INTESTINAL FLAGELLATES OF RATS 137 



causing some to colonize themselves in the small intestine and 

 others in the cecum? 



(b) For flagellates in the encysted state: (i) what are the 

 chemical and physical properties of the cyst membrane for each 

 species? (2) what conditions are necessary for survival of the 

 cyst stage outside the host? (3) how long may the cyst survive 

 outside the host under favorable circumstances? (4) what effect, 

 if any, does gastric juice and its constituents have on the cyst 

 stage? (5) where and how does excystation normally occur? (6) 

 what conditions in the host's digestive tract are favorable or un- 

 favorable to excystment and the survival of excysted animals? Heg- 

 ner (1927Z7) believes that moisture and a temperature of 37° C. 

 for several hours are the only factors necessary to bring about 

 excystation of intestinal protozoa of man. Perhaps the same is true 

 of those in the rat. According to Hegner (1927a?) cysts of Giardia 

 Imnhlia, when injected into the stomach of a rat were found (i) 

 to show motility sooner when placed at 40° C. than control cysts, 

 (2) the percentage of viable cysts was greater in the controls, 

 indicating that some had been killed, and (3) that movement and 

 excystation were to some extent inhibited. Cysts remained viable 

 for as long as six hours in the rats' stomachs. 



In regard to the defenses of the host several problems suggest 

 themselves: (i) what habits of the host tend to increase or mini- 

 mize the likelihood of contamination of food or drink with viable 

 stages of intestinal flagellates from feces? (2) what variations in 

 the character of the host's secretions occur which might affect the 

 survival of ingested flagellates? (3) does the host have any nat- 

 ural resistance to invasion other than that implied by the effect 

 of secretions? (4) does the host develop an acquired immunity, 

 either partial or complete, as a reaction to the invasion of any of 

 the flagellates? (Lavier, 192 1, states that Deschiens infected mice 

 with Giardia from man and that the mice developed an immunity 

 to this species of Giardia, and Davis (1926) believes that young 

 trout develop an immunity to Octomitus salmonis.) (5) Having 

 successfully invaded the normal habitat in the host, what condi- 

 tions of the host's digestive tract affect its survival? (The relation 

 to the diet of the host is treated in Chapter XI.) 



The food habits of the intestinal flagellates of rats have not 

 been thoroughly investigated. T. muris and T. minuta do not 

 usually contain solid food bodies but contain vacuoles filled with 



