402 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



it might apparently be said that they are all parasites. But under 

 habitual conditions an animal kills and devours its prey ; does away 

 with it, in fact. This kind of life is called " predatism ," and it is 

 properly distinguished from parasitism. The parasite, contrary to 

 the predator, feeds on the organism at whose expense it lives, but 

 without destroying it ; it exploits its victim methodically, so to speak, 

 by deflecting in its own favor a part of the victim's energy, thus 

 causing the victim more or less harm and exercising an influence 

 which is more or less pathogenic but which is sometimes perfectly 

 tolerated. 



Between predatism and parasitism there will be found in nature 

 a continuous series of intermediates. Sometimes, also, two- beings 

 are associated in a relationship of dependence which is to a certain 

 degree an exploitation of one by the other, without there being, how- 

 ever, any direct borrowing. This is the condition in commensalism. 

 Nereilepas fucata, a polychete annelid which is always found at the 

 apex of gastropod shells (such as Buccinum) inhabited by hermit 

 crabs {Pagv/rus heriihardus) , profits by the current of water which 

 the crab produces and, moreover, robs the crab, when eating, of some of 

 its food. The annelid is not parasitic, ij; is commensal. But if many of 

 these associations can be easily labeled as commensalism, the limits 

 between this category and those of parasitism are difficult to draw. 

 Certain creatures, such as many infusorians, like the Urceolarians, 

 the Trichodinians and the Vorticellae make their homes, and neces- 

 sarily, on other animals ; they are epizoarians ; still commensals, bor- 

 rowing directly from the animal which carries them locomotion and 

 often the conditions of aeration and renewal of water assured by 

 the functioning of the latter's gills. Thus there is no real boundary 

 between commensalism and parasitism. 



One of the characters of parasitism is the fixity and necessity of 

 the relations between host and parasite. A true parasite can not 

 go through its life cycle without the aid of its host ; and lives at its 

 expense. These associations are therefore fixed and more or less 

 intimate in degree. But it is not always easy to say of such an 

 association that it exists to the detriment of one of the partners; 

 there are well-defined cases in which it can be demonstrated that for 

 both there is a physiological advantage and sometimes a necessity in 

 carrying out the partnership. Thus are formed complexes of two 

 organisms for mutual benefit and close reciprocal interdependence; 

 they are designated under the name of " S}^mbiosis." In the two 

 kingdoms there are now known a fair number of well-defined cases. 



There exist, however, all transitions leading from parasitism to 

 symbiosis. No exact line between them can be drawn. And here our 

 subject touches on a matter of immediate interest. The publication, 



