546 



HOLOTHUROIDEA. 



archenteron (fig. 249 Vpv.), and is called by Selenka the vaso- 



peritoneal vesicle. It gives origin to the epithelioid lining of 



the body cavity and water-vascular system of the adult 1 . In the 



parts now developed we have the rudiments of all the adult organs. 



The mouth and anal involutions (after the separation of the 



vaso-peritoneal vesicle) meet and unite, a constriction indicating 



their point of junction (fig. 248 B). Eventually the former gives 



A B C 



7r 



f, 



FIG. 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 peri- 

 toneal vesicle; pi. left peritoneal vesicle; w.v. water- vascular vesicle; /. dorsal pore 

 of water- vascular system ; ms. muscle cells. 



rise to the mouth and oesophagus, and the latter to the re- 

 mainder of the alimentary canal 2 . 



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 direction, and from about its middle sends a narrow 

 diverticulum towards the dorsal surface of the body, where an 



1 The origin of the vaso-peritoneal vesicle is not quite the same in all the species. 

 In Holothuria tubulosa it is separated from the csecal end of the archenteron ; the 

 remainder of which then grows towards the oral invagination. In Cucumaria the 

 archenteron forks (fig. 249) ; and one fork forms the vaso-peritoneal vesicle, and the 

 other the major part of the mesenteron. 



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

 derived from the stomoclaeal invagination. 



