s. hepatic sinus ; inf. jug. r. inferior jugular vein or sinus ; jug. v. jugular vein or sinus ; lat. v. lateral vein ; liv. liver ; 1. card, x left cardinal sinus ; I. port. v. left renal portal vein ; neph. kidney ; r. card. s. right cardinal sinus ; r. port. v. right renal portal vein. 158 ZOOLOGY SECT. capsule. The diencephalon (ZH) is comparatively small ; its roof is very thin, while the floor is composed of two thickish masses L;ol Tro VII Gp IV HE ZH Mil FIG. 821. — Brain of Scyllium canicula. A, dorsal view ; B, ventral view ; C, lateral view. F. rfio. fossa rhoinboidalis (fourth ventricle) ; Gj>, epiphysis ; ////, cerebellum ; HS. H, hypo- physis ; L. ol. olfactory bulb ; Mff, mid-brain ; Ntf, medulla oblougata ; Sr, saccus vasculosus* Tro, olfactory peduncle ; UL, lobi inferiores ; VII, prosencephalon ; ZH, diencephalon ; //, optic nerves; ///, oculomotor ; IV, j>athetic ; V, trigoniinal ; VI, abducent; VII, facial; VIII, auditory ; IX, glossopharyngeal ; A", vagus. (From Wiedershcirn's Coinp. Anatomy.) —the optic thalami. Attached to the roof is a slender tube the epiphysis cerebri or pineal body (Gfp.), which runs forwards and PHYLUM CHORD ATI 159 terminates in a slightly dilated extremity fixed to the membranous part of the_ roof of the skull. Projecting downwards from its floor are two rounded bodies, the lobi inferiores (UL\ which are dilated portions of the infundibulum . Behind these give off a thin- walled vascular outgrowth — the saccus vasculosus (Sv.) Attached to the infimdibulum and extending backwards from it is a thin- walled sac — the pituitary body or hypophysis cerebri (//$), having on its ventral surface a median tubular body attached at its posterior end to the floor of the skull. In front of the infun- dibulum, and also on the lower surface of the diencephalon, is the optic chiasma, formed by the decussation of the fibres of the two optic nerves. The mid-brain (MB) consists of a pair of oval optic lobes dorsally, and ventrally of a band of longitudinal nerve-fibres corresponding to the crura cerebri of the higher vertebrate brain. The cerebellum (HH) is elongated in the antero-posterior direction, its anterior portion overlapping the optic lobes, and its posterior the medulla oblongata. Its surface is marked with a few fine grooves. The medulla oblongata (NH), broad in front, narrows posteriorly to pass into the spinal cord. The fourth ventricle or fossa rhomboidalis (F. rho.) is a shallow space on the dorsal aspect of the medulla oblongata covered over only by a thin vascular membrane, the choroid plexus : it is wide in front and gradually narrows posteriorly. At the sides of the anterior part of the fourth ventricle are a pair of folded ear-shaped lobes, the corpora resti- formia. The fourth ventricle or metaccele (Fig. 822, meta.) is continuous behind with the central canal of the spinal cord. It gives off an epiccele above, and in front is continuous with a narrow passage, the iter or mesoccule (iter.), which opens anteriorly into a wider space, the diaccele or third ven- tricle (dia), occupying the interior of the diencephalon. From this opens in front a median prosoccele, which gives off' a pair of paraccdes (para.) extending into the two lateral portions of the prosencephalon. From the anterior enlargements of the olfactory bulbs already mentioned spring numerous fibres which constitute the first pair of cerebral nerves and enter the olfactory capsules. Between the mela, S_'L>.-Hemiscyllium. The brain viewed from the dorsal side, the roofs of the various ventricles removed so as to show the relations of the cavities (semi-diagrammatic). cer, dilatation from which the epi- coele is given off ; dia, diacoele, pointing to the opening leading into the infundibulum ; iter. iter or inesociele ; meta. metacrele ; opt. optoc(ule ; imra. paraccelc ; pros. prosocuele ; rh. rhmoccele. 160 ZOOLOGY SECT. two olfactory lobes two small nerves, the terminal or pre- olfactory, arise from the prosencephalon : they are the nerves of ordinary sensation for the interior of the olfactory sacs. From the optic chiasma the two optic nerves (Figs. 821, 823, 823 bis, II) run outwards through the optic foramina into the orbits, each perforating the sclerotic of the corresponding eye and terminating in the retina. The third, fourth, and sixth pairs of nerves have the sp.co FIG. 823. — Scyllium catulus. — Dissection of the brain and spinal nerves from the dorsal surface. The right eye has been removed. The cut surfaces of the cartilaginous skull and spinal column arc dotted. The buccal branch of the facial is not represented ; d,\—d.^ branchial clefts ; ep. epiphysis ; ex. rect. external rectus muscle of the eye- ball ; gl. ph. glossopharyngeal ; hor. can. horizontal semicircular canal ; hy. mnd. V If. hyomandibular portion of the facial ; inf. old. inferior oblique muscle ; int. rect. internal rectus muscle ; lat. vag. lateral branch of vagus ; mx. V. maxillary division of the trigeminal ; olf. cps. olfactory capsule ; olf. s. olfactory sac ; op/t. V. VII. superficial ophthalmic branches of trigeminal and facial ; path, fourth nerve ; pi. VII. palatine branch of facial ; sp. co. spinal cord ; spir. spiracle ; s. rect. superior rectus muscle ; s. olb. superior oblique ; vag. vagus ; vest, vestibule. (From Marshall and Hurst.) general origin and distribution which has already been described as universal in the Craniata (p. 104). The trigeminal (Figs. 821,823, 823 bis, V) arises in close relation to the facial. As it passes into the orbit it swells into a ganglion —the Gasserian. Its chief branches are three in number. The first given -off is the superficial ophthalmic (Fig. 823, oph. V\ Fig. 823, bis, V op.), which runs forwards through the orbit above the origin of the recti muscles, and in very close relation with the ophthalmic branch of the facial. Anteriorly it breaks up into branches distributed to the integument of the dorsal surface of xin PHYLUM CHORD ATA 161 the snout.1 The main trunk of the nerve then runs forwards and outwards across the floor of the orbit, and divides into two branches, the maxillary and mandibular, or second and third divisions of the trigeminal. The former (mx. V) supplies the skin of the ventral surface of the snout, the latter (mnd. V) the skin and muscles of the lower jaw. Of the branches of the facial, the ophthalmic runs through the orbit in close relation to the superficial ophthalmic branch of the trigeminal, and is distributed to the sensory and ampullary canals of the snout region ; the buccal runs forwards in intimate relation with the maxillary division of the trigeminal, and breaks up into branches which are distributed to the sensory canals and ampullae of the region of the snout ; the palatine (pi. VII, VIIp.) runs to the roof of the mouth ; the main body of the nervre — hyomandibular nerve (hy. mnd. VII, Vllhy.) — then runs outwards close to the edge of the hyomandibular cartilage and behind the spiracle, eventually becoming distributed to the muscles between the spiracle and the first branchial cleft : a small external mandi- bular branch (VH.e.m.) comes off from it and goes to the lateral and ampullary canals of the lower jaw. The eighth or auditory nerve passes directly into the internal ear, and breaks up into branches for the supply of its various" ^parts. The glossopharyngeal (gl. ph. IX.} perforates the posterior part of the auditory region of the skull, and, after it reaches the exterior, passes to the first branchial cleft, where it bifurcates, one branch passing to the anterior, and the other to the posterior wall of the cleft. The last nerve of the series — the pneu^no gastric or •vagus (vag., X) — is a large nerve which emerges from the skull by an aperture situated between the auditory region and the foramen magnum. It first gives off a series of four branchial branches, each of which bifurcates to supply the anterior and posterior borders of the last four branchial clefts. The- lateralis nerve (/at. vag., X.I.) is usually referred to as a branch of the vagus since it runs in intimate connection with the trunk of that nerve for some distance, but it has a distinct origin in the medulla : after becom- ing separated from the vagus trunk it runs along beneath the peritoneum opposite the lateral line, which it supplies, to the posterior end of the body. The rest of the vagus runs backwards to divide into cardiac branches for the heart arid gastric branches for the stomach. - l In most Elasmobranchs a nerve of considerable size — the ophthalmicus pro- fniHliix (Fig. 779) arises from the dorsal and anterior part of the Gasserian ganglion, and is usually regarded as a branch of the trigeminal. It runs forwards over the external rectus muscle and under the superior rectus, and perforates the pre-orbital process to end in the integument of the snout. Among other branches it givefc off ciliary branches to the iris : these are joined by the ciliary brunches of the oculomotor. An ophthalmicus profundus is not present in Scyllium in the adult condition. VOL. II L ZOOLOGY SECT It will be observed that the system of neuromast organs (lateral line and ampullary organs) are supplied by nerve-fibres which pass out in various branches of -the facial and in the vagus (lateralis) : all these fibres originate in a centre in the medulla, the acustico- lateral centre, common to them and the fibres of the auditory nerve. The spinal cord is a cylindrical cord which extends from the foramen magnum, where it is continuous with the hind-brain, backwards throughout the length of the neural canal, enclosed by the neural arches of the vertebras. As in the Craniata in general (see p. 98), it has dorsal and ventral longitudinal fissures and xin PHYLUM CHORDATA 163 a narrow central canal, and gives origin to a large number of paired spinal nerves, each arising from it by two roots. Organs of Special Sense. — The olfactory organs are rounded chambers enclosed by the cartilage of the olfactory capsules of the skull, and opening on the exterior by the external nares on the ventral surface of the head. The interior has its lining membrane raised up into a number of close-set ridges running out from a median septum. The fibres of the olfactory nerves terminate in cells of the epithelium covering the surface of these ridges. The eye has the general structure already described as char- acterising the Craniata in general (p. 109). The sclerotic is cartilaginous, the choroid has a shining metallic internal layer or tapetum, and the lens is spherical. There are the usual eye- muscles, the two obliques situated anteriorly, the four recti posteriorly, not embracing the optic nerve. There are no eyelids. The ear consists only of the membranous labyrinth (Fig. 788), equivalent to the internal ear of higher Craniata, the middle and outer ear being absent. The membranous labyrinth consists of the vestibule and three semicircular canals. The former, which is divided into two parts by a constriction, communicates by a narrow passage — the endolymphatic duct or aqueductus vestibuli — with the exterior, in the position already mentioned. Of the three semicircular canals, the anterior and posterior are vertical and the external horizontal, as in Craniata in general. Each has an ampulla, that of the anterior and external canals situated at their anterior ends, and that of the posterior canal, which is the largest of the three and forms an almost complete circle, at its posterior end. In the fluid (endolymph} in the interior of the vestibule are suspended, in a mass of gelatinous connective-tissue, numerous minute calcareous particles or otoliths, giving it a milky character. The sensory canals of the integument running along the lateral line and over the head contain special nerve-endings (neuromasts), and doubtless function as organs of some special sense (see p. 108). The same probably holds good of a number of unbranched canals arranged in groups situated on the anterior portion of the trunk and on the head, and being particularly numerous in the neigh- bourhood of the snout. These are dilated internally into vesicles, the ampullce, provided with special nerve-endings. Urinogenital Organs. — In the female there is a single ovary (Fig. 817, ov.\ an elongated, soft, lobulated body, lying a little to the right of the middle line of the abdominal cavity, attached by a fold of peritoneum, the mesoarium. On its surface are rounded elevations of various sizes, the Graaftan follicles, each containing an ovum of a bright yellow colour. There are two oviducts (Miillerian ducts) entirely unconnected with the ovaries. Each oviduct (Figs. 817 and 824, ovd.) is a greatly elongated tube extending throughout the entire length of the L 2 161 ZOOLOGY SECT. -m.d els FIG. 824.— The urinogenital organs of Scyllium canicula from the ventral side. A, male, and B, female. Only the anterior end of the gonad is represented in each figure, and except that in B both kidneys are shown, the organs of the right side only are drawn. In A the seminal vesicle and sperm-sac are dissected away from the kidneys and displaced outwards, and the ureters inwards. -'. its aperture into the urino- genital sinus; f.s. spermary ; u. y. s. urinogenital sinus; m: ureters; u/. their apertures into the urinogenital sinus ; u. a. urinary sinus. (From Parker's Practical Zoology.) XIII PHYLUM CHORDATA 165 abdominal cavity. In front the two unite behind the pericardium to open into the abdominal cavity by a wide median aperture (ovd1.). At about the point of junction of the middle and anterior thirds is a swelling marking the position of the shell- gland (sh. gl.) The posterior part dilates to form a wide uterine chamber, and in Scy Ilium the two unite to open into the cloaca by a common aperture situated just behind the opening of the rectum, while in Hemiscyllium they remain distinct and have separate cloacal openings. Each kidney consists of two parts, anterior and posterior. The former (Fig. 817, r. meson, Fig. 824, &') is a long narrow ribbon of soft reddish substance, which runs along throughout a great part of the body-cavity at the side of the vertebral column, covered by the peritoneum. The posterior portion (r. metan, Jc) is a compact, lobulated, dark- red body, lying at the side of the cloaca, continuous with the anterior portion ; like the latter it is covered over by the peritoneum. Both portions have their ducts. Those of the anterior are narrow tubes, which run over its ventral surface and become dilated behind to form a pair of elongated chambers, the urinary sinuses (Fig. 825, ur. sin.), uniting behind into a median sinus (med. ur. sin.), opening into the cloaca by a median aperture situated on a papilla, the urinary papilla. The ducts of the posterior portion, the ureters, which are usually from four to six in number, open into the urinary sinuses. In the male (Fig. 824) there are two elongated, soft, lobulated testes,*e&ch attached to the wall of the abdominal cavity by a fold of peritoneum — the mesorchium. From each testis anteriorly, a small number of efferent ducts pass to the anterior end of a long, narrow, strap-shaped body, which corresponds to the vestigial anterior portion of the kidney in the female. This is the epi- didymis ; the duct, spermiduct or vas deferens, runs along the entire length of the non-renal part of the kidney, or " Leydig's gland" and, where it leaves the latter posteriorly, becomes a wide tube, which opens into the urinogenital sinus (u. g. s.), a median chamber projecting into the cloaca. Posteriorly the spermiduct dilates to form a wide thin- walled sac, the vesicula seminalis. Closely applied to the latter is a thin- walled elongated sac, the sperm-sac. Anterioly the sperm-sac narrows to a blind extremity ; posteriorly the right and left sperm-sacs combine to form the urinogenital sinus. The posterior part of the kidney has the same character as in the female ; its ducts, usually five in number on each side, open into the urinogenital sinus, some of the most anterior first uniting to form a common tube. The sinus has a median aperture into the general cavity of the cloaca situated on the summit of a prominent urinogenital papilla. The oviducts (Miillerian ducts) of the female are represented in the male by vestiges of their anterior portions (m.d). The entire kidney is sometimes regarded as a 166 ZOOLOGY SECT. mesonephros, but the posterior portion, developed entirely behind the part which, in the male, takes part in forming the epididymis, and having its own ducts, is sometimes looked upon as fore- shadowing the metanephros of the higher Vertebrates. The ripe ovum, rupturing the wall of its Graafian follicle, escapes into the abdominal cavity, whence it reaches the interior of one of the oviducts; there it is fertilised by sperms received •nted.ur.sirv FIG. 825.-Hemiscyllium. Right kidney and urinary sinus of female, med. ur. sinus, median urinary sinus ; neph. kidney ; -ur. sinus, right urinary sinus. FIG. 826.— Dog- fish, egg-case. Dean.) (After from the male in, the act of copulation, and then becomes enclosed in a chitinoid case or shell (Fig. 826) secreted by the shell-gland. 2. — DISTINCTIVE CHARACTERS AND CLASSIFICATION. The Elasmobranchii are Pisces in which the cartilaginous cranium is never ossified by replacing-bones, and in which investing- bones are not developed in connection either with the cranium or the pectoral arch. The skull is hyostylic, except in some of the Protoselachii, in which it is amphistylic (p. 78). The dermal fin-rays are " horny " ; at their bases the fins are supported by cartilaginous pterygiophores which are never very numerous. The XIII PHYLUM CHORDATA pelvic arch is a distinct cartilage. There is nearly always an exoskeleton which is of the placoid type. The intestine has a spiral or a scroll-like valve. A cloaca is present into which both the rectum and the ducts of the urinary and reproductive systems open. There is never an operculum in recent Elasmobranchs, and only rarely in fossil forms. The inter-branchial septa are of considerable breadth, and the gill-filaments are attached to them throughout their entire extent. A mandibular spiracular gill is only exceptionally present as a fully developed organ; it is represented usually by a vestige (pseudobranch). A conus arteriosus is always developed ; it is rhythmically contractile, and in its interior are several transverse rows of valves. The optic nerves form a chiasma. The ova are very large ; with one possible exception they are always fertilised internally. The oviducts are not continuous with the ovaries, but open by wide mouths into the body-cavity. Fio s-27.— Restoration of Cladoselache fyleri, lateral and ventral views. (After Dean.) ORDER 1. — CLADOSELACHII (PLEUROPTERYGII). Extinct Shark-like Elasmobranchs in which both pectoral and pelvic fins had much wider bases of attachment than in existing forms. There is an exoskeleton of small denticles. The notochord was persistent : there are calcified neural and haemal arches, but no intercalary cartilages. The caudal fin is heterocercal. Claspers are absent. The gill-openings were apparently protected by a fold of skin. The teeth are of the nature of placoid denticles. The lateral line was represented by an open groove. This order comprise only one known representative — Cladose- lache— from the lower Carboniferous rocks of America. 168 ZOOLOGY SECT. ORDER 2. — PLEURACANTHEI (ICHTHYOTOMI). Extinct Shark-like Elasmobranchs in which the skeleton of the pectoral fin was constructed on the type of the so-called archi- pterygium, i.e. consisted of an elongated, segmented central axis bearing two rows of jointed rays. The notochord was persistent ; in- tercalary cartilages were present in addition to neural and haemal arches. The caudal fin is diphy- cercal. Claspers were present. There was no opercular fold, and the teeth resemble those of other Elasmobranchs. Placoid scales are not known to have been present, but the skull is protected by roof- ing dermal ossifications. This order, like the last, in- cludes only one satisfactorily known genus — Pleuracanthus (Fig. 828) — of Carboniferous and Per- mian age. ORDER 3. — ACANTHODEI. Extinct Elasmobranchs (Fig. 829) having the anterior margin of each fin supported by a stout spine. The tail is heterocercal. There were no claspers. There is a placoid exoskeleton of small den- ticles. An operculum was not present. The notochord was per- sistent, with neural and haemal arches. Calcified plates are pre- sent in relation to the jaws and to the roof of the skull. The teeth are few and large, numer- ous and minute, or altogether absent. The lateral line was in the form of an open groove. ORDER 4. — SELACHII. Living and extinct Elasmo- branchs in which the skeleton of the paired fins is never of the nature of an archipterygium. The notochord is more or less completely replaced by vertebrae, and there xin PHYLUM CHORDATA 169 is a series of intercalary cartilages. The caudal fin is nearly always heterocercal. Claspers are always developed. A distinct opercular fold is never present. FIG. 829.— Acanthodes wardi. (Restored, after Dean.) Sub-Order a. — Protoselachii. . Selachii in which the spinal column is uncalcified, and the centra are very imperfectly developed ; there are more than five branchial arches. Except in Clilamydoselachus, the palatoquadrate develops a process by which it articulates with the postorbital region of the skull. This sub-order includes the Notidanidce (Hexanchus and Heptan- chus), and Chlamydoselachus (Fig. 830), as well as, probably, many fossil forms. FIG 830 — Chlamvdoselaclms anguineus. (From the Cambridge Natural History, after GUnther.) Sub-Order 1. — Euselachii. Selachii in which the spinal column is partly or completely calcified. There are only five branchial arches. The palatoquad- rate has no postorbital articulation with the skull. Section a. — Squalida. Euselachii with fusiform body and well-developed caudal fin. The pectorals are of moderate size. A ventral fin is present. The vertebra of the anterior part of the spinal column are not fused together. The branchial apertures and the spiracle are situated laterally. This section comprises all the recent Sharks and Dog-fishes, with the exception of the Protoselachii. 170 ZOOLOGY SECT. Section ft. — Eajida. Enselachii with dorso- ventral ly compressed body, and, usually, feebly developed caudal fin. The pectorals are of great size, the pelvics usually small. A ventral fin is usually absent. The verte- brae of the anterior region are fused together. The branchial apertures are ventral, the spiracles dorsal. This section comprises all the recent and extinct Rays (Skates, Thorn-backs, Sting- Rays, Electric Rays, Saw-fish Rays). 3. — GENERAL ORGANISATION. External Characters. — In general shape most Sharks (Fig. 831) are somewhat fusiform and slightly compressed laterally. In the Rays (Fig. 832), on the other hand, there is great dorso-ventral compression. The head is in many cases produced forwards into a long rostrum, which is of immense F-io. 831.— Porbeagle Shark (lamna cornubica). (From Dean's Fishes.) length and bordered with triangular teeth in -the Saw-fish Shark (Pristiophorus) and Saw-fish Ray (Pristis). In the Hammerhead Shark (Sphyrna or Zygcena} the anterior part of the head is elongated transversely. There are well-developed median and paired fins. The caudal fin is well developed, and, as a rule, strongly heterocercal in the Sharks and shark-like Rays, feebly developed in most of the latter group. The dorsal and ventral fins are large in the Sharks, the former completely divided into two : in the Rays the dorsal fin is usually small, and the ventral absent. The paired fins are very differently developed in the two groups. In the Sharks both pairs are well developed, the pectoral being the larger. In the Rays the pectoral fins are extremely large, very much larger than the pelvic, fringing the greater part of the length of the flattened body, and becoming prolonged forwards on either side and even in front of the head, so that the animal presents the appearance of a broad fleshy leaf. In all recent Elasmobranchs the male has, connected with the XIII PHYLUM CHORDATA 171 pelvic fins, a pair of grooved appendages — the claspcrs or ptcry go- podia — which subserve copulation. The mouth is situated on the ventral surface of the head, usually a considerable distance from the anterior extremity. In front of each angle of the mouth on the ventral surface is the opening of one of the olfactory sacs, each of which is frequently connected by a groove — the naso-buccal groove — with the mouth-cavity. Behind the mouth, on the dorsal surface in the Rays, and at the side in the Sharks, is the spiracle. Along the sides of the neck in the Sharks, and on the ventral surface in the Rays, there is on either side a row of slit- like apertures- — the branchial slits or branchial clefts. These are usually five in number on each side ; but in Hexanchus and Chlamydose- lackus there are six, and in ffeptanckus seven. In Chlatny- doselachus (Fig. 830) a fold comparable to a rudimentary operculum extends back over the first branchial cleft, and is continuous across the middle line ventrally ; in the remainder of the sub-class no such structure is repre- sented. A large cloacal open- ing is situated just in front of the root of the tail, and a pair of small openings placed in front of it — the abdominal pores — lead into the abdom- inal cavity. When the integument de- velops any hard parts, as 4s the case in the majority of the Elasmobranchs, they take the form, not of regular scales, as in most other fishes, but of numerous hard bodies (Fig: 833) which vary greatly in shape, are usually extremely minute, but are in some cases developed, in certain parts of the surface, into prominent tubercles or spines. When these hard bodies are, as is commonly the case, small and set closely together in the skin, they give the surface very much the character of a fine file ; and the skin so beset, known as " shagreen," was formerly used for various polishing purposes in the arts. This is the placoid form of exoskeleton, to which reference has been already made. Each of the hard bodies has the same structure as a tooth, being composed of dentine, capped with an enamel-like layer, and FIG. 832.— Sting-Ray (Urotophus cruciatus). (After Gunther.) 172 ZOOLOGY SECT. supported on a base of a substance somewhat resembling the bony cement or crusta petrosa of the tooth. The skeleton is composed of cartilage, with, in many cases, deposition of calcareous matter in special places — notably in the jaws and the vertebral column. The entire spinal column may be nearly completely cartilaginous (Hexanchus and Heptanckus), but usually the centra are strengthened by radiating or concentric lamellae of calcified tissue ; or they may be completely calcified. They are deeply amphiccelous, the remains of the notochord persisting in the large inter-central spaces. Inter- calary pieces (Fig. 834, /c.) are in- terposed between both superior and inferior arches. In the Rays (Fig. 835) the anterior part of the spinal column becomes converted into a continuous solid cartilaginous and calcified mass — the anterior verte- bral plate (a. v.p.*). As in Fishes in general, two regions are distinguish- able in the spinal column — the pre-caudal and the caudal, the latter being characterised by the possession of inferior or haemal arches. In the pre-caudal region short ribs may be developed, but these are sometimes rudimentary or entirely absent. In the Sharks pterygiophores, sometimes jointed, fused at their bases with the haBmal spines, support the ventral lobe of the caudal fin, and the dorsal lobe of the same fin is supported by a series of pterygio- phores resembling produced neural spines, but only secondarily related to the spinal column, and some- times also divided by joints. The dorsal and ventral fins are sometimes supported by similar pterygiophores ; but in many cases the cartilaginous supports of these fins consist, in whole or in FIG. 833.— Dermal denticles of Centre - calceus, slightly magni- (From Gegenbaur's Comparative Anatomy.) WK FIG. 834.— Portion of the spinal column of Scymnus. Ic. Ob, neu rtebrata.) intercalary cartilages ; Ob, neural arches ; WK, centra. (From Wiedersheim's Vert xni PHYLUM CHORDATA 173 part, of expanded plates of cartilage. The marginal portions of the unpaired fins beyond the limits of the endoskeleton are supported by dermal fibre-like structures (ceratotrichici), composed of elastin. The skull is an undivided mass of cartilage, hardened, in many cases, by deposition of calcareous matter, but not containing any true bony tissue. It consists of a cartilaginous case for the pro- tection of the brain and the organs of special sense. The struc- ture of this cartilaginous brain-case as it occurs in the Dog-fish has already been described. The main differences observable in the different families are connected with the size and form of the rostrum. In the Rays the lower lip of the foramen magnum is deeply excavated for the reception of a short process, the so-called odontoid process, which projects forwards from the anterior vertebral plate, and on either side of this is an articular surface — the occi- pital condyle — for articulation with corresponding surfaces on that plate. In the Sharks the skull is not so definitely marked off from the spinal column. The apertures of the aqueductus vesti- buli in the Rays are not situated in a median depression such as is observable in the Dog-fish and in all the Sharks. The articular surface in the auditory region for the hyomandibular is sometimes borne on a projecting process, sometimes on the general level of the lateral surface. Sometimes in the Rays there is a smaller articulation behind for the first branchial arch. The upper and lower jaws — the palatoquadrate and MeckeVs car- tilage— are connected with the skull through the intermediation of a hyomandibular cartilage (Fig. 813, hy. mn. ; Fig. 835, h. m.). The skull is thus of the hyostylic type as regards the mode of suspension of the jaws. In the Sharks the palatoquadrate has a process (absent in the Rays) for articulation with the base of the skull in the pre-orbital region. In Hexanchus and Heptan- chus (Fig. 836) there is in addition to this a prominent post- orbital process of the palatoquadrate for articulation with the postorbital region of the skull (ampTiistylic arrangement). Ces- tracion is also in a sense amphistylic ; the palatoquadrate is firmly united with the skull, articulating with a groove on the base, and the hyomandibular takes only a small share in the suspension of the jaws. At the sides of the mouth in all Elasmo- branchs are a series of labial cartilages, usually two pairs above and one pair below. Attached to the hyomandibular is a thin plate of cartilage — the spiracular (Fig. 835, sp.) — which supports the anterior wall of the spiracle. The hyoid arch proper is in most of the Elasmobranchs con- nected at its dorsal end with the hyomandibular — sometimes at its distal extremity, sometimes near its articulation with the skull ; but in some Rays it is not so related, but articulates separately and independently with the skull behind the hyo- mandibular, and in the genera Hypnos and Trygonorhina it articu- 174 ZOOLOGY SECT. lates with the dorsal portion of the first branchial arch. In the Sharks the hyoid is usually relatively massive ; in the Rays it is smaller, and in most cases closely resembles the branchial arches, and bears similar cartilaginous rays ; a larger or smaller median element, or basihyal, is present in all cases. There are always five pairs of branchial arches except in Hexanchus and Chlamydoselachus, which have six, and Heptanchus, lab bas.br FIG. 835. — Skeleton of Sting-Ray (Urolophus testaceus), ventral view. n. r. />. anterior vertebral, plate ; bat. br. basibranchial plate ; br. 1 — br.$ branchial arches. (The branchial rays are not represented, the round dots indicating their articulations with the arches.) cl. skeleton of clasper ; h. m. hyomandibular ; hy, hyoid arch , lab, labial cartilage ; lift, ligament connect- ing the hyomandibular with the palatoquadrate and Meckel's cartilage ; M<-k. Meckel's carti- lage ; ins. pt. rnesopterygiinn, and int. pt. metapterygium of pectoral fin ; rut. pt'. metapterygium of pelvic fin ; nas. nasal cartilage ; pal. palatoquadrate ; pect. pectoral arch ; pi. pelvic arch ; pro. pt. propterygium ; xp. spiracular cartilage. in which there are seven. Their dorsal ends are free in the Sharks, articulated with the anterior vertebral plate of the spinal column in most Rays. Externally they bear a series of slender cartilaginous branchial rays. The median ventral elements of the branchial arches are usually more or less reduced, and in some xin PHYLUM CHORDATA 175 cases are represented by a single basi-branchial plate (Fig. 835, bas. br.). In the Rays the fifth branchial arch articulates with the pectoral arch, a connection which is absent in the Sharks. A series of slender cartilages, probably modified branchial rays — the extra- branchial cartilages — absent as such in some Dog-fishes and Rays, support the branchial apertures. The pectoral arch (Figs. 815, 835, pect.) consists of a single cartilage, with, however, in most of the Sharks, a mesial flexible portion by which it is divided into right and left halves. Each lateral half consists of a dorsal scapular, and a ventral cora- coid part, the two being separated by the articular surfaces for the basal cartilages of the fin. In the Rays, but not in the Sharks, the dorsal ends of the pectoral arch are connected with the anterior ft. orb \ .,,.„, ,„..., FIG. 836.— Lateral view of the skull of Heptanchus. mck. Meckel's cartilage ; pal.qu. palato- quadrate ; pt. orb. postorbital process of the cranium, with which the palatoquadrate articulates. (After Gegenbaur.) vertebral plate of the spinal column by a distinct articulation, the portion of the arch on which the articular surface is situated sometimes forming an independent cartilage (supra-scapula). In Heptanchus a small median ventral element may represent the sternal apparatus of the Amphibia. The basal pterygiophores of the pectoral fin are typically three, pro-, iii'-xo-, and meta-ptwygium (Figs. 815 and 835), but there are some- times four, and the number may be reduced to two. The pro- and meta-pterygia are divided in the Rays (Fig. 835) into several seg- ments, and the former articulates, through the intermediation of a cartilage termed the antorbital, with the olfactory region of the skull. The pelvic arch (pi.) is usually, like the pectoral, a single cartilage, but in some exceptional cases it consists of two lateral portions. 176 ZOOLOGY SECT. In some cases a median epipubic process projects forwards from the pelvic arch, and frequently there is on each side a pre- pubic process. A lateral iliac process, which becomes highly developed in the Holocephali, is sometimes represented, and may attain considerable dimensions. The pelvic fin has usually two basal cartilages, representing the pro- and meta-pterygia, but the former is often absent. In the male special cartilages attached to the metapterygia support the claspers. With the basal car- tilages of both pectoral and pelvic fins are connected a number of jointed cartilaginous fin-rays supporting the expanse of the fin. The arrangement of the muscles is simple. The trunk-muscles are divided into a pair of dorsal and a pair of ventral divisions, each composed of mairv myomeres with intercalated myocommata (Fig. 759, p. 70), following a metameric arrangement. The ventral part, where it forms the muscles of the wall of the abdominal cavity, is composed externally of obliquely running fibres, and represents one of the two oblique muscles of the abdomen of higher forms. Mesially this passes into a median band of longitudinally running fibres corresponding to a primitive rectus. The muscles of the limbs are distinguishable into two main sets — those inserted into the limb-arch and those inserted into the free part of the appendage. The latter, according to their insertion, act as elevators, depressors, or adductors. A series of circular muscles pass between the cartilages of the visceral arches, and when they contract, have the effect of contracting the pharynx and constricting the apertures. A set of muscles pass between the various arches and act so as to approximate them; and abroad sheet of longitudinal fibres divided into myomeres extends forwards from the shoulder-girdle to the visceral arches. Electric organs — organs in which electricity is formed and stored up, to be discharged at the will of the Fish — occur in several Elasmobranchs. They are best developed in the Electric Rays (Torpedo and Hypnos, Fig. 837) in which they form a pair of large masses running through the entire thickness of the body, between the head and the margin of the pectoral fin. A network of strands of fibrous tissue forms the support for a number of vertical prisms, each divided by transverse partitions into a large number of com- partments or cells. Numerous nerve-fibres pass to the various parts of the organ. These are derived mainly from four nerves, which originate from an electric lobe of the medulla oblongata, with a branch from the trigeminal. By means of the electric shocks which they are able to administer at will to animals in their immediate neighbourhood, the Torpedo-Rays are able to ward off the attacks of enemies and to kill or paralyse their prey. In the other Rays in which the electric organs are developed, they are comparatively small organs situated at the sides of the root of the XIII PHYLUM CHORDATA 177 tail. In all cases the cells are formed from metamorphosed muscular fibres. Luminous organs by the agency of which a phosphorescent light is produced occur on the surface of a few Elasmobranchs. Digestive System. — Teeth are developed on the palato- quadrate and on Meckel's cartilage. They are arranged in several parallel rows, and are developed from a groove within the margin of the jaw, successive rows coming to the front, and, as they are FIG. 837. — A Torpedo - Ray with the electric organs dissected out. On the right the surface only of the electric organ (o.e.)is shown. On the left the nerves passing to the -organ are shown. The roof of the skull is removed to bring the brain into view. br. branchiae ; ./', spiracle ; o, eyes ; tr. trigeminal ; tr', its electric branch ; v. vagus ; /, fore-brain ; //, mid- brain ; ///, cerebellum ; IV, electric lobe. (From Gegenbaur.) worn out, falling off and being replaced by others. In the Sharks the teeth are usually large, and may be long, narrow, and pointed, or triangular with serrated edges, or made up of several sharp cusps ; in the Rays, however, the teeth are more or less obtuse, sometimes, as in the Eagle-Rays, forming a continuous pavement of smooth plates covered with enamel, adapted to crushing food consisting of such objects as Shell-fish and the like. VOL. IT M 178 ZOOLOGY SECT. The Sharks have a prominent tongue supported by the median basi- hyal ; this is entirely or almost entirely absent in the Rays. The various divisions of the enteric canal are similar in all the members of the class to what has already been described in the case of the Dog-fish. A spiral valve is always present in the large intestine, though its arrangement varies considerably in the different families. In some cases (e.g. Carcharias), the fold is not a spiral one, but, attached by one edge in a nearly longitudinal line to the intestinal wall, is rolled up in the shape of a scroll. A caecum occurs in Lcemargus, The rectum always terminates in a cloaca, into which the urinary and genital ducts also lead. There is always a voluminous liver and a well-developed pancreas. A thyroid lies in the middle line behind the lower jaw. A representative of the thymus lies on either side, a little below the upper angles of the branchial clefts. The respiratory organs of the Elasmobranchii always have the general structure and arrangement already described in the case of the Dog-fish. In the Rays the water of respiration is taken in mainly through the spiracles ; in the Sharks through the mouth. In addition to the gills supported on the hyoid and branchial arches there is also in the Notidanidas a gill on the mandibular side of the spiracular cleft — the spiracular gill — represented in many others by a rete mirabile or network of blood-vessels. In Sclache (the Basking Shark) there are a series of slender rods, the gill-rakers, which impede the passage outwards through the branchial clefts of the small animals on which those Sharks feed. Blood-system. — The heart has, in all essential respects, the same structure throughout the group. The conus arteriosus is always contractile, and contains several rows of valves. The general course of the circulation is the same in all (see p. 90), with some variation in the precise arrangement of the vessels. In some of the Rays the ventral aorta and the roots of the afferent vessels are partly enclosed in the cartilage of the basi-branchial plate. The brain attains a much higher stage of development than in the Cyclostomata. The fore-brain greatly exceeds the other divisions in size. In Scymnus, there are two widely-separated parencephalic lobes or cerebral hemispheres containing large lateral ventricles. In other genera there is at most, as in the Dog- fish, a median depression of greater or less depth, indicating a division into two lateral portions. In Scy Ilium, as already pointed out, there is a median prosocoele which gives rise anteriorly to two lateral ventricles, or paracoeles, and the same holds good of Ehina and Acanthias. In most Rays there is only a very small prosocoale without anterior prolongations; in Myliobatis this is absent. The olfactory bulbs are of great size, in some cases with XITI PHYLUM CHORDATA 179 short and thick, in others longer and narrower, stalks. In Scyllium, Rhina, and Acanthias, as well as in Scymnus, they contain ventricles ; in the Rays they are solid. The diencephcdon is of moderate extent. On its lower aspect are a pair of rounded loin inferiores, which are of the nature of dilatations of the infundibulum, and a saccus vasculosus, which is a diverticulum of the infundibulum ; directly below the saccus vasculosus lies the hypophysis. The epiphysis is long and narrow. In the hind-brain the cerebellum is relatively greatly elongated and overlaps the optic lobes and sometimes also the diencephalon in front, while behind it extends over the anterior part of the medulla oblongata. It usually contains a cerebellar ventricle or cpicoele. The medulla is elongated in the Sharks, shorter and more triangular in the Rays. The Electric Rays are characterised by the presence of the electric lobes, rounded elevations of the floor of the fourth ventricle. Organs of Sense. — Integumentary sense-organs (neuromasts, p. 107) are highly developed in the Elasmobranchs. They are supplied, as already mentioned, by branches of the nerves of what is known as the lateral system, comprising, in addition to the lateralis, nerves in relation with the facial and sometimes the glossopharyngeal. These integumentary sense-organs occur in the interior of a continuous system of closed tubes, the sensory tubes, more rarely of open grooves. The chief canals of this system are a lateral-line canal, running along the middle of each side of the body, which is continuous with certain canals in the head: these communicate with the exterior at intervals by small pores. In addition to the canals of the lateral line system there are a number of isolated canals, the ampidlary canals, with neuromasts contained in terminal enlargements or ampullce: these, which are peculiar to the Elasmobranchs, are most numerous about the snout region. Of similar essential character are the vesicles of Savi which occur in the Electric Rays. The olfactory organs are a pair of cavities opening on the lower surface of the head, a little distance in front of the mouth, and enclosed by the cartilaginous olfactory capsules of the skull. Their inner surface is raised up into a number of ridges on which the fibres of the olfactory nerves are distributed. The eye has a cartilaginous sclerotic, arid is in most cases attached to the inner wall of the orbit by means of a cartilaginous stalk. A fold of the conjunctiva resembling the nictitating membrane, or third eyelid of higher Vertebrates, occurs in some Sharks. The ear consists of the membranous vestibule, which is partly divided into two (utriculus and sacculus), from which arise the three semicircular canals with their ampulla?, and also the aqueductus vestibuli or endolymphatic duct — which opens on the exterior on the dorsal M 2 180 ZOOLOGY SECT. surface of the head. In the Rays the semicircular canals form almost complete circles and open separately into the vestibule by narrow ducts. Urinogenital Organs. — The kidneys, as already noticed in the account given of the Dog-fish, differ somewhat in their relations in the two sexes. In the male the anterior portion persists in the epididymis, and its duct becomes the spermiduct, while the posterior portion, which is the functional kidney, has a duct or ducts — the ureter or ureters — of its own. In the female there is no direct connection between the reproductive and renal organs ; the anterior portion of the kidney may be functional, and its duct persists, opening along with those of the posterior portion. In the male the ^ureters open into a median chamber — the urinogenital sinus — which extends into the cloaca, and receives also the spermi- ducts : it communicates with the general cavity of the cloaca by a median opening situated on a papilla — the urinogenital papilla. In the female there is a median urinary sinus, into which the ureters open, or the latter may open separately into the cloaca. Save in certain exceptional cases (e.g. Scyllium), there are two ovaries, varying considerably in form, but always characterised towards the breeding season by the great size of the follicles enclosing the mature ova. The oviducts (Miillerian ducts) are quite separate from the ovaries. The right and left oviducts come into close relationship anteriorly, being united in the middle on the ventral surface of the oesophagus, where each opens by a wide orifice into the abdominal cavity, or both open by a single median aperture. The following part of the oviduct is very narrow ; at one point it exhibits a thickening, due to the presence in its walls of the follicles of the shell-gland. Behind this is a dilated portion which acts as a uterus, and this communi- cates with the cloaca through a wide vagina. A considerable number of the Elasmobranchii are viviparous, and in these the inner surface of the uterus is beset with numerous vascular villi, while the shell-gland is small or vestigial. The testes are oval or elongate : the convoluted epididymis is connected with the anterior end by efferent ducts, and from it arises the vas deferens. The latter is dilated near its opening into the urinogenital sinus to form an ovoid sac — the vesicula seminalis. A sperm-sac is sometimes present, opening close to the aperture of the vas deferens. The Miillerian ducts are vestigial in the male. Impregnation is internal in all the Elasmobranchs with the possible exception of Lasmargus (the Greenland Shark), the claspers acting as intromittent organs by whose agency the semen is transmitted into the interior of the oviducts. In all the Elasmobranchs the ova are very large, consisting of a XIII PHYLUM CHORDATA 181 large mass of yolk-spherules held together by means of a network of protoplasmic threads, with, on one side, a disc of protoplasm — the germinal disc. The process of maturation is similar to that observable in holoblastic ova; one polar body is thrown off in the ovary, the other apparently at impregnation. The ripe ovum ruptures the \\all of the enclosing follicle and so passes into the abdominal cavity to enter one of the oviducts through the wide abdominal opening. Impregnation takes place in the oviduct, and the impregnated ovum in the oviparous forms becomes sur- rounded by a layer of semi-fluid albumen and enclosed in a shell of keratin secreted by the shell-gland. The shell varies in shape somewhat in the different groups : most commonly, as in many Dog-fishes (Fig. 826), it is four- cornered, with twisted filamentous appendages at the angles, by means of which it becomes at- tached to sea-weeds and the like. In the Skates the filaments are absent. In the Port Jackson Sharks (Cestracion, Fig. 838) it is an ovoid body, the wall of which presents a broad, spiral flange. The young Elasmobranch goes through its development enclosed in the shell, until it is fully formed, when it escapes by rupturing the latter. In the viviparous forms the ovum undergoes its develop- ment in the uterus, in which for the most part it lies free — except in some Mustelidoe and Car- chariidse, in which there is a close connection between the yolk-sac of the embryo and the wall of the uterus, folds of the former interdigitating with folds of the latter, and nourishment being thus conveyed from the vascular system of the mother to that of the foetus by diffusion. In some of the viviparous forms a distinct, though very delicate shell, some- times having rudiments of the filaments, is formed, and is thrown off in the uterus. In the genera Rhinclatus and Trygonorhina, which are both viviparous, each shell encloses not one egg, but three or four. Laemargus is said to differ from all the rest of the Elasmobranchii in having the ova fertilised after they have been deposited, as well as in the small size of the ova. Fio. 838.— Egg-case of Cestracion galeatus. (After Waite.) 182 ZOOLOGY SECT. Development. — Segmentation is meroblastic,1 being confined to the germinal disc, which, before dividing, exhibits amoeboid movements. While segmentation is going on in the germinal disc there appear a number of nuclei, the source of which is not certain, in the substance of the yolk. When segmentation is complete, the blastoderm appears as a lens-shaped disc, thicker at one end— the embryonic end. It is found to consist of two layers of cells — an upper layer in a single stratum, and a lower layer several cells deep. A segmentation-cavity appears early among the cells of the lower layer ; the lower-layer cells afterwards disappear from the floor of this, the cavity then coming to rest directly on the yolk. An in-folding (Fig. 839) now begins at the thickened embryonic edge of the blastoderm, which here becomes continuous with the cells of the lower layer. The cavity (al), at first very small, formed below this in-folding is the rudiment of the archenteron, Fio. 839. — Longitudinal section through the blastoderm of a Fristiurus embryo before the medullary groove has become formed, showing the beginning of the process of infolding or invagination. al. archenteron ; ep. ectoderm ; er. embryonic rim ; /;?. niesoderm. (After Balfour.) and the cells lining this cavity above, which form a definite layer, partly derived from the in-folded ectoderm, partly from the cells of the lower layer, are the beginning of the definite endoderm. The edge of the in-folding, entitled the embryonic rim is obviously the equivalent of the dorsal lip of the blastopore in Amphioxus. The endoderm and its underlying cavity soon grow forwards towards the segmentation-cavity. Under the latter appears a floor of lower-layer cells, but the cavity soon becomes obliterated as the archenteron develops. After the formation of the embryonic rim a shield-like embryonic area is distinguishable in front of it, with two folds bounding a groove — the medullary groove. The mesoderm becomes estab- lished at about the same time. It is formed from two separate and distinct sources (Fig. 840). Along the edge of the embryonic rim appears a horizontal groove-like depression : this — the external ccelomie lay (c.l).1) — marks the line of origin of the peripheral part of the mesoderm (m.s.1), which grows inwards from it as a plate of cells between the ectoderm and the endoderm. The central part of the embryonal mesoderm (m.s.2) is developed from the endoderm at a point immediately external to the rudiment of the notochord : here also a slight groove — the internal 1 Except in one species of Cestraciou. XIII PHYLUM CHORDATA 183 ccelom.ic bay (c,.b.2) — is distinguishable, and from this a plate of mesoderm cells grows outwards. Eventually the peripheral and central plates of mesoderm come into contact and coalesce to form a continuous sheet on each side of the middle line. Though the mesodermal rudiments, peripheral and central, contain no cavities, the grooves (coslomic bays) from which their development takes its origin, may represent the cavities of the coelomic sacs of Amphioxus. As the blastoderm extends over the yolk, the edge forms a ridge continuous with the embryonic rim. The latter assumes the form of FIG. 840.— FriSturus, transverse section of blastoderm, showing the formation of the mesoderm. ?)/>• I. dorsal lip of blastopore ; <•. 6.1 external ccelomic bay ; c. !>.% internal coelomic bay ; ec. ectoderm ; en. endoderm ; m. /. medullary fold ; HI. pr. medullary groove ; ws.l external rudiment of mesodirm; wi*.2 internal rudiment of mesoderm; nc. notochord ; y. yolk; yl'.n. yolk nuclei. (From O. Hertwig, after Rabl.) two prominent caudal swellings (Fig. 842, cd.}. The medullary groove meanwhile deepens, and its edges grow over, as in Amphi- oxus and the Urochorda, so as to form a canal (Fig. 841, G\ Fig. 843). The union takes place first in the middle, the anterior and posterior parts (Fig. 843, neur.) remaining open for a while. When the posterior part closes, it does so in such a way that it encloses the blastopore, and there is thus formed, as in Amphioxus and Ascidians, a temporary passage of communica- tion betsveen the medullary canal and the archenteron — the neurenteric passage. The ectoderm gives rise, as in Vertebrates in general, not only to the epidermis and the central nervous system, but also to the peripheral nervous system, the lining membrane of the olfactory sacs, the lens of the eye, and the lining membrane of the auditory labyrinth, of the mouth, and of the outer portions of the cloaca and gill-clefts. 184 ZOOLOGY The notoeliord (Fig. 841, eh.} is developed as a cord of cells derived from the lower layer. Each lateral sheet of mesoderm, soon after its formation by the coalescence of the peripheral and central rudiments, becomes divided by the development of a horizontally directed cleft-like space in its interior. The inner part of each sheet then separates from the outer by the formation of a longitudinal fissure. The former, which is known as the vertebral plate, becomes divided by a series of trans- verse fissures into a number of squarish masses, the prot overt cbrce or FIG. 841.— Diagrammatic longitudinal sections of an Elasmobranch embryo. A, section of the young blastoderm with segmentation-cavity enclosed in the lower layer cells; B, older ' blastoderm with embryo in which endoderni and mesoderm are distinctly formed, and in which the alimentary slit has appeai-ed. The segmentation-cavity is still represented as being present, though by this stage it has in reality disappeared. C, older blastoderm with embryo in which the neural canal has become formed and is continuous posteriorly with the alimentary canal. Ectoderm without shading ; mesoderm and also notoeliord black with clear outlines to the cells ; endoderm and lower-layer cells with simple shading, al. alimentary cavity ; ch. notoeliord ; ep, ectoderm ; m. mesoderm ; n. nuclei of yolk ; n.c neurocoele ; sfi. segmentation-cavity ; x. point where ectoderm and endoderm become continuous at the posterior end of the embryo. (From Balfour.) mesodermal somites. The outer part forms a broad plate, the lateral plate. The lateral plate consists of two layers, a dorsal or somatic, and a ventral or splanchnic, and the cavity between them is the beginning of the ccelome. The protovertebrae send off cells round the notochord to form the bodies of the vertebra, the remainder giving rise to the muscles of the voluntary system. An isthmus of mesoderm cells (nephrotome), which still connects each protovertebra with the lateral plate and contains a prolongation of the cavity, gives rise to the pronephric duct and tubules. The PHYLUM CHORDATA 185 Fro. S42.— Embryo of Scyllium canicula with the tail- swellings well marked and the medullary groove just beginning. U. e. edge of blastoderm ; M. p. blastopore ; r -^T^ — ¥**~ bLe, of mesodermal seg- ments appear, the cells of which give rise to the muscles of the branchial, hyoid, and mandibular arches, and probably also of the palatoquadrate and the eye. By degrees the body of the young Fish becomes moulded on the blastoderm. This is effected by the formation of a system of folds, anterior, posterior, and lateral, which grow in- wards in such a way as to separate off the body of the embryo from the rest of the blastoderm enclosing the yolk. As the folds approach one another in the middle, underneath the embryo, they come to form a con- striction connecting the body of the embryo with the yolk enclosed in the extra-embryonic part of the blastoderm. The pro- cess may be imitated if FIG. 843. — Embryo of a Ray with the medullary groove closed except at the hind end. The notched em- bryonic part of the blastoderm has grown faster than the rest and come to project over the surface of the , yolk. bl. e. edge of blastoderm ; hd. head ; neur. un- enclosed part of the neurocoele. (After Sedgwick.) necting the pinched-off portion with we pinch off a portion of a ball of clay, leaving only a narrow neck con- the rest. The body of the embryo thus becomes folded off from the yolk-sac and comes to 186 ZOOLOGY SECT. be connected with it only by a narrow neck or yolk-stalk (Fig. 844). The head and tail of the young Fish soon become differentiated, and a series of involu- tions at the sides of the neck (Fig. 845) form the branchial clefts and spiracle. A number of very delicate filaments (Figs. 845, 846) grow oat from these apertures and be- come greatly elongated ; these are the pro- visional gills, which atrophy as the develop- ment approaches com- pletion, their bases alone persisting to give rise to the permanent gills. The great development of these gill-filaments in the embryos of some vivi- parous forms suggests that, in addition to their respiratory functions, they may also serve as organs for the absorption of nutrient fluids secreted by the villi of the uterine wall.1 The fins, both paired and unpaired, ap- pear as longitudinal ridges of the ectoderm Dping egg of an ibryo, the blnsto- FIG. 844. — Three views of the develo Elasmobranch, showing the embry derm, and the vessels of the yolk-sac. The shaded part (W.) is the blastoderm, the white part the un- covered yolk. A, young stage with the embryo still attached at the edge of the blastoderm ; B, older stage with the yolk not quite enclosed by the blasto- derm ; C, stage after the complete closure of the yolk. a. arterial trunks of yolk-sac ; bl. blastoderm ; v. venous trunks of yolk-sac ; y. point of closure of the yolk-blastopore ; x, portion of the blastoderm out- side the arterial sinus terminalis. (From Balfour.) enclosing mesoderm. In some Elasmobranchs the paired fins are at first represented on each side by a continuous ridge or fold, which only subse- quently becomes divided into anterior and pos- terior portions — the rudi- ments respectively of the rctoral and pelvic fins. buds from the proto- 1 In a species of Trygon a number of the villi of the uterus project into the pharynx of the fetus through the spiracles, and nourishment is probably received by this means. XIII PHYLUM CHORDATA vertebrae : these, the muscle-buds, give rise to the fin-muscles ; at first, from their mode of origin, they present a metameric Fie. $45 — Side view of head of embryo of Scyllium canicula, with the rudiments of the gills on the first and second branchial arches, eye, eye ; ni. >>rn. mid-brain; mnd. mandible; WM. nasal sac. (After Sedgwick.) spir — Fio. $46.— Side view of the head of Scyllium canicula at a somewhat later stage. The gill- filaments have increased in number and are present on the mandibular arch. nnci. angle of the jaw ; hy. hyoid ; in. brn. mid-brain ; ntix. nasal sac ; sjrir. spiracle. (After Sedgwick.) arrangement, but this is in great measure lost during develop- ment. Ethology and Distribution. — The habits of the active, fierce, and voracious Sharks, which live in the sui face- waters of the sea, waging war on all and sundry, contrast strongly with those of the more sluggish Rays, which live habitually on the bottom, usually in shallow water, and feed chiefly on Crustaceans and Molluscs, with the addition of such small Fishes as they can capture. As a group, the Elasmobranchs, more particularly the Sharks, are distinguished by their muscular strength, the activity of their movements, and also by the acuteness of their senses of sight and smell. The only deep water Elasmobranch known is a species of Ray, which extends to a depth of over GOO fathoms. None of the Elasmobranchs are of very small size, and com- prised among them are the largest of living Fishes : the harm- less Basking Sharks (Selache) sometimes attain a length of 35 feet or more, the formidable Great Blue Shark (Carcharodon) some- times reaches 40 feet, and some of the Rays also attain colossal dimensions. In this respect, however, recent Sharks and Rays are far behind some of the fossil forms, some of which, if their general dimensions were in proportion to the size of their teeth, must have reached a length of as much as sixty feet. The earliest fossil remains of Elasmobranch Fishes that have been found occur in rocks belonging to the Upper Silurian period. Throughout the Palaeozoic epoch the Elasmobranchs constituted a 188 ZOOLOGY SECT. very important section of the fauna — a large proportion of the fish- remains that have been found in Palaeozoic formations being the remains of Elasmobranchs, mainly in the form of spines and teeth. Most of the Palaeozoic Elasmobranchs were characterised by a great development of the exoskeleton. The teeth differ from those of existing forms in being provided with broad bases by means of which they articulated together, and in various groups there is a union of the teeth by the coalescence of their bases so as to form broad crushing plates. A similar union is not uncommon between the parts of the general exoskeleton, a good many Palaeozoic Sharks having been encased in an armour of solid plates formed by such a coalescence. In the endoskeleton there is to be observed among the fossil Elasmobranchs a gradual advance in the degree of calcification of the spinal column from the Palaeozoic forms onwards, the Protoselachii alone among exist- ing forms representing in this respect the condition which seems to have prevailed in the most ancient members of the class. The group (Cestracionts) now represented by two or three species of Port Jackson Sharks seems to have been very abundant in Palaeozoic times. The extinct Pleuracanthea, together with Cladoselachus, which, as briefly stated in the sketch of the classification, differ from the other known members of the class in the structure of the fins and other points, range from the Devonian to the Permian, and are perhaps also represented in the Trias. Sub-Class II. — Holocephali. The existing representatives of the Holocephali are included under the single family ChwMeridce, containing three genera— Chimcera, Callorhynchus, and Harriotta. Even taking in fossil forms, the group is a very small one ; it agrees in many funda- mental characteristics with the Elasmobranchii, and is sometimes included in that sub-class. Of the recent genera, Chimaera, the so-called " King of the Herrings" (Fig. 847, A) is found on the coasts of Europe, Japan, and Australia, the west coast of North America, and at the Cape of Good Hope ; Callorhynchus (B*) is tolerably abundant in the South Temperate seas; Harriotta is a deep-sea form. External Characters. — The general form of the body is Shark- like, but the large, compressed head and small mouth are strikingly different from the depressed, shovel-shaped head and wide mouth of most Selachians. The mouth is bounded by lip-like folds, two of which (B, l.f,,l.f'\ placed laterally and supported by labial carti- lages, resemble the folds in which the premaxillae and maxillae of many Bony Fishes are enclosed : a third fold, external to and concentric with the mandible, is also supported by labial cartilages XIII PHYLUM CHORDATA 189 and has the appearance of a second or external lower jaw. In Chimera the snout is blunt, in Harriotta long and pointed ; in Callorhynchus it is produced into a rostrum, from the end of which depends a large cutaneous flap (B, tc) abundantly supplied with nerves and evidently serving as an important tactile organ. A still more important difference from Elasmobranchs is the possession of only a single external branchial aperture (br. ap.), owing to the fact that a fold of skin, the opcrculwn (op,), extends fr.cl Km. S47.— A, Chimacra monstrosa ; B, Callorhynchus antarcticus. a. d. anterior clasper ; a. d.' pouch for its reception ; br. ap. branchial aperture ; <•. /. caudal fin ; c. /.' its whip like prolongation ; d. /. 1, d. /. 2, dorsal fins ; //•. d. frontal clasper; l.f., I. /.' labial folds ; 1. I. lateral line ; na. ap. nasal aperture ; op. operculum ; pet. f. pectoral fin ; ptg. ptei-ygopodia ; pe.f. pelvic fin ; t. teeth ; tc, tactile flap ; v.f. ventral fin. (A, after Cuvier.) backwards from the region of the hyoid arch and covers the true gill-slits, which thus come to open into a common chamber situated beneath the operculum and communicating with the exterior by a single secondary branchial aperture placed just anterior to the shoulder-girdle. Equally characteristic is the circumstance that the urinogenital aperture is distinct from and behind the anus, there being no cloaca. There are two large dorsal fins (d.f.l}d.f.(^ and a small ventral (v. /.) ; the caudal fin (c./.) is of the ordinary heterocercal type 190 ZOOLOGY SECT. in the adult Callorhynchus, but- in the young (Fig. 853) the extremity of the tail proper is not upturned, and the fin-rays are arranged symmetrically above and below it, producing the form of tail-fin called dipliy cereal. In Chimsera the dorsal lobe of the tail may be produced into a long whip-like filament (c.f.). The pectoral (pct.f.) and pelvic (pv.f.) fins are both large, especially the former. In the male there is a horizontal slit (B, a. d.f) situated a little in front of the pelvic fins ; it leads into a shallow glandular pouch, from which can be protruded a peculiar and indeed unique apparatus, the anterior clasper (A,a.cL), consisting of a plate covered with recurved dermal teeth, to which is added, in Callo- rhynchus, a plate rolled upon itself to form an incomplete tube. The use of this apparatus is not known. A rudiment of the pouch occurs in the female, although the clasper itself is absent. The male possesses, in addition, a pair of the ordinary pterygopodia or posterior claspers (ptg.\ and is furthur distinguished by the presence of a little knocker-like structure, the frontal clasper (fr. el.), on the dorsal surface of the head. In Harriotta the claspers are poorly developed, and the frontal clasper is absent. The lateral line (I. I.) is an open groove, and there are numerous sensory pits, arranged in curved lines, on the head. The skin is smooth and silvery, and bears for the most part no exoskeletal structures. There are, however, delicate, recurved dermal teeth on the anterior and frontal claspers, and the first dorsal fin is supported by an immense bony spine or dermal defence (sp.). In the young, moreover, there is a double row of small dermal teeth along the back. Endoskeleton. — The vertebral column consists of a persistent notochord with cartilaginous arches. In Chimera there are calcified rings (Fig. 848, c. r.) embedded in the sheath of the notochord. The anterior neural arches are fused to form a high, compressed, vertical plate, to which the first dorsal fin is articulated. The cranium (Figs. 849 and 850) has a very characteristic form, largely owing to the compression of the region between and in front of the large orbits, which are separated from the cranial cavity only by membrane in Callo- rhynchus (Fig. 850, or.)] in Chimera they lie above the level of the cranial cavity and are separated from one another by a median vertical partition of fibrous tissue (Fig. 849, i. o. s). At first sight the palatoquadrate, or primary upper jaw, appears to be absent, but a little consideration shows it to be represented by a triangular plate (pal. quJ) which extends downwards and outwards from each side of the cranium and presents at its apex a facet for the articulation of the mandible. The palatoquadrate is therefore fused with the cranium and furnishes the sole support for the lower jaw; in a word the skull is autostylic. The pituitary fossa (Fig. XIII PHYLUM CHORDATA 191 71. Sp Tt.CL c.r rich nch.sh 850, s. t.) is very deep and inclined backwards; on the ventral surface of the basis cranii is a pit (pt.) for the extra-cranial portion of the pituitary body. The posterior portion of the cranial cavity is very high ; the anterior part — containing most of the fore-brain — is low and tunnel-like, and has above it a cavity of almost equal size (Nv. 5 0'.) for the ophthalmic branches of the fifth nerves. The greater part of the membranous labyrinth is lodged in a series of pits on the side-walls of the cranium (a.s.c., p.s.c.), and is separated from the brain by membrane only. The occipital region articulates with the verte- bral column by a single saddle-shaped surface or condyle (pc. en.). There is a great development of labial cartilages, particularly noticeable being a large plate which, in Callorhyn- chus, lies just externally to the mandible, nearly equal- ling it in size and having the appearance of a second- ary or external jaw. In Callorhynchus the snout is supported by three cartilagi- nous rods growing forward from the cranium, of which one (r.) is median and dorsal and represents the rostrum ; these, as well as the great lower labial are represented by comparatively small structures in Chimsera (Fig. 849, lb?). The hyoid resembles the branchial arches in form and is little superior to them in size. Above the epihyal (Fig. 849, e. hy.) is a small cartilage (ph. hy.\ evidently serially homologous with the pharyngo- branchials, and therefore to be considered as a pharyngokyal. It represents the hyomandibular of Elasmobranchs, but, having no function to perform in the support of the jaws, it is no larger than the corresponding segments in the succeeding arches. Long cartilaginous rays (op. r.} for the support of the operculum are attached to the ceratohyal. The first dorsal fin is remarkable for having all its pterygio- phores fused into a single plate, which articulates with the B n.ei FIG. 848.— Chimaera monstrosa. A, transverse section of the vertebral eolunm ; B, lateral view of the same. c. r. calcified ring ; /t. r. haemal ridge ; int. intercalary piece ; n. a. neural arch ; nch. position of notochordal tissue ; nch. sh. sheath of notochord ; n. sp. neural spine. (After Hasse.) 192 ZOOLOGY SECT. coalesced neural arches already referred to. The remaining fins are formed quite on the Elasmobranch type, as is also the shoulder girdle. The right and left halves of the pelvic arch are separate from one another, being united in the middle ventral line by ligament only; each presents a narrow iliac region and a broad flat pubo-ischial region perforated by two apertures or fenestrae closed by membrane, one of them of great size in Callorhynchus. -/. pharyngohyal ; p. a. c. position of posterior semicircular canal ; qu. quadrate region ; r, rostrum. (After Hubrecht.) The skeleton of the anterior clasper articulates with the pubic region. Digestive Organs. — The teeth (Fig. 850) are very character- istic, having the form of strong plates with an irregular surface and a sharp, cutting edge. In the upper jaw there is a pair of small vomerine teeth (vo. t.) in front, immediately behind them a pair of large palatine teeth (pal. t.), and in the lower jaw a single pair of large mandibidar teeth (mnd. t.). They are composed of vaso- dentine, and each palatine and mandibular tooth has its surface slightly raised into a rounded elevation of a specially hard sub- stance, of whiter colour than the rest of the tooth, and known as a tritor (tr.). The stomach is almost obsolete, the enteric canal passing in a straight line from gullet to anus ; there is a well- developed spiral vahe in the intestine. Respiratory Organs. — There are three pairs of holobranchs or complete gills borne on the first three branchial arches, and two hemibranchs or half-gills, one on the posterior face of the hyoid, XIII PHYLUM CHORDATA 193 the other on the anterior face of the fourth branchial arch. The fifth branchial arch is, as usual, gill-less, and there is no cleft between it and its predecessor. The gill-filaments are fixed in their whole length to an interbranchial septum, as in Elasmo- branchs. The small heart resembles that of the Dog-fish in all essential respects, being formed of sinus venosus, auricle, ventricle, and conus arteriosus, the last with three rows of valves. Nv. s.'o' rtch °ccn FIG. 850.— Callorhynchus antarcticus, sagittal section of skull ; the labial cartilages are removed, a. s. c. apertures through which the anterior semicircular canal passes from the cranial cavity into the auditory capsule ; e. 1. d. aperture for endolymphatic duct ; mck. c. Meckel's cartilage ; mnd. t. mandibular tooth ; nch. notochord ; Nv. 5, trigeminal foramen ; Nv. 5. o. foramen for exit of ophthalmic nerves , Nv. 5.'o', canal for ophthalmic nerves with apertures of entrance and exit ; Nv. 10, vagus foramen ; oc. en. occipital condyle ; or. fenestra separating cranial cavity from orbit ; pal. qu. palatoquadrate ; pal. t. palatine tooth ; pn. position of pineal body ; pt. pit for extra-cranial portion of pituitary body ; p. s. c. apertures through which the posterior semicircular canal passes into the auditory capsule ; qu. quadrate region of palatoquadrate ; r. rostrum ; sac. depression for sacculus ; s. t. sella turcica ; fr» tritor ; to. t. vomerine teeth. The brain (Fig. 851), on the other hand, is very unlike that of Scyllium, but presents a fairly close resemblance to that of Scymnus. The medulla oblongata (med. obi.) is produced laterally into large frill-like restiform bodies (cp. rst.\ which bound the hinder half of the cerebellum (cblm.). The diencephalon (dien.) is extremely long, trough-shaped, and very thin-walled, without pronounced optic thalami ; it is continued without change of diameter into a distinct prosencephalon, which gives off the cerebral hemi- spheres (n'b. li.) right and left. The combined diacoele and proso- ccele (di. civ.) are widely open above in a brain from which the VOL. n N 194 ZOOLOGY SECT. membranes have been removed (A), but in the entire organ (B) are roofed over by a conical, tent-like cJioroid plexus (ch. plx. 1). The cavities of the small, spindle-shaped hemispheres (crl. A.) com- municate with the third ventricle by wide foramina of Monro Q ccL.obl FIG. 851.— Callorhynchus antarcticus. A, dorsal view of brain after removal of the mem- branes ; B, side view with the membranes in place, cblin. cerebellum ; ch. plx. 1, choroid plexus of fore-brain, and ch. plx. 2, of hind-brain ; cp. rst. corpus restiforme ; cp. str. corpus striatum ; crb. h. cerebral hemisphere ; di. coe. diacoele ; dien. diencephalon ; for. M. foramen of Monro ; lb. inf. lobus inferior; med. obi. medulla oblongata ; mt. me. metaccele ; Nr. 3, optic nerve ; Nv. 5, trigeminal ; Nv. 8, auditory ; Nv. 10, vagus ; olf. 1. olfactory bulb ; olf. p. olfactory peduncle ; opt. 1. optic lobe ; pn. b. pineal body ; pn. s. pineal stalk ; ply. pituitary body. (for.M.), partly blocked up by hemispherical corpora striata (cp. str.). Each hemisphere is continued in front into a delicate thin-walled tube, the olfactory peduncle (olf. p.), bearing at its extremity a compressed olfactory bulb (olf. /.). XIII PHYLUM CHORDATA 195 The optic nerves (Nv.2) form a chiasma. The pineal body (pn. &.) is a small rounded vesicle borne on a hollow stalk (pn. s.) which runs just outside the posterior wall of the tent-like choroid plexus. The pituitary body (pty.} consists of intra- and extra- cranial portions, the former lodged in the sella turcica, the latter in the pit already noticed on the ventral or external face of the skull-floor (Fig. 850, pt.). In advanced embryos the two are united by a delicate strand of tissue. Urinogenital Organs. — The kidneys (Fig. 852, kd.) are lobed, deep-red bodies, like those of the Dog-fish, but shorter and stouter. In the male they are much longer than in the female; the anterior portion is massive, and consists mainly of a mass of true renal tubules; it is in- distinctly divided into segments: the posterior portion is narrower and also indistinctly segmented ; from both parts arise a number of ducts (mesonephric ducts) the majority of which open into the vas deferens, while the last six open into the urinogenital sinus. In the female the ducts all open into a rounded median diverticulum of the cloaca, the urinary bladder or urinary sinus, situated between the two ovi- ducal apertures. The female reproductive organs are also constructed on the Elasmo- branch pattern, and are chiefly noticeable for the immense size 'of the shell-glands and of the uteri. But the male organs present certain quite unique characters. The testcs (ts.) are large ovoid bodies the tubules of which apparently do not contain FIG. 852.— Callorhynchus antarcticus. A, male urinogenital organs of left side- ventral aspect ; B, anterior part of vesicula seminalis in section, d. cloaca; epid. epi- didymis ; kd. kidney ; muL d. Miillerian duct ; j sph. spermatophores ; ts. 'testis ; u. y. x. ririnogenital sinus ; v. df. vas de- ferens ; vs. sem. vesicula scrninalis. (A, after lledeke.) N 2 196 ZOOLOGY SECT. fully developed sperms, but only immature sperm-cells. These latter are probably passed through vasa efferentia into the vas deferens, which is coiled in a highly complicated manner to form a body of considerable size, commonly termed the epididymis, closely applied to the surface of the anterior part of the kidney. In this the sperms become aggregated into sp&rmatopkoresm the form of small ovoidal capsules surrounded by a resistant membrane and full of a gelatinous substance in which bundles of sperms are imbedded. The lower end of the vas deferens (v. df.) is dilated to form a large cylindrical vesicula seminalis (v. sem.) imperfectly divided into compartments by transverse partitions (7?) and filled with a greenish jelly. The spermatophores (sph.) are passed into these compartments and finally make their way through the central passage into the urinogenital sinus (11. g. $.). The vestigial Mullerian ducts (mul. d>) are much more fully developed than in the Dog-fish : they are complete, though narrow, tubes opening in front by a large common aperture into the ccelome, and behind connected with the urinogenital sinus. Development. — Internal impregnation takes place, and the oosperm becomes surrounded, as in the Dog-fish, by a horny egg- shell secreted by the shell-glands. The egg-shell of Callorhynchus (Fig. 853) is of extraordinary size — about 25 cm. in length, or fully five-sixths as long as the abdominal cavity — and the elongated chamber for the embryo is surrounded by a broad, flat expansion covered on one side with yellow hair-like processes, and giving the shell a close resemblance, doubtless protective, to a piece of kelp. Nothing is known of the early development : the advanced embryo has elongated gill-filaments (br. f.j projecting through the branchial aperture, a diphycercal tail, and a curiously lobed and nearly sessile yolk-sac (yk. s.). Fossil remains of Holocephaliare known from the lower Jurassic rocks upwards. As might be expected, they consist mostly of teeth and" of dorsal fin-spines, but in some cases, and notably in Squaloraja, practically the whole of the skeleton is preserved. Sub-Class III.— Teleostomi. In this sub-class are included all the commonest and most familiar Fishes, such as the Perch, Pike, Mackerel, Cod, Sole, Herring, Eel, Salmon, etc., as well as the so-called " Ganoid " Fishes, such as the Sturgeon, Bony Pike (Lepidosteus), and Bow-fin (Amia) of North America, and the Folypterus of the Nile. They are distinguished from Elasmobranchs and Holocephali by having the primary skull and shoulder-girdle complicated by the addition of investing bones, and by possessing bony instead of horn-like fin- rays. The gills are covered by an operculum ; the anus is distinct VOL. II 197 N 2* 198 ZOOLOGY SECT. from the urinary and genital apertures ; and the brain has in most cases no cerebral hemispheres, but an undivided prosence- phalon with a non-nervous roof. 1. EXAMPLE OF THE SUB-CLASS. — THE BROWN TROUT (Salmo f arid). The Brown Trout is common in the rivers and streams of Europe, and has been acclimatised in other parts of the world, notably in Australia and New Zealand. It varies greatly in size according to the abundance of food and the extent of the water in which it lives : it may attain sexual maturity, and therefore be looked upon as adult, at a length of 18 — 20 cm. (seven or eight inches), but in large lakes it may grow to nearly a metre in length. Other species of Salmo, such as the Salmon (S. salar}, the Lake Trout (S.ferox), the American Brook Trout (S. fontinalis), are common in the Northern Hemisphere and differ only in details from S. fario. External Characters. — The body (Fig. 854) is elongated, com- pressed, thickest in the middle, and tapering both to the head and FIG. 854.— Salmo fariO. a. L adipose lobe of pelvic fin ; tin. anus ; c. /: caudal fin ; tL f. 1, first dorsal fin ; d. /. 2. second dorsal or adipose fin ; L 1. lateral line ; op. operculum ; pet. f. pectoral fin ; i>c. f. pelvic fin ; r. / ventral fin. (After Jardine.) tail. The mouth is terminal and very large : the upper jaw is supported by two freely movable bones, the prcmaxilla (Fig. 855, pmx.) in front and the maxilla (mx.) behind, both bearing sharp curved teeth arranged in a single row. When the mouth is opened a row of palatine teeth is seen internal and parallel to those of the maxilla, and in the middle line of the roof of the mouth is a double row of wmerine teeth. The lower jaw (md.} is mainly supported by a bone called the dentary and bears a row of teeth : on the throat each ramus of the mandible is bounded mesially by a deep groove. The floor of the mouth is produced into a prominent tongue (£.) bearing a double row of teeth. In old males the apex of the lower jaw becomes curved upwards like a hook. The large eyes have no eyelids, but the flat cornea is covered by PHYLUM CHORDATA 199 pop pmac a transparent layer of skin. A short distance in front of the eye is the double nostril (nal, no2), each olfactory sac having two external apertures, the anterior one (via1) provided with a flap- like valve. There is no external indication of the ear. On each side of the posterior region of the head is the operculum (Fig. 854, op.) or gill-cover, a large flap which, when raised, displays the gills; between it and the flank is the large crescentic gill- opening, from which the respiratory current makes its exit. The operculum is not a mere fold of skin, as in Holocephali, but is supported by four thin bones the out- lines of which can be made out through the skin ; they are the oper- cular (Fig. 855, op.), preopercular (p. op.), sub-opercular (s. op), and inter - opercular (i. op) ; the latter is attached to the angle of the mandible. The ventral portion of the operculum is produced into a thin membran- ous extension, the branchiostegal mem- brane (br. m), supported by twelve flat, overlapping bones, the branchiostegal rays. The narrow area on the ventral surface of the throat which separates the two gill-openings from one another is called the isthmus. The gills, seen by lifting up the operculum, are four red, comb-like organs, each having a double row of free gill-filaments ; alternating with the gills are the five vertically elongated gill-slits, opening into the mouth. The Trout breathes by the drawing in of water through the mouth and its passage outwards through the gill-slits. The inspiration or inward movement of the water is effected by the opercula being moved outwards, the space internal to them thus being widened, and water flowing in through the open mouth to fill the vacuum, the branchiostegal membrane at the same time closing the gill-opening and thus preventing the water from flowing in from behind. Expiration is brought about by the opercula moving inwards and forcing the water out. Owing to the action of a pair of transversely directed membranous folds, the respiratory valves, one attached to the roof, the other to the floor of the mouth, which are so directed as to become expanded and block the passage when water presses on them from behind, the water is compelled to make its exit through the gill-slits. FIG. 855.— Head of female Salmo fario. Ir. m. branchio- stegal membrane ; i. op. interopercular : mnd. mandible : mx. maxilla ; nal, anterior, aud no?, posterior external nostril ; op. opercular; pet. /. pectoral fin; pmx. pre- maxilla ; p. op. preopercular ; s. op. subopercular ; t. tongue. 200 ZOOLOGY On the ventral surface of the body, at about two-thirds of the distance from the snout to the end of the tail, is the anus (Fig. 854, an.)', behind it is the urinogcnital aperture, of almost equal size and leading into the urinogcnital sinus, into which both urinary and genital products are discharged. The region from the snout to the posterior edge of the operculum is counted as the head', the trunk extends from the operculum to the anus ; the post-anal region is the tail. There are two dorsal fins: the anterior dorsal (Fig. 854, d.f. 1) is large and triangular, and is supported by thirteen bony fin-rays ; the posterior dorsal (d.f. 2) is small and thick, and is devoid of bony supports : it is distinguished as an adipose fin. The caudal fin (c.f.) is the chief organ of locomotion ; it differs markedly from that of most Elasmobranchs in being, as far as its external appear- ance is concerned, quite symmetrical, being supported by fin-rays which radiate regularly from the rounded end of the tail proper ; such outwardly symmetrical tail-fins are called homocercal. There is a single large ventral fin (v.f.) supported by eleven rays. The pectoral fin (pet. /.) has fourteen rays and is situated, in the normal position, cldfee behind the gill-opening, but the pelvic fin (pv. /.) has shifted its position and lies some distance in front of the vent : it is supported by ten rays, and has a small process or adipose lobe (a. I.) springing from its outer edge near the base. The body is covered by a soft, slimy skin through which, in the trunk and tail, the outlines of the scales can be seen; on the head and fins the skin is smooth and devoid of scales. A well-marked lateral line (I. I.) extends along each side from head to tail, and is continued into branching lines on the head. The skin is grey above, shading into yellowish below, and is covered with minute black pigment-spots which, on the sides and back, are aggregated to form round spots two or three milli- metres in diameter. In young specimens orange-coloured spots are also present. Skin and Exoskeleton. — The epi- dermis contains unicellular glands, from which the mucus covering the body is secreted, and pigment-cells, to which the colours of the animal are due. The scales (Fig. 856) are lodged in pouches of the dermis and have the form of fiat, FIG. 856.— scale of Saimo fario. nearly circular plates of bone marked a. anterior portion covered by . , • • T i • overlap of preceding scales; with concentric lines, but having no b. free portion covered only by TT 11 i • v pigmeiited epidermis. Jlaversian canals, lacunae, or canalicun. They have an imbricating arrangement, overlapping one another -from before backwards, like the tiles of a house, in such a way that a small three-sided portion (&) of each scale comes to lie immediately beneath the epidermis, while -XIII PHYLUM CHORDATA 201 the rest (a) is hidden beneath the scales immediately anterior to it. Besides the scales, the fin-rays belong to the exoskeleton, but will be most conveniently considered in connection with the endoskeleton. Endoskeleton. — The vertebral column shows a great advance on that of the two previous classes in being thoroughly differentiated into distinct bony vertebrae. It is divisible into an anterior or abdominal region and a posterior or caudal region, each containing about twenty- eight vertebrae. A typical abdominal vertebra consists of a dice-box-shaped centrum (Fig. 857, ON.) with deeply concave anterior and pos- terior faces, and perforated in the centre by a small hole. The edges of the centra are united by ligament and the biconvex spaces between them are filled by the remains of the notochord ; there are also articulations be- tween the arches by means of little bony processes, the zygapo- •physes (N. ZYG., H. ZYG.). To the dorsal surface of the centrum is attached, by ligaments in the anterior vertebrae, by ankylosis or actual bony union in the pos- terior, a low neural arch (N. A.), which consist in the anterior vertebrae of distinct right and left moieties, and is continued above into a long, slender, double neural spine (N. SP.), directed upwards and backwards. To the ventro-lateral region of the vertebra are attached by ligament a pair of long, slender pleural ribs (R.) with dilated heads, which curve downwards and backwards between the muscles and the peritoneum, thus encircling the abdominal cavity. In the first two vertebrae they are attached directly to the centrum, in the rest to short downwardly directed bones, the parapophyses (PA. PH.) immovably articulated by broad surfaces to the centrum. At the junction of the neural arch with the centrum are attached, . also by fibrous union, a pair of delicate inter-muscular bones (i. M. B.), which extend outwards and backwards in the fibrous septa between the myomeres. The first and second abdominal vertebrae bear no ribs. In the last three the neural spines (B; N. SP.) are single. i Fio. 857.— Salmo fario. A, one of the anterior, and B, one of the posterior ab- dominal vertebrae ; C, one of the anterior, and D, one of the posterior caudal vertebrae. CN. centrum ; 1MB. intermuscular bone ; HA. haemal arch ; H. SP. haemal spine ; H. ZYG. haemal zygapophysis ; N. A. neural arch ; BT. SP. neural spine ; N. ZYG. neural zygapophysis ; FA. PH. parapophysis ; R. pleural rib. 202 ZOOLOGY SECT. In the caudal vertebrae the outgrowths corresponding to the parapophyses are fused with the centrum and unite in. the middle ventral line, forming a 'haemal arch (C, H. A.), through which the caudal artery and vein run. In the first six caudals each haemal arch bears a pair of ribs (R.); in the rest the arch is produced downwards and backwards into a hcemcd spine (D, H. SP.). The centra as well as the arches of the vertebras are formed entirely from the skeletogenous layer, and not from the sheath of the notochord as in Elasmobranchs (see pp. 73 and 147). The posterior end of the caudal region is curiously modified for the support of the tail-fin. The hindmost centra (Fig. 858, CN.) have their axes not horizontal, but deflected upwards, and following the last undoubted centrum is a rod-like structure, the urostyle (UST), consisting of the partly ossified end of the noto- chord, which has thus precisely the same upward flexure as in the Dog-fish. The neural and haemal . spines (N. SP., H. SP.) of the last five vertebras are very broad and closely connected with one another, and are more numerous than the centra; and three or four haemal arches are at- tached to the urostyle. In this way a firm vertical plate of bone is formed, to the edge of which the caudal fin-rays (D.F.R) are attached fan- wise in a symmetrical manner. It will be obvious, however, that this homocercal tail-fin is really quite as uhsymmetrical as the heterocercal .fin of the Dog-fish, since, its morphological axis being constituted by the notochord, nearly the whole of its rays are, in strictness, ventral. . The skull (Fig. 859) is an extremely complex structure, composed .of mingled bone and cartilage. The cartilage has no superficial mosaic of lime-salts such as we find in many Elasmobranchs, but certain portions of it are replaced by bones, and there are in addition numerous investing bones developed in the surrounding connective-tissue. As in the Dog-fish, the skull may be divided into cranium, upper and lower jaws, with their suspensory apparatus, and hyoid and branchial arches. The cranium (Fig. 860) is a somewhat wedge-shaped structure, its apex being directed forwards. At first sight the distinction between .replacing and investing bones is not obvious, but after maceration or boiling certain flat bones (the paired parietals, CN H.ZYG HSP FIG. 858.— Salzno fario, caudal end of vertebral column. CN. centrum ; 1). F. R. dermal fin-rays ; H. SP. haemal spine ; H. ZVG. haemal zyga- pophysis ; N. SP. neural spine ; N. ZYG. neural zygapophysis ; UST. urostyle. XIII PHYLUM CHORDATA 203 frontais, FR, and nasals, NA., and the unpaired supra-ethmoid, S. ETH.} can be easily removed from the dorsal surface ; and two unpaired bones (the parasphenoid, PA. SPIT,, and vomer, VO.} from the ventral surface. These are all investing bones : they are simply attached to the cranium by fibrous tissue, and can readily be prised off when the latter is sufficiently softened by maceration or boiling. We thus get a distinction between the cranium as a S'phvt par socc ... epifft -op -Siihop dent FIG. 859.— Salmo, the entire skull, from the left side. art. articular ; branchiost. branchio- stegal rays ; dent, den'tary ; epiot. epiotic ; eth. supraethmoid ; fr. frontal ; hyom. hyo- mandibular ; intop. intefopercular ; Jug. jugal ; mpt. mesopterygoid ; mtpt. metaptery- goid ; mx. maxilla ; nas. nasal ; o. subprbitals ; op. opercular ; pal. palatine ; par. parietal ; pmx. premaxilla ; praop. preopercular ; pt. pterygoid ; pter. pterotic ; Quad, "quadrate; xotc. eupraoceipital ; sphot. sphenotic ; xubop. suboporcular ; sympl. symplectio ; Zunge, basi- hyal. (From Wiedersheim's Vertebrata.) whole, or secondary crani^lm, complicated by the presence of invest- ing bones, and the primary cranium or chondrocranium, left by the removal of these bones and corresponding exactly with the cranium of a Dog-fish. The primary cranium contains the same regions as that of Scy Ilium. Posteriorly is the occipital region, surrounding the foramen magnum, presenting below that aperture a single concave occipital condyle for the first vertebra, and produced above into an occipital crest. The auditory capsules project outwards from the occipital region, and between them on the dorsal surface of the 204 ZOOLOGY SEC'T. skull are paired oval fontanelles (fon.*) closed in the entire skull by the frontal bones. The posterior region of the cranial floor is pro- duced downwards into paired longitudinal ridges, enclosing be- tween them a groove which is converted into a canal by the apposition of the parasphenoid bone and serves for the origin of the eye -muscles. In front of the auditory region the cranium is exca- vated on each side by a large orbit, a vertical plate or inter orbital septum (OR. SPH.) separating the two cavities from one another. In front of the orbital region the cranium broadens out to form the olfactory capsules, each excavated by a deep pit (olf. 5.) for the olfactory sac, and anterior to these is a blunt snout or rostrum. The occipital region is formed as usual from, the parachordals of the embryonic skull, the auditory region from the auditory capsules, and the rest of the cranium from the trabeculse. The replacingbones, formed as ossifications in the chondrocranium> correspond in essentials with the typical arrangement already de- scribed (p. 79). In the occipital region are four bones; the lasioccipital (B. oc.), forming the greater part of the occipital condyle and the hinder region of the basis cranii or skull-floor : the exoccipitals (EX. oc.), placed one on each side of the foramen magnum and meeting both above and below it ; and the supra- occipital (s. oc.) forming the occipital crest already noticed. Each auditory capsule is ossified by five bones — i.e., two more than the typical number (p. 79) ; the pro-otic (PR. OT.) in the anterior region of the capsule, uniting with its fellow of the opposite side in the floor of the brain-case, just in front of the basioccipital ; the opisthotic, in the posterior part of the capsule, external to the exoccipital; the splienotic (SPH. OT.), above the pro-otic and forming part of the boundary of the orbit ; the pterotic (PT. OT), above the ex-occipital and opisthotic, forming a distinct lateral ridge and produced behind into a prominent pterotic process ; and the epiotic (EP. OT.), a small bone, wedged in between the supra- and ex-occipitals and pterotic, and produced into a short epiotic process. On the external face of the auditory capsule, at the junction of the pro-, sphen-, and pter-otics, is an elongated facet (h.m) covered with cartilage and serving for the articulation of the hyomandibular. The trabecular region of the cranium contains six bones. Im- mediately in front of the conjoined pro-otics, and forming the anterior end of the basis cranii, is a small unpaired Y-shaped bone, the basisphenoid (B. SPH.). Above it, and forming the anterior parts of the side-walls of the brain-case, are the large paired alisphenoids (AL. SPH.). In the interorbital septum is a median vertical bone, representing fused orbitospJienoids (OR. SPH.). Lastly, in the posterior region of each olfactory capsule, and forming part of the boundary of the orbit, is the eeto-ethmoid {EC. ETH.) XITI PHYLUM CHORDATA 205 The investing bones already referred to are closely applied to the roof and floor of the chondrocranium, and modify its form considerably by projecting beyond the cartilaginous part, and con- cealing apertures and cavities. The great frontals (FU.) cover the greater part of the roof of the skull, concealing the f bntanelles, and furnishing roofs to the orbits. Immediately behind the frontals is a pair of very small parietals (PA.) ; in front of them is an unpaired E8R4 Fio. 860.— Salxno fario. Disarticulated -skull with many of the investing bones removed. The cartilaginous parts are dotted, fon. fontanelle ; h. m. articular facet for hyomandibular ; Mck. C. Meckel's cartilage ; off. s. hollow for olfactory sac. Replacing bones— AL. SPH. alisphenoid ; ART. articular ; B. BR.l, first basibranchial ; B. HY. basihyal • B. OC. basioccipital ; BR. 5, fifth branchial arch; B. SPH. basisphenoid ; C. BR.l, first ceratobranchial ; C. HY. ceratohyal ; EC. ETH.-ecto-ethmoid; E. BR.l, first epi- branchial; E. HY. epi-hyal ; EP.OT. epiotic ; EX. OC. ex-occipital ; H. BR. 1, first hyp( > branchial ; H. HY. hypohyal; HY. M. hyomandibular; I. HY. interhyal ; MS. PTG. mesopterygoid ; MT. PTG. metapterygoid ; OR. SPH. orbitosphenoid ; PAL. palatine; PH. BR.l, first pharyngobranchial ; PTG. pterygoid ; PT. OT. terotic ; QU. quadrate ; S. OC. supraoccipital ; 8FH.OT. sphenotic ; SYIK. symplectic. nvesting bones— ANG. angular ; DNT. dentary ; PR. frontal ; JU. jugal. ; MX. maxilla ; NA. nasal ; PA. palatine ; PA. SPH. parasphenoid ; PMX. premaxilla ; VO. vomer. supra-ethmoid (S. ETH), to the sides of which are attached a pair of small nasals (NA). On the ventral surface is the large para- sphenoid {PA. SPH.), which forms a kind of clamp to the whole cartilaginous skull floor ; and in front of and below the parasphenoid is the toothed vomer (VO). Encircling the orbit is a ring of scale- like bones, the suborbitals (Fig. 859, o).* In the jaws, as in the cranium, we may distinguish between primary and secondary structures. The primary upper -jaw or 206 ZOOLOGY SECT. palatoquadrate is homologous with the upper jaw of the Dog-fish, but instead of remaining cartilaginous, it is ossified by five replac- ing bones : the toothed palatine (PAL.) in front, articulating with the olfactory capsule: then the pterijgoid (PTG.) on the ventral, and the mesopterygoid (MS. PTG.) on the dorsal edge of the original cartilaginous bar: the quadrate (QU.) at the posterior end of the latter, furnishing a convex condyle for the articulation of the lower jaw : and projecting upwards from the quadrate the metapterygoid (MT. PTG.). These bones do not, however, enter into the gape, and do not therefore constitute the actual upper jaw of the adult fish : external to them are two large investing bones, the premaxilla (PMX.) and the maxilla (MX.), which together form the actual or secondary upper jaw ; they both bear teeth. A small scale-like bone, ihejugal (JU.) is attached to the posterior end of the maxilla. The lower jaw is similarly modified. Articulating with the quadrate is a large bone, the articular (ART.), continued forwards by a narrow pointed rod of cartilage : the latter is the unossified distal end of the primary lower jaw or Meckel's cartilage ; the articular is its ossified proximal end, and therefore a replacing bone. Ensheathing Meckel's cartilage and forming the main part of the secondary lower jaw is a large toothed investing bone, the dentary (DNT.), and a small investing bone, the angular (ANG.) is attached to the lower and hinder end of the articular. The connection of the upper jaw with the cranium is effected partly by the articulation of the palatine with the olfactory region, partly by means of a suspensorium formed of two bones separated by a cartilaginous interval : the larger, usually called the hyoman- dibular (HY. M.), articulates with the auditory capsule by the facet already noticed, and the small pointed symplectic (SYM.) fits into a groove in the quadrate. Both bones are attached by fibrous tissue to the quadrate and metapterygoid, and in this way the suspensorium and palatoquadrate together form an inverted arch, freely articulated in front with the olfactory, and behind with the auditory capsule, and thus giving rise to an extremely mobile upper jaw. As its name implies, the hyomandibular (to- gether with the symplectic) is commonly held to be the upper end of the hyoid arch and the homologue of the hyomandibular of Elasmobranchs, but there is some reason for thinking that it really belongs to the mandibular arch, and corresponds with the dorsal and posterior part of the triangular palatoquadrate of Holocephali : a perforation in the latter would convert it into an inverted arch having the same general relations as the upper jaw plus suspensorium of the Trout, but fused, instead of articulated, with the cranium at either extremity. The hyoid cornu is articulated to the cartilaginous interval between the hyomandibular and symplectic through the inter- xin PHYLUM CHORDATA 207 mediation of a small, rod-like bone, the interhyal (i. HY.), which perhaps represents the hyomandibular of Elasmobranchs. It is ossified by three bones : an epihyal (E. HY.) above, then a large ceratohyal (c. HY.), and below a small double hypohyal (H. HY.). The right and left hyoid bars are connected by a keystone-piece, the unpaired, toothed basihyal (B. HY.), which supports the tongue. Connected with the hyomandibular and hyoid cornu are certain investing bones serving for the support of the operculum. The opcrcular (Fig. 859, op.) is articulated with a backward process of the hyomandibular, the preopercular (pra. op.} lies outside the posterior border of the hyomandibular and quadrate, and clamps them together ; the subopercular (sub. op.) is below and internal to the opercular; and the interopcrcular (int. op.) fits between the lower portions of the three preceding bones, and is attached by ligament to the angle of the mandible. The ten sabre-shaped IrancTiiostegal rays (branchiost.) are attached along the posterior border of the epi- and cerato-hyal, and below the basihyal is an unpaired bone, the basi-branchiostegal or urohyal. There are five branchial arches, diminishing in size from before backwards. The first three present the same segments as in the Dog-fish : pharyngdbranchial (PH. BR.) above, then epibranchial (E. BR.), then a large ceratobrancliial (c. BR.), and a small hypo- branchial (H. BR.) below. The right and left hypobranchials of each arch are connected by an unpaired basibranchial (B. BR.). All these segments are ossified by replacing bones, and the basi- branchials are connected with one another and with the basihyal by cartilage, so as to form a median ventral bar in the floor of the pharynx. In the fourth arch the pharyngo-branchial is unossified, and the hypobranchial absent, and the fifth arch (BR, 5) is reduced to a single bone on each side. Small spine-like ossifications are attached in a single or double row along the inner aspect of each of the first four arches : these are the gill-rakers ; they serve as a sieve to prevent the escape of food by the gill-slits. The comparison of this singularly complex skull with the com- paratively simple one of the Dog-fish is much facilitated by the examination of the skull of a young Trout or Salmon. In the latter, at about the second week after hatching, the only ossifications present are a few investing bones ; when these are removed we get a purely cartilaginous skull (Fig 861), exactly comparable with that of an Elasmobranch. There is a cranium devoid of replacing bones and divisible only into regions; the upper jaw is an unossified palato- quadrate (PL Pt., M. Pt., Qn.) and the lower jaw (Mel:.} a large Meckel's cartilage ; the suspensorium is an undivided hyoman- dibular (HM.\ and the hyoid and branchial arches are unseg- mented. The first dorsal and the ventral fins are supported each by a triple 208 ZOOLOGY SECT. S.Or T.C Pa.ch. IIM Brl idt. 861.— Skull of young Salmon, second week after hatching • the investing bones removed. Au. auditory capsule ; Br. 1, first branchial arch ; Ch. notochord ; C. Hy. hyoid cornu ; Fo. fontanelle ; G. Hy. basihyal ; H. Hy. hypohyal ; H. M- hyomandibular ; 1. Hy. interhyal ; fl, I'2, labial cartilages ; Mck. Meckel's cartilage ; M. PL metapterygoid region of primary upper jaw ; Pa. ch. parachordal ; PI. Pt. palato- pterygoid region; Qu. quadrate region; S.Or. supraorbital region of cranium ; Sy. symplectic region of suspeusorium ; T. C'r. cranial roof ; Tr. trabecula ; II, optic foramen ; V, trigeminal foramen. (From Parker and Bettany's Morphology of the SI- nil.) set of pterygiophores, so that the fin-skeleton is multiserial, as in the Dog-fish. The proximal series con- sists of slender bony rays — the interspin- ons bones (Fig. 862, PTG. 1 ; Fig. 865, PTG.), lying in the median plane, be- tween the muscles of the right and left sides, and more numerous than the myonieres of the regions in which they occur. Their distal ends are broadened, and with them are connected the second series (PTG. 2) in the form of small dice-box shaped bones ; to these, finally, are attached small nodules of carti- lage (pig. o] forming the third series of radials. The dermal fin- rays or Icpidotrichia (D.F.R.), which lie in the substance of the fin itself, are slender bones, jointed like the antennas of an Arthro- pod, and mostly branched in the sagittal plane (Fig. 865, D.F.E.). Each is formed of distinct right and left pieces (Fig. 862), in close contact for the most part, but diverging below to form a forked and dilated end, which fits over one of the cartilaginous nodules (ptg.3). In the caudal fin (Fig. 858) the dermal rays (D.F.R.) are similarly seated on the broad haemal arches of the posterior caudal vertebrae. The second dorsal or adi- pose fin has no bony support. The shoulder-girdle (Fig. 863), like the skull, consists of a primary shoulder -girdle, homologous with that of a Dog-fish, and of several investing bones. The primary shoulder- . Y. -ri- i • /• i r v girdle in the young Fish is formed of dis- tinct right and left bars of cartilage, which do not unite with one another ventrally. In the adult each bar is ossified by three bones, a scapula (SCP.), situated dorsally to the DF.R FIG. 8<>2.— Salmo fano. A dsrmai fin-ray with its XIII PHYLUM CHORDATA 209 glenoid facets, and developed partly as a replacing, partly as an investing bone ; a coracoid (COR.), situated ventrally to the glenoid facet, and a meso-coracoid (MS. COR.), situated above tbe coracoid and anterior to the scapula. Externally to these is found a very large investing bone, the clavicle (CL.}, extending down- wards under the throat : its dorsal end is connected by means of a supra-clavicle (S. CL.), to a forked bone, the post-temporal (P.TM.), one branch of which articulates with the epiotic, the other with the pterotic process. To the inner surface of the clavicle are attached two flat scales of bone (P. CL'.), with a slender rod-like COR. PTG.I FIG. 8<53.— Salmo fario. Left half of shoulder-girdle and pectoral fin, from the mier surface. CL. clavicle ; COR. coracoid ; D.F. R. dermal fin-rays ; WIS. COR. mesocoracoid ; P.CL., P. CL'. post-clavicles; PTG.I, proximal, and pty. %, distal pterygiophores ; P. TM. post- temporal ; £ CL. supraclavicle ; SCP. scapula. post-clavicle (P. CL.) passing backwards and downwards among the muscles. The structure of the pectoral fin is very simple. Articulated to the posterior border of the scapula and coracoid are four dice-box- shaped bones, the proximal pterygiophores or radials (PTG. 1), followed by a row of small nodules of cartilage (ptg. 2) repre- senting distal pterygiophores. The main body of the fin is supported by dermal fin-rays, which resemble those of the median fins, and have their forked ends seated upon the distal pterygio- phores : the first ray, however, is larger than the rest, and articulates directly with the scapula. There is no pelvic girdle, its place being taken by a large, flat triangular bone, the basipterygium (Fig. 864, B. PTG.), probably representing fused proximal pterygiophores : to its posterior border 210 ZOOLOGY SECT. BPTG are attached three partly ossified nodules, the distal pterygio- phores (PTG), and with these the dermal fin-rays are articulated. The adipose lobe of the pelvic fin is supported by a small scale-like bone. The muscles of the trunk and tail are arranged, as in the Dog-fish, in zigzag myo- meres : there are small muscles for the fins, and the head has a complex musculature for the movement of the jaws, hyoid, operculum, and branchial arches. The coelome is divisible into a large ab- domen (Fig. 865) containing the chief viscera, and a small pericardial cavity, situated below the branchial arches, and containing the heart. Digestive Organs. — The mouth (Figs. 854 and 865) is very large and has numerous small, recurved, conical teeth, borne as already mentioned, on the premaxilla3, maxillae, pala- tines, vomer, dentaries and basihyal. They obviously serve merely to prevent the escape of the slippery animals used as food and are of no use for either rending or chewing. The pharynx (ph.) is perforated on each side by four vertically elongated gill-slits, fringed by the bony tooth-like gill-rakers. Each gill-slit is V-shaped, the epihyal being bent upon the ceratohyal so that the dorsal and ventral moieties of the branchial arches touch one another when the mouth is closed. The pharynx leads by a short gullet (gut) into a U-shaped stomach (st.) consisting of a wide cardiac and a narrow pyloric division : between the latter and the intestine is a ring-shaped pyloric valve. The intestine passes at first forwards as the duodenum (du.), then becomes bent upon itself (int.) and passes backwards, without convolution, to the anus (an.). Its posterior portion has the mucous membrane raised into prominent annular ridges which simulate a spiral valve. The liver (Ir.) is imperfectly divided into right and left lobes, and there is a large gall-bladder (g. U.). Opening into the duodenum are about forty blind glandular tubes, the pyloric cceca (py. c.\ There is a large spleen (spl.) attached by peritoneum to the fundus of the stomach. The stomach, duodenum, and pyloric ca3ca are surrounded by loose folds of peritoneum loaded with fat. Lying below the kidneys and extending the whole length of the abdominal cavity is the air-bladder (a.bl.), a thin-walled sac serving as an organ of flotation. Anteriorly its ventral wall presents a small aperture leading, by a short pneumatic duel FIG. 864.— Salxno fario. Skeleton of left pelvic fin, dorsal aspect. B. FTG. basiptery- gium ; Z>. F. R. dermal fin -rays ; PTG. distal pterygiophores. XIII PHYLUM CHORDATA 211 b > =3 -3'- * ji * "§ s * " h^ ISf.S"! f^srll ^ •** IS G ^ " — &m 'g^^c^^ ^i-j*! iKall slllii i* ., >>s as- 212 ZOOLOGY SECT. (pn. d.) into the oesophagus on the dorsal side somewhat to the right of the middle line. Respiratory Organs. — There are four pair of gills each with a double row of branchial filaments united proximally but having their distal ends free : interbranchial septa are practically obsolete (see Fig. 771). The gills are borne on the first four branchial arches, the fifth arch bearing no gill. On the inner surface of the operculum is a comb-like body, the pseudo-branchia, formed of a single row of branchial filaments, and representing the vestigial gill (hemibranch) of the hyoid arch. Circulatory Organs. — The heart (Fig. 865) consists of sinus venosus, auricle (au.) and ventricle (v.). There is no conus arteriosus, but the proximal end of the ventral aorta is dilated to form a bulbus aortce (b. a.), a structure which differs from a conus in being part of the aorta and not of the heart ; its walls do not contain striped muscle, and are not rhythmically contractile. In accordance with the atrophy of the hyoid gill there is no afferent branchial artery to that arch, but a hyoidean artery springs from the ventral end of the first efferent branchial and passes to the pseudobranch. The right branch of the caudal vein is continued directly into the corresponding cardinal, the left breaks up in the kidney, forming a renal-portal system. There are no lateral veins, but the blood from the paired fins is returned to the cardinals. The red blood-corpuscles are, as in other fishes,, oval nucleated discs. Nervous System, — The brain (Fig. 866) is very different from that of Elasmobranchs, and is in many respects of a distinctly lower type. The cerebellum (If. If.) is very large, and bent upon itself. The optic lobes (M.H.) are also of great size, and corresponding with them on the ventral surface are large bean-shaped lobi inferiores (U.L.). The diencephalon is much reduced, and, indeed, is indicated dorsally only as the place of origin of the pineal body (6r. p.) : ventrally it is produced into the lobi inferiores with the infundibulum between them giving attachment to the pituitary body (Hyp.). Hence, seen from above, the small undivided prosencephalon ( V.H.) comes immediately in front of the mid-brain : it has a non-nervous roof (Pall.) and its floor is raised into prominent corpora striata (BG., Bas. G.). The olfactory bulbs, situated in close apposition with the prosence- phalon without intervening olfactory peduncles or olfactory tracts such as are present in Scy Ilium (L.ol.), are nearly as large as the corpora striata, and each contains a small cavity or rhinoccele in communication with the undivided prosoccele. Three transverse bands of fibres connect the right and left halves of the fore-brain, an anterior commissure joining the corpora striata, a posterior commissure, situated just behind the origin of the pineal body, and an inferior commissure in front of the infundibulum. The pineal XIII PHYLUM CHORDATA 213 body (bp.) is rounded and placed at the end of a hollow stalk : a shorter offshoot of the roof of the diencephalon may perhaps SgpM FIG. 866 -Salmo fario. Dorsal (A), ventral (B), and lateral (C) views of brain. BG., Bas. G. corpora stSata; ch, crossing of optic nerves; G. p pineal body; HE. cerebellum ; Hyp, pituitary body Inf. infundibulum ; L. oL olfactory bulbs ; Med, spmal cord ; ME. optic lobes , NH. medulla oblongata ; Pall, pallium; Sv. saccus vasculosus ; Tr. Opt optic i tracts ; U, L lobi inferiores ' VII, prosencephalon ; r~X, cerebral nerves ; XII, 1, first spinal (hypoglossal) nerve ; 2, second spinal nerve. . (From Wiedersheim's Vertebrata.) represent a rudimentary pineal eye. Behind the pituitary body is a saccus vasculosus (s, v.): The optic nerves do not form a chiasma, but simply cross one another, or decussate (Ch-\ on VOL. II ° 214 ZOOLOGY SEC'T. C-71 cTi.alcL leaving the brain, the right nerve going to the left, and the left nerve to the right eye. Sensory Organs. — The most distinctive feature of the olfactory sac is the possession of two small apertures, the anterior provided with a valve. The eye (Fig. 867) has a very flat cornea (en.} with which the globular lens (I.} is almost in contact, so that the aqueous chamber of the eye is extremely small. Between the cartilaginous sclerotic (scl.} and the vascular choroid (ch.} is a silvery layer or argentea (arg.) which owes its colour to minute crystals in the cells of which it is composed. In the posterior part of the eye, between the choroid and the argentea, is a thickened ring-shaped structure (ch. gld.) surrounding the optic nerve, and called the choroid gland : it is not glandular, but is a complex network of blood-vessels, or rete mira- l)ile. It is supplied with blood by the efferent artery of the pseudobranch. Close to the entrance of the optic nerve a vascular fold of the choroid, the falciform pro- cess (pr. gl.) pierces the retina, and is continued to the back of the lens where it ends a knob, the cam- panula Halleri (cp. hal.), which contains smooth muscular fibres. The falci- form process with the cam- panula Halleri take an im- portant part in the process of accommodation by which the eye becomes adapted to forming and receiving images of objects at various distances. Accommodation in the Fish is effected, not by an alteration in the curvature of the lens as in higher Vertebrates, but by changes in its position, by which it becomes more approximated towards, or further withdrawn from, the retina. In bringing about these changes of position, the structures in question appear to play the principal part. The aibditory organ (Fig. 868) is chiefly remarkable for the large size of the otoliths (ot. 1 — 3}. They are three in number; one, called the sagitta (st. 1), is fully 6 mm. in length, and almost fills the sacculus : another, the asterisciis (ot. 2), is a small granule lying in the lagena or rudimentary cochlea : the third, the lapillus (ot. 3), is placed in the utriculus close to the ampulla of the anterior and horizontal canals. £7* Fig. $67. — Salrno fario. Vertical section of eye (semi-diagrammatic), arg. argentea ; ch. choroid ; ch. gld. choroid^gland ; en. cornea ; cp. hi. cam- panula Halleri; ir.jp.ris ; I. lens; opt. n. optic nerve ; pg. pigmentary layer ; pr. fl. processus falciformis ; gel. scl^otic (dotted). XIII PHYLUM CHORDATA 215 Urinogenital Organs.— The kidneys (Fig. 865, M.t and Fig. 869, J?) are of great size, extending the whole length of the dorsal wall of the abdomen, above the air-bladder, and partly fused together in the middle line. They are derived from the meso- nephros of the embryo. Their anterior ends (Fig. 865, Jed, Fig. 869, K) are much dilated and consist in the adult of lymphatic tissue, thus ceasing to discharge a renal function. The ureters (mesonephric ducts, ur.) unite into a single tube, which is dilated to form a urinary Madder (Fig. 865, u. U., Fig. 869, v.\ and discharges into the urinogenital sinus. The gonads are of great size in the sexually mature fish. The testes (Fig. CL.S.C p.s.c Fir;. 868.— Salmo fario. The right auditory organ, from the inner side ; the otoliths are shown separately below. a. s. c. anterior semicircular canal ; aud, nv. auditory nerve ; h. s. c. horizontal canal ; ot. 1—3, otoliths ; p. s. c. posterior canal ; sac. sacculus ; ut. utriculus. FIG. 869.— Salmo fario. The kidneys andj adjacent parts. d, precaval vein ; R (to the right) kidney; R (to the left), degenerate anterior portion of kidney ; rr, efferent renal vein ; s. subclavian vein ; u, ur, ureter ; v, bladder. (From Gegenbaur's Com- parative Anatomy.) 865, ts.~) are long, smooth, pinkish, paired organs, extending the whole length of the abdominal cavity ; each is continued pos- teriorly into a duct (v. df.) which opens into the urinogenital sinus, and the homology of which with the ducts of the primitive nephridial system is still uncertain. The ovaries are also of the full length of the abdominal cavity and are much wider than the testes : they are covered with peritoneum on their inner or mesial faces only, and the numerous ova, which are about 4 mm. in diameter, are discharged when ripe from their outer faces into the ccelome. There are no oviducts, but the anterior wall of the urinogenital sinus is pierced by a pair of genital pores o 2 216 ZOOLOGY SECT. through which the ova make their way to the exterior. There is reason for thinking that these pores are to be looked upon as degenerate oviducts, and in no way homologous with the abdominal pores of Elasmobranchs. Development. — Impregnation is external, the male shedding his milt or seminal fluid on the newly-laid eggs. The ovum is covered by a thick membrane, the zona radiata, perforated by an aperture, the micro- pyk, through which the sperms rind access : it is formed of a super- ficial layer of proto- plasm surrounding mass of transparem fluid yolk of a pah yellow colour. At one pole the protoplasm ac- cumulates to form an elevated area or ger- minal disc, in which seg- mentation takes place (Fig. 870, A, £)i the same way as Elasmobranchs, except that, owing to th( smaller proportion o yolk, the resulting bias toderm (bl.) and th< embryo formed there from are proportionally _— — -—^^f-.) much larger, and the yolk - sac (y. s.) corre spondingly smaller, than in the two previous classes. Epiboly takes place as in Elasmo- branchs, the blastoderm gradually growing round and enclosing the yolk (C-F). The embryo (emb.) arises as an elevation growing forwards from the thickened edge of the blastoderm, and, as it increases in length, appears as a clear colourless band (H, cmb.) winding round the yellow yolk, and kept in close contact with it by the enclosing zona radiata. There is no open medullary groove, the nervous system being formed, as in Lampreys, from a fold of ectoderm the walls of e.rrtb n y.s y.s FIG. 870. — Nine stages in the development of Salrno fario. A— H, before hatching ; I, shortly after hatch- ing, bl. blastoderm ; emb. embryo ; r. thickened edge of blastoderm ; y. s. yolk-sac. (A— G after Henneguy.) xni PHYLUM CHORDATA 217 which are in apposition so as to form a keel-like ridge. The endoderm and mesoderm are formed as a result of a process of infolding of the posterior edge of the blastoderm (Fig. 871). FIG. 871.— Longitudinal section of blastoderm of Salmo, at about the stage represented in D of Fig. 870. ec. ectoderm ; en+ms, infolding giving rise to endoderm and mesoderm. (After O. Hertwig.) Gradually the head and tail become free from the yolk, and at the time of hatching the yolk-sac (I, y. s.) is a shoe-shaped body sessile upon the ventral surface of the transparent embryo. 2. — DISTINCTIVE CHARACTERS AND CLASSIFICATION. The Teleostomi are Pisces in which the primary cranium is always complicated by the addition of investing bones, of which a pair of parietals and one of frontals above, and unpaired vomer and parasphenoid below, are the most constant. The chondro- cranium is always more or less ossified by replacing bones, including, a supraoccipital, and the upper and lower jaws are both bounded by investing bones. The jaws are connected with the cranium through the intermediation of a hyomandibular, which is probably not homologous with the similarly named element of Elasmobranchs. The dermal fin-rays are formed of bone, and are supported by pterygiophores which may be either cartilaginous or bony, but which always show a great reduction in number as compared with the homologous structures in Elasmobranchs. The primary shoulder- girdle is complicated by the addition of investing bones, of which a large clavicle is the most constant. The pelvic girdle is vestigial or absent. The pelvic fins usually undergo a forward displacement, their position being either abdominal i.e., between the anus and the pectoral region, or thoracic, i.e., in the pectoral region, or jugular, i.e., under the throat. A dermal exoskeleton is usually present. The intestine may or may not have a spiral valve ; the anus is distinct from, and placed in front of, the urinary and genital apertures. The gills are covered by an operculum supported by investing bones, and the interbranchial septa are reduced or absent, so that the gill-filaments are partially or wholly free ; the hyoidean gill is reduced or absent. The conus 218 ZOOLOGY SECT. I arteriosus is sometimes present, sometimes absent ; when it is absent there is a large bulbus aortas formed as a dilatation of the ventral aorta. The prosencephalon has usually a non-nervous roof; the optic nerves either form a chiasma or simply decussate. The ova are small ; the gonoducts are either continuous with the gonads, or open anteriorly into the ccelome, or are absent : in the latter case the sexual products pass out by genital pores ; true abdominal pores may be present in addition. Segmentation of the egg is either entire or discoidal ; development is sometimes accompain< by a metamorphosis. The Teleostomi are classified as follows : — ORDER 1. — CROSSOPTERYGII. Teleostomi in which the pectoral fin consists of a rounded basal lobe supported by endoskeletal structures and fringed by dermal ^ br. TTV pe-lj3 FIG. 872.— Polypterus bichir. A, entire animal; B, ventral view of throat, an. anus ; br. m.' branchiostegal membrane ; c. f. caudal fin ; d. f. dorsal finlets ; jug. pi. jugular plates ; na. nostril ; pet. f. pectoral fin ; pv. f. pelvic fin ; v. f. ventral fin. (After Cuvier.) rays. There are no branchiostegal rays. The vertebral column is well ossified, and the caudal fin is diphycercal. The pelvic fins are abdominal. A spiral valve and a conus arteriosus are present, and the optic nerves form a chiasma. The only existing members of this order are several species of Polypterus (Fig. 872) from the Congo and Upper Nile, and Ccdamichthys calabaricus from Old Calabar. ORDER 2. — CHONDROSTEI. Teleostomi in which the paired fins have no basal lobe, but their whole free portion is supported by dermal rays. There are few replacing bones in the skull, and the primary shoulder-girdle is unossified. The vertebral column consists of a persistent notochord with cartilaginous arches, and its anterior end is fused with the cranium. Branchiostegal rays are few or absent. The tail is heterocercal. The pelvic fins 'are abdominal. A spiral valve, conus arteriosus, and optic chiasma are present. XIII PHYLUM CHOEDATA 219 This order includes the Sturgeons (Acipenser, Fig. 873, and Scaphirhynchus) found in the rivers of Europe, Asia, and North pclj FIG. 873.— Acipenser ruthenus (Sturgeon), b. barbels ; c. j. caudal fin ; d.j. dorsal fin ; pet. f. pectoral fin ; pv. f. pelvic fin ; sc. scutes ; v. f. ventral fin. (After Cuvier.) America ; the curious spoon-billed Polyodon, from the Mississippi ; and Psephurus from the rivers of China. ORDER 3. — HOLOSTEI. Teleostomi in which the paired fins have no basal lobe. The chondrocranium is well ossified by replacing bones and investing bones : branchiostegal rays are present. The vertebral column FIG. 874.— Lepidosteus platystomus (Bony Pike), c. /. caudal fin \-d.f. dorsal fin; /f. fulcra ; 1. 1. lateral iline ; pet, /. pectoral fin ; pv. f, pelvic fin ; v, f. ventral fin. (After Cuvier.) consists of bony vertebrae, and the tail is heterocercal or nearly homocercal. The pelvic fins are abdominal. A reduced spiral valve, a conus arteriosus, and an optic chiasma are present. This order includes the Gar-pike or Bony Pike (Lepidosteus, Fig. 874), from the fresh waters of North and Central America and FIG. sTf..— Amia calva (Bow-fin). A, the entire animal ; B, ventral view of throat, br. in. branchiostegal nanil rune ; r. f. caudal fin; d.f. dorsal fin ; jvy. pi. jugular plate; pct.f. pectoral fin ; pv. /. pelvic fin ; v. f, ventral fin. (After Giinther.) 220 ZOOLOGY SECT. Cuba, and the Bow-fin or Mud-fish (Amia calva, Fig. 875), from the rivers of the United States. Orders 1 — 3 are frequently grouped together as the sub-class Ganoidei, and, although such a group is an artificial one, it will often be convenient to refer to these fishes as " Ganoids." They are all small and numerically insignificant groups at the present day, but formed the whole of the Teleostomian fauna in the Palaeozoic and the greater part of the Mesozoic epoch. ORDER 4. — TELEOSTEI. Teleostomi in which the paired fins have no basal lobe. The skull is well ossified both by replacing and investing bones: branchiostegal rays are present. The vertebral column is well ossified : the tail is homo- or diphycercal. There is no spiral valve except as a vestige in one genus. The conus arteriosus is absent except as a vestige in a small number of genera : a large bulbus aortae is present. The optic nerves never form a chiasma and usually simply decussate. The vast majority of existing Teleostomi are included in this order, which is divided into six sub-orders as follows : — Sul-order a. — Physostomi. Teleostei in which the air-bladder, when present, has an open pneumatic duct. All the fin-rays are jointed, or the dorsal and S./T7T/ FIG. 876.— Rita buchanani, one of the Siluroids. &. barbel ; d.f. r. 1, first dorsal fin-ray; d. J. 2, adipose fin ; pet. 1. r. 1, first pectoral fin-ray; pv. /. pelvic fin; v.f. ventral fin. (After Day.) the pectoral are armed each with an anterior ossified spine, and the pelvic fins, when present, are abdominal in position. Including the Cat-fishes or Siluroids (Fig. 876), Carp, Gudgeon, Loach, Pike, Salmon and Trout (Fig. 854), Smelt, Grayling, Herring, Anchovy, Eels, &c, XIII PHYLUM CHORDATA Sub-order b. — Anacanfhini. 221 Teleostei in which the air-bladder, when present, has, except in one species, no pneumatic duct. The rays of the unpaired and of FIG. 877.— Gadus morrhua (Cod), an. anus ;c.f. caudal fin ; d.f. 1—3, dorsal fins ; mx. maxilla ; pet. f. pectoral fin ; pmx. premaxilla ; pv. f. pelvic fin ; v.f. 1 and 2, ventral fins. (After Cuvier.) the pelvic fins are all jointed, and the pelvic fins are either thoracic or jugular. Including the Cod (Fig. 877), Haddock, Whiting, Hake, Ling, and the Pleuronectida3 or Flat-fishes (Fig. 882), such as the Sole, Flounder, Turbot, &c. Sub-order c. — Acanthoptcri. Teleostei in which the air-bladder, when present, has usually no meumatic duct. More or fewer of the rays of the dorsal, ventral, FIG. 878.— Sebastes percoides. l>r. m. branchiostegal membrane ; d.f. spiny portion of dorsal fin ; i/. ;'.' soft portion ; mx. maxilla; op. opercular ; /»<•/./'. pectoral fin ; p.mx. premaxilla; /*/•. f>/>. i)reopercular ; pv. f. pelvic fin ; v.f. spiny portion of ventral fin; v. /'soft portion. (After Richardson.) 222 ZOOLOGY SECT. and pelvic fins are unjointed, and have the form of strong spines. The right and left bars of the fifth branchial arch are usually not fused. This immense group includes the greater number of marine fishes (Fig. 878), as well as many fresh-water forms : the Perch, Stickleback, Sea-bream, Mullet, Mackerel, and Gurnard may be specially mentioned. ^lib-order d. — Phctryngognathi. Teleostei in which the right and left bars of the fifth branchial arch are fused to form a single bone in the floor of the FIG. 879.— tabrichthys psittacula (Wrasse), d. f hard dorsal ; d. /.' soft dorsal ; lp. lips ; pct.f. pectoral fin ; pi: f. pelvic fin ; v. f. ventral fin. 13, inferior pluuyngeal bone of Labrichthys. (A, after Richardson ; B, after Owen.) mouth (Fig. 879, B). The remaining characters are as in Acanthopteri. Including the Wrasses (Fig. 879) and their allies. Sub-order e. — Plectognathi. Teleostei having no pneumatic duct. The exoskeleton, when present, takes the form of bony plates or spines. The gill-opening is very narrow. The mouth is very small, and the premaxilla and maxilla are united. The pelvic fins are absent or represented by spines. XIII PHYLUM CHORDATA 223 This is a small sub-order, including the File-fishes, Globe-fishes, Sun-fishes and Goffer-fishes (Fig. 880). FIG 880 — Ostracion (Coffer-fish), br. ap. branchial aperture ; d.f. dorsal fin ; pct.f. pectoral fin ; v.. f. ventral fin. (After Day.) Sub-order f. — Lopliobmnchii. Teleostei having no pneumatic duct. The gills are not comb- like, but have their filaments arranged in tufts (Fig. 881, B). The B FIG 881 —Hippocampus (Sea-horse). In B the operculum is removed to show the gills, 'br. ap. branchial aperture ; brd. p. brood-pouch ; d. /. dorsal fin ; g. gills ; pet. f. pectoral fin. (From Claus and Gunther.) 224 ZOOLOGY SECT. branchial aperture is very small. The exoskeleton consists of bony plates arranged segmentally. This is also a very small sub-order, including only the Sea- horses (Fig. 881), Pipe-fishes and their allies. Sub-orders b—f are frequently grouped together as Physoclisti, distinguished from Physostomi by the closed air-bladder. Systematic Position of the Example. Salmo fario is one of several species of the genus Salmo, belong- ing to the family Salmonidce, of the sub-order Physostomi and the order Teleostei. The absence of a spiral valve and of a conus arteriosus, the presence of a bulbus aorta3, and the decussation of the optic nerves indicate its position among the Teleostei. It belongs to the Physostomi in virtue of possessing a pneumatic duct, none but jointed fin-rays, and abdominal ventral fins. The characters which place it among the Salmonidae are the presence of an adipose fin and of pseudobranchiae, the absence of oviducts, and the fact that the maxilla enters into the gape of the mouth. The genus Salmo is distinguished by its small scales, well-developed conical teeth, absent on the pterygoids, a short ventral fin with fewer than fourteen rays, numerous pyloric appendages, and compara- tively large ova. The distinctive characters of the various species of Salmo depend upon comparatively minute points, such as the relative proportions of various parts, arid are often difficult of determination owing to individual variations correlated with different environments. In S. fario the posterior margin of the operculum is evenly curved, the maxilla is longer than the snout, and the vomerine teeth are in a double series and persist throughout life. 3. GENERAL ORGANISATION. External Form. — The typical form of the Teleostomi is very fairly represented by that of the Trout (Fig. 854) — a long, com- pressed body, nearly half of which is formed by the tail, pointed anterior and posterior ends, a large vertical tail-fin, a head of mode- rate size, and a terminal mouth. Such a form is eminently fitted for rapid progression through the water. But from this characteristic fish-form there are many striking deviations. The body may be greatly elongated and almost cylindrical, as in the Eels ; or of great length and strongly flattened from side to side, as in the Ribbon- fishes ; or the head may be of immense proportional size and strongly depressed, as in certain shore-fishes, such as the " Fishing- frog " ; or, as in the beautiful Reef-fishes, the whole body may be as high as it is long. The mouth sometimes has a xiii PHYLUM CHORDA T A 225 ventral position, as in Elasmobranchs, with the snout prolonged over it. This is the case, for example, in the Sturgeons (Fig. 873) ; in the allied Polyodon the snout takes the form of a horizon- tally flattened shovel-like structure, about one-fourth the length of the body. On the other hand, in the ground-feeding " Star-gazers " and some other Acanthopteri the lower jaw is underhung like that of a bull-dog, and the mouth becomes dorsal in position. A beak may be produced by the prolongation of the upper jaw, as in the Sword-fish, or of the lower jaw, as in the Half-beak or Gar-fish, or of both jaws, as in the Bony-Pike (Fig. 874). Such a projection is not to be confounded with the snout of the Sturgeon or Polyodon, being formed by the elongation of the bones of the jaws (premaxilla, maxilla, dentary, &c.), whereas in the two Chondrostean forms referred to it is the anterior region of the cranium which is prolonged. Still another form of "snout" is produced in many Teleostei by the great mobility of the jaws, allowing of their protrusion in the form of a short tube. In the Wrasses or " lip-fishes" the mouth is bounded by fleshy lips (Fig. 879, /p.). Tactile processes or barbels sometimes arise from the head ; the most familiar example is that on the chin of the Cod and Haddock (Figs. 873 and 877 &.). An operculum is always present, and is supported by a variable number of investing bones; it is con- tinued below into a branchiostegal membrane (Fig. 855, br. m.), which, except in Crossopterygii and the Sturgeons, is supported by bony rays. In Polypterus a pair of bony jugular plates (Fig. 872, B, jug. pi.) are placed at the lower end of the branchiostegal membrane, between the rami of the mandible : Amia has a single plate (Fig. 875, B, jug. pi.) in the same position. Spiracles are present only in Polypterus (Fig. 887) and some Sturgeons. The commonest number of median fins is two dorsals, one caudal, and one ventral, but this number may be increased or diminished (Figs. 877 and 879), or there may be a continuous median fin ex- tending along the back and round the end of the tail to the anus. The dorsal fin is sometimes partly or wholly represented by a series of small finlets (Fig. 872). The tail fin may be diphycercal, heterocercal, or homocercal, and is usually the chief organ of progression. But in the Sea-horse (Fig. 881) there is no caudal fin, and the tail is prehensile, being used in the position of rest to coil, in the vertical plane, round sea-weeds, &c. : when swimming it hangs downwards, having no lateral movement, and locomotion is effected by the vibration of the dorsal fin. The dermal-rays of the caudal fin are always jointed, as in the Trout, but in most of the Acanthopteri and Pharyngognathi more or fewer of the foremost rays of the dorsal, ventral and pelvic fins are unjointed, forming spines (Figs. 878 and 879, d. /.)> some- times large and strong enough to recall the dermal defences of 226 ZOOLOGY SECT. some Sharks and of Holocepliali (Fig. 776, d.f. r. 1 pct,f. r. 1). In Polypterus (Fig. 872) each finlet is supported along its anterior edge by a strong spine, to which the soft rays are attached. The anterior dorsal fin may attain an immense size, and is subject to some curious variations. In the Fishing-frog or Angler (LopJiius) its foremost rays are elongated and bear lobes or lures by which small fishes are attracted as to the bait on a fishing-line. In the Sucking-fish (Echeneis) the anterior dorsal is modified into an adhesive disc by means of which the fish attaches itself to the bodies of Sharks and Turtles. The portion of the paired fins visible externally is usually very thin, and supported entirely by dermal rays. But in the Crosso- pterygii (Fig. 872) the rays form a fringe round a thick basal lobe, which is supported by endoskeletal structures (vide infra). This condition of things forms an approach to the structure met with in Elasmobranchii and Holocephali. The pectorals vary con- siderably in size, and in the Flying-fishes (Exo&xtus, Dactylopterus) form large, wing-like expansions, capable of sustaining the animal in its long flying leaps into the air. In the Butterfly-fish (Gasterochisma) the pelvic fins are similarly modified. In many Fishes the pelvics are reduced to filaments or scales, and in some cases a sucking-disc is developed in connection with them. The pectorals always retain their normal position, just behind the gill-clefts, but the pelvics often become more or less shifted forwards from the typical position beside the vent. The change in position is least in the three " ganoid " orders (Figs. 872-875) and in the Physostomi (Fig. 854 and 855), in which they are usually between the middle of the abdomen and the anus, and are said to be abdominal in position; but in a large proportion of the fishes in the remaining orders of Teleostei they come to be placed almost beneath the pectorals (Fig. 879,pv.f.\ when their position is called thoracic, or on the throat (Fig. 877), when they are said to be jugular in position. A very remarkable deviation from the typical form occurs in the Flat-fishes (Pleuronectidse), a family of Anacanthini. The body (Fig. 882) is very deep and strongly compressed : the fish habitually rests on the bottom, in some species on the right, in others on the left side, partly covering itself with sand, and occasionally swimming with a curious undulating movement.' The under side is usually pure white, the upper side dark. The eyes (r.e, I.e.) are both on the upper or dark-coloured side, and the skull is distorted so as to adapt the orbits to this change of position. The abdominal cavity is very small, the anus placed far forward, and the dorsal and ventral fins are sometimes continuous. Young Flat-fishes swim in the ordinary vertical position, but after a time they lie on one side and assume the adult peculiarities, the eye on the lower side gradually rotating until it reaches the upper surface. xin PHYLUM CHORDATA 227 Many Shore-fishes exhibit protective characters, the tints and markings of the skin being harmonised with those of the rocks, sea- weeds, &c., among which they live. The effect may be heightened by fringes and lobes of skin, resembling sea-weed, and often giving the fish a most grotesque appearance. The colours are often adaptable : Trout, for instance, alter their colour by the contraction or expansion of their pigment-cells, according to whether the streams in which they live have a muddy or a sandy bottom. In some Shore-fishes, such as those of the coral reefs, the colours are of the most brilliant description ; vivid reds, blues, and yellows, spots or stripes of gold or silver, are common, and although the combination of tints may sometimes seem to our eye rather crude and glaring, they appear to be distinctly protec- tive, harmonising with the brilliant hues of the Coral Polypes Fir,. 882.— Pleuronectes cynoglossus (Craig-fluke), from the right side. d. j. dorsal fin ; 1. e. left eye ; pet. f. pectoral fin ; pv. f. pelvic fin ; r. e. right eye ; v. /. ventral fin. (After Cuvier.) and other members of the reef fauna. Pelagic fishes, such as the Mackerel and Herring, are usually steely-blue above, white beneath. Many deep-sea Teleostei are phosphorescent : in some of these definite luminous organs (Fig. 883) are arranged in longitudinal rows along the body, each provided with a lens and other accessory parts, like those of the eye, the whole organ having the character of a minute bull's-eye lantern. Some species of the same order, such as the Weaver (Trackings), possess poison-glands, opening either on one of the dorsal spines, or on a spinous process of the operculum, or, as in the Cat-fishes (Siluridse), on the spine of the pectoral fin. . Exoskeleton. — In many Teleostomi, such as Polyodon and the Eels, the skin is devoid of hard parts, but in most cases a 228 ZOOLOGY SECT. dermal exoskeleton is present. In Amia and in the majority of Teleostei this takes the form, as in the Trout, of scales, rounded plates of bone imbedded in pouches of the derm and (overlapping one another from behind forwards. When the free border of the FIG. 883.— Stomias boa. The white dots are the luminous organs. (From Hickson, after Filhol.) scales presents an even curve, as in Amia and most Physostomi and Anacanthini, they are called cycloid scales (Fig. 856) ; when, as in most Acanthopteri, the free edge is produced into small spines (Fig. 884, A), they are distinguished as ctenoid scales. Usually the integument is continued as a thin layer over the surface of the scales, but in a good many cases this investment is absent. In exceptional cases the scales may be so large and strong as to form a rigid armour. In the Sturgeon (Fig. 873) there is a strong armour, formed of stout bony plates, or scutes, produced into enamelled spines and articulating with one another by suture. Scutes are also found in many Siluroids (Fig. 876) and in Lophobranchii (Fig. 881) and some Plectognathi (Fig. 880); while in other Plectognathi the exo- skeleton takes the form, as in the File-fishes, of minute spines like the shagreen of Sharks, or, as in many Globe- fishes, of long, outstanding, bony spines. Lastly, in Poly- pterus and Lepidosteus are found rhomboid or ganoid scales (Fig. 884, B), in the form of thick, close-set, rhom- boidal plates formed of bone, covered externally by a layer of enamel-like material (ganoiri) and joined together by pegs and sockets. In many Ganoids the anterior fin-rays of both median and paired fins bear a row of spine-like scales called fulcra (Fig. 874,/.). FIG. 884.— A, ctenoid scale ; B, ganoid scales. (After Glinther.) XIII PHYLUM CHORDATA 229 Endoskeleton. — In the Sturgeon the vertebral column (Fig. 886 WS.) consists of a persistent notochord with cartilaginous arches, and is fused anteriorly with the cranium. In the remaining orders bony vertebrae are present ; the centra are biconcave, except in' some Eels, in which the anterior face is flat or even convex, and in Lepidosteus, in which the anterior face is distinctly convex. Vertebrae of this form, i.e. having the centrum convex in front and concave behind, are called opisthoccelous. Ribs are usually present : in Polypterus each vertebra has two pairs, a dorsal pair (Fig. 885, R, I — V) of considerable length, running between the dorsal and ventral muscles, and a short ventral pair (f) between the muscles and the peritoneum: the former answer to the ribs of Elasmobranchs, -IT ..V FIG. 885. — Anterior end of vertebral column of Polypterus. PS. parasphenoid ; R. I — V, dorsa ribs ; WK, centra ; t, ventral (pleural) ribs. (From Wiedersheim's Comparative Anatomy.) the latter to the ribs (pleural ribs) of the remaining Teleostomi, which are always placed immediately beneath the peritoneum. There may be one or more sets of intermuscular bones, attached either to the neural arch (epineurals), to the centrum (epicentrals), or to the ribs (epiphurals), not preformed in cartilage, but developed as ossifications of the intermuscular septa. The posterior end of the vertebral column is turned up in the Sturgeons, Lepidosteus, and Amia, resulting in a heterocercal tail-fin: in Amia, however, the fin-rays are so disposed that the fin appears almost symmetrical. Among Teleostei the tail-fin is very usually homocercal, as in the Trout, with a more or less disguised asymmetry : in many cases in the adult the development of the large, fan- shaped, posterior haamal arches completely hides the upturned end VOL. II P 230 ZOOLOGY SECT. of the notochord, and in some the spinal column ends simply in a somewhat compressed centrum around which the fin-rays are symmetrically disposed ; such truly symmetrical tail-fins are called diphycercal. In the structure of the skull the Chondrostei make the nearest approach to Elasmobranchs. The cranium (Fig. 886) is an un- divided mass of cartilage with a few isolated replacing bones. The roofing investing bones lie in the dermis, so as to be practi- cally superficial, and behind pass insensibly into the scutes covering the trunk : the fact that these bones (parietals, frontals, &c.) are exoskeletal structures is here perfectly obvious. The same is the case in Polypterus (Fig. 887), in, which, however the replacing bones are better developed. Jn Lepidosteus and Amia, FIG. 886. — Skull of Sturgeon, with the investing bones removed, a, pharyngo-branchials ; AF, antorbital process ; AR. articular ; 6. epibranchial ; c. ceratobranchial ; C, notochord ; Cop. basibraiichials ; d, hypobranchial ; De. dentary ; GK, auditory capsule ; EM. hyomandibular ; ^^hyoid coriiu ; Ih. interhyal ; Md. mandible ; Na. nasal capsule ; Gb. neural arches ; ••jMvbital process ; PQ. palatoquadrate ; Ps. Ps'. Ps". parasphenoid ; Psp. neural spines ; ^Pquadrate ; R. rostrum ; Ri. ribs ; Sp. N. foramina for spinal nerves ; Sy. symplectic ; WS, vertebral column ; n, vagus foramen ; / — V, branchial arches. (From Wiedersheim's Com- parative Anatomy.) and especially the latter, the skull resembles that of the Trout in all essential respects, the main differences consisting in the absence of certain bones, such as the supraoccipital, and in the presence of additional investing bones. Among Teleostei it is only in the Physostomi that the investing bones remain separable from the ehondrocranium in the adult; in the remaining orders, e.g. in the Cod, Haddock, or Perch, they become grafted on to the chondrocranium and so closely united with the replacing bones that they can be removed only by pulling the whole skull to pieces ; most of the original cartilage frequently disappears in the adult and the cranium thus becomes a firm bony mass in which no dis- tinction between replacing and investing bones is discernible. The varying size of the gape, which is so noticeable a feature in the Teleostomi, depends upon the inclination of the suspensorium : in wide:mouthed Fishes (Fig. 877) the axis of the hyomandibular XIII PHYLUM CHORDATA 231 and suspensorium is nearly vertical or even inclined backwards; in small-mouthed forms (Fig. 880) it is strongly inclined forwards, and the length of the jaws is proportionately reduced. In the branchial arches the pharyngo-branchials of each side are very commonly fused, and constitute what are called the superior pharynycal bones : the re- duced fifth branchial bars, or inferior pharyngeal bones, bite against them. The Pharyngognathi are dis- tinguished by having the inferior pharyngeal bones united into a single bony mass of characteristic form (Fig. 879, B). The gill- rakers are often very highly developed, and may form a mesh capable of retaining even microscopic organisms. In the shoulder- girdle, as in the skull, the Chon- drostei approach the Elas- mobranchs. There is a primary shoulder - girdle consisting of large paired cartilages, not united in the middle ventral line, and unossified : each is covered externally by a large scute- like investing bone, the clavicle. In the remaining Ganoids and in Teleostei, the primary shoulder-girdle is reduced in size and is usually ossified by two bones, a dorsal scapula and a ventral coracoid: some- times, as in the Trout, there may he an additional ossi- fication, the mesocoracoid. Additional investing bones — supra -clavicle, post-clavicle, &c. — are added, and one of them, the post-temporal, serves to articulate the shoulder-girdle with the skull (Fig. 863). In the skeleton of the pectoral fin it is the Crossopterygii which approach most nearly to Elasmobranchs. In Polypterus (Fig. the basal lobe of the fin is supported by a rod-like ossified p 2 Fig. 887.— Skull of Polypterus, from above F, frontal-; M. maxilla ; NA. nasal ; Na. nostril ; Op. opercular ; Orb. orbit ; P. parietal. The remaining letters points to less important investing bones. The arrow is passed into the spiracle. (From Wiedersheim's Comparative Anatomy.) 232 ZOOLOGY SECT. propterygium (TV), a broad cartilaginous, partly ossified, meso- tp pterygium (MS), and an ossified metapterygium (MT) ; to these, two rows of elongated radials (Ra, fia1) are articulated fan wise, and these in their turn give attachment to the fin-rays (FS). In all the re- maining orders the basal ia (pro-, meso-, and meta-pterygium) are absent, and the endoskeleton of the fin consists only of a single or double row of radials (Fig. 863). In Polypterus there is a vestigial pelvic girdle (Fig. 889, BP) in the form of a small rhomboidal cartilage to which the anterior ends of the basalia (Has1) are attached : thus in the structure of the posterior ex- tremities also, the Crossopterygii are the most primitive of the Teleo- stomi. In all the remaining orders the pelvic girdle appears to be atrophied. The pelvic fin is sup- ported by a single bone of variable form (Fig. 864, BSTG) and apparently representing a lasale, i.e. a structure arising from the fusion of proximal pterygiophores. Between its posterior end and the dermal rays irregular nodules, repre- senting radials, may be interposed. The distinction between hard or unjointed fin-rays, or spines, and soft or jointed fin-rays has already been referred to. The first ray of the dorsal and pectoral fins some- times, e.g. in Siluroids (Fig. 876), has the form of a very strong spine articulated by a bolt-and-shackle joint, i.e. by the interlocking of two rings. In some cases the first dorsal spine springs from the skull. The texture of the bones is sub- ject to wide variation : in some Acanthopteri they are very thick and strong, in some places almost like ivory ; while in the Lump-fish (Cydopterus), the huge Sunfish (Orthagoriscus), and in many deep-sea forms, such as the Eibbon- Fio. 888.— Pectoral fin of Polypterus. FS. dermal rays ; MS. mesoptery- gium ; MT. metapterygium ; NL, nerve-foramina ; Os*. ossification in mesoptcrygiurn ; Pr. propterygium ; Xa. first radials ; Ra'. second radials. At * the bony marginal rays meet and shut off the middle region from the shoulder-girdle. (From Wicder- sheim's Comparative Anatomy.) BP FlG fW 9.— Pelvic fin of young Poly- pterus. Ap. part of basale ; Has*. basale ; BP. pelvic cartilages (fused in adult) ; Rad. radials. (From Wie- dersheim.) XIII PHYLUM CHORDATA 233 fishes (Regcdccus and Trachypterus), the amount of mineral matter is so small that the bones are easily cut with a knife and weigh astonishingly little when dry. Electric Organs. — Three genera of Teleostomi possess electric organs, the Electric Cat-fish (Malapterurus), one of the Siluridae, found in the fresh waters of tropical Africa, the Electric Eel (Gymnotus), a Physostome occurring in Brazil and the Guyanas, and an American Star-gazer of the A genus Astroscopes. In Malapterurus the electric organ ex- tends over the whole body, beneath the skin ; in Gymnotus (Fig. 890) there are two pairs of batteries in the ventral half of the greatly elongated tail; in Astroscopus the electric organs are situated on the upper surface of the head just behind the eyes. As in the Elas- mobranchs, the elec- tric organs are formed by modification of muscular tissue. Digestive Organs. — Some Teleostomi are toothless ; but in most instances teeth are present, and may be developed on the premaxilla, maxilla, palatine, pterygoid, vomer, dentary, basi- hyal, and superior and inferior pharyngeal bones. It is character- istic of most Teleostei, with the exception of Physostomi, that the maxilla is edentulous and does not enter into the gape (Fig. 878). In a large majority of species the teeth are small, conical, and recurved, suitable for preventing the struggling prey from slipping out of the mouth, but quite unfitted for either tearing or crushing. In some Fishes, such as the Pike, the teeth are hinged backwards so as to offer no resistance to the passage of the prey towards the gullet, but effectually barring any movement in the other direc- FIG. 890.— Gymnotus electricus, showing the extent of the electric organ (E). Fl, ventral fin. B, small portion of tail, in section. DM. DM.' dorsal muscles ; E. E.' electric organ ; Fl, ventral fin ; H. skin ; LH, caudal canal ; Sep. fibrous septum ; VM. VM'. ventral muscles ; WS, WS', vertebral column, with spiral nerves. (From Wiedersheim's Comparative Anatomy.) 234 ZOOLOGY SECT. FIG. 891.— Premaxillse of Sargus, showing teeth. (After Owen.) tion. In many deep-sea Fishes (Fig. 883) the teeth are of immense size and constitute a very formidable armature to the jaws. Many instances occur in which there is a marked differentiation of the teeth, those in the front of the jaws (Fig. 891) being pointed or chisel-edged, and adapted for seizing, while the back teeth have spherical surfaces adapted for crush- ing. In the Wrasses (Fig. 879, B) strong crushing teeth are developed on the pharyngeal bones. In the Globe-fishes the teeth are apparently reduced to one or two in each jaw, but each " tooth" in this case really consists of numerous calcified plates fused together. The teeth may be either simply embedded in the mucous membrane so as to be de- tached when the bones are macer- ated or boiled, or they may be implanted in sockets of the bone, or ankylosed to it. They are formed of some variety of dentine, and are often capped with enamel. Their succession is perpetual, i.e. injured or worn-out teeth are replaced at all ages. In some species the enteric canal shows little differentiation into regions, but, as a rule, gullet, stomach, duodenum, ileum, and rectum are more or less clearly distinguishable. The stomach is generally Y-shaped, but its cardiac region may be prolonged into a blind pouch ; it is often very distensible, allowing some of the deep-sea Teleostei to swallow Fishes as large as themselves. In the Globe-fishes the animal can inflate the gullet with air, when it floats upside down on the surface of the water. The Ganoids have a spiral valve in the intestine, which is very well developed in Polypterus and the Sturgeon, vestigial in Lepidosteus (Fig. 893, sp. v.) and Amia : it is absent in all Teleostei, except possibly in Chirocentrus, one of the Physostomi. The liver is usually large ; a pancreas may be present as a compact gland, as in Elasmobranchs, or may be widely diffused between the layers of the mesentery, or in part surrounded by the liver. Pyloric cceca are commonly present, and vary in number from a single one to two hundred. The anus is always distinct from, and in front of, the urinogenital aperture. Respiratory Organs. — The gills are usually comb-like, as in the Trout, the branchial filaments being free, owing to the atrophy of the interbranchial septa. In the Sturgeon, however, the septa are fairly well developed, reaching half-way up the filaments, so that the latter are free only in their distal portions ; this arrange- ment is obviously intermediate between the Elasmobranch and Teleostean conditions. The most striking deviation from the normal structure occurs in Lophobranchii, in which the gill- XIII PHYLUM CHORDATA 235 filaments are replaced by curious tufted processes (Fig. 881, B, #.). As a rule gills (holobranchs) are developed on the first four branchial arches, but the fourth is frequently reduced to a hemi- branch, and further reduction takes place in some cases. The pseudobranch or vestigial hyoidean gill may either retain the characteristic comb-like structure, as in the Trout, or may be reduced, as in the Cod, to a gland-like organ formed of a plexus of blood-vessels and called a vaso-ganglion or rete mirabile. In most Teleostomi the mechanism of respiration is similar to what has already been described in the case of the Trout, and respiratory FIG. 802.— A, Anabas scandens (Climbing Perch). B, dissection of head, showing accessory respiratory organ. (A, after Cuvier ; B, after Giinther.) valves 'are developed in the mouth-cavity. But there are con- siderable differences in details, more especially as regards the relative importance of the opercula and the branch iostegal membranes in carrying on the movements of inspiration and expiration. In addition to the gills some Teleostei possess accessory organs of respiration. In Amphipnous, an Indian Physostome, the gills are poorly developed and are functionally replaced by a vascular sac occurring on each side of the body and opening in front into the first (hyobranchial) gill-cleft. Such sacs are physiologically, though not morphologically, lungs. In the Climbing Perch (Anabas) of the Oriental Region (Fig. 892) the superior pharyngeal bones are developed into folded plates (B) covered with vascular 236 ZOOLOGY SECT. mucous membrane and capable of retaining water for a consider- able period : the Fish is able to traverse the land, and is even said to climb trees, holding on alternately by the spines of its pre-operculnm and of its ventral fins. It has become so thoroughly a land- animal that it is drowned if immersed in water. In the little armoured Siluroid Callichthys, anal respiration takes place, air being drawn into and expelled from the rectum. Lastly, in the curious little goggle-eyed Periophthalmus of the In- dian and Pacific Oceans the tail-fin seems to serve as a respiratory organ, being kept in the water while the Fish perches on a rock. The air-bladder retains its connec- tion with the pharynx or the gullet in Ganoids and Physostomes ; in the other Teleostei the pneumatic duct atrophiesin the adult and the bladder becomes a shut sac. In Poly pterus it consists of two lobes, alarge left and a smaller right. The pneu- matic duct is always connected with the dorsal wall of the pharynx or gullet except in Polypterus, in which the aperture is ventral, and in some Physostomes, such as the Herring, in which it is con- nected with the'stomach. The bladder is sometimes divided into compartments or produced into lateral offshoots : in Amia, Lepidosteus (Fig. 893, a. &.), and Polypteruo its wall is sacculated or raised into anastomosing ridges, enclos- ing more or less well-marked chambers and thus resembling a lung. In Poly- pterus its lung-like character is en- hanced by its division into two com- partments by a longitudinal partition, as well as by the ventral position of the opening of the pneumatic duct and by the blood being conveyed to it by a pair of pulmonary arteries given off from the last pair of epibranchial arteries, as in the Dipnoi. The air-bladder seems to be capable of acting as a sort of accessory respiratory organ ; it has been found that in a Perch, asphyxiated Fig. 893. — Digestive organs and air- bladder of Lepidosteus. a. anus ; a. b. air-bladder ; a. b' its aperture in the pharynx ; b. d. aperture of bile-duct . c. pyloric caeca ; g. b. gall-bladder ; hp. d. hepatic duct; Ir. liver ;py. pylor- ic valve ; s. spleen ; sp. v. spiral valve; si. stomach. (From Wieder- sheiin's • Comparative Anatomy, after Balfour and Parker.) XIII PHYLUM CHORDATA 237 in stagnant water, the oxygen in the bladder, which normally amounts to 20 or 25 per cent., is entirely absorbed and replaced by nitrogen and carbonic acid. Its normal function however, is hydrostatic, i.e. it serves to keep the Fish of the same specific gravity as the water. The specific gravity of the Fish as a whole, falling or rising as it must on account of the increase or decrease of pressure at various depths as the Fish descends or ascends causing greater or less compression of the gases in the air-bladder, olf.l opt. I FIG. 894.— Horizontal section of posterior portion of head and anterior end of air-bladder in Pseudophycis bachus, one of the Gadidye or Cods (semi-diagrammatic), a. thickened portion of air-bladder fitting into fenestra in posterior wall of auditory capsule ; a. bl. air- bladder ; au. cp. outer wall of auditory capsule ; au. cp'. inner (membranous) wall ; b, hollow offshoots of air-bladder ; cp, str. corpora striata ; crb. cerebellum ; memb. lab. mem- branous labyrinth ; olf. 1. olfactory bulbs ; olf. p. olfactory peduncles (olfactory tracts) ; op. operculum ; opt. 1. optic lobes ; vg. gn. vaso-ganglia. can be brought to approximate to that of the surrounding water by increase or decrease in the quantity of the contained gas. This is brought about by secretion or absorption, often by means of vaso-ganglia or red glands (Fig. 894, vs. gn). These are elevations of the wall of the bladder, abundantly supplied with blood, and containing tubular glands which open into the cavity of the bladder. In Fishes with a pneumatic duct the red glands are absent, but in Eels their place is taken by red bodies 238 ZOOLOGY SECT. olf.l of similar appearance, but with non-glandular epithelium. In some forms with closed air-bladder the anterior end of the organ is forked, and each branch (Fig. 894, a) fits closely against a membranous space in the posterior wall of the auditory capsule, while laterally it extends outwards in the region of the shoulder-girdle, and comes to lie immediately beneath the skin ; in this way varying pressures on the surface of the body are transmitted through the air in the bladder to the auditory organ. In the Carps and Siluroids a chain of bones connects the air-bladder with the auditory organ, forming the Weberian apparatus, the function of which, as of the simpler arrangement described above, is probably " to bring directly to the consciousness of the Fish the varying tensions of the gaseous contents of the air-bladder, due to the incidence of varying hydrostatic pressures." The structure of the heart forms one of the most striking differences between the three Ganoid orders and the Teleostei. In Ganoids there is a muscular conns arteriosus with rows of valves, as in Elasmobranchs ; in Teleostei a vestige of the conus containing two rows of valves has been found in Albula (Physostomi), and similar vestiges occur in several other genera of the same sub-order, but in all the rest of the order it is entirely- unrepresented. On the other hand, Tele- ostei always have a large bulbus aorta?, formed as a dilatation of the base of the ventral aorta. In the brain the cerebellum and optic lobes are usually large ; the diencephalon is well developed in Ganoids, almost obsolete in Teleostei, In the Teleostei and Ganoidei the prosencephalon has the general features which have been described in the account of the brain of the Trout : it is not divided into hemispheres and has a roof which, except in Aniia, is completely non- nervous; its floor consists of a pair of massive corpora striata (Fig. 895, prs. and Fig. 866, BG.). In most instances the olfactory bulbs are in close apposition with the olfactory region of the prosencephalon without the intervention of olfactory stalks or tracts. The Ganoids agree with Elasmobranchs in the fact that the optic nerves form a chiasma, while in Teleostei they simply cross one another or decussate. Here also, however, the distinction is not quite absolute, since in the Herring and some other Physostomes one cbl 7/t.O FIG. 895.— Brain of Lepi- dosteus, dorsal view. cbl. cerebellum ; c. h. olfac- tory part of prosen- cephalon ; di. diencephalon ; m. o. medulla oblongata ; of/. I. olfactory bulbs ; opt. I. optic lobes ; prs. corpora striata. (After Balfour and Parker.) XIII PHYLUM CHORDATA nerve passes through a slit in the other. In some cases the olfactory bulbs spring directly from the prosencephalon, as in the Trout ; in others they are borne on long olfactory peduncles (or olfactory tracts, Fig. 894, olf. p.\ as in the Cod. In some Plecto- gnaths the spinal cord undergoes a remarkable shortening: in a Sun-fish 2^- metres in length and weigh- ing a ton and a half, the cord is only 15 millimetres long, being actually shorter than the brain. Urinogenital Organs. — The kidney (Fig. 865, M) is formed from the meso- nephros of the embryo, and usually attains a great size; the pronephros usually atrophies. The ureter (wr.) is the undivided pronephric duct : it unites with its fellow of the opposite side before opening either directly on to the ex- terior or into a urinogenital sinus. A urinary bladder is formed as a single or double dilatation of the ureter. The right and left kidneys undergo more or less fusion, and their anterior ends are usually converted into adenoid or lym- phatic tissue (kd!)t so that, while re- sembling the rest of the organ in ex- ternal appearance, they do not discharge a renal function. The male organs of Lepidosteus may be taken as an example of those of Ganoids. The testis (Fig. 896, te.) is a paired, lobulated organ, the secretion of which is carried by a large number of vasa efferentia (v. ef.) into a longitudinal canal (/. c.) lying alongside the ureter (ur.). From thisvcanal tubes are given off which communicate with the urinary tubules of the kidney or open directly into the ureter, so that the seminal fluid has to traverse the latter in order to .reach the urinary bladder (R) and make its escape by the common urino- genital aperture (u.g. ap.). In Teleostei there are no vasa efferentia, but the posterior end of the testis is directly con- tinued into a duct (Fig. 865, v. d.) which unites with its fellow of the opposite side and opens either into a urinogenital sinus, as in the Trout, or, as in the Cod, directly on the exterior, between the anus and the urinary aperture. In the Eels the FIG. 896. — Male organs of Lepi- dosteus. bl. bladder ; I. c. longitudinal canal; ts. testis; u.g. ap. urinogenital aperture ; ur. ureter ; v. ff. vasa efferentia. (After Balfour and Parker.) • 240 ZOOLOGY SECT. B seminal fluid escapes into the ccelome and is discharged by genital pores. In most Ganoids the oviducts (Fig. 897, B, ovd.) have funnel-like anterior ends (ovd") opening into the ccelome, while posteriorly (ovd.'} they discharge into the dilated ureters (bl.\ A similar arrangement occurs in the Smelt, one of the Physostomi (Salmonidse), in which the eggs are discharged from the outer or lateral face of the ovary into the open end of the oviduct. But in most Teleostei and in Lepidosteus (Fig. 897, A) the ovary (ovy.} is a hollow sac continued pos- teriorly into the oviduct (ovd.) : the eggs are set free into its cavity from the folds into which its inner surface is produced, and so pass directly into the oviduct without previously entering the ccelome. An ovary of this kind reminds us of the state of things in Arthropods, in which also the ovary is a hollow organ discharging its pro- ducts into its inter- nal cavity, 'whence they pass directly into the continuous oviduct. It was pointed out that the lumen of the ovary in this case was to be looked upon as a shut-off portion of the ccelome : this is certainly the case in Lepi- dosteus and the Teleostei. In the embryo a longitudinal fold grows from the ventral edge of the then solid ovary, and turns upwards along the lateral face of the organ : it is met by a descending fold of peritoneum from the dorsal wall of the abdomen, and by the -U-.ff.CLp FIG. 897.— Female organs of Lepidosteus (A) and Amia (B). a, degenerate anterior portion of kidney ; bl. bladder ; kd. kidney ; ovd. oviduct ; ovd.' aperture of oviduct into bladder ; ovd." peritoneal aperture ; ovy. ovary ; p. peritoneum ; u.g. ap. urinogenital aperture ; ur. ureter. (A, after Balfour and Parker ; B, after Huxley.) XIII PHYLUM CHORDATA 241 union of the two folds a cavity is enclosed, which is the lumen of the ovary. The oviduct is developed as ahackward continuation of these folds of peritoneum, and appears to be quite unconnected with the embryonic renal system, and therefore not to be homologous with the oviducts of Elasmobranchs and Holocephali, which, as we have seen, are Mullerian ducts. In the Salmonidae and the Eels oviducts are absent, and the ova are discharged by genital pores, which are probably to be looked upon as degenerate oviducts. True abdominal pores are present in Ganoids and in some Physostomi. Most Teleostomi are dioecious, but Serranus, one of the Perch family, is hermaphrodite and self-impregnating ; CJirysophrys is hermaphrodite and successively male and female ; and there are many well-known species, such as the Cod and the Herring, which exhibit the hermaphrodite condition as an occasional variation. Reproduction and Development. — Most Teleostomi are oviparous, the eggs being impregnated after they are laid, but in some Teleostei, such as the Viviparous Blenny (Zoarces), internal impregnation takes place ; the young are developed in the hollow ovary and are brought forth alive. Many instances of parental care of the young are known, the most familiar being that of the male Stickleback (G aster osteus\ which constructs a nest of weeds, fastened together by a glutinous secretion of the kidneys, and jealously guards the developing young. In the Sea-horse (Hippo- campus) and the Pipe-fish (Syngnafhus) the young are developed in a pouch (Fig. 881, brd. p.) on the abdomen of the male. In the Siluroid Aspredo the eggs are pressed into the soft spongy skin of the belly and thus carried about by the parent. The ova are always small as compared with those of Elasmobranchs, never exceeding 5 to 10 mm. in diameter, and being usually much smaller. They are rarely pro- tected by an egg-shell. They are produced in immense numbers, a single female sometimes laying several millions : in such cases the mortality among the unprotected embryos and young is immense. The eggs may be pelagic, i.e. so ligntas to float when laid, as in the Cod, Haddock, Turbo t, Sole, &c. ; or demersal, i.e. so heavy as to sink to the bottom, as in the Herring, Sal- mon, TrOUt, &C. In SOme Cases (Chilo- FIG. S9S.— Segmentation in Lepi- Iranclms) they become cemented to the pSSJ)*' (Afte surface of a rock. In all the Ganoids hitherto investigated (Polypterus, Lepidosteus, Amia, and Acipenser), segmentation is complete, but very unequal (Fig. 898) : the megaraeres are immense as compared 242 ZOOLOGY SECT. with the micromeres, and the process may be said to be intermediate between the holoblastic and meroblastic types. In Teleostei, on the other hand, segmentation is always partial and discoidal. The general features of development are much the same as in the Trout, except that in the Sturgeon and Polypterus, as in Craniates in general, there is an open medullary groove which becomes closed in to form a medullary canal. There is frequently a metamorphosis: in Lepidosteus, for instance, the newly hatched young is provided with a sucking-disc, and the proportions of the head are quite different from those of the adult. In the larval Sturgeon provisional teeth are present, and in many Teleostei the young differ from the adult in the presence of large spines, which probably, like the spines in the zoasa-stage of some Crustacea, serve a defensive purpose. The larvae of Eels are strongly compressed, perfectly transparent, and have colourless blood. They are sometimes known as " Glass-fish/' and were formerly placed in the genus Leptocephalus, their real nature being FIG. 899. — Polypterus bichir. Head of advanced larva ; EG. external gill. (From Dean, after Steindachner. ) unknown. The young of the Crossopterygii (or at least Polypterus) have external gills, as in Dipnoi and Amphibia (vide infra), and the same holds good of Colitis, ffeterotis, and G-ymnarchus among the Teleostei. The Geographical Distribution of the Ganoid Teleostomi is curiously limited : they are all essentially fresh-water forms — although some Sturgeons are found in the sea — and are almost exclusively inhabitants of the Northern Hemisphere, and especially of the Holarctic Region. The Chondrostei occur in the rivers of Europe, Asia, and North America : one genus of Sturgeons (Seaphirhynehus) lives in the Mississippi and in the rivers of Central Asia, but not in the intermediate regions : in the same way Polyodon is found only in the Mississippi, while the closely- allied Psephurus is found in the Yangtse-kiang and Hoangho — a striking instance of discontinuous distribution. Amia is found in the fresh waters of the United States ; Lepidosteus extends also into Central America and Cuba. Polypterus lives in the Upper Nile and some other tropical African rivers ; Calamichthys in the Old Calabar River. XIM PHYLUM CHORDATA 243 Among Teleostei the Physostomi are largely, though not exclusively, fresh-water Fish ; the Carps, Eels, Salmonoids, and Siluroids are important examples. The Acanthopteri, Pharyngo- gnathi, and Anacanthini are mostly marine, some being in- habitants of the shores, some pelagic, some abyssal, extending to a depth of nearly 3,000 fathoms. As we have seen, many species are practically terrestrial. All the sub-orders are uni- versally distributed, so that we have to descend to families before meeting with any important facts in geographical distribution. The "Distribution in Time of the Teleostomi is interesting as showing the gradual replacement of the lower or more FIG. 900.— A, restoration of Glyptolepis (Devonian) ; B, Macropoma mantelli (Cretaceous). a. hi. ossified air-bladder ; d.f. 1, d.f. 2, dorsal fins ; h. a. hsemal arches ; jug. pi. jugular plates ; n. a. neural arches ; nch. position of notochord ; pet. f. pectoral fin ; pv. f. pelvic fin ; v. f. ventral fin. (From Nicholson and Lydekker.) generalised members of a group by the higher or more specialised forms. During the whole of the Paleozoic and the greater part of the Mesozoic era the three orders of Ganoids, to-day small and isolated groups, formed the whole of the Teleostomian fauna, and it is not until the Cretaceous period that the Teleostei, the present dominant order, make their appearance. From the Cre- taceous onwards the Ganoids undergo a progressive diminution in numbers, genus after genus and family after family becoming extinct, while a corresponding increase takes place in all the sub- orders of Teleostei. The Crossopterygii make their first appearance in the Devonian 244 ZOOLOGY SECT. period, and between that period and the Cretaceous, include six families and a large number of genera and species. They exhibit (Fig. 900) a very considerable range of variation in external and internal characters. There are usually two dorsal fins, the tail may be diphycercal or heterocercal, the scales rhomboid or cycloid. In some genera, also, there was a persistent notochord (B. nch.), the fossils showing well-preserved neural and ha3mal arches, but no signs of centra. In many cases the interspinous B FIG. 901.— A, Palrconiscus macropomus (Permian) ; B, Platysomus striatus (Permian). (From Nicholson and Lydekker.) bones or proximal pterygiophores of the dorsal fins are fused into a single basal bone. All agree in the possession of lobed fins ; the basal lobe is sometimes so long as to approach the type of structure we shall find to characterise the Dipnoi (vide infra). The Ghondrostei are also largely represented, from the Devonian upwards, and include a great variety of forms, many of which, apart from the heterocercal tail, have a strong external re- semblance to Teleostei (Fig. 901). Some have the characteristic spindle-form of strong-swimming Fishes (A), others the high, compressed form of such shore-fishes as the Reef-fishes (B). XIII PHYLUM CHORDATA 245 Scutes are present in some species, rhomboid scales in others, and in one genus the greater part of the body is covered by cycloid scales, while rhomboid scales occur in the upper part of the tail. The Holostei first make their appearance in the Triassic rocks and are abundant in Secondary and Lower-Tertiary strata. They also (Fig. 902) show a wide diversity in form and structure. The body may be spindle-shaped or high and compressed ; the scales may be rhomboid or c}rcloid, or may exhibit every gradation from rhomboid to cycloid in passing from the trunk to the tail of one FIG. 902.— A Lepidotus maximus (Jurassic). *. scale ; t. teeth. B, Caturus furcatUS (Jurassic). (From Nicholson and Lydekker.) and the same Fish ; the teeth may be sharp and conical, or blunt, rounded, and adapted for crashing. A persistent notochord is present in some species, a well-ossified vertebral column in others. We see, then, that all the orders of Ganoids, during the period of their prime, branched out into diverse forms, adapted to different environments, and often resembling, in a remarkable manner, the divergent forms of Teleostei which fill similar positions at the present day. The Teleostei first appear in the Cretaceous rocks, where many existing families are represented. From this period onwards the three Ganoid orders undergo a progressive diminution in the VOL. II Q 246 ZOOLOGY SKCT number of families, genera, and species, their places being taken by the more highly differentiated Teleostei, until, at the present day, as we have seen, they are reduced to a few scattered forms, mostly confined to fresh waters. Sub-class V.— The Dipnoi. The Dipnoi or Lung-fishes, comprising as their living repre- sentatives only the Queensland Ceratodus or " Burnett Salmon," and the Mud-fishes (Protopterus and Lepidosiren) of certain South African and South American rivers, are fishes of such well-marked and special features that by some zoologists they are separated from the true Fishes arid regarded as constituting a separate class of Vertebrates. One of their peculiar features is indicated by the name Dipnoi. Not only do these animals breathe by means of gills, like ordinary Fishes, but they have a highly-developed apparatus for the respiration of air — a lung or lungs — with an arrangement of the circulation co-ordinated with this, such as is indicated in Polypterus and Amia only among the -Teleostomi. They have bony scales and dermal fin-rays, but the paired fins, unlike those of any other Fishes, with the exception of certain extinct Elasmobranchs, are constructed on the biseriar type (archipterygium, see p. 168). 1. EXAMPLE OF THE CLASS — Ceratodus (Neoceratodus) forsteri. The Ceratodus or " Burnett Salmon " (Fig. 903) is the largest of the Dipnoi, attaining a length sometimes of four or five feet. It occurs at the present day only in the Burnett and Mary Rivers in Queensland, but fossil teeth referred to the same or nearly related genera have been found in abundance in Palaeozoic and Mesozoic beds in Europe, America, the East Indies, Africa, and Australia. Ceratodus forsteri lives in still pools in which the water in the dry season becomes extremely stagnant and overladen with decomposing vegetable matter; and it is only by rising to the surface occasionally, and taking air into its lung, that it is enabled to obtain sufficient oxygen for purposes of respiration. Its food consists of such small animals as live among the water-plants and decaying leaves, and in order to obtain a sufficient amount of such food, it swallows relatively large quantities of vegetable matter, which passes with little or no alteration through its enteric canal. Its movements are for the most part very sluggish, and are chiefly effected by the agency of the tail-fin. The paired fins are employed in steering and balancing and in the ascending and descending movements : owing to their great flexibility they are entirely incapable of supporting the body when the fish is removed XIII PHYLUM CHORDATA 247 from the water, but the pectoral fins may be employed as props when it lies in a resting condition at the bottom. External Characters. - - The body is fish-like (Fig. 903) with a diphycercal caudal fin. The surface is covered with thin, bony, imbri- cated cycloid scales, very large on the head and trunk, somewhat smaller towards the tail end and very much smaller over the fins and the posterior part of the operculum. The limbs have a characteristic shape, being in the form of two pairs of elongated, leaf-like, pointed paddles. The marginal parts of the paired fins and the whole extent of the unpaired or caudal fin are supported by a double series of slender fibre-like unjointed partly ossified dermal rays (camptotrichict), which are much more numerous than the endoskeletal rays and which are covered by small surface- scales. The mouth is situated on the ventral surface of the head, close to the anterior extremity of the snout. The external nares differ from those of other Vertebrates in being situated immediately outside the aperture of the mouth, enclosed within the upper lip. A pair of internal nares opens not far behind them into the anterior part of the mouth-cavity. At the root of the tail, is the cloacal aperture. There is an operculum similar to that of the Teleostomi, with a single slit- like branchial aperture behind it. There are no spiracles. There is a well-marked lateral line. En do skeleton. — The spinal column (Fig. 904) is represented by a persistent notochord, enclosed in a thick fibrous sheath, together with neural and haamal arches. A series of neural or basidorsal cartilages form the bases of the Q 2 248 ZOOLOGY SECT. neural arches, and haemal or basiventral cartilages are similarly related below to a series of pleural ribs in the precaudal region, and to a series of haemal arches in the caudal. These two sets of basal cartilages are not precisely opposite throughout, and regularly alternate for some distance in front. They are embedded in the sheath of the notochord. but no centra are formed, and the noto- chord, though pressed upon above and below by the series of basal cartilages, is not constricted in the usual annular manner. At the posterior end it becomes surrounded by cartilage. The neural and haemal arches are ossified ; each is surmounted by a rod-like neural or haemal spine which forms part of a continuous three-jointed ossified rod, the proximal segment being the spine, and the two others radials. The pleural ribs are curved bony rods extending downwards and somewhat backwards in the body-wall immediately FIG. 904. Ceratodus forsteri. Lateral view of the anterior portion of the skeleton. A, anterior median investing bone of the roof of the skull ; B, posterior median investing bone ; C, inner lateral investing bone ; bax. basal cartilage of the pectoral fin ; br. branchial arches ; dent, tooth of lower jaw ; int. interoperculum ; lam. plate overhanging branchial region ; tuck. Mcckel's cartilage ; occ. rb. occipital rib ; op. operculum ; pal. palatoquadrate ; pet. pectoral arch ; rbs. ribs ; sub. orb. suborbital bones ; sq. so-called squamosal ; sui>i-a-»<: suprascapula. outside the peritoneal membrane, like the pleural ribs of the Teleo- stomes. The first pair (Fig. 904. occ. rb.\ thicker and straighter than the rest, are connected with the skull in its vertebral portion. The skull (Figs. 904, 905, and 906) consists of an undivided mass of cartilage, devoid of fontanelles, narrowest between the orbits, and broadening before and behind ; posteriorly it is prolonged into a plate (lam.} overhanging the branchial region. Embedded in the cartilage of the posterior part are a pair of small replacing bones which appear to represent the most anterior of the spinal elements fused with the skull. On the upper surface are two unpaired (A and B), and four paired (G and sq.~) investing bones, the homologies of all of which are undetermined. Premaxillae, maxillae, and nasals are XIII PHYLUM CHORDATA 249 absent. On the ven- tral surface is a large investing bone (Fig. 906, P. Sph.) represent- ing the parasphenoid of the Teleostomi. In front is a, pair of small upper labial or nasal cartilages. A palato- quadrate cartilage (Fig. 904, £>«/.), firmly fixed to the side-wall of the cranium, gives attach- ment to the mandible, so that the skull is autostylic; the quad- rate element is distinct in the larva and in- dependently developed. In front the palato- . quadrate contains a palatopterygoid ossifi- cation which forms the support for the large composite tooth of the upper jaw. art FIG. 905.— Ceratodus forsteri. Dorsal view of the skull. A, anterior median investing bone ; art. articular surface for second fin-ray ; B, posterior median investing bone ; C, inner lateral investing bone ; lab. labial car- tilages ; lam. process projecting over gills ; op. oper- culum ; pr. orb. preorbital process of chondrocranium ; sb. orb. suborbital bones ; *q. outer lateral investing bone. (After Huxley.) FIG. 906. -Ceratodus forsteri. Ventral view of the skull, c, occipital rib ; d, palatine teeth ; d', vomerine teeth ; na. anterior and posterior nares ; P. palatine region of palatopterygoid; P. *ph. para- sphenoid ; Pt. pterygoid ; Qu. quadrate region ; Vo. vomer. (From Dean, after Giinther.) The hyomandibular is only represented by a small vestige. Opercular (op.} and interoper- cular (int.) bones support the operculuni. The mandible con- sists of Meckel's cartilage with an angular bone behind, and a large splenial, which bears the tooth, in front. The dentary is vestigial. The hyoid (hy.) and branchial arches (5r.) are cartil- aginous. Of the latter, four are completely developed, and a fifth is represented by a vestige. There are no branchial rays, but the branchial arches bear a series of gill-rakers with cartilaginous supports. The pectoral arch (Fig. 904, pet.) is a stout cartilage with two pairs of investing bones, the clavicles on the coracoid, and the cleithra on the scapular regions. 250 ZOOLOGY SECT. The latter are connected with the skull by post-temporals. The skeleton of the pectoral fin consists of a stout basal cartilage (bos.), an elongated, tapering, central axis made up of a number of short cartilaginous segments, and two rows of jointed cartilaginous rays extending out on either side of the axis so as to support the middle part of the expanse of the fin. The pelvic arch is a single cartilage, produced forwards into an elongated rod-like epipubic process (Fig. 907). The skeleton of the pelvic fin is similar to that of the pectoral. Digestive Organs. — The teeth (Fig. 906) are of a remark- able and characteristic shape. There are two pairs of large compound teeth of similar character, one pair (the palatine, d), on the roof of the mouth (palatopterygoid bone) and the other (splenial) on the lower jaw. Each is a curved plate with the convex border, which is directed inwards and somewhat backwards, FIG. 007.— Ceratodus forsteri. Pelvic arch and skeleton of pelvic fin. (After Gttnther.) entire ; while the concave border presents a series of six or seven vertical, ridge-like projections or cusps. In addition to these, there are, in front of the palatine pair, a pair of much smaller, simple, somewhat chisel-like vomerine teeth (cT) placed close together and directed vertically. In the embryo each tooth is represented by a number of separate denticles which subsequently coalesce. In the enteric canal the chief feature of special interest is the presence, throughout the length of the intestine, of a spiral valve similar to that of the Elasmobranchs and Ganoids. The rectum opens into a small cloaca. A pair of abdominal pores open just behind this. Organs of Respiration. — Ceratodus combines aquatic respira- tion by means of gills similar to those of ordinary fishes, with aerial respiration by means of a lung. There are four pairs of gills, each consisting of a double row of gill-filaments supported on the branchial arches. A rudimentary XIII PHYLUM CHORDATA 251 hyoidean gill or pseudobranch is present as well. The lung (Fig 908) is an elongated median sac connected by a pneumatic duct with a muscular chamber or vestibule opening into the oesophagus on its ventral side by a slit-like aperture or glottis. The internal surface of the lung is sacculated, and a regularly-arranged series of blind pouches opens out of the main central cavity. This lung of Cera- todus corresponds morphologically to the air-bladder of Ganoids and Teleosts, but differs from it in its blood-supply and consequently in its function, being supplied with blood by a special paired pulmonary artery (as is also the case in Poly- pterus) and acting as an important organ of respiration. Blood-Vascular System. — Co- ordinated with the existence of a lung and distinct pulmonary circu- lation, is a complication in the struc- ture of the heart. The sinus venosus is imperfectly divided into two parts, and the cavity of the auricle is divided into two by an incomplete septum in the form of a ridge. The venous blood enters the right-hand division of the sinus venosus and passes thence through the right-hand division of the auricle to the ventricle ; the pulmonary vein, by which the blood is returned from the lung, passes through the sinus, and its blood reaches the ventricle through the left-hand division of the auricle. There are no auriculo-ventricular valves guarding the opening between the auricle and the ventricle. A contractile conus arteriosus is present, and has a remarkable spirally-twisted form ; in its interior are four longitudinal rows of valves, one of which is modified to form an incomplete longitudinal septum. The channel on the left side of this septum, which receives the blood of the pulmonary vein, is in communication in front with the first two aortic arches (afferent branchials), that on the right with the last two. The blood-vessels (Fig. 909) present an arrangement which is intermediate in some respects between that which has been already described as observable in the Elasmobranchs and that which will be found to characterise the Amphibia. The four afferent branchial arteries (off.) take their origin close together, immediately FIG. 908.— Cera todus forsteri. Posterior half of the lung with the ventral wall slit up so as to show the interior. (After Gunther.) 252 ZOOLOGY SECT. in front of the conns, so that a ventral aorta can hardly be said to exist. Each branchial arch has two efferent branchial arteries. A hyoid artery (hy. art.) is connected dorsally and vent- rally with the most anterior of these. The eight efferent vessels unite in pairs to form fonr epibranchial arteries (epi.). The rposl.cctr l.posl.car — l.posl.carcl FIG. 909.— CeratodUS forsteri. Diagrammatic view of the heart and main blood-vessels, as seen from the ventral surface, aff. 1, 2, 3, 4, afferent vessels ; 1 br,2br, 3 br, 4 br, position of gills ; c. «. conus arteriosus ; !. 4, epibranchial arteries ; hit. art. hyoidean artery ; L /-. r. postcaval vein ; /. ant. car. left anterior carotid artery ; 1. aur. left auricle ; ;. br. v. left brachial vein ; ?. jug. r. left jugular vein ; L post. car. left postei-ior carotid artery ; 1. post. card, left cardinal vein ; I. put art. left pulmonary artery ; ?. sc. i?. left subscapular vein ; r. ant. car. right anterior carotid artery; r. aur. right auricle ; r. br. v. right brachial vein; r. jug. right jugular vein; '/•. post. car. right posterior carotid ; r. pul. art. right pulmonary artery ; 7% sc. r. right subscapular vein ; vent, ventricle. (After Baldwin Spencer.) latter unite dorsally to form a main trunk, which combines with the corresponding trunk of the opposite side to form the median dorsal aorta (d. «.). The head is supplied by carotid branches given off from the first epibranchial (L post. car. and r. post, car.), and from the hyoidean arteries (I. ant. car. and r. ant. car), and the latter also gives off a lingual artery to the tongue. From XIII PHYLUM CHORDATA 253 V rh the last (fourth) epibranchial artery arises the pulmonary artery (I. pul. art. and r. pul. art.), carrying blood to the lung. There are two ductus Cuvieri or precawals (d. c.), as in the Dog- fish (p. 156). The right is formed by the union of jugular (I. jug. v. and r. jug. v.\ IracMal (I. br. v. and r. br. v.), and subscapular veins (l.se. v. and r. sc. v.}. The left receives in addition a left cardinal vein (I. post. card.). A large lateral cutaneous vein, running superficially along the side of the body, opens into the subscapular. A large postcaval vein (i. v. c.) brings back the greater portion of the blood from the posterior parts of the body ; it is situated somewhat to the right of the middle line, and opens into the sinus venosus between the two hepatic veins. A postcaval occurs in the Dipnoi alone amongst Fishes, but is universal in all the higher classes. Posteriorly the cardinal and the postcaval are formed by the bifurcation of a median caudal vein; close to its origin each receives the efferent renal veins bringing back the blood from the kidney. The blood from the pelvic fin is brought back by an iliac vein which divides into two branches — pelvic and renal portal. The former, running for- wards and inwards, unites mesially with the corresponding vessel of the opposite side to form a median abdominal vein — a vessel universal in the Amphibia, and perhaps corresponding to the lateral veins of the Elasmobranchs ; it opens into the sinus venosus. The other branch is the renal portal vein ; after receiving tributaries from the posterior region of the body ifc passes to the corresponding kidney.1 Brain.— The whole brain (Fig. 910) is enclosed in a tough and thick mem- brane, which becomes glandular in two positions — on the roof of the diaccele, and on that of the metacoele. In the former position this glandular development of the enclosing membrane, or chor&id plexus, passes downwards into the diacoele and is developed into a spongy mass which is prolonged forwards to the anterior end of the prosencephalon. The prosencephalon (pros.) presents two elongated hemispheres, which are completely 1 How far this arrangement combines Fish-like and Amphibian characters will be best understood at a later stage. Fir,. 010.— Brain of Ceratodus forsteri, dorsal view. nvd. auditory nerve ; clil. cerebellum ; fiic. facial nerve ; yl. glosso- pharyngeal ; me1. p.) ; C, later stages in which the blastopore (bl. p.) has taken the form of a ring-like groove enclosing the yolk-plug (ylk. pi.) ; Z>, stage in which the narrow medullary groove (hip. sut.) has appeared with the rudiment of the medullary folds (med.) ; E, stage in which the medullary folds (med.) have become well developed ; h\ later stage with well-formed head with two visceral arches (rise.) and rudiments of eye (, upper jaw is well in front of the eye, the axis of the suspensorium being inclined forwards and the mandible very short, in correspondence with the small size of the tadpole's mouth. The quadrate is fused by its pedicle with the trabe- cular region, the otic process (ot. pr) which unites it with the auditory capsule being- formed later. Behind the suspensorium are distinct hyoid (c. hy) and branchial (br. 1 — 4) arches supporting the gills by which the tadpole breathes. As development goes on, the axis of the suspensorium is rotated backwards, producing the wide gape of the adult, and the stout palatopterygoid region of the subocular arch (pal. ptg) gradually assumes the slender proportions it has in the adult. The greater part of the hyoid arch gives rise to the anterior cornua of the adult hyoid-apparatus, the body of which is formed from the basi- hyal and basi-branchials, and its posterior cornua probably from the fourth branchial arch. The columella is developed inde- pendently, but may perhaps represent a pharyngo-hyal or dorsal segment of the hyoid arch. The stapes is a detached portion of the outer wall of the auditory capsule. Thus, with the assumption of purely aerial respiration, the complex branchial skeleton is reduced to a simple structure for the support of the tongue. The shoulder-girdle has essentially the structure already de* br, FIG. 924.— Skull of Tadpole, au. cp. auditory capsule ; br. 1 — A, branchial arches ; c. hy. ceratohyal ; col. columella ; mck, Meckel's cartilage ; olf. cp. olfactory capsule ; opt. for. optic foramen ; or. pr. orbital pro- cess of suspensorium ; ot. pr. otic process ; pal. ptg. palato-pterygoid bar ; qu. quadrate ; stp. stapes (After Marshall, slightly altered.) 272 /OOLOGY SECT. scribed (p. 84) in general terms as characteristic of the pentadactyle Craniata. The scapula (Fig. 925, S, Fig. 926, scp) is ossified, and Fio. 925.— Bana esculenta. The shoulder girdle from the ventral aspect. Cartilage dotted. Co. coracoid ; Co', epicoracoid ; Cl. clavicle ; Ep, omosternum ; G. glenoid cavity ; Fe. fenestra between clavicle and coracoid ; KG. cartilage separating scapula and clavicle ; Kn. xiphi- sternum ; m, junction of epicoracoids ; S. scapula; St. sternum. (From Wiedersheim's Comparative Anatomy.) is connected by its dorsal edge with a suprascapula (Fig. 922. s. SCP, Fig. 926, s. scp) formed partly of bone, partly of calcified cartilage, and developed from the dorsal region of the embryonic shoulder-girdle. The coracoid (Fig. 925, Co., Fig. 926, cor.) is also ossified, but the pro- curacoid is represented by a bar of cartilage having an investing bone, the clavicle (Cl), closely applied to it. The suprascapula overlaps the anterior vertebrae ; the coracoid and procoracoid are connected ventrally by a car- FIG. 926.— Bana. Diagrammatic transverse section through the" shoulder-girdle. cor. tllage, the epicOTOCOid (Fig. coracoid; ep. cor. epicoracoid; gL glenoid 025 Co' Fior. 926 eft. COT] cavity ; ku. humerus ; scp. scapula ; .<*. scp. supra-scapula ; r. 3, third vertebra. (From Parker's Practical Zoology.) which is in close contact with its fellow of the opposite side in the middle ventral line, so that the entire shoulder-girdle (Fig. 926), like that of the Dog-fish, forms a single inverted arch. Passing forwards from the anterior ends of the united epi- XIII PHYLUM CHORDATA coracoids is a rod of bone tipped by a rounded plate of cartilage, the omostermim ; and passing backwards from their posterior ends is a similar but larger bony rod, the sternum (sty, also tipped by a cartilaginous plate, to which the name xiphisternum (Kn) is applied. These two structures are the first indication of a sternum we have yet met with, with the possible exception of the median ventral element of the shoulder-girdle of Heptanchus (p. 175). The omosternum is developed as paired forward extensions of the epicoracoids which undergo fusion : the sternum and xiphisternum arise as paired rods lying posterior to the epicoracoids, and subsequently uniting with one another. This sternal apparatus of the Frog (and of the Amphibia in general) differs developmentally from the structures in the higher Vertebrates to which the same name is applied — the latter being formed from separated-off portions of embryonic ribs (costal sternum). The fore-limbs deviate from the typical structure (p. 82) chiefly in the fusion of the radius and ulna into a singie radio-ulna (Fig. 922, RA. UL), and in the presence of only four complete digits with a vestigial one on the radial side. In all probability the latter represents the pollex, and the complete digits are the second to the fifth of the typical hand. Six carpals only are present, the third, fourth, and fifth digits articulating with a single bone which has apparently arisen by the fusion of the third, fourth, and fifth distalia and of at least one centrale. The pelvic girdle (Fig. 927) is very peculiarly modified ; it resembles in form a Bird's " merrythought," consisting of two long, curved bars articulating in front with the transverse processes of the sacral vertebra (Fig. 922) and uniting posteriorly in an irregular vertical disc of mingled bone and cartilage which bears on each side a deep, hemispherical acetabulum (6r) for the articulation of the thigh-bone. The curved rods are the ilia (II. , P) ; they expand posteriorly and unite with one another in the median plane to form the dorsal portion of the disc and about one-half of the acetabulum. The posterior portions of the disc and acetabulum are furnished by the ischia (Is), fused with one another in the sagittal plane, their ventral portions by the similarly united pubes (Kri). The ilium and ischium are formed of true bone, the pubis of calcified cartilage ; the union of the elements in the median plane is called the sympJiysis. In- the larva the ilium is vertical, but during development it bccgmes lengthened FIG. 927.— Bana csculenta. Pelvic girdle from the right side. G, acetabulum ; It, P, ilium ; Is. ischium ; Kn, pubis. (From Wiedersheim's Comparative Anatomy.) 274 ZOOLOGY SECT. and at the same time rotated backwards, thus bringing the articulation of the hind-limbs as far back as possible. In the hind-limb the tibia and fibula are fused to form a single tibio-fibula (Fig. 922, TI. FI), and the two bones in the proximal row of the tarsus — the tibiale or astragalus (AST) and the fibulare or calcaneum (CAL) — are greatly elongated and provide the leg with an additional segment. There are three tarsals in the distal row, one of which appears to represent the centrale, another the/ first distale, and the third the fused second and third distalia. There are five well-developed digits, and on the tibial side of the first is a spur-like structure or calcar (c), formed of three bones, a meta- tarsal and two phalanges : such an additional digit is called a pre-hallux. All the long bones of the limbs consist of the shaft formed of true bone and of extremities of calcified cartilage. The distinction is a very obvious one, both in the freshly-prepared and in the dried skeleton. The muscular system has undergone great modifications in correspondence with the complex movements performed by the limbs. The dorsal muscles of the trunk are no longer divisible into myomeres, but take the form of longitudinal or oblique bands (extensores dorsi, &c.), lying partly above the vertebrae, partly between the tranvserse processes, partly between the ilia and the urostyle. The ventral muscles are differentiated into a paired median band, the rectus abdominis (Fig. 928, ret. abd) with longi- tudinal fibres, and 'a double layer of oblique fibres — dbliquus externus (obi. ext) and internus (obi. int) — extending from the vertebral column to the recti. Both the extensor dorsi and the rectus abdominis are traversed at intervals by transverse bands of fibrous tissue, the inscriptions tendincw (ins. ten), but the segments thus formed do not correspond with the embryonic myomeres. The right and left recti are united by a longitudinal band of tendon> the linea alba (I. alb). The muscles of the limbs are numerous and complex, each seg- ment having its own set of muscles by which the various move- ments of which it is capable are performed. There are muscles passing from the trunk to the limb-girdles ; from the trunk or the limb-girdles to the humerus and femur; from the humerus and femur to the radio-ulna and tibio-fibula; from the fore-arm or shank to the digits; and from one segment of a digit to another. For the most part the limb-muscles are elongated and more or less spindle-shaped, presenting a muscular portion or belly \vhich passes at either end into a tendon of strong fibrous tissue serving to fix the muscle to the bones upon which it acts. The relatively fixed end of a muscle is called its origin, the relatively movable end its insertion, e.g. in the gastrocnemius muscle of the calf of the leg (ffstr) the proximal end attached to the femur is the origin, the distal PHYLUM CHORDATA 275 end attached to the foot the insertion. According to their action muscles are divided into flexors which bend, and extensors which . (.»:>s.— Rana esculenta. The muscles from the ventral aspect. On the left side (right of figure) many of the superficial muscles have been cut and reflected to show the deep layer. add. brev. adductor brevis ; add. long, adductor longus ; add. mag. adductor magnus ; del. del- toid ; ext. cr. extensor cruris ; ext. trs. extensor tarsi ; FE. femur; gn. hy. genio-hyoid ; r/str. gastrocnemius ; liy.gl. hyoglossus ; ins. ten. inscriptio tendinea ; I. alb. linea alba ; iiiy.hy. mylo-hyoid ; obi', int. obliquus interims ; obi. ext. obliquus externus ; o.st. omosternum ; p. c. hy. posterior cornu of hyoid ; pet. pectoralis ; pctn. pectineus ; per. peronaeus ;, ret. abd. rectus abdomiiiis ; rect. int. maj. rectus interims major ; rect. int. min. rectus intern us minor ; sar. sartorius ; sb. mt. sub-mentalis ; sem. ten. semi-tendinosus; tib. ant. tibialis anticus; tib. post, tibialis posticus ; XT. FI. tibiofibula ; vast. int. vastus interims ; x. st. xiphisterimm. 276 ZOOLOGY straighten, one part upon another; adductors which draw towards, and abductors which draw away from, the middle line ; elevators which raise, and depressor^ which lower, a part, such as the lower jaw. The names of the muscles may have reference to their position, e.g. pcctoralis (pet.), the principal muscle of the chest ; or to their form, e.g. biceps, the two-headed muscle ; or to their action, e.g. flexor tarsi; or to their origin and insertion, e.g. coraco- humeralis. Digestive Organs. — The mouth leads into a wide luccal cavity having in its roof the internal or posterior nares (Fig. 929, p. no,.), cp. ad. corpus adiposum ; crb. h. cerebral hemisphere ; d. ly. s. dorsal lymph sinus ; du. duo- denum ; ep.cor. epicoracoid ; ens. t. Eustachian tube ; , § '§ 1 5«aS4*;4w£i*«!i.2i I54|jii..|3«isla|||85! J c ^ • t3 " g 8 .it *s ii AS 2. . — t 3 &o^ a o > o fat^-s s ^ 3 3 ft"S -M « ^?:^i-^'^a" ^ K*P- cS 3 j.9'9 o as WJ«3<3,J Vfti we? • - - a -^ rfall'^i^5?^-*'-"^ , iiPli?:ii^i!liiisi I Wlj!iM»H"ilk ; o 9 S «J*fi;H 5 "XTi 6 •*" 9 "S. a IT »5 a .s-3 » il •a g >» > ° " ^ ^Ss^lS aside and makes its way into the bulbus aortse. Here again the course taken is that of least resistance : owing to the presence of the carotid labyrinth the passage of blood into the carotid trunks is less free than into the wide, elastic, systemic trunks. These will 284 ZOOLOGY SECT. therefore receive the next portion of blood, which, the venous blood having been mostly driven to the lungs, will be a mixture of venous and arterial. Finally, as the pressure rises in the systemic trunks, the last portion of blood from the ventricle, which, coming from the left side, is arterial, will pass into the carotids and so supply the head. The red blood-corpuscles are, like those of Fishes, oval, nucleated discs. The lymphatic system (Fig. 934) is very well developed, and is remarkable for the dilatation of many of its vessels into immense lymph-sinuses. Between the skin and muscle are large subcutaneous sinuses (Fig. 929, v. ly. s.), separated from one another by fibrous partitions, and the dorsal aorta is surrounded by a spacious subvertcbral sinus. The lymph is pumped into the veins by two pairs of lymph-hearts, one situated beneath the supra-scapulae, the other beside the posterior end of the urostyle. Nervous System. — The brain (Fig. 935) has a very small cerebellum, large optic lobes, a well-developed diencephalon, and large hemispheres and olfactory bulbs, the latter fused in the median plane. The corpora striata, or basal ganglia of the cerebral hemispheres, are connected together, as in all Vertebrates, by an anterior* commissure (com, below, lower line), above which is another commissure (com, below, upper line) partly^r£p*^senting the hippocampal commissure of the brain of Reptiles and Mammals. The metaccele is covered by a thick choroid plexus : the mesocoele is divisible into a median passage or iter (i.\ and paired optocceles (opt.v.) in the optic lobes : the paracosles are large cavities each communi- cating with a rhinoccele in the corresponding olfactory bulb. The pineal body is vestigial in the adult, a lobe of the anterior choroid plexus, with a vestige of the stalk (pin), taking the position which it usually occupies : in the larva it is found outside the skull and immediately beneath the skin. The first spinal nerve performs the function of the hypoglossal (Fig. 935), supplying the muscles of the tongue : it passes out between the first and second vertebrae. The spinal cord is short and ends in a delicate filament, the filum terminals. In correspondence with the number of vertebras there are only ten pairs of spinal nerves, of which the second and third unite to form a brachial plexus giving off the nerves to the fore-limb, while the seventh to the tenth join to form a lumbo-sacral plexus giving off the nerves to the hind-limb. Sensory Organs. — The olfactory sacs have each two openings : the anterior naris or external nostril and the posterior naris (Fig. 929, p. na.) or internal nostril, which opens into the mouth immediately external to the vomer. The eye and the auditory organ have the usual structure, but in connection with the latter there is an important accessory organ XIII PHYLUM CHORDATA 285 of hearing not hitherto met with. Bounded externally by the tym- panic membrane and internally by the outer wall of the auditory C/fl /bin Clfl Meet, cbl FIG. 985. — Brain of Rana. A, from above"; B, from below ; C, from the side ; D, in longitudina 1 vertical section. C'li, cerebellum ; Cer. H, cerebral hemispheres ; ch. pLv1, anterioV and fh. /. hy. body of hyoid ; buc. cav. cavity of pharynx; ch. plx. choroid plexus; col. columella; •eus.t. Eustachian tube; feii. ov. fenestra ovalis; med. obi. medulla oblongata ; tnemb. lab. membranous labyrinth ; mnd. mandible ; Nv. VIII. auditory nerve ; o. st. ornosternum ; ptg. pterygoid ; qu. ju. quadrato-jugal ; stp. stapes ; tymp. cav. tympanic cavity ; tymp. m. tympanic membrane. with the pharynx obviates undue compression of the air in the tympanic cavity. There seems little doubt that the tympano- Eustachian passage is homologous with the first or hyomandibular gill-cleft, although, in the Frog, it is formed independently of the clefts and never opens on the exterior. Urinogenital Organs.— The kidneys (Figs. 937 and 938, N.) are flat, somewhat oval bodies, of a dark red colour, lying in the posterior region of the ccelome. On the ventral face of each is an elongated, yellow adrenal, and irregularly scattered mphrostomcs occur in considerable numbers on the same surface ; .these do not, however, communicate with the urinary tubules, but with the renal veins, and serve to propel the lymph from the coelome to the venous system. The ureters (Ur.) pass backwards XIII PHYLUM CHORD ATA 287 CvAo FK from the outer borders of the kidneys and open into the dorsal wall of the cloaca (CL). The kidney is developed from the mesonephros of the embryo, the ureter from the mesonephric duct. In the larva a large pronephros is present and is, for a time, the functional kidney. Opening into the cloaca on its ventral side is an organ (Fig. 929, U.) mentioned in the general account of the Craniata (p. 120), but here actually met with for the first time. It is a bilobed, thin-walled, and very delicate sac into which the urine passes by gravitation from the cloaca when the anus is closed. The sac is a urinary bladder, but, as it is quite different morphologically from the organ of the same name in Fishes, which is a dilatation of the ureter, it is dis- tinguished as the aUantoic Madder. The testes (HO) are white ovoid bodies lying immediately ventral to the anterior ends of the kid- neys, to which they are attached by folds of peritoneum. From the inner edge of each pass a number of delicate vas effercntia which enter the kidney and be- come connected with the urinary tubules. The spermatic fluid is thus passed into the urinary tubules and carried off by the ureter, which is therefore a urino- genital duct in the male Frog. A vesicula seminalis (Fig. 929, vs. sera.) opens by numerous small ducts into the outer side of the ureter. Attached to the testis are lobed bodies of a bright yellow colour, the fat-bodies (FK}. The waries (Fig. 938, 00.) are large folded sacs on the surface of which the black-and-white ova project. A fat-body is attached to each. The oviducts (Od.) are greatly convoluted tubes, the narrow anterior ends of which open into the ccelome by small apertures (0£.) placed close to the bases of the lungs. Their posterior ends are wide and thin-walled -(Ut.\ and open into the cloaca (P). The ova break loose from the surface of the ovary and enter the ccelomic apertures of the oviducts, the walls of which are glandular and secrete an albuminous fluid having the property of swelling up in water. The eggs receive a coating of this substance FIG. 937.— Rana esculenta. Urino- genital organs of the male. Ao. dorsal aorta ; Cl. cloaca ; Cv. postcaval vein ; FK, fat bodies ; HO, testes ; N, kidneys ; S, apertures of ureters into cloaca ; Ur. ureters. (From Wiedersheim's Com- parative Anatomy.) 288 ZOOLOGY SECT. XIII as they pass down the oviducts and are finally stored up in the thin-walled posterior portions of those tubes, which, in the breed- ing season, become immensely dilated and serve as uteri. Development. — The eggs are laid in water in large masses ; each has a black and a white hemisphere, the former always directed upwards, and is surrounded by a sphere of jelly. The egg is telolecithal, the protoplasm being mainly accumulated on the pigmented hemisphere, while the white hemi- sphere is loaded with yolk. During oviposition the male sheds his sper- matic fluid over the eggs, and the sperms make their way through the jelly and impregnate them. In a short time the jelly swells up and becomes thereafter impermeable to the sperms. Segmentation begins by a vertical furrow dividing the oosperm into two cells (Fig. 939, A), and soon followed by a second vertical furrow at right angles to the first (B), and then by an equatorial furrow placed nearer the black than the white pole (C). Thus the eight-celled embryo consists of four smaller black cells and four larger white cells. Further divisions take place (D), the black cells dividing rapidly into micromeres (mi.\ the white, more slowly, into megameres (nig} : as in previous cases, the presence of yolk hinders the process of segmentation. The pig- mented micromeres (D — F, mi.) give rise to the ectoderm, which is many-layered : the megameres (ing.} contribute to all three layers and are commonly called yolk-cells. During the process of seg- mentation [a Uastoccde (E, U. ccel} or segmentation-cavity appears in the upper hemisphere. FIG. 938.— Rana esculenta. Urinogenital organs of the female. N, kidneys ; Oil. oviduct ; Ot, its coeloinic aperture ; Or. left ovary (the right is removed) ; P, cloacal aperture of oviduct ; S, S', cloacal apertures • of ureters ; Ut, uterine dilatation of oviduct. (From |g Wiedersheim's Comparative Anatomy.) p-^^p .y/f FK;. fi:iO. — Development of the Frog. A— F, segmentation ; G, overgrowth of ectodei-m ; H, I, establishment of germinal layers; J, K, assumption of tadpole-form and establishment of nervous system, notochord, and enteric canal ; L, newly-hatched tadpole, bl.ccel. blastocoale ; Up. hip', blastoporc ; //r1. br-. gills ; br. d. branchial arches ; c. eye ; cct. ectoderm ; end. endoderm ; ent. enteron ; /. br. fore-brain ; h. br. hind-brain ; m. br. mid-brain ; md. /. medullary fold ; md. '. medullary groove ; mes. mesoderm ; mg. megameres ; mi. micromeres ; nr/i. notochord ; n. e. c. neurciiteric canal; pcdm. proctodseum ; pty. pituitary invagina- titdi ; ret. commencement of rectum ; sk. sucker ; sp. cd. spinal cord ; st.dm. stomodanun ; t. tail ; ?/£. yolk cells ; ?/A-. pi. yolk plug. (A— D, F— H, and J from Ziegler's models : E, I, K, and L after Marshall.) 290 ZOOLOGY SECT The black now begins to encroach on the white hemisphere ; cells, budded from the yolk-cells, take on the character of ectoderm, acquire pigment, and gradually extend the black area until it covers the whole embryo except a small patch, known as the yolk- plug (G, H, yk. pL), at what will become the posterior end. This process is obviously one of epiboly : the margin of ectoderm cells surrounding the yolk-plug represents the blastopore. The archenteron (I, ent.~) arises by a split taking place among the yolk-cells, beginning at the edges of the blastopore and gradually extending forwards : the process is probably supplemented by a limited amount of invagination of the ectoderm. The archenteron is at first a very narrow cleft, but soon widens considerably: for a long time it does not actually communicate with the exterior, the blastopore being filled up with the yolk-plug. As the archen- teron extends forwards the blastoccele gradually disappears. The yolk-cells soon become differentiated into a layer of endoderm cells (I, end.) immediately surrounding the archenteron, and several layers of mesoderm cells (mes.). Ventrally, however, a large mass of yolk-cells (K, yk.) remains undifferentiated and serves as nutri- ment to the growing embryo. The edges of the lower margin of the blastopore now begin to approach one another, and uniting in the median plane, give rise to a vertical groove, the primitive groove. In the meantime medullary folds (H, tnd.f.) appear and mark the dorsal surface : they are at first widely separated, but gradually approach one another and close over the medullary groove (md. gr.), thus giving rise to the central nervous system. Posteriorly they become continuous with the lips of the blastopore, so that when the neural groove becomes closed in behind, the archenteron, as in Amphioxus, communicates with the neuroccele by a neurenteric canal (K, n. e. c.). The embryo soon begins to elongate ; one end is broad, and, becoming separated by a slight constriction, is marked out as the head: the other end is bluntly pointed and is the rudiment of the tail (£.). On the ventral surface of the root of the tail &procto- dceum (pcdm.) appears and communicates with the archenteron. The head and tail become more distinctly marked off from the trunk. A pit — the stomodmwi (J — L, st. dm.) — appears on the antero -ventral surface of the head, and, immediately behind it, a semilunar area with raised edges, the sucker (sk.). At each side of the head two branched processes appear : they are the external gills (br1., br2.), and the regions from which they arise mark the positions of the first and second branchial arches. The embryos are now hatched as tadpoles. They swim freely in the water or adhere to weeds by means of their suckers (Fig. 940, 1). They are still blind and mouthless, the stomodaeum not having yet communicated with the archenteron. Soon a third pair of external gills appears on the third branchial arch, and the PHYLUM CHORDATA 291 first two pairs increase greatly in size (2, ,2a) : the stomodseum joins the archenteron, gill-slits (branchial clefts) are formed between the branchial arches, and the eyes appear. The mouth is small, bounded by lips beset with horny papillae and provided with a pair of horny jaws. The enteric canal grows to a great length and is coiled like a watch-spring, and the tadpole browses upon the water- weeds which form its staple food. Soon the external gills show signs of shrivelling, and at the same time internal gills, like those of Fishes, are developed in the 40.— Rana temporaria. Stages in the life-history, from the newly-hatched Tadpoles (1) to the young Frog (8). •>* is a magnified view of 2. (From Mivart.) branchial clefts. A fold of skin, the operculum, appears on each side, in front of the gills, growing from the region of the hyoid arch, and extending backwards until the gill-slits and external gills are covered and there is only a single small external branchial aperture on each side, as in Holocephali (3, If). On the right side the operculum soon unites with the body- wall so as to close the branchial aperture, but on the left side the opening remains for a considerable time as the sole exit of the water. At this time the tadpole is to all intents and purposes a Fish. 292 ZOOLOGY SECT, The Jungs now appear', and the larva is for a time truly amphibious, rising periodically to the surface to breathe air : the single branchial aperture, however, soon closes, and henceforth respiration is purely aerial. In the meantime the limbs are developed. The hind-limbs appear as little rounded buds, one on each side of the root of the tail (5). The fore-limbs arise beneath the operculum and are therefore hidden at first ; soon, however, they emerge by forcing their way through the operculum. As the limbs increase in size the tail undergoes a progressive shrinking (6-8} . The mouth widens by the backward rotation of the suspensorium, the in- testine undergoes a relative diminution in length, and vegetable is exchanged for animal diet. The little, tailed Frog can now leave the water and hop about upon land ; its tail is soon completely absorbed, and the metamorphosis is complete. 2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. The Amphibia are Craniata which, in nearly all cases, possess gills either in the larval state only or throughout life, and which usually breathe by lungs in the adult condition. The skin is glandular, and there may or may not be a bony dermal exoskeleton. When unpaired fins are present, they are never supported by fin- rays. The paired appendages, when present, are pentadactyle limbs : the digits are usually devoid of claws. The skull is autostylic and is articulated with the first vertebra by paired occipital condyles borne on the exoccipitals. The basioccipital arid supraoccipital are usually, and the basisphenoid is always, absent: there is a large parasphenoid and there are " well-developed paraquadrates (squamosals). In the branchiate forms large hyoid and branchial arches persist throughout life : in the non-branchiate species these structures undergo more or less degeneration and give rise to the hyoid-cartilage. The heart has a sinus venosus, right t and left auricles, a single ventricle, and aconus arteriosus; the aortic arches arise from a bulbus aortas or abbreviated ventral aorta. The cardinal veins undergo more or less degeneration and are practically replaced by an unpaired postcaval vein. There is a renal portal system, part of the returning blood from the posterior parts of the body going through it, the rest through the hepatic portal system by an abdominal vein which represents fused lateral veins. The red corpuscles are oval and nucleated and are often of unusual size. The lymphatic system is well developed. In the brain the small 'size of the cerebellum is noticeable. The olfactory sacs open into the mouth by posterior nares. The outer wall of the auditory capsule is pierced by a fenestra ovalis into which is inserted a cartilaginous stapes : the stapes may be connected by a columella xm PHYLUM CHORDATA 293 with a tympanic membrane. The efferent ducts of the testis open into the urinary tubules, and the mesonephric duct of the male is a urinogenital duct. In the female the mesonephric ducts become the ureters, and the oviducts are pronephric ducts with ccelomic apertures. The pronephros is the functional kidney in the larva, the mesonephros in the adult. There is an allantoic bladder. Development is usually accompanied by a metamorphosis, the young being hatched in the form of a branchiate larva. The Amphibia are classified as follows: — ORDER 1. URODELA. Amphibia which retain the tail throughout life. There are usually two pairs of limbs of approximately equal size. The order is conveniently divided into — a. Perennibranckiata, which retain the gills throughout life : including the American Necturus, the blind Proteus of the under- ground caves of Carniola in Dalmatia, and the Eel-like Siren of North America. b. Derotremata, in which the gills are lost in the adult, but there is usually a persistent gill-cleft : including the Newt-like Cryptobranchus and the Eel-like AmpMuma from North America,, and the Giant Salamander, Megalobatrachiis, of China and Japan. c. Myctodem, the Salamanders and Newts, in which the gills | are lost ancT the gill-clefts closed in the adult : including the common Newts or Efts (Molge), the Spotted and Black Sala- manders (Salamandra) of the European Continent, and the .American AmUystoma, the sexually mature larva of which is the well-known Axolotl. ORDER 2. — ANURA. Amphibia having no tail in the adult condition. The trunk is short and broad, and the hind-limbs greatly exceed the fore-limbs in size. Gills and gill-slits are never present in the adult. Including the Frogs and Toads. ORDER 3. GYMNOPHIONA. Snake-like Amphibia having neither limbs nor tail. A dermal exoskeleton is present. There are no gills or gill-slits in the adult. Including the Csecilians (Ccecttia, Epicrium, &c.). ORDER 4. STEGOCEPHALA. Extinct tailed Amphibia, often of great size, having usually two pairs of limbs and a well-developed dermal exoskeleton. The group ranges from the Permian to the Trias. VOL. II T 294 ZOOLOGY SECT. Systematic Position of the Example. The genus Rana belongs to the family Ranidce, which with three other families constitutes the series Firmisternia, of the sub-order Phaneroglossa and order Anura. The absence of a tail and the presence of two pairs of limbs, of which the posterior are larger than the anterior, place the genus among the Anura. The presence of a tongue and of distinct paired Eustachian tubes separates the Phaneroglossa from the Aglossa (Pipa and Xenopus), a small group of Toads in which the tongue is absent and the Eustachian tubes have a common median opening. The Firmisternia are distinguished by having the coracoids joined by a common epicoracoid cartilage in contra- distinction to the Arcifera (Tree-frogs, Toads, &c.), in which the epicoracoids overlap one another. The Ranidaa are distinguished from the other families of Firmisternia by having teeth in the upper jaw and the transverse processes of the sacral vertebrae not dilated. R. temporaria is distinguished from R. esculenta by its smaller size and brown colour, by the large black patch in the tympanic region, and by the absence of external vocal sacs in the male. 3. GENERAL ORGANISATION. The Amphibia are specially interesting as illustrating the transition from the water-breathing to the air-breathing type of Craniate structure. The lower forms retain their gills throughout life, but possess lungs in addition : in the higher the gills occur only in the larval state, and the adult breathes exclusively by the lungs and skin, becoming transformed from an aquatic into a terrestrial animal. At the same time further adaptations to land- life take place, the most important being the modification of the blood-vessels consequent'on the disappearance of the gills, the loss of median fins, and the strengthening of the limbs to support the weight of the body. External Characters. — An excellent example of the lower Urodela with persistent gills is afforded by the great North Ameri- can Water-newt, Necturus maculatus (Fig. 941). The animal attains a length of 30 cm. (more than a foot) ; the elongated trunk is separated by a slight constriction from the depressed head, and passes insensibly into the compressed tail, which is bordered by a continuous median fin unsupported by fin-rays. The limbs are small and weak in proportion to the size of the body, and in the ordinary swimming attitude are directed backwards, more or less parallel to the sagittal plane, the upper arm and thigh taking a direction backwards and slightly upwards, the fore-arm and hand and the shank and foot extending backwards and downwards. Each limb thus presents an external or dorsal arid XIII PHYLUM CHORDATA 295 an internal or ventral surface, an anterior or pre-axial border which terminates in the first digit and a posterior or post-axial border which terminates in the last digit. The eyes are small a s * ? 1 I and have no eyelids, there is no tympanic membrane, and the mouth is wide and bordered by thick lips. On each side of the neck are two gill-slits (br. cl. 1, br. d. 2) leading into the pharynx, the first between the first and second branchial arches, the other T 2 290 ZOOLOGY SECT. between the second and third. From the dorsal end of each of the three branchial arches springs a branched external gill (ln\ 1 — br. 3). Very similar in its external characters is the blind, cave-dwelling Proteus', and Siren (Fig. 942) differs mainly in its elongated eel- like body and in the absence of hind-limbs. All three genera are percnnilraneJiiate or persistent-gilled. The remaining IJrodela are often called caducibmnchiatc or deciduous-gilled, and furnish a complete series of transitions from dcrotr evictions forms which, while losing the gills, retain the gill-clefts, to xnlnmnndrinc forms in which all trace of branchiate FIG. 044.— Salamandra maculosa. (After Cuvier.) organisation disappears in the adult. In Amphhuna (Fig. 943) the body is eel-like and the limbs are ex- tremely small : there are no gills in the adult, but two pairs of gill-openings are retained throughout life. In Crypt olyranelms there is a single branchial aperture, sometimes present on the left side only ; but, as in the previously mentioned genera, four branchial arches are retained. In Megalobatrachus, the Giant Sala- mander of Japan and China, all trace of gill-slits disappears, but two branchial arches persist. Lastly, in the Salamanders, such as the spotted Salamander (Salamandra maculosa, Fig. 944) of Europe, and the common British Newts (Molge), the adult has 110 trace either of gills or gill-slits, and the branchial arches are much reduced. The limbs, also, in the terrestrial Salamanders, stand out from the trunk, and have the soles of the feet and hands applied to the ground with the toes directed forwards, so as to support the weight of the body. More- over, all trace of the median fin disappears, the tail becoming nearly cylindrical. In the Anura the body is always Frog-like, the head being XIII PHYLUM CHORDATA 297 large and depressed, with a very wide mouth and large tympanic membranes, the trunk short, the tail absent, and the hind- much larger than the fore-limbs. In the Toads, such as the common British Bufo vulgaris, and most Tree-frogs, the webs between the hind-toes are reduced or absent, and in many species of Hyla the toes end in rounded sucking-discs. In the Gymnophiona (Fig. 945) the body is greatly elongated and snake-like, the head is small and not depressed, and the limbs are absent. There is no tail, the anus (an.) being at the posterior end of the body on the ventral surface. The Stegocephala, or Labyrintho- donts as they are frequently called, were mostly salamander-like, having long tails and well-developed limbs : some, however, were snake-like and limbless, and probably retained their external gills B C oecilia pachynema. A, anterior extremity from the right side ; B, posterior ; vmity from beneath, an. anus. (After Boulenger.) throughout life. They varied in length from 10 centimetres to .Several metr TKe'skin of Amphibia is soft and usually slimy owing to the vtion of the cutaneous glands, which is sometimes poisonous. -orne forms, such as Bufo and Salamandra, there are large swell- - on the sides of the head, formed of aggregated glands and called parotoids. In the larvae of both Urodela and Anura, and in the adult aquatic Urodeles lateral sense-organs are present, and impressions on the cranial bones show these organs to have been well developed in the Stegocephala. The colour of the skin is often y brilliant : the Spotted Salamander is yellow and black, and many Frogs are green and gold, scarlet and black, and so on. The green colour of Tree-frogs is protective, serving to conceal them among the foliage of the plants on which they live. The brilliant and strongly contrasted hues of the spotted Salamander and of some frogs are instances of " warning colours " ; the animals are inedible owing to the acrid secretion of their cutaneous glands, and their conspicuous colours serve to warn off the Birds and other 298 ZOOLOGY SECT. animals which would otherwise devour them. A red and blue Nicaraguan Frog is said to show no sign of fear of the Frog-eating Birds, while the edible and more plainly coloured species are in constant danger. In many Tree-frogs the brightness of the coloration varies with changes in the intensity of the light. In many Toads the skin is dry and covered with warts. An exoskeleton is present in many Gymnophiona in the form of small dermal scales, and in some Anura in the form of bony plates beneath the skin of the back. In the Stegocephala a very complete armour of bony scutes was present, sometimes covering the whole body, sometimes confined to the ventral surface. In a Urodele, Onychodactylus, and in the South African Toad, Xenopus, small pointed horny claws are present on the digits. With these exceptions the skin is devoid of hard parts. Endoskeleton. — The vertebral column is usually divisible into a cervical region, containing a single vertebra devoid of transverse processes ; an abdominal or thoraco-himbar region, containing a variable number of vertebrae with transverse processes and often with ribs ; a sacral region, containing a single vertebra, the large transverse processes — or the ribs — of which give attachment to the ilia ; and a caudal region, forming the skeleton of the tail. In the Gymnophiona the caudal region is very short, and there is no sacrum : in the Anura the caudal region is represented by a single rod-shaped bone, the urostyle. The total number of vertebras may reach 250 in Urodela and Gymnophiona: in Anura there are only nine vertebrae and a urostyle. In the lower Urodela (Fig. 946, A and B) the centra are bi- concave as in Fishes : they consist of dice-box-shaped shells of bone, lined at either end by cartilage (Jvk\ which is continuous between adjacent vertebra?. The bony shell is developed before the cartilage appears, so that the vertebrae are, in strictness, investing bones. The neural arches, on the other hand, are far more perfectly developed than in any Fish, and have well-formed zygapophyses, which articulate with one another by synovial joints. The Gymnophiona also have biconcave vertebras, but in the higher Urodela (Fig. 946, C and D) and the Anura absorption of cartilage takes place between adjacent centra in such a way that the convex end of one fits into the concave end of the next, forming a cup-and-ball joint. In the higher Urodela the convexity is on the anterior, the concavity on the posterior face of each centrum (Z>), and the vertebras are said to be ophisthocalous : in the Anura they are usually, as in the Frog, proccelous. In the Stegocephala there is great diversity in the structure of the vertebral column. There may be well - developed dice-box-shaped centra, or the neural arches may be simply perched upon a persistent notochord surrounded by incomplete XIII PHYLUM CHORDATA 299 hoops of bone, twice as numerous as the arches, and alternately dorsal and ventral in position. The former represent centra, the latter inter centra, or ossifications alternating with the centra on the ventral region of the notochord. The first or cervical vertebra bears paired articular surfaces for the condyles of the skull, and between them the anterior face of -Li3t Fio. 946.— Longitudinal "sections of vertebral centra of A, Ranidens ; B. Amblystoma ; C, Spelerpes ; and D, Salamandrina. Ch. notochord ; C'tf'.intra- -vertebral cartilage and fat-cells ; Gk, convex anterior face of centrum ; Gp, concave posterior face ; Jvk. inter- vertebral cartilage ; K, superficial bone of centrum ; Ligt. inter-vertebral ligament ; Mh, marrow-cavity ; X, transverse process ; S, intra-vertebral constriction. (From Wiedersheim's Comparative Anatomy.) the centrum gives off, in Urodela, a projection called the odontoid process. The Urodela, moreover, have ribs articulating with the transverse processes of the abdominal and sacral vertebrae : they are short bones, forked proximally, and the compressed transverse processes are correspondingly divided. The sacral ribs of Urodeles give attachment to the ilia, and the caudal vertebra? bear ha3mal arches. The skull of Urodela differs from that of the Frog in many 300 ZOOLOGY SECT. XIII u important respects, the most striking of which is the fact that the trabecula3 do not meet either below the brain to form a basis cranii or above it to form a cranial roof. Thus, when the investing bones are removed, the cranium (Fig. 947) is completed above and below in the parachordal or occipital region only : anterior to this it has side walls, but no roof or floor, there being above a huge superior cranial fontanelle, and below an equally large basi-cranial fontanelle, the former covered, in the entire skull, by the parietals and frontals, the latter by the parasphenoid. In the perennibranchiate forms Necturus and Proteus the trabecula3 remain, even in the adult, as narrow cartilaginous bars, and the chondrocranium is actu- ally of a lower or more em- bryonic type than that of any other Craniata, with the possible exception of Cyclo- stomata. In the Urodela, moreover, the parietals (Fig. 948, P) and frontals (F) are separate, the parasphenoid (Ps) is not T-shaped, the palatine and vomer are sometimes repre- sented by a single bone (ft.), and the palatine, when dis- tinct, bears teeth. The sus- pensorium is inclined for- wards, as in the tadpole, not backwards, as in the adult Frog. The hyoid arch is large, and its dorsal end may bo separated as a hyomandi- bular. There are three or four branchial arches which are large in the perenni- branchiate forms, but undergo more or less reduction in caducibranch species, never, however, forming such a simple structure as that seen in the Frog. The stapes has no columella attached to it, and, in correspondence with this, there is no tympanic cavity or membrane. In the Anura there is a very wide range of variation in the skull. Among the most important points are the presence, in a few species, of small supra- and basi-occipitals, and the fact that in others the roofing investing bones are curiously sculptured and so strongly developed as to give the skull a singularly robust appearance. In the Gymnophiona (Fig. 949) very little of the original car- tilage remains in the adult state, but the investing bones are PR.OT EX.OC TIC ft. FIG. 947.— Proteus anguinus. The chondro- cranium from above. ant. antorbital process ; EX.OC. exoccipital and epiotic ; hy.md. hyomandibular ; i.n. inter-nasal plate ; nch. riotochord ; ot. pr. otic process ; peNK, olfactory capsule ; OB, auditory capsule; OS, spheneth- moid (orbitosphenoid); Osp. supraoccipital region ; P, parietal ; Pa. ascending process of suspen- sorium ; ped. pedicle ; Pf. prefrontal ; Pmx. premaxilla ; Pot. otfc process of suspensorium ; Pp. palatine process of maxilla ; Ps. parasphenoid ; Pt. pterygoid bone^ ; Itc. pterygoid cartilage ; Rt, foramen for ophthalmic branch of trigeminal ; Qu. quadrate ; Squ. para- quadrate (squamosal) ; St. stapes ; Vo. vomer ; Vop. vomero-palatine ; Zt process of inter- nasal plate ; II, optic foramen ; V, trigeminal foramen ; VII, facial foramen. (From Wiedersheim's Comparative Anatomy.) -Prf Fir;. L— -A, right side of shoulder-girdle of Salamandra ; B, shoulder-girdle and sternum of Amblystoma (Axolotl) from the ventral aspect, a, b, processes of scapula ; C (in B), c-oracoid ; (.'/. procoracoid ; Co. (in A), coracoid ; G. (in A), glenoid cavity; L, its cartila- ginous edge ; Pf (in B), glenoid cavity ; S. scapula ; SS. supra-scapula ; st. sternum ; *, t, nerve foramina. (From Wiedersheirn's Comparative Anatomy.) (p. 250) is noteworthy. In Anura the pelvic girdle resembles that of the Frog. Attached to the anterior border of the pubic region there occurs in many Urodela and in Xenopus a rod of cartilage, forked in front, 304 ZOOLOGY SECT. FIG. 952.— Pelvic girdle of Salamandra. a, b, processes of epipubis ; Ep. epipubis ; Fo. ob- turator foramen ; G. acetabuluru ; U. ilium ; /x. ischium ; P. pubis ; Sy. pubo-ischiatic syrn- physis ; *, processes of pubis present in some Urodeles. (From Wiedersheim.) the epipubis (Ep). It is developed independently of the pelvis, and its relations to that structure are very similar to those of the sternum to the shoulder-girdle; it has, in fact, been proposed to call it a pelvi-sternum. The limbs of Urodela differ from the typical structure already described only in de- tails : there are usually four digits in the fore-limb and five in the hind-limb. In Anura the limbs are modified by the fusion of the radius and ulna and of the tibia and fibula, arid by the great elongation of the two proximal tarsals. A prehallux is fre- quently present. Myology. — In the lowei Urodela the muscles of th< trunk and tail occur in th< form of typical myomeres like those of Fishes. In the higher forms the myomeres become converted into longitudinal dorsal bands — the extensors of the back, paired ventral bands — th< recti abdominis, and a double layer of oblique muscles, covering tin flanks. Digestive Organs. — The teeth are always small and ankylosed to the bones : they may be singly or doubly pointed. They occur most commonly on the premaxilla?, maxillae, and vomers, but may also be developed on the dentaries, palatines, and, in one instance, on the parasphenoid. In many Anura, such as the Common Toad, teeth are altogether absent. In some of the Stegocephala, such as Mastodonsaurus, the teeth are extraordinarily complex in structure, the tissues being folded in such a way as to produce in section a complex tree-like pattern. It is from this circumstance that the term Labyrinthodont, often applied to the Stegocephala, is derived. The enteric caned is divisible into buccal cavity, pharynx, gullet stomach, small intestine, rectum, and cloaca. ' The stomach am duodenum together form a U-shaped loop in which the pancreas lies. The tongue in many Urodeles is fixed and immovable, like that of a Fish : in most Anura it is free behind, as in the Frog but in Xenopus arid Pipa (hence called Aglossa) it is absent. Respiratory Organs. — With very few exceptions Amphibia possess external gills in the larval state, and, in the perenni branchiate Urodela, these organs are retained throughout life xiii PHYLUM CHORDATA 305 They are branched structures, abundantly supplied with blood, and springing from the dorsal ends of the first three branchial arches. The epithelium covering them is ectodermal, so that they are cutaneous and not pharyngeal gills, and are of a totally different nature from the so-called external gills of the embryos of Elasmo- branchii and Holocephali, which are only the filaments of the internal gills prolonged through the branchial apertures. Internal gills are developed only in the larva? of Anura. They appear as papilla? on the outer borders of the branchial arches below the external gills. They closely resemble the internal gills of Fishes and appear to be homologous with them, although it seems probable that their epithelium is ectodermal. In most adult Amphibia lungs are formed as outgrowths of the ventral wall of the pharynx. The right and left lungs com- municate with a common laryngo-tracheal chamber, supported by the cartilages of the larynx and opening into the mouth by a longitudinal slit, the glottis. In the more elongated forms, such as Siren, Amphiuma, and the Gymnophiona, the laryngo-tracheal chamber is prolonged into a distinct trachea or wind-pipe, sup- ported by cartilages. In many species of Salamanders the lungs are absent, and respiration is exclusively cutaneous and pharyngeal. Circulatory Organs. — The heart always consists of a sinus venosus, right and left auricles, ventricle, and conus arteriosus. The sinus venosus opens into the right auricle, the pulmonary veins enter the left, and the two are separated by a septum auricularum which forms a complete partition in Anura, but in Urodela and Gymnophiona is more or less fenestrated, i.e. formed of a network of muscular strands with intervening spaces. The conus arteriosus has no longitudinal valve in the lower Urodela and the Gymnophiona, but is separated both from the ventricle and from the bulbus aortse by transverse rows of valves. In the perennibranchiate Urodela and in the larva? of the air- breathing forms the circulation is essentially like that of a Fish. The bulbus aortse (Fig. 953, A, I. 00.), which represents an abbre- viated ventral aorta, gives off four afferent branchial arteries (of. br. a. 1 — 4), three to the external gills, and a fourth which curves round the gullet and joins the dorsal aorta directly. From each gill an efferent branchial artery brings back the purified blood, and the efferent arteries unite, in a somewhat irregular way, to form the dorsal aorta (d. ao.~). Each afferent with the corresponding efferent artery constitutes an aortic arch. Short connecting branches unite the afferent and efferent arteries of each gill, carotids (ext. car., int. car.) arise from the first efferent artery, and, when the lungs appear, a pulmonary artery (pul. a.) is given off from the dorsal portion of the fourth aortic arch of each side. In those Urodela which in the adult condition are devoid of gills, when the latter atrophy (B) the first aortic 306 ZOOLOGY SECT. arch loses its connection with the dorsal aorta, and becomes tlie carotid trunk ; the second increases in size, forming the main factor of the dorsal aorta, and becomes the systemic trunk ; the third undergoes great reduction, and the fourth becomes the pulmonary artery, its dorsal portion retaining its connection with the systemic trunk in the form of a small connecting branch, the ductus Botalli (d. lot.'). In the Anura, as we have seen (p. 280), the third arch vanishes completely and there is no ductus Botalli. As to the venous system, the Urodela exhibit very clearly the transition from the Fish-type to the condition already described in the Frog. The blood from the tail is brought back by a caudal &*" FIG. 053.— Heart and chief arteries of Salamandra. A. larva ; B, adult, a/, br. a. 1—4, afferent branchial arteries ; b. ao. bulbus aortse ; car. gl. carotid labyrinth ; c. art. conus arteriosus ; d. ao. dorsal aoi'ta ; d. hot. ductus Botalli; ex.br. 1 — 3, external gills ; <.'/. <•«./'. external carotid ; int. car. internal carotid ; 1. an. left auricle ; Ing. lung ; pi. plexus, giving rise to carotid labyrinth ; ynil. a. pulmonary artery ; r. aw. right auricle ; v. ventricle. (Altered from Boas.) vein (Fig. 954, Gaud. V.) which, on reaching the coelome, divides into two renal portal veins, one going to each kidney. From the kidney the blood is taken, in the larva, into paired cardinal veins, each of which joins with the corresponding /w#w/«r to form aprecaval vein. In the adult the anterior portions of the cardinals undergo partial atrophy, becoming reduced to two small azygos veins (Card, post.} which receive the blood from the region of the back : their posterior portions unite and are continued forwards by a new unpaired vein, the poslcaval (V. cava inf.), which, joined by the hepatic veins, pours its blood into the sinus venosus. The iliac vein from the hind-leg divides into two branches : one joins the renal portal, the other, representing the lateral vein of Elasmobranchs, unites with its fellow in the middle ventral line to form the abdominal vein (Aid. V.) and joins the hepatic portal, its blood, after XIII PHYLUM CHORDATA 307 Cut™. MerJPft.Kr. w.t: Mia FK;. 0"t4.— Salamandra maculosa. Venous system, diagrammatic, from the ventral aspect. Abd. V. abdominal vein ; Card. ant. (Jv.y.), jugular vein ; Card. post. (Az.), azygos vein ; Cutid. V. caudal vein ; Cut. m, left musculo-cutaneous vein; Cut. m, the same on the right side (partly removed) ; D, intestine ; Duct. Cuv. precaval vein ; H. heart ; Jug. ext. external jugular ; Jug. int. internal jugular ; lg. V. mesenteric vein ; L. pft. hepatic portal system ; L. V. hepatic vein ; N, kidney ; Nier. Pft. Kr. renal portal system ; Sin. ven. sinus venosus ; Si'.M. subclavian vein ; V. adv. branches of renal portal vein ; V. Cava inf. postcaval ; V. iliaca, iliac vein; V. n-n. renal veins ; *, cloacal veins; t, branch of iliac to renal portal vein ; f t, lateral vein. (From Wiedersheim's Comparative Anatomy.) 308 ZOOLOGY SECT. traversing the capillaries of the liver, being returned by the hepatic vein into the post-caval. The red corpuscles are oval and nucleated, and are remarkable for their unusual size. Those of Amphiuma are the largest known, being about JT mm. in diameter, or eight times that of a human red corpuscle. Nervous System and Sense-Organs.— The brain of Urodela differs from that of the Frog in its more elongated and slender form, in the comparatively small size of the optic lobes, and in the non-union of the olfactory lobes. The olfactory sacs always open into the mouth by posterior nares situated behind or external to the vomers. The eye has no lids in the lower forms and is de- generate in the cave-dwelling Proteus and in some Gymnophiona. The Urodela, the Gymnophiona, and some Anura have no tympanic cavity or membrane, and no columella; there is, however, a stapes, (Figs. 948, 949) in the form of a nodule of cartilage inserted in the fenestra ovalis. In the perennibranchiate Urodeles and in the larvae of the air-breathing forms lateral-line sense-organs are present. There was an extensive lateral line system, leaving its impress on the bones of the skull, in the Stegocephala. Urine-genital Organs. — In the Urodela the kidneys (Fig. 955, N) are much elongated and are divided into two portions, a broad posterior part, the functional kidney (G-N), and a narrow anterior sexual part connected in the male with the efferent ducts of the testis. Numerous ducts leave the kidney and open into the Wolffian (mesonephric) duct [Ig. ( Z7r.)], which thus acts as a ureter in the female, as a urinogenital duct in the male. The oviduct \mg, (Od.)] is developed from the Miillerian duct, a rudiment of which (mg.t mgf.) occurs in the male. In the Gymnophiona the kidneys extend the whole length of the coalome, and in the young condition are formed of segmentally arranged portions, each with a nephrostome and a glomerulus, as in Myxinoids (see p. 141). A pronephros is present in the larva, but disappears in the adult. In some Gymnophiona the cloaca can be protruded and acts as a penis. Reproduction and Development. — External impregnation takes place in Anura, but in many Urodela the sperms are aggregated into spermatophores by glands in the wall of the cloaca, and these, being deposited on the body of the female, are taken into the cloaca and effect internal impregnation. Several curious instances of parental care are known. A number of different species of Frogs and Toads construct nests or shelters of leaves or other materials in which the eggs are deposited and in which the young are developed. In the Obstetric Toad (Alytes obstetricans) of Europe the male winds the strings of eggs — formed by the adhesion of their gelatinous investment — round his body and thighs, where they are retained until the PHYLUM CHORDATA 309 tadpoles are ready to be hatched. In Rhinoderma darwini^ a little South American Frog, they are transferred by the male to his immense vocal sacs, which extend over the whole ventral surface, and there hatched. In another Anuran, Nototrema (Fig. 956), there is a pouch on the back of the female in which Ov.~~ Fit;. (.C).">. — Diagrams of urhiogeuital organs of male (A) and female (B) Urodele. a, collecting tubes; GN, sexual portion of kidney; Ho, testis; Iff. (Ur.) Wolffian duct (ureter); mg, mg'. vestigial Miillerian duct of male; ing. (Od), oviduct; N, non-sexual portion of kidney ;_ Oc. ovary; Ve, vasa efferentia; t, longitudinal canal. (From Wiedersheim's Comparative Anatomy, after Spengel.) . ^ the eggs are stored, .the young being hatched in some species as tadpoles, in others in the adult or Frog-form. In the Surinam Toad (Pipa americana, Fig. 957) the skin on the back of the female becomes soft and spongy during the breeding season : the eggs are placed on it by the male, and each sinks into a little pouch VOL. II U 310 ZOOLOGY SECT. of skin covered by a gelatinous film. The ernbryos; which have a large yolk-sac, develop in these pouches ; they never possess FIG. 956.— IfotOtrema marsupiatum. Female, with pouch opened. (From Mivart.) external gills, and are hatched in the adult form. In the case of several species the tadpoles are carried about by the female, adhering to her dorsal surface by suckers or by a viscid secretion. FIG. 957.— Pipa americana. Female. (From Mivart.) Another Anuran, Pseud-is paradoxa, is remarkable for the fact that the tadpole is many times larger than the adult. Some Salamanders (S. maculosa and S. cttra) and a species of Cascilia are viviparous. In the Black Salamander (S. atra\ though XIII PHYLUM CHORDATA 311 many eggs are developed, only two larvae survive, one in each oviduct, these being nourished in later stages by means of the remainder of the eggs. The larva in this species possesses long FK.. '.LOS.— Ichthyophis glutinosa, xl. 1, a nearly ripa embryo, with gills, tail-fin, and still with a considerable amount of yolk ; 2, female guarding her eggs, coiled up in a hole underground ; 3, a bunch of newly-laid eggs ; 4, a single egg, enlarged, schematised to show the twisted albuminous strings or chalazae within the outer membrane, which surrounds the white of the egg. (After P. and F. Sarasin.) plume-like external gills during its existence in the oviduct, shedding them before birth. If, however, the unborn young is removed from the oviduct and placed in water, it swims about like an ordinary aquatic larva, losing its long gills and developing a new and shorter set. Most Gymnophiona lay their eggs in burrows, but the larvaB in some cases lead an aquatic life for a time, and during this period possess, like tadpoles, a tail with a tail-fin which afterwards undergoes absorption. The larvae of most Gymnophiona have long external gills (Fig. 958). A very interesting case of pcedogenesis is furnished by the Axolotl (Amblystoma tigrinuiri). This animal frequently under- goes no metamorphosis, but breeds in the gilled or larval state FK;. '.to1.'.— Amblystoma tigrinum. Larval or Axolotl stage. (From Mivart.) (Fig. 959). But under certain circumstances the gills are lost, the gill-slits close, and a terrestrial salamandrine form is assumed. It is to the branchiate stage that the name Axolotl properly applies ; U '2 312 ZOOLOGY SECT. before the metamorphosis was discovered its connection with Amblystoma was not suspected, and it was placed in a distinct genus, Siredon, among the Perennibranchiata. Segmentation of the egg in the Anura and Urodela is always com- plete but unequal. In Pipa and Alytes there is a large quantity of food -yolk, and the developing embryo lies on the surface of a large yolk-sac. In the Gymnophiona the eggs, which are singularly like those of a Bird, are of large size and segmentation is partial, the formation of segments at the pole of the egg opposite that at which the formation of the embryo begins only taking place at the stage of gastrulation : the embryo is coiled over the surface of the yolk as in the Trout. Distribution. — The Urodela are almost exclusively Pala3arctic and Nearctic forms, occurring in North America, Europe, Asia, and North Africa : a few species extend south wards into the Neotropical and Oriental regions. The Gymnophiona, on the other baud, are mainly southern, occurring in the Neotropical, Ethiopian, and Oriental regions, but are absent in Australasia and the Pacific Islands. The Anura are almost universally distributed, and are abundant in all the greater zoo-geographical regions : they are, however, represented in New Zealand only by a single species (Liopelma Jiochstetteri). very locally distributed, and are absent in most Oceanic islands, a fact due to the fatal effects of salt water upon the eggs and embryos of Amphibia as well as upon the adults. Remains of Stegocephala are found in considerable abundance from the Carboniferous to the Trias, and one genus extends into the Lower Jurassic, after which period the order apparently became extinct. The Urodela and Anura are not known until the Eocene, and no fossil remains of Gymnophiona have been found. Mutual Relationships. — The perennibranchiate Urodela are undoubtedly the lowest of existing Amphibia ; they lead up, through such forms as Amphiuma, with persistent gill-slits but deciduous gills, to the Land Salamanders, in which a purely terrestrial form is assumed. The Stegocephala exhibit a parallel series of modifications, some of them being perennibranchiate, others caducibranchiate. Their skull is more complex than that of the Urodela, but their vertebral column never reaches the same degree of specialisation as that of the Land Salamanders, and in some cases shows a lower grade of organisation than in any existing Amphibia. Both in their skeleton and in the distribution of their lateral sense-organs they show some affinity with the Crossopterygii. The Anura are a very specialised group : their development indicates their derivation from branchiate tailed forms, but there is no palseonto- logical evidence on this point. XTTI PHYLUM CHORDATA 313 CLASS IV.— REPTILIA. Reptiles, Birds, and Mammals are associated together as having in common certain features in which they differ from lower Vertebrates. The most important of these is the occurrence in all three classes of certain embryonic membranes termed the amnion and the cdlantoist to be described subsequently. The term Amniota is, accordingly, frequently used for the group formed by these three highest classes of the Vertebrata. The classes Reptilia and Aves are much more closely allied with one another than either of trfem is with the Mammalia ; and the two first are sometimes associated together under the title of Sauropsida. The following are some of the most salient features of the Sauropsida when compared with the other Vertebrates : — The integument always gives rise to important and characteristic exoskeletal structures in the form of scales or feathers; the derm is may or may not take part in the formation of an exoskeleton. The skull is well ossified : it rarely in the adult state contains a distinct parasphenoid. There is a single occipital condyle borne on the basioccipital. The basisphenoid is a well-developed bone. The mandible articulates with the skull through the intermediation of a quadrate, and consists of five or six bones on each side. The ankle-joint is an articulation between the proximal and distal divisions of the tarsus. As in the Amphibia, there is a cloaca into which the rectum and the renal and reproductive ducts open. The heart consists of two auricles and a ventricle which is sometimes incompletely, sometimes completely, divided into two parts. Branchiae are never present at any stage. The mesonephri are never the functional renal organs of the adult, but are always replaced by metanephri. Both an amnion and an allantois are present in the embryo, the latter becoming highly vascular and acting as a temporary embryonic organ of respiration. The class Reptilia comprises four orders having living repre- sentatives, in addition to a number of extinct groups. In the Mesozoic period the class reached its maximum both in the number of its representatives and the size which many of them attained ; at that period they were very unmistakably the dominant class of the Animal Kingdom. In the Tertiary period they underwent a decline, while the Birds, and, in a yet higher degree, the Mammals, were gaining a preponderance over them. The living Reptiles are the Lizards and Chamaaleons, the Tuataras, the Snakes, Tortoises and Turtles, and the Crocodiles and Alligators. Though horny scales are not by any means present in all the Reptiles, their occurrence as a complete covering is characteristic of the group and almost peculiar to it. When scales are not ;U4 ZOOLOGY SECT. present, the epidermis is always hardened and cornified so as to form plates of horny material, such as the horny plates of the Tortoises, which protect the underlying parts from injury and desiccation. Bony plates are frequently present as well. Inmost respects the internal structure of the Reptilia shows a very decided advance on that of the Amphibia. The skull, as well as the pectoral and pelvic arches, are more completely ossified, and both vascular and nervous systems show a higher grade of organisation. 1. EXAMPLE OF THE CLASS.— A LIZARD (Lacerta). The most striking external differences between the Lizard (Fig. 960) and the Frog are the covering of scales, the comparative smallness of the head, and the presence of a distinct neck, the great FIG. 960.— Lacerta Viridis. (After Brehm.) length of the caudal region, the shortness of the limbs, and the approximate equality in length of the anterior and posterior pairs. The anterior limbs are situated just behind the neck, springing from the trunk towards the ventral surface. The fore-limb, like that of the Frog, is divided into three paits, the upper-arm or brachium, the fore-arm or anti-brachinm, and the hand or manus ; there* are five digits provided with horny claws, the first digit or pollex being the smallest. The hind-limbs arise from the posterior xiii PHYLUM CHORDATA 315 end of the trunk towards the ventral aspect; each, like that of the Frog, consists of three divisions — thigh or femur, shank or cms, and foot or pes. The pes, like the manus, terminates in five clawed digits, of which the first or hallux is the smallest. The head is somewhat pyramidal, slightly depressed : the openings of the external nares are situated above the anterior extremity. The mouth is a wide slit-like aperture running round the anterior border of the head. At the sides are the eyes, each provided with upper and lower opaque, movable eyelids and with a transparent third eyelid or nictitating membrane, which, when withdrawn, lies in the anterior angle of the orbit. Behind the eye is a circular brown patch of skin — the tympanic membrane — corresponding closely to that of the Frog, but somewhat sunk below the general level of the skin. The trunk is elongated, strongly convex dorsally, flatter at the sides and ventrally. At the root of the tail on the ventral surface, is a slit-like transverse aperture — the anus or cloacal aperture. The tail is cylindrical, thick in front, gradually tapering to a narrow posterior extremity ; it is nearly twice as long as the head and trunk together. There is an exoskeleton of horny scales covering all parts. These are formed from folds of the dermis each covered with a thick horny epidermal layer. Tn size they differ in different positions. On the dorsal surface of the trunk they are small, hexagonal, and indistinctly keeled. On the ventral surface they are larger and are arranged in eight longitudinal rows. Immediately in front of the cloacal aperture is a large pre-anal plate. A collar-like ridge of larger scales surrounds the throat. On the tail the scales are elongated, keeled, and arranged in regular transverse (annular) rows, giving the tail a ringed appearance. Oh the surface of the limbs the scales of the pre-axial (radial or tibial) side are larger than those of the post-axial (ulnar or fibular). The scales on the upper surface of the head (head-shields) are large, and have a regular and characteristic arrangement. Endoskeleton. — The vertebral column is of great length and made up of a large number of vertebrae. It is distinctly marked out into regions, a cervical of eight vertebras, a thoraco-lumbar of twenty-two, a sacral of two, and a caudal of a considerable, but indefinite number. A vertebra from the anterior thoracic region (Fig. 961, A, B) presents the following leading features. The centrum (cent.) is elongated and strongly proccdous, i.e. the anterior surface is concave, the posterior convex ; the neural arch bears a short neural spine (sp.). There are pre- and post-zygapophyses (pr. zy, pt. zy), the former with their articular surfaces directed upwards, the latter downwards. On each side at the junction of centrum and neural arch is a facet — the capitular facet — for the articulation of a rib. The cervical vertebrae in general are similar in essential respects to those of the trunk, but are somewhat shorter. M10 ZOOLOGY SECT. The first two. however, differ greatly from the others. The first is the atlas (C, D). It has no distinct centrum, but is in the form of a ring ; ventrally on its anterior face it bears a smooth articular facet for the occipital condyle of the skull. It consists of three distinct ossifications, one ventral, the others dorso-lateral : the latter do not quite meet dorsally, being separated by a space bridged over by mem- brane. The second or /-I cen.1 < JC E neit-r > ,lig cent lot vent FIG. 961.— Vertebrae of Lizard. A, anterior, B, posterior, view of a thoracic vertebra ; C, lateral, D, anterior, view of atlas vertebra ; E, lateral view of axis. cent, centrum ; hyp. hypapophysis of axis ; hit. lateral piece of atlas ; lift, ligamentous band dividing the ring of the atlas into two ; neur. neural arch of atlas ; o<* rt.orb s.orb FIG. 962.— Skull of Lacerta agilis. A, from above.; 15, from below ; C, from the side. any. angular ;. art. articular; ban. cc. basioccipital ;• las. ptg. basipteiygoid processes ; /;./>•. s'ph. basisphenoid ; col. epipterygoid ; cor. coronary ; .) articulating with the frontal. Just behind the postorbital is a supra-temporal bone (supra tl), in close relation to which are the para-quadrate (sq) and squamosal (supra. £2), the former bending forwards and upwards to form with the post- orbital the superior temporal arch. At the anterior extremity of the snout is a median bone formed by the coalescence of the two premaxillce (p. mx) ; this bears the four anterior teeth of each side. On each side behind the premaxilla is the maxilla (max), consisting of two portions, an alveolar bearing. all the rest of the teeth, and a palatine extending inwards on the roof of the mouth, together with an ascending process articulating with the nasal and pre-frontal above. Articulating behind with each maxilla is a jugal (ju\ which forms the posterior half of the ventral boundary of the orbit. The quadrate (qu) articulates movably with the parotic process, and bears at its distal end the articular surface for the mandible. In the anterior portion of the roof of the mouth, articulating in front with the premaxillaa and maxillae, are the vomers (vom). Behind and embracing them posteriorly are the flat palatines (pal). The elongated pterygoids (pt.g) articulate in front with the posterior extremities of the palatines : behind each articulates with the corresponding basi-pUrygoid process (has. ptg) of the basi- sphenoid, and sends back a process which becomes applied to the inner face of the quadrate. A stout bone which extends between the maxilla externally and the pterygoid internally is termed the transverse bone or ecto-pterygoid (trans). Extending nearly verti- cally downwards from the pro-otic to the pterygoid is a slender rod of bone, the epi-pterygoid (col). The columella is a small rod partly composed of cartilage and partly of bone, the outer end of which is fixed into the inner surface of the tympanic membrane, while the inner is attached to a small aperture, the fenestra ovalis, in the outer wall of the auditory capsule between the pro-otic and the opisthotic. Certain depressions or fossae and apertures or foramina are to be observed in the skull. The foramen magnum, the parietal foramen, and the orbits have been already mentioned.. The posterior temporal fossa is situated on either side of and above the foramen magnum, bounded above and externally by the roofing bones, and on the inner side by the bones of the occipital region. The inferior temporal fossa is bounded internally by the pterygoid, and is separated from the palatine foramen by the transverse. The lateral temporal fossa is the wide space in the side wall of the skull behind the orbit ; the bony bar which limits it above is the superior temporal arch ; a bony inferior temporal or qudrato- jugal arch is here absent. The tympano-eustachian fossa, situated in the auditory region, is bounded by the bones of that region together with the quadrate. The posterior or 320 ZOOLOGY SECT. interned nnres are bounded posteriorly by the palatines. The anterior or external nasal aperture is situated at the anterior extremity of the skull bounded by the nasals and premaxillae. Each ramus of the mandible consists of six bony elements in addition to the slender persistent MecJcel's cartilage. The proximal element is the articular (art) which bears the articular surface for the quadrate, and is produced backwards into the angular process. The angular (ang) is a splint-like bone covering the ventral edge and the lower half of the outer surface of the articular. The supra- angular (s. any) overlies the dorsal edge and upper half of the outer surface of the same bone. The dentary (dent) forms the main part of the distal portion of the mandible, and bears all the mandi- bular teeth. The splenial is a flat splint applied to the inner face of the dentary. The coronary (cor), a small, somewhat conical bone, forms the upwardly directed coronoid process immedi- ately behind the last tooth. All these, with the exception of the articular, are investing bones. The hyoid apparatus (vide Fig, 968; b. hy) consists (1) of a median cartilaginous rod, the ~basi-hyal, (2) of the anterior cornua, elongated cartilaginous rods which, connected ventrally with the basi-hyal, curve round the gullet and end in close relation with the ventral surface of the auditory capsule, (3) of the middle cornua, rods of cartilage ossified at their proximal ends, and (4) of the posterior cornua, cartilaginous rods arising from the posterior edge of the basi-hyal and passing backwards and outwards. The middle cornua are vestiges of the first, the posterior of the second, branchial arch. In the pectoral arch (Fig. 963), the coracoids are flat bones articulating with the antero -lateral border of the sternum, and bearing the ventral half of the glenoid cavity (glen) for the head of the humerus ; a cartilaginous epicoracoid (ep. cor) element lies on the inner side of the procoracoid and coracoid ; a large gap or fenestra divides each into a narrow anterior portion — the procoracoid (pr. cor), and a broader posterior portion, the coracoid proper (cor). The scapulcv (sc) articulate with the outer ends of the coracoids, and each bears the dorsal half of the glenoid cavity. Dorsally the scapulae become expanded, and each has connected with it a thin plate of partly calcified cartilage — the suprascapula (supra, sc), which extends inwards towards the spinal column on the dorsal aspect of the body. An element not hitherto met with, except in the Stegocephala (p. 302), is the interclavicle or episternum (epist), a cross-shaped investing bone, the stem of which is longi- tudinal and is in the posterior portion of its extent closely applied to the ventral surface of the anterior part of the sternum, while the cross-piece is situated a little in front of the scapula. The clavicles (cl) are flat curved bones articulating with one another in the middle line and also with the anterior end of the interclavicle. The bones of the fore-limb consist of a proximal bone or humerus, a middle xm PHYLUM CHORDATA 321 division composed of two bones — the radius and ulna, and a distal division or manus. In the natural position of the parts the humerus is directed, from the glenoid cavity with which it articulates, backwards, upwards and outwards ; the radius and ulna pass from their articulation with the humerus downwards and slightly forwards, while the manus has the digits directed forwards and outwards. When the limb is extended at right angles to the long axis of the trunk, it presents, like that of the Frog, dorsal and ventral surfaces, and pre-axial and post-axial borders. In this position the radius is seen to be pre-axial, the ulna post- axial. In the natural position the pre-axial border of the humerus l'i< . '. '( tf.— Pectoral arch and sternum of Lacerta agilis. d. clavicle; cor. coracoid ; cj>. cor. epicoracoid ; epist. episternum ; ylen. glenoid cavity for head of humerus ; •j>i: '-or. procoracoid ;— 1-1.— ?-*. first to fourth sternal ribs ; «t. scapula ; st. sternum ; su.pm. si:, suprascapula. (After Hoffmann.) is external, and the distal end of the forearm is rotated in such a way that, while the pre-axial border looks forwards and out- wards at the proximal end, it faces directly inwards at its distal end, the manus being rotated so that its pre-axial border looks inwards. The humerus is a long bone consisting of a shaft and two ex- tremities, each of the latter being formed of an epiphysis of calcified cartilage, the proximal rounded, the distal (trochlea) pulley-like with two articular surfaces, one for the radius and the other for the ulna. The radius is a slender bone consisting, like the humerus, of a shaft and two epiphyses; the distal extremity has a concave articular surface for the carpus., and is produced pre-axially into a radial stylo id process. The proximal end of the ulna is produced into an upwardly directed process — the olecranon : the distal end bears a 322 ZOOLOGY SECT. TV FIG. 064. — Carpus of Lacerta agilis, (left) from above. R. radius ; U. ulna ; c. centrale ; i. interme iium ; r. radiale ; u. ulnare: 1—5, the five distal carpals : t, pisiform ; / — V, the five metacarpals. (From Wieder- shehn's Comparative Anatomy.) convex articular surface for the carpus. The carpus (Fig. 964) is composed of ten small polyhedral or rounded carpal bones. These consist of a proximal row containing three, viz., the radiale (•?*), ulnare (u\ and intermedium (i), of a centrale (c), and of a distal row of five (1-5) ; with an accessory or pisiform (f) bone attached to the distal epiphysis of the ulna on its post-axial side. The first digit or pollex consists of a metacarpal and two phal- anges, the second of a metacarpal and three phalanges, the third of a meta- carpal and four phalanges, the fourth of a metacarpal and five phalanges, and the fifth of a metacarpal and three phalanges. The number of phalanges in the first four digits is, therefore, one more than the serial number of the digit. The pelvic arch (Fig. 965) consists of two triradiate bones, the ossa innominiata, each ray being a separate bone. On the outer side at the point from which the rays diverge is a concave articular surface — the acetctbulum (Ac) — for the head of the femur. From the region of the ace tabu him one of the rays, the ilium (/), a compressed rod, passes upwards and backwards to arti- culate with the sacral region of the spinal column. A second ray — the pubis (P) — passes downwards and forwards to meet its fellow in the middle lino, the articulation being termed the pubic sympJiysis. In the middle line in front, between the anterior ends of the pubes, is a small nodule of calcified cartilage, the epipubis (Ccp\ The third ray or ischimu (Is) runs downwards and backwards, and articulates with its fellow in the ischiatic symphysis, the ventral FIG. 965.— Pelvis of Lacerta vivipara, from the ventral side. Ac. acetabulum ; Ctp. epi-pubis. Fo'. foramen for obturator nerve (obtutator foramen); Hp. h. hypo-ischium ; /. ilium; 1 1, process representing the pre-acetabular pai-t of the ilium ; Is. ischium ; P. pubis ; PP. pre-pubis. (From Wie'dersheim's Comparative Anatomy.) xin PHYLUM CHORDATA 323 ends of the two bones being separated by a plate of calcified carti- lage. Between the pubes and ischia is a wide space divided by a median ligament (Ig) into a pair of apertures, and a smaller aperture in each pubis is the obturator foramen (Fo'}. A small rod of bone, the os cloacae, or hypo-isekium (Up. Is), passes backwards from the ischiatic symphysis and supports the ventral wall of the cloaca. The hind-limb consists, like the fore-limb, of three divisions ; these are termed respectively the proximal or femur, the middle or cms, and the distal or pes. The proximal division consists of one bone, the femur ; the middle division of two, the tibia and fibula; the distal of the tarsal and metatarsal bones and the phalanges. When the limb is extended at right angles with the trunk, the tibia is pre-axial and the fibula post-axial : in the natural position of the parts the pre-axial border is internal in all three divisions of the limb. The femur is a stout bone consisting of a shaft and two epiphyses. The proximal epiphysis develops a rounded head which fits into the acetabulum ; near it on the pre-axial side is a prominence, the lesser troehanter, and a nearly obsolete prominence on the post-axial side represents the greater trochanler. The distal extremity is pulley-shaped, with internal and external prominences or condyles for articulation with the tibia ; immediately above the external condyle is a prominence or tuberosity for articulation with the fibula. The tibia is a stout, curved bone, along the anterior (dorsal) edge of which runs a longitudinal ridge; the cnemial ridge : the proximal extremity pre- sents two articular surfaces for the condyles of the femur. The fibula is a slender bone, the proximal end articulating with the external tuber- osity of the femur, the distal with the tarsus. The tarsus (Fig. 966) comprises only three bones in the adult, one large proximal bone, the tiUo-fibv,lare FlG. 966.-Tarsus of Lacerta agiiis. (tb.fb\ arid two smaller distal (tars. /?• ,fibula; tb- «biaj A.tb-{b- iib\°- ... . fibulave ; tars. dist. distal tarsals. dlSt). Each digit Consists Of a meta- (After Gegenbaur.) tarsal bone and phalanges, the number of the latter being respectively two, three, four, five, and three. The first and second metatarsals articulate with the tibial side of the tibio-fibulare, the rest with the distal tarsals. Digestive System. — The upper and lower jaws, forming the boundary of the aperture of the mouth, are each provided with a single row of small conical teeth, and there is a patch of similar teeth (palatine teeth] on the palatine. On the floor of the mouth- cavity is the tongue, a narrow elongated fleshy organ, bifid in front. 324 ZOOLOGY SECT. OB- 1 MD The stomach (Fig. 967, M, Fig. 968, St) is a cylindrical organ but little wider than the oesophagus, and with thick muscular walls. At the point where the small intestine joins the large intestine or rectum, the latter is produced into a short ccecum (Fig. 969, Gee). The liver (/r) is divided into right °? and left lobes, and a gall-bladder (Fig. 967,£,£.; Fig. 968, ^ ; Fig. 969, gM) lies at the lower margin of the right lobe. The pancreas (pii) is situated in the loop between the stomach and first part of the small intestine or duodenum (du). The stomach is attached to the body-wall by a fold of peritoneum, the mesoff aster, the small intestine by a fold termed the mesentery, the rectum by a meso-rectum. From the dorsal surface of the liver to the stomach extends a thin fold, the gastro-hepatic omentum ; and this is continued backwards as the duodeno-hepatic omentum, connect- ing the liver with the first portion of the small intestine. Vascular System. — The heart is enclosed, like that of the Frog, in a thin transparent membrane, the pericardium. It consists of a sinus venosus, right and left auricles, and an incompletely divided ven- tricle. The sinus venosus (Fig. 968, s.- v.), into which the large veins open, is thin walled, and has a smooth inner surface. From it a sinu-auricular aperture, guarded by a two-lipped valve, leads to the right auricle. The auricles have their inner surfaces raised up into a network of muscular ridges, the musculi pectinati. Both auricles open into the cavity of the ven- tricle, the aperture of communication, or auriculo-ventricular aper- ture, being divided into two by the auricular septum, and guarded by the auriculo-ventricular valve, consisting of two semilunar flaps. The ventricle (Fig. 968, v. ; Fig. 969, vent.} has very thick spongy walls and a small cavity, divided into two parts by an incomplete muscular partition. From the part of the ventricular cavity to Pn El FIG. 967.— Lacerta agilis. General view of the viscera in their natural relations. Bl. urinary bladder ; Ci. post- caval vein ; ED, rectum ; GB. gall- bladder ; H. heart ; Lg. Lg'. the lungs ; M, stomach ; MD, small intestine ; Oe. oesophagus ; Pn. pancreas ; 2V. trachea. (From Wiedersheim's Comparative Anatomy.) XIII PHYLUM CHORDATA 325 the right of the partition arises the pulmonary artery; from the part to the left are given off the right and left aortic arches. When the two auricles contract, the blood from the right 1. an, -I — c FIG. 968.— Lacerta viridis. Dissection from the ventral aspect showing the alimentary;, circulatory, respiratory and uriiiogenital organs (nat. size). The liver (Ir.) is divided longi- tudinally and its two halves displaced outwards ; the alimentary canal is drawn out to the animal's left ; the cloaca with the urinary bladder and posterior ends of the vasa deferentia is removed, as also is the right adipose body. a. co. anterior cornu of hyo d ; az. azygos or cardinal vein ; b. hy. body of hyoid ; c. caudal vein ; c. ad. adipose body ; c. m. creliaco- mesenteric artery ; cce. caecum ; cr. carotid artery ; d. ao. dorsal aorta ; du. duodenum ; e. ju. external jugular vein ; ep. epididymis ; epg. epigastric vein ; /. a. femoral artery ; f. v. femoral vein; g. b. gall-bladder; i.jv. internal jugular vein; U. ileum ; i. m. inferior mesenteric arteries; /(.-.kidney; I. ao. left aortic arch ; I. au. left auricle ; Ig. lungs ; Ir. liver ; m. co. middle cornu of hyoid : p. a. pulmonary artery ; pc. pericardium ; p. co. posterior cornu of hyoid ; pn. pancreas ; pi. pelvic vein ; pt. c. postcaval vein ; pt. v. poital vein ; p. v. pulmonary vein ; r. rectum ; r. au. right auricle ; r. It. a. right hepatic artery ; sc. sciatic vein ; sd. a. subclaviau artery ; scl. r. subclavian vein ; spl. spleen ; st. stomach ; s. v. sinus venosi s ; th. thyroid gland ; tr. trachea ; ts. testis ; v. ventricle. (From Parker's Zootomy.) VOL. II X 326 ZOOLOGY SECT, xm auricle (venous blood) tends to run more to the right-hand portion of the cavity of the ventricle, while that from the left auricle (arterial) occupies the left-hand portion. When the ventricle begins to contract, its walls come in contact with the dorsal and ventral edges of the ventricular partition, thus qom- pleting the separation of the right-hand part of the cavity, containing venous blood, from the left-hand part, containing arterial and mixed blood; and the further contraction results in the driving of the venous blood through the pulmonary artery to the lungs and of the rest through the aortic arches to the head and body. (Vide Fig. 1002.) From the right aorta arise the carotid arteries (Fig. 968, cr. ; Fig. 969, car. art.), and each runs for some distance parallel with the corresponding aortic arch, with which it anastomoses distally (the connecting part being termed the ductus Botalli), having previously given off the carotid artery proper, by means of which the blood is carried to the head. The two aortic arches curve backwards round the oesophagus, one on the right hand and the other on the left, and meet in the middle line dorsally to form the median dorsal aorta (Fig. 968, d. ao. ; Fig. 969, dors. aort.). From the right arch, just in front of the junction, arise the two subclavian arteries (Fig. 968, s. cl. a.), right and left, each running outwards to the corresponding fore-limb. From the dorsal aorta the first important branch given off is the coeliaco-mesenteric (c. in.). This shortly divides into two trunks, a cceliac (Fig. 969, cod. .a.) supplying the stomach, spleen, pancreas, duodenum, and left lobe of the liver, and an anterior mesenteric supplying the posterior part of the small intestine. Three small posterior mesenteric arteries given off further back supply the large intestine. Pos- teriorly, after giving off renal and genital branches, and a pair of large iliacs to the hind-limb, the dorsal aorta is continued along the tail as the caudal artery (Fig. 969, caud. art). Throughout its length, in addition to the larger branches mentioned, the dorsal aorta gives off a regularly-arranged series of pairs of small vessels, the intercostal and lumbar arteries, giving off branches that enter the neural canal and others that supply the muscles and integument. The vpnous blood from the tail is brought back by means of a caudal -vein (Fig. 968, c.). This bifurcates at the base of the tail to form the two pelvic (lateral) veins (pi.) ; these unite to form the median epigastric or abdominal (ep.g.), which eventually enters the left lobe of 'the liver. Entering the pelvic veins are the femoral and sciatic veins from the hind-limb. Arising from the pelvic are the renal portal veins distributed to the substance of the kidneys. The efferent renal veins, carrying the blood from the kidneys, combine to form a pair of large trunks, which soon unite to form the median postcavaL The postcaval runs forwards x 2 328 ZOOLOGY 8ECT. towards the heart, and, after receiving the wide hepatic vein from the liver, enters the sinus venosus. Two precavals, right and left, carry the blood from the anterior extremities and the head to the sinus venosus. The right precaval is formed by the union of the internal and external jugular and the subdaman. On the left side the precaval is formed by the union of internal jugular and subclavian, the left external jugular being absent. The liver is supplied, as is other Vertebrates, by a hepatic portal system of vessels, blood being carried to it by a portal vein, formed by the union of gastric, pancreatic, splenic, and mesenteric veins. The adipose bodies (Fig. 968, c. ad.} are two masses of fat of somewhat semilunar shape in the posterior part of the abdominal cavity, between the peritoneum and the muscles of the body-wall. The thyroid is a whitish, transversely-elongated body on the ventral wall of the trachea, a short distance in front of the heart. The spleen (Figs. 968 and 969, spl.) is a small red body lying in the mesogaster, near the posterior end of the stomach. Organs of Respiration. — A slit-like aperture, the glottis, situated behind the tongue, leads into a short chamber, the larynx, the wall of which is supported by cricoid and arytenoid cartilages. From the larynx an elongated cylindrical tube, the trachea, passes backwards on the ventral side of the neck. Its wall is supported by a large number of small rings of cartilage, the tracheal rings. Posteriorly the trachea bifurcates to foim two similar but narrower tubes, the bronchi, one entering each lung. The lung (Fig. 968, Iff) is a fusiform sac, the inner lining of which is raised up into a net- work of delicate ridges, having the appearance of a honeycomb ; these ridges are much closer and more numerous towards the anterior than towards the posterior end of the lung. The brain (Figs. 970 and 971) presents all the parts that have been described in the brain of the Frog (p. 284), with some minor modifications. The two cerebral hemispheres (parencephala) (Fig. 970, c.h.) are oval bodies, somewhat narrower in front than behind, closely applied together. Each is prolonged anteriorly into the corresponding olfactory peduncle or tract, somewhat dilated in front to form the olfactory lull (plf.} from which the olfactory nerve arises. In the interior of each is a cavity, the lateral ventricle or paracoele, sending a prolongation forwards into the ol- factory bulb, and communicating behind by a small aperture, the foramen of Monro (D, /. m.\ with the diaccele (v.3). Through the foramen of Monro there passes into each paraccele a vascular process of pia mater, the choroid plexus (ch. p.} : immediately above and behind this is a hippocampal commissure (c.p.a.) connect- ing together two areas known as hippocampi, one on the mesial surface of each hemisphere. On the floor of each paracoele is a XIII PHYLUM CHORDATA 329 thickened mass of nerve-matter, the and between them passes a transverse band of nerve-fibres, the anterior commis- sure (a.c.). The dien- cephalon is a small rounded lobe between the paracoeles and the mid-brain, and con- taining a laterally compressed cavity, the diacwle (v. 3). Its roof is extremely thin. Its lateral walls are formed of two thicken- ings, the optic ihalami, behind which passes a transverse band, the posterior commissure (p c.), Behind and below the thalami are the optic tracts (o. t.) continued into the optic nerves. Be- hind the optic tracts the floor is produced downwards into a tubular process, the infundibulum (inf.), ending below in a rounded body, the pituitary liody or hypo- physis (pty.). The roof is produced into a median process, the epiphysis (Fig. 970, D, pn; Fig. 971,^), which is divided into two parts, one of which has connected with its distal extremity an eye-like structure, the parietal organ or pineal eye (Fig. 971, pa), lying in the parietal foramen, in the velum transversum (v. t.), a m.o FIG. 970.— Brain of Lacerta viridis. A, from above with the left parencephalou (c. h.) and optic lobe (o. I.) opened. B, from beneath. C, from the left side. D, in longitudinal vertical section, a. c. anterior commissure ; aq. s. aque- duct of Sylvius ; ck-cerebellum ; c. c. crura cerebri ; c. h. cerebral hemispheres ; ch. p. choroid plexus ; c. s. corpus striatum ; /. m. foramen of Monro ; inf. iiifundibulum ; m. o. medulla oblongata ; o. c. optic chi'asma ; o. I. optic lobes ; olf. olfactory bulbs with their peduncles or tracts ; o. t. optic tracts; o. v. apeiture between aqueduct of Sylvius and optic ventricle ; p. c. posterior commissure • jpn. pineal body ; metaco?ie ; Zootomy.) xviv, , jj. (,. pwoucllUt lsULUUU0BUl C? , ody ; pty. pituitary body ; v3, diaccele ; y4, I— XTI, cranial nerves. (From ParkeFs' In front of the epiphysis, transverse fold of the thin .330 ZOOLOGY roof of the brain marking the anterior limit of the dien- cephalon, is another commissure, the aberrant commissure (c.p.p.\ which connects together the posterior and dorsal parts of the parencephala ; this is not represented either in the Frog or in higher Vertebrates. The mid-brain consists dorsally of two oval o^/^'c /o&fs (corpora bigemina, Fig. 970, o. Z.) and ventrally of a mass of longitudinal nerve-fibres, the crura cerebri (V. e.), passing forwards to the fore-brain. Each optic lobe contains a cavity (optoccele) communicating with the iter, a narrow passage leading from the diaccele to the metacoele. The cerebellum (cb.) is, like that of the Frog, of small size, being a small antero-posteriorly XIII PHYLUM CHORDATA 331 flattened lobe overlapping the anterior portion of the metacoele. The me/bAwephcdon (medulla oblongata, m. o.\ broad in front, tapers behind to where it passes into the anterior portion of the spinal cord. The metaccele is a shallow space on the dorsal aspect of the medulla oblongata, overlapped in front for a short distance by the cerebellum, and behind covered only by the pia mater, containing a network of vessels, the choroid plexus of the metaccele (Fig. 971, pch.). At the point where medulla oblongata and spinal cord meet is a strong ventral flexure. The spinal cord is continued backwards throughout the length of the neural canal, becoming slightly dilated opposite the origins of the two pairs of limbs and tapering greatly towards the posterior end of the tail. The cerebral nerves resemble those of the Frog as regards their origin and distribution in most respects, the principal difference being that a spinal accessory is intercalated in front of the hypo- glossal, and that the hypoglossal arises from the medulla oblongata, not from the spinal cord, and is therefore a cerebral nerve. The nasal cavities (Fig. 972) open at the extremity of the snout by the external nares, and into the cavity of the mouth by a pair of slit-like internal nares situated near the middle line of the palate. The external aperture opens into a sort of vestibule, beyond which is the nasal or olfactory cavity proper, containing a convoluted turbinal bone over which the mucous membrane extends. Opening into each nasal cavity, near the internal opening, is Jacobsons organ (J. «/.), an oval sac with strongly pigmented walls supported by cartilage. The eye has a cartilaginous sclerotic having a ring of small bones (Fig. 973) supporting it externally. There is a cushion-like pecten or vascular pigmented process similar to the struc- ture of the same name occurring in Birds (see below, Class Aves), projecting into the inner chamber of the eye. In essential structure the rest of the eye agrees with that of the Craniata generally as already described. Two glands lie in the orbit, the lacrymal and the Har- derian. The ear consists of two principal parts, the middle ear or tympanum, and the internal ear or membranous laby- rinth. The former is closed externally by the tympanic membrane, FIG. 972. — Transverse section of the nasal region of the head of Lacerta to show the relations of Jacobson's or- gans. Z>, nasal glands ; /. /. Jacob- son's organs ; N. N. nasal cavities. (Prom. Wiedersheim's Comparative Anatomy.) FIG. 973.— Ring of ossicles in sclerotic of eye of Lacerta. (After Wiedersheim.) 332 ZOOLOGY SECT. mn the position of which has been already mentioned. It communicates with the cavity of the mouth by the Eustachian passage, which is narrower and longer than in the Frog. The inner wall of the tympanic cavity is formed by the bony wall of the auditory region of the skull, in which there are two fenestrse — the fenestra ovalis and the fenestra rotunda. The columella stretches across the cavity from the tympanic membrane, and is fixed internally into the membrane covering over the fenestra ovalis. The parts of the membranous labyrinth (Fig. 974) are enclosed by the bones of the auditory region : between the membranous wall of the labyrinth and the surrounding bone is a small space containing fluid, the peri- lymph. The labyrinth itself consists of the utriculus with the three semicircular canals and the sacculus with the lagena (cochlea). The utriculus (u.) is a cylindrical tube, bent round at a sharp angle : the semicircular can- als (ca.} ce., cp.) are arranged as in Vertebrates in general (p. 115). A narrow tube, the ductus endolympha- ticus, leads upwards towards the roof of the skull and ends blindly in the dura mater. The sacculus is large and rounded. The lagena (/.) forms a flattened, not very prominent, lobe, and is of simple form. Urinary and Reproductive Systems. — The kidneys (Figs. 975 and 976, &.) are a pair of irregularly shaped, dark red bodies, each consisting of two lobes, anterior and posterior, situated in close contact with the dorsal wall of the posterior portion of the abdominal cavity, and covered with peritoneum on their ventral faces only. Their posterior portions, which are tapering, are in close contact with one another. Each has a delicate duct, the ureter, open- ing posteriorly into the cloaca. A urinary (allantoic) bladder (&/.), a thin-walled sac, opens into the cloaca on its ventral side. In the male the testes (Fig. 975, t.) are two oval white bodies, that on the right side situated just posterior to the right lobe of the liver, that on the left somewhat further back. Each testis is attached to the body- wall by a fold of the peritoneum, the mes- orchium (ms. a). The epididymis (ep.) extends backwards from the inner side of each testis, and passes behind into a narrow convoluted op FIG. 974. — Membranous labyrinth of Lacerta viridis, viewed from the outer side. aa. an- terior ampulla ; ac. auditory nerve ; ade. opening of the ductus endolymphaticus ; ae. external ampulla ; ap. posterior ampulla ; br. basilar branch of nerve ; ca. anterior semicircular canal ; ce. external semicircular canal ; cp. posterior semicircular canal \cv.s. canal connecting utriculus and sacculus ; de. ductus endolymphaticus ; I. lagena ; mb. basilar membrane ; raa, rae, rap. rl. branches of aud.tory nerve ; s. sacculus ; ss. com- mon canal of communication between anterior and posterior semicircular canals and utricle ; u. utriculus. (From Wiedersheim's Comparative Anatomy, after Retzius.). PHYLUM CHORDATA 333 tube, the vas deferent or spermiduct (v. d.), which opens into the terminal part of the corresponding ureter. A pair of vascular eversible copulatory sacs (p. p.), which when everted are seen to be of cylin. lg, fold of peritoneum sup- porting epididymis ; c/.1 anterior and cl.2 pos- terior divisions of the cloaca ; cp. epididymis ; /,-. kidney ; mso. mesorchium ; p copulatory organs, of which the right is shown retracted (//) and the left everted (p) ; r.m. retractor muscle of latter ; r. ridge separating anterior and posterior divisions of cloaca ; ret. rectum ; ret', its opening into the cloaca ; t. testis ; u, g. urinogenital papilla and aperture ; v. d. vas deferens. ;(From Parker's Zootomy.) od FIG. 976.— Female urinogenital organs of Lacerta viridis. The ventral wall of the cloaca, the urinary bladder, the posterior end of the left oviduct, and the peritoneal investment of the left ovary and oviduct are removed, b. Ig. broad ligament; d.1 anterior, and c-L2 posterior divisions of the cloaca ; k. kidney ; ms. o. mesoaiiutn ; od. left oviduct ; od'. its peritoneal aperture ; od". aperture of right oviduct into the cloaca ; ov. ovary ; ur. aperture of ureter. (From Parker's Zootomy.) marking the position of the ova. They are situated a little further back than the testes, and each is attached to the body- wall by a fold of the peritoneum, the mesoarium (ms. o.). The oviducts (od.) are thin-walled, wide, plaited tubes which open in front into the cavity of the body (od'.), while behind they communicate with the posterior part of the cloaca, their opening (od".) being distinct from, and a little in front of, those of the ureters. A fold of the peritoneum, the broad ligament (b. lg.)} attaches the oviduct to the body-wall. 234 ZOOLOGY SECT. 2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. The Reptilia are cold-blooded Sauropsida (p. 313), with a horny epidermal skeleton of scales, and frequently with an armour of dermal bony plates. The centra of the vertebrae have spheroidal articular surfaces. There are usually only two vertebrae in the sacral region. The episternum, when present, always remains distinct from the clavicles. The floor of the acetabulum is often completely ossified. The pubes and the ischia usually meet in ventral symphyses. The metatarsals do not become ankylosed, The mandible, as well as several bones of the upper jaw, very usually bear teeth. The optic lobes are situated on the dorsal aspect of the brain. The ventricle is rarely divided by a complete partition. There is always a paired aortic arch in the adult. ORDER I. — SQUAMATA. Reptilia in which the surface is covered with horny scales, sometimes with the addition of dermal ossifications. The opening of the cloaca is transverse in direction. There is a pair of eversible copulatory sacs in the male. The vertebrae are nearly always procoelous. The sacrum, absent in the Ophidia and some Pythonomorpha, consists of two vertebrae in the Lacertilia. The ribs have simple vertebral extremities. The quadrate is usually movably articulated with the skull. There is no inferior temporal arch. The nasal apertures of the skull are separate. The limbs, when present, are sometimes adapted for terrestrial locomotion (Lacertilia), sometimes for swimming (Pythonomorpha). The teeth are acrodont or pleurodont (see below). The lungs are simple sacs. There is always a wide aperture of communication between the right and left divisions of the ventricular cavity. The optic lobes are approximated, and the cerebellum is extremely small. Sub-Order a. — Lacertilia. Squamata in which, as a rule, the limbs are present and are adapted for walking. The mouth is capable of being opened to only a moderate extent. The maxillae, palatines, and pterygoids are incapable of free movement. The rami of the mandible are firmly united at the symphysis. There are nearly always movable eyelids and a tympanum. A sternum and an episternum are present. Including all the Lizards, such as the Skincs, Geckos, Monitors, Iguanas, Amphisbamians, Chamaeleons, and other groups. Sub-Order b. — Ophidia. Squamata with long narrow body, devoid of limbs. The mouth is capable of being opened to form a relatively very wide gape by divarication of the jaws. The maxillae, palatines, and ptery- XIII PHYLUM CHORDATA 335 golds are so articulated as to permit of free movement. The rami of the mandible are connected together only by elastic fibres at the symphysis, so that they are capable of being widely separated. There is no separate supra-temporal ossification. Sternum and episternum are absent. Movable eyelids and tympanum are absent. Including all the Snakes — Vipers, Rattlesnakes, Sea-Snakes, Fresh-water Snakes, Tree-Snakes, Blind-Snakes, Pythons, and Boas. Sub-Order c. — Pyihonomorpha. Extinct Squamata with elongated Snake-like body, provided with limbs which take the form of swimming-paddles. The skull resembles that of the Lacertilia; a supra- temporal helps to suspend the quadrate. The union of the rami of the mandible was ligamentous. There is, as a rule, no sacrum, the ilia not articulating with the spinal column. ORDER II. — RHYNCHOCEPHALIA. Lizard-like, scaly Reptiles with well-developed pentadactyle limbs adapted for walking. The opening of the cloaca is trans- verse. There are no copulatory sacs. The vertebra are amphi- ccelous, sometimes enclosing vestiges of the notochord. The sacrum consists of two vertebrae. Numerous intercentra are present. The ribs have simple vertebral extremities, and are provided with uncinates (see below). There is a system of abdominal ribs. The quadrate is immovably fixed to the other bones of the skull. There are both upper and lower temporal arches. The rami of the mandible are united by ligament. There is a sternum. The teeth are acrodont. The lungs, heart, and brain resemble those of the Squamata. This order comprises only a single living genus, Sphenodon or Hatteria, together with a number of fossil forms. ORDER III. — CHELONIA. Reptilia having the body enclosed in a shell of bony plates, consisting of a dorsal carapace and a ventral plastron, partly of dermal, partly of endoskeletal origin. There is usually on the surface an epidermal exoskeleton of horny plates. The vertebrae and ribs of the thoracic region are firmly fused with the bony carapace, into the composition of which they enter. The quad- rate is immovably united with the skull. The nasal apertures in the skull coalesce into one. The limbs are sometimes terminated by clawed digits adapted for terrestrial locomotion, sometimes modified into the shape of flippers. There are no teeth, and the jaws have a horny investment. The lungs are compound sacs. 336 ZOOLOGY In essentials the heart and brain resemble those of the Squamata. There are no copulatory sacs, but a median penis. This order includes the Land Tortoises, Soft Tortoises, River and Mud Tortoises, and the Turtles, besides a number of fossil forms. ORDER IV. — THEROMORPHA. Extinct Reptiles with amphicoelous vertebrae sometimes enclosing remnants of the notochord, with a sacrum composed of from two to six vertebrae, and with ribs having bifid vertebral extremities. The quadrate is not movable. The limbs are adapted for walking. The pubes and . ischia are united. The teeth, which are usually, though not always, present, are highly differentiated and lodged in sockets This order comprises a large number of extinct Reptiles, which are grouped in the four sub-orders, Anomodontia, Placodontia, Pareiosauria, and Theriodontia (Fig. 1011). ORDER V. — OROCODILIA. Reptiles in which the dorsal surface, or both dorsal and ventral surfaces, are covered with rows of sculptured bony scutes supporting horny scales. The vertebral centra are either amphiccelous, flat at each end, or proccelous. The anterior thoracic vertebrae have elongated and bifid transverse processes. The sacrum consists of two vertebrae. The ribs are bifid at their vertebral ends. The quadrate is immovable. A sternum is present, and there is a series of abdominal ribs. The limbs are adapted for walking. The teeth are lodged in sockets. The lungs are com- pound sacs. The ventricle of the heart is completely divided in recent forms. The opening of the cloaca is elongated in the direction of the long axis of the body. There is a median penis. This order includes among living forms the true Crocodiles,, the Gavials, the Alligators, and Caimans. ORDER VI— SAUROPTERYGIA. Extinct aquatic Reptiles with elongated neck, small head, short tail, and usually flipper-like limbs. The centra are slightly amphicoelous or quite flat. The sacrum is composed of either one or two vertebrae. The cervical ribs are bifid, the thoracic simple. The quadrate bone is immovable. There is no sternum. The teeth are situated in sockets (Fig. 1012). ORDER VII. — ICHTHYOPTERYGIA. Extinct aquatic Reptiles, with large head, without neck, and with elongated tail and completely flipper-like limbs. The centra are amphiccelous, and there is no sacrum. The ribs are bifid at their vertebral ends. The quadrate is immovable. The premaxillae are drawn out to form an elongated rostrum. xni PHYLUM CHORDATA 337 There is no sternum, but there is a series of abdominal ribs. The teeth are lodged in a common groove. The integument is naked (Fig. 1015). ORDER VIII. — DINOSAURIA. Extinct terrestrial Reptiles with elongated limbs, having the surface sometimes naked, sometimes provided with a bony armour. The centra are usually amphicoalous. The sacrum consists of from two to six vertebra?. The ribs are bifid. A sternum is present. The quadrate is fixed. The pelvis usually resembles that of a Bird, the ilium being expended fore and aft, and the pubis, as well as the ischium, directed backwards. The teeth are lodged in sockets, and usually have compressed crowns (Fig. 1016). ORDER IX. — PTEROSAURIA. Extinct Reptiles, the structure of which is greatly modified in adaptation to a flying mode of locomotion. The vertebrae are procoelous, the neck elongated. The sacrum contains three to six vertebrae. The anterior thoracic ribs are bifid. The skull resembles that of a bird in its general shape and in the obliteration of the sutures, but not in more essential features. There is a ring of sclerotic bones. The quadrate is immovable. There is a sternum. The fore-limbs are modified to act as wings by the great enlarge- ment of the post-axial digit, for the support of a fold of skin. The posterior limbs are weak and have four or five digits. The teeth are implanted in sockets. In the brain the optic lobes were widely separated by the cerebellum, and the latter bore a pair of lateral processes or flocculi (Figs. 1018-1020;. Systematic Position of the Example.. There are twenty known species of the genus Lacerta, occurring in Europe, Asia, Africa and North America. Lacerta is a member of the sub-order Lacertilia of the order Squamata. The flattened and elongated tongue with notched apex places it in the section Leptoglossa3 of that sub-order. Among the Leptoglossae the family Lacertidae, which comprises Lacerta and a number of other genera, is characterised by the presence of dermal bony supra-orbital and supra-temporal plates, by the presence of small granular or wedge-shaped scales, and of pleurodont conical teeth, excavated at the base. The chief distinctive marks of the genus Lacerta are the presence of comparatively large shields on the head and on the ventral surface, the arrangement of the scales of the trunk in transverse rows which become circular zones or rings on the tail, the development of a collar-like band of larger scales round the neck, and the laterally-compressed falciform claws, grooved on the lower surface. 338 ZOOLOGY SECT;. 3. GENERAL ORGANISATION OF RECENT REPTILIA. External Features. — In external form, as in some other respects, certain of the Lacertilia exhibit the least specialised condition to be observed among the living Reptilia. Lacerta is such a central type, and the general account of that Lizard which has just been given applies in all the points of cardinal importance to a large proportion of the Lacertilia. Modifications take place, however, in a variety of different directions. Of such the following are a few of the chief. The tail region is usually, as in the example, FIG. 077.— Chamaeleon vulgaris, xf. (From the Cambridge Natural History.') extremely long and tapering ; but in some groups of Lizards it is comparatively short and thick ; and in others it is depressed and expanded into a leaf-like form. In the Chameleons (Fig. 977) the long and tapering tail is used as a prehensile organ, the coiling of which round branches of the trees in which the animal lives aids in maintaining the balance of the body in climbing from branch to branch. In the limbs there is likewise a considerable amount of varia- tion in the different groups of the Lacertilia. Moderately long pentadactyle limbs like those of Lacerta are the rule. In the Chameleons (Fig. 977) both fore- and hind-limbs become prehensile xin PHYLUM CHORDATA 339 by a special modification in the arrangement and mode of articula- tion of the digits. In these remarkable arboreal Reptiles the three innermost digits of the manus are joined together throughout their length by a web of skin, and the two outer digits are similarly united : the two sets of digits are so articulated that they can be brought against one another with a grasping movement analogous to the grasping movement of a Parrot's foot or of the hand of Man. A similar arrangement prevails in the pes, the only difference being that the two innermost and three outermost digits are united. In some groups of Lacertilia, on the other hand, such as the Blind- Worm (Anguis), limbs are entirely absent, or are represented only by mere vestiges ; and numerous intermediate gradations exist between these and forms, such as Lacerta, with well-developed limbs. The limbless Lizards (Fig. 1)78) bear a very Fie. 978.— Pygopus lepidopus, with scale-like vestiges of hind-limbs. (After Brehm.) close resemblance to the Snakes, not only in the absence of the limbs, but also in the general form of the body and the mode of locomotion. The body of a Snake is elongated, narrow and .cylindrical, usually tapering towards the posterior end, sometimes with, more usually without, a constriction behind the head. In the absence of limbs the beginning of the short caudal region is only indicated by the position of the cloacal opening. The fore-limbs are never represented even by vestiges ; in some Pythons there are in- conspicuous vestiges of hind-limbs in the form of small claw-like processes. The mouth of the Snake is capable of being very widely opened by the free articulation of the lower jaw, and it is this mainly which distinguishes it from the snake-like Lizards. But other, less conspicuous, points of distinction are the absence of movable eyelids in the Snake, and also the want of a tympanum. - Sphenodon or Hatteria, the New Zealand Tuatara (Fig. 979), the only living representative of the Rhynchocephalia, is a Lizard-like 340 ZOOLOGY SECT. XIII PHYLUM CHORDATA Reptile with a well-developed laterally-compressed tail, and pentadactyle extremities, very similar to those of a typical Lizard. The upper surface is covered with small granular scales, and a crest of compressed spine-like scales runs along the middle of the dorsal surface. The lower surface is covered with transverse rows of large squarish plates. In the Chelonia (Fig. 980) the body is short and broad, enclosed in a hard " shell " consisting of a dorsal part or carapace and a ventral part or plastron. These are in most cases firmly united, aper- tures being left between them for the head and neck, the tail and the limbs. The neck is long and mobile ; the tail short. The limbs are fully developed though short. In some (land and fresh-water FIG. 980.— Grecian Tortoise (Testudo grceca). (After Brehm.) Tortoises) they are provided each with five free digits terminating in curved horny claws ; in the Turtles the digits are closely united together, and the limb assumes the character of a " flipper " ^or swimming paddle. The cloacal aperture is longitudinal. The Crocodilia, the largest of living Reptiles, have the trunk elongated and somewhat depressed, so that its breadth is much greater than its height. The snout is prolonged, the neck short, the tail longer than the body and compressed laterally. The limbs are relatively short and powerful, with five digits in the manus and four in the pes, those of the latter being partly or completely united by webs of skin. The eyes are very small ; the nostrils placed close to the end of the snout and capable of being closed by a sphincter muscle. The cloacal aperture is a longi- tudinal slit. The dorsal and ventral surfaces are covered with thick, squarish horny scales, often sculptured or ridged, those VOL. II Y 342 ZOOLOGY SECT. of the dorsal surface of the tail developed into a longitudinal crest. Integument and Exoskeleton. — Characteristic of the Squa- rnata is the development of horny plates which cover the entire surface, overlapping one another in an imbricating manner. These differ considerably in form and arrangement in different groups ; sometimes they are smooth, sometimes sculptured or keeled. Sometimes they are similar in character over all parts of the surface ; usually there are specially developed scales — the head shields — covering the upper surface of the head. In the majority of Snakes the ventral surface is covered with a row of large transversely elongated scales, the ventral shields. In some Lizards (ChamaBleons and Geckos) the scales are reduced and modified into the form of minute tubercles or granules. In some Lizards special developments of the scales occur in the form of large tubercles or spines. Underlying the horny epidermal scales in some Lizards (Skincoids) are series of dermal bony plates. In the integument of the Geckos are numerous minute hard bodies which are inter- mediate in character between cartilage and bone. In the Snake-like Amphisbasnians there are no true scales, with the exception of the head shields, but the surface is marked out into annular bands of squarish areas. In addition to the modification of the scales, the integument of the Chamseleons is remarkable for the changes of colour which it undergoes, these changes being due to the presence in the dermis of pigment-cells which contract or expand under the influence of the nervous system, in a way that reminds one of the integument of the Cephalopoda. Less conspicuous and rapid changes of colour take place in Anguis and in some Snakes. In the Chelonia scales, when developed, are confined to the head and neck, the limbs and the tail ; but in all of them, with the exception of the Soft Tortoises, both dorsal and ventral surfaces are covered by a system of large horny plates. A series of horny head-shields usually cover the dorsal surface of the head. Beneath the horny plates of the dorsal and ventral surfaces are the bony carapace and plastron, partly composed of dermal bones, but so intimately united with elements derived from the endoskeleton that the entire structure is best described in connection with the latter (vide infra). In the Crocodilia, the whole surface is covered with horny plates or scales, each usually marked with a pit-like depression about the ce'ntre. Underlying each of these, which are of epidermal deriva- tion, is a thick pad of dermal connective-tissue which, in the case of the dorsal scales, is replaced by a bony scute. In the Caimans thin scutes also occur under the ventral scales. A periodical ecdysis or casting arid renewal of the outer layers of the horny epidermis takes place in all the Reptilia with PHYLUM CHORDATA 343 the exception of the Crocodiles. Sometimes this occurs in a fragmentary manner ; but in Snakes and many Lizards the whole comes away as a continuous slough. Endoskeleton. — The vertebne are always fully ossified. Only in the Geckos and Sphenodon (Fig. 981) are the centra amphiccelous with remnants of the notochord in the inter- central spaces In most of the others the centra are proccelous, a ball-like convexity on the posterior surface of each centrum projecting into a cup-like concavity on the anterior face of the next. In Sphenodon and the Geckos a series of wedge-shaped discs (intercentra} are intercalated between the vertebrae of the cer- vical, part of the thoracic, and the caudal regions. The paired bones of the inferior arches (chevron bones) are attached to these bones when they are present. In the Lizards in general and the Crocodiles, there are inferior processes (hypapo- physes), perhaps representing intercentra, situated below the centra of the anterior cervical vertebrae. In Chamaeleons, Sphenodon, and the Crocodiles there is a median bone, the pro- <(ll(ts (Fig. 985, 0), intercalated between the atlas and the occipital region of the skull. In the Snakes and in Iguanas, in addition to the ordinary articulating processes or zygapophyses, there are peculiar articular surfaces termed zygotyhcncs and zyyantra (Fig. 982). The zygosphene FKJ. 981.— Vertebra of Sphenodon, show- ing the amphiccelous centrum (£,). (After Headley.) Jit.*. Kic. !>S2. — Vertebra of Python, anterior and posterior views, n. s. neural spine; p. z. pre- zygapophyses ; {>t. z. post-zygapophysis ; t. p. trans verse processes ; 2. a. zygantrum ; z.s. zygo- sphuiie. (After Huxley.) is a wedge-like process projecting forwards from the anterior face of the neural arch of the vertebra, and fitting, when the vertebra? are in their natural positions, into a depression of corresponding form — the zygantrum — on the posterior face of the neural arch of the vertebra in front. To this arrangement, as well as to the Y 2 344 ZOOLOGY SECT. deeply concavo-convex centra, the extraordinary flexibility and strength of a Snake's back-bone are due. The various regions of the spinal column are well marked in most of the Lizards, in the Rhynchocephalia, in the Chelonia, and in the Crocodilia (Fig. 983). In the Snakes and many of the snake-like Lizards only two regions are distinguishable — pre-caudal and caudal. In the others there is a sacral region comprising two vertebrae, both of which have strong processes (sacral ribs) for articulation with the ilia. The first and second vertebrae are always modified to form an atlas and axis. Intercentra are present throughout the spinal column in Sphenodon, and these are represented in the Crocodilia by cartilaginous rings or discs — the intervertebral discs (Fig. 985 IS) : in the other orders the inter- centra are of more irregular occurrence, but in the Lacertilia (as well as in Sphenodon and the Crocodilia) there are a series of sub- vertebral chevron bones in the caudal region. Ribs are developed in connection with all the vertebrae of the pre-sacral or pre-caudal region ; in the caudal region they are usually replaced by inferior arches ; but Sphenodon, the Chelonia and Crocodilia have caudal ribs which become fused with the bodies of the vertebrae. In the Lacertilia only a small number (three or four) of the most anterior of the thoracic ribs are connected with the sternum by cartila- ginous sternal ribs ; the rest are free, or are connected together into continuous hoops across the middle line. In the so-called Flying Lizards (Draco} a number of the ribs are greatly produced, and support a pair of wide flaps of skin at the sides of the body, acting as wings, or rather as parachutes. In Sphenodon (Fig. 984) and Crocodilia (Fig. 983) each rib has connected with it posteriorly a flattened curved cartilage, the uncinute. In the Chelonia (Fig. 986) the total number of vertebras is always smaller than in the members of the other orders. The cervical ribs are small and fused with the vertebrae. The cervical and the caudal are the only regions in which the vertebrae are movable upon one another. The vertebras of the trunk, usually ten in number, are immovably united with one another by means of fibro-cartilaginous intervertebral discs. Each of the neural spines, from the second to the ninth inclusively, is flattened and fused with a flat plate of dermal origin the neural plate (Fig. 987, V\ and the row of plates thus formed constitutes the median portion of the carapace. The ribs (R) are likewise immovable ; a short distance from its origin each passes into a large bony dermal costal plate ((?), and the series of costal plates uniting by their edges form a large part of the carapace on either side of the row of neural plates. The carapace is niade up of the neural and costal plates supplemented by a row of marginal plates (Figs. 986 and 987, M) running along the edge, and nuchal (Nu) and pyyal (Py) plates situated respectively in front of and behind XTTI PHYLUM CHORDATA 345 346 ZOOLOGY the row of neural plates. The neural plates are absent in some cases (Chdodina). Po FIG. 985.— Anterior vertebrae of young Crocodile. A. atlas; Ep. axis; IS. intervertebral discs; 0. pro-atlas; Ob. neural arches-; Po. odontoid bone; Ps. •• spinous processes ; Pt. transverse processes ; R. R.^ R.'* ribs ; s. arch of--atlas ; v.. median piece of atlas ; WK. centra. (From Wicdersheim's Comparative Anatomy.) Fio. 986. — Cistudo lutaria. Skeleton seen from below ; the plastron has been removed and is represented on one side. C. costal plate ; Co. coracoid ; e. entoplastron (episUrnum); Ep. epiplastron (clavicle?); F. fibula; Fe. femur; H. humerus ; Hup. hyoplastron ; Itpp; hypoplastrou ; Jl. ilium ; Js. ischium ; M. marginal plates ; Ku. nuchal plate ; PI. pubis ; Pro. procoracoid or process of scapula ; Py. pygal plates ; R. radius ; Sc. scapula ; T. tibia ; U. ulna ; Xp. xiphiplastron. (From Zittel.) The bony elements of the plastron of the Chelonia are an anterior and median plate, or entoplastron, and four pairs of plates xin PHYLUM CHORDATA 347 which are termed in their order from before backwards epiplastra, hyoplastm, hypoplastra and xiphiplastra. The median element, probably corresponds to the episternum of other Reptiles, the first pair (epiplastra) to the clavicles, the others pro- C ^^^^^~V bably being of the same character as the abdominal ribs of the Crocodilia. The carapace of the Luth or Leather-backed Turtle (Dermatochelys or is distinguished Flo osT.-Chelone midas. Transverse section of from thnt, of the rest of skeleton. C. costal plate; £1 centrum ; M. mar- ginal plate ; P. lateral element of plastron ; R. rib ; the Order in being COm- V. expanded neural plate. (After Huxley.) posed of numerous poly- gonal discs of bone firmly united together, and in not being con- nected with the endoskeleton ; in the plastron the median bone is absent. Carapace and plastron are firmly fixed together by bony union in most instances, but sometimes the connection is ligarnentous. The sternum in the Lacertilia is a plate of cartilage with a bifid posterior continuation. In the Ophidia and Chelonia it is absent. In the Crocodilia it is a broad plate with a posterior continuation which bifurcates posteriorly. A series of ossifications — the abdominal ribs — lie in the wall of the abdomen in the Crocodilia (Fig. 983, Sta), and similar ossifica- tions occur also in the Monitors and in Sphenodon. A.S already noticed, the posterior elements of the plastron of the Chelonia are probably of a similar character. In the skull ossification is much more complete than in the Amphibia, the primary chondrocranium persisting to a considerable extent only in some Lizards and in Sphenodon ; and the number of bones is much greater. The parasphenoid is reduced, and its place is taken by the large basioccipital, basisphenoid, and pre- sphenoid. A fairly typical Lacertilian skull has been described in the case of Lacerta. Its principal characteristic features are the presence of an interorbital septum, the presence'of the epipterygoid and the mobility of the quadrate. The last of these features it shares with that of the Ophidia. The epipterygoid is not universal in the Lacertilia, being absent in the Geckos, the Amphisbasnians, and the Chamas- leons. The skull of the Chameleons has a remarkable helmet- like appearance owing to the development of processes of the squamosal and occipital regitfns, which unite above the posterior part of the cranial roof. The skull of the Amphisbenians differs from that of other Lacertilia and approaches that of Snakes in the absence of an interorbital septum. 348 ZOOLOGY SECT. In the skull of the Ophidia (Fig. 988) orbitosphenoidal and alisphenoidal elements are absent, their places being taken by downward prolongations of the parietals and frontals. In the substance of the mesethmoid are two cartilaginous tracts (Fig. 989, B, T) which are the persistent trabecula^ of the foetal skull. The interorbital septum is absent, and the cranial cavity is prolonged forwards to the ethmoidal region. The palatines (PI) are movably articulated with the base of the skull ; as in -Vo Fav Fio. £7oo.— Skull of Colubrine Snake (TropidonotUS natrix). A, from above ; B, from below. Ag. angular ; Art. articular ; Bp. basioccipital ; B*. basisphenoid ; Ch. internal nares ; Cor.c. occipital condyle ; l)t. dentary ; Eth. ethmoid ; F. frontal ; F. post-orbital ; For. Fen jnestra occipital condyle ; Dt. dentary, ovalis ; M. Maxilla ; N. nasal ; 01. exoccipital ; Osp. f upraoccipital taking the place of orbito-sphenoid ; P. parietal ; Pe. periotic ; P. f. prefrontal ; PL palatine ; Pmx. premaxilla ; Pt. pterygoid ; 01. exoccipital ; Qu. quadrate ; SA. supra-angular ; Squ. squamosal ; 7'x. transverse ; Vo. vorner ; 77, optic foramen. (From Wiedersheim's Comparative Anatomy.) the Lizards, they are widely separated from one another, and do not develop palatine plates. They are movably articulated behind withthepterygoids(P£), and the latter, through the intermediation of the slender transverse bones (Ts), with the maxillae. The premaxillae are very small (in some venomous Snakes entirely absent), and when present usually fused together. The maxillae (Mx), usually short, articulate by means of a movable hinge-point with the conjoined lacrymal and pre-frontal (La), which, in turn, is movably connected with the frontal. The long and slender quadrate (Qu) is freely articulated with the posterior end of the elongated squamosal. The rami of the mandible, likewise long and slender, XITI PHYLUM CHORDATA 349 are not united anteriorly in a symphysis, but are connected together merely by clastic ligamentous tissue, so that, when the mouth of the Snake is opened to allow of the entry of the relatively large prey, which it swallows whole, they are capable of being widely separated from one another. The Typhlopida; differ from the rest of the Ophidia in having the maxilke immobile, the quadrate more closely connected with the skull, and the rami of the mandible united by a fibre-cartilaginous symphysis. The skull of Sphenodon (Fig. 990) differs very considerably from that of the Lizards. There is a large supra-temporal fossa bounded by the parietal, post-orbital (Pt. /), and squamosal, and separated below by a bar of bone (superior temporal arch}, formed Fin. 989.— A, lateral view of skull of Rattlesnake (Crotalus). B. 0. basioccipital ; B. S. basisphenoid ; E. 0. exoccipital ; F. 0. fenestra ovalis ; La. conjoined lacryroal and pre- frontal ; L.f. articulation between lacrymal and frontal ; Mn. mandible; MX. maxilla; Nit. nasal ; PI. palatine ; Pmp. premaxilla ; P. Sph. presphenoid ; Pt. pterygoid ; QH. quadrate ; Srj. squamosal ; //, 7, foramina of exit of the second and fifth cranial nerves ; B, transverse section at point lettered B in Fig. A ; T. trabeculsw. (After Huxley.) of processes of the two last-mentioned bones and of the post- frontal, from a still larger space — the lateral temporal fossa. The latter is bounded below by a slender bony bar (the inferior temporal arch), formed of the long narrow jugal («7i^),with a small quadrato- jugal, by which the jugal is connected with the quadrate (Q). The lateral temporal fossa is separated from the orbit in front by a bar of bone formed of the jugal and post-orbital, and is bounded behind by a posterior temporal arch formed of the parietal arid squamosal. The quadrate (Qii) is immovably fixed, wedged in by the quadrato- jugal, squamosal and pterygoid. The premaxillse (Pmx) are not fused together, but separated by a suture. There is a broad palate formed by the plate-like vomers, palatines, and pterygoids. 350 ZOOLOGY SECT. XIII In the Chelonia (Figs. 991, 992) all the bones, including the quadrate, are solidly connected together. Transverse bones (ectopterygoids), lacrymals, orbitosphenoids and alispbenoids are absent. The place of alisphenoids is taken to a certain extent by vertical downward plate-like extensions of the parietals, the Pmx Pmx Prf Fin. 990.— Skull of Sphenodon. A, Dorsal ; B, ventral ; C, left-sided view of skull of Sphenodon, xf. Col. Columella auris ; Con atomy.) FIG. 99'2.— Ventral view of the skull of Chelone myda8.--&s.ibasi-sphenoid ; fr. frontal ; ;'. jugal 7/1. maxilla , ob. basioccipital ;ol. exoccipital ; op. opisthotic ; os. supraoccipital ; pal. palatine '/mi-, parietal ; ph, post-frontal ; pri'r. prefrontal ; pt. pterygoid ; prm. premaxilla ; q. quadrate f[j. quadrato-jugal ; sq. squamosal ; v. vomer. (After Hoffmann.) .352 ZOOLOGY SECT. roof formed of expansions of the post-frontals (ph) parietals (par) and squamosals (sq.) with the jugal (/) and quadrato-jugal (q.f). The immovably fixed quadrates (Fig. 992, qu, and Fig. 991, q) are modified to afford a part or the whole of the rim for the support of the tympanic membrane. The occipital condyle is triobed. The vomer (v) is unpaired. The palatines (pal) are approximated and give off palatino plates, which for a short distance cut off a nasal passage from the cavity of the mouth. Nasals are usually ab- sent as separate bones. The pre- maxillae are very small. The rami of the mandibles are stout, and are firmly united together at the symphysis. In the Crocodiles (Figs. 993, 994), as in the Chelonia, the quadrate (Qu) is firmly united with the other bones of the skull. There is a membranous and car- tilaginous interorbital septum. There are no distinct orbito- sphenoids, but alisphenoids are well developed. The orbit is separated from the lateral tem- poral fossa by a stout bar situated somewhat below the surface, and formed of processes from the post-frontal, jugal and ectoptery- goid. The lateral temporal fossa is bounded below, as in Spheno- don, by an inferior temporal arch composed of jugal and quadrato- jugal. The frontals are early united into one, and the same holds good of the parietals. An ectopterygoid is present. Both palatine (PI) and pterygoid (Pt),. as well as maxillae, develop palatine plates in the roof of the mouth, cutting off a nasal passage of great length from the cavity of the mouth, the posterior nares (ch) being situated far back towards the posterior end of the cranial base. The nature of the articulation between the mandible and the quadrate is such that movement is re- stricted to the vertical plane, and lateral displacement is further provided against by the development of a broad process of the pterygoid against which the inner surface of the mandibular FIG. 903.— Skull of Crocodilus porosus, dorsal view, x about J. Col, buttress connecting the post-frontal with the jugal and ectopterygoid ; F. frontal ; Jg. jugal ; MX. maxilla ; Na. nasal ; P. parietal ; Pm. premaxilla ; Po.f. post-frontal ; Pr.f. prefrontal ; Q. quadrate ; Qj. quadrato- jugal ; R. characteristic ridge on the prefrontal bone ; Sq. squamosal ; T. per- foration in the premaxilla caused by a pair of lower incisor teeth. (After Gadow.) XTIT PHYLUM CHORDATA rarnus plays, an arrangement which occurs also in most Lacertilia. In accordance with their purely aerial mode of respiration, the •rixri'i'al arches are much more reduced in the Reptil.ia than in the Amphibia in general. The only well-developed post-mandibular arch is the hyoid, and even this may undergo considerable reduction (Ophidia). The branchial arches, except in so far as they contribute to the formation of the laryngeal cartilages, are not represented in the adult, with the exception of most^Chelonia, in which the first and second branchial arches persist as cornua of the hyoid. There is little variation in the structure of the limb-arches and skeleton of the limbs in the different groups of Lacertilia. The pelvic arch is distinguished in the Lacertilia in general by its slender character ; and the pubes and ischia are, as in fact 'is the case throughout the class, separated from one another by wide ischio-pubic foramina — a feature which markedly distinguishes the reptilian pelvis from that of the Amphibia. In limbless forms the pectoral arch may be present or may be absent. In the Ophidia all trace of limbs is as a rule, absent ; but in some Pythons vestiges of hind-limbs are to be detected in the form of two or three small bones which support a small horny claw. In Sphenodon (Fig. 984) there is a foramen above the outer and one above the inner condyle of the hum ems. There are eleven carpal elements, of which there are four, including a pisiform, in the proxi- mal row, two centrals, and five in the distal row. The pubes are united in a symphysis, in front of which is a cartilaginous epipubis. A large oval foramen intervenes between the ischium and the pubis. A cartilaginous Tiypo-iscliium is attached to the ischia behind. In the tarsus the tibial and fibular elements are. distinct, though firmly united. The intermedium and the centrale are firmly fixed to the tibiale. There are three distal tarsal bones. In the Chclonia (Fig. U86) the intcrclavicle (episternum) and Coec FIG. 994.— Ventral view of the skull of young Crocodile. CVt, posterior nares ; C'occ. occipital con iyle ; Jg. jugal ; M. maxilla (palatine process ) ; Ob. basioccipital ; Orb. orbit; PL palatine ; Pmx. premaxillai ; Pt. pterygoid ; Qj. quadrato- jugal ; Qu. quadrate. (From Wiedershcim's Com- parative Anatomy.) 3.54 ZOOLOGY SECT. clavicles are absent, unless, as is probable, the former be represented by the median element of the plastron and the latter by the first lateral pair. The entire pectoral arch is a tri-radiate structure of which the most ventral and pos- -P7i* FIG. 995.— Tarsus of Emys europaca (right side) from above. J-\ fibula ; T. tibia ; (i.)f. t. c. the united tarsals of the proximal row ; Ph'. first phalanx of the fifth digit; 1—4. distal tarsals; I—V, metatarsals. (From Wiedersheim's Com- 2iamtive Anatomy.) terior ray, ending in a free ex- tremity, is the coracoid ; while the other two are the scapula and a process, sometimes regarded as representing the procoracoid, given off on the inner side of the scapula near its glenoid end. The bones of the carpus have nearly the typical arrangement, consist- ing, as in Lizards, of a proximal row of three, a distal row of five, and a centrale between the two. The pelvis resembles that of Lacertilia, except that it is broader and shorter. Both pubes and ischia meet in ventral symphyses, and epipubic arid hypo-ischial cartilages may be present. In the tarsus (Fig. 995) there is usually a single proximal bone and four dis- talia. There are never more than two phalanges in any of the digits. In the Grooodiiia also the clavicle is absent, but there is an episternum. The number of carpal ele- ments is reduced, the largest being two proximal bones, the radiale and the ulnare (Fig. 996, r, u.J). On the ulnar side of the latter is a small accessory bone (pisiform f). The pelvic arch (Fig. 997) differs some- what widely from that of other living Reptiles, and the parts have been variously interpreted. Two bones (P), which are usually regarded as the pubes, extend from the region of the acetabula forwards and inwards, but, though they become closely approximated anteriorly, do not meet in a symphysis. Between and in front of their anterior extremities, which are tipped with cartilage, extends a membrane (M) with which are connected in front the last pair of abdominal ribs (Jfir). The posterior ends of the pubes are cut off from the acetabulurn by the interposition of a pair of bones which may be parts of the ilia, but are separately ossified. The ischia extend downwards and somewhat backwards from the acetabula and are fixed together ventraliy (at Sy.) but Fin. 99(5.— Carpus of young Alligator. C. centrale (?) ; R. radius ; U. ulna ; r. radiale ; u. ulnare ; 1 — 5, the five distal carpals(not. yet ossified) ; 1 and ~1 united into one, and also 3, 4 and 5 ; t, pisiform ; 7 — P, the five metacarpals. (From Wiederfheim's Com- jini'ntlf'e Anatomy.) XIII PHYLUM CHORD AT A 355 there is no true symphysis, as their extremities remain cartilaginous A hypo-ischium is not present. In the tarsus (Fig. 998) there are two proximal bones — an astray (do-scaphoid and a ctdrirncum — the latter having a prominent calcaneal process, .and two distal tarsal bones, together with a thin plate of cartilage supporting the first and second metatarsals. The missing fifth digit is re- presented by a rudimentary metatarsal. vs# Digestive Organs. — The \ form and arrangement of the '. ' : us.— Tarsus of Crocodile (right side) from above. F. fibula ; T. tibia ; t i. c. the astragalus, formed of the united tibiale, intermedium and centrale ; /. tibulare (caleaneum) ; 1 — 3, united first, second and third distal tarsals ; 4, fourth tarsal ; /— / V, first to fourth metatarsals ; F?, fifth distal tarsal and fifth metatarsal. (From Wiedersheim's Comparative Anatomy.) teeth already described in the account of Lacerta prevail in the majority of Lizards. In some of them the palatine teeth are absent. The teeth are sometimes fixed by their bases to the summit of the ridge of the jaw (acrodont forms), sometimes fixed by their sides to the lateral surface of the ridge (pleurodont) ; they are never embedded in sockets in any recent form. A Mexican Lizard, Heloderma, differs from all the rest in having teeth which are grooved for the ducts of poison-glands. In the Snakes (Figs. 988, 989) teeth are rarely developed on the premaxilla3, but are present on the maxillae, palatines and pterygoids, as well as the dentary of the mandible. They may be of the same character through- out, solid, elongated sharp-pointed teeth, which are usually O. ',i'.»7.— Pelvis of young Alligator, ventral assert, tt, fibrous band passing between the pubic and ischiadic syniphyses ; lilt, last pair «i abdominal ribs; F. obturator foramen; ft. acetabulum ; II. ilium ; Is. ischium ; M. fibrous membrane between the anterior ends of the two innominate bones and the last pair of abdominal ribs ; P. pubis ; Sy. ischiadic symphysis ; 1, II, first and second sacral vertebrae. (From Wiedersheim's Comparative Anatomy.) 356 ZOOLOGY SECT. strongly recurved, so that they have the character of sharp hooks, their function being rather to hold the prey and prevent it slipping from the mouth while being swallowed than to masticate it. Non-venomous Snakes possess only teeth of this character. In the venomous Snakes more or fewer of the maxillary teeth assume the character of poison-fangs. These are usually much larger than the ordinary teeth and either grooved or perforated by a canal for the passage of the duct of the poison-gland. In the Vipers (Fig. 989) there is a single large curved poison-fang with small reserve-fangs at its base, these being the only teeth borne by the maxillae, which is very short ; in the venomous Colubrine Snakes the poison-fangs are either the most anterior or the most posterior of a considerable range of maxillary teeth. In the Vipei s the large poison-fang is capable of being rotated through a considerable angle, and moved from a nearly horizontal position, in which it lies along the roof of the mouth embedded in folds of the mucous membrane, to a nearly vertical one, when the Snake opens its mouth to strike its prey. The rotation of the maxilla is brought about by the backward or forward move- ment of the pterygoid with the palatine and transverse. In Sphenodon (Fig. 990) there are pointed, triangular, laterally- compressed teeth, arranged in two parallel rows, one along the maxilla, the other along the palatine. The teeth of the lower jaw, which are of similar character, bite in between these two upper rows, all the rows becoming worn down in the adult in such a way as to form continuous ridges. Each premaxilla bears a prominent, chisel-shaped incisor, represented in the young animal by two pointed teeth. In the young Hatteria a tooth has been found on each vomer — a condition exceptional among Reptiles. In the Chelonia, teeth are entirely absent, the jaws being invested in a horny layer in such a way as to form a structure like a Bird's beak. The Orocodilia have numerous teeth which are confined to the pre-maxillae, the maxilla, and the dentary. They are large, conical, hollow teeth devoid of roots, each lodged in its socket or alveolus (thecodont), and each becoming replaced, when worn out, by a successor developed on its inner side. A bifid tongue like that of Lacerta occurs in several families of Lacertilia. Others have a thick, short tongue, undivided in front and often provided with two long appendages behind. The Monitors (Fig. 999, A) have forked retractile tongues like those of Snakes. The tongue of the Chameleons is an extremely remark- able organ ; it is of sub-cylindrical form with an enlarged extremity, and is so extensile that it is capable of being darted out to a distance sometimes equalling, or even exceeding, the length of the trunk ; this protrusion can be effected with lightning-like rapidity ; and it is in this way that the animal catches the Insects which Mil PHYLUM CHORDATA 357 constitute its food. The tongue in Snakes is slender and bifid, capable of being retracted into a basal sheath, and highly sensi- tive, being used chiefly as a tactile organ. The tongue of the Crocodilia (C) is a thick, immobile mass extending between the rami of the mandible. In some of the Chelonia (B) the tongue is immobile ; in others it is protrusible, sometimes bifid. In the enteric canal of the Reptiles the principal special features to be noticed are the muscular gizzard-like stomach of the Crocodilia, the presence of a rudimentary cascum at the junction of small and large intestines in most Lacertilia and in the Ophidia, ,. i in1. i.— A, tongue of Monitor indicus. B, tongue of Emys europaea. C, tongue of Alligator. L, glottis ; M, mandible ; Z, tongue ; ZS, tongue -sheath. (From Wiedershehn's Comparative Anatomy.) and the presence of numerous large cornified papillaB in the oesophagus of the Turtles. Organs of Respiration. — The Reptiles all have an elongated trachea, the wall of which is supported by numerous cartilaginous rings. The anterior part of this is dilated to form the larynx, the wall of which is supported by certain special cartilages — the cricoid and the arytenoids. The trachea bifurcates posteriorly to form two bronchi, right and left, one passing to each lung. In some of the Chelonia its lumen is divided internally by a vertical septum. The lungs of the Lacertilia and Ophidia are of the simple sac-like character already described in the case of the Lizard. In some the lung is incompletely divided internally into two portions — an anterior respiratory part with sacculated walls, and a posterior part with smooth, not highly vascular walls, having mainly the func- tion of a reservoir. The only additional complication to be speci- ally noted is the presence in the Chanueleons (Fig. 1000) of a VOL. n z 358 ZOOLOGY SEC'T. number of diverticula or air-sacs which are capable of being inflated, causing an increase in the bulk of the animal which doubtless has an effect on assailants. In the snake-like Lizards the right lung is larger than the left, and in the Amphisbaenians the latter is entirely aborted. In the Snakes a similar reduction or abortion of the left lung is observable. In the Crocodilia and Chelonia the lungs are of a more complex character, -As -Ac FIG. 1001.— Heart of Lacerta muralis, ventral view. A, A. auricles ; A p. pulmonary artery ; As, As', subelaviaii arteries ; C'i, post-caval ; J. jugular vein ; Ra, aortic arches (made up on cither side of two embrj-unic arches, 1 and :.') ; V. ventricle ; Vp, pulmonary vein ; Vs, sub- clavian vein. (From Wiedersheirn's Comparative Anatomy.) being divided internally by septa into a number of chambers. Organs of Circulation. — In the heart (Fig. 1001) the sinus venosus is always distinct, and is divided into two parts b}r a septum ; its aperture of communication with the right auricle is guarded by valves. There are, as in the Amphibia, al- ways two quite distinct auricles, the right receiving the venous blood from the body, the left the oxygenated blood brought from the lungs by the pulmonary veins. But a vital point of difference between FIG. 1000. — Lungs of Chamceleon. T. trachea. (From Wiedersheirn's Com- parative Anatomy.) XIII PHYLUM CHORDATA 359 FIG. 1002. -Diagram of heart of Turtle. a, incomplete ventricular septum ; C. p. cavum pulmonale ; C.v. cavum venosum ; L. A. left auricle ; L. Ao. left aortic arch ; P. A. pulmonary artery ; R. A. right auricle ; R. Ao. right aortic arch ; s, arrow showing the course of blood in left aorta ; t, in right aorta ; v. v'. auriculo-ventricular valves ; w, arrow showing the course of blood in left auriculo-ventricular aperture ; .»:, in right ; y, between cavum venosum and cavum pulmonale ; z, in pulmonary artery. (After Huxley.) the heart of the Reptile and that of the Amphibian is that in the former the ventricle is always more or less completely divided into right and left portions. In all the JMo Lacertilia, Ophidia and Chelonia (Fig. 1002) the structure is essen- tially what has been described in Lacerta, the ventricular septum being well-developed, but not com- pletely closing off the left-hand portion of the cavity of the ven- tricle from the right (cavum pul- monale). The left-hand portion, which is much the larger, is further imperfectly divided into two parts —the cavum arteriosum on the left and the cavum venosum on the right — by the two elongated flaps of the auriculo-ventricular valve, which project freely into the cavity of the ventricle. From the cavum pulmonale arises the pulmonary artery, and from the cavum venosum, the right and left aortic arches. When the auricles contract the cavum venosum becomes filled with venous blood from the right auricle, the cavum arteriosum with arterial blood from the left auricle ; the cavum pulmonale be- comes filled with venous blood which flows into it past the edges of the incom- plete septum. When the ventricle contracts, its walls come in con- tact with the edge of the septum, and the cavum pulmonale is thus cut off from the rest of the ventricle. The further contrac- tion consequently re- sults in the venous blood of the cavum pulmonale being driven out through the pulmonary artery to the lungs, while the blood which remains in the ventricle (arterial and mixed) is compelled to pass out through the z 2 r.au.r. i> •& fit. a. FIG. 1003.— Heart of Crocodile with the principal arteries (diagrammatic). The arrows show the direction of the arterial and venous Currents. I. aort. left aortic arch ; l.aur. left auricle; l.aur. vent.ap, left auriculo-ventri- cular aperture ; I. car. left carotid ; L sub. left subclavian ; /. r, ,>t. left ventricle ; put. art. pulmonary artery ; r. aort. right aortic arch; '/-. aur. right auricle; r.aur.vent.ap. right auriculo-ventricular aperture ; r. car. right carotid ; r.ntft. sight subclavian; r.vent. right ventricle. (From Hertwig s Lehrbuch.) 360 ZOOLOGY SECT. aorta. But in the Crocodilia (Fig. 1003) the cavity is com- pletely divided, so that there we may speak of distinct right and left ventricles. From the right arises the pulmonary artery and the left aortic arch : from the left the right aortic arch only. The right and left arches cross one other, and where their walls are in contact is an aperture — the foramen Panizzce — placing their cavities in com- munication. The brain of Reptiles is some- what more highly organised than that of the Amphibia. The brain substance exhibits a distinction into superficial grey layer or cor- tex containing pyramidal nerve- cells, and central white medulla, not observable in lower groups. The cerebral hemispheres are well developed in all, and there is a hippocampus (see below in the description of the brain of the Rabbit, and of that of the Mam- mals in general) in the shape of a specially modified region of the dorsal and mesial walls of each hemisphere, represented less dis- tinctly in the Amphibia ; a com- missure— the hippocampal — con- nects the hippocampi of opposite sides, and is dorsal to the chief cerebral commissure — the an- terior commissure. The mid-brain consists usually of two closely- approximated oval optic lobes ; rarely it is divided superficially into four. The cerebellum is always of small size, except in the Crocodilia (Fig. 1004), in which it is comparatively highly developed, and consists of a median and two lateral lobes. Sensory Organs. — In most Lacertilia, but not in the Ophidia, the nasal cavity consists of two parts — an outer or vestibule, and an inner or olfactory chamber — the latter having the sense-cells in its walls, and containing a turbinal bone. In the Turtles each nasal chamber is divided into two passages, an upper and a lower, Med FIG. 1004.— Brain of Alligator, from above. B. ol. olfactory bulb ; G. p, epi- physis ; HH, cerebellum ; Med, spinal cord ; MH, optic lobes ; NH, medulla oblongata ; VH, cerebral hemispheres ; /— XI, cerebral nerves; ], 2, first and second spinal nerves. (From Wieder- shcim's Comparative Anatomy.) xm PHYLUM CHORDATA 361 and the same holds good of the hinder part of the elongated nasal chamber of the Crocodilia. Jacobson's organs (Fig. 97*2) are present in Lizards and Snakes, absent in Chelonia and Crocodilia in the adult condition. The eyes are relatively large, with a cartilaginous sclerotic in which a ring of bony plates (Fig. 973) is developed in some cases. The muscular fibres of the iris are striated. A pecten is present in most. Most Reptiles have both upper and lower eyelids and nicti- tating membrane. The greater number of the Geckos and all FIG. 1005.— Section of the pineal eye of Sphenodon punctatum. g, blood-vessels ; h, cavity of the eye filled with fluid ; A-, capsule of connective-tissue ; /. lens ; m. molecular layer of the retina ; r. retina ; st. stalk of the pineal eye ; x, cells in the stalk. (From Wiedersheim's Comparative Anatomy, after Baldwin Spencer.) the Snakes constitute exceptions, movable eyelids being absent in both of these groups ; in the former the integument passes uninter- ruptedly over the cornea with a transparent spot for the admission of the light ; in the Snakes there is a similar modification, but the study of development shows that the transparent area is derived from the nictitating membrane which becomes drawn over the cornea and permanently fixed. In the Charnaeleons there is a single circular eyelid with a central aperture. 362 ZOOLOGY SECT. The middle ear is absent in the Snakes, though a columella auris is present, embedded in muscular and fibrous tissue and attached externally, in some cases at least, to the middle of the quadrate. Developed in close relation to the epiphysis there is in many Lizards (Lacerta, Varanus, Anguis, Amphibolurus and others) and in Sphenodon, a remarkable eye-like organ — the parietal organ or pineal eye (Fig. 1005), which is situated in the parietal foramen of the cranial roof immediately under the integument, and covered over by a specially modified, transparent scale. The pineal eye is developed from a hollow outgrowth of the roof of the diencephalon in front of the epiphysis ; the distal end of this becomes constricted off as a hollow sphere while the remainder is converted into a nerve. The wall of the hollow sphere becomes divergently modi- fied on opposite sides; the distal side gives rise to a lens-like thickening (/), the proximal forms a membrane several layers in thickness — the retina, (r.) : the whole is enclosed in a capsule of connective-tissue (k). The nerve degenerates before the animal reaches maturity, so that the organ would appear — though evidently, from its structure, an organ of sight — to have now entirely or nearly lost its function. Reproductive Organs. — The description already given of the reproductive organs of the Lizard (p. 332) applies, so far as all the leading features are concerned, to all the Lacertilia and to the Ophidia ; in Hatteria the copulatory sacs are absent. In the Crocodilia and Chelonia, instead of the copulatory sacs there is a median solid penis attached to the wall of the cloaca, and a small process or clitoris occurs in a corresponding position in the female. Though fertilisation is always internal, most Reptilia are oviparous, laying eggs enclosed in a tough, parchment-like or calcified shell. These are usually deposited in holes and left to hatch by the heat of the sun. In the Crocodiles they are deposited in a rough ne?t and guarded by the mother. In all cases development has only progressed to a very early stage when the deposition of the eggs takes place, and it is only after a more or less prolonged period of incubation that the young, fully formed in every respect, emerge from the shell and shift for themselves. Many Lizards, however, and most Snakes are viviparous, the ova being developed in the interior of the oviduct, and the young reaching the exterior in the completely-formed condition. Development. — In all the Reptilia the segmentation is meroblastic, being confined to a germinal disc of protoplasm situated on one side of the yolk. This divides to form a patch of cells which gradually extends as a two-layered sheet, the blasto- derm, over the surface of the ovum. The upper of the two layers is the ectoderm, the lower the yolk-endoderm ; the latter is the equivalent of the mass of yolk-cells of the Frog, and the shallow XIII PHYLUM CHORDATA 363 space between it and the yolk represents the segmentation-cavity. As the blastoderm extends (Fig. 1006) it becomes distinguishable into a central clearer area — areapellucida (a. pel.) — and a peripheral whitish zone — area o-paca (a. op.). On the former now appears an Fio. 1006.— A— D, early stages in the development of the Alligator. A, stage with em- bryonic shield, primitive knot and blastopore ; B, considerably later stage in which the medullary groove has become formed, together with the head-fold of the embryo and the head-fold of the amnion ; C, somewhat later stage with well-developed medullary folds and medullary groove ; D, later stage in which the medullary groove has become partly closed in by the medullary folds and in which six pairs of protovertebrse have become developed. amn. amnion ; a. op. area opaca ; a. pel. area pellucida ; Up. blastopore ; emb. s. embryonic shield; f.br. fore-brain; h.br, hind-brain; hd.f. head-fold; rn. br. mid-brain; med. medullary folds ; prot. v. protovertebrse ; pr.st. primitive knot. (After S. F. Clarke.) elliptical thickened patch — the embryonic shield (emb. s.) — which is formed by the ectoderm cells in this region assuming a cylindrical form while remaining flat J elsewhere. Behind the embryonic shield appears a thickening, due to a proliferation of the ectoderm 364 ZOOLOGY cells — the so-called primitive knot (pr. .y their overlap, and the body is thus completely covered with a thick, very light, and non-conducting investment. AJII PHYLUM CHORD AT A 385 In the wings and tail certain special arrangements of the feathers are to be distinguished. When the wing is stretched out at right angles to the trunk, twenty-three large feathers (Fig. 1022) are seen to spring from its hinder or post-axial border : these are the remiges or wing-quills. Twelve of them are connected with the ulna and are called cubitals or secondaries (cb. rmg). The rest are known as primaries: seven of these are attached to the m eta- carpal region, and are hence called metacarpals (mtcp. rmg.\ the remaining four or digitals to the phalanges of the second and third digits. These are again distinguished into a single ad- digital (ad. dg. rmx.) connected with the single phalanx of the third digit (Fig. 1034, ph.o], two mid-digitals (md. dg. rmg.) with the proximal phalanx of the second digit (Fig. 1034, ph.2\ and two pre-digitals (pr.dg.rmg.) with its distal phalanx (Fig. 1034, ph.2'). A special tuft of feathers on the anterior border of the wing, arising from the pollex (Fig. 1034, ph.l), forms the a/ a spuria (al. sp.). The spaces which would otherwise be left between the bases of the remiges are filled in, both above and below, by several rows of upper and under wing-coverts. In the tail there are twelve long rectrices (Fig. 1022, ret.) or tail-quills, springing in a semicircle from the uropygium ; their bases are covered, as in the wing, by upper and under tail-coverts. The whole feather- arrangement is known as the pterylosis. Endoskeleton. — The vertebral column is distinguished from that of most other Craniata by the great length and extreme mobility of the neck, the rigidity of the trunk-region, and the short- ness of the tail. As in Reptilia, the cervical passes almost insensibly into the thoracic region, and the convention is again adopted of counting as the first thoracic (Fig. 1027, th.v. 1\ the first vertebra having its ribs united with the sternum. There are fourteen cervical vertebrae, the last or last two of which have double-headed ribs (cv. r.), each having its proximal end divisible into the head proper articulating with the centrum of the vertebra, and a tubercle with the transverse process : their distal ends are free, not uniting with the sternum. In the third to the twelfth there are vestigial ribs (Fig. 1028, rb.\ each having its head fused with the centrum, and its tubercle with the transverse process. The whole rib thus has the appearance of a short, back ward ly-directed transverse pro- cess perforated at its base ; the perforation transmits the vertebral artery, and is called the vertebrarterial foramen (vrb.f.) The centra of the cervical vertebraB differ from those of all other Vertebrata in having saddle-shaped surfaces, the anterior face (Fig. 1028, A) being concave from side to side and convex from above downwards, the posterior face (B) convex from side to side and concave from above downwards. Thus the centrum in sagittal section appears opisthocoelous, in horizontal section proccelous. This peculiar form of vetebra is distinguished as heterocodous. 386 ZOOLOGY SECT. The centra articulate with one another by synovial capsules each traversed by a vertical plate of cartilage, the meniscus, with a 3t FIG. 1027. — Columbalivia. The bones of the trunk, acr.cor. acrocoracoid ; a. A Fin. 1031.— Sagittal section of a Bird '8 skull (diagrammatic). Replacing bones— AXi.SFH. alisphe- noid; ART. articular ; B.OC. basioccipital ; B.SPH. basisphenoid ; EP.OT. epiotic; EX.OC. exoccipital; 1VI.ETH. mesethmoid; OP.OT. opisthotic ; ORB.SPH. orbito- sphenoid ; PR.OT. pro-otic ; QU. quadrate ; S.OC. supraoccipital. Investing bones — ANG. angular; B. I'M P. basi- temporal ; COR. coronary; DNT. dentary ; FR. frontal ; JU. jugal ; LCR. lacrymal ; MX. maxilla ; NA. nasal ; PA. parietal ; PAL. palatine ; PMX. pre- maxilla; PTG. pterygoid ; QU. JU. quadrate- jugal ; RS T. rostrum ; S. ANG. supra-angular; SPL. splenial ; SQ. squamosal ; VO. vomer. fie. fos. floccular fossa; nix. pal. pr. rnaxillo palatine process ; opt. for. optic foramen ; orb. pr. orbital process ; ot. pr. otic process ; pty.fos. pituitary fossa. lacrymals (Fig. 1030, lc.t Fig. 1031, LOR), and the nasals (na, na', na", NA) are forked bones each furnishing both an inner and an outer boundary to the corresponding nostril. The premaxillse (p.mx., PMX) are united into a large triradiate bone which forms practically the whole of the upper beak. The maxillae (mx., MX), on the other hand, are small, and have their anterior ends produced inwards into spongy maxillo- palatine pro- cesses (Wig. 1030, mx.p., Fig. 1031, mx.pal.pr). The slender posterior end of the maxilla is continued backwards by an equally slender jugal (ju., JU) and quadra to-jugal (QU. JU) to the quadrate. The latter (qu., QU.) is a stout, three-rayed bone articulating by two facets on its otic process (ot. pr.) with the roof of the tympanic VOL. II B B 390 ZOOLOGY SECT. cavity, sending off an orbital process (orb. jpr.) from its anterior mar- gin, and presenting below a condyle for articulation with the man- dible ; it is freely movable upon its tympanic articulation, so that the lower jaw has a double joint as in Lizards and Snakes. The palatines (pi, PAL.) have their slender anterior ends anky- losed with the maxilla, their scroll-like posterior ends articulating with the pterygoids and the rostrum. The pterygoids (pt.} PTG). are rod-shaped and set obliquely ; each articulates behind with the quadrate, and, at about the middle of its length, with the basi- pterygoid process, a small facetted projection of the base of the rostrum. There is no vomer in the Pigeon. The mandible of the young Bird consists of a replacing bone, the articular (ar., ART.), and four investing bones, the angular (an., ANG-.), supra-angular (s.an., S.ANG.), dentary (d., I>NT.), and splenial (SPL.), all having the same general hh i relations as in the Lizard. The ky oid-apparatus (Fig. 1032), is of characteristic form, having an arrow- shaped body (b. ky.) with a short pair b.br.z s.sl :_ ..._ c br st e.st. i. sb. e/b.br FIG. 1033.— Columba livia. The colurnclla auris (magnified). The cartilaginous parts are dotted. e. st. extra-stapedial ; i. st. infra-stapedial ; s. st. supra-stapedial ; st. stapes. (From Parker's Zootomy.) FIG. 1032.— Columba livia. Hyoid apparatus. The cartilaginous parts are dotted, b. br. 1, basi-branchials ; b.hy. basi-hyal ; c.br. cerato-branchial; c. hy. hyoid cornu ; ep. br. epi- branchiaL of anterior cornua (c. ky.) derived from the hyoid arch, and a long pair of posterior cornua (c.br., ep.br.) from the first branchial. The columella (Fig. 1033) is a rod- shaped bone ankylosed to the stapes, and bearing at its outer end a three- rayed cartilage or extra-columella (e.st , i.st., s.st.) fixed to the tympanic membrane. The shoulder-girdle (Fig. 1027) is quite unlike that of other Craniates. There is a pair of stout, pillar-like coracoids (cor.) articulating with deep facets on the anterior border of the sternum, and directed upwards, forwards, and outwards. The dorsal end of each is produced into an acro-coracoid process (acr. cor.), and below this, to the posterior aspect of the bone, is attached by ligament a sabre-shaped scapula (scp.) which extends backwards over the ribs, and includes, with the coracoid, an acute angle, the coraco-seapular XIII PHYLUM CHORD ATA 391 angle. The glenoid cavity (gl. cv.) is formed in equal proportion by the two bones ; internal to it the scapula is produced into an acromion process. In front of the coracoids is a slender V-shaped bone, thefurcula (fur.) or " merrythought," the apex of which nearly reaches the sternum, while each of its extremities is attached by ligament to the acromion and acro-coracoid processes of the corresponding side in such a way that a large aperture, the foramen triosseum (f. trs.)t is left between the three bones of the shoulder- girdle. The furcula is an in- vesting bone and represents fused clavicles and inter- clavicle. Equally characteristic is the skeleton of the fore-limb. The humerus (Fig. 1034, hu.} is a large, strong bone, with a greatly expanded head and a prominent ridge for the in- sertion of the pectoral muscle. In it, as in all the other long bones, the extremities as well as the shaft are formed of true bone. The radius (ra.) is slender and nearly straight, the utna stouter and gently curved. There are two large free carpals, a radiate (ra!) and an ulna-re (ul.1), and articu- lating with these is a bone called the carpo-metacarpus (cp.nitcp.) consisting of two rods, that on the pre-axial side strong and nearly straight, that on the postaxial side slender and curved, fused with one another at both their proximal and distal ends ; the proximal end is produced, pre-axially, into an outstanding step-like process. The study of development shows that this bone is formed by the union of the distal carpals with three metacarpals (Fig. 1035), the second and third of which are the two rod-like portions of the bone, the first the step-like projection. Articulating with the first metacarpal is a single pointed phalanx (ph. 1) ; the second B B 2 FIG. 1034.— Columba livia. Skeleton of the left wing, cp.mtcp. carpo-metacarpus ; hu. ; humerus ; ph. 1, phalanx of first digit ; ph. 2', ph. f", phalanges of second digit ; ph.3, phalanx of third digit ; pn.for. pneumatic foramen. ra. radius ; ra'. radiale ; ul. ulna ; ul'. ulnare. 392 ZOOLOGY SECT. metacarpal bears two phalanges, the proximal one (ph.%') produced postaxially into a flange, the distal one (ph.®") pointed ; the third metacarpal bears a single pointed phalanx (ph. S). The pelvic girdle (Fig. 1027) resembles that of no other Vertebrate with the exception of some Dinosaurs. The ilium (il.) is an immense bone attached by fibrous union to the whole of the synsacrum and becoming ankylosed with it in the adult. It is divisible into pre- acetabular and post-acetabular portions of approximately equal size. As usual it furnishes the dorsal portion of the acetabulum, and on the posterior edge of that cavity is produced into a process, the antitrochanter (ci.tr.), which works against the trochanter, a process of the femur. The ventral portion of the acetabulum is furnished in about equal proportions by the pubis and ischium (Fig. 1030) : it is not completely closed by bone, but is perforated by an aperture covered by membrane in the recent state. Both pubis and ischium are directed sharply backwards from their dorsal or acetabular ends. The ischium (is.) is a broad bone, ankylosed posteriorly with the ilium, and separated from it in front by an ischiadic foramen (Fig. 1027, is. for. ;Fig. 1036, i.s.f.). T\\Q piibis (pu.) is a slender, curved rod, parallel with the ventral edge of the ischium, and separated from it by an obturator notch (Fig. 1027, oU.n. ; Fig. 1036, ob.f.). Neither ischium nor pubis unites ven- trally with its fellow to form a sympbysis. In the hind-limb the /<5WWM'(Fig.l037,/«.)is a comparatively short bone. Its proximal ex- tremity bears a pro- minent trochanter (tr) and a rounded head (hd.), the axis of which is at right angles to the shaft of the bone ; so that the femur, and indeed the whole limb, lies in a plane parallel with the FIG. 1035.— Columbalivia. Lert manus of a nestling. The car- , tilaginous parts are dotted. cp. 1, radiale ; cp. 2, ulnarc ; mcp. 1, 3, 3, metacarpals ; ph. 1, phalanx of first digit ; ph. 2, ph. 3', -phalanges of second digit ; ph. 3, phalanx of third digit ; ra. radius; ul. ulna. (From Parker's Zootomy.) il a.tr ,-s.f. 'S pit Fir,. 103(3.— Columba livia. Left innominate of a nest- ling. The cartilage is dotted, etc. acetabulum ; a. tr. anti-trochanter ; il. pre-acetabular, and if. post-aceta- bular portion of ilium; is. ischium ; i. s. f. ischiadic foramen; ob.f. obturator notch; pu. pubis. (From Parker's Zootomy.) XIII PHYLUM CHORDA.TA 393 fid sagittal plane of the trunk, and is not directed outwards as in Reptiles. Its distal end is produced into pulley -like condylcs. There is a small sesamoid bone (i.e., a bone developed in a tendon), the patella (pat.), on the extensor side of the knee-joint. Articulating with the femur is a very long bone, the tibio- tarsus (ti.ts.) produced on the anterior face of its proximal end into a large cnemial process (cn.pr.) for the insertion - of the extensor muscle of the thigh. Its proximal articular surface is slightly hollowed for the condyle of the femur, its distal end is pulley-like, not concave like the corresponding extremity of the fiat cn.pi ti.ls — -/*' mtli ph.* FIG. 103S.-Columba livia. Part of left foot of an un- hatched embryo (magnified). The cartilage is dotted. mtl. %, second, mtl. £>, third, and mtl. k, fourth meta- tarsal ; li. tibia ; tl. 1, proximal tarsal cartilage ; tl. 2, distal tarsal cartilage. (From Parker's Zootomy.) tibia of other Arnniota. The study of development shows that the pulley- like distal end of the bone (Fig. 1038, tl.l) consists of the proximal tarsals — astragalus and calcaneum — which at an early period unite with the tibia and give rise to the compound shank- bone of the adult. The fibula (fi.) is very small, much shorter than the tibia, and tapers to a point at its distal end. Following the tibio-tarsus is an elongated bone, the tarso-metatarsus (Fig. 1037, ts. mtts), presenting at its proximal end a concave surface for the tibio-tarsus, and at its distal end three distinct pulleys for the articulation of the three forwardly- Fio. 1037.— Columba livia. Bones of the left hind-limb, m. ///•. cnemial process ; fe. femur ; fi. fibula ; hd. head of femur ; mtts. 1, first metatarsal ; pat. patella ; ph.l, phalanges of first digit ; ph.U, phalanges of fourth digit ; ti. ts. tibio-tarsus ; ts. mtts. tarso-meta- tarsus : tr. trochaiiter. 394 ZOOLOGY SEPT. directed toes. In the young Bird the proximal end of this bone is a separate cartilage (Fig. 1038, tl.2), representing the distal tarsals, and followed by three distinct metatarsals, belonging respectively to the second, third, and fourth digits. Thus the ankle-joint of the bird is a meso-tarsal joint, occurring, as in the Lizard, between the proximal and distal tarsals, and not, as in Mammals (q.v.), between the tibia and the proximal tarsals. To the inner or pre-axial side of the tarso-metatarsus, near its distal end, is attached by fibrous tissue a small irregular bone, the first rnetatarsal (mtts. f). The digits have the same number of phalanges as in the Lizard, the backwardly-directed hallux two, the second or inner toe three, the third or middle toe four, and the fourth or outer toe five. In all four digits the distal or ungual phalanx is pointed and curved, and serves for the support of the horny claw. It will be observed that every part of the Bird's skeleton presents characteristic and indeed unique features. The vertebral column, the skull, the sternum, the ribs, the limb-girdles, and the limbs themselves are all so highly specialised that there is hardly a bone, except the phalanges of the toes and the free caudal vertebrae, which could possibly be assigned to any other Vertebrate class. A further peculiarity is the fact that the larger proportion of the bones contain no marrow, but are filled during life with air, and are therefore said to be pneumatic. The cavities of the various bones open externally in the dried skeleton by apertures called 'pneumatic foramina (Fig. 1034, pn.fr.), by which, in the entire bird, they communicate with the air-sacs (vide infra). In the Pigeon the bones of the fore-arm and hand, and of the leg, are non- pneumatic. Muscular System. — As might naturally be expected, the muscles of the fore-limb are greatly modified. The powerful downstroke of the wing by which the bird rises into, and propels itself through the air, is performed "bytlnepectoralis (Fig. 1039,_pc£.), an immense muscle having about one-fifth the total weight of the body ; it arises from the whole of the keel of the sternum (car. st.\ from the posterior part of the body of that bone (cp.st.\ and from the clavicle (cl.), filling nearly the whole of the wedge-shaped space between the body and the keel of the sternum and forming what is commonly called the "breast" of the Bird. Its fibres converge to their insertion (pet.") into the ventral aspect of the humerus (ku., huf.) which it depresses. The elevation of the wing is per- formed, not, as might be expected, by a dorsally placed muscle, but by the subclavius (sb. civ.), arising from the anterior part of the body of the sternum, dorsal to the pectoralis, and sending its tendon (sb. civ.) through the foramen triosseum to be inserted into the dorsal aspect of the humerus. In virtue of this arrange- ment, the foramen acting like a pulley, the direction of action of the muscle is changed, the backward pull of the tendon XITI PHYLUM CHORDATA 395 raising the humerus. There are three tensorcs patagii (tns. Ig., tns. br., tns. ace.), the action of which is to keep the pre-patagium tensely stretched when the wing is extended. A similar muscle (tns. m.p). acts upon the post-patagium. The muscles of the digits are naturally much reduced. The muscles of the neck and tail are well developed ; those of the back are practically atrophied, in correspondence with the im- mobility of that region. In the leg certain of the muscles are modified to form the -perching mechanism. The toes are flexed by two sets of tendons, deep and superficial. The deep ten- dons of the three forwardly-directed digits are formed by the pet $~Mtn*4 s.br tna.acc I cxt.cjy.rd 1 ^ — ""^E^V / /''£-• t^^^n ^f/^^ Fm. 1039.— Coluxnba livia. The principal muscles of the left wing ; the greater part of the pectoralis (pet.) is removed, car. st. carina sterni ; cl. furcula ; cor. coracoid ; cor. br. br. coraco- brachialis brevis ; cor. br. Ig. coraco-brachialislongus ; cp. st. corpus sterni ; ext. cp. rd. extensor carpi radialis ; ext.cp. ul. extensor carpi ulnaris ; fl. cp. ul. flexor corpus sterni ; gl. c. glenoid cavity ; hu. head of humerus ; hu'. its distal end ; pet. pectoralis ; pet', its cut edge ; pet", its insertion ; prn. br. pronator brevis ; prn. Ig. pronator longus ; pr. ptgm. pre-patagium ; jit. ptgm. post-patagium ; sb. civ. subclavius ; sb. civ', its tendon of insertion passing through the foramen triosseum, and dotted as it goes to the humerus ; tns. ace. tensor accessorius ; tns. br. tensor brevis ; tns. Ig. tensor longus ; tns. m. p. tensor membranee posterioris alse. trifurcation of the tendon of a single muscle, the peronceus medius, that of the hallux is derived from a separate muscle, the flexor perforans, which is joined by a slip from the peronseus medius. Thus a pull upon one tendon flexes all the toes. When the leg is bent, as the bird settles to roost, the flexion of the tarso- metatarstis on the shank puts the flexor tendons on the stretch as they pass over the mesotarsal joint, and by the pull thus exerted the toes are automatically bent round the perch by the simple action of flexing the leg. They are kept in this position while the Bird is asleep by the mere weight of the body. The action is assisted by a small but characteristic muscle, the ambiens, which 396 ZOOLOGY SECT. . i.orb.sjy arises from the pubis, passes along the inner surface of the thigh, and is continued into a long tendon which comes round to the outer side of the knee, enclosed in a special sheath, and, continuing down the leg, joins the superficial flexors of the digits. Digestive Organs. — The mouth (Fig. 1040) is bounded above and below by the horny beaks, and there is no trace of teeth. The tongue (tng) is large and pointed at the tip. The pharynx leads into a th.V.f lh.v.s wVttP# '*jsr"6 r.t/nl fed FIG. 1040.— Columba liyia. Dissection from the right side. The body-wall, with the vertebral column, sternum, brain, &c., are in sagittal section ; portions of the gullet and crop are cut away and the cloaca is opened ; nearly the whole of the ileum is removed, and the duodenum is displaced outwards, a. ao. aortic arch ; M. 1, bd. 2, bile-ducts ; b.fabr. bursa Fabricii ; cbl. cerebellum ; ecu. right caecum ; cpdm. coprodseum ; cr. cere ; crb. h. left cerebral hemi- sphere ; crp. crop ; cr. v. 1, first cervical vertebrae ; di.cce. diaccele ; dnt. dentary ; dvn. duodenum ; eus. ap. aperture of Eustachian tubes ; giz. gizzard (dotted behind the liver) ; gl. glottis ; gul. gullet ; Urn. ileum ; i. orb. sp. inter-orbital septum ; M. right kidney ; Ing. right lung ; Ir. liver (right lobe) ; na. bristle passed from nostril into mouth ; obi. xep. oblique septum ; ol. g. oil-gland ; pcd. pericardium ; pinx. premaxilla ; pn. pancreas ; pn. b. pineal body ; pnd. 1 — 3, pancreatic ducts ; pr. cv. right pre-caval ; prdm. proctodseum ; prvn. proventriculus (dotted behind liver) ; pt. cv. post-caval ; pty. b. pituitary body ; p't/ft. xt. spygostyle ; r. a«. right auricle ; r. br. right bronchus ; ret. rectum ; r. vnt. right ventricle ; p. cd. spinal cord ; spl. spleen (dotted behind liver) ; s. rhb. sinus i-homboidalis ; s. scr. syn- sacrum ; st. carina sterni ; syr. syrinx ; th. v. 1, first, and th. v. 5, fifth thoracic vertebra ; tng. tongue ; tr. trachea ; ts. right testis ; ur. aperture of left ureter ; urdm. urodseum ; r. 0,gl.) is situated just behind the root of the tongue, and leads into the larynx, which is supported by cartilages — a cricoid divided into four pieces, and paired arytenoids — but does not, as in other Vetebrates, function as the organ of voice. The anterior part of the trachea (tr.) has the usual position, ventral to the gullet; but further back it is displaced to the left by the crop, becoming ventral once more 398 ZOOLOGY SECT. as it enters the body-cavity, where it divides into the right (r. br.) and left bronchi. The rings supporting the trachea are not cartilaginous but bony, as also is the first ring of each bronchus, those of the trachea completely surrounding the tube, those of the bronchi incomplete mesial ly. At the junction of the trachea with the bronchi occurs the characteristic vocal organ, the syrinx (syr.)} found in no other class. The last three or four rings of the trachea (Fig. 1041, tr.), and the first or bony half-ring of each bronchus (br.), are modified to form a slightly dilated chamber, the tympanum, the mucous membrane of which forms a cushion-like thickening on each side. At the junction of the bronchi a bar of cartilage, the pessulus, extends dorso-ventrally and sup- 8y f* ^i ports an inconspicuous fold of mucous mem- brane, the membrana semilunaris. The mem- branous inner walls of the bronchi form the internal tympaniform membranes. A pair of intrinsic syringeal muscles arise from the sides of the trachea and are inserted into the syrinx, and a pair of sterno-tracheal muscles arise from the sternum and are inserted into the trachea. The voice is produced by the vibration of the semi- lunar membrane : its pitch is altered by changes in the form of the tympanum produced by the action of the muscles. The lungs (Figs. 1040 and 1041, Ing.) are very small in comparison with the size of the Bird, and are but slightly distensible, being solid, spongy organs, not mere bags with sacculated walls as in Amphibia and many Reptiles. Their dorsal surfaces fit closely into the spaces between the ribs, and have no peritoneal covering : their ventral faces are covered by a strong sheet of fibrous tissue, the pulmonary aponeurosis or pleura (Fig. 1042, B, pul.ap.), a special development of the peritoneum. Into this membrane are inserted small fan- like costo-pulmonary muscles, which arise from the junction of the vertebral and sternal ribs. The bronchus, on entering the lung, is continued to its posterior Irr" FIG. 1041.— Columba livia. The lungs with the posterior end of the trachea, ventral aspect. a. in. aperture of anterior thoracic air-sac ; b.r. principal bronchus ; W. br". br'". secondary bronchi ; p. aperture of abdominal air-sac ; p. a. pulmonary artery entering lung ; p. in. aperture of posterior thoracic air-sac ; p. v. pulmonary vein leaving lung ; sb. b. aperture of inter- clavicular air-sac ; sp. b. aperture of cervical air-sac ; sy. syrinx ; tr. trachea. (From Parker's Zootomy.) PHYLUM CHORDATA 39!) lllll - ilji ••" 6 .„ be <0 rt 0 O' *S- lls fifjfj i^lil! •ijiiii 400 ZOOLOGY SECT. end (Figs. 1041 and 1042), dividing into two branches, each of which enters a bladder-like air-sac, formed as a dilatation of the mucous membrane of the bronchus. One of these, the abdominal air-sac (Fig. 1042, A, aid. a. s), lies among the coils of the intestine, the other, or posterior thoracic air-sac (post. th. a. s), is closely applied to the side-wall of the body. The bronchus also gives off, near its entrance into the lung, three short branches, one of which becomes connected with an anterior thoracic air-sac (ant. th. a. s), situated just in front of the posterior thoracic ; another with an inter clavicular air -sac (int.clav. a.s), which is median and unpaired, and connected with both lungs ; the third enters a cervical air-sac (cerv. a. s.) placed at the root of the neck. Each side of the inter- clavicular gives off an axillary air-sac, lying in the arm-pit. All these sacs are paired except the interclavicular, which is formed by the fusion of right and left moieties. The sacs are in communi- cation with the pneumatic cavities of the bones. The ventral or free walls of the thoracic air-sacs of each side are covered by ^ sheet of fibrous tissue, the oblique septum (obi. sept.), which is continued forwards to the pericardium, and is united with its fellow of the opposite side in the middle dorsal line : it divides the ccelome into two compartments ; one containing the lungs with the interclavicular and thoracic air-sacs, the other (abd. cav.) the heart, liver, stomach, intestine, etc., with the ab- dominal air-sacs. Besides the branches to the air-sacs, the main bronchus gives off secondary bronchi, and these branch again, sending off tubes which give rise to a system of fine branching and anastomosing tubules, the " lung-capillaries," which make up the main substance of the lung. When the Pigeon is standing, the alternate elevation and de- pression of the sternum, produced partly \)y the abdominal, partly by the intercostal muscles, causes an alternate enlargement and diminution of the capacity of the ccelome, and thus pumps air in and out of the lungs. Daring flight, when the weight is supported by the wings, and the sternum is thus rendered relatively im- movable, the same effect seems to be produced by the elevation and depression of the back. In either case the inspired air rushes through the lungs into the air-sacs and thence by diffusion into the pneumatic cavities of the bones. Thus, while in other animals a certain amount of unchanged or residual air is always left in the lungs after each expiration, in Birds the residual air is confined to the air-sacs and to the smaller branches of the bronchi, every respiratory movement drawing a current of fresh or tidal air through the lungs. As a result of this the aeration of the blood is very complete and its temperature correspondingly high. It is worthy of notice that Birds agree with Insects, the only other typically aerial class, in having the inspired air distributed all PHYLUM CHORDATA 401 over the body, so that the aeration of the blood is not confined to the limited area of an ordinary respiratory organ. Circulatory Organs. — The heart (Fig. 1040, ht.) is of great proportional size, and, like that of the Crocodile, consists of four chambers — right and left auricles, and right and left ventricles. There is no sinus venosus, that chamber being, as it were, absorbed into the right auricle (Fig. 1043, A, r.cm.). The right ventricle (Fig. 1043, B) partly encircles the left, the former having a crescentic, the latter a circular cavity in tranverse sections. The left auriculo-ventricular valve has the usual membranous structure, consisting of two flaps connected with the wall of the ventricle by Irr.v pc.v r.vn. Kit;. 1043. — A, heart of the Pigeon, dorsal aspect, a. ao. arch of aorta ; l>r. a. brachial artery ; br. v. brachial vein ; c. c. common carotid ; ju. jugular ; I. au. left auricle ; I. p. a. left pulmonary artery ; I. vn. left ventricle ; pc. v. left pre-caval ; ptc. post-caval ; p. v. pulmonary veins ; r. au, r. au'. right auricle ; r. p. a. right pulmonary artery ; r. pr. c. right pre-caval ; r. vn. right ventricle. B, heart of a Bird with the right ventricle opened ; L. V. septum ventriculorum ; R. V. right ventricle ; V. right auriculo-ventricular valve. (A, from Parker's Zootomy ; B, from Headley's Birds.) tendons, but the corresponding valve of the right side (R. V.) is a large muscular fold, very characteristic of the class. The right auricle receives the right and left pre-cavals (r.prc., pc. v.) and the post-caval (ptc.) ; the left four large pulmonary veins (2). v.). The left ventricle (Fig. 1044, /. vn.), as in the Crocodile, gives origin to the right aortic arch (a. ao.), but the right ventricle (r. vn.) gives off only one trunk, the pulmonary artery, which soon divides into two (r.p.a., /.|>.«,). The left aortic arch is absent in the adult, and it is the right alone which is continued into the dorsal aorta. The result of this is that the systemic arteries receive pure arterial blood from the left side of the heart, and the only mingling of aerated and non-aerated blood is in the capillaries. This is perhaps the most important physiological advance made by Birds over Reptiles. 402 ZOOLOGY SECT. SC.O, r.p cjnv Fiu. 1044. Columba livia. The heart and chief blood-vessels, ventral aspect, a. ao. arch of aorta ; a. m. a. anterior mesenteric artery ; a. r. e. afferent renal veins ; a. r. v'. vein bringing blood from pelvis into renal portal system ; br. a. brachial artery ; br. v, brachial vein ; c. caudal artery and vein ; c. c. common carotid artery ; c. m. v. coccygeo-mesenteric vein, dis- placed to the right ; cce. a. ccelific artery ; d. ao. dorsal aorta ; e. c. external carotid artery ; epg. epigastric vein ; e. r. v. efferent renal vein ; /. a. femoral artery ; /. v. femoral vein ; h v. hepatic vein ; i. c. internal carotid artery ; i'.il. internal iliac artery and vein ; i.m. internal mammary artery and vein ; in. a. innominate aftery ; i. v. iliac vein ; ju. jugular vein ; ju'. anastomosis of jugular veins ; I. au. left auricle ; I. p. a. loft pulmonary artery ; I. pre. left pre-caval vein ; I. vn. left ventricle ; pc. left pectoral arteries and veins ; p<: a. right pectoral artery ; pc. v. right pectoral vein ; p. m. «. posterior mesenteric artery ; pt<: post-caval vein ; ra. 1, ra. 3, ra. 3, renal arteries ; r. au. right auricle ; r.p. renal portal vein, on the left side of the figure, supposed to be dissected so as to show its passage through the right kidney ; r.p. a. right pulmonary artery; r.pr.v. right pre-caval vein; r. v. renal vein ; r.vii. right ventricle ; sc. a. sciatic artery ; sc. v. sciatic vein ; sd. a. subclavian artery ; v,: vertebral artery and vein. (From Parker's Zootomy.) XIII PHYLUM CHORDATA 403 The aortic arch curves over the right bronchus to reach the dorsal body-wall, and then passes directly backwards as the dorsal aorta (d. ao.). Owing to the immense size of the pectoral muscles the arteries supplying them are of corresponding dimensions, ' and the right and left innominate arteries (in. a.), from which the carotids (c. c.), subclavians (br. a.), and pectorals (pc. a.), arise, are actually larger than the aorta itself beyond their origin. In correspondence with the position of the legs, the femoral (/. a.) and sciatic (sc. a.) arteries arise very far forward : the caudal artery (c.) is naturally small. The most characteristic feature in the disposition of the circulatory organs is the almost complete disappearance of the renal portal system. There are two renal portal veins (r.p.) formed by the bifurcation of the caudal ; but each, instead of breaking up into capillaries in the kidney, sends off only a few small branches (a. r. v.) which apparently carry blood to that organ, the main vein passing forwards, through the substance of the kidney, and joining the femoral vein (f. v.) from the leg to form the iliac vein (i. v.) which, uniting with its fellow of the opposite side, forms the post-caval (pt. c.). Thus the main part, at any rate, of the blood from the caudal and pelvic regions is taken directly to the heart, and not through the renal capillaries as in most Fishes and all Amphibians and Reptiles. At the point of bifurcation of the caudal veins a large coccygeo- mesenteric vein (c. in. v.) comes off, and, running parallel with the rectum, from which it receives tributaries, joins the portal vein. The abdominal vein of Amphibians and Reptiles appears to be represented, in part at least, by the epigastric vein (epg.\ which returns the blood, not from the ventral body-wall, but from the great omentum, a fold of peritoneum, loaded with fat, lying ventral to the intestine and gizzard : the epigastric discharges into the hepatic vein. The red blood-corpuscles are oval and nucleated. The tempera- ture of the blood is unusually high— over 38° C. (100° F.). Nervous System. — The brain (Fig. 1045) completely fills the cranial cavity, and is remarkable for its short, broad, rounded form. The medulla ollongata (m. o.) has a well-marked ventral flexure, as in the Lizard. The cerebellum (c5.) is of great size, and has a large median portion and two small lateral lobes oiflocculi (/.) ; the surface of the middle lobe is marked by grooves passing inwards in a radiating manner and carrying with them the grey matter, the extent of which is thus greatly increased. The metaccele (Fig. 1046, v.4) is completely hidden by the cerebellum, and the latter is solid, having, no epiccele. The hemispheres (c.h.) extend back- wards to meet the cerebellum, and the optic lobes (o. I.) are thereby pressed outwards so as to ta^e up a lateral instead of the usual dorsal position: these are of rounded form, and each 404 ZOOLOGY SECT. contains an optoccele (Fig. 1046,0.0.) opening from a narrow passage, the iter, which represents the original cavity of the mid-brain. A further result of the extension of the hemi- spheres and cerebellum respectively backwards and forwards is that no part of the diencephalon (the.} appears externally except on the ventral surface : elsewhere it is seen only when the hemispheres are pressed aside. It contains a narrow vertical o.t Pin. 1045.— Columba livia. The brain ; A, from above ; B, from below ; C, from the left side. cb. cerebellum ; c. h. cerebral hemispheres ; /. flocculus ; inf. infundibulum ; m. o. medulla oblongata ; o. 1. optic lobes ; o. t. optic tracts ; pn. pineal body ; II — XIII, cerebral nerves ; sp. 1, first spinal nerve. (From Parker's Zootomy.) cavity, the diacceh ( V. 3), bounded laterally by the optic thalami, and communicating on each side by the foramina of Monro (/. m.) with the paraccdes or cavities of the hemispheres. The corpora striata (c. s.) are of immense size, and form the great mass of the hemispheres : the dorsal portions of the latter, forming the roofs of the paracoeles, are very thin. Hippocampi are absent. The anterior commissure is, as in lower Verte- brates, the chief commissure of the fore-brain. The olfactory bulbs (olf.) are extremely small, in correspondence with the poorly xrn PHYLUM CHORDATA 405 developed olfactory organ : on the other hand, the optic nerves and tracts are of unusual size. The spinal cord (Fig. 1040, sp. cd.) presents large brachial and lumbar enlargements from which the nerves of the fore- and hind-limbs respectively are given off. In the lumbar enlargement there is a divergence of the dorsal columns of the cord converting the central canal into a wide, diamond-shaped cavity, the sinus C.7l f* olf FIG. 1046.— Columba livia. The brain. A, with the cavities opened from above; B, in sagittal section, a. c. anterior commissure ; cb. cerebellum ; c. h. cerebral hemispheres ; c. s. corpus striatum ; /. TO. foramen of Mpnro ; inf. infundibulum ; TO. o. medulla oblongata ; o. c. commissure of optic lobes ; o. ch. optic chiasma ; o. 1. optic lobes ; olf. olfactory bulbs ; o. v. optoccele ; p peduncles of cerebellum ; p. c. posterior commissure ; pn. pineal body ; the. diencephalon ; v, 3, diacuele; v.lt, metacoele. (From Parker's Zootomy.) rho'inboidalis (s. rJib.\ bounded above only by the membranes of the cord. Sensory Organs. — The olfactory organs are paired chambers in the base of the beak, separated from one another by the meseth- moid and bounded externally by the ecto-ethmoid. The latter is produced inwards into three scroll-like processes, the turbinals, which greatly increase the surface of mucous membrane. The anterior portion of the cavity, including the anterior turbinal, is covered by laminated epithelium and serves as a vestibule; VOL. II C C 406 ZOOLOGY SKCT. its posterior portion, including the middle and posterior turbinals, is invested by the one-layered epithelium of the Schneiderian » cl.p FIG. 1047.— Columba livia. The eye. A, in sagittal section ; B, the entire organ, external aspect, en. cornea ; ch. choroid ; c.l. pr. ciliary processes ; ir. iris ; I. lens ; opt. nv. optic nerve ; pet. pecten ; rt. retina ; sd. sclerotic ; scl.pl. sclerotic plates. (After Vogt and Yung.) membrane to which the fibres of the olfactory nerve are distributed. The eye (Fig. 1047) is not even approximately globular, but has the form of a biconvex lens. Sclerotic bony plates (B., scl.pl.) are present, and there is a large pecten (pet), in the form of a plaited and strongly pigmented membrane, projecting into the cavity of the eye from the entrance of the optic nerve. The pecten is of nervous character, and is in all probability a sensory organ having some function connected with the process of accommodation. The auditory organ (Fig. 1048) is chiefly distinguished from that of Reptiles by the great development of the cochlea (lag). The anterior canal (SB) is of great size, and the whole membranous labyrinth is closely invested by a layer of dense ivory- like bone, which can be isolated by cutting away the surrounding spongy bone, and is then seen to form a sort of model of the contained organ, to which the name bony labyrinth is applied. The tympanic cavity and columella have the same arrangement FIG. 1048.— Columba livia. The right membranous laby- rinth, outer aspect. FA, am- pulla of posterior canal ; FB, posterior canal ; HA, ampulla of horizontal canal ; HB, hori- ^ontal canal ; lag. cochlea or lagena ; mr. membrane of Be issuer ; j)b, basilar part of cochlea ; S. sacculus ; SA, am- pulla of anterior canal ; SB, anterior canal. (From Wieders- heim, after Hasse.)' XIII PHYLUM CHORDATA 407 as in the Lizard ; the narrow Eustachian tubes open by a common aperture (Fig. 1040, cits, ap.) on the roof of the pharynx. Urinogenital Organs.— The Honeys (Fig. 1040, M, Figs. 1049, and 1050, k) have a very characteristic form. Each is a flattened organ divided into three main lobes and fitted closely into the hollows of the pelvis. It is formed from the metanephros, the large mesonephros or Wolffian body, which forms the embryonic kidney, undergoing atrophy. The ureters (ur.) are narrow tubes passing directly backwards to open into the urodseum or middle compartment of the cloaca. The testes (Figs. 1040, and 1049, ts.) are ovoid bodies, varying greatly in size according to the season, attached by peritoneum to elf ur 1049.— Columba livia. Male urino- genital organs, adr. adrenal ; d. 2, uro- daeum ; cl. 3, proctodseum ; k. kidney ; ts. testis, that of the right side displaced ; ur. ureter ; ur'. aperture of ureter ; vd. vas deferens ; vd'. its cloacal aperture ; v. s. vesicula seminalis. (From Parker's Zootomy.) FIG. 1050.— Columba livia. Female urino- genital organs, cl. 2, urodaeum ; cl. 3, procto- daeum ; k. kidney ; 1. od. left oviduct ; /. od'. its cloacal aperture ; I. od". its ccelomic funnel ; 1. od'". its coelomic aperture ; ov. ovary ; 7-. od. right oviduct ; r. od' . its cloacal aperture ; ur. ureter ; ur'. its cloacal aperture. (From Parker's Zootomy.) the ventral surfaces of the anterior ends of the kidneys. From the inner border of each goes off a convoluted vas deferens (vd.), which passes backwards, parallel with the ureter, to open into the urodasum on the extremity of a small papilla. The posterior end of the spermiduct is slightly enlarged to form a vesicula seminalis (v.s.). There is no copulatory organ. The female organs (Fig. 1050) are remarkable for the more or less complete atrophy of the right ovary and oviduct. The left ovary (ov.) is a large organ in the adult Bird, its surface studded with follicles or ovisacs, varying in size from about 15 mm. in diameter downwards, and each containing a single ovum. The left oviduct c c 2 408 ZOOLOGY SECT. (/. od.) is long and convoluted; its anterior end is enlarged to form a wide, membranous, coelomic funnel (/. od.") into which the ripe ova pass on their liberation from the ovisacs ; the rest of the tube has thick muscular walls, lined with glandular epithelium, and opens into the urodseum. A fair-sized vestige of the right oviduct (r. od.) is found in connection with the right side of the cloaca, and a more or less extensive vestige of the right ovary is frequently present. Internal impregnation takes place. As the ova or "yolks" pass down the oviduct they are invested with the secretions of its various glands ; first with layers of albumen or " white," next with a parchment-like, double shell-membrane, and lastly with a white calcareous shell. They are laid, two at a time, in a rough nest, and are incubated or sat upon by the parents for fourteen days, the temperature being in this way kept at about 38° to 40° C. (100° to 101° F.). At the end of incubation the young Bird is sufficiently developed to break the shell and begin free life. It is at first covered with fine down, and is fed by the parents with a secretion from the crop, the so-called "Pigeon's milk." 2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. Aves are Craniate in which the epidermal exoskeleton takes the form of feathers over the greater part of the body, of a rhampho- theca or horny sheath to the beak, and of claws on the digits of the foot and sometimes of the hand. In the standing position the body is entirely supported on the hind-limbs, the articulations of which are thrown forward. The fore-limbs are modified to form wings, usually provided with large feathers for the support of the body during flight. The cervical and free thoracic vertebrae are usually heteroccelous, but may be proccelous or amphicoelous. The sacral vertebrae are fused with the lumbar and with more or fewer of the posterior thoracic and anterior caudal to form a synsacrum for the support of the ilia. The posterior caudal vertebrae are usually fused to form a pygostyle around which the tail-quills are arranged in a semicircle. The bones of the skull undergo early ankylosis. There is a single, rounded, occipital condyle ; the united premaxillae form nearly the whole of the upper jaw ; and the lower jaw is composed originally of five or six bones in each ramus, and is supported by a freely articulated quadrate. The vertebral ribs are double-headed, provided with bony uncinates, and articulate with the bony sternal ribs by synovial joints. The sternum is broad, and is typically produced into a longitudinal ventral keel, having a separate centre of ossification. The coracoid is usually more or less pillar-like, the scapula is sabre-shaped, and the clavicles and interclavicle unite to form a furcula. Except in one extinct xin PHYLUM CHORDATA 409 species the distal carpals and the metacarpals are united to form a carpo-metacarpus. There are usually only three digits in the wing, which probably represent the first, second, and third of the typical hand. The ilium is of great size, having iarge pre- and post- acetabular portions. The acetabulum is perforated in the dry bone. The pubis and ischium are directed backwards, and, except in one case of each, there is neither pubic nor ischiadic symphysis. The head of the femur is at right angles to the shaft. The proximal tarsals are fused with the tibia to form a tibio-tarsus ; the fibula is much reduced. The distal tarsals are fused with the second, third, and fourth metatarsals to form a tarso-metatarsus ; the first metatarsal is free. The fifth digit of the typical foot is absent. In all tertiary and recent Birds teeth* are absent. The gullet is frequently dilated into a crop, and the stomach is usually divided into proventriculus and gizzard. The junction between the large and small intestines is marked by a pair of caaca. The lungs are spongy and non-distensible. The bronchi give off branches which open on the surface of the lung into thin-walled air-sacs, and these in their turn usually communicate with pneumatic cavities in more or fewer of the bones. The voice is produced in a syrinx situated at or near the junction of the trachea with the bronchi. The heart is four-chambered, the right auriculo-ventricular valve is muscular, and the right aortic arch alone is present in the adult. The renal portal system is vestigial. The red blood-corpuscles are oval and nucleated. The temperature of the blood is high (about 38° C.). The optic lobes are displaced laterally owing to the meeting of the large cerebral hemispheres and cerebellum. The lumbar region of the spinal cord has a sinus rhomboidalis. The olfactory organ is usually poorly developed. The eye is usually large, and has sclerotic plates and a pecten. The auditory organ has a large, curved cochlea. The kidney is three-lobed, and is developed from the metanephros, the mesonephros undergoing atrophy. There is no urinary bladder. The ovary and oviduct of the right side are more or less completely atrophied. Birds are all oviparous, and the large ovum, containing much food -yolk, becomes invested with albumen, a double shell-mem- brane, and a calcareous shell, in its passage down the oviduct. The embryo has an amnion, an allantois, and a large yolk-sac. The newly-hatched young may be either well covered with down and able to run or swim and to obtain their own food, in which case they are said to be precocious ; or may be more or less naked and dependent for a time upon the parents for their food supply, when they are non-precocious. There is no general agreement with regard to the classification of Birds. Owing to the singular uniformity of the class in essential 410 ZOOLOGY SECT. matters of structure, the vast and bewildering diversity in detail, and the puzzling cross-relationships between group and group, the splitting up of the class into orders is a matter of great difficulty and one upon which hardly two ornithologists are agreed. The following scheme will probably answer the present purpose sufficiently well. Sub -class I.— Archseornithes. Mesozoic Birds : have no ploughshare bone (pygostyle), but a long tail of many vertebra, having the rectrices arranged in two rows, one on each side of it. The carpals and inetacarpals are probably free and the hand has three, clawed digits. Teeth are present in both jaws. Including the single genus Arch&opteryx with two species, known only from three specimens found in the Upper Jurassic rocks of Bavaria. Sub-class II.— Neornithes. Birds in which the greatly shortened tail usually ends in a pygostyle, around which the rectrices, when present, are arranged in a semicircle. Except in a few extinct forms, there are no teeth. The metacarpals are fused with the distal carpals to form a carpo- metacarpus. Except in one instance, not more than two digits of the hand bear claws, and in nearly all cases claws are absent in the manus. Division A. — Ratitae. Flightless Neornithes, usually of large size, having no hooked barbules to the feathers, so that the barbs are free. Apteria are usually absent in the adult. The rectrices are absent or irregularly arranged, and the pygostyle is small or undeveloped. The sternal keel is vestigial or absent. The coracoid and scapula are com- paratively small and completely ankylosed ; the acrocoracoid pro- cess is vestigial, and the coraco-scapular angle approaches two right angles. The wing is reduced in size and may be vestigial or absent. There are large basi-pterygoid processes developed from the basi- sphenoid. The vomer is large and broad. The quadrate articu- lates with the skull by a single or partially divided facet. The male has a penis. The young are precocious. ORDER 1. — MEGISTANES. Including the Emus (Dromwus) and Cassowaries (Casuarius). XI J I PHYLUM CHORDATA 411 FIG. 1051.— Apteryx australis, with egg. (From a specimen in the Royal College of Surgeons, London.) OKDER 2. — APTERYGES. Incl tiding only the Kiwis (Apteryx, Fig. 1051). ORDER 8. — DINORNITHES. Including the Moas (Dinornithidce, Fig. 1069). ORDER 4. — RHE^E. Including the South American Ostriches (Ehea). ORDER 5. — STRUTHIONES. Including the true Ostriches (Struthw). ORDER 6. — ^EPYORNITHES. Including only the post-Pliocene Madagascan geiieia and Mullerornis. 412 ZOOLOGY SKCT. FIG. 1052.— Apteryx australis. Skeleton. (From a specimen in the British Museum, Natural History.) ORDER 7. — GASTORNITHES. Including Gastornis and other genera from the Eocene of Europe. Division B. — Carinatae. Neornithes in which, with the exception of some flightless species, the sternum has a keel, the coracoid and scapula are not ankylosed, the acrocoracoid and usually the furcula are well developed, and the coraco-scapular angle is less than a right angle. There is a pygo- style around which the rectrices are arranged. The quadrate usually articulates with the skull by two facets. The barbs of the feathers have booklets.1 1 Except, perhaps, in Hesperornis. xiii PHYLUM CHORDATA 413 OKDER 1. — STEREORNiTHES.1 Including Phororhacos, Dryornis, and other genera from the Eocene of South America. FIG. 1053.— Hesperornis regalis. The restored skeleton. (After Marsh.) ORDER 2. — ODONTOLCLE. Including Hesperornis 2 (Fig. 1053), a large diving and swimming Bird from the Cretaceous of North America, and other less known genera. 1 Recent investigations indicate that this is not a natural group, but that its various genera will have to be distributed amongst various orders both of Ratitre and of Carinatse. 2 Hesperornis is perhaps more nearly related to the Ratitee. 414 ZOOLOGY SECT. ORDER 3. — ICHTHYORNITHES. Including LMhyornis (Fig. 1054) and Apatornis. Tern-like Birds from the Cretaceous of North America. FIG. 1054.— Ichthyornis Victor. The restored skeleton. (After Marsh.) ORDER 4. — PYGOPODES. Including the Divers (Colymbus) and the Grebes (Podicipes). ORDER 5. — IMPENNES. Including the Penguins (Aptenodytcs, Eudyptes, &c., Fig. 1055). XIII PHYLUM CHORDATA 415 FIG. 1055.— Eudyptes antipodum. (After Bullcr.) ORDER 6. — TURBINARES. Including the Petrels, such as the Albatrosses (Diomedea), Storm- petrels (Oceanites), Fulmars (Fulmarus), Shearwaters (Pnjfinus), &c. ORDER 7. — STEGANOPODES. Including the Boatswain-bird (Phaethon), Gannets (Sula), Cor- morants or Shags (Phalacrocorax). Frigate-bird (Fregata), and Pelicans (Pelecanus). ORDER 8. — HERODIONES. Including the Herons (Ardea, &c.), Storks (Ciconia, &c.), Ibises (Ibis), Spoonbills (Platalea), and Flamingoes (PJmnicopterus). 416 ZOOLOGY SECT. ORDER 9. — ANSERES. Including the Ducks (Anas, &c.), Geese (Anser), Swans (Cygnus), and Mergansers (Mergus); and the Screamers (Palamedca and Chauna). ORDER 10. — ACCIPITRES. Including the diurnal Birds of prey, such as the Eagles (Aquila), Falcons (Falco), Vultures (Vultur, &c.), and Sec- retary Bird (Gypogeranus). The American Vultures or Turkey- buzzards (Cathartes), are sometimes placed in a distinct order. ORDER 11. — CRYPTURI. Including only the Tinamous (Tinamus, &c.). ORDER 12. — GALLING. Including the Fowls (Gallus), Pheasants (Phasiamis), Grouse (Tetrao), and other Game-Birds ; Curassows (Craz), Brush-turkeys (Megapodius), Hemipodes or Button-quails (Turnix), and the Hoatzin (Opisthocomus). ORDER 13. — GRALL.E. Including the Rails (Rallus, Ocydromus,&c.\ tlie flightless Giant Rail (Aptornis), the Cranes (€frust &c.), the Bustards (Otis), &c. ORDEK 14. Including the Gulls (Larus) and Terns (Sterna), and the Auks (Alca and Pratercula). ORDER 15. — LIMICOL.E. Including the Plovers (Charadrius, &c.), Oyster-catchers (Hcematopus), Curlews (Limosa), Jacanas (Parra), &c. ORDER 16. — PTEROCLETES Including the Sand-grouse (Pterocles and Syrrhctptes). ORDER 17. — COLITMB^E. Including the Pigeons and Doves (Columla, Turtur, &c.), Crowned Pigeons (Goura), and the extinct flightless Dodo (Didus) and Solitaire (Pezophaps). ORDER 18. — PsiTTACi.2 Including the Parrots (Psittacm, &c.), Parrakeets (Platycercus), Cockatoos (Gdccdua\ Lories (Lorius), and Macaws (Ara). ORDER 19. — STRIGES. Including the Owls (Strigidce). 1 Sometimes united with the next two orders under the designation Gharadrii- formes. 2 Sometimes combined with the Cuckoos, xm PHYLUM CHORDATA 417 ORDER 20. — PICARI.E. A somewhat heterogeneous group including the Cuckoos (Gucu- lidcv), Plantain-eaters (Musophagidce), Rollers (Coraciidce), Motmots (Momotidce), Kingfishers (Ahedinidce), Bee-eaters (Meropidce), Hoopoes ( Upupidm), Goat-suckers (Caprimulgi), Swifts (Cypselidce), Humming Birds (Trochilidce), Colies (Colii\ Trogons (Trogones), Woodpeckers, and Hornbills (Pici), &c. * ORDER 21. — PASSERES. Including the Lyre-birds (Menura), Larks (Alauiidce), Starlings (Stumidce), Finches (Fringillidce), Swallows (Hirundinidce), Black- birds and Thrushes (Turd/idee), Birds of Paradise (Paradiseidw), Crows (Corvidai), &c. Systematic Position of the Example. The numerous species of Columba belong to the family Columbidce, of the order Columbce. The following are the chief characters of the Columbae : — There are eleven primary remiges, the first very small ; the skull is schizognathous (see p. 428) ; the oil-gland has no tuft of feathers ; the vomer is vestigial; there is a large crop; the caeca are vestigial ; and the young are non-precocious. Of the two families of Columbse, the Columbidce, or Doves and Pigeons, are distinguished from the JDididce, including the Dodo and Solitaire, by the power of flight and the accompanying typical carinate characters of the sternum and shoulder-girdle. In Columba there are twelve rectrices; the second primary remex is longer than the sixth, and the proximal portion of the tarso- metatarsus is feathered. 3. GENERAL ORGANISATION. In respect of range of structural variations, the entire class of Birds is hardly the equivalent of a single order of Reptiles. Among existing Birds, the Emu and the Raven, which may be said to stand at opposite ends of the series, present nothing like the anatomical differences to be found between a common Lizard and a Chamaeleon, or between a Turtle and a Tortoise. Hence in dividing the class into orders, we find none of those striking dis- tinctive characters which separate the orders of Fishes, Amphibia, and Reptiles, but have to be content with characters which in other groups would be considered insignificant, such as details in the structure of the skull and sternum, in the arrangement of the muscles of the wing and leg, in the form of the foot, and in the peculiarities of the newly-hatched young. It is for this reason that in the preceding classification no diagnoses of the orders are given : to define them adequately would involve a degree of ana- tomical detail quite beyond the scope of the present work. 418 ZOOLOGY SECT. The differences between the two avian sub-classes, the Arcliseor- uithes and the Neonithes, are, however, of a far more fundamental nature ; and as Archaeopteryx, the sole representative of the first of these groups, is a unique form, and perhaps the best example of an undoubted link between two classes — Reptiles and Birds — it. will be convenient to deal with it separately. Sub-Class I,— Archaeornithes. Only two specimens of Archceopteryx litkographica have hitherto been found, both in the finely-grained lithographic limestone of FIG. 1056.— Archaeopteryx lithographica. From the Berlin specimen, c. carpal ; d, furcula ; co. coracoid ; h. hunierus ; r, radius ; sc. scapula ; u. ulna ; I — IV, digits. PHYLUM CHORDATA 419 Solenhofen, Bavaria, belonging to the Upper Jurassic period. The Bird (Fig. 1056) was about the size of a Crow, and in both fossils not only are the bones preserved, but also many of the feathers. The most striking feature in the organisation of the Bird is the fact that the tail is composed of about 18 — 20 free caudal vertebra3, gradually tapering to the distal end as in a Lizard. The rectrices are arranged in two rows, one on each side of the caudal vertebrae, forming a long tail quite unlike that of any existing Bird. The centra probably had biconcave faces. In addition to cervical and thoracic ribs (which were apparently devoid of uncinates), there were abdominal ribs, like those of Sphenodon and Crocodiles. The skull (Fig. 1057) is proportionately large, with rounded brain -case and strong jaws, in each of which is a series of conical Fin. 1057. — Archaeopteryx lith.ograph.ica. The skull, showing teeth and sclerotic plates. (From Headley, after Dames.) •teeth lodged in sockets. There is no trace of sternum in either specimen, and the coracoids (co.) are only partially visible : the scapulae (se.) are slender, curved bones, and there is a U-shaped furcula (cl.). FK;. 1058. —Archaeopteryx lithographica. The left manus. c. carpal ; d. 1, first digit ; #, second digit ; 3, third digit ; m. m. metacarpals ; r. radius ; u. ulna. (From Headley, after Dames.) The bones of the upper- and fore-arm are of the normal avian character: only one carpal is certainly known (Fig. 1058, c.) : it apparently belongs to the distal row, and is closely applied to the 420 ZOOLOGY SECT. first and second metacarpals. Three digits (d. 1, 2, 3) are clearly visible in the more perfect of the two specimens — that in the Berlin Museum — the metacarpals of which are usually stated to be all free, in which case there is no carpo-metacarpus as in other Birds, and the hand approaches the normal reptilian type. The number of phalanges follows the usual reptilian rule, two in the first digit, three in the second, and four in the third, and the ungual phalanx of all three digits is claw-shaped and doubtless supported a horny claw. The remiges, like the retrices, are in a wonderful state of pre- servation (Fig. 1056), and are divisible, as usual, into primaries or metacarpo-digitals, and secondaries or cubitals. The primaries were probably attached to the second, or to the second and third, of the digits just described. The pelvis and the hind-limb have the usual avian character. The tibia and fibula are separate. The foot consists of a slender tarso-metatarsus and four digits, the hallux being small and directed backwards. In addition to the wing- and tail-quills already referred to, there are remains of contour-feathers at the base of the neck and of wing-coverts. Moreover, the rectrices are continued forwards by a series of large feathers which extend for some distance along the sides of the body; and a row of similar but smaller feathers is attached along both anterior and posterior faces of the tibio-tarsus. A second species of Archa30pteryx, which has been named Archceopteryx siemensii, has been found more recently in the same locality. Sub -Class II,— Neornithes. External Characters.— In the general build of the body the Neornithes differ from Archseopteryx chiefly in the shorter and stouter trunk, and in the point of articulation of the hind-limbs being thrown forward, so as to be almost directly below the centre of gravity of the body : the animal is thus enabled without effort to support itself on the legs alone. In a word Birds are essentially bipedal, the only exception being the young of the Hoatzin (Opisthocomus), which uses its wings in climbing. The neck is always well developed, and is often, as in the Swan and Flamingo, of immense proportional length. The cranial por- tion of the head is usually not large, but the beak may attain extraordinary dimensions, and exhibits a wide range of form. It may be extremely short and wide for catching Moths and other flying Insects, as in Swifts and Goatsuckers; short and conical for eating fruit, as in Finches; strongly hooked for tearing the bodies of animals, as in Birds of Prey, or for rending fruits of various kinds, as in Parrots ; long, conical, and of great strength, as in Storks ; slender and elongated, as in Swifts, Ibises, and Curlews ; broad and XIII PHYLUM CHORDATA 421 flattened for feeding in mud, as in Ducks and Geese; expanded at the end as in Spoonbills ; immensely enlarged as in Hornbills and Toucans. It is most commonly bent downwards at the tip, but may be straight or curved upwards, as in the Avocet, or bent to one side, as in the New Zealand Crook-billed Plover. It is some- times, ns in the Toucans, brilliantly coloured, and there may also be bright coloration of the cere, as in the Macaws, and of naked spaces on the head, as in the Cassowaries. In the latter the head is pro- duced into a great horny prominence or " casque," supported by an elevation of the roof of the skull. The cere is frequently absent. The nostrils are placed at the base of the beak except in Apteryx, in Avhich they are at the tip. The essential structure of the wing — apart from its feathers — is very uniform. As a rule, all three digits are devoid of claws, FIG. 1050.— A, Wing of nestling of Opisthocomus ; B, Wing of adult Apteryx; both from the inner (ventral) aspect, cb. 1, first cubital remex ; dg. 1, dg.2.dg. 3. digits; pr.ptgm. pre-patagium ; pt. ptgm. post-patagium. (A, after Pycraft ; IJ, after T. J. Parker.) as in the Pigeon, but the Ostrich has claws on all three digits; Rhea on the first and sometimes on the second and third ; the Cassowary, Emu, and Kiwi (Fig. 1059, B) on the second; the Crested Screamer (Ohauna) and two other species, and, as a rare abnormality, the Common Fowl and Goose, on the first. With these exceptions, the hand of the adult bird has lost all the characters of a fore-foot ; but in the young of the Hoatzin (Opisthocomus^ claws are present on the first two digits (Fig. 1059, A), which are sufficiently mobile to be used in climbing. Besides the true claws, horny spurs are sometimes present on the carpo-metacarpus. There is almost every gradation in the proportional length of the hind-limb, from Birds in which nothing but the foot pro- jects beyond the contour feathers — and even the toes may be VOL. II D D 422 ZOOLOGY feathered, to the long-legged Storks and Cranes, in which the distal part of the tibio-tarsus is covered with scales as well as the foot. In aquatic forms a fold of skin or web is stretched between the toes, sometimes including all four digits, as in the Cormorants; sometimes leaving the hallux free, sometimes forming a separate fringe to each digit, as in the Coots and Grebes. As to the toes themselves, the commonest arrangement is for the hallux to be directed backwards, and Nos. 2, 3, and 4, forwards ; but in the Owls No. 4 is reversible, i.e., can be turned in either direction, and in the Parrots, Woodpeckers, &c., it, as well as the hallux, is permanently turned backwards. In the Swifts, on the other hand, all four toes turn forwards. The hallux is frequently vestigial or absent, arid in the Ostrich No. 4 has also atrophied, producing the characteristic two-toed foot of that Bird. Pterylosis. — With the exception of the Penguins, most Car- inatee have the feathers arranged in distinct feather-tracts or cd.pl FIG. 1060. — A, pterylosis of GypaetUS (Bearded Vulture); B, of Ardea (Heron), al. pt. wing- tract ; c. pt, head-tract ; cd. pt, caudal tract ; cr. pt, crural tract ; cv.apt. cervical apterinm ; hu. pt, humeral tract : Uil. apt, lateral apterium ; p. d. p., p. d. p'. powder-down patches ; sp. pt, spinal tract ; v. apt, ventral apterium ; v. pt, ventral tract. (After Nitsch.) pterylse, separated by apteria or featherless spaces. These are commonly much more distinct than in the Pigeon (Fig. 1060), and their form and arrangement are of importance in classification. In the Ratitae, apteria are usually found only in the young, the adult XIII PHYLUM CHORDATA 423 having a uniform covering of feathers. The Ratitae, also, have nothing more than the merest trace of booklets on the barbules, so that the barbs do not interlock, and the vanes of the feathers are downy or hair-like. In the Penguins the wing-feathers are degenerate and scale-like. Many Birds are quite naked when hatched, but in most cases the body is more or less completely covered by a temporary crop of feathers, the nestling-downs, of various forms, but always having a short axis, soft, loose barbs, devoid of interlocking apparatus, and, except in the Emu, having no after-shaft (vide infra). They are succeeded, as already described, by the permanent feathers. Many Birds, such as the Swan, possess down-feathers or plumulce throughout life, in- terspersed among and hidden by the contour feathers or pennce. In the Heron and some other Carinatae are found powder-down patches (Fig. 1060, B, p. d. p, p. d. p'), areas of downs, the ends of which break off and make a fine dust. Semi-plumes are downs with a well- developed axis: filoplumes, as we have seen (Fig. 1023, B), have an elongated axis and vestigial vexillum. In many Birds there springs from the under side of the quill, near the superior umbilicus, a second vane, the after-shaft (Fig. 1061), usually smaller than the main shaft, but some- times of equal size. Both among Carinatse and Ratitse we find genera with double- shafted feathers and allied forms in which the after-shaft is rudimentary or absent. The feathers are always shed or " moulted " at regular intervals, as a rule annually. The old feathers drop out and new ones are formed from the same pulps. The colours of feathers present great variety. Black, brown, red, orange, and yellow colours are due to the presence of definite pigments, i.e., are absorption-colours. White, and in some cases yellow, is produced by the total reflection of light from the spongy, air-contain- ing substance of the feather, there being, as in nearly all other natural objects, no such thing as a white pigment. Blue, violet, and in some cases green, are produced by the light from a brown D D 2 FIG. 1061.— Feather of Casuarius (Casso- wary), showing after- shaft and disconnected barbs. (From Headley.) 424 ZOOLOGY SECT. pigment becoming broken up as it passes through the super- ficial layer of the feathers in its passage to the eye : no blue or violet pigments occur in feathers, and green pigments are very rare. The beautiful metallic tints of many birds are entirely the result of structure, owing their existence to a thin, transparent, superficial layer, which acts as a prism : in such feathers the colour changes according to the relative position of the Bird and of the eye of the observer with regard to the source of light. There is also infinite variety in the general coloration of Birds. In many the colouring is distinctly protective, harmonising with the environment, and even changing with the latter — as in the Ptarmi- gan, which is greyish-brown in summer, white in winter, the former hue helping to conceal the Bird among herbage, the latter on snow. Frequently, as in Pheasants and Birds of Paradise, the female alone is protectively coloured, while the male presents the most varied and brilliant tints, enhanced by crests, plumes or tufts of feathers on the wings, elongated tail, &c., &c. These have been variously explained as " courtship colours " for attracting the female ; as due simply to the exuberant vitality of the male Bird ; or as helping to keep the number of males within proper limits by rendering them conspicuous to their enemies. Such ornaments as the bars and spots on the wings and tail of many gregarious birds, such as Plovers, fully exposed only during flight, and often widely different in closely allied species, have been explained as "recognition marks," serving to enable stragglers to distinguish between a flock of their own and one of some other species. Skeleton. — The vast majority of Birds have saddle-shaped or heteroccelous cervical and thoracic vertebrae, but the thoracic verte- bras are opisthoccelous in the Impennes (Penguins), the Gavias (Gulls), and the Limicolas (Plovers, &c.), while in the Ichthyornithes alone they are biconcave. The spaces between adjacent centra are traversed by a meniscus with a suspensory ligament, as in the Pigeon (p. 386). The number of vertebrae is very variable, especi- ally in the cervical region, where it rises to twenty-five in the Swan and sinks to nine in some Song-birds. There is very com- monly more or less fusion of the thoracic vertebras, and the formation of a syn-sacrum by the concrescence of the posterior thoracic, lumbar, sacral, and anterior caudal vertebras, is universal. The posterior cervical and anterior thoracic vertebras commonly bear strong hypapophyses or inferior processes for the origin of the great flexor muscles of the neck. The number of sacral vertebras varies from one to five. A pygostyle, formed by the fusion of more or fewer of the caudal vertebras, is of general occurrence, but is small and insignificant, or absent, in the Ratitas. The ribs are always double-headed : the sternal ribs are ossified, not merely calcified, and are united with the vertebral ribs by XIII PHYLUM CHORDATA 425 sy no vial joiuts. Ossified imcinates are nearly always present, and usually become ankylosed to the vertebral ribs. What rnay be considered as the normal type of sternum, is a broad plate, concave dorsally from side to side, and produced ventrally into an antero-posterior keel which is ossified from a distinct centre (Fig. 1062, A, os. 1). The posterior edge of the bone is either entire (I)), or presents on each side of the keel one or two more or less deep notches (A, B) or foramina (C). In the Ratitue Fi<;. 1 Of •>•>.— Sterna of various Birds. A, Gallus (common Fowl, young); B, Turdus (Thrush) ; C, Vultur (Vulture) ; D, Procellaria (Petrel) ; E, Casuarius (Cassowary), ant. Int. pr. anterior lateral process ; cur. earina ; <•(. clavicle ; cor. coracoid ; fan. fontanelle ; fur. fureula ; obi . Int. pr. oblique lateral process ; o.v. paired ossification of sternum in K ; o,s. .7. carinal ossifi- cation in A; os.:t, on. 3, lateral ossifications; post, metl.pr. posterior median pi-ocess ; post. lat. pr. posterior lateral process; pr.cor. pro-coracoid ; scp. scapula; sp. spina stern i. (A and E after W. K. Parker ; B, C, and D, from Bronn's Ihierreich.) (E) the keel is either absent or reduced to the merest vestige, and there is no trace of the carinal ossification in the young. External to the coracoid grooves, the anterior edge of the sternum is pro- duced into larger or smaller antero-lateral processes {ant. lat. pr.) ; in the Emu these are of great size and are closely applied to the pericardium. It was upon the characters of the raft-like sternum that the group Ratitae was founded ; but the difference between them and the Carinatse in this respect is not absolute, the ratite condition 426 ZOOLOGY SECT. having been acquired by many Carinatae which have lost the power of flight. The keel is very small in Ocydromus, Notornis, and Aptornis, three flightless Rails — the last extinct — from New Zealand, and is practically absent in the Dodo (Didus) and Solitaire (Pezophaps) — two gigantic extinct Pigeons from Mauritius and Rodriguez, in the Kakapo or Ground-parrot (Stringops) of New Zealand, in the extinct Giant-goose (Cnemiornis) from the same FIG. 10C3. — Eudyptes pachyrlxynclius (Penguin). Skeleton. (From a photograph by A. Hamilton.) country, and in Hesperornis. The absence of the carina may therefore be considered as an adaptive modification of no signifi- cance as indicating affinity. The entire order of Penguins (Impennes) and the extinct Great Auk (Alcd impennis) are also flightless, but their wings, instead of being functionless, are modified into powerful swimming-paddles (Fig. 1063). There has therefore, in these cases, been no reduction either of the pectoral muscles or of the carina. XIII PHYLUM OHORDATA 427 The skull of Birds is generally remarkable for its huge orbits separated by a thin interorbital septum, and for the comparatively small size of the ethmoid bone and the turbinals. The most striking exception is afforded by the Kiwi (Apteryx] in which the orbits (Fig. 1064) are small and indistinct, while the olfactory chambers (Ec. EtJi) extend backwards between the eyes; the orbits being therefore separated from one another by the whole width of the organ of smell. The same thing occurs, to a less degree, in the Moas. In its essential features the skull is remarkably uniform throughout the class. The rounded form of the brain-case, more or less concealed externally by ridges for the attachment of muscles ; the upper beak composed mainly of great triradiate NvV .oc.pr ;. 10ti4.— Apteryx mantelli. Skull of a youn^ specimen, side view. The cartilaginous parts are dotted. Al.sph. alisphenoid ; Ang. angular ; Cn. 1, Cn. 2, condyle of quadrate ; Dent. dentary ; '/. fa:, r. descending processes of nasal and frontal ; Ec.Eth. ecto-ethmoid ; Ex. Col. extra-columella ; Ex. oc. exoccipital ; Jv. jugal ; Lac. lacrymal ; Inc. for. lacrymal foramen ; Xn. nasal; ;m.. n/>. nasal aperture; Xr. If. Iff, 1 F, optic foramen, transmitting also the 3rd and 4th nerves ; Jfv. F', foramen for orbito-iiasal nerve ; Nv. VII, for facial ; Oc. Cn. occipital c' iidyle ; PH. parietal ; Puf. palatine : pa. oc. pr. par-occipital process ; Pmx. pre- maxilla ; Pr. ot. pro-otic ; Qu. Ju. quadrato-jugal ; Qu. quadrate ; Qu. (orb. pr.) orbital process of quadrate ; S.orb.F. supra-orbital foramen ; -Sty. squamosal. (After T. J. Parker.) premaxillse ; the single, small, rounded occipital .condyle; the slender maxillo-jugal arch; the large parasphenoidal rostrum;' the freely articulated quadrate, with its otic, orbital, and articular processes ; the absence of the reptilian post-frontals ; and the early ankylosis of the bones ; all these characters are universal among Birds. There are, however, endless differences in detail, some of which, connected with the bones of the palate, are of importance in classification. In the Katitae and the Tinamous (Crypturi) there are large basi-pterygoid processes (Fig. 1065, B,ptg. pr} springing, as in Lizards, from the basisphenoid, and articulating with the ptery- goids near their posterior ends. The vomer ( Vo) is large and broad, and is usually connected posteriorly with the palatines (Pal), which do not articulate with the rostrum. The maxillo-palatine 428 ZOOLOGY SECT. processes are comparatively small, and do not unite with one another or with the vomer. This arrangement of the bones of the palate is called dromceogna- thous. In many Carinata3, e.g. the Pigeon and the Fowl, the basi-pterygoid processes are either absent or spring from the base of the rostrum. The vomer is small and pointed, or may be absent, and the palatines articulate posteriorly with the rostrum. The maxillo-palatines do not unite with one another. These peculiarities charac- terise the schizo gnathous arrangement. In the Pas- seres a similar arrangement obtains, but the vomer is broad and truncated instead of pointed in front. This gives the wgitho gnathous ar- rangement. Lastly in the Storks, Birds of Prey, Ducks and Geese, &c., the maxillo- palatines (Fig. 1066, mx.p) fuse with one another in the middle line, often giving rise to a flat, spongy palate and producing the desmo- gnathoiLS arrangement. The most specialised form of skull is found in the Parrots (Fig. 1067). In many Birds the nasals and the ascending process of the premaxilla are very thin and elastic where they join the skull, and there is an unossified space in the process; Host. .rostrum ; _*. Oc. (^praoccipital) ; rnesethmoid, SO that the upper beak is capable of a considerable amount of movement in the vertical plane. In Parrots there is a true joint between the upper beak and the skull, allowing of that movement of the former which is so striking in the living &o° FIG. 1065.— Apteryx mantelli. Skull of young specimen, from below. The cartilaginous parts are dotted. B. Oc. basioccipital ; B. ptg. pr. basi-pterygoid process ; B. Tmp. basitemporal ; EC. Eth. ecto-ethmoid ; Eus. T. Eustachian tube ; Ex. Col. extra-columella ; Ex. oc. exoccipital ; Int. car. carotid foramen ; MX. maxilla ; Nv. VII, foramen for facial : Nv. IX, X, for glossopharyn- geal and vagus ; Nv. XII, for hypoglossal ; Oc. en. occipital condyle ; Oc. for. foramen magnum ; Pal. palatine ; pa. oc. pr. par-occipital process ; Pmx. premaxilla ; Ptg. pterygoid ; Qu. (orb. pr.) orbital process of quadrate; Qu.(ot.pr.) otic (supraoccipital) ; •S. Orb. F. supra-orbital foramen ; Sq. squamosal ; Vo. vomer. (After T. J. Parker.) XIII PHYLUM CHORDATA is 429 Bird. When the mandible depressed, the contraction of the digastric muscle causes a for- ward movement of the lower end of the quadrate, which pushes forwards the rnaxillo-jugal bar and the palatines and ptery- goids, the latter sliding upon the rostrum. Both the maxillae and the palatines are articulated in front with the premaxilla, and together push it upwards; in this way depression of the lower, produces an automatic raising of the upper, jaw. The great size and strength of both premaxilla and mandible are remarkable, as also is the fact that the orbit is completely surrounded by bone, a backward process of the lacrymal being joined beneath it by a for- ward process of the frontal. The mandible contains in the young Bird the six bones on each side characteristic of Reptiles ; the coronary is, however, often absent. As a rule the head of the quadrate articulates with the roof of the tympanic cavity by a single facet in Ratita3,by a double facet in Carinatoe. The Jiyoid always agrees in essential respects with that of the Pigeon ; in the Woodpecker the posterior cornua are curved round the head and at- tached to the skull in the neighbour- hood of the right nostril, a very flexi- ble and protrusible tongue being pro- duced. The structure of the shoulder-girdle furnishes one of the FIG. 10G7.-Skull of Ara (Maca^ (Prom a photograph by ^^ fundamenfcal FIG. 1006.- Anas bOSChas (Duck). Ventral view of Skull, a. p.f. anterior palatine foramen ; b. o. basioccipital ; b. pg. basi- pterygoid process ; b. s. basispherioid ; b. t. basi-teinporal ; e. o. exoccipital ; eu. aper- ture of Eustachian tube ;/. in. foramen mag- num; i. c. internal carotid foramen ; j. jugal ; mx maxilla ; mx. p. maxillo-palatine pro- cess ; oc. c. occipital condyle ; pi. palatine ; p. n. posterior nares ; px. premaxilla ; q. quadrate; q.j. quadrato-jugal ; v. vorner ; IX, X, foramen for ninth and tenth nerves ; XII, for twelfth nerve. (From Wieders- heirn's Vertebrata.) 430 ZOOLOGY SECT. distinctive characters between Ratitge and CarinataB, bnt, as with the sternum, the differences are adaptive and not of phylogenetic significance. In most Carinatse both coracoid and scapula are large and united with one another by ligament; the coracoid has an acrocoracoid, and the scapula an acromion process ; the coraco-scapular angle is acute ; and there is a furcula. In the RatitaB the coracoid (Fig. 1068, cor.) and scapula (scp.) are much reduced in proportional size and are ankylosed with one another ; the acrocoracoid (cicr. cor.) and acro- mion- («C7\) processes are reduced or absent; the coraco-scapular angle approaches two right angles; and there is no furcula, although separ- ate vestiges of clavicles are present in the Emu and Cassowary. In some of the Moas (Pachyornis, &c.) the shoulder-girdle is wholly ab- sent. But, as in the case of the sternum, the distinction is not ab- solute. In Hesperornis, the Dodo, the Solitaire, Aptornis, Notornis, Ocydromus, and Cnemiornis, the bones of the shoulder-girdle are proportionally small, the coraco- scapular angle exceeds 90°, and in some cases, such as certain Parrakeets and Owls, the furcula is feeble, or represented by paired vestiges, or absent. Curiously enough, considering that increase in the coraco-scapular angle is usually correlated with diminished powers of flight, it also slightly exceeds 00° in the Albatross and some of its allies. In most adult Birds the procoracoid is reduced to a process on the dorsal end of the coracoid, but in the Ostrich and in the embryo of Apteryx it is well developed and separated by a fenestra from the coracoid. A small bone, the accessory scapula, is sometimes found on the outer side of the shoulder joint. The variations in the structure of the wing are mostly matters of proportion, but a remarkable flattening of all the bones is very characteristic of Penguins (Fig. 1063), which are further dis- tinguished by the presence of a sesamoid bone, the patella ulnaris, taking the place ofthe olecranon-process. In the Emu and Kiwi the first and third digits of the normal wing have atrophied during de- velopment, the middle one alone remaining. In the Moas (Fig. 1069) no trace of a wing has been found, and in one species only is ithere even a trace of the glenoid cavity. In the embryos of several Birds an additional digit has been found on the ulnar or postaxial Fig. 1068.— Apteryx mantelli. left shouder-girdle. A, anterior ; B, literal (outer) surface, acr. acromirn ; .) — the whole pubis being singularly like that of a Dinosaur. In the Ostrich alone the pubes unite in the middle ventral line to form a symphysis : Rhea presents the unique peculiarity of a dorsal symphysis of the ischia, just below the vertebral column: in the Emu the posterior end of the pubis FIG. 1070.— Sterna wilsoni (Tern). Fore-limb of embryo, dg. 1 — It, digits ; hu. humerus ; ra. radius ; id. ulna. (After Leighton.) Fio. 1071.— A . 1071.— Apteryx australis. Left innominate, a. acetabulum ; il. ilium ; is. ischium ; p. pectineal process ; j)!. pubis. (From Wiedersheim's Comparative Anatomy, after Marsh.) gives off a slender process, which extends forwards close Ho the ventral edge of that bone and probably represents the epi-pubis of Reptiles. XIII PHYLUM CHORDATA 433 S'v'llll?f FIG. 1072.— Callus bankiva (common Fowl). Innominate of a six days' embryo. Jl. ilium ; Js. ischium ; pb. pubis ; pp. pectineal process. (From Wiedersheim's Comparative Anatomy, after Johnson.) The bones of the hind-limb are very uniform throughout the class, but the form of the tarso-meta tarsus of Penguins is worthy of notice. It is short and wide, its three con- stituent metatarsals, though fused, are clearly distinguish- able throughout their whole length, and the resemblance to the homologous part in Iguanodon is very striking. In the embryo (Fig. 1073) a vestige of the fifth digit (Mt. tsl. 5) has been found in the form of a small rod of cartilage on the postaxial or fibular side. One or two free cen- tralia may occur in the meso- tarsal joint. The skeleton is always more or less pneumatic, but there is no definite relation between pneumaticity and power of flight. A very usual arrangement is for all the bones to contain air except those of the fore-arm and hand, shank and foot. But in Apteryx, Pen- guins, and some Song-birds the skull alone is pneumatic, while in the Hornbill every bone in the body contains air. Myology. — As might be in- ferred from a study of the skele- ton, the muscles of flight undergo a great reduction, often amount- ing to complete atrophy, in the Ratitas ; and to a less degree in the flightless Carinatae. The pre- Mttsl.i ^TW\ ^|W" \ sence or absence of an ambiens Mtf 7 -r*Jf P vl \ an<^ °^ certa^n otner muscles in * 2 ( ? i % ] ^e ^e£ anc^ *n ^e w^n& furnisn \ ^ / characters of considerable classi- \^ I / ficatory importance. Digestive Organs. — In all FIG. io73.-APteryx oweni. Left hind- existing Neornithes the iaws are limb of embrvo. dorsal asrject. dtst. *? - . . «* . , covered by a horny beak and there are no teeth. But that teeth were present in the more primitive Birds, and have gradually been lost during the evolution 1073.— Apteryx oweni. Left hind- limb of embryo, dorsal aspect, dist. distal e ; Fe. femur ; Fib. fibula ; Jib. fibu- lare ; Mt. tsl. 1—5, metatarsals ; Tib. tibia ; tib. tibiale. (After T. J. Parker.) 434 ZOOLOGY SECT. of the recent orders, seems certain from the fact that the Cretaceous Birds were toothed. In Hesperornis (Fig. 1053) there are long conical teeth in both jaws, set in a continuous groove. In Ichthyornis (Fig. 1054) the teeth are theeodont, like those of the Crocodile, each being placed in a distinct socket. In Gastornis and in Odontopteryx, an extinct carinate form allied to the Anseres, the margins of the bony jaws are produced into strong, pointed, tooth-like prominences. Supposed vestigial teeth have been discovered in the young of some Parrots. In the enteric canal the chief variations have to do with the size of the crop and of the caeca, with the gizzard, and with the coiling of the intestine. In grain-eating Birds the gizzard has thick muscular walls and is lined by a thickened horny epithelium, as in the Pigeon : in flesh-eaters, such as Gulls, Petrels, Hawks, and Owls, it is thin- walled and lined with epithelium of the ordinary character. In the Common Fowl and many other Birds the casca are of great length. A gall-bladder is usually present : the spleen is always small. The tongue may be pointed, as in the Pigeon ; very long and protrusible, as in Woodpeckers ; short arid thick, as in Parrots ; or modified for honey-sucking by the tip being produced either into a brush-like organ or into paired sucking tubes. There are variously situated buccal glands, to some of which the name salivary is often applied. Respiratory and Vocal Organs. — The rings of the trachea are always ossified : the tube is frequently deflected to one side by the crop, as in the Pigeon, and may undergo such an increase in length as to extend beneath the skin of the abdomen, or even into the keel of the sternum. The syrinx is either tracheo-lronchial, as in the Pigeon — i.e., formed by the distal end of the trachea and the proximal ends of the bronchi, or is exclusively tracheal or ex- clusively bronchial. In singing Birds it is complex, and is provided with numerous muscles — five or six pairs — for altering the tension of the vibrating membrane. The lungs are always firmly fixed to the dorsal body-wall by a pulmonary aponeurosis, and are but slightly distensible. The general arrangement of the air sacs has been described in the Pigeon (p. 400) : in Apteryx the abdominal air-sacs are small, and are completely enclosed by the oblique septum, so as not to extend into the abdominal cavity among the viscera. The bronchi send off branches at right angles. The Circulatory Organs agree in all essential respects with those of the Pigeon : their most characteristic features are the large size of the heart, the muscular right auriculo-ventricular valve, the atrophy of the left aorlic arch, and the vestigial character of the renal portal system. The red blood-corpuscles are always oval and nucleated. Nervous System and Sense-Organs. — The brain is also xiir PHYLUM CHORDATA 435 very uniform in structure, being characterised by its short, rounded hemispheres, large folded cerebellum produced forwards to meet the hemispheres, and laterally placed optic lobes. In the embryo the optic lobes have the normal dorsal position, and the whole brain resembles that of a Reptile. In Apteryx, in correlation with the reduction of the eyes, the optic lobes are very small, and are situated on the under side of the brain. Above the anterior commissure is a small bundle of fibres which has been considered as the homologue of the hippocampal commissure of Mammals, but hippocampi are not developed. Apteryx is distinguished by the high development of the olfactory chamber, which extends from the tip of the beak to the level of the optic foramina : the turbinals are large and complex, and there is a vestige of the cartilage of Jacobson's organ. The small eye of Apteryx differs from that of all other Birds in the absence of a pecteu, although a vestige of that organ occurs in the embryo. The structure of the auditory organ is very uniform throughout the class. Urinogenital Organs. — In these, also, the general agreement with the Pigeon is very close, the most characteristic feature being the more or less complete atrophy of the right ovary and oviduct. The Megistanes, Rhese, Anseres, and some other Birds have & penis in the form of a thickening of the ventral wall of the cloaca: it has a groove on the dorsal surface acting as a sperm-channel, and its distal end is invaginated, in the position of rest, by an elastic ligament. In the Ostrich there is a solid penis, like that of Chelonia and Crocodiles: it can be retracted into a pouch of the cloaca. Development. — The process of development in Birds has been most thoroughly \vorked out in the Common Fowl, but enough is known of the embryology of other Birds to show that the differences are comparatively unimportant. The ovum is always large owing to the great quantity of food-yolk ; the protoplasm forms a small germinal disc at the upper pole. Im- pregnation is internal, and, as the oosperm passes down the oviduct, it is coated by successive secretions from the oviducal glands. It first receives a coat of thick, viscid albumen (Fig. 1074, alb.)9 which, as the egg rotates during its passage, becomes coiled at either end into a twisted cord, the ckalaza (ch.\ Next, more fluid albumen (alb!) is deposited layer by layer, then a tough, parchment-like shell-membrane (sh. m.), and finally a calcareous shell (sh.). The shell-membrane is double, and, at the broad end of the egg, the two layers are separate and enclose an air-cavity (a.). The shell may be white or variously coloured by special pigments : it consists of three layers, and is traversed by vertical pore-canals, which are unbranched in the Carinatse and in Apteryx, branched in the other Ratitse. The eggs may be laid on the bare ground or on the rocks by the 436 ZOOLOGY SECT. sea-shore, as m Penguins and Auks, or on the ledges on inaccessible cliffs, as in the Sooty Albatross (Diom-edea fuliginosa) ; but as a rule a nest is constructed for their reception by the parent Birds. This may be simply a hole in the sand, as in the Ostrich ; a mere clearing on the hill-side surrounded by a low wall of earth, as in the Wandering Albatross (Diomedea exulans) ; or a cylinder with excavated top, built of grass, earth, and manure, as in the Molly- mawks (Diomedca melanoplirys, etc.). It may take the form of a burrow, as in many Petrels, Kingfishers, and Sand-martins, or it may be more or less elaborately built or woven of sticks, moss, leaves, hair, or feathers, showing every stage of constructive skill, sh.m. alb FIG. 1074.— OallUS bankiva (domestic Fowl). Semi-diagrammatic view of the egg at tin- 1 iim* of laying, «. air-space; alb. dense layer of albumen ; alb', more fluid albumen ; bl. Mast it- derm ; ch. i-hala/a ; sh. shell ; sh. m. shell-membrane ; sh. m. 1, xA. //;. .', its two layers separated to enclose air-cavity. (From Marshall's Embryology, slightly altered.) from the rude contrivance of sticks of the Pigeon and Eagle, to the accurately constructed cup- or dome- shaped nests of many familiar Passeres. In the Tailor-Bird (Orthotomus) it is formed of leaves sewn together, the beak acting as needle : in a Malayan Swift (Collocalia) it is largely built of the secretion of the Bird's buccal glands. The number of eggs laid varies from 15 — 18 in the Partridge, to a single one in many Sea-birds and in the Kiwi. As a rule the size of the eggs bears some proportion to that of the Bird, the smallest being those of Humming-birds, the largest those of the Moas and of ^Epyornis : but in Apteryx the egg is of disproportionate size — as large as a Swan's or an Albatross's, the Kiwi itself being no larger than a Barn-door Fowl. xm PHVLIM I'HOKPATA 4:17 Segmentation takes place during the passage of the egg down the oviduct, and results, as in Reptiles, in the formation of a blttsfn- (Fig. 1074, />/.) occupying a small area on the upper pole of the yolk. After the egg is laid, the process of development is arrested unless the temperature is kept up to about 37° to 40° 0.: this is usually done by the heat of the body of the parent Birds, one or both of which sit upon, or incubate, the eggs until the young are hatched ; but in the Australian mound-makers (Mega- j>odi(i^ the eg^s are buried in heaps of decaying vegetable matter, the decomposition of which generates the necessary heat. In the newly-laid egg the blastoderm is divisible, as in Reptiles, into two parts, a central, clear area ^clhicida (Fig. 1075, ar.pl.) and a peripheral area opaea (ar. op.\ and is formed of a superficial Fin. 107.V — OallUB bankiva. Two stages in the development of the blastoderm : HI'. o/>. area opaea ; Of*. /••/. area pellueida : /<d. <'>'. medullary groove ; no it. mes<>dernt, indieated l>y dotted outline and deeper shade ; /•/•. am. pro-anuu'on ; /»/-. at. primitive streak ; protovertebnr. (I'roni Marshall's Embryology, in part after Duval.) ectoderm having below it a somewhat irregular aggregation of yolk-endoderm cells. On the surface of the area pellucida, as in the Reptiles, appears an c/nhri/onit- shield, the formation of which is due to the elonga- tion of the ectoderm cells in a vertical direction. A primitive knot (p. o(.)4) is not recognisable, and in most Birds there is no imagination. In the posterior part of the area pellucida behind the embryonic shield appears a longitudinal opaque band, the jtrimiticc xtrcak. and along the middle of this is formed a groove, the 'primitive groove. The latter represents the blastopore of the Reptiles, and there is no archenteric cavity. It is by active pro- liferation of cells along the course of the primitive streak, which represents the coalescent lips of the blastopore, leading to the formation of masses of new cells that grow out laterally and VOL. II E E 438 ZOOLOGY SECT. forwards into the space between the ectoderm and yolk-endoderm that the foundations of the mesoderm are formed, In the anterior primitive streak-region the primitive knot of the Reptiles is repre- sented by a close union, for a short space, of all three layers. In front of this, below the head end of the embryonic shield, as in Reptiles, is a thickening of the yolk-endoderm — the protochordal plate, or head-process of the endoderm. This passes behind into the endoderm underlying the primitive streak. As there is no invagination in Birds in general, there is no primitive endoderm, and the definitive endoderm is formed solely from the yolk-endoderm underlying the embryonic shield. The notochord is formed by an axial modification of the endoderm cells along the primitive knot (anterior primitive streak) region and the protochordal plate. In the latter is formed the anterior or head-part of the notochord, and from it are derived also the mesoderm of the head and the endodermal lining of the head- part of the enteric canal. Immediately in front of the primitive streak the medullary groove (med. gr.) appears, and the medullary folds which bound it on the right and left -diverge posteriorly, so as to embrace the anterior end of the primitive streak, in just the same way as they embrace the blastopore in Amphioxus. Both primitive streak and medullary groove lie at right-angles to the long axis of the egg, the broad end of the latter being to the embryo's right. In some Birds there is an invagination at the anterior end of the primitive groove, resulting in the formation of a neurenteric canal. The blastoderm gradually extends peripherally so as to cover the yolk, and thereby becomes divisible into an embryonic portion, from which the embryo is formed, and an extra-embryonic portion which invests the yolk-sac, and takes no direct share in the forma- tion of the embryo. The extension of the ectoderm and endoderm takes place regularly and symmetrically, but the extra-embryonic mesoderm, while extending equally in the lateral and posterior regions, grows forwards in the form of paired extensions, which afterwards unite, so that for a time there is an area of the blastoderm in front of the head of the embryo, formed of ectoderm and endoderm only, and called the pro-amnion (pr. airi). At an early period the vertebral plate, or dorsal portion of meso- derm bounding the medullary groove, becomes segmented into protovertebrse (Fig. 1075, B, pr. v.), and the lateral plate or ventral portion of the same layer splits into somatic and splanchnic layers with the ccelome between (Fig. 1078, B). The notochord (nek.) is developed in the middle line below the medullary groove : some- times it arises directly from the endoderm, as in most of the lower forms, sometimes the mesoderm is formed as a continuous plate, the axial portion of which is subsequently divided off as the notochord. XIII PHYLUM CHORD ATA 439 Gradually the embryo becomes folded off from the yolk-sac, as in other large-yolked eggs ; but, owing apparently to the confined space in which it is enclosed, it soon turns over so as to lie with its left side against the yolk, and its right side facing the shell (Fig. 1077). The body (Fig. 1076, A) becomes strongly flexed so as to bring the head and tail into contact, and the head soon acquires a proportionally immense size, with very large projecting eyes. At first the head is quite like that of one of the lower vertebrate embryos, with protuberant brain -swell ings (/. br., m. br., h. ~br.\ large square mouth, ventrally placed nostrils connected by grooves with the mouth, and three or four pairs of gill-slits. As in fZTL FIG. 107(5.— Gallus bankiva. Two stages in the development of the embryo, all. allantois ; am. cut edge of amnion ; an. anus ; au. ap. auditory aperture ; au. s. auditory sac ; /. br. fore- brain ; /I ?. fore-limb ; h. br. hind-brain ; h. I. hind-limb ; ht. heart ; Jiy. hyoid arch ; m. b. mid- braiu ; mn. mandibular arch ; na. nostril ; t. tail. (After Duval.) f< Reptiles, there is never any trace of gills. In the Ostrich and Apteryx, as well as in some Carinatse, an opercular fold grows backwards from the hyoid arch, and covers the second and third branchial clefts. Soon the margins of the mouth grow out into a beak (Fig. 1076, B), the clefts close, with the exception of the first, which gives rise to the tympano-eustachian passage, and the head becomes characteristically avian. The limbs are at first alike in form and size (A, /. /., h. /.), and the hands and feet have the character of paws, the former with three, the latter with four digits ; but gradually the second digit of the hand outgrows the first and third, producing the characteristic avian manus (B), E E 2 440 ZOOLOGY SECT, xin while the metatarsal region elongates and gives rise to the equally characteristic foot. At the same time feather-papillae make their appearance, arranged in narrow and well-defined ptervlse. At an early period capillaries appear in the extra-embryonic blastoderm between the opaque and pellucid areas, and give rise all FIG. 1077.— Gallus bankiva. Egg with embryo and foetal appendages. «. air-space ; nil. allantois ; «/>t. aranion ; ar. rase, area vasculosa ; emb. embryo ; yk: yolk-sac. (After Duval.) to a well-defined area vasculosa (Fig. 1077, ar. vase.) : they are sup- plied by vitelline arteries from the dorsal aorta, and their blood is returned by vitelline veins which join the portal vein and take the blood, through the liver, to the heart. The vascular area gradually extends until it covers the whole of the yolk-sac : its vessels take an important share in the absorption of the yolk by the embryo. Before the embryo has begun to be folded off from the yolk the rudiment of one of the two characteristic embryonic membranes — the amnion, has appeared. A crescentic amniotic fold arises (Fig. 1078, A, am.f.\ in front of the head-end of the embryo, from the region of the pro-amnion : it consists at first of ectoderm only, the mesoderm not having yet spread into the pro-amnion. The fold is soon continued backwards along the sides of the body (B) and round the tail (A), but in these regions (am./.') it consists from the first of ectoderm plus the somatic layer of mesoderm, i.e., it is a fold of what may be called the extra-embryonic body-wall. The cavity is a prolongation of the space between the somatic and splanchnic layers of mesoderm, i.e., is an extension of the extra- embryonic ccelome. The entire amniotic fold gradually closes in above (C), forming a double-layered dome over the embryo. Its inner layer, formed of ectoderm internally and mesoderm externally, is the amnion (am.), the cavity of which becomes filled with a watery amniotic fluid, serving as a protective water-cushion to the enclosed embryo* FIG. 1078.— Diagrams illustrating the development of the foetal membranes of a Bird. A, early stage in the formation of the amnion, sagittal section ; B, slightly later stage, transverse section ; C, stage with completed amnion and commencing allantois ; D, stage in which the allantois has begun to envelop the embryo and yolk-sac. The ectoderm is represented by a blue, the endoderm by a red line ; the mesoderm is grey. all. atlantois ; all', the same growing round the embryo and yolk- sac ; am. amnion ; am.f., am./', amiiiotic fold ; an. anus ; br. brain ; ecel. coelome ; ccel'. extra-em- bryonic coelome ; lit. heart ; ms.ent. mesenteron ; mth. mouth ; nch. notochord ; »p. cd. spinal cord : ar. m. serous membrane ; umb. d. umbilical duct ; vt. m. vitelline membrane ; yk. yolk-sac. VOL. II E E 2* 442 ZOOLOGY SECT. Its outer layer, formed of ectoderm externally and mesoderm in- ternally, is the serous membrane (sr. m.) : it comes to lie just beneath the vitelline membrane, with which it subsequently fuses. The second of the embryonic membranes, the allantois, is developed as an outpushing of the ventral wall of the mesenteron at its posterior end (C, all.), and consists, therefore, of a layer of splanchnic mesoderm lined by endoderm. It has at first the form of a small ovoid sac having the precise anatomical relations of the urinary bladder of Amphibia (Fig. 1076, A, all.) It increases rapidly in size (Fig. 1077, all.) and makes its way, backwards and to the right, into the extra-embryonic ccelome, between theamnion and the serous membrane (Fig. 1078, C, D). Arteries pass to it from the dorsal aorta, and its veins, joining with those from the yolk-sac, take the blood through the liver to the heart. Next, the distal end of the sac spreads itself out and extends all round the embryo and yolk- sac (D, all'.), fusing, as it does so, with the serous and vitelline membranes, and so coming to lie immediately beneath the shell- membrane. It finally encloses the whole embryo and yolk-sac together with the remains of the albumen, which has by this time been largely absorbed. The allantois serves as the embryonic respiratory organ, gaseous exchange readily taking place through the porous shell; its cavity is an embryonic urinary bladder, excretory products being discharged into it from the kidneys. At the end of incubation the embryo breaks the shell, usually by means of a little horny elevation or caruncle at the end of the beak. By this time the remainder of the yolk-sac has been drawn into the ccelome, and the ventral body-walls have closed round it. On the shell being broken, respiratory movements begin, the aperture is enlarged, and the young bird is hatched and begins a free life. In the Ratitee, Anseres, Gallinas, and some other Birds the young when hatched are clothed with a complete covering of down or of feathers, and are precocious, being able from the first to run about and feed themselves ; such Birds are called Prcecoces or Nidifugce. In the higher types, such as the Rapacious Birds, Pigeons, and Passeres, the young are at first either quite naked, blind and helpless, or covered with mere patches of soft down, so that they require to be fed and kept warm by the parents ; these non-precocious forms are called Altrices or Nidicoloe. In many Sea-birds, such as Petrels, Gulls, and Penguins, the young have a complete covering of woolly down, but remain in the nest for a prolonged period, sometimes until the full size is attained. Distribution. — The Ratitae furnish an interesting case of dis- continuous distribution. Struthio occurs in Africa and South- western Asia, Rhea in South America, Dromseus in Australia, Casuarius in Australia, New Guinea, and some of the other Austro- Malayan islands, and Apteryx in New Zealand. Thus, taking recent forms only, each of the great Southern land-masses contains xiii PHYLUM CHORDATA 443 one order of Ratitae not found elsewhere ; the Struthiones are Ethiopian, but extend also into the adjacent part of the Palaaarctic region, the Rhese Neotropical, and the Megistanes Australasian. ^Epyornis, the affinities of which appear to be with the Megis- tanes, occurs only in Madagascar, where it has become extinct within — geologically speaking — comparatively recent times. Taking the scattered distribution of the above-mentioned Ratitse into consideration, one of the most remarkable facts in distri- bution is the occurrence, in the limited area of New Zealand, of no fewer than six genera and between twenty and thirty species of Dinornithidae or Moas, some of which became extinct so short a time ago that their skin, flesh, feathers, dung, and egg-shells are preserved. Among the CarinataB the Penguins are exclusively southern, occurring only in the South Temperate and Arctic Oceans. They may be said to be represented in the Northern Hemisphere by the Puffins and Auks, one of which, the now extinct Great Auk or Gare- fowl (Alca impennis) was actually impennate, its wings being con- verted, as in the Penguins, into paddles. The Crypturi (Tinamous) are exclusively Neo-tropical, the Humming-birds American, the Birds of Paradise and Bower-birds Australian and Austro-Malayan. Amongst negative facts, the Psittaci or Parrots are characteristic- ally absent in the Palsearctic and most of the Nearctic region, the Finches in the Australasian region, as well as in New Zealand and Polynesia, and the Starlings in both regions of the New World. Birds are comparatively rare in the fossil state : their powers of flight render them less liable to be swept away and drowned by floods and so embedded in deposits at the mouths of rivers or in lakes. Up to the Cretaceous period, Archseopteryx, from the Lower Jurassic, is the only Bird known. In the Cretaceous of North America toothed Birds of the orders Odontolcse and Ichthyornithes make their appearance, while in the Eocene numerous interesting forms occur, including the Gastornithes and the Stereornithes. Ethology. — It is impossible here to do more than allude, in the briefest way, to the immense and fascinating group of facts relating to the instincts, habits, and adaptations found in the class Aves. Their social instincts, their song, their courtship-customs, the wonderful advance in the parental instinct, leading to diminished mortality in the young, are all subjects for which the reader must be referred to the works on general Natural History men- tioned in the Appendix. The same applies to the puzzling subject of migration, which will be referred to in the Section on Distribution. Phylogeny. — That Birds are descended from Reptilian ances- tors, that they are, as has been said, "glorified Reptiles" seems as certain as anything of the kind can well be. Apart from the 444 ZOOLOGY SECT. direct evidence afforded by Arch?eopter37x and by the numerous avian characteristics of Dinosauria and Ornithosauria, the indirect evidence of anatomy and embryology is very strong. The single occipital condyle, the six bones to each mandibular ramus, the ankle- joint between the proximal and distal tarsals, the number of phalanges in the digits of the foot, the epidermal exoskeleton partly taking the form of scales, the meroblastic egg with large food-yolk, the amnion, and the respiratory allantois, are all characters common to Birds and Reptiles and not found together — indeed for the most par.t not found at all — in any other class. For this reason Reptiles and Birds are often conveniently grouped together, as already stated (p. 313), as Sauropsida. It seems probable that the earliest Birds could fly, and that their evolution from Reptilian ancestors was directly connected with the assumption of aerial habits. It is not unlikely that these ances- tors possessed a patagium, like that of Ornithosauria, and that, as the scales of the fore-limb developed into feathers, this organ was gradually reduced to the small pre- and post-patagia of the exist- ing Bird's wing. What was the nature of the reptilian ancestor is a question as yet quite unsolved. It can hardly have been a Pterodactyle, since in that order the modification of the fore-limb has proceeded on entirely different lines from those which charac- terise Birds ; it cannot well have been a Dinosaur, since we hav^e no evidence that any member of that order was arboreal, or showed the least tendency on the part of the fore-limb to assume the wing- form. Nevertheless the skull and brain of Ornithosauria and the pelvis and hind-limb of many Dinosauria show such approximation to avian characters as can hardly be without significance. Probably the earliest Birds were all, in the etymological sense, Carinatae, i.e., had the sternum provided with a keel for the attach- ment of the pectoral muscles. Probably, also, they all possessed teeth, and had diverged into well-marked orders before those organs were lost. The Odontolcae, for instance, have their nearest allies in the Divers (Pygopodes), while the Ichthyornithes resemble the Terns, members of the widely separated order Gaviaa. In several existing types of Carinataa the power of flight is wanting, and in all such cases it is practically certain that Sight- lessness is due to the degeneration of the wings : in other words, that the ancestors of the Penguins, Great Auk, Dodo, Weka (Ocydromus), Kakapo (Stringopsj, &c., were ordinary flying Birds. In the Penguins and the Great Auk the wings have simply under- gone a change of function, being converted into paddles, and con- sequently the only parts of them which have degenerated are the feathers ; but in the other forms referred to, the wing has become more or less functionless, and hence has diminished in size, while the partial atrophy of the muscles has resulted in a more or less complete reduction of the carina sterni and furculum and an increase of the coraco-scapular angle. Now it is by an exaggeration of these XIII PHYLUM CHORDATA 445 peculiarities that the Ratitae are distinguished from the Carinatae, and there is every reason for thinking that they also are the de- scendants of flying Birds, and that their distinctive characters — absence of locking apparatus in the feathers, keel-less sternum, wide coraco-scapular angle, &c. — are all due to degeneration correlated with disuse of the wings. From the fact that the dromaeognathous skull is more reptilian than any other type, it woukl seem that the Ratitse diverged early from the carinate stock. From the fact that, in the structure of the skull and pelvis, the Ostrich and Rhea are widely separated both from one another and from the Austral- asian Ratitae, it . seems probable that the three orders of Ratitae arose independently from primitive Carinatae, and that the entire division is to be looked upon as a convergent Q? polyphyletic group, owing its distinctive characters, not to descent from a common ancestor, but to the independent acquisition of similar characters under the influence of like surroundings. The question of the phylogeny of the orders of Carinatae is far too complex to be discussed here. Suffice it to say that the Ichthy- ornithes, Odontolcae, Impennes, Pygopodes, and Crypturi are to be looked upon as the lowest or most generalised orders, while the highest or most specialised are the Psittaci, the Accipitres, the Striges, the Picariae, and especially the Passeres. Among the latter the Corvidse (Crows) are probably the most exalted members of the class (Fig. 1079). PASSERES GAVIAE COLYMBI \ICHTHyORNITHES N! OOONTOLCA GALLINAE MEGISTANES ORNITHOSAURIA OINOSAURIA FIG. 1079. -Diagram illustrating the Relationships of the chief groups of Birds. 446 ZOOLOGY SECT. CLASS VI,— MAMMALIA. The class Mammalia, the highest of the Vertebrata, comprises the Monotremes and Marsupials, the Hoofed and Clawed Quadru- peds, the Whales and Porpoises and Sea-Cows, the "Rodents, Bats and Insectivores, the Lemurs and Apes, and the Human Species. All Mammals, though many are aquatic, are air-breathers through- out life, lungs being, as in Reptiles and Birds, the sole organs of respiration. The blood of Mammals has a high temperature, resembling in that respect the blood of Birds, and differing from that of Reptiles and Amphibia. The scales of Reptiles and the feathers of Birds are replaced in Mammals by peculiar epidermal structures, the hairs, usually developed in such quantities as to form a thick, soft covering or fur. The young are nourished after birth by the secretion of mammary or milk-glands. 1. EXAMPLE OF THE CLASS— THE RABBIT (Lepus cuniculus). External Characters.— The Rabbit (Fig. 10SO) is a four- footed or quadrupedal animal, having the whole surface of its FIG. ).— Lepus cuniculus. Lateral view of skeleton with outline of body, body covered with soft fur. The head bears below its anterior extremity the mouth, in the form of a tranverse slit bounded by soft lips. The upper lip is divided by a longitudinal cleft, running backwards to the nostrils, and exposing the chisel-shaped incisor teeth. Behind the incisor teeth the hairy integument projects on each side into the cavity of the mouth. At the end of the snout, above the mouth, are the nostrils, in the shape of two oblique slits. The large eyes, situated at the sides of the head, have each three eyelids, an upper and a lower hairy lid, and an anterior hairless third eyelid or nictitating membrane, supported by a plate of cartilage. Vibrissce — very long stiff hairs — are scattered above and below the xm PHYLUM CHORD ATA 447 ' eyes and on the snout. Behind the eyes and a little nearer the summit of the head, are a pair of very long flexible and movable external ears or pinna?. These are somewhat spout-shaped, expand- ing distally, and are usually placed vertically with the concavity directed laterally and somewhat forwards, leading to the external auditory opening. The neck is a distinct constriction, but rela- tively short as compared with the neck of the Pigeon. The trunk is distinguishable into thorax in front and abdomen behind. On the ventral surface of the abdomen in the female are four or five . pairs of little papillae — the teats. At its posterior end, below the root of the tail, is the anal opening, and in front of this in the male is the penis, with a small terminal urinogenital aperture, and with the testes, each in a prominent scrotal sac, at the sides : and in the female the opening of the vulva. In the space (perinceum) between anus and penis or vulva are two bare, depressed areas of skin into which open the ducts of certain glands— the perinceal glands — with a secretion having a strong and characteristic odour. The tail is very short and covered with a tuft of fluffy fur. The fore- and hind-limbs, both of which take part in locomotion and in supporting the weight of the animal, differ considerably in size — the fore-limbs being much shorter than the hind-limbs. Both have the same general divisions as in the Lizard. The upper arm is almost completely hidden by the skin being applied closely against the side of the body. The manus is provided with five digits, each terminating in a horny claw. The thigh "is also almost hidden by the skin ; the pes has four digits only, all pro- vided with claws. Skeleton. — The spinal column of the Rabbit is divisible, like that of the Pigeon and the Lizard, into five regions — the cervical, the thoracic, the lumbar, the sacral, and the caudal. In the cervical region there are seven vertebrae ; in the thoracic twelve or some- times thirteen, in the lumbar seven, or sometimes six, in the sacral four, and in the caudal about fifteen. The centra of the vertebras in a young Rabbit consist of three parts — a middle part which is the thickest, and two thin discs of bone — the epiphyses — anterior and posterior, applied respectively to the anterior and posterior faces of the middle part or centrum proper. Between successive centra in an unmacerated skeleton are thin disc-like plates of nbro-cartilage — the inter-vertebral discs. The transverse processes of all the cervical vertebrae, except the seventh or last, are perforated by a canal, the vertebrarterial canal, for the passage of the vertebral artery. The first vertebra or atlas (Fig. 1081, A) resembles the corresponding vertebra of the Pigeon in being of the shape of a ring without any solid centrum like that of the rest. On the anterior face of its lateral portions are two concave articular surfaces for the two condyles of the skull. The second vertebra or axis (A and £) bears on the anterior face of its 448 ZOOLOGY SECT. centrum a peg-like process — the odontoid process (od.) — which fits into the ventral part of the ring of the atlas : it has a compressed spine (sp.), produced in the antero-posterior direction ; its transverse processes are short and perforated by a canal for the vertebral artery. All the cervical vertebrae except the last have their trans- verse processes bifurcated into dorsal and ventral lamellae. The seventh differs from the others in having a more elongated neural spine, in having its transverse processes simple and without per- foration for the vertebral artery, and in the presence on the posterior edge of the centrum of a little concave, semi-lunar facet. The thoracic vertebrae (C) have elongated spines which are mostly directed backwards as well as upwards. The transverse processes are short and stout; each bears near its extremity a small smooth articular surface or tubercular facet for the tubercle of a rib. On met cent fac. Fir,. 1081.— Lepus cuniculus. A, atlas and axis, ventral aspect. ' the ethmo-tiurbinals (e. S^-and with its inferior edge articulates^Mang median bone, with a pair^oMelica^e lateral wings, the vomer\^). None p£these.with the exc&g&vu of the cribriform plate, ta^fce^n^harj^mTfh e bmmtring^rthe cavity of the cranium. Roofing overTrTe^part of the cranial cavity the walls and floor of which are formed by the sphenoid elements, is a pair of investing bones, the parietals (Fig. 1082, pa.\ and further forward is another pair, the frontals (/?*.). The parietals are plate-like bones, convex externally, concave internally, which articulate with the supraoccipital behind by a transverse serrated suture, the lamb- doidal. The right and left parietals articulate together by means of a somewhat wavy suture, the sagittal; in front a transverse serrated suture, the coronal, connects them with the frontals. Between the supra-occipital and the parietals is a median osssi- fication or inter-parietal (int. pa) of varying extent. The frontals are intimately united along the middle line by means of the frontal suture. Laterally their orbital plates form an important part of the upper portion of the inner wall of the orbit ; above this, over each orbit, is a curved, somewhat crescentic process, the supra-orbital process. Between the alisphenoid below, the parietal and frontal above, the frontal and orbitosphenoid in front, and the parietal behind, is a broad bone (sq.\ the superior margin of which is bevelled off : this is the squamosal. It is produced in front into a strong zygomatic process, which curves outwards, then downwards, and finally forwards, to unite with the jugal in the formation of the zygomatic arch. Below the root of the process is a hollow, the glenoid fossa. Behind, the squamosal gives off a slender process, the post-tympanic process (p. t. sq.) which becomes applied to the outer surface of the periotic. Between the occipital and parietal bones, below and behind the XIII PHYLUM CHOftDATA 453 squamosal, are the tympanic and periotic bones. The tympanic forms the bony part of the wall of the external auditory meatus ; below it is dilated to form a process (ty. lid.) projecting on the under surface of the skull — the India tympani. The periotic is a bone of irregular shape, its internal (petrous) portion (Fig. 1088, peri) enclosing the parts of the membranous labyrinth of the internal ear, and externally presenting two small openings — the fenestra ovalis and fenestra rotunda — visible only when the tym- panic is removed ; internally it bears a depression, the floccidar fossa, for the lodgment of the flocculus of the cerebellum. Part of the periotic (mastoid portion) is seen on the exterior of the skull between the tympanic and exoccipital. The periotic and tympanic are not ankylosed together, and are loosely connected with the surrounding bones, being held in position by the post- tympanic processes of the squamosal. Between the tympanic and periotic are two foramina of importance — the stylomastoid, which transmits the seventh cerebral nerve, and the Eustachian aperture, at which the Eustachian tube opens. Roofing over the olfactory cavities are two flat bones— the nasals (nas.) — each having on its inner surface a very thin, hollow process, the naso-turbinal. In front of the nasals are the premaxillae (p. max) — large bones which form the anterior part of the snout, bear the upper incisor teeth, and give off three processes — a nasal, a palatine (pal.p. max), and a maxillary. The maxillae (max.*), which form the greater part of the upper jaw, and bear the pre-molar and molar teeth, are large, irregularly-shaped bones, the outer surfaces of which are spongy. They give off internally horizontal processes — the palatine processes (rial, max) — which unite to form the anterior part of the bony palate. Between the premaxillse and maxillae and the palatines on the lower surface of the skull is a large, triangular opening divided into two — the anterior palatine foramina — by the palatine processes of the premaxilla3. On the outer surface of each maxilla, above the first pre-molar tooth, is a foramen — the infra-orbital (i.o. fo) — through which the second division of the fifth nerve passes. A strong process which is given off from the outer face of each maxilla, and turns outwards and then backwards to unite with the zygomatic process of the squamosal and thus complete the zygomatic arch, is a separate bone in the young — the malar orjugal (/?£.)• The maxilla? help to bound the nasal cavities externally, and with each is connected on its inner aspect a pair of thin scroll-like bones — the maxillo-turbincds (Fig. 1083, mx. <$.). The rest of the narrow bony palate, forming the roof of the mouth and the floor of the nasal cavities, is formed by the palatine plates of the palatine bones (pal.}. The pterygoids (p.) are small irregular bones, each of which articulates with the palatine in front and with the pterygoid process of the alisphenoid behind. The lacrymals (ler.} are small 454 ZOOLOGY SECT. bones, one situated in the anterior wall of each orbit and perforated by a small aperture — the lacrymal foramen. In the interior of the skull (Fig. 1083) are three cavities, the two olfactory or nasal cavities, right and left, in front, and the cranial cavity behind. The former are separated from one another by a median partition or septum, partly cartilaginous, partly bony, formed, as above described, by the mesethmoid. Each contains the turbinals or turbinated bones of its side ; it opens on the exterior by- the large external nasal aperture, and behind it communicates with the cavity of the mouth by the posterior nasal aperture. The cranial cavity has its walls moulded to a considerable extent on the surface of the contained brain, and, in consequence, there FIG. 1083,— Lepus cuniculus. Skull in longitudinal vertical section. The cartilaginous nasal septum is removed, a. sph. alisphenoid ; e. oc. exoccipital ; e. tb. ethmo-turbinal ; nth. ethmoid ; fl. fossa for flocculus of brain ; i. incisors ; mx tb. maxillary turbinal ; n. tb. iiaso-turbinal ; pal', palatine portion of the bony palate ; peri, periotic (petrous portion) ; p. sph. presphenoid ; sph. f. sphenoidal fissure ; st. sella turcica, or depression in which the body lies ; I. point .at which the olfactoryriervcs leave the skull; II. optic oramen ; \ mn. foramen for _mandibular division of trigerninal ; VII. for facial nerve; d spinal access iced Zoology.) VIII. for auditory nerve ; IX, X, XI, for glossopharyngeal, vagus, and spinal accessory ; XII. for hypoglossal. Other letters as in Fig. 1082. (From Parker's Practic are to be recognised concavities in the former corresponding with the prominent portions of the latter. These concavities are termed the fossae, and they consist of the cerebellar fossa behind and the cerebral fossa in front, with the inconspicuous olfactory fossa in the frontal region. The mandible, or lower jaw, consists of two lateral halves or rami, which are connected with one another in front by a rough articular surface or symphysis, while behind they diverge like the limbs of a letter V. In each ramus is a horizontal portion (anterior) which bears the teeth, and a vertical or ascending portion, which bears the articular surface or condyle (condJ) for articulation with the glenoid cavity of the squamosal ; in front of the condyle is the compressed coronoid process. The angle where the horizontal and ascending processes meet gives off an inward projection or angular process (any. pro.). ' xiii PHYLUM CHORDATA 455 The hyoid consists, in addition to separate vestigial slylo-hyals , of a stout, thick body or basi-hyal, a pair of small anterior cornua or ccrato-liyals, and a pair of longer, backwardJy-directed cornua or thyro-liyals. The auditory ossicles, contained in the cavity of the middle ear, and cut off from the exterior, in the unmacerated skull, by the tympanic membrane, are extremely small bones, which form a chain extending, like the columella auris of the Pigeon, from the tympanic membrane externally to the fenestra ovalis internally. There are three of these auditory ossicles — the stapes, which corre- sponds to the columella of the Pigeon ; the incus, and the malleus, the latter with a slender process — the processus gracilis. In addition there is a small disc-like bone, the orbicular, which is attached to the incus. The elements of the pectoral arch are fewer than in the Lizard. There is a broad, thin, triangular scapula, the base or vertebral edge of which has a thin strip of cartilage (the supra-scapular cartilage) continuous with it. Along the outer surface runs a ridge — the spine ; the spine ends below in a long process — the acromion-process — from which a branch process or metacromion is given off behind. The part of the outer surface of the scapula in front of the spine is the pre-'spinous or pre-scapular fossa, the part behind is the post-spinous or post-scapular fossa. At the narrow lower end of the scapula is a concave surface — the glenoid cavity— into which the head of the burner us fits, and immediately in front of this is a small inwardly curved process — the coracoid process — which is represented by two separate ossifications in the young Rabbit. A slender rod — the clavicle — lies obliquely in the region between the pre-sternum and the scapula, but only extends a part of the distance between the two bones, and in the adult is only connected with them through the intermediation of fibrous tissue. The skeleton of the fore-limb is more readily comparable with that of the Lizard than with that of the Bird ; but there is a difference in the position of the parts owing to the rotation back- wards of the distal end of the humerus, all the segments being thus brought into a plane nearly parallel with the median vertical plane of the body, with the pre-axial border directed outwards, and the original dorsal surface backwards. The radius and ulna are fixed in the position of pronation, i.e., the distal end of the radius is rotated inwards, so that, while the proximal end is external to the ulna, the distal end becomes internal, and the digits of the manus are directed forwards. At the proximal end of the humerus are to be recognised: (1) A rounded head for articulation with the glenoid cavity of the scapula ; (2) externally a greater and (3) internally a lesser tuberosity for the insertion of muscles ; (4) a groove, the l)icipital groove, between the two tuberosities. On the anterior surface 456 ZOOLOGY SECT. rad of the proximal portion of the shaft is a slight ridge, the deltoid ridge. At the distal end are two articular surfaces, one large and pulley-like — trochlea — for the ulna; the other smaller — capitellum — for the radius : laterally are two prominences or condyles, an internal and an external. The radius and ulna are firmly fixed together so as to be in- capable of movement, but are not actually ankylosed. The radius articulates proximally with the humerus, distally with the scaphoid and lunar bones of the carpus. The ulna presents on the anterior aspect of its proximal end a deep fossa, the greater sigmoid cavity, for the trochlea of the humerus ; the prominent process on the proximal side of this is the olecranon process. Distally it articulates with the cuneiform. The carpal bones (Fig. 1084), nine in number, are all small bones of irregular shape. Eight of these are arranged in two rows — a proximal and a distal ; the ninth, centrale (cent.), lies between the two rows. The bones of the proximal row are — taken in order from the inner to the outer side, scaphoid (sc.), lunar (or semi-lunar) (lun.), cuneiform (cun.), and pisiform. Those of the distal row are, reckoned in the same order, trapezium (trpm.\ trapezoid (trpz.), magnum (mag.), and vmciforni (unc.J- The five metacarpals are all small, but relatively narrow and elongated bones, the first being smaller than the rest. Each of the five digits has three phalanges, except the first, which has only two. The distal (ungual) phalanges are grooved dorsally for the attachment of the horny claws. The pelvic arch (Fig. 1085) contains the same elements as in the Pigeon, but the union of the ilium with the sacrum is less intimate, the acetabulum is not perforated, and the pubes and ischia of oppo- site sides unite ventrally in a symphysis (sy.}. The three bones of the pelvis — ilium, pubis and ischium — are separate ossifications in the young Rabbit; but in an adult animal complete fusion takes place between the bones. The ilium and ischium meet in the acetabulum or articular cavity, which they contribute to form, for the head of 1 The homologies of these bones are not quite certain, but are very probably as follows: — scaphoid = radiale, lunar = 1st centrale, cuneiform = intermedium, pisiform = ulnare, centrale = 2nd centrale, trapezium = 1st distale, trapezoid = 2nd distale, magnum = 3rd distale, uncif orm = 4th and 5th distalia. FIG. 1084.— Lepus cuniculus. Distal end of fore-arm and carpus, dorsal view, the bones partly separated, cent, cen- trale ; cun. cuneiform ; lun. lunar ; mag. magnum ; md. radius ; sc. scap- hoid ; trpz. trapezoid ; trpm. trapezium ; utn. ulna ; unc. unciform ; / — V, bases of metacarpals. (After Krause.) XIII PHYLUM CHORDATA 457 the femur, but the remainder of the cavity is bounded, not by the pubis, but by a small intercalated ossification — the cotyloid bone. The ilium (il.) has a rough surface for articulation with the sacrum. Between the pubis (pubJ) in front and the ischium (iscA.) behind is a large aperture — the obturator foramen (obtJ). The femur is rotated forwards when compared with that of the Lizard, so that the limb is nearly in the same plane as the fore-limb, and the pre- axial border is internal and the originally dorsal surface anterior. The femur has at its proximal end a prominent head for articulation with the acetabulum ; external to this a prominent process — the great trochanter, and internally a much smaller — the lesser trochanter, while a small process or third trochanter is situated on the outer border a little Fi^i r below the great trochanter. At its curt FIG. 1085.— Lepus cuniculus. Innominate bones and sacrum, ventral aspect, acet. acetabulum ; il. ilium ; isclt. ischium ; obt. obturator foramen ; pub. pubis ; Kitrr. sacrum; sy. symphysis. IV Jff FIG. 1080.— Lepus cuniculus. Skeleton of pes. ast. astragalus ; col. calcaneum ; cub. cuboid ; cun. cuneiforms ; nav. navi- cular. distal end are two prominences or condyles, with a depression between them. Two small sesamoids or fcibellce are situated opposite the distal end on its posterior aspect ; and opposite the knee-joint, or articulation between the femur and the tibia, is a larger bone of similar character — the knee-cap or patella. The tibia has at its proximal end two articular surfaces for the condyles of the femur ; distally it has also two articular surfaces, one, internal, for the astragalus, the other for the calcaneum. The fibula is a slender bone which becomes completely fused distally with the tibia. The tarsus (Fig. 1086) consists of six bones of irregular shape, VOL. II F F 458 ZOOLOGY SECT. arranged in two rows, one of the bones — the navicular (nav.) — being intercalated between the two rows. In the proximal row are two bones — the astragalus (ast.*) and the calcaneum, (cal.) — both articulating with the tibia; the calcaneum presents behind a long calcaneal process. The distal row contains three bones, the meso- cuneiform, edo-cuneiform and cuboid (cub.*) ; the ento-cuneiform, which commonly forms the most internal member of this row in other Mammals, is not present as a separate bone.1 There are four metatarsals, the hallux or first digit being vestigial and fused with -the second metarsal in the adult. The proximal end of the second is produced into a process which articulates with the navicular. Each of the digits has three phalanges, which are similar in character to those of the manus. The ccelome of the Rabbit differs from that of the Pigeon and Lizard in being divided into two parts by a transverse muscular partition, the diaphragm. The anterior part, or thorax, contains the heart and the roots of the great vessels, the lungs and bronchi, and the posterior part of the oesophagus. The posterior part, or abdomen, contains the stomach and intestine, the liver and pancreas, the spleen, the kidneys, ureters and urinary bladder, and the organs of reproduction. Digestive Organs. — The teeth (Fig. 1082) are lodged in sockets or alveoli in the premaxillas, the maxillae, and the mandible. In the premaxillas are situated four teeth — the four upper incisors. Of these the two anterior are very long, curved, chisel-shaped teeth, which are devoid of roots, growing throughout life from per- sistent pulps. Enamel is present as a thick layer on the anterior convex surface only, which accounts for the bevelled-off character of the distal end — the layer of enamel being much harder than the rest, which therefore wears more quickly away at the cutting extremity of the tooth. Along the anterior surface is a longitudinal groove. The second pair of incisors of the upper jaw are small teeth which are lodged just behind the larger pair. In the lower jaw are two incisors, which correspond in shape with the anterior pair of the upper jaw, the main difference consisting in the absence of the longitudinal groove. The remaining teeth of the upper jaw are lodged in the maxillae. Canines, present in most Mammals as a single tooth on each side, above and below, are here entirely absent, and there is a considerable space, or diastema,- as it is termed, between the incisors and the teeth next in order — the pre-molars. Of these there are three in the upper jaw and two in the lower. They are long, curved teeth with persistent pulps like the incisors, the first smaller than the others and of simple shape, 1 In all probability the horaologies of these bones are as follows : — astragalus = tibiale + intermedium, calcaneum •= fibulare, navicular = centrale, ento-cunei- form = 1st distale, meso-cuneiform = 2nd distale, ecto-cuneifonn=3rd distale, cuboid = 4th and 5th distalia. XIII PHYLUM CHORDATA 459 the rest grooved longitudinally on the outer side and with two transverse grooves, bounded by ridges, on their crowns. The first pre-molar of the lower jaw has two grooves and three ridges ; the second is similar to those of the upper jaw. Behind the pre- molars are the molars, three on each side both in the upper and lower jaws. These are similar to the upper pre-molars, except the last, which is small and of simple shape. Opening into the cavity of the mouth, or buccal cavity, are the ducts of four pairs of salivary glands — the parotid, the infraorbital, the submaxillary (Fig. 1088, s. mx. gl.\ and the mblingucd (s. gL). On the floor of the mouth is the muscular tongue, covered with a mucous membrane which is beset with many papillae, on certain of which the taste-buds (p. 108) are situated. The roof of the mouth is formed by the hard palate, which is crossed by a series of transverse ridges of sepl.carl 71CLS mazc.trb, its mucous membrane. Posteriorly the hard palate passes into the soft palate, which ends behind in a free pen- dulous flap in front of the opening of the posterior nares. At the anterior end of the palate is a pair of openings — the na*o- palatine or anterior palatine canals, leading into the nasal cham- bers, and into them open a pair of tubular structures — the organs of Jacol)son (Fig. 1087, /<•&.) — enclosed in car- tilage and situated on the floor of the nasal cavities. Behind the mouth or buccal cavity proper is the pharynx, which in the Rabbit is not sharply marked off from the buccal cavity, but begins where the hard palate ends. The pharynx is divided into two parts, an upper or nasal division, and a lower or buccal division, by the soft palate. The passage of the pos- terior nares is continuous with the nasal division, at the sides of which are the openings of the Eustachian tubes. The nasal F F 2 Icxdcl Jcb FIG. 1087.— Lepus cuniculUS. Vertical section through the anterior part of the nasal region of the head. inc. section of larger incisor tooth ; jcb. lumen of Jacobson's organ, surrounded by cartilage ; Icr. . a.) ; the duodenum is spread out to the right of the subject to show the pancreas (pn.) ; the branches of the bile-duct (c. b. d ), portal vein (p. v.)j and hepatic artery (h. a.), are supposed to be traced some distance into the various lobes of the liver, a. in. a anterior mesenteric artery ; cau. caudate lobe of liver with its artery, vein and bile-duct ; c. b. d. common bile-duct ; cd. st. cardiac portion of stomach ; c. il. a. common iliac artery ; cce. a. coeliac artery ; cy. a. cystic artery ; cy. d. cystic duct ; d. ao. dorsal aorta ; du. proximal, and du'. distal limbs of duodenum ; du. a. duodenal artery ; du, h.a. (in A), duodeno-hepatic artery ; g. a. gastric artery and vein ; g. b. gall- bladder ; h. a. hepatic artery ; b. d. left bile-duct ; I. c. left central lobe of liver, with its artery, vein and bile-duct ; /. g. v. lieno-gastric vein ; I.- 1. lateral lobe of liver with its artery, vein and bile-duct ; ms. branch of mesenteric artery and vein to duodenum ; ms. r. meso- rectum ; m. v. chief mesenteric vein ; ves. oesophagus ; p. m. a. posterior mesenteric artery ; p. m. v posterior mesenteric vein ; pn. pancreas ; pn. d. pancreatic duct ; p. v. portal vein ; py. st. pyloric portion of stomach ; ret. rectum ; r. c. right central lobe of liver, with artery, vein and bile-duct ; spy. Spigelian lobe of liver with its artery, vein and bile-duct ; spl. spleen ; sp. a. splenic artery. (From Parker's Zootomy.) 462 ZOOLOGY SECT. marked by a spiral constriction, indicating the presence in its interior of a narrow spiral valve. At its extremity is a small, fleshy, finger-like vermiform appendix. The intestine, like that of the Pigeon, is attached throughout its length to the dorsal wall of the abdominal cavity by a mesentery, or fold of the lining membrane or peritoneum. The liver is attached to the diaphragm by a fold of the peri- toneum. Its substance is partly divided by a series of fissures into five lobes. A thin-walled (/all-bladder lies in a depression on its posterior surface. The common bile-duct (ct>.d.\ formed by the union of cystic duct from the gall-bladder and hepatic ducts from the various parts of the liver, runs to open into the duodenum near the pylorus. The pancreas (pn.) is a diffused gland in the fold of mesentery passing across the loop of the duodenum. Its single duct, the pancreatic duct (pn. d.), opens into the distal limb of the loop. Circulatory Organs. — The heart (Fig. 1090) is situated in the cavity of the thorax, a little to the left of the middle line, and lies between the two pleural sacs enclosing the lungs. Between the pleural sacs is a space, the media- stinum (Fig. 1093). This is divisible into four parts, the anterior, the dorsal, the middle, and the ven- tral. In the anterior part lie the posterior part of the trachea, the neigh- bouring parts of the oeso- phagus and of the thoracic duct of the lymphatic system, the roots of the o-vpnt nr<-pn'p<3 anrl tV»PVPi'n<* §real arter of the pre-Caval SVStem, , , * , ami the phrenic, „„«*«:„ Qnr1 r»f>ia gaStriC, ana OtHe Jj^ the dorsal part are situated the posterior part of the oesophagus, the thoracic part of the dorsal aorta, the pneumogastric nerve, the azygos vein, and the thoracic duct. The middle part is the widest, and lodges the heart and roots of the aorta and pulmonary artery enclosed in the pericardium, the posterior portion of the pre-caval veins, the phrenic nerves, the terminal part of the azygos vein, and the roots of the lungs. The ventral part contains only areolar tissue with the thymus gland. The pericardial membrane enclosing the FIG. 1090.— Lepus cuniculus. Heart, seen from the right side, the walls of the right auricle and right ventricle partly removed so as to expose the cavities, ao. aorta; J. ov. fossa ovalis ; l.pr. c. open- ing of left precaval; m. pap. musculi papillares ; pt c. postcaval ; pt. c'. opening of postcaval ; r.pr.c. right precaval ; r. put. right pulmonary artery ; sem. v. semiluuar valves ; tri. v. tricuspid valve. PHYLUM CHORDATA 467 / aort «J ./y cavity of the thorax. Each lung lies in a cavity, the plcural sac, lined by a pleural membrane. The right and left pleural sacs are separated by a considerable interval owing to the development in the partition between them of a space, the mediastinum, in which, as already explained, lie the heart and other organs. The lung is attached only at its root, where the pleural mem- brane is reflected over it. / In this respect it differs widely from the lung of the Bird. It differs also in its minute structure. The bronchus, entering at the root, divides and subdivides to form a ramifying system of tubes, each of the ulti- mate branches of which, or terminal bronchioles , opens into a minute chamber or infundi- bulum, consisting of a central passage and a number of thin- walled air-vesicles or alveoli given off from it. A group of these infundi- bula, supplied by a single bronchiole, which divides within it to form the terminal bronchioles, is termed a lobule of the lung. In shape the lung may be roughly described as conical with the apex directed forwards. The base, which is concave, lies, when the lung is distended, in contact with the convex anterior surface of the diaphragm. The outer or costal surface is convex in adaptation to the form of the side-wall of the thorax ; the internal surface is concave. Ductless Glands. — The spleen is an elongated, compressed, dark-red body situated in the abdominal cavity in close contact with the stomach, to which it is bound by a fold of the peritoneum. The thymus, much larger in the young rabbit than in the adult, is a soft mass, resembling fat in appearance, situated in the ventral division of the mediastinal space below the base of the heart. The thyroid is a small, brownish, bilobed, glandular body situated in close contact with the ventral surface of the, larynx. FIG. 1 093.— Lepus cuniculus. Diagram of a trans- verse section of the thorax in the region of the ven- tricles to show the relations of the pleurae (dotted line), mediastinum, etc. The lungs are contracted. aort. dorsal aorta ; az. v. azygos vein ; cent, centrum of thoracic vertebra ; I. Ing. left lung ; I. pi. left pleural sac ; I. vent, left ventricle ; my. spinal cord ; as. oesophagus ; par. per. parietal layer of pericardium ; pt. cav. post-caval, close to its entrance into right auricle ; r. Ing. right lung ; r. pi. right pleural cavity; r. vent, right ventricle ; st. sternum ; vise. per. visceral layer of pericardium ; v. med. ventral mediastinum. 468 ZOOLOGY SKCT. Nervous System. — The neural cavity, as in the Pigeon, con- tains the central organs of the cerebro-spinal nervous system— the brain and spinal cord. The brain (Figs. 1094-1096) of the Rabbit contains the same principal parts as that of the Pigeon, m I ly i v p.v. vi vii ix xii FIG. 1094.— Lepus cuniculus. Brain. A, dorsal, B, ventral, C, lateral view. b. o. olfactory bulb ; cb', median lobe of cerebellum (vermis) ; cb". lateral lobe of cerebellum ; cr. crura cerebri ; ep. epiphysis ;/. b. parencephala ;/ p. longitudinal fissure ; h.b. cerebellum ; hp. hypo- physis ; m. b. mid-brain (corpora quadrigemina) ; md. medulla oblongata ; p. v. pons Varolii ; i — xii, cerebral nerves. (From Wiedersheim's Comparative Anatomy.) with certain differences, of which the following are the most im portant. The surface of the cerebral hemispheres or parencephala (Fig. 1094, /. &., Fig. 1095, c.h.), which are relatively long and narrow, presents certain depressions or snlci, \vhich, though few and in- XIII PHYLUM CIIORDATA 469 distinct, yet mark out the surface into lobes or convolutions not distinguishable in the case of the Pigeon or the Lizard. A slight depression — the Sylvian fissure — at the side of the hemisphere separates off a lateral portion, or temporal lobe (Fig. 1096, c. A,2.), from the rest. There are very large club-shaped olfactory bulbs at the anterior extremities of the cerebral hemispheres, and behind each on the ventral surface of the hemisphere is the corresponding — olf. st.l. C.TS FIG. 1005.— Lepus cuniculus. Two dissections of the brain from above (nat. size). In A the left parencephalon is dissected down to the level of the corpus callosum : on the right the lateral ventricle is exposed, In B the cerebral hemispheres are dissected to a little below the level of the anterior genu of the corpus callosum ; only the frontal lobe of the left hemisphere is retained ; of the right a portion of the temporal lobe also is left ; the velum interpositum and pineal body are removed, as well as the greater part of the body of the f ornix, and the whole of the left posterior pillar ; the cerebellum is removed with the exception of a part of its right lateral lobe. a. co. anterior commissure ; a fo. anterior pillar of fornix ; ft. pn. anterior peduncles of cerebellum; b.fo. body of fornix; eft1, superior vermis of cere- bellum; d*2. its lateral lobe; c. gn. corpus geniculatum ; c. h. cerebral hemisphere; ch.pl. choroid plexus ; cp. d corpus callosum ; cp. s. corpus striatum ; c. rs. corpus restiforme ; d. p. dorsal pyramid ; ft. flocculus ; lip. m. hippocampus ; m. co. middle commissure ; o. I1. anterior, arid o. T2. posterior lobes of corpora quadrigemina ; o. th. optic thalamus ; o. tr. optic tract ; p. co. posterior commissure ; p.fo. posterior pillar of fornix ; .pn. pineal body ; pd. pn. peduncle of pineal body ; p. pn. posterior peduncles of cerebellum ; p. va. fibres of pons Varolii forming middle peduncles of cerebellum ; sp. lu. septum lucidum ; at. I stria longi- tudinal is ; t. x. tsvnia .semicircular is ; v. vn. valve of Vieussens ; i-3, third ventricle ; v4, fourth ventricle. (From Parker's Zootomii.) olfactory tract leading back to a slight rounded elevation, the tuberculum olfoctorium. Connecting together the two hemispheres is a commissural structure — the corpus callosum (Figs. 1095, 1096, cp. d.) — not present in the Pigeon ; this runs transversely above the level of the lateral ventricles. Examined in transverse section, i.e., in a longitudinal section of the brain (Fig. 1096), the corpus callosum is seen to bend slightly downwards, forming what is termed the genu\ posteriorly it bends downwards. .and forwards, 470 ZOOLOGY SECT. forming the splenium, which passes forwards and is united, below the corpus callosum, with another characteristic struc- ture of a commissural nature — the fornix (b.fo) — a narrow median strand of longitudinal fibres, which bifurcates both anteriorly and posteriorly to form the so-called pillars of the fornix (anterior and posterior, a. fo., p. /. — Odontoceti. Cetacea in which the prernaxilbe are narrow, and the nostrils far back, as in the Mystacoceti. The nasals are reduced and do not. roof over the nasal cavities. The tympanic bones are not scroll-like, and do not become fused with the periotics. The rami of the mandible are united at the symphysis. Baleen-plates are never present, and teeth are developed and are usually very numerous and homodont. This sub-order comprises the Porpoises (Phoccena), Dolphins (Delpliinus and others), and Killers (Oreo) ; the Sperm- whales (Pliyseter and Cogia)-, the Bottle-nosed Whales (Hyperoodon) ; and Beaked Whales (Mesoplodori). ORDER 3. — SIRENIA. Aquatic Eutheria with moderate-sized head and fish-like, de- pressed, fusiform body, with the pectoral limbs paddle-like, the pelvic absent, and with a horizontally expanded tail-fin. There is no vertical dorsal fin. There is a very thick wrinkled integument devoid of or with only a scattered covering of hairs. The snout is not greatly elongated, and the nostrils open by a pair of valvular XIII PHYLUM CHORDATA 481 apertures on its upper surface. The cervical vertebrae (of which there are only six in the Manatee) are not fused. A clavicle is absent. There is a distinct, though small, articulation between the humerus and the bones of the forearm. There are never more than three phalanges in any of the digits. The pelvis is represented by a pair of vertically situated vestiges. The anterior part of the palate and the symphysis of the mandible (which is prolonged) are covered with rugose, horny plates. The epiglottis and arytenoids are not prolonged as they are in the Cetacea. The brain is com- paratively small, and the convolutions are not highly developed. The testes are abdominal. The teats are two in number and pectoral in position. The uterus is two-horned. The placenta is noD-deciduate and zonary. This order includes among recent forms only the living Dugong (ffalicore) and Manatee (Manatus), and the recently extinct Rhytina. ORDER 4. — UNGULATA. Terrestrial, chiefly herbivorus, Eutheria, with the fur abundant or scanty, with the terminal phalanges, on which the weight of the body usually rests, nearly always invested in solid horny hoofs. The teeth are heterodont and diphyodont; the canines usually absent or small, and the premolars and molars well developed, with broad crowns having tuberculated or ridged surfaces. The clavicle is absent; the humerus has no foramen over the inner condyle : the scaphoid and lunar of the carpus are always distinct. The villi of the placenta are diffuse or gathered into patches — the cotyledons. SECTION 1. — UNGULATA VERA. Ungulata in which the feet are always unguligrade, with never more than four functional digits. The os magnum of the carpus articulates with the scaphoid. The testes are contained in a scrotum. The teats are usually four, and situated far back, never exclusively thoracic in position. The uterus is two-horned, The allantois is large, the placenta non-deciduate, and the villi diffuse or gathered into cotyledons. This section comprises all the typical Ungulates. Sub-order a. — Perissodactyla. Ungulata vera in which the third toe of both manus and pes is larger than the others and symmetrical in itself, and in which there is a tendency to reduction of the others. The femur has a third trochanter. The tibial articular surface of the astragalus is pulley-shaped ; the distal surface flat and more extensively related with the navicular than with the cuboid ; the calcaneum 482 ZOOLOGY SECT. Joes not articulate with the fibula. The premolars and molars are complexly folded, and the posterior premolars usually resemble the molars in size and pattern. The stomach is simple ; the caecum large. There is never a gall-bladder. The teats are situated in the groin, and the placenta is diffuse. This sub-order includes the Horses, Asses, and Zebras (Eguidce), the Tapirs (Tapir us), and the Rhinoceroses (Rhinoceros}. Sub-order I. — Artiodactyla. Ungulata vera in which the third and fourth digits of both in anus and pes form a symmetrical pair, and in which the others are usually absent or vestigial. The femur has no third troclianter. The tibial surface of the astragalus is flat, the distal surface articulates largely with the cuboid, and the calcaneum has a flat articular surface for the fibula. The premolars are smaller than the molars. The stomach is almost always complex, and the caecum is small. The teats are few and situated in the groin, or numerous and extending along the abdomen. The placenta is diffuse or cotyledonary. This sub-order includes the Ruminants — such as the Camels (Camelidai), Oxen (Bovidce), Sheep (Oms) t Goats (Capra), Antelopes, Giraffes (Giraffa), and Deer (Cervidcv)-, and the Non-Ruminants, viz., the Pigs (Sus), Peccaries (Dicotyles), and Hippopotami (Hippopotamus). SECTION 2. — SUBUNGULATA. Ungulata in which the feet may be plantigrade, and there may be five functional digits. The magnum of the carpus does not articulate with the scaphoid, at least in living forms. Sub-order a. — Hyracoidea. Small Subungulata with furry integument, with four completely formed digits in the fore-foot (the pollex being vestigial), and three in the hind-foot (the hallux being absent and the fifth digit vestigial). The uugual phalanges of the four complete digits of the fore-foot are small, somewhat conical and flattened ; that of the second digit of the hind-foot is deeply cleft, and has a long, curved claw; the rest of the digits of the hind-foot have broad nails. There are no canines, and in the upper jaw in the adult there is only a single pair of incisors, which resemble those of the Rodents in their elongated, curved form and in growing from persistent pulps. The thoracic and lumbar vertebras are very numerous (28-30), twenty-one or twenty-two bearing ribs. The tail is very short. Clavicles are absent. There is a centrale in the carpus. The stomach is divided into two parts by a con- striction. The large intestine has connected with it a pair of large xiir PHYLUM CHORDATA 483 supplementary caeca. There is no gall-bladder. The testes do not descend into a scrotum. There are six teats, four in the groin and two in the axillae. The villi surround the placenta in a broad band (zonary placenta). This sub-order includes only a single family, the Hyracidce, with two genera, Hyrax and Dendrohyrax. Sub-order b. — Proboscidea. Large Subungulata with greatly thickened integument scantily furnished with hair ; with massive limbs, each having five com- plete digits united by skin, but each terminating in a distinct hoof; and with the nose produced into a long, flexible and prehensile proboscis or trunk, at the end of which the external nares are situated. In existing forms only a single pair of incisors is present, situated in the upper jaw, and developed into enormous tusks. There are no canines, and the molars are large and transversely ridged. The stomach is simple. The testes do not descend into a scrotum. There are two teats, situated on the thorax. The uterus is two-horned, the placenta deciduate and zonary. This sub-order includes among existing forms only the Elephants (Elcphas). ORDER 5. — CARNIVORA. Mainly carnivorous Eutheria with furry integument, with never less than four well-developed digits in the manus and pes, all provided with claws, which are frequently more or less retractile. The pollex and hallux are never capable of being opposed to the other digits. The clavicle is frequently absent, and, when present is never complete. There is often a foramen over the inner con- dyle of the humerus. The scaphoid and lunar of the carpus are always united, and there is never an os centrale. The Carnivora are diphyodont and heterodont, and the teeth are provided with roots. The incisors, usually three pairs in the upper and three in the lower jaw, are small and chisel-shaped. The canines are usually large, conical, curved, and pointed. The premolars and molars are usually compressed and trenchant, especially the most anterior. The stomach is simple ; the caecum, when present, is small. The brain is usually highly developed, and the cerebral hemispheres always convoluted. The teats are abdominal. The uterus is two-horned ; the placenta deciduate and nearly always zonary. Sub-order a. — Carnivora vera. Carnivora which have the limbs nearly always adapted for a terrestrial existence, with all the digits usually provided with claws, which may be retractile into a sheath. The first digit of 484 ZOOLOGY HECT. * the manns and the first and fifth of the pes are never longer than the others. One tooth on each side in each jaw — the last pre- molar in the upper jaw and the first molar in the lower, is always modified to form the carnassial or sectorial tooth with a cutting edge which bites against the edge of the opposed tooth. This sub-order comprises the Cats (Felidai), Civets ( Viverri', duct of sweat- gland ; SM, Malpighian layer. (From Wiedersheim's Comparative Anatomy.) PHYLUM CHORDATA 489 the one sort very numerous and forming the soft fur, and those of the other consisting of longer and coarser hairs scattered over the surface. Examples of a hairy covering of this kind are seen in the case of the Platypus and the Fur- Seals. A hair, like a feather, is formed from the epi- dermis. The first rudi- ment of a developing hair (Fig. 1102) usually takes the form of a slight down- wardly projecting out- growth — the hair-germ (grin.) — from the lower mucous layer of the epi- dermis, beneath which there is soon discernible a condensation of the dermal tissue to form the rudiment of a ft air-papilla (pp.). In some Mammals, however, the dermal papilla makes its appear- ance before the hair-germ. The hair-germ, which con- sists of a solid mass of epidermal cells, elongates, and soon its axial portion becomes condensed and cornified to form the shaft of the hair, while the more peripheral cells go to form the lining of the hair-follicle, becoming ar- ranged in two layers, the inner and outer root-sheaths (.sA1.,^2.). The epidermal cells in immediate con- tact with the hair-papilla retain their protoplasmic character and form the FIG. 1101.— Longitudinal section through a hair (dia- grammatic). A/>, band of muscular fibres inserted into the hair-follicle ; Co. dermis ; F. external longitudinal, and f". internal circular fibrous layer of follicle ; Ft, fatty tissue in the dermi.s ; Gfl, hyaline membrane between the root-sheaili and the follicle ; HBD, sebaceous gland ; HP. hair-papilla with vessels in its.interior ; M. medullary substance (pith) of the hair ;' 0-, cuticle ; R, cortical layer ; Sc, horny layer of epidermis ; Sch. hair-shaft ; SM, Malpighian layer of epidermis ; IKS, JFS', outer and inner layers of root-sheath. (From Wiedershcim's Comparative Anatomy.) hair-bulb (bib.), by the activity of which the further growth of the hair is effected. Soon the upper end of the hair-shaft grows out beyond the surface of the epidermis, and the projecting part eventually becomes much VOL. IE H H 490 ZOOLOGY SECT. longer than that which lies embedded in the follicle. At the same time the follicle grows downwards into the dermis. During its growth the hair is nourished by the blood-vessels in the dermal hair-papilla, which projects into its base. Modifications of the hairs are often found in certain parts. Such modified hairs are the elongated hairs of the tails of some Mammals, e. g., most Ungulates ; the eye-lashes of the eye-lids, which are stronger than the ordinary hairs ; and sensitive hairs or vibrissce about the snout. In some Mam- mals the hairs in part assume the form of spines, viz., in Echidna, the Hedge- hogs, and the Porcu- pines. The coating of hairs is scanty in some Mammals, and is vir- tually absent in the Cetacea and Sirenia. In such cases the skin is greatly thick- ened, as in the Ele- phants, &c. ; or, as in the Cetacea, an un- derlying layer of fat performs the function as a heat- FIG. 1102.— Four diagrams of stages in the development of a hair. A, earliest stage in one of those Mammals in which the dermal papilla appears first ; B, C, D, three stages in the development of the hair in the human embryo. W6. hair-bulb ; cm. horny layer of the epidermis ; foil, hair- follicle ; grm. hair-germ ; h. extremity of hair projecting on the surface ; muc. Malpighiau layer of epidermis ; pp. dermal papilla ; seb. developing sebaceous glands ; shl. sh2, inner aths. and outer root-sheat (After Hertwig.) preserving covering. In Manis (Fig. 1115) the greater part of the surface is covered with large, rounded, overlapping horny scales of epi- dermal origin, similar in their mode of development to those of Reptiles. A similar phenomenon is seen in the integument of the tail of Anoma- lurus — a Flying Rodent. The Armadillo (Fig. 1114) is the only Mammal in which there occurs a bony dermal cxoskeleton (vide infra). Also epidermal in their origin are the horny structures in the form of nails, claws, or hoofs, with which the terminations of the digits are provided in all the Mammalia except the Cetacea. And the same holds good of the horny portion of the horns of Ruminants. The horns of the Rhinoceros are also epidermal, and have the XIII PHYLUM CHORDATA 491 appearance of being formed by the agglutination of a number of hair-like horny fibres. Cutaneous glands are very general in the Mammalia, the most constant being the sebaceous glands (Figs. 1100, D, and 1101, HBD), which open into the hair-follicles, and the sweat glands (Fig. 1100, SD). In many Mammals there are, in addition, in various parts of the body, aggregations of special glands secreting an odorous matter. The mammary glands, by the secretion of which the young are nourished, are specially developed cutaneous glands. In the g.m. el. FIG. I HI:,. Echidna hystrix. A, lower surface of brooding female; B, dissection showing a dorsal view of the rnarsupium and mammary glands ; t t, the two tufts of hair projecting from the mammary pouches from which the secretion flows ; bin, brood-pouch or marsupium ; d. cloaca ; , Wallaby (Halmaturus) before lactation ; c, Opossum (Didelphys) before lactation ; d, Opossum during lactation ; this diagram stands also for Rodents and Man ; e, embryonal, and /, full-grown cow. 1, integumentary wall ; 2, mam- mary area, the broken line represents the mammary pouch ; 3, milk-ducts. (After Max Weber.) may have a wide central canal. The number and situation of the teats varies in the different groups, and has been noticed in the synopsis of the characters of the orders and sub-orders (pp. 478 to 487). The two genera of the Prototheria, Ornithorhynchus and Echidna, differ somewhat widely from one another in general appearance. The former (Fig. 1105) has the surface covered with a close, soft fur, and has the upper jaw produced into a depressed muzzle, not unlike the beak of a duck, covered with a smooth, hairless integument, which forms a free fold or flap at the base. The eyes are very small, and there is no auditory pinna. The legs are short, and the five digits end in strong claws, and are con- nected together by a web, so that the limbs are equally adapted for burrowing and for swimming. The tail is elongated and XIII PHYLUM CHORDATA 493 depressed, and is covered with fur. The male has a sharp-pointed, curved spur on the inner side of the foot, having the duct of FIG. 1105.— Duck -Bill (Ornithorhynchusunatinus). .(After Vogt and Specht.) a gland opening at its apex. Echidna (Fig. 1106) has the body covered above with strong, pointed spines, between which are coarse hairs ; the lower surface is covered with hair only. The jaws are produced into a rostrum, which is much narrower than that of Ornithorhynchus. The eyes are small, and there is no auditory pinna. The limbs are short and powerful. There are five toes on each foot, each ending in a very strong claw, by means of which Fir;. HOG. -Spiny Ant-Eater (Echidna aculeaia). (After Vogt and Specht.) the Echidna is able to burrow with rapidity. There is a spur on the inner side of the hind foot, larger in the male than in the female. The tail is vestigial. 494 ZOOLOGY SECT. The Opossums (Didelphyidce, Fig. 1107) are arboreal, rat-like Marsupials, with elongated naked muzzle, with well-developed, though clawless, opposable hallux, and elongated prehensile fail. A marsupium is sometimes present, but is absent or incomplete in the majority. One species — the Water Opossum — has the toes webbed. The Dasyu- ridce (Australian Na- tive Cats, Tasmanian Devil, Thylacine, &c.) often have the pollex rudimentary, the foot four-toed, the hallux, when present, small and clawless, and the tail non - prehensile. There is a well-de- veloped , marsupium. The Native Cats (Fig. 1108) and their near allies are cat-like animals, the largest equal in size to a Domestic Cat, some no larger than Rats or Mice ; the Tasmanian Devil has a more thickset body ; the Thylacine has a remark- able resemblance in general shape, as well as in size, to a Wolf. The Banded Anteater (Myrmecdbius) is devoid of the marsupium. FIG. 1107.— Virginian Opossum (Didelpliys virginiana). (After Vogt and Specht.) FIG. 1108.— Dasyure (Dasyurus viverrinus}. (After Vogt and Specht.) The Bandicoots (Peramelidce) are burrowing Marsupials, the size of whicb varies from that of a large Rat to that of a Rabbit. They have an elongated, pointed muzzle, and, in some cases, large auditory pinnse. The tail is usually short, sometimes long. XIII PHYLUM CHOKDATA 495 Fio. 1109. -Rock Wallaby (Petrogale xanthopus). (After Vogt and Specht.) The first and fifth digits of the fore-feet are vestigial or absent, the remaining three nearly equally developed. In the hind-foot the fourth toe is much longer and stouter than the others, while the second and third are small and slender and united together by a web of skin, and the first is vestigial or absent. The marsupium .has its opening directed backwards. FIG. 1110.— Marsupial Mole (Notoryctes typhlops). (From the Cambridge Natural History.) Notoryctes,the Marsupial Mole (Fig. 1110), is a small, burrowing Marsupial, with short and powerful limbs, each with five toes, 496 ZOOLOGY the third and fourth of the fore-foot provided with remarkable, large, flat, triangular claws. The tail is short and covered with bare skin. An auditory pinna is absent and the eyes are vestigial. The pouch opens backwards. The Wombats (Phascolomyidce, Fig. 1111) are large, heavy, thick-bodied, burrowing animals, with short, flattened heads, short, thick limbs, provided with strong claws on all the digits except the hallux, and with the second, third and fourth toes of the hind- foot partly connected together by skin. The tail is very short. The Kangaroos and their allies (Macropodidee, Fig. 1109) are adapted, as regards their limbs, for swift terrestrial locomotion. They have a relatively small head and neck, the fore -limbs small, and each provided with five digits ; the hind-legs long and powerful ; rapid progression is effected by great springing leaps, with the body inclined forwards and the fore-limbs clear of the ground. The foot is narrow and provided with four toes, the hallux being- absent ; the two inner (second and third) small and united together by integument, while the middle toe is very long and powerful. The tail is very long, and usually thick. There is a large marsupium. The Tree-Kangaroos differ from the FIG. 1111.— Wombat (Pkascolomys wombat). (From the Cambridge Natural History.) ordinary Kangaroos in their shorter and thicker hind-limbs, in which the second and third toes are nearly as large as the fourth. The Phalangers (Phalangeridce) are climbing Marsupials which have both fore- and hind-feet prehensile ; the second and third toes of the hind-foot slender and united by a web, as in the Kangaroo, but the hallux, which is nailless, opposable to them ; xin PHYLUM CHORDATA 497 the fourth and fifth nearly equal The tail is well developed and prehensile. A number of Phalangers (Flying Phalangers) are provided with lateral folds of skin extending from the fore- to the hind-limbs and, acting as a parachute, enabling the animal, as in Fir;. 1112.— Koala (Pliascolarctos cinereus). (After Vogt and Spccht.) the Flying Squirrels, to perform flying leaps from tree to tree. The Koalas (Fig. 1112) differ from the Phalangers mainly in the relatively thicker body and the vestigial tail. The Sloths (Brady podidce, Fig. 1113) are more completely adapted, in the structure of their limbs, to an arboreal life than any other group of the Mammalia. They have a short, rounded head, with small pinnae, and long, slender limbs, the anterior much longer than the posterior, with the digits, which are never more than three in number, long, curved, and hook-like, adapted for enabling the animal to hang and climb, body downwards, among the branches of trees. In the three-toed Sloth there are three toes in both maims arid pes ; in the two-toed Sloth there are only two in the manus, three in the pes. The tail is rudimentary. The body is covered with long, coarse hairs, which differ from those of other Mammals in being longitudinally fluted. On these hairs grows abundantly an alga, the presence of which gives a greenish tinge to the fur. The ordinary Anteaters (MyrmecopJiagidce) have a greatly elong- ated snout, with the mouth as a small aperture at its extremity, small eyes, and the auditory pinna sometimes small, sometimes well developed. There are five digits in the fore-foot, of which the third has always a very large, curved and pointed claw, render- 498 ZOOLOGY SECT. ing the manus an efficient digging organ. The toes of the hind- foot, four or five in number, are sub-equal, and provided with moderate-sized claws. In walking, the weight of the body rests on the dorsal surfaces of the second, third and fourth digits of the manus and on a thick callous pad on the extremity of the fifth, and, in the pes, on the entire plantar surface. The tail is always very long, and is sometimes prehensile. The body is covered with long hair. In the Two-toed Anteater (Cydoturus) the muzzle is FIG. 1113.— TJnau, or Two -Toed Sloth (Cholvpui didactylus). (After Vogt and Specht.) short ; there are four toes in the manus, of which the second and third only have claws, that of the third being the longer ; the pes has four sub-equal, clawed toes, forming a hook not unlike the foot of the Sloths ; the tail is prehensile. In the Armadillos (Dasypodidce, Fig. 1114) the head is com- paratively short, broad and depressed. The number of complete digits of the fore-foot varies from three to five; these are pro- vided with powerful claws, so as to form a very efficient digging PHYLUM CHORDATA 499 organ. The hind-foot always has five digits with smaller claws. The tail is usually well developed. The most striking external feature of the Armadillos is the presence of an armour of bony dermal plates ; this usually consists of a scapular shield of closely- united plates covering the anterior part of the body, followed by a FIG. 1114.— TatU Armadillo (Dasypus sexcinctus). (After Vogt and Specht.) series of transverse bands separated from one another by hairy skin, and a posterior pelvic shield. In the genus Tolypeutes these bands are movable, so that the animal is enabled to roll itself up into a ball. The tail is also usually enclosed in rings of bony plates, and a number protect the upper surface of the head. In the Scaly Anteaters (Manis, Fig. 1115) the head is produced into a short, pointed muzzle. The limbs are short and strong, with -^ FIG. 1115.— Scaly Anteater (Manis gigantea). (From the Cambridge Natural History. five digits in each foot. The upper surface of the head and body, the sides of the latter, and the entire surface of the tail, are covered with an investment of rounded, horny, epidermal scales. The lower surface is covered with hair, and there are a few coarse hairs between the scales. In walking, the weight rests on the upper and outer side of the fourth and fifth toes of the manus and ou the sole of the pes. 500 ZOOLOGY SECT. The Aard-varks (Orycteropus, Fig. 1116) have a thick-set body, the head produced into a long muzzle with a small tubular mouth, Fio. 1116.— Aard-vark (Orycteropus capensis). (After Vogt and Specht.) the pinna? of great length, the tail long and thick. The fore-limbs are short and stout, with four toes, the palmar surfaces of which are placed on the ground in walking. The hind-limb is five-toed. The surface is covered by a thick skin with sparse hairs. FIG. 1117.— Killer (Orca gladiator). (After True.) The Cetacea (Fig. 1117), among which are the largest of existing Mammals — some reaching a length of 80 or 90 feet — are xin PHYLUM CHORDATA 501 characterised by the possession of a fusiform, fish-like body, tapering backwards to the tail, which is provided with a hori- zontally expanded caudal fin divided into two lobes or "flukes," and a relatively large head, not separated from the body by any distinct neck. A dorsal median fin is usually present. The fore- limbs take the form of flippers, with the digits covered over by a common integument, and devoid of claws; the hind-limbs are absent. The mouth is very wide; the nostrils are situated on the summit of the head, and the auditory pinna is absent. Hairs are completely absent, or are represented only by a few bristles about the mouth. In the Whale-bone Whales the nostrils have two external slit-like apertures ; in the toothed Whales, Porpoises, and Dolphins, on the other hand, the t\vo nostrils unite to open by a single, crescentic, valvular aperture. In the Sirenia also the body is fish-like, with a horizontal caudal fin, the fore-limbs flipper- like, the hind-limbs absent, and the integument almost hairless. But the body is distinctly depressed, and the head is by no means so large in proportion as in the Cetacea and has a tumid truncated muzzle, not far back from the extremity of which the nostrils are situated. There is no dorsal fin. The eyes are small, the pinna) of the ears absent.^ The digits are in some cases provided with claws. In the Ungulata vera the claws or nails are replaced by thick, solid masses, the hoofs, investing the ungual phalanges and bearing the weight of the body. The number of digits is more or less reduced, and the limbs as a whole are usually specially modified to act as organs of swift locomotion over the surface of the ground, their movements being restricted, by the nature of the articulations, to antero-posterior movements of flexion and extension. The metacarpal and metatarsal regions are relatively very long. In the Artiodactyla the third and fourth digits of each foot form a symmetrical pair. In the Ruminants vestiges of the second and fifth digits are also commonly present ; but these are usually not functional, never reaching the ground, though in the Reindeer they are better developed than in the others, and have the effect of preventing the foot from so readily sinking in the snow. In the Camels the third and fourth digits alone are present. The Giraffes are distinguished from the other Ruminants by the enormous length of the neck. Characteristic of the Ruminants, though absent in the Camels and some others, are the cephalic appendages known as horns and antlers. The horns of the Hollow-horned Ruminants (Oxen, Sheep, Goats, Antelopes), sometimes developed in both sexes, sometimes only in the males, are horny sheaths supported by bony cores which are outgrowths of the frontal bones. In the Giraffe the horns, which are short and occur in both sexes, are bony structures covered with soft skin, and not at first attached by bony union to the skull, though 602 ZOOLOGY SECT. subsequently becoming firmly fixed. Between them is a short, rounded, median bony protuberance on the frontal region of the skull. The antlers of the Deer, which, except in the case of the Reindeer, are restricted to the male sex, are bony growths covered only while immature by a layer of skin, the " velvet," provided with very soft, short fur. Antlers are shed annually, and renewed by the growth of fresh vascular bony tissue from the summit of a pair of short processes of the frontal bones, the pedicles. Even- tually when the antlers are full grown, a ring-like thickening of the bone — the " burr " — appears round the base of the antler, and constricts the blood-vessels, so tbat the substance of the antler becomes converted into dry, dead bone; the skin shrivels and is peeled off. The antler is shed by the absorption of the bone immediately beneath the burr. The pinnas of the ear of the Ruminants are well-developed. The tail is sometimes elongated, and provided with a terminal leash of long coarse hairs ; sometimes short and bushy. The entire surface, with the exception of the end of the muzzle, which is naked, is always covered with a close coat of longer or shorter hairs. In the Pigs the legs are relatively short, and the two lateral toes of both manus and pes are fully developed, though scarcely reaching the ground. The surface is covered with a scanty coat of coarse bristles. There is a truncate, mobile snout, the anterior end of which is disc-shaped and free from hairs. The pinna? are large; the tail is rather long, narrow and cylindrical, provided with a terminal tuft of strong hairs. A remarkable feature of the males is the development of the canine teeth of both jaws into large, upwardly-curved tusks. In the Peccaries, which resemble the Pigs in most of the features mentioned, the points of the upper tusks are directed downwards. In the Hippopotami (Fig. 1117, bis), the body is of great bulk, the limbs very short and thick, the head enormous, with a transversely expanded snout, prominent eyes, and small pinnae. The tail is short and laterally compressed. The toes are four in each manus and pes, all reaching the ground. The surface is naked, with only a few hairs in certain positions ; the skin is of great thickness. In the Perissodactyles the third digit is either the only complete one in both fore- and hind-foot (Horses), or there are only three digits — second, third, and fourth — in each (Rhinoceroses), or there are four in the fore-foot and three in the hind (Tapirs). The " Horses " (Eqiddce, Fig. 1118) have the distal divisions of the limbs slender, the metacarpals and metatarsals nearly vertical to the surface of the ground ; the single hoof massive and with a broad lower surface. Though the head is elongated, the nasal region is not produced into a proboscis. The tail is short or moderately long, and is either beset throughout with a large number of very XIII PHYLUM CHORDATA 503 long, coarse hairs, or with a tuft of such specially developed hairs at the extremity. A mane of similar large hairs usually runs Fig. 1117, bis.— Hippopotamus (Hippopotamus amphiOiut), (From the Cambridge Natural History.) FIG. 1118.— Burchell's Zebra (Equus burchelti). (From the Cambridge Natural History.) along the dorsal surface of the neck. There is a wart-like callosity above the wrist, and in the true Horses a second a little below the heel or " hock," 504 ZOOLOGY The Tapirs (Fig. 1119) have the body more massive than the Horses, and the limbs, especially the distal segments, shorter and FIG. 11 19. -American Tapir (Tapirus terrestris). (From the Cambridge Natural History.) stouter. The nasal region is produced into a short proboscis. The surface is beset with a scanty covering of hairs. The tail is vestigial. FIG. 1120.— Indian Rhinoceros (Rhinoceros indicus). (From the Cambridge Natural History.) In the Rhinoceroses (Fig. 1120) the body is extremely massive, the limbs short and stout, each digit provided with a hoof-like xnr PHYLUM CHORDATA 505 nail. There is a short, soft muzzle. Either one or two remark- able median horns are borne on the nasal region, not attached directly to the skull : these are epidermal structures which are formed of a dense aggregation of slender fibre-like elements. The eyes are small, the auditory pinnae well developed. The surface is devoid, or nearly devoid, of hairs, and the skin is enormously thick and in some species thrown into deep folds. The tail is narrow and of moderate length. The Hyraxes are small, somewhat Rabbit-like animals, with slender limbs and vestigial tail. There are four functional digits in the mantis and three in the pes, all provided with short, flat nails, except the innermost of the pes, which has a curved claw. The body is covered with soft fur. The Elephants, the largest of existing terrestrial Mammals, have the limbs much more typically developed than in the true Ungu- lates, there being five comparatively short digits, enclosed in a common integument in each foot, all of them in the fore-, and three or four in the hind-foot, terminating in broad, flat nails, or hoofs. The weight of the body is borne on an integumentary pad forming the sole of the foot. The limbs are very stout and pillar-like, and the thigh and leg when at rest are in a straight line, instead of being, as in the Ungulata vera, placed nearly at right angles to one another — a circumstance which gives a charac- teristic appearance to the hind-quarters. The nasal region is produced into a proboscis or "trunk," a mobile cylindrical ap- pendage, longer than the rest of the head, at the extremity of which the nostrils are situated. There is in the male a pair of immense tusks — the incisors of the upper jaw. The eyes are small, the pinnae of the ears enormous. The tail is small, and has a tuft of hairs at its extremity. The skin is very thick, and provided with only a scanty hairy covering. In the Carnivora vera the typical number of digits is sometimes present ; or, more usually, there are five in the fore-, and four in the iind-foot, or four in each. The extremities of the digits are pro- vided with compressed, curved claws, which may be very long and sharp, when they are capable, when not in use, of being retracted into a sheath of skin situated at their bases ; or relatively short and blunt, when they are incompletely, or not at all, retractile. The Dogs (Canidce) and Cats (Felidce) are digitigrade, the Bears ( Ursidce) and allied groups plantigrade. The Otters (Lutra) differ from the rest in having short limbs with the toes connected by webs of skin. The Pinnipedia (Fig. 1121) have the proximal segments of the limbs short, so that the arm and thigh and nearly all the fore-arm and leg, are enclosed in the common integument of the trunk ; the manus and pes are elongated. The Earless Seals (Phocidce) are much more completely adapted to an aquatic life than the Eared VOL. II II 506 ZOOLOGY SECT. Seals (OtariidcB) and Walruses (Trichechidce), being unable to flex the thigh forwards under the body so that the hind-limbs may aid in supporting the weight, and thus being only able to drag themselves along very awkwardly when on dry land. The pinna of the ear is absent in the Earless Seals and Walruses, well developed in the FIG. 1121.— Seal (P/ioca vitulina) Eared Seals. The surface in all is covered with a thick, soft fur. In the Fur-Seals there are two kinds of hairs — those of the one kind being longer and coarser, and scattered through the more numerous shorter and finer hairs composing the fur proper. A remarkable feature of the Walruses is the presence of a pair of large tusks — the enlarged canine teeth — projecting downwards from the upper jaw. Some of the Rodents (Beavers, Water- Voles) are aquatic and some (Squirrels and Tree-Porcupines) are arboreal, while others (the majority of the order) lead a terrestrial life, and are active burrowers ; they are on the whole a very uniform group, and exhibit few such remarkable modifications as are to be observed in some of the other orders of Mammals. They are nearly all furry animals, with plantigrade or semiplantigrade limbs, usually five-toed. The tail is usually elongated, and may be naked or covered with fur ; but sometimes, as in the Rabbits and Hares, it is very short. A few special modifications, however, have to be noted in certain families of Rodents. The Flying Squirrels have on each side a fold of skin, the patagium, which serves as a parachute. The African Flying Squirrels (Anomalurus) are remarkable also on account of the presence of a series of overlapping horny scales on the lower surface of the basal part of the tail. The Jerboas (Dipus) and their allies are characterised by the great relative length of the hind-limbs — the mode of locomotion of these remarkable Rodents being by a series of leaps not unlike those of the Kangaroo — and by the reduction of the number of the toes to three in some of them. The Porcupines (Hystricidd) have numerous elongated spines or " quills " among the hairs of the XIII PHYLUM CHORDATA 507 dorsal surface, and some of them have prehensile tails. The Agoutis and the Capybara (ffydrochcerus) have hoof-like claws (Dasyprocta), the latter having webs between the digits. The Insectivora are, in general, small, furry, burrowing Mammals with plantigrade limbs and an elongated muzzle. But there is a considerable range of modification within the order in adaptation to different modes of life. The Colugos (Galeopithecus, Fig. 1122) have a fold of skin (patagium) extending along each side of the neck and body and continued between the hind-legs, enclosing the tail ; FIG. 1122.— Galeopithecus. (After Vogt and Specht.) the fore-and hind-feet are both webbed, and the tail is prehensile. The Hedgehog (Erinaceus) has the surface beset with pointed spines. The Moles (Talpa) and their allies, which are active burrowers, have the limbs very short and stout and provided with extremely strong claws. The jumping Shrews (Macroselididce) have slender limbs adapted to progressing by leaps on the surface of the ground. The Chiroptera (Fig. 1123) are the only Mammals which are capable of active flight. The fore-limbs have the segments greatly elongated, especially the fore-arm and the four ulnar digits, and I I 2 508 ZOOLOGY SECT. these support a thin fold of the integument which stretches to the hind-limbs and constitutes the wing. A fold (interfemoral membrane) also extends between the hind-limbs, and may or may not involve the tail. The pollex is much shorter than the other digits, is directed forwards, and terminates in a well developed, curved claw ; in the Megachiroptera, but not in the Microchiroptera, the second digit also has a claw ; the other digits are always claw- less. The position of the hind-limbs is peculiar, and, in walking, the knee is directed backwards, instead of forwards as in other Mammals ; the five digits of the foot are all provided with claws. So com- plete is the adaptation of the limbs to the purpose of flight that Bats are only able to shuffle along with great difficulty on the ground ; though with the aid of their claws they are able to climb and to suspend themselves from branches of trees by the hind -feet. In the Megachiroptera the muzzle is nearly always elongated, and the pinna of the ear simple; while in the FIG. 1123.— Bat (Synotus barbastellus). (After Vogt and Specht.) Microchiroptera the muzzle is short, the pinna usually compli- cated by the presence of an inner lobe or tragus and often pro- duced into remarkable arborescent appendages, and the nose also often provided with elaborate leaf-like or arborescent lobes. The surface is usually covered with soft fur, except in one group of Microchiroptera in which the integument is practically naked. The tail is sometimes short, sometimes well developed ; in the latter case it may or may not be involved in the tail-membrane. In the Lemurs and their allies (Prosimii) the body is slender, and the limbs adapted for an arboreal existence. The hallux is divergent from the other digits of the foot and opposable to them, and the same holds good, in some cases, of the pollex. In some, all the digits are provided with claws, in others all but the hallux. More commonly all the digits have flat nails, except the second of the pes, which always has a claw. The eyes are very large. The muzzle is sometimes elongated, sometimes short; the nostrils are slit-like. The tail is sometimes absent or short ; more XIII PHYLUM CHORDATA 509 The usually it is greatly elongated, but it is never prehensile, surface is always covered with soft fur. Of the Anthropoidea, the Hapalidse or Marmosets are small, squirrel-like animals with all the digits except the hallux pro- vided with pointed claws, with the pollex incapable of opposition, the tail non-prehensile, and without cheek-pouches or callous patches over the ischia. The Cebidse resemble the Hapalidse in the negative characters of the absence of ischial callosities and of cheek-pouches, and of the power of opposition in the hallux. But the limbs are much longer, the digits are all provided with flat nails, and the tail is frequently prehensile. The Cercopithecidse FIG. 1124.— Gorilla. (From the Cambridge Natural History.) all have brightly-coloured, bare, callous patches of skin (callosities) over the ischia, and most of them have cheek-pouches for the storage of food. All the digits are provided with flat nails. The tail may be long, or short, or absent; when present it is never prehensile. The pollex, when developed, is always opposable to the other digits. In the Simiidse or Man-like Apes (Fig. 1124) a tail is never developed, and there are no cheek-pouches; ischial callosities are present only in the Gibbons. The Gibbons can walk in an upright position, without the assistance of the fore- limbs; in the others, though, in progression on the surface of the ground, the body may be held in a semi-erect position with the weight resting on the hind-limbs, yet the assistance of the long 510 ZOOLOGY SECT fore-limbs acting as crutches is necessary to enable the animal to swing itself along. Endoskeleton. — The spinal column of Mammals varies in" the number of vertebrae which it contains, the differences being mainly due to differences in the length of the tail. The various regions are very definitely marked off. In the cervical region the first two vertebras are modified to form the atlas and axis. Owing to the absence of distinct cervical ribs, the posterior cervical vertebrae are much more sharply marked off from the anterior thoracic than is the case in Reptiles and Birds. The vertebras of the cervical region have double transverse processes (or a transverse process perforated at the base by a foramen) in all except the last. The lower portion of the transverse process in certain cases (e.g., seventh and sometimes some of the others in Man) arises from a separate ossification, and this is regarded as evidence that the lower part, even when not independently ossified, represents a cervical rib. Seven is the prevailing number of vertebrae in the cervical region ; there are only three exceptions to this — the Manatee, Hoffmann's Sloth, and the three-toed Sloth (cf. pp. 525 and 534). The number of thoracic and luiiibar vertebras is not so constant ; usually there are between nineteen and twenty-three. Hyrax has a larger number of thoraco-lumbar vertebras than any other Mammal — from twenty-nine to thirty-one. The thoracic vertebrae have ribs which are connected, either directly or by intermediate ribs, with the sternal ribs, and through them with the sternum. Each rib typically articulates with the spinal column by two articulations — one articular surface being borne on the head and the other on the tubercle. The tubercle articulates with the transverse process, and the head usually with an articular surface furnished partly by the vertebra with which the tubercle is connected, and partly by that next in front ; so that the head of the first thoracic rib partly articulates with the centrum of the last cervical vertebra. In all the Mammalia in which the hind-limbs exist — that is to say, in all with the exception of the Sirenia and the Cetacea — there is a sacrum consisting of closely united vertebras, the number of which varies in the different orders. The caudal region varies greatly as regards the degree of its development. In the caudal region of many long-tailed Mammals there are developed a series of chevron "bones — V-shaped bones, which are situated opposite the inter- vertebral spaces. The centrum of each vertebra ossifies from three centres1 — a middle one, an anterior, and a posterior. The middle centre forms the centrum proper ; the anterior and posterior form the epiphyses. The epiphyses are almost entirely absent in the Monotremes, and in the Dugong (Sirenia) have not been detected. Between 1 Usually the two centres of ossification which form the neural arches also contribute to the formation of the bony centrum. XIII PHYLUM CHORDATA 511 successive centra are formed a series of discs of fibro-cartilage — the intervertebral discs — represented in lower Vertebrates only in Crocodiles and Birds. The anterior and posterior surfaces of the centra are nearly always flat. The sternum consists of a number of segments — the presternum in front, the mesosternum, or corpus sterni, composed of a number of segments or stemebrce, in the middle, and the xipJiisternum behind. The sternum is formed in the foetus in great part by the separating off of the ventral ends of the ribs. Some of the Cetacea and the Sirenia are exceptional in having a sternum composed of a single piece of bone. The sternal ribs, by which the vertebral ribs are connected with the sternum, are usually cartila- ginous, but frequently undergo calcification in old animals, and in some cases early become completely converted into bone. The skull of a Mammal (Fig. 1125) contains the same chief elements and presents the same general regions as that of the Sauropsida; but exhibits certain special modifications. A number of the bones present in the skull of Sauropsida are not represented, or, at all events, not certainly known to be represented definitely by separate ossifications in the Mammalia. Such are the supra- orbital, the pre-frontal, the post-orbital, the ecto-pterygoid and the quad rat o-jugal. The bones of the skull, with the exception of the auditory ossicles, the lower jaw, and the hyoid, are all immovably united together by means of sutures. The palatine bones develop palatine plates separating off a posterior nasal passage from the cavity of the mouth, a condition found among the Sauropsida only in the Crocodilia, and, to a less extent in the Chelonia and some Lizards. True pterygoids are only known to occur in the Monotremes, the elements to which that name is usually given in other Mammals apparently repre- senting a part of the parasphenoid. The zygomatic arch is a strong arch of bone formed partly of the squamosal, partly of thejugal and partly of the maxilla: in position it represents the upper temporal arch of Amphibia and Sauropsida, but is differently constituted (see p. 349). The orbit in the skull of some Mammals is completely enclosed by bone, constituting a well-defined cavity ; in others it is not completely surrounded by bone behind, and so communicates freely with the temporal fossa, which lies behind it. The periotic bones (pro-otic, opisthotic, and epi-otic) become completely fused together in the skull of Mammals. Part of the periotic mass sometimes projects on the exterior at the hinder part of the lateral region of the skull, and is the mastoid portion ; the rest is commonly called the petrous portion of the periotic, and encloses the parts of the internal ear — the mastoid portion containing only air-cells. The tympanic bone, which perhaps represents the quadrato-jugal of Sauropsida, sometimes forms a long tube, sometimes a mere ring of bone ; in other cases it not 512 ZOOLOGY SECT. only gives rise to a tube for the external auditory meatus, but also forms the lulla tympani, a dilated bony process containing a cavity. The occipital region presents two condyles for the articulation of the atlas. The mandible consists in the adult of one bone, the equivalent ^ «r-* ^Jllfj?!! IW^^EllSoo- 2f|ifjlf I«.^T' . j§s 2 of the dentary of Sauropsida, on each side — the two rami, as they are called, being in most Mammals closely united at the symphysis. The mandible articulates with ;'an articular surface formed for it by the squamosal bone, below vthe posterior root of the zygomatic arch. xm PHYLUM CHORDATA 513 The hyoid consists of a body and two pairs of cornua — anterior and posterior ; of these the anterior pair are usually longer, and consist of several bones, the most important and most constant of which is the stylohyal, connected usually with the periotic region of the skull. The posterior cornua or thyro-hyals are in most cases much smaller. The ratio borne by the capacity of the cranial cavity to the extent of the facial region varies greatly in the different orders. The greater development of the cerebral hemispheres in the higher groups necessitates a greater development of the corre- sponding cerebral fossa of the cranium. This is brought about by the bulging upwards, forwards, and backwards of the cranial roof, resulting in a great modification in the primitive relations of cer- tain of the great planes and axes of the skull (Fig. 1126). Taking as a fixed base line the basi-cranial axis — an imaginary median line running through the basioccipital, basisphenoid, and presphenoid bones, we find that the great expansion of the cerebral fossa in the higher Mammals leads to a marked alteration in the relations to this axis (1) of the occipital plane or plane of the foramen magnum ; (2) of the tentorial plane or plane of the tentorium cerebelli (a transverse fold of the dura mater between the cerebral hemi- spheres and the cerebellum) ; and (3) of the ethmoidal plane or plane of the cribriform plate of the ethmoid. In the lower Mammals (A) these are nearly at right angles to the basi-cranial axis. In the higher groups, by the bulging forwards and back- wards of the cranial roof, the occipital and tentorial planes incline backwards and the ethmoidal forwards, until all three may become approximately horizontal. At the same time there is produced a change in the relations of the basi-cranial axis to the basi-facial axis — a line passing along the axis of the face between the meseth- moid and the vomer. In the lower forms the angle (cranio-facial) at which the basi-facial axis, when produced, meets the basi-cranial, is an exceedingly open one ; in the higher forms, owing to the downward inclination of the facial region, this angle decreases in size, though it is never reduced to less than a right angle. The pectoral arch of the Theria has fewer distinct elements than that of the Sauropsida. The coracoid, which in the latter is a large bone, taking a share at its dorsal end in the bounding of the glenoid cavity, and at its ventral end articulating with the sternum, is never present, in the adult, as a distinct bone. In the young of many Mammals it appears to be represented by a small ossification which enters into the glenoid facet ; but this very soon coalesces with the scapula. The coracoid process — which is a separate ossification in the young Mammal, and. though in most instances completely fusing with the scapula and with the smaller coracoid element, is sometimes recognisable as a distinct element up to a late period (many Marsupials, Sloths) — appears to VOL. II I I 2* 514 ZOOLOGY SECT. correspond to the bone called epicoracoid in the Prototheria (vide infra). In foetal Marsupials the coracoid is represented by a well-developed cartilaginous element meets the rudiment of the sternum. rhich extends inwards and XIII PHYLUM CHORDATA 515 In the scapula a spine is nearly always developed, and usually ends in a freely-projecting acromion-process. It is developed, unlike the main body of the scapula, without any antecedent formation of cartilage, and is perhaps to be compared with the deithrum, an investing bone occurring in some Amphibia and Reptilia (p. 302). A clavicle is well developed in many Mammals, but is incomplete or absent in others; its presence is characteristic of Mammals in which the fore-limbs are capable of great freedom of movement. In the embryo of the Theria there is, in the position of the clavicular bar, a bar of cartilage, which coalesces with its fellow in the middle line. The cartilaginous tract thus formed segments into five portions — a median, which coalesces with the pre-sternum, two small inner lateral, which unite with the clavicles or are converted into the sterno-clavicular ligaments, and two long outer lateral, which give rise to the clavicles. The median and inner lateral portions appear to correspond to the episternnm of Reptiles and Prototheria. An additional small cartilage may represent the inner portion of the procoracoid of Amphibia. A piece of cartilage at the outer end of the clavicle proper is sometimes distinguishable — the meso-scapular segment. In the carpus there are four proximal bones — scaphoid, lunar, cuneiform, and pisiform. The scaphoid corresponds to the radiale of the typical carpus (p. 83) ; the lunar perhaps represents a second centrale that occurs in some Amphibia ; the cuneiform is probably the intermedium, and the pisiform the ulnare. The centrale is present sometimes as a distinct ossification ; the five distal carpals are represented by the trapezium, trapezoid, magnum, and unciform, the last being the equivalent of the fourth arid fifth distalia. There are never more than five digits, and in many forms the number is greatly reduced ; only in certain Cetacea does the number of phalanges in a digit ever exceed three. The three elements of the pelvic arch unite to form a single bone, the innominate. The ilia unite by broad surfaces with the sacrum ; the pubes, and sometimes the ischia, unite in a symphysis. All three may take a share in the formation of the acetabulum, but the pubis is usually shut out by a small cotyloid bone. In the shank the inner or tibial element is always the larger; the fibula may be vestigial. A large sesamoid bone — the patella — is almost universally formed in close relation to the knee-joint. , In the tarsus there are two proximal bones, the astragalus and calcaneum, the latter undoubtedly corresponding to the fibulare of the carpus of lower Vertebrates, and the proximal part of the former to the intermedium and its distal portion to the proximal of the two central elements present in the tarsus of some Amphibia. The scaphoid or navicular represents the second 516 ZOOLOGY SECT, xin central bone, and the distal row of tarsals is represented by the cuboid and the three cuneiforms. The external form of the limbs and the mode of articulation of the bones vary in the various orders of the Mammalia, in accordance with differences in the mode of locomotion. In most the habitual attitude is that which is termed the quadrupedal — the body being supported in a horizontal position by all four limbs. In quadru- pedal Mammals the manus and pes sometimes rest on the ventral surfaces of the entire metacarpal and metatarsal regions as well as on the phalanges — when the limbs are said to be plantigrade ; or on the ventral surfaces of the phalanges only (digitigrade) ; or on the hoofs developed on the terminal phalanges (unguligrade). Many quadrupeds have the extremities prehensile, the hand and foot being converted into grasping organs. This is most marked in those that pass the greater part of their life among the branches of trees, and in the Sloths the modification goes so far that both hands and feet are converted into mere hooks by means of which the animal is enabled to suspend itself, body downwards, from the branches of trees. Certain Mammals, again, have their limbs modified for locomo- tion through the air. The only truly flying Mammals are the Bats and the so-called " Flying Foxes," in which the digits of the fore- limb are greatly extended so as to support a wide, delicate fold of skin constituting the wing. In other so-called flying Mammals, such as the Flying Squirrels and Flying Phalangers, there is no active flight, and the limbs undergo no special modification ; the flying organ, if it may be so termed, in these cases being merely a parachute or patagium in the form of lateral flaps of skin ex- tending along the sides of the body between the fore- and hind- limbs. Further, there are certain groups of swimming Mammals. Most Mammals, without any special modification of the limbs, are able to swim, and some of the quadrupeds, such as the Tapirs and Hippopotami, spend a great part of their life in the water. But there are certain Mammals in which the limbs are so specially modified to fit them for an aquatic existence — assuming the form of flippers or swimming paddles — that locomotion on land becomes almost, if not quite, impossible. Such are the Whales and Porpoises, the Dugongs and Manatees, and, in a less degree, the Seals and Walruses. Skeleton of Prototheria. — In the Prototheria (Fig. 1127) the epiphyses of the vertebrae are not well developed in the Platypus, being represented only in the caudal region, and they appear to be absent in Echidna. In both genera there is the normal number of vertebrae in the cervical region. The odontoid process long remains separate from the centrum of the axis. The cervical transverse processes are separately ossified, and only completely ol G. 1127.— Skeleton of male Ornithorhynchus. Ventral view. The right foie-limb has been separated and turned round so as to bring into view the dorsal surface of the maims : the lower jaw is removed, ace. tors, accessory tarsal bone supporting the spur; ant. pal. for. anterior palatine foramen; all. atlas; ast. astragalus ; ax. axis ; bs.oc. basi-occipital ; bs.sph. basi-sphenoid ; calc. calcaneum ; cbd. cuboid ; cerv. rb, cervical rib ; clav. clavicle ; cond. for. foramen above inner condyle of humerus ; cor. coracoid ; cun. cuneiform of carpus ; dent, position of horny teeth ; ect.cun. ecto-cuneiform ; ent.cun. ento-cuneiform ; ep.cor. epicoracoid ; ep. pb. epipubis ; fb. fibula ; fern, femur ; for. mag. foramen magnum ; glen, glenoid cavity of shoulder- joint and glenoid cavity for mandible ; 1mm. humerus ; il. ilium ; in. cond. inner condyle of humerus ; inf. orb. .for. points to position of infra-orbital foramen ; infr. proc. inferior processes of caudal vertebrae ; int. rbs. intermediate ribs ; isch. ischium ; mag. magnum of carpus ; max. maxilla ; max.for. maxillary foramen ; in i tat. I, first metatarsal ; metal. V, fifth metatarsal ; nas. cart, nasal cartilage; obt. obturator foramen; ol. olccranon ; out. cond. outer condyle of humerus ; pal. palatine ; pat. patella ; post. pal. for. posterior 1 palatine foramen ; pr.max. premaxilla ; pr. st. presternum ; pter. pterygoid ; pub. pubis ; rad. radius ; scap. scapula ; graph, scaphoid of tarsus ; scaph. lun. scapho-lunar ; ses. sesamoid bones of wrist and ankle ; >. tarsal horny spur ; sq. squamosal ; tib. tibia ; trd. trapezoid ; trm. trapezium ; track, maj. greater 518 ZOOLOGY SECT. unite with the vertebrae at a late period, sutures being traceable in all but very old animals. Zygapophyses are absent in the cervical region. There are nineteen thoraco-lumbar vertebras in both genera. The transverse processes are short, and the ribs do not articulate with them, but only with the sides of the centra. In the sacrum of Echidna there are three or four, in that of Platypus two, united vertebras. The caudal region differs con- siderably in its development in the two genera. In Echidna the tail is very short, the vertebras depressed, with no inferior spines, but with about five subvertebral bones, which differ from ordinary chevron bones in being mere flat nodules. In the Platypus the tail is long, and the number of caudal vertebras is twenty or twenty- one ; each has a distinct inferior spinous process (infr. proc). The sternum consists of a pre-sternum and three keeled sternebras : in Echidna, but not in Platypus, there is a xiphisternum. The most remarkable feature of the sternal apparatus in the Prototheria is the presence of a T-shaped epi-sternum (interclavicle, epist.) corre- sponding to that of Reptiles. The sternal ribs are ossified, and are connected with the vertebral ribs by imperfectly ossified inter- mediate ribs (int. rbs.*). The skull of the Monotremes differs widely from that of other Mammals. The bones early become fused together, so that it is difficult to trace their exact boundaries. The brain-case is larger and more rounded in Echidna than in the Platypus, in accordance with the larger size of the brain in the former genus. In both genera there is a pterygoid (investing) bone not separately represented in higher Mammals, corresponding to the pterygoid of lower Vertebrates. The parasphenoid represents the lateral parts of the parasphenoid of lower Vertebrates and the inner lamella of the pterygoid process (usually regarded as the pterygoid) of higher Mammals. Perforating the posterior root of the zygomatic arch is a canal, comparatively wide in the full-grown Ornithorhynchus, narrow in Echidna — the temporal canal — which is not present in higher Mammals, and apparently represents the post-temporal fossa of Reptiles (see p. 319). In Echidna (Fig. 1128) the squamosal extends further forwards, and the posterior root of the zygomatic arch is more anterior than in Mammals in general. The zygoma is very narrow, and there is no rudiment of post-orbital processes : the jugal is absent as a separate ossification. The alveolar border of the maxilla (max.} is narrow and devoid of teeth. The nasal and premaxillary region of the skull is drawn out into a long, narrow rostrum. Near the anterior end of this is a rounded opening, the external nasal opening, which is entirely bounded by the pre- maxillas — the nasals not extending so far forwards. An aperture in the nasal septum corresponds to an actual perforation by which the nasal cavities are in direct communication in the living XIII PHYLUM CHORDATA 519 animal. The pterygoids (pt.) are in the form of flat plates con- tinuous with the bony palate ; they extend back so as to form a part of the walls of the tympanic cavities. The tympanic (ty.) is an imperfect ring which does not become united with the periotic. The mandible consists of very narrow, styliform rami, which are not firmly united at the symphysis. The condyle (cond.) is narrow, oc.cond Fio. 1128. — Echidna aculeata. Ventral view of skull and right rarnus of mandible, a ng. angle of mandible ; aud. oss. auditory ossicles ; cond. condyle of mandible ; cor. coronoid process ; max. maxilla ; oc. cond. occipital condyle ; pal. palatine ; p. max. premaxilla ; pt. pterygoid ; sg. squamosal ; ty. tympanic ring. rather more elongated antero-posteriorly than transversely. There are very slight rudiments of the angle and of the coronoid process (cor.). In the Platypus (Fig. 1127) the zygoma is stouter than in Echidna, and there is a post-orbital process which is formed by the jugal. The maxillary root of the zygoma develops a process 620 ZOOLOGY SECT. which supports the horny teeth (dent.) of the upper jaw. The nasal and premaxillary region is expanded into a rostrum, which is much broader than in Echidna. The premaxillse (pr. max.) diverge from one another anteriorly, and then curve inwards again, partly enclosing a large space in which the nostrils are situated, and which is covered over in the recent state by the tough but sensitive, hairless integument investing the cartilage of the rostrum, the latter being continuous with the nasal septum. In this space between the premaxillse is situated a dumb-bell shaped bone (x) which appears to be of the nature of an anterior vomer. The pterygoid (pter.) is much smaller than in Echidna, and does not extend so far back as the tympanic cavity. The mandible has its*rami stouter than in Echidna; they meet for a short distance anteriorly, and then again diverge slightly. The condyle is much larger than in Echidna, and is elongated transversely. In front of it is a broad process bearing the horny tooth. It is in the shoulder girdle that we find perhaps the most striking peculiarities of the skeleton of the Prototheria. There is a T-shaped episternum (epist.), as already stated, similar to that of Reptiles, the median limb articulating behind with the pre- sternum and the cross-piece closely applied to the clavicles. There are two short and broad coracoids (cor.) articulating internally and behind with the presternum, and, externally, uniting with the scapula to form the glenoid cavity. In front of the acr, ^w^. v coracoid is a flat plate, the epicoracoid (ep.cor.). The scapula (Fig. 1129) is very unlike that of other Mammals. There is a FIO. ii29.-0uter surface of well-developed acromion process (acr.) with left scapula of ornitho- which the clavicle articulates ; this ter- rhynchus. acr. process • • r i corresponding to acromion; inmates the anterior border, so that the gSj^m^SSlSr^ ^tter would appear to correspond to the corresponding to the spine; spine of the scapula of other Mammals : x, slight ridge which bounds f . . _ the surface of origin of the this is confirmed by the arrangement of tertSy?* the scapular muscles. The anterior part of the inner surface is in reality the pre- spinous fossa ; the anterior portion of the outer surface the post- spinous fossa ; and the part behind this, separated from it by a slight ridge, together with the posterior portion of the inner surface, is the subscapular fossa. The humerus is of remarkable shape, with greatly expanded extremities — especially in Echidna — and prominent tuberosities and condyles. In the carpus the scaphoid and lunar are united ; there is no separate centrale. There is a radial sesamoid, and two very large palmar sesamoids, which are sometimes united. xin PHYLUM CHORDATA 521 In the pelvis there is a very long symphysis in which pubes and ischia take an almost equal share. The acetabulum is per- forated in Echidna. There is a large cotyloid in Ornithorhynchus, larger than the pubis. With the anterior border of the pubes are articulated a pair of large epipubic or " marsupial " bones (Fig. 1127, ep. pb.}. The femur has expanded extremities with prominent external and internal trochanters. There is a large ossified patella (pat.). The fibula (fb.) has at its proximal end a remarkable compressed process which ossifies from a separate centre, and resembles the olecranon of the ulna. In the tarsus there are the usual bones. In the Platypus the astragalus and calcaneum are firmly united, and an accessory ossification (ace. tars.) on the inner side in the male bears the tarsal spur. The metatarsals are short and broad, as are all the phalanges except the last. Skeleton of Metatheria. — In the Marsupials the inferior arch of the atlas (Fig. 1130) is often incompletely ossified, a gap being left in the prepared skeleton ; sometimes the gap becomes closed in by the ingrowths of the lateral parts of the arch, sometimes a small separate ossi- fication is developed filling up the opening. In the trunk there are always nineteen verte- brae. The transverse processes FIQ. iiso.—Atias of Kangaroo, of the thoracic vertebras are always well developed, and the ribs articulate with them as well as with the bodies. Prominent metapophyses and anapophyses are developed ; these are largest in the lumbar region. Only one sacral vertebra is present in most Marsupials ; in some a second is ankylosed with it. The caudal region varies greatly in length. It is short in the Koala and the Wombat, long in the Opossums, Dasyures, Phalangers and Kangaroos (Fig. 1131). Chevron bones are generally present, except in the Koala and the Wombat, In the skull (Figs. 1132-1134) the brain-cavity is relatively small, with the cerebellar fossa entirely behind the cerebral. The pituitary fossa is not distinct, and there are no clinoid pro- cesses. The zygoma is complete, but the orbit is not completely bounded by bone behind. The jugal extends beneath the squamosal- root of the zygoma to form part of the outer wall of the glenoid fossa. The lacrymal foramen is usually on the anterior margin of the orbit, sometimes on the face. The palate usually presents vacuities in its posterior portion. The " pterygoid " (p. 511) is always small. The alisphenoid is large, and forms the anterior boundary of the tympanic cavity ; in the Kangaroos (Fig. 1133, all) it extends 522 ZOOLOGY .SECT backwards so as to join the paroccipital process, \vliich is greatly elongated. When an auditory bulla is developed, it is formed by this bone, the tympanic being always small, and never ahkylosed to neighbouring bones. The internal carotid artery perforates the orb cbd ' "% FIG. 1131.— Skeleton of Wallaby (Halmai. * ^utdabatus). The scapula is represented as raised somewhat higher than it would be*i*. yfie natural relations of the parts ; the head of the femur has been separated from the acetabulum. acet. acetabulum ; acr. acromion process ; ast. astragalus ; cole, calcaneum ; cbd. cuboid ; chev. chevron bones ; cl. clavicle ; cun. cuneform of carpus ; epi. epipubis ; fb. fibula ; fern, femur ; hd. head of femur ; hu. humerus ; il. ilium ; isch. ischium ; obt. obturator foramen ; orb. orbit ; pis. pisiform ; pub. pubis ; rad. radius ; r&i. first rib ; r&w. iast rib ; sc. scapula ; st. sternum ; tb. tibia ; troch. great trochauter of femur ; uln. ulna ; unc. unciform ; IV. fourth toe. basisphenoid. The optic foramen is not separate from the sphenoidal fissure. In all except Tarsipes the angle of the mandible sends inwards a remarkable process (ang.\ and is said to be inflected. In inspectoral arch of the Marsupials the coracoid process is, XIII PHYLUM CHORDATA 523 as usual, developed from a special bony centre, and a distinct suture is often recognisable between it and the scapula unti] a Icr- fr s.o e FIG. 1132.— Skull of Dasyurus (lateral view). al.sph. alisphenoid ; ang. angular process of mandible ; /r. frontal ; ju. jugal ; Icr. lacrymal ; max. maxilla ; nas. nasal ; oc..cond. occipital condyle ; orb. sph. orbitosphenoid ; par. parietal ; par. oc. paroccipital process ; p. max. pre- maxilla ; s. oc. supra-occipital ; sq. squamosal ; sq'. zygomatic process of squamosal. eac.oc bas.oc FIG. 1133.— Skull of Rock Wallaby (Petrogale penicillata) (ventral view). Letters as in Fig. 1132, except ali. alisphenoid. In addition, bas. oc. basi-occipital ; bas. sph. basi-sphenoid ; ex.oc. ex-occipital ; pal. palatine ; pt. so-called pterygoid ; ty. tympanic. comparatively late stage. In the young condition (when the foetus is attached to the teat), the coracoid is comparatively extensive and 524 ZOOLOGY SECT. reaches the presternum ventrally. A clavicle is always present, except in the Bandicoots, bat may be incomplete. There is never a distinct centrale in the carpus. In the Opossums the ilium has the primitive form of a straight, three-sided rod. In the Kangaroos (Fig. 1131, il.) it is still simple and three-sided, but somewhat curved outwards ; in the rest it is more or less com- pressed. In nearly all the Marsupials there is a pair of epipubic or marsupial bones (Fig. 1131, epi.) — elongated and compressed bones which articulate posteriorly with the anterior edge of the pubes : in the Thylacine they are represented only by small unossified fibro-cartilages. In the leg the fibula is always well- developed. In the young condition of some Marsupials there is an accessory element situated outside the fibula at its proximal FIG. 1134.— Skull of Wombat (Phascolomys wombat) (lateral view). Letters as in Fig. 1132. In addition, ext. aud. opening of bony auditory meatus ; cond. condyle of mandible. end : this apparently corresponds to a bone known as the para- fibula which occurs in some Lacertilia. In the Phalangers (Fig. 1135) and the Koala there is always a considerable range of move- ment between the fibula and the tibia, comparable in some degree to the movements of pronation and supination of which the radius and ulna are capable in many Mammals. The foot (Figs. 1135, 1136), as already stated in the account of the external characters, presents a much greater range of modification than the manus. Skeleton of Edentata, — In the Armadillos more or fewer of the cervical vertebras are ankylosed together both by their bodies and by their neural arches. In the lumbar region the meta- pophyses are greatly prolonged — longer, than the transverse pro- cesses— and support the bony carapace. A remarkable peculiarity of the spinal column in the Armadillos is the fusion of. a number of the anterior caudal vertebrae with the true sacrals to form the long sacrum, containing as many as ten vertebras altogether (Fig. 1146). The caudal region is of moderate length; there are XIII PHYLUM CHORDATA 525 numerous chevron bones. In Manis, Orycteropus and Myrmeco- phaga none of the neck- vertebrae are united. In the posterior thoracic and the lumbar regions of Myrmecophaga there are deve- loped complex accessory articulations between the vertebrae. The sacrum contains, in addition to the true sacral vertebrae, a number derived from the caudal region, a condition which occurs also in Oryc- teropus. In the Sloths none of the cervical vertebrae are FIG. 1136. — Bones of right foot of Kangaroo (Macropus bennetii.) a. astragalus ; c. calcaneum ; cb. cuboid ; e3. ento-cuneiform ; n. navicular. (After Flower.) ankylosed together ; but in the three-toed Sloths there is an important divergence from ordinary Mammals in the number of vertebrae in the cervi- cal region, there being nine or ten instead of seven; while in one species of two-toed Sloth (Ckolcepus hqffmanni) there are only six. The neural spines of all the vertebrae are very short. A number of the anterior caudal vertebrae are united firmly, though not quite fused, with one another and with the true sacrals. In the Armadillos the sternal ribs, which are sub-bifid at their FK;. 1135. — Bones of leg and foot of Phalanger. ast. astragalus ; calc. calcaneum ; cub. cuboid ; ect. cun. ccto-cuneiform ; ent. cun. ento-cuneiform ; fb. fibula ; mes. cun. meso-cuneiform ; nav. navicular : tib. tibia. (After Owen.) VOL. II K K 526 ZOOLOGY SECT. sternal ends, are ossified, and articulate with the sternum by means of well-developed synovial articulations. In the American Ant- eaters there are similar synovial joints, and the sternal ends of the sternal ribs are completely bifid. In the Sloths the sternum is long and narrow, and there are no synovial joints. In front the par nets S.OC FIG. 1137.— Skull of Armadillo (Dasypus sexcinctus.) Letters as in Figs. 1132—1134. In addition, peri, periotic. sternal ribs are ossified and completely united with the vertebral ribs, but behind they are separated from the latter by intermediate ribs which are less perfectly ossified. In the Armadillos the skull (Fig. 1137) is broad and flat, the facial region triangular. The tympanic (ty.} is in some developed into a bulla. The bony auditory meatus is in some cases elongated. The zygoma is complete. The "pterygoids" are small, and do not par occ.cond FIG. 1138.— Skull of Anteater (Myrmecopliaga), lateral view. aL spli. alisphenoM ; cond. condyle of mandible ; cor. coronoid process of mandible ; ex. oc. exoccipital ; ext. and. external auditory meatus ; fr. frontal ; ju. jugal ; Icr. lacrymal ; max. maxilla ; nas. nasal ; occ. cond. occipital condyle ; pal. palatine ; par. parietal ; p. max. premaxilla ; s.oc. supraoccipital ; sq. squamosal ; ty. tympanic. develop palatine plates. The mandible has a well-developed ramus with a prominent coronoid process and a well-marked angular process. In the American Anteaters (Figs. 1138 and 1139) the skull is extremely long and narrow — the facial region being drawn out into XIII PHYLUM CHORD AT A 527 b.oc ex.oc a long, narrow rostrum, with the external nares at its extremity. The olfactory fossa3 are greatly developed. The rostrum is composed of mes- ethmoid, vomer, maxillae, and nasals —the premaxillas being very small. The zygoma is incomplete, and the orbit is not closed behind by bone, the post-orbital processes of the frontal being entirely absent. The " pterygoids" (ptcr.\ in all but Cyclo- turus, develop palatine plates. There is no bony auditory meatus. The mandible is entirely devoid of ascending rarnus — consisting of two long and slender horizontal rami, with a very short symphysis. In the Sloths (Fig. 1140) the cranial region is elevated and rounded, the facial short; the frontal region is elevated, owing to the development of extensive frontal air-sinuses. The premaxillaB are small, and not firmly connected with the maxilla?, so that they are commonly lost in the macerated skull. The jugal (Ju.) develops a strong zygomatic process which Ur na,s s.oc FIG. 1139.— Skull of Anteater (Myrme- cophaffa), ventral view. Letters as in Fig. 1138. In addition, b.oc. basi- occipital ; glen, glenoid surface for mandible ; pter. " pterygoid." Fio. 1140.— Skull of Three-toed Sloth (Bradypus tridactylus). Letters as in Fig. 1 bifurcates behind into two branches, neither of which is connected with the rudimentary zygomatic process of the squamosal, so that the K K 2 528 ZOOLOGY SECT. pl.SC zygomatic arch remains incomplete. There are, at most, the rudi- ments of post-orbital processes of the frontals. The " pterygoids " develop vertical laminae and form no palatine plates. The ascending ramus and coronoid process of the mandible are both well developed. In the American Anteaters and Armadillos, the bones of the fore-limb are short and powerful. The scapula in the Anteaters is broad and rounded ; the anterior border unites with the coracoid process so as to convert the coraco-scapular notch into a foramen, In the middle of the spine there is a triangular process : a ridge on the post-spinous fossa presents the appear- ance of a second spine. The fibres of origin of the subscapularis muscle extend on to the outer surface as far forward as this ridge, so that the part of the outer surface behind the ridge corresponds to a part of the subscapular fossa, which in other Theria is co-extensive with the inner surface. Except in Cycloturus the clavicles are rudimentary. All the carpal bones are distinct. In the Armadillos the scapula (Fig. 1141) has an extremely prolonged acromion (acr.), sometimes articulating with the humerus. A ridge (spr.) representing a second spine is present. The clavicle is well developed. The humerus is short and powerful, with well-developed pro- cesses and ridges, and with a foramen above the inner condyle (entepicondylar foramen). The carpus consists of the ordinary eight bones. In the Sloths (Fig. 1142) the arm- bones are comparatively long and slender. A coraco-scapular foramen is formed as in the Anteaters. In the three-toed Sloths (Fig. 1143) the acromion (acr.) is at first connected with the coracoid process, but becomes reduced and loses the connection ; in the two-toed Sloth the connection persists. The clavicle (cl.) is not directly connected internally with the sternum ; externally it is connected with the coracoid process — a condi- observed in no other Mammal. The humerus is very long and slender ; so are the radius and ulna, which are capable of a certain amount of movement in pronation and supination. In the carpus (Fig. 1144) the trapezoid and magnum are united in Bradypus, distinct in Cholcepus: in the former the trapezium is usually fused with the rudimentary first metacarpal. The first pr.se cor FIG. 1141.— Shoulder-girdle of Arma- dillo (Dasypus sexcinctus). acr. acromion ; cor. coracoid process ; pr.sc. pre-spinous fossa'; pt. sc. post- spinous fossa ; gp. spine ; gp'. ridge probably marking the anterior limit of origin of the subscapularis muscle. tion XIII PHYLUM CHORDATA 529 and fifth metacarpals are represented only by rudiments. The proximal phalanges of the three digits are early ankylosed with the corresponding metacarpals, so that it might readily be supposed that one of the ordinary bones of each digit was absent. 530 ZOOLOGY SECT. The pelvis of the American Anteaters is elongated, with a short symphysis pubis. The ischia unite with the spinal column. There is no third trochanter. The tibia and fibula are nearly straight, rad. acr FIG. 1143.— Shoulder-girdle of Three-toed Sloth (Bradypus tridactylus). acr. aero- mion ; cl. clavicle ; cor. coracoid. FIG. 1144.— Right nianus of Three -toed Sloth. cun. cuneiform; Inn. lunar; me1, first meta- carpal ; mc5. rudiment of fifth metacarpal ; pis. pisiform ; rad. radius ; sc. scaphoid ; trd. m. trapezoid and magnum united ; uln. ulna ; unc. unciform. and parallel with one another. In Cycloturus the pes is modified to form a climbing organ. In the Sloths the pelvis is short and wide; the spines of the ischia unite with the anterior caudal vertebrae so that sacro- sciatic foramina are formed as in Anteaters. The femur is long and slender; it is devoid of third trochanter. The tibia and fibula are also long and slender. At its distal end (Fig. 1145) the fibula develops a peg-like process (V) which fits into a depression in the outer face of the astragalus. The calcaneal process is ex- tremely prolonged in Bradypus, in which there is a tendency to ankylosis between the tar- sal bones, and the proximal phalanges ankylose with the metatarsals. In the Armadillos the pelvis (Fig. 1146) is extremely long, and both ilia and ischia are firmly fused with the spinal column. The femur has a pro- minent third trochanter. The bones of the pes (Fig. 1147) are normal. rrtetal.1 FIG. 1145.— Pes of Three-toed Sloth, ast. astragalus ; calc. calcaneum ; cbd. cuboid ; fb. fibula ; mesoc. mesocuneiform ; metafi. vestige of first metatarsal ; metat5. vestige of fifth metatarsal ; nav. navicular ; tib. tibia ; x, peg-like process at distal end of fibula. XIII PHYLUM CHORDATA 531 -In the Cetacea (Fig. 1148) the cervical Skeleton of Cetacea. region (cerv.) is al- ways very short, and the constituent verte- bras are often com- pletely fused to- gether into a con- tinuous bony mass, or the atlas alone may be separated from the rest ; but sometimes all the vertebras are com- plete and separate. In the latter case they have small arches and long transverse processes consisting of two nar- row bars with a wide space between them. The epiphyses are very distinct discs which often remain separate from the bodies up to a late period. The neural spines are well de- veloped. The zygapophyses are not well developed, and are absent in the posterior portion. In the absence of hind-limbs there is no sacral region. The caudal region consists of numerous vertebrae be- neath which, opposite the inter- vertebral spaces, are a series of chevron bones (chev.). In the Whale-bone Whales only one pair of ribs articulates with the sternum, and none articulate with the bodies of the vertebras, but only with the transverse pro- cesses. In the Toothed Whales only a small number are connected with the sternum, sometimes through the intervention of inter- mediate ribs, and a few of the anterior only, in most cases, arti- culate with the bodies of the vertebras ; but in some a greater I FIG. 1146. — Pelvis and sacrum of Armadillo (Dasypus sex- cinctus). ac. acetabulum ; it. ilium ; iscli. ischium ; obt.for. obturator foramen; pect. tub. pectineal tubercle ;pub. pubis. cat FIG. 1147.— Pes of Armadillo (Dasypus sexcinctus). ast. astragalus ; col. cal- caneum ; cbd. cuboid ; ect. ecto-cunei- form ; ent. ento-cuneiform ; mes. meso- cuneiform ; nav. navicular. ZOOLOGY number articulate with both transverse processes and bodies by distinct tubercles and heads. The sternum varies in shape. Sometimes it consists of a pre- sternum and a series of several sternebrse without xiphister- num; sometimes (Fig. 1149) it FIG. 1149.— Sternum of Rorqual (Balanojitera musculus). (After Flower.) is a continuous plate of bone, occasionally with median notches or fontanelles. In the skull (Fig. 1150), the brain-case is rounded, the jaws greatly elongated, often unsym- metrical. The parietal s (Pa) do not meet in the middle line above in most Cetacea, being- separated by the supraoccipital (SO.) and an interparietal (IP.)', there is thus no sagittal suture. A large supra-orbital plate is developed from the frontal. There are large and stout zygo- matic processes of the squamosal, but the jugals are extremely small. In all the recent forms the maxilla (Mx.) is very large and extends backwards to over- lap a good deal of the frontal, and forwards nearly to the ex- tremity of the snout ; while the prem axillae (P. Mx.\ which are XIII PHYLUM CHORDATA 533 long, narrow bones, bound but a very small part of the oral border of the upper jaw. The nasals (Na.) are very small. The tym- panic bone is very large, and is sometimes fused with the periotic (Mystacoceti) sometimes not (Odontoceti). The lower iaw is remarkable for the absence of an ascending ramus. The scapula in most of the Cetacea is very broad and flat, ex- panded into the shape of an open fan. The spine is usually situated close to the anterior border, sometimes coalescent with it. The acromion is curved and flat, the coracoid also compressed and parallel with the acromion. In some, both acromion and coracoid are absent. Fr FIG. 1150.— Skull of Dolphin (Globiocephalue'), sagittal section, a. angle of mandible ; an. external nares ; AS. alisphenoid ; bh. basihyal ; BO. basioccipital ; BS. basisphenoid ; cd. condyle of mandible; cp. coronoid process; Ex.0, exoccipital ; Fr. frontal; IP. inter- pariotal ; ME. mesethmoid ; MX. maxilla ; Na. nasal ; Pa. parietal ; Per. periotic ; PI. pala- tine ; P. MX. premaxilla ; pn. posterior nares ; PS. presphenoid ; Pt. "pterygoid " ; sh. stylo- hyal ; -SO. supraoccipital ; Sq. squamosal ; th. thyro-hyal ; Vo. vomer. (After Flower.) There is never any trace of a clavicle. The humerus is short and very stout ; the head freely movable in the glenoid cavity ; the distal articulating surfaces are flat and oblique, meeting at an angle. The proximal ends of the radius and ulna are so firmly united with the humerus as to allow of very little movement ; at the distal end there are no synovial membranes. The manus is extremely modified. There are no synovial joints ; the carpus is in some (Whale-bone Whales) almost entirely cartilaginous, as also are the metacarpals and phalanges — the cartilages being coalescent or separated by intervals of fibrous tissue : in some of the carpal elements bone 534 ZOOLOGY SECT. is deposited. In the toothed Whales the carpal s are com- pletely ossified, and are of poly- gonal form : the phalanges are also ossified, with incomplete synovial articulations. In the Cetacea there are sometimes t five digits, sometimes only four : more or fewer have considerably more than the normal number of phalanges — sometimes as many as fourteen. The second digit is usually the longest. Vestiges of the pelvis are pre- sent in the form of a pair of long narrow bones (Fig. 1148, pelv.) which lie parallel with the spinal column some little distance be- low the region where the chevron bones begin : these appear to re- present the ischia. A second pair of smaller bones which lie close to these in the Whale-bone Whales are apparently vestiges of the femora, and there may be additional vestiges representing the tibiae. Skeleton of Sirenia. — In the Sirenia (Fig. 1151) the cervical vertebrae do not coalesce, with the exception of two of them in the Manatee. In the Manatee there are only six cervical ver- tebrae, and the neural arches are sometimes incomplete. In the trunk the thoracic vertebrae are numerous; all have distinct facets for the heads of the ribs, and well developed zygapo- physes. The caudal vertebrae are numerous and depressed, with wide transverse processes The ribs are numerous, but few of them are connected with the sternum. The sternum is a broad bone not composed of distinguish- able segments. XIII PHYLUM CHORDATA 535 The skull (Fig. 1152) is characterised by its extreme hardness. The cranial cavity is rather long and narrow as compared with that of the Cetacea. Although the supraoccipital (SO.) is pro- duced forwards on the upper surface of the skull for a considerable distance, it does not separate the parietals (Pa.) from one another. The frontals develop broad supra-orbital plates. The zygoma is stout. As in the Cetacea, the external nares are very wide, but they are relatively further forwards. The nasals are rudimentary. PMx ExQ 1152. — Section of skull of Manatee (Manatus senegalensis). Letters as in Fig. 1150. In addition, ET. ethmo-turbiiial ; Ty. tympanic. (After Flower.) 'he tympanic and periotic are readily separable from the other bones. There are enormous premaxillse in the Dugong. The mandible has a well-developed ascending ramus and coronoid process (cp.). The scapula of the Sirenia is much more like that of the terrestrial Mammals than is that of Cetacea, but is nearer that of the Seals; it is narrow and curved backwards. The spine is situated about the middle ; the acromion is directed downwards. The coracoid is fairly large, and of a conical shape. The clavicle is absent, as in the Cetacea. The skeleton of the arm also departs less from the ordinary mammalian type than in the Cetacea. The 536 ZOOLOGY SECT. radius and ulna are ankylosed at their extremities. The carpus has seven bones in the Manatee : the pisiform is absent. In the Dugong coalescence takes place between the carpal bones, so that the number of ossifications is reduced in the adult. There are five digits, all of which possess the normal number of phalanges. The pelvis is represented by a pair or more of vestiges widely separated from the spinal column, and having a vertical position : they probably represent the ilia. Skeleton of the Ungulata. — In general, the centra of the Ungulata are more or less distinctly opisthoccelous. The odontoid process of the axis (Fig. 1153) has a peculiar spout-like form in the majority of the Ruminants, and in a less marked degree in the FIG. 1153.— Axis of Red Deer (Cervus elaphus). A, lateral view ; B, dorsal view. e_p. epiphysis of centrum ; od. odontoid process ; pt. z. post-zygapophysis ; sp. neural spine ; trans, trans- verse process. Horses and Tapirs : in the Chevrotains, the Pigs and the Pro- _boscidea it is conical. In the Ruminants the cervical vertebrae present a median keel below, produced in the posterior part of the region into a process. The development of the cervical neural spines varies : in most they are elongated and compressed ; but in the Horses they are almost completely absent, and in the Elephants they are all small, with the exception of the last. The number of thoraco-lumbar vertebras is nearly always nineteen in the Artiodactyles, twenty-three in the Perissodactyles and in the Proboscidea. Hyrax has a larger number of trunk-vertebra— twenty-eight to thirty — than any other terrestrial Mammal. The transverse processes of the lumbar vertebrae are nearly always elongated, flattened, and directed outwards, or outwards and slightly forwards. Usually there is a single wide sacral vertebra united with the ilia, ankylosed with which behind are a varying number xiii PHYLUM CHORDATA 537 of narrow caudal vertebras. There are never chevron bones in the caudal region of any existing Ungulate. In all the Ungulata the sternebrse are distinct. As a general rule the presternum is narrow, sometimes (Horses and Tapirs) greatly compressed laterally, while the mesosternum is broad; but in the Rhinoceros the mesosternum is no broader than the presternum. Among the Perissodactyle Ungulates the skull of the Horse (Fig. 1154) is elongated, especially in the facial region; the axis of the skull, or the line from the anterior margin of the premaxillas to the lower edge of the foramen magnum, is nearly straight, and both the occipital plane and ethmoidal plane are nearly perpen- dicular to it. The supraoccipital (SO.) has a prominent transverse To, Mtt, FIG. 1154.— Side view of posterior parts of skull of Horse (Equus cdballus). AS. alisphenoid ; Ex 0. exoccipital ; Fr. frontal ; g.f. glenoid fossa ; Ma, jugal ; oc. occipital condyle ; Pa. parietal ; pp. paroccipital process ; Per. periotic ; p. g. postglenoid process of squamosal ; p.t. posttympanic process; SO. supraoccipital ; Sq. squamosal; th. tympano-hyal ; Ty. tympanic. (After Flower.) crest ; and in front of this the temporal ridges which limit the tem- poral fossa above, unite to form a median longitudinal sagittal crest, running along the course of the sagittal suture. The exoccipital develops a prominent, downwardly-directed, paroccipital process (pp). The tympanic (Ty.) is small and, with the periotic (Per.), is only loosely connected with the neighbouring bones, being held in place mainly by a post-tympanic process developed from the squamosal. A considerable part of the periotic (mastoid portion) appears on the surface of the skull between this and the exocci- pital. The tympanic forms a tubular auditory meatus, but is not expanded into a bulla. The glenoid fossa is extended transversely, and is bounded behind by a post-glenoid process. The orbit, which is relatively small, is completely surrounded by bone. The nasals are large, and are separated from the premaxillaB in a great part of their extent. The mandible has a large ascending ramus, and a coronoid process which rises high above the level of the condyle ; 538 ZOOLOGY SECT. the latter is elongated transversely in co-ordination with the form of the glenoid cavity. The skull of the Rhinoceros differs from that of the Horse mainly in the presence of large air-cells in the supraoccipital and parietal bones, and in the orbit not being separated by bone from the temporal fossa, except in the two-horned Asiatic species. The postglenoid process equals or exceeds the paroccipital ; the mastoid does not appear on the surface, owing to the post-tympanic process of the squamosal extending backwards to articulate with the ex- occipital and concealing it from view. The skull of the Tapirs resembles that of the Rhinoceros in most respects. As in the latter, the orbits are not completely bounded by bone behind. The nasal openings are very large, and extend backwards above the orbits, separated from them only by a thin plate. The nasals are very prominent, and the inferior and lateral boundaries of the nasal apertures are formed entirely by the maxillae. There are large post-glenoidal and post-tympanic pro- cesses; the latter is united with the paroccipital process. The mandible differs from that of the other Perissodactyles chiefly in the prominent incurved angle. In the Ruminant Artiodactyles (Fig. 1155) the facial region is more or less bent downwards on the basi-cranial axis, and, while the occipital plane is nearly perpendicular to the latter, the ethmoidal plane is nearly horizontal. There are prominent paroccipital processes (pp). The tympanic (Ty), which may or may not be ankylosed with the periotic, forms a tubular auditory meatus and sometimes a distinct bulla. The mastoid appears for a short space on the surface, between the squamosal and the exoccipital. The frontals usually bear a pair of processes, more or less prominent, for the support of the horns, and between these a transverse ridge frequently extends. The orbit is completely encircled by bone, and has a prominent margin. The nasals are elongated and the premaxillaB slender. The condyle of the mandible is broad and flat ; the horizontal ramus usually rather slender, and expanded in front for the lodgment of the incisors. In the Pigs, as in the Ruminants, the facial region is bent downwards. There is a prominent transverse occipital crest at the junction of the supraoccipital and parietals; but the temporal ridges do not meet in the middle to form a sagittal crest such as occurs in the skull of the Horse. There are prominent paroccipital processes. There is a large, but compressed, bulla tympani ; the auditory meatus is very long, directed upwards and outwards, and is surrounded by the post-glenoidal and post-tympanic processes, which are in contact with one another. The mastoid is rudi- mentary, and does not appear on the outer surface of the skull. The frontal develops a short postorbital process ; but this does not meet the zygoma, so that the bony margin of the orbit is XIII PHYLUM CHORDATA 539 incomplete behind. The facial region as a whole is elongated and laterally compressed. The nasals are long and narrow, and the premaxillse send backwards long processes on each side of them. A peculiar bone — the pre-nasal — is developed in the nasal septum. The condyle of the mandible is transversely elongated ; the coronoid process very small. The skull of the Hippopotamus differs from that of the Pig mainly in the proportions of the various parts. The cranial cavity is relatively small, and the face large. The orbits are almost FIG. 1155.— Section of skull of Sheep .(Ovis aries). AS. alisphenoid ; bh, basihyal ; BO. basioccipital ; £S. basisphenoid ; cd. condyle ; ch. ceratohyal ; cp. coronoid process ; cli. epihyal ; EO. exoccipital ; ET. ethmo-turbinal ; Fr. frontal ; ME. mesethmoid ; M T. maxil- lary turbinal ; MX. maxilla ; Na. nasal ; OS. orbito-sphenoid ; Pa. parietal ; PL palatine ; Per. periotic; P. MX. premaxilla ; P.S. pre-spbenoid ; Pt. "pterygoid" ; s. h. stylohyal ; SO. supraoccipital ; pp. paroccipital process ; th. thyrohyal ; ty. ••tympanic ; Vo, vomer. (After Flower. ) tubular, and are almost, or quite, encircled by bone. The face is laterally contracted in front of the orbit and again expands anteriorly. The mandible is extremely massive ; anteriorly the symphysial portion is greatly expanded to support the large incisor and canine teeth. In the Hyracoidea (Fig. 1156) the skull shows affinities with Rodents and also with Perissodactyles. The zygomatic arch is stout : it is formed mainly by the jugal (/%), which forms part of the glenoid fossa. The postorbital processes may meet to bound the 540 ZOOLOGY SECT. orbit behind ; the upper one is formed from the parietal (par). The facial region is comparatively short. The premaxillse (p. max) are not greatly developed. There are distinct paroccipital pro- cesses (p. oc,). The periotic and tympanic are ankylosed together, par p.m FIG. 1156.— Skull of Hyrax. Letters as in FIG. 1132. In addition, int. par. interparietal ; ty. tympanic. The suture between the frontal and parietal has been by an error made to run behind the post^orbital process. ME FIG. 1157.— Section of skull of African Elephant (Elephas africanus), to the left of the middle line. a. n. anterior nares ; ME. mesethmoid ; p. n. posterior nares ; Vo. vorner. (After Flower.) but not to the squamosal. The tympanic (ty.} forms a bulla with a spout-like prolongation. In the Proboscidea (Fig. 1157) the bones of the skull are of enormous thickness, the inner and outer tables being separated by XIII PHYLUM CHORDATA 541 extensive air-cells. The sutures are early obliterated. Paroccipital and postglenoidal processes are absent. The tympanic forms a large, rounded auditory bulla ; but the external auditory meatus is bounded chiefly by the post-tympanic process of the squamosal. The mastoid portion of the periotic does not appear on the surface. The orbit is not completely separated by bone from the temporal fossa. The nasal aperture is situated far back, and looks upwards and forwards, almost as in the skull of some of the Cetacea. The chief characteristic of the mandible is its prolongation forwards into a spout-like process at the symphysis. In the Ungulata vera the scapula (Fig. 1158) is never very broad ; the spine is usually near the middle. Neither the acromion nor the coracoid process is very prominent ; some- times, as in the Horse, the former is ab- sent. A clavicle is never present. In the Ruminants, as in some other Mammals, the vertebral portion of the scapula re- mains cartilaginous, forming the so- called supra-Scapular cartilage (ss). In Pigs and some Perissodactyles, though there is no acromion, there is a tri- angular process about the middle of the spine. The humerus is short and stout, the radius always well developed, the ulna in some (Pigs, Hippopotami, Tapirs, and Rhinoceroses), well developed, in others (the Horses and the Ruminants), incom- plete. The first digit is always absent. There is never a centrale. The trape- zium and magnum unite in most of the Ruminants. In the Perissodactyla the third digit in both the fore- and hind-foot is sym- metrical in itself. In the Rhinoceroses the second and fourth are also present, and in the Tapirs (Fig. 1159) the fifth of the fore-foot is developed as well. The Horses (Fig. 1160) present the greatest reduction in the number of the digits observable in any Mammal, the third being the Only functional digit in each foot. Its elongated metacarpal or metatarsal (sometimes called the cannon lone) has in apposition with it laterally a pair of splint- like vestiges which represent the metacarpals or metatarsal s of the second and fourth digits. In the Artiodactyla, on the other hand, the third and fourth digits form a symmetrical pair. In VOL. II L L FIG. 1158.- Right scapula of Red Deer (Cervus elaphus). a. acromion ; a/. prescapular fossa ; c. vestigial coracoid pro- cess ; gc. glenoid cavity ; pf. post-scapular fossa ; sp. spine ; ss. imperfectly ossified supra- scapular portion. (After Flower.) 542 ZOOLOGY SECT. FIG. 1159.— Bones of the manus of Tapir (Tapirus indicus). c. cuneiform ; I. lunar ; in. magnum ; P. pisiform ; R. radius ; s. scaphoid ; td. trapezoid ; tm. trapezium ; U. ulna ; u. unciform. (After Flower.) FTG. 1160. — Bones of the manus of Horse (Equus caballus). c. cuneiform ; I. lunar ; m. magnum ; p. pisiform ; R. radius ; s. scaphoid ; td. trapezoid ; u. unciform ; //, IV, vestigial second and fourth meta- carpals. (After Flower.) FIG. 1161.— Bones of manus of Pig (Sus scrofa). c. cuneiform ; 1. lunar ; m. mag- num ; R, radius ; s. scaphoid ; td. trapez- oid ; ton. trapezium ; U. ulna ; u. unci- form. (After Flower.) FIG. 1162.— Bones of manus of Red Deer (Cervus elaphus). m?. m5. vestigial second and fifth metacarpals. R. radius. (After Flower.) XIII PHYLUM CHORDATA 543 the Ruminant Artiodactyles (Fig. 1162) the metacarpals or meta- tarsals of these digits unite to form a single elongated bone, the cannon bone. The pelvis of most Ungulata is greatly elongated. The ilia are wide transversely, the syniphysis is very long, involving a part of the ischia as well as the pubes. In the Perissodactyla, but not in the Artiodactyla, there is a well-marked third trochanter. In some Ungulates (Rhinoceroses, Tapirs, Pigs, Hippopotami), the fibula is distinct, though slender. In the Horse it is represented FIG. 1103. — Dorsal surface of right tarsus of Horse (Equus caballus). «. astragalus ; c. cal- caneum ; cb. cuboid ; c'2. united nicso- and ento-cuneiform ; c3. ecto-cimeiform ; n. navicular ; a. scaphoid ; mil, IV, vestigial second and fourth metatarsals; ///, third metatarsal. (After Flower.) FIG. 1164.— Dorsal surface of right tarsus of Red Deer (Cervus elaphus). a. astragalus ; c. cal- caneum ; cb. cuboid ; c3. conjoined ecto- and meso-cuneiform ; mill, mir, third and fourth metatarsals ; n. navi- cular. (After Flower.) FIG. 1165.— Dorsal surface of right tarsus of Pig (Sus scrofa). a. astra- galus ; c. calcaueum ; cb. cuboid ; cs, ecto- cuneiform ; c2. meso- cuneiform ; mil— V. metatarsals ; n. navi- cular. (After Flower.) by a vestige. In the Ruminants it is represented only by a small vestige, the malleolar lone, which articulates with the distal end of the tibia. The structure of the foot exhibits a close parallelism to that of the manus. The tarsal bones are closely dove-tailed together, and articulate with one another by flat surfaces. The hallux is never developed. In the Perissodactyla the third digit is sym- metrical in itself. In the Rhinoceros and Tapirs the second and fourth digits are also completely developed ; but in the Horses (Fig. 1163) they are represented only by splint-like vestiges of their metatarsals, the metatarsal of the third digit forming an L L 2 544 ZOOLOGY SECT. elongated " cannon bone/' like the metacarpal of the third digit of the manus. In the Rhinoceroses and Tapirs all the usual tarsal bones are present; in the Horses the ento-cuneiform and meso-cuneiform are united. In the Artiodactyles the third and fourth digits form a symmetrical pair as in the manus; and in the Ruminants (Fig. 1164) their metatarsals unite to form a cannon bone. In most Ruminants there are no vestiges of the second and fifth digits. In the Pigs (Fig. 1165) all the tarsal bones are present. In most Ruminants the cuboid and navicular are united ; in the Camels these bones are distinct, but the ento- cuneiform is wanting. In the Hyracoidea the scapula is triangular, like that of the Ungulata vera, and the spine is moderately developed and most prominent in the middle. There is a large supra-trochlear fora- men. The radius and ulna are complete, but often ankylosed. In the carpus there is a centrale between the scaphoid and the trapezoid. There are five digits, the first very small, and in some represented only by a vestigial metacarpal. In the femur an indistinct ridge-like elevation is to be regarded as representing the third trochanter. The foot resembles that of the Rhinoceros in having three digits developed ; but there is a small bone representing the fifth metatarsal, and the ungual phalanx of the second is cleft. In the Proboscidea the coracoid-process is small. The acromion presents a recurved process or metacromion, as in Rodents. The clavicle is absent. The radius and ulna are permanently fixed in the prone condition. The manus is short and broad, the carpals are squarish, with flat articular surfaces. There is no centrale ; five digits are present. The pelvis has its long axis nearly vertical. The iliac crest is directed transversely, and is greatly expanded; the iliac and gluteal surfaces look almost directly forwards and backwards. The pubes and ischia are com- paratively small. The femur is very long as compared with that of the Ungulata vera. There is no third trochanter. The fibula is complete. The foot is short and broad, somewhat smaller than the manus. Skeleton of the Carnivora. — In the Carnivora the atlas is very large, with wing-like lateral processes. The neural spine of the axis is elongated and compressed, the odontoid conical. The other cervical vertebra have small spines and large transverse processes. There are twenty or twenty-one thoraco-lumbar vertebras. The most anterior thoracics have long, slender, back- wardly-sloping spines. In the posterior thoracics large meta- pophyses and anapophyses are developed. The transverse pro- cesses of the lumbar vertebrae are extremely long and the spines short. The sternum is long and narrow, composed usually of eight or nine pieces. The sternal ribs are almost uncalcified. XIII PHYLUM CHORDATA 545 FIG. 1166.— Skull of Tiger (Felis tigris). Blainville.) (After In the skull of the Carnivora vera (Figs. 1166 and 1168) there are prominent sagittal and lambdoidal crests. The temporal fossae are very deep; the orbits are not separated from them by bone. The relative development of the facial region varies in the different groups ; in the Bears and their allies, and in the Dogs, it is elongated ; in the Cats it is very short. The zygoma is strong and greatly arched outwards. The glenoid cavity is in the form of a transverse groove, to the shape of which the transversely elongated con- dyle is adapted. In the Cats there is a large, rounded tympanic bulla (Fig. 1167), the cavity of which is divided into two parts — an- terior and posterior — by a septum, the anterior con- taining the auditory ossicles and the opening of the Eustachian tube; the bony auditory meatus is short: the paroccipital is closely applied to the posterior surface of the tympanic bulla. In the Dogs the septum of the bulla is incom- plete, the auditory meatus short, and the paroccipital pro- cess not applied to the bulla. In the Bears and their allies (Fig. 1169), the bulla is usually less dilated, and the septum is absent or only re- presented by a ridge, while the bony audi- tory meatus is elon- _ gated. FIG. 1167. -Section of the left auditory bulla of Tiger (Felis "^® ^^l11111. *n titjris). a. aperture of communication between the two the Pinnipedia \£ Iff. chambers into which the cavity of the bulla is divided; -, thy-»\ • TL J ' J a.m. external auditory rneatus ; b.oc. basioccipital ; PL 1172) IS broad and periotic; a. septum between the two chambers; Sq. rnnnf{*t\ rathpr r»nm- squamosal. (After Flower.) lOUnGCd, latner COm 546 ZOOLOGY SECT. pressed from above downwards. The orbits are large and approach near to one another. In the Carnivora vera the spine of the scapula is situated at Jm FIG. 1168.— Lateral view of skull of Dog (Canis familiaris). C. occ. occipital condyle ; F.frontal ; F inf. infra-orbital foramen ; Jg. jugal; Jm. premaxilla ; L. lacrymal ; M. maxilla; M.aud. external auditory meatus ; Md. mandible; N. nasal; P. parietal; Pal. palatine; Pjt. zygomatic process of squamosal ; Pt. " pterygoid " ; Sph. alisphenoid ; Sq. squamosal ; Sq. occ. supraoccipital ; T. tympanic. (From Wiedersheim's Comparative Anatomy.) about the middle of the outer surface of the bone. The acromion is usually well developed, sometimes with a metacromion. The coracoid process is very small. The clavicle is never complete, FIG. 1169.— Section of the left auditory bulla and surrounding bones of a Bear ( Ursus ferox). a. m. external auditory meatus ; B. 0. basioccipital ; e. Eustachian tube ; Sq. squamosal ; T. tympanic ; t. tympanic ring. (After Flower.) sometimes entirely absent. There is a supra-condyloid foramen in the Cats and some of the other groups, not in the Dogs or Bears. XIII PHYLUM CHORDATA 547 The scaphoid and lunar are united (Fig. 1170). There is no centrale. Usually a radial sesamoid is present. There are five digits, though the pollex may be reduced in size, as in the Dog, and it is vestigial in the Hyaena. The pelvis is long and narrow. In the tarsus all the ordinary bones are developed. The hallux is fully formed in the Bears, &c., FIG. 11TO. — Carpus of Bear (Ursus amen- canus). c. cuneiform ; m. magnum ; p. pisiform ; r. s. radial sesamoid ; ft. I. scapho-lunar ; til. trapezoid ; tm. tra- pezium ; u. imciform. (After Flov/er.) FIG. 1171.— The phalanges of the middle digit of themanus of the Lion (Felia leo). ph\ proxi- mal phalanx; plA middle phalanx ;ph3. ungual phalanx ; a, the central portion forming the internal support to the horny claw ; b, the bony lamina reflected around the base of the claw. (After Flower.) but shorter than the other digits, In the Cats and Dogs it is represented only by a vestige of the metatarsal. In the Pinnipedia (Fig. 1172) both acromion and coracoid are short, and the scapula is curved backwards ; there is no clavicle. The bones of the fore-limb are short and stout ; the humerus has a prominent deltoid crest; there is no foramen above the inner condyle. The ulna is greatly expanded at its proximal, the radius at its distal end. The manus is broad and expanded. The scaphoid and lunar are united to form a scapho-lunar. The ungual phalanges are nearly straight, slender, and pointed. The ilia are short; the symphysis pubis is short and without firm union of the bones. The femur is short, thick, and flattened. The fibula and tibia are commonly ankylosed proximally. The calcaneum is short and usually without a distinct calcaneal process ; the lateral digits are usually the longest. Skeleton of the Rodentia. — Among the Rodents the Jerboas are exceptional in having the cervical vertebrae ankylosed. Gene- rally, as in the Rabbit, the transverse processes of the lumbar vertebrae are elongated. As in the Ungulata, the sacrum usually consists of one broad anterior vertebra followed by several narrower ones. The caudal region varies in length in the different families ; in some it is very short, but it is elongated in many (the Porcupines, Squirrels, and Beavers). The sternum of the Rodents has a long and narrow body ; sometimes there is a broad presternum ; the posterior end is always expanded into a cartilaginous xiphisternum. 548 ZOOLOGY SECT. The skull is elongated, narrow in front, broader and depressed behind. The nasal cavities are very large, especially in the Porcupines, with air sinuses in the upper part. In some the optic foramina fuse into one. An interparietal is often present. Paroccipital processes are developed. The orbit and the tem- poral fossa are always continuous. The nasal bones are large, and the nasal apertures are ter- minal or nearly so. The premaxillae are always very large. A remark- able feature of the skull is the presence in many of a large opening cor- responding to the infra- orbital foramen. The middle part of the zygoma is formed by the jugal; the latter often helps to bound the glenoid cavity, as in the Marsupials. The palate is short, and the anterior palatine foramina large. The periotic and tym- panic may become ank}T- losed together, but not to the neighbouring bones. The coronoid process of the mandible is sometimes rudiment- ary or absent ; the angle is often produced into a process. The scapula of the Rodentia is generally long and narrow. The spine sometimes has a XIII PHYLUM CHORDATA 549 metacromion process and a long acromion. The coracoid process is small. The clavicle varies as regards its development. Vestiges of the sternal end of the coracoid are sometimes distinguishable. There is considerable variation in the bones of the arm and fore- arm. The radius and ulna are in most instances distinct, though in close and firm apposition. The scaphoid and lunar are usually united ; the centrale is sometimes present, sometimes absent. The pelvis and femur vary greatly. Sometimes there is a third trochanter. The fibula is sometimes distinct, sometimes fused with the tibia. In the Jerboa the inetatarsals of the three digits are fused together. Skeleton of the Insectivora. — The neural spine of the axis is usually well developed, that of the remaining cervical vertebrae small or obsolete. The number of trunk-vertebrae varies in the different families frorn eighteen to twenty-four, and there is also great variation in the development of the various processes. The caudal region varies in its length ; frequently it has chevron bones. The presternum is expanded, the mesosternum composed of distinct, narrow sternebraa. The skull (Fig. 1173) varies greatly in the different families, in the higher forms approaching that of the Lemurs, with com- paratively large cerebral fossae, large orbits with complete or Firt. 1173. — Skull of Tenrec (Centetes ecaudatus). Jr. frontal ; max. maxilla ; pa. parietal ; p. max. premaxilla ; sq. squamosal. (After Dobson.) nearly complete bony rims, well developed zygoma, and a tympanic bulla and tubular auditory meatus. In the others the cranial capacity is less, and the orbits and temporal fossa3 are completely continuous ; the zygoma is incomplete, and the tympanic does not usually form a bulla. The pectoral arch also varies a good deal in the different families of the Insectivora. In the true Moles and their allies there is a remarkable bone of cuboid shape articulating ventrally with the presternum and dorsally with the humerus, and only 550 ZOOLOGY SECT. connected by a ligamentous band with the scapula. Its mode of formation from a mass of cartilage — to the anterior face of which the clavicle, formed as usual in membrane, becomes applied — proves that this bone represents a procoracoid as well as a clavicle. In other Insectivora this bone is not developed, and the clavicle is a distinct, long and slender bone, but vestiges of the inner or ventral ends of the coracoid and procoracoid may be recognisable. Sometimes there is a distinct bone intervening between the outer end of the clavicle proper and the acromion process. The humerus usually has a supracondylar foramen. In the Moles this is absent, and their humerus is remarkable in other respects, being short, greatly expanded at the extremities, with a prominent deltoid ridge, and with two synovial articular surfaces at the proximal end, one for the glenoid cavity of the scapula, the other for the coraco-clavicle. The radius and ulna are completely developed in all, and are usually distinct, but some- times fused distally. In the carpus the scaphoid and lunar sometimes coalesce, sometimes remain distinct ; an os centrale is usually present. In the Moles the manus is extremely broad, the breadth being increased by the presence of a large, curved, radial sesamoid. In the pelvis the symphysis pubis is in some cases elongated, in others short, and sometimes absent, the pubes remaining separated by a wide median ventral cleft. A third trochanter is sometimes represented by a ridge. The fibula usually, though not always, fuses distally with the tibia. Skeleton of the Chiroptera (Fig. 1174). — The cervical region of the vertebral column is characterised by the absence of any distinct neural spines, and the same holds good to a less extent of the trunk-vertebraB ; the transverse processes of the lumbar region are also rudimentary. The tail varies in development : when it is elongated the component vertebras are long, cylindrical centra without processes. Sagittal and occipital crests are developed in the skull of some species. The facial region is rather elongated, especially in the Megachiroptera (Fig. 1175). Post-orbital pro- cesses of the frontal are present or absent : the zygoma is long and slender : the malar is small and applied to the outer surface of the zygoma. The long and narrow nasals are in some cases united ; the premaxillse are small. The mandible has an angular process in the Microchiroptera, not in the Megachiroptera. The segments of the sternum are sometimes distinct, sometimes united : the presternum has a mesial keel developed in co-ordination with the great size of the pectoral muscles. The sternal ribs are ossified. The scapula is large and oval in shape : the spine is near the anterior margin : the post-scapular fossa has ridges for the origin XIII PHYLUM CHORDATA 551 of muscular fibres : the spine has a well developed acromion. The coracoid is elon- gated and in some cases bifurcated. The clavicle is long. The pro-coracoid is repre- sented by a separate ossification ; there are rudiments of the sternal end of the coracoid between the clavicle and the first rib. The humerus, radius, and ulna are all elongated, and the ulna is reduced, being sometimes only represented by the proximal end, anky- losed with the radius. A large sesamoid is developed in the tendon of the triceps muscle near the ole- cranon process of the ulna. In the carpus the scaphoid and lunar are united : sometimes also the cuneiform is united with these : the pisi- form is small. There is no centrale. The ungual phalanges are absent in the nail- less digits. The pel- vis is small, and a symphysis pubis is often wanting. The fibula is sometimes well-developed, some- times rudimentary. The tuber calcanei..is an inwardly curved r^rocess of the calca- 552 ZOOLOGY SECT. neum, attached to which by means of ligamentous fibres is a slender rod of bone or cartilage, the calcar, which supports the inter-femoral membrane. FIG. 1175.— Skull of Pteropus fuscus. (After Blainville.) Skeleton of the Primates. — The atlas is ring-like, the odontoid sub-conical, The spines of the cervical vertebrae are usually well-developed and simple : in Man they are short — with the exception of the seventh — and bifid : in some they are trifid. The number of thoraco-lumbar vertebra is usually nineteen, but only seventeen in Man, the Gorilla and Chimpanzee, sixteen in the Orang; in some Lemurs it may be twenty-three or twenty-four. The number of sacral vertebrae varies from two to five. The sacral region of Man, which comprises five ankylosed vertebras, differs from that of other Primates in its greater relative breadth and in its backward curvature; it forms a well-marked angle where it joins the lumbar region — the sacro -vertebral angle — scarcely recognisable in other Mammals. The number of caudal vertebrae varies with the length of the tail — from four to about thirty- three. In Man there are only four vestigial caudal vertebrae, anky- losed together to form the coccyx. In all those forms in which the tail is well developed chevron bones are present. The human skull (Fig. 1176) presents a marked contrast in certain respects to that of other Mammals, but in many points is approached by that of the other Primates, more especially by that of the Simiidae. One of the most important characteristics of the human skull is the large size of the brain-case, the cubic contents of the cranial cavity averaging 1500 cubic centimetres in the male of white races. This great development is most marked in that part of the cavity which lodges the cerebral hemispheres, in adapta- tion to the large dimensions of which the cranium bulges out both anteriorly and posteriorly to such an extent that the entire length XIII PHYLUM CHORDATA 553 of the cavity greatly exceeds that of the basi-cranial axis. A re- sult of the posterior bulging of the brain-case is that the foramen magnum (f.m) is no longer situated at the posterior extremity of the skull as in other Mammals, but assumes a position further forwards towards the middle of the base. The anterior expansion, causing a strong arching forwards of the frontal region, brings about an alteration in the position of the ethmoidal plane, which, instead of being perpendicular or inclined to the basi-cranial axis, becomes AS OS FIG. 1176. — Skull of Man. Letters as in Fig. 1155. In addition, a. angle of mandible ; c. g. crista galli, a process of the mesethmoid ; sh. styloid process ; st. sella turcica. (After Flower.) horizontal, and the cribriform plate forms the middle part of the floor of the anterior extension of the cranial cavity. The fossa for lodgment of the cerebellum lies entirely beneath the posterior t)ortion of the cerebral fossa : the olfactory fossa is comparatively small. (See Fig. 1126, Z>.) The outer surface is smooth and rounded, devoid of any prom- nent ridges or crests. The occipital crest of lower Mammals s represented merely by a rough, raised line — the superior curved ine of the occiput. The paroccipital processes are only 554 ZOOLOGY SECT. represented by slight eminences — the jugular eminences. There is no auditory bulla ; the mastoid portion of the periotic pro- jects downwards as a prominent mastoid process. The periotic, tympanic, and squamosal early fuse into one bone — the temporal lone. The post-glenoid process is very slightly developed. The whole facial region is relatively small. The orbits, which are of moderate size, are directed forwards ; the bony margin is com- plete, and a plate of bone, developed partly from the jugal, partly from the alisphenoid, almost completely cuts it off from the temporal fossa, leaving only a small aperture of communication — the spheno-maxillary fissure. The frontal suture usually early dis- appears. The nasals rarely become fused. The suture between the premaxillse and the maxillaB becomes obliterated at an early stage, so that the entire upper jaw appears to consist of a single bone. A peculiar spine, the nasal spine, is developed in the middle line below the nasal opening. The most marked feature of the mandible is the presence of a prominence, the mental prominence, in the lower part of the symphysial region ($.). The stylo-hyal nearly always becomes fused — together with the tympano-hyal — to the periotic and tympanic, giving rise to a slender process— the styloid process (sh.') — projecting downwards from the base of the skull. None of the other Primates have a cranial capacity approaching that of Man ; and those modifications in the shape of the skull which are the concomitants of the great development of the brain in the human species are accordingly not recognisable, or are much less strongly marked. The various fossae of the cranium, as a rule, however, occupy the same relative positions as in Man ; the cerebellar fossa is entirely beneath the cerebral ; and the ethmoidal plane and that of the foramen magnum (occipital plane) are usually both horizontal or nearly so. In all the Simiidse, with the exception of the Orang, the frontals meet in the middle line below, over the presphenoid. In many Monkeys the outer surface of the cranium is smooth and free from prominent ridges ; but in the Baboons, the Orangs,the Gorilla, and the Chimpanzee (Fig. 1177), there are strongly developed occipital, sagittal, and supra-orbital ridges, usually much more prominent in the male than in the female, and increasing in size with age. The paroccipital processes are always rudimentary, but there are well-marked post-glenoid pro- cesses. The mastoid does not form a distinct mastoid process. In the Cebidte and Hapalidae alone is there a tympanic bulla. The entire facial region is relatively larger than in Man ; the pre- maxillo-maxillary region is always more prominent, and in the Baboons projects forwards as a distinct muzzle. The orbit is separated from the temporal fossa as in Man. The nasals are usually ankylosed in the adult. The nasal spine is never developed. The suture between the premaxilla and the maxilla only becomes xin PHYLUM CHORDATA 555 obliterated, if at all, in old individuals. The mental prominence of the mandible is never developed, the anterior surface of the symphysial region sloping backwards and downwards from the bases of the incisor teeth. The stylo-hyal never gives rise to an ossified styloid process. In the skull, as in many other respects, the Lemurs occupy an intermediate position between the higher Primates and the lower orders of Mammals. The occipital and ethmoidal planes are usually vertical. The tympanic forms a large bull a. The orbits, which are large, are usually separated from the temporal fossa only by a narrow rim of bone. The lacrymal foramen is situated on the face outside the margin of the orbit. The facial FIG. 1177.— Skull of Chimpanzee (Anthropopithecus troglodytes). (After Blainville.) region is usually elongated, and may form a prominent muzzle. In all the Primates the clavicle is present and complete, and in the scapula the spine, acromion, and coracoid process are well developed. In Man and the higher Apes the glenoid border of the scapula is much longer than the coracoid border. In the lower Monkeys, on the other hand, these borders are nearly equal. The humerus is comparatively long and slender ; the tuberosities and ridges are not, as a rule, very strongly developed. In Man and the Simiidae the bone is twisted around its long axis ; in the lower forms this torsion is absent. In Man and the higher Apes the foramen above the inner condyle is absent ; it is present in many of the American Monkeys and in most Lemurs. Characteristic of the ulna of Man and the higher Apes is the small upward extension of the olecranon process. The radius and ulna are distinct in all ; 556 ZOOLOGY SECT. in the higher forms the shafts of the two bones are bent outwards, so that there is a wide interosseous space, and there is consider- FIG. 1178.— Skeleton of Orang (Simla satyrus). (After Blainville.) able freedom of movement in pronation and supination. In the carpus (Fig. 1179) the scaphoid and lunar are always distinct, and a centrale is present in all except some of the Lemurs, the Gorilla, Chimpanzee, and Man. A pisiform is present, and in most a radial sesamoid. As compared with that of the other Primates, the carpus of Man is short and broad ; the trapezium has a saddle-shaped articular surface turned somewhat inwards. In Man, the Chimpanzee, Gorilla, and Orang, the carpus articulates exclusively with the radius; in all the others it articulates also with the ulna. In Man the pollex has a remarkable and characteristic free- dom of movement in opposition to the other digits. The human pelvis is remarkable for its relative breadth, for the expanded form of the ilia and the deep concavity of their FIG. 1179.— Carpus of Baboon (Cynocephalus anubis). ce. cen- trale ; c. cuneiform ; ?. lunare; m. magnum ; p. pisiform ; r.s. radial sesamoid. ; s. scaphoid ; id. trapezoid ; tm. trapezium ; u. unciform. (After Flower.) XIII PHYLUM CHORDATA 557 inner surfaces, and for the shortness of the pubic symphysis. In the higher Apes some of these features are recognisable, though less pronounced ; but in the lower the ilia are long and narrow, and usually curved outwards ; in the Old-world Monkeys the tuberosities of the ischia are strongly everted and roughened for the attachment of the ischial callosities. The tibia and fibula are well-developed and distinct in all. In nearly all, the hallux, owing to the form and direction of the articu- lation between it and the internal cuneiform, is opposable to the other digits, converting the foot into a grasping organ. The human foot (Fig. 1180) is distinguished from that of the other Man Fin. 1180.— Foot of Man, Gorilla and Orang drawn the same absolute length, to show the difference in proportions. The line a' a' indicates the boundary between tarsus and meta- tarsus ; V b', that between the latter and the proximal phalanges ; and c' c' bounds the ends of the distal phalanges, as. astragalus ; ca. calcaneuni ; sc. scaphoid. (After Huxley.) Primates by the absence of .this power of opposition and by the relative length of the tarsus, which exceeds that of the metatarsus. Digestive Organs. — Teeth are present in nearly all Mammals, but in some they are wanting in the adult condition (e.g., Whale- bone Whales and Platypus). In Echidna teeth are not present even in the young. In some of the Anteaters teeth are developed in the foetus and are thrown off in utero — the adult animal being devoid of them. VOL. II M M 558 ZOOLOGY SECT. Teeth, as already explained in the general account of the Craniata (p. 86), are developed partly from the epidermis and partly from the underlying dermis. In the Mammals each tooth is lodged in a socket or alveolus in the jaw. The part of the tooth developed from the epidermis is the enamel; the remainder of the tooth — dentine, cement and pulp — being formed from the subjacent meso- dermal tissue. Along the oral surface of the jaw is formed a ridge- like ingrowth of the ecto- derm— the dental lamina (Fig. 1182, lam. ). The posi- tion of this is indicated externally by a groove — the dental groove (gr.). From this a bud is given off in the position to be occu- pied by each of the teeth. The bud becomes con- stricted off as a conical cap of cells — the enamel-organ — which remains in con- tinuity with the dental ridge only by a narrow isthmus. This cap-like form is brought about by the development of a papilla of condensed dermal tissue — -the dental papilla (pap.), which pushes up- wards against the enamel- organ. On the surface of this papilla, in contact with the enamel-organ, the cells (odontoblasts) become ar- ranged into a layer having the appearance of an epithelium — the dentine- forming layer. The cells of the enamel-organ form two layers, of which that in contact with the dental papilla assumes the character of a layer of long cylindrical cells — the enamel-membrane (en. m.) ; the more superficial layer consists of cubical cells. Between the FIG. 1181. — Diagrammatic sections of various forms of teeth. I, incisor or tusk of Elephant with pulp- cavity persistently open at base ; II, human incisor during development, with root imperfectly formed, and pulp-cavity widely open at base ; III, completely formed human incisor, with pulp-cavity opening by a contracted aperture at base of root ; IV, human molar with broad crown and two roots ; V, molar of the Ox, with the enamel covering the crown deeply folded, and the depressions filled up with cement ; the surface is worn by use, otherwise the enamel coating would be continuous at the top of the ridges. In all the figures the enamel is black, the pulp white, the dentine represented by horizontal lines, and the cement by dots. (After Flower and Lydekker.) XIII PHYLUM CHORDATA 559 two the remaining cells of the enamel-organ become modified to form a kind of connective tissue — the enamel-pulp (en. pip.}. The connective-tissue immediately surrounding the entire rudiment of the tooth becomes vascular and forms a distinct en. en,. pip La,m' a,lv Lam FIG. 1182.— Two stages in the development of the teeth of a Mammal (diagrammatic sections). alv. bone of alveolus ; dent. s. dental sac ; en. m. enamel-membrane ; en. pip. enamel-pulp ; gr. dental groove ; lam. dental lamina ; lam', part of dental lamina which grows downwards below the tooth -germ ; pap. dental papilla. (After O. Hcrtwig.) investment — the dental sac (dent, s.); from this blood-vessels extend into the papilla. Calcification begins by the formation of a cap of dentine (Fig. llSSjdent) produced by the dentine-forming cells, and of a layer of enamel (en.) on the sur- face of this, produced by the cells of the enamel- membrane. To these additional layers are added until the crown of the tooth becomes fully d eveloped. The substance of the den- tal papilla gives rise to the pulp. As the tooth elongates, it pro- jects on the surface and eventually breaks through the mucous membrane of the gum, the remains of the enamel-organ becoming thrown off'. The cement Hertwig.) is formed by the ossi- fication of the connective-tissue surrounding the tooth-papilla. In the teeth of most Mammals distinct roots are formed, each with a minute opening leading into the pulp-cavity (Fig. 1181, ///-— V) ; but in some there are no roots, the pulp-cavity being M M 2 FIG. 1183.— Diagrammatic section showing the develop- ment of the milk- and permanent teeth of Mammals. alv. bone of alveolus ; dent, dentine ; dent. &. dental sac ; en. layer of enamel ; en. m. outer layer of enamel-organ of milk-tooth ; en. m2. enamel-membrane of permanent tooth ; en. pip. enamel-pulp of milk-tooth ; (jr. dental groove ; lam. dental lamina ; n. neck connecting milk- tooth with lamina ; pap. dental papilla of milk-tooth ; pap2, dental papilla of permanent tooth. (After O. Hert 560 ZOOLOGY SECT. open below (7), and the tooth constantly growing from the base as it becomes worn away at the crown ; such teeth are said to have persistent pulps. Usually Mammals have two distinct sets of teeth developed, the milk and permanent dentitions, but sometimes there is only one, and accordingly we distinguish diphyodont and monopJiyodont Mammals : in nearly all of the latter, however, another set are developed, though they early become absorbed or remain in the condition of functionless vestiges ; and in a considerable number of groups it has been found that more than two sets of teeth dc Ac elm .3 FIG. 1184.— Milk- and permanent dentition of upper (I) and lower (II) jaw of the Dog (Cants familiaris), with the symbols by which the different teeth are commonly designated. (After Flower and Lydekker.) are formed, only one, or at most (in diphyodont forms) two, of these sets becoming fully developed. The milk-teeth in Mam- mals with typical diphyodont dentition sometimes disappear at an early stage, and sometimes do not become replaced by the permanent teeth till long after birth. Some Mammals have the teeth almost indefinite in number, e.g., the Dolphins and Porpoises, in which they are all uniform (homodont) and not divided into sets (Fig. 1185). In the typical dentition there are forty-four teeth, viz., three incisors on each side, one canine and seven pre- molars and molars above and below. The incisors (Fig. 1184, i.) of the upper jaw are to be distinguished as being the teeth that are lodged in the premaxilJa3 ; the incisors of the lower jaw are XIII PHYLUM CHORDATA 561 the teeth that are placed opposite to these. The upper canine (s.) is the most anterior tooth of the maxilla situated on or immediately behind the premaxillo-maxillary suture, and has usually a charac- teristic shape. The lower canine is the tooth which bites in front of the upper canine. The pre-molars (p.) are distinguished from the molars by having milk predecessors (d.m.), but the first pre- molar is, except in the Marsupials, nearly always a persistent milk-tooth ; the molars (m.) have no teeth preceding them, and are sometimes looked upon as persistent teeth of the first set. FIG. 1185. — Upper and lower teeth of one side of the mouth of a Dolphin (Lagenorhynchus), illustrating the homodont type of dentition in a Mammal. (After Flower and Lydekker.) The various sets of teeth are also usually distinguishable by their shape. As a rule the incisors have cutting edges ; the canines are pointed and conical ; the pre-molars and molars have broad surfaces with ridges and tubercles for crushing the food, and may have from two to four roots. The simplest form of molar tooth (which occurs, however, only in certain extinct forms) is that of a simple cone, or a cone with two small accessory processes or cusps. Almost as primitive is the type of tooth termed triconodont (likewise occurring only in a few extinct Mammals), in which there are three equal conical cusps set in a straight line, the upper teeth biting on the outer side of the lower From the triconodont is derivable the trituber- culate molar, in which the free surface of the tooth presents three cusps or tubercles arranged in a triangle, the apex of which is internal in the upper, external in the lower jaw. In the upper molar the inner cusp is termed the protocone, the antero-external the paracone, and the postero-external the metacone. These terms are modified in the case of the molars of the lower jaw, the equivalent of the protocone, here external, being termed the protoconid and the others paraconid and metaconid respectively. This trituberculate type of molar is usually complicated by various additions and modifications — accessory cusps being added, together with ridges or folds connecting the cusps together. The resulting complex tooth may be modified to act as a cutting (secodoni) or a crushing (bunodont) molar. A modification of the bunodont molar is brought about by the cusps, instead of retaining their conical form, being drawn out into the shape of crescents (selenodont). ZOOLOGY SECT. The number of the various sets of teeth in the jaws is con- veniently expressed by a dental formula, in which the kind of tooth (incisor, canine, pre-molar, molar) is indicated by the initial letter (£, c.y p., m.\ and the whole formula has the arrangement of four vulgar fractions, in each of which the numerator indicates the teeth of the upper, the denominator those of the lower jaw. Thus : . 3'3 1-1, 4-4 3-3 ^F3'cTl^^m>3 = 44- or, in a simpler form, since the teeth of the right and left sides are always the same — FIG. 1186.— Teeth of Bandicoot (Perameles). (After Owen.) Echidna has no teeth at any stage. In Ornithorhynchus teeth are present in the young and are functional for a time, but they are thrown off when the animal is about a year old : vestiges of an earlier dentition have been detected, and the function of teeth is per- formed in the adult by broad horny plates, one on each upper and one on each lower jaw. The Marsupials have the milk-dentition in a degenerate condition. Germs of milk-teeth are de- veloped, but with the exception of one — the last pre-molar — and in some cases of canine and incisors, these remain in an imperfect state of development, though they persist, as function- less vestiges, to a comparatively late stage. . In the adult dentition of the Marsupials the number of in- cisors in the upper and lower jaws is always dissimilar except in Phascolomys. With regard to the arrangement of these teeth, the order falls into two series, termed respectively the dipro- todont and the polyprotodont. Fjo 1187>_Front view of skull of Koaia In the former (FlgS. 1 187-1188) (Phascolarctos cinereus), illustrating dipro- v . e . . todont ar.d herbivorous dentition. (After the two anterior incisors are Flower.) XIII PHYLUM CHORDATA 563 large and prominent, the rest of the incisors and the canines being smaller or absent. On the other hand, in the polyprotodont forms FIG. 1188.— Teeth of Great Kangaroo (Man-opus mayor). (After Owen.) FIG. 1189.— Front view of the skull of Tasmania!! Devil (Sarcophilus ursinus), showing polyprotodont and carnivorous dentition. (After Flower.) FIG. 1190. -Teeth of upper jaw of Opossum (Diddphys marsupialis}, in all of which there is no succession except in the last pre-molar, the place of which is occupied in the young animal by a molariform tooth represented in the figure below the line of the otHer teeth. (After Flower and Lydekker.) (Figs. 1189, 1190), which are all more or less carnivorous, the incisors are numerous and sub-equal and the canines large. There are typically three pre-niolars and four molars. A good example 564 ZOOLOC4Y SKCT. of the diprotodont arrangement is the Kangaroo (Macropus, Fig. 1188), which has the dental formula — The canine is very small and is early lost. Of the polyprotodont forms the Australian Dasyure or Native Cat (Fig. 1132) has the formula — 4 1 9 4 ' and the American Opossum (Didelphys) (Fig. 1190)— .5134 *' Z'C'T'^' Q'm'l, = o0' 4 1 6 4 The Edentata, as noticed in the outline of the classification, though not by any means all toothless, always have some defect in the dentition ; when teeth are present in the adult the anterior series are absent and the teeth are imperfect, wanting roots and devoid of enamel. The tooth-characters differ widely in the different groups. In the Sloths there are five teeth above and four below on each side; no second series is known. In the American Anteaters there are no teeth in the adult. In the Armadillos, on the other hand, the teeth are numerous, though simple and rootless, and in one genus at least, two series occur. In the Scaly Anteaters there are no teeth. In the Cape Ant- eaters (Fig. 1191), again, there are numerous teeth which are heterodont and diphyodont, and have a peculiar structure, being perforated by numerous minute, parallel, vertical canals ; the pulp FIG. 1191.— Section of lower jaw and teeth of Orycteropus. (After Owen.) of each tooth, entire at its base, is divided distally into a number of parallel columns. In the Ungulata the dentition is heterodont and diphyodont, and the teeth are very rarely devoid of roots. In the Artiodactyla the pre-molars and- molars differ from one another in pattern; the. XIII PHYLUM CHORDATA 565 first upper pre-molar is almost always without a milk predecessor. The Pigs (Fig. 1192) are among the very few recent Mammalia FIG. 1192.— Left lateral view of the dentition of *the Boar {Sits scrofa), the roots of the teeth being exposed. (After Flower and Lydekker.) which possess what has been referred to as a typical dentition : the formula of the completed dentition is — .3143 The incisors of the upper jaw are vertical, those of the lower greatly inclined forwards. The canines are greatly developed, especially in the male, and grow from persistent pulps ; both the upper and lower are bent upwards and outwards and work against one another in such a manner that the upper wears on its anterior and external surface, the lower at the extremity of the posterior. The pre-molars are compressed, with longitudinal cutting edges, and the molars provided with numerous tubercles or cusps arranged for the most part in transverse rows (bunodont type). The formula of the milk dentition is — In the typical Ruminants there are no teeth on the prernaxillaB, the incisors of the lower jaw and the canines, which resemble them in shape, biting against a thickened callous pad on the opposed surface of the upper jaw, and the upper canines are also usually absent ; there are three pre-molars and three molars in both upper and lower series, all characterised by the presence of column-like 566 ZOOLOGY SECT. vertical folds of enamel, the interstices between which may be filled up with cement, and the worn surface of the tooth presenting a pattern of the selenodont type (Fig. 1181, V). In the Camels there are a pair of upper incisors and a pair of large canines in each jaw. In the Perissodactyla the molars and pre-molars form a con- tinuous series of large teeth with ridged or complexly-folded crowns, the posterior pre-molars often differing little in size and structure from the molars. In the Horse (Fig. 1193) the formula is — .314 3 ,, but the first pre-molar is a small tooth which soon becomes lost, and may belong to the milk-dentition. A fold of the enamel Fio, 1193.— Side view of skull of Horse with the bone removed so as to expose the whole of the teeth, c. canine ; Fr. frontal ; i±. i^. is. incisors ; L. lacrymal ; Ma. jugal ; MJC. maxilla ; m1. 7ii2. m3. molars ; Na. nasal ; oc. occipital condyle ; Pa. parietal ; p. ml. situation of the vestigial first pre-molar, which has been lost in the lower, but is present in the upper jaw ; pm2. pms. pm*. remaining pre-molars ; PMx. pre-maxilla ; pp. par-occipital process ; Sq. squamosal. (After Flower and Lydekker.) dips downwards (i.e., towards the root) from the extremity of the incisor teeth like the partly inverted finger of a glove ; the canines are small in the female, and may not appear on the sur- face. There is a wide interval in both jaws between the canines and pre-molars. The pre-molar and molar teeth present a com- plicated pattern due to folds of the enamel, which differ in their arrangement in the upper and lower jaws ; their roots become completed only at a late period. XIII PHYLUM CHORDATA 567 In the Hyracoidea the dental formula is — .10 4 3 *-2'C'0'|7-4>™-3=34- The upper incisors are not unlike the larger pair of the Rabbit in shape, though prismatic and pointed, instead of compressed and chisel-like ; they grow from persistent pulps. The outer incisors are elongated, inclined forwards, and trilobed at the extremities. The pre-molars and molars form a continuous series, separated by an interval from the incisors, and in pattern closely resemble those of some of the Perigsodactyla. The Elephants have an extremely specialised dentition. There are no canines and no lower incisors. The single pair of upper incisors are developed into the enormous tusks (Fig. 1181, /), which grow continuously from persistent pulps throughout the FIG. 1194. — Grinding surface of a partially worn right upper molar of the African Elephant (Elephas africanus). (After Owen.) life of the animal ; they are of elongated conical form, and usually become curved. The tusks are composed of solid dentine, enamel occurring only on the apices and becoming early worn away. The molars (Fig. 1194) are very large, and their worn surfaces are marked with prominent transverse ridges ; there are six molars altogether on each side, but only one or two are functional at once, the more posterior moving forward and taking the place of the more anterior as these become worn out. Fia. 1195.— Left lower jaw of foetus of Balaenoptera ro strata, inner aspect, showing teeth ; natural size. (After Julin.) When teeth are developed in the Cetacea they are nearly always numerous, homodont, and monophyodont : in the Sperm- whales they are confined to the lower jaw. In the Whalebone 568 ZOOLOGY SECT. fl Whales, though teeth are developed in the foetal condition (Fig. 1195), they become lost either before or soon after birth, and they are succeeded in the adult by the plates of baleen or whalebone (Fig. 1196), which, in the form of numerous triangular plates, hang vertically downwards from the palate. Of the Sirenia, the Dugong and Manatee have a heterodont denti- tion ; in Rhytina teeth were absent. In the two former Sirenians there are incisors and molars with a wide diastema between them. In the Manatee there are two rudimentary incisors on each side, both in the upper and the lower jaw ; these dis- appear before the adult condition is reached. There are altogether eleven molars on each side above and below, but not more than six of these are in use at once, the more anterior when worn out being succeeded by the more posterior. They have enamelled crowns with transverse ridges, and are preceded by milk- teeth. In the Dugong there are no incisors in the mandible of the adult, and only one tusk-like pair in the upper jaw, large in the male — in which they grow from persistent pulps — little developed in the female, and remaining concealed in their sockets. In the young there are rudimentary incisors in the mandible, and also a rudimentary second pair in the upper jaw. There are either five or six molars on each side, both in the upper and lower jaws. These are cylindrical teeth, devoid of enamel, and with persistent pulps. In the Carnivora vera (Fig. 1197) the dentition is complete, heterodont and diphyodont, and all the teeth are provided with roots. The incisors are relatively small, chisel-shaped teeth ; there are nearly always three of them on each side, in both upper and lower jaws. The canines are always large and pointed. The presence of carnassials, consisting of the last pre-molar in the upper, and the first molar in the lower, jaw, is universal. In front of the carnassial the teeth are compressed and pointed ; behind it they have broad surfaces. In the Cat family (Felidce) the formula is— FIG. 1196.— Section of m G. 1196.— section ol upper jaw, witn baleen-plates, of Balaenoptera. (After Owen.) .31 1 PHYLUM CHORD ATA 569 The lower carnassial is thus the last of the series. In the Dogs (Canidse) the formula is usually — and in the Bears (Ursidaa) it is the same. Q In the Pinnipedia there are always fewer than ^ incisors, and o carnassials are not developed. The pre-molars and molars have a Fi<;. 1107.— Left lower carnassial teeth of Carnivora. 7, Felis ; //, Canis ; III, Herpestes ; /r, Lutra ; r, XKeles ; VI, Ursus. 1, anterior lobe (paraconid) of blade ; 2, posterior lobe (protoconid) of blade ; 3, inner cusp (metaconid) ; 4, talon (hypoconid). (After Flower and Lydekker.) compressed, conical, pointed form. The prevailing dental formula of the Seals is — 4 44 »•;=»• In the Walrus the adult formula is — 1130 The upper canines take the form of large, nearly straight tusks. In the large order of the Rodents the dentition is remarkably uniform, and, in all its general characters, resembles what has already been described in the Rabbit. But the second, smaller 570 ZOOLOGY SECT. pair of incisors of the upper jaw is present only in the Hares and Rabbits ; the number of pre-molars and molars varies from — 02^ 33 p.-m.-to p.^m.p and they may develop roots. In the Insectivora the dentition is heterodont, complete, and diphyodont. AlFthe teeth are" rooted. There are never fewer than two incisors on either side of the lower jaw. The canines are not of large size. The crowns of the molars are beset with pointed tubercles. In the Chiroptera the dentition is complete, and the teeth are all rooted. There is a milk-series which differs entirely from the permanent teeth. In the insectivorous Chiroptera (Bats) the molars are provided with pointed cusps, while in the frugivorous forms (" Flying Foxes ") they are longitudinally grooved or excavated. In the Primates the teeth are heterodont and diphyodont, and always form roots. There are almost invariably two incisors on each side in each jaw, and, in all but the Hapalidas, three molars. The dental formulae of the various families have been given in the synopsis of the classification. The dentition of Man differs from that of the rest of the order in the teeth forming a continu- ous series not interrupted by a diastema, and in the comparatively small size of the canines. The mouth in Mammals is bounded by fleshy lips. On the floor of the mouth is situated the tongue, which is usually well developed, but varies in size and shape in different orders. Its surface is covered with papillae of different forms, in association with certain of which are the special end-organs of the nerves of taste — the taste-buds. The roof of the mouth is formed in front by the hard palate, consisting of the horizontal palatine plates of the maxillary and palatine bones covered with mucous membrane. Behind the hard palate projects backwards the soft muscular fold of the soft palate, also with taste-buds, which divides the cavity of the pharynx into two chambers, an upper and a lower. In front of the opening leading from the lower division of the pharynx into the larynx, is a cartilaginous plate — the epiglottis — of which rudiments only are found in certain lower Vertebrates. The oesophagus is always a simple straight tube. The stomach varies greatly in different orders, being sometimes simple, as in the majority of Mammals, sometimes divided into chambers, as in the Cetacea and the Ruminants. In the majority of Mammals the stomach is a simple sac, as in the Rabbit (p. 460). But in certain groups it is complicated by the development of internal folds, and may be divided by con- strictions into a number of different chambers. The complication XIII PHYLUM CHORDATA 571 of this organ reaches its extreme limit in the ruminant Ungulata and in the Cetacea. In a typical Ruminant (Fig. 1198, E, Fig. 1199), such as a sheep or an Ox, the stomach is divided into four chambers — the rumen or paunch, the reticulum, the psalterium, C De IL FIG. 1198.— Different forms of the stomach in Mammals. A, Dog; -B, Mus decumanus ; (', Mus musculus ; J), "Weasel ; E, scheme of the ruminant stomach, the arrow with the dotted line showing the course taken by the food ; F, human stomach ; G, Camel ; //, Echidna aculeata ; /, Bradypus tridactylus. A. (in E and (?) abomasum ; Ca. cardiac end ; Cma, greater curvature ; Omi, lesser curvature ; Du. duodenum ; MB, csecum ; 0, psalterium ; Oe. oesophagus ; P. pylorus ; R. (to the right in Fig. E) rumen ; R (to the left in Pig. E) reticulum ; Sc. cardiac division ; Sj>, pyloric division ; W. Z, water-cells. (From Wiedersheim's Comparative Anatomy.) and the abomasum, or rennet-stomach. The first of these (Fig. 1199, V) is much larger than the rest; its mucous membrane is beset with numerous short villi. The reticulum (c), which is much smaller than the rumen, has its mucous membrane 572 ZOOLOGY SECT. raised up into a number of anastomosing ridges, giving its wall the appearance of a honeycomb with shallow cells. From the aperture by which the reticulum communicates with the rumen .to that with which it communicates with the psalterium, runs a groove bounded by a pair of muscular ridges, which are capable of closing together in such a way as to convert the groove into a canal. The mucous membrane of the psalterium (d) is raised up into numerous longitudinal leaf-like folds. The abomasum (e), smaller than the rumen, but larger than the reticulum, has a smooth, vascular and glandular mucous membrane. The oeso- phagus opens into the rumen close to its junction with the reticulum. The herbage on which the Ruminant feeds is swal- lowed without mastication, accompanied by copious saliva, and passes into the rumen and reticulum, where it lies until, having finished feeding, the animal begins ruminating or chewing the FIG. 1199.- Stomach of Ruminant opened to show the internal structure, a, oesophagus ; b, rumen ; c. reticulum ; d. psalterium ; e, abomasum ; /, duodenum. (After Flower and Lydekker.) cud. In this process the sodden food is returned in rounded boluses from the rumen to the mouth, and there undergoes mastication. When fully masticated it is swallowed again in a semi-fluid condition, and passes along the groove into the reti- culum, or over the unmasticated food contained in the latter chamber, to strain through between the leaves of the psalterium and enter the abomasum, where the process of digestion goes on. In some Ruminants the psalterium is wanting. In the Camels (Fig. 1198, G) the stomach is not so complicated as in the more typical Ruminants, there being also no distinct psalterium, and the rumen being devoid of villi ; both the rumen and the reticulum have connected with them a number of pouch-like diverticula (w. z.\ the openings of which are capable of being closed by sphincter muscles ; in these water is stored. In the Cetacea the stomach is also divided into compartments. In the Porpoise (Fig. 1200) the oesophagus (a) opens into a spacious XIIT PHYLUM CHORDATA 573 paunch (&), the cardiac compartment of the stomach, with a smooth, thick, mucous membrane. This is followed by a second chamber (c), of considerably smaller dimensions, with a glandular mucous membrane, which is thrown into a number of complex folds. A long and narrow third, or pyloric, compartment (d, e) follows upon this, terminating in a constricted pyloric aperture, beyond which the beginning of the intestine is dilated into a bulb. A caecum, situated at the junction of the large and small intes- tines, is usually present, but varies greatly in extent in the different orders and families. It is much larger in vegetable-feeding than in carnivorous forms, and among the former it is those that have FIG. 1200. — Diagrammatic section of the stomach of the Porpoise, a, oesophagus ; 6, left or cardiac compartment ; c, middle compartment ; d and c, the two divisions of the right, or pyloric compartment ; /. pylorus ; g, duodenum, dilated at its commencement ; h. bile- duct. (After Flower and Lydekker.) a simple stomach, such as the Rabbit, that have the largest caecum. Hyrax differs from all the rest of the class in having a pair of supplementary caeca situated some distance down the large intestine. A cjocum is absent in the Sloths, some Cetacea, and a few Garni vora. The Prototheria resemble Reptiles, Birds, and Amphibia, and differ from other Mammals, in the presence of a cloaca, into which not only the rectum, but the urinary and genital ducts open. In the Marsupials, a common sphincter muscle surrounds both anal and urinogenital apertures; in nearly all the Eutheria (cf. p, 447) the apertures are distinct, and separated from one another by a considerable space — the perinccum. VOL. II N N 574 ZOOLOGY SECT. FIG. 1201. — Diagrammatic plan of the liver of a (posterior surface), c. caudate lobe ; cf. cystic fissure ; dr. The liver (Fig. 1201) consists of two parts or main divisions, right and left, incompletely separated from one another by a fissure termed the umbilical, owing to its marking the position of the fcetal umbilical vein. Typically each of these main divisions is divided by a fissure into two parts, so that right lateral (rl.) and right central (re.), and left lateral (II.) and left central (lc.) lobes are distinguishable. When a gall-bladder is pre- sent, as is the case in the majority of Mam- mals, it is attached to, or embedded in, the right central lobe. A fissure, the portal, through which the portal vein and hepatic artery pass into the substance of the liver, and the hepatic vein passes out, crosses the right central lobe near the anterior border. ductus venosus ; g. gall-bladder ; lc. left central lobe ; The DOStcaval lies 111 contact with, or em- bedded in, the right lateral lobe near its anterior border, and, given off from this lobe between the postcaval and the portal fissure, is a small lobe, of varying extent — the Spigclian. The term caudate lobe is applied to a process of the right lateral lobe, of considerable extent in most Mammals, having the post- caval vein in intimate relation to it, and often closely applied to the kidney. A gall-bladder is usually present but is absent in the Cetacea, the Perissodactyle Ungulata, the Hyracoidea, and some Rodents. Vascular System. — The blood of Mammals is warm, having a temperature always of from 35° to 40° C. The red corpuscles are non-nucleated : in form they are most usually biconcave discs, always circular in outline except in the Camelidaa, in which most of them are elliptical. The lymphatic system of vessels is very highly developed, ramifying richly throughout all parts of the body. In the course of this system occur numerous lymphatic glands. The special part of the lymphatic system of vessels (lacteals) which ramify in the wall of the intestine and absorb the fatty matters of the food, combine with the lymphatic vessels from the hind-limbs and body to form a receptacle — the reapta- culum chyli — from which a tube — the thoracic duct — which may II. left lateral lobe ; II f. left lateral fissure ; p. portal vein al lo Spigeli u. umbilical vein ; vc, postcaval. (After Flower and entering trans verse fissure ; re. right central lobe; rl. right lian lobe ; lateral lobe ; rlf. right lateral fissure ; s. S u. umbilica Lydekker.) xin PHYLUM CHOftDATA 575 be double, runs forwards to open into the base of one of the great veins of the precaval system by a valvular aperture. The general statements which have been given with regard to the heart of the Rabbit (p. 462) hold good for the Mammalia in general. The sinus' venosus is never distinct from the right auricle ; of its valves, which are more completely retained in the Edentata than in the other orders, the right gives rise to the Eustachian valve — a membranous fold, often fenestrated in the adult, extending from the right wall of the postcaval to the edge of the foramen ovale (annulus ovalis); while the left becomes merged in the auricular septum, helping to complete the annulus ovalis behind. Each auricle has an auricular appendix. The right auriculo-ventricular aperture has a three-lobed (tricuspid) valve, and the left a two-lobed (bicuspid or mitral), with chordae tendinese and musculi papillares. In all Mammals the openings of the pulmonary artery and aorta are provided with three-lobed semilunar valves. The single aortic arch, situated in all Mammals on the left side, varies greatly in the way in which it gives off the main arterial trunks. Sometimes a single large trunk passes forward from the arch of the aorta and gives rise to both carotids and both sub- clavians. Sometimes there are two main trunks — right and left innominate arteries — each giving rise to the carotid and subclavian of its own side. Sometimes there is a right innominate giving off right carotid and right subclavian, the left carotid and left subclavian coming off separately from the arch of the aorta ; or, as in the Rabbit, an innominate may give origin to. the right subclavian and both carotids, the left subclavian alone coming off separately. In Monotremes and Marsupials, in most Ungulates, and in the Rodentia, Insectivora, and Chiroptera, both right and left precavals persist ; in the others the left aborts, its vestige giving rise to the coronary sinus. In the Monotremes the openings of all three cavals are provided with valves, only vestiges of which exist in the other groups. In the Monotremes all the pulmonary veins open by a common trunk. In the Metatheria and Eutheria the four veins sometimes open separately, sometimes the two veins of each side unite to form a single lateral trunk. The following are some of the principal variations in the struc- ture of the heart which occur in the different groups of Mammals. In the Monotremes there is a deep fossa representing the fossa ovalis in the auricular septum. The tricuspid valve in Ornitho- rhynchus consists of two membranous and two fleshy portions ; the mitral valve is wholly membranous. In Echidna the tricuspid valve is completely membranous, and consists of two portions — a larger and a smaller. In the Marsupials the fossa ovalis and amiulus ovalis aie absent; in the uterine foetus of the Kangaroo N N 2 676 ZOOLOGY SECT. the auricles communicate by a fissure, but all trace of this becomes lost before the adult stage is reached. In the Cetacea, Eustachian and Thebesian valves are both absent. In some of the Cetacea the apices of the ventricles are separated by a slight depression. In the Sirenia there is a corresponding, but much deeper and wider, cleft, so that the apex of the heart is distinctly bifid. In the Ungulata, Eustachian and coronary valves are both absent ; in some there is a cartilage or a bone — the os cordis — often double, at the base of the heart. The Eustachian valve is absent in most of the Carnivora. In the Pinnipedia, an aperture of communication between the auricles often persists in the adult. The organs of respiration resemble those of the Rabbit in the general features mentioned on p. 466. In the Cetacea, the epiglottis and arytenoids are prolonged to form a tube, which extends into the nasal chambers and is em- braced by the soft palate, so that a continuous passage is formed, leading from the nasal chambers to the larynx, and giving rise to the condition of intra-narial epiglottis. In all the remaining orders a similar condition occasionally occurs — the epiglottis being produced upwards into the respiratory division of the pharynx behind the soft palate. In foetal Marsupials, in which the intra- narial condition is very complete, it is obviously associated with the passive absorption of the milk, while breathing is being carried on continuously through the nostrils. Some Cetacea and Artiodac- tyla, &c., are exceptional in having a third bronchus, which passes to the right lung anteriorly to the ordinary bronchus of that side and to the pulmonary artery. In connection with various parts of the respiratory system, there are cavities containing air. The connection of the tympanic cavity with the pharynx by means of the Eustachian tubes has been already mentioned. Air-sinuses, connected with the nasal chambers, extend into the bones of the skull, especially into the maxillae and frontals, where they may reach large dimensions and are known as the maxillary antra and frontal sinuses. Air-sacs are also developed in connection with the larynx in many of the Apes. Nervous System. — The brain of Mammals (Fig. 1202) is distinguished by its relatively large size, and by the large size and complex structure of the ceretoal hemispheres of the fore- brain. The cerebral hemispheres of opposite sides are connected together across the middle line in all Mammals, except the Mono- tremes and Marsupials, by a band of nerve tissue termed the corpus callosum — a structure not present in the Sauropsida. The hemi- spheres, in all but certain of the lower and smaller Mammals, are not smooth, but marked by a number of grooves or sulci separating winding ridges or convolutions. The lateral ventricles in the XIII PHYLUM CHORDATA 577 interior of the hemispheres are of large size and somewhat complex form. The optic lobes, which are relatively small, are divided into four parts, and are hence called the corpora quadrigemina. The Rol Fit:. 1202. -Brain of Dog. A, dorsal ; B, ventral ; C, lateral aspect. B. ol. olfactory bulb ; Cr. ce. crura cerebri ; Fi. p. great longitudinal fissure ; Hff, HH', lateral lobes of cerebellum ; Hyp. hypophysis ; Med. spinal cord ; NH, medulla oblongata ; Po. pons Varolii ; VH. cerebral hemispheres ; IFv, middle lobe (vermis) of cerebellum ; I — XII. cerebral nerves. (From Wiedersheim's Comparative Anatomy.) pineal body is always a small, gland-like structure. Connecting together the lateral parts of the cerebellum, which, in the higher Mammals, attains a high degree of development, is a transverse flattened band — the pons Varolii (Po.) — crossing the hind-brain on its ventral aspect. 578 ZOOLOGY SECT. -venl.3 FIG. 1203.— Brain of Echidna aculeata, sagittal section, ant. com, anterior commissure ; cbl. cere- bellum ; c. mam. corpus mammillare ; col. forn. column of the fornix ; c. qu. corpora quadrigemina ; crur. crura cerebri ; gang. liab. habenular ganglion ; hip. com. hippocampal commissure ; liyj>o. hypo- physis ; med. medulla oblongatn ; mid. com. middle commissure ; olf. olfactory bulb ; opt. optic chiasma ; tub. olf. tuberculum olfactorium ; vent. 3, third ventricle. In the Monotremes and Marsupials (Figs. 1203, 1204) there is no corpus callosum, while the anterior commissure (ant. com.) is of , , relatively large size, and, &- t hip.com unlike the corresponding 7nid.com \ / commissure in lower Ver- tebrates, contains fibres connecting together areas of the non-olfactory regions (neo-pallinm) of the hemi- spheres. The hippocampi extend along the whole length of the lateral ven- tricles. The layer of nerve- cells in each hippocampus gives origin, as in Eutheria, to numerous fibres, which form a layer on the sur- face, the alveus, and become arranged in a band — the tamia hippocampi. In the Eutheria, as we have seen in the case of the Rabbit, the tasnise unite mesially to form the body of the fornix (see p. 470). In the Monotremes and Marsupials, on the other hand, there is no such union ; the fibres, of the tasnia £ 7tip.com run towards the foramen of Monro, where they become divided into several sets. Of these one set, constituting the great majority of the fibres, pass into the hippocampus of the opposite side, giving rise to a hip- pocampal commis- sure (hip. corn,., cf. Figs. 935 and 971), the great develop- ment of which readily leads to its being mistaken for a corpus eallosum. The fibres entering into the formation of this commissure corre- spond, however, not to the fibres of the corpus callosum, which "771071 cbl meet FIG. 1204.— Sagittal section of brain of Rock Wallaby (Petro- l/ale penicillatii). ant. com. anterior commissure ; cbl. cere- bellum ; c. mam. corpus mammillare ; c. qu. corpora quadri- gemina ; crur. crura cerebri ; epi. epiphysis, with the pos- terior commissure immediately behind ; /. Mon. position of foramen of Monro; \ip. com. hippocampal commissure, consist- ing here of two layers continuous behind at the splenium, somewhat divergent in front where the septum lucidum ex- tends between them ; hypo, hypophysis ; med. medulla ob- longata ; mid. com. middle commissure ; olf. olfactory bulb ; opt. optic chiasma ; vent. 3, third ventricle. PHYLUM CHORDATA 570 is the commissure of the nee-pallium, but, as proved by their mode of origin, to the fibres of the fornix, and they connect together only the hippocampi, the fasciae dentatce, or specialised lower borders of the hippocampi, and an area of the hemisphere in front of the anterior commissure (pre-commissuml area} : they thus constitute an olfactory or arcJiipallial commissure, since all these parts belong to the olfactory region or archipallium of the hemispheres. In the Mouotremes (Fig. 1203) the hippocampal commissure is only very slightly bent downwards at its posterior extremity. In most Marsupials (Fig. 1204) it bends sharply round posteriorly and runs forward again, becoming thus folded into two layers, dorsal and ventral, continuous with one another at a posterior bend or splenium, similar to the splenium of the corpus callosum. The dorsal layer of the hippocampal commissure becomes almost completely replaced in cbl Fro. 1205.— Brain of Ornithorhynchus anatinus, dorsal view (natural size) ; ell. cerebellum ; olf. olfactory bulbs. Fm. 1206.— Brain of Echidna aculeata, dorsal view (natural 'size). the Eutheria by the fibres of the corpus callosum, and the ventral part persists in the shape of the psalterium or lyra. In Ornithorhynchus (Fig. 1205) the hemispheres are smooth ; in Echidna (Fig. 1206) they are tolerably richly convoluted. Both genera, but more particularly Echidna, are characterised by the enormous development of the parts of the hemispheres (archi- pallium) connected with the olfactory sense. In the lower Mar- supials there are no convolutions (Notoryctes, Koala, Phalangers), while in the higher the convolutions are numerous, though the sulci are not very deep (Macropus, Fig. 1207). Among the Eutheria there is a great range in the grade of development of the brain, from the Rodents and lower Insectivores to the higher Primates. In the lower types of Mammalian brain the cerebral hemispheres are relatively small, do not overlap the cerebellum, arid have ZOOLOGY SECT FIG. 1207.— Brain of Kangaroo (Macropits major). (After Owen.) smooth, or nearly smooth, surfaces. In the higher types the relative development of the hemispheres is immense, and their backward extension causes them to cover over all the rest of the brain, while the cortex is thrown into numerous complicated con- volutions separated by deep sulci (Fig. 1208). This develop- ment of the cerebral hemispheres reaches its maximum in Man. The organs of special sense have the same general structure and arrangement as in the Sauropsida. Jacdbsons organs, which in the Sauropsida consti- tute such important accessory parts to the olfactory apparatus, are well developed only in the lower groups of Mammals. The olfactory mucous membrane is of great extent, owing to the de- velopment of the convoluted ethmo-turbinal bones over which it extends. In the toothed Cetacea alone among Mammals do the nasal chambers lose their sensory functions — the olfactory nerves being vestigial or absent. The organs of taste are taste- buds in the mucous membrane covering certain of the papillae on the surface of the tongue. In essential structure the eye of the Mammal resembles that of the Vertebrates in general (see p. 109). The sclerotic is com- posed of condensed fibrous tissue. The pecten of the eye of Birds and Reptiles is absent. In most Mam- mals there are three movable eyelids, two, upper and lower, opaque and usually covered with hair, and one anterior, translu- cent, and hair-less — the nictitating membrane. The secretions of a lacrymal, a Harderian and a series of Meibomian glands moisten and lubricate the surface of the eye-ball and its lids. In Moles, and certain other burrowing FIG. 1208.— Dorsal view of brain of Gray's Whale (Coyia grayi). (After Haswcll.) XIII PHYLUM CHORDATA 581 Insectivores and Rodents, and in Notoryctes among the Marsupials, the eyes are imperfectly de- veloped and functionless. The ear of a Mammal is more highly developed than that of other Vertebrates, both in respect of the greater complexity of the essential part — the mem- branous labyrinth — and in the greater development of the accessory parts. A large external auditory pinna, supported by cartilage, is almpst invariably present, except in the Monotremata, Cetacea, and Sirenia. This is a widely open funnel, of a variety of shapes in different groups, having the function of collecting the waves of sound. By the action of a system of muscles it is usually capable of being turned about in different directions. FIG. 1209. — Sagittal section through the nasal and buccal cavities of the human head. 7, //, ///, the three olfactory ridges formed by the turbinals ; be, entrance to the mouth ; Ig. tongue ; os, open- ing of Eustachian tube ; ««,', frontal sinus ; sn", sphenoidal sinus ; r. i, atlas vertebra ; v. ii, axis vertebra. (After Wiedersheim.) Fio. 1210. — Parts of the Human ear (diagrammatic). Cell, cochlea; E. Eustachian tube; Ex. outer opening of ear ; L. labyrinth ; M. tympanic membrane ; N. entrance of auditory nerve ; 01- /-. oo. primitive ova. (From Hertwig, after Waldeyer.) these, surrounded by smaller unmodified cells of the epithelium, sinks into the stroma of the ovary, in which it becomes embedded, the small cells forming a Graafian follicle (foil.) which encloses it. cap ov FIG. 1-214.— Two stages in the development of the Graafian follicle. A, with the follicular fluid beginning to appear ; B, after the space has largely increased, caps, capsule ; disc. cumulus proligerus ; memb. membrana granulosa ; ov. ovum ; sp. space containing fluid. (After Hertwig.) Spaces filled with fluid soon appear among the follicle cells (Fig. 1214, Asp.), and these eventually coalesce to form a single cavity. This cavity, which in some Mammals is crossed by strings of xin PHYLUM CHORD ATA 587 cells, separates an outer layer of the follicle cells — the membrana granulosa (me-nib.) — from the mass — cumulus proligerus (disc.) — sur- rounding the ovum, except on one side where they coalesce. A basement membrane is formed externally to the follicle cells, and the stroma around this becomes vascular, and forms a two- layered investment for the follicle. The cells immediately surrounding the ovum become arranged as a definite layer of cylindrical cells — the corona radiata. A thick membrane — the zona radiata — perforated by numerous radially arranged pores, into which project processes from the cells of the corona, invests the ovum ; and in many, if not in all, there is beneath this a delicate vitelline membrane. As the ovum increases in size, granules of yolk become distinguishable among the protoplasm. As the ovum approaches maturity the fluid — liquor folliculi— -in the cavity of the follicle increases in quantity, so that the follicle becomes greatly distended. The follicle has meanwhile approached the surface of the ovary, on which it comes to project as a rounded prominence. Eventually the middle region of the projecting part of the wall of the follicle thins out and ruptures, setting free the ovum, which passes into the Fallopian tube. On the way along the Fallopian tube impregnation takes place, and, after becoming enclosed in an envelope of albumen, the ovum passes onwards to the uterus, there to undergo its development. In the place of the discharged ovum there is left a space which becomes filled with connective-tissue to form a body known as the corpus luteum. If the ovum should become fertilised and proceed to develop in the uterus, the corpus luteum increases in size and persists for a considerable time : if no development takes place it disappears comparatively quickly. With the absence of food-yolk are connected most of the differ- ences observable between the early stages of the development of a higher Mammal (Fig. 1215) and the corresponding stages in the development of a Reptile or Bird. One of the most striking of these is in the mode of segmentation. In the case of the large ovum of the Bird, as we have seen, the segmentation is of the incomplete or meroblastic type, being confined to a small disc of protoplasm — the germinal disc — on one side of the ovum. In the Mammals, on the other hand, except in the Moaotremes, segmentation is complete or holdblastic, the entire ovum taking part in the process of seg- mentation. The segmentation is nearly or quite regular, the cells into which the ovum divides being of equal, or approxi- mately equal, size. The result, in the Eutheria, is the formation of a sphere of cells, which soon become distinguishable into an outer layer and a central mass, the inner cell-mass or embryonal knot. In the Marsupials, so far as known, the stage of a solid cellular sphere or morula does not occur, a central cavity being present from the outset. In the Eutheria, by imbibition of 588 ZOOLOGY SECT. eel mb.encL perv.encl perLend FIG. 1215. — Diagram representing sections of the embryo of a Mammal at successive stages in the segmentation and formation of the layers. A and B, formation of enclosing layer (trophoblast) and inner cell-mass destined to give rise to the embryo ; 6', blastodei mic vesicle with embryonic cell- mass separated from trophoblast except on one side ; D, blastodermic vesicle in which peripheral and embryonal por- tions of endoderm have become established : the break here represented on each side between the two does not occur. E, stage in which the embryonal ectoderm has broken through the trophoblast and become joined to it peripherally. liquid, a cavity, which is formed in the interior of the sphere, increases rapidly in size. The stage now reached is called the blastodermic vesicle. During the growth in size of the internal cavity the central mass of cells remains in contact with one side only of the outer layer, where it spreads out as a stratum several cells deep. From it are derived the embryonal ecto- derm and the entire endoderm of the vesi- cle. The outer layer is apparently the equiva- lent of the extra- embryonal ectoderm of the Bird and Rep- tile, and has been termed the trophoblast or trophoblastic ecto- derm, because of the part which it plays in the nutrition of the foetus. Immediately beneath it, through- out its extent, a thin layer of flattened cells appears — the peri- pheral endoderm — this is continuous with a similar layer formed on the inner surface of the embryonic cell- mass — the embryonal endoderm. The rest of the cell-mass gives rise to the embryonal ectoderm. The part of XIII PHYLUM CHORDATA 589 the trophoblast, known as the covering layer or Banker's layer, lying over this embryonal ectoderm, has a widely different fate in different Eutheria : it may thin out and disappear. A primitive knot and embryonic shield are formed as in Reptiles. The primitive knot has simply the appearance of the somewhat enlarged anterior extremity of & primitive streak (Fig. 1216, pr.), which is developed very much in the same way as in the Bird, its formation being due to the same cause as in the latter, viz., active proliferation ot cells leading to the development of the begin- nings of the mesoderm. A dark median streak, the head- process, appears in front of the primitive knot, and in some FIG. 1216. — Embryonic area of a seven days' embryo Rabbit, ag, embryonic area ; o, place of future vascular area ; pr, primitive streak ; rf, medullary groove. (From Balfour, after Kolliker.) Mammals there is an invagination on the surface of the former leading to the formation of a neurenteric canal and of a noto- chordal canal which gives rise to the rudiment of the posterior part of the notochord. In the region of the anterior part of the primitive streak — the primitive knot and the head-process, the mesoderm coalesces with the endoderm; but there does not appear to be any breaking through into the underlying space such as occurs in Reptiles (p. 3G5). A medullary groove (rf) and canal are formed in front of the primitive streak, and a row of protovertebraa (Fig. 1217) make their appearance on each VOL. II O O 590 ZOOLOGY SECT. side of the former. The embryo becomes folded off from the blastoderm as in the Bird, and at length the body of the young Mammal becomes constricted off from the " yolk-sac " or umbilical vesicle, so that, ultimately, the two come to be con- nected only by a narrow yolk-stalk (Figs. 1218 and 1219): the yolk-sac is a thin-walled sac containing a coagulable fluid in place vet FIG. 1217. — Embryo Rabbit, of about nine days, from the dorsal side, ab, optic vesicle ; a/, fold of aninion ; as, area opaca ; ap, area pellucida ; h, hz, heart ; h', h", h'", medullary plate in the regions of the future fore-, mid-, and hind-brain respectively ; hh, and hh1'', hind-brain ; mh, mid-brain ; ph, pericardial section of body-cavity ; pz, lateral zone ; rf, medul - lary groove ; stz, vertebral zone ; me, proto vertebrae ; vd, anterior part of mesenteron ; vh, fore-brain ; ro, vitelline vein. (From Balfour, after Kolliker.) of yolk. A vascular area early becomes established around the embryo on the wall of the yolk-sac. The most important of the points of difference between a Mammal and a Bird, as regards the later part of the history of the development, are connected with the fate of the foetal membranes. The amnion is in many Mammals developed in the same way as in the Bird, viz. : by the formation of a system of folds of the extra-embryonal somatopleure which arise from the blastoderm XIII PHYLUM CHORDATA 59] eh FIG. 1218. — Five diagrammatic sections illustrating the formation of the foetal membranes of a Mammal. In 1, 2, 3, 4 the embryo is represented in longitudinal section. 1, Embryo with zona pellucida, blastodermic vesicle, and embryonic area ; 2, embryo with commencing formation of yolk-sac and amnion ; 3, embryo with atniiion about to close ; 4. embryo with villous chorion, larger allantois, and mouth and anus ; 5, embryo in which the mesoderm of > the allantois has extended round the inner surface of the chorion and united with it to form the foetal part of the placenta ; the cavity of the allantois is aborted ; «, ectoderm of embryo ; a', ectoderm of non-embryonic part of the blastodermic vesicle ; ah. amniotic cavity ; al. allautois ; am. amnion ; cli. foetal part of placenta ; chz, placental villi ; d, investing membrane ; d'. processes of investing membrane ; dd, embryonic endodertn ; df. area vasculesa ; dy, stalk of umbilical vesicle ; ds, cavity of umbilical vesicle ; e. embryo ; hit, pericardia! cavity ; i, non-embryonic endoderm ; Kk, cavity of blastodermic vesicle ; Kn. head-fold of amnion ; j/t. embryonic mesoderm ; m'. non-embryonic rnescderm ; /•, space between investing membrane and amnion ; ah, chorion ; an, tail-fold of amnion ; at, sinus terminalis ; vl. ventral body- wall. (From Balfour, after Kb'lliker.) o o 2 592 ZOOLOGY SECT. around the embryo, and grow upwards and inwards, eventually meeting in the middle over the body of the embryo, and uniting in such a way as to form two layers. Of the two layers thus formed the outer, consisting of trophoblastic ectoderm and somatic mesoderm, simply constitutes a part of the extra- embryonic somatopleure which forms a complete investment for OF TA AX OL YS AN EK FIG. 1219.— A Babbit embryo and blastodermic vesicle at the end of the tenth day. The embryo is represented in surface view from the right side, the course of the alimentary canal being indicated by the broad dotted line ; the blastodermic vesicle is shown in median longitudinal section. The greater part of the tail has been removed. AN', pro-amnion ; AX. cavity of amnion ; C. extra-embryonic portion of coelome ; E. ectoderm ; E'. thickened ectoderm by which the vesicle is attached to the uterus and from which the foetal part of the placental is derived ; EGT. ectodermal villi ; El. auditory vesicle ; EK, ectodermal villi ; GF. fore-gut ; GH. hind-gut ; GT. mid-gut ; H. eiidoderm ; OL. lens of eye ; R. heart ; SI. sinus] terminalis ; TA. allantoic cavity ; YS. yolk-sac. (From Marshall, in part after Van Beneden and Julin.) the entire sphere, and is known as the chorion (Fig. 1218, 2 and 3). In the account of the development of the Bird it has been referred to as the false amnion or serous membrane. The inner layer or true amnion, as in the Bird, forms the wall of a cavity— the amniotic cavity (4 and 5, ah) — which becomes tensely filled with fluid (the liquor amnii) over the body of the embryo ; this serves the purpose of protecting the delicate embryo from the xiii PHYLUM CHORDATA 598 effects of shocks. As in the case of the Bird, the folds giving rise to the am n ion and chorion or serous membrane may consist from the first (except the head-fold, which, being formed from the proamnion, consists solely of ectoderm and endoderm) of somatic mesoderm as well as ectoderm (trophoblast) : or mesoderm may extend into them later, so that, either from the first, or as a result of outgrowth which takes place subsequently, the serous membrane contains mesoderm as well as ectoderm. The ectodermal cells — trophoblast cells — of the chorion may enter into close relationship with the mucous membrane of the wall of the uterus, and send out processes or primary villi (Fig. 1219, EK) by means of which the ovum becomes intimately attached, and by means of which perhaps nourishment is absorbed. In certain Mammals the history of the amnion is very different from that above described. In the Hedgehog (Fig. 1220), for FIG. 1220. — A — C, diagram illustrating the formation of tin amnion and trophoblast in the Hedgehog. Only the ectoderm is represented. A, early stage in which the amniotic cavity has appeared, roofed over by chorionic ectoderm ; B, later stage in which the amniotic ectoderm is growing up below the chorionic from the edges of the ectodermal floor ; C", stage in which the amniotic ectodenn completely roofs over the cavity. (After Hubrecht.) example, a cavity appears in the ectoderm of the embryonic area ; this is destined to give rise to the cavity of the amnion. The ectoderm which forms its roof is entirely trophoblastic or chorionic ; that which forms its floor is partly destined to become amniotic ectoderm, partly embryonal ectoderm. After the meso- derm has begun to become differentiated, the margins of the amniotic part of this ectodermal floor (B} begin to grow upwards, giving rise to a layer which extends over the roof on the inner side of the chorionic ectoderm and eventually (C) forms a complete layer — the ectodermal layer of the amnion. In the Mole (Talpa) spaces appear in the layer of ectoderm of the embryonal area, and these subsequently coalesce to form a single cavity — the primitive amniotic cavity, but this has only a temporary existence, the amnion arising later by the formation of a series of folds. In Mus, Arvicola, and others (Fig. 1221, A), 594 ZOOLOGY SECT. the amnion is developed from a series of folds of the ectoderm which arise beneath the trophoblast. In other Mammals (B) the amnion arises in the manner already described, and the portion of the trophoblast immediately overlying the embryonic part of the ectoderm eventually disappears. The allantois has, in all essential respects, the same mode of development as in the Bird, arising in most cases as a hollow outgrowth from the hinder part of the alimentary canal ; this, growing out into the space (extra-embryonal ccelome) between the chorion and the amnion, becomes in all the Eutheria applied to the former, and unites with it to contribute towards the formation of the placenta. But in some cases the allantois does not at first contain a cavity, and in some (Primates) the ao FIG. 1221.— Diagram illustrating the mode of formation of the amnion in various Mammals. A, commencing formation of the amnion in Mus, Arvicola, etc. The asterisk marks what corresponds to the portion of the trophoblast overlying the embryo in Fig. 1221, C ; B, mode of formation of the amnion in many Mammals. The portion of the trophoblast indicated by the asterisk in A disappears before the, amniotic folds make their appearance. (After Hubrecht.) severance between the amnion and the chorion is not com- pleted, and the allantois arises from the outset in continuity with the latter. Sometimes, as in the Kabbit (Fig. 1219), the union between the allantois (TA) and the chorion is limited to a comparatively small part of the extent of the latter, but in most instances the allantois spreads over the entire inner surface of the chorion, and becomes united with it throughout its entire extent (Fig. 1218). Villi, into which mesoderm with blood-vessels penetrates, grow out from the surface of the chorion and are received into depressions or crypts in the mucous membrane of the uterus, which undergoes profound modification. The villi become branched and enter into intimate union with the uterine mucous membrane, so that a close connection is established between the vascular system of the foetus and that of the parent. The term placenta is applied to the entire structure by means of which this connection is brought about ; the parts derived from xiii PHYLUM CHORDATA 595 the embryo are termed the foetal placenta, those developed from the wall of the uterus the maternal placenta. In some Mammals the union between the two is not very close, so that at birth no part of the uterine mucous membrane is thrown off; such a placenta is said to be non-dtciduate (semi-placenta}. In other Mammals the union is closer, and at birth a part of the hypertrophied mucous membrane is thrown off in the form of a decidua ; such a placenta is termed deciduate (placenta vcra). In the Mole and the Bandicoot not only is there no decidua thrown off, but the foetal placenta with the distal portion of the allantois does not pass out after the foetus, but remains, and is broken up or absorbed in the uterus. Such a condition has been termed contra- deciduate. In one of the simplest forms of placenta — the discoidal — found in the Rabbit and other Rodents (Fig. 1219), the yolk-sac extends over the surface of the serous membrane and fuses with it, except in a small area on the dorsal side of the embryo. In this small area the allantois becomes applied to the chorion and coalesces with it ; and from the membrane so formed vascular villi grow out, and are received into the uterine crypts. In most Mammals, however, as already stated, the allantois becomes applied to the chorion throughout its entire extent, and thus completely encloses the embryo. Villi may be developed from all parts except the poles : when this condition persists in the fully- formed placenta, the term diffuse is applied. Sometimes the diffuse condition is temporary, and the completed placenta has villi disposed in a broad band or zone (zonary placenta). Sometimes the villi are grouped together in patches or cotyledons (cotyledonary placenta). In Man and the Apes the villi become secondarily restricted to a disc-shaped area of the chorion situated on the ventral side of the embryo (meta-discoidal placenta). (Gf. pp. 479-486.) In many Mammals the yolk-sac, through the medium of the chorion, enters into a close relationship with the uterine wall, and a connection, the so-called yolk-sac placenta, is established, through which nourishment can be conveyed to the embryo ; but this rarely persists after the true (allantoic) placenta has become established. The stalk of the yolk-sac, with the corresponding narrowed part of the allantois and the vessels which it contains, forms the umbilical cord by which the foetus is connected at the navel or umbilicus with the yolk-sac and placenta. This is enclosed in a sheath formed by the ventral portion of the amnion. The part of the allantois which remains within the cavity of the body develops into the urinary bladder, together with a cord — the urachus— connecting the bladder with the umbilicus. The developmental history of the Marsupials differs from that 593 ZOOLOGY SECT. of the Eutheria not only in the non-occurrence, as already mentioned, of the solid morula-like stage, but also in the fact that the layer corresponding to the embryonal cell-mass is never enclosed by the trophoblast — Rauber's layer being absent ; and in the transitory character, or entire absence, of an allantoic placental connection be- tween the foetus and the uterine mucous membrane. The intra-uterine develop- ment of the foetus is abbreviated, and birth takes place when the young animal is still relatively very small and has many of the parts incompletely formed. In this helpless condition the young Marsupial is placed by the mother in the marsupium, where it remains for a time as a mam- mary foetus (Fig. 1222), hanging passively to the teat, to which the mouth becomes firmly adherent. The milk is expressed from the mammary gland by the con- traction of a muscle, the cremaster, and passes down the gullet of the foetus, which is enabled to breathe unobstructedly through the nostrils by the establishment of a continuous passage from the nasal cavities to the larynx, as already described (p. 576). In all the Marsupials, so far as known, the embryo is covered over, except in a limited area, by the compressed and expanded coel ij Kangaroo attached to the teat. (Natural size.) coel FIG. 1223 —Diagram of the embryo and foetal mem- branes of Hypsipryxnnus rufescens. all. allantoic cavity ; amn. amnion ; amn. c. cavity of amnion ; ccel. extra-embryonic ccelome ; ser. serous membrane; yk.s. yolk-sac. (After Sem on.) FIG. 1224. — Diagram of the embryo and foetal membranes of Phascolarctos cinereus. Letters as in Fig. 1154. (After Semon.) yolk-sac. In the majority (Fig. 1223) the allantois (all.) is small, and is completely enclosed with the embryo in the yolk-sac. In the Koala, however (Fig. 1224), it stands out and. becomes XIII PHYLUM CHORDATA FIG. 1225. — Diagram of the embryo and placenta of Perameles obesula. Letters as in Fig. 1154. In addition — all. s. allantoic stalk ; mes. mesen- chyme of outer surface of allantois fused with mesenchyme of serous membrane ; s. t. sinus terminalis ; ut. uterine wall. (After J. P. Hill.) closely applied to the chorion over the small area not covered by the yolk-sac ; but no vascular villi are developed. In the Native Cat (Dasyurus) there is a well-developed yolk- sac placenta. Only in the Bandicoots (Fig. 1225), so far as known, is the out- growth of the allantois to the chorion followed by the establishment of an in- timate relationship between the latter and the uterine wall, with the formation of interlocking ridges and de- pressions, the whole con- stituting a placenta of the same essential character as that of the Eutheria, though devoid of actual villi. The Prototheria, unlike all the rest of the Mammalia, are oviparous. In Echidna only a single egg, as a general rule, is laid in a season. This is placed in a temporary marsupium, formed as already described (p. 491) in the mammary region of the ventral surface. The young animal soon emerges from the egg, and remains enclosed in the marsupium till it reaches an advanced stage of develop- ment. Ornithorhynchus develops no marsupium, and the two eggs which it produces are deposited in its burrow. In Echidna the egg-shell is com- posed of keratin ; in Ornitho- rhynchus it contains carbonate of lime. The ova of the Pro- totheria (Fig. 1226) are very much larger than those of other Mammals, their greater dimen- sions being due to the presence S 12-20 -.<, biastuia stage of one of the Of a large proportion of food yolk. Theria. B, transition stage between _.. r . - J _ the momia and biastuia in a Mono- The segmentation, unlike that of ^1 the Theria, is meroblastic, 598 ZOOLOGY SECT. A " primitive knot/' the nature of which is doubtful, appears on the blastoderm, and an invagination is formed, the cavity of which subsequently opens below into the cavity of the yolk-sac (cf. Reptilia, p. 365) and which becomes the neurenteric canal. Con- temporaneous with this invagination is a primitive streak whicli is separated from the "primitive knot" by a distinct interspace in which the blastoderm is not specially differentiated. Geographical Distribution. — The Monotremes are entirely confined to Australia, Tasmania, and New Guinea. The Mar- supials are most abundantly represented in the Australian region, the greater number of the Australian families and genera being restricted to the Australian continent and to Tasmania, though several genera extend to New Guinea and some of the neighbouring islands. The family Didelphyidse, or Opos- sums, inhabits South America and extends into the southern part of North America; and a single genus, Cmnolestes, of a family usually stated to be related to the Australian Dipro- todonts, has been comparatively recently found in South America. The Edentates are most numerously represented in South and Central America, the true Anteaters, the Sloths, and the Arma- dillos being all inhabitants of that region. But the Scaly Ant- eaters and the Aard-varks (Cape Anteaters) are denizens of the Old World; the former inhabiting Southern Africa and South- Eastern Asia, the latter being confined to Africa. The Cetacea are cosmopolitan in their distribution : the great majority are marine, but some ascend rivers, and a few are exclu- sively fluviatile, inhabiting the rivers of South America and South-Eastern Asia. The distribution of the Sirenia is somewhat restricted. The recently extinct Ehytina inhabited Behring's Straits. The Manatee is confined to the Atlantic coasts of South America and of Africa, living chiefly in the larger rivers. The Dugong occurs on the east coast of Africa, in the Red Sea, the Indo-Malayan islands, and the northern coast of Australia. The Ungulata occur in all the great regions, with the exception of the New Zealand, Polynesian, and Australian. Oxen are, with the exception of the American Bison, natives of the Palsearctic, Ethiopian, and Oriental regions. Wild Sheep, with the exception of one African and one North American species, are confined to the Nearctic and Oriental regions. Goats are also mainly Nearctic and Oriental. Antelopes are confined to the Old World, and are by far more numerous in the Ethiopian than in other regions. The Prongbucks are Nearctic; the Giraffes exclusively Ethiopian. Deer are widely distributed in the Nearctic, Neotropical, Palsearctic, and Oriental regions, but are absent from the Ethiopian, The Camels are xiii PHYLUM CHORDATA 599 natives of the Old World ; the Llamas of the Neotropical region. Wild species of Pigs are widely distributed in the Old World and are absent in the New ; while the Peccaries are confined to the Nearctic and Neotropical regions. Hippopotami are confined to Africa. The Horses, including the Zebras and Asses, are restricted at the present day, as regards their natural distribution, to the Old World, though they abounded also in America in the Pleistocene period. Rhinoceroses are Oriental and Ethiopian. Tapirs have a singular distribution, one species occurring in the Malay Archi- pelago, and the rest in the Neotropical region. Hyraxes are con- fined to Africa, Arabia, and Syria. Of the Elephants, one species is confined to the Oriental, the other to the Ethiopian regions, but fossil remains prove that in Pleistocene times the range of the Elephants, and their gigantic extinct allies, the Mammoths, was very much wider, and extended over Northern Africa and the entire Palaearctic region. Only one fossil species has been found in America. Carnivora, if we leave out of account the Australian Dingo or Native Dog, are absent in the Australian, Polynesian, and New Zealand regions, but range over all the other geographical pro- vinces. The Cats and the Dogs are found in all parts of this extensive area : the Hyaenas are restricted to the Western part of the Oriental region and the warmer parts of the Holarctic and the Ethiopian. The Civets are most abundant in Africa, Mada- gascar, and South-Eastern Asia, but occur also in the Southern parts of Europe ; and many of the smaller groups have a yet more restricted range. Bears have a wide distribution, but are absent from the whole of the Ethiopian region. The majority of the Pinnipedia are found in the Arctic and Antarctic regions and in the temperate zones of both hemi- spheres, few ranging into the tropics. The Walruses are almost exclusively Northern, while the Eared Seals and Earless Seals are almost equally abundant in the Northern and Southern hemispheres. The Rodents have a wider range than any others of the orders of land Mammals, and occur in all parts of the globe, though they are poorty represented in Australia and Madagascar. The Rodents reach their greatest development, as regards the number of families, in South America, in which region occur also the majority of the largest members of the order. Insectivora are absent in the Australian, Polynesian, and New Zealand regions, and in South America ; but occur in all the other provinces. The Chiroptera are world-wide in distribution, occurring in greatest abundance in tropical and warm temperate zones. The Flying " Foxes " (Pteropidae) are absent from the Nearctic and 600 ZOOLOGY SECT. Neotropical regions, and the Vampire Bats occur exclusively in the latter. The distribution of the Lemurs is remarkable ; they occur only in Madagascar, a limited part of South Africa, Southern India and Ceylon, some of the islands of the Malay Archipelago, and the Philippines. The headquarters of the group is the island of Madagascar, of which they constitute one half of the entire Mammalian fauna. Of the other groups of Primates, the Marmosets (Hapalidae) and the Cebidas are exclusively American ; the Cercopithecidse Palse- arctic, Oriental and Ethiopian, with a single species in Mada- gascar. Of the Simiida3 the Gibbons occur only in South-Eastern Asia and the Malay Peninsula ; the Orangs only in Borneo and Sumatra ; the Gorilla and Chimpanzee in certain parts of Western Equatorial Africa. Geological Distribution. — The earliest fossil-remains of Mammals have been found in strata of Upper Triassic and of Jurassic age in Europe and America. These remains consist almost exclusively of jaws and teeth, and, as the latter differ widely from those of existing Mammals, there is frequently great difficulty, in the absence of remains of the other hard parts, in determining the affinities of these Mesozoic forms. Some of the Triassic and Jurassic Mammalian molar teeth are con- structed on the most primitive form of the triconodont type, which has already been referred to (p. 561) as being the primitive form in the class, having three cones or cusps in a longitudinal row. In Dromatherium and its allies each molar has a single main cusp with two smaller accessory cusps. There is no decisive evidence as to the affinities of these primitive triconodont Mam- mals, but they may be provisionally set down as allied to the Prototheria. Of the remainder of the Mesozoic Mammals some were probably Prototheria, others Metatheria, while others again may have been Insectivores. Most of them fall into two main groups. The type of dentition presented by the members of one of these groups (Fig. 1227) is more nearly allied to that of the Polyprotodont Marsupials (p. 478) than to any other. In the other group (Multi- tulerculata) (Fig. 1228) there is a superficial resemblance to the Diprotodont Marsupials ; a single chisel-shaped incisor is present on each side of the lower jaw, and one large, and sometimes one or two smaller, on each side of the upper. A wide diastema separates these from the pre-molars. The molars present a number of variously-arranged small tubercles. In some cases the pre-molars have a pattern similar to that exhibited by the molars, but in others they have a cutting edge which may be serrated or obliquely grooved. The fact that the vestigial molar teeth of Ornitho- XIII PHYLUM CHORDATA 601 rbynchus come nearer in pattern to those of certain of the Multituberculata than to those of any other known group is looked upon as evidence that the affinities of the latter are rather with the Prototheria than with the Metatheria. In the American FIG. l_'_'7.— Phascolotherium bucklandi. Inner view of right ramus of mandible. (After Owen.) Cretaceous beds in which these teeth are most abundant a number of limb-bones have also been found, some of which show evidence of Monotreme characteristics. Fossil remains of Mammals belonging to the Cretaceous age are known only from certain limited beds in North America. But in deposits of the succeeding Tertiary period there have been found the remains of an extensive and varied Mammalian fauna. The earlier Tertiary Mammals in many cases present features which ..-i FKI. 1-Ji'S.— Plagiaulax becklesi. Mandible with teeth. (After Owen.) enable us without hesitation lo refer them to one or other of the existing orders ; but when this is the case there is nearly always to be recognised an absence, or a less advanced development, of some of the more salient characteristics ; in other words, the earlier Tertiary Mammals, when referable to existing orders, are less highly specialised than the living representatives of these orders. No less significant is the fact that these early Tertiary representa- 602 ZOOLOGY SECT. tives of existing orders had the cavity of the brain-case nearly always much smaller in proportion to the other dimensions than in living forms. But many are not so readily referable to existing orders, sometimes owing to their possessing marked special features of their own, sometimes owing to their combining characteristic features of two or more living orders. Through the series of Tertiary and Post-Tertiary formations it is possible to trace a gradual development, from the early generalised, to the existing specialised genera, and in some instances by such gradual transitions, that the actual course of the evolution can be followed stage after stage. There is only space here for a very brief review of this extensive and remarkable Tertiary and Post-Tertiary Mammalian fauna. No remains of Prototheria are known from the Tertiary, and it is only when we come to the Post-Tertiary (Pleistocene) that we meet with fossil representatives of the group. These, which have been found only in Australia, differ little from the existing Echidna and Ornithorhynchus. Of the Marsupials the Opossums (Didelphyidae) of America are FIG. 1229. — DiprotOdon australis. (From a restoration of the skeleton by Prof. E. C. Stirling in the Adelaide Museum.) represented not only in Cretaceous, Tertiary and Pleistocene deposits in that continent, but in beds of Tertiary age in Europe. In addition, in certain European deposits of Eocene age, there occur teeth and jaws which may be Marsupial in character, but the affinities of which are uncertain ; and in Tertiary deposits of South America XIII PHYLUM CHORDATA 603 have been found numerous remains of Diprotodonts belonging to a group represented by a single surviving genus, Ccenolestes. These South American Diprotodonts appear, so far as is known, to have differed from the Australian Diprotodonts in the absence of the characteristic syndactylism of the latter. The remainder of the fossil Marsupials hitherto discovered are of Pleistocene age, Fin. 1230.— Nototherium mitchelli. Side view of skull. (After Owen.) and have nearly all been found in Australia. The Australian Pleistocene Marsupials are for the most part referable to existing families and even genera, representing both the Diprotodont and the Polyprotodont sections; but some differ widely from existing forms. One of these, Diprotodon (Fig. 1229), was the largest known Marsupial, and reached the dimensions of a Rhinoceros ; it occupies a position intermediate between the Phalangers and the Kangaroos. Unlike the latter, Diprotodon had the limbs of approximately equal size, and adapted for walking : both maims and pes were pentadactyle with very small sub-equal digits. Nototherium (Fig. 1230), somewhat smaller than Diprotodon, but also of large size, seems to connect together Diprotodon, the Wombats, and the Phalangers. Thylacoleo (Fig. 1231) is an extinct genus referable to the Phalanger family, 604 ZOOLOGY SECT. and characterised by an extremely modified dentition, the only functional teeth being a single pair of large incisors in the middle FIG. 1231.— Thy lacoleo carnifex. Side view of skull. (After Flower.) in both upper and lower jaws, with a single elongated trenchant pre-molar on each side both above and below. Among the Edentata the majority of fossil, as of existing forms have been found in South America. But the family of the Cape Anteaters, at the present day confined to j3outh Africa, is proved, by the discovery of remains in the Pliocene of the island of Samos in the Eastern Mediterranean and in the Eocene of Southern France, to have formerly had a wider distribution. The American fossil Edentata, all of Pleistocene age, comprise, in addition to extinct genera of Armadillos (some of gigantic FIG. l232.-Glyptodonclavipes. (After Owen.) size) and one genus of Sloths, representatives of two extinct families, the Qtyptodontidee and the Meg cither iidce.1 The former (Fig. 1232) are large Edentates resembling the Armadillos in 1 Recent remains stated to belong to a Megatherium have been found in South America. XIII PHYLUM CHORDATA 605 the presence of a bony dermal carapace and a bony investment for the tail ; but in the Glyptodontidae the carapace has no movable rings, so that the animal could not roll itself up, and there is FIG. 1233.— Mylodon robustus. (Restoration, after Owen). usually a ventral bony shield or plastron, never present in the Armadillos. Glyptodonts occur in North as well as in South America. The Megatheriidse (Fig. 1233) are Edentates, mostly of enormous size and massive build, which combine certain of the features now characteristic of the Anteaters (Myrmecophagidae) and the Sloths (Bradypodidse) respectively, the spinal column and limbs allying them with the former, and the crania and the teeth with the latter. The Cetacea are represented in the Tertiary (Eocene and Miocene) of Europe, Egypt, and North America, by an extinct FIG. 1234.— Squalodon. Three of the lower true molars. (After Flower.) sub-order — Archceoceti or Zeuglodonta, comprising only one known genus — Zeuglodon. Zeuglodon differs from existing Cetacea mainly in the possession of rooted, heterodont teeth, and in the VOL. ir p p 606 ZOOLOGY SECT. position of the narial aperture, which is situated comparatively far forwards ; the limbs are not known : there were irregular dermal bony plates. The remains of both Whale-bone Whales and Toothed Whales occur abundantly in Pliocene deposits, some belonging to extinct, others to existing, genera. Toothed Whales occur also in Miocene formations, and there, as well as in the Pliocene and Pleistocene of Europe, North America, New Zealand, and Australia, are represented by an extinct family, the Squalodtmtidce (Fig. 1234), with heterodont dentition. The order Sirenia is first met with in the Eocene, and was repre- sented in that and succeeding periods by several extinct genera, of which Halitherium is the best known. These were characterised by the possession of upper incisors, in some cases of canines, of enamelled pre-molars and molars, of a milk-dentition, and of small vestiges of femora. The family of the Dugongs is represented by a form nearly allied to the existing genus in the Pliocene of France, and probably by another genus in the Tertiary of California. The family of the Manatees is not known to be represented by any fossil forms. The recently extinct Rhylina (" Steller's Sea-Cow "), which lived within historic times in the Behring's Straits, was the largest known member of the order, and sometimes attained a length of seven or eight metres. The Tertiary Ungulata comprise an immense number of forms, including numerous extinct families, into an account of which it would be going beyond the scope of the present work to enter. In the Artiodactyle series there is to be traced a pro- gressive union and coalescence of the third and fourth metacarpals to form the cannon-bone, a progressive reduction of the lateral digits, and a progressive development of horns or of tusks — absent or rudimentary in the earlier representatives of the sub-order. In the Perissodactyle series the reduction of the lateral toes reaches its maximum in the existing genus Equus. 'The history of this reduction, together with the development of other characteristic features, can be traced from pentadactyle forms with simple molars through a long series of gradations to the monodactyle Horses with their complexly folded molars. Similar genealogies, though not always so complete, can be traced for the Tapirs and Rhinoceroses, and for the Deer, Camels, and Pigs. The order Proboscidea was represented in Tertiary and Pleisto- cene times, not only by forms allied to those now living — though sometimes, as in the Mammoths, of much greater size — but also by an extinct family, the Dinotheridce (Fig. 1235) (Miocene and Pliocene of Europe and India), which possess a pair of downwardly- directed tusks in the lower jaw. The genus Pyrotherium, from the Patagonian Tertiary deposits, was a primitive Ungulate, with a pair of short tusk-like incisors in the lower jaw, which may have been related to the Proboscidea : and Maeritherium from the XITI PHYLUM CHORDATA 607 Eocene of Egypt, is probably also a primitive member of the same group. The Hyracoidea were represented in the Pliocene by an extinct genus (Pliohyrax) ; and Archceohyrax from the Patagonian Tertiary is perhaps also an allied form. A separate sub-order, the Condylarthra, has been established for a number of Eocene Ungulates, which differ somewhat widely from all the other members of that group, and approach the Carnivora in some respects, though having certain resemblances to Fi<;. 1-23.J.— Dinotherium giganteum. Side view of skull. y6th natural size. (From Zittei's Paleontology, after Kaup.) the Hyracoidea. The pre-molars and molars are short and usually bunodont,the pre-molars being simpler than the molars, the latter sometimes tritubercular, like those of many of the Carnivora; the incisors and canines also sometimes resemble those of the Carni- vora. The humerus differs from that of the other Ungulata, and resembles that of the Carnivora, in the presence of a foramen over the inner condyle. The arrangement of the carpals corres- ponds with what is observable in the Hyracoidea and also, as in the latter group, the femur has a third trochanter. The limbs are usually pentadactyle, with pointed ungual phalanges. The astragalus has, as in the Carnivora, a uniformly rounded distal articular p P 2 608 ZOOLOGY SECT. surface. The fibula does not articulate with either the astragalus or the calcaneum. Another extinct primitive sub-order of the Ungulata is the Aiiiblypoda, the members of which have been found, like the Condylarthra, in the Eocene of North America and of Europe. These resemble the Condylarthra and Hyracoidea in the scaphoid being opposite the trapezoid ; both magnum and unciform articulate with the lunar. The fibula articulates with the calcaneum ; the cuboid articulates with both the astragalus and the calcaneum ; The feet are short, pentadactyle, and plantigrade. Canines are present in both upper and lower jaw ; the pre-molars and molars are short and lophodont in type. Dinoceras and Tinoceras are characterised by the process of remarkable bony prominences on the upper surface of the skull. In the Tertiary deposits of Patagonia and Bolivia have been found the remains of another group of extinct Ungulates of low organisation — the Litopterna, These had the distal segments of the limbs elongated and constructed on the perissodactyle type, the number of the digits varying from five to one, the third being always the largest. The carpal and tarsal bones do not interlock as in the existing Ungulata vera. There is no foramen above the inner condyle and clavicles are absent. A third trochanter is present. The brain- case is very much smaller than in the existing Ungulata vera. The dentition is complete or nearly so ; the pre-molars and molars short and provided with roots. The Ancylopoda are another group of primitive extinct Ungulates. The remains of some of these have only been found in the Pata- gonian Tertiary deposits, but others had a wide range both in the Old and New Worlds. They all differ from other groups of Ungulates in the form of the limbs: the weight of the body appears to have been borne on the outer edge of the manus and pes, and the digits were evidently provided, not with true hoofs, but with pointed claws. The teeth resemble those of the Perissodactyla. The Typotheria, and Toxodontia are two groups of extinct Ungulata the representatives of which have only so far been found in the South American Tertiary formations. The former differ from ordinary Ungulates in the possession of a clavicle and the presence of a foramen above the inner condyle of the humerus. The ulna and fibula are complete, and there are either-fbur or five fully formed digits in each foot. The Toxodontia approach nearer the normal Ungulate type, the clavicle and the foramen over the inner condyle being both absent. They have a massive skull and short, stout limbs, each with three digits. The true Carnivora of the Tertiary period are, as compared with those of the present time, remarkable for the abserrce-^fLthe well- marked distinction into groups such as are now to be recognised ; xni PHYLUM CHORDATA 609 numerous intermediate forms connect together the Dogs, Civets, Cats, Bears, and Weasels, which, in the existing fauna, appear separated from one another by differences of the most strongly marked character. Several extinct families are recognised, and one extinct order — the Creodonta. The latter present resemblances to the Insectivora on the one hand, and to the Polyprotodont Marsupials on the other, such as would appear to indicate a relationship with both of those groups. A group of Eocene Mammals of uncertain affinities are the Tillodontia (Fig. 1236), which by some have been elevated to the FIG. 123r>.— TillOtherium fodiens. Left lateral view of skull. (From Flower, after Marsh.) rank of a distinct order. The Tillodontia appear to unite in a remarkable degree, in skull and dentition, ungulate, rodent, and carnivorous characteristics. The Rodents were represented in the Tertiary period by all, or nearly all, the principal groups existing at the present day, together with several extinct families. Some of the Tertiary Eodents attained a much larger size than any living members of the order. Among the Tertiary Insectivora, in addition to representatives of existing families, are a number of extinct forms. Through these it is possible to connect the living Insectivora with the Creodont Carnivora on the one hand, and with the Prosimii on the other. Chiroptera, not differing widely from existing forms, occurred as early as the Eocene. Of the Primates, Prosimii occur from the Eocene onwards. A single extinct family is known, comprising Lemuroids which bear a closer resemblance to Insectivora than do the living members of the order. Of the Anthropoidea,, the Hapalidae and CIO ZOOLOGY SFXT. Cebidne are only represented in the Miocene (Eocene ?) and Pleistocene of South America; the Cercopithecidre in the Miocene and Pliocene of Europe and the Pliocene and Pleistocene of India by extinct genera (Mcsopithecus, &c.) and by species of the existing genera Macacus, Semnopithecus, and Cynocephalus, and in the Pliocene of India, France, and Italy by species of extinct genera. Among the Simhdae the Gibbons occur in the Miocene of France, the Pliocene of Germany, and the Pleistocene of Borneo. An extinct genus, Dryopithecus, occurring in the Miocene of Europe, is perhaps related to the Gorilla; and a species of Orang (Simia), together with a form allied to the Chimpanzee, occur in the Indian Pliocene. In deposits of late Pliocene or early Pleistocene age in Java have been found the remains of an Anthropoid (Pithecanthropus) which has been supposed to be an intermediate. form between the man-like Apes and Man. Traces of the existence of Man in the form of flints of undoubted human manufacture have been found in the Miocene of India ; but any such evidences are extremely rare until we reach the Pleistocene. THE MUTUAL RELATIONSHIPS or THE CHORDATA. IN discussing the relationships of the various groups of Chordata, it will be convenient to begin with Fishes, and to work from them upwards and downwards. The question of the inter-relationships of the various groups of Fishes is a very puzzling one. As in other cases of the kind, there are three lines of evidence to be kept in mind — anatomical, embryological, and pal»3ontological — the last being, always, when available, the final court of appeal. With regard to anatomical evidence, it seems fairly obvious that Fishes having neither limbs nor jaws are more primitive than forms in which those structures are present, unless undoubted evidence of degeneration can be produced : that a purely cartilaginous skeleton is more primitive than a bony one, and a notochord than a vertebral column, however simple; that a brain with distinct cerebral hemispheres is more advanced than one having an undivided prosencephalon ; that an autostylic skull, being due to the concrescence of originally distinct parts, is more specialised than a hyostylic skull; that the loss of the spiracle and the presence of an operculum and of a highly differentiated hyoid arch, are evidences of specialisation, as also are the presence of air-bladder or lung, spiral valve, conus arteriosus, or copulatory xin PHYLUM CHORDATA 611 In embryology, eggs with much food-yolk are to be looked upon as more modified than those with little, unless there is distinct evidence of reversion towards an alecithal condition. Any special contrivances for the nourishment and protection of the embryo, obviating the necessity for the production of immense numbers of eggs, are also marks of advance. On both these lines of evidence the lowest place may safely be assigned to the Cyclostomes. In spite of the similarity of the lateral cartilage of the Lamprey to Meckel's cartilage, there is no real evidence that the ancestors of the class had either jaws or limbs, and the most reasonable theory is that they are the descendants — highly specialised in certain respects in accordance with their peculiar mode of life — of a primitive craniate stock. With regard to the two largest groups of Pisces — the Elasmo- branchii and the Teleostei — the evidence from anatomy and embryology is conflicting. The Teleostei take the highest place in virtue of their skeleton, operculum, air-bladder, and gills, as well as in their extraordinary adaptability to various environments ; but the Elasmobranchs reach a distinctly higher grade of organi- sation in their enteric canal, heart, brain, and urinogenital organs, as well as in their large and well-protected eggs. The anatomy of Ganoids seems to show, however, that the spiral valve, conus arteriosus, and typical oviducts (Miillerian ducts) have been lost in the course of the evolution of the Teleostei, and that the simpler structure of these organs in that order is actually a concomitant of their extreme specialisation. The Holocephali and the Dipnoi, while agreeing with Elasmo- branchs in many important respects, show an advance in the presence of an autostylic skull and of an operculum, while the Dipnoi rise above all other Fishes in possessing not only lungs like Polypterus, but also posterior nares and a partially divided auricle. The lung appears to have been derived from an air- bladder with pneumatic duct opening on the ventral wall of the pharynx, as in Polypterus ; by the dorsal shifting of the duct and its final atrophy the closed air-bladder of the higher Teleostei has arisen. Coming to the results of Palaeontology, many striking and unexpected facts have recently come to light. There is reason to believe that Palseospondylus is a Cyclostome, but one with well- developed vertebrae ; from which it must be assumed either that the vertebral column of existing members of the class is degenerate, or that Palseospondylus is a highly specialised offshoot of the primitive Cyclostome stock, in which a vertebral column had been independently acquired. The latter conclusion seems the more probable, and is supported by the fact that in all three orders of 612 ZOOLOGY SECT. Ganoids there are some species with a persistent notochord, others with well ossified vertebrae ; the conclusion being that the vertebral column is a polyphyletic structure — that is, has been evolved independently in various groups in accordance with similar conditions. Among extinct Elasmobranchs the Acanthodea and Pleura- can thea had bones investing the cranium, and Cladoselache had 110 claspers. These facts seem to indicate as a probable an- cestor of the Teleostomi and Dipnoi — the two sub-classes with ossified skeleton — a generalised Elasmobranch in which fusion of dermal ossicles into investing bones had begun, but in which the special reproductive phenomena of the existing members of the group — internal impregnation and few, large, well-protected eggs —had not yet been acquired. The origin of the Dipnoi from such a source is rendered more probable by the possession of the characteristic biserial fin or "archipterygium " by Pleuracanthus. The Holocephali and the existing Elasmobranchs may be considered as having arisen from the same primitive stock along diverging lines of descent. There is, however, at present no evidence to trace or to explain the fusion of the palatoquadrate with the cranium to form the characteristic autostylic skull of the Holo- cephali and Dipnoi. The connection of the Ostracodermi with the better-known groups of Fishes is very uncertain. It has been proposed to class them with Cyclostomata on account of the absence — as far as our present knowledge goes — of jaws and limbs, and attempts have been made to show affinities with the Xiphosura and with larval Tunicates. They seem, however, to be undoubted Fishes, but with no clear relationship to any existing group. The Arthrodira appear to be most closely allied to the Dipnoi. The question of the origin of Fishes from lower forms is involved in the greatest obscurity. Practically the only assistance in the solution of the problem is furnished by Amphioxus, which seems to indicate as the ancestral stock of Vertebrates, fish-like animals having a skeleton in the form of a notochord, fin-rays, buccal cartilages, and branchial rods ; a barely differentiated brain ; no heart, but a contractile ventral vessel below the pharynx and a dorsal vessel immediately beneath the notochord ; colourless blood ; separate nephridia of the annulate type ; a ccelome developed as an enteroccele; metamerically arranged gonads devoid of ducts; and alecithal eggs. The forward extension of the notochord, the immense pharynx, the very numerous gill-slits, and the atrium, are very probably characters special to the Acrania; but even putting them aside as of no phylogenetic importance, it is obvious that this group must have sprung from a point very low down the chordate stem. The morphological differences between Amphioxus xiii PHYLUM CHORDATA 613 and a Hag are, in fact, of a more fundamental character than those between a Hag and a Mammal. Still lower must have been the point of origin of the Urochorda, with the notochord confined to the tail, the dorsal mouth, the absence of myomeres and of nephridia, and with only exceptional and ill-defined traces of segmentation. The huge pharynx with its innumerable stigmata is undoubtedly a secondary character; but the atrium, endostyle, dorsal lamina, and peripharyngeal bands seem undoubtedly to indicate an affinity with the Acrania. So also do the earlier stages of development ; but the later stages, and especially the mode of origin of the atrium, are quite different in the two cases. The propriety of including the Hemichorda among the Chordata is still sul> judice. Allowing that any single organ may have a polyphyletic origin — i.e., may arise independently in different groups in accordance with similar needs, it seems highly improb- able that three such peculiar and characteristic structures as notochord, hollow dorsal nervous system, and gill-slits, can have arisen together more than once in the history of animals ; and if it could be shown with certainty that these three characters were all present in the Hemichorda their place in the chordate phylum would be assured. But the cavity or cavities in the dorsal nerve- cord of Balanoglossus are inconstant, and are very different from the neuroccele of Urochorda and Vertebrata, which from the first extends through the whole length of a well-defined dorsal nervous system. In Cephalodiscus and Rhabdopleura, moreover, there is no trace of any such cavity. The pharyngeal diverticulum of the Hemichorda, also, is a very different thing from the notochord of Urochorda and Vertebrata, nothing in the structure or development of which gives the slightest indication that it originally arose as a forward outgrowth of the anterior portion of the mid-gut. The diverti- culum of Hemichorda is, in fact, obviously a support to the per- sistent prostomium of a fixed or sluggish animal, while that of Urochorda and Vertebrata forms a strengthening axis — either to the tail alone or to the whole body — of an active, elongated, animal swimming by lateral movements of the tail ; and there seems to be no reason why two such different structures should not have had an independent origin. The supposed double " notochord " of Actinotrocha, the larva of Phoronis, is even more problematical. Far more significant are the gill-slits, but even their evidence is hardly conclusive, since they are absent in Rhabdopleura and Actinotrocha, and in Cephalodiscus are a single pair of apertures having apparently no respiratory function. In Balanoglossus, however, they are very numerous and increase in number with the 614 ZOOLOGY SECT. growth of the animal, as in Araphioxus, and the division of each by a " tongue " is very similar in the two cases. Further homologies have been suggested by comparing the snout of Amphioxus with the proboscis or pre-oral lobe of Hemichorda and its pre-oral pit with the proboscis-pore. On the whole, although it is by no means certain that the "chordate" peculiarities of the Hemichorda may not have been independently evolved, it is convenient to retain them in the present phylum, pending further knowledge of their true affinities. Various zoologists have supported the view that the nearest relatives of the Chordata among the Invertebrata were forms characterised, like all but the lowest of the former phylum, by the occurrence of metamerism, i.e., Annulata or Arthropoda; while others have looked upon the metamerism of the Chordata as independently evolved. Among the non-inetameric groups which have been supposed to have been ancestral to the Chordata are the Nemertines — the proboscis-sheath being compared with the notochord and the proboscis itself with the pituitary invagi na- tion. But this view has received little support. On the other hand, the supporters of the Annulate ancestry have been more numerous — some seeing in the Chastopoda, others in the Hirudinea, the nearest Chord ate relatives. The metamerism of the Annulata is regarded as supporting this theory, as well as the structure and arrangement of their circulatory, nervous, and excretory systems. The notochord might have been supposed to have originated in an Annulate from a typhlosole or from an intestinal diverticulum or siphon. But a very serious difficulty is met with when we proceed to try to derive the Chordate nervous system from that of the Annulata. Not only must the ancestral Annulate, in order to become transformed into the primitive Chordate, have reversed its habitual position, so that its neural surface became dorsal instead of ventral ; but its mouth, which pierced the central part of the nervous system, must have been abolished, while a new one was developed on the haemal side of the head. Elaborate comparisons have been instituted between the brain of Cyclostomes and Fishes and those of Crustacea and Xiphosura, and it has been sought to explain the neuroccele as the discarded Arthropod enteric canal. But if Amphioxus and the Urochorda, to say nothing of the Hemichorda, are branches from some low part of the chordate stem — a fact it seems hardly possible to doubt — it is obvious that there can be no direct connection with the highly specialised classes referred to. If, for instance, the lower Craniata sprang either from a Chaetopod-like or from a Limulus-like ancestor, Amphioxus and the Tunicates must either xni PHYLUM CHORDATA 615 have no connection at all with Vertebrates, or must have under- gone a quite inconceivable amount of degeneration. There appears, in fact, to be a good deal of evidence supporting the view that the first developed Chordates may not have had any such pronounced metamerism as Annulates and Arthropods possess, and it becomes possible to look for the immediate ancestral form among less highly organised Invertebrate groups. But on this matter, though there is abundant scope for ingenious speculation, no finality can be said to have been reached, and the ancestry of Vertebrates still remains an unsolved problem. With regard to the higher classes, Amphibians may be held to have arisen from a Fish-type allied to the Dipnoi, the resem- blances of which to the Amphibia are so great as to lead some authors to place them in a distinct class intermediate between Fishes and Amphibia. The chief difficulty in this case — and it is a serious one — is the derivation of the pentadactyle limb from the Fish's fin, a transformatian of which no satisfactory explanation is at present offered either by anatomy, embryology, or pala3ontology. Reptiles may be considered to have arisen from a generalised Amphibian stock, but there is no direct evidence on this point ; and, apart from purely theoretical considerations, there is nothing to show how or why gills vanished so completely as to leave no trace of their existence apart from the branchial clefts, or by what steps the allantoic bladder became precociously enlarged into an embryonic respiratory organ. The precise mode in which the pro- tecting amnion arose is also very doubtful, though from theoretical considerations it has been argued that its development in the Hedgehog (p. 593) indicates a more primitive condition than obtains in the other Mammalia or even in Sauropsida. Birds appear to be undoubtedly derived from true Reptiles, although nothing is known of the actual ancestral form. In spite of the enormous adaptive differences between the warm-blooded, feathered, bipedal Bird, and the cold-blooded, scaly, quadrupedal Reptile, the connection between the two is far closer than between any other two vertebrate classes. Mammals also appear to have had a reptilian origin : the numerous reptilian characters of the Monotremata certainly point in this direction, and the reproductive processes of that group help us to understand the stages by which the large-yolked egg of the ancestral form, with the embryo developed outside the body, may have given place to the secondarily alecithal egg of the typical Mammal, giving rise to a foetus developed within the uterus, and nouiished by a complete placenta. Though palaeontology does not reveal to us the v actual reptilian progenitors of the Mammalia, yet, as already pointed out, there are some of the 616 ZOOLOGY SECT. Anomodontia (Theriodonta) which have pronounced mammalian resemblances in skeleton and teeth. The following diagram may serve as a rough illustration of the view set forth in the preceding pages — MAMMALIA AMPHIBIA TELEOSTOMI \ vl DIPNOI I/HOLOCEPHALI V / EXISTING I s' ELASMOBRANCHII PRIMITIVE ELASMOBRANCHII CYCLOSTOMATA OSTRACODERMI ACRANIA — - -> UROCHORDA HEMICHORDA FIG. 1237.— Diagram illustrating the Mutual Relationships of the Chordata. ON THE MUTUAL RELATIONS OF THE PHYLA OF ANIMALS. It will be advantageous in concluding our survey of the animal kingdom to sum up with a few remarks as to the phylogeny of the primary groups or phyla, since that of the sub-divisions of each phylum has already, in nearly every instance, been discussed. It cannot be too strongly emphasised that in the majority of cases it is useless to seek for the ancestors of any animal among existing forms. As far as we know, most living species are culmina- tions— terminal branches of the great tree, not leading directly to any other form, but connected only at the fork of a branch. It is, as a rule, only among fossils that actual ancestral forms are to be looked for ; hence the area of strict phylogenetic investigation is very limited, and in most instances the only evidence is to be sought in anatomy and embryology. Not only are most existing species culminations, and therefore xiii PHYLUM CHORDATA 617 off the direct line of ancestry of other species, but, as far as we can judge, the same is true of most genera and families, of a large majority of orders and classes, and even of most phyla. It would certainly seem that existing Chordata, Mollusca, Arthropoda, Annulata, Echinodermata, Nemathelminthes, and Platyhelminthes, are all independent branches of the animal tree, having no con- nection with one another save through the trunk. There are, however, existing groups which seem to represent actual stages in the existence of others. For instance, it can hardly be doubted that Amphibia are derived from Fishes and Birds from Reptiles; that if we could discover the unknown ancestors of those classes they would be classed definitely among Pisces and Reptilia respectively, though probably not belonging to any known order. In the same way everything seems to point to the conclusion that all the higher phyla must have passed through some kind of coelenterate stage, and, before that, some kind of protozoan stage, so that these two phyla may be said to represent actual steps in the evolution of the higher forms. It is therefore legitimate to assume, in the absence of direct evidence, that the ancestors of both the Ccelenterata and the Porifera were unicellular or "non- cellular " forms, i.e., to be classed among the Protozoa, and that the ancestors of the nine higher or triploblastic phyla were diploblastic forms, i.e., to be classed among the Ccelenterata. Most, if not all, of the triploblastic phyla appear to be terminal or culminating groups. There is no reason for thinking that either of the three highest phyla — the Chordata, Mollusca, and Arthropoda — ever passed through a stage which, if known, would be classed among Platyhelminthes, Nemathelminthes, Echinodermata, or Annulata. The wide occurrence of the trochophore, or some similar larval form, seems, however, to indicate a certain bond of union. The typical trochophore of Annulata and Mollusca, the echinopsedium of Echinoderms, the ciliated larva of Molluscoida, the tornaria of Balanoglossus, and the adult Rotifer, present, amid endless diversity in detail, common characters which, in the absence of better evidence, may be considered as indications of remote affinity. The Arthropoda alone among the higher phyla are devoid of even this slender connection with lower forms : there is no indi- cation throughout the phylum of anything approaching to a trochophore ; the crustacean nauplius is quite sui generis ; and the larval forms of Insects and Arachnids simply suggest a homonomously segmented ancestor. This suggestion is supported by Peripatus, the cilia, hollow appendages, nephridia, and ladder- like nervous system of which seem to point to its derivation from a segmented " worm " not far removed from the annulate type. 618 ZOOLOGY SECT. XIII In accordance with these conclusions the mutual relationships of the phyla may be expressed in a diagrammatic form as follows : — CHORDATA ARTHROPODA MOLLUSCA MOLLUSCOIDA TROCHELMINTHES \ NEMATHELMINTHES .... ~~~\ PLATYHELMINTHES\\\ COELENTERATA PORIFERA PROTOZOA FIG. 1238.— Diagram illustrating the Mutual Relationships of the Phyla. SECTION XIV DISTRIBUTION IN discussing the various groups of animals, the subject of their geographical and geological distribution has in every case been referred to, and the reader will already haveTroiiced how strikingly the different parts of the earth's surface at the present day, and the different periods of its geological history, differ from one an- other in respect of their animal inhabitants. In order to bring forward the facts of distribution more prominently, the present section will be devoted to this important subject. 1. GEOGRAPHICAL DISTRIBUTION The facts and ideas of which the subject of Geographical Dis- tribution takes cognisance are clearly brought out 'by comparing the fauna of Great Britain with that of the most distant of her colonies, New Zealand, including, in each case, the adjacent islands. The two countries are not widely different in size. The climate of each is temperate, Great Britain extending from about 50° to 60° north latitude, and having a mean annual temperature of about 48° F., New Zealand extending from about 34° to 48° south latitude, and having a mean annual temperature of 55°. Both contain mountainous regions, forests and arable land. The climate of both is humid, the rainfall of Great Britain being from about 25 to 30 inches in flat, 40 to 80 inches or more in mountainous districts, while the average rainfall for the whole of New Zealand is about 50 to 55 inches. The physical conditions of the two countries are thus very similar, the chief differences being the far higher summer temperature of the northern part of New Zealand, and the far lower winter temperature of the northern part of Great Britain. But when we come to compare the faunae of the two countries these similarities disappear. In Great Britain there are about 620 ZOOLOGY forty species of native land Mammals, including Ungulata (Wild Cattle, Red Deer, Fallow Deer, &c.), Carnivora (Fox, Badger, Wild Cat, Stoat, Marten, &c.), Rodentia (Squirrel, Rabbit, Hare, &c.), Tnsectivora (Hedgehog, Shrew, and Mole), and fifteen species of Bats. Moreover, within the historic period, the Wolf, Bear, Wild Boar, Reindeer, and Beaver were among the wild animals of Britain. In New Zealand, on the other hand, the only land Mammals found in the islands previous to the advent of Europeans were two species of Bats (Gkalinolobus morio and Mystacina tuberculata), the Maori Dog. which was certainly introduced by the Maories who colonised New Zealand from some of the Pacific Islands not many centuries ago, and the Maori Rat (Mus maorum), which perhaps owes its introduction to the same source. With the excep- tion, then, of Seals, two Bats, and perhaps a Rat, are the only native Mammals of New Zealand. The Birds of the two countries also offer a marked contrast. Great Britain has no native Ratitaa; in New Zealand there are now living five species of Apteryx, while within the historic period — three or four hundred years ago or less — there lived in the two islands half-a-dozen genera, and some five -and -twenty species of Moas (Dinornithidse). Great Britain has no Parrots; New Zealand has two species of Nestor, seven Parrakeets of the genus Cyanorhamphus, and the extraordinary Ground Parrot or Kakapo (Stringops). On the other hand the Finches (Fringillidse) are wholly absent in New Zealand, but abundantly represented in Great Britain. Moreover, New Zealand is remarkable for the large number of flightless Birds included in its fauna: besides the Kiwis and Moas, there are Stringops (Ground-Parrot) ; Ocy- dromus, Cabalus, and Notornis (Rallidse) ; Nesonetta (the flightless Duck of the Auckland Islands) ; besides the extinct Giant Goose (Cnemiornis) and Giant Rail (Aptornis). In Great Britain none of the Birds are actually flightless. As to Reptiles the difference is less striking, but is still suffi- ciently marked, the most important facts being the entire absence of Snakes in New Zealand and the presence of Sphenodon, the only existing representative of the Rhynchocephalia. Lizards occur in both countries, and in both Crocodilia are wholly absent, and Chelonia occur only as occasional visitants. Great Britain possesses four species of Tailed Amphibia (Newts), and the same number of Frogs and Toads. In New Zealand Urodeles are entirely absent, and there is only a single and rare species of Frog (Liopelma Jiochstetteri). The fresh-water Fishes of Britain are numerous and varied ; the most important are the various species of Salmo (Salmon and Trout), the Perch, Carp, Grayling, Pike, Eels, &c. In New Zealand there are only Eels, a small salmonoid, Retropinna, not found else- xiv DISTRIBUTION 621 where, several species of Galaxias—an exclusively Australasian, South African, and South American Physostome — and a small indigenous genus (Neochama) of the same family. The differences between the marine Fishes, though obvious enough, are less fundamental, a fair proportion of the New Zealand shore-fishes belonging to the same families, and in some cases even to the same genera and species, as those of Britain. Among Mollusca the fresh-water Unio (fresh-water Mussels), are found in both countries, but New Zealand has no species of the common genus Helix (Land-snail), abundant in Great Britain, and its molluscan fauna generally is very peculiar. The Insect-fauna of New Zealand is remarkable for the paucity of Butterflies — sixteen species against about seventy in Britain — - and for the abundance of Moths, mostly belonging to the Micro- lepidoptera and the Geometrina. The occurrence of Peripatus in New Zealand furnishes another strong point of contrast. Amongst fresh-water Crustacea, the British Astacus is represented by an allied genus Paranephrops. Among marine Crustacea many genera are common to the two countries, but there are numerous peculiar forms, and it is worthy of mention that the New Zealand species of Palinurus belongs to a more generalised type than the British species, having no stridulating organ. The British Earthworms all belong to the familiar Lumbricidce (including Lunibricus) and Cryptodrilidce ; in New Zealand both these families are absent, and the majority of the Earthworms belong to the Megascolecidse, including the genera Acanthodrilus, Octochcetus, &c. Lastly, there are found in New Zealand nearly forty species of Land Planarians and one terrestrial Nemertean ; neither of these groups is represented in the land-fauna of Britain. That these striking differences are quite independent of climate, food, &c. — in other words, that the environment in the one country is in no way inimical to the fauna of the other, is shown by the zoological history of New Zealand since its colonisation. Apart from domestic animals, the Brown Rat (Mus decumanus) and the House-Mouse (Mus domesticus) are now as common in New Zealand as in Britain ; the Rabbit has become a plague, barely kept in check by constant effort stimulated by severe legislative enactments ; Deer flourish as well in the mountains of Otago as in those of Scotland ; the Birds first noticed by a visitor to the settled districts of the colony will probably be the Sparrow, Black- bird, Thrush, Starling, and Goldfinch; and Trout have become so thoroughly acclimatised in the streams and lakes, that in some districts the poorer settlers, like the British apprentices of old, decline to eat them. We thus learn to distinguish between the native or indigenous fauna of a, country, and the introduced fauna VOL. II Q Q 622 ZOOLOGY SECT. which owes its existence to human agency; in comparing the faunae of any two countries, the latter element must of course be carefully eliminated. The contrast between our two selected countries is further emphasised when we compare the fauna of each with that of the nearest continental area — the fauna of Great Britain with that of the continent of Europe, the fauna of New Zealand with that of Australia, With the exception of the Red Grouse (Lagopus scoticus) perhaps the Coal Tit (Parus britannicus), and the Long-tailed Tit (Parus rosea), and some fifteen species of fresh-water Fishes, the British Vertebrates are all found on the European continent. On the other hand, there are many European species of Mammals, Birds, Reptiles, Amphibians, and Fishes which do not occur in Great Britain. The British fauna may, in fact, be described as an isolated and somewhat impoverished section of the general European fauna. Now contrast New Zealand with Australia. Of the two New Zealand Bats, one (Chalinolobus morio) occurs also in Australia, the other (Mysiacina tuberculata) is endemic, that is, found nowhere else. Australia, instead of having a Mammalian fauna com- prising only two Bats and a doubtful Rat, possesses a large number of characteristic Mammals, all except the Bats, Rats, and Mice, and the Dingo (Canis dingo) being either Monotremes or Marsupials. Out of about 200 species of New Zealand Birds, fully 100 are endemic; of the rest only about 50 are found in Australia. Conversely Australia has a large number of charac- teristic Birds, belonging to families wholly unrepresented in New Zealand, such as the Birds of Paradise and Bower Birds (Par- adiseidce), Cockatoos (Cacatuidce), Mound-makers (Megapodiidce), the Lyre-Bird (Menura), the Emu. and the Cassowary. Among Rep- tiles, Sphenodon, i.e., the entire order Rhynchocephalia, is endemic in New Zealand, as also is the little Gecko Naultinus, while a small genus of Lizards, Lygosoma, is common to the two countries. Australia, on the other hand, besides possessing a large number of Lizards, such as the Monitors, is remarkable for the great number of its Snakes, a group quite unrepresented in New Zealand, and has two species of Crocodiles and several of Chelonia. Among Fishes, the presence of Ceratodus in Australia is specially remarkable. The fresh -water Crayfishes of each country are endemic, those of New Zealand belonging to the genus Paranephrops, those of Australia to Astacopsis and Engmus. The majority of the Australian Earthworms belong to the familes Perichcctidce and Cryptodrilida:, the latter including the Giant Earthworm of Gippsland (Mcgascolides) ; the Megascolecidas are represented, but are not dominant, as in New Zealand. XIV DISTRIBUTION 623 Thus, while the zoological resemblances between Great Britain and the Continent of Europe are so close as almost to amount to identity, there is more difference, zoologically speaking, between New Zealand and Australia than between Europe and North America, or between England and Japan. FIG. l-23Jt.— Map showing the shallow bank connecting the British Isles with the continent. The light tint indicates a depth of less than 100 fathoms ; the figures show the depth in fathoms. (From Wallace.) The reason of this remarkable contrast is not far to seek. Geologically speaking, Great Britain is a recently detached portion of Europe, having been united with it during the latter part of the Glacial period (Pleistocene), and, at the present moment, an elevation of the bed of the English Channel to the amount of 2GO feet, would bring about a re-union (Fig. 1239). Prior to this Q Q 2 624 ZOOLOGY SECT. union, moreover, it was largely submerged, so as to leave no trace of its Pliocene fauna. Thus, the British animals are all migrants from Europe, isolated by the post-glacial separation from Europe, and the absence of certain European forms is due to the fact that the separation took place too early to allow of complete migration. New Zealand, on the other hand, instead of being separated from the nearest continent by 21 miles of shallow sea, is divided from Australia by 1,000 miles of ocean — the Tasman Sea — varying from 2,000 to 2,600 fathoms (12,000-15,600 feet) in depth. It is almost certain that there was never any direct connection between 160 170 ISO 100 110 120 1130 140 150 |I50 JI60 1170 180 FIG. 1240. — Map showing depths of sea around Australia and New Zealand. The light tint indicates a depth of less than 1,000 fathoms ; the dark tint indicates a depth of more than 1,000 fathoms. (From Wallace.) the two countries, and the only indication of even an indirect con- nection is afforded by the existence of an area of comparatively shallow sea — i.e., under 1,000 fathoms — stretching between the North Island of New Zealand on the one hand, and Northern Australia and New Guinea on the other (Fig, 1240). It would take therefore, an upheaval of over 6,000 feet to join the two countries, and it may be taken as certain that if there ever was a direct connection, either by continuous land or by a chain of islands, such connection could not have been later than the early part of the Mesozoic era. It must also be noted that while the British fauna is related xiv DISTRIBUTION 625 exclusively to that of Europe, the New Zealand fauna presents not only Australian but also Polynesian and South American affinities. Some of the Birds are represented by the same or closely allied species in New Caledonia, while the land Molluscs and Insects, the Fresh-water Lamprey (Geotria), and the Earthworms, show South American affinities. Still more remarkable is the fact that a little fresh-water Fish, Galaxias attemiatus, occurs not only in New Zealand and Tasmania, but in the southern extremity of South America and in the Falkland Islands. In this con- nection it is interesting to find that there is a submerged bank of less depth than the surrounding ocean — under 2,000 fathoms — passing westwards from South America, and including many of the Pacific Islands ; and an area, also of less than 2,000 fathoms, in the Antarctic Ocean, sending offshoots northwards. The first of these may possibly indicate a former westward extension of South America, the second a former Antarctic land-area, perhaps more or less directly connected with the existing southern conti- nents. The whole question is quite unsettled and extremely obscure, and is complicated by the fact that in one respect the New Zealand fauna shows Ethiopian affinities. There have been discovered in the Chatham Islands, a small group about 400 miles to the east of New Zealand, the remains of a long-beaked Eail (Diaphora/irteryx), evidently not long extinct, the nearest ally of which is the R-ed Bird (Aphancijptcryx) of Mauritius, known to have been exterminated by human agency. Moreover, the great Ratite Birds, the ^Epyornithidse, of Madagascar, show undoubted affinities with the Dinornithidae. The foregoing comparison of the faunas of Great Britain and New Zealand leads us to the consideration of certain fundamental conceptions of zoo-geography. Insular Faunas. — We notice, in the first place, the striking contrast between the fauna of an island which has been recently detached from a great continental area, and that of an island which has remained isolated for an immense and unknown period. In the one case the fauna has a strictly continental character, there having been insufficient time for modification since the sepa- ration took place. In the other case immigration has taken place from various sources over a vast period of time, during which modification has taken place to a sufficient extent to give rise to new or endemic species. Habitat,, Range, and Station. — Each kind of animal has, as a rule, its own habitat, fresh-water in one case, the sea between tide-marks in another, marsh, forest, snow-clad peaks, and so on. A similar habitat may characterise whole genera and even orders. Keeping always to its own habitat, the range of an animal may .extend over avast area. The marsh-loving Curlew, for. instance, 626 ZOOLOGY SECT. is found all over the world; the Cormorants (PJialacrocornx), Gulls (Larus), some Ducks (Anas), &c., are also cosmopolitan or world- wide in distribution. On the other hand, the range of a species, genus, or order may be restricted to a single limited district. The genus Liopelma (New Zealand Frog) is found only in a small area in the Auckland district ; the species Salmo kUlinensis (Loch Killin Char) occurs only in Loch Killin in Inverness-shire ; the Order Rhynchocephalia is confined to New Zealand. The entire range may be broken up, as it were, into a number of stations, depending sometimes on habitat, sometimes on unknown causes ; the Tuatara, for instance, is found at present only in some half dozen small islets, each of which is a station, while the whole of them constitute the range of the species. Barriers. — A barrier in zoo-geography implies any permanent obstacle to the dispersal of a species. For instance, the Tasman Sea is an impassable barrier to the passage of most animals be- tween Australia and New Zealand, strong-flying birds being the only species able to cross it. On the other hand, the number of Birds able to pass so narrow a strait as the British Channel is very considerable, while still narrower arms of the sea may be crossed by a large proportion of Mammals, Insects, &c. Thus barriers are of unequal importance according to the particular animals under discussion ; wide deserts and lofty snow-covered mountain ranges are impassable to most species; to some even a narrow river or arm of the sea is unsuperable. Means of Dispersal. — Most Mammals and many Reptiles can swim across rivers and arms of the sea if not too broad ; thus narrow straits and rivers are of no significance as barriers to the dispersal of many animals. On the other hand, rivers of even moderate breadth are" insuperable barriers to Monkeys, which are unable to swim. Aerial animals can overcome many of the barriers impassable to flightless forms. Birds of strong flight often pass over immense stretches of ocean. For instance, a Cuckoo (Eudynamis taitensis) habitually winters in Fiji and other Pacific islands, and spends its summer in New Zealand, traversing the enormous distance of 1,500 miles twice a year. Many Insects also are able to fly great distances, especially when carried by gales. Mechanical dispersal is an important factor in the case of many animals. Small Crustacea and Molluscs may be carried great distances in the mud adhering to the feet of Birds. Infusoria, the eggs of Rotifers, and other microscopic forms may be transported in the dried condition by wind. Birds and Insects are frequently blown out to sea and carried for immense distances, and Mammals, Reptiles, &c., may be widely distributed by being carried on drift- wood, or on floating islands or "rafts," formed of large masses of matted vegetation, such as are often detached by storms in the xiv DISTRIBUTION 627 tropics. Finally, the dispersal of many fixed or shore-haunting animals is ensured by their free-swimming larvae Importance of the various Groups of Animals in Zoo- Geography. — In close dependence on the means of dispersal we have the fact that the various groups of animals are of very unequal value in the study of distribution. The greater the facilities for the transport of any species across a given barrier, the less significance will attach to its occurrence on both sides of the barrier. Conversely, when a species, having few or no facilities for dispersal, is found on opposite sides of an important barrier, the natural conclusion is either that the barrier is of comparatively recent formation, and that the two areas separated by it were once, so to speak, in zoological continuity, or that the species in question is a very ancient one, and was widely dispersed at a time when the arrangement of the land-surface was very different from what it is at the present day. For instance, the occurrence of strong-flying Birds, such as Gulls and Cormorants, in widely separated countries, is a fact of no significance in determining the mutual relationships of the faunas of those countries. But the occurrence of the same species of Fresh-water Crayfish — to which the narrowest arm of the sea is an insuperable barrier — in Great Britain and the European Continent, is explained only by the fact — of which there is independent evidence — that the English Channel is of recent formation. And when we find the various species of Peripatus dotted over the earth's surface in an apparently casual manner, we are forced to the conclusion that this genus must formerly have been very widely and continuously distributed and subsequently exterminated over the greater part of its range ; since it is hardly possible to conceive of either the adult or the young of this creature, living in rotten wood in the recesses of the forest, having been transported between Australia and New Zealand, or between Africa and the West Indies. Speaking generally, then, it may be said that discontinuity in the distribution of a species or other group is evidence of its antiquity. In addition to Peripatus, the Dipnoi and the Tapirs may be mentioned as examples. It will be seen that terrestrial and fresh-water animals are of more importance, from the point of view of zoo-geography, than marine forms. Among the inhabitants of the sea, littoral species are of greater significance than pelagic or abyssal. Amongst land animals, those which are unable to swim, and those which cannot survive immersion in salt water, are of more importance than strong swimmers, or than such forms as are able to live for a pro- longed period on drift-wood, or in mud attached to the feet of Birds. In connection with what has been said above about there being no special significance to be attached to the distribution of certain 628 ZOOLOGY SECT. strong-flying Birds, it must be remarked that this is by no means true of migratory Birds. Many British Birds, such as the Swallow, Cuckoo, Swift, &c., spend the summer in England, the winter in South Europe or Africa. One of the New Zealand Cuckoos winters in Australia, the others in Fiji or some other Pacific Islands. Birds capable of such feats of flight might, one would think, soon overspread the globe ; yet, as a matter of fact, each species is found to keep strictly to its own definite line of migration, even across 1,000-1,500 miles of sea. Having now indicated the general character of the facts and problems connected with the subject of zoo-geography, we may proceed to give some account of the Zoo-geographical Regions into which the land-surface of the earth is divided (see Fig. 1241 and Map). It must be borne in mind that the determination of these regions depends largely upon the classes of animals upon which stress is laid, the peopling of any given portion of the earth by a particular class depending upon the time during which it has been in exist- ence and its means of dispersal. Thus regions founded upon the distribution of Mollusca will differ from those depending on Eeptiles or on Birds. .The regions adopted here are mainly founded on the distribution of Birds and Mammals. The whole of Europe, Africa and Arabia north of the Tropic of Cancer, and the whole of Asia except India, Burmah, Siam, and South-east China, together with Japan, Iceland, the Azores, and the Cape de Verde Islands, are so similar in their animal pro- ductions as to form a single division of the earth's surface called the Palaearctic Region. This region is bounded on the north, West, and east by ocean, but its southern limits are at first sight less obvious. It appears strange, for instance, that Northern Africa and Arabia should be included in this region, the Mediterranean being, as it were, ignored as a boundary. But the facts show that the great line of sandy deserts in the region of the Tropic of Cancer^ the Sahara in Africa, and Roba el Khali in Arabia, form a far more efficient barrier to the dispersal of species than the Mediterranean, and it is probable that there was direct land connection between Europe and North Africa during the Pleistocene period. In Asia the Himalayas form an effective barrier, which has existed since Tertiary times, between Thibet and India; an ill-defined line of country following the course of the Indus continues the boundary south-west to the shores of the Arabian Sea; and another ill-defined area passing south of the Yang-tse-Kiang, and travelling northward to Shanghai, constitutes the eastern end of the southern boundary of the region. None of the larger groups of animals, no orders or even families, are absolutely confined to this region, the characteristics of which it is difficult to define -without descending to genera and species. The Moles (Talpidce), Sheep and Goats (Ovidce), and Dormice • the xiv DISTRIBUTION 629 Pheasants, Robins, Magpies and many other Birds, are highly characteristic, and many species of Deer, Oxen, and Antelopes, Rodents. Passerines and other Birds, Reptiles, Amphibia — including Proteus — and fresh-water Fishes, are endemic. The Palaearctic region includes, as we have seen, nearly all the northern portion of the eastern hemisphere ; the corresponding part of the western hemisphere, viz., North America, with Green- land, constitutes the Nearctic Region. It also is bounded by the ocean on its northern, eastern, and western sides, while in the south an ill-defined tract of country, passing between Cape San Lucas on the west and the Rio Grande Del Norte on the east, separates it from the Neotropical region. The Nearctic differs from the Palsearctic region in the possession of several characteristic Mammals, such as Opossums (Didelpliyidce), the Skunk, Racoon, &c. ; many Birds, such as the Blue-jays, and Turkey-buzzards, &c. ; Reptiles, such as Rattlesnakes and Iguanas; Amphibia, including the Axolotl, Necturus, Siren, and other large Urodeles; and numerous fresh-water Fishes, in- cluding Amia, Lepidosteus, Polyodon, and Scaphirhynchus. Only three entire families are endemic, two of Rodents, and one of Passerines. On the other hand, the resemblances between the two northern regions are very close. Both possess Wild Cats, Hyenas, Foxes, Weasels, Bears, Elk, Deer, Wild Oxen, Beavers, Voles, Squirrels, Marmots, and Hares, the species of the one region being all closely al lied to, and sometimes identical with, those of the other. Thrushes, Wrens, Tits, and Finches are also common to the two regions, and generally speaking, the differences between them are, as we shall see, nothing like so striking as those between either of them and the region or regions bounding it to the south. Hence the Palsearctic and Nearctic regions are sometimes grouped together as a single Holarctic Region. In the southern regions the characteristic features are. much more striking. The Ethiopian Region is constituted by the whole of Africa and Arabia south of the Tropic of Cancer, together with Madagascar, Mauritius, Bourbon, Rodriguez, and the Seychelles. The region is bounded by sea on the west, south, and east, but on the north it is perfectly continuous with the PalaBarctic region, and it certainly seems a very remarkable fact, until we remember what an impassable barrier is afforded by a sandy desert of great extent, that there should be more difference between the faunae of northern and central Africa, than between those of England and Japan, or of Alaska and Florida. Among the animals most characteristic of the Ethiopian region and not found elsewhere are the Gorilla, the Chimpanzee, several 630 ZOOLOGY SECT. Baboons, and the large majority of Lemurs, including the curious Aye-aye (Chiromys) ; several peculiar Insectivora, such as the Golden Moles (Ckrysochforid&), and the River Shrew (Pota- mogale) ; the African Elephant, the Hippopotamus, two or three species of Rhinoceros, the Zebras and Quaggas, the Giraffe and Okapi, and more than seventy species of Antelopes ; the Aardvark (Orycteropus), one of the most singular types of Ede-ntata; the Plantain-eaters (Musiphagidce), the Secretary Bird (Serpentarius), and many other families and genera of Birds ; numerous snakes and other Reptiles, and several fresh- water Fishes, including the Dipnoan Protopterus, and the Ganoid Polypterus. The Lion, Leopard, and Ostrich are also character- istic, although not actually endemic, since the two former extend into the Palsearctic and Oriental regions, while the Ostrich occurs in Arabia and Syria. Almost equally remarkable are the negative peculiarities of the region, and especially the absence of Bears, Deer, and Oxen, and the extreme paucity of Goats, Sheep, true Pigs (Sus), and Shrews. The great island of Madagascar is characterised by the immense number of Lemurs, the absence of Monkeys, and the poverty of its carnivorous and ungulate fauna, the Lions, Antelopes, &c.t of the African continent being all absent. Most of its Mammals are endemic, only three out of twenty-eight (including Bats) being found in Africa. The Birds, also, are quite different from those of the African continent. It shows affinities with America in the presence of a peculiar family of Insectivora (Centetidai), otherwise found only in the West Indies, and of certain Snakes ; and its relationships with India are so marked that it has been proposed to account for them by assuming the former existence of a land connection, in Jurassic and Cretaceous times, extending north- eastward across the Indian Ocean and represented at the present day by the Seychelles and other neighbouring islands. In the opinion of some authorities these peculiarities entitle Madagascar and the adjacent islands to rank as a distinct zoo-geographical region. The Oriental Region consists of India, Burmah, Siam, south- eastern China, and certain islands of the East Indian Archipelago, including Sumatra, Java, Borneo, and the Philippines. As we have seen, it is separated from the Palasarctic region by the Himalayas, continued on the west by a tract of country following the course of the Indus, and on the east by a region curving at first southwards and finally northwards to Shanghai. The south- eastern boundary is an imaginary line, known as Wallace's line, which passes between the small islands of Bali and Lombok, then through the Straits of Macassar, between Borneo and Celebes, and finally to the east of the Philippines. The islands to the north- xiv DISTRIBUTION 631 west of this line — conveniently distinguished as the Indo-Malayan Islands — belong to the Oriental region, those to the south-east — • the Ausiro- Malay an Islands — to the Australian region. Curiously enough, the zoological differences between the two groups of islands are more marked between Bali and Lombok, separated by a deep channel of only about twenty miles in width, than between Borneo and Celebes, separated by the whole width of the Straits of Macassar. The most characteristic members of the Oriental fauna are the Orang-utan (Simia), the Gibbons (Hylolates and Siamanga), and numerous Lemurs ; the Tiger, which, however, extends into the Palatal ctic region, and several Bears and Civets ; the Indian Elephant, the Indian Tapir, three species of Rhinoceros, and the Chevrotains or Mouse-deer (Tragulidai) ; and several large and handsome Gallinaceous Birds, such as the Peacock, Argus Pheasant and Jungle-fowl. The resemblances to the Ethiopian Region are numerous and striking, among the most important being the presence of the Elephant, Rhinoceros, the higher Apes, Lemurs, and Manis. On the other hand, the presence of Deer and Bears furnish a characteristic difference. The Australian Region includes Australia, Tasmania, and the Austro-Malayan Islands as defined above, from Celebes and Lombok on the west, to the Solomon Islands on the east, the most important of them being the immense island of Papua or New Guinea. New Zealand and Polynesia are very generally included in this region, but it is more convenient, on the whole, to treat them apart. The most striking feature of the region is the almost total absence of Eutheria, the Mammalian fauna belonging mainly to the Marsupials and Monotremes. The last-named order is en- tirely confined to this region, while Marsupials occur elsewhere only in America. The only exceptions are the Dingo or Aus- tralian Wild Dog, which is probably indigenous, the universally distributed groups of Rats, Mice, and Bats, and, in some of the islands bordering on the Oriental region, Deer, Civets, and Pigs. The abundance of Marsupials is very remarkable, all the leading groups of that sub-class, with the exception of the Didelphyida3, or American Opossums, and Ccenolestes, being strictly endemic. Equally striking is the number and peculiarity of the endemic Birds, the most important of which are the Emus and Cassowaries, the Mound-makers or Brush Turkeys (Talegallus, &c.), the Birds of Paradise and Bower-birds, the Lyre-bird (Menura), the Cockatoos and Brush-tongued Lories. The great number and variety of Parrots, Kingfishers, and Pigeons is also a marked feature, as also is the absence of Pheasants, Woodpeckers, Finches, and other 632 ZOOLOGY SECT. Birds abundant in the Oriental region. Snakes, Lizards, and Frogs are abundant, and in the rivers of Queensland occurs Ceratodus, one of the three existing genera of Dipnoi. The New Zealand Region comprises the three islands of New Zealand (North, South, and Stewart's Islands), together with Norfolk, Lord Howe, and the Kermadec Islands to the north, the Chatham Islands to the east, and the Bounty, Antipodes, Auckland, Campbell, and Macquarie Islands to the south. The characteristics of the New Zealand fauna have already been dealt with in some detail. The total absence of land Mammals, with the exception of two Bats and a Rat, the latter probably introduced ; the large proportion of endemic Birds, many of which are flightless; the exclusive possession of more than half the known genera, and of a large majority of the species of Ratita?, and of the entire order Rhynchocephalia ; the total absence of Ophidia, Chelonia, and Crocodilia ; the paucity of Lacertilia and the almost total absence of Amphibia; all these faunal characters combine to make New Zealand one of the best marked and most peculiar, tracts on the earth's surface. One or two facts must be mentioned with regard to the smaller islands of the region. In Norfolk Island there existed until recently a flightless Rail, Notornis alba, belonging to a genus the only other species of which lives or lived in New Zealand. In Phillip Island, close to Norfolk Island, Nestor productus formerly occurred, a member of an endemic New Zealand family of Parrots. In Lord Howe Island there is a species of the endemic New Zealand flightless Rail Ocydromus. These three facts all point to a former partial or complete land connection between New Zealand and the Islands in question. The remaining islands are closely related to New Zealand, but with greatly impoverished fauna?. In Macquarie Island, the southernmost land outside the Antarctic circle, there has recently been discovered an Earthworm with distinct South American affinities. The Polynesian Region embraces the numerous groups of islands lying within the tropics to the east and north of the Austro-Malayan Islands. The most important groups are New Caledonia, the New Hebrides, Fiji, the Friendly Islands, Samoa, the Society Islands, and the Sandwich Islands. They are all typical oceanic islands, that is, they are of volcanic origin, have no stratified rocks, and show no indication of former connection with any continental area. In correspondence with their isolated position, the faunas of these islands, although exhibiting great variety from one group to another, all agree in the absence of Land Mammals, except Bats, and — with one or two exceptions— of Amphibia, in the small xiv DISTRIBUTION 633 total number of species, and in the very large proportion of endemic species. The islands have evidently been peopled by waifs and strays from other lands, at periods so remote that most of the immigrants have assumed the character of distinct species, or even, especially in the isolated Sandwich Islands, of distinct genera. On the whole, the affinities of the Polynesian fauna are dis- tinctly Australian ; they present, however, certain American char- acteristics, especially in the occurrence of Lizards, belonging to the American family of the Iguanida?, in Fiji. Amongst the most notable endemic forms are the Dodo-like Pigeon, Didnnculus, in Samoa ; the Kagu ( Rhinochetus), a remarkable genus of Gralla?, in New Caledonia, and the Drepanidce, a family of Passerines allied to the American Greenlets, in the Sandwich Islands. Polynesia cannot be said to form a well-defined region, the islands composing it being united largely on the ground of convenience. In the Neotropical Region we have once more an immense tract of land, presenting such well-defined faunal characteristics as make it one of the best-marked of all the zoo-geographical regions. And this in spite of the fact that it is in free connection with the Nearctic region, the two being separated by an ill-defined tran- sition-region formed by the northern part of Mexico. The Neo- tropical region includes, therefore, the tropical part of North America, as well as the whole South American Continent, the Antilles or West Indies, the Galapagos Islands, the Falkland Islands, and Juan Fernandez. Both geological and zoological evidence point to a complete separation of the two Americas during the Miocene and Pliocene periods. The endemic animals of the region are very numerous and characteristic. They include, among Mammalia, the Prehensile- tailed Monkeys (Cebidce) and the Marmosets (Hapalidce) ; the Chinchillas and Cavies, two peculiar families of Rodents ; the Jaguar ; the Llamas, and Peccaries, and a species of Tapir ; the Sloths, Armadillos and Ant-eaters — three entire families of Eden- tata. The Opossums (Didelphyidce) are also very characteristic, though not actually endemic since they extend into the Nearctic region. A single Diprotodont Marsupial (Casnolestes) has been found in the extreme south. Among Birds the chief endemic forms are the three species of Rhea, constituting the entire order Rhe;e; the Tinamous, forming the order Crypturi;the Toucans, Screamers, Oil-bird (Stcatornis), Hoatzin (Opisthocomus), and many others. The Humming-birds, although extending into the Nearctic Region, are a characteristic group. Boas, Rattlesnakes, Iguanas, Crocodiles, and Caimans are abundant, and among the fresh-water Fish are the Electric Eel (Gymnotus), and Lepidosiren, one of the three existing genera of Dipnoi. 634 ZOOLOGY SECT. The negative characteristics of this region are also very remark- able. Except in Central America and the West Indies, there are no Insectivora ; Civets, Oxen, Sheep, Antelopes, and true Swine (Suince) are altogether absent, and there are very few species of Deer; Crows and Ravens are also practically unrepresented. In the West Indies there are no Edentata, Monkeys, or Car- nivora, and there occurs a peculiar Insectivore, Solenodon, belong- ing to the Centetidse, otherwise found only in Madagascar. The Galapagos Archipelago, a group of oceanic islands, about 600 miles to the west of the continent, have at the most two Mammals, a Bat and a Mouse ; their Birds are very different from those of the mainland, and include many endemic species ; and among the Reptiles are the gigantic Tortoises (Testudo), of which different species occur in the various islands. The general relations of the zoo-geographical regions may be expressed in a diagrammatic form as follows :— PALAEARCTI C N E A R C T I C ORIENTAL POLYNESIAN ETHIOPIAN AUSTRALIAN — NEW ZEALAND NEOTROPICAL FIG. 1241. — Diagram showing the general relations of the zoo-geographical regions. 2. BATHYMETRICAL DISTRIBUTION. The foregoing pages have given a brief sketch of the facts con- nected with geographical or horizontal distribution. We now turn to bathy metrical or vertical distribution — the facts concerning the distribution of animals at various depths of the sea or of lakes, and at various heights of the land. The region of greatest abundance of marine life, as regards both the number of genera and species and of individuals, is the littoral or shore-region. The rocks left, dry by the retreating tide, the rock-pools exposed at low water, and the forests of kelp at the limit of low tide or a few fathoms below, possesses an extra- ordinarily rich and abundant fauna, including all the Calcareous Sponges and a large proportion of the remaining groups, Hydroid Zoophytes, Sea-anemones and Corals, Echinoderms, Turbellaria, Nennertinea, Polychoata, Polyzoa, Brachiopods, decapod Crustacea, Pelecypods, Gastropods, Octopods, and Teleostei. Numerous examples of other groups — Protozoa, the lower Crustacea, Insects, xiv DISTRIBUTION G35 and Elasmobranchs — are also littoral, and Penguins, Seals, and Sirenia may be included in the list. Next in abundance to the littoral is the pelagic or ocean-surface fauna, including animals which live habitually on the surface or at slight depths of the ocean, often far from land. Amongst them are many Foraminifera, such as Globigerina and Hastigerina, the Radiolaria, the Siphonophora, the majority of Medusae, both hydrozoan and scyphozoan, the whole class of Ctenophora, many Entomostraca and Schizopods, the hemipterous Insect Halobates, the Pteropoda, Heteropoda, and some other Gastropods, such as Glaucus, most Cephalopods, Pyrosoma and the Salps, numerous Teleosts, such as Herrings, Flying-fish, Mackerel, &c., the greater number of Sharks, and the majority of Cetacea. The pelagic Invertebrates are mostly distinguished by great transparency, and by being either colourless or of a blue or violet hue. Pelagic Fishes are usually grey or steel-blue above, white beneath, presenting none of the brilliant colours, varied mark- ings, and extraordinary forms so often found among Shore- fishes. It must be remembered that many littoral animals are pelagic in the larval condition, or during some phase of their life-history, e.#.,.many Sponges, fixed Hydrozoa and Actinozoa, Echinoderms, Annulata, Mollusca, Crustacea, and Fishes. The abyssal or deep-sea fauna is far more abundant than might be supposed from the physical conditions — immense pres- sure and absence of light and of vegetation. In most parts of the world the bed of the ocean, at depths from 400 to 2,000—2,500 fathoms, is formed of a greyish mud called globigerina-ooze, consist- ing largely of the shells of Foraminifera, such as Globigerina, Orbulina, &c., which have for the most part sunk to the bottom after death. Below 2,500 fathoms the sea-bottom is formed of a red clay, in which shells are absent, having apparently been dissolved during their descent to the greater depth. Living on the sea-bottom, and most abundant on the globigerina- ooze, are representatives of many groups of animals : Sponges, especially Hcxactinellida ; a few Medusas and Corals ; examples of all classes of Echinoderms, Stalked Crinoids, and Holothurians being especially abundant ; Crustacea, particularly Schizopods and Prawns; and Teleostei. Crabs, Molluscs, and Annulata are rare. Many abyssal animals are blind, including several of the Crustacea; many others are phosphorescent, and thus supply their own light in an otherwise dark environment. The deep-sea Teleosts are often of very grotesque appearance, with immense heads, wide mouths furnished with long, pointed teeth, extremely distensible stomachs, and phosphorescent organs arranged in rows along the body (see Fig. 883). Other forms, such as the Ribbon- fish (Regalecus), attain a great size, and are toothless. When 636 ZOOLOGY SECT. brought to the surface, the expansion of the gases in the interior of the deep-sea Teleosts often bursts the air-bladder, and produces a general disintegration of the tissues. Plankton, Nekton, and Benthos. — Besides being arranged with regard to their relations to the shore, the surface of the ocean, and its bed, marine animals are also conveniently classified on the basis of their capacity for movement. Many forms, such as Medusaa, Siphonophora, Ctenophora, Salps, and numerous pelagic larvae are carried along passively by oceanic currents, their own powers of progression being of the feeblest. Such animals together constitute the Plankton, or " drifting-fauna." Others swim actively by means of fins or other appendages, such as the pelagic Teleosts and Elasmobranchs, Schizopods, Prawns, and Squids — they form the Nekton, or " swimming-fauna." Others again, have no natatory organs, and are either permanently fixed, like Zoophytes and Stalked Crinoids, or move by creeping over the sea-bottom, like Starfishes, Holothurians, Chaetopods, &c. ; such forms constitute the Benthos, or " bottom-fauna." The Fresh-water Fauna presents certain characteristic features, arid is divisible into flumatile forms, inhabiting streams and rivers, and lacustrine forms, inhabiting lakes. It is very rich in Lobosa, Heliozoa, Flagellata, and Infusoria, but contains very few Foraminifera and no Kadiolaria. Among Sponges there is only a single fresh-water family, the Spongillidae : among Hydrozoa only four genera, Hydra, Cordylophora, Limnocodium, and Limno- cnida ; and among Actinozoa and Ctenophora not a single species. There are also no fresh-water Echinoderms or Brachiopods, but many Turbellaria, a few Nementinea and numerous Nematoda. Among Polyzoa one genus of Endoprocta, the whole of the Phylactolasmata, and one or two genera of Gynmolaamata, are fresh-water forms ; so also are many of the OligochaBta, e.g., Nais and Tubifex, but very few Polycha3ta. Fresh- water Entomostraca are numerous and abundant, and belong to all orders except Cirripedia ; among Malacostraca there are only some Amphipods and Isopods, Anaspides and its allies, and Fresh-water Shrimps, the various genera of Fresh-water Crayfishes, and a few Crabs. The larvaB of many Insects are aquatic, and there are several aquatic Spiders. Pelecypods and Gastropods furnish abundant fluviatile and lacustrine forms, although belonging to comparatively few genera ; Cephalopods, on the other hand, are wholly absent from fresh-waters, as also are the Tunicata. Among Fishes there are several species of Lampreys, and numerous Teleostei, the Siluroids and Salmonida3 being especially characteristic. There are no fresh-water Elasmobranchs, with the exception of one or two genera of Sting-Rays in the rivers of tropical America ; but the Ganoids are a characteristic fresh-water group, although some xiv DISTRIBUTION 637 forms, such as the Sturgeons, migrate to the sea at certain seasons. The Dipnoi are exclusively fluviatile, or live in swamps caused by river overflow, and the perennibranchiate Amphibia, as well as the larvae of the caducibranchiate forms, are characteristic members of the fresh-water fauna. Many Chelonia and Crocodiles, such Birds as Ducks and Grebes, and such Mammals as Otters, the Hippopotamus, and Ornithorhynchus, may also be included in the fresh-water fauna, and some Dolphins are purely fluviatile. The animal inhabitants of large lakes, like those of the sea, are divisible into littoral, pelagic, and deep-water, and the pelagic forms are, in this case also, characterised by their extreme transparency. Mention must also be made of animals dwelling in deep subterranean caves, shut off from sunlight, such as Proteus, the blind Urodele of the caves of Carniola, the blind Fish (AmUyopsis spdceus) of the Mammoth caves of Kentucky, numerous Insects, &c. These, like abyssal species, are blind, and usually colourless, and are obviously specialised derivatives of the ordinary fresh-water or land-fauna. In the Terrestrial Fauna, also, we find certain groups pre- ponderant, others absent or nearly so. A terrestrial Amoeba has been described, and the Mycetozoa are all terrestrial, but no other Protozoa, nor any Sponges, Ccelentrates, or Echinoderms. Among Platyhelminthes we have the numerous species of Land-Planarians and the Land-Nemertines, and among Chastopods nearly the whole of the Earthworms. Several Crustacea are more or less completely adapted to terrestrial life, such as the Woodlice, Land- crabs, Cocoa-nut Crab, and Burrowing Crayfish. The Onychophora and Myriapoda are characteristic land-animals, so also are most Arachnida and many Insects. Among the Mollusca the only terrestrial forms are the majority of pulmonate Gastropoda. Among Fishes the Climbing Perch, Periophthalmus, and some others are imperfectly adapted to life on land, and the Caducibranch Urodeles, the Anura, and the Gymnophiona are all terrestrial or semi- terrestrial. The Lacertilia, Sphenodon, the majority of Snakes, and the Tortoises are land-animals, and so also are many Birds, including all the Ratitse, the Crypturi, Galling, &c., and the vast majority of Mammals. Among terrestrial animals, those which habitually live on the open ground must be distinguished from arboreal forms, such as Tree-Kangaroos, Sloths, and Monkeys, which pass their lives among the branches of trees, and from cryptozoic forms, which live under stones, logs of wood, &c., such as Land-Planarians, Peripatus, Centipedes, and Woodlice. Lastly, we have the Aerial Fauna, including animals capable of sustaining themselves for an indefinite period in the air, such VOL. II R R 63& ZOOLOGY SECT. as most Insects, the large majority of Birds, and Bats. The Flying Fishes, Flying Dragons (Draco), Flying Phalangers, Flying Squirrels, and Flying Lemur (Galeopithecus) are semi-aerial. The majority of land-animals live at or near the sea-level, and as \ve ascend mountains the fauna undergoes a gradual impoverish- ment as the snow-line is approached. The higher ranges of all great mountains have a characteristic Alpine Fauna. In the European Alps, the Chamois (Rupicapra), Alpine Hare (Lepus variabilis), and Marmot (Arctomys marmot), may be specially mentioned ; in the Himalayas, Yaks (Poephagus), Musk-deer (Moschus), Goats and Ibexes (Capra), besides abundant Birds and Insects ; in the Andes, the Condor (Sarcorhamphus) ; in the New Zealand Alps, the rapacious Kea or Mountain Parrot (Nestor notabilis). 3. GEOLOGICAL DISTRIBUTION In considering the distribution of animals in past time, we are met at the- outset with the difficulty that our knowledge of the subject is, and must always remain, very imperfect and fragmentary. With few exceptions, only calcified, silicified, or strongly chitinised parts are preserved in the fossil state, so that whole classes of animals are absolutely unknown in that condition, and of the rest our whole information depends upon the more or less imperfect skeleton. Moreover, it is only under very favour- able circumstances that even the hard parts are preserved ; the chances are usually in favour of the animal being devoured or disintegrated before there is time for it to be silted over with mud or sand. And, lastly, many rocks have been so altered by the internal heat of the earth as to destroy any organic remains they may once have contained. Thus while palaeontology furnishes us with the only sure test of phylogenetic speculation, it is a test which, more often than not, is incapable of application, owing to the extreme imperfection of many parts of the geological record. It is in the oldest of the stratified rocks that this imperfection is most severely felt. In the Laurentian period, forming the base of the sedimentary series (see Vol. I., p. 7), no animal or vegetable remains are known. In certain Canadian serpentine rocks belonging to this period there is found a remarkable structure which, under the microscope, bears a certain resemblance to the supplementary skeleton, with its canal-system, of an immense Fora- minifer. On the assumption that it was the fossilised remains of a member of this order, it was called Eozoon canadense, but recent researches seem to show conclusively that the supposed fossil is of purely mineral origin. Radiolarians and Foraminifera have been described from the Pre-Cambrian rocks of Brittany, but the nature of the bodies in question has not yet been established beyond dispute. xtv DISTRIBUTION 630 There are thus no undoubted fossil animals until the Cam- brian period, where many existing groups appear to start suddenly into being. We find Radiolaria, Sponges, Graptolites, Polyzoa, Brachiopoda, Edriasteroidea, Carpoidea, Asteroidea, Chsetopoda (worm-tubes), Phyllocarida, Ostracoda, Trilobites, the generalised Insects known as Pal^eodictyoptera, iso- and hetero- myarian Pelecypoda, Gastropods ( Prosobranchs and Pteropods), and tetrabranchiate Cephalopods (Orthoceras, &c.) — all, it will be noticed, marine forms, with the exception of Insects. Proceeding a stage onwards we find in the Silurian period, in addition to the above groups, Foraminifera, Actinozoa (rugose Corals), Ophiuroids, Echinoids, Crinoids, Cirripedes, Scorpions, Eurypterida, Amphineura, Scaphopoda, Elasmobranchii, and Ostra- codermi. Thus, in the two earliest fossiliferous systems are found repre- sentatives of all the skeleton-forming phyla, i.e., of all but Platyhelminthes, Nemathelminthes, and Trochelminthes. And, as far as our present knowledge goes, there is no indication of any connecting link between one phylum and another, the primary divisions of the animal kingdom having been apparently as well characterised at that enormously distant epoch as at the present day. Obviously all the older or more generalised animal types, which, if we reason from analogy, we must suppose to have preceded the present well marked phyla, have been destroyed by metamorphic action or otherwise, without leaving a trace of their existence. The Devonian period is remarkable for its abundant remains of Fishes; Crossopterygii, Chondrostei, and Dipnoi appear for the first time, and all three groups of Ostracodermi are abundant. Decapod Crustacea, of the macrurous or Shrimp-type, also make their appearance. In the Carboniferous period, notable for its immense forest-flora, there is a great development of air- breathing forms, such as Insects, Arachnids (Spiders), and Myriapods, as well as Stegocephala, the earliest amphibious Vertebrates. In the Permian rocks true air-breathing Verte- brates first make their appearance in the form of the reptilian orders, Theromorpha, Sauropterygia, and Rhynchocephalia. This period is also remarkable for the occurrence of Ceratodus, the oldest still existing genus of Vertebrates. Thus by the end of the Pakeozoic era, every important class of animals capable of leaving fossil remains is represented, with the exception of Mammalia and Birds. Moreover, the Trilobites, the Eurypterida, the Pal anodic tyoptera, and the Ostracodermi come to an end during this era, no remains of them being known in rocks of secondary age. In the succeeding Mesozoic era, the Triassic period intro- duces existing orders of Insects — Orthoptera, Neuroptera, and R 11 2 640 ZOOLOGY SECT. Coleoptera, as well as Xiphosura, siphoniate Pelecypoda, opistho- branchiate Gastropods, and dibranchiate Cephalopods (Belemnites). The Palaeozoic types of Tetrabranchs (Orthoceras, &c.) have nearly disappeared, and the Ammonites have become important. Among Vertebrates are found Holostei, Chelonia, Ichthyopterygii, Croco- dilia, and Dinosauria, the latter especially being a very prominent group, as well as several Mammalia (Microlestes, Hypsiprym- nopsis, &c.) of uncertain affinities. In the Jurassic period the two highest orders of Insects, Hymenoptera and Lepidoptera, are known for the first time, as well as the reptilian Ornithosauria, and the earliest known Bird (Archccopteryx). There are also several small Mammals (Pla- giaulax, Amphitherium, Phascolotherium, &c.) belonging either to the Prototheria or to the Metatheria, but occurring in Europe and North America, where there are at present — with the excep- tion of the Opossums — no representatives of either order. This seems to indicate that the lower Mammals originated in the northern hemisphere and spread southwards. In the Cretaceous period the Crabs — the most specialised of the higher Crustacea — and the Teleostei — the most specialised of Fishes — make their appearance. Of the last-named group, several Cretaceous genera survive and flourish to the present day, e.g., Clupea (Herring), Esox(Pike), Osmerus (Smelt), and Beryx. Ophidia are known for the first time, and Pythonomorpha, Dinosaurs, and Ornithosaurs are important. Mammals are practically unknown, but among Birds the Odontolcse -and the Ichthyornithes are characteristic. By the end of the period five entire groups of Reptiles — the Sauropterygia, Ichthyopterygia, Pythonomorpha, Dinosauria, and Ornithosanria — have become extinct, none of them being known to extend into Tertiary times. Except in California and Patagonia there is a well-marked break between the Cretaceous and the Eocene periods, the fauna of the latter having a comparatively modern character. The Pelecypods and Gastropods belong to existing families and even to existing genera, and Belemnites have almost, and Ammonites quite, dis- appeared. The Fishes all belong to existing types ; Stegocephala have given place to Urodela and Anura, and none of the Reptiles belong to extinct orders. Among Birds, the Penguins, Gulls, Rails, Owls, Picarians (Kingfishers, &c.) and Passeres have appeared, as well as the extinct orders Stereornithes and Gastornithes, and the goose-like Odontopteryx. But the most noticeable feature of the period is the rise and differentiation of the Mammalia. Among existing orders the Marsupialia (Opossums), Cetacea (Zeuglodon), Sirenia (Prorastomus, Eosireri), Ungulata, Carnivora, Insectivora, Chiroptera, and Primates (Lemurs) appear for the first time, as well as the extinct orders Creodonta, Condylarthra, Amblypoda, and Tillodontia, together xiv DISTRIBUTION 641 with the Dinocerata, none of which extend beyond the Eocene period. In the lower Eocene none of the Mammals belong to exist- ing genera, but in the upper Eocene are found DidelpJiys (Opossum), Rhinoceros, Viverra (Civet), Mmtela (Weasel), and possibly Canis. The period is also remarkable for the number of annectent or linking forms. There are, for instance, species connecting Dogs with Bears and with Civets, Civets with Hyaenas, Hyaenas with Cats, Pigs with Pecora, Deer with Chevrotains, Tapirs with Rhino- ceroses and with Horses, and so on. It is perfectly clear that the orders, sub-orders, and families of Mammalia, as we now under- stand them, were, during the Eocene period, becoming gradually differentiated from common ancestral forms. In the Miocene period the Proboscidea (Elephant and Mastodon) make their appearance, as well as Gibbon-like Anthropoidea (Pliopithecus, Hylobates and other genera), and some other Anthro- poidea. The first definite evidences of the existence of Man, in' the shape of worked flints, have been found in the Indian Miocene. Many existing families have arisen, such as Hedge- hogs, Shrews, arid Moles ; Mice, Rabbits, and Porcupines ; Whales and Dolphins ; Tapirs, Hippopotami, Swine, and Antelopes ; and species of Felis and Canis. The Rhinoceroses of the period still have no horns, and the antlers of the Deer are small or absent. The Tapir-like ancestors of the Equidse found in the Eocene have developed into more Horse-like forms, and the ancestors of the Camels (Poelrotherium) still retain upper incisors and distinct metacarpals. Numerous Diprotodont Marsupials lived in South America during this or the preceding period. The Pliocene fauna has a still more modern aspect, a large proportion of the animals composing it belonging to existing genera, although most of the species are extinct. Complex antlers have appeared in the Deer, horns in the Rhinoceroses, and tusks in the Pigs. The occurrence of Giant Tortoises (Testudo) in the Pliocene of both Palsearctic and Nearctic regions, and of a Chimpanzee and a true Ostrich (Strnthio) in deposits of this age in India and the Crimea, indicates the northern origin of these forms. Indeed it seems probable that most of the higher Verte- brata, except Penguins and the New-World Edentates, have originated in the Holarctic region. In the Pleistocene period many existing species have made their appearance, but their geographical distribution is very diffe- rent from that of the present day. For instance, the European fauna includes many forms now confined to the Ethiopian and Oriental regions, such as Apes, large Felidae, Hyenas, Tapirs, Rhinoceroses, Hippopotami, Horses, and Elephants, all of which appear to have been driven southwards by the cold of the Glacial epoch. In some parts of the world the Pleistocene fauna includes remarkable and often gigantic forms now extinct — most notable 042 ZOOLOGY SECT. XIV being the great Edentates (Megatherium, Mylodon, Glyptodon, &c.) of South America, the gigantic Marsupials (Diprotodon,Nototherium) of Australia, and the great flightless Birds (Dinornis, <&pyornis, &c.) of Madagascar and New Zealand. Nesopithecus, which occurs in the Pleistocene of Madagascar, is either a Monkey-like Lemur or a true Monkey : if it be the latter, its occurrence indicates a closer affinity between that island and Africa than their existing faume would indicate. Pithecanthropus, found in beds of late Pliocene or early Pleistocene age in Java, was perhaps a connect- ing link between the other Anthropoids and Man. The Pleistocene passes insensibly into the Recent period, which has also witnessed some important zoological changes, especially the extinction of many interesting animal forms, for the most part by human agency. Among these may be particularly noticed Steller's Sea-cow (Rhytina), the Great Auk, the Dodo and Solitaire, several flightless Rails (Aptornis, Notornis, Aphanctptcryx, &c.), the Philip Island Parrot, and, above all, the whole great race of Moas. SECTION XV THE PHILOSOPHY OF ZOOLOGY IN dealing with the structure and development of the various groups of animals, there has been occasion not infrequently to refer incidentally to various subjects of a general nature, such as evolution, heredity, and the like. Such topics, dealing, not with the concrete facts of the science, but with abstract generalisations deduced from the facts, may be grouped together under the general heading of the Philosophy of Zoology. The generalisations forming the subject-matter of the philosophy of zoology may, in some instances, be so clearly and directly deducible from the data concerned, that it is scarcely possible for anyone conversant with the facts to refuse credence to the generalisation. But in other cases the conclusion is a matter of probability only, and one conclusion or another may be regarded as the more probable, according to the estimate formed of the relative importance to be attached to different sets of the facts or to different aspects of the facts. This will become clearer as we proceed ; but at the outset it should be distinctly understood that what follows is not to be looked upon in the same light as the statements regarding the known phenomena of animal life which constitute the main sub- stance of the preceding sections. Nearly all the subjects now to be touched upon are, to a greater or less extent, matters in which there may be variety of opinion among those conversant with the phenomena; they are all subjects which will bear discussion from various sides ; but, as discussion is here almost out of the question, it is possible to give little more than a brief statement of some of the current views on these questions as an introduction to the study of works specially dealing with them.1 The Philosophy of Zoology, or the Philosophy of Biology (for it is here almost impossible to treat Zoology apart from its com- panion science of Botany), aims at an explanation of the facts of 1 See Appendix II, Nos. 2, 3, 4, 13, 14, 15, 16, 18, 32, 38, 46, 47, 48, 53, 55, 56, 60, 62, 63, 65, 66, 68, 644 ZOOLOGY SECT. the science. It is observed that an animal possesses a certain structure, develops in a certain way, has certain affinities with other animals, has a certain geographical and geological range ; and the attempt is made to find a satisfactory explanation of these facts. Evolution. — Of these facts there is, to all intents and purposes, but one explanation requiring consideration here. The animal- and plant-life of the globe has come to be as it now is by a process of evolution which has been going on continuously from an early period in the history of the earth to the present time. The plant- and animal-worlds, in other words, have been evolved by a gradual process of development, in the course of which the higher forms have originated from the lower. Evidence bearing on this doc- trine has already been encountered in abundance—in fact the theory of evolution has to be looked upon as in many respects a guiding principle in i the study of our science ; and it has, accordingly, been necessary in many parts of previous sections to take its truth for granted. In discussing the relations of the various phyla to one another, the relations of the various classes of each phylum, and the position of the described examples within the classes ; in referring to the homologies borne by the organs of the members of one class to those of the members of another, it has been necessary to assume the truth of a theory of evolution. For the evidence, then, in favour of a doctrine of evolution the reader is referred to the substance of previous sections, where it will be found on almost every page. For his guidance some land- marks may, however, be here pointed out. Anatomical and Embryological Evidence. — A consider- able body of the evidence in favour of the view that the higher animals have been derived from lower forms is obtained from the provinces of comparative anatomy and embryology. The 'mere fact that we are able conveniently' to express the resem- blances and differences in structure between different groups by the construction of such genealogical trees as have been given in some of the previous sections tells strongly in favour of a theory of descent ; for, though it is by assuming evolution that such diagrams are constructed, the resemblances which they represent point strongly to common ancestry. A theory of evolution explains also the fact that there is running through a whole series of forms — let us say Fishes, Amphibians, Reptiles, Birds, and Mammals— a common type of structure, in which the same essential parts, though perhaps differently modified in accordance with differences in function, are to be found in the same mutual relations. It would be difficult, on any other view of the facts, to explain, for example, the occurrence in the wing of the Bird and of the Bat, the flipper of the Whale, and in the fore-foot xv THE PHILOSOPHY OF ZOOLOGY 645 of the Horse, of essentially the same bony elements. More difficult still would it be to explain the cases in which what is a functionally active and important part in one animal is to be found- though only as a mere vestige, apparently quite useless — in an allied form. Very many instances of this phenomenon will be found in the previous chapters. The wing of the Pigeon is an efficient organ of flight ; in the New Zealand Kiwi or Apteryx it is a vestige, not visible externally, being covered over by the feathers and wholly without function ; yet this vestige possesses essentially the same bony framework and the same muscles as the complete and functional wing of the Pigeon. Again, the teeth of the Rabbit are parts essential to the welfare and the very existence of the animal, and persist throughout life ; while in the Whalebone- Whale teeth are indeed developed in the foetal condition, but are thrown off before or shortly after birth, never being of any use for mastication or any other purpose. The conclusion that seems to follow from these facts is that it is at least highly probable that the Kiwi has vestiges of wings because it is descended from birds which, like the Pigeon, possessed functionally useful wings ; and that the Whalebone- Whale has teeth in the foetal state because it is descended from ancestors which possessed teeth in the adult condition. The fact that the embryos of animals of one great phylum or class present a great resemblance to one another, and that, the nearer the adult forms are in structure, the closer, usually, is the similarity in their developmental stages, tells strongly in favour of a theory of common descent. Thus the nauplius-stage is found in a considerable number of groups of Crustacea, but it is only between members of families whose structure is closely similar that there is a very near correspondence in the precise character of the nauplius and in the stages which the larva subsequently passes through. Evidence of an allied character is afforded by the fact that in the course of its development one of the higher animals some- times appears to exhibit in successive stages features which are permanent in forms lower in the scale. Thus the embryo of a Mammal presents at an early stage visceral arches and clefts com- parable to the branchial arches and clefts of a Fish, and has a blood-circulation in accordance with this ; while at a later stage it exhibits in these particulars some resemblance to an Amphibian, later on to a Reptile, and only when development is further advanced takes on its special Mammalian characters. Again, we have seen that such an Amphibian as the Frog is, in its early condition as a tadpole, to all intents and purposes a Fish. Such phenomena may be explained, according to the theory of evolution, by the supposition that the successive stages in the development of the individual animal tend to reproduce, though in a very 646 ZOOLOGY BKCT. abbreviated and often greatly modified shape, the stages through which the group to which the animal belongs has passed in the course of its evolution from lower forms. This supposition — the " biogenetic law," or " recapitulation theory," as it is termed — though it cannot be accepted without great modifications and reservations, yet covers a number of facts which distinctly demand a process of evolution for their explanation.1 The phenomenon of retrograde metamorphosis observable in many animals, for the most part parasitic in the adult condition, also affords evidence in favour of evolution. It would be difficult to give any other explanation than that afforded by a theory of descent, of the life-history of such animals as Sacculina (Vol. I., p. 599), the parasitic Copepoda (p. 598), or the Ascidians (Vol. II., p. 37). The relatively high organisation of the larva of Sacculina, for example, with its well-marked Crustacean features, can only be explained on the supposition that the shapeless, unsegmented adult has been derived by a process of retrograde development from more normally constructed ancestors. Most Birds and Mammals, and many animals of lower groups, exhibit a more or less strongly marked sexual dimorphism, the males differing from the females in various other respects besides the character of the sexual organs. Such differences can only be explained on the supposition that they are the result of a gradual process of modification brought about in accordance with the more special adaptation of each sex to its special functions. Palaeontological Evidence. — A second body of evidence in favour of a theory of evolution comes from the side of Palae- ontology. It might, perhaps, on first considering the subject, be supposed that, had a process of evolution taken place, we ought to be able to find in the rocks belonging to the various geological formations a complete series of animal- and plant-remains repre- senting all the stages in the evolution of the highest forms from the lowest. Beginning with those strata in which evidence of life first appears, we ought, it might be supposed, to be able to trace upwards, through all the series of fossil -bearing strata, continuous, unbroken lines of descent showing the gradual evolution of all the various forms of plant- and animal-life. But such a supposition 1 As an instance of the difficulties in the way of the acceptance of the biogenetic law as such, it may be pointed out here that the ovum is by no means equivalent to the simple cell with which the phylogenetic series must be supposed to have begun. On the contrary, the ovum of one of the higher animals must be an extremely complex structure, and in reality widely different from the Protozoan which, according to the biogenetic law, should be its prototype. The ovum of the higher animal is, it is true, a single cell ; but it is a cell which comprises potentially within itself the entire complex adult organism, and is thus essentially an entirely different thing from the unicellular Protozoan. The same holds good of later developmental stages : they may resemble the adult condition of lower groups ; but they differ from the latter in the same way as the ovum differs from the Protozoan, xv THE PHILOSOPHY OF ZOOLOGY 647 would leave out of account the extreme incompleteness of the record of the history of life on the globe which is preserved to us in the rocks. In the first place, there are many groups of animals and plants which, owing to the absence of any hard supporting parts, are incapable of leaving any recognisable trace of their former existence in the form of fossils. Again, even in the case of such as have such hard parts, the conditions necessary for their preservation in deposits destined to be converted into rock cannot be of very frequent occurrence ; and many forms might fail to be preserved simply owing to the non-occurrence of such conditions. In the case of land-animals, such as Mammals or Reptiles, for example, when one of them dies, it is for the most part torn to pieces, and even the bones destroyed by various carnivorous and carrion-feeding creatures. Only now and again would it happen that, by becoming buried in a morass, or swept away by a flood and buried under alluvial deposits, such forms might be preserved. Again, great thicknesses of sedimentary strata, sometimes con- taining fossils, can be shown to have become removed by the agencies of denudation, or the various forces — such as the action of waves, tides and currents in the sea, of rain and fresh-water streams on the land — by which rock-masses are constantly, where exposed, being worn away ; while other rocks, subjected to the pressure of enormous superincumbent masses, and perhaps acted upon by intense heat and other agents of change, have been com- pletely metamorphosed — their mineral constituents having become re-arranged and what organic remains they may have contained completely destroyed. Moreover, of the fossil-bearing rocks that remain unaltered, only a small part can be said to have been thoroughly explored for fossil-remains. Yet, notwithstanding these causes of imperfection in the record of the succession of life on the earth preserved to us in the rocks, there is sufficient evidence to enable us to judge of the general character of the faunae (and florae) of the various geological periods. It is manifest, from what has already been stated throughout the earlier sections with regard to the geological history of each phylum and class, that there has been a general progress in successive eras from the simple to the more complex ; the higher forms have, so far as the recorded facts enable us to judge, come into existence later than the lower. The Yertebrata may be taken as an example. There is no evidence of the existence of the highest class — the Mammalia — earlier than the Triassic period of the Mesozoic era. The case of the Birds appears at first sight anomalous : Birds appear for the first time in deposits of Jurassic age, and are therefore more recent than the oldest Mam- mals. Birds are, however, very highly specialised Vertebrates, and, should it be proved that they appeared at a time when primitive 648 ZOOLOGY SECT. Mammals already existed, the separate evolution of the two classes from lower forms would afford a sufficient explanation. Reptiles extend as far back as the Permian. Amphibians, in the shape of the Sfcegocephala, first appeared in the Devonian; while all the earliest vertebrate remains in the Cambrian and Silurian forma- tions appear to belong to the class of the Fishes. Within each of these classes a progress is usually traceable from older, more generalised types, along diverging lines, to the various specialised forms existing at the present day. In some cases, however — notably in the Amphibia, Reptilia and Aves — the orders first represented have become entirely extinct, and have been succeeded by others that made their appearance on the scene at a comparatively late period. In certain cases among the Mammalia a number of closely related stages have been discovered, showing, taken in their chronological order, a gradually increasing specialisation of struc- ture. One of the best known examples of this is that of the Horse, to which attention is directed in the section on the' fiammalia (p. 606). No fewer than five parallel series of horse- like Perissodactyles are traceable, which developed and culminated separately, the culminating member — viz., the genus Equus — of one only of these series surviving to the present day. And there are other families of Mammals, chiefly among the Ungulates (the family of the Pigs and various families of Ruminants) in which an equally complete history has been made out. The direct evidence of the evolution of the Invertebrates is, in general, very imperfect. Some existing types of a comparatively highly-organised character are to be recognised among the fossil remains in the oldest formations— the Cambrian — in which definite organic structures, if we except a few Radiolaria andvForamiriifera, are traceable. There is no trace of primitive fossil members of the various invertebrate phyla, and the highly organised air- breathing Arthropods are represented both by Scorpions and by Insects as far back as the Silurian. Such remarkably complete geological histories as have been traced in some of the Mammalia are extremely rare in the Invertebrates. Such direct evidence, however, as is obtainable, points to the probability of evolution, and it may be inferred that the absence of primitive generalised representatives of the invertebrate phyla is most probably due to the imperfect character of the geological record. The Lamarckian Theory. — Supposing it to be regarded as proved that the organic world has come to be as we find it by a process of gradual evolution, we have next to inquire by what agencies this process of development has been brought about. A sketch of the history of thought on this subject will be given in the section on the history of Zoology, and it will not be necessary here to refer to more than the most important points. xv THE PHILOSOPHY OF ZOOLOGY 649 The first noteworthy attempt to solve the problem regarding the nature of the forces by means of which evolution has taken place was made, long before evolution was generally accepted among men of science, by Lamarck in his Philosophic. Zoologique, published in 1809. Lamarck's view was that evolution of new forms has taken — and is taking — place, in great measure owing to the direct action of the conditions of life on the organism, but still more owing to the use and disuse of organs. The surroundings or environment of the animal or plant produce a direct effect on the individual — bring about slight modifications in one direction or another, and these slight differences are transmitted by inherit- ance to the next generation—such slight modifications going on, generation after generation, producing eventually a marked effect on the characters of the organism. The chief agencies that might be supposed to act in this way are climate, the nature of the country, and food. But, in addition to these, Lamarck attributes considerable influence to the use and disuse of organs. The exercise of a part tends to increase its size and efficiency, and such increase may be and frequently is, according to Lamarck, transmitted to the succeeding generation. In this way, in the course of a number of generations, very great changes might be brought about. To take an example which is often quoted, Lamarck accounts for the great length of the neck of the Giraffe as compared with other .Ruminants by the supposition that it has been brought about by continuous efforts made by the animals through a long series of generations to reach higher and higher among the foliage of the trees from which they derive their main subsistence. Similarly, the disuse of a part, in Lamarck's view , gradually leads to its diminution, and perhaps ultimately to its complete disappearance. In this way he would explain the dis- appearance of the hind-limbs in the Cetacea, of both pairs of limbs in the Snakes, of the olfactory nerves in aquatic Mammals, and so on. Whether differences which are produced in the in- dividual organism by surrounding conditions or by its own efforts may be transmitted by inheritance to succeeding generations is not yet a settled point : we shall have again to refer to this question — the question of the inheritance of acquired characters — at a later stage. That such inheritance, if it takes place, could account for the development of all the various groups of animals and plants is not held by many biologists at the present time. Darwinian Theory. — It is to Charles Darwin that we owe the most thorough and consistent explanation of evolution that has hitherto been put forward — the explanation known as the theory of Natural Selection. The development of this theory and the share taken in it by Wallace will be sketched in the historical section. The two main supports of Darwin's theory are two sets of biological phenomena known respectively as the struggle for existence 650 ZOOLOGY SECT. 'and variation, both of which have to be understood before it is possible to grasp the theory of natural selection. Struggle for Existence. — In order that it may flourish, there are necessary for every species of plant and animal certain con- ditions. The plant must find a place with soil containing certain constituents, and with a certain degree of moisture and of sunlight. For spots presenting the necessary favourable conditions there is constantly going on a competition between individual plants of one species and between the members of different species. The nature of this struggle is well seen when a piece of garden-ground is allowed to run to waste, Its surface is soon overgrown by weeds of a variety of kinds, which kill out some of the original garden-plants. By and by the more hardy weeds kill out and replace such weaker forms as may first have obtained a footing, till an entirely new set of weeds may take the place of those that first appeared. Again, it was shown by Darwin that in turf which is kept cut close a much greater number of plants are enabled to grow than is the case if the turf is allowed to grow freely. If the turf is not kept cut some of the stronger plants gain predominance and kill out weaker forms. In a space of turf on which Darwin experimented, no less than half of the species present in the turf when kept pretty closely shaven perished when it was allowed to grow freely. Plants, however, have not only to compete with one another for space and light and nourishment. They have also numerous animal foes to contend with. A large proportion of young seed- ling plants are destroyed by various Insects and by Snails and Slugs. One of Darwin's experiments bearing on this point was to clear and dig up a small plot of ground and watch the fate of the seed- ling plants that sprang up on it : he found as a result that some four-fifths were destroyed by Insects, Snails, and Slugs. But it is not the lower forms of animals alone that are thus destructive to plants. Many of the Mammalia, particularly, as we should expect, the herbivorous Ungulata, exercise a strong influence in this way. Cattle, and Goats especially, sometimes produce a marked effect on the flora of a country. The introduction of Goats has been observed gradually to destroy the forests of certain districts — the seedling plants being eaten as they appear, and thus no young trees being developed to take the place of those dying from old age or other causes. The mere enclosing of a piece of moorland by means of a fence was observed by Darwin to have resulted in the growth of a number of trees. In the unen- closed parts the young trees were never able to make any headway against the cattle by which they were constantly being browsed down. Among animals, with which we are here more particularly con- cerned, as well as among plants, a struggle for existence goes on on all xv THE PHILOSOPHY OF ZOOLOGY 651 sides. To begin with, before there is any struggle for existence in the strict sense, there is — particularly in lower groups — a very great indiscriminate destruction of ova and young embryos. Most lower animals produce ova in great number — hundreds, more often thousands and tens of thousands, annually. Only a few of these reach maturity ; a large proportion are destroyed indiscriminately at one stage or another of their development, some failing to reach a spot favourable for their development, others becoming the food of other animals. But such of the young as are less adapted to escape the various dangers to be encountered, and less fitted to procure the necessary food, are more likely to be destroyed. This is one phase — and the most important, perhaps, of all — of the struggle for existence among animals. But there is also a struggle for existence, not only between individual animals of the same kind, but between animals of different kinds. This struggle, in so far as it relates to the competition for food and shelter, is more severe between nearly-related species ; for in such a case the food and the favourable conditions required are the same, or nearly so, in the two competitors. But there is also a struggle for existence of a constant and severe kind which goes on between carnivorous animals and the animals on which they prey — a struggle in which the defensive qualities of the latter, such as swiftness, power of eluding observation, power of resisting attack and the like, are opposed to the predatory powers of the former. Variation. — It was by observing this struggle for existence constantly going on in nature, taken in connection with the phenomenon of variation, that Darwin was led to his principle of Natural Selection as accounting for evolution. Variations in domestic animals and cultivated plants are observed to take place in various directions. Taking advantage of this, man has been able to select, in the animals which he has domesticated and the plants which he has cultivated, those qualities which seemed most likely to be useful to him ; he has thus been able to produce, from one and the same original wild stock, widely different varieties specially adapted for different purposes. Thus from one wild species of plant of the order Cruciferw — viz., Brassica oleracea — have apparently been produced all the varieties of cabbage, cauliflower, broccoli, Brussels-sprouts, anil other forms, each with a peculiar and strongly- marked growth of its own. All the domestic vegetables afford us instances of the same thing, and so do all the cultivated fruits. The crab-apple or wild apple, for example, was the original of all the varieties of apple, amounting to about a thousand, cultivated at the present day — varieties presenting in many cases very great differences in size, colour, texture, flavour, time of ripening, and other qualities. In cultivated flowers, the same holds good in an even higher degree. The instances of variation observable among domestic animals 652 ZOOLOGY SECT. are still more striking. The domestic Dog, for example, exhibits a large number of very marked varieties. Though all these seem to be fertile with one another, and to produce fertile offspring, it is generally supposed that they have been derived from several wild species with more or less hybridisation. But the enormous differences which are to be observed between some of the varieties have been produced to a great extent under domestication. These are not all mere superficial differences, but involve also the proportions and shape of the parts of the skeleton. The difference in the form of the skull and in the proportions of the bones of the limbs between a Greyhound and a Bulldog, for example, are very remarkable — so great, in fact, that if they were found to occur between two wild forms they would justify a zoologist in referring the two to distinct genera. Sheep and Cattle, Pigs and Horses, present similar, though not perhaps quite so strongly-marked, varieties. One of the most remarkable cases of variation under domestication, and one to which Darwin paid a good deal of attention, is that of the domestic Pigeon. Of this, there are a considerable number of varieties, known to fanciers as pouters, fantails, carriers, tumblers, and so on ; and it appears to be almost certain that these are descended from one wild species — the blue Rock-pigeon. These varieties, and many more that might be mentioned, have been produced by man selecting those forms that tended to vary in a desired direction — have been produced, that is to say, by artificial selection, sometimes consciously exercised, sometimes, no doubt, unconsciously. This process has had a long period of time for its operation, many of our domestic animals and plants having been the objects of care and cultivation in Egypt and Western Asia certainly several thousand years ago ; in many cases the wild forms from which they were developed appear to have become totally extinct. But variation occurs among animals and plants not only under domestication ; it occurs also in a state of nature. Evidence of this has already been adduced in the account of certain of the examples of the various phyla ; and in the examination of specimens of these in the laboratory the student can hardly have failed to notice the occurrence of individual differences not due to differences in sex or age in animals of all classes. In this respect, in the strength of the tendency to individual variation, there is a very great inequality between different species of animals, some being extremely variable, some comparatively stable. Variations of external parts have naturally, from the greater ease with which they may be observed, attracted most attention, but the ex- amination of the internal parts in large numbers of individuals of the same species, when it has been carried out, has shown that variations in internal organs are also of great frequency. xv THE PHILOSOPHY OF ZOOLOGY 653 Among the Protozoa, the Foraminifera are characterised by numerous and marked variations — so marked as " to include, not merely those differential characters which have usually been accounted specific, but also those upon which the greater part of the genera of this group have been founded, and even, in some instances, those of its orders!' The Mollusca vary also very frequently and extensively, especially in the form and markings of the shell ; and of some of the species which have been most completely studied in this respect a very large number of more or less strongly marked varieties have been recorded. Many of the Crustacea are also extremely variable in coloration and in the length and proportions of the various appendages. But, among the Arthropoda, it is in the Insecta, and more especially the Lepidoptera, that we find the most striking instances of variation. In the Vertebrata, also, variations in colour and proportions, as well as in internal organs, occur frequently in all classes. Mutations. — It has been shown by de Vries that variations are not always of a comparatively minute character. According to de Vries, in addition to the small fluctuations 'on which Darwin mainly relied for the phylogenetic development of organisms, there are others which occur comparatively rarely, and are of a much more striking character. These larger variations, which de Vries distinguishes by the name of mutations, take the form of the sudden appearance of differences equivalent to the formation per saltum of new species; and it is by the successive appearance of such steps or leaps, and not by the more gradual process of Darwinian variation, that, according to de Vries, progress from the lower towards the higher is effected. The new forms developed in this sudden way may live side by side with the old, and thus isolation is not necessary for their perpetuation. The weakness of this doctrine of mutations is that it is founded on an extremely limited number of observed cases — confined in fact to species of the Phanerogamous genus (Enothera (Evening Primrose). . Natural Selection. — According to Darwin's theory of Natural Selection, nature, i.e., the conditions under which the organism exists, selects certain variations as they arise, very much as the breeder or the gardener selects variations in domestic animals or cultivated plants. Let us see how this selection is carried on. We have seen that there is going on, on all sides, a struggle for existence. It is at first difficult to realise the intensity of this struggle, for there is little appearance of it on the surface. If we consider, however, that a large proportion of living things prey on living things of other groups, and when we bear in mind the extremely small proportion which, in most cases, the surviving individuals of any group bear to the number of young produced, we come to understand that this struggle for existence must be general and intense. VOL. II S S 654 ZOOLOGY SECT. Now in the case of a species living under tolerably uniform and stable conditions as regards climate, food-supply, and the like, the effects of this struggle would be the survival of the fittest. Of the young produced, only a small proportion (in most cases) reach maturity ; some of these surviving forms have survived, perhaps, because they have happened to escape being preyed upon by enemies, while others have succumbed ; but there can be little doubt that, in the long run, such individuals will survive as are best fitted to cope with the conditions to which they are subjected — such as are swiftest, let us say, in escaping pursuit ; or such as, by their special shade of colour or the nature of their markings, elude the observation of an enemy ; or such as by reason of their thicker covering, can better endure extremes of cold. Such surviving individuals would, it is assumed, transmit their special properties to their progeny, and there would thus be a gradual approximation towards a better adaptation of the species to its surrounding conditions by virtue of this " survival of the fittest." Let us suppose the conditions to change. Gradual changes in climate and other conditions are known to take place owing to subsidence or elevation of the land. But conditions might be changed in many other ways : some animal or plant previously used as food might become exterminated ; or a new enemy might find its way into the district inhabited by the species. Then such individuals as presented variations which enabled them better to cope with the new surroundings would have the advantage over the others, and would have a much better chance of surviving and leaving progeny. The useful variations thus produced and trans- mitted to the progeny would tend to increase, generation after generation, until a form sufficiently distinct to be regarded as a new species had become developed from the original one. The process of survival of the fittest has a reverse side, which has been termed the elimination of the unfit. Of the varieties that appear some are less completely adapted to their surroundings than the majority, and these (the conditions remaining the same) tend to become destroyed owing to their unfitness to cope with their environment. The result of this process of elimination (apart altogether from the selection of progressive variations by which evolution, according to the theory, proceeds) is to keep up a certain standard of efficiency in the organs of the members of the species. Under certain conditions this sustaining influence, as we may term it, of natural selection may be suspended ; the organism may be placed under conditions in which natural selection acts with reduced effect or does not act at all. There is, under such circum- stances, no " elimination of the unfit " ; and, as a result, fit and unfit survive indiscriminately, inter-breed and produce offspring, the ultimate outcome in the course of generations being a gradual deterioration in the whole race. xv THE PHILOSOPHY OF ZOOLOGY 655 This suspension of the influence of natural selection, with its re- sults, has been termed cessation of selection, or panmixia. Panmixia acts more commonly on single organs than on the entire organism. Thus, if, owing to some change in surrounding conditions, an organ is no longer useful, it is no longer kept up to the previous degree of efficiency by the elimination of the individuals in which the organ in question is imperfectly developed, and, as these cross with one another, offspring is produced in which the organ is below the efficient standard ; by a continuance of this process through a series of generations, it is supposed that the organ gradually dwindles in size, and may altogether disappear. Thus at that stage in the ancestral history of the Cetacea in which they had come to adopt a purely aquatic mode of life and no longer visited the shore, the hind-limbs, being no longer of service, would no longer be maintained by natural selection, and would gradually decrease in size until, finally, they entirely disappeared. In the case of these, as of many other rudimentary organs, however, it is probable that natural selection played a positive part in bringing about their diminution. Under the conditions supposed, the possession of hind-limbs would probably be an actual disadvantage to the animal, acting as an impediment to the swift progression through the water, and interfering with the free movements of the tail ; and varieties with diminished hind-limbs would, therefore, possess an advantage over their fellows in the struggle for existence. There would then be, in a sense, a positive reversal of selection. Darwin's theory of selection is concerned mainly with the small individual variations which are observed to occur, more frequently in some species, more rarely in others. Such variations are so slight and unimportant that it is difficult to understand how they could be of sufficient life-and-death value to give the indivi- duals in which they occur sufficient advantage in the struggle for existence to enable them to survive, when others in which they are absent perish. Failing the extermination of the unmodified individuals, unless the appearance of the variation should happen to be coincident with the occurrence of other factors leading to the isolation of the individuals possessing the new variation from the stock in which they originated, the new variety would tend to become swamped by inter-crossing with the latter and would fail to be perpetuated. If, however, the individuals in which the new modification occurs should by some means — such as migration beyond a geographical barrier of some kind, or by the nature of the variation itself — be preserved from inter-crossing with the stock, then, without the extermination of the latter being a necessary condition — without, that is to say, a life-and-death struggle, the new form might be preserved unaltered and perpetu- ated as a new and distinct variety, which further changes similarly brought about might raise to the rank of a species. s s 2 656 ZOOLOGY SECT. Detailed study of the geographical distribution of species and varieties in certain regions, more especially in the United States of America, has afforded much support to the view that the develop- ment of new forms takes place as a result of the appearance of varieties differing slightly from the parent stock and their isolation from the latter by geographical barriers; and by some writers evolution is even supposed to have proceeded solely, or almost solely, in this way with little, or entirely without, aid from natural selection. Another kind of isolation might be supposed to take the place of geographical in preventing the inter-crossing of new varieties with the original stock. By means of sexual or physiological isolation, i.e., lay the form in question becoming varied in such a way that it does not readily inter-breed with the main stock, the new variety may be as effectively isolated as if separated from it by a geographical barrier. That allied species and sub-species differ in their geographical range, and are often separated by geographical barriers ; and that a physiological barrier may be set up between allied forms owing to union between them being impossible or sterile, are facts of great importance in the study of evolution, and the details of such cases are necessary data of the science. But that the whole of organic nature should have been evolved by variation and isolation alone seems to be highly improbable : such a view takes no account of progressive adaptation and orthogenetic development, and it would seem to call for the formation of an impossible succession of barriers. A special phase of Natural Selection is distinguished under the title of Sexual Selection. By means of Sexual Selection it is attempted to explain the greater part of the secondary differences between the sexes which are so striking in many groups of animals. The special part which each sex has to play in the fertilising and deposition of the ova, in protecting and procuring food for the young, requires qualities, both anatomical and psychical, of a more or less widely divergent character in the male and female. Between the males of animals of many groups, contests frequently take place, and this affords us an explanation of the presence or special development in many cases in that sex of various offensive and defensive weapons — horns, tusks, and the like. Similarly, we are able to understand the greater vigour, in the majority of cases, of the male, with concomitant greater intensity of coloration, and the development of various ornaments and excrescences not present in the female. In many groups of Insects, and in a large propor- tion of Birds, sexual differences in coloration are very marked. These are, in some instances, to be traced to the necessity for different protective resemblances required in the two sexes owing to different habits, or to the necessity for protective colorations and markings in the female and not in the male. In the case xv THE PHILOSOPHY OF ZOOLOGY 657 of Birds, when the sexes differ, as they do in a large proportion of the species, the male has always more brilliant coloration, and often possesses also special crests or frills, wattles and the like, not present or less developed in the female. The greater obscurity of the colouring of the female Bird appears to be adapted to rendering her less conspicuous to enemies, such as Birds of Prey, while sitting on the nest; and, in cases where the females are brightly coloured, the nest is covered over above, or is constructed in a hole in the ground. The brilliant colouring and other features distinguishing the males of many Birds may be in great part the by-product of higher vitality, and may thus be the indirect outcome of natural selection leading to the more vigorous males obtaining an advantage in contest with rivals. It is possible, also, that the choice of the female in selecting a mate may have been a factor in bringing about the special modifications in question. But the evidence which has been adduced for any such selection on the part of the female of a mate with some slight superiority in brilliancy of colouring, or in the development of crests and the like, over his rivals, is insufficient, and many observations tend to show that selection of this kind, though it may occur, is exceptional. In any case, as a general explanation of secondary sexual characters, sexual selection is not at the present time very widely accepted as adequate. Protective and aggressive Resemblance and Mimicry.— One of the most important of the phenomena which are well ex- plained by the theory of natural selection and which may, therefore, well be taken as affording evidence in favour of that theory, are the phenomena of protective resemblance, warning characters and of mimicry. In innumerable cases among all classes of animals there are found instances of a resemblance between the animal and its ordinary natural surroundings, which has the effect of rendering it inconspicuous and unlikely to attract the observation of an enemy, or of its prey. Such a resemblance is brought about sometimes merely by colour, very often by the arrangement of the colour in a pattern, this being frequently accompanied by modifications of shape, including sometimes the development of special excrescences or appendages. In some cases of protective resemblance the colour, and even the markings, change with a change of the surroundings. For details of such cases reference must be made to special works. Many Insects present elaborate markings which give them a close resemblance to a tuft of lichen or moss, a twig, a leaf, or other object, and resemblances of an equally striking character occur in other classes. Some animals, more especially certain Insects, are protected by their nauseous character against being devoured by animals that would otherwise prey upon them ; but often, no doubt, such nauseous Insects are attacked and killed before their unpalatable 658 ZOOLOGY SECT. character is detected. It is thus manifestly of advantage to such animals that they should be readily recognisable, and should thus be passed over: and in many such cases the coloration is bright and conspicuous, or the animal is rendered conspicuous by other means (warning characters). By mimicry is meant a superficial resemblance borne by one animal to a member of a different group. The best-known examples of mimicry occur among the Insects. It is manifestly of advantage to a Butterfly belonging to a group which is not nauseous to be readily mistaken for a nauseous form with conspi- cuous warning colours and markings, and this appears to be the explanation of many cases of mimicry. Similarly, a variety of flower-frequenting Dipterous Insects which have no sting or other weapon, bear a remarkable resemblance to Bees or Wasps, belonging to a distinct order (the Hymenoptera) — the resemblances embracing, not only shape, colour, markings, and development of " hairs " on certain parts, but the movements of the wings and other parts and the humming sounds emitted, so that, on a superficial inspection, the mimicry appears complete. Heredity. — The various characteristics of a plant or animal are transmitted to the succeeding generation. In the highest groups of animals this transmission is effected only through the intermediation of the sexual cells — ova and sperms — since they alone are capable of giving rise to a new generation. But in lower organisms .the faculty of reproduction is more widely diffused among the component parts ; in some lower multi-. cellular plants each and every cell is capable of taking on the function of reproduction and giving rise to progeny similar in all respects to the parent ; in other words, every cell in such cases must contain germinal substance. In other, somewhat higher, forms the germinal substance, though still widely diffused, may not be present, or capable of becoming active, in all parts, and may be confined to the cells of one or other of the layers. In the vegetable kingdom, even amongst the highest forms, the germinal substance can be shown to be widely diffused throughout the plant. Thus in many flowering plants, if we cut a shoot into lengths, the pieces are all capable of giving rise under suitable treatment to complete plants with flowers containing reproductive cells; and in many cases a leaf, or a portion of, one, is capable of a similar development. In many animals a similar wide distribution of the germinal material may be shown to prevail. This appears most strikingly in forms that multiply by budding. In Hydra, for example, any part of the body seems capable of giving off buds, and in the buds, after they have become separate, ova and sperms are developed from the cells of the ectoderm. A similar phenomenon is to be observed in other Ccelenterates and in the Polyzoa and the Composite Ascidians, and also in certain cases xv THE PHILOSOPHY OF ZOOLOGY 059 among the Platyhelrninthes and Annulata. In all these, and in other cases that might be mentioned, the germinal substance is not confined to the reproductive cells — new reproductive cells being capable of being formed from the substance of the cells of various tissue-layers. The phenomena of regeneration are important in connection with this question of the site of the germinal substance. Many members, not only of the lowest phyla, but of the Echinodermata, the Annulata, the Arthropoda, the Mollusca and the Chordata, are able, as has been repeatedly mentioned, to replace by a process resembling budding, parts that have been broken off: some of the cells of the adult body must, therefore, in these cases retain in a certain degree the faculty of reproduction, and must contain germinal substance. The germinal substance concerned in regene- ration, must, it is of importance to note, be capable of being stimulated into activity in a certain definite direction by an influence brought to bear upon it from without. In the Vertebrata the power of regeneration, if we leave out of account the various epidermal structures, is exceptional ; and where it occurs (most Amphibia, some Reptiles) it is confined to the limbs, jaws, lens of the eye, or the tail. In the highest Vertebrates there is no power of regenerating such parts when lost, and the capacity for reproduction is confined to the sexual cells. A remarkable persistency characterises these reproductive cells. By their means there are handed down from one generation to another, with little alteration, all the characteristics of the species of plant or animal. This special faculty of the reproductive cells is the faculty of heredity. Heredity does not imply absolute fixedness of all the character- istics inherited by one generation from its predecessor. On the contrary, as already pointed out, variations are constantly present- ing themselves. Some of the variations which animals exhibit are a direct result of the action of surrounding conditions, or of the use or disuse of parts, on the fully developed animal ; we can in some cases actually cause the animal to change to a more or less marked extent by placing it under different conditions. Another set of variations produced by the action of external influences, on the organism only appears if the action takes place in the course of development at one stage or another between the oosperm and the adult. Of the occurrence of both these forms of variation we have direct and positive evidence. It is a familiar fact that increased exercise of a part tends to an increase in the bulk of its muscles. The colours and markings of certain Fishes can be altered at will (of course within certain limits) by changing the material on the bottom of the aquarium in which they are confined; the colours of many Caterpillars may be altered by 660 ZOOLOGY SECT. changing the colour of their surroundings. A third set of varia- tions probably also occur, though direct evidence is wanting— namely, variations which may arise within the sexual cells before the union of ovum and sperm, or which may result from that union. The former two sets of variations are generally spoken of as " acquired characters " — new characters acquired during the lifetime of the individual — but their nature would be more clearly indicated by terming them extrinsic variations, as contrasted with the intrinsic variations forming the last group. The extrinsic variations being brought about by the action of external conditions, their causes are very various. In every such case the organism responds to some persistent external influence by undergoing some more or less persistent change. Mutilations, the rapid mechanical removal or destruction of parts, are here, by the terms of the above definition, excluded from the class of variations altogether, since, though the change involved is frequently permanent, it is effected by an influence which is temporary in its character. This, as will be seen, is of importance in connection with the next question we have to deal with — the inheritance of acquired characters. Can acquired characters or extrinsic variations be transmitted by inheritance ? That they can be is of the essence of Lamarck's doctrine of development, which, in fact, may be described as, a theory of development by means of the inheritance of extrinsic variations, or, as it is sometimes called, use-inheritance. But the maintenance of the view that extrinsic variations may be transmitted is not inconsistent with the acceptance of natural selection as a true cause of evolution. Evolution might be supposed to be due to the selection and inheritance of both intrinsic and extrinsic variations. From the nature of the case, evidence in favour of the inheritance of extrinsic variations on the one hand, and the occurrence of intrinsic on the other, is extremely difficult to obtain. One or the other must occur, or there would be no evolution. But to prove in any given case that a change is due to the one factor rather than to the other, is extremely difficult. When a character not present in the parents appears in the offspring, there is, to begin with, great difficulty in proving that it is really new : characters not present in the parents are known to be frequently inherited from a more or less remote ancestor. But, if we suppose it to be established that the character is a new one (and absolutely new characters must appear, or we should have no evolution), then it would require a very accurate knowledge of all the circumstances to enable us to be certain whether the appearance of the character is not due to the action of some external influence on the parent, either during development or in the adult state, rather than to a change arising within the reproductive cells. Instances are frequently xv THE PHILOSOPHY OF ZOOLOGY 661 brought forward which have been supposed to afford evidence of the transmission of mutilations from parent to offspring ; but such a transmission must, from the nature of the case, always be extremely difficult to prove, and the majority, at least, of such cases are found, on a careful analysis, to be capable of other inter- pretations. On the other hand, though well-established cases of the inheritance of mutilations would greatly support the doctrine that acquired characters are transmissible, the negative results that have attended certain experiments on mutilations are of little value in the direction of proving that extrinsic variations cannot be transmitted, since, as has already been pointed out, such experiments in mutilation cannot be said to reproduce the con- ditions under which an extrinsic variation might be supposed to be transmitted ; the mutilation is instantaneous ; the variation must be supposed to be the result of long-continued action, which it might be expected, would have a sufficiently profound effect to engraft it permanently on the organism. It should be pointed out here that there is no absolutely hard and fast line to be drawn between the intrinsic and extrinsic variations, since changes in the sexual cells may very well be due, directly or indirectly, to influences exerted from without. The material from which reproductive cells may subsequently be fashioned is, in plants and in many animals, in such close and intimate union — so far as can be seen — with the other proto- plasmic elements of the organism, that it seems highly probable that influences affecting the latter may in many cases affect also the former. Another question that presents itself in connection with heredity is : Can any special part of the germ-cell be fixed upon as the part specially concerned in hereditary transmission ? Is it by means of the nucleus alone that transmission takes place, or does the cytoplasm take a share in the process ? The complicated changes which the nucleus undergoes during mitotic division (Vol. I. p. 16), with the definite form and (for each species) constant number of the chromosomes, and their precise halving during the process, tell strongly in favour of the view that the nucleus is the vehicle of transmission rather than the, apparently, less highly differentiated cytoplasm. But such evidence is far from amounting to positive proof. To determine this point many series of ernbryological experi- ments have been carried out. Should it prove possible to fertilise by means of a sperm an ovum from which the nucleus had previously been removed, and as a result to obtain an embryo, it might be possible, by a process of exclusion, to get some light on the influence exerted on normal development by the nucleus of the ovum. For such experiments sperms and ova of the same species are not well adapted, since the differences between the individuals 662 ZOOLOGY SECT. of a species, especially in the early stages, are not of a sufficiently strongly -marked character to permit of any definite conclusions being arrived at regarding inheritance from one parent as against the other. Eecourse has, therefore, been had to crossing between distinct species or distinct genera, or even between the members of distinct classes. In such experiments the Echinoderms have proved capable of affording the most convenient material, and have been very largely made use of. It has been found to facilitate suc- cessful crossing between distinct kinds of Echinoderms if the ova experimented with are first treated by certain methods which have been found to prepare the way for the process of artificial parthenogenesis briefly referred to in the Intro- duction (Vol. I., p. 21). The ova so treated are then shaken violently in a tube, with sea-water, until they become to some extent broken up. From the fragments such large pieces as are found not to contain nuclei are picked out, and the sperms of the second kind of Echinoderm are added to the water in which these non-nucleated fragments are contained. In many cases it is found as a result that a sperm enters a non-nucleated fragment and an embryo becomes formed. It now has to be determined how far this embryo resembles and differs from the embryos of the -two species from which the ovum and the sperm respectively have been derived. This is always a matter attended with a considerable amount of difficulty, since such embryos can rarely be reared beyond the gastrula stage. But evidence seems to have been obtained by means of this method that the nucleus has not a monopoly in the transmission of the parental characters, since in such experiments maternal features do appear in the embryo, and these, in the absence of a female nucleus, must have been transmitted by the agency of the cytoplasm. Evidence tending in the same direction has been obtained as a result of experiments on the ova of Ctenophora (Vol. I., p. 217). If, before these are fertilised, a definite area of the cytoplasm be removed without the nucleus being interfered with in any way, the embryo which develops after fertilisation presents deficiencies in the organs of definite areas corresponding to the parts which have been removed. It has been urged in connection with the question of heredity, that what is transmitted from generation to generation is not so much matter as energy. The quantity of matter is always relatively small ; the important fact appears to be that this relatively small particle carries with it potential energy sufficient to effect the structural changes which precede the beginning of the process of assimilation, and to at least initiate that process. But we can hardly imagine a succession of complicated and very definite changes of structure, such as are involved in the develop- ment of an animal, taking place unless the germinal matter or xv THE PHILOSOPHY OF ZOOLOGY 663 germ-plasm in which they originate has a correspondingly com- plicated and definite structure. The oosperm, having the faculty of reproducing the entire animal without (in many cases) any further influence emanating from the parent, must contain within itself something to represent each of the parts — even each group of cells — of the adult body. The oosperm of a Frog, for example (p. 288), simple though its structure appears to be, must contain potentially within itself all the characteristics of the adult animal, and not only these, but the characteristics of each successive stage in the formation of the tadpole and its metamorphosis into the adult Frog. Attempts have been made to explain how it is that the reproductive cells acquire this reproductive capacity. One of the most interesting of these is a theory which is termed pangenesis, the origination of which is due to Darwin. According to this theory, the cells of the various parts of the body throw off minute ultra-microscopic particles or " gemmules," and these find their way by various channels to the developing reproductive cells, in which they accumulate until each reproductive cell contains gemmules repre- senting all parts of the body. When development takes place each gemmule develops into the part corresponding to that from which it has been derived. If this theory afforded a true explanation of the facts of repro- duction, there would necessarily be accumulated in the ovum gemmules representing, not only every part of the body of the adult, but also every stage in the development of the embryo, and (since we see ancient ancestral characters occasionally reverted to) something to represent the special peculiarities of former generations. Now, it is a question if such an accumulation of gemmules, each necessarily several times the size of a chemical molecule, would not form a mass very much larger than an ovum. Such a doctrine would, moreover, hardly appear to be necessary in order to explain the facts. The accumulation in the ovurn of the hereditary tendencies (as we may call them) may only in part take place during the life-time of the individual : a good part of them — all, perhaps, except such as have been more recently acquired — might be contained in the ready-formed germinal material handed down from previous generations. Against a hypothesis of pangenesis such as was formulated by Darwin, the mode of reproduction of many plants tells more strongly perhaps than any of the facts derived from the animal kingdom. Many of the higher flowering plants, for example, are capable of being propagated by means of a cutting of the stem or root, or even by a leaf. As the new plant developed from the cutting gives rise to flower and fruit, the cutting must contain germinal matter ; and germinal matter must, therefore, be diffused throughout the cells of such a plant. Pangenesis, unmodified, 664 ZOOLOGY SECT. would require that in such a case a large proportion of the ordinary cells of the plant should receive gemmules derived from all parts. It is a moot point whether it is possible that any influence (such as is pre-supposed in pangenesis) can pass from the cells of the various parts of the body to the ova — whether there can be any communication of substance carrying with it tendencies to be transmitted to the next generation. It is certain, however, that an influence of a centrifugal character is exerted by the sexual cells. The absence of ovaries or testes has, in many cases, a marked effect on certain of the characters — an effect on the development and form of certain of the parts. This is seen not only in higher animals (Mammals and Birds), but also among some lower forms. In certain Crabs, for example, the presence of Sacculina, a parasitic Rhizocephalaii nourished at the expense of the testes, which become destroyed, produces a very marked alteration in some of the external features. But, while this is the case, an influence exerted in the opposite direction — an influence transmitted from the other parts to the germ-cells, has not been proved, and from the nature of the case perhaps cannot be directly proved. Such an influence, it is hardly necessary to add, must be pre-supposed if we assent to the doctrine of the in- heritance of acquired characters. On this problem certain experiments which have been made on the transplantation of ovaries promise to throw light. It has been found possible to remove the ovaries in early stages from in- dividuals of one variety of animal and to transplant them into individuals of another variety without impairing the reproductive functions. This has been done most successfully in the case of certain breeds of Domestic Fowls. The ovaries of young hen-chicks of two distinct breeds have been interchanged, and the effects on the offspring investigated. The results which have been obtained so far seem to place it beyond doubt that the eggs from the trans- planted ovary do not develop exactly as they would had the latter remained till maturity in the body of a Fowl of the variety from which the ovary was originally taken : the progeny show unmistakable traces of an influence exerted on them by the individual, of a distinct variety, in which they were brought to maturity : they present some of the features of their " mother by transplantation." This seems to prove the possibility of the trans- mission from the body or soma to the germ-cells, during their growth, of influences of a definite character destined to bring about definite effects on the offspring ; and this department of research in experimental embryology would appear to promise to show that there actually exists a mechanism such as the trans- mission of acquired characters in normal heredity seems to demand. xv THE PHILOSOPHY OF ZOOLOGY 665 Rules or Laws of Heredity. — As in the case of the experi- ments referred to on p. 661, by which it has been sought to determine the respective values of nucleus and cytoplasm as bearers of the hereditary qualities, so also in the majority of the experiments designed to determine the laws by which the in- heritance of paternal and maternal characters is regulated, it has been necessary to have recourse, not to fertilisation between individuals of the same variety, but to crossing between distinct varieties or between distinct species with the production of bastards or mongrels (variety- or species-crosses), since it is only in cases in which the differences between the maternal and paternal characters are strongly marked that it is possible to trace these characters in the descendants. It is found that in such experiments in crossing between varieties the result may take one or other of the three following courses. — In the first place the progeny may be intermediate in character between the two parents : the paternal and maternal characters, that is to say, may be mixed or blended without any tendency to the predominance of either : a cross between a black and a white variety, for example, in such a form of hybridisation becomes a grey. In a second set of cases, which are the rarest of all, the offspring exhibit paternal characters in one part or set of parts and maternal in another : thus a cross between a black variety and a white yields a piebald or a mottled black-and-white. This is the so-called mosaic form of bastard-inheritance. Lastly there are certain cases in which there is no blending and no mosaic formation, but the paternal and maternal characters remain separate, and appear, or fail to appear, in the progeny in a certain regular order. This is the alternating or Mendelian form of bastard-inheritance. It is with this last form of bastard-inheritance that the theory of Mendel, a product of the middle of the last century, but only comparatively recently re-discovered and given due recognition, is concerned. Mendel's original experiments were mainly with varieties of garden-peas. When he crossed two varieties he found that the offspring all presented the characters of one of the parent forms. Thus in the case of a cross between a tall and a short variety — whether the short was fertilised by the pollen of the tall or the tall fertilised by the pollen of. the short — the progeny were tall : in a cross between yellow-seeded and white-seeded varieties, the products were all yellow-seeded. Of the two opposed characters, that which reappears in the offspring is known as the dominant, that which does not appear as the recessive character : the varieties presenting these characters are accordingly spoken of respectively as either dominant or recessive. When the hybrids, all showing the dominant character, produce a second generation by self-fertilisation, there is a re-appearance of the recessive character 606 ZOOLOGY SECT. in a certain fixed proportion of the progeny. In this second generation one quarter are pure recessives, one quarter pure dominants, and the rest of mixed character. The pure recessives in following (inbred) generations always remain true to the recessive character — the dominant having evidently become eliminated from their constitution : and the same holds good, mutatis mutandis, for the dominants. The intermediate forms present the dominant character, but when inbred they behave exactly like the original hybrid, that is to say, the progeny consist of pure dominants (a quarter) pure recessives (a quarter) and mixed or intermediates (a half). In future generations these proportions are regularly maintained. Such is the nature of the fundamental experiments of the Mendelian system. For further developments, the details of cases among animals, of cases of the crossing of varieties differing from one another in more than one character, of cases in which a blend- ing of the parental characters appears in a proportion of the progeny, &c., as well as the practical applications in stock-raising and horticulture, reference may be made to the works mentioned in the Appendix. Orthogenesis. — The fact that, as shown by the evidence afforded by both existing and extinct forms, organisms vary in such a way as to follow definite lines leading to special adaptations (and some- times to excessive development of parts), is held not to be explainable by a theory of selection of slight variations the direction of which is under no known guidance. This defmiteness in the direction of evolution — orthogenesis — is a fact in nature of which there is evidence on all sides. In nearly all groups and nearly all systems of organs there is evidence of progressive de- velopment such as cannot be supposed to be due simply to the fortuitious appearance and selection of the Darwinian variations. A nervous system, simple, diffused and superficial in the lower members of a group, becomes more highly elaborated, more complex, more deeply placed, in the higher. Light-perceiving organs, mere groups of pigmented elements, and nerve-cells with refractive bodies, in the lower forms, become represented in the higher by complex eyes with elaborate mechanism for the reception of images of external objects. Appendages undergo progressive modification, so that each pair becomes specially adapted for the performance of particular functions. Moreover, parallel and evidently independent lines of orthogenetic development are in many cases traceable in separate groups. As examples may be mentioned the series of stages in the development of .complex eyes from simple rudiments, which are observable in the Annulata, the Arthropoda and the Chordata : the parallel and quite unconnected stages in the reduc- tion of the digits, leading to the greater perfection of the limbs as running organs, to be traced in the Perissodactyle and Artiodactyle xv THE PHILOSOPHY OF ZOOLOGY 667 series of the Ungulata. In many cases such orthogenetic develop- ment has led to excessive growth of parts — growth beyond the requirements of the organism — an excess which has sometimes, apparently, led to extinction. Since natural selection has been judged by many to be inade- quate to account for such straightforward progress in organisms and organs, various other theories of orthogenesis have been put forward. Such of these as merely postulate the existence in organisms of a principle or tendency to develop towards a more perfect condition, fail to reach the standard of admissible scientific theories. Such a supposed tendency is not merely analogous to the tendency of the young organism to grow into the adult form — a phenomenon sufficiently difficult to account for by any nexus of causes and effects known to us : it must be something much more, since it must be, not merely a tendency to repeat what has been received in inheritance, but must be in a highly important degree prophetic — must, in fact, be a tendency to develop to a point beyond that to which the hereditary impulse reaches. Other theories of orthogenesis rely upon the action of the environment for bringing about the results observed : these have to encounter the same fundamental difficulty as the Lamarckian theory itself — the difficulty of explaining how changes in the parents due to the effects of the environment can be impressed on the germ-cells in such a way as to become transmitted to the next generation. But many of those who concern themselves with the study of evolution at the present day, while admitting the absence of any satisfactory theory of the mode of transmission to the germ-cells of changes of organisation in the adult of the nature of acquired characters, are yet inclined to the view that such a transmission does occur, and that without it it is quite impossible to account for the definite adaptive developments that have taken place, especially since there seems to be no convincing explanation, apart from such transmission, of the mode of origin of the first beginnings of structures destined, when further developed, to be of vital im- portance to the organism, but in their early stages not of sufficient value to be capable of determining its survival or extinction.1 1 For an ingenious theory of the causes of the initiation and early stages of changes of organisation, see the account of Weismann's theory of germinal *t /erf ion in his work on the Evolution Theory (1904), and his article in Darwin and Modern Science (see Appendix). SECTION XVI THE HISTORY OF ZOOLOGY ZOOLOGY, like other branches of Natural Science, has had two lines of progress, observation and generalisation. Without accurate and detailed knowledge of the facts and phenomena of animal life and structure, all theories of classification or of origin are so much idle speculation : in the absence of the philo- sophic spirit suggesting hypotheses of greater or less magnitude, the mere accumulation of facts is an empirical and barren study. Zoology as a science, therefore, can hardly be said to have existed until a sufficient proportion of the facts relating to animals had been observed and recorded accurately and systematically, and until some attempt had been made to classify these facts and to arrange animals into larger and smaller groups according to some definite plan. This being the case, it may be said that the common knowledge of animals possessed by mankind in all ages, and constantly being developed and extended by lovers of external nature and by anatomists working from the medical standpoint, first became scientific and evolved itself into a system some 200 years ago, when John Ray, an English clergyman, first grasped the idea of species, and published the earliest classification of animals founded upon anatomical characters. Although soon overshadowed by the greater genius of Linnaeus, Ray may safely be called the father of modern zoological science, the only serious precursor of his Synopsis methodica animalium, published in 1693, being the voluminous De differentiis animalium of Edward Wotton, printed nearly 150 years earlier. But although Zoology, as a science, was practically non-existent up to the period referred to, much valuable knowledge of animals had been accumulated, and was, as it were, merely waiting to be systematised. As in other branches of knowledge, the first steps were taken by the Greeks, and, in philosophical grasp, the zoological writings of Aristotle (384-322 B.C.) are far in advance of those of all other students of the subjects up to the times of Wotton and Ray. His treatises, especially The History of Animals, The Generation of Animals, and The Parts of Animals, contain an xvi THE HISTORY OF ZOOLOGY 669 immense body of facts, many of them singularly accurate, others as curiously incorrect, a circumstance which no one will wonder at who, with all modern resources at his elbow, has tried to break fresh ground in any department of Zoology. Although he propounds no definite system of classification, he clearly recognises many of the more important animal groups, or, as he calls them, " genera." Vertebrata, for instance, are spoken of as animals with blood (evatpa) and Invertebrates as animals without blood (avai^a\ colourless blood not being recognised as such. Among animals with blood are included Viviparous Quadrupeds (Mam- mals), Birds, Oviparous Quadrupeds (Reptiles and Amphibians), Cetaceans, and Fishes: among bloodless forms, Malakia or soft animals (Cephalopods), Malacostraca or soft animals with shells (the higher Crustaceans), Entoma (Insects, Arachnids, Myriapods, and the higher Worms), and Ostracodermata or shelled animals (Echinoderms, Cirripedes, Pelecypods, Gastropods, and Tunicates). Starfishes, Medusae and Sponges are also referred to. In the then existing state of knowledge it was impossible that even so profound a philosopher as Aristotle could erect a science of Zoology. No standard of nomenclature was established ; there was no clear idea of what constitutes a species : in matters of structure, no distinction was drawn between nerves and tendons : in physiology the vessels and tendons were looked upon as the organs of movement, the muscles being considered as mere packing. Obviously, anything like real progress was barred by ignorance of animal structure and function, and it was absolutely necessary that exact anatomical knowledge should precede anything ap- proaching to successful generalisation. It is, therefore, hardly to be wondered at that, up to the time of Ray, scientific Zoology owes more to those anatomists and physiologists whose main object was to advance the study of Medicine, than to the naturalists in the ordinary sense of the word. With the exception of the works of Galen (born A.D. 130), which contain numerous observations on the anatomy of Mammals, anatomy, as well as Zoology in the broad sense, was practically at a standstill from the time of Aristotle to the sixteenth century, when Vesalius, by his observations, chiefly on the human subject, raised anatomy to a degree of accuracy hitherto undreamt of; and Goiter, Bellonius, and Fabricius ab Aquapendente resumed the study of comparative anatomy, dormant since Aristotle. Somewhat later — in 1645 — Severino published his Zootomia democret&a, the first book devoted exclusively to the general subject of comparative anatomy. During the same period the general knowledge of animals was increasing, and a distinct epoch is marked by the learned, and, for the time, exhaustive Historia animalium of Conrad Gesner, published in 1551-58, and consisting of 4,500 folio pages, with VOL. II T T 070 ZOOLOGY SECT numerous illustrations, some of them of considerable merit, some wonderfully inaccurate ; some depicting various fabulous monsters, such as Winged Dragons, many-headed Hydras, and crowned Basilisks, the existence of which was not yet thoroughly dis- credited. The work is, however, rather an encyclopedia than the exposition of a science : it contains no general ideas ; there is still no conception of the subordination of groups, and no exact naming either of animals as a whole or of their various parts. Five chief groups of animals are recognised : Viviparous Quadrupeds, Oviparous Quadrupeds, Birds, Aquatic Animals, and Serpents. Within these divisions the various animals are described without any attempt at grouping. Among Aquatic Animals, for instance, Fishes, Amphibia, Cetacei, Molluscs, Crustacea, Echinodermata, and Sea-serpents are included. • In the seventeenth century great strides were made both in knowledge of structure and function, in generalisation, and in methods of investigation. Especially famous and fruitful — indeed one of the greatest scientific events of all time — was the discovery of the circulation of the blood, made by William Harvey in 1616, and announced in 1628 in a small pamphlet Exercitatio anatomica de Motu Cordis et Sanguinis. He demonstrated fully, partly by dissections, partly by experiments on living animals, the action of the heart as a pumping mechanism, the nature of its valves and of those of the veins, the presence of blood — not air, as was then supposed — in the arteries, the cause of the pulse, and the whole course of the circulation so far as it could be known previous to the discovery of the microscopic capillaries. Of hardly less importance is Harvey's embryological work : he made extended observations on the development of the Chick and in his Bxercita- tioms de Generatione Animalium (1657), 'declared that all living things arise from a primordium, or ovum, and propounded the doctrine of epiyenesis, according to which development is a process of gradual differentiation of the primordium, whereby " out of the inorganic arises the organic, out of the similar the dissimilar." The primordium itself he considered might " proceed from parents, or arise spontaneously, or out of putrefaction." Harvey worked with no optical aid beyond a simple lens, and it is not surprising that his results are incomplete and often in- accurate. His successors had the advantage of the compound microscope, invented by Hans and Zacharias Janssen about 1590-1600, and sufficiently improved during the course of the seventeenth century to be an important engine of research in the hands of the earliest microscopists, Malpighi in Italy, Leeuwen- hoek and Swammerdam in Holland, Robert Hook and Nehe- miah Grew in England. Malpighi made numerous histological discoveries, with some of which — such as the Malpighian capsules of the kidneys and the Malpighian vessels of Insects, his name is xvi THE HISTORY OF ZOOLOGY 671 still associated. He was also the first to study the development of the Chick under the microscope, and was one of the earliest- supporters of the theory Q£ pre-formation, l according to which all the parts and organs of the adult are present in the germ, so that there is no differentiation, but only an unfolding. Leeuwenhoek discovered blood-corpuscles, striated muscle-fibres, dentinal canals, arid epiderm-cells, observed the circulation of the blood in the tadpole's tail, and described many of the lesser forms of life, such as Infusoria, Rotifers, and Hydra. Swammerdam investigated the anatomy of Insects and Molluscs, and the metamorphosis of Insects, and described the three " sexes " of Bees. The researches of Hook and Grew were mainly botanical ; both they and Malpighi discovered in the tissues of plants little spaces with firm walls and full of fluid ; these they called cells, thus taking the first step in the structural analysis of the higher organisms. Another discovery of fundamental importance was made in 1677 when Louis de Hamen observed and described the sperms of animals. These were at first thought to be the young, which only required to be nourished in the egg to grow into the embryo or foetus, and were therefore considered to disprove the theory of the ovulists — such as Harvey, who made the egg the origin of the new generation — in favour of that of the spermatists, who believed the whole material to be furnished by the male parent. Belonging also to this period are Redi's experiments on genera- tion, in which he began the work of establishing the doctrine of biogenesis, according to which organisms originate only from pre- existing organisms, and of demolishing that of abiogenesis, or " spontaneous generation," which, maintained from the time of Aristotle onwards, held that Flies, Lice, Worms, and other animals were directly generated in mud, putrefying flesh, dung, &c., having, therefore, no living progenitors. Redi's contribution to this question lay in proving, for the first time, that the maggots, " bred " in putrefying meat, were the products of eggs laid thereon by Flies. Thus the seventeenth century saw a great advance in the knowledge of animal structure and function, and the way was paved towards a rational classification. As we have already seen, Ray, towards the end of the century, gave zoology as a whole a scientific form ; he first grasped the ideas of species and of specific characters, acknowledged anatomy as the basis of classification, and introduced a greatly increased precision in the definition of species and other groups, and in terminology. He had, however, no clear idea of genera, his genera being rather what we now call orders or families, and he showed an undue conservatism in 1 Often known as the theory of evolution. As, however, the latter word is now universally used in a different sense, it is advisable to drop it in this connection, and to employ the synonym pre-formation. T T 2 672 ZOOLOGY SECT. retaining, as far as possible, the groups of Aristotle. His general classification of animals is as follows : I. Animals with (red) blood [ Vertebrata]. 1. Respiration pulmonary. A. Heart with two ventricles. (a) Viviparous. i. Aquatic [Cetacta]. ii. Terrestrial [other Mammalia]. (/>). Oviparous [Birds]. B. Heart with one ventricle. Viviparous Quadrupeds and Serpents, [i.e., Eeptilia and Amphibia]. 2. Respiration branchial [Fishes']. II. Animals without (red) blood [Invertebrata]. 1. Majora. A. Mollia [Cephalopoda]. B. Crustacea. C. Testacea [Gastropoda and Pdecypoda]. 2. Minora. Insecta [Insecta, Arachnida, Myriapoda, and Vermes]. It will be noticed, that while the classification of Vertebrates is fairly natural, being founded upon the rock of anatomy, the arrangement of Invertebrates is no advance upon that of Aristotle : the two main divisions depend upon mere size ; and Crustacea, separated from the rest of the Arthropoda, are interposed between Cephalopods and the remaining Mollusca. In association with Ray must be mentioned his friend and fellow-worker Francis Willughby, who made extensive contributions to Zoology. The eighteenth century saw the imperfect efforts of Ray developed, and in some respects perfected, by Carl Linne, or Linnaeus, universally recognised as the founder of modern systematic Zoology — or more accurately Biology, since his reforms equally affected Botany. Born in Sweden in 1707, two years after Ray's death, he published the first edition of his S^stema Natures y in 1735, as a small pamphlet. The twelfth edition (1766- 68) was in three volumes, and was the last to receive the author's corrections, but from materials left at his death in 1778 an authoritative (thirteenth) edition in ten volumes was prepared by J. F. Gmelin. It was LinnaBus who first recognised the value of groups higher than species — genera, orders, classes, &c., and employed them in a definite and uniform way, with due subordination of one to the other; it was he who invented linomial nomenclature, the advantage of which in promoting precision in systematic work it is impossible to over-estimate. He gave each species xvi THE HISTORY OF ZOOLOGY 673 a brief diagnosis in Latin, so that any naturalist versed in his system could recognise whether an animal or plant which came under his notice was already described or not. In this way he, as it were, pigeon-holed the facts of Biology, and so made the deter- mination of the proper place of any new fact a comparatively simple matter. By universal consent, the Sy sterna Natures is taken as a starting-point by systematists. It is customary to place after the name of a species the initial or abbreviated name of the writer by whom the species was first distinguished and named. For instance, the Bass, a common British Teleost, was named Perca labrax by Linnaeus. In 1828, Cuvier and Valenciennes, in their great work on Fishes, recognised that it was generically distinct from the Perch, and, retaining the generic name Perca for the latter, called the Bass Labrax lupus. In 1860, further investigations into the Perch family necessitated placing it in the genus Morone, and, according to the law of priority, the specific name lupus gives place to labrax, the latter having been applied by Linnaeus. The Bass is therefore correctly called Morone labrax, Linn., the more usual name, Labrax lupus, Cuv. and Val., becoming a synonym. In deciding all such questions of priority, the tenth edition (1758) of the Sy sterna Naturce is taken as a starting point: all species distinguished by Linnaeus, and not subsequently split up into two or more species, are dis- tinguished by the abbreviation L. or Linn. For instance, Canis familiaris Linn, is the Domestic Dog, Passer domeshcus Linn. the House Sparrow : and names given by the older naturalists are neglected unless endorsed by Linnaeus. In many respects the system of Linnaeus was eminently artificial ; he relied too much on single characters in classification, and did not take the totality of structure into sufficient consideration. He divided the animal kingdom into the following six classes : — 1. Mammalia. 2. Aves. 3. Amphibia [including Reptilia and Amphibia']. 4. Pisces. 5. Insecta [including all the Arthropoda]. 6. Vermes [including Mollusca, Worms, Echinoderms, Cwlen- terata, and Protozoa']. It will be seen that all the classes are of natural groups, with the exception of the last, but that they are far from being of even approximately equal value. The first four are what we still call classes, but there is no attempt to unite them into a single group of higher order ; and in this respect the classification of Linnaeus falls behind that of Ray, who recognised the phylum Vrertebrata under the name of animals with blood. The fifth class, on the other hand — that of Insecta — is the equivalent of an entire phylum, 674 ZOOLOGY SKCT. while under the head of Vermes are included all the phyla re- cognised at the present day, except Chordata and Arthropoda. Other naturalists of the eighteenth century must be briefly referred to. Bonnet introduced the idea of a " scale of beings " (tchelle des tires), conceiving the true classification to be a linear one, passing in a single series from the lowest to the highest forms. This conception was opposed by Pallas, who introduced the true conception of representing the relationships of the various groups under the form of a much-branched tree. Spal- lanzani made numerous investigations on reproduction, and, together with Bonnet, Buffon, and Haller, strongly supported the doctrine of pre-formation already referred to. Haller summed up the position by stating emphatically that there was no such thing as development or differentiation, no part of the body being made before another, but all parts simultaneously created. It followed, as a natural corollary from this view, that the germ destined to give rise to an animal — i.e., the ovum according to the ovulists, the sperm according to the spermatists— contained within itself the germ of the next generation, that of the next, and so on, ad infinitum, so that the first created male or female of each species contained within its sperms or ova the germs of all future generations, enclosed one within the other, like a nest of Chinese boxes. Buffon, as the result of numerous experiments, came to the conclusion that the ovary secretes a seminal fluid containing moving particles analogous to sperms, and, from this erroneous observation, framed a theory which is an interesting anticipation of Darwin's Pangenesis (p. 663) — namely, that organic particles, derived from all parts of the body, occur in the seminal fluids of the two sexes, and that the union of these in the uterus " determines them to arrange themselves as they were in the individuals which furnished them." The theory of pre-formation (as then understood) was practically demolished, and that of epigenesis, or new formation, established on a firm basis, by Caspar Friedreich Wolff, who, at the age of twenty-six — in 1759 — gave the most, accurate account of the development of the Chick hitherto known, and showed clearly that there was no pre-formation of the various parts, but a gradual differentiation from a layer of organised particles, or, as we should now say, from a cellular blastoderm. Another great eighteenth century name is that of John Hunter, the most profound comparative anatomist and physiologist of his time. He was not a zoologist in the narrow sense of classifier, but his exquisite investigations on the various systems of organs and their functions throughout the animal kingdom furnished the science with a foundation of wide and exact knowledge which was of far more importance than the most cunningly devised system of classification. Important anatomical investigations were also xvi THE HISTORY OF ZOOLOGY 675 made during this period by Vicq d'Azyr, who enunciated the principle of serial homology ; by Peter Camper, who investigated the pneumaticity of the bones of Birds, and was the first to apply exact methods of measurement to the human skull ; by Alexander Monro, who greatly advanced our knowledge of the anatomy of Fishes ; and by Poll, whose Tcstacea utriusque Sicilice is the most famous of the older works on Mollusca. And in the domain of out-door zoology — the study of the actual life of animals with but little regard to their structure or classification, or to the broader scientific questions connected with them — special mention must be made of Gilbert White, whose Natural History and Antiquities of Selborne is a classic both in science and letters. The latter part of the eighteenth century is also specially re- markable for the publication of the earliest scientific speculations on the origin of species. The idea of evolution is to be found in the works of more than one of the great Greek and Roman philosophers, such as Empedocles (495 — 415 B.C.) and Lucretius (99 — 55 B.C.); and the writings of some of the Fathers of the Church, such as Augustine (353 — 430) and Thomas Aquinas (1225 — 1274) seem to show that they had no objection to " derivative creation/' or evolution under direct Divine superin- tendence. But by about the middle of the sixteenth century, the idea of the immutability of specially created species had hardened into a dogma which it was unsafe to question ; and, this state of things continuing, the earliest of the great evolutionists, Buffon, felt himself obliged to qualify all his speculations with a declara- tion, sincere or ironical, of his belief that species were immutable. Linnaeus, reckoning all higher groups as subjective, contended for the real existence of species, saying " we recognise as many species as were originally created/' and this opinion was held by the vast majority of naturalists, not only of his own time, but up to within forty or fifty years of the present day. Buffon, born in the same year (1707) as Linnaeus, was, in his methods and ideas, the exact opposite of his great systematising contemporary. He wrote charming accounts of the external characters and habits of animals, but declined to classify them, on the ground that all arrangements of the kind were arbitrary, and that it was easier, more useful, and more agreeable to con- sider the lower animals in relation to ourselves. On this principle, he begins his Histoire naturelle with Man, then takes up the various domestic Mammals, and afterwards proceeds to consider the less familiar forms. But he was essentially a philosophical zoologist ; besides enunciating a theory of heredity, he grasped the idea of homology, endeavoured to explain the facts of geo- graphical distribution, and, in a tentative and guarded way, admitted the mutability of species, and advanced a hypothesis of their origin. His speculations refer mainly to the modification, 676 ZOOLOGY SECT. or, as he calls it, degeneration, of domestic animals, and he sums up his position as to the factors of the process by saying " the temperature of the climate, the quality of nutriment, and the ills of slavery, these are the three causes of change, of alteration, and of degeneration in animals." In other words, he supports the theory of the direct action of the environment. A bolder and more consistent evolutionist than Buffon was his contemporary, Erasmus Darwin (1731 — 1802), grandfather of the author of the Origin of Species. As a competent critic has said, " he was the first who proposed and consistently carried out a well-rounded theory with regard to the development of the living world." In his Zoonomia, published in 1794-6, after summarising the extraordinary adaptations to be seen in the animal kingdom, he asks, " Would it be too bold to imagine that all warm-blooded animals have arisen from one living filament [he was a spermatist] which the great First Cause endued with animality, with the power of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions, and associations ; and thus possessing the faculty of continuing to improve by its own inherent activity, and of delivering down those improvements by generation to its posterity, world without end ? " And a little later he inquires : " Shall we conjecture that one and the same kind of living filament is and has been the cause of all organic life ? " He anticipated Lamarck in the importance he attached to the principle of use and disuse, expressed his belief in the inheritance of acquired characters, and recognised the import- ance of sexual selection. The study of Zoology was also greatly advanced during the eighteenth century by the voyages of Cook, Bougainville, and others. New countries were explored, the peculiarities of their faunae recorded, and valuable data accumulated for the study of distribution. In this connection the names of Sir Joseph Banks, Solander, and the two Forsters — all attached to Cook's exped- tions, of Sparrmann, and of Sir Hans Sloane, may be specially mentioned. The last-named was one of the greatest of collectors, and the founder of the British Museum. The beginning of the nineteenth century was a period of great zoological activity, distinguished by the work of some of the most prominent leaders of the science. J. B. P. A. de Lamarck (1744-1829) was not only a distin- guished general zoologist and palaeontologist, but may also be looked upon as the chief of the pre-Darwinian evolutionists. In his Philosophic Zoologique, published in 1809, he completely rejected the idea of the fixity of species, and endeavoured to explain the trans- formation of one form into another by the operation of known causes; of these he attached most importance to the principle of use and disuse, and he was a firm believer in use-inheritance. He was a uniformitarian in Geology, believing that the history of the earth XVI THE HISTORY OF ZOOLOGY 677 and of its past inhabitants is to be explained by the action of the causes seen in operation to-day, and not by invoking great catastrophes or cataclysms by which changes of vast magnitude were suddenly produced. He considered, also, that the trans- formation of species took place by slow, orderly changes, Nature requiring only matter, space, and time in order to effect her various changes. He introduced the terms Vertebrata and Invertebrata, and, in the same year as Treviranus (1802), proposed the term Biology for the whole science of living things. Lamarck at first believed in a linear classification of animals, but afterwards adopted the earliest known branching or phylogenetic classification — a crude attempt, but interesting as being the first of its kind. It is as follows : — Worms [flat and round Worms] Infusoria Polypes [including Rotifers, Polyzoa, Actinozoa, Crinoids, and some In- fusoria] Racliaria [including Echinoderms and some Worms and Ctelenterates] Annelids [Annulata, &c.] Cirripedes Mollusca Fishes Reptiles Insects Arachnids Crustacea Amphibious Mammals [Sirenia and Pinnipedia] \ Ungulate Mammali Unguiculate Mammals [Edentata, Rodentia,'Marsupialia, Insectivora, Carnivora, hiroptera, and Primates]. 678 ZOOLOGY SECT. The hypothesis of evolution was also supported by Lamarck's contemporary, Etienne Geoffrey St. Hilaire, who denied use- inheritance and considered the direct action of the environment as the sole cause of transformation. He also differed from Lamarck in believing in the occurrence of sudden changes, e.g., in the possibility of the emergence of a fully formed Bird from a Reptile's egg. In systematic zoology he established the orders Mono- tremata and Marsupialia : the members of the latter group had hitherto been distributed among Rodents and Primates. Another keen supporter of evolution was the great poet Goethe (1739-1832), who also introduced the word Morphology, and made important contributions to the- department of science thus named. He propounded the vertebral theory of the skull, presently to be referred to (p. 680), recognised the importance of vestigial organs, and predicted the presence of a premaxilla in Man — the absence of that bone in the adult human skull being hitherto considered as distinctively separating the genus Homo from the other Primates. That the views of Lamarck and the other evolutionists produced so little effect upon contemporary science is largely due to the great and far-reaching influence of Georges Cuvier (1769-1832), one of the greatest of comparative anatomists, whose views dominated zoological science for half-a-century. He propounded the fruitful principle of correlation, according to which peculiarities in one part of the body are always associated Avith equally characteristic features in other parts — e.g., the ruminating stomach with cloven hoofs. He rejected the idea of a scale of being or unity of type, and, in his great work, the Rtyne Animal, abandoning the linear classification, divided animals into four Branches (em- branchements), each with its own plan of organisation and inde- pendent of the rest. This conception, though not absolutely correct, marked a great advance in classification, as the following table shows. Branch 1. VERTEBRATA. „ 2. MOLLUSCA [including Tunicata, Brachiopoda, and Cirripedia. as well as the true Mollusca]. „ 3. ARTICULATA [including Arthropoda and Annulata]. „ 4. RADIATA [including Echinodermata, Polyzoa, Nemat- helminthes, Platyhelminth.es, Coelenterata, Sponges, and Protozoa. The Rotifera are placed among the Protozoa, and Bacteria and the Pedicel laria? of Echinoderms are also included]. Here, it will be seen, the Vertebrata as a whole, and not the separate classes of that phylum, are considered as the equivalent of one of the great invertebrate sub-divisions : the Linnaean Vermes are broken up, Mollusca being elevated to the rank of a xvi THE HISTORY OF ZOOLOGY 679 primary sub-division, and the articulated worms associated with Arthropods ; while Echinoderms are grouped with Ccelenterata on account of their radial symmetry, and the imperfectly understood lower Worms, Sponges, and Protozoa are included in the same branch. Cuvier may also be said to have created the science of Palaeon- tology by his investigations of the Tertiary Mammalia of France. As long ago as the sixth century B.C., Xenophanes had recognised fossils as the actual remains of animals, but the usual view was that they were merely mineral productions ; and one of the earliest observers in modern times to perceive their true nature was Scheuchzer, at the beginning of the eigthteenth century, who considered them as evidences of a universal deluge. Cuvier, as well as the English geologist William Smith (1769-1839), showed that the older fossils belonged to entirely different species, genera, and even families, from the animals existing at the present day, the differences being greater in the deeper than in the more superficial formations. In this way the idea of a de- finite succession of life in time was introduced. Cuvier and his followers rejected, however, the notion of any genetic connection between the inhabitants of successive geological periods, and con- sidered that the fauna of each epoch was exterminated by some catacylsm or convulsion of nature, and the earth subsequently re-peopled by a fresh creative act. This catastrophic view of the history of the earth received its death-blow in 1830-33, when Sir Charles Lyell (1797-1875) published his Principles of Geology — next to the Origin of Species the most famous con- tribution to natural science in modern times. By insisting on the evidences for continuity in the history of the earth, he pre- pared men's minds for the idea of continuity in the history of its living inhabitants, and thus, more than any of the older evolutionists, paved the way for the reception of Darwin's views. Apart from .the work of Cuvier, the most important con- tributions to Zoology during the first half of the nineteenth century are in the domains of histology and embryology. In 1838 the cell-theory, according to which all parts of the body are built up either of cells or of tissues derived from cells, was pro- pounded first for plants by Schleiden and shortly afterwards for animals by Schwann. Both, however, had an erroneous concep- tion of the cell, considering the cell-wall as its essential part- whence the name cellula, a small chamber. But in 1846 the " plant-slime," observed by Schleiden in the interior of the cell, was investigated with great thoroughness by von Mohl and was called by him protoplasm, a name originally used by Purkinje in 1840, for the substance of which the youngest embryos of animals are composed Albert Kblliker and others proved that animal-cells existed in which no cell-wall was present, and Dujardin showed 680 ZOOLOGY SECT. that Amoebae and other lowly organisms were formed entirely of protoplasm, or, as he called it, sarcode. These discoveries paved the way for the generalisations of Max Schultze and De Bary, that the essential constituent of the cell is protoplasm, and that the protoplasm of animals and plants is identical. In embryology, the most important work of this time was that of K. E. von Baer, who, in 1827, discovered the ovum of Mammals. He also described three primary germ-layers — ectoderm, meso- derm, and endoderm — in the Vertebrate embryo, and showed that histological differentiation, or the formation of the permanent tissues from embryonic cells, proceeds hand in hand with morpho- logical differentiation or the evolution of organs. He was thus led to enunciate what is known as von Baers law, that development is a progress from the general to the special, and to frame the generalisation that embryos of animals belonging to various classes closely resemble one another in their earlier stages, but diverge more and more as development proceeds. His investiga- tions led him to support Cuvier's view of the division of the animal kingdom into distinct and clearly separated types or branches. It was during this period also that the real meaning of fertilisa- tion was discovered, and the controversy between ovulists and spermatists finally set at rest. Artificial fertilisation had been tried in the last century, but up to 1842 the greatest physiologist and most accurate anatomist of his time, Johannes Muller, was unable to state positively whether or not the sperms were parasitic animalcules. But in 1843 Martin Barry observed the union of ovum and sperm in the Rabbit, and three years later Kolliker proved that the sperms were developed from the cells of the testis. The period under consideration also saw the development of a school of speculative or deductive zoology. In 1790 Goethe con- ceived the idea that the skull of Vertebrates is made of modified vertebrae — in other words, that the skull is the highly differentiated anterior end of the backbone. This theory, which may be taken as a type of morphological speculation in the pre- evolutionary period, was re-enunciated and greatly elaborated in 1807 by Lorenz Oken, whose conclusions are worthy of mention, if only to show the dangers of the deductive method in natural science, and the lengths to which unbridled speculation may carry a presumably sane man. He did real service by demonstrating the secondary segmentations of the bony skull ; the occipital segment being his "ear vertebrae," the parietal his "jaw vertebrae," and the frontal his " eye vertebrae." But he clearly went beyond the limits of legitimate speculation when he contended that the nasal cavity is a cephalic thorax and the mouth a cephalic abdomen ; that the bones of the upper jaw are hornologues of the fore-limbs, the lower jaw of the hind-limbs, and the teeth of the digits. xvi THE HISTORY OF ZOOLOGY 681 About the middle of the century the vertebral theory, freed from the most obvious absurdities of Oken, was resuscitated and developed by Sir Richard Owen (1 803-93) in his Report on the Archetype and Homologies of the Vertebrate Skeleton, published in 1846. He also founded his generalisations on the structure of the adult or late embryonic skeleton in the higher groups, neglecting the unsegmented crania of Cyclostomes and Elas- mobranchs, and of the higher Vertebrate embryo. In his view, the limb-girdles are modified ribs, the shoulder-girdle belonging to the " occipital vertebra/' while the limbs themselves are " diverging appendages," or uncinates. Owen's chief services to Zoology were, however, his numerous and brilliant anatomical researches, such as those on Nautilus, on Apteryx, and on the structure and hornologies of the teeth in the entire vertebrate series ; and his palaeontological investigations, especially those on Archaeopteryx,onthe fossil Mammals of Australia, and on the Dinornithidoe and other flightless Birds. His conclusion from the examination of a single fragmentary femur, that there had existed in New Zealand a Bird larger and heavier than the Ostrich — a fact then practically unknown — forms one of the most famous stories in natural history. His contributions to classification were not happy ; he took the nervous system as the basis of his larger divisions, classifying Mammals, for instance, according to the presence or absence of a corpus callosum and of convolutions, and placing Man in a separate sub-class as the supposed sole possessor of a posterior cornu and hippocampus minor. He rendered great service to philosophical Zoology by pointing out the distinction between homology arid analogy, and by the publication of his great text-book on the Anatomy and Physiology of Vertebrates. The chief successor of Cuvier in France was Henri Milne- Edwards (1800-85), who enunciated the principle of the division of physiological labour, and modified the classification of Cuvier in several particulars.- He separated Tunicates from Mollusca proper and united them with Polyzoa under the name of Molluscoida, and he divided Vertebrates into Allantoidea and Anallantoidea, according to the presence or absence of an allantois ; in so doing he took the important step of separating Amphibia from Reptiles, a step in which De Blainville had been his only precursor. His learned Lemons de I'Anatomie et de la Physiologic compares is a storehouse of information on the structure and functions of animals. It was not until about the middle of the century that further increase in the knowledge of the lower animals resulted in the gradual dismemberment of Cuvier's unnatural Branch Radiata. Frey and Leuckart established the group Coelenterata, and placed Echinoderms apart ; Wiegmann removed Rotifera from 682 ZOOLOGY SF.OT. Protozoa to Vermes ; Vaughan Thomson defined tfie Polyzoa, and Rudolphi, Leuckart, and von Siebold showed that the Flat-worms were in no sense Zoophytes. Sponges were con- sidered by some as polypes, by others as plants ; the current of water flowing in at the pores and out at the oscula was discovered by Robert Grant about 1820 : later Bowerbank demonstrated the presence of cilia, and the full proof of their animal nature was made by the researches of Lieberktihn and Carter. The Fora- minifera were classed as Cephalopoda until the 'thirties, when Dujardin determined their proper place by the discovery of the living protoplasmic body. Other important advances in classifica- tion were the separation of Cirripedia from Mollusca by Vaughan Thomson, and the withdrawal from intestinal worms of the parasitic Copepoda and of the Pentastomida. The Infusoria have also had a chequered history. Ehrenberg in his magni- ficent work Die Infusionsthiere, looked upon the food-vacuoles as stomachs, and described a complex enteric canal connecting them ; it is, therefore, not surprising that he considered them as belong- ing to the same group as Rotifers. Louis Agassiz, as late as 1859, considered Paramoecium, Opalina*, &c., to be the young of Planarians and Trematodes and Vorticella to be a Polyzoan, and it was only by the researches of Stein and others that the class of Infusoria was fully established as a natural group of unicellular organisms. The Swiss zoologist, Agassiz (1807-73), referred to in the preceding paragraph, is interesting, not only as one of the foremost naturalists of his time and the founder of the large and active school of zoologists in the United States, where he spent the latter part of his life, but also as the last great biologist to maintain the fixity of species. In his Essay on Classification, published, curiously enough, in the same year (1859) as the Origin of Species, he supports the proposition that the various subordinate groups of animals, from phyla to species, are not mere "devices of the human mind to classify and arrange our knowledge in such a manner as to bring it more readily within our grasp and facilitate further investigations," but that they " have been instituted by the Divine Intelligence as the categories of His mode of thinking." In other words, that in our classifica- tions we " have followed only, and reproduced, in our imperfect expressions, the plan whose foundations were laid in the dawn of creation." In 1859 occurred what may fairly be called the most important event in the history of biological science, the publication of Charles Darwin's Origin of Species. The evolutionary theories of Buffon, Erasmus Darwin, Lamarck, and Geoffroy St. Hilaire had produced little effect upon contemporary zoology; and Robert Chambers's Vestiges of Creation (1844), although exciting xvi THE HISTORY OF ZOOLOGY 683 great interest, was too crude and speculative to make many converts among men of science. But Darwin had the advantage of being, not only a philosopher, but a naturalist in the broadest sense — a systematist with a sufficient knowledge of anatomy, thoroughly conversant with the breeding of domestic animals and cultivated plants, a keen observer of external nature, both organic and inorganic, and with unrivalled experience as a traveller. It is not surprising, therefore, that the wealth of illustration, the close reasoning, and the philosophic spirit of the Origin, converted the whole scientific world to the general doctrine of transformism within twenty years. The theory of Natural Selection, the Survival of the Fittest, or the Preservation of Favoured Races in the Struggle for Life, was first grasped by Darwin in 1838, but was not published until 1858, when it was announced simultaneously by himself and by Alfred Russel Wallace. Both these authors had, however, been anticipated by W. C. Wells in 1813, and by Patrick Matthew in 1831. Darwin's other works, especially The Variations of Animals and Plants under Domestication and The Descent of Man, rank among the most important contributions to philosophical Biology. With them must be mentioned the luminous Principles of Biology of Herbert Spencer, who consistently upheld the direct action of the en- vironment as a factor in evolution. Wallace, on the other hand, is a pure selectionist, while Darwin held "that natural selec- tion has been the main but not the exclusive means of modification." The additions to zoological knowledge made by the voyagers of the eighteenth century have been referred to ; even more important are the numerous great scientific expeditions of the nineteenth and twentieth centuries. Among the most prominent of these are the voyages of the French ships Astrolabe, Uranie, Eonite and Geographe, in which researches were carried on by Peron and La Sueur, Quoy and Gaimard, Eydoux and Souleyet, and Hombron and Jacquinot, and given to the world in splendidly illustrated folios. Still more famous is the voyage of H.M.S. Beagle (1831-36), in which Darwin gained his extraordinarily wide and accurate knowledge of natural history, and the narration of which is published in his Naturalist's Voyage. Other celebrated voyages are those of H.M.S. Rattle- snake (1846-50), of which T. H. Huxley was assistant-surgeon ; of H.M.SS. Erebus and Terror, accompanied by Sir J. D. Hooker ; of the American " Wilkes " expedition, with J. D. Dana as naturalist, and of the Austrian frigate Novara. But the most famous and complete of all scientific voyages was that of H.M.S. Challenger, in 1872-76, the five years' cruise of which was marked by discoveries of great importance by the scientific staff, Sir Wyville Thomson, John Murray, H. N. Moseley, 684 ZOOLOGY SECT. and Willemoes-Suhm, while the zoological material collected on the voyage was worked out by the leading zoologists in all parts of the world, and the results published in thirty handsome and fully-illustrated quarto volumes. On board the United States cruisers Blake and Albatross Alex. Agassiz made several cruises in the Gulf of Mexico (1877-80 and 1891), the Tropical Pacific (1899-1900), and the Eastern Pacific (1904-1905), the results of which have been in part published in The Memoirs and Bulletin of the Museum of Comparative Zoology of Harvard College. The results of the German Deep Sea Expedi- tion of 1898, under the leadership of C. Chun, have been appear- ing in a long series of Memoirs since 1892 ; and those of the German Plankton Expedition (1889) since the same year, under the editorship of V. A. C. Hensen. An important scientific expedition to the Dutch East Indies was the cruise of the Siboga (1899-1900) under the directorship of Max Weber. In land- travel, numerous journeys, and especially those of A. R. Wallace in the Malay Archipelago and Brazil, and of H. W. Bates in Brazil, have not only added immensely to our knowledge of the genera of the countries visited, but have enriched the science with the ideas of protective and aggressive characters, of mimicry, and of the relations of organism to environment generally. The establishment of Zoological gardens in different parts of the world — notably in Paris and London, Stellingen, Berlin, Hamburg, and New York — has added greatly to our knowledge both of the habits and of the anatomy of animals, and a similar advance in the investigation of marine animals has followed upon the establishment of Zoological Stations or Marine Laboratories in various countries. The earliest and most impor- tant of these is the Naples Station, founded in 1870 by Anton Dohrn. The results of the researches there carried on form the most elaborate and sumptuous series of zoological monographs ever published. Other stations with similar aims have been instituted in all parts of the world : of these may be mentioned those of Plymouth, Wimereux, Roscoff, Banyuls-sur-Mer, Kiel, Helder, Trieste, Woods Holl, and Tokyo : to these have been added in some countries stations for the study of the fresh-water and terrestrial faunas. The establishment of Zoological (or Biological) Laboratories in connection with Universities is also a work of the last forty years, and has had an important influence both in diffusing a knowledge of the science and in stimulating research. Even more recent is the complete change of view as to the functions and arrangement of a Museum. Formerly it was looked upon as a collection of curiosities, in which everything was to be exhibited to the public. Now, thanks in great measure to Sir W. H. xvi THE HISTORY OF ZOOLOGY 685 Flower in England, and Browne Goode in America, special collections are formed for study and research, while the cases accessible to the public are gradually becoming a series of actual illustrations of zoological science, in which not only the principles of classification, but the chief facts of structure, life-history, and habit are strikingly and adequately shown. Such reforms in the arrangement of museums and the advancement of their usefulness in many directions have been the objects aimed at by the Museums Association, an organisation of those interested in museums in all parts of the world, founded in 1889. During the second half of the last century, Zoology as a whole has been greatly influenced by the writings of Thomas Henry Huxley and of Ernst Haeckel. Huxley (1825-1895) was the first to point out the homology of the ectoderm and endoderrn of Coelenterates with the two primary germ -layers of the vertebrate embryo. He also introduced the word zooid, demolished the vertebral theory of the skull, and placed the anatomy of the fossil Ganoids upon a satisfactory footing, as well as making many other important contributions to animal morphology. His Elements of Comparative Anatomy (1864) forms an important landmark in the history of modern Zoology, as giving the views of one of the keenest, most logical, and least speculative of biologists just before the time when the various improved histological and embryological methods began to revolutionise the science. Huxley's " eight primary categories or groups " are as follow : — VERTEBRATA. MOLLUSCA. ANNULOSA MOLLUSCOIDA [including Arthropoda and Annulata]. [including Brachiopoda, Polyzoa and ANNULOIDA Tunicata]. [including Echinodermata, Rotifera, CCELENTERATA. Flatyhelminthes and Nemathelminthes]. INFUSORIA [including Infusoria proper and Mastigophora]. PROTOZOA [including Rhizopoda, Sporozoa, and Porifera]. The lower " Worms " are associated with Echinoderms, on account of the resemblance of the adult Rotifers, as well as of the larvae of certain Flat Worms, to the echinopsedium. Sponges are placed among the Protozoa, in accordance with the view that they are to be looked upon as colonies of unicellular zooids. Infusoria are separated from the remaining Protozoa, because the conjugation observed in them was misinterpreted, the meganucleus being considered as an ovary, the micronucleus as a testis. Haeckel, apart from his elaborate and beautiful researches on the Radiolaria, Calcareous Sponges, and Hydrozoa, is remarkable as the first modern zoologist to attempt the classification of VOL. II U U 680 ZOOLOGY XKCT. animals on a frankly evolutionary basis. We owe to him the terms phylogeny and ontogeny, csenogenesis and palingenesis, and the fruitful " gastrsea-theory," according to which the gastrula is the ancestral form of all the Metazoa. His classifications take the form of genealogical trees, and he was the first to employ the method of introducing hypothetical ancestral forms, wherever they might be wanted to complete the connection between known groups. He may be said, in fact, to have founded a school of deductive zoology, the phylogenetic speculations of which are often as ingenious and suggestive as they are transient. The student must, however, bear in mind that Archi-molluscs, Ideal Craniates, and Pro-mammalia are mere figments of the imagina- tion, and have no more real existence than the " Divine Arche- types " of an earlier school of thought. One result of the new views on species, very obvious in the writings of both Huxley and Haeckel, was the marked alteration in the position assigned to Man in the animal series. Linnaeus considered Homo as a genus of his order Primates, equivalent to Simia, Lemur, &c. ; but Cuvier took the retrograde step of erecting a distinct order, Bimana, to contain Man alone, the Apes and Lemurs forming the order Quadrumana. Ehrenberg went further, and divided the Animal Kingdom into Nations, i.e., Mankind, and Animals. Even as late as 1857 Owen, as we have already seen, made a distinct subclass, Archencephala, for Man, the remaining Primates being included with the other higher mammalian orders in the sub-class Gyrencephala. This view of the isolated position of Man was connected with the theory of his late appearance in time, and the fact of his co-existence with the Mammoth and other extinct Mammals, first proved by Boucher de Perthes in 1836 by the discovery of flint axes 20-30 feet below the present surface, was for many years almost universally denied. But LyelPs Antiquity of Man (1863) placed the geological evidence on a sound footing, and the same was done for the morphological evidence by Huxley, who, in his Mans Place in Nature (1863), summed up the position by the statement, now universally conceded, "that the structural differences which separate Man from the Gorilla and the Chimpanzee are not so freat as those which separate the Gorilla from the lower Apes." inally, Darwin, in his Descent of Man (1871), discussed the question from every point of view, and concluded that " Man still bears in his bodily frame the indelible stamp of his lowly origin." It was also during the third quarter of last century that the old doctrine of Abiogenesis or Spontaneous Generation, first assaulted by Redi, but maintained by many naturalists from Aristotle to Haeckel, was finally disposed of. The accurate methods of Louis Pasteur, Lord Lister, John Tyndall, and others, proved conclusively that the Bacteria, Monads, and other xvi THE HISTORY OF ZOOLOGY 687 lowly organisms which occur in putrefying substances, do not arise de novo, but are the product of germs in the floating dust of the air by the exclusion of which putrefaction may be absolutely prevented. During the last quarter of a century the progress of Zoology has been profoundly influenced by the improvements in micro- scopical methods, especially by the invention and perfection of the microtome, the method of serial section-cutting, and the various ways of preserving, imbedding, and staining tissues. The microtome began as a simple contrivance for holding small objects firmly while sections of them were cut by hand with a razor or other knife, and has developed into the various modern forms of the instrument in which the knife is fixed in a plane parallel to the surface of the object, and the latter is raised mechanically by small and equal increments as the sections are cut. In this way perfectly regular sections are obtained of an even thickness not exceeding the diameter of a cell. The method of imbedding began by simply holding an object, too small or too soft to be grasped by the fingers, between two pieces of carrot or pith, and has gradually been evolved into the present method of complete impregnation with paraffin or celloidin, by means of which imbedding material and object form a homogeneous mass. Simple preservation in alcohol has given place to elaborate fixing methods by means of chromic, picric, or osmic acids, platinum chloride, corrosive sublimate, etc., and gradual hardening in alcohols of increasing strength. Similarly, direct staining with an ammoniacal solution of carmine has developed into innumerable methods of differential staining, mostly with aniline dyes, by which the various tissues and the constituents of the cell — chromatin, centrosomes, etc. — are clearly brought into view. By the serial methods successive sections of an embryo or small animal are mounted in regular order, so that the organs, tissues, etc., can be traced through the series. In this way the dislocation of parts produced by dissection is avoided, organs are seen in absolutely natural relations, and parts quite undiscernible either by dissection or by microscopic examination of the whole animal or of dissociated parts of it, are clearly brought into view : the study of the structure upon which the sections are intended to throw light may be further facilitated by the fashioning of models in wax, reconstructed by putting together reproductions of the sections enlarged to scale. Morphological inquiry has, in fact, been brought within measureable distance of a precision limited only by the imperfections of our eyes and optical instruments. Similar accuracy in the topographical anatomy of the larger Animals, including Man, has been attained by freezing the whole 4 subject and cutting it into sections with a saw. These improved methods have necessitated a re-examiriation by u U 2 688 ZOOLOGY SECT. their aid of every group in the animal kingdom, and, as a result, our knowledge of the structure of many animals, especially, of the lower forms, of complex organs such as the vertebrate brain, of embryology, and of the minute structure of cells and tissues has been completly revolutionised. Specially remarkable is the advance in our knowledge of the Protozoa, Sponges, Actinozoa, Echino- derms and Amphioxus. The new light which has been thrown on the affinities of Balanoglossus, Rhabdopleura, and Cephalodiscus is also worthy of special mention. Probably the greatest of comparatively recent embryological triumphs, belonging to the earlier part of the period now under discussion, is Kowalewsky's discovery of the notochord and hollow nervous system of the Tunicate larva, which resulted in the removal of the Urochorda from Molluscoida to Chordata, and in breaking down the sharp line between Vertebrates and Invertebrates. But perhaps the most remarkable result of improved micro- scopical technique is the rise and development of a distinct department of histology, known as cytology, dealing with the minute structure of the protoplasm and nuclei and the various intra-cellular phenomena, such as mitosis. Our knowledge of this subject is entirely a product of the last thirty-five years, and is due in great measure, in the first instance, to the researches of W. Fleming, E. Strasburger, and E. van Beneden. A modification of the cell-theory has also been necessitated by the proof that many animal tissues do not consist of dis- tinct cells, but of a continuous mass of protoplasm with more or less regularly arranged nuclei, and are therefore strictly not multicellular but non-celhdar. As certain Protozoa, such as the Mycetozoa - and Opalina, are also non-cellular, con- taining numerous nuclei in an undivided mass of protoplasm, the distinction between Protozoa and Metazoa appears to be less absolute than it was at one time considered. The advance in paleontology during the same period has also been immense. In particular the researches of E. D. Cope, O. C. Marsh, and others in America, have added whole orders to Zoology — the Odontolca3, Ichthyornithes, Stereornithes, Ambly- poda and Dinocerata — and have resulted in the discovery of many new and strange forms among the Dinosauria, Elasmobranchs, Ganoids, and other groups, and in the tracing of the pedigrees of the Equidae, Camelidse, and other Mammalian families. Im- portant, though less striking discoveries have also been made among the fossil faunae of Europe, India, South Africa, and Australia ; while among Invertebrates the attempts to trace the pedigree of the Ammonites and Brachiopods are specially note- worthy. In embryology an important landmark is furnished by F. M. Balfour's Comparative Embryology (1880-81); in distribution, by xvi THE HISTORY OF ZOOLOGY 689 A. R. Wallace's Geographical Distribution of Animals (1876), each the first complete treatise on the subject in question. The zoo- geographical regions adopted by Wallace were originally proposed by P. L. Sclater in 1857. Similar landmarks for Zoology as a whole are Huxley's Anatomy of Vertebrated Animals (1871) and Anatomy of Invertebrated Animals (1877), Carl Gegenbaur's Ele- mentsof Comparative Anatomy (English edition, 1878), Claus's Text- Book of Zoology, (1st English edition, 1884-5), Ray Lankester's Notes on Embryology and Classification (1877), and the same author's articles in the Encyclopaedia Britannica (9th edition). Both Glaus and Gegenbaur retain Vermes as a primary division ; Lankester was the first to split up that unnatural assemblage into distinct phyla, and to include Balanoglossus and the Tunicata among Vertebrates, and Xiphosura and Eurypterida among Arachnida. He also associated Rotifers and Chsetopods with Arthropoda, and placed Hirudinea among the Platyhelminthes. A later develop- ment of the same author's views on morphology and classification is embodied in his Treatise on Zoology, of which six volumes have now been published (see Appendix, 42). Of inestimable value in the advancement of the embryology of Vertebrates is the com- prehensive Handbuch (1901-1906) of O. Hertwig, with sections by various other embryologists. The student who is interested in the permutations and com- binations of modern classification may be referred to the works just quoted as well as to the numerous text-books published of late years. The most important point to notice in this connection is the breaking down of the sharp boundaries between the four Cuvierian Branches and a return to something like the conception of unity of type, expressed, however, not as a linear series, but as a branch-work with the most complex and often puzzling inter- relations. Among the numerous recent contributions to philosophical Zoology it must suffice to mention the works on heredity and kindred subjects of August Weismann, the most prominent member of the ultra-Darwinian school, who deny use-inheritance and rely upon natural selection as the main, if not the sole, factor in evolution. The opposite view, which accepts the truth of use- inheritance, is mainly supported by the American school of Neo- Lamarckians. Weismann has also resuscitated the theory of pre-formation under a modern form. He considers that the various parts of the adult organism are represented in the chromatin (germ-plasm) of the sex-cells by ultra-microscopic particles or determinants. These and allied topics are comprehen- sively treated from a different standpoint by O. Hertwig in his Allgemeine Biologie (1909). In a brief sketch like the present it is impossible to do more than refer, in general terms and without mention of names, to the 690 ZOOLOGY SECT, xvi vast amount of work now being done in every department of Zoology. The output of original research is greater than at any former time and is increasing rapidly, and every important addition to our knowledge necessitates a more or less thorough reconsidera- tion of the general and special problems of morphology and classification. Attention, must, however, be drawn to the researches of the last few years in the departments of experimental and statistical Zoology. Exact observations and systematic experiments on comparative physiology, on the precise nature of the action of external conditions, on the physiology of the cell, on the conditions influencing the development and growth of the embryo, on the limits and characteristics of individual variation, and on heredity, are the fields of study in which it may safely be said that the greatest promise of the future lies. APPENDIX ZOOLOGICAL LITERATURE. THE following are lists of some of the publications which will be most useful to the student of Zoology, the first comprising books giving directions for practical work in the laboratory ; the second, general works on the entire subject or on one or other of its main sub-divisions ; the third, publications in which are given the titles, and in some cases abstracts of the contents, of new works as they appear. More or less comprehensive bibliographical lists are given in several of the works referred to in the second list (Nos. 8, 19, 29, 33, 40, 42, 54, 57, 67, 69, 70, 73, 76). I. Books bearing specially on Laboratory work. 1. APATHY, S. Mikrotechnik der thierischen Morphologic, 1896-1901. 2. BROOKS, W. K. Handbook of Invertebrate Zoology, 1890. [Amoeba, Paramcecium, Yorticella, Calcareous Sponge, Zoophyte, Anthomedusa, Leptomedusa, Starfish, Sea-urchin, Embryology and Metamorphosis of Echinoderms, Earthworm, Leech, Crab, Crayfish or Lobster, Metamorphosis of Crab, Cyclops (including metamorphosis), Grasshopper, Mussel, Development of Lamelli- branchs, Squid, Development of Squid.] 3. EHRLICH, P., KRAUSE, R., Moss, M., ROSIN, H., WEIGERT, K. Encyclopaedic der mikroskopischen Technik, 2 vols.? 2nd edition, 1910. 4. FOSTER, M., and BALFOUR, F. M. Elements of Embryology, 2nd edition, by A. Sedgwick and W. Heape, 1883. [Chick and Rabbit.] . 5. FRIEDLANDER, R. Mikroskopische Technik, 6th edition, 1900. 6. HOWES, G. B. Atlas of Practical Elementary Zootomy, 1902. [Amoeba, Stentor, Parana oecium, Spirostomum, Vorticella, Hydra, Fresh-water Mussel, Snail, Earthworm, Crayfish, Frog.] 692 APPENDIX 7. HUXLEY, T. H. and MARTIN, H. N. A Course of Practical Instruction in Elementary Biology, 2nd edition, by G. B. Howes and D. H. Scott, 1888. [Amoeba, Vorticella, Paramcecium, Opalina, Hydra, Earthworm, Crayfish, Mussel, Snail, Frog.] 8. KUKENTHAL, W. Leitfdden fur das zoologische Praktikum, 4. Aufl., Jena, 1907. 9. LEE, A. B. Microtomists' Vade Mecum. 6th edition, 1905. [A compendium of laboratory methods.] 10. MARSHALL, A. M. The Frog : an Introduction to Anatomy, Histology, and Embryology, 6th edition, 1896. 11. MARSHALL, A. M. and Hurst, C. H. Practical Zoology, 6th edition, 1905. [Amoeba, Vorticella, Paramoecium, Hydra, Liver-Fluke, Leech, Earthworm, Crayfish, Cockroach, Freshwater Mussel, Snail, Amphioxus, Dogfish, Pigeon, Rabbit.] 12. PARKER, T. J. A Course of Instruction in Zootomy, 1884. [Lamprey, Skate, Cod, Lizard, Pigeon, Rabbit.] 13. PARKER, T. J. and PARKER, W. N. An Elementary Course of Practical Zoology, 2nd edition, 1908. [Part I — Frog; Part II — Amoeba, Sphserella, Paramoecium, Vorticella, Opalina, Monocystis, Hydra, Obelia, Earthworm, Nereis, Crayfish, Fresh-water Mussel, Amphioxus, Dogfish, Rabbit; Histology, Embryology, etc.] 14. VOGT, C. and JUNG, E. Traite d'Anatomie comparee pratique, 2 vols., 1883-94 (also a German edition). [Amoeba, Fora- minifer (Polystomella), Actinosphserium, Radio! arian (Actino- metra), Paramoecium, Dicyema, Calcareous Sponge, Alcyonium, Aurelia, Hydra, Ctenophore (Bolina), Tsenia, Distomum, Turbellarian, Nemertean Leech, Ascaris, Sipunculus, Rotifer (Brachionus), Earthworm, Lobworm (Arenicola), Feather- star, Starfish, Sea-urchin, Holothuriari, Polyzoan (Plumatella), Brachiopod, Mussel, Snail, Pteropod, Cuttle-fish, Crayfish, Centipede, Cockchafer, Spider, Salpa, Simple Ascidian, Amphioxus, Lamprey, Perch, Frog, Lizard, Pigeon, Rabbit.] 15. WILDER, B. G. and GAGE, S. H. Anatomical Technology [Cat.]. 3rd edition, 1892. II. General Works and Text-books. 1. BALFOUR, F. M. Treatise on Comparative Embryology, 2 vols., 1880-81. Works, Memorial Edition, 4 vols., 1885. 2. BATESON, W. Materials for the Study of Variation, 1894. 3. BATESON, W. Mendel's Principles of Heredity, 1909. 4. BEDDARD, F. Animal Coloration, 1892. 5. BEDDARD, F. Text-book of Zoogeography, 1895. 6. BOAS, J. E. V. Lehrbuch der Zoologie, 1894. English Translation, 1894. APPENDIX 693 7. BONNET, R. Lehrbuch der Entwickelungsgeschichte, 1907. 8. BRONN, H. G. Klassen und Ordnungen des Tierreichs. Protozoa (BUTSCHLI), 1880-89; Porifera (YOSMAER), 1887; Turbellaria (GRAFF) ; Trematoda (BRAUN) ; Cestoda (BRAUN) ; Nemertina( BURGER); Tunicata (SEELIGER); Asteroidea (LuDWiG and HAMANN); Ophiuroidea (HAMANN); Echinoidea (HAMANN) ; Crinoidea (HAMANN) ; Holothuroidea (LUDWIG) ; Mollusca (KEFERSTEIN), 1862-66; Amphineura (SIMROTH) ; Gastropoda (SIMROTH) ; Crustacea (GERSTACKER) ; Myriapoda (VERHOEFF) ; Amphibia (HOFFMANN), 1873- 78 ; Reptilia (HOFFMANN), 1890 ; Aves (GADOW). 1891 ; Mammalia (GIEBEL and LECHE). 9. Cambridge Natural History. Vol. I. Protozoa (HARTOG, M.) ; Porifera (SOLLAS, I. B. J.) ; Coelenterates and Ctenophora (HiCKSON, S. J.) ; Echinoderms (MACBRIDE, E. W.), 1906. Yol. II. Flatworms and Mesozoa (GAMBLE, F. W.) ; Nemertines (SHELDON, L.); Threadworms and Sagitta (SHIPLEY, A. E.) ; Rotifers (HARTOG, M.) ; Polychaet Worms (BENHAM, W. B.) ; Earthworms and Leeches (BEDDARD, F. E.) ; Gephyrea and Phoronis (SHIPLEY, A. E.) ; Polyzoa (HARMER, S. F.). Vol. III. Molluscs (CooKE, A. H.) ; Brachiopods (recent), (SHIPLEY, A. E.) ; Brachiopods (fossil), (REED, F. R. C.). Vol. IV. Crustacea (SMITH, G.) ; Trilobites (Wooos, H.) ; Intro- duction to Arachnida and King-Crabs (SHIPLEY, A. E.) ; Eurypterida ( WOODS, H.) ; Scorpions, Spiders, Mites, Ticks, etc. (WARBURTON, C.) ; Tardigrada (SHIPLEY^ A. E.) ; Pentastomida (SHIPLEY, A. E.) ; Pycnogonida (THOMPSON, D'A. W.). Vol. V. Peripatus (SEDGWICK, A.) ; Myriapods (SINCLAIR, F. G.) ; Insects, Part I. (SHARP, D.). Vol. VI. Insects, Part II. (SHARP, D.). Vol. VII. Hemichordata (HARMER, S. F.) ; Ascidians and Amphi- oxus (HERDMAN, W. A.) ; Fishe's (BRIDGE, T. W. and BOULENGER, G. A.). Vol. VIII. Reptiles and Amphibia (GADOW, H.). Vol. IX. Aves (EVANS, A. H.). Vol. X. Mammalia (BEDDARD, F. E.). 10. BROOKS, W. K. The Foundations of Zoology, 1899. 11. CARUS, V. Geschichte der Zoologie, 1872. (French Translation, Histoire de la Zoologie, 1880.) 12. CUVIER, G. Regne Animal. Illustrated edition, 1849. 13. Darwin and Modern Science. Essays in commemoration of the Centenary of Charles Darwin, 1909. 14. DARWIN, C. Origin of Species, 6th edition, 1880. 15. DARWIN, C. Descent of Man, 1882. 16. DARWIN, C. Animals and Plants under Domestication, 2 vols., 1888. 17. DEAN, B. Fishes, Living and Fossil, 1895. 694 APPENDIX 18. DELAGE, Y. L'Heredite et les grandes problemes de Biologie gdnerale, 2nd edition, 1903. 19. DELAGE, Y. et HEROUARD, E. Traite de Zoologie Concrete. I. La Cellule et les Protozoaires, 1896. II. Spongiaires et Coelenteres, 1899. III. Echinodermes, 1903. Y. Vermidiens, 1897. VII. Prochordts, 1898. 20. DUVAL, M. Atlas d Embryologie, 1889. 21. EIMER, T. Die Entstehung der Arten, 1888. English Trans- lation, Organic Evolution, 1889. 22. FLOWER, W. H. Osteology of the Mammalia, 1885. 23. FLOWER, W. H., and LYDEKKER, R. Mammals, Living and Extinct, 1891. 24. GAUDRY, J. Les enchainements du monde animal dans les temps geologiques, 1878. 25. GEGENBAUR, C. Elements of Comparative Anatomy. English Translation, 1878. 26. GEGENBAUR, C. Vergleichende Anatomie der Wirbelthiere, 2 vols., 1898 and 1901. 27. HATSCHEK, B. Lehrbuch der Zoologie. 28. HEILPRIN, A. The Distribution of Animals, 1887. 29. HERTWIG, O. Lehrbuch der Entwickelungsgeschichte der Wirbel- thiere, 8th edition, 1906. English Translation of 3rd German edition, 1892. 30. HERTWIG, O. Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbelthiere. Sections by various writers, 3 vols., 1901-1906. 31. HERWIG, O. The Biological Problem of To-day, 1896. 32. HERTWIG, O. Allgemeine Biologie, 3rd edition, 1909. 33. HERTWIG, R. Lehrbuch der Zoologie. 9th edition, 1910. English Translation of 3rd edition. 34. HUXLEY, T. H. The Anatomy of Invertebrated Animals, 1877. 35. HUXLEY, T. H. The Anatomy of Vertebrated Animals, 1871. 36. JORDAN, D. S., and KELLOGG, V. L. Evolution and Animal Life, 1907. 37. KEIBEL, F. Normentafeln zur Entwickelungsgeschichte der Wirbeltiere. [Pig, Fowl, Ceratodus, Li/ard, Rabbit, Deer, Tarsius, Nycticebus, Man, Lapwing.] 1897-1909. 38. KELLOGG, V. L. Darwinism To-day, 1907. 39. KINGSLEY, J. S. Text-book of Vertebrate Zoology, 1899. 40. KORSCHELT, E., and HEIDER, K. Text-book of the Embryology of Invertebrates. English edition, 4 vols., 1895. 41. LANG, A. Lehrbuch der vergleichenden Anatomie der wirbellosen Thiere, 1894; English Translation, Comparative Anatomy of Invertebrates, 1891-6. Second German edition, Protozoa, 1901, Mollusca, 1900. 42. LANKESTER, E. R. A Treatise on Zoology. I. Protozoa (LISTER, MINCHIN, HICKSON). II. Porifera and Ccdentera (MiNCHiN, APPENDIX 695 FOWLER, BOURNE). III. Echinoderma (BATHER). IV. Platy- helmia, Mesozoa and Nemertini (BENHAM). "VII. Crustacea (CALMAN). IX. Cyclostomes and Fishes (GOODRICH). 1900-09. 43. LILLEY, F. Embryology (Chick). 44. LYDEKKER, R. Royal Natural History. 6 vols. 1894-96. 45. MARSHALL, A. M. Vertebrate Embryology, 1893. (Amphioxus, Frog, Chick, Rabbit, Human Embryo.) 46. MORGAN, C. L. Animal Life and Intelligence, 1891. 47. MORGAN. C. L. Habit and Instinct, 1896. 48. MORGAN, T. H. Experimental Zoology, 1906. 49. NICHOLSON, H. A., and LYDEKKER, R. Manual of Palaeontology, 2 vols., 1889. 50. OPPEL, A. Lehrbuch der vergleichenden mikroskopischen Anatomie der Wirbeltiere, 1896-1904. 51. OSBORN, H. F. From the Greeks to Darwin, 1894. 52. OWEN, R. Anatomy of Vertebrates, 4th edition, 1871. 53. POULTON, E. B. Essays on Evolution, 1908. 54. ROLLESTON, G., and HATCHETT-JACKSON, W. Forms of Animal Life, 1888. 55. RABL, C. Geschichte der biologischen Theorien, 2 vols., 1909. 56. ROMANES, G. J. Darwin and after Darwin, 2 vols., 1892 and 1895. 57. SEDGWICK, A. A Student's Text-Book of Zoology. Vol. I. Protozoa to Chcetognatha, 1898. Vol. II. Amphioxus and Vertebrata, 1905. Vol. III. Tunicata, Enteropneusta, Echino- dermata, Arthropoda, 1909. 58. SCHNEIDER, K. C. Lehrbuch der vergleichenden Histologie der Thiere, 1902. 59. SCHNEIDER, K. C. Vorlesungen uber Tierpsychologie, 1909. 60. THOMSON, J. A. Heredity, 1908. 61. VERWORN, M. Allgemeine Physiologie, 5th edition, 1909 ; English translation, General Physiology, 1899. 62. VRIES, H. DE. Die Mutationstheorie, 2 vols., 1901. English translation by Farmer and Darbishire, 1910. 63. WAGNER, M. Die Entstehung der Arten durch rdumliche Sonderung, 1889. 64. WALLACE, A. R. Geographical Distribution of Animals, 2 vols., 1876. 65. WALLACE, A. R. Darwinism, 1889. 66. WALLACE, A. R. An Examination of Weismannism, 1893. 67. WEBER, M. Die Saugetiere, 1904. 68. WEISMANN, A. The Evolution Theory, 2 vols., 1904. 69. WIEDERSHEIM, R. Vergleichende Anatomie der Wirbeltiere, 7th edition, 1909. 70. WIEDERSHEIM, R., and PARKER, W. N. Comparative Anatomy of Vertebrates, 1907. 71. WILLEY, A. Amphioxus and the Ancestry of the Vertebrates, 1894. 696 APPENDIX 72. WILSON, E. B. The Cell in Development and Inheritance, 2nd edition, 1900. 73. WOODWARD, A. S. Outlines of Vertebrate Palceontology, 1898. 74. ZIEGLER, E. H. Lehrbuch der Entwickelungsgeschichte der niederen Wirbeltiere, 1902. 75. ZITTEL, K. VON. Handbuch der Paldontologie, 5 vols., 1876-93. English translation, 2 vols., 1902. 76. ZITTEL, K. VON. Grundzuge der Palaontologie, 2 Aufl., 1903. II. Periodical Publications containing Current Bibliographies, etc. 1. American Naturalist, fortnightly. Contains original articles, usually of a popular character, and summaries of recent researches. 2. Anatomischer Anzeiger, fortnightly. Original papers (mostly short) and current bibliography. 3. Annee Biologique, annual. Abstracts of works and papers on general Biology. 4. Biologisches C entralblatt, fortnightly. Contains, in addition to original articles, resumes of recent publications. 5. Ergebnisse und Fortschritte der Zoologie. Resumes of the present state of knowledge on special subjects. 6. International Catalogue of Scientific Literature. Zoology and General Biology. Obtainable in sections. Full classified lists of all current writings with indication of new species and reference to new genera. 7. Journal oj the Royal Microscopical Society, bi-monthly. Abstracts of papers on nearly all departments of Zoology. 8. Zoological Record, annual. Bibliographical lists with some synopses of contents. 9. Zoologischer Anzeiger, fortnightly. Original papers (mostly short preliminary notices), and current bibliography (Bibliographia Zoologica, also to be had separately.) 10. Zoologischer Jahresbericht, annual. Published by the Naples Zoological Station. Consists (1) of bibliographical lists, (2) of abstracts of the contents of most of the papers, 1879— 11. Zoologisches Zentralblatt, fortnightly. Consists (1) of occasional summaries and criticism of recent work on special subjects, (2) of abstracts of individual books or papers in all depart- ments of Zoology. INDEX All numbers refer to pages : words in italics are names of families, genera and species : words in thick type are names of higher divisions : words in small capitals are names of examples. Numbers in thick type are numbers of pages on which there are figures : an asterisk after a number indicates a definition of the term or of the group. A ARD-VARKS, 479, 604— See Orycteropus Abdomen — Mammals, 72*, 447 Abdominal cavity, of Craniata, 72 Abdominal pore, 67, 68, 146, 250 Abdominal ribs, 302, 347, 419 Abdominal vein, 281, 326 Abducent nerve, of Craniata, 105 Abductor muscles, 276 Abiogenesis, 671* Abomasum, 571* Abyssal fauna, 635* Acanthias, 178 Acanthodei, 168* Acanthodes, 169 Acanthodrilus, 621 Acanthopteri, 221*, 225, 228, 232, 243 Accessory nerve, of Craniata, 104, 106 Accessory scapula, 430 Accipitres, 416, 445 Acetabulum, of Craniata, 83, 84* Acipenser, 219 — See Sturgeon Acipenser ruthenus, 219 Acrania, 44 — See Amphioxus Acrocoracoid process, 390 Acrodont, 355* Acromion process, of Pigeon, 391 ; Rab- bit, 455— See Pectoral arch Acustico-lateral centre, 162 Ad-digital, 385 Adductor muscles, 276 Adelochorda, 2* : Affinities, 13 Adhesive papilla, of Ascidian larva, 34, 37 Adipose bodies, of Frog, 287 : Lizard, .328 Adipose fin, 200* Adipose lobe, 200 Adrenals, of Craniata, 120 : Frog, 286 : Pigeon, 397 : Rabbit, 473 ^Egithognathous, 428* jflpyornis, 411, 436, 443, 642 jEpyornithes, 411 Aerial fauna, 637 Afferent branchial arteries, 92 : of Am- phioxus, 51 — See Vascular system Affinities — See Relationships After-shaft of Feather, 381, 423 Agamidce, 369 Agassiz, Alex., 684 Agassiz, Louis, 682 Aggressive resemblance, 657 Aglossa, 294*, 304 Agoutis, 507 Air-bladder, 90* Air-bladder, of Trout, 210, 211 : Teleos- tomi, 236 Air-sacs, of Chamseleon, 358 : Pigeon, 399, 400 : Birds, 434 Air-space, of Bird's egg, 436 Ala spuria, 385* Alar membrane, 380* Alaudidce, 417 Albatrosses, 415, 430, 436 Albula, 238 Albumen, of Bird's egg, 408, 435 Alca, 416 Alca impennis, 426, 443 Alcedinidce, 417, 436 Ali-sphenoid, 79*, 80— See Skull Allantoic bladder, of Craniata, 120* Allantois, Reptiles, 313, 365 : Bird, 441, 442 : Rabbit, 475 : Mammalia, 591 Alligator, 336, 354, 355, 357, 360, 368, 370 Alpine fauna, 638 Altrices, 442* 697 698 INDEX Alveoli, 459 Alveoli, of lung, 467 Alveus, 578* Alytes obstetricam, 308, 312 Amarcecium, 23 Ambiens muscle, 395 Amblyopsis spelceus, 637 Amblypoda, 608 Amblystoma, 293, 299, 303, 311, 312 American Ant-eaters, 479, 497, 525, 526, 527, 528, 529, 564, 598 Amia, 219, 220, 225, 228, 229. 230, 234, 236, 238, 240, 241, 242 Amia calva, 219 Ammoccetes, 137* Amnion, of Reptiles, 365: Birds, 440, 441 : Rabbit, 475 : Mammals, 591 Amniota, 313* Amphibia, 66, 264 : Example, 264 : Dis- tinctive characters and classification, 292 : General organisation, 294 : Ex- ternal characters, 294 : Exoskeleton, 298 : Endoskeleton, 298 : Myology, 304 : Digestive organs, 304 : Respira- tory organs, 304 : Circulatory organs, 305 : Nervous system and sense organs, 308 : Urinogenital organs, 308 : Repro- duction and development, 308 : Distri- bution, 312 : Mutual relationships, 312 Amphibolous, 362, 366 Amphicoelous, 146* Amphioxides, 44, 60, 64 Ampkioxididce, 44 AMPHIOXUS, 44 : External features, 45 : Body- wall, 46 : Skeleton, 47, 46, 49 : Digestive and respiratory organs, 46, ' 47, 49 : Atrium, 49 : Ccelome, 50 : Blood-system, 51 : Excretory organs, 52, 53 :* Nervous system, 53, 54, 55 : Sensory organs, 54, 55 : Reproductive organs, 55 : Development, 56, 57, 58, 59, 60, 61, 62, 63, 64 : Distribution, 64 : Distinctive characters, 64 : Affinities, 65 Amphipnous, 235 Amphisbaenians, 334, 342, 347, 358, 365, 369 Amphistylic, 78*, 173 Amphitherium, 640 Amphiuma, 293, 295, 296, 305, 308, 312 Amphiuma tridactyla, 295 Ampullae, 115,* 179— See Ear Ampullary canals, 179 Anabas scandens, 235 Anacanthini, 221*, 226, 228, 243 Anapophyses, of Rabbit, 448* Anas, 416, 421, 428, 429, 626 Anas boschas, 429 Anatomical evidence of Evolution, 644 Anchinia, 23 Anchovy, 220 Ancylopoda, 608 Angler, 226 Anguidce, 369 Anguis, 339, 342, 362, 369 Angular, 206 Angular process, of mandible, 454— See Skull Angulo-splenial, 270, 269 Ankylosis, 201* Annular cartilage, of Lamprey, 127 Annulus ovalis, 463 Annulus tympanicus, of Frog, 271, 286 Anomalurus, 490, 506 Anomodontia, 336 Anser, 416, 421, 428 Anseres, 416, 435, 442 Ant-eater, American, 479, 497 Ant-eater, Banded, 494 Ant-eater, Scaly — See Manis Ant-eater, Two- toed, 498 Ant-eaters, 479 Ant-eaters, Cape, 479— See Orycteropus Antelopes, 482, 501, 598 Anterior clasper, 190 Anterior commissure, 212 Anterior vertebral plate, 172*, 174 Anihropithecus, 487 — See Chimpanzee Anthropoidea, 486* Anthropopithecus troglodytes, 555 Antiarcha, 263, 264 Antibrachium, 314* Antitrochanter, 392* Antlers, 501, 502 Antorbital, 175* Anura, 293*, 294,296, 297, 298, 300, 302, 303, 304, 306, 308, 309, 312 Anwiella, 38 Aorta, 92: Amphioxus, 51— See Vascular system Aortic arches, 95 — See Vascular system Apatornis, 414 Apes, 508 Apical plate, of Tornaria, 8, 9 Aphanapteryx, 625, 642 Appendicularia, 21, 22, 24, 25, 29, 30, 31, 42,43 Appendicidariidce, 22 Appendix, vermiform, of Rabbit, 462 Aptenodytes, 414 Apteryges, 411 Apteria, 384*, 422 Apteryx, 411, 430, 433, 434, 435, 436, 439, 442 Apteryx australis, 411, 412, 421, 432 Apteryx mantelli, 427, 428, 430 Apteryx oweni, 433 Aptornis, 416, 426, 430, 620, 642 Aqueduct of Sylvius, 100* Aqueductus vestibuli, 148, 163, 179 Aqueous chamber, of Eye, 112* Aqueous humour, 112* Aquila, 416, 428, 436 Aquinas, Thomas, 675 INDKX 699 Ara, 416, 429 Arachnoid membrane, 103 Arbor vitse, of Rabbit, 472 Arboreal fauna, 637 Archaeoceti, 605 Archceohydrax, 607 Archceopteryx lithoyraphica, 410, 418, 419, 443, 444, 445, 640 Archceopteryx siemtnni, 420 Archaeornithes, 410*,' 418 Archeiiteron, 290 Archipallium, 579* Archipterygiuni, 168*, 257 Arcifera, 294* Arctomys marmot, 638 Ardea, 415, 422, 423 Area opaca, 363, 437 Area pellucida, 863, 437 Area vasculosa, 440 Argeritea, 214* Aristotle, 668 Armadillos, 479, 490, 498, 499, 524, 525, 526, 528, 530, 531, 564, 598, 604 Arteries, 92 — See Vascular system Arthrodira, 260 Articular, 80, 81*— See Skull Artiodactyla, 482*, 501, 536, 538, 541, 543, 544, 564, 576 Arvicola, 594 Arytenoids, of Lizard, 328 : Reptilia, 357 : Pigeon, 397 : Rabbit, 466 ASCIDIA, 14: Body- wall and Atrial cavity, 15, 16: Pharynx, 16, 17, 18, 19 : Enteric canal, 19 : Blood system, 18. 19 : Nervous system, 20, 21 : Neural gland, 21 : Excretory system, 21 : Reproductive system, 21 : Sj'stematic position, 24 : Development, 32 Ascidiacea, 23* Ascidiae compositae, 23*, 26 Ascidiae simplices, 23*, 25 Ascidia mammillata, 37 Ascidians, 14 Ascidiidce, 24 Aspredo, 241 Asses, 482, 599 Astacopsis, 622 Astacus, 621 Asteriscus, 214*, 215 Astragalo-scaphoid, 355 Astragalus, 274, 458 — See Limb-skeleton Astroscopus, 233 A teles, 487 Athecata, 371 Atlas, 316* Atrial canals, of Appendicularia, 24 Atrial cavity of Ascidia, 15, 18 Atrial siphon, 15, 18 Atrial lobes, of Doliolum, 27 Atriopore, of Amphioxus, 46, 45, 49 Atrium, of Ascidia, 15, 36 : Amphioxus, 49 Auditory capsules, of Craniata, 75* Auditory nerve, 104, 105 — See Brain Auditory organ — See Ear Auditory ossicles, 455— See Ear Auditory region, of Craniata, 76* Augustine, 675 Auks, 416, 436, 443, 444 Autostylic, 78*, 190 Auricle, 90— See Heart Auricular appendix, 463 Australian region, 631 Aves, 66, 313, 378 : Example, 378 : Distinctive characters and classifica- tion, 408 : General organisation, 417 : Archseorriithes, 418 : External char- acters of Neornithes, 420 : Pterylosis, 422 : Skeleton, 424 : Myology, 433 : Digestive organs, 433 : Respiratory and vocal organs, 434 : Circulatory organs, 434 : Nervous system and sense organs, 434 : Urinogenital organs, 435 : Development, 435: Dis- tribution, 442 : Ethology, 443 : Phyto- geny, 443 Avocet, 421 Axis, basi-cranial, 513* Axolotl, 293, 311 Aye-Ayes, 486 Axis, 316 Azygos vein of Rabbit, 464 Azygos veins of Urodela, 306, 307 B BABOONS, 487, 514, 554, 556 Baer, K. E., von, 680 Balcena, 480 Balcenidce, 480 Balcenoptcra muscidus. 532 Balcc.noptera rostrata, 567, 568 Balanoglossus, 2, 43 : External characters, 2, 3, 4 : Ccelome, 3 : Digestive organs, 5 : Notochord or cesophageal diverti- culum, 4, 6 : Blood-vascular system, 4, 6: Nervous system, 4, 5,7: Repro- ductive system, 7 : Development, 7, 8, 9 : Metamorphosis, 8, 9 Baleen, 480*, 568* Baleen whales, 480 Balfour, F. M., 688 Banded ant-eater, 494 Bandicoots, 478, 494, 524, 562, 597 Banks, Sir J., 676 Barbel, 146 Barbels, in Teleostomi, 225 Barbs, of feather, 382* Barbules, of feather, 382* Barriers, 626 Barry, Martin, 680 Basale, 232 Basalia, of Craniata, 82* 700 INDEX Basal plate, of Craniata, 76*, 77 Basi-branchial, 150*, 207 Basi-branchial plate, 174, 175 Basi-branchiostegal, 207 Basi-cranial axis, 513* Basi-cranial fontanelle, 125, 127 Basi-dorsal, 147 Basi-facial axis, 513* Basi-hyal, 77, 78*— See Skull Basi-occipital, 79*, 80— See Skull Basipterygium, 209, 210 Basi pterygoid processes, of Birds, 390, 427 : of Lizard, 318, 319 Basis cranii, 76*— See Skull Basi-sphenoid, 79*, 80— See Skull Basi-temporals, 388, 389 Basking sharks, 178, 187 Bastards, 665* Bates, H. W., 684 Bathy metrical distribution, 634 Bats, 485, 508, 516, 570— See Chiroptera Bdellostoma, 138, 139, 141, 142, 143 Bdellostoma forsteri, 139 Bdellostoma stouti, 142 Beak in Teleostomi, 225 Beak of pigeon, 379 : of Neornithes, 420 Beaked whales, 480 Bear, 484, 505, 545, 546, 547, 569, 582, 599 Beavers, 484, 506, 547 Bee-eaters, 417 Bellonius, 669 Belly, of muscle, 274* Belodon, 370 ' Beneden, E. van, 688 Benthos, 636* Beryx, 640 Biceps muscle, 276 Bicipital groove, 455 Bile, 87* Bile ducts, 88— See Liver Biogenesis, 671* Birds— See Aves Birds of Paradise, 417, 424, 622, 443 Blackbirds, 417 Bladder, urinary, of Craniata, 120 : Trout, 215 : Teleostomi, 239 : Rabbit, 473 : Mammals, 582 Blainville, 681 Blastocoele, 288* Blastodermic vesicle, 588 Blastula, of Amphioxus, 56, 57 Blenny, 241 Blind-snakes, 335, 367 Blind-worm, 339, 342, 362, 369 Blood, 97— See Vascular system Blood corpuscles, 52, 97— See Vascular system Blood vessels— See Vascular system Boar, 565 Boas, 335, 367 Boatswain-bird, 415 Body-cavity — See Crelome Body, of vertebra, 74* Body- wall, of Amphioxus, 46 : of Craniata, 69 Boltenia, 25 Bombinator, 302 Bones, of Craniata, 78 Bonnet, 674 Bony labyrinth, 406 Bony Pike, 196, 219, 225 Botryllus, 23, 25, 26 Bottle-nosed whales, 480, 516 Boucher de Perthes, 686 Bougainville, 676 jBovidce, 482 — See Oxen Bowerbank, 682 Bower-birds, 443 Bow-fin, 196, 219, 220 Brachial plexus, 284* Brachium, 69, 314 Bradypodidw, 479— See Sloths Bradypus tridactylus, 527, 528. 529, 530, 571 Brain, Amphioxus, 54 : of Craniata, 98, 100: Lamprey, 131, 132: Dog-fish, 157, 158, 159, 160: Elasmobranchii, 178: Holocephali, 193, 194: Trout, 212, 213 : Teleostomi, 238 : Ceratodus, 253 : Frog, 284, 285 : Amphibia, 308, Lizard, 328, 329, 330 : Reptilia, 360 : Pigeon, 403, 404, 405: Aves, 434: Rabbit, 468-472 : Mammals, 576-580 Brain-case of Craniata, 76 Branchia, of Salpa, 27*, 28 Branchiae, of Amphioxus, 48: Lamprey 130 : Dog-fish, 154 : Elasmobranchii, 178: Holocephali, 192: Trout, 212: Teleostomi, 235 : Ceratodus, 251 : Tad- pole, 290 : Amphibia, 296, 304 Branchiae, external, 258, 289, 290, 304 : internal, 291, 305 Branchial apertures, of Amphioxus, 48, 49 : of Craniata, 67, 88* Branchial arches, of Craniata, 77* Branchial basket, of Lamprey, 126-128 Branchial clefts, 2*— See branchial slits Branchial filaments, of Craniata, 89* Branchial junctions, Amphioxus, 49 Branchial lamella?, of Amphioxus, 48, 49 Branchial nerves, of Craniata, 104, 106 — See Brain Branchial rays, 150, 174 Branchial rods, of Amphioxus, 48, 49 Branchial slits, of Balanoglossus, 3, 4 : Amphioxns, 48, 49, 62 : of Cephalo- discus, 10 Branchio-cardiac vessel, 20, 18 Branchiostegal membrane, 199, 225 Branchiostegal rays, 199, 203 Branchiostoma, 44 — See Amphioxus BranchiostomidcK, 44 INDEX 701 Brassica oleracea, 651 Broad ligament, 333 Bronchi, of Lizard, 328 : of Pigeon, 397 : of Rabbit, 466 Bronchioles, 467 Brown funnels, of Amphiojcus, 49, 52 Brush-turkeys, 416 Buccal cavity, of Cruniata, 84 Buccal funnel, of Lamprey, 124 : Myxint, 139 Buccal glands, 87* : Pigeon, 397 : Birds, 434 Budding, in Cephalodiscus, 10 : in Ascidians, 25 : Doliolum, 39 : Salpa, 41 Buffon, 674, 675 Bnfo vul'jaris, 297 Bulbus aortee, 92, 130, 212, 280 Bulla tympani, 453, 512 Bunodont, 561* Burchell's zebra, 503 Burnett Salmon, 246, 247 Burr, of antlers, 502* Bursa Fabricii, 397* Bustards, 416 Butterfly -fish, 226 Button-quails, 416 a C ' ABALUS, 620 Cacatua, 416 CbcoftMcZee, 622 Cadophore, 39,* 40, 41 Caducibranchiata, 296*, 300, 312 Chilians, 264, 293 Caecum, 324, 327, 460, 573 Caimans, 336, 342, 370 Calcumickthys, 218, 242 Calamus, 381 Calamus scriptorius 472* Calcaneum, 267, 274, 457, 458 -See Limb-Skeleton Calcified cartilage, 78* Calcar, 267, 274, 485, 552 Callichthys, 236 Callithrix, 487 Callorhynchus, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197 Callosities, ischial, 509* Cambrian, 639 Camelida:, 482— See Camels Camels, 482, 501, 544, 566, 571,572, 598 Campanula Halleri, 214* Camper, Peter, 675 Camptotrichia, 247* Cantdce, 484, 505— See also Canis and Dogs Canines, of Rabbit, 458— See Teeth Canis, 484 VOL. II Canis dinyo, 622 Canis familiaris, 546, 560, 571, 576 Cannon bone, of Horse, 541, 542 : Ruminants, 543, 544 Cape Ant-eaters, 479, 500, 525, 564, 598, 604 Capibara, 507 Capillaries, 51, 92, 93— See Vascular system Capitellum, 456 Capitular facet, 315, 449 Capitulum, 448 Capra, 482, 501, 638 Caprimulgida, 417, 420 Capuchin Monkeys, 487 Capybara, 507 Carapace, of Cheloriia, 341, 347 Carboniferous, 639 Carcharias, 178 Carchariidce, 181 Carcharodon, 187 Cardiac nerve, of Cramata, 104, 106 Cardiac sac, of Balanoglossus, 4, 7 Cardiac vein, 281, 282 Cardinal veins, 93— See Vascular system Cardio-visceral vessel, 18, 20 Carina sterni, 388, 425 Carinatae, 412*, 423, 425, 426, 428, 429, 430, 433, 435, 439, 443, 444, 445 Carnivora, 483*, 544, 545, 546, 547, 573, 576, 599, 608 Carnivora vera, 483*, 505, 545, 546, 568 Carotid arteries, 92— See Vascular svstem „ Carotid gland, 280* Carp, 220, 238, 243 Carpals, of Craniata, 82*, 83— See Limb- skeleton Carpo-metacarpus, 391 Carriers, 378 Carter, 682 Cartilage-bones, 78* Cartilages of San tori ni, 466* Caruncle, 442 Casque, of Cassowary, 421 Cassowaries, 410, 421, 423, 425, 430, 432, 442 Castoridce, 484 Casuarius — See Cassowaries Cat-fishes, 220, 227, 228, 232, 233, 236, 238, 241, 243 Catharfies, 416 Cats, 484, 505, 545, 546, 547, 568, 569, 599 Caturus furcatus, 245 Caudal ganglion, of A ppendicularia, 30 Caudal swellings, 183 Caudal vein, 93, artery, 92— See Vascular system Caudate lobe, 574* Cavum arteriosum, 359* Cavum pulmonale, 359* X X 702 INDEX Cavum venosum, 359* Oebidcv, 487*, 500, 554, 600, 633 Cebus, 487 Cell, 671 Ccll-theory, 678* Cement, 85, 86 Centetida, 630 Centetes tcaudatuy, 549 Central canal, 98*, 99 Centralia, of Craniata, 83* Centrale, of Mammals. 515 — See Limb- Skeleton Uentrophoru* calcens, 172 Centrum, of Craniata, 74* Cephalaspis lyelli, 263 Cephalaspis, 262, 263 Cephalaspis murchisoin, 263 Cephalochorda, 44 Cephalodiscus, 2, 9, 10, 11, 12 Cerato-branchial, of Craniata, 77, 78* — See Skull CERATODUS FORSTKRI, 246 : External characters, 247 : Endo-skeleton, 247 : Digestive organs, 250 : Organs of re- spiration, 250 : Blood -vascular system, 251 : Brain, 253 : Urino-genital organs, 254, 255 : Development, 254 Cerato-hyal, 77, 78*, 455— See Skull Cerato-trichia, 151*, 173 Cercopithecidtr, 487*, 509, 600 Cere, 380*, 421 Cerebellum, 100 Cerebral, commissures of Frog, 284 : Lizard, 328 — See Brain Cerebral flexure, of Craniata, 103 Cerebral ganglion, of Appendieularia, 30 Cerebral hemispheres, 100 See Brain Cerebral nerves, 103 — See Brain Cerebral vesicle, Amphioxus, 49, 53 Cerebro-spinal cavity, of Craniata, 70, 71 Cerebro-spinal fluid, 102 Cervical ribs, 317 Cervidie, 482, 502 Ccrvus elaphus, 536, 541, 542, 543 Gestracion, 181 Cestradon galeatus, egg -case, 181 Cestracionts, 188 Cetacea, 479*, 490, 500, 510, 511, 515, 531-534, 567, 571, 572, 573, 574, 576, 581, 582, 598, 605 Chalaza, 435 Chalinolobus morio, 620, 622 ChamcuUo vulgaris, 338 Chameleons, 334, 338, 342, 343, 347, 356, 357, 358, 361, 365, 366, 369 Chambers, Robert, 682 Chambers, of eye, 111 Charadriiformes, 416 Jharadrius, 416, 424 Chauna, 416, 421 Chelodina, 346 Chdone midas, 347, 311 Chelonia, 335*, 341, 342, 344, 346, 347. 350, 351, 352, 353, 356, 357, 358, 359, 361, 362, 368, 369, 371 Chevron-bones, 316, 343 Chevrotains, 536 Chilobranchus, 241 Chimccra, 188, 189, 190. 191, 192, 193, 194, 195, 196 (Jhimaeridae ,188 Chimpanzees, 487, 552, 554, 555, 556, 600 Chirocen/rn#, 234 Chiromyv, 630, 486 Chiroptera, 485*, 507, 550, 551. 570, 575, 599, 609 Chlamydosaurn*, 366 Chlamydoselachus, 169, 171, 174 Cholctpm didactylm, 498 Chol, 261*, 262 Ccelome, of Balanoglossus, 3, 5 : Asci- dia, 22; Amphioxus, 49, 50, 58, 59: Craniata, 69, 123 : Trout, 210, 211 : Rabbit, 458 Coelomic bays, 183 Cvnolestes, 598, 603, 631, 633 Coffer-fishes, 223 Cogia, 480, 580 Coiter, 669 Colies, 417 Coiii, 417 Collar, of Balanoglossus, 3, 4 Collar-pores, 4, 10 Collocalia, 436 Colon, of Dogfish, 152 : of Rabbit, 460 Colours, of feathers, 423 Colours, courtship, 424 Colubrine Snakes, 356 Colugos, 507 Columba, 416 COLUMBA LIVIA, 378 : External charac- ters, 379, 380 : Exoskeleton, 381, 382, 383, 384: Endoskeleton, 385-394 : Mus- cular system, 394, 395 : Digestive or- gans, 396, 397 : Ductless glands, 397 : Respiratory and vocal organs, 397, 398, 399, 400: Circulatory organs, 401,402, 403 : Nervous system, 403, 404, 405 : Sensory organs, 405, 406 ; Urmogeni- tal organs, 407, 408 : Systematic posi- tion, 417 Columbse, 416, 417*, 436, 442 ColumbidtK, 417 Columella auris, of Frog, 269, 271 ; Lizard, 319 : Reptilia, 362 : Pigeon, 390 Columns carnew, 463 Colymbus, 414, 444, 445 Commissures, 329, etc, aberrant, 330 Composite Ascidian — See Ascidue Com- positaz Condylar foramen, 450 Condylarthra, 607 Condyle, of mandible, 454 : of skull, 450 Cones of eye, 111 Contour feathers, 383* Contra-deciduate, 595 Conns arteriosus, 90 Cook, Captain, 676 Coots, 422 Cope, E. D., 688 Coprodceum, of Pigeon, 397* Copulatory sacs, 333 CoraciidcK, 417 Coraco-humeralis, 276 Coracoid, of Craniata, 84* — See Pectoral arch Coraco -scapular angle, 390* Corium— See Dermis Cormorants, 415, 422 Cornea, 110* Cornu, hvoid of Craniata, 78— See Skull Cornual cartilage, of Lamprey, 126 Coronal suture, 317* 452 Corona radiata, 587* Coronary, 320 Coronary arteries, 464 Coronoid process, Lizard, 320 — See Skull of Mammals, 454 Corpora bigemina — See Optic lobes Corpora cavernosa, 474, 475 Corpora quadrigemina, 471, 577 — See Brain of Mammals Corpora restiformia, of Dogfish, 159 : Holocephali, 193 — See Brain Corpora striata, of Craniata, 103*— See Brain Corpus callosum, 469, 576 Corpus geniculatum, 471* Corpus luteum, 587* Corpus mammillare, 471 — See Brain of Mammals Corpus spongiosum, 474, 475 Corpus sterni, 511 Corpus trapezoideum, of Rabbit, 472 Corpus uteri, 584 Cortex, of hair, 489 : of Kidney, 582 Corcidce, 417, 445 Costal plate, 344* Costal sternum, 273* Costo-pulmonary muscles, 398 Cotyledonary placenta, 595 Cotyledons, 481* Cotyloid, 457, 515 Cowper's glands, 474 Craig-fluke, 227 Cranes, 416, 422 Cranial cavity, of Craniata, 70 Cranial nerves -See Cerebral nerves, 103 Craniata, 44 : Classification, 65 : External •-, characters, 67, 68, 69, 70 : Body-wall x x 2 704 INDEX and internal cavities, 69, 70, 71: Skeleton, 72 84 : Digestive organs, 84, 85, 86, 87 : Respiratory organs, 88, 89 : Blood -vascular system, 90-97 : Lymphatics, 97 : Nervous system, 98, 99, 100: Sensory organs, 107-116: Urinogenital organs, 117, 118, 119, 120 : Development, 121 : metamerism, 122 : Distinctive characters, 123 Cranium of Craniata, 76— See Skull Crax, 416 Credontia, 609 Cremaster, 596 Cretaceous, 640 Cribiform plate, 452 Cricoid, of Lizard, 328; Reptilia, 357: Pigeon, 397 : Rabbit, 466 Cristse acoustics?, 115*, 116 Crocodilia, 336*, 341, 342, 343, 344, 345, 346, 347, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 368, 369, 370, 371 Crop, of Pigeon, 396 Crossopterygii, 218*, 225, 226, 229-230, 231, 232, 234, 235, 236, 238, 239, 241, 242, 243 Crotalus, 349 Crowned pigeons, 416 Crows, 417, 445 Crura cerebri, of Craniata, 102— See Brain Crusta petrosa, 172* Cryptobranchus, 293, 296 Cryptodrilid(K, 621, 622 Cryptozoic fauna, 637* Crypts, of uterus, 476, 594 Crypturi, 416, 427, 432, 443, 445 Ctenoid scales, 228* Cubitals, 385* Cuboid, 458 Cuckoos, 417 Cuculidce, 417 Cumulus proligerus, 587* Cuneiform, 456, 515, 516 Currasows, 416 Curlews, 416, 420 Cutaneous glands, of Mammals, 491 Cuvier, 678, 679, 686 Cyanorhamphus, 620 Cyathozooid, 39* Cycloid scales, 228* Cyclomyaria, 22*, 27, 39, 40 Cydopterus, 232 Cyclostomata, 66, 123: Example, 124: Distinctive characters and classifica- tion, 137 : Comparison of Myxinoids with Lamprey, 138 : General remarks, Oydoturus, 498, 527, 528, 530 Cyywu, 416, 420, 423 Cynocephalus, 487, 514, 554, 556, 610 Cynocephcdus anubis, 556 , 417, 420, 422 Cystic duct, of Craniata, 88* JL/ANA, J. D., 683 Darwin, Charles, 649, 682, 683 Darwin, Erasmus, 676 Darwinian theory, 649 Dactylopterm,- 226 D'Azyr, V., 675 DasypodidcK, 479 — See Armadillos Dasyprocta, 507 Dasypus sexcinctu*, 499, 526, 528, 531 Dasyures, 478, 494 Dasyurida, 478, 494, 521, 523, 564, 597 Dasyurus, 523 Ddsyurus viverrinus, 494 Decidua, 476, 595 Deciduate, 476, 595 De Bary, 680 Deer, 482, 502 Deer, Red, 536, 541, 542, 543 Delphinus, 480, 501, 560, 561 Deltoid ridge, 456* Demersal eggs, 241* Dendrohyrax, 483 Dental formula, 562* Dental groove, 558 Dental lamina, 558 Dental papilla, 86*, 558 Dental sac, 559 Dentary, of Craniata, 80, 81* Dentine, 85*, 86 Dentition— See Teeth DePerthes, B.,686 Depressor muscles, 276 Dermal defences, 190 Dermal fin-rays, 81* Dermal teeth, 145 Dermalochdys, 347, 368, 370 Dermis, of Amphioxus, 46 : Craniata, 69 Derotremata, 293*, 296 Desmognathous 428* Determinants, 689* Development, of Balanoylosxus, 7, 8, 9 : Ascidian, 31, 32, 33, 34, 35, 36, 37, 38: Pyrosoma, 39 : Doliolum, 39, 40 : Safyxt, 41: Amphi'xus, 56-65: Craniata, 121 : Lamprey, 135, 136, 137 : Elasmo- branchii, 182, 183, 184, 185, 186, 187 : Holocephalii 196 : Trout, 216, 217 : Teleostomi, 241 * Geratodux, 254, 255 : Frog, 288, 289 290, 291 : Amphibia, 308, 309, 310, 311 : Reptilia, 362, 363, 364, 365 : Aves, 435-442 : Rabbit, 475. 476 : Mammals, 586-598 Devonian, 639 Diactele, 100 INDEX 705 Diaphorapteryx, 625 Diaphragm, '458 Diaphragm, of Craniata, 72 Diastema, 458 Diazona, 23 Dicotyles, 482, 599 Didelphyidce, 478, 492, 495, 521, 524, 563, 584, 598, 602, 629, 633 Didelphys dorsigera, 583 Didelphys marsupialis, 563, 564 Didelphys virginiana, 494 Dididai, 417 Diduncidus, 633 Didus, 416, 417, 430, 444 Diencephalon, of Craniata, 100* Diffuse placenta, 595 Digestive system, of Balanoylosms, 4, 5 : Ascidia, 19 : Appendicularia, 29 : Sim- ple Ascidians,29: Composite Ascidians, 29 : Salpa, 29 : Doliolum, 29 : Crani- ata, 84 : Lamprey, 128 : Myxine, 141 : Dogfish, 152, 153 : Elasmobranchii, 177 : Holocephali, 192 : Trout, 210, 211 : Teleostomi, 233 : Ceratodus, 250 : Frog, 276, 277 : Amphibia, 304 : Lizard, 323, 324 : Reptilia, 355, 356 : Pigeon, 396, 397 : Aves, 433, 434 : Rabbit, 458 : Mammals, 557 Digitals, 385* Digitigrade, 37o Digits, 69— See Limbs Dingo, 599, 63 J Dinoceras, 608 Dinornis robust HH, 431, «42 Dinornithes, 411, 427, 430, 436 Di)iornithid(e, 411, 443 Dinosauria, 337*, 374, 375 Dinotheridtr, 606 Dinotherium giganteum, 607 Diomedea, 415, 430, 436 Diphycercal, 190*, 230 Diphyodont, 476* Diploblastic, 617 Dipneumona, 257* Dipnoi, 66, 246: Example, 246: Dis- tinctive characters and classification, 257 : General remarks, 257 DipodidcK, 484, 547, 549 Diprotodon australis, 602, 603, 642j Diprotodont, 562 Diprotodontia, 478*, 496, 602 J)il>terus, 260 J)ipux, 506 Discoidal placenta, 595 Dispersal, 626 Distalia, of Craniata, 83* Distinctive characters and classification of Acrania, 64 : Craniata, 66, 123 : Cyclostomi, 137: Elasmobranchii, 166; Teleostomi, 217 : Dipnoi, 257 : Am- phibia, 292 : Reptilia, 334: Aves, 408 : Mammals, 476 Distribution, of Acrania, 64 Distribution, geological, 638 Distribution, geological, of Cyclostomi, 143 : Elasmobranchii, 187 : Holoce- phali, 196 : Teleostomi, 243 : Dipnoi, 257 : Amphibia, 312 : Reptilia, 370, 371 : Aves, 443 : Mammals, 600 Distribution, geographical, 619 Distribution, geographical, of Cephalo- discus, 12 : of Rhabdopleura, 12 : of Urochorda, 42 : of Cyclostomi, 143 ; Holocephali, 188 : Teleostomi, 242 : Dipnoi, 257 : Amphibia, 312 : Reptilia, 369, 370 : Aves, 442 : Mammalia, 598 Divers, 414, 444 Diverticulum, cesophageal, of Balano- glossus, 4, 6 Dodo, 416, 417, 426, 430, 444, 642 Dogfish, 70— See Scyllium and Hemis- cy Ilium Dogfishes, 169-188 Dogs, 484, 505, 514, 545, 546, 547, 560, 569, 577, 599 Dohrn, Anton, 684 Dolchinia, 23 Doliolida?, 23 Doliolum, 23, 27, 29, 30, 31, 39, 40, 43 Dolphins, 480, 501, 560, 561 Dominant characters, 665* Dorsal aorta, 92— See Vascular system orsal fissure, 98*, 99 Dorsal lamina, 1 8, 19 Dorsal shield, Pteraspis, 261 Dorsal tubercle, of Ascidia, 20, 21 Dorsal root, of spinal nerve, 91* Doves, 416 Down-feathers, 381, 383% 423 Draco, 366, 344, 369, 638 Draco volans, 366 Drepanaspidce, 261, 262* Drepanaspis gemnndenensis, 262 Drepanidce, 633 Drorneeognathous, 428* Drom&us, 410, 421, 423, 425, 430, 432, 442 Dromatheriurti, 600 Dryopithecus, 610 Dryornis, 413 Duck-Bill, 478— See Ornithorhynchns Ducks, 416, 421, 428, 429 Ductless glands of Crapiata, 88, 120 Ductus Botalli, 304, 306* Ductus Cuvieri, 156* 157 Ductus endolymphaticus, 332 Dugong, 481, 511, 516, 534, 535, 536, 568, 598, 606 Dujardin, 679, 682 Dumb-bell-shaped bone, 520 Duodenum, 152* : of Pigeon, 397 : Ral- r^bit, 460 Duplicidentata, 488 Dura mater, 103* 706 INDEX E E AGLES, 416, 428, 436 Eagle-rays, 177 Ear, of Craniata, 115 : of Lamprey, 134 : Myxine, 141 : Dog-fish, 163 : Elasmo- branchii, 179 : Trout, 214, 215 : Frog, 284,286: Lizard, 331, 332: Reptilia, 362 : Pigeon, 380, 406 : Aves, 435 : Rabbit, 453, 472 : Mammals, 581, 582 Eared Seals, 484, 506, 599 Earless Seals, 484— See Phocida> Ecdysis, 342 Echtneis, 226 Echidna, 478, 490, 491, 492, 493, 516, 518, 519, 520, 521, 557, 562, 575, 579, 597 Echidna acideata, 493, 519, 571, 578, 579 Echidna hystrix, 491 Ectocuneiform, 458 — See Linib-sketeton Ecto-ethmoids, of Craniata, 79*, 80 — See Skull Ectopterygoid, 318, 319 Edentata, 479*, 524-530, 557, 564, 575, 584, 598, 604 Edestomurus, 377 Eels, 196, 220, 224, 227, 229, 237, 239, 241, 242, 243 Efferent branchial arteries of Amphioxus, 51 — See Vascular system, 92 Efts, 293, 296 Egg — See Development Egg-shell, of Dog-fish, 166 : of Elasmo- branchs, 181 : Holocephali, 196 : Rep- tiles, 368 : Birds, 435 : Prototheria. 587 Ehrenberg, 682, 686 EJjeoblast, 41* Elasmobranchii, 66, 144 : Example, 144 : Distinctive characters and classifica- tion, 166: External characters, 170: Integument, 171 : Skeleton, 172 : Muscles, 176 : Electric organs, 176 : Luminous organs, 177 : Digestive system, 177 : Respiratory organs, 178 : Blood system, 178 : Brain, 178 : Organs of sense, 179 : Urinogenital organs, 180 : Development, 182 : EthoWv and distribution, 187 Elastin, 151* Electric Cat-fish, 233 Electric Eel, 233 Electric lobe, 176, 179 Electric organs, 176, 233 Electric rays, 170, 177, 179 Elephant, African. 540, 567 Elephants, 483, 490, 505, 558, 567, 599 Eiephas, 483 Elepha* africannx, 540, 567 Elevator muscles, 276 Elimination, 654 Embryological evidence of evolution, 644 Embryonal knot, 587* Embryonic membranes, of Bird, 440, 441 Embryonic rim, 182* Embryonic shield, 363*, 437 Empedocles, 675 Emus, 410, 421, 423, 425, 430, 432, 442 Emys europcm, 351, 354, 357 Enamel, 85, 86 Enamel membrane, 558 Enamel organ, 86*, 558 Enamel pulp, 559 Encephalocoele, 49, 53 Endemic, 622* Endochondral ossification, 78* End-buds, 106 Endolymph, 115* Endolymphatic duct, 115* Endoskeleton — See Skeleton Endostyle, of Tornaria, 9 : Ascidia, 17, 19 : Appendicnlaria, 24 : Doliohim, 27 : of Amphioxus, 46, 48, 62, 63 Ascidian larva, 37, 38 EngcKus, 622 Entepicondylar foramen, 528* Enteric canal— See Digestive Organs Enterocoele, of Amphioxnt, £9 Enteropneusta, 2*, 3, 4, 5, 6, 7, 8, 9 Entocuneiform, 458 — See Limb-skeleton Entoplastron, 346* Eocene, 640 Eotherium, 640 Eozoon canademe, 638 Epencephalon, 100 Ependyme, 102* Epi-branchial, of Craniata, 77, 78* Epiboly, 136*, 216, 290 Epicentrals, 229* Epiccele, 100, 179 Epicoracoid, 272 : Prototheria, 520 Epicrium, 293 Epidermis, of Amphioxu*, 46 : Crani la, 69 Epididymis, 165, 332, 474 Epigastric vein, 326 Epiglottis, of Rabbit, 460 : Mammalia, 576 Epiglottis, intra-narial, 576 Epigonichthys, 44 Epi-hyal, of Craniata, 77, 78*, 207 Epineurals, 229* Xpi-otic, of Craniata, 79*, 80 Kpipharyngeal groove, Amphioxus, 48 Epiphyses, of Craniata, 84* : Rabbit. 447 : Mammals, 510 Epiphysis, (cerebri), of Craniata, 102* Epiplastron, 347* Epipleurals, 229* Epipterygoid, 318, 319, 347 Epi -pubic bones, 521*, 524 Epipubic process, 176 Epi-pubis, 304, 322: Birds, 432 INDEX 707 Episternum, pf Lizard, 320, 321 : Rabbit. 449 : Prototheria, 517 Equidat, 482, 502, 503 Equus caballn*, 537, 542, 543, 566 Eqnus burche/li, 503 Erinaceida, 485 — See Hedgehogs Esox, 640 Ethiopian region, 629 Ethmoidal plane, 513* Ethmo-turbinals, 452*— See Skull Ethology, of Elasmobram-hii, 187 : Cera- todus, 246 : Reptiles, 365 : Birds, 443 Euchorda, 43*, 44 Eudynamis taifensix, 626 Eiidyptes, 414 Eudyptes antipodnm, 415 Eudyptes pachyrhynchus, 426 Euselachii, 169* Eustachian aperture, 453 Eustachian valve, 463 Eustachian tubes, of Frog, 276 : Rabbit, 453 Eutheria, 479*, 575, 578, 579, 584, 585, 587 Evolution, 644 Excretion, organs of, in Ascidia, 21 : Simple Ascidians, 30 : Amphioxim, 52, 53 — See Urinogenital Organs Ex-occipital, 79*, 80— See Skull Exocrjftus, 226 Exoskeleton, of Craniata, 72 Expiration, 199* External coelomic bay, 182*, 183 Extensores dorsi muscles, 274 Extensor muscles, 275 External auditory meatus, 380 External characters, of Bafanoglossus, 1 : Aseidia, 14: Craniata, 67 : Lamprey, 124 : Dog-fish, 144 : Elasmobranchii, 170: Holocephali, 188: Trout, 198: Teleostomi, 224 : Ceratodus, 247 : Frog, 265: Amphibia, 294: Lizard, 314: Reptilia, 338: Pigeon, 379, 380 : Aves (Neornithes), 420 : Rabbit, 446 : Mammalia, 488 External elastic membrane, 72, 73 External gills, 89* 242 External rectus muscle of eye, 113, 114 Kxtra-branchials, 159, 175 Extra-columella, 271, 390 Extremity, of long bone, 84* Eydoux, 683 Eye, of Salpa, 30 : of Amphioxus, 54, 55 : Craniata, 109 : Dog-fish, 163 : Elasmobranchii, 179: Trout, 214: Flat-fish, 226 : Frog, 284 : Amphibia. 308 : Lizard, 315, 331 : Reptilia, 361 : Pigeon, 380, 406 : Aves, 435 : Rabbit, 446, 472 : Mammals, 580 Eye, development, 111 Eyelids- Frog, 265 : Lizard, 315 Eve-muscles, 113 X1 ABELLJE, 457 Fabricius ab Aquapendente, 669 Facial ganglion, of Craniata, 105 Facial nerve, of Craniata, 104, 105 Falciform process, 214* Falco, 416, 428 Falcons, 416, 428 Fallopian tubes, of Rabbit, 475 Fan-tails, 378 Fasciae dentato?. 579" Fat-bodies, of Frog, 287 Faunas, 619 Feathers of Pigeon, 379, 381, 382, 383, 384: Arclweopteryx, 419, 420: Neor- nithes, 423 Feather-follicle, 383, 384* Feather-germ, 383, 384* Feather papilla, 383 Feather-pulp, 383, 384* Feather-tracts, 384" Fdida>, 484, 505— See also Felt* and Cats Felis ho, 547 Felis tigris, 545 Felting, of hair, 488 Femur, 82*, 83— See Limb-skeleton Fenestra ovalis, 269, 286, 332— See Ear Fenestra rotunda, 332, 453 Fibula, 82*, 83— See Limb-skeleton Fibulare, of Craniata, 83* File- fishes, 223, 228 Filo-plumes, 383*, 381, 423 Filum terminale, 284* Fimbria, 470 Finches, 417, 420, 443 Finlets, 225 Fins, of Amphioxus, 45 : Craniata, 8l : Lamprey, 125: Cyclostomi, 138: Dog- fish, 145 : Elasmobranchs, 175 : Holo- cephali, 189, 191 : Trout, 200 : Teleo- stomi, 217 : Teleostei, 225 : Ceratodn*, 247 : Dipnoi, 258 Fins, development of, 186 Fin-rays, of Amphioxus, 47: Craniata, 69 : Dogfish, 151 : Teleostomi, 231 : Teleostei, 225 Fire-toad, 302 Firmistemia, 294* Fishing-frog, 224, 226 Flamingoes, 415, 420 Flanges, of feather, 382' Flat fishes, 221, 226 Fleming, W., 688 Flexor muscles, 275 Flexor perforans, 395 Flexor tarsi, 276 Flippers, 501 Floccular fossa, 453 Flocculi, 472— See Brain Flounder, 221 708 INDEX Flower, W. H., 685 Fluviatile fauna, 636 Flying-fish, 226 Flying Foxes, 485, 516, 551, 570, 599 Flying Lizard, 344, 366, 369, 638 Flying Mammals, 516 Flying Phalangers, 516 Flying Squirrels, 497, 506, 516 Fretal membranes, of Mammals, 590, 591 Follicle cells, of Ascidian, 32 ; Salpa, 41 Follicular membrane, of Amphioxus, 56 Fontanelles, of Craniata, 76*, 77 : Lamprey, 125: Dogfish, 148: Trout, 204 : Frog, 269 Foot, 69— See Hind -limb Foramen, of Monro, 100— See Brain Foramen, ischiatic, 392 Foramen magnum, of Craniata, 76*, 77 — See Skull Foramen ovale, of heart, 463 Foramen Panizza^, 360* Foramen triosseum, 391 Foramina, inter vertebral, of Craniata. 74 Foramina (nerve), of Craniata, 76* — See Skull Foramina, pneumatic, 394 Fore-arm, 69, 314 Fore-brain, of Craniata, 100— See Brain Fore-limb, 69 Fore-kidney, 117*, 119 Fornix, of Rabbit, 470 — See Brains of Mammals Forster, 676 Fossa, glenoid, of skull, 452 : Pre- spinous, of scapula, 455 : Post-spinous, of scapula, 455 Fossa ovalis, 463 Fossa rhomboidalis, 159*, 158 Fossa?, of cranium, 454 Fourth ventricle, 100— See Brain Fowls, 416, 425, 428, 433, 436, 437, 439, 440 Fratercula, 416 Fregata, 415 Fresh-water fauna, 636 Fresh-water Snakes, 335, 367 Frey, 681 Frigate-bird, 415 Fringillidfjp, 417, 420, 443 Frogs, 264, 293, 297, 308— See Anura Frontal clasper, 190, 189 Frontal segment, of Craniata, 80* Frontal sinuses, 576 Frontal suture, 317*, 452 Frontals, of Craniata, 79*, 80— See Skull Fronto-parietals, 270, 269 Fulcra, 219, 228 Fulmars, 415 Fulmarus, 415 Fur, 506 Furcula, 391 Fur Seals, 506 G G AD US MORRHUA, 221 Gaimard, 683 Galaxias, 621, 625 Galen, 669 Galeopithecus, 638, 507 OcUesciUTUs planiceps, 371 Gall-bladder, of Craniata, 88*, 71 : of Dogfish, 152 : of Birds, 433 : Rabbit, 462 Gallinae, 416, 442. 445 Gallns, 416 Gallus hankiva, 425, 428, 433. 436, 437. 439, 440 Game birds, 416 Ganglia habenulae, 131 Ganglion, coeliac, 472 Ganglion impar, 472 Gannets, 415 Ganoidei, 220*, 226, 228, 231, 234, 236, 238, 239, 240, 241, 242, 243, 245 Ganoid scales, 228* Ganoin, 228 •flare-fowl, 443 Gar-fish, 225 Gar-pike, 219 Gasserian ganglion, 104, 160 Gasterochixma, 226 Gasterosteus, 241 Gastornifi, 412, 434 Gastornithes, 412, 443 Gastrrea theory, 686 Gastric glands, 85* Gastric juice, 85* Gastric nerve, of Craniata, 104, 106 Gastrocnemius, muscle, 274, 275 Gastro-cutaneous pores, 6 Gastrula, of Amphioxus, 57 : Craniata, 121 Gaviatus, 492, 522 Hainen, Louis de, 671 Hammer-head shark, 170 Hand, 69, 314 Hapale, 486, 509, 600 Hapalidce, 486, 509, 554, 570, 600, 633 Harderian gland, 331, 580 Hares, 484 Harriotta, 188, 189, 190 Harvey, William, 670 Hatschek, groove of, 55 Hatschek's nephridium, 60, 62 Hatteria, 335, 339, 340, 343, 344, 345, 347,.'349,=353, 356, 361, 362 368, 369 Hawks, 434 Hawk's-bill, 370 Head-shields, of Lizard, 315, 342 Head, of Craniata, 67 Heart, of Balanoglossus, 6 : Ascidian, 18, 19, 36 : Craniata, 90, 91 : Lamprey, 130 : Dog-fish, 154 : Elasmobranchii, 178 : Holocephali, 193 : Trout, 212 : Teleo- stomi, 238 : Ceratodus, 251 : Frog, 278, 279 : Amphibia, 305 : Lizard, 324, 325 : Reptilia, 358, 359 : Pigeon, 401 : Aves, 434 : Rabbit, 462 : Mammals, 575 Hedgehogs, 485, 490, 507, 593 Helix, 621 Heloderma, 355 Helodermida, 369 Hemibranch, 89* Hemichorda, 2* : Affinities, 13 Hemimyaria, 23* Hemijwdes, 416 HEMISCYLLIUM, MODESTUM, 144 : General external features, 144 : Skeleton, 146 : Enteric canal, 152 : Organs of respira- tion, 154 : Blood- sy stein, 152 : Nervous system, 157 : Organs of special sense, 163 : Urogenital organs, 163 710 INDEX Henson, 684 Hepatic artery, 94— Sec Vascular system Hepatic csecum, Amphioxus, 48, 49 Hepatic ducts, of Craniata, 88* Hepatic portal system, 44, 51, 52, 94— See Vascular system Hepatic portal vein, 94— See Vascular system Hepatic vein, Amphioxus, 52 Htptanchw, 169, 171, 172, 173, 174, 175 Heredity, 658, 665 Herodiones, 415 Herons, 415, 422, 423 Herpetfe.*, 569 Herring, 196, 220, 227, 236, 238, 241 Hertwig, 0. , 689 ff&tporornis, 413, 426, 430, 434 Heaporornis regalis, 413 Heterocercal, 145* Heterocoelous, 385* Heterodont, 477* Heterodontua — See Cestrarion Heterostraci, 261, 262 Heterotis, 242 Hexanchus, 169, 171, 172, 173, 174 Hilaire, E. G. St., 678 Hilus, 473 Hind-brain, of Craniata, 100— See Brain Hind kidney, 117— See Metanephros Hind-limb, 69— See Limbs Hip-girdle, of Craniata, 84 Hippocampal commissure, of Frog, 284 : Lizard, 328, 330: Reptilia, 360 : Birds, 435 : Mammals, 578 Hippocampal sulcus, 470 Hippocampus, 223, 241, 360, 470— See Brain Hippopotamus, 482, 502, 503, 541, 516, 539, 543, 599 Hippopotamus amphilriux, 503 Hirundinidw, 417 Hoatzin, 416, 420, 421. 633 Hock, 503* Hoffman's Sloth, 510 Holarctic region, 629 Holoblastic 587 Holobranch, 89* Holocephali, 66, 188 : External charac- ters, 188 : Endoskeleton, 190 : Diges- tive organs, 192 : Respiratory organs, 192 : Heart, 193 : Brain, 193 : Urino- genital organs, 195 : Development, 196 : Fossil remains, 196 Holostei, 219*, 225, 228, 229, 230, 234, 236, 238, 239, 240, 241, 242, 245 Hombrom, 683 Hominidw, 487* Homocercal, 200 Homodont, 477* Homo sapiens, 487— See Man Hoofs, 501 Hook, Robert, 670 Hooker, J. D., 683 Hooklets, of feather, 382* Hoopoes, 417 Hornbills, 417, 421, 433 Horns, of Ruminants, 501 ; of Rhino- ceros, 505 Horses, 482, 502,503, 536, 537, 541, 542, 543, 544, 564, 599, 606 House, of, Appendicularia, 22*, 24 Howling monkeys, 487 Human species, 487 Humerus, 82*, 83 Humming, of Craniata, 76 Humming-birds,* 417, 436, 443 Hunter, John, 674 Huxley, T. H., 683, 685, 686, 689 Hyana, 484, 547, 599 Hyainidie, 484, 547, 599 ffydrochcerua, 507 Hyla, 297, 298, 302 Hytobates, 487, 631, 641 Hyoid arch, of Craniata, 77*— See Skull Hyoid bone, 81* Hyoid cornu, of Craniata, 78* 81, 80 Hyoidean artery, 212 Hyomandibular, of Craniata, 77, 78*, 80, 81 : Dogfish, 149 : Elasmobranchii, 173 : Trout, 204 : Teleostomi, 230 Hyomandibular nerve, of Craniata, 105 — See Brain Hyoplastron, 347* Hyostylic, 78*— See Skull Hypapophysis, 316* Hyperoodon, 480, 516 Hypnos, 173, 176 Hypo-branchial, of Craniata, 77, 78 * Hypoglossal nerve, of Craniata, 104, 10U — See Brain Hypo-hyal, of Craniata, 77, 78* Hypo-ischium, 322, 323, 353 Hypophysis, of Avidia, 20, 21 : Amphi- oxus, 55 : Craniata, 102— See Brain Hypoplastron, 347* Hypsiprymnus rufescenx, 596 Hystricidce 484— See Porcupines Hyracidce, 483, 505 Hyracoidea, 482*, 539, 544, 567, 574, 599, 607 Hyrar, 483, 510, 536, 540, 573 1 J. SI 8, 415, 420 Ibises, 415, 420 Ichthyophis glutinosa, 301, 311 Ichthyopterygia, 336* 373 Ichthyornis, 414, 434 Ichthyornis victor, 414 INDEX 711 Ichthyornithes, 414, 424, 443, 444, 445 Icththyomyzon, 138, 143 Ichthyosaurus, 373 Ichthyotomi, 168* Iguanas, 334, 343, 366, 369 lyitanodon, 374 lynanodon berntmirtensia, 374 Iguanodon mantdli, 375 Iliac process, 176 Iliac region, 84*, of Craniata— See Limb- skeleton Iliac vein, 94 : artery, 92 Ilium, 84*— See Pelvic arch Impennes, 414, 424, 426, 436, 445 Incisors, of Rabbit, 458 Incubation, 408 Incus, 455, 582 Indigenous fauna, 621* Inferior oblique muscle of eye, 113, Inferior rectus muscle of eye, 113, 114 Inferior temporal arch, 319 Inferior temporal fossa, 319 Inferior umbilicus, 381 Infra-orbital glands, of Rabbit, 459 Infundibulum, of Craniata, 102— See Brain Infundibulum, of lung, 467 Inguinal canal, 474 Innominate arteries, 155 Innominate, 515— See Pelvic arch of Mammals Inscriptiones tend i new, of Frog, 274, 275 Insectivora, 484*, 507, 549, 550, 570, 575, 581, 599, 609 Inspiration, 199* Insular faunas, 625 Integument, of Craniata, 69 : Lamprey, 125: Dog-fish, 145: Elasmobranchii, 171: Holocephali, 190: Trout, 200: Teleostomi, 224: Ceratodus, 247: Frog, 266 : Amphibia, 297 : Lizard, 315 : Reptilia, 342 : Pigeon, 381, 382, 383 : Aves, 423 : Rabbit, 446 : Mam- malia, 488 Inter-branchial septa, of Craniata, 88*, 89 : Dogfish, 154 Intercalary pieces, 172 Intercentra, 299, 317* Interclavicle, 302— See also Episternum Inter-costal arteries, 464 Interdorsal plate, 147* Inter-hyal, 207 Intermedium, of Craniata, 83*— See Limb-skeleton Intermuscular bones, 201 Internal ccelomic bay, 182*, 183 Internal rectus muscle of eye, 113, 114 Inter-neural plate, 147* Inter-opercular, 199, 207 Inter-orbital region of skull, 76* Inter-orbital septum, 204, 317 Inter-parietal, 452 Inter-renal bodies, 120* Inter-spinous bones, 208 Inter- vertebral discs, of Crocodilia, 344 : Rabbit, 447* : Mammals, 511 Intervertebral foramina, 266 Intestinal glands, 85* Intestine, of Craniata, 85 Intra-narial epiglottis, 576 Intrinsic muscles of syrinx, 398 Introduced fauna, 621* Investing bones, 78* Iris, lip* Ischiadic foramen, 392 Ischiatic symphysis, 322* Ischium, of Craniata, 83, 84*— See Pelvic- arch Isthmus, 199* Iter, 284— See Brain .1 fj ACANAS, 416 Jacobson's organ, of Craniata, 109 : of Lizard, 331 : of Reptilia, 361 : of Rabbit, 459 : of Mammals, 580 Jacquinot, 683 Janssen, Hans, and Zacharias, 670 Jaws, of Craniata, 77*, 81— See Skull Jerboas, 484, 506, 547, 549 Jugal, 205, 206, 453 Jugular eminence, 554* Jugular plate, 225 Jugular veins, 93 — See Vascular system .Jumping Shrews, 507 Jurassic, 640 K K. .AGU, 633 Kakapo, 426, 444, 620 Kangaroos, 479, 496, 521, 524, 525, 563, 564, 575, 596 Kea, 638 Keel, of sternum, 388 Kidneys— See Excretion, organs of Kidney, development of, 118, 119 Killers, 480, 500 Kingfishers, 417, 436 King of the Herrings, 188 Kiwis, 411, 430, 433, 434, 435, 436, 439, 442 Koalas, 497, 514, 521, 524, 562, 579, 596 Kolliker, A., 679, 680 Kowalewsky, 688 Kowahvskia, 30 712 INDEX I JABIA MAJORA, 475 Labial cartilages, of Craniata, 77, 78* : Dog-fish, 150 Labrichthy* pxittacula, 222 } Labyrinth, membranous — See Ear: caro- tid, 280 Labyrinthodonts, 264, 297, 304 LACERTA, External features, 314, Exo- skeleton, 315 : Endo-skeleton, 315-23 : Digestive system, 323, 324 : Vascular system, 324, 325, 326, 328 : Organs of respiration, 328 : Brain, 328, 329, 330 : Spinal cord, 331 : Organs of special sense, 331, 332 : Urinary and repro- ductive systems, 332, 333: Systematic position, 337 Lacerta muralis, 358 Lacertida, 337*, 369 Lacertilia, 334*, 337, 338, 339, 342, 343, 344, 347, 348, 349, 353, 356, 357, 358, 359, 360, 361, 362, 364, 365, 369, 370 Lacrymal bone, 318 Lacrymal gland, of Lizard, 331 : Mam- malia, 580 Lacteals, 98* Lacustrine fauna, 636 Lvmargus, 178, 180, 181 Lagena, 214, 332— See Ear Lagenorhynchus, 561 Layomyidai, 488 Lagopus scoticus, 622 Lamarck, 648, 676, 677 Lamarckian theory, 648 Lambdoidai suture, 452 Lamina perpendicularis, 452* Lamina terminalis, of Craniata, 103* — See Brain Lamna cornubica, 170 Lampern, 124 Lamprey— See Petromyzon Lanarkia spinosa, 262 Lancelot— See Amphioxus Land tortoises, 336, 341, 368, 370 Languets, 26*, 30 Lankester, E. R., 689 Lapillus, 214*, 215 Larks, All Larus, 416, 434, 442, 626 Larvacea, 22* Laryngeal nerves, of Craniata, 106 Laryngo-tracheal chamber, 278, 305 Larynx, of Lizard, 328 : Reptilia, 357 : Bird, 397 : Rabbit, 466 La Sueur, 683 Lateral line, 107* : Petromyzon, 125 : Dog-fish, 145 : Ceratodus, 247 : Trout, 200 : Holocephali, 190 : Amphibia, 308 Lateral line canal, 145 : Elasmobranchs. 179 Lateral line organs, 107*, 179, 308 Lateral nerve, of Craniata, 106 Lateral plate, of Amphioxits, 61 Lateral plate, of mesoderm, 121 Lateral post-frontal, 318, 319* Lateral temporal fossa, 319 Lateral vein, 94 Lateral ventricle, of Craniata, 100— See Brain Laurentian, 638 Leather- backed Turtle, 347 Leeuwenhoek, 670 Lemur, 486 — See Prosimii Lens, 111, 110 Lens-capsule, 111 Lens involution, 113 Lepidosiren, 246, 257, 258, 259 Lepidosteus, 219, 225, 228, 229, 230, 234, 236, 238, 239, 240, 241, 242 Lepidoateu* platystomn*, 219 Lepidotricha, 208* Lepidotus maximus, 245 Leporidcv, 484, 488 Leptocephalus, 242 Leptoglossce, 337* LEPUS CUNICULUS, 446 : External charac- ters, 447, 446 : Skeleton, 447-458 : Ccelome, 458 : Digestive organs, 458- 462: Circulatory organs, 462-466: Respiratory organs, 466, 467 : Ductless glands, 467 : Nervous system, 468, 469, 470, 471, 472: Organs of special sense, 472 ; Urinogenital organs, 473, 479 : Development, 475 : Systematic- position, 488 Lepus variabilis, 638 Leuckart, 681, 682 Leucocytes, 97 Leydig's gland, 165* Lieberkuhn, 682 Lienogastric artery, 156, Limbs, of Craniata, 69, 82: Dog-fish, 145, 146 : Elasmobranchii, 170 : Holo- cephali, 189 : Trout, 200 : Teleostomi, 226: Ceratodus, 247: Dipnoi, 258: Frog, 265 : Amphibia, 304 : Lizard, 314 : Reptilia, 338, 339, 341 : Pigeon, 380: Aves (Neornithes), 421, 422: Rabbit, 447 : Mammalia, 516 Limb-girdles, of Craniata, 83, 84— See Pectoral arch and Pelvic arch Limb-skeleton, of Craniata, 81, 82: Dog- fish, 150: Elasmobranchii, 175: Holo- cephali, 192: Trout, 208: Teleostomi, 231 : Dipnoi, 250 : Frog, 273, 274 : Amphibia, 304: Lizard, 321, 322, 323 : Reptilia, 353, 354, 355: Pigeon, 391, 392, 393 ; Aves, 430, 433 : Rabbit, 455, 456, 457 : Mammalia, 515 : Proto- theria, 520: Metatheria, 524, 525: Edentata, 528, 529, 530 : Cetacea, 533 : Sirenia, 535 : Ungulata, 541 : Car- INDEX 713 nivora, 547 : Roderitia, 549 : Insecti- vora, 550 : Chiroptera, 551 : Primates, 555, 556, 557 Limicolae, 416, 424 Limoaa, 416, 420 Linea alba, of Frog, 274, 275 Ling, 221 Lingual cartilage, of Lamprey, 127, 126 Linmeus, 672, 673, 675, 686 Lion, 547 Liopelma hochxletteri, 312, 620, 626 Lip-fishes, 225 Liquor amnii, 592* Liquor folliculi, 587* Lister, Lord, 686 Litopterna, 608 Littoral fauna, 634* Liver, of Ascidians, 29 : Euchorda, 44 : Amphioxns, 48, 49 : Craniata, 87*, 71: Petrani'izon, 129, 130 : Myxine, 141 : Dog-fish, 152: Elasmobranchii, 178: Trout, 210, 211 : Teleostomi, 234 : Frog, 277 : Lizard, 324 : Pigeon, 397 : Aves, 433: Rabbit, 462: Mammals, 574 Lizards — See Lacerta and LacertUia Llamas, 599 Loach, 220 Lobi inferiores, 159*, 158, 179 Lophius, 226 Lophobranchii, 223*, 228, 234 Lories, 416 Loriux, 416 Lucretius, 675 Lumbo-sacral plexus, 284* LumbricidjK, 621 Lumbricus, 621 Luminous organs, 177, 227, 228 Lump-fish, 232 Lunar, 456 Lung-fishes— See Dipnoi Lungs, of Craniata, 89, 71 : Ctratodus, 251 : Frog, 277 : Tadpole, 292 : Lizard, 328: Amphibia, 305: Reptilia, 357: Pigeon, 398 : Aves, 434 : Rabbit, 466 : Mammals, 576 Luth, 347, 368, 370 Lutra, 569 Lutridw, 484, 505 Lyell, Sir C. , 679, 686 Lygosoma, 622 Lymphatic gland, 98 Lymphatics, 97, 284, 283, 574 Lymph, 97* Lymph capillaries, 97 Lymph-hearts, 98, 284 Lymph-sinuses, 97, 284 Lymph -space. 47, 52 Lymph-vessels— See Lymphatics L}Tmphatic glands, 98 Lyra, 470* Lyre-birds, 417, 622, 631 M |M ACACUS, 610, 487 Macaques, 487 Macaws, 416, 421, 429 Mackerel, 222, 227, 196 MacropodidtKt 479 — See Kangaroofe Macropoma mantelli, 243 Macropus — See Kangaroos Macropus bennettii, 525 Macropus major, 563, 579, 580 Macroacelididce, 507 Maculae acustica3, 115* Madagascar, fauna, 630 Magnum, 456 — See Limb-skeleton of Mammals Malapterurus, 233 Malar, 453 Malleolar bone, 543* Malleus, 455, 582 Malpighi, 670 Malpighian capsules, 118*, 117 Malpighian capsule, of Bdtllostoma, 141 Mammae — See Teats Mammalia, 67, 313, 446 : Example, 446 : Distinctive characters and classifica- tion, 476 : Integument and general external features, 488-510: Endo- skeleton, 510-516 : Skeleton of Proto- theria, 516-521 : Metatheria, 521-524 : Edentata, 524 : Cetacea, 531 : Sirenia, 534 : Ungulata, 536 : Carnivora,, 544 : Rodentia, 547 : Insectivora, 549 : Chiroptera, 550 : Primates, 552 : Digestive organs, 557 : Vascular system, 574 : Organs of respiration, 576 : Nervous system, 576 : Organs of special sense, 580 : Urinogenital or- gans, 582 : Development, 586 : Geo- graphical distribution, 598 : Geological distribution, 600 Mammary foatus, 596 Mammary glands, 491 Mammary pouch, 491, 492 Mammoths, 599, 606 Man, 487, 510, 514, 552, 553, 554, 555, 556, 557, 570, 571 Manatee, 481, 510, 516, 534, 535, 536, 568, 598 Manatus, 481, 535 Manatus senegalensis, 535 Mandible, of Craniata, 81*— See Skull Mandibular arch, of Craniata, 77* Mandibular nerve, of Craniata, 104 Manidai, 479, 499, 525, 564, 598 Mania, 490, 499, 525, 582 Manis gigantea, 499 Mantle of Ascidia, 15, 16 Manubrium sterni, 449* Manus, 69, 314 Maori Dog, 620 Maori Rat, 620 714 INDEX Marginal plates, 344* Mceritherium, 606 Marmosets, 486 — See Ha,palid« Marrow, 84* Marsh, 0. C., 688 Marsupialia, 478*, 492, 513, 514, 5-21, 522, 523, 524, 561, 562, 573, 575, 576, 578, 579, 582, 583, 584, 587, 595, 596, 598, 602 Marsupial bones, 521*, 524 Marsupial Mole, 495, 579, 581 Marsupium, 478*, 491 Mastodonsaurus, 304 Mastoid, 511 Matthew, Patrick, 683 Maturation of ovum, of Amphioxu*, 56 Maxilla, of Craniata, 80, 81*— See Skull Maxillary antra, 576 Maxillary nerve, of Craniata, 104 Maxillo-turbinals, 453 Meckel's cartilage, of Craniata, 77*, 80, 81 : of Dog-fish, 149 : ot Elasmo- branchs, 173 Mediastinum, 462*, 467 Medulla oblongata, 100* —See Brain Medullary canal, 34, 35 Medullary folds, of Ascidian, 34 : of Amphioxm, 57, 58 Medullary groove, of Craniata, 34, 98 Medullary keel, 136 Medullary plate, of Ascidian, 34 : of Amphioxus, 57, 58 Megachiroptera, 485*, 508, 550 Megalobatrachus, 293, 296 Megameres, 288* Megapodius, 416. 437 Megapodiidw, 622 Jfegasolide*, 622 Megatheriidw, 604, 605 Megatherium, 641 Megistanes, 410, 435, 443, 445 Meibomian glands, 580 Melet, 569 Membrana granulosa, 587* Membrana semilunaris, 398 Membrane bones, 78* Membranous cochlea, 472 Membranous labyrinth, 115, 116— See Ear Membranous vestibule, 115* Meniscus, 386* Mental prominence, 554* Mento-meckelian, 270, 269 ^fenura, 622, 631, 417 Mergansers, 416 Mergus, 416 Meroblastic, 587 Meropidce, 417 Merrythought, 391 Mesencephalon— See Mid-brain, 100 Mesenteric artery, 92 Mesenteries, dorsal and ventral, 3, 5 Mesentery, of Craniata, 71, 80* Mesethmoid, 76*, 79, 80*— See Skull Mesoarium, of Dog-tish, 163 : Lizard. 333 Mesoccele. 100 — See Brain Meso-coracoid, 209 Mesocuneiform, 458 — See Limb-skeleton of Mammals Mosoderm, formation in Craniata, 121 Mesodermal segments, of Craniata, 122, 121 Mesogaster, 324* Mesonephric ducts, of Craniata, 117* Mesonephros, of Craniata, 117*, 119 Mesonephros, of Cyclostomi, 138 Mesopithecns, 610 Mesoplodon, 480 Mesopterygiutn, 151*, 175 Meso-pterygoid, 206, 205 Meso- rectum, 324* Mesorchium, of Dog-tish, 165* : of Lizard, 332 Mesoscapular segment, 515* Mesosternum, 511 Meso-tarsus joint, 394* Metacarpals, of Craniata, 82*, 83 Metacarpals (feathers), 385 Metacoele, 100 — See Brain Metacone, 561* Metaconid, 561* Metacromion, 455 Meta-discoidal placenta, 595 Metagenesis of Thaliacea, 22 Metamerism, 122* Metamorphosis, of BcLlanoylonsus, ft, 9 : of Ascidian, 23, 31 : Frog, 290 Metamorphosis, retrogressive, of As- cidian, 14, 37, 38 Metanephric ducts, of Craniata, 117*, 119 Metanephros, of Craniata, 117*, 119 Metapleure, of Amphioxus, 45, 63 Metapophyses, of Rabbit, 448* Metaptervgium, 151*, 175 Metapterygoid, 205, 206 Metatarsals, of Craniata, 83" Metatheria, 478*— See Marsupialia Metencephalon, 100* — See Brain Mice, 484 MicrocMroptera, 485*, 508, 550 Microlestes, 640 Micromeres, 288* Micropyle, 216 Mid-brain, of Craniata, 100 Mid-digitals, 385 Mid-kidney, 117 Milne-Edwards, H., 681 Mimicry, 657 Minimus, if Craniata, 83* Miocene, 641 Mitral valve, 463 Moas, 411, 427, 430, 436 INDEX 715 Mohl, Von, 679 Molars, of Rabbit, 459 Mole, Marsupial, 495 Moles, 485, 507, 549, 550, 580, 593 Molye, 293, 296 Molgulidae, 38 Momotidce, 417 Mongrels, 665* Monitors, 334, 347, 356. 357, 366, 369 Monkeys — See Primates Monophyodont, 476* Monopneumona, 257* Monotremata, 478*, 510, 575, 576, 578, 579, 581, 582, 584, 587, 597, 598 Monotremes — See Monotremata Monro, Alexander, 675 Mordacia, 124, 138, 143 Monckitft, 638 Moseley, H. N., 683 Motmots, 417 Moulting, of feathers, 423 Mucous canals, 161 Mucous membrane, 85* Mud-fishes, 219, 220, 246 Mud tortoises, 336 Miiller, Johannes, 680 Miillerian duct, 119, 120* — See Repro- ductive system Mulleromis, 411 Mullet, 222 Multituberculata, 600, 601 M-tu-idce, 484 Murray, John, 683 Mut, 594 Muscle buds, 187* Muscle-plates, 47 Muscles, of Amphioxn*, 46 : Lamprey, 128 : Elasmobranchii, 176 : Trout, 210: Frog, 274, 275: Amphibia, 304: Pigeon, 394, 395 : Aves, 433 Muscular layer, of Craniata, 70 Muscular system, of Amphioxu-s, 46 Musculi papillares, 463 Musculi pectinati, 324, 463 Musculo-cutaneous vein, 280, 281 Mus decumanus, 571, 621 Mus domesticus t 621 Museums Association, 685 Mus niaorum, 620 Mus muxc.ulus, 571 Musiphayidce, 630 MueophagidcB ,417 Mustelidce, 181, 484, 571 Mustelus antarcticti*, 70 Mutations, 653* Mycetes, 487 Myctodera, 293* Myelencephalon, 100 Myliobatis, 178 Mylodon robustua, 605, 641 Myocoele, 61 Mvocommas, of Amphioxus, 46 : Crani- ata, 70 Myomeres, of Amphioxus, 46, 49 : Crani- ata, 70 Myytaciua tuberculata, 620, 622 Mystacoceti, 480*, 533, 534 Myrmecobiua, 494 Afyrmecophaffa, 525, 526, 527 Myrmecophagidee, 479 Myxine, 138, 139, 140, 141, 142 Myxine ylufinosa, 139 Myxinoidei, 138* 1\| AKES — See Olfactory organ Nasals, 80* Nasal spine, 554* Naso-buccal groove, 146 Naso-palatine canals, 459* Naso-turbinals, 453 Native Cats, 478 -See Dasyures Natural selection, 649, 653* Nanltinus, 622 Navicular,458— See Limb-skeleton (Mam- malia) Nearctic region, 629 Neck, of Craniata, 67 Necturus, 293, 294, 295, 300, 302, 303 Necturus maculatus, 294, 295 Nekton, 636* Neoceratodus, 246 Neochama, 621 Neo-pallium, 578 Neornithes, 410*, 420, 433 Neotropical region, 633 Nephridium — See Excretion, organs of Nephrostome— See Excretion, organs of Nephrotome, 184 Nerve components, 116 Nerve-foramina : Craniata, 76 Nerves, of Amphioxus, 54, 55 : Craniata, 103 — See under Brain and Spinal cord Nervi terminales, 103* Nervous system, of Balanoglossus, 7 : Ascidia, 20,21: Urochorda, 30: Amphi- oxu8, 53, 54: Craniata, 98, 99, 100- 106 — See under Brain and Spinal cord Nesonetta, 620 Nesopitheciis, 642 Nestling-downs, 423* Nestor, 620 Nestor notabilis, 638 Nestor productus, 632 Nests, of Birds, 436: of Stickleback, 241 Neural arch — See Vertebra Neural canal, 53 Neural cavity, of Craniata, 70, 71 Neural gland, 21 Neural plate, 147* INDEX Neural spine, 147 Neural tube, Craniata, 73* Neurenteric canal, 34, 58, 59, 290 Neurenteric passage, 183 Neurocale, 2*, 34, 35, 49, 53, 57, 58 Neuroglia, 98* Neuromast-organs, 107*, 108 Neuromasts, 107*, 108 Neuron, 44, 49, 50, 53, 57, 58. 98 Neuropore, 34, 54, 55, 58, 61, 62 Newts, 264, 293, 296 New Zealand, comparison of its physical conditions and fauna with .those of Great Britain, 619 New Zealand region, 632 Nictitating membrane, of Elasmobranchii, 179 : Frog, 265 : Lizard, 315 : Pigeon, 380 : Rabbit, 446 : Mammalia, 580 Nidicoke, 442* Nidifugae, 442* Non-Ruminants, 482* Nostril, 109 Notidanidce, 169, 178 Notochord, 1*: Balanoglossus, 6 : Cepha- lodiscus, 10, 12 : Rhabdopleura, 12, 13 : Appendicularia, 24 : Ascidian larva, 34 : Amphioxus, 47, 49, 58, 59 : Crani- ata, 71, 72 — See Vertebral column Notochordal sheath, 47, 72, 73 Notochordal tissue, 47, 72, 73 Nolornis, 426, 430, 620, 642 Notornis alba, 632 Notoryctes, 495, 579, 581 Notoryctes typhlops, 495 Nototherium, 642 Nolotherium mitchelli, 603 Nototrema marsupium, 309, 310 Nuchal plates, 344* Nucleus, of Salpge, 29 * Nurse, Doliolum, 39*, 41 0 0 BLIQUE SEPTUM, 400 Obliquus externus, 274, 275 Obliquus internus, 274, 275 Obstetric Toad, 308 Obturator foramen, 457 Obturator notch, 392 Occipital condyle Occipital plane, 513* Occipital region, 76* Occipital segment, 80* Oceanic Islands, 632* Oceanites, 415 Octacnemidce, 27 Octacnemus, 27, 28, 29 Octochcetus, 621 Octopods, 634 Oculomotor nerve, 104 : ganglion, 104 Ocydromus, 416, 426, 430, 444, 620, 632 Odontoblasts, 86*, 558 Odontoceti, 480*, 501, 531, 533, 534, 580, 606 Odontoid process, of Amphibia, 299 : Lizard, 316 Odontolcse, 413, 443, 444, 445 Odontopteryx, 434 (Esophageo-cutaneous duct, Myxinoid. 140 Oikophura, 22, 24 Oil-bird, 633 Oil-gland, 380" Oken, Lorenz, 680 Old-world Monkeys, 557 Olecranon, 321 Olfactory bulb, 100, 157, 212 Olfactory capsules, 75* Olfactory nerve, 103 Olfactory peduncle, 194, 239 Olfactory tracts, 239 Olfactory lobe, 100*— See Brain Olfactory lobe, median : Amphioxus, 54 : Craniata, 100 Olfactory organ, of Amphioxus, 54, 55 : Craniata, 109* : Lamprey, 132, 133, 134 : Myxinoids, 139 : Dog-fish, 163 : Elasmobranchii, 179 : Trout, 214 : Ceratodus, 254 : Frog, 284 : Amphibia, 308 : Lizard, 331 : Reptilia, 360 : Pigeon, 405 : Mammalia, 580 Olfactory region, of Skull, 76* Olfactory ventricle, 100*— See Brain Olivary body, 472* Omentum duodeno-hepatic, 324 Omentum gastro-hepatic, 324 Omentum, great, 403 Omosternum, 272, 273 Onychodactylus, 298 Opercular, 199 Operculum, of Balanoglossus, 4 : Crani- ata, 67 : Holocephali, 189 : Trout, 199 : Teleostomi, 225 : Dipnoi, 247, 249 : Tadpole, 291 Ophidia, 334*, 339, 342, 343, 344, 347, 348, 353, 357, 358, 359, 360, 361, 362, 367, 368, 370 Opisthoccelous, 229*, 298 Opisthocomus, 416, 420, 421, 633 Opisthotic, 79*, 80— See Skull Opossums, 478, 492, 495, 521, 524, 563, 584, 598, 602 Opthalmic nerve, 104, 105 Opthalmicus profundus, 161 Optic capsules, 75* Optic cup, 112, 113* Optic chiasma, 159 Optic foramen, 452* Optic lobes, 102* — See Brain Optic nerve, 103 Optic thalamus, 102* — See Brain Optic vesicle 112* Optic ventricle, 102* — See Brain INDEX 717 Optocoele, 102*— See Brain Oral cirri of Amphioxus, 45 * 49 Oral hood-, 45* Oral lobes of Doliolum, 27 Oral siphon, 15, 18 Orang, 487, 552, 554, 556, 557, 600 Ora serrata, 110* Orbicular, of Rabbit, 455 Orbit, 76* Orbito-sphenoid, 79*, 80— See Skull Orai, 480 Orca gladiator, 500 Oriental region, 630 Ornithorhynchus, 478, 489, 492, 493, 516, 517, 518, 519, 520, 521, 557, 562, 575, 579, 597 Ornithorhynchus anatinus, 493, 579 Ornithosauria— See Pterosauria Orthagoriscm, 232 Orthoceras, 640 Orthogenesis, 666* Ort-hotomus, 436 Orycteropodidce, 479 — See Cape Ant- eaters Orycteroput, 525, 564, (530 Orycteropua capenxi*, 500 Os cloaca), 322, 323 Os cordis, 576* Osmerus, 640 Ossa innominata, 322* Ossicula auditus, of Rabbit, 455 Ossification, centres of, 79 Osteocranium, 81* Osteo-dentine, 85* Osteostraci, 262 Ostracion, 223 Ostracodermi, 261*, 262, 263, 264 Ostrich, 411, 421, 422, 430, 432, 435, 436, 439, 442, 445 Otariidce, 484, 506, 599 Otis, 416 Otocyst, 25, 30, 35 Otoliths, 116 Otters, 484, 505 Ovarian artery, 92 : vein, 94 Ovidce, 482, 628— See Sheep Ovis aries, 539 Ovulists, 671* Owen, R., 681,686 Owls, 416, 422, 430, 434, 445 Oxen, 482, 501, 571, 582, 598 Oyster-catchers, 416 _L ^DOGENESIS IN AXOLOTL, 311 Pachyornis, 430 Facinian corpuscles, 107* Paltearctic region, 628 Palceohatteria, 371 VOL. II Palceoniscns macropomus, 244 Palajontological evidence of evolution, 646 Palcaospondylus ganni, 143 Palamedea, 416 Palate, hard and soft, 459, 570 Palatine, 81*— See Skull Palatine nerve, 105 Palatine teeth, Holocephali, 191 : Trout, 210, &c. Palato-pterygoid, 250 Palato-quadrate, 77*, 80— See Skull Palinurus, 621 Pallas, 674 Pallium, 103* Pancreas, 71, 87* — See Digestive system Pancreatic juice, 87* Pangenesis, 663* Panmixia, 655 Parachordals, 75* Paraccele, 100*— See Brain Paracone, 561* Paraconid, 561* Paradiseidce, 417, 424, 443, 622 Parafibula, 524 Paramyxine, 143 Paranephrops, 621, 622 Paraphysis, 102* Parapineal eye, 102* Parapophyses, 201* Paraquadrate, 270* Parasphenoid, 80*— See Skull Pareiosauria, 336 Parencephalon, 100*— See Brain Parietal, 79*, 80— See Skull Parietal foramen of Stegocephala, 30 Parietal organ, 102*, 329, 361, 362 Parietal segment, 80* Paroccipital, 450 Parotic processes, 317 Parotid gland, 459 Parotoid glands, 297* Parra, 416 Parrakeets, 416, 430 Parrots 416, 420, 422, 428, 434, 443 Partridge, 436 Parus britannicus, 622 Parus rosea, 622 Passeres, 417, 428, 436, 442, 445 Pasteur, Louis, 686 Patagium, 506* 516 Patella, 393 Patella ulnaris, 430 ^ Paunch — See Rumen Pecarries, 482, 599 Pecten, of Reptilia, 331, 361 : Pigeon, 406 : Birds, 435 Pectoral arch, of Cranatia, 83, 84* : Dog- fish, 151* : Elasmobranchii, 175 : Trout, 209 : Teleostomi, 231 : Ceratodus, 248, 249: Frog, 271, 272: Amphibia, 302, 303 : Lizard, 320, 321 : Reptilia, 353, Y Y INDEX 354 : Pigeon, 386, 390, 391 : Aves, 429, 430 (Neornithes) : Rabbit, 455 : Mam- malia, 513 : Prototheria, 520 : Meta- theria, 522 : Edentata, 528, 529, 530 : Cetacea, 533 : Sirenia, 535 : Ungulata, 541 : Carnivora, 546 : Bodentia, 548 : Insectivora, 549 : Chiroptera, 550 : Primates, 555 Pectoral fin— See Fin Pectoral fin, skeleton of— See Limb- skeleton Pectoralis muscle, 276 Pelagic eggs, 241* Pelagic fauna, 635* Pelagic fishes, 227 Pelicans, 415 Pelicanus, 415 Pelvic arch, of Craniata, 83, 84* : Dog-fish, 152 : Elasmobranchii, 175 : Holocephali, 192 : Trout, 209, 210 : Teleostomi, 232 : Ceratodus, 250 : Frog, 273 : Amphibia, 302, 303, 304 : Lizard, 322, 323 : Reptilia, 353, 354, 355 : Pigeon, 392 : Aves, 432 : Rabbit, 456, 457 : Mammalia, 515 : Prototheria, 521 : Metatheria, 524 : Edentata, 530 : 529: Cetacea, 534: Sirenia, 536: Ungulata, 543 : Carnivora, 547 : Ro- dentia, 549 : Insectivora, 550 : Prim- ates, 556 Pelvic fin— See Fin Pelvic fin, .skeleton of — See Limb-skeleton Pelvic vein, 281 Pelvis of kidney, 473* Pelvi-stcrnum, 304* Penguins, 414, 422, 423, 430, 433, 442, 443, 444 :^H%J — See Urinogenital organs 423* Pentadactyle limb, 69*, 82, 83 : Skeleton of, 83 : Origin of, 615 'c/es. 562 Perameles obesula, 597 Peramelidte, 478— See Bandicoots Perch, climbing, 235 Perch, 196, 222, 230, 236 Perching mechanism, 395 Perennibranchiata, 293*, 294, 300, 304, 305, 308, 312 >ranehial cavity— See A trial cavity ericardial cavity, pericardium, 71, 72* Pericardio-peritoneal canal, 154 ichcetidc?, 622 Perichondrial ossification, 79* Pehchondrium, 79* Perichordal tube, 73* Pcrilymph, 116* Peri meal glands, of Rabbit, 447, 474 Perinseuni, 447, 474, 573 Periophthalmus, 236 Periosteal ossification, 79* Periotic bone, 453, 511 Peripharyngeal bands, of Appendicularia, 24 : of Amphioxus, 48 Peripharyngeal groove, 18 Peripharyngeal ridge, 18 Perissodactyla, 481* 502, 536, 537,541, 543, 566, 574 Peritoneum of Craniata, 71, 72 Permian period, 639 Peron, 683 Peroneeus medius, 395 Persistent pulps, 560* Pes — See Hind-limb Pessulus, 398 Petrels, 415, 425, 434, 436, 44-2 Petrogale penicillata, 523, 578, Petrogale xanthopns, 495 PETROMYZON, external characters, 124 : Skeleton, 125, 126, 127 : Muscles, 128 : Digestive organs, 128, 129 : Respira- tory organs, 129, 130 : Circulatory system, 130 : Nervous system, 131, 132 : Sensory organs, 132, 133, 134 : Urinogenital organs, 134, 135 : De- velopment, 135, 136, 137 Petromyzon branchial™, 124 Petromyzon fluviatilis, 124 Petromyzon marinus, 124 Petromyzontes, 138* Pezophaps, 416, 417, 426, 43<) Phcenicopterus, 415, 420 Phaethon, 415 Phalacrocorax, 415, 422, (521 > Phokmgeridce, 479, 496, 497, 516, 521, 524, 525, 579 Phalangers, 479 — See Phalangeridse Phalanges, 83* — See Limb Phaneroglossa, 294 Pharyngeai bones, superior and inferior, 231* Pharyngo-branchial, 77, 78* — See Skull PharyngognatM, 222*, 225, 231, 243 Pharyngo-hyal, 77, 78*— See Skull Pharynx— See Digestive organs Phascalarctos cinereus, 497, 562, 596 Phascolomyidce, 479, 521, 524, 562 Phascolomys, 562, 583 Phascolomys wombat, 496, 524 Phascolotherium bucklandi, 601, 640 Phasianits, 416, 424 Pheasants, 416, 424 Phoca vitulina, 506, 548 PAocajwa,»480, 501, 516, 532, 560, 572, 573 Phoccena, communis, 532 Phocidw, 484, 505, 506, 548, 599 Phororhacos, 413 Phorozooid, 40*, 41 Phrenic veins, 466 Phylogeny, of Birds, 443 Physeter, 480, 516, 507 Physoclisti, 224* Physostomi, 220*, 226, 228, 230, 233, 234, 235, 236, 238, 240, 241, 243, 245 INDEX 719 Pia mater, 103* Picarise, 417, 445 Picas, 488 Pici, 417 Pigeons, 416 — See Columba Pigeon's mi Ik, 408 Pigs, 482, 502, 536, 538, 541, 542. 543, 544, 564, 599 Pike, 196, 220, 223 Pineal apparatus, of Craniata, 67, 102*, 114* : Petromyzon, 132 Pineal body, 102 : Pineal organ, 102* Pineal eye, of Lizards, 329, 330, 361, 362 Pinna of ear, 447, 481 Pinnipedia, 484*, 505, 506, 516, 545, 547, 569, 576, 599 Pipit americana, 294, 304, 309, 310, 312 Pipe-fish, 224, 241 Pisces, 66, 144 : Appendix, 261 Pisiform, 322, 456 -See Carpus /'ithr.ranfhropus, 610, 642 Pituitary body, of Amphioxns, 55 : Craniata, 71, 72, 87, 102 Pituitary body, extra-cranial portion (Gallorhynchus), 195 Pituitary diverticulum, 71, 87* Pituitary foramen, 450 Pituitary fossa, 450 Pituitary pouch, Petromyzon, 132, 133 : Myxinoids, 139 Placenta, Salpa, 31, 41: Rabbit, 476: Mammalia, 594 Placodontia, 336, 372 Placoid scales, 145, 168, 171, 172 Placula, 32* Pl'njiaulax becklesi, 601, 640 Plankton, 636* Plantain-eaters, 417 Plantigrade, 516* Plastron, 341, 347 P/afalea, 415, 421 Pfatycercut, 416, 430 Platypus — See Ornithorhynchus Platytfomus striatus, 244 Plectognathi, 222*, 228, 239 Pleistocene period, 641 Plesiosaurus macrocephalux, 372, 373 Pleura, 398 Pleuracanthei, 168*, 188 Pleuracanthus chicheui, 168 Pleural ribs, 201 Pleurodont, 355* Pleuronectes cynoglossus, 227 Plewvnectidce, 221, 226 Pleuropterygii, 167* Pliocene period, 641 P/.iohydrax, 607 P/iopithectift, 641 Ploughshare-bone — See Pygostyle Plovers, 416, 424 Plumuhe, 423 Pneumatic duct, of Trout, 210, 211: Teleostomi, 236 Pneumaticity of bones, Pigeon, 394 : Aves, 433 Pneumogastric, 106, 161 Podicipes, 414, 422 Poebotherium, 641 Poephagm, 638 Poison -glands, in Teleostei, 227 : Ophidia, 355, 356, 367 Poli, 675 Pollex, 83* Polynesian region, 632 Polyodon, 219, 225, 227, 242 Polyphyletic, 445* Polyprotodont, 562* Polyprotodontia, 478* Polypteru* bichir, 196, 218, 225, 226, 229, 230, 231, 232, 234, 235, 236, 241, 242, 630 Pons Varolii, 472*, 577 Porcupines, 484, 490, 506. 547, 548 Porpoises, 480— See Phocaana Port Jackson Shark, 181, 188 Portal veins, 156 Post-anal gut, 85* Post-axial, 295*, 315 Post-caval, 280 Post-clavicle, 209 Posterior commissure, 212 Posterior temporal fossa, 319 Post-orbital, 318 Post-patagium, 380* Post-temporal, 209 Potamogale, 630 Pouters, 378 Powder-down-patches, 423* Prsecoces, 442* Precocious, 409* Prae-oral pit, 61 Prge-axial, 295*, 315 Pre-anal plate, 315 Precaval vein, 93 Pre-commissural area, 579* Pre-formation, 671*, 674 Pre-f rental, 317, 318 Pre-hallux, 274, 304 Premaxilla, 80, 81— See Skull Premolars, 458— See Teeth Pre-nasal, 539* Pre-nasal region, 76* Pre-olfactory nerves, 160 Pre-opercular, 199 Pre-oral pit, Amphioxus, 60, 61 Pre-patagium, 380* Pre-pubic process, 176 Prepuce, 474* Pre-sphenoid, 79*, 80— See Skull Presternum, 449 Primates, 485*, 492, 552-557, 584, 599, 609 Y Y 2 • 720 INDEX Primitive groove, 290, 437 Primitive knot, 364* Primitive streak, of Aves, 437 Prixtiophorus, 170 Pristis. 170 * Pristiurus, 182, 183 Pro-amnion, 438* Pro-atlas, 343* Proboscidea, 483*, 536, 540, 544, 606 Proboscis, of Balanoglossus 3, 4 : Cepha- lodiscus, 10, 11, 12 : RhalKlopleura, 13 Proboscis-cavity, 3, 4, 12, 13 Proboscis-pore, 3, 4, 10, 12 Proboscis-skeleton, 4, 6 Procellaria, 425 Processus gracilis, 455* Procoelous, 266* Pro-coracoid, 84*, 320 Proctodseum — See Digestive s\7stem Pronatiori, 455 Pronephric duct, 117*, 119 Pronephros, 117*, 119 Prongbuck, 598 Pro-otic, 79*, 80— See Skull Propterygium, 151*, 175 Prosencephalon, 100*, 157— See Brain Prosimii, 486*, 508, 552, 555, 556, 570, 600 Prosocosle, 100 — See Brain Prostate, 474 Protective resemblance, 657* Proteus, 293, 296, 300, 308, 637 Proteus anguineus, 300 Protochordal plate, 364, 365*, 438 Protocone, 561* Protoconid, 561* Protoplasm, 679 Protopterus, 246, 257, 258, 259, 630 Protoselachii, 169* Prototheria, 477*, 491, 492, 514, 515, 516-521, 573, 597, 602 Protovertebra, 58, 61, 122, 184 Protriton, 302 Proventriculus, of Pigeon, 3^6, 397* Psalterium, of brain, 470* : of stomach, 571* Psammapilidium, 26 Psammosteidcv, 261* Psephurus, 219, 242] Pseudis paradoxa, 310 Pseudobranchia, of Dogfish, 154: Elasmo- branchii, 178 : Trout, 212 : Teleostomi, 235 : Ceratodus, 251 Pseudocoele, 470* Pseudophycis backus, 237 Psittaci, 416, 445 Pxlttaciw, 416, 420, 422, 428, 434, 443 Ptarmigans, 424 Pteraspidff, 261, 262* Pteraspis rostrata, 261 Pterichthys, 263, 264 Pterobranchia, 3*, 9, 10, 11, 12, 13 Pt erodes, 416 Pterocletes, 416 Pterodactyles — See Pterosauria Pterodactyl-it*, 376 PteropidcK, 485— See Flying Foxes Pteropus J'USCMS, 552 PteropHsjubatux, 551 Pterosauria, 337*, 375, 376, 377 Pterotic, 204, 205 Pterygiophores, 71, 81* — See Fins Pterygoid, 80, 81* — See Skull Pterygopodia, 171* Pteryla?, 384* Pterylosis, of Pigeon 384, 385* : Aves, 422 Ptychodera, 5 Pubic symphysis, 322* Pubis, 83, 84*— See Pelvic arch Pubo-ischial region, 84* Puffins, 443 Pu/inns, 368, 415 Pulmonary aponeurosis, 398*, 399 Pulmonary artery and vein, 95 : nerve, 106 Pupil, of eye, 110 Purkinje, 679 Pygal plates, 344* Pygochord, 6 Pyyopidw, 369 Pygopodes, 414, 444, 445 f'yyopu* lepidopm, 339 Pygostyle, of Pigeon, 387* Pyioric c«ca, of Trout, 210, 211 : Teleo- stomi, 234 Pyioric valve, 210 Pylorus 152* Pyrosoma, 23, 28, 29, 31, 39, 43 Pyrosomata, 23* Pyrosomidw, 23 Pyrotherium, 606 Pythonomorpha, 335*, 377, 378 Pythons, 335, 339, 343, 353, 367, 368 Q , 77, 78*, 80, 81 — See Sku Quadrate jugal, 270, 2&6 Quadratojugal arch, 319 Quadrupedal, 516* Quagga, 630 Quills, 506* Quoy, 683 R R JABBITS, 484 — See Lepus ouniculus Rachis, 381*— See Feather Radiale, 83*— See Limb-skeleton Radialia, radial cartilages, 81*, 82— See Limb-skeleton INDEX 721 Radio-ulna, 273, 267 Radius, 82*, 83— See Limb-skeleton Rails, 416, 426 Rajida, 170* Rallus, 416, 426 RANA TEMPORARIA and R. ESCULENTA, 264 : External characters, 265 : Endo- skeleton, 266, 267-274 : Muscular sys- tem, 274, 275 ; Digestive organs, 276, 277 : Respiratory organs, 277 : Circula- tory organs, 278-284 : Nervous system, 284, 285: Sensory organs, 284, 285: Urinogenital organs, 286, 287, 288 : Development, 288, 289, 290, 291 : Systematic position, 294 Rana pipiens, 265^ Range, 625* Ranidte, 294*' Ranidens, 299 Rapacious Birds, 442 Ratitee, 410*, 423, 425, 427, 429, 430, 433, 442, 445 Rats, 484 Rattlesnakes, 335, 349 367,| Rauber's layer, 589* Raven, 417 Ray, John, 668, 6711 Rays, 170-187 Recapitulation Theory, 646 Recent period, 642 Receptaculum chyli, 574 Recessive characters, 665* Recognition-marks, 424 Rectal gland, Dogfish, 154? Rectrices— See Pterylosis, 385| Rectus abdominis, 274, 275 Red-bodies, 237* Red Deer, 536, 541, 542, 54* Red-glands, 237* I Redi, 671 Reef-fishes, 224, 244? Regalecus, 233, 635 5 Regeneration, 659* Reindeer, 501, 502, Relationships of Adelochorda, 13, 43 : Urochorda, 42 : Amphioxus, 65 : Cyclo- stomata, 142 : Amphibia, 312 : Aves, 443 : Chordata, 610 : Phyla of Animals, 616 Remiges— See Pterylosis, 385* Renal artery, 92 : vein, 93 Renal organs— See Excretion, organs of, and Urinogential organs Renal portal system, 94*— See Vascular system Replacing bones, 78* Reprod active organs of Balanoglossus, 7 : Cephalodiscus, 11, 12 : Ascidia, 21 : Urochorda, 31 : Amphioxus, 55 — See Urinogenital organs Reptilia, 313 : Example, 314 : Distinc- tive characters and classification, 334 : External features, 338 : Integument and exoskeleton, 342 : Endoskeleton, 343 : Digestive organs, 355 : Organs of respiration, 357 : Organs of circulation, 358 : Brain, 360 : Sensory organs, 360 : Reproductive organs, 362 : Develop- ment, 362 : Ethology, 365: Geographical distribution, 369 : Geological distribu* tion, 370 : Extinct groups of reptiles, 371 : Relationships, 610 Respiration, organs of, Amphioxus, 48 : Craniata, 88 : Petromyzon, 129, 130 : Chiloscy Ilium, 154 ; Elasmobranchii, 178 : Holocephali, 192 : Trout, 212 : Teleostomi, 234 : Ceratodus, 250, 251 : Frog, 277: Amphibia, 304: Lizard, 328 : Reptilia, 357, 358 : Pigeon, 397. 398, 399, 400 : Aves, 434 : Rabbit, 466 : Mammalia, 576 Respiratory, heart, 96* Respiratory tube of Petromyzon, 128, 129, 130 Respiratory valves, of Trout, 199 Restiform bodies, 193 Rete mirabile, 178* Reticulum, 571* Retina, Craniata, 110*, 111 Retropinna, 620 Reversal of selection, 655* Rhabdopleura, 2, 9, 12, 13, 613 Rhamphorhynchus, 377 Rhamphothieca, 408* Rhea, 411, 421, 430, 442, 445 Rhe«, 411, 435, 443 Rhina, 178 Rhinencephalon, 100* — See Brain Rhinobatux, 181 Rhinoceros 482, 490, 502, 504, 537, 538, 541, 543, 544, 599 Rhinoceros indicus, 504 Rhinochetns, 633 Rhinocoele, 100*— See Brain Rhinoderma darwini, 309 Rhomboid scales, 228 Rhynchocephalia, 335*, 339, 344, 371 Rhytina, 481, 568, 598, 606, 642 Ribbon-fishes, 224, 232 Ribs, of Craniata, 71, 74 : Dogfish, 147 : Teleostomi, 229 : Urodela, 299 : Lizard, 316 : Reptilia, 344, 345, 346, 347 : Pigeon, 385 : Aves, 424 : Rabbit, 449 : Mammalia, 510 : Edentata, 526 : Cetacea, 531 : Sirenia, 534 : Garni vora, 544 : Chiroptera, 550 : Rabbit, 449 Rita buchanani, 220 River tortoises, 336, 341, 368 Rock -pigeon, 378 Rock Wallaby, 495, 523, 578 Rodentia, 484*, 492, 547, 548, 549, 569, 574, 575, 581, 595, 599, 609 Rods and cones, of retina, 111* Rollers, 417 722 INDEX Rorqual, 532 Rostrum of skull, Craniata, 76* : Dog- fish, 147 : Trout, 204 : Pteraspis, 261 : Avea, 388 Rudolphi, 682 Rumen, 571* Ruminants, 482*, 490, 501, 502, 536, 541, 543, 544, 566, 571, 572 Rupicapra, 638 )ACCULUS, 115* Saccus vasculosus, 102*, 158, 159'", 179, 213 Sacral ribs, 316 Sacral vertebra, 268 Sacro-vertebral angle, 552* Sacrum, 449* Sagitta, 214*, 215 Sagittal crest, 537* Sagittal suture, 452 Salamanders, 264, 293, 296, 297, 301, 303, 304, 305, 306, 307, 310, 312 Salamandra, 296, 307, 310, 301, 293, 297, 303, 304, 306 Salamandra atra, 301, 310 Salamandra maculosa, 296, 297, 307, 310 Salamandrina, 299 Saliva, 87* Salivary glands, 87*— See Digestive system SALMO FARIO, 198 : External characters, 198, 199, 200 : Skin and exoskeleton, 200: endoskeleton, 201, 209 : Muscles, 210 : Coslome, 210 : Digestive organs, 210 : Air-bladder, 210, 211 : Respiratory organs, 212 : Circulatory organs, 212 : Nervous system, 212, 213: Sensory organs, 214, 215 : Urinogenital organs, 215 : Development, 216, 217 : System- atic position, 224 Salmo ferox, 198 Salmo fontinalix, 198 Salmo killinensis, 626 Salmo salar, 198 Salmon, 196, 207, 220, 241 Salmonidw, 224, 241, 243 Salpa, 27, 28, 29, 30, 31 Salpa democratica, 28 Salpidw, 23 Sand-grouse, 416 Sand-martins, 436 Sand-grouse, 416 Sand-pride, 124 Sarcophilus ursinus, 563 Sarcorhamphus, 638 Sargiis, 234 Sauropsida. 313*, 334 Sauropterygia, 336*, 372, 373 Saw-fish rays. 170 Saw-fish shark, 170 Scala tympani, 472 Scala vestibuli, 472 Scales, 200 Scales, of Pigeon, 380 Scaly Anteater, 499— See Manida± Scaphirhynchnv, 219, 242 Scaphognathus, 376 Scaphoid, 456 — See Limb-skeleton of Mammalia Scapula, 84* — See Pectoral arch Scapula, accessory, 430 Scapular region, 84* -See Pectoral arch Scheuchzer, 679 Schizoccele, 122* Schizognathous, 428* Schleiden, 679 Schneiderian membrane, 109*, 406 Schultze, Max, 680 Schwann, 679 Scincidse — See Skinks Sciuridce, 484, 506, 547 Sclater, P. L., 689 Sclerotic, 75*, 110 Sclerotic plates, of Stegocephala, 302: Lizard, 331 : Reptilia, 361 : Pigeon, 406 Screamers, 416 Scroll- valve of Elasmobraiichii, 178 Scrotal sac of rabbit, 447, 474 Scrotum, 584 Scutes, of Teleostomi, 228 : Stegocephali, 298 : Reptiha, 341 : Armadillos, 499 Scyllium canicula, 144 — See Hemis- cyllium Scymnus, 172, 178 Sea-bream, 222 Sea-cows, 480— See Sirenia Sea-horse, 223, 224, 225, 241 Seals, 484 Sea-snakes, 335, 367 Sea-squirts, 14 Sea-turtles, 368 Sebaceous glands, 488, 491 Sebastes percoidet, 221 Secodont, 561* Secondary cranium, 203 Secretary-bird, 630, 416, 428 Segmental duct, 118*, 119 Selache, 178, 187 Selachii, 168* Selenodont, 561* Sella turcica, 79*, 80, 195 Semicircular canals, 115* Semi-lunar, 456 Semi-lunar valves, 279, 278 Semi-plumes, 423* Semnopithecus, 610 Sense-vesicle, 35, 36, 37, 38 Sensory organs : Amphioxus, 54, 55 Craniata, 106— See Ear, Eye, Latera line, Olfactory organ INDEX 723 Sep», 365 •Septum auricularum, 278, 279*, 305 Septum lucidum, 470 Serous membrane, of Birds, 441, 442 : Mammalia, 476 Serpentarius, 630 Serranus, 241 Sesamoid bone, 393* Severino, 669 Sexual selection, 656* Shaft of long bone, 84* Shagreen, 171 Shags, 415, 422 Shank, 69— See Hind-limb Sharks, 169-188 Shearwaters, 368, 415 " Sheep, 482, 501, 539, 571, 598 Shell of Chelonia, 341 ; of Pigeon, 408 Shell-gland, Dogfish, 164, 165 : Elaamo- branchs, 180 Shell-membrane, 408, 435 Shore-fishes, 227 Shoulder-girdle — See Pectoral arch Shrews, 485, 507 Sid manga, 631 Siebold, 682 Silurian period, 639 Siluroids, 220, 227, 228, 232, 233, 236, 238, 241, 243 Simla, 631, 487 Simia satyrus, 556, 657 Simiidce,, 487*, 509, 552, 554, 600 Sinuses, 93, 156* Sinus rhomboidalis, 405 Sinus venosus, 90— See Heart Siphons, oral and atrial, of Ascidia, 15, 16 Siredon, 312 Siren, 293, 295, 296, 305 Sirenia, 480*, 490, 501, 510, 511, 534, 535, 536, 568, 576, 581, 598, 606 Siren lacertina, 295 Skates, 170 Skeletogenous layer, 73* Skeleton of Craniata — See Skull, Verte- bral column, Ribs, Sternum, Pectoral arch, Pelvic arch, Limb-skeleton Skin, Craniata, 69 Skincs, 334, 342, 365, 369 Skull of Craniata, 75, 76, 77, 78, 79, 80 : Petromyzon, 125, 126, 127 : Myxinoids, 140: Dogfish, 147, 148: Elasmo- branchii, 172, 173, 174: Holocephali, 190, 192, 193 : Trout, 202, 203, 204, 205, 206 : Teleostomi, 230 : Ceratodus, 248, 249: Frog, 268, 269: Amphibia, 299, 300, 301, 302: Lizard, 317, 318, 319: Reptilia, 347-353 : Pigeon, 388, 389 : Archaeopteryx, 419 : Birds, 427-429 : Rabbit, 449-454 : Mammalia, 51 1, 512: Prototheria, 517, 518, 619 : Metatheria, 521-524 : Edentata, 526, 527 : Cetacea 532, 533 : Sirenia, 535 : Ungulata, 537- 540 : Carnivora, 545, 546 : Rodentia, 548 : Insectivora, 549 : Chiroptera, 550, 551 : Primates, 552-555 Slime-Eels, 138 Sloane, H., 676 Sloths, 479, 488, 497, 498, 510, 513, 516, 525, 526, 527, 528, 529, 530, 564, 573, 598 Smelt, 220, 240, 640 Smith, William, 679 Snakes, 335— See Ophidia Snakes, venomous, 367 Soft palate, 459*, 570 Soft tortoises, 336, 342 Solander, 676 Sole, 183, 196, 221, 241 Solenocytes, 52, 53 Solenodon, 634 Solitaire, 416, 417, 426, 430, 642 Somatic motor fibres, 116* Somatic nerves, 99* Somatic sensory fibres, 116* Soricidce, 485, 507 Souleyet, 683 South American Ostrich — See Rhea Spallanzani, 674 Sparrmann, 676 Hpelerpes, 299 ~-f] Spencer, Herbert, 683 Spermatic artery, 92 : vein, 94 Spermatists, 671* Spermatophores, of Holocephali, 195, 196 ; Amphibia, 308 Sperm-sac, of Elasmobrauchii, 165, 180 Sperm Whales, 480, 516, 567 Sphargis, 347, 368, 370 Sphenethmoid, 269 Sphenodon punctatum, 335, 339. 340, 343, 344, 345, 347, 349, 353, 356, 361, 362, 368, 369 Sphenoidal fissure, 452 Spheno-maxillary fissure, 554* \ . Sphenotic, 204, 205 Sphyrna, 170 Spider Monkeys, 487 Spigelian lobe, 674* Spinal accessory nerve, 331 Spinal column — See Vertebral column Spinal cord, of Urochorda, 35 : Amphi- oxus, 54 : Craniata, 98, 99 Spinal nerves of Craniata, 98 Spines, Spinous fin-rays of Teleostomi, 225 Spiny Anteater, 478— See Echidna Spiracle, of Dogfish, 146 : Teleostomi, 225 Spiracular cartilage, 173, 174 Spiracular gill, 154*, 178 Spiral valve of Petromyzon, 130, 134: Dogfish, 152, 153: Elasmobranchs, 178: Teleostomi, 218 : Ceratodus, 250 724 INDEX Splanchnic nerves, 100* Spleen, 88* Splenial— See Skull, 320* 318 Splenial teeth, 250 Splenium, 470*, 579* Spontaneous generation— See Abiogenesis Spoonbills, 415, 421 Spurs, 421 Squalida, 169* Squalodon, 605 Squalodontidce, 606 Squctloraja, 196 Squamata, 334* Squamosal, 80, 81*— See Skull Squamous suture, 449* Squirrel Monkeys, 487 Squirrels, 484, 506, 547 Stapes, of Frog, 269, 271 : Urodela, 300 : Rabbit, 455 Star-gazers, 225, 233 Starlings, 417, 443 Station, 625* Statocyst — see Otocyst Steatornis, 633 Steganopodes, 415 Stegocephala, 264, 293*, 297, 298, 302, 304, 308, 312 Stein, 682' Steller's Sea Cow, 606 Stereornithes, 413, 443 Sterna, 416, 432, 444 Sternal rib— See Rib Sternebrse, 449*, 511 Sterno-tracheal muscles, 398 Sternum, abdominal, 347 Sternum, of Craniata, 74 : Heptanchus, 175 : Frog, 272, 273 : Amphibia, 302 303 : Lizard, 317, 321 : Reptilia, 347 Pigeon, 387 : Birds, 425 : Rabbit, 449 Mammalia, 511 : Prototheria, 517, 518 Edentata, 526 : Cetacea, 532 : Sirenia, 534 : Ungulata, 537 : Carnivora, 544 : Rodentia, 547 : Insectivora, 549 : Chiroptera.^550 St. Hilaire, E. G., 678 Stickleback, 222, 241 Stigmata of Ascidia, 16*, 17* Sting-rays, 170, 171 Stolon, Salpa, 28, 41 : (Doliolum), 39,40: Rhabdopleura, 12 Stomach— See Digestive organs Stomata, 97* Stomias boa, 228 Stomodyeum — See Digestive system Storks, 415, 420, 422, 428 Storm-petrels, 415 Strasburger, E., 688' Stratum corneum — See Skin tratum malpighii — See Skin Striges, 416, 422, 430, 434, 445 Strigidse, 416 Stringops, 620, 426, 444 Stroma of ovary, 120 Struthio, 411, 421, 422, 430, 432, 435, 436 439, 442, 445 Struthiones, 411, 443 Sturgeon, 196, 219, 225, 228, 229, 230, 234, 241, 242 Sturnida:, 417, 443 Styliform cartilages, 126, 128 Stylo-glossus, 450* Stylo-hyal, 450*, 513 Stylo inastoid, 453 Styloid process, 126, 321, 554* Struggle for existence, 649, 650* Sub-atrial ridge, 63*, 64 Subclavian vein, 156 : artery, 92 Subclavius, 394 Sub-cutaneous sinus, 284 Sub-intestinal vein, 52, 94 Sub-lingual gland, 459 Sub-maxillary gland, 459 Sub-mucosa, 85* Sub-neural gland, Ascidia, 21 : Uro- chorda, 30 Sub-ocular arch, 126 Sub-opercular, 199 Sub-orbitals, 203, 205 Subungulata, 482* Sub- vertebral sinus, 284 Sucker of tadpole, 290 Sucking-fish, 226 Suince, 634 Sula, 415 Sun-fish, 223, 232, 239 Superior curved line, 553* Superior oblique muscle of eye, 113, 114 Superior rectus muscle, 113, 114 Superior temporal arch, 319* Supra -angular, 318, 320*— See Skull Supra-clavicle, 209 Supra-ethmoid, 203, 205 Supra-occipital, 79*, 80— See Skull Supra-orbitals, 318 Supra-renals, 120* Supra- scapula : Supra-scapular cartilage, 175 — See Pectoral arch Supra- temporal, 318, 319 Surinam Toad, 309, 310, 312 Survival of the fittest, 654* Sue, 482^See Pigs Suspensorium, 78*, 268, 269 Suspensory ligament, of eye, 110, 111 : Bird, 386, Sus scrofa, 542, 543, 565 Sutures, 449* Swallow, 417 Swammerdam, 670 Swan, 416, 420, 423 Sweat-glands, 488, 491 Swift, 417, 420, 422 Swim-bladder— See Air-bladder Sword-fish, 225 Sylvian fissure, 469* INDEX 725 Sympathetic nerves : Craniata, 99* : Rabbit, 472 Symplectic, 205, 206 Symphysis, 149* Synapticula, of Balanglossus, 5 : Amphi- oxus, 49 Syngnathut, 241 Si/not its barbastellus, 508 Syn-sacrum of Pigeon, 387" Syrinx of Pigeon, 398 : Aves, 434 Syrrhaptes, 416 Systemic heart, 96 _L ADPOLE, 290, 291, 292 : Skull, 271 : Aortic arches, 280, Tjenia hippocampi, 470, 578 Tail : Ascidian larva, 36 ; Amphioxus, 46 : Craniata, 67 Tail coverts— See Pterylosis, 385 Tailor-bird, 436 Talegallu*, 631 Talpa, 507 Taljridie, 485, 628— See Moles Tapetum, 163 Tapirs, 482, 502, 504, 516, 536, 537, 538, 541, 542, 543, 544, 599 Tapirus, 482 Tapir ux indicia, 542 Tfi/i/rns terrestris, 504 Tarsals, of Craniata, 83* — see Limb- skeleton Tarsipes, 522 Tarsius, 486 Tarso-metatarsus, 380*, 393* Tasmanian Devil, 494, 563 Taste-buds, 459, 570, 580 » Taste, organ of, Craniata, 108*, 109 Tatu, 499 Teats of Rabbit, 447 : Mammalia, 492* Tee Tees, 487 Teeth, of Craniata, 85, 86 : Petromyzon, 124, 128: Myxine, 139: Elasmobranchs, 177: Holocephali, 192, 193: Trout, 210 : Teleostomi, 233 : Ceratodus, 250 : Frog, 276 : Amphibia, 304 : Lizard, 323: Reptilia, 355, 356 : Archteopteryx, 419 : Rabbit, 458 : Mammalia, 557- 570 Tender, 369 Telencephalon, 100 Teleostei, 220*, 226, 227, 228, 229, 230, 234, 236, 238, 239, 240, 241, 242 Teleostomi, 66, 196 ; Example, 198 : Distinctive characters and classifica- tion, 217: External form, 224: Exo- skeleton, 227 : Endoskeleton, 229 : Electric organs, 233 : Digestive organs, 233 : Respiratory organs, 234 : Air- bladder, 236 : Heart, 238 : Brain, 238 : Urinogenital organs, 239 : Reproduc- tion and development, 241 : Geographi- cal distribution, 242 : Distribution in time, 243 Temporal canal, 518* Tendon, of muscle, 274* Tensores patagii, 395 Tentorial plane, 513* Terrestrial fauna, 637 Terns, 416, 432, 444 Tenrec, 549 Tentacles of Pterobranchia, 9, 11, 13 : Myxine, 139 : Ascidia, 16. 19 Test-cells of ovum, 32 Test of Urochorda, 15, 16 YW?