THE PROTOZOA 227 



Parasitic Protozoa. — And now we come to what are the most 

 important of all Protozoa, the parasitic species. The easiest of 

 these to obtain for purposes of study is known as Monocystis 

 and lives in the reproductive organs (seminal vesicles) of the 

 earthworm. It is about j-J-j of an inch long and moves about 

 somewhat like an Ameba. A very characteristic stage in its 

 life history is that of the formation of spores (Fig. 136, A). 

 Many of these spores are formed by a single animal, and each 

 spore grows into a full-grown individual. Protozoa that repro- 

 duce in this way are known as Sporozoa. 



The Malarial Parasite. — Perhaps the best-known Sporozoon 

 is that which causes malarial fever and goes by the scientific 

 name Plasmodium vivax. An account of how this parasite is 

 transmitted from one person to another by mosquitoes has al- 

 ready been given (Chap. IX, p. 88). The spores when they 

 are injected into the blood by the bite of the mosquito are 

 slender, spindle-shaped bodies (Fig. 136, B). Each spore pene- 

 trates a blood corpuscle, becomes ameboid in shape, and feeds 

 upon the substance of the corpuscle until it is demolished. The 

 nucleus then divides several times, forming twelve or sixteen 

 daughter nuclei, each of which becomes the center of a new spore. 

 Soon the corpuscle wall breaks and the spores escape. This 

 breaking down of the corpuscles causes a chill. The new spores 

 enter other corpuscles and pass through a similar series of stages. 



If the blood of a malarial fever patient is sucked up by an 

 Anopheles mosquito, part of the spores become eggs in the stom- 

 ach of the insect and part of them produce spermatozoa. 

 The eggs are fertilized by the spermatozoa; the fertilized eggs 

 become spindle-shaped and bore their way into the wall of the 

 stomach, where they form little tumorlike swellings. In each 

 of these a great many spores arise. They break out finally (Fig. 

 136, B) and make their way into the salivary glands of the mos- 

 quito and are then ready to be injected into the blood of another 

 human being. 



Pathogenic Protozoa. — Protozoa that cause diseases, like 



