Buccal or Pharynx Esopliagus Stomach 

 oral cavity / / | 



Intestine 



Salivary gland 

 (paired) 



Digestive gland 

 (paired) 



Fig. 1 7.1 . Schematic diagram of the gen- 

 eralized digestive system in mvertebrates. 



serve as food. These reactions tend to become more elaborate, or to be served 

 by more elaborate structural specializations, as the size and complexity of 

 animals increase. Holozoic animals ingest many kinds of food, and their di- 

 gestive mechanisms are correspondingly adapted, both structurally and 

 functionally, to their individual necessities. And yet, in all animals that 

 have digestive tracts, the digestive systems show obvious similarities; in fact, 

 the digestive system has much the same fundamental structure in all animals 

 that have both mouth and anus. In contrast, the reproductive systems of the 

 different phyla show great diversity of structure, although there are remark- 

 able functional parallels. Thus, to return to our figure of speech, the 

 problem of developing an efficient reproductive system has been solved in a 

 great variety of ways. 



The comparative review to be undertaken in the present chapter furnishes 

 data for consideration by the student of evolution, although little reference 

 will be made to this aspect of the subject. It should become evident, how- 

 ever, that homologous structures may be modified for diverse functions, and 

 that structures of varied embryonic origins may be adapted to common func- 

 tional purposes. 



Systems Related to Metabolism 



Digestive Systems. Members of the phylum Protozoa perform all of 

 their vital activities at the unicellular level, and their structural specializations 

 for various functions are often referred to as organelles. In the more complex 

 protozoans, such as the paramecium, there, is a "system" of organelles 

 specialized for various aspects of nutrition. The cytostome or cell mouth, the 



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