MULTICELLULAR ANIMALS 157 



Digestive Enzymes. Since animal food consists essentially 

 of three main types, carbohydrates, fats, and proteins, there are three 

 main types of digestive enzymes required in conditioning the food. 

 The DIASTASES act on sugars and starches; the lipases act on the 

 fats; the proteolytic enzymes digest the proteins. Their relative 

 abundance depends on the type of food that forms the chief in- 

 gredient of the diet. In Man the digestive system contains all three 

 types of enzymes apparently equally important, for the human food 

 supply is most \'aried. Accessory to the activity of the enzymes is the 

 presence in the digestive system of certain other agents. For ex- 

 ample, the PEPSIN, a proteolytic enzyme in the human stomach, op- 

 erates best in the presence of acid; hence the fluid of the stomach 

 contains a quantity of hydrochloric acid. The percentage of HCl 

 is considerably greater in the stomach of the dog, for in this animal 

 it serves also to decalcify bone, which forms a part of the diet. 



Absorption of Foods. All organisms are so adapted to their 

 environment, including their food supply, as to solve and to take 

 advantage of the principles and laws operating in non-living sys- 

 tems. Digested food passes out of the digestive system according to 

 the principles of diflfusion through a membrane and consequently 

 the properties of the walls of the digestive tract are those of semi- 

 permeable membranes, which have been previously discussed (p. 

 32). In the passage of any substance through a membrane, three 

 factors are concerned: (i) the condition of the penetrating sub- 

 stance; in other words, a food to be digested must be in the form of 

 molecules that are small enough to pass through the pores of the 

 membrane from a region of higher to a region of lower concentra- 

 tion; (2) the area; the greater the area of contact the more will pass 

 through in a given time. In the case of digested foods, the amount 

 that will pass through within a given time thus increases with an 

 increase in the area of the walls of the digestive canal. In the earth- 

 worm the area for absorption is very greatly increased by a struc- 

 ture, the TYPHLosoLE (Fig. 104), which is an extensive inward fold 



