Ophiurida

The Ophiurida are an order of echinoderms belonging to the class Ophiuroidea, containing most living brittle stars. They use bursae for respiration and excretion, and their arm shields—both dorsal and ventral—are typically well developed. Their arms are unbranched and cannot coil vertically.

While most individuals have five arms, some may have four or six as an abnormality, and certain species naturally bear six or seven arms. The madreporite is located on the oral surface, and the digestive glands are confined to the central disc. Arm movement is side-to-side via ball-and-socket joints.

Tropical species often have colors that contrast with their surroundings, while others tend to be camouflaged; their biochromes do not include echinochromes. There is no agreed-upon subdivision of the order, with several competing classification schemes proposing different suborders and infraorders, such as Ophiomyxina, Ophiurina, Hemieuryalina, Chilophiurina, Gnathophiurina, Ophiodermatina, and Ophiolepidina, or alternatively Chilophiurina, Laemophiurina, Ophiothricidae, and Ophiurina. Ophiurida are found worldwide in oceans at various depths, and most are herbivores or detritus feeders.