CHAP, v SOCIAL LIFE OF ANIMALS 73 



partnerships. The " beef-eater " birds (By,phagus) perch 

 on cattle and extract grubs from the skin ; a kind of 

 ploA r er (Pluvianus cegyptius) removes leeches and other 

 parasites from the back of the crocodile, and perhaps 

 " picks his teeth," as Herodotus alleged ; the shark is 

 attended by the pilot-fish (Naucrates ductor), who is 

 shielded by the shark's reputation, and seems to remove 

 parasites from his skin. 



Especially among marine animals, we find many almost 

 constant associations, the meaning of which is often 

 obscure. Two gasteropods Rhizochilus and Magilus 

 grow along with certain corals, some barnacles are com- 

 mon on whales, some sponges and polyps are always 

 found together, without there being in any of these cases 

 either parasitism or partnership. But when we find a 

 little fish living contentedly inside a large sea-anemone, 

 or the little pea-crab (Pinnotheres) within the horse-mussel, 

 the probable explanation is that the fish and the crab 

 are sheltered by their hosts and share their food. They 

 are not known to do harm, while they derive much 

 benefit. They illustrate " commensalism," which means 

 eating at the same table. 



But the association between crabs and sea-anemones 

 affords a better illustration. One of the hermit-crabs of 

 our coast (Pagurus prideauxii) has its borrowed shell 

 always enveloped by a sea-anemone (Adamsia palliatd), 

 and Pagurus berrihardus may be similarly ensheathed by 

 Adamsia rondeletii. Mobius describes two crabs from 

 Mauritius which bear a sea-anemone on each claw, and 

 in some other crabs a similar association occurs. It 

 seems that in some cases the crab deliberatelv chooses its 



V 



ally and plants it on its shell, and that it does not leave 

 it behind at the period of shell-changing. Deprived of 

 its polyp companion, one was seen to be restlessly ill at 

 ease until it obtained another of the same kind. The 

 use of the sea-anemone as a mask to the crab and also 

 perhaps as aid in attack or defence is obvious ; on the 

 other hand, the sea-anemone is carried about by the 

 crab and may derive food from the crumbs of its bearer's 



*. 



repast. 



