236 
The air-bladder, however, is not only absent in many families, 
but it may be present or deficient among species of the same 
genus. One form of British mackerel, Scomber colias, possesses 
this organ, while the S. scomber has none. This is by no means 
peculiar to European genera. Thus among the Asiatic species 
of the marine Polynemus the P. paradiseus and P. tetradactylus are 
without air-bladders, while such exist in P.indicus and P. plebejus. 
In such forms as swim near the surface the air-bladder is 
generally of a comparatively small size; while in those which 
live near the bottom, as the flat fishes, Plewronectide, it is as a 
rule absent. In species possessing this organ, should it become 
ruptured from any cause, permitting the contained gas to 
escape, the fish has by some authors been stated to sink 
to the bottom, and unable to re-ascend, a conclusion some 
experiments have failed to establish. On the other hand some 
forms which have been hooked or netted at great depths and 
suddenly brought to the surface, without having time to 
compress or partially empty their air-bladders, the contained 
gas being no longer weighted down by a mass of superimposed 
water, expands rapidly, causing the organ to burst, or else 
forcing the stomach and upper portion of the alimentary canal 
into the fishes mouth.* 
The chief use of this organ+ in Teleostean fishes are two— 
(1). 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 accom- 
panied with the power of renewing, expelling, compressing, 
or dilating its gaseous contents, so that it can rise or fall as 
necessity occurs. (2). The second use is acoustic, it being 
partially or entirely employed for hearing, by means of various 
* 
A fish may remain at the bottom of the water due to the very fact of 
the pressure of the column of water on the air contained in the bladder.” 
MULLER. 
+ I purposely omit such instances in which it might be employed, for 
respiration or in the production of sounds, constituting what has been termed 
“* fishes voices.” 
