Animalculum signifies a little animal, and therefore the term might be applied to every animal which is considerably inferior in size to ourselves. It has been customary, however, to distinguish by the name of animalcula only such animals as are of a size so diminutive, that their true figure cannot be discerned without the assistance of glasses; and more especially it is applied to such as are altogether invisible to the naked eye, and cannot even be perceived to exist, but by the aid of microscopes.
By the help of magnifying glasses we are brought into a kind of new world; and numberless animals are discovered, which, from their minuteness, must otherwise for ever have escaped our observation: and how many kinds of these invisibles there may be, is yet unknown; as they are observed of all sizes, from those which are barely invisible to the naked eye, to such as resist the action of the microscope, as the fixed stars do that of the telescope, and with the best magnifiers hitherto invented, appear only as so many moving points.
The smallest living creatures our instruments can shew, are those that inhabit the waters; for, though possibly animalcula equally minute, or perhaps more so, may fly in the air, or creep upon the earth, it is scarce possible to obtain a view of them; whereas, water being transparent, and confining the creatures within it, we are enabled, by applying a drop of it to our glasses, to discover with ease a great part of its contents, and in a space barely visible to the naked eye, often perceive a thousand little creatures, all full of life and vigour.
By the animalcula infusoria are meant, not the larvæ of those insects which in their first state are inhabitants of water, and afterwards become winged insects, as the gnat, &c. Baker, and many other writers on the subject, have often confounded these, and hence entered into a train of reasoning contrary to fact and experience. The animalcula infusoria take their name from their being found in all kinds either of vegetable or animal infusions; if seeds, herbs, or other vegetable substances, be infused in water, it will soon be filled with an indefinite number of these minute beings. There is a prodigious variety in their forms; some perfectly resemble the bell-polype; others are round or oblong, without any, at least apparent, members; some resemble a bulb with a long taper tail; some are nearly spherical; the greater part are vesicular and transparent. Those most generally found in every drop of ditch water are mere inflated bladders, with a small trace of intestines in the center; the next are a flat kind, with a number of legs under the belly.
Motion seems to be their great delight; they pervade with equal ease and rapidity, and in all forms and directions, the whole dimensions of the drop, in which they find ample space for their various progressions, sometimes darting straight forward, at other times moving obliquely, then again circularly: they know how to avoid with dexterity any obstacles that might obstruct their progress. Hundreds may be seen in a drop of water in constant action, yet never striking against each other. If at any time the clusters prove so thick as to impede any of their motions, they roll and tumble themselves over head, creep under the whole range, force their way through the midst, or wheel round the cluster, with surprizing swiftness; sometimes they will suddenly change the direction in which they are moving, and take one diametrically opposite thereto. By inclining the glass on which the drop of water is laid, it may be made to move in any direction; the animalcula in the drop will swim as easily against the stream as with it.
If the water begin to evaporate, and the drop to grow smaller, they flock impetuously towards the remaining part of the fluid; an anxious desire of attaining this momentary respite of life is very visible, as well as an uncommon agitation of the organs by which they imbibe the water. These motions grow more languid as the water fails, till at last they entirely cease.
Animalcula and insects will support a great degree of cold, but both one and the other perish when it is carried beyond a certain point. The same degree of heat that destroys the existence of insects, is fatal to animalcula; as there are animalcula produced in water at the freezing point, so there are insects which live in snow.
If the smallest drop of urine be put into a drop of water where these animalcula are roving about, apparently happy and easy, they instantly fly to the other side, but the acid soon communicating itself to this part, their struggles to escape are increased, but the evil also increasing, they are thrown into convulsions, and soon expire.
Among animalcula, as in every other part of nature, there is constantly a certain proportion preserved between the size of the individuals and their number. There are always fewest amongst the larger kinds, but they increase in number as they diminish in size, till of the last, or lowest to which our powers of magnifying will reach, there are myriads to one of the larger. Like other animals, they increase in size from their birth till they have attained their full growth. When deprived of food, they grow thin and perish; and different degrees of organization are to be discovered in their structure.
The birth and propagation of these microscopic beings is as regular as that of the largest animals of our globe; for though their extreme minuteness prevents us, in most cases, from seeing the germ from which they spring, yet we are well assured, from numerous observations, that the manner in which they multiply is regulated by constant and invariable laws.