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 j and it may, therefore, be 
considered as the first rudimentary appearance of an aerial 
respiratory apparatus. 
With regard to its function, in Fishes, it appears prima- 
rily to bo 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, which have the 
same habits, are provided with one. Again, of surface- 
fishes, our common Mackerel lias 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 water. 
The blood, as already observed, is cold; that is, it com- 
