574 



PISCES FISHES. 



recall Crossopterygii ; in their cartilaginous skeleton 

 and persistent notochord they are 

 primitive ; in their lung, heart, in- 

 ferior vena cava, multicellular skin- 

 glands, and eggs they approach 

 Amphibians. 



The Dipnoi are physiologically 

 transitional between Fishes and 

 Amphibians, having, for instance, 

 acquired lungs while retaining gills, 

 but it does not follow that they 

 are morphologically transitional. 

 They are intermediate, but that is 

 not to say that they are the connect- 

 ing links. 



Ceratodus. The genus Cera- 

 todus is abundantly represented by 

 fossils in the Mesozoic beds of 

 Europe, America, Asia, and Aus- 

 tralia, but the living animal is now 

 limited to the basins of the Burnett 

 and Mary rivers of Queensland 

 (see Fig. 6). Like that other old- 

 fashioned animal the duckmole, 

 Ceratodus frequents the still, deep 

 places of the river's bed, the 

 > SSCE so-called "water -holes." At the 

 ~ 3 bottom of these it lies sluggishly, 

 occasionally rising to the surface 

 to gulp in air. Its diet was for- 

 merly supposed to be exclusively 

 vegetarian, but Semon holds that 

 it crops the luxuriant vegetation of 

 the river-banks only for the sake 

 of the associated animal life 

 larvae and eggs of insects, worms, 

 molluscs, amphibians, and fishes. 

 Though Ceratodus is quite unable 

 to live out of water, its air-breath- 

 ing powers enable it to exist in 



water which is laden with sand or rotten vegetable matter. 



$ S 



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