124 PROCEEDINGS OP THE ACADEMY OP [1885. 



ON THE AIR-BLADDER OF FISHES. 

 BY CHARLES MORRIS. 



The generally accepted explanation of the use of this singular 

 organ, that it serves to enable the fish to readily rise and sink in 

 the water, while it is in all probability true in a measure, has 

 undoubtedly been too greatly extended. It is usually offered as 

 applying generally to fishes with an air-bladder, with little 

 regard to the fact that in many cases the air-bladder is too small 

 to serve anj r useful purpose as a gravity organ. This being 

 the case, some further examination into its functions and organic 

 relations seems not amiss. 



Cuvier tells us that " the most obvious use of the swim-bladder 

 is to keep the animal in equilibrium with the water, or to increase 

 or reduce 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." This is, however, not always the case, for in many 

 cases the bladder is provided with compressing muscles, and, as 

 Van Der Hoeven says : " In many fishes it is difficult to show 

 how they are in a condition to expand the bladder and to rarefy 

 the air." Cuvier says further : " With regard to the presumed 

 assistance which the swim-bladder affords in respiration, it is :i 

 fact that, when a fish is deprived of that organ, the product of 

 carbonic acid b}^ the branchiae is very trilling ; but there is no 

 sufficient foundation for assuming that it oilers any analogy to 

 the lungs." This is no doubt true as regards the usual condition 

 of the organ. It may perform some function in facilitating the 

 exchange of gases in the blood, but this is not a direct 

 respiratory function. In some cases, however, its function is 

 directly respiratory, and in a few instances it constitutes an 

 actual lung, closely approaching the Batrachian lung in organ- 

 ization. 



A similar view is offered by the latest writers. Giinther, in 

 his "Study of Fishes," remarks that "this organ serves to 

 regulate the specific gravity of the fish, to aid it in maintaining 

 a particular level in the water, in rising or sinking, in raising the 

 front part of its body or depressing it as occasion ma\ require." 

 This theory is based on hypothesis, since it would be no easy 



' 



