42 LIFE AND HER CHILDREN. 
one egg, the whole sponge is one single individual ; yet, 
when grown up it is not a mere mass all doing the same 
work, as in the simplest animals, for it has learnt the 
secret of division of labour; and while one set of cells, 
those forming the smooth slime, are busy taking in 
food, the other and whip-like cells are foraging for this 
same food and sweeping away the refuse ; and, be- 
tween these two, a special layer of smooth cells is 
employed in building up the skeleton which supports 
the whole body. 
If we knew only the grown-up sponge, we might 
look upon it as a society of two kinds of slime- 
animals living together and building a common house. 
But when we consider that each whole sponge comes 
from a single egg, growing and dividing like one of 
the eggs of the higher animals, and that any piece 
of a sponge-animal is able to settle and grow up 
into a perfect sponge with the two kinds of cells, we 
see that these animals have made a great step never 
again to be forgotten by the children of Life. They 
have learned to form in one body two kinds of cells 
with different duties, which, by their mutual labour, 
carry on in one being the work of life. 
We are now, I hope, able to picture to ourselves 
the sponge growing upon the rocks in deep water, or 
sometimes in shallow pools, or between the tide- 
marks, looking like a smooth mass of slime of 
different shapes, with holes invariably open as long 
as it is under water, but closed (as we shall find on 
the English shores) if the sponge is by chance left 
high and dry by the tide. We can imagine to our- 
selves the small fountains of water spouting from 
