Glimpses of Ocean Life 24

Glimpses of Ocean Life 24

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.]

 

 

 

 

CHAPTER XIV.

 

Doris, Eolis, &c.

 

 

'The inhabitants of the watery element were made for wise men to

contemplate and fools to pass by without consideration.'--ISAAK

WALTON.

 

 

 

 

XIV.

 

 

One fine morning during the month of January, on peering into my

largest tank I perceived, attached to the upper portion of the marble

arch (centre piece), a peculiar object that had evidently been

deposited during the preceding night, but by whom or by what means I

knew not. It resembled a fungoid growth, or riband of flesh, plaited up

and attached at one edge to the stone. At every undulation of the water

the object moved to and fro with an extremely graceful motion.

 

By careful and close examination it was seen to be covered with a film,

that gradually expanded until it burst, and thus gave means of escape

to thousands of minute white granules. On submitting these to the

microscope, a most wondrous sight met my astonished eyes. Each dot or

granule proved to be a transparent shell, resembling the periwinkle or

rather the nautilus in shape; containing an animal whose excited and

rapid movements were amusing to witness. From out the opening of the

shell appeared now and again two rings of cilia. When these organs

were about to be put into action, they reminded me of two circular

tubes of gas connected together, and each containing innumerable

perforations, which were sometimes suddenly and entirely lit up by a

torch being applied to one end.

 

The _cilia_ may be distinctly seen to play at a certain point, and then

gradually extend round the circumference of the rings. When the whole

are in full action their movements are so extremely swift _as to appear

devoid of motion_, and thus bear a resemblance to rings of flame.

 

The result of the movements of the cilia was always evident in

the vigorous evolutions of the little embryos, for the microscope

filaments, while in action, caused the animal to roll about in all

directions in a confined circle. When this envelope burst, the little

nautiline dashed out, and then--then it was of little use attempting to

get a view of the animal again, so rapid and violent were its movements

to and fro, never resting for one instant on any spot, and least of

all the spot wished. By the aid of blotting-paper, I sometimes reduced

the quantity of water in the watch glass; and the animal, thus being

compelled to confine its evolutions to a narrower stage, was more often

within the field of view.

 

On visiting the sea-shore shortly after the discovery of the egg

cluster just described, I perceived attached to numberless stones and

large boulders thick clusters, composed of the self-same objects!

Beside them were lying confused heaps of _sea-slugs_, evidently

exhausted with their hatching exertions. Anything more repulsive to

the eye than those animal heaps exhibited it would be difficult to

conceive. Yet, at the same time, I know of no sight more pleasing than

to watch the _Doris_ in its healthy state, gliding along with outspread

plume on the under surface of the water, or up the sides of the tank,

more especially if it be observed through a powerful hand lens.

 

These remarks will perhaps convey some new information to the young

naturalist, embracing as they do the leading facts connected with the

wondrous embryotic development of many marine animals. The reader will

already be prepared to learn that the vivacious little animal, moving

by aid of cilia and enclosed in a _shell_, was in reality the youthful

stage of that slow creeping gasteropod the _Doris_, which, in its

mature form, is possessed of no cilia, nor any shelly covering whatever.

 

I should not have introduced this subject so familiarly did I not feel

anxious to make my readers aware how easy it is for each of them to

conduct experiments in the early stages of embryotic development, and

to gain practical evidence of the wonders which this study unfolds.

 

'What,' eloquently asks Mr. Lewes, 'can be more interesting than to

watch the beginnings of life, to trace the gradual evolution of an

animal from a mass of cells, each stage in the evolution presenting

not only its own characteristics, but those marks of affinity with

other animals which make the whole world kin? To watch the formation

of the blood-vessels, to see the heart first begin its tremulous

pulsations, to note how life is from the first one incessant struggle

and progress--these keep us with fascinated pertinacity at our studies.'

 

The remarkable fact above mentioned, of the young of the Nudibranchiate

Gasteropoda being furnished with a shell is exhibited not only in

Doris, but in Eolis, Tritonia, Aplysia, &c., while the embryos of the

Purpura, Nerita, Trochus, &c., are likewise in their youthful state

furnished with cilia, by the agency of which these animals swim freely

about in their native element.

 

There is one exception to this, which occurs in _Chiton_, the early

stage of which has recently been shown by the observations of Mr.

Clarke and Professor Loven to be peculiar, and more resembling that

of an annelid than of a mollusc. In this case the animal can scarcely

be said to undergo a metamorphosis; for the embryo, even within the

egg, has nearly the form of the parent, and the appearance of the

shell-plates is a mere matter of development.

 

I have never attempted to count the ova that were contained in any

single riband of Doris spawn, in fact I considered the task an

impossibility; but at a rough calculation, I concluded there would not

be less than a million. Dr. Darwin, however, travelling in the Falkland

Isles, met with a riband of spawn from a white Doris (the animal itself

was three and a half inches long), which measured twenty inches in

length, and half an inch in breadth! and by counting how many balls

were contained in a tenth of an inch in the row, and how many rows in

an equal length of riband, this gentleman reckoned that upon a moderate

computation there could not be less than six millions of eggs. Yet, in

spite of such amazing fecundity, this Doris was not common. 'Although,'

says Dr. Darwin, 'I was searching under the stones, I saw only seven

individuals. No fallacy is more common with naturalists, than that the

numbers of an individual species depend on its powers of propagation.'

 

This apparent paradox is not difficult of explanation when we consider

the number of enemies which are always hovering near, and ready with

hungry mouths to snap up the infant embryos as soon as they begin

to show signs of vitality. The Hermit-Crabs are especially fond of

Doris spawn, so much so, indeed, that the writer could never retain

any for hatching purposes while any of the Paguri were near. Mr.

Peach says they (the young Dorides) have myriads of enemies in the

small _Infusoria_, which may be noticed, with a powerful microscope,

hovering round them, and ready to devour them the instant weakness or

injury prevents their keeping in motion the cilia, which serve both

for locomotion and defence. Let them cease to move, a regular attack

is made, and the animal is soon devoured; and it is interesting to

observe several of the scavengers sporting with the empty shell, as if in derision of the havoc they have made.