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 



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; i. intestine; a. anus; I.e. longitudinal 

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

 toneal vesicle; //. 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 caecal 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. 



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

 derived from the stomodscal invagination. 



