Radiole

A radiole (plural: radioles) is a calcareous spine found on sea urchins. These spines articulate with tubercles on the test (shell) of the sea urchin, allowing for movement and defense. The size, shape, and arrangement of radioles vary significantly between different species of sea urchins and can be useful for identification. The radioles are attached to the test via a ball-and-socket joint, and muscles control their movement. While primarily serving as a protective mechanism against predators, radioles also assist in locomotion, burrowing, and camouflage.