SWIMMING BLADDER OF FISHES. 431 



former bulk, and restored the power of de- 

 scending. 



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 Pleuroncetes. They 

 are chiefly inhabitants of sand-banks, or other 

 situations where they are comparatively sta- 

 tionary, seldom moving to a distance, or rising 

 much in the water ; and when they do so, it is 

 with manifest effort, 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 pectoral fins, may 

 be compared to wings, their vertical action on 

 the water being similar in effect to the cor- 

 responding 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 



