ECHINODERMATA. 



471 



fairly advanced the part of the body with the provisional skeleton forms an 

 elongated rod-like process attached to the developing disc. It becomes 

 eventiially absorbed. 



Echinoidea. The Echinus larva (fig. 263) has a Pluteus form 

 like that of the Ophiuroids, and in most points, such as the presence of 

 the anal lobe, the ciliated band, the provisional skeleton, etc., develops 



FIG. 264. Two LARVAE OF STEONGTLOCENTBUS. (From Agassiz.) 

 m. rnoutb; a. anus; o. oesophagus; d. stomach; c. intestine; v . and v. ciliated 

 ridges; w. water-vascular tube; r. calcareous rods. 



in the same manner. The chief difference between the two Pluteus 

 forms concerns the development of the lateral arms. These, which 

 form the most prominent arms in the Ophiuroid Pluteus, are entirely 



FIG. 265. LATERAL AND VENTRAL VIEW OF A LARVA OF STRONGYLOCENTRUS. 

 (From Agassiz.) General references as in fig. 264. 



b. dorsal opening of madreporic canal; e'. posterior arms; e'". anterior arms; 

 f' T . anterointernal arms. 



