428 



EMBRYOLOGY 



do not bud forth from it until later (Semon). The parts of 

 the nervous system, Avhich at first lie superficially, are 

 finally overgrown by the rest of the ectoderm, and since 

 mesenchyma cells crowd in over them, they come to lie at a 

 still greater depth. 



The ciliated band, which, according to Semon, encircles the mouth, 

 changes its position during the raetamoriihosis of the larva by coming to 

 lie wholly in the stomodsBum. Here its cells are said to spread them- 

 selves out on the wall and constitute the epithelium. 



Our previous account of the internal organization of the 

 Holothurian larva was confined to the formation of the 

 intestine, the two enterocoelic sacs, which extended between 



the intestine and the body-wall, 

 and the hydrocoele. We saw 

 the latter growing around the 

 stomoda?um of the larva as a 

 five-lobed structure. It already 

 represented the fundament of 

 the water- vascular ring and the 

 five tentacles of the Holothurian. 

 Five secondary evaginations of 

 the water-vascular ring arise 

 between the five primary ten- 

 tacles, and at first are also 

 directed upwards (Fig. 205 B). 

 Later, however, five of the ten 

 evaginations of the water-vas- 

 cular ring now present bend 

 over the calcareous arches and 

 grow out backwards (Figs. 206 

 and 207), so that there are now five tentacles and five radial 

 vessels (Semon). The question now arises whether it is the 

 five vessels first developed (the so-called primary tentacles) 

 which bend over backwards, and thus correspond to the 

 i-adial vessels of other Echinoderms — as seems most natural, 

 though this has been denied — or whether it is the five vessels 

 of the second group, which correspond to radial vessels. As 

 to the homologies of the ambulacral vessels in the different 



Fig. 206. — Pupal stage of the 

 larva of Synapta digitata (after 

 Skmon). ed, proctodfeum ; ent, en- 

 terocoele; in, oral funnel; u-, water- 

 va.scular ring with evaginations 

 forwards (tentacles [tj) and back- 

 wards (radial vessels and Polian 

 vesicles [ji]). 



