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230 LUCINIM.
According to Valenciennes, the animal of Lucina has but
a single gill-leaflet on each side. Forbes and Hanley
state that there can be little doubt that two lamellse
on the same side are so united as to appear like a single
gill. Deshayes has shown that the two gills when
divided contain four rows of vessels, and that they
occupy as much space as all the gills in other Conchiferous
mollusks. As I do not pretend to be, like
Sidrophel, " old dog at physiology," I can say no more
than that I hope the question may be sooner or later
set at rest. The foot is a remarkable and peculiar
it looks like a
organ. When at rest and contracted,
shrivelled worm, and is doubled up within the shell ;
but
when the period for action has arrived, the wrinkles
disappear, and in a marvellously
short time the foot
expands and is drawn out to three times the length of
the whole body. It is thus rendered firm and more
capable
of muscular exertion. This distention is effected
by means of aquiferous ducts or canals, which permeate
the tissue of the foot, the water being introduced at its
base through a wide funnel opening directly into the
visceral cavity. A similar organization of the foot has
been noticed in the Cephalophorous mollusks, especially
in some of the Muricidce, Naticida, and Bullidae, which
have the same habit of burrowing as the Lucinidce.
Another peculiarity which characterizes the present
family is the elongation of the anterior adductor muscle ;
and it is easy to recognize the fossil species by the nature
of the scar or mark inside the shell. The ligament is
more or less external in all the genera but Loripes,
which has an internal cartilage concealed within the
hinge-line and occupying an oblique groove, as in Kellia.
The Lucinida inhabit sandy mud and gravel, in which
they shelter themselves. Very many recent species of