Musculoskeletal system Brittle star

a field of soft coral callogorgia sp. brittle star symbionts

like echinoderms, ophiuroidea possess skeleton of calcium carbonate in form of calcite. in ophiuroids, calcite ossicles fused form armor plates known collectively test. plates covered epidermis, consists of smooth syncytium. in species, joints between ossicles , superficial plates allow arm bend side, not bend upwards. however, in basket stars, arms flexible in directions.

both ophiurida , euryalida (the basket stars) have 5 long, slender, flexible, whip-like arms, 60 cm in length. supported internal skeleton of calcium carbonate plates referred vertebral ossicles. these vertebrae articulate means of ball-in-socket joints, , controlled muscles. fused plates correspond parallel ambulacral plates in sea stars , 5 paleozoic families of ophiuroids. in modern forms, vertebrae occur along median of arm.

the ossicles surrounded relatively thin ring of soft tissue, , 4 series of jointed plates, 1 each on upper, lower, , lateral surfaces of arm. 2 lateral plates have number of elongated spines projecting outwards; these provide traction against substrate while animal moving. spines, in ophiuroids, compose rigid border arm edges, whereas in euryalids transformed downward-facing clubs or hooklets. euryalids similar ophiurids, if larger, arms forked , branched. ophiuroid podia function sensory organs. not used feeding, in asteroidea. in paleozoic era, brittle stars had open ambulacral grooves, in modern forms, these turned inward.

in living ophiuroids, vertebrae linked well-structured longitudinal muscles. ophiuroida move horizontally, , euryalina species move vertically. latter have bigger vertebrae , smaller muscles. less spasmodic, can coil arms around objects, holding after death. these movement patterns distinct taxa, separating them. ophiuroida move when disturbed. 1 arm presses ahead, whereas other 4 act 2 pairs of opposite levers, thrusting body in series of rapid jerks. although adults not use tube feet locomotion, young stages use them stilts , serve adhesive structure.