VERTEBHATA. 



311 



needed, and such as they have are without external parts, 

 the sound being obliged to pass through the cranium. 

 Taste and smell are blunted, and touch is nearly confined 

 to the lips. 



The class yields to no other in the number and variety 

 of its forms. It' includes nearly one half of all the ver- 

 tebrated species. So great is the range of variation, it is 

 difficult to frame a definition which will characterize all the 

 finny tribes. It may be said, however, that Fishes are the 

 only backboned animals having median fins (as dorsal and 

 anal) supported by fin-rays, and whose limbs (pectoral and 

 ventral fins) do not exhibit that threefold division (as thigh, 

 leg, and foot) found in all other Vertebrates." 9 



The form of Fishes is admirably adapted to the element 

 in which they live arid move. Indeed, Nature nowhere 

 presents in one class such elegance of proportions with 

 such variety of form and beauty of color. The head is 





ABC 



PIG. 283. Scales of Fishes: A, cycloid scale (Salmon); B, ctenoid scale (Perch); C, 

 placoid scale (Ray) ; D, gauoid scales (Amblypteru^a, upper surface ; b, under 

 surface, showing articulating processes. 



disproportionately large, but pointed to meet the resist- 

 ance of the water. The neck is wanting, the head be- 

 ing a prolongation of the trunk. The viscera are closely 

 packed near the head, and the long, tapering trunk is left 

 free for the development of muscles which are to move 

 the tail the instrument of locomotion. The biconcave 

 vertebrae, with intervening cavities filled with elastic gel- 

 atine, are designed for rapid and versatile movements. The 

 body is either naked, as in the Lamprey, or covered with 



