OF FISH IN GENERAL. 



llemarks on the air-bladder. 



through them, until the animal has drawn the 



o 



proper quantity of air from it; then the bony 

 covers open, and give it a free passage, by which 

 means also the gills are again opened, and admit, 

 a fresh quantity of water. If the fish be pre- 

 vented from the free play of its gills, it soon falls 

 into convulsions, and expires. 



Notwithstanding this plausible explanation of 

 the respiration offish, it still remains a difficulty 

 not easy to be solved, What is done with this 

 air ? There seems to be no receptacle for con- 

 taining it, except the air-bladder or swim, which 

 many modern philosophers are of opinion is only 

 to enable the fish to rise or sink at pleasure, and 

 not destined to answer any vital purpose. 



It should be observed that the air-bladder is ^ 

 vesicle found in the bodies of all spinous, or bony 

 fish, though not in those of the cartilaginous or 

 cetaceous kind, situated towards the back of the 

 fish, opening to the maw or gullet, and composed 

 of one, two, or three divisions. It is thought 

 that the animal possesses a power of distending 

 or contracting this bladder, and of consequence 

 becoming specifically lighter or heavier than the 

 fluid in which it swims, and thence to rise to the 

 top, or sink to the bottom of the water at plea- 

 sure; and that such is its use, seems deducible 

 from the following experiment: A carp being 

 placed in an air-pump, and the air exhaused, the 

 fish soon swelled to such a degree that his eyes 

 itarted from his head, and the bladder burst by 



VOL. V. NO. 30. B 



