ECHINODERMATA 



445 



L 



^Mhesive disc. At this stage of attachment the larva lies 

 ^^ith its entire ventral surface on the object to which it 

 ^^ttaches itself. At first it still possesses its typical ciliation, 

 but that is soon lost. At the same time its shape changes! 

 the anterior end, with which the larva attaches itself and 

 which subsequently grows out into the stem, diminishing in 

 size and the opposite end becoming broader. The club- 

 shaped larva now rises from its support, to which only the 

 small end remains united. Accordingly we now designate 

 the club-shaped portion, which becomes the calyx, as the 

 upper part, the narrowed portion as the lower part, of the 

 larva (Fig. 218). 



Fig. 218.—^ to C, early stages of development of the attached larva of Antedon 

 rosacea (after J. Baerois). Development of the vestibule (F) by invagination of 

 the ectoderm (ect). D, intestine; Ls, subambulacral, Lv, visceral, body cavity; 

 S, stalk of the larva ; T, tentacular vessel ; x, stone canal (?). 



The most important change which takes place in the 

 larva after this metamorphosis of its external shape affects 

 its ventral surface. The wide pit whicK is found there, and 

 which is called the larval mouth, becomes obliterated during 

 the attachment of the larva, but a new invagination of the 

 ectoderm takes place at the same spot, which is deeper than 

 the pre-existing one. Here also, as in the region of the 

 "larval mouth," the ectoderm is greatly thickened (Fig. 

 218^). The invagination soon enters into relationships 

 with the internal organs, for its upper margin extends out 



