INTESTINAL FLAGELLATES OF RATS 135 



cyst C. mcsn'di by Kofoid and Swezy (1920) and by Hegner 

 (1923^7). ^ 



Hexamitits mitris forms cysts in the lower part of the small 

 intestine and cecum. Binary and sometimes multiple fission appear 

 to take place within the cysts soon after they are formed. Com- 

 parable cysts for H. pulchcr have not been observed in the rat 

 but probably occur. 



Giardia encysts in the lower part of the small intestine and 

 cecum of rats and the cysts may be found in the cecum, colon, 

 and feces. Boeck (1919) found that there is a cycle of encyst- 

 ment of Giardia in rats with maximum numbers occurring at in- 

 tervals of about seven days. A somewhat similar cycle was noted 

 for the Giardia of man by Porter (1916). As already noted, both 

 binary and multiple fission have been described for the cyst stage 

 of Giardia. 



The significance of encystment has been much in dispute. In 

 these intestinal flagellates the cyst stage serves for the transfer of 

 the animals from one host to another and also serves to permit 

 reproduction by fission unhampered by the necessity of actively 

 coping with the environment. It is an interesting biological specu- 

 lation as to whether cyst formation has evolved primarily to serve 

 the purposes of reproduction, reorganization or revitalization, or 

 to serve primarily a protective function during transition through 

 an environment unfavorable to normal vegetative activity. Since 

 cysts for present day protozoa serve not only these two functions 

 but may also serve as a retreat during the digestion of food, it is 

 impossible to settle the problem as to whether any one of these 

 functions had priority in evolution. 



HOST-PARASITE RELATIONS 



I. Relation to normal host environment 



In order for any intestinal flagellate to establish itself in the 

 digestive tract of a rat it must first gain entrance to that part 

 which is its normal habitat and then be able to maintain itself in 

 that situation. 



In discussions of host-parasite relations the invasive powers of 

 the parasite are often spoken of in contrast to the means of pro- 

 tection possessed by the host. As Hegner (1927a) has pointed 

 out, intestinal protozoa gain entrance to a new host through the 



