132 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



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. Under unfavorable conditions, such as dry surround- 

 ings, 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 harmful 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 multiplication 

 or continuous existence independent 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, and, as in the coccidia 

 (page 889), the presence of such a stage is often absolutely essential 

 in the life history of a parasite. 



PARASITISM 



A parasite is an organism which is, at some time, directly dependent 

 upon another, usually, a larger organism. 



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

 to organisms belonging to the animal kingdom, parasites may be 

 either animal or vegetable; bacteria and fungi, which live at the 

 expense of other living beings, are parasites just as the disease-pro- 

 ducing protozoa and the biting insects which transmit them are 

 parasites. 



Most parasites are simple organisms, low in the scale of life. They 

 nourish themselves without exertion, at the expense of their hosts, and 

 as might be expected, their unemployed organs, such as the sensory 

 locomotory and seizing appendages, by means of which food is usually 

 obtained, gradually disappear; degeneration always occurs in an 

 organism which assumes a parasitic mode of life. 



Organisms, such as the malarial parasite, which are wholly de- 

 pendent for existence upon their hosts, are called obligatory parasites; 

 those which are not, such as the infusoria usually found in the stomach 

 of herbivorous animals, are facultative parasites. Facultative parasites 

 often feed upon organic material provided by the host, and not upon 



