ON THE AIR-BLADDERS OF FISH. 103 
or less enveloped in bone, as observed upon in the Loaches, and, like 
the Cyprinide, a chain of ossicles passes forwards to the internal 
ear. Finding that the majority of these fishes with bone 
investing their air-vessel were found near the mountains, at once 
led me to conjecture that such must have something to do with 
sound. The number of genera existing in India and Burma 
in fresh water 1s twenty-five, out of which fourteen have the air- 
bladder more or less enclosed in bone, and, as all are ground-feeders, 
it appears probable that one reason at least is to prevent undue 
pressure on the organ when at great depths. 
The chain of auditory ossicles connecting the air-bladder with 
the internal ear, as far as I know,* is confined to the fresh-water 
Cyprinide, Characinide, and Siluride, all of which are Physo- 
stomous; while that having the lowest organization, its opercular 
pieces being incomplete, is the Siluride, and which alone appears 
to have marine representatives. Whether further investigation 
will confirm this, time alone must show, but if such should prove 
to be the rule, it would seem that this forms a connecting-link 
between such as are marine with those which belong exclusively 
to the fresh waters. 
Among the marine Physostomous forms, all that have been 
examined, I believe, and in which communications between this 
organ and internal ear exist, have such by means of cecal 
prolongations from the air-bladder and not by a chain of auditory 
ossicles, which appear to be wanting in marine fishes. This 
leads one to enquire whether there are any fresh-water fishes that 
have the connection between these organs, as seen in sea-fish. 
The Perch has no auditory ossicles, and I should consider it of 
marine origin; and the same conclusion may be arrived at 
regarding Trout.and the anadromous Salmon. 
In conclusion I think it may be affirmed that the air-bladder 
in fishes is the homologue of the lung of the superior vertebrate 
forms ; that in some of the higher sub-classes it serves as an 
accessory respiratory organ depurating the blood; that in the 
majority of Teleostean fishes it is employed for one or both of 
the following purposes:—as a float, enabling its owner, by 
compressing or dilating it, to sink or rise to any required level in 
the water; while, secondly, it assists hearing by communicating 
* Most probably this chain of ossicles will be found in some other fresh-water 
families, but which I have not yet had the opportunity of investigating, 
