262 



ANALYSIS OF THE ENVIRONMENT 



sitic queen is able to develop a brood in a 

 mixed colony to which she contributes little 

 or nothing. The nest parasitism of European 

 cuckoos and of American cowbirds illus- 

 trates a somewhat similar situation among 

 birds. 



Representatives of many different phyla 

 grow as epicoles (epibionts) on the shells 

 or on the skin of others without becoming 

 noticeably parasitic and without contribut- 

 ing anything to the well-being of the ani- 

 mals on which they perch. A basically simi- 

 lar, though more intimate, relationship exists 

 when one organism lives within the body 

 of another without otherwise becoming a 

 parasite : a whole microcommunity of plants 

 and animals lives in the canal system of 

 sponges, and the intestinal fauna and flora 

 of ruminants and other mammals is largely 

 nonparasitic. The Pinnotheres, that lives in 

 the mantle cavity of certain sea mussels, has 

 already been cited (p. 244); the crab 

 steals food collected by the host moUusk, 

 but does little if any known injury. 



In conclusion, it is important to note that 

 the parasitic habit is a specialized, more or 



less intimate ecological relationship between 

 two kinds of organisms in which each forms 

 an important, often a critical, portion of the 

 environment of the other symbiont. In the 

 small-scale ecological community of host 

 and its parasites, the host shows many re- 

 semblances to the dominant species in large- 

 scale ecological communities in that the 

 host also receives the full impact of the 

 more generalized environment and so modi- 

 fies it that associated organisms can thrive 

 under conditions that they could not other- 

 wise tolerate. 



So many animals support a great variety 

 of species of parasites, and individual meta- 

 zoans may harbor such great populations of 

 the smaller parasites, as to justify the state- 

 ment that parasitic animals approach, and 

 perhaps outnumber, the nonparasitic in in- 

 dividuals if not in species. Without examin- 

 ing this proposition more closely, it is evi- 

 dent that the higher animals live in an en- 

 vironment in which parasites and disease- 

 producing organisms form one of the most 

 important of the biotic factors. 



