404 THE WORLD OF ANIMAL LIFE 



Where our sea-coast cliffs are composed of limestone or chalk, 

 we may have noticed that the rocks are perforated by a great 

 number of tunnels, each of which is sufficiently large to admit a 

 man's thumb. These are the burrows of the Piddock or Pholas, 

 as we have called it. 



Let us suppose that we have before us a piece of chalk which 

 has been pierced by these creatures, and that we have carefully 

 split it open with a mallet and chisel. There, lying in its tunnel, 



is the piddock or pholas that made 

 it — a mollusc with a double shell, 

 hinged firmly together at the back. 

 Its shell has a number of raised lines 

 which cross and recross one another, 

 causing its surface to resemble that 

 of a rasp. 



It is by means of this shell that 

 the tunnels are made. When boring 

 a hole in the chalk the pholas takes 

 Piddocks a firm hold with its " foot ", or that 



part of the body which can be pushed 

 out from between the shells and used in locomotion, and then 

 begins to turn slowly from side to side. The sharp edges of 

 the valves soon cut a hollow, which is quickly enlarged by the 

 action of their rasp-like surfaces upon it. And before long the 

 animal is buried in the rock. 



The piddock has a siphon of the same character as that of the 

 cuttle-fish, through which a jet of water can be expelled with 

 no little force. So, when the burrow becomes clogged by the 

 particles of stone or chalk, all that the animal has to do is to 

 eject a jet of water, which washes out the tunnel, and removes 

 the obstruction. 



Upon some parts of our coasts the work of the pholas has been 

 extraordinary. 



That the piddock or pholas has helped the sea very materially 

 to undermine and destroy the cliffs upon its margin is unquestion- 

 able. For it drives its tunnels through and through the rock, until 

 it reduces them almost to the condition of a honey-comb. Then 



