ECHINODERMATA 431 



structure. Additional tentacles and feet are developed in 

 the san;e way as those which we have already learned about, 

 namely, by evaginations of the parts of the water-vascular 

 system already formed. The ciliated bands are said by 

 Semon to disappear owing to their cells spreading themselves 

 out over the entire surface of the larva, and substituting 

 themselves for the originally flat body epithelium, which now 

 makes way for a thick epithelial layer. With progressing 

 growth the mouth and anus of the larva are shifted fi-om 

 the more ventral position toward the anterior and posterior 

 ends of the animal respectively. 



Now that we have followed the Holothurian larva in its development 

 as far as the young animal, there remain for consideration only a few 

 important internal developmental pi'ocesses, which relate to the deriva- 

 tion of the middle germ-layer. As we were obliged in the previous 

 descrii^tion to take into consideration various forms, so are we under the 

 same necessity in the following statements, which for the greater part we 

 take from the works of Selenka and Semon. 



Having treated of the origin of the mesenchyma cells in the first 

 division of the chapter, we intentionally left the subject for the time 

 being. By far the greater part of these cells become connective tissue. 

 Whereas they are usually api^lied to the inner surface of the ectoderm as 

 isolated cells, they are accumulated on both sides of the proctodeum in 

 larger groups, which give rise to the calcareous spherules and calcareous 

 wheels. They also occur in large numbers in the region of the stone 

 canal and of the water-vascular ring, forming in one place the well- 

 known calcareous deposits and in the other the calcareous ring surround- 

 ing the oesophagus. The mesenchyma cells give rise by multiplication 

 to a kind of cutis under the entire ectoderm (Metschnikoff, No. 37). 

 Underneath the ciliated baud they form, according to Semon, groove-like 

 sheaths, which probably serve as supports for the ciliary apparatus. 

 Fissures in the mesenchyma are said to give rise to the blood-vessels of 

 the Holothurian. Thus the vessels accompanying the intestine first 

 make their api^earance as lacunar spaces in the mesenchyma lying 

 dorsad and ventrad of the intestine. The blood cells, on the contrary, 

 are said to have been detached from the walls of the hydro-enterocoele 

 and to have taken part in the formation of these vessels. These free 

 cells, which are found in the body cavity as well as in the ambulacral 

 vessels and blood-vessels, would therefore, according to this view, not 

 arise from the primitive mesenchyma (Semon). 



Of all the musculature only that of the stomodseum originates from 

 the mesenchyma. It persists, being carried over from the larva to the 

 young animal (Selenka). The rest of the musculature arises partly 



