Pvn H,i I'UitlS

262 CARDITID^.
Zostera marina. I have also seen them at Balta clus-
tering in vast profusion round the base of the pegtopshaped
plants of Himanthalia lorea, or the common
strapweed; and my friend Mr. Norman brought me
one day a small hand-net nearly full of them, which he
had captured by sweeping the seaweeds at low water on
the Whalsey Skerries. There must have been millions
in this heap. The animal is a fearless or hardy little
creature, and very active. When put into a saucer of
sea-water,
it immediatelv stretched out its white and
slender foot, attaching the point to the surface of the
vessel, and walked about with tolerable rapidity. I
have observed it in a rock-pool, after having thus shifted
its quarters, drawing in the foot and mooring itself to a
but almost
stone, or leaf of a seaweed, by its strong
transparent byssus. Under a microscope the gills may
be seen through the shell, when it is of a lighter colour
and therefore more transparent than usual, to flap like
those of a fish. It swims freely in an inverted position,
the beaks of the shell downwards, and the point of the
foot expanded. The motion of swimming seems to be
effected by a series of muscular contractions parallel with
the line of progress, the point only of the foot being so
employed. The gills are then distinctly visible through
the open folds of the mantle, and consist of two pairs ;
the outer pair are rather larger and higher than the
inner pair.
The mantle appears to have two folds, the
inside one being the smaller. When the animal is
creeping, the foot is folded inwards for its whole length.
It is then filled with water through a wide and open
slit in the heel or base, and becomes of a tubular or
cylindrical shape. Several individuals being confined
in the same vessel, after restlessly and actively moving
about for some time, assembled in groups, young and