136 THE BIOLOGY OF THE SEA-SHORE 



kind becomes more intimate, and has important consequences. 

 For instance, there are certain small crabs of the genus 

 Hapalocarcinus, which become profoundly modified in form 

 owing to the fact that they pass the greater portion of their 

 life confined in small cavities in coral colonies. At an early 

 stage the crab settles between two coral branchlets, and so 

 affects their growth that they broaden, and later coalesce to 

 form a so-called gall, which is about the size of a large nut. 

 Each of these appears to be occupied by a soHtary female 

 crab. The male is free-living (cf. Pinnotheres, p. 145) and 

 visits the female while the galls are still open. The crab 

 does not feed on the coral, but probably lives on small 

 organisms drawn in with the water current (Potts, 19 14). 

 Reference has already been made to gall formation in the 

 common seaweed Ascophyllum nodosum, due to young forms 

 of Mytiliis edulis occupying the air vesicles. The growing 

 valves cause the tissues to split. The plant is stimulated 

 to respond, and abnormal form results. 



The classical example of commensalism is provided by 

 the hermit crab of British coasts Eupagtirus prideauxii, and 

 the sea-anemone Adamsia palliata. The anemone serves to 

 conceal the crab, and may find its reward for doing so in the 

 scraps of food let drop by the Crustacean. No doubt, also, 

 the presence of the anemone is an aid to the crab in attack 

 and defence. Another species of hermit crab, Eiipagurus 

 Pubescens, almost invariably has the shell in which it is living 

 entirely concealed by the sponge Suherites domuncula. On 

 rare occasions (in three cases out of several thousand, accord- 

 ing to Elmhirst) another species, E. bernharduSy is found 

 with the same sponge. As we have already noticed, the 

 top whorl of the shell of the last-named species is tenanted 

 by the worm Nereilepas fucata. This worm, in fact, was 

 probably the first tenant of the empty shell and tolerates the 

 subsequent advent of the crab because of the food which the 

 latter procures (Hornell, 1892). 



Experimental analysis of the commensalism of anemones 

 and hermit crabs tends to show that this association, in 

 certain cases at least, is not a chance one. Brunelli (1910), 



