Inhabitants Of My Pool

Turning then to the diagram (Fig. 6) 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 horny 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 [pg 375] more clearly if you put a little rice-flour, very minutely powdered and colored by carmine, into the water; for you can trace these red atoms into some round spaces 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.

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 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 w of 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. 6), 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. These are so small that two thousand of them could lie side by side in an inch; [pg 376] 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 contracting and expanding within them.

FIG. 7. LIVING DIATOMS.

a, Cocconema lanceolatum. b, Bacillaria paradoxa. c, Gomphonema marinum. d, Diatoma hyalina.

But if there are so many thousands of mouths to feed, on the tree-like Sertulariæ 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 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, called Diatoms, which live both in salt and fresh water, are single cells feeding and growing just like those we took from the water-butt, only that instead of a soft covering they build up [pg 377] 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, and now I have brought a few living ones that we may learn to know them. The diagram (Fig. 7) shows the chief forms you will see on the different slides.

The first one, Sacillaria paradoxa (b, Fig. 7), 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.

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. 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.

FIG. 8. 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.

Another species of Diatoma (Fig. 8) 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 [pg 378] 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 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.

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