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Fishes. The lowest class of vertebrate animals, cold-blooded, and breathing by means of gills through life. They are wholly adapted for living in the water. The shape of the body is such as to give rise to the least possible friction in swimming, and thus to admit of rapid locomotion in water. To this end also, as well as for purposes of defense, the body is usually covered with a coating of scales. The limbs, when present, are always in the form of fins, but one or both pairs may be wanting; the anterior or fore limbs are known as the pectoral fins, and the posterior or hind limbs as the ventral fins. Besides the fins which represent the limbs, fishes possess other fins placed in the middle line of the body; one or two of these run along the back, and are known as the dorsal fins, one or two lie on the belly, near the vent, known as the anal fins, and a broad fin at the extremity of the spinal column is called the caudal or tail fin. The tail fin is always set vertically in fishes, so as to work from side to side, and is the chief organ of progression; it differs altogether from the horizontal expansion which constitutes the tail of whales, dolphins, dugongs, and manatees - animals which belong to the class of mammals. In the form of the tail, fishes exhibit two very distinct types of structure, termed respectively the homocercal and the heterocercal type of tail. The homocercal tail is the one which most commonly occurs in existing fishes; it is characterized by the fact that the two lobes of the tail are equal, and the spinal column stops short at its base. In the heterocercal tail, on the other hand, found in many fossil specimens of the fish class, the spinal column is prolonged into the upper lobe of the tail, so that the tail becomes unequally lobed. All the fins are supported by bony spines, or rays, which are of two kinds, termed respectively spinous rays and soft rays. Further, to aid in supporting themselves at varying depths in the water, most fishes are provided with a sac containing gas, situated above the alimentary tube, and known as the air or swimming bladder, by the filling or emptying of which the fish is rendered heavier or lighter in comparison with the surrounding water.