sentences:  They have a rigid, usually spherical body bearing moveable spines, which gives the class the name Echinoidea (from the Greek "ekhinos", spine).The name "urchin" is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs.The name is derived from Old French "herichun", from Latin "ericius", hedgehog.