204 SUB-CLASS (AND ORDER) TELEOSTEI. 



narchus the efferent vessels of the third and fourth branchial 

 arches go exclusively to the air bladder (Hyrtl). The veins 

 join either the system of the posterior cardinal, or the hepatic, 

 or the portal. 



The air-bladder is extraordinarily variable in its occurrence. 

 It is entirely absent in some families, e.g. Bhnnidae, Pleuro- 

 nectidae, Symbranchidae. It may be present in some genera of 

 a family and absent in others, or even in different species of 

 the same genus.* 



Several functions f have been ascribed to the air-bladder ; it 

 has been said to be hydrostatic, a resonator, sound producing, 

 and respiratory. There can be but little doubt that it is a 

 hydrostatic organ : % its function appears to be to keep the 

 weight of the fish equal to the weight of the volume of water it 

 displaces. Thus if the fish smks, its body is compressed and 

 the specific gravity is increased. To meet this the air-bladder 

 slowly secretes gas, which distends the bladder and so restores 

 the specific gravity of the fish to its former point. Further, 

 when the fish rises, its air-bladder becomes distended and its 

 specific gravity diminishes. The fish consequently has some 

 difiiculty m preventing its body rising to the surface. To meet 

 this, the superfluous gas is slowly absorbed and the air-bladder 

 becomes reduced in bulk so that the specific gravity of the fish 

 returns to its normal point. In the Ostariophysi the reduction 

 of pressure causes the fish to expel the gas through its pneu- 

 matic duct, but this does not always occur in other fishes with 

 pneumatic duct, though possibly it does so in some cases. In 

 fishes without pneumatic duct the only way in which the super- 

 fluous gas can be removed is by absorption. With regard to 

 the process of secretion, it takes place so slowly that it would 

 not be worth while for a flsh to change its depth unless it meant 

 the change to be of some duration. Moreover, Biot,§ and more 

 recently Moreau, have shown that the gas secreted is mostly 

 oxygen. The gas in the air-bladder of fishes taken near the 

 surface contains nitrogen, oxygen, and a trace of carbonic acid 

 (not more than 1 or 2 per cent.). The nitrogen in such cases is 



* See Stannius, Handbuch, 2nd edit. he. cit., p. 221. 



t Vide W. Sorensen, in Journal of Anato^ny and Physiology, 29, 1895, 

 p. 109, et acq. 



X A. Moreau, Recherches exp. s. 1. functions de la vessie natatoire, 

 Ann. d. Sci. Nat., 4, 1876. ' 



§ Mimoires d. I. Societe d'Arcueil, 1, 1807. 



