JANUARY.
Visit to the ponds--Confervae--Spirogyra
quinina--Vorticella--Common Rotifer--Three divisions of
Infusoria--Phytozoa--Protozoa--Rotifera--Tardigrada--Meaning of
these terms--Euglenae--Distinction between animals and
vegetables--Description of Vorticellae--Dark ground
illumination--Modes of producing it--The Nucleus of the
Vorticellae--Methods of reproduction--Ciliated Protozoa--Wheel
bearers or Rotifers--Their structure--The common Rotifer--The
young Rotifer seen inside the old one--An internal
nursery--"Differentiation" and "Specialisation"--Bisexuality of
Rotifers--Their zoological position--Diversities in their
appearance--Structure of their Gizzard--Description of Rotifers.
The winter months are on the whole less favorable to the collection of
microscopic objects from ponds and streams than the warmer portions of
the year; but the difference is rather in abundance than in variety, and
with a very moderate amount of trouble, representatives of the principal
classes can always be obtained.
On a clear January morning, when the air was keen, but no ice had yet
skinned over the surface of the water, a visit to some small ponds in an
open field not far from Kentish Town provided entertainment for several
days. The ponds were selected from their open airy situation, the
general clearness of their water, and the abundance of vegetation with
which they were adorned. Near the margin confervae abounded, their
tangled masses of hair-like filaments often matted together, almost with
the closeness of a felted texture. At intervals, minute bubbles of air,
with occasionally a few of greater size, indicated that the complex
processes of vegetable life were actively going on, that the tiny plants
were decomposing carbonic acid, dexterously combining the carbon--which
we are most familiar with in the black opaque form of charcoal--to form
the substance of their delicate translucent tissues, and sending forth
the oxygen as their contribution to the purification of the adjacent
water, and the renovation of our atmospheric air. This was a good sign,
for healthy vegetation is favorable to many of the most interesting
forms of infusorial life. Accordingly the end of a walking-stick was
inserted among the green threads, and a skein of them drawn up, dank,
dripping, and clinging together in a pasty-looking mass. To hold up a
morsel of this mass, and tell some one not in the secrets of pond-lore
that its dripping threads were objects of beauty, surpassing human
productions, in brilliant colour and elegant form, would provoke
laughter, and suggest the notion that you were poking fun at them, when
you poked out your stick with the slimy treasure at its end. But let us
put the green stuff into a bottle, with some water from its native
haunt, cork it up tight, and carry it away for quiet examination under
the microscope at home.
Here we are with the apparatus ready. We have transferred a few threads
of the conferva from the bottle to the _live box_, spreading out the
fine fibres with a needle, and adding a drop of water. The cover is then
gently pressed down, and the whole placed on the stage of the
microscope, to be examined with a power of about sixty. A light is
thrown somewhat obliquely by the mirror through the object, the focus
adjusted, and a beautiful sight rewards the pains. Our mass of conferva
turns out to contain one of the most elegant species. Fine hair-like
tubes of an organic material, as transparent as glass, are divided by
partitions of the same substance into cylindrical cells, through which a
slender ribbon of emerald green, spangled at intervals with small round
expansions, is spirally wound. We shall call it the Spiral Conferva, its
scientific name being _Spirogyra quinina_. Some other species, though
less elegantly adorned, make a pleasing variety in the microscopic
scene; and appended to some of the threads is a group of small crystal
bells, which jerk up and down upon spirally twisted stalks. These are
the "Bell Flower Animalcules" of old observers, the _Vorticellae_, or
Little Vortex-makers of the present day. Other small creatures flit
about with lively motions, and among them we observe a number of green
spindles that continually change their shape, while an odd-looking thing
crawls about, after the manner of certain caterpillars, by bringing his
head and tail together, shoving himself on a step, and then repeating
the process, and making another move. He has a kind of snout, behind
which are two little red eyes, and something like a pig-tail sticks out
behind. This is the Common Wheel-bearer, _Rotifer vulgaris_, a
favourite object with microscopists, old and young, and capable, as we
shall see, of doing something more interesting than taking the crawl we
have described.
A higher power, say one or two hundred, may be conveniently applied to
bring out the details of the inhabitants of our live box more
completely; but if the glasses are good, a linear magnification of sixty
will show a great deal, with the advantage of a large field, and less
trouble in following the moving objects of our search.
Having commenced our microscopic proceedings by obtaining some Euglenae,
Vorticellae, and a Rotifer, we are in a position to consider the chief
characteristics of three great divisions of infusoria, which will often
engage our attention.
It is well known that animalcules and other small forms of being may be
found in _infusions_ of hay or other vegetable matter, and hence all
such and similar objects were called _Infusoria_ by early observers.
Many groups have been separated from the general mass comprehended under
this term, and it is now used in various senses. The authors of the
'Micrographic Dictionary' employ it to designate "a class of microscopic
_animals_ not furnished with either vessels or nerves, but exhibiting
internal spherical cavities, motion effected by means of cilia, or
variable processes formed of the substance of the body, true legs being
absent." The objection to this definition is, that it to some extent
represents theories which may not be true. That nerves are absent _all
through the class_ is an assumption founded merely upon the negative
evidence of their not having been discovered, and the complete absence
of "vessels" cannot be affirmed.
In the last edition of 'Pritchard's Infusoria,' to which some of our
ablest naturalists have contributed, after separating two groups, the
Desmids, and the Diatoms, as belonging to the vegetable world, the
remainder of the original family of infusoria are classified as
_Phytozoa_, _Protozoa_, _Rotifera_, and _Tardigrada_. We shall explain
these hard names immediately, first remarking that the Desmids and the
Diatoms, concerning whom we do not intend to speak in these pages, are
the names of two groups, one distinctly vegetable, while the other,
although now generally considered so, were formerly held by many
authorities to be in reality animal. The Desmids occur very commonly in
fresh water. We have some among our Confervae. They are most brilliant
green, and often take forms of a more angular and crystalline character
than are exhibited by higher plants. The Diatoms are still more common,
and we see before us in our water-drop some of their simplest
representatives in the form of minute boats made of silica (flint) and
moved by means still in dispute.
Leaving out the Desmids and Diatoms, we have said that in Pritchard's
arrangement the views of those writers are adopted who divide the rest
of the infusoria into four groups, distinguished with foreign
long-tailed names, which we will translate and expound. First come the
_Phytozoa_, under which we recognise our old acquaintance _zoophyte_
turned upside down. _Zoophytes_ mean animal-plants, _Phytozoa_ mean
plant-animals. We shall have by-and-bye to speak of some of the members
of this artificial and unsatisfactory group, and postpone to that time a
learned disquisition on the difference between animals and plants, a
difference observable enough if we compare a hippopotamus with a
cabbage, but which "grows small by degrees, and beautifully less," as we
contemplate lower forms.
After the _Phytozoa_ come the _Protozoa_, or first forms in which
animality is distinctly recognised. Under this term are assembled
creatures of very various organization, from the extreme simplicity of
the _Proteus_ or _Amoeba_, a little lump of jelly, that moves by
thrusting out portions of its body, so as to make a sort of extempore
legs, and in which no organs can be discerned,[2] up to others that are
highly developed, like our _Vorticellae_. This group is evidently
provisional, and jumbles together objects that may be widely separated
when their true structure and real affinities are discerned.
[2] In some kinds and in some stages of growth this is not strictly
true.
Following the _Protozoa_, come the _Rotifera_, or Wheel-bearers, of
which we have obtained an example from our pond, and whose
characteristics we shall endeavour to delineate when our specimen is
under view; and last in the list we have the _Tardigrada_,
"Slow-steppers," or Water Bears, queer little creatures, something like
new-born puppies, with a double allowance of imperfect feet. These,
though somewhat connected with the rotifers, are considered to belong to
a low division of the arachnida (spiders, &c.).
[Illustration: --_a_, motile; and _b_, resting condition of Euglenae.]
Feeling that we must be merciful with the long-tailed words and
explanations of classification, we reserve further matter of this kind
for the opportunities that must arise, and direct our attention to
living forms by watching the _Euglenae_ which our water-drop contains. We
have before us a number of elegant spindle-shaped bodies, somewhat
thicker in front than behind, and in what may be called the head there
glitters a brilliant red speck, commonly called an _eye-spot_, although,
like the eyes of potatoes, it cannot see. Round this eye-spot the
tissues are clear, like glass; but the body of the creature is of a rich
vegetable green, which shines and glistens as it catches the light. Some
swim rapidly with a rollicking motion, while others twist themselves
into all manner of shapes. Now the once delicate spindle is oddly
contorted, now it swells out in the middle, like a top, and now it rolls
itself into a ball. The drawings will afford some idea of these protean
changes, but they must be seen before their harlequin character can be
thoroughly appreciated. Some of the specimens exhibit delicate lines
running lengthwise, and taking a spiral twist as the creature moves
about; but in none can any mouth be discerned, and their antics,
although energetic and comical, afford no certain indications of either
purpose or will. What are they? animals or vegetables? or something
betwixt and between?
The first impression of any casual observer would be to declare in
favour of their animality; but before this can be settled, comes the
question, what is an animal, and how does it differ from a vegetable?
and upon this the learned do by no means agree. One writer considers the
presence of _starch_ in any object a proof that it belongs to the
dominions of Flora, while another would decide the issue by ascertaining
whether it evolves oxygen and absorbs carbon, as most plants do, or
whether it evolves carbon and absorbs oxygen, as _decided_ animals do.
Dr. Carpenter asserts that the distinction between _Protophyta_ and
_Protozoa_ (first or simplest plants and animals), "lies in the nature
of their food, and the method of its introduction, for whilst the
_Protophyte_ obtains the materials of its nutrition from the air and
moisture that surround it, and possesses the power of detaching oxygen,
hydrogen, carbon, and nitrogen from their previous binary combinations,
and of uniting them into ternary and quaternary organic compounds
(chlorophyll, starch, albumen, &c.), the simplest _Protozoa_, in common
with the highest members of the animal kingdom, seems utterly destitute
of any such power, makes, so to speak, a stomach for itself in the
substance of its body, into which it injects the solid particles that
constitute its food, and within which it subjects them to a regular
process of digestion."
Unfortunately it is very difficult to apply this simple theory to the
dubious objects which lie on the border-land of the animal world, and no
other theory that has been propounded appears to meet all cases. Some
naturalists do not expect to find a broad line of demarkation between
the two great divisions of living things, but others characterise such
an idea as "unphilosophical," in spite of which, however, we incline
towards it.
Mr. Gosse, whose opinion is entitled to great respect, calls the
_Euglenae_ "animals" in his 'Evenings with the Microscope;' but from the
aggregate of recorded observations it seems that they evolve oxygen, are
coloured with the colouring matter of plants, reproduce their species in
a manner analogous to plants, and have in some cases been clearly traced
to the vegetable world. It is, however, possible that some _Euglenae_
forms may be animal and others vegetable, and while their place at
nature's table is being decided, they must be content to be called
_Phytozoa_, which, as we have before explained, is merely _Zoophyte_
turned upside down.
Some authorities have thought their animality proved by the high degree
of contractility which their tissues evince. This, however, cannot go
for much, as all physiologists admit contractility to belong to the
vegetable tissues of the sensitive plant, "Venus' Fly-trap," &c., and a
little more or less cannot mark the boundary between two orders of
being.
We shall have occasion again to notice the _Protophytes_, and now pass
to the _Protozoa_, of which we have a good illustration in the
_Vorticella_ already spoken of. In the group before us a number of
elegant bells or vases stand at the end of long stalks, as shown at the
top of the frontispiece, while round the tops of the bells, the
vibrations of a wreath or cilia produce little vortices or whirlpools,
and hence comes the family name. This current brings particles of all
sorts to the mouth near the rim of the bells, and the creature seems not
entirely destitute of power to choose or reject the morsels according to
its taste. Every now and then the stalk of some specimen is suddenly
twisted into a spiral, and contracted, so as to bring the bell almost to
the ground. Then the stem gracefully elongates again, and the cilia
repeat their lively game.
The general effect can be seen very well by a power of about sixty
linear, but one of them from one to two hundred is necessary to bring
out the details, and a practised observer will use still more
magnification with good effect. They should be examined by a moderately
oblique light, or most of the cilia are apt to be rendered invisible,
and also by _dark ground_ illumination. This may be accomplished in a
well-made microscope by turning the mirror quite out of the plane of the
axis of the instrument, that is to say, on one side of the space the
body would occupy if it were prolonged. By this means, and by placing
the lamp at an angle with the mirror, that must be learnt by experiment,
all the light that reaches the eye has first passed through the object,
and is refracted by it out of the line it was taking, which would have
carried it entirely away. Or the object may be illuminated by an
apparatus called a _spotted lens_, which is a small bull's-eye placed
under the stage, and having all the centre of its face covered with a
plaster of black silk. In this method the central or direct rays from
the mirror are obstructed, but those which strike the edge of the
bull's-eye are bent towards the object, which they penetrate and
illuminate if it is sufficiently transparent and refractive. Another
mode of dark ground illumination is by employing an elegant instrument
called a _parabolic illuminator_, which need not be described.
[Illustration: Vorticella, with posterior circlet of cilia in process of
separation, 300 linear.--_Stein._]
[Illustration: Vorticella in process of self-division. A new frontal
wreath in formation in each of the semi-lunar spaces.]
Different specimens and species of _Vorticellae_ vary in the length of
their bells from one three or four thousandth to one hundred and
twentieth of an inch, and when they are tolerably large, the dark ground
illumination produces a beautiful effect. The bells shine with a pearly
iridescent lustre, and their cilia flash with brilliant prismatic
colours.
[Illustration: Vorticella microstoma, showing alimentary tube, ciliated
mouth, and formation of a gemma at the base, 300 linear.--_Stein._]
[Illustration: Vorticella microstoma, the encysted animal protruding
through a supposed rupture of the tunic.]
The _Vorticellina_ belong to the upper division of the _Protozoa_--the
_ciliata_, or ciliated animalcules, and they have a mouth, an
oesophagus, and an orifice for the exit of their food.
Many observers used to ascribe to those creatures a complete intestinal
canal, but such an apparatus is now believed not to exist in any of the
Infusoria. Food particles, after leaving the oesophagus, are thrust
forward into the sarcode, or soft flesh, and any cavity thus formed acts
as a stomach.
The bells or cups are not, as might be fancied from a casual inspection,
open like wineglasses at the top, but furnished with a retractile disk
or cover, on which the cilia are arranged. Their stalks are not simple
stems, but are hollow tubes, which in the genus Vorticella are furnished
with a muscular band, by whose agency the movements are principally
made.
Some of the Vorticellids will be observed to leave their stalks, having
developed cilia round their base, and may be seen to swim about in the
enjoyment of individual life. They are also capable of becoming
_encysted_, that is, of secreting a gelatinous cover.
[Illustration: Encysted Vorticella, showing the obliteration of special
organs by the advancement of the process.--_Pritchard._]
These changes are exhibited in the annexed cuts, which are copied from
known authorities. By careful observation of the bodies of Vorticellids,
a contractile vesicle may be observed, which appears to cause a movement
of fluids, that is probably connected either with respiration or
secretion.
Another piece of apparatus in this family, but not confined to it, is
the so-called _nucleus_, which in this case is of a horseshoe shape and
granular texture, and greater solidity than the surrounding parts. The
functions of this organ formed the subject of various conjectures, but
it is now generally held to be an ovary.
[Illustration: Vorticella microstoma, in process of encystment, 300
linear; in the last the inclosing tunic is plainly developed.--_Stein._]
In common with many of the lower animals, the Vorticellids have three
ways of multiplying their race. One by _fission_, or division of their
bodies: another by _buds_, somewhat analogous to those of plants; and
another by reproductive germs. These processes will come again under our
notice, and we shall leave the Vorticellids for the present by observing
that if they are fed with a very small quantity of indigo or carmine,
the vacuoles or spaces, into which their nutriment passes, will be
clearly observed. Ehrenberg thought in these and similar creatures that
every vacuole was a distinct stomach, and that all the stomachs were
connected by an intestinal canal; hence his name _Polygastrica_, or
many stomached. In these views he has not been followed by later
observers, and it is probable he was misled, partly by pushing the
process of reasoning from the analogies of higher animals much too far,
and partly by the imperfection of the glasses he employed.
[Illustration: Rotifer vulgaris.--A, mouth, or gizzard; B, contractile
vesicle.--_Micrographic Dictionary._ N.B.--When the cilia and tail part
are retracted, and the body shortened, the creature assumes an obtuse
oval form.]
Having thus briefly considered the Vorticellids we must turn to the
wheel-bearer, who belongs to a higher race than even the ciliated
_Protozoa_. We left her crawling about with her snout or proboscis
protruded, but now she has moored herself by her tail-foot, pulled in
her nose, and put out two groups of cilia, which look like revolving
wheels, and a little below them is seen a gizzard in a state of active
work. After a little while she swims away with her wheels going, and her
tail, forked at the end, is found to be telescopic, or capable of being
pulled in and out. As the cilia play, the neighbouring water is
agitated, and the multitudes of small objects are brought by the
whirlpools within her ravenous maw. But the strangest thing of all is
that inside her body is seen a young one; in this case a large and fine
infant, which, like "a chip of the old block," imitates the parental
motions, thrusts forth its cilia and works its gizzard.[3] In other
genera the eggs are hatched externally, but this one is ovoviparous, and
carries its nursery inside.
[3] This was met with in the summer, but is described here to avoid
repetition. I do not know whether the eggs are hatched in very cold
weather.
A very slight investigation is sufficient to show that in the
wheel-bearer we have made a great advance towards a higher organization
than we discovered in the preceding creatures. We witness what the
learned call a "differentiation" of parts and tissues, and a
"specialization" of organs. The head is plainly distinguishable from the
body, the skin or integument is distinctly different from the internal
tissues, behind the eyes we can detect a nervous ganglion or miniature
brain, the gizzard is a complicated piece of vital mechanism, such as we
have not met with before, and in various parts of the transparent inside
we see organs to which particular functions are assigned.
It was at one time thought that Rotifers were hermaphrodite--uniting
both sexes in one body--but that idea is now generally abandoned, for in
many species the males have been discovered, and the fair sex may be
gratified to hear that they are without doubt the "inferior animals."
Their function is simply to assist the female in producing young, and as
this can be quickly accomplished, their lives are short, and they are
not supplied with the gizzard and digestive apparatus, which their
lady-loves possess. Much discussion has taken place as to the rank which
the Rotifers hold in the animal kingdom, some naturalists thinking them
relations of the crabs, and others believing them to belong to the
family of the worms. Professor Huxley, who adopts the latter view, which
has the most friends, groups the lower _Annulosa_ together under the
name of _Annuloida_, in which he includes _Annelides_, or worms of
various kinds, the _Echinodermata_ (or "spine skins," among which are
the star-fish and sea hedgehogs), and some other families. He considers
the Rotifers to be "the permanent forms of Echinoderm larvae." This does
not mean that they were ever produced by Echinoderms, and had their
development checked, but that they resemble them in organization, and
illustrate a general law, observable in animated beings, namely, that
the lower creatures are like the imperfect stages of higher animals, and
that all things are formed according to general principles, and exhibit
a uniformity of plan.
Mr. Gosse adopts a different view, and while admitting a connection
between the Rotifers and the worms, adduces important reasons for
associating them with the insects.
Leaving zoologists to settle their position, we may remark that the
Rotifers form a very numerous family, presenting very great diversities
of structure, some of the most interesting of which we shall meet with
in the course of our rambles; but they all possess a gizzard, which,
though differing in complexity, is throughout formed upon the same
principle, and that we must now explain.
We have called the masticatory apparatus of the Rotifers a _gizzard_;
but Mr. Gosse, who has done most to elucidate its structure, contends
that it is a _mouth_; and in some species it is frequently protruded,
and used like the mouth of higher animals. Taking one of the most
typical forms of this organ, and drawing our illustrations from Mr.
Gosse's admirable paper in the "Transactions of the Royal Society," we
may describe it, when completely developed, as consisting of three
lobes, having a more or less rounded form. The eminent naturalist we
have named calls the whole organ the _mastax_, and states that it is
composed of dense muscular fibre. The tube which leads down to it he
designates the "buccal (mouth) funnel," and the tube that issues from
it, and conveys the food to the digestive sac or stomach, he calls the
_oesophagus_, in conformity with the nomenclature applied to creatures
whose mouths are in the usual place. Inside the mouth-gizzard are placed
two organs, which work like hammers, and which Mr. Gosse therefore names
_mallei_. The hammers work against a sort of anvil, which is called
_incus_, the Latin for that implement. Each hammer consists of two
portions articulated by a hinge joint. The lower portion, the
_manubrium_, or handle, gives motion to the upper portion, which from
its shape is named the _uncus_, or hook. The _unci_ are furnished with
finger-like processes of teeth, which vary in number. There are five or
six in the best developed specimens. These hooks or teeth work against
each other, and against the _incus_, or anvil, which consists of
distinct articulated portions, of which the principal are two _rami_, or
branches, jointed so that they can open and close like a pair of shears.
These two rest upon the third portion, which is called the _fulcrum_.
Some faint idea of the working of the toothed hammers may be obtained by
rubbing the knuckles of both hands together, but the motion is more
complicated, and the _rami_ play their part in the trituration of the
food. Mr. Gosse states that when an objectionable morsel has got as far
as this mouth-gizzard, "it is thrown back by a peculiar scoop-like
action of the _unci_, very curious to witness." The foregoing diagram
will help the reader to comprehend this description, but no opportunity
should be lost for viewing this remarkable organ busy at work in the
living animals.
[Illustration: Gizzard of Notomata.]
The respiration of the Rotifers is supposed to be effected by the
passage of water through vessels running round them, and called the
"water vascular system," and in addition to their eyes, which often
disappear in adult specimens, the organ we described as standing out
like a pig-tail, as our acquaintance crawled along, is thought to act as
an _antenna_, or feeler, and brings its possessor in further relation to
the external world. It is also called the _calcar_, or spur, and is
furnished with cilia or bristles at its extremity.
Sometimes the particles swallowed by the Common Rotifer are large enough
for their course to be traced, but there is frequently a great commotion
and grinding of the gizzard, without any appreciable cause, although
doubtless something is taken in, and when the creature is tired, or has
had enough, we see both head and tail retracted, and the body assumes a
globular form. In another chapter, when viewing a Philodine, we shall
see how in the family to which the Common Rotifer belongs, the gizzard
departs from the perfect type.