CHAPTER XXIV 



ANIMAL PARASITISM 

 (By Sewell H. Hopkins, Texas A. and M. College) 



SOCIAL RELATIONS OF ANIMALS 



It has been explained in the previous chapter that no animal is 

 ever entirely independent of others, since all plants and animals 

 are influenced, directly or indirectly, by all the other organisms in 

 the community. Most animals, however, can and do catch and eat 

 their own food, and such animals are said to be ''free-living." 

 But there are thousands of species which depend either completely 

 or partially on others to provide them with a livelihood. The vary- 

 ing degrees of dependence are called commensalism, mutualism, and 

 parasitism. The term symbiosis is applied to all cases of two different 

 kinds of animals living together, and thus includes commensalism, 

 mutualism and parasitism. 



In commensalism,- one animal receives all of the benefit from the 

 association while the other is neither benefited nor harmed. The 

 jackal which follows the tiger and cleans up the carcass of the prey 

 when the tiger has eaten his fill, the small fishes which accompany 

 sharks and feed on the scraps wasted by the shark in feeding, and 

 the oyster crab which lives inside the oyster's shell and feeds on the 

 organisms brought in by the oyster's feeding movements, are examples 

 of commensalism. 



Mutualism is the kind of symbiosis in which both animals receive 

 benefit from their association. One species of hydra (Hydra viridis) 

 is green in color because a certain species of green alga lives within 

 its cells; the alga receives protection and some nourishment from 

 its host, while the hydra benefits from the food manufactured by 

 the green plant. A case of mutualism so far developed that the 

 two animals cannot live separately is the relationship between 

 wood-eating termites and their intestinal protozoa. The termite 

 cannot digest the wood which it eats; the protozoa in the termite's 

 intestine break down the wood into a form in which it can be used 

 by the host ; on the other hand, the protozoa are absolutely depend- 

 ent on the termite for food and the proper environment; neither 

 termite nor protozoan can live without the other partner. 



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