(subphylum Mastigophora), any of a group of protozoans, mostly uninucleate organisms, that possess, at some time in the life cycle, one to many flagella for locomotion and sensation. (A flagellum is a hairlike structure capable of whiplike lashing movements that furnish locomotion.) Many flagellates have a thin, firm pellicle (outer covering) or a coating of a jellylike substance. Reproduction is either asexual (usually by longitudinal splitting) or sexual. The flagellates are divided taxonomically into two classes, those resembling plants, Phytomastigophorea (see phytoflagellate), and those resembling animals, Zoomastigophorea (see zooflagellate).
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