382 



ZOOLOGY 



SECT 



an opening, the blastopore, 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 (D and E), 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 

 rise subsequently to an intermediate mass of tissue, the 

 mesenchyme. 



The cavity in the gastrula is early distinguishable into two 

 parts (Fig. 319, 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 



Lorv.org B 



A 



ect. 



Lari/.offf 



&(bm^ 



enC 



arcfo 



FIG. 319.- Later stages in the development of the larva of Asterina gibbosa. A, newly 

 hatched larva, ventral surface, with the beginning of the larval organ at the anterior end and 

 with the larval mouth. B, dorsal half of an embryo of the same age as A. C, somewhat 

 older larva with larger larval organ, the ectoderm of the left side removed to expose the 

 alimentary canal and the walls of the body-cavity, arch, archenteron ; bl. p. blastopore ; 

 ect. ectoderm ; ent. enterocoele ; larv. mo. larval mouth ; larv. org. pre-oral lobe ; stom. 

 stomoda'um. (From Ziegler's models.) 



larval anus ; the latter is termed the enterocoele (ccelome). The wall 

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

 swellings, the right and left enteroccslic 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 

 in such a way as to give rise to the coelome of the adult. The 

 anterior undivided part (anterior ccelome). forms the coslorne of a 

 conspicuous larval structure, the pre-oral lobe, and it eventually 

 becomes cut off from the right and left pouches, giving off on 



