XVI 



ECHINODERMATA 



509 



In a well-developed larva of this age, the body consists of a 

 conical mass which contains the larval stomach and intestine, 

 and from which spring the post-oral arms, and of an oral lobe 

 containing the mouth and oesophagus, from which spring the antero- 

 lateral arms. , , 



The development during the second week of larval life presents 

 many resemblances to the development of the Ophiopluteus. Thus 

 the coelomic sacs divide into anterior and posterior portions, the 

 latter applying themselves to the stomach and the former to the 

 oesophagus, to which they supply constrictor muscles. A pair of 



Fig. 385. — Young larvae of Mchinus escvlentus viewed from the dorsal surface. (Original. ) 



A, prism larva, three days old. B, four-armed Echinopluteus larva, seven days old. a.l.a, anterior 

 lateral arm ; a.l.r, antero-lateral rod ; hr, body-rod ; eiLad^ adoral band of cilia ; eil.long, longitudinal 

 band of cilia ; coe, coelomic vesicle ; h.r, horizontal rod ; l.coe, left coelomic sac; oes, oesophagus ; p.o.a, 

 post-oral arm ; p.o.r, post-oral rod ; r.Goe, right coelomic sac ; st, stomacli ; stom, stomodaeum. 



postero-dorsal arms are developed, but these, unlike their homologues 

 in the Ophiopluteus, are supported by calcareous rods developed from 

 spicules totally independent of the primary calcareous spicules. 



About the end of the second week rudiments of a fourth pair of 

 arms appear; these are situated at the sides of the loop of the 

 longitudinal ciliated band which overhangs the mouth, the prae-oral 

 loop in fact. These are the prae-oral arms ; they are supported by 

 the forks of an independent V-shaped calcareous centre termed the 

 dorsal arch, which is situated in the mid-dorsal line above the 

 region of the oesophagus. 



The larva has thus five independent calcareous centres and eight 



