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.
The motions of the Vorticella do not seem much affected by the stalk
losing hold of its attachment; but the result of such an accident
taking place is that the cilia cause the animal to swim through the
water, trailing its thread behind it, and the contraction of the latter
merely causes it to be drawn up to the head.
There are various species of Vorticellæ. That just described is the
simplest, consisting merely of a hemispherical ciliated cup, attached
to a single thread. It is barely visible to the naked eye. But there
is a compound species which I have this year found to be extremely
abundant in my aquarium,--whose occupants, both large and small, it
excels in singularity and beauty. In structure it is to the simple
Vorticella what a many-branched zoophyte is to an _Actinia_. My
attention was first drawn to the presence of this creature by observing
some pebbles and fronds of green ulva thickly coated with a fine
flocculent down. On closer inspection this growth appeared to consist
of a multitude of feathery plumes, about one-sixteenth of an inch in
height, and individually of so fine and transparent a texture as to be
scarcely discernible to the unassisted sight. On touching one with the
point of a fine needle it would instantly shrink up into a small but
dense mass, like a ball of white cotton--scarcely so large as a fine
grain of sand. In a few seconds it would again unfold and spread itself
out to its original size. By carefully detaching a specimen with the
point of a needle or pen-knife, and transferring it, along with a drop
of water upon a slip of glass, to the stage of the microscope, a sight
was presented of great wonder and loveliness:--
'The more I fixed mine eye,
Mine eye the more new wonders did espye!'
Let the reader imagine a tree with slender, gracefully curved, and
tapering branches thickly studded over with delicate flower-bells
in place of leaves. Let him suppose the bells to be shaped somewhat
between those of the fox-glove and convolvolus, and the stem, branches,
bells, and all, made of the purest crystal. Let him further conceive
every component part of this singular structure to be tremulous with
life-like motion, and he will have as correct an idea as words can give
of the complex form of this minute inhabitant of the deep. Moreover,
while gazing at it through the microscope, the observer is startled
by the sudden collapse of the entire structure. The lovely tree has
shrunk together into a dense ball, in which the branching stem lies
completely hidden among the flower-bells--themselves closed up into
little spherules, so closely packed together that the entire mass
resembles a piece of herring-roe. This contraction is so instantaneous
that the mode in which it is accomplished cannot be observed until the
tree is again extended. As the re-extension takes place very slowly, we
are enabled to observe that each branchlet has been coiled in a spiral
form, like the thread of the simple Vorticella previously described;
and also that the main stem, above the lowest branch, was coiled up in
the same way, but not so closely, and that the part below the lowest
branch had, curiously enough, remained straight. Sometimes, in large
and numerously branched specimens, one or two of the lowest members
do not contract at the same time with the rest, but do so immediately
afterwards, as if they had been startled by the shrinking movements of
their neighbours. Sometimes these lowest branches will contract alone,
while all the others remain fully extended,--a fact that would almost
seem to indicate that they possessed an independent life of their own.
In the accompanying engraving I have attempted faithfully to portray
one of these wonderful creatures. Fig. 1 represents it fully extended,
while Fig. 2 indicates its collapsed form. There is another curious
circumstance which I have fortunately observed in connection with this
Vorticella, a description of which will perhaps be interesting to the
reader. I allude to the casting off of what may be called the fruit
of the tree. When this event takes place, the buds (or fruit) dart
about with such rapidity, that it is almost impossible to keep them
in the field of view for the briefest space of time. A represents the
enchanted fruit hanging on the tree; B shows it as it swims about.
[Illustration: Fig. 1. and Fig. 2.]
Although not exactly fruit, it is, no doubt, the means by which
the Vorticellæ are propagated, for it is known that many fixed
zoophytes, and even some plants, produce free swimming germs or spores,
which afterwards become fixed, and grow up into forms like those
which produced them. In some of the branching zoophytes (_Coryne_,
_Sertularia_, &c.), the germs are exactly like little medusae, being
small, gelatinous cups fringed with tentacula, by means of which they
twitch themselves along with surprising agility. In this Vorticella,
however, it is more like one of the ciliated Infusoria. The first one
that I saw attached I conceived to be a remarkably large bell, with
its mouth directed towards me, but the cilia with which it appeared
to be fringed were unusually large and distinct. The movements of
these appendages being comparatively slow, it was most interesting
to watch them as they successively bent inwards and rose again, like
the steady swell of a tidal wave, or an eccentric movement in some
piece of machinery, making a revolution about twice in a second, and
in the opposite direction to the hands of a clock. Suddenly the tree
contracted, when, to my surprise, I observed the bell, which not an
instant before appeared attached, now floating freely in the water,
its ciliary movements not being in the least interrupted. Presently,
however, they became brisker, the bell turned over on its side, and,
ere the tree had again expanded, darted out of view, not, however,
before I had remarked that it was not a bell, but a sphere flattened
on one side, and having its circular ring of cilia on the flat side,
with only a slight depression in the middle of it. There also appeared
to be a small granular nucleus immediately above this depression, the
rest of the body being perfectly transparent. I afterwards saw several
others attached to the tree, each seated about the centre of a branch;
but none of these were so fully developed. They were like little
transparent button mushrooms, and had all more or less of a nucleus
on the side by which they were attached. On only one of these did I
detect any cilia.
Mr. Gosse, in his 'Tenby,' gives a picture of an animal exceedingly
like what I have described; but from his account of it, there seems to
be some doubt of their identity. He calls it '_Zoothamnium spirale_,'
because the insertions of the branches were placed spirally around the
main stem, like those of a fir-tree. In my specimens the branches were
set alternately on opposite sides of the main trunk, and the whole was
curved like a drooping fern leaf or an ostrich feather, the bells being
mostly set on the convex side.
In conclusion, let me mention that it is an error to suppose, as many
persons do, that putrid water alone contains life. Infusoria occur,
as before hinted, in the clear waters of the ocean, in the water that
we drink daily, and also in the limpid burn that flows through our
valleys, or trickles like a silver thread down the mountain side.[1]
[1] Ehrenberg states that Infusoria are in a higher state of
organization when taken from pure streams than from putrid waters.
'Where the pool
Stands mantled o'er with green, invisible,
Amid the floating verdure millions stray.
Each liquid too, whether it pierces, soothes,
Inflames, refreshes, or exalts the taste,
With various forms abounds. Nor is the stream
Of purest crystal, nor the lucid air,
Though one transparent vacancy it seems,
Void of their unseen people. These, concealed
By the kind art of forming Heaven, escape
The grosser eye of man.'
Let it be remembered, too, that Infusoria, when found in either do not
themselves constitute the impurity of fresh or salt water; they merely
act as 'nature's invisible scavengers,' whose duty it is to remove
all nuisances that may spring up; and most unceasingly do these tiny
creatures labour in the performance of their all-important mission of
usefulness.