1, ventral aspect, with animal; 2, dorsal side of shell showing accessory valves
The animal inhabiting the shell is somewhat wormlike in general form, and the mantle lobes are united in front—that is at the lower end of the shell as it lies in the burrow—except that an opening is left for the protrusion of the short foot. The siphons are united and much elongated, so that they protrude beyond the mouth of the burrow when the animal is active; the gills are narrow, and extend into the exhalent siphon; and the anterior 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 it 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 undoubtedly 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 burrow 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 débris. 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 sediment 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 into pieces with a hammer, thus laying open the burrows so that the molluscs fall from their places.
The Common Piddock (Pholas dactylus) may be identified by the illustrations, and the other members of the family may be recognised at once by the similarity in structure and habit. The principal species are the Little Piddock (P. parva), the shell of which is wider in proportion to the length, with only one accessory valve; and the White Piddock (P. candida), also with a single accessory. In all the above the foot is remarkable for its ice-like transparency.
Fig. 133.—Pholas dactylus, interior of Valve; and Pholadidea with Animal