142 The Under-Water World 



shell is wedge-shaped the better to pene- 

 trate sand and gravel, being prolonged 

 into a tube made in two longitudinally 

 divided pieces united by a hinge in the 

 case of the razor shell. 



In the wood-and stone-boring bivalves, 

 which present a problem as yet unsolved, 

 the delicate shell is file-like, the " foot " is 

 short and muscular, whilst certain acids 

 are brought to bear upon the substance 

 penetrated. The animal will drive for- 

 ward blindly through the hardest stone 

 other than marble and granite, and if it 

 comes into contact with another borer 

 tunnelling at right angles to its road, 

 burrows through its fellow-mollusc into 

 the rock or wood on the other side. In 

 the old days of " wooden walls " terrible 

 havoc was wrought by the " ship worm/' 

 a bivalve that as it tunnels lines the 

 burrow with carbonate of lime, and in 

 the early eighteenth century the coast of 

 Holland was threatened with inundation 



