CHAPTER 1 7 



THE DIGESTIVE SYSTEM 



The business of procuring food and of 

 extracting the energy that resides in it is 

 one of the most important activities con- 

 fronting animals. This is true because all 

 animal activities require energy which can 

 be obtained only by releasing it from the 

 food where it is stored. All animals, from 

 amoeba to man, have made special provi- 

 sions for bringing about this conversion. 

 The first steps in this complex energy- 

 releasing mechanism are ingestion and di- 

 gestion of food. 



A large variety of means have been 

 devised by animals to accomplish these 



ends. The amoeba engulfs its food to form 

 a tiny vacuole in which the food is finally 

 dissolved or digested and absorbed into the 

 surrounding cell protoplasm (Fig. 17-1). 

 In ascending the animal scale — to hydra 

 with its spacious gastrovascular cavity, to 

 the earthworm with its internal tube, to the 

 vertebrate with a more elaborate tube — we 

 see the evolution of a complex digestive ap- 

 paratus of higher animals. Whereas the 

 digestive enzymes in the earthworm are 

 secreted solely from the walls of the tube, 

 in the vertebrate there are, in addition, 

 special glands some distance away from the 



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