SEA-SHELLS ON THE SEASHORE
ANY hard coat or covering enclosing a softer material is called a
"shell"--thus we speak of an egg-shell, a nut-shell, a bomb-shell,
and the shell of a lobster. But there is a special and restricted use
of the word to indicate as "true" and "real" shells the beautiful
coverings made for their protection by the soft, mobile animals called
Molluscs. These animals expand and contract first this and then that
region of the body by squeezing the blood within it (by means of the
soft muscular coat of the sac-like body) into one part or another
in turn. There is not enough blood to distend the whole animal, and
accordingly one part is swollen out and protrudes from the shell,
whilst another shrinks as the blood is propelled here or there by the
compressing muscular coat. These creatures are the Molluscs, a name
which has come into general use (and has even served as the title for
a stage-play), as well as being the zoologist's title for the great
division of animals which they constitute.
They are sometimes called "shell-fish," but this is no good as a
distinctive name--since it is applied in the fish-trade to lobsters,
crabs, and shrimps as well as to Molluscs. Lobsters, crabs, and shrimps
are Crustacea, and totally different in their architecture and their
mechanism from Molluscs. Familiar examples of Molluscs are the oyster,
the mussel, the various "clams," and, again, the snails, periwinkles,
whelks, and limpets. It is the shells of these animals which are
"true" shells in the sense in which the word is used by "collectors"
of shells, and in the sense in which we speak of "the shells of the
seashore." These shells are usually very hard, solid things, made up
of layers of lime-salts and horny matter mixed, and they remain for a
long time undestroyed, washed about by the currents of the sea, and
thrown up on to the beach, after the soft, oozy creature which formed
them--chemically secreted them on its soft skin--has decomposed and
disappeared. They are readily distinguished into two sorts--(1) those
which are formed in pairs, or "bivalves," each member of the pair being
called a "valve"; and (2) those which are single, or "univalves," often
spirally twisted, as are those of snails and whelks, but sometimes
cap-like or basin-like, as are the shells of the limpets. There is not
so great a difference between bivalve and univalve shells as there
seems to be at first sight. For if you examine the pair of shells of
a mussel or a clam when they are quite fresh, you will find that the
valves are joined together by a horny, elastic substance, and are, in
fact, only one horny shell, or covering, which is made hard by lime
deposited on the right and on the left, as two plates or valves, but
is left soft and uncalcified along a line where these two valves meet,
so as to allow them to move and gape, as it were, on an elastic hinge.
It is the fact that the two valves of the shell of the bivalve, lying
right and left on its body, correspond to the single shell of the snail
or limpet, which differs from the bivalve-shell in not being divided
along the back by a soft part into right and left pieces. That there
is this real agreement between bivalve and univalve molluscs is quite
evident when we examine the soft animal which forms the shell and is
protected by it.
[Illustration: FIG. 19.--Some British Marine Bivalve Molluscs.
a, The smaller Piddock, Pholas parva, which bores into chalk, clay, and
hard rock. Natural size.
b, The Razor-shell, Solen siliqua. The muscular foot is seen protruding
from the shell. One-third the natural size, linear.
c, Venus verrucosa. Natural size.
d, Cardium echinatum. Two-thirds the natural size, linear.
e, Pinna pectinata, the "cappy longy." One-fifth of the natural size,
linear.]
Though "shells" are often numerous on parts of the seashore, some
beaches (as, for instance, at Falmouth, at the mouth of the Eden of St.
Andrews, and at Herm in the Channel Islands) being so placed in regard
to the currents and waves of the sea that great quantities of shells
of dozens of species are thrown up, and even "make up" the beach, yet
there are not so very many Molluscs which live commonly on the shore
between tide-marks. The shells which are accumulated as shell-beaches
have come from animals which lived in quantity at depths of ten or
twenty fathoms, whence they can be brought up alive by the dredge.
There are, however, certain bivalves and certain univalves which are
commonly to be found in the living state between tide-marks. You will
not find the oyster there on our own coast, but in Australia they have
picnic parties where every guest provides himself with a hammer and a
bottle of vinegar and a pepper-pot, and at low tide proceeds to chip
the oysters off the rocks on which they grow tightly fixed, and to eat
them "right away" before they have time to lose their good temper and
sweetness! In Jamaica they show you oysters apparently growing on trees
high up in the air, but they are dead, having attached themselves to
the branches of a young tree which dipped into the water. Once fixed
there, they were unable to move as the tree grew and carried them up
with its branches above the sea-level.
The only bivalve at all common and visible to the eye between
tide-marks is the common or edible sea-mussel, which is attached
in purple clusters to the rocks (as in North Cornwall), or forms a
wide-spreading pavement, called a "scalp," of as much as an acre in
extent, on which thousands of mussels lie side by side. But by digging
in the sand and mud between tides there are other living bivalves to be
found, which burrow more or less deeply. The razor-shell (Fig. 19, b)
is one of these (see p. 80). Often (as at Teignmouth and Barmouth) we
find "cockles" buried in the sand, and those delicate, smooth bivalves
not an inch long, white outside and purple within, which are made into
soup at Naples and are called "vongoli," but have no English name.
Other "clams" (Tapes, which is eaten in France, even in Paris, and Mya,
and Scrobicularia which lives in black mud) may be dug up, but they are
devoid of English names because we do not eat them; hence I have to
speak of them by their Latin scientific names. As to univalves, there
are three which are found almost everywhere on our coasts where there
are rocks, namely, the periwinkles (one species of which actually
lives above high tide-mark), the limpet, and the dog-whelk. A small
species of top-shell or trochus is also very common, and so is the
chiton, or armadillo-shell, which, though really the most primitive
and nearest representative of the ancestors of all univalve molluscs,
yet has its own shell of a very peculiar character (sometimes with
very minute eyes--true eyes--dotted about on it), and always divided
transversely to its length (not right and left) into eight separate
pieces, which, indeed, seem to be really separate, independent little
shells, corresponding to eight segments like the segments of a shrimp
or an earth-worm.
Let us now compare the soft animal of one of the bivalves--say
the common cockle--with the soft animal to which a univalve shell
belongs--say the limpet. They can be kept alive and watched in a
finger-glass of sea-water, and can be removed from their shells and
examined more closely--by killing them by dipping them for half a
minute into very hot (not boiling) water. Both these molluscs--like
all others--adhere tightly at one place to the shell. They cannot be
removed from it alive, and make a new shell or creep back into the old
one, as can some worms (_e.g._ the serpula) and other creatures which
form a hard shell to live in. Certain muscles of the soft mollusc
are so closely fixed to the shell that they must be torn in order to
separate it. These muscles draw the two valves of the bivalve together,
and shut it tight. You can verify this whenever the oyster-man "opens"
an oyster for you. When at rest the shells gape, being kept open by the
horny, elastic hinge-piece. Some bivalves (for instance, the common
scallop, or pilgrim's shell, which can often be dredged in shallow
water, and of which a large kind is sold in the London fish shops)
actually swim in the sea-water by aid of this mechanism, the shells
opening by elasticity and being closed by the muscle joining one to the
other, at rapid intervals, flapping like the wings of a butterfly.
In the univalves the attachment of the muscle to the shell gives a
fixed point for all the movements of the animal. The limpet has a
well-marked head and neck--a pair of sensitive tentacles, and a small
pair of dark-coloured eyes. The mouth is at the end of a sort of short
snout. Just within the mouth, and capable of being pushed forwards to
the level of the lips, is a most extraordinary rasp. It consists of a
long ribbon, beset with fine horny teeth--very sharp and complicated in
pattern. The ribbon extends far back into the body, and is worn away by
constant use at the orifice of the mouth. It grows forward, like one
of our finger-nails, as it wears out, and a new, unworn portion takes
the place of that worn away. It is constantly in use to rasp and bring
into the mouth the particles of the seaweed on which the limpet feeds.
It is easy to remove this rasping ribbon with a needle or pen-knife,
and examine it with a microscope. Every one of the hundreds of kinds of
univalve molluscs has this ribbon-rasp, and its teeth are of different
patterns in the various kinds. It is worked by very powerful little
muscles, backwards and forwards, and is strong enough in the whelks to
bore a round hole into other shells (for instance, that of the oyster),
when the whelk proceeds to eat the soft animal, whose protecting shell
has been thus penetrated. Some of the large marine snails produce a
poisonous secretion from the mouth, which renders their attack with
the ribbon-rasp all the more deadly to other marine creatures. The
cuttle-fishes and octopods, which are molluscs too, possess, like the
univalve limpets, snails, and whelks, this terrible ribbon-rasp in
the mouth. It is an indication of a common parentage or ancestral
relationship in the forms which possess it.
The cockle (Fig. 19, d), to which we now turn, has not got a
ribbon-rasp, nor anything of the kind. It has a mouth with four
flapper-like lips, but no projecting head, no eyes, no biting
mechanism, nor have any of the bivalves, excepting a few which like the
scallop have a series of eyes on the edge of the soft mantle or flap
which lines the shell. This constitutes a greater difference between
bivalves and the univalves than does the shape of the shell. They are
a very quiescent, peaceful lot, feeding on microscopic floating plants
(diatoms and such), which are drawn to the mouth by currents of water
set going by millions of vibrating hairs arranged on four soft plates
hanging under the protecting arch of the shell, and called in the
oyster--in which bivalve most people know them--the "beard."
The limpet adheres to rocks by a great disk-like mass of muscle, which
is called "the foot." It is really the whole ventral surface, and it
can loosen its hold, and, by curious ripples of contraction, cause
the animal to creep or glide over the rock. At low tide the limpet is
exposed to the air, and remains motionless, but when the tide is up
it makes a small excursion in search of food, never going more than a
foot or two from the spot which it has chosen, and returning to it,
so that in the course of time it actually wears away a sort of cup or
depression at this spot--if the rock is not of exceptional hardness.
The word "foot" is applied to the ventral disk-like surface of the
limpet, because in many univalves this region becomes drawn out, and
is connected by a comparatively narrow and nipped-in stalk or pillar
with the rest of the animal. This occurs in the univalves which have
large spiral shells, into which the whole of the soft animal can be
deeply withdrawn, which is not the case with the limpet. You may find
on the shore at Torquay a sea-snail (Natica), in which the animal is
quite invisible, drawn far up into the shell. Place this in sea-water
and watch it. Soft semi-transparent lobes begin to issue from the
mouth of the shell, part of the soft distensible foot appears swelling
out and growing bigger and bigger, and soft folds spread out from the
mouth of the shell, and gently creep over it, and completely envelop
it; the foot begins to grip the bottom of the vessel, and the animal
"crawls." At last, swelling out from the other folds of soft but tense
"molluscan" substance, the head and its tentacles emerge. Touch the
animal and it shrinks rapidly, disappearing into the shell.
It used to be thought (about twenty-five years ago) that the molluscs
expand their bodies in this manner by taking water, through definite
apertures provided with valves, into their blood, and that, having
thus swelled themselves out, they could shrink and reduce themselves
by pouring out again the in-taken water. The behaviour of some other
marine animals, namely the sea-anemones, which really do act in this
way, made this explanation of the swelling and shrinking of molluscs
seem probable. It was also known that the star-fishes and sea-urchins
actually do take in the sea-water into a system of vessels connected
with their wonderful sucker-bearing tentacles. But it turned out on
close examination that the molluscs do not take in or shed out water in
this way. A hole, which was thought to let in water into the blood of
sea-snails, was shown to be only the opening of a great slime-gland.
In the case of some bivalves which have red-blood corpuscles, I showed
that the blood is never made paler, nor are the red corpuscles shed
during the great distensions and contractions of the body. Measurements
were made to determine the removal of water from a glass jar by an
expanding sea-snail, and it was found that none is removed or taken
up; in fact, the whole of what is very often an astonishingly large
and bulky distension of the foot, or of lobes of the body, and the
subsequent rapid shrinking of the same parts, depend entirely on the
blood being injected from the rest of the body into the swelling part,
and squeezed from it into the depleted region when the swollen part
shrinks again. The firm, opaque shell hides from view the change of
shape of the concealed body, and we see only the distended foot or
other lobes which project from the shell.
The cockle has a "foot" of a very curious scythe-like shape, usually
carried bent up between the two valves of the shell. Those who
rightly like to confirm statements about unfamiliar animals can do
so by buying a cockle or two at the fishmonger's. Some bivalves (the
Noah's-ark-shell, called "Arca," and a few others) have a great flat
foot, like that of the univalves, and crawl about on it. But in most
bivalves it is curiously elongated and modified, for the purpose of
burrowing into sand by vigorous strokes, and in some it is suppressed
altogether, as in the oyster. The cockle is remarkable for the fact
that when placed on a board or a rock it will give such a vigorous kick
with its bent foot as to throw itself up a yard or so into the air. A
naturalist (Stutchbury) dredging in Port Jackson, Australia, many years
ago was overjoyed at discovering in his net three specimens of a very
peculiar kind of cockle (Trigonia), which was till then only known in
the fossil state from the oolite strata of Europe. He placed the three
novelties on the seat of his boat, and was looking at other things
when he heard a click-like sound, then another. He turned his head
and saw that two of his newly-discovered "living fossils" had jumped
overboard, and had the pleasure of seeing the third perform the same
feat!