200 THE SEA SHORE 



adductor muscle, being very near the umbones, serves the double 

 purpose of adductor and ligament. 



Such are the general distinguishing features of this family, all 

 the species of which burrow into stone or other material. Those 

 more commonly met with on our coasts belong principally to the 

 genus Pholas, and are popularly known as Piddocks. 



It was long a puzzle as to how the fragile piddocks could 

 excavate the tubular burrows in which they live, and, since their 

 shells are so thin that 5t seemed almost impossible for hard stones 

 to be ground away by them, it was suggested that the rocks were 

 excavated by the action of an acid secretion. This, however, would 

 not account for the formation of holes in sandstone and other 

 materials which are insoluble in acids ; and, as a matter of fact, 

 no such acid secretion has ever been discovered. The boring is un- 

 doubtedly done by the mechanical action of the rasp-like shell, 

 which is rotated backwards and forwards, somewhat after the 

 manner of a brad-awl, though very slowly, by the muscular action 

 of the foot of the animal. 



Piddocks are found principally in chalk and limestones, though, 

 as before hinted, they are to be seen in sandstones and other rocks, 

 the material in any case being, of course, softer than the shell that 

 bores it. The largest holes and the largest specimens are to be 

 found in chalk and other soft rocks ; while the piddocks that bur- 

 row into harder material are unable to excavate to the same extent 

 and are, as a consequence, more stunted in their growth. The 

 burrowing is continued as long as the animal grows, the hole being 

 always kept at such a depth that the shell is completely enclosed ; 

 and not only this, for when the rock is soft, and the surface is 

 worn down by the sea, the piddock has to keep pace with this 

 action, as well as to allow for its increase in size. 



As a result of the rasping action of the pholas shell on the 

 surrounding rock the space hollowed out becomes more or less 

 clogged with debris. This is ejected at intervals by the sudden 

 contraction of the foot of the animal, which brings the shell quite 

 to the bottom of the burrow, thus causing the water with its sedi- 

 ment to shoot upwards, 



It is not usually an easy matter to obtain perfect specimens of 

 the pholas by simply pulling them from their burrows, the shells 

 being so thin and fragile, and the mouth of the burrow being often 

 narrower than the widest part of the shell. The best plan is to 

 chip away the rock with the aid of a mallet and chisel, or to break it 



