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A drop of water is a world to the minute beings that inhabit it. We place it under the microscope, and are astonished to fluid that it is a world teeming with life, and that the forms of life are not less beautiful and scarcely less varied than those of the higher beings that dwell in the great world around us, and therein exercise their wonderful instincts.

There are various instincts at work amongst the tiny inhabitants of that little world, though, on account of the apparent simplicity of their structure, we are often inclined to think their actions more the result of mechanical than of instinctive power, perhaps only because our microscopes are not even yet perfect enough to reveal all the complexity which really exists.

I once had the good fortune to observe an incident, which I do not think has ever been recorded before, in which two of the inhabitants of a drop of water—the Rotifer and the Volvox—played a very curious part. Whether what I saw was the result of instinct, as I would fain believe, or whether it was mere caprice or a chance action, I must leave my readers to judge for themselves.

All who read Science Gossip do not profess to be learned naturalists, and they will perhaps like to know what sort of creatures these Rotifers and Volvoces are. The first is undoubtedly an animal, and one of the most active that you will find in a drop of water; darting about here and there, poking his nose into every corner, rolling about like a porpoise, and, Proteus-like, assuming various forms. He is put by naturalists into the lowest division of the animal kingdom,—the division Radiata; and, by some, into the lowest class of that division,—the class Infusoria,^[1] which consists entirely of minute animalcules, some of which are so low in the scale of creation as to have only one organ, and that a stomach; the whole animal, in fact, being a stomach, with an opening at one end for a mouth, and sometimes not even that.

The Infusoria, however, are not all of them of quite so simple a structure as this; and amongst them are a number of species of our friends the Rotifer, so called because around their mouth they have two or more sets of cilia or hairs placed in circles, and which, being in constant motion, have the appearance of revolving wheels. They have the power of stretching themselves out and retracting like worms; and their tail acts as a claw, by which they can anchor themselves to a leaf or anything in the water; and they can likewise draw in their wheels at pleasure. Under the microscope there is seen, within their transparent bodies, a complicated arrangement of digestive organs. Here is a drawing of one of the Rotifers.

The Volvox is a very different object. It is just large enough to be seen with the naked eye, when the water containing it is held up to the light, and appears like a green speck floating in the water; indeed, after some practice in looking for minute object, it is possible to fish a single one out of a wine-glassful of water with a teaspoon. A low power of the microscope is sufficient to reveal one of the most beautiful objects that it is possible to conceive,—a globe of the most delicate green-colour, formed of a transparent membrane, which is marked with a network of fine lines, with darker green spots where the lines cross. Within are seen smaller Volvoces precisely similar to the parent, and sometimes within these, smaller ones still.

The Volvox, by means of a constant rolling motion, moves slowly through the water from place to place. When viewed with a higher power, the green spots are found to be bunches of delicate hairs, and probably the constant motion of these propels the Volvox through the water.

1. ↑ Others place the Rotifers in the class Annelidæ; others again in the class Crustaceæ.