Let us now examine some odd-looking animals called hydroids, or
sea-firs, which grow in the ocean, firmly rooted upon the bottom, or
attached to shells and stones.
[Illustration: FIG. 1.--HYDROIDS GROWING ON A SHELL.]
The tall branches in Fig. 1 are hydroids growing upon the shell of a
dead mussel. A barnacle, too, has lived and died on this pretty shell,
and little sea-weeds cluster around its remains.
We can scarcely imagine animals that are more unlike jelly-fish than
these slender branches are; and yet the wonderful story I have to tell
you will show them to be so closely related that we could not study the
life of one without the other.
Long graceful sprays of hydroids are often thrown on shore by the tide,
and as they resemble plants much more than animals, they are generally
mistaken for sea-weeds. Many persons gather them for decorating brackets
and hanging baskets. We frequently see bunches of them arranged in
sea-shells, and offered for sale in our shops. The shop-keeper would
probably not know them by any other name than sea-weed. Still, they are
animals, and we can mostly recognize them by their yellow, horny
appearance, and by the numerous joints on their stems.
In looking at one of these sprays with a microscope you will find each
little point on the stem to be in reality a dainty cup, which when alive
contained a hungry animal. Should you find a piece freshly washed up
from the ocean, it would be well to place it in a glass jar filled with
sea-water, and after allowing it to remain perfectly still for a while,
it may perhaps show you, if it is yet alive, how it has been accustomed
to pass the quiet hours in its native home.
You will find each cup occupied by a soft animal, with a mouth in the
centre opening directly into the stomach. Hydroids, you see, are higher
in the scale of life than sponges, for they possess mouths and stomachs.
As we watch, the body of the animal will rise up in the cup, and from
around the mouth will gradually creep out slender thread-like feelers,
which may be extended quite a distance, or drawn up at will entirely
within the body of the animal. You will, of course, wish to use the
proper name for these feelers. They are called tentacles, and they
evidently serve to produce currents of water toward the mouth, and to
bring the required food. In this way the little animals live, day after
day and year after year, patiently waving their tentacles, and waiting
for the food that is sure to come.
Do you still ask what connection there is between these demure little
animals and the jolly jelly-fish? We shall soon see.
The hydroids have grown by budding and branching somewhat as plants do.
Occasionally pear-shaped cups much larger than those we have looked at
are formed on the stem. These large cups are called spore-sacs. They
contain the substances which, later, will grow into eggs; and at the
proper time they fall off. After resting awhile, and throwing out cilia
and tentacles, these spore-sacs swim gayly away, and, strange to relate,
they are hence-forth known by the name of jelly-fish!
[Illustration: FIG. 2.--HYDROID MAGNIFIED, SHOWING SPORE-SACS.]
In Fig. 2 you will see a spray of hydroid magnified which shows two
spore-sacs. In the species which is represented here (the Sertularia)
the spore-sacs do not fall off, but they burst and discharge the eggs
which they contain.
These jelly-fish now lead active lives, and as they dart and swim about
in the water no one would suspect that they had any relation to the
plant-like animals with which we started, yet it is supposed that most
hydroids have this wonderful history.
Forgive us, jelly-fish, forgive us, hydroids, if in our ignorance we
have ever cast an indifferent glance upon you. We did not know your
charming secret, and we should never have guessed it, for the lives we
lead are so different from yours. Now that we have learned your secret,
we shall certainly tell it to the boys and girls, that they may help us
enjoy it.
Jelly-fish produce eggs, from which are born little floating bodies.
These after a time fasten themselves to some stick or stone, and grow by
budding until they become the elegant feathery branches which we must
now call hydroids. The young of nearly all animals resemble their
parents, but the children of jelly-fish, you see, are very different
from the jelly-fish itself. In the next generation, however, we shall
find jelly-fish again.
Most of the plant-like objects which we are accustomed to see growing
near the shore are in reality hydroids. Has it ever puzzled you to know
the difference between plants and these low forms of animal life? One
very important difference is that most plants can procure their food
directly from the soil, whereas animals are obliged to feed upon living
substances, or those which have at some time been alive, as vegetables
and animals.
Hydroids grow in all parts of the ocean, in deep water as well as near
the coast. Some of them are three feet high. One branch may contain a
hundred thousand distinct animals, the only communication between them
being a circulation of fluid through the hollow stems. In this way each
branch constitutes a family which has sprung originally from the same
little egg. Some varieties never grow tall, and as they occur in patches
over rocks and shells, they resemble thick beds of moss.
[Illustration: FIG. 3.--JELLY-FISH (AURELIA AURITA), WITH YOUNG IN
VARIOUS STAGES.]
The little hydroids which we see hanging from the under side of a rock
in Fig. 3 produce jelly-fish in a different manner from the one I have
described, although it is equally remarkable. This hydroid has no buds
or branches, but the main tube of the body divides itself into a number
of rings or plates, until the whole animal looks somewhat like a pile of
tiny saucers with scalloped edges. Finally the upper plate begins to
twist and squirm until it loosens itself from the pile, and floats off
to lead the gay and independent life of a jelly-fish. It is followed by
the other plates in their turn, each making a separate animal. These new
jelly-fish eat greedily and grow fast, forming some of our largest
varieties.
We can form but little idea of the immense numbers of animals thus
leading quiet contented lives, and drawing from the surrounding water
all that is needed for their support. They can not go in search of food,
and they take only such as floats toward them. Still, they seem to have
some choice in the matter, as they reject from their mouths any food
they are not suited with. Many of these curious animals are glowing with
bright colors, and surrounded as they are with a great variety of
plants, they give to the bottom of the ocean a marvellous beauty.
Does it not seem strange that the slender, delicate sprays of which we
have been speaking are really animals; and more than that, the children
of jelly-fish? A little girl once exclaimed, on hearing of these
wonderful changes that happen in the life of hydroids, "Why, it seems
almost like a fairy tale!"