12 

 PARASITISM 



Parasitism is a very strange mode of life, and as the nematodes 

 include some of the most successful, as well as most important, 

 parasites in the animal kingdom, it is convenient to discuss here 

 some of the generalisations that can be made on the subject. 



A parasite is generally formally defined as an organism which 

 lives in or on the body of another to the detriment of the latter, but 

 any such definition brings one into trouble at once. It is best to 

 make definitions, if such must be made, of the relationships rather 

 than of the relatives. It is clear as soon as one studies the animal 

 kingdom that most species of animal stand in some sort of special 

 relation to one or more other species, and what we are trying to 

 do is to set limits and give names to these relations. (A similar 

 argument can be apphed to plants, but it is not relevant here.) 



The simplest relationship is that where one animal merely 

 uses another as a base on which to stand, as barnacles often grow 

 on the shells of Hmpets, and the ciHate Kerona runs about on 

 Hydra. So far as is known neither has any effect on the other. 

 The nomenclature of this sort of relationship is imperfect, but 

 on the analogy of epiphytism it might be called epizoonism, 

 and the animal which is supported an epizoon. A stage beyond 

 this is commensalism, where two animals live in close physical 

 proximity so as to be mutually helpful ; since the partnership 

 is equal, each member may be called a commensal. The small 

 hermit crab Eupagiirus pridemtxi often carries on its mollusc shell 

 an anemone, Adamsiapalliata, which finally dissolves the shell and 

 takes its place, so saving the crab from having to move house ; it 

 also protects the crab by its nematocysts. In return it acquires the 

 advantages of locomotion and receives as food scraps which fall 

 from the mouth of the crab. The syrphid flies which scavenge in the 

 nests of bees, and the aphides tended by ants, are other examples. 



A partnership for mutual benefit where the advantages and 

 association are intimately physiological is called symbiosis, and 

 the partners are symbionts. It is often probable that the benefits 

 are rather one-sided. Though the relation between Chlorohydra 

 viridissima or the corals and their green Algae is generally called 

 symbiosis, it is not known what advantage the coelenterates 

 derive from the plants. They seem not to use the carbohydrate 



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