BCHINODERlfATA. 



551 



formed, and become the future body cavity ; but their lumens remain distinct. 

 Originally appearing as lateral outgrowths, the right one assumes a dorsal 

 position and sends a prolongation into the stalk (fig. 252 rp'), and 

 the left one assumes first a ventral, and then an oral position (fig. 

 252 //). 



The third outgrowth of the archenteron gives rise to the water-vascular 

 vesicle. It first grows round the region of the future cesophagus and so 

 forms the water-vascular ring. 



wv r/> rp 



FIG. 151. LONGITUDINAL SECTION THROUGH 

 AN ANTEDON LARVA. (From Carpenter; after 

 Gbtte.) 



al. mesenteron ; wv, water-vascular ring ; 

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

 toneal vesicle ; rp'. continuation of right vesicle 

 into the stalk ; st. stalk. 



The wall of the ring then 

 grows towards the body wall 

 so as to divide the oral (left) 

 peritoneal vesicle into two 

 distinct vesicles, an anterior 

 and a posterior, shewn in fig. 

 2 53 IP' and lp. Before this 

 division is completed, the 

 water-vascular ring is pro- 

 duced in front into five pro- 

 cesses the future tentacles 

 (fig. 252, TUV) which project 

 into the cavity of the oral 

 vesicle (lp). After the oral 

 peritoneal space has become 

 completely divided into two parts, the anterior dilates (fig. 253, lp'} greatly, 

 and forms a large vestibule at the anterior end of the body. This vestibule 

 (lp^ next acquires a communication with the mesenteron, shewn in fig. 253 

 at m. The anterior wall of this vestibule is finally broken through. By this 

 rupture the mesenteron is placed in communication with the exterior by the 

 opening at m, while at the same time the tentacles of the water-vascular ring 

 (/) project freely to the exterior. Such is Gotte's account of the pne-oral 

 body space, but, as he himself points out, it involves our believing that the 

 lining of the diverticulum derived from the primitive alimentary vesicle 

 becomes part of the external skin. This occurrence is so remarkable, that 

 more evidence appears to me requisite before accepting it. 



The formation of the anus occurs late. Its position appears to be the 

 same as that of the blastopore, and is indicated by a papilla of the mesente- 

 ron attaching itself to the skin on the ventral side (fig. 253, an}. It event- 

 ually becomes placed in an interradial space within the oral disc of the adult. 

 The water-vascular ring has no direct communication with the exterior, but 

 the place of the madreporic canal of other types appears to be taken in 

 the larva by a single tube leading from the exterior into the body cavity, the 

 external opening of which is placed on one of the oral plates (vide p. 571) in 

 the next interradial space to the right of the anus, and a corresponding 

 diverticulum of the water-vascular ring opening into the body cavity. The 

 line of junction between the left and right peritoneal vesicles forms in the 

 larva a ring-like mesentery dividing the oral from the aboral part of the body 



