ECOLOGY, COMMENSALISM AND PARASITISM 353 



Polysaprobic types finally live in waters with little free oxygen 

 but with sulphuretted hydrogen, carbonic acid and other products 

 of putrefaction advertized by their foul odors. In this group are 

 all of the open sewage Protozoa (see p. 357), as well as some of 

 the sapropelic fauna (Lauterborn) which live in a medium quite 

 free from oxygen at the bottom of ponds where ill-smelling gases 

 (methane, carbonic acid, sulphuretted hydrogen, etc.) accumulate. 

 These are anaerobic forms some of which, characterized by fan- 

 tastic shapes, are unable to live under aerobic conditions (see 

 p. 356). 



2. Semi-terrestrial Protozoa.— Semi-terrestrial protozoa may be 

 found in moss, sphagnum, etc. (many types of testate rhizopods 

 and a few flagellates and ciliates). 



Not only food and oxygen but relative alkalinity and acidity are 

 also determining factors in the life of given types. Acanthocystis 

 aculeata, for example, lives well with a hydrogen-ion concentration 

 (pH) of 8.1 but dies in a less alkaline medium with pH 7.4 (Stern, 

 1924). Other forms may live in a distinctly acid medium and 

 some may live in waters having a wide range of pH. In standing 

 waters with decomposed matter at the bottom, the pH at different 

 levels is variable which accounts in part for the sequence of forms 

 in a hay infusion (Woodruff, 1912; Bresslau, 1926, et al.) and the 

 segregation of specific types at different levels. 



With the varying conditions to which Protozoa are adapted and 

 under which they live and thrive, it is probable that some types are 

 more readily adaptable to a parasitic mode of life than others. 

 Anaerobic forms, for example, are already partially adapted. 



3. Soil-dwelling Protozoa.— It is to be expected that an occasional 

 water-dwelling form of Protozoa should be found in the soil, par- 

 ticularly where moisture abounds. It is also possible that coprozoic 

 forms under suitable conditions in the soil might have a more or 

 less extended life. The so-called soil Protozoa, therefore, might 

 well include representative genera and species of both water-dwelling 

 and coprozoic forms. 



Modern studies of representative soils from all over the world, 

 including all types, have demonstrated, however, that the great 

 numbers of Protozoa found cannot be accounted for on any such 

 casual basis. In this connection Sandon (1927) states that soil 

 forms constitute a fairly well-defined group, with characteristic 

 functional needs and cannot well be regarded as an accidental 

 collection. 1 



Sandon's conclusion is supported by the facts of distribution which 

 he adduces. These apply to all groups of Protozoa of which he 



1 Sandon, H.: The Composition and Distribution of the Protozoan Fauna of the 

 Soil, London, 1927, p. 63. 

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