SWIMMING BLADDER OF FISHES. .'583 



of the organization, for a purpose so definite and 

 unequivocal ! 



In several tribes of fishes there is a canal (c d), 

 called the ductus pneumaticus, establishing a com- 

 munication between this bladder and the stomach, 

 or the gullet (o); so that by compressing the blad- 

 der, a quantity of air may be forced out, and a very 

 sudden increase of 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 grovelling 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 become 

 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 bladder 

 to its 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 Pleuronectes. They are chiefly inha- 

 bitants 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 effort ; for their 

 ascent must be accomplished entirely by the con- 

 tinued beating and flapping of the water with their 



