CHAPTER 8 



THE DIGESTIVE SYSTEM 



The complex coordination of activity of the skeletal and muscular 

 systems is to a great extent responsible for the ability of the animal 

 to move about in its environment in search of food, shelter, to escape 

 from its enemies, and to search for mates. Of these activities, the 

 search for food takes the greater share of the time and energy of the 

 animal. Inasmuch as the food is constantly being used for the pro- 

 duction of energy and for repair, the search is continuous. 



Food when first obtained is not suitable for use by the cells of 

 the body. It must undergo a series of changes by which it is trans- 

 formed from complex to simple compounds which can then be as- 

 similated. 



Once the food is changed to simpler compounds, it then can be 

 incorporated into the cells themselves. This building-up process is 

 known as anabolisin; while the utilization of the assimilated foods to 

 produce heat or energy is catabolism. The sum total of these two 

 processes is metabolism. Thus the cells of the body are in a constant 

 dynamic state of activity. 



Those animals of less complex construction often do not possess 

 special structures for the intake, preparation, and transportation of 

 their food. With increasing size and complexity, specialized digestive 

 organs become imperative. At first this may consist of simply a cavity 

 in which both digestion and distribution take place. Further compli- 

 cations of food getting and utilization of solid matter require elaborate 

 accessory mechanisms for handling, mastication, and distribution. These 

 mechanisms are variously developed among the members of the dif- 

 ferent phyla; perhaps the most complex are those found among the 

 vertebrates and arthropods. 



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