HOW SPONGES LIVE. 45 
but against the smaller but dangerous animals, which 
might be washed into it. In another sponge the spic- 
ules point towards the mouth at the top, so that any 
creature which has got in can be easily thrown out 
but one trying to get in would be spiked directly. 
Lime - sponges 
are to be found in 
most parts of the 
world, and some of 
them are very beau- 
tiful from the ar- 
rangement of their 
spicules. But these 
look, after all, like 
mere rough attempts 
at spike - building 
when compared with 
the wonderful spic- 
ules which are made 
by the flint -build- Spicules of flint found in the flesh of 
ing sponges. flint -sponges. Real size a mere speck, 
Fig. I5 shows almost invislble to the naked e y e> 
only a very few of the forms of flint spicules which 
are known. They look, under the microscope, as 
if the sponge -animal were an artist trying how 
many curious patterns he could invent ; and yet 
Dr. Bowcrbank has shown that each of these 
shapes has some special use, either in keeping out 
enemies, in supporting the sponge, or in spiking and 
entangling the smaller animals which form the food 
of the sponge-animal. Often as many as from three 
to seven different shapes may be found in one single 
sponge, forming by their combinations intricate and 
