Shell-Bearers 



tunnellers that they are the cause of considerable damage 

 in one way and another. 



The piddock, an exceedingly common bivalve, with a 

 delicate shell whose surface is somewhat reminiscent of 

 that of a file, is, despite its fragile shell, a rock burrower. 

 The precise method by which this creature makes its 

 home in the rock is not known, but it is probably accom- 

 plished by the foot, aided by the rasp-like surface of the 

 fore part of the shell. Once within the burrow, the mollusc 

 never leaves it, and so for the remainder of its life is 

 amply protected against its enemies. That this industrious 

 and notorious bivalve plays a considerable part in the 

 erosion of our coasts, especially where the coastal cliffs 

 are formed of chalk or limestone, there can be no doubt. 

 So riddled with holes do the cliffs become, where the 

 piddock is common, that the destructive action of the 

 waves is far more effective than would be the case were 

 the cliffs undamaged. 



There are other molluscs possessed of this strangely 

 unaccountable habit of boring into rocks. One of them is 

 absolutely impartial as to the kind of rock it attacks. 

 Kentish rag, clay ironstone and Portland stone all come 

 alike to the mollusc, for it tunnels them all with pits five 

 or six inches long. It does not appear to have any sense 

 of direction, for its borings point all ways, in striking 

 contrast to those of the piddock, which are always 

 approximately vertical and parallel to one another. As 

 may be guessed, one result of this indiscriminate boring 

 is that the tunnel of one animal frequently opens into that 

 of its neighbour. But the molluscs are absolutely devoid 

 of all sense of politeness, and they simply bore on and on, 

 with the result that one individual will surely bore through 

 the shell and body of its neighbour. 



Of all the burrowing molluscs by far the worst enemy 

 of mankind is the shipworm, which makes tunnels of no 

 inconsiderable dimensions along the grain of wood-piles, 

 the bottoms of wooden ships, etc. The adult shipworm 



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