ECHINODERMATA 



427 



band grow together into five rings, wliicli surround the 

 larva like the hoops of a barrel (Fig. 206). This is the 

 so-called pupal stage, which is also assumed by some Holo- 

 thurians {e.g. Gucumaria) without passing through the 

 Auricularia form. This stage is remarkable owing to its 

 resemblance to the larva of Antedon, with which it has in 

 common even the number of the ciliated rings. 



o 



According to Sejion, we may imagine that the rearrangement and loss 

 of continuity in the ciliated band are the result of the migration of the 

 band together with the adjacent body epithelium, probably in con- 

 sequence of internal processes of growth. 



In the metamorphosis of the ciliated band we have not 

 yet considex-ed the parts lying near the mouth, which do 

 not share in the formation of the external ciliation of the 

 pupal stage. Parts of the longitudinal and transverse 

 portions of the ciliated band approach very close to the 

 region of the mouth-opening (Fig. 205 A). After the 

 breaking up of the ciliated band, four parts can be dis- 

 tinguished, which closely surround the mouth, and finally 

 form a continuous ring about it. They gradually move 

 more into the infundibular depression which leads to the 

 mouth-opening. By a marked narrowing of the funnel 

 they come to lie inside the larva, and are employed to 

 clothe the tips of the five anteriorly directed evaginations 

 of the hydrocoele (therefore for the formation of tentacles) 

 (Fig. 205 B). The nerve bands have moved down into the 

 funnel even before the parts of the ciliated band have, and 

 are to a certain extent forced down by them (Fig. 205 A, 

 n), for the nerve bands occupied a position nearer to the 

 mouth-opening than the ciliated bands. Their free ends 

 then unite at the bottom of the f annel and there form the 

 nerve-ring of the Synapta (Semon). It was precisely those 

 four parts of the ciliated band that moved into the funnel 

 which were united to the nerve bands by means of nerve 

 fibres. Probably this connection is retained during the 

 metamoi'phosis, and as soon as the ciliated band has covered 

 the five tentacles the five large tentacular nerves are es- 

 tablished on the nerve-ring, whereas the five radial nerves 



