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.

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.

All the specimens were males, and the puzzle was to find out how it reproduced itself. After a few seasons had passed I determined to solve this problem. I made the guess that perhaps the jelly-fish were budded off from a fixed weed-like polyp growing in the depths of the tank—as is the case with many of the marine jelly-fishes. I remember that one leading member of the council, which still presides over the destinies of the Botanic Gardens, confided to me in a hushed whisper his belief that Providence created this new jelly-fish year by year in the tank in honour of the august patroness of the Botanic Society—Her Royal Highness the Duchess of Teck. I was obliged to make an end of this flattering theory when I discovered, after long searching with my assistant—attached to the rootlets of floating water weeds a minute three-branched polyp (Fig. 4), from which, as we subsequently were able to observe, the jelly-fish were pinched off as tiny spheres about one-sixteenth of an inch in diameter. No females of this jelly-fish were ever discovered. The polyps lived on from year to year, and budded off each season a swarm of pretty but futile male jelly-fish. They ripened and died on attaining a diameter somewhat less than that of a shilling. There were many most interesting points made out as to their structure, mode of feeding, and growth. You could keep them in a tall glass jar supported over a small gas-jet (they lived best at a temperature of 80° Fahr.), and they would swim up by a series of strokes to the top of the water, and then drop like little parachutes through the eighteen inches of depth to the bottom—taking in water-fleas and such food on the way—and immediately would start upwards again. I used to take them alive in my pocket corked up in a test-tube to show to friends.

After they had disappeared from the tank in Regent’s Park (owing to some unhappy cleaning of the tank) they suddenly, in 1903, appeared—it seems incredible—at Sheffield! Then they briefly showed up in 1905 at Munich, and at Lyons had been captured in 1901—always in a tepid water-lily tank! We never could make out where they came from originally. Of course, the polyp must have been brought into the tank with some bundle of water plants from a tropical lake or river, but we never had any indication as to when or which.

Since the days of the fresh-water jelly-fish of Regent’s Park, which was called (a name, but why should it not have a name?) Limnocodium Sowerbii—a jelly-fish of about the same size (Fig. 5) but very different in shape and tentacles—was discovered in the great African fresh-water lake Tanganyika—in enormous numbers, and was named Limnocnida TanganyikÆ. Only five years ago the same jelly-fish was discovered in the Victoria Nyanza, and a little earlier in backwaters of the Niger. It is a curious and significant fact bearing upon the history of these three areas of fresh water connected with the three greatest African rivers—the Congo, the Nile, and the Niger—that the same little jelly-fish is found in all of them.

And now we have just been reminded of Limnocodium, the Regents Park jelly-fish, from a remote and unexpected source. A thousand miles up the Yang-tse-Kiang River, in China, in the province of Hupi, the Japanese captain of a river steamer, plying there and belonging to a Japanese company, captured ten jelly-fish in the muddy waters of the river. He brought them home, preserved, I suppose, in alcohol or formalin, and they have been described by Dr. Oka, a Japanese zoologist of Tokio, in a publication bearing the Latin title Annotationes ZoologicÆ niponenses, issued in December 1907. European sea captains have not rarely been ardent naturalists, but I think the Japanese is the first captain of a river steamboat who has discovered a new animal on his beat. I have not heard of Mississippi steamboat captains amusing themselves in this way—other rivers, other tastes.

Dr. Oka describes the jelly-fish thus brought to him as a Limnocodium, differing in a few details from that of Regent’s Park, so that he distinguishes this Chinese species as Limnocodium Kawaii, naming it after the naturalist captain, who must have a rare taste for picking up strange and new things, and a rare goodwill in bringing them home with him. So here is another fresh-water jelly-fish, for it is not the same as the Regent’s Park one, though closely like it. Possibly Limnocodium is an Asiatic genus, and the original Sowerby’s Limnocodium will be found in another Chinese river. But it may prove to be South American, as is the water-lily Victoria regia.

A very small fresh-water jelly-fish was found some twelve years ago—in 1897—in the Delaware River at Philadelphia, United States, and was lately described by the well-known naturalist, Mr. Potts. It was budded off from a very minute polyp resembling that found in the Regent’s Park, but the jelly-fish was totally different from Limnocodium. Only four or five specimens of this jelly-fish have ever been seen, and the Philadelphian naturalists ought certainly to look it up again.

An account of the Philadelphian jelly-fish and of other fresh-water jelly-fishes, with illustrative plates, will be found in the Quarterly Journal of Microscopical Science, 1906. Mr. Charles Boulenger has, in the same Journal, 1908, described yet another fresh-water jelly-fish from the Fayoum Lake in Egypt.