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: (i) 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). J^vdoid 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 

 the scales develop into 

 large protective spines, 

 3ftS^I> 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. 



F IG- 37 g. Chromatophores in skin of The Colors are due to 



upper side of a freshly killed flounder, Pleuro- pig men ts within special 



nectes ftesus. Black bodies represent black J 



Chromatophores; gray bodies, yellow; small dermal Cells, Called Chro- 



gray plates, iridocytes. (From the Cambridge mato phores, or to reflec- 

 Natural History, after Cunningham and Mac- 



Munn.) 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 



