A class of Pyrrophyta, comprising algae that are unicellular and have two flagella (thread-like structures) of unequal length. Most dinoflagellates belong to this class and the cysts (dinocysts) are useful in biostratigraphy. The organisms have two biological stages. (a) In the motile (thecate) stage (see theca) the organism may have either a flexible cell wall or a rigid, armoured one, and it maintains itself in the water by active movement of the flagella. The cell surface bears two furrows, each holding one flagellum. The transverse furrow is called the ‘cingulum’, the longitudinal one the ‘sulcus’. The cingulum divides the cell into an anterior epitheca and a posterior hypotheca. The apex of the epitheca is sometimes extended to form an apical horn. (b) In the cyst stage the organism is dormant. When encystment occurs a two-layered cyst wall (phragma) is formed. Proximate cysts develop in the cyst wall, in contact with the wall from the motile stage. Chorate cysts develop deeper in the motile cell and are linked to the cell by processes. Cavate cysts are those where the two layers of the wall are separated by cavities. The organism leaves the cyst by an opening (archaeopyle) when conditions change. There are three important orders: Gymnodiniales, Peridiniales, and Dinophysiales.