1892.] Contributions to the Anatomy of Fishes. 149 



and, so far as the point is concerned, these Fishes must resemble the 

 typical Physoclisti. (5.) On the other hand, even if it be admitted that 

 some Physostomi, without the Weberian mechanism, can liberate gas 

 through the ductus pneumaticus, it is nevertheless not difficult 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 peripheral nervous system by the diffused 

 pressure exerted by a distended air-bladder on the surrounding 

 organs, instead of in a particular afferent nerve by a stimulus applied 

 to a localised sensory epithelium through the Weberian mechanism, 

 as is the case with the Ostariophyseae. The indefinite character of 

 the stimulus in the former would certainly militate against any 

 delicacy in the responsive process of pressure 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, as we have suggested, this is the great advantage 

 which the Weberian mechanism confers upon all Fishes that 

 possess it. 



Certain structural adjuncts in connexion with the air-bladder of 

 the Siluridae may also be considered from a physiological point of 

 view. These are, (1) the lateral cutaneous areas ; (2) the " elastic- 

 spring " apparatus of Miiller ; (3) the extrinsic muscles of the Pime- 

 Iodina3 ; and (4), the distinctive features of the air-bladder and 

 Weberian mechanism in the Siluridse as compared with other Ostario- 

 physese. 



1. The lateral cutaneous areas probably enable variations in the 

 size of the anterior chamber of the air-bladder, the result of pressure 

 variations, more promptly to modify the volume and therefore the 

 specific gravity of the Fish, and consequently ensure a corresponding 

 increase in the delicacy of the responsive processes involved in pres- 

 sure adjustment. 



2. Miiller held the opinion that the " elastic-spring " apparatus is 

 a mechanism for the condensation and rarefaction of the gases in 

 the air-bladder. We cannot agree with Miiller that the elastic springs 

 can have any share in dilating the air-bladder or rarefying the gases 

 which it contains, and it is doubtful if the apparatus can possibly 

 give the Fish any power of directly compressing the bladder except 

 under certain conditions, viz., when the anterior chamber becomes 

 distended through the diminution of pressure which occurs during 

 ascent in the water, coincidently with the forward or outward move- 

 ments of the two springs as the result of the voluntary reflex con- 



