410 FOSSIL INVERTEBRATES/Porifera Figure Nomenclature of common megascleres or main body spicules, which may range from a fraction of a millimetre to several millimetres in length Shapes and compositions of megascleres are important elements in classification Triactines, for example, are commonly calcareous and representative of the Calcarea, and hexactines and derivative spicules, such as stauractines and pentactines, are siliceous and characterize the Hexactinellida