ACALEPHÆ, OR SEA NETTLES.
"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.
[Illustration: Fig. 84. Æquerea violacea, natural size (Milne Edwards).]
_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.
[Illustration: Fig. 85. Aurelia aurita (Lamarck). Cyanea aurita
(Cuvier). One-third natural size.]
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.
[Illustration: Fig. 86. Hydra vulgaris. 1. Hydra with ova and young,
unhatched. 2. Hydra of natural size attached to a piece of floating
wood. 3. Egg ready to burst its shell.]
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.
[Illustration: Fig. 87. Hydra viridis (Trembley). 1. Hydra magnified,
bearing an embryo ready to detach itself. 2. Animal, natural size. 3.
Bud much magnified. 4. Bud, natural size.]
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."
[Illustration: Fig. 88. Chrysaora Gaudichaudi.]
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.
[Illustration: Fig. 89. Rhizostoma Cuvieri.]
_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.
* * * * *
Having presented to the reader certain characteristic types of Medusadæ,
we proceed to offer some general remark upon the organization and
functions of these strange creatures. We have, in short, selected these
types because they have been special objects of anatomical and
physiological study to some of our best naturalists.
[Illustration: Fig. 90. Rhizostoma Aldrovandi.]
The Medusæ have no other means of breathing but through the skin. We
remark all over the body of these zoophytes certain cutaneous
elongations, disposed so as to favour the exercise of the breathing
function. Certain marginal fringes of extended surface, as well as the
tentacle, are the special seats of the apparatus. The organs of
digestion also present arrangements peculiar to themselves; the mouth is
placed on the lower part of the body, and is pierced at the extremity of
a trumpet-like tube, hanging sometimes like the tongue of a bell. The
walls of the stomach, again, are furnished with a multitude of
appendages, which have their origin in the cavity of the organ, and
which are very elastic. The stomach, furnished with these vibratile
cells, appears to secrete a juice whose function is to decompose the
food and render it digestible.
[Illustration: Fig. 91. Cassiopea Andromeda (Tilesius).]
In some of the Medusadæ the central mouth is absent altogether. With the
Rhizostoma, for instance, the stomachal reservoir has no inferior
orifice; it communicates laterally with the canals which descend through
the thickness of the arms, and open at their extremities through a
multitude of small mouths. These are the root-like openings from which
the animals derive their name of Rhizostoma, from the Greek words
ῥίζα, root, and στόμα, mouth.
[Illustration: Fig. 92. Cephea Cyclophora.]
The arms of the Rhizostoma are usually eight in number, the free
extremities of each being slightly enlarged: in these arms many small
openings or mouths occur, which are the entrances to so many ascending
canals communicating with larger ones, as the veins do in the higher
animals: the common trunk canal is thus formed, which directs itself to
the stomach, receiving in its way thither all the lateral branches.
A very distinct circulation exists in the Medusæ. The peripheric part of
the stomach suffers the nourishing liquid which has been elaborated in
the digestive cavity to pass: this fluid then circulates through
numerous canals, the existence of which have been clearly traced.
It is also a singular fact, that organs of sense seem to have been
discovered in these Medusæ, which early observers believed to be
altogether destitute of organization. "During my sojourn on the banks of
the Red Sea," says Ehrenberg, in his work on the _Medusa aurita_,
"although I had many times examined the brownish bodies upon the edge of
the disk of the Medusæ, it is only in the month past that I have
recognized their true nature and function. Each of these bodies consists
of a little yellow button, oval or cylindrical, fixed upon a thin
peduncle. The peduncle is attached to a vesicle, in which the microscope
reveals a glandular body, yellow when the light traverses it, but white
when the light is only reflected on it. From this body issue two
branches, which proceed towards the peduncle or base of the brown body
up to the button or head. I have found that each of these small brown
bodies presents a very distinct red point placed on the dorsal face of
the yellow head; and when I compare this with my other observations of
similar red points in other animals, I find that they greatly resemble
the eyes of the Rotifera and Entomostraca. The bifurcating body placed
at the base of the brown spot appears to be a nervous ganglion, and its
branches may be regarded as optic nerves. Each pedunculated eye presents
upon its lower face a small yellow sac, in which are found, in greater
or smaller numbers, small crystalline bodies clear as water." The
presence of a red pigment in very fine grains is an argument in favour
of the existence of visual organs in these zoophytes, for the small
crystals disseminated in the interior of the organ would no doubt
perform the part of refracting light which is produced by crystalline in
the eyes of vertebrated animals. Moreover, it is found that there are
marginal corpuscles analogous to these brown spots in other species of
Medusæ. They are of a palish yellow, or quite colourless, and enclose
sometimes a single, sometimes many calcareous corpuscles. When they are
colourless, some naturalists have rather taken them for ears reduced to
their most simple expression.
The Medusæ are not absolutely destitute of nervous system. We have seen
that they have ganglions, and probably optic nerves. Ehrenberg also
states that they have ganglions at their base, which furnish them with
nervous filaments.
Without entering further into the details of their delicate and
complicated structure, we shall pause briefly on their mode of
reproduction. We shall find here physiological phenomena so remarkable
as to appear incredible, had not the researches of modern naturalists
placed the facts beyond all doubt. "Which of us," says M. de
Quatrefages, "would not proclaim the prodigy, if he saw a reptile issue
from an egg laid in his court-yard, which afterwards gave birth to an
indefinite number of fishes and birds? Well, the generation of the
Medusæ is at least as marvellous as the fact which we have imagined."
Let us note, for example, what takes place with the Rose Aurelia, a
beautiful Medusa, of a pale rose colour, with nearly hemispherical disk,
from four to five inches in diameter, whose edge is furnished with short
russet-brown tentacles; taking for our guide the eloquent and learned
author of the "Metamorphosis in Men and Animals," M. de Quatrefages.
The Medusa, designated under the name of Rose Aurelia, lays eggs which
are characterised by the existence of three concentric spheres. These
eggs are transformed into oval larvæ, covered with vibratile cells,
having a slight depression in front. They swim about for a short time
with great activity, much like the infusoria, which they strikingly
resemble in other respects.
At the end of forty-eight hours the movements decrease. Aided by the
depression already noted, the larvæ attaches itself to some solid body,
fixing itself to it at this point by the assistance of a thick mucous
matter. A change of form soon takes place: it becomes elongated; its
pedicle is contracted, and its free extremity swells into a club-like
shape. An opening soon presents itself in the centre of this extremity,
through which an internal cavity appears. Four little mammals have now
appeared on the edge, which are elongated in the manner of arms. Others
soon follow: these are the tentacles of a polyp: the young infusoria has
become a polyp!
The polyp increases by buds and shoots, just like a strawberry plant,
which throws out its slender stems in all directions, covering all the
neighbouring ground.
The young Medusa lives some time under this form. Then one of the polyps
becomes enlarged and its form cylindrical. This cylinder is divided into
from ten to fourteen superposed rings. These rings, at first smooth,
form themselves into festoons, and separate into bifurcated thongs; the
intermediate lines become channeled. The animal now resembles a pile of
plates, cut round the edges. In a short time each ring is stirred at the
free edge of its fringe: this becomes contractile. The rings are
individualised. Finally, these annular creatures, obscure in their
lives, isolate themselves. When detached, they begin to swim: from that
time they have only to perfect and modify their form. From being flat,
they become concave on the one side and convex on the other. The
digestive cavity--the gastro-vascular canals--become more decided; the
mouth opens, the tentacles are elongated, the floating marginal cirri
become more and more numerous; and now, after all these metamorphoses,
the Medusa appears: it perfectly resembles the mother.
TUBULARIDÆ.