d FARM ANIMALS 
acted upon by a digestive fluid that prepares it for ab- 
sorption. The cavity in hydra, in which digestion goes 
on, corresponds to the cavity inside of a simple sponge. 
5. Corals and jellyfish——The structure of the body is 
still more complex in the coral and jellyfish. In these ani- 
mals cells are grouped to do special work. Thus, some cells 
serve as muscles, others as nerves, others digest the food, 
and still others are distinct reproductive cells. The sponges 
were slightly higher in form than the 
amoeba, the hydra than the sponges, 
the corals and jellyfish than the hydra. 
And while there is still greater diver- 
sity, the same structural and physiologi- 
cal complexity proceeds on up the scale 
through all the higher forms of animal 
life. 
6. Worms.—There are many aquatic 
worms that swim. Most worms crawl. 
They do this by means of successive 
Siu) contractions of successive parts of the 
Corats muscular wall of the elongated body. 
at pollen aaiccn They have no legs such as caterpillars 
nae have. The common earthworm, the 
plowman, is one of the higher forms. 
1 
“The earthworm not only dwells in the soil, but is in a sense the 
manufacturer of soil, since the fertility of the earth depends greatly 
upon the work of earthworms. They pass the soil through their 
bodies, digesting the organic particles they find in it, and thereby 
loosen the soil, reduce it to a state of fine division, and render it 
more fit to support the growth of plants.” 
The tapeworm draws nourishment through the skin, and therefore 
has no need of an alimentary canal. The liver fluke is flat. The 
adult form infests the sheep’s liver. There it lays eggs, which after 
a time find their way into water. Unless a certain water snail takes 
them up they die. Housed for a while in these snails, the eggs now 
hatch, escape and finally settle on plants. If eaten by sheep the route 
is repeated. Gréat damage is done the sheep industry by these worms 
