VIII 



ECHINODERMA TA ONTOGENY 



539 



neural tuft; the neural plate flattens out, and the larval nervous system 

 completely degenerates. 



The ectoderm cells continue to yield an intermediate substance. Many of them 

 sink below the surface, the consequence being that the distinction between the 

 body epithelium and the mesenchymatous cutis is entirely obliterated. 



The vestibule becomes completely constricted off, the last remains of the aperture 

 of invagination closing. At the same time it shifts entirely to the posterior end 

 of the larva, the end which now freely projects, twisting through an angle of 90, so 

 that its thickened epithelial floor, which before lay parallel to the principal axis, now 



r-3 



d 



ba. 



FIG. 440. Young attached larva of An- 

 tedon, forty-eight hours old, from the left 

 side (after Seeliger). The vestibule 1ms be- 

 come entirely constricted otr, but the distinc- 

 tion between the calyx and the stalk is not yet 

 pronounced. Imi-bn^, Basals ; or r or 3 , orals ; 

 Hi, infrabasals ; 1, pedal plate ; 2, parietal 

 canal ; 3, hydrocoel outgrowths ; 4, vestibule ; 

 5, intestinal vesicle ; G, left cuelom sac ; 7, right 

 cttlom sac ; 8, calcareous joints of the stalk. 



"bets 



01*2 



post 



Fi<;. 4"iii. Young attached larva of 

 Antedon, forty -eight hours after being 

 hatched, from the left side (after Seeliger). 

 co, Stalk; ca, calyx; bai-ba 3 , basals; nr]-'"':;- 

 orals ; il>, infrabasals of the. left side ; 1, pedal 

 plate ; 2, hydropore ; 3, left coelom ; 4, right 

 crelom ; 5, joints of the stalk. 



lies at right angles to it. The larval body becomes club-shaped, the anterior body 

 forming the handle of the club. The vestibule, which continues to increase in size, 

 comprises the entire posterior part of the club (the calyx) ; it becomes pentagonal. 

 and imprints the same shape upon the whole posterior part of the bod} 7 , and thus 

 first determines the radiate structure (Figs. 450, 451, and 452). 



The anterior end of the larva becomes the apical end of the stalk, the pos- 

 terior end becoming the oral side of the calyx of the attached Pentacrinus-like 

 larva. 



The hydrocoel undergoes the same twisting and shifting as the vestibule, beneath 



