112 FISHES, 



specific gravity of their bodies. When bv their ab- 

 dominal muscles they press the air contained in it,' 

 the bulk of their body is diminished, their weight, 

 compared with that of the water, is increased, and 

 they consequently sink. If they want to rise, they 

 relax the pressure ot the muscles, the air-bladder 

 again acquires its natural size, the body is rendered 

 more bulky, and they ascend towards the surface. 

 This bladder lies in the abdomen, along the course 

 of the back- bone : in some fish it is single, and in 

 others double ; but in the latter case the two parts 

 communicate by a small canal. The air appears to 

 be conveyed into it from the blood, by means of ves-^ 

 sels appropriated to the purpose, and it can be dis- 

 charged thence either into the stomach or the mouth. 

 -x-Those fish that are without air-bladders have much 

 less facility in elevating themselves in the water. 

 The greater part of them remain at the bottom, un- 

 less the form of their body enables them to strike the 

 water downwards with great force. This the Rays 

 do with their large pectoral fins, which are some- 

 times, and not improperly called wings, since the 

 means which these fishes use in elevating themselves 

 are precisely the same as those employed by birds in 

 flying.— When the bladder of a fish is burst it is 

 never afterwards able to rise. From a knowledge 

 of this fact, the fishermen, after taking a quantity 

 of Cod-fish, are able to keep them alive for a con- 

 siderable time in their well-boats. They perforate 

 the sound or air-bladder with a needle, disengage 

 the enclosed air, and then throw them into the well, 

 where they immediately sink to the bottom. With- 



