THE FISHES 155 



rays. These are the unpaired fins, as distinguished from 

 the paired ones, which correspond to the limbs of the higher 

 vertebrates. In the bass or perch the latter are of much 

 service in swimming, and are also most important organs in 

 directing the course of the fish upward or downward, or for 



Pf 



FIG. 97. Yellow perch (Perca flavescens). df, dorsal fins ; pc, pectoral fin ; pf, pelvic 

 fin ; v, ventral fin. 



aiding the tail in changing the course from side to side ; 

 or they may be used to support the animal as it rests upon 

 the bottom in wait for food ; and, finally, they may serve to 

 keep the body suspended at a definite point. 



In addition to an internal skeleton the bass or perch, 

 like the greater number of fishes, is more or less enclosed 

 and protected by an external one, consisting of a beautifully 

 arranged series of overlapping scales, which afford protec- 

 tion to the underlying organs, and at the same time admit 

 of great freedom of movement. These usually consist of a 

 horny substance, to which lime is sometimes added, and 

 are peculiar modifications of the skin, something like the 

 feathers, nails, and hoofs of higher forms. 



147. The air-bladder. Xaturally a fish's body is heavier 

 than the water in which it lives, and there are reasons for 

 thinking that the air-bladder (Fig. 106, a.U.) acts in the 



