BALANTIDIUM COLI I SPECIFICITY 



material; and Ziemann (1925) obtained only negative 

 results in an attempt to transmit the infection from the 

 chimpanzee to the cat. 



General discussion. From the evidence available it 

 seems certain that the balantidia occurring in pigs, man 

 and certain other primates all belong to one species, 

 Balantidium coli. This is in marked contrast to the situa- 

 tion that exists in the case of most of the other human 

 protozoa, which normally live only in one species of verte- 

 brate host, man. Does B. coli possess any morphological, 

 physiological or life-history peculiarities that account for 

 this weak host-parasite specificity? 



It has been pointed out above that B. coli resembles 

 free-living ciliates morphologically; that is, it has not 

 become appreciably modified by its parasitic habit. For 

 example, it takes in solid food particles by means of a 

 well-developed ingesting apparatus ; is supplied with two 

 active contractile vacuoles ; and swims about freely, there 

 being no organs of attachment, undulating membranes 

 nor obvious modifications of the locomotor organelles so 

 characteristic of certain other parasitic protozoa. The 

 fact that B. coli does not exhibit morphological and 

 physiological peculiarities associated with its parasitic 

 habit indicates that it is very resistant to changes 

 in the environment; that is, it is able to withstand 

 successfully a great range in the factors of its en- 

 vironment, such as temperature, density and chemical 

 nature of the medium and food supply. The observations 

 of Rees (1927) indicate that B. coli is really more re- 

 sistant than most other human protozoa in spite of the 

 view that ciliates, as Siitterlin (1921) has shown in his 



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