8 PROTOZOOLOGY 



has been found widely only in many species of anopheline mosqui- 

 toes; T. opacita has, on the other hand, been found exclusively in 

 culicine mosquitoes, although the larvae of the species belonging to 

 these two genera live frequently in the same body of water (Kudo, 

 1924, 1925). By observing certain intestinal Protozoa in some mon- 

 keys, Hegner (1928) obtained evidence on the probable phylogenetic 

 relationship between them and other higher mammals. The relation 

 of various Protozoa of the wood-roach to those of the termite, as 

 revealed by Cleveland and his associates (1934), gives further proof 

 that the Blattidae and the Isoptera are closely related. 



Study of a particular group of parasitic Protozoa and their hosts 

 may throw light on the geographic condition of the earth which 

 existed in the remote past. The members of the genus Zelleriella are 

 usually found in the colon of the frogs belonging to the family Lepto- 

 dactylidae. Through an extensive study of these amphibians from 

 South America and Australia, Metcalf (1920, 1929) found that the 

 species of Zelleriella occurring in the frogs of the two continents are 

 almost identical. He finds it more difficult to conceive of convergent 

 or parallel evolution of both the hosts and the parasites, than to 

 assume that there once existed between Patagonia and Australia a 

 land connection over which frogs, containing Zelleriella, migrated. 



Experimental studies of large Protozoa have thrown light on the 

 relation between the nucleus and the cytoplasm, and have furnished 

 a basis for an understanding of regeneration in animals. In Protozoa 

 we find various types of nuclear divisions ranging from a simple 

 amitotic division to a complex process comparable in every detail 

 with the typical metazoan mitosis. A part of our knowledge in 

 cytology is based upon studies of Protozoa. 



Through the efforts of various investigators in the past fifty 

 years, it has now become known that some 25 species of Protozoa 

 occur in man. Entamoeba histolytica, Balantidium coli, and four 

 species of Plasmodium, all of which are pathogenic to man, are 

 widely distributed throughout the world. In certain restricted areas 

 are found other pathogenic forms, such as Trypanosoma and Leish- 

 mania. Since all parasitic Protozoa presumably have originated 

 in free-living forms and since our knowledge of the morphology, 

 physiology, and reproduction of the parasitic forms has largely been 

 obtained in conjunction with the studies of the free-living organ- 

 isms, a general knowledge of the entire phylum is necessary to under- 

 stand these parasitic forms. 



Recent studies have further revealed that almost all domestic 

 animals are hosts to numerous parasitic Protozoa, many of which 



