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. By ob- 

 serving certain intestinal Protozoa in some monkeys, Hegner ob- 

 tained 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, gives further proof that the Blattidae and the Isoptera 

 are of the common origin. 



Study of a particular group of parasitic Protozoa and their hosts 

 may throw light on the geographic condition of the earth in the 

 remote past. The members of the genus Zelleriella are usually found 

 in the colon of the frogs belonging to the family Leptodactylidae. 

 Through an extensive study of these amphibians from South Amer- 

 ica and Australia, Metcalf found that the species of Zelleriella occur- 

 ring 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 numerous parasitic Protozoa 

 occur in man (Kudo, 1944). Entamoeba histolytica, Balantidium coli, 

 and three 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 

 Leishmania. 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 the parasitic forms. 



Recent studies have further revealed that almost all domestic 

 animals are hosts to numerous parasitic Protozoa, many of which 



