A small class of biflagellate unicellular algae (cryptomonads) in the chlorophyll a–c phyletic line (Chromophycota). In protozoological classification, these organisms constitute an order, Cryptomonadida, of the class Phytomastigophora. Cryptomonads are 4–80 micrometres long, ovoid or bean-shaped, and dorsoventrally flattened. The cell is bounded by a moderately flexible periplast comprising the plasmalemma and underlying rectangular or polygonal proteinaceous plates. A tubular invagination (gullet, groove, or furrow) traverses the ventral cytoplasm and opens just below the apex of the cell. A pair of subequal flagella, which are covered with hairs and small scales, arise from the centre or apical end of the gullet.