JELLY-FISHES.
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 (Acalephæ), 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 Acalephæ 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.
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 medusæ 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.
In the first section, the animals are furnished with a disk or umbrella of varying shape, which serves as a float, beneath which hang certain processes connected with the functions of prehension and digestion. In this division are included some of the best-known forms. The creature, in this case, propels itself by the alternate contraction and expansion of its disk, thus striking the water, and driving itself forward. These movements take place at regular intervals, and serve a double purpose. They not only propel, but at the same time drive the water over the lower surface of the disk. Here is situated a complicated net-work of vessels, and the fluids of the body are thus exposed to the influence of oxygen, and receive the needed aeration. The stroke of the disk, therefore, is not only a locomotive, but also a respiratory act. The jelly-fishes of this section move as they breathe, and breathe as they move. Hence the name which has been given them—Pulmonigrades. We find the same admirable economy of resources amongst the lower animalcules. The cilia which propel them secure the aeration of the system.
It is evident that the motive apparatus in this section of the Acalephæ is but a feeble one. It only avails in calm weather. When the sea is agitated, the jelly-fish is driven helplessly along. It cannot choose its path. As its food, however, is everywhere abundant around it, and it has no business that should lead it in one direction more than another, there is no great hardship in this.