24 PROTOZOOLOGY 



Pringsheim, Hall, Loefer, Johnson, and others, found that sodium 

 acetate may increase or decrease the growth rate of various Phyto- 

 mastigina subject to the hydrogen-ion concentration of the culture 

 media. 



Food. The kind and amount of food available in a given body 

 of water also controls the distribution of Protozoa. The food is 

 ordinarily one of the deciding factors of the number of Protozoa 

 in a natural habitat. Species of Paramecium and many other holo- 

 zoic protozoans cannot live in waters in which bacteria or minute 

 protozoans do not occur. If other conditions are favorable, then the 

 greater the number of food bacteria, the greater the number of these 

 protozoans. Didinium nasutum feeds almost exclusively on Para- 

 mecium, hence it cannot live in the absence of the latter ciliate. 

 As a rule, euryphagous protozoans are widely distributed and 

 stenophagous forms are limited in their distribution. 



Some protozoans inhabit soil of various types and localities. Un- 

 der ordinary circumstances, they occur near the surface, their maxi- 

 mum abundance being found at a depth of about 10-12 cm. (Sandon, 

 1927). It is said that a very few protozoans occur in the subsoil. 

 Here also one notices a very wide geographical distribution of ap- 

 parently one and the same species. For example, Sandon found 

 Amoeba proteus in samples of soil collected from Greenland, Tristan 

 da Cunha, Gough Island, England, Mauritius, Africa, India, and 

 Argentina. This amoeba is known to occur in various parts of North 

 America, Europe, Japan, and Australia. The majority of Testacea 

 inhabit moist soil in abundance. Sandon observed Trinema enchelys 

 in the soils of Spitzbergen, 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 organ- 

 isms. The Sporozoa are made up exclusively of parasites. The rela- 

 tionships between the host and the protozoan differ in various ways, 

 which make the basis for distinguishing the associations into three 

 types as follows: commensalism, symbiosis, and parasitism. 



Commensalism is an association in which an organism, the com- 

 mensal, is benefited, while the host is neither injured nor benefited. 

 Depending upon the location of the commensal in the host body, 

 the term ectocommensalism or endocommensalism is used. Ecto- 

 commensalism 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 



