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PROFESSORS T. W. BRIDGE AND A. C. HADDON 
With reference to one of these forms Wev^enbergh (40, pp. 91, 92), remarks tliat 
from its “ground ” habitat and peculiar shape an air-bladder is unnecessary and con- 
sequently has entirely atrophied. Weyenbergh’s statement that the air-bladder is 
absent in this species is certainly inaccurate, since the results of Reissner’s investi- 
gations, which we are able to confirm, have demonstrated its existence in a rudimentaiy 
condition, but his remarks as to the uselessness of the organ in the light of the 
peculiar habits of the fish are worthy of note. 
But although it is possible in the case of many Siluridm abnormal es to prove the 
association of a “ground” habit with the possession of a rudimentary air-bladder in 
the probable relation of cause and eifect, there are a few genera in which this is much 
more difficult. Bagarius, Silondia, Aiha, Eutropiichthys and Calloriiystax are neither 
inhabitants of mountain streams or torrents, nor are they in any sense amphibious, 
but, on the contrary, frequent the larger rivers of the low country and plains of 
India, and beyond this we have not been able to obtain any precise information as to 
their habitat or mode of life. The non-utility of the air-bladder as a hydrostatic 
organ in several of these Siluroids is very obvious from its extremely small size, and 
notably in Bagarius Yarrellii, where each of the two air-sacs into which the bladder 
is divided was only equal to the size of a garden pea in a Fish weighing ten pounds. 
In all such instances as these we can only suggest that here, as in many other cases, 
it is highly probable that the rudimentary condition of the air-bladder will eventually 
be found to be correlated with a purely ground life. Hence, it may be concluded 
that in the case of many Siluridm ahnormales there is a certain amount of evidence 
which strongly favours the view that the degenerate condition of the air-bladder is 
directly caused by reversion to a “ ground ” life, and also that, by analogy, this is 
likewise true of the remaining species and genera where, from ignorance of their 
habits, no trustworthy evidence on this point can at present be obtained. If our 
contention should ultimately prove to be correct, it will furnish a strong argument in 
favour of the conclusion which, on other grounds, we have already arrived at, viz., 
that the Weberian mechanism is physiologically related to the normal hydrostatic 
function of the air-bladder, and not to audition, or to the perception of varying 
atmospheric pressures. 
The susceptibility of the air-bladder to change of habitat on the part of its 
possessor is well shown by the variations which occur within the limits of the same 
genus. Two species of Cryptopterus {C. micropus and C. hexnpterus) have rudi- 
mentary air-bladders, while all the remaining species of the genus that came under 
our notice had those organs unusually well-developed. The genus Pimelodus exhibits 
an even more interesting illustration of the influence of a change of habit on tlie 
condition of the air-bladder. In two species of this genus [P. pulclier and P. sapo) 
the air-bladder itself is not only rudimentary but lacks even a trace of the compressor 
and tensor tripodis muscles which are so characteristic of the normal Pimelodinae. 
* TTyposfnmus plecosfomus, Cuv. et Vai., = Flecostomus bicirrliosas, Gron’OV. 
