HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



phozoites may also withstand the conditions in mouth, 

 stomach, and small intestine (Hegner, 1926c). Active 

 trophozoites from the pig were injected into the stomach 

 of guinea-pigs. In one guinea-pig killed one hour after 

 such an injection balantidia normal in appearance and 

 actively swimming were found in all parts of the stom- 

 ach and small intestine and in the cecum. In another 

 guinea-pig killed four days after an injection, no balan- 

 tidia were found in the stomach and small intestine but 

 a few were present in the cecum. These resembled the 

 specimens injected but may have been inhabitants of the 

 cecum before the experiment was begun. Apparently the 

 trophozoites may pass through the stomach and small in- 

 testine unharmed and set up an infection in the cecum 

 but this has not been definitely proved. Similar results 

 have been obtained by Rees ( 1927). 



Excystation. Passive resistance includes all conditions 

 that are unfavorable to excystation. In order to set up an 

 infection encysted specimens must excyst or they are car- 

 ried directly out of the body. When and how excystation 

 occurs is not known. This process might be prevented if 

 the cysts are carried through the intestine too quickly, 

 for example under more or less diarrheic conditions; 

 hence a certain degree of stasis is probably necessary. 

 Other factors that may play a role in excystation are 

 temperature, the degree of moisture and the character 

 of the digestive fluids. 



Tlie intestinal environment. Another type of passive 

 host resistance is the character of the digestive contents, 

 especially as regards their effect upon the physiological 

 processes of the parasite and upon its food supply. Tro- 



176 



