HOW SPONGES LIVE. 41 
always pouring along the small canals, bringing air 
and food to each cell along the road, while the bad 
water out of which the slime has taken all the oxy- 
gen, and into which it has thrown the hard parts and 
refuse of its food, is driven out at the large holes, 
carrying away with it all that is hurtful and useless. 
Fig- 13- 
Section of an ordinary bath sponge. Murie. 
a a a, Small holes where the water enters, b b, Large holes where 
it flows out. c c, Chambers with whip-cells which drive the water on. 
I, A chamber enlarged showing the cells. 2, 3, Different forms of 
whip-cells. 
And now we can understand why the sponge, 
though a mere slime-animal, is classed as the pioneer 
of the second division of living animals ; because in it 
quite a new plan of structure has begun. Starting from 
