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CHAPTER II.
The molluscs proper—the Pholas family—the piddock—uses of the animal—general depiction of the shell—its rough surface and its apparent use—the "accessory valves," and their shape and object—similarity of structure in the Pholas and the Ascidians—method of boring—variety in size and average dimensions—colour of the piddock shell—method of procuring living specimens—various species of British piddocks, and their distinguishing peculiarities—the Pholadidea—why separated from Pholas—its curious structure; the cup and the fringe—presumed object of the cup—delicacy and fragility of the shell—consequent difficulty in procuring perfect specimens—average size of the shell—consequent difficulty in procuring specimens—average size of the shell—the ship-worms—the siphons and the pallets—value of the pallets in determining species—ravages of the ship-worm—its general structure and mode of boring—its metamorphosis when young—remarkable specimen of the ship-worm's tunnel—the wood-piddock.
We now pass to the Molluscs proper, and begin with those which have some resemblance in structure to the Ascidians.
The first group in our list is that which includes the various species of Pholas, the boring-shells which are so plentiful upon our rocky or chalky shores, and which are popularly known among fishermen as Piddocks. As the shell is better known than the animal, it will be first described.
At Plate I., fig. 12, is shown the common Piddock (Pholas dactylus), as it appears in the hole which it has excavated. The shell is beautifully white, and its whole surface is covered with deep grooves, sweeping in a bold hexagonal curve, and crossing each other so as to make the outer surface into a shelly rasp. The shells gape slightly above, and rather more below.
