ON THE ANATOMY OF FISHES. 
299 
the gas itself under such conditions will constitute a sufficient expulsive force and 
render any special mechanism for the purpose altogether superfluous. The more 
obvious value of the second alternative lies in the fact that, while it may prove to be 
a method of adjustment even more rapid than the first, it also possesses the con- 
tingent economical advantage that as no gas has been lost by mechanical liberation, 
there will be no demand on the activity of the walls of the air-bladder for the 
secretion of additional gas when the Fish returns to a deeper level and greater 
pressure. On tliis view the processes of pressure adjustment will not consist in the 
actual expulsion of gas and its subsequent slow replacement by secretion, but rather 
in the volumetric changes of a fairly constant quantity of gas, which are affected by 
the action of the “ elastic spring ” apparatus, and are always of the opposite character 
to those which the external hydrostatic pressure tends to produce. Of the two 
alternatives the second seems to us the more feasible, but experiment alone can 
determine the accuracy of either, while it may prove the falsity of both. 
In a limited sense our second suggestion is in accordance with Muller’s theory, at 
least to the extent that condensation may be produced by the action of the elastic 
springs, although, as we think, for a different purpose than that which the theory 
maintains, but his view that the mechanism enables the Fish to ascend through the 
rarefaction of the contained gases must, for the reasons previously given, be regarded 
as entirely without foundation, and, to say the least, it is extremely doubtful if con- 
densation can be of much, if any, use in facilitating descent. But apart from the 
special objections to the theory there are others of a more general character, based on 
the apparent uselessness of any special mechanism for causing the rise or fall of the 
Fish in the water. It has been previously pointed out that to Fish in approximate 
equilibrium the slightest action of the ordinary locomotor organs is quite sufficient to 
produce either ascent or descent, and the existence of an elaborate mechanism for 
varying the internal capacity of the air-bladder and the volume of the enclosed gases 
with this object is altogether unnecessary. These objections to Muller’s theory 
will apply with equal force whether it be presumed that condensation, or the actual 
expulsion of gas, is the prime agent in promoting descent. On the other hand the 
unquestionable importance to the Fish of rapid and accurate methods of pressure 
adjustment is highly suggestive that the “ elastic spring ” mechanism is solely related 
to this function. We prefer, therefore, to regard the mechanism as a contrivance for 
expediting the process of pressure adjustment when the Fish is exposed to a rapid 
reduction of pressure during quick ascent, which, far more than descent, necessitates 
both rapidity and accuracy in the readjustment processes, and this view may reason- 
ably be adapted whichever of the two alternatives is accepted as to the precise 
method employed. Should our views on this point prove to be correct, it would be 
reasonable to anticipate that of all the Ostariophysem these Siluridm are best (pialified 
for rapid and extensive vertical locomotion under conditions the most favourable to 
2 2 
