8 HANDBOOK OF PROTOZOOLOGY 



parasites such as Gregarinida, Coccidia, etc., encystment is 

 followed by the production of numerous spores which become 

 the sources of infection when excreted. In the Protozoa which 

 inhabit organs other than those of the digestive tract, the re- 

 sistant spores may or may not be produced. Plasmodium vivax, 

 one of the three species of malarial parasites of man, lives in 

 the circulatory system and is transmitted from man to man by 

 anopheline mosquitoes only. Therefore, it does not produce 

 spores. Cnidosporidia, which are cell or tissue parasites, produce 

 typical resistant spores, which become set free usually through 

 wounds or disintegration of the host body after death and 

 which serve for infection of new host animals. 



The cysts of both free-living and parasitic Protozoa are 

 carried from place to place by the wind, very often attached 

 to soil particles, decayed leaves, twigs, etc., or by insects, 

 birds, and other animals. When the cysts encounter a proper 

 environment in the water or in a specific host animal, the con- 

 tents germinate and the organisms once more assume their 

 active and trophic phase. 



Relationship of Protozoology to Other Biological Sciences 



A brief consideration of the relationship of protozoology 

 to other branches of biology and its possible applications may 

 not be out of place here. Since the Protozoa are single-celled 

 animals manifesting the characteristics common to all living 

 things, they have been studied by numerous investigators with a 

 view to discovering the nature and mechanism of various 

 phenomena, the sum-total of which is known collectively as 

 vitality. Though the investigators generally have been dis- 

 appointed in the results, inasmuch as the assumed simplicity 

 of unicellular organisms has proved to be offset by the com- 

 plexity of their cell-structure, nevertheless any discussion of 

 biological principles today must take into account the informa- 

 tion obtained from studies of Protozoa. It is now commonly 

 recognized that adequate information on various types of Pro- 

 tozoa is prerequisite to a thorough comprehension of biology 

 and a proper application of biological principles. 



Practically all students agree in holding that the higher 

 types of animals have been derived from organisms which 



