The first thing that arrests our attention in these creatures is the extreme delicacy and tenuity of their substance.  The jelly-fish is chiefly made up of fluid.  A quantity of water and a thin membranaceous film, these are its chief component parts.  Professor Owen has ascertained that a large individual, weighing two pounds, when removed from the sea, will be represented, when the fluid which it contains is drained off, ’by a thin film of membrane not exceeding thirty grams in weight.’  Naturalists have commonly described the jelly-fish as being little more than ‘coagulated water’ and the description is correct.

And yet these masses of film and fluid, floating at the mercy of wind and wave, possess powers which we should hardly associate with so simple a structure, and can accomplish works of which we should little suspect them.  Delicate and defenceless as they appear, they can capture fishes of large size, and digest them with ease and rapidity.  Some of them are in truth formidable monsters.  Professor E. Forbes gives the following humorous description of the destructive propensities of some medusae which he had captured in the Zetland seas:—­’Being kept,’ he says, ’in a jar of salt-water with small crustacea, they devoured these animals, so much more highly organised than themselves, voraciously; apparently enjoying the destruction of the unfortunate members of the upper classes with a truly democratic relish.  One of them even attacked and commenced the swallowing of a Lizzia octopunctata, quite as good a medusa as itself.  An animal which can pout out its mouth twice the length of its body, and stretch its stomach to corresponding dimensions, must indeed be “a triton among the minnows;” and a very terrific one too.  Yet is this ferocious creature one of the most delicate and graceful of the inhabitants of the ocean—­a very model of tenderness and elegance.’

The jelly-fishes are all, in their adult state, locomotive beings.  They float freely and incessantly through the ocean, either impelled by their own efforts, or driven by storm and billow.  They for the most part frequent the open seas, and shun the shore, their delicate frames being endangered by the perennial strife between land and water.  Being designed for constant motion, for the navigation of the great waters, their entire organisation is adapted to such a mode of life.  We find amongst those ocean-floaters the greatest perfection and variety of locomotive apparatus; and they have been divided into sections, according to the modifications of this portion of structure which they exhibit.  We shall endeavour to give a popular account of the leading peculiarities of each, and note the most interesting points in the history of the tribe.