34 INTRODUCTION TO PROTOZOA 



rotifers and small crustaceans it replaces parthenogenesis, 

 though either endomixis or parthenogenesis apparently may con- 

 tinue indefinitely with conditions favorable. 



Another phenomenon which is often, though not always, 

 associated with sexual reproduction is encystment, i.e., the de- 

 velopment of an impervious enclosing capsule in which the 

 delicate protozoan cell is able to resist extremely adverse en- 

 vironmental conditions, such as very high or low temperatures, 

 drouth, presence of injurious substances, lack of oxygen, etc. 

 The degree of protection afforded by encystment can bo judged 

 from the fact that encysted amebse exist in considerable numbers 

 on the sun-baked sands of Egypt. Encystment ma}^ take place 

 whenever environmental conditions become unfavorable, or as a 

 normal stage of existence following sexual reproduction, thus 

 being comparable with the impervious shelled eggs of man}'^ 

 higher animals, or sometimes as a step preliminary to some form 

 of asexual reproduction. Nearlj^ or quite all parasitic protozoans 

 which are not transmitted by an intermediate host adapt them- 

 selves for passive transfer from one host to another by encystment. 



A full understanding of the significance and limitations of the 

 sexual and asexual phases of the life histories of parasitic Proto- 

 zoa is of great importance, since means of control and prevention 

 often hinge on these points. In many species of protozoan para- 

 sites a different host is required for the sexual portion of the 

 life historj^ than that utilized for asexual reproduction, though 

 this is not true, in general, of the intestinal parasites. Some 

 species, although normally utilizing a second host for the 

 sexual reproduction, are apparently aljle at times to pass from 

 host to host without the intervention of an intermediate host of 

 different species. This is true, for instance, of the sleeping 

 sickness trypanosome, T. gamhiense, which is normally trans- 

 mitted by a tsetse fly, Glossina palpaUs, as an intermediate host, 

 but which is thought to be capable of direct transmission by 

 sexual intercourse as well. It is interesting to note also that, 

 according to observations made by Gonder on trypanosomes 

 (quoted by Nuttall), characters such as immunity to certain 

 drugs, acquired by protozoans and maintained through thousands 

 of asexual generations in vertebrate hosts, may be blotted out at 

 a stroke in the invertebrate host by the sexual process which 

 presumably occurs there. The great significance of this is 



