THE SCHOOL OF THE SHORE 23 
ture 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these are very quickly withdrawn as we ap- 
proach. 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 
