The substance of these animals seems to have hardly more density or solidity than their native element. I remember showing one to a friend who had never seen such an animal before, and after watching its graceful motions for a moment in the glass bowl where it was swimming, he asked, "Is it anything more than organized water?" The question was very descriptive; for so little did it seem to differ in substance from the water in which it floated that one might well fancy that some drops had taken upon themselves organic structure, and had begun to live and move. It swims by means of rapid contractions and expansions of its disk, thus impelling itself through the water, its tentacles floating behind it and measuring many times the length of the body. The disk is very convex, as will be seen by the wood-cut; four tubes radiate from the central cavity to the periphery, where they unite in a circular tube around the margin and connect also with the four tentacles; from the centre of the lower surface hangs the proboscis, terminating in a mouth. Notwithstanding the delicate structure of this little being, it is exceedingly voracious. It places itself upon the surface of the animal on which it feeds, and, if it have any hard parts, it simply sucks the juices, dropping the dead carcass immediately after; but it swallows whole the little Acalephs of other Species and other soft animals that come in its way. Early in summer these Jelly-Fishes drop their eggs, little transparent pear-shaped bodies, covered with vibratile cilia. They swim about for a time, until they have found a resting-place, where they attach themselves, each one founding a Hydroid stock of its own, which will in time produce a new brood of Medusae.