BIOLOGY OF HOST-PARASITE RELATIONS 



ber of intermediate stages. That they have arisen many 

 times is indicated by their wide distribution among the 

 phyla in the animal kingdom. The evolution of para- 

 sitism is one of the most interesting of all biological prob- 

 lems and, as has been pointed out by several writers, 

 parasites offer particularly favorable material for the 

 study of the course of evolution, since parasites undoubt- 

 edly originated from free-living organisms from which 

 they have become differentiated by a sort of superim- 

 posed evolution, and in many cases the free-living ances- 

 tors of these parasites still exist. 



One very striking effect of the parasitic habit is that 

 generally called degradation. This term implies that the 

 parasite has degenerated, but although some of the parts 

 of the parasite undergo degeneration, others become 

 more highly developed. It seems better therefore to speak 

 of the parasitic condition as a specialization rather than 

 a degradation or regression, especially since most para- 

 sites are marvelously adapted to their mode of life. 



II. General Account of the Biology of Host-Parasite 

 Relations between Man and His Intestinal Protozoa 



Before discussing the various species of intestinal pro- 

 tozoa of man in detail it seems desirable to present a gen- 

 eral account which will serve more or less as an outline. 

 The subject matter has been arranged as nearly as 

 possible in the order of the series of events that occur 

 during an infection, and illustrative material has been 

 taken principally from data available regarding intes- 

 tinal protozoa. 



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