PLANKTON is the name which is now given to the drifting (and usually microscopic) life contained in the waters of the seas, lakes, ponds and streams. Its study was begun, just prior to 1845, by the great German zoologist, Johannes Muller, in the sea round Heligoland. Associated with Willer, in 1845, were his pupils, Max Muller, Busch, Wilms, Wagener and others. Ernst Haeckel, as a student of 20 years of age, was with J. Miiller, at Heligoland, in 1854, and began to study the plankton. "When," said Muller, "you have once entered this pelagic, magic world you will not easily leave it again." Rapid extension of plankton investigation followed, both in Germany and in Great Britain, and the great names associated with this branch of 19th century marine biology are Heinrich Muller, A. Kolliker, C. Gegenbaur, Leuckart, Carl Vogt, T. H. Huxley and McIntosh.
In 1873-76 the "Challenger" made her voyage of circumnaviga tion and great collections of plankton were made in all the seas of the world. Many other voyages have since been made but there has been none that did so much work as was carried out during those three years under the direction of Wyville Thomson and John Murray. Another equally great landmark was the "Plankton Expedition" carried out by the "National." Victor Hensen, of the University of Kiel, led this expedition (which worked in the Atlantic ocean) and with him were associated Brandt, Schiitt, Fischer and KrUmmell. All subsequent work on the plankton has been dominated by the methods elaborated by these workers and by the extraordinarily fertile ideas of Hensen, and these methods, with their results, make up the modern science of hydrobiology.
large numbers of these microscopic organisms and sometimes they are so abundant that the water may be coloured, or turbid, and they may even tint snow and ice. A practical distinction is made between the Macroplankton, which can be caught by filter ing the water through very finely woven silk cloth, and the Microplankton, which are so small that they escape through the pores of the finest fabrics that can be employed. Later on we shall consider more fully the methods of collecting the plankton. A further practical distinction, in discussing distribution, is that between the Transitory Plankton and the Permanent Plankton. The former category includes all those organisms which have a brief, pelagic, drifting stage in their life history, after which they develop into actively swimming, large animals (the Nekton), or into rooted or sedentary plants and animals (the Benthos). The latter category includes a multitude of species of organisms which live throughout their lives as planktonic, passively drifting individuals. It is customary to distinguish between Oceanic and Neritic Plankton, the former inhabiting water which is far from land and the latter being practically restricted to the zone of shallow water near the Continental coasts. Then there are many plankton organisms that prefer to live near the surface of the sea and these we call the Epiplankton, while there are others that habitually live in the deeper layers of the ocean : these make up the Bathyplankton. Finally the different oceans, seas, zones of latitudes, etc., tend to harbour characteristic plankton com munities, so that a naturalist who is very familiar with plankton investigations can often say where an unknown sample has been collected.