soft bodies; but it is not at all like the shell of a Snail, or other
_mollusc_. The Snail's shell is like the little boy's suit which is
altered and made bigger as the boy grows. The crab's covering is a suit
which cannot be altered. It must be thrown away, and replaced by a
larger one.
The body of the shell-builder is wrapped in a soft covering, a kind of
outer coat, which is called the _mantle_. Now this mantle is one of
Nature's cleverest inventions. It is able to take the substance called
_lime_ from the food of the animal, and to use it as building stuff.
[Illustration: PRECIOUS WENTLETRAP.]
The shell is built to fit the soft body. When a Periwinkle is hatched
from the egg, it is as big as a pin's head. It eats and grows, and the
shell must therefore be made larger. So the mantle is stretched out, and
it puts a film of lime to the edge of the shell. Bit by bit the shell is
thus added to by the wonderful mantle. Look at a snail's shell, and
notice the lines which show how many times the little house has been
made larger.
Each kind of shell-builder has its own style of building. If you go to a
museum and examine the shells gathered from all over the world, you are
surprised at their wonderful shapes, markings and colours. Another
surprising thing is their size. Some are enormous, so large that they
make good washing-basins. Others are so small that you can hardly see
them. Each one was made by the folds of the mantle of the animal that
lived in it.
In our coloured pictures you see many different kinds of shells, some of
them built by uni-valve molluscs and some by bi-valve molluscs.
Wherever there are weeds along the shore you can find whole armies of
the Periwinkle--the "Winkle" we all know so well. It browses there,
among the weeds, just as its cousin, the land Snail, browses on your
cabbages. You must have seen the little door with which the Periwinkle
closes the entrance to his house. The land Snail does not own a door,
but he makes one when he goes to sleep for the winter.
The Periwinkle crawls on a broad, slimy foot, which is put out from the
shell. It is stretched on this side or that, and so draws him and his
home in any direction. There are two sensitive feelers in front of his
head; and behind these are two short stalks, on each of which is a tiny
eye. If alarmed, the Periwinkle can shorten his body, and pull it back
into its shell, closing the entrance with the horny door.
But the strangest part of him is the tongue. It is not for tasting, but
for rasping. It is like a long, narrow ribbon, on which are hundreds of
tiny points, all sloping backwards. They are arranged three in a row.
The Periwinkle rasps the seaweed with his tongue, and so scrapes off his
dinner. Of course the teeth wear away.
[Illustration: COWRIES.]
But only part of the toothed ribbon is used at a time, so there are
plenty of teeth behind the worn ones, ready to take their place.
The shell, as we have seen, is made of _limestone_. But the teeth are
made _of flint_. This is a hard substance, so hard that it is used for
striking sparks.
Now we will look at a shell-builder, the Whelk, who uses his flinty
tongue in quite another fashion. The Whelk does not care for a vegetable
dinner. He prefers to eat other molluscs--he is carnivorous, a
flesh-eater; but these other molluscs do not wait to be eaten. As the
enemy draws near they retire into their shells, and shut themselves up
as tight as they can. The Whelk, however, is a clever burglar; he knows
how to make a way into the hardest of shelly houses.
His front part--we might call it a nose--will stretch out to a fine
point; and it contains a rasping tongue even harder than that of the
Periwinkle. He sets to work. Moving the rasp up and down, he drills a
neat round hole in the shell of the animal he is attacking. No shell is
safe from him; and no tool could make a neater hole.
When you next gather shells on the beach, look at them closely; in some
you will see where Mr. Whelk, the burglar, has been at work. He needs
but a small entrance to enable him to suck out his helpless prey at his
ease. Is it not strange that this creature, with a body as soft as your
tongue, should earn its living by breaking into houses made of hard
shell!
There are other molluscs which find their meals in this strange manner,
and many others which, like the Periwinkle, feed more easily on seaweed.
One of these, the Limpet, you can always be sure of finding at low tide;
indeed, there are so many Limpets on the rocks that it would be hard
_not_ to see them. You will know, if you have tried to force a Limpet
from its hold on the rock, how very tightly it clings. It is as if the
shell were glued or cemented by its edges.
Yet there is no glue or cement used, but only a simple dodge. The Limpet
has a broad "foot," which almost fills up the opening of its shell. Like
the foot of the Snail, it is used when the animal wishes to take a walk;
but it serves another purpose too. It can be used as a sucker; and it is
this which enables the Limpet to cling so firmly to its rock.
When the tide is out, the Limpet clings to the rock, its soft body
tucked safely away in the shell. Its feeding time comes when the water
covers the rocks once more. Then the Limpet's shell may be seen to tilt
up, and a foot, and a head with feelers and eyes, come out. The Limpet
crawls to the seaweed and begins to browse, using a rasp like that of
the Periwinkle. It then crawls back to its own place on the rock. In
time this resting-place becomes hollowed out, and the Limpet's shell
fits into the groove thus made.
Limpets are useful as bait for fish. The Whelk and Periwinkle are
gathered in immense numbers, and are used by us for food. Perhaps you
have seen the egg-bundle of the Whelk. It contains many eggs when first
laid in the sea. Each egg is as big as a pin's head. They swell in the
water, until the yellowish bundle is three times as large as the Whelk
that laid it. You often see the empty bundle blown by the wind along the
shore.
EXERCISES
1. Give the names of two bi-valve molluscs.
2. What is the Periwinkle's shell made of?
3. Describe how the Periwinkle eats seaweed.
4. How does the Whelk obtain its food?
5. Give the names of three one-shelled molluscs.
LESSON XI.
SHELLS AND THEIR BUILDERS (2)
THE MUSSEL AND OYSTER.
As everyone knows, the Mussel and the Oyster live between two hinged
shells. In the last lesson we called them _bi-valve molluscs_, which is
only another way of saying "soft-bodied animals with two shells." Have
you ever opened an Oyster? It is a tug-of-war, your skill and strength
against the muscles of the animal inside the tight shells.
Like the Periwinkle and other shell-builders, these creatures owe their
strong houses to a wonderful _mantle_; but in this case the mantle is in
two pieces instead of one. You can imagine the Periwinkle's mantle as a
tube enclosing the animal's body. The mantle of the Mussel or the Oyster
is in two pieces; and each half forms its own shell.
The Snail, and other one-shelled molluscs, poke their heads out of the
shell when feeding or moving. Oysters and their two-shelled cousins
cannot do this, for the simple reason that they have no heads!
In some places you see that the rocks at low tide are covered with
Mussels. In dense black masses they cling to the rocks; and, though
heavy waves bang them like so many hammers, they stick tight. Little
Mussels and big ones, they form a mass so thick that baby crabs and
other creatures use them as a hiding-place. On the piers and groynes,
and the woodwork of the harbour, you can see other clusters of Mussels;
they are placed where the high tide covers them.
Have you noticed how the Mussel anchors himself? He uses a bunch of
threads, like so many cables or tiny ropes. It is interesting to know
how these threads are made.
The Mussel is, as a rule, a stay-at-home, but he can move from place to
place if he likes. He has a long, slender foot which can be pushed out
of the shells. Now the threads are fixed by the foot, just where the
Mussel wishes to anchor himself. They are made from a liquid which forms
in the body of the creature. This liquid hardens in the water so that it
can be pulled out into long, fine threads.
Our ordinary Mussels do not make very long threads, but those of some
kinds are so long that they can be woven into silky purses or stockings.
The Mussel which makes such long anchor-threads might be called "the
silkworm of the sea."
If the Mussel is such a stay-at-home, how does he find his food? The
answer is, that the food comes to him, brought by the ever-moving water.
There are countless specks floating in the sea, mostly specks of
vegetable stuff. These settle on the floor of the sea, just as dust
settles on our house-floors; and the waves wash this "sea-dust" hither
and thither. The Mussel or Oyster, with shells gaping wide open, is
bound to get some of this food with the water which enters the shells.
The Oyster has no "foot," and is fixed in one place nearly all its life.
It is an interesting animal; and one of such value as food, that
hundreds of thousands of Oysters are reared in special "beds," and sent
to the market at the proper season. Our British Oysters were famous even
in the time of the Romans; they were carefully packed and sent to Rome,
and, at the Roman feasts, surprising quantities of them were eaten.
Many sea-animals have wonderfully large families, but the Oyster, with
its millions and millions of eggs, beats most of them. Strangely enough,
its eggs are not sent into the sea at once, but are kept between the
Oyster's shells until they hatch. Needless to say, these babies are very
small indeed, else their nursery could not contain them all Though so
small that thousands of them together look more like a pinch of dust
than anything else, yet each one has two thin shells; so that, if you
eat the parent Oyster, they grate on your teeth like sand. Oysters, at
this time, are "out of season"--that is, unfit for food.
At the right moment, the Oyster gets rid of its numerous family. It
opens its shells, then shuts them rapidly; and, each time this happens,
a cloud of young Oysters is puffed out like smoke. Now these mites must
fend for themselves in a sea full of foes.
They have no defence, and countless numbers of them are gobbled up by
crabs, anemones, and others. If this did not happen, the sea would soon
be paved with Oysters.
For a time, the baby Oysters--which are known as "spat"--are able to
swim here and there. In rough weather they are driven far into the deeps
of the ocean, and lost. The rest of them, before they have been free for
two days, settle on the bed of the sea--sometimes on their own parents;
and there they remain for life. Only a very few out of each million
become "grown-ups"--the rest are eaten by enemies, or smothered in mud
or sand. In a year or so they are as big as half-a-crown. In five years
they are fine, fat grown-up Oysters--that is to say, if they have not
been dredged up from their bed and sent to market.
Their shells open and shut like a trap. You may have seen a picture of
an inquisitive mouse trapped by an Oyster. Thinking to have a nice taste
of Oyster, the mouse had poked its head into the open shells, but they
were snapped together, and the mouse was firmly held in the trap.
Between the hinge of the two shells there is a pad, which acts like an
elastic spring, and forces the shells open. The Oyster can close them by
means of a strong muscle. They are its only defence, so it closes them
at the least hint of danger.
Even these thick walls are sometimes of no avail, as we saw in our talk
on "Five-fingered Jack." We saw how the starfish forces the shells open
with the help of its strong tube-feet. The whelk and his cousins know
how to bore a hole in the shell, and suck out the helpless Oyster. Then
there are certain sponges, with the strange habit of making holes in
shells, and living in and on them. Sometimes the Oysters are stifled in
their "beds" by other Oysters settling and growing over them. Thick
masses of Mussels may cling to them and suffocate them. And grains of
sand sometimes get in the hinges of their shells, so that they cannot
close up the house when they wish.
Like the other animals which are useful as food, Oysters have been
carefully studied and cultivated by man for many, many years. The story
of the Oyster-beds is a long and interesting one.
Oysters feed in rather a strange way. You may have looked inside the
shells and seen two delicate dark-edged fringes, known as the "beard."
This fringe is the Oyster's gills or breathing arrangement. Trace the
"beard" as far as the hinge of the shells, and you see the mouth with
its white lips. If you could watch the creature having its dinner, you
would see a constant stream of water flowing over the gills and towards
the mouth.
What makes the water move in that way? The gills are covered with very
tiny lashes, like little hairs. There are so many of them that, as they
keep moving, they force the water along, over the gills and towards the
mouth. In this way the Oyster breathes the air which is in the water;
but not only that. As we have already noticed, there is a kind of
"vegetable dust" in the sea. This is driven to the Oyster's mouth and
swallowed. The Oyster, fixed in its "bed," unable to hunt for food, thus
makes its dinner come to it. What a strange use for a "beard"! It not
only serves as lungs, but also helps the animal to catch its "daily
bread"!
Another mollusc used as food is the Cockle, and its shell is one of the
commonest found along the shore, especially near sandy places. It lives
in sand, and can bury itself so quickly that you would have to use your
spade with all your might in order to keep pace with this little
shell-fish. Where Cockles have buried themselves you will see spurts of
water and sand, showing where they are busy down below in the wet sand.
Besides being so skilful at digging, the Cockle is a first-rate jumper.
If left on the beach, it jumps over the sand, towards the sea, in the
funniest way. It is strange to see a quiet-looking shell suddenly take
to hopping and jumping like an acrobat.
To perform this astonishing feat the Cockle makes use of its foot, which
is worked by very strong muscles. It is large and pointed, and bent: if
the Cockle wishes to move quickly, it stretches out its foot from
between the shells, as far as it will go. Then, by using all its power,
it leaps backwards or forwards in a surprising manner.
There are many other interesting molluscs, besides those we have looked
at. The Piddock, or Pholas, is a smallish, rather delicate one, with a
soft foot. But this foot is a most wonderful boring tool, fitted with a
hard file. Hard rocks and wood are perforated by these little molluscs.
Indeed, they are a positive danger, for they pierce the wooden piles of
piers, and weaken them. They cannot pierce through iron, however, and so
iron plates or nails are used to protect the piles from their
onslaughts. You will often see stones and rocks riddled by the Piddock
as if they were as soft as cheese. Chalk, sandstone, or oak, it is all
the same to the Piddock, which rasps them away with its file. When the
points of this strange instrument are worn out with all this hard wear,
a new set takes their place.
EXERCISES
1. How does the Mussel anchor itself?
2. Describe how the shells of the Oyster are opened and closed.
3. What is the food of the Mussel?
4. Of what use is the "beard" of the Oyster?
5. Why is the Oyster called a bi-valve?
6. Why is the Oyster sometimes unfit for use as food?