Photosynthetic organisms of the order dinoflagellida (for botanists Dinophyceae). They are aquatic and have 2 flagella lying in grooves in an often elaborately sculptured shell or pellicle that is formed from plates of cellulose deposited in membrane vesicles. The pellicle gives some dinoflagellates very bizarre shapes. Their chromosomes lack centromeres and may have little or no protein and may perhaps be intermediate between pro and eu karyote types, hence the group has been termed mesokaryotic. The nuclear membrane persists during mitosis. They are very abundant in marine plankton. Gymnodinium and Gonyaulax, that causes red tide, produce toxins that if accumulated by filter feeding molluscs can be fatal. Another common genus is Peridinium.