CHARACTERISTICS OF ASSOCIATIONS 5 



that of the host plants which, in turn, are subject to physical 

 controls. Other animals have become carnivorous, and 

 by virtue of this habit have escaped from the fettering control 

 of a host plant, but must pursue or search out their prey, 

 or perish. 



A still closer relationship is that between host and 

 parasite. While many parasites are more or less adaptable 

 and may even exist for a time as freerliving forms (facultative 

 parasites), others again are associated entirely with a particular 

 species of host (obligate parasites), the dependence of the 

 parasite thus being extreme. Special devices, such as a 

 complex life-history or enormous fertility, are then necessary 

 to maintain the relationship unbroken. Most intimate 

 association of all, however, is that known as symbiosis. 

 This is a partnership for mutual benefit between an animal 

 and a simple chlorophyll-bearing plant, or between two 

 plants (fungus and alga). A typical shore example of such 

 an association is that between the Planarian worm Convoluta 

 roscqffensis and a green flagellate alga, in which the worm 

 feeds on the reserve material manufactured by the plant, 

 and the latter has the advantage of a plentiful supply of 

 nitrogenous material and carbon dioxide. 



A somewhat analogous association for mutual benefit is 

 that known as CommensaUsm. The partnership here is 

 not nearly so close, being simply one of habit, and though 

 of very frequent occurrence is not essential to the forms 

 concerned. A familiar example of this is afforded by the 

 hermit crab Pagiirus prideaiixii and the sea anemone 

 Adamsia palliata, the latter helping to conceal the crab 

 and profiting by the food morsels it drops. 



We thus see that the inter dependence between plant 

 and animal and among animals themselves is frequently 

 very close. But when we add to this the influence of the 

 environment - in favouring the survival of forms specially 

 adapted to it, it becomes evident that in any well-defined 

 area certain forms suited to that area, or with life- 

 relationships to one another, will be found to occur together 

 forming communities or associations. Plant associations, 



