Most people nowadays know a jelly-fish when they see one—and recognise that it is eminently a product of the sea—one sees them washed up on the seashore, soft discs of transparent jelly of the size of cheese-plates (Fig. 2). They have a mouth in the centre of the disc, often at the end of a depending trunk, like the clapper of a bell. Some have tentacles, sometimes yards long, which sting like nettles. They also have eye-spots, an internal system of canals and muscles which enable them to swim by causing the edge of the disc or bell to contract and expand in alternate strokes. There are hundreds of kinds of marine jelly-fish varying in size from a sixpence to that of a dinner table, and until twenty-five years ago none were known to live in ponds, lakes, or rivers. Although they often are carried up estuaries, and may stay for a time in brackish water, or even in fresh water, none were known which really lived and bred in fresh water. They were regarded, as are star-fishes and sea-urchins, as distinctively marine, and debarred by the delicacy of their watery jelly-like substance from tolerating the change from sea water to fresh water as a permanent thing. All fresh-water animals—fishes, shell-fish, cray-fish, worms, and polyps—are derived from closely similar marine animals, are in fact sea-things which have suffered a change, and been able to stand it. [Illustration: FIG. 2.—The common jelly-fish (_Aurelia aurita_) one-third the natural size; _or_, one of the four arms or fleshy tentacles surrounding the diamond-shaped mouth; _Tc_, one of the eight eye-bearing tentacles at the edge of the disc; _GP_, opening of one of the four sub-genital pouches, which bring sea-water close to the ovaries and spermaries, which, however, do not open into these pouches; _x_ and _y_, outline of the sub-genital pouches seen through the jelly.] These being our preconceptions about jelly-fish, great was the excitement when, in 1880, hundreds of beautiful little jelly-fish were suddenly discovered briskly expanding and contracting, rising and sinking in the water of a large fresh-water tank in the middle of London (Fig. 3). You never know who or what may turn up in London. A badger, a green parakeet, a whale, an African pigmy, an Indian scorpion, and a voice worth ten thousand a year, have all, to my knowledge, been stumbled upon unexpectedly at different times in the highways of London. A new jelly-fish was perhaps one of the least expected “casual visitors.” It was found in the large tank four feet deep in which the great tropical water-lily—the Victoria regia—and other tropical water plants are grown in the Botanic Gardens, Regent’s Park. It came up by hundreds every year for some ten years after its first appearance, dying down in six weeks or so each season.