512 



INVEETEBRATA 



CHAP. 



METAMORPHOSIS OF THE EGHINOPLUTEUS 



The metamorphosis may be said to begin about the fifteenth day, 

 when the ciliated epaulettes have been formed and the madreporic 

 vesicle is making its appearance. The hydrocoele has now the form of 

 a vesicle of a somewhat flattened oval shape when seen in cross-section, 

 with a wall consisting of a single layer of columnar cells. On the 

 left side of the larva, between postero-dorsal and post-oral arms, an 



FiQ. 387. — Echinopluteus larva of Echinus esculentus twenty-three days old, viewed from 

 the dorsal surface, (Original.) 



Names as in previous figure. In addition, p.cil.ep, posterior ciliated epaulette. 



invagination of the ectoderm can be seen above the spot where the 

 left hydrocoele is situated. This invagination is termed the amniotic 

 invagination ; it is lined by cubical cells, which contrast with the 

 flattened cells of the general ectoderm on the one hand and the highly 

 columnar cells of the hydrocoele on the other. The amniotic invagina- 

 tion grows inward till its floor becomes flattened against the wall of 

 the hydrocoele. 



As a consequence of the changes which these two adpressed organs 



