Ophiurida
The Ophiurida are an order of echinoderms within the class Ophiuroidea. It includes the vast majority of living brittle stars. Characteristics. Ophiurida have bursae for respiration and excretion, and dorsal and ventral arm shields are present and usually well developed. Arms are unbranched and incapable of coiling vertically. Most are five-armed, some with 4 or 6 arms as an abnormality, but others properly bear six or seven arms. The madreporite is on the oral surface. The digestive glands are entirely within the central disc. They move their arms side to side by means of ball-and-socket joints . Tropical species tend to contrast color from the environment, but most others prefer to blend in. These biochromes do not include echinochromes. Systematics and phylogeny. There is currently no consensus as to the subdivision of the Ophiurida. The order has been divided into the following suborders and infraorders Another classification scheme divides it into the following suborders: Another classification scheme divides it as: Ecology. Ophiurida have a world-wide distribution range and are found in oceans in different depths. Most of them are herbivores or detritus feeders.