We inscribe at the head of this paper the popular name of a class of beings, which, though simple in their organisation, are full of interest to the zoologist, and attractive to the common observer from the singularity or beauty of their forms, and, in many cases, the brilliancy of their colouring. The ocean, throughout its wide extent, swarms with myriads of gelatinous creatures--some microscopic, some of large dimensions--which deck it with the gayest colours by day, and at night light up its dreary waste with 'mimic fires,' and make it glow and sparkle as if, like the heavens, it had its galaxies and constellations. These are the jelly-fishes, or sea-nettles (_Acalephae_), as they are often called, from the stinging properties with which some of them are endowed. The commoner forms are well known, for the beach is often strewn with the carcasses of the larger species. On fine days in summer and autumn, whole fleets of these strange voyagers appear off our coasts. Their umbrella-shaped, transparent disks float gracefully through the calm water, and their long fishing-lines trail after them as they move onward. At times, multitudes, almost invisible to the naked eye, tenant every wave, and give it by night a crest of flame; while other kinds measure as much as a yard in diameter. The _Acalephae_ present the greatest variety of form and colour, as well as of size, but they are all of the most delicate structure, frail, gelatinous, transparent. Some are so perfectly colourless, that their presence can with difficulty be detected in the water. The following description, by Professor E. Forbes, applies to a large proportion of the species:--'They are active in their habits, graceful in their motions, gay in their colouring, delicate as the finest membrane, transparent as the purest crystal.' The poet Crabbe has characterised them well in the following passage:-- 'Those living jellies which the flesh inflame, Fierce as a nettle, and from that the name; Some in huge masses, some that you might bring In the small compass of a lady's ring; Figured by hand divine--there's not a gem Wrought by man's art to be compared to them; Soft, brilliant, tender, through the wave they glow, And make the moonbeam brighter where they flow.' 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.