Taxonomy Brittle star

the 1900 extant species placed in 230 genera, grouped in 3 orders living: oegophiurida, phrynophiurida, , ophiurida. also, paleozoic order exists, stenurida.

the relationships among ophiuroids , other echinoderms provide enduring problem in invertebrate evolution. developmental , other studies based on modern organisms imply asteroids , ophiuroids not closely related within echinoderms. stenurid morphology, in contrast, suggests close common ancestry two; nature of ambulacral plates important, general form transitional.

stenurida (extinct)

this paleozoic (ordovician–devonian) order, bearing double row of plates (ambulacra) abut across arm axis either directly opposite 1 or offset. in contrast, modern ophiuroids have single series of axial arm plates termed vertebrae. in stenurids, in modern ophiuroids, lateral plates present @ sides of ambulacrals, , prominent lateral spines typical. stenurids lack dorsal , ventral arm shields found in ophiuroids. proximal ambulacral pairs can partially separated, forming buccal slit, expansion of mouth frame. arms of stenurids slender , flexible, of others broad , comparatively stiff. central disk varies little larger juncture of arms expansion extends of length of arms. content of order poorly established, , fewer 10 genera known.