INTRODUCTION 9 



are responsible for serious infectious diseases. Some of the forms 

 found in domestic animals are morphologically indistinguishable 

 from those occurring in man. Balantidium coli is considered as a 

 parasite of swine, and man is its secondary host. Knowledge of 

 protozoan parasites is useful to medical practitioners, just as it is 

 essential to veterinarians inasmuch as certain diseases of animals, 

 such as southern cattle fever, dourine, nagana, blackhead, coccidio- 

 sis, etc., are caused by Protozoa. 



Sanitary betterment and improvement are fundamental re- 

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

 is safe drinking water. The majority of Protozoa live freely in various 

 bodies of water and some of them are responsible, if present in suffi- 

 ciently large numbers, for giving certain odors to the waters of 

 reservoirs or ponds (p. 114). But these Protozoa which are occasion- 

 ally harmful are relatively small in number compared with those 

 which are beneficial to man. It is generally understood that bacteria 

 live on various waste materials present in the polluted water, but 

 that upon reaching a certain population, they would cease to multi- 

 ply and would allow the excess organic substances to undergo de- 

 composition. Numerous holozoic Protozoa, however, feed on the bac- 

 teria and prevent them from reaching the saturation population. 

 Protozoa thus seem to help indirectly in the purification of the water. 

 Protozoology therefore must be considered as part of modern sani- 

 tary science. 



Young fish feed extensively on small aquatic organisms, such as 

 larvae of insects, small crustaceans, annelids, etc., all of which de- 

 pend largely upon Protozoa and Protophyta as sources of food sup- 

 ply. Thus the fish are indirectly dependent upon Protozoa as food 

 material. On the other hand, there are numbers of Protozoa which 

 live at the expense of fish. The Myxosporidia are almost exclusively 

 parasites of fish and sometimes cause death to large numbers of com- 

 mercially important fishes (Kudo, 1920) (p. 648). Success in fish- 

 culture, therefore, requires among other things a thorough knowl- 

 edge of Protozoa. 



Since Russel and Hutchinson (1909) suggested some forty years 

 ago that Protozoa are probably a cause of limitation of the numbers, 

 and therefore the activities of bacteria in the soil and thus tend to 

 decrease the amount of nitrogen which is given to the soil by the 

 nitrifying bacteria, several investigators have brought out the fact 

 that in the soils of temperate climate various sarcodinans, flagellates 

 and less frequently ciliates, are present and active throughout the 

 year. The exact relation between specific Protozoa and bacteria in 



