The ship cuts through the ocean. It rises like a lofty fortress from the illimitable blue plain, with the white clouds on the far horizon. No land has been in sight for days. Yesterday a poor wind-borne butterfly rested on the deck. To-day it is gone, and all is sea. Then they suddenly appear silently in the blue mirror: mysterious discs, red as the anemones on a Roman meadow in spring, golden as the autumn leaves on a dark pond in the park, then blue, like a lighter blue floating on the general azure. They are the medusæ. At one time the ship sails through a whole swarm of them—thousands, hundreds of thousands, millions, a veritable milky way of coloured stars. On the next day they have all gone. No inhabitant of the ocean seems to be so close to it as this creature. The whole animal is only a shade more substantial than the water. You take it out, and try to catch hold of it. It stings your hand like a nettle: that is its one weapon. But it is already destroyed, melted away, a formless nothing. You put it on a piece of blotting-paper, and it dries up into the spectral outline of a shadow, a tiny “fat-spot,” summary of its whole existence. Yet this soap-bubble of the water is a real animal. Its transparent body is shaped like a bell, and moves through the water by regular contraction and expansion, like the lung in breathing. Where the clapper of the bell should be, we find a stomach, with a mouth for eating, hanging down from the curved upper part. At the edge of the curved surface are many long fibrils that close on the approaching prey and paralyse it by their sting. Then it thrusts it into its mouth and swallows the object into the stomach. The medusa is, of course, a very lowly creature, but it is much more advanced in organisation than the tiny radiolarian. The radiolarian consists of a single cell. The medusa is a cell-state, a community of countless cells with a division of labour amongst them. Some of the cells form the wall of the bell, some the stinging threads, some the devouring and digesting stomach. In this the medusa comes nearer to man than the radiolarian. Some of the cells see to the reproduction of the medusa. Ova and spermatozoa are detached from the cell-community of the medusa’s body, blend together, and thus form the germ of a new medusa. In most cases the process is curious enough. From the germ-cell we get at first, not a real medusa, but a polyp that attaches itself to the ground, a little creature that may be remotely compared to the pretty water-lilies that meet the eye in an aquarium. Then the polyp produces something like a plant that grows buds, the real medusæ; it may produce these out of its substance as buds, and they then float away like detached flowers, or (in other species) it may gradually change itself into a chain of medusæ, of which the uppermost is detached first, then the next, and so on.