24 THE HAUNTS OF LIFE 
rock as the animal patiently scrapes. The 
crystals in their turn are worn away by the 
rock; but they are continually being renewed. 
Thus by means of this adaptation the boring 
Pholas secures three things: first, the neces- 
sary moisture and shelter from sun or frost 
to keep it alive while the tide is out; second, 
security from being dashed to pieces by the 
waves; and third, relative safety from the 
attacks of hungry enemies. 
Molluscs are not the only animals that have 
learnt these ways of protecting themselves. 
Many worms burrow in the sand, and a few 
bore into the rocks. Flexible worms cannot 
have shells in the strict sense, for shells are 
made by the living skin; but they often build 
round their bodies sheltering tubes of lime or 
of grains of sand, or bits of shell or other sub- 
stances cemented together, and they with- 
draw their delicate tentacles into these when 
danger threatens. Fragments of the different 
kinds of tubes made by different kinds of 
worms may often be picked up on the shore. 
Crabs and some of their relatives have also 
the habit of burrowing in the sand, shovelling 
it aside with their great claws or with their 
legs. ‘The big crabs, indeed, do not need this 
