FISHES. 3 



gills, through 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 ofi' 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 



