270 DIMYARIA,—CARDIADZA. 
abrupt elbow near the middle. These animals in- 
habit 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 1s moved by an immense number of 
muscles of the most elaborate structure and arrange- 
ment, 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 re- 
peated, until the animal is sufficiently buried. 
But the foot can be made an instrument of loco- 
motion, of quite another kind. The Cockle is a 
vigorous leaper, having been known to clear the 
eunwale 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 repre- 
sented 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, 
