2, _Sphacelaria filicina._ 3, _Polysiphonia urceolata._ 4, _Corallina officinalis._]
Lastly, the lovely red threadlike weeds, such as this _Polysiphonia 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 cla.s.ses 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 tiny plant 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 pa.s.s 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 among them. Yet the truth is, that while the coralline (1, Fig. 66) is a plant, the other specimen (2) which is called _Sertularia filicula_, is an animal.
[Ill.u.s.tration: Fig. 66.
Coralline and Sertularia, to show likeness between the animal Sertularia and the plant Coralline.
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 pa.s.sed 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 gla.s.s 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 gla.s.s 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 pocket lens 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.
[Ill.u.s.tration: Fig. 67.
_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 h.o.r.n.y 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 gla.s.s, which wave about in the water. If you shake the gla.s.s 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 pa.s.sed, 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 is dotted 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-fir _Sertularia cupressina_ grows 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 cla.s.s 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-gla.s.s 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 vase _hc_ being the h.o.r.n.y covering 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 s.p.a.ces called _vacuoles_ which are dotted over the body of the animal, and are really globules of watery fluid in which the food is probably partly digested.
[Ill.u.s.tration: Fig. 68.
_Thuricolla folliculata_ and _Chilomonas amygdalum_. (Saville Kent.)
1, Thuricolla erect; 2, retracted; 3, dividing. 4, _Chilomonas amygdalum_. _hc_, h.o.r.n.y carapace. _cv_, Contractile vesicle. _v_, Closing valves.]
You will notice, however, one round clear s.p.a.ce (_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 point about 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 valves _v_, _v_ of h.o.r.n.y 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 these _Infusoria_, 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 s.p.a.ce (_cv_) contracting and expanding within them.
But if there are so many thousands of mouths to feed, on the tree-like _Sertulariae_ as well as in all these _Infusoria_, where does the food come from?
Partly from the numerous atoms of decaying life all around, and the minute eggs of animals and spores of plants; but besides these, the pool is full of minute living plants--small jelly ma.s.ses 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, called _Diatoms_, which live both in salt and fresh water, are single cells feeding and growing just like those we took from the water-b.u.t.t (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 gla.s.ses (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.
[Ill.u.s.tration: Fig. 69.
Living diatoms.
_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.
[Ill.u.s.tration: 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.]
The next two forms, _a_ and _c_, 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) called _Diatoma vulgare_, is a very simple and common form, and will help to explain how these plants grow.
The two flinty valves _a_, _b_ inside the cell are not quite the same size; the older one _a_ is larger than the younger one _b_ and fits over it like the cover of a pill-box. As the plant grows, the cell enlarges and forms two more valves, one _c_ fitting into the cover _a_, so as to make a complete box _ac_, and a second, _d_, back to back with _c_, fitting into the valve _b_, and making another complete box _bd_. This goes on very rapidly, and in this plant each new cell separates as it is formed, and the free 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 acc.u.mulating 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 acc.u.mulated 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 inhabitants of 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 Fl.u.s.tra, looks like a crumpled drab-coloured seaweed, but is really composed of many thousands of grottos, the homes of tiny sea-animals.
[Ill.u.s.tration: 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_.]
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 a transparent 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 (_m_ 2, Fig.
71), and from it, inside the ball, hangs a long bag or stomach, which opens below into a cavity c, from which two ca.n.a.ls branch out, one on each side, and these divide again into four ca.n.a.ls 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 ca.n.a.ls all over the body. The smaller tubes which branch out of these ca.n.a.ls 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 gla.s.s. 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 short _tendrils_ are 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 cla.s.sed with the jelly-fish; but it really is most nearly related to sea-anemones, as it has a true central cavity which acts as a stomach, and many other points in common with the _Actinozoa_. We cannot help wondering, as the little being glides. .h.i.ther 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 pa.s.s 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 ma.s.ses of the Fl.u.s.tra, 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 gla.s.s 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.