INTRODUCTION 13 



and interesting forms. This work was followed by Stokes' The Fresh- 

 water Infusoria of the United States, which appeared in 1888. 

 Butschli (1880-1889) established Sarcodina and made an excellent 

 contribution to the taxonomy of the then-known species of Protozoa, 

 which is still considered as one of the most important works on gen- 

 eral protozoology. The painstaking researches by Maupas, on the 

 conjugation of ciliates, corrected erroneous interpretation of the 

 phenomenon observed by Balbiani some thirty years before and gave 

 impetus to a renewed cytological study of Protozoa. The variety in 

 form and structure of the protozoan nuclei became the subject of in- 

 tensive studies by several cytologists. Weismann (1881) put into 

 words the immortality of the Protozoa. Schaudinn contributed much 

 toward the cytological and developmental studies of Protozoa. 



In the first year of the present century, Calkins in the United 

 States and Doflein in Germany wrote modern textbooks on protozo- 

 ology dealing with the biology as well as the taxonomy. Calkins 

 initiated the so-called isolation pedigree culture of ciliates in order to 

 study the physiology of conjugation and other phenomena connected 

 with the life-history of the ciliates. Recently application of bacteria- 

 free culture technique to certain free-living flagellates and ciliates has 

 brought to light hitherto unknown facts regarding nutritional re- 

 quirements of these organisms. 



Today the Protozoa are more and more intensively and exten- 

 sively studied from both the biological and the parasitological sides, 

 and important contributions appear continuously. Since all parasitic 

 Protozoa appear to have originated in free-living forms, the com- 

 prehension of the morphology, physiology, and development of the 

 latter group is obviously fundamentally important for a thorough 

 understanding of the former group. 



Compared with the advancement of our knowledge on free-living 

 Protozoa, that on parasitic forms has been very slow. This is to be ex- 

 pected, of course, since the vast majority of them are so minute that 

 the discovery of their presence has been made possible only through 

 improvements in the microscope and in technique. 



Here again Leeuwenhoek seems to have been the first to observe 

 a parasitic protozoan, for he observed, according to Dobell, in the 

 fall of 1674, the oocysts of the coccidian, Eimeria stiedae, in the con- 

 tents of the gall bladder of an old rabbit; in 1681, Giardia intestinalis 

 in his own diarrhceic stools; and in 1683, Opalina and Nyctotherus in 

 the gut contents of frogs. The oral Trichomonas of man was ob- 

 served by O. F. Miiller (1773) who named it Cercaria tenax (Dobell, 

 1939). There is no record of anyone having seen Protozoa living in 



