294 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
the light of such contradictory and mutually destructive evidence no satisfactory 
conclusion seems possible, but two alternative suggestions may be made. If the 
ductus pneumaticus cannot from any cause or other be used for the mechanical 
liberation of gas, then, in these Fishes, as in the typical Physoclisti, gaseous 
secretion and absorption must be the only methods of pressure adjustment. On the 
other hand, even if it be admitted that some Physostomi without Weber’s mechanism 
can liberate gas through the pneumatic duct, it is nevertheless not dilScult to see how 
it may be that the process is of little use to them for pressure adjustment. The 
completeness of the control exercised over the liberation of the necessary amount of 
gas will largely depend on the perfection of the reflex mechanism employed in the 
process, and in all the Fishes now under consideration the necessary afferent 
impulses must be initiated in the general peripheral nervous system by the dilfused 
pressure exerted by a distended air-bladder on the surrounding organs, instead of in 
a particular afferent nerve by a stimulus applied to a localized sensory epithelium 
through a highly specialized arrangement of movable ossicles as is the case with 
the Ostariophysese. The indefinite character of the stimulus in the former would 
certainly militate against any delicacy in the process of adjustment, even if it did 
not altogether prevent the possibility of any such adjustment. The more perfect 
afferent mechanism of the Ostariophysese conditions a more effective control over the 
function of the pneumatic duct, and a greater capacity for regulating the processes 
involved in pressure adjustment, and it may also be that this is the precise advantage 
which the Weberian mechanism confers upon all Fishes that possess it. 
It may be admitted that this suggestion does not necessarily preclude the possible 
use of the duct as a safety valve in certain cases, but it does negative the probability 
that any effective control can be exercised over the liberation of gas in ordinary 
locomotion. 
We cannot conclude this section of our paper without remarking that it would be 
extremely interesting to compare the behaviour of a Physostomous Fish without a 
Weberian mechanism with one in which this mechanism is present, when exposed both 
to rapid and gradual variations of pressure. A careful and experimental comparison 
of the methods and rate of adjustment in the two types under such conditions should 
go a long way towards the discovery of the true function and utility of the Weberian 
apparatus. 
There yet remain certain points in connection with the air-bladder of the Siluridae 
which it is desirable should be discussed from a physiological point of view ; these 
are (a) the lateral cutaneous areas ; (b) the “ elastic-spring ” apparatus of Muller ; 
(c) the extrinsic muscles of the Pimelodinae ; and (d) the distinctive features of the 
air-bladder and Weberian mechanism in the Siluridae as compared with other 
Ostariophyseae. 
(a.) The lateral cutaneous areas . — The close relation between the lateral walls of 
the anterior chamber of the air-bladder and the adjacent superficial skin, which is so 
