LUNG-FISH 



375 



a right and a left lung. An important point which has emerged from 

 the study of its development is that for a time it shows a distinctly 

 lop-sided arrangement as in Polypterus, the morphologically right lung 

 being larger than the left. Correlated with this the lung apparatus as 

 it grows backwards becomes twisted round the right side of the alimentary 



.- oip 



Fig. i6i. 



Ceratodus. A, Ventral surface of skull of adult ; B, roof of mouth of young specimen ; 

 C, teeth of roof of mouth of a young specimen. (A, from Zittel's Palaeontology t after Giinther ; 

 B and C, from Graham Kerr's Embryology , after Semon.) olf.^, Anterior uaris; olj.^, posterior 

 naris ; /, compound tooth. 



canal until it lies completely dorsal and reversed in position, the original 

 right lung being now on the left side. In Lepidpsiren and Protopterus 

 the lagging behind of the original left lung is only temporary, it soon 

 goes ahead actively with its growth and the two lungs come to be equal 

 in size. In Ceratodus, however, no such recovery in the growth activity 

 of the original left lung appears to take place : on the contrary it 



