40 LIFE AND HER CHILDREN. 
or main thoroughfares, while the slimy sponge-body is 
spread out as a thin film along them all. In this 
way it is possible for the sea-water to reach right 
throughout the whole body of the sponge along the 
various canals. But if this water only lay still from 
day to day, no fresh food could be brought, and the 
whole would become stagnant and bad. The animal 
cannot feed or even breathe unless a constant fresh 
supply of water, full of oxygen and living beings, is 
driven through the canals. 
How is this to be done ? 
At first sight it seems as, if the young sponge 
were behaving very foolishly in this matter, for no 
sooner has it settled down than it draws in all the 
whip -like hairs outside its body which we should 
have thought would be useful for driving in food, and 
becomes a mass of smooth slime-cells with large and 
small holes scattered here and there. Still, as the 
water goes on pouring out at the big holes (see Fig. 
1 2), it is clear that it must be going in somewhere; and 
on cutting open the living sponge and watching it 
at work the secret appears. Here and there through- 
out the narrow canals of the skeleton are to be found 
little chambers, like two saucers face to face (i,Fig. 1 3), 
and in these are arranged in rows a number of whip- 
like cells, exactly like those which were before outside 
the sponge. It is the whips in these cells which do 
the work required. Waving ceaselessly to and fro, 
they drive the water before them always in one direc- 
tion, so that it is drawn in at the small holes (a a, 
Fig. 1 3) and driven out at the large ones (b &). By 
means of this wonderful contrivance fresh sea-water 
full of oxygen and living plants and animals is 
