We inscribe at the head of this paper the popular name of a class of
beings, which, though simple in their organisation, are full of
interest to the zoologist, and attractive to the common observer from
the singularity or beauty of their forms, and, in many cases, the
brilliancy of their colouring. The ocean, throughout its wide extent,
swarms with myriads of gelatinous creatures--some microscopic, some of
large dimensions--which deck it with the gayest colours by day, and at
night light up its dreary waste with 'mimic fires,' and make it glow
and sparkle as if, like the heavens, it had its galaxies and
constellations. These are the jelly-fishes, or sea-nettles
(_Acalephæ_), as they are often called, from the stinging properties
with which some of them are endowed. The commoner forms are well
known, for the beach is often strewn with the carcasses of the larger
species. On fine days in summer and autumn, whole fleets of these
strange voyagers appear off our coasts. Their umbrella-shaped,
transparent disks float gracefully through the calm water, and their
long fishing-lines trail after them as they move onward. At times,
multitudes, almost invisible to the naked eye, tenant every wave, and
give it by night a crest of flame; while other kinds measure as much
as a yard in diameter. The _Acalephæ_ present the greatest variety of
form and colour, as well as of size, but they are all of the most
delicate structure, frail, gelatinous, transparent. Some are so
perfectly colourless, that their presence can with difficulty be
detected in the water.
The following description, by Professor E. Forbes, applies to a large
proportion of the species:--'They are active in their habits, graceful
in their motions, gay in their colouring, delicate as the finest
membrane, transparent as the purest crystal.' The poet Crabbe has
characterised them well in the following passage:--
'Those living jellies which the flesh inflame,
Fierce as a nettle, and from that the name;
Some in huge masses, some that you might bring
In the small compass of a lady's ring;
Figured by hand divine--there's not a gem
Wrought by man's art to be compared to them;
Soft, brilliant, tender, through the wave they glow,
And make the moonbeam brighter where they flow.'
The first thing that arrests our attention in these creatures is the
extreme delicacy and tenuity of their substance. The jelly-fish is
chiefly made up of fluid. A quantity of water and a thin membranaceous
film, these are its chief component parts. Professor Owen has
ascertained that a large individual, weighing two pounds, when removed
from the sea, will be represented, when the fluid which it contains is
drained off, 'by a thin film of membrane not exceeding thirty grams in
weight.' Naturalists have commonly described the jelly-fish as being
little more than 'coagulated water' and the description is correct.
And yet these masses of film and fluid, floating at the mercy of wind
and wave, possess powers which we should hardly associate with so
simple a structure, and can accomplish works of which we should little
suspect them. Delicate and defenceless as they appear, they can
capture fishes of large size, and digest them with ease and rapidity.
Some of them are in truth formidable monsters. Professor E. Forbes
gives the following humorous description of the destructive
propensities of some medusæ which he had captured in the Zetland
seas:--'Being kept,' he says, 'in a jar of salt-water with small
crustacea, they devoured these animals, so much more highly organised
than themselves, voraciously; apparently enjoying the destruction of
the unfortunate members of the upper classes with a truly democratic
relish. One of them even attacked and commenced the swallowing of a
_Lizzia octopunctata_, quite as good a medusa as itself. An animal
which can pout out its mouth twice the length of its body, and stretch
its stomach to corresponding dimensions, must indeed be "a triton
among the minnows;" and a very terrific one too. Yet is this ferocious
creature one of the most delicate and graceful of the inhabitants of
the ocean--a very model of tenderness and elegance.'
The jelly-fishes are all, in their adult state, locomotive beings.
They float freely and incessantly through the ocean, either impelled
by their own efforts, or driven by storm and billow. They for the most
part frequent the open seas, and shun the shore, their delicate frames
being endangered by the perennial strife between land and water. Being
designed for constant motion, for the navigation of the great waters,
their entire organisation is adapted to such a mode of life. We find
amongst those ocean-floaters the greatest perfection and variety of
locomotive apparatus; and they have been divided into sections,
according to the modifications of this portion of structure which they
exhibit. We shall endeavour to give a popular account of the leading
peculiarities of each, and note the most interesting points in the
history of the tribe.
In the first section, the animals are furnished with a disk or
umbrella of varying shape, which serves as a float, beneath which hang
certain processes connected with the functions of prehension and
digestion. In this division are included some of the best-known forms.
The creature, in this case, propels itself by the alternate
contraction and expansion of its disk, thus striking the water, and
driving itself forward. These movements take place at regular
intervals, and serve a double purpose. They not only propel, but at
the same time drive the water over the lower surface of the disk. Here
is situated a complicated net-work of vessels, and the fluids of the
body are thus exposed to the influence of oxygen, and receive the
needed aeration. The stroke of the disk, therefore, is not only a
locomotive, but also a respiratory act. The jelly-fishes of this
section move as they breathe, and breathe as they move. Hence the name
which has been given them--_Pulmonigrades_. We find the same admirable
economy of resources amongst the lower animalcules. The cilia which
propel them secure the aeration of the system.
It is evident that the motive apparatus in this section of the
_Acalephæ_ is but a feeble one. It only avails in calm weather. When
the sea is agitated, the jelly-fish is driven helplessly along. It
cannot choose its path. As its food, however, is everywhere abundant
around it, and it has no business that should lead it in one direction
more than another, there is no great hardship in this.
In this section are included some of the most beautiful, as well as
common of the tribe. The forms of the umbrella are often most lovely,
and present an astonishing variety. As an example of the beauty which
they sometimes display, we may refer to a species which resembles an
exquisitely formed glass-shade, ornamented with a waved and tinted
fringe. The most perfect grace of form, the transparency of the
crystal, and colour as delicate as that of the flower, combine to
render this frail being one of the loveliest of living things.
In another section, locomotion is effected by a modification of
ciliary apparatus. We have a familiar example in the _Beroe_ of our
own seas, a most attractive little being, and a prime favourite with
naturalists, who have described its habits and celebrated its beauty
with enthusiasm. We shall not soon forget the delight with which we
first made acquaintance with this graceful little rover. While
rambling along the shore in quest of marine animals, our attention was
arrested by a drop of the clearest jelly, as it seemed to be, lying on
a mass of rock, from which the tide had but just receded. On
transferring it to a phial of sea-water, its true nature was at once
revealed to us. A globular body floated gracefully in the vessel,
scarcely less transparent than the fluid which filled it. Presently it
began to move up and down within its prison-house, and the paddles by
means of which the beroe dances along its ocean-path were distinctly
visible. These paddles are nothing more or less than cilia of a
peculiar kind, ranged in eight bands upon the surface of the body.
They are set in motion at the will of the animal, and their incessant
strokes propel it swiftly through the water. By stopping some of its
paddles, and keeping others in play, the beroe can change its course
at pleasure, and so wander 'at its own sweet will,' through the
trackless waste. Beauty waits upon the course of this little crystal
globe. The grace and sprightliness of its movements must strike the
commonest observer. As the sunlight falls upon its cilia, they are
'tinted with the most lovely iridescent colours;' and at night they
flash forth phosphoric light, as though the little creature were
giving a saucy challenge to the stars.
The beroe is a most active being, its habits conforming to the
organisation with which it is endowed. Such an array of paddles
prophesies of a mercurial temperament and an energetic character. It
can, however, anchor itself and lie by when occasion offers. It is
provided with two long cables, prettily set with spiral filaments or
tendrils, by means of which it can make fast to any point. When not in
use, it can retract them, and stow them away in two _sacs_ or pouches
within the body, where they may be seen coiled up, through the
transparent walls. The mouth is a simple opening at one pole of the
globular body. No arms are needed. The beroe is spared the labour and
uncertainty of the chase. As it dances gaily along, streams of water,
bearing nutritive particles, pass through the orifice into its
stomach.
In this creature, as in many of the lower animals, there is a
remarkable power of retaining vitality after the most serious
injuries; nay, in portions actually severed from the body, it will
continue for some time. Mr Patterson, in his excellent _Introduction
to Zoology_, mentions that on one occasion he divided a fragment of
the body of a beroe, lately taken from the shore and shattered by a
storm, 'into portions so minute that one piece of skin had but two
cilia attached to it, yet the vibration of these organs continued for
nearly a couple of days afterwards!' But we must leave the beroe,
charmer though it be.
Another member of this section--the _Ciliograde acalephæ_, as they are
called--is the Girdle of Venus, which resembles a ribbon in form, and
is sometimes five or six feet in length, covered with cilia, and
brilliantly phosphorescent. This must be one of the most beautiful of
the _fireworks_ of the ocean.
The jelly-fishes of another section are furnished with one or more
air-bags, which assist them in swimming, and hence bear the name of
_hydrostatic acalephæ_. In the Portuguese man-of-war (_Physalia_), the
bag is large, and floats conspicuously on the surface of the water.
From the top of it rises a purple crest, which acts as a sail, and by
its aid the little voyager scuds gaily before the wind. But should
danger threaten--should some hungry, piratical monster in quest of a
dinner heave in sight, or the blast grow furious--the float is at once
compressed, through two minute orifices at the extremities a portion
of the air escapes, and down goes the little craft to the tranquil
depths, leaving the storm or the pirate behind. In one species
(_Cuvieria_), the floats are numerous and prettily ranged round the
margin of the body. Resting on these, the creature casts about its
long fishing-lines, and arrests the passing prey.
One more section remains to be noticed. The jelly-fishes which belong
to it have a rudimentary skeleton--a plate which supports the soft,
circular body. From the lower part of the body hang numerous tentacles
(_cirri_), amidst which the mouth is placed. Probably these
multitudinous arms assist in locomotion; and, hence the name of the
family, _Cirrigrades_. Amongst the creatures of this division we meet
with some very interesting locomotive apparatus. There are some of
them by no means obliged to trust to their oars alone--they have also
sails. The _Velella_, large fleets of which visit our seas at times,
has a plate (the mast) rising from its bluish disk or deck, covered
with a delicate membrane (the sail) of snowy whiteness, by means of
which it traverses the ocean. This sail, it has been noticed, 'is set
at the same angle as the lateen-sail' of the Malays. We cannot doubt
that it is admirably suited to its purpose, and the Malays may be
proud of having nature as a voucher for their contrivance.
We find in another species a still more perfect rigging. In it
(_Rataria_) the crest is supplied with muscular bands, by means of
which the sail can be lowered or raised at pleasure. These adaptations
of structure are full of interest. Nothing can be more admirable than
the sailing-gear of these little creatures. They have to traverse the
surface of the ocean amidst all diversities of weather. Paddles alone
would not suffice for them. They must be enabled to take advantage of
the winds. Sails, therefore, are added, and the mightiest agents in
nature are commissioned to speed the little voyagers on their way.
We have already mentioned that some of the jelly-fishes possess the
power of stinging. Only a few of the larger species, however, seem to
be thus endowed; and the name sea-nettle is by no means applicable to
the class as a whole. The poisonous fluid which produces the
irritating effect on the skin, and no doubt paralyses the creatures
upon which the jelly-fish feeds, is secreted by the arms. By means of
its poison-bearing tentacles, the soft, gelatinous medusa is more than
a match for the armed crustacean and the scale-clad fish. We take from
Professor Forbes the following graphic description of one of the
stinging species:--'The _Cyanæa capillata_ of our seas is a most
formidable creature, and the terror of tender-skinned bathers. With
its broad, tawny, festooned, and scalloped disk, often a full foot or
more across, it flaps its way through the yielding waters, and drags
after it a long train of ribbon-like arms, and seemingly interminable
tails, marking its course when its body is far away from us. Once
tangled in its trailing "hair," the unfortunate who has recklessly
ventured across the graceful monster's path too soon writhes in
prickly torture. Every struggle but binds the poisonous threads more
firmly round his body, and then there is no escape; for when the
winder of the fatal net finds his course impeded by the terrified
human wrestling in its coils, he, seeking no contest with the mightier
biped, casts loose his envenomed arms, and swims away. The amputated
weapons severed from their parent body vent vengeance on the cause of
their destruction, and sting as fiercely as if their original
proprietor itself gave the word of attack.'
We now approach the most extraordinary portion of the history of
these creatures. Recent investigations have brought to light the most
interesting facts respecting their reproduction and development. It is
now known that the young jelly-fish passes through a series of
transformations before reaching its perfect state.
At certain seasons, eggs are produced within the body of the parent in
appropriate ovaries, where they are retained for a time. They are then
transferred to a kind of marsupial pouch, analogous to that of the
kangaroo, where their development proceeds. After passing through
certain changes here, the egg issues from the maternal pouch as an
oval body, clothed with cilia--an animalcule in external aspect, and
as unlike its parent as can well be imagined. For awhile the little
creature dances freely through the water, and leads a gay, roving
life; but at last it prepares to 'settle;' selects a fitting locality;
applies one extremity of its body to the surface of stone or weed, and
becomes attached. And now another change passes over it. The cilia, no
longer needed, disappear. A mouth is developed at the upper extremity
of the body, furnished with a number of arms. Gradually this number
increases, and the jelly-fish now appears in the disguise of a polype,
which feeds voraciously on the members of the class from which it has
itself so lately emerged. At this point there is a halt. The medusa
remains in its polype state for some months. At the expiration of this
term, a strange alteration in its appearance begins to take place.
Rings are formed round its body, from ten to fifteen in number. These
gradually deepen, until at length it is literally cut up into a number
of segments, which rest one upon the other--their upper margins
becoming elevated, and divided into eight lobes. It is, in fact, a
pile of cup-shaped pieces, very loosely connected together. A little
later, these pieces free themselves successively, and the sedate
polype disappears in a company of sprightly young medusæ. These
beings, indeed, still differ in some respects from the adult animal;
but the differences gradually vanish, and we have the perfect
jelly-fish as the final result of this extraordinary series of
transformations.
Similar observations have been made respecting other tribes amongst
the lower animals, and some interesting generalisations have been
founded upon them, into which, however, it is not our present purpose
to enter.
The _Acalephæ_ are the principal agents concerned in the production of
the beautiful phenomena of phosphorescence. The minute species--mere
gelatinous specks--swarm at times by countless myriads in the waters
of the ocean, and make its surface glow with 'vitalised fire.' The
waves, as they curl and break, sparkle and flash forth light, and the
track of the moving ship is marked by a lustrous line. 'In the torrid
zones between the tropics,' says Humboldt, 'the ocean simultaneously
develops light over a space of many thousand square miles. Here the
magical effect of light is owing to the forces of organic nature.
Foaming with light, the eddying waves flash in phosphorent sparks over
the wide expanse of waters, where every scintillation is the vital
manifestation of an invisible animal world.' Beneath the surface
larger forms are seen, brilliantly illuminated, and lighting up the
mystic depths of the sea. Fiery balls and flaming ribbons shoot past;
and submarine moons shine with a soft and steady light amidst the
crowds of meteors. 'While sailing a little south of the Plata on one
very dark night,' says Mr Darwin, 'the sea presented a wonderful and
most beautiful spectacle. There was a fresh breeze; and every part of
the surface, which during the day is seen as foam, now glowed with a
pale light. The vessel drove before her bows two billows of liquid
phosphorus, and in her wake she was followed by a milky train. As far
as the eye reached, the crest of every wave was bright; and the sky
above the, horizon, from the reflected glare of these livid flames,
was not so utterly obscure as over the vault of the heavens.' Even in
our own seas very beautiful displays of phosphorescence may be
witnessed. On fine summer nights, a soft, tender light plays round the
boat as it moves onward, and the oars drop liquid fire. For how much
of beauty are we indebted to these living specks of jelly?
Of the extreme minuteness of some of the species, an idea may be
formed from the fact, that 110,000 might be contained in a cubic foot
of water. We can say nothing with certainty as to the cause of the
phosphorescence of the medusæ, and shall not trouble our readers with
mere speculations.
The jelly-fishes furnish us with a striking illustration of the
profusion of life in the ocean. Provision has indeed been made for
securing in all the realms of our globe the largest possible amount of
sentient being, and consequently of happiness. And to each tribe a
definite part is assigned--a special mission is intrusted. None can be
spared from the economy of nature. The shoals of microscopic medusæ
store up in their own tissues the minute portions of nutritious matter
diffused through the waters, and supply food for the support of higher
organisms. All the tribes of animated beings are dependent one upon
another. That the greatest may enjoy its existence and fulfil its
work, the least must hold its place and discharge its function. They
co-operate unconsciously to secure the unity and harmony of a system
which is designed to promote alike the interests of each and all of
them.