Читать книгу The Vertebrate Skeleton - Sidney H. Reynolds - Страница 10

Fig. 5. Skull of a male Chimaera monstrosa (after Hubrecht).

1. nasal capsule.	6. auditory capsule.
2. cartilaginous appendage to	7. interorbital septum.
the fronto-nasal region.	8. mandible articulating with
3. erectile appendage.	an outgrowth from the posterior
4. foramen by which the	part of the palato-pterygo-quadrate.
ophthalmic nerves leave the orbit.	9. teeth.
5. foramen by which the	10. labial cartilage.
ophthalmic branch of the Vth nerve	II. III. V. VII. IX. X. foramina
enters the orbit.	for the passage of cranial nerves.

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.