HOW SPONGES LIVE. 43 
the larger openings, and carrying off the refu 
the inside of the sponge, and \vc can fai 
the small chambers buried in the canals with tl 
active inmates lashing the water onwards in it 
through the whole mass. 
But we have yet to consider the ski I ' n 
living animal, and why so much time and la; 
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 ma 
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 arc a few sponges made entirely o{ 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 pro! 
become degraded and have lost their skeleton, and I 
are clearly under a disadvantage, for the walls o{ slime 
are forced to be much thicker, and food cannot i\ 
them so easily ; and besides this, when we iv: 
how many sea-animals feed on living slii 
not but sec that these sponges offer a very t< 
feast. Comparatively large animals, sue!. 
and fish, will take big mouthfuls out of them, V 
Water-fleas and smaller sea-worms which are 
through their canals, are quite as n 
slime as the slime is to eat them. But if ' 
can otter a very tough and unpalatal 
can prick its enemies 1 mouths with 
they will not be so ready to b 
so it comes to pass that we find in s] 
the most CUrioUS w eaponS i 
