CERATIUM

genus of single-celled aquatic organisms common in fresh water and salt water from the Arctic to the tropics. Its members are armoured dinoflagellates. (A dinoflagellate has two unlike flagella and has both plant and animal characteristics.) The genus is a member of the kingdom Protista, which incorporates microorganisms known as algae and protozoans. The cell is flattened horizontally and contains chromatophores with yellow, brown, or green pigments. The theca, or armour, is composed of many textured plates that form one anterior horn and usually two posterior horns; these horns slow the sinking of the organism. Genus members' body form varies according to salinity and temperature indicators, the spines tending to be short and thick in cold, salty water and long and thin in less salty, warmer water. Members of the genus form an important part of the plankton found in northern seas.