HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



Two extremes of host-parasite specificity may be illus- 

 trated by the relations which have been found in the 

 writer's laboratory to exist between ( i ) the giardias of 

 mammals and (2) the herpetomonad flagellates of flies. 

 Within the past few years we have been carrying on a 

 series of investigations (Simon, 1921, 1922; Hegner, 

 1922a, 1922b, 1923b, I924d, 1925c) which seem to indi- 

 cate that the giardias found in each species of host differ 

 specifically from those found in every other species of 

 host and only in a few cases is more than one species of 

 host infected by one species of giardia. Thus morpholog- 

 ically distinct species have been described from tadpoles, 

 house mice and rats, field mice, rabbits, cats, dogs, 

 guinea-pigs, and ground squirrels as well as from cer- 

 tain birds and reptiles. Here then is an example of very 

 rigid host-parasite specificity. 



In contrast to this are the results of Becker's (1923) 

 studies on the herpetomonad flagellates that live in the 

 intestine of flies. Investigators previous to Becker's work 

 assumed that each species of fly was infected with its 

 own peculiar species of herpetomonad and hence when a 

 new species of fly was found to be infected the organism 

 was given a new specific name. Becker carried on ex- 

 periments with six species of muscoid flies belonging to 

 six different genera and found that each species could 

 be infected with herpetomonads from each of the other 

 five species. Because of these results and of the fact that 

 no morphological differences could be observed between 

 the various so-called species, Becker concludes that the 

 flagellates from these six species of flies are all of the 

 same species, — that first described from the house fly, 



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