Marvels of Pond-life.

by Henry J. Slack.

INTRODUCTION.

As this little book is intended to be no more than an introduction to an agreeable branch of microscopical study, it is to be hoped it will not require a formal preface; but a few words may be convenient to indicate its scope and purpose.

The common experience of all microscopists confirms the a.s.sertion made by Dr. Goring, that the most fascinating objects are living creatures of sufficient dimensions to be easily understood with moderate magnification; and in no way can objects of this description be so readily obtained, as by devoting an occasional hour to the examination of the little ponds which are accessible from almost any situation. A complete volume of pond lore would not only be a bulky book--much bigger than the aldermanic tomes which it is the fashion to call "Manuals,"

although the great stone fists in the British Museum would be required to grasp them comfortably,--but its composition would overtask all the philosophers of our day. In good truth, a tea-spoonful of water from a prolific locality often contains a variety of living forms, every one of which demands a profound and patient study, if we would know but a few things concerning it.

To man, then, is a vast and a minute. Our minds ache at the contemplation of astronomical immensities, and we are apt to see the boundless only in prodigious ma.s.ses, countless numbers, and immeasurable s.p.a.ces. The Creative Mind knows no such limitations; and the microscope shows us that, whether the field of nature's operation be what to our apprehension is great or small, there is no limit to the exhibition of marvellous skill. If the "undevout astronomer" be "mad,"

the undevout microscopist must be still more so, for if the matter be judged by human sense, the skill is greater as the operation is more minute; and not the sun itself, nor the central orb round which he revolves, with all his attendant worlds, can furnish sublimer objects of contemplation, than the miraculous a.s.semblage of forces which make up the life of the smallest creature that the microscope reveals.

There is an irresistible charm in the effort to trace _beginnings_ in nature. We know that we can never succeed; that each discovery, which conducts back towards some elementary law or principle, only indicates how much still lies behind it: but the geologist nevertheless loves to search out the first or oldest traces of life upon our globe; and so the microscopist delights to view the simplest exhibitions of structures and faculties, which reach their completion in the frame and mind of man.

That one great plan runs through the whole universe is now an universally accepted truth, and when applied to physiology and natural history, it leads to most important results.

The researches of recent philosophers have shown us that nature cannot be understood by studying the parts of animals with reference merely to their utility in the economy of the creature to which they belong. We do, indeed, find an admirable correspondence between structures and the services they perform; but every object in creation, and every part of it, is in harmonious relation to some grand design, and exhibits a conformity to some general mode of operation, or some general disposition and direction of forces, which secures the existence of the individual or the species, and at the same time works out the most majestic schemes. Microscopic researches, such as are within the reach of millions, offer many of the most beautiful ill.u.s.trations of these truths; and although the following pages are confined to such objects as are easily obtainable from ponds, and relate almost exclusively to the Infusoria, the Rotifers, the Polyps, and the Polyzoa, it is hoped that they will a.s.sist in a.s.sociating a few of the highly suggestive reasonings of science, with one of the most pleasurable recreations that human ingenuity has devised.

After a preliminary chapter, which is intended to a.s.sist the young microscopist in some technical matters, that could not be conveniently introduced into the text, the observations are distributed in chapters, corresponding with the twelve calendar months. This arrangement was suggested by the author's diary of operations for the year 1860, and although it by no means follows that the months in which particular creatures were then discovered, will be those in which they will be most readily found in other years, it was thought advantageous to give a real account of an actual period of microscopic work, and also that the plan would facilitate a departure from the dry manner of a technical treatise. The index will enable any one to use the book for the purpose of reference, and it will be observed that the first chapter in which any member of a group of creatures is introduced, is that in which a general description of the cla.s.s is given. The ill.u.s.trations are taken from drawings made by the wife of the author from the actual objects, with the exception of a few instances, in which the authority is acknowledged. The sketches were made _especially for beginners_, and the rule followed, was not to introduce any details that could not be seen at one focus, and with the simplest means: more elaborate representations, though of the highest value to advanced students, are bewildering at the commencement.

The ponds referred to are all either close to, or within a moderate distance of, London;[1] but similar objects will in all probability be obtained from any ponds similarly situated, and the descriptions and directions given for the capture of the minute prey will be found generally applicable. Care has been taken throughout to explain the most convenient methods of examining the objects, and although verbal descriptions are poor subst.i.tutes for the teachings of experience, it is hoped that those here given will remove some difficulties from a pursuit that no intelligent person can enter upon without pleasure, or consent to abandon when its elementary difficulties have been mastered, and the boundless fields of discovery are opened to view. Let not the novice be startled at the word "discovery." It is true that few are likely to arrive at new principles or facts which will inscribe their names upon the roll of fame; but no one of ordinary powers can look at living objects with any considerable perseverance, without seeing much that has never been recorded, and which is nevertheless worthy of note; and when the mind, by its own exertions, first arrives at a knowledge of new truth, an emotion is felt akin to that which more than recompenses the profoundest philosopher for all his toil.

[1] Many are now (1871) destroyed by the progress of building.

CHAPTER I.

PLAIN HINTS ON MICROSCOPES AND THEIR MANAGEMENT.

Powers that are most serviceable--Estimated by focal length--Length of body of microscope and its effects--Popular errors about great magnification--Modes of stating magnified power--Use of an "Erector"--Power of various objectives with different eye-pieces--Examination of surface markings--Methods of illumination--Direct and oblique light--Stage aperture--Dark ground illumination--Mode of softening light--Microscope lamps--Care of the eyes.

The microscope is rapidly becoming the companion of every intelligent family that can afford its purchase, and, thanks to the skill of our opticians, instruments which can be made to answer the majority of purposes may be purchased for three or four guineas, while even those whose price is counted in shillings are by no means to be despised. The most eminent English makers, Wales, and Tolles, in America, and Hartnack, in Paris, occupy the first rank, while the average productions of respectable houses exhibit so high a degree of excellence as to make comparisons invidious. We shall not, therefore, indulge in the praises of particular firms, but simply recommend any reader entering upon microscopic study to procure an achromatic instrument, if it can be afforded, and having at least two powers, one with a focus of an inch or two thirds of an inch, and the other of half or a quarter. Cheap microscopes have usually only one eye-piece, those of a better cla.s.s have two, and the best are furnished with three, or even more.

The magnifying power of a compound microscope depends upon the focal length of the object-gla.s.s (or gla.s.s nearest the object), upon the length of the tube, and the power of the eye-piece. With regard to object-gla.s.ses, those of shortest focal length have the highest powers, and the longest eye-pieces have the lowest powers. The body of a microscope, or princ.i.p.al tube of which it is composed, is, in the best instruments, about nine inches long, and a draw tube, capable of being extended six inches more, is frequently useful. From simple optical principles, the longer the tube the higher the power obtained with the same object-gla.s.s; but only object-gla.s.ses of very perfect construction will bear the enlargement of their own imperfections, which results from the use of long tubes; and consequently for cheap instruments the opticians often limit the length of the tube, to suit the capacity of the object-gla.s.ses they can afford to give for the money. Such microscopes may be good enough for the generality of purposes, but they do not, with gla.s.ses of given focal length, afford the same magnifying power as is done by instruments of better construction. The best and most expensive gla.s.ses will not only bear long tubes, but also eye-pieces of high power, without any practical diminution of the accuracy of their operation, and this is a great convenience in natural history investigations. To obtain it, however, requires such perfection of workmanship as to be incompatible with cheapness. An experienced operator will not be satisfied without having an object-gla.s.s at least as high as a quarter, that will bear a deep eye-piece, but beginners are seldom successful with a higher power than one of half-inch focus, or thereabouts, and before trying this, they should familiarise themselves with the use of one with an inch focus.

It is a popular error to suppose that enormous magnification is always an advantage, and that a microscope is valuable because it makes a flea look as big as a cat or a camel. The writer has often smiled at the exclamations of casual visitors, who have been pleased with his microscopic efforts to entertain them. "Dear me, what a wonderful instrument; it must be immensely powerful;" and so forth. These e.j.a.c.u.l.a.t.i.o.ns have often followed the use of a low power, and their authors have been astonished at receiving the explanation that the best microscope is that which will show the most with the least magnification, and that accuracy of definition, not mere increase of bulk, is the great thing needful.

Scientific men always compute the apparent enlargement of the object by _one_ dimension only. Thus, supposing an object one hundredth of an inch square were magnified so as to appear one inch square, it would, in scientific parlance, be magnified to "one hundred diameters," or one hundred linear; and the figures 100 would be appended to any drawing which might be made from it. It is, however, obvious that the length is magnified as well as the breadth; and hence the magnification of the whole surface, in the instance specified, would be one hundred times one hundred, or ten thousand: and this is the way in which magnification is popularly stated. A few moments' consideration will show that the scientific method is that which most readily affords information. Any one can instantly comprehend the fact of an object being made to look ten times its real length; but if told that it is magnified a hundred times, he does not know what this really means, until he has gone through the process of finding the square root of a hundred, and learnt that a hundredfold magnification means a tenfold magnification of each superficial dimension. If told, for example, that a hair is magnified six hundred diameters, the knowledge is at once conveyed that it looks six hundred times as broad as it is; but a statement that the same hair is magnified three hundred and sixty thousand times, only excites a gasping sensation of wonder, until it is ascertained by calculation that the big figures only mean what the little figures express. In these pages the scientific plan will always be followed.

If expense is not an object, a binocular instrument should be purchased, and it is well to be provided with an object-gla.s.s as low as three or even four inches focus, which will allow the whole of objects having the diameter of half an inch or more to be seen at once. Such a low power is exceedingly well adapted for the examination of living insects, or of the exquisite preparations of entire insects, which can now be had of all opticians. Microscopes which have a draw tube can be furnished with an _erector_, an instrument so called because it erects the images, which the microscope has turned upside down, through the crossing of the rays. This is very convenient for making dissections under the instrument; and it also gives us the means of reducing the magnifying power of an object-gla.s.s, and thus obtaining a larger field. The erector is affixed to the end of the draw tube, and by pulling it out, or thrusting it in, the rays from the object-gla.s.s are intercepted at different distances, and various degrees of power obtained.

A binocular microscope is most useful with low powers from two thirds upwards. A new form, devised by Mr. Stephenson, acts as an erector, and is very valuable for dissections. It works with high powers.

Beginners will be glad to know how to obtain the magnifying power which different objects require, and it may be stated that, with a full-sized microscope, a two-inch object-gla.s.s magnifies about twenty-five diameters with the lowest eye-piece; a one-inch object-gla.s.s, or two thirds, from fifty to sixty diameters; a half-inch about one hundred; a quarter-inch about two hundred. The use of deeper eye-pieces adds very considerably to the power, but in proportions which differ with different makers. One instrument used by the writer has three eye-pieces, giving with a two thirds object-gla.s.s powers of sixty one hundred and five, and one hundred and eighty respectively; and with a fifth two hundred and forty, four hundred and thirty, and seven hundred and twenty, which can be augmented by the use of a draw tube.

It has been well observed that the illumination of objects is quite as important as the gla.s.ses that are employed, and the most experienced microscopists have never done learning in this matter. Most microscopes are furnished with two mirrors beneath the stage, one plane and one concave. The first will throw a few parallel rays through any transparent object properly placed, and the latter causes a number of rays to converge, producing a more powerful effect. The first is usually used in daylight, when the instrument is near a window (one with a north aspect, out of direct sunlight, being the best); and the second is often useful when the source of illumination is a candle or a lamp. By varying the angle of the mirror the light is thrown through the object more or less obliquely, and its quant.i.ty should never be sufficient to pain the eye. Few objects are seen to the best advantage with a _large_ pencil of perfectly direct light, and the beginner should practise till the amount of inclination is obtained which produces the best effect.

It is advisable that the hole in the stage of the microscope should be large--at least an inch and a half each way--so that the entrance of oblique rays is not obstructed, and it is desirable that the mirror, in addition to sliding up and down, should have an arm by which it can be thrown completely out of the perpendicular plane of the body of the instrument. This enables such oblique rays to be employed as to give a dark field, all the light which reaches the eye being _refracted_ by the object through which it is sent. The opticians sell special pieces of apparatus for this purpose, but though they are very useful, they do not render it less desirable to have the mirror mounted as described.

Most microscopes are furnished with a revolving diaphragm, with three holes, of different sizes, to diminish the quant.i.ty of light that is admitted to the object. This instrument is of some use, and offers a ready means of obtaining a very soft agreeable light for transparent objects, viewed with low powers. For this purpose cut a circular disk of India or tissue paper, rather larger than the biggest aperture; sc.r.a.pe a few little pieces of spermaceti, and place them upon it, then put the whole on a piece of writing-paper, and hold it a few inches above the flame of a candle, moving it gently. If this is dexterously done, the spermaceti will be melted without singeing the paper, and when it is cold the disk will be found transparent. Place it over the hole in the diaphragm, send the light through it, and the result will be a very soft agreeable effect, well suited for many purposes, such as viewing sections of wood, insects mounted whole, after being rendered transparent, many small water creatures, etc. Another mode of accomplishing this purpose is to place a similarly prepared disk of paper on the flat side of a bull's-eye lens, and transmit the light of a lamp through it. This plan may be used with higher powers, and the white opaque light it gives may be directed at any angle by means of the mirror beneath the stage.

An ordinary lamp may be made to answer for microscopic use, but one of the small paraffine lamps now sold everywhere for eighteen-pence is singularly convenient. It is high enough for many purposes, and can easily be raised by one or more blocks. A paraffine lamp on a sliding stand is still more handy, and all the better for a hole with a gla.s.s stopper, through which the fluid can be poured.

Many people fancy that the eyes are injured by continual use of the microscope, but this is far from being the case if reasonable precautions are taken. The instrument should be inclined at a proper angle, all excess of light avoided, and the object brought into focus before it is steadily looked at. Most people solemnly shut one eye before commencing a microscopic examination; this is a practical and physiological mistake. Nature meant both eyes to be open, and usually resents a prolonged violation of her intentions in this matter. It requires but a little practice to keep both eyes open, and only pay attention to what is seen by that devoted to the microscope. The acquisition of this habit is facilitated, and other advantages gained, by a screen to keep out extraneous light. For this purpose take a piece of thin cardboard about nine inches square, and cut a round hole in it, just big enough to admit the tube of the microscope, about two inches from the bottom, and equidistant from the two sides. Next cut off the two upper corners of the cardboard, and give them a pleasant-looking curve. Then cover the cardboard with black velvet, the commonest, which is not glossy, answers best, and your screen is made. Put the hole over the tube of the microscope, and let the screen rest on the little ledge or rim which forms an ornamental finish to most instruments. A piece of cork may be gummed at the back of the screen, so as to tilt it a little, and diminish its chance of coming into contact with that important organ the nose. This little contrivance adds to the clearness and brilliancy of objects, and is a great accommodation to the eyes.

One more oculistic memorandum, and we have done with this chapter. Do not stare at portions of objects that are out of focus, and consequently indistinct, as this injures the eyes more than anything. Remember the proverb, "None so deaf as those that won't hear," which naturally suggests for a companion, "None so blind as those that won't see." It is often impossible to get every object in the field in focus at one time;--look only at that which is in focus, and be blind to all the rest. This is a habit easily acquired, and is one for which our _natural_ microscopes are exceedingly grateful; and every judicious observer desires to keep on the best terms with his eyes.

CHAPTER II.

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"--Bis.e.xuality 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 princ.i.p.al cla.s.ses 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 ma.s.ses 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 ma.s.s. To hold up a morsel of this ma.s.s, and tell some one not in the secrets of pond-lore that its dripping threads were objects of beauty, surpa.s.sing 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 ma.s.s of conferva turns out to contain one of the most elegant species. Fine hair-like tubes of an organic material, as transparent as gla.s.s, are divided by part.i.tions 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 gla.s.ses 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 ma.s.s comprehended under this term, and it is now used in various senses. The authors of the 'Micrographic Dictionary' employ it to designate "a cla.s.s 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 cla.s.s_ is an a.s.sumption 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 cla.s.sified 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 a.s.sembled 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.).

[Ill.u.s.tration: --_a_, motile; and _b_, resting condition of Euglenae.]

Feeling that we must be merciful with the long-tailed words and explanations of cla.s.sification, 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 gla.s.s; 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?