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of the organ to sustain, and at wiU also to elevate the body of the fish in the 

 water, has been proved by observing, what has been tried, that when the bladder 

 is burst the fish grovel at the bottom; and also that Flounders, Soles, and Skates, 

 which have no air-vessels, seldom rise in the water, and then with effort." And 

 he says, "This power is derived by the contraction of the bladder when the fish 

 descends, and the expansion when it rises." This has doubtless been the general 

 beUef, but Mr. Carpenter states that Vhia is an error, as, by experiments made, 

 the fish stiU retains the power of raising or lowering itself in the water after the 

 organ has been removed. 



In order to reconcile these statements, we must remember the great distinc- 

 tion before noticed between fish that have closed sacs, and those that have sacs 

 provided with an outward communication. Now when the sac is closed, and full of 

 air, it seems impossible to eject the air and so loose the buoyancy, though certainly 

 the compression of the bladder and therefore of the air may have the same 

 effect in a degree, but in these cases it would seem that the air vessels are given 

 feimply to fulfil passive purposes, and that the fish being thus made buoyant, as 

 before said, has no weight to Hft, and is able to raise and depress itself by the 

 smallest amount of muscular exertion. When there is a pneumatic duct 

 attached, it is evident that the fish, by muscular action, can discharge the air 

 through the duct, and is thus enabled to lessen its specific gravity, and pass mora 

 rapidly downwards through the water. Many of our fresh-water fish of prey, 

 the Pike for instance, have very strong ligaments connecting the air-bladder 

 throughout its length with their ribs, and thus by sudden movements of the 

 body have instantaneous control over that organ. The Pike, as is well known, 

 remains for a length of time stationary, or slowly moving in mid-water, until, 

 sighting its prey, it makes a rapid dari; in pursuit, and thus must find such an 

 immediate control over its own specific gravity of the greatest use. As a general 

 rule in aU cases where there is a pneumatic duct there is also a power given by 

 muscles or ligaments to act directly on the air bladder and lessen its contents. 



The changes of temperature to which shallow water is so manifestly 

 liable must often increase very much the size of the air-vessel, and would seem 

 in itseH sufficient to prove a source of great discomfort to the fish. It is 

 probably due to this cause that in hot weather, especially when it has come on 

 suddenly, the fish in ponds are observed moving about in shoals upon the 

 surface in a thoroughly Ustless way, refusing to bite, or take any interest in 

 things in general. The provision of an air-duct in fresh water fishes, to wHch 

 of course these remarks chiefly apply, would more or less quickly give them the 

 required relief. 



The Air vessel has also been considered to be a rudimentary organ of 

 respiration. Fish constitute the lowest order of the vertebral branch of the 

 animal kingdom, and many anatomists have sought to give to the air vessel the 

 character of a homologue of a limg. Professor Owen considers it to be this, and 

 Dr. Eoget says " it is regarded by many of the German naturaUsts as having 



