HO W SPONGES LIVE. 
47 
Fig. 17. 
anchored to the bottom by long flint threads, often 
several feet long, looking like the finest spun glass. 
And now we find the 
sponge-animal advancing 
yet a step farther, and 
beginning no longer to 
build entirely with lime 
and flint taken from the 
water, but to manufacture 
its own material. We all 
know that the spider spins 
its web of threads of 
gum formed in its body, 
and that the silk of the 
silkworm is made in the 
same manner, and now we 
have to learn that the 
sponge - animal with its 
simple slime cells can do 
this too ! For all the 
sponges which we use are 
made of fine fibres, which 
prove, when examined, to 
differ very little chemically 
from the silk of the silk- 
worm. These fibres have 
been secreted by the slime- 
animal out of its food, and 
by crossing and re-cross- ,, , . t , 
. . . Venus Basket.* The skeleton of a 
ing them in all directions flint-sponge. 
it forms the soft elastic 
skeleton of the toilet sponge. Yet they are not woven 
carelessly or without purpose, for we have seen that they 
* Ettpleclella speciosa. 
