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.
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.