COMMENSALISM AND PARASITISM. 155 



of the partnership falls entirely to one of 

 them. 



In a third set of cases one of the associates 

 feeds upon the blood or tissues of the other 

 without rendering it any service in return, and 

 consequently inflicts either temporary or per- 

 manent injury. These are cases of parasitism. 

 In such an association the animal that inflicts 

 the injury is called the " parasite," and the one 

 that receives it, the " host." 



One difficulty the naturalist has to contend 

 with in trying to use these terms correctly is 

 that of finding out whether in any particular case 

 an injury is inflicted or not ; another is that of 

 determining whether those animals should be 

 called parasites which injure, alter, or destroy 

 the tissues of their hosts without feeding upon 

 them. 



A few cases will throw more light upon the 

 subject than any further discussion of the diffi- 

 culties surrounding the application of these 

 terms. 



One of the commonest Corals to be found 

 upon the coral-reefs of both the Old and New 

 world is one called Millepora. In the Mille- 

 pores of the Pacific region we very frequently 

 find a number of Barnacles (called Pyrgoma 

 milleporce) so deeply buried in the substance of 

 the Coral that their presence is indicated only by a 

 small oval hole on the surface. There can be little 

 doubt that in the course of the growth of these 

 Barnacles they distort, if they do not actually 

 destroy, some of the connecting canals of the 

 Coral in their immediate neighbourhood, but 



