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



traction of their protractor muscles. We do not think with Miiller 

 that condensation is of use in facilitating descent ; for a Fish in ap- 

 proximate 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. Sorensen's view that the mechanism 

 is related to the production of voluntary sounds by the forcible expul- 

 sion of air through the pneumatic duct does not seem to us well 

 founded, first, because the " elastic springs " are only able to forcibly 

 compress the air-bladder during ascent, and, secondly, because sounds 

 could only be produced in that way at the expense of a considerable 

 disturbance of the normal equilibrium of the Fish in the water and of 

 its locomotor activity. Two views may be taken as to the precise 

 mode in which this singular mechanism is of practical utility : 

 (1) the compression of the air-bladder may assist the action of the 

 ductus pneumaticus in producing a more rapid ejection of gas during 

 ascent ; or (2) by condensation alone may counteract the effects of a 

 too low specific gravity, and, at the same time, economise the con- 

 tained gases. The latter alternative, in our opinion, is the more 

 probable one. 



3. The extrinsic muscles (compressor muscles) of the Pimelodinse 

 in all probability have a function similar to that of the " elastic- 

 spring " apparatus. The function of the tensor tripodis muscle is 

 probably to limit the violent excursions of the tripus, which other- 

 wise would certainly take place when the bladder is forcibly com- 

 pressed by the contraction of the compressor muscles. 



4. The conclusion suggested by a comparison of the anatomical 



relations of the air-bladder and its associated skeletal structures is 



that, physiologically considered, the most important distinctive 

 features of the Weberian mechanism in the Siluridoe as compared 

 with other Ostariophysese are mainly related to the air-bladder, which 

 in the former attains its maximum degree of specialisation and 

 delicacy as an organ adapted for the registration of varying hydro- 

 static pressures. 



The Condition of the Air-bladder in the Siluridce Abnormales. 

 The most noteworthy features in the structure of the air-bladder in 

 the various genera of Siluridae abnormales are : the absence of lateral 

 chambers; the partial or complete constriction of the anterior 

 chamber into two diminutive laterally situated air-sacs, which may 

 lose all connexion not only with each other, but also with the 

 oasophagus ; and the occasional atrophy of the fibres by which the 

 lateral walls of the bladder are normally connected with the tripodes. 



