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protoctist Any member of the kingdom Protoctista (formerly Protista), it includes such widely differing groups as algae (including large seaweeds), amoebas and other protozoa, slime moulds, and downy mildews. The term was introduced to overcome the difficulty of distinguishing such organisms from the true plant and animal kingdoms (Plantae and Animalia). This kingdom may include organisms with many nuclei within one cell wall (coenocytes).