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common name for echinoderms belonging to the class Ophiuroidea. The name is derived from their habit of breaking off arms as a means of defense. New arms are easily regenerated. They are also called serpent stars because of the snakelike movements of the five mobile, slender arms.

Brittlestars can be distinguished from sea starssea star,
also called starfish, echinoderm of the class Asteroidae, common in tide pools. Sea stars vary in size from under 1-2 in. (1.3 cm) to over 3 ft (90 cm) in diameter. They are commonly dull shades of yellow or orange, but there are many brightly colored ones as well.
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, or starfish, by their rounded central disk, sharply set off from the arms. They have the water-vascular system and tube feet common to all echinoderms; unlike sea stars, brittlestars lack open grooves (ambulacral grooves) on the lower surface of the arms, and the tube feet serve as tactile organs. Also unlike sea stars, brittlestars walk with their arms; only some species use the tube feet for locomotion. Each arm contains a series of jointed, bonelike internal calcite plates, or ossicles, which determine the freedom of arm movements. The body and arms of brittlestars are also protected by calcite plates, which in some species consist of arrays of microlenses that focus light onto a nerve bundle, acting like a compound eye. Brittlestars can move quickly and in any direction.

Individuals are relatively small, usually less than 1 in. (2.5 cm) across the central disk, although the arms may be quite long. They are inconspicuous and often nocturnal, living under rocks, among seaweed, or buried in the sand. All are marine species, feeding on detritus and small living or dead animals. The arms move the larger food masses to the mouth, where they are fragmented by a complex jaw apparatus. Tube feet move smaller particles to the mouth. As a rule, sexes are separate, and fertilization occurs in the open sea after gametes have been discharged. A characteristic armed larval stage, the ophiopluteus, undergoes a profound metamorphosis to produce the rayed adult form.

About 2,000 species of ophiuroids are known, and a number are common along American coasts. Brittlestars are classified in the phylum EchinodermataEchinodermata
[Gr.,=spiny skin], phylum of exclusively marine bottom-dwelling invertebrates having external skeletons of calcareous plates just beneath the skin. The plates may be solidly fused together, as in sea urchins, loosely articulated to facilitate movement, as in sea
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, class Ophiuroidea.

References in periodicals archive ?
2008) showed a reduction in larval survival and size, as well as increased larval abnormalities in the brittlestar Ophiothrix fragilis at pH 7.
In these waters they feed on worms, crustaceans, brittlestars and sand eels.
Brittlestar species that don't sense light don't have the arrays, the researchers found.
The population structure of the brittlestar Ophiura sarsi Lutkin in the Gulf of Maine and its trophic relationship to American plaice (Hippoglossoides platessoides Fabricius).
The brittlestar, on the other hand, makes its microlenses using a "bottom up" approach, in which successive layers of calcite are deposited onto an organic template in intricate patterns to form perfect crystalline lenses at the temperature of seawater.
Finally they've found the answer: The brittlestar doesn't have eyes--it is an eye
Already, expedition scientists feel they have discovered new species of invertebrates, including at least one new species of brittlestar.
The analysis of bony structures in the arms of the brittlestar Ophiocoma wendtii showed the presence of a regular array of spherical microstructures that look like lenses.
into the pink anemones and brittlestars of a coral reef
The Irish Sea also has lots of different habitats including honeycomb reefs made up of living worms and muddy beds home to sea urchins, Dublin Bay prawns and brittlestars.
Huge beds of brittlestars, a kind of starfish, which lie in dense numbers on the gravel seabed, waving their fragile arms to catch food.