IX 



PHYLUM ECHINODERMATA 



359 



cavity increases by further divisions, and the whole surface becomes 

 covered with vibra^ile cilia. A process of intiagination then 

 follows; one side of the blastula becomes pushed inwards to form 

 a doubled-walled cup QT gastrula (C) opening on the exterior by 

 an opening, the blastopore, which, at first very wide, gradually 

 becomes narrowed. At the same time the shape of the larva alters, 

 becoming somewhat elongated, the blastopore, lying at first midway 

 between the two poles, afterwards gradually drawing nearer to 

 what becomes the lower or ventral pole. 



Of the two layers of the gastrula (D and E] the outer is the 

 ectoderm, the inner the endoderm ; between them is a space, at first 



B 



arch 



Fin. 287. Early stages in the development of a Starfish (Asterina ibbosa). .-(, eight-celled 

 stage; B, stage of about thirty-two rolls seen in section; c. gastrula stage; D. section of 

 early gastrula; E, section of later gastrula. /</<. archenteron ; iifuxtm: lil, stocosle ; /<///. 

 blastopore; ecf. ectoderm; t/c'. endoclcrm. (Modified after Ludwig.) 



filled with gelatinous matter, in which cells soon appear, giving 

 rise subsequently to a third or middle layer, the mesoderm. 



The cavity in the gastrula early becomes distinguishable into 

 two parts (Fig. 288, B) that part into which the blastopore leads 

 (arch), and a wider terminal part (cnf) ; the former becomes the 

 posterior part of the alimentary canal of the larva, the blastopore 

 becoming the anus ; the latter is termed the r///c/v.wv, '<. The wall 

 of the enteroccele becomes thinner, and it gives off two lateral 

 swellings, the right and left cnf< '/'< < fie pouches (C, cnt], which 

 become closely applied to the sides of the larval alimentary canal : 

 the left pouch soon becomes larger than the right. The entero- 

 ccele then becomes completely closed off from the enteric canal. 

 Towards the anterior end of the larva there is now formed a new 

 invagination from the surface, the stom.otkc/rtii, which meets the 



