THE SEA SHORE 45 



association called " commensalism," which may be defined 

 as an external partnership between two different animals 

 usually for their mutual benefit. On the shore the most 

 striking example is furnished by the association between the 

 hermit crabs and different kinds of anemones and sponges. 

 One species of hermit, Eupagurus prideauxi, is always found 

 with its body enfolded by an anemone, Adamsia palliata, 

 which resembles a sausage-shaped bag pushed in at either 

 end, to form, at the upper end the stomach cavity, the mouth 

 of which is surrounded by tentacles, and at the lower end 

 the much larger cavity occupied by the soft body of the 

 crab. A mollusc shell is always present in the first place and 

 to this the crab is attached. The common shore hermit, E. 

 bernhardus, which lives in shells of all sizes up to those of the 

 whelk, usually carries on the shell a large anemone, Sagartia 

 parasitica, while in the upper whorls of the shell there is 

 often a worm called Nereilepas. Yet a third, smaller 

 hermit, E. piibescens, is often almost obscured by ths 

 relatively large masses of a sponge which almost invariably 

 grows on the shell. This definite association between the 

 hermit and the anemones and sponge is clearly not 

 accidental, in the former case the anemone probably helps 

 to protect the hermit which, in turn, provides the anemone 

 with scraps of food, in the latter case the sponge may 

 provide protection by camouflage and itself receive 

 food. 



Other examples of commensalism are provided by the 

 gall crabs which live in coral. The young female crab 

 settles down between two branches of the coral, which as 

 a result, broaden and finally unite above the crab, forming 

 a gall within which the crab lives, feeding, not on the coral, 

 but on particles brought in by the water. The male of 

 the species remains outside the coral. The pea crab, 

 which lives in bivalves and sea squirts, is a further example. 



