PODAXONIA 



179 



an introvert, is begun as soon as the coelomic cavity appears in the 

 mesodermic bands. Then the retractor muscles, whose formation 

 was described above, begin to act and to pull in the whole apical 

 region. 



Up till the end of the sixth day there is a freely projecting 

 flattened prostomium, ciliated on its under surface. In the second 

 week this prostomium grows out into a dorso-lateral extension on 

 each side, on which ciliated tentacles are developed. Beneath the 

 mouth a ciliated under-lip is formed. 



ret.v 



nepk retd 



retd 



FIG. 307. Young specimens of 

 Phascolosomn, youldii after the 

 metamorphosis. (After Gerould. ) 



A, young worm six and a half days 

 old. B, young worm thirty days old. 

 Letters as in preceding figure. In ad- 

 dition : neph, nephridium ; ten, ridge-like 

 outgrowth of prostomium, on which 

 ciliated tentacles of the adult appear. 



Both species of Phascolosoma then develop a circle of hooks round 

 the base of the introvert ; these can still be made out as minute 

 hooks in the adult P. vulgare, but in P. gouldii they entirely dis- 

 appear. The papillae, so characteristic of the adult skin, appear as 

 oval clusters of ectoderm cells which project slightly inwards towards 

 the coelom. 



Large yellow cells (chloragogen cells) are disposed in lines along 

 the coelom. The two nephridia (neph, Fig. 307) appear to originate 

 as solid ectodermal ingrowths, in each of which a cavity appears later. 

 They come to open into the coelom by the intervention of cells of 



