Dinoflagellates of the supergroup Alveolata and their characteristic alveoli.

(a) Sac-shaped membranous vesicles known as alveoli lie beneath the plasma membrane of a dinoflagellate, along with trichocysts ready for discharge. (b) The surface of Peridiniopsis berolinensis appears smooth because the alveoli seem empty. Two types of flagella are seen on this freshwater dinoflagellate. One flagellum coils around a cellular groove; as it moves, this flagellum causes the cell to spin. By contrast, a straight flagellum extends from the cell, acting as a rudder to determine the direction of backward or forward movement. (c) The alveoli of the marine dinoflagellate genus Ornithocercus contain cellulose cell-wall plates.