ECHINODERMATA. 



547 



opening to the exterior becomes formed (fig. 248 B, />.). The 

 diverticulum becomes the madreporic canal, and the opening 

 the dorsal pore. 



The vaso-peritoneal vesicle next divides into two, an an- 

 terior vesicle (fig. 248 B, ^v.^>.}, from which is derived the 

 epithelium of the water-vascular system, and a posterior (fig. 

 248 B,/.7'.), which gives rise to the epithelioid lining of the body 

 cavity. The anterior vesicle (fig. 248 C, zv.v.) becomes five- 

 lobed, takes a horseshoe-shaped form, and grows round the 

 oesophagus (fig. 256, w.v.r). The five lobes form the rudiments 

 of the water-vascular prolongations into the tentacles. The 

 remaining parts of the water-vascular system are also developed 

 as outgrowths of the original vesicle. Five of these, alternating 

 with the original diverticula, form the five ambulacral canals, 

 from which diverticula are produced into the ambulacral feet ; a 

 sixth gives rise to the Polian vesicle. The remaining parts of 

 the original vesicle form the water-vascular ring. 



We must suppose that eventually the madreporic canal loses 

 its connection with the exterior so as to hang loosely in the 

 interior, though the steps of this process do not appear to 

 have been made out. 



original 



hinder peri- 

 grows rapidly, 



The 



toneal vesicle 

 and divides into two (fig. 248 C, 

 //. and //".), which encircle the 

 two sides of the alimentary 

 canal, and meet above and 

 below it. The outer wall of 

 each of them attaches itself to 

 the skin, and the inner one to 

 the alimentary canal and water- 

 vascular system ; in both cases 

 the walls remain separated 

 from the adjacent parts by a 

 layer of the amoeboid cells 

 already spoken of. The cavity 

 of the peritoneal vesicles be- 

 comes the permanent body 

 cavity. Where the walls of 



-ME 



. Fid. 



FIG. 249. LONGITUDINAL SECTION 

 THROUGH AN EMBRYO OF CUCUMARIA 

 DOLIOLUM AT THE END OF THE FOURTH 



DAY. 



Vpv. vaso-peritoneal vesicle; ME. 

 mesenteron ; Blp., Ptd. blastopore, proc- 

 todreum . 



352 



