. Fishes. Fishes. The Exterior of the Fish 21 essentially like cycloid scales. These are found in the garpike and in many genera of extinct Ganoid and Crossopterygian fishes. In the Hne of descent the placoid scale preceded the ganoid, which in turn was followed by the cycloid and lastly by the ctenoid scale. Bony scales in other types of fishes may have noth- ing structurally in common with ganoid scales or plates, however great may be the superficial resemblance. The distribution of scales on the body may vary exceedingly. In some fishes the scales ^'°" 14.—Cycloid are arranged in very
. Fishes. Fishes. The Exterior of the Fish 21 essentially like cycloid scales. These are found in the garpike and in many genera of extinct Ganoid and Crossopterygian fishes. In the Hne of descent the placoid scale preceded the ganoid, which in turn was followed by the cycloid and lastly by the ctenoid scale. Bony scales in other types of fishes may have noth- ing structurally in common with ganoid scales or plates, however great may be the superficial resemblance. The distribution of scales on the body may vary exceedingly. In some fishes the scales ^'°" 14.—Cycloid are arranged in very regular series; in others they are variously scattered over the body. Some are scaly everywhere on head, body, and fins. Others may have only a few lines or patches. The scales may be everywhere alike, or they may in one part or another be greatly modified. Sometimes they are transformed into feelers or tactile organs. The number of scales is always one of the most valu- able of the characters by which to distinguish species. Lateral Line.—The lateral line in most fishes consists of a series of modified scales, each one provided with a mucous tube extending along the side of the body from the head to the caudal fin. The canal which pierces each scale is simple at its base, but its free edge is often branched or ramified. In most spiny-rayed fishes it runs parallel with the outline of the back. In most soft-rayed fishes it follows rather the outline of the belly. It is subject to many variations. In some large groups {Gohiidcc, Pcecihida;) its surface structures are entirely wanting. In scale- less fishes the mucous tube lies in the skin itself. In some groups the lateral line has a peculiar position, as in the flying- fishes, where it forms a raised ridge bounding the belly. In many cases the lateral line has branches of one sort or another. It is often double or triple, and in some cases the whole back and sides of the fish are covered with lateral lines and their ramific
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