p- .3- a m a A TEXT BOOK OF ZOOLOGY A TEXT-BOOK OF ZOOLOGY BY T. JEFFEEY PARKER, D.Sc., F.R.S. PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO. DUNEDIN, N.Z. AND WILLIAM A. HASWELL, M.A., D.Sc., F.R.S. PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF SYDNEY. N.S.W IN TWO VOLUMES VOL. II WITH ILLUSTRATIONS iotUion MACMILLAN AND CO., LIMITED NEW YORK: THE MACMILLAN COMPANY 1897 All rights reserved V RICHARD CLAY AND SONS, LIMITEI., LONDON AND BUNGAV. CONTENTS SECTION XIII PAGE PHYLUM CHORDATA . Sub-phylum and Class I. Adelochorda . 1 Sub-phylum and Class II. TTrochorda . H 1. Example of the Class Ascidia . 12 2. Distinctive Characters and Classification . 18 Systematic Position of the Example . . 20 3. General Organisation . 20 Sub-phylum III. Vertebrata 37 Division A. ACRANIA 38 ,, B. CRANIATA 58 Class I. Cyclostomata . 115 1. Example of the Class Petromyzon . . 116 2. Distinctive Characters and Classification . .128 3. Comparison of the Myxinoids with the Lamprey . . 129 4. General Remarks ... . . 132 Class II. Pisces . 134 Sub-class I. Elasmobranchii . . . 134 1. Example of the Class Scyllium canicula or Chiloscyllhnn fuscum 135 2. Distinctive Characters and Classification . 154 3. General Organisation .... .157 Sub-class II. Holocephali . 173 vi CONTENTS PHYLUM CHORDATA Cant in ued. Class II. Pisces Continued. PAOB Sub-class III. Teleostomi ... . 183 1. Example of the Class Salnio 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 Ceratudus 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 Rana temporaria .... 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 Coliimba Una . . 351 2. Distinctive Characters and Classification . ... 380 Systematic Position of the Example . . . 389 .'5. General Organisation . . 389 Sub-class I. Archreornithes .... . 390 II. Neornithes . . 392 Class VI. Mammalia 417 1; Example of the Class L?iix run imhis 417 "2. Distinctive Characters and Classification . . . 447 Systematic Position of the Example . . 460 General Organisation . 400 The Mutual Relationships of the Choi-data . .575 The Mutual Relationships of the Phyla of Animals . . 580 CONTENTS vii SECTION XIV PAGE DISTRIBUTION ... . 58o 1. Geographical Distribution . 583 '2. Bathy metrical Distribution . 598 M. Geological Distribution . . i\(Y> SECTION XV THE PHILOSOPHY or ZOOLOGY . . 007 SECTION XVI THE HISTORY OF ZOOLOGY . . . 628 APPENDIX Guide to Modern Zoological Literature . 651 INDEX <>5<> LIST OF ILLUSTRATIONS FIG. PAGE 664. Balanoglossus ... 2 665. ,, anterior end 4 666. ,, development . . 5 667. Tomaria . 6 668. ,.'... 6 669. Cephalodiscus, gelatinous investment . 7 670. ,, zooid 671. ,, sagittal section . 9 672. Rhabdopleura . 10 673. Ascidia . 12 674. ,, 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 democratica, ventral view 24 685. ,, lateral view .... 24 686. Pyrosoma ... 25 687. ,, part of section . .25 688. Development of Clavellina, early stages . . 28 689. ,, ,, 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 . . . 36 696. Amphioxus lanceolatus .... 39 x LIST OF ILLUSTRATIONS FIG. PAfiE 697. Amphioxus lanceolatus, transverse sections of pharyngeal and in- testinal regions . . ... 40 698. ,, ,, anatomy, diagrammatic 43 699. ,, ,, transverse section of pharyngeal 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 '.."") 734. Diagram of cerebral and anterior spinal nerves . 98 735. Organs of touch . 101 731). 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 Kl(. 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 ... .111 747. 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 . . 117 751. . . . 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. ,, ,, auditory organ 126 757. ,, ,, 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. PaliBeospoiidylus gunni ... . 133 766. Chiloscyllium modestum . ... 135 767. ,, vertebras . . 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 cornubica .... .... 157 785. Urolophus cruciatus . . 158 786. Centrophorus calceus, dermal denticles . . . 159 787. Scymnus, spinal column .... 159 788. Urolophus testaceus, skeleton 160 789. Heptanchus, skull 16 xii LIST OF ILLUSTRATIONS FIG. PAGE 790. Torpedo, showing electric organ . . . 163 791. Cestracioii galeatus, egg-case . ... 167 792. Pristiurus, section of blastoderm . . . 168 793. Elasmobranch 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 Callorhyiichus ... . . 174 800. ,, vertebral column ... . . 176 801. ,, skull . .177 802. Callorhyiichus antarcticus, skull . ... 178 803. ,, ,, brain . 180 804. ,, ,, male urinogenital organs . . 181 805. ,, ,, embryo in egg-shell ... . 182 806. Salmo fario . 184 807. ,, ,, head . 185 808. ,, scale 186 809. ,, ,, vertebra .... . .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. Acipeiiser ruthenus .... . . 203 825. Lepidosteus platystomus ... . . . 203 826. Amia calva . 204 827. Rita buchaiiani . . 205 828. Gadus morrhua . . 205 829. Sebastes percoides . . . 206 830. Labrichthys psittacula . 206 831. Ostracion . 207 832. Hippocampus . 208 833. Pleuronectes cynoglossua . .211 834. Stomias boa 212 835. Ctenoid and ganoid scales . . . . 212 836. Polypterus, part of vertebral column . .... 213 837. Sturgeon, skull . 214 LIST OF ILLUSTRATIONS xiii FAt:E 838. Polypterus, skull . 215 839. ,, pectoral tin . 216 839 /,/.s. ,, pelvic fin 216 840. Gymnotus electricus 217 841. Sargus, teeth . 218 842. Anabas scandens 843. Lepidosteus, digestive organs . 220 844. Pseudophycis bachus, relation of air-bladder to auditory organ . 221 845. Lepidosteus, brain .... 846. ,, male organs 847. ,, and Amia v female organs . 224 848. segmentation . 225 -' 1117. Perameles, teeth . 5:50 1118. Phascolarctos cinereus, front view of skull . . . 530 LIST OF ILLUSTRATIONS xix FIG. l'\f,K 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 1123. Sus scrofa, teeth . 533 1124. Equus caballus, skull and teeth . 534 1125. Elephas africanus, molar teeth . . 535 1126. Bakenoptera rostrata, lower jaw of foetus, 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. Ornithorhynchus 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 cuniculus, 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 amnioii and trophoblast .... 560 1152. Formation of amnioii in Mammalia 560 1153. Macropus, mammary foetus 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 bucklaiidi, 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 PAGE 1167. Tillotherium fodiens, skull 574 1168. Diagram illustrating the mutual relationship of the Chordata . 580 11(il) - ,, Phyla of animals . 582 1170. Map showing depths of sea between the British Isles and the < 'ontinenb 587 1171. -Maj. showing depths of sea between New Zealand and Australia . 588 1172. Map of tin- \V<> rid 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 notoclwrd. 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 neuroccde, 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 Entcropneusta was applied. There seems reason to believe, however, that two remarkable deep-sea animals RluMopleura and Ccpludodiscus- though not close allies of Balanoglossus, may yet be sufficiently nearly related to it to justify their being placed in the same class. VOL. II B ZOOLOGY SECT. FIG. 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. %>o) rarely by two. Its narrow posterior part or " neck ' is strength- ened by a layer of cartilage-like or cliondroid 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 collar 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 life. The most anterior are in some species overlapped by a posterior prolongation of the collar called the operculum. A pair of longi- xin 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 coeca. 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 coelome of the trunk is divided into two lateral closed cavities by a vertical partition (dorsal and ventral mesenteries). Digestive Organs. --The mouth (Fig. 665, wo.) 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 synapticidce 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. skel.) 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 brob div isc nl ris uent.v ctcra.v FIG. 665. Balanoglossus Diagrammatic sagittal action of anterior end. rare?, s. cardiac sac; div. diverticulum (supposed iiotochord) ; dors. n. dorsal nerve strand; i/o/u. ,W. dorsal sinus ; .l. proboscis skeleton ; vent. n. ventral nerve strand ; emit. 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 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 imagination 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 (B) the body of the embryo becomes divided into three parts an anterior, a middle and a posterior these Fig. 666. Development of BalanOglOSSUS. J, 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.) 6 ZOOLOGY SECT. ca. rd. ,s cil.i Fig. 667. Tornaria. Dorsal view. an. anus ; card. s. cardiac sac ; cil. r. post-oral ciliated band ; cil. r 2 . posterior ciliated ring ; eye, eye-spots on apical plate ; prob. car. proboscis cavity; prob. po. proboscis pore. (After Spengel.) 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 of 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 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 onTHstri'ptirm ivofo Fig. r>68. Tornaria. Lateral view. Lettering as in Fig. 607 ; in addition, mo. mouth. (After Spengel.) card.s -ft rob. fie XIII PHYLUM CHORDATA 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 endostyle to be subsequently met with in the Tunicata (p. 14). The 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 RhaMopUum - - 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- pores, 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 Rhabclopleura 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.) 8 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 ; . ,-,. ':--. .- Fio. 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 coalesceiit ; this area is usually, though t/it Vi .. 671. Cephalo discus. Diagram of longitudinal section, a. anus ; bc^. column of pro- boscis ; be-, coelom of collar ; bc$. ccelom of trunk ; int. intestine ; nch. supposed notochord ; n. s. nerve-strand; ces. oesophagus; 01: ovary ; ocd. oviduct ; ph. pharnyx ; p. p. proboscis pore ; us. proboscis ; st. stomach ; stk: stalk. (After Harrner.) 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- nogiossus, and thus to be homologous with the nobochord of the Chordata. The posterior end of the body is drawn out into a sort of stalk on which the buds are developed (Fig*. 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 B Fig. 072. Rhabdopleura. A, Entire zooid. a, mouth ; I, anus; c, stalk of zooid ; (7, pro- boscis ; e, intestine ;/, anterior region of trunk ; r/, one of the tentacles. (After Ray Lankester.) B, Diagrammatic longitudinal section a little to one side of the median line, anv.s, aims : lie 1 , ccelome of proboscis ; lc 2 . ctelome of collar ; between be 1 , and be'*, is the diverticulum ; bc'3. coelome of trunk ; int. intestine ; moi'th, 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. xin PHYLUM CHORDATA II 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 Chor- dates, they are at least a greatly modified branch, taking its origin from the base of the Chordate tree. The presence of the pre- sumed rudimentary representative of a notochord and of the gill- slits seems to point in this direction. It should, however, be stated that by some of those zoologists by whom the members of this group have been most closely studied, their chordate affinities are altogether denied. If the 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. 396), and, though this likeness between the larvae 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 Salpge 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.ap at rap mant 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 ; mftnt. mantle ; ne. cm. nerve-ganglion ; ces. ap. aperture of resophagus ; 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. tilQ,or.siph. 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 ccelome, 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 i.l 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 endostylc is applied. The cells covering the endostyle are large FIG. 675. Ascidia, a single mesh of the branchial sac, seen from the inside, i. I. internal longi- tudinal bar ; 1. r. 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 teal slo m. FIG. 676. Ascidia, diagram of longitudinal section from the left-hand side, the test and mantle removed, atr. cac. atrial cavity; atr. siph. atrial siphon; br. car. branchio-cardiac vessel card. vise, cardio-visceral vessel ; gonod. gonoduct ; lit. heart ; hyp. hypophysis ; mant. mantle n. gn. nerve-ganglion ; ens. oesophagus ; or. ovary ; rect. rectum ; stir/, stigmata ; stom. stomach tent, tentacles ; test, testis ; tr. r. transverse vessel ; rent. r. ventral vessel ; rise. In-, 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-pTiaryngeal 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. bl.u -per ti lest the gland cells of the enclostyle 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. The oesophagus (ces.) leads from the pharynx (near the posterior end of the dorsal lamina) to the stomach (stom.) which, together with the intestine, lies embedded in the mantle on the left-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 typlilosole. 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, Jit.) is a simple muscular sac, situated near the stomach in a pericardium forming part of the primitive coelome. It's mode of pulsation is very remarkable. The contractions are of a peristaltic character, and follow one another from one end of the heart to the other for a certain time ; then follows a short pause, and, wher 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 Iranchio- cardiac vessel (br. car.), e-pt. FIG. 677. Ascidia, transverse section, bl. r. blood vessels ; dors. lam. dorsal lamina; epi. epidermis; end. endostyle ; C/n. ganglion ; hyp. hypophysis ; mus. muscular layer of wall of body; peribr. peribraiichial cavity; ph. pharynx; test. test ; xas. tr. vascular trabeculse. (After Julin.) XIII PHYLUM CHORDATA 17 71V 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- visceral (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 viscero- branchial vessel, which runs along the middle of the dorsal wall of the pharynx externally to the dorsal lamina, and communicates with the dorsal ends of the series of transverse branchial vessels. In addition to these principal vessels there are numerous lacuna? 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 tl f .e 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 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 . ) VOL. II C . 18 ZOOLOGY SECT. duct (Fig. 678, dct.) 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 forma 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 coelome. 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 retrogress! vely 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 coelome 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 xm 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.' liere 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.' lie 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 ;he exterior by the atrial aperture. There is usually an alterna- tion of generations ; there may or may not be availed larval stage. Sid i-O i -de i ' a . Ci/dom yaria. 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, Ancliinm, and Dolcliinia. Sub-Order I. Hcmimyaria. 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 Octacncmidce the latter comprising only the aberrant deep-sea genus Odacnemus, 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 cvlinder. . This sub-order comprises only one family, the Pyrosomidoe, 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 ^ 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 l>. Ascidice composites. 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 Ascidise simplices. The AscidiidaB 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. i 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 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 FIG. 079. Appendicularia (Oikopleura) in (From Herdmau, after Fol.) House." or.ap geal bands. 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 at Hal 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 tunie 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- ruito 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 ; iioto. notochord ; cts. oesophagus ; or. up. oral aper- ture ; oto. otocyst ; peri. Id. peripharyngeal band ; ph. pharynx ; Us. testis ; stig. one of the stigmata ; stoin. 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, lany.). 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 t/ 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 Botryllus (Fig. 681) they are arranged in star-shaped, radiating sets around a common cloacal chamber into which the at rial 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 periph CTlfi slom 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 Avail alone is usually perforated by stigmata (stig.). An endostyle (end.) is present, and a peri -pharyngeal band : but there is P h 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. Sctlpa (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. te. 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 ; Id. heart ; Inip. hypophysis ; lang. languets ; ;,tant. mantle ; or. ap. oral aperture ; or. ovary ; periph. peri- pharyngeal baud ; ph. pharynx ; net. rectum ; stom. stomach ; t>-. testis; tent, tentacles; tst. test, or common gelatinous mass ; T. d. vas' deferens. (After Herdman.) 24 ZOOLOGY SECT. 7ttu,s.bds on ap air up stom. FIG. 683. Doliolum. Diagram of the sexual form. atr. ap. atrial aperture surrounded by lobes ; atr. cay. atrial cavity ; d. tbc. dorsal tubercle ; end. endostyle ; Id. heart ; int. intestine ; mus. bds. muscular bands ; ne. gn. nerve-ganglion ; or. ap. oral aperture ; or. ovary ; peri, bd . peripharyngeal band ; ph. pharynx ; stig. stigma ; stoni. stomach ; test, testis. (After Herd- man.) e,nct or.ap ort, FIG. 684. Salpa democratica, asexual form, ventral view. atr. ap. atrial aperture ; l>mnch y dorsal lamina ; end. endostyle ; ht. heart ; mus, bds. mviscular 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. in. I FIG. 685. Salpa, semi-diagrammatic lateral view. an. anus ; atr. ap. atrial aperture ; branch, dorsal lamina ; dors. tubl. dorsal tubercle ; ht. heart ; hyp. hypophj^sis ; lang. languet ; mus. bds* muscular bands ; ne.gn. nerve ganglion ; or. ap. oral aperture ; or. ovary in ovisac ; stom. stomach (After Herdman.) XIII PHYLUM CHORDATA 25 B FIG. 086. Colony of Pyrosoma. A, side view ; B, end view. (After Herdnian.) 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 produced into eight 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 oeso- phagus^ 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 COlllpOSlte lOrillS the ar- colony. atr. ap. atrial aperture ; or. ap. oral aper- ran^ement of the Darts is ture ; pioc. processes of test on outer surface of . . colony; ph. pharynx; stol. stolon on which are de- the Same in all CSSeiltial veloped buds giving rise to new zooids ; tent, tentacles. , v i (After Herdman.) respects as in the simple. proc tent stol ZOOLOGY SECT. In the Salpse and in Doliolum and Octacnemus the alimentary canal forms a relatively small dark mass the so-called nucleus -towards the posterior end of the body; it consists of oeso- phagus, stomach, and intestine, the anal aperture being situated in the peribranchial or atrial part of the internal cavity. The heart in all has the simple structure already described in the simple Ascidian. In Appendicularia its wall consists of only two cells. In Oikopleura it is apparently absent. The nervous system in Appendicularia consists of a cerebral ganglion (Fig. 680, ne. gn.) at the side of the mouth on the dorsal side, of a dorsal nerve which passes from this to a caudal ganglion (ne. gn.) at the root of the tail, and of a caudal nerve (ne'.) which passes from this to the extremity of the tail, presenting at intervals slight enlargements from which nerves are given off. An otocyst (oto.) and a pigment-spot are placed in close relation to the cerebral ganglion, and close to it also is a tubular process opening into the branchial sac and evidently representing the duct of the sub- neural gland of the simple Ascidian. In the simple Ascidians, as we have seen, there is a single flattened ganglion, representing the cerebral ganglion of Appendicularia, situated between the oral and atrial apertures ; and the same holds good of the com- posite forms. Many of the simple Ascidians have pigment-spots, probably of a sensory character, around the oral and atrial aper- tures. In Salpa and Doliolum there is also a single ganglion (Figs. 683, 684 and 685, nc. gn.} situated dorsally, giving off nerves to the various parts of the body. In Salpa there is an eye of a simple character and an otocyst placed in close relation to the ganglion in addition to eye-like bodies devoid of pigment : in Doliolum these are absent, but pigment spots occur in the lobes surrounding the oral opening. A subneural gland and duct are present in both these genera. In the simple Ascidian we have seen that the renal organ consists of a number of large clear vesicles situated in the loop of the intestine and devoid of duct. In some forms the terminal portion of the spermiduct has glandular walls in which concretions of uric acid have been found. The sub-neural gland is by some zoologists looked upon as perhaps having an excretory function. Reproductive system.- -The Urochorda are hermaphrodite. Ovary and testis are in all cases simple organs placed in close relation with one another. In Appendicularia (Fig. 680) they are situated in the aboral region of the body. In the simple Ascidians, they may be either single or double, and their ducts, sometimes very short, sometimes more elongated, open close together into the atrial cavitv. In Pyrosoma there are no gonoducts, the ovary, which contains only a single ovum, and the testis being lodged in a diverticulum of the peribranchial cavity. In Salpa also the ovary contains usually only a single ovum : ovary and testis lie in xiii PHYLUM CHORDATA 27 close relation to the alimentary canal in the " nucleus," and their short ducts open into the peribranchial cavity. In Doliolum the elongated testis and oval ovary have a similar position to that which they occupy in Salpa, but the ovary consists of a number of ova. Development and Metamorphosis. Usually impregnation takes place after the ova have passed out from the atrial cavity. But in a few simple and many compound forms impregnation takes place in the atrium, and the ovum remains there until the tailed larval stage is attained. In. certain composite forms there is a coalescence of the embryo with the wall of the atrium, forming a structure analogous to the placenta of the Mammals and desig- nated by that term. Self-impregnation is usually rendered im- possible by ova and sperms becoming mature at different times ; but sometimes both become ripe simultaneously, and self-im- pregnation is then possible. A somewhat complicated series of membranes invests the ovum. The immature ovarian ovum is enclosed in a layer of flat cells the primitive follicle cells derived from indifferent cells of the ovary. On the surface of this is developed a structureless basal membrane. The follicle cells increase by division and soon form a sphere of cubical cells. Certain of the cells migrate into the interior of the sphere so as to form a layer on the surface of the ovum. Others penetrate into the latter so as to lie in the superficial strata of the yolk. The layer of cells on the surface of the ovum are termed the testa cells : they afterwards develop on the outer surface a thin structureless layer, the chorion. Meantime, external to the follicle cells, between them and the basal membrane, has appeared a layer of flattened epithelial cells ; this, with the basal membrane, is lost before the egg is discharged. In all the simple Ascidians, with the exception of the few in which development takes place internally, the protoplasm of tho follicle cells becomes greatly vacuolated, so as to appear frothy, and the cells become greatly enlarged, projecting like papillae on the surface and buoying up the developing ovum. Segmentation is complete and approximately equal, but in the eight-cell stage four of the cells are smaller and four larger. The smaller, situated on the future ventral side, are the beginnings of the ectoderm ; the four larger form the endoderm, but also perhaps give origin to a number of small ectoderm cells. A small segmentation- cavity (Fig. 688, A, scg. cav.*) appears early. A curvature of the embryo then supervenes, so that the side on which the larger cells are situated becomes concave, and the larger cells thus become invaginated within the smaller, obliterating the segmenta- tion cavity, the result being the formation of a gastrula stage (B) with an archenteron. The blastopore, at first very wide, gradually becomes narrowed to a comparatively small rounded aperture (6') 28 ZOOLOGY SECT. which at the same time changes its position until it becomes placed at what is destined to be the posterior end of the dorsal surface. The embryo elongates in the direction of the future long axis. The dorsal surface becomes recognisable by being flatter, while the ventral remains convex. The ectoderm cells bordering the blasto- pore become distinguished from the rest by their more cubical B eel eel noto nerv end end. eel ecf FIG. 688. Early stages in the development of Clavellina. A, flattened blastula; B. early gastrula ; C, approximately median optical section of more advanced gastrula in which the blastopore has become greatly reduced and in which the first rudiment of the notochord is discernible ; D, similar view of a later larva in which the medullary canal has begun to be closed in posteriorly. M. p. blastopore ; ect. ectoderm ; end. endoderm ; rued. can. medullary canal ; nerr. cells destined to give rise to the nerve-cord ; neur. neuropore ; noto. notochord ; seg. cav. segmentation cavity. (A and B from Korschelt and Heider after Seeliger, C and D after Van Beneden and Julin.) shape ; these cells, which form the earliest rudiment of the ner- vous system, become arranged in the form of a plate the medul- lary plate on the dorsal surface. On the surface of this plate appears a groove the medullary groove bounded by right and left medullary folds, which pass into one another behind the blasto- pore. At the same time a number of small cells of the inner layer in the neighbourhood of the blastopore form a ring round that opening, and then extend forwards in the form of a plate below the medullary plate. The middle portion of this subsequently forms xin PHYLUM CHORDATA 29 the rudiment of the posterior portion of the notochord ; the lateral parts go to form the caudal part of the mesoderm. The medullary folds grow upwards and inwards over the medul- lary groove, and unite together (D), the union beginning behind and progressing forwards, in such a way as to form a canal, the neuro- coile, in the hinder portion of which is the opening of the blastopore. In this process of closing-in of the medullary groove the fold which passes round behind the blastopore takes an important part, growing forwards over the posterior part of the canal. The blasto- pore thus enclosed in the medullary canal persists for a time as a small opening the neur enteric canal by which the neuroccele and enteric cavity are placed in communication. At the anterior end of the medullary canal, owing to its incomplete closure in this region, there remains for a time an opening the neuropore (Fig. 689, neur.') leading to the exterior. The embryo (Fig. 689, .#) now becomes pear-shaped, the narrow part being the rudiment of the future tail. As this narrow part elongates the part of the enteric cavity which it contains soon disappears, coming to be represented only by a cord of endoderm cells. In the anterior wide part of the embryo the mesoderm (mes.) arises by the formation of paired outgrowths, which arise from the dorsal wall of the archenteron. A row of endoderm cells between the two sets of outgrowths represent the rudiments of the trunk part of the notochord ; they become arranged to form a cylindrical cord. The caudal region increases in length rapidly, and the anterior or trunk region, at first round, becomes oval. At its anterior end there appear three processes of the ectoderm, the rudiments of the ttilhesive papilke (Fig. 690, adh,), organs by which the larva subse- quently becomes fixed. The ectoderm cells at an early stage secrete the rudiments of the cellulose test ; in the caudal region this forms longitudinal dorsal and ventral flaps having the function of unpaired fins. The medullary canal becomes enlarged at its anterior end. A vesicular outgrowth from this enlarged anterior portion forms the sense-vesicle (sens. ves.). The posterior narrow part forms the caudal portion of the central nervous system (spinal cord). Masses of pigment in relation to the sense-vesicle early form the rudiment of the two larval sense-organs, otocyst and eye. The part behind this presents a thickened wall with a narrow lumen. This is known as the ganglion of the trunk. The rudiment of the hypo- physis early appears as a ciliated diverticulum (cil. gr.) of the anterior end of the archenteron. The embryonic alimentary canal consists of two regions, a wide region situated altogether in front of the notochord, and a nar- rower portion situated behind in the region of the notochord. The wider anterior part gives rise to the pharynx ; the posterior part 30 ZOOLOGY SECT. to the oesophagus, stomach, and intestine. The mouth-opening is formed shortly before the escape of the embryo from the egg: an Tries 710 to TTted.CClTt neur ndto FIG. 680. Later stages in the development of Clavellina. A, approximately median optical section of a larva in which the medullary canal (neuroccele) has become enclosed throughout, communicating with the exterior only by the neuropore at the anterior end, and with the archenteron by the neurenteric canal ; J3, larva with a distinct rudiment of the tail and well- formed mesoderm layer and notochord. Letters as in preceding figure; in addition, //a*. niesoderm. (After Van Beneden and Julin.) ectodermal invagination is formed at the anterior end, and an endodermal diverticulum from the archenteron grows out to meet it ; the two coalesce, and the oral passage is thus formed. XIII PHYLUM CHORDATA 31 The first beginnings of the atrial cavity appear about the same time as a pair of imaginations of the ectoderm which grow inwards and form a pair of pouches, each opening on the exterior by an opening. There is some difference of opinion as to some points in the history of these atrial pouches. According to one account each gives off a diverticulum inwards towards the pharynx, while from the latter a pair of diverticula grow outwards to meet them ; the two sets of diverticula subsequently meet and unite to form a pair of passages, one leading from each atrial pouch to the pharynx ; these form the first pair of stigmata. The atrial pouches then extend round the pharynx until they form a narrow space completely surrounding it, the cavities of the two pouches coalescing, and a number of perforations of the pharynx placing its cavity in direct communication with the surrounding space. According to another account two endodermal diverticula from the primitive pharynx grow out and open into the atrial pouches ; these diverticula subsequently become greatly expanded and grow round the pharynx to form the peri-pharyngeal space. It will be observed that, while according to the former of these two views the peri-pharyngeal space is ectodermal in origin, according to the latter it is endodermal. The two openings of the atrial pouches subsequently coalesce to form one the permanent atrial aperture. It will be useful now, at the cost of a little repetition, to sum- marise the various characteristics of the larval Ascidian at the FIG. 690. Free-swimming larva of Ascidia mammillata, lateral view. tuUi. adhesive papilla? ; all. alimentary canal ; utr. atrial aperture ; cil. (jr. ciliated groove ; e/"/. endostyle ; ej/e, eye ; rued, nerve cord ; noto. notochord ; oto. otocyst ; sens. res. sense-vesicle ; stirj. earliest stigmata. (From Korschelt and Heider, after Kowalewsky.) stage when it escapes from the egg and becomes free-swimming (Fig. 690). In general shape it bears some resemblance to a minute tadpole, consisting of an oval trunk and a long, laterally- compressed tail. The tail is fringed with a caudal fin, which is merely a delicate outgrowth of the thin test covering the whole of the surface ; running through the delicate fringe are a series of striae, presenting somewhat the appearance of the fin-rays of a Fish's fin. In the axis of the tail is the notochord (noto.\ which at this stage consists of a cylindrical cord of gelatinous substance 32 ZOOLOGY SECT. enclosed in a layer of cells. Parallel with this runs, on the dorsal side, the narrow caudal portion of the nerve-cord, and at the sides are bands of muscular-fibres. In the trunk the nerve-cord is dilated to form the ganglion of the trunk, and, further forwards, expands into the sense-vesicle (sens, ves.) with the otocyst (oto.) and eye (eye). The enteric canal is distinguishable into pharynx, oeso- phagus, stomach and intestine. The pharynx opens on the exterior by the mouth : in its ventral floor the endostyle (end.) has become developed ; its walls are pierced by stigmata, the number of which varies ; a ciliated sac (cil. gr.) opens into it below the trunk part of the nerve-cord. The atrial cavity has grown round the pharynx, and opens on the exterior by a single aperture only (atr.). The heart and pericardial cavity have become developed. In this tailed free-swimming stage the larva remains only a few hours ; it soon becomes fixed by the adhesive papilla?, and begins to undergo the retrogressive metamorphosis by which it attains the adult condition. The chief changes involved in the retrogressive metamorphosis (Fig. 691 ) are the increase in the number of pharyngeal stigmata, the diminution, and eventually the complete disappearance, of the tail with the contained notochord and caudal part of the nerve- cord, the disappearance of the eye and the otocyst, the dwindling of the trunk part of the nervous system to a single ganglion, and the formation of the reproductive organs. Thus, from an active, free-swimming larva, with well-developed organs of special sense, .and provided with a notochord and well-developed nervous system, there is a retrogression to the fixed inert adult, in which all the parts indicative of affinities with the Vertebrata have be- come aborted. The significance of these facts will be pointed out when we come to discuss the general relationships of the Chordata. In some simple Ascidians, and in the composite forms in which development takes place within the body of the parent, the meta- morphosis may be considerably abbreviated, but there is always, so far as known, a tailed larva, except in one genus of the simple forms (Molgula), in which the tailed stage is wanting. In Pyrosoma development is direct, without a tailed larval stage, and takes place within the body of the parent. The ovum contains a relatively large quantity of food-yolk, and the seg- mentation is meroblastic. A process, developed at an early stage, elongates to form the so-called stolon, which divides, by the forma- tion of constrictions, into four parts, each destined to give rise to a zooid ; and this group of tetrazooids, as they are termed, gives rise by budding to an entire colony. The development of Doliolum is, in all essential respects, very like that of the simple Ascidians. There is total segmentation, followed by the formation of an embolic gastrula ; the larva (Fig. 692) has a tail with a notochord (noto.), and a body in which the characteristic muscular bands soon make their appearance. By XIII PHYLUM CHORDATA 33 and by the tail aborts, and two processes, one postero-dorsal. the other ventral, known respectively as the dorsal (dors, st.) and ventral reel C stol end FIG. 691. Diagram of the metamorphosis of the freetailed larva into the fixed Ascidian. A, stage of free-swimming larva ; B, larva recently fixed ; C, older fixed stage, adh. adhesive papillae ; atr. atrial cavity ; ctl. a,: ciliated groove ; end. endostyle ; lit. heart ; mcd. ganglion of trunk ; 71. gn. nerve-ganglion ; noto. notochord ; or. oral aperture ; net. rectum ; sens. res. sense vesicle ; stig. stigmata ; stol. stolon ; t. tail. (From Korschelt and Heider, after Seeliger.) stolons (vent, st.), grow out from the body of the larva. On the latter are formed a number of slight projections or buds. These VOL. II D 34 ZOOLOGY SECT. become constricted off, and in the form of little groups of cells, each consisting of seven strings of cells with an ectodermal investment, creep over the surface of the parent (Fig. 693, c, and Fig. 694) till they reach the dorsal stolon, to which they FIG. 692. Doliolum, late stage in the development of the tailed larva, atr. ap. atrial aperture ; dors. st. dorsal stolon ; end. endostylc ; lit. heart ; ne. gn. nerve-ganglion ; noto. notochord ; or. ap. oral aperture ; vent. st. ventral stolon. (After Uljanin.) become attached. The dorsal stolon soon becomes elongated, and the bud-like bodies attached to it multiply by division and deve- lop into zooids. As the long chain of zooids thus established becomes farther developed, the parent Doliolum (Fig. 694) loses its branchias, its endostyle and its alimentary canal ; at the same ne. dors.st or.ap vent. si FIG. 693. Doliolum, lateral view of asexual stage, showing the early development of the buds. atr. ap. atrial aperture ; dors, st. dorsal stolon ; e. embryos passing over the surface from the ventral stolon to the dorsal ; hi. heart ; ne. gn. nerve-ganglion ; or. ap. oral aperture ; vent. st. ventral stolon. (After Uljanin.) time the muscle-bands increase in thickness, and the nervous system attains a higher development, until the whole parent comes to resemble, in a certain sense, the nectocalyx of a Siphono- phore (Vol. I. p. 147), its exclusive function being by its contrac- tions to propel the colony through the water. XIII PHYLUM CHORD ATA 35 The zooids of the dorsal stolon consist of two sets, differing from one another in position and in future history the lateral zooids and the median zooids. The lateral zooids serve solely to carry on the nourishment and respiration of the colony, and do not undergo any further development. The median zooids, on the other hand, become detached, and each develops a ventral stolon. On this are found buds which have either migrated with the rest from the ventral stolon of the parent or have become de- veloped in situ. Each of these buds develops into a sexual Doliolum. The succession of stages in the life-history of Doliolum thus briefly sketched will be seen to succeed one another in the following order : (1) sexual form ; (2) tailed larva developed sexually from (1) ; (3) first asexual form the direct outcome of (2) ; (4) second asexual form developed on the dorsal stolon of (3) from buds originating on the ventral stolon ; (5) the young of the sexual form (1) which are developed on the stolon of 4 from buds which were either formed there, or derived originally from the ventral stolon of 3. Salpa, like Doliolum, presents a remarkable alternation of genera- tions. In the sexual form, which has already been described, only one ovum becomes developed. The testis becomes mature later than the ovum, and the latter is impreg- nated by sperms from the testis of an individual of an older chain. The development is direct and takes place within the body of the parent, the embryo as it grows projecting into the branchial cavity. The nourishment of the developing embryo (Fig. 695) is effected by the formation of a structure the placenta through which a close union is brought about between the vascular system of the parent and that of the embryo. The placenta of Salpa is partly formed from follicle-cells and ectoderm cells of the embryo, partly from the cells of the wall of the oviduct. Segmentation is complete. The study of the earlier stages is complicated by the very remarkable and unusual circum- D 2 dors.slol lal.bds me.d.bds FIG. 694. Doliolum, dorsal view of the posterior part of the body of an asexual zooid showing the course taken by the buds (tm>>.) over the sur- face from the ventral stolon (cent. . atrial aperture ; &/. branchia ; cil. . stolon ; stom. stomach. (From Korschelt and Heider, after Salensky.) 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 by 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- phioxus and had not yet acquired the distinctive higher character- istics of the Craniates. The nearest existing ally of the Urochorda among lower forms is probably Balanoglossus. The similarity in the character of the pharynx or anterior segment of the enteric canal, 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 development of the central part of the nervous system. But the alliance 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. AVe have seen that the fundamental characters of the Chordata are the presence of a notochord, of a dorsal hollow nervous system, and of 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 neuroccele 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. In 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 neurocoele. 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-Avail 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 are 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 Mammals. DIVISION A. ACRANIA. The division Acrania contains a single family, the Bro.nchios- tomidce, containing two genera. Brancliiostoma (usually known by the name of one of its sub-genera, Ampliioxus), and Asi/nunetron. The differences between the genera and species are comparatively insignificant, and the following description will deal exclusively with the best known and most thoroughly investigated species, the Lancelet or Amphioxus, Amphioxus lanceolatus, found in the 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 (cent. /.), as far as the spot where the oval gives place to the triangular transverse section. The portion of the continuous an, or.hcL. B mj/om Jors.fr dorsf mj/om cir , , or.hd FIG. 696. Amphioxus lanceolatus. A, ventral, B, side view of the entire animal- em, anus ; atrp. atriopore ; cd. /. caudal fin ; cir. cirri ; dors. /'. dorsal fin ; dors. /. r. dorsal fin rays ; gon. gonads ; rntpl. metapleure ; mt/om. myonieres ; nch. uotochord ; or. hd. oral hood ; vtiit. /. ventral fin ; vent. /. r. 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 distinguished as the caudal fin (cd. /.). In the anterior two-thirds of the body there is no median ventral fin, but at the junction of each 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 hood encloses a cup-shaped cavity or vestibule, at the bottom of which is the mouth (Fig. 698, mth). Immediately in front of the anterior termination of the ventral fin and partly enclosed by the meta- pleures is a rounded aperture of considerable size, the atriopore (atrp), and a short distance from the posterior extremity of the body is the anus (an), placed unsymmetrically on the left side of 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 B a neu ncJi c myom- \ CLO inl co&l at 7 FIG. 697. Amphioxus lanceolatus. A, transverse section of the pharyngeal region. a, dorsal aorta ; b, atrium ; c, notochord ; co. coalome ; e. endostyle ; g. gonad ; kb, branchial lamellte ; M, pharynx ; 1. liver ; my. myomere ; n, nephridium ; r, neuron ; sn. spinal nerves ; sp. gill-slits. B, transverse section of the intestinal region, air. atrium ; cosl. crelome ; d. ao. dorsal aorta ; int. intestine ; myom. myomere ; nch. notochord ; neii. neuron ; s. int. r. sub-intestinal vein. (A, from Hertwig, after Lankester and Boveri ; B, partly after Rolph.) are provided with sensory hairs. The epithelium of the buccal cirri presents at intervals regular groups of sensory cells, some of them bearing stiff sensory hairs, others cilia. Beneath the epi- dermis is the dermis, formed mainly of gelatinous connective tissue. The muscular layer (my, myom.) is remarkable for exhibiting metameric segmentation. It consists of a large number about sixty of muscle-segments or myomeres, separated from one another by partitions of connective tissue, the myocommas, and having the appearance, in a surface view, of a series of very open V's with their apices directed forwards (Figs. 696 and 698). Each myomere xiii PHYLUM CHORDATA 41 is composed of numerous flat, striated muscle plates, arranged longi- tudinally, so that each is attached to two successive myo- cornmas. In virtue of this arrangement the body can be bent from side to side with great rapidity. The myomeres of the right and left sides of the body are not opposite to one another, but have an alternate arrangement. A special set of transverse muscles (Fig. 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 notoclwrdal 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 hood is supported by a ring (Fig. 698, sk.) of carti- laginous consistency, made up of separate rod-like pieces arranged end to end, and corresponding in number with the cirri. Each piece sends an offshoot into the cirrus to which it is related, furnishing it with a skeletal axis. The pharynx is supported by delicate oblique rods of a chitinoid material, the gill-rods (br. r.). These will be most conveniently discussed in connection with the pharynx itself. The dorsal fin is supported by a single series, and the ventral fin by a double series, of Jin-rays (dors. f. r., vent. f. ?*.), short rods of connective tissue, each contained in a cavity or lymph space. Digestive and Respiratory Organs.- -The mouth (mth.), as already mentioned, lies at the bottom of the vestibule or cavity of the oral hood (or. hd.). It is a small circular aperture surrounded by a membrane, the 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 of the 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. el.), 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 lands, and join the anterior end of the endostyle. / From the ventral region of the anterior end of the intestine is given off a blind pouch, the liver (Ir.) or hepatic caecum, which extends forwards, to the right of the pharynx : it is lined with glandular epithelium and secretes a digestive fluid. The gill-slits (br. cl.) are long, narrow clefts, nearly vertical in the expanded condition, but very oblique in preserved and con- tracted specimens hence the fact that a large number of clefts always appear in a single transverse section (Fig. 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 lamellce (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 for the 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 simple and forked. The forked bars are the primary (br. r. 1), those with simple ends the secondary (br. r. 2} branchial rods, and the lamellas in which they are contained are similarly to be distinguished as primary lamellce (br. sep. 1) and secondary or tongue lamellce (br. sep. 3). In the young condition the two clefts between any two primary lamella are represented by a single aperture : as development proceeds a down- XIII 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 \J CD 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, 1).). 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 (atr p.) ; and is continued backwards by a blind, pouch-like extension (atr'.) lying to the right of the intes- tine (Fig. 697, B, atr.). The whole cavity is lined by an atrial epi- thelium of ectodermal origin. As in Ascidia, the cilia lining the gill- clefts produce a current setting in at the mouth, - ,3-3-22... !!!- .3 g c .-S 2 ;, ~ 44 ZOOLOGY SECT. 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 water. FIG. 699. Amphioxus lance Olatus. Diagrammatic transverse section of the pharyn- geal region, passing on the right through a primary, on the left through a secondary branchial lamella. o. dorsal aorta ; e, derm ; ec endostylar portion of coelome ; /. fascia or investing layer of myomere ; fh, compartment containing fin-ray; g. gonad ; gl. glomerulus ; A-, branchial artery ; M, pharynx ; lit, combined atrial and coelomic wall (ligamentum denticu- latum) ; m. myomere ; nit. transverse muscle ; n. nephridium ; of, metapleural lymph-space ; j>, atrium ; sc, co3lome ; */, ventral aorta ; sk, sheath of notochord and neuron ; v.f, spaces in ventral wall. (From Korschelt and Heider, after Boveri and Hatschek.) CcBlome. Owing to the immense size of the atrium the body cavity, which is a true coelome, is much reduced. It is represented, in the pharyngeal region, by paired cavities (Fig. 698, cod., Fig. 697, A, co., Fig. 699, sc.) lying one on either side of the dorsal region of xm PHYLUM CHORDATA 45 the pharynx above 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. atr'.), 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 blood forwards. From it are given off, on each side, lateral branches, the afferent efbra fl" w\> P, / j j _ f j jr., jf IMLJ:H nn 11 r; /; g // r/ // gg irtt brcl ph CLfbr.a. e,p.porl.t/ FIG. 700. Diagram of the vascular system of Amphioxus. /. b,\ a. afferent branchial arteries ; cp. intestinal capillaries ; :. 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 (//>s), and its cavity as the myoccele or muscle-cavity : the ventral section is called the lateral plate, and its cavity forms a segment of the coelome. The ventral plates now unite with one another in pairs below the enteric canal, their cavities becoming continuous : at the same time the cavities of successive ventral plates are placed in communication with one another by the absorption of their XIII PHYLUM CHORDATA oo 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 unsegmentecl coalorae of the adult, their walls giving rise to the coelomic epithelium. At the same time the cells of the splanchnic layer of the protovertebrae 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 protovertebra?. An outpushing of the splanchnic layer, at about the level of the ventral sur- face of the notochord, grows upwards between the myomere 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 protovertebrse 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 neuroccele by the t/ still open neuropore (np.y. 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, and gradually shift over to the right side : at first they corre- spond with the myomeres, so that the segmentation of the pharynx is part of the general metamerism of the body. Altogether fourteen clefts are t*1 ir ---e\t 56 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 fold 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 larvae showing the develop- ment of the atrium, up. atriopore ; A-, gill-slits ; //'. left metapleural fold ; m. mouth ; rf. right metapleural fold ; ?', prae-oral pit. (From Korschelt and Heider, after Lankester and Willey.) pleural 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, 67) appears, and, the two sub-atrial ridges meeting and coalescing, a canal (B, p) is formed immediately below the ventral body-wall. This canal is the commencement of the atrium : it is at first quite narrow, but gradually extends upwards on each side (C, pi) 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 stomodaeum. 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. t/ 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- FIG. 711. Amphioxus lanceolatus. Diagrammatic transverse sections of three larvae to show the development of the atrium, ao. aorta ; c, derniis ; d, intestine ; /. fascia ; /A, cavity for dorsal fin-ray ; m. myomere ; n. nerve-tube ; p, atrium ; sf. metapleural folds ; si, sub-intestinal vein ; si; sheath of notochord and neuron ; */. sub-atrial ridge ; sp. coelome. (From Korschelt and H eider, after Lankester and Willey.) 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, #), 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, New 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 note-chord 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 coelome 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, and it was shown that in many fundamental points, notably in the formation of the nervous system and the notochord, there was the closest resemblance between the two. The likeness was further emphasised by the presence in both forms of an endostyle, an epipharyngeal groove, and peripharyngeal bands, and of an atrium, and by the obvious homology of the 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 xin 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 . Elasmobrancliii, Including the Sharks and Rays. Sub-class 2. Holocepliali, Including only the Cat-fish (Chimcera) and the Elephant-fish ( Callorhynclius). Sub-class 3. Tclcostomi, Including the bony Fishes, such as Perch, Cod, Trout, &c. and the Sturgeons and their allies. Sub-Class 4. Dipnoi, 1 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, xin CLASS IV. REPTTLIA, Including Lizards, Snakes, Crocodiles, Turtles, and Tortoises. CLASS V. AYES, 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. l>r. 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 aperture-. On the ventral surface at the junction of the trunk and tail is the anus (<>n.). Distinct urinary and genital apertures, or a single 62 ZOOLOGY SECT. urine-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 coelome. 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 (#./.), 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 condition there is some- times found to be a low ridge (r.) connecting the pectoral and pelvic fins of each side with one 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 divisioiis- 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 pcntado.ctyle limit thus formed is very characteristic of all the N. m^-& 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 ; A"<. granule-cells ; S, vertical, and ir, horizontal bun- dles of connective tissue. (From Wiedersheim's XIII PHYLUM CHORDA TA 63 higher Vertebrata. 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 layer, the epidermis (Fig. 713, JEp.\ derived from the ectoderm of the embryo, and an inner or con- nective-tissue layer, the dermis (Co), of mesodermal origin. The epidermis is always many-layered; the cells of the lower layers, forming the stratum Malpigliii, being protoplasmic and capable of active multiplication, while those of the superficial layers often become flattened and horny, and constitute the stratum cor- ncurii. 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 myomeres (Fig. 714, mym.), separated from one another by partitions of con- nective tissue, or myocommas (myc.), 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 t .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 mvomere, moreover, is t, 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 an extremely complex arrangement. 64 ZOOLOGY SECT, xm In the trunk, as shown by a section of that region, the muscles form a definite layer beneath the skin and enclosing the coelome (Fig. 715, A and C, ccel.). 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 haemal canal (h. 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 coelome 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 coelomic epithelium bounding the cavity, and thus forming the innermost layer of the body-wall. In Fishes the coelome is divided into two chambers, a large abdominal cavit// containing the chief viscera, and a small forwardly-placed pc/'i- cardial 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 coelome 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 cornificatioii 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 = - x .- -- ?. ?. -\ =~~ ~ ~ . % - 1 s ; " S - . "" 5 "~ ^ ~~ ~ 5 ^ j2 * OVS ." . 3 j *> "^ fi 8 e 1 -8 ** g SS'^W^'JJCjJg ? ' "r^ f i' o - -r '^ | " - "t.-r x ^ s c !!-?:; ^ > "C TV > 3 >> ^ ^ "^ o . 2 . f. ^ ' J/r''^ ^ ^* '^- ^ v llJlJ^is^ i^ o . H o .- o o GC 4 ~ ? j; ^ _ c VOL. II F 66 ZOOLOGY SECT. sp.cd 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 near the middle of the brain im- mediately behind a peculiar organ, the pituitary l>o. 1 10, foramina for cerebral nerves ; /. rostrum ; 5. t. sella turcica or pituitary fossa. Cartilage bones AL.SPH . alisphenoid ; ART. articular : B. BR. basi-branchial ; B. HY. basi-hyal ; B. OC.- basi-occipital ; B. SPH. basi-sphenoid; C. BR. cerato-bran- chial ; C. HY. cerato-hyal ; EC. ETH. ecto-ethmoicl ; 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. ptervgoid ; QU. quadrate; 8. OC. supra-occipital. Membrane bones DNT. dentary; FR. frontal ; 3IX. maxilla; XA. nasal; PA. parietal; PA.* PH. parasphe- noid ; PM.X. prernaxilla ; Sty. squamosal ; VO. vomer. frontals (FR), placed in front of the parietals, and often connected below with the orbito-sphenoids. A pair of nasals (JV14) 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 vomer ( VO) which may be double in front, and the paraspJienoid (PA. SPH) 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 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. 3) through or immediately behind the orbito-sphenoids, the fifth nerves (Nv. o) 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 cliondrocranium , 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 palate-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 of 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 MeckeVs 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 dcntary (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 kno\vn as the squamosal (SQ). In the hyoid arch a cartilage bone, the liyomandibular (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 7o 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 FIG." 721. Diagram of three stages in the development of the pelvic fins. In A the anterior pterygiophores on the right side (Rail), have united to form a basal cartilage (Bat.) ; in B the basaiia (Bas.) are fully formed and are uniting at * to form the pelvic girdle ; in C the pelvic girdle (G) is fully constituted, and at t has segmented from the basale on the right side. Cl. cloacal aperture. 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 basalia (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 A CL HU B PU pcor n in \J T SS FIG. 722. Diagrams of the fore (A) and hind (B) limbs with the limb-girdles, actb. acetabulum ; ,-,ta.) these the dentine is formed in successive layers which gradually accumulate between the layer of odontoblasts and the inner or invaginated layer of the enamel organ. The lower, or proximal, part of the papilla remains uncalcified and forms the tooth-pulp. The enamel is formed by the deposition of successive layers of calcific matter from the inner or invaginated layer of the enamel organ, the cement by the ossification of the tissue immediately surrounding the papilla. Thus the tooth is partly of ectodermal, partly of mesodermal, origin. 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, A, pty. s.\ which grows upwards and, losing its connection with the mouth, becomes attached to the ventral surface of the brain as the pituitary body (pty. 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,Zr.)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, I.) 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. 7 15, A,/>?i.) 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. IT G FIG. 725. Diagram of structure of liver. 6, a small branch of hepatic duct ; b', its ultimate termination in the intercellular spaces; c, blood capillaries; I, liver cells, (trom Huxley's Physiology.) 82 ZOOLOGY SECT. the gall-bladder (g. />.), in which the bile is stored. We thus have one or more hepatic ducts conveying the bile from the liver and meeting with a cystic duct from the gall-bladder, while from the junction a common bile duct leads into the intestine. Another important and characteristic organ in the abdomen of Craniata is the spleen (spl.\ a gland-like organ of variable size and shape, attached to the stomach by a fold of peritoneum, but having no duct. It is formed of a pulpy substance containing numerous red blood-corpuscles, many of them in process of dis- integration : dispersed through the pulp are masses of leucocytes which multiply and pass into the veins. Two other ductless glands are formed in connection with the enteric canal. The thyroid (tJid.) 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 Fir,. 726. Diagrammatic horizontal section of the pharyngeal region of a Craniate : on the left are shown three gill-pouches (g. p.) with fixed branchial filaments (/>/./.) and separated by inter-branchial septa 0'. be. s.) ; on the right one hemibranch (km. In:) and two holobrauchs (hi. b,:) with free fila- ments, covered by an operculum (op.). Ectoderm dotted, endoderm striated, mesoderm evenly shaded, visceral bars (c. l>.) 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 holobrancTi (hi. fo\) is the morphological equivalent of two half-gills, hemibranchs, 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, Ms) : 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 Chaatopods 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 coelome. 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 (cm.), 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 conus 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 aortcv, 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 XIII PHYLUM CHORDATA 85 -*J - O - __ II 1.11 1 . > - el* ^Si = -^ (It 3t?5 li a*! .-- t; '-i ^- > ^'- in 8,,, . 05 ;-; e.-S "" s s .r,-S O 3>^ ^a ai.^g-3 ^-^ 2 o s-3 ^s :o ri ^ sen ><% ^ " ^^ "* . CJ ~ r-J fl ^ rt ho-- S -^^ o C a: " -^ .5 = bo-'-t 3 ^ t>>~ o-P - - rt '" 5 S . 3 S 2 " ' |^|'^. 3 _ :~ ^ O ~ O 3 w ^ .2 - * " ~ u_d _^ . ^ *~< . o . - u t> ^ J r^ ~ " -' ^ u-r "- y S. ? "S r fin -. s^slS"! f < ~ ^V-4-^ ^ -4J is .^ ^ ;_ T*. 3k-|^ 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. ,). 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 sulclaman (scl. a.) to the pectoral fins, the coeliac (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 xm PHYLUM CHOKDATA 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 prccaval vein (pr. cv. v. ) which passes directly downwards and enters the sinus venosus. The blood from the tail returns by a caudal vein (ccL v.), lying immediately below the caudal artery in the haemal canal of the caudal vertebrae (Fig. 715, D). On reaching the ccelome 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 jcoeliac. 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 (h. v.\ into the sinus venosus. The hepatic, unlike the renal, portal system, is of universal occurrence in the Craniata. In the embryo there is a sub-intestinal vein, corresponding with that of Amphioxus, and lying beneath the intestine and the post- anal gut. Its posterior portion becomes the caudal vein of the adult, its anterior portion one of the factors of the hepatic portal vein. To sum up : the circulatory organs of the branchiate Craniata consist of (a) a muscular organ of propulsion, the heart, provided with valves and driving the blood into (b) a set of thick-walled, elastic, afferent vessels, the arteries, from which it passes into (c) a network of microscopic vessels or capillaries, which permeate the 0,0 i III br.a a.brc. ft U FIG. 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 ; ac. auricle ; c. a. conus arteriosus ; (/. ao. dorsal aorta ; e. br. a. efferent branchial arteries ; /;. p. r. hepatic portal vein ; k. <. hepatic vein ; Ic. lacteals ; ly. lymphatics ; _/>/. tv. r. pre-caval veins ; /. p. r. renal portal veins ; s. r. sinus venosus ; c. ventricle ; c. 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 (&) 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. t-SO, 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 (A 1 ), 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 -AU Acd 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; A ,11. vitelline arteries ; , ventral aorta ; C, C' 1 , carotid arteries ; D, pre-caval veins ; Ic, ", iliac arteries ; HC, cardinal veins ; KL, gill- clefts ; R. A. S,S l , roots of dorsal aorta ; M, sub-claviau arteries ; Sb l , sub-clavian veins ; V. Ventricle ; VC, jugular vein ; r//<, vitelline veins. (From Wiedersheinrs V'.i-t<.bi-ata.) 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 B FIG. 730. Diagram of the heart A, in an Amphibian ; B, in a Crocodile. A, right auricle ; A', left auricle ; Ap, pulmonary artery ; li , pulmonary vein ; RA, aortic arches ; V, ventricle ; V, left ventricle ; V,V, and Ve, Fe, 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 Wood 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 A B Tin 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 ccelome 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 lacteal s (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 neurocosle 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) ; OAving 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, 8). In both grey and white matter the nervous elements are supported by a non-nervous tissue called neuroglia, formed of branched cells. From the cord the spinal nerves are given off. They arise in pairs from the sides of the cord, and agree in number with the myomeres. Each nerve arises from the cord by two roots, a 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 XIII 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, b, 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 ccelome. 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,/. 7?.), mid-brain (m. b.) and hind-brain (h. b.). Soon a hollow outpushing grows forwards from the first vesicle (B, prsen), and the third gives off a similar hollow outgrowth (cblm.) from its dorsal surface. The brain now consists of five divisions : the prosencephalon (prs. en.) and the diencephalon (dien.), derived from the fore-brain : the mid-brain or mesencephalon (m. b.) which remains unaltered : and the epencephalon, or cerebellum (cblm.), and the metencephalon, or medulla oblongata (med. obi.) 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 prosocode, third ventricle or diaccele, mid-ventricle or mcsoccele, 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, theforainen of Monro (/. m.). Moreover, each hemisphere gives off a forward prolongation, the olfactory lobe or rhinencephalon (olf. I.), containing an olfactory ventricle or rhinoccele (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 epiccele 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 optocoeles (G. opt. cce.) : the median portion of the mesocoele is then called the iter (I) or aqueduct of Sylvius. In the diencephalon the sides XTII PHYLUM CHORDATA 95 " - 13 o i; , o ! r- a: x > ""^ ,-t O ^^' ~H **H o - a o o r^3 """! -I- !- C "^ 2 ~ti o P ri' |S^ -/. i ^ ^ ^^ - ^? B -$ __ ,^ PQ '^ ' S _ , 3 O i - (D i % . ~^ -^ * g .;^ i'S. ^2 ^ VT' -2 * ^ vi O O PH 33 O O O QC (-1 O ' 5l-S O . . GC **< O r\ -^ O . g^-^ ~ " 3 .s .2 ' c3 ^ "* 2S > ^ _c _o -^ * Mlte & l|_- sTrT ^ -^ o A "sS * *x ? ^5 "S o 3 S.2 ^ O s ^ ^ s s o-ilto T3 * -2 = i ' S -SS s?a +^ S <> . =o c6 h, -= y: o O O' T) o IS 8 =3-^ S ^ 5-.S = ? r- g = O^ I SSsS CO ' CO r~< C or: -J rO c 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 (cpendyme) 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 (jm.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. &.), 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 into 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 and between the diverging posterior ends of the trabeculse. 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 xin PHYLUM CHORDATA 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 clioroid 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 brain, containing a prolongation of the 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 cms 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 sj/m br. 7 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. }>. deep ophthalmic ; VI, abducent ; VII, facial ; VII. /;, hyomandibular branch ; VII. p, palatine branch ; VIII, auditory ; IX, glossopharyngeal ; X, vagus ; X. br. 1 5, branchial branches ; X. c, cardiac branch ; X. g, gastric branch ; X. 1, lateral branch ; XI, accessory ; XII, hypoglossal. au. auditory organ; br. 1 7, branchial clefts; cblm. cere- bellum ; c. ffii. ciliary ganglion ; c. It. cerebral hemispheres ; d. dorsal branch of spinal nerve ; d. /. dorsal root ; c. eye ; gn. d. r. ganglion of dorsal root ; m. l>. mid-brain ; mcd. obi. medulla oblongata ; mth. mouth ; na. olfactory sac; o. I. olfactory lobe; pn. b. pineal body ; pn. e. pineal eye ; */>. c. spinal cord ; */,>. 1 3, ventral branches of spinal nerves ; sum. sympathetic nerve ; $1/111. [in. sympathetic ganglion ; v. 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 & facial ganglion. It has two chief branches, a palatine (VII. p.}, which passes in front of the mandibulo-hyoid gill-cleft, and supplies the mucous membrane of the palate, and a hyomandibular (VII. h.\ which passes behind the same cleft and sends branches to the lower jaw, and to the hyoid arch. In 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 Vertebra ta 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 Vertebrata, 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. l>r. 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. /) 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 hypoglossal (XII.) arises from the ventral aspect of the medulla oblongata, after the manner of the ventral root of a spinal nerve. It is purety 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 Avill 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, and 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- XIII 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 C * -A* FIG. 735. A, tactile spot from skin of Frog, a, touch-cells ; I, epidermis ; N, nerve. B, tactile corpuscle from dermal papilla of human hand, a, connective-tissue investment ; 6, touch- cells ; /!, />/, >>", '/>'", nerve. C, Pacinian corpuscle from beak of Duck. A, A', neuraxis ; JK, central knob and surrounding cells; L,Q, investing layers; A*S,niedullary sheath of nerve. (From Wiedersheini's Vtrtebrata.) 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 (?) 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 trigeminal and glosso- pharyngeal. The olfactory organ is typically a sac-like imagination 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 Schneiderian 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 B R tt a Jff FIG. 73i5. A, diagram of the organs of the lateral line in a Fish, e, lateral line : ,'yrt''.h,-ntn.) communicates with the cavity of the mouth b} r 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 Jacobson's Organ: it is supplied by the olfactory and trigeminal nerves. The paired eye is a mmv 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). Tt A 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; d, sub-mucosa ;) e. epithelium of tongue; m, sensory processes; />, internal sensory cells shown in upper bud. (From Fester and Shore's Physiology.) 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 i 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 choroid (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- F IG . 738. Epithelial cells of olfactory mucous membrane. A, of 'Lamprey; B, of Salamander. E, inter- stitial cells ; R, olfactory cells. (From Wiedersheim's Vertebrata.) 104 ZOOLOGY SECT. e.c. v e- C.R C.CJ Ch/ 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 (JR.) covered on its outer or choroiclal surface with a layer of black pig- ment (P. J). It extends as far as the outer ends of the ciliary processes where it appears to end in a wavy line, the or a serrata (0. S.) : actually, however, it is con- tinued as a verv delicate / membrane (p. c. E~) over the ciliary processes and the posterior face of fhe 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 (r.~). 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 t> transparent biconvex lens (Fig. 739, Z.), 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 licmment towards the iris ;iml allow the lens to assume its normal Set' FIG. 730. Diagrammatic horizontal section of the eye of -Man. c. cornea; cli. choroid (dotted); C.P. ciliary processes ; e. c. epithelium of cornea ; e. cj. conjunctiva ; /. o. yellow spot ; /. iris ; L, lens ; ON. optic nerve ; OS. ora serrata ; o ./:, optic axis ; p. c. R, anterior non-visual portion of retina ; P.E. pigment ed epithelium (black) ; .K. retina ; sp. 7. suspensory ligament ; Si- 1. sclerotic ; 7'. 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 b}- 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 b)' 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 nu n.c d n.c o.n FIG. 740. Diagram of the retina, the supporting structures to the left, the nervous and epithelial elements to the right ; d. fibrous supporting structures ; fir. r/r'. 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; /. rods and cones. (From Wiedersheim's Fertebrata.) 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 ayers 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, I.) 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. c 1 .), 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 opt. si FIG. 741. Early (A) and later (B) stages in the development of the eye of a Craniate. dun. diencephalon ; in <. I. invagination of ectoderm to form lens ; I. lens ; opt. c. outer layei of optic cup; opt. c'. inner layer; opt. st. optic stalk; opt.r. optic vesicle; pit. 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 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 oblique (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 (7.) 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 in vagi nation of the VI FIG. 74-2. Muscles of the eye of a Skate and their nerves (semi-diagrammatic). ///. oculo- motor nerve ; IV, trochlear ; VI, abducent. e. r. external rectus ; in. o. inferior oblique ; 'm. r. inferior rectus; i. r. internal rectus; or. wall of orbit ; s. o. superior oblique ; s. /. 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 lo^e^ its connection with the external ectoderm, becoming a closed sac surrounded by mesoderm. At first simple, it soon become* 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.) . "-: V-V '*. St Fi,.. 743.-Section of the pineal eye of Hatteria. g, blood-vessel ; h, cavity of eye, filled with fluid ; A-, connective tissue capsule ; L lens ; M. molecular layer of retina ; r, layer of rods and cones ; st, nerve ; x, cells in nerve. (From Wiedersheim's Vvrtelrata, after Baldwin Spencer.) >ry epithelium ; a. h. auditory hairs ; c. part of semicircular canal : cr. crista acustica ; ct. connective tissue; c. <', epithelium; />. nerve ; u. junction with utriculus. (From Foster and Shore's Physiology.) 110 ZOOLOGY SECT. tween it and the cartilage is a space, filled by a fluid called perilymph, which acts as a buffer to the delicate organ floating in it. Urinogenital Organs. In all Craniata there is so close a connection between the organs of renal excretion and those of reproduction that the two systems are conveniently considered together as the urinogenital organs. Speaking generally, the excretory organ consists of three parts, all paired and situated along the dorsal wall of the ccelome ; the fore-kidney or pronephros (Fig. 715, A, p. nph.\ the mid-kidney or mesonephros (ms. npli.}, and the hind-kidney ox metanephros (int. nph.). Each of these is provided with a duct, the pro- (pn. d.), meso- (msn. d.), and meta-nephric (int. n. d.) ducts, which open into the cloaca. The gonads (gon.) lie in the coelome suspended to its dorsal wall by a fold of peritoneum : in some cases their products are discharged into the coelome and make their exit by abdominal pores, but more usually the pronephric duct in the female assumes the functions of an oviduct and the mesonephric duct in the male those of a spermiduct. The pronephros is almost always functionless in the adult, and usually disappears altogether. The mesonephros is usually the functional kidney in the lower Craniata, in which, as a rule, no metanephros is -developed, and the mesone- phric duct, in addition to carrying the seminal fluid of the male, acts as a ureter. In the higher forms the mesonephros atrophies, and the metanephros is the functional kidney, the metanephric duct becoming the ureter. The kidney meso- or meta-riephros of the adult is a massive gland of a deep red colour made up of convoluted urinary tubules (Fig. 746), separated from one another by connective tissue con- taining an abundant supply of blood vessels. The tubules are lined by a single layer of glandular epithelial cells (B, C) and each ends blindly in a globular dilatation, the Malpighian capsule (A, gl.\ lined with squamous epithelium. In many of the lower Craniata, a branch goes off from the tubule, near the Malpighian capsules, and, passing to the ventral surface of the kidney, ends in a ciliated funnel-like body (Fig. 747, nst.), resembling the nephrostome of a worm, and, like it, opening into the ccelome. At their opposite ends the tubules join with one another, and finally discharge into the ureter. The renal arteries branch extensively in the kidney, and give off to each Malpighian capsule a minute afferent artery (Fig. 746, A, v. a.) : this pushes the wall of the capsule before it, and breaks up into a bunch of looped capillaries, called the glomerulus, sus- pended in the interior of the capsule. The blood is carried off from the glomerulus by an efferent vessel (v. e.), which joins the general capillary system of the kidneys, forming a network over the PHYLUM CHORDATA 111 urinary tubules: finally, the blood is returned from this network to the renal vein. The watery constituents of the urine are separated from the blood in traversing the glomerulus, and, flowing down the tubule, take up and dissolve the remaining constituents urea, uric acid, &c. which are secreted by the cells of the tubules. The development of the kidney reveals a resemblance to the nephriclia of worms which would hardly be suspected from its adult structure. The pronephros (Fig. 747, A, p. nph.) originates a.- two or three coiled tubes formed from mesoderm in the body- wall at the anterior end of the coelome : thev are arranged meta- i/ O merically and each opens into the coelome by a ciliated funnel B FIG. 746.- -A, part of a urinary tubule with blood-vessels, ai, artery ; gl, Malpighian capsule con- taining glomerulus ; r. veinlet returning blood from capillary network (to the right) to vein ri ; TO afferent vessel oi glomerulus ; re, efferent vessel. B, longitudinal, and C, transverse sections of urinary tubules, a, secreting part of tubules ; b, conducting part of tubules ; c. capillaries ; n. nuclei. (From Foster and Shore's Physiology.) (nst.). Obviously such tubes are mesonepJiridia : their chief pecu- liarity is that their outer ends do not open directly on the exterior, but into a longitudinal tube, the archinephric or segmental duct (sg. d.\ which passes backwards and discharges into the cloaca. It seems probable that this arrangement is to be explained by supposing that the nephridia originally opened externally into a longitudinal groove, which, by the apposition of its edges, was converted into a tube. All three nephridia of the pronephros open, by their ciliated funnels, into the narrow anterior end of the coelome, into which projects a branch of the aorta ending in a single large glomerulus. The pronephros soon degenerates, its nephridia losing their connection with the segmental duct (B), but in the meantime fresh nephridia appear in the segments posterior to the pro- nephros, and together constitute the mesonephros or Wolffian body 112 ZOOLOGY SECT. (B, ms. nph.) from which the permanent kidney is formed in most of the lower Craniata. The mesonephric nephridia open at one CLTL Fic. 747. Diagrams illustrating the development of the urinogeuital organs of Craniata. A, development of proiiephros and segmental duct ; B, atrophy of pronephros, development of mesonephros ; C, differentiation of pro- and meso-nephric ducts ; D, development of meta- nephros, male type; E, female type. al. bl. allantoric bladder; an. anus; cl. cloaca; gon. gonad ; int. intestine; //i.e. Malpighian capsule; ms.n.tl. mesonephric duct; //<*. ////A. mesonephros ; mt. n. '. metanephric duct ; mt. nph. metanephros ; nst. nephrostomc ; or ovary ; j). n. d. pronephric duct ; p. nph. pronephros ; sg. (?. segmental duct ; t. testis ; <. (. vasa efferentia. end into the segmental duct (sg, rf.), at the other, by ciliated funnels (nst.), into the ccelome ; a short distance from the funnel XIII PHYLUM CHORDATA 113 each gives off a blind pouch which dilates at the end and forms a Malpighian capsule (m. 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 Wolffidn duct (C, ms.n.d.): the other has no connection with the nephridia, but opens into the coelome 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 Wolffian 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, mt. 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 (mt. nph.), which becomes the permanent kidney, and a metane- phric duct (mt. 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 allantoic bladder (al. U.\ which serves the same purpose although morphologically an entirely different structure. The gonads (gon.) are developed as ridges growing from the dorsal wall of the coelome, and covered by ccelomic epithelium, from the cells of which, as in so many of the lower animals, the ova and sperms are derived. The test is 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 coeloine- 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 ncJt e-nt msd B pr.v msd FIG. 748. Transverse section of earlier (A) and later (B) embryos of Frog. cal. ccelome ; col', pro- longation of ccelome into protovertebra ; ent. mesenteron ; mal. gr. medullary groove; //<.>/. mesoderm; m-k. notochord ; pro. protovertebra ; g. d. segmen tal duct; sow. somatic layer of mesoderm ; sp. c. spinal cord ; spl. splanchnic layer of mesoderm ; ?/A-. 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 ccelome 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, mscL) 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 PHYLUM CHORDATA 115 -\ stem 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 mesodemnal 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 coelome (ccel.), which is thus a schizoccele or cavity holloAved 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 (ccel') may pass into each protovertebra, but such an arrangement is temporary. From the dorsal portions of the protovertebras the myomeres are formed, from their ventral portions the vertebrae. 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 denned 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. fluvin- tilis), about 60 cm. in length : and the Sand-pride, or lesser- fresh-water Lamprey (P. Iranchialis), 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 Gfeotria, in the rivers of Chili, Australia, and Xew 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 marinus. Ventral (A), lateral (B), and dorsal (C) views of the head.. /'/. cl. 1, first gill-cleft ; ina:. f. buccal funnel : ."'.". : , eye ; //. <\ annular cartilage ; an. c. auditory capsule ; lr. b. 1 7, verti- cal bars of branchial basket ; In: ct. 1 7, external branchial clefts ; en. c. cornual cartilage ; c/-. /. cranial roof; 1. <: 1 4, longitudinal bars of branchial basket ; /;/. c. lingual cartilage; m. >: <: median ventral cartilage ; nn. ap. nasal aperture ; nch. notochord ; Nc. 2, foramen for optic nerve ; off. c. olfactory capsule ; pc. c. pericardia! cartilage ; p. . oc. a. sub-ocular arch ; st. p, styloid process ; st>/. c. 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, 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 trabeculse, and surrounding pos- teriorly the fore-end of the notochord. Immediately in front of the termination of the notochord is a large aperture, the ba si- cranial fontanelle (b. cr.f.), due to the non-union of the posterior ends of the trabecula? ; through it passes the pituitary pouch, pre- sently to be referred to (Fig. 754), on its way from the olfactory sac to the ventral surface of the notochord. Lateral walls extend upwards from each side of the basal plate, but the roof of the cranium is formed by membrane except at one point, where a narrow transverse bar (cr. r.) extends across between the side-walls and furnishes a rudimentary roof. United with the posterior end of the basal plate are the auditory capsules (au. c), and the side- walls are pierced with apertures for the cerebral nerves (Nv. i.. Xv. o, Nv. 8.). So far the skull is thoroughly typical, though in an extremely simple or embryonic condition ; the remaining parts of it differ ;i good deal from the ordinary structure as described in the preceding section, and are in many cases very difficult of interpretation. The olfactory capsule (plf. 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,s&. 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.), which 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 trabeculae. 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 support- 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 flTl.C CL.d.C B - TIC/I p.lat.c crv.c FK.. 751. Petromyzon marinus. Dorsal (A), ventral (B), and sectional (C), views of skull. The cartilaginous parts are dotted, a. . . pi. basal plate; o. t\ comual cartilage ; (/. ;-. cranial roof ; tin. np. nasal aperture ; ndi. notochord ; JY<-. 1, olfactory nerve ; NI-. -2, ':, and S, foramina for the optic, trigeminal, and auditory nerves ; J\v. 5', fifth nerve ; olf. <. olfactory capsule ; p. ' s ST; K -S ~J2 S< 3 K -5 2 ^ o * i ^X T ""*" '"' t *" H O =-3 g rt -"- : ^.^So^y.-g t-Js-2 x = SPSl "- 2|- S^^ - 5 CK , v o -" G . *" -P i s c I M "' a - 39 S-=i s.-5 ' +3 a SP-^"3 ^ O 3 D --H C-^i-^^-sj^i-. B. 2 2 5 -5 ^ 5 ~ - * ffl "es" n ic 2^ C6 Q.tW-W S = S 4S " g * T 1 - i ^r?z ^sF^Nl 3 o .-a, -J -/-. ^i - _ v 3 -| f- - | a T.-t ggg d ^ s ^ I all llfl aM" o ^-^ N ;-s ^.. 11 O OS 03 ^ "5 r3 K 1 3 > -f-2 S'^^-fc 5..i = '- ^ 8 -8 3 aj N -^ .a 3 . s " w ^ . o > s -3 -i f > -n ' 01 / o . 2 - c ~ - / sS|-HE,,-5-b ^ ? 9-! '"^PM o ac 3 - -t> c3 ^ = =! a ^-^^3 t^ = ^, c -^-c^-i ^ v -N .a * -.; = ^ - a^ . o 3-3 . ."^3 ^.3 ^ a s a i -C o -"-^ ' 3 =->_ s H C ,-, S a.j"* -g b S^ - t ^ 7 -~.Z ^course and is hence known as the spiral valve. There is no continuous mesentery, but a number of narrow supporting bands. 122 ZOOLOGY SECT, The liver (Fig. 752, lr.) is a large one-lobed organ, and is peculiar from the fact that there is neither gall-bladder nor bile-duct in the adult, except as an individual variation, although both are present in the larva. There is a small gland opening into the intestine which may represent a pancreas : the spleen is absent. Paired glands imbedded in the muscles of the head, and opening into the mouth, are known as " salivary glands." Respiratory Organs.- -The Lampreys differ from all other Vertebrata in the fact that the gills do not open directly into the enteric canal in the adult, but into a respiratory tube (Fig. 752, r.t.) lying below the gullet. This is a wide tube opening in front into the buccal cavity, and ending blindly a short distance in front of the heart : in the larva it communicates behind with the intestine, and is, in fact, the pharynx, the gullet of the adult being not yet developed ; but at the time of metamorphosis it loses its con- nection with the intestine, and the gullet is developed as a forward extension of the latter an entirely new formation. The respiratory organs are typical gill-pouches (br. 5) : they have the form of biconvex lenses, and are separated from one another by wide inter- branchial septa. In the larva an eighth cleft has been found in front of the first of the adult series. Circulatory System. --The auricle (au.) lies to the left of the ventricle (v.) and receives blood from a small sinus venosus (s. v.\ There is no conus arteriosus, but the proximal end of the ventral aorta presents a slight dilatation or bulbus aorta:. Both afferent and efferent branchial arteries supply each the posterior hemi- branch of one gill-pouch and the anterior hemibranch of the next : they are thus related to the gills, not to the gill-pouches. In addition to the paired jugulars (ju.) there is a median ventral inferior jugular win (i. ju.} returning the blood from the lower parts of the head. There is no renal-portal system, the two branches of the caudal vein being continued directly into the cardinals (cd.). The red blood-corpuscles are circular nucleated discs. There is a large system of lymphatic sinuses. Nervous System. --In the brain the small size of the cerebellum (Fig. 753, crb.) is remarkable : it is a mere transverse band roofing over the anterior end of the metaccele. The optic lobes (opt. I. ) are very imperfectly differentiated, 'and the central region of the roof of the mid-brain is formed merely of a layer of epithelium, giving rise to an aperture (ell. pi. 2) when the membranes of the brain are removed, but covered in the entire organ by a vascular thickening of the pia or choroid plexus. On the dorsal surface of the diencephalon are two masses of nervous matter, the ganglia kabenula 1 , the right (r. (jn. hi.) much larger than the left (/. gn. hi.) : they are connected with the pineal apparatus. Below the dien- cephalon is a small flattened pituitary body (Fig. 754, pty. I.). In front of the diencephalon are paired bean-like masses, each con- XIII PHYLUM CHORDATA sisting of a small posterior portion, the cerebral liemispherc'\ci'l). h.), and a larger anterior portion, the olfactory lobe (olf. I.}. The diacoele communicates in front with a small prosocosle or common fore- ventricle, which is roofed over by a choroid plexus (d. pi. 1) and from which a transverse passage goes off on each side and tful. otf.l pn. cK-pl .1 L&ns.Hb med-.obl - m-cd.obl Fir;. 7">3. Petromyzon marinus. Dorsal (A) and ventral (B) views of brain, ch.pl. 1, an- terior choroid plexus forming roof of pros- and diencephaloii ; cli. pi. 2, aperture in r roof of mid-brain exposed by removal of middle choroid plexus ; cli. pi. 3, metacoele exposed] by removal of posterior choroid plexus ; crb. cerebellum ; crb. li. cerebral hemispheres ~~cr. crb. crura cerebri ; dicn. diencephaloii ; i/if. infuiidibuluni ; I. /. 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, no.' . Fig. 754, pty. _/;>.) 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, befoiv the stomodseum (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 CHOBDATA 125 opens just outside the stoinpdseum 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 infimdibulum (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. rich olfs Ttch aldm FIG. 755. Petromyzon. Diagrams of four stages in the development of the olfactory and! pituitary sacs. cnt. mesenteron ; iiij. infundibulum; /. Ip. lower lip; -,ich. iiotochord ; f>/f. *. olfactory sac; pn. pineal body; ptii. s. pituitary sac; ft ZOOLOGY SECT. a.s.c P .s.c sac FIG. 75G. Auditory sac of Petromyzon. . s. <\ anterior semicircular canal ; uv.d. n. auditory nerve; end. s. endolymphatic sac ; p. s.c. posterior canal ; sac. sacculus ; vt,: 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.s.), 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 gonad (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. in ), containing much yolk. In the bias- tula stage (D) the segmentation cavity or blast ocoele (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 blastoccele 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 n.ca d.ao ts ur int. FIG. 7a7. Petromyzon mar inns Transverse section of abdomen. <. urino- genital papilla; c. i. .<. eiidolym- phatic sac ; .. c. semicircular canal ; i.. parachordal and auditory region ; t. p. trabecular re- gion ; x. backward processes of skull. (After Traquair.) 134 ZOOLOGY SECT. parachordals and auditory capsules. Just in advance of the anterior region is a ring-shaped opening surrounded by cirri (c.), and con- sidered to be the nasal aperture. The posterior region gives off paired plates (#.) which may perhaps correspond with the dorsal longitudinal bars of the branchial basket in the Lamprey. CLASS II. PISCES. The Pisces, including the cartilaginous and bony Fishes and the Dipnoi, are Craniata which have the organs both of re- spiration and of locomotion adapted for an aquatic mode of life. The chief, and in the majority the only, organs of respiration are the gills, which are in the form of series of vascular processes attached to the branchial arches and persisting throughout life. The organs of locomotion are the paired pectoral and pelvic fins, and the unpaired dorsal, ventral, and caudal ; these are all supported by fin-rays of dermal origin. A dermal exoskeleton is usually present. In the endoskeleton the notochord is usually more or less com- pletely replaced by vertebra? ; there is a well-developed skull and a system of well-formed visceral arches, of which the first forms upper and lower jaws, the latter movably articulating with the skull, and both nearly always bearing teeth. There is frequently an air- bladder, which in certain exceptional cases acquires the function of a lung or chamber for breathing air. The hypophysis is not in any way connected with the nasal chambers, and lies within the cranial cavity. There is a pair of nasal chambers which only exceptionally communicate internally with the mouth-cavity. The auditory labyrinth contains the three typical semicircular canals. The kidney is a persistent mesonephros. * Sub-Class I. Elasmobranchii. The sub-class Elasmobranchii com prises the Sharks, Dog-fishes, and Rays. The skeleton of these fishes, like that of the Cyclo- siomata, is composed essentially of cartilage, and, though there may be ossifications in the substance of the cartilage, distinct bones, such as are found in all higher groups, with the exception of the Holocephali, are not present. The dermal fin-rays, supported on the cartilaginous skeleton of the fin, are of horn-like constitution. There is never (in recent forms) an operculum or gill-cover. There is a cloaca, the external opening of which serves as a common outlet for the rectum and the renal and reproductive ducts. Among some of the fossil representatives of this group are to be found the most primitive of all known Fishes. xiii PHYLUM CHORDATA 135 1. EXAMPLE OF THE SUB-CLASS : THE Doo-FiSH (Scyllmiu- canicula or Chiloscyllium fuscum). General external features.- -The general shape of the body (Fig. 766) may be roughly described as fusiform ; at the anterior, or head, end it is broader and depressed; posteriorly it tapers gradually and is compressed from side to side. The head termi- nates anteriorly in a short blunt snout. The tail is .narrow and bent upwards towards the extremity. The colour is grey with brown markings, or dark-brown above, lighter underneath. The entire surface is covered closely with very minute hard placoid scales or dermal teeth somewhat larger on the upper surface than on the lower. These are pointed, with the points directed some- what backwards, so that the surface appears rougher when the hand is passed over it forwards than when it is passed in the opposite direction. When examined closely each scale is found to be a minute spine situated on a broader base. The spine consists T W FIG. 7C-G. Dog-Fish (Chiloscyllium mode stum). Lateral view. (After Gun ther.) of dentine covered with a layer of enamel ; the base is composed of bone, and the whole scale has thus the same essential structure as a, tooth. Along each side of the head and body runs a faint depressed longitudinal line or slight narrow groove the lateral line. As in Fishes in general, two sets of fins are to be recognised the unpaired or median fins, and the paired or lateral. These are all flap-like outgrowths, running vertically and longitudinally in the case of the median fins, nearly horizontallv in the case of the lateral : / *** they are flexible, but stiffish, particularly towards the base, owing to the presence of a supporting framework of cartilage. Of the median fins two the dorsal are situated, as the name indicates, on the dorsal surface : they are of triangular shape ; the anterior, which is the larger, is situated at about the middle of the length of the body, the other a little further back. The caudal fringes the tail : it consists of a narrower dorsal portion and a broader ventral, continuous with one another round the extremity of the tail, the latter divided by a notch into a larger, anterior, and a smaller, posterior lobe. The tail is heterocercal, i.e., the posterior extremity of the spinal column is bent upwards and lies in the dorsal portion of the caudal fin. The ventral or so-called anal fin is situated on 136 ZOOLOGY SECT. the ventral surface, opposite the interval between the anterior and posterior dorsals (in Beryllium)] it resembles the latter in size and shape. Of the lateral fins there are two pairs, the pectoral and the pelvic. The pectoral are situated at the sides of the body, just behind the head. The pelvic, which are the smaller, are placed on the ventral surface, close together, in front of the middle of the body. In the males the bases of the pelvic fins are united together in the middle line, and each has connected with it a chispcr or copulatory organ. The latter is a stiff rod, on the inner and dorsal aspect of which is a groove leading forwards into a pouch-like depression in the base of the fin. The mouth a transverse, somewhat crescentic opening is situated on the ventral surface of the head, near its anterior end. In front and behind it is bounded by the upper and lower jaws, each bearing several rows of teeth with sharp points directed back- wards. The nostrils are situated one in front of each angle of the mouth, with which each is connected by a wide groove the nciso- buccal groove. In Chiloscyllium the outer edge of the groove is prolonged into a narrow subcylindrical appendage the barbel. A small rounded aperture, the spiracle placed just behind the eye -leads into the large mouth cavity, or pharynx. Five pairs of slits running vertically on each side of the neck the branchial slits also lead internally into the mouth cavity. A large median opening on the ventral surface at the root of the tail, between the pelvic "fins, is the opening leading into the cloaca, or chamber forming the common outlet for the intestine and the renal and reproductive organs. A pair of small depressions, the abdominal pores, situated behind the cloacal opening, lead into narrow passages opening into the abdominal cavity. The skeleton is composed entirely of cartilage, with, in certain places, depositions of calcareous salts. As in Vertebrates in general, we distinguish two sets of elements in the skeleton the axial set and the appendicular, the former comprising the skull and spinal column, the latter the limbs and their arches. The spinal column is distinguishable into two regions the region of the trunk and the region of the tail. In the trunk region each vertebra (Fig. 767, A) consists of a centrum (cent.), neural arch (new.), and transverse processes (tr.~). In the caudal region there are no transverse processes, but inferior orhamal arches (B, haem.) take their place. The centra of all the vertebrae are deeply biconcave or amphicwlous, having deep conical concavities on their anterior and posterior surfaces. Through the series of centra runs the noto- chord, greatly constricted in the centrum itself, dilated in the large spaces formed by the apposition of the amphicoelous centra of adjoining vertebrae, where it forms a pulpy mass. The concave anterior and posterior surfaces of the centra are covered by a dense XIII PHYLUM CHORDATA 137 B *P ru,ur haem FIG. 707. Chiloscy Ilium, vertebrae. A, end view t trunk vertebra, cent, centrum; nev.r. neural plate and process ; sp. neural spines ; >. ribs ; tr. pro?. transverse processes. , lateral view of the same. h&iii. haemal arch; neur. neural arch. C, transverse section of a centrum, showing radiating lamella f bone. calcified layer, and eight radiating lamella of bone (6*) run longi- tudinally through the substance of the centrum itself. The centra, unlike those of the higher forms, are developed as chondrifications of the sheath of the notochord into which cells of the skeletogen- ous layer have migrated (p. 66). Each neural arch consists of a pair of rod- like neural pro- cesses, which form the sides, and two pairs of compressed ne ural plates (one placed opposite the centrum, the other or intercalary cartilage, op- posite the interval be- tween adjoining centra) (Fig. 768), which form the roof of the arch, together with usually two nodules the representatives of neural spines (sp.~) which form the keystones. The transverse processes are very short : connected with each of them is a cartilaginous rudimentary rib (r.) about half an inch in length. The cranium (Fig. 768) is a cartilaginous case, the wall of*which is continuous throughout, and not composed, like the skulls of higher Vertebrates, of a number of distinct elements (bones) fitting in together. At the anterior end is a rostrum, consisting in Scyllium of three cartilaginous rods converging as they extend forwards and meeting at their anterior ends. At the sides of the base of this are the olfactory capsules (olf.) thin rounded cartilaginous sacs opening widely below the cavities of the two capsules being separated from one another by a thin septum. The part of the roof of the cranial cavity behind and between the olfactory capsules is formed, not of cartilage, but of a tough fibrous membrane, and the space thus filled in is termed the anterior fontanelle : in contact with the lower surface of the membrane is the pineal body, to be afterwards mentioned in the account of the brain. Each side-wall of this part of the skull presents a deep concavity the orbit over which is a ridge-like prominence, the supra-orbital crest, terminating anteriorly and posteriorly in obscure processes termed respectively the prcG-orbital and post-orbited processes. Below the orbit is a longitudinal infra-orbital ridge. Behind the orbit is the auditory region of the skull a mass of cartilage in which the parts of the membranous labyrinth of the internal ear are embedded. On the upper surface of this posterior 138 ZOOLOGY SECT. portion of the skull are two small apertures situated n a mesial depression. These are the openings of the aqueductus vestibuli (endolympliatic ducts), leading into the vestibule of the membranous labyrinth. Behind this again is the occipital region, forming the posterior boundary of the cranial cavity, and having in the middle a large rounded aperture the foramen magnum through which the spinal cord contained in the neural canal and protected by the neural arches of the vertebrae, becomes continuous with the brain, lodged in the cranial cavity. On either side of this is an articular surface the occipital condylc for articulation with the spinal column. A number of smaller apertures, or foramina, chiefly for the passage of nerves, perforate the wall of the skull. Behind and to path neur inlerc \tr '.p.br.5 FIG. 768. Chiloscy Ilium, lateral view of skull with visceral arches and anterior part of spinal column ; the branchial rays are not represented. The skull and hyoid arch are somewhat drawn downwards, so that the hyoid and first branchial arch are not exactly in their natural relations. 6r.i l>r$ branchial arches ; ccr. Juj. cerato-hyal ; ep. br. epibranchials ; gl. aperture for glosso-pharyngeal nerve ; b. hy. basi-hyal ; hy. mn. hyo-mandibular ; interc. intercalary plates ; Mck. Meckel's cartilage ; nev.r. neural processes ; olf. olfactory capsule ; oc. foramen for oculo-motor ; opt. optic foramen ; pal. q. palato-quadrate ; path, foramen for 4th nerve ; j>li.br.i first pharyngo-branchial ; ph. br.5 fifth pharyngo-branchial ; ^p. neural spines; tr. transverse processes and ribs ; in. foramen for trigeminal nerve. the outer side of the anterior fontanelle is the aperture for the ophthalmic branch of the fifth, or trigeminal, nerve. Piercing the inner wall of the orbit are foramina through which the optic nerves, or second pair of cranial nerves (opt.)', the oculo-motor (oc.), or third ; the pathetic, or fourth (path.)', the trigeminal, or fifth; the abducent, or sixth; and the facial, or seventh, gain an exit from the interior of the cranial cavity. Just behind the auditory region is the foramen for the glosso-pharyngeal, and in the posterior wall of the skull, near the foramen magnum, is the foramen for the vagus. In close connection with the cranium are a number of cartilages XIII PHYLUM CHORDATA 139 composing the visceral arches (Figs. 768 and 769). These are in- complete hoops of cartilage, mostly segmented, which lie in the sides and floor of the mouth-cavity or pharynx. The first of these forms the upper and lower jaws. The upper jaw, or palato-quadrate ( pal. q.}, consists of two stout rods of cartilage firmly bound to- gether in the middle line and bearing the upper (or anterior) series of teeth. The lower jaw, or Meckel's cartilage (Mck.\ likewise con- sists of two stout cartilaginous rods firmly united together in the middle line, the union being termed the symphysis. At their outer rn.ck hyp.b, ce.r.br.3 cer.br: t c&r.br.s ph.br. Kn;. 709. Chiloscy Ilium, ventral view of the visceral arches. Letters as in preceding figure. In addition, l>. l . basi-branchial plate ; ccr. In: cerate-branchiate ; hyp. In: hypo-branchials. ends the upper and lower jaws articulate with one another by a movable joint. In front the upper jaw is connected by a ligament with the base of the skull. Immediately behind the lower jaw is the liyoid arch. This con- sists of two cartilages on each side, and a mesial one in the middle below. The uppermost cartilage is the hyo-mandibular (hy. mn.) : this articulates by its proximal end with a distinct articular facet on the auditor}' region of the skull ; distally it is connected by ligamentous fibres with the outer ends of the palato-quadrate 140 ZOOLOGY SECT. and Meckel's cartilage. The lower lateral cartilage is the cerato- liyal (cer. hi/.). Both the hyo-mandibular and cerato-hyal bear a number of slender cartilaginous rods the branchial rays of the hyoid arch. The mesial element, or basi-hyal (b. hy.\ lies in the floor of the pharynx. Behind the hyoid arch follow the branchial arches, which are five in number. Each branchial arch, with exceptions to be presently noted, consists of four cartilages. The uppermost of these pharyngo-branchial (ph. br. l -ph. br. 5 ) lie in the dorsal wall of the pharynx, not far from the spinal column ; the pharyngo-branchials of the last two arches are fused together. The next in order the epibranchials (ep. br.) with the exception of those of the last arch, bear a number of slender cartilaginous rods the branchial rays which support the walls of the gill-sacs ; and the next the cerato-branchials (cer. br.) are, with the same exception, similarly provided. The hypo-branchials (hyp. br.), which succeed these, are absent in the case of the first and fifth arches. In the middle line on the floor of the pharyngeal cavity is a mesial cartilage the basi-branchial (Fig. 769, b. br.) which is connected with the ventral ends of the third, fourth, and fifth arches. Three pairs of slender curved rods the extra-branchials lie superficial to the second, third, and fourth branchial arches, along the borders of the corresponding branchial clefts. Two pairs of delicate labial cartilages lie at the sides of the mouth, and a couple at the margins of the openings of the olfactory capsules. The skeleton of all the fins paired and unpaired presents a considerable degree of uniformity. The main part of the expanse of the fin is supported by a series of flattened segmented rods, the pterygiophores or cartilaginous fin rays, which lie in close apposition : in the case of the dorsal fins these are calcified along their axes. At the outer ends of these are one or more rows of polygonal plates of cartilage. On e#ch side of the rays and polygonal cartilages are a number of slender horny fibres of dermal origin. In the smaller median fins there may be an elongated rod of cartilage constituting the skeleton, or cartilage may be entirely absent. In the pectoral fin (Fig. 770) the fin rays are supported on three basal cartilages articu- lating with the pectoral arch. The latter (pect.) is a strong hoop of cartilage incomplete dorsally, situated immediately behind the last of the branchial arches. It consists of a dorsal, or scapular, and a ventral, or coracoid portion, the coracoid portions of oppo- site sides being completely continuous across the middle line, while the scapular are separated by a wide gap in which the spinal column lies. Between the two portions are the three arti- cular surfaces for the three basal cartilages. The coracoid portions are produced forwards in the middle line into a flattened process supporting the floor of the pericardial cavity in which the heart is lodged. The three basal cartilages of the fin are named, XIII PHYLUM CHORDATA 141 the only ray two r. respectively, the anterior, pro-ptcrygium (pro.), the middle, meso- pt&rygiurn (meso.), and the posterior, metapterygium (mcta). Of these the first is the smallest, and the last the largest : first bears one large the other bear twelve or more, diffe- rently arranged in the two genera. The pelvic fin (Fig. 771) has only a single basal cartilage (met a.) articulating with the pelvic arch, with which also one or two of the fin rays articu- late directly. The pelvic arch (pelv.) is a nearly straight bar of cartilage which runs transversely across the ventral surface of the body, just in front of the cloacal opening. Enteric canal (Fig. 772). -The mouth leads into a very wide cavity, the pharynx, into which open at the sides the internal apertures of the branchial clefts and of the spiracle. From this runs backwards a short wide tube -the oesophagus (ces) which passes behind into the stomach. The stomach is a U-shaped organ, with a long left limb continuous with the oesophagus, and a short right passing into the intestine. At the pylorus (pyl) the point where the stomach passes into the intestine is a slight constriction, followed by a thickening. The FIG. 770. Chiloscy Ilium, pectoral arch and fin. d. r. dermal horny rays ; meso. mesopterygium ; meta. metapterygium ; pect. pectoral arch ; ^/-o. propterygium. FIG. 771.- fiii. Chiloscyllium, pelvic arch and pelvic ifta. metapterygium ; pelv. pelvic arch. 142 ZOOLOGY SECT. XIII intestine consists o two parts small intestine or duoden urn, and large intestine. The former is very short, only an inch or two in length. The latter is longer and very wide ; it is divisible into two portions the colon (col.) in front and the rectum (red.) behind. The former is very wide and .is characterised by the presence in its interior of a spiral valve, a fold of the mucous mem- brane which runs spirally round its interior, and both retards the too rapid passage of the food and affords a more extensive surface for absorption. The rectum differs from the colon in being narrower and in the absence of the spiral valve ; it opens behind into the cloaca. There is a large liver (liv.) consisting of two elongated lobes. A rounded sac the gall-bladder (g.U.) lies embedded in the left- lobe at its anterior end. The duct of the liver the bile-duct (b. dct.) -runs from the liver to the intestine. Proximally it is connected with the gall-bladder, and by branch-ducts with the right and left lobes of the liver. It opens into the commencement of the colon. The pancreas (pancr.) is a light-coloured compressed gland con- sisting of two main lobes with a broad connecting isthmus, lying in the angle between the right-hand limb of the stomach and the small intestine. Its duct enters the wall of the small intestine and runs in it for about half an inch, opening eventually at the point where the small intestine passes into the colon. Connected with the rectum on its dorsal aspect is an oval gland, -the rectal gland (red. gl) about three-quarters of an inch in length. The spleen (spl.) is a dark-red or purple body attached to the con- vexity of the U-shaped stomach and sending a narrow lobe along the right-hand limb. The organs of respiration in the Dog-fish are the gills, situated in the five gill-pouches. Each gill-pouch (Fig. 773) is an antero- posteriorly compressed cavity opening internally into the pharynx and externally by the gill-slit. The walls of the pouches are sup- ported by the branchial and hyoid arches with their rays, the first pouch being situated between the hyoid and first branchial arches, the last between the fourth and fifth branchial arches. On the anterior and posterior walls of the pouches are the gills, each hemibranch consisting of a series of close-set parallel folds or plaits of highly vascular mucous membrane. Separating adjoining gill-pouches, and supporting the gills, are a series of broad intcrbranchial septa, each containing the corresponding branchial arch with its con- nected branchial rays. The most anterior hemibranch is borne on the posterior surface of the hyoid arch. The last gill-pouch differs from the rest in having gill-plaits on its anterior wall only. On the anterior wall of the spiracle is a rudimentary gill the pseudo- branch or spiracular gill in the form of a few slight ridges. Blood system. The heart is situated in the pericardia! cavity.. ^ 3 r2 '-> -T3 "-r 1 ' ^"S^^So 5 ^ Ti ~ - *=i .S 3 ~ '- ^n*z$%* '- 9. 2 & S? 3 -fr! _ ,i3c< r-T*? >^-^ 5 ** -f- > -* ' r* i> 5 2"i^"| o "S s r.~ -t ~ * ? 1 | ' :; 'yT g ^-'~5'-=-5-3 j < f ?*( SJ.sbS5: : ^ ^ r-^ ^r - - : ^j *- . ^ -* fcO i-H -f _~* \^ ' ^-3 K i^^c'- o g >, JH n P. ? t ^^- S a 8 3? t| 'd o >' 73 ^ ^,-g 3 "i^^l o b-^ -H SH . y y o 73 "r; y S -4J 3C -5 S J ^ *> ^j r* *z >.^4 2 > r- O O ^H" " ^3 x^ {JO W 5^2 3^3 o 3 .2 . .3 z ~l " ^ . A bc^ ,2 5. o 144 ZOOLOGY SECT. on the ventral aspect of the body, in front of the pectoral arch, and between the two series of branchial pouches. Its dorsal wall is supported by the basi -branchial cartilage. Placing it in communi- cation with the abdominal cavity is a canal the pericardio-peri- toneal canal. The heart (Fig. 772) consists of four chambers sinus venosus (sin. ven.), auricle (aur.), ventricle (vent.), and conus arteriosus (con.}, through which the blood passes in the order given. The sinus renosus is a thin-walled, transverse, tubular chamber, into the ends of which the great veins open. It opens into the auricle by an aperture, the sinu-auricular aperture. The auricle is a large, tri- angular, thin-walled chamber, situated in front of the sinus veno- sus and dorsal to the ventricle. Its apex is directed forwards, and its lateral angles project at the sides of the ventricle : it commu- nicates with the ventricle by a slit-like aperture guarded by a two- lipped valve. The ventricle is a thick- walled, globular chamber, forming the most conspicuous part of the heart when looked at from the ventral surface. From it the conus arteriosus runs forwards as a median stout tube to the anterior end of the peri- cardial cavity, where it gives off the ventral aorta. It contains two transverse rows of valves, anterior and posterior, the former consisting of three, the latter of three or four. The ventral aorta (Fig. 774) gives origin to a series of paired afferent branchial arteries (br.v.), one for each branchial pouch. In Scyllium the two most posterior arise close together near the beginning of the ventral aorta, the third pair a little further forwards. The ventral aorta then runs forwards a little distance and bifurcates to form the two innominate arteries, right and left, each of which in turn bifurcates to form the first and second afferent vessels of its side. In Chiloscyllium (Fig. 774) the arrangement is some- what different. From the gills the blood passes by means of the efferent branchial arteries. These efferent vessels form a series of loops, one running around the margin of each of the first four internal branchial clefts : a single vessel runs along the anterior border of the fifth branchial cleft and opens into the fourth loop. The four main efferent branchial vessels run inwards and backwards from the loops under cover of the mucous membrane of the roof of the mouth to unite in a large median trunk the dorsal aorta. From the first efferent vessel, that from the first or hyoidean gill, arises the carotid artery, which runs forwards and bifurcates to form the internal and external carotid arteries, supplying the head with arterial blood. A hyoidean artery arises further out from the same vessel, Fin. 773. --Chiloscyllium. Branchial sac exposed from the outside. XIII PHYLUM CHORDATA 145 I. card, a liv and, after giving off branches to the pseudobranch, passes into the cranial cavity and joins the internal carotid of the opposite side. The dorsal aorta runs backwards throughout the length of the body cavity, giving off numerous branches, and is continued as the caudal artery, which runs in the canal enclosed by the inferior arches of the caudal vertebrae. The first pair of branches are the subclavian, for the supply of the pectoral fins ; these are given off between the third and fourth pairs of efferent ar- teries. The next large branch is the un- paired cceliac (Fig. 772, cceL): this runs in the mesentery and divides into branches for the supply of the stomach and liver, the first part of the in- testine, and the pan- creas. The anterior mesenteric artery, also median, supplies the rest of the intestine and gives off branches to the reproductive organs. The lieno- gastric supplies part of the stomach, the spleen, and part of the pancreas. The pos- terior mesenteric is a small vessel mainly supplying the rectal gland. A pair of small renal arteries carry a small quantity of arterial blood to the kidneys, and a pair of iliac arteries, likewise of small size, supply the pelvic fins. In addition to VOL. II L ccuul.v FIG. 774. Chiloscyllium. Diagrammatic representation of the ventral aorta and afferent branchial arteries, and of the chief veins, alt. alimentary canal ; br. cJ-br. r.5 afferent branchial arteries ; caud. r. caudal vein ; Jet. c. ductus Cuvieri ; Jtf. heart ; /./*. port. i\ hepatic portal vein ; hep. s. hepatic sinus ; inf. jv.. epiphysis ; si. rtct. external rectus muscle of the eye-ball; gl. ph. glossopharyngeal ; hor. can-, horizontal semicircular canal; , liy. mnd. VII. ', hyomandibular portion of the facial; . spiracle ; s. rtct. superior rectus muscle ; . olh. superior oblique ; xay. vagus ; vest, vestibule. (From Marshall and Hurst.) the neural arches of the vertebrae. As in the Craniata in general (see p. 92), it has dorsal and ventral longitudinal fissures and a narrow central canal, and gives origin to a large number of paired spinal nerves, each arising from it by two roots. Organs of Special Sense. --The olfactory organs are rounded chambers enclosed by the cartilage of the olfactory capsules of the skull, and opening on the exterior by the external nares on the ventral surface of the head. The interior has its lining membrane raised up into a number of close-set ridges running out from xin PHYLUM CHORDATA 151 a median septum. The fibres of the olfactory nerves terminate in cells of the epithelium covering the surface of these ridges. The eye has the general structure already described as char- acterising the Craniata in general (p. 103). The sclerotic is cartilaginous, the choroicl has a shining metallic internal layer or tapctnm, and the lens is spherical. The eyeball is attached to the inner wall of the orbit by a cartilaginous stalk. There are the usual eye-muscles, the two obliques situated anteriorly, the four recti posteriorly. There are no eyelids. The ear consists only of the membranous labyrinth equivalent to the internal ear of higher Craniata, the middle ear and the outer ear being absent. The membranous labyrinth consists of the vestibule and three semicircular canals. The former, which is divided into two parts by a constriction, communicates by a narrow passage- the aqiieductus vestibuli with the exterior, in the position already mentioned. Of the three semicircular canals, the anterior and posterior are vertical and the external horizontal, as in Craniata in general. Each has an ampulla, that of the anterior and external canals situated at their anterior ends, and that of the posterior canal, which is the largest of the three, and forms an almost complete circle, at its posterior end. In the fluid (endolymph) in the interior of the vestibule are suspended, in a mass of gelatinous connective tissue, numerous minute calcareous particles or otoliths, giving it a milky character. The mucous canals of the integument contain special nerve- endings, and doubtless function as organs of some special sense. The same probably holds good of a number of minute canals situated on the anterior portion of the trunk, and on the head, being particularly numerous in the neighbourhood of the snout. These are dilated internally into vesicles, the ampullce, provided with special nerve-endings. Urino-genital Organs. In the female there is a single ovary (Fig. 773, or.), an elongated, soft, lobulated body, lying a little to the right of the middle line of the abdominal cavity, attached by a fold of peritoneum, the mcsoarium. On its surface are rounded elevations of various sizes, the Graafian follicles, each containing an ovum of a bright yellow colour. There are two oviducts (Mltllerian ducts) entirely unconnected with the ovaries. Each oviduct (Fig. 773, ovd.\ Fig. 778) is a greatly elongated tube extending throughout the entire length of the abdominal cavity. In front the two unite behind the pericardium to open into the abdominal cavity by a wide median aperture (abd. ap.). At about the point of junction of the middle and anterior thirds is a slight swelling marking the position of the shell- gland (sli. gld.). The posterior part dilates to form a wide chamber, and in Scyllium the two unite to open into the cloaca by a common aperture situated just behind the 152 ZOOLOGY SECT. abd ajb -sftgl opening of the rectum, while in Chiloscyllium they remain distinct and have separate cloacal openings. Each kidney consists of two parts, anterior and posterior. The former (Fig. 773, r. meson.} is a long narrow ribbon of soft reddish sub- stance, which runs along throughout a great part of the length of the body-cavity at the side of the vertebral column, covered by the peritoneum. The posterior portion (r. metan.) is a compact, lobulated, dark-red body, lying at the side of the cloaca, continuous with the anterior por- tion ; like the latter, it is covered over by the peritoneum. Both portions have their ducts. Those of the anterior are narrow tubes, which run over its ventral surface and become dilated behind to form a pair of elongated chambers, the urinary sinuses (Fig. 779, ur. sin), which unite into a median sinus (mcd. ur. sinus.), opening into the cloaca by a median aperture situated on a papilla, the urinary papilla. The ducts of the posterior portion, the ureters, which are usually from four to six in num- ber, open into the urinary sinuses. In the male there are two elongated, soft, lobulated tcstes, each attached to the wall of the abdominal cavity by a fold of peritoneum the mesorchium. From each testis efferent ducts pass to the anterior end of a long, narrow, strap-shaped body, which corresponds to the anterior portion of the kidney in the female. This is the rpididymis, the duct of which is a convo- luted tube running along the entire length of the mesonephros, and where it leaves the latter posteriorly becoming a wide tube the vas defcrens or spermiduct which opens into a special median com- partment of the cloaca, the urino-gcnital sinus. Posteriorly the spermiduct dilates to form a wide thin-walled sac, the vesi- < ula seminalis. Closely applied to the 1 jitter is a thin- walled elongated sac, the sperm-sac. Anteriorly the sperm-sac narrows to a blind extremity. Posteriorly the right and left sperm-sacs combine to form the urine-genital sinus. The posterior part of the kidney has the same character as in the female ; its ducts, usually live in number i/ -clo.ajj ur.ajb K.. 77*. Chiloscyllium. Oviducts. ali. },. common abdominal aperture of ovi- ducts ; do. dp, cloacal aper- ture ; sit. i/l>i, shell-gland; /'./. ti/t>. urinary papilla. XIII PHYLUM CHORDATA 153 on each side, open into the urino-genital sinus. The latter has a median aperture into the general cavity of the cloaca situated on the summit of a prominent urino-genital papilla. The oviducts (Mulleriaii ducts) of the female are repressnted in the male by rudiments of their anterior portions. The entire kidney is some- times regarded as a mesonephros. but the posterior portion, de- veloped entirely behind the part which is converted in the male into the epididymis, and having its own ducts, is sometimes neph med.,ur.sin, FIG. 779. Chiloscyllium. Right kidney and urinary sinus of female, ma!. *'/. sii>. i'.--, median urinary sinus; neph, kidney; v.i-. */'/><'.$, right urinary sinus. FIG. 780. Dog-fish, egg-case. (After Dean.) looked -upon as corresponding to the metanephros of the higher Vertebrates. The ripe ovum, rupturing the wall of its Graafian follicle, escapes into the abdominal cavity, whence it reaches the interior of one of the oviducts ; there it becomes fertilised by sperms received from the male in the act of copulation, and then becomes enclosed in a chitinoid case or shell (Fig. 780) secreted by the shell-gland. 154 ZOOLOGY SECT. 2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. The Elasmobranchii are Pisces in which the cartilaginous cranium is never ossified by cartilage-bones, and in which mem- brane-bones are not developed in connection either with the cranium or the pectoral arch. The skull is hyostylic, except in some of the Protoselachii, in which it is amphistylic. The dermal fin-rays are horny ; they are supported by cartilaginous pterygio- phores which are never very numerous. The pelvic arch is a dis- tinct cartilage. There is nearly always an exoskeleton, which, when present, is of the placoid type. The intestine has a spiral or a scroll- like valve. There is a cloaca into which both the rectum and the ducts of the urinary and reproductive systems open. There is never an operculum in recent Elasmobranchs, and only rarely in fossil forms. The inter-branchial septa are of considerable breadth, and the gill-filaments are attached to them throughout their entire extent. A spiracular gill is only exceptionally present as a fully developed organ ; it is represented usually by a vestige (pseudo- branch). A conus arteriosus is always developed ; it is rhythmically contractile, and in its interior are several transverse rows of valves. The optic nerves form a chiasma. The ova are very large : they are usually fertilised internally. The oviducts are not continuous with the ovaries, but open by wide mouths into the body-cavity. . 781. Restoration of Cladoselache fyleri. lateral and ventral views. (Restored, after Dean.) ORDER 1. CLADOSELACHE A. Extinct Shark-like Elasmobranchs in which both pectoral and pelvic fins had much wider bases of attachment than in existing XIII PHYLUM CHORDATA 155 forms. The notochord was persistent : there were cartilages. The caudal fin was strongly heterocercal. Claspers were absent. The gill-openings were apparently protected by a fold of skin. The teeth were of the nature of placoid denticles. The lateral line was represented by an open groove. This order comprises only one known representative Cladose- lache from the lower Carbon- iferous rocks of America. ORDER 2. PLEURACAXTHEA. no intercalary Extinct Shark-like Elasmo- branchs in which the skeleton of the pectoral fin was con- structed on the type of the archipterygium, i.e. consisted of an elongated segmented central axis bearing two rows of jointed rays. The notochord was per- sistent, but intercalary cartilages were present. The caudal fin was diphy cereal. Claspers were present. There was no opercular fold, and the teeth resembled those of other Elasmobranchs. There were no placoid scales, but the skull was protected by roofing dermal bones. This order, like the last, in- cludes only one satisfactoril y known genus Pleuraca nthus of Carboniferous and Permian age. ORDER 3. ACAXTHODEA. Extinct Elasmobranchs having the anterior margin of each fin supported by a stout spine. The tail was heterocercal. There were probably membrane-bones on the roof of the skull. The 156 ZOOLOGY SECT. teeth were few and large. The lateral line was in the form of an open groove. FIG. 783. Acanthodes wardi. (Restored, after Dean.) ORDER 4. SELACHIL Living and extinct Elasmobranchs in which the skeleton of the paired fins is never of the nature of an archipterygium. The notochord is more or less completely replaced by vertebrae, and there is a series of intercalary cartilages. The caudal fin is nearly always heterocercal. Claspers are always developed. A distinct opercular fold is never present. Sub- Order a. Protosela chii. Selachii in which the spinal column is unossified, and the centra are very imperfectly developed : there are more than five branchial arches. The palato-quadrate develops, except in Chlamydoselachus, a process by which it articulates with the post-orbital region of the skull. This sub-order includes the Notidanidce (Hevanchus and Septan- chns), and Clilamydosdaclius, as well as, probably, many fossil forms. Sub- Order ~b. Euscla chi i. Selachii in which the spinal column is partly or completely ossified. There are only five branchial arches. The palato-quad- rate has no post-orbital articulation with the skull. Section a. Squalida, Euselachii with fusiform body and well-developed caudal fin. The pectorals are of moderate size. A ventral fin is present. The vertebrae of the anterior part of the spinal column are not fused together. The branchial apertures and the spiracle are situated laterally. This section comprises all the recent Sharks and Dog-fishes, with the exception of the Protoselachii. Section /3. Eajida. Euselachii with dorso-ventrally compressed body, and, usually, feebly developed caudal fin. The pectorals arc of great size, the XIII PHYLUM CHORDATA 157 pelvics usually small. A ventral fin is usually absent. The verte- bras of the anterior region are fused together. The branchial apertures are ventral, the spiracles dorsal. This section comprises all the recent and extinct Rays (Skates, Thorn-backs, Sting-Rays, Electric Rays, Saw-fish Rays). 3. GENERAL ORGANISATION. External Characters. In general shape the Sharks (Fig. 784), for the most part, are somewhat fusiform and slightly com- pressed laterally. In the Rays (Fig. 785), on the other hand, there is great dorso-ventral compression. The head is in many cases pro- duced forwards into a long rostrum, which is of immense length and bordered with triangular teeth in the Saw-fish Shark (Pristio- pliorus) and Saw-fish Ray (Pristis). In the Hammerhead Shark the anterior part of the head is elongated transversely. There are well-developed median and paired fins. The caudal fin is well developed, and, as a rule, strongly heterocercal in the FIG. 784. Shark (Lamna cornubica). (From Dean's Fishes.) Sharks and Shark-like Rays, feebly developed in most of the latter group. The dorsal and ventral fins are large in the Sharks, the former completely divided into two : in the Rays the dorsal fin is usually small, and 'the ventral absent. The paired fins are very differently developed in the two groups. In the Sharks both pairs are well developed, the pectoral being the larger. In the Rays the pectoral fins are extremely large, very much larger than the pelvic, fringing the greater part of the length of the flattened body, and becoming prolonged forwards on either side and even in front of the head, so that the animal presents the appearance of a broad fleshy leaf. In all recent Elasmobranchs the male has, connected with the pelvic fins, a pair of grooved appendages the daspers or ptery go- podia which subserve copulation. The mouth is situated on the ventral surface of the head, usually a considerable distance from the anterior extremity. In front of 158 ZOOLOGY SECT. each angle of the mouth on the ventral surface is the opening of one of the olfactory sacs, each of which is connected by a groove the naso-buccal groove with the mouth-cavity. Behind the mouth, on the dorsal surface in the Rays, and at the side in the Sharks, is the spiracle. Along the sides of the neck in the Sharks, and on the ventral surface in the Rays, is on either side a row of slit-like aper- tures- the branchial slits or branchial clefts. These are usually five in number on each side ; but in Hexanclms and Chlamydosc- ktchus there are six, and in Heptanclius seven. In Cklamydosclachus a fold comparable to a rudimentary operculum extends back over the first branchial cleft, and is continuous across the middle line ventrally ; in the remainder of the sub-class no such structure is represented. A large cloacal opening is situated just in front of the root of the tail, and a pair of small openings placed in front of it- -the abdominal pores lead into the ab- dominal cavity. When the integument develops any hard parts, as is the case in the majority of the Elasmobranchs. they take the form, not of regular scales, as in most other fishes, but of numerous hard bodies which vary greatly in shape, are usually ex- tremely minute, but are in some cases developed, in certain parts of the surface, into prominent tubercles or spines. When these hard bodies are, as is commonly the case, small and set closely together in the skin, they give the surface very much the character of a fine file ; and the skin so beset, known as " shagreen," is used for various polishing purposes in the arts. This is the placoid form of exoskeleton, to which reference has been already made. Each of the hard bodies has the same structure as a tooth, being- composed of dentine, capped with enamel, and supported on a bony base, representing the cement or crusta petrosa of the tooth. The skeleton is composed of cartilage, with, in many cases, deposition of bony matter m special places notably in the jaws and the vertebral column. The entire spinal column may be nearly completely cartilaginous (Rcxanchus and Hcptanchus), but usually l"i<;. 785. Sting-Ray (Urolopliv.s cruciatus). (After Giinther.) XIII PHYLUM CHORDATA 159 the centra are strengthened by radiating or concentric lamellae of bone ; or they may be completely ossified. They are deeply amphiccelous, the remains of the notochord persisting in the large inter-central spaces. Intercalary pieces (Fig. 787, Ic.} are interposed between both superior and inferior arches. In the Rays (Fig. 788) the anterior part of the spinal column becomes converted into a continu- ous solid cartilaginous and bony mass the anterior vertebral plate (a.vjj.) As in Fishes in general, two regions are distinguishable in the spinal column the prcerCaudal and the caudal, the latter being distinguished by the possession of inferior or haemal arches. In the prse-eaudal region short ribs may be developed, but these are some- times rudimentary or entirely ab- sent. In the Sharks pterygio- phores, sometimes jointed, fused at their bases with the haemal spines, support the ventral lobe of the caudal fin, and the dorsal lobe of the same fin is supported by a series of pterygiophores resembling produced neural spines, but only secondarily related to the spinal column and sometimes also divided by joints. The dorsal and ventral fins are sometimes supported by similar ptery- giophores ; but in many cases the car- tilaginous supports of these fins consist, in whole or in part, of expanded plates of cartilage. The skull is an undivided mass of cartilage, hardened, in many cases, by deposition of osse- ous matter, but not containing any separate bony elements. It consists of a cartilaginous case for the protection of the brain and the organs of special sense. The struc- ture of this cartilaginous brain-case as it occurs in the Dog-fish has FIG. 786. Dermal denticles of Centro- phorus calceus, slightly magni- fied. (From Gegenbaur's Compare fir, Anatomy.) WK Fict. 787. Portion of the spinal column of Scymnus. Ic. intercalary cartilages ; Ob, neural arches ; WK, centra. (From Wiedersheim.) 160 ZOOLOGY SECT. already been described. The main differences observable in the different families are connected with the size and form of the rostrum. In the Rays the lower lip of the foramen magnum is deeply excavated for the reception of a short process, the so-called odontoid process, which projects forwards from the anterior vertebral lab has, FIG. 788. Skeleton of Sting:- Ray (Uroloplius testaceus), ventral view. a. r. p. anterior vertebral-plate ; bos. br. basi-branchial plate ; br.i br$ branchial arches. The branchial rays are represented as having been removed, the round dots indicate their articulations with tlae arches, cl. skeleton of clasper ; h. m. hyomaudibular ; A//, hyoid arch ; lab. labial cartilage ; lig. ligament connecting the hyomandibular with the palato-quadrate and Meckel's cartilage ; Mck. Meckel's cartilage ; ins. pt. mesopterygium ; int. pt. metapterygium of pectoral fin ; mt. pt'. metapterygium of pelvic fin ; nan. nasal cartilage ; -pal. palato-quadrate ; pcct. pectoral arch ; pi. pelvic arch ; pro. pt. propterygium ; up. spiracular cartilage. plate, and on either side of this is an articular surface the occi- pital condyle for articulation with corresponding surfaces on that plate. In the Sharks the skull is fused with the spinal column. The apertures of the aqueductus vestibuli in the Rays are not situated in a median depression such as is observable in the Dog- fish and in all the Sharks. The articular surface in the auditory xm PHYLUM CHORDATA 161 region for the hyo-mandibular is sometimes borne on a projecting process, sometimes on the general level of the lateral surface. Sometimes in the Rays there is a smaller articulation behind for the first branchial arch. The upper and lower jaws the palato-quadrate and MeckeVs car-, tilagi are connected with the skull through the intermediation of a hyo-mandibular cartilage (Fig. 768, liy. mn. ; Fig. 788, h. m.). In the Sharks the palate-quadrate has a process (absent in the Ravs ) for articulation with the base of the skull. In Hexanchus , and Heptanchus (Fig. 789) there is a prominent post-orbital pro- cess of the palato-quadrate for articulation with the post-orbital region of the skull (amphistylic arrangement). At the sides of '. or If Fir;. 780. Lateral view of the skull of Heptanchus. ruck. Meckel's cartilage ; pal.-qu. palato quadrate ; pt. orb. post-orbital process of the cranium, with which the palato-quadrate articulates. (After Gegenbaur.) the mouth are a series of labial cartilages, usually two pairs above and one pair below. Attached to the hyo-mandibular is a thin plate of cartilage the spiracular (Fig. 788, sp.) which supports the anterior wall of the spiracle. The hyoid arch is in most of the Elasmobranchs connected at its dorsal end with the hyo-mandibular, sometimes at its distal extremity, sometimes near its articulation with the skull ; but in some Rays it is not so related, but articulates separately and independently with the skull behind the hyo-mandibular, and in the genera Hypnos and Trygonorliina it articulates with the dorsal portion of the first branchial arch. In the Sharks the hyoid is usually relatively massive ; in the Rays it is smaller, and in most cases closely resembles the branchial arches, and bears similar cartilaginous rays ; a larger or smaller median element, or basi-hyal, is present in all cases. VOL. II M 162 ZOOLOGY SECT. There are always five pairs of branchial arches except in Hexanchus and Chlamydoselachus, which have six, and Heptanchus, in which there are seven. Their dorsal ends are free in the Sharks, articulated with the anterior vertebral plate of the spinal column in most Rays. Externally they bear a series of slender cartilaginous branchial rays. The median ventral elements of the branchial arches are usually more or less reduced, and in some cases are represented by a single basi-branchial plate (Fig. 788, las. &?'.). In the Rays the fifth branchial arch articulates with the pectoral arch, a connection which is absent in the Sharks. A series of slender cartilages the extra-branchial cartilages absent as such in the Dog-fishes and Rays, support the branchial apertures. The pectoral arch (Figs. 770, 788, pect.} consists of a single cartilage, with, however, in most of the Sharks, a mesial flexible portion by which it is divided into right and left halves. Each lateral half consists of a dorsal, or scapular, and a ventral, or cora- coid, part, the two being separated by the articular surfaces for the basal cartilages of the fin. In the Rays, but not in the Sharks, the dorsal ends of the pectoral arch are connected with the anterior vertebral plate of the spinal column by a distinct articulation, the portion of the arch on which the articular surface is situated sometimes forming an independent cartilage (supra-scapula). In Heptanchus a small median ventral element may represent the sternal apparatus of the Amphibia and higher Vertebrata, The based pterygiophores of the pectoral fin are typically three, pro-, meso-, and meta-pterygium (Figs. 770 and 788), but there are some- times four, and the number may be reduced to two. The pro- and meta-pterygia are in the Rays divided (Fig. 788) into several seg- ments, and the former articulates, through the intermediation of a cartilage termed the ant- orbital, wiih the olfactory region of the skull. The jpdfcic arch (pi) is usually, like the pectoral, a single cartilage, but in some exceptional cases it consists of two lateral portions. In some cases a median cpipubic process projects forwards from the pelvic arch, and frequently there is on each side a y//Y>- pubic process. A lateral iliac process which becomes highly developed in the Holocephali is sometimes represented, and may attain considerable dimensions. The pelvic fin has usually two basal cartilages, representing the pro- and meta-pterygia, but the former is often absent. In the males special cartilages attached to the meta-pterygia support the claspers. With the basal car- tilages of both pectoral and pelvic fins are connected a number of jointed cartilaginous fin-rays supporting the expanse of the fin. The arrangement of the muscles is simple. The trunk muscles are divided into a pair of dorsal and a pair of ventral divisions, each composed of many myomeres with intercalated myocommata (Fig. 714, p. 63), following a metameric arrangement. The ventral part, where it forms the muscle of the wall of the abdominal cavity XIII PHYLUM CHORDATA tr. ae. is composed externally of obliquely running fibres, and represents one of the two obh'qt'.r muscles of the abdomen of higher forms. Mesially this passes into a median band of longitudinally running- fibres corresponding to a primitive rcctus. The muscles of the limbs are distinguishable into two main sets those inserted into the limb arch and those inserted into the free part of the appendage. The latter, according to their insertion, act as elevators, depress- ors, and adductors. A series of circular muscles pass between the cartilages of the visceral arches, and when they contract have the effect of con- tracting the pharynx and constricting the apertures. A set of muscles pass between the various arches and act so as to ap- proximate them ; and a broad sheet of longi- tudinal fibres divided into myomeres ex- tends forwards from the shoulder girdle to the visceral arches. Electric organs -organs in which electricity is formed and stored up, to be discharged at the will pi the Fish OCCUr FIG. TOO. A Torpedo-Ray with the electric organs dissected ill Several Fl-ism out - On the right the surface oiilv of the electric organ (ot) rrn is shown - On the left the nerves passing to the organ are brancllS. I hey are dissected out. The roof of the skull is removed to bring the i * _j -I * i brain into view. l,r. branchife ; /, spiracle; o, eyes; tr. developed 111 trigeminal ; tr'. its electric branch ; <. vagus ; /, fore-brain ; thp "Flon-H-io "Ro^- n > mid-brain; ///, cerebellum; ir, electric lobe. (From Ullfc; ttiU S Gegenbaur.) (Torpedo and Hypnf>*) (Fig. 790) in which they form a pair of large masses running through the entire thickness of the body, between the head and the margin of the pectoral fin. A network of strands of fibrous tissue forms the support for a number of vertical prisms, each divided by transverse partitions into a large number of com- partments or cells. Numerous nerve-fibres pass to the variou parts of the organ. These are derived mainly from four nerv M 2 164 ZOOLOGY SECT. which originate from an electric lobe of the medulla oblongata, with a branch from the trigeminal. By means of the electric shocks which they are able to administer at will to animals in their immediate neighbourhood, the Torpedo-Kays are able to ward off the attacks of enemies and to kill or paralyse their prey. In the other Rays in which the electric organs are developed, they are comparatively small organs situated at the sides of the root of the tail. In alf cases the cells are formed from metamorphosed muscular fibres. Digestive System. Teeth are developed on the palato- quadrate and on Meckel's cartilage. They are arranged in several parallel rows, and are developed from a groove at the back of the jaw, successive rows coming to the front, and, as they become worn out, falling off and becoming replaced by others. In the Sharks the teeth are usually large and may be long, narrow, and pointed, or triangular with serrated edges, or made up of several sharp cusps: in the Rays, however, the teeth are more or less obtuse, sometimes, as in the Eagle-Rays, forming a continuous pavement of smooth plates covered with enamel, adapted to crushing food consisting of such objects as Shell-fish and the like. The Sharks have a prominent tongue supported by the median basi- hyal ; this is entirely or almost entirely absent in the Rays. There are no salivary glands. The various divisions of the enteric canal are similar in all" the members of the class to what has already been described in the case of the Dog-fish. A spiral valve is always present in the large intestine, though its arrangement varies considerably in the different families. In some cases (e.g. Carcharias) the fold is not a spiral one, but, attached by one edge in a nearly longitudinal line to the intestinal wall, is rolled up in the shape of a scroll. A ccecum occurs in Lsemargus. The rectum always terminates in a cloaca, into which the urinary and genital ducts also lead. There is always a voluminous liver and a well-developed pancreas. A thyroid lies in the middle line behind the lower jaw. A representative of the thymus lies on either side, a little below the upper angles of the branchial clefts. The respiratory organs of the Elasmobranchii always have the general structure and arrangement already described in the case of the Dog-fish. In addition to the gills supported on the hyoid and branchial arches there is also in the Notidaniclse a gill in the spiracular cleft the spiracular gill represented in many others by a rete miralnle or network of blood-vessels. In Selache (the Basking Shark) there are a series of slender rods, the gill-rakers, which impede the passage outwards through the branchial clefts of the small animals on which those Sharks feed. Blood System.- -The licart has in all essential respects the same structure throughout the group. The conns arteriosus is xiii PHYLUM CHORDATA 105 always contractile, and contains several rows of valves. The > general course of the circulation is the same in all (see p. 145). with some variation in the precise arrangement of the vessels. In some of the Ravs the ventral aorta and the roots of the afferent t, vessels are partly enclosed in the cartilage of the basi-branchial -L / plate. The brain attains a much higher stage of development than in the Cyclostomata. The fore-brain greatly exceeds the other divisions in size. In Scymnus there are two widely-separated parencephalic lobes or cerebral hemispheres containing large lateral ventricles. In other genera there is at most, as in the Dog- fish, a median depression of greater or less depth, indicating a division into two lateral portions. In Scyllium, as already pointed out, there is a median prosoccele which gives rise anteriorly to two lateral ventricles, or paracoeles, and the same holds good of Rhina and Acanthias. In the Rays there is only a very small i/ t^ t/ prosoccele without anterior prolongations. The olfactory lobes are of great size, with in some cases short and thick, in others longer and narrower, stalks. In Scyllium, Rhina, and Acanthias, as well as in Scymnus, they contain ventricles : in the Ravs they are solid. i/ . t t/ The dieneephalon is of moderate extent. On its lower aspect are a pair of rounded lobi inferior es, which are of thenature of a bilobed dilated infundibulum, and a median saccus infundibuli and saccus vasculosus, both diverticula of the infundibulum ; directly behind the saccus vasculosus lies the hypophysis. The cpiphysis is long and narrow. In the hind-brain the cerebellum is relatively greatly elongated, and overlaps the optic lobes and sometimes also the dieneephalon in front. Behind it extends over the anterior part of the medulla oblonyata. It usually contains a cerebellar ventricle or cpiccelc. The medulla is elongated in the Sharks, shorter and more triangular in the Rays. The Electric Ravs are characterised by the / t/ t/ presence of the electric lobes, rounded elevations of the floor of the fourth ventricle. Organs of Sense. --The sense-organs of the integument* &re almost always simple or enclosed in branched canals, the mucous or sensory canals, and are supplied by the lateral branch of the vagus, and by branches of the trigeminal, facial, and glosso- pharyngeal. On the head and anterior trunk region larger or smaller canals occur having a number of dilatations the ampullce, filled with gelatinous matter given off from them ; in these are nerve-endings. Some Sharks are exceptional in the presence of an open groove for the lodgment of the integumentary sense- organs. The olfactory organs are a pair of cavities opening on the lower surface of the head, a little distance in front of the mouth, and enclosed by the cartilaginous olfactory capsules of the skull. 160 ZOOLOGY SECT. Their inner surface is raised up into a number of ridges on which the fibres of the olfactory nerves are distributed. The eye has a cartilaginous sclerotic and is in most cases attached to the inner wall of the orbit by means of a cartilaginous stalk. A fold of the conjunctiva corresponding to the nictitating membrane, or third eyelid of higher Vertebrates, occurs in some Sharks. The ear consists of the three semicircular canals with their ampullae ; of the membranous vestibule, which is partly divided into two ; and of a canal the aqueductus vestibuli or endolymrjhatic duct which opens on the exterior on the dorsal surface of the head. In the Rays the semicircular canals form almost complete circles and open separately into the vestibule by narrow ducts. Urino-genital Organs. --The kidneys, as already noticed in the account given of the Dog-fish, differ somewhat in their relations in the two sexes. In the male the anterior portion persists as the epididymis, and its duct becomes the spermiduct, while the posterior portion, which is the functional kidney, has a duct, the ureter, of its own. In the female there is no direct connection between the reproductive and renal organs ; the anterior portion of the kidney may be functional, and its duct persists, opening along with those of the posterior portion. In the male the ureters open into a median chamber the urino-genital sinus a special compartment of the cloaca, which receives also the spermi- ducts : this communicates with the general cavity of the cloaca by a median opening situated on a papilla the urino-genital papilla. In the female there is a median urinary sinus, into which the ureters open, or the latter open separately into the cloaca. Save in certain exceptional cases (e.g. the Dog-fish), there are two ovaries, varying considerably in form, but always characterised towards the breeding season by the great size of the Graafian follicles enclosing the mature" ova. The oviducts (Mlillerian ducts) are quite separate from the ovaries. The right and left oviducts come into close relationship anteriorly, being united in the middle on the ventral surface of the oesophagus, where each opens by a wide orifice into the abdominal cavity, or both open by a single median aperture. The following part of the oviduct is very narrow ; at one point it exhibits a thickening, due to the presence in its walls of the follicles of the shell-gland. Behind this is a dilated portion which acts as a -uterus ; and this communi- cates with the cloaca through a wide vagina. A considerable number of the Elasmobranchii are viviparous, and in these the inner surface of the uterus is beset with numerous vascular villi, while the shell-gland is small or vestigial. The testes are oval or elongate: the convoluted epididymis is connected with the anterior end by efferent ducts, and passes posteriorly into the vas deferens. The latter is dilated near its XIII PHYLUM CHORDATA 167 opening into the urino-genital sinus to form an ovoid sac--the vcswula scmincdis. A large thin-walled sperm-sac is. sometimes present, opening close to the aperture of the vas deferens. The Mlillerian ducts are vestigial in the male. Impregnation is internal in all the Elasmobranchs with the ex- ception of Lsemargus (the Greenland Shark), the claspers acting as intromittent organs by whose agency the semen is transmitted into the interior of the oviducts. In all the Elasmobranchs the ova are very large, consisting of large mass of yolk-spherules held together by means of a network of protoplasmic threads, with, on one side, a disc of protoplasm- the germinal disc. The process of maturation is similar to that observable in holoblastic ova ; one polar globule is thrown off in the ovary, the other apparently at impregnation. The ripe ovum ruptures the wall of the enclosing- follicle and so passes into the ab- dominal cavity to enter one of the oviducts through the wide ab- dominal opening. Impregnation takes place in the oviduct, and the impregnated ovum in the oviparous forms becomes sur- rounded by a layer of semi-fluid albumen and enclosed in a chitin- ous shell secreted by the shell- gland. The shell varies in shape somewhat in the different groups : most commonly, as in the Dog- fishes (Fig. 780),it is four-cornered, with twisted filamentous append- ages at the angles, by means of which it becomes attached to sea- weeds and the like. In the Skates the filaments are absent the Port Jackson Sharks (Cestracion) (Fig. 791) it is an ovoid body, the wall of which presents a broad spiral flange. Enclosec the shell, the young Elasmobranch goes through its develop- ment until it is fully formed, when it escapes by rupturing the eggshell. In the viviparous forms the ovum undergoes its development in the uterus, in which for the most part it lies free, except in some Mustclidce, in which there is a close connection between the yolk-sac of the foetus and the wall of the uterus, fold* of the former interdigitating with folds of the latter, and nourish- ment beino- thus conveyed from the vascular system of the motne FIG 701. Egg-case of Cestracion galeatus. (After Waite.) 168 ZOOLOGY SECT, to that of the foetus. In Mustelus antarcticus the uterus is divided by septa into several compartments, each containing a single foetus. In some of the viviparous forms a distinct, though very delicate, shell, sometimes having rudiments of the filaments, is formed, and is thrown off in the uterus. In the genera Khinobatus and Trygonorhina, which are both viviparous, each shell encloses, not one egg, but three or four. Lsemargus appears to differ from all the rest of the Elasmobranchii in having the ova fertilised after they have been deposited as well as in the small size of the ova. Development. Segmentation is meroblastic, being confined to the germinal disc, which, before dividing, exhibits amoeboid movements. While segmentation is going on in the germinal disc there appear a number of nuclei, the source of which is not certain, in the substance of the yolk. When segmentation is complete the blastoderm appears as a lens-shaped disc, thicker at one end- the embryonic end. It is found to consist of two layers of cells, -an upper layer in a single stratum, and a lower layer several cells deep. A segmentation-cavity appears early among the cells of the lower layer ; the lower-layer cells afterwards disappear from the floor of this, the cavity then coming to rest directly on the yolk. An in-folding of the blastoderm (Fig. 792) now begins at the thickened embryonic edge of the blastoderm, which here becomes FIG. 792. Longitudinal section through the blastoderm of a Pristiurus embryo before the medullary groove has become formed, showing the beginning of the process of infolding or invagination. al. archenteron ; Lp. ectoderm; <:,: embryonic rim; >/i. mesodenn. (From BaJfour.) continuous with the cells of the lower layer. The cavity (al), at first very small, formed below this in-folding is the rudiment of the archenteron, and the cells lining this cavity above, which form a definite layer, partly derived from the in-folded ectoderm, partly from the cells of the lower layer, are the beginning of the definite endoderm. The edge of the in-folding, entitled the embryonic rim, is obviously the equivalent of the dorsal lip of the blastopore in Amphioxus. The endoderm and its underlying cavity soon grow forwards towards the segmentation cavity. Under the latter appears a floor of lower-layer cells, but the cavity soon becomes obliterated as the archenteron develops. After the formation of the embryonic rim a shield-like tmbryonic area is distinguishable in front of it, with two folds bounding a groove the medullary groom. The mesoclerm becomes esta- blished at about the same time. It is formed from the lower- XIII PHYLUM CHORDATA layer cells and assumes the form of a pair of independent plate s one on either side of the middle line of the body. t/ As the blastoderm extends over the yolk the edge forms a ridge continuous with the embryonic rim. The latter assumes the form of two prominent caudal swellings (Fig. 794, cd.). The medullary groove meanwhile deepens, and its edges grow over, as in Amphi- oxus and the Urochorda, so as to form a canal (Fig. 793, (7: Fig. 795). The union takes place first in the middle, the anterior FIG. 793. Diagrammatic longitudinal sections of an Elasmobranch embryo. A, section of the young blastoderm with segmentation-cavity enclosed in the lower layer cells. B, older blastoderm with embryo in which endoderni and mesoderm are distinctly formed, and in which the alimentary slit has appeared. The segmentation-cavity is still represented as being present, though by this stage it has in reality disappeared. C, older blastoderm with embryo in which the neural canal has become formed and is continuous posteriorly with the alimentary canal. Ectoderm without shading ; niesotlerm and also notochord black with clear outlines to the cells ; endoderm and lower layer cells with simple shading, al. alimentary cavity; rh. notochord; tp, ectoderm; m. mesoderm; n. nuclei of yolk; nc. neuroccele ; <;/. segmentation-cavity ; x. point where ectoderm and endoderm become continuous at the posterior end of the embryo. (From Balfonr.) and posterior parts (Fig. 795, neurJ) remaining open for a while. When the posterior part closes, it does so in such a way that , it encloses the blast opore, and there is thus formed, as in the Ascidian, a temporary passage of communication between the medullary canal and the archenteron the neurenteric passage. The ectoderm gives rise, as in Vertebrates in general, not only to the epidermis and the central nervous system, but also to the peripheral nervous system, the lining membrane of the olfactory sacs, the lens of the eye, and the lining membrane of the auditory labyrinth, of the mouth and gill clefts, and of the cloaca. 170 ZOOLOGY SECT. FIG. 7'.'4. Embryo of Scyllium canicula with the tail- swellings well marked and the medullary groove just beginning. 1>I. <. edge of blastoderm ; bl. p. blastopore ; ><>. caudal swellings ; //. head. (After Sedgwick.) The notochord (Fig. 793, ch.) is developed as a cord of cells derived from the lower layer. Each of the two plates of mesoderm soon divides into two layers, somatic and splanchnic, with a cavity between them. The inner part of each separates off from the rest and becomes divided by transverse fissures into a series of squarish masses, the proto-vertebrce, the outer part forming a broad plate, the lateral -plate. The splanchnic layer of the proto- vertebrse sends off cells round the notochord to form the bodies of the vertebrae, the re- mainder giving rise to the muscles of the voluntary system. The isthmus between the lateral plate and the protovertebrse, con- taining a prolongation of the cavity, gives rise to the pronephric duct and tubules. The lateral plates eventually unite ventrally, and their cavi- ties coalesce to form the body cavity. The parts derived from the meso- derm are the system of voluntary muscles, the e/ dermis, the inter-mus- cular connective tissue, the endoskeleton, the mUSCUlar and Connective- Fic , 795._Enibryo of a Ray with the medullary groove tissue layers Of the all- closed except at the hind end. The notched em- , bryonic part of the blastoderm has grown faster than mentaiy Canal, tile VaS- the rest and come to project over the surface of the i , A i j j_v yolk. bl. e. edge of blastoderm ; hd. head ; nevr. un- S}'S enclosed pai't of the ncnroccele. (After Sedgwick.) generative organs. The segmentation of the mesoderm does not at first extend into the There is some uncertainty as to the germinal layer from which the internal lining membrane of the heart and blood-vessels is derived. In Acanthias vulgar is, if not in others, it seems to be derived from the endoderm, and the entire vascular system is to be looked upon as a separated-off part of the archenteron. hd. XIII PHYLUM CHORDATA 171 head, but, on the formation of the gill-clefts, a series of mesodermal segments appear, the cells of which give rise to the muscles of the branchial, hyoid, and mandibular arches, and probably also of the palato-quadrate and the eye. By degrees the body of the young fish becomes moulded on the blasto- derm. This is effected by the formation of a system of folds, anterior, posterior, and lateral. which grow inwards in such a way as to separate off the body of the em- bryo from the rest of the blastoderm enclosing the yolk. As the folds ap- proach one another in the middle, underneath the embryo, they come to form a constriction con- necting the body of the embryo with the yolk e/ t/ enclosed in the extra- embryonic part of the blastoderm. The process may be imitated if we pinch off a portion of a ball of clay, leaving only a narrow neck connecting the pinched-off portion with the rest. The body of the embryo thus be- comes folded off from the yolk-sac and comes to be connected with it only by a narrow neck or yolk- stalk (Fig. 796). The head and tail of the young Fish soon be- come differentiated, and a series of involutions at the sides of the neck (Fig. 797) form the branchial clefts and spiracle. A number of very delicate long filaments (Fig. 797) grow out from these apertures : these are the provisional gills, which atrophy as the development FIG. 796. Three views of the developing egg of an Elasmobranch, showing the embryo, the blasto- derm, and the vessels of the yolk-sac. The shaded part (bl.) is the blastoderm, the white part the un- covered yolk. A, young stage with the embryo still attached at the edge of the blastoderm. B, older stage with the yolk not quite enclosed by the blasto- derm. C, stage after the complete closure of the j-olk. ft. arterial trunks of yolk-sac ; bl. blastoderm ; r. venous trunks of yolk-sac ; ?/, point of closure of the yolk blastopore ; ./:, portion of the blastoderm out- side the arterial sinus terminalis. (From Balfour.) 172 ZOOLOGY SECT. approaches completion, their bases alone persisting, to give rise to the permanent gills. The great development of these gill- filaments in the embryos of some viviparous forms suggests that in addition to their respiratory functions they may also serve as organs for the absorption of nutrient fluids secreted by the villi of the uterine wall. 1 The fins, both paired and unpaired, appear as longitudinal ridges of the ectoderm enclosing mesoderm. In some Elasmobranchs the paired fins are at first represented on each side by a continuous ridge or fold, which only subsequently becomes divided into anterior and posterior portions the rudiments respectively of the pectoral and pelvic fins. Into these folds penetrate a series of buds from the prot overt ebras ; these, the- m.Jir-n. m.brn Gi pir -r FIG. 797. Side view of head of embryo of Scy Ilium canicula, with ^the rudiments of the gills on the first and second branchial arches, eye, eye ; in. brn. mid -brain ; mnd. mandible ; nas. nasal sac. (After Sedgwick.) FIG. 70S. Side view of the head of Scyllium canicula at a somewhat later stage. The gills have increased in number and are present on the mandibular arch. anr/. angle of the jaw ; hj/. hyoid ; m. brn. mid-brain ; ,w*. nasal sac ; spir. spiracle. (After Sedgwick.) muscle-buds, give rise to the fin-muscles ; at first, from their mode of origin, they present a metameric arrangement, but this is in great measure lost during development. Ethology and Distribution. The habits of the active, fierce, and voracious Sharks, which live in the surface-waters of the sea, making war on all and sundry, contrast strongly with those of the more sluggish Rays, which live habitually on the bottom, usually in shallow water, and feed chiefly on Crustaceans and Molluscs, with the addition of such small Fishes as they can capture. As a group, the Elasmobranchs, more particularly the Sharks, are distinguished by their muscular strength, the activity of their movements, and also by the acuteness of their senses of sight and smell. The only deep-water Elasmobranch known is a species of Ray, which extends to a depth of over 600 fathoms. 1 In a species of Trygon the nutrient secretion is stated to pass into the enteric canal through the spiracles. xiii PHYLUM CHORDATA 173 Xone of the Elasmobranchs are of very small size, and com- prised among them are the largest of living Fishes. The harm- less Basking Sharks (Selache) sometimes attain a length of 35 feet or more, the formidable Great Blue Shark (Carclmrodon) some- times reaches 40 feet, and some of the Rays also attain colossal dimensions. In this respect, however, recent Sharks and Rays are far behind some of the fossil forms, some of which, if their general dimensions were in proportion to the size of their teeth, must have attained a length of as much as sixty feet. The earliest fossil remains of Elasmobranch Fishes that have been found occur in rocks belonging to the Upper Silurian period. Throughout the Palaeozoic Epoch the Elasmobranchs constituted a very important section of the fauna a large proportion of the fish- remains that have been found in palaeozoic formations being the remains of Elasmobranchs, mainly in the form of spines and teeth. Most of the palaeozoic Elasmobranchs were characterised by a great development of the exoskeleton. The teeth differ from those of existing forms in being provided with broad bases by means of which they articulated together, and in various groups there is a union of the teeth by the coalescence of their bases so as to form broad crushing plates. A similar union is not uncommon between the parts of the general exoskeleton, a good many palaeozoic Sharks having been encased in an armour of solid plates formed by such a coalescence. In the endoskeleton there is to be observed among the fossil Elasmobranchs a gradual advance in the degree of calcification of the spinal column from the palaeozoic forms onwards, the Protoselachii alone among exist- ing forms representing in this respect the condition which seems to have prevailed in the most ancient members of the class. The group (Cestracionts) now represented by two or three species of Port Jackson Sharks seems to have been very abundant in palaeozoic times. The extinct Pleuracanthea, together with Cladoselachus, which, as briefly stated in the sketch of the classification, differ from the $/ other known members of the class in the structure of the fins and other points, range from the Devonian to the Permian, and are perhaps also represented in the Trias. Sub-Class II. Holocephali. The existing representatives of the Holocephali are included under the single family Chimceridce, containing three genera- Chimcem, Callorhynchus, and Harriotta. Even taking in fossil forms, the group is a very small one ; it agrees in many funda- mental characteristics with the Elasmobranchii, but presents so many important differences that it cannot well be included in that sub-class. Of the recent genera, Chimaera, the so-called 174 ZOOLOGY SECT. " King of the Herrings " (Fig. 799, A) is found on the coasts of Europe and Japan, the west coast of North America, and at the Cape of Good Hope ; Callorhynchus (B) is tolerably abundant in the South Temperate seas ; Harriotta is a deep-sea form. External Characters.- -The general form of the body is Shark- like, but the large, compressed head and small mouth are strikingly different from the depressed, shovel-shaped head and wide mouth of most Selachians. The mouth is bounded by lip-like folds, two of to' FIG. 799. A, Chimaera mcmstrosa B, Callorhynchus antarcticus. a. d. anterior clasper ; a. cl.' pouch for its reception; br. ap. branchial aperture; c. j. caudal fin; c. /.' its whip-like prolongation; d. /. 1, d. /. S, dorsal fins;/;-, ct. frontal clasper ; ?./'., /./'/labial folds ; I. ?. lateral line ; no. ap. nasal aperture ; op. operculum ; pet. /. pectoral fin ; i>tf>. pterygopodia ; pc. /. pelvic fin ; t. teeth ; tc. tactile flap ; r. ^. ventral fin. (A, after Cuvier.) which (B, /./., /./.'), placed laterally and supported by labial carti- lages, resemble the folds in which the premaxillaB and maxillas of Bony Fishes are enclosed : a third fold, external to and concentric with the mandible, is also supported by labial cartilages and has the appearance of a second or external lower jaw. In Chimera the snout is blunt, in Harriotta long and pointed ; in Callo- rhynchus it is produced into a rostrum, from the end of which depends a large cutaneous flap (B, tc) abundantly supplied with nerves and evidently serving as an important tactile organ. xiii PHYLUM CHORDATA 175 A still more important difference from Elasmobranchs is the possession of only a single external branchial aperture (&/. a p. ). owing to the fact that a fold of skin, the oferculum (op.}, extends backwards from, the region of the hyoid arch and covers the true gill-slits, which thus come to open into a common chamber situated beneath the operculum and communicating with the exterior by a single secondary branchial aperture placed just anterior to the shoulder-girdle. Equally characteristic is the circumstance that the urino-genital aperture is distinct from and behind the anus, there being no cloaca. There are two large dorsal fins (d.f. l,d.f. 2) and a small ventral (v. /.) ; the caudal fin (c. /.) is of the ordinary heterocercal type in the adult Callorhynchus, but in the young (Fig. 805) the extremity of the tail proper is not upturned, and the fin-rays are arranged symmetrically above and below it, producing the form of tail-fin called diphycercal. In Chimsera the dorsal lobe of the tail may be produced into a long whip-like filament (c./'.). The pectoral (pct.f.) and pelvic (pv.f.) fins are both large, especially the former. In the male there is a horizontal slit (B, a. d.') situated a little in front of the pelvic fins ; it leads into a shallow glandular pouch, from which can be protruded a peculiar and indeed unique apparatus, the anterior clasper (A, a. cL), consisting of a plate covered with recurved dermal teeth, to which is added, in Callo- rhynchus, a plate rolled upon itself to form an incomplete tube. The use of this apparatus is not known ; as it lies in the line of the hypothetical continuous lateral fin, and as the cartilages which support it articulate with the pelvic girdle, it seems possible that it may have arisen as a portion of the lateral fin, which has atrophied in all other Craniata. If this be so, it must be looked upon as a third or intermediate paired appendage. A rudiment of the pouch occurs in the female, although the clasper itself is absent. The male possesses, in addition, a pair of the ordinary pterygopodia or posterior claspers (ptg& and is further dis- tinguished by the presence of a little knocker-like structure, the frontal -clasper (fr. neural spine. (After Haasse.) portion of the pituitary body. The posterior portion of the cranial cavity is very high ; the anterior part containing most of the fore-brain is low and tunnel-like, and has above it a cavity of almost equal size (Nv. 6 l m. cerebellum ; clt. pi jr. 1, choroid plexus of fore-brain; ch. r ^.c. 2, of hind-brain; cp. rst. corpus restiforme ; cp. xtr. corpus striatum ; crb. k. cerebral hemisphere ; di. coc. diaccele ; (.Hen. diencephalon ; for. M. foramen of Monro ; ll>. inf. lobus inferior ; med. ol/l. medulla oblongata ; tut. coc. metacosle ; Nr. 2, optic nerve ; i\V. .5, trigerninal ; ^Y/-. 8. auditory ; Ar. 10, vagus ; olj. I. olfactor}- lobe ; olf. p. olfactory peduncle ; opt. I. optic lobe ; pn. b. pineal bodj T ; pn. s. pineal stalk ; ptij. pituitary body. finally make their way through the central passage into the urino-genital sinus (u. g. s.). The vestigial Miillerian ducts (MuL d.) are much more fully developed than in the Dog-fish : they are complete, though narrow, tubes opening behind into the urino- XIII PHYLUM CHORDATA 181 genital sinus (3IuL d.") and in front by a large common aperture (Mid. d!} into the coelome. Development. Internal impregnation takes place, and the oosperm becomes surrounded, as in the Dog-fish, by a horny egg- shell secreted by the shell-glands. The egg-shell of Callorhynchus -vs. se.m Mul.d FIG. 804. Callorhynchus antarcticus. A, male urine-genital organs, ventral aspect ; the left testis is removed and the left vesicula seminalis displaced ; B, anterior part of vesicula seminalis in section. , DO o * o t* ^* C"^ r "^ 1^ o- /5 O a: > K . 0! | ' o I o o O o CO tt u. .SECT. XIII PHYLUM CHORD ATA 183 piece of kelp. Nothing is known of the early development : the advanced embryo has elongated gill-filaments (br. /.) projecting through the branchial aperture, a diphycercal tail, and a curiously lobed and nearly sessile yolk-sac (yk. s.). Fossil remains of Holocephali are known from the lower Jurassic rocks upwards. As might be expected, they consist mostly of teeth and of dorsal fin-spines, but in some cases, and notably in Squaloraja, practically the whole of the skeleton is preserved. Sub-Class III. Teleostomi, In this sub-class are included all the commonest and most familiar Fishes, such as the Perch, Pike, Mackerel, Cod, Sole, Herring, Eel, Salmon, etc., as well as the so-called " Ganoid " Fishes, such as the Sturgeon, Bony Pike (Lepidosteus) and Bow-fin (Amid] of North America, and the Polypterus of the Nile. They are distinguished from Elasmobranchs and Holocephali by having the primary skull and shoulder-girdle complicated by the addition of membrane bones, and by possessing bony instead of horn-like fin- rays. The gills are covered by an operculum ; the anus is distinct from the urinary and genital apertures ; and the brain has no cerebral hemispheres but an undivided prosencephalon. 1. EXAMPLE OF THE SUB-CLASS- -THE BROWN TROUT (Salmo fario). The Brown Trout is common in the rivers and streams of Europe, and has been acclimatized in other parts of the world, notably in New Zealand. It varies greatly in size, according to the abundance of food and the extent of the water in which it lives : it may attain sexual maturity, and therefore be looked upon as adult, at a length of 18 20 "cm. (seven or eight inches), but in large lakes it may grow to nearly a metre in length. Other species of Salmo, such as the Salmon (S. solar), the Lake Trout (S. ferox), the American Brook Trout (S. fontinalis), are common in the Northern Hemisphere and differ only in details from S. fario. External Characters.- -The body (Fig. 806) is elongated, com- pressed, thickest in the middle, and tapering both to the head and tail. The mouth is terminal and very large : the upper jaw is supported by two freely movable bones, the premaxiMo (Fig. 807, pmx.) in front and the maxilla (mx.) behind, both bearing sharp curved teeth arranged in a single row. When the mouth is opened a row of palatine teeth is seen internal and parallel to those of the maxilla, and in the middle line of the roof of the mouth is a double row of vomerine teeth. The lower jaw (md.) is mainly supported by a bone called the dentary and bears a row of teeth : on the throat 184 ZOOLOGY SECT. each ramus of the mandible is bounded mesially by a deep groove. The floor of the mouth is produced into a prominent tongue (t.) bearing a double row of teeth. In old males the apex of the lower jaw becomes curved upwards like a hook. The large eyes have no eyelids, but the flat cornea is covered by a transparent layer of skin. A short distance in front of the eye is the double nostril (na 1 , no?), each olfactory sac having two- apertures, the anterior one (na 1 ) provided with a flap-like valve. There is no external indication of the ear. On each side of the posterior region of the head is the operculum (Fig. 806, op.) or gill-cover, a large flap which, when raised, displays the gills : between it and the flank is the large crescentic gill- opening, from which the respiratory current makes its exit. The operculum is not a mere fold of skin, as in Holocephali, but is- supported by four thin bones the outlines of which can be made it FIG. 806. Salmo fario. a. 1. adipose lobe of pelvic fin ; an. anus ; c. /. caudal fin ; d. /. 1, first dorsal ; d.f. 3, second dorsal or adipose fin ; 1. L lateral line ; op. operculum ; pet. /'. pectoral fin ; pv. f. pelvic fin ; r. /. ventral fin. (After Jardine.) out through the skin; they are the opercular (Fig. 807, op.), pre- opercular (p. op.), sub-opercular (s. op.), and inter-opercular (i op.) : the latter is attached to the angle of the mandible. The ventral portion of the operculum is produced into a thin membranous extension, the branchiostegal membrane (br. m.), supported by twelve flat, overlapping bones, the branchiostegal rays. The narrow area on the ventral surface of the throat which separates the two gill-openings from one another is called the isthmus. The gills, seen by lifting up the operculum, are four red, comb-like organs, each having a double row of free gill filaments ; alternating with the gills are the five vertically elongated gill-slits, opening into the mouth. On the ventral surface of the body, at about two-thirds of the distance from the snout to the end of the tail, is the anus (Fig. 806, an.) ; behind it is the urino-genitcd aperture, of almost equal size and leading into the urino-genital sinus, into which both urinary and genital products are discharged. .XIII PHYLUM CHORDATA 185 The region from the snout to the posterior edge of the operculum is counted as the head ; the trunk extends from the operculum to the anus ; the post-anal region is the tail. There are two dorsal fins : the anterior dorsal (Fig. 806, d.f. 1) is large and triangular, and is supported by thirteen bony fin-rays : the posterior dorsal (d. f. 2) is small and thick, and is devoid of bony supports : it is distinguished as an adipose, fin. The cainl"! tin (c.f.) is the chief organ of locomotion ; it differs markedly from that of Elasmobranchs in being, as far as its external appearance is concerned, quite symmetrical, being supported by fin-rays which radiate regularly from the rounded end of the tail proper ; such outwardly symmetrical tail-fins are called h.omocercaL There is a single large central fin (v. /.) supported by eleven rays. The pectoral fin (pet. f.) has fourteen rays and is situated", in the normal position, close behind the gill-open- ing, but the pelvic fin (pv.f.) has shifted its position and lies some distance in front of the vent : it is sup- ported by ten rays and has a small pro- cess or adipose lobe (a. I.) springing from its outer edge near the base. The body is covered by a soft, slimy skin through which, in the trunk and tail, the outlines of the scales can be seen; on the head and fins the skin is smooth and devoid of scales. A well-marked lateral line (I, I.) extends along each side from head to tail. The skin is grey above, shading into yellowish below, and is covered with minute black pigment spots which, on the sides and back, are aggregated to form round spots two or three millimetres in diameter. In young specimens orange-coloured spots are also present. Skin and Exoskeleton. The epidermis has no stratum corneum ; it contains unicellular giands, from which the mucus covering the body is secreted, and pigment cells, to which the colours of the animal are due. The scales (Fig. 808) are lodged in pouches of the dermis and have the form of flat, nearly circular plates of bone marked with concentric lines, but having no Haversian canals, lacunae, or canaliculi. They have an imbricating arrangement, overlapping one another from before backwards, m/ici FIG 807. -Head of female Salmo fario. L>,: ,, ( . branchio- stegal membrane ; i. op. inter-opercular ; ,nnd. mandible ; mx. maxilla; ,uii, anterior, and /i-, posterior nostril; "/>. opercular ; pet. /. pectoral fin ; pnix. premaxilla ; ji. oji. pre-opercular ; s. op. sub-opercular ; t. tongue. 186 ZOOLOGY SECT. FIG. SOS. Scale of Salmo fario. a. anterior portion covered by overlap of preceding scales ; b, free portion covered only by pigmented epidermis. -N.SP like the tiles of a house, in such a way that a small three- sided portion (b) of each scale comes to lie immediately beneath the epidermis, while the rest (a) is hidden beneath the scales immediately anterior to it. Besides the scales, the fin-rays belong to the exoskeleton, but will be most conveniently considered in con- nection with the endoskeleton. En do skeleton. --The vertebral column shows a great advance on that of the two previous classes in being thoroughly differentiated into distinct bony vertebrae. It is divisible into an anterior or abdominal region and a posterior or caudal region, each con- taining about twenty-eight vertebrae. A typical abdominal vertebra consists of a dice-box-shaped centrum (Fig. 809, CN.) with deeply concave anterior and posterior faces, and perforated in the centre by a small hole. The edges of the centra are united by liga- ment and the biconvex spaces between them are filled by the remains of the notochord ; there are also articulations between the arches bv means of little / bony processes, the zygapophyses (x. ZYG., H. ZYG.). To the dorsal surface of the centrum is at- tached, by ligaments in the anterior vertebrae, by ankylosis or actual bony union in the posterior, a low neural arch (x. A.), which consists in the anterior vertebra? of distinct right and left moieties and is continued above into a long, slender, double neural spine (x. SP.), directed upwards and backwards. To the ventro- lateral region of the vertebra are attached by ligament a pair of long, slender ribs (R.) with dilated heads, which curve downwards and backwards be- tween the muscles and the peritoneum, thus encircling the abdominal cavity. In the first two vertebrae they are attached directly to the centrum, in the PA.PH l-'ic. 801'. Salmo fario. A, one of the anterior, and B, one of the posterior ab- dominal vertebra? ; C, one of the anterior, and D, one of the posterior caudal vertebra-. CN. centrum; 1MB, inter-muscular bone; HA. haemal arch; H. SP. haemal spine; H. ZYG. haemal zygapophysis ; N. A. neural arch ; N. SP. neural spine ; N. ZYG. neural zygapophysis ; PA. PH. parapophysis ; R. rib. XIII PHYLUM CHORDATA 187 UST rest to short downwardly directed bones, the pdrapophyses (PA. PH.) immovably articulated by broad surfaces to the centrum. At the junction of the neural arch with the centrum are attached, also by fibrous union, a pair of delicate inter -muscular bom-* (i. M. B.) which extend outwards and backwards in the fibrous septa between the myomeivs. The first and second abdominal vertebra? bear no ribs. In the last three the neural spines (x. SP.) are single. In the caudal vertebrae the outgrowths corresponding to the parapophyses are fused with the centrum and unite in the middle ventral line, forming a haemal arch (C, H. A.), through which the caudal artery and vein run. In the first six caudals each haemal arch bears a pair of ribs (R.), in the rest the arch is produced downwards and backwards into a hccmal spine (D, H. SP.). The centra as well as the arches of the vertebrae are formed entirely from the skeleto- t/ genous layer, and not from the sheath of the notochord as in Elasmo- branchs (see pp. 66 and 137). The posterior end of the caudal region is curi- ously modified for the support of the tail-fin. The hindmost centra (Fig. 810, ex.) have their axes not horizontal but deflected upwards, and following the last un- doubted centrum is a rod-like structure, the n.rostylc (UST), consisting of the partly ossified end of the noto- chord, which has thus precisely the same upward flexure as in the Dog-fish. The neural and haemal spines (N.SP., H.SP.) of the last five vertebrae are very broad and closely connected with one another, and are more numerous than the centra ; and three or four haemal arches are attached to the urostyle. In this way a firm vertical plate of bone is formed, to the edge of which the caudal fin-rays (D.F.It.} are attached fan-wise in a symmetrical manner. It will be obvious, however, that this homocercal tail-fin is really quite as unsymmetrical as the heterocercal fin of the Dog-fish, since, its morphological axis being constituted by the notochord, nearly the whole of its rays are, in strictness, ventral. *, The skull (Fig. 811) is an extremely complex structure, com- posed of mingled bone and cartilage. The cartilage has no super- ficial mosaic of lime-salts such as we find in Elasmobranchs, but CN H.1YG HSP D.FJl FIG. 810. Salmo fario, caudal end of vertebral column. CN. centrum; D. F. R. dermal fin-rays; H. SP. haemal spine ; H. ZYG. haemal zyga- pophysis ; N. SP. neural spine ; N. ZYG. neural zygapophysis ; UST. urostyle. 188 ZOOLOGY SECT. certain portions of it are replaced by cartilage bones, and there are in addition numerous membrane-bones developed in the surround- ing connective tissue. As in the Dog-fish, the skull may be divided into cranium, upper and lower jaws, with their suspensory apparatus, and hyoid and branchial arches. The cranium (Fig. 812) is a somewhat wedge-shaped structure its apex being directed forwards. At first sight the distinction between cartilage and membrane-bones is not obvious, but after Sphot SOCb dent FIG. 811. Salmo fario, the entire skull, from the left side. ,-a,ichioist. brauchiostegal rays; . opereular ; pnl. palatine ; JHI.: parietal; pmx. pre-maxilla; praop. pre-opercular ; pt. pterygoid ; ptn: pterotic ; Q"
    .','< . basi- hyal. (From Wiedersheim's Vertebrata.) maceration or boiling certain flat bones (the paired parietal*, PA., frontals, FR., and nasals, NA., and the unpaired supra-ethmoid, S.ETH.) can be easily removed from the dorsal surface ; and two unpaired bones (the parasphenoid, PA. SPff., and vomer, VO.) from the ventral surface. These are all membrane-bones: they are simply attached to the cranium by fibrous tissue, and can readily be prised off when the latter is sufficiently softened by maceration or boiling. We thus get a distinction between the cranium as a whole, or secondary cranium, complicated by the presence of mem- XIII PHYLUM CHORDATA 189 brane-bones, and the primary cranium or chondrocranium, left by the removal of these bones and corresponding exactly with the cranium of a Dog-fish. The primary cranium contains the same regions as that of Scyllium. Posteriorly is the occipital region, surrounding the foramen magnum, presenting below that aperture a single concave occipital condi/h for the first vertebra, and produced above into an SPH.OT BR4 BUY Fi<;. S12. Salmo fario. Disarticulated skull with many of the membrane bones removed. The cartilaginous parts are dotted, fon. fontanelle ; h. m. articular facet for hyomandibular ; J/c/c. C. Meckel' s cartilage; olf. s. hollow for olfactory sac. Cartilage bones AL. SPH. alisphenoid ; ART. articular ; B. K R.I, first basi-branchial ; B. HY. basi-hyal ; B. OC. basi-occipital ; BR.5, fifth branchial arch; B.SPH. basi-sphenoid ; C. BR.l, first cerato-branchial ; C. HY. cerato-hyal ; EC. ETH. ecto-ethmoid ; E. BR.l, first epi- branchial ; E. H Y. epi-hyal ; EP.OT. epiotic ; EX. OC. ex-occipital; H. BR.l, first hypo-branchial; H. HY. hypo-hyal ; HY. M. hyo-mandibular ; I. HY. inter-hyal ; MS. PTG. meso-pterygoid ; MT. PTG. meta-pterygoid ; OR. SPH. orbito-sphenoid ; PAL. palatine; PH. BR.l, first pharyngo-brauchial ; PTG. pterygoid ; PT.OT. pterotic ; QU. quadrate ; S. OC. supra-occipital ; SPH.OT. sphenotic ; SYIXE. symplectic. Membrane bones A KG. angular; DNT. dentary ; FR. frontal; JU. jugal; MX maxilla; NA . nasal ; PA. palatine ; PA. SPH. para-sphenoid ; PMX. pre-maxilla ; VO. vomer. occipital crest. The auditory capsules project outwards from the occipital region, and between them on the dorsal surface of the skull are paired oval fontanelles (fon.) closed in the entire skull by the frontal bones. The posterior region of the cranial floor is pro- duced downwards into paired longitudinal ridges, enclosing be- tween them a groove which is converted into a canal by the apposition of the parasphenoid bone and serves for the origin of the 190 ZOOLOGY SECT, eye-muscles. In front of the auditory region the cranium is exca- vated on each side by a large orbit, a vertical plate or interorlital septum (OR. SPH.) separating the two cavities from one another. In front of the orbital region the cranium broadens out to form the olfactory capsules, each excavated by a deep pit (olf. s.) for the olfactory sac, and anterior to these is a blunt snout or rostrum, The occipital region is formed as usual from the parachorclals of the embryonic skull, the auditory region from the auditory cap- sules, and the rest of the cranium from the trabeculse. The cartilage bones, formed as ossifications in the chondrocranium r correspond in essentials with the typical arrangement already de- scribed (p. 72). In the occipital region are four bones; the basi-ocdpital (B. oc.), forming the greater part of the occipital conclyle and the hinder region of the basis cranii or skull-floor : the ex-occipital s (EX. oc.), placed one on each side of the foramen magnum and meeting both above and below it ; and the supra- occipital (s. OC.) forming the occipital crest already noticed. Each auditory capsule is ossified by five bones i.e., two more than the typical number (p. 72) ; the pro-otic (PR. OT.) in the anterior region of the capsule, uniting with its fellow of the opposite side in the floor of the brain case, just in front of the basi-occipital ;. the opisthotic, in the posterior part of the capsule, external to the ex-occipital ; the spkenotic (SPH. OT.), above the pro-otic and forming part of the boundary of the orbit ; the pterotic (PT. OT.),. above the ex-occipital and opisthotic, forming a distinct lateral ridge and produced behind into a prominent pterotic process ; and the epiotic (EP. OT.), a small bone, wedged in between the supra- and ex-occipitals and pterotic, and produced into a short epiotic process. On the external face of the auditory capsule, at the junction of the pro-, sphen-, and pterotics, is an elongated facet (h.m.) covered with cartilage and serving for the articulation of the hyo-mandibular. The trabecular region of the cranium contains six bones. Im- mediately in front of the conjointed'pro-otics, and forming the anterior end of the basis cranii, is a small unpaired Y-shaped bone, the basi-sphenoid (B. SPH.). Above it, and forming the anterior parts of the side-walls of the brain-case, are the large paired alisphenoids (AL. SPH.). In the interorbital septum is a median vertical bone, representing fused orbitospherwids (OR. SPH.). Lastly, in the posterior region of each olfactory capsule, and forming part of the boundary of the orbit, is the ccto-cthmoid (EC. ETH.). The membrane bones already referred to are closely applied to the roof and floor of the chondrocranium, and modify its form considerably by projecting beyond the cartilaginous part, and con- cealing apertures and cavities. The great frontals (FR) cover the greater part of the roof of the skull, concealing the fontanelles, and XIII PHYLUM CHORDATA 191 furnishing roofs to the orbits. Immediately behind the frontals is a pair of very small parietals (PA.}, in front of them is an unpaired supra-ethmoid (S. ETH.}, to the sides of which are attached a pair of small nasals (NA.). On the ventral surface is the large para- sphenoid (PA. SPH.), which forms a kind of clamp to the whole cartilaginous skull floor; and in front of and below the parasphenoid is the toothed vomer ( VO.}. Encircling the orbit is a ring of scale- like bones, the sub-orbitals. (Fig. 811, o.}. In the jaws, as in the cranium, we may distinguish between primary and secondary structures. The primary upper jaw or palato-guadrate is homologous with the upper jaw of the Dog-fish, but instead of remaining cartilaginous, it is ossified by five carti- lage bones : the toothed palatine (PAL.) in front, articulating with the olfactory capsule : then the pterygoid (PTG.) on the ventral and the meso-pterygoid (MS. PTG.) on the dorsal edge of the original cartilaginous bar : the quadrate (QU.) at the posterior end of the latter, furnishing a convex condyle for the articulation of the lower jaw : and projecting upwards from the quadrate the meta-pterygoid (MT. PTG.). These bones do not, however, enter into the gape, and do not therefore constitute the actual upper jaw of the adult fish : external to them are two large membrane bones, the premaxilla (PMX.) and the maxilla (MX.}, which together form the actual or secondary upper jaw ; they both bear teeth. A small scale-like bone, the j-v.gal (JU.) is attached to the posterior end of the maxilla, The lower jaw is similarly modified. Articulating with the quadrate is a large bone, the articular (ART.) continued forwards by a narrow pointed rod of cartilage : the latter is the unossified distal end of the primary lower jaw or Meckel's cartilage, the articular is its ossified proximal end, and therefore a cartilage bone. Ensheathing Meckel's cartilage and forming the main part of the secondary lower jaw is a large toothed membrane bone, the dcntary (DNT}, and a small membrane bone, the angular (ANG} is attached to the lower and hinder end of the articular. The connection of the upper jaw with the cranium is effected partly by the articulation of the palatine with the olfactory region, partly by means of a suspensorium formed of two bones separated by a cartilaginous interval : the larger, usually called the liyo- mandibular (HY. M.), articulates with the auditory capsule by the facet already noticed, and the small pointed symplectic (SYM.), fits into a groove in the quadrate. Both bones are attached by fibrous tissue to the quadrate and metapterygoid, and in this way the suspensorium and palato-quadrate together form an inverted arch, freely articulated in front with the olfactory, and behind with the auditory capsule and thus giving rise to an extremely mobile upper jaw. As its name implies, the hyo-mandibular (to- gether with the symplectic) is commonly held to be the upper 192 ZOOLOGY SECT. end of the hyoid arch and the homologue of the hyo-mandibular of Elasmobranchs, but there is some reason for thinking that it really belongs to the mandibular arch, and corresponds with the dorsal and posterior part of the triangular palato-quadrate of Holocephali : a perforation in the latter would convert it into an inverted arch having the same general relations as the upper jaw plus suspensorium of the Trout, but fused, instead of articulated, with the cranium at either extremity. The hyoid cornu is articulated to the cartilaginous interval between the hyo-mandibular and symplectic through the inter- mediation of a small, rod-like bone, the inter-hyal (i. HY.), which perhaps represents the hyo-mandibular of Elasmobranchs. It is ossified by three bones : an epi-hyal (E. HY.) above, then a large cerato-liyal (c. HY.), and below a small double hypo-hyal (H. HY.). The right and left hyoid bars are connected by a keystone-piece, the unpaired, toothed lasi-hyal (B. HY.), which supports the tongue. Connected with the hyo-mandibular and hyoid cornu are certain membrane-bones serving for the support of the operculum. The opercular (Fig. 811, op.) is articulated with a backward process of the hyo-mandibular, the pre-opercular (pra-op) lies outside the posterior border of the hyo-mandibular and quadrate, and clamps them together ; the sub-opercular [sub-op.) is below and internal to the opercular ; and the inter -opercular (int. -op) fits between the lower portions of the three preceding bones, and is attached by ligament to the angle of the mandible. The ten sabre-shaped Immchiostegal rays (brancliiost) are attached along the posterior border of the epi- and cerato-hyal, and below the basi-hyal is an impaired bone, the basi-branchiostegal or uro-Jiyal, There are five branchial arches, diminishing in size from before backwards. The first three present the same segments as in the Dog-fish : pliaryngo-lranchial (PH. BR.) above, then epi-branchial (E. BR.), then a large cerato-branchial (c. BR.), and a small hypo- Imnchial (H. BR.) below. The right and left hypobranchials of each arch are connected by an unpaired basi-branchial (B. BR.). All these segments are ossified by cartilage bones, and the basi- branchials are connected with one another and with the basi-hyal by cartilage, so as to form a median ventral bar in the floor of the pharynx. In the fourth arch the pharyngo-branchial is unossi- fied, and the hypo-branchial absent, and the fifth arch (BR.5) is reduced to a single bone on each side. Small spine-like ossifica- tions are attached in a single or double row along the inner aspect of each of the first four arches : these are the gill-rakers ; they serve as a sieve to prevent the escape of food by the gill-slits. The comparison of this singularly complex skull with the com- paratively simple one of the Dog-fish is much facilitated by the examination of the skull of a young Trout or Salmon. In the XIII PHYLUM CHORDATA 193 Pa.ch, l Jf.Uy Mck Sy FIG. 813. Skull of young Salmon, second week after hatching ; the membrane bones removed. Au. auditory capsule ; Br. 1, first branchial arch ; Ch. notochord ; C. Hy. hyoid cornu ; Fo. fontanelle ; G. Hy. basi-hyal ; H. Hi/, hypo-hyal ; H. J7. hyomandibular ; /. Hy. iuter-hyal ; /i, 1-, labial cartilages ; Mc/c. Meckel's cartilage ; 37. Pt. meta-pterygoid region of primary upper jaw; Pa. ch. parachordal ; PI. Pt. palato- pterygoid region; Qu. quadrate region; S.Or. supra-orbital region of cranium ; Sv. symplectic region of suspensorium ; T. Ci\ cranial roof ; Tr. trabecula ; 77, optic foramen ; V, trigeminal foramen. (From Parker and Bettauy's Morphology of th>. SI- t'll.) latter, at about the second week after hatching, the only ossifica- tions present are S0 r a few membrane bones; when these are removed we get a purely cartilagin- ous skull (Fig. 813), exactly comparable with that of an Elas- mobranch. There is a cranium devoid of cartilage bones and divisible only into regions : the upper jaw is an unossified palato-quadrate (PL PL, M. Pt., Qu.) and the lower jaw (Mck.) a large Meckel's cartilage ; the sus- pensorium is an undivided hvo-man- t, dibular (HM.J, and the hyoid and branchial arches are unsegmented. The first dorsal and the ventral fins are supported each by a triple set of pterygiophores, so that the fin-skeleton is multiserial, as in the Dog-fish. The proxi- mal series consists of slender bony rays the interspinous bones (Fig. 817, PTG. ; Fig. 814, PTG.l), lying in the median plane, between the muscles of the right and left sides, and more numerous than the myomeres of the regions in which they occur. Their distal ends are broadened, and with them are con- nected the second series (PTG.2) in the form of small dice-box shaped bones ; to these, finally, are attached small nodules of cartilage (ptg.3) forming the third series of radials. The dermal fin-rays (D.F.K), which lie in the substance of the fin itself, are slender bones, jointed like the antennas of an Arthro- pod, and mostly branched in the sagittal plane (Fig. 81*1 , D.F.H.). Each is formed of distinct right and left pieces (Fig. 814), in close contact for the most part, but diverging bejow to form a forked and dilated end, which fits over one of the cartilaginous nodules (ptg.3). In the caudal fin (Fig. 810) VOL. II O BF.R FIG. 814. Salmo fario. A dermal fin-ray with its supports. D.F.R. dermal fin-ray ; PTG.l, proximal pterygiophore (inter-spin - ous bone); PTG 2, middle pterygiophore ; ptg.3, dis- tal pterygiophore (cartila- ginous). 194 ZOOLOGY SECT. the dermal rays (D.F.E.) are similarly seated on the broad haemal arches of the posterior caudal vertebrae. The second dorsal or adipose fin has no bony support. The shoulder girdle (Fig. 815), like the skull, consists of a primary shoulder girdle, homologous with that of a Dog-fish, and of several membrane bones. The primary shoulder-girdle in the young Fish is formed of distinct right and left bars of cartilage, which do not unite with one another ventrally. In the adult each bar is ossified by three bones, a scapula (SOP.) situated dorsally to the glenoid facets, and developed partly as a cartilage, partly as a membrane bone ; a coracoid (COR.), situated ventrally to the glenoid facet, and a COR. mesocoracovl (MS. COR.) situ- ated above the coracoid and an- terior to the sca- pula. Externally to these is found a very large membrane bone, the clavicle (CL.), extending down- wards under the throat : its dorsal end is connected by means of a supra-davide (S. CL.} to a forked bone, the post-temporal (P. TM.), one branch of which articu- lates with the epiotic, the other with the pterotic process. To the inner surface of the clavicle are attached two flat scales of bone (P. CL'.), with a slender rod-like post-clavicle (P.CL.) passing backwards and downwards among the muscles. The structure of the pectoral fin is very simple. Articulated to the posterior border of the scapula and coracoid are four dice-box shaped bones, the proximal pterygiophores or radials (PTG.1), followed by a row of small nodules of cartilage (ptg. 2) repre- senting distal pterygiophores. The main body of the fin is supported by dermal fin-rays, which resemble those of the median fins, and have their forked ends seated upon the distal pterygio- phores: the first ray, however, is larger than the rest, and articulates directly with the scapula. FIG. 815. Salmo fario. Left half of shoulder-girdle and pectoral fin, from the inner surface. CL. clavicle ; COR. coracoid ; D. F. R. dermal fin-rays ; MS. COR. meso-coracoid ; P. CL.,P. CL'. i><>st- clavicles ; PTG.1, proximal ; t>t a *i Q* 10 " Z-S -y i 2 U Vi j ^ X O O. >>32 SO - "" ' v: ^ . '- -i ~ - , _ P = X o " o a > . -2 e " J S s .- "^ ^- . ** ** ^^ >^^ r- r*- -^ ~ X - SH a .. o >- xni PHYLUM CHORDATA 197 abdominal cavity is the air-bladder (a. bl.), a thin-walled sac serving as an organ of flotation. Anteriorly its ventral wall presents a small aperture leading, by a short pneumatic dud (pn.d.), into the pharynx. Respiratory Organs.- -There are four pairs of gills each with a double row of branchial filaments united proximally but having their distal ends free : interbranchial septa are practically obsolete (see Fig. 726). The gills are borne on the first four branchial arches, the fifth arch bearing no gill. On the inner surface of the operculum is a comb-like body, the pseudo-hranchia, formed of a single row of branchial filaments, and representing the vestigial gill (hemibranch) of the hyoid arch. Circulatory Organs.- -The heart (Fig. 817) consists of sinus venosus, auricle (au.), and ventricle (v.). There is no conus arteriosus, but the proximal end of the ventral aorta is dilated to form a bulbus aortce (b. a.\ a structure which differs from a conus In being part of the aorta and not of the heart ; its walls do not contain striped muscle, and are not rhythmically contractile. In accordance with the atrophy of the hyoid gill there is no afferent branchial artery to that arch, but a liyoidean artery springs from the ventral end of the first efferent branchial and passes to the pseudobranch. The right branch of the caudal vein Is continued directly into the corresponding cardinal, the left breaks up in the kidney, forming a renal portal system. There are no lateral veins, but the blood from the paired fins is returned to the cardinals. The red blood corpuscles are, as in other fishes, oval nucleated discs. Nervous System.- -The brain (Fig. 818) is very different from that of Elasmobranchs, and is in many respects of a distinctly lower type. The cerebellum (H. H.) is very large, and bent upon itself. The optic lobes (M. H.) are also of great size, and corresponding with them on the ventral surface are large bean-shaped lobi inferiores ( U. Z.). The diencephalon is much reduced, and, indeed, Is indicated dorsally only as the place of origin of the pineal body (G-.p.): ventrally it is produced into the lobi inferiores with the infundib ulum between them giving attachment to the pituitary body (Hy. p.). Hence, seen from above, the small undivided prosencephalon (V.H.) comes immediately in front of the mid-brain : it has a non-nervous roof or pallium (Pall.) and its floor is raised into prominent corpora striata (JB. G.,Bas. dr.). The olfactory lobes (Z. ol.) are nearly as large as the corpora striata, and each contains a small cavity or rhinocoele in communication with the undivided prosoccele. Three transverse bands of fibres connect the right and left halves of the fore-brain, an anterior commissure joining the corpora striata, a posterior commissure situated just behind the origin of the pineal body, and an inferior commissure in front of the infundibulum. The pineal body (b.p.) 198 ZOOLOGY SECT. is rounded and placed at the end of a hollow stalk : a shorter offshoot of the roof of the diencephalon may perhaps represent^ a rudimentary pineal eye. Behind the pituitary body is a saccus- L.o'l B YJT ~\m L.ol. if?* FIG. 818. Salmo fario. Dorsal (A), ventral (B), and lateral (C) views of brain. EG, or ax G. corpora striata ; '/,, crossing of optic nerves ; G. j>, pineal body; HH. cerebellum; Jf>/j>. pituitary body ; I, if. infunclibulum ; L. ol. olfactory lobes ; M, t >t. optic tracts ; U, L, lobi inferiores ; VII, prosencephalon ; 7 A", cerebral nerves ; XII. 1. tirst spinal (hypoglossal). nerve ; 2, second spinal nerve. (From Wiedersheim's /" ,-t,-. iris; J. lens; opt. n. optic nerve ; p.) wind- ing round the yellow yolk, and kept in close contact with it by the enclosing zona radiata. There is no open medullary groove, the nervous system being formed, as in Cvclo- i/ stomes, from a fold of ectoderm, the walls of which are in appo- sition. Gradually the head and tail become free from the yolk, and .at the time of hatching the yolk-sac (I, y, s.) is a shoe-shaped body sessile upon the ventral surface of the transparent embryo. errib em. T/.S FIG. 822. Nine stages in the development of Salmo fario. A H, before hatching ; I, shortly after hatch- ing, bl. blastoderm ; emb. embryo ; ?, thickened edge of blastoderm ; ys. yolk-sac. (A G after Henneguy.) 2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. The Teleostomi are Pisces in which the primary cranium is always complicated by the addition of membrane bones, of which a pair of parietals and one of frontals above, and unpaired vomer and parasphenoicl below, are the most constant. The chondro- cranium is always more or less ossified by cartilage bones, and the upper and lower jaws are both bounded by membrane bones. The 202 ZOOLOGY SECT, jaws are connected with the cranium through the intermediation of a hyomandibular, which is probably not homologous with the similarly named element of Elasmobranchs. The dermal fin- rays are formed of membrane bone, and are supported by pterygiophores which may be either cartilaginous or bony, but which always show a great reduction in number as compared with the homologous structures in Elasmobranchs. The primary shoulder-girdle is complicated by the addition of membrane bones, of which a large clavicle is the most constant. The pelvic girdle is vestigial or absent. The pelvic fins usually undergo a forward displacement, their position being either abdo- minal, i.e. between the anus and the pectoral region, or thoracic, i.e.. in the pectoral region, or jugular, i.e. under the throat. A dermal exoskeleton is usually present. The intestine may or may not have a spiral valve : the anus is distinct from, and placed in front of, the urinary and genital apertures. The gills are covered by an operculum supported by membrane bones, and the interbranchial septa are reduced or absent, so that the gill-filaments are partially or wholly free ; the hyoidean gill is reduced or absent. The conus arteriosus is sometimes present, sometimes absent ; when absent there is a large bulbu s aortse formed as a dilatation of the ventral aorta. The prosencephalon has a non-nervous roof; the optic nerves either form a chiasma or simply decussate. The ova are small : the gonoducts are either continuous with the gonads, or open anteriorly into the ccelome, or are absent : in the latter case the sexual products pass out by genital pores; true abdominal pores may be present in addition. Segmentation of the egg is either entire or discoidal : development is sometimes accompanied by a metamorphosis. The Teleostomi are classified as follows :- ORDER 1. CROSSOPTERYGII. Teleostomi in which the pectoral fin consists of a rounded basal lobe supported by endoskeletal structures and fringed by dermal no. bf. m, FIG. 823. Polypterus bichir. A, entire animal ; B, ventral view of throat, "/'.anus; In: nt. branohiostegal membrane; c./. caudal tin; <'t..f. pectoral fin; j><: /. pelvie fin ; v. /. ventral fin. (After ( 'uvier.) -" *{f B Juff.pl rays. There are no branehiostegal rays. The vertebral column is well ossified, and the caudal fin is diphycercal. The pelvic fins are XIII PHYLUM CHORDATA 203 abdominal. A spiral valve and a conns arteriosus are present, and the optic nerves form a chiasma. The only existing members of this order are Polyptcrus Incliir (Fig. 823), from the Upper Nile, and Calamoichtliys calabaricus from Old Calabar. ORDER 2. CHONDROSTEI. Teleostomi in which the paired fins have no basal lobe, but their whole free portion is supported by dermal rays. There are few FIG. 824. Acipenser ruthenus (Sturgeon). I. barbels; c.f. caudal fin ; *;./.), when their position is called XIII PHYLUM CHORDATA 211 thoracic, or on the throat (Fig. 828), when they are said to be jugular in position. A very remarkable deviation from the typical form occurs in the Flat-fishes (Pleuronectidce), a family of Anacarithini. The body (Fig. 833) is very deep and strongly compressed: the fish habitually rests on the bottom, in some species on the right, in others on the left side, partly covering itself with sand, and occasionally swim ming with a curious undulating movement. The under side is usually pure white, the upper side dark. The eyes (r.e, I.e.) are both on the upper or dark-coloured side, and the skull is distorted so as to adapt the orbits to this change of position. The abdominal cavity is very small, the vent placed far forward, and the dorsal le. tFio. 833. Pleuronectes cynoglossus (Craig-fluke), from the right side. and yellows, spots or stripes of gold or silver, are common, and, although the combination of tints may sometimes seem to our p 2 212 ZOOLOGY SECT. eye rather crude and glaring, they appear to be distinctly pro- tective, harmonising with the brilliant hues of the Coral Polypes and other members of the reef fauna. Pelagic fishes, such as the Mackerel and Herring, are usually steely-blue above, white beneath. Many deep-sea Teleostei are phosphorescent : in some of these definite luminous organs (Fig. 834) are arranged in longitudinal FIG. 834. Stomias boa. The white dots are the luminous organs. (From Hickson., after Filhol.) rows along the body, each provided with a lens, like that of the eye, the whole organ having thus the characters of a minute bull's-eye lantern. Some species of the same order, such as the Weaver (Trachinus), possess poison- glands, opening either on one of the dorsal spines, or on a spinous process of the operculum, or, as in the Cat-fishes (Siluridse), on the spine of the pectoral fin. Exoskeleton. In many Teleostomi, such as Polyodon and the Eels, the skin is devoid of hard parts, but in most cases a dermal exoskeleton is present. In Amia and in the majority of Teleostei this takes the form, as in the Trout, of scales, rounded plates of bone imbedded in pouches of the derm and overlapping one another from behind for- wards. When the free border A ^gsnz B of the scales presents an even curve, as in Amia and most Physostomi and Anacanthini, they are called cycloid scales (Fig. 808) ; when, as in most Acanthopteri, the free edge is produced into small spines (Fig. 835, A) they are dis- tinguished as ctenoid scales. In exceptional cases the scales may be so large and strong as to form a rigid armour. In the Sturgeon (Fig. 824) there is a strong armour, formed of stout bony plates, or scutes, i'i<;. 635. A, ctenoid scale; B, ganoid (After Giinther.) XIII PHYLUM CHORDATA 213 produced into enamelled spines and articulating with one another by suture. Scutes are also found in many Siluroids (Fig. 827) and in Lophobranchii (Fig. 832) and some Plectognathi (Fig. 831), while in the Plectognathi the exoskeleton takes the form, as in the File-fishes, of minute spines like the shagreen of Sharks, or, as in many Globe-fishes, of long, outstanding, bony spines. Lastly, in Polypterus and Lepidosteus are found rhomboid or ganoid scales (Fig. 835, B), in the form of thick, close-set, rhomboidal plates formed of bone, covered externally by a layer of enamel or ganoin, and joined together by pegs and sockets. In many Ganoids the anterior fin-rays of both median and paired fins bear a row of spine-like scales called fulcra (Fig. 825,/.). Endoskeleton. In the Sturgeon the vertebral column (Fig. 837 WS.) consists of a persistent notochord with cartilaginous arches FIG. 630. Anterior end of vertebral column of Polypterus. PS. parasphenoid ; R. IV } dorsal ribs ; W~K, centra ; t, ventral ribs. (From Wiedersheim's Comparative Anatomy.) and is fused anteriorly with the cranium. In the remaining orders bony vertebrae are present ; the centra are biconcave, except in some Eels, in which the anterior face is flat or even convex, and in Lepidosteus, in which the anterior face is distinctly convex. Vertebrae of this form, i.e. having the centrum convex in front and concave behind, are called opistlwcozlous. Bibs are usually present: in Polypterus each vertebra has two pairs, a dorsal pair (Fig. 836, R, I- -T 7 ') of considerable length, running between the dorsal and ventral muscles, and a short ventral pair (f ) between the muscles and the peritoneum : the former answer to the ribs of Elasmo- branchs, the latter to the ribs of the remaining Teleostomi, which 214 ZOOLOGY SECT, are always placed immediately beneath the peritoneum. There may be one or more sets of intermuscular bones, attached either to the neural arch (epineurals), to the centrum (epicentrals), or to the ribs (epipleurals). The posterior end of the vertebral column is turned up in the Sturgeons, Lepidosteus, and Amia, resulting in a heterocercal tail -fin : in Amia, however, the fin-rays are so disposed that the fin appears almost symmetrical. Among Teleostei the tail-fin is rarely as obviously unsymmetrical as in the Trout : usually in the adult the development of the large, fan- shaped, posterior hsemal arches completely hides the upturned end of the notochord, and in some cases the spinal column ends simply in a somewhat compressed centrum around which the fin-rays FIG 837. Skull of Sturgeon, with the membrane bones removed. . pharyngo-branehial : 4F antorbital process ; AR. articular ; b. epibranchial ; c. cerato-branchial ; 'C, notochord ; Cop. basi-branchials ; d, hypobranchial ; DC. deiitary ; GK, auditory capsule ; H3L hyomandibular : h;i hyoid cornu ; Ih. inter-hyal ; 3I<>. mandible; Na. nasal capsule; Gb, neural arches PF. post-orbital process ; PQ. palato-quadrate ; Ps. Ps'. Ps". parasphenoid ; Psp. neural spines : Qu quadrate ; R. rostrum ; Rl. ribs ; Sp. N. foramina for spinal nerves ; Sy. sympleetic ; If to, vertebral column; n, vagus foramen; IV, branchial arches. (From Wiedersheuns 1'iardtire Anatomy.) are symmetrically disposed ; such truly symmetrical tail-fins are called diphycercal. In the structure of the skull the Chondrostei make the nearest approach to Elasmobranchs. The cranium (Fig. 837) is an un- divided mass of cartilage with a few isolated cartilage bones. The roofing membrane bones lie in the derrnis, so as to be practi- cally superficial, and behind pass insensibly into the scutes covering the trunk : the fact that these bones (parietals, frontals, &c.) are exoskeletal structures is here perfectly obvious. The same is the case in Polypterus (Fig. 838), in which, however, the cartilage bones are better developed. In Lepidosteus and Amia, and especially the latter, the skull resembles that of the Trout in all essential respects, the main differences consisting in the absence of certain bones, such as the supra-occipital, and in the presence of additional membrane bones. Among Teleostei it is only in the Physostomi that the membrane bones remain separable XIII PHYLUM CHORDATA 215 from the chondrocranium in the adult ; in the remaining orders, e.g. in the Cod, Haddock, or Perch, they become grafted on to the chondrocranium and so closely united with the cartilage bones that they can be removed only by pulling the whole skull to pieces : most of the original cartilage frequently disappears in the adult, and the cranium thus be- comes a firm bony mass in which no distinction be- tween cartilage and mem- brane bones is discernible. The varying size of the i i gape, which is so noticeable a feature in the Teleostomi depends upon the inclina- tion of the suspensorium ; in wide-mouthed Fishes (Fig. 828) the axis of the hyomandibular and suspen- sorium is nearly vertical or even inclined backwards : small-mouthed forms in (Fig. 831) it is strongly in- clined forwards and the length of the jaws is pro- portionately reduced. In the branchial arches the pharyngo-branchials of each side are very commonly fused, and constitute what are called the superior pkaryngeal bones: the re- duced fifth branchial bars, or inferior pliaryngeal bones, bite against them. The Pharyngognathi are dis- tinguished by having the inferior pharyngeal bones united into a single bony mass of characteristic form (Fig. 830, B). The _ gill- rakers are often very highly . . developed, and may form a mesh capable of microscopic organisms. In the shoulder-girdle, as in the skull, the Chondrostei approach the Elasmobranchs. There is a primary shoulder-girdle sisting of large paired cartilages, not united in the middle ve line, and unossified : each is covered externally by a large FlG- S38. Skull of Polypterus, from above. t. frontal ; M. maxilla ; XA. nasal ; Sn. nostril ; Op. opercular ; Orb. orbit P. parietal. The remaining letters point to ISss important membrane bones. The arrow is passed into the spiracle. (From Wiedersheim's Coniparatio. A ,>' ZOOLOGY SECT. *(* like membrane-bone, the clavicle. In the remaining Ganoids and in Teleostei, the primary shoulder- girdle is reduced in size and is usually ossified by two bones, a ** */ dorsal scapula and a ventral coro- coid : sometimes, as in the Trout, there may be an additional ossifica- tion, the meso-coracoid. Additional membrane bones supra-clavicle, post-clavicle, &c. are added, and one of them, the post- temporal, serves to articulate the shoulder- girdle with the skull (Fig. 815). In the skeleton of the pectoral fin it is the Crossopterygii which ap- proach most nearly to Elasmobranchs. In Polypterus (Fig. 839) the basal lobe of the fin is supported by a rod-like ossified propterygium (Pr), a broad cartilaginous mesoptery- gium (MS), and an ossified meta- pterygium (MT) : to these, two rows of elongated radials (Ra, Ra l ) are articulated fan-wise, and these in their turn give attachment to the fin-rays (F$). In all the re- maining orders the basalia (pro-, meso-, and metapterygium) are absent, and the endoskeleton of the fin consists only of a single or double row of radials (Fig. 815). In Polypterus there is a vestigial pelvic girdle (Fig. 839 Us, BP) in the form of a small rhomboidal cartilage to which the anterior ends of the basalia (Bets 1 ) are at- tached : thus in the structure of the posterior extremities also, the 'Crossopterygii are the most primi- tive of the Teleostomi In all the remaining orders the pelvic girdle is atrophied. The pehic fin is sup- port, -d by a single bone of variable form (Fig. 816, BSTG) and re- presenting a lasale, i.e. a structure arising from the fusion of proxi- mal pterygiophores. Between its posterior end and the dermal rays irregular nodules, representing- radials, may be interposed. \. -^''.Pectoral fill of Polypterus. FS. dermal rays; MS. mesoptery- gium ; MT. metapterygium ; NL, nerve-foramina ; Ox*, ossification in mesopterygium ; Pr. propterygium ; R". rh-st radials; A"', second radials. (From Wiedersheim's C'oiitpa,-ui;.--> Anatomy.) BP FIG. 839 6z.-^Pelvic fin of young Poly- pterus. A,>. ].art of basale ; 3oi. basale ; BP. pelvic cartilages (fused in adult); Cep. epipubis ; Hn,i. radial-. (From \Vicdcrsheiui.) XIII PHYLUM CHORDATA 217 The distinction between hard or unjointed fin-rays, or spines, and soft or jointed fin-rays has already been referred to. The first ray^of the dorsal and pectoral fins sometimes, e.g. in Siluroids (Fig. 827), has the form of a very strong spine articulated by a bolt-and-shackle joint, i.e. by the interlocking of two rings. In some cases the first dorsal spine springs from the skull. The texture of the bones is subject to wide variation : in some Acanthopteri they are very thick and strong, in some places almost like ivory, while in the Lump-fish (Ci/dopterus}, the hug-e O 12 1 //I J.7 " ounnsh (Ortlw.goms- cus\ and in many deep-sea forms, such as the Ribbon-fishes (Regalecus and Trac- hypterus), the amount of mineral matter is so small that the bones are easily cut with a knife and weigh astonishingly little when dry. Electric organs. Two genera of Teleo- stomi possess electric organs, the Electric Cat-fish (Malapter- urv.s), one of the Sil- uridae, found in the fresh waters of tropi- cal Africa, and the Electric Eel (Gym- iiotns), a Physostome occurring in Brazil and the Guyanas. In Malapterurus the electric organ ex- tends over the whole body, beneath the skin ; in Gymnotus (Fig. 840) there are two pairs of batteries in the ventral half of the greatly elongated tail. Digestive organs. Some Teleostomi are toothless ; but in most instances teeth are present, and may be developed on the premaxilla, maxilla, palatine, pterygoid, vomer, dentary, basi- hyal, and superior and inferior pharyngeal bones. It is character- istic of most Teleostei, with the exception of Physostomi, that the maxilla is edentulous (Fig. 829) and does not enter into the gape. IT.... ,. S40. Gymnotas electricus, showing the extent of the electric organ (E). Fl, ventral fin. B, small portion of tail, in section. DM. DM.' dorsal muscles ; E. E'. electric- organ ; Fl, ventral fin ; H, skin ; LH, caudal canal ; Sep. fibrous septum; VM. VM'. ventral muscles; WS, WS', vertebral column, with spinal nerves. (From Wiedersheim's Comparative Anatomy.) 218 ZOOLOGY SECT. Fir:. 841. Premaxillpe of Sargus, showing teeth. (After Owen.) In a large majority of species the teeth are small, conical, and recurved, suitable for preventing the struggling prey from slipping out of the mouth, but quite unfitted for either tearing or crushing. In some Fishes, such as the Pike, the teeth are hinged backwards so as to offer no resistance to the passage of the prey towards the gullet, but effectually barring any movement in the other direc- tion. In many deep-sea Fishes (Fig. 834) the teeth are of immense size and constitute a very formidable armature to the jaws. Many instances occur in which there is a marked differentiation of the teeth, those in the front of the jaws (Fig. 841) being pointed or chisel-edged, and adapted for seizing, while the back teeth have spherical surfaces adapted for crushing. In the Wrasses (Fig. 830, B) strong crushing teeth are developed on the pharyngeal bones. In the Globe-fishes the teeth are apparently reduced to one or two in each jaw, but each " tooth in this case really consists of numer- ous calcified plates fused together. The teeth may be either simply imbedded in the mucous mem- brane so as to be detached when the bones are macerated or boiled, or they may be implanted in sockets of the bone, or ankylosed to it. They are formed of some variety of dentine, and are often capped with enamel. Their succession is perpetual, i.e. injured or worn-out teeth are replaced at all ages. In some species the enteric canal shows little differentiation into regions, but, as a rule, gullet, stomach, duodenum, ileum, and rectum are more or less clearly distinguishable. The stomach is generally V-shaped, but its cardiac region may be prolonged into a blind pouch : it is often very distensible, allowing some of the deep-sea Teleostei to swallow Fishes as large as themselves. In the Globe-fishes the animal can inflate the gullet with air, when it floats upside down on the surface of the water. The Ganoids have a s/>/W valve in the intestine, which is very well developed in Polypterus and the Sturgeon, vestigial in Lepidosteus (Fig. 843, y. r.) and Amia : it is absent in all Teleostei, except possibly in Chirocentrus, one of the Physostomi. The liver is usually large ; a pancreas may be present as a compact gland, as in Elasmobranchs, or may be widely diffused between the layers of the mesentery. Pyloric cceca are commonly present, and vary in number from a single one to two hundred. The anus is always distinct from, and in front of, the urino-genital aperture. Respiratory organs.- -The gills are usually comb-like, as in the Trout, the branchial filaments being free, owing to the atrophy XIII PHYLUM CHORDATA 219 of the interbranchial septa. In the Sturgeon, however, the septa are fairly well developed, reaching half-way up the filaments, so that the latter are free only in their distal portions ; this arrange- ment is obviously intermediate between the Elasmobranch and Teleostean conditions. The most striking deviation from the normal structure occurs in Lophobranchii, in which the gill- filaments are replaced by curious tufted processes (Fig. 832. B y.). As a rule gills (holobranchs) are developed on the first four branchial arches, but the fourth is frequently reduced to a hemi- branch, and further reduction takes place in some cases. The FIG. 842. A. Anabas scandens (Climbing Perch). B, dissection of head, showing accessory respiratory organ. (A, after Cuvier ; B, after Giinther.) pseudobranch or vestigial hyoidean gill may either retain the characteristic comb-like structure, as in the Trout, or may be reduced, as in the Cod, to a gland-like organ formed of a plexus of blood vessels and called a vaso- ganglion or rcte mirabile. In addition to the gills some Teleostei possess accessory organs of respiration. In Amphipnous, an Indian Physostome, the gills are poorly developed and are functionally replaced by a vascular sac occurring on each side of the body and opening in front into the first (hyo-branchial) gill-cleft. Such sacs are physiologically, though not morphologically, lungs. In the Climbing Perch (Anabas) of the Oriental Region (Fig. 842) the superior pharyngeal bones are developed into folded plates (B) covered with vascular 220 ZOOLOGY 8ECT. --fr.lt 1 - st lr-~\ .-(Z.b mucous membrane and capable of retaining water for a consider- able period : the Fish is able to traverse the land, and is even said to climb trees, holding on alternately by the spines of its pre-operculum and of its ventral fins. It has become so thoroughly a land animal that it is drowned if immersed in water. In the little armoured Siluroid Callichthys, anal respiration takes place, air being drawn into and expelled from the rectum. And, lastly, in the curious little goggle- eyed Periophthcdmus of the Indian and Pacific Oceans the tail-fin seems to serve as a respiratory organ, being kept in the water while the Fish perches on a rock. The air-bladder retains its connec- tion with the gullet in Ganoids and Physostomes ; in the other Teleostei the pneumatic duct atrophies in the adult, and the bladder becomes a shut sac. The pneumatic duct is always connected with she dorsal wall of the gullet except in Polypterus, in which the aperture is ventral, and in some Physostomes, such as the Herring, in which it is connected with the stomach. The bladder is sometimes divided into compartments or produced into lateral offshoots : in Amia, Lepidosteus (Fig. 843, . 5.), and Polypterus its wall is sacculated or raised into anastomosing ridges, enclosing more or less well- marked chambers and thus resembling a lung. In Polypterus its lung-like character is enhanced by its division into two compartments by a longitu- dinal partition, as well as by the ven- tral position of the opening of the pneumatic duct. The air-bladder seems able to act as a sort of accessory respiratory organ ; it has been found that in a Perch, asphyxiated in stagnant water, the oxygen in the bladder, which normally amounts to 20 or 25 per cent., is entirely absorbed and replaced by nitrogen and carbonic acid. Its normal I FIG. 43. Digestive organs and air- bladder of Lepidosteus. . anus ; <>. />. air-bladder; a.b'. its aperture in the pharynx ; l>. ,/. aperture of bile-duct; c. pyloric cteca ; (i. b. gall-bladder ; lr/i. >i. hepatic duct ; l;\ liver ; pi/, pylo- ric A'alve ; s. spleen ; sp. v. spiral valve ; *t. stomach. (FromWieder- sheiia'.s Comparatiice Anatomy.) XIII PHYLUM CHORDATA 221 function, however, is hydrostatic, i.e. it serves to keep the Fish of the same specific gravity as the water. Variations in pressure as the Fish ascends or descends are regulated by absorption or secretion of gas, often by means of vase-ganglia or red glands (Fig. 844, cs. gn. ) in the walls. These are elevations of the wall of the bladder, abundantly supplied with blood, and having tubular glands which open into the cavity of the bladder and secrete a fluid of unknown function. In Fishes with a pneumatic duct the red jjlands an- opt. I FIG. 844. Horizontal section of posterior portion of head and anterior end of air-bladder in Pseudophycis bachus. one of the Gadidse or Cods (semi-diagrammatic), a, thickened portion of air-bladder fitting into fenestra in posterior wall of auditory capsule ; a. bl. air- bladder ; au. cp. outer wall of auditory capsule; an. cp.' inner (membranous) wall; l>. hollow offshoots of air-bladder ; cp. str. corpora striata ; crb. cerebellum ; mcmb. lab. mem- branous labyrinth ; olf. 1. olfactory lobes ; olj. p. olfactory peduncles ; op. operculum ; opt. I. optic lobes ; cs. gn. vaso-ganglia. absent, but in Eels their place is taken by red bodies of similar appear- ance but with non-glandular epithelium. In some forms with closed air-bladder the anterior end of the organ is forked, and each branch (a) fits closely against a membranous space in the posterior wall of the auditory capsule, while laterally it extends outwards in the region of the shoulder-girdle, and comes to lie immediately beneath the skin ; in this way varying pressures on the surface of the body are transmitted through the air in the bladder to the auditory 222 ZOOLOGY SECT. jors organ. In the Carps and Siluroids a chain of bones connects the air-bladder with the auditory organ, forming the Weberian apparatus, the function of which, as of the simpler arrangement described above, is probably " to bring directly to the consciousness of the Fish the varying tensions of the gaseous contents of the air- bladder, due to the incidence of varying hydrostatic pressures." The structure of the heart forms one of the most striking differences between the three Ganoid orders and the Teleostei. In Ganoids there is a muscular conus arteriosus with rows of valves, as in Elasmobranchs ; in Teleostei a vestige of the conus containing two rows of valves has been found in Albula, one of the Herring family, but in no other member of the order. On the other hand, Teleostei always have a large bulbus aorta?, formed as a dilatation <>f the base of the ventral aorta. In the brain the cerebellum and optic lobes are large ; the diencephalon is well developed in Ganoids, almost obsolete in Tele- ostei. In Ganoids there is an unpaired prosencephalon, which may be produced into lobes (Fig. 845, prs.) and has a non- nervous roof, giving off anteriorly a pair of cerebral hemispheres (cJi.) into which the prosoccele is continued as a pair of lateral ventricles or paracoeles ; thus the fore-brain of Ganoids presents many re- semblances to that of the Lamprey. In Teleostei (Fig. 818) there are no cerebral hemispheres, but only an undivided pro- sencephalon with a non-nervous roof or pallium, and with its floor raised into large rounded corpora striata. The Ganoids agree with Elasmobranchs in the fact that the optic nerves form a chiasma, while in Teleostei they simply cross one another or decussate. Here also, however, the dis- tinction is not quite absolute, since in the Herring and some other Physostomes one nerve passes through a slit in the other. In some cases the olfactory lobes spring directly from the prosencephalon, as in the Trout ; in others they are borne on long olfactory peduncles, (Fig. 844, olf. p.], as in the Cod. In some Plectognaths the spinal cord undergoes a re- markable shortening : in a Sun-fish 2J metres in length and weighing a ton and a half, the cord is only 15 millimetres long, being actually shorter than the brain. Urino-genital Organs. The Icidncy (Fig. 817, M) is formed from the inesonephros of the embryo and usually attains a great cbl 771.O Fio. 845. Brain of Lepi dosteus. dorsal view. cl>/ . cerebellum ; c. h. cere- bral hemispheres'; blongata ; olf. I. olfactory lobes ; opt. I. optic lobes ; />/*. lobes of prosence- phalon. (After Balfour and Parker.) XIII PHYLUM CHORDATA 223 size l.c ur- the pronephros usually atrophies. The ureter (ur.) is the undivided segmental duct : it unites with its fellow of the opposite side before opening either directly on to the exterior or into a urine-genital sinus. A urinary bladder is formed as a single or double dilatation of the ureter. The right and left kidneys undergo more or less fusion, and their anterior ends are usually converted into adenoid or lymphatic tissue (ltd'.), so that, while resembling the rest of the organ in ex- ternal appearance, they do not discharge a renal function. The male organs of Lepidosteus may be taken as an example of those of Ganoids. The testis (Fig. 846, ts.) is a paired lobulated organ, the secretion of which is carried by a large number of vasa efferentia (v. ef.) into a longitudinal canal (I. c.) lying alongside the ureter (ur.). From this canal tubes are given off which communicate with the urinary tubules of the kidney, so that the seminal fluid has to traverse these tubules in order to reach the urinary bladder (bl.) and make its escape by the common urinogenital aperture (u.g. ap). In Teleostei there are no vasa efferentia, but the posterior end of the testis is directly continued into a duct (Fig. 817, v. d.) which unites with its fellow of the opposite side and opens either into a urino-genital sinus, as in the Trout, or, as in the Cod, directly on to the exterior, between the anus and the urinary aper- ture. In the Eels the seminal fluid escapes into the coelome and is dis- charged by genital pores. In most Ganoids the oviducts (Fig. 847, B, ovd.) have funnel-like anterior ends (ovd.") opening into the coelome, while posteriorly (ovd/) they discharge into the dilated ureters (bl.). A similar arrangement occurs in the Smelt, one of the Phy sostomi, in which the eggs are discharged from the outer or lateral face of the ovary into the open end of the oviduct. But in most Teleostei and in Lepidosteus (Fig. 847, A) the ovary (ovy.) is a hollow sac continued posteriorly into the oviduct (ovd.) : the eggs are set free into its cavity from the folds into which its inner surface is produced, and FIG. 84(3. Male organs of Lepi dosteus. bl. bladder ; I. c. longitudinal canal ; ts. testis ; u.g. ap. urine-genital aperture ; ur. ureter ; r. ef. vasa efferentia. (After Balfour and Parker.) 224 ZOOLOGY SECT. 3 Ad ur so pass directly into the oviduct without previously entering the ccelome. An ovary of this kind reminds us of the state of things in Arthropods, in which also the ovary is a hollow organ discharg- ing its products into its internal cavity, whence they pass directly into the continuous oviduct. It was pointed out that the lumen of the ovary in this case was to be looked upon as a shut-off portion of the coelome : this is certainly the case v in Lepidosteus and Teleostei. In the embryo a longitu- dinal fold grows from the ventral edge of the then solid ovary, and turns upwards along the lateral face of the organ : it is met by a descending fold of peritoneum from the dorsal wall of the abdomen, and by the union of the two . folds a cavity is enclosed, which is the lumen of the ovary. The oviduct is developed as a backward continua- tion of these folds of peritoneum, and appears to be quite unconnected with the embryonic ne- phridial system, and therefore not to be homologous with the oviducts of Elasmobranchs and Holocephali, which, as we have seen, are Miillerian ducts. In the Salmonida? and the Eels oviducts are absent, and the ova are discharged by genital pores, which are probably to be looked upon as degenerate oviducts. True abdominal pores are present in Ganoids and in some Physostomi. Most Teleostomi are dioecious, but Serranus, one of the Perch family, is hermaphrodite and self-impregnating, ovcL'~ FIG. 847. Female organs of Lepidosteus (A) and Amia (B). a, degenerate anterior portion of kidney ; W. bladder ; M. kidney ; ovd. oviduct ; onl. 1 aperture of oviduct into bladder ; ovd." peritoneal aperture ; ory. ovary ; />. peritoneum ; u.g. ap. urine-genital aperture ; ur. ureter. (A, after Balfour and Parker ; B, after Huxley.) xin PHYLUM CHORDATA 225 arid there are many well-known species, such as the Cod and the Herring, which exhibit the hermaphrodite condition as an occa- sional variation. Reproduction and Development. Most Teleostomi are oviparous, the eggs being impregnated after they are laid, but in some Teleostei, such as the Viviparous Blenny (Zoarces), internal impregnation takes place ; the young are developed in the hollow ovary and are brought forth alive. Many instances of parental care of the young are known, the most familiar being that of the male Stickleback (Gasterosteus), which constructs a nest of weeds, fastened together by a glutinous secretion of the kidneys, and jealously guards the developing young. In the Sea-horse (Hippo- campus) and the Pipe-fish (Syngnatlms) the young are developed in a pouch (Fig. 832, brd. p.) on the abdomen of the male. In the Siluroid Aspredo the eggs are pressed into the soft spongy skin of the belly and thus carried about by the parent. The ova are always small as compared with those of Elasmobranchs, never exceeding 5 to 10 mm. in diameter, and being usually much smaller. They are rarely protected by an egg-shell. They are produced in immense numbers, a single female sometimes laying several millions : in such cases the mortality among the unprotected embryos and young is immense. The eggs may be pelagic, i.e. so light as to float when laid, as in the Cod, Haddock, Turbot, Sole, &c., or demersal, i.e. so heavy as to sink to the bottom, as in the Herring, Salmon, Trout, &c. In some cases they become cemented to the surface of a rock. In all the Ganoids hitherto investigated segmentation is com- plete, but very unequal (Fig. 848) : the megameres are immense as compared with the micromeres, and the process may be said to be inter- mediate between the holoblastic and meroblastic types. In Teleostei, on the other hand, segmentation is al- ways partial and discoidal. The general features of development are much the same as in the Trout, except that in the Sturgeon there is an open medul- lary groove. There is frequently a metamorphosis : in Lepidosteus, for in- stance, the newly hatched young 1 is j j ..i J T . T J 1 . 1 FIG. 848. Segmentation in Lepi- provided with a sucking-disc, and the dosteus. (After Baifom- and proportions of the head are quite dif- ferent from those of the adult. In the larval Sturgeon provisional teeth are present, and in many Teleostei the young differ from the adult in the presence of large spines, which probably, like the spines in the zosea-stage of some Crustacea, serve a defensive purpose. The larvae of Eels are VOL. II Q 226 ZOOLOGY SECT. strongly compressed, perfectly transparent, and have colourless blood. They are sometimes known as " Glass-fish/' and were formerly placed in the genus Leptocephalus, their real nature being unknown. The Crossopterygii (or at least Polypterus) are unique Fio."S49. Polypterus bichir. Head of advanced larva ; E. G. external gill. (From Dean, after Steindachner.) in the sub-class in possessing, on each side, a single external gill, as in Dipnoi and Amphibia (vide infra). The Geographical Distribution of the Ganoid Teleostomi is curiously limited : they are all essentially fresh-water forms- although some Sturgeons are found in the sea and are almost exclusively inhabitants of the Northern Hemisphere, and especially of the Holarctic Region. The Chondrostei occur in the rivers of Europe, Asia, and North America: one genus of Sturgeons (Scaphirhynchus) lives in the Mississippi and in the rivers of Central Asia, but not in the intermediate regions: in the same way Polyodon is found only in the Mississippi, while the closely- allied Fsephurus is found in