Pvn H,i I'UitlS

ISOCARDIA. 301
peated at unequal intervals during the whole time my
specimens were under examination, but at shorter inter-
vals on receiving fresh supplies of sea-water, when, I
suppose, food (its quality I could not ascertain) was
more abundant. The animal appears to be insensible
both to sound and light, as the presence or absence of
either did not at all interrupt its movements; but its
sense of feeling appeared to be very delicate : minute
substances being dropped into the orifice of the mantle
instantly excited the animal, and a column of water
strongly directed expelled them from the shell. With
so much strength was the water in some instances
ejected,
that it rose above the surface of three inches of
superincumbent fluid. Animal small in proportion to
its shell, occupying when dead barely
a third of the
space enclosed in the valves. Its mantle is slightly
attached to the shell and to the epidermis at the margin,
and appears to be kept distended, and in contact with
the interior of the valves, bv the included water. The
valves fit so closely that the animal can remain two
days or more without permitting a single drop
of fluid
to escape. Locomotion is very confined ; it is capable,
with the assistance of its foot, which it uses in the same
manner (but in a much more limited degree) as the
Cardiacea, of fixing itself firmly in the sand, generally
choosing to have the umbones covered by it, and the
orifices of the tubes of the mantle nearly perpendicular.
Uesting in this position on the margin of a sand-bank,
of which the surrounding soil is mud, at too great a
depth to be disturbed by storms, the Isocardia of our
Irish Sea patiently collects its food from the surround-
ing element, assisted in its choice by
the current it is
capable of creating by the alternate opening and closing
of its valves. Some of the specimens that had been