10 
OF FISHES IN GENERAL. 
of weight, according to its depth, the fifties, by thus va- 
rying their fpecific gravity, can poife themfelves in any 
part of it *. That this is the ufe of, the air bladder, 
fome naturalifts pretend to have afcertained by experi- 
ment : An incifion made into this organ, by which the 
air is allowed to .eicape, obliges the fillies, after fome ef- 
forts, to fink to the bottom of the water. This opi- 
nion feems to receive confirmation from the manners of 
thofe kinds where it is wanting. All flounders and 
ground-fiftes are deflitute of afwimming bladder ; and, by 
confequence, lie eonttantly at the bottom ; while the ce- 
taceous and cartilaginous orders, which often fupport 
themfelves at the furface, without the aid of this in- 
flrurnent, are fupplied with lungs, which ferve the fame 
purpofe, by admitting the air, in much greater quantity, 
into the cavity of their bodies. 
So far this matter feemcd to be clear ; but, upon ex- 
amining this lingular organ, it appeared to be fupplied 
with no mufcles that could enable the animal to contract 
or dilate it, in a voluntary manner. Two other opinions 
have, therefore, been formed concerning the ufe of the 
fvvimming bladder. Dr. Needham obferved, that in the 
greater part of fiftes, whether this organ was fingle, as 
in fome fpecies, or double, as in others, it was furniflred 
with a duct or canal that opened into the llomach : 
Hence he concluded, that its ufe was to collect the air 
fecreted from the blood, and to convey it into the llo- 
mach, where it affified the procefs of digellionf. In 
this manner, he endeavoured to account for the extraor- 
dinary voracity of fillies, and their uncommon powers of 
digellion. 
No 
* tyilloubii Ichthyol. p. 13. f De formato ch. vi. 
