BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



253 



the nematocyst-bearing acontia (which ex- 

 tend outside the body when its opening is 

 contracted) are especially developed. 



The association of the hydroid Hydrac- 

 tinia sodalis with Eiipaguriis constans is 

 notable as an independent instance of sub- 

 stitution of a structure formed by the sessile 

 guest for the snail shell house, and is fur- 

 ther remarkable for the differential place- 

 ment of the special defensive polyps at the 

 open edge of the structure. 



The crab-sponge association is a common 

 one, exemplified by the hermit crab Paguris- 

 tes maculatus with the sponge Hircinia vari- 

 abilis and by Dromia vulgaris with Suberites 

 dromuncula. 



Mutualism between animal and animal 

 has been mentioned as frequently explain- 

 able as a derivation from commensahsm. In 

 animal-animal mutuaUsts the size relations 

 vary remarkably. At one extreme are the 

 microscopic protozoa of herbivorous mam- 

 mals and of certain cellulose-digesting in- 

 sects, associated with at least macroscopic 

 animal "hosts;" the disparity of size is con- 

 siderable, though much less when both 

 members of the pair are macroscopic, as in 

 the ox-tickbird relation. In the crustacean- 

 sea anemone or crustacean-sponge relation- 

 ship, the more passive of the pair of species 

 may actually outweigh the more active part- 

 ner. There are equally great disparities in 

 the populations of the respective mutuaUsts, 

 greatly in favor of the smaller partner in 

 the case of intestinal protozoa, reducing to 

 approximately the one-one relation as the 

 sizes becomes subequal, except that in social 

 forms, like the ants and termites, mutuahst 

 nest-inhabiting beetles and other termito- 

 philes and myrmecophiles are enormously 

 in the minority as compared with the host 

 populations. 



The transition from nest commensalism 

 and social parasitism to be found in the 

 nest inhabitants attracted to the organized 

 colonies of ants and termites presents an- 

 other clear indication of the origin of mu- 

 tualism from a predatory-prey, a parasite- 

 host, or a host-guest relation. These rela- 

 tions arise in the extremely well-defined and 

 evidently favorable biotic environment af- 

 forded by the stabilization of microclimate 

 and food supply in their nests by the so- 

 cieties of ants and termites. The evolution- 

 ary trend for the nest inhabitants to offer 

 a counter-benefit to their hosts results in 



numerous extreme types of obhgate mutual- 

 ism (see p. 718). 



The relation of ant and aphid and of ant 

 and other symbionts has often been com- 

 pared to that between man and his domestic 

 animals. In general, the development of 

 domestic animals in association with human 

 societies, as well as the relation of man with 

 certain cultivated plants, represents an ap- 

 proach to mutuaUstic relations. That these 

 are at best loose is shown by the capacity 

 of a great many domestic animals to revert 

 to self-sustenance in feral life, no less than 

 by the capacity of man himself to become 

 "feral." 



PARASITISM 



At this point in our development of eco- 

 logical generalizations under the heading of 

 Symbiosis we are concerned with parasites 

 primarily as part of the biotic environment 

 of their hosts and with the hosts as a major 

 part of the total environment of their para- 

 sites. The relations between the two are al- 

 ways intimate and may be exact and cru- 

 cial. Primarily, the operational aspects of 

 these relations need to be considered in the 

 present connection. Population, community, 

 and evolutionary phases of parasitism will 

 be discussed in later sections (pp. 379-386 

 and 701-704). 



Parasitism is a form of symbiosis in which 

 a small organism Uves on or in or with and 

 at the expense of a larger animal or plant. 

 The parasite obtains noteworthy aid in the 

 form of food, shelter, protection, or trans- 

 port. It not only does not give due return, 

 but is more or less harmful to its host. In a 

 narrower usage, parasitism is restricted to 

 those cases in which the parasite hves on 

 or in and at the expense of its host's body. 



Parasitism, commensalism, and mutual- 

 ism, the three main types of symbiosis, are 

 distinguished from each other on the basis 

 of benefits received and harm inflicted; 

 these are relations that often have demon- 

 strable positive or negative survival values. 

 If neither associate is harmed and at best 

 only one benefits, we are dealing with com- 

 mensalism; if both associates are benefited, 

 the relation is mutualism; if one is harmed, 

 it is parasitism. The distinction of these 

 categories is on the basis of short-run, oper- 

 ational values. Often, in the absence of pre- 

 cise information, judgment must be sus- 

 pended or estimates must be made that re- 

 semble, more or less closely, the value judg- 



