274 



GENERAL ZOOLOGY 



around the base of the flagellum. The flagella wave inward 

 and cause a current of water to flow into the sponge through 

 the minute dermal pores and along the incurrent canals, 

 past the collar-cells into the excurrent canals. Water passes 



FIG. 136. Fresh-Water Sponge. Upper 

 portion of figure natural size; lower 

 portion enlarged 



from the excurrent canals 

 into the cloaca, a large 

 chamber just below the os- 

 culum, and thence flows 

 out of the sponge through the osculum (Fig. 137, d). 



Food consisting of microscopic plants and animals is drawn 

 in at the dermal pores and carried along by the current. 

 Very small particles are taken into certain cells along the 

 canals and digested there, as in the endodermal cells of Hydra. 

 Larger particles are surrounded by ivandering cells and 

 digested little by little. Digested food passes by osmosis to 

 all the cells of the sponge. 



