410 



EMBRYOLOGT 



itself from the rest, and unites with the dorsal wall of the 

 larva. As its further development shows, this abstricted 

 portion of the archenteron corresponds to the vaso-peritoneal 

 vesicle of the other Echinodermata (Fig. 179). It communi- 

 cates directly with the outside world by means of the dorsal 

 pore, just as in certain Astei'oidea. It is not until this stage 

 that a constriction makes its appearance in the postei-ior 



Fig. 191. 

 Fig. 194. — Optical longi. 

 tudinal section of an Auricu- 

 laria larva of Holothuri'i tubu- 

 losa (after Selenka). A, 

 auus; D, intestine; i!, entero- 

 ccBle ; H, hydrocoele ; M, 

 mouth ; P, dorsal pore ; W, 

 ciliated band. 



Fig. 195. 

 Fig. 195. — Longitudinal section of a larva 

 of CiicHdiaria doliolum, somewhat diagram- 

 matic (after Selenka). A, anus; Am, ambu- 

 lacral (radial) vessels; D, intestine; E, entero- 

 coeles; F, feet; M, mouth ; P, dorsal pore, 

 leading through the stone canal to the water, 

 vascular ring, Wr ; T, tentacular vesicles ; 

 Wv, water- vascular ring. 



third of the vaso-peritoneal vesicle (in HolotMiria tubulosa), 

 which divides the vesicle into the hydrocoele, connected with 

 the dorsal pore, and the less extensive enteroccele (Fig. 194 



