648 PHYLUM CHORDATA : CLASS PISCES — FISHES 



the latter becomes included in the buccal cavity and forms 

 the internal nares, whilst the former, remaining outside, 

 forms the external nares. 



Development. — The eggs exhibit total unequal seg- 

 mentation. In Ceratodiis the development is direct, 

 without any larval stage. In Protopterus and Lepidosiren 

 there is a metamorphosis, the larva resembling a young 

 Amphibian or young Polypterus, with four pairs of large 

 feathery external gills, richly supplied with blood, and 

 bearing a glandular area or '' cement-organ " on the 

 underside of the head. 



