HOW SPONGES LIVE. 43 
the larger openings, and carrying off the refuse from 
the inside of the sponge, and we can fancy we see 
the small chambers buried in the canals with their 
active inmates lashing the water onwards in its course 
through the whole mass. 
But we have yet to consider the skeleton of the 
living animal, and why so much time and labour 
should be spent in forming it. There are two 
reasons why a solid framework is useful to the 
sponge-animal. First, it supports the large mass of 
soft slime, and enables it to spread itself out in thin 
layers, so as to touch the water in the canals ; and, 
secondly, it protects it from enemies. 
There are a few sponges made entirely of slime, 
the canals and thoroughfares being in the slime itself ; 
and in these, when the animal dies and decays, nothing 
solid is left behind. But such sponges have probably 
become degraded and have lost their skeleton, and they 
are clearly under a disadvantage, for the walls of slime 
are forced to be much thicker, and food cannot reach 
them so easily ; and besides this, when we remember 
how many sea-animals feed on living slime, we can- 
not but see that these sponges offer a very tempting 
feast. Comparatively large animals, such as shrimps 
and fish, will take big mouthfuls out of them, while the 
water-fleas and smaller sea-worms which are carried 
through their canals, are quite as ready to eat the 
slime as the slime is to eat them. But if the sponge 
can offer a y^ry tough and unpalatable mouthful, or 
can prick its enemies' mouths with a sharp point, 
they will not be so ready to take a second bite ; and 
so it comes to pass that we find in sponges some of 
the most curious weapons imaginable. 
