44 
LIFE AND HER CHILDREN. 
Fig. 14. 
The sponges we use are by no means the first 
attempts at sponge-skeletons ; on the contrary, they 
represent the highest art in sponge-building. The 
simplest kind of sponges build their skeletons of 
lime and flint, as did the earlier slime-animals. Fig. 
14 is a picture of a lime-sponge. Here the outer 
layer of sponge -flesh has taken in lime and built 
up with it a number of little pointed spikes or 
spicules, which lie buried in the 
slime. The rest of the sponge is 
composed entirely of the sponge- 
animal, the outer cells being smooth 
and the inner ones whip-like, so 
that water and food are drawn in 
at the small holes in the sides, 
while the refuse is driven out at the 
large hole in the top. 
Now suppose that a fish attacks 
this sponge, instead of a mouthful 
of soft slime he will bite upon a 
number of minute sharp points 
which he will carry away sticking 
to the soft lining of his mouth, 
and the next time he sees such a 
sponge growing, he will hesitate 
before touching it. In some 
Sponge with lime sponges these lime-thorns are so 
spicules forming the arranged that they lie flat against 
skeleton over the living ^ sponge when ft J S still, but form 
flesh. Haeckel* ' 
a complete hedge of spikes round 
the holes when it is taking in water, showing that it is 
not only against the fish that it is protecting itself, 
* Ascetta primordialis. 
