The dinoflagellates are classified within the phylum Dinophyta. The 550 genera are typically unicellular and have armoured cell walls composed of distinctive thickened thecal plates, although some genera are naked. About half of the dinoflagellates are colorless and heterotrophic. The pigmented species are usually golden brown in color. There are several thousand species, of which only a couple hundred are found in freshwater habitats.   Armored dinoflagellates have thick thecal plates that fit tightly together to form a continuous covering for the cell. The regions between plates are known as sutures. As the cell grows, material is added to the edges of the plates to enlarge them, forming intercalary or growth bands.   The apical portion of the cell is known as the epitheca, and the posterior portion is the hypotheca. A groove called the cingulum encircles the cell, and a smaller groove called the sulcus extends posteriorly from the cingulum. Two flagella emerge from a pore where the sulcus and cingulum meet on the ventral surface of the cell. The transverse flagellum fits within the cingulum, while the longitudinal flagellum lies within the sulcus.