Davekport — Cristatella. 



PLATE XI. 



Figs. 97-99. Vertical sections, showing three successive stages in the degeneration 

 of the roof to form tlie atrial opening, of. atr., and development of the 

 parieto-vaginal muscles, x 390. 

 " 100. Lnte stage in the development of the ectoderm, showing its extreme 

 modification between adult polypides. x 390. 



