PROTOZOA AND OTHER ANIMALS 895 



general agreement on the mutualistic character of the latter. To com- 

 plete this series, relationships of strict parasitism should be discussed 

 from the standpoint of the physiology of the parasite and the effect on 

 the host. The absence of this is readily compensated for, however, in 

 the abundant literature of parasitology; and examples of parasitism are 

 described in almost all groups considered in this chapter. 



As is apparent, the material is grouped under certain headings; 

 but in every section data bearing on various topics in symbiotic rela- 

 tionships will be found. To group all facts under specific topics would 

 involve much dislocation in other respects; and it has seemed prefer- 

 able to preserve systematic continuity to a considerable extent. 



Accidental and Facultative Parasitism 



In connection with the origin of host relationships, it is of interest 

 to consider instances in which organisms can develop in both free- 

 living and symbiotic habitats. Accidental and facultative parasitism, 

 therefore, come up for primary consideration. Facultative parasites, as 

 opposed to obligate parasites, are able to live either associated with 

 hosts or not, but parasitism is a natural occurrence in the bionomics of 

 the species. Accidental parasitism is that of a naturally free-living 

 species, which happens through some accident to become parasitic. 

 Although the two types are not identical, they are obviously closely 

 related and categorical separations are not attempted in the discussion. 



Mercier and Poisson (1923) pointed out that those forms that are 

 most ubiquitous and are preadapted to varied modes of nutrition have 

 the best chance of surviving in the new medium into which they are intro- 

 duced accidentally. For such forms Giard (1880) used the term "in- 

 choate parasitism"; Giard had reference to the incomplete and tempo- 

 rary parasitism illustrated by the occurrence of geophilids in the nasal 

 cavities of man. 



These conditions are met by the ciliate Glaucoma pyrijormis. Under 

 natural conditions G. pyrijormis ingests bacteria. Hetherington (1933) 

 stated that it is one of the commonest fresh-water Protozoa, appearing 

 in the early stages of the usual infusions of hay, wheat, or lettuce if 

 they are inoculated with pond water. (At that time he named the ciliate 

 Colpidium campylum, but later [1936] reported that it is G. pyrijormis.') 



