Taking first the external features, we note that the body is generally covered with scales, sometimes very large and distinct, but often so small and closely set that they are not visible without careful examination; indeed they are often so small, and so thoroughly embedded in the slimy skin as not to be discovered without the aid of a microscope. When the scales have unbroken edges and overlap one another they are said to be cycloid, but when the projecting edges are toothed or serrated, giving a roughness to the touch, they are described as ctenoid. Sometimes the scales are modified into bony plates or little isolated bony granules, and in either case they are practically identical in structure with teeth, consisting as they do of dentine, capped with a little harder substance resembling enamel.
Fig. 227.—The Pilchard
1. Dorsal fin. 2. Pectoral fin. 3. Pelvic fin. 4. Ventral or anal fin. 5. Caudal fin.
We often observe a row of scales, of a different nature from those covering the body generally, running along each side of a fish from near the eye to the end of the tail; and these constitute what is called the lateral line. If we examine these scales closely, we observe that each one is pierced by a hole that communicates with a little sac beneath containing a gelatinous material, and in which a nerve tendril terminates. The presence of the nerve filament evidently denotes that the scales in question, with the little sacs beneath them, are organs connected with sensation, and it is also believed that they have something to do with the secretion of the slimy mucus that covers the scales of the body.
The mouth of a fish is generally situated on the extreme front of the head, but occasionally, as in the sharks and rays, quite on the under side. If it contains a tongue at all, this organ is of small size and simple structure; thus it is highly probable that the sense of taste is very feeble in these animals, and this is just what one might expect when one remembers that fishes never retain their food in the mouth for any length of time, but simply bolt it without any attempt at mastication.
The arrangement and nature of the teeth are very variable. Often they are developed on the membrane of the mouth only, in which case they are generally renewed as fast as they are worn down, but sometimes they are persistent structures more or less embedded in the bone of the jaws. In some cases teeth are altogether wanting, but in others they are situated not only on the jaws, but also on the tongue, the roof of the mouth, and even on the bony arches that support the gills.
A glance at the fins of a typical fish will suffice to show that they may be divided into two groups—the paired fins, representing the two pairs of limbs in most of the higher animals, and the median fins occupying the middle line of the body. The former comprise the two pectoral fins that correspond with our arms, and are attached to the bones of the pectoral or shoulder girdle; and the pelvic fins, corresponding with the lower extremities. The pectorals, too, are present in nearly all fishes, while the pelvic pair are rather more frequently absent than the pectorals.
The medial fins comprise the dorsal, the ventral, and the caudal or tail-fin, and are not to be regarded as limbs, but rather mere outgrowths of the skin. They are not directly connected with any part of the main bony framework of the body, though they are generally jointed with a series of bones (interspinal bones) that run between processes of the vertebral column. The dorsal and ventral fins are often divided into two or more parts, and the tail fin is commonly distinctly forked.