652 The Outline of Science 



which they have commandeered for the shelter of their flabby tails. 

 There may be three or more sea-anemones on the top of a big shell. 

 The advantages to the hermit-crab are that the anemones mask 

 its real nature and that they can sting. In certain crabs a sea- 

 anemone is actually fixed on each of the great claws, as if the 

 crustacean made a weapon of the polyp. The advantages to the 

 sea-anemones are that they are carried about and that they get 

 morsels from the hermit-crab's meals, which are many. In this 

 mutually beneficial partnership there are several points of much 

 interest. Thus a hermit-crab deprived of its partner was seen to 

 stalk about restlessly, ill at ease until it obtained another of the 

 same kind. When a hermit-crab has grown too large for its 

 borrowed shell it has to flit. This means leaving the sea-anemones 

 behind, but the hermit-crab has been seen removing them from the 

 relinquished shell and establishing them on the new one. A sea- 

 anemone removed from its partner was seen, after a while, to 

 fasten itself to the leg of a passing hermit-crab and gradually 

 move on to the top of the shell. In certain cases the sea-anemone 

 is never seen apart from the hermit-crab, and vice versa. 



Getting away from marine animals, we may notice that asso- 

 ciations which may be called commensalism are well known among 

 insects. Thus Mr. William Beebe has recently described a minute 

 blind cockroach (Attaphila) that lives in the subterranean nests 

 of the Atta leaf -cutter ants. They clean the bodies of the giant 

 soldier-ants and seem to do no harm in the nest. We need not 

 refer to other instances of commensalism which have been 

 mentioned elsewhere in this work. We have also had occasion to 

 refer to examples of symbiosis. 



Symbiosis 



The term symbiosis, which simply means living together 

 (syn, together, and bios, life), has been earmarked for mutually 

 beneficial internal partnerships between two organisms of dif- 

 ferent kinds. Thus a green Alga lives inside the little marine 



