These minim animals may be said almost to be universally dispersed; they inhabit the sea, the rivers, and other waters; are supposed to float in the air; they are found in the blood and urine; in the tartar of the teeth; in animal substances; in vinegar; in paste; in vegetable substances; in fruits, seeds, and grain; in sand; amongst tiles; in wells; on mountains, &c. Their numbers are infinite; hundreds of thousands may be seen in a single drop of water; their minuteness is extreme, some being not more than 100 part of a line in length, and yet these atoms of animals have a mouth and several stomachs.

Let a man, says Dalyell, the translator of Spallanzani, conceive himself in a moment conveyed to a region where the properties, and the figure and motions of every animal are unknown. The amazing varieties of these will first attract his attention. One is a long slender line; another an eel or serpent; some are circular, elliptical, or triangular; one is a thin flat plate; another like a number of reticulated seeds; several have a long tail, almost invisible; or their posterior part is terminated by two robust horns; one is like a funnel; another like a bell, or cannot be referred to any object familiar to our senses. Certain animalcules can change their figure at pleasure: sometimes they are extended to immo

1 PLATE I. FIG. 3.

derate length, then almost contracted to nothing; sometimes they are curved like a leech, or coiled like a snake; sometimes they are inflated, at others flaccid; some are opaque while others are scarcely visible from their extreme transparence. No less singular is the variety of their motions;-several swim with the velocity of an arrow, so that the eye can scarcely follow them; others appear to drag their body along with difficulty, and move like the leech; and others seem to exist in perpetual rest; one will revolve on its centre, or the anterior part of its head; others move by undulations, leaps, oscillations, or successive gyrations ;-in short, there is no kind of animal motion, or other mode of progression, that is not practised by animalcules.

Their organs are equally various. Some appear to take their food by absorption, having no mouth, to this tribe belong what have been called vinegar eels; others have a mouth and several stomachs, but no orifice for the transmission of their excrements; others, again, have both a mouth and anal passage, and what is wonderful, in such minute creatures, sometimes as many as forty or fifty stomachs; though many are without eyes, others are furnished with these useful organs, some having one, others two, others three, and others four; some

1

have processes resembling legs. In the second Class of these animals, the Rotatories, to which the wheel-animalcules belong, the internal organization approaches to that of the higher classes, for they exhibit the rudiments of a nervous system; their alimentary canal is simple; they have a branching dorsal vessel, but without a systole and diastole; their pharynx is usually furnished with mandibles, which are sometimes armed with teeth. The mouth of the majority, especially amongst the rotatories, is fringed with ray-like bristles, which Cuvier thinks are connected with their respiration. This circumstance of a circle of rays surrounding the oral orifice, is found in the polypes and several other animals of a higher grade. Their use in the present instance, I speak more particularly of the wheel-animalcules, is by their rotation to produce a current in the water to the mouth of the animal, bringing with it the still more minute beings which constitute its food.

These invisible inhabitants of the visible world created an early interest in inquisitive minds; Dr. Henry Power, and after him the celebrated Hooke, about the middle of the seventeenth century, or earlier, noticed, what were called vinegar eels.1 Sir E. King, in the Philosophical Transactions, described some experi

1 Vibrio Anguilla.

ments on the animalcules found in pepper water; and, subsequently, Mr. Harris made observations upon a variety of these minute creatures. The subject was afterwards taken up by various writers, both here and on the continent. Amongst these none was more eminent than Spallanzani. O. F. Müller, who seems to have been the first who treated the subject systematically, embodied these animals in a Class by the name of Infusories. He was followed by Bruguiere and Lamarck, who divided it into Orders and Sections. But the system of these zoologists has for the most part been set aside by Ehrenberg, a Prussian naturalist, before-mentioned, who devoted ten years of his life to the investigation of these animals, for which he was particularly qualified by his previous studies and employment, the anatomy of the Molluscans of the Red Sea, by which he had been accustomed to the use of microscopes and micrometers. His researches on the Infusories, during Baron de Humboldt's last journey, extend to more than fifty degrees of longitude, and fourteen degrees of latitude; he went as far as Dongola in Africa, and the Altai mountains in Asia, and examined these animals in a great variety of situations. He found them on Mount Sinai; swarms of various species in the wells of the Oasis of Jupiter

1 Infusoria.

Ammon; and at a considerable depth in some Siberian mines, in places entirely deprived of light.

He considers them, it should seem, as forming a Sub-kingdom, which he denominates Plantanimals. This sub-kingdom he divides into two Classes. The first, from the number of stomachs, with which the genera belonging to it are furnished, he names, Polygastrica, or many-stomached, probably, to contrast with De Blainville's name before-mentioned. The second class he calls Rotatories,' consisting of the ciliated Polypes of Lamarck; each of these classes he subdivides into two parallel orders, the first containing those that are naked, and the second those that are loricated, or covered with some kind of shell.

In the first of these classes, the Polygastrics, the animals recede further from the organization of the higher tribes, and approach nearer to that of vegetables; but in the second, as I before observed, rudiments of the organization of those tribes make their appearance. Many of the former are known to derive their nutriment from vegetable substances, but what the majority subsist upon is not certainly known;-but the latter class, the Rotatories, are ascertained to be predaceous, as above stated. Their mode of

1 Phyto-zoa.

3 Rotatoria.

5 See Appendix, note 20.

2 PLATE I. FIG. 1.

4 PLATE I. FIG. 2.