Glimpses of Ocean Life 23

Glimpses of Ocean Life 23

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.