The mouth is supported by definite jaws.

ICHTHYOPSIDA.

The epiblastic exoskeleton is generally unimportant, the mesoblastic exoskeleton is usually well developed.

The notochord with its membranous sheath (1) may remain unmodified, or (2) may be replaced by bone or cartilage derived from the skeletogenous layer, or (3) may be calcified to a varying extent.

The first vertebra is not homologous throughout the whole series and so is not strictly comparable to the atlas of Sauropsids and Mammals.

The centra of the vertebrae have no epiphyses. The skull may be (a) incomplete and membranous, or (b) more or less cartilaginous, or (c) bony. Membrane bones are not included in the cranial walls, and there are large unossified tracts in the skull. When membrane bones are developed in connection with the skull, a large parasphenoid occurs. The basisphenoid is always small or absent. The skull may be immovably fixed to the vertebral column, or may articulate with it by a single or double occipital condyle. When the occipital condyle is double, it is formed by the exoccipitals, and the basi-occipital is small or unossified. The mandible may be (a) cartilaginous, (b) partially ossified, or (c) membrane bones may be developed in connection with it,—if so, there is usually more than one membrane bone developed in connection with each half.

There are at least four pairs of branchial arches present during development. The sternum, if present, is not costal in origin.

Class I. Pisces.

The exoskeleton is in the form of scales, which may be entirely mesoblastic or dermal in origin (e.g. cycloid and ctenoid scales), or may be formed of both mesoblast and epiblast (e.g. placoid and ganoid scales). Large bony plates may be derived from both these types of scale. In general fish with a greatly developed dermal armour have the endoskeleton poorly developed; and the converse also holds good.

The integument of the dorsal and ventral surfaces is commonly prolonged into longitudinal unpaired fins, supported by an internal skeleton. These fins are distinguished according to their position as dorsal, caudal and anal fins. The dorsal and anal fins are used chiefly as directing organs, the caudal fin is however a most important organ of propulsion.

Three types of tail are found in fishes, viz.:—

1. The diphycercal, in which the axis is straight and the tail is one-bladed and symmetrical, an equal proportion of radiale[29] being attached to the upper and lower surfaces of the axis.

2. The heterocercal, in which the tail is asymmetrical and the axis is bent upwards, the proportion of radiale or of fin-rays attached to its upper surface being much smaller than that attached to its lower surface.

3. The homocercal, in which the tail though externally symmetrical, so far resembling the diphycercal type, is internally really heterocercal, the great majority of the radiale or of the fin-rays being attached to the lower surface of the axis.

The cranium in the simplest cases (e.g. Selachii) forms a cartilaginous box enclosing the brain and sense organs; in bony fishes it is greatly complicated. When palatine or pterygoid bones are present they are formed by the ossification of cartilage; in Sauropsida and Mammalia they are laid down as membrane bones. There is no tympanic cavity or auditory ossicle in relation to the ear.

There are two principal types of suspensorium by means of which the jaws are attached to the cranium:—

(1) The Autostylic. This is the primitive condition in which the mandibular arch articulates with the base of the cranium in front of the hyoid and in a similar manner.

(2) The Hyostylic. In this case the mandibular arch becomes connected with the hyomandibular and supported by the hyoid arch. These terms are more fully discussed in Chapter VIII.

There is always an internal framework supporting the gills; it usually consists of the hyoid arch and five, rarely six or seven, pairs of branchial arches. The limbs are represented by two pairs of fins, the pectoral and the pelvic; they are not divided into proximal, middle and distal portions. The ribs do not unite with a median ventral sternum, or meet in the mid-ventral line in any other way in the trunk region.

Order I. Elasmobranchii.

The exoskeleton is in the form of placoid scales which are sometimes so numerous as to give the whole skin a rough surface forming shagreen. In some cases the placoid scales are enlarged to form plates or spines capped or coated with enamel. These spines may be imbedded in the flesh in front of the paired or unpaired fins, or may be attached to the tail. They are specially characteristic of the suborder Acanthodii. The endoskeleton is cartilaginous and true bone is never found. Much of the skeleton, especially of the vertebral column, is however often calcified, this being especially well seen in the anterior part of the vertebral column of Rays (Raiidae). In living forms cartilaginous biconcave vertebrae are always well developed, but in some extinct forms the notochord persists unconstricted. Neural and haemal arches are however always developed; they sometimes remain separate, sometimes fuse with the centra. Ribs are often wanting and when present are often not separated off from the vertebrae. The cranium is a simple cartilaginous box whose most prominent parts are the capsules which enclose the sense organs. The skull is sometimes immovably fixed to the vertebral column, sometimes articulates with it by means of two condyles. There is no operculum and no representative of the maxilla or premaxillae. The teeth are very variable. Large pectoral and pelvic fins always occur.

The Elasmobranchii may be divided into four suborders:—

(1) Ichthyotomi.

(2) Pleuropterygii.

(3) Selachii.

(4) Acanthodii.

Suborder (1). Ichthyotomi[30].

The members of this suborder range from the Devonian to the Permian and so have long been extinct.

The endoskeletal cartilage has granular calcifications evenly distributed throughout it. The notochord is unconstricted, but the neural and haemal arches are well-developed, and the neural spines are long and slender. There is a continuous dorsal fin with separate basalia and radiale. The tail is diphycercal, and the pectoral fins are typical archipterygia[31]. The pelvic fins of the male are prolonged to form claspers.

The best known of these primitive Elasmobranchs are the Pleuracanthidae.

Suborder (2). Pleuropterygii.

This suborder was formed for the reception of Cladoselache, an Elasmobranch found in the Lower Carboniferous of Ohio[32].

The exoskeleton is in the form of small, thickly-studded dermal denticles. The vertebral centra are unossified, and the tail is strongly heterocercal. There were certainly five, perhaps seven gill slits, and the suspensorium is apparently hyostylic. The paired fins are, according to the view which derives them by concentration from continuous lateral folds, the most primitive known (see p. 129) and claspers are absent.

Suborder (3). Selachii.

Cartilaginous or partially calcified biconcave vertebrae are always well developed; they constrict the notochord intervertebrally. The neural and haemal arches and spines are stout and intercalary cartilages (interdorsalia) are present. The tail is heterocercal, but in some cases (Squatina) approaches the diphycercal condition. In most cases the suspensorium is hyostylic, the jaws being attached to the cranium by means of the hyomandibular, and the palato-pterygo-quadrate bar not being fused to the cranium. There are generally five pairs of branchial arches, and gill rays are borne on the posterior surface of the hyoid arch, and on both the anterior and posterior surfaces of the first four branchial arches. The Notidanidae differ from most Selachians in two respects, first as regards the suspensorium,—Meckel's cartilage articulating directly with the palato-pterygo-quadrate bar, and not being connected with the hyoid arch; and secondly as regards the number of branchial arches,—six pairs occurring in Hexanchus and seven in Heptanchus.

The pectoral fins are without the segmented axis of the archipterygium. In most cases they are sharply marked off from the body and lie almost at right angles to it; but in the Rays they have the form of lateral expansions in the same plane as the body, from which they are not sharply marked off. The pelvic fins in the male bear long grooved cartilaginous rods which are accessory copulatory organs or claspers.

There are two principal groups of Selachii, the Squalidae or Sharks and Dogfish, and the Batoidei or Skates and Rays. The Squalidae have the shape of ordinary fish, the pectoral fins are vertically placed and the body ends in a powerful heterocercal tail. The Batoidei have flattened bodies owing to the great size and horizontal position of the pectoral fins. The tail is long and thin and is often armed with spines. The teeth in Selachii differ much in character in the different forms, and are always arranged in numerous rows. They are generally pointed and triangular or conical in the Squalidae, while in the Batoidei they are often broad and flattened.

Suborder (4). Acanthodii.

The fishes included in this group are all extinct and in some respects are intermediate between Elasmobranchii and Ganoidei. The body is elongated and closely covered with small scales consisting of dentine enamelled at the surface. The notochord is persistent and the calcification of the endoskeletal cartilage is only superficial. The tail is heterocercal. The jaws bear small conical teeth, or in some cases are toothless. The skeleton of all the fins differs from that of modern Elasmobranchs in having the cartilaginous radiale much reduced, and the fins are nearly always each provided with an anterior spine, which except in the case of the pectoral fins is merely inserted between the muscles. These spines are really enormous dermal fin-rays; the pectoral fin-spine is articulated to the pectoral girdle.

The suborder includes many well-known extinct forms like Acanthodes and Diplacanthus; it ranges from the Devonian to the Permian.

PISCES, HOLOCEPHALI.

Order II. Holocephali.

This order includes a single suborder only.

Suborder. Chimaeroidei.

These singular fish have the skin smooth and in living forms almost or quite scaleless. The palato-pterygo-quadrate bar and hyomandibular are fused to the cranium, and Meckel's cartilage articulates directly with the part corresponding to the quadrate. The skull is distinctly articulated with the spinal column, the notochord is persistent and unconstricted, and the skeletogenous layer shows no trace of metameric segmentation, though in the neural arches this segmentation is readily traceable. The neural arches of the first few vertebrae are fused together and completely surround the notochord, while they do not in other parts of the body. The tail is diphycercal. Of the living genera, in Callorhynchus there is no trace of calcification in the skeletogenous layer, while in Chimaera rings of calcification are found, there being three to five for each vertebra as indicated by the foramina for the exit of the spinal nerves. The pelvic fins are produced into claspers. Besides the living genera Chimaera, Harriotta and Callorhynchus a fair number of fossil forms are known, e.g. Ischyodus.

Order III. Ganoidei.

The fishes included under the term Ganoidei form a very heterogeneous group, some of which closely approach the Dipnoi, others the Elasmobranchii, others the Teleostei. The great majority of them are extinct, only eight living genera being known; these are all inhabitants of the northern hemisphere, and with the exception of Acipenser, which is both fluviatile and marine, are entirely confined to fresh water.

The following is a list of the living genera of Ganoids with their respective habitats:—

Acipenser. Rivers and seas of the northern hemisphere.

Scaphirhynchus. Mississippi and rivers of Central Asia.

Polyodon (Spatularia). Mississippi.

Psephurus. Yan-tse-kiang, and Hoangho.

Polypterus. Rivers of tropical Africa.

Calamoichthys. Some rivers of West Africa.

Lepidosteus. Freshwaters of Central and North America and Cuba.

Amia. Rivers of Carolina.

The exoskeleton is very variable, thus the body may be:—

(a) Naked or with minute stellate ossifications as in the Polyodontidae. (b) Partially covered with large detached bony plates as in Scaphirhynchus and Acipenser. (c) Entirely covered with rhomboidal ganoid scales as in Lepidosteus, Polypterus, Palaeoniscus and many extinct forms. (d) Covered with rounded scales shaped like the cycloid scales of Teleosteans as in Amia. (e) Having the trunk and part of the tail covered with rhomboidal scales, and the remainder of the tail with rounded scales as in Trissolepis.

The teeth also are very variable. The endoskeleton shows every stage of transition from an almost entirely cartilaginous state as in Acipenser to a purely bony state as in Lepidosteus. Sometimes, as in Acipenser, the notochord persists, and its sheath is unsegmented; sometimes, as in Lepidosteus, there are fully formed vertebrae. The tail may be heterocercal, as in Acipenser, or diphycercal as in Polypterus. The cartilaginous cranium is always covered with external membrane bone to a greater or less extent, and the suspensorium is markedly hyostylic. The pectoral girdle is formed of two parts, one endoskeletal and cartilaginous, corresponding with the pectoral girdle of Elasmobranchs, and one exoskeletal and formed of membrane bones, corresponding with the clavicular bones of Teleosteans. The pelvic fins are always abdominal. The fins often, as in Polypterus, have spines (fulcra) attached to their anterior borders.

The order Ganoidei may be divided into three suborders:

(1) Chondrostei. Living genera Acipenser, Scaphirhynchus, Polyodon and Psephurus.

(2) Crossopterygii. Living genera Polypterus and Calamoichthys.

(3) Holostei. Living genera Lepidosteus and Amia.

Suborder (1). Chondrostei.

The skull is immovably fixed to the vertebral column. By far the greater part of the skeleton is cartilaginous. The notochord is persistent and unconstricted, its sheath is membranous, but cartilaginous neural and haemal arches are developed. Intercalary pieces (interdorsalia) occur between the neural arches, and similar pieces (interventralia) between the haemal arches. The cranium is covered with membrane bone, and teeth are but slightly developed. The tail is heterocercal. Gill rays occur on the hyoid arch, and the gills are protected by a bony operculum attached to the hyomandibular. The skin (1) may be almost or quite naked, (2) may carry bony plates arranged in rows, or may be covered (3) with rhomboidal scales, or (4) partly with rhomboidal, partly with cycloidal scales.

Suborder (2). Crossopterygii.

The exoskeleton has the form of cycloidal or rhomboidal scales. The condition of the vertebral column differs in the different genera. Sometimes, as in Polypterus, there are well-developed ossified vertebrae; sometimes, as in many extinct forms, the notochord persists and is unconstricted. The tail may be diphycercal or heterocercal. The pectoral and sometimes the pelvic fins consist of an endoskeletal axis bearing a fringe of dermal rays.

Suborder (3). Holostei.

The exoskeleton has the form of cycloidal or rhomboidal scales. The notochord is constricted and its sheath is segmented and ossified, forming distinct vertebrae, which are generally biconcave, sometimes opisthocoelous (Lepidosteus). The cartilaginous cranium is largely replaced by bone, and in connection with it we find not only membrane bone, but cartilage bone, as the basi-occipital, exoccipitals, and pro-otic are ossified. The tail is heterocercal. The suspensorium resembles that of Teleosteans, consisting of a proximal ossification, the hyomandibular, which is movably articulated to the skull and a distal ossification, the symplectic. The two are separated by some unossified cartilage. The cartilaginous upper and lower jaws are to a great extent surrounded and replaced by a series of membrane bones.

Order IV. Teleostei.

The exoskeleton is sometimes absent but generally consists of overlapping cycloid or ctenoid scales. Bony plates are sometimes present, as in the Siluridae, or the body may be encased in a complete armour of calcified plates, as in Ostracion. Enamel is however never present, and the plates are entirely mesodermal. The skeleton is bony, but in the skull much cartilage generally remains. The vertebral centra are usually deeply biconcave, and the tail is of the masked heterocercal type distinguished as homocercal. In the skull the occipital region is always completely ossified, while the sphenoidal region is generally less ossified. The skull has usually a very large number of membrane bones developed in connection with it. The teeth vary much in character in the different members of the order, but are as a rule numerous and pointed, and are ankylosed to the bone. The suspensorium is hyostylic and the jaws have much the same arrangement as in the Holostei. There are five pairs of branchial arches, of which all except the last bear gill rays. A series of dermal opercular bones is developed in connection with these arches. The pectoral girdle consists almost entirely of dermal clavicular bones. The pelvic girdle has disappeared, its place being taken by the enlarged and ossified dermal fin-rays of the pelvic fins.

The group includes the vast majority of living fish (see p. 33).

Order V. Dipnoi.

The exoskeleton is of two types; dermal bones are largely developed in the head region, while the tail and posterior part of the body may be naked or may be covered with overlapping scales. The cranium remains chiefly cartilaginous, the palato-pterygo-quadrate bar is fused with the cranium, and the suspensorium is autostylic. The gill clefts are feebly developed and open into a cavity covered by an operculum. The notochord is persistent and unconstricted, and the limbs are archipterygia. The pelvic fins are without claspers.

Suborder (1). Sirenoidei[33].

The head has well developed membrane bones. The trunk is covered with overlapping scales and bears no bony plates. Three pairs of teeth are present, two in the upper and one in the lower jaw, the two principal pairs of teeth are borne on the palato-pterygoids and splenials, while the third pair are found in the vomerine region. The tail is diphycercal in living forms. In the extinct Dipteridae it is heterocercal. The pectoral girdle includes both membrane and cartilage bones. The pelvic girdle consists of a single bilaterally symmetrical piece of cartilage.

This suborder is represented by the living genera Ceratodus, Protopterus and Lepidosiren, and among extinct forms by the Dipteridae and others.

Suborder (2). Arthrodira.

Bony plates are developed not only on the head but also on the anterior part of the trunk, where they consist of a dorsal, a ventral, and a pair of lateral plates which articulate with the cranial shield. The posterior part of the trunk is naked. The tail is diphycercal. The jaws are shear-like, and their margins are usually provided with pointed teeth whose bases fuse with the tissue of the jaw and constitute dental plates. There seem to have been three pairs of these plates, arranged as in the Sirenoidei, the principal ones in the upper jaw being borne on the palato-pterygoids. Small pelvic fins are present, but pectoral fins are unknown.

The Arthrodira occur chiefly in beds of Devonian and Carboniferous age. Two of the best known genera are Coccosteus from the European Devonian and Dinichthys, a large predatory form from the lower Carboniferous of Ohio.