Chapter VIII: Acalephæ, or Sea Nettles (1)

"In nova fert animus mutatis dicere formas corpora."--OVID, MET.

The class Acalephæ, from ἀκαλήφη, a nettle, so called from the stinging properties which many of them possess, include a great number of radiate animals of which the Medusæ are the type. They form the third class of Cuvier's zoophytes. The Acalephæ, forming the first order, are characterised as floating and swimming in the sea by means of the contraction and dilation of their bodies, their substance being gelatinous, without apparent fibres.

The great genus Medusa is characterised by having a disk, more or less convex above, resembling a mushroom or expanded umbrella--the edges of the umbrella, as well as the mouth and suckers, being more or less prolonged into pedicles, which take their place in the middle of the lower surface; they are furnished with tentacula, varying in form and size, which have given rise to many subdivisions, with which we need not concern ourselves.

The substance of the disk presents an uniform cellular appearance internally, but the cellular substance being very soft, no trace of fibre is observable. Taken from the sea and laid upon a stone, a Medusa weighing fifty ounces will rapidly diminish to five or six grains, sinking into a sort of deliquescence, from which Spalanzani concluded that the sea-water penetrated the organic texture of its substance, and constituted the principal volume of the animal. Those which have cilia round their margins have also cellular bands running along their bases, and most of the projectile and extensile tentacula and filaments have sacs and canals containing fluids at their roots. Suckers are also found at the extremities, and along the sides of these tentacles in several genera are suckers, by which they are able more securely to catch their floating prey, or to anchor themselves when at rest. The indications of nerves or nervous system are too slight to be received as evidence, although Dr. Grant observed some structure which he thought could only belong to a nervous system, and Ehrenberg thought he observed eyes in _Medusa aurita_, as well as a nervous circle formed of four ganglion-like masses disposed round the mouth. But most naturalists seem to be of opinion that touch is the only sense of which any conclusive proof can be advanced.

Here we behold a class of bell-shaped semi-transparent organisms, which float gracefully in the sea--a great family of soft, wandering animals, constituted in a most extraordinary manner. They look like floating umbrellas, breeches, or, better still, floating mushrooms, the footstalk replaced by an equally central body, but divided into divergent lobes at once sinuous, twisted, and fringed, so that one is at first tempted to take them for a species of root. The edges of the umbrella or mushroom are entire or dentate, sometimes elegantly figured, often ciliate, or provided with long filiform appendages which float vertically in the water.

Sometimes the animal is uncoloured, and limpid as crystal; sometimes it presents a slightly opaline appearance, now of a tender blue, or of a delicate rose colour; at other times it reflects the most brilliant and vivid tints.

In certain species the central parts only are coloured, showing brilliant reds and yellows, blues or violets, the rest being colourless. In others the central mass seems clothed in a thin iridescent or diaphanous veil, like the light evanescent soap-bubble, or the transparent glass shade which covers a group of artificial flowers.

The Acalephæ are animals without consistence, imbued with much water, so that we can scarcely comprehend how they resist the agitation of the waves and the force of the currents; the waves, however, float without hurting them, the tempest scatters without killing them. When the sea retires, or they are withdrawn from their native waters, their substance dissolves, the animal is decomposed, they are reduced to nothing; if the sun is ardent, this disorganisation occurs in the twinkling of an eye, so to speak.

When the Medusæ travel, their convex part is always kept in advance, and slightly oblique. If they are touched while swimming, even lightly, they contract their tentacula, fold up their umbrella, and sink into the sea. Like Ehrenberg, M. Kölliker thought he discovered visual and auditory organs in an _Oceania_, and Gegenbauer thought he detected them in other genera, such as _Rhizostoma_ and _Pelagia_. The eyes are said to consist of certain small, hemispherical, cellulose, coloured masses, in which are sunk small crystalline globules, the free parts of which are perfectly naked. The supposed auditory apparatus is seated close to these organs; they are small vesicles filled with liquid; the eyes having neither pupil nor cornea, and the ears without opening or arch.

But it is in their reproduction that these evanescent beings present the most marvellous phenomena. At one period of the year the Medusæ are charged with numbers of very minute eggs, of the most lively colours, which are suspended in large festoons from their floating bodies. In some cases these eggs develop themselves grafted to their bodies, and are only detached at maturity. In other cases the larvæ produced bear no resemblance to the mother; they are elongated and vermiform, broad at their extremity; we speak of the microscopic _leeches_, which have vibrating cilia, scarcely perceptible, by which they execute the most lively motions. At the end of a certain time they are transformed into polyps, and furnished with eight tentacula. This preparatory sort of animal seems to possess the faculty of reproduction by means of certain buds or tubercles which develop themselves on the surface of the body, and also by filaments which start up here and there, so that a single individual originates a numerous colony. This polyp is subjected to a transformation still more remarkable; its structure becomes complex, its body articulate, and it seems to be composed of a dozen disks piled one upon the other, like the jars of a voltaic pile; the upper disk is convex, and is separated from the colony after a convulsive effort; it becomes free, and an excessively small, star-like Medusa is the result; every disk, that is, every individual, is isolated one after the other in the same manner.

Thus of the sexual zoophytes which propagate their kind according to the usual laws; but others engender young which have no resemblance to the parent zoophyte at all: in this respect they are neuter, that is, non-sexual, or _agamous_. These are produced by budding, or fissiparity, from individuals like themselves. They can also give sexual distinctions; but before this change takes place the creature, which was simple, is transformed into a composite animal, and it is from its disaggregation that individuals having sexual organs are produced, the process being that which has been called _alternate_ generation. It goes on in a perfectly regular manner, although it is a fact that the young never resemble their mothers, but their grandmothers.

This great family of Zoophytes Gosse divides into--

_Discophora_, having the body in the form of a circular disk, more or less convex and umbrella-shaped, moving by alternate contractions and expansions of the disk.

_Ctenophora_, body cylindrical, moving by means of many parallel rims of cilia set in longitudinal lines on the surface.

_Sophonophora_, body irregular, without central digestive cavity like the others, having sucking organs, and moving by means of a contractile cavity, or by air-vessels.

The Discophora are again subdivided into _Gymnophthalmata_, having the eye-specks uncovered or wanting, a great central digestive cavity, circulating vessels proceeding to the margin quite simple or branched; and _Steganophthalmata_ having the eye-specks protected by membranous hoods, or lobed coverings, circulating vessels much ramified, and united with a network. Of the Gymnophthalmata we have an example in _Æquerea violacea_ (Fig. 84), in which the disk is slightly convex, glass-like in appearance, and furnished all round with very short, slender, thread-like, violet-coloured tentacula; with circulating vessels, eight in number, quite simple, and ovaries placed on them; peduncle wide, expanding into many broad and long fringed lobes. The Steganophthalmata include the _Medusadæ_ proper, in which the umbel is hemispherical, with numerous marginal tentacles, eight eyes covered by lobes, four ovaries, four chambers, four fringed arms, with a central and four lateral openings. _Aurelia aurita_ (Fig. 85) is here represented as a type of the group; it is plentiful in the Baltic, and has been carefully studied by the Swedish naturalists. Rosenthal has made its anatomy his special study. Sars has also made it the subject of observations. In the same group we find the _Pelagia cyanella_ of Péron, whose body is globose, scolloped with eight marginal tentacles, peduncles ending in four leaf-like, furbelowed arms, united at the base, having four ovaries, and appendages to the stomach, without orifices.

The _Pelagia_, as the name implies, belong to the deep sea. _P. noctiluca_ has a transparent, glass-like disk, of a reddish-brown colour and warty appearance. It is found in the Mediterranean, about the coast near Nice, and is still more plentiful on the coast of Sicily, and on the African coast. Another species, _P. panopyra_, is very common in the Atlantic and Pacific, between the Tropics. The naturalist Lesson met whole banks of them in the equatorial ocean, about the twenty-seventh degree north latitude and the twenty-second degree west longitude. During the night, this species emits a brilliant phosphoric light, and living individuals, which Lesson succeeded in preserving, exhibited great luminosity in the dark. This medusa is remarkable for its semi-spherical disk, slightly depressed, umbilicate at the summit, a little compressed at the edges, and densely bristling on the surface with small elongated warts, but regularly festooned along the edges. In colour it is a delicate rose.

* * * * *

The animals which constitute this class of Zoophytes, and, in former times, so curious and so imperfectly known, were designated _Polypomedusæ_, in order to remind us that at one time they were called Medusæ, and at others ranged among the Polyps. It has, however, been recently discovered that, shortly after they issue from the egg, these zoophytes show themselves in the form of polyps, and that, at a later period, they assume the animal form, to which we give the name of medusæ. These animals are, then, true proteans: hence the very considerable difficulty of studying them--difficulties which have long reduced naturalists to despair. Even now their history is too obscure and too complicated to justify us in presenting it, except in its general features. We shall, therefore, content ourselves here with a description of the best known species of the class only--those, namely, which have particularly attracted the attention of naturalists, and which are, at the same time, of a nature to interest our readers.

The class of Discophoræ may be divided into four orders or families, namely:--

I. THE HYDRAIDÆ, having single, naked, gelatinous, sub-cylindrical,
but very contractile stems, mutable in form, mouth encircled with
a single series of granulous filiform tentacula.

II. SERTULARIADÆ, plant-like and horny, rooted and variously
branched, filled with semi-fluid organic pulp, the polyps contained
within sessile cells disposed along the sides of the main stem or
branchlets, but never terminal.

III. MEDUSADÆ. Umbel hemispherical, with marginal tentacula; having
eight eyes covered by lobes, four ovaries, four cells, four fringed
arms, a central opening, and four lateral openings.

IV. SIPHONOPHORA, having the animals double, and bell-shaped, one
fitting into the cavity of the other; in _Dyphyes_ the animal has a
large air-vessel with numerous tentacula; in _Physalia_, the animal
stretches over a cartilaginous plane.

The true form of the Medusa does not appear in the two first
orders.

HYDRAIDÆ.

The Hydraidæ are, according to modern naturalists, Discophoræ arrested in their development. They comprehend the single genus Hydra, of which many species are known, whose habits and metamorphoses it will be our object to particularise.

_Hydra vulgaris_ inhabits stagnant ponds and slowly-running waters. It is of an orange-brown or red colour, the intensity of the colour depending on the nature of its food, becoming almost blood-red when fed on the small crimson worms and larvæ to be found in such places. M. Laurent even succeeded in colouring them blue, red, and white, by means of indigo, carmine, and chalk, without any real penetration of the tissue, the buds from them acquiring the same colour as the mother, while the colour of the ova retains its natural tint, even when the Hydra mother has been fed with coloured substances during the progress of this mode of reproduction. The tentacula, usually seven or eight in number, never exceed the length of the body, tapering insensibly to a point.

_Hydra viridis_, the fresh-water polyp, being more immediately within the sphere of our observation, naturally presents itself to our notice. It is common in ponds and still waters. It was noticed by Pallas, who was of opinion that offspring was produced from every part of the body. De Blainville, on the contrary, was of opinion that offspring was always produced from the same place; namely, at the junction of that part which is hollow and that which is not. Van der Höven, the Leyden professor, agrees with Pallas, and Dr. Johnston's opinions accord with Pallas. The green Hydra is common all over Europe, inhabiting brooks filled with herbage--attaching itself particularly to the duckweed of stagnant ponds, and more especially to the under surface of the leaf. The animal is reduced to a small greenish tubular sac, closed at one of its extremities, open at the other, and bearing round this opening from six to ten appendages, very slender, and not exceeding a line in breadth. The tubulous sac is the body of the animal (Fig. 87). The opening is at once its mouth and the entrance to the digestive canal; the appendages, the tentacula or arms.

The Hydras have no lungs, no liver, no intestines, no nervous system, no heart. They have no organ of the senses, except those which exist in the mouth and the skin. The arms or branches are hollow internally, and communicate with the stomach. They are provided with vibratile cells, furnished with a great number of tuberosities disposed spirally, and containing in their interior a number of capsules provided each with a sort of fillet. These threads, which are of extreme tenacity, are thrown out when the animal is irritated by contact with any strange body. We may see these filaments wrapping themselves round their prey, sometimes even penetrating its substance, and effectually subduing the enemy. The green Hydra has thus a very simple organisation. Nevertheless, it would be a mistake to say the animal was imperfect, for it possesses everything necessary for its nourishment and for the propagation of its species.

There are learned men who have composed hundreds of volumes, who have published whole libraries--naturalists and physicists who have written more than Voltaire ever penned, but whose names are utterly forgotten. On the other hand, there are some who have left only two or three monograms, and yet their names will live for ever. Of this number is the Genevois, A. Trembley. This writer published in 1744 a "Memoir on the Fresh-water Polyps." In this little work he recorded his observations on some of these animals of smallest dimensions. He limited himself even to two sets of experiments: he turned the fresh-water polyp outside in, and he multiplied it by cutting it up. These experiments upon this little creature, which few persons had seen, have sufficed to secure immortality to his name. Trembley was tutor to the two sons of Count de Bentinck. He made his observations at the country-house of the Dutch nobleman, and he had, as he assures us, "frequent occasion to satisfy himself, in the case of his two pupils, that we can even in infancy taste the pleasures derivable from the studies of Nature!" Let us hope that this thought, uttered by a celebrated naturalist, who spoke only from what he knew himself, may remain engraved on the minds of our younger readers.

Trembley established by his observations, a thousand times repeated, that _Hydra viridis_ can be turned outside in, as completely as a glove may be, without injury to the animal, which a day or two after this revolution resumes its ordinary functions. Such is the vitality of these little beings, that what was once the outer surface soon fulfils all the functions of a stomach, digesting its food, while the intestinal tube expanding its exterior performs all the functions of an outer surface; it absorbs and respires. But we shall leave Trembley to relate his very remarkable experiments. "I attempted," he says, "for the first time to turn these polyps inside out in the month of July, 1741 but unsuccessfully. I was more successful the following year, having found an expedient which was of easy execution. I began by giving a worm to the polyp, and put it, when the stomach was well filled, into a little water which filled the hollow of my left hand. I pressed it afterwards with a gentle pinch towards the posterior extremities. In this manner I pressed the worm which was in the stomach against the mouth of the polyp, forcing it to open--continuing the pinching pressure until the worm was partly pressed out of the mouth. When the polyp was in this state I conducted it gently out of the water, without damaging it, and placed it upon the edge of my hand, which was simply moistened, in order that the polyp should not stick to it. I forced it to contract itself more and more, and, in doing so, assisted in enlarging the mouth and stomach. I now took in my right hand a thick and pointless boar's bristle, which I held as a lancet is held in bleeding. I approached its thicker end to the posterior extremity of the polyp, which I pressed until it entered the stomach, which it does the more easily since it is empty at this place and much enlarged. I continued to advance the bristle, and, in proportion as it advanced, the polyp became more and more inverted. When it came to the worm, by which the mouth is kept open on one side, and the posterior part of the polyp is passed through the mouth, the creature is thus turned completely inside out; the exterior superficies of the polyp has become the interior."

The poor animal would be justified in feeling some surprise at its new situation--disagreeably surprised we may add, for it makes every imaginable effort to recover its natural position, and it always succeeds in the end. The glove is restored to its proper form. "I have seen polyps," says Trembley, "which have recovered their natural exterior in less than an hour." But this would not have served the purpose of our experimenter. He wished to know if the polyps thus turned outside in could live in this state; he had consequently to prevent it from rectifying itself, for which purpose a needle was run through the body near the mouth--in other words, he impaled the creature by the neck.

"It is nothing for a polyp only to be spitted," says Trembley. It is in fact a very small thing, as we shall see, for thus reversed and spitted they live and multiply as if nothing had happened.

"I have seen a polyp," says this ingenious experimenter, "turned inside out, which has eaten a small worm two days after the operation. I have fed one in that state for more than two years, and it has multiplied in that condition.

"Having experimented successfully myself, I was desirous of having the testimony of others capable of forming opinions on the subject. M. Allamand was persuaded to put his hand to the work, which he did with the same success I had met with. He has done more, having succeeded in permanently turning specimens which had been previously turned, and which continued to live in their re-inverted state; he has seen them eat soon after both operations; finally, he has turned one for the third time, which lived some days, but perished without having eaten anything, although it did not appear that its death was the result of the operation."

We have said that the _Hydra viridis_ has neither brain, nervous system, heart, muscular rings, lungs, nor liver; the organs of the senses--namely, those of sight, hearing, and of smell--have also been denied them. Nevertheless, they act as if they possessed all these senses. Oh Nature! how hidden are thy secrets, and how the pride of man is humbled by the mysteries which surround thee--by the spectacles which strike his eyes, and which he attempts in vain to explain!

Trembley states that the fresh-water polyps, having no muscular ring, can neither extend nor contract themselves, nor can they walk. If touched, or if the water in which they are immersed is suddenly agitated, they are certainly observed to contract more or less forcibly, and even to inflect themselves in all directions; and by this power of extension, of contraction and inflection, they contrive to move from place to place; but these movements are singularly slow, the utmost space they have been observed to traverse being about eight inches in the twenty-four hours.

Painfully conscious of his powers of progression, however, he has found means of remedying it, and the aquatic snail is his steed; he creeps upon the shell of a Planorbis, or Limnæa, and by means of this improvised mount he will make more way in a few minutes than he would in a day by his own unassisted efforts.

The _Hydra viridis_, although destitute of organs of sight, are nevertheless sensible of light; if the vase containing them is placed partly in shade and partly in the sun, they direct themselves immediately towards the light; they appreciate sounds; they attach themselves to aquatic plants and other floating bodies. Without eyes, without brain, and without nerves, these animals lie in wait for their prey, recognize, seize, and devour it. They make no blunder, and only attack where they are sure of success. They know how to flee from danger; they evade obstacles, and fight with or fly before their enemies. There are, then, some powers of reflection, deliberation, and premeditated action in these insignificant creatures; their history, in short, is calculated to fill the mind with astonishment.

Trembley insists much upon the address which the Hydra employs to secure its prey: by the aid of its long arms, small animals, which serve to nourish it, are seized, for it is carnivorous, and even passably voracious. Worms, small insects, and larvæ of dipterous insects are its habitual prey. When a worm or woodlouse in passing its portals happens to touch them, the polyp, taking the hint, seizes upon the wanderer, twining its flexible arms round it, and, directing it rapidly towards its mouth, swallows it. Trembley amused himself by feeding the Hydra, while he observed the manner in which it devoured its prey. "When its arms were extended, I have put into the water a woodlouse or a small worm. As soon as the woodlouse feels itself a prisoner it struggles violently, swimming about, and drawing the arm which holds it from side to side; but, however delicate it may appear, the arm of the polyp is capable of considerable resistance; it is now gradually drawn in, and other arms come to its assistance, while the polyp itself approaches its prey; presently the woodlouse finds itself engaged with all the arms, which, by curving and contracting, gradually but inevitably approach the mouth, in which it is soon engulfed." Frédol also notices a singular fact. "The small worms, even when swallowed by the polyp," he says, "frequently try to escape; but the ravisher retains them by plunging one of its arms into the digestive cavity! What an admirable contrivance, by which the worms are digested while the arm is respected!"

The food of the fresh-water Hydra influences the colour of their bodies in consequence of the thinness and transparency of their tissues; so that the reddish matter of the woodlouse renders them red, while other food renders them black or green, according to its prevailing colour!

The multiplication of these creatures takes place in three different ways: 1. By eggs. 2. By buds, after the manner of vegetables. 3. By separation, in which an individual may be cut into two or many segments, each reproducing an individual.

We shall only say a few words on the first mode of reproduction. The eggs, according to Ehrenberg, come to maturity in the _H. viridis_ at the base of the feet, where the visceral cavity terminates. They are carried during seven or eight days, and determine by their fall the death of the animal. When the Hydra has laid its eggs, according to M. Laurent, it gradually lowers itself until it covers them with half its body, which, spreading out and getting proportionably thin, passes into the condition of a horny substance, that glues the eggs disposed in a circle round the body to plants and other foreign substances. She ends her career by dying in the midst of her ova.

Trembley has studied with great care the mode of reproduction by budding--a process which seems to prevail in the summer months. The buds which are to form the young polyp appear on the surface of the body as little spherical excrescences terminating in a point. A few steps further towards maturity, and it assumes a conical and finally a cylindrical form. The arms now begin to push out at the anterior extremity of the young animal; the posterior extremity by which it is attached to the mother contracting by degrees, until it appears only to touch her at one point. Finally, the separation is effected, the mother and the young acting in concert to produce the entrance of this interesting young polyp into the world. Each of them take with their head and arms a strong point of support upon some neighbouring body; and a small effort suffices to procure the separation: sometimes the mother charges herself with the effort, sometimes the young, and often both.

When the young polyp is separated from the mother, it swims about, and executes all the movements peculiar to adult animals. The entrance into life and maturity takes place with these beings at one and the same moment. Infancy and youth are suppressed in this little world.

So long as the young polyp remains attached to the mother, she is the nurse; by a touching change, the young polyp nurses her in his turn. In short, the stomach of the mother and her young have communication; so that the prey swallowed by the parent passes partially into the stomach of her progeny. On the other hand, while still attached to the mother, the little ones seize the prey, which they share in their turn with their parent by means of the communication Nature has arranged between the two organisms.

In the course of his experiments Trembley states another fact still more remarkable.

Upon a young polyp still attached to its parent he observed a new polyp or polypule, and upon this unborn creature was another individual. Thus three generations were appended to the parent, who carried at once her child, her grandchild, and great-grandchild.

"In observing the young polyps still attached to their parent," says Trembley, "I have seen one which had itself a little one which was just issuing from its body; that is to say, it was a mother while yet attached to its own parent. I had in a short time many young polyps attached to their parents which had already had three or four little ones, of which some were even perfectly formed. They fished for woodlice like others, and they ate them. Nor is this all. I have seen a mother-polyp which had carried its _third generation_. From the little one which she had produced issued another little one, and from this a third."

Charles Bennet, the naturalist of Geneva, says wittily, that a polyp thus charged with all its descendants constitutes a living genealogical tree.

We have just spoken of turning polyps inside out! If one of these creatures is thus operated upon while it bears its young on the surface of its body, such of them as are sufficiently advanced continue to increase; although they find themselves in this sudden manner imprisoned in an internal cavity, they re-issue subsequently by the mouth. Those less advanced at the moment of reversal issue by little and little from the maternal sac, and complete their career of development on the newly-made exterior.

The third and most extraordinary mode of reproduction in the polyps has been discovered by Trembley in the case of the green Hydra. So surprised was this naturalist at the strange anomalies which surrounded these creatures, that he began to have doubts, and gravely to ask the question, Was this polyp an animal? Is it a plant?

In order to escape from this state of indecision, it occurred to him to cut a Hydra into pieces. Concluding that plants alone could reproduce themselves by slips, he waited the result of the experiment for the conclusion he sought. On the 25th of November, 1740, he cut a polyp into sections. "I put," he tells us, "the two parts into a flat glass, which contained water four or five lines in depth, and in such a manner that each portion of the polyp could be easily observed through a strong magnifying glass. It will suffice to say that I had cut the polyp transversely, and a little nearer to the anterior. On the morning of the day after having cut the polyp, it seemed to me that on the edges of the second part, which had neither head nor arms, three small points were issuing from these edges. This surprised me extremely, and I waited with impatience for the moment when I could clearly ascertain what they were. Next day they were sufficiently developed to leave no doubt on my mind that they were true arms. The following day two new arms made their appearance, and, some days after, a third appeared, and I could now trace no difference between the first and second half of the polyp which I had cut."

This is assuredly one of the most startling facts belonging to natural history. Divide a fresh-water polyp into five or six parts, and at the end of a few days all the separate parts will be organized, developed, and form so many new beings, resembling the primitive individual. Let us add, that the polyp which should thus have lost five-sixths of its body, the mutilated father of all this generation, remains complete in itself; in the interval, it has recuperated itself and recovered all its primitive substance.

After this, if a _Hydra vulgaris_ wishes to procure for itself the blessings of a family, it has only one thing to do: cut off an arm; if it desire two descendants, let it cut the arm in two parts; if three, let it divide itself into three; and so on _ad infinitum_. "Divide one of the animals," says Trembley, "and each section will soon form a new individual in all respects like the creature divided." "A whole host of polyps hewn into pieces," says Frédol, "will be far from being annihilated." "On the contrary," we may say, in our turn, "its youth will be renewed, and multiplied in proportion to the number of pieces into which it has been divided." "The same polyp," says Trembley, "may be successively inverted, cut into sections, and turned back again, without being seriously injured."

If a green Hydra is cut into two pieces, and the stomach is cut off in the operation, the voracious creature will, nevertheless, continue to eat the prey which presents itself. It gorges itself with the food, without troubling itself with the loss which it has sustained; but the food no longer nourishes it, for it merely enters by one opening, passes through the intestinal canal, and escapes by the other. It realizes Harleville's pleasantry of M. de Crac's horse, in the piece of that name, which eats unceasingly, but never gets any fatter.

All these instances of mutilation, resulting in an increase of life, are very strange. The naturalists to whom they were first revealed could scarcely believe their own eyes. Réaumur, who repeated many of Trembley's experiments, writes as follows: "I confess that when I saw for the first time two polyps forming by little and little from that which I had cut in two, I could scarcely believe my eyes; and it is a fact that, after hundreds of experiments, I never could quite reconcile myself to the sight."

In short, we know nothing analogous to it in the animal kingdom. About the same period Charles Bennet writes: "We can only judge of things by comparison, and have taken our ideas of animal life from the larger animals; and an animal which we cut and turn inside out, which we cut again, and it still bears itself well, gives one a singular shock. How many facts are ignored, which will come one day to derange our ideas of subjects which we think we understand! At present we just know enough to be aware that we should be surprised at nothing."

Notwithstanding the philosophic serenity which Bennet recommends, the fact of new individuals resulting from dividing these fresh-water polyps was always a subject of profound astonishment, and of never-ending meditation.

SERTULARIADÆ.

All Hydraidæ, with the exception of the Hydra and a few other genera, are marine productions, varying from a few lines to upwards of a foot in height, attaching themselves to rocks, shells, sea-weeds, and corallines, and to various species of shell-fish. Many of them attach themselves indiscriminately to the nearest object, but others show a decided preference. _Thuiaria thrya_ attaches itself to old bivalves; _Thoa halecuia_ prefers the larger univalves; _Antennularia antennina_ attaches itself to coarse sand on rocks; _Laomedea geniculata_ delights in the broad frond of the tangle; _Plumularia catherina_ attaches itself in deep water to old shells, corallines, and ascidians, growing in a manner calculated to puzzle the naturalist, as it did Crabbe, the poet, who writes of it:--

"Involved in sea-wrack, here you find a race
Which science, doubting, knows not where to place;
On shell or stone is dropp'd the embryo seed,
And quickly vegetates a vital breed."

_Sertularia pumila_, on the other hand, loves the commoner and coarser wracks. "The choice," says Dr. Johnston, "may in part be dependent on their habits, for such as are destined to live in shallow water, or on a shore exposed by the reflux of every tide, are, in general, vegetable parasites; while the species which spring up in deep seas must select between rocks, corallines, or shells." There seems to be a selection even as to the position on the rocks. According to Lamouroux, some polyps always occupy the southern slopes, and never that towards the east, west, or north; others, on the contrary, grow only on these exposures, and never on the south, altering their position, however, according to the latitude, and its relation to the Equator.

The _Sertulariadæ_ have a horny stem, sometimes simple, sometimes so branching that they might readily enough be mistaken for small plants, their branches being flexible, semi-transparent, and yellow. Their name is derived from _Sertum_, a bouquet. Each Sertularia has seven, eight, twelve, or twenty small panicles, each containing as many as five hundred animalcules; thus forming, sometimes, an association of ten thousand polyps. "Each plume," says Mr. Lister, in reference to a specimen of _Plumularia cristata_, "might comprise from four to five hundred polyps;" "and a specimen of no unusual size now before me," says Dr. Johnston, "with certainly not fewer cells on each than the larger number mentioned, thus giving six thousand as the tenantry of a single polypidom, and this on a small species." On _Sertularia argentea_, it is asserted, polyps are found on which there exist not less than eighty to a hundred thousand.

Each colony is composed of a right axis, on the whole length of which the curved branches are implanted, these being longest in the middle. Along each of these branches the cells, each containing a polyp, are grouped alternately. The head of the animal is conical, the mouth being at the top surrounded by twenty to twenty-four tentacles. These curious beings have no digestive cavity belonging to themselves; the stomach is common to the whole colony--a most singular combination, a single stomach to a whole group of animals! Never have the principles of association been pushed to this length by the warmest advocates of communism.

Certain species belonging to the colony, which seem destined to perpetuate the race, have not the same regular form. Destitute of mouth and tentacles, they occupy special cells, which are larger than the others. The entire colony is composed exclusively of individuals, male or female. "We have traced _Sertularia cupressina_ through every stage of its development," say Messrs. Paul Gervais and Van Beneden. "At the end of several days, the embryos are covered with very short vibratile cells; their movement is excessively slow; then, from the spheroid form which they take at first, they get elongated, and take a cylindrical form, all the body inclining lightly sometimes to the right, sometimes to the left. The vibratile cells fading afterwards, the embryo attaches itself to some solid body, a tubercle is formed, and the base extends itself as a disk. At the same time that the first rudiments of the polyp appear, the disk-like tubercle throws out on its flanks a sort of bud, and a second polyp soon shows itself; its surface is hardened; the polyp appears in its turn, and the same process of generation is repeated; a colony of _Sertulariadæ_ is thus established at the summit of a discoid projection. At the end of fifteen days the colony, which has been forming under our eyes, consists of two polyps and a bud, which already indicates a third polyp. The sea-cypress, as this species is called, is robust, with longish branches decidedly fan-shaped, the pinnæ being closer and nearly parallel to each other. The cells form two rows, nearly opposite, smooth and pellucid. The branches in some specimens are gracefully arched, bending as it were under the load of pregnant ovaries which they carry, arranged in close-set rows along the upper side of the pinnæ. They are found in deep water on the coast of Scotland, and as far south as the Yorkshire coast and the north of Ireland. The cells, which are the abode of the polyps, are not always alike in their distribution. Sometimes they are ranged on two sides, sometimes on one only. Sometimes they are grouped like the small tubes of an organ, at other times they assume a spiral form round the stem, or they form here and there horizontal rings round it."

MEDUSADÆ.

The Medusæ comprehend, not only the animals so designated in the days of Cuvier under that name, but also the polyps known as _Tubulariadæ_ and _Campanulariadæ_.

If we walk along the sea shore, after the reflux of the tide, we may often see, lying immovable upon the sands, disk-like, gelatinous masses of a greenish colour and repulsive appearance, from which the eye and the steps instinctively turn aside. These beings, whose blubber-like appearance inspires only feelings of disgust when seen lying grey and dead on the shore, are, however, when seen floating on the bosom of the ocean, one of its most graceful ornaments. These are Medusæ. When seen suspended like a piece of gauze or an azure bell in the middle of the waves, terminating in delicate silvery garlands, we cannot but admire their iridescent colours, or deny that these objects, so forbidding in some of their aspects, rank, in their natural localities, among the most elegant productions of Nature. We could not better commence our studies of these children of the sea than by quoting a passage from the poet and historian Michelet: "Among the rugged rocks and lagunes, where the retiring sea has left many little animals which were too sluggish or too weak to follow, some shells will be there left to themselves and suffered to become quite dry. In the midst of them, without shell and without shelter, extended at our feet, lies the animal which we call by the very inappropriate name of the _Medusa_. Why was this name, of terrible associations, given to a creature so charming? Often have I had my attention arrested by these castaways which we see so often on the shore. They are small, about the size of my hand, but singularly pretty, of soft light shades, of an opal white; where it lost itself as in a cloud of tentacles--a crown of tender lilies--the wind had overturned it; its crown of lilac hair floated about, and the delicate umbel, that is, its proper body, was beneath; it had touched the rock--dashed against it; it was wounded, torn in its fine locks, which are also its organs of respiration, absorption, and even of love.... The delicious creature, with its visible innocence and the iridescence of its soft colours, was left like a gliding, trembling jelly. I paused beside it, nevertheless: I glided my hand under it, raised the motionless body cautiously, and restored it to its natural position for swimming. Putting it into the neighbouring water, it sank to the bottom, giving no sign of life. I pursued my walk along the shore, but at the end of ten minutes I returned to my Medusa. It was undulating under the wind; really it had moved itself, and was swimming about with singular grace, its hair flying round it as it swam; gently it retired from the rock, not quickly, but still it went, and I soon saw it a long way off."

Of all the zoophytes which live in the ocean there is none more numerous in species or more singular in their matter, more odd in their form, or more remarkable in their mode of reproduction, than those to which Linnæus gave the name of Medusa, from the mythical chief of the Gorgons.

The seas of every latitude of the globe furnish various tribes of these singular beings. They live in the icy waters which bathe Spitzbergen, Greenland, and Iceland; they multiply under the fires of the Equator, and the frozen regions of the south nourish numerous species. They are, of all animals, those which present the least solid substance. Their bodies are little else than water, which is scarcely retained by an imperceptible organic network; it is a transparent jelly, almost without consistence. "It is a true sea-water jelly," says Réaumur, writing in 1701, "having little colour or consistence. If we take a morsel in our hands, the natural heat is sufficient to dissolve it into water."

Spallanzani could only withdraw five or six grains of the pellicle of a medusa weighing fifty ounces. From certain specimens weighing from ten to twelve pounds, only six to seven pennyweights could be obtained of solid matter, according to Frédol. "Mr. Telfair saw an enormous medusa which had been abandoned on the beach at Bombay; three days after, the animal began to putrefy. To satisfy his curiosity, he got the neighbouring boatmen to keep an eye upon it, in order to gather the bones and cartilages belonging to the great creature, if by chance it had any; but its decomposition was so rapid and complete that it left no remains, although it required nine months to dissipate it entirely."

"Floating on the bosom of the waters," says Frédol, "the Medusa resembles a bell, a pair of breeches, an umbrella, or, better still, a floating mushroom, the stool of which has here been separated into lobes more or less divergent, sinuous, twisted, shrivelled, fringed, the edges of the cap being delicately cut, and provided with long thread-like appendages, which descend vertically into the water like the drooping branches of the weeping willow."

The gelatinous substance of which the body of the Medusa is formed is sometimes colourless and limpid as crystal; sometimes it is opaline, and occasionally of a bright blue or pale rose colour. In certain species the central parts are of a lively red, blue, or violet colour, while the rest of the body is of a diaphanous hue. This diaphanous tissue, often decked in the finest tints, is so fragile, that when abandoned by the wave on the beach, it melts and disappears without leaving a trace of its having existed, so to speak.

Nevertheless, these fragile creatures, these living soap-bubbles, make long voyages on the surface of the sea. Whilst the sun's rays suffice to dissipate and even annihilate its vaporous substance on some inhospitable beach, they abandon themselves without fear during their entire life to the agitated waves. The whales which haunt round the Hebrides are chiefly nourished by Medusæ which have been transported by the waves in innumerable swarms from the coast of the Atlantic to the region of whales. "The locomotion of the Medusæ, which is very slow," says De Blainville, "and denotes a very feeble muscular energy, appears, on the other hand, to be unceasing. Since their specific gravity considerably exceeds the water in which they are immerged, these creatures, which are so soft that they probably could not repose on solid ground, require to agitate constantly in order to sustain themselves in the fluid which they inhabit. They require also to maintain a continual state of expansion and contraction, of systole and diastole. Spallanzani, who observed their movements with great care, says that those of translation are executed by the edges of the disk approaching so near to each other that the diameter is diminished in a very sensible degree; by this movement a certain quantity of water contained in the body is ejected with more or less force, by which the body is projected in the inverse direction. Renovated by the cessation of force in its first state of development, it contracts itself again, and makes another step in advance. If the body is perpendicular to the horizon, these successive movements of contraction and dilatation cause it to ascend; if it is more or less oblique, it advances more or less horizontally. In order to descend, it is only necessary for the animal to cease its movements; its specific gravity secures its descent."

It is, then, by a series of contractions and dilatations of their bodies that the Medusæ make their long voyages on the surface of the waters. This double movement of their light skeleton had already been remarked by the ancients, who compared it to the action of respiration in the human chest. From this notion the ancients called them _Sea Lungs_.

The Medusæ usually inhabit the deep seas. They are rarely solitary, but seem to wander about in considerable battalions in the latitudes to which they belong. During their journey they proceed forward, with a course slightly oblique to the convex part of their body. If an obstacle arrests them, if an enemy touches them, the umbrella contracts, and is diminished in volume, the tentacles are folded up, and the timid animal descends into the depths of the ocean.

We have said that the Medusæ constitute in the Arctic seas one of the principal supports of the whale. Their innumerable masses sometimes cover many square leagues in extent. They show themselves and disappear by turns in the same region, at determinate epochs--alternations which depend, no doubt, on the ruling of the winds and currents which carry or lead them. "The barks which navigate Lake Thau meet," says Frédol, "at certain periods of the year with numerous colonies of a species about the size of a small melon, nearly transparent--whitish, like water when it is mixed with a shade of aniseed. One would be tempted to take these animals at first for a collection of floating muslin bonnets."

The Medusæ are furnished with a mouth placed habitually in the middle of the neck. This mouth is rarely unoccupied. Small molluscs, young crustaceans, and worms, form their ordinary food. In spite of their shape, they are most voracious, and snap up their prey all at one mouthful, without dividing it. If their prey resists and disputes with it, the Medusa which has seized it holds fast, and remains motionless, and, without a single movement, waits till fatigue has exhausted and killed its victim, when it can swallow it in all security.

In respect to size, the Medusæ vary immensely. Some are very small, while others attain more than a yard in diameter. Many species are phosphorescent during the night.

Most Medusadæ produce an acute pain when they touch the human body. The painful sensation produced by this contact is so general in this group of animals, that it has determined their designation. Until very recently all the animals of the group have been, after Cuvier, designated under the name of Acalephæ, or sea nettles, in order to remind us that the sensation produced is analogous to that occasioned by contact with the stinging leaves of the nettle.

According to Dicquemare, who made experiments on himself in this matter, the sensation produced is very like that occasioned by a nettle, but it is more violent, and endures for half an hour. "In the last moments," says the abbé, "the sensation is such as would be produced by reiterated but very weak prickings. A considerable pain pervaded all the parts which had been touched, accompanied by pustules of the same colour, with a whitish point." "The sea-bladder," says Father Feuillée, "occasions me, on touching it, a sudden and severe pain, accompanied with convulsions."

"During the first voyage of the _Princess Louise_ round the world," to quote Frédol, "Meyen remarked a magnificent physalia, which passed near the ship. A young sailor leaped naked into the sea, to seize the animal. Swimming towards it, he seized it; the creature surrounded the person of its assailant with its numerous thread-like filaments, which were nearly a yard in length; the young man, overwhelmed by a feeling of burning pain, cried out for assistance. He had scarcely strength to reach the vessel and get aboard again, before the pain and inflammation were so violent that brain fever declared itself, and great fears were entertained for his life."

The organization is much more complicated than early observers were disposed to think it. During many ages naturalists were inclined to imagine, with Réaumur, that the Medusæ were mere masses of organized jelly, of gelatinized water. But when Courtant Dumeril tried the experiment of injecting milk into their cavities, and saw the liquid penetrating into true vessels, he began to comprehend that these very enigmatical beings were worthy of serious study--the study of subsequent naturalists, such as Cuvier, De Blainville, Ehrenberg, Brandt, Makel-Eschscholtz, Sars, Milne Edwards, Forbes, Gosse, and other modern naturalists, who have demonstrated what richness of structure is concealed under this gelatiniform and simple structure in the Medusæ; at the same time they have revealed to us most mysterious and incredible facts as connected with their metamorphoses.

Among the Medusæ proper, the most common are Aurelia, Pelagia, and Chrysaora. In the latter, _C. Gaudichaudi_ (Fig. 88), the disk is hemispherical, festooned with numerous tentacles, attached to a sac-like stomach, opening by a single orifice in the centre of the peduncle, with four long, furbelowed, unfringed arms. Gaudichaudi's chrysaora is found round the Falkland Islands. The disk forms a regular half-sphere, very smooth, and perfectly concave, forming a sort of canopy in the shape of a vault. The circle which surrounds it is divided into sections by means of vertical lines, regularly divided, of a reddish-brown colour, which forms an edging to the umbrella-like disk. Twelve broad regular festoons form this edging. From the summit of these lobes issue twelve bundles of very long, simple, capillary tentacles, of a bright red. The peduncle is broad and flat, perforated in the middle, to which are attached four broad foliaceous arms.

RHIZOSTOMA.

The Medusæ which bear the name of _Rhizostoma_ have the disk hemispherically festooned, depressed, without marginal tentacles, peduncle divided into four pairs of arms, forked, and dentated almost to infinity, each having at their base two toothed auricles. Such is _Rhizostoma Cuvieri_ of Péron (Fig. 89), the disk of which is of a bluish-white, like the arms, and of a rich violet over its circumference. This beautiful zoophyte is found plentifully in the Atlantic, living in flocks, which attain a great size. It is common in the month of June on the shores of the Saint Onge; in August on the English coast; and along the strand of every port in the Channel they are seen in the month of October in thousands, where they lie high and dry upon the shore, on which they have been thrown by the force of the winds.

Such also is _R. Aldrovandi_ (Fig. 90), which appears all the year round in calm weather. It is an animal much dreaded by bathers. It possesses an urticaceous apparatus, which produces an effect similar to the stinging-nettle when applied to the skin. If the animal touches the fisherman at the moment of being drawn from the water, it is apt to inflame the part and raise it into pustules.

_Cassiopea_ and _Cephea_ are two other types belonging to the same group. In _Cassiopea Andromeda_ (Fig. 91), belonging to the first, the disk is hemispherical, but much depressed, without marginal tentacles or peduncle, but with a central disk, with four to eight half-moon-shaped orifices at the side, and throwing off eight to ten branching arms, fringed with retractile sucking disks. _Cephea Cyclophora_, Péron (Fig. 92), is another very remarkable form of these strangely-constituted organisms.

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