I have made several attempts to keep medusae alive in an aquarium, but have only recently been successful. A medusae when first placed in an aquarium swims actively about, but in a few hours it sinks to the bottom apparently tired out. After an interval of rest it takes another swim and again sinks to the bottom. This is repeated until the medusa becomes completely exhausted; then it stays at the bottom and slowly dies. In spite of every attention, plenty of clean sea-water, plenty of copepods, and a suitable temperature, I found that my medusae often used to die within a day of their capture.
When I have been watching medusae at the surface of the sea, I have noticed that they simply float along with the tide without often pulsating the umberlla. In my bell-jars the water was perfectly motionless, so that a medusa had to pulsate its umbrella in order to keep afloat, and as soon as the pulsations stopped it began to sink. There are some species, like those belonging to the Bougainvillidae, which live longer in confinement, as they are able to poise themselves in the water by the extension of their tentacles and remain motionless for long periods, but even these finally reach the bottom of the bell-jar, and a long period at the bottom ends in death.
It appeared to me that to keep medusae alive in an aquarium it was necessary to have the water in motion so that a medusae could float about just as it does in the sea, without having constantly to pulsate its umbrella.