ECONOMIC IMPORTANCE 
183 
This sac is worked forward and as it passes the openings 
of the reproductive organs, eggs and the sperms from another 
worm are pushed into it. The sac continues to move forward 
and finally leaves the worm as a closed capsule. This 
capsule contains eggs, sperms, and fluid food. After the 
fusion of the eggs and sperms, the resulting embryonic 
worms begin to feed upon the fluid food in the capsule ; later 
they feed upon one another until but one may remain eventu¬ 
ally to bore or eat its way to the earth outside. From now 
on the food of the young worm is the 
soil. 
The earthworm is an example of an ani¬ 
mal which has both ovaries and spermaries. 
168. Self-protection. — The habit earth¬ 
worms have of remaining in their burrows 
during the daytime is their chief means 
of protection. During the daytime birds 
and other animals seek them out as food. 
They are perfectly helpless when captured. 
When a robin tries to pull an earthworm 
from its burrow, the projecting setae pre¬ 
vent the worm from being pulled out. 
Sometimes the body breaks under the 
strain but this does not kill the worm as 
the portion remaining in the burrow soon regrows the 
lost parts. During the winter or a dry season earthworms 
burrow deep into the ground and collect in groups for 
protection. 
169. Economic Importance. — The value of earthworms 
to agriculture is too great to be overestimated. In burrow¬ 
ing their way through the soil they leave passageways for 
water and air to enter, thus assisting plants to grow. They 
bring the fertile, swallowed soil to the surface. When the 
total number of earthworms is considered, it is obvious that 
they are the great natural cultivators of the soil. 
Figure 169. — Dero, 
a Common Fresh¬ 
water Annelid. 
