PREVIOUS INTERPRETATIONS OF CNIDARIA 



139 



observe in the ontogenies of numerous Eumetazoa that their 

 nephridia are developed at least partly by way of the invagi- 

 nation of the skin layer. Later these protonephridia have been 

 influenced first by the polymerization, and subsequently, ac- 



S^Av>^- ^^. 





\ >^ 



Fig. 23. Excretorial-emunctorial organelle of a ciliate (Pycnotbrix 



monocystoides) . A, habit. B, the ciliated end of the canal. (From 



Dogiel.) 



cording to circumstances, by the oligomerization which has 

 frequently led to a complete disappearance of the nephridial 

 organs. Here we are interested in the fact that, as far as we 

 can judge on the basis of our present knowledge, the small 

 acoelous Turbellaria do not possess — and this without an 



