298 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
the anterior chamber which is prevented in all other Siluridse by the absolute rigidity 
of the processes in question, but it is at the same time equally clear that the “ elastic 
spring ” apparatus cannot possibly give the Fish any poAver of directly compressing 
the air-bladder, except under certain conditions, viz., when the anterior chamber 
becomes distended through the diminution of pressure which occurs in movements of 
ascent, coincidently with the forward movement of the two springs as the result of 
the voluntary or reflex contraction of their protractor muscles. Under such circum- 
stances the mechanism potentially acquires the power to modify the capacity of the 
aii’-bladder, for the subsequent relaxation of the muscles will at once enable the 
springs, through the force of their own recoil, to exert their full strength in com- 
pressing both the air-bladder and its gaseous contents.'^ If, therefore, the elastic 
springs can have no share in promoting ascent by rarefaction, and can only be called 
into action under conditions in which the distension of the air-bladder from pressure 
reduction is a necessary factor, the possibility that the mechanism is related to pres- 
sure adjustment is at once suggested. Tentatively adopting this suggestion it would 
seem that two alternative views may be taken as to the precise mode in which this 
singular mechanism is of practical utility : — 
(1.) That the compression of the air-bladder, under the conditions and by the 
means already indicated, will effectually aid the action of the ductus pneumaticus in 
producing a more rapid and forcible ejection of gas, and at the same time secure the 
more rapid readjustment of the Fish to the reduced pressure of a more superficial 
level. 
(2.) The advantage of a more rapid readjustment to diminished pressure may be 
gained by means of the “ elastic spring apparatus ” without necessarily involving a 
mechanical liberation of gas. The mere compression of the air-bladder would seem 
to be perfectly competent to prevent over-distension under a diminishing external 
pressure by simply condensing the contained gases, and so counteract the possible 
evil results of a too low specific gravity. If it be admitted that the Ostariophysere 
can control, or, if need be and occasion requires, entirely prevent the escape of gas 
through the pneumatic duct, it may easily be the case that pressure adjustment in 
those Siluridse with an “ elastic spring ” mechanism can be more readily brought 
about by condensation without the actual expulsion of gas. 
The sole advantage of the first alternative method is that it will secure a more 
rapid adjustment during ascent than is probably the case in most other Ostario- 
physeoe ; on the other hand we incline to the opinion that the augmented tension of 
* Should this view of the mode of action of the “ elastic spring ” apparatus prove correct, it will be 
difficult to see how the mechanism can have anything to do with the production of voluntary sounds, as 
suggested by Sorensen (36), inasmuch as the Fish would only be able to exercise its vocal powers under 
conditions involving pressure reduction during ascent from a deeper to a more superficial level. Under 
such conditions only does it seem likely that the contained gases would be expelled with sufficient force 
to produce any definite or characteristic sounds. 
