PHYLUM ECHINODERMATA 



359 



cavity increases by further divisions, and the whole surface becomes 

 covered with vibratile cilia. A process of imagination then, 

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

 a doubled-walled cup or 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 

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



B 



cis-ch 



FIG. 287. Early stages in the development uf a Starfish (Asterina gibbosa). A, eight-celled 

 stage ; B, stage of about thirty-two cells seen in section ; C, gastrula stage ; D, section of 

 early gastrula ; E, section of later gastrula. arch, archenteron ; l>!nxto<\ blast<>c<ele ; <>lji. 

 blastopore; ect. ectoderm; <:,i<i. endoderm. (Modified after Ludwig.) 



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

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



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 (enf) ; the former becomes the 

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

 becoming the anus ; the latter is termed the enteroccele. The wall 

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

 swellings, the right and left enteroccelic pouches (G, ent), which 

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

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

 cosle 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 stomodccum, which meets the 



