EVERY piece of water, besides containing large plants and animals which are readily visible to the naked eye, harbours a more or less considerable number of minute forms, which pervade all the layers of the water in varying amount, and collectively constitute the plankton, or pelagic life. The most important difference between the plankton and the remaining flora and fauna of our waters lies in the fact that all the organisms which compose it are free-floating during the greater part of their life. Practically all the pelagic plants belong to the group of the algie, and their minute size, of course, suits them well to a floating existence. A certain number of them are motile (e.g. Volvox, Gonium, Pandorina, &c.), and these are able actively to maintain themselves in their position in the water; but the large majority are non-motile, and all these forms are slightly heavier than water, and consequently tend to sink; they develop diverse mechanisms, by means of which their power of flotation is increased. The most important of these are:—assumption of a flat plate-like shape (Pediastrum, Merismopedia, many Desmids); development of numerous delicate processes from the body of the plant (Stephanodiscus, Richteriella); arrangement of the individuals of a colony in a more or less stellate manner (Asterionella, some Tabellarias); assumption of delicate acicular shape (Synedra); formation of fat in the cell (many Diatoms and Cyanophycee), and so on.