10 HANDBOOK OF PROTOZOOLOGY 



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

 invariably found in the frogs of the family Leptodactylidae. 

 By an extensive study of these amphibians from South America 

 and Australia, Metcalf has 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 ani- 

 mals. 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 protozoan cells. 



Through the studies of various investigators in the past 

 thirty years, it has now become known that numerous parasitic 

 Protozoa occur in man. 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 on the morphology, physiology and reproduction of 

 the parasitic forms has largely been obtained by study of the 

 free-living organisms, a general knowledge of the entire phylum 

 is necessary to an understanding of the parasitic forms. 



Recent studies have further revealed that almost all domes- 

 tic animals are hosts to numerous parasitic Protozoa, many of 

 which are responsible for serious infectious diseases. Many of 

 the forms found in domestic animals are morphologically indis- 

 tinguishable from those in man. Balantidium coli is now gen- 

 erally considered as a parasite of swine, and man is its secondary 

 host. Knowledge of protozoan parasites is useful to medical 

 practioners, just as it is essential to veterinarians inasmuch as 

 certain diseases in animals, such as Texas fever, dourine, nagana, 

 cocciodiosis, blackhead, etc., are caused by protozoans. 



