374 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



The scales of the Lung-fish are rounded and overlap like the cycloid 

 scales of Teleosts. They are largest in Ceratodus, smallest and deeply 

 buried in the skin in Lepidosiren. 



The teeth are in the young Ceratodus (Fig. i6i, B) simple pointed 

 cones like those of a young shark, but as they develop irregular strands 

 of bone spread out from their bases so that they are connected together 

 into groups (Fig. i6i, C). With further development the spongework 

 of modified bone which connects the denticles of each group increases in 

 extent and compactness, assumes a ridged form, and constitutes the 



Lung-fishes. 



Fig. i6o. 

 .\, Ceratodus ; B, Protopterus ; C, Lepidosiren, 



characteristic " tooth " of the adult (Fig. i6i. A, t), the sharp points of 

 the original denticles soon disappearing. 



The branchial apparatus is of the usual fish type — ^the spiracle having 

 disappeared and the remaining clefts being overlapped by an operculum. 

 In Lepidosiren the respiratory lamellae are reduced to irregular little 

 projections. 



The feature of the Dipnoi which probably in great part accounts for 

 their having been able to survive, in spite of the pressure of competing 

 and more highly evolved fishes, is the high development of the lung as 

 a breathing organ. In the young Dipnoan the lung arises in normal 

 fashion as a downgrowth from the floor of the pharynx which grows 

 backwards, and in the case of Lepidosiren and Protopterus forks to form 



