CHAPTER XI 

 NUTRITION 



It is a great satisfaction for me to know when regaling on my humble 

 fare that I am putting in motion the most beautiful machinery with 

 which we have any acquaintance. — Dickens. 



Among the single-celled animals, such as Amoeba and Para- 

 mecium, nutrition is reduced to its simplest terms. The food ma- 

 terial enters the cell and is acted upon by substances formed by 

 the protoplasm in its vicinity: the food is chemically changed, 

 or digested, so that it becomes available for the use of the cell. 

 In Hydra a special layer of cells, the endoderm, is largely devoted 

 to digestion. Although some of the endoderm cells actually en- 

 gulf small particles of food and digest them within the cell (intra- 

 cellular digestion), the major part of digestion is brought 

 about within the enteric cavity by secretions from the endoderm 

 cells. Digestion of the latter type (intercellular) is characteris- 

 tic of the Earthworm and all higher animals. 



We have considered the form and supporting structures of the 

 body wall of a typical Vertebrate — the outer tube which sur- 

 rounds and contains the viscera — and therefore we recall that 

 through this outer tube, just as in the case of the Earthworm and 

 Crayfish, there runs from mouth to anus a second or inner tube, 

 the digestive tract, or alimentary canal. 



The alimentary canal is essentially a tubular chemical labora- 

 tory which passes the food on by its own muscular activity from 

 one part to another. Each of these regions, in turn, supplies the 

 chemical reagents which it uses both for changing the food into 

 a soluble form so that it can pass through the walls and be dis- 

 tributed to the cells of the organism as a whole, and also for mak- 

 ing it suitable for use by these cells. Indeed, the complex food 

 materials which enter the human mouth run the gauntlet of a 

 whole series of digestive fluids. 



Although the various kinds of food eaten by animals differ 

 widely in their chemical composition, nevertheless the process 



150 



