HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



animals, but that every one who studies protozoa is not 

 necessarily a protozoologist. Thus, many of the inves- 

 tigators who have added valuable data to our knowledge 

 of the protozoa were or are biologists who have employed 

 protozoa in their researches because these organisms 

 seemed to be favorable for the study of problems of gen- 

 eral biological significance. Both types of investigators 

 are necessary in order to build up protozoology as a 

 science. 



2. HOMOLOGIES AND ANALOGIES BETWEEN FREE-LIVING 

 AND PARASITIC PROTOZOA 



Parasitic protozoa are often considered by biologists 

 apart from free-living species as though there existed 

 some more fundamental difference between them than 

 that of habitat. A comparison of the structure, life- 

 cycles, habitats and activities of the free-living and para- 

 sitic protozoa prove, however, that the same principles 

 govern both types of organisms. 



The activities of all animals may be separated into ( i ) 

 those necessary for the maintenance of the individual, 

 and (2) those necessary for the maintenance of the race. 

 The individual must be able to protect itself in its envir- 

 onment, to escape enemies, to reach a favorable situation 

 in which to live, to find, capture, ingest, digest and as- 

 similate food, to egest undigested material, to secrete 

 protective substances, digestive juices, etc., to carry on 

 respiration and to excrete waste products. Races are 

 maintained by the asexual reproduction of the individuals 

 of which they are composed or by sexual reproduction 

 or by both of these processes. 



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