INTRODUCTION 3 



protozoa and parasitic protozoa in lower animals was built up be- 

 fore similar genera and species were recorded from man. For 

 example, free-living amoebae were well known before amoebae were 

 discovered in the intestine of human beings. 



The study of morphology, classification and life-cycles occupied 

 the time of most of the early investigators of the protozoa. As 

 more became known about the group, however, protozoa were 

 more frequently employed for the study of genetics, animal be- 

 havior, physiology, sex, ecolog>% evolution and other subjects and 

 experiment was combined with observation in their investigation. 

 A review of the literature of protozoology that has appeared dur- 

 ing the past few years indicates that although large numbers of 

 new species are being described every year the principal investiga- 

 tions are devoted to experimental work and much of this work is 

 aided by statistical methods. Probably no one can predict the direc- 

 tion protozoological investigations will take in the future but the 

 trends seem to point toward more fundamental studies in the field 

 of host-parasite relations. 



One of the most interesting of all problems in protozoology is 

 that of the evolution of the parasitic habit. Certain postulates 

 regarding this subject seem rather definite and simple, such as, for 

 example, that parasitic protozoa evolved from free-living protozoa 

 and that the ectoparasitic habit evolved before the endoparasitic 

 habit. Protozoan parasites offer particularly favorable material 

 for the study of the course of evolution since they have become 

 differentiated from their free-living ancestors by a sort of super- 

 imposed evolution. 



The literature of protozoology is widely scattered. Information 

 on the parasitic species of protozoa is contained in books, mono- 

 graphs and articles published in journals of various sorts. Some 

 of the books are devoted to both free-living and parasitic species 

 and others to parasitic species only. Books on tropical medicine all 

 contain sections on the protozoa that produce disease in man, 

 especially in tropical countries. A large number of journals contain 

 articles on parasitic protozoa but not one of these is devoted 

 entirely to this subject. Those that contain a considerable number 

 of articles and seem to be of greatest use on this account are listed 

 together with the more important text and reference books at the 

 beginning of the Bibhography. 



