Astrolithium (astro-lith-ee-um) is one of the more commonly encountered acantharea. These star-shaped organisms used to be grouped with polycystines and phaeodarea as radiolaria. These three groups all have inorganic skeletons to which the cytoplasm attaches, usually have an inner condensed area of cytoplasm and an outer more open region, and have a star-like morphology. Like other acantharea, the spines of Astrolithium are made of strontium salts which readily dissolve when the cells die. There are 20 radiating spines which are linked together at the centre of the cell. This image has been taken with polarised light so the the crystalline appearance of the arms is more evident. Phase contrast with polarised light.