ADAPTATIONS 387 



they begin as a widely open median groove, on the ventral side 

 of the pharynx, and this outgrowth as it extends backwards 

 divides into two horns, which almost from the first are unequal 

 in length. The development in Dipnoi has yet to be studied. 



Thus on the whole whether originally dorsal or ventral it 

 seems most probable that the air-bladder had nothing to do in 

 its evolution with gill-sacs, but was an outgrowth of the gullet 

 or stomach, behind the heart, between this organ and the liver. 

 If air was swallowed for respiratory purposes it would seem 

 most probable that it would cause a dilatation of the dorsal 

 side from its tendency to ascend. In any case as a part of the 

 alimentary canal behind the gills, the bladder would naturally 

 receive its arteries from the dorsal aorta or dorsal branchial 

 arteries, and the evidence of Polypterus and the Dipnoi is in 

 favour of the ventral connection with the gullet being the more 

 primitive. 



Among Teleostei with open air-bladders there are some 

 cases in which the respiratory function of the organ is retained, 

 and in fact the majority of the fresh-water forms at least seem 

 to have the power of respiring air to some extent, as we see in 

 the common gold-fish. Species of Erythrinus, one of the 

 Characinidag, living in fresh waters in Brazil, were found by 

 Jobert to die of asphyxia if the pneumatic duct were ligatured. 

 The same result occurred also in Sudis gigas, one of the Scope- 

 lidae. 



The hydrostatic function of the air-bladder, that is to say 

 its effect on the buoyancy of the fish, especially in the sea where 

 there is a great range of depth, involves some interesting physi- 

 cal principles which are not generally appreciated. Every one 

 knows that when fishes are caught and brought to the surface 

 even from depths of only a few fathoms, the air in the bladder 

 expands to such a degree that the stomach of the fish is often 

 pushed out through the mouth. When the pressure on a gas 

 is diminished by a half, the gas expands till its volume is 

 doubled : at a depth of only five fathoms in the sea, the pres- 

 sure on a fish is double the atmospheric pressure on the surface, 

 and increases by one atmosphere for every five fathoms of depth. 

 It is easy, therefore, to understand the great force of expansion 

 exerted by the gas enclosed in the air-bladder when a fish is 

 suddenly brought to the surface. The effect of the air-bladder 



