300 THE BIOLOGY OF THE SEA-SHORE 



occurred Littorina littorea (large), L. obtusata (small), Patella 

 and Carcinus. Some stone work then intervened and on 

 the reef and in the pools immediately beyond the rocks were 

 still decidedly oily, and the fauna was rich . Over forty species 

 of invertebrates were collected within a few yards and in some 

 cases the specimens were of very large size and peculiarly 

 coloured as compared with those of unaffected rocks near by. 



It has also been shown by several workers that the green 

 alga Ulva latissima grows more rapidly in polluted than in 

 pure water (see Chap. II, p. 45). 



Injurious Animals. — Some other invertebrates may be a 

 source of loss on shell-fish beds, as for instance Cliona — the 

 boring sponge. The mollusc Crepidula fornicata may 

 increase on oyster-beds to such an extent as to enter into 

 serious competition with the oysters by reducing their food 

 supply ; while mussel-beds may be ravaged by hordes of 

 star-fish {Asterias rubens)^ or dog whelks {Purpura lapillus), 

 both of which feed upon the shell-fish. 



The Sea-shore in relation to Human Culture. — The 

 foregoing pages will have served to convey some idea of the 

 economic value of the sea-shore at the present day. The 

 importance of this area to human affairs cannot be correctly 

 gauged, however, by the figures we have given, as to minds 

 accustomed to modern industrial undertakings these will 

 always appear small. It is not too much to say that without 

 the food supply, represented chiefly by molluscs, offered by 

 the shore the development of early civilisations would have 

 been much retarded. Quite apart from food, however, 

 the role played by shells in human life has been very great. 

 In many parts of the world littoral shells were, and are still, 

 employed as a medium of exchange. One of the commonest 

 forms put to this purpose is the money-cowrie {Cyprcea 

 fnoneta), still current in West Africa. Among the coastal 

 tribes of North- West America the elephant-tusk shell 

 {Dentalium indianorum) was the standard of value until under 

 the Hudson Bay Company it was superseded by blankets. 

 The " wampum " of the eastern coast of North America 

 consisted of strings of beads formed from the shells of 



