ECOLOGY 23 



Greenland, England, Japan, Australia, St. Helena, Barbados, 

 Mauritius, Africa, and Argentina. 



The parasitic Protozoa 



Some Protozoa belonging to all groups live on or in other or- 

 ganisms. The Sporozoa are made up exclusively of such forms. 

 The relationships between the host and the protozoan differ in 

 various ways, which make the basis for distinguishing the associa- 

 tions into three types as follows : commensalism, symbiosis, and 

 parasitism. 



The commensalism is an association in which an organism, the 

 commensal, is benefited, while the host is neither injured nor 

 benefited. Depending upon the location of the commensal in the 

 host body, the ectocommensalism or endocommensalism is used. 

 The ectocommensalism is often represented by Protozoa which 

 may attach themselves to any aquatic animals that inhabit the 

 same body of water, as shown by various species of Chonotricha, 

 Peritricha, and Suctoria. In other cases, there is a definite rela- 

 tionship between the commensal and the host. For example, 

 Kerona polyporum is found on various species of Hydra, and the 

 ciliates placed in Thigmotricha (p. 560) are inseparably associated 

 with certain species of the mussels. 



The endocommensalism is often difficult to distinguish from 

 the endoparasitism, since the effect of the presence of the com- 

 mensal upon the host cannot be easily understood. On the whole, 

 the protozoans which live in the lumen of the alimentary canal of 

 the host may be looked upon as endocommensals. These proto- 

 zoans use undoubtedly part of the food material which could be 

 used by the host, but they do not invade the host tissue. As 

 examples of endocommensals may be mentioned: Endamoeba 

 hlattae, Lophomonas hlattaru7n, L. striata, Nyctotherus ovalis, etc., 

 of the cockroach; Entamoeba coli, lodamoeba butschlii, Endolimax 

 nana, Dientamoeba fragilis, Chilomastix mesnili, Giardia intesti- 

 nalis, etc., of the human intestine; numerous species of Proto- 

 ciliata of Anura, etc. Because of the difficulties mentioned above, 

 the term parasitic Protozoa, in its broad sense, includes the 

 commensals also. 



The symbiosis on the other hand is an association of two species 

 of organisms which is of mutual benefit. The cryptomonads be- 

 longing to Chrysidella ("zooxanthellae") containing yellow or 



