448 



COLLEGE ZOOLOGY 



in search of food; and the first dorsal fin of the sucker- fish, 

 Remora (Fig. 400), forms a sucker for the attachment of its 

 possessor to a shark or turtle. 



Scales. — The scales of fishes form a protecting exoskeleton. 

 They are of three principal types: (1) ganoid, (2) cycloid, and 

 (3) ctenoid. Ganoid scales are usually rhombic in shape (Fig. 

 371, B). They have a superficial covering of dentine called 

 ganoin. Ganoid scales occur in most of the Chondrostei and 

 Holostei, and these are often called ganoid fishes. Cycloid 

 and ctenoid scales are arranged in overlapping rows as described 

 for the perch (p. 434). Cycloid scales (Fig. 371, C) are nearly 

 circular with concentric rings about a central point. Ctenoid 

 scales (Fig. 371, A) are similar to cycloid scales, but the part 

 which extends out from under the neighboring scales bears small 



spines. In many fishes 





m 



^j* 





Vwr 



■ &. 





the scales develop into 

 large protective spines, 

 or may fuse to form 

 bony plates. 



Color. — The general 

 impression is that fishes 

 are not brightly colored, 

 but many of them, espe- 

 cially in tropical waters, 

 are exceedingly brilliant. 

 The colors are due to 

 pigments within special 

 dermal cells, called chro- 

 matophores, or to reflec- 

 tion and iridescence re- 

 sulting from the physical 

 structure of the scales which contain crystals of guanin (irido- 

 cytes, Fig. 378). The pigments are red, orange, yellow, or 

 black, but other colors may be produced by a combination of 

 chromatophores; for example, yellow and black when blended 



Fig. 378. — Chromatophores in skin of 

 upper side of a freshly killed flounder, Pleuro- 

 nectes flesus. Black bodies represent black 

 chromatophores; gray bodies, yellow; small 

 gray plates, iridocytes. (From the Cambridge 

 Natural History, after Cunningham and Mac- 

 Munn.) 



