They're echinoderms, members of the same club as sea urchins , sea cucumbers , sea lilies and sand dollars. A sea star's skeleton is made up of many plates that move like flexible joints. When sea stars are born, they're symmetrical. That means their right and left sides are exactly the same. But as they grow, they eventually develop five similar parts.
Some sea stars, like the purple sea star of the Gulf of Maine, have nine or ten arms.
If they lose an arm, or even most of their body, they are can grow those sections back. So long as one fifth of the central disk and at least one arm remains, the sea star can completely recover. Regeneration can take up to a year. The brittle star, a relative of the sea star, is named for its fragile appendages, which can easily break off if caught by a predator or under a rock tossed by the crashing tides. On the end of each arm is a small pigment eye spot that is sensitive to light.