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character of the Cyprinidje, and some of this family have also a connection 

 between the anterior lobe and the acoustic organs by a chain of vesicles. Some 

 of the Cyprinoids have also air vessels in three divisions, placed one behind the 

 other, and in some of the Gurnards, a fish of the order of Acanthopteri, the air 

 vessel has three lobes placed side by side, while in others it is only bilohate. 

 In this species there are many various forms of air vessels. 



The nature of the air or gas conttiined in these bladders has been the 

 subject of repeated inquiry. Monsieur Fourcroy found some azote in the vessel 

 of a Carp, while Dr. Priestly states that he found in those of several fish oxygen 

 mixed with a considerable quantity of another gas, of which he had not found 

 out the nature. Dr. Brodbett examined some Sword fish, and found pure 

 nitrogen. Lacepede examined some Tench, and found hydrogen gas. YarreU 

 states that various chemists have found the air in different fishes to consist of 

 nitrogen, oxygen, and carbonic acid, the nitrogen in gi-eater jwoportion, and the 

 oxygen smaller, than in atmospheric air. In marine fishes the oxygen is in 

 excess, varying from 40 to 87 per cent., depending on the depth at which the 

 species usually remain. Fish, like the Gurnards, having closed air bladders are 

 the best subjects for this examination, because the vessels can be removed from 

 the interior of the fish without any of their contents being lost. In the air 

 bladders of all sea fish a gi-eater proportion of oxygen is found. Biot detected 

 as much as 87 per cent, of oxygen in the air bladder of deep-sea Mediterranean 

 fish. Professor Owen states generally that oxygen predominates in the air 

 bladders of aU sea fish living at great depths ; and that in most fresh-water 

 fishes the air bladder is filled with nitrogen, mixed with a very small quantity 

 of oxygen, and a trace of carbonic acid. According to Hunibolt there is 4 per 

 cent, of oxygen, and 9G per cent of nitrogen in the air bladder of the Gymnotus, 

 and Dr. Davy found in the air bladder of a Salmon in fresh water a little 

 carbonic acid, 10 per cent, of oxygen, the remainder being nitrogen. Carpenter 

 says that "the gas which the air bladder contains is composed of the same 

 elements as atmospheric air, namely, oxygen, nitrogen, and carbonic acid, but 

 these are mixed in proportions that are very liable to variation." 



From this conclusion, it has been argued that the excess of oxygen in the 

 bladders of sea fish is given for a special purpose, and intended to serve as a sub" 

 stitute for the deficiency of oxygen in the sea water itself. There is some weight 

 to be attached to this argument, and independently of the different physiological 

 proijerties of the gases, it is strengthened by the consideration of their different 

 specific gravity. The specific gravity of fresh water being much less than that 

 of sea water, being as 1,000 to nearly 1,030, and nitrogen being a much hghter 

 gas than oxygen, fresh-water fishes are supplied with the more buoyant gas ; and 

 thus in each the degree of buoyancy is adjusted to the specific gravity of the 

 water in which the fish lives. 



One evident purpose of the air-vessel is to lessen the specific gi'avity of the 

 body of the fish, which is in itself heavier than the quantity of water whose place 



