CHAPTER XXI 



Social Life of Animals 

 Association of Different Species 



Living Free. — Free living forms are those that are at liberty to 

 range independently. They feed upon animals, and are then called 

 carnivorous^ or they feed upon plants, usually destroying them 

 although not necessarily causing their death, and are called herbiv- 

 orous. 



Commensalism. (Lat. com^ together; and mensa, table.) — 

 Commensalism as we shall treat it consists in an association for 

 mutual benefit less permanent than symbiosis. Commensalism 

 may be for protection, transportation ov food. 



The best known example of commensalism is the case of the 

 hydroid colony living on the shell of a hermit crab. As the hydroid 

 furnishes the crab with camouflage and protects it from enemies by 

 the use of its stinging cells, it renders some service in return for 

 transportation and for particles oi food broken up by the crab. 



Symbiosis. (Gr. syn, together; and bios, life.) — The term 

 symbiosis has by some been applied to all intimate associations 

 between organisms (including parasitism); by others to relations 

 between organisms in which benefits are mutual; and by still others 

 to plants living within the body of animals in a presumably beneficial 

 relationship. Until recently, zoologists considered symbiosis in 

 the broad sense, holding that it refers to ani7nals or plants living 

 together for mutual benefit. It is now applied to relationship be- 

 tween plant and animal; animal and animal; or plant and plant; it 

 always being understood that no parasitism occurs. Lull calls it 

 the most intimate association for ynutual benefit, an extremely close 

 commensalism. We shall consider symbiosis in the latter sense. 



Symbiosis may be between a green chlorophyll-bearing plant 

 and an animal; between a chlorophyll-bearing plant and a colorless 

 one; or between two animals. Bacteria living in the digestive tracts 

 of animals are to be considered as an example of symbiosis since they 

 seem beneficial. Certain ruminants, apes, and South American 

 rodents are able to digest cellulose by means of the Ophryoscolecidae, 



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