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 
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.—Naturally 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, @.01.) acts in the 
