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!
Fig. 2.—Hydroid magnified, showing Spore-Sacs.