DECEMBER.
Microscopic Hunting in Winter--Water-bears, or Tardigrada--Their
comical behaviour--Mode of viewing them--Singular gizzard--A
compressorium--Achromatic condenser--Mouth of the
Water-bear--Water-bears' exposure to heat--Soluble
albumen--Physiological and chemical reasons why they are not
killed by heating and drying--The Trachelius ovum--Mode of
swimming--Method of viewing--By dark-ground
illumination--Curious digestive tube with
branches--Multiplication by division--Change of form immediately
following this process--Subsequent appearances.
There is always satisfaction in finding a work accomplished; but the
attempt to delineate some of the marvels of minute creation has been a
pleasant one, and we approach the completion of our task of recording a
_Microscopic Year_ with something like regret. The dark, dirty December
of the great metropolis may not seem a promising time for field
excursions, but some ponds lie near enough to practicable roads and
paths to render an occasional dip in them, not of ourselves, but of our
bottles--an easy and not unpleasant performance; and if the weather is
unusually bad, we can fall back upon our preserves in bottles and tanks,
which seldom fail to afford something new, as we have been pretty sure
to bring home some undeveloped germs with our stock of pond-water and
plants, and even creatures of considerable size are very likely to have
escaped detection in our first efforts at examination.
When objects are not over abundant, as is apt to be the case in the cold
months, it is well to fill a large vial with some water out of the
aquarium or other large vessel, and watch what living specks may be
moving about therein. These are readily examined with a pocket-lens, and
with a little dexterity any promising creature can be fished out with
the dipping-tube. It is also advisable to shake a mass of vegetation in
a white basin, as the larger infusoria, &c., may be thrown down; and
indeed this method (as recommended by Pritchard) is always convenient.
Even so small a quantity of water as is contained in a glass cell,
appropriated to the continual examination of polyps or polyzoa, should
be frequently hunted over with a low power, as in the course of days and
weeks one race of small animals will disappear, and another take their
place.
Following these various methods in December, we obtained many specimens;
but the most interesting was found by taking up small branches of the
Anacharis with a pair of forceps, and putting them into a glass trough
to see what inhabitants they might possess. One of these trials was
rewarded by the appearance of a little puppy-shaped animal very busy
pawing about with eight imperfect legs, but not making much progress
with all his efforts. It was evident that we had obtained one of the
_Tardigrada_ (slow-steppers), or Water-Bears, and a very comical amusing
little fellow he was. The figure was like that of a new-born puppy, or
"unlicked" bear cub; each of the eight legs were provided with four
serviceable claws, there was no tail, and the blunt head was susceptible
of considerable alteration of shape. He was grubbing about among some
bits of decayed vegetation, and from the mass of green matter in his
stomach, it was evident that he was not one of that painfully numerous
class in England--the starving poor.
[Illustration: Water-Bear.]
A power of one hundred and five linear, obtained with a two-thirds
object-glass, and the second eye-piece, enabled all his motions and
general structure to be exhibited, and showed that he possessed a sort
of gizzard, whose details would require more magnification to bring out.
Accordingly the dipping-tube was carefully held just over him, the
finger removed, and luckily in went the little gentleman with the
ascending current. He was cautiously transferred to a Compressorium,[24]
an apparatus by which the approach of two thin plates of glass can be
regulated by the action of a spring and a screw; and just enough
pressure was employed to keep him from changing his place, although he
was able to move his tiny limbs. Thus arranged, he was placed under a
power of two hundred and forty linear, and illuminated by an achromatic
condenser,[25] to make the fine structure of his gizzard as plain as
possible. It was then seen that this curious organ contains several
prominences or teeth, and is composed of muscular fibres, radiating in
every direction. From the front of the gizzard proceed two rods, which
meet in a point, and are supposed to represent the maxillae or jaws of
insects, while between them is a tube or channel, through which the food
is passed. The mouth is _suctorial_, and the two horny rods, with their
central piece or pieces, are protrusile. They were frequently brought as
far as the outer lips (if we may so call the margins of the mouth), but
we did not witness an actual protrusion, except when the lips
accompanied them, and formed a small round pouting orifice. The skin of
the animal was tough and somewhat loose, and wrinkled during the
contractions its proprietor made. The interior of the body exhibited an
immense multitude of globular particles of various sizes in constant
motion, but not moving in any vessels, or performing a distinct
circulation.
[24] The best forms of this instrument are made by Messrs. R. & J. Beck,
the glass plates being held in their places by flat-headed screws, and
not by cement. This plan was devised by the author, and makes it easy to
renew the glasses when broken.
[25] The achromatic condenser is a frame capable of supporting an
object-glass, lower than that employed for vision, through which the
light passes to the object in quantities and directions determined by
stops of various shapes. The appearances mentioned can be seen without
it, though not so well.
My specimens had no visible eyes, and these organs are, according to
Pritchard's book, "variable and fugacious." The same authority remarks,
"In most vital phenomena they very closely accord with the rotatoria;
thus like these they can be revived after being put into hot water at
113 deg. to 118 deg., but are destroyed by immersion in boiling water. They may
be gradually heated to 216 deg., 252 deg., and even 261 deg. It is also by their
capability of resuscitation after being dried that they are able to
sustain their vitality in such localities as the roofs of houses, where
at one time they are subjected to great heat and excessive drought, and
at another are immersed in water."
When vital processes are not stopped by excess of temperature, as is the
case with the higher animals, the power of resisting heat without
destruction depends upon the condition of the albumen. Soluble albumen,
or, as it should be called, _Albuminate of Soda_ (for a small quantity
of that alkali is present and chemically united with it), after having
been _thoroughly dried_, may be heated without loss of its solubility;
although if the same temperature was applied before it was dry, that
solubility would be destroyed, and it would no longer be a fit
constituent of a living creature. As Dr. Carpenter observes, this fact
is of much interest in explaining the tenacity of life in the
Tardigrada.
The movements of the water-bears, although slow, evince a decided
purpose and ability to make all parts work together for one common
object; and as might be expected from this fact, and also from the
repetition of distinct, although not articulated limbs, they are
provided with a nervous apparatus of considerable development, in the
shape of a chain of a ganglia and a brain, with connecting filaments.
From these and other circumstances naturalists consider the Tardigrada
to belong to the great family of _Spiders_, of which they are,
physiologically speaking, _poor relations_. Siebold says "they form the
transition from the Arachnoidae to the Annelides."[26] Like the spiders
they cast their skin; and, although I was not fortunate enough to
witness this operation--called in the language of the learned _ecdysis_,
which means putting its clothes off--I found an empty hide, which,
making allowance for the comparative size of the creatures, looked tough
and strong as that of a rhinoceros, and showed that the stripping
process extended to the tips of the claws. The 'Micrographic Dictionary'
states that the Tardigrada lay but few eggs at a time, and these are
"usually deposited during the ecdysis, the exuviae serving as a
protection to them during the process of hatching." Thus Mrs. Water-Bear
makes a nursery out of her old skin, a device as ingenious as
unexpected. The water-bears are said to be hermaphrodites, but this is
improbable.
[26] 'Anatomy of the Invertebrata,' Burnett's trans., p. 364.
The _Plumatella repens_, described in a former chapter, was kept in a
glass trough, to which some fresh water was added every few days, taken
from a glass jar that had been standing many weeks with growing
anacharis in it. One day a singular creature made its appearance in the
trough; when magnified sixty diameters it resembled an oval bladder,
with a sort of proboscis attached to it. At one part it was
longitudinally constricted, and evidently possessed some branched and
complicated internal vessel. The surface was ciliated, and the neck or
proboscis acted as a rudder, and enabled the creature to execute rapid
turns. It swam up and down, and round about, sometimes rotating on its
axis, at others keeping the same side uppermost, but did not exhibit the
faintest sign of intelligence in its movements, except an occasional
finger-like bend of the proboscis, upon which the cilia seemed thicker
than upon the body. It was big enough to be observed as a moving white
speck by the naked eye, when the vessel containing it was held to catch
the light slantingly; but a power of one hundred and five was
conveniently employed to enable its structure to be discerned. Under
this power, when the animal was resting or moving slowly, a mouth was
perceived on the left side of the proboscis, which was usually, though
not always, curved to the right. The mouth was a round or oval orifice,
and when illuminated by the parabola, its lips or margin looked
thickened, and of a pale blue, and ciliated, while the rest of the body
assumed a pinkish pearly tint.
Below the mouth came a funnel-shaped tube or oesophagus, having some
folds or plaits on its sides, and terminating in a broad digestive tube,
distinct from the nucleus, and ramifying like a tree. The constriction
before mentioned, which was always seen in certain positions, although
it varied _very considerably_ in depth and width, drew up the integument
towards the main trunk of the digestive tube, and thus the animal had a
distinct ventral and dorsal side. The branches of the tube stopped
somewhat abruptly just before reaching the surface, and were often
observed to end in small round vacuoles or vesicles.
[Illustration: Trachelius ovum (slightly flattened).]
At the bottom of the bladder, opposite the mouth, in some specimens were
large round cavities or cells, filled with smaller cells, or partially
transparent granules. These varied in number from one to two or three,
and were replaced in other specimens by masses that did not present the
same regular form or rounded outline. In one instance an amorphous
structure of this kind gradually divided itself, and seemed in the
course of forming two cells, but the end of the process was
unfortunately not seen. The annexed drawing will readily enable the
animal to be recognised. It shows the mouth very plainly, and a current
of small particles moving towards it. The oesophagus terminates in a
digestive tube, like the trunk of a tree, from which numerous branches
spring. This arrangement is probably analogous to that of the
phlebenterous mollusks described by Quatrefages, in which the
ramifications of the stomach answer the purpose of arteries, and convey
the nutrient fluid to various parts of the body. It is also likely that
they minister to the function of respiration.
The cilia on the surface, which are arranged in parallel lines, are best
observed when the animal is slightly flattened in a live-box; but this
process produces a considerable derangement in the relative position of
the internal parts, and they can only be well seen when it is immersed
in plenty of water, and is polite enough to stand still, and submit his
digestive economy to a steady gaze. The only way to succeed in this
undertaking is to have a large stock of patience as well as a convenient
cell or trough. The table must be kept steady, and the prisoner watched
from time to time, and at last he will be found ready for display.
Pritchard says this animal, whose name is _Trachelius ovum_, is an
inhabitant of stagnant bog water, and has been found encysted. My
specimens could not be called plentiful, but for several weeks I could
generally find two or three, by filling a four-ounce vial from the glass
jar, and examining its contents with a pocket-lens. If none were
present, another dip was made, and usually with success.
One evening I caught a good specimen by means of the dipping-tube, and
cautiously let it out, accompanied by a drop of water, on the glass
floor of the live-box. A glance with the pocket-lens showed all was
right, and the cover was very gently put on, but it had scarcely touched
the creature when it became crumpled up and in confusion. On one or two
former occasions I had been unfortunate enough to give my captives a
squeeze too much, with the usual result of a rupture of their
integuments and an escape of globules and fluids from the regions
within. Now, however, there was no such rupture and no such escape, but
instead of a smooth, comely surface, my Trachelius had lost all title to
his specific designation, _ovum_, for instead of bearing any resemblance
to an egg, it was more like an Irishman's hat after having a bit of a
"shindy" at Donnybrook Fair.
I was greatly puzzled with this aspect of things, and still more so when
my deranged specimen twirled and bumped about with considerable
velocity, and in all directions. Presently a decided constriction
appeared about half-way below the mouth and proboscis, and in transverse
direction. The ciliary motion became very violent in the lower half just
below the constriction, while the proboscis worked hard to make its half
go another way. For some minutes there was a tug of war, and at length
away went proboscis with his portion, still much crumpled by the fight,
and left the other bit to roam at will, gradually smooth his puckers,
and assume the appearance of a respectable well-to-do animalcule.
[Illustration: _Trachelius ovum_, three hours after division.]
Three hours after the "fission" the proboscis half was not unlike the
former self of the late "entire," but with diminished body and larger
neck; while the remaining portion had assumed a flask form, and would
not have been known by his dearest acquaintance. The portraits of the
_dis-United States_ were quickly taken, and, as bed-time had arrived,
they were left to darkness and themselves. The next morning a change had
come over the "spirit of their dream." Both were quiet, or sedately
moving, and they were nearly alike. The proboscis fellow had increased
and rounded his body, and diminished his nose; while Mr. Flask had grown
round also, and evinced an intention of cultivating a proboscis himself.
Twenty-seven hours after the separation, both had made considerable
progress in arranging and developing their insides, which had been
thrown into great confusion by the way in which the original animal had
been wrenched in half, and in both a granular mass was forming opposite
the mouth end. The proboscis portion, which may perhaps be termed the
_mother_, was more advanced than her progeny, but both had a great deal
to do if they meant to exhibit the original figure, and develop a set of
bowels as elegantly branched. Whether they would have succeeded or not
under happier circumstances I cannot tell, but unfortunately the Fate
who carries the scissors cut short their days.
In all other animalcules in which I had observed the process of
multiplication by self-division, it seemed to go on smoothly, and with
no discomfort to either the dividend or the quotient, and it may be that
in the fission of the _Trachelius ovum_ I witnessed what the doctors
would call a bad case. Indeed it may have been prematurely brought on,
and aggravated by the squeeze in the live-box. It is, however, probable,
from the stronger texture and greater organic development of this
animalcule, that it does not divide so easily as the softer and simpler
kinds.
Frequent examination of this animalcule has created a strong doubt in my
mind whether it is rightly placed in our "systems." My own impression is
that it belongs to a higher class.