200 GENERAL ZOOLOGY 



For illustration, we may suppose that an earthworm has 

 swallowed a small piece of potato which contains a great deal 

 of starch. In the intestine the starch is changed physically by 

 being made liquid or partially so, and it is changed chemically 

 by the enzyme diastase into a sugar-compound. In the latter 

 state, what was once starch passes readily through the wall of 

 the intestine. A proteid substance like iVeat-juice cannot pass, 

 except in a slight degree, through the intestine until it has 

 been changed chemically ; this is done by the enzyme trypsin. 

 The changed substance is called a peptone. It is not likely 

 that earthworms in nature ever need to digest considerable 

 quantities of fat, but in experiments fats have been fed to 

 earthworms. Results followed similar to those obtained in 

 many other animals which usually consume fats. The enzyme 

 steapsiri separates the fats into compounds known as glycerin 

 and fatty acid. The fatty acids combine chemically with the 

 alkali in the digestive tract, resulting in compounds similar 

 to soap ; the process is therefore called saponification. When 

 the proteids are changed to peptones, the starch to sugar, and 

 the fats to glycerin and soaps, then these organic foods are 

 ready to pass with any inorganic foods, as, for example, 

 salt and water, through the' wall of the intestine. 



Absorption. Digested food passes through the intestine- 

 wall and mixes with the body-fluids, in accordance with a 

 principle known in physics as osmosis (see Fig. 101). In the 

 experiment illustrated in the figure there is a salt-solution 

 in the sac made of sheep's intestine (Fig. 101, 4), and pure 

 water in the beaker outside. The experiment is supposed 

 to have been started with the salt water and the pure water 

 at the same level. Within a half-hour the level of the salt- 

 solution has risen several inches. Besides, some of the salt- 

 solution has passed through the membrane into the pure water. 

 It is one of the phenomena of osmosis that a greater amount 

 ^of liquid passes from the less dense to the more dense solution 



