148 Profs. T. W. Bridge and A. C. Haddon. [June 16, 



duct exhibit some structural analogies to the gall-bladder and cystic 

 duct of Mammalia, and bearing in mind that the absence of intrinsic 

 muscles in the walls of the bladder is associated with the fact that the 

 tension of the contained gases under the influence of reduced hydro- 

 static pressure will supply the needful expulsive force, it is by no 

 means improbable that a close physiological parallelism may also 

 exist with regard to the escape of their respective contents. 



Whatever may be the precise nature of the controlling mechanism, 

 the advantage to the Fish of some method of carefully graduated 

 adjustment to pressure variations is sufficiently obvious. Without 

 any form of regulatory control, and with an open ductus pneumaticus 

 in free communication with the exterior, it may be surmised that the 

 escape of gas would be continuous and unchecked, and might even 

 involve a more or less complete exhaustion of the gas in the bladder 

 as the pressure diminished, .with the contingent disadvantage that the 

 normal equilibrium of the Fish in the water would be greatly dis- 

 turbed, and a considerable demand be made on the secretive activity 

 of the bladder for the subsequent restoration of the gas. On the 

 other hand, the existence of a controlling mechanism would ensure 

 that only so much gas will be evolved as may suffice to maintain the 

 Fish in a plane of equilibrium, and, at the same time, secure the 

 needful economy in the liberation of the gas. A further advantage 

 in the speedy adjustment to alterations in pressure is that there will 

 be less expenditure of energy in the performance of ordinary move- 

 ments, inasmuch as the Fish can readily find a plane of least effort ; 

 otherwise it would have to counteract a too feeble, or an increased 

 specific gravity, by additional muscular effort. In the light of these 

 considerations, but with the qualification which the absence of direct 

 experimental evidence necessitates, we conclude that the Weberian 

 mechanism not only confers on all Fishes that possess it an excep- 

 tional capacity for freedom of locomotion in the vertical direction, 

 but also entails the contingent advantage that all movements will be 

 effected with the maximum economy of muscular effort and tissue 

 metabolism. 



With regard to those Physostomi which have no Weberian 

 mechanism, the evidence as to their ability to make use of the ductus 

 pneumaticus as a means of pressure adjustment is very conflicting, 

 and, moreover, it is not even certain that in all cases the duct is in 

 free communication with the exterior. The frequent presence of retia 

 mirabilia would also suggest bhat gaseous secretion and absorption are 

 important factors in pressure adjustment. In the light of such con- 

 tradictory evidence, no satisfactory conclusion is, at present, possible, 

 but two alternative suggestions may be made, (a.) If the ductus 

 pneumaticus cannot, from any cause, be used for pressure adjustment, 

 gaseous secretion and absorption must be the only methods employed, 



