214 



PRACTICAL ZOOLOGY 



attached to rocks just below low-tide mark. Its body wall is 

 folded as shown in Figure 128, B. This folding increases the 

 amount of surface of the body wall and consequently the num- 

 ber of cells. The result is a greater number of flagella, an in- 

 creased current of water, and more food. 



M 



Fig. i2g. 

 A, spicules ; B, spongin ; C, 



— Parts of the bodies of sponges. 



a reproductive body or ge'mmule ; 

 (After various authors.) 



D, collar cells. 



Flow of Water in Fresh-water Sponge. — The fresh-water 

 sponge is comparatively complex. Water passes through the 

 pores (Fig. 128, C, DP) into a cavity (SD) just beneath the 

 outer wall, then by way of incurrent canals (In) into chambers 

 lined with flagellated cells ( C) , and from here through excurrent 

 canals (Ex) into the gastral cavity (PG) and out through the 

 osculum (O). 



