Most boys and girls like corals. They are so common and easily obtained
that I hope each of you will lay aside your reading just here, and hunt
up a piece, no matter how small, that we may examine it carefully, and
see what we can find out about it. You must find, however, a piece of
the natural coral, just as it was brought up out of the sea, and not an
elegant and polished piece such as is made into ear-rings and brooches
and long strings of beads to adorn the necks of ladies and little folk.
What makes this bit of natural coral so rough? The first glance will
convince you that those curious pits and little cups on the surface mean
something; and when we remember that all the corals which reach us are
the skeletons of former living animals, they interest us at once.
Few of us, perhaps, will ever be so fortunate as to see living corals,
since they grow principally in the deep water of warm oceans. The higher
the temperature, the greater the variety and profusion of the coral.
During life the skeleton is covered with soft flesh, the surface of
which is thickly studded with star-like animals called polyps. In this
way millions of polyps are sometimes clustered together in one
community. As they wave their delicate tentacles of white, green, or
rose color, they are said to be very beautiful, especially if seen in
the bright sunlight through water that is clear and still.
[Illustration: Fig. 1.--BRANCHING CORAL ALIVE, WITH POLYPS EXPANDED.]
In Fig. 1 is shown a piece of living coral with the polyps expanded. The
flesh has been removed from the upper branch on the left that we may see
the skeleton. Let us suppose that the specimens we have selected for
study are of this kind. Each of the tiny cups on the surface was once
the frame-work of a separate polyp, and we shall find that its interior
is divided by a number of partitions which do not quite reach the
centre. Look into the cups with your microscopes,[2] and you will find
them very beautiful. One set of partition-walls reaches almost to the
centre, and between these walls are shorter ones. These give us a clew
to the kind of animal that has lived here, and they will at once remind
you of the partitions in the sea-anemone, as shown in Fig. 2 in the
article on "Sea-Anemones," published in YOUNG PEOPLE No. 143. Indeed,
the whole structure of a coral polyp is similar to that of an anemone,
and we can now easily imagine the stomach of the polyp hanging down in
the opening left between those delicate partitions. Coral polyps differ
from sea-anemones, however, in three important ways--they have hard
skeletons, they can not move about, and they usually grow in clusters.
[2] A Coddington lens, which is inexpensive, is a useful thing to
possess. It can be carried in the pocket; and if we have it always with
us, we may find new beauties wherever we go.
When young, coral polyps are little jelly-like animals which swim about
in the water. After they have chosen a resting-place, and the stomach
and tentacles have grown, hard particles of lime, which they have drawn
in from the sea-water, settle in their flesh to form a circular cup as
well as the partitions inside. In this way the polyps soon acquire a
solid frame, the soft parts being the stomach, the fringe of tentacles,
and the fleshy mass covering the skeleton and the internal partitions.
They can draw the tentacles entirely within the body, as the anemone
does. Like the anemone they also have lasso-cells for capturing their
food.
Should it be a branching coral whose history we are tracing, it will now
begin to bud from the sides. The buds will grow into branches, throwing
out other buds, somewhat as plants do, until we have an elegantly
branching colony. Each bud is a new polyp, and remains attached to the
branch from which it sprang. Although the polyps in such a community
have separate mouths and stomachs, there is a close connection between
them, and a free circulation of fluids through the soft flesh.
As in other families one generation passes away and another takes its
place, so in large branches of coral the lower and older portions may be
dead, and living polyps will be found only at the ends of the branches.
Corals seem to be delicate creatures, as they will not flourish under
adverse circumstances. They require water of a certain depth, and they
die immediately if exposed to the sun or to cold weather.
Besides increasing by budding, corals increase rapidly by eggs. Their
eggs are pear-shaped, transparent bodies, covered with cilia, which are
in constant motion, and which row the jelly-like lumps through the
water. The parents, you remember, are firmly rooted to some object, but
their little ones are gifted for a time with the power of motion. They
may well enjoy the privilege while it lasts, for it is their only chance
of exploring their ocean home. Presently they must settle down like
other sedate corals. But it is in this manner that the young polyps are
distributed through the ocean instead of growing in a crowded colony
around the parent.
You will often hear coral spoken of as having been built by an insect,
and you will see at once that this is far from correct. Coral polyps are
very different from insects, and their skeletons grow, much as ours do,
inside of the animal; so we can not say they have been built. All such
animals as coral polyps, which have the mouth in the centre, with other
parts radiating from it, are called "Radiates."
Besides these branching corals which resemble trees and shrubs, some
grow in solid masses without sending off branches. Others assume the
shape of graceful vases; all of these are gayly decked with star-like
polyps of varied colors. Does it not seem to you as if the ocean was one
vast store-house of beautiful things?
[Illustration: Fig. 2.--MUSHROOM CORAL.]
The mushroom coral (Fig. 2) looks indeed like a large mushroom, although
you will notice that the leaflets are on the upper surface instead of
being underneath, as they are in the vegetable mushroom. This coral is
the skeleton of one huge polyp, and we see the depression in the centre
corresponding to the little cups on most other corals.
[Illustration: Fig. 3--ORGAN-PIPE CORAL.]
The organ-pipe coral consists of lovely crimson tubes standing upright,
and connected at short distances by thin flat plates, which give it the
appearance of being several stories in height. These plates may be
distinctly seen in Fig. 3. When alive, a little polyp protrudes from the
top of each tube, and being of a bright purple color, it makes a
striking contrast with the crimson tube.
[Illustration: Fig. 4.--FRAGMENT OF RED CORAL WITH POLYPS.]
Red coral, which is used for jewelry, grows in a bushy form on rocks at
the bottom of the Mediterranean and Red Seas. The fleshy mass of this
coral is colored red by the numerous red spicules it contains, while the
polyps are pure white. The whole resembles a pretty red shrub spotted
over with sparkling white flowers. The spicules in the centre of the
branches form a solid stem, which takes a fine polish. Underneath the
flesh the surface of the coral is marked with deep grooves, which are
canals for the circulation of water. These grooves are shown at both
ends of the branch in Fig. 4. They are always removed in polishing.
Red coral is generally obtained by fishermen, who drop into the water
heavy wooden crosses to which strong nets are attached. As the boat
moves slowly forward, the crosses are raised and lowered to break off
the coral branches. The apparatus is then lifted from the water, and the
fragments of coral which have become entangled in the net are carefully
removed. There are many shops in Italy where the coral is polished and
cut into various ornaments. Delicate rose-colored corals are considered
very choice and elegant, but the natives of India prefer blood-red ones,
which contrast finely with their dark rich complexions. Corals are their
favorite ornaments, and large quantities are imported every year.