2 RESEARCH IN PROTOZOOLOGY 



as material for the study of certain biological phenomena. Never- 

 theless such investigators add much to our knowledge of the 

 protozoa and the results of their work constitute a large part of 

 our body of knowledge of this group of animals. 



Certain protozoa are of great importance to medical men, espe- 

 cially those interested in tropical medicine, and to veterinarians 

 since these organisms may be pathogenic parasites of man or 

 domesticated animals. Hence we find that diseases in man and 

 other animals due to these protozoa are being investigated in medi- 

 cal and veterinary schools. In these investigations attention is 

 directed primarily to the effects of the parasitic invasion on the 

 host and the studies of the parasites themselves are undertaken 

 with particular reference to their relations to the host. 



A third type of investigator interested in parasitic protozoa in- 

 cludes public health men who are interested in the prevention and 

 control of protozoan diseases that occur in endemic or epidemic 

 form. The point of view of these investigators differs from that 

 of the other two types just described since they are primarily 

 interested in the transmission of human protozoa and in methods 

 of protection and control. The term host-parasite relations has been 

 employed (Hegner, 1926) to include all of the phases of proto- 

 zoology of interest to zoologists, medical men, veterinarians and 

 public health workers. A brief review of the subject of host- 

 parasite relations is presented in Chapter II. 



Protozoology is a comparatively new science. Because of their 

 small size protozoa were not studied until after the microscope 

 was invented. Free-living protozoa were discovered by the great 

 Dutch microscopist Leeuwenhoek (1632- 1723). In 1676 Leeuwen- 

 hoek prepared his findings in the form of letters which he sent 

 to the Royal Society of London. Five years after his discovery 

 of free-living protozoa he announced the discovery in his own 

 stools of animalcules which, as Dobell (1920) has pointed out, 

 must have been flagellates belonging to the species Giardia lamblia. 

 Most of the early work on the protozoa was carried on with free- 

 living species but parasitic forms were described from time to 

 time. These did not create any considerable attention, however, 

 until certain species were shown to be pathogenic to domesticated 

 animals or to man. The discovery of such a pathogenic protozoon 

 has always been followed immediately by a flood of investigations. 

 As a rule a considerable body of knowledge' regarding free-living 



