20 THE COMMERCIAL SEA FISHES OF 



cause, permitting the contained gas to escape, the fish 

 sinks to the bottom. On the other hand, some forms 

 which have been hooked or netted at great depths, and 

 suddenly brought to the surface without having had time 

 to compress or partially empty their air-bladders, the con- 

 tained gas being no longer weighted down by a mass of 

 super-imposed water, expands rapidly, causing the organ 

 to burst, or else forces the stomach and upper portion of 

 the alimentary canal out of the fish's mouth. 



The chief uses of this organ in teleostean or bony fishes 

 are two : (i) a hydrostatic, or for flotation, which serves, 

 by contracting or distending its capacity, to condense or 

 rarify the contained gases, giving it the mechanical function 

 of enabling its possessor to maintain a desired level in the 

 water, and which is accompanied with the power of renew- 

 ing, expelling, compressing, or dilating its gaseous con- 

 tents, so that it can rise or fall as necessity occurs. (2) The 

 second use is acoustic, it being partially or entirely em- 

 ployed in hearing, by means of various modes of connection 

 with the internal ear, mostly by tubular prolongations of 

 the air-bladder to the internal ear, or by a connecting 

 chain of auditory ossicles. 



If we examine the Physostomi we find that, as regards 

 the air-bladder, they hold a place between the Physoclisti 

 on one hand, and the Dipnoids and Ganoids on the other. 

 If we examine the carps, Cyprinidce, the Characinidce, and 

 likewise the sheat-fishes, Siluridce, we find that a chain of 

 auditory ossicles connects the air-bladder with the internal 

 ear (instead of a tube filled with gas and a prolongation of 

 the air-bladder as found in Physoclisti). Among the carps 

 we find in the ground-feeding loach a curious modification 

 of this organ ; its posterior two-thirds, or its hydrostatic 

 portion, are deficient, the air-bladder being in the form of 



