MASTICATION 



285 



the cytoplasm. Into this vacuole the cytoplasm empties digesting 

 fluids, which act upon the food. The digested food is absorbed by the 

 surface of the vacuole, and the non-digestible parts of the food are thrown 

 out of the cytoplasm at a definite part of the surface of the cell. When 

 the vacuole is thus emptied, it disappears (Fig. 245). 



ect. 



end. 



FIG. 246. Part of the body of a sponge, Grantia. 

 mes., mesoderm; ect., ectoderm; end., endoderm; 

 spi., spicules. The black objects in the round 

 bases of some endodermal cells are food particles. 



FIG. 247. A sectional diagram of a 

 water canal of the sponge Sycon 

 gelatinosum. cil.d.c., ciliated diges- 

 tive cells. (After PARKER and HAS- 

 WELL.) 



In the sponges there is a differentiation of the cells into ectodermal, 

 endodermal, and mesodermal tissues. The endodermal cells line the 

 internal cavities and form the alimentary tissue of the sponge. These 

 cells are collared cells, each bearing a 'flagellum (Fig. 246). The water, 

 bearing food, is brought into the endodermal canals at certain places 

 and driven out at other places by the actions of the flagella (Fig. 247). 



