The endoskeleton of echinoderms consists of a meshwork of calcite. Using light and electron microscopy, this study investigates a cortex covering the arms of crinoids. In Metacrinus rotundus, it consists of massive calcite and has a regular pattern of ridges and holes. The cortex is covered by thin extensions of epidermal cells whose cell bodies are located in the holes. The cells carry intracuticular cilia and seem to have contact with axons connecting to the central nervous system. The cilia probably have sensory function. We compared three other species of living stalked crinoids and two species of stalkless crinoids and found that they have a similar cortex with varying surface patterns, possibly due to various modes of life. The cortex of arms with its pattern seems to be a species-specific characteristic of crinoids. The ridges of the cortex might influence drag caused by currents or serve to facilitate current detection.