In all parts of the world, and in most waters where aquatic plants in a healthy state abound, these invisible creatures may be met with, and not only in stagnant pools, but in running streams and the broad ocean. Among water-plants these little beings find shelter and food; therefore, when water is brought from these localities, some of the vegetation peculiar to the pool or stream should be procured at the same time. They swarm among duckweed. Many are found also in clear shallow pools, particularly in the spring. When a pond is observed to have a stratum of dust on the surface, or a thin film, it will generally be found almost entirely composed of living creatures. This dust-like appearance consists nearly exclusively of species of the most beautiful colors, such as _Pandorina_, _Gonium_, &c. A shining film of various colors is also occasionally seen on standing water: this is composed of Infusoria; a red appearance being often given to water by some species, and by others a yellowish hue. Sheets of water often assume an intense green, from the presence of many of these minute bodies. Lakes have been known to change their color very mysteriously, and to have caused some alarm in the superstitious; but it is now known to arise from Infusoria, as they are attracted to the surface by the sun in the middle of the day, and descend as that luminary declines--thus the lake will be clear, morning and evening, and turbid, or of different colors, in the course of the day. If stalks of flowers are steeped for a few days in water, it will be found to swarm with life; even a few dead leaves, or a bit of dry hay, will produce the same effect. At first monads will appear; these will be succeeded by specimens of the genera _Paramecium_, _Amoeba_, and those of the class _Rotatoria_. I have tried these experiments, and always with success. If the infusion be kept a few weeks (particularly that formed with leaves), one peculiar kind of animalcule will swarm to a most astonishing degree, so that a drop will contain hundreds, so close together that they form quite a crowd, and yet all are in a state of activity, and feeding from the vegetable matter disengaged from the decaying leaves. They are not even confined to these localities, for lakes and rivers, the fluids found in animals and vegetables, strong acids, and also the briny ocean, are full of these interesting creatures. One kind of phosphorescence (an appearance which is so often observed by the seaside and at sea) is occasioned by some species; and, when we remember that this luminosity often extends for miles, we are lost in astonishment at the immensity of their numbers.