FISH IN GENERAL. 9o 



its relative weight, and thereby cause it to ascend or sink in 

 proportion as that organ is dilated or compressed. For this 

 purpose the fish contracts the ribs, or allows them to expand. 

 Certain we are, that when the air-bladder is burst, the animal 

 remains at the bottom, turns up the belly, and becomes defi- 

 cient in the powers of motion. A curious phenomenon is 

 observed in fishes caught with hook and line at great depths, 

 and drawn up suddenly : for the air contained in their swim 

 bladders expands, as they are ascending, more rapidly than 

 they can counteract by compression, and either it bursts, and 

 the abdomen becomes inflated, or by expanding forces the 

 oesophagus and stomach out at the mouth. With regard to 

 the presumed assistance which the swim bladder affords in 

 respiration, it is a fact that when a fish is deprived of that 

 organ, the production of carbonic acid by means of the bran- 

 chiae, is very trifling : but there is no sufficient foundation for 

 assuming that it offers any analogy to lungs. There are how- 

 ever grounds for admitting, that in many species, in whom 

 the anterior part of the swim bladder approaches the auditory 

 apparatus, an additional power of hearing is thereby conferred, 

 and small bones considered as analogous with the malleus 

 incus and stapes, appear to be in contact with it in several 

 Gangetic fisbes. 



But whatever the use and importance of this organ may be, 

 there remains the difficulty of explaining why it should have 

 been denied to a great number of species, not only of those 

 whose habits require a constant residence on the floor of the 

 waters, such as rays and pleuronectes, but also to others who 

 do not appear to yield to any in velocity and activity of life. 

 Such, for instance, as mackerel, for in this case the presence 

 or absence of the swim bladder is not even in conformity with 

 the other organs of the fish, there being a species (scomber 

 pneumatophorus) very similar in other respects to the common 

 mackerel, and yet provided with an air bladder, although no 



