SWIMMING BLADDER OP FISHES. 29.Q 



specific gravity produced; followed, of course, by a quick 

 descent. When, by any accident, the air bladder has been 

 opened, or has burst, so that all the air has escaped, the fish 

 is seen to grovel at the bottom, lying on its back, and can 

 never afterwards rise to the surface. On the other hand, it 

 occasionally happens that a fish which has remained too long 

 at the surface of the sea, exposed to the scorching rays of a 

 tropical sun, suddenly finds itself retained against its will at 

 the surface, because the bladder has beconie over distended 

 by the heat, and resists all the efforts which the animal can 

 make to compress it. It thus continues floating, until the 

 coolness of the night has again condensed the air in the blad- 

 der to its former bulk, and restored the power of descending. 

 Some tribes of fish are totally unprovided with an air- 

 bladder. This is the case with the flounder, the sole, and 

 other genera of a flat shape, forming the family of Pleuron- 

 cetes. They are chiefly inhabitants of sand-banks, or other 

 situations where they are comparatively stationary, seldom 

 moving to a distance, or rising much in the water; and 

 when they do so, it is with manifest eflfort, for their ascent 

 must be accomplished entirely by the continued beating and 

 flapping of the water with their expanded pectoral fins. It 

 is only the larger fish of this form, such as rays, which have 

 very voluminous and powerful pectoral fins for striking the 

 water downwards with considerable force, that can rise with 

 facility without the assistance of an air-bladder. In these, 

 the lateral fins, which are enormous expansions of the pec- 

 toral fins, may be compared to wings, their vertical action 

 on the water being similar in eflfect to the corresponding 

 movements of a bird, when it rises vertically in the air. 

 Those fishes which swim rapidly, and frequently ascend and 

 descend in the water, are, in general, provided with the 

 largest air-bladders. 



In studying the varieties presented by the forms of the 

 fins in different tribes of fishes, we find the same constant 

 relation preserved with the particular situations and circum- 

 stances in which they are placed. The dorsal fins, which 



