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PROFESSORS T. W. BRIDGE AND A. C. HADDON 
The conclusion to which these facts seem irresistibly to point is still further 
strengthened by the condition of the air-bladder and Weberian mechanism in the 
Sihiridae abnormales, but as we shall subsequently have occasion to refer to these 
Siluroids at some length, we need now only say that in a large number of cases the 
degenerate and rudimentary condition of these structures is associated with the 
purely “ ground ” habit of these Fishes. It is well known that the assumption of a 
ground habit is in many Fishes accompanied by the degeneration, or even total dis- 
appearance, of the air-bladder, which, under such conditions, becomes useless as a 
hydrostatic organ, even if its retention is not distinctly harmful to the species, and 
hence its liability to degenerate to the condition of a more or less rudimentary 
structure, or become completely suppressed. The correlation of a similar habit of 
life with degeneration of the air-bladder and Weberian mechanism in the particular 
case of the Siluridre abnormales, seems to us to furnish almost conclusive evidence 
that both organs are solely related to the normal hydrostatic function. As an 
accessory to audition, or even as a register of varying atmospheric pressures, or, 
indeed, as fulfilling any purpose other than the hydrostatic function, the air-bladder 
and Weberian mechanism might still be of some utility to these Fishes in spite of 
their change of habit, and therefore need not degenerate, but the fact that these 
structures do degenerate under such conditions seems to us to be only reconcilable 
with the conclusion at which on other grounds we have arrived. 
Considered solely from an anatomical standpoint, and with the qualification which 
the absence of direct experimental evidence imperatively demands, it may be 
affirmed that Hasse’s theory is correct, and that the Weberian mechanism is a 
pressure register serving to acquaint the Fish with the varying tensions or volumes 
of the gases contained within its air-bladder, the result of corresponding variations 
of the hydrostatic pressure. As it is inconceivable that such pressure variations can 
arise from any other cause than the ascent or descent of the Fish in the water in the 
course of the oi’dinary locomotor movements, it may be further concluded that the 
Weberian mechanism is related to some form of pressure adjustment. 
3. If the conclusion at which we have arrived be admissible, the further question — 
What advantage does the Fish derive from the possession of a pressure register by 
which it becomes acquainted with the varying degrees of hydrostatic pressure to 
which it is exposed ? or. In what way does the animal respond to the sensory impulses 
communicated to its central nervous system by the Weberian mechanism and the 
co-adapted parts of the internal ear ? is one to which it is very difficult to return a 
satisfactory answer. That the Weberian mechanism is of great functional importance 
to the Fish possessing it admits of no doubt. It is extremely improbable that so 
complicated and highly specialized a mechanism would have been evolved did it not 
confer some exceptional advantage upon its possessors, and that this is the case seems 
to be clearly demonstrated by the significant fact that the presence of a Weberian 
mechanism is characteristic of nearly all the dominant families of freshwater Teleostei. 
