And here I may mention the evident use of these wonderful beings. They appear wherever decaying animal or vegetable substances are found in water, and are extremely useful in destroying what would otherwise taint the air with noxious gasses and smells. Minute algæ also assist in preserving the purity of the water in which they live; they serve as food, also, to animals higher in the scale of creation than themselves. Captain Sir James Ross, in his Antarctic Voyage, speaking of a small fish found by him in the South Seas, and stating by what means it and many others are fed, says, "All are eventually nourished and sustained by the minute infusorial animalcules, which we find filling the ocean with an inconceivable multitude of the minutest forms of organic life." We may infer from this, the immense importance of the Infusoria in the scale of existence, for although only remotely supporting the higher animals, yet the want of them would be greatly felt. Ehrenberg states, that a single drop of water may hold five hundred millions of the smallest animalcules. What, then, can be the population of a lake or of the ocean? I have watched specimens of the genera _Floscularia_, _Vorticella_, and _Stentor_, for hours at a time, and they have never ceased to feed on minute portions of animal and vegetable substances, brought to them by the current they are enabled to make in the water; others eagerly pursue their prey, or feed on the decaying vegetable matter floating about: indeed, the appetite of these little creatures seems insatiable. Many genera have a strong chewing apparatus, like a mouth armed with teeth. All seem employed in the same way, though using different methods. Much decaying matter must thus be taken away by this insatiable, though miniature army, provided for the purpose. They, in their turn, afford sustenance to aquatic insects, which are again preyed on by fishes; and thus food is prepared for more highly organized animals, and lastly for man. Animalcules have never been observed to rest, or at least to sleep; but this may be partly owing to the light necessarily used in viewing them, which forms an artificial sunlight, exciting their powers of motion: they may rest during darkness, when they can not be seen by us. Many are only attracted to the surface of the water by the light of the sun, and are difficult to be obtained on a dull day; they are, however, not much affected by cold or heat, for they are procurable in winter as in summer, though not in such profusion: they are found even under thick ice, and I have frequently broken, in severe frost, the frozen surface of a pond, and, inserting a bottle, have obtained some most interesting kinds. Many of the _Polygastrica_ will bear a great degree of cold, even more so than those of the class _Rotatoria_, whose organization is of a higher order.