2i8 LARVAL DEVELOPMENT 



region (Fig. 287), is to be noted the prominence of the 

 mouth cavity, and the enlarged gill arches, showing by 

 this time the outbudding branchial filaments. In the 

 stage of Fig. 288, the larva begins to appear shark-like ; 

 the fins are longer and more noticeable, the anus has 

 appeared, and the branchial filaments by continued growth 

 protrude at all gill openings. The external gills thus 

 acquired are seen in a later stage (Fig. 289) to have 

 disappeared ; they have aided, however, as Beard, Turner, 

 and others have shown, in absorbing nutriment, and must 

 be looked upon as an especial organ of the larval life of 

 the animal. Fig. 289 illustrates a final larval stage : in it 

 there appear all of the structures of the adult outward form, 

 e.g. shagreen, fin spines, nictitating membrane, anterior 

 and posterior nasal openings. This larva has been esti- 

 mated to be about a year older than that of Fig. 284. 



III. Larval Liing-fish 



The larval history of the lung-fish, Ceratodus, as recently 

 described by Semon, seems to offer characters of excep- 

 tional interest, uniting features of Ganoids with those of 

 Cyclostomes and Amphibians. 



The newly hatched Ceratodus (Fig. 290) does not 

 strikingly resemble the e^rly larva of shark (Fig. 284). 

 No yolk sac occurs, and the distribution of the yolk 

 material in the ventral and especially the hinder ventral 

 region is suggestive rather of lamprey or amphibian ; it 

 is, in fact, as though the quantum of yolk material had 

 been so reduced that the body form had not been con- 

 stricted off from it. The caudal tip in this stage appears, 

 however, to resemble that of the shark, and as far as can 

 be inferred from surface views a neurenteric canal persists. 

 Like the shark there then exists no unpaired fin ; the 



