Page:Natural History, Mollusca.djvu/282

270 abrupt elbow near the middle. These animals inhabit sandy or muddy shores, in which they live immersed at a moderate depth, or covered only by the tide at high-water. The powerful and versatile organ known as the foot is admirably adapted for the situations in which they reside. This organ in the present family is developed to a very great size, and is moved by an immense number of muscles of the most elaborate structure and arrangement, as will be seen by the preceding engraving of the dissected foot in a native species, Cardium rusticum.

In order to enable the animal to burrow in the sand, the foot, by means of its conical point, is thrust beneath the surface. When stretched to its utmost, it is bent into a hook, the whole organ is then forcibly contracted, and the animal, shell and all, is thus dragged a little way down into the yielding sand. This process is successively repeated, until the animal is sufficiently buried. But the foot can be made an instrument of locomotion, of quite another kind. The Cockle is a vigorous leaper, having been known to clear the gunwale of a boat when laid on the bottom boards. In order to effect this, the end of the foot bent into a hook is pressed firmly against the plane on which the animal is lying, in the position represented in the following engraving. The foot is then stiffened, when a sudden spring-like action of the elongating muscles jerks the animal into the air, with a force and agility that cannot fail to surprise those who witness it for the first time.

The Cockles are widely distributed, being found in nearly every sea, but their great metropolis is in the Indian Ocean, whence, as from a centre,