I. INHABITANTS OF A DROP OF WATER
The most unobserving stroller through the forest or by the seashore can not fail to be impressed by the abundance and variety of animal life; yet the forms visible to the naked eye really constitute but a fraction of the vast horde which makes up what we call life.
In the year 1901 a strange phenomenon appeared off the coast of southern California. The ocean assumed a reddish muddy hue which was traced for four hundred miles up the coast and from one to twenty miles offshore; hence, at a conservative estimate, the reddish color occupied a space of ten thousand square miles. At night it assumed a greenish light, and when the wind rose and formed whitecaps, each became a blaze of light, and the ocean as far as the eye could reach was converted into a mass of seeming flame. The sands of the beach gave out flashes of light when touched; the footsteps of dog or man on the sands left an imprint of vivid light, and figures made on the sands with a finger or stick stood out in lines of light. Ten thousand square miles of phosphorescent light; ten thousand square miles of living beings, each so minute that it was almost if not quite invisible to the human eye. Who could estimate the individuals in one square mile, one square foot, or even a drop of this reddish water? This illustrates the fact that the greater number of the earth's population are unseen, even though not invisible to the unaided eye.
These minute animals are as interesting as the larger forms. Equipped with a microscope, we are prepared to explore the regions in which they live and observe their habits. A favorite hunting ground for this small game is some long-standing water in which plants have been growing. Placing some of this, with the green scraping of the glass, on the slide, we shall soon make out, moving mysteriously along, something which resembles the white of an egg, an atom of slime or jelly. Now it stops and throws out parts of itself which seem to fuse together again; now it is long, now short and compact, again circular. You almost believe it is a mere atom of slime, yet it is an animal which eats and lives its life cycle in a drop of water, one of the lowest of all animals.
It is called Amœba (Fig. 1), and although it is hardly a hundredth of an inch in diameter, yet if we devote some time to it we shall find that it is a very remarkable animal. Thus if it wishes to move in any given direction, a portion of the body becomes a seeming leg and protrudes in that direction, the rest of the body following, drawn along in some mysterious manner. If it wishes to eat, it is not obliged to twist around to bring the food or victims opposite the mouth, as a mouth forms there and then; the Amœba merely glides around it and covers it up.
We may even notice a difference in the parts. Thus the center calls to mind ground glass; it is blurred or granular, while around the edges is a little border which is transparent, like ordinary window glass. So the Amœba is a minute mass of jelly inclosed in a layer a little clearer.
Fig. 1.—Amœba proteus, with the pseudopodia (false feet) protruded, enlarged 200 diameters (after Leidy): n, nucleus; c, contractile vesicle; v, one of the larger food-vacuoles; en, the granular endosarc; ec, the transparent ectosarc; a, cell of an Alga taken in as food (other cells of the same Alga are obliquely shaded).]
Floating in the granular portion is seen a minute round body called the nucleus, clearer than the fluid in which it rests, and not far away another clear, circular body, which from time to time contracts or sometimes disappears in a marvelous fashion, but always returns. This is called the contracting vesicle, and here our discoveries end, so far as organs and structure are concerned, as these are nearly all that have been found; yet the Amœba eats, doubtless sleeps, and grows.
Fig. 2.—Amœba eating: Pv, contracting vesicle.]
We may watch it at its dinner. When a victim is found, an animal smaller than itself, out shoots a little cape or extension from the clear rim, creeping slowly up the side of the animal; and if we watch very closely, we shall see the thicker portion of the Amœba, that which calls to mind ground glass, running or flowing into it. Then another false foot, as it is called, slowly creeps out on the opposite side and joins its companion, more of the ground-glass matter slides or pours into this, filling it out, and, presto! the two arms merge one into the other. The victim has been swallowed and is now being digested [(Fig. 2, d)].
Fig. 3.—Amœba separating.]
That this minute atom has its likes and dislikes is evident, for if the food is too large, or not to its taste, it retracts, or even draws away from it after it has swallowed it. The shells of its victim, if it has them, are rejected in a manner equally simple; the Amœba flows away from them. Jar it with a needle point and it contracts, showing that it can be irritated. At times the body is seen to divide and two Amœbæ are formed (Fig. 3), each of which has a separate existence from then on. Such is one of the lowest of all animals. It is made up of but a single cell. All the members of the other great branches of the animal kingdom and the higher plants are made up of many cells; hence we see that the Amœba is the simplest and lowest of all animals.
Fig. 4.—Ciliated Infusoria: A, Bursaria; B, Nyctotherus; C, Amphileptus; D, Ceratium; E, Monosiga; f, flagellum; n, nucleus; c, contractile vesicle.
In looking into our drop of water our attention has perhaps been distracted by other animals. In point of fact, it is very difficult to keep the eye on this mass of slime in its slow movements, for about it, over it, and constantly bumping into it are countless other forms whose motions convey the impression that life to them is very active. The most numerous are little objects (Fig. 4) resembling hats or bells, which go rushing along, bumping aimlessly into all others, and always in a hurry. Around the edge of the bell or hat are numerous hairs (cilia) which are really locomotive organs by which the little animals whirl themselves along. Near them we see numbers of similar objects, each one forming the cup of a seeming flower, each having a long stem. These are Bell Animalcules (Fig. 5) or Vorticellæ, among the most beautiful and graceful of all the minute animals, but much higher in the scale of life, as they have a permanent mouth and form. Among them, swimming rapidly, comes a giant by contrast, the Paramœcium (Fig. 6) or Slipper Animalcule, so called from its resemblance to a slipper. It, too, is a higher form than Amœba, as it has a permanent shape; yet in other ways it is as simple as Amœba. The Paramœcium has a marvelous array of oars which cover its body. Under the glass they look like eyelashes or whips, and by their rapid movements they drive the animal along. On the side is the mouth opening, into which the animal fans minute animals, and they can be seen swept along by the irresistible current, caught by the mouth if desirable, or tossed off if not to the taste of the wonderful living slipper. After glancing at the drop of water for a few moments the observer is convinced that here is a world in itself, with a population growing, increasing, developing, devouring its prey, and in such multitudes that the mind can not grasp the figures.
Fig. 5.—A, Stentor; B, Vaginicola; C, group of Vorticellæ; D, bud of Vorticella.
Fig. 6.—Paramœcium: e, mouth; v, contracting vacuoles.
Fig. 7.—Nummulites.
If the reader visits Egypt and climbs the pyramids, he will be impressed by these the greatest works of mankind. If a small portion of the stone from which they are constructed is placed under the glass, it will be found in many instances made up almost entirely of beautiful shells (Fig. 7). These are the shells of an amœba-like animal known as a Nummulite, which lived millions of years ago, and whose fossil remains formed the stone from which the early Egyptians in turn built the great piles or monuments of their kings. Man is powerful, but in this instance one of the most insignificant of animals made his work possible. These shells are of great beauty and variety (Fig. 8). Many are perforated, and through the minute holes are seen the false feet of the Amœba reaching out for food. They illustrate the boundless resources of nature, and suggest that the very lowest of creatures are not too insignificant to be dressed in most beautiful garbs, as all these forms vie with one another in the delicacy of their designs (Fig. 9) and the grace of their shapes. Some of these shelled forms are giants two inches across. All these minute shells perform a marvelous work in building up the crust of the earth, forming the bottom of deep seas and the platforms of coral reefs. The chalk cliffs of England are composed of shells of unestimated millions (Fig. 10), which were once dropped upon the bottom of a deep sea and piled upward until some were crushed into a shapeless mass of lime, others retained their shapes (Fig. 11); and all, by some convulsion, were afterward lifted high into the air.
Fig. 8.—Flint-shelled Radiolarian.
Fig. 9.—Flint-shelled Polycystina.
Fig. 10.—Foraminifera from Atlantic ooze.
The entire ocean is as thickly populated with these atoms as the drop of fresh water. The shelled forms are as constantly dying, the shells falling or sinking in a continual rain of shells upon the bottom, piling up eternally. Who shall estimate their countless numbers? It is believed that these minute shells are as abundant down to a depth of six hundred feet as they are at the surface. There are more than sixteen tons of limy shells floating in the uppermost one hundred fathoms of every square mile of the ocean. These facts convey an idea of one way in which the earth is growing—increasing in size but not in weight, as these delicate creatures merely secrete the carbonate of lime which forms their shells. They take it from the surrounding water of which it has been a part.
Fig. 11.—Section of English chalk cliff. Highly magnified. Bottom of an ancient sea.
It would be of great interest to learn exactly how these humble creatures take lime from the water and produce shells of such marvelous beauty: to learn why one is of lime and others, like the Radiolarians, are of silica; why some live at the surface and are free swimmers, while others creep about in the ooze. When the deep-sea explorers first began to dredge, they found over vast areas a peculiar mud or ooze, and investigation showed that it was formed almost entirely of the shells of a certain minute shell maker, from which it was named the Globigerina ooze. Finding these vast banks or beds of mud at this depth is suggestive that the deepest seas may yet be filled by the dropping of this invisible rain; but as the average depth of the ocean is nearly if not quite three miles, many centuries must pass before this will be accomplished.
Fig. 12.—Noctiluca. Highly magnified.
The marvelous phosphorescent light previously described came from a minute armored form known as Peridinium, but even this is exceeded by the glories of a little monad called Noctiluca (Fig. 12). It is a giant of the tribe, and is visible to the naked eye, being almost as large as the head of a pin, and resembling a currant in shape. It has a single hairlike organ or lash, supposed to be a locomotive organ, by which it whirls itself through the water. Of all the light givers of the sea this is the most common, some of its species being found in every sea, and where they are, it is necessary only to splash the water to cause a blaze of light to follow. A French naturalist placed on record the fact that so brilliant was the light occasioned by this minute organism in African waters that he read by their light standing on a beach where a heavy surf came pounding in upon the sand. The light of this little creature is remarkable not only for its vividness but for its many different tints. Now it is a fitful vivid green, again the water is a blaze of yellow light, or orange. At such times, when a ship is plowing along, the light is so brilliant that the sails and rigging are brilliantly illumined, casting weird shadows.
Some Noctilucæ emit a clear blue light, but when the same animal is disturbed it appears white with green and blue flashes of great beauty and intensity—a telling illustration of the boundless, and marvelous, resources of nature. Many interesting experiments have been tried with these dainty light givers. A tube fifteen millimeters in diameter was filled with them, and by shaking this novel lamp a printed page was read a foot distant; yet when a delicate thermometer was thrust into the fiery mass, the mercury was not affected in the slightest, showing that here was that wonder of wonders—vivid light without heat—a secret which man has vainly endeavored to wrest from nature. The vast number of these minute creatures can be realized when it is said that the ship Magenta sailed nearly five hundred miles among swarms of Noctilucæ, which gave splendid displays of phosphorescent light at night. Sometimes the light emitted was milky white; again it was green, or blue, the different species possessing different colors.