102 A NATURAL HISTORY OF THE SEAS 



become embedded in coral formations often so completely 

 that only the outer margin of the shell protrudes. 



To this order belongs the largest shell mollusc, the 

 Giant Clam (Tridacnia gigas) of the Barrier Reef, which may 

 attain a length of four or five feet and a weight of a 

 thousand pounds. 



When a large specimen is at rest with its shells agape 

 at low tide it forms a very effective man-trap and gruesome 

 stories of persons thus seized and held by the ankle are 

 only too well founded. 



In Australia the shells are used by the natives as foot- 

 baths and to gather rain water. 



A remarkable relation of the Clams is the Razor Shell 

 {Solen) of our coasts. By means of a muscular foot the 

 animal burrows vertically at a great rate and is often 

 used for food or bait. 



The Piddocks (Pholas) have thin sculptured shells 

 reinforced over the hinge with several extra plates. Though 

 so frail these shells easily tunnel the hardest limestone 

 until they lie entirely concealed. The animal by means 

 of its powerful feet creates a rowing motion, which causes 

 the numerous fine teeth and sharply-edged facets of the 

 shell to act as a file, whilst an acid secretion may also be 

 brought into play. 



The allied Rock Borers {Saxkavd) drive straight burrows, 

 and should a fellow mollusc lie across their path it is un- 

 hesitatingly sacrificed in the steady onward movement. 



The Ship Worms (^Teredo) are borers of historic interest 

 since in the days of wooden ships they helped to sink more 

 vessels than did storms at sea or enemy cannons. The 

 ancients so well knew the devastating effects of these 



