These are not only an interesting but an almost indispensably useful portion of the inhabitants of every well regulated aquarium. They serve to keep down the confervas, to remove decaying vegetable matter; and when well supplied with these they multiply very rapidly, and their eggs and young form excellent food for the fishes. The variety that may be found is very large, and every stagnant pond swarms with them. We would advise our readers to select a few of all the kinds that can be found, work out their names by studying books on the subject, and carefully watch their habits. Some of them have a curious habit of swimming on their backs on the surface of the water. They adhere to the smooth surface of the glass by "suction," creep along it with a curious motion, and literally "mow" off the confervee with their tongues. All this can be easily seen through the glass by means of a good lens. The eggs, also, are most interesting objects. They are frequently deposited on the glass, and can then be studied very readily by means of a microscope. After a short time the young snail can be seen distinctly in the egg, and its motions easily observed. For tanks of fair size the fresh-water mussel is an interesting and beautiful inhabitant. Its pearl-white mantle and gorgeously colored shell are features which even the most unenthusiastic must admire.