84 FROM FISH TO PHILOSOPHER 



It is an interesting postscript to the evolution of lungs 

 that the air bladder continues to be used as a lung only 

 in the Dipnoi and a few primitive forms such as 

 Poltfpterus of the Nile and the bowfin and gar pike of 

 North American rivers. Among most of the higher fishes, 

 this once lifesaving device has been converted to a swim 

 bladder, which may or may not be sealed off (if sealed 

 off it is filled with oxygen secreted from the blood) , and 

 which serves as a hydrostatic organ to enable the fish to 

 live at a convenient depth (as in the common trout and 

 salmon); or it has disappeared entirely (except for a 

 transient appearance in the embryo) in many fishes- 

 such as the mackerel, which is specialized for speed, 

 the flounder, which is specialized for bottom life, and the 

 lumpsucker, which is specialized for suctorial attach- 

 ment to rocks. 



