Sea urchins are the Class Echinoidea of the Phylum Echinodermata. Like the rest of the Echinoderms, literally "spike-skinned", they are entirely marine. They are usually globe-shaped, and protected by calcareous plates and spines.p65 Urchin is an old word for hedgehog, and in many foreign languages these animals are called "sea hedgehogs".

Like other echinoderms they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive 'tube feet'. The symmetry is not obvious in the living animal, but is easily visible in the dried test.

Sea urchins mostly feed on algae and small animals. They have a special chewing apparatus called Aristotle's lantern, after the Greek philosopher Aristotle who was fascinated by sea urchins. With this apparatus they can scrape organisms stuck to the surface over which the urchin is moving.

Spines
The spines are long and sharp in some species, and protect the urchin from predators. The spines inflict a painful wound when they penetrate human skin, but are usually not dangerous. Echinoids also have pincer-like pedicellaria all over their body between the spines. The job of these is to deal with anything (like larvae) settling on the test (outer shell).p101 It is not known if the spines are venomous (unlike the pedicellariae between the spines, which are venomous).

Typical sea urchins have spines that are  in length,  thick, and not terribly sharp. Diadema antillarum, familiar in the Caribbean, has thin, potentially dangerous spines that can reach  long.

Teeth

Sea urchins have five jaws with one tooth each.  The teeth grow throughout the sea urchin's life.  The teeth of the pink sea urchin chip against each other to stay sharp.

Ecology

Sea urchins feed mainly on algae, but can also feed on sea cucumbers, and a wide range of invertebrates such as mussels, polychaetes, sponges, brittle stars and crinoids. Sea urchins are one of the favorite foods of sea otters and wolf eels. Without predators to eat them, urchins breed in large numbers and eat almost all the nearby kelp, creating an urchin barren, devoid of macroalgae and associated fauna. Sea otters have re-entered British Columbia, dramatically improving coastal ecosystem health by eating sea urchins.

Fossil record and evolution 
The first echinoid fossils are from the Lower Ordovician period. The earliest forms had flexible tests, with plates that could slide over each other. Echinoids were a relatively unimportant part of the biota in the Palaeozoic. Only one group survived the P/Tr extinction event to form the basis of all later echinoids. The group which survived into the Triassic, the Cidaroids, radiated into all other modern groups, which are known as the Euechinoids.

The break with perfect symmetry, which happened in the Jurassic, gave them definite front and back ends. This opened up new habitats, in particular, the burrowing habitat. Sand dollars and heart urchins have been highly successful. The living echinoids are now important members of the biota, especially in shallow and in-shore waters.

Sea urchins as food 
Some humans eat the reproductive organs of sea urchins (which they call roe). This is popular in Korea. In Japan, this kind of sushi is called uni.

References 

Echinoderms