410 



EMBRYOLOGY 



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 Echiiiodermata (Fig. 179). It communi- 

 cates directly with the outside woi^ld by means of the dorsal 

 pore, just as in certain Asteroidea. It is not until this stage 

 that a constriction makes its appearance ia the posterior 



Fig. 191.. 



Fig. 191. — Optical longi- 

 tudinal section of an Auricii- 

 laria larva of Holothurii tabu- 

 losa (after Selbnka). A, 

 auus ; D, intestine; JB, onteni- 

 ccBle ; H, hydrncojle ; M, 

 mouth ; P, dorsal pore ; W, 

 ciliated band. 



Fig. 195. 



Fig. 195. — Longitudinal section of a larva 

 of Citctii?iana (JolioIu7a, Bomewhat diagram- 

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

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

 coeles; l\ feet; 31, mouth ; /', di)rsal pore, 

 leading through the stone canal to the water- 

 vascular ring, Wv ; T, tentacular vesicles ; 

 IK)-, water- vascular ring. 



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

 which divides the vesicle into the hydrocoele, connected with 

 the dorsal pore, and the less extensive enteroccele (Fig. 19-1 



