446 EMBRYOLOGY 



toward the upper pole of the larva, and thus the floor of the 

 pit comes to lie over against the internal organs (Fig. 218 B). 

 At the same time the opening of the invagination narrows, 

 and is finally entirely closed and [the invaginated layer] 

 detached. In this way the invaginated part of the ectoderm 

 comes to lie inside as a closed sac, and since it follows still 

 more the tendency existing from the beginning, it moves 

 quite to the upper end of the larva (Fig. 218 G). This sac 

 subsequently changes in sueh a way that its floor overlies 

 the evaginations of the water-vascular system (tentacular 

 vessels), and its roof unites with the mesenchyraa and the 

 outer ectodermal lamella to form the roof of the vestibule 

 (Figs. 219 and 220) — the chamber on the floor of which the 

 mouth subsequently arises, and the roof of which disappears 

 to set free the tentacles. However, before the beginning of 

 these processes, which bring the larva nearer to its per- 

 manent shape, important changes take place in the internal 

 organs. 



Just as the Antedon larva, with its five ciliated rings, recalls the cask- 

 like shape of the Holothurian larva, so, too, the development of the 

 vestibule and the investment of the tentacles by its floor show a certain 

 resemblance to the fomiation in the Holothurian larva of the vestibule 

 in which the tentacles lie (comp. p. 427). Here, as there, it is a de- 

 pression of the ectoderm which forms the vestibule and supplies the 

 external covering of the tentacular vessels. In both cases the process 

 takes place in the region of the mouth, which, however, exhibits a 

 different position in regard to the ciliated rings. 



We left the internal organs at a stage of development at 

 ■which the two enterocoeles and the hydrocoele lay at the 

 side of the saccular intestine. The latter, which at first lies 

 ventrad of the intestine, moves, with the metamorphosis of 

 the larva into the pentacrinoid form, to a position over the 

 intestinal sac (Figs. 218 and 219), and grows out into the 

 shape of a horseshoe, its two arms finally uniting into a ring. 

 At the same time there are formed five upward evaginations, 

 which are covered over by the ectodermal cell-layer which 

 forms the floor of the vestibule (Figs. 218 G and 219). The 

 prolongation of the hydrocoele, which was recognizable even 



