the scallops. Scallops of the genus Chlamys are equally poor swimmers and, unlike adult Pecten, spin small byssal threads for attachment to the bottom.
Burrowing in sand and mud is accomplished by the foot of the bivalve. The principle is the same in all digging clams. The foot is slowly protruded with the pointed tip wriggling down into the mud. During this extension the end of the foot is kept small, but when it reaches its greatest extension the end is suddenly swelled into a great bulb and the whole foot becomes very rigid. This is accomplished by injecting blood into the foot. The bulbous end serves as an anchor while the clam withdraws the foot and pulls its entire shell deeper into the mud. In the case of the Razor Clam, this action is accompanied by a jet of water against the mud ahead. The dislodged mud is washed up the sides of the shell and out the burrow. The action is similar to the pile driver that opens a way for the pile by a somewhat similar stream of water.
Because of its long, powerful foot the Razor Clam is capable of leaping. Generally, when this clam is lying on the surface of the mud, the foot is bent back under the shell and is then suddenly made rigid with the result that it is straightened out with great rapidity. In some cases the animal may turn itself end over end.
Many types of clams are especially adapted to boring into hard clay, shale, sandstone and concrete. The Date Mussels, Lithophaga, possess acid-secreting glands as an aid to penetrating limestone. The shell of the clam would of course be dissolved by this acid were it not for the thick, protective covering of periostracum. The Saxicave Borers, Hiatella, may live attached by a byssus on surfaces that they cannot penetrate, or they may bore into soft rock. The boring of the adults is wholly mechanical and is accomplished by rubbing the edges of their shell valves against the rock. Hiatella stays near enough to the surface to allow its siphons to protrude just outside its cone-shaped burrow. In three years the burrow is only three fourths of an inch in length and, after eight years of constant grinding and new gowth of shell, it is only one and a half inches in length.
The shipworms burrow long distances into the wood but retain contact with the “outside world” by means of the long fleshy, tube-like extension of the body. Boring is accomplished by the two valves at the anterior end of the tunnel. The denticulated ridges of the shell are the cutting tools, and the foot and muscles aid in rotating the shells back and forth. Burrowing may progress at the rate of as much as four inches a month. In the genus Bankia, at the posterior end of the worm-like animal there are two tiny, feathery pallets. These are used to plug the entrance of the burrow, thus giving protection from enemies, changes in salinity or other adverse conditions. When the shipworm is undisturbed, the pallets are drawn inside and the siphons extended into the water for breathing and feeding.
