The Conchifera differ from the Molluscous animals that bear shells,
in a very great degree; for, although the substance of the body is
soft, unlike the Mollusca it is inarticulate, always enclosed in a
shell of two valves, without head or eyes,—a mouth, if it may be so
called, concealed from view, and without any hard parts, and the whole
body enveloped in a large mantle, or hood, formed of two thin lobes,
generally perfectly free, but at times united in front; these are the
principal distinguishing characters of this class.
In earlier systems, when shells were classed without much reference
to the animals that inhabited them, the only distinction made was the
number of pieces of which the shell was formed, and they were arranged
under the heads of _univalves_, of one piece, _bivalves_, with two
pieces, and _multivalves_, with more than two pieces. This arrangement
was inconvenient, as, in some cases, it separated animals that
otherwise agreed with each other. With respect to the bivalve shells,
however, this objection does not hold good, as they all contain animals
belonging to the class Conchifera.
The individuals of this class appear to be deprived of all the senses
except that of feeling. Their powers of motion have been so well
described by Dr. Roget, in his _Bridgewater Treatise_, that we cannot
do better than extract a portion from that interesting work.
[Illustration: Valves of the _Unio Batava_, with the Connecting
Ligament.]
The two valves of the shell of the Conchifera are united at the back by
a hinge-joint, often very artificially constructed, having teeth that
lock into each other; and the mechanism of this articulation varies
much in different species. The hinge is secured by a substance of great
strength.
During the life of the animal, the usual and natural state of its shell
is, that of being kept open for a little distance, so as to allow of
the ingress and egress of the water necessary for its nourishment
and respiration; but, as a security against danger, it was necessary
to furnish the animal with the means of rapidly closing the shell,
and retaining the valves in a closed state. These actions, being
only occasional, yet requiring considerable force, are effected by a
muscular power, for which purpose sometimes one, sometimes two, or even
a greater number of strong muscles are placed between the valves, their
fibres passing directly across from the inner surface of the one to
that of the other, and firmly attached to both. They are named, from
their office of bringing the valves towards each other, the _adductor
muscles_.
[Illustration: Section of an Oyster, showing the situation of the
Hinge, L., the Adductor Muscle, A, and the transverse direction of its
Fibres, with respect to the Valves.]
The simple actions of opening and closing the valves, are capable of
being converted into a means of retreating from danger, or of removing
to a more commodious situation, in the case of those bivalves which are
not actually attached to rocks, or other fixed bodies.
Diquemarc long ago observed, that even the Oyster has some power of
locomotion, by suddenly closing its shell, and thereby expelling the
contained water with a degree of force, which, by the reaction of the
fluid in the opposite direction, gives a sensible impulse to the heavy
mass. He notices the singular fact, that Oysters which are attached to
rocks occasionally left dry by the retreat of the tide, always retain
within their shells a quantity of water sufficient for respiration, and
that they keep the valves closed till the return of the tide; whereas,
those Oysters which are taken from greater depths, where the water
never leaves them, and are afterwards removed to situations where they
are exposed to these vicissitudes, of which they have had no previous
experience, improvidently open their shells after the sea has left
them; and, by allowing the water to escape, soon perish.
[Illustration: _The Cardium, or Cockle._]
Many bivalve Mollusca are provided with an instrument shaped like a
leg and foot, which they employ extensively for progressive motion. In
the _Cardium_, or cockle, this organ is composed of a mass of muscular
fibres, interwoven together in a very complex manner, and which may
be compared to the muscular structure of the human tongue; the effect
in both is the same, namely, the conferring a power of motion in
all possible ways; thus it may be readily protruded, retracted, or
inflected at every point.
The _Solen_, or razor-shell fish, has a foot of a cylindrical shape,
tapering at the end, and much more resembling in its form a tongue
than a foot. In some bivalves, the dilatation of the foot is effected
by a curious hydraulic mechanism; the interior of the organ is formed
of a spongy texture, capable of receiving a considerable quantity of
water, which the animal has the power of injecting into it, and of thus
increasing its dimensions.
The foot of the _Mytilus edulis_, or common mussel, can be advanced
to the distance of two inches from the shell, and applied to any
fixed body within that range. By attaching the point to such body,
and retracting the foot, this animal drags its shell towards it, and
by repeating the operation successively on other points of the fixed
object, continues slowly to advance.
This instrument is of great use to such shell-fish as conceal
themselves in the mud or sand, which its structure is then peculiarly
adapted for scooping out. The cockle continually employs its foot for
this purpose: first, elongating it, directing its point downwards, and
insinuating it deep into the sand, and next, turning up the end, and
forming it into a hook, by which, from the resistance of the sand, it
is fixed in its position, and then the muscles, which usually retract
it, are thrown into action, and the whole shell is alternately raised
and depressed, moving on the foot as on a fulcrum. The effect of
these exertions is to drag the shell downwards. When the animal is
moderately active, these movements are repeated two or three times in a
minute. The apparent progress is at first but small, the shell, which
was raised on its edge at the middle of the stroke, falling back on
its side at the end of it; but when the shell is buried so far as to
be supported on its edge, it advances more rapidly, sinking visibly
at every stroke, till nothing but the extremity of the tube can be
perceived above the sand.
By a process exactly the inverse of this, that is, by doubling up the
foot, and pushing with it downwards against the sand below, the shell
may be again made to rise by the same kind of efforts which before
protruded the foot. By this process of burrowing, the animal is enabled
quickly to retreat when danger presses, and when this is past, it can,
with equal facility, emerge from its hiding-place.
The _Cardium_ can also advance at the bottom of the sea, along the
surface of the soft earth, pressing backwards with its foot, as a
boatman impels his boat onwards by pushing with his pole against
the ground in a contrary direction. It is, likewise, by a similar
expedient, that the Solen forces its way through the sand, expanding
the end of its foot into the form of a club.
The _Tellina_ is remarkable for the quickness and agility with which it
can spring to considerable distances, by first folding the foot into a
small compass, and then suddenly extending it, while the shell is, at
the same time, closed with a loud snap.
The _Pinna_, or marine mussel, when inhabiting the shores of
tempestuous seas, is furnished, in addition, with a singular apparatus
for withstanding the fury of the surge, and securing itself from
dangerous collisions, which might easily destroy the brittle texture of
its shell. The object of this apparatus is, to prepare a great number
of threads, which are fastened at various points to the adjacent rocks,
and then tightly drawn by the animal, just as a ship is moored in a
convenient station, to avoid the buffeting of the storm. The foot of
this bivalve is cylindrical, and has, connected with its base, a round
tendon, of nearly the same length as itself, the office of which is to
retain all the threads in firm adhesion with it, and concentrate their
power on one point. The threads themselves are composed of a glutinous
matter, prepared by a particular organ. They are not spun by being
drawn out of the body, like the threads of the silkworm, or of the
spider, but they are cast in a mould, where they harden, and acquire a
certain consistence before they are employed. This mould is curiously
constructed; there is a deep groove which passes along the foot, from
the root of the tendon to its other extremity, and the sides of this
groove are formed so as to fold and close over it, thereby converting
it into a canal. The glutinous secretion, which is poured into this
canal, dries into a solid thread; and, when it has acquired sufficient
tenacity, the foot is protruded, and the thread it contains is applied
to the object to which it is to be fixed, its extremity being carefully
attached to the solid surface of that object. The canal of the foot is
then opened along its whole length, and the thread, which adheres by
its other extremity to the large tendon at the base of the foot, is
disengaged from the canal. Lastly, the foot is retracted, and the same
operation is repeated.
Thread after thread is thus formed, and applied in different directions
around the shell. Sometimes the attempt fails, in consequence of
some imperfection in the thread; but the animal, as if aware of the
importance of ascertaining the strength of each thread, on which its
safety depends, tries every one of them as soon as it has been fixed,
by swinging itself round, so as to put it fully on the stretch; an
action which probably also assists in elongating the thread. When once
the threads have been fixed, the animal does not appear to have the
power of catting or breaking them off. The liquid matter, out of which
they are formed, is so exceedingly glutinous as to attach itself firmly
to the smoothest bodies. It is but slowly produced, for it appears that
no Pinna is capable of forming more than four, or at most five threads,
in the course of a day and night. The threads which are formed in
haste, when the animal is disturbed in its operations, are more slender
than those which are constructed at its leisure. In Sicily, and other
parts of the Mediterranean, these threads have been manufactured into
gloves, and other articles, which resemble silk.
The number of muscles by which the shells are moved have caused this
Class to be divided into two orders: the Bimusculosa, in which there
are two pair of muscles to perform this office; and the Unimusculosa,
with only one pair.
_ORDER UNIMUSCULOSA._
The Conchiferous animals which possess but one pair of muscles, are
much more limited in number than those which possess two or more; but
they contain in their ranks several well-known and useful species, as,
for instance, the oyster, the mussel, and the animal which produces the
oriental pearl.