554 



PISCES FISHES. 



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 connecting links. 



Ceratodus. --The genus Cera- 

 todus is abundantly represented 

 by fossils in the Mesozoic beds 

 of Europe, America, Asia, and 

 Australia, but the living animal is 

 now limited to the basins of the 

 Burnett and Mary rivers of 

 Queensland. Like that other old- 

 fashioned animal the duckmole, 

 Ceratodus frequents the still deep 

 places of the river's bed, the so- 

 called " water -holes." At the 

 bottom of these it lies sluggishly, 

 occasionally rising to the surface 

 to gulp in air. Its diet was 

 formerly supposed to be exclus- 

 ively vegetarian, but Semon holds 

 that it crops the luxuriant vegeta- 

 tion 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-breathing powers enable it to 

 exist in water which is laden with 



FIG. 269 Skeleton of Cera- sanc j O r rotten vegetable matter. 

 todus fin. From Gegen- 

 baur. Ceratodus sometimes attains a length 



a., Central axis ; ;-., radials ; /*., of 6 ft " The bod y is elongated and com- 

 basal piece. pressed, and bears a continuous vertical 



fin. The paired fins are trowel-like, 



with a median jointed axis, from which rays project on each side. 

 There are five gill-clefts, four internal gills, and a hyoid half-gill. 

 There are no external gills. 



The swim-bladder or lung is single. It is supplied with blood from 

 the fourth aortic arches, as is the swim-bladder of Polyptcrus 

 and Ainia. It arises venlrally, but lies dorsally, and is divided 

 into compartments. 



