(11-) 



The "nephridial pit" is of ectodermal orxcin. 



The mesoderm arises from the endoderm. Archeiiteri c diver- 

 ticula are not present in the larva of P. architecta but there is 

 a sac-like formation of mesoderm cells in the anterior end which 

 forns t!:e lining of the preoral lobe and wi:ich gives rise to a 

 mesentery between the lobe and collar cavities. 



The linii'c of the collar cavity does not arise froiii a mes- 

 odermal sac. It is formed by isolated mesoderm cells which ar- 

 range themselves on the somatic wall leaving the splanctoic wall 

 practically without any lining. 



In the larva of P. architecta, the mesodermal lining of the 

 trunk cavity is complete, covering both the somatic and splanch- 

 nic walls arid it seems probable that it arises from cells forming 

 the base of the nephridial diverticula. There is a mesentery 

 between the cavities of the trunk and collar, 



A stomodaeum saio. proctodaeum aru not present. The blasto- 

 pore becomes the mouth, the anus arises quite late in the early 

 life of the embryo and the rectum is for/red as an outgrowth of 

 the blind end of the archenteron. 



The nephridial canals, at least;, have their origin in a sin- 

 [ gle median pit which soon branches into two intercellular tubes. 



I have noL found any evidence that the excretory cells of the ne- 

 ^ phridia are forxred from free mesoderm cells attaching themselves 



