THE ORIGIN OF MAN 43 



Nowhere in the sea has any animal an additional 

 organ for extracting oxygen directly from the 

 air, excepting in such stocks as are known to 

 have had land-living ancestors. 



There is in all fishes above the sharks a struc- 

 ture known as the swimming bladder or air blad- 

 der, which is structurally a sac-like outgrowth, 

 single or paired, of the alimentary canal. There 

 are, however, two exceptions to this rule, first in 

 those fishes which because they inhabit swiftly 

 running waters, have developed quick darting 

 movements and have therefore lost the air blad- 

 der; and, second, in the flounders that lie upon 

 the bottom awaiting their prey. The principal 

 function of this structure is to give buoyancy to 

 the animals, and it is controlled by muscles in 

 such a way as to permit its possessor to remain 

 at any desired level in the water. The air blad- 

 der is the homologue of the lungs of the terres- 

 trial vertebrates and is utilized by the so-called 

 lung fishes as a respiratory organ supplementary 



