PLANKTON, a term given by Haeckel to the life, both animal and plant, which occurs floating at the top of lakes and seas, in contrast to the forms of the shores or bottoms. The vegetable element in the plankton is largely diatoms while the animals show a much greater variety. In the fresh waters the plankton is very inconsiderable as compared with that of the sea and to the latter most of these statements apply. The marine plankton consists of both young and adult animals. The adults are largely protozoans, jellyfish, tunisates (especially Satpa, Doliolum, Pyrosoma, Appeisdicularia), together a few worms. The young include repre sentatives of almost every group of marine animals for the larva of many shore or bottom animals are free-swimming for a time. One inal-ked feature of these surface forms is their :Zit It transparency and almost total absence of col ir, except the black pigment in the eyes of the eyed forms.
The plant life of the oceanic plankton in cludes diatoms, bacteria, blue-green alga and similar low organisms. The first are especially conspicuous in very cold waters. In some cases the prevailing organism is so abundant that the water is distinctly tinted by it, as in the Red Sea, where a red alga is especially numerous. Many of the plants have powers of locomotion well developed, a feature peculiar to low forms of plant life. The fresh-water vegetable plank
tons are less noteworthy than the marine be cause of their smaller extent, the much smaller number of species generally represented, etc. These formations are of particular interest from an economic point of view because they constitute the pastures of the deep. The minute animals feed more or less upon the plant life, great numbers of free-swimming, surface inhabiting marine animals feed upon the plants or the animals or both, and are in turn the food of other pelagic animals or birds. At certain seasons the leading forms of the plank ton disappear and others take their places. Generally they sink to lower levels to reappear after a more or less definite time. To the student of zoology the plankton is of great in terest both from its importance as a food sup ply for marine forms and as a means of ob taining the larva: of many animals. It is studied by collecting the life in a net of bolt ing cloth drawn along the surface and then ex amining the catch with the microscope. The plankton varies greatly from day to day, and is markedly more abundant at night than in the day. Many of the forms, notably the medusa, worms and young crustacea. are markedly phos phorescent and to them is due the light in a vessel's wake.