100 STBUCTUEE OP FISHES. 



a Fish or as a Reptile, so complete is the mixture of charac- 

 ters which it presents. 



88. The class of PISHES is distinguished from all other 

 Vertebrata, by the adaptation of the animals composing it to 

 breathe by means of water in their adult state, so as to be 

 capable of living in that element only. Like Reptiles, they 

 are oviparous and cold-blooded ; and in these characters they 

 differ completely from the "Whales and other Mammals, 

 which are, like them, inhabitants of the great deep, but which 

 are warm-blooded, viviparous, and air-breathing animals. 

 There is a simple external character, by which the members 

 of the two classes may be at once distinguished. The animals 

 of the "Whale tribe are, like fishes, chiefly propelled through 

 the water by means of a flattened tail ; but in the former the 

 tail is flattened horizontally, so that its downward stroke may 

 serve to bring the animal to the surface to breathe ; whilst in 

 Fishes it is flattened vertically, that its strokes from side to 

 side may simply propel the fish through the water. A 

 flattening or compression of the body is seen more or less 

 in almost all fishes, and is intimately connected with the 

 nature of their motion through the element they inhabit ; as 

 it serves the double purpose of diminishing the resistance 

 which is offered to their progress, and of increasing the extent 

 of the oar-like surface, by the lateral stroke of which the 

 body is propelled forwards (Chap. xn.). This stroke is given 

 by- a series of muscles of great power, which pass from the 

 prolonged extensions of one vertebra to those of another, and 

 altogether make up the principal part of the bulk of the 

 animal. The fins which represent the limbs are not so much 

 used in propelling the Fish, as in changing its direction 

 either laterally or vertically. Thus in the lowest group of 

 the Vertebrated series, the act of motion is chiefly performed 

 by the vertebral column itself, instead of being committed to 

 the limbs, as in Mammals, Birds, and most Reptiles. The 

 larger number of Fishes swim with great activity ; and their 

 lives may be said to be passed in seeking their subsistence 

 and in flying from their enemies. 



89. Fishes are for the most part very voracious, and their 

 food consists in great part of the members of their own class. 

 In seeking it, they appear to be chiefly guided by the sight ; 

 for their eyes are usually large and highly developed, while 



