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that this organ always contains a greater partxof oxygen 
whenever the animal is in a normal condition, that'this oxygen 
disappears little by little if the animal cannot any longer derive 
it in the midst of its surroundings, and it perishes asphyxiated. 
The air-bladder, excluding those forms which respire air, is 
generally found after death tightly distended with gas, and 
this consists chiefly of nitrogen in the fresh-water forms, and 
oxygen in marine genera, this latter substance augmenting in 
proportion in sea fishes in accordance with the depth at which 
the fish is captured. It has formed a subject of considerable 
discussion as to how this gas is generated, but, as in those 
classes in which the air-bladder is a closed sac, (Physoclisti) 
it is as well seen as in others possessing a pneumatic tube, 
(Physostomi) one cannot resist believing that the gas must be 
eliminated from the blood-vessels lining the interior of the 
organ. Probably the gland serves the special purpose of 
removing superfluous gas or any deletetious substance, while 
the pneumatic tube is not employed to admit the ingress of 
air, but acts as a safety-valve when the organ is too tightly 
distended. 
But the reason why the air-bladder is refused the title of 
“Jung” in most fishes is because it is usually supplied with 
blood from the adjacent arteries of the body not direct from 
the heart, and returns venous blood into the general circulation. 
In Ceratodus, however, although there is no special vessel 
employed to convey blood to the air-bladder, still we observe 
this organ possessing a higher functional development than 
exists in Acipenser for instance, as the returned blood is purified 
and subsequently conveyed direct to the heart, which arrange- 
ment does not obtain in Amia or Lepidosteus, both of which 
fish evidently use their air-bladders for respiratory purposes. 
Lepidosiren is doubtless the highest known form of living fish, 
forming a transitional link between Amphibia and Fishes. It 
is allied to Amphibia, as it possesses a heart composed of three 
chambers, two auricles and a ventricle; it has lungs (the air-blad- 
der) which receives venous blood direct from the heart, to which 
it returns it arterialised; it likewise has rudimentary external 
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