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 



