INTRODUCTION 9 



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 practitioners, 

 just as it is essential to veterinarians inasmuch as certain diseases in 

 animals, such as Texas fever, dourine, nagana, blackhead, coccidio- 

 sis, etc., are caused by protozoans. 



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. 100). 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 polluted water, but that 

 upon reaching a certain population, they would cease to multiply 

 and would allow the excess organic substances to undergo decompo- 

 sition. 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. Protozo- 

 ology therefore must be considered as part of modern sanitary 

 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. Success in fish-culture, therefore, requires 

 among other things a thorough knowledge of Protozoa. 



Since Russel and Hutchinson suggested some thirty 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 de- 

 crease 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 climates Protozoa are present com- 

 monly and active throughout the year. The exact relation between 

 specific protozoans and bacteria in the soil is a matter which still 

 awaits future investigations, although numerous experiments and 



