THE GENUS BALANTIDIUM 233 



Rees and Jameson with balantidia in cultures containing starch 

 suggest a method of studying the physiology of the parasite. The 

 carrier condition in the host would have to be investigated in some 

 animal, such as the monkey, that is susceptible and sometimes ex- 

 hibits symptoms. 



e. Encystation. Cysts are said to be rare in man and monkeys 

 but common in the pig. Rees (1927) reports cysts as rare in pigs 

 in Baltimore during February and Alarch. Perhaps the number of 

 cysts formed varies with the seasons. Culture methods afford an 

 opportunity to determine factors that are responsible for encysta- 

 tion. 



f. Host-Parasite Specificity. A^arious problems of host-parasite 

 specificity may be studied with the aid of B. coll. What species of 

 hosts can be infected by the various species of balantidia and under 

 what conditions ? What relation has the age of the host to suscepti- 

 bility or resistance? How can the apparent infectivity of B. coli to 

 man, monkeys and guinea-pigs be explained? Are there two 

 species, varieties or races of Balantid'uiin in the pig {B. coli and 

 B. siiis) ? Does the diet of the host have an influence on suscepti- 

 bility? Is the size of the infective dose important? 



