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PROFESSORS T. W. BRIDGE AND A. C. HADDON 
which vary with different species. A Fish without the mechanism relies solely upon 
the relatively slow processes of gaseous secretion and absorption for the readjustment 
of the volume of gases contained within its air-bladder to pressure variations, and for 
reasons already given we have arrived at the conclusion that such processes are of 
little value in ordinary locomotor movements involving rapid changes of level and 
pressure, although possibly of considerable advantage when such changes are incurred 
in the course of periodic or other migrations. It will follow, therefore, that in 
ordinary locomotor movements attended by rapid changes of level any departure from 
the normal plane of equilibrium must be accompanied by an increase or decrease of 
specific gravity to an extent proportional to the amount of the pressure variation. 
Hence in all such movements a corresponding increase of muscular exertion must take 
place which will necessarily be the greater in proportion as the variation of level 
carries the Fish away from its normal plane of least efibrt, and the result is the same 
whether the Fish has to counteract by muscular effort the effect of a too feeble specific 
gravity, or to sustain itself against an increase of specific gravity. On the contrary, 
all Fishes that have a Weberian mechanism possess also a greater capacity for pres- 
sure adjustment, and consequently at nearly all levels and pressures will retain a 
normal plane of least effort, with the correlative advantage that all their normal 
movements, however rapid, and whatever may be the plane in which they take place, 
will be executed with a minimum expenditure of muscular energy. We may fairly 
conclude, therefore, that the Weberian mechanism not only confers upon all Fishes 
that possess it an exceptional capacity for freedom of locomotion in all directions, and 
especially in the vertical plane, but also entails the contingent advantage that all 
movements will be efiected with the maximum economy of muscular effort and tissue 
metabolism. 
This seems to be about as far as the data at present at our disposal will warrant us 
in pursuing this discussion as to the function of the Weberian mechanism. The 
evidence at present obtainable is strongly in favour of the presumption that this 
singular mechanism is directly related to the hydrostatic function of the air-bladder ; 
nevertheless, we cannot but feel that the tentative suggestions which we have 
ventured to make as to its precise utility, while by no means inconsistent with them, 
yet derive but little positive support from any morphological or experimental data at 
present known. We have ventured to discuss this question at some length in order 
that the issues involved may be clearly understood, and in the hope that it will receive 
the attention of the experimental physiologist, in whose hands the final solution of 
the problem must rest. 
So far our remarks have been confined to the discussion of the methods by which 
pressure adjustment is effected in the Physoclisti and Ostariophysese, and there still 
remain for consideration a few Physostomi (e.^., Clupeidee, Salmonidae, Mursenidae, 
&c.), in which the air-bladder is said to be provided with an open ductus pneumaticus, 
