WATER FRONTAGE 85 



Water Frontage in Lower Animals. — Among the 

 one-celled animals no special transportation system exists, 

 for reasons already described. In somewhat higher ani- 

 mals, such as hydra, it will be remembered, the component 

 cells are arranged to form an outer protective and an 

 inner digestive layer surrounding the central digestive 

 cavity. Food material, entering the sac, is circulated by 

 means of flagella borne on some of the cells, and is thus 

 brought into intimate relation with the entire inner layer, 

 by which it is taken up. The excess of nutritive material 

 diffuses to the outer cells. No special transportation 

 system exists, and in this respect hydra resembles its 

 relatives, the sea anemones, jelly-fishes and the more 

 distantly related sponges where the diffusion stream also 

 supplies the needs of scattered cells between the outer 

 and inner body layers. 



Fig. 14. — Diagram of the Digestive System of a Flatworm. 

 Mouth in center of figure. 



The Flatworm Transportation System. — The flat- 

 worms are small animals, usually not over an inch in 

 length, frequently beautifully colored and abundantly 

 represented among the rocks along the seacoast and to a 

 less extent in fresh water. The body is flat and leaf-like, 

 as their name suggests, and on its under surface bears the 

 mouth opening. This leads into a central cavity from 

 which intestinal branches extend through the animal to 

 its outer margins where each ends blindly (Fig. 14). 



