Life of Plants and Animals 215 



able adaptations of external characters to the habits 

 and mode of life of the animal. 



An animal has been compared to a plant turned 

 inside out. By this is meant that the roots of plants, 

 serving for absorption, are spread out and perma- 

 nently fastened to the soil, from which the plant is 

 nourished. In animals, the absorbing villi, corre- 

 sponding to roots or root-hairs, are turned inward, 

 thus coming in contact with the food material of the 

 alimentary canal. Thus the animal might be said 

 to carry the soil around with it. The alimentary 

 canal is thus an adaptation to enable the animal to 

 move from place to place to get its food. The free- 

 dom of movement thus secured is necessary to the 

 higher development which the animal attains. Some 

 fixed forms of animals, like the sponge, hydra, corals, 

 etc., are comparatively low. Their alimentary canal 

 is, a mere bag with a single primitive opening, the 

 mouth. 



Digestion. Food, while in the alimentary canal, 

 is really outside the body. To be useful as food it 

 must be absorbed. The alimentary canal" is, in all 

 cases, lined by a layer of cells, the epithelium. In 

 the primitive forms this epithelium is variously folded 

 so as to produce pouches either simple or more com- 

 plex. They are the glands of the alimentary canal 

 concerned in digestion. The principal ones are: 

 (i) the salivary glands at the entrance to the alimentary 

 canal, variously developed according to the food of 

 the animal; (2) the liver, the largest and most con- 

 stant gland in the animal kingdom, being well devel- 

 oped in mollusks and all animals above them. To 

 these are added the pancreas and gastric glands or 

 glands corresponding to them in the higher vertebrates. 



Even the simple tubular glands of the alimentary 

 canal secrete fluids that either serve to soften the food 

 or else produce such chemical changes as will prepare 



