20-J 



PORIFERA 



layer of pavement epithelium which is easily destroyed. This epithelium 

 (earlier called ectoderm) and the connecti\e tissue (mesoilerm) are now 

 regarded as a common layer, 'iiicsccloiicnn,' since the pavement ej)ithelium 

 is often genetically oulv connecti\'e -tissue cells \\'hich have spread o\cr 



"^^%?5,'( ( , 



Fig. 159. Fig. 160. 



Fig. 15Q. — Ascon stage o£ 5yfa«(/ru (after Maas). c, entoderm; m, mcseclodcrm; 

 o, osculuni; p, ]iores. 



Fig. iOo. — Section of \vall of Sycuiuh'ti ra[>ho}uis (after Scltulze). c, epithelium; 

 en, coliared llagellale cells; m, mesodernt \\ith eonneelive-lissue cells; 0, eggs; st^ 

 calcareous spicules. 



Fig. 161. — Section of Pldh'im (afler F. F. Seliul'eV f, canals leading from ampull-E 

 to cloacal tubes; c, anipull;e; (/, afferent canals; o, oscukun. 



the surface. On the other hand, there is a distinctly differentiated ento- 

 derm in the shape of a one htyered llagellate epithelium lining the stomach, 

 the cells of which (en) recall the Choanoflagellata (p. 184), since they 

 ha\'e collars surrounding the llagella. The taking of food is accom- 

 plished l)y the collared cells, its distrihutioir by the ama-hoid cells. 



