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



fers upon its possessor, there are certain attendant disadvantages, of 

 which, perhaps, the most important is the restriction of freedom of 

 locomotion in a vertical direction, the result of the slowness with 

 which the necessary secretion or absorption of gas takes place. 



The conclusions embodied in the preceding sections relate more 

 particularly to the Physoclisti, by far the largest group of Teleostean 

 Fishes, but it may be pointed out that in a general way they apply 

 also to the Physostomi, with, however, the qualification that in the 

 great majority of the latter group the mechanical liberation of gas 

 through the ductus pneumaticus takes the place of absorption as a 

 means of adjustment to reduced hydrostatic pressures. 



From the conclusions established by Moreau and Charbonnel-Salle 

 it is obvious that the varying pressures to which the gases contained 

 in the air-bladder a,re exposed constitute an important factor in the 

 physiology of locomotion in Fishes, and hence, in the absence of any 

 other tenable hypothesis as to its function, there is a strong antecedent 

 probability in favour of the view which Hasse was the first to suggest, 

 vi z . : that the object of the Weberian mechanism is to bring 

 directly to the consciousness of the Fish the varying tensions of the 

 gaseous contents of the air-bladder, due to the incidence of varying 

 hydrostatic pressures. The late Dr. Sagemehl also adopted Basse's 

 view, at least, so far as to regard the mechanism as a register of 

 pressure variations, but with this important modification, that it is 

 not hydrostatic but atmospheric pressure which the Fish is thereby 

 enabled to appreciate. There are, however, certain grave objections 

 to Sagemehl's ingenious theory. 



To a Fish at a depth of, say, six feet below the surface of the water, 

 a variation of atmospheric pressure sufficient to raise or depress a 

 column of mercury in a barometer to the extent of half an inch will 

 only involve a variation of pressure amounting to less than one- tenth 

 of the already existing hydrostatic pressure ; and even this trifling 

 difference will become relatively smaller as the depth at which the 

 Fish lives becomes greater, while the ascent or descent of the Fish in 

 the water to the extent of only seven inches would certainly mask any 

 variation of atmospheric pressure to the extent indicated, seeing that 

 the animal can have no power of differentiating the effects due to the 

 incidence of the two pressures. A barometrical variation of even 

 half an inch takes place but slowly, and rarely occurs in less time 

 than several hours, and consequently could only be appreciated as dis- 

 tinct from hydrostatic pressure if the Fish remained at exactly the 

 same depth in the water during the whole time that the barometrical 

 variation was in progress. The maximum range of variation in atmo- 

 spheric pressure, as measured by the barometer, is about two inches, 

 but such variations occur only at considerable intervals of time, and 

 then may take hours to accomplish. Even in this extreme case the 



