The organisms which make up this "pelagic" fauna and flora may be grouped into two classes, which may be termed the "swimmers," or Necton, and the "drifters," or Plankton. The former include the larger and more active animals, such as fish, whales, and the like, whose movements are more or less independent of the movements of the water; the latter comprise the plant-life and the floating or feebly swimming animals that drift at the mercy of waves and currents. A great deal of attention has been given in recent years to the study of the plankton, and it has come to be recognized as filling a very important place in the balance of life in the sea. In the sea, as on land, all the animals are ultimately dependent on plants for their food. The larger and more conspicuous sea-weeds which grow on the sea-bottom, however, can only flourish in comparatively shallow water, and the region which they occupy forms only a narrow fringe round the land-masses of the globe. It is only necessary to look at a map of the world, showing the depth of the sea, to realize what an insignificant part of the area of the oceans contributes in this way to the food-supply of marine animals. The microscopic plant-life of the plankton, however, makes up for the individual minuteness of its constituents by their incalculable numbers. The lowly organisms known as "diatoms," familiar to the microscopist from the beauty of their flinty skeletons, are among the most numerous and important of these, and they are associated with a great variety of other single-celled algæ and allied organisms, some of them so minute that they pass through the finest silk plankton-nets, and have to be sought for by special methods of collection recently devised for the purpose. All these organisms possess the green colouring matter (chlorophyll) that enables them to live, as the higher plants do, on the carbon dioxide and other substances dissolved in the water. The smaller animals of the plankton feed on these vegetable organisms, and in their turn serve as food for larger animals. The Herring, the Mackerel, the gigantic Basking Shark, and the still more gigantic Greenland Whale, all feed directly on the animal plankton, and we have already seen that the animals of the deep sea depend entirely on the same source of food-supply. Further, very many of the bottom-living animals of shallow water swim at the surface in the early stages of their life, and feed on the other plankton animals and plants. Indeed, it is no exaggeration to say that "all fish is diatom" in the same physiological sense as "all flesh is grass," and the study of the plankton is thus of practical importance as well as of scientific interest.