Starfishes are a familiar part of any day spent at the seaside. With their five fleshy 'arms', they are as much a part of the tide-pool panorama as crabs and mussels. Less familiar are the brittle-stars, with their spiny, snake-like arms.
Brittle-stars (or ophiuroids) are quite different from regular starfishes (technically, asteroids). The fleshy arms of asteroids are often more-or-less continuous with the central part of the body, and are relatively immobile. Asteroids don't move their arms to get around. Instead, they use thousands of tiny, extensible 'tube feet', borne on the undersurfaces of the arms, to carry the whole animal along. The arms of ophiuroids make a complete contrast. Rather than plump and fleshy, they are thin and almost solid, joined to a discrete, disc-like body and supported on an internal 'backbone' of calcite blocks, articulated just like the vertebrae in our backbone. The arm of an ophiuroid can coil like a snake: armed with five of these, the brittle-star is a surprisingly lithe and mobile mover.
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