CRUSTACEANS
We now come to a very important class of animals, which includes the
crabs, the lobsters, the shrimps, and the prawns. These creatures,
together with the mollusks, are often called shell-fish, although the
"shell" of a crab is not in the least like the shell of an oyster, for
example, or like that of a whelk, or a snail. It is only a sort of crust
upon the skin, made chiefly of carbonate of lime. That is why these
animals are called _crust_aceans; and instead of growing, like true
shells, this coat never increases in size at all.
But crabs and lobsters grow? Yes: but not as other animals do, a little
every day. They only grow, as a rule, once a year; and they get a whole
twelvemonth's growth into about two days!
When, in warm weather, the proper time approaches, they hide away in
some crevice among the rocks, where none of their enemies are likely to
find them. This is because they are going to throw off their so-called
shells; and they know that when these are gone they will be deprived of
their natural armor, and of their weapons too, and so will be quite at
the mercy even of foes much smaller than themselves. Then a very strange
thing happens. Part of their flesh actually turns to water! Sometimes,
if you happen to take up a crab in a fish-market, and shake it, you will
hear water swishing about inside it. This is a "watery" crab, and is not
good to eat; for it was just about to change its "shell" when it was
caught. A good deal of its flesh has actually turned to water.
Now this always happens a few days before the "shell" is thrown off; and
the animal wriggles and twists about inside it, in order to loosen the
attachments which bind it to its body. It also rubs its feelers against
its legs, and its legs against one another, in order to loosen their
hard coverings in the same way. This goes on, perhaps, for three or four
days. Then, suddenly, the "shell" splits across, and the animal,
with a tremendous effort, springs right out of it, while the "shell"
closes up again, and looks just as it did before. One might really think
that there were two crabs instead of only one.
For some little time the animal now lies perfectly still. It is
exhausted by its efforts, and its muscles are so cramped that they feel
quite hard to the touch. This cramp soon passes off, however; and then
at once the animal begins to grow. It grows very fast. Indeed, you can
almost _see_ it grow, for a whole year's increase in size has to
take place in about forty-eight hours. Then a fresh crust is gradually
formed upon the skin, and two or three days later the animal is once
more clad in a coat of mail, and is ready to leave its retreat and face
its enemies. For a whole twelvemonth after this it grows no bigger. But
at the end of that time the process is repeated, and so on, year after
year, until at last the animal reaches its full size.
FORMS OF CRUSTACEANS
The bodies of the crustacean animals are made up of a number of rings,
or segments, like those of the insects. But there are always twenty of
these rings, instead of thirteen; six forming the head, while there are
eight in the thorax and six in the hind body.
Then--again like the insects--crustaceans have feelers, or antennæ, upon
their heads. You can see these very well indeed in a lobster or a
shrimp. But instead of having one pair of these organs, as insects have,
they always possess two pairs. And it is rather curious to find that at
the base of the front pair there are two little organs which seem to be
ears, specially formed for hearing in the water, while at the base of
the second pair are two other little organs which seem to serve as a
nose, specially made for smelling in the water.
And--once more like the insects--crustaceans have to pass through
several different forms before they reach the perfect state. They are
hatched in the first place from eggs, which the mother animal carries
about with her for some little time firmly fastened to the hairs
of the swimmerets, which we find under the hind part of her body. You
will often find a shrimp with quite a large bunch of these eggs; and if
you look at them carefully with a good strong magnifying-glass, you will
see that they are all glued down to hairs.
Inside each of these eggs an odd little creature is formed, which is
called the nauplius. Sometimes it is hatched while still in that state,
and swims about through the water. But in almost all the higher
crustaceans a change takes place before it leaves the egg, and it
appears at last in the form of a zoëa.
This is a kind of crustacean caterpillar, and a very odd little creature
it is. A great naturalist once described it as an animal "with goggle
eyes, a hawk's beak, a scorpion's tail, a rhinoceros' horn, and a body
fringed with legs, yet hardly bigger than a grain of sand!" Certainly it
does not look in the least like the crab, or lobster, or shrimp into
which it is going, by and by, to turn. And it swims in the oddest way
possible, by turning endless somersaults in the water!
These zoëas are sometimes found in immense shoals, the surface of the
sea being quite thick with them for miles. And they are useful little
creatures, for they feed on the tiny scraps of decaying matter which are
always floating about in the sea, just as tadpoles and gnat-grubs do in
ponds, thus helping to keep the water pure. But a very great number of
them are devoured by whales. For when whalebone-whales are hungry, they
swim with open mouths through a shoal of these little creatures, and
then strain them out of the water by means of the whalebone fringe which
hangs down from the upper jaw.
After a time the zoëa throws off its skin and appears in quite a
different form. It is now called a megalopa, or big-eyed creature,
because it has very large eyes, which are usually set on foot-stalks,
and project to quite a long distance from the sides of the head. And as
the zoëa is a kind of crustacean caterpillar, so the megalopa is a kind
of crustacean chrysalis. It generally has a long, slender body, made up
of several joints. And it swims by flapping this to and fro in the
water.
CRABS
First among the crustaceans come the crabs, of which there are a great
many different kinds. They are distinguished by having the tail tucked
under the body, and firmly soldered, so to speak, to the "shell" on
either side.
You can find several kinds of these creatures by hunting among the rocks
on the sea-shore when the tide is out. There is the common shore-crab,
for example, which is green in color. It is generally to be found hiding
under masses of growing seaweed. But sometimes you may see it prowling
about in search of prey. It is wonderfully active, and will even pounce
upon the sandhoppers as they go skipping about, just as a hunting-spider
will pounce upon flies, seldom or never missing its aim. It will catch
flies, too, leaping upon them when they settle, and shutting them up, as
it were, in a kind of cage formed by its legs. Then it pokes one claw
carefully into this cage, seizes the prisoners, pulls them to pieces,
and pokes the fragments into its mouth.
Swimming about in the pools, too, you may often find a fiddler-crab,
which is so called because its movements in the water rather remind one
of a man who is playing the violin. You will find that its hind legs are
very much flattened, and are fringed with stiff hairs, so that they may
be used as oars. In fact, the animal rows itself through the water. Both
these crabs, sad to say, are cannibals, and are always ready to attack
and devour their own kind.
Then there is the edible crab, or blue crab, which is common on many
parts of our coasts. The edible crab of Europe is somewhat different.
You are not likely to meet with the larger examples, which live in
deeper water. But even the smaller ones can give a very sharp nip with
their great claws, and you will find it as well to be very careful in
handling them. The best plan is to seize them with the thumb and finger
just behind these claws, then they are perfectly harmless. The larger
crabs, which sometimes weigh as much as twelve pounds, are extremely
powerful, and in more than one case a man has been killed by
them, having been seized by the wrist as he was groping among the rocks,
and held in a grip from which he could not break away until he was
drowned by the rising tide.
These crabs are captured by means of crab-pots, made of basketwork,
which have the entrance so formed that while the crabs can easily enter,
they cannot possibly get out again. These pots are baited with pieces of
fresh fish, and are then weighted with stones, and lowered to the bottom
of the sea among the rocks, at a depth of from three to about twenty
fathoms. They are also caught on lines baited with meat. No hook is
needed, for the crab clings to the meat till it reaches the surface of
the water, when it must be flung into the boat or somehow captured
quickly, before it has time to let go and sink.
Some crabs live on dry land, sometimes at a distance of two or three
miles from the sea, which they only visit at intervals. Among these are
the famous calling-crabs, found in many of the warmer parts of the
world. These crabs obtain their name from the fact that one of the great
claws of the male is very much larger than the other. So big is it,
indeed, that it has to be held aloft over the body when the animal is
running, in order to prevent it from losing its balance and toppling
over. And as soon as the crab begins to move this huge claw is jerked up
and down, just as if the creature were "calling," or beckoning, to its
companions. The calling-crabs live in burrows in the sand, which are
often placed as close to one another as those in a rabbit-warren.
HERMIT-CRABS
Next we come to those small, curious creatures known as hermit-crabs,
which form a kind of connecting link between the crabs and the lobsters,
for their tails, instead of being firmly soldered down underneath their
bodies, are quite free.
But the odd thing about these animals is that their tails have no shelly
covering. The front part of the body is protected by a coat of mail,
just as it is in all the other crabs; but the hind part is quite bare
and soft. The consequence is that a hermit-crab is always very
nervous indeed about his tail. He is dreadfully afraid that one of his
many enemies may creep up behind, and bite it when he is not looking. So
he always tucks it away in an empty shell like that of a whelk or a
sea-snail, which he drags about with him wherever he goes!
You may often find these curious crabs by hunting for them in the pools
among the rocks at low water. The crab always sits just inside the
entrance of the shell, which he closes and guards with one of his great
claws. And if you try to pull him out, you will find that you are quite
unable to do so, for he has a pair of strong pincers at the end of his
body, by which he holds the shell so firmly that you can tear him in two
without forcing him to loose his grip.
Sometimes you will find that a sea-anemone has fastened itself to the
edge of a shell in which a hermit-crab is living. This is a great
advantage to the crab; for while there are many fishes which would be
quite ready to crunch him up, shell and all, no fish will ever meddle
with a sea-anemone. So as long as the anemone remains on his shell he is
perfectly safe.
And this plan is also a great advantage to the anemone, which is sure to
get plenty of food without any trouble. For when the crab finds the dead
body of some small creature, and begins to pull it to pieces, a quantity
of small fragments is sure to float upward in the water. And the anemone
catches them with its spreading tentacles and feeds upon them.
THE ROBBER-CRAB
One of the most extraordinary crustaceans is this, which is found in
many of the islands in the Indian Ocean. It is like the hermit-crabs in
some ways, but the tail is covered with shelly plates, just like the
rest of the body; and instead of living in shells in the sea, it lives
in deep burrows on dry land.
But the oddest thing of all with regard to this crab is its food. What
do you think it feeds upon? Cocoanuts! That seems impossible, doesn't
it? One would imagine that the crab could never get the nuts open. But
it manages in this way: First of all, it pulls away the fibers from that
end of the nut at which the three eyeholes are situated. With
one of its stout claws it then hammers away at one of these till it
breaks its way through. And finally, after allowing the milk to run
away, it pokes its hind claws, which are very slender indeed, through
the opening and picks out the white fleshy part of the nut a little
piece at a time.
It is said, too, that this crab sometimes opens a nut by poking the
smaller joint of one of its claws into the hole, and then striking it
over and over again upon a big stone.
The burrow of the robber-crab is rather a deep one, and is nearly always
situated beneath the roots of a tree. And at the end of the burrow is a
large chamber, in which the crab piles up a quantity of cocoanut fiber
to serve as a bed.
LOBSTERS
Of course you know the lobster very well by sight; and perhaps you know
that until it is boiled it is black, not red. But do you know how it
swims? If so, you know that it has two different ways of swimming. When
it is not in a hurry it swims slowly forward by means of its swimmerets,
of which it has five pairs under the hinder part of its body. But if it
is startled or alarmed it swims swiftly backward by means of its tail.
If you look at a lobster's tail, you will see that it is very broad and
flat, and that on either side of it are two plates, which are quite as
flat, and, if anything, are rather broader. So, when these are spread,
the tail looks like a fan. And the animal swims by first stretching out
its body almost straight, and then doubling it suddenly with all its
force. As it does so, the tail and the tail-plates spread out, and act
very much like a broad oar. And the result is that the lobster darts
swiftly backward through the water. Shrimps and prawns swim in exactly
the same way.
Lobsters are very quarrelsome creatures, and are constantly fighting;
and it very often happens that in these battles they pull off one
another's limbs. They seem to feel very little pain, however, from such
an injury, and before very long new legs begin to grow in place of the
old ones, so that in course of time the wounded creatures are as perfect
as ever.
Sometimes lobsters will throw off their limbs when they are not attacked
at all. They do so, for example, if they are suddenly frightened; and it
is said that if a heavy gun is fired near the surface of the water,
every lobster for a long way round will shed its great claws in alarm.
You will notice, on looking at a lobster, that one of the great claws is
a good deal smaller than the other; and sometimes people think that this
is a new claw which is growing in place of one that has been lost, and
that it has not yet reached its full size. This, however, is a mistake,
for one of the claws is always much bigger than the other; and the
reason is that they are used for different purposes. The larger claw is
intended as a weapon, and with this the lobster fights. But the smaller
one is chiefly employed as a kind of anchor, by means of which the
animal can cling to the weeds or rocks at the bottom of the sea.
Lobsters are caught in pots made of basketwork, just as crabs are. But
they are not nearly so dainty as crabs, and do not mind whether the bait
is fresh or putrid. They are always very much attracted, too, by any
object that glitters, and many a lobster has been lured to its death
merely by one or two oyster-shells hung up inside the pot in such a
manner as to show the shining pearly interior.
CRAYFISH
The crayfish is a kind of fresh-water lobster, which is found commonly
in many parts of the world, and numerously in the central and southern
parts of the United States. Most species hide all day long under the
projecting edges of big stones, or in holes in the bank, only coming out
after nightfall to search for food. The British crayfish is said to be
particularly fond of the deserted burrow of a water-vole, and as it sits
inside it always guards the entrance with its great claws, striking
fiercely at any enemy which may be bold enough to come within reach.
One, at least, of the American kinds sinks its own burrows, in the form
of round holes in the soil of damp meadows. These holes go down to
water, which the animal cannot live long away from; and a part of the
soil dug out is piled about the mouth of the hole in a little
tower or chimney, sometimes several inches high.
In Europe crayfish are eaten and regarded as a delicacy when properly
cooked; and there is no reason why the American ones should not be
equally good, but they are rarely if ever used as food by us. The flesh
tastes like that of the lobster, but is more tender.
SHRIMPS AND PRAWNS
These are really only tiny lobsters, and if you examine them carefully
you will find that their bodies are made in exactly the same way. They
swim, too, by means of their tails, and dart about so swiftly that it is
almost impossible to follow their movements. You may often find them in
numbers in the pools which are left among the rocks by the retreating
tide. But as they are almost colorless until they are boiled, it is very
difficult to see them, and they look just like shadows darting to and
fro in the water.
You can easily tell a prawn from a shrimp, for the beak which projects
in front of its head is covered with sharp points, which are almost
exactly like the teeth of a saw. It feeds upon the bodies of the various
small creatures which die by millions every day. In this way it helps to
keep the water of the sea pure. It feeds in a curious way, tearing off
tiny scraps of flesh with the little pincers at the tips of the second
pair of legs, and poking them into its mouth one after another. The
sides of these limbs are covered with hairs, so that they form little
brushes; and with these the prawn carefully cleans its body and limbs,
rubbing off every little speck of dirt which may happen to cling to
them.
SANDHOPPERS
You can hardly walk along a sand shore when the tide is rising without
seeing sandhoppers leaping and twisting about in thousands. If you turn
over a bunch of seaweed which has been flung up by the waves just above
high-water mark, you are almost sure to find forty or fifty of
these odd little creatures hiding under it. In some ways they are rather
like shrimps. But they differ from them in having their eyes set on the
head itself, instead of on little foot-stalks projecting from it. And
they have no carapace, or shelly shield, covering the middle part of the
body.
How do these creatures hop? By first doubling up their bodies, and then
straightening them out again with a kind of jerk. It is exactly
opposite, in fact, to the way in which shrimps and lobsters swim.
Sandhoppers do not follow the retreating tide, but bury themselves in
the sand very soon after the waves have ceased to break over them. Even
when the surface of the sand is quite dry you can find their burrows by
stamping with your foot, when a number of little round holes will
suddenly open all round you.
These creatures have wonderfully sharp little teeth, and if you allowed
a swarm of them to rest for a little while on your handkerchief you
would most likely find that it was full of tiny holes when you took it
up. They will eat almost anything, either animal or vegetable, and are
quite as useful as the shrimps and prawns in helping to keep the
sea-water pure. But they have a great many enemies, for sea-birds,
land-birds, crabs, and all sorts of other creatures, destroy them
literally in millions.
THE FRESH-WATER SHRIMP
This shrimp is very much like the sandhopper in some ways. You may find
it in numbers in almost any small stream or rivulet. It hides under
stones, or in little crevices in the bank, darting out now and then to
seize one of the tiny creatures upon which it feeds, and then hurrying
back with it to its retreat. When it is in the water it travels along by
a series of jerks; sometimes swimming with its back uppermost, and
sometimes on one side. But if it is placed on dry ground it is perfectly
helpless, for its legs are not nearly strong enough to carry it, and the
only result of its struggles is to turn it round and round in a
screw-like manner without forcing it forward at all.
WOODLICE
These odd little creatures are really crustaceans, although they belong
to quite a different group from that about which you have just been
reading. They simply swarm in all damp places. Under logs, in heaps of
decaying leaves, and under the bark of dead trees, they are always
extremely plentiful, and you may also find them in hundreds in cellars
and outhouses. There are several different kinds, one of which rolls
itself up into a ball when it is touched or alarmed. This is called the
pill-woodlouse, or pill-armadillo. Another one is remarkable for the
fact that the mother carries her little ones about with her in a pouch
underneath her body for some little time after they are born.
BARNACLES
You would hardly think that barnacles were crustaceans, would you? Yet
they are; though certainly they are very unlike any of those about which
we have been telling. You can find them in countless thousands upon the
rocks which are left bare by the tide at low water, and very often the
hulls of ships are so covered with them that the vessels have to be
taken into dry dock and thoroughly cleaned before they are fit to start
upon a voyage.
These animals fasten themselves down to their hold by a kind of
foot-stalk, which is firmly attached by a very strong cement. The upper
part of the body becomes covered with a shell, consisting of several
pieces, or valves; and between these, six odd little limbs can be poked
out at will. These limbs are very hairy, and are always waving about, so
as to sweep into the mouth any tiny scraps of food which may be floating
in the water.
There are a great many kinds of barnacles, some of which look very much
like acorns, and grow to a considerable size. These are known as
acorn-barnacles. And there is another, shaped rather like a piece of
round tube, which burrows into the skin of whales, in which it spends
all the remainder of its life! Sometimes it bores its way down
so far that it actually reaches the blubber.
The young of these strange creatures pass through several
transformations, just like those of the lobster and the crab. First,
there is a nauplius, then a zoëa, and then a megalopa, all of which swim
freely about in the water, never fastening themselves down until they
are ready to pass into the perfect form.