id OF FISHES IN GENERAL. 



of weight, according to its depth, the fiflies, by thus va» 

 rying their fpecific gi-avitj, can poife themfelves in any 

 part of it *. That this is tlie 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 efcape, obliges the fifties, after fome ef- 

 forts, to fink to the bottom of the water. This opi- 

 nion feems to receive confirmation from the manners of 

 tbofe kinds where it is wanting. All flounders and 

 ground-fillies are deftitute of a fwimmmg bladder ; and, by 

 comequence, lie conftantly at the bottom ; while the ce- 

 taceous and cartilaginous orders, which often fupport 

 themfelves at the furface, without the aid of this in- 

 Urument, are fupplied v/ith 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 

 fwimming bladder. Dr. Ncedhayn obferved, that in the 

 greater part of fiflies, whether this organ was fingle, as 

 in fome fpecies, or double, as in others, it was furniflied 

 wuth a duft or canal that opened into the ftomach : 

 Hence he concluded, that its ufe v»ras to colIe(rt the air 

 fecreted from the blood, and to convey it into the llo- 

 mach, where it afnfted the procefs of digefiion f . Ii^ 

 this manner, he endeavoured to account for the extraor- 

 dinary voracity of fifties, and their uncommon powers of 

 digeftion. 



No 



• Willoubii Ichthyol. p. 13. f De formato fcEtu, ch. vi. 



