HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



terms in common use ; to these the following definitions 

 may be of value. It is perhaps desirable first to distin- 

 guish between parasitism and predatism. A parasite is 

 an organism that lives on or in and at the expense of 

 another organism without immediately destroying it. 

 A predaceous animal also lives at the expense of other 

 animals but kills them directly and devours them. There 

 are in nature a continuous series of intermediate stages 

 between parasitism on the one hand and predatism on the 

 other. 



The term symbiosis was proposed by deBary in 1879 

 for the constant, intimate and mutually beneficial associa- 

 tion of two organisms. Etymologically, symbiosis means 

 simply "living together" and hence should include para- 

 sitism, mutualism, commensalism and all other types of 

 consociation, but the term now implies the permanent as- 

 sociation of two specifically distinct organisms so de- 

 pendent on each other that life apart is impossible. When 

 the association is less intimate but each partner benefits 

 the other the term mutualism is sometimes employed. The 

 terms commensalism and inquilinism are often used for 

 still looser associations. Commensalism is applied to the 

 regular association of two definite species of organisms 

 which "eat together at the same table" but not at each 

 other's expense. Very similar in meaning is inquilinism, 

 which is used to describe the condition where one animal 

 lives with another as a co-tenant but usually not at its 

 expense. 



The origin of these various types of association is, of 

 course, not definitely known, but can be inferred without 

 much difficulty because of the existence of a large num- 



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