148 THE SEAS 



these worms all possess horny bristles which may assist 

 them in burrowing, it is probable that the greater part of 

 this work is done by chemical means. 



But, as in the case of the wood borers, the largest and most 

 efficient rock borers are bivalve Molluscs. Of these the 

 largest is the familiar " Piddock " (Plate 55) which is a 

 stout bivalve with a white, spiny shell sometimes as much 

 as six inches long. It is distantly related to the Shipworm 

 and more nearly to the other wood-boring Molluscs. It 

 bores by the rasping action of its shell, the rows of spines 

 with which this is furnished gradually cutting into the rock 

 until a burrow of anything up to one foot in length, in 

 the largest specimens, is constructed. The Piddock is 

 indifferent as to the nature of the stone into which it bores, 

 it has been found in limestone, sandstone, shale, mica- 

 schist, peat and, very occasionally, wood. In specimens 

 which bore into soft material the spines are long and pointed, 

 in those taken from harder rock they are round and blunt. 

 During the operation of boring, the head of the burrow 

 is held by the sucker-shaped foot, as in the Shipworm, 

 while the shell twists and rocks on this fulcrum. There 

 are limits, apparently, to the efficiency of this apparatus be- 

 cause the Piddock is never found boring in the very hardest 

 type of rocks. Moreover, though the head of the burrow 

 is certainly hollowed out by the mechanical action of the 

 shell, the hind end of the burrow also increases in width 

 as the animal grows so that it appears that there must be 

 some kind of chemical action for only the soft, fleshy siphons 

 are in contact with the rock in this region. A peculiar 

 feature of the Piddock is the fact that, though it lives 

 hidden away in rock, it is luminescent (see Chapter VIII). 



There are a number of different species of Piddocks, 

 all belonging to the genera Pholas, Barnea and Pholadidea, 

 which are very common, while there are a number of rather 



