254 



water, according to the quantity of its contents." 

 Perhaps one mean of diminishing the specific 

 gravity of the bodies of fishes is the quantity of 

 fat with which they in general abound ; but the 

 principal instrument for effecting this is undoubt- 

 edly their air-bladder. This gives to their back a 

 sufficient degree of lightness to enable it to re- 

 main upward, else this part of the body, which is 

 the heaviest, would draw after it the rest, and 

 the animal, lying on its back, would be incapable 

 of performing any motions of progression, as 

 happens when the bladder is burst or punctured. 

 Hence, too, such fishes as naturally want an air- 

 bladder generally remain in the mud at the bot- 

 tom of the water. Constrictor muscles expel 

 the air which it contains, and force it into the 

 stomach or oesophagus, when the animal wishes 

 to sink ; but this expulsion becomes impractica- 

 ble if the air undergoes considerable expansion 

 from any cause, and resists the compression that 

 is applied to it, as when fishes are contained in 

 a vessel which is placed under the exhausted re- 

 ceiver of an air-pump. Hence, also, during the 

 fry time, fishes, after remaining long on the sur- 

 face of the water, exposed to the heat of the sun, 

 become unable to sink, and are easily caught. 

 But not only was it necessary to supply fishes 

 with the power of sinking to the bottom or float- 

 ing on the surface of the water the means of 

 supporting themselves and of making progress in 

 the water were also to be provided. According- 

 ly their tail, moved by powerful muscles, may be 



