THE SCHOOL OF THE SHORE 23 



moisture and safety. The animal which lives 

 in the familiar razor-shell of our shores burrows 

 straight downwards with its foot into the sand, 

 where it lies so well concealed that it is not 

 easy to discover it alive. For even if we see 

 above the sand the little jet of water it shoots 

 up from time to time through its breathing 

 tube, the animal burrows away so rapidly that 

 it is not easy to reach it. 



Still more remarkable is the fact that some 

 bivalves, such as the one known to fishermen 

 as the piddock, burrow, or rather bore, into 

 the rocks themselves. We can see their holes 

 very easily, especially when the rock is one of 

 the softer kinds such as sandstone, and we 

 may sometimes see the breathing tubes of the 

 piddock at the mouth of its long burrow ; 

 but these are very quickly withdrawn as we 

 approach. The hole is too narrow and long 

 for a crab's claw or a bird's bill, and the animal 

 can only be got at by breaking open the rock. 



This mollusc uses its "foot" for burrowing 

 into the rock just as its sand-burrowing rela- 

 tives do ; but there is a difference in the foot. 

 Though it looks quite soft, it has, embedded in 

 its muscular substance, a layer of sharp, hard 

 crystals, and these slowly wear away the rock 



