GENERAL ZOOLOGY 



probably gave rise to the Amphibia and so to the land vertebrates before it 

 became extinct. Another line, the ray-finned fishes, was ancestral to the bony 

 fishes of the present, in which the primitive lung was transformed into a 

 hydrostatic organ, the swim bladder. 



It is significant in this connection that the most primitive of existing 

 ray-finned fishes, such as Polyptems (Fig. 18.11), the ''bichir" of the Nile, 

 retains the lung in its original function. Another and independent survival 

 of the primitive lung appears in the three genera of lungfishes or Dipnoi, 

 Ceratodus of Australia, Protoplerus of South Africa, and Lepidosiren of 

 South America. In these, the lung functions in respiration as an accessory 

 to the gills, particularly in times of drought and stagnation of the water 

 in which they live. In North America, the sturgeon, Scaphyrhynchus, the 

 paddlefish, Pnlyodon, the gar pike, Lepidosteus, and the bowfin, Amia, are 

 ray-finned fishes of a primitive type. The more specialized ray fins include 



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