(sim-buy-o-din-ee-um). Dinoflagellates which are usually encountered as endosymbionts of invertebrates. They can also be maintained in culture. They can produce gymnodinioid cells. There is an circumferential groove (the girdle or cingulum) which wraps around the cell, and a longitudinal groove which extends from the point of flagellar insertion towards the back of the cell. The circumferential flagellum and the trailing flagellum are evident. Plastids with chlorophylls a and c. Differential interference contrast.