Echinoderm adults are radially symmetrical invertebrates with three unique features: five-part symmetry; an internal calcium carbonate skeleton; and a distinctively divided coelom, including a water vascular system. Many have spines, shields, hooks, and scales on both oral and aboral (away from the mouth) surfaces and arms. The echinoderm coelom includes a body coelom as well as coelom derivatives - a water vascular system and one or more closed, fluid- or tissue-filled duct systems called the hemal and perihemal systems, which have an array of functions.