ECOLOGY 33 



the parasites are destroyed during the developmental phase of the 

 infection and that after a variable length of time, resistance to the 

 parasites often develops in the host, as the parasites disappear from 

 the peripheral blood and symptoms subside, though the host still 

 harbors the organisms. In malarious countries, the adults and chil- 

 dren show usually a low and a high rate of malaria infection respect- 

 ively, but the latter frequently do not show symptoms of infection, 

 even though the parasites are detectable in the blood. Apparently 

 repeated infection produces tolerance which can keep, as long as the 

 host remains healthy, the parasites under control. There seems to be 

 also racial difference in the degree of immunity against Plasmodium 

 and Trypanosoma. 



As to the mechanism of immunity, the destruction of the parasites 

 by phagocytosis of the endothelial cells of the spleen, bone marrow 

 and liver and continued regenerative process to replace the de- 

 stroyed blood cells, are the two important phases in the cellular de- 

 fense mechanism. Besides, there are indications that humoral de- 

 fense mechanism through the production of antibodies is in active 

 operation in infections by Plasmodium knowlesi and trypanosomes 

 (Taliaferro, 1926; Maegraith, 1948; Culbertson, 1951). Immunity 

 (Taliaferro, 1941). 



With regard to the origin of parasitic Protozoa, it is generally 

 agreed among biologists that the parasite in general evolved from 

 the free-living form. The protozoan association with other organ- 

 isms was begun when various protozoans which lived attached to, 

 or by crawling on, submerged objects happened to transfer them- 

 selves to various invertebrates which occur in the same water. 

 These Protozoa benefit by change in location as the host animal 

 moves about, and thus enlarging the opportunity to obtain a con- 

 tinued supply of food material. Such ectocommensals are found 

 abundantly; for example, the peritrichous ciliates attached to the 

 body and appendages of various aquatic animals such as larval in- 

 sects and microcrustaceans. Ectocommensalism may next lead to 

 ectoparasitism as in the case of Costia or Hydramoeba, and then 

 again instead of confining themselves to the body surface, the Pro- 

 tozoa may bore into the body wall from outside and actually acquire 

 the habit of feeding on tissue cells of the attached animals as in the 

 case of Ichthyophthirius. 



The next step in the evolution of parasitism must have been 

 reached when Protozoa, accidentally or passively, were taken into 

 the digestive system of the Metazoa. Such a sudden change in 

 habitat appears to be fatal to most protozoans. But certain others 



