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6 PROTOZOOLOGY 



neborn; Kimbell) and in P. hursaria (Jennings) will probably 

 assist in bringing to light many genetic problems of Protozoa 

 which have remained obscure in the past. 



Parasitic Protozoa are limited to one or more specific hosts. 

 Through studies of the forms belonging to one and the same 

 genus or species, the phylogenetic relation among the host ani- 

 mals may be established or verified. The mosquitoes belonging 

 to the genera Culex and Anopheles, for instance, are known to 

 transmit avian and human Plasmodium respectively. They are 

 further infected by specific microsporidian parasites. For in- 

 stance, Thelohania legeri has been found widely in many species 

 of anopheline mosquitoes only; T. opacita has, on the other hand, 

 been found in culicine mosquitoes, although the larvae of the 

 species belonging to these two genera live frequently in the same 

 body of water. By observing certain intestinal Protozoa in some 

 monkeys, Hegner obtained evidence of the probable phylogenetic 

 relationship between them and other higher mammals. The re- 

 lation of various Protozoa of the wood-roach to those of the ter- 

 mite, 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 amphibi- 

 ans from South America and Australia, Metcalf 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 con- 

 vergent 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 Zelleri- 

 ella, 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 gradations of nuclear division ranging 

 from a simple amitotic division to a complex process comparable 

 in every detail with the typical metazoan mitosis, so that a great 

 part of our knowledge of cytology is based upon studies of pro- 

 tozoan cells. 



