INTRODUCTION 7 



Through the studies of various investigators in the past forty 

 years, it has now become known that numerous parasitic Protozoa 

 occur in man. Entamoeba histolytica, Balantidium coli, and three 

 species of Plasmodium, all of which are pathogenic to man, are 

 ^\'idely distributed throughout the world. In certain restricted 

 areas are found other pathogenic forms, such as Trypanosoma 

 and Leishmania. Since all parasitic Protozoa presumably have 

 originated in free-living forms and since our knowledge of the 

 morphology, physiology and reproduction of the parasitic forms 

 has largely been obtained through studies of the free-living 

 organisms, a general knowledge of the entire phylum is necessary 

 to understand the parasitic forms. 



Recent studies have further revealed that almost all domestic 

 animals are hosts to numerous parasitic Protozoa, many of which 

 are responsible for serious infectious diseases. Many of the forms 

 found in domestic animals are morphologically indistinguishable 

 from those occurring in man. Balantidium coli is now generally 

 considered as a parasite of swine, and man is its secondary host. 

 Knowledge of protozoan parasites is useful to medical practi- 

 tioners, just as it is essential to veterinarians inasmuch as certain 

 diseases in animals, such as Texas fever, dourine, nagana, black- 

 head, coccidiosis, etc., are caused by protozoans. 



Sanitary betterment and improvement are fundamental re- 

 quirements in the modern civilized world. One of man's necessi- 

 ties is safe drinking water. The majority of Protozoa live freely 

 in various bodies of water and some of them seem to be responsi- 

 ble, if present in sufficiently large numbers, for giving certain 

 odors to the waters of reservoirs or ponds (p. 95). But these 

 Protozoa which are occasionally harmful are relatively small in 

 number compared with those which are beneficial to man. It is 

 generally understood that bacteria feed on various waste materi- 

 als present in polluted water, but that upon reaching a certain 

 population, they would cease to multiply and would allow the 

 excess organic substances to undergo decomposition. Numerous 

 holozoic Protozoa, however, feed on the bacteria and prevent 

 them from reaching the saturation population. Protozoa thus 

 seem to help indirectly in the purification of the water. Proto- 

 zoology therefore must be considered as an important part of 

 modern sanitary science. 



Young fish feed extensively on small aquatic organisms, such 



