There are roughly 1900 extant species in 230 genera, grouped in the three orders currently living: Oegophiurida, Phrynophiurida, and Ophiurida. There is also a Paleozoic order, the Stenurida.The relationships among ophiuroids and all other echinoderms provide an enduring problem in invertebrate evolution. Developmental and other studies based on modern organisms imply that asteroids and ophiuroids are not closely related within the echinoderms. Stenurid morphology, in contrast, suggests a close common ancestry for the two; the nature of the ambulacral plates is important, but even their general form is transitional.[edit]Stenurida (extinct)This is a Paleozoic (OrdovicianDevonian) order, bearing a double row of plates (ambulacra) that abut across the arm axis either directly opposite one another or slightly offset. In contrast, modern ophiuroids have a single series of axial arm plates termed vertebrae. In stenurids, as in modern ophiuroids, lateral plates are present at the sides of ambulacrals, and prominent lateral spines are typical. Stenurids lack the dorsal and ventral arm shields that are found in most ophiuroids. Proximal ambulacral pairs can be partially separated, forming a buccal slit, an expansion of the mouth frame. The arms of some stenurids are slender and flexible, but those of others are broad and comparatively stiff. The central disk varies from little larger than the juncture of the arms to an expansion that extends most of the length of the arms. The content of the order is poorly established, and fewer than 10 genera are known.

http://en.wikipedia.org/wiki/Brittle_star#Taxonomy

One of the most emblematic animals from Antarctic waters is the euryalid Astrotoma agassizii Lyman, 1875. Gigant, long-lived, with a disk diameter of up to 70 mm and an age which could overpass 100 years; Dahm (1996) calculated, by the growth rings of the arm vertebrae, the age of 90 years for a specimen of disc

less than 50 mm.

Astrotoma is a predator, who climbs up the gorgonians from Primnoella and Thouarella genus, waits patiently for the arrival of copepods which will be caught by its ambulacral tentacles Dearborn et al (1986). His rough and tough skin defends it from the nematocysts that the polyps of the Gorgon throw without success. Astrotoma grabs the gorgonia with its modified arm spines and the hooklets that cover the skin of its arms arranged in rings.

http://brittlestars.wordpress.com/page/2/

http://www.racerocks.com/racerock/eco/taxalab/ariolr.htm

http://www.seaworld.org/animal-info/animal-bytes/animalia/eumetazoa/coelomates/deuterostomes/echinodermata/brittle-stars.pdf