158 
THE SIMPLER METAZOA 
sponges. In others, they are of silica and form the glassy 
sponges. Sometimes fresh-water sponges grow in the water 
mains of cities and towns, causing the pipes to become 
clogged. 
The more important sponges have a skeleton made up of 
a hornlike substance which is flexible. This is the sponge 
of commerce, great quantities of which are gathered from the 
sea by divers and by dredges. The living tissues are allowed 
to decay, and the skeletons are then washed and dried. 
Some are bleached to form the white sponges. The sponges 
of best quality come from the Mediterranean Sea and the 
Red Sea. 
The value of the sponge fishery in the whole world annually 
amounts to about $3,500,000. 
137. Relation to Other Animals. — No animal is known to 
eat the sponge. Sponges themselves feed on minute particles 
of food, which are carried in by the currents of water pro¬ 
duced by the cilia of the endoderm. Some marine animals 
use the porous body of the sponge as a retreat. 
Certain sponges live in close relationship to higher forms 
of animals. One kind is always found growing on the legs 
of crabs. The movement of the crab carries the sponge to 
water richer in oxygen and food, and the crab is hidden from 
its enemies by its sponge covering. Each animal gains by 
this inter-relationship. Where two such animals as the crab 
and sponge live in this way the relationship is known as 
symbiosis (sym-bl-o'sis: Greek, syn, with; bios, life). 
SUMMARY 
The transition from simple Protozoa, through the Colo¬ 
nial Protozoa, to the Metazoa is simple and direct. In 
gonium and volvox, the beginning of division of labor is 
noticed; that is, one part of the body becomes dependent 
on another part for certain definite things. For example, 
one cell is devoted to securing food, while another produces 
