A large and extremely diverse class of biflagellate algae (dinoflagellates) in the chlorophyll a–c phyletic line (Chromophycota). In protozoological classification, these organisms constitute an order, Dinoflagellida, of the class Phytomastigophora. Many taxonomists emphasize the distinctness of dinoflagellates by placing them in a separate division (Pyrrophyta or Pyrrhophyta) or even in a separate kingdom (Mesokaryota). More than 1200 species are known, classified into 18 orders and 54 families. Most are microscopic, but a few reach a diameter of 2 mm (0.08 in.). Cell shape is highly variable, with many planktonic species having elaborately modified surfaces. Dinoflagellates occur in marine, brackish, and fresh waters, frequently producing algal blooms. They may be benthic as well as planktonic, and a few are colonial. Ameboid, palmelloid, coccoid (with or without a gelatinous sheath), and filamentous forms are also known.