430 THE MECHANICAL FUNCTIONS. 



condensed, the specific gravity of the whole body 

 is increased, and the fish sinks to the bottom. On 

 relaxing the same muscles, the air recovers its 

 former dimensions, and the fish is again ren- 

 dered buoyant. Can there be stronger evidence 

 of design than the placing of this hydrostatic 

 apparatus, acting upon philosophical principles, 

 in the interior of the organization, for a purpose 

 so definite and unequivocal ? 



In several tribes of fishes there is a canal 

 (c D) establishing a communication between 

 this bladder and the stomach, or the gullet (o) ; 

 so that by compressing the bladder, 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 to grovel at the bottom, lying on its 

 back, and can never afterwards rise to the sur- 

 face. On the other hand, it occasionally hap- 

 pens 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 re- 

 tained 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 



