(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. The outermost region of cytoplasm attaches to the arms with contractile ligaments or myonemes about halfway along their length. The cells often have an orange or brown colour because of the presence of symbiotic dinoflagellates. Phase contrast.