68 The Study of Animal Life parti 



Coherence and harmony and mutual helpfulness of 

 parts — whether these be organs, tissues, or cells — are 

 certainly facts in the life of individuals ; we have now 

 to see how far the same is true of the larger life in 

 which the many are considered as one. 



I. PartnersMps. — Animals often live together in strange 

 partnerships. The " beef-eater " birds (Btiphagus) perch 

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

 {Phivianus agyptitis) 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 reputa- 

 tion, 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 common on whales, 

 some sponges and polypes are alVays found together, with- 

 out there being in any of these cases either parasitism or 

 partnership. But when we find a little fish living con- 

 tentedly inside a large sea -anemone, or the little pea-crab 

 {Pinnotheres) within the horse-mussel, the probable explana- 

 tion 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 one 

 kind of " commensalism," or of 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 {Pagurtis prideauxii) has its borrowed shell 

 always enveloped by a sea-anemone {Adamsia palliaid), 

 and Pagurus bernhardus may be similarly ensheathed by 

 Adatnsia 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 deliberately chooses its ally and 

 plants it on its shell, and that it does not leave it behind at 

 the period of shell -changing. Deprived of its polype com- 

 panion, one was seen to be restlessly ill at ease until 

 It obtained another of the same kind. The use of the sea- 



