Vital Animal Life Processes 



All animals must carry on certain vital life processes if they are to 

 continue to live and reproduce their kind. The animal may be a tiny 

 one-celled organism, such as an amoeba ; a larger mass of protoplasm, 

 such as a jellyfish ; or a highly complicated aggregation of cells, such 

 as a human being — all must share these basic reactions. First we shall 

 consider the life process known as digestion which is necessary for 

 normal animal nutrition. 



Digestion 



Food, as it is taken into the animal body, is seldom in a form that 

 can be used by the cells. For instance, if instead of eating your break- 

 fast this morning you had ground it finely and injected it into your 

 veins, you not only would have gotten no nourishment, but it would 

 have killed you. The same food obtained through the normal channel 

 of your mouth is now circulating through your veins and nourishing 

 the cells of your body. Somewhere between the eating and the ab- 

 sorption into the blood, chemical changes took place in that food which 

 made it usable by your body. Enzymes are the agents which ac- 

 complish this transformation. We shall find that in some of the 

 simpler forms of animal life the raw food particles are taken into the 

 cells and digestion is accomplished by enzymes within these cells. 

 This is known as intracellular digestion. In more complex animals 

 there are special organs for the production of these enzymes, and they 

 are released into other organs such as a stomach or intestine where 

 digestion takes place. The digested food is then absorbed and dis- 

 tributed to the cells of the animal's body. This is extracellular di- 

 gestion. 



We use the term ingestion to refer to the intake of food into the 

 body. Most animals have a mouth for ingestion, but we shall find 

 some simple animals which can engulf food at any part of the body. 

 After digestion there is a residue of matter which could not be broken 

 down into soluble form by the enzymes. Since this residue is of no 

 value to the animal, it must be expelled from the body — this is egestion, 



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