FISHES 



19 



far the greatest number of species have scales of one of two types, the 

 cycloid or the ctenoid scales. These are thin rounded plates which cover 

 the body more or less completely and are imbricated, that is, arranged 

 in overlapping rows like the shingles on a roof, the free ends being 

 directed backwards. Cycloid scales characterize the more primitive 

 of the bony fishes, as the trout and salmon, and have a smooth hinder 

 margin. Ctenoid scales have a more or less roughened surface and a 

 dentate, serrate or spiny free hinder margin; they characterize the more 

 specialized bony fishes, as the perch and the bass. In some species both 

 types of scales are present, and in some species scales which are inter- 

 mediate between the two. Certain ganoid fishes, as the garpike, have 



Fig. 5. — Scales of fishes: a, ctenoid scale (hinder end of scale at the right); b, ganoid 

 scales; c, cycloid scale (hinder end of scale above) {modified from Cambridge Natural History 

 and Jordan's Guide 10 the Study of Fishes). 



ganoid scales, which are rhombic in form and are not imbricated, while 

 certain ganoids, as the sturgeon, and certain teleosts, as the sticklebacks, 

 have bony plates on the sides of the body. 



In the description of a fish the average number of scales (Fig. 6) 

 occurring in the lateral line between the head and the caudal fin is 

 given. The number, also, occurring in a dorso-ventral series, between 

 the lateral line and the back and the lateral line and the belly, is 

 usually given. Thus the expression "scales 5-46-11" means that 46 

 scales occur in the lateral line, or if no lateral line is present in a lateral 

 series between the head and the caudal fin, that 5 scales occur in a dorso- 

 ventral series between the forward end of the dorsal fin and the lateral 

 line and 11 scales between the lateral line and the anus or the forward 

 end of the anal fin. These figures are never absolute for all the indi- 

 viduals of a species, but are liable to vary a sixth from the average 

 given. 



Color. — Fishes are among the brightest of animals. Their colors 

 cover a wide range, and a statement of the markings of a specimen is 



