202 



PORIFERA 



layer of pavement epithelium which is easily destroyed. This epithelium 

 (earlier called ectoderm) and the connective tissue (mesoderm) are now 

 regarded as a common layer, 'mesectoderm,' since the pavement epithelium 

 is often genetically only connective -tissue cells which have spread over 



FIG. 159. FIG. 160. 



FIG. 159. Ascon stage of Sycandra (after Maas). e, entoderm; m, mesectoderm; 

 o, osculum; p, pores. 



FIG. 160. Section of wall of Sycandra raphanus (after Schulze). e, epithelium; 

 en, collared flagellate cells; m, mesoderm with connective-tissue cells; o, eggs; st, 

 calcareous spicules. 



<* d 



FIG. 161. Section of Plakiua (after F. E. Schulze). c, canals leading from ampullae 

 to cloacal tubes; e, ampullae; d, afferent canals; o, osculum. 



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



j 



derm in the shape of a one-layered flagellate epithelium lining the stomach, 

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

 have collars surrounding the flagella. The taking of food is accom- 

 plished by the collared cells, its distribution by the amoeboid cells. 



