94 UNDERHill: 



the animal cotitinues to lead a free and independent, often 

 solitary existence, a communion of life's interests may be 

 established between two animal organisms which causes us to 

 surmise that this association is founded upon some mutual 

 advantage in the strife. To such association the general term 

 symbiosis has been applied, and each of the organisms con- 

 cerned is referred to as a symbiont. Though there is by no 

 means a uniformity in the use of these terms by zoologists, it 

 will serve here to subdivide symbiosis into the three categories, 

 mutualism, commensal ism and parasitism. In the first there 

 is a reciprocal advantage derived from the union ; in the 

 second but one symbiont is benefitted though the other suffers 

 no harm, while in the third division we have one receiving an 

 advantage to the detriment of the animal which it invades. 

 There is, however, no sharp line of demarcation between 

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

 determine in some cases whether one or both symbionts are 

 benefitted by 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, often 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 posi- 

 tion 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 is benefitted, in its turn, by being carried about by 

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



Such associations, however, are not always of mutual 

 advantage, and may 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 



