MOLLUSKS
The class of the mollusks is a very large one, for at least fifty
thousand different kinds of these creatures are already known, while new
ones are constantly being discovered. They may be described as
soft-bodied, boneless animals, which are enclosed in a tough muscular
skin called the mantle. And they are divided into five orders, the first
of which includes the singular creatures known as squids, or cuttles.
You may sometimes find these animals hiding in the pools which are left
among the rocks when the tide goes out; and you can recognize them at
once by the long, fleshy tentacles, or arms, which spring from the upper
part of the head. Some of them have ten of these arms, and are called
decapods; the rest have only eight and are known as octopods. And the
lower surface of each arm is furnished with a row of circular suckers,
the grip of which is so powerful that the tentacle may even be torn in
two without causing it to release its hold. Indeed, if quite a small
cuttle were to seize you with one of its arms, you would not find it at
all easy to make it let go again without killing it.
The cuttles employ these suckers for two purposes. In the first place,
they use them in walking. When a cuttle is crawling along at the bottom
of the sea it pushes one or two tentacles forward, takes firm hold of a
rock or a large stone with the suckers underneath them, pulls up the
body, and then thrusts them forward again. And in the second place, they
use them in catching their prey. Quite large victims are often seized by
cuttles, and when once the deadly suckers have fastened upon them there
is no hope of escape. In spite of their struggles one tentacle after
another comes closing in, till they are completely surrounded by the
long, slimy arms, soft almost as jelly, yet strong as steel. Then they
are pushed down against the sharp, strong beak, by which they are
quickly torn in pieces.
On the upper part of the head of the cuttle there is another
curious organ known as the siphon, which consists of two tubes lying
side by side together, like the barrels of a double-barreled gun. This
organ is used in three different ways.
First, it is used in breathing. The cuttles, like the fishes, breathe
water, by means of gills. These gills lie inside the head, and the water
passes down to them through one of the siphon-tubes, and then out again
through the other.
Next, it is used in swimming. When cuttles are not in a hurry they crawl
along by means of their long tentacles, as we told you just now. But if
they are startled, or alarmed in any way, they fold all their tentacles
together in a straight line, fill both the siphon-tubes with water, and
then squirt it out again as hard as they possibly can. The result is, of
course, that they are driven rapidly backward by the recoil, just like
the dragonfly grub, of which we have read.
But the third use of the siphon-tubes is the most curious. If you
discover a small cuttle hiding in a rock-pool, you will very likely find
that the water all round it suddenly grows dark as night, just as if a
quantity of ink had been poured into it. The fact is this. Inside its
body the cuttle has a bag filled with a quantity of a deep-black liquid
called sepia. This bag is surrounded by strong bands of muscle, and
opens into the siphon-tubes. So, you see, when the animal suddenly
contracts the muscular bands, the sepia is squirted out through the
siphon into the water, which is immediately darkened for some little
distance all round. And under cover of the darkness the animal escapes.
The eggs of the cuttle are laid in a very curious way, for they are
fastened by little stalks to a stem of seaweed, so that they look very
much like a bunch of grapes. Fishermen, indeed, nearly always speak of
them as "sea-grapes."
The cuttles which are found in the British seas are always quite small.
But in some parts of the ocean these creatures grow to a giant size.
Fragments of the tentacles of an enormous cuttle, for instance, have
been found lying on the coast of Newfoundland; and by careful
calculation it was shown that if the animal to which they belonged had
stretched them out at right angles to its body, they would actually have
measured more than eighty feet from tip to tip!
These huge creatures seem to form the principal food of the
spermaceti-whale.
THE CHAMBERED NAUTILUS
This animal is a near relation of the cuttles. It lives in a shell,
which cannot increase in size. The mollusk itself grows, however, and
soon becomes too big to live in its home; so it forms a second and
larger compartment outside the first one. Time after time this happens,
till at last the shell consists of about thirty-six chambers, only the
outside one being inhabited by the nautilus.
This shell is often more than a foot in diameter. But if you were to see
it when it is first taken out of the sea you would never think that it
was a shell at all. Indeed it looks much more like a big shapeless lump
of blubber, for the animal covers it entirely with its muscular mantle,
so that the shell itself is completely concealed.
Very little is known of the habits of the chambered nautilus, for it
lives at the bottom of the sea, at a depth of two or three hundred
fathoms. It is found in various parts of the Indian and Pacific oceans.
GASTROPODS
A great many well-known creatures belong to this large group, first upon
the list being the slugs. We need not describe these animals, but
perhaps you will be surprised to hear that they have shells! These
shells are very small, however, and are entirely covered over by the
mantle, so that they cannot be seen unless the body is dissected.
Slugs have the most wonderful power of stretching out and drawing up
their bodies. You may see one of these creatures crawling about on a
damp evening, and measuring fully five inches in length. But at the
slightest touch it begins to contract, and in a few seconds it is just a
shapeless lump, scarcely half as long as it was before. The odd little
tentacles are drawn back into the head, and the head is drawn back into
the body so that if you did not happen to know what it was you
might easily mistake it for a pebble.
On the right-hand side of a slug's body, as it crawls along, you will
notice a rather large and almost round hole. This is the entrance to the
breathing-organs, which lie just behind the head and underneath the
mantle.
During the daytime slugs remain in hiding, lying behind the loose bark
of dead trees, or under logs and large stones, or in heaps of decaying
leaves. And if the weather is very hot and dry they do not come out even
at night, for they very soon die if they are deprived of moisture. But
on warm, damp evenings they travel for long distances in search of food,
which is almost entirely of a vegetable character. In Europe every
gardener knows what injury they do to gardens there, but in America the
slugs are practically harmless.
A good many different kinds of slugs are found in Great Britain. The
largest of all is the great gray slug, which often grows to a length of
more than six inches. Then the black slug is very common in many parts
of the country. It is not always black, however, for one may often find
examples which are brown, or yellow, or gray, or even white. The milky
slug, which has a thick creamy slime, is plentiful everywhere. And
sometimes one may dig up a very curious slug--testacella--which feeds on
earthworms, and follows them down to the very bottom of their burrows.
When the weather is cold, this slug makes a kind of cocoon of earth and
slime, and lies fast asleep inside it, often for many months at a time.
SNAILS
In many ways snails are very much like slugs, but they have a shell
large enough to contain the entire body when the animal withdraws inside
it. Several hundred different kinds of snails are found in North
America, and many more in other parts of the world, varying in size from
that of a small pinhead to that of a big walnut. Some are exceedingly
numerous, others so rare and singular in their living-places that they
are highly prized by conchologists. All snails lay eggs, usually in damp
soil; and if you will turn over an old log in the woods in
summer, you will be almost certain to find some of the minute shining
globules. When winter draws near all the snails go into hiding, and they
have a most curious way of closing the entrances to their shells by
making little doors across them, composed partly of slime and partly of
very small fragments of earth. This is in order to prevent the frosty
air from getting in and killing them. But it would never do, of course,
to keep all the air out, for in that case they would be unable to
breathe. So they always leave a tiny hole in the middle of each door,
through which just enough air can pass to prevent them from being
suffocated.
Among the largest of all is the edible snail, which is largely used for
food in many parts of Europe and is imported into the United States and
pickled, to be eaten by those who like this delicacy.
Most of the gastropod mollusks, however, live in the water, some
inhabiting ponds and streams, while others dwell in the sea.
In almost every brook and every ditch, for example, you may find
water-snails of different kinds. Some are quite flat, and some are
conical and pointed. Some are as large as land-snails, and some are so
tiny that they are almost always overlooked. Most of them feed upon
decaying leaves, and they have an odd way of traveling by floating
upside down at the surface of the water, each with its broad fleshy
"foot" expanded, so as to convert themselves into tiny boats. You may
sometimes see quite a fleet of these little creatures being carried
along by the stream. But if you throw a stone into the water they all
sink down to the bottom at once, and do not resume their journey until
many hours or even days afterward.
The eggs of this snail are laid in long jelly-like ribbons, which are
generally fastened either to the stems and leaves of water-plants, or
under the edges of large stones lying at the bottom of the stream. A
very large number of gastropods live in the sea. One of the best known
of these is the whelk, of which one reads in all books of English
natural history. On almost every sandy and shingly beach, in Western
Europe, one may find it lying about in hundreds; and even in large
inland towns one often sees whelks for sale, both in fishmongers'
shops and on barrows at the corners of the streets. Its eggs are
one of the curiosities of the sea-beach--small, yellowish-white objects
about the size of peas, made of tough, parchment-like skin, and fastened
together in bundles about as big as cricket-balls. You may often find
these bundles on the shore in dozens; and most likely you will wonder
how the whelk ever managed to lay a batch of eggs a good deal bigger
than itself.
But the fact is that the eggs of the whelk are just like those of the
frog. When they are first laid they are very tiny; but the tough skin of
which they are made is very elastic, so that it will stretch almost like
a piece of india-rubber. Besides this, it has the curious property of
allowing water to soak in from the outside, but not to pass out again.
So as soon as the eggs are dropped into the sea they begin to swell, and
before very long they are quite twenty or thirty times as large as they
were when they were first laid.
We do not have these whelks in North America, but we have a variety of
small gastropods, whose shells are sometimes rough and coiled in a
spiral form, sometimes round like land-snails, and of various sizes. One
of them is the purpura, which has many ribs, and broad dark and light
stripes running spirally. The purpura of the Mediterranean is famous for
the purple dye obtained from its body; but our species yields such a dye
also in small quantity. This was the dye anciently known as Tyrian
purple. It is contained in a little bag behind the throat, which holds
just one small drop of liquid, and no more. And if you were to see it
you would never think that it was dye at all, for it looks only like
rather yellowish water. But if it is squeezed out on a sheet of white
paper, and laid in the sunshine, it very soon begins to change color.
First it becomes green, then blue, and then purple. And it is really the
dye which the ancient Romans valued so highly that no one who did not
belong to the royal family was allowed to dress in purple raiment.
BORERS
In many parts of our eastern coast occur in great numbers two or three
kinds of small, rough, spiral gastropods, called borers by the
fishermen, who hate them because of the great number of oysters they
kill. Each of these spends its whole life in seeking and devouring other
shell-bearing mollusks. It kills and eats these in a very curious way.
Like all the gastropods, it possesses what we call a tooth-ribbon--that
is, a narrow strip of very tough gristle in its mouth; set with row upon
row of sharp, notched, flinty teeth. There are some times more than six
thousand of these teeth, and although they are so small that they cannot
be seen without the aid of a powerful microscope, they are nevertheless
very formidable. For every tooth is hooked, with the points of the hook
directed toward the throat.
The tooth-ribbon is used in this way: When a borer meets with a victim,
it fastens itself to it by means of its fleshy, muscular "foot." Then it
bores a round hole through its shell, as neatly as if it had been
pierced by a drill. And then it pokes the tooth-ribbon down into the
body of the creature inside, and draws it back again. As it does so, of
course the hooked teeth tear away little bits of the victim's flesh. The
borer swallows these, and then pokes down its tooth-ribbon once more.
And so it goes on, over and over again, until the shell of its victim
has been completely emptied, when it goes off to look for another.
PERIWINKLES
These are common on rocky parts of the coast, and you may find them
crawling about on the weed-covered rocks in thousands when the tide is
out. They have tooth-ribbons just like that of the borer, but they do
not use them in the same way, for they feed only upon seaweeds. And they
are remarkable for having the foot divided by a kind of groove, which
runs right down the middle. When a periwinkle crawls, it moves first one
side of this foot forward, and then the other side, so that although it
has no legs it may really almost be said to walk.
THE COWRY
One of the prettiest of the gastropod shells, is that of the cowry, in
some parts of Africa used as money. It would seem strange to
earn one's living just by picking up money on the sea-shore, wouldn't
it? And perhaps you might think that every one who lived near those
parts of the coast where cowries are found must be very well off. But
then sixteen hundred of these shells are only worth about a quarter of a
dollar, so that you would have to hunt for a very long while and stoop a
great many times in order to obtain sufficient even to buy food. And it
must be very awkward to have to carry several sacks of money when one
goes out marketing! Many of them, however, are extremely beautiful.
LIMPETS
Commoner still are the limpets, which you may find in thousands clinging
to the rocks that are left bare when the tide goes out. They fasten
themselves down by means of the broad, fleshy foot, which acts as a big
sucker. And so firmly do they hold that it is almost impossible to pull
them away.
After a time, the edges of a limpet's shell cut a circular groove in the
rock to which it clings, so that even the sea-birds cannot drive their
beaks underneath and force it from its hold. And though, when the tide
is up, the mollusk will wander to a distance of two or even three feet
in search of food, it always seems to return to its resting-place before
the retreating waves again leave the rock uncovered.
AMPHINEURANS
This order of mollusks contains the curious creatures which are known as
chitons. These may be described as sea-armadillos, for they are covered
with a kind of shelly armor, consisting of a series of plates, and can
roll themselves up into balls, in order to protect themselves from the
attacks of their enemies.
One of these mollusks is called the prickly chiton, because it is
covered all over with sharp spines, like a hedgehog. It grows to a
length of nearly six inches. But long before it reaches its full size
the spines are rubbed off, so that a large example of this
creature is nearly always perfectly bare. The chitons live among muddy
rocks at low-water mark, and are not common outside the tropics or in
shallow water.
The order of the amphineurans is quite a small one, and so is that of
the scaphopods, which consists only of the tooth-shells, which are very
common on the sandy coasts of the Northern Pacific, and look rather like
very tiny elephants' tusks. The Indians of the Puget Sound region used
to string them as ornaments, and valued them highly.
BIVALVES
The order of the bivalves is a very large and important one. All these
creatures have their shells made of two parts, or valves, which are
fastened together by means of a hinge. They have no heads, and the
mantle forms a kind of flap on either side of the body. They are found
both in fresh and salt water. Every one knows the "fresh-water clams,"
or mussels, which abound in our lakes and rivers. In the central and
southern parts of the United States they are exceedingly numerous and of
many kinds, some rough, others smooth. All are lined with
mother-of-pearl, and pretty buttons and other ornaments are made from
them. Moreover, pearls are very frequently discovered inside their
shells, and sometimes they are of great value.
THE PEARL-OYSTER
Pearls are obtained chiefly, however, from the pearl-oyster, which is
found in warm seas in many parts of the world, the principal fisheries
being in Ceylon, the Persian Gulf, the South Sea Islands, and off the
northeast coast of Australia. They are deposited by the mantle, and it
is most likely that they are really due to a grain of sand, which has
lodged inside the shell and set up irritation. Indeed, it has been found
that if small objects, such as tiny stones, are forced between the
valves of one of these oysters, they become covered with layers of pearl
in a very short time. The best mother-of-pearl is also obtained from the
shells of the pearl-oyster.
OYSTERS
The ordinary oyster belongs to another family of bivalves, in which one
part of the shell is a good deal larger than the other.
The early life of this mollusk is very curious. The spawn is known as
spat, and is produced in enormous quantities. This spat looks at first
like very fine gray dust, and remains for some little time within the
shells of the parent. But one day in early summer the oyster opens its
valves a little way, and squirts it out like a cloud into the water. For
a few weeks the little oysters are able to swim, and they generally
travel backward and forward with the tide. But after a while they attach
themselves to some object at the bottom of the water, and there they
remain without moving any more for the rest of their lives.
One would think that, since a family of oysters is so enormously large,
these creatures must be the most plentiful mollusks in the sea. But by
far the larger number are destroyed by other creatures before they are
able to settle down; while even after that they have a great many
enemies. We have already told you how mischievous starfishes are in the
oyster-beds. Then borers and dog-whelks are almost equally troublesome,
and besides these there is a curious kind of sponge, called the cliona,
which burrows into the shells of the mollusk and gradually destroys
them, sometimes actually causing them to fall to pieces.
BLACK MUSSELS
Two or three kinds of black mussels live in vast numbers on almost all
coasts, clinging to rocks and submerged timber. The way in which a
mussel fastens itself to its hold is very curious, for instead of
turning the whole of the foot into a big sucker, as the limpet does, it
spins a number of very strong threads from that part which lies nearest
to the hinge; and every one of these threads is separately fastened to
the support, so that the creature is moored down, as it were, by a kind
of cable. These threads are known as the byssus, and hold so firmly that
it is not at all easy to pull them away. Some of these mussels
are good to eat, but are not as much used in the United States as in
Europe.
THE COCKLE
This is another very well-known bivalve. Its heart-shaped shells,
covered with low ridges, you must know by sight. It is one of the
burrowing mollusks, spending its life buried in sandy mud. It is
especially common at the mouths of large rivers, where enormous
quantities are collected to serve as human food. And its large muscular
foot is not only used in digging, but also enables it to leap to a
considerable height. It is to this family that the quahog or hard clam
of our markets belongs.
RAZOR-SHELLS
These, too, are inhabitants of the mud, and if you want to find their
burrows all that you have to do is to visit a patch of sandy mud when
the tide is out, and stand quietly watching it. Before long you are sure
to see a little jet of water spurt out of the mud to a height of three
or four inches. Now this water has been squirted out of the siphon-tubes
of a razor-shell, and if you walk to the spot, treading very carefully,
you will find a tiny hole in the mud. This is the entrance to the
burrow, and if you want to get the animal out, the best way to do so is
to drop a little salt down the hole. For it is a very strange fact that
although the razor cannot live in mud at the bottom of fresh water, it
does not like pure salt at all, and is sure to come up to the surface
and try to get rid of it. But if you fail to seize it at once it will
retreat to the very bottom of its burrow, and no amount of salt will
persuade it to come up again. The soft clam, which is sold in our
markets in such enormous quantities, is a near relative of the razor.
THE PIDDOCK
One of the most wonderful of all the bivalves is the piddock, as it is a
boring mollusk, living buried in the solid chalk or limestone.
If you should examine the rocks which are left bare at low water along
the shore of the Mediterranean, or some other warm sea, you would often
find that they are pierced by numbers of rather large round holes. These
are the entrances to the burrows of piddocks; and if you could split the
rock open you would find several of these creatures lying in their
tunnels.
Sometimes, when they are boring, their burrows become choked up behind
them with the material which they have scraped away. Then they just
squirt out a jet of water from their siphon-tubes, and so wash the
passage clear.
It is really owing to the work of the piddocks that chalk and limestone
cliffs are so much cut away by the sea. The waves by themselves can do
very little in this way. For when they wash up against the face of the
cliff they leave the spores of seaweeds behind them; and these very soon
grow and cover the whole surface with a mantle of living green, which
almost entirely prevents the cliff from being worn away. But the
piddocks drive their burrows into the rock just below the surface of the
water, boring backward and forward till it is completely honeycombed by
their tunnels, which only have just the thinnest of walls left between
them. Then the sea washes into the burrows, and breaks these walls down,
so that the whole foundation of the cliff is cut away. Very soon, of
course, there is a landslip, and hundreds of tons of chalk or limestone,
as the case may be, come falling down. Then the piddocks begin working
again a little farther back, and the process is repeated; and so on over
and over again.
On many parts of the south coast of England long stretches of rocks run
ever so far out into the sea, and are only partly left bare at low
water. Those rocks were once the bases of cliffs, which the piddocks and
the waves together have cut away. And it even seems almost certain that
the Strait of Dover was cut in this manner, and that if it had not been
for the labors of the piddocks, carried on day after day for thousands
upon thousands of years, Great Britain even now would not be an island,
but would still form part of the continent of Europe, as we know that it
did in ages long gone by!
THE TEREDO
There is a bivalve mollusk which burrows into submerged timber, such as
the hulls of wooden ships, or the beams of piers and jetties. This is
called the teredo, or ship-worm, and certainly it does look much more
like a worm than a mollusk, for it has a cylinder-shaped body something
like a foot in length, with a forked tail, while the shell only covers
just a little part at one end. How it burrows into the wood nobody quite
knows. It is generally supposed to do so by means of the foot. But in a
very short time it will honeycomb a great beam of timber with its
burrows, which it always lines with a kind of shelly deposit, weakening
it to such a degree that at last it gives way beneath the slightest
pressure.
Like a great many other mollusks, the teredo passes through a kind of
caterpillar stage before it reaches its perfect form. While it is in
this condition it is able to swim freely about in the water, and looks
rather like a very tiny hedgehog, being almost globular in shape, and
covered all over with short projecting hairs. It is by means of the
action of these hairs upon the water that it is able to swim.