IX 



PHYLUM ECHINODERMATA 



397 



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

 4 to 6) or a Brachiolaria. The former has a series of bilaterally 



PIG. 315. Diagrams of the development of the larvre of Echinoderms : 1, Primitive form of 

 Echiuoderru larva; 2 aud 3, Development of an At<rici'liti-in (Holothuroidea) ; 4, 5, and 0, 

 Development of a Bipinnaria (Asteroidea) ; 7, S, and 9, Development of a Plti.ti'i'n (Echinoidea 

 and Ophiuroidea). (From Leuckart and Nitsche's Diagi'ams.) 



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 



