A piddock shell may measure over 15 cm. It is whitish or pale grey in colour, but often discoloured by bits of whatever it has been boring into; for it is a most persistent and versatile borer. ‘The common piddock bores into sand, peat, marl, wood, including submerged tree trunks, shale, slate, chalk, red sandstone and schists . . .’ The purpose of this activity is to provide itself with a safe shelter, which it achieves at the cost of making itself a prisoner for life, communicating with the outside world and feeding by the sole means of its double siphon.
