Ophiuroids, or brittle stars, are star-shaped echinoderms with five slender arms radiating from a distinct, circular, central disk (Figs 7.5 and 7.6). The arms are extremely flexible and are formed of specialized vertebrae-like plates. The mouth is on the lower surface at the center of the disk.
Ophiuroids gather food with their arms. They are very mobile and are able to coordinate their arm movements to allow for relatively rapid crawling and swimming. Some species live in shallow water, although most prefer deeper water environments (below 500 m) where there are fewer predators.
Arm: extremely flexible. Interlocking ossicles surround a central muscle. Two rows of tube feet occur on the undersurface of the arm
Disk: contains the viscera. The madreporite is on the oral surface of the disk
The fossil record of ophiuroids is poor. The skeleton fragments easily and complete specimens are extremely rare. Ophiuroids originated in early Ordovician times and their skeletal structure has remained generally unchanged since then. Modern ophiuroids are very diverse and form the largest living echinoderm class.
Fig. 7.5 Ophiuroid morphology.
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