_Examination._--The Infusoria must be examined during life; for they are so altered by preservative liquids that they cannot be well preserved. The shells of those that are provided with them may be kept simply dried upon a slide, and in this way a few will retain their form, and the cilia of all may be more easily distinguished; the vacuoles may also then be seen very distinctly. When they are confined in a small quantity of water and are about to die, a curious phenomenon may be observed in them, a number of oil-like sarcodic globules exuding from the body, and within these, vacuoles may often be seen to form spontaneously. The Infusoria may be collected in small phials; but it is difficult to keep them, as they form the food of the Entomostraca, the Rotatoria, and the larvæ of insects; so that their enemies are very numerous, and they soon disappear. * * * * * CLASSIFICATION.--Before leaving the subject of living bodies, it may be well to make a few remarks upon their systematic relation as defined by classification. All natural bodies are referable to one of three great kingdoms, viz. the Animal, the Vegetable, or the Mineral Kingdom. The bodies belonging to the latter seldom come under the notice of the microscopic observer, as they are mostly visible to the naked eye, and their minute structure is the same as that of the larger masses. The general structure of the members of the vegetable and animal kingdoms has been illustrated in the preceding pages. These bodies are distinguished from those of the mineral kingdom by their vital power of appropriating surrounding matters to their own nutrition and growth--this power being exercised by their organs, or, in the lowest forms, by any portion of their substance. Hence animals and vegetables or plants are termed organic bodies, while minerals are termed inorganic bodies, as having no organs; and the material of which organic bodies consist is termed organic matter, that of minerals being inorganic matter. But in both animals and plants inorganic matter is mixed with the organic matter, having been taken up or absorbed from the inorganic kingdom, although it does not usually exist in its characteristic condition, which is that of crystals, _i. e._ angular solids, as crystals of Epsom salts, &c. The individual members of the animal and vegetable kingdoms are systematically divided into certain groups, and these into successively smaller groups until we arrive at the species. These groups are founded upon the possession of certain points of resemblance by their members, forming the distinguishing characters, and their names are significant and definite. They usually run as follows, the larger and higher groups standing first in order:--Kingdoms, Subkingdoms, Classes, Orders, Families, Tribes, Genera, and Species. But it must be observed that the term species has a different value from that of the other terms; for the