ECHINODERMA TA . 



459 



The blastopore completely closes, and is not converted into the per- 

 manent anus. The archenteron gives rise to the epithelioid lining of both 

 body cavity and water-vascular 

 system. These parts do not, 

 however, appear as a single or 

 paired outgrowth from the ar- 

 chenteron, but as three distinct 

 outgrowths which are not form- 

 ed contemporaneously. Two of 

 them are first formed, and be- 

 come the future body cavity; 

 but their lumens remain dis- 

 tinct. Originally appearing as 

 lateral outgrowths, the right 

 one assumes a dorsal _ position fl/ _ mesenteron; m ,. wate r-vascular ring; 



LONGITUDINAL SECTION THROUGH 

 LARVA. (From Carpenter ; after 



J_ 



and sends a prolongation into 

 the stalk (tig. 252 rp), and the 

 left one assumes first a ventral, 

 and then an oral position (fig. 

 252 lp). 



The third out- 

 growth of the archen- 

 teron gives vise to the 

 water- vascular v esicle. 

 It first grows round 

 the region of the fu- 

 ture oesophagus and so 

 forms the water-vas- 

 cular ring. The wall 

 of the ring then grows v--. 

 towards the body wall 

 so as to divide the oral 

 (left) peritoneal vesicle 

 into two distinct ves- 

 icles, an anterior and 

 a posterior, shewn in 

 fig. 253, lp and lp. Be- 

 fore this division is 

 completed, the water- 

 vascular ring is pro- 

 duced in front into a 

 series of processes 

 the future tentacles 

 (tig. 252, wv) which 

 project into the cavity 

 of the oral vesicle (Ijj). 

 After the oral peri- 

 toneal space has be- 

 come completely di- 

 vided into two parts, 

 the anterior dilates 



FIG. 252. 

 AN ANTEDON 

 Gotte.) 



dl. mesenteron; wv. water- vascular 

 lp. left (oral) peritoneal vesicle; rp. right peri- 

 toneal vesicle; rp'. continuation of right vesicle 

 into the stalk; st. stalk. 



</. 



FIG. 253. LONGITUDINAL SECTION THROUGH THE CALYX 

 OF AN ADVANCED PENTRACRINOID ANTEDON LABVA WITH 

 CLOSED VESTIBULE. (From Carpenter ; after Gotte.) 



ae. epithelium of oral vestibule; m. mouth; al. me- 

 senteron; are. rudiment of permanent anus; lp. posterior 

 part of left (oral) peritoneal sack; lp . anterior part of 

 left (oral) peritoneal sack; wr. water- vascular ring; t. 

 tentacle; mt. mesentery; rp. right peritoneal sack; rp' . 

 continuation of right peritoneal sack into the stalk; r. roof 

 of tentacular vestibule. 



