The Fins correspond to the limbs of other vertebrates. The "anterior," or first pair, are called the "pectoral" fins, and are analogous to the arms of man and the fore-limbs of other animals. The hind-fins in fishes, known as "ventral" fins, are often wanting, and when present are less developed than the pectoral fins, and less fixed in their position. The "median" or "vertical" fins, situated on the back, are characteristic of fish, and extend more or less from the head to the tail. The fins of some fish are soft and flexible; in others rigid spines, or a combination of the two. By a wonderful mechanical contrivance the rigid spines can be raised or lowered at pleasure. In swimming, the fins enable fish to maintain their upright position, the centre of gravity being in their backs. The expansion and contraction of the fins enable a fish to ascend or descend in the water.
The Tail is placed vertically, and strikes the water from side to side. It possesses great muscular power, and is the chief organ of progression in a fish. Its action is similar to that of the rudder of a ship, turning the body to the right or to the left. When moved with a quick vibratory motion it acts like a screw-propeller, the fish darting forward with a speed proportionate to the force exerted. Two very distinct types of tail are found. In the one, common to most fish, the tail is composed of two nearly equal lobes, and is termed "homocercal." In the other type, represented by the sharks and by many extinct forms, the upper lobe is much longer than the lower, the tail in this case being called "heterocercal."
The Bodies of Fish are mostly covered with horny scales; but in some, as the eel and carp, scales are altogether wanting, or exist in so minute a form as to be almost invisible. The scales preserve the fish from injury by the pressure or friction of the water, or the sudden contact with pebbles, rocks or seaweed. Where the scales are very minute or absent, the bodies, as in sand-fish, are covered with a mucous secretion, which answers the same purpose as scales. When thin, horny, flexible, circular or elliptical in shape, like those of the salmon or herring, the scales are called "cycloid." When in detached plates, sometimes furnished with projecting spines, as in the case of the shark, they are called "placoid." Thin, horny, flexible scales and comb-like projections on the hinder margin, like those of the perch, are termed "ctenoid." Bony scales overlaid with hard polished enamel, a type represented by the sturgeon, and by many fossil fish, are known as "ganoid."
The Respiration of Fish is aquatic, and is effected by the comb-like organs, branchiæ, or gills, situated on each side of the neck. The gills are composed of delicate filaments, in which the blood is exposed to the aerating influences of the external water. The water, after being taken into the mouth by a process resembling swallowing, passes over the gills, where it gives up its oxygen, and is ejected from the
