6 GENERAL CONSIDERATIONS 



and flagellates, and some testate rhizopods, may be found here, but one 

 must be an optimistic collector, and an opportunist, to get good results. 



It is somewhat the same with soil-dwelling Protozoa, among which it 

 is to be expected that water- dwelling forms would occasionally be found 

 and interpreted as casual soil-dwelling types. Sandon (1927), however, 

 has shown that there is a characteristically well-defined group of forms 

 living in this environment, although, as would be expected, there is a 

 wide difference in soils, both as to depth of occurrence of Protozoa and 

 chemical make-up. In arable soils, Sandon finds that not only are they 

 most abundant, but that they live at a depth of four to five inches and, 

 for the most part, are bacteria eaters. He, with other observers, has 

 described some seventy-five species of flagellates as fairly common; and 

 about eighty-five species of rhizopods and ciliates which are less com- 

 mon. There is not much evidence that life in the soil leads to any particu- 

 lar type of morphological adaptations, but there is a possibility that species 

 adapted to life deep in the soil are already partly anaerobic, and such 

 forms may more easily become parasitic. 



Lackey (1925) enumerates no less than nineteen common forms of 

 Protozoa which live in the sewage of Imhof tanks, while five common 

 species of rhizopods, four of ciliates, and about twenty less common 

 species of flagellates are occasionally found. 



So-called coprozoic forms are rarely segregated, but may be found 

 more or less sporadically almost anywhere on the earth's surface. These 

 are Protozoa which are taken into the digestive tract by all animals, and, 

 as encysted forms, become stored up in the intestine until they are passed 

 out with the feces. In water, such cysts accompanied by nutrient material 

 from the feces, develop into adult and active forms which may be mis- 

 taken by the unwary as entozoic parasites. So great and complete is the 

 specificity of internal parasites, however, that such cysts, while not repre- 

 senting parasites of the host providing the feces, may nevertheless be 

 cysts of active parasites of other hosts which now pass with disastrous 

 results, into the digestive tract of a new definitive host by which they 

 have been eaten with contaminated food and water. 



In numbers of species there is little doubt that the free-living and 

 water-dwelling Protozoa stand first, but the parasitic forms make a 

 close second, for no type of animal is free from the possibility of in- 

 fection by one or more species of parasitic Protozoa. 



