ii4 THE MOLLUSCA 



small and but feebly developed, but in those molluscs which 

 burrow deeply in the sand or mud, or excavate holes in hard sub- 

 stances, it has become exceedingly strong and is often of con- 

 siderable size. In the Solen, or Razor-shell, the foot is the most 

 remarkable feature of the mollusc ; by means of its powerful 

 foot the Solen bores a deep, perpendicular tunnel in the ground, 

 and in this retreat it lives, ascending and descending by the expan- 

 sion and contraction of its extraordinary and accommodating 

 foot, which can be stretched out to a great length or drawn com- 

 pletely within the shell, while the tip may be pointed to furnish 

 an efficient boring instrument or swelled out into a thick knob 

 to enable the mollusc to wedge itself firmly in its burrow. 



The Pholas, or Piddock, bores into rocks, wood, mud, or sand. 

 It is a small mollusc, with a white-ridged shell which is strong, 

 although very thin. Its foot is white and soft, and almost as 

 transparent as ice, but at the tip is a layer of sharp, flint crystals, 

 converting it into a perfect file. With this instrument the Pholas 

 rasps away with such persistence that the hardest substance in 

 time becomes worn away ; and as the crystals are worn down 

 with hard work they are replaced by a new set. 



The Teredo, or Ship-worm, is another curious boring mollusc. 

 It tunnels chiefly into timber, and does a great deal of damage 

 to piles, breakwaters, etc., which are not protected by metal. 

 The Teredo is a strange-looking creature, with a long, worm-like 

 body enclosed in a thin shelly tube, having a very small bivalve 

 shell attached to the thicker end, which serves to protect the 

 mouth and the internal organs of the animal. Some large species 

 of Ship-worm are quite a foot in length ; others not more than 

 6 inches. 



The Cockle has a large, muscular foot, pointed at the tip and 

 bent in the middle. This is used not for burrowing or boring, 

 but for locomotion. By its aid the Cockle progresses over the 

 ground by a series of astonishing leaps and bounds. These mol- 

 luscs frequent sand-banks on the margin of the sea, and when 

 the water retreats, leaving them uncovered, they bury themselves 

 in the sand. When the sea returns the Cockles emerge and hurry 

 down to meet the incoming tide, hopping and bounding along on 

 their sturdy bent " foot " in the most amusing manner. 



The largest of the bivalves are the great Clams (Tridacnidse) ; 



