Inter-Relations of Living Creatures 651 



hide, and the pilot-fish that accompanies the shark. There is noth- 

 ing hard and fast in our grouping of these associations, and what 

 we have called partnership passes insensibly into something more 

 definite. Thus the little tiny pea-crab often lives within the horse- 

 mussel, finding shelter and apparently food as well. 



It is naturally difficult to draw a firm line where shelter stops 

 and some sort of co-operation begins. There is a brilliant Indian 

 Ocean fish, about two inches long, called Amphiprion, that lives 

 in association with a large reef -anemone (Discosoma). It lives 

 among the tentacles of the anemone and retires into the food- 

 cavity on the slightest alarm. It dies when it is removed from the 

 sea-anemone. As Mr. Banfield says in his delightful My Tropic 

 Isle (1910), "it is almost as elusive as a sunbeam, and most dif- 

 ficult to catch, for if the anemone is disturbed it contracts its folds 

 and shrinks away, offering inviolable sanctuary." The benefit to 

 the little fish is plain enough it finds shelter and crumbs; but 

 is there any benefit on the other side? Many sea-anemones are 

 in the habit of stinging and seizing small fishes which intrude in- 

 quisitively or incautiously, but Discosoma does not seem to do 

 this to Amphiprion. It has been suggested that the brilliantly 

 coloured fish serves as a lure ; it seems more likely that the move- 

 ments of the fish in and about the sea-anemone keep up useful 

 currents of water. 



2 



Commensalism 



In a previous section reference has been made to an external 

 partnership, mutually beneficial, between two quite different 

 kinds of animals, which is usually spoken of as "commensalism," 

 meaning eating at the same table (con, together, and mensa, a 

 table) : the word is almost the same as companionship (eating the 

 same bread). Fine instances are found in the associations be- 

 tween hermit-crabs and sea-anemones. Certain kinds of hermit- 

 crabs place sea-anemones on the back of the buckie or other shells 



