Flagellata, a group of Infusoria (q.v.) belonging to the division Mastigophora (q.v.), and including those forms which swim by the action of one or more flagellas. A flagellum is a whip-like filament which can be lashed backward and forward by the animal. The two best-known members of the group are Noctiluca, which is the principal cause of the phosphorescence of the sea, and Euglena, which often occurs in such abundance in stagnant pools as to colour the water green. The members of the group are all microscopic in size.