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. 



C D 



FIG. I2g. Parts of the bodies of sponges. 



A, spicules; B, spongin ; C, a reproductive body or gemmule ; D, collar cells. 



(After various authors.) 



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 (0). 



