THE PROTOZOA 



skeletons of Radiolaria and Foraminifera almost identical with those of 

 present-day species. This fact indicates that these forms, and so perhaps 

 many other Protozoa, have remained almost unchanged for half a billion 

 years or more. 



Parasitic Rhizopoda. Although most of the rhizopods are free-living 

 animals, a smaller number of the known species are parasitic. One of these, 

 Entamoeba histolytica, causes the serious disease of humans called amoebic 

 dysentery, or amoebiasis. Entamoeba histolytica is found in humans chiefly in 

 the large intestine, where the active feeding stages, or trophozoites, destroy 

 the mucous membrane and invade the submucosa. The ulcers of the in- 

 testinal lining thus formed become infected with bacteria, as well as with 

 amoebas. The food of the parasite consists of the tissues it destroys, to- 

 gether with large numbers of red corpuscles. In advanced cases of amoe- 

 biasis, the parasite may gain access to the intestinal blood vessels; in the 

 blood stream, the parasites are carried to the liver, lungs, and brain. Al- 

 most any part of the body may be thus invaded, the amoebas then causing 

 serious abscesses in the secondary sites. Hence, the symptoms of amoebiasis 

 are not limited to intestinal disturbances. 



The trophozoites (Fig. 8.8) may be observed in the freshly discharged feces 

 of the host. They become increasingly sluggish as the feces cool and die with- 

 in a few hours. The feces of an infected individual also contain large numbers 

 of encysted stages of the parasite, which are smaller cells encased in cysts 

 and having typically four nuclei. The cysts can survive for some time out- 

 side the host and are moderately resistant to heat and cold. They can be 

 killed by the pasteurization process in milk and by boiling in water. Viable 

 cysts are transferred to the intestine of a new host via the mouth and digestive 

 tract in contaminated food and drink, and to a lesser extent by other means. 

 Reproduction in the trophozoite stage occurs by binary fission. In the en- 



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