CHAPTER 39 



Parasitism 



The relationship between two species of organisms in which one species 

 lives in or on the body of the second and at its expense is termed para- 

 sitism. The species that derives benefit from the relationship, and 

 usually cannot survive otherwise, is called the parasite, and the species 

 which is injured or affected adversely in some way is called the host. This 

 relationship is distinguished from mutualism (p. 766), in which both 

 species derive some benefit from the association and cannot survive in 

 nature without it. The term symbiosis has been used with several dif- 

 ferent meanings in the past, but it is now widely used as a general term 

 to indicate a persistent physical association between two different species 

 of animals, plants or micro-organisms without special connotation of 

 harm or of benefit to the host species. 



Green plants, fungi, bacteria and viruses, as well as animals of many 

 different phyla, may be parasites. There are animals parasitic on plants, 

 and plants which are parasites of animals. 



361 . Origin of Parasitism 



The ecologic relationship of parasitism may arise by any of several 

 evolutionary paths. Predation, commensalism or competition between 

 species for food may develop into parasitism. Animals which are saprozoic 

 or bacterial feeders are to some extent adapted beforehand to living in 

 the digestive tract and can become parasites directly on their first contact 

 with the host species. 



Predation and Parasitism. ^Vhen predation evolves into para- 

 sitism, the diet is usually changed from small prey to a large host species. 

 The mites, for example,' which are small relatives of the spiders include 

 many predators that hunt down and kill small arthropods, sucking out 

 their body juices. Some of these attack large prey and, in the process of 

 removing a full meal, do not kill the prey. These have taken the first 

 step toward parasitism. Still other mites not only do not remove enough 

 juice to kill the host at one meal, but remain on the host between meals 

 so that much of their life is spent there. These are fully evolved parasites. 

 The predaceous mites generally attack small arthropods; the parasitic 

 mites attack larger arthropods and vertebrates. 



Leeches show a similar progression from predation to parasitism. 



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