PROTOZOA AND OTHER ANIMALS 893 



host without invading its substance. There are few instances among 

 Protozoa in which injurious effects of this type have been proved. 



There are only two ways in which a protozoon can directly benefit 

 a larger animal. It may contribute its own body to be used in the nutri- 

 tion of the host; but such a relationship would not constitute mutualism 

 unless some essential substance not otherwise obtained is thus supplied. 

 Another way is action upon the food materials such as to make a sub- 

 stance usable that otherwise would not be. The latter is the situation in 

 the only proved instance of significant mutualism between Protozoa 

 and their hosts — that between flagellates and certain termites, as well 

 as Cryptocercus punctulatus. Another, indirect, benefit might be con- 

 ferred by aid in controlling an injurious organism or substance, but no 

 instance of that is known with certainty. 



In all its ramifications, the problem of the symbiotic relationships 

 between Protozoa and other animals is far too large for concise treat- 

 ment. It has been considered in various textbooks of protozoology, par- 

 ticularly in the chapter on "Ecology, Commensalism, and Parasitism" 

 in Calkins (1933) and in Doflein-Reichenow ((1927-29). Protozoan 

 relationships are discussed, together with associations in other groups 

 of animals, in CauUery (1922) and Grasse (1935). An important gen- 

 eral article on the subject is that by Wenrich (1935) ; and a general dis- 

 cussion was published by Fantham (1936). Various aspects have been 

 discussed by Hegner (1924, 1926a, 1926b, 1926c, 1928, 1937), by 

 Metcalf (1923, 1929), by Cleveland (1926, 1934), by contributors to 

 Hegner and Andrews (1930), by Becker (1932, 1933), and by Kirby 

 (1937). 



In undertaking to make a contribution to the subject, the author recog- 

 nized two possible paths of approach. Either he could attempt to make 

 a comprehensive scrutiny of the entire expanse of pertinent information, 

 or he could explore in as much detail as possible certain chosen fields 

 of inquiry. The former approach would lead to a generalized account, 

 with selected and perhaps original illustrations; and it would in large 

 part reiterate existing, readily accessible, sometimes commonplace con- 

 cepts. The latter course, although less exhaustive, permits selection for 

 more detailed consideration of certain representative topics; that is the 

 course which has in the main been followed here. 



In the ecology of symbiotic relationships, an introductory chapter is 



