Acalephæ.
(MEDUSÆ, OR JELLY-FISH.)
'And now your view upon the ocean turn,
And there the splendour of the waves discern;
Cast but a stone, or strike them with an oar,
And you shall flames within the deep explore;
Or scoop the stream phosphoric as you stand,
And the cold flames shall flash along your hand,
When lost in wonder, you shall walk and gaze
On weeds that sparkle, and on waves that blaze.'
XIII.
There are certain narrow-minded persons who raise objections to men
of science prying into the secrets of nature, and profanely, as they
think, attempting to explain the design and purpose of the great
Creator.
But to the intelligent and right thinking man, no employment could
be found more elevating or ennobling than this; and whether he be a
fellow-worker himself, or merely an approving observer of the labours
of others, still he feels, and conscientiously believes in the words of
Milton, that--
"The desire which tends to know
The works of God, thereby to glorify
The great Workmaster, leads to no excess
That merits blame, but rather merits praise
The more it seems excess."
When such a one contemplates the atmosphere, for instance, with its
'wonderful phenomena of clouds, rain, and sunshine, that alternately
shield, moisten, and warm the face of the earth, he feels awed by the
grandeur of the exquisite system of machinery by which such beautiful
results are accomplished. To him also the sea, with its physical
geography, becomes as the main-spring of a watch; its waters, and its
currents, and its salts, and its inhabitants with their adaptations,
as balance wheels, cogs, and pinions, and jewels. Thus he perceives
that they too are according to design; that they are the expression of
one thought, a unity with harmonies, which one intelligence only could
utter.' To his eye all created things possess an interest doubly great,
not only from their marvellous structure, but from the mission they are
destined to fulfil in this lower world.
What peculiar mission the Acalephæ (which we are now about to consider)
were destined to fulfil it has long puzzled men of science to explain.
Nor can this be wondered at, when we remember the amazing number of
these creatures, and also the extreme delicacy of their structure.
Some indeed appear almost as if they were formed by the sportive
combination of air and water, as if the sea-breeze ruffling the face of
ocean caused bubbles innumerable to arise, which becoming mysteriously
endowed with life, thenceforth existed as Medusæ.
They have, indeed, frequently been spoken of as 'animated sea-water,'
or 'living jelly.' These expressions seem most appropriate when we
remember, that if one of these creatures be placed upon a plate of
glass, and allowed to remain exposed to the sun's rays, the only
thing that will remain to testify to the existence of this singularly
graceful object is a thin film, that a stroke of the sponge or finger
will remove in an instant.
The most satisfactory explanation that has been offered as to the use
and purpose of the Medusæ is, that _they serve as the principal food of
whales and other Cetacea_. To these marine monsters--frequently found
from 70 to 110 feet long--we can imagine a few hundreds of jelly-fish
would be considered a small meal. The supply, however, is ever equal to
the demand, as we shall see hereafter.
I may here be permitted to explain that, in most large fishes, the
jaws are completely filled with formidable teeth, as in the shark,
for instance. This rapacious monster--which has been aptly termed the
tiger of the sea by us, and which the French, in allusion to the deadly
character of its habits, have named Requin, or Requiem, the rest or
stillness of death--possesses a most marvellous dental apparatus.
Its teeth are not, as might be supposed, fixed in sockets, but attached
to a cartilaginous membrane. The teeth, in fact, are placed one
behind the other in a series of rows; the first of which, composed of
triangular cutting teeth, stands erect and ready for use. But as the
membrane continues to grow and advance forward, it slowly perishes,
and the teeth drop off, their place being taken by the next row which
formerly stood second. These, in the course of time, are succeeded by
a third series, which are again followed by others.
Now, whales possess no such weapons. Their enormous mouths are not
filled with 'tusks or grinders, but fitted instead with vast numbers of
oblique laminæ of a softer substance, usually denominated whalebone,
which is admirably adapted for the crushing and masticating of soft
bodies.'
To give an idea of the amazing extent of the harvests of 'whale food,'
as the Medusæ are termed, that abound in various parts of the ocean,
we need only quote the evidence of various navigators on the subject.
One (Lieut. Maury), for example, states, that on the coast of Florida
he met with a shoal of these animals, that covered the sea for many
leagues, through which his vessel, bound for England, was five or six
days in passing. The most singular part of the story is that, on his
return some sixty days after, he fell in with the same shoal off the
Western Islands, and here again he was three or four days in getting
clear of them.
The Western Islands here mentioned are, it seems, the great resort for
whales; and 'at first there is something curious to us in the idea
that the Gulf of Mexico is the harvest field, and the Gulf Stream
the gleaner which collects the fruitage planted there, and conveys
it thousands of miles off to the living whales at sea. But, perhaps,
perfectly in unison is it with the kind and providential care of that
great, good Being who feeds the young ravens when they cry, and caters
for the sparrow.'
But Dr. Scoresby, in his work on the Arctic Regions, by aid of figures
conveys the most vivid idea of the myriads of these creatures that
float in the bosom of the ocean. This writer discovered that the
olive-green colour of the waters of the Greenland sea was caused by the
multitudes of jelly-fish contained therein. On examination he found
that 'they were about one-fourth of an inch asunder. In this proportion
a cubic inch of water must contain 64; a cubic foot, 110,592; a cubic
fathom, 23,887,872; and a cubical mile, 23,888,000,000,000,000! From
soundings made in the situation where these animals were found, it
is probable the sea is upwards of a mile in depth; but whether these
substances occupy the whole depth is uncertain. Provided, however, the
depth which they extend be but 250 fathoms, the above immense number
of one species may occur in a space of two miles square. It may give
a better conception of the amount of Medusæ in this extent, if we
calculate the length of time that would be requisite with a certain
number of persons for counting this number. Allowing that one person
could count 1,000,000 in seven days, which is barely possible, it would
have required that 80,000 persons should have started at the creation
of the world to complete the enumeration at the present time! What a
prodigious idea this fact gives of the immensity of creation, and of
the bounty of Divine Providence, in furnishing such a profusion of life
in a region so remote from the habitations of man. But if the number of
animals be so great in a space of two miles square, what must be the
amount requisite for the discolouration of the sea through an extent of
perhaps 20,000, or 30,000 square miles.'
These creatures may be appropriately termed the glow-worms of the
ocean, for it is to them that the phosphorescence of the sea is mainly
attributable.
Sir Walter Scott, in his poem of the 'Lord of the Isles,' thus alludes
to this phenomenon:--
'Awaked before the rushing prow,
The mimic fires of ocean glow.
Those lightnings of the wave.
Wild sparkles crest the broken tides,
And, flashing round the vessel's sides,
With elfish lustre lave;
While far behind their livid light
To the dark billows of the night
A gloomy splendour gave.'
Hugh Miller also gives a beautiful prose description of the luminosity
of our own seas, but we must resist the temptation to introduce it here.
The appearance of the Greenland Seas is principally owing to the
presence of the minute species of Acalephæ, but there are many others
that grow to an immense size. Specimens of these may be frequently seen
cast on the sea-beach by the force of the waves. When in their native
element they form the swimmer's dread, owing to a peculiar stinging
power which they possess.
The Medusæ have been divided into groups, and distinguished according
to their different organs of locomotion. The common idea is that all
jelly-fishes are like mushrooms or miniature umbrellas. Such, it is
true, is their general form, but others abound both in our own and
in foreign seas, that possess a totally different appearance. For
instance, some move by means of numerous cilia, or minute hairs that
are attached to various parts of their bodies. By the exercise of these
organs the creatures glide through the water, and hence they are called
_ciliograde Acalephæ_.
One of the most remarkable examples of this class is seen in the
Girdle of Venus (_Cestum veneris_). 'This creature is a large, flat,
gelatinous riband, the margins of which are fringed with innumerable
cilia, tinted with most lively irridescent colours during the day,
and emitting in the dark a phosphorescent light of great brilliancy.
In this animal, too, which sometimes attains the length of five or
six feet, canals may be traced running beneath each of the ciliated
margins.'
This animal, as it glides rapidly along, has the appearance of an
undulating riband of flame. Most likely it is the species to which
Coleridge alludes in the following passage:--
'Beyond the shadow of the ship
I watched the water snakes
They moved in tracks of shining white,
And when they reared, the elfish light
Fell off in heavy flakes.
* * * * *
Within the shadow of the ship
I watched their rich attire--
Blue, glossy green, and velvet black,
They curled and swam; and every track
Was a flash of golden fire.'
Another of this class is the common Beroë (_Cydippe pileus_); its body
is melon-shaped, and covered over by rows or bands of cilia, placed
similarly to the treads on a water wheel, one above another. These are
entirely under the will of the little gelatine. It can use each or
all of them, and thus row itself along at pleasure. But perhaps the
most singular portion of this creature is what has been termed its
fishing apparatus, though by some writers it is considered merely to
be the means by which the Beroë anchors its body to any desired spot.
It consists of two exceedingly slender filaments or streamers, which
measure many times the length of the Beroë itself. Some writers, again,
fancy that these organs are used to propel the animal. This must be an
erroneous notion, for if they were cut off, the creature would still
continue to move with the same power as before. Nay more, if the little
Cydippe be cut into pieces, and the ciliated bands be attached to
each fragment, the latter will swim about with the same power as when
connected with the entire animal.
From the filaments here described, others more slender still depend at
regular intervals, which curl up like vine tendrils upon the principal
stem. The whole can be spontaneously elongated or slowly withdrawn
within the body of the Beroë, where they lie enclosed in two sheaths
until again required for use.
These interior 'sheaths,' which resemble in shape the drone of a
bag-pipe, are easily seen, being almost the only parts which are not
perfectly transparent. They are whitish in colour, and semiopaque.
(Plate 6 contains a sketch of the Beroë, drawn from nature.)
I may mention that the paddles, with their comb-like array of cilia,
flap successively in regular order from the top to the bottom of each
row. This wave-like movement takes place simultaneously in all the
rows, when the animal is in full vigour.
The organs of progression in the _Pulmonigrade_ Acalephæ, as their name
imports, bear certain resemblance to the lungs in respiration. They
move by the expansion and contraction of their umbrella-shaped bodies.
Graceful and elegant indeed are the motions of these creatures. I have
seen small specimens about the size of a sixpence, advance, in three
springs, from the bottom to the top of a large vase in which they were
confined.
In descending they turn over and allow themselves to sink gradually as
if by their own weight.
The third division of the Acalephæ is termed _Physograde_. The most
common member of this group is the _Physalus_, so well known to all
sailors under the name of the Portuguese Man-of-War. It is buoyed up
by air bladders--in fact, its entire body appears as one bladder, which
the animal is enabled to contract or expand at will. At first glance
the _Physalus_ appears to belong to quite a different family--suffering
under some maltreatment; for from its lower side, what seem a number of
entrails, of all shapes and sizes, hang down. When the upper surface
or crest of its swimming bladder projects above the waves, it has a
beautiful appearance, spangled with rays of purple, blue, and gold.
This formation acts as a kind of sail, by means of which the creature
is enabled to glide along with considerable speed.
This Physalus is a somewhat mysterious being, and zoologists have not
as yet been able to determine many points connected with its structure
and development.
The _Cirrigrade_ Acalephæ, too, are a singular family. They exhibit a
higher stage of development than those already alluded to, and possess
a kind of skeleton embedded within their gelatinous bodies.
The _Porpita_ and _Velella_ are examples of this class, but for
detailed descriptions I must refer the reader to larger works which
treat on the subject.
I cannot conclude this brief and imperfect sketch of the Acalephæ
without noticing their singular mode of reproduction. Nothing can
appear more marvellous than this process when first brought before
one's attention. It far excels the wildest dreams of fiction; and were
it not so well authenticated by naturalists who have devoted labour and
valuable time to gain ocular demonstration of the fact, we might well
hesitate to believe the statements laid before us in their works.
For example, a Polype, as _Hydra Gelatinosa_ or _Hydra Tuba_(found
on buoys, oyster shells, &c., long submerged), will, it may be in a
simple aquarium, produce a number of small objects which, on being
examined through the microscope, are found to be, not young Polypes,
but Jelly-fish! In process of time, the latter, by a wondrous law of
nature, will produce in their turn, not Medusæ, but Polypes!
'Imagine,' says Mr. Lewes, 'a lily producing a butterfly, and the
butterfly in turn producing a lily, and you would scarcely invent
a marvel greater than this production of Medusæ was to its first
discoverers. Nay, the marvel most go further still, the lily must first
produce a whole bed of lilies like its own fair self before giving
birth to the butterfly, and this butterfly must separate itself into a
crowd of butterflies, before giving birth to the lily.'
Let me now, by entering briefly into detail, endeavour to make the
reader acquainted with the leading features of this mysterious subject,
known as 'the alternation of generations.'
The adult Medusæ, then, gives birth to a number of oval gemmæ or buds,
appropriately so called by most writers, which appear like minute
jelly bubbles, covered with numberless vibratile cilia. These organs,
ten thousand times more delicate, we may imagine, than the eyelashes
of some infant member of fairy land, are ever in constant motion. The
currents produced thereby serve to propel the little animal to some
stray pebble or stalk of sea-weed, situated at a respectful distance
from its gelatinous relative. On some such object the young bud
attaches itself, and proceeds to vegetate.
The body gradually lengthens, and becomes enlarged at its upper
extremity; from this portion of the animal four arms appear surrounding
a kind of mouth. The arms lengthen, and are soon joined by four others.
These organs, as also the inner surface of the lips and of the stomach,
are covered with cilia, and become highly sensitive. They are used
in the same manner as the tentacula of the Actiniæ, namely, for the
capture of food. There is this difference, be it observed, between the
two animals, that while the infant Medusæ labours incessantly to gain
its daily meals, the zoophyte remains still, and trusts to chance for
every meal that it enjoys.
Fresh sets of arms continue to be developed successively upon the
little jelly fish, until the whole amount in number to twenty-five or
thirty. 'And the body, originally about the size of a grain of sand,
becomes a line, or the twelfth part of an inch in length.'
Thus far there appears nothing particularly striking or improbable in
the history of the Medusæ; the next stage, however, exhibits matter for
our 'special wonder.'
The young Acaleph now throws off its animal existence, and sinks into a
plant or compound polype.
The lower part of the body swells, and from thence, what may be termed
a _stolen_, is thrown out. On the upper surface of the stolen one
and even two buds are often formed. 'As the bud enlarges it becomes
elongated, and bends itself downwards to reach the surface of the stone
to which the elongated extremity adheres; after this the attached end
is gradually separated from the body of the parent. When thus detached,
a small opening presents itself at its upper end, its interior
gradually becomes hollowed out, and cilia grow upon it, and tentacula
begin to sprout around the mouth, exactly in the same manner as in the
buds formed on the upper surface of the stolens.'
Thus, from a single bud numberless other buds are formed, each being
endowed with equally prolific powers. If the parent be cut in half
transversely, the cut will close in, attach itself to some object, and
produce stolens and buds! If cut longitudinally, and the cut edges
be allowed to touch each other, they will again adhere, and exhibit
no trace of their ever having been divided. If the cut edges of each
division be not kept apart they will approximate and adhere together,
and thus two separate animals will be produced, each gifted with the
power of throwing out stolens and buds with the same prodigality as if
they had never been disunited!
How long this budding process of necessity continues we cannot tell. It
may be only during the winter season. These creatures in their perfect
condition are generally found crowding our seas during the summer
months; most probably, therefore, as Sars and Steenstrup state, it is
at the commencement of spring that they undergo the last portion of
this 'transformation strange.'
Still, this cannot be taken as a general rule. Dr. Reid, who for a
period of two years kept colonies of Medusæ, and assiduously watched
the various stages of their development, found that the larvæ of
one colony, which was obtained in September 1845, did not split
transversely into young Medusæ in the spring of 1846, as he expected
them to do, but continued to produce stolens and buds abundantly.
On the other hand, the larvæ of the other colonies, which this
gentleman obtained in July, began to yield young Medusæ about the
middle of March. This process takes place in the following manner:
A 'bud' having arrived at maturity, it becomes 'cylindrical,'
considerably elongated, and much diminished in diameter, its outer
surface being marked with a series of transverse wrinkles.
These wrinkles, or rings, which frequently amount to thirty or forty
in number, are first formed at the top, and slowly extend downwards.
Gradually as these furrows become deeper, the tentacula waste away, and
upon the margin of the upper ring eight equi-distant rays are formed.
The process continuing, in the space of a fortnight or so each groove
or ring is in like manner furnished with rays. The Medusæ now present
an appearance exactly resembling a series of cups piled up one within
the other. Strange to state, each little cup becomes eventually endowed
with life! As the uppermost segment is completely developed, it rests
upon the slender lips of the one beneath. It then glides off from its
old resting-place, and swims freely about in the water. Quickly it
aspires to the rippling surface above, and by a series of graceful
evolutions accomplishes its object. Once among the dancing waves and
exposed to the rays of a cheering sun, our little Medusa assumes its
complete form; and as a beautiful _Modecra formosa_, it may be destined
at some time or other to be the prize of an ardent zoologist, who, I
venture to assert, could not compliment it in more poetical language
than Professor Forbes has already done. This delightful author,
describing the little gem in question, says, 'It is gorgeous enough
to be the diadem of sea fairies, and sufficiently graceful to be the
night-cap of the tiniest and prettiest of mermaidens.' Or as an adult
_Cyanea capillata_, our once insignificant jelly-bag may perhaps
appear, and by an exercise of its urticating powers, send some unhappy
swimmer smarting and trembling to his home.
While the Medusæ column proceeds to throw off from its uppermost part
living segments of itself, its lower half, or stem, continues to grow,
but does not become ringed, for as the budding process ceases, the last
formed cup rests on newly-formed tentacula! Then again stolens are
thrown out, on which young Medusæ are formed, as before described.
Contemplating such mysteries as these, the mind becomes bewildered and
the spirits humbled.
'Imagination wastes its strength in vain,
And fancy tries and turns within itself,
Struck with the amazing depths of Deity.'
The above may be deemed one of the most interesting zoological theories
that has ever been promulgated in modern times. It was founded by
Chamisso, and termed the 'alternation of generation,' but was much
improved and extended by the researches of Steenstrup. Professor Owen,
however, had previously reduced the theory to a fixed and definite
scientific form, under the title of '_Parthenogenesis_.' Another
author, not viewing the Medusæ in the various stages of development
as an aggregation of individuals, 'in the same sense that one of
the higher animals is an individual,' proposes that each Medusa be
considered as an individual, developed into so many 'zooids.'
Into this abstract question, of course, I cannot enter. The reader who
would wish to know more of the subject than I have faintly shadowed
forth in this chapter, may consult Steenstrup's Memoir, published
by the Ray Society; Dr. Reid's admirable papers in the 'Magazine of
Natural History' 2d series; Lewes' 'Sea-Side Studies;' and the learned
works of Professor Owen.
[Illustration: MEDUSÆ IN VARIOUS STAGES OF DEVELOPMENT.]