100 THE ZOOLOGIST. 
be furnished with a distinct valve precluding the inlet or exit of 
atmospheric air. 
In short, among the Teleosteans the air-bladder exists as a 
closed sac of diverse forms, Physoclisti, in the spiny-rayed 
Acanthopterygians, the spineless Anacanthini, the tufted-gilled 
Lophobranchii, and the hard-jawed Plectognathi; whilst in the 
remaining orders, the pneumatic duct remains patent, as in the 
whole of the Physostomi, excluding the Family Scombresocide. 
In such forms as swim near the surface this organ is mostly 
of a comparatively small size, while in those which live near the 
bottom, as the flat-fishes, Plewronectide, it is, as a rule, absent. 
When this organ is ruptured the fish mostly sinks, turning on its 
back and remaining at 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 time to compress 
or partially empty their air-bladder, the contained gas, being no 
longer weighed down by superincumbent water, rapidly expands, 
causing the organ to burst, or else forcing the stomach and 
upper portion of the alimentary canal into the fish’s mouth. 
The air-bladder is generally found after death tightly 
distended with gas, which consists mostly of nitrogen in fresh- 
water forms, and oxygen in marine genera. It has formed a 
subject of discussion as to how this gas is generated, but as in 
those classes in which this organ is a closed sac (Physoclisti), it 
is perceived as well as in others possessing a pervious pneumatic 
duct (Physostomi), one cannot resist believing that the gas may 
be eliminated in most instances from the blood-vessels lining its 
interior. Probably the gland which is so apparent in most of the 
Physoclisti serves for the purpose of removing superfluous gas or 
any deleterious substances, acting in fact as a depurating organ. 
The chief uses of this organ in Teleostean fishes appear to 
be two. (1) Hydrostatic, or for flotation, serving the mechanical 
purpose of enabling its possessor to maintain a desired level in 
the water, and which is usually accompanied with the power of 
removing, 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 modes of connection with the internal ear. 
Among the Physoclisti, the majority of which are marine 
forms, we find this organ as a closed sac, the greatest length of 
