510 PISCES FISHES. 



as the compressed air is again permitted to expand, the creature 

 becoming more buoyant rises towards the surface. 



In the Perch, and many other fishes, this organ is entirely 

 closed, so that there is no escape for the contained air; and in such 

 it has been found that if they are suddenly brought up by means 

 of a line from any great depth, the gas being no longer compressed 

 by the weight of the column of water above, and having no exit, 

 bursts the swimming-bladder, and sometimes distends the abdomen 

 to such an extent, that it pushes the stomach and oesophagus into 

 the fish's mouth. 



In other cases, however, a provision is made apparently with a 

 view of obviating such an accident, and a kind of safety-valve pro- 

 vided, through which the air may be permitted to escape : thus, in 

 the Carps a tube communicates between the interior of the air- 

 bladder and the oesophagus, and in the Herring a similar commu- 

 nication is met with between this organ and the stomach. 



The gas which fills the air-bladder has been found in many 

 cases to be nearly pure nitrogen, but in fishes that live at a great 

 depth Messrs. Configliacchi * and Biot ascertained that oxygen 

 was substituted, whence it has been presumed that this apparatus 

 was in some way or other an auxiliary in respiration ; and some 

 authors have even gone so far as to see in the swimming-bladder 

 the representative of the lungs of aerial Vertebrata. But, however 

 this may be, the gas enclosed is indubitably a product of secretion, 

 being derived either from the lining membrane of the viscus, or 

 from a glandular structure which may frequently be distinctly 

 pointed out in its interior. 



Cuvier justly observes, that, whatever opinions may be enter- 

 tained relative to the use of the air-bladder, it is difficult to 

 explain how so considerable an organ has been refused to so many 

 fishes, not only to those which ordinarily remain quiet at the bot- 

 tom of the water, as Skates and Flat-fishes, but to many others 

 that apparently yield to none either in the rapidity or facility of 

 their movements, such as the Mackerel, for instance ; yet even 

 while the common Mackerel (Scomber scomber) has no air-blad- 

 der, a very nearly allied species (Scomber pneumatophorus) is 

 provided with one, and of this many other instances might be 

 adduced. 



(544.) From the circumstances under which fishes seize and 

 swallow their prey, it must be evident that they are incapable of 



* Suir analisi dell' aria contenuta nella vesica natatoria del Pesci. Pavia, 1809. 4to. 



