PROTOZOA. 75 



processes. This is evident from a consideration of the phenomena 

 of autogamy and parthenogenesis, which have just been mentioned. 

 Reproduction occurs in both these processes without a preceding union 

 of two sexually differentiated cells. 



The DEVELOPMENTAL CYCLE of a protozoon is the history of the 

 processes through which it may pass in the period intervening between 

 each fertilizing act. In many of the pathogenic protozoa, an alternation 

 of generations occurs; that is, states of being of the parasite in which an 

 asexual method of reproduction occurs, alternate with states of being 

 in which reproduction is effected by sexual methods. The developmental 

 cycle is often complicated by binary division, which may occur at any 

 point, by cyst formation, and by the intervention of a second host as a 

 necessary factor for the existence of a part of the cycle. An alternation 

 of generations occurs in the life cycle of the parasite which produces 

 malaria (Fig. 125) ; this parasite is one of the most important of the path- 

 ogenic protozoa. While it is in the body of its mammalian host, man, 

 it multiplies by binary division and by schizogony; it multiplies, sexually, 

 within the body of its invertebrate host, a mosquito. The host in which 

 the adult, sexual stages of the parasite occur, in this instance the mosquito, 

 is said to be the definitive host; hosts harboring the parasite while it is 

 in other stages are called intermediate hosts. 



ENCYSTMENT. When unfavorable circumstances, such as drying, 

 occur, many protozoa are able to surround themselves by a resistant 

 cyst and to enter upon a resting stage of indefinite length. The cyst 

 protects them from hurtful influences and, surrounded by it, they remain 

 in a resting state until favorable circumstances come about once more. 

 The power of forming resistant cysts plays an important part in the 

 life-history of many parasitic protozoa; it is especially so with those 

 protozoa which have become so specialized that continued existence 

 outside of their appropriate host has become impossible for them. It 

 is often through the formation of cysts that an infection by a protozoon 

 is spread ; then, as in the coccidia, the presence of such a stage is abso- 

 lutely essential in the life history of the parasite. 



PARASITISM. 



A parasite is a living being which is, at some time, directly dependent 

 upon another, usually a stronger being. 



Although the word parasite is often used as though it referred only 



