PHYLUM ECHINODBRMATA 



389 



a double-walled cup or gastrula (G) opening on the exterior by 

 an opening, the Mastopm-e, which, at first very wide, gradually 

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

 alters, so as to be somewhat elongated, the blastopore, lying at first 

 midway between the two poles, afterwards gradually drawing 

 nearer to what becomes the posterior end. 



Of the two layers of the gastrula (I) and U), the outer is the 

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

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



Fig. 313. — Early stages in the development of a Starfish (Asterina gibbosa). A, eight-celled 

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

 early gastrula ; E, section of later gastrula. arch, archenteron ; blastoc. blastocoele ; hl^p. 

 blastopore ; ect. ectoderm ; end. endoderm. (Modified after Ludwig.) 



rise subsequently to an intermediate mass of tissue, the 

 mesenchyme. 



The cavity in the gastrula is early distinguishable into two 

 parts (Fig. 314, B) — that part into which the blastopore leads 

 {arch), and a wider terminal part {ent) ; the former becomes the 

 stomach and intestine of the larva, the blastopore giving rise to the 

 larval anus ; the latter is termed the enterocosle (coslome). The wall 

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

 swellings, the right and left e^iteroccelic pouches (C, ent), which 

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

 left pouch is soon seen to be larger than the right. The entero- 

 ccele is subsequently completely closed off from the enteric 

 canal. It now consists of three parts, an anterior undivided part, 

 and the two pouches, right and left. Of the latter the left grows 

 more rapidly than the right : both extend posteriorly in the space 

 between the enteric canal and the body-wall to coalesce posteriorly 



