118 University of California Publications in Zoology \yoi.. 19 



conjecture at this time. It is very probable that this species is different 

 from G. microti although their structure is very similar. If it is a new 

 species then it is very probable that there is simply a difference in the 

 rate of development between G. microti and this new species. The 

 first stages of development are similar in all respects and there is 

 reason to believe that the following stages are also similar. The greatest 

 difference is in the location of the different stages in the various 

 regions of the intestine. The fact that binary cysts were found in 

 the small intestine leads one to believe that the early stages of 

 development, including the formation of the two-zooid somatella in 

 this new species, were more rapid than the corresponding rate in the 

 formation of these stages in G. microti, but that after the cysts had 

 gone through these stages of binary fission the rest of the develop- 

 ment was slower or arrested when compared with G. microti, which 

 continued to pass through all the other stages of binary fission even 

 to the completion of plasmotomy within the cyst. 



On the other hand, the presence of cysts in the faeces in an early 

 stage of binary fission might be accounted for by a more rapid rate 

 of peristalsis in the rat compared with the rate of peristalsis in the 

 meadow mouse. In order to test this hypothesis the cysts which were 

 found in the faeces were incubated at 31° C for several days contin- 

 ously and examined at intervals of one, four, and five days. The 

 temperature of 31° G was chosen because it was lower than the 

 optimum of most of the bacteria found in the faeces. This would tend 

 to prevent excessive proliferation on the part of the bacteria which 

 would hasten the death of the encysted flagellates. This temperature 

 is considerably lower than the body temperature of the rat, but it has 

 been used for the cultivation of Protozoa with some degree of success. 



Upon the examination of the cysts at the intervals stated above no 

 noticeable changes in the development within the cysts were found. 

 The condition of the body within the cyst was identical with that when 

 the cysts were defaecated. The cysts decreased markedly in their 

 number, which may be due to their death through the agency of the 

 bacteria. The incubation failed to cause any further development 

 but it cannot be said that this experiment disproves the supposition 

 of the effect of peristalsis. Incubation with higher temperatures and 

 with cultures of cysts free from bacteria should be tried before con- 

 demning the hypothesis. 



