64 INTRODUCTION. 



in place of lungs in tliis capacity, many hshes are 

 provided with an organ commonly known as the 

 air or swim-bladder, to which they owe more, in 

 this respect, than most other animals do to their 

 lungs. The principal use of this bladder, however, 

 appears to be, not so much that of rendering the 

 body of fishes unifoi-mly buoyant, but to modify 

 this buoyancy as occasion may require. 



It is for the same purpose of diminishing their 

 specific gravity, that the cetaceous tribes — the bones 

 of which, unlike those of most fishes, are in general 

 lighter than water — are furnished with a prodigious 

 quantity of fat ; for it must be remembered that they 

 require, not merely to be kept at any given level be- 

 low the water, but to be raised again to the surface, 

 as often as in the pursuit of their prey, or from any 

 other cause, they had dived below it. This is a prin- 

 cipal use of the enonnous quantity of oil which is 

 found in these animals, contained, in most part, in 

 what is called the blubber, immediately under the 

 skin, and constituting the train-oil of commerce. The 

 cetaceous animals, also, have no proper air-bladder ; 

 but their lungs, which are generally continued in 

 an elongated form along the spine, instead of being 

 confined, as in the other mammiferous animals, to 

 the plane of the proper chest, serve, in some mea- 

 sure, as a substitute for that organ. 



Fishes are of nearly the same specific gravity as wa- 

 ter, and consequently they have little or no ten- 

 denc}'-, at any given level, either to rise to the surface, 

 or sink to the bottom, but can move either upwards 



