Anatomy Brittle star

1 anatomy

1.1 water-vascular system
1.2 nervous system
1.3 digestion
1.4 respiration
1.5 musculoskeletal system

anatomy

asteriacites, trace fossil of ophiuroid; carmel formation (middle jurassic), near gunlock, utah; scale bar 10 mm.

of echinoderms, ophiuroidea may have strongest tendency toward five-segment radial (pentaradial) symmetry. body outline similar of starfish, in ophiuroids have 5 arms joined central body disk. however, in ophiuroids, central body disk sharply marked off arms.

the disk contains of viscera. is, internal organs of digestion , reproduction never enter arms, in asteroidea. underside of disk contains mouth, has 5 toothed jaws formed skeletal plates. madreporite located within 1 of jaw plates, , not on upper side of animal in starfish.

the ophiuroid coelom reduced, particularly in comparison other echinoderms.

green brittle star - ophiarachna incrassata


water-vascular system

the vessels of water vascular system end in tube feet. water vascular system has 1 madreporite. others, such euryalina, have 1 per arm on aboral surface. still other forms have no madreporite @ all. suckers , ampullae absent tube feet.

nervous system

the nervous system consists of main nerve ring runs around central disk. @ base of each arm, ring attaches radial nerve runs end of limb. nerves in each limb run through canal @ base of vertebral ossicles.

most ophiuroids have no eyes, or other specialised sense organs. however, have several types of sensitive nerve endings in epidermis, , able sense chemicals in water, touch, , presence or absence of light. moreover, tube feet may sense light odors. these found @ ends of arms, detecting light , retreating crevices.

digestion

the mouth rimmed 5 jaws, , serves anus (egestion) mouth (ingestion). behind jaws short esophagus , large, blind stomach cavity occupies of dorsal half of disk. ophiuroids have neither head nor anus. digestion occurs within 10 pouches or infolds of stomach, ceca, unlike in sea stars, never extend arms. stomach wall contains glandular hepatic cells.

ophiuroids scavengers or detritivores. small organic particles moved mouth tube feet. ophiuroids may prey on small crustaceans or worms. basket stars in particular may capable of suspension feeding, using mucus coating on arms trap plankton , bacteria. extend 1 arm out , use other 4 anchors. brittle stars eat small suspended organisms if available. in large, crowded areas, brittle stars eat suspended matter prevailing seafloor currents.

in basket stars, arms used rhythmically sweep food mouth. pectinura consumes beech pollen in new zealand fjords (since trees hang on water). eurylina clings coral branches browse on polyps.

respiration

gas exchange , excretion occur through cilia-lined sacs called bursae; each opens between arm bases on underside of disk. typically ten bursae found, , each fits between 2 stomach digestive pouches. water flows through bursae means of cilia or muscular contraction. oxygen transported through body hemal system, series of sinuses , vessels distinct water vascular system.

the bursae main organs of excretion, phagocytic coelomocytes collecting waste products in body cavity , migrating bursae expulsion body.

musculoskeletal system

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.