NUTRITION IN ANIMALS 157 



B. DIGESTION 



Among the single-celled animals such as Paramecium, 

 digestion is reduced to its simplest terms. The food material 

 enters the cell and is acted upon directly by substances 

 formed by the protoplasm (endoplasm) in its vicinity. In 

 Hydra a special layer of cells, the endoderm, is largely de- 

 voted to digestion. Although some of the endoderm cells 

 actually engulf small particles of food and digest them within 

 the cell (INTRACELLULAR DIGESTION), the major part of 

 digestion is brought about within the enteric cavity by secre- 

 tions from the endoderm cells. Digestion of the latter type 

 (INTERCELLULAR) is characteristic of all higher animals and 

 reaches its full development in the Vertebrates. 



The alimentary canal is essentially a tubular chemical 

 laboratory which passes the food on by its own muscular 

 activity, known as PERISTALTIC CONTRACTIONS, from one com- 

 partment to another. Each of these compartments, in turn, 

 supplies the chemical reagents which it uses for changing 

 the food into a soluble form so that it can pass through the 

 walls to be taken up by the circulatory system and finally 

 distributed to the cells of the organism as a whole. The 

 complex food materials which enter the human mouth run 

 the gauntlet of a whole series of digestive fluids. The sali- 

 vary glands in the mouth secrete an enzyme which chemically 

 modifies the starches; the gastric glands of the stomach 

 supply the gastric juice containing enzymes which act on 

 proteins, and free hydrochloric acid which renders the 

 stomach contents acid in reaction; while glands in the intes- 

 tinal walls, and the pancreas collectively supply other enzymes 

 which act on proteins, carbohydrates, and fats in a medium 

 made alkaline chiefly by certain substances from the liver. 



Turning now to the origin of the chemicals which bring 



