Sea urchins belong to the order of echinoderms. The correct hedgehogs body shape is almost round, and they are constructed strictly five-way radial symmetry. The body of sea urchins usually nearly spherical, ranging in size from 2-3 cm to 30 cm; with a row of calcareous plates. Plates usually still connected and form a dense shell (shell), not allowing anyone to change form. Needles are sea urchins for movement, nutrition and protection. Some species are poisonous, as connected with a particular poisonous glands. Needle poisonous sea urchin is very dangerous as it will break easily isastream in the body, causing irritation and accelerating the spread of the venom.
Mouth and anus at the sea urchins are located in the center of the lower part of the body. Eat sea urchins all in a row: algae, sponges, bryozoans, molluscs, small starfish. And sea urchins themselves are prey for sea lions, lobsters, birds, sea stars (adults) and sea otters. Catching urchin, sea otter or long turns in his legs to flatten the needles, and then eats; or splits hedgehog stone... on your chest!
The flat sea urchins (also called irregular urchins) needles too, but they are small and thick, so that the surface of their bodies as if covered with velvet. Irregular urchins body shape flattened, and they have distinct anterior and posterior ends of the body.
Flat sea urchins living in shallow water, sandy beaches, nothing to bite into. So they swallow the heavy soil particles, creating a ballast for stability. And still the waves often carry them ashore.
If you were on the beach, you've probably seen rounded, with a characteristic pattern in the form of 5 petals that resemble coins, shells of these urchins. In America they are called "sand dollars".