COMMENSALISM AND PARASITISM. 147 



is the Hermit-crab. From the open mouth of 

 what is apparently an empty shell a bundle of 

 claws and legs may be seen to protrude ; turn 

 the shell over and it will scamper away into the 

 deeper parts of a rock pool. This is an association 

 of a living Crab with the shell of an animal that is 

 dead ; but if the Hermit-crab be extracted it will 

 be seen that it has a soft and twisted tail, quite 

 unlike that of the shore Crabs, and that it could 

 not possibly live for any length of time with- 

 out the shelter and protection afforded by the 

 shell that it has appropriated to itself. The 

 Hermit-crab in the course of its life increases in 

 size, and when it gets too big for the shell it is 

 living in, it goes in search of another a little bit 

 larger and changes, until at last it attains to the 

 size and dignity that requires a full-grown Whelk 

 shell. 



In the waters of our coast just beyond the low 

 tide mark we often find that the shell containing 

 a Hermit-crab bears a Sea-anemone which be- 

 longs to a species rarely found anywhere except- 

 ing in association with a Hermit-crab. Moreover, 

 the Anemone is always seated in a definite 

 position on. the shell, so that its mouth is turned 

 towards the jaws of the Hermit-crab when it is 

 extended, enabling it to catch any morsels of 

 food that escape the mouth of its comrade. 

 When the Hermit-crab has grown too large for 

 its shell, and moves into a new one, the Anemone 

 moves too, and takes up the same position on the 

 new shell that it occupied on the old one, and 

 the companionship is continued in this manner 

 throughout life. 



