further examination, it was discovered that there existed animalculæ in stagnant pools, in rivers, and likewise in the sea, of a more perfect structure; and Professor Ehrenberg ascertained that Infusoria, which previously were scarcely considered as organized beings, had an internal structure resembling that of higher animals. Some colouring matter, as carmine, was thrown into water which contained infusoria, and it was found that they swallowed it and conveyed it into internal cavities or stomachs. These it has now been found exist in large numbers in almost every known genus, and some single animalcules possess as many as two hundred stomachs, from which circumstance they received the name of Polygastrica. Professor Grant asserts that nearly five hundred millions are contained in a single drop of water, that is, as many as there are individuals of our own race on the earth; and it is most surprising, that though so minute, the majority possess eyes, have an acute sense of taste, the power of distinguishing, and that they pursue and seize their prey, and in spite of their great number in a single drop of water, they avoid coming in contact with each other while swimming. These movements are effected by minute hair-like filaments disposed frequently around the mouth. They possess no proper skeleton, but there are parts which give them support. Some of these animals exude on their surface a secretion which agglutinates foreign particles that are floating around them, which serve as a partial covering. The majority of these animalcules possess an alimentary canal with an oral and an anal orifice. Ehrenberg discovered in the greater number of the species nerves, muscles, intestines, teeth, and different kinds of glands resembling in structure those of higher animals. Their power of reproduction is so great, that from one individual a million were produced in ten days; on the eleventh day there were four millions, and on the twelfth sixteen millions. They serve as food to the higher classes, especially of Zoophytes. They themselves are the most voracious of all living beings.

"We are more perplexed," says Professor Buckland, "in attempting to comprehend the organization of the minutest infusoria than that of a whale; and one of the last conclusions at which we arrive is a conviction that the greatest and most important operations of nature are conducted by the agency of atoms too minute to be either perceptible by the human eye, or comprehensible by the human understanding."

A few years since Ehrenberg made the singular and unexpected discovery, that the ashes and pumice in which Pompeii is buried, consists of siliceous cases of microscopic infusoria of freshwater origin, and he found afterwards that several beds of tuff and pumiceous conglomerate near Hochsimmer on the Rhine consisted likewise in a great measure of siliceous cases of infusoria. The Patagonian pumiceous tuffs contain the remains of marine animalcules, which differ in their figure from the

elongated forms of freshwater animalcules. The impalpable dust which is known to fall sometimes out of the atmosphere in the midst of the Atlantic, and has been collected from ships upon which it fell, contains infusoria with siliceous shields, and siliceous tissue of plants. This dust is considered to have been raised by the wind or harmattan, which prevails during certain months in Africa and was carried into the air'.

They have been found at the bottom of the sea in a living state and in fossil forms. When I arrived therefore in Barbados, I did not fail to collect samples of mud and marls for the purpose of examination, which I forwarded to Professor Ehrenberg, and in which he found this new class, to the general description of which I shall devote the following remarks2.

Two parts are to be considered in the structure of the Polycystina, namely, the siliceous shield or cuirass, and the soft internal parts which are inclosed by it. This siliceous shield is not peculiar to the new group: it had been previously observed that the animalcules called Bacillaria possessed a transparent siliceous covering (lorica silicea), which is apparently composed of pure silica. In these organized bodies the shield is divided by longitudinal lines, but in Polycystina it consists of several transverse articulations containing two apertures, and covering in the recent species a gelatinous substance of a brown olive colour. Professor Ehrenberg considers that they possess a distinct animal structure consisting of vessels, but without a heart and pulsation, and are provided with a single tubular intestinal canal. The shield possesses an anterior and a posterior aperture; the former is lattice-like or fenestrate, and the latter is open.

The structure of these animalcules is very peculiar; they differ from the Bacillaria not only in their external appearance, but also in their internal anatomy. The siliceous shell connects them with the Polygastrica, in which the intestinal structure is radiated; but the transverse articulation and the cellular arrangement of their structure point to a connexion with that section which has not a radiated, but a tubular formation of their internal organs, which possess however calcareous and no siliceous shields. Professor Ehrenberg infers from the physiological organization of the whole numerous group a close analogy to the moss-animalcules (Bryozoa) and chiefly to Polythalamia. Nevertheless the character of

1 It is now considered that this dust comes in a great measure from South America. Samples of soil which my brother and myself forwarded to Professor Ehrenberg from Guiana, contained two species of infusoria which he had observed in the atmospheri cal dust, while he did not find any species in the different kinds of dust which he examined, that he knew to belong to Africa.

2 Dr. Davy, Inspector-General of Army Hospitals, observed already in a discourse delivered before the Agricultural Society of Barbados in July 1846, that some of the rocks abound in siliceous skeletons of infusoria.