22 THE ECOLOGY OF ANIMALS 



Endoparasites : 



In liver : 1 tapeworm larva 



In stomach : 1 roundworm 



In small intestine : 3 roundworms 



2 tapeworms 



3 flatworms 

 2 flagellates 

 1 ciliate 



1 amoeba 



2 coccidians 

 In coecum : 1 ciliate 



In kidney : 1 spirochaete 

 In blood : 1 trypanosome 



It will turther be noticed that the same parasite may 

 occupy a different position in the body when larval 

 and when adult. It is interesting to note that two 

 other surveys of mammal parasites have given similar 

 figures, e.g. 50 species of parasites for the suslik or 

 ground squirrel {Citellus pygmaeus) of south-east 

 Russia (Sassuchin and Tiflov, 1932) and 27 for English 

 rats {Baitus norvegicus) (Ballbur and others, 1922). 

 We now begin to perceive the sort of way in which 

 animal communities can be grouped into various 

 grades of unit : the host-population of one species, 

 the community of a major habitat such as a plant 

 association, the community of the hfe zone or of the 

 sere (that complex of habitats which is related by 

 ecological succession — seep. 37). The chief principle 

 which is beginning to emerge from this analysis of 

 ecological surveys is that in any fairly limited area 

 only a fraction of the forms that could theoretically 

 do so actually form a community at any one time. 

 The list of, say, soil animals from pasture land all 

 over England may run into thousands, but in one 

 field it will not usually exceed a hundred. ' Many 

 are called, but few are chosen.' The animal com- 

 munity really is an organized community in that it 

 apparently has ' hmited membership '. An import- 



