Dinoflagellates include flagellates with and without plastids or perhaps with symbionts. Mostly with two flagella, but a few multiflagellated or aflagellated forms also exist. Flagella insert on the ventral face of the cell in most species, and have a paraxial rod associated with the axonemes. One flagellum, termed longitudinal, is directed posteriorly along the sulcus, a longitudinal groove in the body. The second flagellum is directed circumferentially around the cell and lies within a groove termed the cingulum or girdle and has a spiral beat pattern. The position and arrangement of the sulcus and cingulum, together with cell shape, provide the basic taxonomic criteria for many dinoflagellates. Most dinoflagellates have a very distinctive haploid nucleus (dinokaryon) containing large chromosomes which are permanently condensed. Mitosis takes place within the nuclear envelope with the aid of an extra-nuclear spindle. The alveoli have been referred to as amphiesma, and may or may not include organic plates. Species with thick plates are referred to as 'armoured'. The patterns of dinoflagellate plates, known as tabulation, can be useful in identification at various levels. Sexual reproduction has been described in a number of dinoflagellates. About half of the dinoflagellates have chloroplasts. Includes a variety of taxa associated with producing toxins, includes also symbionts of coral reefs.