THE DEVELOPMENT OF THE HOLOTIIURIDEA. 



473 



before the oesophageal invagination reaches it. The anterior 

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

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

 Salenka terms a vaso-peritotieal vesicle. This 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 ambulacral 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 Miil- 

 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 



Fig. 137.— Development of a Holothurid. (After MiilJer.)— A, early condition of the 

 larva (Auricularia) : g, the dorsal pore of h, the ambulacra! sac. B, later staee: 

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

 tions ; ?', calcareous body. C, youn? Holothurid with circular ciliated bands : g, 

 madreporic canal ; /', Polian vesicle. 



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

 the mesoblastic cells which appear to be developed from the 



