EC HI NO DERUfA TA. 4W 



the mouth and oesophagus, and the latter to the remainder of the 

 alimentary canal 1 . 



The vaso-peritoneal vesicle undergoes a series of remarkable 

 changes. After its separation from the archenteron it takes up a 

 position on the left side of this, elongates in an antero-posterior direc- 

 tion, and from about its middle sends a narrow diverticulum towards 

 the dorsal surface of the body, where an opening to the exterior 

 becomes formed (fig. 248 B, p.}. The diverticulum becomes the 

 madreporic canal, and the opening the dorsal pore. 



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

 vesicle (fig. 248 B, w.v.), from which is derived the epithelium of the 

 water-vascular system, and a posterior (fig. 248 B, p.v.), which gives 

 rise to the epithelioid lining of the body cavity. The anterior vesicle 

 (fig. 248 C, w.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 



O c* O 



B 



Fio. 248. THREE STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA 

 VIEWED FROM THE SIDE IN OPTICAL SECTION. (After Selenka.) 



m. mouth; oe. oesophagus; st. stomach; ?'. intestine; a. anus; I.e. longitudinal 

 ciliated band; v.p. vaso-peritoneal vesicle ; p.v. peritoneal vesicle; p.r. right peritoneal 

 vesicle; pi. left peritoneal vesicle; w.v. water-vascular vesicle; p. dorsal pore of water- 

 vascular system; ms. muscle cells. 



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. 



1 There appears to be some uncertainty as to how much of the larval oesophagus is 

 derived from the stomodaeal invagination. 



