BIOLOGY R G 2, LIBRARY G CONTENTS SECTION XIII PAGE PHYLUM CHORDATA 1 lub-phylmn and Class I. Adelochorda ... . . . 1 .b-phylum and Class II. TIrochorda H 1. Example of the Class — Ascidia 12 '. Distinctive Characters and Classification 18 hematic Position of the Example 20 .•ner-1 Organisation 20 Sub-phylum III. Vertebrata 37 Division A. ACRANIA 38 ,, B. CRANIATA . 58 I. Cyclostomata 115 1. Example of the Class — Petromyzon llfi 2. Distinctive Characters and Classification 128 • parison of the Myxinoids with the Lamprey .... 129 4. General Remarks 132 II. Pisces 134 'lass I. Elasmobranchii 134 IxM.mple of the Class — Scyllium canicula or Chilosctfttwm fusciim ... . 135 Distinctive^" and Classification 154 organisation 157 II. Holocephali 173 vi CONTENTS PHYLUM CHORDATA — Continued. Class II. Pisces — Continued. PAOB Sub-class III. Teleostomi 183 1. Example of the Class — Salmo fario 183 2. Distinctive Characters and Classification 201 Systematic Position of the Example 207 3. General Organisation . . . . . ' 209 Sub-class IV. Dipnoi 229 1. Example of the Class— Ceratodus forsteri . . . . . 230 2. Distinctive Characters and Classification 239 3. General Remarks 240 Appendix to Pisces — The Ostracodermi . . . . • . . 243 Class III. Amphibia 245 1. Example of the Class — Eana temporaries 245 2. Distinctive Characters and Classification 271 Systematic Position of the Example . . . . . . 273 3. General Organisation . . 273 Class IV. Reptilia . . 291 1. Example of the Class— Lacerta 292 2. Distinctive Characters and Classification . . . - . .311 Systematic Position of the Example 315 3. General Organisation of Recent Reptilia 315 4. Extinct Groups of Reptiles 344 Class V. Aves .350 1. Example of the Class — Columba lima 351 2. Distinctive Characters and Classification . . . • . . . 380 Systematic Position of the Example 389 3. General Organisation 389 Sub-class I. Archseornithes . . . . . . . . 390 ,, II. Neornithes 392 Class VI. Mammalia . v . . . .417 1. Example of the Class — Lepus cunicidus 417 2. Distinctive Characters and Classification 447 Systematic Position of the Example 460 3. General Organisation 460 The Mutual Relationships of the Chordata 575 The Mutual Relationships of the Phyla of Animals 580 CONTENTS vii SECTION XIV PAGE IBUTION ; 583 1. Geographical Distribution . 583 i Jathymetrical Distribution . 598 • 'ological Distribution 602 SECTION XV THE PHILOSOPHY OF ZOOLOGY 607 SECTION XVI THE HISTORY OF ZOOLOGY 628 A P P E N DIX— Guide to Modern Zoological Literature . . 651 INDEX ... 656 LIST OF ILLUSTRATIONS FIG. PAGE HIS , - 2 664. Balanojossi 665. ,, anterior end .4 666. ,, development 5 667. Tornaria 6 668. „ 6 669. Cephalodiscus, gelatinous investment 7 670. ,, zooid 671. ,, sagittal section ... ... 672. Rhabdopleura 10 673. Ascidia 674. ,, anatomy 13 675. ,, mesh of branchial sac 14 676. ,, diagrammatic longitudinal section 15 677. ,, transverse section 16 678. ,, hypophysis, ganglion, and associated parts .... 17 679. Appendicularia . 21 680. ,, diagram . 21 681. Botryllus violaceus 22 682. Composite Ascidian, diagram of zooid 23 683. Doliolum . 24 684. Salpa clemocratica, ventral view 24 685. ,, lateral view . . . .24 686. Pyrosoma . 25 687. ,, part of section . 25 688. Development of Clavellina, early stages 689. ,, ,, later ,, . . 30 690. Larva of Ascidia mammillata 31 691. Metamorphosis of Ascidian, diagrammatic 33 692. Doliolum, tailed larva 34 693. ,, asexual stage, lateral view ..... .34 694. ,, ,, ,, dorsal ,, 35 695. Salpa, late stage of development 36 Amphioxus laiiceolatus 39 LIST OF ILLUSTRATIONS 697. Aruphioxus lanceolatus, transverse sections of pharyiigeal and in- testinal regions 40 698. ,, ,, anatomy, diagrammatic 43 699. ,, ,, transverse section of pharyiigeal region, diagrammatic ... 44 700. ,, ,, diagram of vascular system .... 45 701. ,, ,, nephridium 47 702. ,, ,, brain and cerebral nerves .... 48 703. ,, ,, anterior portion of neuron .... 49 704. ,, ,, segmentation of the oosperm . . 50 705. ,, ,, formation of gastrula 51 706. ,, ,, development of notochord, neuron, and mesoderm . . . . . . .52 707- ,, ,, advanced embryo 53 708. ,, ,, young larva . . _ . . 54 709. ,, ,, more advanced larva . . .™ . . 55 710. ,, ,, development of atrium 56 711. ,, ,, ,, ,, transverse sections. 57 712. Ideal Craniate 61 713. Section of skin of Fish 62 714. Muscular system of Dogfish ... 63 715. Ideal Craniate, anatomy . . • . . .65 716. Vertebral column of embryo, transverse section .... 66 717. Diagram illustrating segmentation of vertebral column ... 67 718. Elements of embryonic cranium 68 719. Diagrams of cartilaginous skull • . .70 720. Diagrams of bony skull 73 721. Development of pelvic fins, diagram . . . . . . .75 722. Diagrams of limbs and limb-girdles 76 723. Transverse section of intestine 79 724. Structure and development of tooth 80 725. Structure of liver, diagrammatic 81 726. Diagram of gills 83 727. Diagram of vascular system of Fish 85 728. Diagram of circulation in a Fish 88 729. Diagram of vascular system of embryo of air-breathing Vertebrate 89 730. Diagram of heart of Amphibian and Crocodile 90 731. Blood corpuscles of Frog and Man . 91 732. Transverse section of spinal cord 93 733. Diagrams of Craniate brain .95 734. Diagram of cerebral and anterior spinal nerves 98 735. Organs of touch . . . 101 736. Organs of the lateral line 102 737. Taste-buds 103 738. Olfactory cells 103 739. Section of eye . .104 740. Diagram of retina . . . . . 105 741. Development of eye . . .106 LIST OF ILLUSTRATIONS xi FIG PAGE 742. Muscles and nerves of eye 107 743. Pineal eye of Hatteria 108 744. Organ of hearing 109 745. Section of Ampulla 109 746 Urinary tabule HI 747. Diagrams of urinogenital organs . . .112 748. Development of mesoderm in Frog 114 749. Petromyzon marinus, external views of head 116 750. ,, „ skull, with branchial basket .... H7 751. „ „ „ 119 752. ,, ,, dissection of female 121 753. ,, ,, brain 123 754. ,, ,, ,, with olfactory and pituitary sacs . . 124 755. "^^^ " development of olfactory and pituitary sacs. 125 756. ^H ^ , , auditory organ . 126 757. v, ,, transverse section of abdomen . . . 126 758. ,, ,, urinogenital sinus and related parts . . 127 759. ,, development 127 760. , , fluviatilis, head of larva 128 761. Head of Myxine and of Bdellostoma 130 762. Myxine glutinosa, dissection 131 763. ,, auditory organ . . . 132 764. Bdellostoma, kidney 132 765. Palseospondylus gunni 133 766. Chiloscyllium modestum . . . . 135 767. ,, vertebrae 137 768. ,, skull 138 769. ,, visceral arches 139 770. ,, pectoral arch and fin 141 771. ,, pelvic arch and fin 141 772. , , lateral dissection 143 773. ,, branchial sac 144 774. ,, blood-vessels 145 775. Scyllium canicula, brain 147 776. Chiloscyllium, brain 148 777. Scyllium canicula, cranial nerves and brachial plexus . . . 150 778. Chiloscyllium, oviducts 152 779. ,, right kidney and urinary sinus 153 780. Dog-fish, egg-case 153 781. Cladoslache fyleri .154 782. Pleuracanthus ducheni 155 783. Acanthodes wardi ..." 156 784. Lamna comubica 157 785. Urolophus cruciatus 158 786. Centrophorus calceus, dermal denticles 159 787. Scymnus, spinal column 159 788. Urolophus testaceus, skeleton 160 789. Heptanchus, skull . ... 161 xii LIST OF ILLUSTRATIONS FKi. ^AGE 790. Torpedo, showing electric organ 163 791. Cestracion galeatus, egg-case 167 792. Pristiurus, section of blastoderm 168 793. Elasrnobraiich embryo, sections 169 794. Scyllium canicula, embryo 170 795. Ray, embryo 170 796. - Elasmobranch embryo with yolk-sac 171 797. Scyllium canicula, head of embryo 172 798. „ ,, ,, ,, later stage 172 799. Chimsera and Callorhynchus 174 800. ,, vertebral column 176 801. ,, skull 177 802. Callorhynchus antarcticus, skull . . . . . . . 178 803. ,, „ brain ^ . . 180 804. ,, ,, male urinogenital organs . Me? ^^. . 181 805. ,, ,, embryo in egg-shell . . ™ . 182 806. Salmofario .184 807. ,, ,, head 185 808. ,, ,, scale . . . . . . . . . . .186 809. ,, ,, vertebrae 186 810. , , , , caudal end of vertebral column 187 811. ,, „ skull 188 812. ,, ,, ,, disarticulated 189 813. ,, salar, skull of young individual . _ 193 814. ,, fario, fin-ray 193 815. ,, ,, shoulder-girdle and pectoral fin . . . . 194 816. ,, ,, pelvic fin 195 817. ,, ,, side dissection 196 818. „ „ brain 198 819. „ eye 199 820. ,, ,, auditory organ . . . 199 821. ,, ,, urinary organs 200 822. ,, ,, development 201 823. Polypterus bichir 202 824. Acipenser ruthenus 203 825. Lepidosteus platystomus 203 826. Amia calva 204 827. Rita buchanani 205 828. Gadus morrhua 205 829. Sebastes percoides 206 830. Labrichthys psittacula .206 831. Ostracion 207 832. Hippocampus 208 833. Pleuronectes cynoglossus 211 834. Stomiasboa 212 835. Ctenoid and ganoid scales . 212 836. Polypterus, part of vertebral column . 213 837. Sturgeon, skull 214 LIST OF ILLUSTRATIONS xv FIO. PACJK 932. Tropidonotus natrix, skull ... 324 933. Crotalus, skull 325 934. Hatteria ,, . . . . 326 935. Emys europrea, skull 327 936. Chelone midas 327 937. Crocodile, skull 328 938. Emys europrea, tarsus , . . . 329 939. Alligator, carpus 329 940. ,, pelvis 330 941. Crocodile, tarsus 330 942. Monitor, Emys, and Alligator, tongues 331 943. Chameleon, lungs 332 944. Varanus, heart 333 945. Turtle, diagram of heart 334 946. CrocodileJ^rt . . . . .334 947. Alligator, brain . . 335 948. Hatteria, pineal eye 336 949. Alligator, early development 338 950. Rattlesnake, poison apparatus 340 951. Belodon, skull 343 952. Galesaurus planiceps, skull 344 953. Plesiosaurus macrocephalus 345 954. „ pectoral arch 345 955. , , pelvic arch 345 956. Ichthyosaurus communis . 346 957. Iguanodon bernissartensis 347 9*58. ,, mantelli, teeth 347 959. Pterodactylus spectabilis 348 960. Scaphognathus, skull 349 961. Rhamphorhynchus 349 Edestosaurus 350 Columba livia, external form 352 ,, ,, feathers 353 Structure of feather . 354 966. Development of feather 356 967. Columba livia, pterylosis 357 968. ,, ,, bones of trunk 358 969. ,, ,, cervical vertebra 359 970. ,, ,, sacrum of nestling . . 359 971. ,, ,, skull of young specimen! 360 972. Diagram of Bird's skull . . 361 973. Columba livia, hyoid apparatus . 362 ,, ,, columella auris 362 ,, ,, bones of left wing 363 ,, ,, manus of nestling 364 ,, ,, innominate of nestling 364 ,, ,, bones of hind -limb 365. ,, ,, foot of embryo . . 366 xvi LIST OF ILLUSTRATIONS KIG. PAOK 980. Columba livia, muscles of wing 367 981. ,, ,, dissection from right side .... 369 982. ,, ,, lungs and trachea 370 983. Diagram of air-sacs of a Bird 372 984. Columba livia, heart 373 985. ,, ,, vascular system % 375 986. ,, ,, brain 376 987. ,, ,, dissections of brain 377 988. „ „ eye 378 989. ,, ,, auditory organ 379 990. , , , , urinogenital organs, male 380 991. ,, ,, ,, ,, female 380 992. Apteryx australis 384 993. Hesperornis regalis, skeleton 385 994. Ichthyornis victor ^^^: • • ^86 995. Eudyptes antipodum £ Mk . . 387 996. Archseopteryx lithographica ^^fi . , 390 997. ,, ,, skull 391 998. ,, ,, manus 391 999. Opisthocomus and Apteryx, wings 393 1000. Gypaetos and Ardea, pterylosis 394 1001. Casuarius, feather 395 1002. Gallus, Turdus, Vultur, Procellaria, and Casuarius, sterna . . 397 1002 bis. Eudyptes pachyrhynchus, skeleton 398 1003. Apteryx mantelli, skull of young specimen, side view . . . 399 1004. ,, ,, ,, ,, ,, dorsal view . . 400 1005. Anas boschas, skull . .401 1005 bis. Ara ararauna, skull 401 1006. Apteryx mantelli, shoulder-girdle 402 1007. Dinornis robustus, skeleton . . 403 1008. Sterna wilsoni, fore-limb of embryo 404 1009. Apteryx australis, left innominate 404 1010. Gallus bankiva, innominate of embryo . . . . . . 405 1011. Apteryx oweni, hind-limb of embryo 405 1012. Gallus bankiva, egg at time of hatching . . * 408 1013. ,, ,, blastoderm 409 1014. ,, ,, two embryos 411 1015. ,, ,, egg with embryo and embryonic appendages . 411 1016. ,, ,, diagrams of development of embryonic mem- branes 413 1016 bis. Diagram illustrating the Relationships of the chief groups of Birds 417 1017. Lepus cuniculus, skeleton with outline of body .... 418 1018. ,, ,, vertebra 419 1019. ,, ,, skull 423 1020. ,, ,, shoulder-girdle with part of sternum . . . 426 1021. ,, ,, carpus with distal end of fore-arm . . . 427 1022. sacrum and innominates . 428 LIST OF ILLUSTRATIONS xvii FIG. PAGE 1023. Lepus cuniculus, skeleton of pes 429 1024. ,, ,, nasal region, vertical section . .... 430 1025. ,, ,, lateral dissection of head, neck, and thorax . 431 1026. ,, ,, digestive organs 432 1027. „ heart .434 1028. ,, ,, vascular system 436 1029. ,, ,, larynx 437 1030. ,, ,, transverse section of thorax . . . . . 438 1031. ,, „ brain 439 1032. ,, ,, dissections of brain 440 1033. ,, ,, sagittal section of brain 441 1034. ,, ,, urinogenital organs . . . • . . . . 444 1035. ,, ,, female organs (part) 445 1036. ,, ,, diagrammatic section of advanced embryo . . 446 1037. Section of human skin 460 1038. Longit$Lin!U section of hair 461 1039. Development of hair 462 1040. Echidna hystrix, with pouch and mammary glands . >.-''. . 463 1041. Diagrams of development of nipple 464 1042. Ornithorhynchus anatinus 465 1043. Echidna aculeata 465 1044. Didelphys virginiana . . . 466 1045. Dasyurus viverriiius 466 1046. Petrogale xanthopus 467 1047. Phascolarctus cinereus 468 1048. Choloepus didactylus 469 1049. Dasypus sexciiictus . . . 470 1050. Manis pentadactyla 470 1051. Orycteropus capensis 471 1052. Orca gladiator 472 1053. Phoca vitulina 475 1054. Galeopithecus 477 1055. Synotus barbastellus 477 1056. Diagram of Mammalian skull ........ 480 1057. Sagittal sections of Mammalian skulls, diagrammatic . . . 482 1058. Ornithorhynchus, skeleton . . 486 1059. Echidna aculeata, skull 487 1060. Ornithorhynchus, scapula ... 488 1061. Kangaroo, atlas 489 1062. Halmaturus ualabatus, skeleton 490 1063. Dasyurus, skull 491 1064. Petrogale penicillata, skull 491 1065. Phascolomys, skull 492 1066. Phalanger, bones of leg and foot 493 1067. Macropus bennettii, bones of foot 493 1068. Dasypus sexciiictus, skull . . . . . . . . 494 1069. Myrmecophaga, skull, lateral 494 1070. ventral . 495 LIST OF ILLUSTRATIONS PACK 1071. Bradypus tridactylus, skull 495 1072. Dasypus sexcinctus, shoulder-girdle 496 1073. Bradypus tridactylus, skeleton . 497 1074. ,, ,, shoulder-girdle 498 1075. ,, ,, manus 498 1076. „ „ pes 498 1077. Dasypus sexcinctus, pelvis 499 1078. ,, „ pes 499 1079. Phocsena communis, skeleton . . , 500 1080. Balaenoptera musculus, sternum 500 1081. Globiocephalus, skull 501 1082. Halicore australis, skeleton . . . 502 1083. Manatus senegalensis, skull 503 1084. Cervus elaphus, axis 504 1085. Equus caballus, posterior part of skull 505 1086. Ovis aries, skull . ....... . . .*'.•. .507 1087. Hyrax, skull 508 1088. Elephas africanus, skull 508 1089. Cervus elaphus, scapula 509 1090. Tapirus indicus, manus . 510 1091: Equus caballus ,, . . . . 510 1092. Sus scrofa ,, 510 1093. Cervus elephas ,, 510 1094. Equus caballus, tarsus 511 1095. Cervus elaphus ,, 511 1096. Sus scrofa ,, . 511 1097. Felis tigris, skull 513 1098. ,, ,, section of auditory bulla 513 1099. Canis lupus, skull 514 1100. Ursus ferox, section of auditory bulla 514 1101. ,, americamis, carpus . . . 515 1102. Felis leo, digit . . . 515 1103. Phoca vitulina, skeleton . . . 516 1104. Centetes ecaudatus, skull 517 1105. Pteropus jubatus, skeleton . . 519 1106. ,, fuscus, skull 520 1107. Homo sapiens, skull 521 1108. Anthropopithecus troglodytes, skull 523 1109. Simia satyr us, skeleton . 524 1110. Cynocephalus anubis, carpus 524 1111. Homo, Gorilla, and Simia, foot 525 1112. Various forms of teeth, sections 526 1113. Development of Mammalian teeth 527 1114. „ „ „ „ . . . . . . 527 1115. Canis familiaris, milk and permanent dentitions .... 528 1116. Lagenorhynchus, teeth . 529 1117. Perameles, teeth 530 1118. Phascolarctos cinereus, front view of skull 530 LIST OF ILLUSTRATIONS xix PAGE 1119. Macropus major, teeth 531 1120. Sarcophilus ursinus, front view of skull 531 1121. Didelphys marsupialis, teeth . . 531 1122. Orycteropus, section of lower jaw and teeth 532 11.23. Sus scrofa, teeth 533 1124. Equus caballus, skull and teeth 534 1125. Elephas africanus, molar teeth . . 535 1126. Balrenoptera rostrata, lower jaw of foatus, with teeth . . . 535 1127. ,, section of upper jaw, with baleen . 536 1128. Lower carnassial teeth of Carnivora 537 1129. Different forms of stomach in Mammalia 539 1130. Stomach of Ruminant 540 1131. ,, „ Porpoise 541 1132. Liver of Mammal, diagrammatic 541 1133. Canis familiaris, brain . . 545 1134. Echidna- aculeata, sagittal section of brain . . . . . 546 1135. Petrogale penicillata 546 1136. Omithorhynchus anatinus, brain 547 1137- Echidna aculeata, brain 547 1138. Macropus major ,, 547 1139. Cogia greyi ,, 548 1140. Homo sapiens, sagittal section of nasal and buccal cavities . . 548 1141. „ ,, ear 549 1142. Female organs of Marsupials 551 1143. Uteri of Eutheria 553 1144. Homo, sagittal; section of ovary 554 1145. Development of Graafian follicle 554 1146. Segmentation of Mammalian oosperm 555 1147. Lepus cuiiiculus, embryonic area 556 1148. ,, ,, embryos 557 1149. Formation of foetal membranes of Mammal 558 1150. Lepus cuniculus, embryo with membranes 559 1151. Erinaceus, formation of amnion and trophoblast .... 560 1152. Formation of amnion in Mammalia 560 1153. Macropus, mammary fcetus 562 1154. Hypsiprymnus rufescens, embryo and foetal membrane . . . 563 1155. Phascolarctos cinereus ,, ,, ,, ,, . . . 563 1156. Perameles obesula ,, ,, placenta .... 563 1157. Theria and Monotremata, blastula . 564 1158. Phascolotherium bucklandi, mandible 566 1159. Plagiaulax becklesi, mandible 567 1160. Diprotodon australis, skeleton . . 568 1161. Nototherium mitchelli, skull 569 1162. Thylacoles carnifex 569 1163. Glyptodon clavipes, skeleton 570 1164. Mylodon robustus 571 1165. Squalodon, teeth 571 1166. Dinotherium giganteum, skull 573 xx LIST OF ILLUSTRATIONS FIG. PAGE 1167. Tillotherium fodiens, skull 574 1168. Diagram illustrating the mutual relationship of the Chordata . 580 1169. „ „ ,, ,, ,, „ ,, Phyla of animals . 582 1170. Map showing depths of sea between the British Isles and the Continent 587 1171. Map showing depths of sea between New Zealand and Australia . 588 1172. Map of the World showing Zoo-geographical Regions . .. . 592 1173. Diagram illustrating the relations of the Zoo-geographical Regions 598 ZOOLOGY SECTION XIII PHYLUM CHORDATA. THE Phylum Chordata comprises all the Vertebrate animals (Fishes, Amphibians, Reptiles, Birds, and Mammals) together with the Urochorda or Ascidians and the Adelochorda or Balanoglossus and its allies. The name Chordata is derived from one of the most important 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 cells, typically developed from the endoderm, extending along the middle line on the dorsal side of the enteric cavity, and on the ventral side of the central nervous system. It becomes enclosed in a firm sheath, and forms an elastic supporting structure. ) In the Vertebrata (with the exception of Amphioxus and the Lampreys and Hag-fishes) 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 throughout life, by a system of clefts — the branchial clefts : and a third is the almost universal presence at all stages, or only in the larva, of a cavity or system of cavities, the neuroccele, in the interior of the central nervous system. SUB-PHYLUM AND CLASS I,— ADELOCHORDA. Until quite recently a single genus, Balanoglossus, was the only known representative of a class to which the name JEnteropneusta was applied. There seems reason to believe, however, that two remarkable deep-sea animals1 — Rhabdopleura and Cephalodiscus — though not close allies of Balanoglossus, may yet be sufficiently nearly related to^i-t to justify their being placed in the same class. VOL. II B ZOOLOGY SECT. Fro. ri 664. — Balanoglossus. En- tire : animal, br. branchial region ; co. collar ; gen. genital ridges ; hep. prominences formed by hepatic coeca ; pr. proboscis. (After Spengel.) External Characters. — Balano- glossus (Fig. 664) is a soft-bodied, cylindrical, worm-like animal, the sur- face of which is uniformly ciliated. It is divisible into three regions ; in front there is a large club-shaped hollow organ — the proboscis (pr.) ; immedi- ately behind the proboscis and en- circling 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. Numerous glands in the integument secrete a viscid matter to which grains of sand adhere in such a way as to form a fragile temporary tube. The proboscis (Fig. 665, prob.) has muscular walls ; its cavity opens on the exterior usually by a single minute aperture — the proboscis pore (prb. po.) — rarely by two. Its narrow posterior part or " neck " is strength- ened by a layer of cartilage-like or chondroid tissue, which supports the blood-vessels. The collar is also mus- cular, and contains one cavity or two (right and left) separated from one another by dorsal and ventral mesen- teries, and completely cut off from the proboscis cavity. The collar cavity and also that of the proboscis are crossed by numerous strands of con- nective tissue of a spongy character. The collar cavity communicates with the exterior by a pair of cottar pores — ciliated tubes leading into the first gill-slit or first gill-pouch. On the dorsal surface of the an- terior part of the trunk is a double row of small slits — the gill-slits (Fig. 664, br.) — each row situated in a longi- tudinal furrow; these slits increase in number throughout li!eT~ The~^most anterior are in some species overlapped by a posterior prolongation of the collar called the operculum. A pair of longi- xm PHYLUM CHORDATA 3 tudinal ridges — the genital ridges (gen.) — not recognisable in some species, extend throughout a considerable part of the length of the body both behind and in the region of the gill-slits (branchial region); these are formed by the internally situated gonads. Behind the branchial region are two rows of prominences (hep.) formed by the hepatic cceca. The trunk is irregularly ringed, this ringing, which is entirely superficial and does not correspond to an internal segmentation, being most strongly marked behind. The ccelome of the trunk is divided into two lateral closed cavities by a vertical partition (dorsal and ventral mesenteries). Digestive Organs. — The mouth (Fig. 665, 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. 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 lamellae, a median and two lateral; the median which is bifurcate at the end, lies in the septum or interval between two adjoining gill-sacs; the two lateral lie in the two neighbouring tongues. In some species a number of slender transverse rods — the synapticulce — connect together the tongues and the adjoining septa. The posterior part of the alimentary canal is a nearly straight tube with, in its middle part, paired hepatic cceca, which bulge outwards in the series of external prominences already mentioned. Posteriorly it terminates in an anal aperture situated at the posterior extremity of the body. Throughout its length it lies between the dorsal and ventral divisions of the vertical partition, which act as mesenteries. Skeleton. — In front the dorsal wall of the anterior portion of the alimentary canal gives off a diverticulum (div.), the lumen of which extends nearly to the anterior end. This diverticulum consists of epithelium with gland cells and of a sort of retiform connective tissue ; it has been supposed to be homologous with the notochord of the typical Chordata. In close relation with this on its ventral surface is the proboscis-skeleton (prob. sJcel.) which consists of a median part, of an hour-glass shape, with a tooth- shaped process, bifurcating behind into two flattened bars which lie in the anterior region of the collar 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 situated in the anterior part of the collar and the neck of the proboscis. From the pos- terior part of the sinus is given off a vessel which bifurcates to B 2 ZOOLOGY SECT. supply the proboscis. In communication with the sinus are a number of vessels of a bilateral plexus — the glomerulus — situated at the anterior end of the alimentary diverticulum. From the poste- rior 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 continuous behind with the ventral vessel. The dorsal sinus, having no definite walls .ft rob div dcrs.v FIG. 665. — Balanoglossus 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 ; prob. proboscis ; prob.po. proboscis pore ; prob. skel. proboscis skeleton ; vent. n. ventral nerve strand ; vent. v. ventral vessel. (After Spengel.) is not contractile ; but a closed sac, the cardiac sac (card, s.), derived from the heart of the larva and 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 below the epidermis — the thickening being enclosed on both sides by a layer of cells which passes into XIII PHYLUM CHORDATA -fir 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 .neurocoele, opening in front and behind ; in others a closed canal ; in most a number of separate cavities. Between the collar and the trunk the dorsal and ventral strands are connected by a ring- like thickening. 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 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. 666) differs in dif- ferent species. In some it is com- paratively direct ; in others there is a metamorphosis. Impregnation is ex- ternal. Segmenta- tion is complete and fairly regular ; re- sulting in the for- mation of a blastula which is at first rounded, then flat- tened. 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 becomes closed up, and the ectoderm and endoderm become com- pletely separate. The embryo becomes elongated and a transverse groove (gr.) appears (A) : the mouth is formed by an invagination in the position of the groove. The anus is developed in the position formerly occupied by the blastopore. Before the mouth appears there are formed two diverticula of the archenteron which become completely separated off, their cavities subsequently giving rise to the cavities of the proboscis and of the collar and the body cavity of the trunk. By the appearance of a second transverse groove (2?) the body of the embryo becomes divided into three parts — an anterior, a middle and a posterior — these Fig. 666.— Development of Balanoglossus. A, stage of the formation of the first groove (gr.). B, stage in which the second groove has appeared, and the first gill slit has become developed ; co. collar ; g. si. gill slit ; pr. proboscis. (After Bateson.) ZOOLOGY SECT. rob.caxr being the beginnings respectively of the proboscis, the collar and the trunk. The branchial region be- comes marked off' by the appearance of a pair of apertures —the first pair ol branchial slits (g. si.} — and other pairs subsequently de- velop behind these. In the species that undergo a metamorphosis the embryo assumes a larval form termed Tornaria. This (Figs. 667 and 668) is somewhat like an Echinoderm larva, with a pair of cili- ated bands, one of which is considered prse-oral, and the other post-oral, and an inde- pendent circlet of strong cilia at the posterior end. At the anterior end, in the middle of the prse-oral lobe, is an ectodermal thickening — the apical plate — containing nerve-cells and eye-spots and, like v -L , k ' . 7 'Carct.s Fig. 667.— Tornaria. Dorsal view. an. anus ; card. s. cardiac sac ; cil. r. post-oral ciliated band ; cil. r-. posterior ciliated ring ; eye, eye-spots on apical plate ; prob. car. proboscis cavity; prob. po. proboscis pore. (After Spengel.) the apical plate of a trochosphere, constitut- ing the nerve-centre of the larva: this disap- pears in the adult. There is a short ali- mentary canal with mouth and anus. The ciliated bands become lost ; an outgrowth is formed to give rise to the proboscis, and a constriction separates I — -firob.fic — i-nb Fig. 668. — Tornaria. Lateral view. Lettering as in Fig. 6*37 ; in addition, mo. mouth. (After Spengel. XIII PHYLUM CHOKDATA it from the collar; the hinder part becomes elongated and narrow to form the body of the worm ; a series of perforations from the exterior give rise to the branchial pouches. A band of thickened epithelium has been described as developed on the wall of the oesophagus and has been supposed to correspond to the structure termed endostvle to be subsequently met with in the Tunicata (p. 14). TKe collar-fold is formed by the separa- ting off of the deeper portion of the ectoderm along the middle line : or, in other species, by a sinking down of the whole thickness of the layer, which becomes cut off to form a medullary plate with its edges overlapped by the ectoderm. Usually associated with Balanoglossus are two aberrant animals — Cephalodiscus and Rhabdopleura — formerly re- garded as Polyozoa. These both resemble Balanoglossus in having the body divided into three parts or regions — a proboscis, with a proboscis cavity, a collar with a collar- cavity communicating with the exterior by a pair of collar- poreSj and a trunk with two distinct lateral cavities ; and in the presence of a structure re- sembling a notochord with the •same relations to the nervous system as in Balanoglossus. They both differ from Balano- glossus in having the aliment- ary canal bent on itself so that the anal opening is situated not far from the mouth; in the presence of tentacles arising from the collar; and in the comparatively small size of the proboscis. Cephalodiscus, moreover, has only a single pair of apertures which may be regarded as representing the gill-slits ; while in Rhabdopleura such openings are entirely absent. Both forms occur in associations or colonies secreting a common case or investment. Both occur at considerable depths in the sea. Cephalodiscus has an investment (Fig. 669) in the form of a FIG. 669.— Cephalodiscus. Gelatinous investment. (After Mclntosh.) ZOOLOGY SECT. branching gelatinous structure, which is beset with numerous short filiform processes, and contains a number of cavities occupied by zooids. The latter (Fig. 670) are not in organic continuity, so that though enclosed in a common investment FIG. 670.— Cephalodiscus. Entire zooid. (After Mclntosh.) they do not form a colony in the sense in which the word is used of the Polyzoa or the Hydroid Zoophytes. They have this feature in common with such a colony that they multiply by the formation of buds ; but these become detached before they XIII PHYLUM CHORDATA are mature. With the collar region are connected a series of twelve arms or tentacles, each beset with numerous very fine filaments and containing a prolongation of the collar cavity. The proboscis (Fig. 671, ps.) is a shield-shaped lobe overhanging the mouth ; its cavity communicates with the exterior 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 region is on each side a small area in which the body- wall and that of the pharynx are coalescent ; this area is usually, though VIG. 671. — Cephalodiscus. Diagram of longitudinal section, a. anus ; &C1. column of pro- boscis ; &c2. coelom of collar ; bc$. ccelom of trunk ; int. intestine ; nch. supposed notochord ; n. s. nerve-strand ; oes. oesophagus ; ov. ovary ; ovd. oviduct ; ph. pharnyx ; p. p. proboscis pore ; ps. proboscis ; st. stomach ; stk. stalk. (After Harmer.) not always, perforated by an opening — the gill-slit. A nerve- strand containing nerve fibres and ganglion cells is situated on the dorsal side of the collar and is prolonged on to the dorsal sur- face 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 Bala- noglossus, and thus to be homologous with the nofcochord of the Ohordata. The posterior end of the body is drawn out into a sort of stalk on which the buds are developed (Fig1. 670). A pair of 10 ZOOLOGY SECT. ovaries (ov.) lie in the trunk cavity ; and there is a pair of ovi- ducts (ovd.*) (originally supposed to be eyes) lined by elongated pigmented epithelium. Rhabdopleura (Fig. 672) occurs in colonies of zooids organically connected together, and enclosed in, though not in organic con- tinuity with, a system of branching membranous tubes. The be* Fig. 672. — Rhabdopleura. A, Entire zooid. a, mouth ; b, anus ; c, stalk of zooid ; (/, pro- boscis ; «, intestine ; /, anterior region of trunk ; g, one of the tentacles. (After Ray Lankester.) B, Diagrammatic longitudinal section a little to one side of the median line, anus, anus-; fee1, ccelome of proboscis ; lc%. ccelome of collar ; between fee1, and bc'2. is the diverticulum ] be*. coalome of trunk ; int. intestine ; mouth, mouth ; r. rectum. (After Fowler.) collar region bears a pair of arms or tentacles, each carrying a double row of slender filaments — the whole supported by a system of firm internal (cartilaginous ?) rods. The " notochord " and the nervous system resemble those of Cephalodiscus. A single testis has been found, opening on the exterior by a pore situated near the anus. The female reproductive organs have not been discovered. xiii PHYLUM CHORDATA 11 Affinities. — The inclusion of the Adelochorda 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 Ch'or- dates, they are at least a greatly modified branch, taking its origin from the base of the Chorcfate 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 Adelochorda are primitive Chordates the fact is of special interest that among lower forms they show remarkable resemblances in some points to a phylum — that of the Echinodermata — 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. J96A and, though this likeness between the larvse does not establish a near connection, it suggests, at least, that an alliance exists. Between Actinotrocha, the larva of Phoronis (vol. i. p. 330) and Tornaria there are some striking points of resemblance ; and the discovery in the former of a pair of diverticula resembling the " notochord " of the Adelochorda lends support to the view that Phoronis is nearly related to the present group. 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. 12 ZOOLOGY SECT. 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. 673), 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 aper- ture, and some little distance from it 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 float- ing particles, to be flowing steadily in at the former and out of the latter. When the ani- mal is removed from the water both apertures become narrowed, so as to be almost com- pletely closed, by the contraction of sphincters of muscular fibres which surround them. At the same time the walls of the body contract, streams of water are forced out through the apertures, and the bulk becomes considerably 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. 674, test). This proves when analysed to consist largely of the substance cellulose, which has already been referred to (vol. i. p. 14) as a characteristic component of the tissues of plants, and which is rare in its 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. The cells which form it, however, seem to be chiefly derived, not from the ectoderm, but from the underlying mesoderm, from which they migrate through the ectoderm to the outer surface. These for- mative cells of the test are to be found scattered through its substance. Running through it also are 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 FIG. 673. — Ascidia, entire animal seen from the right-hand side. (After Herd- man.) XIII PHYLUM CHORDATA 13: .two channels by a longitudinal septum which, however, does not completely divide the terminal bulb. Through these tubes (which are of the nature of blood-vessels) blood circulates, passing along one channel, through the terminal bulb, and back through the other channel. When the test is divided (Fig. 674) the soft wall of the body or mantle (mant.), as it is termed, comes into view ; and the "body is. or.ctp fcypd alrap FIG. 674.— Dissection of Ascidia from the right-hand side. The greater part of the test and mantle has been removed from that side so as to bring into view the relations 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. hypophysis ; hyp. d. duct of hypophysis ; mant. mantle ; ne. gn. nerve-ganglion ; oes. ap. aperture of oesophagus ; or. ap. oral aperture ; ph. pharynx ; stom. stomach ; tent, tentacles ; test, test. (After Herdman.) 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. 676, or. sipk. atr. siph.). These are continuous at, 14 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. Within the body-wall is a cavity, the atrial or peri- branchial cavity (atr. cav.), communicating with the exterior through .the atrial aperture : this is not a coelome, being formed by involu- tion from the outer surface, and probably lined by a prolongation of the ectoderm. Pharynx. — The oral aperture leads by a short and wide oral passage into a chamber of large dimensions, the pharynx or branchial chamber (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 stigmata (Fig. 676, stigm.) arranged in transverse rows. Through these the cavity of the pharynx communicates with the atrial or peribranchial cavity, which completely surrounds il_ 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. 675) 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. It has been already mentioned that the atrial cavity does not completely surround the pharynx on one side. This is owing to the fact that on the side in question, which is ventral in position, the wall of the pharynx is united with the mantle along the middle line {Fig. 677). Along the line of adhesion the inner surface of the pharynx presents a thickening in the form of a pair of longitudinal folds separated by a groove (end.). To this structure, consisting of the two ventral longitudinal folds with the groove between them, the term endostyle is applied. The cells covering the endostyle are large iFio. 675.— Ascidia, a single mesh of the branchial sac, seen from the inside, i. I. internal longi- tudinal bar ; I. v. fine longitudinal vessel ; p. p'. papillae projecting inwards from the branchial bar ; sg. stigma ; tr. transverse vessel. (After Herdman.) XIII PHYLUM CHORDATA 15 cells of two kinds — ciliated cells and gland cells — the former beset at their free ends with cilia, the action of which is to drive floating particles that come within their influence outwards towards the oral aperture, the latter secreting and discharging a viscid mucous matter. Anteriorly the endostyle is continuous with a ciliated ridge which runs circularly round the anterior end of the pharynx. tent test Ir.v mant •ve.nl. v slam. FIG. 676. — Ascidia, diagram of longitudinal section from the left-hand side, the test and mantle removed, atr. cav. atrial cavity ; atr. siph. atrial siphon ; br. car. branchio-cardiac vessel ; card. vise, cardio-visceral vessel ; gonod. gonoduct ; ht. heart ; hyp. hypophysis ; mant. mantle ; n. gn. nerve-ganglion ; oes. oesophagus ; ov. ovary ; rect. rectum ; stiff, stigmata ; stom. stomach ; tent, tentacles ; test, testis ; tr. v. transverse vessel ; vent. v. ventral vessel ; vise. br. viscero- branchial vessel. (After Herdman.) In front of this circular ridge, and running parallel with it, sepa- rated from it only by a narrow groove, is another ridge of similar character ; these are termed the peri-pharyngeal 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 runs along the middle of the dorsal surface of the pharynx to the opening of the oesophagus. The mucus secreted by 16 ZOOLOGY SECT. 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 these for- wards to the peri- branchial groove, around which they pass to the dorsal lamina, and the cilia of the cells of the latter drive them backwards to the opening of the oeso- phagus. Some little dis- tance in front of the anterior peri- pharyngeal ridge, at the inner or pos- terior end of the oral siphon, is a circlet of delicate tentacles (Fig. 674' tent.). Enteric Canal. FIG. 677. — Ascidia, transverse section, bl. v. blood vessels ; dors. lam. dorsal lamina ; epi. epidermis ; end. endostyle ; gn. ganglion ; hyp. hypophysis ; mus. muscular layer of wall of body ; peribr. peribranchial cavity ; ph. pharynx ; test. test ; vas. tr. vascular trabeculse. (After Julin.) The OeSOpha^US (CBS.) 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-hand side. The stomach is a large fusiform sac with tolerably thick walls. The intestine 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 delicate tubules which ramify over the wall of the intestine is supposed to be of the nature of a digestive gland. The Ascidian has a well-developed blood system. The heart (Fig. 676, ht.) is a simple muscular sac, situated near the stomach in a pericardium forming part of the primitive ccelome. Its mode of pulsation is very remarkable. The conEractions 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 contractions begin again, they have the opposite direction. Thus the direction of the current of blood through the heart is reversed at regular intervals. At each end of the heart is given off a large vessel. That given off ventrally, the branchio-cardiac vessel (br. car.).. XIII PHYLUM CHORDATA 17 runs along the middle of the ventral side of the pharynx below (externally to) the endostyle, and gives off a number of branches which run 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 mscero- Iranchial 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 cardio- visceral vessels, ramify, as already stated, in the substance of the test. The direction 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 trans- verse vessels, after undergoing oxy- genation 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 con- tractions take the opposite direction, the course of this main current of the blood is reversed. The cavity of the heart and vessels is derived from the blastocoele or primary body-cavity of the embryo. The nervous system is of an ex- tremely simple character. There is a single nerve-ganglion (Figs. 374 and 376, ne. gn., and 378 gn.) which lies between the oral and atrial apertures, embedded in the mantle. This is elongated in the dorso-ventral direc- tion, 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 gland — the sub- neural gland (Figs. 674, 676, hyp. ; Fig. 678, gld.)— which there is evidence for correlating with the hypophysis of the Craniata. A VOL. II C FIG. 678.— Ascidia. Hypophysis, nerve-ganglion and associated parts as seen from below, dct. duct of hypophysis ; dors. lam. dorsal lamina ; gld. subneural gland ; gn. ganglion ; hyp. hypophysis ; nv. nv. nerves ; periph. peri- pharyngeal band. (After Julin.) 18 ZOOLOGY SECT. duct (Fig. 678, dot.) runs forward from it and opens into the cavity of the pharynx ; the termination of the duct is dilated, and this terminal dilatation is folded on itself in a complicated way to form a tubercle, the dorsal tubercle, which projects into the cavity of the pharynx. The excretory system is represented by a single mass of clear vesicles, without a duct, lying in the second loop of the intestine. In the interior of these are found concretions containing uric acid. Reproductive system. — The sexes are united. The ovary and the testis are closely united together, and lie on the left-hand side of the body in the intestinal loop. Each of them contains a a cavity which, like the pericardium and the cavities of the nephridial vesicles, forms a part of the original ccelome. Con- tinuous with the cavity of each is a duct — oviduct or spermiduct, as the case may be — which opens into the atrial cavity close to the anus. The development of the Ascidian is described below (p. 27). 2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. The Urochorda are Chordata in which the notochord is confined to the tail region, and, in all but the Larvacea, is found only in the larva. The adults, which for the most part are retrogressively metamorphosed, in other respects besides the abortion of the notochord, are sometimes sessile, sometimes free and pelagic ; they frequently form colonies (fixed or free) by a process of budding, and in some instances exhibit a well-marked alternation of gene- rations. The body is enclosed in a test consisting largely of cellulose. The proximal part of the enteric canal (pharynx) is enlarged to form a spacious sac with perforated walls acting as an organ of respiration. There is a simple heart and a system of sinuses, the cavities of which are remains of the blastoccele. The ccelome is represented, apparently, only by the pericardium and by spaces in the interior of the gonads and of the renal organ. The sexes are united. The larva is always free-swimming, and is nearly always provided with a caudal appendage. Three orders of Urochorda are recognised :— ORDER 1. — LARVACEA. Free-swimming pelagic Tunicata with a caudal appendage, supported by a skeletal axis or notochord. The test is represented by a relatively large temporary envelope, the " house," formed with great rapidity as a secretion from the surface of the ectoderm and frequently thrown off and renewed. The pharynx has only two stigmata which lead directly to the exterior. There is no atrial xin PHYLUM CHORDATA 19 or peribranchial cavity. The principal nerve-ganglion gives off a nerve cord with ganglionic enlargements running to the tail, along the dorsal aspect of which it passes to the extremity. There is no reproduction by budding, and development takes place without metamorphosis. This order contains only a single family, the Appendiculariidce, with five genera, including Appendicularia and Oikopleura. ORDER 2. — THALIACEA. Free-swimming Tunicata, sometimes simple, sometimes colonial, never provided with a caudal appendage in the adult condition. The test is a permanent structure. The muscular fibres of the body- wall are arranged in complete or interrupted ring-like bands, or diffusely. The pharynx has either two large or many small stigmata leading into an atrial cavity which communicates with the exterior by the atrial aperture. There is usually an alterna- tion of generations ; there may or may not be a tailed larval stage. Sub-Order a. — Cydomyaria. Thaliacea with a cask- shaped body, having the oral and atrial apertures at opposite ends, and surrounded by a series of complete rings of muscular fibres. This sub-order contains only one family, the Doliolidce, with the three genera, Doliolum, Anchinia, and Dolchinia. Sub-Order b. — Hemimyaria. Thaliacea with a more or less fusiform body, with sub-terminal oral and atrial apertures. The muscular fibres are arranged in bands which do not form complete rings. There are two families — the Salpidce and the Octacnemidce — the latter comprising only the aberrant deep-sea genus Octacnermis, which seems to be fixed and not free-swimming like the rest of the order. Sub-Order c. — Pyrosomata. Thaliacea which reproduce by budding, so as to give rise to hollow cylindrical colonies, open at one or both ends, having the zooids embedded in the gelatinous wall in such a manner that the oral apertures open on the outer, the atrial on the inner, surface of the cylinder. This sub-order comprises only one family, the Fyrosomidce, with one genus, Pyrosoma. ORDER 3. — ASCIDIACEA. Mostly fixed Tunicata, either simple or forming colonies by a process of budding, and, in the adult condition, never provided c 2 20 ZOOLOGY SECT. with a tail. The test is a permanent structure, usually of con- siderable thickness. The muscular fibres of the mantle (body- wall) are not arranged in annular bands. The pharynx is large, and its walls are perforated by numerous stigmata leading into a surrounding atrium or peri-branchial cavity, which communicates with the exterior by an atrial aperture. Many form colonies by a process of budding ; and most undergo a metamorphosis, the larva being provided with a caudal appendage supported by a notochord similar to that of the Larvacea. Sub-Order a. — Ascidice. simplices. Ascidians in which, when colonies are formed, the zooids are not embedded in a common gelatinous mass, but possess distinct tests of .their own. They are nearly always permanently fixed and never free-swimming. Including all the larger Ascidians or Sea-Squirts. Sub-Order b. — Ascidice compositor. Fixed Ascidians which form colonies of zooids embedded in a common gelatinous material without separate tests. This order includes Botryllus, Amarcecium, Diazona, and a number of other genera. Systematic position of the Example. The genus Ascidia, of which there are very many species, is a member of the family Ascidiidce of the Ascidias simplices. The Ascidiidae differ from the other families of simple Ascidians by the union of the following characters : — The body is usually sessile, rarely elevated on a peduncle. The oral aperture is usually 8-lobed and the atrial 6-lobed. The test is always of gelatinous or cartilaginous consistency. The wall of the pharynx is not folded ; the tentacles are simple and filiform. The gonads are placed close to the intestine. The genus Ascidia is characterised by having the oral and atrial apertures not close together, by the dorsal lamina being a continu- ous undivided fold, and by the ganglion and sub-neural gland being situated at a little distance from the dorsal tubercle. 3. GENERAL ORGANISATION. General Features. — Appendicularia (Fig. 679), which^may be taken as an example of the Larvacea, is a minute transparent animal, in shape not unlike a tadpole, with a rounded body and a long tail-like appendage attached to the ventral side. At the extremity of the body most remote from the tail is the aperture XIII PHYLUM CHORDATA 21 FIG. 679.— Appendicularia (Oikopleura) in (From Herdman, after Fol.) House." of the mouth. This leads into a tolerably wide pharynx (Fig. 680, ph.), in the ventral wall of which is an endostyle similar to that of the simple Ascidian, but com- paratively short. Round the pharynx there run two bands covered with strong cilia— fheperipha ryn- geal lands. On the ventral side of the pharynx there are two ciliated openings — the stigmata (stig.) — which communi- cate with the exterior by short passages — the atrial canals, situated on either side behind the anus. The axis of the tail is occupied by a cylindrical rod — the notochord (noto.). A remarkable peculiarity of Appendicularia is the power which it possesses of secreting from the surface a transparent envelope (Fig. 679) in the interior of which the animal can move freely. This structure — the house as it is called — is soon thrown off, and a new one developed in its stead. It represents the test or tunic of the simple Ascidian, though it does not appear to contain cellulose. Among the simple Ascidians there is a considerable degree of uniformity of struc- ture, and there is not much that need be added here to the ac- count given of the example. The shape varies a good deal : it is sometimes cylindri- cal, sometimes globu- lar, sometimes com- pressed ; usually sessile and attached by a broad base, often with root-like processes, but in other cases (e.g. Boltenia) elevated on a longer or shorter stalk. Most are solitary ; but some multiply by budding, stolons being given off on which new zooids are developed. The test varies considerably in consistency, being some- noto FIG. 680.— Diagram of Appendicularia from the right- hand side. an. anus ; ht. heart ; int. intestine ; ne. nerve ; ne.' caudal portion of nerve ; ne. gn. principal nerve- ganglion ; ne.gn.'ne. gn." first two ganglia of nerve of tail ; noto. notochord ; as. oesophagus ; or. ap. oral aper- ture ; oto. otocyst ; peri. bd. peripharyngeal band ; ph. pharynx ; tes. testis ; stig. one of the stigmata ; stom. stomach. (After Herdman.) 22 ZOOLOGY SECT. times almost gelatinous, transparent or translucent, sometimes tough and leathery, occasionally hardened by encrusting sand- grains or fragments of shells, or by spicules of carbonate of lime. The apertures always have the same position and relations, varying only in their relative prominence. The pharynx varies in its size as compared with the rest of the internal parts, in the position which it occupies with regard to the various parts of the alimen- tary canal, and in the number and arrangement of the stigmata. The tentacles are sometimes simple, sometimes compound; and the dorsal lamina may or may not be divided up into a system of lobes or languets (Fig. 682, lang.}. In the composite Ascidians, as mentioned in the summary, the zooids are. embedded in a common gelatinous mass. The gela- tinous colony thus formed is sometimes flat and encrusting, sometimes branched or lobed, sometimes elevated on a longer or shorter stalk. In certain forms (Psammapilidium) the gelatinous substance is hardened by the in- clusion in it of numerous sand- grains. The arrangement of the zooids presents great differences. Sometimes they occur irregularly dotted over the entire surface without exhibiting any definite arrangement ; sometimes they are arranged in rows or regular groups; in Botrylhis (Fig. 681) they are arranged in star-shaped, radiating sets around a common cloacal chamber into which the atrial apertures of the zooids lead, while the oral apertures are towards their outer ends. In essential structure the zooids of such colonies (Fig. 682) resemble the simple Ascidians. In the free-swimming pelagic Doliolum (Fig. 683) the shape is widely different from that of the ordinary fixed forms. The body is cask-shaped, surrounded as by hoops by a series of annular bands of muscular fibres (mus. Ms.). The oral and atrial apertures (or. ap.,atr. ap.) instead of being situated near together at the same end of the body, are placed at opposite extremities, and the relations of the various organs have undergone a corresponding modification. The test is thin and transparent. Surrounding each opening is a series of lobes — the oral and atrial lobes — in which there are sense-organs; and the first and last of the FIG. 681.— Botryllus violaceus. or. oral apertures ; cl. opening of common cloacal chamber. (After Milne-Edwards.) XIII PHYLUM CHORDATA 23 ,*, CTlft— slant muscular hoops serve as sphincters for the two orifices. The oral aperture leads into a wide pharyngeal sac (ph.), occupying at least the anterior half of the body; its pos- terior wall alone is usually perforated -perish ^p ' ^ cl by stigmata (stig.). An endostyle (end.) is present, and a Ci-pharyngeal d; 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 back- wards out of the branchial sac. Salpa (Figs. 684- 685) is nearly allied to Doliolum in its external features and internal struc- ture. It has a fusi- form body, usually somewhat com- pressed 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 separ- ated from one another only by an obliquely running vascular band, which represents the dorsal lamina of the fixed Ascidians, and is frequently termed the branchia. til — FIG. 682.— Diagram of a zooid of a colony of Composite Ascidians, in which the zooids are in pairs, as seen in a vertical section of the colony, an. anus ; at. atrium ; at'. atrium of adjoining zooid ; cl. cloaca common to the two zooids ; end. endostyle ; gld. digestive gland ; gn. nerve- ganglion ; ht. heart ; hyp. hypophysis ; lang. languets ; mant. mantle ; or. ap. oral aperture ; ov. ovary ; periph. peri- pharyngeal band ; ph. pharynx ; rect. rectum ; stom. stomach ; te. testis ; tent, tentacl mass ; x. d. vas deferens. tst. test, or common gelatinous (After Herdman.) 24 7/iu.s.bds or. a. ft SECT. ctlrctp peri.bd. FIG. 683. — Doliolum. Diagram of the sexual form. atr. ap. atrial aperture surrounded by lobes ; atr. cav. atrial cavity ; d. tbc. dorsal tubercle ; end. endostyle ; lit. heart ; int. intestine ; mus. Ms. muscular bands ; ne. gn. nerve-ganglion ; or. ap. oral aperture ; or. ovary ; peri. M . peripharyngeal band ; ph. pharynx ; stiff, stigma ; stom. stojnach ; test, testis. (After Herd- man.) or. dp FIG. 684.— Salpa democratica, asexual form, ventral view. atr. ap. atrial aperture ; blanch, dorsal lamina ; end. endostyle ; Id. heart ; mus. Ms. muscular bands ; ne. gn. nerve-ganglion ; proc. processes at the posterior end ; sens. org. sensory organ ; stol. stolon. (After Vogt and Jung.) or.a.p stom. Int FIG. 685. — Salpa, semi-diagrammatic lateral view. an. anus ; atr. ap. atrial aperture ; branch, dorsal lamina ; dors. tubl. dorsal tubercle ; ht. heart ; hyp. hypophysis ; lancj. languet ; mus. bds, muscular bands ; ne.gn. nerve ganglion ; or. ap. oral aperture ; ov. ovary in ovisac ; stom. stomach (After Herdman.) XIII PHYLUM CHORD ATA 25 Octacnemus, allied to Salpa, appears to be fixed, and has the oral and atrial aper- A tures towards one end of the body, which is somewhat discoid, with its margin into eight B FIG. 686.— Colony of Pyrosoma. A, side view ; B, end view. (After Herdmaii.) produced tapering processes. Pyrosoma (Fig. 686) is a colonial Tunicate, the colonies of which are of a cylindrical form, with an orifice at one end and usually closed at the other. The oral apertures (Fig. 687, or. ap.) of the zooids are situated on the outer surface of the cylinder on the extremities of 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 brilliantly phos- phorescent. The enteric canal in Appendicularia (Fig. 680) consists, in addition to the pharynx, of a narrow reso- phagus,a bilobed stomach, and a straight intestine (int.) which opens directly by an anal aperture (an.) situated on the ventral side. In Oikopleura the intestine is absent. The alimentary canal of the simple Ascidians has al- ready been described, and there are few differences of consequence in the various families ; in the FIG. 687.-Part of a section through a Pyrosoma Composite forms the ar- colony. air. ap. atrial aperture ; or. ap. oral aper- rangement of the parts IS ture ; p'toc. processes of test on outer surface of , ' . ,, . , colony ; ph. pharynx ; stol. stolon on which are de- the Same in all CSSential veloped buds giving rise to new zooids ; tent, tentacles. r^-norta a « in th P . atrial aperture ; br. branchia ; cil. yr. ciliated groove ; tbl. elseo- blast ; end. endostyle ; n. gn. nerve-ganglion ; «•*. oesopha- gus ; or. ap. oral aperture ; jwric. pericardium ; pi. placenta ; rect. rectum ; xtol. stolon ; stom. stomach. (From Korschelt and Heider, after Salensky.) stance that during segmentation there is a migration inwards of some of the cells of the follicle and of the wall of the oviduct, which enter the segmenting ovum arid pass among the blastomeres. There is uncertainty as to what part these inwardly-migrating cells play in the de- velopment of the embryo. According to one observer they «rfxtf$S8»l £^m*^nMtV*&>> act merely as car- riers of nourishment, and become broken up and eventually completely absorb- ed ; according to another they actu- ally displace the blastomeres and give rise to the greater part of the embryo. There is no tailed larval stage, and the embryo develops the muscle-bands and all the characteristic parts of the adult while still enclosed within the body of the parent and nourished by means of the placenta. This sexually-developed embryo, however, does not give rise to a form exactly like the parent, but to one which differs from the latter in certain less important features and notably in the absence of reproductive organs. The sexually formed embryo, in other words, forms an asexual generation. It- escapes to the exterior and becomes free-swimming (Fig. 684). After a time there is developed a process or stolon (stol.), on the surface of which are formed a number of bud-like projections. These increase in size as the stolon elongates and each eventually assumes the form of a sexual Salpa. The stolon with the Salpae attached becomes separated off and swims about as a chain of zooids in which the reproductive organs become developed. Distribution, etc. — The pelagic forms are, as is the case with most pelagic organisms, of very wide distribution, and none of the genera are confined to particular oceanic areas. The fixed forms, both simple and composite, are also of world-wide distribution ; they are much more abundant in the southern hemisphere than in the northern — the composite forms attaining their maximum in the South Pacific area. The depth to which the pelagic forms extend has not been determined. Fixed forms occur at all depths, but are much more numerous in shallow water than in deep, and xin PHYLUM CHORDATA 37 at great depths are comparatively poorly represented, the simple forms extending to a greater depth than the composite. Several genera of pedunculated simple forms seem to be confined to very great depths. Though placed so high in the animal series, the Urochorda exhibit very low functional development. This is chiefly connected with the sessile condition of most of them. The movements per- formed b}^ an Ascidian are slow and very limited in character, being confined to contractions of the mantle ; when the animal is detached such contractions may be sometimes observed to result in a slow creeping locomotion. Even in the free forms the move- ments are limited to the contractions, of the tail muscles in Appendicularia, of the muscle-bands of the body- wall in Doliolum, by which swimming is effected. The mode of obtaining food resembles that which has already been described in the case of the Pelecypoda (Vol. I. p. 640), the currents which subserve respiration also bringing in microscopic organic particles to the mouth. Affinities. — That the Urochorda are degenerate descendants of primitive Chordates admits of little doubt; the history of the development of the Ascidians, taken in connection with the occur- rence of permanently chordate members of the group (Appendicu- laria and its allies), is quite sufficient to point to this conclusion. But the degree of degeneration which the class has undergone — the point in the line of development of the higher Chordata from which it diverged — is open to question. According to one view the Urochorda are all extremely degenerate, and have descended from ancestors which had all the leading features of the Craniata ; according to another the ancestors of the class were much lower than any existing Craniate — lower in the scale than even Am- 'is — and had not yet acquired the distinctive higher character- of the Craniates. The nearest existing ally of the Urochorda _;• lower forms is probably Balanoglossus. The similarity in iiaracter of the pharynx or anterior segment of the enteric anal, perforated by branchial apertures, is alone sufficient to point to such a connection ; and further evidence is afforded by the occurrence of a notochord in both, and by the similarity in the pment of the central part of the nervous system. But the e is by no means a close one, and Balanoglossus and its allies can only be looked upon as very remotely connected with the .stock from which the Urochorda are descended. SUB-PHYLUM III.— VERTEBRATA. We have seen that the fundamental characters of the Chordata are the presence, of a notochord, of a dorsal hollow nervous system, and ( »f a pharynx perforated by apertures or gill-slits. In none of the lower Chordata, however, are these structures found in a 38 ZOOLOGY SECT. typical condition, at least in the adult. In Balanoglossus, Cephalo- discus, and Rhabdopleura. the " notochord " is rudimentary, and in nearly all Tunicata it is present only in the embryo. In Rhabdopleura the gill-slits are absent, and in that genus as well as in Cephalodiscus and the adult Tunicata the nervous system is represented by a single solid nerve-centre or ganglion, the neurocoele being absent. In Balanoglossus, moreover, there is a ventral as well as a dorsal nerve-cord, and it is only in the anterior portion of the latter that the neurocoele is represented. Jn the Vertebrata, on the other hand, what have been called the three fundamental chordate peculiarities are fully and clearly developed. There is always a distinct notochord extending as a longitudinal axis throughout the greater part of the elongated body, and either persisting throughout life, or giving place to an articulated vertebral column or backbone. The central nervous system remains throughout life in the form of a dorsal nerve- tube or neuron, containing a longitudinal canal or neuroccele. And the pharynx is always perforated, either throughout life or in the embryonic condition, by paired branchial apertures or gill- slits. In addition to these characters the mouth is ventral and anterior, the anus ventral and posterior; the muscular layer of the body-wall is segmented, and the renal organs arise as meso-nephridia. Moreover there is always an important digestive gland, the liver, developed as a hollow outpushing of the gut, and distinguished by the fact that the blood from the intestine circu- lates through it before passing into the general current, thus giving rise to what is called the hepatic portal system of blood vessels. There ^re two primary sub-divisions of Vertebrata of very unequal size. DIVISION A. — ACRANIA. Including only the little fish-like Lancelets. DIVISION B. — CRANIATA. Including Fishes, Amphibians, Reptiles, Birds, and ]\i DIVISION A.— ACRANIA. The division Acrania contains a single family, the Bran tomidce, containing two genera, Branchiostoma (usually known by the name of one of its sub-genera, Amphioxus\ and ^l>y// The differences between the genera and species are comparati* insignificant, and the following description will deal ex( with the best known and most thoroughly investigated the Lancelet or Amphioxus, Ampliioxus lanceolatus, found in English Channel, the North Sea, and the Mediterranean. XIII PHYLUM CHORDATA 39 Amphioxus is a small transparent animal, occurring near the shore and burrowing in sand ; its length does not exceed 5 '8 cm, or less than two inches. Its form will be obvious from Fig. 696 and from the transverse sections, Fig. 697, A and B. The body is elongated, pointed at either end, and compressed. The anterior two-thirds is roughly triangular in transverse section, presenting right and left sides, inclined towards one another, above, and a convex ventral surface. The posterior third is nearly oval in section, the right and left sides meeting above and below in a somewhat sharp edge. Extending along the whole of the dorsal border is a median longitudinal fold, the dorsal fin (dors. /.) : this is continued round the posterior end of the body and extends forwards, as the ventral fin (vent. /.), as far as the spot where the oval gives place to the triangular transverse section. The portion of the continuous or.hd. FIG. 696. —Amphioxus lanceolatus. A, ventral, B, side view of the ei^fcire animal. an. an itriopore ; cd. /. caudal fin ; dr. cirri ; dors. /. dorsal fin ; dors. /. )•. dorsal fin ; -nit pi, metapleure ; myom. myomeres ; nch. uotochord ; or. lid. oral hood \. . f. ventral fin ; cait. /. /•. ventral fin rays. (After Kirkaldy.) median fold which extends round the pointed posterior extremity of the body is somewhat wider than the rest and may be distinguish d as the caudal fin (cd.f.). In the anterior two-thirds- of the body there is no median ventral fin, but at the junction of ea.ch lateral with the ventral surface is a paired longitudinal fold, the 'metapleure (mtpl.), which extends forwards to the oral hood mentioned in the next paragraph. Below the pointed anterior extremity is a large median aperture surrounded by a frill-like membrane, the oral hood (or. hd.), the edge/ of which is beset with numerous tentacles or cirri (dr.). The oral liood « -in- 1. is« ,»s a cup-shaped cavity or vestibule, at the bottom of which is tli (Fig. 698, mth). Immediately in front of the anterior termination of the ventral fin and partly enclosed by the meta- pleures is ;i rounded aperture of considerable size, the atriopore (otrp), and a short distance from the posterior extremity of the H>jly is the anus (an), placed unsymmetrically on the left side of i>dy is 40 ZOOLOGY SECT. the ventral fin. The post-anal portion of the body is dis- tinguished as the tail. Amphioxus ordinarily lives with the greater part of the body buried in sand, only the anterior end with the expanded oral hood protruding. It also swims in the vertical position, and frequently lies on one side on the sand : it burrows, head fore- most, with great rapidity. A current of water is constantly passing in at the mouth and out at the atriopore. Body- wall. — The body is covered with an epidermis (Fig. 697) formed|of a single layer of columnar epithelial cells, some of which A T&- air FIG. 697. — Amphioxus lanceolatus. A, transverse section of the pharyngeal region. a, dorsal aorta ; b, atrium ; c, notochord ; co* ccelome ; e. endostyle ; g. gonad ; kb, branchial lamellae ; • M, pharynx r^i. liver; my. mypmere ; n, iiephridium ; /-, neuron; .-,>. spinal nerves; sp. gill-slits. \T3, transverse section of the intestinal region, utr. atriuii ccelome; d. uo. dorsal aorta'; int. intestine; mi/om. myomere ; nch. neuron ; s. int. v. sub-intestinal vein. (A, from'Hertwig, after Laiikester ai ; tly after Rolph.) are provided with sensory hairs. The epitheliurr cirri presents at intervals regular groups of sensory >rne of them bearing stiff sensory hairs, others cilia. Eene- i-pi- dermis is the dermis, formed mainly of gelatinous connective tissue. •' t The muscular layer (my, myom.) is remarkable for exhibiting metameric segmentation. It consists of a large number — abo it sixty — of muscle-segments or myomcres, separated from one another by partitions of connective tissue, the myocomma*, and having the appearance, in a surface view, of a series of very - w i \ h their apices directed forwards (Figs. 696 and 698). E; ch moT xiii PHYLUM CHORDATA 41 is composed of mi Aerous flat, striated muscle plates, arranged longi- tudinally, so thai each is attached to two successive myo- commas. In virtue of this arrangement the body can be bent from side to side witn great rapidity. The myomeres of the right and left sides of the liody are not opposite to one another, but have an alternate arrangement. A special set of transverse muscles (Fig. 697, 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 notochord (Figs. 697 and 698, 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 left myomeres. It is composed of a peculiar form of cellular tissue, known as notochordal tissue, formed of large vacuolated cells extending from side to side of the notochord, and having the nuclei confined to its dorsal 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 resistent character to the vacuoles of its component cells being tensely filled- with fluid, a condition of turgescence being thus produced. The oral ;-' supported by a ring (Fig. 698, sk.) of carti- laginous < ticy, 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 ])h;<; "ed by delicate oblique rods of a chitinoid material, the . ?'.). These will be most conveniently discussed in the pnarynx itself. The dorsal fin is supported b e series, and the ventral fin by a double series, of jin-rni ., vent.f. r.\ short rods of connective tissue, each contain^ avity 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 h ). It is a small circular aperture surrounded by a membmuo, the velum (vl.) which acts as a sphincter, and has its free edge produced into a number of velar tentacles (vl. t.). 42 ZOOLOGY SECT. The mouth leads into the largest section GM jhe -enteric canal, the pharynx (ph.), a high, compressed chamber/ extending through the anterior half of the body. Its walls are perforated 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. 697, A, e.), lined by ciliated epithelium containing groups of gland-cells. Like the homologous organ in Ascidia (p. 14), 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 epi-pharyngeal 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 peri-pharyngeal 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 coecum, 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. 697, A, sp.)~ The clefts are more numerous than the myomeres in the adult, but correspond with them in the larva : hence they are funda- mentally metameric, but undergo an increase in number as growth proceeds. The branchial lamella? (Fig. 698, br. scp., Fig. 697, A, kb.) or por- tions of the pharyngeal wall separating the clefts from one another, are covered by an epithelium which is <>r tL^ most part endo- dermal in origin, and 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 chitinoid branchial rods (br. r.) already referred to. These are narrow bars united with one another dorsally by loops, but ending below in free extremities which are alternately :lmple a,nd forked.. The forked bars are the primary (br. r. 1), those with simple ends the secondary (br.r. 3) branchial rods, and the lamellae in which they are contained are similarly to be distinguished a > primary lamellae (br. sep. 1) and secondary or tongue lamellae (br. ,sr/< .-?). In the young condition the two clefts between any two pr.mary lamellaa are represented by a single aperture : as development, proceeds a down- XI PHYLUM CHORDATA 43 growth takes place from the dorsal edge of the aperture, forming, as in Balanoglossus (p. 3), a tongue which extends downwards, dividing the original cleft into two, and itself becoming a secondary lamella. A further complication is produced by the for- mation of transverse branchial junctions con- necting the primary septa with one another at tolerably regular intervals. The Atrium.— The gill-clefts lead into a wide chamber occupy- ing most of the space between the body-wall and the pharynx and called the atrium (Fig. 698; atr. ; Fig. 697, A, &.). It is crescentic in section, surrounding the ventral and lateral re- gions of the pharynx, but not its dorsal por- tion. It ends blindly in front ; opens extern- ally, behind the level of the pharynx, by the atriopore (atrp.) ; and is continued backwards by a blind, pouch-like extension (atr'.) lying to the right of the intes- tine (Fig. 697, B, (ftr.l The whole cavity is lined by an atrial epi- thelium of ectod origin. As in A- the cilia linir. clefts pro< i • setting in at t 44 ZOOLOGY entering the pharynx, passing thence by the gill-slits into the atrium, and out at the atriopore. The current, as in Tunicata and Balano- glossus, is both a respiratory and a food current, the animal feeding passively on the minute organisms in' the surrounding watery . FIG. 699.— Amphioxus lav.ceolatus. Diagrammatic tran- ion of the pharyn- geal region, passing on the i _rht through a primary, on the left thr- ; lary branchial lamella, ao. dorsal aorta; '•. >lerm ; ec endostylar portion of . t'aseia or investing layer of myomere ; jli . ut containing fin-ray; y. gouad ; (//. glomerulns ; /•. iDranchial artery ; &/, pj. 'cd atrial and ctelomic wall (ligamentum denticu- latum); m. myomere; int. trap 10 ; />.. nephridium ; or, metapleural lymph-sp;n-o ; p, atrium ; sc, coalome ; si, ventral aorta ; .- nt>tochord and reuron ; uf, spaces in ventral wall. (From Korschelt and Heider, aft-/r liatschek.) Coelome. — Owing to the immci cavity, which is a true coslome, is much re< i in the pharyngeal region, by paired cavitit A, co., Fig. 699, sc.) lying one on either sid trium the body is represent^ 1 . '., Fig. 69 7, don of xin PHYLUM CHORDATA 45 1 i. the pharynx ^abo vre the atrium, and connected by narrow canals in the primary branchial lamellae (Fig. 699, right side), with a median longitudinal space below the endostyle (Fig. 699, ec.). In the intestinal region it is much reduced on the right side, being displaced by the backward extension of the atrium (Fig. 697, B, atr., Fig. 698. air'.), but is well developed on the left side : a forward extension of it surrounds the liver (Fig. 697, A, /.). The whole series of spaces is lined by ccelomic epithelium. Blood-System. — The blood-vessels of Amphioxus are all of one kind, but, owing to certain undoubted homologies with the more complex vessels of the Craniata (see below), some of them receive the name of arteries, others of veins. Lying in the ventral wall of the pharynx, below the endostyle, is a median longitudinal vessel, the ventral aorta (Fig. 700, v. ao} Fig. 699, si.) ; it is contractile and drives the bjood forwards. From it are given off, on each side, lateral branches, the afferent ef.br.a. cL.cn ^_ »»> > / ^f ™ int FIG. 700. — Diagram of the vascular system of Amphioxus. . intestinal capillaries ;§ //. no. paired dorsal aortae ; d. ao.' median dorsal aorta ; (f. In: a. efferent, branchial arteries ; /« />. j)ort. r. hepatic portal vein ; hep. r. hepatic vein ; iiit. intestine ;//•. liver ; ±>lt. pharynx ; .s. iiit. r. sub-intestinal Vein: branckiffl a rf erics (Fig. 700, af. br. a. ; Fig. 699, &), which pass up the primary branchial lamellae and communicate by cross- branches with similar vessels (cif. br. a'.) in the secondary or tongue lamellae. The blood is exposed, while traversing these vessels, to the aerating influence of the respiratory current, and leaves the branchial lamellae dorsally by efferent branchial arteries (ef. br. a.) which open on each side into paired longitudinal vessels, the right and left dorsal aortce (d. ao.), lying one on each side of the epipharyngeal groove. Anteriorly both dorsal aortse are continued forwards to the region of the snout, the right being much dilated ; posteriorly they unite with one another, behind the level of the pharynx, into an urijfaired dorsal aorta (d. ao'.), which extends backwards in the middle\lme, immediately below the notochord and above the intestine. \ The unpaired dorsal aorta sends off branches to the intestine, in the walls of which they break up to form a network of microscopic vessels or cupill«ru \ (cp.\ From these the blood is collected and 46 ZOOLOGY SECT poured into a median longitudinal vessel, the nub-intestinal rein (Figs. 697, B, and 700, s. int. v.), lying beneath the intestine : in this trunk the blood flows forwards, and, at the origin of the liver, passes insensibly into a hepatic portal vein (hep. port, v.), which extends along the ventral side of the liver and breaks up into capillaries in that organ. From the liver the blood makes its way into a hepatic vein (Iwp. v.), which extends along the dorsal aspect of the digestive gland, and, turning downwards and forwards, joins the posterior end of the ventral aorta. It will be seen that the vascular system of Amphioxus consists essentially of (a) a dorsal vessel represented by the paired and unpaired dorsal aortse, (b) a ventral vessel represented by the sub-intestinal vein and the ventral aorta, and (c) commissural vessels represented by the afferent and efferent branchial arteries and the intestinal capillaries. So far the resemblance to the vascular system of Annulata is tolerably close ; but two important differences are to be noted. The blood in the ventral vessel travels forwards, that in the dorsal vessel backwards — the precise opposite of what occurs in Worms, and the ventral vessel is broken up, as it were, into two parts, by the interposition in its course of the capillaries of the liver, so that all the blood from the intestine has to pass through that organ before reaching the ventral agrta. This passage of the intestinal blood through the vessels of the liver constitutes what is called the hepatic porj&l system, and is eminently characteristic of Vertebrata. The blood is colourless, and appears to contain no leucocytes. It is not confined to the true blood vessels just described, but occurs also in certain cavities or lymph-spaces, the most important of which are the cavities in the dorsal and ventral fins containing the fin-rays (Fig. 699, //*.), and paired canals in the metapleures (of.}. Excretory Organs. — The principal organs of excretion are about ninety pairs of peculiarly modified nephridia(Fig.698,neph.) situated above the pharynx and in relation with the main V coelomic cavities. Each nephridium (Fig. 701) is a bent tube consisting of an anterior vertical and a posterior horizontal limb. The vertical limb opens by a wide aperture into the coelome : the horizontal limb has several coelomic apertures, one at its posterior end, the others on its dorsal surface. On the ventral surface of the horizontal limb, opposite a secondary branchial lamella, is a single aperture bearing long cilia and opening into the atrium : this corresponds with the nephridiopore or external apertures of the typical nephridium. With the coelomic apertures are connected peculiar thread-like cells with knobbed extremities. An excretory function has also been assigned to a single pair of organs called the brown funnels (Fig. 698, 6r./.),also situated on the dorsal aspect of the pharynx at its ]»<•• u>nor end. Their wide, backwardly directed ends open he -itrium ; their PHYLUM CHORDATA 47 narrow anterior ends probably communicate with the ccelome. There are also groups of columnar excretory cells on the floor of the atrium. Nervous System. — The central nervous system is a rod-like organ the neuron or dorsal nerve-tube (Fig. 697, A, n. ; B. neu., Figs. 698, 699), contained within and completely filling a median longitudinal neural canal which lies immediately above the noto- chord. It is roughly triangular in transverse section : anteriorly it ends abruptly some distance behind the anterior end of the FIG 701. — Amphioxus lance olatus. A, nephridium of the left side with part of the wall of the pharynx. (From Willey, after Boveri.) k ^fctochord, while posteriorly it tapers to a point over the hinder ™d of the latter. It is traversed by an axial cavity, the neuro- ccele (Fig. 698, cent, c.), connected with the mid-dorsal region by a longitudinal cleft. At the fore-end of the nerve-tube the neurocoele becomes greatly dilated, forming a considerable cavity, the encephaloca>le or cerebral ventricle (Fig. 698, en. cos., Fig. 702, cv.), and a little behind this the dorsal fissure widens out above to form a trough-like dorsal dilatation (dil.) covered only by the delicate connective tissue sheath which invests the whole nerve- 48 ZOOLOGY SECT. tube. The anterior end of the neuron, containing these two cavities, is to be looked upon as the brain, although not dis- tinguishable externally from the remaining portion or spinal cord. The anterior and dorsal region of the brain is produced into a small hollow pointed pouch which comes into relation with the olfactory organ and is called the median olfactory lobe. In its posterior and ventral region a depression has been described which appears to correspond with the infundibulum of the Craniata (vide infra). The neuron is mainly composed of longitudinal nerve-fibres with abundant nerve cells mostly grouped around the neuroccele. At Fir,. 702. — Amphioxus lanceolatus. A, brain and cerebral nerves of a young speci- men; B, transverse section through neuropore; C, behind cerebral ventricle; D, through dorsal dilatation, ch. notochord ; cv. cerebral ventricle ; dil. dorsal dilatation ; e. eye-spot ; np. neuropore ; olf. olfactory pit ; 7, //, cerebral nerves. (From Willey, after Hatschek.) intervals giant nerve-cells occur, multipolar cells of immense proportional size, connected with nerve-fibres of unusual size, the giant fibres. The latter appear to correspond with the giant fibres of Chastopods (Vol. I. p. 438) which, however, have no nervous function and are mere supporting structures. The peripheral nervous system consists of N the nerves given off from the neuron. They are divisible into two sets, the first consisting of two pairs of cerebral nerves (Fig. 702, ,1. nd II.) arising from the brain, the second of a large numbe^ of spinal nerves arising from the spinal cord. The cerebral \ierves take their PHYLUM CHORDATA 49 origin in front of the first myomere, the first from the anterior extremity of the brain, the second from its dorsal region : they are both distributed to the snout, their branches being pro- vided towards their extremities with numerous ganglia containing nerve-cells. The spinal nerves are segmentally arranged, and, in correspondence with the disposition of the myomeres, those of the right and left sides arise alternately, and not opposite one another (Fig. 703). In each segment there are two nerves on each side, a dorsal nerve, arising by a single root from the dorsal aspect of the spinal cord, and a ventral nerve- arising by numerous separate fibres : the dorsal nerves supply the skin and the trans- verse muscles and are therefore both sensory and motor, the ventral nerves are purely motor, supplying the myomeres. Sensory Organs. — At the level of the anterior end of the brain is a narrow ciliated depression, the olfactory pit (Fig. 702, olf.} opening externally on the left side of the snout and connected at its lower end with the median olfactory lobe. This structure is supposed to be an organ of smejl : in the larva its cavity is in direct communication with the neurocoele through an aperture, the neuropore (np.), which becomes closed in the adult. There is some reason for thinking that the olfactory pit answers to the liypo- physis or pituitary body of Urochorda and Craniata (pp. 17 and 96). The organ of sight is an unpaired pigment spot (e) in the front wall of the brain : it is therefore a median cerebral eye. There is no .lens or other accessory apparatus. Smaller pigment spots occur in Hir spinal cord throughout the greater part <>f its length, below the neurocoele. There is no trace of auditory organ. A peculiar structure, the groove of II < >ii the roof of the oral hood, is si t'» have a sensory function, and in i-^air of taste. Lastly, the sensory cells on the buccfil C|| Knose organs jan important tactile function. Reproductive Organs. — dhe sexes are separate, but there is no (list -i part from the irgans of reproduction, between male and femalj The gonads (Fid. 698, gon., Figs. 697, A, and 699, g.) arc ,,'IMMM. uty-six pairs pf pouches arranged metamerically along th< ' bou -\\all, and projecting into the atrium so as largely to fill upb its cavity. The inner -or mesial face of each pouch is VOL. ii* I E FIG. 703.— Ampbioxus lanceolatus. An- terior portion of neuron from above, showing nerves. (From Willey, after Schneider.) zoo; covered by atrial epithelium pushed inwards by the gr^ the gonad; within this, and completely surrounding the repro- ductive organ, is a single layer of epithelium which is't shown by development to be ccelomic, Hence each gonad is surfoundbd by a closed coelomic sac. -/ When ripe the inner walls of the gonadic pouches burst, and the ova or sperms make their way into the atrium and thence i . 7U4.— Amphioxus lancelolatus. .Segmentation stage, (C) from above ; H, vertical section of G ; K, vertical (From Korschelt and Heider, after Hatschek.) by the atriopore to the external water where place. The laid eggs are covered by a thin formed of flattened cells. Development. —Only one polar body has beta 704, A). After impregnation the follicukr n ur-celled stage I. takes wane, Fig. PHYLUM CHORDATA 51 from the oosperm leaving a wide space around the latter. Seg- mentation is complete, there being very little yolk : it begins by a meridional cleft dividing the oosperm into two (B), and followed by a second cleft, also meridional, at right angles to the first (C, D). Next, an equatorial cleavage takes place, the embryo coming to be formed of eight cells (E), of which the four be- longing to the upper hemisphere, distinguished by the presence of the polar cell, are smaller than the lower four. Further meridional and equatorial divisions take place, and the embryo becomes a Uastula (I, K), enclosing a spacious blastocoele, and having the cells on its lower pole larger than the rest. Invagination then takes place (Fig. 705, A), the lower pole of the blastula becoming gradually pushed in until the whole lower hemisphere is in complete contact with the upper hemisphere and the blastocoele obliterated (B). The gastrula thus formed A FIG. TOO.— Amphioxus lanceolatus. Three stages in the formation of the gastrula (From Korschelt and Heider, after Hatschek.) is at first basin-shaped, having a very wide blastopore, but its cavity gradually deepens, and the blastopore is reduced to a com- paratively narrow aperture (C). At the same time the aspects of the body are marked out : the Dorsal surface becomes flattened, the ventral convex ; the blastopore marks the posterior end and is distinctly dorsal in position. Cilia are developed from the ectoderm cells and by their vibration cause the embryo to rotate within its membrane. The ectoderm cells forming the median portion of the flattened dorsal surface now become differentiated and sink below the rest, giving rise to the medullary plate (Fig. 706, A, mp). The ordinary ectoderm cells on each side of this plate rise up as a pair of longitudinal medullary folds (kb), extend towards the middle line and unite (B, h b), covering over the medullary plate. The latter bends upwards at the sides so as to become trough-like instead of flat (C), and, its two sides coming in contact with one another ;il><>vt\ the plate is converted into a tube, the neuron (D, n\ E 2 52 ZOOLOGY SECT. enclosing a central canal, the ne-urocaele, continued dorsally into a narrow cleft. The medullary folds extend behind the blastopore so that when they unite the latter aperture opens into the neurocoele by a neurenteric canal (Fig. 707, A, en). Anteriorly the folds remain apart up to a late period so that the neurocoele opens externally in front by a wide aperture, the neuropore (Figs. 707, 708 and 709, np).' While the central nervous system is thus being formed, the endoderm sends out dorsally a paired series of offshoots, the D hb FIG 706.— Amphioxus lanceolatus. Four stages in the development of the notochord nervous system, and mesoderm. ak, ectoderm ; ch, notochord ; dh, cavity of archenteron ; hb, ridge of ectoderm growing over medullary plate ; ik, endoderm ; Hi, coeleme ; mk, coelomic pouch ; mk1, parietal layer of mesoderm ; mk'2, visceral layer ; nip. medullary plate ; n. nerve- tube ; ns, protovertebra. (From Korschelt and Heider, after Hatschek.) ccelomic pouches (Fig. 706, mk) arranged metamerically. In this way segmentation is established, and it is at this period that the embryo ruptures its containing membrane and begins free existence. Before long the coelomic pouches separate from the archenteron and take on the form of a series of closed coelomic sacs (Fig. 706, C, D), lying between ectoderm and endoderm. From the walls of these sacs the mesoderm is derived, their XIII PHYLUM CHORDATA 53 cavities become the coelome, which is therefore an enteroccele, like that -of Sagitta and the Echinodermata. While the coelomic sacs are in course of formation a median groove appears along the dorsal wall of the archenteron (Fig. 706, B, C, c$) : it deepens, loses its tubular character, and becomes a solid rod, the notochwd (D, ck\ lying immediately beneath the nerve-tube. The ordinary endoderm cells soon unite beneath it and so shut it off from the archenteron. It will be seen that the notochord, like the neuron, never exhibits any trace of seg- mentation. At its first formation it stops short of the anterior VK.. 707.— Amphioxus lanceolatus. Embryo. A, from the side ; B, in horizontal section, cd, ectoderm ; en, neurenteric canal ; dh, archenteron ; ik, endoderm ; ml-, meso- dermal folds ; w /neural tube ; v.if, archenteron ; us, first ccelomic pouch ; ush, coelomic cavity ; V, anterior ; //, posterior end. (From Korschelt and Heider, after Hatschek.) end of the archenteron : its final extension to the end of the snout is a subsequent process. New coslomic 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. 708, 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 : it forms a direct communication between the pharynx and the exterior, like the stigmata of Appendicularia (p. 21) : there is at present no trace of the atrium. The anterior end of the archenteron has meanwhile grown out 54 ZOOLOGY KELT. into a pair of pouches, which become shut off as closed sacs : of these the right gives rise to the coelome of the head (7i), the left to a depression called the prw-^raljgit (w), from which the groove of Hatschek is afterwards formed. On the floor of the archenteron, in the neighbourhood of the mouth, a depression appears, which gives rise to a structure called the club-shaped gland (k\ Posteriorly the neurenteric canal closes and the anus appears. We left the mesoderm in the form of separate paired coelomic 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. 706, D, mkl) in contact with the external ectoderm, and a splanchnic layer (mk2) in contact cn B "-**"*-• TTIT Ef * ch FIG. 70S.— Amphioxus lanceolatus. A, young larva; B, anterior end more highly magnified, c, provisional tail-fin; ch, notochord ; cn, neurenteric canal;, d, enteric canal; h, coelome of head ; k, club-shaped gland ; k', its external aperture ; ks, first gill-slit ; /«. niuuth ; mr. nerve-tube ; np. neuropore ; sv. sub-intestinal vein ; v:, prse-oral pit. (From Korschelt and Heider, after Hatschek.) 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 ccelomic sac (Fig. 706, D), separating it into a dorsal and ventral portion, The dorsal section is distinguished as the protovertebra < its cavity as the myooaele or muscle-cavity : the ventral section is called the lateral plate, and its cavity forms a segment * >\' the coelome. The ventral plates now unite with one another in pairs 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 •>! their XIII PHYLUM CHORDATA 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 unsegmeiited coalome of the adult, their walls giving rise to the coslomic epithelium. At the same time the cells of the splanchnic layer of the protovertebrse become converted into muscular fibres, which nearly fill the myocoele, and give rise to the myomeres : the myocommas arise from the adjacent anterior and posterior walls of the protovertebrse. An outpushing of the splanchnic layer, at about the level of the ventral sur- face of the notochord, grows upwards between the n.yomere externally and the notochord and nerve-tube intern- ally: from the cells lining this pouch the connective-tissue sheath of the notochord and nervous system arises, and perhaps also the fin-rays. From the parietal layer of the protovertebra3 arises 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. 709, c). As growth proceeds, new segments are added behind those already formed, the notochord grows forwards to the an- terior end of the snout, and the eye- spot (au.) and olfactory pit appear, the latter as an ectodermal pit which com- municates with the neurocoele by the still open neuropore (np.). The mouth (m.) attains a relatively immense size, still remaining on the left side. Additional gill-slits appear behind the one already mentioned : they all make their appearance near the middle ventral line, an3~gractually 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 50 ZOOLOGY SECT. 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 sides. At first each gill-slit is simple, but before long a fold grows down from its dorsal edge, and, proceeding ventrally, divides the single aperture into two: this Ibid is the secondary or tongue 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- ap FIG. 710.— Amphioxus lanceolatus. Ventral aspect of three larva; showing the develop- ment of the atrium, ap. atriopore ; 1; gill-slits ; //. left metapleural fold ; m. mouth ; rf. right metapleural fold ; w, prse-oral pit. (From Korschelt and Heider, after Lankester and Willey.) plmiral folds (Fig. 710, If. rf., Fig. 711, 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 ridge (Fig. 711, A, si) appears, and, the two sub-a trial 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, but gradually extends upwards on each side (C, p) until it attains its full dimensions. It is open, at first, both 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. 710, B and C), and is finally completely closed. In this way the gill-slits come to open, XIII PHYLUM CHORDATA 57 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. 709, fi), 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 proto- •A £ FIG. 711.— Amphioxus lanceolatus. Diagrammatic transverse sections of three larvae to show the development of the atrium, ao. aorta ; c, dermis ; d, intestine ; /. fascia ; fit, cavity for dorsal fin-ray ; m. myomere ; n, nerve-tube ; p, atrium ; sf. metapleural folds ; x!. sub-intestinal vein ; si; sheath of notochord and neuron ; si. sub-atrial ridge ; sp. ccelome. (From Kprschelt and Heider, after Lankester and Willey.) c vertebrae in the form of pouches, which gradually assume their permanent form. The development of the nephridia is ' not known, but an organ, considered to be a provisional nephridium (Fig. 709, ie), is formed in the mesoderm of the first metamere,^ and opens into the pharynx : it disappears in the adult. Distribution. — Amphioxus has been found in the North Atlantic and Mediterranean, on the west coast of North America, the East Indies, the east coast of South America, Australia, Now Zealand, and the .Malayan Islands. Asymmetron was first known from the Bahamas, and a second closely allied species has been found in the Louisiade Archipelago. As might be expected, no fossil remains of the group are known. 58 ZOOLOGY SECT, Distinctive Characters. — The Acrania may be defined as Vertebrata 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 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 ; the 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 enteroccele. Affinities. — Amphioxushas 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 real suspicion of " invertebrate " affinities until the development both of Amphioxus and the Urochorda was worked out, arid 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 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 no evidence, and all recent investigations and especially the discovery of the nephridia, have tended to bring the Acrania nearer to the Craniate Vertebrata, and to remove them urther from the lower Chordata. DIVISION B.— CRANIATA, The group of Craniate Vertebrata includes all those animals known as Fishes, Amphibians, Reptiles, Birds, and Mammals, or, in xiii PHYLUM CHORDATA 59 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. 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. — CYCLOSTOMATA, Including the Lampreys and Hags. CLASS II. — PISCES, Including the true Fishes, which are again divisible into Sub-class 1. — Elasmobranchii, Including the Sharks and Rays. Sub-class 2. — Holocephali, Including only the Cat-fish (Ghimcera) 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?- Including the Amphibious Fishes or Mud-fishes. CLASS III. — AMPHIBIA, Including Frogs, Toads, Newts, and Salamanders. 1 The animals included in Classes I and II are all "Fishes" in the broad sense of the word 60 ZOOLOGY SECT, xni CLASS IV. — REPTILIA, Including Lizards, Snakes, Crocodiles, Turtles, and Tortoises. CLASS V. — AVES, Including Birds. CLASS VI. — MAMMALIA, Including Hairy Quadrupeds, Seals, Whales, Bats, Monkeys, and Man. External Characters. — The body of Craniata (Fig. 712) 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 coelome 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 no essential organs. Between the head and trunk there is frequently a narrow region or neck, into which the coelome 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 terrestrial forms it 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 Cyclostomata the single nostril — leading to the organs of smell. Farther back, on the sides of the head, are the large paired eyes (e.}, and on the dorsal surface there is sometimes more or less indication of a vestigial median or pineal sense organ (pn. e.), which may take the form of an eye. Posterior to the paired eyes are the auditory organs (au.), the position of which is indicated in the higher forms by an auditory aperture. On the sides of the head, behind the mouth, are a series of openings, the gill-slits or external branchial apertures (e. br. a. 1 — 7) : they are never more than seven in number, and in air- breathing forms disappear more or less completely in the adult. In the higher Fishes a fold called the operculum (Fig. 726, op.) springs from the side of the head immediately in front of the first gill-slit and extends backwards, covering the branchial apertures. On the ventral surface at the junction of the trunk and tail is the anus (an.). Distinct urinary and genital apertures, or a single 62 ZOOLOGY SECT. urino-genital aperture, are sometimes found either in front of or behind the anus, but more commonly the urinary and genital ducts open into the termination of the enteric canal, or cloaca, so that there is only a single egestive opening, known as the cloacal aperture. On either side of this there may be a small abdominal pore (ah. p.) leading into the ccelome. In Fishes and some Amphibians, the trunk and tail are produced in the middle dorsal line into a vertical fold or median fin, which is continued round the end of the tail and forwards in the middle line to the anus. Frequently this continuous fin becomes broken up into distinct dorsal (d.f. 1 and &), ventral (v.f.), and caudal (c.f.) fins, which may assume very various forms : in the higher classes all trace of median fins disappears. Fishes also possess paired fins. Immediately posterior to the last gill-slit is a more or less horizontal outgrowth, the pectoral fin (pct.f.), while a similar but smaller structure, the pelvic fin (pv.f.), arises at the side of the anus. In the embryonic E8 condition there is some- times found to be a low ridge (r.) connecting the pectoral and pelvic fins of each side with one £b another, and from this and other considerations there is reason for think- ing that the paired fins are detached and en- larged portions of a continuous lateral fin, having similar anatomi- cal relations to the meta- >pleural folds of Amphi- oxus. In all Craniata above Fishes, i.e., from Am- phibia upwards, the paired fins are replaced by fore- and hind-limbs (f.L, h.L), each consist- ing of three divisions — upper-arm, fore-arm, and hand in the one case ; thigh, shank, and foot in the other. Both hand and foot normally terminate in five fingers or digits, and the pvn&adaetyle limb thus formed is very characteristic of all the Co FIG. 713.— Diagrammatic vertical section of the skin of a Fish. B, unicellular mucous glands ; Co, derm ; Ep. epiderm ; F. fat ; G, blood-vessels ; Ko, goblet-cells ; Ko, granule-cells ; S, vertical, and W, horizontal bun- dles of connective tissue. (From Wiedersheim's Fertebrata.) PHYLUM CHORDA TA 63 higher Yertebrata. The paired fins or limbs, as the case may be, are the only lateral appendages possessed by Vertebrates. Body- wall and Internal Cavities. — The body is covered externally by a skin consisting of two' layers, an outer or epithelial Liver, the epidermis (Fig. 713, Ep. derived from the ectoderm of the embryo, and an inner or con- nective-tissue layer, the derm-is (Co), of mesodermal origin. The epidermis is always many-layered, the cells of the lower layers, forming the stratum Malpighii, being protoplasmic and capable of active multiplication, while those of the superficial layers often become flattened and horny, and constitute the stratum cor- ncum. Glands are often present in the skin in the form of tubular or flask-shaped in-pushings of the epidermis or of isolated gland- cells ( B). Beneath the skin comes the musrular layer. This is always highly developed, and, in the lower Craniata, has the same general arrangement as in Am- phioxus, i.e. consists of zig-zag muscle-segments or myqmeres (Fig. 714, m?/m.), separated from < )IK.' another by partitions of con- nective tissue, or myofimmmas (nine.}, and formed of longitudin- ally disposed muscle-fibres. The myomeres are not placed at right angles to the long axis of the body, but are directed from the sagittal plane outwards and back- wards, and are at the same time convex in front and concave be- hind, so as to have a cone-in- cone arrangement (Fig. 715, C). Each myomere, moreover, is divisible into a dorsal (d. m.) and a ventral (v. m.) portion. In the higher groups this segmental arrangement, though present in the embryo, is lost in the adult, the myomeres becoming converted into more or less longitudinal bands, having ail extremely complex arrangement. 64 ZOOLOGY SECT, xin In the trunk, as shown by a section of that region, the muscles form a definite layer beneath the skin and enclosing the ccelome (Fig. 715, A and C, cod.). The muscular layer, as in Amphioxus, is not of even diameter throughout, but is greatly thickened dorsally, so that the coelome is, as it were, thrown towards the ventral side. Its dorsal portion, moreover, is excavated by a canal, the neural or cerebro-spinal cavity (c. s. c), in which the central nervous system is contained, and the anterior portion of which is always dilated, as the cranial cavity, for the brain. Thus a transverse section of the trunk has the form of a 'double tube. In the head, neck, and tail, (B, D), the coelome is absent in the adult, and the muscles occupy practically the whole of the interval between the skin and the skeleton, presently to be referred to : in the tail, however, there is found a hcemal canal (Ji. c.) containing connective tissue, and representing a virtual backward extension of the ccelome. The fins, or fore- and hind-limbs, are moved by longitudinal muscles ^ derived from those of the trunk. All the voluntary or body- muscles of Craniata are of the striped kind. The ccelome is lined by peritoneum (C, pr.), a membrane con- sisting of an outer layer of connective tissue, next the muscles, and an inner layer of ccelomic epithelium bounding the cavity, and thus forming the innermost layer of the body-wall. In Fishes the ccelome is divided into two chambers, a large abdominal cavity containing the chief viscera, and a small forwardly-placed pen- car dial cavity (A. pc.) containing the heart, and lined by a de- tached portion of peritoneum known as the pericardium. In Mammals there is a vertical muscular partition, the diaphragm, dividing the ccelome into an anterior chamber or thorax, containing the heart and lungs, and a posterior chamber or abdomen containing the remaining viscera. Skeleton. — The hard parts or supporting structures of Craniata fall into two categories, the exoskeleton and the endoskeleton. The exoskeleton consists of bony or horny deposits in the skin, and may be either epidermal or dermal,. but is never, like the armour of an Arthropod or the shell of a Mollusc, cuticular. The epidermal exoskeleton is always formed by the cornification or conversion into horn of epidermal cells, and may take the form of scales — as in Reptiles — feathers, hairs, claws, nails, horns, and hoofs. The > dermal exoskeleton occurs in the form of either bony or horn-like deposits in the derm, such as the scales and fin-rays of Fishes, and the bony armour of the Sturgeon, Crocodile, or Armadillo. Some recent researches tend to show that the dermal exoskeleton may be ectodermal and not mesodermal in its ultimate origin. The endoskeleton, or " skeleton " in the ordinary sense of the word, forms one of the most complex portions of the body, and presents an immense range of variation in the different classes and orders. As in Amphioxus, the axis of the entire skeletal system 1 o g'S "g -c -c 3 ^ is «-s2.3,3SG.3~ •r > & o " ^-r1 " ^ ^ -e a o . ^5 S?-3 MrJiSI i^iiiiil ^O.^o5sd9gs i^'llll^ ?S?ll3l |ls.as«lb^ l.^^.^ilr: •5l35fi'ft^S >.fti -•?-"> I^s||||^l Il3-|4s iri-^s -.•lirj^gs Sf.N^^8 S»"ss«=!'5.-a| 5 «lia*|8,!i;i ^lal^^l VOL. II 66 ZOOLOGY SECT. is formed by the notochord (Fig. 715, nch.), an elastic rod made of peculiar vacuolated cells (Fig. 716, nch.), resembling the pith of plants, and covered by a laminated sheath (sh. nch.), with an external elastic membrane (el. m.) around it. The whole sheath is a cuticular product of the superficial notochordal cells (nch. c.). i.e., is developed as a secretion from their outer or free surfaces. The notochord lies in the middle line of the dorsal body-wall between the cerebro-spinal cavity above and the coelome below : it is usually de- veloped, as in the lower Chordata, from a median longitudinal out- growth of the dor- sal wall of the gut. Posteriorly it ex- tends to the end of the tail, but in front it always stops short of the anterior end of the head, ending sp.cd c.c the middle of the brain im- mediately behind a peculiar organ, the pituitary body p.cJb FIG. 716. — Semi-diagrammatic transverse section of the vertebral column of a- craniate embryo ; c. c. central canal ; el. m. ex- ternal elastic membrane ; 7t. ?•. haemal ridges ; n. c. neural tube ; nch. notochord ; nch. c. notochordal cells ; p. c. t. perichordal tube ; sh. nch. sheath of notochord ; sk. c. skeletogenous cells migra sp. cd. migrating into notochordal sheath ; sk. I. skeletogenous layer ; . spinal cord. (Modified from Balfour and Gadow.) which will be re- ferred to again in treating of the digestive organs and of the nervous system. The ex- tension of the nervous system in front of the noto- chord is one of the most striking differences between the Craniata and Amphioxus, in which it will be remembered the notochord is prolonged to a considerable distance beyond the anterior end of the nerve-tube. In the majority of Craniata the notochord is a purely embryonic structure, and all but the anterior end of it is replaced in the adult by the vertebral column, the structure to which the entire vertebrate sub-phylum owes its name. The cells of mesoderm surrounding the notochord become concentrated around the sheath and give rise to the skeletogenous layer (Fig. 716, sk. L), some of the cells of xiii PHYLUM CHORDATA 67 which (sk. c.) may migrate into the sheath itself. In this way the notochord becomes surrounded by a cellular investment which soon takes on the structure of cartilage, and may be called the perichordal tube (Fig. 716, p.c.t., and Fig. 717, c.n.t). The skeletogenous layer also grows upwards and outwards, and gives rise to an inverted tunnel of cartilage, the neural tube (n.c., n.t), enclosing the cerebro-spinal cavity and connected below with the perichordal tube ; and paired hcemal ridges (h.r.) of cartilage standing out from the sides of the perichordal tube into the muscles : in the region of the tail these unite below to enclose the hcemal canal (h.t.) already referred to. Actually, however, the vertebral column thus constituted is from the first more or less broken up into segments, and in the higher forms is replaced by a chain of bones called vertebra; which follow one another from before i.v.f tt.C I *J -p — • — • — - f JL Mil FIG. 717. — Diagram illustrating the segmentation of the vertebral column, c. n. t. perichordal tube; h. r. haemal ridge; h. t. haemal tube; i. r. f. inter-vertebral foramen; n. t. neural tube ; nch. notochord. The dotted lines indicate the segmentation into vertebras. backwards, beginning a short distance behind the anterior end of the notochord and extending to the extremity of the tail. A vertebra consists essentially of the following parts : (1) a centrum or body (Fig. 715, C, en.) lying below the spinal canal in the position formerly occupied by the notochord and perichordal tube, and arising either in the skeletogenous layer proper, or in the notochordal sheath after its invasion by skeletogenous cells ; (2) a neural arch (n. a.) which springs from the dorsal surface of the centrum and encircles the spinal canal, representing a segment of the neural tube; and (3)" a pair of transverse processes (t.p.) which extend outwards from the centrum among the muscles and represent segments of the haemal ridges : to them are often attached ribs which extend downwards in the body-wall, some- times between the dorsal and ventral muscles (r1), sometimes immediately external to the peritoneum (r). In the anterior part of the ventral body- wall a cartilaginous or bony sternum or breast- bone may be developed : in the Amphibia it is an independent structure ; in the higher classes it is formed by the fusion of some F 2 ZOOLOGY SECT. 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 hcemal arch (Fig. 715, 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 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 vertebrae are equal in number to the myomeres, but are arranged alternately with them, the fibrous parti- tion between two myomeres abutting against the middle of a vertebra, so that each muscle-segment acts upon two adjacent verteJbrae. Thus, the myomeres being meta- meric or segmental struc- tures, the vertebrae are inter- ^ • segmental. In connection with the anterior end of the noto- chord, where no vertebrae are formed, there are developed certain elements of the skull I or cephalic skeleton, a struc- ture eminently characteristic of the whole craniate divi- sion, and to the possession of which, indeed, it owes its name. The skull makes its first appearance in the embryo in the form of paired cartilaginous plates, the pamchvrdals (Fig. 718,_pc), lying one on each side of the anterior end of the notochord (nch) and thus continuing forward the line of vertebra) 71 ch, in FIG. 718. — The elements of the cranium embryo Salmon from above, au. c. auditory capsule ; nch. notochord ; pc. parachordal ; ptii. position of pituitary body ; tr. trabecula. (From a model by Ziegler.) xiii PHYLUM CHORDATA 69 centra. In front of the parachordals are developed a pair of curved cartilaginous rods, the trabeculce (tr), which underlie 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 as outgrowths of the trabeculse, and are therefore continuous with those structures from the first. The auditory cap- sules are in some cases formed as outgrowths of the parachordals, in others arise as independent cartilages, each of which, however, soon unites with the parachordal of its own side. As development goes on, the trabecuhe and parachordals become fused into a single basal plate (Fig. 719, B, b. cr.) 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. 719, B, for. mag.), through which the spinal cord becomes continuous with the brain. ; an auditory region formed by the two outstanding auditory capsules (au. cp.) ; and a trabecular region, including all the rest. The latter is again divisible into an inter-orbital region, between the orbits or eye-sockets; an olfactory region, constituted by the olfactory capsules (A, olf. cp.), and by a median vertical plate, the mescthmoid (B. ms. eth.), which separates them from one another ; and a pre- 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 fontanelles (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 70 ZOOLOGY SECT. 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, trabeculse, and auditory capsules — there enter into the composition m.cth B b.br.s FIG. 719. — A, diagram of cartilaginous skull ; B, cranium in sagittal section, au. cp. auditory capsule ; b. br. 1 — 5, basi-braiichials ; b. cr. basis cranii ; b. /«/>/. basi-hyal ;• c. br. cerato- branchial ; c. hy. cerato-hyal ; ep. br. epi-branchial ; ep. hy. epi-hyal ; fon. fontanelle ; for. mag. foramen magnum; h. br. hypo-branchial; h. liy. hypo-hyal ; hy. m. hyomandibular ; lb. 1 — It, labial cartilages; mck. c. Meckel's cartilage; m. ctk. mesethmoid; nv. 1—10, foramina for cerebral nerves ; olf. cp. olfactory capsule ; pal. qv. palato-quadrate ; ph. br. pharyngo- branchial ; r. rostrum ; s. t. pituitary fossa or sella turcica. 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. 715, B,rs. &.). The corresponding right and left bars become united with one another below by an unpaired cartilage (Fig. 719, A, 1}. br.), forming a visceral arch, and the unpaired ventral pieces 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 segmental arrangement, being formed of parts arranged in an antero-posterior series, whereas in the cranium there is no indication whatever of segmentation. There is, however, no exact correspondence between xin PHYLUM CHORDATA 71 the segments of the visceral skeleton and the metameres. The visceral arches vary in number from four to nine : the foremost 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 Crariiata 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. 719, 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 jaw : Meckel's cartilages similarly extend along the lower or posterior margin of the mouth and unite in the middle line, forming the lower jaw. The quadrate (qu) 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 Polychsetous 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 (ep.hy), cerato-hyal (c.hy), and hypo-hyal (h.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- diliuJar 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 suspensorium, furnished by the quadrate, and a hyoidean suspensorium 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 articula- tion of both mandibular and hyoid arches with the skull. The branchial arches become divided transversely into dorso- ventral segments called respectively pharyngo-branchial (ph. br.), epi-branchial (ep.br.), cerato-branchial (c.lr.), and hypo-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. 72 ZOOLOGY SECT. In connection with and always superficial to the rostrum, olfactory capsules, and jaws are frequently found labial cartilages (Ib. 1-4), which sometimes attain a great size. In the lower 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, to calcified cartilage, but in all the higher forms true ossification takes place, the cartilaginous skull becoming complicated, and to a greater or less extent replaced, by distinct bones. Of these there are two kinds, cartilage and membrane bones. Cartilage bones begin by the deposition of minute patches of calcific matter in the cartilage itself: these centres of ossification are not dis- tributed irregularly, but have definite positions, constant in the whole series of higher Craniata. As development proceeds, they may be replaced by ossification, starting in the perichondrium, or layer of connective tissue surrounding the cartilage, and gradu- ally invading the latter. But in either case the bones in question are preformed in cartilage, i.e. replace originally cartilaginous parts. In the case of membrane bones centres of ossification appear, also in constant positions, in the fibrous tissue outside the cartilage : they may remain quite independent of the original cartilaginous skull and its cartilage 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 membrane and cartilage bones is lost in the adult. The membrane 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 cartilage 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 basi-occipital (Fig. 720, 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 opistJiotic (OP. OT) behind, and an epiotic (EP. OT) over the arch of the posterior semicircular canal (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 on each side with the basi-sphenoid are paired bones, the alisphenoids (AL. SPH), which help to furnish the side walls of the interorbital region. The basi-sphenoid is continued forwards by another median bone, the pre-splunoid (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. XIII PHYLUM CHORDATA 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 cartilage bones furnish floor and side walls only. This deficiency is made good by tAVO pairs of membrane bones, the parietals (PA), formed immediately in front of the supra-occipital, and usually articulating below with the alisphenoids, and the PROT FIG. 720. — 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 ; cartilage bones are marked in thick type, membrane bones in italics ; mck. c. Meckel's cartilage ; Nv. 1 — 10, foramina for cerebral nerves ; r. rostrum ; s. t. sella turcica or pituitary fossa. Cartilage bones— AL.S PH. 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. H Y. epi-hyal ; EX. OC. ex-occipital ; H. BR. hypo-branchial ; H. HY. hypo-hyal ; HY.M. hyomandibular ; M, ETH. mesethmoid; OP.OT. opisthotic ; OR. SPH. orbito-sphe- noid ; PAL. palatine; PH. BR. pharyngo-branchial ; PR.OT. pro-otic; PR. SPH. pre-sphenoid; PTG. pterygoid ; QU. quadrate; S. OC. supra-occipital. Membrane bones —DNT. dentary; FR. frontal; MX. maxilla; NA. nasal; PA. parietal; PA. SPH. parasphe- noid ; PM.X. premaxilla ; SQ. squamosal ; VO. vomer. f rentals (FR), placed in front of the parietals, and often connected 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 membrane bones make their appearance, the vonier ( VO) — which may be double — in. front, and the para-sphenoid (PA. SPff) behind. The result of the peculiar arrangement of cartilage and mem- brane, 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), 74 ZOOLOGY SECT. formed by the basi-occipital below, the ex-occipitals at the sides > and the supra-occipital above ; 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 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 cartilage bones, and the secondary cranium modified by the super-addition of membrane bones. A similar distinction may be drawn between the primary and secondary jaws. The primary upper jaw or palato-quadrate be- comes ossified by three chief cartilage bones on each side, the palatine (A. PL) in front, then the pterygoid (PTG), and the quad- rate (QU) behind, the latter furnishing the articulation for the .lower jaw or mandible. In the higher classes the primary upper jaw does not appear as a distinct cartilaginous structure, and the palatine and pterygoid are developed as membrane bones. The secondary upper jaw is constituted by two pairs of membrane bones, the premaxilla (PMX) 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 o the primary lower jaw ossifies to form a cartilage bone, the articular (ART), by which the mandible is hinged ; the rest of it remains as a slender, unossified Meckel's cartilage (Mck. C), which may dis- appear entirely in the adult. The secondary lower jaw is formed by a variable number of membrane 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 membrane bone formed external to the latter, and known as the squamosal (SQ). In the hyoid arch a cartilage bone, the hyomandibular (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 XIII PHYLUM CHORDATA 75 branchial arches undergo more or less complete atrophy, the whole gill- bearing apparatus becoming reduced to a small hyoid bone serving for the support of the tongue. The skeleton of the median fins is formed of a single row of cartilaginous rays or pterygiophores (Fig. 715, 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, formed either of bone or of a horn-like material, and developed in the Bas FIG. 721.— Diagram of three stages in the development of the pelvic fins. In A the anterior pterygiophores on the right side (Rad), have united to form a basal cartilage (Bas.); in B the basalia (Baft.) 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. derm along the free edge of the fin. The latter are clearly exoskeletal structures. As already mentioned, the paired fins are probably to be looked upon as the detached and enlarged anterior and posterior portions of a continuous lateral fin the intermediate portion of which has disappeared. Both pectoral and pelvic fins are supported by pterygiophores or radialia (Fig. 721, Had), the basal or proximal ends of which are articulated with stout cartilages, often replaced 76 ZOOLOGY SECT. by cartilage bones, the lasalia (Bas\ 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, 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 humerus (Fig. 722, A, HU), or upper arm bone in the fore-limb, the femur (B, 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 SCP B FIG. 722. — Diagrams of the fore (A) and hind (B) limbs with the limb-girdles, actb. acetabulum : grLglenoid cavity ; p. cor. procoracoid ; / — V, digits. Cartilage bones — cn.l, en. 2, centralia : COR. coracoid; dst. 1—5, distalia ; FE. femur; FI, fibula; fi. fibulare ; HU. huinerus : III. ilium ; int. intermedium; IS. ischium ; mtcp. 1 — 5, metacarpals ; mt.ts.l — 5, metatarsals ; ph. phalanges ; PU. pubis ; RA. radius ; ra. radiale.; TI. tibia ; ti. tibiale ; UJL. ulna ; ul. ulnare. Membrane bone — CL. clavicle. 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, metacarpals (mtcp) or hand-bones, and phalanges (ph) or finger- bones, in the fore-limb ; tar sals or ankle-bones, metatarsals (B, 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 radiale (A, ra), intermedium (int), and ulnare (ul), those of the middle row the first and second centralia (en. 1. xni PHYLUM CHORDATA 77 en. 2), those of the third row the five distalia (dst. 1-5), the separate elements being distinguished by numbers, counting from the anterior or radial edge of the limb. In the tarsus the bones of the first row are known respectively as tibiale (B, 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- earpals (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 (A, i) is distinguished as the pollex or thumb, that of the hind-limb (B, 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 structures called the limb- gir dies ; 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 in-growths of the primitive fin-skeleton (Fig. 721). 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 glcnoid surface (Fig. 722, A, 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, the procora- coid (p. cor}, and a posterior, the coracoid proper. Each of these regions commonly ossifies, a cartilage bone, the scapula (SCP), appearing in the scapular region, another, the coracoid (COR), in the coracoid region, while in relation with the procoracoid is formed a bone, the clavicle (CL), largely or entirely developed from membrane. 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 acetabulum (B, 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 ischi^lm. Each region is replaced in the higher forms by a cartilage 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 shcf/t, 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 78 ZOOLOGY SECT. fatty tissue called marrow. The extremities become simply calcified in the lower forms, but in the higher a distinct centre of ossification may appear in each, forming the epipliysis, which finally becomes ankylosed to the shaft. Digestive Organs. — The enteric canal is divisible into buccal cavity (Fig. 715, A, buc. c.), pharynx (ph.), gullet, stomach (st.), and intestine (int.), the latter sometimes communicating with the exterior by a cloaca (cl.), which receives the urinary and genital ducts. The buccal cavity is developed from the stomodseum of the embryo : the proctodseum gives rise to a very small area in the neighbourhood of the anus, or, when a cloaca is present, to its external portion : all the rest of the canal is formed from the mesenteron, and is therefore lined by an epithelium of endo- dermal 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 a compara- tively narrow 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 be taken to indicate that the anus has shifted forwards in the course of evolution. The epithelium of the buccal 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 (Fig. 723, E) underlaid by a loose layer of connective tissue, the sub-mucosa (Z) ; epithelium and sub-mucosa together constitute the mucous mem- brane. The mucous membrane of the stomach and sometimes of the intestine contains close-set tubular glands (D) ; those of the stomach, the gastric glands, secrete gastric juice, which acts upon the proteid portions of the food only ; the intestinal glands digest proteids, starch, and fats. Outside the mucous membrane are layers of unstriped muscle, usually an internal circular (M') and an external longitudinal (M ) layer. Externally the intra-coelomic portion of the canal is invested by peritoneum (£) formed of a layer of connective tissue next the gut and a single-layered coelomic 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 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 XIII PHYLUM CHORDATA 79 cement. The main bulk of the tooth is made of dentine (Fig. 724, B,'£Z?), 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 in its structure and mode of FIG. 723. — A, semi-diagrammatic transverse section of the intestine of a Craniate ; B, two epithelial cells, highly magnified. B, visceral layer of peritoneum ; D, tubular glands ; E, columnar epithelium (magnified at B, a) ; El, the same with amoeboid processes (magnified at B, b) ; G, Qi, blood-vessels ; L, lymph-follicles ; Ll— L3, Ly, lymph-cells ; Lym, lacteals ; M, longitudinal muscular layer ; M', circular muscular layer ; N, nutritive matters in cavity of intestine being ingested by wandering lymph-cells ; Sa, striated border of epithelial cells ; z, sub-mucosa ; Zv villus. (From Wiedersheim's VerUbmta.) development to bone. 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 (ZC) coats that portion of the tooth which is embedded in the tissues of the jaw, 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 80 ZOOLOGY SECT. 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. 724, B) the deep layer of the buccal epithelium becomes invaginated and grows inwards or 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 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 odontoblasts (0). From ZS zc zir-A FIG. 724. — A, longitudinal section of a tooth, semi-diagrammatic. PH, pulpcavity ; PH', opening of same ; ZB, dentine ; ZC, cement ; ZS, enamel. B, longitudinal section of developing tooth ; Bg, 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 Vertebrata.) 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 calcine 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. In some Fishes the scales or elements of the dermal exo- skeleton, pass insensibly into the teeth over the ridges of the XIII PHYLUM CHORDATA 81 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 mtuitary diverticulum (Fig. 715, K,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. b.). It may correspond with the sub-neural gland of Urochorda. In terrestrial Craniata buccal glands are present, opening by ducts into the mouth : the most important of these are the race- mose salivary glands which secrete a digestive fluid, saliva, capable 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 is (Fig. 715, A, Ir.) 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 re- stored to the blood in the form of sugar. The liver is formed of a mass of polyhedral cells (Fig. 725, /.) with minute intercellu- lar spaces which receive the bile secreted from the cells and from which it passes to the ducts (b). The pancreas (Fig. 715, FIG. 725. — Diagram of structure of liver, b, a small branch of hepatic duct ; b', its ultimate termination in the intercellular spaces; c, blood capillaries ; I, liver cells, (trom Huxley's Physiology.) 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 the liver (b. d.) generally gives off a blind offshoot ending in a capacious dilatation, VOL. II G 82 ZOOLOGY SECT. the gall-bladder (g. 7>.), 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 Hie duct leads into the intestine. Another important and characteristic organ in the abdomen of Craniata is the spleen (spl.)t 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, which, as will be remembered, is an open groove on the ventral side of the pharynx. 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 functions of both thyroid and thymus are very imperfectly understood. 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. 715, C, mes.) and having the usual relation to the parietal and visceral layers of 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. 715, 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 inter-branchial septa (Fig. 726, i. br. s). The mucous membrane forming the anterior and posterior walls of the pouches is raised up into a number of horizontal ridges, the branchial filaments ' (br.f.), 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, XIII PHYLUM CHORDATA 83 i.br.s FIG. 72(5.— Diagrammatic horizontal section of the pharyngeal region of a Craniate : on the left are shown three gill-pouches (g. p.) with fixed branchial filaments (br.f.) and separated by inter-branchial septa (i. br. s.) ; on the right one hemibranch (hm. 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. 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 em- bedded in the inner or pharyngeal side of an inter - branchial 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 anterior set of the next. In the higher Fishes, such as a Trout or Cod, the inter-branchial septa become reduced to narrow bars enclosing the visceral arches (right side of Fig. 726), 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 holobranck (hi. br.) is the morphological equivalent of two half-gills, hemibmnchs, or sets of branchial filaments, belonging to the adjacent sides of two consecutive 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 type are found. These are the external gills (Fig. 886, bis) : they are developed as branched outgrowths of the body- wall in immediate relation with the gill-slits, and differ from the internal gills just described in having an ectodermal epithelium. They are, there- fore, comparable with the gills of Chaetopods or Crustacea. Lungs (Fig. 715, A, Ig) are found in all Craniata, from the Dipnoi upwards. They are developed as a hollow outpushing from the ventral wall of the pharynx, which 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 G 2 84 ZOOLOGY SECT. 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 pharynx, but, except in two 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. 715 and. 727) is a muscular organ contained in the pericardial cavity and composed of three chambers, the sinus venosus (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. Sometimes a fourth chamber, the comes arteriosus (c. art.\ is added in front of the ventricle. The various chambers are separated from one another by valvular apertures (Fig. 728) which allow of the flow of blood in one direction only, viz. from behind forwards, or from sinus to auricle, auricle to ventricle, and ventricle to conus. The heart is made of striped muscle — the only involuntary muscle in the body 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 ventral aorta (Figs. 715, B, and 727, v. ao.\ formed of fibrous and elastic tissue and unstriped muscle, and lined with epithelium. At its origin, which may be dilated, forming a bulbus aortce, 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. 728, 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 PHYLUM CHORDATA 85 \ 86 ZOOLOGY SECT. 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 (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. 728, 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 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 xiii - PHYLUM CHORDATA 87 containing valves. In some cases the veins become dilated into spacious cavities called sinuses ; but sinuses without proper walls, such as occur in many Invertebrates, are never found in the Craniata. The veins from the head join to form large, paired jugular veins (j. 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 preeaval 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 vertebrae (Fig. 715, 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. 727, r; p. v.}. In the kidneys they break up into capillaries (Fig. 728, K), their blood mingling with that brought by the renal arteries and being finally discharged into the cardinals by the renal veins (r. v). Thus the blood from 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 vein ; 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 vein (scl. v.) 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 there breaks up into capillaries, its blood mingling with that brought to the liver by the hepatic artery (h. a.), a branch of the coeliac. 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. 728, 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 88 ZOOLOGY SECT. that the afferent vein terminates, as it originates, in capillaries. After circulating through the liver the blood is poured, by hepatic veins (Ji. 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 ct.eto a.brci PIG. 728.- -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 ; c. a. con us arteriosus ; d. ao. dorsal aorta ; e. br. a. efferent branchial arteries ; h. p. r. hepatic portal vein ; /<. c. hepatic vein ; Ic. lacteals ; ly. lymphatics ; pr. cv. v. pre-caval veins ; r. p. r. renal portal veins ; s. r. sinus venosus ; v. ventricle ; v, ao. ventral aorta. The arrows show the direction of the current. 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 tubes by which it is returned to the heart. Thus the general scheme of the circulation is simple : the arteries 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 interposition 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 respiratory capillaries, the efferent arteries taking their origin in those capillaries after the manner of veins ; and (b) by the interposition of two important blood-purifying organs, the liver and the kidney, in the course of the returning current, as a result of which we have veins acting XIII PHYLUM CHORDATA 89 as both afferent and efferent vessels of the hepatic and renal capillaries, 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. 729, Ah.). With the ap- pearance of the lungs, however, a very fundamental change occurs in the blood-system. The last aortic arch of each side give off a pulmonary artery (Fig. 730, 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 directly : there it enters the left side of the auricle, in which a vertical . partition is developed, separat- ing a left auricle (A1), 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 the ven- tricle also becomes divided into right and left chambers (B.), and we get a four-chambered heart, having right and left auricles, and right and left ventricles : at the same time the sinus venosus ceases to exist as a distinct chamber. The left auricle receives aerated blood from the lungs and passes it into the left ventricle, whence it propelled FIG. 729. — Diagram of the vascular system in the embryo of an air-breathing Craniate. A, dorsal aorta and auricle ; Ab, aortic arches ; Acd, caudal artery; All. allantoic arteries; Am, vitelline arteries ; £, ventral aorta ; C, C1, carotid arteries ; D, pre-caval veinsv; Jc, E, iliac arteries ; HC, cardinal veins ; KL, gill- clefts ; £. A. S, -S1, roots of dorsal aorta ; Sb, sub-clavian arteries ; Sb}, sub-clavian veins ; V. Ventricle ; VC, jugular vein ; Vm, vitelline veins. (From Wiedersheim's Vertebrata.) 90 ZOOLOGY SECT. 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 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 modifications 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 V V*' . u A m RA FIG. 730.— Diagram of the heart A, in an Amphibian ; B, in a Crocodile. A, right auricle ; A', left auricle ; Ap, pulmonary artery ; lr, pulmonary vein ; RA, aortic arches ; V, ventricle ; V\ left ventricle ; V, 7, and Ve, Ve, pre- and post-cavals. (From Wiedersheim's Vertebrata.) Worms. The sub-intestinal vein, heart and ventral aorta together form a ventral vessel, the dorsal aorta a dorsal vessel, and the aortic arches commissural vessels. The heart is therefore to 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. There seems to be some reason for thinking that the caudal, hepatic-portal, and hepatic veins represent detached portions of the original ventral vessel, .while the lateral veins may be compared with the lateral vessels of some Annulates. The blood 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 blood corpuscles (Fig. 731), which occur floating in the plasma in addition to, and in far greater numbers than, the leucocytes. They usually have xiii PHYLUM CHORDATA 91 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- nucleated and usually circular. They do not perform amoeboid movements. The colour of the blood varies with the amount of oxygen taken up by the haemoglobin. 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 FIG. 731.— Surface and edge views of red-blood corpuscles of Frog (A) and Man (B). nu. nucleus. (From Parker's Biology.) 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.g., 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. 728, 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 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 course 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 (Ic.) 92 ZOOLOGY SECT. 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 neurones formed, its lumen forms the neuroccde 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 becomimg 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. 715, cs. 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 vertebrae, and in front by the cranium (Fig. 715, B-D). The spinal cord (Fig. 732) is a thick-walled cylinder, continuous in front with the brain. It is transversed 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 sub- stance of the cord itself being formed from the deeper layers. The dorsal surface of the cord is marked by a deep, narrow, longitudinal groove, the dorsal fissure (#), the ventral surface is similarly scored by a 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 by 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, 6'). 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 dorsal and a ventral. The dorsal root (Fig. 734, 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 PHYLUM CHORDATA 93 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 arches of adjacent vertebrae. Soon after its emergence it divides into two chief divisions, the dorsal (d.) and ventral (sp. 1, &c.) nerves. The spinal nerves supply the muscles and skin of the trunk and limbs, and are therefore spoken of as somatic nerves. FIG. 732.— 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, 6, dorsal horn of grey matter ; c, Clarke's column ; e. ventral horn. (From Huxley's Physiology.) 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. 734, 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 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 sympathatic nerves supply the enteric canal and its glands, the heart, blood-vessels, &c., and are therefore denominated splanchnic nerves. 94 ZOOLOGY SECT. 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. 733). Constrictions appear in the dilated part and divide it into three bulb-like swellings or vesi- cles, the fore-brain (A,/. &.), 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 (eblm.) from its dorsal surface. The brain now consists of five divisions : the prosencephalon (prs. en.) and the diencephalon (dien.), derived from the fore-brain : the mid-brain or mesencephalon (m. b.) which remains unaltered : and the epencephalon, or cerebellum (cblm.\ and the metencephalon, or medulla oblongata (med.obl.) derived from the hind-brain. 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 ventricles, all communicating with one another and called respectively the fore- ventricle or prosoccele, third ventricle or diaccele, mid-ventricle or mesoccele, cerebellar ventricle or epiccele, and fourth ventricle or metaccele. In 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 hemispheres or parencephala (I-L, c.h), each containing a cavity, the lateral ventricle or paraccele (pa. cce) which communicates with the diacoele (di. cce.) by a narrow passage, the foramen of Monro (f. m.). Moreover, each hemisphere gives off a forward prolongation, the olfactory lobe or rhinencephalon (olf. I.), containing an olfactory ventricle or rhinocosle (rh. cce.) : when there is an undivided pro- sencephalon the olfactory lobes (C, D, olf. I.) spring from it. In the embryo of some forms there is a median unpaired olfactory lobe, like that of Amphioxus. 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- sisting of a single layer of epithelial cells, assuming the character therefore of a purely non-nervous epithelial layer (ependyme). In the cerebellum the thickening takes place to such an extent that the epicoele is usually obliterated altogether. In the mid-brain the ventral wall is thickened in the form of two longitudinal bands, the crura cerebri (cr. crb.), the dorsal wall in the form of paired oval swellings, the optic lobes (opt. I.): extensions of the mesocoele into the latter form the optic ventricles or optocosles (G. opt. cce.) : the median portion of the mesoccele is then called the iter (I) or aqueduct of Sylvius. In the diencephalon the sides XTH PHYLUM CHORDATA 95 „ , . :-5 a AO • •5 2 o 8 g£ •- iillill-i C8 o * 3»3 O B^Issi .2 W -g Sv3 8 -9 ~» ^ ^> ^ •sl^siifi- <^ a^.j^ti j5 lrgi«^|.-i s III! l^ii cj o tU-g 96 .ZOOLOGY SECT. become thickened forming paired masses, the optic thalami (D, F, L, o. th.), the roof remains for the most part in the con- dition of a thin membrane (ependyme) composed of a single layer of cells, but part of it gives rise to a very peculiar adjunct of the brain, the pineal apparatus. This originates as a narrow hollow outgrowth, the epiphysis. The epiphysis is frequently double, one portion being in front of the other, and the two parts may be widely separated. From this, or when it is double, from one of its portions, a diverticulum is developed, which becomes constricted off in the Lampreys and some Reptiles to form an eye-like body, the pineal eye (pn.e.); some- times a second, less fully formed parapineal eye may be formed from another part of the epiphysis. In most adult Vertebrates the epiphysis is represented by a gland-like structure, the pineal body (pn. 5.), connected with the roof of the diencephalon by a hollow or solid stalk. The term paraphysis is sometimes applied to an out- growth of the roof of the fore-brain developed in front of the epiphysis in the hinder region of the prosencephalon. The floor of the diencephalon grows downwards intox a funnel-like pro- longation, the infundibulum (inf.): with this the pituitary diver- ticulum of the pharynx (p. 81) comes into relation, and there is formed, partly from the dilated end of the diverticulum, partly from the extremity of the infundibulum, a gland-like structure, the pituitary body or hypophysis (pt.) always situated immediately in front of the anterior extremity of the notochord arid between the diverging posterior ends of the trabeculaB. In cases where cerebral hemispheres are not developed, the roof or pallium of the undivided fore-brain is reduced to a layer of epithelium (D and E. pal.) : its floor is thickened so as to form large paired masses, the corpora striata (c. s.). When hemispheres are developed the corpora striata form the floors of the two lateral ventricles (L. c. s.), and the roof (pallium) of each is formed of nervous tissue. In such cases the front wall of the diencephalon remains very thin, and is distinguished as the lamina terminalis (I. t.): this is the actual anterior extremity of the central nervous system, the cerebral hemispheres being lateral outgrowths. In the preceding description the brain has been described as if its parts were in one horizontal plane, but, as a matter of fact, at a very early period of development the anterior part becomes bent down over the end of the notochord, so that the whole organ assumes a retort-shape, the axis of the fore-brain being strongly inclined to that of the hind-brain. The bend is known as the cerebral flexure : it is really permanent, but, as the hemispheres grow forward parallel to the hind-brain and the floor of the mid- and hind-brain thickens, it becomes obscure, and is not noticeable in the adult. The brain, like the spinal cord, is composed of grey and white matter, but the grey matter either forms a thin superficial layer an PHYLUM CHORD AT A 97 or cortex, as in the hemispheres and cerebellum, or occurs as ganglionic masses surrounded by white matter. The whole cerebro-spinal cavity is lined with a tough membrane, the dura mater, and both brain and spinal cord are covered by a more delicate investment, the pia, mater : the space between the two contains a serous fluid. In the higher forms there is a delicate arachnoid membrane outside the pia, and in many cases the regions of the pia in immediate contact with the thin epithelial roofs of the diencephalon and medulla become greatly thickened and very vascular, forming in each case what is known as a choroid plexus. From the brain are given off cerebral or cranial nerves : these, like the spinal nerves, are paired, but, unlike them, are strictly limited in number, the number being constant, at least within very narrow limits : there are ten pairs in Fishes and Amphibians, twelve in Reptiles, Birds, and Mammals. The first or olfactory nerve (Fig. 734, I.) is rather a bundle of fibres than a single nerve : it arises from the olfactory lobe, and supplies the organ of smell, i.e., the epithelium of the olfactory sac (see below). It is therefore a purely sensory nerve. The second or optic nerve (II.) arises from the ventral region of the diencephalon, just in front of the infundibulum. It differs from all the other nerves in being originally a hollow out-pushing of the brair£ containing a prolongation of tne diaccele (see Fig. 741). It supplies\the retina or actual organ of sight, and is therefore a purely .sensory nerve. The third or oculomotor nerve (III.) arises from the crus cerebri or ventral region of the mid-brain. In its course is a ganglion, the oculomotor or ciliary ganglion (c. gn.). It supplies four out of the six muscles of the eye-ball (see below, Fig. 742), viz., the superior, inferior, and internal recti, and the inferior oblique (Fig. 742, III.), as well as the ciliary muscles and muscles of the iris in the interior of the eye. It is therefore a purely motor nerve. The fourth or trochlear nerve (Figs. 734 and 742, IV.) arises from the dorsal surface of the brain at the junction of the mid-brain with the medulla oblongata. It is a very small and purely motor nerve, supplying only the superior oblique muscle of the eye. The fifth or trigeminal nerve (Fig. 734;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. In some instances each root, or the dorsal root only, has a ganglion near its origin, in others the two roots enter a single Gasserian ganglion (g. gn.) 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 VOL. II H 98 ZOOLOGY SECT. two branches, a superficial (V. o. s. and a deep V. o. p) : it 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. The ophthalmic nerve is connected by a branch with the ciliary ganglion. The sixth or abducent (Figs. 734 and 742, 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 FIG. 734. Diagram of the cerebral and anterior spinal nerves of a Craniate. I, olfactory nerve; II, optic; III, oculomotor; IV, trochlear ; V. trigeminal ; V. o. .s. superficial ophthal- mic branch ; V. o. p. deep Ophthalmic ; VI, abducent ; VII, facial ; VII. h, hyomandibular branch; VII. p, palatine branch; VIII, auditory ; IX, glossopharyngeal ; X, vagus ; ~K.br. 1 — 5, branchial branches ; X. c, cardiac branch ; X. c/, gastric branch ; X. 1, lateral branch ; XI, accessory ; XII, hypoglossal. au. auditory organ ; br. 1 — 7, branchial clefts ; cblm. cere- bellum ; c. (id. ciliary ganglion ; c: h. cerebral hemispheres ; d. dorsal branch of spinal nerve ; d. r. dorsal root ; e. eye ; gn. d. r. ganglion of dorsal root ; m. l>. mid-brain ; med. obi. medulla oblongata ; milt, mouth ; na. olfactory sac; o. I. olfactory lobe; pn. b. pineal body; ?>/(./:. pineal eye ; sj>. c. spinal cord ; «j.>. 1 — 3, ventral branches of spinal nerves ; *//,/<. sympathetic nerve ; sinn. gn. sympathetic ganglion ; r. r. ventral root. 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 accom- modation of the eye for varying distances is effected. The seventh or facial ^Fig. 734, 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 a fa cial 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 aquatic Vertebrata an ophthalmic branch is given off from the trunk of the nerve, and usually xiii PHYLUM CHORDATA 99 accompanies the 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 glossopharyngeal (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 arid 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 Yertebrata, in which gills are absent, the glossopharyngeal sends a gustatory nerve to the tongue and supplies the pharynx. 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. Ir. 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, and a lateral nerve (X. I) which passes backwards along the' side of the body and supplies the cutaneous sense- organs (see below). 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 laryngeal nerves. to the larynx. The above mentioned ten nerves are all that exist in most of the lower Craniata : the eleventh or accessory nerve (XI.) appears first in Reptiles. It arises by numerous roots from the anterior part of the spinal cord, passes forward, between the dorsal and ventral roots of the spinal nerves, and finally leaves the medulla just behind the vagus. It is thus a spinal nerve as regards its origin, H 2 100 ZOOLOGY SECT. a cerebral nerve as regards its final exit. It is purely motor supplying certain of the muscles of the shoulder. The twelfth or hypoglcssal (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. It will be noticed that there are facts in connection with the cerebral nerves which suggest that they, like the spinal nerves, have a segmental value, and indicate that the head of a Vertebrate, like that of an Arthropod, is composed of fused metameres. For instance, the nerves to the gills have a regular segmental arrange- ment, and the conclusion is obvious that each visceral arch repre- sents a metamere, the seventh, the ninth, arid the branchial branches of the tenth being the corresponding segmental nerves. But it has been shown that at an early period of development the mesoderm of the head becomes divided into a number (9-19) of distinct segments, like those which give rise to the myomeres of the trunk and tail, and it is by no means certain that there is any precise correspondence between this original segmentation of the head and the segmentation of the pharynx which gives rise to the gills and associated structures. It has been stated that the first head-metamere gives rise to the superior, inferior, and internal rectus muscles of the eye, the second to the superior oblique, and the third to the external rectus. If this be so, the third, fourth, and sixth are true segmental nerves, and the anomalous fact of three out of ten nerves being devoted to the supply of the eye- muscles is satisfactorily explained. It seems tolerably certain that the third, fourth, sixth and twelfth nerves correspond to ventral roots of spinal nerves — they are all motor, and, except the fourth, arise from the ventral region of the brain : the fifth, with the exception of its motor root, and the seventh and eighth, ninth and tenth appear to correspond to dorsal roots. 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. 735, A) are dermal nerve-cells occurring 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- PHYLUM CHORDATA 101 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 organs of the lateral line. Extending along the sides of the trunk B FIG. 735.— A, tactile spot from skin of Frog, a, touch-cells ; l>, epidermis ; N, nerve. B, tactile corpuscle from dermal papilla of human hand, a, connective-tissue investment ; b, touch- cells ; ,>, ??,', n", ri", nerve. C, Pacinian corpuscle from beak of Duck. A, A', neuraxis ; JK, central knob and surrounding cells; L,Q, investing layers ; AS, medullary sheath of nerve. (From Wiedersheim's Vertebrata.) and tail is a longitudinal streak, due to the presence either of an open groove or of a tube sunk in the epidermis, and continued on to the head in the form of branching grooves or canals (Fig. 736, A). The organs are lined with epithelium, (B), some of the cells of which (b) have the rod-like form characteristic of sensory cells, and are 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, having at first a metameric arrangement, also occur in the aquatic Amphibia. The sense of taste is lodged in the tongue, the epithelium of which contains end-buds (Fig. 737) similar to those of the skin and supplied by the gustatory branches of the trigemirial and glosso- pharyngeal. The olfactory organ is typically a sac-like invagination of the skin of the snout, anterior to the mouth, and communicating with 102 ZOOLOGY SECT. the exterior by an aperture, the external nostril. It is paired in all Craniata, except Cyclostomes, in which there is a single olfactory sac, supplied, however, by paired olfactory nerves. The sac is lined by the olfactory mucous membrane or Schneidericm membrane, the epithelium of which contains peculiar, elongated sensory cells (Fig. 738), their free ends often produced into hair-like processes. In the Dipnoi and all higher groups the posterior end of each sac / R A FIG. 736.— A, diagram of the organs of the lateral line in a Fish. .;-, lateral line ; a—d, its continuation on the head. B, organ of the lateral line in a tailed Amphibian (semi- diagrammatic), a, epidermic cells, through which are seen b, sensory cells ; <-, sensory hairs ; N, nerve ; R, hyaline tube. (From Wiedersheim's Vertebrate,.) communicates with the cavity of the mouth by an aperture called the posterior nostril, and a similar communication occurs in the case of the unpaired organ of the Hags. In many air-breathing Vertebrates there is formed an offshoot from the olfactory organ, which, becoming separated, forms a distinct sac lined with olfactory epithelium and opening into the mouth. This is Jacobsons Organ : it is supplied by the olfactory and trigeminal nerves. The paired eye is a more or less globular structure, lying in XIII PHYLUM CHORDATA 103 the orbit, and covered externally by a thick coat of cartilage or of dense fibrous tissue, the optic capsule or sclerotic (Fig. 739, scL). FIG. 737. — A, vertical section of one of the papilla; of the tongue of a Mammal. <7, sub- mucosa; c. epithelium; n. nerve-fibres; t. taste-buds. B, two taste-buds, c. covering cells shown in lower bud; (?, sub-mucosa ;>! /.'. epithelium of tongue; HI, sensory processes; it, internal sensory cells shown in upper bud. (From Foster and Shore's Physioiocjy.) On the outer or exposed portion of the eye the sclerotic is replaced by a transparent membrane, the cornea (c.), formed of a peculiar variety of connective tissue, and covered on both its outer and inner faces by a layer of epithelium. The curvature of the cornea is not the same as that of the sclerotic, so that the whole external coat of the eye has the character of an opaque spherical case — the sclerotic, having a circular hole cut in one side of it and fitted with a transparent window — the cornea. The latter is almost flat in Fishes, but bulges outwards in terrestrial Vertebrates. Lining the sclerotic is the second coat of the eye — the clwroid (ch.) — formed of connective tissue abundantly supplied with blood vessels. At the junction of sclerotic and cornea, it becomes continu- ous with a circular membrane (7), placed behind but at some distance from the cornea and called the iris. This latter is strongly pigmented, the colour of the pigment varying greatly in different species, and giving, as seen through the transparent cornea, the characteristic colour of the eye. The iris is perforated in the centre by a circular or slit-like aperture, the pupil, which, in the entire eye, appears like a black spot in the middle of the coloured portion. Except in Fishes, the pupil can be enlarged by the action of a set of radiating unstriped muscle-fibres con- Fio. 73S.— Epithelial cells of olfactory mucous membrane. A, of Lamprey ; B, of Salamander. /•:. inter- stitial cells ; R, olfactory cells. (From Wiedersheim's Vcrtebrata.) 104 ZOOLOGY SECT. e.c e.cj tained in the iris, and contracted by a set of circular fibres; and the anterior or outer portion of the choroid, where it joins the iris, is thrown into radiating folds, the ciliary processes (C. P.), containing unstriped muscular fibres, the ciliary 'muscle. Lining the choroid and forming the innermost coat of the eye is a delicate semi- transparent membrane, the retina (E.) covered on its outer or choroidal surface with a layer of black pig- ment (P. 2£). It extends as far as the outer ends of the ciliary processes where it appears to end in a wavy line, the ora serrata (0. S.) : actually, however, it is con= tinued as a very delicate membrane (p.c.E) over the ciliary processes and the posterior face of the iris. The optic nerve (ON.)" pierces the sclerotic and choroid and becomes con- tinuous with the retina, its fibres spreading over the inner surface of the latter. Microscopic examination shows that these fibres, which form the innermost layer of the retina (Fig. 740, o. n.\ turn outwards and become connected with a layer of nerve-cells (n. c.). External to these come other layers of nerve-cells and granules, supported by a framework of delicate fibres, and finally, forming the outer surface of the retina proper, a layer of bodies called, from their shape, the rods and cones (?•.). These are placed perpendicularly to the surface of the retina, and their outer ends are imbedded in a single layer of hexagonal pigment cells, loaded with granules of the black pigment already referred to. Immediately behind and in close contact with the iris is the transparent biconvex lens (Fig. 739, L.), formed of concentric layers of fibres each derived from a single cell. The lens is enclosed in a delicate capsule, attached by a suspensory ligament (sp. /.) to the ciliary processes. The suspensory ligament exerts a pull upon the elastic lens so as to render it less convex than when left to itself ; when the ciliary muscles contract they draw the suspensory ligament towards the iris and allow the lens to assume its normal FIG. 739. — Diagrammatic horizontal section of the eye of "Man. c. cornea; clt. choroid (dotted); C.P. ciliary processes"; e. c. epithelium of cornea ; c. cj. conjunctiva; /.. o. yellow spot; /.iris; L, lens ; ON. optic nerve ; OS. ora serrata ; o — x, optic axis ; p. c. R, anterior non-visual portion of retina ; P.E. pigmented epithelium (black) ; R. retina ; sp. L suspensory ligament ; Scl. sclerotic ; V. H, vitreous. (From Foster and Shore's Physiology.) XIII PHYLUM CHORDATA 105 curvature.. It is in this way that the accommodation of the eye to near and distant objects is effected. The space between the cornea in front and the iris and lens behind is called the anterior chamber of the eye, and is filled by a watery fluid — the aqueous humour. The main cavity of the eye, bounded in front by the lens and the ciliary processes and for the rest of its extent by the retina, is called the posterior chamber, and is filled by a gelatinous substance, the vitreous humour ( V. H.). The cornea, aqueous, lens, and vitreous together constitute the dioptric apparatus of the eve, and serve to focus the rays of light FIG. 740.— Diagram of the retina, the supporting structures to the left, the nervous and epithelial elements to the right ; a—d. fibrous supporting structures ; gr. gr'. granular layers ; n.c. n.c'. n.c". n.c'". nerve cells; nu. nuclear layer of rods and cones; o.n. fibres of optic nerve; r. rods and cones. (From Wiedersheim's Vertebrata.) from external objects on the retina. The iris is the diaphragm by which the amount of light entering the eye is regulated. The percipient portion or actual organ of sight is the retina, or, more strictly, the layer of rods and cones. The great peculiarity of the vertebrate eye, as compared with that of a Cephalopod (Vol. I, p. 720), to which it bears a close superficial resemblance, is that the sensory cells form the outer instead of the inner layer of the retina, so that the rays of light have to penetrate the remaining layers before affecting them. 106 ZOOLOGY SECT. The mode of development of the eye is as characteristic as its structure. At an early stage of development a hollow outgrowth— the optic vesicle (Fig. 741, A, opt. v) — is given off from each side of the diencephalon (dien.). It extends towards the side of the head, where it meets with an in-pushing of the ectoderm (inv. I.) which deepens and forms a pouch, and finally, separating from the ectoderm, a closed sac (B, /.) with a very small cavity and thick walls. This sac is the rudiment of the lens : as it enlarges it pushes against the optic vesicle, and causes it to become invaginated (£), the single-layered optic vesicle thus becomes converted into a two- layered optic cup (opt. c., opt. c1.), its cavity, originally continuous with the diaccele, becoming obliterated. The invagination of the vesicle to form the cup does not take place symmetically, but obliquely from the external (posterior) and ventral aspect of the vesicle, so that the optic cup is incomplete along one side where there is a cleft — the choroid fissure — afterwards more or less completely closed by the in-vl FIG. 741.— Early (A) and later (B) stages in the development of the eye of a Craniate. dien. diencephalon ; inv. 1. invagination of ectoderm to form lens ; I. lens ; opt. c. outer layer of optic cup ; opt. c'. inner layer ; opt. st. optic stalk ; opt. r. optic vesicle ; ph. pharynx ; pty. pituitary body. (Altered from Marshall.) union of its edges. The outer layer of the optic cup becomes the pigmentary layer of the retina : from its inner layer the rest, of that membrane, including the rods and cones, is formed. The stalk of the optic cup occupies, in the embryonic eye, the place of the optic nerve, but the actual fibres of the nerve are formed as backward growths from the nerve-cells of the retina to the brain. During the formation of the lens, mesoderm grows in between the pouch from which it arises and the external ectoderm'; from this the main substance of the cornea and its inner or posterior epithelium are formed, the adjacent ectoderm becoming the external epithelium. Mesoderm also makes its way into the optic cup, through the choroid fissure, and becomes the vitreous. Lastly, the mesoderm immediately surrounding the optic cup is differenti- ated to form the choroid, the iris, and the sclerotic. Thus the paired eye of Vertebrates has a threefold origin : the sclerotic, choroid, iris, vitreous, and the greater part of the cornea XIII PHYLUM CHORDATA 107 i.r are mesodermal : the lens and external epithelium of the cornea are derived from the ectoderm of the head : the retina and optic nerve are developed from a hollow pouch of the brain, and are therefore, in their ultimate origin, ectodermal. The sensory cells of the retina, the rods and cones, although not directly formed from the external ectoderm, as in Invertebrates, are ultimately traceable into the superficial layer of ectoderm, since they are developed from the inner layer of the optic vesicle, which is a prolongation of the inner layer of the brain, which is continuous, before the closure of the medullary groove, with the ectoderm covering the general surface of the body. The eye-ball is moved by six muscles (Fig. 742). Four of these arise from the inner wall of the orbit, and pass, diverging as they go, to their insertion round the equator of the eye. One of them is dorsal in position, and is called the superior rectus (s. r.) a second ventral, the inferior rectus (in. r.), a third anterior, the anterior or internal rectus (i.r.), and a fourth posterior, the posterior or external rectus (e.r). The usual names (in- ternal and external) of the two last-named muscles origin- ate from their position in Man, where, owing to the eye look- ing forwards instead of out- wards, its anterior surface be- comes internal, its posterior surface external. The two re- maining muscles usually arise from the anterior (in Man inner) corner of the orbit, and are inserted respectively into the dorsal and ventral surface of the eye-ball. They are the superior (s. o.) and inferior obliqiie (i. o. ) muscles. The median or pineal eye (Fig. 743), is formed, in certain cases, from the distal end of the epiphysial diverticulum already men- tioned. It has the form of a rounded capsule, the outer or anterior portion of the wall of which is a lens (/.) formed of elongated cells, while its posterior portion has the character of a retina (M, r). The latter has a layer of nerve fibres on its outer, and one of rod-like visual elements (r.) on its inner sur- face : it thus agrees with the usual types of Invertebrate retina, and not with that of the paired eye. The organ of hearing, like that of sight, presents quite peculiar features. It arises in the embryo as a paired imagination of the VI FIG. 741*. — Muscles of the eye of a Skate and their nerves (semi-diagrammatic). ///. oculo* motor nerve ; / V, trochlear ; VI, abducent. e. r. external rectus ; in. o. inferior oblique ; in. r. inferior rectus ; i. /•. internal rectus ; or. wall of orbit ; s. o. superior oblique ; .?. /•. superior rectus. 108 ZOOLOGY SECT. ectoderm in the region of the hind-brain, a shallow depression being formed which deepens and becomes flask-shaped, and finally, as a rule, loses its connection with the external ectoderm, becoming, a closed sac surrounded by mesoderm. At first simple, it soon becomes .divided by a constriction into dorsal and ventral com- partments, The dorsal compartment is differentiated into an irregular chamber, the utriculus (Fig. 744, u.), and, usually, three tubes, the semicircular canals. Of these two, the anterior (ca.) >/,. mesonephros ; mt. n. d. metanephric duct ; mt. nph. metanephros ; nst. nephrostorne ; or ovary; p.n.d. pronephric duct; p. nph. pronephros; sg. d. segmental duct; t. testis : <•. e. vasa effereutia. end into the segmental duet (sg. rf.), at the other, by ciliated funnels (nst.), into the coelome : a short distance from the funnel xni PHYLUM CHORDATA 113 each gives off a blind pouch which dilates at the end and forms a Malpighian capsule (in. c.), and a branch from the aorta entering it gives rise to a glomerulus. In some forms the archinephric duct now becomes divided by a longitudinal partition into two tubes : one retains its connection with the mesonephros and is known as the mesonephric or Wolffian duct (C, ms.n.d.): the other has no connection with the nephridiaj but opens into the ccelome in the region of the vanishing pronephros : it is the pronephric or Mullerian duct (p. n. d.). In some Craniata the Mullerian appears quite independently of the Wolffiaii duct : the latter is then simply the segmental duct after the union with it of the mesonephric tubules. In the higher Vertebrata, from Reptiles to Mammals, a diverti- culum (D, E, int. n. d.) is given off from the posterior end of the Wolffian duct, which grows forwards and becomes connected with the hindmost nephridia. In this way is formed a metanephros (int. nph.\ which becomes the permanent kidney, and a metane- phric duct (int. n. d.\ which becomes 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 cdlantoic bladder (al. bl.}, 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. c.\ 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 open ends of the Mullerian ducts (E, p. 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 ccelome and escape by the genital pores, and in many teleo- stean or bony Fishes, the ovary is a hollow organ, as in Arthro- poda, 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, occa- sionally, as~an abnormality. In close connection with the urinogenital organs are found VOL. II I 114 ZOOLOGY SECT. certain "ductless glands," the adrenals or supra-renal bodies. They are developed partly from ridges of the dorsal wall of the coelorne — i.e., from mesoderm, partly from the sympathetic ganglia. There may be numerous adrenals segmen tally arranged, or a single pair. Their function is quite unknown, but their abundant blood-supply points to their possessing a high physiological importance. Development. — The ova of 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 e-nl Triad, FIG. 748.— Transverse section of earlier (A) and later (B) embryos of Frog. ccel. ecelome ; ccel'. pro- longation of cojlome into protovertebra ; ent. mesenteron ; mad. fir. medullary groove ; nisd. mesoderm ; ncli. notochord ; pro. protovertebra ; sg. d. segmen tal duct ; som. somatic layer of mesoderm ; sp. c. spinal cord ; spl. splanchnic layer of mesoderm ; yk. yolk cells. (After Marshall.) combination of in-pushing and over-growth: the 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. 748, A, msd.) lying one on each side of the middle line, where they are separated from one another by the medullary tube (md. gr.) 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 primi- tive germ-layers. Each mesoderm band becomes differentiated into a dorsal portion, the vertebral plate, bounding the nervous xin PHYLUM CHORDATA 115 system and notochord, and a ventral portion, the lateral plate, bounding the mesenteron. The vertebral plate undergoes meta- meric segmentation, becoming divided into a row of squarish masses, the protovertebrce or mesodermal segments (B, pr. v.) : the lateral plate splits into two layers, a somatic (som.) adherent to the ectoderm, a splanchnic (sp/.) to the endoderm. The space between the two is the ccelome (ccel.), which is thus a schizoccele or cavity hollowed out of the mesoderm and is at no stage in com- munication with the mesenteron, like the coelomic pouches of Amphioxus. A dorsal offshoot of the coelome (cod') may pass into each protovertebra, but such an arrangement is temporary. From the dorsal portions of the protovertebra? 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. Distinctive characters. — The Craniata may be defined as Vertebrata 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. There is no atrium. The liver is large, massive, and not obviously tubular. There is a muscular chambered heart, and the blood contains red corpuscles. The nephridia (mesonephridia) 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 a single pair of gonads, and the reproductive products are usually discharged by ducts derived from the nephri- •dial 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 a schizoccele. CLASS I — CYCLOSTOMATA, The Cyclostomata, or Lampreys and Hags, are eel-like Fishes, 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. I 2 116 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. fluma- tilis), about 60 cm. in length : and the Sand-pride, or lesser fresh-water Lamprey (P. branchialis), not exceeding 30 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. 749) are nearly cylindrical, the tail-region compressed or flattened from FIG. 749. — Petromyzon xnarinus. Ventral (A), lateral (B), and dorsal (C) views of the head.. br. cl. 1, first gill-cleft ; buc. f. buccal funnel ; <=?/(-, eye ; /nth. mouth ; na. ap. nasal aperture :, V. papillae ; pn. pineal area ; fi. t-. &. teeth of buccal funnel ; t*. teeth of tongue. (After W. K. Parker.) side to side. At the anterior end, and directed downwards, is a large basin-like depression, the buccal funnel (hue./.), surrounded with papillae (p.) and beset internally with yellow, horny teeth (tl — t3). At the bottom of the funnel projects the end of the tongue (t*), also bearing teeth, arid 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 trans- parent area of skin (pn.} indicates the position of the pineal organ.. The paired eyes have no eyelids, but are covered by a transparent, area of skin. The gill-slits (br. cl. 1) are seven pairs of small aper- tures on the sides of the head, the first a little behind the eyes. On the ventral surface, marking the junction between trunk and XIII PHYLUM CHORDATA 117 tail, is the very small emus (Fig. 758, a.), lying in a slight depres- sion and having immediately behind it a small papilla pierced at its extremity by the y/rinogenital aperture (z.). It has been sug- gested that a pair of ridges, lying one on each side of the anus, represent vestiges of pelvic fins ; otherwise there is no trace of paired appendages. Two dorsal fins and a caudal fin are present, the second dorsal being continuous with the caudal. Lampreys live on small Crustacea, Worms, and other aquatic organisms, but also 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 olf.c Nu* br.b.i br.b.s brb.a I •(-••* l.c.1 alat.c l.C.3 FIG. 750. — Petromyzon marinus. Skull, with branchial basket and anterior part of verte- bral column. The cartilaginous parts are dotted, a. d. c. anterior dorsal cartilage ; a. lot. c. anterior lateral cartilage ; a;>. r. annular cartilage ; aw. c. auditory capsule ; br. b. 1—7, verti- cal bars of branchial basket ; br. d. 1 — 7, external branchial clefts ; en. c. coruual cartilage ; c/-. ,•. cranial roof ; I. c. l—U, longitudinal bars of branchial basket ; l<>. c. lingual cartilage ; •m. v. c. median ventral cartilage ; na. ap. nasal aperture ; nch. notochord ; Ni: 2, foramen for optic nerve ; olf. c. olfactory capsule ; pc. c. pericardia! cartilage ; p. d. c. posterior dorsal cartilage ; p. lat. c. posterior lateral cartilage ; ••*/>. oc. a. sub-ocular arch ; st. p, styloid process ; stii. r. styliform cartilage ; t. teeth. (After W. K. Parker.) 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 segmental sense organs take the form of a double lateral line 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. 750, 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 and bounding the spinal 118 ZOOLOGY SECT. canal on each side : they are rudimentary neural arches. For the rest of its extent the spinal canal is enclosed only by tough r pigmented connective tissue. The cranium also exhibits a very primitive type of structure. Its floor is formed by a basal plate, (Fig. 751, ~b.pl.), made by the union of the parachordals and trabecula3, 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. /.), due to the non-union of the posterior ends of the trabeculaB ; through it passes the pituitary pouch, pre- sently to be referred to (Fig. 754), on its way from the olfactory saV- 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 of the basal plate are the auditory capsules (cm. c), and the side- walls are pierced with apertures for the cerebral nerves (Nv. £„ 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. 750 and 751, sb. oc. a.), from the fact that it affords a support to the eye. From its posterior end a slender styloid process (st. 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 up- wardly directed plate, the posterior lateral cartilage (p.lat.c.),w~hich probably answers to the primary lower jaw, or Meckel's cartilage. 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 trabeculse. 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 Amphibia (see below). Also belonging to the series of labial cartilages are the paired anterior lateral cartilages (a. I. c.) and the great ring-shaped annular cartilage (an. c.) which supports the edge of the buccal funnel. XIII PHYLUM CHORDATA 119 The tongue is supported by a long unpaired lingual cartilage (Fig. 750, Ig. c.), which probably answers to the basi-hyal or a.d.c nci.a n a. cip olfc Nus p.lal.c crv.c Fie;. 751.— Petromyzon marinus. Dorsal (A), ventral (B), and sectional (C), views of skull. The cartilaginous parts are dotted, a. .. <: auditory capsule; >>. <-,-. f. basi-cranial fontanelle ; /;. pi. basal plate; en. c. cornual cartilage ; o-. r. cranial roof ; na. ap. nasal aperture ', ncli. notochord ; Nr. 1, olfactory nerve ; •>, and 8, foramina for the optic, trigeminal, and aviditory nerves; Nr. 6', fifth nerve ; off. c.'olfactory capsule ; p. . I. olfactory lobes ; opt. I. optic lobes ; r. gn. hb. right ganglion habenulaj. (After Ahlborn.) divides into two branches, a rhinocoele going directly forwards into the olfactory lobe, and a paracosle backwards into the hemisphere. The pineal apparatus consists of three vesicles placed in a vertical series : the dorsal -most of these is the vestigial pineal eye (Fig. 754, pn. e.) : it has a pigmented retina, a flat and imperfectly formed lens, and is connected with the right ganglion habenulse. The 124 ZOOLOGY SECT. middle vesicle (pn.) is in connection with the small left ganglion habenulse. The optic nerves differ from those of the higher classes in the fact that each passes directly to the eye of its own side. The spinal cord (Figs. 752 and 757, my.) is flattened and band- like. The dorsal roots of the spinal nerves alternate with the ventral and do not unite with them to form a trunk. There is no sympathetic. The hypoglossal is the first spinal nerve. Sensory Organs. — The external nostril (Fig. 752, wa,* Fig. 754, na. ap) leads by a short passage into a rounded olfactory sac (Fig. 752, na, Fig. 754) placed just in front of the brain and having its posterior wall raised into ridges covered by the olfactory or FIG. 754. — Petromyzon. Side view of brain with olfactory and pituitary sacs in section. cblm. cerebellum ; crb. ft. cerebral hemisphere; dlen. diencephalon ; /. fold in nasal tube; gl. nasal glands ; inf. infundibulum ; 1. gn. hb. left ganglion habenulse ; med. obi. medulla oblongata; na. ap. nostril; nch. notochord; Nv. 1, olfactory nerve ; ' Nv. 3, optic"; Nv. 3, oculo- motor ; Nv. k, trochleaf ; 'Nv. 5, trigeminal; Nv. 6, abducent ; Nv. 7, facial ; Nv. 8, auditory ; Nv. 10, vagus ; Nv. 12, hypoglossal ; olf. cp. olfactory capsule ; olf. I. olfactory lobe ; olf. m. m. olfactory mucous membrane ; opt. I. optic lobe ; pn. middle pineal body ;' pri. inferior pineal body ; pn. e. pineal eye ; pty. 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.) Schneiderian membrane (Fig. 754, olf. m. m.). From the bottom of the sac is given off a large pituitary pouch (Fig. 752, na', Fig. 754, pty. p.) which extends downwards and backwards, be- tween the brain and the skull-floor, passes through the basi-cranial 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 the stomodaeum (Fig. 755, 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 XIII PHYLUM CHORDATA 125 opens just outside the stomodseum 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 and pituitary invaginations become sunk in a common pit (B), which, by the growth of the immense upper lip (up.l.\ is gradually shifted to the top (C, D) of the head, 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 Ticfl Fiii. 755. — Petromyzon. Diagrams of four stages in the development of the olfactory and pituitary sacs. ent. mesenteron ; inf. infundibulum ; 1. Ip. lower lip ; nch. nptochord ~ off. s. olfactory sac ; pn. pineal body ; pty. s. pituitary sac ; stdm. stomodaeum ; u. Ip. upper lip. (Altered from Dohrn.) give rise to the pituitary body (Fig. 754, pty. &.). Thus the entire nasal passage of the Lamprey, including its blind pouch, is a, persistent pituitary sac into which the single olfactory organ opens. Moreover, 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. 756) is remarkable for having only two semicircular canals, corresponding to the anterior (a.s.c.) and posterior (p.s.c.) of the typical organ. Urinogenital Organs. — The kidneys (Figs. 757 and 758, k.) are long strap-shaped bodies developed from the mesonephros of the 120 ZOOLOGY SECT. a.s.c jy.s.e sac FIG. 750.— Auditory sac of Petromyzon. a. s. c. anterior semicircular canal ; and. n. auditory nerve ; end. s. endolymphatic sac ; p.s.c. posterior canal ; sac. sacculus ; lit,-, utriculus. (After Retzius.) embryo. 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 segmental duct. The ureters open posteriorly into a small urino-genital sinus (Fig. 758, u.g.8.), placed just behind the rectum, and opening, by a urino- genital papilla (u.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 aper- tures, the genital pores (y), which place its cavity in communication with the coelome. The gonacl (Fig. 752, ov, Fig. 758, 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 in the testis of the male. The reproductive products are shed into the coelome .and make their way by the genital pores into the urmogenital sinus, and so to the surrounding water, where impregnation takes place. Development. — The oosperm is telolecithal, having a considerable accumulation of yolk in one hemi- sphere ; in correspondence with this segmentation is complete but un- equal, the morula consisting of an upper hemisphere of small cells or micromeres (Fig. 759, mi. m.), free from yolk, and of a lower hemisphere of large cells or megameres (mg. m), containing much yolk. In the bias- tula stage (D) the segmentation cavity or blastocoele (bid.) is situated nearer to the upper than to the lower pole. The gastrula is not formed by invagination (E), but a cavity appears among the cells of the upper pole and becomes the archenteron, its aperture being the blastopore (C and E, Up.}. The un- symmetrical positions of the blastocoele and archenteron are due to the comparatively rapid division of the micromeres as compared with that of the inert yolk-cells. The blastopore becomes the anus cL.ao cd int Fi<;. 757.— Petromyzon marinus. Transverse section of abdomen. '•«/. cardinal veins ; (/. no. dorsal aorta ; f. r. fin-rays ; /. t. fibrous tissue of spinal canal ; int. intestine, the line pointing to the spiral valve ; k. kidneys ; h/. sub-vertebral lymph sinus ; in. body muscles ; //. Petromyzon marinus. The urino-geiiital sinus with posterior end of intestine and part of left kidney, a. anus ; 'int. intestine ; 1:. left kidney ; r. rectum ; v. rr. p. urino- genital papilla; i>.