430 



EMBRYOLOGY 



at about tlie middle of its course, and here deposits a semi- 

 lunar calcareous ridge, which must be looked upon as cor- 

 responding to the madreporic plate. Where it rests upon 

 the stone canal, the latter is cut through by a constriction, 

 and one half henceforth hangs down from the ring-canal 

 free in the body cavity, whereas the other half is gradually 

 obliterated. 



Externally the pupal stage now approaches more the 

 adult Holothurian, owing to the fact that the first two feet, 

 the development of which is to be traced to evaginations of 

 the corresponding radial vessel, make their appearance on 

 the posterior part of the ventral surface (Figs. 207 F and 

 208/). At the eame time the tentacles also advance in their 

 development. We saw in Synapta that a part of the ciliated 

 band moved down into the oral funnel to supply the ecto- 

 dermal covering of the tentacular vessels, which consists 

 partly of sensory cells. The oral fnnnel then closed to an 

 extremely narrow fissure, and there was thus formed a 

 kind of vestibule (comp, the corresponding processes in the 

 development of the vestibule of Antedon, p. 446). The 

 tentacles, to" which the nerve-ring is still joined, lie in the 

 vestibule. The nerve-ring lies at the point where the calca- 

 reous ring, — the supporting apparatus of the tentacles, con- 

 sisting at first of five and later of ten rods, — is attached to 

 the tentacles. When the tentacles have reached the neces- 

 sary development, they are extended out through the fissure, 

 which widens again (Fig. 208), and the young Holothui'ian 

 now moves both by means of those ciliated bands which still 

 remain, and by means of adhesion with the tentacles and. 



feet when the latter are present. 

 In Synapta, as is known, the 

 feet are not developed, even the 

 radial vessels degenerating. 



The shape of the Holothurian 

 would thus be attained if the 

 young anima.1 did not lack the 

 larger number of tentacles and 

 feet, and if its body-covering 

 already possessed its permanent 



Fin. 208.— Holothurian larva wit-h 

 ciliated bands, extended tentacles 

 (T), and developing feet (;) (after 

 JoH. Müller). 



