DEVELOPMENT OF ANTEDON. 



157 



but deeper and in the axis of the body is a row of stem-joints. 

 The anterior of these is the dorsocentral (Fig. 114, 7), which 

 becomes the terminal joint of the stem. The stem-joints 

 rapidly increase in number, the new pieces being added at the 

 posterior (proximal or calycine) end of the row. A little later, 

 on the seventh day, the underbasals (ib), three in number (rarely 

 four or five), are formed. They lie in front of the basals and a 

 good deal deeper and eventu- 

 ally fuse with the top (posterior) 

 stem joint to form the centro- 

 dorsal plate. 



At about this stage the larva 

 hatches and undergoes its brief 

 free-swimming life. It then 

 attaches itself (p. 153), the ciliary 

 rings, preoral tuft and apical 

 plate atrophy, and the anterior 

 end begins to become narrower 

 and longer, and to mark itself 

 out as the stem from the pos- 

 terior end, which becomes the 

 calyx. In fact the larva be- 

 comes club-shaped, the swollen 

 free end forming the rudiment 

 of the calyx (Fig. 114). 



The vestibular depression 

 (p. 153) on the ventral surface 

 becomes deeper and converted 

 into an ectodermal invagination 

 which occupies the greater part of the ventral surface, remaining 

 open for some time anteriorly. It eventually closes, and soon 

 after attachment shifts on to the posterior end of the larva (Fig. 

 115, 5). The hydrocoel follows this shift of the vestibule and 

 lies at the posterior end immediately beneath the floor of the 

 vestibule (Fig. 114, 3). It is still an open horseshoe, the opening 

 being towards the water-pore ; but its ends have approximated 

 and its five lobes, each of which soon becomes trilobate, pushing 

 up the ectoderm of the floor of the vestibule, project into the 

 vestibular cavity as the primary tentacles. Five additional pairs 

 of tentacles are formed later at the base of these (Fig. 115, 7). 



FIG. 114. Young attached larva o 

 Antedon from the left side (from Lang, 

 after Seeliger). The vestibule is closed. 

 ba 1-3 basals ; or 13 orals ; ib under- 

 basals ; 1 dorsocentral plate ; 2 anterior 

 coelom ; 3 lobes of hydrocoel ; 4 vestibule ; 

 5 enteron ; 6 left, 7 right posterior 

 coelom ; 8 stem-joints ; v. ventral ; d dorsal 

 surface of the larva. 



