CHAPTER XXIII 



COMMBNSALISM AND PARASITISM 



269. Association between animals of different species. 



The living together and mutual help discussed in the last 

 chapter concerned in each instance a single species of ani- 

 mal. All the various members of a pack of wolves or of a 

 community of ants are individuals of the same species. 

 But there are many instances of an association of individ- 

 uals of different kinds of animals. In many cases of an 

 association of individuals of different species one kind 

 derives great benefit and the other suffers more or less 

 injury from the association. One kind lives at the expense 

 of the other. This association is called parasitism. In 

 some cases, however, neither kind of animal suffers from 

 the presence of the other. The two live together in har- 

 mony and presumably to their mutual advantage. In some 

 cases this mutual advantage is obvious. This kind of asso- 

 ciation is called commensalism or symbiosis. 



270. Commensalism. A curious example of commensal- 

 ism is afforded by the different species of Remoras (Echenei- 

 didce) which attach themselves to sharks, barracudas, and 

 other large fishes by means of a sucking disk on the top of 

 the head (Fig. 201). This disk is made by a modification 

 of the dorsal fin. The Remora thus attached to a shark 

 may be carried about for weeks, leaving its host only to 

 secure food. This is done by a sudden dash through the 

 water. The Remora injures the shark in no way save, per- 

 haps, by the slight check its presence gives to the shark's 

 speed in swimming. 



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