Petunculus.
a. Foot.
The foot of the lamellibranchiates presents a great variety of form, and is found in various degrees of development, gradually passing into a rudimentary state, until finally it is completely wanting in the oyster family. In most of those which live at large it is strong and muscular, serving either as an excellent spade for speedy concealment in the sand when an enemy approaches, or to dig a furrow into which the animal forces itself partially, and then advances slowly by making slight see-saw or balancing motions, or even to jump along with tolerable rapidity. Thus, the common Cockle protrudes its foot to its utmost length, bending it and fixing it strongly against the surface on which it stands; then by a sudden muscular spring it throws itself into the air, and, by repeating the process again and again, hops along at a pace one would hardly expect to meet with in a shell-bound mollusc.
Even some of those which have but a very rudimentary foot, incapable of subserving locomotion, are able to move from place to place by the sudden opening or shutting of their valves. In this manner the scallop, which inhabits deep places, where it lies on a rocky or shelly bottom, swims or flies through the water with great rapidity, and the file or rasp mussel, a closely related genus, principally occurring in the Indian Ocean, glides so swiftly through the water that the French naturalists Quoy and Gaimard were hardly able to overtake it.
Cockle.
a. Foot.
In the stone or wood-boring bivalves the functions of the foot with regard to locomotion are much more limited than in the Cockle, or Tellina, as they merely consist in moving the animal up and down in the cavity where it has fixed its residence. In the Razor-Shells, which will sometimes burrow to the depth of two feet, and very rarely quit their holes, the cylindrical foot, no longer fit for horizontal locomotion, serves the animal for rising or sinking in the sand, for when about to bore, it attenuates it into a point, and afterwards contracts it into a rounded form so as to fix it by its enlargement when it desires to rise.
In places where the razor-shells abound, they are sought after as bait for fish, and taken in spite of their mole-like facility of concealment, for when the tide is low, their retreat is easily recognised by the little jet of water they eject when alarmed by the motion of the fishermen above. Having thus detected their burrow, the wily enemy who is well aware that, though inhabiting the salt water, the Solen does not like too much of a good thing, merely throws some salt into the hole, which, sadly irritating the nerves of the poor creature, generally brings it to the surface. He must, however, be very quick in grasping it firmly, for should he fail, the animal speedily sinks again into the sand and will remain there, being either insensible to the additional irritation or its instinct of self-preservation teaching it to remain beneath.
The pholades, which have very delicate milk-white valves, burrow holes in limestone or sandstone rocks, though occasionally they content themselves with houses of clay or turf. How creatures invested with shells as thin as paper and as brittle as glass are able to work their way through hard stone has long been a puzzle to naturalists, some of whom asserted that they attained their object by means of an acid solvent, others that they bored like an auger by revolving; but recent investigations have discovered that their short and truncated foot is the chief instrument they use in their mining operations, being provided at its base with a rough layer of sharp crystals of flint, which, when worn off, are soon replaced by others, and act as excellent files.
