2 PARASITES OF THE DOMESTIC ANIMALS 



dependent, often solitary existence, on the other hand, a communion of 

 life's interests may be estabhshed between two organisms which, it is 

 surmised, is founded upon some nmtual advantage in the strife. To 

 such association the general term s>Tiibiosis has been applied and each 

 of the organisms concerned is referred to as a SAaubiont. Though there 

 is by no means a uniformity in the appHcation of terms referring to the 

 symbiotic relationship, a usage is adopted here that seems best defined, 

 and by which s>mibiosis is subdivided into the three categories, (1) mu- 

 tualism, (2) commensalism, and (3) parasitism. In the first there is a 

 reciprocal advantage derived from the union; in the second but one 

 s\Tnbiont is benefited though the other suffers no harm, while in the 

 third division one receives an advantage to the detriment of the animal 

 or plant which it invades. There is, however, no sharp line of demarca- 

 tion between these three states of living together, and it may be difficult 

 to determine in some cases whether one or both symbionts receives 

 benefit from the union, or whether one is or is not injured by it. 



One of the more obvious examples of mutualism is the case of the- 

 hermit crab and the sea anemone. This crab selects a shell, as that of 

 the whelk, for its habitation, from the opening of which it projects only 

 its head and claws. On the surface of the shell may often be found a sea 

 anemone fastened near the opening with its mouth and tentacles in the 

 vicinity of the crab's head. The anemone in this position not only in a 

 measure serves to conceal the hermit crab from its enemies, but the 

 creature that would prey upon the crab must first reckon with the 

 dangerous stinging threads with which the tentacles of the anemone are 

 armed. The anemone, in its turn, is benefited by being carried about 

 by the crab and aided in this way in obtaining its food. 



Such associations are not always of mutual advantage, and maj' be 

 more in the nature of an invasion of one animal upon or within the 

 body of another, the invading animal alone deriving benefit, while the 

 animal upon which the association is forced, though not benefiting, 

 ma}^ in no way suffer from it. A familiar form of this living together 

 (commensalism) is the little crab so commonly found in the shell of the 

 oyster. The oyster is not harmed by its presence, but the crab is bene- 

 fited by the protection which the shell affords. Another more curious 

 example of such association is afforded among the vertebrates by the 

 species of Remora, or suck fishes, which have the first dorsal fin modified 

 into a sucking disk on top of the head. By means of this disk it attaches 

 itself to a shark or other large fish, and is thus carried about, detaching 

 itself only to secure food. Its benefit from such association is in being- 

 carried to new feeding grounds without effort of its own, and in the 

 shelter from its enemies which the body of the larger fish may afford. 

 The host, on the other hand, cannot be benefited, nor does it seem to 

 suffer by the presence of its uninvited guest. 



