Amphidinium (am-fee-din-ee-um) testudo 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. This photograph is of the ventral side and shows the recurrent flagellum. Differential interference contrast.