540 MANUAL OF ELEMENTARY ZOOLOGY 



internal structure. ■ Owing to their nematocysts they arc 

 distasteful to fishes, as is shown by the fact that they will 

 not serve for bait. They cannot pursue food, but must 

 wait till it comes within reach of their tentacles. Now 

 certain kinds of sea-anemones are found on the shells of 

 hermit crabs. Here they are never molested by the 

 owner of the shell, and benefit by the constant change of 

 feeding ground and by fragments of food which are let fall 

 by the hermit crab, to obtain which some of them stand 

 with the mouth on the lower side of the shell, which their 

 base enwraps. In return the crab obtains protection from 

 fish, which are kept from eating it by the stinging powers of 

 the anemone. Within the shell there is often found a 

 species of Nereis, which with its horny jaws robs the food 

 out of the very pincers of the crab, without, so far as can 

 be seen, conferring any benefit in return, and must 

 therefore be regarded as a parasite. 



The assemblage of animals which dwells in any given 

 locality or kind of locality — its fauna — is, as will 

 be gathered from the foregoing pages, no collec- 

 tion of independent units, but a complex system of beings 

 in constant interaction with one another and with their 

 surroundings, both living and lifeless. We may recognise 

 geographical faunas, which belong to localities, and (ecological 

 faunas, which belong to kinds of locality. These, of course, 

 are cross divisions, for most local areas have parts of different 

 kinds — as an island may have mountains, plains, and 

 streams — and most kinds of local conditions turn up again 

 and again in different places — as, for instance, do those of 

 fresh-water ponds. The kinds of animals which make up 

 a geographical fauna are determined by the past history of 

 the locality (see p. 523) — as, for instance, by land bridges 

 which have in earlier ages allowed the immigration of certain 

 animals but have disappeared before others were evolved, — 

 and such a fauna has no features which are common to all 

 its members except in so far as it may happen to be also 

 an oecological fauna, which it will be when the locality is 

 throughout of one type. But an cecological fauna has 

 usually well-marked common features which appear in 

 different forms in all its members, however unrelated they 

 be in evolution, and enable them to live in the conditions 



