THE DEVELOPMENT OF THE HOLOTHURIDEA. 



473 



before the oesophageal invagination reaches it. The anterior 

 part of the archenteron gives off a C2ecal process which, be- 

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

 Salenka terms a vaso-peritoneal vesicle. Tnis vesicle changes 

 its position to the left side of the alimentary canal, and then 

 sends a narrow, duct-like diverticulum toward the dorsal re- 

 gion of the ectoderm, which eventually coalesces with the 

 latter, 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 amhulacral sac — remains 

 connected with the exterior by the duct, and constitutes the 

 foundation of the whole of the ambulacral 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" (wurstformige KOrper) observed by Mul- 

 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 w^all is formed be- 

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





Fig. 137.— Development of a Ilolothurid. (After Miiller.)— A, early condition of the 

 Jarv-i {AurlculaHa) : (/, the dorsal pore of h, the ainhulacral sac. B. later stajre: 

 c', intestine ; g, dorsal pore ; /, f\ circu'ar ambulacral ves-^el with its prolonga- 

 tions ; z, calcareous body. C, youn<? Holothuria with circular ciliated bands: <7, 

 madreporic canal ; /', Poliaa vesicle. 



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

 the mesoblastic cells which appear to be developed from the 



