The genus Amphidinium (am-fee-din-ee-um) is a commonly encountered and speciose member of the dinoflagellates. There is a circumferential groove on the cell surface that runs around the anterior half of the cell, and a second one running along the length of the body. There is a flagellum in each groove. The asymmetrical location of the horizontal groove gives this genus its name (cf. Gymnodinium). Swimming species, with plastid and red inclusion. Differential interference contrast.