FEBRUARY.
Visit to Hampstead--Small
ponds--Water-fleas--Water-beetle--Snails--Polyps--Hydra
viridis--The dipping-tube--A glass cell--The Hydra and its
prey--Chydorus sphaericus and Canthocamptus, or friends and their
escapes--Cothurnia--Polyp buds--Catching Polyps--Mode of viewing
them--Structure of Polyps--Sarcode--Polyps stimulated by
light--Are they conscious?--Tentacles and poison
threads--Paramecium--Trachelius--Motions of Animalcules, whether
automatic or directed by a will--Their restless character.
It has been a bitterly cold night, and as the sun shines on a clear keen
morning, and glistens in the hoar-frost which covers the trees, it might
seem an unpropitious time for visiting the ponds, in search of
microscopic prey. We will, however, try our luck, and take a brisk trot
to the top of Hampstead Heath, where the air is still keener, and the
ice more thick. Arriving at the highest point, London appears on one
side enveloped in its usual great coat of smoke, through which St.
Paul's big dome, with a score or two of towers and steeples, can be
dimly made out; while looking towards Harrow-on-the-Hill, or Barnet, we
see the advantage of country air in the sharpness with which distant
objects cut the blue sky. We leave the large ponds for another time,
and hunt out the little hollows among the furze and fern. One looks
promising from the bright green vegetation to be discovered under the
sheet of ice, which is almost firm enough to bear human weight.
Breaking a convenient hole we hook up some of the water-plants, and
place them in a wide-mouthed vial, which we fill with water, and
cursorily examine with a pocket-lens. Some water-fleas briskly skipping
about, and a beautiful little beetle, with an elegant dotted pattern on
his brown back, and a glistening film of air covering his belly, show
that we have not been unsuccessful, although we must wait till we get
home to know the extent of our findings, among which, however, we can
only discern the graceful spiral shell of a small water-snail, the
_Planorbis_.
Arriving at home the bottle was left undisturbed for some hours in a
warm light place, and then on being examined several specimens of that
beautiful polyp, the _Hydra viridis_, were seen attached to the glass,
and spreading their delicate tentacles in search of prey. One of the
polyps is carefully removed by the _dipping-tube_, a small glass tube,
open at both ends. The forefinger is placed upon the top, and when the
other end is brought over the object the finger is raised for an
instant, and as the water rushes in the little hydra comes too, and is
placed in a glass cell, about half an inch wide, and one tenth of an
inch deep. These cells are obtained from the opticians, and cemented
with varnish or marine glue to an ordinary glass slide. After an object
has been placed in one of them, a little water is taken up in the
dipping-tube, and the cell filled until the fluid stands in a convex
heap above its brim. We then select around glass cover, and press it
gently on the walls of our cell. A few drops of superfluous water
escape, and we have the cell quite full, and the cover held tight by
force of the capillary attraction between the water and the glass.
[Illustration: _Hydra viridis_ with developed young one, and bud
beginning to sprout.]
The polyp deposited in one of these water cages is then transferred to
the stage of the microscope, and its proceedings watched. At first it
looks like a shapeless mass of apple-green jelly. Soon, however, the
tail end of the creature is fixed to the glass, the body elongates, and
the tentacles (in this case eight) expand something after the manner of
the leaves of a graceful palm.
By accident two small Water Fleas were imprisoned with the polyp, and
one (a shrimp-like looking creature, carrying behind her a great bag of
eggs) came into contact with the tentacles, and seemed paralysed for a
time. The hydra made no attempt to convey the captive to its mouth, but
held it tight until another Water Flea, a round merry little fellow
(_Chydorus sphaericus_), came to the rescue, and assisted _Canthocamptus_
to escape by tugging at her tail. This friendly action may not have been
prompted by the intelligence which seemed to suggest it, but those who
have kept tame soldier-crabs and prawns in an aquarium, will not be
indisposed to attribute to the crustaceans more brains than they have
usually credit for. It must, however, be confessed that the subsequent
conduct of Mrs. Canthocamptus did not indicate the possession of much
prudence, for she learnt no lesson from experience, but repeatedly swam
against her enemy's tentacles, suffered many captures, and only escaped
being devoured through the indifference, or want of appetite, which the
polyp evinced.
[Illustration: A, _Canthocamptus minutus_; B, _Chydorus sphaericus_; C
and D, Capsules and poison-thread of polyp; E, _Tricodina pediculus_,
side view and under view; F, _Kerona polyporum.--Microg. Dict._]
On the body of the _Canthocamptus_ were some small transparent vases or
bottles, containing living objects, which sprang up and down. These
were members of the _Vorticella_ family, called _Cothurnia_, and will
be hereafter described.
[Illustration: _Hydra viridis_, in various shapes.]
Watching the hydra it was curious to note the changes of form which
these creatures are able to assume. Now the tentacles were short and
thick, and the body squat; now the body was elongated, like the stem of
a palm tree, and the tentacles hung gracefully from the top. From some
of the polyps little round buds were growing, while other buds were
already developed into miniature copies of the parent, and only attached
by a slender stalk. In a few days many of these left the maternal side,
fixed their own little tails to the glass, and commenced housekeeping on
their own account.
Polyps may be obtained at all times of the year by bringing home
duckweed, conferva, and other water-plants from the ponds. Some hauls
may be unsuccessful, but if one pond is not propitious others should be
tried. The plants should be put in a capacious vessel of water, and
placed in the light, where, if polyps be present, they will show
themselves within twenty-four hours, either attached to the sides of the
glass, or hanging from the plants, or suspended head downwards from the
upper film of the water. They are elegant objects, and may be kept
without difficulty for some weeks. After being confined in a small
quantity of water for purposes of examination, they should be carefully
replaced in the larger vessel, and may thus be used again and again
without suffering any injury. A low power--a three or two-inch glass--or
a one-inch, reduced by employing the erector--is the most convenient for
examining the whole creature, but higher powers are necessary to make
out its minute structure. They should be viewed with direct and oblique
light, as transparent and also as opaque objects. In the latter case the
"Lieberkuhn," or polished silver speculum, is convenient, and if the
microscope is not furnished with Lister's dark wells, a small piece of
black paper may be stuck behind the object, by simply wetting it with
the tongue.[4]
[4] The side silver reflector is useful for illuminating such objects.
Although the polyps are remarkable for the simplicity of their
organization, they do not the less exhibit the wonderful nature of
animal life. Their bodies are composed of the substance, called
_sarcode_, in which is imbedded a colouring matter resembling that in
the leaves of plants; every part possesses irritability and
contractility, and they are very sensitive to the stimulus of light. The
outer layer of their bodies is harder than the inner layer. These
layers are severally called _ectoderm_ and _endoderm_. They may be cut
and grafted like trees, and if turned inside out, the new inside digests
and assimilates as well as the old. Whether any form of consciousness
can belong to creatures which have no distinct nervous system is open to
doubt, but it would seem probable from their movements that food and
light afford them something like a pleasurable sensation in a very
humble degree. If we were sufficiently acquainted with the secrets of
molecular combination we might discover that the various functions of
these simple organisms were discharged by different _particles_,
although it is only in higher creatures that muscular particles are
aggregated into muscles, or nerve particles into nerves.
Having examined the general appearance and proceedings of the hydra, let
us cut off a tentacle, or take a small specimen and gently crush it by
pressing down the cover of the live box, and place the object so
prepared under a power of about three hundred linear. If we then
illuminate it with a moderate quantity of oblique light, we shall
discover round the edge of the tentacle a number of small cells or
capsules, from some of which a very slender wire or thread will be
emitted.[5] These are the stinging organs of the polyp, and resemble
those which Mr. Gosse has so ably elucidated in the sea anemones. Some
writers have endeavoured to show that they are not stinging organs at
all, but so large an amount of evidence to the contrary is accumulated
in Mr. Gosse's 'Actinologia Britannica,' that no reasonable doubt
remains. The stinging capsules of the polyp are shown in the annexed
sketch, and also the way in which they are employed, for it fortunately
happened that on exposing one of the hydras to pressure in the live box,
a small worm (_Anguillula_) escaped, which had been pierced with the
minute weapons which are supposed to convey a poison into the wound. The
authors of the 'Micrographic Dictionary' think that the prongs of the
forks, which will be seen to point upwards in the sketch,[6] are
springs, and occupy a reversed position in the capsule cells, and that
their function is to throw out the threads. However this may be, the
polyps, and similarly endowed creatures, have the power of darting out
their poison threads with considerable force, and Mr. Gosse found that
the anemone was able to pierce a thick piece of human skin.
[5] See page 34, C and D.
[6] See page 38.
[Illustration: Anguillula pierced by stinging organs of the _Hydra
viridis_.]
The same excellent observer attributes the emission of the anemone
poison threads, which he considers hollow, to the injection of a fluid.
In their quiescent state, he thinks they are drawn in, like the finger
of a glove, and are forced out as the liquid enters their slender tubes.
Possibly the polyp stinging organs may have the same structure.
Notwithstanding their dangerous weapons, polyps are often infested with
a parasite, the _Trichodina pediculus_, as shown in Fig. E, page 49, and
it must happen that either this visitation is not disagreeable, or that
the Trichodina is not influenced by the poison.
As the plants in the bottles decayed, some of the animalcules died off
and others appeared. In one bottle, containing decaying chara,
_Paramecia_ abounded. The _Paramecia_, of which there are various
species, have always been favourite objects with microscopists. The
Germans call them "slipper animalcules," and they vary in size from
1--96"[7] to 1--1150". They are flat rounded-oblong creatures, with a
distinct integument or skin, "through which numerous vibratile cilia
pass in regular rows."[8] They are furnished with a distinct mouth, and
adult specimens exhibit star-shaped contractile vesicles in great
perfection.
[7] The usual mode of giving dimensions is by fractions thus expressed:
1--96" means one ninety-sixth of an inch.
[8] 'Micrographic Dictionary.'
The swarm of specimens before us belong to one species, _Paramecium
aurelia_, the _Chrysalis animalcule_, and they crowd every portion of
the little water-drop we have taken up, and examined with a power of
about one hundred linear. When they are sufficiently quiet a power of
about four hundred may be used with advantage, and Pritchard recommends
adding a little indigo and carmine to the water, in order to see the
cilia more clearly, or rather to render their action more plain. The
cilia are disposed lengthwise, and Ehrenberg counted in some rows sixty
or seventy of them, making an aggregate of three thousand six hundred
and forty organs of motion in one small animated speck. This number
seems large, but although we have never performed the feat of counting
them, we should have expected it to prove much greater. Unlike most
animalcules they are susceptible of being preserved by drying upon
glass, and we subjoin a figure from Pritchard, of one thus treated, in
which the star-shaped vesicles are clearly seen. These curious organs
communicate with other vessels, and, as we have previously stated, are
probably connected with respiration and excretion.
[Illustration: Paramecium aurelia. A dried specimen showing the
vesicles.--_Pritchard._]
The genus _Paramecium_ is now confined to those creatures which exhibit
rows of longitudinal cilia of uniform length, which are destitute of
hooks, styles, or other organs of motion than the cilia, which have a
lateral mouth, and no eye-spots. One mode of increase is by division,
which may be easily observed; another is through the formation of true
eggs as traced by Balbiani.
Another of the treasures from the pond was a species of _Trachelius_, or
long-necked ciliated animalcule, which kept darting in and out of a
slimy den, attached to the leaf of a water-plant. The body was stout and
fish-shaped, the tail blunt, and the neck furnished with long
conspicuous cilia, which enabled the advancing and retreating movements
to be made with great rapidity. The motions of this creature exhibit
more appearance of purpose and design than is common with animalcules,
but in proportion as these observations are prolonged, the student will
be impressed with the difficulty of assuming that anything like a
reasoning faculty and volition, is proved by movements that bear some
resemblance to those of higher animals, whose cerebral capacities are
beyond a doubt. It is, however, almost impossible to witness motions
which are neither constant nor periodic, without fancying them to be
dictated by some sort of intelligence. We must, nevertheless, be
cautious, lest we allow ourselves to be deceived by reasoning so
seductive, as the vital operations of the lowest organisms may be merely
illustrations of blind obedience to stimuli, in which category we must
reckon food, and until we arrive at forms of being which clearly possess
a ganglionic system, we have no certainty that a real will exists, even
of the simplest kind; and perhaps we must go still higher before we
ought to believe in its presence.
Ehrenberg was much struck with the restless character of many
infusoria--whether he looked at them by day or by night, they were never
still. In fact their motions are like the involuntary actions which take
place in the human frame; and if attached to their bodies we observe
cilia that never sleep, the living membrane of some of our own organs,
the nose, for example, is similarly ciliated, and keeps up a perpetual
though unconscious work.