X. DIGESTION IN THE LOWER ANIMALS.' 



The process of digestion (as a preliminary step in the 

 clianging of food into living matter or protoplasm) is to 

 be met with wherever we find living cells. In man the 

 system has reached its most complex development, but if 

 we examine the various gi-oups of animals, we find that in 

 every group the process of changing food into soluble sub- 

 stances by the aid of enzymes is quite as important and 

 necessary a step in metabolism as it is in man, though the 

 organs which perform this action may be much simpler of 

 structure. 



It is interesting, then, to study the methods of digestion 

 in the lower forms, in a comparative way, from the light 

 they throw upon the development of the complex human 

 system. In the pages that follow a few types have been 

 selected to bring out the manner in which the human sys- 

 tem has been developed from lower types. 



Digestion in one-celled animals. — In animals like the 

 amceba and paramcecium (one-celled animals) the food is 

 taken into the cell in the form of solid particles. In 

 this respect the protozoans differ from all higher animals. 

 Digestion proper, therefore, in these animals occurs in 

 the cjiioplasm of the cells themselves, and to distinguish 

 it from that of other forms is called intracellular digestion. 



' Chapters X, XIV, XVII, XX, XXII, XXIV, XXVII, and XXXI, 

 are intended for study in connection with a course in zoology. In con- 

 nection with laboratory work in zoology they furnish necessary data 

 for comparison. When not preceded by such laboratory work they 



should be omitted. 



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