Fig. 7.—A common British
Jelly-fish.Aurelia aurita, usually as
large as a breakfast-plate
and often larger.
The animals to which the name "jelly-fishes" is now more or less strictly applied are (as that fine zoologist Aristotle knew) in their structure closely similar to the sea-anemones, but even simpler. They are called the Medusæ by naturalists. Their disk-like bodies are largely formed by a jelly-like material, on the surface of which are stretched delicate transparent skin, nerves, and delicate muscles, whilst in the middle of the disk, on the surface which faces downwards as the creature floats, is the mouth, leading into a relatively small pouched cavity excavated in the jelly, from which a delicate system of canals is given off, and radiates in the jelly of the disk. There is, as in the sea-anemones, only one continuous cavity. The edge of the disk is beset with fine, sensitive tentacles, sometimes many feet in length, and the lips of the mouth are often drawn out into a sort of depending trunk, or into four large tapering lobes or lips of jelly, which, with the longer tentacles, are used for seizing prey. The commonest jelly-fish on our coast—so common as to be "the" jelly-fish par excellence—is often to be seen left on the sands by the receding tide or slowly swimming in quiet, clear water at the mouth of a river in enormous numbers. It is known as "Aurelia" ([Fig. 7]). It is as big as a cheese-plate, and the four pouches connected with the stomach are coloured pink or purple, and appear in the middle of the circular plate of jelly, like a small Maltese cross. The reproductive particles (germ-cells and sperm-cells) are produced in that coloured region, and escape by the mouth. There is a fringe of fine, very short tentacles round the edge of the disk, and they, as well as the great lobes of the mouth, are provided with innumerable coiled-up stinging hairs or "thread-cells," similar to those of the sea-anemones, which led Aristotle to call both groups "sea-nettles." Eight stalked eyes are set at equal intervals around the disk.
Usually accompanying the floating crowd of the common and abundant Aurelia are a few specimens of a very unpleasant kind of Medusa of a turbid appearance, often called "slime balls" by fishermen, from six inches to a foot in diameter. It is known to naturalists by the name "Cyanæa capillata." The tentacles on the edge of the disk of this kind of jelly-fish are very long and elastic, stretching to several feet, even yards, in length, and are provided with very powerful stinging hairs. The tentacles not infrequently become coiled around the body of a bather; the stinging hairs are shot out of the little sacs in which they are rolled up, and the result may be very painful to the person stung in this way and even dangerous. There are two other common large jelly-fish on the English coast, one called "Chrysaora" (Fig. 8), with a wheel-like pattern of brown pigment on the disk, and the other with the mouth lobes very large and bound together like a column.
Fig. 8.—A common British
Jelly-fish.Chrysaora hysoscella, usually twice
as big as the figure.
The common Aurelia is remarkable for the fact that the young which hatch from its eggs attach themselves to stones and rocks on the sea bottom, and grow into little white tube-like polyps, about half an inch long, quite unlike their parent, with a crown of small tentacles surrounding the mouth, whilst they are fixed by the opposite end of the body. Then a very curious thing happens. The little polyp becomes nipped at intervals across its length, so that it looks like a pile of saucers—a dozen or more. And then the top saucer swims away as a minute jelly-fish, the next follows, and so on, so that, in the course of an hour or two, the whole pile separates into a number of freely swimming young, each of which gradually grows into a full-sized Aurelia. I have only once had the chance of witnessing this beautiful sight, and that was many years ago in a tank at the Zoological Gardens (they have no such tanks now), where the polyp-like young (called "Hydra tuba") spontaneously put in an appearance, and proceeded to break up into piles of little disks, which separated and swam off as one watched them. The French poet, Catulle Mendés, imagined a world where the flowers flew about freely and the butterflies were fixed to stalks. His fancy is to some degree realized by the swimming away of the young jelly-fish from their stalks. There are a host of very minute jelly-fish, measuring when full grown only half an inch or less in diameter. They originate as buds from small branching polyps, one kind of which is common on oyster-shells, and is called "the herring-bone coralline." The dried skins of these coralline polyps (which are horny) are often to be picked up with masses of seaweed on the seashore after a storm. The little jelly-fish are the ripe individuals of the polyps, and produce eggs and sperm which grow to be polyp-trees. These, again, after growing and branching as polyps, give rise to little jelly-fish here and there on the tree, which in most kinds (though not in all) break off and swim away freely.