There are about 2,000 species of brittle stars which are close relatives of sea stars. They consist of two major groups - brittle stars and basket stars. Most are found at depths greater than 500 metres to greater than six kilometres although some species can tolerate brackish water which is very unusual for Echinoderms. They normally move slowly along the sea bed.

Most Brittle stars have five long slender, whip-like arms which can 60 centimetres in length on the largest species although most are under 2.5 cm in diameter. They usually become sexually mature at around two years, become fully grown in 3 to 4 years, and live up to five years.

Brittle stars have a skeleton made of calcium carbonate, found as the mineral calcite which forms small bone-like structures. These "bones" are fused together to form an armour-like shell.

In Brittle stars all the five arms (with tbe feet) are attached to the central disc which contains all the body organs. The mouth, on the underside of the body, is surrounded by five jaws. The mouth also acts as the anus, food is digested in 10 pouches or infolds of the stomach pouch.

Most Brittle stars are scavengers or detrivores eating decaying matter and plankton. Some are predators, pushing their stomach out through their mouth to digest their prey. Basket stars are suspension feeders, using the mucus coating on their arms to trap plankton and bacteria. They extend one arm out and use the other four to anchor them in place.

Brittle stars have no brain, eyes or any other specialised sense organs. However, they have several types of nerve ending in their skin and can sense chemicals in the water, touch, and even the presence or absence of light.

Most Brittle stars are either males or females and fertilisation takes place in the water. In some species, the eggs develop in sack-like body cavities called bursae. In a few species the embryos are fed by nutrients which pass through the walls of the bursae.

Brittle stars can regenerate lost arms or arm segments and use this to escape predators, such as some gastropods, some fish, crabs and shrimps and other echinoderms like starfish.They are also vulnerable to attacks by parasitesincluding protozoans, nematodesand algae.

They do not, like sea stars, depend on tube feet, which are mere sensory tentacles without suction. Brittle stars move fairly rapidly by wriggling their arms which are highly flexible and enable the animals to make either snake-like or rowing movements.

References

http://en.wikipedia.org/wiki/Brittle_star
http://ezinearticles.com/?Brittle-Stars---Why-Are-They-Brittle?&id=3964833
www.enchantedlearning.com/subjects/invertebrates/echinoderm/Brittlestar.shtml