XVI 



ECHINODEKMATA 



537 



remaining fragments of the longitudinal ciliated band then join with 

 each other so as to form five transversely arranged ciliated rings. 

 Of these the fifth and most posterior is formed by the union of the 

 fragments formed from the two postero-lateral processes. The next, 

 i.e. the fourth ciliated ring, is formed by the union of four fragments, 

 two derived from the remains of the postero-dorsal processes, two from 

 the lateral fragments of the anal loop. The next, i.e. the third ciliated 

 ring, arises from the union of the fragments produced by the inter- 

 mediate dorsal processes. 



A B 



slam 



FIG. 399. Diagrams of the anterior aspect of metamorphosing larvae of Synapta digital*/ 

 in order to show the changes undergone by the longitudinal ciliated band. (After Bury.) 



A, before metamorphosis into the pupa. B, C, during metamorphosis into the pupa. In C the 

 mouth has moved to the left. D, after the metamorphosis into the pupa has been almost completely 

 accomplished. I, fragment which gives rise to the first transverse ciliated band of the pupa. Ha, 

 II/, I If, three fragments which give rise to the second transverse ciliated band of the pupa. III, 

 lllb, two fi-agincnts which give rise to the third transverse ciliated band of the pupa. OR, four frag- 

 ments which give rise to the oral ring of the pupa. ;>, apical thickening of the larva; xtom, .stomodaeum, 

 which becomes the atrium ; mp, madreporic pore. 



The second ciliated ring arises from the union of three fragments, 

 two b^ing the remnants of the antero-dorsal processes, and the third 

 is the remnant of the right prae-oral process derived from the right 

 side of the prae-oral loop. The first ciliated ring which immediately 

 surrounds the opening of the atrium is derived from the left prae-oral 

 process (Figs. 398, 399, 400). 



By the shrinkage of the prae-oral portion of the larva the opening 

 of the atrium and the band surrounding it come to be situated at 



