XIX. FISHES 



Branch XII. — Chordata {continued) 



Class. — Pisces 



The fishes constitute the largest class of the vertebrates. 

 They var}' greatly in shape, size, and general appearance 

 and are found everywhere over the earth. Three thousand 

 species are now known to occui- in North America alone. 



An Example of the Class — the Perch 



General form. — The body of the perch is long, deep, and 

 thin from side to side and has a wedge-shaped head and a 

 somewhat rounded tail ending in a fin, the principal organ 

 of locomotion. There is no neck for the head is joined 

 directly to the trunk. Therefore, the body is divided into 

 three regions: head, trunk, and tail — the latter comprising 

 that portion of the body beyond the anal opening. 



Scales. — The liody, except a part of the head and the 

 fins, is covered with small scales that overlap each other 

 like shingles and constitute the exoskelcton. The scales 

 are horny outgrowths of the true inner skin, and they are 

 covered with a thin, slimy mcmlii-anc, the epidctiiils. The 

 free, posterior edge of a scale is I'ounded, while the anterior 

 edge, by which the scale is attachetl, is scalloped like the 

 shell of certain mollusks. 



Eyes. — Tlicre are two large eyes, one on each side of the 

 head. They have no eyelids, but a thin, transparent skin 



208 



