The Sponge. 9 



it takes in through the tabes. It takes in water through 

 the small tubes that we see, and the tiny ones that we 

 cannot trace carry it all over the sponge. When the 

 sponge has taken from the water the very smallest plants 

 and animals, which are its food, and has given carbonic 

 acid in exchange for oxygen, then the water passes out 

 through the large tubes. Bat as only the most minute 

 plants and animals can pass through the microscopic tubes 

 without danger of choking them up, a thin, porous skin 



Fig, 3. 



Fio. 4. 



Fig. 5. 



like a delicate sieve covers the whole sponge except the 

 two or three large openings. But why does no water 

 enter at these ? Because there is always a current flowing 

 out from them. 



Id little eacs (Fig 2*} all over the sponge are cells (a) bearing each 

 a microscopic whip (c), alwajs lashiDg the water and producing the 

 currents that carry the foul water out through the large tubes as fresh 

 streams come in through the small ones. In these cells, too, the food 

 is digested. Though the outward current keeps the large tubes 

 open, yet if a living sponge is disturbed, it will contract so forcibly 

 as to close even these openings. 



* Figs. 2 3, and 4 are highly magnified, while Fig. 6 is much reduced 

 from the natural size. 



