22 The Exterior of the Fish 



essentially like cycloid scales. These are found in the garpike 

 and in many genera of extinct Ganoid and Crossopterygian 

 fishes. In the line 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 

 FIG. 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 (Gobiidce, 

 Pceciliidce) 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 

 ramifications. Sometimes peculiar sense-organs and occasionally 

 eye-like luminous spots are developed in connection with the 

 lateral line, enabling the fish to see in the black depths of the 

 sea. These will be noticed in another chapter. 



The Lateral Line as a Mucous Channel. The more primitive 



