Fig. 62.Fig. 62. η Lyræ. A double-binary star. Each couple revolves, and the couples probably also revolve round each other. (After Chambers.)η Lyræ. A double-binary star. Each couple revolves, and the couples probably also revolve round each other. (After Chambers.)
η Lyræ. A double-binary star. Each couple revolves, and the couples probably also revolve round each other. (After Chambers.)
Let us turn the telescope for a short time upon a few of the double stars and we shall have a great treat, for one of the most interesting facts about them is that both stars are rarely of the same colour. It seems strange at first to speak of stars as coloured, but they do not by any means all give out the same kind of light. Our sun is yellow, and so are the Pole-star and Pollux; but Sirius, Vega, and Regulus are dazzlingwhite or bluish-white, Arcturus is a yellowish-white, Aldebaran is a bright yellow-red, Betelgeux a deep orange-red, as you may see now in the telescope, for he is full in view; while Antares, a star in the constellation of the Scorpion, which at this time of year cannot be seen till four in the morning, is an intense ruby red.
Plate II.Plate II. COLOURED DOUBLE STARS. Illustration: γ Andromedæ. Illustration: ε Boötis. Illustration: δ Geminorum. Illustration: α Herculis. Illustration: β Cygni. Illustration: η CassiopeiæCOLOURED DOUBLE STARS.
Plate II.
It appears to be almost a rule with double stars to be of two colours. Look up at Almach (γ Andromedæ), a bright star standing next to Algol the Variable in the sweep of four bright stars behind Cassiopeia (see Fig. 58). Even to the naked eye he appears to flash in a strange way, and in the telescope he appears as two lovely stars, one a deep orange and the other a pale green, while in powerful telescopes the green one splits again into two (Plate II.) Then again, η Cassiopeæ, the sixth star lying between the two large ones in the second V of Cassiopeia, divides into a yellow star and a small rich purple one, and δ Geminorum, a bright star not far from Pollux in the constellation Gemini, is composed of a large green star and a small purple one. Another very famous double star (β Cygni), which rises only a little later in the evening, lies below Vega a little to the left. It is composed of two lovely stars; one an orange yellow and the other blue; while ε Boötis, just visible above the horizon, is composed of a large yellow star and a very small green one.[5]
There are many other stars of two colours even among the few constellations we have picked out to-night,as, for example, the star at the top of the tailboard of Charles's Waggon and the second horse Mizar. Rigel in Orion, and the two outer stars of the belt, α Herculis, which will rise later in the evening, and the beautiful triple star (ζ Cancer) near the Beehive (see Fig. 54), are all composed of two or more stars of different colours.
Why do these suns give out such beautiful coloured light? The telescope cannot tell us, but the spectroscope again reveals the secrets so long hidden from us. By a series of very delicate experiments, Dr. Huggins has shown that the light of all stars is sifted before it comes to us, just as the light of our sun is; and those rays which are least cut off play most strongly on our eyes, and give the colour to the star. The question is a difficult one but I will try to give you some idea of it, that you may form some picture in your mind of what happens.
We learnt in our last lecture (p. 131) that the light from our sun passes through the great atmosphere of vapours surrounding him before it goes out into space, and that many rays are in this way cut off; so that when we spread out his light in a long spectrum there are dark lines or spaces where no light falls.[6]Now in sunlight these dark lines are scattered pretty evenly over the spectrum, so that about as much light is cut off in one part as in another, and no one colour is stronger than the rest.
Dr. Huggins found, however, that in coloured stars the dark spaces are often crowded into particular parts of the long band of colour forming the spectrum;showing that many of those light-rays have been cut off in the atmosphere round the star, and thus their particular colours are dimmed, leaving the other colour or colours more vivid. In red stars, for example, the yellow, blue, and green parts of the spectrum are much lined while the red end is strong and clear. With blue stars it is just the opposite, and the violet end is most free from dark lines. So there are really brilliantly coloured suns shining in the heavens, and in many cases two or more of these revolve round each other.
And now I have kept your attention and strained your eyes long enough, and you have objects to study for many a long evening before you will learn to see them plainly. You must not expect to find them every night, for the lightest cloud or the faintest moonlight will hide many of them from view; and, moreover, though you may learn to use the telescope fairly, you will often not know how to get a clear view with it. Still, you may learn a great deal, and before we go in I want to put a thought into your minds which will make astronomy still more interesting. We have seen that the stronger our telescopes the more stars, star-clusters, and nebulæ we see, and we cannot doubt that there are still countless heavenly bodies quite unknown to us. Some years ago Bessel the astronomer found that Sirius, in its real motion through the heavens, moves irregularly, travelling sometimes a little more slowly than at other times, and he suggested that some unseen companion must be pulling at him.
Twenty-eight years later, in 1862, two celebratedopticians, father and son, both named Alvan Clark, were trying a new telescope at Chicago University, when suddenly the son, who was looking at Sirius, exclaimed, "Why, father, the star has a companion!" And so it was. The powerful telescope showed what Bessel had foretold, and proved Sirius to be a "binary" star—that is, as we have seen, a star which has another moving round it.
It has since been proved that this companion is twenty-eight times farther from Sirius than we are from our sun, and moves round him in about forty-nine years. It is seven times as heavy as our sun, and yet gives out so little light that only the keenest telescopes can bring it into view.
Now if such a large body as this can give so very faint a light that we can scarcely see it, though Sirius, which is close to it, shines brightest of any star in the heavens, how many more bodies must there be which we shall never see, even among those which give out light, while how many there are dark like our earth, who can tell?
Now that we know each of the stars to be a brilliant sun, many of them far, far brighter than ours, yet so like in their nature and laws, we can scarcely help speculating whether round these glorious suns, worlds of some kind may not be moving. If so, and there are people in them, what a strange effect those double coloured suns must produce with red daylight one day and blue daylight another!
Surely, as we look up at the myriads of stars bespangling the sky, and remember that our star-sun has seven planets moving round it of which one atleast—our own earth—is full of living beings, we must picture these glorious suns as the centres of unseen systems, so that those twinkling specks become as suggestive as the faint lights of a great fleet far out at sea, which tell us of mighty ships, together with frigates and gunboats, full of living beings, though we cannot see them, nor even guess what they may be like. How insignificant we feel when we look upon that starlit sky and remember that the whole of our solar system would be but a tiny speck of light if seen as far off as we see the stars! If our little earth and our short life upon it were all we could boast of we should be mites indeed.
But our very study to-night lifts us above these and reminds us that there is a spirit within us which even now can travel beyond the narrow bounds of our globe, measure the vast distances between us and the stars, gauge their brightness, estimate their weight, and discern their movements. As we gaze into the depths of the starlit sky, and travel onwards and onwards in imagination to those distant stars which photography alone reveals to us, do not our hearts leap at the thought of a day which must surely come when, fettered and bound no longer to earth, this spirit shall wander forth and penetrate some of the mystery of those mighty suns at which we now gaze in silent awe.
[1]Reproduced in the Frontispiece with Mr. Roberts's kind permission. The star-halo at the top of the plate is caused by diffraction of light in the telescope, and comes only from an ordinary star.
[1]Reproduced in the Frontispiece with Mr. Roberts's kind permission. The star-halo at the top of the plate is caused by diffraction of light in the telescope, and comes only from an ordinary star.
[2]The Story of the Heavens.
[2]The Story of the Heavens.
[3]In Fig. 54 the sickle alone comes within the picture.
[3]In Fig. 54 the sickle alone comes within the picture.
[4]For Almach see Fig. 58, it has been accidentally omitted from this figure.
[4]For Almach see Fig. 58, it has been accidentally omitted from this figure.
[5]The plate of coloured stars has been most kindly drawn to scale and coloured for me by Mr. Arthur Cottam, F.R.A.S.
[5]The plate of coloured stars has been most kindly drawn to scale and coloured for me by Mr. Arthur Cottam, F.R.A.S.
[6]See No. 1 in Table of Spectra, Plate I.
[6]See No. 1 in Table of Spectra, Plate I.
ornate capital i
n our last lecture we soared far away into boundless space, and lost ourselves for a time among seen and unseen suns. In this lecture we will come back not merely to our little world, nor even to one of the widespread oceans which cover so much of it, but to one single pool lying just above the limits of low tide, so that it is only uncovered for a very short time every day. This pool is to be found in a secluded bay within an hour's journey by train from this college, and only a few miles from Torquay. It has no name, so far as I know, nor do many people visit it, otherwise I should not have kept my little pool so long undisturbed. As it is, however, for many years past I have had only to make sure as to the time of low tide, and put myself in the train; and then, unless the sea was very rough and stormy, I couldexamine the little inhabitants of my miniature ocean in peace.
The pool lies in a deep hollow among a group of rocks and boulders, close to the entrance of the cove, which can only be entered at low water; it does not measure more than two feet across, so that you can step over it, if you take care not to slip on the masses of green and brown seaweed growing over the rocks on its sides, as I have done many a time when collecting specimens for our salt-water aquarium. I find now the only way is to lie flat down on the rock, so that my hands and eyes are free to observe and handle, and then, bringing my eye down to the edge of the pool, to lift the seaweeds and let the sunlight enter into the chinks and crannies. In this way I can catch sight of many a small being either on the seaweed or the rocky ledges, and even creatures transparent as glass become visible by the thin outline gleaming in the sunlight. Then I pluck a piece of seaweed, or chip off a fragment of rock with a sharp-edged collecting knife, bringing away the specimen uninjured upon it, and place it carefully in its own separate bottle to be carried home alive and well.
Now though this little pool and I are old friends, I find new treasures in it almost every time I go, for it is almost as full of living things as the heavens are of stars, and the tide as it comes and goes brings many a mother there to find a safe home for her little ones, and many a waif and stray to seek shelter from the troublous life of the open ocean.
You will perhaps find it difficult to believe thatin this rock-bound basin there can be millions of living creatures hidden away among the fine feathery weeds; yet so it is. Not that they are always the same. At one time it may be the home of myriads of infant crabs, not an eighth of an inch long, at another of baby sea-urchins only visible to the naked eye as minute spots in the water, at another of young jelly-fish growing on their tiny stalks, and splitting off one by one as transparent bells to float away with the rising tide. Or it may be that the whelk has chosen this quiet nook to deposit her leathery eggs; or young barnacles, periwinkles, and limpets are growing up among the green and brown tangles, while the far-sailing velella and the stay-at-home sea-squirts, together with a variety of other sea-animals, find a nursery and shelter in their youth in this quiet harbour of rest.
And besides these casual visitors there are numberless creatures which have lived and multiplied there, ever since I first visited the pool. Tender red, olive-coloured, and green seaweeds, stony corallines, and acorn-barnacles lining the floor, sea-anemones clinging to the sides, sponges tiny and many-coloured hiding under the ledges, and limpets and mussels wedged in the cracks. These can be easily seen with the naked eye, but they are not the most numerous inhabitants; for these we must search with a magnifying-glass, which will reveal to us wonderful fairy-forms, delicate crystal vases with tiny creatures in them whose transparent lashes make whirlpools in the water, living crystal bells so tiny that whole branches of them look only like a fringe of hair, jelly globesrising and falling in the water, patches of living jelly clinging to the rocky sides of the pool, and a hundred other forms, some so minute that you must examine the fine sand in which they lie under a powerful microscope before you can even guess that they are there.
Fig. 63.Fig. 63. Group of seaweeds (natural size). 1, Ulva Linza. 2, Sphacelaria filicina. 3, Polysiphonia urceolata. 4, Corallina officinalis.Group of seaweeds (natural size).1,Ulva Linza.2,Sphacelaria filicina.3,Polysiphonia urceolata.4,Corallina officinalis.
1,Ulva Linza.2,Sphacelaria filicina.3,Polysiphonia urceolata.4,Corallina officinalis.
So it has proved a rich hunting-ground, where summer and winter, spring and autumn, I find some form to put under my magic glass. There I can watch it for weeks growing and multiplying under my care; moved only from the aquarium, where I keep it supplied with healthy sea-water, to the tiny transparent trough in which I place it for a few hours to see the changes it has undergone. I could tell you endless tales of transformations in thesetiny lives, but I want to-day to show you a few of my friends, most of which I brought yesterday fresh from the pool, and have prepared for you to examine.
Fig. 64.Fig. 64. Ulva lactuca, a green seaweed, greatly magnified to show structure. (After Oersted.) s, Spores in the cells. ss, Spores swimming out. h, Holes through which spores have escaped.Ulva lactuca, a green seaweed, greatly magnified to show structure. (After Oersted.)s, Spores in the cells.ss, Spores swimming out.h, Holes through which spores have escaped.
s, Spores in the cells.ss, Spores swimming out.h, Holes through which spores have escaped.
Let us begin with seaweeds. I have said that there are three leading colours in my pool—green, olive, and red—and these tints mark roughly three kinds of weed, though they occur in an endless variety of shapes. Here is a piece of the beautiful pale green seaweed, called the Laver or Sea-lettuce,Ulva Linza(1, Fig. 63), which grows in long ribbons in a sunny nook in the water. I have placed under the first microscope a piece of this weed which is just sending out young seaweeds in the shape of tiny cells, with lashes very like those we saw coming from the moss-flower, and I have pressed them in the position in which they would naturally leave the plant (ss, Fig. 64.).[1]You will also see on this slide several cells in which these tiny sporessare forming, ready to burst out and swim; for this green weed is merely a collection of cells, like the single-celled plants on land. Each cell canwork as a separate plant; it feeds, grows, and can send out its own young spores.
This deep olive-green feathery weed (2, Fig. 63), of which a piece is magnified under the next microscope (2, Fig. 65), is very different. It is a higher plant, and works harder for its living, using the darker rays of sunlight which penetrate into shady parts of the pool. So it comes to pass that its cells divide the work. Those of the feathery threads make the food, while others, growing on short stalks on the shafts of the feather make and send out the young spores.
Fig. 65.Fig. 65. Three seaweeds of Fig. 63 much magnified to show fruits. (Harvey.) 2, Sphacelaria filicina. 3, Polysiphonia urceolata. 4, Corallina officinalis.Three seaweeds of Fig. 63 much magnified to show fruits. (Harvey.)2,Sphacelaria filicina.3,Polysiphonia urceolata.4,Corallina officinalis.
2,Sphacelaria filicina.3,Polysiphonia urceolata.4,Corallina officinalis.
Lastly, the lovely red threadlike weeds, such as thisPolysiphonia urceolata(3, Fig. 63), carry actual urns on their stems like those of mosses. In fact, the history of these urns (see No. 3, Fig. 65) is much the same in the two classes of plants, only that instead of the urn being pushed up on a thin stalk as in the moss, it remains on the seaweed close down to the stem, when it grows out of the plant-egg, and the tinyplant is shut in till the spores are ready to swim out.
The stony corallines (4, Figs. 63 and 65), which build so much carbonate of lime into their stems, are near relations of the red seaweeds. There are plenty of them in my pool. Some of them, of a deep purple colour, grow upright in stiff groups about three or four inches high; and others, which form crusts over the stones and weeds, are a pale rose colour; but both kinds, when the plant dies, leaving the stony skeleton (1, Fig. 66), are a pure white, and used to be mistaken for corals. They belong to the same order of plants as the red weeds, which all live in shady nooks in the pools, and are the highest of their race.
My pool is full of different forms of these four weeds. The green ribbons float on the surface rooted to the sides of the pool and, as the sun shines upon it, the glittering bubbles rising from them show that they are working up food out of the air in the water, and giving off oxygen. The brown weeds lie chiefly under the shelves of rocks, for they can manage with less sunlight, and use the darker rays which pass by the green weeds; and last of all, the red weeds and corallines, small and delicate in form, line the bottom of the pool in its darkest nooks.
And now if I hand round two specimens—one a coralline, and the other something you do not yet know—I am sure you will say at first sight that they belong to the same family, and, in fact, if you buy at the seaside a group of seaweeds gummed on paper, you will most likely get both these amongthem. Yet the truth is, that while the coralline (1, Fig. 66) is a plant, the other specimen (2) which is calledSertularia filicula, is an animal.
Fig. 66.Fig. 66. Coralline and Sertularia, to show likeness between the animal Sertularia and the plant Coralline. 1, Corallina officinalis. 2, Sertularia filicula.Coralline and Sertularia, to show likeness between the animal Sertularia and the plant Coralline.1,Corallina officinalis.2,Sertularia filicula.
1,Corallina officinalis.2,Sertularia filicula.
This special sertularian grows upright in my pool on stones or often on seaweeds, but I have here (Fig. 67) another and much smaller one which lives literally in millions hanging its cups downwards. I find it not only under the narrow ledges of the pool sheltered by the seaweed, but forming a fringe along all the rocks on each side of the cove near to low-water mark, and for a long time I passed it by thinking it was of no interest. But I have long since given up thinking this of anything, especially in my pool, for my magic glass has taught me that there is not even a living speck which does not open out into something marvellous and beautiful. So I chipped off a small piece of rock and brought the fringe home, and found, when I hung it up in clear sea water as I have done over this glass trough (Fig. 67) and looked at it through the lens, that each thread of the dense fringe, in itself not a quarter of an inch deep, turns out to be a tiny sertularian with at least twenty mouths. You can see this with your pocketlens even as it hangs here, and when you have examined it you can by and by take off one thread and put it carefully in the trough. I promise you a sight of the most beautiful little beings which exist in nature.
Fig. 67.Fig. 67. Sertularia tenella, hanging from a splint of rock over a water trough. Also piece enlarged to show the animal protruding.Sertularia tenella, hanging from a splint of rock over a water trough. Also piece enlarged to show the animal protruding.
Sertularia tenella, hanging from a splint of rock over a water trough. Also piece enlarged to show the animal protruding.
Come and look at it after the lecture. It is a horny branched stem with a double row of tiny cups all along each side (see Fig. 67). Out of these cups there appear from time to time sixteen minute transparent tentacles as fine as spun glass, which wave about in the water. If you shake the glass a little, in an instant each crystal star vanishes into its cup, to come out again a few minutes later; so that now here, now there, the delicate animal-flowers spread out on each side of the stem, and the tree is covered with moving beings. These tentacles are feelers, which lash food into a mouth and stomach in each cup, where it is digested and passed, through a hole in the bottom, along a jelly thread which runs down the stem and joins all the mouths together. In this way the food is distributed all over the tree, which is, in fact, one animal with many feeding-cups. Some day I will show you one of these cups with the tentacles stretched out and mounted on a slide, so that you can examine a tentacle with a very strong magnifying power. You will then see that it isdotted over with cells, in which are coiled fine threads. The animal uses these threads to paralyse the creatures on which it feeds, for at the base of each thread there is a poison gland.
In the larger Sertularia (2, Fig. 66) the whole branched tree is connected by jelly threads running through the stem, and all the thousands of mouths are spread out in the water. One large form called the sea-firSertularia cupressinagrows sometimes three feet high, and bears as many as a hundred thousand cups, with living mouths, on its branches.
The next of my minute friends I can only show to the class in a diagram, but you will see it under the fourth microscope by and by. I had great trouble in finding it yesterday, though I knew its haunts upon the green weed, for it is so minute and transparent that even when the weed is in a trough a magnifying-glass will scarcely detect it. And I must warn you that if you want to know any of the minute creatures we are studying, you must visit one place constantly. You may in a casual way find many of them on seaweed, or in the damp ooze and mud, but it will be by chance only; to look for them with any certainty you must take trouble in making their acquaintance.
Turning then to the diagram (Fig. 68) I will describe it as I hope you will see it under the microscope—a curious tiny, perfectly transparent open-mouthed vase standing upright on the weed, and having an equally transparent being rising up in it and waving its tiny lashes in the water. This is really all one animal, the vasehcbeing the hornycovering or carapace of the body, which last stands up like a tube in the centre. If you watch carefully, you may even see the minute atoms of food twisting round inside the tube until they are digested, after they have been swept in at the wide open mouth by the whirling lashes. You will see this more clearly if you put a little rice-flour, very minutely powdered and coloured by carmine, into the water; for you can trace these red atoms into some round spaces calledvacuoleswhich are dotted over the body of the animal, and are really globules of watery fluid in which the food is probably partly digested.
Fig. 68.Fig. 68. Thuricolla folliculata and Chilomonas amygdalum. (Saville Kent.) 1, Thuricolla erect; 2, retracted; 3, dividing. 4, Chilomonas amygdalum. hc, Horny carapace. cv, Contractile vesicle. v, Closing valves.Thuricolla folliculataandChilomonas amygdalum. (Saville Kent.)1, Thuricolla erect; 2, retracted; 3, dividing. 4,Chilomonas amygdalum.hc, Horny carapace.cv, Contractile vesicle.v, Closing valves.
1, Thuricolla erect; 2, retracted; 3, dividing. 4,Chilomonas amygdalum.hc, Horny carapace.cv, Contractile vesicle.v, Closing valves.
You will notice, however, one round clear space (cv) into which they do not go, and after a time you will be able to observe that this round spot closes up or contracts very quickly, and then expands again very slowly. As it expands it fills with a clear fluid, and naturalists have not yet decided exactly what work it does. It may serve the animal either for breathing, or as a very simple heart, making the fluids circulate in the tube. The next interesting pointabout this little being is the way it retreats into its sheltering vase. Even while you are watching, it is quite likely it may all at once draw itself down to the bottom as in No. 2, and folding down the valvesv,vof horny teeth which grow on each side, shut itself in from some fancied danger. Another very curious point is that, besides sending forth young ones, these creatures multiply by dividing in two (see No. 3, Fig. 68), each one closing its own part of the vase into a new home.
There are hundreds of theseInfusoria, as they are called, in my pond, some with vases, some without, some fixed to weeds and stones, others swimming about freely. Even in the water-trough in which this Thuricolla stands, I saw several smaller forms, and the next microscope has a trough filled with the minutest form of all, called a Monad (No. 4, Fig. 68). These are so small that 2000 of them would lie side by side in an inch; that is, if you could make them lie at all, for they are the most restless little beings, darting hither and thither, scarcely even halting except to turn back. And yet though there are so many of them, and as far as we know they have no organs of sight, they never run up against each other, but glide past more cleverly than any clear-sighted fish. These creatures are mostly to be found among decaying seaweed, and though they are so tiny, you can still see distinctly the clear space (cv) contracting and expanding within them.
But if there are so many thousands of mouths to feed, on the tree-likeSertulariæas well as in all theseInfusoria, where does the food come from?
Partly from the numerous atoms of decaying lifeall around, and the minute eggs of animals and spores of plants; but besides these, the pool is full of minute living plants—small jelly masses with solid coats of flint which are moulded into most lovely shapes. Plants formed of jelly and flint! You will think I am joking, but I am not. These plants, calledDiatoms, which live both in salt and fresh water, are single cells feeding and growing just like those we took from the water-butt (Fig. 29, p. 78), only that instead of a soft covering they build up a flinty skeleton. They are so small, that many of them must be magnified to fifty times their real size before you can even see them distinctly. Yet the skeletons of these almost invisible plants are carved and chiselled in the most delicate patterns. I showed you a group of these in our lecture on magic glasses (p. 39), and now I have brought a few living ones that we may learn to know them. The diagram (Fig. 69) shows the chief forms you will see on the different slides.
Fig. 69.Fig. 69. Living diatoms. a, Cocconema lanceolatum. b, Bacillaria paradoxa. c, Gomphonema marinum. d, Diatoma hyalina.Living diatoms.a, Cocconema lanceolatum.b, Bacillaria paradoxa.c, Gomphonema marinum.d, Diatoma hyalina.
a, Cocconema lanceolatum.b, Bacillaria paradoxa.c, Gomphonema marinum.d, Diatoma hyalina.
The first one,Bacillaria paradoxa(b, Fig. 69), looks like a number of rods clinging one to another in a string, but each one of these is a single-celled plant with a jelly cell surrounding the flinty skeleton. You will see that they move to and fro over each other in the water.
Fig. 70.Fig. 70. A diatom (Diatoma vulgare) growing. a, b, Flint skeleton inside the jelly-cell. a, c and d, b, Two flint skeletons formed by new valves, c and d, forming within the first skeleton.A diatom (Diatoma vulgare) growing.a,b, Flint skeleton inside the jelly-cell.a,candd,b, Two flint skeletons formed by new valves,candd, forming within the first skeleton.
a,b, Flint skeleton inside the jelly-cell.a,candd,b, Two flint skeletons formed by new valves,candd, forming within the first skeleton.
The next two forms,aandc, look much more like plants, for the cells arrange themselves on a jelly stem, which by and by disappears, leaving only the separate flint skeletons such as you see in Fig. 16. The last form,d, is something midway between the other forms, the separate cells hang on to each other and also on to a straight jelly stem.
Another species of Diatoma (Fig. 70) calledDiatoma vulgare, is a very simple and common form, and will help to explain how these plants grow. The two flinty valvesa,binside the cell are not quite the same size; the older oneais larger than the younger oneband fits over it like the cover of a pill-box. As the plant grows, the cell enlarges and forms two more valves, onecfitting into the covera, so as to make a complete boxac, and a second,d, back to back withc, fitting into the valveb, and making another complete boxbd. This goes on very rapidly, and in this plant each new cell separates as it is formed, and thefree diatoms move about quite actively in the water.
If you consider for a moment, you will see that, as the new valves always fit into the old ones, each must be smaller than the last, and so there comes a time when the valves have become too small to go on increasing. Then the plant must begin afresh. So the two halves of the last cell open, and throwing out their flinty skeletons, cover themselves with a thin jelly layer, and form a new cell which grows larger than any of the old ones. These, which are spore-cells, then form flinty valves inside, and the whole thing begins again.
Now though the plants themselves die, or become the food of minute animals, the flinty skeletons are not destroyed, but go on accumulating in the waters of ponds, lakes, rivers, and seas, all over the world. Untold millions have no doubt crumbled to dust and gone back into the waters, but untold millions also have survived. The towns of Berlin in Europe and of Richmond in the United States are actually built upon ground called "infusorial earth," composed almost entirely of valves of these minute diatoms which have accumulated to a thickness of more than eighty feet! Those under Berlin are fresh-water forms, and must have lived in a lake, while those of Richmond belong to salt-water forms. Every inch of the ground under those cities represents thousands and thousands of living plants which flourished in ages long gone by, and were no larger than those you will see presently under the microscope.
These are a very few of the microscopic inhabitantsof my pond, but, as you will confuse them if I show you too many, we will conclude with two rather larger specimens, and examine them carefully. The first, called the Cydippe, is a lovely, transparent living ball, which I want to explain to you because it is so wondrously beautiful. The second, the Sea-mat or Flustra, looks like a crumpled drab-coloured seaweed, but is really composed of many thousands of grottos, the homes of tiny sea-animals.
Fig. 71.Fig. 71. Cydippe Pileus. 1, Animal with tentacles t, bearing small tendrils t´. 2, Body of animal enlarged. m, Mouth. c, Digestive cavity. s, Sac into which the tentacles are withdrawn. p, Bands with comb-like plates. 3, Portion of a band enlarged to show the moving plates p.Cydippe Pileus.1, Animal with tentaclest, bearing small tendrilst´. 2, Body of animal enlarged.m, Mouth.c, Digestive cavity.s, Sac into which the tentacles are withdrawn.p, Bands with comb-like plates. 3, Portion of a band enlarged to show the moving platesp.
1, Animal with tentaclest, bearing small tendrilst´. 2, Body of animal enlarged.m, Mouth.c, Digestive cavity.s, Sac into which the tentacles are withdrawn.p, Bands with comb-like plates. 3, Portion of a band enlarged to show the moving platesp.
Let us take the Cydippe first (1, Fig. 71). I have six here, each in a separate tumbler, and could have brought many more, for when I dipped my net in the pool yesterday such numbers were caught in it that I believe the retreating tide must just have left a shoal behind. Put a tumbler on the desk in front of you, and if the light falls well upon it you will see atransparent ball about the size of a large pea marked with eight bright bands, which begin at the lower end of the ball and reach nearly to the top, dividing the outside into sections like the ribs of a melon. The creature is so perfectly transparent that you can count all the eight bands.
At the top of the ball is a slight bulge which is the mouth (m2, Fig. 71), and from it, inside the ball, hangs a long bag or stomach, which opens below into a cavity c, from which two canals branch out, one on each side, and these divide again into four canals which go one into each of the tubes running down the bands. From this cavity the food, which is digested in the stomach, is carried by the canals all over the body. The smaller tubes which branch out of these canals cannot be seen clearly without a very strong lens, and the only other parts you can discern in this transparent ball are two long sacs on each side of the lower end. These are the tentacle sacs, in which are coiled up the tentacles, which we shall describe presently. Lastly, you can notice that the bands outside the globe are broader in the middle than at the ends, and are striped across by a number of ridges.
In moving the tumblers the water has naturally been shaken, and the creature being alarmed will probably at first remain motionless. But very soon it will begin to play in the water, rising and falling, and swimming gracefully from side to side. Now you will notice a curious effect, for the bands will glitter and become tinged with prismatic colours, till, as it moves more and more rapidly these colours,reflected in the jelly, seem to tinge the whole ball with colours like those on a soap-bubble, while from the two sacs below come forth two long transparent threads like spun glass. At first these appear to be simple threads, but as they gradually open out to about four or five inches, smaller threads uncoil on each side of the line till there are about fifty on each line. These shorttendrilsare never still for long; as the main threads wave to and fro, some of the shorter ones coil up and hang like tiny beads, then these uncoil and others roll up, so that these graceful floating lines are never two seconds alike.
We do not really know their use. Sometimes the creature anchors itself by them, rising and falling as they stretch out or coil up; but more often they float idly behind it in the water. At first you would perhaps think that they served to drive the ball through the water, but this is done by a special apparatus. The cross ridges which we noticed on the bands are really flat comb-like plates (p, Fig. 71), of which there are about twenty or thirty on each band; and these vibrate very rapidly, so that two hundred or more paddles drive the tiny ball through the water. This is the cause of the prismatic colours; for iridescent tints are produced by the play of light upon the glittering plates, as they incessantly change their angle. Sometimes they move all at once, sometimes only a few at a time, and it is evident the creature controls them at will.
This lovely fairy-like globe, with its long floating tentacles and rainbow tints, was for a long time classed with the jelly-fish; but it really is most nearlyrelated to sea-anemones, as it has a true central cavity which acts as a stomach, and many other points in common with theActinozoa. We cannot help wondering, as the little being glides hither and thither, whether it can see where it is going. It has nerves of a low kind which start from a little dark spot (ng), exactly at the south pole of the ball, and at that point a sense-organ of some kind exists, but what impression the creature gains from it of the world outside we cannot tell.
I am afraid you may think it dull to turn from such a beautiful being as this, to the grey leaf which looks only like a dead dry seaweed; yet you will be wrong, for a more wonderful history attaches to this crumpled dead-looking leaf than to the lovely jelly-globe.
First of all I will pass round pieces of the dry leaf (r, Fig. 72), and while you are getting them I will tell you where I found the living ones. Great masses of the Flustra, as it is called, line the bottom and sides of my pool. They grow in tufts, standing upright on the rock, and looking exactly like hard grey seaweeds, while there is nothing to lead you to suspect that they are anything else. Yesterday I chipped off very carefully a piece of rock with a tuft upon it, and have kept it since in a glass globe by itself with sea-water, for the little creatures living in this marine city require a very good supply of healthy water and air. I have called it a "marine city," and now I will tell you why. Take the piece in your hand and run your finger gently up and down it; you will glide quite comfortably from the lower tothe higher part of the leaf, but when you come back you will feel your finger catch slightly on a rough surface. Your pocket lens will show why this is, for if you look through it at the surface of the leaf you will see it is not smooth, but composed of hundreds of tiny alcoves with arched tops; and on each side of these tops stand two short blunt spines (see 2, Fig. 72), making four in all, pointing upwards, so as partly to cover the alcove above. As your finger went up it glided over the spines, but on coming back it met their points. This is all you can see in the dead specimen; I must show you the rest by diagrams, and by and by under the microscope.
Fig. 72.Fig. 72. The Sea-mat or Flustra (Flustra foliacea.) 1, Natural size. 2, Much magnified. s, Slit caused by drawing in of the animal a.The Sea-mat or Flustra (Flustra foliacea.)1, Natural size. 2, Much magnified.s, Slit caused by drawing in of the animala.
1, Natural size. 2, Much magnified.s, Slit caused by drawing in of the animala.
First, then, in the living specimen which I have here, those alcoves are not open as in the dead piece, but covered over with a transparent skin, in which, near the top of the alcove just where the curve begins, is a slit (s2, Fig. 72). Unfortunately the membrane covering this alcove is too dense for you to distinguish the parts within. Presently, however, if you are watching a piece of this living leaf in a flat water-cell under the microscope, you will see the slit slowly open, and begin to turn as it were inside out, exactlylike the finger of a glove, which has been pushed in at the tip, gradually rises up when you put your finger inside it. As this goes on, a bundle of threads appears, at first closed like a bud, but gradually opening out into a crown of tentacles (a, Fig. 72), each one clothed with hairs. Then you will see that the slit was not exactly a slit after all, but the round edge where the sac was pushed in. Ah! you will say, you are now showing me a polyp like those on the sertularian tree. Not so fast, my friend; you have not yet studied what is still under the covering skin and hidden in the living animal. I have, however, prepared a slide with this membrane removed (see Fig. 73), and there you can observe the different parts, and learn that each one of these alcoves contains a complete animal, and not merely one among many mouths, like the polyp on the Sertularia.
Fig. 73.Fig. 73. Diagram of the animal in the Flustra or Sea-mat. 1, Animal protruding. 2, Animal retracted in the sheath. sh, Covering sheath. s, Slit. t, Tentacles. m, Mouth. th, Throat. st, Stomach. i, Intestine. r, Retractor muscle. e, Egg-forming parts. g, Nerve-ganglion.Diagram of the animal in the Flustra or Sea-mat.1, Animal protruding. 2, Animal retracted in the sheath.sh, Covering sheath.s, Slit.t, Tentacles.m, Mouth.th, Throat.st, Stomach.i, Intestine.r, Retractor muscle.e, Egg-forming parts.g, Nerve-ganglion.
1, Animal protruding. 2, Animal retracted in the sheath.sh, Covering sheath.s, Slit.t, Tentacles.m, Mouth.th, Throat.st, Stomach.i, Intestine.r, Retractor muscle.e, Egg-forming parts.g, Nerve-ganglion.
Each of these little beings (a, Fig 72) living in its alcove has a mouth, throat, stomach, intestine, muscles, and nerves starting from the ganglion of nervous matter, besides all that is necessary for producing eggs and sendingforth young ones. You can trace all these under the microscope (see 2, Fig. 73) as the creature lies curiously doubled up in its bed, with its body bent in a loop; the intestinei, out of which the refuse food passes, coming back close up to the slit. When it is at rest, the top of the sac in which it lies is pulled in by the retractor muscler, and looks, as I have said, like the finger of a glove with the top pushed in. When it wishes to feed, this top is drawn out by muscles running round the sac, and the tentacles open and wave in the water (1, Fig. 73).
Look now at the alcoves, the homes of these animals; see how tiny they are and how closely they fit together. Mr. Gosse, the naturalist, has reckoned that there are 6720 alcoves in a square inch; then if you turn the leaf over you will see that there is another set, fixed back to back with these, on the other side, making in all 13,440 alcoves. Now a moderate-sized leaf of flustra measures about three square inches, taking all the rounded lobes into account, so you will see we get 40,320 as a rough estimate of the number of beings on this one leaf. But if you look at this tuft I have brought, you will find it is composed of twelve such leaves, and this after all is a very small part of the mass growing round my pool. Was I wrong, then, when I said that my miniature ocean contains as many millions of beings as there are stars in the heavens?
You will want to know how these leaves grew, and it is in this way. First a little free swimming animal, a mere living sac provided with lashes, settles down and grows into one little horny alcove, with its livecreature inside, which in time sends off from it three to five buds, forming alcoves all round the top and sides of the first one, growing on to it. These again bud out, and you can thus easily understand that, in this way, in time a good-sized leaf is formed. Meanwhile the creatures also send forth new swimming cells, which settle down near to begin new leaves, and thus a tuft is formed; and long after the beings in earlier parts of the leaf have died and left their alcoves empty, those round the margin are still alive and spreading.
With this history we must stop for to-day, and I expect it will be many weeks before you have thoroughly examined the specimens of each kind which I have put in the aquarium. If you can trace the spore-cells and urns in the seaweeds, observe the polyps in the Sertularia, and count the number of mouths on a branch of my animal fringe (Sertularia tenella); if you make acquaintance with the Thuricolla in its vase, and are fortunate enough to see one divide in two; if you learn to know some of the beautiful forms of diatoms, and can picture to yourselves the life of the tiny inhabitants of the Flustra; then you will have used your microscope with some effect, and be prepared for an expedition to my pool, where we will go together some day to seek new treasures.