LOWER COASTAL ANIMALS 121 





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Sand patterns Jormed by small crabs on the beach. India. 



which in some mysterious way it always finds its way back with unfaihng 

 accuracy. 



Below these animals there is always a zone of oysters in warmer waters, 

 if there is shelter from rough sea. A corresponding zone of mussels is 

 found in colder latitudes, both south and north. A photograph taken on 

 Campbell Island south of New Zealand, and showing both limpets and 

 common mussels, could have been from the west coast of Sweden or 

 Norway. But the beating of heavy surf on the rocks is too much for these 

 bivalves. They get their food by filtering all manner of minute organisms 

 from the water which passes through their gills. To do this their shells 

 must be open, and shells cannot be kept open in heavy surf without com- 

 ing apart. And so, both in the Tropics and in colder regions, we find 

 instead of these a broad white strip inhabited by barnacles. These are 

 crustaceans which have attached themselves to rocks or floating objects 

 and have become surrounded by a conical shell cemented to the object. 

 They have feathery tentacles which they protrude through a slit at the 

 top of the shell. When the tentacles are withdrawn the slit closes, and by 

 this means the creature can withstand the roughest surf. How far up the 

 tidal zone both oysters and barnacles can live depends on the length of 

 time they need to be immersed in order to catch their food. 



