What is the name of the structure that forms the arm of a starfish?

The structure that forms the arm of a starfish is called a ray. This term specifically refers to the elongated, radiating appendages that extend from the central disk of the starfish. Each ray is part of the starfish's body and plays a crucial role in its movement and feeding. Starfish typically have five rays, although some species can have many more.

The other terms do not accurately describe the starfish's arm structure. "Fin" refers to a limb used by fish and other aquatic animals for swimming, which is not applicable to starfish, as they do not swim in the same way. "Appendage" is a general term for any limb or body part that extends from the main body; while it could loosely apply to the rays of a starfish, it lacks the specificity needed. "Claw" refers typically to a sharp, pinching appendage found in creatures like crabs and lobsters and does not relate to the structure of a starfish.