The first class of the Vertebrata is that of the Fishes (Pisces), which may be broadly defined as including Vertebrate animals which are provided with gills throughout the whole of life; the heart, when present, consists (except in Dipnoi) of a single auricle and a single ventricle ; the blood is cold ; the limbs, when present, are in the form of fins, or expansions of the intrgument; and there is neither an amnion nor allantois in the embryo, unless the latter is represented by the urinary bladder.

In form, Fishes are adapted for rapid locomotion in water, the shape of the body being such as to give rise to the least possible friction in swimming. To this end also, as well as for purposes of defence, the body is usually enveloped with a coating of scales developed in the inferior or dermal layer of the skin; whereas the epidermis is represented only by the slimy mucus covering the exterior of the animal. The more important modifications in the form of these dermal scales are as follows: I. Cycloid scales (fig. 247, a), consisting of thin, flexible, horny or bony scales, circular or elliptical in shape, and having a more or less completely smooth outline. These are the scales which are characteristic of the most of the ordinary bony fishes. II. Ctenoid scales (fig, 247, b), also consisting of thin horny plates, but having their posterior margins fringed with spines, or cut into comb-like projections. III. Ganoid scales, composed of an inferior layer of bone, covered by a superficial layer of hard polished enamel (the so-called "ganoine"). These scales (fig. 247, e) are usually much larger and thicker than the ordinary scales, and though they are often articulated to one another by special processes, they only rarely overlap. IV. Placoid scales, consisting of detached bony or dentinal grains, tubercles, or plates, of which the latter are not uncommonly armed with spines (fig. 247, c and d).

Fig. 247.   Scales of different fishes. a Cycloid scale (Pike); b Ctenoid scale (Perch) ; c Placoid scale (Thornback); d Placoid scale of Rhina; e Ganoid scales (Palaeoniscus).

Fig. 247. - Scales of different fishes. a Cycloid scale (Pike); b Ctenoid scale (Perch) ; c Placoid scale (Thornback); d Placoid scale of Rhina; e Ganoid scales (Palaeoniscus).

In most fishes there is also to be observed a line of peculiar scales, forming what is called the "lateral line." Each of the scales in this line is perforated by a tube leading down to a longitudinal canal which runs along the side of the body, and is connected with cavities in the head. The function of this singular system has been ordinarily believed to be that of secreting the mucus with which the surface of the body is covered; but this is certainly erroneous, and it seems to be more probably sensory in function, and to be connected with the sense of touch.

As regards their true osseous system or endoskeleton, Fishes vary very widely. In the Lancelet there can hardly be said to be any skeleton, the spinal cord being simply supported by the gelatinous notochord, which persists throughout life. In others the skeleton remains permanently cartilaginous; in others it is partially cartilaginous and partially ossified; and, lastly, in most modern fishes it is entirely ossified, or converted into bone.* Taking a bony fish (fig. 248) as in this respect a typical example of the class, the following are the chief points in the osteology of a fish which require notice:

* The so-called "bone" of the skeleton of Fishes is only occasionally true osseous tissue. In a great many instances it is a homogeneous or tubular, bone-like substance, or it may resemble genuine dentine.

Fig. 248.   Skeleton of the common Perch (Perca fluviatilis). p One of the pectoral fins ; v One of the ventral fins ; a Anal fin, supported upon interspinous bones (i); c Caudal fin ; d First dorsal fin ; d' Second dorsal fin, both supported upon interspinous bones ; i i Interspinous bones ; r Ribs ; s Spinous processes of vertebrae; h Haemal processes of vertebrae.

Fig. 248. - Skeleton of the common Perch (Perca fluviatilis). p One of the pectoral fins ; v One of the ventral fins ; a Anal fin, supported upon interspinous bones (i); c Caudal fin ; d First dorsal fin ; d' Second dorsal fin, both supported upon interspinous bones ; i i Interspinous bones ; r Ribs ; s Spinous processes of vertebrae; h Haemal processes of vertebrae.

The vertebral column in a bony fish consists of vertebrae, which are hollow at both ends, or biconcave, and are technically said to be "amphicoelous." The cup-like margins of the vertebral bodies are united-by ligaments, and the cavities formed between contiguous vertebrae are filled with the gelatinous remains of the notochord. This elastic gelatinous substance acts as a kind of ball-and-socket joint between the bodies of the vertebrae, thus giving the whole spine the extreme mobility which is requisite for animals living in a watery medium. The ossification of the vertebrae is often much more imperfect than the above, but in no case except that of the Bony Pike (Lepidos-teus) is ossification carried to a greater extent than this. In this fish, however, the vertebral column is composed of "opis-thocoelous " vertebrae - that is, of vertebrae, the bodies of which are concave behind and convex in front. The entire spinal column is divisible into not more than two distinct regions, an abdominal and a caudal region. The abdominal vertebrae possess a superior or neural arch (through which passes the spinal cord), a superior spinous process (neural spine), and two transverse processes to which the ribs are usually attached. The caudal vertebrae (fig. 248) have no marked transverse processes, but, in addition to the neural arches and spines, they give off an inferior or haemal arch below the body of the vertebrae, and the haemal arches carry inferior spinous processes (haemal spines).