1892.] Contributions to the Anatomy of Fishes. 145 



atmospheric pressure variation might be negatived by a variation of 

 level in the water to extent of twenty-seven inches, or more or less 

 completely masked by similar movements of still less extent during 

 ordinary locomotion, or by the rise or fall of the tide in the case of the 

 few estuarine or marine species. It may also be urged in opposition 

 to the theory that there is no evidence that the Siluridse, or any other 

 Ostariophyseae, are in any way different from other Fishes in being 

 specially susceptible to atmospheric pressure variations, or that they 

 possess any special capacity for anticipating impending changes in 

 the weather. Sagemehl's theory being untenable, Hasse's view only 

 remains. The general structure of the air-bladder, the mode of inter- 

 connection of the different Weberian ossicles, and their relations to 

 the air-bladder and auditory organ, as well as the relations inter se of 

 the two last-mentioned structures, are perfectly consistent with this 

 theory, against which no anatomical objections can be urged, and 

 equally inconsistent with any other at present suggested. 



III. That the Weberian mechanism is of great functional importance 

 to the Fish possessing it admits of no doubt. It is extremely im- 

 probable that so complicated and highly specialised a mechanism 

 would have been evolved did it not confer some exceptional advantage 

 upon its possessor, and that this is the case seems to be clearly de- 

 monstrated by the significant fact that the presence of the mechanism 

 is characteristic of nearly all the dominant families of fresh-water 

 Teleostei. The precise utility of the mechanism is, nevertheless, a 

 very difficult problem. Assuming the correctness of Hasse's theory, 

 it is inconceivable that the pressure variations, which it is the func- 

 tion of the Weberian apparatus to register, can arise from any other 

 cause than the ascent or descent of the Fish in the water during 

 ordinary locomotion, and this at once suggests that the advantage of 

 the mechanism is directly related to some form of pressure adjust- 

 ment. In dealing with this aspect of the question it becomes neces- 

 sary to first consider the methods of pressure adjustment in Fishes in 

 general. 



Gaseous secretion and absorption are highly important factors in 

 the adjustment of the volume of the gases contained in the air-bladder 

 to variations of hydrostatic pressure. The conditions under which 

 these processes take place have been experimentally investigated by 

 Moreau, who has demonstrated that when the air is exhausted from 

 the air-bladder (by means of an air-pump in the case of Physostomous 

 Fishes, or by puncture in the Physoclisti) it takes from several hours 

 to several days to restore the abstracted air by secretion and for the 

 Fish to regain its normal liberty of movement. The rate of absorp- 

 tion is in fairly close agreement with that of secretion. It is obvious 

 that the rapidity with which these processes take place is an impor- 

 tant factor in determining how far they are likely to be available as 



VOL. LII. L 



