FISHES. 



261 



in the Axolotes and the Newts, in which this organ is a 

 simple bladder, and in the Frogs and Toads, in which it 

 is subdivided into large cells; and it may, therefore, be 

 considered as the first rudimentary appearance of an aerial 

 respiratory apparatus. ^ P <>-~:.t 



^With regard to its function, in Fishes, it appears prima- 

 rily to be connected with swimming. Being condensed 

 by muscular pressure, or allowed to expand, it renders the 

 body either heavier or lighter; and thus enables the fish 

 to swim at any height in the water according to its plea- 

 sure. In general, the roving and surface-swimming species 

 are furnished with the organ in question, while such as 

 haunt the bottom are destitute of it ; and this arrange- 

 ment well agrees with the function we have ascribed to 

 it. Yet it cannot be denied that there are some most 

 unaccountable deviations from this rule. Thus, the Gur- 

 nards, which are ground-fishes, are well furnished with 

 bladders ; the two British species of Surmullet are deprived 

 of the organ; while the Tropical species, w T hich have the 

 same habits, are provided with one. Again, of surface- 

 fishes, our common Mackerel has no bladder, while the 

 Spanish and Coly Mackerels, which have exactly the same 

 manners, are each furnished with one. 



In many cases the bladder is hermetically sealed, but, 

 in some instances, it communicates by a tube with the 

 stomach, or the gullet. The air which it contains appears 

 to be a secreted gas; it is found to be, in some cases, 

 oxygen, and in others, strange to say, nearly pure nitrogen. 

 The former gas occurs chiefly in species that live in very 

 deep watei\ 



The blood, as already observed, is cold ; that is, it com- 



