Sea urchins are small, globe-shaped animals with well developed spines. They are very closely related to Sand dollars. The name "urchin" is an old name for the round spiny hedgehogs that they look like. They are found on the sea floor in all oceans, most commonly in intertidal habitats and on shallow reefs. They are nocturnal, hiding in crevasses during the day and coming out at night to feed.

Sea urchins have tube feet between their spines which are attached to the animal's water vascular system. Their tube feet allow them to move about slowly looking as if they are walking on stilts.

They are usually from 3 to 13 centimetres with the largest species up to 36 centimetres in diameter. The internal organs are enclosed in a hard "shell" made of fused plates of calcium carbonate covered by a thin skin. The spines, often long and sharp, serve as protection from predators. The spines can cause a painful wound to humans. The largest spines can be 10 to 30 centimetres long They are attached to the "shell" with ball-and-socket joints (as in human shoulders) and can point in any direction.

Sea urchins are sensitive to touch, light, and chemicals in the water. They do not have a brain. Most sea urchins have five pairs of external gills, placed around the mouth. Their moutha are genrally made up of five calcium carbonate teeth or jaws containing a fleshy tongue-like structure.

Most sea urchins are grazers, feeding mainly on algae, but some are predators feeding on sea cucumbers, and other invertebrates including mussels, polychaetes, sponges, brittle stars and crinoids. Sea urchins are one of the favorite foods of sea otters and is also the main source of nutrition for wolf eels. Other predators include octopuses and triggerfish, which bite off the spines allowint them fish to crack open the shell of the urchin. They have a pharynx, oesophagus, intestines and a rectum.

Sea urchins are male or female and, in most species, the eggs are released into the water. It can take several months before the larva settle to the bottom and become adults. In some species, it can take up tp five years for adults to grow to their maximum size.

References

http://en.wikipedia.org/wiki/Sea_urchin
www.reefed.edu.au/home/explorer/animals/marine_invertebrates/echinoderms/sea_urchins
www.marineparks.wa.gov.au/fun-facts/sea-urchins.html