Solitary, motile, bean-shaped cells without metabolic movement and with the dorsal surface often convex. Two heterokont flagella inserted side by side on the ventral surface. Ventral furrow subtending the flagellar insertion, extending obliquely anteriorly. A single, bright yellow chloroplast, lacking a stigma and with a pyrenoid transected by a pair of thylakoids. A multilayered external cell covering present.