Marvels of Pond-life Part 7

[25] The achromatic condenser is a frame capable of supporting an object-gla.s.s, lower than that employed for vision, through which the light pa.s.ses to the object in quant.i.ties 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 to 118, but are destroyed by immersion in boiling water. They may be gradually heated to 216, 252, and even 261. 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 alb.u.men. Soluble alb.u.men, or, as it should be called, _Alb.u.minate of Soda_ (for a small quant.i.ty 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 const.i.tuent 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 repet.i.tion 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 circ.u.mstances 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 gla.s.s trough, to which some fresh water was added every few days, taken from a gla.s.s 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 a.s.sumed 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.

[Ill.u.s.tration: 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 ma.s.ses 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 a.n.a.logous to that of the phlebenterous mollusks described by Quatref.a.ges, 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 gla.s.s 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 gla.s.s 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 t.i.tle 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 "s.h.i.+ndy" at Donnybrook Fair.

I was greatly puzzled with this aspect of things, and still more so when my deranged specimen twirled and b.u.mped 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 a.s.sume the appearance of a respectable well-to-do animalcule.

[Ill.u.s.tration: _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 a.s.sumed 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 ma.s.s 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 circ.u.mstances 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 cla.s.s.

CHAPTER XIII.

CONCLUSION.

The creatures described in the preceding pages range from very simple to highly complicated forms, and in describing them some attention has been paid to the general principles of cla.s.sification. The step is a wide one from the little ma.s.ses of living jelly that const.i.tute Amoebae to the Rotifers, supplied with organs of sensation--eyes, feelers (calcars), and the long cilia in the Floscularians, which seem to convey impression like the whiskers of a cat--together with elaborate machinery for catching, grinding up, and digesting their prey, and which are also well furnished with respiratory and excretory apparatus, ovaries, &c. In the polypi and polyzoa may be observed those resemblances in appearance which induced early naturalists to group them together, and also the wide difference of organization which marks the higher rank to which the latter have attained. Amongst the ciliated infusoria important gradations and differences will also be noticed, some having only one sort of cilia, others two sorts, and others, again, supplied, in addition to cilia, with hooks and styles. No perfectly satisfactory cla.s.sification of the infusoria has yet been devised, and the life history of a great many is still very imperfectly known. On the whole, the tendency of research is to place many of them higher than they used to stand after Ehrenberg's supposition of their having a plurality of distinct stomachs, &c., was given up. Balbiani and others have shown numerous cases of their forming their eggs by a process a.n.a.logous to that of higher animals. Some really are, and others closely resemble, the larval conditions of creatures higher in the scale, and the contracted vesicle with its channel bears resemblance to what is called the "water vascular system" of worms.

Zoological cla.s.sification depends very much on morphology, that is, the tracing of particular structures, or parts, through all their stages, from the lowest to the highest forms in which they are exhibited. In this way the swimming bladder of a fish is shown to be a rudimentary lung, though it has no respiratory functions, and Mr. Kitchen Parker has found in the imperfect skull of the tadpole a rudimentary appearance of bones belonging to the human ear. The comparative anatomist, after a wide survey of the objects before him, arranges them into groups. He asks what are the characteristic things to be affirmed concerning all the A's that cannot be said of all the B's; or of all the C's that marks their difference from the A's or the D's. Careful investigation upon these methods shows affinities where they were not previously expected--birds and reptiles being close relations, for example, instead of distant connections--and they lessen the value for purposes of cla.s.sification of peculiarities that might have been deemed of the highest importance.

Professor Huxley divides the vertebrates into ITHYCOIDS, comprising fishes and amphibia, which, besides other characteristics, have gills at some period of their existence; SAUROIDS (reptiles and birds), which have no gills, and possess certain developmental characteristics in common; and, lastly, MAMMALS. The Insecta, Myriopoda, Arachnidae, and Crustacea, he remarks, "without doubt present so many characters in common as to form a very natural a.s.semblage. All are provided with articulated limbs attached to a segmented body skeleton, the latter, like the skeleton of the limbs, being an 'exoskeleton,' or a bordering of that layer which corresponds with the outer part of the vertebrates.

In others, at any rate in the embryonic condition, the nervous system is composed of a double chain of ganglia, united by longitudinal commissures, and the gullet pa.s.sed between two of these commissures. No one of the members of these four cla.s.ses is known to possess vibratile cilia. The great majority of these animals have a distinct heart, provided with valvular apertures, which are in communication with a peri-visceral cavity containing corpusculated blood." These four cla.s.ses have const.i.tuted the larger group or "province" of _Articulata_ or _Arthropoda_. Professor Huxley thinks that, notwithstanding "the marked differences" between the Annelida (worms) and the preceding Arthropods (joint-foots), their resemblances outweighing them--"the characters of the nervous system, and the frequently segmented body, with imperfect lateral appendages of the Annelida, necessitates their a.s.semblage with the Arthropoda in one great division, or sub-kingdom, of ANNULOSA."

Tracing a.n.a.logies between the Echinodermata (sea urchins, star-fish, &c.) and the Scolecida (intestinal worms), he places them together as _Annuloida_.

Cephalopoda, Pteropoda, Pulmo-gasteropoda, and Branchio-gasteropoda, having resemblances of nervous system, and "all possessing that remarkable buccal apparatus, the Odontoph.o.r.e," are placed together by him as ODONTOPHORA. The Odontoph.o.r.es (tooth-bearers) are familiar to microscopists as the so-called _palates_ of mollusca. Placing with the above the lamellibranchial mollusks (mollusks with gills formed of lamellae or little plates), Ascidioida (ascidians), Brachiopoda (lamp-sheds), and Polyzoa, in spite of their differences, he forms another great group, ANNULOIDA.

The Actinozoa (anemonies, &c.) and the Hydrozoa (polyps) const.i.tute the COELENTERA of Frey and Leuckart. "In all these animals," says Professor Huxley, "the substance of the body is differentiated into those histological elements which have been termed cells, and the latter are previously disposed in two layers, one external and one internal, const.i.tuting the ectoderm and endoderm. Among animals which possess this histological structure the Coelenterata stand alone in having an alimentary ca.n.a.l, which is open at its inner end and communicates freely by this aperture with the general cavity of the body," and "all (unless the Ctenophora should prove a partial exception to the rule) are provided with very remarkable organs of offence or defence, called thread-cells or nematocysts." In describing the Polyps we have given ill.u.s.trations of these weapons.

The remaining cla.s.ses, which have been roughly a.s.sociated as _Protozoa_, must evidently be rearranged. Sponges, Rhizopods (Amoebae, &c.), and Gregarines, have strong resemblances, but recent researches may place the former higher. The Infusoria comprehend creatures too various to remain under one head, and very many of them too highly organized to be called "protozoons," or first life-forms.

Those who wish to pursue this subject further may consult Professor Huxley's 'Elements of Comparative Anatomy,' from which the preceding quotations have been taken.

A system of cla.s.sification founded upon anatomical and developmental considerations frequently differs considerably from one we might arrive at if all the creatures were arranged according to the perfection of their faculties and the extent and accuracy of their relations to the external world. Such a cla.s.sification would not in any way supersede the former, but it would prove very instructive and offer many valuable suggestions. Some years since, Professor Owen proposed to divide the Vertebrates according to the perfection of their brains, but other anatomists did not find his divisions sufficiently coincident with facts. Very little has been done towards an exact science of human phrenology. The difficulties remain pretty much as they were many years ago, and our comparative phrenology, if we may use such a term, is in a very imperfect state. When we come to the lower animals we do not know what peculiarities of the brain of an ant make it the recipient of a higher instinct, or give its possessor greater capacities for dealing with new and unexpected difficulties than are possessed by most other insects, and if any reader has a marine aquarium, and will make a few experiments in taming prawns, and watching their proceedings, he will discover symptoms of intelligence beyond what the structure of the creature would have led him to expect.

Animals usually possess some one leading characteristic to which their general structure is subordinated. Man stands alone in having the whole of his organization conformed to the demands of a thinking, ruling brain. To pa.s.s at once to the other extreme, we observe in the lower infusoria a restless locomotion, probably subservient to respiration, but utterly inconsistent with a well developed life of relation, or with manifestations of thought. The life of an animalcule may be summed up as a brief and restricted, but vigorous organic energy, and if the amount of change which a single creature can make in the external world, is inconceivably small, the labours of the entire race alter the conditions of a prodigious amount of matter. Microscopic vegetable life is an important agent in purifying water from the taint of decomposing organisms. By evolving oxygen it brings putrescent particles under the influence of a species of combustion, which, though slow, is as effectual as that which a furnace could accomplish. In this way minute moulds burn up decaying wood.

Microscopic animal life helps the regenerative process, and, together with the minute vegetable life, restores to the organic system myriads of tons of matter, which death and decay would have handed over to the inorganic world. In a very small pond or tank the quant.i.ty of this kind of work is soon appreciable, and if we reflect on the amazing amount of water all over the globe, including seas and oceans, which swarm with infusoria, the total effect produced in a single year must seem considerable, even when compared with that portion of the earth's crust that is subject to alteration from all other causes put together. If we add to the labour of the Infusoria those of other creatures whose organization can only be discovered by the microscope, and take in the foraminifera, polyps, polyzoa, &c., we shall have to record still larger obligations to minute forms of living things. The coral polyp builds reefs that const.i.tute the chief characteristic of certain regions in the Pacific; foraminifera are forming or helping to form strata of considerable extent, while diatoms are making deposits many feet in thickness, composed of myriads of their silicious sh.e.l.ls, or adding their contributions of silex, very large in the aggregate, to all sedimentary rocks. Testimony of this kind of work is found by the navigator who examines the ice in arctic seas, and it comes up with soundings from the ocean depths.

On the surface of the earth the amount of change produced is equally remarkable, although it leaves less permanent traces behind. As a rule no decomposition of organized matter takes place, no death of plants or animals, without infusorial life making its appearance, and disposing of no small portion of the spoil. Even in our climate the ma.s.s of matter thus annually affected is very large; but what must it not be in moist tropical lands, where every particle seems alive, and the race of life and death goes on at a speed, and to an extent scarcely conceivable by those who have not witnessed it.

Thus, if we look at the world of minute forms which the microscope reveals, there opens before us a spectacle of boundless extent. We see life manifested by the specks of jelly containing particles not aggregated into structure, and we see it gradually ascending in complexities of organization. In creatures whose habits and appearance seem most remote from our own, we find the elementary developments of the organs and powers that const.i.tute our glory, and give us our power.

Such studies a.s.sist us to conceive of the universe as a Cosmos, or Beautifully Organized Whole; and, although we cannot tell the object for which a single portion received its precise form, we trace everywhere relations of structure to means of existence and enjoyment, and are led to the conviction that all the actions and arrangements of the organic or inorganic worlds are due to a definite direction and co-ordination of a few simple forces, which implicitly and unerringly obey the dictates of an Omniscient Mind.