COMMENSALISM. 9 



whicli van Beneden first applied the term " commensalism." Here we 

 have creatures that live within the bodies of larger animals, like para- 

 sites, to which they are generally very similar in organization ; never- 

 theless, they are not true parasites, inasmuch as they do not feed upon 

 the juices and tissues of their host, but share its food or live upon the 

 refuse of its body. Although there are several instances of commen- 

 salism among the lower aquatic animals, we do not find any in man 

 and the domestic animals, which form the subject of the present 

 treatise, it being supposed, of course, that the conception of the term 

 is not extended to such parasites as live upon the internal excretory 

 products, instead of the living tissues of their host. If it could be 

 definitely proved that certain intestinal worms (such as Oxyuris 

 curvula of the horse) really feed upon the undigested food of their 

 host,^ this statement would need some limitation; but it would at 

 the same time tend to show that commensalism is connected with 

 true parasitism by numerous transitional stages, as we have already 

 seen that the free and parasitic modes of existence are connected. 



1 'Dujardin, Ann. Sci. Nat., S(5r. 3, t. xv., p. 302, 1851. 



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