146 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



cavities filled with elastic gelatin, are designed for 

 rapid and versatile movements. The body is either 

 naked, as in the bullhead (Ameiurus), or covered with 

 polished, overlapping scales, as in the perch. Rarely, 

 as in the sturgeon, it is defended by bony plates, or by 



ABC V^r D 



FlG. 119. Scales of fishes: A, cycloid scale (Salmon); B, ctenoid scale (Perch); C, 

 placoid scale (Ray) ; D, ganoid scales (Amblypterus} a, upper surface; b, under 

 surface, showing articulating processes. 



minute, hard spines, as in the shark. Scales with smooth, 

 circular outline are called cycloid ; those with notched or 

 spiny margins are ctenoid. Enameled scales are ganoid, 

 and those with a sharp spine, like those of the shark, 

 are placoid. 



The vertical fins (dorsal, anal, and caudal) are peculiar 

 to fishes. The dorsal vary in number, from one, as in 



FIG. 120. Bluefish (Pomatomus saltatrix) 



the herring, to three, as in the cod ; and the first dorsal 

 may be soft, as in the trout, or spiny, as in the perch. 

 If the dorsals are cut off, the fish reels to and fro. The 



