HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



these different host species was taken for granted. Ben- 

 sen, in 1908, however, distinguished three species which 

 we know now as G. lamhlia, from man, G. miiris, from 

 rats and mice, and G. diiodenalis from the rabbit. Practi- 

 cally all of those who accepted Bensen's work still be- 

 lieved that cross-infection occurred. Many investiga- 

 tors have carried on cross-infection experiments, but cir- 

 cumstances in many cases rendered the results obtained 

 of little value principally because of the difficulty of ob- 

 taining clean experimental animals. Even as recently as 

 1923 (Galli-Valerio) the possibility of human infection 

 with giardias from rats and mice has been urged. 



Species of giardias in lower animals. Careful morpho- 

 logical studies carried out especially during the past dec- 

 ade indicate that the giardias of lower animals differ 

 from those in man specifically and that each species of 

 animal is infected with its own pecuHar species of giardia. 

 Cross-infection experiments when properly controlled 

 indicate that this specificity is very rigid ; hence the mem- 

 bers of this genus furnish very favorable material for 

 studies of host-parasite specificity. Up to the present time 

 the following giardias have been described, chiefly on the 

 basis of morphology and host association. 



