THE DEVELOPMENT OF TIIK lloi,<)THURIDK.\. 473 



before the cesophageal invagination reaches it. The anterior 

 part of the archenteroii gives off a caecal process which, be- 

 coming a sac, is constricted off from the archenteron as what 

 Salenka terms a vaso-peritoneal vesicle. This vesicle changes 

 its position to the left side of tin- alimentary eanal, and then 

 sends a narrow, duct-like di vertical um toward the dorsal re- 

 o-ion of the ectoderm, which eventually coalesces with the 

 latler, the cavity of the diverticulum opening on the exterior 

 by a rounded pore. The vaso-peritoneal vesicle now divides 

 into two portions, one of which the ambulacral sac remains 

 connected with the exterior by the duct, and constitutes the 

 foundation of the whole of the ambulacra! system of vessels ; 

 while the other the peritoneal sac gives rise to the peri- 

 toneum. The former becomes five-lobed, grows round the 

 gullet, and gives rise to the tentacular and ambulacral canals 

 with the Polian vesicle, or vesicles ; while the duct, de- 

 taching itself from the dorsal wall, becomes the madreporic 

 canal. 



The latter divides into two vesicles, which arrange them- 

 selves at the sides of the stomach. The stomach takes on 

 a more cylindrical shape, and these vesicles become the " sau- 

 sage-shaped bodies " (wurstfo'rrnige KOrper) observed by Mai- 

 ler (Fig. 136, A). They gradually increase in size, and, grow- 

 ing round the alimentary canal, unite above and below it. 

 Thus a cylindrical cavity with a double wall is formed be- 

 tween the endoderm and the ectoderm. The inner wall of the 



1'ifj. 137. Development of a Holothurid. (After Miiller.) A, early condition of the 

 larvi (Ayrisutatia): g, the dorsal pore of A, the ambulacral sac. B. later stasre: 

 c', intestine : g, dorsal pore; /, f, circular ambulacral vessel with its prolonga- 

 tions ; , calcareous body, c, youn-r Holothuria with circular ciliated bands: ar, 

 madreponc canal ; /', Poliau vesicle. 



cavity applies itself to the alimentary canal, and, aided by 

 the mesoblastic cells which appear to be developed from the 



