PLANKTON (Neo-Lat., from Gk. irXcrytcros, pl«nktos, wandering. from irXcfletp,plazein, to wan der). A collective term for the free floating or swimming organisms in oceans, lakes, or rivers. The oceanic plankton is more interesting and im portant than that of lakes and of rivers, inasmuch as it forms the basis for all oceanic life. Among the more important forms which are found in the plant plankton are blue-green algae, bacteria, and diatoms. The colors of certain waters are due to plankton organisms. For example, the Red Sea is colored by a red alga. Diatoms are found in especial abundance in the colder seas. Among the inure interesting structures and life habits of the plankton plants there may be noted a high devel opment of adaptations for active movements through the water, a type of structure which is found in plants only among the lower forms. An other very striking feature of the plankton is its periodicity; at various seasons its constitution varies widely and the plankton organisms are known to rise and sink more or less periodically.
The mechanism involved in these movements is not known.
In zotilop• the term is restricted to the pelagic life which drifts, the actively swimming surface forms constituting a separate assemblage—the 'nekton.' It consists mainly of 'jellyfishes' (Medusan, Siphonophora, and Ctenophora), as eidian, especially the salp[e (q.v.), and a great variety of pelagic larvae and minute crustacea with feeble powers of locomotion that are carried along almost passively by the oceanic currents. Much of this occurs in innumerable flocks, and sometimes tinges large areas of the sea by its color, or causes them to glow with phoshores cenee; and it furnishes food to many surface-feed ing pelagic animals.