ANIMAL LIFE 



353 



BASE 

 FIG. 314. The Hydra. 



Hydra. The hydra is a fresh-water animal which 

 resembles a hollow bag. Food is carried into the animal 

 by little tentacles which grasp the food and carry it 

 toward the mouth (Figure 314). 

 The body wall of the hydra is 

 made up of two layers of cells. 

 The inner layer serves to digest 

 the food, while the outer layer 

 serves as a protective covering 

 for the inner layer. Between 

 these two layers of cells are some 

 muscular fibers and nerve cells 

 which enable the animal to re- 

 ceive sensations and to move the 

 parts of its body, or to change its 

 position. 



Worms. Earthworms must be classified among our 

 most useful animals (Figure 315). They break up the 

 soil, thus allowing water and air free passage through it. 

 They also carry large quantities of deep soil to the surface, 

 thus mixing the soil and making it more fertile. The 

 damage they do to growing plants is small compared with 



the good they do 

 in the soil. 



Earthworms fur- 

 nish an interesting 

 laboratory study. 

 They may be kept 

 for some time in a 



box filled with soil. Notice how the worm moves. It 

 has two layers of muscles which provide for its move- 

 ments : an outer layer which passes in a circular direction 

 around the body, and another longitudinal set which runs 



FIG. 315. An Earthworm. 



