FISHES. 
3 
gills, throiigli orifices for the purpose, called the 
gill-openings. The breathing apparatus is pro- 
tected by large bony plates. 
In most of the Bony Fishes there is found a 
membranous bladder, commonly of a lengthened 
form, placed along the body between the spine 
and the bowels. It is filled with air, and is well- 
known as the air-bladder, or swimming-bladder. 
It varies in appearance ; sometimes, as in the 
Hedgehog-fishes (Diodon), and their allies, it is 
two-lobed, more rarely it is double ; in some 
genera, as in the Electric Eels, and the Carp 
family, it is divided by a transverse partition, 
which, in the latter, allows of intercommunication 
through a narrow orifice. In one of the Catfish 
family {Pangasius) it is divided into four com- 
partments. In many species there are closed or 
blind tubular processes sent off’ from various parts 
of the surface ; and in others it is subdivided 
into many irregular cells. From this structure 
it appears evident that the air-bladder is the lin- 
gering remnant of the lungs of air-breathing 
animals. 
In some instances this bladder is found to be 
connected with the organs of hearing; but its 
chief function is the regulation of the specific 
gravity of the animal, aiding it in rising or sink- 
ing in the water, or maintaining any particular 
depth that its exigencies may require. In ge- 
neral, those species that swim at the surface, or 
that rove freely through the water, are furnished 
with this organ, while deep-water fishes are des- 
titute of it ; but there are many unaccountable 
exceptions. The air contained is found to vary 
in its character ; but in marine fishes oxygen 
