Glimpses of Ocean Life 4

Glimpses of Ocean Life 4

CHAPTER II.

 

A Glance at the Invisible World.

 

 

'There is a great deal of pleasure in prying into this world of

wonders, which Nature has laid out of sight, and seems industrious to

conceal from us.... It seems almost impossible to talk of things so

remote from common life and the ordinary notions which mankind receive

from the blunt and gross organs of sense, without appearing extravagant

and ridiculous.'--ADDISON.

 

 

 

 

II.

 

 

It is hardly possible to write upon marine zoology without either

more or less alluding to those many objects, invisible to the naked

eye, which call for the use of the microscope; and it seems equally

difficult for any one who has been accustomed to this instrument

to speak in sober terms of its wonderful revelations. The lines of

Cowper, as the youngest student in microscopic anatomy will readily

acknowledge, present no exaggerated picture of ecstasy:--

 

'I have seen a man, a worthy man,

In happy mood conversing with a fly;

And as he through his glass, made by himself,

Beheld its wondrous eye and plumage fine,

From leaping scarce he kept for perfect joy.'

 

It is proper, however, to notice that a serious objection has been

urged against the use of the microscope by young persons, namely, the

injurious effects of its habitual use upon the eyesight.

 

So far as my experience goes, I cannot deny that this objection is

well founded. Since I have begun to use the instrument, I am obliged,

if I wish to view distinctly any distant object, to distort my eyes

somewhat to the shape of ill-formed button-holes puckered in the

sewing. Some individuals, I am aware, foolishly affect this appearance,

from the notion that it exhibits an outward and visible sign of their

inward profundity of character. In my own case this result may have

arisen from my having worked principally at night or in the dusk.

'As to the sight being injured by a continuous examination of minute

objects,' writes Mr. Clark, a most scientific naturalist, 'I can truly

say this idea is wholly without foundation, if the pursuit is properly

conducted; and that, on the contrary, it is materially strengthened by

the use of properly adapted glasses, even of high powers; and in proof

I state, that twenty years ago I used spectacles, but the continued

and daily examination of these minutiæ (_foraminifera_) has so greatly

increased the power of vision, that I now read the smallest type

without difficulty and without aid. The great point to be attended to

is not to use a power that in the least exceeds the necessity; not

to continue the exercise of vision too long, and never by artificial

light; and to reserve the high powers of certain lenses and the

microscope for important investigations of very moderate continuance.

The observant eye seizes at a glance the intelligence required;

whilst strained poring and long optical exertions are delusive and

unsatisfactory, and produce those fanciful imaginations of objects

which have really no existence. The proper time for research after

microscopic objects is for _one_ hour after breakfast, when we are in

the fittest state for exertion.'

 

Mr. Lewes, again, speaking to the same point, viz., the eyes being

injured by microscopic studies, says:--'On evidence the most conclusive

I deny the accusation. My own eyes, unhappily made delicate by

over-study in imprudent youth, have been employed for hours daily

over the microscope without injury or fatigue. By artificial light,

indeed, I find it very trying; but by daylight, which on all accounts

is the best light for the work, it does not produce more fatigue than

any other steadfast employment of the eye. Compared with looking at

pictures, for instance, the fatigue is as nothing.'

 

In spite of the foregoing assertions, I feel it my duty to caution the

student against excess of labour. Let him ride his hobby cautiously,

instead of seeking to enrol his name among the martyrs of science,

of whom the noble Geoffry St. Hilaire, M. Sauvigny, and M. Strauss

Dürckheim, are noted modern examples. Each member of this celebrated

trio spent the latter part of his existence in physical repose, having

become totally blind from intense study over the microscope. But

setting aside the evils of excess, we must bear witness to the intense

delight which this pursuit affords when followed with moderation.

 

"'Tis sweet to muse upon the skill displayed

(Infinite skill!) in all that _He_ has made:

To trace in Nature's most minute design

The signature and stamp of power divine.

Contrivance intricate, expressed with ease,

_Where unassisted sight no beauty sees_."

 

As my aim is merely to give the reader a taste of the subject, and

whet his appetite for its more extensive pursuit at other sources, I

shall confine my remarks to a few of those creatures which are readily

to be found in any well-stocked aquarium. The number of animalculæ

and microscopic zoospores of plants, invisible to the naked eye,

with which such a receptacle is filled, even when the water is clear

as crystal, is truly marvellous. These animals mostly belong to the

class _Infusoria_, so called from their being found to be invariably

generated in any _infusion_, or solution of vegetable or animal matter,

which has begun to decay. Now, the water in an aquarium which has

been kept for any length of time necessarily becomes more or less

charged with the effete matter of its inhabitants, which, if allowed to

accumulate, would soon render the fluid poisonous to every living thing

within it. This result is happily averted by the Infusoria, which feed

upon the decaying substances in solution, while they themselves become

in their turn the food of the larger animals. Indeed, they constitute

almost the sole nutriment of many strong, muscular shell-fish, as

pholas, mussel, cockle, &c.; and doubtless help to maintain the life

of others, such as actiniæ, and even crabs, which, as is well known,

live and grow without any other apparent means of sustenance. Thus

the presence of Infusoria in the tank may be considered a sign of its

healthy condition, although their increase to such an extent as to give

a milky appearance to the water, is apt to endanger the well-being

of the larger, though delicate creatures. The peculiar phenomenon

alluded to arises from decaying matter, such as a dead worm or limpet,

which should be sought after and removed with all possible speed. The

whereabouts of such objectionable remains will be generally indicated

by a dense cloud of Infusoria hovering over the spot. The milkiness,

however, although it may look for the time unsightly, is ofttimes

the saving of the aquarium 'stock.' When these tiny but industrious

scavengers have completed their task of purification, they will cease

to multiply, and mostly disappear, leaving the water clear as crystal.

I believe it is the absence or deficient supply of Infusoria that

sometimes so tantalizingly defeats the attempts of many persons to

establish an aquarium. Pure deep-sea water, although never without

them, often contains but very few, hence great caution is necessary not

to overstock the tank filled with it, otherwise the animals will die

rapidly, although the water itself appears beautifully transparent.

 

Of Infusoria there are many species. They are nearly all, at one stage

or other of their existence, extremely vivacious in their movements;

so much so, indeed, that it becomes a matter of difficulty to observe

them closely. Some have the power of darting about with astonishing

velocity, others unceasingly gyrate, or waltz around with the grace of

a Cellarius; while not a few content themselves by, slug-like, dragging

their slow length along. The last are frequently startled from their

propriety and aplomb by the rapid evolutions of their terpshicorean

neighbours. Some, again, grasping hold of an object by one of their

long filaments, revolve rapidly round it, whilst others spring, leap,

and perform sundry feats of acrobatism that are unmatched in dexterity

by any of the larger animals.

 

I may here observe that the motions and general structure of many of

the microscopic forms of vegetation, so much resemble those of some

of the infusoria, that it has long puzzled naturalists to distinguish

between them with any degree of certainty. The chief distinction

appears to lie in the nature of their food. Those forms which are truly

vegetable can live upon purely inorganic matter, while the animals

require that which is organized. The plants also live entirely by the

absorption of fluid through the exterior, while the animalculæ are

capable of taking in solid particles into the interior of the body.

Their mode of multiplication, and the metamorphoses they undergo, are

much alike in both classes, being, during one stage of their existence,

still and sometimes immovably fixed to stones, sea-weed, &c., and at

another freely swimming about. Notwithstanding the similarities here

stated, the appearance of certain of the species is as various as it

is curious. One of the commonest species of the Infusoria (_Paramecium

caudatum_) is shaped somewhat like a grain of rice, with a piece

chipped out on one side, near the extremity of its body. It swims about

with its unchipped extremity foremost, rotating as it goes. During the

milky condition of the water (before alluded to), these creatures swarm

to such a degree, that a single drop of the fluid, when placed under

the microscope, appears filled with a dense cloud of dancing midges.

Another (_Kerona silurus_) may be said to resemble a coffee-bean, with

a host of _cilia_, or short bristles, on the flat side. These are used

when swimming or running. But perhaps the most singular and beautiful

of all the infusorial animalcules are the _Vorticellæ_, which resemble

minute cups or flower-bells, mounted upon slender retractile threadlike

stalks, by which they are moored to the surface of the weeds and

stones. They are called Vorticellæ on account of the little vortices

or whirlpools which they continually create in the water, by means of

a fringe of very minute cilia placed round the brim of their cups.

These cilia are so minute as to require a very high microscopic power

to make them visible, and even then they are not easily detected, on

account of their extremely rapid vibration, which never relaxes while

the animal is in full vigour. On the other hand, when near death,

their velocity diminishes, and ample opportunity is afforded for

observing that the movements consist of a rapid bending inwards and

outwards, over the edge of the cup. This is best seen in a side view.

The action is repeated by each cilium in succession, with such rapidity

and regularity that, when viewed from above, the fringe looks like the

rim of a wheel in rapid revolution. A similar appearance, produced

by the same cause, in another class of animalcula, of much more

complex structure than the Vorticellæ, has procured for it the name of

_Rotifera_, or wheel-bearers. The result of this combined movement of

the cilia is, that a constant stream of water is drawn in towards the

centre of the cup, and thrown off over the sides, when, having reached

a short distance beyond the edge, it circles rapidly in a small vortex,

curling downwards over the lips. These currents are rendered evident

by floating particles in the water. The possession of these vibratile

cilia is not peculiar to this class of animals; indeed, there is good

reason to believe that there is scarcely a living creature, from the

lowest animalcule, or plant germ, up to man himself, that is not

provided with them in some part or other. In many of these Infusoria

the cilia constitute the organs of locomotion; while in the higher

forms they serve various other purposes, but chiefly that of directing

the flow of the various internal fluids through their proper channels.

But the peculiar and perhaps most wonderful organ of the Vorticella,

is its stalk or mooring thread. This though generally of such extreme

tenuity as to be almost invisible with ordinary microscopes, yet

exhibits a remarkable degree of strength and muscular activity in

its movements, which apparently are more voluntary than those of the

cilia. Its action consists of a sudden contraction from a straight to

a spiral form with the coils closely packed together, by which the

head or bell is jerked down almost into contact with the foot of the

stalk; after a few seconds the tension seems gradually relaxed, the

coils are slowly unwound, and the stalk straightens itself out. This

action takes place at irregular intervals, but it is seldom that more

than a minute elapses between each contraction. It (the contraction)

invariably happens when the animal is touched or alarmed, and is,

consequently, very frequent when the water swarms with many other

swimming animalcula. When it takes place the flower-bell generally

closes up into a little round ball, which opens out again only when

the stalk becomes fully extended. From this we might almost infer that

some animalcule, or other morsel of food, had been seized and retained

within the cup; moreover, that the contraction of the stalk assisted in

securing or disposing of the prey. This, however, is uncertain.