SEA-URCHINS, STARFISHES, AND SEA-CUCUMBERS
Next in order to the crustaceans comes a group of animals which live in
the sea, and which are known as echinoderms, which simply means
spiny-skins. This group includes the sea-urchins, the starfishes, and
the sea-cucumbers.
[Illustration: LIFE ON THE SEA-BOTTOM.
1. Sticklebacks. 2. Carp. 3, 5, 6, 13, 17. Sea-Anemones.
4. Shrimps. 7. Prawn. 8. Fiddler Crab. 9. Starfish.
10. Sea-horses. 11. Edible Mussels. 12. Serpula Worm.
14. Hermit-Crab in Whelk's Shell. 15. Sea-urchins. 16. Rock Crab.
18. Polyzoan (Flustra). 19. Corallines (Gorgonia).]
SEA-URCHINS
You can find a good many of these creatures when you go to the seaside,
by hunting about on the beach at low water. In some places on rocky
coasts sea-urchins are very common. Sometimes they are known as
sea-eggs, and in many countries they are actually boiled and eaten as
food, just as we eat the eggs of fowls and ducks. And their shells are
so thickly covered with spines that they look just like little hedgehogs
which have rolled themselves up into balls.
When the animal is alive it can move these spines at will, each of them
being fastened to the shell by a ball-and-socket joint, just like those
which we described to you when we were telling about the vertebræ of the
snakes. But after it has been dead for a few days they are nearly always
knocked off by the action of the waves, so that the shell is left quite
smooth and bare.
By means of these spines a sea-urchin can bury itself in the sand at the
bottom of the sea in a very short time, only just a little funnel-shaped
pit being left to show where it is hiding. And in some of the larger
kinds they are really formidable weapons, for they grow to a length of
eight or ten inches, and are so sharp and strong that they can actually
pierce the sole of a stout shoe. Besides this, they have poison-glands
connected with them, so that they can easily inflict a really serious
wound.
In the shell of a sea-urchin are a number of little holes, through which
the animal pokes out most curious sucker-like feet when it wants
to climb about over the rocks. By means of the suckers on the upper part
of the shell it often clings to small stones, which it sometimes gathers
up in such numbers as to conceal itself entirely from sight.
Just inside the mouth of the urchin are five very large chisel-like
teeth. These are formed just like the front teeth of the rodent animals,
and grow as fast as they are worn away.
Sea-urchins are not numerous on the Atlantic shores of North America,
because these shores are not rocky except in the cold north. One small
flat kind, however, occurs in the deep waters off this coast, and its
cases are often cast up on the beaches and are called sand-dollars. On
the Pacific coast, however, sea-urchins are common and well known; and
the Indians of the northwest coast boil them and eat them greedily.
STARFISHES
More plentiful on both coasts, and extremely numerous and harmful in all
the bays and sounds from Florida to Maine, are the starfishes, or
fivefingers, as the oystermen call them. But although they are so
abundant, very few people seem to know what curious creatures they are.
The starfish has hundreds of little sucker-like feet, just like those of
the sea-urchin. You cannot see these, as a rule, because the starfish
keeps them tucked away inside its skin. But when it wants to use them it
can poke them out in a moment.
If you want to look at these odd little feet, the best way to do so is
to take a live starfish, put it at the bottom of a pool of sea-water,
and then wait patiently for ten minutes or a quarter of an hour. By the
end of that time you are almost sure to see that the animal is slowly
moving. Then snatch it out of the water, turn it upside down, and you
will see hundreds of little white objects waving about on the lower
surface of its body. These are its feet, and if you look at them through
a good strong magnifying-glass, you will see that they are shaped just
like wine-glasses, each having a kind of fleshy cup at the end of a
slender stem. And at the end of the cup is the sucker.
In the very middle of the lower part of the body of a starfish is its
mouth. This is generally rather large, for the animal feeds chiefly on
shell-bearing animals which it swallows whole, shells and all. Then,
when it has digested the bodies of its victims, it turns their empty
shells out again through its mouth. That is an odd way of feeding, isn't
it? But sometimes it feeds in an odder way still, for when it finds a
creature which is too big to be swallowed, it will actually turn its own
digestive organs out of its mouth, wrap them round its victim, hold them
there until it is digested, and then drag them in again and go off to
look for another victim!
Starfishes eat a great many oysters in this way. So many do they destroy,
indeed, that they are the very worst foes with which oyster-fishers have
to deal, and the damage done by them in one single oyster-bed on the
coast of North America is estimated at no less than fifty thousand
dollars every year.
There are a great many different kinds of starfishes. One, for example,
has twelve rays instead of five, and looks very much like a live
sunflower. This is called the sun-star. Another has its five rays all
joined together by webbing, very much like the toes on a duck's foot,
and is known as the bird's-foot star. It is a very handsome creature,
for while the greater part of its body is bright yellow, it has a broad
band of crimson running all the way round the outer margin, and another
stripe of the same color down the outer edges of each ray, while the
membrane between them is fringed with yellow hairs. But you are not very
likely to find it, for it lives in rather deep water, and is hardly ever
caught except by means of that useful net which is called a dredge.
Odder by far than any of these, however, are the brittle-stars, which
owe their name to their extraordinary habit of breaking themselves to
pieces! They nearly always do this if they are touched or alarmed. In
fact, they are so ready to do so that it is very difficult indeed to
obtain a perfect brittle-star for a museum. The creature just gives a
kind of shudder, and its five rays all drop off and break up into little
pieces, all that is left of the animal being just the central disk. But
it does not appear to suffer any pain, and loses hardly any blood.
And before very long new rays grow in the place of the old ones,
so that in a few weeks' time the starfish is just as perfect as ever!
The brittle-stars have five very long and very slender rays, which are
generally fringed on either side with yellow hairs. And these rays are
hardly ever still, but twist and writhe and curl about so actively that
they really look almost like so many centipedes! It is by no means so
numerous as the fivefinger, and is so easily broken that it is hard to
find a whole one on the beach.
Very curious, too, is the basket-star, which at first sight does not
look like a starfish at all. The reason is that, close to its body, each
of the five rays divides into two. Then each of the branches divides
into two again, and each of those into two more, and so on over and over
again, till sometimes there are more than eighty thousand little arms
altogether!
The basket-star catches its prey by means of these wonderful rays, which
it wraps all round it in the form of a circular basket. It is not at all
a common creature, and is only found in deep water.
But perhaps the oddest of all these creatures is the rosy feather-star,
which actually grows on a stalk while it is young, and looks just like a
flower with its petals spread. The stalk, which is fastened down to a
rock at the bottom of the sea, is made up of a great number of tiny
joints, and grows longer and longer. And when it reaches its full length
the animal breaks itself free and swims away, leaving the stem behind.
The rosy feather-star lives in rather deep water, from which it is
sometimes brought up by means of the dredge. It can crawl about on the
ground by means of its sucker-like feet, and can swim through the water
with some little speed. And very often, to save itself trouble, it will
cling by means of its rays to a piece of floating wood, and allow itself
to be carried for long distances by the waves.
In Great Britain these may often be found near shore, but the American
feather-stars all live in very deep water. They are all that remain of a
large class of such animals which abounded in the very ancient seas,
whose fossil remains are called stone-lilies.
SEA-CUCUMBERS
These are really relations of the starfishes, although they do not look
in the least like them; for they closely resemble the vegetable after
which they are named. In front of the slit at one end of the body,
however, which serves as a mouth, there is a feathery tuft. This
consists of delicate little tentacles, or feelers, by means of which the
animal fishes for its food, and which can be drawn back inside the body
when it is not hungry. And if it were not for this tuft one really might
almost mistake the animal for a grayish-white cucumber.
We saw just now that the brittle-star breaks off its own rays at the
slightest alarm. But the sea-cucumber, in this way, is even odder still,
for if it eats anything which disagrees with it, as it sometimes does,
it turns all its digestive organs out of its mouth, cuts them off, and
allows them to float away! Then for three or four months it is very
little else than a bag of empty skin, with just a slit at one end and a
tuft in front of it. But at the end of that time new digestive organs
begin to grow in the place of the old ones, and very soon the
sea-cucumber is as perfect as ever!
Isn't that a remarkable way of curing indigestion?
Some of the sea-cucumbers grow to a very great size. One indeed, when
fully grown, is nearly six feet long. And in China they are largely used
as food, under the name of trepang, and are looked upon as a great
dainty.