PHYLUM ECHINODERMATA 



397 



In the Asteroidea the larva is either a Bipinnaria (Fig. 315, 

 4 to 6) or a BracTiiolaria, The former has a series of bilaterally 



Fin. 315. Diagrams of the development of the larvae of Echinoderms : 1, Primitive form of 

 fichinoderm larva; 2 and 3, Development of an Ani-i<-nla.,-ia (Holothuroidea) ; 4, 5, and 0, 

 Development of a Bipinnaria (Asteroidea) ; 7, 8, and >, Development of a Pluteus (Echinoidea 

 and Ophiuroidea). (Prom Lenckart and Nitsche's Diagrams.) 



arranged processes or arms ; the latter has, in addition, three 

 processes not developed in the course of the ciliated ridges. The 

 Bipinnaria is usually free-swimming, but sometimes, as in the 



