Kitabı oku: «Chambers's Edinburgh Journal, No. 423», sayfa 3
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.
In this section are included some of the most beautiful, as well as common of the tribe. The forms of the umbrella are often most lovely, and present an astonishing variety. As an example of the beauty which they sometimes display, we may refer to a species which resembles an exquisitely formed glass-shade, ornamented with a waved and tinted fringe. The most perfect grace of form, the transparency of the crystal, and colour as delicate as that of the flower, combine to render this frail being one of the loveliest of living things.
In another section, locomotion is effected by a modification of ciliary apparatus. We have a familiar example in the Beroe of our own seas, a most attractive little being, and a prime favourite with naturalists, who have described its habits and celebrated its beauty with enthusiasm. We shall not soon forget the delight with which we first made acquaintance with this graceful little rover. While rambling along the shore in quest of marine animals, our attention was arrested by a drop of the clearest jelly, as it seemed to be, lying on a mass of rock, from which the tide had but just receded. On transferring it to a phial of sea-water, its true nature was at once revealed to us. A globular body floated gracefully in the vessel, scarcely less transparent than the fluid which filled it. Presently it began to move up and down within its prison-house, and the paddles by means of which the beroe dances along its ocean-path were distinctly visible. These paddles are nothing more or less than cilia of a peculiar kind, ranged in eight bands upon the surface of the body. They are set in motion at the will of the animal, and their incessant strokes propel it swiftly through the water. By stopping some of its paddles, and keeping others in play, the beroe can change its course at pleasure, and so wander 'at its own sweet will,' through the trackless waste. Beauty waits upon the course of this little crystal globe. The grace and sprightliness of its movements must strike the commonest observer. As the sunlight falls upon its cilia, they are 'tinted with the most lovely iridescent colours;' and at night they flash forth phosphoric light, as though the little creature were giving a saucy challenge to the stars.
The beroe is a most active being, its habits conforming to the organisation with which it is endowed. Such an array of paddles prophesies of a mercurial temperament and an energetic character. It can, however, anchor itself and lie by when occasion offers. It is provided with two long cables, prettily set with spiral filaments or tendrils, by means of which it can make fast to any point. When not in use, it can retract them, and stow them away in two sacs or pouches within the body, where they may be seen coiled up, through the transparent walls. The mouth is a simple opening at one pole of the globular body. No arms are needed. The beroe is spared the labour and uncertainty of the chase. As it dances gaily along, streams of water, bearing nutritive particles, pass through the orifice into its stomach.
In this creature, as in many of the lower animals, there is a remarkable power of retaining vitality after the most serious injuries; nay, in portions actually severed from the body, it will continue for some time. Mr Patterson, in his excellent Introduction to Zoology, mentions that on one occasion he divided a fragment of the body of a beroe, lately taken from the shore and shattered by a storm, 'into portions so minute that one piece of skin had but two cilia attached to it, yet the vibration of these organs continued for nearly a couple of days afterwards!' But we must leave the beroe, charmer though it be.
Another member of this section—the Ciliograde acalephæ, as they are called—is the Girdle of Venus, which resembles a ribbon in form, and is sometimes five or six feet in length, covered with cilia, and brilliantly phosphorescent. This must be one of the most beautiful of the fireworks of the ocean.
The jelly-fishes of another section are furnished with one or more air-bags, which assist them in swimming, and hence bear the name of hydrostatic acalephæ. In the Portuguese man-of-war (Physalia), the bag is large, and floats conspicuously on the surface of the water. From the top of it rises a purple crest, which acts as a sail, and by its aid the little voyager scuds gaily before the wind. But should danger threaten—should some hungry, piratical monster in quest of a dinner heave in sight, or the blast grow furious—the float is at once compressed, through two minute orifices at the extremities a portion of the air escapes, and down goes the little craft to the tranquil depths, leaving the storm or the pirate behind. In one species (Cuvieria), the floats are numerous and prettily ranged round the margin of the body. Resting on these, the creature casts about its long fishing-lines, and arrests the passing prey.
One more section remains to be noticed. The jelly-fishes which belong to it have a rudimentary skeleton—a plate which supports the soft, circular body. From the lower part of the body hang numerous tentacles (cirri), amidst which the mouth is placed. Probably these multitudinous arms assist in locomotion; and, hence the name of the family, Cirrigrades. Amongst the creatures of this division we meet with some very interesting locomotive apparatus. There are some of them by no means obliged to trust to their oars alone—they have also sails. The Velella, large fleets of which visit our seas at times, has a plate (the mast) rising from its bluish disk or deck, covered with a delicate membrane (the sail) of snowy whiteness, by means of which it traverses the ocean. This sail, it has been noticed, 'is set at the same angle as the lateen-sail' of the Malays. We cannot doubt that it is admirably suited to its purpose, and the Malays may be proud of having nature as a voucher for their contrivance.
We find in another species a still more perfect rigging. In it (Rataria) the crest is supplied with muscular bands, by means of which the sail can be lowered or raised at pleasure. These adaptations of structure are full of interest. Nothing can be more admirable than the sailing-gear of these little creatures. They have to traverse the surface of the ocean amidst all diversities of weather. Paddles alone would not suffice for them. They must be enabled to take advantage of the winds. Sails, therefore, are added, and the mightiest agents in nature are commissioned to speed the little voyagers on their way.
We have already mentioned that some of the jelly-fishes possess the power of stinging. Only a few of the larger species, however, seem to be thus endowed; and the name sea-nettle is by no means applicable to the class as a whole. The poisonous fluid which produces the irritating effect on the skin, and no doubt paralyses the creatures upon which the jelly-fish feeds, is secreted by the arms. By means of its poison-bearing tentacles, the soft, gelatinous medusa is more than a match for the armed crustacean and the scale-clad fish. We take from Professor Forbes the following graphic description of one of the stinging species:—'The Cyanæa capillata of our seas is a most formidable creature, and the terror of tender-skinned bathers. With its broad, tawny, festooned, and scalloped disk, often a full foot or more across, it flaps its way through the yielding waters, and drags after it a long train of ribbon-like arms, and seemingly interminable tails, marking its course when its body is far away from us. Once tangled in its trailing "hair," the unfortunate who has recklessly ventured across the graceful monster's path too soon writhes in prickly torture. Every struggle but binds the poisonous threads more firmly round his body, and then there is no escape; for when the winder of the fatal net finds his course impeded by the terrified human wrestling in its coils, he, seeking no contest with the mightier biped, casts loose his envenomed arms, and swims away. The amputated weapons severed from their parent body vent vengeance on the cause of their destruction, and sting as fiercely as if their original proprietor itself gave the word of attack.'
We now approach the most extraordinary portion of the history of these creatures. Recent investigations have brought to light the most interesting facts respecting their reproduction and development. It is now known that the young jelly-fish passes through a series of transformations before reaching its perfect state.
At certain seasons, eggs are produced within the body of the parent in appropriate ovaries, where they are retained for a time. They are then transferred to a kind of marsupial pouch, analogous to that of the kangaroo, where their development proceeds. After passing through certain changes here, the egg issues from the maternal pouch as an oval body, clothed with cilia—an animalcule in external aspect, and as unlike its parent as can well be imagined. For awhile the little creature dances freely through the water, and leads a gay, roving life; but at last it prepares to 'settle;' selects a fitting locality; applies one extremity of its body to the surface of stone or weed, and becomes attached. And now another change passes over it. The cilia, no longer needed, disappear. A mouth is developed at the upper extremity of the body, furnished with a number of arms. Gradually this number increases, and the jelly-fish now appears in the disguise of a polype, which feeds voraciously on the members of the class from which it has itself so lately emerged. At this point there is a halt. The medusa remains in its polype state for some months. At the expiration of this term, a strange alteration in its appearance begins to take place. Rings are formed round its body, from ten to fifteen in number. These gradually deepen, until at length it is literally cut up into a number of segments, which rest one upon the other—their upper margins becoming elevated, and divided into eight lobes. It is, in fact, a pile of cup-shaped pieces, very loosely connected together. A little later, these pieces free themselves successively, and the sedate polype disappears in a company of sprightly young medusæ. These beings, indeed, still differ in some respects from the adult animal; but the differences gradually vanish, and we have the perfect jelly-fish as the final result of this extraordinary series of transformations.
Similar observations have been made respecting other tribes amongst the lower animals, and some interesting generalisations have been founded upon them, into which, however, it is not our present purpose to enter.
The Acalephæ are the principal agents concerned in the production of the beautiful phenomena of phosphorescence. The minute species—mere gelatinous specks—swarm at times by countless myriads in the waters of the ocean, and make its surface glow with 'vitalised fire.' The waves, as they curl and break, sparkle and flash forth light, and the track of the moving ship is marked by a lustrous line. 'In the torrid zones between the tropics,' says Humboldt, 'the ocean simultaneously develops light over a space of many thousand square miles. Here the magical effect of light is owing to the forces of organic nature. Foaming with light, the eddying waves flash in phosphorent sparks over the wide expanse of waters, where every scintillation is the vital manifestation of an invisible animal world.' Beneath the surface larger forms are seen, brilliantly illuminated, and lighting up the mystic depths of the sea. Fiery balls and flaming ribbons shoot past; and submarine moons shine with a soft and steady light amidst the crowds of meteors. 'While sailing a little south of the Plata on one very dark night,' says Mr Darwin, 'the sea presented a wonderful and most beautiful spectacle. There was a fresh breeze; and every part of the surface, which during the day is seen as foam, now glowed with a pale light. The vessel drove before her bows two billows of liquid phosphorus, and in her wake she was followed by a milky train. As far as the eye reached, the crest of every wave was bright; and the sky above the, horizon, from the reflected glare of these livid flames, was not so utterly obscure as over the vault of the heavens.' Even in our own seas very beautiful displays of phosphorescence may be witnessed. On fine summer nights, a soft, tender light plays round the boat as it moves onward, and the oars drop liquid fire. For how much of beauty are we indebted to these living specks of jelly?
Of the extreme minuteness of some of the species, an idea may be formed from the fact, that 110,000 might be contained in a cubic foot of water. We can say nothing with certainty as to the cause of the phosphorescence of the medusæ, and shall not trouble our readers with mere speculations.
The jelly-fishes furnish us with a striking illustration of the profusion of life in the ocean. Provision has indeed been made for securing in all the realms of our globe the largest possible amount of sentient being, and consequently of happiness. And to each tribe a definite part is assigned—a special mission is intrusted. None can be spared from the economy of nature. The shoals of microscopic medusæ store up in their own tissues the minute portions of nutritious matter diffused through the waters, and supply food for the support of higher organisms. All the tribes of animated beings are dependent one upon another. That the greatest may enjoy its existence and fulfil its work, the least must hold its place and discharge its function. They co-operate unconsciously to secure the unity and harmony of a system which is designed to promote alike the interests of each and all of them.
